Developments in Petroleum Science, 26
hydraulic proppant fracturing and gravel packing
DEVELOPMENTS I N PETROLEUM SC...
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Developments in Petroleum Science, 26
hydraulic proppant fracturing and gravel packing
DEVELOPMENTS I N PETROLEUM SCIENCE Advisory Editor: G.V. Chilingarian Volumes 1 , 3 , 4 , 7 and 13 are out of print. 2. W.H.FERTL ABNORMAL FORMATION PRESSURES T.F. YEN and G.V. CHILINGARIAN (Editors) 5. OIL SHALE 6. D.W. PEACEMAN FUNDAMENTALS OF NUMERICAL RESERVOIR SIMULATION 8. L.P.DAKE FUNDAMENTALS OF RESERVOIR ENGINEERING 9. K.MAGARA COMPACTION AND FLUID MIGRATION M.T. SILVIA and E.A. ROBINSON 10. DECONVOLUTION OF GEOPHYSICAL TIME SERIES IN T H E EXPLORATION FOR OIL AND NATURAL GAS 11. G.V. CHILINGARIAN and P. VORABUTR DRILLING AND DRILLING FLUIDS 12. T.D. VAN GOLF-RACHT FUNDAMENTALS OF FRACTURED RESERVOIR ENGINEERING 14. G. MOZES (Editor) PARAFFIN PRODUCTS 15A O.SERRA FUNDAMENTALS OF WELL-LOG INTERPRETATION 1. T H E ACQUISITION OF LOGGING DATA
15B O.SERRA FUNDAMENTALS OF WELL-LOG INTERPRETATION
2. T H E INTERPRETATION OF LOGGING DATA
16. R.E. CHAPMAN PETROLEUMGEOLOGY 17A E.C. DONALDSON, G.V. CHILINGARIAN and T.F. YEN ENHANCED OIL RECOVERY, I
FUNDAMENTALS A N D ANALYSES
18A A.P. SZILAS PRODUCTION AND TRANSPORT OF OIL AND GAS A. FLOW MECHANICS A N D PRODUCTION
second completely revised edition
18B A.P. SZILAS PRODUCTION AND TRANSPORT OF OIL AND GAS
B. GATHERING A N D TRANSPORTATION second completely revised edition
19A G.V. CHILINGARIAN, J.O. ROBERTSON Jr. and S. KUMAR SURFACE OPERATIONS IN PETROLEUM PRODUCTION, I 19B G.V. CHILINGARIAN, J.O. ROBERTSON Jr. and S. KUMAR SURFACE OPERATIONS IN PETROLEUM PRODUCTION, I1 A.J. DIKKERS 20. GEOLOGY IN PETROLEUM PRODUCTION 21. W.F. RAMIREZ APPLICATION OF OPTIMAL CONTROL THEORY TO ENHANCED OIL RECOVERY E.C. DONALDSON, G.V. CHILINGARIAN and T.F. YEN (Editors) 22. MICROBIAL ENHANCED OIL RECOVERY 23. J. HAGOORT FUNDAMENTALS OF GAS RESERVOIR ENGINEERING 24. W. LITTMANN POLYMER FLOODING N.K. BAIBAKOV and A.R. GARUSHEV 25. THERMAL METHODS OF PETROLEUM PRODUCTION
Developments in Petroleum Science, 26
hydraulic proppant fracturing and gravel packing DETLEF MADER Paul-Hindemith-Ring4B 0 - 6 0 5 4 Rodgau 1 (Jiigesheim),F . R . G .
ELSEVIER -Amsterdam - Oxford - New York - Tokyo 1989
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands
Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655 Avenue of the Americas New York, NY 10010, U.S.A.
ISBN 0-444-87352-x (Vol. 26) ISBN 0-444-41625-0 (Series)
0Elsevier Science Publishers B.V., 1989 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./Physical Sciences & Engineering Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Printed in The Netherlands
V
P r e f a c e H y d r a u l i c p r o p p a n t f r a c t u r i n g i s a r e l a t i v e l y young t e c h n i q u e o f hydrocarbon pay s t i m u l a t i o n which has been i n v e n t e d o n l y a b t . 40 y e a r s ago (CLARK 1948, K H R I S T I A N O V I C H & ZHELTOV 1955) and which e n t e r e d t h e f o c u s o f a t t e n t i o n by r e s e r v o i r and t e c h n i c a l e n g i n e e r i n g p a r t i c u l a r l y d u r i n g t h e l a s t t e n y e a r s s i n c e t h e i n v e n t i o n o f s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h ceramic proppants t h a t can w i t h s t a n d t h e h i g h c l o s u r e s t r e s s e s e s p e c i a l l y i n deep t i g h t gas-beari n g sandstone pay f o r m a t i o n s (COOKE 1976, 1977). The p o s s i b i l i t y o f now t r e a t i n g a l s o h i g h - p r e s s u r e hydrocarbon-bearing i n t e r v a l s where n a t u r a l sand crushes as a consequence o f e l e v a t e d c l o s u r e s t r e s s t r i g g e r e d c o n s i d e r a b l e expansion o f h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n which c u l m i n a t e d i n v a r i o u s MHF camp a i g n s p r e d o m i n a n t l y i n USA and Europe where up t o 1 M i o . g a l o f f l u i d s and more than 6 Mio. l b s (3,000 t ) o f proppants have been pumped i n t o l a r g e - s c a l e f r a c t u r e s w i t h i n a s i n g u l a r jumbo t r e a t m e n t and p r o p p a n t q u a n t i t i e s o f 200 500 t p e r j o b a r e q u i t e common i n l a r g e - s c a l e h y d r a u l i c r e s e r v o i r s t i m u l a t i o n . The f i r s t f i e l d a p p l i c a t i o n o f s i n t e r e d b a u x i t e ( w h i c h was t h e e a r l i e s t a r t i f i c i a l high-qua1 i t y p r o p p a n t ) i n Western Europe o n l y d a t e s back almost e x a c t l y t e n y e a r s (ERDOEL-ERDGAS AKTUELL 1978 a ) . Since t h a t time, n o t o n l y t h e cumul a t i v e q u a n t i t y o f proppants pumped a n n u a l l y downhole was p r o g r e s s i v e l y i n c r e a s i n g , b u t a l s o t h e amount o f t e c h n i c a l l i t e r a t u r e on h y d r a u l i c proppant f r a c t u r i n g was e x p o n e n t i a l l y growing. Much hope f o r f u t u r e booming o f hydrocarbon app r a i s a l and development was based on h y d r a u l i c proppant f r a c t u r i n g o f low-perm e a b i l i t y gas and o i l r e s e r v o i r s p r e d o m i n a n t l y d u r i n g t h e l a s t f i v e y e a r s , and whole e x p l o r a t i o n and a p p r a i s a l d r i l l i n g s t r a t e g y concepts i n p e t r o p h y s i c a l l y p o o r e r f a c i e s b e l t s o f sedimentary b a s i n s were founded on t h e c e r t a i n t y o f i n e v i t a b l e s t i m u l a t i o n b e f o r e w e l l c o m p l e t i o n i n t h e golden y e a r s o f t h e h y d r o c a r bon i n d u s t r y i n t h e l a t e 1970's and e a r l y 1980's due t o back-up by a h i g h o i l p r i c e and a s t r o n g US $. The y e a r o f p u b l i c a t i o n o f t h i s book c e l e b r a t e s t h e 2 5 t h a n n i v e r s a r y o f hydrocarbon e x p l o r a t i o n and h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n i n t h e N o r t h Sea w h i c h i s t h e o i l and gas b a s i n w i t h t h e g r e a t e s t p o t e n t i a l i n Western Europe, t h e 4 0 t h j u b i l e e o f commercial e x e c u t i o n o f h y d r a u l i c proppant f r a c t u r i n g t r e a t m e n t s i n t h e o i l and gas w o r l d , and f o r m y s e l f 10 y e a r s p r o f e s s i o n a l e x p e r i e n c e as r e s e r v o i r g e o l o g i s t i n hydrocarbon i n d u s t r y .
Hydraulic proppant fracturing and 1986 oilllgas industry crisis The f u l l s i g n i f i c a n c e , s e n s i t i v i t y and f a t e o f h y d r a u l i c p r o p p a n t f r a c t u r i n g f o r t h e whole p e t r o l e u m i n d u s t r y c o m p r i s i n g p r o d u c t i o n , s e r v i c e and s u p p l y companies, as w e l l as t h e dependency o f e x p l o r a t i o n and e x p l o i t a t i o n f r o m t h e p o l i t i c a l scenery and i t s s t a b i l i t y , however, was most markedly i l l u s t r a t e d by t h e 1986 c r i s i s when t h e o i l p r i c e n e a r l y suddenly c o l l a p s e d ( a n d e f f e c t i v e f o r Europe i n a d d i t i o n t h e US $ exchange r a t e almost a t t h e same t i m e c a t a s t r o p h i c a l l y dropped), and t h e s u s c e p t i b i l i t y o f e s p e c i a l l y h y d r a u l i c p r o p p a n t f r a c t u r i n g t o d e t e r i o r a t i o n s o f t h e g e n e r a l economical framework was v i o l e n t l y u n d e r l i n e d by t h e f a c t t h a t i n c o n t r a s t t o d r i l l i n g and c o n v e n t i o n a l c o m p l e t i o n which a l ways remain a t a c e r t a i n minimum l e v e l , s t i m u l a t i o n can a t l e a s t i n some areas be t o t a l l y suspended i f no l o n g e r p r o f i t a b l e . The c r i s i s came f a s t e r than expected and h i t t h e i n d u s t r y worse t h a n anyone had p r e d i c t e d and t h u s was a pronounced shock t o awake dreaming r e s e r v o i r e n g i neers, e u p h o r i c hydrocarbon m a r k e t i n g people and a l s o p a r t i a l l y r e a l i t y - d e c o u p l e d s e r v i c e and s u p p l y companies by a b r u p t l y v i s u a l i z i n g t h a t c r i t i c a l l y chang i n g i n t e r a c t i o n s between s e v e r a l p a r t s o f t h e w o r l d ' s p o l i t i c a l network and f i n a n c i a l f a b r i c and t h e r e s u l t i n g consequences f o r t h e d i f f e r e n t economical systems can n o t o n l y f o r c e t o s e r i o u s l y m o d i f y s c i e n t i f i c and t e c h n i c a l models and p l a y s as w e l l as t o suspend complete s t r a t e g i c a l concepts o f energy supply, b u t can a l s o t u r n commercial businesses almost o v e r n i g h t f r o m p r o f i t i n t o l o s s and t h u s a l s o t h r e a t t h e s o c i a l s t r u c t u r e o f whole i n d u s t r y branches.
VI
MY
involvement in the stimulation market
I happened t o e n t e r t h e h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n s c e n e r y a s Marketing Manager Proppants Europe j u s t when t h e 1986 o i l i n d u s t r y c r i s i s was a l m o s t a t i t s peak which c o n v e r t e d the a n t i c i p a t e d p r o g r e s s i v e sound and reasoned approach t o my new p r o f e s s i o n a l d u t y i n t o a sudden j u m p i n t o the c o l d w a t e r of a c o l l a p s e d d r i l l i n g and t r e a t i n g m a r k e t . The n e c e s s i t y of q u i c k l y s t a b i l i z i n g and r e v i v i n g t h e Western European h y d r a u l i c f r a c t u r i n g and g r a v e l packing mark e t f o r proppant a p p l i c a t i o n and the urgency of performing market e x t e n s i o n e s p e c i a l l y towards E a s t e r n Europe, Southern Europe and Northern A f r i c a ( p a r t s o f which were h i t h e r t o a l m o s t w h i t e p a t c h e s on t h e s t i m u l a t i o n map) c o n f r o n t e d me w i t h t h e need of r a p i d c o l l e c t i o n of l a r g e amounts of i n f o r m a t i o n d u r i n g cons u l t i n g d i s c u s s i o n s , e v a l u a t i o n s of f r a c t u r i n g p o t e n t i a l and demand, and recommendations of a p p l i c a t i o n of proppant t y p e s and g r a i n s i z e s depending on t h e r e q u i r e m e n t s of t h e r e s e r v o i r t y p e s and t h e i r g e o l o g i c a l s t r u c t u r e s when v i s i t i n g hydrocarbon p r o d u c t i o n and s e r v i c e companies, a s well a s reviewing t h e e x t e n s i v e l i t e r a t u r e , and f a s t p r o c e s s i n g of the a s s e s s e d m a t e r i a l i n t o r e v i s e d mark e t i n g s t r a t e g y c o n c e p t s based on g e o l o g i c a l , r e s e r v o i r e n g i n e e r i n g , t e c h n i c a l and commercial a s p e c t s w i t h the u l t i m a t e goal of q u i c k l y i n c r e a s i n g the a r e a l proppant s a l e s l e v e l .
The i n t e r n a t i o n a l marketing a c t i v i t y and the n e c e s s a r y a c c e l e r a t i o n of d i r e c t approach a l s o r e q u i r e d me t o l e a r n a c o u p l e of a d d i t i o n a l languages i n o r d e r t o have more working i n s t r u m e n t s f o r independent a c t i o n d u r i n g my p r o f e s s i o nal r e o r g a n i z a t i o n and t o e n a b l e a d d i t i o n a l t r a n s p a r e n c y with r e s p e c t t o compet i t i o n , e s p e c i a l l y when r e a l i z i n g the f a v o u r a b l e c o n s t e l l a t i o n f o r expansion of t h e s t i m u l a t i o n market i n E a s t e r n Europe a t the time of p o l i t i c a l and economic a l r e s t r u c t u r i n g ( p e r e s t r o i k a ; GORBATCHEV 1 9 8 7 ) . In t h i s c r i t i c a l s i t u a t i o n , f o r t u n a t e l y the u n d e r s t a n d i n g o f g e o l o g i c a l mod e l s and r e s e r v o i r e n g i n e e r i n g p r i n c i p l e s a s well a s e x p l o r a t i o n and p r o d u c t i o n problems of the main g a s - b e a r i n g f o r m a t i o n s with t h e g r e a t e s t p o t e n t i a l of hyd r a u l i c f r a c t u r i n g and thus l a r g e s t proppant consumption in Western Europe being t e r r e s t r i a l s a n d s t o n e s of R o t l i e g e n d , C a r b o n i f e r o u s and B u n t s a n d s t e i n was enhanced by t h e e x p e r i e n c e which I g o t from my own independent s e d i m e n t o l o g i c a l , p e t r o g r a p h i c a l and p a l a e o e c o l o g i c a l r e s e a r c h on c o n t i n e n t a l r e d bed format i o n s (MADER 1 9 8 5 ) . Valuable improvement of t h e s c i e n t i f i c and t e c h n i c a l knowledge was a l s o achieved by i n c o r p o r a t i n g t h e background from my e a r l i e r p r a c t i c a l p r o f e s s i o n a l work a s Area P r o d u c t i o n G e o l o g i s t r e s p o n s i b l e f o r an o i l - and g a s - f i e l d r e g i o n in Germany FRG. Without a l l t h i s e x p e r i e n c e , the n e c e s s a r y speedy a c q u i s i t i o n of t h e r e q u i r e d knowledge l e v e l would c e r t a i n l y n o t have been p o s s i b l e .
Planning o f the present proceedings volume The more o r l e s s e v i d e n t slow r e v i v a l of the s t i m u l a t i o n a c t i v i t y i n Western Europe i n 1987 with d e f i n i t e p l a n s f o r e x t e n s i v e h y d r a u l i c proppant f r a c t u r i n g campaigns p a r t i c u l a r l y i n t h e North Sea mainly in 1988 - 1990 and beyond, a profound r e o r g a n i z a t i o n of the European marketing s t r a t e g y of t h e USA-based major proppant s u p p l i e r s a t the same time, and my independent work a s an i n t e r n a t i o nal c o n s u l t a n t f o r petroleum geology and r e s e r v o i r e n g i n e e r i n g t r i g g e r e d t h e i d e a t o compile a r e p o r t i n c l u d i n g review, s t a t u s and f o r e c a s t of h y d r a u l i c proppant f r a c t u r i n g and g r a v e l packing p a r t i c u l a r l y i n Europe and a d j o i n i n g i n ternational markets. This elaboration o r i g i n a l l y intended t o not only sketch the v a r i o u s consequences of t h e c r i s i s , b u t a l s o t o i n d i c a t e a s u i t e of p o s s i b i l i t i e s of improvement of the s t i m u l a t i o n b u s i n e s s i n the next y e a r s i n an o u t look f o c u s s i n g on marketing and economical a s p e c t s t h a t a l t h o u g h being of subj e c t i v e n a t u r e a c c o r d i n g t o my view and a s s e s s m e n t , r e l a t e s n e v e r t h e l e s s t o t h e demand of t h e customers w i t h which I have been c o n f r o n t e d d u r i n g my i n v e s t i g a t i o n s o f t h e f r a c t u r i n g and p a r t i c u l a r l y the proppant market.
V II C o m p i l i n g t h e r e p o r t , however, soon r e v e a l e d t h a t t h e m a r k e t i n g and economic a l q u e s t i o n s a r e i n t i m a t e l y i n t e r w o v e n w i t h v a r i o u s t e c h n i c a l p o i n t s , and t h e r e f o r e I decided a l s o t o g i v e a t e c h n i c a l overview o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g as w e l l as s t i m u l a t i o n m o n i t o r i n g by predominantl y w e l l l o g g i n g . T h i s expansion a l s o r e q u i r e d t o renounce f r o m t h e a r e a l l i m i t a t i o n t o Europe and t o process i n f o r m a t i o n and l i t e r a t u r e on t h e o r i e s and a p p l i c a t i o n s a l l o v e r t h e globe, w i t h t h e emphasis on t e c h n o l o g i c a l r e v i e w and summar y h a v i n g been p u t on t h e USA which i s p r o b a b l y t h e most mature s t i m u l a t i o n mark e t o f t h e w o r l d . A t t h e t i m e o f b e g i n n i n g t o w r i t e down my e x p e r i e n c e and s t a r t i n g t o e v a l u a t e t h e l i t e r a t u r e , however, I c o u l d never imagine t h a t t h i s p r o j e c t c o u l d e v e r end up i n such a voluminous c o m p i l a t i o n as p r e s e n t e d now t o the readership.
Design o f
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Several monographs and t e x t b o o k s on h y d r a u l i c proppant f r a c t u r i n g and r e l a t e d r e s e r v o i r s t i m u l a t i o n have a l r e a d y appeared s i n c e t h e i n t r o d u c t i o n o f t h e t r e a t m e n t t e c h n o l o g y t o t h e e x p l o r a t i o n and p r o d u c t i o n i n d u s t r y . W h i l e t h e c l a s s i c a l monograph by HOWARD & FAST (1970 a ) has been t h e s t a n d a r d r e f e r e n c e f o r more than t h r e e decades s i n c e i t s f i r s t e d i t i o n , i n t h e l a s t y e a r s o t h e r compil a t i o n s have been p r e s e n t e d w h i c h a r e m a i n l y proceedings volumes and r e v i e w c o l l e c t i o n s (USATCHOV 1986, ECONOMIOES & NOLTE 1987, GIDLEY 1988). Summaries o f t e c h n o l o g i c a l advances o f h y d r a u l i c f r a c t u r i n g have n o t o n l y been p u b l i s h e d as books, b u t a l s o as l o n g e r j o u r n a l a r t i c l e s (VEATCH 1983, VEATCH & M O S C H O V I D I S 1986) and congress l e c t u r e s (FUHRBERG 1983). Most o f these c o m p i l a t i o n s , howe v e r , have been w r i t t e n by r e s e r v o i r o r t e c h n i c a l e n g i n e e r s and c o n c e n t r a t e on aspects of f r a c t u r i n g mechanics, f l u i d rheology, p r e s s u r e e v o l u t i o n , f r a c t u r e geometry, t r e a t m e n t m o d e l l i n g and s i m u l a t i o n . I n a d d i t i o n , m a r k e t i n g approaches h i t h e r t o h a v i n g been c a r r i e d o u t by s u p p l y and s e r v i c e companies were c h i e f l y performed by commercial r e p r e s e n t a t i v e s w i t h f r e q u e n t l y i n s u f f i c i e n t t e c h n i c a l and s c i e n t i f i c background and above a l l n o t enough u n d e r s t a n d i n g o f t h e p a r t i c u l a r pay i n t e r v a l s and t h e i r g e o l o g i c a l and e n g i n e e r i n g c h a r a c t e r i s t i c s . T h i s i s t h e reason why I became convinced t h a t i t would be more than d e s i r a b l e t o f i l l t h e gap and d i s c u s s t h e s u b j e c t s o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g f r o m t h e s t a n d p o i n t o f a r e s e r v o i r g e o l o g i s t w i t h emphasis b e i n g p u t on t h e r e l a t i o n s h i p s between proppants and hydrocarbon-bearing f o r m a t i o n s , t h e r e b y u n d e r l i n i n g t h e s i g n i f i c a n c e o f p r o p p a n t t y p e and g r a i n s i z e s e l e c t i o n a c c o r d i n g t o t h e s e d i m e n t o l o g i c a l and p e t r o g r a p h i c a l n a t u r e o f t h e pay zone t o be t r e a t e d . Many p a r t s o f t h e d i s c u s s i o n t h u s focus on proppant c h o i c e as a f u n c t i o n o f granulometry, t h i c k n e s s , v e r t i c a l and l a t e r a l c o n t i n u i t y , consol i d a t i o n , cementation and g e o m e t r i c a l f a b r i c o r g a n i z a t i o n o f t h e r e s e r v o i r format i o n s t h a t a r e t h e t a r g e t s o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g . Proppant aspects a r e t h e main t o p i c o f t h e p r e s e n t book which has t h e c o n c e p t i o n of a proceedings volume i n s t e a d o f t h e l a y o u t o f a t e x t b o o k , and a r e i l l u s t r a t e d n o t o n l y from s c i e n t i f i c and t e c h n i c a l view, b u t a l s o f r o m m a r k e t i n g and economical s i d e . Throughout t h e t e c h n i c a l e v a l u a t i o n , emphasis i s p u t on s k e t c h i n g t h e s u i t e o f r e s e r v o i r types t r e a t e d by h y d r a u l i c f r a c t u r i n g , c o m p r i s i n g low- and h i g h - p e r m e a b i l i t y sandstone, carbonate, c h a l k , d i a t o m i t e , c o a l , s h a l e and c r y s t a l l i n e r o c k s ( g r a n i t e ) which a l l r e q u i r e d i f f e r e n t f r a c t u r e geometry as w e l l as proppant q u a n t i t i e s , types and g r a i n s i z e s f o r t e c h n i c a l l y s u c c e s s f u l and economically p r o f i t a b l e s t i m u l a t i o n .
Selection o f aspects discussed in the text The p o s s i b i l i t i e s o f p r o p p a n t a p p l i c a t i o n i n h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a r e v e r y widespread and q u i t e complex. D i s c u s s i n g t h e t o p i c f r o m a l l t h e i m p o r t a n t v i e w p o i n t s p r o v i d e s an endless s u i t e o f i n t e r e s t i n g aspects. I n o r d e r t o r e s t r i c t t h e p r e s e n t e l a b o r a t i o n t o a more o r l e s s p r a c t i c a l , w e l l - o r -
VIII ganized summary w i t h o u t l o o s i n g t h e r e d l i n e , no d e t a i l e d r e v i e w o f t h e e x t e n s i v e s c i e n t i f i c and a p p l i e d l i t e r a t u r e i n f u l l coverage was i n t e n d e d , b u t t h e compiled o v e r v i e w was i n i t i a l l y scheduled m a i n l y as an enhanced and upgraded exp e r i e n c e r e p o r t d e r i v i n g b a s i c a l l y f r o m my p r o f e s s i o n a l d u t y as M a r k e t i n g Manag e r Proppants r e s p o n s i b l e f o r Western and E a s t e r n Europe and r e s u l t i n g f r o m my own assessment ' J f t h e demand and s u p p l y s i t u a t i o n o f t h i s m a r k e t . D u r i n g course o f t h e l i t e r a t u r e research, however, numerous s i g n i f i c a n t a r t i c l e s on v a r i o u s aspects o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g have been encounter e d and e v a l u a t e d which I decided t o p r e s e n t and summarize i n an as much as poss i b l e condensed f o r m and above a l l reworked w i t h my own ideas wherever my knowledge p e r m i t t e d , and t o b r i n g t h i s i n t e r w o v e n synopsis t o t h e a t t e n t i o n o f t h e r e a d e r s h i p . T h a t i s why t h e s i z e o f t h e volume s t a r t e d t o expand p r o g r e s s i v e l y , w i t h f i n a l l y o n l y t h e d e a d l i n e f o r p u b l i c a t i o n having been a b l e t o s t o p t h e growth. C e r t a i n l y t h e task o f l i m i t i n g t h e t o t a l volume o f t h e book t o an economicalbe discussed, w i t h t h e r e f o r e t h e s u i t e o f i t e m s h a v i n g been r e s t r i c t e d t o more o r l e s s p r o p p a n t - r e l a t e d s u b j e c t s , and t h u s f o r example aspects o f s t i m u l a t i o n f l u i d s , f r a c t u r i n g models, m a t r i x a c i d i z i n g and a c i d f r a c t u r i n g , r e s e r v o i r and f r a c t u r e s i m u l a t i o n , f r a c t u r e geometry and mechanics, p r e s s u r e e v a l u a t i o n , w e l l t e s t i n g , f l o w regimes and p r o d u c t i o n p l a n n i n g i n f r a c t u r e d f o r m a t i o n s , and o t h e r s c o u l d o n l y be t r e a t e d v e r y b r i e f l y i n c o n n e c t i o n w i t h p r o p p a n t q u e s t i o n s and were n o t examined i n more d e t a i l , because these t o p i c s have been a l r e a d y discussed i n more e x t e n s i o n i n v a r i o u s o t h e r c o m p i l a t i o n s p u b l i s h e d s o f a r t o which r e f e r e n c e i s made. I p u t t h e emphasis on such t e c h n i c a l and economical asp e c t s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g where t y p e and s i z e o f proppants p l a y t h e key r o l e i n d e s i g n and a p p l i c a t i o n and w h i c h a c c o r d i n g t o my f e e l i n g have h i t h e r t o n o t been condensed i n t o a c t u a l r e v i e w s and summaries. l y s t i l l a c c e p t a b l e s i z e meant i n e v i t a b l y c a r e f u l s e l e c t i o n o f t h e t o p i c s t o
The most i n t e r e s t i n g theme f o r me as a g e o l o g i s t o f e d u c a t i o n was t o document s i g n i f i c a n c e and n e c e s s i t y o f d i f f e r e n t f r a c t u r e d e s i g n f o r v a r i o u s pay f o r m a t i o n t y p e s t h a t a r e p r o s p e c t i v e f o r gas, o i l o r w a t e r and which demand hyd r a u l i c f r a c t u r i n g s t i m u l a t i o n f o r achievement o f e c o n o m i c a l l y f e a s i b l e product i o n r a t e s , w i t h t h e a p p l i c a t i o n spectrum o f h y d r a u l i c t r e a t i n g i n c l u d i n g lowand h i g h - p e r m e a b i l i t y sandstone, carbonate, c h a l k , d i a t o m i t e , c o a l , s h a l e and c r y s t a l l i n e ( g r a n i t e ) r e s e r v o i r s . The mentioned r e l a t i o n s h i p s a r e t h e reason why I have t o beg t h e r e a d e r f o r u n d e r s t a n d i n g i f some s p e c i a l t o p i c s c o u l d n o t have been r e s p e c t e d i n t h i s proceedings volume which a c c o r d i n g t o i t s t i t l e o n l y p r e s e n t s a c h o i c e o f aspects, b u t does n o t want t o c l a i m t o be r e g a r d e d as a t e x t b o o k and a l s o was n o t w r i t t e n w i t h t h e i n t e n t i o n t o p r o v i d e s u f f i c i e n t coverage t o be a b l e t o r e a c h t h e s t a t u s o f a t e x t b o o k .
Hierarchical s t r u c t u r e o f the t e x t The o r g a n i z a t i o n o f t h e t e x t i n a complex s t r u c t u r e r e q u i r e d s u b d i v i s i o n o f t h e main c h a p t e r s i n t o v a r i o u s h i e r a r c h i c a l o r d e r s o f subchapters, s e c t i o n s and s u b s e c t i o n s . A s a consequence o f c o m p a r a t i v e l y s m a l l amounts o f i l l u s t r a t i o n s and t a b l e s b r e a k i n g t h e v a s t d e s e r t o f t h e voluminous t e x t , I have p u t s p e c i a l emphasis on d e t a i l e d s p l i t t i n g o f t h e t e x t i n t o numerous paragraphs i n a h i e r a r c h i c a l manner f o r t h e purpose t o g u i d e t h e r e a d e r e a s i l y t h r o u g h t h e v a r i o u s c h a p t e r s and t h e whole book. S i x main t e x t c h a p t e r s a r e d i s t i n g u i s h e d compris i n g proppant s e l e c t i o n , m a r k e t i n g and economics, R o t l i e g e n d s t i m u l a t i o n i n Europe, h y d r a u l i c p r o p p a n t f r a c t u r i n g , g r a v e l p a c k i n g and sand c o n t r o l , and p r o p ped f r a c t u r e and g r a v e l pack m o n i t o r i n g . The b e g i n n i n g o f each o r d e r o f s u b d i v i s i o n c o n t a i n s a l s o a s h o r t g e n e r a l i n t r o d u c t i o n i n t o t h e v a r i o u s t o p i c s which are discussed i n the sections o f the h i e r a r c h i c a l l y next i n f e r i o r l e v e l i n o r d e r t o connect t h e i n d i v i d u a l p a r t s o f d i f f e r e n t h i e r a r c h i c a l stage and t o l a y a r e d l i n e t h r o u g h t h e whole t e x t . The main t a b l e o f c o n t e n t s p r e s e n t s o n l y
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IX book i n c l u d i n g t h e f i r s t t h r e e h i e r a r c h i c a l o r d e r s o f s u b d i v i s i o n , whereas t h e a s s o c i a t e d t a b l e s o f c o n t e n t s assigned t o each c h a p t e r f o l l o w i n g t h e i n t r o d u c t o r y summary p r o v i d e t h e f u l l s p l i t t i n g i n t o s e c t i o n s , and w h i l e s e r v i n g as a det a i l e d l i s t o f t o p i c s , t h e y f u l f i l l a t t h e same t i m e t h e t a s k o f b e i n g a sequent i a l s u b j e c t index by a l l o w i n g t o search f o r a l o t o f key words i n t h e sequence o f headers and subheaders. F u r t h e r a s s i s t a n c e f o r r a p i d access t o i n f o r m a t i o n on s p e c i a l t o p i c s i s p r o v i d e d by t h e a l p h a b e t i c a l s u b j e c t i n d e x a t t h e end o f t h e book.
Compilation of 1 i terature references The c o m p i l a t i o n o f t h e abundant r e f e r e n c e s c o n f r o n t e d me w i t h a n o t h e r p r o blem. I n c o n t r a s t t o many g e o l o g i c a l ( p a r t i c u l a r l y s e d i m e n t o l o g i c a l ) j o u r n a l s which c o n t a i n m a i n l y l o n g e r a r t i c l e s t h a t t r e a t p r e d o m i n a n t l y b r o a d e r s u b j e c t s i n g r e a t e r d e p t h and p r e s e n t a d e t a i l e d b i b l i o g r a p h y a t t h e end w h i c h can o f t e n be quoted as a whole, most o f t h e p u b l i c a t i o n s on h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g and r e l a t e d e n g i n e e r i n g and t e c h n i c a l aspects a r e s h o r t SPE ( S o c i e t y o f Petroleum E n g i n e e r s ) papers t h a t a r e s p l i t up i n t o v e r y d e t a i l e d t o p i c s and comprise b r i e f e x p e r i e n c e r e p o r t s o f l a t e s t s c i e n t i f i c , t e c h n i c a l and econom i c a l knowledge c o m p i l e d v e r y a c t u a l l y f o r s p e c i a l s u b j e c t symposia and s p e c i a l i s t s meetings and workshops, and t h e r e f o r e i n c l u d e i n t h e m a j o r i t y o f cases o n l y a few r e f e r e n c e s t o some v e r y c l o s e l y r e l a t e d a r t i c l e s . Looking a t t h e f l o o d o f papers s u r g i n g a n n u a l l y f r o m t h e s u i t e o f SPE meetings and v i s u a l i z i n g t h e s h o r t t i m e between i n d i v i d u a l conferences and sharp d e a d l i n e s f o r paper subm i s s i o n and acceptance, i t i s easy t o understand t h a t t h e p u b l i c a t i o n s c o u l d o n l y be p r a c t i c a l s t a t u s r e p o r t s and w i t h a v e r y s u b o r d i n a t e amount o f except i o n s , i t c o u l d n o t be t h e i n t e n t i o n o f t h e a u t h o r s t o p l u n g e a b i t deeper i n t o t h e t e c h n i c a l l i t e r a t u r e , b u t l i m i t a t i o n had t o be made t o a few p r a c t i c a l r e f e rences c o r r e s p o n d i n g t o t h e a c t u a l w o r k i n g environment. The p r e s s u r e on t h e i n d i v i d u a l r e s e a r c h e r s t o produce r e s u l t s almost w i t h o u t i n t e r r u p t i o n t h a t i s e x e r t e d by i m p a t i e n t s u p e r v i s o r s and managers, c o m p e t i t i o n between a m b i t i o u s s p e c i a l i s t s f o r j o b promotion, p r e s t i g i o u s behaviour, and above a l l t h e b a r e r o c k b o t t o m demand t o p r o v e e f f e c t i v i t y and i n d i s p e n s i b i l i t y as a base f o r s a l a r y u p g r a d i n g and j o b s e c u r i t y p a r t i c u l a r l y i n t h e t i m e o f cat a s t r o p h i c a l s t a f f l a y o f f campaigns i n a s h r i n k i n g and r e s t r u c t u r i n g h y d r o c a r bon i n d u s t r y f o r c e s t h e p e o p l e t o m a i n t a i n c o n t i n u o u s o u t p u t o f news i n f o r m o f numerous i n t e r n a l r e p o r t s and a reasonable amount o f c o n t r i b u t i o n s t o meetings and workshops which i n terms o f c o n t e n t s a r e o f t h e l a t e s t knowledge l e v e l s , b u t t h e r e f o r e n e c e s s a r i l y i n f a c t do n o t r e p r e s e n t more t h a n b r i e f s t a t u s r e p o r t s . Thus t h e i n f o r m a t i o n on h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i s e x t r e m e l y s c a t t e r e d i n t h e l i t e r a t u r e by t h e d i v i s i o n i n t o a v a s t amount o f i n d i v i d u a l papers w i t h l i m i t e d c r o s s - r e f e r e n c i n g , and a more o r l e s s comprehens i v e c o m p i l a t i o n o f t h i s broad s u i t e o f q u o t a t i o n s r e s u l t s i n an enormous r e f e rence l i s t .
Organization of the bib1 iography The b i b l i o g r a p h y , however, i s o f t e n t h e most v a l u a b l e source o f i n f o r m a t i o n f o r many r e a d e r s which a r e l o o k i n g f o r s p e c i a l i t e m s t h a t c o u l d n o t have been t r e a t e d i n s u f f i c i e n t d e t a i l i n t h e main t e x t . I n o r d e r t o serve as a source o f m a t e r i a l f o r f u r t h e r p l u n g i n g i n t o s p e c i a l i z e d s u b j e c t s , a good o r g a n i z a t i o n o f t h e r e f e r e n c e c o l l e c t i o n was i n e v i t a b l e , and a l s o t h e l i t e r a t u r e c o m p i l a t i o n was c o n s i d e r e d t o be a m a j o r s e c t i o n o f t h e book. T h i s c o n c l u s i o n i s p a r t i c u l a r l y v a l i d i n view o f t h e p r e s e n t volume b e i n g an e s p e c i a l l y pronounced example o f my a l r e a d y r e p e a t e d e x p e r i e n c e t h a t a f t e r thorough w o r k i n g t h r o u g h t h e l i t e r a t u r e , t h e r e f e r e n c e l i s t o f t h e f i n a l paper i s o f t e n l o n g e r than t h e f i r s t o r i g i n a l d r a f t o f t h e whole t e x t . No one, however, i n c l u d i n g m y s e l f c o u l d e v e r f o r e s e e t h a t t h e voluminous l i t e r a t u r e c o m p i l a t i o n c o u l d grow so much as t o a l most r e a c h t h e s i z e o f a l i t t l e book i t s e l f .
X Therefore I decided t o n o t o n l y present t h e l i t e r a t u r e l i s t i n a l p h a b e t i c a l o r d e r o f r e f e r e n c e s and t o i n c l u d e t h e r e i n as much q u o t a t i o n s as I f e l t necessary f o r proper quoting o f the state-of-the-art o f the individual topics, but a l s o t o p r o c e s s t h e assemblage f o r s p e c i f i c e v a l u a t i o n f o l l o w i n g d e t a i l e d g o a l s o f a c q u i s i t i o n . Thus t h e b i b l i o g r a p h y a t t h e end o f t h e book i n c l u d e s a l s o a s u b j e c t key c l a s s i f i c a t i o n t o a l l o w i t s easy consultance independent f r o m t h e t e x t , w i t h t h e p o s s i b i l i t y o f accessing t h e t o p i c - r e l a t e d a r t i c l e s by key words a n d / o r a u t h o r s names. The o v e r w h e l m i n g amount o f t h e p a p e r s w h i c h a r e c o n t a i n e d i n t h e s u b j e c t k e y c l a s s i f i c a t i o n ( t h a t i n f a c t r e p r e s e n t s some t y p e o f i n d e x t o t h e r e f e r e n c e l i s t ) i s a l s o d i r e c t l y r e f e r r i n g t o t h e t r e a t e d s u b j e c t s and a r e t h e r e f o r e a l s o i m p l e m e n t e d and c i t e d i n t h e t e x t . I t i s my s i n c e r e hope t h a t t h e r e a d e r w i l l n o t b e shocked b y t h e tremendous r e f e r e n c e l i s t w h i c h e x ceeds b y f a r w h a t i s n o r m a l f o r such a t y p e o f p r o c e e d i n g s volumes, b u t t h a t acknowledge i s made t o a c o m p r e h e n s i v e s o u r c e o f l i t e r a t u r e q u o t a t i o n s p r o v i d i n g a b r o a d s u i t e o f s p e c i a l i s t s w i t h f u r t h e r m a t e r i a l f o r more d e t a i l e d e v a 1u a t i o n .
Choice and presentation o f illustrations A n o t h e r a i m o f t h e enormous r e f e r e n c e l i s t and t h e accompanying s u b j e c t k e y c l a s s i f i c a t i o n i s t o p r o v i d e a p o s s i b i l i t y t o g e t access t o numerous i l l u s t r a t i o n s and g r a p h i c a l p r e s e n t a t i o n s o f v a r i o u s r e l a t i o n s h i p s . W h i l e a t l e a s t p a r t s o f t h e t e x t c o u l d be w r i t t e n i n t h e f o r m o f a d r a s t i c a l summary o r abs t r a c t t o i n c l u d e as many i n d i v i d u a l p a p e r s as p o s s i b l e i n a h i g h l y p r o c e s s e d and condensed f o r m , f i g u r e s needed a n o t h e r a p p r o a c h . I n v i e w o f t h e o b v i o u s lim i t a t i o n o f t h e p r e s e n t book t o a s t i l l handy volume ( a n d t h e s i z e t h a t has t u r n e d o u t a t t h e e n d i s c e r t a i n l y r e p r e s e n t i n g t h e a b s o l u t e maximum), no r e a s o n was seen why v a l u a b l e p r i n t i n g space s h o u l d be w a s t e d b y r e p e a t i n g i l l u s t r a t i o n s w h i c h a r e c o n t a i n e d i n numerous e a r l i e r p u b l i c a t i o n s and have p a r t i a l l y a l r e a d y been once o r s e v e r a l t i m e s c o p i e d i n o t h e r summaries, o v e r v i e w s and textbooks. Emphasis was t h e r e f o r e p u t on p r o p e r r e f e r e n c i n g o f t h e t e x t i n o r d e r t o e n a b l e t h e r e a d e r t o l o o k f o r f i g u r e s and f u r t h e r g r a p h i c a l i n f o r m a t i o n i n t h e o r i g i n a l l i t e r a t u r e . Advantage c o u l d be made i n t h i s r e s p e c t o f t h e e x c e l l e n t s t o r a g e and access s y s t e m o f t h e numerated SPE p a p e r s w h i c h a l l o w s t h e i n t e r e s t e d i n d i v i d u a l f a s t and u n c o m p l i c a t e d g r a s p i n g o f t h e a d d i t i o n a l i n f o r m a t i o n b y h a v i n g an e x t e n s i v e l i b r a r y on s t a n d - b y and c a l l a t h i s d i s p o s i t i o n f o r f a s t s e r v i c e b y j u s t q u o t i n g SPE p a p e r number and c r e d i t c a r d number. R e a s o n a b l e comp i l a t i o n s o f numerous i l l u s t r a t i o n s r e l a t i n g t o m o s t o f t h e d i s c u s s e d t o p i c s a r e a l s o c o n t a i n e d i n t h e above m e n t i o n e d t e x t b o o k s and monographs as w e l l as o v e r v i e w j o u r n a l a r t i c l e s . Thus t h e f i g u r e s , t a b l e s and p l a t e s i n t h e p r e s e n t p r o c e e d i n g s volume were l i m i t e d t o such i l l u s t r a t i o n s w h i c h d e r i v e d f r o m my own c o n c e p t i o n and p r e s e n t m a t e r i a l and i n f o r m a t i o n t h a t p r e v i o u s l y has n o t been p u b l i s h e d i n r e l a t e d form, and w h i c h i s n e c e s s a r y t o u n d e r l i n e and t o f u r t h e r comment t h e a s p e c t s t h a t a r e d i s c u s s e d c h i e f l y f r o m my e x p e r i e n c e and k n o w l e d g e based o n own i n v e s t i g a t i o n s and r e s e a r c h .
Processing o f the material As much o f t h e i n f o r m a t i o n w h i c h I have r e c e i v e d o r a l l y d u r i n g my i n t e r n a t i o n a l a t t e n t i o n o f t h e s t i m u l a t i o n m a r k e t b y c o n s u l t i n g and i n t e r v i e w i n g numerous s p e c i a l i s t s i n v a r i o u s c o u n t r i e s i s c o n f i d e n t i a l and c a n n o t be p u b l i s h e d i n w r i t t e n o r d r a f t e d f o r m , many f a c t s c o u l d o n l y be s k e t c h e d i n t h e p o s s i b l e gener a l way. W h i l e d e s c r i p t i o n i n t h e t e x t c a n s t i l l make r e a s o n a b l e a d v a n t a g e o f i n d i r e c t e x p r e s s i o n , o n l y v e r y l i m i t e d o b l i t e r a t i o n o f i n f o r m a t i o n f o r t h e sake o f p r o t e c t i n g c o n f i d e n t i a l m a t e r i a l i s p o s s i b l e i n i l l u s t r a t i o n s , and t h e r e f o r e i n a d d i t i o n t o t h e r e a s o n s d i s c u s s e d i n e a r l i e r s e c t i o n s , f i g u r e s and t a b l e s i n t h e p r e s e n t p r o c e e d i n g s volume had t o be r e s t r i c t e d t o a v e r y s m a l l q u a n t i t y .
XI I n o r d e r t o address t h e i n s i d e r p r o p e r l y and s t i l l m a i n t a i n t h e c o n f i d e n c e b a r r i e r t o t h e o u t s i d e r , many aspects have been f o r m u l a t e d i n such a way i n t h e t e x t t h a t t h e i n s i d e r understands what i s expressed, and a l s o t h e o u t s i d e r g e t s h i s i n f o r m a t i o n , a l t h o u g h access t o c o n f i d e n t i a l m a t e r i a l i s i n h i b i t e d . Respecti n g t h e c o n f i d e n c e l e v e l when p r e p a r i n g t h i s summary, however, I w i s h t o express t h e e x p e c t a t i o n t h a t t h e performed a n a l y s i s w i l l n e v e r t h e l e s s be o f c o n s i d e r a b l e importance f o r a l l o w i n g t h e r e a d e r t o g e t a r e a s o n a b l y q u i c k overview on what i s happening where and when i n t h e s t i m u l a t i o n scenery, and h o p e f u l l y t h e r e p o r t w i l l a l s o c o n t r i b u t e a l i t t l e b i t t o promote h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g p a r t i c u l a r l y i n Europe and o t h e r i n t e r n a t i o n a l mark e t s d u r i n g t h e n e x t few y e a r s . According t o t h e r e l a t i v e u n d e r r e p r e s e n t a t i o n o f i l l u s t r a t i o n s o f any t y p e w i t h r e s p e c t t o t h e tremendous amount o f t e x t , I c o n s i d e r e d i t t o be i n d i s p e n s i b l e t o break t h e t e x t i n t o as much chapters, subchapters, paragraphs and items ( i n t o t a l a h i e r a r c h i c a l system o f seven h e a d l i n e o r d e r s has been emp l o y e d ) as p o s s i b l e , i n o r d e r t o p r o v i d e n o t o n l y a b e t t e r overview on t h e l i s t o f t o p i c s and t o c o n v e r t almost a l r e a d y t h e t a b l e o f c o n t e n t s i n t o a s e q u e n t i a l s u b j e c t index o f t h e book ( t h a t i s a s s i s t e d by t h e a l p h a b e t i c a l s u b j e c t index a t t h e end o f t h e volume), b u t a l s o t o f a c i l i t a t e r e a d i n g o f t h e pages by p r o v i d i n g i n t e r r u p t i o n s o f t h e o t h e r w i s e endless c o n t i n u i n g t e x t d e s e r t . F o r t h e p u r pose o f keeping t h e r e d l i n e , each c h a p t e r c o n t a i n s a b r i e f o u t l i n e o f t h e cont e n t s o f i t s subchapters, and c r o s s r e f e r e n c e s t o o t h e r s e c t i o n s i n t h e book d i s c u s s i n g r e l a t e d items o r o t h e r aspects o f p a r t i c u l a r s u b j e c t s have been f m plemented as o f t e n as p o s s i b l e .
Acknowledgements o f sources of practical information I t i s my s i n c e r e i n t e n t i o n t o thank a l l t h e persons w i t h whom I s a t down f a c e t o face d i s c u s s i n g v a r i o u s problems o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n t h e hydrocarbon e x p l o r a t i o n and p r o d u c t i o n areas which t h e y r e p r e s e n t and a r e r e s p o n s i b l e f o r i n t h e i r companies, f o r t h e i r h o s p i t a l i t y and e s p e c i a l l y f o r t h e i r g e n e r a l l y open mind, c o n f i d e n c e and w i l l i n g c o o p e r a t i o n t o g i v e me g e n e r o u s l y t h e necessary d i r e c t p r a c t i c a l i n f o r m a t i o n f o r n o t o n l y mak i n g them s p e c i f i c t e c h n i c a l and m a r k e t i n g recommendations and s a l e s p r o p o s a l s f o r s o l u t i o n o f t h e i r problems and s a t i s f a c t i o n o f t h e i r demand, b u t a l s o f o r e n a b l i n g me t o g e t a r a p i d o v e r v i e w o f t h e p r e d o m i n a n t l y European s t i m u l a t i o n market by p u t t i n g t o g e t h e r t h e i n d i v i d u a l p i e c e s o f d a t a and s p l i n t e r e d meani n g s t o a mosaic which forms t h e base f o r t h e i n t e g r a t e d c o m p i l a t i o n o f t h i s r e view.
I n a d d i t i o n t o my p r o f e s s i o n a l d u t i e s , I would l i k e t o t a k e a l s o t h i s opport u n i t y t o acknowledge t h e h e l p which I r e c e i v e d f r o m numerous i n d i v i d u a l s by welcoming me f o r some t a l k i n g and p r o v i d i n g me w i t h m a t e r i a l , and t o express my g r a t e f u l n e s s f o r t h e good c o o p e r a t i o n f o r mutual b e n e f i t by r e t u r n i n g my general i z e d e v a l u a t i o n and i n t e r p r e t a t i o n t o g e t h e r w i t h r e s u l t s f r o m my own i n v e s t i g a t i o n s and r e s e a r c h i n t h e form o f t h e p r e s e n t proceedings volume w i t h my v e r y b e s t compliments, thanks and g r e e t i n g s . I do hope t h a t through t h i s t e c h n i c a l and m a r k e t i n g r e v i e w and s t a t u s r e p o r t , I can o f f e r some c o n t r i b u t i o n s t o t h e s o l u t i o n o f v a r i o u s s t i l l open q u e s t i o n s i n o r d e r t o h e l p t o achieve d more widespread d i s t r i b u t i o n and e s p e c i a l l y c o n s i d e r a b l e expansion o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n Western and E a s t e r n Europe i n t h e near f u t u r e (when h o p e f u l l y t h e general economical framework i s a g a i n more p r o m i s i n g ; u n f o r t u n a t e l y I am a f r a i d t h a t e x c e p t o f a few key p o l i t i c a l people nobody has any p o s s i b i l i t y t o i n f l u e n c e t h e main c o n t r o l l i n g f a c t o r s ) t o t h e s c i e n t i f i c and commercial b e n e f i t o f a l l o f us t o g e t h e r .
XI I
Acknowledgements o f pub1 icat ion Looking f o r a p u b l i s h e r w i t h t h e aims on f a s t d e c i s i o n on acceptance and speedy p r i n t i n g i n o r d e r t o r e s p e c t t h e v e r y a c t u a l n a t u r e o f m a r k e t i n g s t a t u s r e p o r t and p r e d i c t i o n as w e l l as t e c h n i c a l summary and r e v i e w which t h e r e f o r e need r a p i d d i s t r i b u t i o n among t h e t e c h n i c a l and economical s p e c i a l i s t s i n o r d e r t o be s t i l l r e a s o n a b l y u p - t o - d a t e upon appearance o f t h e book ( p a r t i c u l a r l y i n view o f t h e permanent p o s s i b i l i t y o f r a p i d l y changing economical framework cond i t i o n s w i t h o u t e x p e c t a t i o n o r p r i o r n o t i c e , and i n l i g h t o f t h e almost u n i n t e r r u p t e d surge o f t e c h n i c a l p u b l i c a t i o n s t h a t b r i n g s p r o g r e s s and i n n o v a t i o n n e a r l y e v e r y q u a r t e r o f t h e y e a r ) , I found i n v a l u a b l e s u p p o r t by O r . M a r t i n Tanke (Department o f E a r t h Sciences, E l s e v i e r Science P u b l i s h e r s ] who v e r y q u i c k l y guaranteed me p u b l i c a t i o n o f t h e volume i n t h e s e r i e s " Developments i n P e t r o leum Sciences " i n camera-ready f o r m which enabled me t o b r i n g t h e m a n u s c r i p t on t h e l a t e s t l e v e l s t i l l v e r y s h o r t l y b e f o r e p r i n t i n g . I am v e r y g r a t e f u l t o M a r t i n f o r h i s u n d e r s t a n d i n g o f my r e q u i r e m e n t s e s p e c i a l l y i n terms o f q u a n t i t y o f pages and f o r h a v i n g g i v e n me a l l t h e necessary c o n f i d e n c e and freedom t o f i n i s h t h e volume a l o n g t h e l i n e s o f my i n t e n t i o n . My s a t i s f a c t i o n and happyness cannot be overempahsized i n view o f t h e f a c t t h a t a l t h o u g h t h e f i n a l s i z e o f t h e book c e r t a i n l y has c o n s i d e r a b l y shocked M a r t i n because i t was more than t w i c e as much as he expected and as was c o n f i r med by c o n t r a c t , he d i d n o t h e s i t a t e t o a c c e p t t h e volume f o r p u b l i c a t i o n i n t h e s u b m i t t e d f o r m and w i t h o u t d e l a y made a l l t h e necessary i n t e r n a l p r e p a r a t i o n s f o r speedy p r o c e s s i n g . P a r t i c u l a r acknowledgement deserves t h e c o o r d i n a t i n g engagement o f M a r t i n i n c o n n e c t i o n w i t h t h e good c o o p e r a t i o n f o r r e a l i z i n g my dream t o c a t c h t h e most s u i t a b l e p u b l i c a t i o n d a t e by f i l l i n g i n t h e gap b e t ween t h e annual SPE meeting i n October 1988 when f o r t h e l a s t t i m e a g r e a t e r amount o f brand-new t e c h n i c a l i n f o r m a t i o n was r e c e i v e d f o r i n c o r p o r a t i o n i n t o t h e t e x t , and March 1989 when a f t e r t h e w i n t e r break t h e p r i n t e d book c o u l d be p r e s e n t e d a t t h e f i r s t SPE s p e c i a l i z e d meetings on l o w - p e r m e a b i l i t y gas r e s e r v o i r s and h y d r a u l i c p r o p p a n t f r a c t u r i n g .
Acknowledgements o f manuscript preparation D u r i n g t h e s t r e s s y p e r i o d o f f i n a l e d i t i n g o f t h e t e x t and p r e p a r a t i o n o f t h e camera-ready m a n u s c r i p t i n t h e above mentioned s l o t minus t h e t i m e r e q u i r e d by t h e p u b l i s h e r f o r p r o d u c t i o n , I r e c e i v e d i n v a l u a b l e s u p p o r t and unrenounc a b l e a s s i s t a n c e f r o m my dear f r i e n d s E l z b i e t a Zawadzka ( M a r i n e I n s t i t u t e , Gdahsk/Poland) and M a l g o r z a t a T r u s z e l ( G e o l o g i c a l I n s t i t u t e , Sosnowiec/Poland) who devoted t h e i r whole p h y s i c a l and mental w o r k i n g c a p a c i t y t o me i n o r d e r t o f i n i s h t h e p r o j e c t i n t h e r e q u e s t e d t i m e and i n a good shape. W i t h o u t t h e s a c r i f i c i n g and h e a r t y h e l p o f E l a and p a r t i c u l a r l y M a l g o s i a w i t h t h e time-consuming work o f t e x t p r o c e s s i n g and camera-ready p r i n t i n g d u r i n g c o u n t l e s s hours which t h e y spent p a t i e n t l y and i n s p i t e o f heavy work w i t h e x c e l l e n t humour a t t h e computer i n my o f f i c e , i t would a b s o l u t e l y n o t have been p o s s i b l e t o meet t h e t e c h n i c a l l y and e c o n o m i c a l l y most d e s i r a b l e d e a d l i n e f o r d e l i v e r y o f t h e f i n a l m a n u s c r i p t as agreed w i t h t h e p u b l i s h e r . As f am a l o n e r e s p o n s i b l e f o r a l l t h e c o n t e n t s o f t h e p r e s e n t p r o c e e d i n g s volume and because I need t o keep u p - t o - d a t e f o r t h e p r e p a r a t i o n o f a r e v i s i o n f o r an e v e n t u a l second e d i t i o n , I would a p p r e c i a t e t o r e c e i v e t h e comments o f t h e r e a d e r s h i p s t h r o u g h c r i t i c a l d i s c u s s i o n and complimentary i n f o r m a t i o n . I would a l s o l i k e t o express my s i n c e r e hope t h a t t h e p r e s e n t book forms a val u a b l e c o n t r i b u t i o n t o t h e p r o g r e s s i v e u n d e r s t a n d i n g and r e a l i z a t i o n o f hydraul i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n v a r i o u s p a r t s o f t h e w o r l d and a wide spectrum o f hydrocarbon r e s e r v o i r s .
Rodgau 1 (Jugesheim) , January 1989
D e t l e f Mader
XI11
o f C o n t e n t s Preface ................................................................... Table o f contents ........................................................ A b s t r a c t ................................................................ T a b l e
1. P r o p p a n t
S e 1 e c t i o n
V XI11 XXI
.....................................
1
...........................
1
1.0.
Summary and associated t a b l e o f contents
1.1.
Introduction ....................................................... 8 1.1.1. T i g h t gas r e s e r v o i r s and MHF s t i m u l a t i o n ................... 8 1.1.2. E v o l u t i o n o f h y d r a u l i c f r a c t u r i n g technology and o i l p r i c e 11 h i s t o r y ................................................... 1.1.3. S i g n i f i c a n c e of proppant s e l e c t i o n ........................ 11 1.1.4. Special aspects o f the European s t i m u l a t i o n market ........ 12 1.1.5. O r g a n i z a t i o n of t h e review and s t a t u s r e p o r t .............. 13
1.2.
H i s t o r i c a l development o f hydraulic proppant f r a c t u r i n g . . . . . . . . . . . 1.2.1. General aspects ........................................... 1.2.2. L i m i t a t i o n s o f n a t u r a l sand ............................... 1.2.3. R e s t r i c t i o n s o f a l t e r n a t i v e propping m a t e r i a l s b e f o r e the i n v e n t i o n o f s y n t h e t i c ceramic proppants .................. 1.2.4. Impact o f e a r l y s y n t h e t i c ceramic proppants ............... 1.2.5. S i g n i f i c a n c e o f advanced s y n t h e t i c ceramic proppants . . . . . . 1.2.6. Impact o f resin-coated proppants .......................... 1.2.7. P o s s i b l e f u t u r e i n n o v a t i o n ................................ 1.2.8. V e r t i c a l vs . h o r i z o n t a l f r a c t u r e o r i e n t a t i o n ..............
20 22 23 23 26 28
1.3.
Proppant types and grain sizes .................................... 1.3.1. Provenance o f sand and proppants .......................... 1.3.2. M i n e r a l o g i c a l composition and c l o s u r e s t r e s s r e s i s t i v i t y .. 1.3.3. Chemical composition and a c i d s o l u b i l i t y .................. 1.3.4. Grain s i z e s and c o n d u c t i v i t y per U S $ ..................... 1.3.5. G r a i n shapes ..............................................
32 32 33 34 36 38
1.4.
Proppant selection f o r hydraulic f r a c t u r i n g and gravel packing .... 1.4.0. S i g n i f i c a n c e o f proppant t e s t i n g f o r q u a l i t y improvement .. 1.4.1. Surface morphology and i n t e r n a l m i c r o p o r o s i t y . . . . . . . . . . . . . 1.4.2. Crushing behaviour and c r y s t a l l i n e s t r u c t u r e . . . . . . . . . . . . . . 1.4.3. F r i c t i o n angle and g r a i n shape ............................ 1.4.4. F l u i d s a l i n i t y and mechanochemical s t a b i l i t y . . . . . . . . . . . . . . 1.4.5. F l u i d temperature and h o t b r i n e a g g r e s s i v i t y . . . . . . . . . . . . . . 1.4.6. Equipment abrasion and g r a i n hardness ..................... 1.4.7. S p e c i f i c g r a v i t y and f l u i d suspension p r o p e r t i e s . . . . . . . . . . 1.4.8. G r a i n s i z e and embedment .................................. 1.4.9. Manufacturing process and p e l l e t composition . . . . . . . . . . . . . . 1.4.10. C o n d u c t i v i t y d i s c o u n t ..................................... 1.4.11. Proppant m i x i n g ........................................... 1.4.12. Computer programs f o r proppant s e l e c t i o n .................. 1.4.13. Proppant s e l e c t i o n g u i d e l i n e s .............................
40 40 41 42 43 45 47 47 50 52 53 55 72 81 87
1.5.
Proppant applications ............................................. 1.5.1. O i l - and g a s - f i e l d a p p l i c a t i o n s ........................... 1.5.2. N o n - o i l - and g a s - f i e l d a p p l i c a t i o n s .......................
90 90 93
1.6.
Conclusion and o u t l o o k
............................................
96
14 16 18
XlV
2. H a r k e t i n g
a n d
E c o n o m i c s ...........................
97
..........................
97
2.0.
Summary and a s s o c i a t e d t a b l e o f c o n t e n t s
2.1.
Introduction
2.2.
Economical i n f l u e n c e s on h y d r a u l i c proppant f r a c t u r i n g 2.2.1. I n f l u e n c e s o f h y d r o c a r b o n p r i c e changes . . . . . . . 2.2.2. I n f l u e n c e s o f p r o p p a n t p r i c e changes . . .
2.3.
S t i m u l a t i o n market d i f f e r e n c e s between USA and Europe . . . . . . . . . . . . 2.3.;. Genera: a s p e c t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. F r e q u e n c y o f d r i l l i n g and s t i m u l a t i o n . . . . . . . . . . . . . . . . 2.3.3. S u c c e s s / f a i l u r e r a t i o and c o n s o r t i a 1 s t r u c t u r e . . . . . . . 2.3.4. O p e r a t i n g m e n t a l i t y and t r e a t m e n t c o s t . . . . . . . . . . . . . . . . . . . 2.3.5. U S S exchange r a t e d e c l i n e . . . . . . . . . . ........... 2.3.6. P r o p p a n t m a r k e t i n g s t r a t e g y ..............................
166 166
Areal marketing assessment o f h y d r a u l i c proppant f r a c t u r i n g . . . . . . 2.4.0. General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. W e s t e r n Europe 1977 - 1985 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. W e s t e r n Europe 1986 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3. E a s t e r n Europe u n t i l 1986 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4. S t a t u s i n E a s t e r n Europe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5. W e s t e r n Europe 1987 - 1990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6. E a s t e r n Europe 1987 - 1990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
173 173 174 192 201
............
228
2.4.
2.5.
.....................................................
C o n c l u d i n g assessment o f p r o p p a n t m a r k e t i n q p o t e n t i a l
3 . R o t l i e g e n d
S t i m u l a t i o n
i n
E u r o p e
106
...
.
168 169 170
203 216 223
229
3.0.
Summary and a s s o c i a t e d t a b l e o f c o n t e n t s
.........................
229
3.1.
Introduction ..................................................... 3.1.1. General aspects 3.1.2. Economical aspec ........... .......... 3.1.3. Factors influen gas r e s e r v o i r s
236 236 237
3.2.
D i s t r i b u t i o n o f main f i e l d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................ 3.2.1. Vertical distribution 3.2.2. Horizontal d i s t r i b u t i o ................ ........................ 3.2.3. Other aspects . . . . . . . . . .
240 241 248 249
3.3.
H i s t o r i c a l development o f prod 3.3.1. B r i t i s h N o r t h Sea . . . . 3.3.2. Germany FRG . . . . . . . . . . 3.3.3. D u t c h N o r t h Sea .............. 3 . 3 . 4 . O t h e r a r e z s ..................
3.4.
P e r m e a b i l i t y p r e d i c t i o n and e f f e c t i v e p e r m e a b i l i t y 3.4.1. T i g h t r e s e r v o i r p e r m e a b i l i t y range . . . . . . . 3.4.2. P e r m e a b i l i t y d e t e r m i n a t i o n and p r e d i c t i o n 3.4.3. Temporary damage and r e s t o r a t i o n o f permea
3.5.
240
r a c t u r i n g . . . . . . . . . . . . . . 249 ....................... 250 ........... . . . . . . . . . 251 ........... . . . . . . . . . 252 ........................ 252
...............
P e r m e a b i l i t y d e t e r i o r a t i o n and c l a y mineral d i s t r i b u t i o n 3.5.1. C l a y m i n e r a l o g y and m o r p h o l o g y . . . 3.5.2. Permeability deterioration .......
...
253
253
. . . 257 . . . 259
.........
259
...
262
. . . 260
XV 3.6.
3.7.
Minimum pre-fracturing gas production r a t e and poroperm properties 266 3.6.1. P r e - f r a c t u r i n g economical f e a s i b i l i t y e s t i m a t i o n . . . . . . . . . 266 3.6.2. D i f f e r e n c e s between e x p l o r a t i o n and development w e l l s . . . . 267 Stimulation cost and f r a c t u r i n g p o l i c y ........................... 267 F r a c t u r i n g p o l i c y ........................................ 268 P r i c i n g s c e n a r i o ......................................... 269 F r a c t u r i n g t r e a t m e n t sequence s t r a t e g y . . . . . . . . . . . . . . . . . . . 269
3.7.1. 3.7.2. 3.7.3.
3.8.
Philosophy o f offshore and onshore exploration well f r a c t u r i n g ... 270 3.8.1. P h i l o s o p h y o f o f f s h o r e e x p l o r a t i o n w e l l f r a c t u r i n g . . . . . . . 270 3.8.2. P h i l o s o p h y o f onshore e x p l o r a t i o n well f r a c t u r i n g ........ 272
3.9.
Implications o f f r a c t u r i n g philosophy on proppant selection ...... 273 3.9.1. E x p l o r a t i o n s t r a t e g y and s t i m u l a t i o n t e s t i n g . . . . . . . . . . . . . 273 3.9.2. F i e l d e x p e r i e n c e and development concepts . . . . . . . . . . . . . . . . 274 3.9.3. A p p r a i s a l d r i l l i n g s t r a t e g y concepts and e x p e r i e n c e . . . . . . 275
3.10. Horizontal and v e r t i c a l differences o f f r a c t u r i n g p o t e n t i a l . . . . . . 276 3.10.1. P e r m e a b i l i t y ............................................. 276 . . . . . . . . . . . . . . . . . . . 280 3.10.2. Gas c o m p o s i t i o n .......................
................... 3.11. Water s e n s i t i v i t y ............................ ................... 3.11.1. General aspects ...................... 3.11.2. F o a m - f r a c t u r i n g s t i m u l a t i o n .......... ................... 3.11.3. C r o s s l i n k e d a c i d systems . . . . . . . . . . . . . ................... ................... 3.11.4. Methanol systems ..................... 3.11.5. L i q u i d carbon d i o x i d e p r o p p a n t f r a c t u r ng . . . . . . . . . . . . . . . . 3.12. Other asoects
.................................
281 281 283 289 289 290
. . . . . . . . . . . . . . . . . . . 292
3.12.1. i a t e r s a t u r a t i o n ......................................... 3.12.2. Gas demand ............................................... 3.12.3. R e s e r v o i r performance ....................................
292 293 293
3.13. Sedimentary structures o f f l u v i a l . aeolian and l a c u s t r i n e deposits i n Rotliegend and Buntsandstein i l l u s t r a t i n g d i s t r i b u t i o n o f reser v o i r heterogeneities and natural fractures ....................... 3.13.0. I n t r o d u c t i o n ............................................. 3.13.1. Angular u n c o n f o r m i t i e s and basement f o l d i n g . . . . . . . . . . . . . . 3.13.2. C r o s s - s t r a t i f i c a t i o n i n a e o l i a n dune sands and f l u v i a l channel b a r d e p o s i t s ..................................... 3.13.3. Gravel a g g l o m e r a t i o n s and mud drapes i n a l l u v i a l - f a n b r e c c i a s and f l u v i a l channel sandstones ...................... 3.13.4. N a t u r a l f r a c t u r e s and sedimentary h e t e r o g e n e i t i e s i n f l u v i a l channel and f l o o d p l a i n sandstones and mudstones . . . . . 3.13.5. G r a i n - s i z e d i s t r i b u t i o n changes and e r o s i o n a l s u r f a c e s i n b r a i d e d - r i v e r channel sandstones and conglomerates ....... 3.13.6. Synsedimentary d e s i c c a t i o n f r a c t u r i n g i n l a c u s t r i n e and f l u v i a l f l o o d p l a i n mudstones and n a t u r a l p r o p p i n g o f c r a c k 3.13.7. L a c u s t r i n e and f l o o d p l a i n mud drapes i n f l u v i a l channel and a l l u v i a l p l a y a sandstones ............................ 3.13.8. N a t u r a l f r a c t u r e s and sedimentary h e t e r o g e n e i t i e s i n c r o s s - s t r a t i f i e d a e o l i a n dune and sheet sands as w e l l as f l u v i a l channel sands .................................... 3.13.9. Sedimentary and d e f o r m a t i o n a l f e a t u r e s o f l a c u s t r i n e mud drapes i n f l u v i a l channel, overbank and a l l u v i a l p l a y a sandstones ............................................... 3.13.10.Root tubes and carbonate c o n c r e t i o n s i n c a l c r e t e p a l a e o s o l 3.13.11.Natural f r a c t u r e o r i g i n and d i s t r i b u t i o n i n v a r i o u s r e s e r v o i r r o c k s ...............................................
I-
294 294 295 299 302 305 308 310 314 318 321 324 328
XVI 4 . H y d r a u l i c
P r o p p a n t
F r a
4.0.
Summary and a s s o c i a t e d t a b l e o f c o n t e n t s .
4.1.
I n t r o d u c t i o n ......................... 4.1.1. P o s s i b i l i t i e s o f expanding the h y d r a u l i c proppant f r a c t u r i n g m a r k e t i n t h e n e a r f u t u r e ...................... 4.1.2. O u t l i n e o f t h e t e c h n i c a l r e v i e w and s t a t u s r e p o r t . . . . . . . . 356
4.2.
Containment o f f r a c t u r e propagation i n t h i n sandstones . . . . . . . . . . . 357 4.2.1. A r e a l d i s t r i b u t i o n o f p o t e n t i a l r e s e r v o i r s . . . . . . . . . . . . . . . 358 4.2.2. Technical aspects o f f r a c t u a t i o n ................ 358 4.2.3. F r a c t u r e p r o p a g a t i o n ...... ............. . . . . . 388 4.2.4. R e s e r v o i r h e t e r o g e n e i t y and py . . . . . . . . . . . . . . . 402 4.2.5. C o m b i n a t i o n o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g _ . .410 4.2.6. E c o n o m i c a l s i g n i f i c a n c e .................................. 411
4.3.
Proppant s a t u r a t i o n and conductivity improvement 4.3.1. General aspects . . . . . . . . . . . . 4.3.2. Selection o f proppant type . ................. ................. 4.3.3. P r o p p a n t embedment and f r a c t 4.3.4. P r o p p a n t t r a n s p o r t and f r a c t u r e damage . . . . . . . . . . . . . . . . . . . 4.3.5. U l t r a - h i g h f r a c t u r e c o n d u c t i v i t y achievement . . . . . . . . . . . . . 4.3.6. Uniform vs. changing f r a c t u r e c o n d u c t i v i t y ...............
413 414 425 460 462
o f unconventional gas stimulation ........................ 463 T i g h t g a s s a n d s t o n e s ......_... Geopressured r e s e r v o i r s . . . . . . . . . . . . . _ . . . . . 466 Coal seams . . . . . . . . . . . . . . . . . S h a l e s ..................... High-temperature r e s e r v o i r s . . . . . . . . . . . . . . . 489 491 Gas c o n d e n s a t e r e s e r v o i r s ................................
4.4.
Targets 4.4.1. 4.4.2. 4.4.3. 4.4.4. 4.4.5. 4.4.6.
4.5.
Hydraulic proppant f r a c t u r i n g o f carbonate rocks . . . . . . . . . . . . . . . . . 492 4.5.1. E x t e n s i o n o f d r a i n a g e p a t h and c o n d u c t i v i t y c o n t r a s t . . . . . 492 4.5.2. C o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r e s ... 4.5.3. P o s s i b i l i t i e s o f a p p l i c a t i o n i n Europe . . . . . . . . . . . . 4.5.4. C h a l k s t i m u l a t i o n ........................................ 509 4.5.5. Sandstone a c i d i z i n g _ _ . _ _ . . _ . . . . . . . . . . .
4.6.
High-permeability r e s e r v o i r f r a c t u r i n g ........................... 4.6.1. Fracture-formation conductivity contrast .......... 4.6.2. P r o p p a n t g r a i n s i z e and c o n c e n t r a t i o n . . . . . . . . . . . . . 4.6.3. P r o p p a n t / f l u i d s l u r r y m i x i n g t y p e s ....................... 4.6.4. F r a c t u r e d e s i g n c h a r a c t e r i s t i c s ..........................
533 534 536
4.7.
Geothermal f r a c t u r i n g . . . ... . .. . 537 4.7.1. Geological aspect . . . . . . . . . . . . . . 537 4.7.2. Geothermal r e s e r v o i r d i s t r i b u t i o n and e x p l o i t a t i o n . . . . . . . 538 ......... 539 4.7.3. E x p l o s i v e and n o n - p r o p p a n t f r a c t u r i n g . 4.7.4. Proppant f r a c t u r i n g .........__.._..... 4.7.5. Communication f r a c t u r i n g ................................. 547 4.7.6. F r a c t u r e s y s t e m e n l a r g e m e n t b y h e a t e x t r a c t i o n . . . . . . . . . . . 548 4.7.7. Shear s t i m u l a t i o n v s . t e n s i l e f r a c t u r i n g . . . . . . . . . . . . . . . . . 549
4.8.
M i n i f r a c t u r i n g and other small-scale f r a c t u r i n g . . . . . . . . . . . . . . . . . . 4.8.1. M i n i f r a c t u r e , m i c r o f r a c t u r e and MHF d e f i n i t i o n and p r i n .................................... c i p l e s ... 4.8.2. P r e - f r a c t u r e p r e s s u r e breakdown t e s t and f r a c t u r e paramet e r determination ........................................ 4.8.3. F o r m a t i o n damage a r o u n d t h e w e l l b o r e ..................... 4.8.4. F i e l d e x p e r i m e n t s and mHF v s . MHF ........................
552
555 568 571
XVII P i l o t s t i m u l a t i o n o f marginal r e s e r v o i r s ................. H o r i z o n t a l w e l l o r d r a i n h o l e d r i l l i n g and f r a c t u r i n g ..... Uptake c a p a c i t y enhancement o f i n j e c t i o n w e l l s ........... Combination o f n a t u r a l and a r t i f i c i a l f r a c t u r e s . . . . . . . . . . Late-stage r e f r a c t u r i n g o f o l d w e l l s ..................... Combination o f a c i d and proppant f r a c t u r i n g ..... ......... F r a c t u r e l e n g t h and h e i g h t o p t i m i z a t i o n . . . . . . . . . ........
573 575 592 597 619 628 631
........ Oil-reservoir f r a c t u r i n g ................................ ........ 4.9.1. General aspects ................................. C o n d u c t i v i t y c o n t r a s t ........................... ........ 4.9.2. ........ Economical f e a s i b i l i t y .......................... 4.9.3. Steam-drive e f f e c t s on f r a c t u r e propagation ..... ........ 4.9.4. Cold water i n j e c t i o n e f f e c t s on h y d r a u l i c f r a c t u r ng ..... 4.9.5.
640 640 641 642 643 657
4.8.5. 4.8.6. 4.8.7. 4.8.8. 4.8.9. 4.8.10. 4.8.11. 4.9.
4.10. C m u n cation f r a c t u r i n g ......................................... Conventional storey-wise f r a c t u r i n g ...................... 4.10.1 4.10.2 A p p l i c a t i o n i n p r o d u c t i o n d r i l l i n g ....................... I n f l u e n c e o f p e r f o r a t i o n arrangement ..................... 4.10.3 4.10.4 A p p l i c a t i o n i n a p p r a i s a l d r i l l i n g ........................ 4.10.5 F r a c t u r e i n t e r f e r e n c e ....................................
659 660 662 664 665 666
.............. 4.11. Fracturing o f deviated wells ...................... 4.11.1. F r a c t u r e propagation and o r i e n t a t i o n ...... .............. 4.11.2. Proppant s t r a t i f i c a t i o n and f l u i d l a y e r i n g .............. 4.11.3. Hydrocarbon p r o d u c t i o n and f l o w regimes ... .............. 4.11.4. Improvements o f f r a c t u r e e f f e c t i v i t y i n dev ated w e l l s ...
668 668 671 671 673
.............. 4.12. Fracture damage ................................... 4.12.1. General aspects ........................... .............. .............. 4.12.2. Proppant s e t t l i n g ......................... 4.12.3. Proppant flowback ......................... .............. .. 4.12.4. Proppant c r u s h i n g ........................................ 4.12.5. F l u i d aspects ............................................
673 673 675 683 691 696
4.13. Aggressive f r a c t u r i n g design ..................................... 4.13.1. Progress i n s t e a d o f r o u t i n e .............................. 4.13.2. R i s k y approach o f marginal r e s e r v o i r s ....................
696 697 697
.......................................................
698
4.14. Conclusion 5. G r a v e 1 5.0.
5.1.
5.2.
5.3.
..........
699
......................... Introduction ............................................... ...... 5.1.1. R e s e r v o i r compaction spectrum r e q u i r i n g gravel pack ng . . . 5.1.2. Sand s t a b i l i z a t i o n and p r o d u c t i v i t y maintenance ... ...... 5.1.3. Organization o f the review and s t a t u s r e p o r t ...... ...... Gravel types and g r a i n sizes .................................... 5.2.1. Sand p r o d u c t i o n and i t s c o n t r o l .......................... 5.2.2. Gravel type .............................................. 5.2.3. Gravel g r a i n s i z e ........................................ 5.2.4. Gravel g r a i n surface and shape ...........................
699
Gravel packing canpletion f l u i d s and gravel saturation . . . . . . . . . . . 5.3.1. F l u i d v i s c o s i t y and g r a v e l c o n c e n t r a t i o n ................. 5.3.2. Transport and packing c a p a c i t y o f the s l u r r i e s . . . . . . . . . . . 5.3.3. Gravel s e t t l i n g d i m i n u t i o n i n the c a r r i e r f l u i d .......... 5.3.4. Gel surface f i l t r a t i o n from polymer s o l u t i o n s ............
734 735 741 746 748
P a c k in g
a n d
S a n d
C o n t r o 1
Summary and associated t a b l e o f contents
710 710 711 711 711 712 718 721 734
XVIII 5.3.5.
C o m b i n a t i o n o f g r a v e l p a c k i n g and h y d r a u l i c p r o p p a n t f r a c t u r i n g ................................................... 749
5.4. Reservoir stability and fluid dynamics ........................... 5.4.1. 5.4.2. 5.4.3. 5.4.4. 5.4.5. 5.4.6.
F o r m a t i o n p r e s s u r e and f l u i d c o m D o s i t i o n . . . . . . . . . . . . . . . . . F l u i d dynamics ...................... .................... Rock mechanics ...................... .................... P e r f o r a t i o n t u n n e l m o r p h o l o g y . . . . . . . .................... Water c u t i n t h e h y d r o c a r b o n s . . . . . . . .................... D i f f e r e n c e s between h y d r a u l i c f r a c t u r ng and g r a v e l p a c k i n g ................................. ....................
5.5.
Areal distribution of potential reservoirs
5.6.
Particle transport through perforatio
5.7.
5.5.1. 5.5.2. 5.5.3. 5.5.4.
5.6.1. 5.6.2. 5.6.3. 5.6.4. 5.6.5. 5.6.6. 5.6.7. 5.6.8.
P e r f o r a t i o n p a c k i n g and p a r t i Impact o f f l u i d v i s c o s i t y . . . Achievement o f p e r f o r a t i o n t u Influence o f perforation tech I n f l u e n c e o f p e r f o r a t i o n geom Influence o f reservoir sectio Gravel prepacking o f p e r f o r a t O t h e r a s p e c t s ................. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Steam-drive enhanced oil recovery
5.7.1. 5.7.2. 5.7.3. 5.7.4. 5.7.5.
...
5.9.
757 757
. . 776
...........
C o n s o l i d a t e d p a c k i n s t a l l a t i o n ............... ........... Resin-coated g r a v e l placement . . . . . . . . . . . . . . . . . . . . . . . . . . . Gravel packing o f deviated w e l l s . . . . . . . . . . . . . ........... Gravel packing w i t h l i n e r v i b r a t i o n . . . . . . . . . . . . . . . . . . . . . C o n c e n t r a t e d g r a v e l s l u r r y pumping . . . . . . . . . . . . . . . . . . . . . . D e n s i t y s l u r r y and foam g r a v e l p a c k i n g . . . . . . . . . . . . . . . . . . V i s c o u s p o l y m e r g r a v e l p a c k i n g and d e p o s i t b u i dup . . . . . . . P r e v e n t i o n o f p r o p p a n t f l o w b a c k by f r a c t u r e t a 1 - i n . . . . . . ........... P u d d l e p a c k i n g ................................
Other sand control methods ....................................... 5.9.1. 5.9.2. 5.9.3. 5.9.4. 5.9.5. 5.9.6.
757
776
Sintered Bauxite ......................................... N i c k e l - c o a t e d sand ....................................... A p p l i c a t i o n p o t e n t i a l i n Europe . . . ... ...... Gravel d e n s i t y ........................................... A l c a l i n i t y r e d u c t i o n o f t h e aqueous i n j e c t e d phase . . . . . . .
5.8. Special gravel pack applications ...................... 5.8.1. 5.8.2. 5.8.3. 5.8.4. 5.8.5. 5.8.6. 5.8.7. 5.8.8. 5.8.9.
...........
General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Centres o f gravel packing . . . Gravel packing p o t e n t i a l i n E I m p a c t o f r e s i n - c o a t e d sand and p r o p p a n t s
751 751 753 754 756 756
Resin consolidation ...................................... H y d r o x y - a l u m i n u m and e l e c t r o l e s s n i c k e l c o n s o l i d a t i o n . . . . S i l i c a cementation ....................................... Warm a i r c o k i n g c o n s o l i d a t i o n ............................ S t a b l e sand a r c h f o r m a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pre-pack l i n e r s and m u l t i w i r e - w r a p p e d s c r e e n s . . . . . . . . . . . .
776 777 778 778 779
781 781 783 789 800 803 804 804 806 807 807 807 819 820 820 821 824
5.10. Influence o f completion type on gravel packing . . . . . . . . . . . . . . . . . . . 826 5.10.1. I n s i d e g r a v e l p a c k i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 . 1 0 . 2 . Open h o l e g r a v e l p a c k i n g ................................. 5 . 1 0 . 3 . F a c t o r s i m p r o v i n g g r a v e l pack e f f i c i e n c y . . . . . . . . . . . . . . . . . 5.10.4. L i n e r placement .......................................... 5 . 1 0 . 5 . W e l l b o r e damage i n g r a v e l p a c k e d h o l e s . . . . . . . . . . . . . . . . . . .
827 829 831 831 831
XIX
...........................
832 832 834 834 837
............................
843
.........................
843
5.11. Formation damage during gravel packing 5.11.1. 5.11.2. 5.11.3. 5.11.4.
P e r f o r a t i o n t u n n e l p l u g g i n g .............................. O r i l l i n g mud l o s s ........................................ Completion f l u i d l o s s .................................... F o r m a t i o n damage removal by a c i d i z i n g ....................
6.Propped F r a c t u r e a n d G r a v e 1 P a c k M o n i t o r i n g 6.0.
Summary and a s s o c i a t e d t a b l e o f c o n t e n t s
6.1.
Introduction
.....................................................
852
6.2.
Fracture detection and determination .............................
853 853 854 909 926 926 939 941 942
Photon l o g g i n g ........................................... Gamma-ray l o g g i n g ........................................ Neutron l o g g i n g .......................................... O t h e r g r a v e l pack l o g g i n g techniques ..................... T e l l - t a l e screen ......................................... Mechanical p r o p e r t i e s l o g ................................ C a l i p e r l o g g i n g ..........................................
..............................
943 943 947 949 951 952 953 958
7 . B i b 1 i o g r a p h y ................................................
959
6.2.0. 6.2.1. 6.2.2. 6.2.3. 6.2.4. 6.2.5. 6.2.6. 6.2.7.
General aspects o f f r a c t u r e h e i g h t m o n i t o r i n g . . . . . . . . . . . . Well l o g g i n g methods ..................................... S e i s m i c a l and g r a v i m e t r i c a l methods ...................... Magnetometrical methods .................................. S t i m u l a t i o n d a t a a n a l y s i s ................................ F r a c t u r e m i g r a t i o n a n a l y s i s .............................. I n f l a t a b l e i m p r e s s i o n packers ............................ Photography and imagery ..................................
6.3. Gravel pack checking and evaluation 6.3.1. 6.3.2. 6.3.3. 6.3.4. 6.3.5. 6.3.6. 6.3.7.
..........................
959
7.1. Introduction ......................................................
963
7 . 0 . Summary and a s s o c i a t e d t a b l e o f c o n t e n t s
........................................ 964 Overview o f h y d r a u l i c f r a c t u r i n g and economical framework . 965 F r a c t u r e d e s i g n and e v a l u a t i o n ............................ 976 F r a c t u r e f e a t u r e s ......................................... 990 Proppant f e a t u r e s ........................................ 1007 T e c h n i c a l , f l u i d and temperature f e a t u r e s ................ 1013 F i e l d case s t u d i e s ....................................... 1032 S p e c i a l a p p l i c a t i o n s o f h y d r a u l i c f r a c t u r i n g . . . . . . . . . . . . . 1039 Gravel p a c k i n g and sand c o n t r o l .......................... 1052 P e t r o p h y s i c a l e v a l u a t i o n f o r h y d r a u l i c f r a c t u r i n g and g r a 1061 v e l pack d e s i g n ..........................................
7.2. Subject key classification 7.2.1. 7.2.2. 7.2.3. 7.2.4. 7.2.5. 7.2.6. 7.2.7. 7.2.8. 7.2.9.
...................................................
1063
....................................................................
1205
7.3. Reference list Index
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xx I A b s t r a c t H y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g a r e common s t i m u l a t i o n and s t a b i l i z a t i o n t r e a t m e n t s d u r i n g completion, t e s t i n g and e x p l o i t a t i o n o f hydrocarbon r e s e r v o i r s i n o i l and gas e x p l o r a t i o n and p r o d u c t i o n i n d u s t r y . The p u r pose o f n a t u r a l sand o r s y n t h e t i c proppants o f d i f f e r e n t t y p e and g r a i n s i z e i s i n h y d r a u l i c f r a c t u r i n g t o s u p p o r t t h e c r a c k in o r d e r t o keep i t open a g a i n s t t h e c l o s u r e s t r e s s a c t i n g i n pay zone d e p t h and t o m a i n t a i n a h i g h l y - c o n d u c t i v e d r a i n a g e p a t h through t h e t i g h t r e s e r v o i r r o c k m a t r i x f o r o i l and gas f l o w i n g t o t h e borehole, and i n g r a v e l p a c k i n g t o p l u g t h e p e r f o r a t i o n t u n n e l s and t o b u i l d a gravel mantle along the borehole w a l l i n order t o f i l t e r the hydrocarbons f l o w i n g i n t o t h e w e l l b o r e and p r e v e n t pay zone sand f r o m moving. H y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g l e a d t o enhanced o i l and gas r e c o v e r y f r o m l o w - p e r m e a b i l i t y and weakly-cemented t o l o o s e f r i a b l e sandstones, w i t h t h e t r e a t m e n t s i m p r o v i n g hydrocarbon f l o w r a t e s and m i n i m i z i n g i n f l u x o f d i s t u r b i n g sand i n t o t h e borehole, r e s p e c t i v e l y . The p r e s e n t proceedings volume i s o r g a n i zed i n t o seven c h a p t e r s c o m p r i s i n g p r o p p a n t s e l e c t i o n , m a r k e t i n g and economics, R o t I i e g e n d s t i m u l a t i o n i n Europe, h y d r a u l i c proppant f r a c t u r i n g , g r a v e l p a c k i n g and sand c o n t r o l , propped f r a c t u r e and g r a v e l pack m o n i t o r i n g , and b i b l i o graphy.
P r o p p a n t
S e l e c t i o n
Proppant t y p e and g r a i n s i z e s e l e c t i o n i s t h e key element o f b o t h h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g design, because n a t u r a l sand o r s y n t h e t i c p r o p p a n t s a r e t h e o n l y m a t e r i a l l e f t i n p l a c e downhole a f t e r t e r m i n a t i o n o f t h e oper a t i o n and a r e t h e c r i t i c a l agents whose performance decides on success o r f a i l u r e o f the job.
Proppant types and properties The f i v e s t a n d a r d p r o p p a n t t y p e s which a r e a v a i l a b l e a t t h e moment i n t h e hydrocarbon s t i m u l a t i o n market f o r d i f f e r e n t p r i c e s a r e n a t u r a l q u a r t z sand, synthetic intermediate-strength low-density alumina silicate (ceramic) proppant, i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina oxide and silicate proppant, h i g h - s t r e n g t h b a u x i t e p r o p p a n t and h i g h - s t r e n g t h z i r c o n i a - s i l i c a t e p r o p p a n t . The spectrum o f c u r r e n t l y c o m m e r c i a l l y marketed m a t e r i a l s f o r f r a c t u r e s u p p o r t i s completed by sand and proppants c o a t e d by s i n g l e o r double r e s i n p e l l i c l e s o f p r e c u r e d o r c u r a b l e n a t u r e . The d i f f e r e n t p r o p p a n t t y p e s and g r a i n sizes are discussed along the l i n e s o f t h e i r mineralogical composition and c l o s u r e s t r e s s r e s i s t i v i t y , chemical c o m p o s i t i o n and a c i d s o l u b i l i t y , g r a i n s i z e s and c o n d u c t i v i t y p e r U S $, and g r a i n shapes. C a r e f u l c h o i c e o f p r o p p a n t t y p e and g r a i n s i z e a c c o r d i n g t o c l o s u r e s t r e s s o f t h e r e s e r v o i r , b o t t o m h o l e temperature, c o m p o s i t i o n of t h e f o r m a t i o n f l u i d s and p e r m e a b i l i t y c o n t r a s t b e t ween pay zone and i n f i l l e d f r a c t u r e a r e e c o n o m i c a l l y e s s e n t i a l due t o t h e f a c t t h a t i n deep t i g h t gas w e l l s , t h e proppant c o s t can r e a c h up t o two t h i r d s o f t h e t o t a l massive h y d r a u l i c f r a c t u r i n g (MHF) expenses. Aspects r e q u i r i n g c o n s i d e r a t i o n f o r p r o p p a n t s e l e c t i o n a r e s u r f a c e morphology and i n t e r n a l m i c r o p o r o s i t y , c r u s h i n g behaviour and c r y s t a l l i n e s t r u c t u r e , f r i c t i o n a n g l e and g r a i n shape, f l u i d s a l i n i t y and mechanochemical s t a b i l i t y , f l u i d temperature and h o t b r i n e a g g r e s s i v i t y , equipment a b r a s i o n and g r a i n hardness, s p e c i f i c g r a v i t y and f l u i d suspension p r o p e r t i e s , g r a i n s i z e and embedment, m a n u f a c t u r i n g process and p e l l e t composition, c o n d u c t i v i t y d i s c o u n t and proppant m i x i n g . Computer programs which model investment and revenue f o r var i o u s p r o p p a n t t y p e s and g r a i n s i z e s and g i v e a f o r e c a s t o f t h e expected product i o n i n c r e a s e and i t s n e t p r e s e n t v a l u e and d i s c o u n t e d cash f l o w a i d i n s t i m u l a t i o n p l a n n i n g by p r o v i d i n g c r i t e r i a f o r p r o p p a n t s e l e c t i o n f r o m b o t h t e c h n i c a l and economical aspects.
XXII
Proppant applications W h i l e t r a d i t i o n a l l y i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a s i l i c a t e and o x i d e p r o p p a n t s as we1 1 as h i g h - s t r e n g t h b a u x i t e p r o p p a n t s have been u t i 1 i z e d in t h e c u m u l a t i v e l a r g e s t amounts f o r m a s s i v e h y d r a u l i c f r a c t u r i n g o f deep t i g h t gas r e s e r v o i r s , t h e m a r k e t i s more and more s h i f t i n g t o w a r d s a p p l i c a t i o n o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y a l u m i n a s i l i c a t e p r o p p a n t s w h i c h have t h e a d v a n t a g e s o f a p p r o a c h i n g t h e s p e c i f i c g r a v i t y o f n a t u r a l sand and b e i n g c o n s i d e r a b l y c h e a p e r t h a n t h e o t h e r s y n t h e t i c p r o p p a n t t y p e s . As m o s t o f t h e p r e s e n t l y s t i m u l a t e d r e s e r v o i r s a r e i n such d e p t h r a n g e s where t h e c l o s u r e s t r e s s r e s i s t i v i t y o f alumina s i l i c a t e proppants would be s t i l l s u f f i c i e n t even i n l i g h t o f maximum p r e s s u r e drawdown d u r i n g l o n g - t e r m f i e l d p r o d u c t i o n , c o n s u m p t i o n o f t h e l i g h t e s t t y p e of s y n t h e t i c p r o p p a n t s i s e x p e c t e d t o p r o g r e s s i v e l y i n c r e a s e during the next years. High-strength z i r c o n i a - s i l i c a t e proppants represent a t e c h n i c a l l y f a v o u r a b l e p r o d u c t i f s t a y i n g r e a s o n a b l y b e l o w i t s s t a b i l i t y boundary a t which i t instantaneously f a i l s t o dust i n c o n t r a s t t o bauxite-based propp a n t s w h i c h b r e a k i n t o h a l v e s and q u a r t e r s t h a t r e t a i n o f t e n s u f f i c i e n t r e s i dual c o n d u c t i v i t y , b u t the elevated p r i c e prevents z i r c o n i a - s i l i c a t e proppants from a c h i e v i n g a l a r g e r market share. S y n t h e t i c proppants a r e a l s o used o u t s i d e o f h y d r o c a r b o n e x p l o r a t i o n and e x p l o i t a t i o n e s p e c i a l l y as h e a t - e x c h a n g e p e l l e t s in c u p o l a f u r n a c e s i n t h e s t e e l f o u n d r y , a l u m i n a and g l a s s i n d u s t r y , as d u s t - a b s o r p t i o n o r - f i l t e r i n g p e l l e t s i n t h e gas p u r i f i c a t i o n i n d u s t r y , and f o r f l u i d and gas f i l t r a t i o n , s h o t - b l a s t i n g and g r i n d i n g , and c o n s t i t u e n t s o f h i g h - s t a b i l i t y cements.
M a r k e t i n g
a n d
E c o n o m i c s
I n t e r m s o f m a r k e t i n g and e c o n o m i c a l a s p e c t s , s i n c e t h e i n v e n t i o n and f i r s t s u c c e s s f u l f i e l d a p p l i c a t i o n o f s y n t h e t i c s i n t e r e d a l u m i n a o x i d e ( c e r a m i c ) and f u s e d z i r c o n i a - s i l i c a t e p r o p p a n t s a b t . 10 y e a r s ago, t h e h y d r a u l i c s t i m u l a t i o n m a r k e t as w e l l as t h e g r a v e l p a c k i n g scene has been r e p e a t e d l y i n f l u e n c e d i n b o t h p o s i t i v e and n e g a t i v e manner b y changes o f h y d r o c a r b o n p r i c e s , U S $ e x change r a t e ( w i t h r e s p e c t t o DM and L ) and p r o p p a n t p r i c e s . V a r i o u s e c o n o m i c a l i n f l u e n c e s on h y d r a u l i c proppant f r a c t u r i n g a r e o u t l i n e d b y comparing o i l p r i c e e v o l u t i o n w i t h U S $ exchange r a t e h i s t o r y and p r o p p a n t p r i c e development, w i t h s p e c i a l comments o f f e r e d o n p r e s e n t and f u t u r e consequences o f t h e 1986 o i l p r i c e c o l l a p s e f o r t h e f r a c t u r i n g m a r k e t . An o u t l i n e i s a l s o g i v e n on t h e i m p a c t o f t h e oil p r i c e h i s t o r y on t h e gas m a r k e t and i t s e v o l u t i o n . The m a j o r c r i s i s o f o i l and gas i n d u s t r y i n e a r l y 1986 h i t t h e h y d r a u l i c f r a c t u r i n g mark e t segment i n Europe much s t r o n g e r t h a n t h a t i n USA, w i t h t h e m o s t i m p o r t a n t s t i m u l a t i o n m a r k e t d i f f e r e n c e s b e i n g based o n f r e q u e n c y o f d r i l l i n g and s t i m u l a tion, s u c c e s s / f a i l u r e r a t i o and c o n s o r t i a 1 s t r u c t u r e , o p e r a t i n g m e n t a l i t y and t r e a t m e n t c o s t , i n c r e a s e d c a n c e l l a t i o n r a t e s o f j o b s , U S $ exchange r a t e dec l i n e , and p r o p p a n t m a r k e t i n g s t r a t e g y .
Impact o f the oil price collapse and the
US $
exchange rate drop
The oil p r i c e c o l l a p s e and t h e US $ exchange r a t e d r o p a r e e x p e c t e d t o l e a d i n f u t u r e t o d e c l i n i n g r e s e r v e s due t o s u s p e n s i o n o f e x p l o r a t i o n and d e v e l o p ment ( i n c l u d i n g s t i m u l a t i o n ) , r e g a i n i n g power o f t h e OPEC, and p e r s i s t i n g mark e t u n c e r t a i n t y due t o o f oil p r i c e i n s t a b i l i t y and v o l a t i l i t y . The m o s t s i g n i f i c a n t p r e s e n t consequences o f t h e o i l p r i c e c r a s h and t h e U S $ b r e a k a r e changes i n f r a c t u r i n g s t r a t e g y , r e d u c t i o n o f c a p i t a l e x p e n d i t u r e , stimulation boat and d r i l l i n g r i g unemployment, and r e s t r u c t u r i n g o f o p e r a t i n g and s e r v i c e compan i e s a l o n g w i t h b u d g e t c u t t i n g and s t a f f l a y o f f . The i m p a c t o f t h e o i l p r i c e d r o p on t h e gas m a r k e t i s a t l e a s t i n p a r t s o f Europe c o n s i d e r a b l y weakened o r compensated b y f i r m p l a n n i n g o f gas d e l i v e r y w h i c h i s s e c u r e d b y c o n t r a c t s and t h u s t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g a c t i v i t y i n t h e S o u t h e r n N o r t h Sea g a s f i e l d b e l t i n g o i n g t o become t h e a b s o l u t e h i g h l i g h t o f s t i m u l a t i o n s i n c e t h e
XXIII b e g i n n i n g o f t r e a t m e n t campaigns i n Europe d e s p i t e t h e u n f a v o u r a b l e economical framework c o n d i t i o n s , and w i l l a l s o r e p r e s e n t t h e c u l m i n a t i o n o f proppant consumption so f a r i n t h i s area. I n comparison t o t h e USA and o t h e r p a r t s o f t h e world, t h e u n d e r s t a n d i n g o f t h e a r e a l market i s i n Europe p a r t i c u l a r l y c o m p l i c a t e d due t o d i f f e r e n t economic a l systems and r e s u l t i n g d i f f e r e n t commercial c a p a c i t i e s due t o t h e q u e s t i o n o f c u r r e n c y c o n v e r t i b i l i t y and n a t i o n a l o r g a n i z a t i o n o f companies i n r e l a t i o n s h i p t o t r a d e o f f i c e s and m i n i s t r i e s , as w e l l as due t o v a r i o u s m e n t a l i t i e s and languages. The p r e s e n t s t i m u l a t i o n market i s c h a r a c t e r i z e d by r e o r g a n i z a t i o n i n b o t h Western and E a s t e r n Europe. W h i l e i n Western Europe some m a j o r t i g h t gas development campaigns a r e based on t h e i n e v i t a b i l i t y t o s t i m u l a t e most o f t h e w e l l s by massive h y d r a u l i c p r o p p a n t f r a c t u r i n g t h e r e b y f o l l o w i n g a concept which has a l r e a d y been f r e q u e n t l y a p p l i e d i n many l o w - p e r m e a b i l i t y gas patches i n t h e USA, i m p r o v i n g economical p o s s i b i l i t i e s and more a d m i n i s t r a t i v e independence i n E a s t e r n Europe a l o n g t h e l i n e s o f a m a j o r r e s t r u c t u r i z a t i o n as w e l l as t h e s h i f t i n g emphasis f r o m d r i l l i n g and f a s t c o m p l e t i o n t o i n c r e a s i n g e x p l o i t a t i o n o f r e s e r v e s i n e x i s t i n g w e l l s a l l o w s l o w l y p r o g r e s s i n g m a t u r i n g o f hydraul i c p r o p p a n t f r a c t u r i n g a l s o i n t h i s area.
Aspects o f areal hydraulic proppant fracturing potential The account o f p a s t , p r e s e n t and f u t u r e a r e a l h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l i n t h e European market w i t h comparisons t o USA and o t h e r p a r t s o f t h e w o r l d focusses on massive s t i m u l a t i o n o f t i g h t gas sandstones i n R o t l i e g e n d and C a r b o n i f e r o u s i n B r i t i s h Southern N o r t h Sea and Germany FRG which a r e c h a r a c t e r i z e d by t h e overwhelming share o f p r o p p a n t consumption. F u r t h e r h i g h l i g h t s a r e massive t r e a t m e n t s on T e r t i a r y sediments i n Yugoslavia where t o g e t h e r w i t h t h e mentioned f o r m a t i o n s i n Germany FRG t h e h i t h e r t o l a r g e s t j o b s have been c a r r i e d o u t i n terms o f p r o p p a n t q u a n t i t y and f l u i d volume. The impact o f t h e o i l p r i c e c r a s h i s p a r t i c u l a r l y i l l u s t r a t e d a l o n g t h e l i n e s o f unemployment o f d r i l l i n g r i g s and s t i m u l a t i o n boats, i n c r e a s e d c a n c e l l a t i o n r a t e s o f j o b s , s t a f f l a y o f f and company merging, and business l o s s f o r proppant s u p p l i e r s . T a i l - i n p r o p p a n t s t a g i n g r e p r e s e n t s a m a j o r t o o l o f c o s t containment i n s t i m u l a t i o n j o b s i n t h e B r i t i s h Southern N o r t h Sea where t h e g r e a t e s t amount o f f r a c t u r i n g a c t i v i t y i s t a k i n g p l a c e i n t h e coming y e a r s . The h i g h e s t s i g n i f i c a n c e f o r p r o m o t i o n o f hyd r a u l i c r e s e r v o i r s t i m u l a t i o n i n E a s t e r n Europe i s t h e r e o r g a n i z a t i o n ( p e r e s t r o i k a ) i n t h e USSR which t o g e t h e r w i t h i n c r e a s i n g f i n a n c i a l p o s s i b i l i t i e s i n t h e s a t e l l i t e c o u n t r i e s such as w o r l d bank c r e d i t s and e x p o r t revenues enable t o r e a l i z e numerous p r o j e c t s i n t h e near f u t u r e .
R o t l i e g e n d
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i n
E u r o p e
R o t l i e g e n d ( a n d t o s u b o r d i n a t e amounts a l s o C a r b o n i f e r o u s and B u n t s a n d s t e i n ) gas r e s e r v o i r s a r e h i g h - p o t e n t i a l examples o f l o w - p e r m e a b i l i t y ( t i g h t ) gas sands which have c o n s i d e r a b l e economical s i g n i f i c a n c e as gas r e s o u r c e s i n p r e s e n t and f u t u r e , and because t h e y r e p r e s e n t t h e c e n t r e o f massive h y d r a u l i c f r a c t u r i n g (MHF) a c t i v i t y i n Europe and have seen some o r t h e l a r g e s t s t i m u l a t i o n o p e r a t i o n s c a r r i e d o u t w i t h alumina o x i d e and s i l i c a t e proppants w o r l d wide, t h e y a r e c h a r a c t e r i z e d as a case h i s t o r y i n more d e t a i l . R o t l i e g e n d t i g h t gas development by massive t r e a t m e n t s i n t h e f a c i e s b e l t between t h e m a r g i n a l a e o l i a n dune and f l u v i a l b r a i d e d - r i v e r t o s h e e t - f l o o d sandstones and t h e cent r a l p l a y a and l a c u s t r i n e mudstones i n s e v e r a l p a r t s o f t h e N o r t h Sea w i l l be t h e h i g h l i g h t o f h y d r a u l i c proppant f r a c t u r i n g d u r i n g t h e n e x t f i v e y e a r s i n Europe and t h u s a l s o t h e c e n t r e o f proppant consumption i n t h i s area. R o t l i e g e n d (and a l s o C a r b o n i f e r o u s ) gas r e s e r v o i r f r a c t u r i n g i s sketched along aspects o f d i s t r i b u t i o n o f main f i e l d s i n h o r i z o n t a l and v e r t i c a l d i r e c t i o n , h i s t o r i c a l development o f p r o d u c t i o n and f r a c t u r i n g , p r o p p a n t consumpt i o n , sedimentary-petrophysical and geochemical z o n a t i o n o f f r a c t u r i n g poten-
XXIV t i a l , p e r m e a b i l i t y p r e d i c t i o n and e f f e c t i v e p e r m e a b i l i t y , p e r m e a b i l i t y d e t e r i o r a t i o n and c l a y m i n e r a l d i s t r i b u t i o n , minimum p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e and poroperm p r o p e r t i e s , d i f f e r e n c e s between e x p l o r a t i o n and development w e l l s , s t i m u l a t i o n c o s t and f r a c t u r i n g p o l i c y , p h i l o s o p h y o f o f f - and onshore e x p l o r a t i o n w e l l f r a c t u r i n g , i m p l i c a t i o n s o f f r a c t u r i n g p h i l o s o p h y on p r o p p a n t s e l e c t i o n , a p p r a i s a l d r i l l i n g s t r a t e g y concepts and experience, h o r i z o n t a l and v e r t i c a l d i f f e r e n c e s o f f r a c t u r i n g p o t e n t i a l i n terms o f p e r m e a b i l i t y and gas composition, w a t e r s e n s i t i v i t y , w a t e r s a t u r a t i o n , gas demand and r e s e r v o i r p e r formance. The i l l u s t r a t i o n o f f r a c t u r i n g performance i n c l u d e s a l s o comments on s t i m u l a t i o n b o a t a v a i l a b i l i t y and c a p a c i t y , s p o t vs. b a t c h f r a c t u r i n g , e x p l o r a t i o n / a p p r a i s a l vs. development w e l l s i n terms o f p r o p p a n t s e l e c t i o n and c a n c e l l a t i o n r i s k , and impact o f p o l i t i c a l - e c o n o m i c a l systems on s t i m u l a t i o n r e q u i r e m e n t . An o u t l i n e o f s e d i m e n t o l o g i c a l and p e t r o p h y s i c a l e v o l u t i o n o f v a r i o u s t i g h t gas sandstones i s a l s o i n c o r p o r a t e d i n t o t h e d i s c u s s i o n , and foam f r a c t u r i n g as a s p e c i a l t e c h n i q u e f o r w a t e r - s e n s i t i v e r e s e r v o i r s i s p r e s e n t e d more g e n e r a l l y . V a r i o u s sedimentary s t r u c t u r e s o f f l u v i a l , a e o l i a n and l a c u s t r i n e d e p o s i t s i n R o t l i e g e n d and B u n t s a n d s t e i n i l l u s t r a t i n g d i s t r i b u t i o n o f r e s e r v o i r h e t e r o g e n e i t i e s and n a t u r a l f r a c t u r e s as w e l l as b e i n g o f c o n s i d e r a b l e s i g n i f i c a n c e f o r f l o w p a t t e r n s o f pay f l u i d s and p r o p a g a t i o n o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s a r e p r e s e n t e d i n p h o t o g r a p h i c a l p l a t e s and accompanied by d e t a i l e d c a p t i o n s t h a t can even be e v a l u a t e d independent f r o m t h e main t e x t .
H y d r a u l i c
P r o p p a n t
F r a c t u r i n g
The account o f h y d r a u l i c p r o p p a n t f r a c t u r i n g f o c u s s i n g on t e c h n o l o g i c a l p e r s p e c t i v e s o f p o t e n t i a l e x t e n s i o n o f t h e s t i m u l a t i o n market i n t h e near f u t u r e i n c l u d e s i l l u s t r a t i o n o f v a r i o u s aspects o f containment o f f r a c t u r e p r o p a g a t i o n i n t h i n sandstones, proppant s a t u r a t i o n and c o n d u c t i v i t y improvement w i t h pumpi n g o f up t o u l t r a - h i g h c o n c e n t r a t i o n s , t a r g e t s o f u n c o n v e n t i o n a l gas s t i m u l a t i o n , h y d r a u l i c p r o p p a n t f r a c t u r i n g o f carbonate rocks, h i g h - p e r m e a b i l i t y r e s e r v o i r f r a c t u r i n g , geothermal f r a c t u r i n g , m i n i f r a c t u r i n g (mHF) and o t h e r s m a l l s c a l e f r a c t u r i n g , o i l - r e s e r v o i r f r a c t u r i n g , communication f r a c t u r i n g and f r a c t u r e i n t e r f e r e n c e . f r a c t u r i n s o f d e v i a t e d w e l l s . f r a c u r e damage, and aggress i v e f r a c t u r i n g design.
Fracture containment and propagation Containment o f f r a c t u r e p r o p a g a t i o n i s p a r t i c u l a r l y s g n i f i c a n t f o r a v o i d i n g t h e o r i q i n o f communication t h r o u q h t h e c r a c k t o under- and/or o v e r l y i n q w a t e r o r o t h e r - t y p e o f hydrocarbon-bearing h o r i z o n s , o v e r p r e s s u r e d o r o t h e r t r o u b l e making beds, and t h e ground w a t e r s t o r e y i n case o f v e r y s h a l l o w r e s e r v o i r depth. Another d e c i s i v e impact i s w a s t i n g o f s t i m u l a t i o n m a t e r i a l and energy f o r opening and p l u g g i n g o f a d j o i n i n g b a r r e n i n t e r v a l s a t t h e expense o f propag a t i o n and i n f i l l i n g o f t h e c r a c k i n t h e p r o s p e c t i v e h o r i z o n a c c o r d i n g t o des i g n t o achieve t h e necessary l e n g t h and c o n d u c t i v i t y f o r p r o f i t a b l e r e s e r v o i r d r a i n a g e . The most i m p o r t a n t aspects o f containment o f f r a c t u r e p r o p a g a t i o n i n t h i n sheet sandstones a r e s i n g l e vs. m u l t i p l e f r a c t u r i n g and d i v e r t i n g t e c h n i ques, t r e a t m e n t parameter adjustment, h o r i z o n t a l s t r e s s d i f f e r e n c e s and f r a c t u r e geometry, c o n f i n i n g s t r e s s c o n t r a s t , m u l t i p l e - z o n e f r a c t u r e m i g r a t i o n , sel e c t i v e proppant placement, v e r t i c a l f r a c t u r e growth blockage by buoyant o r s e t t l i n g d i v e r t e r s , r e s e r v o i r e l a s t i c i t y vs. p l a s t i c i t y ; r e s e r v o i r h e t e r o g e n e i t y f r o m s e d i m e n t o l o g i c a l , p e t r o p h y s i c a l and r o c k mechanical p o i n t o f view; inf l u e n c e o f g e o l o g i c a l d i s c o n t i n u i t i e s on f r a c t u r e e x t e n s i o n and morphology, and c o m b i n a t i o n o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g .
xxv
Proppant saturation and conductivity inprovement Proppant c o n c e n t r a t i o n i s one o f t h e m a j o r t o o l s f o r c o n d u c t i v i t y improvement, w i t h m u l t i l a y e r f r a c t u r e p r o p p i n g w i t h h i g h s a t u r a t i o n o f m a t e r i a l b e i n g t h e o n l y p o s s i b i l i t y o f s i g n i f i c a n t r e d u c t i o n o f f l o w c a p a c i t y damage by propp a n t c r u s h i n g and embedment i n many h a r d and s o f t r e s e r v o i r r o c k s , r e s p e c t i v e l y . M u l t i l a y e r high-concentration propping also helps t o maintain s u f f i c i e n t f r a c t u r e w i d t h and t h e r e b y t o a v o i d t h e occurrence o f non-Darcy f l o w . U l t r a h i g h f r a c t u r e c o n d u c t i v i t y can be achieved a p a r t f r o m o p t i m i z a t i o n o f p r o p p a n t s a t u r a t i o n b y s e l e c t i n g a h i g h e r - q u a l i t y proppant t y p e and by a p p l i c a t i o n o f l a r g e r p r o p p a n t g r a i n s i z e s . The s i g n i f i c a n c e o f proppant c o n c e n t r a t i o n i s ill u s t r a t e d by d i s c u s s i n g p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e , proppant t r a n s p o r t and f r a c t u r e damage, g e l f i l t e r cakes d e p o s i t i o n on f r a c t u r e w a l l s , f l u i d v i s c o s i t y and c a r r i e r c a p a c i t y , p r o p p a n t d e n s i t y l f l u i d v i s c o s i t y and/or d e n s i t y r e l a t i o n s h i p , o v e r p r e s s u r e c a l i b r a t e d s t i m u l a t i o n design, impact o f delayed crosslinking, f l u i d h e a t c a p a c i t y and r e s e r v o i r temperature, f l u i d l e a k o f f containment, f r a c t u r e w i d t h vs. p r o p p a n t b r i d g i n g , m i x i n g o f round and a n g u l a r p r o p p a n t g r a i n s , and u n i f o r m vs. changing f r a c t u r e c o n d u c t i v i t y .
Targets o f unconventional gas stimulation The main t a r g e t s o f u n c o n v e n t i o n a l gas s t i m u l a t i o n a r e t i g h t gas sandstones, geopressured r e s e r v o i r s , c o a l seams, shales, h i g h - t e m p e r a t u r e pay zones, and gas condensate-bearing i n t e r v a l s . V a r i o u s f r a c t u r i n g s t i m u l a t i o n t y p e s a r e o u t l i n e d f o r the d i f f e r e n t r e s e r v o i r rock types. While c r e a t i o n o f long f r a c t u r e s d e e p l y p e n e t r a t i n g i n t o t h e pay and supported by i n t e r m e d i a t e - t o h i g h - s t r e n g t h proppants t h a t a r e r e s i s t i n g t o h i g h c l o s u r e s t r e s s e s a r e t h e b e s t approach t o t i g h t gas sandstones and c l a s s i c a l massive h y d r a u l i c t r e a t m e n t i s t h e most f e a s i b l e means o f a c q u i s i t i o n o f gas r e s e r v e s , e x p l o s i v e and non-proppant n i t r o g e n gas f r a c t u r i n g have proven t o be a t l e a s t p a r t i a l l y successive i n s h a l e s t i m u l a t i o n , a l t h o u g h i n many cases h y d r a u l i c proppant f r a c t u r i n g i s a l s o t h e most sup e r i o r t e c h n i q u e i n t h e l a t t e r t y p e o f pay f o r m a t i o n s . Coal seams a r e c h a r a c t e r i z e d by s p e c i a l mechanical p r o p e r t i e s which l e a d t o e l e v a t e d t r e a t i n g pressur e s and f r e q u e n t l y r e s t r i c t t h e volume o f f l u i d and proppants t h a t can be i n s e r t e d i n t o s h o r t and wide c r a c k s . F r a c t u r i n g s t r a t e g i e s a r e m a i n l y designed acc o r d i n g t o t h i c k n e s s and i n t e r v a l spacing o f c o a l seams i n t h e s t r a t i g r a p h i c a l column. High-temperature r e s e r v o i r s p r o v i d e p a r t i c u l a r c h a l l e n g e s t o thermal f l u i d and p r o p p a n t s t a b i l i t y . Aspects o f o t h e r sources o f deep and u n c o n v e n t i o n a l gas such as a b i o g e n i c gas, subducted gas, gas condensate and gas h y d r a t e a r e a l s o b r i e f l y mentioned.
Carbonate and high-permeability sandstone proppant fracturing H y d r a u l i c p r o p p a n t f r a c t u r i n g o f b o t h sandstone and carbonate r o c k s i s cons i d e r e d t o be t h e most s i g n i f i c a n t s t i m u l a t i o n technique i n l o w - p e r m e a b i l i t y r e s e r v o i r s r e g a r d l e s s o f t h e i r c o m p o s i t i o n . W h i l e t i g h t sandstone pay zones a r e a l r e a d y s i n c e l o n g t i m e t r e a t e d by massive h y d r a u l i c f r a c t u r i n g a l l around t h e world, l o w - p e r m e a b i l i t y carbonate r e s e r v o i r s have n o t y e t been f u l l y understood as p r o f i t a b l e t a r g e t s f o r h y d r a u l i c proppant s t i m u l a t i o n . The main reason f o r t h e e f f e c t i v i t y o f p r o p p a n t f r a c t u r i n g i n hydrocarbon-bearing i n t e r v a l s o f any t y p e i s t h e c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r r o c k m a t r i x and p r o p p a n t package i n t h e f r a c t u r e . While h i t h e r t o m a i n l y a c i d i z i n g and a c i d f r a c t u r i n g have been c a r r i e d o u t i n carbonate r e s e r v o i r s i n Europe, s p r e a d i n g a p p l i c a t i o n o f proppant f r a c t u r i n g p a r t i a l T y i n c o m b i n a t i o n w i t h a c i d f r a c t u r i n g i s c o n s i d e r e d t o be t h e o n l y reasonable p o s s i b i l i t y t o s i g n i f i c a n t l y i n c r e a s e t h e o u t p u t f r o m t h i s t y p e o f hydrocarbon-bearing s e c t i o n s i n t h e n e x t y e a r s . Aspects o f hyd r a u l i c p r o p p a n t f r a c t u r i n g o f carbonate r o c k s a r e summarized a l o n g t h e l i n e s o f e x t e n s i o n o f d r a i n a g e p a t h and c o n d u c t i v i t y c o n t r a s t , open vs. propped f r a c t u r e s , c o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r e s , and c o m b i n a t i o n o f hyd r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g . The comparative d i s c u s s i o n o f a c i d i z i n g
XXVI s t i m u l a t i o n and p r o p p a n t f r a c t u r i n g a l s o i n c l u d e s comments on a d v a n t a g e s and drawbacks o f f r a c t u r e p r o p p i n g , c o n d u c t i v i t y improvement b y a c i d o p t i m i z a t i o n , enhancement o f a c i d p e n e t r a t i o n d i s t a n c e , a c i d r e t a r d a t i o n , and a c i d f l u i d - l o s s control.
I n a s i m i l a r way as c a r b o n a t e s , a l s o m o d e r a t e - and h i g h - p e r m e a b i l i t y sands t o n e r e s e r v o i r s c a n be s u c c e s s f u l l y s t i m u l a t e d b y h y d r a u l i c p r o p p a n t f r a c t u r i n g by c r e a t i n g a s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t between p a y zone m a t r i x and p r o p p a n t wedge i n t h e f r a c t u r e , w i t h t h e v a r i a b l e s f o r i n c r e a s i n g c o n d u c t i v i t y c o n t r a s t b e i n g type, g r a i n s i z e and c o n c e n t r a t i o n o f t h e p r o p p a n t s u s e d f o r plugging o f the crack.
Chalk fracturing One m a j o r t a r g e t o f c a r b o n a t e p r o p p a n t f r a c t u r i n g i s t h e C r e t a c e o u s c h a l k o i l r e s e r v o i r i n t h e N o r t h Sea w h i c h i s i l l u s t r a t e d as a c a s e s t u d y i n more det a i l . The c h a l k r e q u i r e s f r e q u e n t l y c o m b i n a t i o n s o f e i t h e r a c i d and p r o p p a n t f r a c t u r i n g , o r f r a c t u r i n g and g r a v e l p a c k i n g f o r t e c h n i c a l l y s u c c e s s f u l and e c o n o m i c a l l y p r o f i t a b l e s t i m u l a t i o n . W h i l e some c h a l k t y p e s a r e s t a b l e and e x t e n s i v e l y n a t u r a l l y f r a c t u r e d , o t h e r c h a l k s a r e s o f t and f l o w i n g as a consequence o f o v e r p r e s s u r i n g and u n d e r c o m p a c t i o n and b e l o n g t o t h e m o s t c o m p l i c a t e d r e s e r v o i r s i n t e r m s o f t e c h n i c a l l y s u c c e s s f u l and e c o n o m i c a l l y p r o f i t a b l e s t i m u l a t i o n . S p e c i a l c h a l k f r a c t u r i n g t e c h n i q u e s i n c l u d e t i p s c r e e n o u t , i n t e r f a c e , dend r i t i c and w a t e r f r a c t u r i n g . The c o m b i n a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g i s f u r t h e r s k e t c h e d a l o n g t h e l i n e s o f v i s c o u s f i n g e r i n g a c i d i z i n g and wormhole g r o w t h , w h i s p e r f r a c t u ring, f o r m a t i o n breakdown and c l o s e d f r a c t u r e a c i d i z i n g . The b e s t s o l u t i o n a r e f r e q u e n t l y s h o r t w i d e p r e c i s e f r a c t u r e s i n f i l l e d by h i g h p r o p p a n t c o n c e n t r a t i o n s p l a c e d i n m u l t i l a y e r s . Comments a r e a l s o o f f e r e d on r e s e r v o i r c o m p a c t i o n and f i e l d s u b s i d e n c e w h i c h i s a m a j o r p r o b l e m accompanying s t i m u l a t i o n and e x p l o i t a t i o n o f c h a l k f i e l d s and w h i c h has i n t h e l a s t y e a r s a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n t h r o u g h s p e c t a c u l a r p l a t f o r m j a c k u p o p e r a t i o n s . An o u t l i n e o f s a n d s t o n e a c i d i z i n g i s a l s o p r o v i d e d f o c u s s i n g on r e a c t i o n t y p e s and p e n e t r a t i o n d i s t a n c e o f a c i d . S p e c i a l methods f o r e f f e c t i v i t y enhancement a r e d e l a y e d a c i d r e a c t i o n and i n - s i t u a c i d g e n e r a t i o n .
Geothermal fracturing Geothermal f r a c t u r i n g i s c h i e f l y c o n f r o n t e d w i t h t h e p r o b l e m o f f l u i d s t a b i l i t y i n t h e h o t r e s e r v o i r s . W h i l e h i t h e r t o i n many c a s e s b o t h c o n v e n t i o n a l and s t e m - i n d u c e d e x p l o s i v e f r a c t u r i n g w i t h o u t i n s e r t i o n of p r o p p a n t s have been app l i e d as s t i m u l a t i o n t e c h n i q u e s o f g e o t h e r m a l r e s e r v o i r s , improvement o f f l u i d r e s i s t i v i t y , t e m p o r a r y f o r m a t i o n c o o l i n g b y pumping of l a r g e p r e - p a d and p a d volumes, a p p l i c a t i o n o f w a t e r as f r a c t u r i n g f l u i d and enhanced p r o p p a n t p l a c e m e n t t e c h n i q u e s have a l l o w e d t o c o n s i d e r p r o p p a n t f r a c t u r i n g a l s o as a c o m p e t i t i v e s t i m u l a t i o n technique i n geothermal r e s e r v o i r s . G i a n t - s c a l e w a t e r f r a c t u r i n g encompasses t h e h i t h e r t o b y f a r l a r g e s t amounts o f f l u i d pumped d u r i n g c o u r s e o f an i n d i v i d u a l s t i m u l a t i o n j o b . Communication f r a c t u r i n g c o n n e c t i n g i n j e c t i o n and p r o d u c t i o n w e l l s i s o f p a r t i c u l a r s i g n i f i c a n c e i n g e o t h e r m a l p a y s i n c o n t r a s t t o t h e i r u n d e s i r e d n a t u r e i n o i l - and g a s - b e a r i n g f o r m a t i o n s . Enhancement o f d r a i n a g e i s a c h i e v e d b y f r a c t u r e s y s t e m e n l a r g e m e n t b y h e a t e x t r a c t i o n as a consequence o f c o l d w a t e r i n j e c t i o n i n t o h o t d r y r o c k s . N u c l e a r f r a c t u r i n g i s a p o s s i b i l i t y o f e s p e c i a l l y l a r g e - s c a l e e x p l o s i v e r e s e r v o i r r u b b l i z a t i o n and i s a b l e t o overcome t h e drawbacks o f more s t a n d a r d e x p l o s i v e f r a c t u r i n g t e c h n i q u e s c o m p r i s i n g s h o r t f r a c t u r e e x t e n s i o n and i n t e n s i v e m a t r i x p u l v e r i z a t i o n . An o u t l i n e i s a l s o g i v e n on s h e a r s t i m u l a t i o n v s . t e n s i l e f r a c t u r i n g .
XXVII
Mini f ract ur ins and other small -scale fracturing M i n i f r a c t u r i n g (mHF) o r comparable s m a l l e r - s c a l e t r e a t m e n t s a r e v e r y s i g n i f i c a n t f o r t h e i n i t i a l approach o f many m a r g i n a l and p a r t i a l l y s t i l l uncommercial r e s e r v o i r s i n t h e near f u t u r e and a r e assessed as b e i n g one o f t h e m a j o r methods t o m a i n t a i n o r even a c c e l e r a t e t h e t e c h n o l o g i c a l e v o l u t i o n o f h y d r a u l i c f r a c t u r i n g d e s p i t e t h e p r e s e n t l y u n f a v o u r a b l e economical c o n s t e l l a t i o n . F o l l o w i n g d e f i n i t i o n of m i n i - , m i c r o - and massive f r a c t u r e s , m i n i f r a c t u r i n g and i t s importance i s d i s c u s s e d along t h e l i n e s o f p r e - f r a c t u r e breakdown t e s t and f r a c t u r e parameter d e t e r m i n a t i o n , f o r m a t i o n damage around t h e w e l l b o r e , f i e l d exper i m e n t s and mHF v s . MHF, p i l o t s t i m u l a t i o n o f m a r g i n a l r e s e r v o i r s , a p p l i c a t i o n o f r e s i n - c o a t e d proppants i n u n s t a b l e pay zones, h o r i z o n t a l o r d r a i n h o l e d r i l l i n g and f r a c t u r i n g , uptake c a p a c i t y o f o i l - f i e l d w a t e r i n j e c t i o n and w a s t e - l i q u i d d i s p o s a l w e l l s , c o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r e s , l a t e stage r e f r a c t u r i n g o f o l d w e l l s , c o m b i n a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g , and f r a c t u r e l e n g t h and h e i g h t o p t i m i z a t i o n . The main s i g n i f i c a n c e o f m i n i f r a c t u r e c a l i b r a t i o n experiments comprises det e r m i n a t i o n o f c l o s u r e s t r e s s and f l u i d - l o s s c o e f f i c i e n t which a r e i m p o r t a n t parameters f o r r e d u c t i o n o f s c r e e n o u t f a i l u r e r a t e s . V a r i o u s m i n i f r a c t u r e t e s t s and m i c r o f r a c t u r e breakdown procedures as w e l l as proppant vs. non-proppant m i n i f r a c t u r i n g a r e discussed. S h o r t p r e c i s e propped m i n i f r a c t u r e s a r e s u i t a b l e f o r b r e a k t h r o u g h o f f o r m a t i o n damage haloes around t h e w e l l b o r e , c o s t c o n t a i n ment o f s t i m u l a t i o n o f v a r i o u s r e s e r v o i r types, and p i l o t approach o f m a r g i n a l pays. S p e c i a l techniques a r e t i p screenout f r a c t u r i n g and t a i l - i n o f r e s i n - c o a t e d proppants f o r p r e v e n t i o n o f embedment and flowback. M o d i f i e d m i c r o - and m i n i f r a c t u r i n g analysis f o r overpressure c a l i b r a t e d s t i m u l a t i o n design are also ou t l ined.
Horizontal borehole and water-injection well fracturing H o r i z o n t a l w e l l f r a c t u r i n g i s most p r o f i t a b l e i n c o m b i n a t i o n w i t h r a t h e r than as a l t e r n a t i v e t o h o r i z o n t a l h o l e d r i l l i n g , and p a r t i c u l a r l y thorough access t o t h e r e s e r v o i r m a t r i x can be achieved by m u l t i p l e p a r a l l e l v e r t i c a l f r a c turing o f horizontal wellbore trajectories creating a suite o f s l i c e s intersecti n g t h e pay w h i c h i s most s u c c e s s f u l i f t h e h o r i z o n t a l b o r e h o l e i s d r i l l e d i n accordance w i t h t h e g e o s t r e s s f i e l d . O i l - f i e l d w a t e r - f l o o d and w a s t e - l i q u i d d i s posal w e l l s a r e c h a r a c t e r i z e d by almost permanent o p e r a t i o n above f r a c t u r e p r e s s u r e which r e s u l t s n o t o n l y i n r e s e r v o i r p a r t i n g , b u t a l s o i n keeping t h e c r e a t e d c r a c k s open as l o n g as i n j e c t i o n a t e l e v a t e d p r e s s u r e s i s g o i n g on, w i t h t h e r e f o r e i n most cases p r o p p i n g o f t h e f r a c t u r e s n o t b e i n g r e q u i r e d . D i s t i n c t i o n has t o be made between c o n t r o l l e d f r a c t u r i n g f o l l o w i n g g o a l s o f improvement o f w e l l i n j e c t i v i t y , and a c c i d e n t a l f r a c t u r i n g which can provoke submature f l u i d b r e a k t h r o u g h and t h u s d e t e r i o r a t e sweep e f f i c i e n c y . D i s p o s a l f r a c t u r i n g i s i n some areas t h e o n l y p o s s i b i l i t y t o c r e a t e s u f f i c i e n t uptake c a p a c i t y f o r safe underground removal o f waste l i q u i d s f r o m d r i l l i n g , c o m p l e t i o n and hydrocarbon p r o c e s s i n g .
Naturally- jointed reservoir st irnulat ion H y d r a u l i c proppant f r a c t u r i n g o f n a t u r a l l y j o i i i t e d r e s e r v o i r s focusses on i n t e r a c t i o n between n a t u r a l and h y d r a u l i c c r a c k s . P e r m e a b i l i t y c o n t i n u i t y and s t r e s s s e n s i t i v i t y a r e i m p o r t a n t parameters c o n t r o l l i n g i n t e r f r a c t u r e communic a t i o n . Other aspects which a r e d i s c u s s e d i n c l u d e f r a c t u r e i n t e r f e r e n c e and sup e r i m p o s i t i o n , f r a c t u r e divergence, temporal n a t u r a l f r a c t u r e damage, dual f l u i d l e a k o f f ( c o m p r i s i n g s t a n d a r d l o s s t o m a t r i x and a c c e l e r a t e d l o s s t o h a i r l i n e f i s s u r e s w h i c h open up i n an advanced stage o f t h e j o b ) , and proppant p l u g ging o f natural f r a c t u r e s . A special s t i m u l a t i o n technique f o r n a t u r a l l y cracked r e s e r v o i r s i s d e n d r i t i c f r a c t u r i n g which comprises e n t e r i n g and b a l l o o n i n g of t h e p r e - e x i s t i n g j o i n t s by t h e h y d r a u l i c t r e a t m e n t f l u i d s and c o n n e c t i n g
XXVIII t h e whole system t o t h e w e l l b o r e . Propping o f n a t u r a l c r a c k s o n l y l e a d s t o accel e r a t i o n o f p r o d u c t i o n , b u t does n o t e n a b l e t o i n c r e a s e t h e r e c o v e r a b l e r e s e r ves, as no a d d i t i o n a l d r a i n a g e area is c r e a t e d . Aspects o f t a i l o r e d p u l s e l o a d i n g and shear s l i p p a g e vs. t e n s i l e f a i l u r e a r e a l s o discussed.
Refracturing R e f r a c t u r i n g o f a l r e a d y e a r l i e r s t i m u l a t e d b o r e h o l e s and o r i g i n a l f r a c t u r i n g o f i n f i l l w e l l s i n l a t e stage o f r e s e r v o i r p r e s s u r e d e p l e t i o n and hydrocarbon e x p l o i t a t i o n is a s u i t a b l e means o f cheap m o b i l i z a t i o n o f a d d i t i o n a l o i l and gas q u a n t i t i e s , because o l d b o r e h o l e s a r e a l r e a d y c o m p l e t e l y a m o r t i z e d o r w r i t t e n - o f f and t h e o r i g i n a l f r a c t u r e r e p r e s e n t s a weakness p l a n e which a l l o w s eas i e r r e o p e n i n g a t l o w e r i n j e c t i o n p r e s s u r e s . Removal o f t h e o l d p r o p p a n t package has s o f a r n o t t u r n e d o u t t o be a m a j o r problem. R e f r a c t u r i n g c a n d i d a t e w e l l c h o i c e has t o f o l l o w t h e e x p e r i e n c e o f t h e o r i g i n a l s t i m u l a t i o n j o b , and p r o p p a n t s e l e c t i o n f o r r e f r a c t u r i n g p a r t i a l l y a l s o depends on t h e aims o f accel e r a t i o n vs. i n c r e a s i n g r e c o v e r a b l e r e s e r v e s . The approach o f r e f r a c t u r i n g cand i d a t e c h o i c e i s p a r t i a l l y comparable t o t h a t o f m i n i f r a c t u r e c a l i b r a t i o n t e s t e v a l u a t i o n . The main advantage o f renewed f r a c t u r i n g f o l l o w i n g an o r i g i n a l t r e a t m e n t independent f r o m t h e t i m e t h a t has passed i n between b e n e f i t s f r o m r e c o v e r y enhancement independent f r o m r e s e r v o i r d e p l e t i o n stage. S p e c i a l methods a r e combinations between e i t h e r h y d r a u l i c and e x p l o s i v e o r a c i d and p r o p p a n t fracturing. Concerning f r a c t u r e l e n g t h and h e i g h t o p t i m i z a t i o n , t h e most i m p o r t a n t p a r a meters i n f l u e n c i n g d e s i g n a r e r e s e r v o i r p e r m e a b i l i t y , w e l l spacing, f r a c t u r e c o n d u c t i v i t y and proppant d i s t r i b u t i o n . G e n e r a l l y l o n g and s h o r t f r a c t u r e s a r e r e q u i r e d i n low- and h i g h - p e r m e a b i l i t y r e s e r v o i r s , r e s p e c t i v e l y . N e t p r e s e n t val u e o p t i m i z a t i o n r e v e a l s t h a t e x c e s s i v e f r a c t u r e l e n g t h does n o t r e s u l t i n a d d i t i o n a l i n v e s t m e n t r e t u r n , b u t i s r a t h e r a waste o f c a p i t a l e x p e n d i t u r e , because beyond an optimum f r a c t u r e l e n g t h , hydrocarbon p r o d u c t i o n r a t e and volume r e mains a t a p l a t e a u and t h e r e f o r e t h e payout is d e t e r i o r a t i n g .
Oil-reservoir fracturing O i l - r e s e r v o i r f r a c t u r i n g i s i n c o n t r a s t t o many cases o f gas-pay zone s t i m u l a t i o n c h a r a c t e r i z e d by h i g h e r - p e r m e a b i l i t y r o c k t r e a t i n g w h i c h p u t s emphasis on achievement o f a s i g n i f i c a n t c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r r o c k mat r i x and proppant package i n t h e f r a c t u r e . The h i g h e r v i s c o s i t y o f many o i l s f r e q u e n t l y r e q u i r e s t o s e l e c t coarse t o v e r y coarse proppants i n o r d e r t o achieve t h e necessary c o n d u c t i v i t y and f l o w c a p a c i t y . Most o i l r e s e r v o i r s demand s t i m u l a t i o n by s h o r t wide f r a c t u r e s t h a t a r e i n f i l l e d w i t h p r o p p a n t m u l t i l a y e r s and which i n some cases r e p r e s e n t n o t v e r y much more t h a n an enhanced g r a v e l pack. A s p e c i a l e f f e c t i s t h e p o s s i b i l i t y o f c o m b i n a t i o n o f h o r i z o n t a l and v e r t i c a l f r a c t u r i n g due t o changing g e o s t r e s s regime by thermal r e s e r v o i r expansion i n s t e a m - d r i v e enhanced o i l r e c o v e r y o p e r a t i o n s . O t h e r p o s s i b i l i t i e s o f changing f r a c t u r e o r i e n t a t i o n f r o m v e r t i c a l t o h o r i z o n t a l o r a l t e r i n g f r a c t u r e azimuth d i r e c t i o n t o c r e a t e networks o f p e r p e n d i c u l a r c r a c k elements a r e d i s t u r b i n g impact o f a nearby h y d r a u l i c f r a c t u r e which provokes a s t r e s s h a l o o r b e l t , and thermal r e s e r v o i r c o n t r a c t i o n by i n j e c t i o n o f c o l d w a t e r d u r i n g secondary w a t e r - f l o o d r e c o v e r y . W h i l e f r a c t u r e o r i e n t a t i o n changes f r o m v e r t i c a l t o h o r i z o n t a l a r e common d u r i n g s t a n d a r d steam-drive o p e r a t i o n , f r a c t u r e a z i muth changes can be achieved by staggered steam i n j e c t i o n i n t o o f f - t r e n d w e l l pairs.
Comnunication fracturing Communication f r a c t u r i n g w i t h c o n n e c t i o n o f e i t h e r two superimposed r e s e r v o i r s t o r e y s separated by a t i g h t b a r r i e r o r two n e i g h b o u r i n g w e l l s is an impor-
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t a n t a l t e r n a t i v e t o s i d e t r a c k d r i l l i n g i n f i e l d a p p r a i s a l and t o c o n v e n t i o n a l i n t e r v a l - w i s e f r a c t u r i n g w i t h subsequent commingling o f s e p a r a t e l a y e r s . Connect i o n o f i s o l a t e d sand b o d i e s i n l e n t i c u l a r r e s e r v o i r s by communication f r a c t u r i n g l i n k s much more d i s c r e t e sand u n i t s t o t h e b o r e h o l e than c o u l d be achieved by d r i l l i n g . F r a c t u r e system and w e l l p a t t e r n p l a n n i n g has t o be made a c c o r d i n g t o c r a c k azimuth as p r e d e t e r m i n e d by t h e i n - s i t u s t r e s s c o n s t e l l a t i o n , and sand l e n s a x i s o r i e n t a t i o n . D u r i n g c o n n e c t i o n o f separated r e s e r v o i r s t o r e y s and f i e l d d r a i n a g e o p t i m i z a t i o n s t i m u l a t i o n , c a u t i o n has t o be e x e r c i s e d t h a t unwant e d f r a c t u r e i n t e r f e r e n c e i s n o t s h o r t - c i r c u i t i n g t h e planned d r a i n a g e areas and t h u s d e t e r i o r a t i n g t h e e f f e c t o f t h e i n d i v i d u a l f r a c t u r e s f o r c h a n n e l l i n g and d e l i v e r y o f hydrocarbons t o t h e w e l l b o r e s . I n c o n t r a s t t o o i l and gas r e s e r v o i r s , f r a c t u r e i n t e r f e r e n c e and b o r e h o l e c o n n e c t i o n i s s i g n i f i c a n t l y enhancing a c c e s s i b l e pay volume and f l u i d c y c l i n g e f f e c t i v i t y i n geothermal r e s e r v o i r s .
Deviated-well fracturing F r a c t u r i n g o f d e v i a t e d w e l l s s u f f e r s f r o m t h e drawback o f l i m i t e d communicat i o n i n t e r v a l between a h i g h l y - i n c l i n e d o r h o r i z o n t a l b o r e h o l e and a v e r t i c a l f r a c t u r e i n t e r s e c t i n g i t . The most f a v o u r a b l e c o n s t e l l a t i o n comprises a w e l l bore t r a j e c t o r y being o r i e n t e d p a r a l l e l t o the p r i n c i p a l i n - s i t u stress d i r e c t i o n which a l l o w s g e n e r a t i o n o f an e x t e n s i v e s i n g u l a r v e r t i c a l f r a c t u r e t h a t most e f f e c t i v e l y d r a i n s t h e r e s e r v o i r a l o n g t h e h i g h - a n g l e t o h o r i z o n t a l boreh o l e p a t h . The o t h e r extreme i s an o r i e n t a t i o n o f t h e w e l l b o r e t r a j e c t o r y p e r p e n d i c u l a r l y t o t h e p r i n c i p a l i n - s i t u s t r e s s d i r e c t i o n which p e r m i t s c r e a t i o n o f m u l t i p l e v e r t i c a l f r a c t u r e s c u t t i n g orthogonal through the highly-deviated w e l l b o r e l i k e a s e t o f p a r a l l e l s l i c e s . A l l t h e s o l u t i o n s between these two end members o f a c o n t i n u o u s spectrum s u f f e r f r o m p r o d u c t i v i t y l o s s due t o l i m i t e d borehole/fracture contact length. O t h e r f e a t u r e s i n h i g h l y - i n c l i n e d b o r e h o l e s a r e proppant s t r a t i f i c a t i o n and f l u i d l a y e r i n g , and a l s o an i n c r e a s e d p r o b a b i l i t y o f non-Darcy f l o w . The drawback o f l i m i t e d communication i n t e r v a l between w e l l b o r e and f r a c t u r e a p p l i e s e q u a l l y f o r h i g h l y - d e v i a t e d t o h o r i z o n t a l deep b o r e h o l e s c r o s s i n g g e n t l y - i n c l i ned r e s e r v o i r s and s u b v e r t i c a l s h a l l o w w e l l s t r a v e r s i n g s t e e p l y - d i p p i n g pay f o r m a t i o n s . The b e s t improvement o f t h e i n d i s p e n s i b l y s i t u a t i o n o f extended-reach d r i l l i n g f r o m o f f s h o r e p l a t f o r m s i n a s t a r - l i k e p a t t e r n would be achievement o f a S-shaped b o r e h o l e t r a j e c t o r y w i t h a g a i n decreasing t h e i n c l i n a t i o n p r i o r t o i n t e r s e c t i n g t h e r e s e r v o i r . An o u t l i n e o f hydrocarbon p r o d u c t i o n and f l o w r e g i mes i s a l s o g i v e n .
Fracture damage F r a c t u r e damage i s t h e w o r s t d e t e r i o r a t i o n o f a s t i m u l a t i o n j o b , and f a c t o r s l e a d i n g u l t i m a t e l y t o t r e a t m e n t f a i l u r e a r e o u t l i n e d by commenting on p r o p p a n t s e t t l i n g , p r o p p a n t flowback, p r o p p a n t c r u s h i n g and f l u i d aspects. Proppant s e t t l i n g i s i n f l u e n c e d by c l u s t e r e d p r o p p a n t t r a n s p o r t and thus h i n d e r e d segregat i o n o f t h e p a r t i c l e s i n t h e s l u r r y . Proppant t r a p p i n g i n t h e c r a c k can be accel e r a t e d by f o r c e d f r a c t u r e c l o s u r e . The d e c i s i v e n e s s o f p r o p p a n t banking depends on g r a i n s i z e d i s t r i b u t i o n and f l u i d s t r a t i f i c a t i o n i n t h e r e s e r v o i r , w i t h d i s t i n c t i o n h a v i n g t o be made between coarsening- and f i n i n g - u p w a r d s c y c l o t h e m as w e l l as homogeneous pays and f o r m a t i o n s d i v i d e d by an o i l - w a t e r - c o n t a c t i n t o l o w e r and upper p o r t i o n s . Proppant flowback d u r i n g w e l l cleanup can be avoided above a l l by s t a b i l i z a t i o n o f t h e package by a t a i l o f r e s i n - c o a t e d proppants which p r o v i d e s an aggregated wedge b l o c k i n g t h e f r a c t u r e mouth. O t h e r p o s s i b i l i t i e s o f p r o p p a n t flowback c o n t r o l a r e p r e s s u r e d e p l e t i o n a s s i s t a n c e by c a r e f u l f l u i d p r o d u c t i o n , l i m i t a t i o n o f e x p l o i t a t i o n r a t e s and choke s i z e , and avoidance o f e x c e s s i v e p r e s s u r e drawdown. Proppant c r u s h i n g by shock l o a d i n g and s t r e s s c o r r o s i o n c r a c k i n g a r e m a i n l y consequences o f u n s u i t a b l e c o m p l e t i o n and o f f t a k e p r a c t i c e s and can be reaso-
xxx n a b l y p r e v e n t e d b y o p e r a t i o n w i t h l o w p r o d u c t i o n r a t e s and l o w f l o w i n g p r e s s u r e . The b e s t i n s u r a n c e a g a i n s t p r o p p a n t c r u s h i n g , however, i s s e l e c t i o n o f a proppant type t h a t i s able t o s u f f i c i e n t l y withstand a l l the possible r e s e r v o i r p r e s s u r e drawdowns d u r i n g f i e l d l i f e e s p e c i a l l y in t h e i m m e d i a t e s u r r o u n d i n g s o f t h e w e l l b o r e where f l u i d v e l o c i t y and p r e s s u r e d e p l e t i o n c u l m i n a t e . The i n v e s t m e n t f o r a b e t t e r p r o p p a n t t y p e and t h u s t h e i n s u r a n c e a g a i n s t f a i l u r e o f t h e a g e n t s u p p o r t i n g t h e f r a c t u r e p a y s o f f many t i m e s d u r i n g c o u r s e o f l o n g term f i e l d e x p l o i t a t i o n . Production c y c l i n g by repeated w e l l s h u t - i n leads t o irrecoverable f r a c t u r e c o n d u c t i v i t y diminution unless a proppant w i t h high enough c l o s u r e s t r e s s r e s i s t i v i t y a l s o f o r peak l o a d i n g has been chosen and s h o u l d t h e r e f o r e be a v o i d e d .
Aggressive fracturing design F o r t h e s a k e of t h e n e c e s s a r y p r o g r e s s i n u n c o n v e n t i o n a l r e s e r v o i r s t i m u l a t i o n w i t h i n n o v a t i v e t e c h n i q u e s , an a g g r e s s i v e f r a c t u r i n g d e s i g n i s recommended which, however, has n e c e s s a r i l y t o c o n s i d e r t h e r i s k o f some drawback by f a i l u r e s i n p i l o t t r e a t m e n t s o f m a r g i n a l r e s e r v o i r s . The d a n g e r o f p r e m a t u r e s c r e e n o u t t e r m i n a t i o n o f c h a l l e n g i n g j o b s i s p a r t i c u l a r l y h i g h when w o r k i n g w i t h e x c e s s i v e p r o p p a n t c o n c e n t r a t i o n s and e x t r a o r d i n a r i l y l a r g e p r o p p a n t g r a i n s which, however, a r e t h e m o s t e f f e c t i v e ways o f f u r t h e r i n c r e a s i n g t h e c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r r o c k and p r o p p a n t wedge i n f i l l i n g t h e f r a c t u r e . C o n c l u s i o n i s made t h a t g i v e n an a g a i n more s t a b l e o i l p r i c e a t a r e a s o n a b l e l e v e l and p a r t i a l l y a l s o an a g a i n s t r o n g e r US $, t h e r e a r e many g r o w i n g s t i m u l a t i o n and s t a b i l i z a t i o n m a r k e t s w i t h i n c r e a s i n g demand o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n t h e n e a r f u t u r e i n v a r i o u s p a r t s o f t h e w o r l d .
G r a v e l
P a c k i n g
a n d
S a n d
C o n t r o l
The s p e c t r u m o f sand c o n t r o l methods encompasses m e c h a n i c a l g r a v e l p a c k i n g , c h e m i c a l r e s e r v o i r c o n s o l i d a t i o n and m e c h a n i c a l s t a b l e sand a r c h f o r m a t i o n . Grav e l p a c k i n g i s t h e m o s t commonly a p p l i e d and m o s t r e l i a b l e method o f sand c o n t r o l i n w e l l s where w e a k l y - c e m e n t e d o r e v e n l o o s e f r i a b l e sands a r e n o t s u f f i c i e n t l y s t a b l e enough t o r e s i s t t o f l u i d f o r c e s d u r i n g h y d r o c a r b o n p r o d u c t i o n . D i s t i n c t i o n has t o be made between r e s e r v o i r s p r o d u c i n g sand f r o m t h e v e r y beg i n n i n g o f h y d r o c a r b o n o f f t a k e , and p a y s where sand s t a r t s t o f l o w i n t o t h e w e l l b o r e when r i s i n g w a t e r c u t changes f l u i d v i s c o s i t y o r p r o g r e s s i v e e x p l o i t a t i o n l e a d s t o d e c l i n i n g p r e s s u r e w h i c h b o t h l o w e r i n t e g r i t y and s u p p o r t o f t h e g r a i n f a b r i c o f t h e r o c k m a t r i x . Some o v e r p r e s s u r e d and u n d e r c o m p a c t e d c h a l k o i l r e s e r v o i r s a l s o r e q u i r e g r a v e l packing p a r t i a l l y i n combination w i t h hydraul i c fracturing.
Standard gravel packing The s i g n i f i c a n c e o f c a r e f u l s e l e c t i o n o f t y p e and g r a i n s i z e o f t h e g r a v e l pack m a t e r i a l f o r m e c h a n i c a l c o n t r o l o f pay sands i s o u t l i n e d b y d i s c u s s i n q a s p e c t s o f h y d r o c a r b o n p r o d u c t i v i t y v s . sand c o n t r o l , r e s e r v o i r d e p t h and L l u s u r e s t r e s s in p e r f o r a t i o n t u n n e l s , g r a v e l g r a i n s i z e c h o i c e v s . f o r m a t i o n g r a i n size, a b s o l u t e s t o p p a g e o f m o v i n g and f l o w i n g r e s e r v o i r sand, i m p a c t o f g r a v e l pack hardware, f l u i d v i s c o s i t y and g r a v e l c o n c e n t r a t i o n , c h e m i c a l f o r m a t i o n c o n s o l i d a t i o n v s . m e c h a n i c a l g r a v e l p a c k i n g , and c o m b i n a t i o n o f g r a v e l p a c k i n g and h y d r a u l i c f r a c t u r i n g . Comments a r e a l s o o f f e r e d on g r a v e l - l e s s h a r d w a r e i n s t a l l a t i o n w i t h b u i l d i n g o f a n a t u r a l g r a v e l pack a r o u n d s c r e e n o r l i n e r b y i n t r u d i n g f o r m a t i o n sand, t r i p l e - w r a p p e d s c r e e n u t i l i z a t i o n where t h e d i f f e r e n t w i r e - w r a p l a y e r s a r e s c h e d u l e d t o s t o p p r o g r e s s i v e l y f i n e r g r a i n s i z e s , and lin e r l e s s g r a v e l p l a c e m e n t . G r a v e l p a c k i n g c o m p l e t i o n f l u i d s and g r a v e l s a t u r a t i o n a r e i l l u s t r a t e d a l o n g t h e l i n e s o f f l u i d v i s c o s i t y and g r a v e l c o n c e n t r a t i o n , t r a n s p o r t and p a c k i n g c a p a c i t y o f t h e s l u r r i e s , and g r a v e l s e t t l i n g d i m i nution in the c a r r i e r f l u i d .
XXXI The e f f e c t of a t a i l - i n t e r m i n a l stage i n h y d r a u l i c f r a c t u r i n g which p l a c e s c o a r s e r proppants near t h e c r a c k mouth t h a t keep i n p l a c e t h e f i n e r g r a i n s o f t h e main l o t i s comparable t o t h a t o f a g r a v e l pack, and a row o f n a r r o w l y - s p a ced g r a v e l - f i l l e d p e r f o r a t i o n t u n n e l s approaches t h e s i g n i f i c a n c e o f a propped m i n i f r a c t u r e . R e s e r v o i r s t a b i l i t y and f l u i d dynamics, and p a r t i c l e t r a n s p o r t through p e r f o r a t i o n s a r e o t h e r i m p o r t a n t t o p i c s f o r f u r t h e r c h a r a c t e r i z a t i o n o f t h e s i g n i f i c a n c e o f g r a v e l packing, w i t h emphasis b e i n g on f o r m a t i o n p r e s s u r e and f l u i d composition, p e r f o r a t i o n p a c k i n g and p a r t i c l e b r i d g i n g , achievement o f p e r f o r a t i o n t u n n e l p l u g g i n g , and i n f l u e n c e o f p e r f o r a t i o n t e c h n i q u e and geom e t r y . Steam-drive enhanced o i l r e c o v e r y r e q u i r e s s p e c i a l a d j u s t m e n t o f t h e g r a vel-pack m a t e r i a l i n o r d e r t o r e s i s t t o t h e h o t a l c a l i n e a g g r e s s i v e m i l i e u , w i t h s i n t e r e d b a u x i t e and n i c k e l - c o a t e d sand having t u r n e d o u t t o be s u f f i c i e n t l y s t a b l e , whereas n a t u r a l q u a r t z sand i s c o n s i d e r a b l y d i s s o l v e d .
Special gravel packing S p e c i a l g r a v e l pack a p p l i c a t i o n s a r e c o n s o l i d a t e d pack i n s t a l l a t i o n , r e s i n c o a t e d g r a v e l placement, g r a v e l p a c k i n g o f d e v i a t e d w e l l s w i t h t h e e f f e c t o f g r a v e l duning, g r a v e l p a c k i n g w i t h l i n e r v i b r a t i o n , c o n c e n t r a t e d g r a v e l s l u r r y pumping, d e n s i t y s l u r r y and foam g r a v e l packing, and v i s c o u s polymer g r a v e l packing. C o n s o l i d a t e d pack i n s t a l l a t i o n and r e s i n - c o a t e d g r a v e l placement have t h e advantage o f a g g r e g a t i n g t h e p e r f o r a t i o n t u n n e l i n f i l l i n g s t o s t a b l e g r a v e l wedges and t h e r e f o r e a g r a v e l m a n t l e seaming t h e b o r e h o l e w a l l and h o l d i n g t h e p e r f o r a t i o n h o l e p l u g s i n p l a c e i s n o t r e q u i r e d , w i t h t h i s s p e c i a l t y p e o f app l i c a t i o n t h u s r e s t r i c t i n g t h e o p e r a t i o n t o o u t s i d e g r a v e l p a c k i n g which l e a v e s t h e w e l l b o r e f r e e o f hardware and a l s o c o n s i d e r a b l y reduces t h e r e q u i r e d g r a v e l q u a n t i t y . C o n s o l i d a t e d pack i n s t a l l a t i o n and r e s i n - c o a t e d g r a v e l placement a r e e s p e c i a l l y s u i t a b l e f o r o l d p r o d u c t i o n w e l l s and steam-drive i n j e c t i o n boreholes. Gravel compaction and overcoming o f t h e phenomenon o f g r a v e l duning i n d e v i a t e d w e l l s can be a t t a i n e d by f l o w r e s i s t a n c e c r e a t e d by f l e x i b l e b a f f l e s and rubber disks, l a r g e - d i a m e t e r washpipes, p e r f o r a t i o n p r e p a c k i n g , s t r i n g r o t a t i o n , l i n e r v i b r a t i o n , low p a r t i c l e d e n s i t y l c a r r i e r f l u i d d e n s i t y r a t i o , i n v e r t e d s l u r r y c i r c u l a t i o n , g r a v e l - l e s s s l o t t e d l i n e r s and prepacked screens. L i n e r v i b r a t i o n compensates i n i t i a l undercompaction o f t h e g r a v e l by t r i g g e r i n g s e t t l i n g i n t o a hexagonal package which a l l o w s t o u t i l i z e c o a r s e r g r a v e l g r a i n s i zes, and t h e absence o f a f t e r - p a c k s e t t l i n g does n o t n e c e s s i t a t e a r e s e r v e b l a n k s e c t i o n above screen and p e r f o r a t e d i n t e r v a l . Concentrated g r a v e l s l u r r y pumping p r o f i t s f r o m t h e t a c k i f y i n g a c t i o n o f t h e i n t e r m i x e d m i n o r amounts o f r e s i n . Viscous polymer g r a v e l p a c k i n g i s a s s o c i a t e d w i t h b u i l d u p o f n o d u l a r g r a v e l d e p o s i t s a t t h e p e r f o r a t i o n s and a n n u l a r sediments on t h e screen. Comments a r e a l s o o f f e r e d on puddle p a c k i n g which i s a method f o r i n f i l l i n g o f b o r e h o l e w a l l i r r e g u l a r i t i e s generated by e a r l i e r sand p r o d u c t i o n o r e x p l o s i v e s h o o t i n g i n o l d e r w e l l s and t h u s i s a s i g n i f i c a n t r e p a i r t e c h n i q u e i n open b o r e h o l e sections.
Other sand control methods O t h e r sand c o n t r o l methods comprise r e s i n c o n s o l i d a t i o n , hydroxy-aluminum stabilization, e l e c t r o l e s s n i c k e l p l a t i n g , s i l i c a cementation by s i l i c a l o c k and S o l d e r g l a s s methods, warm a i r coking, s t a b l e sand a r c h f o r m a t i o n , and pre-pack l i n e r s and m u l t i w i r e - w r a p p e d screens. The s p e c i f i c f e a t u r e s o f chemical r e s i n c o n s o l i d a t i o n o f t h e f o r m a t i o n sand a r e i l l u s t r a t e d a l o n g t h e l i n e s o f a c t i v a t i o n , c a t a l y z a t i o n and c u r i n g o f t h e r e s i n . Performance o f r e s i n c o n s o l i d a t i o n can be achieved by prepacking, p r e h e a t i n g and p r e f l u s h i n g o f t h e r e s e r v o i r . S t a b l e sand a r c h f o r m a t i o n can t a k e p l a c e a l r e a d y n a t u r a l l y i n f o r m a t i o n s w i t h s u f f i c i e n t g r a i n f r i c t i o n due t o rough p a r t i c l e s u r f a c e s o r i n r e s e r v o i r s i n g r e a t e r d e p t h w i t h h i g h e r overburden s t r e s s , b u t can a l s o be s e m i a r t i f i c i a l l y
XXXII i n t r o d u c e d o r enhanced b y p a c k e r i n f l a t i o n . P r e - p a c k l i n e r s t h a t a r e p r e p a r e d w i t h a s s i s t a n c e o f v i b r a t i o n and m u l t i w i r e - w r a p p e d s c r e e n s c a n b e u s e d f o r g r a v e l p a c k i n g and pump p r o t e c t i o n . The i n f l u e n c e o f c o m p l e t i o n t y p e on g r a v e l p a c k i n g i s o u t l i n e d by s k e t c h i n g f e a t u r e s o f i n s i d e g r a v e l p a c k i n g such as s i g n i f i c a n c e o f p e r f o r a t i o n t u n n e l p l u g g i n g , and open h o l e g r a v e l p a c k i n g such as b o r e h o l e u n d e r r e a m i n g , p o r t c o l l a r i m p l e m e n t a t i o n and pack w a s h i n g . F o r m a t i o n damage d u r i n g g r a v e l p a c k i n g i s i n t r o d u c e d b y t h e p l a c e m e n t o f t h e m a n t l e i t s e l f as w e l l a s b y t r a p p i n g o f f l u i d - l o s s a d d i t i v e s a t t h e i n t e r f a c e between g r a v e l and r e s e r v o i r w h e r e t h e y a r e k e p t i n p l a c e b y t h e p a c k . Removal o r f o r m a t i o n damage b y a c i d i z i n g p r i o r t o g r a v e l p a c k i n g w i t h p a r t i a l u t i l i z a t i o n o f t h e g r a v e l i t s e l f as d i v e r t i n g a g e n t i s u s u a l l y l e a d i n g t o s a t i s f a c t o r y r e s u l t s , whereas a c i d i z i n g a f t e r hav i n g i n s t a l l e d t h e g r a v e l pack r e s u l t s i n s e l e c t i v e a c i d p e n e t r a t i o n t h r o u g h t h e p e r m e a b l e g r a v e l i n f i l l i n g t h e p e r f o r a t i o n t u n n e l s as w e l l as t o g r a v e l unc o n s o l i d a t i o n and d i s s o l u t i o n and s h o u l d t h e r e f o r e be a v o i d e d whenever p o s sible.
P r o p p e d F r a c t u r e a n d G r a v e l P a c k M o n i t o r i n g P r o p p e d f r a c t u r e and g r a v e l pack m o n i t o r i n g b y w e l l l o g g i n g , s e i s m i c a l and o t h e r methods i s n o t o n l y a means o f c h e c k i n g q u a l i t y o f p e r f o r m a n c e o f t h e s t i m u l a t i o n and s t a b i l i z a t i o n o p e r a t i o n s a f t e r t e r m i n a t i o n o f t h e j o b , b u t i s a l s o a t e c h n i q u e o f l o n g - t e r m b e h a v i o u r c o n t r o l b y r e p e t i t i o n r u n s and g i v e s even i m p o r t a n t i m p a c t on f u t u r e p l a n n i n g and d e s i g n o f f r a c t u r i n g t r e a t m e n t s and g r a v e l pack p l a c e m e n t s b y d e l i v e r i n g d i r e c t i n f o r m a t i o n f r o m t h e s u p e r v i s i o n o f e x i s t i n g p r o p p e d f r a c t u r e s and i n s t a l l e d g r a v e l packs, r e s p e c t i v e l y .
Propped f r a c t u r e height monitoring F r a c t u r e d e t e c t i o n and d e t e r m i n a t i o n by w e l l l o g g i n g a i m i n g c h i e f l y o n h e i g h t m o n i t o r i n g c a n be c a r r i e d o u t b y t e m p e r a t u r e , c o n v e n t i o n a l and s p e c t r a l gamma-ray, t h e r m a l decay t i m e , a c o u s t i c and n o i s e l o g g i n g . M u l t i p l e - t r a c e r d i s c r i m i n a t i o n b y gamma-ray s p e c t r o s c o p y a l l o w s t o d i s t i n g u i s h s e v e r a l t r e a t m e n t s t a g e s and e n a b l e s d e t e r m i n a t i o n o f p r o v e n a n c e of r a d i a t i o n f r o m f r a c t u r e d f o r m a t i o n , cement s h e a t h o r b o r e h o l e f l u i d . T e m p e r a t u r e l o g g i n g i s t h e c h e a p e s t and s i m p l e s t f r a c t u r e h e i g h t m o n i t o r i n g t e c h n i q u e , b u t i s sometimes d i s t u r b e d b y t h e r m a l c o n d u c t i v i t y a n o m a l i e s of t h e r e s e r v o i r w h i c h l e a d t o warm n o s e s i n t h e t e m p e r a t u r e r e c o r d . The a n o m a l i e s c a n b e v e r i f i e d and c o r r e c t f r a c t u r e h e i g h t i n t e r p r e t a t i o n c a n be made b y c o m b i n a t i o n o f t e m p e r a t u r e w i t h n o i s e o r gamma-ray l o g g i n g . R a d i o a c t i v i t y d e t e c t i o n b y c o n v e n t i o n a l s i n g l e - o r d o u b l e - m a r k e r gamma-ray l o g g i n g o r advanced m u l t i p l e - t r a c e r gamma-tay s p e c t r o s c o p y c a n be p e r f o r m e d using three d i f f e r e n t types o f sources comprising n a t u r a l l y r a d i o a c t i v e propp a n t s , a r t i f i c i a l l y t a g g e d p r o p p a n t s ( i n c l u d i n g c o a t i n g s and f u l l g r a i n s ) and s y n t h e t i c m a r k e r s i n t e r m i x e d i n t o f r a c t u r i n g f l u i d o r s l u r r y . Gamma-ray s p e c t r o s c o p y i s a b l e t o d i f f e r e n t i a t e between s e v e r a l i n s e r t e d t r a c e r s and t h u s p e r m i t s t o d i s t i n g u i s h v a r i o u s f l u i d and p r o p p a n t s t a g e s a s w e l l as s e p e r a t e layers i n m u l t i p l e - i n t e r v a l treatments i f s e l e c t i v e tagging o f the i n d i v i d u a l l o t s i s c a r r i e d o u t , and i s t h u s t h e m o s t m a t u r e t e c h n i q u e o f f r a c t u r e h e i g h t m o n i t o r i n g . C o m b i n a t i o n o f t e m p e r a t u r e and c o n v e n t i o n a l gamma-ray l o g g i n g i s t h e m o s t p r o f i t a b l e s o l u t i o n f o r c o s t c o n t a i n m e n t i n m u l t i w e l l f i e l d campaigns b y p r o v i d i n g t h e p o s s i b i l i t y o f m u t u a l c a l i b r a t i o n i n some k e y w e l l s and l a t e r r u n n i n g o n l y one o f t h e r e g i s t r a t i o n s . Thermal decay t i m e l o g g i n g a l l o w s a l s o m o n i t o r i n g o f a c i d s t i m u l a t i o n due t o t h e a c i d e f f e c t o n p u l s e d n e u t r o n c a p t u r i n g . D i s t i n c t i o n c a n b e made between p r o p p e d f r a c t u r e h e i g h t as r e f l e c t e d b y gamma-ray l o g g i n g o f p r o p p a n t r a d i o a c t i v i t y , c r e a t e d f r a c t u r e h e i g h t as document e d b y t e m p e r a t u r e l o g g i n g and gamma-ray m o n i t o r i n g o f f l u i d r a d i o a c t i v i t y , and
X X X I II permeable ging.
or
e f f e c t i v e f r a c t u r e h e i g h t as i n d i c a t e d by thermal decay t i m e l o g -
Propped fracture azimuth mapping P o s s i b i l i t i e s o f f r a c t u r e azimuth mapping a r e c a l i p e r and a c o u s t i c l o g g i n g , b o r e h o l e t e l e v i e w e r , f o r m a t i o n microscanner, and s e i s m i c a l and g r a v i m e t r i c a l methods, w i t h some o f t h e means a l s o r e v e a l i n g i n f o r m a t i o n on f r a c t u r e w i d t h . A c o u s t i c l o g g i n g employs m a i n l y r e a d i n g and p r o c e s s i n g o f t h e c i r c u m f e r e n t i a l l y t r a v e l l i n g R a y l e i g h and S t o n e l e y waves, w i t h v a r i o u s techniques o f waveform mod e l l i n g and i n t e r p r e t a t i o n e x i s t i n g . C a l i p e r l o g g i n g r e c o r d s b o r e h o l e e l l i p t i c i t y o r o v a l i z a t i o n as a response t o t h e i n - s i t u s t r e s s d i s t r i b u t i o n , w i t h t h e maj o r e c c e n t r i c i t i e s i n d i c a t i n g the d i r e c t i o n o f p r i n c i p a l h o r i z o n t a l stresses and t h u s r e f l e c t i n g f r a c t u r e azimuth. Borehole t e l e v i e w e r and f o r m a t i o n m i c r o scanner p e r m i t d i r e c t o b s e r v a t i o n o f f r a c t u r e s by v i s u a l i z a t i o n i n c o n t r a s t t o i n d i r e c t i n t e r p r e t a t i o n of o t h e r measurement methods. D i s t i n c t i o n can be made between r e c o g n i t i o n o f t h e n a t u r a l f r a c t u r e system i n t h e r e s e r v o i r r o c k and e v a l u a t i o n of h y d r a u l i c a l l y generated f r a c t u r e s i n t e r s e c t i n g b o t h pay zone mat r i x and p r e - e x i s t i n g j o i n t assemblages. The most s u i t a b l e techniques f o r d e t e c t i o n o f n a t u r a l f r a c t u r e s a r e b o r e h o l e t e l e v i e w e r , f o r m a t i o n microscanner, d i p meter, dual l a t e r o l o g , and s o n i c l o g g i n g w i t h f u l l waveform a n a l y s i s . The s u i t e o f d i f f e r e n t f r a c t u r e i n d i c a t o r s which a r e a l l s u b j e c t t o v a r i o u s d i s t u r b i n g i n f l u e n c e s i s b e s t e v a l u a t e d by i n t e g r a t i o n i n t o a m u l t i p l e - s o u r c e f r a c t u r e probab i l i t y program. Mechanical p r o p e r t i e s l o g g i n g and f r a c t u r e m i g r a t i o n a n a l y s i s a r e i m p o r t a n t p r e d i c t i v e t o o l s o f f r a c t u r e m o d e l l i n g and design. S e i s m i c a l , g r a v i m e t r i c a l and g e o e l e c t r i c a l methods m a i n l y aim on f r a c t u r e azimuth mapping, a l t h o u g h some techniques such as v e r t i c a l s e i s m i c p r o f i l i n g can a l s o be a p p l i e d f o r f r a c t u r e h e i g h t m o n i t o r i n g . M i c r o s e i s m i c f r a c t u r e r e g i s t r a t i o n can be performed by a c t i v e o r p a s s i v e r e c o r d i n g , w i t h most o f t h e m i c r o earthquake e v e n t s b e i n g c r e a t e d by t h e c l o s i n g f r a c t u r e . Enhanced i n t e r p r e t a t i o n can be achieved by combining microearthquake and p r e s s u r e p u l s e documentat i o n . A n e l a s t i c a l s t r a i n r e c o v e r y a n a l y s i s o f c o r e s i s a s u i t a b l e method f o r p r e d i c t i o n o f f r a c t u r e azimuth. D i s t i n c t i o n o f v e r t i c a l and h o r i z o n t a l hydraul i c f r a c t u r e s can be performed by t i l t m e t e r a n a l y s i s o f s u r f a c e d e f o r m a t i o n s , whereas f o r n a t u r a l c r a c k s t h e s e p a r a t i o n mode o f s h a l l o w and deep dual l a t e r o l o g r e c o r d s i s t h e most i n d i c a t i v e i n s t r u m e n t . O t h e r g e o p h y s i c a l t e c h n i q u e s f o r c h i e f l y f r a c t u r e azimuth mapping a r e a c o u s t i c emission, h y d r a u l i c impedance, h y d r a u l i c resonance and magnetometry. F u r t h e r f r a c t u r e m o n i t o r i n g can be p e r f o r med by s t i m u l a t i o n d a t a a n a l y s i s , i n f l a t a b l e impression packers, and p h o t o g r a phy and imagery. F r a c t u r e p r e s s u r e i n t e r p r e t a t i o n does n o t o n l y a l l o w t o c a l c u l a t e f r a c t u r e h e i g h t and l e n g t h , b u t i s a l s o an i n s t r u m e n t o f premature screeno u t t e r m i n a t i o n f o r e c a s t and t h u s d e t a i l e d o b s e r v a t i o n o f t h e p r e s s u r e e v o l u t i o n d u r i n g course o f t h e t r e a t m e n t enables screenout p r e v e n t i o n by p e r m i t t i n g remedy a c t i o n s i n t i m e .
Gravel pack evaluation Gravel pack checking and e v a l u a t i o n can be performed by photon, gamma-ray and n e u t r o n l o g g i n g and can be f u r t h e r checked by u t i l i z i n g d a t a from t h e t e l l t a l e screen. Photon and n e u t r o n l o g g i n g employ t h e d e n s i t y change generated by g r a v e l i n s e r t i o n i n t o t h e borehole, whereas gamma-ray l o g g i n g has t o r e c o r d nat u r a l g r a v e l o r a r t i f i c i a l g r a v e l and/or f l u i d r a d i o a c t i v i t y . C a l i b r a t i o n w i t h t h e end members o f 0 % g r a v e l i n t h e b l a n k s e c t i o n above t h e screen and 100 % g r a v e l i n t h e i n t e r v a l o f h i g h e s t i n s t r u m e n t c o u n t enable t o p e r f o r m q u a n t i t a t i v e g r a v e l pack e v a l u a t i o n by n e u t r o n and gamma-ray l o g g i n g w i t h e x p r e s s i o n o f percentages o f g r a v e l d e n s i t y i n t h e surveyed w e l l b o r e s e c t i o n . S p e c i a l r e q u i r e ments f o r d i s c r i m i n a t i o n o f g r a v e l p a c k i n g stages o r l e v e l s can be s o l v e d by gamma-ray spectroscopy o f m u l t i p l e t r a c e r s . I n t e r p r e t a t i o n c o n f i d e n c e can be enhanced by c o m b i n a t i o n o f n e u t r o n and gamma-ray l o g g i n g , and p r o g r e s s i v e s e t t -
XXXIV l i n g o f gravel a f t e r i n s t a l l a t ging. Repair o f detected voids b i n a t i o n which allows synthet shaking, w i t h repeated l o g g i n g pack have d i s a p p e a r e d .
on c a n be m o n i t o r e d b y t i m e - l a p s e r e p e t i t i o n l o g c a n be c a r r i e d o u t b y t h e p h o t o n - v i b r a p a c k comc improvement o f g r a v e l s e t t l i n g b y v i b r a t i o n and and e x c i t i n g b e i n g done u n t i l t h e v o i d s i n t h e
The t e l l - t a l e s c r e e n r e c o g n i z e s c o m p l e t i o n o f g r a v e l i n s e r t i o n once i t becomes c o v e r e d w i t h g r a v e l and t h u s s e r v e s as a r e l i a b l e i n d i c a t o r when t o s t o p pumping. M e c h a n i c a l p r o p e r t i e s l o g g i n g a l l o w s d e t e c t i o n o f s t r o n g and weak sands and h e l p s i n c a n d i d a t e s c r e e n i n g f o r t h e n e c e s s i t y o f sand c o n t r o l t r e a t m e n t s . W h i l e i n s i n g l e - w e l l o p e r a t i o n s t h e c o s t f o r an i n t e g r a t e d sand p r o d u c t i o n f o r e c a s t a n a l y s i s c o u l d be h i g h e r t h a n t h e expenses f o r sand c o n t r o l j o b s , c o n s i d e r a b l e c o m p l e t i o n c o s t c o u l d be saved i n m u l t i w e l l f i e l d campaigns when m e c h a n i c a l p r o p e r t i e s l o g g i n g and e v a l u a t i o n a l l o w s t o d i s t i n g u i s h between s t a b l e sands w h i c h do n o t l e a d t o sand i n f l u x and u n s t a b l e sands t h a t w o u l d r e s u l t i n sand i n t r u s i o n i n t o t h e b o r e h o l e u n l e s s c o n t r o l l e d b y m e c h a n i c a l g r a v e l packing o r chemical formation c o n s o l i d a t i o n .
B i b l i o g r a p h y The e x t e n s i v e l i t e r a t u r e c o m p i l a t i o n i s p r o c e s s e d i n a b i b l i o g r a p h y w h i c h c o n s i s t s o f t w o p a r t s . The f i r s t s e c t i o n g i v e s a s u b j e c t k e y c l a s s i f i c a t i o n o f t h e r e f e r e n c e s i n t o v a r i o u s s p e c i a l t o p i c s o f m a i n l y t e c h n i c a l and r e s e r v o i r eng i n e e r i n g n a t u r e , and t h e second d i v i s i o n c o n t a i n s t h e r e f e r e n c e l i s t w h i c h i n c l u d e s a l l t h e q u o t a t i o n s i n a l p h a b e t i c a l o r d e r o f t h e a u t h o r s names. The m a i n t o p i c headers i n t h e s u b j e c t key c l a s s i f i c a t i o n a r e overview o f h y d r a u l i c f r a c t u r i n g and e c o n o m i c a l framework o f h y d r o c a r b o n d r i l l i n g and s t i m u l a t i o n , f r a c t u r e d e s i g n and e v a l u a t i o n , f r a c t u r e f e a t u r e s , p r o p p a n t f e a t u r e s ; technical, f l u i d and t e m p e r a t u r e f e a t u r e s ; f i e l d c a s e s t u d i e s , s p e c i a l a p p l i c a t i o n s o f hyd r a u l i c f r a c t u r i n g , g r a v e l p a c k i n g and sand c o n t r o l , and p e t r o p h y s i c a l e v a l u a t i o n f o r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g d e s i g n . The r e f e r e n c e l i s t cont a i n s more t h a n 3,500 q u o t a t i o n s o f p a p e r s on t e c h n i c a l and e c o n o m i c a l a s p e c t s of h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g as w e l l as a s s o c i a t e d comp l e t i o n and s t i m u l a t i o n t e c h n i q u e s .
List of figures Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
1 : F r a c t u r e s u p p o r t b y s p h e r i c a l s t r e s s - r e s i s t a n t p r o p p a n t s . . . . . . . . . 15 2 : C e n t r e s of a c t i v i t y i n p a s t and f u t u r e o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n E u r o p e . . _ . _ _ . 175 3 : Schematical g e o l o g i c a l cross-section o f t h r c o l u m n w i t h h y d r a u l i c f r a c t u r i n g p o t e n t i a l i n W e s t e r n E u r o p e .... 179 4 : C e n t r e s o f m a s s i v e h y d r a u l i c p r o p p a n t f r a c t u r i n g (MHF) in R o t l i e 181 gend and C a r b o n i f e r o u s i n Germany FRG ........................... 5 : D i s t r i b u t i o n o f o i l and gas f i e l d s w i t h h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g p o t e n t i a l i n Europe . . . . . . . . . . . . . . . . . . . 180 6 : C e n t r e s o f m a s s i v e h y d r a u l i c p r o p p a n t f r a c t u r i n g (MHF) i n R o t l i e gend and B u n t s a n d s t e i n i n t h e N o r t h Sea . . . . . . . . . . . . . . . . . . . . . . . . . 181 7 : Proppant d i s t r i b u t i o n types w i t h i n h y d r a u l i c f r a c t u r e s 185 8 : C e n t r e s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l pac t h e N o r t h Sea . . . . . . . . . . . . . 195 9 : R o t l i e g e n d and C a r b o n i f e r o u 239 p r o p p a n t f r a c t u r i n g i n N o r t h w e s t e r n Europe ...................... 10 : C e n t r e s o f m a s s i v e h y d r a u l i c p r o p p a n t f r a c t u r i n g i n R o t l i e g e n d , C a r b o n i f e r o u s and B u n t s a n d s t e i n i n N o r t h Sea and o n s h o r e M i d d l e ... 239 Europe .................... 11 : S c h e m a t i c a l g e o l o g i c a l c r o s ari column w i t h h y d r a u l i c f r a c t u r i n g p o t e n t i a l i n W e s t e r n 243 12 : P r o p p a n t d i s t r i b u t i o n t y p e s w i t h i n h y d r a u l i c f r a c t u r e s . . . . . . . . . . 242
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List o f tables Tab. Tab. Tab. Tab. Tab. Tab. Tab. Tab. Tab. Tab. Tab. Tab.
1 : Proppant types and grain sizes and their application in petroleum reservoir stimulation and other energy industry processes . . . . . . . . 21 2 : Mineralogical and chemical composition o f proppants for hydraulic 31 fracturing and gravel packing .................................... 3 : Mechanical properties and p r o d u c t i o n / m a n u f a c t u r i n g of proppants . . . . . . . . . . . . . . . . . . . 35 for hydraulic fracturing and gravel packing 4 : Significance o f proppants in petroleum reser r stimulation and other energy industry processes . . . . . . . . . . . . . . . . . 5 : Centres of past and future activity of hydraulic ring in Europe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 6 - 9 : Hydraulic proppant fracturing and gravel packing in Europe. Past and future stimulation potential in important hydrocarbon fields in selected European countries 6 : ................................... 187 7 : ................................................. 191 8 : Part 3 .............................. ........ 200 205 9 : Part 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 : Lithostratigraphy and palaeoenvironmental evolution of Permian (Rotliegend) and Triassic (Buntsandstein) in South Oevon/England 332 11 : Centres o f past and future activity of gravel packing in Europe . 759
List o f plates P1. P1. P1. P1. P1. P1. P1. P1. P1. P1. P1.
1 : Angular unconformities and basement folding ..................... 297 2 : Cross-stratification in aeolian dune sands and fluvial channel bar deposits .................................................... 301 3 : Gravel agglomerations and mud drapes in alluvial-fan breccias and 303 fluvial channel sandstones ...................................... 4 : Natural fractures and sedimentary heterogeneities in fluvial channel and floodplain sandstones and mudstones . . . . . . . . . . . . . . . . . 307 5 : Grain-size distribution changes and erosional surfaces in braided-river channel sandstones and conglomerates . . . . . . . . . . . . . . . . . . 309 6 : Synsedimentary desiccation fracturing in lacustrine and fluvial floodplain mudstones and natural propping of cracks . . . . . . . . . . . . . 313 7 : Lacustrine and floodplain mud drapes in fluvial channel and alluvial playa sandstones ........................................... 315 8 : Natural fractures and sedimentary heterogeneities in cross-stratified aeolian dune and sheet sands as well as fluvial channel 319 sands ........................................................... 9 : Sedimentary and deformational features of lacustrine mud drapes in fluvial channel, overbank and alluvial playa sandstones . . . . . . 323 10 : Root tubes and carbonate concretions in calcrete palaeosols . . . . . 327 11 : Natural fracture origin and distribution in various reservoir 329 rocks ...........................................................
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1 . P r o p p a n t 1.0.Summary
a n d
Selection a s s o c i a t e d
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c o n t e n t s
H y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g a c t i v i t y i n Europe i s o f d i f f e r e n t s i g n i f i c a n c e i n t h e western and e a s t e r n p a r t s o f t h e area, and has a g a i n o t h e r emphasis i n t h e USA, due t o g e o l o g i c a l ( m a i n l y sedimentological-petrophys i c a l ) , r e s e r v o i r e n g i n e e r i n g , t e c h n o l o g i c a l , m a r k e t i n g and economical reasons. The f i v e b a s i c proppant t y p e s which a r e c o m m e r c i a l l y a v a i l a b l e a t t h e moment i n t h e hydrocarbon s t i m u l a t i o n market f o r d i f f e r e n t p r i c e s a r e n a t u r a l q u a r t z sand, s y n t h e t i c i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y alumina s i l i c a t e ( c e r a m i c ) proppant, i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e p r o p pant, h i g h - s t r e n g t h h i g h - d e n s i t y b a u x i t e proppant, and h i g h - s t r e n g t h l o w - d e n s i t y z i r c o n i a - s i l i c a t e proppant ( t h e d i f f e r e n t m a t e r i a l s a r e l i s t e d i n a sequence o f i n c r e a s i n g s p e c i f i c g r a v i t y , c l o s u r e s t r e s s r e s i s t i v i t y and p r i c e ) . C a r e f u l s e l e c t i o n o f t y p e , g r a i n s i z e and t o m i n o r amounts a l s o g r a i n shape o f t h e propp a n t a c c o r d i n g t o c l o s u r e s t r e s s o f t h e r e s e r v o i r , b o t t o m h o l e temperature, comp o s i t i o n o f t h e f o r m a t i o n f l u i d s and p e r m e a b i l i t y c o n t r a s t between pay zone and i n f i l l e d f r a c t u r e a r e e c o n o m i c a l l y e s s e n t i a l due t o t h e f a c t t h a t i n deep gas w e l l s , proppant c o s t can r e a c h up t o two t h i r d s o f t h e t o t a l massive h y d r a u l i c f r a c t u r i n g (MHF) expenses. In deep l o w - p e r m e a b i l i t y t i g h t gas r e s e r v o i r f r a c t u r i n g , proppant q u a n t i t i e s o f 100 - 500 t a r e f r e q u e n t l y needed and some jumbo j o b s even r e q u i r e more m a t e r i a l i n t h e range o f 500 - 1,500 t, w i t h some superg i a n t t r e a t m e n t s needing even r e c o r d l o t s up t o 3,000 t, and p r o p p a n t c h o i c e i s t h u s t h e most c r i t i c a l p a r t o f d e s i g n i n g s t i m u l a t i o n o p e r a t i o n s . Since t h e f i r s t h y d r a u l i c p r o p p a n t f r a c t u r i n g j o b s i n 1947, t h e t e c h n i q u e has e v o l v e d dur i n g course o f f o u r decades f r o m small o p e r a t i o n s on s i n g l e - z o n e c o m p l e t i o n s t o massive t r e a t m e n t s on m u l t i p l e - i n t e r v a l i n s t a l l a t i o n s , w i t h t h e main stages o f s t i m u l a t i o n f o c u s h a v i n g been f o r m a t i o n damage b r e a k t h r o u g h and s m a l l - s c a l e sand f r a c t u r i n g i n t h e l a t e 1940's t h r o u g h e a r l y 1970's, t i g h t gas r e s e r v o i r MHF t r e a t m e n t w i t h p r o g r e s s i v e l y i n c r e a s i n g amounts o f s y n t h e t i c p r o p p a n t s i n t h e l a t e 1970's t h r o u g h e a r l y 1980's, and h i g h - p e r m e a b i l i t y pay zone s t i m u l a t i o n f r o m t h e e a r l y 1980's onwards. The e v o l u t i o n o f h y d r a u l i c f r a c t u r i n g t e c h nology i s c h a r a c t e r i z e d by p r o g r e s s i v e l y i n c r e a s i n g proppant q u a n t i t i e s and c r a c k wing l e n g t h f o r l o w - p e r m e a b i l i t y t i g h t gas r e s e r v o i r s t i m u l a t i o n as w e l l as s u c c e s s i v e l y r i s i n g proppant c o n c e n t r a t i o n and f r a c t u r e w i d t h i n moderatet o h i g h - p e r m e a b i l i t y o i l and gas pay t r e a t m e n t . Since t h e i n n o v a t i o n and inaugur a t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g s h o r t l y a f t e r t h e end o f t h e Second World War, t o d a t e almost one m i l l i o n j o b s have been c a r r i e d o u t w o r l d w i d e . The major t e c h n o l o g i c a l b r e a k t h r o u g h has been achieved by i n v e n t i o n and market i n t r o d u c t i o n o f i n t e r m e d i a t e - and h i g h - s t r e n g t h s y n t h e t i c proppants i n 1976 which a l l o w e d h y d r a u l i c s t i m u l a t i o n o f deep h i g h - p r e s s u r e r e s e r v o i r s t h a t were p r i o r t o a v a i l a b i l i t y o f c l o s u r e - s t r e s s - r e s i s t a n t proppants u n a c c e s s i b l e t o f r a c t u r e t r e a t m e n t s . W h i l e n a t u r a l q u a r t z sand f a i l s by c r u s h i n g o f t h e g r a i n s a l r e a d y between 3,000 and 5,000 p s i c l o s u r e s t r e s s which r e s t r i c t s i t s a p p l i c a t i o n t o r e l a t i v e l y shallow reservoirs, synthetic intermediate-strength low-density alumina s i l i c a t e proppants have almost t h e same s p e c i f i c g r a v i t y as sand, b u t r e s i s t t o c l o s u r e s t r e s s e s up t o 8,000 p s i . I n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e p r o p p a n t s ( s t a b i l i t y up t o 10,000 p s i ) and h i g h s t r e n g t h h i g h - d e n s i t y alumina o x i d e o r l o w - d e n s i t y z i r c o n i a - s i l i c a t e proppants ( r e s i s t a n t up t o 15,000 p s i ) a r e t h e s u i t a b l e m a t e r i a l s f o r i n f i l l i n g o f f r a c t u r e s i n deep h i g h - p r e s s u r e r e s e r v o i r s and have i n Europe d u r i n g t h e l a s t t e n y e a r s been f r e q u e n t l y a p p l i e d i n MHF o p e r a t i o n s i n t i g h t g a s - b e a r i n g R o t l i e gend, C a r b o n i f e r o u s and B u n t s a n d s t e i n sandstones i n Germany FRG and N e t h e r l a n d s onshore as w e l l as B r i t i s h and Dutch Southern N o r t h Sea o f f s h o r e , and i n T e r t i a r y sandstones and conglomerates i n Yugoslavia, and have i n t h e USA been used i n many t i g h t gas sand r e s e r v o i r s i n v a r i o u s h y d r o c a r b o n - p r o s p e c t i v e b a s i n s . Res i n - c o a t i n g o f s y n t h e t i c proppants o r n a t u r a l sand r e s u l t s i n a g g r e g a t i o n o f t h e i n d i v i d u a l g r a i n s t o a s t a b l e package i n u n c o n s o l i d a t e d r e s e r v o i r s and i s a s u i t a b l e method f o r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g o f u n s t a b l e s o f t o i l - b e a r i n g c h a l k s and sands i n t h e Cretaceous and J u r a s s i c o f t h e Norwegian
2 and s u b o r d i n a t e l y a l s o t h e B r i t i s h N o r t h e r n N o r t h Sea as w e l l as i n C r e t a c e o u s and T e r t i a r y i n v a r i o u s c o u n t r i e s o n t h e European c o n t i n e n t . D o u b l e r e s i n c o a t i n g o f p r o p p a n t g r a i n s w i t h an o u t e r c u r a b l e p e l l i c l e b o n d i n g p a r t i c l e s t o g e t h e r and an i n n e r p r e c u r e d f i l m p r o v i d i n g a d d i t i o n a l s t r e n g t h o f t h e m a t e r i a l combines t h e b e n e f i t s o f a s e l f - c o n s o l i d a t i n g p r o p p a n t w i t h t h e a d v a n t a g e s o f a h i g h - s t r e n g t h p r o p p a n t by p r o v i d i n g h i g h c l o s u r e s t r e s s r e s i s t i v i t y , permanent b o n d i n g a b i l i t y and l o w d e n s i t y t o g e t h e r a t r e a s o n a b l e c o s t . T a i l - i n o f r e s i n c o a t e d p r o p p a n t s s t a b i l i z e s t h e g r a i n assemblage i n t h e p r o x i m a l p a r t o f t h e c r a c k and i n h i b i t s f r a c t u r e damage b y e v a c u a t i o n a t i t s m o u t h . T a i l - i n d e s i g n w i t h a s m a l l e r l o t o f c o a r s e r ( m a i n l y 16/20 o r 12/20) and f r e q u e n t l y h i g h e r - q u a l i t y p r o p p a n t s r e p r e s e n t i n g t h e t e r m i n a l s t a g e o f a f r a c t u r e t r e a t m e n t and f o l l o w i n g a l a r g e r m a i n q u a n t i t y o f f i n e r ( i n most c a s e s 20/40) and o f t e n l o w e r q u a l i t y p r o p p a n t s has t h e a d v a n t a g e o f a s c e r t a i n i n g maximum f r a c t u r e c o n d u c t i v i t y i n t h e i m m e d i a t e b o r e h o l e v i c i n i t y where r e s e r v o i r drawdown i s g r e a t e s t b y h i g h e r p r o p p a n t p e r m e a b i l i t y and c l o s u r e s t r e s s r e s i s t a n c e a t l o w e r o v e r a l l c o s t , as w e l l as o f f o r m i n g a p s e u d o - g r a v e l pack i n h i b i t i n g p r o p p a n t f l o w b a c k f r o m f r a c t u r e mouth t o w e l l b o r e and t h u s p r e v e n t i n g s t i m u l a t i o n r e s u l t damage b y f r a c t u r e e v a c u a t i o n . E x p e r i m e n t a l approaches f o r p o s s i b l e f u t u r e i n n o v a t i o n o f p r o p p a n t t y p e s o f a g a i n enhanced t e c h n i c a l p e r f o r m a n c e and i m p r o v e d economic a l f e a s i b i l i t y i n o r d e r t o a d j u s t t o t h e i n c r e a s i n g l y d i v e r s i f i e d spectrum o f a p p l i c a t i o n f o c u s on p r o t o t y p e m a t e r i a l s based on s e v e r a l c e r a m i c o x i d e s , c o r d i e r i t e and s i l i c o n c a r b i d e , and l o o k m a i n l y i n t o s p e c i f i c g r a v i t y l o w e r i n g b y fabrication o f p a r t i a l l y hollow o r p a r t i c l e s with high internal microporosity. C o n c e r n i n g t h e c u r r e n t l y c o m m e r c i a l l y a v a i l a b l e and f i e l d - e x p e r i e n c e d p r o p p a n t t y p e s , c r y s t a l l i n e a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s t h a t a r e p r o d u c e d b y s i n t e r i n g have r o u g h s u r f a c e and more i n t e r n a l m i c r o p o r o s i t y i n c o m p a r i s o n t o the b r i t t l e glassy z i r c o n i a - s i l i c a t e proppants which are manufactured by f u s i n g and have smooth s u r f a c e and l e s s i n t e r n a l m i c r o p o r o s i t y . On t h e o t h e r hand, t h e b r i t t l e glassy z i r c o n i a - s i l i c a t e proppants tend t o f a i l almost suddenly i n a cat a s t r o p h i c manner by b r e a k a g e i n t o t i n y s p l i n t e r s when t h e b o u n d a r y o f c l o s u r e s t r e s s r e s i s t a n c e i s reached, whereas t h e l e s s b r i t t l e and p a r t i a l l y e v e n d u c t i l e c r y s t a l l i n e a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s a r e s p l i t i n t o h a l v e s and q u a r t e r s i n a more c o n t i n u o u s manner when a p p r o a c h i n g t h e b o u n d a r y o f m e c h a n i c a l s t a b i l i t y . V a r i o u s consequences o f t h e d i f f e r e n t m e c h a n i c a l and c h e m i c a l p r o p e r t i e s o f t h e p r o p p a n t t y p e s o f d i f f e r e n t m i n e r a l o g i c a l c o m p o s i t i o n and c r y s t a l l o g r a p h i c a l s t r u c t u r e are discussed along the l i n e s o f r e s i s t i v i t y against c l o s u r e s t r e s s , t e m p e r a t u r e , and c o m p o s i t i o n o f b r i n e s . The numerous a s p e c t s i n f l u e n c i n g p r o p p a n t c h o i c e a r e i l l u s t r a t e d a l o n g t h e l i n e s o f f r i c t i o n a n g l e and g r a i n shape, f l u i d s a l i n i t y and m e c h a n i c a l s t a b i l i t y , f l u i d t e m p e r a t u r e and h o t b r i n e a g g r e s s i v i t y , e q u i p m e n t a b r a s i o n and g r a i n h a r d n e s s , s p e c i f i c g r a v i t y and f l u i d s u s p e n s i o n p r o p e r t i e s , g r a i n s i z e and embedment, and m a n u f a c t u r i n g p r o c e s s and p e l l e t c o m p o s i t i o n . P r o p p a n t c o n d u c t i v i t y as measured d u r i n g l a b o r a t o r y t e s t i n g has t o be c o n s i d e r a b l y d i s c o u n t e d i n f i e l d a p p l i c a t i o n s as a c o n s e quence o f embedment o f b r i t t l e p r o p p a n t s i n t o t h e f o r m a t i o n , p l u g g i n g o f p o r e s by f r a c t u r i n g g e l r e s i d u e s and f l u i d - l o s s f i l t e r cakes, d i s c o n t i n u i t i e s i n c r a c k g e o m e t r y , and s t r e s s c o r r o s i o n o f p r o p p a n t s by l e a c h i n g and c r u s h i n g . Embedment i n t o g e l f i l t e r c a k e s on f r a c t u r e w a l l s and p r e c i p i t a t i o n o f g e l r e s i dues i n t h e p o r e space a r e t h e m o s t s e r i o u s t y p e s o f p r o p p a n t p e r m e a b i l i t y d e t e r i o r a t i o n a p a r t f r o m g r a i n c r u s h i n g . M i x i n g o f p r o p p a n t t y p e s and g r a i n s i z e s has i n m o s t c a s e s c o n s i d e r a b l e d e l e t e r i o u s e f f e c t s on p r o p p a n t package c o n d u c t i v i t y , w h i l e m i x i n g o f d i f f e r e n t g r a i n shapes c a n be b e n e f i c i a l f o r i n c r e a s i n g the f r i c t i o n angle o f t h e m a t e r i a l . S e l e c t i o n o f propp( t y p e and g r a i n s i z e has t o be made o n t h e base o f c a l c u l a t i o n o f c o n d u c t i v i t y p e r US $ v s . c o s t o f t h e h y d r a u l i c f r a c t u r i n g o p e r a t i o n and p a y - o u t by i n c r e a s e d h y d r o c a r b o n p r o d u c t i o n d u r i n g a c e r t a i n t i m e , w i t h c o m p u t e r programs f o r t h e e c o n o m i c a l a s s e s s ment o f p r o p p a n t c h o i c e , p e r f o r m a n c e p r e d i c t i o n and f e a s i b i l i t y m o d e l l i n g b e i n g a v a i l a b l e i n t h e m a r k e t . Economical p r o f i t a b i l i t y a n a l y s i s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g has t o be based o n n e t p r e s e n t v a l u e c a l c u l a t i o n , w i t h t h e m o s t i m p o r t a n t v a r i a b l e s i n f l u e n c i n g i n v e s t m e n t r e t u r n b e i n g p r e - and p o s t - f r a c t u r e h y d r o c a r b o n p r o d u c t i o n r a t e s as w e l l as p r o p p a n t t y p e and q u a n t i t y . The m o s t important t e c h n i c a l aspects o f proppant s e l e c t i o n are c l o s u r e stress, surface morphology, c r u s h i n g behaviour, f r i c t i o n angle, f l u i d s a l i n i t y , f l u i d tempera-
3 t u r e , e q u i p m e n t a b r a s i o n and s p e c i f i c g r a v i t y , whereas i n t h e e c o n o m i c a l f e a s i b i l i t y e v a l u a t i o n , p r o p p a n t p r i c e has t h e d o m i n a n t r o l e p a r t i c u l a r l y i n l i g h t o f the currently quite d i f f e r e n t p r i c i n g o f the various materials. I n addition t o o i l - and g a s - f i e l d a p p l i c a t i o n s o f p r o p p a n t s f o r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g , c e r a m i c p r o p p a n t s a r e u s e d i n o t h e r m a r k e t segments as h e a t - e x change p e l l e t s i n c u p o l a f u r n a c e s i n s t e e l f o u n d r y , a l u m i n a and g l a s s i n d u s t r y , as d u s t a b s o r p t i o n o r f i l t e r i n g p e l l e t s i n t h e gas p u r i f i c a t i o n b r a n c h , f o r f l u i d and gas f i l t r a t i o n , f o r s h o t b l a s t i n g w i t h h a r d e n i n g o f s t a i n l e s s s t e e l b y p e e n i n g r a t h e r t h a n c u t t i n g , f o r g r i n d i n g , as c o n s t i t u e n t s o f h i g h - s t a b i l i t y cements and f o r v a r i o u s o t h e r p u r p o s e s . H y d r a u l i c f r a c t u r i n g i s i n b o t h USA and Europe a l s o i n t h e f u t u r e r e m a i n i n g t o be t h e m o s t s i g n i f i c a n t a p p l i c a t i o n o f t h e d i f f e r e n t t y p e s and g r a i n s i z e s o f t h e p r e s e n t l y c o m m e r c i a l l y a v a i l a b l e p r o p p a n t s , and e x p e r i m e n t a l approaches c a r r i e d o u t i n t h e l a s t y e a r s e v e n sugg e s t t h e p o s s i b i l i t y o f d e v e l o p m e n t o f new p r o p p a n t t y p e s o f enhanced t e c h n i c a l p e r f o r m a n c e and b e t t e r e c o n o m i c a l f e a s i b i l i t y i n c a s e o f a d e q u a t e demand b y t h e market.
1.1.
Introduction
1.1.2. 1.1.3.
1.1.4. 1.1.5.
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tion ............ 1.1.1.1. S i g n i f i c a n c e o f t i g h t r e s e r v o i r f r a c t u r i n g f o r f u t u r e gas s u p p l y . . .............. 1.1.1.2. Hydrocarbon a 1.1.1.3. Economic i n c e n t i v e s o f t i g h t g a s r e s e r v o i r f r a c t u r i n g 10 1.1.1.4. P r o p p a n t q u a n t i t i e s a p p l i e d i n MHF s t i m u l a t i o n E v o l u t i o n o f f r a c t u r i n g t e c h n o l o g y and o i l p r i c e h i s t o r y 11 Significance o f proppant s e l e c t i o n ........................... 1.1.3.1. I m p o r t a n c e o f f r a c t u r i n g f o r f e a s i b i l i t y assessment . . . . . . . . . . 11 o f hydrocarbon f i e l d s . . . . . . . . . . . 1.1.3.2. C e n t r a l r o l e o f p r o p p a n t placemen treatment ................................. S p e c i a l a s p e c t s o f t h e European s t i m u l a t i o n O r g a n i z a t i o n o f t h e r e v i e w and s t a t u s r e p o r t . . . . . . . . . . . . . . . . . 13 1.1.5.1. E x p e r i e n c e e v o l u t i o n and d e r i v a t i o n o f i n f o r m a t i o n 13 1.1.5.2. S u b d i v i s i o n o f t h e book i n t o seven c h a p t e r s . . . . . . . . 13
1.2. H i s t o r i c a l development o f hydraulic proppant f r a c t u r i n g . . . . . . . . . . . . . . 14 1.2.1. General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.2.1.1.
1.2.2.
1.2.3.
1.2.4.
B a s i c aims and p r o c e d u r e s o f h y d r a u l i c f r a c t u r i n g jobs ............................................... 1.2.1.1.1. Fracture propping ...................... 1.2.1.1.2. Stimulation goals . . . . . . . . . . . . . 1.2.1.2. E v o l u t i o n o f main f r a c t u r i n g design concepts . . . . . . . 1.2.1.2.1. F o r m a t i o n damage b r e a k t h r o u g h and s m a l l s c a l e sand f r a c t u r i n g . . . . . . . . . . . . 1.2.1.2.2. T i g h t gas r e s e r v o i r MHF s t i m u l a t i o n w i t h s y n t h e t i c p r o p p a n t s .................... 1.2.1.2.3. H i g h - p e r m e a b i l i t y p a y zone f r a c t u r i n g . . 1.2.1.2.4. S i g n i f i c a n c e o f f r a c t u r e parameter resol u t i o n ................................. L i m i t a t i o n s o f n a t u r a l sand ... .......... 1.2.2.1. Closure stress r e s i s t i v i t y i n t e r v a l . . . . . . . . . . . . . . . . 1.2.2.2. C l o s u r e s t r e s s v s . m a t r i x p e r m e a b i l i t y and o f f t a k e
........................................
Restrictio a t i v e propping materials before the inv e n t i o n o f s y n t h e t i c ceramic proppants .......... 1 . 2 . 3 . 1 . G l a s s beads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3.2. M e t a l s h o t s and w a l n u t h u l l s . . . . . . . . . . . . . . . Impact o f e a r l y s y n t h e t i c ceramic proppants 1.2.4.1. Sintered bauxite invention trigger boom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4.2. Importance o f proppant t e s t i n g f o r q u a l i t y improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16 16 16 17 17 17 18 18 18 19 19 20 20 20 22 22 22
4
1.2.5. 1.2.6.
1.2.7.
1.2.8.
1.3.
Proppant types and grain sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 1 . 3 . 1 . Provenance o f sand and . . . . . . . . . . . . . . . . . . . 32 1 . 3 . 1 . 1 . N a t u r a l sand .... . . . . . . . . . . . 32 1.3.2. 1.3.3. 1.3.4.
1.3.5.
1.4.
S i g n i f i c a n c e o f advanced s y n t h e t i c c e r a m i c p r o p p a n t s . . . . . . . . . 23 Impact o f resin-coated proppants . . . . . . . . . . . . . . . . . . 23 . . . . . . . . . . . . . . . . . 24 1 . 2 . 6 . 1 . R e s i n t y p e s and f u n c t i o n 1.2.6.1.1. R e d u c t i o n o f p o i n t l o a d i n g a t g r a i n con24 tacts .................................. 1.2.6.1.2. S p e c i f i c g r a v i t y and p r o p p a n t p r i c e l o 24 wering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.6.2. R e a c t i o n s o f r e s i n c o a t i n g s t o c l o s u r e s t r e s s and 24 temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.6.2.1. Closure stress ......................... 25 ........... . 25 1.2.6.2.2. Temperature . . . . . . . . . . . . . . . . . . . . 25 1.2.6.3. D o u b l e r e s i n c o a t i n g combin 1.2.6.3.1. F u n c t i o n s o f i n n e r p r e c u r e d and o u t e r 26 curable f i l m s .......................... 1.2.6.3.2. P r e s s u r e and t e m p e r a t u r e r e a c t i o n o f r e 26 s i n p e l l i c l e s .......................... Possible future innovation . . _ . . . . . . 26 1 . 2 . 7 . 1 . P r o p p a n t p a r t i c l e c o m p o s i t i o n t y p e s . . . . . . . . . . . . . . . . 27 1.2.7.2. Specific gravity lowering . . . . . . . . . . . . . . . . . . . . 27 1.2.7.3. S t r e n g t h r e t a i n m e n t o f h o l l o w s p h e r e s . . . . . . . . . . . . . . 28 1.2.7.4. P r o p p a n t f a b r i c a t i o n methods . . . . . . . . . . . . . . . . . . . . . . . 28 V e r t i c a l vs. horizontal f r a c t u r e o r i e n t a t i o n . . . . . . . . . . . . 28 1 . 2 . 8 . 1 . F r a c t u r e o r i e n t a t i o n and r e s e r v o i r d e p t h . . . . . . . . . . . 29 1 . 2 . 8 . 2 . F r a c t u r e o r i e n t a t i o n and g e o t e c t o n i c a l s e t t i n g . . . . . 2 9 1.2.8.2.1. E x t e n s i o n a l v s . c o m p r e s s i o n a l t e c t o n i c s 29 1.2.8.2.2. Comparative s i g n i f i c a n c e o f h o r i z o n t a l and v e r t i c a l f r a c t u r e s . . . . . . . . . . . . . . . . . 2 9 u r e i n t e r s e c t i o n and r e s e r v o i r d r a i ..... ........................... 30 1 . 2 . 8 . 4 . C o e x i s t e n c e and t r a n s i t i o n o f h o r i z o n t a l and v e r t i 30 cal fractures ......................................
1.3.1.2. Synthetic pro .... M i n e r a l o g i c a l c o m p o s i t i o n and c l o s u r e s t r e s s r e s i s 1.3.2.1. A l u m i n a o x i d e and s i l i c a t e p r o p p a n t s . . . 1.3.2.2. Z i r c o n i a - s i l i c a t e proppants . . . . . . . . . . . . Chemical c o m p o s i t i o n and a c i d s o l u b i l ........... 1.3.3.1. Chemical c o m p o s i t i o n _ _ . _ _ _ ........... 1.3.3.2. Acid s o l u b i l i t y . . . . . . . . . . . G r a i n s i z e s and c o n d u c t i v i t y p e r US $ . . . . . . . . . . . . . . . . . . . . . . . . 1 . 3 . 4 . 1 . C o n d u c t i v i t y improvement by g r a i n s i z e s e l e c t i o n v s . i n c r e a s i n g screenout r i s k . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . 3 . 4 . 2 . Commercially a v a i l a b l e proppa in si ...... G r a i n shapes . . . . . . . . . . . . . . . . . . . . . . . . . . ..... _ _ _ _
34 36 36 37 38 .38 _
Proppant selection for hydraulic fracturing and gravel packing . . . . . . . 40 1.4.0. 1.4.1. 1.4.2. 1.4.3.
S i g n i f i c a n c e o f p r o p p a n t t e s t i n g f o r q u a l i t y improvement . . . . . S u r f a c e m o r p h o l o g y and i n t e r n a l m i c r o p o r o s i t y . . . . . . . . . . . . . . . . ........ 1 . 4 . 1 . 1 . L a b o r a t o r y e x p e r i m e n t s and r e s u l t s 1 . 4 . 1 . 2 . F i e l d t e s t i n g and e x p e r i e n c e . . . . . . C r u s h i n g b e h a v i o u r and c r y s t a l l i n e s t r u c t u r e . . . . . . . . . . . . . . . . . 1.4.2.1. F a i l u r e o f m i c r o c r y s t a l l i n e and g l a s s y p r o p p a n t t y p e 1.4.2.2. I n t e r n a l proppant s t r u c t u r e ........................ ............. F r i c t i o n a n g l e and g r a i n shape . . . . 1.4.3.1. Proppant backflow o u t o f 1 . 4 . 3 . 2 . S t a b i l i t y enhancement o f t h e p r o p p a n t package . . . . . . 1.4.3.2.1. Proppant backflow p r e v e n t i o n i n hydrau. l i c fracturing ................... 1.4.3.2.2. Proppant evacuation i n f r a c t u r i n g vs.
40 41 41 42 42 42 43 44 44
5 gravel packing . . . .... ... F l u i d s a l i n i t y and mechanochemical s t a b i ................. 1 . 4 . 4 . 1 . B r i n e c o m p o s i t i o n and proppant d i s s o l u t i o n . . . . . . . . . 1.4.4.1.1. Low and h i g h f l u i d s a l i n i t i e s . . . . . . . . . . 1.4.4.1.2. Proppant s u r f a c e s t a b i l i z a t i o n . . . . . . . . . 1.4.4.2. Formation water s a l i n i t y . . . . . . . . 1.4.4.3. Formation water aggressivity vs. 1.4.5. F l u i d temperature and h o t b r i n e a g g r e s s i v i t .......... 1.4.5.1. Steam-drive enhanced o i l r e c o v e r y . 1.4.5.2. Deep t i g h t gas r e s e r v o i r s . . . . . . . 1 . 4 . 6 . Equipment a b r a s i o n and g r a i n hardness . . . . . . . . . . . . . . . . . . . . . . . . 1.4.6.1. E r o s i o n a l b e h a v i o u r o f d i f f e r e n t proppant t y p e s 1.4.6.1.1. Sintered bauxite . . . . . . . . . . . . . . . . . . . 1.4.6.1.2. Z i r c o n i a - s i l i c a t e proppant . . . . . . . . . . . . . 1.4.6.2. Equipment e r o s i o n by proppants . . . . . . . . . . . . . . . . . . . . . 1.4.6.2.1. Diameter changes . . . . . . . . . . . . . . . . . . . . . . . 1.4.6.2.2. Threaded c o n n e c t i o n s 1.4.6.2.3. P r e v e n t i o n o f proppan 1.4.6.2.3.1. Replace i n t e g r a l connections 1.4.6.2.3.2. J o i n t s and t u b u l a r s o h i g h e r w e i g h t and grade . . 1.4.6.3. Proppant damage by equipment . . . . . . . . . . 1 . 4 . 7 . S p e c i f i c g r a v i t y and f l u i d suspension p r o p e r t i e s 1 . 4 . 7 . 1 . F l u i d v i s c o s i t y and proppant s e t t l i n g . . . . . . . . . . . . . . .... 1.4.7.2. S p e c i f i c g r a v i t y vs. tap density 1 . 4 . 8 . G r a i n s i z e and embedment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . 4 . 8 . 1 . R e s e r v o i r c o m p o s i t i o n and p r o p p a n t r e q u i r e m e n t . . . . . 1.4.8.2. F a c t o r s i n f l u e n c i n g proppant embedment . . . . . . . . 1 . 4 . 9 . M a n u f a c t u r i n g process and p e l l e t c o m p o s i t i o n . . . . . . . . . . . . . . . . . 1 . 4 . 9 . 1 . Alumina o x i d e and s i l i c a t e proppants . . . . . . . . . . . . . . . 1.4.9.1.1. Grinded powder p a r t i c l e s i z e vs. p e l l e t integrity .............................. 1.4.9.1.2. S i n t e r i n g temperature and k i l n e n v i r o n ment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.9.1.3. A r t i f i c i a l p e l l e t formation vs. natural p i s o l i t e p r o c e s s i n g .................... 1.4.9.2. Z i r c o n i a - s i l i c a t e proppants . . . . . . . . . . . . . . . . . . . . . . . . 1.4.9.3. N a t u r a l sand . . . . . . . . . . . . . . . . 1.4.10. Conductivity discount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.10.1. Laboratory vs. f i e l d c o n d u c t i v i t y discrepancy . . . . . . 1 . 4 . 1 0 . 1 . 1 . Proppant b e h a v i o r and r e a c t i o n t o s t r e s s 1 . 4 . 1 0 . 1 . 2 . Proppant g r a i n s i z e e f f e c t s . . . . . . . . . . . . 1 . 4 . 1 0 . 2 . Impact o f e x p e r i m e n t a l t e c h n 1.4.10.2.1. General aspects 1 . 4 . 1 0 . 2 . 2 . Proppant g r a i n s 1 . 4 . 1 0 . 2 . 3 . Proppant c o n c e n t r a t i o n . . . . . . . . . . . . . . . . . 1.4.10.3. I m p l i c a t i o n s o f longterm laboratory proppant t e s t i n g 1 . 4 . 1 0 . 3 . 1 . Gel r e s i d u e damage and a c i d d i s s o l u t i o n 1.4.10.3.2. S t r e s s c o r r o s i o n c r a c k i n g . 1 . 4 . 1 0 . 3 . 2 . 1 . Proppant sen aqueous s o l u t i o n s . . . . . . . . 1 . 4 . 1 0 . 3 . 2 . 2 . G r a i n weakening by s t r e s s i n t e n s i f i e d erosion . . . . . . 1 . 4 . 1 0 . 3 . 3 . E f f e c t s o f h i g h temperature, h i g h b r i n e s a t u r a t i o n and h i g h c l o s u r e s t r e s s . . . . . 1.4.10.3.3.1. S i l i c a s a t u r a t i o n . . . . . . . . 1 . 4 . 1 0 . 3 . 3 . 2 . Oxygen c o n c e n t r a t i o n . . . . . 1 . 4 . 1 0 . 3 . 3 . 3 . R e s e r v o i r c o n d i t i o n spectrum approach . . . . . . . . . . . . 1.4.10.3.4. E f f e c t s o f pay r o c k s l a b s and p r o p p a n t concentration . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4.4.
44 45 45 45 46 47 47 48 48 49
50 51 51 52 52 53 53 53 53 54 54 54 55 56 56 57
58 58 59 60 60 60 61 61 61 62 62
6 1 . 4 . 1 0 . 3 . 4 . 1 . W a l l s l i p and m e t a l p i s t o n v s . f i l t e r c a k e and c o r e 62 slab ..................... 1 . 4 . 1 0 . 3 . 4 . 2 . F r a c t u r e p o r o s i m e t r y . . . . _ 63 1 . 4 . 1 0 . 3 . 5 . Gel f i l t e r c a k e o n f r a c t u r e w a l l . . . . . . . 63 1 . 4 . 1 0 . 3 . 5 . 1 . P r o p p a n t embedment . . . . . . _ 63 1.4.10.3.5.2. Reduction o f e f f e c t i v e f r a c t u r e w i d t h . . . . . . . . . . . 64 1 . 4 . 1 0 . 3 . 5 . 3 . P r o p p a n t p e r m e a b i l i t y damage by f i l t e r - c a k e b u i l d up . . . . . . . . . . . . . . . . . . . . . . . 64 1 . 4 . 1 0 . 3 . 5 . 4 . P r o p p a n t c o n c e n t r a t i o n and f l u i d t y p e . . . . . . . . . . . . . . . 65 1 . 4 . 1 0 . 3 . 5 . 5 . Gel f i l t e r c a k e s i n f r a c t u r i n g and g r a v e l p a c k i n g 65 1 . 4 . 1 0 . 3 . 5 . 6 . Removal o f g e l f i l t e r c a k e b y combined f l u i d - l o s s add i t i v e and g e l b r e a k e r . . . 65 1.4.10.3.6. G e l l i n g agent concentration 1 . 4 . 1 0 . 3 . 6 . 1 . G e n e r a l a s p e c t s . . . . . . . . . . 66 1 . 4 . 1 0 . 3 . 6 . 2 . Cleanup f l o w r a t e and c l o s u r e s t r e s s . . . . . . . . . . . . . . 66 1 . 4 . 1 0 . 3 . 6 . 3 . P r o p p a n t g r a i n s i z e and b r e a k e r c o n c e n t r a t i o n . . . _ 67 1 . 4 . 1 0 . 3 . 6 . 4 . Hydrocarbon-based cleanup f l u i d s f o r proppant package permeabi 1 it y i m p r o v e ment . . . . . . . . . . . . . . . . . . . . . 67 1 . 4 . 1 0 . 4 . Recommended c o n d u c t i v i t y c o r r e c t i o n f a c t o r s _ . . . . . . . 67 1 . 4 . 1 0 . 4 . 1 . I n f l u e n c e o f t e s t i n g e q u i p m e n t _ _ . _ . _ _68 _ _ 1 . 4 . 1 0 . 4 . 2 . I m p a c t o f b r i n e c o m p o s i t i o n . . . . . . . . . . . . 68 1 . 4 . 1 0 . 4 . 2 . 1 . C l a y s t a b i l i z a t i o n b y KCl water i n f i e l d stimulation a p p l i c a t i o n . . . . . . . . . . . . . . 69 1 . 4 . 1 0 . 4 . 2 . 2 . KC1 s o l u t i o n s as l a b o r a t o r y t e s t i n g f l u i d s . . . . . . . . 69 1 . 4 . 1 0 . 4 . 3 . C o n d u c t i v i t y improvement _ _ . . . . . . . . . . . . _ 69 1.4.10.4.3.1. Stimulation treatment o v e r d e s i g n . . . . . . . . . . . . . . _ 70 1 . 4 . 1 0 . 4 . 3 . 2 . Demand f o r h i g h e r p r o p p a n t q u a l i t y and s a t u r a t i o n . . . 70 1.4.10.4.4. P r a c t i c ctivity data . . .......... . . . . . . . . , 71 1.4.10. l e n t f l o w 71 1.4.10.4.4.2. Conductivity necessity f o r cleanup vs. production . . . 71 1 . 4 . 1 0 . 5 . Performance experience i n longterm f i e l d a p p l i c a t i o n 71 1.4.10.5.1. R o t l i e g e n d a s i n Germany 72 1.4.10.5.2. N o r t h China . . . . . . . . . . . . , 72 1.4.11. Proppant mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 1.4.11.1. Mixing o f proppant types . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 1 . 4 . 1 1 . 1 . 1 . M i x i n g o f n a t u r a l sand and s y n t h e t i c proppants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 1.4.11.1.1.1. Mixing o f s i n t e r e d bauxite o r g l a s s beads w i t h sand . 73 1.4.11.1.1.2. Conductivity d e t e r i o r a t i o n b y p r o p p a n t m i x i n g . . . . . . . 73 1 . 4 . 1 1 . 1 . 2 . M i x i n g o f d i f f e r e n t sand t y p e s . . . . . . . . . 74 1 . 4 . 1 1 . 2 . M i x i n g o f p r o p p a n t g r a i n s i z e s . . . . . _ . . . . . . . . . . . . . . _74 1 . 4 . 1 1 . 2 . 1 . M i x i n g o f c o a r s e p r o p p a n t s and f i n e 100 mesh sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 1.4.11.2.1.1. A p p l i c a t i o n spectrum o f
7 ... 100 mesh sand . . _ _ _ _ _ . . 75 Preventing permeability d e t e r i o r a t i o n . . . . . . . . . . . . 76 1 . 4 . 1 1 . 2 . 1 . 3 . 100 mesh sand as e x c l u s i v e o r p a r t i c i p a n t p r o p p a n t . . 77 1 . 4 . 1 1 . 2 . 1 . 4 . P r o v o k i n g p e r m e a b i l i t y des t r u c t i o n . . . . . . . . . _ . . . . . .78 1.4.11.2.2. M i x i n g o f various coarse proppant g r a i n sizes . . ................................ 79 M i x i n g o f p r o p p a n t g r a i n shapes .................... 79 1 . 4 . 1 1 . 3 . 1 . Enhanced c o n d u c t i v i t y o f r o u n d - a n g u l a r 80 sand b l e n d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . 4 . 1 1 . 3 . 1 . 1 . C o n d u c t i v i t y improvement and packaqe s t a b i l i t y . . . . 8 0 1 . 4 . 1 1 . 3 . 1 . 2 . C r u s h i n g b e h a v i o u r .-. _ . . . 8 0 1 . 4 . 1 1 . 3 . 2 . B e n e f i t f o r t h e h y d r o c a r b o n s t i m u l a ion . . . . 81 i n d u s t r y .......................... programs f o r p r o p p a n t s e l e c t i o n . _ _ _ . . . . . . . . . . . . . . 8 1 General e c o n o m i c a l c r i t e r i a . . . . . . . . . . . . . . . . . . . . . . . 82 1 . 4 . 1 2 . 1 . 1 . I n v e s t m e n t r e t u r n and p r o f i t a b i l i t y _ . . . 82 1 . 4 . 1 2 . 1 . 2 . D e s i g n o p t i m i z a t i o n and success d e f n i _ . . . a3 t i o n ............................... N e t o r e s e n t v a l u e c a l c u l a t i o n . . . . . . . . . . . . . . . . . ._ . . . 83 S i m u i a t i o n t e c h n i q u e s and i n p u t d a t a . . . . . . . . . . . . . . . 84 1.4.12.3.1. Cost e f f e c t i v e n e s s o f v a r i o u s proppant t y p e s , s i z e s and q u a n t i t i e s . . . . . . . _ . . . . 84 1.4.12.3.2. Evolution o f p o s t - f r a c t u r e w e l l p e r f o r mance f o r e c a s t . . . . . . . . . . . . . . . . . 84 S i g n i f i c a n c e f o r economical l i t y m o d e l l i n g . . 85 1 . 4 . 1 2 . 4 . 1 . P r o p p a n t q u a l i t y and d r a i n a g e e f f i c i e n c y . . . . . . . . _ _ . _ . . . .85 . differences ..... 1 . 4 . 1 2 . 4 . 2 . E a r l y and l a t e p r o d u c t i o n h i s P r o p p a n t volume v s . w e i g h t . . . . . . . . . . . . . . . . . . . . . . . . . 86 Other aspects . . . . . . . ...... 1 . 4 . 1 2 . 6 . 1 . P r o p p a n t volume m i n i m i z a t i o n and d e s i g n o p t i m i z a t i o n . . . . 87 1.4.12.6.2. Proppant s e l e c t i o n vs. payo u t m a x i m i z a t i o n . . . . . . . . . . . 87 selection guidelines . ..... . . . . a7 88 N a t u r a l sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intermediate-strength low-density alumina s i l i c a t e proppants ......................... Intermediate-strength high-density 88 s i l i c a t e proppants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High-strength high-density alumina oxide proppants . 89 High-strength l o s i t y z i r c o n i a - s i l i c a t e proppants . . . . . . . . . . ............................... 89
1.4.11.2.1.2.
1.4.11.3.
1 . 4 . 1 2 . Computer 1.4.12.1.
1.4.12.2. 1.4.12.3.
1.4.12.4.
1.4.12.5. 1.4.12.6.
1.4.13. Proppant 1.4.13.1. 1.4.13.2. 1.4.13.3. 1.4.13.4. 1.4.13.5.
1.5.
Proppant applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . 5 . 1 . O i l - and g a s - f i e l d a p p l i c a t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5.2.
90 90 1.5.1.1. H i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s . . 9 1 1.5.1.1.1. S t a n d a r d g r a i n s i z e s _ . . . . . . . . . . . . . _ _ . .9. 1 1.5.1.1.2. Coarser g r a i n s i z e s .................... 91 1.5.1.2. L o w - d e n s i t y a l u m i n a s i l i c a t e p r o p p a n t s . . . . . . . . . . . . . 92 1.5.1.3. Z i r c o n i a - s i l i c a t e p r o p p a n t s . . . . . . . . . . . . . . . . . . . . . . . . 92 N o n - o i l - and g a s - f i e l d a p p l i c a t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . 93 1.5.2.1. F o u n d r y h e a t exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 1.5.2.1.1. P e l l e t t y p e . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 1.5.2.1.2. Pellet grain size .... 94 1.5.2.1.3. A l u m i n a and g l a s s i n d u t r y h e a t exchange 95 1.5.2.2. M e c h a n i c a l gas p u r i f i c a t i o n . . _ . . . 95 1.5.2.2.1. O p e r a t i o n a l s t a g e . . . . . . . . . . . . . . . . . . . . . 95 .
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8 1.6.
1.5.2.2.2. Cleaning stage . . . . . . . . . . . . . . . . . . . . . . . . . C o n c l u s i o n and o u t l o o k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95 96
1.1. Introduction H y d r a u l i c p r o p p a n t f r a c t u r i n g o f p r e d o m i n a n t l y l o w - p e r m e a b i l i t y o i l and gas r e s e r v o i r s has become a w i d e l y a p p l i e d t e c h n o l o g y o f h y d r o c a r b o n p a y s t i m u l a t i o n i n order t o increase recoverable reserves, t o accelerate production by i n c r e a s i n g r a t e s o r t o even p e r m i t o f f t a k e a t a l l . The p r o m o t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g t o an o u t s t a n d i n g t e c h n i q u e o f w e l l s t i m u l a t i o n w h i c h a l l o w s a c q u i s i t i o n o f enormous gas r e s e r v e s i n h i t h e r t o e c o n o m i c a l l y u n e x p l o i t a b l e deep t i g h t gas s a n d s t o n e ( c f . s e c t i o n 4 . 4 . 1 . ) , shale ( c f . section 4 . 4 . 4 . ) and c o a l ( c f . s e c t i o n 4 . 4 . 3 . ) p a y zones t o o k p a r t i c u l a r l y p l a c e d u r i n g t h e p a s t dozen o f y e a r s s i n c e s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s had been i n v e n t e d and i n t r o d u c e d t o t h e p e t r o l e u m i n d u s t r y i n l a t e 1 9 7 6 (COOKE 1 9 7 6 ; COOKE, GIDLEY & MUTT1 1 9 7 7 ; ATTEBERRY, TUCKER & R I T Z 1 9 7 9 ; COOKE & GIDLEY 1 9 7 9 ) . The s i g n i f i c a n t and d e c i s i v e i m p a c t o f h i g h - q u a l i t y man-made p r o p p a n t s on t h e booming o f h y d r a u l i c f r a c t u r i n g i s t h a t t h e y c o u l d w i t h s t a n d h i g h e r c l o s u r e s t r e s s e s and t h e r e f o r e a l l o w e d t o p r o c e e d t o h i g h e r - p r e s s u r e d r e s e r v o i r s i n g r e a t e r d e p t h s t h a n was p r e v i o u s l y p o s s i b l e when o n l y n a t u r a l sand as a p r o p p i n g a g e n t was a v a i l a b l e t h a t s t a r t s t o c r u s h a l r e a d y i n m o d e r a t e f o r m a t i o n d e p t h and t h u s i s u n s u i t a b l e t o s u p p o r t f r a c t u r e s i n deep p r o s p e c t i v e i n t e r v a l s o f t h e g e o l o g i c a l column. The f o l l o w i n g i n t r o d u c t o r y r e m a r k s b e g i n w i t h a s h o r t c h a r a c t e r i z a t i o n o f t i g h t gas r e s e r v o i r s and m a s s i v e h y d r a u l i c p r o p p a n t f r a c t u r i n g (MHF) s t i m u l a t i o n w h i c h has t u r n e d o u t d u r i n g t h e l a s t 10 y e a r s t o be t h e m o t o r o f h y d r o c a r bon w e l l s t i m u l a t i o n a c t i v i t y and t h e r e f o r e i s t h e k e y t o t h e w h o l e d i s c u s s i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g b y r e p r e s e n t i n g t h e t e c h n i c a l l y and e c o n o m i c a l l y m o s t s i g n i f i c a n t a p p l i c a t i o n o f w e l l t r e a t m e n t . MHF s t i m u l a t i o n i s a l s o t h e p r o c e d u r e where t h e v a r i o u s p r o p e r t i e s o f d i f f e r e n t p r o p p a n t t y p e s have t h e h i g h e s t s i g n i f i c a n c e i n t e r m s o f t e c h n i c a l p e r f o r m a n c e and e c o n o m i c a l f e a s i b i l i t y o f t h e operation. A f t e r a succeeding o u t l i n e o f e v o l u t i o n o f h y d r a u l i c f r a c t u r i n g t e c h n o l o g y and o i l p r i c e h i s t o r y as w e l l as i m p o r t a n c e o f p r o p p a n t s e l e c t i o n and f o l l o w i n g some comments on s p e c i a l a s p e c t s o f t h e European s t i m u l a t i o n m a r k e t , t h e o r g a n i z a t i o n o f t h e r e v i e w and s t a t u s r e p o r t i s b r i e f l y s k e t c h e d .
1.1.1. Tight gas reservoirs and MHF stimulation H y d r a u l i c proppant f r a c t u r i n g enables t o complete hydrocarbon e x p l o r a t i o n and p r o d u c t i o n w e l l s w i t h an o r i g i n a l e f f e c t i v e r e s e r v o i r p e r m e a b i l i t y i n t h e m i c r o d a r c y r a n g e and has t h u s p e r m i t t e d t o s a t i s f y i n c r e a s i n g demand f o r n a t u r a l gas b y f o c u s s i n g a t t e n t i o n o n u n c o n v e n t i o n a l t i g h t gas h o r i z o n s t h a t were h i s t o r i c a l l y c o n s i d e r e d t o be u n c o m m e r c i a l , b u t w h i c h a r e now r e p r e s e n t i n g i m p o r t a n t resources t h a t a r e already d e l i v e r i n g appreciable shares o f t h e t o t a l gas p r o d u c t i o n and w h i c h w i l l become p a r t i c u l a r l y s i g n i f i c a n t i n t h e f u t u r e when gas s u p p l y f r o m t r a d i t i o n a l r e s e r v o i r s d e c l i n e s more and more (RANDOLPH 1 9 7 4 ; FAST, HOLMAN & COVLIN 1 9 7 7 ; KUUSKRAA, BRASHEAP, OOSCHER & E L K I N S 1 9 7 8 ; AGARWAL, CARTER & POLLOCK 1 9 7 9 ; KUUSKRAA, BRASHEAR, EL:'":C & MORRA 1 9 7 9 ; KUUSKRAA 1980, BAKER 1981, WALLS 1981; AHMED, SCHATZ, ABOU-SAYED & JONES 1 9 8 2 ; BRASHEAR, E L K I N S & HAAS 1 9 8 2 ; MORRA, BRASHEAR & HAAS 1 9 8 2 ; O'SHEA & MURPHY 1 9 8 2 ; O'SHEA, ROSENBERG & BRASHEAR 1 9 8 2 ; KAZEMI 1983; ROSENBERG, O'SHEA, MERCER, MORRA & BRASHEAR 1 9 8 3 ; VEATCH & BAKER 1 9 8 3 ; BRASHEAR, ROSENBERG & MERCER 1984; DOE 1 9 8 4 ; HAAS, BRASHEAR & MORRA 1 9 8 5 ; VEATCH 1986, CLARK 1987, DUDA & HANCOCK 1 9 8 7 , KUUSKRAA & HAAS 1988, NORTHROP 1988; a s p e c t s o f t i g h t gas r e s e r v o i r s a r e a l s o e v a l u a t e d i n s e c t i o n s 3 . 4 . 1 . and 4 . 4 . 1 . ) . The d i s c u s s i o n as f o l l o w s c o n c e n t r a t e s on s i g n i f i c a n c e o f t i g h t gas r e s e r v o i r f r a c t u r i n g f o r f u t u r e gas s u p p l y , h y d r o c a r b o n a c q u i s i t i o n expenses and s t i m u l a t i o n s h a r e , e c o n o m i c a l i n c e n t i v e s o f t i g h t gas r e s e r v o i r f r a c t u r i n g , and p r o p p a n t q u a n t i t i e s a p p l i e d i n MHF s t i m u lation.
9
1.1.1.1. Significance of tight reservoir fracturing for future gas supply Massive h y d r a u l i c p r o p p a n t f r a c t u r i n g (MHF; c f . s e c t i o n 4 . 8 . 1 . 2 . ) i s u s u a l l y t h e most e f f e c t i v e r e s e r v o i r s t i m u l a t i o n t r e a t m e n t i n l o w - p e r m e a b i l i t y h y d r o c a r bon-bearing i n t e r v a l s (ECONOMIDES 1987 b ) , i s i n f a c t t h e o n l y proven economic a l development method f o r t i g h t gas r e s e r v o i r s t o d a t e (VEATCH 1983) and i s e x p e c t e d t o be a b l e t o p r o v i d e a d d i t i o n a l gas r e s e r v e s o f 4 - 8 T c f ( T r i l l . f t 3 ) i n t h e USA i f p r u d e n t l y a p p l i e d (PAI, G A R B I S & HALL 1983). Massive h y d r a u l i c f r a c t u r i n g c o m p r i s i n g l a r g e t r e a t m e n t s i z e s i n terms o f proppant q u a n t i t y , f l u i d volume and c r a c k l e n g t h has been f i r s t a p p l i e d t o deep gas r e s e r v o i r s i n 1975 (RANDOLPH 1974, WORLD OIL 1975 a, SLUSSER & RIECKMANN 1976; c f . s e c t i o n s 1 . 2 . 4 . and 2 . 4 . 1 . ) and s i n c e t h a t t i m e has r a p i d l y become t h e p r o b a b l y most w i d e l y u t i l i z e d t e c h n i q u e i n hydrocarbon s t i m u l a t i o n e s p e c i a l l y i n USA and West e r n Europe. The s i g n i f i c a n c e o f u n c o n v e n t i o n a l t i g h t gas sandstone e x p l o i t a t i o n by MHF s t i m u l a t i o n i s u n d e r l i n e d by t h e p r o g r e s s i v e l y r i s i n g c o n t r i b u t i o n o f t i g h t r e s e r v o i r gas t o t h e t o t a l gas p r o d u c t i o n i n t h e USA f r o m a b t . 5 10 % i n t h e 1980's t o a b t . 30 % and 50 % f o r e c a s t e d f o r t h e y e a r s 2000 and 2020, r e s p e c t i v e l y (BAKER 1981; c f . s e c t i o n 2 . 2 . 1 . 6 . 4 . ) . Independent from, b u t c e r t a i n l y c o n s i d e r a b l y promoted by t h e c h a l l e n g i n g acq u i s i t i o n o f gas i n t i g h t r e s e r v o i r s o f sandstone, s h a l e and c o a l type, t h e use o f h y d r a u l i c p r o p p a n t f r a c t u r i n g f o r s t i m u l a t i o n o f w e l l s i n sandstone and c a r bonate pays i n o i l and gas i n d u s t r y and s u b o r d i n a t e l y a l s o i n geothermal h e a t e x p l o i t a t i o n ( c f . s e c t i o n 4 . 7 . ) has r i s e n s i g n i f i c a n t l y d u r i n g t h e l a s t y e a r s and t h i s t r e n d i s expected t o c o n t i n u e and t o even a c c e l e r a t e , because i t w i l l become i n c r e a s i n g l y i m p o r t a n t t o maximize t h e amount o f o i l and gas t h a t can be produced f r o m a g i v e n w e l l and pay i n t h e near and f a r f u t u r e when many o f t h e h i g h - c a p a c i t y c o n v e n t i o n a l r e s e r v o i r s become p r o g r e s s i v e l y exhausted and d e p l e t e d (McDANIEL & WILLINGHAM 1978).
1.1.1.2. Hydrocarbon acquisition expenses and stimulation share S i g n i f i c a n c e o f h y d r a u l i c p r o p p a n t f r a c t u r i n g f o r i n c r e a s i n g a c c e s s i b l e hydrocarbon r e s e r v e s and o p t i m i z a t i o n o f p r o f i t a b i l i t y o f o i l and gas f i e l d exp l o i t a t i o n i s u n d e r l i n e d by t h e f a c t t h a t d u r i n g t h e l a s t decade, p e t r o l e u m i n d u s t r y i n f r e e w o r l d spent a n n u a l l y more t h a n 100 B i l l . US $ f o r o i l and gas e x p l o r a t i o n and p r o d u c t i o n (OEHME 1986; c f . s e c t i o n 2 . 2 . 1 . 2 . and 2 . 2 . 1 . 5 . 2 . ) . I t i s d i f f i c u l t t o e s t i m a t e t h e share o f s t i m u l a t i o n , b u t i t i s o n l y l o g i c a l t o p o s t u l a t e t h a t i n view o f such h i g h expenses e v e r y e f f o r t s h o u l d be undertaken t o maximize hydrocarbon o f f t a k e by a p p l i c a t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g as a more o r l e s s common p r a c t i c e o f w e l l c o m p l e t i o n i n f i e l d s where t h e o u t p u t can be i n c r e a s e d t h r o u g h t h i s t e c h n i q u e . I n t h e USA which p r o b a b l y r e p r e s e n t t h e most mature o i l and gas r e s e r v o i r s t i m u l a t i o n market, h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t s have t o be c a r r i e d o u t i n 35 - 40 % o f a l l t h e w e l l s t h a t a r e d r i l l e d and completed (VEATCH 1983, OIL GAS JOURNAL 1984; c f . s e c t i o n 2.3.). I n t h e B r i t i s h s e c t o r o f t h e N o r t h Sea which i n c l u d e s t h e Southern R o t l i e gend B a s i n t h a t i s t h e most i m p o r t a n t t i g h t gas p r o v i n c e i n Europe and which i s a t t h e moment i n t h e stage o f advancing t o t h e c e n t r e o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n Europe and t o r e p r e s e n t t h e h i g h l i g h t o f t h e whole w e l l s t i m u l a t i o n h i s t o r y i n t h i s area ( c f . s e c t i o n 2 . 4 . 1 . 2 . ) , a b t . 50 B i l l . t! have been i n v e s t e d i n t o e x p l o r a t i o n and p r o d u c t i o n d u r i n g 25 y e a r s s i n c e t h e d i s c o v e r y o f t h e f i r s t commercial gas f i e l d i n 1965 (OFFSHORE ENGINEER 1986 d ) , w i t h t h i s c a p i t a l e x p e n d i t u r e c o n t a i n i n g a l r e a d y reasonable shares o f s t i m u l a t i o n due t o v a r i o u s b i g campaigns h a v i n g a l r e a d y t a k e n p l a c e i n t h e R o t l i e g e n d t i g h t gas be1 t .
10
1 . 1 . 1 . 3 . Economical incentives o f tight gas reservoir fracturing As a consequence o f w o r l d - w i d e i n c r e a s e d demand f o r n a t u r a l gas and t h e c r e a t i o n o f t i g h t gas i n c e n t i v e p r i c e and t a x i n t h e USA (HAAS, BRASHEAR & MORRA 1985) i n 1978 ( t h u s a l m o s t c o i n c i d i n g w i t h t h e b e g i n n i n g o f c o m m e r c i a l a v a i l a b i l i t y o f i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s ; c f . s e c t i o n 1 . 2 . 4 . 1 . ) , lowp e r m e a b i l i t y r e s e r v o i r s have been r e c o g n i z e d as t h e n e x t m a j o r s o u r c e o f f u t u r e gas s u p p l y p a r t i c u l a r l y i n t h e USA and a l s o i n s e v e r a l p a r t s o f Europe (BAKER 1981; c f . s e c t i o n 2 . 2 . 1 . 6 . 4 . ) and have been r e g a r d e d as h a v i n g t h e p r o s p e c t o f o f f s e t t i n g t h e p r e d i c t a b l e d e c l i n e i n c o n v e n t i o n a l r e s e r v e s (HAAS, BRASHEAR & MORRA 1 9 8 5 ) . The n a t u r e o f t h e two e c o n o m i c a l i n c e n t i v e s , p r o v i d i n g e i t h e r h i g h e r p r i c e s when gas i s i n s h o r t s u p p l y o r t a x c r e d i t s when i t i s n o t , c r e a t e s a s i t u a t i o n w h i c h i s b e n e f i c i a l f o r t i g h t gas e x p l o r a t i o n and e x p l o i t a t i o n . Because t i g h t gas f o r m a t i o n s p r o d u c e p r i m a r i l y o n l y w i t h l o w u n e c o n o m i c a l r a t e s , t h e n e c e s s a r y i n c r e a s e o f p r o d u c t i v i t y has t o be s e c o n d a r i l y a c h i e v e d b y h y d r a u l i c proppant and/or a c i d f r a c t u r i n g ( c f . s e c t i o n 4 . 5 . 1 . ) o r h o r i z o n t a l bor e h o l e d r i l l i n g (SUNG & ERTEKIN 1987; c f . s e c t i o n s 4 . 8 . 6 . 2 . 1 . and 4 . 8 . 6 . 2 . 2 . ) o r a combination o f both. Hydraulic proppant f r a c t u r i n g i s generally considered t o be t h e most v i a b l e method o f i m p r o v i n g r e c o v e r y i n t i g h t f o r m a t i o n s (ECONOMIDES 1987 b ) , and m a x i m i z a t i o n o f t h e e x p l o i t a t i o n e f f e c t i v i t y c a n be a c h i e v e d by i n s t a l a t i o n o f m u l t i f r a c t u r e systems i n h i g h l y - d e v i a t e d t o h o r i z o n t a l d r a i nage we1 s ( c f . s e c t i o n 4 . 8 . 6 . 2 . 4 . ) e s p e c i a l l y i n t h i n r e s e r v o i r s , whereas f o r t h i c k pay zones, d e e p l y p e n e t r a t i n g n o r m a l f r a c t u r e s e m a n a t i n g f r o m v e r t i c a l w e l l s a r e s t i l l t h e optimum s t i m u l a t i o n t e c h n i q u e .
1.1.1.4. Proppant quantities applied in MHF stimulation Enhancement of t h e a b i l i t y t o a c c u r a t e l y p r e d i c t and c o n t r o l t h e g e o m e t r y of h y d r a u l i c f r a c t u r e s i n c r e a s e s t h e chances o f a c h i e v i n g optimum p r o p p e d f r a c t u r e l e n g t h s and h e i g h t s ( c f . s e c t i o n 4 . 8 . 1 1 . ) and t h u s l e a d s t o an a m e l i o r a t e d e c o n o m i c a l i n c e n t i v e f o r d e v e l o p m e n t o f t i g h t gas r e s e r v o i r s (HOLDITCH, ROBINSON, WHITEHEAD & ELY 1 9 8 7 ) . T h i c k t i g h t r e s e r v o i r f o r m a t i o n s ( w i t h p e r m e a b i l i t i e s b e i n g l e s s t h a n 0 . 1 md; PAI, GARBIS & HALL 1983; c f . s e c t i o n 3 . 4 . 1 . 2 . ) containi n g enormous amounts o f gas r e s e r v e s i n USA and Europe t h a t o n l y have t o be liberated from t h e low-permeability m a t r i x r e q u i r e f r e q u e n t l y massive h y d r a u l i c f r a c t u r i n g t r e a t m e n t s (MHF; RANDOLPH 1974; MALONE, STAHL & COULTER 1977; MURPHY & CARNEY 1977; PARKER 1980, 1981; WHITE & DANIEL 1980, 1981; HOLCOMB 1982, PA1 & G A R B I S 1983 a; P A I , G A R B I S & HALL 1983) where p r o p p a n t q u a n t i t i e s o f f r e q u e n t l y u p t o more t h a n 400 - 600 t o r e v e n a b t . 1,000 t a r e pui '.:d w i t h i n a s i n g l e j o b c o m p r i s i n g f l u i d volumes up t o more t h a n 1 M i o . g a l , w i t h some o f t h e jumbo s t i m u l a t i o n s c h a l l e n g i n g t h e p r e s e n t t e c h n o l o g i c a l and o p e r a t i o n a l b o u n d a r i e s . The w o r l d r e c o r d t o d a t e c o m p r i s e s a MHF gas w e l l t r e a t m e n t i n t h e USA w i t h 3,150 t o r 6 . 3 M i a . l b s o f 20/40 n a t u r a l sand as p r o p p a n t s pumped i n a t o t a l f l u i d volume o f a b t . 1 . 5 4 M i o . g a l i n t o a t i g h t gas r e s e r v o i r o f a b t . 300 f t (100 m) p a y t h i c k n e s s i n a b t . 9,300 f t ( 3 , 1 0 0 m) f o r m a t i o n d e p t h i n t h e W i l c o x Sand i n t h e B e n a v i d e s f i e l d ( Z a p a t a c o u n t y , S o u t h T e x a s ) i n 1987 (CONSTIEN, BRANNON & B A N N I S T E R 1988; PITTS 1 9 8 8 ) . I n o r d e r t o c a r r y o u t t h e jumbo j o b , 1 8 h i g h - p r e s s u r e t r u c k s pumped f o r a l m o s t 11 h o u r s and i n t o t a l a b t . 100 p i e c e s o f e q u i p m e n t had t o b e i n s t a l l e d a t t h e w e l l s i t e . The maximum q u a n t i t y o f s y n t h e t i c p r o p p a n t s o f b a u x i t e base h a v i n g been i n j e c t e d so f a r i n t o a l a r g e - s c a l e f r a c t u r e i s a b t . 1.5 M i o . l b s , w i t h t h e main reason f o r t h e s i g n i f i c a n t d i s c r e pancy i n w o r l d r e c o r d q u a n t i t y between n a t u r a l sand and s y n t h e t i c p r o p p a n t s n o t b e i n g s u s p e n s i o n and t r a n s p o r t c h a r a c t e r i s t i c s w i t h i n t h e c a r r i e r f l u i d b u t b e i n g expense o f t h e o p e r a t i o n due t o t h e p r i c e d i f f e r e n c e between n a t u r a l sand and a r t i f i c i a l h i g h - q u a l i t y man-made p r o p p a n t s .
11
1.1.2. Evolution of fracturing technology and oil price history The e v o l u t i o n o f t h e v a r i o u s main types and b a s i c concepts o f h y d r a u l i c f r a c t u r i n g t e c h n o l o g y (VEATCH 1983; c f . s e c t i o n 1.2.1.2.) and d i f f e r e n t g e n e r a t i o n s o f a r t i f i c i a l proppants o f v a r i o u s s t r e n g t h and o f m a i n l y corundum, m u l l i t e and z i r c o n i a c o m p o s i t i o n (DOWELL SCHLUMBERGER 1985, WESTERN PETROLEUM 1985; c f . sect i o n 1.3. and t a b s . 1 - 3) took p l a c e p a r a l l e l t o s e v e r a l d r a s t i c a l o i l p r i c e changes e a r l i e r i n upwards and l a t e r i n downwards d i r e c t i o n (GRIFFITHS 1986, RUNGE 1986) t h a t were i n Europe p a r t i a l l y even accentuated by v a r i a t i o n s o f t h e US $ exchange r a t e ( w i t h r e s p e c t t o DM and t; c f . s e c t i o n 2 . 2 . 1 . 3 . ) . P l a n n i n g and e x e c u t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g j o b s has t h u s been n o t o n l y i n f l u e n c e d by r e s e r v o i r e n g i n e e r i n g , geological-petrophysical and t e c h n i c a l m o d e l l i n g o f optimum hydrocarbon e x p l o i t a t i o n i n terms o f r a t e and durat i o n o f p r o d u c t i o n as a base f o r commitment o f gas s u p p l y c o n t r a c t s ( c f . sect i o n 2 . 2 . 1 . 6 . 2 . ) and t h u s d i s c o u n t e d cash f l o w and p a y - o u t o f investment, b u t p a r t i c u l a r l y d u r i n g t h e l a s t t h r e e y e a r s s i n c e t h e m a j o r o i l p r i c e d e c l i n e (MENGES 1986, OEHME 1986; c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) a l s o by economical e v a l u a t i o n o f t h e f e a s i b i l i t y o f c a r r y i n g - o u t o f t h e o p e r a t i o n s i n l i g h t o f t h e c u r r e n t hydrocarbon p r i c e s c e n a r i o ( t h a t was i n Europe d u r i n g t h e l a s t c o u p l e o f y e a r s negat i v e l y i n f l u e n c e d by t h e US $ exchange r a t e drop; c f . s e c t i o n 2 . 2 . 1 . 3 . ) . Propp a n t p r i c e changes a l s o have s i g n i f i c a n t impacts on f r a c t u r e o p e r a t i o n budget i n g and design, because proppant c o s t comprises t h e h i g h e s t share o f t h e t o t a l expenses o f t h e h y d r a u l i c s t i m u l a t i o n j o b ( c f . s e c t i o n 2 . 2 . 2 . ) .
1.1.3. Significance of proppant selection As a consequence o f d i f f e r e n t t e c h n i c a l p r o p e r t i e s , changing o p e r a t i o n a l p e r formance and d i f f e r e n t p r i c e s o f t h e v a r i o u s proppant types, c a r e f u l s e l e c t i o n a c c o r d i n g t o p r o p p a n t b e h a v i o u r under r e s e r v o i r c o n d i t i o n s ( c l o s u r e s t r e s s , temp e r a t u r e and b r i n e c o m p o s i t i o n ) and economical frame i s o f c o n s i d e r a b l e s i g n i f i cance f o r f e a s i b i l i t y m o d e l l i n g o f t h e h y d r a u l i c f r a c t u r e o p e r a t i o n , because i n deep t i g h t gas w e l l s , p r o p p a n t c o s t can r e a c h up t o two t h i r d s o f t h e t o t a l s t i m u l a t i o n expenses ( f o r example t h e c u m u l a t i v e c o s t f o r a MHF t r e a t m e n t i n a R o t l i e g e n d w e l l i n t h e Sohlingen gas field/Germany FRG i n l a t e 1982 where a b t . 550 t o f proppants had been i n j e c t e d amounted up t o 6 Mio. DM o r 2.5 Mio. US 8 , b u t t h e jumbo j o b o p e r a t i o n p a i d o f f by p r o v i n g a b t . 500 Mio. m 3 gas r e s e r v e s ; JOHN 1983, KLOSE & KRUMER 1983, BLEAKLEY 1984; c f . s e c t i o n 3 . 3 . ) . Some comments a r e o f f e r e d as f o l l o w s on importance o f f r a c t u r i n g f o r f e a s i b i l i t y assessment o f hydrocarbon f i e l d s , and c e n t r a l r o l e o f p r o p p a n t placement i n t h e s t i m u l a t i o n treatment.
1.1.3.1. Importance of fracturing for feasibility assessment of hydrocarbon fields H y d r a u l i c f r a c t u r i n g o p e r a t i o n s do n o t o n l y o f t e n r e p r e s e n t a l a r g e f r a c t i o n o f i n i t i a l w e l l c o s t s , b u t a l s o determine t h e economical v i a b i l i t y o f a p a r t i c u l a r b o r e h o l e o r f i e l d (NEWBERRY, NELSON & AHMED 1985). A s t i m u l a t i o n j o b b e i n g t o o l a r g e can be an unnecessary waste o f c o m p l e t i o n funds, whereas a f r a c t u r e t r e a t m e n t b e i n g t o o small may r e s u l t i n such i n e f f i c i e n t r e s e r v o i r d r a i n a g e t o make a w e l l u n p r o f i t a b l e , and because o f t h i s economical double-edged sword, hyd r a u l i c f r a c t u r i n g j o b s have t o be designed i n such a way as t o o p t i m a l l y dep l e t e t h e pay f o r m a t i o n , w i t h c h o i c e o f p r o p p a n t t y p e and g r a i n s i z e p l a y i n g a key r o l e i n t h i s m o d e l l i n g . A f t e r t h e f r a c t u r e t r e a t m e n t has been c a r r i e d o u t and t h e f l u i d has been p r o duced back, t h e o n l y t h i n g which i s l e f t i n p l a c e i n t h e r e s e r v o i r i s t h e p r o p p a n t package s u p p o r t i n g t h e c r a c k , keeping t h e pathway open t h e r e b y p r o v i d i n g a h i g h - c o n d u c t i v i t y channel t o enable enhanced f l o w o f o i l o r gas t o t h e b o r e h o l e and t h u s c o n t r o l l i n g l o n g - t e r m p r o d u c t i v i t y (HOLDITCH 1984, ANDERSON & PHILLIPS
12 and 4 . 1 3 . ) . This relationship 1986, HALL & LARKIN 1986; c f . s e c t i o n s 1 . 4 . 1 2 . h i g h l i g h t s t h e o u t s t a n d i n g s i g n i f i c a n c e and c r i t i c a l r o l e o f t h e p r o p p a n t i n t h e h y d r a u l i c f r a c t u r i n g o p e r a t i o n and t h u s t h e i m p o r t a n c e o f i t s p r o p e r s e l e c tion.
1.1.3.2. Central role o f proppant placement
in the fracturing treatment
SIEVERT, WAHL, CLARK & HARKIN ( 1 9 8 1 ) summarize t h e i m p o r t a n c e o f p r o p p a n t sel e c t i o n b y u n d e r l i n i n g t h a t a l l t h e t i m e and money s p e n t d e v e l o p i n g and p e r f e c t ing fracturing fluids, t e c h n i q u e s and e q u i p m e n t a r e p e r i p h e r a l t o t h e c e n t r a l o b j e c t i v e o f p l a c i n g p r o p p a n t i n t h e f r a c t u r e i n o r d e r t o c r e a t e a h i g h l y cond u c t i v e f l o w c h a n n e l . Economical a s p e c t s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g a r e a l s o e v a l u a t e d b y VEATCH ( 1 9 8 6 ) . T h i s h i g h r a n k o f p r o p p a n t c h o i c e as t h e m o s t c r i t i c a l p o i n t o f h y d r a u l i c f r a c t u r e t r e a t m e n t d e s i g n (HOLDITCH 1984) f o r t h e d i f f e r e n t a p p l i c a t i o n s i n o i l , gas and w a t e r r e s e r v o i r management i s t h e r e a s o n f o r t h e n e c e s s i t y o f c a r r y i n g o u t a sophisticated m u l t i d i s c i p l i n a r y proppant marketing w i t h i n c l u s i o n o f g e o l o g i c a l ( m a i n l y sedimentological-petrophysical) and r e s e r v o i r e n g i n e e r i n g c o n c e p t s i n t o t h e t e c h n i c a l and c o m m e r c i a l m a r k e t i n g s t r a t e g y . P r o p p a n t s e l e c t i o n r e q u i r e s t e c h n i c a l and r e s e r v o i r e n g i n e e r i n g recommendations i n v i e w o f t h e c o m m e r c i a l l y b e s t n e t p r e s e n t v a l u e p e r i n v e s t e d US $ (BRIM 1986, ANDERSON & PHILLIPS 1987) w h i c h c a n a l s o b e e x p r e s s e d as t h e n e e d o f l o o k i n g f o r a p r o p p a n t t o g i v e maximum c o n d u c t i v i t y p e r US $ (NORTON-ALCOA PROPPANTS 1986, STANDARD O I L PROPPANTS 1986; c f . s e c t i o n s 1 . 3 . 4 . and 1 . 4 . 1 2 . ) . The s i g n i f i c a n c e o f p r o p p a n t c h o i c e i s h i g h l i g h t e d b y t h e f a c t t h a t p a r t i c u l a r l y i n deep w e l l a p p l i c a t i o n , t h e t y p e o f p r o p p a n t u s e d c a n make a d i f f e r e n c e between an e c o n o m i c a l success o r a f a i l u r e o f t h e w h o l e t r e a t m e n t (McDANIEL & WILLINGHAM 1 9 7 8 ) .
1 . 1 . 4 . Special aspects of the European stimulation market W h i l e t h e g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g r e l a t i o n s h i p s and c h a r a c t e r i s t i c s o f o i l and gas pay zones r e q u i r i n g t r e a t m e n t b y h y d r a u l i c p r o p p a n t f r a c t u r i n g a r e b a s i c a l l y more o r l e s s c o m p a r a b l e i n d i f f e r e n t p a r t s o f t h e w o r l d , f u n damental d i f f e r e n c e s e x i s t i n t e r m s o f e c o n o m i c a l and c o m m e r c i a l s e t t i n g b e t ween t h e s t i m u l a t i o n m a r k e t s i n Europe and USA. In c o n t r a s t t o t h e USA w h i c h c a n b e r e g a r d e d as a l a r g e more o r l e s s homogeneous u n i t , E u r o p e i s not o n l y p o l i t i c a l l y a complex and h e t e r o g e n e o u s a g g r e g a t e o f c o u n t r i e s s p e a k i n g a s u i t e o f d i f f e r e n t l a n g u a g e s and h a v i n g a s p e c t r u m o f v a r i o u s m e n t a l i t i e s , w i t h t h e m a j o r d i s t i n c t i o n c o n c e r n i n g e c o n o m i c a l s i t u a t i o n , s t a t u s o f t e c h n o l o g i c a l devel o p m e n t , c o m m e r c i a l c a p a c i t y and f o r e i g n t r a d e a c t i v i t y h a v i n g t o be made b e t ween Western and E a s t e r n Europe, b u t a l s o i n c l u d e s a w i d e r a n g e o f o i l and gas r e s e r v o i r types i n terms o f s t r a t i g r a p h i c a l , palaeogeographical, sedimentologic a l and p e t r o p h y s i c a l c h a r a c t e r i z a t i o n . T h e r e f o r e an assessment o f t h e European a r e a l h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l f r o m b o t h t e c h n i c a l and m a r k e t i n g p o i n t o f v i e w ( w h i c h i s p r e s e n t e d as a r e g i o n a l c a s e s t u d y i n c h a p t e r s 2 and 3 i n t h i s c o m p i l a t i o n ) has t o summarize t h e g e n e r a l g e o l o g i c a l and c o m m e r c i a l scene b y c o m p a r i n g t h e d i f f e r e n t c a s e s and i n t e g r a t i n g n o t o n l y s c i e n t i f i c , e n g i n e e r i n g and t e c h n o l o g i c a l , b u t a l s o e c o n o m i c a l and e v e n i d e o l o g i c a l a s p e c t s . Due t o my t e c h n i c a l and m a r k e t i n g e x p e r i e n c e , p r e f e r e n t i a l r e f e r e n c e i s made w i t h i n t h i s c o m p i l a t i o n t o examples d e r i v i n g f r o m W e s t e r n and E a s t e r n Europe, a l t h o u g h t h e a n a l y s i s and r e v i e w ( w i t h t h e e x c e p t i o n o f t h e a r e a l m a r k e t s t u d y p r e s e n t e d i n c h a p t e r 2 and t h e t e c h n i c a l - e c o n o m i c a l example s k e t c h o f a t i g h t gas r e s e r v o i r f o r m a t i o n i n c h a p t e r 3 ) a p p l i e s f o r o t h e r p a r t s o f t h e w o r l d as w e l l , w i t h m o s t o f t h e c a s e h i s t o r i e s and s p e c i a l c o n c l u s i o n s b e i n g based o n e x p e r i e n c e f r o m t h e USA w h i c h i s p r o b a b l y t h e m o s t i m p o r t a n t m a r k e t f o r h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n t h e w o r l d b o t h i n p a s t and f u t u r e .
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1.1.5. Organization of the review and status report The o r g a n i z a t i o n o f t h e r e v i e w and s t a t u s r e p o r t o f h y d r a u l i c proppant f r a c t u r i n g and g r a v e l p a c k i n g i n t h e p r e s e n t proceedings volume i s sketched a l o n g t h e l i n e s o f e v o l u t i o n o f e x p e r i e n c e and d e r i v a t i o n o f i n f o r m a t i o n as w e l l as s u b d i v i s i o n o f t h e book i n t o seven c h a p t e r s .
1.1.5.1. Experience evolution and derivation o f information The p r e s e n t o u t l i n e g i v e s a r e v i e w and d i s c u s s e s s t a t u s and p e r s p e c t i v e s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g ( a c i d f r a c t u r i n g s t i m u l a t i o n w i t h o u t proppants i s excluded f r o m t h i s account) and g r a v e l p a c k i n g o f o i l and gas r e s e r v o i r s p a r t i c u l a r l y i n Western and E a s t e r n Europe based t o c o n s i d e r a b l e amounts on t h e i n f o r m a t i o n which I have c o l l e c t e d d u r i n g my r e s p o n s i b i l i t y as M a r k e t i n g Manager Europe f o r proppants (MADER 1987). V a r i o u s comparisons o f USA and Europe a r e used i n t h e c h a p t e r s c o n c e n t r a t i n g on m a r k e t i n g and economical aspects f o r g i v i n g more g e n e r a l overviews and l e a d t o t h e m a i n l y t e c h n i c a l s e c t i o n s t h a t r e p r e s e n t common analyses w i t h o u t r e f e r e n c e t o s p e c i f i c example areas and a r e val i d f o r most p a r t s o f t h e w o r l d . Concerning t h e d i v i s i o n s f o c u s s i n g on Europe, many b o t h g e n e r a l and s p e c i f i c comments d e r i v e from t h e e x p e r i e n c e o f d e v e l o p i n g a m a r k e t i n g s t r a t e g y f o r p r o p p a n t s t h a t i n c l u d e s g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g concepts i n v a r i o u s European c o u n t r i e s . Abundant t e c h n i c a l i n f o r m a t i o n has been achieved t h r o u g h e x t e n s i v e l i t e r a t u r e s t u d i e s as w e l l as a t t e n t i o n o f t o p i c a l symposia and congresses. I n a d d i t i o n , numerous o t h e r s e d i m e n t o l o g i c a l , p e t r o p h y s i c a l , e n g i n e e r i n g and t e c h n i c a l f a c t o r s as w e l l as p o i n t s o f f i n a n c e , m e n t a l i t y and i n t e r n a t i o n a l coo p e r a t i o n which I have compiled d u r i n g my f o r m e r p r o f e s s i o n a l t a s k as Area O i l and Gas P r o d u c t i o n G e o l o g i s t as w e l l as d u r i n g my own independent s c i e n t i f i c r e search on d e p o s i t i o n a l e n v i r o n m e n t a l m o d e l l i n g , p a l a e o g e o g r a p h i c a l c o n s t e l l a t i o n , e c o l o g i c a l m i l i e u e v o l u t i o n and d i a g e n e t i c a l h i s t o r y o f Lower Permian ( R o t l i e g e n d ) and Lower T r i a s s i c ( B u n t s a n d s t e i n ) f l u v i a l , a e o l i a n and l a c u s t r i n e r e d bed sediments ( p a r t o f which f o r m some o f t h e most i m p o r t a n t n a t u r a l gas r e s e r v o i r s onshore M i d d l e Europe and o f f s h o r e i n t h e K o r t h Sea; MADER 1985 a ) and v i o l e t c a l c r e t e p a l a e o s o l s (MADER 1985 b ) a r e i n c o r p o r a t e d i n t o t h i s summary r e p o r t . The main work f o r e s t a b l i s h i n g t h i s c o m p i l a t i o n was c a r r i e d o u t s i n c e I am an independent c o n s u l t a n t f o r i n t e r n a t i o n a l p e t r o l e u m geology and r e s e r v o i r engineering.
1.1.5.2. Subdivision of the book into seven chapters The r e v i e w and s t a t u s r e p o r t o f h y d r a u l i c proppant f r a c t u r i n g and g r a v e l p a c k i n g aspects i n m a i n l y USA and Europe i n t h e p r e s e n t p r o c e e d i n g s volume i s d i v i d e d i n t o seven c h a p t e r s . The f i r s t c h a p t e r d i s c u s s e s p r o p p a n t s e l e c t i o n ( c f . tabs. 1 - 4), t h e second s e c t i o n p r e s e n t s m a r k e t i n g and economical asp e c t s , and t h e t h i r d d i v i s i o n r e p o r t s t e c h n i c a l and m a r k e t i n g r e l a t i o n s h i p s o f R o t l i e g e n d and C a r b o n i f e r o u s s t i m u l a t i o n i n Europe, t h e r e b y f o r m i n g a t r a n s i t i o n from t h e e c o n o m i c a l l y o r i e n t e d p a r t s t o t h e t e c h n i c a l l y dominated chapt e r s . The f o u r t h c h a p t e r g i v e s an account o f t e c h n i c a l and e n g i n e e r i n g aspects o f h y d r a u l i c p r o p p a n t f r a c t u r i n g , and t h e f i f t h s e c t i o n focusses on g r a v e l packi n g . The s i x t h u n i t summarizes f r a c t u r e and g r a v e l pack m o n i t o r i n g . W h i l e t h e second d i v i s i o n i n c l u d e s a m a r k e t i n g a n a l y s i s o f Western and E a s t e r n Europe w i t h some comparative assessment o f t h e USA and t h e t h i r d u n i t r e p r e s e n t s an a r e a l i n t e r d i s c i p l i n a r y case s t u d y o f t h e R o t l i e g e n d b a s i n b e i n g t h e most import a n t t i g h t gas r e s e r v o i r p r o v i n c e i n Europe, t h e f i r s t and f o u r t h t o s i x t h sect i o n s a r e g e n e r a l t e c h n i c a l c o m p i l a t i o n s n o t o n l y b e i n g v a l i d f o r Europe and USA, b u t a l s o f o r many o t h e r p a r t s o f t h e w o r l d , a l t h o u g h many o f t h e s e l e c t e d examples a r e i l l u s t r a t e d by e x p e r i e n c e and p r o g r e s s achieved i n Europe and USA.
14 The evaluation of proppant types and grain sizes is introduced by a brief account of historical development of hydraulic proppant fracturing. The summary of proppant choice offers general comments as well as particular references on the European oil- and gas-bearing formations requiring stimulation. The seventh chapter of the present book contains the bibliography which is divided into subject key classification and general reference list. A preliminary outline of some marketing and economical aspects of hydraulic proppant fracturing has been given by MADER (1987), MADER (1988 a) briefly illustrates a selection of the most important technical and reservoir engineering aspects, and a short interdisciplinary characteristic of the Rotliegend as the most significant tight gas pay horizon in Europe and thus the centre of hydraulic proppant fracturing activity in this area both offshore and onshore is provided by MADER (1988 b).
1.2. Historical development o f hydraulic proppant fracturing Hydraulic proppant fracturing is an established stimulation technique since abt. 40 years from the late 1940's onwards. Hydraulic fracturing was introduced to the petroleum industry in 1947 - 1949 (CLARK 1948, FARREY 1953, WILSEY & BEARDEN 1954, KHRISTIANOVICH & ZHELTOV 1955, ZHELTOV & KHRISTIANOVICH 1955) and since the first commercial treatments in 1947 (well Klepper 1 in Hugoton gas field in Grant countylwestern Kansas/USA; CLARK 1948, HOWARD & FAST 1970 a) through early 1949 (WATERS 1980) already more than one hundred thousand operations have been performed until 1955, more than half a million treatments have been executed during the first 20 years until 1968, and almost one million jobs have been carried out world-wide to date since innovation and inauguration of the method shortly after the end of the Second World War (HUBBERT & WILLIS 1957, WATERS 1980; HICKEY, BROWN & CRITTENDEN 1981; VEATCH 1983; VEATCH & MOSCHOVIDIS 1986). Hydraulic proppant fracturing has evolved during course of four decades from small treatments on single-zone completions to massive operations on multiple-interval installations (GRIFFITH & MADISON 1988). During the peak year of 1955, hydraulic stimulation jobs in the USA have been performed at a rate of 4,500 every month, whereas after 17 years of commercial use i n 1965, the rate levelled off to abt. 2,000 actions per month (HOWARD & FAST 1970 a) and later even temporarily declined to abt. 1,500 operations per month. Hydraulic proppant fracturing is a younger reservoir treatment method than acid fracturing which is common since more than 50 years (HOWARD & FAST 1970 a, WATERS 1980, ECONOMIDES 1986). Before the advent of hydraulic proppant fracturing, well stimulation in sandstone reservoirs was frequently done by detonating different types of explosives in the open hole which resulted in rubblization of the formation and radial propagation of fractures for a short distance from the wellbore, thus effectively increasing borehole diameter (WATERS 1980; cf. section 4.7.3.), and to minor amounts also by acidizing or acid fracturing (cf. section 4.5.1. and 4.5.5.), with all these pre-proppant fracturing methods, however, having had only limited success which at the bottom of the line was in many cases far from satisfying. Since its commercial introduction in 1947/1948, hydraulic fracturing has affected every facet of oil and gas industry by altering pipeline construction, changing production practices, killing the nitroglycerin oil well-shooting business, upsetting drilling companies, revolutioning well service spectrum, and giving rise to re-evaluation of many pay sections that had been previously bypassed and/or neglected (HOWARD & FAST 1970 a). Hydraulic proppant fracturing is performed in various reservoir depth ranging from very shallow levels from abt. 500 ft (150 m) in case of muddy finegrained sandstones, shales and chalks bearing oil and/or gas, to very deep storeys in excess of 20,000 ft (6,500 m) in case of tight sandstone, shale and coal seam gas pay horizons. After review of some general points, the following outline mainly focusses on discussion of aspects of limitations o f natural sand, restriction of alternative propping materials before the invention of synthetic ceramic proppants, impact of early synthetic ceramic proppants, significance of advanced ceramic proppants, impact of resin-coated proppants, possible
15
Fig. 1 S c h e m a t i c a l i l l u s t r a t i o n o f s u p p o r t i n g open h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s i n sandstone r e s e r v o i r s by i n f i l l i n g t h e c r a c k s w i t h s p h e r i c a l s t r e s s - r e s i s t a n t p r o p p i n g m a t e r i a l s which keep t h e f r a c t u r e s open and p r e v e n t them f r o m c l o s i n g once h y d r a u l i c p r e s s u r e e x e r t e d d u r i n g t h e s t i m u l a t i o n j o b subsequently d e c l i nes and c l o s u r e s t r e s s o f t h e f o r m a t i o n becomes e f f e c t i v e . T y p i c a l f r a c t u r e supp o r t by p r o p p a n t m u l t i l a y e r s w i t h a p p l i c a t i o n o f p r e f e r e n t i a l l y r o u n d g r a i n s of c o n s i d e r a b l y l a r g e r g r a i n size t h a n t h a t o f t h e r e s e r v o i r f o r m a t i o n p r o v i d e s h i g h - c o n d u c t i v i t y d r a i n a g e p a t h s or superhighways t h r o u g h t h e t i g h t pay zone a l o n g which t h e hydrocarbons can f l o w l i n e a r l y from t h e outward p o i n t s o f t h e r e s e r v o i r t o t h e b o r e h o l e i n s t e a d o f r a d i a l l y as i n u n d i s t u r b e d pays. Proppant c l o s u r e s t r e s s r e s i s t i v i t y has t o be such t h a t c r u s h i n g o f s p h e r i c a l p a r t i c l e s i n t h e f r a c t u r e upon i n c r e a s i n g c l o s u r e s t r e s s w i t h p r o g r e s s i v e r e s e r v o i r d e p l e t i o n i s avoided, w i t h t h i s g o a l b e i n g p r e d o m i n a n t l y achieved by a p p l i c a t i o n o f i n t e r m e d i a t e - and h i g h - s t r e n g t h s y n t h e t i c proppants i n s t e a d of n a t u r a l q u a r t z sand which has o n l y l i m i t e d p r e s s u r e r e s i s t i v i t y . Another i m p o r t a n t c o n s i d e r a t i o n is r e s i s t i v i t y a g a i n s t r e s e r v o i r t e m p e r a t u r e and a g g r e s s i v i t y o f f o r m a t i o n w a t e r which i n some cases may be s a t u r a t e d b r i n e s . S e l e c t i o n o f t y p e and g r a i n s i z e o f t h e p r o p p a n t s t o be pumped i n t o t h e c r a c k has t h e r e f o r e a key r o l e i n h y d r a u l i c f r a c t u r i n g d e s i g n , because i n some l a r g e - s c a l e t r e a t m e n t s , p r o p p a n t c o s t can r e a c h up t o two t h i r d s o f t h e t o t a l s t i m u l a t i o n j o b expenses.
16 f u t u r e i n n o v a t i o n , and v e r t i c a l v s . h o r i z o n t a l f r a c t u r e o r i e n t a t i o n .
1.2.1. General aspects Proper understanding o f t h e n e c e s s i t y o f a p p l i c a t i o n o f v a r i o u s proppant types and t h e s i g n i f i c a n c e o f p r o p p a n t s e l e c t i o n i s f a c i l i t a t e d b y b r i e f l y i n t r o d u c i n g b a s i c aims and p r o c e d u r e s o f h y d r a u l i c f r a c t u r i n g o p e r a t i o n s as w e l l as s k e t c h i n g t h e e v o l u t i o n o f m a i n f r a c t u r i n g d e s i g n c o n c e p t s as f o l l o w s .
1.2.1.1. Basic aims and procedures of hydraulic fracturing jobs Hydraulic proppant f r a c t u r i n g p l a y s a major r o l e i n enhancing petroleum r e s e r v e s and p r o d u c t i o n (VEATCH 1983). The h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n o p e r a t i o n c o m p r i s e s pumping o f a s l u r r y c o n s i s t i n g o f h i g h - v i s c o s i t y f l u i d ( g e l , e m u l s i o n o r foam) and p r o p p a n t g r a i n s a t h i g h r a t e s , v a r i o u s r e l a t i v e c o n c e n t r a t i o n s , and h i g h p r e s s u r e s i n t o t h e f o r m a t i o n where due t o p r o g r e s s i v e l y b u i l d i n g u p o f w e l l b o r e p r e s s u r e f i n a l l y c o m p r e s s i v e e a r t h s t r e s s e s and r o c k t e n s i l e s t r e s s e s a r e exceeded and as a consequence o f r e s e r v o i r f a i l u r e a f r a c t u r e i s h y d r a u l i c a l l y created t h a t propagates p e r p e n d i c u l a r l y t o t h e l e a s t p r i n c i p l e e a r t h s t r e s s ( t h u s t h e f r a c t u r e i s u s u a l l y v e r t i c a l i n w e l l s e x c e e d i n g 3,000 f t o r 1,000 m d e p t h ; c f . s e c t i o n 1 . 2 . 8 . ) . F o l l o w i n g a s k e t c h o f t h e s i g n i f i c a n c e o f f r a c t u r e p r o p p i n g , some s t i m u l a t i o n g o a l s i n v i e w o f r e s e r v o i r p r o d u c t i v i t y enhancement a r e i l l u s t r a t e d .
1.2.1.1.1. Fracture propping The f r a c t u r e opened a t h i g h h y d r a u l i c p r e s s u r e i s k e p t open b y s u p p o r t o f h a r d p r e s s u r e - r e s i s t a n t p r o p p a n t s w h i c h a r e i n f i l l e d i n t o t h e c r a c k and i n h i b i t i t s closure ( c f . f i g . l ) , w i t h thus a high-conductivity a r t i f i c i a l drainage avenue o r s u p e r h i g h w a y o r i g i n a t i n g t h r o u g h w h i c h l a r g e volumes o f h y d r o c a r b o n s c a n e a s i l y and r a p i d l y f l o w f r o m t h e f o r m a t i o n e x t r e m i t i e s t o t h e w e l l b o r e i n a l i n e a r c o n c e n t r a t e d and f u n n e l l e d manner r a t h e r t h a n p r e v i o u s l y i n a r a d i a l d i s p e r s e d and d i v i d e d manner (HOWARD & FAST 1970 a, VEATCH 1983). W i t h o u t s u p p o r t by proppants, h y d r a u l i c a l l y formed f r a c t u r e s tend t o heal o r r e t u r n t o t h e i r i n i t i a l s t a t e a f t e r t h e p a r t i n g p r e s s u r e i s r e l e a s e d (CARROLL & BAKER 1979). H y d r a u l i c c r a c k s have two w i n g s e x t e n d i n g i n o p p o s i t e d i r e c t i o n s f r o m t h e w e l l . F r a c t u r i n g o p e r a t i o n s u s u a l l y c o n s i s t o f s u c c e s s i v e pumping o f f o u r d i f f e r e n t f l u i d stages w i t h d i f f e r e n t r h e o l o g i c a l p r o p e r t i e s comprising pre-pad, pad, p r o p p a n t - l a d e n c a r r i e r and d i s p l a c e m e n t (LEE & OANESHY 1985). The w h o l e s t i m u l a t i o n t r e a t m e n t c a n be d i v i d e d i n t o f o u r phases c o m p r i s i n g f r a c t u r i n g , shut-in, c l e a n - u p and p r o d u c t i o n (AHMED, SCHATZ, HOLLAND, JONES & GREENFIELD 1982). Marked advancements have been made i n m a t e r i a l s , e q u i p m e n t and t r e a t i n g t e c h n i q u e s d u r i n g t h e f o u r decades s i n c e h y d r a u l i c p r o p p a n t f r a c t u r i n g was i n t r o d u c e d and f i r s t a p p l i e d (WATERS 1980; c f . s e c t i o n 1 . 2 . 1 . 2 . ) .
1.2.1.1.2. Stimulation goals H y d r o c a r b o n and w a t e r w e l l s a r e f r a c t u r e d i n o r d e r t o i n c r e a s e t h e r a t e o f f l u i d o f f t a k e a n d / o r r e c o v e r a b l e r e s e r v e s and t o d e c r e a s e t h e p r e s s u r e d r o p a r o u n d t h e w e l l b o r e (SINCLAIR 1980). P r o d u c t i o n i n c r e a s e f o r e c a s t a c h i e v e d b y h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t r e q u i r e s knowledge o f t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e p r o p p e d c r a c k , f r a c t u r e o r i e n t a t i o n , and t h e b e h a v i o u r o f t h e i n s e r t e d p r o p p a n t s u n d e r c l o s u r e s t r e s s and t e m p e r a t u r e . W e l l s t i m u l a t i o n b y f r a c t u r i n g c a n b e p e r f o r m e d e i t h e r b y p r o v i d i n g a f l o w p a t h t h r o u g h a damage zone a r o u n d t h e b o r e h o l e o r b y a l t e r i n g t h e c u r r e n t p a t t e r n i n t h e r e s e r v o i r . S m a l l - v o l u m e f r a c t u r e o p e r a t i o n s overcome w e l l b o r e damage ( c f . s e c t i o n 4 . 8 . 3 . ) and r e s t o r e p r o d u c t i v i t y t o t h e b o r e h o l e b y r e m o v i n g t h e zone o f l o w - p e r m e a b i -
17 l i t y o r h i g h p r e s s u r e d r o p f r o m t h e f l o w p a t t e r n . Large s t i m u l a t i o n t r e a t m e n t s p r o v i d e a much g r e a t e r s u r f a c e f o r f l u i d o f f t a k e ( o r i n o t h e r cases a l s o i n j e c t i o n ; c f . s e c t i o n 4 . 8 . 7 . ) , t h e r e b y i n t e n s i f y i n g a w e l l by a l t e r i n g t h e r e s e r v o i r f l o w p a t t e r n f r o m r a d i a l t o l i n e a r ( c f . s e c t i o n 4 . 1 1 . 3 . ) which i s t h e key r e s u l t o f a f r a c t u r e j o b . H y d r a u l i c proppant s t i m u l a t i o n t e c h n o l o g y r e q u i r e ments a r e m u l t i f a c e t t e d and a r e becoming more complex as t h e r e s e r v o i r format i o n s g e t deeper, h o t t e r and t i g h t e r (VEATCH 1983).
1.2.1.2.Evolution of main fracturing design concepts The h i s t o r i c a l e v o l u t i o n o f main f r a c t u r i n g design concepts i s c l o s e l y l i n ked w i t h t h e t e c h n o l o g i c a l p r o g r e s s o f t r e a t m e n t f l u i d s and p a r t i c u l a r l y p r o p p a n t s and comprises t h e successive p e r i o d s o f f o r m a t i o n damage b r e a k t h r o u g h and s m a l l - s c a l e sand f r a c t u r i n g , t i g h t gas r e s e r v o i r MHF s t i m u l a t i o n w i t h s y n t h e t i c proppants, and h i g h - p e r m e a b i l i t y pay zone f r a c t u r i n g . Some comments on s i g n i f i cance o f f r a c t u r e parameter r e s o l u t i o n a r e a l s o o f f e r e d ( t h e a p p l i c a t i o n spect r u m o f proppants i n h y d r a u l i c f r a c t u r i n g i s summarized i n t a b . 1 ) .
1.2.1.2.1.Formation damage breakthrough and small-scale sand fracturing The f i r s t commercial h y d r a u l i c f r a c t u r i n g t r e a t m e n t s i n t h e l a t e 1940's were s m a l l - s c a l e o p e r a t i o n s w i t h l e s s than 1,000 g a l o f t o t a l f l u i d q u a n t i t y and w i t h l i t t l e amounts o f sand up t o a b t . 5,000 l b s o r even no p r o p p i n g m a t e r i a l a t a l l (DERBY & SMITH 1979). I n t h e 1950's, most h y d r a u l i c proppant f r a c t u r i n g j o b s were c a r r i e d o u t i n o r d e r t o break t h r o u g h t h e f o r m a t i o n damage h a l o around t h e w e l l b o r e and d i d n o t aim on t r u e pay s t i m u l a t i o n f a r t h e r p e n e t r a t i n g i n t o t h e v i r g i n a l r e s e r v o i r (SMITH 1987). Predominantly s h o r t f r a c t u r e s were c r e a t e d w i t h f l u i d volumes o f l e s s t h a n 20,000 g a l and propped w i t h small amounts o f n a t u r a l sand n o t exceeding a b t . 10,000 l b s (ELY 1988). A f t e r an i n i t i a l h i g h phase d u r i n g t h e f i r s t decade o f technology a v a i l a b i l i t y when hydraul i c f r a c t u r i n g r e c e i v e d much p r o m o t i o n by t h e e f f e c t o f b e i n g a brand-new f a s h i o n , t h e a c t i v i t y d e c l i n e d s l o w l y in t h e 1960.s, p a r t i a l l y as a consequence o f t h e low o i l p r i c e a t a t i m e when o c c a s i o n a l l y h i g h - q u a l i t y w a t e r was more e x pensive than o i l . From t h e e a r l y 1970's onwards, n a t u r a l gas began t o r e p l a c e o i l and o t h e r f u e l s i n t h e energy s u p p l y spectrum ( c f . s e c t i o n 2 . 2 . 1 . 6 . 4 . ) and t i g h t gas f r a c t u r i n g s t a r t e d due t o b e t t e r u n d e r s t a n d i n g o f t h e processes and as a consequence o f a v a i l a b i l i t y o f s u i t a b l e m a t e r i a l s and methods, and a l s o p r o p p a n t q u a n t i t i e s and c o n c e n t r a t i o n s q u i c k l y i n c r e a s e d . I n a s i m i l a r way as i n t h e e a r l y y e a r s o f h y d r a u l i c proppant f r a c t u r i n g , even today many o f t h e s t i m u l a t i o n successes w i t h remarkable p r o d u c t i o n i n c r e a ses a r e due t o removal o f s i g n i f i c a n t n e a r - w e l l b o r e damage h a v i n g been p r e s e n t p r i o r t o t h e t r e a t m e n t (ELY 1988; c f . s e c t i o n 4 . 8 . 3 . ) .
1.2.1.2.2.Tight gas reservoir MHF stimulation with synthetic proppants The most s i g n i f i c a n t f a c t o r s t r i g g e r i n g t h e boost o f h y d r a u l i c s t i m u l a t i o n have been t h e f i r s t o i l p r i c e jumps ( c f . s e c t i o n 2.2.1.1.) and t h e i n v e n t i o n o f h i g h - s t r e n g t h ceramic proppants ( c f . s e c t i o n 1 . 2 . 4 . 1 . ) . T i g h t gas f r a c t u r i n g comprising c r e a t i o n o f long cracks i n t e r s e c t i n g the low-permeability r e s e r v o i r and p l u g g i n g o f t h e f r a c t u r e s w i t h h i g h - c o n d u c t i v i t y proppants soon became more and more p o p u l a r and q u i t e q u i c k l y reached i t s golden y e a r s a l r e a d y i n t h e l a t e 1 9 7 0 ' s / e a r l y 1980's when p a r t i c u l a r l y MHF o p e r a t i o n s were c a r r i e d o u t abundantl y e s p e c i a l l y i n USA and p a r t s o f Europe. Since t h i s peak t h e w o r l d w i d e s i g n i f i cance o f t i g h t r e s e r v o i r f r a c t u r i n g d e c l i n e d t o v a r i o u s degrees due t o changes o f t h e economical framework c o m p r i s i n g o i l p r i c e and US $ exchange r a t e , conso-
18 lidation of the market after the initial burst and attractivity of the brandnew fashion, and partial saturation of the market when many treatments had been successfully carried out that had to wait so far until intermediate- and highstrength proppants were available, but is certainly keeping an important role in reservoir development especially with increasing shares of gas in the energy supply spectrum in near and far future.
1.2.1.2.3.High-permeability pay zone fracturing From the early 1980's onwards, moderate-permeability pay formation fracturing (cf. section 4.6.)was becoming more common and in the last years, hydraulic stimulation of high-permeability reservoirs with creation of sufficient conductivity contrast between crack and formation by high proppant saturations and wide fractures (cf. sections 4.3.3. and 4.5.1.1.2.)is progressively spreading out. I n contrast to microdarcy wells where a 10 - 20 times productivity increase is achieved in optimum cases, millidarcy wells only reach a 2 - 5 times productivity increase which, however, is sufficient for feasible exploitation. While in hydraulic proppant fracturing history the main aim was on creation of long cracks in massive stimulation of tight gas pays, the trend in the future is more and more towards greater width and higher conductivity o f shorter fractures particularly in moderate- to high-permeability reservoirs. There are basically four hydraulic fracturing modes comprising confined vertical height and unrestricted horizontal crack extension (slope 1/4 - 1/3), stable height or fluid loss fracture with uncontrolled leakoff signaling hairline fissure opening, flow restriction leading to slurry dehydration o r screenout by proppant bridging (wellbore storage mode) and unstable height crack ( P A I , GARBIS & H A L L 1983; S M I T H 1987).
1.2.1.2.4.Significance of fracture parameter resolution The sensitivity of the predicted and actual results of hydraulic proppant fracturing treatments to the quality of the design depends on both relative operation cost and nature of experience in an individual formation ( V E A T C H 1983). I n some areas, it may be the best and at the bottom of the line also cheapest approach to try a number of alternative fluids, proppants, job sizes and injection procedures in several wells of one group of reservoir exposures in order to arrive at a set o f standard treatment parameters that provide acceptable results. This approach i s particularly suitable to obtain relatively quick and effective results in regions where fracturing operations constitute a relatively small portion of the total drilling and completion expenses, especially applying for high-permeability reservoirs where short cracks are adequate. I n low-permeability formations where deeply penetrating fractures are required, however, resolution of the necessary hydraulic stimulation parameters is very important. I n areas where MHF treatments can account for abt. half of the total well costs, the importance of fracturing is equal or greater than that of development drilling for increasing recoverable reserves (cf. section 2.3.), and here a high degree of crack parameter resolution is essential. A s pects of regional evolution of hydraulic proppant fracturing technology and stimulation concepts are also discussed by ROBERTS (1981).
1.2.2.Limitations of natural sand i n the early days of hydraulic reservoir stimulation, only natural quartz sand from different ancient and recent sources was available for propping of the fractures. I n the first hydraulic fracturing jobs in the late 1940's, no propping agents were used at all, while in the early 1950's, river sand was simply added to the fracturing fluids (FRACFAX 1988 9 ) . I n the 1950's and
19 1960's, d i f f e r e n t sand s o u r c e s were e x p l o i t e d and r e s c r e e n i n g o f n a t u r a l sand p r o v i d e d h i g h e r c o n d u c t i v i t i e s , w i t h O t t a w a and Texas sands h a v i n g become t h e a c c e p t e d i n d u s t r y s t a n d a r d f r a c t u r i n g sands s i n c e t h e 1960's ( c f . s e c t i o n 1 . 3 . 1 . 1 . ) . F o l l o w i n g an o u t l i n e o f t h e c l o s u r e s t r e s s r e s i s t i v i t y i n t e r v a l o f n a t u r a l sand, some a s p e c t s o f c l o s u r e s t r e s s v s . m a t r i x p e r m e a b i l i t y and p r o d u c t i o n r a t e are discussed.
1.2.2.1. Closure stress resistivity interval 20/40 mesh g r a i n s i z e sand i s p r e f e r e n t i a l l y u s e d due t o i t s cheap a c q u i s i t i o n as a b y - p r o d u c t o f g l a s s and f o u n d r y i n d u s t r y sands (WATERS 1 9 8 0 ) . O t h e r more o r l e s s common g r a i n s i z e s d e p e n d i n g o n r e s e r v o i r c o n d i t i o n s c o m p r i s e t h e c o a r s e r 16/20, 1 2 / 2 0 and o c c a s i o n a l l y a l s o 8 / 1 2 and 6/10, as w e l l as t h e f i n e r 30/50, 4 0 / 7 0 and sometimes f o r s p e c i a l a p p l i c a t i o n s a l s o 7 0 / 1 4 0 and 100 mesh ( c f . t a b s . 1 and 4 as w e l l as s e c t i o n 1 . 3 . ) . The l o w c o s t , r e l a t i v e abundance, good s p h e r i c i t y and l o w s p e c i f i c g r a v i t y o f h i g h - q u a l i t y , specially selected sands have made i t a good p r o p p a n t f o r many h y d r a u l i c f r a c t u r i n g t r e a t m e n t s (CUTLER, JONES, SWANSON & CARROLL 1 9 8 1 ) . C r u s h i n g o f t h e sand (HUITT, McGLOTHLIN & McDONALD 1959, TUNN 1971) a t c l o s u r e s t r e s s e s between 3,000 and 5,000 p s i ( d e p e n d i n g on sand q u a l i t y ) s e r i o u s l y l i m i t e d t h e a p p l i c a t i o n o f h y d r a u l i c f r a c t u r i n g i n g r e a t e r d e p t h s ( m o s t commonl y i n e x c e s s o f 3,000 m (10,000 f t ; CUTLER, JONES, SWANSON & CARROLL 1981) dur i n g t h e f i r s t years o f i n d u s t r i a l operation. Closure s t r e s s o r compressive e a r t h s t r e s s i s t h e n u m e r i c a l d i f f e r e n c e between t h e b o t t o m h o l e t r e a t i n g o r f r a c t u r i n g p r e s s u r e and t h e r e s e r v o i r p o r e p r e s s u r e o r b o t t o m h o l e f l o w i n g p r e s s u r e , w i t h t h e l a t t e r b e i n g t h e p r o d u c t o f f r a c t u r i n g g r a d i e n t t i m e s d e p t h . Acc o r d i n g t o f a l l i n g r e s e r v o i r p r e s s u r e upon drawdown d u r i n g c o u r s e o f p r o d u c t i o n h i s t o r y , t h e c l o s u r e s t r e s s i n c r e a s e s p a r t i c u l a r l y a r o u n d t h e w e l l b o r e (ATTEBERRY, TUCKER & RITZ 1979; TUCKER 1979, SARDA 1981, CLARK 1983; c f . s e c t i o n 4.12.4.2.). C l o s u r e s t r e s s r e s i s t i v i t y t e s t i n g o f sand i s c a r r i e d o u t b y TUNN ( 1 9 7 1 ) and SLUSSER & RIECKMANN ( 1 9 7 6 ) .
1.2.2.2. Closure stress vs. matrix permeability and offtake rate The o b v i o u s c o r r e l a t i o n between c l o s u r e s t r e s s and l o s s i n f r a c t u r e f l o w c a p a c i t y i n many e a r l i e r t r e a t m e n t s c a r r i e d o u t w i t h n a t u r a l sand as a p r o p p i n g a g e n t i n d i c a t e s t h a t t h e c r e a t e d f r a c t u r e has p a r t i a l l y o r c o m p l e t e l y c l o s e d due t o e i t h e r embedment o r c r u s h i n g o f t h e p r o p p a n t s (TUNN 1971, TUCKER 1979; c f . s e c t i o n 4 . 3 . 3 . ) . The r a t e o f c l o s u r e s t r e s s i s f u n c t i o n a l l y r e l a t e d t o o r i g i n a l m a t r i x p e r m e a b i l i t y and o f f t a k e r a t e . R e s e r v o i r s o f s u p e r i o r m a t r i x p e r m e a b i l i t y a r e b e t t e r a b l e t o produce a t commercial r a t e s w i t h o u t severe nearw e l l b o r e p r e s s u r e d e c l i n e , whereas b o r e h o l e s w h i c h e x h i b i t v e r y l o w f l o w c a p a c i t i e s o r where h y d r o c a r b o n w i t h d r a w a l i s p e r f o r m e d a t e x c e s s i v e r a t e s e x p e r i e n c e r a p i d bottom hole pressure d e c l i n e near t h e wellbore allowing f r a c t u r e closure due t o p r o p p a n t c o l l a p s e . The r e l a t i o n s h i p between c l o s u r e s t r e s s and p r o p p a n t c r u s h i n g i s a l s o r e f l e c t e d b y p r e s s u r e t r a n s i e n t t e s t i n g r e s u l t s . T h e r e f o r e nowadays t h e g e n e r a l l y a c c e p t e d p r a c t i c e i s t o a v o i d t h e a p p l i c a t i o n o f n a t u r a l sand when c l o s u r e s t r e s s e s i n e x c e s s o f 4,000 - 6,000 p s i have t o b e e x p e c t e d a t any t i m e o f w e l l l i f e (CUTLER, JONES, SWANSON & CARROLL 1981; PAI, GARBIS & HALL 1983; c f . s e c t i o n 1 . 4 . 1 0 . 4 . ) . P r i o r t o i n v e n t i o n and m a r k e t i n t r o d u c t i o n o f i n t e r m e d i a t e - and h i g h s t r e n g t h p r o p p a n t s , deep gas r e s e r v o i r s t i m u l a t i o n p e r f o r m a n c e w i t h n a t u r a l sand has been i m p r o v e d b y pumping h i g h c o n c e n t r a t i o n s w i t h t h e h e l p o f h i g h i n j e c t i o n r a t e s and h i g h f l u i d v i s c o s i t i e s (HOLDITCH & ELY 1973; CALLANAN, CIPOLLA & LEWIS 1983; c f . s e c t i o n 4 . 3 . 2 . 3 . ) . The pump r a t e s and f l u i d v i s c o s i t i e s n e c e s s a r y t o pump h i g h sand s a t u r a t i o n s , however, have sometimes been d e t r i m e n t a l b y c a u s i n g a l a r g e p r e s s u r e d r o p a l o n g t h e f r a c t u r e and p r o m o t i n g e x c e s s i v e h e i g h t g r o w t h i n t o u n p r o d u c t i v e zones (NOLTE 1 9 8 2 ) .
20
1.2.3. Restrict ions o f alternative propping materials before the invention of synthetic ceramic proppants With progressive technological e v o l u t i o n o f hydraulic f r a c t u r i n g stimulat i o n , more and more n a t u r a l m a t e r i a l s o t h e r t h a n q u a r t z sand as w e l l as a r t i f i c i a l p r o d u c t s such as g l a s s beads ( s t a b i l i t y b o u n d a r y a b t . 6,000 p s i ) , i r o n and s t e e l shots, broken w a l n u t s h e l l s , rounded w a l n u t h u l l s , alumina p e l l e t s , mined g a r n e t and c o r d i e r i t e , and p l a s t i c beads o r p o l y m e r s p h e r e s were t r i e d w i t h c h a n g i n g s u c c e s s and e a c h o f t h e s e n o v e l p r o p p a n t s had i t s a d v a n t a g e s and drawb a c k s , w i t h o n l y t h e h i g h - s t r e n g t h a n n e a l e d g l a s s beads s u r v i v i n g u n t i l t h e e a r l y 1970's (WATERS 1980; HICKEY, BROWN & CRITTENOEN 1981; WESTERN PETROLEUM 1 9 8 5 ) . Some f e a t u r e s o f g l a s s beads, m e t a l s h o t s and w a l n u t h u l l s w h i c h were t h e m o s t w i d e s p r e a d a l t e r n a t i v e p r o p p i n g m a t e r i a l s b e f o r e t h e i n v e n t i o n o f synt h e t i c ceramic proppants a r e o u t l i n e d as f o l l o w s .
1.2.3.1. Glass beads G l a s s beads have been c o m m e r c i a l l y u s e d f o r many y e a r s (DAVIS 1 9 7 5 ) , b u t t h e s t r e n g t h o f t h e g l a s s d e c r e a s e s g r e a t l y i n t h e p r e s e n c e o f b r i n e (McGLOTHLIN, HUITT & McKAY 1963; HOWARD & FAST 1970 a, COOKE 1973; HICKEY, BROWN & CRITTENDEN 1 9 8 1 ) . The b r i t t l e g l a s s beads a l s o f a i l i n s t a n t a n e o u s l y i n t o f i n e powd e r - s i z e s p l i n t e r s ( s u c h as a l s o c o l l a p s e o f z i r c o n i a - s i l i c a t e p r o p p a n t s t a k e s p l a c e ; c f . s e c t i o n 1 . 4 . 2 . 2 . ) and n o t i n t o l a r g e r f r a g m e n t s o f h a l f - , q u a r t e r o r s m a l l e r s i z e as does q u a r t z sand when e x c e e d i n g t h e c r u s h r e s i s t i v i t y boundar y o f a b t . 5,000 p s i and c r y s t a l l i n e a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s once t h e i r s t a b i l i t y border i s crossed ( c f . section 1.4.2.1.). Severe c r u s h i n g o f g l a s s beads a l s o o c c u r s i n h o t w a t e r e n v i r o n m e n t as w e l l as i n m u l t i l a y e r g r a i n packages (FRACFAX 1988 9 ) . I n a d d i t i o n , g l a s s beads c a n l o s e much o f t h e i r c o m p r e s s i v e s t r e n g t h i f b e i n g n i c k e d w h i l e m o v i n g t h r o u g h t h e pumps, and g l a s s beads w h i c h a r e s c r a t c h e d c a n be e a s i e r c r u s h e d t h a n sand (HOLDITCH 1 9 8 4 ) . M e t a l l i z e d c o a t i n g s c o n s i s t i n g o f b r a s s o r a l u m i n u m o n g l a s s beads a l s o d i d n o t i m p r o v e t h e s i t u a t i o n . As a consequence o f t h e i r c a t a s t r o p h i c a l c o m p l e t e b r i t t l e f a i l u r e i n w a t e r and b r i n e a t e l e v a t e d c l o s u r e s t r e s s and t e m p e r a t u r e as w e l l as o f t h e i r premium p r i c e , g l a s s beads were l a t e r c o m p l e t e l y w i t h d r a w n f r o m t h e m a r k e t i n 1975 (SINCLAIR 1 9 8 0 ) .
1.2.3.2. Metal shots and walnut hulls M a l l e a b l e m e t a l l i c m a t e r i a l s a r e c o m p a r a t i v e l y e x p e n s i v e and as t h e y c o r r o d e when c o n t a c t i n g h i g h - s a l i n i t y s a l t w a t e r , t h e i r u s e has been v e r y l i m i t e d (COOKE 1973, HOLDITCH 1 9 8 4 ) . A p a r t f r o m c o r r o s i o n , m e t a l and s t e e l s h o t s a r e a l s o t o o dense t o be a p p l i e d as a p r o p p a n t i n h y d r a u l i c f r a c t u r i n g (FRACFAX 1988 9 ) . w i t h t r a n s p o r t and s u s p e n s i o n p r o p e r t i e s i n m o s t o f t h e f r a c t u r i n g f l u i d s b e i n g unacceptable, and e x c e s s i v e s e g r e g a t i o n and b a n k i n g i n t h e f r a c t u r e p r e v e n t i n g a d e q u a t e p r o p p i n g o f t h e c r a c k . N u t s h e l l s d e f o r m w i t h o u t c r u s h i n g , b u t have a l s o n o t t u r n e d o u t t o be a s u i t a b l e m a t e r i a l , because w a l n u t h u l l s a r e o n l y d e f o r m a b l e u n d e r l o w t e m p e r a t u r e s and l o a d . N u t s h e l l s a r e a l s o t o o l i g h t t o be a b l e t o be p a c k e d w i t h s u f f i c i e n t d e n s i t y w i t h i n t h e f r a c t u r e w i t h o u t e l e v a t e d f l u i d loss w h i c h i s d e s i r a b l e i n g r a v e l p a c k i n g w i t h l i g h t w e i g h t m a t e r i a l s ( c f . s e c t i o n 5 . 3 . 3 . ) , b u t i s u n t o l e r a b l e i n h y d r a u l i c f r a c t u r i n g due t o p r o g r e s s i v e l y i n c r e a s i n g s c r e e n o u t r i s k . W a l n u t h u l l s a r e t o d a y u s e d as f l u i d - l o s s additives r a t h e r t h a n a s p r o p p a n t s (SINCLAIR 1980), i n a s i m i l a r way as a p p l i e s f o r 100 mesh sand ( c f . s e c t i o n 1 . 4 . 1 1 . 2 . 1 . ) . G a r n e t g r a i n s a r e t o o b r i t t l e , p l a s t i c p e l l e t s d e f o r m t o o r a p i d l y , and f l u i d i z e d c o k e i s t o o weak (FRACFAX 1988 9 ) . Thus a t t h e b o t t o m o f t h e l i n e none o f t h e a f o r e m e n t i o n e d e a r l y - s t a g e s y n t h e t i c p r o p p i n g a g e n t s p e r f o r m e d s a t i s f a c t o r i l y f o r usage i n deep h i g h - p r e s s u r e r e s e r v o i r f r a c t u r i n g t r e a t m e n t s (COOKE & GIDLEY 1979) and a l l o f them had more
21
1
Y
P E
I
G R A I N
S I Z E
HAPI
PROPPANT TYPES AND GRAIN SIZES AND THEIR APPLICATION IN PETROLEUM RESERVOIR
STIMULATION AND OTHER ENERGY INDUSTRY PROCESSES A
P
P
L
I
C
A
T
I
O
N
I
I Oil 011 production production HYDRAULIC FRACTURING
'ACKING
)THEA
I
Water production and injection
I
Shallow g a s production
I
Moderately deep a a s production
8
production Deep g a s production
8
Oil production
8
Water production a n d i n j e c t i o n
8
Steam-drive enhanced o i l recovery F r a c t u r e fluid-loss a d d i t i v e
I
Geothermal w e l l stimulation
I
Coal seam degasification
I
Foundry cupola furnace h e a t transfer
1 Mechanical ggas a s purification
Tab. 1
22 d i s a d v a n t a g e s such as h i g h p r i c e , h i g h d e n s i t y and l a c k o f s t a b i l i t y w h i c h o v e r compensated t h e b e n e f i t s , w i t h t h u s f o r many y e a r s n a t u r a l q u a r t z sand r e m a i n i n g t o be t h e o n l y r e a l l y a p p l i c a b l e m a t e r i a l f o r h y d r a u l i c f r a c t u r e s u p p o r t (HICKEY, BROWN & CRITTENLfEN 1 9 8 1 ) .
1.2.4.Impact o f early synthetic ceramic proppants The d e c i s i v e b r e a k t h r o u g h i n t h e t e c h n o l o g i c a l e v o l u t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g was a c h i e v e d i n t h e l a t e 1970's w i t h t h e i n v e n t i o n of s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s o f s i n t e r e d b a u x i t e ( c e r a m i c ; COOKE 1976; COOKE, GIDLEY & MUTT1 1977; COOKE & GIDLEY 1979; c f . s e c t i o n 1 . 3 . 2 . 1 . ) and f u s e d z i r c o n i a (SEPR 1983) c o m p o s i t i o n w h i c h - i n c o n t r a s t t o t h e l o w s t r e n g t h sand - c o u l d r e s i s t t o c l o s u r e s t r e s s e s u p t o more t h a n 10,000 15,000 p s i ( c f . s e c t i o n 1 . 3 . ) and t h u s t o g e t h e r w i t h t h e d e v e l o p m e n t o f h i g h - c a p a c i t y modern f r a c t u r i n g f l u i d s opened v a s t p o s s i b i l i t i e s o f s t i m u l a t i o n o f deep h i g h - p r e s s u r e r e s e r v o i r s . Some comments a r e o f f e r e d as f o l l o w s o n s i n t e r e d b a u x i t e i n v e n t i o n t r i g g e r i n g f r a c t u r i n g boom and i m p o r t a n c e o f p r o p p a n t t e s t i n g f o r q u a l i t y improvement.
1.2.4.1.Sintered bauxite invent ion trigger ins fracturing boom S i n t e r e d b a u x i t e was f i r s t a p p l i e d as p r o p p i n g a g e n t f o r h y d r a u l i c t r e a t m e n t i n t h e f i e l d i n t h e USA i n 1976 and as a consequence o f s p e c t a c u l a r s t i m u l a t i o n r e s u l t s t r i g g e r e d a boom o f deep w e l l f r a c t u r i n g i n t h e f o l l o w i n g y e a r s (HICKEY, BROWN & CRITTENDEN 1981), because i t has e n a b l e d t o a c h i e v e s t i m u l a t i o n i n many deep f o r m a t i o n s where i n h i g h - p r e s s u r e r e g i m e s h y d r a u l i c f r a c t u r i n g had been i m p o s s i b l e b e f o r e , and b e t t e r r e s u l t s o f t r e a t m e n t s i n o t h e r s h a l l o w e r h o r i z o n s (WATERS 1 9 8 0 ) . The m a i n a d v a n t a g e s of s i n t e r e d b a u x i t e o v e r t r a d i t i o n a l propping m a t e r i a l s are f i n e p a r t i c l e size, low porosity, high s p h e r i c i t y , good c o r r o s i o n r e s i s t a n c e , and a b i l i t y t o d e f o r m s l i g h t l y u n d e r h i g h l o a d s ( C O O K E 1977; ATTEBERRY, TUCKER & RITZ 1 9 7 9 ) . S i n t e r e d b a u x i t e has e x c e l l e n t c o n d u c t i v i t y up t o 20,000 p s i ( 1 3 9 MPa) c l o s u r e s t r e s s (CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) . While a t t h e beginning o f a r t i f i c i a l h i g h - s t r e n g t h proppant a p p l i c a t i o n mainl y a n g u l a r s y n t h e t i c b a u x i t e - b a s e d p r o p p a n t s were made as b y - p r o d u c t s o f a b r a s i v e m a n u f a c t u r i n g (HARBEN 1978, LARSEN & SMITH 1985; c f . s e c t i o n 1 . 3 . 5 . and 1.4.11.3.2.), l a t e r a l m o s t p e r f e c t l y r o u n d c o r u n d u m / m u l l i t e p r o p p a n t s were p r e s e n t e d w h i c h were p r o d u c e d i n s p e c i a l p l a n t s , w i t h r o u n d n e s s and s p h e r i c i t y , s u r f a c e morphology, c r y s t a l l i t e size, c r u s h r e s i s t i v i t y and s i z e s c r e e n i n g h a v i n g been p r o g r e s s i v e l y i m p r o v e d d u r i n g t h e y e a r s b y o p t i m i z a t i o n o f g r i n d i n g o f r a w m a t e r i a l s , i n t e r m i x i n g o f f l u x media, s i n t e r i n g and f u s i n g t e m p e r a t u r e s and c o n t r o l s i e v i n g . These a m e l i o r a t i o n s were t h e consequence o f s t r o n g l y i n c r e a s i n g q u a l i t y demand b y t h e c u s t o m e r s ( o i l and gas p r o d u c t i o n companies t h r o u g h f i e l d s e r v i c e companies) and a g g r e s s i v e c o m p e t i t i o n between t h e d i f f e r e n t p r o p p a n t p r o d u c t i o n companies.
1.2.4.2.Importance of proppant testing f o r quality improvement The improvement o f p r o p p a n t p r o p e r t i e s and t h e i r m a t c h i n g w i t h s t a n d a r d p a r a m e t e r s was r e i n f o r c e d b y q u a l i t y and p e r f o r m a n c e t e s t i n g campaigns w h i c h were c a r r i e d o u t b y p r o p p a n t m a n u f a c t u r e r s (HICKEY, BROWN & CRITTENDEN 1981; NORMAN, CIPOLLA & WEBB 1983; BROWN & MUCH 1986, COBB & FARRELL 1986, COMALCO 1986, STANDARD OIL PROPPANTS 1986, MUCH 1987, MUCH & PENNY 1987), i n d e p e n d e n t c o n s u l t i n g and r e s e a r c h l a b o r a t o r i e s (AHMED, ABOU-SAYED & JONES 1979; SINCLAIR 1980; CUTLER, ENNISS, JONES & CARROLL 1983; DGMK 1983, 1986; CUTLER, ENNISS, JONES & SWANSON 1985; STIM-LAB 1986, PENNY 1987; PURSELL, HOLDITCH & BLAKELEY 1988), h y d r o c a r b o n p r o d u c t i o n companies (BECQ, ROQUE & SARDA 1984; CHEUNG 1985; ROODHART, KUIPER & D A V I E S 1986) and s e r v i c e companies (McDANIEL & WILLINGHAM 1978;
23 WENDORFF 1982, K I M & LOSACANO 1985; MCDANIEL 1986, 1987, 1988; PARKER & McDANIEL 1987, McDANIEL & PARKER 1988; c f . s e c t i o n 1 . 4 . ) . Fused z i r c o n i a - b a s e d p r o p pants s u f f e r e d i n t h e e a r l y years from appendix-like subsize t o m i n i - s i z e round g r a i n s g r o w i n g l i k e noses and h o r n s o u t o f s t a n d a r d - s i z e p a r t i c l e s ( t h e r e b y a l r e a d y a p r i o r i i n c l u d i n g q u i t e some f i n e s ) , b u t p r o g r e s s i v e q u a l i t y a m e l i o r a t i o n b y o p t i m i z i n g f u s i n g t e m p e r a t u r e s , r e a c t i o n t i m e and r a w - m a t e r i a l p r o c e s s i n g has overcome t h i s drawback.
1.2.5. Significance of advanced synthetic ceramic proppants I n t h e l a t e 1970's and e a r l y 1980's, even two s e c o n d - g e n e r a t i o n a r t i f i c i a l p r o p p a n t t y p e s (CUTLER, JONES, SWANSON & CARROLL 1981; JONES, CUTLER & SWANSON 1981; CUTLER & JONES 1982; CALLAHAN, CIPOLLA & LEWIS 1983; CUTLER, ENNISS, JONES & CARROLL 1983; SPARLIN & HAGEN 1983, FITZGIBBON 1984; CUTLER, ENNISS, JONES & SWANSON 1985) have been i n t r o d u c e d t o t h e h y d r o c a r b o n s t i m u l a t i o n m a r k e t w h i c h b a s i c a l l y c o n s i s t more o r l e s s o f m u l l i t e and m u l l i t e - c o r u n d u m m i x t u r e s , r e s p e c t i v e l y ( c f . s e c t i o n 1 . 3 . ) , and a r e l o w - d e n s i t y and h i g h - d e n s i t y i n t e r m e d i a t e - s t r e n g t h m a t e r i a l s r e s i s t i n g t o c l o s u r e s t r e s s e s u p t o 8,000 p s i and 10,000 p s i , r e s p e c t i v e l y . These t w o i n t e r m e d i a t e - s t r e n g t h p r o p p a n t t y p e s a r e f i l l i n g t h e gap between l o w - s t r e n g t h n a t u r a l sand on t h e one hand and h i g h s t r e n g t h s y n t h e t i c b a u x i t e o r z i r c o n i a on t h e o t h e r hand i n t e r m s o f s p e c i f i c g r a v i t y , c o n d u c t i v i t y , c l o s u r e s t r e s s r e s i s t i v i t y and p r i c e ( c f . t a b s . 1 - 4 ) . Both types o f intermediate-strength proppants are l i g h t e r than h i g h - s t r e n g t h s i n t e r e d b a u x i t e and t h u s a l l o w b e t t e r s u s p e n s i o n i n and t r a n s p o r t b y t h e f r a c t u r i n g f l u i d s and p l a c e m e n t i n t h e c r a c k b y t h e c a r r i e r medium w i t h l e s s s e t t l i n g . The t w o advanced s y n t h e t i c c e r a m i c p r o p p a n t s a r e t h e r e f o r e p e r m i t t i n g t o a c h i e v e a b e t t e r p e r f o r m a n c e o f t h e o p e r a t i o n a t l o w e r c o s t and t h u s c o n s i d e r a b l y b r o a d e n i n g t h e s p e c t r u m o f t e c h n i c a l and e c o n o m i c a l a p p l i c a t i o n b y p r o v i d i n g a w i d e r and more d i f f e r e n t i a t e d scope o f j o b - s p e c i f i c p r o p p a n t c h o i c e .
1.2.6. Impact of resin-coated proppants W i t h t i m e , a l s o t h e t e c h n i q u e o f r e s i n - c o a t i n g (SANTROL 1975, SINCLAIR & GRAHAM 1977; UNDERDOWN, DAS & SPARLIN 1980; GRAHAM, SINCLAIR & BRANDT 1982; UNDERDOWN & DAS 1982; SINCLAIR, GRAHAM & SINCLAIR 1983) o f b o t h n a t u r a l sand and synt h e t i c p r o p p a n t s was d e v e l o p e d i n o r d e r t o a c h i e v e p a r t i c u l a r e f f e c t s i n spec i a l a p p l i c a t i o n s such as f o r m a t i o n o f c o n s o l i d a t e d p r o p p a n t packages i n t h e f r a c t u r e by cementation o f t h e g r a i n s by adhesion o f t h e p l a s t i c a l l y f l o w i n g r e s i n c o a t i n g s a t t h e i r c o n t a c t s t o p r o v i d e s t a b i l i t y and t o m i n i m i z e embedment, and r e l i e v i n g h i g h s t r e s s e s caused b y g r a i n - t o - g r a i n c o n t a c t s , t h e r e b y i m p r o v i n g t h e l o a d - c a r r y i n g c a p a c i t y o f t h e p r o p p a n t pack (SINCLAIR & GRAHAM 1978, KANAT 1980; HICKEY, BROWN & CRITTENDEN 1981; POPE, WILES & P I E R C E 1 9 8 7 ) . The f i r s t f i e l d o p e r a t i o n s w i t h r e s i n - c o a t e d sand arid p r o p p a n t s were c a r r i e d o u t i n 1976 and 1977, r e s p e c t i v e l y (FRACFAX 1988 c , 1988 i ) . W h i l e f o r many y e a r s p r e f e r e n c e i n deep w e l l a p p l i c a t i o n was g i v e n t o i n t e r m e d i a t e - and h i g h s t r e n g t h ceramic proppants, t h e beginning hydrocarbon i n d u s t r y recession i n t h e e a r l y 1980's draw more a t t e n t i o n t o p r o p p a n t c o s t and t h e l e s s e x p e n s i v e r e s i n c o a t e d sand w h i c h had been i n v e n t e d and p a t e n t e d a l m o s t a t t h e same t i m e as s i n t e r e d b a u x i t e and f u s e d z i r c o n i a have been d i s c o v e r e d and i n t r o d u c e d began t o be u s e d i n much l a r g e r q u a n t i t i e s . R e p l a c i n g s i n t e r e d b a u x i t e b y r e s i n - c o a t e d sand can r e s u l t i n a b t . 50 % t o t a l p r o p p a n t c o s t s a v i n g s due t o b o t h c h e a p e r pound p r i c e and h i g h e r pound volume o f t h e l a t t e r p r o d u c t (SINCLAIR, GRAHAM & SINCLAIR 1983; c f . a l s o s e c t i o n 1 . 4 . 1 2 . 4 . 2 . ) . The f o l l o w i n g o u t l i n e i l l u s t r a t e s some p o i n t s o f r e s i n t y p e s and f u n c t i o n s , r e a c t i o n s o f r e s i n c o a t i n g s t o c l o s u r e s t r e s s and t e m p e r a t u r e , and d o u b l e r e s i n c o a t i n g c o m b i n a t i o n .
24
1.2.6.1. Resin types and functions R e s i n - c o a t e d sand and p r o p p a n t s have d e f o r m a b l e r e s i n p e l l i c l e s t h a t polymer i z e a t e l e v a t e d temperatures under c l o s u r e s t r e s s t o form a cohesive h i g h s t r e n g t h l a y e r o f p e r m e a b l e s a n d s t o n e (SINCLAIR 1 9 8 0 ) . C u r a b l e m a t e r i a l r e q u i r e s t i m e a t downhole t e m p e r a t u r e s t o c u r e t h e t h e r m o s e t t i n g r e s i n , whereas p r e c u r e d p r o p p a n t s do n o t have t o r e l y on downhole c o n d i t i o n s t o s e t t h e r e s i n (CUTLER, ENNISS, JONES & CARROLL 1 9 8 3 ) . A s p e c t s o f r e d u c t i o n o f p o i n t l o a d i n g a t g r a i n c o n t a c t s , and s p e c i f i c g r a v i t y and p r o p p a n t p r i c e l o w e r i n g a r e o u t l i ned as f o l l o w s .
1 . 2 . 6 . 1 . 1 . Reduction of point loading at grain contacts The r e s i n p e l l i c l e s s u r r o u n d i n g t h e p r o p p a n t s i n c r e a s e t h e c o n t a c t a r e a b e t ween i n d i v i d u a l g r a i n s w h i c h c a n t h u s w i t h s t a n d h i g h e r c l o s u r e s t r e s s e s b e f o r e c r u s h i n g . W h i l e u n c o a t e d p r o p p a n t s have p o i n t - l i k e c o n t a c t s c o m p r i s i n g l e s s t h a n 10 % o f g r a i n d i a m e t e r , r e s i n - c o a t e d p r o p p a n t s e x h i b i t b r o a d e r c o n t a c t r a n g e s r e a c h i n g up t o 25 - 40 % o f g r a i n d i a m e t e r (SINCLAIR, GRAHAM & SINCLAIR 1 9 8 3 ) . I n a d d i t i o n t o r e d u c t i o n o f p o i n t - t o - p o i n t l o a d i n g caused b y t h e p l a s t i c f l o w o f t h e r e s i n f i l m s , t h e r e s i n p e l l i c l e s b i n d and c o n t a i n c r u s h e d f r a g m e n t s o f t h e sand g r a i n s and t h e r e b y d e c r e a s e t h e amount o f m o b i l e f i n e s w h i c h i m p r o ves c o n d u c t i v i t y , whereas o n t h e o t h e r hand, d e f o r m a t i o n o f t h e r e s i n p e l l i c l e s u n d e r l o a d d e c r e a s e s c o n d u c t i v i t y (CUTLER, ENNISS, JONES & SWANSON 1985). I n c o m p a r i s o n t o p r e c u r e d m a t e r i a l , c u r a b l e p r o p p a n t s have much b r o a d e r c o n t a c t zon e s b y b e t t e r f l o w i n g o f t h e r e s i n , l o w e r i n i t i a l c o n d u c t i v i t y due t o d e f o r m e d r e s i n c o a t i n g s , l o w e r f i n e s g e n e r a t i o n and h i g h e r c o n d u c t i v i t y a t h i g h c l o s u r e stress.
1 . 2 . 6 . 1 . 2 . Specific gravity and proppant price lowering The t y p e o f r e s i n w h i c h i s p r e c u r e d a r o u n d t h e sand g r a i n s i s c r i t i c a l t o t h e f i n a l p r o p p a n t p r o p e r t i e s , a5 a r e amount o f r e s i n and e x t e n t o f c u r e (UNDERDOEIN & DAS 1 9 8 2 ) . The g r e a t e s t a d v a n t a g e o f r e s i n - c o a t e d sand i s t h a t i t s b u l k d e n s i t y i s much l e s s t h a n t h a t o f i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s , and t h a t i t s p r i c e i s c o n s i d e r a b l y c o m p e t i t i v e t o t h e c o s t o f t h e l a t t e r p r o d u c t s . The much l o w e r b u l k d e n s i t y o f r e s i n - c o a t e d sands a l l o w s s i g n i f i c a n t l y b e t t e r s u s p e n s i o n i n many d i f f e r e n t f r a c t u r i n g f l u i d s t h a n p o s s i b l e f o r h e a v i e r i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s , and t h e much c h e a p e r m a t e r i a l r e n d e r s many o p e r a t i o n s e c o n o m i c a l l y more f e a s i b l e and as a consequence o f t h e e l e v a t e d c l o s u r e s t r e s s r e s i s t i v i t y , r e s i n - c o a t e d sand i s an a c c e p t a b l e s o l u t i o n f o r many m a r g i n a l r e s e r v o i r s where e x p e n s i v e s y n t h e t i c p r o p p a n t s a r e d i f f i c u l t t o j u s t i f y a t l e a s t i n p i l o t p r o j e c t s . The o n l y a r t i f i c i a l p r o p p a n t t y p e w h i c h c a n compete w i t h r e s i n - c o a t e d sand i n t e r m s o f b o t h t e c h n i c a l p e r f o r m a n c e and economical f e a s i b i l i t y i s i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y alumina s i l i c a t e proppant.
1 . 2 . 6 . 2 . Reactions of resin coatings
to closure stress and temperature
The d e f o r m a b l e r e s i n c o a t i n g on t h e p r o p p a n t g r a i n s u r f a c e s p r e a d s c r u s h i n g and embedment l o a d s o v e r a g r e a t e r a r e a , t h e r e b y p r e v e n t i n g c r u s h i n g and r e s i s t i n g embedment (SINCLAIR, GRAHAM & SINCLAIR 1 9 8 3 ) . The t o u g h r e s i n f i l m i s c h e m i c a l l y i n e r t i n c r u d e o i l s , b r i n e s and m o s t a c i d s . The t h i n and t e n a c i o u s r e s i n p e l l i c l e c o m p r i s e s l e s s t h a n 5 % o f t h e t o t a l p r o p p a n t w e i g h t and has o n l y a t h i c k n e s s o f a b t . 0 . 0 0 1 i n . Resin-coated proppants c o n s o l i d a t e under f o r m a t i o n s t r e s s e s and t e m p e r a t u r e s b o t h w i t h i n t h e r e s e r v o i r and w i t h i n l a b o r a t o r y t e s t c e l l s . V a r i o u s e f f e c t s o f c l o s u r e s t r e s s and t e m p e r a t u r e and t h e r e a c t i o n s o f r e s i n - c o a t e d p r o p p a n t s a r e i l l u s t r a t e d as f o l l o w s .
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1.2.6.2.1. Closure stress A t high r a t e s o f a p p l i c a t i o n o f closure stress, resin-coated proppants tend t o be s h o c k - l o a d e d ( c f . s e c t i o n 4 . 1 2 . 4 . 1 . ) , because t h e r e i s n o t i m e f o r t h e d e f o r m a b l e p e l l i c l e t o a d j u s t and p r o t e c t t h e p r o p p a n t g r a i n s . I n f i e l d a p p l i c a t i o n , however, c l o s u r e s t r e s s r e t u r n s o v e r a p e r i o d o f h o u r s a f t e r t h e t e r m i n a t i o n o f t h e f r a c t u r e t r e a t m e n t ( c f . s e c t i o n 4 . 1 2 . 2 . 2 . ) , w i t h t h i s g i v i n g t h e def o r m a b l e r e s i n f i l m ample t i m e t o a d j u s t and p r o t e c t t h e g r a i n s (SINCLAIR, GRAHAM & SINCLAIR 1 9 8 3 ) . A t e x t r e m e l y h i g h c l o s u r e s t r e s s e s when a t s o r e s i n - c o a t e d m a t e r i a l s a r e f i n a l l y cracked o r broken, most fragments a r e s t i l l a t t a c h e d t o t h e tenacious r e s i n e n v e l o p e , and m i g r a t i o n o f any f i n e s i s m i n i m i z e d s i n c e t h e r e s i n i s s e c u r e l y bonded t o m o s t o f t h e f r a g m e n t s o f t h e s p l i t p a r t i c l e s ( t h i s e f f e c t i s much l i k e t h a t o f t h e s h a t t e r p r o o f s a f e t y g l a s s used i n c a r w i n d s h i e l d s ; SINCLAIR, GRAHAM & SINCLAIR 1 9 8 3 ) . W h i l e r e s i n c u r i n g a t h i g h r e s e r v o i r t e m p e r a t u r e s and p r e s s u r e s i n deep w e l l s does n o t r e q u i r e e x t e r n a l c a t a l y z a t i o n , r e s i n - c o a t e d proppants are a l s o a p p l i c a b l e i n shallow w e l l s w i t h low formation temperatures and c l o s u r e s t r e s s e s once c u r i n g i s i n i t i a t e d b y an a c t i v a t o r . D i s t i n c t i o n c a n t h u s be made between c h e m i c a l r e s i n b o n d i n g a t l o w t e m p e r a t u r e s and t h e r m a l r e s i n b o n d i n g a t h i g h t e m p e r a t u r e s above a b t . 150 OF.
1.2.6.2.2. Temperature H i g h t e m p e r a t u r e s a r e w e l l t o l e r a t e d b y t h e r e s i n c o a t i n g , as i t i s t i g h t l y a t t a c h e d t o t h e p r o p p a n t s u r f a c e b y a b o n d i n g a g e n t and does n o t come o f f ( S I N CLAIR, GRAHAM & SINCLAIR 1 9 8 3 ) . W h i l e h i g h - t e m p e r a t u r e w a t e r o r b r i n e w i l l a t t a c k f r e s h s i l i c e o u s s u r f a c e s , t h e r e s i n p e l l i c l e i s n o n - r e a c t i v e t o any r e s e r v o i r f l u i d system. T h i c k e r r e s i n f i l m s s u r r o u n d i n g p r o p p a n t g r a i n s l o w e r s p e c i f i c g r a v i t y and i n c r e a s e t r a n s p o r t c a p a b i l i t y o f t h e p r o p p a n t s , w i t h p a r t i a l l y t o t a l s p e c i f i c g r a v i t i e s s i g n i f i c a n t l y l i g h t e r t h a n t h a t o f n a t u r a l sand b e i n g a c h i e v e d when e n v e l o p i n g sand g r a i n s w i t h r e s i n f i l m s . V a r i a t i o n s i n p r o p p a n t g r a i n s i z e as w e l l as r e s i n p e l l i c l e t h i c k n e s s c a n be made i n o r d e r t o o p t i m i z e r e s u l t s o f d i f f e r e n t t y p e s o f t r e a t m e n t s and c a n be t a i l o r e d f o r optimum downh o l e performance. T a i l i n g - i n o f resin-coated proppants i n t h e near-wellbore r e g i o n i n h i b i t s f r a c t u r e e v a c u a t i o n and c o u p l e s b o r e h o l e and p r o d u c t i v e f o r m a t i o n ( c f . section 4.12.3.3.). P r o p p a n t f l o w b a c k p r e v e n t i o n by r e s i n - c o a t e d t a i l - i n i s made r e g a r d l e s s o f t y p e o r amount o f m a i n p r o p p a n t u s e d i n t h e f r a c t u r e . R e s i n - c o a t e d s y n t h e t i c p r o p p a n t s have l o w e r c o n d u c t i v i t i e s t h a n u n c o a t e d ones when s i m i l a r s i z e d i s t r i b u t i o n s a r e t e s t e d (CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) . A l t h o u g h c o n t a i n m e n t o f f i n e s w i t h i n t h e r e s i n p e l l i c l e s i m p r o v e s c o n d u c t i v i t y , r e s i n envelope deformation under l o a d decreases c o n d u c t i v i t y . F l o w o f r e s i n f i l m s , p a r t i c l e r e a r r a n g e m e n t , e l a s t i c d e f o r m a t i o n , and p a r t i c l e c r u s h i n g a l l t e n d t o n a r r o w f r a c t u r e w i d t h as c l o s u r e s t r e s s i s i n c r e a s e d . Res i n - c o a t e d p r o p p a n t s a r e i n l a b o r a t o r y t e s t i n g b e s t e v a l u a t e d as a f u n c t i o n o f t e m p e r a t u r e because o f t h e p o s s i b l e c r e e p o f t h e p o l y m e r i c p e l l i c l e s .
1 . 2 . 6 . 3 . Double resin coating combination The l a t e s t d e v e l o p m e n t i s d o u b l e r e s i n - c o a t i n g o f p r o p p a n t s w i t h an i n n e r p r e c u r e d p e l l i c l e and an o u t e r c u r a b l e e n v e l o p e (SANTROL 1 9 8 6 ) . F u n c t i o n s o f i n n e r p r e c u r e d and o u t e r c u r a b l e r e s i n f i l m s as w e l l as p r e s s u r e and t e m p e r a t u r e r e a c t i o n o f r e s i n p e l l i c l e s a r e o u t l i n e d as f o l l o w s .
26
1.2.6.3.1. Functions o f inner precured and outer curable films The inner precured resin film provides additional strength of the material and the outer curable resin coating bonds the particles together, thus giving a combination of the benefits of a high-strength proppant with the advantages of a self-consolidating proppant and providing high strength, low density, and permanent bonding ability together at reasonable cost (SANTROL 1986). The inner resin pellicle has undergone tempering representing advanced polymerization proc e s s and imparts great strength to the grain for crush resistance. The outer envelope is a rather thin layer of curable resin that uses downhole temperature and fracture closure stress to form a multilayer proppant package which stays in place. The curable resin coating also encapsulates and traps any broken grains or fines. The resin pellicle is so thin that it bonds together only upon contact stress in the fracture, in contrast to earlier submature thicker films which sometimes also bonded together in the wellbore. Maximum conductivity and strength of the proppant is achieved by 3 - 4 % coating of the material. Proppants with curable envelopes are the only ones which do not embed into the reservoir formation. In low-temperature pays (below 130 OF), an activator is required to initiate and speed-up bonding of the resin pellicles. Dual resin-coated sand and proppants were introduced to the hydrocarbon stimulation market in 1984 (FRACFAX 1988 i).
1.2.6.3.2. Pressure and temperature reactions of resin pellicles Comparative closure stress resistivity testing reveals that uncoated ceramic proppants crush up to 6 % and more at 10,000 psi and 300 OF, whereas dual resin-coated proppants of any type at the same conditions do not suffer any crushing and fines generation (FRACFAX 1988 d). Closure stress resistivity testing at 275 OF and 8,000 psi illustrates that double resin-coated proppants of any type are much more stable in monolayers and at proppant loadings of 0.5 - 3 . 0 lbs/ft2 which correspond to frequent actual industry designs in contrast to the API recommendation of 4 lbslft2 for proppant testing (FRACFAX 1988 1). While high-temperature weakens all high- and intermediate-strength ceramic proppants, higher temperatures increase closure stress resistivity of resin-coated proppants. Double resin-coated material is stronger than almost all uncoated proppant types, and the strength contrast becomes more pronounced a s temperature rises (SANTROL 1986). Double resin-coated proppants can be successfully applied up to 25,000 ft reservoir depth and 600 OF bottomhole temperature and bonds together by means of both formation temperature and closure stress. While resin-coated sand can particularly in case of double pellicles successfully replace ceramic proppants up to 12,000 psi closure stress, dual resin-coated ceramic proppants outperform sintered bauxite and all other synthetic types in crush resistance up to closure stresses reached in the hitherto deepest oil and gas wells of the world with highest bottomhole pressures and temperatures.
1.2.7. Possible future innovation In the last years, continuing quality demand and the need of cost containment triggered experimental approaches of developing even once again new proppant types of enhanced technical performance and better economical feasibility in order to adjust to the increasingly diversified application spectrum (CUTLER, JONES, ENNISS & SWARTZ 1983; CUTLER, ENNISS, JONES & SWANSON 1985; DGMK 1986), with some lines o f possible proppant performance improvement including prototype materials based on cordierite, silicon carbide, and several ceramic oxides.
27 E x p e r i m e n t a l approaches even l o o k e d i n t o t h e p o s s i b i l i t y o f m a n u f a c t u r i n g h o l l o w o r p o r o u s p r o p p a n t s i n an a t t e m p t t o r e d u c e s p e c i f i c g r a v i t y o f h i g h - q u a l i t y m a t e r i a l b e l o w t h a t o f sand (CUTLER, JONES, SWANSON & CARROLL 1981; CUTLER & JONES 1982; CUTLER, JONES, E N N I S S & SWARTZ 1 9 8 3 ) , b u t r e v i e w i n g t h e s u i t e of m a t e r i a l s and p r o d u c t i o n t e c h n i q u e s w h i c h have a l r e a d y been t e s t e d , i t has t o be d o u b t e d a t t h e moment t h a t i t w o u l d be p o s s i b l e t o d e v e l o p an u l t r a l i g h t synt h e t i c p r o p p a n t w i t h f l u i d s u s p e n s i o n p r o p e r t i e s and c e r t a i n l y s u f f i c i e n t c l o s u r e s t r e s s r e s i s t i v i t y even b e t t e r t h a n sand o r l o w - d e n s i t y i n t e r m e d i a t e s t r e n g t h a l u m i n a s i l i c a t e p r o p p a n t s a t an a c c e p t a b l e c o s t ( t h e same a p p l i e s f o r s i m i l a r a p p r o a c h e s of g r a v e l p a c k i n g m a t e r i a l ; c f . s e c t i o n 5 . 3 . 3 . 2 . ) . The f o l l o wing discussion o u t l i n e s proppant p a r t i c l e composition types, specific gravity lowering, s t r e n g t h r e t a i n m e n t o f h o l l o w spheres, and p r o p p a n t f a b r i c a t i o n methods.
1.2.7.1.Proppant particle composition types CUTLER, JONES, SWANSON & CARROLL (1981) and CUTLER, ENNISS, JONES & SWANSON ( 1 9 8 5 ) g i v e an o v e r v i e w o f e x p e r i m e n t a l m o d e l l i n g o f l i g h t w e i g h t i n n o v a t i v e p r o p p a n t t y p e s . A n i d e a l p r o p p a n t f o r m a s s i v e h y d r a u l i c f r a c t u r i n g w o u l d have a s p e c i f i c g r a v i t y o f l e s s t h a n 2.0, be a b l e t o w i t h s t a n d a c l o s u r e s t r e s s o f 20,000 p s i ( 1 3 9 MPa), be c h e m i c a l l y i n e r t i n b r i n e a t t e m p e r a t u r e s u p t o 200 OC ( 3 9 0 O F ) , have p e r f e c t s p h e r i c i t y , and have a n a r r o w g r a i n s i z e d i s t r i b u t i o n a t an a c c e p t a b l e p r i c e . T e s t i n g o f v a r i o u s m e t a l s , c e r a m i c s , p l a s t i c s and combinat i o n s o f these t h r e e m a t e r i a l groups has r e v e a l e d t h a t ceramics w i t h o r w i t h o u t p o l y m e r c o a t i n g s r e p r e s e n t t h e o n l y sound a p p r o a c h t o o b t a i n i n g a l o w - c o s t , c o r r o s i o n - r e s i s t a n t , l o w - s p e c i f i c - g r a v i t y and h i g h - s t r e n g t h p r o p p a n t . I n o r d e r t o meet t h e s p e c i f i c a t i o n f o r l o w g r a v i t y , p r o p p a n t s may be f a b r i c a t e d as s o l i d s p h e r e s ( f u l l y d e n s e ) , h o l l o w g l o b u l e s ( f u l l y dense e x c e p t f o r h o l l o w c o r e ) , p o r o u s s p h e r e s ( u n i f o r m f i n e l y d i s p e r s e d and i n t e r c o n n e c t e d m i c r o p o r o s i t y t h r o u g h o u t t h e b e a d ) , and c o m p o s i t e s o f c e r a m i c and p o l y m e r i c m a t e r i a l s . F o r f u l l y dense s o l i d s p h e r e s and h o l l o w g l o b u l e s w i t h a s m a l l c e n t e r v o i d , t h e c o n t a c t zone s t r e s s e s a r e assumed t o be c o n t r o l l i n g , and t h u s t h e c e n t r a l empty space d e c r e a s e s s p e c i f i c g r a v i t y w i t h o u t a f f e c t i n g f r a c t u r e p r e s s u r e . Once a c r i t i c a l v o i d s i z e i s exceeded, t h e l i m i t i n g s t r e s s becomes t h a t a t t h e i n s i d e s u r f a c e , and f u r t h e r i n c r e a s e s i n empty space cliameter t h u s d e c r e a s e f r a c t u r e pressure.
1.2.7.2.Specific gravity lowering S p e c i f i c p r o p p a n t g r a v i t y may be l o w e r e d s i g n i f i c a n t l y b y f a b r i c a t i n g a h o l l o w bead w i t h o u t any l o s s i n s t r e n g t h (CUTLER, JONES, SWANSON & CARROLL 1981; CUTLER, ENNISS, JONES & SWANSON 1985). H o l l o w s p h e r e s , however, a r e n o t a l w a y s s u p e r i o r t o p o r o u s g l o b u l e s , w h i c h i s e c o n o m i c a l l y v e r y i m p o r t a n t , because i t i s much e a s i e r t o f a b r i c a t e p r o p p a n t s w i t h u n i f o r m p o r o s i t y t h a n t o make h o l l o w s p h e r e s . The e x c e s s i v e c o s t of r a w m a t e r i a l s may i n many c a s e s e x c l u d e t h e u s e o f h i g h - p u r i t y materials, although i t i s obvious t h a t high-strength high-qualit y c e r a m i c s may be t h e answer t o m e e t i n g t h e d e s i g n g o a l o f 2 . 0 g/cm3 s p e c i f i c g r a v i t y and 20,000 p s i c l o s u r e s t r e s s . S e v e r a l c a n d i d a t e p r o p p a n t m a t e r i a l s have been i d e n t i f i e d w h i c h show p r o m i s e as l i g h t w e i g h t p r o p p a n t s i n h o l l o w , p o r o u s o r s o l i d f o r m c o m p r i s i n g h i g h - p u r i t y a l u m i n a , m u l l i t e , c o r d i e r i t e and h i g h - s t r e n g t h p o r c e l a i n s , w i t h d e n s i t i e s r a n g i n g f r o m 1 . 8 t o 2 . 5 g/cm3. F r a c t u r e c o n d u c t i v i t y r e s u l t s f o r s p r a y - d r i e d a l u m i n a a r e v e r y e n c o u r a g i n g s i n c e i t a p p e a r s t h a t t h e s p r a y - d r i e d - f l u i d i z e d bed c o n c e p t p r o d u c e s an e a s i l y s i n t e r e d m a t e r i a l w h i c h w i l l have a c o n d u c t i v i t y comp a r a b l e t o s i n t e r e d b a u x i t e i f p r o p p a n t s o f s i m i l a r p a r t i c l e s i z e and shape a r e t e s t e d . Spray d r y i n g a l s o p e r m i t s t h e economical f a b r i c a t i o n o f h o l l o w propp a n t s t h r o u g h an a g g l o m e r a t i o n t e c h n i q u e and c o a t i n g p r o c e s s i n t h e f l u i d i z e d bed.
28
1.2.7.3. Strength retainment o f hollow spheres The m o s t e c o n o m i c a l a p p r o a c h t o s t r e n g t h r e t a i n m e n t o f h o l l o w s p h e r e s app e a r s t o be s p r a y d r y i n g o f t h i n - w a l l e d s m a l l h o l l o w g l o b u l e s i n t o a f l u i d i z e d bed, a g g l o m e r a t i n g s e v e r a l o f t h e s e s p h e r e s t o g e t h e r i n t o a l a r g e r bead and then c o a t i n g the agglomerate c l u s t e r w i t h a s o l i d envelope. S i n t e r e d b a u x i t e p r o d u c e d i n t h i s manner w o u l d have a s p e c i f i c g r a v i t y n e a r t h a t o f sand, b u t r e t a i n i t s h i g h strength, although t h e process i s s t i l l i n experimental stage. B o t h dense and h o l l o w a l u m i n a s p h e r e s have been f a b r i c a t e d b y s p r a y f l u i d i z a t i o n and c o u n t e r c u r r e n t s p r a y d r y i n g (CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) . S p r a y d r y i n g i n t o a f l u i d i z e d bed i s a l s o a v i a b l e method f o r p r o d u c i n g p o rous proppants c h a r a c t e r i z e d by disseminated m i c r o p o r o s i t y throughout t h e g r a i n . L i g h t w e i g h t e x p e r i m e n t a l h o l l o w a l u m i n a p r o p p a n t s have l o w e r c r u s h r e s i s t a n c e t h a n b a u x i t e b u t a r e s u b s t a n t i a l l y s t r o n g e r t h a n sand a l t h o u g h l o w e r i n d e n s i t y , w i t h n o n - c o n c e n t r i c and m u l t i p l e v o i d s w i t h i n i n d i v i d u a l p r o p p a n t s b e i n g t h e main reasons f o r t h e h i t h e r t o inadequate s t r e n g t h . S e l e c t i o n o f p r o p e r s p r a y - d r y i n g p a r a m e t e r s such as d r y e r d e s i g n , n o z z l e t y p e , a d d i t i v e s , temper a t u r e and f l o w r a t e s a r e i m p o r t a n t f o r o p t i m i z a t i o n o f h o l l o w p r o p p a n t f a b r i c a t i o n by spray d r y i n g .
1.2.7.4. Proppant fabrication met hods A w i d e v a r i e t y o f f a b r i c a t i o n methods c a n be u s e d t o make p r o p p a n t s i n c l u ding mix-pelletization, s p r a y d r y i n g , d r o p l e t g e n e r a t i o n and c o a t i n g (CUTLER, E N N I S S , JONES & SWANSON 1 9 8 5 ) . W h i l e t r a d i t i o n a l p r o p p a n t s a r e p r e d o m i n a n t l y m a n u f a c t u r e d b y m i x - p e l l e t i z a t i o n and o c c a s i o n a l l y a l s o b y d r o p l e t g e n e r a t i o n , t h e m o s t p r o m i s i n g e x p e r i m e n t a l a p p r o a c h c o m p r i s e s c o a t i n g o f cheap c e r a m i c rnat e r i a l s and c l u s t e r i n g o f s e v e r a l s m a l l e r s p h e r e s t o l a r g e r p a r t i c l e s . I n t e r m e d i a t e - s t a g e low-density proppants are m u l l i t e proppants f a b r i c a t e d by m i x - p e l l e t i z a t i o n and c a r b o n - c o a t e d a l u m i n a m i c r o s p h e r e s p r o d u c e d by d r o p l e t g e n e r a t i o n . On l a b o r a t o r y s c a l e , h o l l o w p r o p p a n t s have been f a b r i c a t e d w i t h c r u s h r e s i s t a n c e v a l u e s between sand and i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s . M a n u f a c t u r i n g o f e i t h e r h o l l o w beads o r g l o b u l e s w i t h f i n e l y d i s p e r s e d i n t e r c o n n e c t e d p o r o s i t y app e a r s t o have t h e p o t e n t i a l f o r c o m b i n i n g adequate s t r e n g t h w i t h l o w d e n s i t y . I t i s t h e r e f o r e b e l i e v e d t h a t h o l l o w proppants a r e t h e key t o o b t a i n i n g lower s p e c i f i c g r a v i t y proppants a t c o m p e t i t i v e c o s t s .
1.2.8. Vertical vs. horizontal fracture orientation As a consequence o f t h e a v a i l a b i l i t y o f v a r i o u s p r o p p a n t t y p e s o f d i f f e r e n t m e c h a n i c a l and c h e m i c a l p r o p e r t i e s , m i n e r a l o g i c a l c o m p o s i t i o n and p o s s i b i l i t i e s o f i n d u s t r i a l a p p l i c a t i o n t h a t a r e r e f l e c t e d i n d i f f e r e n t p r i c e s , numerous h y d r a u l i c p r o p p a n t f r a c t u r e o p e r a t i o n s have been c a r r i e d o u t d u r i n g t h e l a s t t e n y e a r s , w i t h m a j o r i n f l u e n c i n g f a c t o r s on d e s i g n and e x e c u t i o n b e i n g n o t o n l y proppant performance w i t h r e s p e c t t o t h e i r r e s i s t i v i t y against c l o s u r e s t r e s s and a g g r e s s i v e f o r m a t i o n f l u i d s and t h e i r s u i t a b i l i t y o f a p p l i c a t i o n i n t h e geol o g i c a l and r e s e r v o i r e n g i n e e r i n g framework, b u t a l s o e c o n o m i c a l a s p e c t s o f h y d r o c a r b o n p r i c e ( c f . s e c t i o n Z . Z . I . ) , p r o p p a n t c o s t ( c f . s e c t i o n 2.2.2.),US $ exchange r a t e ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) and c o m m e r c i a l gas p r o d u c t i o n r a t e s , as w e l l as f o r e i g n t r a d e and f i n a n c i n g p a r t i c u l a r l y i n E a s t e r n European c o u n t r i e s (MADER 1987; c f . s e c t i o n 2 . 4 . 4 . ) . O t h e r i m p o r t a n t p o i n t s w h i c h have t o be c o n s i d e r e d a r e minimum r e s e r v o i r p r o p e r t i e s r e q u i r e d f o r e c o n o m i c a l l y p r o f i t a b l e s t i m u l a t i o n s (DIETZEL & ERNST 1987; c f . s e c t i o n 3 . 6 . 1 . 2 . ) . A t t e n t i o n has a l s o t o be p a i d t o t h e q u e s t i o n o f v e r t i c a l v s . h o r i z o n t a l f r a c t u r e p r o p a g a t i o n d i r e c t i o n . The b r i e f o u t l i n e as f o l l o w s s k e t c h e s some a s p e c t s o f f r a c t u r e o r i e n t a t i o n and r e s e r v o i r d e p t h as
29 w e l l as g e o t e c t o n i c a l s e t t i n g , b o r e h o l e / f r a c t u r e i n t e r s e c t i o n and r e s e r v o i r drainage, and c o e x i s t e n c e and t r a n s i t i o n o f h o r i z o n t a l and v e r t i c a l f r a c t u r e s .
1.2.8.1. Fracture orientation and reservoir depth As a consequence o f i n c r e a s i n g overburden s t r e s s w i t h depth and t h e r e f o r e s h i f t i n g r e l a t i o n s h i p between h o r i z o n t a l and v e r t i c a l p r i n c i p a l s t r e s s , t h e r e i s a c o r r e l a t i o n between f r a c t u r e p r o p a g a t i o n d i r e c t i o n and r e s e r v o i r d e p t h . The c r a c k s c r e a t e d by h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s a r e o r i e n t e d v e r t i c a l l y t o l e a s t p r i n c i p a l s t r e s s ( w h i c h i s a p p r o x i m a t e l y equal t o t h e f r a c t u r e c l o s u r e p r e s s u r e ) i n t h e e a r t h ' s c r u s t (HUBBERT & WILLIS 1957, DUNLAP 1963, HAIMSON & FAIRHURST 1969, HOWARD & FAST 1970, VEATCH 1983). As a consequence o f t h e geos t r e s s f i e l d , above a b t . 3,000 f t (1,000 m) t h e c r a c k s a r e g e n e r a l l y h o r i z o n t a l , whereas below t h a t boundary d e p t h p r e d o m i n a n t l y v e r t i c a l f r a c t u r e s e x i s t (SMITH 1965, McGARR & GAY 1978, AGUILERA 1983; ROBINSON, HOLDITCH & WHITEHEAD 1986; o t h e r a u t h o r s assume t h e b o r d e r l i n e between h o r i z o n t a l f r a c t u r e o r i g i n and v e r t i c a l f r a c t u r e growth a t l e a s t i n some areas t o be a l r e a d y i n a d e p t h o f a b t . 1,000 - 2,000 f t o r 300 - 700 m; VEATCH 1983).
1.2.8.2. Fracture orientation and geotectonical setting Concerning g e o t e c t o n i c a l s e t t i n g , t h e l e a s t p r i n c i p a l s t r e s s i s m a i n l y h o r i z o n t a l i n t e c t o n i c a l l y r e l a x e d areas which a r e o n l y s u b j e c t e d t o subsidence and e x t e n s i o n a l ( n o r m a l ) f a u l t i n g , and f r a c t u r i n g t h e r e f o r e occurs along v e r t i c a l planes, whereas i n t e c t o n i c a l s t r e s s f i e l d s where a l s o compressional ( i n v e r s e o r t h r u s t ) f a u l t i n g , s h e a r i n g and f o l d i n g t a k e s p l a c e , t h e ( e x t e r n a l l y generat e d ) h o r i z o n t a l s t r e s s e s may be g r e a t e r than t h e overburden s t r e s s and due t o t h e l e a s t p r i n c i p a l s t r e s s b e i n g v e r t i c a l , h o r i z o n t a l f r a c t u r e s have t o be e x p e c t e d ( s t r e s s e f f e c t s on crack o r i e n t a t i o n and p r e s s u r e s r e q u i r e d t o produce v e r t i c a l and h o r i z o n t a l f r a c t u r e s a r e a l s o discussed by HUBBERT & WILLIS 1957). Some comments a r e o f f e r e d as f o l l o w s on e x t e n s i o n a l v s . compressional t e c t o n i c s as w e l l as on comparative s i g n i f i c a n c e o f h o r i z o n t a l and v e r t i c a l f r a c t u r e s .
1.2.8.2.1. Extens iona 1 vs . compressional tectonics N a t u r a l j o i n t o r i g i n i s a s s o c i a t e d w i t h d i a s t r o p h i s m d u r i n g f o l d i n g and f a u l t i n g where c r a c k s a r e generated a l o n g t h e l i n e o f t h e f a u l t which i n t u r n p r o duce a zone o f d i l a t a n c e ( c f . s e c t i o n 4 . 8 . 1 . 3 . 1 . ) ; deep e r o s i o n o f t h e o v e r b u r den p e r m i t t i n g t h e upper p a r t s t o expand, u p l i f t and f r a c t u r e t h r o u g h planes o f weakness; and volume s h r i n k a g e as i n case o f c o o l i n g o f igneous r o c k s ( c f . p l a t e V1/8 and IX/5-6) and d e s i c c a t i o n of sediments ( c f . p l a t e V1/1-7 and sect i o n 4 . 8 . 8 . 1 . ) . T h r u s t f a u l t i n g i s a p o s s i b i l i t y o f c r e a t i n g h o r i z o n t a l and low-angle i n c l i n e d c r a c k s ( c f . p l a t e X1/3) a l s o i n s h a l l o w d e p t h (AGUILERA 1983). H o r i z o n t a l f r a c t u r e s , however, o f t e n t e n d t o c l o s e owing t o t h e e f f e c t o f overburden. F o r m a t i o n h e a t i n g by steam i n j e c . i r i n i s a s i g n i f i c a n t means o f f u n d a m e n t a l l y changing t h e i n - s i t u s t r e s s s t a t e i n s h a l l o w r e s e r v o i r s f r o m a r e gime f a v o u r i n g o r i g i n o f v e r t i c a l c r a c k s i n c o l d environment t o a framework supp o r t i n g p r o p a g a t i o n o f h o r i z o n t a l f r a c t u r e s i n h o t m i l i e u (HOLZHAUSEN, CARD, RAISBECK & DOBECKI 1985; c f . s e c t i o n 4 . 9 . 4 ) .
1.2.8.2.2. Comparative significance o f horizontal and vertical fractures As most o f t h e h y d r o c a r b o n - b e a r i n g f o r m a t i o n s which a r e s t i m u l a t e d a r e i n depths exceeding t h e boundary o f a b t . 3,000 f t (1,000 m) and a r e i n b a s i n s w i t h o n l y moderate t e c t o n i c a l s t r e s s undergoing s t i l l s l i g h t subsidence and h a v i n g been s u b j e c t e d t o normal f a u l t i n g w i t h o n l y m i n o r t i l t i n g , s h e a r i n g and compress i o n , t h e d i s c u s s i o n as f o l l o w s c o n c e n t r a t e s on v e r t i c a l c r a c k s i n t e r s e c t i n g
30 t h e r e s e r v o i r r o c k s . A s p e c t s o f h o r i z o n t a l f r a c t u r e s a r e emphasized b y CRAWFORD & LANDRUM ( 1 9 5 6 ) , WAHL (1956, 1 9 6 5 ) , LANDRUM & CRAWFORD ( 1 9 5 7 ) , MORRISSON & HENDERSON ( 1 9 6 0 ) , HARTSOCK & WARREN ( 1 9 6 1 ) , WAHL & CAMPBELL ( 1 9 6 3 ) , LOWE & HUITT ( 1 9 6 6 ) , SUN ( 1 9 6 9 ) , GRINGARTEN & RAMEY ( 1 9 7 4 ) , ERDLE ( 1 9 8 1 ) and SUNG & ERTEKIN ( 1 9 8 7 ) . EVANS ( 1 9 8 3 ) and EVANS & HOLZHAUSEN ( 1 9 8 3 ) a l s o comment on d e v e l o p m e n t o f shallow h y d r a u l i c f r a c t u r e s . I d e a l l y , h o r i z o n t a l s t r e s s i s independent from a z i m u t h i n i s o t r o p i c r o c k s w h i c h , however, a r e h a r d l y d e v e l o p e d i n n a t u r e . In a n i s o t r o p i c r o c k s w h i c h a r e t h e r e a l cases, c r a c k o r i e n t a t i o n r u n s p e r p e n d i c u l a r l y t o t h e l e a s t h o r i z o n t a l s t r e s s (LEICHT 1 9 8 5 ) . I n d u s t r i a l p r a c t i c e h a s shown t h a t h o r i z o n t a l f r a c t u r e s c o n s t i t u t e a r e l a t i v e l y l o w p e r c e n t a g e o f t h e s i t u a t i o n s e x p e r i e n c e d t o d a t e (VEATCH 1983) and a r e r a t h e r s p e c i a l c a s e s t h a n b e i n g more common.
1.2.8.3. Borehole/fracture intersection and reservoir drainage I n terms o f r e s e r v o i r drainage, t h e e f f e c t o f h o r i z o n t a l f r a c t u r e s i n v e r t i c a l w e l l s c a n be compared w i t h t h a t o f v e r t i c a l c r a c k s c r o s s i n g h i g h l y - d e v i a t e d o r h o r i z o n t a l b o r e h o l e s ( c f . c h a p t e r 4 . 1 1 . ) . The r e s t r i c t e d c r o s s - s e c t i o n o f t h e f r a c t u r e p l a n e b e i n g i n c o m m u n i c a t i o n w i t h t h e w e l l b o r e t r a j e c t o r y t h a t perf o r a t e s t h e h o r i z o n t a l c r a c k l i k e a s l i c e i n c o n t r a s t t o t h e c o n t i n u o u s and f u l l f r a c t u r e h e i g h t exposure i n case o f i d e a l v e r t i c a l cracks r e s u l t s i n o n l y l i m i t e d d r a i n a g e t h r o u g h t h e f r a c t u r e and v e r y r e s t r i c t e d e n l a r g e m e n t o f e f f e c t i v e wellbore diameter. Therefore except o f special applications, preference i s always g i v e n t o v e r t i c a l cracks f o r t h e reason o f b e t t e r r e s e r v o i r d e p l e t i o n due t o l o n g e r b o r e h o l e / f r a c t u r e c o m m u n i c a t i o n d i s t a n c e i f any i n f l u e n c e c a n be e x e r t e d on c r a c k o r i e n t a t i o n . Some e x t r a o r d i n a r y i n d i c a t i o n s o f t h e s u i t a b i l i t y o f horizontal fractures, p a r t i a l l y i n combination w i t h v e r t i c a l cracks, are outlined i n section 4.9.4.2.
1 . 2 . 8 . 4 . Coexistence and transition
of horizontal and vertical fractures H o r i z o n t a l f r a c t u r e s s t a r t i n many c a s e s t o p r o p a g a t e as v e r t i c a l c r a c k s and t h e n r e o r i e n t t h e m s e l v e s t o become h o r i z o n t a l and t o r u n p e r p e n d i c u l a r l y t o t h e l e a s t p r i n c i p a l s t r e s s (DANESHY 1973 b ) , w i t h i n i t i a l v e r t i c a l c r a c k f o r m a t i o n b e i n g s u b s t i t u t e d b y f o l l o w i n g h o r i z o n t a l f r a c t u r e o r i g i n w h i c h t h e n becomes d o m i n a n t and t h e r e b y s e v e r e l y l i m i t s f u r t h e r g r o w t h o f t h e v e r t i c a l c r a c k (WOOD, PARKIN, YOTAM, HANSON, S M I T H , ABBOTT, COX & O'SHEA 1 9 8 3 ) . I n some c a s e s , a h o r i z o n t a l f r a c t u r e and a v e r t i c a l c r a c k may e v e n k e e p open and p r o p a g a t e s i multaneously, w i t h t h e r a t e o f growth o f t h e h o r i z o n t a l f r a c t u r e being g r e a t e r t h a n t h a t o f t h e v e r t i c a l c r a c k . Mushroom-type f r a c t u r e s c o n s i s t i n g o f a p r i m a r y v e r t i c a l c r a c k t o p p e d b y a s e c o n d a r y h o r i z o n t a l f r a c t u r e l i k e a c a p (EVANS, HOLZHAUSEN & WOOD 1982; FITZ-PATRICK, KARR & O'SHEA 1985; SMITH, REN, SORRELS & TEUFEL 1985) a l s o o c c u r o c c a s i o n a l l y i n d e p t h r a n g e s t h a t a r e t r a n s i t i o n a l f o r b o t h b a s i c c r a c k t y p e s ( c f . s e c t i o n 6 . 2 . 3 . 4 . ) as w e l l as due t o c h a n g i n g c l o s u r e s t r e s s c o n d i t i o n s b y s t e a m - d r i v e enhanced o i l r e c o v e r y o p e r a t i o n s ( c f . sect i o n 4 . 9 . 4 . 2 . ) . N e t w o r k s o f o r t h o g o n a l f r a c t u r e s c o n s i s t i n g o f two i n t e r l o c k i n g systems b e i n g o r i e n t e d p e r p e n d i c u l a r l y t o e a c h o t h e r c a n be c r e a t e d b y a l t e r e d stress f r a c t u r i n g ( c f . section 4.9.4.1.3.1.). Horizontal f r a c t u r e s i n shallow d e p t h a r e r e c o g n i z e d by p a r t i n g g r a d i e n t s g r e a t e r t h a n 1. P r o p p a n t s e l e c t i o n (STEANSON, ELBEL & WENDORFF 1979; CLARK 1983, DOWELL SCHLUMBERGER 1985, MONTGOMERY & STEANSON 1985, NORMAN & FAST 1985, NORTON-ALCOA PROPPANTS 1985, PHILLIPS & ANDERSON 1985, WESTERN PETROLEUM 1985, STANDARD O I L PROPPANTS 1986; c f . s e c t i o n 1 . 4 . ) f o r h y d r a u l i c f r a c t u r e t r e a t m e n t s i s i n f l u e n c e d by a l l t h e m e n t i o n e d f e a t u r e s . I t s k e y s i g n i f i c a n c e i n o p e r a t i o n d e s i g n as a consequence o f t h e c o n s i d e r a b l e s h a r e w h i c h p r o p p a n t c o s t has i n t h e t o t a l MHF expenses r e q u i r e s d e t a i l e d d i s c u s s i o n o f t h e i n d i v i d u a l p r o p p a n t p r o p e r t i e s and p o s s i b i l i t i e s o f p r o p p a n t a p p l i c a t i o n as f o l l o w s .
I
MINERALOGICAL A N D
P R O P P A N T
C O M P O S I T I O N
CHEMICAL COMPOSITION OF P R 0 P P A NTS FOR HYDRAULIC FRACTURING
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STRENGTH l ow s trengt h
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quartz sand l o w - d e n s i t v a lu m in a
strength
h ig h - d e n s it y a lu m in a o x id e and s i l i c a t e proppants
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a lu m in a o x id e p r o p p a n t s
ynthetic strength
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zirconia-silicate proppants
Tab. 2
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C H E M I S T R Y
32
1.3. Proppant types and g r a i n sizes Presently basically five different proppant types for hydraulic fracturing and gravel packing are available in the hydrocarbon stimulation market in various grain sizes and for different prices from several manufacturers : natural quartz sand, synthetic intermediate-strength low-density alumina silicate (ceramic) proppant, intermediate-strength high-density alumina oxide and silicate proppant, high-strength high-density bauxite proppant and high-strength low-density zirconia-silicate proppant (arranged in an order of progressively increasing specific gravity, closure stress resistivity and proppant cost; the main characteristics are compiled in COMALCO 1983, KIM 1983, SEPR 1983, HOLDITCH 1984, DOWELL SCHLUMBERGER 1985, NORMAN & FAST 1985, NORTON-ALCOA PROPPANTS 1985, WESTERN PETROLEUM 1985, BROWN & MUCH 1986, HALLIBURTON 1986, MESSINA 1986, STANDARD O I L PROPPANTS 1986; cf. tabs. 2 - 4). The current proppant prices are increasing in the mentioned sequence from cheap natural quartz sand via moderately expensive synthetic alumina oxide and silicate proppants to expensive artificial zirconia-silicate proppants. Following an overview of the provenance of natural sand and synthetic proppants and the discussion of the various mechanical and chemical proppant properties, aspects of proppant selection for application unde a spectrum of different reservoir conditions are evaluated. The section below includes the outline of mineralogical composition and closure stress resistiv ty, chemical composition and acid solubility, grain sizes and conductivity per US $, and grain shapes of the presently commercially available proppant types
1.3.1. Provenance o f sand and proppants Natural sand and synthetic proppants are exploited and produced, respectively, in various places in the world. While traditionally both types of materials chiefly derived from the USA, in the last decade increasing contributions are also coming from other countries and continents.
1.3.1.1. Natural sand High-quality sands (known as Ottawa Sand and other sands with regional names; WESTERN PETROLEUM 1985) are mined traditionally in the USA, with the main provenances being Jordan Formation in Minnesota, Hickey Formation in Central Texas, St. Peters Formation in Ottawa/Northern Illinois, and Galesville and Ironton Formations in Wisconsin, and smaller quantities also deriving from Eastern Arizona, Central Colorado, Ohio and Northern Carolina (WATERS 1980). Many of the high-quality sands represent ancient aeolian dune sands where effective windblown transport, sorting and deposition has given rise to high quartz content, negligible amount of fines, high grain roundness and sphericity, and very good sorting with narrow range of grain size distribution. Minor and negligible or larger and significant differences exist between excellent quality sands and inferior-quality sands depending on provenance and composition, with the latter group being restricted in application to shallower reservoirs with lower closure stress regimes, whereas the former including first of all Ottawa Sand had in earlier years prior to invention of intermediate- and high-strength ceramic proppants also been applied in deep well fracturing. Brady Sand or Hickory Sand coming from Texas is the most frequently applied proppant in oil and gas industry, with millions of tons having been pumped during the last decades, because in contrast to recent years, the distribution of the top-quality Ottawa Sand formerly suffered from limited supply (SINCLAIR 1980) and shortages in meeting the enormous demand of the boosting oil and gas industry after the first oil price jump in the early 1970's required the delivery of every sand available almost regardless of quality in the hottest pe-
33 r i o d s o f hydrocarbon w e l l s t i m u l a t i o n d u r i n g t h e golden o i l and gas y e a r s . Other n a t u r a l sands o f d i f f e r i n g q u a l i t y come f r o m v a r i o u s European c o u n t r i e s and a r e d e r i v i n g i n t h e M i d d l e and F a r East a l s o f r e q u e n t l y f r o m l o c a l sources. While t h e f i r s t sand used as f r a c t u r e proppant i n t h e f i r s t t r e a t m e n t s i n 1947 and f o l l o w i n g y e a r s was j u s t t a k e n o u t o f Arkansas r i v e r and pumped downhole, l a t e r i n c r e a s i n g q u a l i t y demand r e q u i r e d more and more s o p h i s t i c a t e d mec h a n i c a l p r o c e s s i n g o f t h e n a t u r a l sands f o r t h e purposes o f h y d r a u l i c f r a c t u r e p r o p p a n t and e s p e c i a l l y sand c o n t r o l p a c k i n g g r a v e l where q u a l i t y standards demand p r i m a r i l y f o r narrow g r a i n s i z e d i s t r i b u t i o n , h i g h c o n t e n t o f p r e f e r a b l y m o n o c r y s t a l l i n e q u a r t z , and e x c e l l e n t roundness and s p h e r i c i t y .
1.3.1.2. Synthetic proppants I n t e r m e d i a t e - s t r e n g t h alumina o x i d e and s i l i c a t e ( c e r a m i c ) proppants a r e m a i n l y manufactured i n t h e USA, w i t h minor amounts d e r i v i n g f r o m B r a z i l and o n l y v e r y l i m i t e d q u a n t i t i e s o f h i g h - d e n s i t y m a t e r i a l h a v i n g s o f a r been produced i n Hungary t h a t have n o t y e t reached t h e stage o f economical a p p l i c a t i o n beyond l o c a l f i e l d t e s t s o f t a i l - i n n a t u r e . H i g h - s t r e n g t h s i n t e r e d b a u x i t e p r o p p a n t s a r e n o t o n l y made i n t h e USA, b u t s l o w l y i n c r e a s i n g q u a n t i t i e s a r e coming f r o m A u s t r a l i a and B r a z i l , and t r i a l r u n s on more o r l e s s l a b o r a t o r y s c a l e w i t h o u t y e t r e a c h i n g f i e l d a p p l i c a t i o n have been c a r r i e d o u t i n China ( c f . s e c t i o n 2 . 4 . 6 . 4 . and t a b . 3 ) . While h i g h - s t r e n g t h b a u x i t e proppants i n USA, B r a z i l and China a r e e x c l u s i v e l y made by m i x - p e l l e t i z a t i o n and s i n t e r i n g o f t h e a r t i f i c i a l l y formed p e l l e t s , t h e A u s t r a l i a n b a u x i t e proppants d e r i v e f r o m s c r e e n i n g and c o n d i t i o n i n g o f n a t u r a l spheres coming f r o m a p i s o l i t h i c b a u x i t e d e p o s i t which f o l l o w i n g s i e v i n g and chemical p r o c e s s i n g a r e s i n t e r e d ( c f . s e c t i o n 1 . 4 . 9 . 1 . 3 . ) . H i g h - s t r e n g t h f u sed z i r c o n i a - s i l i c a t e p r o p p a n t s f i n a l l y a r e manufactured i n France. Several o t h e r s u p p l i e r s have s o f a r o n l y s u b m i t t e d l a b o r a t o r y - s c a l e samples f o r p r o p p a n t t e s t i n g . As a r e s u l t o f t h e wide v a r i e t y o f p r o p p a n t t y p e s t h a t a r e a v a i l a b l e today i n t h e market f r o m v a r i o u s producers, t h e American P e t r o l e u m I n s t i t u t e ( A P I ) has e s t a b l i s h e d recommended t e s t procedures f o r s e v e r a l p r o p p a n t p r o p e r t i e s t o d i s t i n g u i s h q u a l i t y and u s e f u l n e s s o f each p r o p p a n t (AMERICAN PETROLEUM I N S T I T U T E 1985).
1.3.2. Mineralogical composition and closure stress resistivity Concerning m i n e r a l o g i c a l c o m p o s i t i o n and c l o s u r e s t r e s s r e s i s t i v i t y o f p r o p p a n t s , d i s t i n c t i o n can m a i n l y be made between alumina o x i d e and s i l i c a t e p r o p p a n t s and z i r c o n i a - s i l i c a t e proppants ( c f . t a b . 2 ) .
1.3.2.1. Alumina oxide and silicate proppants I n terms o f m i n e r a l o g i c a l composition, n a t u r a l sand c o n s i s t s o f m a c r o c r y s t a l l i n e q u a r t z which has no c r y s t a l l o g r a p h i c a l cleavage, b u t a s h e l l y breakage i n t o b o t h l a r g e r fragments and s m a l l e r s p l i n t e r s a t h i g h e r c l o s u r e s t r e s s e s e x ceeding 3,000 - 5,000 p s i . Resin-coated sand may be s t a b l e up t o 7,000 - 8,000 p s i closure stress ( c f . section 1.2.6.). Intermediate-strength low-density alumina s i l i c a t e ( c e r a m i c ) proppants a r e m a i n l y b u i l t up o f m i c r o - t o c r y p t o c r y s t a l l i n e m u l l i t e and c r y s t o b a l i t e d e r i v i n g f r o m c l a y as raw m a t e r i a l . I n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants a r e composed o f m i c r o - t o c r y p t o c r y s t a l l i n e corundum and m u l l i t e coming f r o m a raw m a t e r i a l m i x t u r e o f b a u x i t e and c l a y . H i g h - s t r e n g t h h i g h - d e n s i t y b a u x i t e proppants a r e almost e x c l u s i v e l y c o n s i s t i n g o f corundum w i t h l i t t l e q u a n t i t i e s o f m u l l i t e d e r i v i n g f r o m s i n t e r i n g o f n e a r l y p u r e b a u x i t e ( c f . s e c t i o n 1 . 4 . 9 . 1 . ) . A s a consequence o f c o m p o s i t i o n o f
34 i n t e r l o c k i n g t i n y c r y s t a l s w h i c h g i v e much b e t t e r t o u g h n e s s t h a n a g l a s s y mater i a l and p r o v i d e some d e f o r m a b i l i t y u n d e r l o a d p r i o r t o f a i l u r e and c r u s h i n g , the t h r e e l a t t e r proppant types are predominantly breaking i n t o halves, quart e r s and s u c c e s s i v e l y f i n e r f r a g m e n t s when b e i n g s u b j e c t e d t o c l o s u r e s t r e s s e s beyond 8,000 p s i ( 3 % c r u s h o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y a l u m i n a s i l i cate proppants); 10,000 p s i ( 4 % c r u s h o f i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s ) ; and 15,000 p s i ( 5 % c r u s h o f h i g h s t r e n g t h h i g h - d e n s i t y b a u x i t e p r o p p a n t s ) , w i t h r e a s o n a b l e amounts o f f i n e r s p l i n t e r s o n l y o r i g i n a t i n g d u r i n g advanced f a i l u r e as a consequence o f e x p o s i t i o n t o excessive closure stresses ( c f . section 1 . 4 . 2 . ) . Intermediate-strength p r o p p a n t s c a n be u s e d u p t o c l o s u r e s t r e s s e s o f 12,000 - 13,000 p s i and s i n c e higher pressures are n o t occurring t h a t frequently, t h i s proppant type s u i t e w i l l be i n f o c u s f o r a p p l i c a t i o n d u r i n g t h e coming y e a r s . S i n t e r e d b a u x i t e has e x c e l l e n t c o n d u c t i v i t y up t o 20,000 p s i ( 1 3 9 MPa) c l o s u r e s t r e s s (CUTLER, ENN I S S , JONES & SWANSON 1 9 8 5 ) . N a t u r a l sand becomes a l r e a d y u n s t a b l e a t 3,000 - 5,000 p s i c l o s u r e s t r e s s d e p e n d i n g on q u a l i t y and g r a i n s i z e , b u t i t s r e s i s t i v i t y c a n be e x t e n d e d u p t o more t h a n 10,000 p s i b y r e s i n e n v e l o p i n g , and s i m i l a r l y , r e s i n - c o a t e d s i n t e r e d b a u x i t e can t o l e r a t e c l o s u r e s t r e s s e s up t o 30,000 p s i (SINCLAIR, GRAHAM & S I N CLAIR 1983; c f . s e c t i o n 1 . 2 . 6 . ) .
1.3.2.2. Zirconia-silicate proppants H i g h - s t r e n g t h l o w - d e n s i t y z i r c o n i a - s i l i c a t e proppants f i n a l l y a r e b u i l t up o f m i c r o - t o c r y p t o c r y s t a l l i n e m o n o c l i n i c and c u b i c z i r c o n i a t h a t i s embedded i n a m o d i f i e d a l u m i n a s i l i c a t e amorphous o r g l a s s y phase o r i g i n a t i n g b y f u s i n g o f z i r c o n and q u a r t z . The l a t t e r b r i t t l e f u s e d p r o d u c t ( c f . s e c t i o n 1 . 4 . 9 . 2 . ) behaves l i k e a g l a s s when s u b j e c t e d t o h i g h e r p r e s s u r e s and t h u s a l m o s t a t once c a t a s t r o p h i c a l l y f a i l s by breakage i n t o e x c l u s i v e l y t i n y s p l i n t e r s ( c f . s e c t i o n 1 . 4 . 2 . ) a t c l o s u r e s t r e s s e s beyond a b t . 15,000 p s i ( 2 . 5 % c r u s h ; a l l c r u s h v a l u e s r e f e r t o p r o p p a n t s o f 2 0 / 4 0 mesh g r a i n s i z e ; t h e d a t a i n t h i s c h a p t e r a r e c o m p i l e d f r o m BJ-HUGHES 1982, COMALCO 1983, K I M 1983, SEPR 1983, NORTON-ALCOA PROPPANTS 1985, BROWN & MUCH 1986; MESSINA 1986, 1987; STANDARD O I L PROPPANTS 1 9 8 6 ) . Thus c o l l a p s e o f z i r c o n i a - s i l i c a t e p r o p p a n t s i s p h y s i c a l l y s i m i l a r t o t h a t o f g l a s s beads w h i c h a l s o c r u s h a t once i n t o f i n e powder ( c f . s e c t i o n 1 . 2 . 3 . 1 . ) a l t h o u g h t a k i n g p l a c e a t a much more e l e v a t e d c l o s u r e s t r e s s n i v e a u i n c a s e o f z i r c o n i a s p h e r e s . Up t o 14,000 p s i ( 9 7 MPa), t h e r e a r e e s s e n t i a l l y no c o n d u c t i v i t y d i f f e r e n c e s between z i r c o n i a - s i l i c a t e p r o p p a n t s and s i n t e r e d b a u x i t e p r o p p a n t s (CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) .
I n c o n t r a s t t o t h e a f o r e m e n t i o n e d b o u n d a r y c l o s u r e s t r e s s e s , ROBINSON, HOLD I T C H & WHITEHEAD ( 1 9 8 6 ) c o n c l u d e t h e i r e v a l u a t i o n o f f r a c t u r e damage and i t s p r e v e n t i o n w i t h recommendation o f maximum a l l o w a b l e c l o s u r e s t r e s s e s o f 4,000 p s i f o r sand, 7,000 p s i f o r i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s and 10,000 p s i f o r h i g h - s t r e n g t h p r o p p a n t s . In c o m p a r i s o n w i t h m o s t r e s u l t s o f p r o p p a n t t e s t i n g , however, t h e v a l u e s f o r t h e l a t t e r two p r o p p a n t t y p e s a r e a s s e s s e d as b e i n g c o n s i d e r a b l y t o o p e s s i m i s t i c , b u t c e r t a i n l y when d e s i g n i n g a s u i t a b l e p r o p p a n t t y p e f o r l o n g p r o d u c t i o n h i s t o r y u p t o 20 - 30 y e a r s , i t s h o u l d be p r e f e r r e d t o go a b s o l u t e l y on t h e s a f e s i d e and t o b e t t e r s e l e c t a more e x p e n s i v e p r o p p a n t type t h a t i s r e s i s t i n g t o higher closure stresses r a t h e r than t o r i s k f r a c t u r e damage due t o p r o p p a n t f a i l u r e u n d e r c o n d i t i o n s o f e x c e s s i v e drawdown i n t h e l a t e stage o f r e s e r v o i r d e p l e t i o n .
1.3.3. Chemical composition and acid solubility C o n c e r n i n g c h e m i c a l c o m p o s i t i o n and a c i d s o l u b i l i t y , d i s t i n c t i o n has t o be made between n a t u r a l sand and o t h e r p r o p p a n t s c o n t a i n i n g f r e e s i l i c a i n l a r g e r amounts, and p r o p p a n t t y p e s i n c l u d i n g o n l y m i n o r s h a r e s o f f r e e s i l i c a o r hav i n g a l m o s t t o t a l l y n o n - s i l i c a c o n s t i t u e n t s ( c f . t a b s . 2 and 3 ) .
MECHANICAL PROPERTfES AND PR 0DUCT I0N/ M A NUFACT UR I NG OF PROPPANTS
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1 . 3 . 3 . 1 . Chemica 1 composition N a t u r a l q u a r t z sand i s a l m o s t e x c l u s i v e l y c o n s i s t i n g o f SiO2 and t h u s has a s p e c i f i c g r a v i t y o f 2.62 - 2 . 6 5 g/cm3. S y n t h e t i c i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y a l u m i n a s i l i c a t e p r o p p a n t s a r e b u i l t u p o f 4 6 . 0 - 5 0 . 0 % SiO2, 4 6 . 0 - 5 1 . 0 % A1203, 0 . 7 - 1.1 % Fe203 and 1 . 5 - 2 . 5 % T i 0 2 , w i t h t h e s p e c i f i c g r a v i t y bei n g 2 . 7 0 - 2.75 g/cm3 w h i c h i s o n l y s l i g h t l y more t h a n t h a t o f s a n d . I n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s a r e composed o f 1 2 . 0 - 1 7 . 0 % SiO2, 7 3 . 0 - 7 8 . 0 % A1203, 4 . 5 - 7 . 0 % Fe203 and 2 . 5 - 4 . 0 % T i 0 2 , and have a s p e c i f i c g r a v i t y o f 3 . 1 5 - 3 . 2 7 g/cm3 w h i c h i s c o n s i d e r a b l y h i g h e r t h a n t h a t o f sand and i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y a l u m i n a s i l i c a t e p r o p p a n t s and i s p a r t i a l l y s i m i l a r o r even more e l e v a t e d t h a n t h a t o f h i g h s t r e n g t h l o w - d e n s i t y z i r c o n i a - s i l i c a t e p r o p p a n t s , b u t s t i l l much l o w e r t h a n t h a t o f almost pure bauxite-based high-strength high-density proppants. I n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s a r e t h u s r e presenting a t r a n s i t i o n a l proppant type l i n k i n g several classes o f products b o t h i n t e r m s o f p h y s i c a l p r o p e r t i e s and p r a c t i c a l a p p l i c a t i o n s w h i c h i s u n d e r l i n e d b y t h e f a c t t h a t t h i s proppant t y p e i s t h e h i t h e r t o most f r e q u e n t l y u t i l i zed s y n t h e t i c p r o p p a n t f o r o i l and gas r e s e r v o i r h y d r a u l i c s t i m u l a t i o n . H i g h - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e p r o p p a n t s a r e t y p i c a l l y made up o f - 6 . 0 % S i 0 2 , 8 5 . 0 - 9 0 . 0 % A1203, 4 . 0 - 7 . 0 % Fe203 and 3 . 0 - 4 . 0 % Ti02, w i t h t h e s p e c i f i c g r a v i t y o f 3 . 6 - 3 . 8 g/cm3 b e i n g t h e h i g h e s t o f a l l s y n t h e t i c proppants. High-strength low-density z i r c o n i a - s i l i c a t e proppants c o n s i s t t y p i c a l l y o f 4 4 . 0 % SiO2, 7 . 5 % A1203, 0 . 3 % Fe203, 6 . 5 % T i 0 2 and 4 1 . 5 % Z r 0 2 , w i t h i t s s p e c i f i c g r a v i t y o f 3 . 1 5 - 3 . 1 7 b e i n g even l o w e r t h a n t h a t o f some i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s ( t h e d a t a a r e c o m p i l e d f r o m BJ-HUGHES 1982, COMALCO 1983, K I M 1983, SEPR 1983, NORTON-ALCOA PROPPANTS 1985, BROWN & MUCH 1986, COBB & FARRELL 1986; MESSINA 1986, 1987; STANDARD OIL PROPPANTS 1 9 8 6 ) .
3.0
1.3.3.2.
Acid solubility
In c o n t r a s t t o q u a r t z sand and g l a s s beads w h i c h a r e q u i t e s o l u b l e i n h o t formation b r i n e s , t h e s y n t h e t i c alumina- o r zirconia-based proppants are c o n s i d e r a b l y more r e s i s t a n t . The s o l u b i l i t y i n 1 2 / 3 HCl/HF a c i d ( 3 0 m i n a t 150 O F ) i s f o r i n t e r m e d i a t e - s t r e n g t h low-density alumina s i l i c a t e proppants 1.5 - 2.0 % w e i g h t l o s s , i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s 3 . 3 - 4 . 1 %, h i g h - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e p r o p p a n t s 2 . 0 - 3 . 0 and h i g h - s t r e n g t h l o w - d e n s i t y z i r c o n i a - s i l i c a t e p r o p p a n t s 2 . 5 - 3 . 9 %, i n cont r a s t t o n a t u r a l sand where t h e s o l u b i l i t y r a n g e i s 0 . 5 - 7 . 0 % d e p e n d i n g on m i n e r a l o g i c a l and c r y s t a l l o g r a p h i c a l c o m p o s i t i o n and s t r u c t u r e o f t h e q u a r t z g r a i n s , and b e i n g h i g h e r i n a l c a l i n e e n v i r o n m e n t ( t h e d a t a a r e t a k e n f r o m t h e a f o r e m e n t i o n e d s o u r c e s . CHEUNG ( 1 9 8 5 ) , however, comes t o t h e o p p o s i t e c o n c l u s i o n t h a t e s p e c i a l l y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s and a l s o s i n t e r e d baux i t e a r e s e r i o u s l y d i s s o l v e d and m e c h a n i c a l l y weakened i n a c i d e n v i r o n m e n t , whereas n a t u r a l q u a r t z sand i s t h e most r e s i s t a n t m a t e r i ? ’ ’ : I t h i s p a r t i c u l a r c h e m i c a l m i l i e u , t h e r e b y h a v i n g s t r o n g i m p a c t on p l a n n i n g L : a c i d i z a t i o n o f f r a c t u r e s o r even j o i n t o p e r a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g ( c f . a l s o s e c t i o n 4 . 5 . 1 . . 4 . 5 . 5 . and 4 . 8 . 9 . ) .
1 . 3 . 4 . Grain sizes and conductivity per US B The v a r i o u s p r o p p a n t t y p e s a r e a p p l i e d a c c o r d i n g t o t h e i r d i f f e r e n t r e s i s t i v i t y a g a i n s t c r u s h i n g i n d i f f e r e n t d e p t h s and t h e i r a b i l i t y t o w i t h s t a n d a g g r e s s i v e formation brines o f d i f f e r e n t s a l i n i t y , w i t h proppant s e l e c t i o n n o t o n l y b e i n g i n f l u e n c e d b y a s p e c t s o f c o n d u c t i v i t y and p e r m e a b i l i t y as w e l l as l o n g t e r m s t a b i l i t y o f t h e p r o p p a n t package ( w h i c h i n b i g f r a c t u r e j o b s i s t h e m a j o r s h a r e o f t h e w h o l e c o s t o f t h e o p e r a t i o n ) , b u t a l s o by t h e p r i c e w h i c h r a i s e s
37 w i t h i n c r e a s i n g c l o s u r e s t r e s s r e s i s t i v i t y o f t h e p r o p p a n t . C a r e f u l economical e v a l u a t i o n i s t h e r e f o r e necessary i n e v e r y i n d i v i d u a l case by an assessment o f c o n d u c t i v i t y p e r US $ ( c f . s e c t i o n 1 . 4 . 1 2 . 2 . ) t h a t i s o b t a i n e d when choosing a more expensive proppant t y p e o f h i g h e r s t a b i l i t y and t h u s b e t t e r l o n g - t e r m p e r formance i n view o f p e r m e a b i l i t y . C o n d u c t i v i t y i s p o t e n t i a l l y accentuated by sel e c t i n g a c o a r s e r g r a i n s i z e , compared w i t h i n c r e a s i n g pumping c o s t w i t h r i s i n g s p e c i f i c g r a v i t y and g r a i n s i z e o f t h e proppant, i n c r e a s i n g s c r e e n o u t r i s k when u s i n g h e a v i e r and c o a r s e r proppants, and i n c r e a s i n g equipment a b r a s i o n by h a r d e r proppants (STEANSON, ELBEL & WENDORFF 1979; HOLDITCH 1984, DOWELL SCHLUMBERGER 1985, MONTGOMERY & STEANSON 1985, WESTERN PETROLEUM 1985, HALLIBURTON 1986). Some aspects o f c o n d u c t i v i t y improvement by g r a i n s i z e s e l e c t i o n v s . i n c r e a s i n g s c r e e n o u t r i s k and c o m m e r c i a l l y a v a i l a b l e p r o p p a n t g r a i n s i z e s a r e b r i e f l y o u t l i n e d as f o l l o w s .
1 . 3 . 4 . 1 . Conductivity improvement by grain
size selection vs. increasing screenout risk
G r a i n s i z e s e l e c t i o n o f proppants a l s o has c o n s i d e r a b l e s i g n i f i c a n c e f o r i m p r o v i n g t h e r e s u l t i n g c o n d u c t i v i t y i n t h e f r a c t u r e d r e s e r v o i r . The l a r g e r t h e g r a i n s i z e and t h e h i g h e r t h e s p e c i f i c p r o p p a n t g r a v i t y , t h e g r e a t e r i s t h e r i s k o f m a t e r i a l screenout b e i n g an o n l y reduced uptake o f t h e f o r m a t i o n w i t h r e s p e c t t o design, w i t h a screenout b e i n g a b l e t o be assessed e i t h e r n e g a t i v e l y as i n d i c a t i n g p a r t i a l f a i l u r e o r p o s i t i v e l y as t e s t i f y i n g t o achievement o f maximum proppant s a t u r a t i o n and uptake p o t e n t i a l o f t h e f o r m a t i o n . Screenouts may be b e n e f i c i a l by i n d i c a t i n g t h a t t h e f r a c t u r e i s f u l l y packed f r o m t h e t i p s backwards i n d i r e c t i o n t o t h e w e l l b o r e and should have a h i g h c o n d u c t i v i t y , whereas on t h e o t h e r hand, low q u a n t i t i e s o f i n j e c t e d p r o p p a n t s may r e f l e c t uneven proppant d i s t r i b u t i o n i n t h e c r a c k and d i s c o n t i n u o u s c o n c e n t r a t i o n i n t h e s u r r o u n d i n g s o f t h e w e l l , t h e r e b y n o t matching t h e attempted f r a c t u r e l e n g t h (KOHLHAAS 1982). Aspects o f s c r e e n o u t f a i l u r e a r e a l s o d i s c u s s e d i n s e c t i o n s 4 . 2 . 2 . 2 . 3 . and 6 . 2 . 4 . 2 . 1 . Some more advanced techniques such as t i p screenout f r a c t u r i n g (SMITH, MILLER & HAGA 1985; c f . s e c t i o n 4 . 5 . 4 . 4 . 1 . ) even aim a p r i o r i on l i m i t e d and cont r o l l e d screenout. Together w i t h s o p h i s t i c a t e d s e l e c t i o n o f adequate p r o p p a n t t y p e and g r a i n s i z e , however, a l s o s e r i o u s a t t e n t i o n has t o be p a i d t o p r o p e r c h o i c e o f f r a c t u r i n g f l u i d , because i f h i g h - q u a l i t y proppants a r e used t o g e t h e r w i t h u n s u i t a b l e f l u i d s , t h i s can r u i n c o n s i d e r a b l e amounts o f r e s e r v o i r permeab i l i t y and f r a c t u r e c o n d u c t i v i t y . The s i g n i f i c a n c e o f h i g h - e f f i c i e n c y s t i m u l a t i o n f l u i d s f o r t h e s u c c e s s f u l t r e a t m e n t o f high-temperature, low-permeability r e s e r v o i r s i s d i s c u s s e d by BAUMGARTNER, PARKER, WILLIAMS & WOODROOF (1983); HARMS, GOSS & PAYNE (1984) and WALSER ( 1 9 8 5 ) . I n f l u e n c e s o f f r a c t u r i n g f l u i d p r o p e r t i e s on proppant t r a n s p o r t and c r a c k h e i g h t behaviour a r e d i s c u s s e d by MORALES & ABOU-SAYED (1985) and ACHARYA (1986; c f . s e c t i o n 4 . 3 . 4 . ) . L a r g e r p r o p p a n t g r a i n s i z e s and/or h i g h e r - q u a l i t y p r o p p a n t t y p e s p r o v i d e g r e a t e r f r a c t u r e f l o w c a p a c i t y t h a t i s n o t as a f f e c t e d by r e l e a s e d f o r m a t i o n f i n e s as s m a l l e r l o w e r - q u a l i t y p r o p p a n t s t h a t a r e c r u s h i n g due t o i n c r e a s i n g c l o s u r e p r e s s u r e (CROSS & SHELDON 1977). C o n d u c t i v i t y p e r US $ ( o r n e t p r e s e n t v a l u e p e r US $; B R I M 1986, ANDERSON & PHILLIPS 1987) t h a t i s o b t a i n e d when s e l e c t i n g a p a r t i c u l a r proppant t y p e and g r a i n s i z e has t o be c a l c u l a t e d a g a i n s t t o t a l expenses o f t h e h y d r a u l i c f r a c t u r i n g t r e a t m e n t and p a y - o u t by i n c r e a s e d c u m u l a t i v e hydrocarbon p r o d u c t i o n w i t h i n a reasonable t i m e a f t e r s t i m u l a t i o n ( t h i s economical m o d e l l i n g i s f a c i l i t a t e d by v a r i o u s computer programs f o r proppant c h o i c e and p r e d i c t i o n o f f r a c t u r i n g o p e r a t i o n r e s u l t s ; AGARWAL, CARTER & POLLOCK 1979; CHENG 1984; COB8 1985, 1986). F o l l o w i n g t h e g u i d e l i n e o f proppant c o s t p e r u n i t c o n d u c t i v i t y i s i n f a c t t h e most s t r a i g h t f o r w a r d way o f p r o p p a n t s e l e c t i o n (ANDERSON & PHILLIPS 1986; c f . s e c t i o n 1 . 4 . 1 2 . ) .
38
1 . 3 . 4 . 2 . Comnercially a v a i l a b l e proppant g r a i n sizes The d i f f e r e n t n a t u r a l and s y n t h e t i c p r o p p i n g m a t e r i a l s a r e a v a i l a b l e i n t h r e e s t a n d a r d mesh s i z e s f o r h y d r a u l i c f r a c t u r i n g : 2 0 / 4 0 ( t h e m o s t a b u n d a n t l y u s e d g r a i n s i z e ) , 1 6 / 2 0 and 1 2 / 2 0 . F o r g r a v e l pack a p p l i c a t i o n , sometimes a l s o 30/50, 4 0 / 6 0 o r 40/70, 100 and even 70/140 a r e o f f e r e d (WESTERN PETROLEUM 1 9 8 5 ) , and f o r s p e c i a l t a i l - i n j o b s o r h i g h - v i s c o s i t y o i l r e s e r v o i r t r e a t m e n t s a l s o 16/30, 1 0 / 1 6 o r e v e n 8 / 1 2 and 6 / 1 0 c o u l d be o b t a i n e d (some a s p e c t s o f t h e a p p l i c a t i o n o f c o a r s e r g r a i n s i z e s a r e d i s c u s s e d b y SCHWARTZ 1969; GRUESBECK, SALATHIEL & ECHOLS 1979; BARBER & THEMIG 1985, ROBINSON 1985; PARKER, ADAMS & LIANKUI 1986; BARBY & BARBEE 1987; c f . s e c t i o n 1 . 5 . 1 . 1 . 2 . and t a b . 1 ) . Sand g r a i n s i z e s o t h e r t h a n 2 0 / 4 0 o n l y r e p r e s e n t a b t . 15 % o f t h e t o t a l q u a n t i t y o f p r o p p a n t sand u s e d (WATERS 1 9 8 0 ) . M o s t o f t h e r a r e r g r a i n s i z e s have o n l y been o f f e r e d f r o m sand s u p p l i e r s w h i c h have t h e e a s y way o f m a n u f a c t u r i n g b y s i e v i n g w i t h o u t f u r t h e r t r e a t m e n t , whereas m o s t o f t h e p r o p p a n t p l a n t s need h i g h e r q u a n t i t i e s t o make e c o n o m i c a l l y f e a s i b l e p r o d u c t i o n r u n s . T h i s i s t h e e x p l a n a t i o n why so f a r 100 mesh i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p p a n t has n o t been p r o d u c e d a l t h o u g h i t w o u l d have been needed a l r e a d y a t l e a s t i n a c o u p l e o f c a s e s ( c f . s e c t i o n 4 . 8 . 8 . 3 . 2 . 3 . 5 . ) . The common g r a i n s i z e s o f i n t e r m e d i a t e - t o h i g h - s t r e n g t h p r o p p a n t m a n u f a c t u r i n g a r e 12/20, 16/20, 20/40 and 40/60, w i t h a l l t h e o t h e r f i n e r o r c o a r s e r g r a d e s b e i n g e x c e p t i o n s t h a t a r e p a r t i a l l y o n l y a v a i l a b l e as s p e c i a l o r d e r and a t a h i g h e r p r i c e . E x c e p t i o n s o f t h e l a t t e r c o n s t e l l a t i o n a r e o n l y p r o p p a n t compan i e s t h a t use a n a t u r a l p i s o l i t h i c i n s t e a d o f an e a r t h y b a u x i t e raw m a t e r i a l (COMALCO 1983, 1986; c f . s e c t i o n 1 . 4 . 9 . 1 . 3 . ) o r where a l a r g e r a n g e o f g r a i n s i z e s o r i g i n a t e s d u r i n g t h e e l e c t r i c a l f u s i n g process i n c o n t r a s t t o t h e s i n t e r i n g method (SARDA 1981, SEPR 1983, M E S S I N A 1986; c f . s e c t i o n 1 . 4 . 9 . 2 . ) , w i t h i n b o t h cases o n l y s i e v i n g b e i n g t h e a d d i t i o n a l p r o d u c t i o n s t e p f o r e x o t i c g r a i n sizes.
1 . 3 . 5 . Grain shapes I n t h e e a r l y s t a g e o f h i g h - s t r e n g t h p r o p p a n t a p p l i c a t i o n i n deep gas r e s e r v o i r f r a c t u r i n g , m a i n l y a n g u l a r p r o p p a n t s were o f f e r e d a t t h e m a r k e t w h i c h were s i m p l e b y - p r o d u c t s o f a b r a s i v e m a n u f a c t u r i n g (HARBEN 1978; c f . t a b s . 1 and 4 as w e l l as s e c t i o n 1 . 2 . 4 . ) . I n c r e a s i n g c o n d u c t i v i t y demand b y t h e c u s t o m e r s l a t e r l e d t o i n t r o d u c t i o n o f p r o g r e s s i v e l y b e t t e r rounded p a r t i c l e s o f b o t h n a t u r a l q u a r t z sand and s y n t h e t i c a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s . The b e s t r o u n d n e s s and s p h e r i c i t y i s p r o v i d e d b y z i r c o n i a - s i l i c a t e p r o p p a n t s w h i c h t o g e t h e r w i t h t h e i r smooth s u r f a c e a l m o s t r e p r e s e n t i d e a l g l o b u l e s . P r o p p a n t s t r e n g t h i s g e n e r a l l y a f u n c t i o n o f s p h e r i c i t y f o r a g i v e n s i z e d i s t r i b u t i o n o f t h e same g r a d e o f m a t e r i a l (STEANSON, ELBEL & WENDORFF 1 9 7 9 ) . W h i l e d u r i n g t h e l a s t t e n y e a r s l a r g e amounts o f w e l l - r o u n d e d m a t e r i a l have been consumed f o r h y d r a u l i c s t i m u l a t i o n and g r a v e l p a c k i n g , LARSEN & SMITH ( 1 9 8 5 ) p r e s e n t a r e n a i s s a n c e s t u d y o f a n g u l a r f r a c t u r i n g sand by e m p h a s i z i n g new c o n d u c t i v i t y f o u n d i n b l e n d s o f a n g u l a r and r o u n d g r a i n s ( c f . s e c t i o n 1.4.11.3.). I n t e r m i x i n g o f a n g u l a r g r a i n s may a l s o be a s u i t a b l e means f o r i n c r e a s i n g t h e f r i c t i o n a n g l e o f t h e p r o p p a n t pack t o a v o i d i t s f l o w o u t o f t h e f r a c t u r e a t t h e mouth o f t h e c r a c k n e a r t h e w e l l b o r e ( c f . s e c t i o n 1 . 4 . 3 . ) as w e l l as t o i n c r e a s e s t a b i l i t y o f g r a v e l p l u g g i n g p e r f o r a t i o n t u n n e l s ( c f . secA t lower closure stresses i n shallower reservoirs, angular proption 5.2.4.). p a n t s even t e n d t o have h i g h e r c o n d u c t i v i t i e s t h a n r o u n d ones (WESTERN PETROLEUM 1 9 8 4 ) . C o n c e r n i n g n a t u r a l sand, a n g u l a r sand has much h i g h e r a b s o l u t e p e r m e a b i l i t y t h a n r o u n d sand a t l o w c l o s u r e s t r e s s e s u p t o 2,000 p s i , whereas a t h i g h e r c l o s u r e s t r e s s e s above 2,000 p s i , r o u n d sand i s much s t r o n g e r t h a n a n g u l a r s a n d . In t h e e a r l y y e a r s o f a l t e r n a t e p r o p p a n t m a n u f a c t u r i n g and e x p e r i m e n t a t i o n , v a -
I
I
SIGNIFICANCE OF
PROPPANT H Y D R A U L I C F R A C T U R I N G
PROPPANTS IN
-
PETROLEUM RESERVOIR c
A P P L I C A T I O N
) R A V E L ' A C K I N E
17
O T H E R ' R E A T M E N T S
2
STIMULATION AND OTHER
L
U
% 5 D C
ENERGY INDUSTRY PROCESS
0
z
C
c
U
I P R O P P A N T PROVENANCE
STRENGTH
I
I
I
:yinath
I
T
0 L
a
Y
P
E
quartz sand
I
low-density alumina Intermediate- silicate proppants strength
synthetic
L
1; -
T Y P E
6
.I
high-density alumina oxide and silicate proppants
high-
alumina oxide proppants
strength
zirconia-silicate proppants
D 3
0 n L
s 01
L
-
I
0
I
.I./.
Tab. 4
10R PH OLOGY
40 r i o u s shapes i n c l u d i n g o b l o n g , d o n u t - and p e l l e t - s i z e p a r t i c l e s a p a r t f r o m r o u n d and a n g u l a r g r a i n s have been t e s t e d , b u t e x c e p t o f r o u n d and a n g u l a r g r a i n s , no o t h e r p a r t i c l e shape has t u r n e d o u t t o have any b e n e f i t i n t h e h y d r a u l i c f r a c t u r i n g p r o p p a n t a p p l i c a t i o n (FRACFAX 1988 9 ) .
1.4. Proppant choice for hydraulic fracturing and gravel packing As a consequence o f t h e e v e r i n c r e a s i n g d e v e l o p m e n t o f l o w - p e r m e a b i l i t y o i l and gas r e s e r v o i r s , h y d r a u l i c p r o p p a n t f r a c t u r i n g has a t l e a s t i n t h e USA a l r e a d y become one o f t h e m o s t i m p o r t a n t a s p e c t s o f w e l l c o m p l e t i o n ( c f . s e c t i o n 2 . 3 . ) . Because t h e s t i m u l a t i o n t r e a t m e n t c a n a c c o u n t f o r 10 - 50 % o f t h e t o t a l w e l l c o s t , s i g n i f i c a n t emphasis s h o u l d be p l a c e d on o p t i m i z i n g t h e d e s i g n o f t h e operation, e s p e c i a l l y proppant s e l e c t i o n . Proppant choice i s p a r t i c u l a r l y i m p o r t a n t i n view o f t h e f a c t t h a t t h e o n l y t a n g i b l e element o f t h e a c t u a l hyd r a u l i c f r a c t u r i n g j o b which w i l l c o n t r i b u t e t o w e l l p r o d u c t i v i t y increase i s t h e p r o p p a n t t h a t was pumped i n t o t h e g e n e r a t e d c r a c k , w i t h t h u s p l a c i n g c o r r e c t t y p e , g r a i n s i z e and amount o f p r o p p a n t i n t h e f r a c t u r e f o r a g i v e n s e t o f r e s e r v o i r and p r o d u c i n g c o n d i t i o n s b e i n g e x t r e m e l y c r i t i c a l (HOLDITCH 1984, PHILLIPS & ANDERSON 1985; ROBINSON, HOLDITCH & WHITEHEAD 1986) and even r e p r e senting t h e key element i n h y d r a u l i c s t i m u l a t i o n o p e r a t i o n p l a n n i n g .
I n a d d i t i o n t o t h e d i f f e r e n c e i n t y p e o f f a i l u r e between c r y s t a l l i n e a l u m i n a o x i d e p r o p p a n t s and g l a s s y z i r c o n i a p r o p p a n t s , s e v e r a l o t h e r c o n c l u s i o n s conc e r n i n g p r o p p a n t c h o i c e have emerged w h i c h c o m p r i s e a s p e c t s and consequences o f s u r f a c e m o r p h o l o g y and i n t e r n a l m i c r o p o r o s i t y , c r u s h i n g b e h a v i o u r and c r y s t a l l i n e s t r u c t u r e , f r i c t i o n a n g l e and g r a i n shape, f l u i d s a l i n i t y and mechanochem i c a 1 s t a b i l i t y , f l u i d t e m p e r a t u r e and h o t b r i n e a g g r e s s i v i t y , e q u i p m e n t a b r a s i o n and g r a i n h a r d n e s s , s p e c i f i c g r a v i t y and f l u i d s u s p e n s i o n p r o p e r t i e s , and g r a i n s i z e and embedment. A p a r t f r o m t e c h n i c a l p r o p p a n t s e l e c t i o n , p r o p p a n t p r i c e i s a l s o a v e r y i m p o r t a n t f a c t o r i n t e r m s o f e c o n o m i c a l e v a l u a t i o n . The b r i e f a c c o u n t as f o l l o w s a l s o i n c l u d e s comments on m a n u f a c t u r i n g p r o c e s s and p e l l e t c o m p o s i t i o n , as w e l l as c o n d u c t i v i t y d i s c o u n t and p r o p p a n t m i x i n g . F i n a l l y , t h e s i g n i f i c a n c e o f c o m p u t e r programs f o r p r o p p a n t c h o i c e i s o u t l i n e d , and t h e numerous r e s u l t s a r e summarized i n some g e n e r a l g u i d e l i n e s f o r p r o p p a n t sel e c t i o n . V a r i o u s a s p e c t s o f p r o p p a n t c h o i c e a r e a l s o d i s c u s s e d b y RIXE, HOWARD & FAST ( 1 9 6 3 ) and PHILLIPS & ANDERSON ( 1 9 8 5 ) . The s i g n i f i c a n c e o f p r o p p a n t t e s t i n g f o r assessment and improvement o f p r o p p a n t q u a l i t y and f o r r e l i a b l e p r o p pant s e l e c t i o n according t o r e s e r v o i r conditions i s b r i e f l y o u t l i n e d i n the i n t r o d u c t o r y comments.
1.4.0. Significance of proppant testing for quality improvement I n o r d e r t o o p t i m i z e f r a c t u r e d e s i g n b y s e l e c t i o n o f s u i t a b l e p r o p p a n t s and t o reinforce material quality, laboratory research w i t h experimental t e s t i n g s t a r t e d i n t h e e a r l y days o f s u c c e s s f u l h y d r a u l i c t r e a t m e n t p e r f o r m a n c e ( S I N CLAIR 1 9 8 0 ) . W h i l e i n t h e 1950's and 1960's n o t v e r y many a l t e r n a t i v e m a t e r i a l s t o n a t u r a l sand e x i s t e d , t h e s i t u a t i o n changed i n t h e 1970's when f i n a l l y h i g h s t r e n g t h s y n t h e t i c p r o p p a n t s f o r h y d r a u l i c f r a c t u r i n g o f deep h i g h - p r e s s u r e r e s e r v o i r s were i n v e n t e d , and t h i s d i v e r s i f i c a t i o n o f t h e p r o p p a n t s u i t e a l s o t r i g g e r e d progressive s o p h i s t i c a t i o n o f t e s t i n g procedures along w i t h successiv e l y r i s i n g q u a l i t y demand. D u r i n g c o u r s e o f q u a l i t y assessment and improvement, numerous l a b o r a t o r y t e s t s ( a i m i n g on p r o p p a n t s e l e c t i o n i n l o n g - t e r m s t u d i e s and o n q u a l i t y c o n t r o l i n s h o r t - t e r m checks, w i t h t h e i n t e n t i o n o f d e t e r m i n a t i o n o f p r o p p a n t a c c e p t a b i l i t y u n d e r v a r i o u s r e s e r v o i r c o n d i t i o n s and p a r t i c u l a r l y c l o s u r e s t r e s s ranges; CUTLER, ENNISS, JONES & SWANSON 1985) have been c a r r i e d o u t w i t h t h e d i f f e r e n t p r o p p a n t t y p e s and g r a i n s i z e s u n d e r v a r i o u s e n v i r o n m e n t a l ( p r e s s u r e , temperat u r e , f l u i d c o m p o s i t i o n ) c o n d i t i o n s b y i n d e p e n d e n t c o n s u l t a n t and r e s e a r c h dep a r t m e n t s (AHMED, ABOU-SAYED & JONES 1979; SINCLAIR 1980; CUTLER, ENNISS, JONES
41
& CARROLL 1983; DGMK 1983, 1986; CUTLER, ENNISS, JONES & SWANSON 1985; STIM-LAB 1986, PENNY 1987; PURSELL, HOLDITCH & BLAKELEY 1988) as a consequence o f o r d e r s o f f r a c t u r i n g customers, as w e l l as by p r o p p a n t manufacturers (HICKEY, BROWN & CRITTENDEN 1981; NORMAN, CIPOLLA & WEBB 1983; BROWN & MUCH 1986, COBB & FARRELL 1986, COMALCO 1986, STANDARD OIL PROPPANTS 1986, MUCH 1987, MUCH & PENNY 1987), o i l / g a s p r o d u c t i o n companies (BECQ, ROQUE & SARDA 1984; CHEUNG 1985; ROODHART, K U I P E R & D A V I E S 1986) and s e r v i c e companies (McDANIEL & WILLINGHAM 1978, WENDORFF 1982, K I M & LOSACANO 1985; McDANIEL 1986, 1987, 1988; PARKER & McDANIEL 1987, McDANIEL & PARKER 1988). C l o s u r e s t r e s s r e s i s t i v i t y t e s t i n g o f n a t u r a l sand p r i o r t o i n v e n t i o n o f s y n t h e t i c proppants has been c a r r i e d o u t by TUNN (1971) and SLUSSER & RIECKMANN (1976).
1.4.1. Surface morphology and internal microporosity As a consequence o f t h e v a r i o u s p r o d u c t i o n processes b e i n g s i n t e r i n g and f u s i n g ( c f . s e c t i o n 1 . 4 . 9 . ) and o f t h e d i f f e r e n t i n t e r n a l s t r u c t u r e b e i n g c r y s t a l l i n e and g l a s s y ( c f . s e c t i o n 1 . 4 . 2 . ) , alumina o x i d e p r o p p a n t s have a rough s t r a w b e r r y - l i k e s u r f a c e and a h i g h e r i n t e r n a l m i c r o p o r o s i t y (open p o r o s i t y ; BECQ, ROQUE & SARDA 1984) than z i r c o n i a - s i l i c a t e p r o p p a n t s w h i c h have a smooth s u r f a c e and almost no i n t e r n a l m i c r o p o r o s i t y (SEPR 1983, MESSINA 1986; c l o s e d p o r o s i t y ; BECQ, ROQUE & SARDA 1984; c f . t a b . 4 ) . Comments a r e o f f e r e d as f o l lows on l a b o r a t o r y experiments and r e s u l t s as w e l l as f i e l d t e s t i n g and experience.
1.4.1.1. Laboratory experiments and results C l o s u r e s t r e s s r e s i s t i v i t y i s no m a j o r d i f f e r e n c e between b o t h h i g h - s t r e n g t h c o m p o s i t i o n a l t y p e s o f proppants which a r e b o t h a b l e t o m e c h a n i c a l l y w i t h s t a n d t o more t h a n 10,000 p s i o r 700 b a r b e f o r e s t a r t i n g t o be s i g n i f i c a n t l y crushed (BROWN & MUCH 1986, COBB & FARRELL 1986, STANDARD OIL PROPPANTS 1986), and appar e n t e n i g m a t i c a l l y h i g h d i f f e r e n c e s i n p e r m e a b i l i t y and c o n d u c t i v i t y l o s s o r preservation during long-term flow t e s t s w i t h exposition t o brines o f various c o m p o s i t i o n (DGMK 1983, 1986) a r e i n most cases n o t supported by s u f f i c i e n t r e p r o d u c t i o n r u n s o f t h e t e s t s o r a r e p u t i n t o doubt by t h e use o f n o t p r o p e r l y s t a i n l e s s t e f l o n and h a s t a l l o y equipment and debatable methods o f f l u i d passage t h r o u g h t h e package. Much c o n f l i c t i n g c o n d u c t i v i t y d a t a has been p r e s e n t e d i n t h e l a s t years, w i t h f r e q u e n t l y t h e c h o i c e o f t e s t i n g parameters o r t h e d e s i g n o f t h e t e s t c e l l h a v i n g been r e s p o n s i b l e f o r t h e d i s c r e p a n c i e s (McDANIEL 1986). The amount o f i n t e r n a l m i c r o p o r o s i t y i s i n d i c a t e d by t h e t a p d e n s i t y of t h e material ( c f . section 1.4.7.2.). Interconnected microporosity reaching t o g r a i n s u r f a c e i s g e n e r a l l y u n d e s i r a b l e because i t p r o v i d e s an i n c r e a s e i n c u m u l a t i v e p a r t i c l e s u r f a c e area and t h u s b e t t e r p o s s i b i l i t y f o r a t t a c k by a c i d s and agg r e s s i v e f o r m a t i o n b r i n e s (CUTLER, ENNISS, JONES & CARROLL 1983).
1.4.1.2. Field testing and experience Some o f t h e r e p o r t e d e x t r e m e l y h i g h d e s t r u c t i o n s o f c o n d u c t i v i t y and f l u i d t h r o u g h p u t a r e even c o m p l e t e l y c o n t r a d i c t e d by t h e l o n g - t e r m performance o f var i o u s R o t l i e g e n d and C a r b o n i f e r o u s gas w e l l s where one o r s e v e r a l p a r t s o f t h e p r o s p e c t i v e r e s e r v o i r complex have been f r a c t u r e d up t o 10 y e a r s ago (BRINKMANN, FUHRBERG & SCHUBER 1980; BRINKMANN, KRUMER & R E I N I C K E 1980; BRINKMANN 1982, SCHWARZ & SCHUBER 1982, JOHN 1983, LEICHT 1985; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985; c f . s e c t i o n s 2.4.1.1. and 3.3.2.) and a r e t h u s pronounced f i e l d experiments h i g h l i g h t i n g t h a t proppants o f e i t h e r t y p e do w e l l p e r f o r m and no s i g n i f i c a n t r e d u c t i o n o f gas f l o w has been proven d u r i n g t h e p e r i o d of i n some cases a f u l l decade, w i t h t h e e x p e c t a t i o n i n case o f t h e r e a l i s t i c nat u r e o f t h e p e r m e a b i l i t y d i m i n u t i o n observed i n some o f t h e l a b o r a t o r y t e s t s (DGMK 1983, 1986) r e q u i r i n g t h a t none o f t h e gas w e l l s which were f r a c t u r e d
42 many years ago would be allowed to produce at acceptable rates still today. Subordinate quantities of alumina silicate and oxide proppants which entered the borehole by partial fracture evacuation and were bailed out of some Carboniferous and Rotliegend wells several years after hydraulic stimulation execution exhibit only negligible mechanical and chemical surface alterations with respect to fresh proppant grains, thus confirming the physical consistency of the proppant wedge in the crack which is also reflected by the satisfactory gas production rates (cf. section 1 . 4 . 1 0 . 5 . ) .
1.4.2. Crushing behaviour and crystalline structure Comparability of the results of the long-term flow tests carried out by the various institutions is hampered by differences in size and composition of the test cell, way and period of pressure increase, rate of fluid flux through the proppant package, singular throughput or repeated circulation of the solutions, and temperature and salinity of the fluids. The discussion as follows focusses on failure of microcrystalline and glassy proppant types as well as internal proppant structure.
1.4.2.1. Failure o f microcrystalline and glassy proppant types The difference in crushing behaviour including slow continuous breakage into larger pieces of tougher and partially even ductile crystalline alumina oxide proppants vs. rapid catastrophical failure into tiny splinters of brittle glassy zirconia-silicate proppants seems to favour the latter material at lower closure stresses where preferentially low percentages of the former product start to break into fragments, while zirconia-silicate proppants are still completely stable. The crystalline bauxite proppants, however, are in a much superior situation once the package of glassy zirconia-silicate proppants totally fails into almost dust at the critical closure stress level where the crystalline bauxite proppant package assemblage still has a reasonable conductivity by being crushed predominantly into halves and quarters of grains instead of tiny splinters The fine zirconia-silicate proppant dust can be transported and redistributed by fluids, whereas the coarser fragments of bauxite proppants remain in place and form residual packages that still have quite some conductivity which is sufficient to economically drain many tight gas reservoirs by still providing a reasonable conductivity contrast to the low-permeability rock matrix (cf. sections 4.6. and 4.12.4.). Crystalline bauxite proppants thus exhibit a broad breakage curve in contrast to a fairly well-defined and narrow breakage peak of zirconia-silicate proppants ( M E S S I N A 1987).
1.4.2.2. Internal proppant structure The superior strength characteristics of sintered bauxite are related to its microcrystalline structure which contains many small grain boundaries within each particle separated by an amorphous phase which has subordinate share in total grain composition, with the latter responding to stress by flexing and distributing the load on larger groups of particles, thereby inducing exceptional crush strength and resistance to proppant fragmentation (HICKEY, BROWN & CRITTENDEN 1981). The same applies for other alumina silicate and oxide proppants. The glassy zirconia-silicate proppants, however, consist of monoclinic and cubic zirconia crystallites which are floating in the alumina silicate glassy matrix and therefore due to the rarity to even lack o f crystallite contacts, the material has an overall amorphous structure and thus breaks homogeneously into tiny splinters upon failure. Natural quartz sand which has no cleavage is characterized by shattering and irregular breaking, while polycrystalline synthe-
43 t i c s i n t e r e d b a u x i t e i s c r a c k i n g and s p l i t t i n g i n t o l a r g e r p a r t s o f ( c f . s e c t i o n 1 . 4 . 2 . 1 . ) i n s t e a d o f f a i l i n g i n t o powder.
the
grains
Reviews o f p r o p p a n t performance i n f i e l d a p p l i c a t i o n s w i t h s p e c i a l emphasis b e i n g p u t on assessment o f b e h a v i o u r o f s i n t e r e d b a u x i t e v s . sand p a r t i c u l a r l y i n terms o f c r u s h i n g degree and c o n c e r n i n g n e t p r e s e n t v a l u e p e r i n v e s t e d US $ a r e performed by ATTEBERRY, TUCKER & R I T Z (1979); CARROLL & BAKER (1979), KOHLHAAS (1982), CLARK (1983) and B R I M ( 1 9 8 6 ) . D e t a i l e d comparative e f f e c t i v e n e s s e v a l u a t i o n s o f t h e i n f l u e n c e o f p r o p p a n t t y p e and g r a i n s i z e on w e l l p r o d u c t i v i t y a r e p a r t i c u l a r l y c a r r i e d o u t by McDANIEL & WILLINGHAM (1978); H I C K E Y , BROWN & CRITTENDEN (1981) and NORMAN, CIPOLLA & WEBB ( 1 9 8 3 ) .
1 . 4 . 3 , F r i c t i o n angle and g r a i n shape The rough s u r f a c e o f c r y s t a l l i n e b a u x i t e proppants g i v e s a much b e t t e r f l u i d f l o w through t h e p r o p p a n t package than t h e smooth s u r f a c e o f t h e g l a s s y z i r c o n i a proppants ( c f . a l s o ACHENBACH 1974). The d i f f e r e n t s u r f a c e morphology a l s o r e s u l t s i n d i f f e r e n t f r i c t i o n angles o f t h e proppant package. While a l o w e r f r i c t i o n a n g l e o f t h e g l a s s y z i r c o n i a proppants may f a v o u r b e t t e r emptying o f t h e s i l o s d u r i n g b l e n d i n g o f proppants w i t h t h e f l u i d when c a r r y i n g o u t t h e f r a c t u r i n g s t i m u l a t i o n j o b , w i t h r e s p e c t t o t h e h i g h e r f r i c t i o n angle o f t h e c r y s t a l l i n e b a u x i t e proppant, and assuming t h a t even d e s p i t e d i s p e r s i o n i n t h e f l u i d t h e r e i s s t i l l an advantage o f t h e l o w e r f r i c t i o n a n g l e o f t h e g l a s s y z i r c o n i a proppant f o r e a s i e r r u s h i n g i n t o t h e f r a c t u r e and f o r a l e s s tendency o f b r i d g i n g i n t h e crack, t h e s t a b i l i t y o f t h e proppant package w i t h i n t h e f r a c t u r e d u r i n g subsequent p r o d u c t i o n i s a n o t h e r q u e s t i o n . Some aspects o f p r o p p a n t b a c k f l o w o u t o f t h e f r a c t u r e mouth and s t a b i l i t y enhancement o f t h e p r o p p a n t package a r e b r i e f l y d i s c u s s e d as f o l l o w s .
1.4.3.1.
Proppant backflow out o f the f r a c t u r e mouth
As p r o d u c t i o n o f small amounts o f proppants w i t h t h e gas t e s t i f i e s t o p r o p p a n t l o s s a t t h e f r a c t u r e mouth where t h e f l o w r a t e i s h i g h e s t and c r e a t e s a c h o k e - l i k e e f f e c t due t o p r e s s u r e drawdown and c u r r e n t a c c e l e r a t i o n , t h e lower f r i c t i o n a n g l e r e s u l t s i n d i m i n u i s h e d s t a b i l i t y o f t h e p r o p p a n t package and c o r r e s p o n d i n g t o t h e b e t t e r r u s h i n g - i n t r i g g e r s a l s o an enhanced r u s h i n g - o u t o f t h e smooth-surface m a t e r i a l f r o m t h e f r a c t u r e w i t h r e s p e c t t o t h e r o u g h - s u r f a c e m a t e r i a l (problems o f n e a r - w e l l b o r e p r o p p a n t flowback a r e d i s c u s s e d by S M I T H 1973; ROBINSON, HOLDITCH & WHITEHEAD 1986; c f . s e c t i o n 4 . 1 2 . 3 . ) . Some aspects o f proppant t r a n s p o r t a r e a l s o r e p o r t e d by CLARK & QUADIR (1981); SIEVERT, WAHL, CLARK & HARKIN (1981); AHMED (1984); CLARK, HALVACI, GHAELI & PARKS (1985) and ACHARYA (1986, 1987). SHAH & LEE (1986) comment on f r i c t i o n p r e s sures o f p r o p p a n t - l a d e n h y d r a u l i c f r a c t u r i n g f l u i d s . Proppant l o s s near t h e crack mouth by b a c k f l o w induces f r a c t u r e c l o s u r e o v e r a c e r t a i n d i s t a n c e which o b v i o u s l y has a c o n s i d e r a b l y d e t e r i o r a t i n g e f f e c t on proppant package c o n d u c t i v i t y ( c f . s e c t i o n 4 . 1 2 . 3 . 1 . ) . F r a c t u r e e v a c u a t i o n by proppant escape i n t h e immediate w e l l b o r e v i c i n i t y where p r e s s u r e drawdown and f l o w r a t e s a r e h i g h e s t i s one o f t h e most d e l e t e r i o u s t y p e s o f f r a c t u r e conduct i v i t y damage. I n a d d i t i o n , t h e f l o w - o u t o f proppants ( p a r t i c u l a r l y s i n t e r e d b a u x i t e ) f r o m t h e c r a c k t h r o u g h t h e p e r f o r a t i o n s i n t o t h e hardware-equipped bor e h o l e a l s o erodes t u b i n g s , v a l v e s , chokes and even t h e w e l l h e a d ( s i m i l a r e f f e c t s a r e provoked by i n f l u x o f u n c o n s o l i d a t e d f o r m a t i o n sand i n t o t h e w e l l b o r e t h r o u g h i n s u f f i c i e n t g r a v e l pack f i l t e r s o r i n case o f absence o f mechanical sand c o n t r o l ; c f . s e c t i o n 5 . 2 . 1 . 3 . ) . The g e n e r a l r u l e i s t h a t proppant packages w i t h l o w e r f r i c t i o n a n g l e a r e l e s s s t a b l e b u t a l s o c r e a t e l e s s e r o s i o n , whereas such w i t h h i g h e r f r i c t i o n a n g l e a r e more s t a b l e b u t cause a l s o more e r o s i o n ( q u e s t i o n s o f p a r t i c l e t r a n s p o r t through p e r f o r a t i o n s a r e reviewed by GRUESBECK & COLLINS 1978; c f . a l s o s e c t i o n 5 . 6 . ) . Proppant b a c k f l o w i s b e s t e l i m i n a t e d by r e s i n p e l l i c l e s s u r r o u n d i n g t h e i n d i v i d u a l g r a i n s and c r e a t i n g a s t a b l e aggrega-
44 t e d package (CUTLER, E N N I S S , JONES & CARROLL 1983) w h i c h p r e v e n t s f r a c t u r e e v a c u a t i o n b y k e e p i n g t h e c o u p l i n g o f w e l l b o r e and p r o d u c t i v e f o r m a t i o n (SINCLAIR, GRAHAM & SINCLAIR 1983; c f . s e c t i o n 4 . 1 2 . 3 . 3 . ) . b l i t h i n a p a c k e d bed o f p r o p p d n t s p h e r e s f i l l i n g t h e w i d t h o f t h e f r a c t u r e , c o m p r e s s i v e f o r c e s t e n d t o a r r a n g e t h e beads i n a c l o s e l y p a c k e d s t r u c t r e (CUTLER, JONES, SWANSON & CARROLL 1 9 5 1 ) . The m o s t t y p i c a l i d e a l i z e d p a c k i n g a r r a n g e m c n t i s t h e rhombohedra1 one where each q l o b u l a r p a r t i c l e c o n t d c t s t w e l e n e i g h b o u r s i n s i x p a i r s o f d i a m e t r i c a l c o n t a c t l o a d i n g s (KNUDSEN 1 9 5 9 ) . Wh l e t h i s r e g u l a r i t y i s l i k e l y n o t t o be p r e s e r v e d o v e r l o n g d i s t a n c e s , i t i s be1 i e v e d t h a t t h e l o a d i n g p a t t e r n i s r e p r e s e n t a t i v e o f t h e f o r c e s t r a n s m i t t e d t o each srlhere.
1 . 4 . 3 . 2 . Stabi 1 i t y enhancement o f the proppant package I n t e r m s o f s t a b i l i t y o f t h e p r o p p a n t package i n t h e f r a c t u r e , b a u x i t e p r o p p d n t s w i t h r o u g h s u r f a c e and h i g h e r f r i c t i o n a n g l e s a r e t h u s s u p e r i o r t o z i r c o n i a p r o p p a n t s w i t h smooth s u r f a c e and l o w e r f r i c t i o n a n g l e s . An e v e n b e t t e r s t a b i l i t y b y a h i g h e r f r i c t i o n a n g l e c o u l d be a c h i e v e d b y i n t e r m i x i n g o f a n g u l a r g r a i n s i n t o a p o p u l a t i o n o f w e l l - r o u n d e d p a r t i c l e s , w i t h t h i s t e c h n i q u e , howe v e r , r e q u i r i n g a compromise s o l u t i o n o f c o n d u c t i v i t y r e d u c t i o n v s . s t a b i l i t y enhancement ( a s p e c t s o f c o n d u c t i v i t y o f a n g u l a r b l e n d s o f f r a c t u r i n g sand a r e a l s o d i s c u s s e d by LARSEN & SMITH 1985; c f . s e c t i o n 1 . 3 . 5 . ) . Flowback i s a l s o r e t a r d e d b y a p p l i c a t i o n o f r e s i n - c o a t e d n a t u r a l sand o r s y n t h e t i c p r o p p a n t s (POPE, WILES & P I E R C E 1987; c f . s e c t i o n 4 . 1 2 . 3 . 3 . ) . Coarse t a i l - i n t e r m i n a l l o t s a l s o f u n c t i o n l i k e a g r a v e l pack and h o l d t h e f i n e r m a i n p r o p p a n t q u a n t i t y i n p l a c e b y b r i d g i n g o f t h e s m a l l e r g r a i n s a t t h e i n t e r f a c e between b o t h p o p u l a t i o n s ( c f . s e c t i o n 5 . 8 . 8 . ) . Some comments on s i g n i f i c a n c e o f p r o p p a n t b a c k f l o w p r e v e n t i o n i n h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g as w e l l as p r e v e n t i o n o f p r o p p a n t e v a c u a t i o n a r e o f f e r e c i as f o l l o w s . Tne c o m p a r a t i v e i m p o r t a n c e o f p r o p p d n t e v a c u a t i o n i n h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i s a l s o a s s e s s e d .
1.4.3.2.1. Proppant backflow prevention i n h y d r a u l i c f r a c t u r i n g F r a c t u r e e v a c u a t i o n n e a r t h e w e l l b o r e b y p r o p p a n t f l o w b a c k i s one o f t h e m o s t s e r i o u s t y p e s o f c r a c k damdge ( c f . s e c t i o n 4 . 1 2 . 3 . ) and t h u s n o t o n l y o p t i mum t y p e , g r a i n s i z e and c o n c e n t r a t i o n o f t h e p r o p p a n t have t o be s e l e c t e d , but a l s o cleanup, f l o w b a c k and i n i t i a l h y d r o c a r b o n p r o d u c t i o n o f t h e w e l l have t o be c a r e f u l l y c o n t r o l l e d i n o r d e r t o a v o i d e x t r e m e drawdown w h i c h w o u l d s u b j e c t t h e p r o p p a n t s s u d d e n l y t o v e r y h i g h c l o s u r e s t r e s s e s t h a t m i g h t cause s i g n i f i c d n t p r o p p a n t c r u s h i n g p a r t i a l l y by e x c e s s i v e c l o s u r e s t r e s s , but partially a l s o by shock l o a d i n g (ROBINSON, HOLDITCH & WHITEHEAD 1 9 8 6 ) . ;r c c r t r a s : t 3 h y d r a u : i c f r d c t u r i r g where p r c p p a r t f l c w b a c k c a n be a s e r i o b s p r o b l e m and t h u s z i r c o n i a - s i l i c a t e p r o p p a n t s w i t h l o w e r f r i c t i o n a n g l e have a t r c h n i c a l l y c o m p e t i t i v e d i s a d v d n t a g e w i t h r e s p e c t t o a l u m i n a o x i d e and s i l i c a t e proppants w i t h h i g h e r f r i c t i o n angle, proppant flowback i s o n l y i n s i g n i f i c a n t t o n e g l i g i b l e i n gravel packing.
1.4.3.2.2.
Proppant evacuation i n f r a c t u r i n g vs. gravel packing
The m a i n r e a s o n f o r t h e a t l e a s t p a r t i a l independence o f p a c k i n g g r a v e l f r o m i n t e r g r a i n s t a b i l i z a t i o n i s the f a c t t h a t the p e r f o r a t i o n tunnel plugs are hold i n p l a c e b y t h e i n t e r n a l g r a v e l pack m a n t l e a l o n g t h e b o r e h o l e w a l l w h i c h i n t u r n i s k e p t i n p o s i t i o n by t h e gravel-pack hardware comprising s l o t t e d l i n e r o r w i r e - w r a p p e d s c r e e n ( c f . s e c t i o n 5 . 2 . 3 . 5 . ) . As t h e a n g l e o f r e p o s e o f g r a v e l p d c k i n g m a t e r i a l s s h o u l d be as l o w as p o s s i b l e i n o r d e r t o a l l o w r e a d y i n f i l l i n g 3f a l l d e v i c e s 2nd p o c k e t s in t b e f o r - a t i o n i n c p e n - h o l e c o m D l e t i o n s (COBERLY & WAGNER 1 9 3 8 ) , z i r c o n i a - 5 i l i c a t e p r o p p a n t s w h i c h have t h e l o w e s t a n g l e
45 o f repose o f a l l p r e s e n t l y c o m m e r c i a l l y a v a i l a b l e p r o p p i n g m a t e r i a l s s h o u l d be t o g e t h e r w i t h t h e i r low s p e c i f i c g r a v i t y t h e i d e a l g r a v e l type, b u t u n f o r t u n a t e l y t h e y a r e t o o expensive f o r most o f t h e s a n d - c o n t r o l o p e r a t i o n s ( c f . s e c t i o n 5.2.2.3.3.). On t h e o t h e r hand, however, t h e smooth s u r f a c e o f z i r c o n i a - s i l i c a t e p r o p pants and g l a s s beads does n o t a l l o w easy b r i d g i n g due t o l a c k i n g f r i c t i o n which i s d e s i r a b l e i n h y d r a u l i c f r a c t u r i n g i n o r d e r t o a v o i d premature screeno u t f a i l u r e , whereas t h e rough s u r f a c e o f n a t u r a l sand as w e l l as alumina o x i d e and s i l i c a t e proppants has s u f f i c i e n t f r i c t i o n t o p e r m i t ready b r i d g i n g which i s s u p p o r t i n g p e r f o r a t i o n t u n n e l i n f i l l i n g and s t a b i l i z a t i o n i n g r a v e l packing, b u t i s u n s u i t a b l e i n h y d r a u l i c f r a c t u r i n g as a consequence o f i n c r e a s i n g screenout r i s k .
1.4.4. Fluid salinity and mechanochemical stability Smoother s u r f a c e and lower i n t e r n a l m i c r o p o r o s i t y g i v e t h e g l a s s y z i r c o n i a proppants a p r i n c i p a l l y b e t t e r r e s i s t i v i t y a g a i n s t chemical s o l u t i o n t h a n t h e c r y s t a l l i n e alumina o x i d e proppants, as w e l l as a l o w e r a b r a s i v e e f f e c t t o pumps and equipment t h a n p a r t i c u l a r l y t h e h i g h - s t r e n g t h b a u x i t e proppants w h i c h cause c o n s i d e r a b l e wear and t e a r o f t h e machinery. Some aspects o f b r i n e compos i t i o n and proppant d i s s o l u t i o n , f o r m a t i o n w a t e r s a l i n i t y , and f o r m a t i o n w a t e r a g g r e s s i v i t y v s . proppant s t a b i l i t y a r e s h o r t l y sketched as f o l l o w s .
1.4.4.1. Brine composition and proppant dissolution An i n v e r s e performance o f t h e d i f f e r e n t p r o p p a n t t y p e s w i t h s a l i n i t y l e v e l and pH-value, however, has been recorded, w i t h i n l o w - s a l i n i t y environment and i n a l c a l i n e m i l i e u t h e g l a s s y z i r c o n i a proppant b e i n g i n f e r i o r t o t h e c r y s t a l l i n e b a u x i t e proppant, whereas t h e former w i t h s t a n d s b e t t e r i n h i g h - s a l i n i t y environment and i n a c i d m i l i e u t h a n t h e l a t t e r . Some aspects o f low and h i g h f l u i d s a l i n i t i e s as w e l l as p r o p p a n t s u r f a c e s t a b i l i z a t i o n a r e o u t l i n e d as f o l 1 ows.
1.4.4.1.1. Low and high fluid salinities Proppant t e s t i n g under d i f f e r e n t f l u i d c o n c e n t r a t i o n s r e v e a l e d c o n s i d e r a b l e p e r m e a b i l i t y r e d u c t i o n s o f t h e p r o p p a n t package ( i n extreme cases up t o 10 % r e s i d u a l v a l u e ) i f exposed t o h i g h - s a l i n i t y s o l u t i o n s , whereas o n l y n e g l i g i b l e p e r m e a b i l i t y r e d u c t i o n occurs a t low f l u i d s a l i n i t i e s , w i t h t h e reason b e i n g m a i n l y proppant c o r r o s i o n . The a g g r e s s i v e f l u i d s l e a d t o d i s s o l u t i o n o f p a r t s o f t h e g l o b u l e s , w i t h t h e e f f e c t b e i n g p a r t i a l f l u i d s u p e r s a t u r a t i o n which f i n a l l y r e s u l t s i n r e p r e c i p i t a t i o n o f material i n other p a r t s o f the proppant package t h e r e b y f o r m i n g overgrowths and cements, combined w i t h t h e e f f e c t o f c o l l a p s e o f t h e framework o f t h e r e s i d u a l g l o b u l e fragments, w i t h t h e f a i l u r e o f t h e package b e i n g a f e a t u r e p r i n c i p a l l y comparable t o compaction o f s e d i ments by a l s o r e d u c i n g pore space and t h u s u l t i m a t e l y a l s o p e r m e a b i l i t y ( c f . also section 4.5.4.1.6.). Low s a l i n i t i e s do n o t a l l o w s u p e r s a t u r a t i o n t o o c c u r and t h u s t h e r e i s no r e p r e c i p i t a t i o n p a r t i a l l y o b s t r u c t i n g o r even c o m p l e t e l y p l u g g i n g t h e p o r e s . Perm e a b i l i t y r e d u c t i o n i n h i g h - s a l i n i t y environment i s t r i g g e r e d by b o t h chemical c o r r o s i o n and mechanical compaction due t o pore space d i m i n u t i o n by changing t h e p u n c t u a l c o n t a c t s o f t h e i n i t i a l g l o b u l e s i n t o more a r e a l c o n t a c t s o f t h e s o l u t i o n remnants. F i n a l l y , p r e s s u r e s o l u t i o n a t t h e p u n c t u a l c o n t a c t s a l s o has i t s influence.
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1 . 4 . 4 . 1 . 2 . Proppant surface stabilization In a d d i t i o n , i f h o t w a t e r i s pumped a t once t h r o u g h t h e p r o p p a n t l a y e r , a g e l f o r m s w h i c h p l u g s t h e p o r e s and r e d u c e s t h e p e r m e a b i l i t y . I f t h e w a t e r f i r s t s t a n d s w i t h i n t h e p r o p p a n t package and i s t h e n f l u s h e d t h r o u g h , however, r e c r y s t a l l i z a t i o n t a k e s p l a c e and g e l f o r m a t i o n i s p r e v e n t e d b y i n a c t i v a t i o n o r p r o t e c t i o n o f t h e p r o p p a n t s u r f a c e (ALBERTSEN 1 9 8 5 ) . T h e r e f o r e i n p r a c t i c e n o d a n g e r i s seen t h a t such a g e l c o u l d p l u g t h e p o r e s o f t h e p r o p p a n t package i n f i l l i n g t h e f r a c t u r e , as t h e common p o l i c y i s t o s h u t i n t h e w e l l a f t e r t h e t r e a t m e n t t o a l l o w t h e h i g h - v i s c o s i t y f r a c t u r i n g g e l t o b r e a k i n t o s i m p l e r comp o n e n t s w h i c h f o r m a l o w - v i s c o s i t y f l u i d and a f t e r a few days s t a r t c l e a n - u p and p r o d u c t i o n , w i t h t h e s h u t - i n p e r i o d b e i n g e q u i v a l e n t t o t h e s t a n d i n g t i m e i n t h e t e s t and t h u s s t a b i l i z a t i o n o f t h e s u r f a c e d u r i n g t h a t t i m e p r e v e n t s g e l f o r m a t i o n . A s p e c t o f d i s s o l u t i o n and c o n d e n s a t i o n o f s i l i c a i n v a r i o u s f l u i d s a r e d i s c u s s e d b y THORNTON & RADKE ( 1 9 8 5 ) and UDELL & LOFY ( 1 9 8 5 ) .
1 . 4 . 4 . 2 . Formation water salinity W h i l e i n t h e USA f o r m a t i o n w a t e r s have i n most c a s e s o n l y s l i g h t t o m o d e r a t e s a l i n i t y , considerably aggressive brines w i t h almost saturated s a l i n i t y occur i n t h e R o t l i e g e n d and C a r b o n i f e r o u s deep t i g h t gas r e s e r v o i r s o f Germany FRG (KLOSE & KROMER 1983, ALBERTSEN 1985; c f . s e c t i o n 1 . 4 . 1 0 . 4 . ) and s u r r o u n d i n g c e n t r a l European c o u n t r i e s and a r e a s w h i c h r e q u i r e s e l e c t i o n o f a p r o p p a n t t y p e t h a t i s s t a b l e i n t h i s high-concentration f l u i d environment. High s a l i n i t i e s o f f o r m a t i o n w a t e r s a r e a l s o p r e s e n t i n T e r t i a r y s a n d s t o n e s i n Y u g o s l a v i a (ECONOMIDES, CIKES, PFORTER, UDICK & URODA 1 9 8 6 ) where MHF t r e a t m e n t s c o m p a r a b l e i n s i z e t o t h o s e i n Germany FRG ( a n d p a r t i a l l y e v e n l a r g e r t h a n t h e b i g g e s t j o b s in t h e l a t t e r a r e a ) a r e c a r r i e d o u t ( c f . s e c t i o n 2 . 4 . 3 . 1 . ) . Many p r o p p a n t l a b o r a t o r y s t a b i l i t y t e s t s c a n n o t be a c c u r a t e l y compared w i t h f i e l d c o n d i t i o n s due t o p r e d o m i n a n t a p p l i c a t i o n o f 2 % KC1 s o l u t i o n as e x p e r i m e n t a l f l u i d w h i c h matches t h e c o m p o s i t i o n o f p r e p a d and d i s p l a c e m e n t d u r i n g t h e f r a c t u r i n g t r e a t m e n t ( w h e r e i t i s u s e d as a s t a n d a r d s a f e t y p r e c a u t i o n f o r m i n i m i z a t i o n o f f o r m a t i o n damage and p r o t e c t i o n o f w a t e r - s e n s i t i v e r e s e r v o i r s b y c l a y s t a b i l i z a t i o n ; WATERS 1980, SYDANSK 1984; c f . s e c t i o n 1 . 4 . 1 0 . 4 . 2 . ) , but n o t due t o c o m p o s i t i o n o f a g g r e s s i v e f o r m a t i o n w a t e r s w h i c h become e f f e c t i v e l a t e r when t h e i n v a s i o n b y t h e i n j e c t e d f l u i d s i s compensated and r e p l a c e d , and a l s o t h e f r a c t u r e i s f l u s h e d w i t h h i g h - s a l i n i t y f o r m a t i o n b r i n e s . RENSVOLD ( 1 9 8 3 ) e v a l u a t e s t h e s t a b i l i t y o f sand c o n s o l i d a t i o n r e s i n s in h o t b r i n e s . GRAHAM, MONAGHAN & OSOBA ( 1 9 5 9 ) d i s c u s s t h e i n f l u e n c e o f p r o p p i n g sand w e t t a b i l i t y on p r o d u c t i v i t y o f h y d r a u l i c a l l y f r a c t u r e d o i l w e l l s .
1 . 4 . 4 . 3 . Formation water aggressivity vs. proppant stability F o r m a t i o n w a t e r a g g r e s s i v i t y i s c r i t i c a l t o p r o p p a n t s t a b i l i t y , because e v e n i n gas r e s e r v o i r s w i t h v e r y l o w w a t e r s a t u r a t i o n , some amounts o f f o r m a t i o n wat e r a r e p r o d u c e d t o g e t h e r w i t h t h e gas and t h u s f l o w a l s o t h r o u g h t h e p r o p p a n t package. C o m p l e t e l y d r y gas p r o d u c t i o n w i t h o u t any t r a c e s o f w a t e r r e p r e s e n t s a t h e o r e t i c a l l y i d e a l case, b u t i s i n p r a c t i c e a l m o s t n e v e r a c h i e v e d . As many deep t i g h t gas w e l l s where f r a c t u r e s have been c r e a t e d t o d r a i n t h e s u r r o u n d i n g r e s e r v o i r a r e s c h e d u l e d t o p r o d u c e f o r l o n g t i m e b e i n g u p t o s e v e r a l decades, d u r i n g such a l o n g p e r i o d even n e g l i g i b l e t r a c e s o f w a t e r w i t h i n t h e gas add u p t o s e r i o u s c h e m i c a l c o r r o s i o n i f b e i n g o f such a g g r e s s i v i t y t o be a b l e t o a t t a c k t h e p r o p p a n t s s u p p o r t i n g t h e f r a c t u r e . The s i t u a t i o n i s becoming more a c c e n t u a t e d when gas w i t h h i g h e r w a t e r c o n t e n t (ARENS & B'CHNEL 1977) i s p r o d u c e d . T h e r e f o r e r e a l i s t i c p r o p p a n t t e s t i n g and s e l e c t i o n has t o be based on t h e h i g h e s t s t r e s s e x p o s u r e i n c l u d i n g b o t h maximum r e s e r v o i r p r e s s u r e drawdown and formation b r i n e composition.
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1.4.5. Fluid temperature and hot brine aggressivity W h i l e m o s t o f t h e s y n t h e t i c p r o p p a n t s and n a t u r a l q u a r t z sands have a more o r l e s s c o m p a r a b l e p e r f o r m a n c e u n d e r s t a n d a r d r e s e r v o i r c o n d i t i o n s o f temperat u r e a c c o r d i n g t o t h e r e g i o n a l g e o t h e r m a l g r a d i e n t and m o d e r a t e s a l i n i t y o f t h e formation water ( i n t h i s context n o t respecting closure stress t h a t i s exerted b y h y d r o s t a t i c r e s e r v o i r p r e s s u r e and l i t h o s t a t i c o v e r b u r d e n p r e s s u r e ) , d i f f e rences i n r e a c t i o n a r e e x h i b i t e d a t higher temperatures o f t h e r e s e r v o i r rocks beyond t h e n o r m a l g e o t h e r m a l g r a d i e n t and a t i n c r e a s e d s a l i n i t i e s a n d / o r pH-val u e s o f t h e b r i n e s beyond t h e s a t u r a t i o n o f t h e p a y f l u i d s common i n m o s t f o r m a t i o n s . Some a s p e c t s o f f l u i d t e m p e r a t u r e and h o t b r i n e a g g r e s s i v i t y i n steamd r i v e enhanced o i l r e c o v e r y and deep t i g h t gas r e s e r v o i r s a r e summarized as f o l lows.
1.4.5.1. Steam-drive enhanced oi 1 recovery I n terms o f temperatures, t h e most s i g n i f i c a n t aspect i s t h e behaviour o f p r o p p a n t s u n d e r c o n d i t i o n s o f s t e a m - d r i v e enhanced o i l r e c o v e r y i n h e a v y - o i l f i e l d s when g r a v e l p a c k i n g a n d / o r h y d r a u l i c f r a c t u r i n g i s r e q u i r e d i n t h e steam i n j e c t i o n w e l l s i n such f o r m a t i o n s ( c f . s e c t i o n 5 . 7 . ) . P a r t i c u l a r l y i t 1 c a s e o f an a l c a l i n e h o t e n v i r o n m e n t i n t h e s u r r o u n d i n g s o f t h e steam i n j e c t o r s , n a t u r a l q u a r t z sand as w e l l as a l l t h e s y n t h e t i c p r o p p a n t t y p e s c o n t a i n i n g f r e e q u a r t z ( o r e q u i v a l e n t c r y s t a l l i n e phases o r g l a s s y s i l i c a ) o r a l u m i n a s i l i c a t e s a r e u n s t a b l e due t o s i l i c a d i s s o l u t i o n , whereas s i n t e r e d b a u x i t e c o n s i s t i n g a l m o s t e x c l u s i v e l y o f i n e r t corundum has been p r o v e n t o be t h e s u i t a b l e m a t e r i a l due t o i t s s t a b i l i t y i n t h i s e x t r e m e t e m p e r a t u r e m i l i e u (REED 1980; UNDERDOWN & DAS 1985; c f . a l s o WATKINS, KALFAYAN, WATANABE & HOLM 1985; a s p e c t s o f s t e a m - f l o o d s i l i c a d i s s o l u t i o n a r e a l s o d i s c u s s e d b y THOMAS, YOST & CATHEY 1987; c f . sect i o n 5 . 7 . 1 . ) . A n o t h e r r e s i s t a n t m a t e r i a l w h i c h may even p e r f o r m b e t t e r t h a n s i n t e r e d b a u x i t e u n d e r s t e a m - d r i v e c o n d i t i o n s i s n i c k e l - c o a t e d sand (SACUTA, NGUYEN & KISSEL 1986; c f . s e c t i o n 5 . 7 . 2 . ) .
I n c o n t r a s t t o t h e USA, however, s t e a m - d r i v e enhanced o i l r e c o v e r y i s i n Eur o p e n o t y e t w i d e l y d i s t r i b u t e d ( m a i n l y i n C r e t a c e o u s s a n d s t o n e s i n Germany FRG and N e t h e r l a n d s ; L I L L I E & SPRINGER 1981; PROYER, CHAZITEODOROU, MUSS & ROSSKAMP 1983; L I L L I E & WIETHOFF 1985, REINICKE & WIETHOFF 1987; f o r f u r t h e r r e f e r e n c e s on s t e a m - d r i v e p r o j e c t s i n Europe c f . s e c t i o n 5 . 7 . 3 . ) , and due t o d i f f e r e n t met h o d s o f steam c o n d i t i o n i n g , t h e p r o b l e m o f s i l i c a d i s s o l u t i o n i n an a l c a l i n e e n v i r o n m e n t i s i n Europe n o t so p r o n o u n c e d as i n t h e USA ( c f . s e c t i o n 5 . 7 . 5 . ) .
1.4.5.2. Deep tight gas reservoirs C o n s i d e r a b l y e l e v a t e d t e m p e r a t u r e s i n t h e r a n g e o f 120 - 240 OC a r e a l s o p r e s e n t i n deep R o t l i e g e n d and C a r b o n i f e r o u s gas w e l l s i n Germany FRG ( P H I L I P P & R E I N I C K E 1982, JOHN 1983, KLOSE & KRUMER 1983, BLEAKLEY 1984; c f . f i g . 3 and s e c t i o n s 2 . 4 . 1 . 1 . and 3 . 3 . ) and i n deep T e r t i a r y g a s and c o n d e n s a t e w e l l s i n Yug o s l a v i a (ECONOMIDES, CIKES, PFURTER, UDICK & URODA 1986; c f . s e c t i o n 2 . 4 . 3 . 1 . ) which c r e a t e together w i t h t h e e x t r a o r d i n a r i l y h i g h formation water s a l i n i t i e s ( p a r t i a l l y s a t u r a t e d b r i n e s ) and h i g h p r e s s u r e s a v e r y h o s t i l e e n v i r o n m e n t and r e q u i r e stable m a t e r i a l s t o r e s i s t n o t o n l y t o the high closure stresses, b u t a l s o t o t h e chemically aggressive m i l i e u ( c f . s e c t i o n 1.4.4.2.). Aspects o f t h e r m a l s t a b i l i t y o f r e s i n - c o a t e d p r o p p a n t s i n h o t b r i n e s a r e d i s c u s s e d b y RENSVOLD ( 1 9 8 3 ) .
1.4.6. Equipment abrasion and grain hardness The e c o n o m i c a l f e a s i b i l i t y m o d e l l i n g o f a h y d r a u l i c f r a c t u r i n g o p e r a t i o n h a s i n terms o f proppant c h o i c e n o t o n l y t o i n c l u d e r e s e r v o i r - r e l a t i n g features,
48
but also to respect handling and maintenance questions. The following outline focusses on erosional behaviour of different proppant types, equipment erosion by proppants, and proppant damage by equipment.
1.4.6.1. Erosional behaviour of different proppant types The different mechanical properties of the various proppant types of different mineralogical composition result in different hardness and thus abrasive effect which has its influence on tanks, pumps and tubes of the equipment during course of stimulation operation execution. While the more expensive zirconia-silicate proppants (cf. section 1.3.2.2.) have a relatively gentle treating behaviour to the equipment (as a consequence of their smooth surface and their low specific gravity), the cheaper alumina oxide and silicate proppants (cf. section 1.3.2.1.) are considerably abrasive particularly in case of high alumina oxide concentration. Some aspects of sintered bauxite and zirconia-silicate proppants are outlined as follows.
1.4.6.1.1. Sintered bauxite The most severe destruction of the operating equipment is caused by sintered bauxite (which was originally developed as a by-product of abrasive manufacturing; cf. section 1.2.4.1.), because corundum is one of the mineralogically hardest phases. Careful evaluation has thus to be made in terms of calculation of total treatment cost, with the performance of sintered bauxite in the fractured reservoir having to be compared with the additional expenses by wear and tear of the pumping machinery. Flowback of sintered bauxite can be extremely hazardous, because it may result not only in cutting out of valves and Christmas trees, but also give rise to blowouts and fires when large quantities of sintered bauxite are returned with the well fluids (OSBORNE, McLEOD & SCHROEDER 1981; the same applies for sand influx through insufficient gravel packs or in case of lacking sand control; ANAND & JONES 1977; cf. section 5.2.1.3.1.).Resin-coated sand which i s in many applications a suitable replacement for intermediate- and high-strength synthetic proppants (cf. section 1.2.6.) has an excellent operational behaviour by almost not causing any hardware damage (SINCLAIR, GRAHAM & SINCLAIR 1983).
1.4.6.1.2. Zirconia-silicate proppants Particularly in case of frequent stimulation jobs in deep high-pressure reservoirs, it may be desirable to use the more expensive zirconia-silicate proppants which are (apart from resin-coated sand) the most protecting material for the pumping equipment, rather than applying the cheaper alumina oxide and silicate or sintered bauxite proppants and accept high cost for repair or even replacement of tanks, blenders, channels and strings after almost every individual large-scale operation. Equipment abrasion by hard aggressive proppants has thus significant impact on proppant selection. Low- and high-density alumina silicate and oxide intermediate-strength proppants perform much better and are considerably less destructive to the equipment than high-density bauxite highstrength proppants, but are definitely more affecting the fracturing hardware than the extremely protecting zirconia-silicate proppants or also resin-coated sand.
1.4.6.2. Equipment erosion by proppants MHF jobs typically require high injection rates and high proppant concentrations pumped at high pressures (PAI, GARBIS & HALL 1983). Larger proppant amounts pumped for longer periods of time can cause serious erosion problems in
49 t r e a t m e n t l i n e , t u b i n g , and i n and around t h e w e l l h e a d a r e a . I n terms o f p r o p p a n t types, s i n t e r e d b a u x i t e has t h e g r e a t e s t d e s t r u c t i v e impact and i s a b l e t o c o m p l e t e l y r u i n equipment when pumped f o r l o n g e r t i m e s due t o i t s h i g h a b r a s i v e ness. H i g h - d e n s i t y alumina s i l i c a t e and o x i d e proppants a l s o o c c a s i o n a l l y e x h i b i t s e r i o u s a g g r e s s i v i t y , whereas a l l t h e o t h e r proppant t y p e s g i v e o n l y i n some cases r i s e t o s u b o r d i n a t e damage and g e n e r a l l y l e a v e t h e m a t e r i a l s i n a l most o r i g i n a l c o n d i t i o n s . The most severe damages o c c u r a t diameter changes and threaded c o n n e c t i o n s which a r e b r i e f l y i l l u s t r a t e d as f o l l o w s . Comments a r e then g i v e n on p r e v e n t i o n o f proppant e r o s i o n by hardware adjustments, whereas p o s s i b i l i t i e s o f i n h i b i t i o n o f m a t e r i a l d e s t r u c t i o n by proppant s e l e c t i o n have a l r e a d y been examined i n t h e p r e v i o u s s e c t i o n .
1.4.6.2.1.
Diameter changes
E r o s i o n o f hardware by a b r a s i v e proppants occurs m o s t l y a t p l a c e s h a v i n g sudden d i a m e t e r changes o r a b r u p t a l t e r a t i o n s i n f l o w d i r e c t i o n such as elbows, tees, chicksans, and threaded c o n n e c t i o n s where n i p p l e d edges r e s t r i c t f l u i d f l o w (PARKER 1980; P A I , G A R B I S & HALL 1983). E r o s i o n problems become more sev e r e w i t h i n c r e a s i n g proppant c o n c e n t r a t i o n and r i s i n g f l u i d v e l o c i t y . A b o r e h o l e i s o l a t i o n t o o l separates t h e expensive w e l l h e a d equipment f r o m h i g h t r e a t ment p r e s s u r e s and t h u s p r o t e c t s i t f r o m e r o s i o n e f f e c t s o f t h e proppant s l u r r y , because i f e r o s i o n a t t h e t r a n s i t i o n o f t u b i n g and b o r e h o l e head would become t o o pronounced, t h e t u b i n g c o u l d r u p t u r e t h e r e b y exposing t h e h i g h p r e s sure t o c a s i n g and w e l l h e a d equipment (GARBIS, BROWN & MAURITZ 1985) which i n t h e w o r s t case would r e s u l t i n w e l l blowout (OSBORNE, McLEOD & SCHROEDER 1981).
1.4.6.2.2,
Threaded connect ions
Proppant e r o s i o n i n t r e a t m e n t l i n e and w e l l h e a d area r e q u i r e s c l o s e w a t c h i n g p a r t i c u l a r l y i n p l a c e s where diameter changes occur and a t threaded c o n n e c t i o n s where t h e edge o f t h e n i p p l e opposes t h e d i r e c t i o n o f f l u i d f l o w (PARKER 1980). Abrupt d i a m e t e r changes between b o r e h o l e head i s o l a t i o n t o o l and t u b i n g cause t u r b u l e n c e i n t h i s area which enhances t h e e r o s i v e e f f e c t o f f l u i d and p r o p p a n t s . Other p l a c e s on t h e w e l l h e a d i s o l a t i o n d e v i c e which a r e s u b j e c t t o e r o s i o n - r e l a t e d f a i l u r e a r e changeover swedge v a l v e s and o t h e r threaded connect i o n s . Changeover swedges a r e r e q u i r e d because most f r a c t u r i n g l i n e s have l a r g e r d i a m e t e r s t h a n t h e b o r e h o l e head i s o l a t i o n device, w i t h damaging e r o s i o n b e i n g c r e a t e d by t u r b u l e n c e a t d i a m e t e r changes and threaded c o n n e c t i o n s .
1.4.6.2.3. Prevention o f proppant erosion Problems w i t h v a l v e s and threaded c o n n e c t i o n s can be improved by t a p e r i n g t h e d i a m e t e r change w i t h a machined i n t e g r a l swedge, w i t h t h u s t h e a l t e r a t i o n i n diameter b e i n g e f f e c t e d over a l o n g e r i n t e r v a l t h e r e b y r e d u c i n g t u r b u l e n c e (PAI, G A R B I S & HALL 1983). Some aspects o f replacement o f t h r e a d e d by i n t e g r a l c o n n e c t i o n s as w e l l as j o i n t s and t u b u l a r s o f h i g h e r w e i g h t and grade a r e d i s cussed as f o l l o w s .
1.4.6.2.3.1.
Replacement o f threaded by i n t e g r a l connections
I n o r d e r t o f u r t h e r d i m i n u i s h t h e danger o f proppant e r o s i o n , threaded conn e c t i o n s should be e l i m i n a t e d i n a l l swedges and v a l v e s and s h o u l d be r e p l a c e d by i n t e g r a l ones. M o n i t o r i n g o f downhole t r e a t i n g p r e s s u r e s by p l a c i n g an open ended t u b i n g i n t h e c a s i n g w i t h o u t a packer i s performed by b o t t o m h o l e operat i o n p r e s s u r e r e c o r d i n g w h i l e pumping t h r o u g h m u l t i p l e e n t r i e s i n t o t h e c a s i n g t u b i n g annulus. As t h e p r o p p a n t - l a d e n f l u i d e n t e r s t h e annulus, t h e t u b i n g i s s u b j e c t e d t o an e r o s i v e c u t t i n g a c t i o n which can e v e n t u a l l y s e p a r a t e i t , w i t h b l a s t j o i n t s h a v i n g i n c r e a s e d w a l l t h i c k n e s s b e i n g a p o s s i b l e compensation o f
50 e r o s i o n i n t h i s c r i t i c a l a r e a . Choke a b r a s i o n c a n be c o n s i d e r a b l y r e d u c e d b y u s i n g remote-controlled a d j u s t a b l e tungsten c a r b i d e elements f o r w e l l cleanup (OSBORNE, McLEOD & SCHROEDER 1 9 8 1 ) . Chokes a r e s e t on a s m a l l o p e n i n g u n t i l p r e sence o r absence o f s i n t e r e d b a u x i t e g r a i n s d e r i v i n g f r o m p r o p p a n t escape due t o f r a c t u r e e v a c u a t i o n c a n be d e t e r m i n e d and choke s i z e c a n p o s s i b l y be i n c r e a sed. H i g h e a r l y drawdown p r e s s u r e s have t o be a v o i d e d and o n l y g r a d u a l l y i n c r e a s i n g choke o p e n i n g i s recommended a s t h e w e l l c l e a n s up and s t a b i l i z e s .
1.4.6.2.3.2. Joints and tubulars of higher weight and grade I n o r d e r t o m i n i m i z e o r e l i m i n a t e p r o p p a n t e r o s i o n , t h e t o p one o r t w o j o i n t s o f t u b i n g s h o u l d be o f h i g h e r w e i g h t and g r a d e t o combat t h e e f f e c t s o f t u r b u l e n c e - i n d u c e d e r o s i o n (PAI, GARBIS & HALL 1 9 8 3 ) . The t r e a t i n g l i n e diamet e r i s u s u a l l y much l a r g e r t h a n t h a t o f t h e w e l l h e a d i s o l a t i o n d e v i c e , t h u s r e q u i r i n g t h e u s e o f a changeover swedge. When t h e o p e r a t i o n i s p e r f o r m e d u s i n g t h e t r i p l e - e n t r y t e c h n i q u e b y pumping v i a t u b i n g and t u b i n g - c a s i n g a n n u l u s (NALL, CAMPBELL & BONEY 1983; P A I , GARBIS & HALL 1 9 8 3 ) , t h e h i g h - c o n c e n t r a t i o n p r o p p a n t - l a d e n s l u r r y e n t e r i n g t h e a n n u l u s a t h i g h v e l o c i t y p r o d u c e s tremendous e r o s i o n a l a c t i o n on t h e t u b i n g and may e v e n t u a l l y c u t i t . Thus h i g h - s t r e n g t h and w e i g h t b l a s t j o i n t s w i t h g r e a t e r w a l l t h i c k n e s s e s s h o u l d be u s e d i n t h i s c r i t i c a l a r e a t o w i t h s t a n d e r o s i o n caused b y i m p a c t w i t h h i g h - v e l o c i t y p r o p p a n t p a r t i c l e s . HOOVER & ADAMS ( 1 9 8 8 ) r e p o r t u t i l i z a t i o n o f a s t a t i c t u b i n g s t r i n g w h i c h n o t o n l y r e d u c e s f r i c t i o n and v i b r a t i o n t o minimum l e v e l s , b u t a l s o v i r t u a l l y e l i m i n a t e s e r o s i o n o f t u b u l a r and s t a n d a r d w e l l h e a d e q u i p m e n t . S i m i l a r e r o s i o n a l and d e s t r u c t i o n a l e f f e c t s as p r o v o k e d b y a b r a s i v e p r o p p a n t s a r e t r i g g e r e d b y r e s e r v o i r sand f l o w i n g i n t o t h e w e l l i n c a s e o f absence o r i n s u f f i c i e n c e o f g r a v e l p a c k i n g ( c f . s e c t i o n 5 . 2 . 1 . 3 . ) .
1.4.6.3. Proppant damage by equipment I n an i n v e r s e a p p r o a c h t o t h e a f o r e m e n t i o n e d , e f f e c t s o f pumping e q u i p m e n t on s a n d - l a d e n s l u r r i e s a r e d i s c u s s e d b y ROLL, HIMES, EWERT & DOERKSEN ( 1 9 8 6 ) . A s p e c t s o f g r a v e l p a c k i n g sand q u a l i t y a r e s t u d i e d b y ZWOLLE & D A V I E S (1983; c f . s e c t i o n 5 . 2 . 3 . 5 . 2 . ) , and WENOORFF, BURKE & G A R V I S ( 1 9 7 9 ) emphasize t h e i n f l u e n c e o f h a n d l i n g e q u i p m e n t on f r a c t u r i n g sand q u a l i t y . A n o n l y i n s i g n i f i c a n t amount o f f i n e s i s g e n e r a t e d b y p r o p p a n t g r a i n damage when pumping p r o p p a n t - l a den f l u i d s w i t h c e n t r i f u g a l and p o s i t i v e d i s p l a c e m e n t pumps and t h r o u g h c r o s s o v e r t o o l s a t l o w pumping r a t e s such as t h o s e u s e d i n g r a v e l p a c k i n g , b u t s i g n i f i c a n t amounts o f f i n e s may o r i g i n a t e when p r o p p a n t - l a d e n s l u r r i e s p a s s t h r o u g h s m a l l o r i f i c e s such as chokes a t h i g h pump r a t e s and p r e s s u r e s l i k e t h o s e u s e d i n h y d r a u l i c f r a c t u r i n g (ROLL, HIMES, EWERT & DOERKSEN 1 9 8 6 ) . A g e n e r a l r e d u c t i o n i n g r a i n s i z e c a n o c c u r i f p o o r - q u a l i t y sands a r e a p p l i e d , a l t h o u g h t h i s does n o t n e c e s s a r i l y l e a d t o f i n e s g e n e r a t i o n . G r a i n b r e a k a g e c a n be d i r e c t l y c o r r e l a t e d t o sand and p r o p p a n t q u a l i t y (ZWOLLE & DAVIES 1983) b y a f f e c t i n g f i r s t c h i e f l y inhomogeneous p a r t i c l e s w h i c h have i n h e r e n t m i c r o f i s s u r e s and o n l y secondly a t higher s t r e s s l e v e l s s w i t c h i n g t o r e a l crushing o f o r i g i n a l l y top-quality grains.
1.4.7. Specific gravity and fluid suspension properties Proppant s e l e c t i o n i s i n terms o f r i s k o f t r e a t m e n t f a i l u r e by screenout a l s o e s p e c i a l l y i n f l u e n c e d by s p e c i f i c proppant g r a v i t y . While r e s i s t i v i t y t o c l o s u r e s t r e s s h a s t o p p r i o r i t y and s t a b i l i t y a g a i n s t f o r m a t i o n b r i n e s i s a l s o e s s e n t i a l , many c a s e s o f a p p l i c a t i o n l e a v e t h e c h o i c e between t w o p r o p p a n t t y pes o f d i f f e r e n t s p e c i f i c g r a v i t y i n t h e advanced s t a g e o f p r o p p a n t s e l e c t i o n . I r r e s p e c t i v e o f f a c t o r s such as p r o p p a n t p r i c e ( c f . s e c t i o n 2 . 2 . 2 . ) and e q u i p ment wear and t e a r b y p r o p p a n t a b r a s i v e n e s s ( c f . s e c t i o n 1 . 4 . 6 . ) , s p e c i f i c g r a v i t y o f t h e p r o p p i n g a g e n t s i s an i m p o r t a n t q u e s t i o n i n h y d r a u l i c f r a c t u r i n g
51 and g r a v e l p a c k i n g . The h e a v i e r t h e proppant, t h e more v i s c o u s c a r r i e r f l u i d s a r e r e q u i r e d f o r reasonable and s a f e t r a n s p o r t and placement o f t h e proppant i n t h e crack, and a l s o t h e g r e a t e r t h e danger o f screenout by t o t a l p l u g g i n g o f t h e f r a c t u r e by p r o p p a n t b r i d g i n g and/or s l u r r y s t i c k i n g b e f o r e a c h i e v i n g t h e goal o f performance matching d e s i g n o f t h e o p e r a t i o n ( c f . s e c t i o n 4 . 3 . 4 . 2 . ) . Some aspects o f f l u i d v i s c o s i t y and p r o p p a n t s e t t l i n g as w e l l as s p e c i f i c g r a v i t y v s . t a p d e n s i t y a r e i l l u s t r a t e d as f o l l o w s .
1 . 4 . 7 . 1 . Fluid viscosity and proppant settling H e a v i e r proppants a r e much e a s i e r t e n d i n g t o s e t t l e i n t h e s t i m u l a t i o n f l u i d s b e f o r e t h e f r a c t u r e c l o s e s t o t r a p them ( c f . s e c t i o n 4 . 1 2 . 2 . ) . The c o n s i d e r a b l y l o w e r s p e c i f i c g r a v i t y i s one o f t h e major advantages o f h i g h - s t r e n g t h z i r c o n i a - s i l i c a t e proppants i n deep h i g h - p r e s s u r e r e s e r v o i r s where t h i s p r o d u c t can even t e c h n i c a l l y compete w i t h i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants because o f s t i l l b e i n g l i g h t e r ( c f . s e c t i o n 1 . 3 . 2 . and t a b . 3 ) , w i t h t h e c o n s i d e r a b l e p r i c e d i f f e r e n c e between z i r c o n i a - and a l u m i na-based proppants, however, b e i n g t h e o t h e r p a r t o f t h e s t o r y and r e p r e s e n t i n g t h e economical drawback ( c f . s e c t i o n 2 . 2 . 2 . 3 . ) . I n terms o f t h e d i f f e r e n t t y p e s o f alumina o x i d e and s i l i c a t e proppants, p r e f e r e n c e should always be g i v e n t o t h e l i g h t e s t p r o d u c t i f s u f f i c i e n t freedom o f c h o i c e i s p r o v i d e d by t h e r e q u i r e d c l o s u r e s t r e s s r e s i s t i v i t y l e v e l . The reasons f o r t h i s a r e t h a t t h e lower t h e s p e c i f i c proppant g r a v i t y , t h e lower t h e r i s k o f premature screenout f a i l u r e ( c f . s e c t i o n s 4 . 2 . 2 . 2 . 3 . and 6 . 2 . 4 . 2 . 1 . ) , t h e b e t t e r t h e v e r t i c a l h e i g h t containment o f t h e crack by a p p l i c a t i o n o f l o w e r - v i s c o s i t y f l u i d s o p e r a t i n g w i t h l o w e r p r e s s u r e ( c f . s e c t i o n 4 . 2 . 2 . 3 . ) , and t h e cheaper t h e whole operat i o n . P a r t i c l e t r a n s p o r t i n v i s c o u s and v i s c o e l a s t i c f r a c t u r i n g f l u i d s i s a l s o e v a l u a t e d by ACHARYA (1986, 1987), and proppant s e t t l i n g c o r r e l a t i o n s a r e d i s cussed by SHAH ( 1 9 8 6 ) . The g r e a t e s t advantage o f d e c r e a s i n g s p e c i f i c proppant g r a v i t y i s t h a t i d e n t i c a l proppant t r a n s p o r t w i t h r e s p e c t t o t h a t o f h e a v i e r proppants i n h i g h e r v i s c o s i t y f l u i d s can t a k e p l a c e f o r l i g h t e r proppants i n l o w e r - v i s c o s i t y f l u i d s which a r e t h e r e f o r e pumpable a t l o w e r p r e s s u r e (CUTLER, ENNISS, JONES & CARROLL 1 9 8 3 ) . V e r t i c a l p r o p a g a t i o n o f t h e f r a c t u r e can be l i m i t e d by r e d u c i n g f r a c t u r i n g f l u i d p r e s s u r e (SIMONSON, ABOU-SAYEO & CLIFTON 1978; NOLTE 1982; c f . sect i o n 4 . 2 . 2 . 3 . ) . The v i s c o s i t y range o f f r a c t u r i n g f l u i d s makes m i n i m i z i n g o f f l u i d v i s c o s i t y a much more e f f e c t i v e method o f c o n t r o l l i n g p r e s s u r e t h a n l o w e r i n g pumping r a t e , and a l t e r n a t i v e l y , h i g h e r proppant volumes can be i n j e c t e d i n a g i v e n amount o f h i g h - v i s c o s i t y f r a c t u r i n g f l u i d s ( c f . s e c t i o n 4 . 3 . 4 . 2 . ) . L i g h t e r proppants can a l s o be i n s e r t e d i n h i g h e r c o n c e n t r a t i o n s i n t o t h e f r a c t u r e t h a n h e a v i e r p r o p p a n t s w i t h t h e same t y p e o f c a r r i e r f l u i d , and placement o f i d e n t i c a l s a t u r a t i o n s r e q u i r e s o n l y a l o w e r - v i s c o s i t y t r a n s p o r t medium f o r l i g h t e r proppants w i t h r e s p e c t t o t h a t needed f o r d e p o s i t i o n o f h e a v i e r p r o p p a n t s . Another economical advantage o f l i g h t e r proppants i s t h e i r h i g h e r volume p e r w e i g h t u n i t which r e s u l t s i n a t w o - f o l d s a v i n g o f s t i m u l a t i o n expenses, comprisi n g f i r s t t h e cheaper base p r i c e o f l i g h t e r proppants and second t h e r e q u i r e ment o f l e s s w e i g h t u n i t s t o meet a designed v o l u m e t r i c q u a n t i t y o f m a t e r i a l (SINCLAIR, GRAHAM & SINCLAIR 1983; c f . a l s o s e c t i o n s 1 . 2 . 6 . and 1 . 4 . 1 2 . 5 . ) .
1.4.7.2,
Specific gravity
vs.
tap density
I n c o n t r a s t t o s p e c i f i c g r a v i t y which i n d i c a t e s t h e c h a r a c t e r i s t i c s o f t h e whole proppant g r a i n s , t a p d e n s i t y o f t h e m a t e r i a l r e f l e c t s t h e amount o f i n t e r connected m i c r o p o r o s i t y w i t h i n t h e g r a i n s (CUTLER, ENNISS, JONES & CARROLL 1983). I n t e r c o n n e c t e d m i c r o p o r o s i t y does n o t o n l y p r o v i d e a h i g h e r s u r f a c e area f o r t h e a t t a c k o f a c i d s and a g g r e s s i v e b r i n e s , b u t a l s o a f f e c t s proppant t r a n s p o r t b e h a v i o u r i f t h e f r a c t u r i n g f l u i d o r i t s f i l t r a t e can a l s o p e n e t r a t e t h e m i c r o p o r o s i t y network, because t h e proppant w i l l t h e n a c t h e a v i e r t h a n i n d i c a -
t e d b y t a p d e n s i t y I n t e r m e d i a t e - s t r e n g t h p r o p p a n t s have s i g n i f i c a n t l y l o w e r t a p d e n s i t i e s t h a n s i n t e r e d b a u x i t e and appear t o have a d e q u a t e s t r e n g t h f o r t h e m a j o r i t y o f h y d r a u l i c f r a c t u r i n g a p p l i c a t i o n s (CUTLER, E N N I S S , JONES & SWANSON 1 9 8 5 ) .
1.4.8. Grain size and embedment
1.4.8.1. Reservoir composition and proppant requirement S o f t formations r e q u i r e a large-diameter propping agent t h a t w i l l r e s i s t t o embedment ( c f . s e c t i o n 4 . 3 . 3 . ) . Medium- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s need a l a r g e - s i z e proppant i n o r d e r t o p r o v i d e adequate flow c a p a c i t y w i t h a s t r o n g p e r m e a b i l i t y c o n t r a s t ( c f . a l s o s e c t i o n s 4 . 5 . 1 . 1 . 2 . and 4 . 6 . 1 . ) , w i t h t h e o p t i mum b e i n g a w i d e and h i g h l y - c o n d u c t i v e f r a c t u r e i n f i l l e d b y p r o p p a n t m u l t i l a y e r s ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) . Propping o f cracks i n hard rocks necessitates a m a t e r i a l w i t h h i g h compressive s t r e n g t h t o r e s i s t t o t h e h i g h c l o s u r e s t r e s s e x e r t e d , and s i n c e embedment i s i n s i g n i f i c a n t i n h a r d r o c k s w i t h c o n t i n u o u s r e servoir distribution, s m a l l - s i z e p r o p p a n t can be u t i l i z e d w h i c h a l l o w s an i n c r e a s e d pack c o n c e n t r a t i o n i n an o l i g o - t o m u l t i l a y e r f a b r i c c o u n t e r a c t i n g t h e high stress. H i g h l y - l i t h i f i e d h a r d r o c k s a r e u s u a l l y more o r l e s s t i g h t and do n o t r e q u i r e l a r g e f l o w c a p a c i t i e s w i t h extreme p e r m e a b i l i t y c o n t r a s t s , w i t h t h e b e s t treatment design being long, n a r r o w , h i g h l y - p a c k e d f r a c t u r e s (CLARK 1 9 8 3 ) , whereas s o f t t o m o d e r a t e l y - c o n s o l i d a t e d f o r m a t i o n s a r e b e t t e r s t i m u l a t e d w i t h s h o r t e r and w i d e r f r a c t u r e s packed b y p r o p p a n t m u l t i l a y e r s . I f composed o f d i s c r e t e l e n t i c u l a r sand b o d i e s ( c f . s e c t i o n 3 . 4 . 1 . 1 . 1 . ) t h a t a r e s e p a r a t e d b y mudstones, h a r d s a n d s t o n e r e s e r v o i r complexes a l s o s u f f e r f r o m p r o p p a n t embedment w h i c h o c c u r s i n t h e i n t e r v e n i n g mudstone u n i t s .
P r o p p a n t embedment i s i n f l u e n c e d b y p r o p p a n t c o n c e n t r a t i o n , p r o p p a n t s i z e , p r o p p a n t d i s t r i b u t i o n , r o c k t y p e and embedment s u r f a c e , and v a r i a b l e s a f f e c t i n g f r a c t u r e c l o s u r e a r e c l o s u r e pressure, proppant size, proppant d i s t r i b u t i o n , proppant saturation, f o r m a t i o n h a r d n e s s and s u r f a c e t o u g h n e s s (VOLK, RAIHLE, CARROLL & SPEARS 1981; c f . s e c t i o n 4 . 3 . 3 . ) . The r a t e o f f r a c t u r e c l o s u r e depends on t h e p r o p p a n t amount p e r u n i t a r e a o r t h e p e r c e n t p r o p p a n t m o n o l a y e r . P r i n c i p a l l y f r a c t u r e c l o s u r e w i t h a 100 % m o n o l a y e r i s t h e same f o r a 50 % monol a y e r a t h a l f t h e c l o s u r e p r e s s u r e and f o r a 25 % m o n o l a y e r a t one q u a r t e r t h e c l o s u r e p r e s s u r e , r e s p e c t i v e l y . P r o p p a n t embedment i n t o mudstone i s more r d p i d i n i t i a l l y b u t q u i c k l y becomes l i n e a r l y dependent on c l o s u r e p r e s s u r e , w i t h t h e t r a n s i t i c n o c c u r r i n s f a s t e r t q a n t h a t f o r s a n d s t c n e e n b e d i e r , t as a consequence o f t h e f i n e r g r a i n s i t e o f s h a l e s w h i c h o f f e r s s u p p o r t f o r t h e p r o p p a n t soon a f t e r embedment commences. T h e r e f o r e embedment w i t h a c o m p l e t e m o n o l a y e r i s a c t u a l l y l e s s f o r shale, w i t h t h e r a t e o f c l o s u r e o f sandstones dropping a t i n creasing pressure u n t i l i t i s less than shale.
A t h i g h c l o s u r e p r e s s u r e s a f t e r s i g n i f i c a n t embedment has o c c u r r e d , t h e r u b b l i z e d s a n d s t o n e b e i n g pushed ahead o f t h e embedding p r o p p a n t b e g i n s t o f i l l t h e p o r e spaces i n t h e r o c k and t o d i s t r i b u t e t h e l o a d more u n i f o r m l y , w i t h t h e s t r o n g e r s a n d s t o n e m a t r i x u l t i m a t e l y r e s u l t i n g i n a l o w e r p r o p p a n t embedment r a t e t h a n i s p o s s i b l e w i t h s h a l e . The r a t e o f f r a c t u r e c l o s u r e f o r s a n d s t o n e s i n c r e a s e s as p r o p p a n t c o v e r a g e d e c r e a s e s , whereas t h a t f o r s h a l e s i s d i r e c t l y r e l a t e d t o p r o p p a n t c o n t a c t a r e a o r number o f p r o p p a n t p e r u n i t a r e a . S h a l e s
53 propped by a p a r t i a l p r o p p a n t monolayer o f 50 % o r g r e a t e r r e s i s t t o f r a c t u r e c l o s u r e about as w e l l as r e s e r v o i r sandstones, b u t t h e i n t e r a c t i o n w i t h f r a c t u r i n g f l u i d s could s i g n i f i c a n t l y a l t e r the r a t e o f f r a c t u r e closure i n shales. As proppant coverage drops below 50 % o f a monolayer, t h e r a t e o f f r a c t u r e c l o sure i s much more r a p i d than f o r sandstones. A t h i g h e r c l o s u r e s t r e s s e s , f r a c t u r e w i d t h i s almost independent o f s u r f a c e roughness. Embedment aspects o f h i g h - s t r e n g t h proppants i n t o l o w - p e r m e a b i l i t y r e s e r v o i r r o c k s a r e a l s o d i s c u s sed by MUCH (1987) and MUCH & PENNY (1987), and proppant embedment v s . c r u s h i n g i s discussed a l o n g t h e l i n e s o f proppant c o n c e n t r a t i o n i n t h e c a r r i e r f l u i d s i n s e c t i o n 4.3.
1.4.9. Manufacturing process and pellet composition Raw m a t e r i a l c o m p o s i t i o n and m a n u f a c t u r i n g process have an i m p o r t a n t i n f l u e n ce on p r o p p a n t p e l l e t m i c r o s t r u c t u r e and i n t e g r i t y and t h u s s t r e n g t h and p e r f o r mance o f t h e p r o p p a n t s . G e n e r a l l y a v a r i e t y o f f a b r i c a t i o n methods can be used t o make proppants i n c l u d i n g m i x - p e l l e t i z a t i o n , spray d r y i n g , d r o p l e t g e n e r a t i o n and c o a t i n g (CUTLER, ENNISS, JONES & SWANSON 1985; c f . s e c t i o n 1 . 2 . 7 . ) , w i t h f i n a l proppant h a r d e n i n g o f raw agglomerations by s i n t e r i n g o r f u s i n g . D i s t i n c t i o n has t o be made between alumina o x i d e and s i l i c a t e proppants and z i r c o n i a s i l i c a t e p r o p p a n t s . Some comments a r e a l s o g i v e n on n a t u r a l sand.
1.4.9.1. Alumina oxide and silicate proppants Alumina o x i d e and s i l i c a t e proppants a r e manufactured by m i l l i n g o r g r i n d i n g t h e raw m a t e r i a l t o a f i n e powder p a r t i c l e s i z e , f o r m i n g s p h e r i c a l p e l l e t s u s i n g a d i s k p e l l e t i z e r , and f i n a l d e n s i f i c a t i o n ( s i n t e r i n g ) o f t h e p e l l e t s (COBB & FARRELL 1986) by f l u i d i z e d bed a g g l o m e r a t i o n (LUNGHOFER 1984) o r h i g h speed r o t a r y m i x i n g (FITZGIBBON 1984). C r y s t a l phase f o r m a t i o n and d e n s i f i c a t i o n o f the p e l l e t s are t y p i c a l l y c a r r i e d o u t i n a r o t a r y k i l n a t a s i n t e r i n g temperature up t o a b t . 1,500 OC (CUTLER, JONES, SWANSON & CARROLL 1 9 8 1 ) . Some aspects o f g r i n d e d powder p a r t i c l e s i z e v s . p e l l e t i n t e g r i t y , s i n t e r i n g temperat u r e and k i l n environment, and a r t i f i c i a l p e l l e t f o r m a t i o n v s . n a t u r a l p i s o l i t e p r o c e s s i n g a r e d i s c u s s e d as f o l l o w s .
1.4.9.1.1. Grinded powder particle size vs. pellet integrity Changes i n ground powder p a r t i c l e s i z e d i s t r i b u t i o n and mean a f f e c t o t h e r process v a r i a b l e s and f i n a l p e l l e t i n t e g r i t y . The c o r r e c t powder p a r t i c l e s i z e d i s t r i b u t i o n i s e s s e n t i a l f o r o p t i m a l p a c k i n g i n t h e p e l l e t i z a t i o n process and f o r more complete d e n s i f i c a t i o n i n t h e s i n t e r i n g process. F i n e r g r i n d i n g o f t h e raw m a t e r i a l n o r m a l l y r e s u l t s i n a more compact, s t r o n g e r p e l l e t , w i t h t h i s bei n g b e n e f i c i a l i n case o f u t i l i z a t i o n o f lower-grade o r e s where f i n e r g r i n d i n g i s e s s e n t i a l t o produce s t r e n g t h s s i m i l a r t o those o b t a i n e d f r o m h i g h e r - g r a d e o r e s w i t h c o a r s e r g r i n d i n g (COBB & FARRECL 1986). F i n e r p a r t i c l e s i z e s o f t h e p r o p p a n t s always r e s u l t f r o m l o n g e r g r i n d i n g b e f o r e p e l l e t i z a t i o n and n o t f r o m the d i f f e r e n c e i n p e l l e t i r a t i o n t e c h n i q u e (CUTLER, JONES, SWANSON & CARROLL 1981).
1.4.9.1.2, Sintering temperature and kiln environment I n a d d i t i o n t o raw m a t e r i a l g r i n d i n g , s i n t e r i n g i s a l s o c r i t i c a l . The f i n a l c r y s t a l s i z e s depend a l s o on t i m e - t e m p e r a t u r e r e l a t i o n s h i p s as w e l l as atmospher i c a l c o n d i t i o n s i n t h e k i l n where t h e p e l l e t s a r e f i r e d f o r t h e purpose o f m i n e r a l o g i c a l t r a n s f o r m a t i o n o f b a u x i t e and c l a y i n t o corundum and m u l l i t e . K i l n c o n d i t i o n s a r e a l s o r e s p o n s i b l e f o r t h e occurrence o f d i f f u s i o n , r e c r y s t a l l i z a t i o n and g r a i n growth. Raw m a t e r i a l c h e m i s t r y t h u s i n t e r r e l a t e s w i t h manufactur i n g v a r i a b l e s t o d e f i n e proppant m i c r o s t r u c t u r e and t h u s performance o f t h e
54 p r o d u c t (COBB & FARRELL 1 9 8 6 ) . Sphere d i a m e t e r i n c r e a s e s as s i n t e r i n g t e m p e r a t u r e d e c r e a s e s s i n c e t h e beads do n o t s h r i n k as much a t l o w e r t e m p e r a t u r e s (CUTLER, JONES, SWANSON & CARROLL 1 9 8 1 ) .
1.4.9.1.3.Art if icial pellet formation vs. natural pisolite processing E x p e r i e n c e has shown t h a t g r i n d i n g o f r a w m a t e r i a l , f o r m a t i o n o f p e l l e t s , and s i n t e r i n g o f t h e p e l l e t s b y f i r i n g i n t h e k i l n p r o d u c e s much h i g h e r c r y s t a l l i t e i n t e r l o c k i n g and t h u s p r o p p a n t s t r e n g t h t h a n p r o c e s s i n g o f n a t u r a l l y o c c u r r i n g b a u x i t e p e l l e t s i n p i s o l i t h i c d e p o s i t s (COMALCO 1983, 1 9 8 6 ) b y c l e a n i n g and s i e v i n g and subsequent s t r a i g h t f o r w a r d s i n t e r i n g b y f i r i n g i n t h e k i l n . The reason f o r the i n f e r i o r q u a l i t y o f t h e f i r e d n a t u r a l p i s o l i t e s are i n t e r n a l inhomogeneities which provoke a g r e a t e r percentage o f breaking o f g r a i n s d u r i n g f i r i n g , and e v e n i n t h o s e g r a i n s w h i c h p a s s t h e q u a l i t y s p e c i f i c a t i o n a f t e r p r o d u c t i o n r e m a i n as weakness f i s s u r e s t h a t r e s u l t i n e a r l i e r c r u s h i n g o f t h e p r o c e s s e d n a t u r a l p e l l e t s w i t h r e s p e c t t o t h e c o m p l e t e l y s y n t h e t i c p r o p p a n t s . Another f a c t o r i s o f t e n the onion-type l a y e r s t r u c t u r e o f the p i s o l i t e s which i s comparable t o t h a t o f carbonate ooids, w i t h t h e s u i t e o f c o n c e n t r i c a l p e l l i c l e s a l s o i n c l u d i n g p o t e n t i a l i n s t a b i l i t y c e n t r e s and b r e a k i n g e a s i e r away a t e l e v a ted closure stresses ( c f . section 1.3.1.2.). A r t i f i c i a l l y formed p e l l e t s a r e c h a r a c t e r i z e d b y an a l m o s t i s o t r o p i c d i s t r i b u t i o n o f powder p a r t i c l e s w h i c h upon f i r i n g i n t h e k i l n r e s u l t s i n n e a r l y i s o t r o p i c d i s t r i b u t i o n o f corundum and m u l l i t e c r y s t a l l i t e s and t h u s p r o v i d e s t h e g r a i n w i t h v e r y h i g h s t r e n g t h due t o c r y s t a l l i t e i n t e r l o c k i n g i n a l l d i r e c t i o n s .
1.4.9.2.Zi rconia-s i 1 icate proppant s Z i r c o n i a - s i l i c a t e p r o p p a n t s a r e p r o d u c e d b y m e l t i n g c e r a m i c m a t e r i a l s and p r e p a r i n g p r o p p a n t s b y u s i n g a d r o p l e t g e n e r a t o r (CUTLER, ENNISS, JONES & CARROLL 1983; CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) . I n an e f f o r t t o m i n i m i z e s u r f a c e a r e a , t h e m o l t e n d r o p l e t s f o r m i n t o f u s e d s p h e r e s upon c o o l i n g and s o l i d i f i c a t i o n . This procedure almost e l i m i n a t e s generation o f i n t e r n a l m i c r o p o r o s i t y i n t h e amorphous o r g l a s s y groundmass i n c o n t r a s t t o v a r i o u s amounts o f i n t e r s t i t i a l s between i n t e r l o c k i n g c r y s t a l l i t e s i n a l u m i n a s i l i c a t e and o x i d e p r o p p a n t s ( c f . s e c t i o n 1 . 4 . 1 . ) . I n a d d i t i o n , v a r i o u s g r a i n s i z e s a r e p r o d u c e d much e a s i e r t h a n b y s i n t e r i n g o f a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s due t o t h e absence o f f o r m a t i o n o f r a w - m a t e r i a l p e l l e t s o f p r e d e t e r m i n e d s i z e and d i f f i c u l t y o f e x a c t adjustment o f a narrow g r a i n s i z e i n t e r v a l by d r o p l e t generation, and t h e r e f o r e a l s o e x t r e m e g r a i n s i z e s c a n b e p r o d u c e d w i t h l e s s h a n d l i n g p r o blems b y s i m p l e s i e v i n g o f t h e t o t a l m a t e r i a l assemblage a f t e r m a n u f a c t u r i n g . On t h e o t h e r hand, t h e f u s i n g p r o c e d u r e i n c l u d e s much more r e j e c t s as a consequence o f i n t e r n a l i n h o m o g e n e i t i e s , e x t e r n a l a p p e n d i c e s and b r o k e n g r a i n s , w h i c h makes t h e m a n u f a c t u r i n g p r o c e s s a t t h e b o t t o m o f t h e l i n e more e x p e n s i v e and i s a l s o t h e r e a s o n f o r t h e h i g h e r p r i c e o f z i r c o n i a - s i l i c a t e p r o p p a n t s i n c o m p a r i s o n t o a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s . I n t h e c o n t e x t o f p r o d u c t i o n , p a c k a g i n g , h a n d l i n g and d e l i v e r i n g o f p r o p p a n t s , an i m p o r t a n t means t o keep s t a n d a r d s p e c i f i c a t i o n s , t o enhance q u a l i t y m a i n t e n a n c e and t o r e i n f o r c e t h e d e s i g n e d and e x p e c t e d p e r f o r m a n c e l e v e l i s ons i t e q u a l i t y c o n t r o l o f f l u i d s and p r o p p a n t s p r i o r t o t h e f r a c t u r i n g j o b (WENDORFF, BURKE & G A R V I S 1979; ROBINSON 1985; SHAH, LEE & JENSEN 1985; HALL & LARK I N 1986).
1.4.9.3. Natural sand N a t u r a l sand p r o d u c e d f r o m t h e same g e o l o g i c a l f o r m a t i o n c o u l d b e p r i n c i p a l l y e x p e c t e d t o e x h i b i t more o r l e s s homogeneous q u a l i t y d i s t r i b u t i o n and t o show o n l y d i f f e r e n c e s i n g r a i n s i z e a c c o r d i n g t o d e p o s i t i o n a l e n v i r o n m e n t and
55 f a c i e s t y p e . I n r e a l i t y , however, f i r s t o r i g i n a l v a r i a t i o n s i n c l u d e a l s o g r a i n c o m p o s i t i o n and s t a b i l i t y as a consequence o f d i f f e r e n t palaeogeographical p r o venance and second a r t i f i c i a l d i s c r e p a n c i e s a r e i n t r o d u c e d by d i f f e r e n t product i o n , p r e p a r a t i o n and h a n d l i n g techniques o f t h e v a r i o u s companies. P r i m a r y var i a t i o n s o f g r a i n c o m p o s i t i o n and s t a b i l i t y comprise t h e number o f i n d i v i d u a l c r y s t a l s b u i l d i n g up q u a r t z sand g r a i n s , w i t h d i s t i n c t i o n h a v i n g t o be made b e t ween mono-, o l i g o - and p o l y c r y s t a l l i n e aggregates. As l a r g e r g r a i n boundaries a r e always p o s s i b l e weakness planes, m o n o c r y s t a l l i n e p a r t i c l e s c o n s i s t i n g o f t h e rounded and worn fragment o f a s i n g l e q u a r t z c r y s t a l have t h e h i g h e s t s t a b i l i t y , whereas o l i g o c r y s t a l l i n e beads b e i n g c o n s t r u c t e d by a few i n t e r l o c k i n g q u a r t z c r y s t a l s may be s u b j e c t e d t o breakage i n t o h a l v e s o r q u a r t e r s by s l i d i n g and p a r t i n g a l o n g t h e c o n t a c t p l a n e s u n l e s s t h e g r a i n boundaries a r e s u t u r e d o r s t y l o l i t h i c i n s t e a d o f concave-convex. P o l y c r y s t a l l i n e q u a r t z aggregates b e i n g b u i l t up o f numerous t i n y i n t e r l o c k i n g c r y s t a l l i t e s e x h i b i t v e r y h i g h s t a b i l i t y due t o s i m i l a r p r e s s u r e l o a d r e d i s t r i b u t i o n and p a r t i a l i n t e r n a l e q u a l i z a t i o n as r e f l e c t e d by m i c r o c r y s t a l l i n e alumina s i l i c a t e and o x i d e p r o p p a n t s . I n c l u s i o n s such as bubble b e l t s , mica t r a i n s and r u t i l e needle g i r d l e s i n q u a r t z g r a i n s may be a d d i t i o n a l sources o f weakness and breakage by s l i d i n g and p a r ting. Sandstone m a t u r i t y as expressed by percentage o f w e l l - r o u n d e d ( t e x t u r a l matur i t y ) and/or m o n o c r y s t a l l i n e ( m i n e r a l o g i c a l m a t u r i t y ) q u a r t z g r a i n s t h e r e f o r e has an i m p o r t a n t impact on c l o s u r e s t r e s s r e s i s t i v i t y o f t h e f r a c t u r e p r o p p i n g m a t e r i a l , and as sandstone m a t u r i t y i s determined by t h e p a l a e o g e o g r a p h i c a l and p a l a e o t e c t o n i c a l e v o l u t i o n and t o minor amounts a l s o by t h e p a l a e o c l i m a t o l o g i c a l h i s t o r y o f t h e sandstone sequence, s u i t a b i l i t y as f r a c t u r e p r o p p i n g mater i a l i s d i r e c t l y r e l a t e d t o d e p o s i t i o n a l environmental c o n d i t i o n s i n t h e s e d i mentary b a s i n . Secondary e f f e c t s i n c l u d e p r o c e s s i n g parameters i n t h e sandstone q u a r r y o r sand p i t such as m a t e r i a l access by d i g g i n g o r b l a s t i n g , m a t e r i a l d i s i n t e g r a t i o n by l o a d i n g , shaking o r m i l l i n g , m a t e r i a l s e p a r a t i o n by f l o a t i n g , and m a t e r i a l s c r e e n i n g by s i e v i n g . As a consequence o f p o s s i b l e s i g n i f i c a n t i m p a c t o f a l l t h e mentioned f a c t o r s , WENOORFF (1982) recommends t o t e s t samples p r o v i d e d by a l l t h e d i f f e r e n t companies even when coming f r o m t h e i d e n t i c a l geol o g i c a l f o r m a t i o n , because s e p a r a t i o n , c l e a n - u p and s i e v i n g procedures g i v e somewhat d i f f e r e n t p a r t i c l e - s i z e d i s t r i b u t i o n . I n a s i m i l a r way, s y n t h e t i c p r o p p a n t s manufactured by d i f f e r e n t companies may a l s o have d i f f e r e n t s u r f a c e areas and p e r m e a b i l i t i e s .
1.4.10. Conductivity discount The most i m p o r t a n t p r o p e r t y f o r p o s t - o p e r a t i o n a l performance and t h u s e f f e c t i v i t y o f h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n i s proppant c o n d u c t i v i t y (COBB & FARRELL 1986). Crack c o n d u c t i v i t y i s a f u n c t i o n o f p r o p p a n t p r o p e r t i e s ( s t r e n g t h , s i z e , composition, roundness, s p h e r i c i t y , d i s t r i b u t i o n , concentration, content o f f i n e s ) , c l o s u r e s t r e s s l o a d on t h e p r o p p a n t package, p r e s s u r e drawdown and f l u i d f l o w r a t e , f o r m a t i o n p r o p e r t i e s ( p r o p p a n t embedment o r c r u s h i n g c o n d i t i o n s ) , p o t e n t i a l p l u g g i n g f r o m f r a c t u r i n g f l u i d r e s i d u e o r g e l f i l t e r cake, and r e s u l t a n t propped c r a c k w i d t h (MONTGOMERY & SJEANSON 1 9 8 5 ) . W i t h i n t h e l a s t decade, s i g n i f i c a n t advances have been made i n a n a l y s i s o f p r e s s u r e t r a n s i e n t d a t a f o r h y d r a u l i c a l l y f r a c t u r e d w e l l s (McDANIEL 1988). One o f t h e most meaningf u l developments has been methods a l l o w i n g an o p e r a t o r t o c a l c u l a t e p o s t - t r e a t ment e s t i m a t e s o f propped f r a c t u r e l e n g t h and crack c o n d u c t i v i t y . I n many i n stances, however, c a l c u l a t e d f r a c t u r e c o n d u c t i v i t y values have been c o n s i d e r a b l y l o w e r than those p r e d i c t e d by l a b o r a t o r y proppant t e s t i n g , t h u s s t i m u l a t i n g i n d u s t r y i n t e r e s t i n t h e use o f e x p e r i m e n t a l c o n d i t i o n s more c l o s e l y approachi n g i n - s i t u parameters. R e a l i s t i c proppant c o n d u c t i v i t y data are essential t o both hydraulic f r a c t u r i n g t r e a t m e n t d e s i g n and a c c u r a t e f o r e c a s t o f t h e r e s u l t a n t f r a c t u r e d w e l l p r o d u c t i o n when u s i n g a computerized r e s e r v o i r s i m u l a t o r t o model p o s t - s t i m u l a t i o n w i t h d r a w a l c a p a b i l i t y (McDANIEL 1987, 1988). A f t e r d i s c u s s i o n o f some g e n e r a l
56 a s p e c t s i n c l u d i n g l a b o r a t o r y v s . f i e l d c o n d u c t i v i t y d i s c r e p a n c y and i m p a c t o f experimental techniques, the i m p l i c a t i o n s o f long-term proppant s t a b i l i t y t e s t i n g a r e o u t l i n e d , and t h e r e s u l t s a r e t h e n summarized i n recommended c o n d u c t i v i t y c o r r e c t i o n f a c t o r s . F i n a l l y , some examples o f p r o p p a n t p e r f o r m a n c e i n l o n g term f i e l d a p p l i c a t i o n a r e mentioned.
1.4.10.1. Laboratory vs. field conductivity discrepancy Post-fracture pressure analysis frequently calculates proppant c o n d u c t i v i t y and e f f e c t i v e c r a c k l e n g t h s t h a t f a l l s h o r t o f t h e d e s i g n e d v a l u e s (McDANIEL 1986, 1987, 1988; MUCH 1987, PARKER & McDANIEL 1987, McDANIEL & PARKER 1 9 8 8 ) . T r e a t m e n t s c h e d u l e s t h e r e f o r e need t o be m o d i f i e d t o t a k e i n t o a c c o u n t a more r e a l i s t i c v a l u e o f f r a c t u r e c o n d u c t i v i t y u n d e r i n - s i t u c o n d i t i o n s . More a c c u r a t e l a b o r a t o r y m o d e l l i n g o f t h e p r o p p a n t bed e n v i r o n m e n t u n d e r s i m u l a t e d c l o s u r e s t r e s s and t e m p e r a t u r e c o n d i t i o n s i n t h e p r e s e n c e o f g e l f i l t e r c a k e s ( c f . section 1.4.10.3.5. and 4 . 3 . 4 . 1 . 4 . ) w h i c h were f o r m e d u n d e r dynamic c o n d i t i o n s has p r o v i d e d a s u i t a b l e a p p r o a c h t o t h i s p r o b l e m . C o n c e r n i n g l a b o r a t o r y v s . i n s i t u data, SECCOMBE & ANDERSON ( 1 9 8 2 ) r e p o r t t h a t s t a n d a r d i z e d e x p e r i m e n t a l d a t a have t o be d i s c o u n t e d u p t o 90 %, w i t h t h e r e m a i n i n g 1 0 % b e i n g t h e e f f e c t i v e c o n d u c t i v i t y o f t h e p r o p p a n t package i n p l a c e i n t h e f r a c t u r e u n d e r r e s e r v o i r c o n d i t i o n s ( c f . a l s o DGMK 1983, 1 9 8 6 ) . P o s t - f r a c t u r e o p e r a t i o n performance a n a l y s i s t y p i c a l l y c a l c u l a t e s a c o n d u c t i v i t y v a l u e s i g n i f i c a n t l y l o w e r t h a n t h a t p r e d i c t e d b y l a b o r a t o r y measurements (McDANIEL 1986, 1987, 1988; PARKER & McDANIEL 1 9 8 7 ) . HICKEY, BROWN & CRITTENDEN ( 1 9 8 1 ) a l s o comment t h a t p r o p p a n t p e r f o r m a n c e u n d e r downhole c o n d i t i o n s o f t e n does n o t d u p l i c a t e t h e r e s u l t s o f l a b o r a t o r y e v a l u a t i o n s . D r a s t i c a l d i f f e r e n c e s o f c r a c k and p r o p p a n t c o n d u c t i v i t y a r e a l r e a d y e v i d e n t when c o m p a r i n g t h e r e s u l t s o f l o n g - t e r m h i g h - t e m p e r a t u r e and s h o r t - t e r m a m b i e n t - t e m p e r a t u r e laborat o r y t e s t i n g (COOKE 1973; McDANIEL 1986, 1 9 8 7 ) . A c c u r a t e d e s i g n o f f r a c t u r i n g t r e a t m e n t s r e q u i r e s c o n d u c t i v i t y measurements a t s i m u l a t e d r e s e r v o i r c o n d i t i o n s (McDANIEL & PARKER 1 9 8 8 ) . VEATCH ( 1 9 7 7 ) i n t r o d u c e s a c o m p u t e r i z e d method f o r det e r m i n i n g i n - s i t u f r a c t u r e f l o w c a p a c i t y i n l o w - p e r m e a b i l i t y gas-bearing format i o n s , and VLIS, HAAFKENS, SCHIPPER & V I S S E R ( 1 9 7 5 ) p r e s e n t c r i t e r i a f o r p r o p p a n t p l a c e m e n t and f r a c t u r e c o n d u c t i v i t y . Some comments a r e o f f e r e d as f o l l o w s on p r o p p a n t b e h a v i o u r and r e a c t i o n t o s t r e s s as w e l l as on p r o p p a n t g r a i n s i z e effects.
1.4.10.1.1. Proppant behaviour and reaction to s t r e s s The r e a s o n s f o r t h e d i s c r e p a n c y i n f r a c t u r e c o n d u c t i v i t y between l a b o r a t o r y t e s t i n g and downhoTe p e r f o r m a n c e i n c l u d e embedment o f b r i t t l e p r o p p a n t s i n t o t h e f o r m a t i o n , p l u g g i n g o f p o r e s b y f r a c t u r i n g g e l r e s i d u e s and f l u i d - l o s s f i l t e r cakes, d i s c o n t i n u i t i e s i n crack geometry, s t r e s s c o r r o s i o n o f proppants b y l e a c h i n g and c r u s h i n g and o t h e r phenomena n o t t y p i c a l l y measured i n l a b o r a t o r y e x p e r i m e n t s (COOKE 1973, ALMOND & BLAND 1984; McDANIEL 1986, 1987, 1988; PARKER & McDANIEL 1987, McDANIEL & PARKER 1 9 8 8 ) . E n t r a i n m e n t and d e p o s i t i o n o f f i n e p a r t i c l e s i n p o r o u s media a r e a l s o e v a l u a t e d b y GRUESBECK & COLLINS ( 1 9 8 2 ) . S i g n i f i c a n t proppant c o n d u c t i v i t y discounts ( w i t h p o s t - f r a c t u r e c o n d u c t i v i t y o f t e n d i f f e r i n g b y a f a c t o r 0 . 1 t o 0 . 5 t i m e s t h e t e s t d a t a ) a r e a l s o p r o p o s e d b y SOLIMAN, VENDITTO & SLUSHER ( 1 9 8 4 ) ; BRITT ( 1 9 8 5 ) , MONTGOMERY & STEANSON ( 1 9 8 5 ) and McDANIEL (1986, 1987, 1988) as a c o n c l u s i o n o f l a b o r a t o r y t e s t i n g and f i e l d p e r formance e v a l u a t i o n . E x p e r i m e n t a l work showed t h a t when p r o p p a n t s a r e s u b j e c t e d t o c o n c e n t r a t e d b r i n e s , a v e r y r a p i d p e r m e a b i l i t y d e c r e a s e o c c u r s , and even 2 % KC1 s o l u t i o n s r e s u l t i n a q u i c k r a t e o f p r o p p a n t p e r m e a b i l i t y d e c l i n e (COBB & FARRELL 1986, MUCH & PENNY 1987, McDANIEL 1 9 8 8 ) . I n t o t a l , numerous l a b o r a t o r y and f i e l d exper i m e n t s have shown t h a t i n - s i t u f r a c t u r e c o n d u c t i v i t y i s o f t e n c o n s i d e r a b l y l e s s t h a n l a b o r a t o r y - m e a s u r e d v a l u e s (COOKE 1973, 1975; SECCOMBE & ANDERSON
57 1982; CALLANAN, CIPOLLA & LEWIS 1983; NORMAN, CIPOLLA & WEBB 1983; SOLIMAN, VEND I T T O & SLUSHER 1984; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985; L I & ZHU 1986; McDANIEL 1986, 1987, 1988; ROODHART, KUIPER & DAVIES 1986; PARKER & McDANIEL 1987, MUCH & PENNY 1987, POULSEN & SOLIMAN 1987, MCDANIEL & PARKER 1988).
1.4.10.1.2. Proppant grain size effects Comparison o f proppant c o n d u c t i v i t y should be made on t h e base o f as i d e n t i c a l g r a i n s i z e d i s t r i b u t i o n as p o s s i b l e , because f o r example 20/40 mesh mater i a l can e x h i b i t c o n s i d e r a b l y d i f f e r e n t c o n d u c t i v i t i e s i f g r a n u l o m e t r i c a l d i s t r i b u t i o n i n c l u d e s 75 % o r 25 % o f g r a i n s l a r g e r than 30 mesh (CUTLER, ENNISS, JONES & CARROLL 1983). Even between s e v e r a l c a r e f u l l y screened samples o f t h e same proppant type, an e r r o r band of performance d i f f e r e n c e o f a b t . 5 - 10 % can occur, w i t h t h e m a j o r reason f o r sample-to-sample v a r i a t i o n b e i n g p r o p p a n t packing i n the t e s t c e l l , although v a r i a t i o n s i n proppant s i z e d i s t r i b u t i o n s t i l l occur even when t h e p r o p p a n t assemblage i s screened s u b t l y (CUTLER, ENN I S S , JONES & SWANSON 1985). As p r o p p a n t t e s t i n g r e s u l t s can o n l y be s i g n i f i c a n t and r e p r e s e n t a t i v e i f a l l t h e examined proppant l o t s have t h e same g r a i n s i z e d i s t r i b u t i o n i n o r d e r t o enable i d e n t i f i c a t i o n o f m a t e r i a l - c a u s e d d i f f e r e n ces, r e s c r e e n i n g o f p r o d u c t i o n - d e r i v e d s i z e d i s t r i b u t i o n s a r e f r e q u e n t l y necess a r y b e f o r e s t a r t i n g e x p e r i m e n t a l work. F r a c t u r e w i d t h i n t h e t e s t i n g apparatus a l s o has t o be c l o s e l y monitored, because f l o w o f r e s i n c o a t i n g s , p a r t i c l e r e a r rangement, e l a s t i c a l d e f o r m a t i o n s and p a r t i c l e c r u s h i n g a l l t e n d t o narrow c r a c k d i a m e t e r as c l o s u r e s t r e s s i s i n c r e a s e d and t h u s v a r i o u s measurements o f a t e s t s e r i e s may n o t be p r o p e r l y comparable due t o n o t i d e n t i c a l base paramet e r s and c o n d i t i o n s . R e a l i s t i c i n - s i t u p r o p p a n t c o n d u c t i v i t y assessment and p r e d i c t i o n , however, i s e s s e n t i a l f o r c o s t and performance o p t i m i z a t i o n m o d e l l i n g o f h y d r a u l i c f r a c t u r e t r e a t m e n t s w i t h r e a l i s t i c c r a c k performance v a l u e s a l l o w i n g e x p e n s e - e f f e c t i v e s t i m u l a t i o n t r e a t m e n t s and maximum r e t u r n on proppant i n v e s t m e n t (MUCH & PENNY 1987; c f . s e c t i o n 1 . 4 . 1 2 . ) . Long-term proppant l a b o r a t o r y s t a b i l i t y t e s t i n g has t u r n e d o u t t o p r o v i d e u s e f u l i n f o r m a t i o n f o r p r o p e r e v a l u a t i o n o f propp a n t f i e l d performance, b u t e x p e r i m e n t a l procedures and r e s u l t s have t o be v e r y c a r e f u l l y and c r i t i c a l l y assessed. A b s o l u t e g r a i n s i z e has s i g n i f i c a n t impact on measured proppant c o n d u c t i v i t y , because c r u s h i n g o f c o a r s e r grades s t a r t s a t lower c l o s u r e s t r e s s e s t h a n t h a t o f f i n e r f r a c t i o n s (DAVIES & KUIPER 1988) as a consequence o f c o a r s e r g r a i n s h a v i n g fewer c o n t a c t p o i n t s w i t h t h e i r neighbours and t h u s l e s s p o s s i b i l i t i e s o f s t r e s s d i s t r i b u t i o n ( c f . s e c t i o n 4 . 3 . 5 . 1 . ) .
1.4.10.2. Impact of experimental techniques When r e v i e w i n g p r o p p a n t c o n d u c t i v i t y d i s c o u n t as a r e s u l t o f l a b o r a t o r y s t r e s s r e s i s t i v i t y experiments performed i n r a d i a l o r l i n e a r f l o w c e l l s , caut i o n has t o be e x e r c i s e d , as many r e p o r t s o f c a t a s t r o p h i c a l c o n d u c t i v i t y d e t e r i o r a t i o n i n t h e p a s t s u f f e r f r o m inadequate c h o i c e o f t e s t i n g parameters and improper d e s i g n o f t h e e v a l u a t i o n c e l l (McDANIEL 1980, 1987, 1 9 8 8 ) . A f t e r d i s c u s s i o n o f some g e n e r a l aspects, i n f l u e n c e s o f proppant g r a i n s i z e d i s t r i b u t i o n and p r o p p a n t c o n c e n t r a t i o n a r e o u t l i n e d .
1.4.10.2.1. General aspects Many erroneous procedures i n v o l v e d r e c i r c u l a t i o n o f t e s t f l u i d i n c o n t r a s t t o s i n g l e f l o w as happening i n n a t u r e , plumbing o f t h e experiment c e l l , a p p l i c a t i o n o f t o o h i g h oxygen c o n c e n t r a t i o n s o f t h e f l u i d , l a c k i n g p r e - t e s t s i l i c a sat u r a t i o n o f t h e f l u i d t o match c o n d i t i o n s i n sandstone r e s e r v o i r s , usage o f s t e e l p l a t e s i n s t e a d o f r e s e r v o i r r o c k c o r e s l a b s as boundary p l a n e s o f t h e e x p e r i m e n t c e l l , and performance o f t e s t s o n l y a t ambient temperatures, l o w f l u i d s a l i n i t i e s ( i n most cases 2 % KC1 s o l u t i o n s c o r r e s p o n d i n g t o t h e c o m p o s i t i o n o f
58 t h e p r e - p a d and d i s p l a c e m e n t s t a g e s o f t h e f r a c t u r i n g j o b a r e used; c f . SYDANSK 1984; c f . s e c t i o n 1 . 4 . 1 0 . 4 . 2 . ) and s h o r t t i m e s o f c l o s u r e s t r e s s e x p o s i t i o n . McDANIEL (1986, 1987, 1988) emphasizes t h a t a t l e a s t p a r t o f t h e h i t h e r t o p u b l i shed d r a s t i c a l c o n d u c t i v i t y l o s s e s o f p r o p p a n t packages a r e t h e r e s u l t o f i n a d e q u a t e t e s t i n g methods t h a t do n o t s u f f i c i e n t l y a p p r o a c h r e s e r v o i r c o n d i t i o n s .
NORMAN, CIPOLLA & WEBB ( 1 9 8 3 ) c a r r y o u t s p e c i a l damage t e s t s i n o r d e r t o mod e l c o n d u c t i v i t y d e c l i n e i n t h e f r a c t u r e a f t e r p r o p p a n t p l a c e m e n t and c o n c l u d e t h a t t h e d o l l a r amount o f l o s t r e v e n u e due t o d e t e r i o r a t i o n o f f r a c t u r e f l o w c a p a c i t y by proppant f a i l u r e increases w i t h formation p e r m e a b i l i t y . While i n t i g h t r e s e r v o i r s t h e c o n d u c t i v i t y c o n t r a s t between c r a c k and f o r m a t i o n i s n o t s e r i o u s l y d i s t u r b e d unless c a t a s t r o p h i c proppant crushing occurs, f r a c t u r e f l o w c a p a c i t y can i n moderate- t o h i g h - p e r m e a b i l i t y r o c k s a l r e a d y s u f f e r f r o m lower amounts o f p r o p p a n t f a i l u r e and c o m p a r a t i v e l y s m a l l changes i n c o n d u c t i v i t y c a n a l r e a d y cause s i g n i f i c a n t p r o d u c t i o n l o s s e s b y c o n s i d e r a b l y r e d u c i n g c o n d u c t i v i t y c o n t r a s t between f r a c t u r e and r e s e r v o i r m a t r i x . The e f f e c t o f p r o p p a n t p e r formance d e c l i n e , however, c a n be c o n t r o l l e d b y p r o p e r m a t e r i a l s e l e c t i o n , b e cause t h e p r o p p a n t s w i t h t h e h i g h e s t c o n d u c t i v i t y ( w h i c h o n t h e o t h e r hand a r e t h e m o s t e x p e n s i v e o n e s ) show t h e l e a s t r e v e n u e loss due t o f r a c t u r e f l o w c a p a c i t y damage.
1.4.10.2.2.Proppant grain size distribution As t h e s t r e s s l e v e l a t w h i c h s i g n i f i c a n t c r u s h i n g o c c u r s v a r i e s w i t h b o t h p r o p p a n t t y p e and g r a i n s i z e , some o b l i t e r a t i o n o f t r u e c o n d u c t i v i t y c o m p a r i s o n can a l s o a r i s e from t h e v a r i a t i o n o f s i e v e d i s t r i b u t i o n s f o r d i f f e r e n t proppant t y p e s (McDANIEL 1987; c f . s e c t i o n 1 . 4 . 1 0 . 1 . 2 . ) . F o r any g i v e n p r o p p a n t t y p e , t h e measured c o n d u c t i v i t y w i l l be h i g h e r when t h e a v e r a g e p a r t i c l e s i z e i s l a r g e r u n t i l p r o p p a n t c r u s h i n g becomes s i g n i f i c a n t ( c f . s e c t i o n 4 . 3 . 5 . 1 . ) . When f o r i n s t a n c e t w o samples o f 20/40 mesh p r o p p a n t have d i f f e r e n t d i s t r i b u t i o n s o f p a r t i c l e s i z e s w i t h i n t h e n o m i n a l s i e v e r a n g e , measured f r a c t u r e c o n d u c t i v i t y w i l l s h i f t accordingly. Short-time t e s t data u s u a l l y favour the proppant w i t h t h e l a r g e r average p a r t i c l e size, b u t t h i s i s n o t always t h e case f o r l o n g - t e r m c o n d u c t i v i t y measurements. Wide d i f f e r e n c e s i n f r a c t u r e c o n d u c t i v i t y between l o t s o f t h e same p r o p p a n t m a t e r i a l a r e g e n e r a l l y t h e r e s u l t o f v a r i a t i o n s i n p r o p p a n t g r a i n s i z e d i s t r i b u t i o n and p r o c e s s i n g p a r a m e t e r s (CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) .
1.4.10.2.3. Proppant concentration P r o p p a n t s a t u r a t i o n i s a l s o a p a r a m e t e r w h i c h needs a t t e n t i o n (McDANIEL 1987). For accurate d e f i n i t i o n o f the v a r i a t i o n o f permeability vs. proppant c o n c e n t r a t i o n o r f r a c t u r e w i d t h , t e s t s have t o be done a t p r o p p a n t s a t u r a t i o n s b o t h l o w e r and h i g h e r t h a n t h e p r e s e n t i n d u s t r y norm o f 2 l b s / f t 2 ( 1 0 k g / m 2 ) , because many f r a c t u r i n g t r e a t m e n t s u s i n g v i s c o u s f l u i d s r e s u l t i n l a r g e p e r c e n tages o f t h e propped crack l e n g t h w i t h proppant c o n c e n t r a t i o n s o f 1 l b / f t 2 ( 5 kg/m2) o r l e s s , and t h e l o w e r p r o p p a n t s a t u r a t i o n s may be s u b j e c t t o more sev e r e c r u s h i n g a t t h e same s t r e s s l e v e l t h a n t h i c k e r p r o p p a n t b e d s . I n w ! . : o f m u l t i l a y e r p a c k i n g , p r a c t i c a l e x p e r i e n c e has shown t h a t p r o p p a n t c o n c e n t r a t i o n s up t o 3 - 4 l b s / f t 2 ( 1 5 - 20 kg/m2) c a n be p l a c e d w i t h o u t t e c h n i c a l p r o b l e m s i f f l u i d s o f s u f f i c i e n t v i s c o s i t y a r e u s e d ( c f . a l s o s e c t i o n s 4 . 3 . 1 . and 4 . 3 . 4 . ) .
1.4.10.3. Implications o f long-term laboratory proppant testing The e c o n o m i c a l success o f h y d r a u l i c f r a c t u r i n g t r e a t m e n t s depends o n o b t a i n i n g s u f f i c i e n t f r a c t u r e c o n d u c t i v i t y w i t h i n t h e p r o p p a n t bed (McDANIEL & PARKER 1 9 8 8 ) . P r o p p a n t p r o p e r t i e s e x p e r i m e n t a l m o d e l l i n g c a n be c a r r i e d o u t as s h o r t t e r m t e s t i n g , l o n g - t e r m t e s t i n g w i t h o u t g e l f i l t e r cake, and l o n g - t e r m t e s t i n g w i t h gel f i l t e r cake. Long-term proppant s t a b i l i t y l a b o r a t o r y experiments r e -
59 veal t h a t g e n e r a l l y t h e r e i s a r a p i d c o n d u c t i v i t y d e c l i n e d u r i n g t h e f i r s t few weeks o f measurement i n t e r v a l f o l l o w e d by l i t t l e o r no f u r t h e r d e c l i n e d u r i n g t h e remainder o f t h e t e s t p e r i o d . The q u i c k decrease i n t h e f i r s t phase i s i n t e r p r e t e d as b e i n g t h e e f f e c t o f proppant c o n s o l i d a t i o n , compaction and r e o r i e n t a t i o n , whereas t h e slow f u r t h e r d i m i n u t i o n i n t h e second stage i s t h e consequence o f s t r e s s c o r r o s i o n . COBB & FARRELL (1986) r e p o r t a magnitude o f propp a n t c o n d u c t i v i t y d e c l i n e o f 20 % d u r i n g t h e f i r s t two weeks and l e s s t h a n 5 % o f a l o n g - t e r m experiment r u n d u r i n g t h e r e m a i n i n g n i n e weeks, i n d i c a t i n g t h a t s t r e s s c o r r o s i o n and p r e s s u r e d i s s o l u t i o n do n o t s t r o n g l y a f f e c t proppant p e r formance a f t e r c o n s o l i d a t i o n o f t h e package was achieved.
MUCH & PENNY (1987) o u t l i n e t h a t t h e major c o n d u c t i v i t y d e c l i n e o f sand dur i n g t h e f i r s t 20 h o u r s o f l o n g - t e r m t e s t i n g i s due t o c r u s h i n g and r e a r r a n g e ment o f g r a i n s which reduce package p o r o s i t y and r e l e a s e f i n e s t h a t p l u g t h e pore t h r o a t s i n t h e p r o p p a n t assemblage. The remainder o f t h e c o n d u c t i v i t y dec l i n e f r o m 20 hours t e s t i n g t i m e onwards i s p r o b a b l y f i n e s m i g r a t i o n and r e d i s t r i b u t i o n w i t h i n t h e proppant assemblage. Proppant pack impairment i s caused by embedment i n t o t h e f i l t e r cake d e p o s i t e d on t h e f r a c t u r e w a l l and p l u g g i n g o f t h e package w i t h f o r m a t i o n d e b r i s , proppant c r u s h fragments and s t i m u l a t i o n f l u i d r e s i d u e s and p r e c i p i t a t e s (ROODHART, KUIPER & D A V I E S 1986; c f . s e c t i o n 4 . 3 . 3 . ) . PARKER & McDANIEL (1987), McDANIEL (1988) and McDANIEL & PARKER (1988) document t h a t g e l f i l t e r cakes p r e c i p i t a t e d on f r a c t u r e w a l l s p r o b a b l y t r i g g e r t h e h i g h e s t r e d u c t i o n i n proppant c o n d u c t i v i t y due t o p a r t i c l e embedment and i n t e r s t i t i a l p l u g g i n g i n t h e o u t e r l a y e r s o f t h e proppant package w i t h i n t h e c r a c k ( c f . s e c t i o n 4 . 3 . 4 . 1 . 4 . ) . Some aspects o f g e l r e s i d u e damage, a c i d d i s s o l u t i o n , s t r e s s c o r r o s i o n c r a c k i n g ; e f f e c t s o f h i g h temperature, h i g h b r i n e s a t u r a t i o n and h i g h c l o s u r e s t r e s s ; and e f f e c t s o f r e s e r v o i r rock s l a b s and p r o p p a n t c o n c e n t r a t i o n a r e b r i e f l y o u t l i n e d as f o l l o w s . Emphasis i s a l s o p u t on t h e impact o f g e l f i l t e r cakes on f r a c t u r e w a l l s .
1.4.10.3.1. Gel residue damage and acid dissolution Proppant d e g r a d a t i o n as observed i n t h e f i e l d i s m a i n l y i n t e r p r e t e d t o be t h e consequence o f g e l r e s i d u e damage ( w h i c h can decrease proppant p e r m e a b i l i t y by 20 - 80 %; K I M & LOSACANO 1985) and s h o r t - t e r m package c o n s o l i d a t i o n which accounts f o r a b t . 15 - 20 % d e c l i n e . Lower proppant performance l e v e l s than e x p e c t e d f r o m l a b o r a t o r y t e s t i n g can a l s o be t h e i n f l u e n c e o f incomplete s i n t e r i n g o f t h e p a r t i c l e s as evidenced by d e n s i t y d i f f e r e n c e s b e i n g t h e r e s u l t o f m i n o r i n t e r n a l a d d i t i o n a l p r o p p a n t p o r o s i t y t h a t cause t h e g r a i n s t o be more susc e p t i b l e t o s t r e s s c o r r o s i o n and p r e s s u r e s o l u t i o n (COBB & FARRELL 1986). Longterm proppant performance under s i m u l a t e d r e s e r v o i r c o n d i t i o n s i s a l s o r e p o r t e d by MUCH & PENNY (1987) and PENNY ( 1 9 8 7 ) . C r o s s l i n k e d f r a c t u r i n g f l u i d s l e a v e much more r e s i d u e than polymer emulsion f l u i d s . While polymer emulsion f l u i d s p r o v i d e a l o o s e p r o p p a n t package t h a t i s v i r t u a l l y r e s i d u e - f r e e ( c f . s e c t i o n 4 . 3 . 4 . 6 . 4 . ) , u s i n g o f c r o s s l i n k e d f l u i d s produces a p r o p p a n t assemblage i n t h e c r a c k c o n t a i n i n g a l o t o f f i b r o u s m a t e r i a l between t h e g r a i n s which a r e t h e n g l u e d t o g e t h e r (DAVIES & KUIPER 1988). D u r i n g h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t s i n l o w - p e r m e a b i l i t y sandstones, s t i m u l a t i o n f l u i d f i l t r a t e p e n e t r a t e s i n t o t h e r e s e r v o i r f o r m a t i o n w h i l e a f i l t e r cake i s d e p o s i t e d on t h e c r a c k f a c e . F l u i d f i l t r a t e i n v a s i o n i n t o pay zone m a t r i x and m i c r o f i s s u r e s causes impairment o f p e r m e a b i l i t y and gas f l o w due t o w a t e r b l o c k i n g ( c f . s e c t i o n s 3 . 1 1 . 2 . 4 . 2 . and 4 . 8 . 8 . 3 . 1 . 5 . ) , clay swelli n g ( c f . s e c t i o n s 3 . 5 . 2 . and 3 . 1 1 . ) and d i s p e r s i o n o f f i n e s o l i d p a r t i c l e s ent r a i n e d i n the f i l t r a t e . Strength reduction o f the r e s e r v o i r rock m a t r i x f a b r i c a d j a c e n t t o t h e crack f a c e may i n i t i a t e c o l l a p s e and/or p l u g g i n g o f t h e f r a c t u r e w a l l . F i l t e r cake a c c u m u l a t i o n g i v e s r i s e t o p r o p p a n t embedment i n t o t h e draped b o r e h o l e w a l l , and d i s p e r s i o n o f f i n e g e l r e s i d u e causes p l u g g i n g o f t h e pore space o f t h e proppant package i n t h e c r a c k . Proppant c o n d u c t i v i t y i m p a i r ment can be provoked by mechanical d e b r i s from t h e c o l l a p s e d f r a c t u r e w a l l , f i n e fragments f r o m crushed proppant g r a i n s , and p r e c i p i t a t e s f r o m t h e t r e a t -
60 ment f l u i d (ROODHART, K U I P E R & DAVIES 1 9 8 6 ) . GALL & RAIBLE ( 1 9 8 5 ) r e p o r t on mol e c u l a r s i z e s t u d i e s o f d e g r a d e d s t i m u l a t i o n f l u i d p o l y m e r s , and GHAZALI & WILLHITE ( 1 9 8 5 ) comment on p e r m e a b i l i t y r e d u c t i o n i n p r o p p a n t packages u s i n g a l u m i num c i t r a t e l p o l y m e r o p e r a t i o n s . V a r i o u s c o n c e n t r a t i o n s o f HCl/HF a c i d c a n d i s s o l v e s i g n i f i c a n t amounts o f t h e p r o p p a n t g r a i n s w h i c h a l s o s e r i o u s l y i m p a i r s c o n d u c t i v i t y (CHEUNG 1985; c f . s e c t i o n s 4 . 5 . 1 . and 4 . 5 . 5 . ) . H i g h l y - s i l i c e o u s p r o p p a n t t y p e s a l s o s e v e r e l y deg r a d e i n b r i n e s a t e l e v a t e d t e m p e r a t u r e s . Mechanism and t e m p e r a t u r e o f p o l y m e r b r e a k i n g have a l s o an i m p o r t a n t i n f l u e n c e on p r o p p a n t pack f l o w i m p a i r m e n t , w i t h d i f f e r e n c e s o f d e g r e e o f damage e x i s t i n g f o r v a r i o u s base f l u i d s (ALMOND & BLAND 1 9 8 4 ) .
1.4.10.3.2. Stress corrosion cracking A s i g n i f i c a n t aspect f o r explanation o f t h e environment-sensitive behaviour o f p r o p p a n t s i s s t r e s s c o r r o s i o n c r a c k i n g w h i c h o c c u r s i n c a s e o f c o n t a c t o f wat e r o r b r i n e w i t h c r y s t a l l i n e o r amorphous s o l i d s ( H I L L I G & CHARLES 1965, W I E DERHORN & BOLZ 1970, COOKE 1973; CUTLER, ENNISS, JONES & CARROLL 1983; CUTLER, ENNISS, JONES & SWANSDN 1 9 8 5 ) . Water p r o m o t e s s a n d s t o n e f r a c t u r i n g o n l y a t e l e vated temperatures, and t h e d r i e r t h e e n v i r o n m e n t , t h e h i g h e r t h e m a t e r i a l s t r e n g t h . The e a r l i e r c o n d u c t i v i t y d e c r e a s e i n w a t e r as compared t o n i t r o g e n i n p r o p p a n t t e s t i n g r e f l e c t s t h a t p r o p a g a t i o n r a t e s o f s t r e s s c o r r o s i o n c r a c k s depend on t h e e n v i r o n m e n t a t t h e f r a c t u r e t i p , w i t h h i g h e r w a t e r v a p o u r c o n c e n t r a t i o n s i n c r e a s i n g c r a c k g r o w t h r a t e . Some a s p e c t s o f p r o p p a n t s e n s i t i v i t y t o aqueous s o l u t i o n s and g r a i n weakening b y s t r e s s - i n t e n s i f i e d c o r r o s i o n a r e o u t l i ned as f o l l o w s .
1.4.10.3.2.1.Proppant sensitivity to aqueous solutions P r o p p a n t c o n d u c t i v i t y t e s t i n g s h o u l d t h e r e f o r e be c a r r i e d o u t i n aqueous env i r o n m e n t s i n c e t h i s r e p r e s e n t s a w o r s t p o s s i b l e c a s e s i t u a t i o n (CUTLER, ENNISS, JONES & CARROLL 1983; CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) . W h i l e s i n t e r e d b a u x i t e c o n s i s t i n g a l m o s t c o m p l e t e l y o f corundum i s i n s e n s i t i v e t o aqueous s o l u t i o n s , m u l l i t e which i s t h e main c o n s t i t u e n t o f alumina s i l i c a t e proppants and an i m p o r t a n t component o f a l u m i n a s i l i c a t e and o x i d e p r o p p a n t s ( c f . t a b . 2 ) i s s u s c e p t i b l e t o s t r e s s c o r r o s i o n c r a c k i n g . Proppant placement w i t h i n t h e f r a c t u r e d r e s e r v o i r s h o u l d be p r e f e r e n t i a l l y c a r r i e d o u t w i t h h i g h - q u a l i t y foamed o r e n e r g i z e d f l u i d s i n o r d e r t o r e s t r i c t c o n t a c t o f p r o p p a n t s and aqueous phase t o a minimum b y g u a r a n t e e i n g i m m e d i a t e i n v a s i o n o f t h e p r o p p a n t package by f o r m a t i o n gas f o l l o w i n g q u i c k w e l l c l e a n u p due t o t h e e x c e l l e n t f l o w b a c k p r o p e r t i e s o f foamed o r e n e r g i z e d f l u i d s as a consequence o f gas d r i v e ( c f . s e c t i o n 3.11.2.1.
1.4.10.3.2.2.Grain weakening by stress-intensified corrosion When p r o p p a n t s a r e h e l d a t e l e v a t e d c l o s u r e s t r e s s and t e m p e r a t u r e f o r l o n g e r t i m e , t h e d e g r e e o f c r u s h i n g i s i n c r e a s e d by s t r e s s - i n t e n s i f i e d corrosion w h i c h weakens t h e g r a i n s (McDANIEL 1 9 8 7 ) . As w i t h m o s t c o r r o s i o n mechanisms, i n creased temperature environments r e s u l t i n t h e most s t r e s s - i n t e n s i f i e d c o r r o s i o n e f f e c t . Chemical c o m p o s i t i o n , c r y s t a l s t r u c t u r e and p r e s e n c e o f s u r f a c e f l a w s a r e t h e p r i m a r y f a c t o r s c o n t r o l l i n g t h e degree o f a d d i t i o n a l c r u s h i n g r e s u l t i n g f r o m s t r e s s - i n t e n s i f i e d c o r r o s i o n a t a g i v e n c l o s u r e s t r e s s and t e m p e r a t u r e . E x p e r i m e n t a l work r e v e a l s d i f f e r e n t p e r m e a b i l i t i e s o f p r o p p a n t packages i n d i f f e r e n t f l u i d s and a t d i f f e r e n t t e m p e r a t u r e s . T h e r e f o r e p r o p p a n t c o n d u c t i v i t y t e s t i n g c a n o n l y a p p r o a c h t h e r e a l i t y downhole i f c a r r i e d o u t u n d e r r e s e r v o i r t e m p e r a t u r e , p r e s s u r e and f l u i d c o n d i t i o n s (COOKE 1 9 7 3 ) .
61
1.4.10.3.3. Effects of high temperature, high brine saturation and high closure stress McDANIEL (1986) summarizes r e s e a r c h on i n f l u e n c e o f extended t i m e a t d i f f e r e n t t e s t c o n d i t i o n s , v a r i a t i o n between ambient and e l e v a t e d temperature, and e f f e c t s o f b r i n e f l o w a t h i g h temperature and c l o s u r e s t r e s s on p r o p p a n t conduct i v i t y d e c l i n e . While c o n v e n t i o n a l p r o p p a n t t e s t i n g showed t h a t c r u s h i n g , embedment and p l u g g i n g by f i n e s o l i d p a r t i c l e s a r e c o n t r i b u t i n g t o i n - s i t u f r a c t u r e c o n d u c t i v i t y l o s s , h i g h temperature coupled w i t h h i g h c l o s u r e s t r e s s and h i g h b r i n e s a t u r a t i o n s has been p r o v e n t o have more damaging e f f e c t s on l o n g - t e r m p r o p p a n t c o n d u c t i v i t y t h a n a l l t h e combined f e a t u r e s p r e v i o u s l y mentioned together. D r a s t i c a l proppant c o n d u c t i v i t y differences are obtained from long-term h i g h - t e m p e r a t u r e t e s t s and s h o r t - t e r m ambient-temperature experiments (COOKE 1973, McDANIEL 1987), w i t h s i g n i f i c a n t l y lower proppant c o n d u c t i v i t i e s h a v i n g been measured under r e a l i s t i c r e s e r v o i r c o n d i t i o n s t h a n r e p o r t e d f o r e a r l i e r s h o r t - t e r m t e s t i n g (OAVIES & KUIPER 1988). The f o l l o w i n g o v e r v i e w focusses on s i l i c a s a t u r a t i o n and oxygen c o n c e n t r a t i o n as w e l l as on r e s e r v o i r c o n d i t i o n spectrum approach.
1.4.10.3.3 .l. Silica saturation I m p o r t a n t parameters o f p r o p p a n t c o n d u c t i v i t y t e s t i n g a r e a l s o oxygen cont e n t and p r e - t e s t s i l i c a c o n c e n t r a t i o n o f t h e f l u i d (McDANIEL 1986). A t a constant closure stress level, conductivity i s i r r e v e r s i b l y declining w i t h r i s i n g temperature and i s n o t r e c o v e r e d by subsequent c o o l i n g down. C l o s u r e s t r e s s i n creases t h e adverse e f f e c t s o f temperature and water-based f l u i d s i n many p r o p p a n t s . I n a c t u a l downhole f r a c t u r e c o n d i t i o n s i n a sandstone r e s e r v o i r , any w a t e r p r e s e n t would be s a t u r a t e d w i t h s i l i c a due t o exposure t o t h e v a s t amount o f s i l i c a t e s u r f a c e s o f t h e r e s e r v o i r r o c k and c o r r e s p o n d i n g l e a c h i n g t o the l i m i t o f uptake capacity o f the f l u i d . I f a s i l i c a - s a t u r a t e d t e s t f l u i d i s used ( c f . a l s o MUCH & PENNY 1987), many o f t h e p r e v i o u s l y r e p o r t e d c a t a s t r o p h i c a l c o n d u c t i v i t y l o s s e s o f proppants and sand cannot be reproduced. On t h e o t h e r hand, t h e b r i n e used i n f r a c t u r i n g f l u i d s c o n t a i n s no s i l i c a o r alumina, w h i l e t h e b r i n e produced f r o m a sandstone f o r m a t i o n w i l l be s i l i c a - and p r o b a b l y a l s o a l u m i n a - s a t u r a t e d . Recommendation can t h e r e f o r e be made t h a t a r e a l i s t i c c o n d u c t i v i t y t e s t would i n c o r p o r a t e a s i l i c a / a l u m i n a - f r e e b r i n e i n t h e e a r l y stage which i s t h e n f o l l o w e d by a s i l i c a / a l u m i n a - s a t u r a t e d b r i n e i n t h e l a t e r phases, because t h e o r i g i n a l f l o w i n g medium i n t h e r e s e r v o i r i s s w i t c h i n g o v e r t o a silica/alumina-saturated b r i n e a f t e r a p e r i o d o f t i m e (MUCH 1987).
1.4.10.3.3.2. Oxygen concentration The oxygen l e v e l has t o be e x t r e m e l y lowered b e f o r e s i g n i f i c a n t d i f f e r e n c e s i n t e s t r e s u l t s a r e o b t a i n e d , w i t h r e s i n - c o a t e d sand b e i n g no l o n g e r d i s c o l o u r e d o r blackened and t h e frequency o f plumbing leakage b e i n g d r a s t i c a l l y reduced. I n c o n t r a s t t o h a s t e l l o y , s t a i n l e s s s t e e l t e s t c e l l s c o r r o d e when exposed t o heated b r i n e t h a t c o n t a i n s oxygen, and t h e r e f o r e a b s o l u t e l y minimal amounts o f oxygen a r e e s s e n t i a l (MUCH 1987), w i t h even t r a c e q u a n t i t i e s o f oxygen c r e a t i n g m i n u t e c o r r o s i o n o f t h e s t a i n l e s s s t e e l c e l l which can provoke d r a m a t i c a l proppant c o n d u c t i v i t y d e c l i n e by p r e c i p i t a t i o n o f c o r r o s i o n p r o d u c t s w i t h i n t h e proppant package. F r a c t u r i n g f l u i d s c a r r y i n g t h e proppants, however, a r e o f t e n s u p e r s a t u r a t e d w i t h oxygen because o f t h e method o f b l e n d i n g and p r o p p a n t a d d i t i o n , w h i l e i t i s assumed t h a t hydrocarbon r e s e r v o i r s c o n t a i n no oxygen. Accurate m o d e l l i n g of a s t i m u l a t i o n t r e a t m e n t i n a t e s t c e l l t h e r e f o r e r e q u i r e s heated b r i n e c o n t a i n i n g oxygen as t h e i n i t i a l f l o w i n g medium which e x c l u d e s use o f s t a i n l e s s s t e e l c e l l s . L a t e r t h e f l o w medium s h o u l d be reduced t o z e r o
62
oxygen brine after some finite amount of time which would model returned fracturing fluid and formation brine after the oxygen in the solution has reacted with the fracturing gels (oxidizers are commonly added as gel breakers). Thus both oxygen and silica/alumina saturation have to be changed throughout the experiment, with oxygen and silica/alumina concentrations having to be reduced and increased, respectively, during progression of the test and therefore have an inverse correlation. The oxygen-richloxygen-free brine is a very important consideration when evaluating resin-coated proppants which are susceptible to oxidation giving quite different performance results in oxygen-free as opposed to oxygen-rich brines. Hastelloy cells are the ultimate choice for testing proppants containing dissolved oxygen, because hastelloy is almost inert in an oxygen-containing brine as long as fluid pH is controlled. CALLANAN, CIPOLLA & LEWIS (1983) document that exposure to hot brines can reduce proppant conductivity to 5 0 - 75 % . The exposition to hot brine can cause a two- to three-fold permeability reduction in sand and bauxite proppant packs (COOKE 1973). McDANIEL (1987) concludes that increased temperature environments result in the most stress-intensified corrosion effect which weakens the proppant grains and increases the degree of crushing.
1 . 4 . 1 0 . 3 . 3 . 3 . Reservoir condition spectrum approach McDANIEL (1988) performs proppant conductivity testing under the hitherto broadest and most complete set of environmental conditions approaching in-situ reservoir situation as closely as possible, including downhole temperature, extended test time, core slabs and wafers as cell limitations, gel residue in the proppant pack, gel filter cakes from dynamic fluid loss tests on the simulated crack walls, fracture fluid clean-up period, wet gas as flowing medium, varying closure stresses, and non-Darcy flow effects. Considering the whole suite of parameters, the test results allow more accurately modelling of fracturing treatments within a design simulator and better predicting of post-fracturing production using a reservoir simulator.
1 . 4 . 1 0 . 3 . 4 . Effects of pay rock slabs and proppant concentration The traditional conductivity vs. stress evaluation can overestimate proppant performance under downhole conditions (MUCH & PENNY 1987). Conductivity at stress vs. time at temperature provides more meaiiingful data for estimating well performance. Bulk density and porosity values of proppants assessed over time at closure stress give more accurate information than those at no stress and zero elapsed time. Some aspects of wall slip and metal piston vs. filter cake and core slab as well as fracture porosimetry as evaluated as follows.
1 . 4 . 1 0 . 3 . 4 . 1 . Wall slip and metal piston vs. filter cake and core slab Proppant assessment using hard metal pistons as test cell boundaries may introduce a variable wall slip that can cause measured proppant assemblage conductivity to be greater than absolute proppant package conductivity, whereas if performing experiments with medium-hard sandstone core slabs, proppant embedment becomes a significant factor when exceeding 8,000 psi closure stress and reduces the amount of wallslip (cf. sections 1.4.10.3.4.1. and 4.3.4.1.4.). Filter cakes deposited on the fracture walls when crosslinked polymer-gelled fluids leak off into the reservoir formation can reduce proppant package conductivity at 2 lbs/ft2 (10 kg/m2) concentration by up to 50 %, with filter cake accumulation and subsequent proppant embedment decreasing effective width of the supporting material package regardless of proppant type. The various tests have shown that regardless of the environmental setting, ceramic intermediate-
63 s t r e n g t h p r o p p a n t s can p r o v i d e c o n d u c t i v i t i e s which a r e o v e r 1000 % g r e a t e r t h a n those o f sand a t equal p r o p p a n t s a t u r a t i o n s and c l o s u r e s t r e s s c o n d i t i o n s . The p r o p p a n t g r a i n s which a r e i n c o n t a c t w i t h t h e experiment c e l l f a c e o r p l a t e n a r e under g r e a t e s t s t r e s s d u r i n g t h e t e s t , because each bead has fewer c o n t a c t p o i n t s w i t h o t h e r spheres t h a n those f a r t h e r away f r o m t h e p l a t e n s u r f a c e and toward t h e f r a c t u r e c e n t e r t h a t have more c o n t a c t p o i n t s amongst t h e i r neighbours depending on t h e i r random manner o f p a c k i n g (MESSINA 1987). I n s t e e l experiment c e l l s , embedment i n t o t h e p l a t e n i n o r d e r t o r e l e a s e s t r e s s i s n o t p o s s i b l e and t h e r e f o r e s t r e s s e q u a l i z a t i o n f i n a l l y t a k e s p l a c e by g r a i n c r u s h i n g , whereas c o r e s l a b s as bounding f a c e s o f t h e t e s t c e l l a l l o w embedment o f p r o p p a n t g r a i n s and t h u s reduce t h e amount o f p r o p p a n t c r u s h i n g .
1.4.10.3.4.2.
Fracture porosimetry
WENDORFF (1982) i n t r o d u c e s t h e f r a c t u r e p o r o s i m e t e r f o r f a s t and s i m p l i f i e d d e t e r m i n a t i o n o f c r a c k c o n d u c t i v i t y a t downhole s t r e s s e s . P e r m e a b i l i t y , p o r o s i t y and w i d t h o f t h e f r a c t u r e depend on t y p e and amount o f p r o p p a n t s and magnitude o f a p p l i e d s t r e s s and a r e measured between samples o f r e s e r v o i r r o c k t o p r o v i d e a c c u r a t e i n f o r m a t i o n on c r u s h i n g and embedment. As c l o s u r e s t r e s s r i ses, s u r f a c e area i n c r e a s e s , b u t n o t n e a r l y as much as p e r m e a b i l i t i e s d i m i n u i s h . F r a c t u r e p o r o s i t y decreases as l o a d i s a p p l i e d because p r o p p a n t g r a i n s c r u s h and embed i n t h e r o c k m a t r i x . I n competent rock, p r o p p a n t p o r o s i t y i s u s u a l l y reduced by c r u s h i n g , whereas i n s o f t f o r m a t i o n s , proppant embedment coupled w i t h proppant c r u s h i n g r e s u l t s i n v e r y low f r a c t u r e p o r o s i t y . I n most propped f r a c t u r e s , b o t h embedment and c r u s h i n g ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) o c c u r and t h e r e s u l t i n g p o r o s i t y i s a measure o f t h e combined e f f e c t .
1.4.10.3.5.
Gel f i l t e r cake on f r a c t u r e w a l l
PARKER & McDANIEL (1987), McDANIEL (1988) and McDANIEL & PARKER (1988) document t h e e f f e c t o f t h e g e l f i l t e r cake b e i n g d e p o s i t e d on t h e f r a c t u r e w a l l s as t h e f l u i d l e a k s o f f i n t o t h e f o r m a t i o n on c o n d u c t i v i t y o f t h e p r o p p a n t package w i t h i n t h e c r a c k . The main impacts o f t h e g e l f i l t e r cake on t h e f r a c t u r e w a l l a r e proppant embedment and r e d u c t i o n o f e f f e c t i v e f r a c t u r e w i d t h as w e l l as p r o p p a n t p e r m e a b i l i t y damage by f i l t e r - c a k e b u i l d u p . Comments a r e a l s o o f f e r e d on proppant c o n c e n t r a t i o n and f l u i d t y p e as w e l l as on removal o f g e l f i l t e r cake by combined f l u i d - l o s s a d d i t i v e and g e l b r e a k e r . A b r i e f comparison o f g e l f i l t e r cakes i n h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i s a l s o p r o v i d e d .
1.4.10.3.5.1,
Proppant embedment
Dynamic f l u i d l o s s t e s t s i n d i c a t e t h a t i n many cases, average f i l t e r cake t h i c k n e s s e s can be v e r y c l o s e t o proppant g r a i n s i z e f o r 20/40 mesh p a r t i c l e s (PARKER & McDANIEL 1987, McDANIEL 1988). Thus t h e r e i s a p o t e n t i a l t o f i l l t h e proppant p o r o s i t y w i t h f i l t e r cake and l o o s e t h e f l o w along t h e w a l l o f b o t h f r a c t u r e f a c e s and f o r a depth o f one o r more p r o p p a n t g r a i n s on each s i d e o f t h e c r a c k . As proppants embed i n t o t h e f i l t e r cake when t h e f r a c t u r e i s c l o s i n g , t h e y may d i s p l a c e p a r t s o f t h e t h i c k g e l r e s i d u e towards t h e c e n t e r o f t h e p r o p p a n t package w i t h i n t h e c r a c k . T h i s embedment, d e f o r m a t i o n and d i s p e r s i o n o f t h e g e l f i l t e r cake s e r i o u s l y d e t e r i o r a t e s proppant p e r m e a b i l i t y and thus f r a c t u r e c o n d u c t i v i t y . Experimental work has shown t h a t c o n d u c t i v i t y d i m i n u t i o n i s l e a s t w i t h t h e base g e l a l o n e . F u r t h e r r e d u c t i o n i s observed w i t h a d d i t i o n o f l i q u i d h y d r o c a r bon phase f o r f l u i d loss c o n t r o l , and t h e most severe d e c l i n e occurs when u s i n g a s o l i d p a r t i c u l a t e f l u i d l e a k o f f a d d i t i v e . Wet g e l f i l t e r cakes provoke d r a s t i Cal r e d u c t i o n s i n p r o p p a n t c o n d u c t i v i t y even a t low c l o s u r e s t r e s s l e v e l s . The t e s t r e s u l t s f o r Ottawa Sand r e v e a l t h a t t h e range o f c l o s u r e s t r e s s f o r t h e ap-
64 p l i c a t i o n o f n a t u r a l sand i s s i g n i f i c a n t l y l o w e r t h a n t h a t r e q u i r e d f o r many f r a c t u r i n g a p p l i c a t i o n s , w i t h t h e use o f i n t e r m e d i a t e - s t r e n g t h proppants b e i n g a l r e a d y n e c e s s a r y f r o m c l o s u r e s t r e s s v a l u e s o f 4,000 p s i onwards (McDANIEL & PARKER 1 9 8 8 ) .
1.4.10.3.5.2.Reduction of effective fracture width MUCH & PENNY ( 1 9 8 7 ) document t h a t t h e m a i n mechanism o f f r a c t u r e c o n d u c t i v i t y d e t e r i o r a t i o n i s r e d u c t i o n o f t h e e f f e c t i v e w i d t h o f t h e p r o p p a n t package b y
f o r m a t i o n o f a f i l t e r c a k e c o v e r i n g t h e c r a c k w a l l s and embedment o f t h e o u t e r l a y e r s o f t h e p r o p p a n t wedge i n t o t h e f i l t e r c a k e d r a p e . T e s t i n g w i t h r e s e r v o i r c o r e s l a b s p r o v i d e s more r e a l i s t i c c o n d i t i o n s b y a l l o w i n g f l u i d l e a k o f f and c o n s e q u e n t l y o r i g i n o f a f i l t e r cake, whereas p r o p p a n t e v a l u a t i o n u s i n g h a r d m e t a l p i s t o n s i n t r o d u c e a w a l l s l i p v a r i a b l e t h a t causes t h e measured p r o p p a n t conduct , v i t y t o be g r e a t e r t h a n t h e a b s o l u t e v a l u e f o r t h e r e a s o n o f s u p p r e s s i o n o f f i l t e r c a k e d e v e l o p m e n t (MUCH & PENNY 1 9 8 7 ) . W a l l s l i p e f f e c t s i n s t e a d y s h e a r v i s c o s i t y measurements o f h y d r a u l i c f r a c t u r i n g f l u i d s a r e a l s o d e s c r i b e d b y GUILLOT & OUNANO ( 1 9 8 3 ) and KNOLL ( 1 9 8 5 ) . F i l t e r c a k e d e p o s i t i o n and p r o p p a n t embedment r e d u c e t h e e f f e c t i v e w i d t h o f t h e p r o p p a n t package r e g a r d l e s s o f p r o p p a n t t y p e ( c f . a l s o s e c t i o n s 1.4.10.3.4.1. and 4 . 3 . 4 . 1 . 4 . ) .
A d d i t i o n a l e f f e c t s a r e generated by d i s p e r s i o n o f f i n e f r a c t u r i n g f l u i d r e s i d u e i n t h e p r o p p a n t package and non-Oarcy f l o w ( c f . s e c t i o n 4 . 3 . 3 . 3 . ) i n the r e s t r i c t e d p o r e space, as w e l l as w a t e r b l o c k i n g ( c f . s e c t i o n s 3 . 1 1 . 2 . 4 . 2 . and 4 . 8 . 8 . 3 . 1 . 5 . ) and c l a y s w e l l i n g ( c f . s e c t i o n s 3 . 5 . 2 . and 3 . 1 1 . 1 . ) i n t h e r e s e r v o i r f o r m a t i o n a d j o i n i n g t o t h e f r a c t u r e w a l l s (ROOOHART, K U I P E R & DAVIES 1 9 8 6 ) . I n t e r m s o f r e l a t i v e d e t e r i o r a t i o n e f f e c t , c r o s s l i n k e d g e l s show t h e h i g h e s t i m p a i r m e n t because o f f o r m a t i o n o f v e r y t h i c k f i l t e r cakes, and b a t c h m i x e d g e l l e d o i l s e x h i b i t h i g h e r d e l e t e r i o u s i m p a c t t h a n compared w i t h o n - t h e f l y m i x e d s y s t e m s . The l e a s t damaging f r a c t u r i n g f l u i d a p p e a r s t o be t h e p o l y mer e m u l s i o n s y s t e m (ROODHART, KUIPER & OAVIES 1 9 8 6 ) . The s i g n i f i c a n c e o f nonOarcy f l o w e f f e c t s o n t h e a c c u r a c y o f p r o p p a n t c o n d u c t i v i t y m o d e l l i n g i s a l s o emphasized b y COOKE ( 1 9 7 3 ) ; EVANS, HUDSON & GREENLEE ( 1 9 8 5 ) ; EVANS & EVANS ( 1 9 8 6 ) ; MALONEY, GALL & RAIBLE ( 1 9 8 7 ) and McDANIEL ( 1 9 8 8 ) .
1.4.10.3.5.3.Proppant permeability damage by filtercake buildup The f i l t e r c a k e f o r m i n g on t h e c r a c k w a l l s upon l e a k o f f o f h i g h - p r e s s u r e f r a c t u r i n g f l u i d c o n s i s t s m a i n l y o f p o l y m e r and f l u i d - l o s s a d d i t i v e s (DAVIES & KUIPER 1 9 8 8 ) . F i l t e r - c a k e t h i c k n e s s i s d e t e r m i n e d b y t h e p a r t i c u l a r s t i m u l a t i o n f l u i d used, f o r m a t i o n c h a r a c t e r i s t i c s , f r a c t u r e - t o - r e s e r v o i r p r e s s u r e d i f f e r e n ce, and e r o s i o n a l e f f e c t s caused b y s l u r r y b e i n g pumped a l i l n g t h e c r a c k f a c e s . D u r i n g f r a c t u r e c l o s u r e , p r o p p a n t s a r e embedded i n t o t h e f i l t e r cake, m a k i n g i t d i f f i c u l t t o remove t h e c r a c k w a l l d r a p e d u r i n g p r o d u c t i o n . W h i l e c r o s s l i n k e d f l u i d s w i t h d i e s e l as f l u i d - l o s s a d d i t i v e s g e n e r a t e t h i c k f i l t e r c a k e s w h i c h can a l m o s t c o m p l e t e l y b l o c k t h i n f r a c t u r e s , p o l y m e r e m u l s i o n f l u i d s do n o t g i v e significant filter-cake buildup ( c f . section 4.3.4.6.). As p r o p p a n t p e r m e a b i l i t y damage b y c r o s s l i n k e d f l u i d s s u b s t a n t i a l l y depends on f i l t e r - c a k e b u i l d u p , m i n i m i z a t i o n o f t h e phenomenon c a n be a c h i e v e d b y u s i n g an e f f e c t i v e v i s c o s i t y b r e a k e r w h i c h i s p a r t i c u l a r l y n e c e s s a r y i n l o w - t e m p e r a t u r e shallow formations. While inadequate breakers l e a d t o v i r t u a l l y complete l o s s o f p r o p p a n t pack c o n d u c t i v i t y , a g g r e s s i v e b r e a k e r s c h e d u l e s on t h e o t h e r hand c a n p r o v i d e h i g h r e t e n t i o n f a c t o r s i n e x c e s s o f 8 0 %, b u t c a n a l s o g i v e r i s e t o excessive proppant s e t t l i n g i n the f r a c t u r e before closure, w i t h theref o r e a proper balance being required.
65
1.4.10.3.5.4. Proppant concentration and fluid type Proppant s a t u r a t i o n has a s i g n i f i c a n t i n f l u e n c e on r e m a i n i n g f r a c t u r e conduct i v i t y i n t h e presence o f a g e l f i l t e r cake (PARKER & McDANIEL 1987). Low p r o p p a n t c o n c e n t r a t i o n s accentuate p e r m e a b i l i t y d e t e r i o r a t i o n caused by t h e g e l filt e r cake, and t h e r e f o r e h i g h p r o p p a n t s a t u r a t i o n s a r e e s s e n t i a l t o achieve t h e d e s i r e d f r a c t u r e c o n d u c t i v i t y . I n t h e presence o f g e l f i l t e r cakes, t h e permeab i l i t y o f a propped c r a c k i s n o t c o n s t a n t , and d e s i g n s i m u l a t i o n s s h o u l d be a l t e r e d t o a l l o w i n p u t o f c o n d u c t i v i t y d a t a as a f u n c t i o n o f p r o p p a n t c o n c e n t r a t i o n (McDANIEL & PARKER 1988) and t h u s v a r i a b l e c o n d u c t i v i t y f r o m a maximum a t t h e f r a c t u r e mouth t o a minimum a t t h e c r a c k t i p . Gel f i l t e r cakes cause even l o w e r c o n d u c t i v i t y t h a n t h a t measured i n l o n g - t e r m t e s t s w i t h o u t f i l t e r cakes ( c f . s e c t i o n 4 . 3 . 4 . 1 . 4 . ) . D i f f e r e n t t y p e s o f f r a c t u r i n g f l u i d s f o r m f i l t e r cakes t h a t r e s u l t i n v a r y i n g degrees o f damage t o proppant bed c o n d u c t i v i t y . H i g h r e s e r v o i r temperatures may a c t u a l l y l e s s e n t h e s i n g u l a r e f f e c t o f g e l f i l t e r cakes due t o more d e g r a d a t i o n o f t h e g e l r e s i d u e . Another v a r i a b l e i s t h e t y p e o f f i l t e r cake p r e s e n t i n t h e p r o p p a n t pack w i t h f l u i d - l o s s a d d i t i v e s c r e a t i n g a t h i n n e r f i l t e r cake on t h e f r a c t u r e w a l l as a consequence o f r e t a r d e d f l u i d l e a k o f f and t h u s d i m i n i s h e d s o l i d agent seg r e g a t i o n and d e p o s i t i o n . Less f l u i d l e a k o f f which means l e s s g e l l i n g agent i n t h e f i l t e r cake w i l l n o r m a l l y g i v e r i s e t o l e s s p r o p p a n t bed damage. The degree o f g e l d e g r a d a t i o n i n t h e f i l t e r cake i s a l s o o f c o n s i d e r a b l e importance. The r e s u l t s o f t h e s t u d i e s o f McDANIEL (1986, 1987, 1988), PARKER & McDANIEL (1987) and McDANIEL & PARKER (1988) t h u s c l e a r l y show t h a t time, temperature, s t r e s s and presence o f g e l f i l t e r cakes a r e a l l i m p o r t a n t f a c t o r s i n f l u e n c i n g c r a c k c o n d u c t i v i t y . Another v a r i a b l e a f f e c t i n g proppant c o n d u c t i v i t y i s t h e number o f phases p r e s e n t . While p r o p p a n t package c o n d u c t i v i t y i s n o r m a l l y measured w i t h s i n g l e - p h a s e f l o w , a d d i t i o n o f a second o r t h i r d phase reduces e f f e c t i v e p r o p p a n t assemblage p e r m e a b i l i t y t o t h e o r i g i n a l phase s i g n i f i c a n t l y (DAVIES & KUIPER 1988).
1.4.10.3.5.5. Gel filter cakes in fracturing and gravel packing A comparable phenomenon t o g e l f i l t e r cakes o r i g i n a t i n g on f o r m a t i o n s u r f a ces a t f r a c t u r e w a l l s a r e g e l c o v e r s c r e a t e d on t h e r e s e r v o i r f a c e d u r i n g g r a v e l p a c k i n g as a consequence o f f l u i d l e a k o f f and f i l t r a t i o n (TORREST & MARCANO 1986). These g e l covers can r a p i d l y b u i l d up a l o w - p e r m e a b i l i t y drape t h a t domin a t e s system b e h a v i o u r and i n h i b i t s f u r t h e r f l u i d l o s s o f t h e g r a v e l c a r r i e r t h e r e b y d e t e r i o r a t i n g g r a v e l placement and p r e s s u r e response. The l a y e r b u i l d up f r o m c o n c e n t r a t e d s o l u t i o n s would a l s o t e n d t o g i v e f a l s e s i g n a l s o f j o b comp l e t i o n s as pumping p r e s s u r e s r i s e r a p i d l y . Reduced g r a v e l pack q u a l i t y and p e r formance c o u l d r e s u l t f r o m f a i l u r e t o account f o r t h e e f f e c t o f s u r f a c e g e l f i l t r a t i o n ( c f . s e c t i o n s 4 . 3 . 4 . 1 . 4 . and 5 . 3 . 1 . 1 . 2 . 2 . ) .
1.4.10.3.5.6. Removal of gel filter cake by combined f luid-loss additive and gel breaker CANTU & BOYD (1988) p r e s e n t an o r g a n i c a c i d p o l y c o n d e n s a t i o n p r o d u c t b e i n g a p o l y e s t e r which a c t s as combined f l u i d - l o s s a d d i t i v e and g e l b r e a k e r and t h a t has t h e advantage t o d i s i n t e g r a t e i n t o s o l u b l e monomers a t r e s e r v o i r temperat u r e a f t e r t e r m i n a t i o n o f t h e s t i m u l a t i o n t r e a t m e n t , t h e r e b y s e c o n d a r i l y remov i n g t h e f i l t e r cake which has been p r i m a r i l y d e p o s i t e d on t h e f r a c t u r e w a l l s and f i n a l l y l e a v i n g no s u b s t r a t e f o r p r o p p a n t embedment t h u s m a x i m i z i n g f r a c t u r e c o n d u c t i v i t y . Upon thermal d e g r a d a t i o n t o s o l u b l e monomers, t h e o r g a n i c e s t e r exposes i n c r e a s i n g a c i d f u n c t i o n a l groups which break t h e c r o s s l i n k e d and/or l i n e a r f r a c t u r i n g g e l used as s t i m u l a t i o n f l u i d as a consequence o f dec r e a s i n g pH ( c f . a l s o s e c t i o n 4 . 3 . 4 . 6 . 4 . ) .
66 The e x c e l l e n t c l e a n u p o f t h e f i l t e r c a k e w i t h no r e m a i n i n g r e s i d u e i s s u p p o r t e d b y t h e c o n c e n t r a t i o n o f t h e p o l y e s t e r p a r t i c u l a t e s on t h e f r a c t u r e w a l l i n c o n t r a s t t o conventional water-soluble breakers which leak o f f i n t o t h e reserv o i r f o r m a t i o n . The p a r t i c u l a t e p o l y e s t e r has a f l u i d - l o s s a g e n t p e r f o r m a n c e a t l e a s t as good as t h a t o f s i l i c a f l o u r ( c f . s e c t i o n 4 . 3 . 4 . 6 . 2 . ) i n crosslinked and l i n e a r g e l s as w e l l as i n n i t r o g e n foam, and i t s d e c i s i v e a d v a n t a g e w i t h r e s p e c t t o c l a s s i c a l two- o r m u l t i c o m p o n e n t f l u i d - l o s s a d d i t i v e / g e l b r e a k e r s y s tems i s t h a t t h e p o l y e s t e r i s o n l y one a g e n t w i t h a d o u b l e f u n c t i o n , w i t h t h e two s u c c e s s i v e t a s k s b e i n g f u l f i l l e d w h i l e c h a n g i n g f r o m s o l i d p o l y e s t e r t o s o l u b l e m o n o e s t e r upon t h e r m a l a n d / o r c h e m i c a l d e g r a d a t i o n and t h u s l e a v i n g n o p a r t i c u l a t e residues t h a t could impair proppant c o n d u c t i v i t y ( c f . a l s o sections 4 . 3 . 4 . 6 . 4 . and 4 . 8 . 8 . 3 . 2 . 3 . 5 . ) .
1.4.10.3.6. Gelling agent concentration Proppant-pack p e r m e a b i l i t y i s s i g n i f i c a n t l y impaired by t h e f r a c t u r i n g f l u i d , w i t h damage d e g r e e i n c r e a s i n g w i t h r i s i n g g e l l i n g a g e n t s a t u r a t i o n (HAWKINS 1 9 8 8 ) . I n many h y d r a u l i c f r a c t u r e t r e a t m e n t s , t h e g e l l i n g a g e n t i s o f t e n c o n c e n t r a t e d o v e r t e n - f o l d upon c r a c k c l o s u r e . L a b o r a t o r y s t u d i e s i n v e s t i g a t i n g g e l l i n g a g e n t s a t u r a t i o n w i t h i n t h e p r o p p a n t package, p r o p p a n t c o n c e n t r a t i o n , c l o s u r e s t r e s s , s h u t - i n t i m e , and d i s p l a c e m e n t f l u i d and i t s f l o w r a t e r e v e a l t h a t even a t r e l a t i v e l y l o w g e l l i n g a g e n t s a t u r a t i o n s , s i g n i f i c a n t proppant package p e r m e a b i l i t y i m p a i r m e n t o c c u r s . As h i g h d i s p l a c e m e n t r a t e s d u r i n g w e l l f l o w b a c k o n l y m a r g i n a l l y remove t h e damage, h i g h b r e a k e r c o n c e n t r a t i o n s a r e n e cessary t o s i g n i f i c a n t l y reduce p e r m e a b i l i t y impairment. F o l l o w i n g d i s c u s s i o n o f some g e n e r a l a s p e c t s , an o u t l i n e i s g i v e n on c l e a n u p f l o w r a t e and c l o s u r e s t r e s s as w e l l as p r o p p a n t g r a i n s i z e and b r e a k e r s a t u r a t i o n . Comments a r e a l s o o f f e r e d on h y d r o c a r b o n - b a s e d c l e a n u p f l u i d s f o r p r o p p a n t package p e r m e a b i l i t y improvement.
1.4.10.3.6.1.General aspects P a r a m e t e r s a f f e c t i n g p r o p p a n t package p e r m e a b i l i t y have t o be d i v i d e d i n t o p a r a m e t e r s o p e r a t i v e u n d e r a l l c o n d i t i o n s and s u c h a p p l y i n g o n l y when g e l l i n g a g e n t s a r e p r e s e n t (HAWKINS 1 9 8 8 ) . A d d i t i o n o f f r a c t u r i n g f l u i d s t o t h e p r o p p a n t package g r e a t l y c o m p l i c a t e s b o t h e x p e r i m e n t s and a n a l y s i s o f r e s u l t s . F a c t o r s i n f l u e n c i n g p r o p p a n t p e r m e a b i l i t y r e d u c t i o n a r e g d l i n g a g e n t t y p e and c o n c e n t r a t i o n , t y p e and s t a b i l i t y o f c r o s s l i n k e r , and b r e a k e r c o n t e n t . O t h e r i m p o r t a n t g e l l i n g a g e n t p r o p e r t i e s a r e m o l e c u l a r w e i g h t / s i z e and r e s i d u e volume a f t e r a c t i o n o f a given f r a c t u r i n g f l u i d breaker o r thermal degradation. F o r m a t i o n o f an i m m o b i l e r e s i d u e a r i s i n g f r o m i n c r e a s i n g g e l l i n g a g e n t s a t u r a t i o n due t o f l u i d l o s s and c r a c k c l o s u r e a n d / o r as t h e r e s u l t o f b r e a k i n g mechanisms i s a m a j o r s o u r c e o f p e r m e a b i l i t y r e d u c t i o n . As p o l y m e r i c g e l l i n g a g e n t s w h i c h a r e n o t d e g r a d e d t o s m a l l m o l e c u l a r s i z e do n o t p e n e t r a t e i n t o t h e m a t r i x o f formations which are t y p i c a l l y candidates f o r f r a c t u r i n g treatments, t h e g e l l i n g a g e n t i s c o n c e n t r a t e d due t o f l u i d l e a k o f f d u r i n g pumping and f r a c t u r e c l o s u r e and a c c u m u l a t e s w i t h i n t h e p r o p p a n t package w i t h a p p r e c i a b l e r i g i d i t y . F u r t h e r c o m p l i c a t i o n s may a r i s e due t o f o r m a t i o n o f r e s i d u e s as t h e r e s u l t o f chemical o r thermal breaking processes.
1.4.10.3.6.2.Cleanup flow rate and closure stress C o n c e r n i n g c l e a n u p f l o w r a t e , an i n c r e a s e e v e n o v e r a t w e l v e - f o l d span h a s l i t t l e e f f e c t on f i n a l s t e a d y - s t a t e p e r m e a b i l i t y o r c l e a n u p r a p i d i t y (HAWKINS 1 9 8 8 ) . H i g h e r f l o w r a t e s e v e n t e n d t o r e s u l t i n somewhat l o w e r p e r m e a b i l i t y r e covery, w i t h thus e v i d e n t l y n o t d i s l o d g i n g r e s t r i c t i n g species. Over t h e range o f 140 - 200 OF ( t h u s b e l o w o n s e t o f t h e r m a l f l u i d d e g r a d a t i o n ) , temperature has o n l y a m i n o r e f f e c t on f i n a l p e r m e a b i l i t y o r p e r m e a b i l i t y r e c o v e r y o f p r o p -
67 p a n t packages c o n t a i n i n g c o n c e n t r a t e d f r a c t u r i n g f l u i d s w i t h o u t b r e a k e r s , and p e r m e a b i l i t y values a r e o n l y s l i g h t l y improved a f t e r h i g h cleanup f l o w r a t e s . W i t h i n c r e a s i n g s h u t - i n t i m e a t temperature and under c l o s u r e s t r e s s , b o t h f i n a l p e r m e a b i l i t y and p e r m e a b i l i t y r e c o v e r y o f t h e proppant package decrease d r a m a t i c a l l y . T h e r e f o r e a minimal s h u t - i n t i m e i s maximizing p r o p p a n t package p e r meabi 1 it y . I n c r e a s i n g c l o s u r e s t r e s s a l s o s u b s t a n t i a l l y reduces p e r m e a b i l i t y o f t h e proppant assemblage c o n t a i n i n g f r a c t u r i n g f l u i d a l t h o u g h no unusual f l o w p a t t e r n i s p r e s e n t and h i g h f l o w r a t e s have o n l y a m a r g i n a l e f f e c t . I n c r e a s i n g g e l l i n g agent s a t u r a t i o n r a p i d l y reduces proppant package p e r m e a b i l i t y , w i t h use o f h i g h e r p r o p p a n t l o a d i n g s i n c r e a s i n g r e t a i n e d proppant assemblage c o n d u c t i v i t y . F i l t e r cakes d e p o s i t e d on f r a c t u r e w a l l s have i n c r e a s i n g l y g r e a t e r i n f l u e n c e on p e r m e a b i l i t y , w i t h p r o p p a n t c o n c e n t r a t i o n d e c r e a s i n g t o such an amount t h a t f i l t e r cake t h i c k n e s s becomes a s i g n i f i c a n t f r a c t i o n o f t h e c r o s s sectional flow area.
1.4.10.3.6.3.
Proppant grain size and breaker concentration
I n v e s t i g a t i o n o f t h e e f f e c t o f i n c r e a s i n g proppant g r a i n s i z e and n a r r o w i n g d i s t r i b u t i o n by changing f r o m 20/40 t o 2 0 / 2 5 mesh s i z e r e v e a l s t h a t p r o p p a n t package p e r m e a b i l i t y can be s i g n i f i c a n t l y i n c r e a s e d e s p e c i a l l y a f t e r h i g h c l e a n up f l o w r a t e s , w i t h t h u s i n c r e a s i n g proppant g r a i n s i z e b e i n g one o f t h e most s u i t a b l e ways o f i n c r e a s i n g p r o p p a n t p e r m e a b i l i t y and f r a c t u r e c o n d u c t i v i t y under a l l p o s s i b l e l a b o r a t o r y and f i e l d c o n d i t i o n s ( c f . s e c t i o n 4 . 3 . 5 . ) . The degree o f p e r m e a b i l i t y damage r e s u l t i n g f r o m s e l e c t e d f r a c t u r i n g f l u i d s c o n t a i n i n g no b r e a k e r s a t 175 OF p a r a l l e l s t h e i r temperature s t a b i l i t i e s which depends from chemical composition, and proppant packages c o n t a i n i n g f r a c t u r i n g f l u i d s w i t h h i g h g e l l i n g agent c o n c e n t r a t i o n s r e q u i r e h i g h b r e a k e r s a t u r a t i o n s f o r r e a sonable p e r m e a b i l i t y improvement (HAWKINS 1988). A t s a t u r a t i o n s where t h e b r e a k e r i s h i g h l y e f f e c t i v e i n r e d u c i n g f l u i d v i s c o s i t y a t t h e i r o r i g i n a l polymer c o n c e n t r a t i o n , i t i s much l e s s e f f e c t i v e than t h e h i g h l y - e n r i c h e d polymer w i t h i n t h e proppant package, and t h e r e f o r e a b r e a k e r system has t o be d e v i s e d which w i l l m a i n t a i n f l u i d p r o p e r t i e s d u r i n g t h e t r e a t m e n t and improve f r a c t u r e permeab i l i t y . C a u t i o n has t o be e x e r c i s e d , however, because h i g h b r e a k e r s a t u r a t i o n s could g i v e r i s e t o extensive f l u i d degradation during the f r a c t u r i n g treatment.
1.4.10.3.6.4. Hydrocarbon-based cleanup f l u i d s for proppant package permeabi 1i t y improvement Hydrocarbon-based cleanup f l u i d s cause an i n i t i a l decrease i n apparent p e r m e a b i l i t y as a consequence o f i n t e r m i t t e n t m i n o r emulsion g e n e r a t i o n which r a p i d l y d i s s i p a t e s f o l l o w e d by a s i g n i f i c a n t i n c r e a s e . Hydrocarbon-based f r a c t u r i n g f l u i d s have t h e e f f e c t o f v e r y h i g h p r o p p a n t package p e r m e a b i l i t y under a1 c o n d i t i o n s , w i t h hydrocarbon-based s t i m u l a t i o n f l u i d s c l e a r l y c a u s i n g much l e s s p e r m e a b i l i t y r e d u c t i o n b o t h a b s o l u t e l y and as a percentage t h a n aqueous g e l l e d f l u i d s (HAWKINS 1988). T h e r e f o r e hydrocarbon-based t r e a t m e n t f l u i d s should be used whenever f r a c t u r e p e r m e a b i l i t y needs t o be maximized. I n o r d e r t o maximize proppant package p e r m e a b i l i t y when aqueous g e l l e d f r a c t u r i n g f l u i d s have t o be a p p l i e d , emphasis has t o be p u t on m i n i m i z a t i o n o f use o f c r o s s l i n k i n g , polymer c o n c e n t r a t i o n and s h u t - i n time, and m a x i m i z a t i o n o f proppant s a t u r a t i o n / l o a d i n g , p r o p p a n t g r a i n s i z e , and b r e a k e r c o n t e n t o f t h e f l u i d .
1.4.10.4. Recmended conductivity correction factors As a r e s u l t of t h e o u t l i n e d t e s t i n g campaign, McDANIEL (1986) concludes t h a t l a b o r a t o r y measurements of p r o p p a n t c o n d u c t i v i t y a t ambient temperatures and s h o r t t i m e s o f experiment d u r a t i o n a t each c l o s u r e s t r e s s l e v e l a r e t y p i c a l l y t o o o p t i m i s t i c as compared t o more r e a l i s t i c i n - s i t u w e l l c o n d i t i o n s . C o n d u c t i -
68 v i t y c o r r e c t i o n f a c t o r s o f 0 . 5 - 0 . 6 f o r i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s a r e recommended t o compensate a l l t h e d e t r i m e n t a l e f f e c t s b y t h e m e n t i o ned t e s t c o n d i t i o n s when p l a n n i n g f r a c t u r e c o n d u c t i v i t y f o r an a c t u a l s t i m u l a t i o n t r e a t m e n t b y r e c a l c u l a t i n g l a b o r a t o r y d a t a t o i n - s i t u downhole c o n d i t i o n s i n t h e f r a c t u r e d r e s e r v o i r r o c k . MUCH ( 1 9 8 7 ) e v e n r e p o r t s examples where l a b o r a t o r y - d e r i v e d p r o p p a n t c o n d u c t i v i t i e s have t o be r e d u c e d b y 85 - 90 % i n o r d e r t o c o r r e l a t e w i t h h i s t o r y m a t c h c o n d u c t i v i t y v a l u e s f o r t h e r e a s o n s o f damage b y f r a c t u r i n g f l u i d g e l r e s i d u e , p r o p p a n t embedment, p r o p p a n t c r u s h i n g , and non-Darcy f l o w i n t h e p r o p p a n t package. An i m p o r t a n t c r o s s - c h e c k b e f o r e t h e f i n a l o p e r a t i o n i s a m i n i f r a c t u r e c a l i b r a t i o n t r e a t m e n t w h i c h c a n v e r i f y t h e c o r r e c t n e s s o f t h e a s s u m p t i o n s and d i s c o u n t i n g ( c f . s e c t i o n 4 . 8 . 1 . ) . MONTGOMERY & STEANSON ( 1 9 8 5 ) p u t f o r w a r d t h a t i n deep w e l l c o m p l e t i o n s , t h e f i n a l recommended c o n d u c t i v i t y needs t o b e t w o t o t h r e e times h i g h e r than t h e t h e o r e t i c a l l y designed c o n d u c t i v i t y , w i t h t h e r e f o r e o v e r d e s i g n b e i n g n e c e s s a r y t o f i n a l l y a c c o u n t f o r downhole c o n d i t i o n s and r e s u l t s ( c f . s e c t i o n 1 . 4 . 1 0 . 4 . 3 . 1 . ) . F u r t h e r comments o n r e a l i s t i c f r a c t u r e c o n d u c t i v i t y o f p r o p p a n t s p a r t i c u l a r l y as a f u n c t i o n o f r e s e r v o i r t e m p e r a t u r e a r e g i v e n b y McDANIEL (1987, 1 9 8 8 ) , and PENNY ( 1 9 8 7 ) r e p o r t s o n t h e e f f e c t s o f e n v i r o n m e n t a l c o n d i t i o n s and s t i m u l a t i o n f l u i d s on l o n g - t e r m p r o p p a n t c o n d u c t i v i t y . Some a s p e c t s o f i n f l u e n c e o f t e s t i n g e q u i p m e n t , i m p a c t o f b r i n e c o m p o s i t i o n and c o n d u c t i v i t y improvement a r e b r i e f l y o u t l i n e d as f o l l o w s . Comments a r e a l s o o f f e r e d on p r a c t i c a l u s e o f p r o p p a n t c o n d u c t i v i t y d a t a .
1.4.10.4.1. Influence of testing equipment ROBINSON, HOLDITCH & WHITEHEAD ( 1 9 8 6 ) p u t f o r w a r d t h a t because m o s t o f t h e f r a c t u r e c o n d u c t i v i t y t e s t s a r e done w i t h s t e e l p l a t e s , embedment e f f e c t s a r e n o t c o n s i d e r e d and t h e r e f o r e t h e measured d a t a a r e t o o o p t i m i s t i c . U n l e s s u s i n g r e s e r v o i r c o r e s l a b s as b o u n d i n g f a c e s o f t h e s i m u l a t e d f r a c t u r e , t h e c o n d u c t i v i t y v a l u e s s h o u l d be r e d u c e d b y a f a c t o r 2 - 3 . P r o p p a n t c o n d u c t i v i t y i s f u r t h e r d i m i n i s h e d b y t h e p r e s e n c e o f g e l r e s i d u e , f o r m a t i o n f i n e s and o t h e r s o l i d m a t e r i a l s i n t h e pore space. Although t h i s problem v a r i e s according t o format i o n and f l u i d t y p e and u s u a l l y i s n o t as s e r i o u s as c r u s h i n g and embedment, i t has t o be c o n s i d e r e d t h a t s o l i d f i n e p a r t i c l e s can f u r t h e r d e c l i n e f r a c t u r e c o n d u c t i v i t y w h i c h s h o u l d be a c c o u n t e d f o r b y d e c r e a s i n g t h e v a l u e s b y a n o t h e r f a c t o r 2 - 3 i n order t o p r o p e r l y simulate actual r e s e r v o i r conditions. B e t t e r exp e r i m e n t a l c o n d u c t i v i t i e s t h a n t h o s e a c h i e v e d downhole i n t h e p a y f o r m a t i o n a r e a l s o t h e consequence o f w a l l s l i p . f e a t u r e s ( c f . s e c t i o n 1 . 4 . 1 0 . 3 . 4 . 1 . ) . Non-Darc y f l o w phenomena i n t h e p r o p p e d f r a c t u r e r e s u l t i n an e x t r a p r e s s u r e d r o p w h i c h c a n r e d u c e e f f e c t i v e f r a c t u r e f l o w c a p a c i t y b y more t h a n a f a c t o r 3 (DAV I E S & KUIPER 1 9 8 8 ) .
1.4.10.4.2. Impact o f brine composition S i m i l a r o r e v e n h i g h e r d i s c o u n t s w o u l d be n e c e s s a r y f o r l a b o r a t o r y d a t a obt a i n e d f r o m t e s t s i n 2 % KC1 s o l u t i o n s w h i c h m a t c h t h e c o m p o s i t i o n o f t h e p r e pad and d i s p l a c e m e n t s t a g e s i n MHF o p e r a t i o n s and a r e t h e base f l u i d f o r many c r o s s l i n k e d g e l s ( P A I , GARBIS & HALL 1983; BLEAKLEY 1 9 8 4 ) , b u t do n o t a g r e e w i t h t h e p a r t i a l l y s a t u r a t e d a g g r e s s i v e b r i n e s i n R o t l i e g e n d and C a r b o n i f e r o u s r e s e r v o i r r o c k s i n Germany FRG w h i c h become e f f e c t i v e a f t e r t e r m i n a t i o n o f t h e f r a c t u r i n g t r e a t m e n t when t h e i n v a s i o n o f t h e pay h o r i z o n caused b y t h e i n j e c t e d f l u i d s has been r e v e r s e d and a l s o t h e c r a c k i s f l u s h e d w i t h h i g h - s a l i n i t y f o r m a t i o n w a t e r i n s e c t i o n s b e l o w t h e g a s - w a t e r - c o n t a c t , as w e l l as d u r i n g p r o g r e s s i v e d e p l e t i o n when t h e w a t e r c u t i n t h e gas i s s t e a d i l y r i s i n g i n r e s e r v o i r i n t e r v a l s above t h e g a s - w a t e r - b o u n d a r y . The o u t l i n e as f o l l o w s d i s c u s s e s c l a y s t a b i l i z a t i o n b y KC1 w a t e r i n f i e l d s t i m u l a t i o n a p p l i c a t i o n and KC1 s o l u t i o n s as l a b o r a t o r y t e s t i n g f l u i d s .
69
1.4.10.4.2.1. Clay stabilization by KC1 water in field stimulation application 2 % KC1 s o l u t i o n s which are f r e q u e n t l y taken as proppant t e s t i n g f l u i d s are the base composition f o r t h e m a j o r i t y o f s t i m u l a t i o n f l u i d s and are used i n t h e prepad o f h y d r a u l i c f r a c t u r i n g treatments f o r the purpose o f c l a y s t a b i l i z a t i o n and thus p r e v e n t i o n o f excessive f o r m a t i o n damage p a r t i c u l a r l y i n water-sensit i v e r e s e r v o i r rocks as a standard s a f e t y p r e c a u t i o n (BLACK & HOWER 1965; BLACK, RIPLEY, BEECROFT & PAMPLIN 1979; WATERS 1980; NALL, CAMPBELL & BONEY 1983; SYDANSK 1984, MUCH 1987, MUCH & PENNY 1987; c f . s e c t i o n 3.5.2.2.).
2 % KC1 water i n combination w i t h c l a y s t a b i l i z e r s renders the s t i m u l a t i o n f l u i d non-damaging by c o n t r o l l i n g p a r t i c l e sloughing and m i g r a t i o n , and an addit i o n a l s u r f a c t a n t reduces boundary t e n s i o n a t t h e f o r m a t i o n and f r a c t u r e f l u i d i n t e r f a c e t o f a c i l i t a t e treatment cleanup a f t e r s u f f i c i e n t v i s c o s i t y r e d u c t i o n o f the gel by breaking (PA1 & GARBIS 1983 b ) . I n a d d i t i o n t o s t i m u l a t i o n f l u i d s , KC1 can a l s o be a p p l i e d as i n h i b i t o r i n d r i l l i n g mud and cement s l u r r i e s (MESSINA 1986 b ) . While 2 % KC1 water prevents f o r m a t i o n damage by f i l t r a te, 5 % KC1 water i n h i b i t s shale sloughing and helps t o m a i n t a i n good h o l e cond i t i o n s , and 2 - 5 % KC1 polymer systems p r o v i d e v i s c o s i t y and water l o s s control.
1.4.10.4.2.2. KC1 solutions as laboratory testing fluids 2 % K C 1 s o l u t i o n s have f r e q u e n t l y been used as standard t e s t f l u i d s i n labor a t o r y proppant c o n d u c t i v i t y experiments (McDANIEL 1986, 1987, 1988; MUCH & PENNY 1987, PARKER & McDANIEL 1987, PENNY 1987, McDANIEL & PARKER 1988). Since a l a r g e number o f gas w e l l s which are h y d r a u l i c a l l y f r a c t u r e d produce m o s t l y gas a f t e r i n i t i a l cleanup, a b e t t e r approach t o i n - s i t u c o n d i t i o n s i s p r o v i d e d by changing the f l o w medium from 2 % KC1 s o l u t i o n t o wet gas a f t e r some days i n o r der t o s i m u l a t e t h e sequence o f f r a c t u r i n g f l u i d backproduction f o l l o w e d by gas withdrawal. Experimental work r e v e a l s b e t t e r p e r m e a b i l i t y p r e s e r v a t i o n when wet gas i s used as t e s t i n g medium a f t e r a couple o f days than w i t h 2 % KC1 water alone. The d i f f e r e n c e i n remaining proppant c o n d u c t i v i t y i s i n t e r p r e t e d t o be p a r t i a l l y the r e s u l t o f g e l f i l t e r cake dehydration by the wet gas, w i t h t h i s s h r i n k i n g e f f e c t e x e r t e d on the g e l f i l t e r cake i n c r e a s i n g proppant package por o s i t y and p e r m e a b i l i t y (McDANIEL 1988). F u r t h e r i n v e s t i g a t i o n s have t o c o n s i der l i q u i d hydrocarbons t o check accurate m o d e l l i n g o f o i l r e s e r v o i r s i n the experiments. Wet gas as f l o w i n g medium i n t e s t s i n c l u d i n g g e l f i l t e r cakes has a l s o been used by ROODHART, KUIPER & DAYIES (1986). Apart from K C l , c l a y s t a b i l i z a t i o n can a l s o be achieved w i t h KOH (SLOAT & LARSEN 1984, SYDANSK 1984, SLOAT 1988).
Other aspects o f s t a b i l i z a t i o n o f b o t h dispersed and massive c l a y s and prev e n t i o n o f t h e i r m i g r a t i o n d u r i n g f r a c t u r i n g operations and o t h e r r e s e r v o i r treatments a r e presented by McLAUGHLIN, ELPHINGSTONE & HALL (1976) ; CLEMENTA (1977) ; McLAUGHLIN, ELPHINGSTONE, REMINGTON & COATES (1977) ; PETERS & STOUT (1977), THOMAS & CROWE (1978) ; COULTER, COPELANO & HARRISBERGER (1979); WILLIAMS & UNDERDOWN (1980); CALLAWAY, ORTIZ & HOLCOMB (1982); HILL (1982), BORCHARDT & BROWN (1984); BORCHARDT, ROLL & RAYNE (1984) and SLOAT & LARSEN (1984).
1.4.10.4.3. Conductivity improvement POULSEN & SOLIMAN (1987) emphasize t h a t a c o r r e c t i o n f a c t o r o f 0.5 i s b e s t compensating the up t o t e n - f o l d decrease i n proppant c o n d u c t i v i t y from l a b o r a t o r y experiment t o f i e l d execution c o n d i t i o n s , and PHILLIPS & ANDERSON (1985) account f o r proppant performance damage by a f a c t o r o f 0.2 - 0.25. K I M & LOSACANO (1985) a l s o propose a c o r r e c t i o n f a c t o r o f 0.4 - 0.6 t o compensate g e l r e s i d u e
70 p l u g g i n g o f p r o p p a n t p o r e space. SECCOMBE & ANDERSON ( 1 9 8 2 ) s u g g e s t t o m u l t i p l y s h o r t - t e r m proppant t e s t i n g d a t a by a c o r r e c t i o n f a c t o r o f 0 . 1 t o match p o s t f r a c t u r i n g c a l c u l a t i o n s , and SOLIMAN ( 1 9 8 3 ) d e t e r m i n e s p o s t - s t i m u l a t i o n c o n d u c t i v i t i e s o f 0.125 - 0.25 o f t h e values p r e d i c t e d f r o m l a b o r a t o r y experiments. REINICKE, BRINKMANN, SCHWARZ & HUENI ( 1 9 8 5 ) f i n d c o r r e c t i o n f a c t o r s b e i n g i n some c a s e s as l o w as 0 . 0 5 . Some comments a r e o f f e r e d on s t i m u l a t i o n t r e a t m e n t o v e r d e s i g n as w e l l as demand o f h i g h e r p r o p p a n t q u a l i t y and s a t u r a t i o n .
1.4.10.4.3.1. Stimulation treatment overdesign F a i l u r e t o consider t h e v a r i o u s types o f proppant c o n d u c t i v i t y impairment and t o compensate them b y c o r r e c t i o n f a c t o r s may cause d r a m a t i c a l u n d e r d e s i g n o f t h e f r a c t u r i n g t r e a t m e n t , w i t h t h e r e f o r e t h e a c t u a l j o b h a v i n g t o be o v e r d e signed f o r t h e purpose o f a c h i e v i n g t h e d e s i r e d i n - s i t u c o n d u c t i v i t y a f t e r a l l t h e p o s s i b l e f l o w c a p a c i t y d e t e r i o r a t i o n s even i n t h e w o r s t c a s e o f combined a c t i o n o f a l l d e t r i m e n t a l f a c t o r s (ROODHART, KUIPER & DAVIES 1 9 8 6 ) . I n t e r m s o f l a y o u t , a c o n d u c t i v i t y c o r r e c t i o n f a c t o r o f 0 . 5 means t h a t f o r c o n s t a n t f r a c t u r e c o n d u c t i v i t y , b o t h r e q u i r e d p r o p p a n t c o n c e n t r a t i o n a t t h e w e l l b o r e and t o t a l p r o p p a n t amount have a p p r o x i m a t e l y t o be d o u b l e d (POULSEN & SOLIMAN 1 9 8 7 ) . C o n d u c t i v i t y improvement when a p p l y i n g c o r r e c t i o n f a c t o r s i n t h e r a n g e o f 0 . 2 0 . 5 c a n be a c h i e v e d b y i n c r e a s i n g t h e maximum a l l o w a b l e p r o p p a n t s a t u r a t i o n o r by u s i n g a higher c o n d u c t i v i t y proppant o f b e t t e r q u a l i t y and/or l a r g e r g r a i n size. As many l a b o r a t o r y p r o p p a n t t e s t s u n d e r d i f f e r e n t c o n d i t i o n s have s u g g e s t e d t h a t t h e f i n a l i n - s i t u c o n d u c t i v i t y o f t h e p r o p p a n t package w i t h i n t h e f r a c t u r e c a n be as l o w as a b t . 1 0 % o f t h e o r i g i n a l p r o p p a n t c o n d u c t i v i t y ( L I & ZHU 1 9 8 6 ) , p r o p e r c o m p e n s a t i o n o f t h e d e t e r i o r a t i o n s h o u l d be made b y o v e r d e s i g n i n g c r a c k c o n d u c t i v i t y i n s u c h a manner t h a t t h e f r a c t u r e f l o w c a p a c i t y w h i c h i s u l t i m a t e l y l e f t downhole i s a t l e a s t e q u a l t o t h e e x p e c t a t i o n and t o t h e n e c e s s a r y minimum t o e c o n o m i c a l l y f e a s i b l y d r a i n t h e r e s e r v o i r , i f l i o t h i g h e r (ROODHART, KUIPER & O A V I E S 1 9 8 6 ) .
1.4.10.4.3.2.Demand o f higher proppant quality and saturation Using r e a l i s t i c proppant c o n d u c t i v i t y data i n h y d r a u l i c s t i m u l a t i o n design r e v e a l s t h a t i n o r d e r t o a c c o u n t f o r t h e n e c e s s a r y c o n d u c t i v i t y l o s s compensat i o n , h i g h e r p r o p p a n t c o n c e n t r a t i o n s a n d / o r q u a l i t i e s may be r e q u i r e d t o a c h i e v e t h e d e s i r e d f r a c t u r e c o n d u c t i v i t y i n t h e r e s e r v o i r e n v i r o n m e n t (McDANIEL 1 9 8 7 ) . F r a c t u r e o v e r d e s i g n s h o u l d a l s o be made i n t e r m s o f l e n g t h , as many l o w - p e r m e a b i l i t y r e s e r v o i r t r e a t m e n t j o b s have t u r n e d o u t t o a c t u a l l y r e a c h o n l y a b t . 70 % o f t h e d e s i g n e d p r o p p e d c r a c k l e n g t h (HOLDITCH & LEE 1 9 7 9 ) . An u n i q u e and unambiguous h i s t o r y m a t c h can o n l y be o b t a i n e d i f t h e d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t y i s 10 o r g r e a t e r ( c f . s e c t i o n 4.8.11.1.). The a p p l i c a t i o n o f r e a l i s t i c and c o r r e c t e d p r o p p a n t c o n d u c t i v i t i e s f o r f r a c t u r e s t i m u l a t i o n d e s i g n o f t e n r e s u l t s i n t h e n e c e s s i t y t o choose c o n s i d e r a b l y h i g h e r - s t r e n g t h proppants and/or h i g h e r proppant s a t u r a t i o n s and/or a l s o l a r g e r g r a i n s i z e s t h a n o r i g i n a l l y p l a n n e d and e x p e c t e d i n o r d e r t o a d e q u a t e l y d e s i g n and a c c u r a t e l y model t h e f r a c t u r e o p e r a t i o n (McDANIEL 1 9 8 7 ) . A c c u r a t e d e s i g n o f f r a c t u r i n g t r e a t m e n t s r e q u i r e s c o n d u c t i v i t y measurements a t s i m u l a t e d r e s e r v o i r c o n d i t i o n s (McDANIEL & PARKER 1 9 8 8 ) . The c o m p a r i s o n o f s h o r t - t e r m t e s t i n g , l o n g - t e r m t e s t i n g w i t h o u t g e l f i l t e r c a k e s and l o n g - t e r m t e s t i n g i n c l u d i n g g e l f i l t e r c a k e s r e f l e c t s p r o g r e s s i v e c o n d u c t i v i t y d e t e r i o r a t i o n as t h e e x p e r i m e n t a l c o n d i t i o n s a r e more and more a p p r o a c h i n g t h e a c t u a l downhole r e s e r v o i r e n v i r o n m e n t a l s i t u a t i o n . T h e r e f o r e r e l i a b l e f r a c t u r e c o n d u c t i v i t y m o d e l l i n g and d e s i g n has t o b e based on p r o p p a n t t e s t i n g c a r r i e d o u t u n d e r c o n d i t i o n s t h a t a r e as c l o s e l y as p o s s i b l e a p p r o a c h i n g t h o s e p r e s e n t i n t h e f i e l d .
71
1.4.10.4.4. Practical use of proppant conductivity data Proppant c o n d u c t i v i t y d a t a a r e p r a c t i c a l l y used i n two b a s i c a p p l i c a t i o n s i n o i l and gas i n d u s t r y (McDANIEL 1988). The f i r s t one comprises o f f t a k e i n c r e a s e c a l c u l a t i o n s where an average c o n d u c t i v i t y v a l u e o f t h e r e s u l t a n t propped f r a c t u r e i s used t o f o r e c a s t m u l t i f o l d p r o d u c t i v i t y i n c r e a s e o f t h e w e l l (TINSLEY, WILLIAMS, TINNER & MALONE 1969; SDLIMAN 1983), and t h e second one i n c l u d e s comp u t e r i z e d f r a c t u r e d r e s e r v o i r s i m u l a t o r s which can be m o d i f i e d t o i n c o r p o r a t e t h e e f f e c t s o f non-Darcy f l o w . Some aspects o f l a m i n a r vs. t u r b u l e n t f l o w and c o n d u c t i v i t y n e c e s s i t y f o r cleanup vs. p r o d u c t i o n a r e discussed as f o l l o w s .
1.4.10.4.4.1.Laminar vs. turbulent flow H i t h e r t o t y p i c a l l y l a m i n a r - f l o w c o n d u c t i v i t y v a l u e s have been used and no a l lowance i s made f o r t h e i n c r e a s e d f l o w r e s i s t a n c e caused by non-Darcy e f f e c t s as a consequence o f t u r b u l e n c e s . F o r gas o r o i l w e l l s p r o d u c i n g a t v e r y h i g h r a t e s o r e x p l o i t i n g v e r y l o w - v i s c o s i t y o i l , r e s p e c t i v e l y , however, t h e use o f l a minar-flow c o n d u c t i v i t y data r e s u l t s i n overestimating the effectiveness o f the planned s t i m u l a t i o n t r e a t m e n t , and t h e r e f o r e non-Darcy f l o w phenomena i n c l u d i n g t u r b u l e n c e s have t o be taken under c o n s i d e r a t i o n (McDANIEL 1988; c f . s e c t i o n
4.3.3.3.).
Concerning computerized f r a c t u r e d r e s e r v o i r s i m u l a t o r s which can be m o d i f i e d t o i n c o r p o r a t e t h e f e a t u r e s o f non-Darcy f l o w due t o c u r r e n t t u r b u l e n c e s , i t i s e s s e n t i a l t h a t c o n d u c t i v i t y v a l u e s used i n t h e s i m u l a t o r a r e measured under t e s t c o n d i t i o n s which model r e a l i s t i c r e s e r v o i r parameters. A t t e n t i o n has t o be p a i d n o t o n l y on p r o p p a n t t y p e and s i z e , b u t a l s o on l o n g - t e r m exposure t o s t r e s s a t i n - s i t u temperature i n c l u d i n g any v a r i a t i o n s f r o m t h e o r e t i c a l behav i o u r caused by such e f f e c t s as presence o f g e l f i l t e r cakes occupying p a r t s o f t h e proppant bed p o r o s i t y .
1.4.10.4.4.2. Conductivity necessity for cleanup vs. production I n terms o f proppant performance m o d e l l i n g , i t has t o be r e s p e c t e d t h a t t h e optimum f r a c t u r e c o n d u c t i v i t y necessary t o clean-up t h e f o r m a t i o n may be much h i g h e r t h a n t h a t r e q u i r e d t o produce gas f r o m t h e r e s e r v o i r (SOLIMAN & HUNT 1985). T h e r e f o r e c r a c k parameters s h o u l d be e s p e c i a l l y f o r t i g h t gas r e s e r v o i r s designed such t h a t a f r a c t u r i n g t r e a t m e n t w i l l l e a v e enough p r o p p a n t c o n d u c t i v i t y f o r e f f i c i e n t cleanup o f s t i m u l a t i o n f l u i d . McDANIEL (1987, 1988) and PARKER & McDANIEL (1987) a l s o recommend t o p l a c e h i g h e r proppant c o n c e n t r a t i o n s t h a n t h e o r e t i c a l l y necessary i n t h e c r a c k i n o r d e r t o overcome c l o s u r e e f f e c t s , t o achieve s a t i s f a c t o r y i n - s i t u f r a c t u r e c o n d u c t i v i t y and t h u s f u r t h e r improve s t i m u l a t i o n r e s u l t s . D A V I E S & KUIPER (1988) u n d e r l i n e t h a t r e c e n t l o n g - t e r m conduct i v i t y measurements under r e a l i s t i c r e s e r v o i r c o n d i t i o n s have r e v e a l e d a t e c h n i c a l r e q u i r e m e n t f o r s t r o n g e r , more c o n d u c t i v e proppants n o t o n l y a t h i g h e r c l o s u r e s t r e s s e s i n deeper pay zones, b u t a l s o a t lower c l o s u r e s t r e s s e s i n s h a l l o wer p r o s p e c t i v e i n t e r v a l s i n o r d e r t o guarantee optimum i n - s i t u c o n d u c t i v i t y i n s t a l l a t i o n . The l a t t e r r e l a t i o n s h i p a l s o a p p l i e s i f c o a r s e r p r o p p a n t grades a r e used i n an a t t e m p t t o i n c r e a s e f r a c t u r e c o n d u c t i v i t y , because c r u s h i n g o f c o a r ser g r a i n sizes s t a r t s a t lower closure stress l e v e l s than t h a t o f f i n e r g r a i n sizes.
1.4.10.5. Performance experience i n long-term field application A l l t h e e x p e r i m e n t a l l y concluded r e d u c t i o n s o f proppant c o n d u c t i v i t y , however, a r e i n p r a c t i c a l a p p l i c a t i o n i n t h e f i e l d a t l e a s t i n l o w - p e r m e a b i l i t y r e s e r v o i r s i n many cases o f low s i g n i f i c a n c e a t t h e b o t t o m o f t h e l i n e , as t h e r e s u l t i n g i n - s i t u p r o p p a n t c o n d u c t i v i t y i s s t i l l much h i g h e r than f o r m a t i o n p e r m e a b i l i t y and t h u s g i v e s e s p e c i a l l y i n t i g h t sandstones s t i l l a marked c o n t r a s t
72 between r e s e r v o i r r o c k s and p r o p p a n t - f i l l e d d r a i n a g e f r a c t u r e i n a s a t i s f a c t o r y way t o be t h e guarantee f o r l o n g - t e r m gas p r o d u c t i o n a t e c o n o m i c a l l y s u f f i c i e n t and a p p r e c i a b l e r a t e s ( a s evidenced by numerous C a r b o n i f e r o u s and R o t l i e g e n d gas w e l l s i n Germany FRG which have been t r e a t e d up t o 10 y e a r s ago and a r e s t i l l producing a t reasonably f e a s i b l e r a t e s t h a t are n o t s i g n i f i c a n t l y d i f f e r e n t f r o m those achieved s h o r t l y a f t e r t h e o p e r a t i o n o r a r e f o l l o w i n g t h e p r e d i c t e d n a t u r a l r a t e d e c l i n e w i t h p r o g r e s s i v e d e p l e t i o n ) . Some examples f r o m N o r t h China a r e a l s o i l l u s t r a t e d .
1.4.10.5.1. Rotliegend and Carboniferous in Germany FRG M i n o r amounts o f back-produced proppants t h a t were r e c o v e r e d i n some deep R o t l i e g e n d and C a r b o n i f e r o u s gas w e l l s r e v e a l o n l y s u b o r d i n a t e mechanical and chemical a l t e r a t i o n s o f t h e i n j e c t e d p r o p p a n t g r a i n s t h u s c o n f i r m i n g t h e p h y s i c a l s t a b i l i t y o f t h e proppant package i n t h e f r a c t u r e as evidenced by t h e p e r s i s t i n g s a t i s f a c t o r y gas w i t h d r a w a l r a t e s and a l s o t e s t i f y t o o n l y s u b o r d i n a t e o r even n e g l i g i b l e f r a c t u r e e v a c u a t i o n by p r o p p a n t flowback ( c f . a l s o s e c t i o n 1 . 4 . 1 . ) . Examination o f g r a i n s u r f a c e s and shapes r e f l e c t s o n l y m i n o r d i s s o l u t i o n and c r u s h i n g o f some p a r t s o f t h e p a r t i c l e p o p u l a t i o n and t h u s c o n f i r m t h e e x c e l l e n t p r o p p a n t b e h a v i o u r which i s i n d e p e n d e n t l y suggested by t h e p r o m i s i n g hydrocarbon o f f t a k e s r a t e s . COBB & FARRELL (1986) conclude t h a t t h e A P I conduct i v i t y d a t a have t o be d i s c o u n t e d by a b t . 20 % i f t h e w e l l c o n d i t i o n s a r e a t l e a s t as severe as 10,000 p s i c l o s u r e s t r e s s , 93 OC (200 OF) r e s e r v o i r temperat u r e and 2 % KC1 f l u i d s a t u r a t i o n ( c o r r e s p o n d i n g t o t h e prepad f l u s h i n g ) . R E I N I C K E , BRINKMANN, SCHWARZ & HUENI (1985) observe i n R o t l i e g e n d and Carbon i f e r o u s gas w e l l s i n Germany FRG a f r a c t u r e c o n d u c t i v i t y r e d u c t i o n by a f a c t o r two t o f i v e which enables performance o f a good h i s t o r y match i n c l u d i n g p r e s s u r e drop, t h u s a g r e e i n g w i t h GUPPY, CINCO-LEY & RAMEY (1982) t h a t t h e a p p a r e n t c o n d u c t i v i t y o b t a i n e d f r o m b u i l d u p s o l u t i o n s can be up t o 85 % l o w e r t h a n t h e t r u e c o n d u c t i v i t i e s . Cross-flow d u r i n g s h u t - i n t i m e s can be expected t o become s i g n i f i c a n t i f some h o r i z o n s o f t h e f r a c t u r e d r e s e r v o i r s e c t i o n a r e d e p l e t e d p r e f e r e n t i a l l y w i t h i n c r e a s i n g e x p l o i t a t i o n , b u t no c r o s s - f l o w was observed i n p r o d u c t i o n c o n t r o l surveys d u r i n g more than 5 y e a r s . Concerning f i e l d performance i n t e r p r e t a t i o n , R E I N I C K E , BRINKMANN, SCHWARZ & HUENI (1985) r e c o r d a d i f f e rence o f a f a c t o r two t o f i v e o r even g r e a t e r when comparing computed f r a c t u r e c o n d u c t i v i t i e s w i t h s i m u l a t i o n - d e r i v e d o r i n t e g r a l - m e t h o d - d e r i v e d values, r e spectively.
1.4.10.5.2. North China L I & ZHU (1986) document t h a t t h e propped f r a c t u r e c o n d u c t i v i t y as c a l c u l a t e d f r o m t r a n s i e n t p r e s s u r e t e s t d a t a i n t h e range o f 2 - 5 months a f t e r t h e s t i m u l a t i o n j o b i n moderate- t o h i g h - d e p t h o i l r e s e r v o i r s i n N o r t h China i s o n l y 30 - 70 % o f t h e l a b o r a t o r y e x p e r i m e n t - d e r i v e d values, w i t h a minimum o f a b t . 10 % a l s o b e i n g reached i n some cases. An a n a l y s i s o f t h e phenomena i n d i c a t e s t h a t i n a d d i t i o n t o damage caused by scraped f i n e s f r o m t h e f r a c t u r e w a l l by p r o p p a n t - c a r r y i n g f l u i d , m i g r a t i o n o f f o r m a t i o n f i n e s , embedding o r c r u s h i n g o f proppants i n t h e c r a c k , and i n c o m p a t i b i l i t y of t r e a t m e n t f l u i d t o f o r m a t i o n f l u i d , t h e damage generated by degraded f l u i d r e s i d u e has s i g n i f i c a n t impact. F r a c t u r e c o n d u c t i v i t y damage can be e f f e c t i v e l y reduced by i n c r e a s i n g p r o p p a n t c o n c e n t r a t i o n . BAKHTAR, BARTON, RAKOP & JONES (1985) d i s c u s s m o d e l l i n g o f c r a c k p e r m e a b i l i t y around a w e l l d u r i n g d e p l e t i o n . Long-term performance o f h y d r a u l i c f r a c t u r e t r e a t m e n t s i s a l s o r e p o r t e d by ZAHNER & CRAFTON ( 1 9 8 5 ) .
1.4.11. Proppant mixing The c o n v e n t i o n a l a p p l i c a t i o n o f proppants i n h y d r a u l i c f r a c t u r i n g comprises placement o f m a t e r i a l o f unimodal g r a i n s i z e and homogeneous type, b u t a l s o se-
73 v e r a l p o s s i b i l i t i e s e x i s t f o r usage o f d i f f e r e n t proppant t y p e s and g r a i n s i z e s i n one o p e r a t i o n , c o m p r i s i n g t a i l - i n and m i x i n g o f proppants. T a i l - i n i s p e r f o r med by pumping o f separated l o t s o f proppants o f s u c c e s s i v e l y i n c r e a s i n g c l o s u r e s t r e s s r e s i s t i v i t y o r c o n d u c t i v i t y and g r a i n s i z e i n o r d e r t o achieve maximum s u p p o r t i n t h e v i c i n i t y o f t h e w e l l b o r e where t h e h i g h e s t c l o s u r e s t r e s s occ u r s due t o maximum drawdown and h i g h e s t f l o w v e l o c i t i e s (HOWARD & FAST 1970 a; HICKEY, BROWN & CRITTENDEN 1981; c f . a l s o s e c t i o n 2.4.1.2.). While during course o f e x e c u t i o n o f t a i l - i n f r a c t u r i n g j o b s m i x i n g o f d i f f e r e n t p r o p p a n t t y pes and g r a i n s i z e s i s n o t i n t e n d e d and can o n l y o c c u r a c c i d e n t a l l y t o m i n o r amounts a t t h e t r a n s i t i o n s between d i f f e r e n t t r e a t m e n t stages, m i x i n g o f propp a n t s w i t h d i f f e r e n t p h y s i c a l p r o p e r t i e s was f r e q u e n t l y approached e x p e r i m e n t a l l y i n o r d e r t o check i t s s i g n i f i c a n c e f o r p o s s i b l y i n c r e a s i n g o r d e c r e a s i n g p r o p p a n t c o n d u c t i v i t y and s t a b i l i t y above t h e l e v e l achieved by unimodal homogeneous p o p u l a t i o n s (McOANIEL & WILLINGHAM 1978). Comments a r e o f f e r e d as f o l l o w s on t h e e f f e c t s o f m i x i n g o f p r o p p a n t types, g r a i n s i z e s and g r a i n shapes.
1.4.11.1. Mixing o f proppant types The r e s u l t s o f most p r o p p a n t m i x i n g i n v e s t i g a t i o n s r e v e a l t h a t t h e b e s t e f f e c t s i n terms o f c o n d u c t i v i t y a r e o b t a i n e d w i t h unimodal homogeneous d i s t r i b u t i o n s . M i x i n g o f p r o p p a n t t y p e s and g r a i n s i z e s h a r d l y has an i m p r o v i n g impact, i f any. Some aspects o f m i x i n g o f n a t u r a l sand and s y n t h e t i c proppants as w e l l as o f d i f f e r e n t sand t y p e s a r e sketched as f o l l o w s .
1.4.11.1.1. Mixing o f natural sand and synthetic proppants M i x i n g o f n a t u r a l sand w i t h s i n t e r e d b a u x i t e o r g l a s s beads has been tempora-
r i l y been common as a consequence o f expected p o s s i b i l i t i e s o f p r o p p a n t c o s t r e -
d u c t i o n e s p e c i a l l y i n t h e e a r l y 1970's when g l a s s beads p r i o r t o m a r k e t i n t r o d u c t i o n o f s i n t e r e d b a u x i t e were t h e o n l y r e l i a b l e h i g h - s t r e n g t h m a t e r i a l and i n 1976 - 1978 when t h e newly i n v e n t e d s i n t e r e d b a u x i t e was o n l y a v a i l a b l e i n l i m i t e d q u a n t i t i e s a t an e x c e s s i v e p r i c e which was up t o 15 t i m e s t h a t f o r n a t u r a l sand. F o l l o w i n g a g e n e r a l c h a r a c t e r i z a t i o n o f m i x i n g o f s i n t e r e d b a u x i t e o r g l a s s beads w i t h sand, some aspects o f c o n d u c t i v i t y d e t e r i o r a t i o n by p r o p p a n t mixing are outlined.
1.4.11.1.1.1. Mixing o f sintered bauxite or glass bead with sand Concerning m i x i n g o f p r o p p a n t t y p e s o f d i f f e r e n t provenance, McOANIEL & WILLINGHAM (1978) t e s t e d m i x t u r e s o f sand and g l a s s beads o r s i n t e r e d b a u x i t e and found o u t t h a t e x c e p t o f 50 : 50 (and sometimes a l s o 75 : 25) r a t i o s , a l l t h e o t h e r m i x t u r e s had n o t more c o n d u c t i v i t y t h a n sand a l o n e o r even l e s s i n c l o s u r e s t r e s s regimes up t o 8,000 p s i , w i t h t h e p e r m e a b i l i t y o f t h e p r o p p a n t mixt u r e a l s o r a p i d l y d e c l i n i n g w i t h r i s i n g c l o s u r e s t r e s s . I n t h e range o f 6,000 10,000 p s i , a m i x t u r e o f sand and s i n t e r e d b a u x i t e i n a 50 : 50 r a t i o r e v e a l s an average p e r m e a b i l i t y d r o p of a b t . 65 % w i t h r e s p e c t t o p u r e s i n t e r e d b a u x i t e p o p u l a t i o n s , and t h e p r o d u c t i v i t y o f o t h e r m i x t u r e r a t i o s i s even more approachi n g t h a t o f sand a l o n e o r below i n t h e most u n s u i t a b l e cases.
1.4.11.1.1.2. Conductivity deterioration by proppant mixing M i x i n g o f n a t u r a l q u a r t z sand w i t h s y n t h e t i c high-performance proppants i s t h u s n o t an e f f e c t i v e method f o r i n c r e a s i n g f r a c t u r e c o n d u c t i v i t y a t h i g h c l o sure s t r e s s e s , because t h e o r i g i n a l t a r g e t o f s a v i n g t r e a t m e n t c o s t by r e p l a c i n g p a r t s o f t h e expensive man-made p r o p p a n t l o t by cheap sand i s n o t reached, b u t i n c o n t r a s t t o the expectation, the p r o p e r t i e s o f the a r t i f i c i a l high-qualit y proppants a r e s i g n i f i c a n t l y d e t e r i o r a t e d by c o n t a m i n a t i o n w i t h much p o o r e r p e r f o r m i n g sand. A p a r t f r o m a b s o l u t e l y tremendous drops o f c o n d u c t i v i t y l e v e l
74 w i t h r e s p e c t t o p u r e p o p u l a t i o n s o f s y n t h e t i c p r o p p a n t s , o n l y 50 : 50 ( a n d i n some c a s e s a l s o 75 : 2 5 ) m i x t u r e s have r e l a t i v e l y s l i g h t l y h i g h e r p e r m e a b i l i t i e s t h a n sand a l o n e , and a l l t h e o t h e r m i x t u r e s p e r f o r m o n l y e q u a l o r e v e n worse t h a n homogeneous sand p o p u l a t i o n s (McDANIEL & WILLINGHAM 1978, CLARK 1983). I n mixtures w i t h i n f e r i o r material, the b e t t e r physical p r o p e r t i e s o f a s u p e r i o r p r o p p a n t t y p e a r e t h u s n o t o f f s e t , and p r o p p a n t s e l e c t i o n t h e r e f o r e has t o be based o n t h e i n d i v i d u a l c o n d u c t i v i t y and c l o s u r e s t r e s s r e s i s t i v i t y o f u n i m o d a l homogeneous p r o p p a n t p o p u l a t i o n s . I n c o n t r a s t t o t h e a f o r e m e n t i o n e d r e s u l t s , DAVIS ( 1 9 7 5 ) r e p o r t s t a i l - i n and m i x i n g o f g l a s s beads f o r a c h i e v e m e n t o f c o n d u c t i v i t y improvement, h i g h p r o p p a n t c o n c e n t r a t i o n , and p r o p p a n t c o s t r e d u c t i o n . F a v o u r a b l e m i x t u r e s r e s u l t i n g from f i e l d e x p e r i e n c e a r e 7 5 % 20/40 sand and 25 % 1 2 / 2 0 g l a s s beads. M i x i n g w i t h sand i s a c c o r d i n g t o DAVIS ( 1 9 7 5 ) s u i t a b l e f o r d i m i n i s h i n g g l a s s bead quant i t y and t h u s d e c r e a s i n g p r o p p a n t expenses, and has a c c e p t a b l y good p e r f o r m a n c e i n t h e f r a c t u r e i n r e s e r v o i r d e p t h ( t h i s c o n c l u s i o n , however, i s c o n t r a d i c t e d b y t h e r e s u l t s o f McDANIEL & WILLINGHAM 1978 and CLARK 1 9 8 3 ) .
1.4.11.1.2. Mixing of different sand types C o n d u c t i v i t y d e t e r i o r a t i o n s even o c c u r when m i x i n g o f t w o d i f f e r e n t sand t y p e s i s c a r r i e d o u t , because a l s o t h e o r i g i n a l sand p o p u l a t i o n s ( a s s c r e e n e d b y s i e v i n g a f t e r m i n i n g i n t h e n a t u r a l o c c u r r e n c e s ) have d i f f e r e n t p e r m e a b i l i t i e s . A c c o r d i n g t o NORMAN, CIPOLLA & WEBB ( 1 9 8 3 ) , H i c k o r y F o r m a t i o n sand f r o m C e n t r a l Texas i s more p e r m e a b l e w i t h i n f r a c t u r e packages t h a n S t . P e t e r Format i o n sand f r o m N o r t h e r n I l l i n o i s a t l o w e r c l o s u r e s t r e s s e s , whereas i n h i g h e r p r e s s u r e r e g i m e s above a b t . 3,500 p s i , S t . P e t e r sand p e r f o r m s b e t t e r t h a n H i c k o r y sand. The e x p l a n a t i o n o f t h i s phenomenon i s p r o b a b l y t h a t t h e S t . P e t e r sand has a h i g h e r c o n t e n t o f more s t a b l e m o n o c r y s t a l l i n e q u a r t z t h a n t h e H i c k o r y sand and t h u s l e s s g r a i n s c r u s h a t h i g h e r c l o s u r e s t r e s s e s ( c f . s e c t i o n 1 . 4 . 9 . 3 . ) , and t h e H i c k o r y sand c o n t a i n s more f r e q u e n t l y o l i g o - t o p o l y c r y s t a l l i n e q u a r t z i n t e r g r o w t h s and a l s o some f e l d s p a r s and r o c k f r a g m e n t s a f t e r p r o c e s s i n g w h i c h a r e p a r t i a l l y b e t t e r and worse r o u n d e d due t o i m p r o v e d w e a r i n g and i n h e r e n t c l e a v a g e , r e s p e c t i v e l y , t h a n t h e i r r e g u l a r l y b r e a k i n g q u a r t z g r a i n s and t h u s p r o v i d e b e t t e r p r o p e r t i e s a t l o w e r c l o s u r e s t r e s s e s u n t i l o l i go- t o p o l y c r y s t a l l i n e q u a r t z , f e l d s p a r s and r o c k f r a g m e n t s s t a r t t o c r u s h a t reasonable r a t e s .
1.4.11.2. Mixing of proppant grain sizes S i m i l a r l y as m i x i n g o f p r o p p a n t t y p e s , c o m b i n a t i o n o f s e v e r a l p r o p p a n t g r a i n s i z e s has d e t r i m e n t a l e f f e c t s o n f r a c t u r e c o n d u c t i v i t y u n l e s s t h e d i f f e r e n t g r a i n s i z e s a r e n o t i n s e r t e d as an i n t e r s p e r s e d homogeneous d i s t r i b u t i o n , but as t e m p o r a l l y d i s c r e t e l o t s t h a t w i l l r e c e i v e s p a t i a l l y s e p a r a t e p l a c e m e n t i n t h e c r a c k . Although i t i s t h e aim o f q u a l i t y c o n t r o l d u r i n g f r a c t u r i n g t r e a t ments t o a v o i d unwanted c o n t a m i n a t i o n , a c c i d e n t a l m i x i n g c a n o c c u r i f s o l i d f l u i d - l o s s a d d i t i v e s a r e used o r i f m u l t i - s t a g e proppant scheduling w i t h c o a r s e r o r f i n e r t a i l s o r heads as compared t o t h e m a i n body a r e a p p l i e d , r e s p e c t i v e l y . The m o s t p r o n o u n c e d example o f a c c i d e n t a l o r a t t e m p t e d m i x i n g o f g r a i n s i zes i s 100 mesh sand w h i c h i s added t o f r a c t u r i n g t r e a t m e n t s c a r r i e d o u t w i t h 20/40 a n d / o r 1 6 / 2 0 mesh p r o p p a n t s f o r v a r i o u s p u r p o s e s . The f o l l o w i n g a c c o u n t s k e t c h e s p o s s i b l e s o u r c e s o f c o n t a m i n a t i o n b y o u t l i n i n g t h e s i g n i f i c a n c e o f 100 mesh sand and c o a r s e r p r o p p a n t s and t h e drawbacks o f t h e m i x i n g o f t h e s e t w o d i f f e r e n t s i z e c l a s s e s i n h y d r a u l i c s t i m u l a t i o n j o b s . Consequences o f m i x i n g o f v a r i o u s coarse proppant g r a i n s i z e s a r e a l s o analyzed.
75
1.4.11.2.1.
Mixing o f coarse Proppants and f i n e 100 mesh sand
During a f r a c t u r i n g operation, i t i s desirable t o minimize f l u i d l e a k o f f f r o m t h e c r a c k i n t o t h e r e s e r v o i r by adding f i n e l y d i v i d e d s o l i d s t o t h e l i q u i d phase (COOKE 1975). The f u n c t i o n o f p a r t i c u l a t e f l u i d - l o s s a d d i t i v e s i s t o p l u g t h e l a r g e p o r e spaces and/or n a t u r a l c r a c k s and f i s s u r e s o f t h e pay zone r o c k s a t t h e f r a c t u r e w a l l s . The drawback of s o l i d f l u i d - l e a k o f f a d d i t i v e s o f i n s o l u b l e n a t u r e (such as 100 mesh sand i n c o n t r a s t t o c a l c i u m carbonate f l o u r and fine-grained s a l t ; c f . sections 1.4.11.2.1.2.2. and 5 . 1 1 . 4 . ) , however, a r e s i g n i f i c a n t d e t r i m e n t a l e f f e c t s on p r o p p a n t p e r m e a b i l i t y i n t h e crack, t h e p r o blem o f displacement o f t h e p a r t i c l e s f r o m t h e f r a c t u r e f a c e a f t e r d e p o s i t i o n , and t h e q u e s t i o n o f t h e i r m o b i l i t y and freedom t o move t h r o u g h t h e c r a c k when t h e w e l l i s p u t on p r o d u c t i o n . S o l i d f i n e - g r a i n e d f l u i d - l o s s a d d i t i v e s can i n deed cause v e r y s e r i o u s decreases i n proppant p e r m e a b i l i t y and t h u s f r a c t u r e c o n d u c t i v i t y (PYE & SMITH 1973) p a r t i c u l a r l y i n case o f m i x i n g w i t h t h e c o a r s e r p r o p p a n t s . F o l l o w i n g a s h o r t o u t l i n e o f t h e a p p l i c a t i o n spectrum o f 100 mesh sand, i t s p e r m e a b i l i t y d e t e r i o r a t i o n i n case o f i n t e r m i x i n g w i t h f r a c t u r i n g proppants i s sketched, w i t h d i s t i n c t i o n b e i n g made between p r e v e n t i n g and p r o v o k i n g p e r m e a b i l i t y d e s t r u c t i o n . Some aspects o f 100 mesh sand as e x c l u s i v e o r p a r t i c i p a n t proppant are also discussed.
1.4.11.2.1.1.
Application spectrum o f 100 mesh sand
Concerning t h e e f f e c t o f m i x i n g o f d i f f e r e n t g r a i n s i z e p o p u l a t i o n s , t h e app l i c a t i o n o f 100 mesh sand as an a d d i t i v e t o reduce f l u i d l e a k o f f t h r o u g h n a t u r a l c r a c k s i n h y d r a u l i c p r o p p a n t f r a c t u r i n g (THOMPSON 1977, McDANIEL & WILLINGHAM 1978; COOPER, NELSON & SCHOPPER 1983; SCHUMACHER & DICKERMAN 1986) and a c i d f r a c t u r i n g (MILLER & WAREMBOURG 1975, BAILEY & WICKHAM 1984, DOWELL SCHLUMBERGER 1988 d ) , t o scour away n e a r - w e l l b o r e damage by p e r f o r a t i o n e r o s i o n (CRAMER 1987), t o f a c i l i t a t e s l u r r y i n s e r t i o n by a c t i n g as a l e a d - i n proppant a t t h e head o f f r a c t u r i n g t r e a t m e n t s f o r p l u g g i n g o f n a t u r a l and/or i i y d r a u l i c m i c r o j o i n t s and - f i s s u r e s t h e r e b y i n c r e a s i n g w e l l p r o d u c t i v i t y (SINCLAIR 1980, G R I 1988), o r t o l i m i t v e r t i c a l f r a c t u r e h e i g h t p r o p a g a t i o n by p e r f o r m i n g as a b r i d g i n g agent t o c o n t r o l downward growth o f t h e crack (HODGES & PAOLI 1982) has t o be c a r r i e d o u t v e r y c a r e f u l l y , and a l s o good s e p a r a t i o n o f i n d i v i d u a l stages dur i n g t a i l - i n operations i s e s s e n t i a l . 100 mesh sand has been w i d e l y used p a r t i c u l a r l y i n t h e USA und i s u s u a l l y pumped ahead o f t h e sand o r p r o p p a n t l o t t h a t i s designed t o p l u g t h e h y d r a u l i c a l l y c r e a t e d f r a c t u r e f o r t h e purpose o f f r e q u e n t l y combined f l u i d l e a k o f f cont r o l and s u p p o r t i n g s o l i d s l e a d - i n as w e l l as m i c r o p r o p p a n t p l u g g i n g h a i r l i n e f i s s u r e s . H y d r a u l i c p r o p p a n t and a c i d f r a c t u r i n g t r e a t m e n t s u s i n g 100 mesh sand as an a d d i t i v e f o r f l u i d - l o s s c o n t r o l and/or h a i r l i n e crack p r o p p i n g b e n e f i t from achieving higher sustained production, reaching higher concentrations o f f r a c t u r e - s u p p o r t i n g proppants, fewer screenout f a i l u r e s happening, more t r e a t ments s t a y i n g i n zone, and c o n v e n t i o n a l f l u i d - l o s s a d d i t i v e s n o t b e i n g r e q u i r e d (MILLER & WAREMBOURG 1975). Examples o f a p p l i c a t i o n o f 100 mesh sand a r e r e p o r t e d by GWINN & McMANUS (1977); CAMPBELL, HANOLD, SINCLAIR & VETTER (1981); ROBERTS (1981), McMECHAN & CONWAY (1983), BEGNAUO & CLAIBORNE (1985), ZAHNER & CRAFTON (1985), NIEMEYER & REINART (1986) and NOLTE (1988 b; c f . a l s o s e c t i o n s 4 . 8 . 8 . 3 . 1 . 2 . and 4 . 8 . 8 . 3 . 2 . 3 . ) . The use o f 100 mesh, 40/60 mesh o r 70/140 mesh sand i n t h e f i r s t 25 % o f t h e f r a c t u r e t r e a t m e n t r e p r e s e n t i n g a l e a d - i n schedule h e l p s i n f l u i d l e a k o f f cont r o l t h r o u g h h a i r l i n e n a t u r a l f r a c t u r e s and h y d r a u l i c f r a c t u r e i n i t i a t i o n (PAI, G A R B I S & HALL 1983). The f i n e sand a c t s as a secondary proppant and i s a h i g h l y - c o n d u c t i v e means o f t i p p a c k i n g i n secondary h a i r l i n e f i s s u r e s and j o i n t l e t s which do n o t accept l a r g e r p r o p p a n t g r a i n s , t h e r e b y i n h i b i t i n g e x c e s s i v e loss o f h i g h - v i s c o s i t y f l u i d s (LAMBERT, DOLAN & GALLUS 1983). Some aspects o f prevent i n g p e r m e a b i l i t y d e t e r i o r a t i o n i n f r a c t u r e p r o p p i n g , a p p l i c a t i o n o f 100 mesh sand as e x c l u s i v e o r p a r t i c i p a n t proppant, and p r o v o k i n g p e r m e a b i l i t y d e s t r u c -
76 t i o n i n p l u g s e t t i n g f o r h e i g h t g r o w t h l i m i t a t i o n a r e o u t l i n e d as f o l l o w s .
1.4.11.2.1.2.Preventing permeability deterioration I n t e r m i x i n g o f 100 mesh sand w i t h c o a r s e f r a c t u r i n g p r o p p a n t s o f m o s t commonl y 2 0 / 4 0 a n d / o r 1 6 / 2 0 mesh g r a i n s i z e r e s u l t s i n d r a s t i c a l p r o p p a n t package c o n d u c t i v i t y d e t e r i o r a t i o n . Measures f o r p r e v e n t i o n o f d e l e t e r i o u s e f f e c t s a r e add i t i o n o f o n l y l i m i t e d amounts o f 100 mesh sand, p r o p e r s e p a r a t i o n o f 100 mesh sand b y s p a c e r f l u i d c u s h i o n s f r o m t h e m a i n p r o p p a n t l o t , r e p l a c e m e n t o f 100 mesh sand b y f i n e s a l t w h i c h i s s o l u b l e , and u t i l i z a t i o n o f r e s i n - c o a t e d 100 mesh sand o r p r o p p a n t s w h i c h do n o t f l o w o u t o f t h e h a i r l i n e f i s s u r e s i n t o t h e m a i n c r a c k once p l a c e d w i t h i n t h e j o i n t l e t s . The d i s c u s s i o n as f o l l o w s f o c u s s e s on p r o p p a n t p e r m e a b i l i t y d e s t r u c t i o n b y f i n e s , 100 mesh sand v s . f i n e s a l t , and 100 mesh m i c r o p r o p p a n t s f o r h a i r l i n e f i s s u r e s .
1.4.11.2.1.2.1. Proppant permeability destruction by fines I f t h e f i n e f l u i d - l o s s c o n t r o l sand happens t o become i n t e r m i x e d w i t h t h e l a r g e r p r o p p a n t g r a i n s , a l r e a d y a 5 % c o n t e n t o f 100 mesh sand w i t h i n t h e p r o p p a n t package c a n r e d u c e f r a c t u r e p e r m e a b i l i t y u p t o 40 %, and a t c l o s u r e s t r e s ses above 4,000 p s i , c o n t a m i n a t i o n o f t h e p r o p p a n t p o p u l a t i o n b y 20 % f i n e sand c a n a l r e a d y have t h e r e s u l t t h a t p e r m e a b i l i t y o f t h i s m i x t u r e a p p r o a c h e s t h e val u e o f 100 mesh sand a l o n e w h i c h i s c o n s i d e r a b l y l e s s t h a n t h a t o f t h e p u r e proppant population, w i t h thus t h e e f f e c t o f the f r a c t u r i n g treatment almost h a v i n g gone away. A s l i g h t l y b e t t e r s o l u t i o n t o r e d u c e f l u i d l e a k o f f m i g h t be a d d i t i o n o f s i l i c a f l o u r which i s s u f f i c i e n t l y fine-grained n o t t o i n t e r f e r e any more w i t h t h e c o a r s e - g r a i n e d p r o p p a n t s i n v e r y l o w c o n c e n t r a t i o n s , b u t a l s o h e r e c a u t i o n has t o b e e x e r c i s e d , because 3 - 4 X o f f i n e s o f any t y p e c a n i n t h e w o r s t c a s e a l r e a d y d e s t r o y 90 % o f p r o p p a n t c o n d u c t i v i t y (McDANIEL & WILLINGHAM 1978; SMITH 1 9 8 7 ) .
A s a f e t y p r o c e d u r e f o r e f f e c t i v e s e p a r a t i o n i s t h e pumping o f a l a r g e s p a c e r pad o r c u s h i o n between t h e 100 mesh f l u i d - l o s s c o n t r o l sand and t h e f r a c t u r e p r o p p a n t s (BLASIO, BAILEY & BERGTHOLD 1985; BARBY & BARBEE 1 9 8 7 ) . 100 mesh sand has t u r n e d o u t t o be a p a r t i c u l a r l y e f f e c t i v e f l u i d - l e a k o f f s u p e r v i s i o n a d d i t i v e i n c a s e o f v e r y s h o r t c r a c k c l o s u r e t i m e s . 70/140 mesh sand has r e p e a t e d l y p r o v e n t o be l e s s damaging t o f r a c t u r e c o n d u c t i v i t y t h a n s i l i c a f l o u r (LAMBERT, DOLAN & GALLUS 1 9 8 3 ) . I n c o n t r a s t t o f i n e p a r t i c l e s w h i c h p l u g t h e p r o p p a n t package i n t e r s t i c e s , l a r g e r p i e c e s o f d e b r i s c a n p o s s i b l y o b s t r u c t t h e p e r f o r a t i o n h o l e s (OSBORNE, McLEOO & SCHROEDER 1 9 8 1 ) . Examples o f s t r o n g p e r m e a b i l i t y r e d u c t i o n b y e x c e s s i v e amounts o f f i n e p a r t i c l e s i n v a d i n g t h e p r o p p a n t pack a r e a l s o r e p o r t e d b y ROLL, HIMES, EWERT & DOERKSEN ( 1 9 8 6 ) who a l s o c o n c l u d e t h a t i n s p i t e o f t h e b e s t e f f o r t s t o d e s i g n and p e r f o r m h y d r a u l i c f r a c t u r i n g o r g r a v e l p a c k i n g t r e a t m e n t s , e x c e s s i v e f i n e s g e n e r a t i o n and/or i m m i g r a t i o n can d e f e a t t h e purpose o f t h e o p e r a t i o n ( t h e det r i m e n t a l e f f e c t o f e x c e s s i v e f i n e s i s a l s o emphasized b y PENBERTHY & COPE 1979; SHRYOCK, DUNLOP & MILLHONE 1979 and KRUEGER 1 9 8 6 ) .
1.4.11.2.1.2.2. 100 mesh sand vs. fine salt An a l t e r n a t i v e t o 100 mesh sand as f l u i d - l o s s a d d i t i v e f o r b r i d g i n g o f n a t u r a l h a i r l i n e c r a c k s i s f i n e s a l t o f 30/60 a n d / o r 8 0 / 1 2 0 mesh g r a i n s i z e w h i c h p a c k s t o l o w e r p e r m e a b i l i t y i n t h e f r a c t u r e s u n d e r p r e s s u r e t h a n s i l i c a (SCHRIETER & SHAW 1 9 7 8 ) , and t o some e x t e n t a l s o c a l c i u m c a r b o n a t e f l o u r . S a l t has a l s o t h e a d v a n t a g e w i t h r e s p e c t t o sand t h a t i t i s s u b s e q u e n t l y d i s s o l v e d a g a i n t o p e r m i t u n r e s t r i c t e d f l o w from t h e h a i r l i n e cracks which t h u s can c o n t r i b u t e t o h y d r o c a r b o n p r o d u c t i o n , and t h a t i t e l i m i n a t e s pump and e q u i p m e n t damage p r o blems p r o v o k e d b y a b r a s i v e sand. C a l c i u m c a r b o n a t e f l o u r and f i n e - g r a i n e d s a l t
77 have t h e advantage t o be a b l e t o be l a t e r d i s s o l v e d by h y d r o c h l o r i c a c i d and wat e r , r e s p e c t i v e l y , i n c o n t r a s t t o s i l i c a f l o u r and 100 mesh sand which c o u l d o n l y be a t t a c k e d by h y d r o f l u o r i c a c i d t h a t a l s o can have d e t r i m e n t a l e f f e c t s on t h e proppant package ( c a l c i u m carbonate f l o u r and f i n e - g r a i n e d s a l t a r e f r e q u e n t l y u t i l i z e d i n g r a v e l p a c k i n g o p e r a t i o n s because o f t h e advantages m e n t i o ned; c f . s e c t i o n 5 . 1 1 . 4 . ) .
1.4.11.2.1.2.3.100 mesh microproppants f o r hairline fissures On t h e o t h e r hand, s u p p o r t o f h a i r l i n e j o i n t l e t s by 100 mesh m i c r o p r o p p a n t s p r o v i d e s a d d i t i o n a l c o n d u c t i v i t y t o t h e whole f r a c t u r e system c o n t r i b u t i n g t o p r o d u c t i o n , w i t h open m i c r o f i s s u r e s a f t e r p r e s s u r e r e t u r n a t l e a s t p a r t i a l l y bei n g a b l e t o c l o s e again and t o d e t e r i o r a t e t h e e f f e c t o f t h e i n t e g r a t e d network (WARPINSKI 1988 a ) . T h e r e f o r e i n many case5 i t may be d e s i r a b l e i n h y d r a u l i c f r a c t u r i n g t o u t i l i z e 100 mesh sand as a c o m b i n a t i o n o f f l u i d - l o s s agent and m i croproppant f o r e f f e c t i v e l y l i m i t i n g accelerated f l u i d l e a k o f f i n t o h a i r l i n e j o i n t l e t s and t o adequately s u p p o r t t h e m i c r o c r a c k s t o keep them open f o r l a t e r r e s e r v o i r e x p l o i t a t i o n ( c f . s e c t i o n s 4 . 8 . 8 . 3 . 1 . 2 . and 4 . 8 . 8 . 3 . 2 . 3 . ) . As c o n t i nuous a d d i t i o n of 100 mesh sand guarantees p o s i t i v e f l u i d - l o s s c o n t r o l impact t h r o u g h o u t t h e j o b , i n some l o w - p e r m e a b i l i t y t i g h t gas sandstones i t can be f e a s i b l e t o accept m i n o r c o n d u c t i v i t y r e d u c t i o n o f h i g h - q u a l i t y proppants and t o secure f l u i d l e a k o f f l i m i t a t i o n by permanent 100 mesh sand a d d i t i o n u n t i l t h e end o f t h e o p e r a t i o n (WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE 1988). Such small amounts o f 100 mesh sand which do n o t s e r i o u s l y d e s t r o y p r o p p a n t c o n d u c t i v i t y , however, have o n l y l i m i t e d e f f e c t i v i t y i n f l u i d - l o s s r e s t r i c t i o n and can r e a s o n a b l y slow down a c c e l e r a t e d l e a k o f f , b u t cannot s t o p i t e n t i r e l y and t h u s r e m a i n i n g f l u i d - l o s s r a t e s a r e s t i l l v e r y h i g h (WARPINSKI 1988 a ) . I n deeper r e s e r v o i r s w i t h h i g h e r c l o s u r e s t r e s s , c o n s i d e r a b l e improvement o f dual f l u i d l e a k o f f c o n t r o l c o u l d be achieved by 100 mesh i n t e r m e d i a t e - t o h i g h s t r e n g t h s y n t h e t i c proppants as an e f f e c t i v e m i c r o p r o p p i n g agent i n h a i r l i n e c r a c k s , b u t t h i s g r a i n s i z e o f a r t i f i c i a l h i g h - q u a l i t y proppants i s n o t y e t comm e r c i a l l y a v a i l a b l e (NORTHCUTT, ROBERTSON & HANNAH 1988). R e s i n - c o a t e d 100 mesh sand o r proppants have t h e advantage t h a t once p l a c e d w i t h i n t h e m i c r o f i s s u r e s , a g g l o m e r a t i o n o f t h e r e s i n p e l l i c l e s p r e v e n t s flowback o f t h e m i c r o p r o p p a n t s t h u s s t a b i l i z i n g them as s u p p o r t i n g wedges i n t h e h a i r l i n e c r a c k s and i n h i b i t i n g m i x i n g o f 100 mesh p a r t i c l e s w i t h coarse proppants i n t h e main f r a c t u r e branch ( c f . s e c t i o n s 1 . 2 . 6 . and 4 . 8 . 8 . 3 . 2 . 3 . ) .
1.4.11.2.1.3. 100 mesh sand as exclusive or participant proppant F l u i d l e a k o f f m i n i m i z a t i o n i s e s s e n t i a l f o r success o f t h e f r a c t u r i n g t r e a t ment, because e x c e s s i v e f l u i d l o s s a u t o m a t i c a l l y l e a d s t o s c r e e n o u t f a i l u r e ( c f . s e c t i o n s 4 . 2 . 2 . 2 . 3 . and 6 . 2 . 4 . 2 . 1 . ) , b u t a l s o proppant performance must n o t be s e r i o u s l y a f f e c t e d , as d e t e r i o r a t i n g proppant c o n d u c t i v i t y a l s o r e s u l t s i n t e c h n i c a l and economical f a i l u r e o f t h e t r e a t m e n t . EASON (1985) and WARPINS K I (1988 a ) d e s c r i b e combined a c t i o n o f 100 mesh sand as f l u i d - l o s s a d d i t i v e and as m i c r o p r o p p a n t i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s where i t f i r s t c o n t r o l s f l u i d l e a k o f f and l a t e r p r o v i d e s adequate p e r m e a b i l i t y and a c t s as a p r o p p a n t i n t h e n a t u r a l and/or h y d r a u l i c m i c r o c r a c k s once f r a c t u r e b a l l o o n i n g s u b s i d e s . When n a t u r a l l y j o i n t e d r e s e r v o i r s a r e h y d r a u l i c a l l y t r e a t e d , a l a r g e f r a c t u r e i s c r e a t e d b u t many small s i d e f i s s u r e s can open d u r i n g h i g h - r a t e i n j e c t i o n . These m i c r o c r a c k s a r e p r e v e n t e d f r o m c l o s i n g by p l u g g i n g w i t h 100 mesh sand which a l l o w s t h e propped h a i r l i n e j o i n t s t o c o n t r i b u t e t o h i g h e r w e l l p r o d u c t i v i t y due t o i n c r e a s e d s u r f a c e area and f r a c t u r e volume. Some comments on a p p l i c a t i o n o f 100 mesh sand f o r i n f i l l i n g o f h a i r l i n e f i s s u r e s i n a c i d and p r o p p a n t f r a c t u r i n g as w e l l as an o u t l i n e o f t h e u t i l i z a t i o n o f 100 mesh sand as e x c l u s i v e proppant a r e g i v e n as f o l l o w s .
78
1.4.11.2.1.3.1.Application in acid and proppant fracturing 100 mesh sand a s a m i c r o p r o p p a n t i s t h e r e f o r e v e r y b e n e f i c i a l i n h y d r a u l i c p r o p p a n t s t i m u l a t i o n o f n a t u r a l l y c r a c k e d f o r m a t i o n s as w e l l as i n a c i d f r a c t u r i n g (FRACFAX 1988 f ) . 100 mesh b o t h r e g u l a r and r e s i n - c o a t e d sand has a b t . 5 7 d a r c y p e r m e a b i l i t y a t a l m o s t any c l o s u r e s t r e s s w h i c h p r o v i d e s s t i l l a r e a s o nable c o n t r a s t t o the m i l l i - t o microdarcy t i g h t rock m a t r i x . I f microproppant f l o w b a c k has t o be i n h i b i t e d , c u r a b l e r e s i n - c o a t e d 100 mesh sand i s k e p t i n p l a c e b y r e s i n b o n d i n g . 100 mesh m i c r o p r o p p a n t s h e l p t o i n c r e a s e p r o d u c t i o n b e c a u s e t h e y s u p p o r t v e r y f i n e n a t u r a l h a i r l i n e c r a c k s as w e l l as n a r r o w segments o r s i d e b r a n c h e s o f t h e a c i d - o r g e l - i n d u c e d f r a c t u r e . CRENSHAW & JOINES ( 1 9 7 7 ) a l s o emphasize t h e d u a l r o l e o f 100 mesh sand as f l u i d - l o s s a d d i t i v e and m i c r o proppant a l l o w i n g drainage from secondary j o i n t s .
In a c i d f r a c t u r i n g , m i c r o p r o p p a n t s a r e a l s o a b l e t o e x t e n d s t i m u l a t e d p r o d u c t i o n and i n c r e a s e u l t i m a t e r e c o v e r y by p r e v e n t i n g o f p r e m a t u r e c l o s u r e o r p l u g g i n g o f t h e e t c h e d f r a c t u r e morphology, vug spaces, h a i r l i n e s i d e b r a n c h e s and wormhole f e a t u r e s . I n o r d e r t o a c c e p t p r o p p i n g a g e n t , f r a c t u r e w i d t h m u s t be i n the range o f 2 . 5 times t h e proppant diameter, w i t h t h e standard proppant g r a i n s i z e s ( 1 6 / 2 0 and 20/40) f u l f i l l i n g t h i s r e q u i r e m e n t f o r m o s t o f t h e h y d r a u l i c f r a c t u r e s , w h i l e 100 mesh sand f i t s i n t o t h e h a i r l i n e m i c r o c r a c k s (THOMPSON 1 9 7 7 ) . 100 mesh m i c r o p r o p p a n t c a n be r e g a r d e d as p r e p a c k s t a g e ahead o f t h e m a i n s t a g e s o f v a r i o u s t r e a t m e n t s c o m p r i s i n g b o t h p r o p p a n t and a c i d f r a c t u r i n g (MILLER & WAREMBOURG 1 9 7 5 ) .
1.4.11.2.1.3.2.100 mesh sand as exclusive proppant A p a r t f r o m i t s a p p l i c a t i o n as f l u i d - l o s s a d d i t i v e a n d / o r p a r t i a l p r o p p a n t , 100 mesh sand c a n a l s o be e x c l u s i v e l y u s e d as p r o p p a n t f o r r e s e r v o i r s t i m u l a t i o n (THOMPSON 1977; HARPER, HAGAN & MARTINS 1985) i f t h e p l a c e m e n t o f c o a r s e r g r a i n s would b e a r h i g h screenout r i s k , i f o n l y moderate c r a c k c o n d u c t i v i t y i s s u f f i c i e n t such as i n m i n i f r a c t u r i n g b r e a k i n g t h r o u g h n e a r - w e l l b o r e damage ( c f . s e c t i o n 4 . 8 . 3 . ) , o r i f s t i m u l a t i o n c o s t have t o be s e r i o u s l y c o n t a i n e d f o r v a r i o u s r e a s o n s . 100 mesh as w e l l as 40/60 mesh o r 70/140 mesh sand c a n a l s o be u t i l i z e d as f i n e - g r a i n e d l e a d - i n s t a g e o f t h e p r o p p a n t assemblage i n o r d e r t o not only plug h a i r l i n e cracks i n the reservoir matrix a t the fracture t i p , but a l s o t o f a c i l i t a t e proppant placement a t t h e e x t r e m i t i e s o f e s p e c i a l l y v e r y l o n g c r a c k s , w i t h t h e aims b e i n g a c h i e v i n g smooth p r o p p a n t s l u r r y f l o w - i n , good p r o p p a n t c o n c e n t r a t i o n and a v o i d i n g s c r e e n o u t f a i l u r e . M i g r a t i o n o f f o r m a t i o n f i n e s and 100 mesh sand p a r t i c l e s f r o m m i c r o f i s s u r e s i n t o t h e c o a r s e p r o p p a n t package i n t h e m a i n f r a c t u r e c a n be p r e v e n t e d b y a p p l i c a t i o n o f r e s i n - c o a t e d 100 mesh sand w h i c h l o c k s t h e m i c r o p r o p p a n t s t o g e t h e r t o a s t a b l e package t h a t maintains high fracture conductivity. G e n e r a l l y t h e amount o f 100 mesh sand and o t h e r f i n e s o l i d f l u i d - l o s s a d d i t i ves s h o u l d n o t e x c e e d a b t . 1 0 %, because i n case o f h i g h e r s h a r e s o f f i n e s , f r a c t u r e c o n d u c t i v i t y i s s i g n i f i c a n t l y d e c r e a s i n g ( L I & ZHU 1 9 8 6 ) . F r a c t u r e c o n d u c t i v i t y damage c r e a t e d b y h i g h - s t r e n g t h b a u x i t e f i n e s i s l e s s t h a n t h a t gener a t e d b y l o w - s t r e n g t h sand m a t e r i a l s , w i t h a f r a c t u r e c o n d u c t i v i t y o f a b t . 70 % and 30 % o f t h e v a l u e s c a l c u l a t e d f r o m f i e l d t r a n s i e n t p r e s s u r e t e s t s b e i n g r e a ched when u s i n g f i n e b a u x i t e and f i n e sand, r e s p e c t i v e l y , as f l u i d - l o s s a d d i t i ves.
1.4.11.2.1.4.Provoking permeability destruction I n c o n t r a s t t o t h e general aim o f p r e v e n t i n g p e r m e a b i l i t y d e t e r i o r a t i o n o f p r o p p a n t assemblages pumped downhole and i n s e r t e d i n t o h y d r a u l i c a l l y g e n e r a t e d f r a c t u r e s , some s p e c i a l a p p l i c a t i o n s w h i c h p a r t i a l l y a r e d e s i g n a t e d t o i m p r o v e p r o p p a n t p l a c e m e n t and t h u s f r a c t u r e e f f i c i e n c y l o o k on p r o v o k i n g p e r m e a b i l i t y
79 d e s t r u c t i o n o f some l o t s o f t h e s o l i d p a r t i c u l a t e s which a r e d r i v e n i n t o t h e c r a c k by t h e c a r r i e r f l u i d s . Such a s p e c i a l case o f d e s i r e d p e r m e a b i l i t y d e t e r i o r a t i o n i s f o r example t h e a p p l i c a t i o n o f m i x t u r e s o f v a r i o u s p r o p p a n t g r a i n s i z e s as buoyant o r s e t t l i n g d i v e r t e r s ( c f . s e c t i o n 4 . 2 . 2 . 4 . ) f o r sealing the upper o r l o w e r t i p o f a f r a c t u r e e x t e n d i n g f r o m t h e r e s e r v o i r sandstone i n t o t h e o v e r l y i n g mudstone b a r r i e r and t h e r e b y r e s t r i c t i n g v e r t i c a l upwards o r downwards h e i g h t growth o f t h e crack, r e s p e c t i v e l y . NOLTE (1988 b ) r e p o r t s an example where s u c c e s s f u l b l o c k i n g o f t h e upper t i p o f t h e f r a c t u r e has been achieved by a t i g h t f l o w b l o c k c o n s i s t i n g o f a m i x t u r e o f s i l i c a f l o u r and 100 mesh, 20/40 mesh and 10/20 mesh sand which, i f p r o p e r l y s e t i n p l a c e , has v i r t u a l l y no p e r m e a b i l i t y a t a l l . C a r e f u l s e l e c t i v e i n s e r t i o n o f buoyant d i v e r t e r f o r f l o w b l o c k purposes and proppant f o r f r a c t u r e s u p p o r t w i t h d i f f e r e n t f l u i d stages o f d i f f e r e n t v i s c o s i t y and w i t h a d j u s t e d pumping r a t e s i s necessary t o a v o i d i n t e r f e r e n c e o f b o t h s o l i d l o a d s and t o keep t h e p r o p p a n t s o f t h e l a t e r l o t which a r e d e s i g n a t e d f o r c o n d u c t i v i t y c r e a t i o n c l e a n and s e p a r a t e d f r o m t h e g r a i n s i z e m i x t u r e w h i c h i s planned f o r f r a c t u r e s e a l i n g by permeability d e t e r i o r a t i o n ( c f . section 4.2.2.4.1.). Proper d e s i g n o f t h e f l u i d spacer s e p a r a t i n g t h e two stages o f s o l i d m a t e r i a l i n t r o d u c t i o n t h e r e f o r e a c h i e ves key s i g n i f i c a n c e f o r s u c c e s s f u l i n s t a l l a t i o n o f t h e d e s i r e d p r o p p a n t d i s t r i b u t i o n and r e a l i z a t i o n o f t h e attempted f r a c t u r e h e i g h t growth c o n t r o l . Another t y p e o f p e r m e a b i l i t y d e s t r u c t i o n i s p l u g g i n g o f f o r m a t i o n p e r m e a b i l i t y i n t h i e f zones by a c r y l i c epoxy r e s i n systems r e p r e s e n t i n g a s p e c i a l a p p l i c a t i o n o f i n - s i t u chemical r e s e r v o i r c o n s o l i d a t i o n (KNAPP & WELBOURN 1978; Z A I TOUN, KOHLER & GUERRINI 1989; c f . s e c t i o n 5 . 9 . 1 . 7 . ) .
1.4.11.2.2. Mixing o f various coarse Proppant grain sizes A c c i d e n t a l m i x i n g o f 20/40 and 16/20 o r 12/20 proppant g r a i n s a t t h e t r a n s i t i o n s between d i f f e r e n t stages o f t a i l - i n ( c f . s e c t i o n 2 . 4 . 1 . 2 . ) o r l e a d - i n mult i - p r o p p a n t h y d r a u l i c s t i m u l a t i o n o p e r a t i o n s has t h e d e t r i m e n t a l e f f e c t o f p l u g g i n g o f t h e i n t e r s t i t i a l s between t h e c o a r s e r g r a i n s by t h e f i n e r p a r t i c l e s o f t h e bimodal d i s t r i b u t i o n . I n a d d i t i o n , t h e f o u r t i m e s f a s t e r s e t t l i n g v e l o c i t y o f a b t . t w i c e l a r g e r g r a i n s ( a c c o r d i n g t o Stokes' l a w ) may l e a d t o v e r t i c a l sep a r a t i o n o f g r a i n s i z e s b y f a l l i n g o f t h e c o a r s e r g r a i n s f a s t e r down t o t h e b o t tom o f t h e f r a c t u r e t h a n t h e f i n e r g r a i n s (LOCKETT & AL-HABBOOBY 1973, M I R Z A & RICHARDSON 1979), r e s u l t i n g i n o r i g i n o f a v e r t i c a l p r o p p a n t g r a i n s i z e zonat i o n due t o e x i s t e n c e o f d i f f e r e n t l a y e r s composing t h e p r o p p a n t bank, which i s i n c o n t r a s t t o t h e d e s i r e d e f f e c t o f l a t e r a l succession o f b e l t s w i t h proppants o f d i f f e r e n t g r a i n s i z e a l o n g t h e c r a c k . Moreover, g r a v i t a t i o n a l s p l i t t i n g o f t h e p o p u l a t i o n s may r e s u l t i n o r i g i n o f asymmetrical wedges o f c o a r s e r m a t e r i a l t h a t i s n o t i n continuous contact w i t h the r e s e r v o i r along the f u l l height o f t h e f r a c t u r e d i n t e r v a l and t h u s t h e a n t i c i p a t e d homogeneous m a n t l e - l i k e d i s t r i b u t i o n o f t h e t a i l around t h e w e l l b o r e i s n o t accomplished. Aspects o f s e t t l i n g v e l o c i t i e s o f proppants a r e d i s c u s s e d by DUNANO & SOUCEMARIANADIN (1985), K I R K BY & ROCKEFELLER (1985) and ROODHART (1985), and ACHARYA (1986) e v a l u a t e s p a r t i c l e t r a n s p o r t i n v i s c o u s and v i s c o e l a s t i c f r a c t u r i n g f l u i d s . CLARK (1983) summarizes t h a t an i n t e r m i x i n g o f d i f f e r e n t p r o p p a n t q u a l i t y t y pes r e s u l t s i n much l e s s r e s i s t a n c e t o c r u s h i n g o f t h e f r a c t u r e - i n f i l l i n g package w i t h r e s p e c t t o t h a t o f homogeneous m o n o q u a l i t y proppant s u i t e s , whereas an i n t e r m i x i n g o f v a r i o u s proppant s i z e s o r sometimes a l s o shapes causes l e s s f r a c t u r e c o n d u c t i v i t y due t o i n c r e a s e d p a c k i n g e f f i c i e n c y o f t h e p a r t i c l e s .
1.4.11.3. Mixing o f proppant grain shapes While a c c i d e n t i a l o r planned m i x i n g o f proppant t y p e s and g r a i n s i z e s i s u s u a l l y d e t r i m e n t a l t o performance and g i v e s f r e q u e n t l y r i s e t o even worse p r o p e r t i e s o f t h e m i x t u r e t h a n those o f some o f t h e i n g r e d i e n t s alone, designed m i -
80 Xing o f p r o p p a n t g r a i n shapes can e f f e c t i v e l y improve s t i m u l a t i o n t r e a t m e n t p e r formance by p r o v i d i n g h i g h e r c o n d u c t i v i t y and c l o s u r e s t r e s s r e s i s t i v i t y t o t h e c o m b i n a t i o n w i t h r e s p e c t t o t h e c h a r a c t e r i s t i c s o f t h e i n d i v i d u a l components alone. An o u t l i n e o f m i x i n g o f round and a n g u l a r sand g r a i n s and i t s s i g n i f i cance f o r c o n d u c t i v i t y m a x i m i z a t i o n as w e l l as t h e r e s u l t i n g b e n e f i t f o r t h e hydrocarbon s t i m u l a t i o n i n d u s t r y i s g i v e n as f o l l o w s .
1.4.11.3.1. Enhanced conductivity o f round-angular sand blends LARSEN & SMITH (1985) s t u d y t h e i n f l u e n c e o f a n g u l a r vs. round g r a i n s i n 20/40 n a t u r a l sand. O i l and gas i n d u s t r y g e n e r a l l y demands f r a c t u r i n g sand and proppants t o meet h i g h roundness and s p h e r i c i t y s p e c i f i c a t i o n s i n o r d e r t o p r o v i d e t h e optimum g e o m e t r i c a l pack i n t h e f r a c t u r e . Some comments on c o n d u c t i v i t y improvement and package s t a b i l i t y as w e l l as c r u s h i n g b e h a v i o u r a r e o f f e r e d as f o l l o w s .
1.4.11.3.1.1. Conductivity improvement and package stability P e r f e c t roundness and s p h e r i c i t y o f t h e g r a i n s indeed decrease p o i n t p r e s s u r e and reduce c r u s h i n g and f i n e s generated i n t h e proppant assemblage, but t h e optimum g e o m e t r i c a l pack f o r c r u s h r e s i s t a n c e does n o t always o f f e r maximum c o n d u c t i v i t y (LARSEN & SMITH 1985). While a h i g h g r a i n s p h e r i c i t y has t o be m a i n t a i n e d because t h i s e l i m i n a t e s p r o p p a n t p a r t i c l e s w i t h e l o n g a t e d shapes and p r o t r u s i o n s and p r e v e n t s t h e i r f i t t i n g i n t o n a t u r a l v o i d spaces t h e r e b y r e s t r i c t i n g c o n d u c t i v i t y , i t i s n o t always advantageous f o r a l l t h e sand g r a i n s t o be e i t h e r p e r f e c t l y round and smooth o r c o m p l e t e l y a n g u l a r . A l t h o u g h t o t a l l y a n g u l a r g r a i n p o p u l a t i o n s have t h e h i g h e s t p o r o s i t y and p e r m e a b i l i t y i n a l o o s e pack and a t v e r y low t o low c l o s u r e s t r e s s , t h e y possess s u r f a c e s t h a t w i l l break o f f OF c r u s h e a s i l y w i t h any s i g n i f i c a n t c l o s u r e s t r e s s because of uneven p r e s s u r e d i s t r i b u t i o n on t h e s u r f a c e p o i n t s t h a t t o u c h each o t h e r and t h u s a t moderate and h i g h c l o s u r e s t r e s s e s , e n t i r e l y round p r o p p a n t g r a i n p o p u l a t i o n s a r e s u p e r i o r t o w h o l l y a n g u l a r assemblages. I f a c e r t a i n percentage o f a n g u l a r g r a i n s i s i n t e r s p e r s e d i n a v e r y round and s p h e r i c a l sand, however, t h e b e n e f i t o f a h i g h l y porous ;.:gular sand can c r e a t e a h i g h e r c o n d u c t i v i t y o f t h e m i x t u r e t h a n e i t h e r pure round o r homogeneous a n g u l a r g r a i n p o p u l a t i o n s a l o n e possess. The i n c r e a s e d v o i d space o f a comb i n a t i o n o f round and a n g u l a r sands overcomes c r u s h i n g s u s c e p t i b i l i t y and enhances o v e r a l l proppant performance.
1.4.11.3.1.2. Crushing behaviour L a b o r a t o r y experiments have r e v e a l e d t h a t o f a l l m i x t u r e s t e s t e d , a 50 : 50 r o u n d - a n g u l a r b l e n d o f 20/40 mesh sand has on t h e average t h e h i g h e s t r e l a t i v e f r a c t u r e c o n d u c t i v i t y t h r o u g h a l l t h e c l o s u r e s t r e s s l e v e l s up t o 6,000 p s i (LARSEN & S M I T H 1985). Round-angular blends o f 20/40 sand have even more conduct i v i t y a t c e r t a i n c l o s u r e s t r e s s l e v e l s than t h e e x c l u s i v e l y round p r o d u c t which t h e o i l and gas i n d u s t r y has c o n v e n t i o n a l l y r e q u i r e d . The b e n e f i t o f t h e angular grains f o r improving c o n d u c t i v i t y o f the mixtures r e s u l t s from the i n t e r n a l s t r u c t u r e o f round and a n g u l a r q u a r t z g r a i n s . Many a n g u l a r g r a i n s have a u t h i g e n i c i d i o m o r p h i c q u a r t z overgrowths which break o f f f r o m t h e p a r e n t c r y s t a l a t low p r e s s u r e s and c o n s e q u e n t l y t h e generated f i n e s may b l o c k e f f e c t i v e pore space. While t h i s may be d e t r i m e n t a l a t l o w e r c l o s u r e s t r e s s e s , t h e f i n e s may act u a l l y surround and s u p p o r t t h e d e t r i t a l c o r e a t h i g h e r c l o s u r e s t r e s s e s , and by t h i s p r o t e c t i o n a l l o w t h e a n g u l a r g r a i n s t o p e r f o r m b e t t e r than t h e more round g r a i n s as t h e y pass t h r o u g h t h e i r r e g u l a r c r u s h i n g sequence. Round g r a i n s a r e f r e q u e n t l y m o n o c r y s t a l l i n e and r e s i s t t o h i g h c r u s h i n g p r e s s u r e s . When
81 t h e i r f a i l u r e p o i n t i s reached, however, c r u s h i n g o f m o n o c r y s t a l l i n e round g r a i n s i s more severe due t o t h e i r h i g h e r b r i t t l e n e s s t h a n t h a t o f o l i g o c r y s t a l l i n e g r a i n s where some d e f o r m a t i o n can o c c u r by s l i d i n g a l o n g c r y s t a l boundar i e s . I n case o f a n g u l a r g r a i n s , a l a r g e amount o f f r a c t u r e s t r e s s may be r e l i e ved when t h e a u t h i g e n i c euhedral q u a r t z overgrowths break o f f , a l l o w i n g t h e par e n t g r a i n t o r e t a i n i t s i n t e r n a l strength longer. I n a d d i t i o n , a n g u l a r sands t e n d t o pack t i g h t e r t h a n round ones because o f t h e i n t e r l o c k i n g e f f e c t which t h e rough s u r f a c e s have on each o t h e r . I f t h e p r o p e r percentage o f a n g u l a r g r a i n s were i n t e r s p e r s e d i n a p o p u l a t i o n o f round g r a i n s such t h a t each a n g u l a r g r a i n i s k e p t f r o m c o n t a c t i n g any o t h e r , b e n e f i t can be t a k e n f r o m t h e a d d i t i o n a l v o i d space o f t h e a n g u l a r g r a i n s and t h e s i g n i f i c a n t r e d u c t i o n o f t h e i n t e r l o c k i n g e f f e c t w i t h i n t h e m i x t u r e , as w e l l as f r o m t h e i n c r e a s i n g f r i c t i o n p r o v i d i n g h i g h e r s t a b i l i t y o f t h e g r a i n assemblage ( c f . s e c t i o n s 1.3.5. and 1 . 4 . 3 . ) .
1.4.11.3.2. Benefit for the hydrocarbon stimulation industry According t o t h e b e n e f i t o f m i x t u r e s o f round and a n g u l a r sand g r a i n populat i o n s , a b e t t e r e x p l o i t a t i o n o f many sandstone q u a r r i e s and sand p i t s can be performed and l o t s t h a t have been separated i n t h e p a s t as b e i n g below s t a n d a r d and o u t o f s p e c i f i c a t i o n can be i n c o r p o r a t e d i n t o t h e q u a n t i t i e s d e l i v e r e d t o t h e w e l l s i t e s , t h e r e b y s e c u r i n g s u p p l y and h e l p i n g t o a v o i d a r e p e t i t i o n o f ser i o u s f r a c t u r i n g sand shortages ( a s e x p e r i e n c e d i n t h e USA d u r i n g t h e 1970's when o i l and gas w e l l s t i m u l a t i o n boosted a f t e r t h e f i r s t o i l p r i c e jump) i n t h e f u t u r e when h y d r a u l i c s t i m u l a t i o n o f hydrocarbon r e s e r v o i r s reaches a g a i n such a frequency t h a t a l l t h e p r e s e n t l y a v a i l a b l e sources o f n a t u r a l sand and s y n t h e t i c proppants r e a c h t h e l i m i t s o f p r o d u c t i o n and d e l i v e r y p o t e n t i a l (LARSEN & SMITH 1985). As combinations o f round and a n g u l a r n a t u r a l sand have t u r n e d o u t t o have h i g h e r c o n d u c t i v i t y t h a n round p o p u l a t i o n s alone, t h e same can be assumed t o be v a l i d f o r s y n t h e t i c proppants, t h e r e b y p r o v i d i n g a s i g n i f i c a n t t o o l f o r f r a c t u r e f l o w c a p a c i t y m a x i m i z a t i o n and enhancement o f t h e f o r m a t i o n - f r a c t u r e cond u c t i v i t y c o n t r a s t . T h i s may g i v e r i s e t o a renaissance o f a n g u l a r s i n t e r e d baux i t e which was o f f e r e d d u r i n g e a r l y a v a i l a b i l i t y o f s y n t h e t i c h i g h - s t r e n g t h proppants as a s i m p l e b y - p r o d u c t o f a b r a s i v e m a n u f a c t u r i n g ( c f . s e c t i o n s 1.2.4. and 1 . 3 . 5 . ) .
1.4.12. Computer programs for proppant select ion H y d r a u l i c p r o p p a n t f r a c t u r i n g d e s i g n s h o u l d i d e a l l y be a b l e n d o f computer s i m u l a t i o n tempered w i t h f i e l d e x p e r i e n c e (PA1 & GARBIS 1983 b; GARBIS, BROWN & MAURITZ 1985). The expenses f o r a h y d r a u l i c proppant f r a c t u r e t r e a t m e n t a r e composed o f proppant, f l u i d , f i x e d s e t - u p and h y d r a u l i c horsepower c o s t s (ZAHNER & CRAFTON 1985). Due t o t h e f a c t t h a t p r o p p a n t expenses can amount up t o two t h i r d s o f t h e t o t a l s t i m u l a t i o n o p e r a t i o n c o s t , proppant s e l e c t i o n i s n o t o n l y t h e key element o f f r a c t u r e d e s i g n f r o m t e c h n i c a l s t a n d p o i n t i n terms o f c l o s u r e s t r e s s r e s i s t i v i t y , b u t a l s o f r o m economical view c o n c e r n i n g j o b f e a s i b i l i t y and p r o f i t a b i l i t y . W h i l e p r o p p a n t c h o i c e and d e s i g n o p t i m i z a t i o n i n l a r g e r h y d r a u l i c f r a c t u r i n g campaigns i n whole f i e l d s have h i s t o r i c a l l y been f r e q u e n t l y done by t r i a l - a n d - e r r o r e v o l u t i o n , i n t h e l a s t few decades i n c r e a s i n g s o p h i s t i c a t i o n o f computerized m o d e l l i n g has c o n s i d e r a b l y improved t h e p o s s i b i l i t i e s o f a c c u r a t e p l a n n i n g w i t h economical assessment o f payout t i m e and c a s h - f l o w r a t e . I n o r d e r t o o b t a i n h i g h l y c o n d u c t i v e and deeply p e n e t r a t i n g cracks, accur a t e f r a c t u r e d e s i g n i s i m p e r a t i v e (CHARLES, HUDOCK, UDICK & HALL 1983). Proppant s e l e c t i o n f o r t h e i n d i v i d u a l a p p l i c a t i o n s a c c o r d i n g t o r e s e r v o i r c o n d i t i o n s had been t r a d i t i o n a l l y made by t y p e c u r v e a n a l y s i s and e v a l u a t i o n o f p r o p p a n t c h a r a c t e r i s t i c s . I n t h e l a s t years, s e v e r a l computer programs have ap-
82 p e a r e d on t h e m a r k e t w h i c h a r e based on a t y p e - c u r v e - l i k e a l g o r i t h m (AGARWAL, CARTER & POLLOCK 1979) and a l l o w f a s t c a l c u l a t i o n o f v a r i o u s c a s e s o f economic a l f e a s i b i l i t y o f p r o p p a n t i n v e s t m e n t v s . p a y o u t b y enhanced a d d i t i o n a l p o s t f r a c t u r e h y d r o c a r b o n p r o d u c t i o n (COOKE & GIDLEY 1979, HOLDITCH & LANCASTER 1982, CHENG 1984; COBB 1985, 1986; HALLIBURTON 1985; ANDERSON & PHILLIPS 1986, 1 9 8 7 ) . The p r o g r a m s c a n be r u n i n s m a l l p o r t a b l e c o m p u t e r s f i t t i n g i n t o b r i e f c a s e s w h i c h a l l o w c o n v e n i e n t d i s p l a y a t any t i m e and p l a c e and t h u s p e r m i t a i r t r a v e l l i n g t o c u s t o m e r o f f i c e s and t r e a t m e n t e x e c u t i o n s i t e s . M i c r o c o m p u t e r mod e l s f o r t h e d e s i g n o f h y d r a u l i c f r a c t u r e s a r e a l s o p r e s e n t e d b y CLEARY, KECK & MEAR ( 1 9 8 3 ) . F o l l o w i n g d i s c u s s i o n o f some a s p e c t s o f n e t p r e s e n t v a l u e c a l c u l a t i o n , s e v e r a l p o i n t s concerning s i g n i f i c a n c e o f t h e o u t p u t f o r economical f e a s i b i l i t y assessment a r e o u t l i n e d . The e c o n o m i c a l i m p o r t a n c e o f p r o p p a n t s e l e c t i o n and f r a c t u r e design m o d e l l i n g a r e u n d e r l i n e d by an i n t r o d u c t o r y overview o f general econ o m i c a l c r i t e r i a and t h e i r r e l a t i v e i m p o r t a n c e . Comments a r e a l s o o f f e r e d o n p r o p p a n t s e l e c t i o n based o n volume v s . w e i g h t .
1 . 4 . 1 2 . 1 . General economical criteria The m o s t i m p o r t a n t g e n e r a l e c o n o m i c a l c r i t e r i o n i n h y d r a u l i c f r a c t u r i n g d e s i g n m o d e l l i n g where p r o p p a n t c h o i c e p l a y s a k e y r o l e i s n e t p r e s e n t v a l u e which i s present value a f t e r treatment minus present value before t h e o p e r a t i o n m i n u s p r e s e n t v a l u e o f a l l c o s t s o r i n v e s t m e n t s a s s o c i a t e d w i t h t h e j o b (VEATCH 1986). Net p r e s e n t value i s t h e n e t cash f l o w r e s u l t i n g from t h e f r a c t u r i n g treatment, assumes t h e c o s t o f c a p i t a l r a t e , and i s t h e d i f f e r e n c e between p r e s e n t v a l u e o f a l l f u t u r e c a s h f l o w s and c o s t o f a s s e t (BARBA 1 9 8 8 ) . P r e s e n t v a l u e i s t h e c a s h f l o w coming f r o m w e l l p r o d u c t i o n and expense s t r e a m s , and i s t h e a c t u a l v a l u e o f some amount i n t h e f u t u r e a t a s p e c i f i c d i s c o u n t r a t e (VEATCH 1 9 8 6 ) . P a y o u t p e r i o d i s t h e l e n g t h o f t i m e r e q u i r e d f o r t h e c u m u l a t i v e c a s h p o s i t i o n t o r e a c h z e r o , and d i s c o u n t e d p a y o u t i n t e r v a l i s t h e p e r i o d o f t i m e f o r t h e p r e s e n t v a l u e o f t h e c u m u l a t i v e c a s h p o s i t i o n t o a t t a i n z e r o . Ret u r n on i n v e s t m e n t i s t h e r a t i o o f c u m u l a t i v e n e t c a s h f l o w o v e r t h e p r o j e c t l i f e t o t h e maximum c a s h o u t l a y . D i s c o u n t e d r e t u r n on i n v e s t m e n t i s t h e r a t i o o f t h e p r e s e n t value o f cumulative n e t cash f l o w over p r o j e c t l i f e t o p r e s e n t v a l u e o f t h e i n v e s t m e n t f o r t h e t r e a t m e n t . Some a s p e c t s o f i n v e s t m e n t r e t u r n and p r o f i t a b i l i t y as w e l l as d e s i g n o p t i m i z a t i o n and success d e f i n i t i o n a r e b r i e f l y i l l u s t r a t e d as f o l l o w s .
1.4.12.1.1. Investment return and profitability R a t e o f r e t u r n o r p r o f i t a b i l i t y i n d e x i s t h e compound i n t e r e s t r a t e whose d i s c o u n t f a c t o r s w i l l make t h e p r e s e n t v a l u e o f a p r o j e c t ' s n e t c a s h f l o w e q u a l t o z e r o (VEATCH 1 9 8 6 ) . Synonyms f o r p r o f i t a b i l i t y i n d e x i n c l u d e m a r g i n a l e f f i c i e n c y o f c a p i t a l , i n t e r n a l r a t e o f r e t u r n (BAILEY & WICKHAM 1 9 8 4 ) , t r u e y i e l d , and d i s c o u n t e d c a s h f l o w method. O t h e r f a c t o r s a r e u n d i s c o u n t e d p r o f i t / i n v e s t ment r a t i o and b e n e f i t / c o s t r a t i o . The i n t e r n a l r a t e o f r e t u r n method b a l a n c e s i n f l o w s and o u t f l o w s , and o u t p u t s t h e c o s t o f c a p i t a l r e q u i r e d t o o b t a i n t h i s b a l a n c e , t h e r e b y a l l o w i n g t h e o p e r a t o r t o compare t h e p e r f o r m a n c e o f t h e a s s e t s and t h u s b e i n g a more u n i v e r s a l t e c h n i q u e (BARBA 1 9 8 8 ) . E v a l u a t i o n s i n v o l v i n g e c o n o m i c a l a n a l y s i s a r e a l s o s e n s i t i v e t o o t h e r p a r a m e t e r s such as p r o d u c t p r i c e , c o s t e s c a l a t i o n , o p e r a t i n g expenses and i n i t i a l i n v e s t m e n t (KOZIK, B A I LEY & HOLDITCH 1 9 7 9 ) . The m e n t i o n e d t e r m s a r e u s e d t h r o u g h o u t i n d u s t r y f o r e v a l u a t i o n o f t h e econom i c a l v i a b i l i t y o f p r o j e c t s . W h i l e p r e s e n t v a l u e i s u s e d as t h e base c r i t e r i o n f o r e c o n o m i c a l c o m p a r i s o n s , t h e r e i s no r e a s o n why any g i v e n i t e m o f a s e t o f c r i t e r i a s h o u l d be more a p p r o p r i a t e t h a n any o t h e r , w i t h t h e c h o i c e o f p o i n t s f o r m a k i n g e c o n o m i c a l d e c i s i o n s d e p e n d i n g h e a v i l y on t h e f i n a n c i a l g o a l s o f t h e i n v e s t o r . P r e s e n t v a l u e c a l c u l a t i o n s c a n be based on d i s c r e t i z e d ( i n c r e m e n t a l )
83 o r continuous discount f a c t o r s . I t i s important t o note the s i g n i f i c a n c e o f earl y t i m e cash f l o w due t o a c c e l e r a t i o n on p r e s e n t values as w e l l as d i f f e r e n c e s r e s u l t i n g f r o m d i f f e r e n t p r o d u c t i o n d e c l i n e performance. N e t p r e s e n t v a l u e i s t h e u l t i m a t e t e r m used t o i n v e s t i g a t e p r e s e n t v a l u e economics coming f r o m hyd r a u l i c f r a c t u r i n g operations.
1.4.12.1.2. Design optimization and success definition The e c o n o m i c a l l y optimum f r a c t u r e d e s i g n i s t h e one which p r o v i d e s t h e l a r g e s t n e t p r e s e n t v a l u e (WAREMBOURG, KLINGENSMITH, HODGES & ERDLE 1985). The econ o m i c a l l y optimum d e s i g n i s m a i n l y a f f e c t e d by d u r a t i o n o f p r o d u c t i o n f o r e c a s t f r o m which n e t p r e s e n t v a l u e i s c a l c u l a t e d , n e t d i s c o u n t e d e x p l o i t a t i o n r e v e nue, and amount o f investment r e q u i r e d t o achieve t h e d e s i g n o p t i o n . The c r i t i c a l v a r i a b l e s i n t e r a c t and accuracy i s e x t r e m e l y i m p o r t a n t f o r p r o d u c t i o n f o r e c a s t i n g , s t i m u l a t i o n d e s i g n and economical o p t i m i z a t i o n . The c o s t t o determine more a c c u r a t e values f o r t h e c r i t i c a l v a r i a b l e s i s u s u a l l y a v e r y small percentage o f t h e t o t a l w e l l c o s t . E a r l y o p t i m i z a t i o n p r e v e n t s r e p e t i t i v e e r r o r s f r o m o c c u r r i n g i n t h e designs f o r f u t u r e w e l l s . The economical optimum can be assessed p r i o r t o any e x p e n d i t u r e f o r a f r a c t u r e s t i m u l a t i o n t r e a t m e n t . The success o f a h y d r a u l i c f r a c t u r i n g o p e r a t i o n can be q u a n t i f i e d i n e x a c t terms r e g a r d l e s s of t h e volumes o f f l u i d and p r o p p a n t a c t u a l l y pumped. U s i n g a l l t h e a v a i l a b l e technology i n a l o g i c a l e n g i n e e r i n g sequence decreases t h e r i s k a s s o c i a t e d w i t h any s t i m u l a t i o n j o b and t h e p r o d u c t i o n r e s u l t s , and t h e e x p e r i e n c e f a c t o r i s acc e l e r a t e d and enhanced. Another i m p o r t a n t economical c r i t e r i o n i s t h e d e f i n i t i o n o f success o f a s t i m u l a t i o n t r e a t m e n t (GIDLEY 1985). F r a c t u r e s t i m u l a t i o n j o b s a r e u s u a l l y d e s i g ned and j u s t i f i e d on t h e f o l d s o f i n c r e a s e d p r o d u c t i o n based on t h e steady s t a t e o f f t a k e r a t e o f t h e w e l l b e f o r e f r a c t u r i n g (SLUSSER & RIECKMANN 1 9 7 6 ) . L I h i l e i n many cases f r a c t u r i n g o p e r a t i o n s a r e c l a s s i f i e d as successes when i n cremental hydrocarbon p r o d u c t i o n pays o u t e x e c u t i o n c o s t w i t h i n a c e r t a i n t i m e p e r i o d and s a t i s f i e s a p r e d e t e r m i n e d p r o f i t a b i l i t y index, a b e t t e r approach i s assessing a t r e a t m e n t as success i f i t r e s u l t e d i n a n e t p o s i t i v e p r o d u c t i o n r a t e b u i l d u p . E v a l u a t i o n s o f s t i m u l a t i o n j o b success can be made on t h e b a s i s o f i n i t i a l p o t e n t i a l of t h e w e l l , b u t a much b e t t e r approach i s u s i n g a c t u a l and p r o j e c t e d u l t i m a t e r e c o v e r y o f t h e b o r e h o l e (HOEL 1988).
1.4.12.2. Net present value calculation The d e s i g n o f h y d r a u l i c f r a c t u r i n g t r e a t m e n t s has t h r e e b a s i c r e q u i r e m e n t s i n c l u d i n g d e t e r m i n a t i o n o f o i l and gas p r o d u c t i o n r a t e s and c u m u l a t i v e r e c o v e r y volumes t h a t m i g h t be expected f r o m v a r i o u s crack l e n g t h s and c o n d u c t i v i t i e s f o r a g i v e n r e s e r v o i r , l a y o u t o f t h e f r a c t u r e o p e r a t i o n i n o r d e r t o achieve t h e d e s i r e d c r a c k l e n g t h s and c o n d u c t i v i t i e s , and m a x i m i z a t i o n o f economical r e t u r n s (VEATCH 1983, 1986). Revenue e s t i m a t e s a r e developed f o r v a r i o u s f r a c t u r e parameter c o n s t e l l a t i o n s f r o m p r e d i c t i o n r u n s w i t h a r e s e r v o i r performance simul a t o r , whereas a h y d r a u l i c f r a c t u r i n g s i m u l a t o r computes t r e a t m e n t volumes, t y pes o f m a t e r i a l s , and pumping schedules which a r e necessary t o achieve t h e p l a n ned crack geometry. As f r a c t u r e c o n d u c t i v i t y i s p a r t i c u l a r l y i n t i g h t gas r e s e r v o i r s t o g e t h e r w i t h c r a c k l e n g t h t h e most i m p o r t a n t d e s i g n parameter, c a r e f u l proppant s e l e c t i o n f o r t h e b e n e f i t o f a l l r e q u i r e m e n t s has a key p o s i t i o n i n f r a c t u r e p l a n n i n g , and t h e r e f o r e s p e c i a l computer programs have been developed f o r proppant c h o i c e on b o t h t e c h n i c a l and economical background. The wide range o f c o s t and performance o f t h e v a r i o u s c o m m e r c i a l l y a v a i l a b l e proppant t y p e s has r e s u l t e d i n p r o l i f e r a t i o n o f proppant s e l e c t i o n models dur i n g course o f t h e t e c h n o l o g i c a l e v o l u t i o n o f h y d r a u l i c f r a c t u r i n g (PHILLIPS & ANDERSON 1985; ANDERSON & PHILLIPS 1986, 1987). While i n e a r l i e r y e a r s a r a t h e r vague r e l a t i o n s h i p between w e l l d e p t h and p r o p p a n t s t r e n g t h d i c t a t e d t h e c h o i c e o f p r o p p a n t type, more r e c e n t l y computerized models o f v a r y i n g c o m p l e x i t y have
84 been u t i l i z e d which i n c l u d e n e t p r e s e n t v a l u e c a l c u l a t i o n s by comparing c o s t and performance o f t h e s e l e c t e d m a t e r i a l s and economical revenue o f t h e s t i m u l a t i o n j o b . S i m p l e r non-computerized e v a l u a t i o n t e c h n i q u e s a r e based on e i t h e r p r o p p a n t expenses o r t e c h n i c a l m e r i t s o f t h e j o b d e s i g n and do n o t c o n s i d e r p r o d u c t i v i t y o r revenue, and include cost/performance comparisons and nomographs. The purpose o f computerized proppant s e l e c t i o n models i s t o determine how p o s t - f r a c t u r e i n c r e m e n t a l and c u m u l a t i v e p r o d u c t i v i t y i s a f f e c t e d by changes o f type, s i z e and q u a n t i t y o f t h e p r o p p a n t s u p p o r t i n g t h e c r a c k . The t e c h n i c a l c h o i c e s a r e t h e n expressed i n economical terms by a p p l i c a t i o n o f n e t present worth c a l c u l a t i o n s , thereby allowing t o s e l e c t the f r a c t u r i n g design which b e s t f i t s t h e f e a s i b i l i t y performance g o a l s o f t h e w e l l t o be t r e a t e d . The o v e r a l l g u i d e l i n e i s always c o s t o f p r o p p a n t p e r u n i t c o n d u c t i v i t y o r c o n d u c t i v i t y p e r US $ (BRIM 1986, ANDERSON & PHILLIPS 1987; c f . s e c t i o n 1.3.4.1.).
1.4.12.3. Simulation techniques and input data The computer programs f o r p r o p p a n t s e l e c t i o n r e q u i r e e n t e r i n g o f r e s e r v o i r , hydrocarbon, p r o p p a n t and economical d a t a and d e l i v e r an o u t p u t o f p r o p p a n t q u a n t i t y and c o s t , and revenue f r o m hydrocarbon p r o d u c t i o n a f t e r t h e t r e a t m e n t , t h e r e b y e n a b l i n g t o choose t h e most f e a s i b l e case o u t o f v a r i o u s r u n s h a v i n g been made w i t h d i f f e r e n t parameters. I n i t i a l d r i l l i n g and c o m p l e t i o n expenses a r e n o t i n c l u d e d i n t o t h e a n a l y s i s , and e f f e c t s o f income t a x and w i n d f a l l p r o f i t s a r e a l s o o m i t t e d f o r s i m p l i c i t y (NORMAN, CIPOLLA & WEBB 1983). The r e s u l t s o f p r o p p a n t s e l e c t i o n e v a l u a t i o n by computer programs i n d i c a t e t h e e f f e c t o f f r a c t u r e p e r m e a b i l i t y on e s t i m a t e d p r o d u c t i o n i n c r e a s e and o f p r o p p a n t c h o i c e on t h e t r e a t m e n t economics (McDANIEL & WILLINGHAM 1978). Some aspects o f c o s t e f f e c t i v e n e s s o f v a r i o u s p r o p p a n t types, s i z e s and q u a n t i t i e s as w e l l as e v o l u t i o n o f p o s t - f r a c t u r e w e l l performance p r e d i c t i o n a r e o u t l i n e d as f o l l o w s .
1.4.12.3.1. Cost effectiveness of various proppant types,
sizes and quantities
Assuming t h a t a l l h y d r a u l i c f r a c t u r i n g expenses e x c e p t p r o p p a n t c o s t a r e cons t a n t d u r i n g s e v e r a l s t i m u l a t i o n runs, i t i s p o s s i b l e t o c h a r a c t e r i z e t h e c o s t e f f e c t i v e n e s s o f v a r i o u s types, s i z e s and amounts o f proppants by comparing n e t p r e s e n t v a l u e o f f u t u r e p r o d u c t i o n when d i f f e r e n t parameter c o m b i n a t i o n s a r e examined i n t h e program l o o p s (ANDERSON & PHILLIPS 1987), w i t h t h e optimum p r o p p a n t b e i n g t h e one g i v i n g t h e h i g h e s t c u m u l a t i v e n e t p r e s e n t v a l u e . More s o p h i s t i c a t e d programs do n o t s i m u l a t e t h e f r a c t u r e as an e n t i r e wedge, b u t as a numb e r o f d i s c r e t e c e l l s , t h e r e b y a l l o w i n g t o i n c l u d e a l s o p r o p p a n t t a i l - i n s and head-ins i n t o t h e e v a l u a t i o n . The r o u t i n e a p p l i c a t i o n s o f computerized r e s e r v o i r s i m u l a t o r s and proppant s e l e c t i o n models i s p a r t i c u l a r l y i m p o r t a n t i n t h e c u r r e n t economical s t a t e o f t h e hydrocarbon i n d u s t r y and a l l o w s f r a c t u r i n g t r e a t m e n t d e c i s i o n s t o be based e x c l u s i v e l y on w e l l p r o d u c t i v i t y g o a l s o f t h e o p e r a t o r and s p e c i f i c r e s e r v o i r c o n d i t i o n s (ANDERSON & PHILLIPS 1986).
1.4.12.3.2.Evolution of post-fracture well performance forecast As b o t h computer programs and a d d i t i o n a l manual techniques have g o t i n c r e a s i n g l y s o p h i s t i c a t e d w i t h time, r e s u l t i n g a l s o i n more and more r e l i a b l e p r e d i c t i o n s and assessments, t h e r e i s an e v o l u t i o n o f s k i l l i n p r o j e c t i o n s o f p o s t f r a c t u r e w e l l performance (BRIM 1986). I n terms o f computerized f r a c t u r e s i m u l a t i o n , p r o d u c t i o n and s e r v i c e companies have developed v a r i o u s c o m p u t a t i o n a l mod e l s f o r o f f t a k e f o r e c a s t ( f o r example numerical s i m u l a t i o n ; SETTARI 1980, LEICHT 1985). A comparative assessment o f v a r i o u s p r o p p a n t s e l e c t i o n models and t h e i r s i g n i f i c a n t advantages and drawbacks i s c a r r i e d o u t by ANDERSON & PHILLIPS ( 1 9 8 7 ) . Concerning economical e v a l u a t i o n , p r o d u c t i o n r a t e s v a r y w i t h t i m e and t h e r e f o r e c u m u l a t i v e w i t h d r a w a l a f t e r some t i m e p e r i o d s h o u l d be chosen as
85 a base f o r comparison (ELBEL & SOOKPRASONG 1985) which i s i n i n t o many c o m p u t a t i o n a l models.
fact
incorporated
1.4.12.4. Sign if icance for economical feasibi 1 i ty model 1 ins The main advantage o f i n t e g r a t e d computer programs f o r proppant s e l e c t i o n and p r o d u c t i o n f o r e c a s t based on e n t e r i n g o f d e t a i l e d r e s e r v o i r m a t r i x and f l u i d d a t a i s economical c r o s s - c h e c k i n g i n c l u d i n g c o n t r o l whether t h e recommended c h o i c e o f a h i g h e r - s t r e n g t h p r o p p a n t i s j u s t i f i e d by s u f f i c i e n t r e s e r v e s and r e a s o n a b l y f a s t payout o f t h e investment o f t h e s t i m u l a t i o n j o b as d e s i g ned. I t c o u l d happen t h a t r e s e r v o i r c o n d i t i o n s would r e q u i r e a h i g h e r - q u a l i t y and t h u s more expensive proppant which i s , however, n o t supported by o v e r a l l s i z e , pay t h i c k n e s s and hydrocarbon c o n t e n t o f t h e s t r u c t u r e (ROBINSON, HOLDITCH & WHITEHEAD 1986). Comparative e v a l u a t i o n o f p r o p p a n t performance has t o i n c l u d e economical a n a l y s i s , as p r o p p a n t e f f e c t i v e n e s s can be b e s t e s t i m a t e d f r o m t h e r e s u l t i n g p r o d u c t i v i t y i n d e x i n c r e a s e and t h e n e t p r e s e n t v a l u e i n c r e mental i n c r e a s e (NORMAN, CIPOLLA & WEBB 1983). Some comments a r e o f f e r e d as f o l lows on p r o p p a n t q u a l i t y and d r a i n a g e e f f i c i e n c y d i f f e r e n c e s as w e l l as e a r l y and l a t e p r o d u c t i o n h i s t o r y .
1.4.12.4.1. Proppant quality and drainage efficiency differences I f t h e a p p l i c a t i o n o f a more expensive proppant t y p e even i n low c l o s u r e s t r e s s regimes i s e c o n o m i c a l l y f e a s i b l e , i t s h o u l d be c a r r i e d o u t because o f t h e h i g h e r c o n d u c t i v i t y o f t h e b e t t e r - q u a l i t y proppant and t h u s more pronounced p e r m e a b i l i t y c o n t r a s t between f r a c t u r e and f o r m a t i o n (NORMAN, CIPOLLA & WEBB 1983 a l s o r e p o r t t h a t economical a n a l y s i s o f t e n i n d i c a t e s t h a t t h e i n c r e m e n t a l p r e s e n t v a l u e o b t a i n e d by f r a c t u r i n g can be i n c r e a s e d by u s i n g a h i g h - p e r f o r mance p r o p p a n t even though p r o p p a n t expenses a r e c o n s i d e r a b l y h i g h e r ) .
Comparing t h e e f f e c t i v i t y o f sand and i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s i n f i e l d a p p l i c a t i o n s , ROBINSON, HOLDITCH & WHITEHEAD (1986) found t h a t r e s e r v o i r zones t r e a t e d w i t h i n t e r m e d i a t e - s t r e n g t h proppants c l e a r l y had a much l a r g e r c o n d u c t i v i t y t h a n those o p e r a t e d w i t h sand even though j o b s i z e s were about t h e same i n terms o f proppants pumped, t h e i n t e r m e d i a t e - s t r e n g t h p r o p p a n t was i n j e c t e d i n t o much deeper h o r i z o n s o f t h e p r o d u c t i v e column, and sand was pumped i n much h i g h e r c o n c e n t r a t i o n s t h a n i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s . I n cases where i t i s t e c h n i c a l l y and o p e r a t i o n a l l y f e a s i b l e , t h e use o f l a r g e r - s i z e d p r o p p a n t s o f t h e same t y p e may be more cost/performance e f f e c t i v e t h a n u s i n g a more expens i v e proppant t y p e o f t h e same g r a i n s i z e (ANDERSON & PHILLIPS 1986).
1.4.12.4.2. Early and late production history One parameter which f r a c t u r e s i m u l a t i o n computer programs u s u a l l y do n o t i n c o r p o r a t e i s f i e l d s t a t u s d u r i n g course o f e x p l o i t a t i o n h i s t o r y . HOLDITCH (1984) demonstrates t h a t i n e a r l y f i e l d l i f e , f r a c t u r i n g w i t h b a u x i t e p r o p p a n t s r e s u l t s i n h i g h e r g a s - f l o w r a t e s and t h u s a g r e a t e r percentage o f gas product i o n i n comparison t o t h e q u a n t i t i e s o b t a i n e d a f t e r s t i m u l a t i o n w i t h sand. I n v e r s e l y , however, i n l a t e e x p l o i t a t i o n h i s t o r y , t h e average r a t e coming o u t o f a sand-propped c r a c k i s h i g h e r t h a n t h a t d e l i v e r e d by a b a u x i t e - i n f i l l e d f r a c t u r e , because i n t h e l a t t e r case, most o f t h e hydrocarbon r e s e r v e s have a l r e a d y been d r a i n e d o f f i n e a r l y f i e l d l i f e and t h e r e f o r e i n stages o f advanced d e p l e t i o n t h e a l r e a d y c o n s i d e r a b l y d e p l e t e d and exhausted r e s e r v o i r o n l y r e l e a s e s f u r t h e r hydrocarbons a t c o n s i d e r a b l y l o w e r r a t e s . T h i s i n v e r s e p r o f i t a b i l i t y beh a v i o u r o f sand and b a u x i t e proppants d u r i n g l a t e and e a r l y f i e l d l i f e , respect i v e l y , needs adequate c o n s i d e r a t i o n i n d e s i g n i n g t h e p r o p p i n g agent i n t h e range o f l o w e r c l o s u r e s t r e s s e s where t h e danger o f sand c r u s h i n g i s n o t y e t p r e s e n t and t h e o n l y d e c i s i v e q u e s t i o n i s how t o o b t a i n maximum c o n d u c t i v i t y and p r o d u c t i o n d u r i n g t h e whole t i m e o f w e l l o r f i e l d e x p l o i t a t i o n ( c f . a l s o
86 section 4.8.9.4.) P r o p p a n t c o s t f a c t o r s w h i c h have t o be examined f o r e c o n o m i c a l f e a s i b i l i t y o f f r a c t u r e t r e a t m e n t s do n o t o n l y c o m p r i s e p r o p p a n t r e t a i l p r i c e w h i c h i s o n l y one f a c e t o f t h e o v e r a l l expenses o f u s i n g a s p e c i f i c p r o p p a n t , b u t i n c l u d e a l s o number o f p r o p p a n t g r a i n s b y u n i t ( v o l u m e p e r pound), a d d i t i o n a l p o l y m e r l o a d i n g o f t h e f r a c t u r i n g f l u i d t h a t i s necessary t o t r a n s p o r t t h e denser propp a n t , a d d i t i o n a l e q u i p m e n t r e q u i r e d on t h e w e l l h e a d such as s p e c i a l f l o w b a c k man i f o l d s , and a d d i t i o n a l m a i n t e n a n c e c o s t s t h a t a r e caused b y a b r a s i o n and e r o sion (SINCLAIR, GRAHAM & SINCLAIR 1 9 8 3 ) . R e p l a c i n g s i n t e r e d b a u x i t e b y r e s i n c o a t e d sand c a n r e s u l t i n a b t . 50 % t o t a l p r o p p a n t expense s a v i n g s due t o b o t h c h e a p e r pound p r i c e and h i g h e r pound volume o f t h e l a t t e r p r o d u c t ( c f . s e c t i o n s 1 . 2 . 6 . and 1 . 4 . 7 . 2 . ) . The change f r o m a h i g h - d e n s i t y t o a l o w - d e n s i t y p r o p p a n t e n a b l e s r e d u c t i o n o f p o l y m e r l o a d i n g o f t h e c a r r i e r f l u i d u p t o 30 % w i t h o u t a f f e c t i n g proppant transport o r s e t t l i n g c h a r a c t e r i s t i c s .
1.4.12.5.Proppant volume vs. weight T e c h n i c a l and e c o n o m i c a l c o m p a r i s o n o f d i f f e r e n t p r o p p a n t t y p e s h a s t o i n c l u d e an e v a l u a t i o n o f expenses b a s e d on volume v s . w e i g h t , as b o t h p a r a m e t e r s r e s u l t i n d i f f e r e n t o v e r a l l p r o p p a n t c o s t (CUTLER, E N N I S S , JONES & CARROLL 1983; CUTLER, ENNISS, JONES & SWANSON 1 9 8 5 ) . N e g l e c t i n g c o n d u c t i v i t y d i f f e r c n ces between v a r i o u s p r o p p a n t t y p e s , f r a c t u r e d e s i g n s h o u l d b e p e r f o r m e d b a s i n g on t h e c o n c e p t t o pump t h e same volume o f p r o p p a n t s r e g a r d l e s s o f m a t e r i a l dens i t y i n o r d e r t o a c h i e v e t h e same v o l u m e t r i c a l p r o p p a n t c o n c e n t r a t i o n p e r u n i t a r e a w i t h i n t h e c r a c k . I d e n t i c a l u n i t volumes o f d i f f e r e n t p r o p p a n t t y p e s , however, have d i f f e r e n t w e i g h t s as a consequence o f t h e d i f f e r e n t s p e c i f i c g r a v i t i e s o f t h e m a t e r i a l s . As p r o p p a n t s a r e m a r k e t e d on a w e i g h t base, more h i g h e r d e n s i t y m a t e r i a l i s needed t o a c h i e v e t h e same v o l u m e t r i c a l p r o p p a n t s a t u r a t i o n as w i t h l o w e r - d e n s i t y m a t e r i a l . The p r i c e d i f f e r e n c e o f p r o p p a n t s when compar i n g e q u a l w e i g h t s i s t h e r e f o r e i n c r e a s i n g when c o m p a r i n g e q u a l volumes ( c f . section 1.4.7.1.). N e g l e c t i n g p r o p p a n t t r a n s p o r t and c o n d u c t i v i t y d i f f e r e n c e s , t a p d e n s i t y ( c f . section 1.4.7.2.) i s t h e more i m p o r t a n t p a r a m e t e r f o r e c o n o m i c a l e v a l u a t i o n w i t h r e s p e c t t o s p e c i f i c g r a v i t y , because i t d e t e r m i n e s p r o p p a n t c o s t o n a v o lume b a s i s . A g i v e n f r a c t u r e w i l l be f i l l e d b y d i f f e r e n t w e i g h t s f o r p r o p p a n t s w i t h d i f f e r e n t tap densities, w i t h the lower the tap density, the less weight i t t a k e s t o f i l l t h e f r a c t u r e . P r o p p a n t p r i c i n g s h o u l d t h e r e f o r e b e compared o n a volume b a s i s b y m u l t i p l y i n g expenses on a w e i g h t b a s i s b y t a p d e n s i t y t o d e t e r m i n e p r i c e p e r volume.
1.4.12.6.Other aspects An a d d i t i o n a l o r p a r t i a l l y a l s o a l t e r n a t i v e t e c h n i q u e t o c o m p u t e r i z e d f r a c t u r e s i m u l a t i o n m o d e l l i n g a r e h a n d - a p p l i e d i n t e g r a l e v a l u a t i o n methods e s p e c i a l l y f o r p o s t - t r e a t m e n t p e r f o r m a n c e assessment (BRINKMANN, KRUMER & REINICKE 1980; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1 9 8 5 ) . Manual and g r a p h i c a l ways o f d e s i g n i n g and e v a l u a t i n g f r a c t u r i n g s t i m u l a t i o n o p e r a t i o n s a r e c o m p i l e d i n HALLIBURTON ( 1 9 7 1 ) . Type c u r v e s p r o v i d e f o r many c a s e s a r e l a t i v e l y f a s t and i n e x p e n s i v e method as compared t o i t e r a t i v e n u m e r i c a l c o m p u t e r i z e d r e s e r v o i r s i m u l a t o r s (VEATCH 1 9 8 6 ) . A s p e c t s o f p r e - f r a c t u r i n g w e l l d e l i v e r a b i l i t y p r e d i c t i o n a r e a l s o d i s c u s s e d b y S I N H A & FURLONG ( 1 9 7 9 ) . Combined c o m p u t e r f r a c t u r e s i m u l a t i o n and t y p e c u r v e m a t c h i n g i s r e p o r t e d b y ROBERTS ( 1 9 8 1 ) and SOLIMAN, VENDITTO & SLUSHER ( 1 9 8 4 ) . VEATCH ( 1 9 7 7 ) p r e s e n t s c o m p u t e r s t u d i e s f o r d e t e r m i n a t i o n o f i n - s i t u f r a c t u r e f l o w c a p a c i t y i n l o w - p e r m e a b i l i t y gas r e s e r v o i r s . Some a s p e c t s o f p r o p p a n t volume m i n i m i z a t i o n and d e s i g n o p t i m i z a t i o n as w e l l as p r o p p a n t s e l e c t i o n v s . p a y o u t m a x i m i z a t i o n a r e o u t l i n e d as f o l l o w s .
87
1.4.12.6.1. Proppant volume minimization and design optimization POULSEN & SOLIMAN (1987) p r e s e n t a numerical procedure f o r d e t e r m i n i n g p r o p p a n t schedules f o r h y d r a u l i c f r a c t u r i n g t r e a t m e n t s which aims on p r o p p a n t quant i t y m i n i m i z a t i o n and o p t i m i z a t i o n based on crack c o n d u c t i v i t y d i s t r i b u t i o n c u r ves. The procedure i s based on m o d e l l i n g o f f r a c t u r e l e n g t h , f r a c t u r e w i d t h , proppant t y p e and p r o p p a n t c o n c e n t r a t i o n , and i n c l u d e s an economical a n a l y s i s t h a t i s c o n s i d e r i n g c o s t o f each t r e a t m e n t a l t e r n a t i v e , time-dependent product i o n r a t e as p r e d i c t e d by a f r a c t u r e d - w e l l s i m u l a t o r , c u r r e n t and expected o i l and gas p r i c e s , and minimum a t t r a c t i v e r a t e o f investment r e t u r n . The c r i t e r i a o f t h e o p e r a t o r f o r making investment d e c i s i o n s a r e a l s o i m p o r t a n t parameters, w i t h l i m i t e d amount o f c a p i t a l a v a i l a b l e f o r investment r e q u i r i n g t o compare i n cremental b e n e f i t - c o s t r a t i o s between a l t e r n a t i v e s i n an e f f o r t t o maximize t h e r e t u r n on a g i v e n investment, whereas h i g h e r investment p o t e n t i a l a l l o w s compar i s o n o f i n c r e m e n t a l r a t e s o f r e t u r n . The d e s i g n o p t i m i z a t i o n procedure can be used f o r e i t h e r s t a i r - s t e p p e d o r ramped proppant schedules ( c f . a l s o HANDKE 1987; c f . s e c t i o n 4 . 3 . 3 . 5 . ) . The c h o i c e o f approach f o r d e t e r m i n i n g f o l d s o f p r o d u c t i o n i n c r e a s e achieved f r o m f r a c t u r i n g and t h e s e l e c t i o n o f performance d e c l i n e b e h a v i o u r i s v e r y i m p o r t a n t and can be q u i t e c r i t i c a l when d o i n g economical o p t i m i z a t i o n s t u d i e s (VEATCH 1986). The approaches s h o u l d be v e r i f i e d w i t h f i e l d e x p e r i e n c e whenever p o s s i b l e . I t i s o f t e n f e a s i b l e t o develop s h o r t - c u t t r e a t m e n t d e s i g n and c o s t e s t i m a t i o n methods when assessing l a r g e numbers o f designs, w i t h development o f t y p i c a l curves f o r s p e c i f i c f o r m a t i o n s b e i n g a s u i t a b l e t o o l . These t y p i c a l c u r ves can be programmed f o r a u t o m a t i c i n t e r p o l a t i o n and p r o v i d e a method f o r r a p i d l y g e n e r a t i n g massive s e t s o f e s t i m a t e s a t minimal expenses. F r a c t u r e d e s i g n must n o t be l i m i t e d t o s e l e c t i o n o f p r o p p a n t and f l u i d t y p ? s and q u a n t i t i e s , b u t a v e r y i m p o r t a n t and even h i g h l y c r i t i c a l f a c t o r i s w e l l b e h a v i o u r . Well performance p r e d i c t i o n as a consequence o f t h e l a y o u t o f t h e f r a c t u r e t r e a t m e n t has key s i g n i f i c a n c e i n p l a n n i n g o f t h e o p e r a t i o n .
1.4.12.6.2. Proppant selection vs. payout maximization Sometimes c r a c k d e s i g n and p r o p p a n t s e l e c t i o n a c c o r d i n g t o r e s e r v o i r r e q u i r e ment i s d i f f i c u l t t o match w i t h economical f e a s i b i l i t y o r a t l e a s t w i t h maximum p a y o u t . OSBORNE, McLEOD & SCHROEDER (1981) p r e s e n t an example o f economical unf o r t u n i t y where w e l l s w i t h weaker gas p r o d u c t i o n c a p a c i t y due t o p o o r e r r e s e r v o i r f a c i e s a r e t h e ones i n g r e a t e s t need o f a f r a c t u r i n g t r e a t m e n t w i t h e x c l u s i v e l y i n t e r m e d i a t e - o r h i g h - s t r e n g t h proppants, because t h e y a r e t h e most lik e l y t o be s u b j e c t e d t o l a r g e drawdowns d u r i n g e a r l y w e l l l i f e . On t h e o t h e r hand, w e l l s w i t h s t r o n g gas p r o d u c t i o n c a p a c i t y as a consequence o f b e t t e r r e s e r v o i r f a c i e s a r e n o t s u b j e c t e d t o such h i g h drawdowns and u s u a l l y o n l y r e q u i r e a t a i l - i n o f i n t e r m e d i a t e - t o h i g h - s t r e n g t h proppants f o l l o w i n g pumping o f n a t u r a l sand i n t h e main stage, t h e r e b y s a v i n g c o n s i d e r a b l e proppant expenses. I n such cases, t h e f e a s i b i l i t y e v a l u a t i o n has t o be made f o r t h e whole group o f w e l l s o r even f o r t h e e n t i r e f i e l d i n c l u d i n g c r e s t a l and m a r g i n a l areas r a t h e r t h a n comparing t r e a t m e n t c o s t s and p r o d u c t i o n revenues f o r i n d i v i dual w e l l s .
1.4.13. Proppant selection guidelines R e s u l t i n g f r o m t h e d i s c u s s i o n i n t h e p r e v i o u s s e c t i o n s , some g e n e r a l g u i d e l i nes o f r u l e - o f - t h u m b c h a r a c t e r can be g i v e n as a summary f o r p r o p p a n t s e l e c t i o n f o r t h e a p p l i c a t i o n i n o i l and gas w e l l s t i m u l a t i o n i n hydrocarbon i n d u s t r y . A l though t h e f o l l o w i n g c u m - g r a n o - s a l i s g u i d e l i n e s a r e h e l p f u l f o r t h e f i r s t approach o f p r o p p a n t c h o i c e , c a u t i o n has t o be e x e r c i s e d n o t t o overemphasize these g e n e r a l c r i t e r i a due t o t h e i r p a r t i a l l y c o n s i d e r a b l e a b s t r a c t i o n t o even o v e r s i m p l i f i c a t i o n which cannot account f o r a l l t h e s p e c i f i c cases ( c f . McDANIEL 1987). More p r e c i s e and s o p h i s t i c a t e d proppant s e l e c t i o n i n e v i t a b l y r e q u i -
88
res plunging into the detailed assessments which have been briefly outlined in the preceding divisions (the proppant application spectrum in oil and gas reservoir hydraulic fracturing stimulation is summarized in tabs. 1 and 4). The most important characteristics of natural sand, intermediate-strength low-density alumina silicate proppants, intermediate-strength high-density alumina oxide and silicate proppants, high-strength high-density alumina oxide proppants, and high-strength low-density zirconia-silicate proppants are briefly sketched as follows.
1.4.13.1. Natural sand Natural sand is the cheapest of all proppant types and has always been available in nearly unlimited quantities due to widespread occurrences, uncomplicated accessibility and easy processing. Its application, however, is restricted to shallow wells due to its low closure stress resistivity which is the reason for classifying natural sand as low-strength proppant (cf. section 1.3.1.1.). Having been applied since the very beginning of hydraulic fracturing technology (cf. section l . Z . Z . ) , natural sand is nowadays more and more replaced by synthetic high-conductivity proppants in all the cases where no extreme cost containment is necessary, and also in subcritical closure stress regimes man-made intermediate- and high-strength proppants that give better permeability contrast between fracture and formation can be selected. The oil price drop in 1986 has temporarily again focussed attention on the cheap sand wherever applicable as a consequence of pronounced to partially even exaggerated fracture treatment cost containment (cf. section 2.2.1.5.), but increasing demand of hydrocarbon reservoir stimulation for progressive production enhancement will automatically push natural sand into a backseat position in the proppant market during the next years. The various intermediate- and highstrength synthetic proppants which during the last ten years already successively replaced natural sand in many sections of the oil and gas market are briefly discussed as follows.
1.4.13.2. Intermediate-strength low-density alumina silicate proppants Intermediate-strength low-density alumina silicate proppants (cf. section 1.3.2.1.) have the best pumping characteristics o f all synthetic proppants due to their low specific gravity which is comparable to that of sand. The higher closure stress resistivity allows the application of this material in shallow to intermediate-depth reservoirs beyond the pressure boundary of natural sand. Due to the availability of intermediate-strength low-density alumina silicate proppants only since a few years, they have much future potential for application in particularly many marginal oil and gas pay zones by replacement of natural sand in order to profit from the higher conductivity of the synthetic material. Intermediate-strength low-density alumina silicate proppants are also the cheapest of all artificial fracture-supporting materials and are harmless to the operational equipment. Effects of proppant settling are still insignificant for a wide variety o f carrier fluids and a broad spectrum of proppant concentrations. Thus intermediate-strength low-density alumina silicate proppants are the economically most feasible proppant type in any respect if the boundary of closure stress resistivity is not exceeded.
1.4.13.3. Intermediate-strength high-density
alumina oxide and silicate proppants
Intermediate-strength high-density alumina oxide and silicate proppants (cf. section 1.3.2.1.) are mainly applied for hydraulic fracturing of gas reservoirs
a9 i n moderate t o h i g h depth. Being cheaper, l i g h t e r and l e s s a b r a s i v e t h a n s i n t e r e d b a u x i t e , t h e y a r e chosen i n a l l t h e cases where l i g h t w e i g h t s y n t h e t i c propp a n t s a r e no l o n g e r r e s i s t i n g t o t h e c l o s u r e s t r e s s p r o p e r l y , b u t h i g h - s t r e n g t h alumina o x i d e proppants a r e n o t y e t necessary, and t h u s b o t h c o s t premium and disadvantage o f even h i g h e r p a r t i c l e d e n s i t y can be avoided. As i n t e r m e d i a t e s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants a r e a l s o s t i l l n o t v e r y h a r m f u l t o t h e t r e a t i n g equipment, p r o f i t o f t h e i r h i g h e r c o n d u c t i v i t y i s f r e q u e n t l y a l s o made i n boundary cases, and t h e m a t e r i a l i s o f t e n p r e f e r r e d w i t h r e s p e c t t o n a t u r a l sand o r i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y proppants i n o r d e r t o go a b s o l u t e l y on t h e s a f e s i d e and n o t t o r i s k any f a i l u r e o f t h e p r o p p a n t s which have t o r e s i s t t o a broad change o f p r e s s u r e drawdown and b u i l d u p d u r i n g l o n g - t e r m f i e l d p r o d u c t i o n h i s t o r y t h a t can comprise up t o s e v e r a l decades. The s p e c i f i c g r a v i t y i s s t i l l low enough t o a l l o w good pumping b e h a v i o u r w i t h l i t t l e r i s k o f screenout, b u t depending on c a r r i e r f l u i d c o m p o s i t i o n and weight, e f f e c t s o f proppant s e t t l i n g may a l r e a d y become s i g n i f i c a n t . Experience i n t h e p a s t i n Europe has shown t h a t a f t e r i t s i n v e n t i o n and i n t r o d u c t i o n t o t h e market, i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants have become t h e s t a n d a r d m a t e r i a l f o r t i g h t gas r e s e r v o i r s t i m u l a t i o n i n moderate t o h i g h d e p t h ( c f . s e c t i o n 2 . 4 . ) and have i n most a p p l i c a t i o n s t o t a l l y r e p l a c e d h i g h - s t r e n g t h alumina o x i d e proppants which were dur i n g t h e e a r l y y e a r s o f deep gas f r a c t u r i n g t h e e x c l u s i v e s y n t h e t i c proppants a v a i l a b l e i n t h e market ( c f . s e c t i o n 1 . 2 . 4 . 1 . ) .
1.4.13.4. High-strength high-density alumina oxide proppants H i g h - s t r e n g t h h i g h - d e n s i t y alumina o x i d e proppants o r s i n t e r e d b a u x i t e ( c f . section 1.3.2.1.) have been t h e f i r s t s y n t h e t i c proppants t h a t were i n t r o d u c e d t o t h e o i l and gas i n d u s t r y . W h i l e t h e y have been v e r y commonly a p p l i e d i n t h e e a r l y y e a r s o f deep r e s e r v o i r s t i m u l a t i o n , t h e i r market share has been i n c r e a s i n g l y taken o v e r i n t h e l a s t y e a r s by proppants w i t h l o w e r s p e c i f i c g r a v i t y which have a b e t t e r pumping b e h a v i o u r and t h a t a r e o f t e n a l s o cheaper. The h i g h s p e c i f i c g r a v i t y o f s i n t e r e d b a u x i t e does n o t o n l y l e a d t o problems o f p r o p p a n t s e t t l i n g i n l i g h t e r c a r r i e r f l u i d s , b u t a l s o i n c r e a s e s t h e r i s k o f premature screenout t e r m i n a t i o n o f t h e f r a c t u r i n g o p e r a t i o n when u s i n g h e a v i e r t r a n s p o r t media i n o r d e r t o m i n i m i z e o r t o a v o i d s e t t l i n g ( c f . s e c t i o n s 4 . 2 . 2 . 2 . 3 . and 6 . 2 . 4 . 2 . 1 . ) . The h i g h e r r i s k o f t r e a t m e n t f a i l u r e can o n l y p a r t i a l l y be compensated by s p e c i a l p r i c i n g o f s i n t e r e d b a u x i t e which t r a d i t i o n a l l y has been t h e most expensive o f a l l proppants, b u t has been o f f e r e d r e c e n t l y v e r y c o m p e t i t i v e l y i n o r d e r t o t r y t o r e g a i n some l o s t market shares o r due t o t h e a t t e m p t s o f r i v a l i z i n g proppant companies t o i n c r e a s e t h e i r market p e n e t r a t i o n a t t h e e x pense o f t h a t o f t h e i r c o m p e t i t o r s . Another m a j o r disadvantage o f s i n t e r e d b a u x i t e i s i t s c o n s i d e r a b l e a b r a s i v e ness t o t h e t r e a t m e n t equipment ( c f . s e c t i o n 1 . 4 . 6 . ) which f u r t h e r d e t e r i o r a t e s i t s economical f e a s i b i l i t y . The a p p l i c a t i o n o f s i n t e r e d b a u x i t e t h u s seems nowadays t o be more o r l e s s l i m i t e d t o deep h i g h - p r e s s u r e r e s e r v o i r f r a c t u r i n g where a l l t h e o t h e r m a t e r i a l s f a i l ( i n c l u d i n g boundary cases where i t i s essent i a l t o go on t h e a b s o l u t e l y s a f e s i d e f o r u n d i s t u r b e d l o n g - t e r m f i e l d p e r f o r mance), and replacement o f l i g h t e r proppants by s i n t e r e d b a u x i t e i n s h a l l o w e r r e s e r v o i r s can o n l y be achieved by a more t h a n c o m p e t i t i v e p r i c i n g p r o b a b l y coupled w i t h an i n c e n t i v e f o r s u c c e s s f u l placement downhole i n t h e f r a c t u r e . A s p e c i a l u t i l i z a t i o n o f s i n t e r e d b a u x i t e i s h y d r a u l i c proppant f r a c t u r i n g and g r a v e l p a c k i n g i n s t e a m - i n j e c t i o n w e l l s i n enhanced o i l r e c o v e r y p r o j e c t s ( c f . s e c t i o n s 1 . 4 . 5 . 1 . and 5 . 7 . 1 . ) .
1.4.13.5. H igh-st rength low-dens i ty zi rconia-si 1 icate proppant s are
H i g h - s t r e n g t h l o w - d e n s i t y z i r c o n i a - s i l i c a t e proppants ( c f . s e c t i o n 1 . 3 . 2 . 2 . ) an almost i d e a l m a t e r i a l f o r a wide range o f a p p l i c a t i o n s as a consequence
90 o f t h e i r t e c h n i c a l p r o p e r t i e s . The m a t e r i a l has been i n t r o d u c e d t o t h e h y d r a u l i c s t i m u l a t i o n m a r k e t o n l y a c o u p l e o f y e a r s a f t e r s i n t e r e d b a u x i t e has f i r s t been p r e s e n t e d and s u c c e s s f u l l y a p p l i e d i n t h e f i e l d and t h u s z i r c o n i a - s i l i c a t e proppants belong together w i t h s i n t e r e d bauxite t o the o l d e s t synthetic highq u a l i t y p r o p p a n t s ( c f . s e c t i o n 1 . 2 . 4 . ) . Low s p e c i f i c g r a v i t y and h i g h c l o s u r e stress r e s i s t i v i t y o f z i r c o n i a - s i l i c a t e proppants are e x c e l l e n t c h a r a c t e r i s t i c s f o r usage i n s h a l l o w t o deep w e l l s w i t h o u t any p r o b l e m s o f p l a c e m e n t and s e t t l i n g , and t h e v e r y good c o n d u c t i v i t y p r o v i d e s i n a l m o s t a l l t h e c a s e s t h e n e c e s s a r y c o n t r a s t between f o r m a t i o n and f r a c t u r e i n o r d e r t o a l l o w h y d r o c a r b o n f l o w a t economically f e a s i b l e r a t e s . The m a j o r t e c h n i c a l d i s a d v a n t a g e s , however, a r e t h e sudden c a t a s t r o p h i c a l f a i l u r e o f t h e b r i t t l e g l a s s y m a t e r i a l i n t o p o w d e r - l i k e c r u s h i n g r e m n a n t s when t h e b o u n d a r y c l o s u r e s t r e s s i s exceeded ( c f . s e c t i o n 1 . 4 . 2 . ) , and t h e l o w f r i c t i o n a n g l e w h i c h does n o t o n l y g u a r a n t e e a b e t t e r e n t r y o f t h e m a t e r i a l i n t o t h e c r a c k , b u t a l s o an e a s i e r subsequent escape f r o m t h e f r a c t u r e by f l o w b a c k ( c f . section 1.4.3.). A l l the technical benefits of high-strength zirconia-silic a t e proppants ( i n c l u d i n g t h e i r harmless behaviour t o t h e o p e r a t i o n a l e q u i p m e n t ) , however, a r e c o m p l e t e l y compensated and d e t e r i o r a t e d b y t h e h i g h p r i c e o f the m a t e r i a l . High-strength low-density z i r c o n i a - s i l i c a t e proppants are the m o s t e x p e n s i v e o f a l l c o m m e r c i a l l y a v a i l a b l e p r o p p a n t t y p e s and t h e r e f o r e t h e i r market p e n e t r a t i o n i s s e r i o u s l y l i m i t e d d e s p i t e the favourable t e c h n i c a l propert i e s ( c f . section 2.2.2.3.).
1.5. Proppant applications S y n t h e t i c p r o p p a n t s o f v a r i o u s c o m p o s i t i o n and g r a i n s i z e a r e a p p l i e d i n t h e o i l and gas i n d u s t r y f o r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g o f p r o d u c t i o n and i n j e c t i o n b o r e h o l e s i n h y d r o c a r b o n - b e a r i n g r e s e r v o i r s , as w e l l as i n o t h e r m a r k e t segments o u t s i d e h y d r o c a r b o n e x p l o i t a t i o n such as i n t h e s t e e l f o u n d r y i n d u s t r y f o r h e a t exchange and i n t h e gas p u r i f i c a t i o n i n d u s t r y f o r d u s t a b s o r p t i o n and f i l t r a t i o n . The m o s t s i g n i f i c a n t a p p l i c a t i o n s a r e b r i e f l y s k e t c h e d as f o l l o w s ( a summary o f u t i l i z a t i o n o f p r o p p a n t t y p e s and g r a i n s i z e s i n h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n o f o i l and gas r e s e r v o i r s and o t h e r m a r k e t s i s p r o v i ded i n t a b s . 1 and 4 ) .
1.5.1. Oil- and gas-field applications The m a j o r o i l - and g a s - f i e l d a p p l i c a t i o n o f n a t u r a l sand and s y n t h e t i c p r o p p a n t s i s h y d r a u l i c f r a c t u r i n g o f deep l o w - p e r m e a b i l i t y g a s - b e a r i n g s a n d s t o n e s and c a r b o n a t e s and s u b o r d i n a t e l y a l s o s h a l l o w o r deep l o w - t o m o d e r a t e - p e r m e a b i l i t y o i l - b e a r i n g s a n d s t o n e s and c h a l k s ( c f . c h a p t e r 4 ) . G a s - b e a r i n g t i g h t sandstone formations i n v a r i o u s depth i n t e r v a l s are t y p i c a l l y c h a r a c t e r i z e d by 25 100 f t n e t t h i c k n e s s , 3 - 8 % p o r o s i t y ; 1,500 - 15,000 p s i r e s e r v o i r p r e s s u r e and i n - s i t u p e r m e a b i l i t y o f u p t o 1 md (HOLDITCH 1 9 8 4 ) , b u t in m o s t c a s e s b e i n g i n t h e m i c r o d a r c y r a n g e (AGARWAL, CARTER & POLLOCK 1979; ATTEBERRY, TUCKER & RITZ 1979; c f . s e c t i o n 3 . 4 . 1 . ) . P r o p p a n t s e l e c t i o n s h o u l d g e n e r a l l y f o c u s o n g e t t i n g t h e maximum c o n d u c t i v i t y o r t h e optimum n e t p r e s e n t v a l u e p e r i n v e s t e d US $, r e s p e c t i v e l y (BRIM 1986, ANDERSON & PHILLIPS 1987; c f . s e c t i o n s 1 . 3 . 4 . and 1 . 4 . 1 2 . ) . The second m a j o r o i l - and g a s - f i e l d a p p l i c a t i o n i s g r a v e l p a c k i n g o f p r i m a r i l y p o o r l y - c e m e n t e d a n d / o r s e c o n d a r i l y u n s t a b l e , more o r l e s s s h a l l o w o i l - b e a r i n g s a n d s t o n e s and c h a l k s ( c f . c h a p t e r 5 ) . The b r i e f o u t l i n e as f o l l o w s o f f e r s o n l y a f e w comments on t h e most i m p o r t a n t b r a n c h e s o f p r o p p a n t a p p l i c a t i o n w i t h p a r t i c u l a r r e f e r e n c e t o p r o p p a n t t y p e s and g r a i n s i z e s (more d e t a i l s w i t h s p e c i a l emphasis on r e s e r v o i r c o m p o s i t i o n and d i s t r i b u t i o n a r e p r e s e n t e d i n c h a p t e r s 2 and 3 ) . D i s t i n c t i o n i n t h e s h o r t o v e r v i e w i s made between h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s , l o w - d e n s i t y a l u m i n a s i l i c a t e p r o p p a n t s and z i r c o n i a - s i l i cate proppants.
91
1.5.1.1. High-density alumina oxide and silicate proppants I n terms o f a p p l i c a t i o n o f h i g h - d e n s i t y alumina o x i d e and s i l i c a t e p r o p pants, d i s t i n c t i o n can be made between s t a n d a r d g r a i n s i z e s c o m p r i s i n g m a i n l y 20/40 and s u b o r d i n a t e l y 16/20 mesh, and c o a r s e r g r a i n s i z e s i n c l u d i n g 12/20 o r even l a r g e r .
1.5.1.1.1. Standard grain sizes The most a b u n d a n t l y a p p l i e d p r o p p i n g m a t e r i a l i n o i l - and g a s - f i e l d hydraul i c f r a c t u r i n g i s s t i l l 20/40 l o w - s t r e n g t h n a t u r a l sand due t o t h e c u r r e n t dominance o f r a t h e r s h a l l o w w e l l s t h a t r e q u i r e n o t more t h a n a t a i l - i n o f a r t i f i c i a l i n t e r m e d i a t e - t o h i g h - s t r e n g t h p r o p p a n t s o f a b t . 10 - 30 % t h e q u a n t i t y o f t h e p r e v i o u s l y pumped sand, i f man-made m a t e r i a l s a r e used a t a l l . Sand o f 20/40 and a l s o 30/50 o r 40/60 g r a i n s i z e i s t h e dominant m a t e r i a l used f o r g r a v e l p a c k i n g . Next i n t h e r a n k i n g l i s t o f c u m u l a t i v e annual consumption a r e 20/40 and t o l e s s e r amount ( e s p e c i a l l y f o r t a i l - i n a p p l i c a t i o n s ; c f . s e c t i o n 2.4.1.2.3.) a l s o 16/20 s y n t h e t i c i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants i n t h e predominant p a r t o f t h e cases and h i g h s t r e n g t h b a u x i t e proppants i n t h e s u b o r d i n a t e share o f t h i s p o r t i o n . Most o f t h e p a s t and f u t u r e massive h y d r a u l i c f r a c t u r i n g j o b s i n t i g h t deep R o t l i e g e n d (Lower Permian), C a r b o n i f e r o u s and s u b o r d i n a t e l y a l s o B u n t s a n d s t e i n (Lower T r i a s s i c ) g a s - b e a r i n g sandstones i n Germany FRG, N e t h e r l a n d s , B r i t i s h and Dutch Southern N o r t h Sea and a d j o i n i n g areas have been c a r r i e d o u t and w i l l be done w i t h t h i s p r o p p a n t t y p e (MADER 1987; c f . c h a p t e r 2 . 4 . ) . Some s p e c i a l a p p l i c a t i o n s i n c l u d e pumping o f 20/40 l o w - d e n s i t y i n t e r m e d i a t e s t r e n g t h proppants i n t h e e a r l y stage and 20/40 and/or 16/20 h i g h - d e n s i t y i n t e r m e d i a t e - s t r e n g t h and/or h i g h - s t r e n g t h proppants i n t h e l a t e s t a g e o f t h e f r a c t u r i n g o p e r a t i o n f o r t h e reason o f g e t t i n g a s a f e r l e a d - i n o f t h e l i g h t e r propp a n t s w i t h l e s s s u r f a c e f r i c t i o n t h a t m i n i m i z e t h e danger o f premature screeno u t . O t h e r e x t r a o r d i n a r y cases a r e t a i l - i n t r e a t m e n t s w i t h a l a r g e r q u a n t i t y o f 20/40 sand f o l l o w e d by a s m a l l e r p o r t i o n o f 20/40 o r 16/20 i n t e r m e d i a t e - o r even h i g h - s t r e n g t h proppants i n o r d e r t o maximize c l o s u r e s t r e s s r e s i s t i v i t y i n t h e immediate w e l l b o r e s u r r o u n d i n g s where r e s e r v o i r p r e s s u r e drawdown reaches i t s maximum (MADER 1987, 1988 a; c f . c h a p t e r s 2 and 3 ) . A s p e c i a l u t i l i z a t i o n i s a l s o pumping o f e x c l u s i v e l y i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e o r l o w - d e n s i t y alumina s i l i c a t e proppants w i t h a s m a l l e r q u a n t i t y of 20/40 m a t e r i a l f i r s t which i s f o l l o w e d by t h e main l o t o f 16/20 m a t e r i a l ( c f . s e c t i o n 2.4.1.2.3.).
1.5.1.1.2. Coarser grain sizes 12/20 i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y and h i g h - s t r e n g t h proppants a r e used i n some e x t r a o r d i n a r y r e s e r v o i r s t o achieve a p a r t i c u l a r l y pronounced p e r m e a b i l i t y c o n t r a s t between f o r m a t i o n and p r o p p a n t package, w i t h t h e a p p l i c a t i o n i n most cases b e i n g l i m i t e d t o t a i l - i n s d u r i n g advanced stages o f f r a c t u r i n g jobs, because t h e coarse g r a i n s i z e i s c o n s i d e r a b l y i n c r e a s i n g t h e r i s k o f screenout. 6/9, 8/12 and 12/20 m a t e r i a l o f v a r i o u s t y p e ( p a r t i a l l y o n l y as a t a i l - i n c o u p l e d w i t h f i n e r g r a i n s i z e s ) i s a l r e a d y o c c a s i o n a l l y r e q u i r e d (some examples o f coarse p r o p p a n t usage a r e r e p o r t e d by FAST, HOLMAN & COVLIN 1977; BLACK, RIPLEY, BEECROFT & PAMPLIN 1979; GRUESBECK, SALATHIEL & ECHOLS 1979; LAMBERT, DOLAN & GALLUS 1983; BARBER & THEMIG 1985, ROBINSON 1985, NIEMEYER & R E I NART 1986; PARKER, ADAMS & LIANKUI 1986; BARBY & BARBEE 1987) o r c o u l d be an app l i c a t i o n o f h i g h f u t u r e promotion p o t e n t i a l f o r h i g h - v i s c o s i t y o i l production, o i l - f i e l d w a t e r i n j e c t i o n , o i l - and g a s - f i e l d as w e l l as a q u i f e r waste l i q u i d d i s p o s a l ( c f . s e c t i o n 4.8.7.), c o a l seam d e g a s i f i c a t i o n ( c f . s e c t i o n 4 . 4 . 3 . ) , and geothermal h e a t e x p l o i t a t i o n f r a c t u r i n g ( c f . s e c t i o n 4 . 7 . ) . O c c a s i o n a l l y as-
92 s o c i a t e d n o n - o i l - f i e l d u t i l i z a t i o n s c o m p r i s e m i n e r a l l e a c h i n g and p r o d u c t i o n o f aqueous s o l u t i o n s b y w e l l s (BARBY & BARBEE 1 9 8 7 ) . 6 / 9 sand has f o r m e r l y been e x t e n s i v e l y u s e d f o r o i l - and w a t e r - p r o d u c t i o n w e l l g r a v e l p a c k i n g , b u t l a t e r more s o p h i s t i c a t e d d e s i g n t e c h n i q u e s have p r o v e n s m a l l e r - s i z e d g r a v e l t o be s u p e r i o r f o r e f f e c t i v e sand c o n t r o l (SCHWARTZ 1969, S A U C I E R 1974; c f . s e c t i o n 5 . 2 . 3 . ) . I n some cases b e i n g c o a r s e r e s e r v o i r sands o f a e o l i a n o r f l u v i a l h i g h - e n e r g y o r i g i n , c o a r s e 1 2 / 2 0 and 1 6 / 2 0 o r e v e n 8 / 1 2 i n t e r m e d i a t e - and h i g h - s t r e n g t h a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s a n d / o r sand a r e s t i l l nowadays r e q u i r e d f o r g r a v e l p a c k i n g .
1.5.1.2. Low-dens i ty alumina s i 1 icate proppant s 20/40 i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y alumina s i l i c a t e proppant ( c f . sect i o n 1.3.2.1.) which i s only commercially a v a i l a b l e since a b t . 5 years i s pres e n t l y g r e a t l y e x p a n d i n g and has - i n c o m b i n a t i o n w i t h t h e r e s i n - c o a t e d v a r i e t y - c e r t a i n l y a b i g f u t u r e due t o t h e p o s s i b i l i t y o f i t s a p p l i c a t i o n i n many marg i n a l r e s e r v o i r s where so f a r no s u i t a b l e s t i m u l a t i o n t e c h n i q u e h a s been d i s c o v e r e d ( s u c h as o i l - b e a r i n g m a r g i n a l c h a l k s as w e l l as c a r b o n a t e s h e l l and q u a r t z sand s t r e a k s and s h e e t s i n t e r b e d d e d w i t h m e c h a n i c a l l y a n d / o r c h e m i c a l l y u n s t a b l e mudstones; c f . s e c t i o n 4 . 8 . 5 . 1 . ) . The m a j o r a d v a n t a g e o f s y n t h e t i c low-density intermediate- s t r e n g t h alumina s i l i c a t e proppants i s t h e low specif i c g r a v i t y t h a t i s a l m o s t c o m p a r a b l e t o t h a t o f n a t u r a l q u a r t z sand w h i c h r e s u l t s i n a f a v o u r a b l e pumping b e h a v i o u r , c o u p l e d w i t h a c o n s i d e r a b l y h i g h e r c l o s u r e s t r e s s r e s i s t i v i t y t h a n q u a r t z sand. Thus i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y alumina s i l i c a t e proppants are scheduled t o replace low-strength n a t u r a l q u a r t z sand i n i n c r e a s i n g s h a r e s due t o t h e i r b e t t e r c o n d u c t i v i t y and h i g h e r closure stress r e s i s t i v i t y , i f the price difference i s not too high f o r applicat i o n s where t r a d i t i o n a l l y o n l y n a t u r a l q u a r t z sand had been used, b u t a much b e t t e r f r a c t u r e performance can be achieved by u s i n g i n t e r m e d i a t e - s t r e n g t h lowd e n s i t y alumina s i l i c a t e proppants.
1.5.1.3. Zirconia-si 1 icate proppants High-strength low-density z i r c o n i a - s i l i c a t e proppants ( c f . s e c t i o n 1 . 3 . 2 . 2 . ) have t h e m a j o r d i s a d v a n t a g e t o be c o n s i d e r a b l y more e x p e n s i v e t h a n a l u m i n a o x i d e ( c e r a m i c ) p r o p p a n t s ( b o t h i n t e r m s o f volume and w e i g h t ; c f . s e c t i o n 1 . 4 . 1 2 . 5 . ) as a consequence o f t h e d i f f e r e n t p r o d u c t i o n p r o c e s s w i t h c o n s i d e r a b l y l a r g e r amounts o f r e j e c t s f o r t h e r e a s o n o f g r a i n s i z e b e i n g o u t o f s p e c i f i c a t i o n , and due t o t e r m i n a t i o n o f g o v e r n m e n t a l s u p p o r t f o r t h e m a n u f a c t u r i n g company some y e a r s ago. Thus i n c o n t r a s t t o a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s w h i c h have become c h e a p e r d u r i n g t h e l a s t y e a r s and o n l y v e r y r e c e n t l y were a g a i n pushed u p i n p r i c e b u t s t i l l r e m a i n b e l o w t h e f o r m e r l e v e l w h i c h t h e y had d u r i n g many y e a r s o f t h e g o l d e n s t i m u l a t i o n p e r i o d , z i r c o n i a - s i l i c a t e p r o p p a n t s have k e p t t h e i r p r i c e o r e v e n g o t more e x p e n s i v e , t h e r e b y h a v i n g an important marketing disadvantage w i t h respect t o the c o m p e t i t i v e products ( c f . section 2.2.2.3.). The a p p l i c a t i o n o f h i g h - s t r e n g t h l o w - d e n s i t y z i r c o n i a - s i l i c a t e p r o p p a n t s i s t h e r e f o r e more o r l e s s l i m i t e d t o some o n s h o r e a r e a s ( i n c l u d i n g MHF t r e a t m e n t s ) , b u t i s v e r y d i f f i c u l t t o e c o n o m i c a l l y j u s t i f y o f f s h o r e f o r more t h a n t a i l - i n j o b s ( a i m i n g i n many c a s e s on t h e p o s s i b i l i t y o f gamma-ray f r a c t u r e h e i g h t c o n t r o l m o n i t o r i n g by w e l l l o g g i n g o f n a t u r a l proppant r a d i o a c t i c i t y ( c f . s e c t i o n 6 . 2 . 1 . 2 . ) . A t e m p o r a r i l y b e t t e r s i t u a t i o n had a r i s e n i n Europe i n t h e e a r l y y e a r s o f a v a i l a b i l i t y o f s y n t h e t i c h i g h - s t r e n g t h p r o p p a n t s when a h i g h o i l p r i c e and a s t r o n g US $ p e r m i t t e d t r e a t m e n t e x e c u t i o n when t e c h n i c a l l y n e c e s s a r y and n o t when e c o n o m i c a l l y m o s t f e a s i b l e , w i t h i n t h o s e days r e p e a t e d l y having occurred t h a t purchase o f z i r c o n i a - s i l i c a t e proppants from t h e l o c a l s u p p l i e r i n F r a n c e t u r n e d o u t t o be t h e b e t t e r s o l u t i o n a t t h e b o t t o m o f t h e l i n e t h a t t o w a i t f o r a v a i l a b i l i t y o f s i n t e r e d b a u x i t e o r t o pay t h e a i r
93 f r e i g h t premium t o f l y i n t h e m a t e r i a l f r o m t h e p r o d u c t i o n p l a n t i n t h e USA t o t h e w e l l s i t e i n Europe. On t h e o t h e r hand, t h e major t e c h n i c a l advantage o f z i r c o n i a - s i l i c a t e proppants i s t h e low s p e c i f i c g r a v i t y o f t h a t h i g h - s t r e n g t h mater i a l which i s even lower t h a n t h a t o f some o f t h e i n t e r m e d i a t e - s t r e n g t h h i g h d e n s i t y proppants ( c f . s e c t i o n 1 . 3 . 2 . ) . The h i g h p r i c e i n c o m b i n a t i o n w i t h t h e v e r y l i m i t e d mechanical and chemical s t r e s s r e s i s t i v i t y a l s o t r i g g e r e d t h e disappearance o f t h e expensive g l a s s beads ( c f . s e c t i o n 1 . 2 . 3 . 1 . ) i n f o r m e r y e a r s when t h e cheaper and h a r d e r c e r a mic proppants were i n t r o d u c e d t o t h e o i l and gas market.
1.5.2. Non-oil- and gas-field applications 8/12 and 12/20 h i g h - s t r e n g t h b a u x i t e proppants a r e a l s o a p p l i e d o u t s i d e o f t h e o i l and gas market p r e d o m i n a n t l y as heat-exchange ( s t a t i c e n v i r o n m e n t ) and h e a t - t r a n s f e r (dynamic m i l i e u ) p e l l e t s i n c u p o l a f u r n a c e s i n t h e s t e e l f o u n d r y market, f u r n a c e s i n alumina and g l a s s i n d u s t r y , and f u r n a c e s f o r hydrocarbon combustion ( a i m i n g on r e t r i e v a l o f r e s i d u a l h e a t ) , as w e l l as d u s t a b s o r p t i o n o r f i l t e r i n g p e l l e t s i n t h e gas p u r i f i c a t i o n market. Other n o n - o i l - and gasf i e l d a p p l i c a t i o n s o f ceramic proppants o f v a r i o u s t y p e and g r a i n s i z e a r e r e s i n , polymer, w a t e r and gas f i l t r a t i o n ; s h o t b l a s t i n g by h a r d e n i n g s t a i n l e s s s t e e l by peening r a t h e r t h a n by c u t t i n g , f l u i d bed r e a c t i o n s , wear r e s i s t a n c e , metal d e b u r r i n g , f i l l i n g , g r i n d i n g , i n s u l a t i o n and cement c o n s t i t u e n t s , w i t h most o f these market segments a t t h e moment i n Europe s t i l l b e i n g i n an immatur e stage, i n c o n t r a s t t o t h e USA where up t o a few p e r c e n t o f t h e t o t a l annual ceramic p r o p p a n t s a l e s i s made t o i n d u s t r i e s o t h e r than o i l and gas companies already since longer time. As a consequence o f t h e i r s i g n i f i c a n c e i n Europe f o r t h e development o f an i m p o r t a n t second market segment i n t h e coming y e a r s , f o u n d r y h e a t exhange and gas p u r i f i c a t i o n a p p l i c a t i o n s o f s i n t e r e d b a u x i t e proppant p e l l e t s a r e b r i e f l y o u t l i n e d as f o l l o w s .
1.5.2.1. Foundry heat exchange Heat exchange a p p l i c a t i o n o f s i n t e r e d b a u x i t e o f p r e f e r a b l y 12/20 g r a i n s i z e i n c u p o l a furnaces f o r h o t wind p r o d u c t i o n i n s t e e l f o u n d r i e s has been d e v e l o ped i n t h e l a s t t e n y e a r s ( f i r s t i n t r o d u c t i o n t o f o u n d r y i n d u s t r y i n 1976) as a means o f e f f e c t i v e h e a t r e t r i e v a l (KGT 1986). The c u p o l a f u r n a c e i s composed o f an upper gas p a r t and a l o w e r a i r p a r t . I n t h e upper gas p a r t , soaking i s done w i t h c o n s i d e r a b l e gas v e l o c i t y , whereas i n t h e lower a i r p a r t , c o l d a i r i s i n j e c t e d w i t h h i g h p r e s s u r e . Hot b l a s t - f u r n a c e gas comes from t h e a d j o i n i n g b u r n i n g chamber and p e n e t r a t e s a c y l i n d r i c a l tower c o n t a i n i n g s e v e r a l d i s t r i b u t i o n d i s k s which a r e p e r f o r a t e d by numerous small h o l e s and a l s o c o n t a i n some l a r g e r o r i f i c e s . Gas v e l o c i t y i s a d j u s t e d i n a way t h a t t h e proppants o r p e l l e t s a r e k e p t i n suspension as a f l u i d i z e d bed between t h e d i s k s , b u t w i t h t i m e s l o w l y pass downwards t h r o u g h t h e o r i f i c e s t o t h e n e x t d i s k below, whereas t h e gas stream moves upwards. Heat t r a n s i t i o n i s o p t i m i z e d by c o n t r o l l e d r e m a i n i n g t i m e o f t h e p e l l e t s b e t ween t h e d i s t r i b u t i o n d i s k s due t o e x a c t c a l i b r a t i o n o f t h e gas stream. Having taken up s u f f i c i e n t h e a t d u r i n g t h e i r way downwards, t h e p e l l e t s f i n a l l y pass v i a a g a s - t i g h t t r a n s i t i o n from t h e upper h o t gas p a r t i n t o t h e lower c o l d a i r p a r t where t h e y a r e c o o l e d down by t r a n s f e r r i n g t h e i r h e a t t o t h e c o l d wind ( w h i c h i s t h e r e b y heated up and l e a v e s t h e chamber as h o t w i n d g o i n g i n t o t h e foundry process system) and t h e n a r e l i f t e d t o t h e t o p o f t h e c u p o l a f u r n a c e where t h e c i r c u i t s t a r t s a g a i n w i t h e n t r y o f t h e p e l l e t s i n t o t h e d i s k tower o f t h e upper gas p a r t . The heated p e l l e t s f o r m a g a s - t i g h t c l o s u r e between upper gas p a r t and lower a i r p a r t even a t h i g h d i f f e r e n t i a l p r e s s u r e s . E f f e c t s of proppant o r p e l l e t t y p e and g r a i n s i z e a r e d i s c u s s e d i n t h e f o l l o w i n g s e c t i o n s .
94
1.5.2.1.1. Pellet type The e f f e c t i v i t y o f h e a t r e t r i e v a l i n c u p o l a f u r n a c e s t h r o u g h h e a t exchange b y t h e s i n t e r e d b a u x i t e p e l l e t s i s much h i g h e r t h a n i n c o n v e n t i o n a l r e c u p e r a t o r s w h i c h s u f f e r f r o m t h e p r o b l e m o f s i n t e r i n g o f d u s t on b r i c k l i n i n g and p i p e s , whereas t h e p e l l e t s y s t e m i n t h e c u p o l a f u r n a c e i n c l u d e s s e l f - c l e a n i n g by l a t e r removal o f t h e d u s t s i n t e r e d o n t o t h e p e l l e t s u r f a c e d u r i n g c o o l i n g and g r a v i t a t i o n a l s e g r e g a t i o n o f d u s t and p e l l e t s i n t h e l i f t system, t h e r e b y a l s o s a v i n g c o n s i d e r a b l e m a i n t e n a n c e work and expenses. Concerning proppant type s e l e c t i o n , h i g h - s t r e n g t h alumina oxide proppants ( s i n t e r e d b a u x i t e ) a r e much more s u i t a b l e t h a n i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s due t o t h e i r m i n e r a l o g i c a l h o m o g e n e i t y , c h e m i c a l i n e r t n e s s and h i g h e r s p e c i f i c d e n s i t y . The m i n e r a l o g i c a l h o m o g e n e i t y o f s i n t e r e d b a u x i t e b e i n g a l m o s t p u r e corundum p r o v i d e s a b e t t e r p e l l e t s i n t e r i n g d u r i n g m a n u f a c t u r i n g and t h u s h i g h e r m e c h a n i c a l and t h e r m o d y n a m i c a l s t a b i l i t y w i t h a l o w e r p o t e n t i a l o f e x p l o s i o n and c r a c k i n g of g r a i n s d u r i n g r e p e a t e d h e a t i n g and c o o l i n g , r e s p e c t i v e l y ( e x p e r i e n c e has shown t h a t t h e s i n t e r e d b a u x i t e p e l l e t s a r e s u f f i c i e n t l y r e s i s t a n t a g a i n s t breakage i n t h e range o f a p p l i e d c h a n g i n g t e m p e r a t u r e s w h i c h g o up t o 900 - 1,000 o C ) . The c h e m i c a l i n e r t n e s s due t o s c a r c i t y t o a l m o s t l a c k o f s i l i c a p r e v e n t s s o l u t i o n i n t h e h o t a l c a l i n e e n v i r o n m e n t ( s i m i l a r l y as i n s t e a m - d r i v e a r e a s i n o i l f i e l d s when u s e d f o r g r a v e l packing and/or h y d r a u l i c proppant f r a c t u r i n g ; c f . s e c t i o n 5 . 6 . ) and t h u s a l s o enhances p e l l e t s t a b i l i t y . The h i g h e r s p e c i f i c d e n s i t y a m e l i o r a t e s t h e e f f e c t i v e n e s s o f g r a v i t a t i o n a l s e g r e g a t i o n o f p e l l e t s and d u s t i n t h e l i f t s y s t e m where t h e d u s t i s s e p a r a t e d and removed.
1 . 5 . 2 . 1 . 2 . Pellet grain size I n t e r m s o f g r a i n s i z e c h o i c e , 12/20 s i n t e r e d b a u x i t e has so f a r p r o v e n t o be t h e optimum f o r t h e f o u n d r y h e a t exchange a p p l i c a t i o n . S m a l l e r p e l l e t s gener a l l y r e s u l t i n b e t t e r h e a t t r a n s i t i o n t h a n l a r g e r ones, b u t i f t h e y a r e t o o s m a l l , however, gas v e l o c i t y in t h e c u p o l a f u r n a c e i s t o o h i g h and t h e p e l l e t s a r e t r a n s p o r t e d a g a i n s t t h e s t r e a m upwards t o t h e n e x t d i s k and a r e e v e n b l o w n o u t w i t h t h e f l u e gas c u r r e n t ( t h e same happens when p e l l e t s o f t o o l o w s p e c i f i c d e n s i t y a r e used, as e v i d e n c e d by a c c i d e n t a l f i e l d t e s t s w i t h t h e l i g h t e r a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s i n s t e a d o f t h e h e a v i e r s i n t e r e d b a u x i t e p e l l e t s ) . O p t i m i z a t i o n o f t h e p r o c e s s has t o be l i m i t e d t o changes o f p e l l e t s i z e and s p e c i f i c d e n s i t y , because t h e gas v e l o c i t y i s more o r l e s s f i x e d t o g u a r a n t e e t h e r e q u i r e d m e l t i n g c a p a c i t y o f t h e c u p o l a f u r n a c e and i s i n d i r e c t l y p r o p o r t i o n a l t o t h e r a w i r o n t h r o u g h p u t . L a r g e r and h e a v i e r p e l l e t s l e a d t o b e t t e r s e g r e g a t i o n f r o m d u s t and a l s o p r e v e n t l o s s o f m a t e r i a l !,. b l o w o u t a t t h e top. C o n c e r n i n g t h e g a s - t i g h t t r a n s i t i o n between u p p e r gas p a r t and l o w e r a i r p a r t o f t h e c u p o l a f u r n a c e , an u n i m o d a l p e l l e t g r a i n s i z e d i s t r i b u t i o n such as t h e n a r r o w l y - s c r e e n e d 12/20 s i n t e r e d b a u x i t e g i v e s s a t i s f a c t o r y s e p a r a t i o n . A b i m o d a l p e l l e t p o p u l a t i o n w o u l d r e s u l t i n an even b e t t e r t i g h t n e s s due t o p l u g g i n g o f t h e i n t e r s t i t i a l s between t h e c o a r s e g r a i n s b y t h e f i n e g r a i n s , b u t t h e p r o b l e m w o u l d be g r a v i t a t i o n a l s e g r e g a t i o n o f b o t h g r a i n s i z e c l a s s e s i n t h e d i s k t o w e r i n t h e u p p e r gas p a r t b e f o r e t h e assemblage r e a c h e s t h e t r a n s i t i o n a t t h e end o f i t s downwards movement, and a l s o d u r i n g r e c y c l i n g and c l e a n i n g o f t h e g r a i n s which are l i f t e d t o t h e t o p o f t h e d i s k tower system w i t h d i f f e r e n t t r a n s p o r t v e l o c i t i e s due t o t h e i r d i f f e r e n t w e i g h t .
95
1.5.2.1.3. Alumina and glass industry heat exchange A s i m i l a r h e a t exchange a p p l i c a t i o n of s i n t e r e d b a u x i t e p e l l e t s as i n c u p o l a f u r n a c e s i n t h e s t e e l f o u n d r y i n d u s t r y i s performed i n alumina m e l t i n g f u r n a ces, hydrocarbon combustion furnaces, and g l a s s i n d u s t r y f u r n a c e s . W h i l e t h e h e a t exchange a p p l i c a t i o n i s a l r e a d y common p r a c t i c e i n t h e USA s i n c e many y e a r s and t h e whole package o f v a r i o u s n o n - o i l - and g a s - f i e l d proppant a p p l i c a t i o n s comprises up t o a c o u p l e o f p e r c e n t o f t h e t o t a l annual p r o p p a n t s a l e s l e v e l , t h e t e c h n i q u e i s i n Europe s t i l l i n t h e i n i t i a l stages and t h e market segment i s d e v e l o p i n g w i t h i n t h e n e x t few y e a r s t o p r o b a b l y a l s o reach an import a n t second s e c t o r o f usage o f m a t e r i a l w i t h a p o s s i b l y even h i g h e r share o f m a r k e t i n g p o t e n t i a l t h a n i n t h e USA p a r t i c u l a r l y i n l i g h t o f t h e expected b o o s t i n g o f t h e f o u n d r y c u p o l a f u r n a c e market segment.
1.5.2.2. Mechanical gas purification Mechanical gas p u r i f i c a t i o n i s performed i n t h e cement i n d u s t r y w i t h g r a v e l bed f i l t e r s (LURGI 1983) where s i n t e r e d b a u x i t e o r g l a s s bead p e l l e t s o f a b t . 6/10 o r s u b o r d i n a t e l y a l s o 8/12 o r 12/20 g r a i n s i z e a r e needed. S i n t e r e d b a u x i t e has t h e advantage t o be m e c h a n i c a l l y more r e s i s t a n t and t o be c o n s i d e r a b l y cheaper t h a n g l a s s beads. The rough b a u x i t e s u r f a c e i n comparison t o t h e smooth g l a s s s u r f a c e a l s o g i v e s a b e t t e r performance p a r t i c u l a r l y d u r i n g c l e a n i n g o f t h e p e l l e t s f r o m a d h e r i n g d u s t . D i s t i n c t i o n can be made between o p e r a t i o n a l and c l e a n i n g stage o f g r a v e l - b e d - f i l t e r s i n mechanical gas p u r i f i c a t i o n p l a n t s .
1.5.2.2.1. Operational stage A t t h e end o f t h e heat-exchanging r o t a r y k i l n i n t h e cement i n d u s t r y process l i n e , t h e f u r n a c e and t h e n t h e f i l t e r a r e i n s t a l l e d . The f i l t e r i s a combinat i o n between a mechanical s e p a r a t o r ( p r e c l e a n i n g c y c l o n e ) and t h e t u b e - t y p e g r a v e l - b e d f i l t e r ( g r a n u l a t e - t u b e - f i l t e r ; LURGI 1983) where t h e p e l l e t s a r e e n c l o sed between two w i r e n e t s . The g r a v e l - b e d f i l t e r p r i n c i p a l l y has t o f u l f i l l t h r e e d i f f e r e n t f u n c t i o n s i n t h e process : p u r i f i c a t i o n o f t h e gas by separat i o n o f dust, maintenance o f t r a n s p o r t a b i l i t y i n t h e system ( t h e p e l l e t s have t o possess s u f f i c i e n t buoyancy i n t h e gas c u r r e n t ) , and p o s s i b i l i t y o f c l e a n i n g t h e p e l l e t s f r o m t h e a d h e r i n g d u s t . The raw gas i s c l e a n e d f i r s t i n t h e c y c lone, w i t h t h e r e m a i n i n g f i n e d u s t t h e n second b e i n g t r a p p e d i n t h e g r a n u l a t e f i l t e r where i t s t i c k s t o t h e s u r f a c e o f t h e p e l l e t s p r e f e r a b l y i n t h e f i r s t q u a r t e r o f t h e g r a v e l bed t h i c k n e s s . W i t h time, t h e g r a v e l - b e d f i l t e r i s t o o much s a t u r a t e d w i t h d u s t and then c l e a n i n g has t o be made.
1.5.2.2.2. Cleaning stage The whole p l a n t c o n s i s t s o f a b t . e i g h t moduls which a r e r u n n i n g s e p a r a t e l y . The o p e r a t i o n i n c l u d e s simultaneous p u r i f i c a t i o n o f t h e gas i n a b t . f i v e t o s i x moduls, and t h e o t h e r moduls a r e d u r i n g t h a t t i m e i n c l e a n i n g stzr;e. Once t h e y a r e p u t back on stream, o t h e r moduls a r e taken o u t f r o m p u r i , i c a t i o n and a r e cleaned. C l e a n i n g i s performed by b l o w i n g compressed a i r i n t o t h e modul and i n s t a l l i n g a v e n t i l a t o r w i t h a c y c l o n e a t t h e t o p . The d u s t - b e a r i n g p e l l e t granul a t e i s blown upwards where g r a v i t a t i o n a l s e p a r a t i o n t a k e s p l a c e , w i t h t h e heav i e r p e l l e t s f a l l i n g back t o t h e ground on t h e g r a v e l - b e d f i l t e r and t h e l i g h t e r d u s t b e i n g blown o u t . The g r a i n s i z e o f t h e p e l l e t s i n t h e g r a v e l bed i s determined by t h e s p a c i n g o f the holes i n the w i r e nets t h a t enclose the granulate. Sintered bauxite i s more s u i t a b l e t h a n i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants, because t h e s p e c i f i c a l l y denser m a t e r i a l has a b e t t e r s e g r e g a b i l i t y a t c l e a n i n g stage ( s i m i l a r l y as t h e p e l l e t s i n t h e l i f t i n g system o f t h e c u p o l a furnace; c f . s e c t i o n 1 . 5 . 2 . 1 . ) . I f t h e y a r e t o o heavy, t h e gas c u r r e n t
96 r a t e has t o be r e a d j u s t e d . I n c o n t r a s t t o t h e d y n a m i c a l s t r e s s i n t h e above d e s c r i b e d g r a v e l - b e d f i l t e r , o t h e r systems o p e r a t e w i t h p e l l e t s t h a t a r e s u b j e c t e d t o o n l y s t a t i c a l stress.
1 . 6 . Conclusion and outlook H y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g o f o i l - and g a s - b e a r i n g f o r m a t i o n s w i t h v a r i o u s s p e c i f i c p r o b l e m s c a n be done w i t h f i v e d i f f e r e n t p r o p p a n t t y p e s w h i c h a r e a v a i l a b l e i n s e v e r a l g r a i n s i z e s and a t d i f f e r e n t p r i c e s i n t h e mark e t . Depending on m i n e r a l o g i c a l and c h e m i c a l c o m p o s i t i o n as w e l l as m e c h a n i c a l and c h e m i c a l p r o p e r t i e s , t h e v a r i o u s p r o p p a n t s c a n be a p p l i e d i n d i f f e r e n t p r e s sure, t e m p e r a t u r e and b r i n e r e g i m e s , w i t h c l o s u r e s t r e s s b e i n g t h e m o s t i m p o r t d n t f a c t o r c o n t r o l l i n g p r o p p a n t s e l e c t i o n . The p r e d o m i n a n t a p p l i c a t i o n i n Europe comprises intermediate- t o high-strength proppants (and p a r t i a l l y a l s o sand i n c o m b i n a t i o n w i t h o r i n some c a s e s e v e n as r e p l a c e m e n t o f s y n t h e t i c p r o p p a n t s ) f o r h y d r a u l i c f r a c t u r i n g o f R o t l i e g e n d and C a r b o n i f e r o u s deep t i g h t g a s b e a r i n g s a n d s t o n e s i n Germany FRG and N e t h e r l a n d s o n s h o r e and B r i t i s h and D u t c h S o u t h e r n N o r t h Sea o f f s h o r e . The m a i n p r o p p a n t a p p l i c a t i o n s i n h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a r e i n t h e f i e l d s o f o i l p r o d u c t i o n , w a t e r o f f t a k e and i n j e c t i o n , s h a l l o w and deep g a s w i t h d r a w a l , and s t e a m - d r i v e enhanced o i l r e c o v e r y . O t h e r p r o p p a n t u t i l i z a t i o n s a r e i n t h e m a r k e t segments o f f r a c t u r i n g f l u i d - l o s s a d d i t i v e s , g e o t h e r m a l w e l l s t i m u l a t i o n and h e a t e x t r a c t i o n , c o a l seam d e g a s i f i c a t i o n , f o u n d r y c u p o l a f u r n a c e h e a t t r a n s f e r , and m e c h a n i c a l gas purification. Based on t h e p r e s e n t d i s c u s s i o n o f a s p e c t s o f p r o p p a n t c h o i c e f o r v a r i o u s app l i c a t i o n s f r o m p r e d o m i n a n t l y t e c h n i c a l p o i n t o f v i e w , a more d e t a i l e d a c c o u n t o f m a r k e t i n g and e c o n o m i c a l a s p e c t s o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g w i t h d e t a i l e d r e v i e w , s t a t u s , f o r e c a s t and p e r s p e c t i v e s o f s t i m u l a t i o n a c t i v i t y i n W e s t e r n and E a s t e r n Europe i s g i v e n i n c h a p t e r 2, t e c h n i c a l and m a r k e t i n g a s p e c t s o f R o t l i e g e n d and C a r b o n i f e r o u s s t i m u l a t i o n in E u r o p e a r e s k e t c h e d i n sect i o n 3 as a t r a n s i t i o n f r o m t h e e c o n o m i c a l l y i n f l u e n c e d d i v i s i o n s t o t h e t e c h n i c a l l y c o n c e n t r a t e d p a r t s , and v a r i o u s t e c h n i c a l and r e s e r v o i r e n g i n e e r i n g p o s s i b i l i t i e s o f p r o m o t i n g and e n h a n c i n g t h e f r a c t u r i n g p o t e n t i a l , p a r t i c u l a r l y i n m a r g i n a l o i l - and g a s - b e a r i n g f o r m a t i o n s , a r e o u t l i n e d i n c h a p t e r 4. S e c t i o n 5 p r e s e n t s an a c c o u n t o f g r a v e l p a c k i n g , and d i v i s i o n 6 summarizes h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g m o n i t o r i n g b y w e l l l o g g i n g and o t h e r t e c h n i ques. The b i b l i o g r a p h y i s c o m p i l e d as s u b j e c t k e y c l a s s i f i c a t i o n and as g e n e r a l reference l i s t i n chapter 7 . H y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g o f o i l and a a s r e s e r v o i r s i s c o n s i d e r e d t o be a g r o w i n g b r a n c h o f t h e s t i m u l a t i o n m a r k e t i n E u r o p e and w i l l a l s o i n t h e f u t u r e r e m a i n t h e m o s t i m p o r t a n t m a r k e t segment o f a p p l i c a t i o n o f t h e v a r i o u s t y p e s o f c u r r e n t l y c o m m e r c i a l l y m a n u f a c t u r e d and d i s t r i b u t e d p r o p p a n t t y p e s and g r a i n s i z e s , w i t h t h e l a r g e s t q u a n t i t i e s o f m a t e r i a l b e i n g c o n t i n u e d t o be consumed b y MHF t r e a t m e n t s o f deep t i g h t gas sands. E x p e r i m e n t a l a p p r o a c h e s d u r i n g t h e l a s t y e a r s have e v e n d e m o n s t r a t e d t h a t p r o v i d e d an a d e q u a t e demand b y t h e m a r k e t , some new p r o p p a n t t y p e s o f enhanced t e c h n i c a l p e r f o r m a n c e and b e t t e r e c o n o m i c a l f e a s i b i l i t y may be d e v e l o p e d i n t h e coming y e a r s t o a d j u s t t o an i n c r e a s i n g l y d i v e r s i f i e d s p e c t r u m o f f i e l d a p p l i c a t i o n .
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2 . M a r k e t i n g 2.0.Summary
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H y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n Western and E a s t e r n Europe t a k e s p l a c e i n d i f f e r e n t t y p e s o f r e s e r v o i r s and under d i f f e r e n t g e o l o g i c a l , eng i n e e r i n g , t e c h n i c a l , marketing, economical and p o l i t i c a l c o n d i t i o n s . I n compar i s o n t o t h e USA, g e n e r a l i n f l u e n c e s on h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g such as changes o f hydrocarbon p r i c e s and proppant p r i c e s have i n Europe more s e r i o u s and a l s o more d i v e r s i f i e d consequences f o r t h e o v e r a l l s t i m u l a t i o n scen e r y . The impacts o f r e d u c t i o n o f hydrocarbon p r i c e s and p r o p p a n t p r i c e s d u r i n g t h e l a s t few y e a r s a r e i n Europe even accentuated by t h e d e c l i n e o f t h e US $ exchange r a t e ( w i t h r e s p e c t t o o t h e r w o r l d g u i d e c u r r e n c i e s such as DM and L ) . The t e c h n o l o g i c a l e v o l u t i o n o f m a n u f a c t u r i n g o f v a r i o u s p r o p p a n t t y p e s d u r i n g t h e l a s t t e n y e a r s took p l a c e p a r a l l e l t o s e v e r a l m a j o r changes o f t h e h y d r o c a r bon p r i c e s c e n a r i o and t h e US $ exchange r a t e s c e n a r i o i n e i t h e r d i r e c t i o n . The m a j o r o i l p r i c e drop i n l a t e 1 9 8 5 / e a r l y 1986 h i t t h e ( m a i n l y Western) European s t i m u l a t i o n m a r k e t much s t r o n g e r than t h a t i n t h e USA due t o v a r i o u s reasons c o m p r i s i n g amount o f d r i l l i n g r i g s , frequency o f s t i m u l a t i o n jobs, o p e r a t i n g m e n t a l i t y and p h i l o s o p h y , p r o p p a n t p r i c e a t t h e b o t t o m o f t h e l i n e , c o m b i n a t i o n w i t h f a l l i n g US $ value, and p r o p p a n t m a r k e t i n g a c t i v i t y . The o i l p r i c e h i s t o r y o f t h e l a s t 20 y e a r s comprises t h e 1973 and 1979 o i l p r i c e e x p l o s i o n s , t h e 1986 o i l p r i c e c o l l a p s e , and t h e 1987 and 1988 o i l p r i c e weakness. W h i l e p r i o r t o t h e i n v e n t i o n and market i n t r o d u c t i o n o f h i g h - s t r e n g t h s y n t h e t i c proppants i n 1976 p o s s i b i l i t i e s o f s t i m u l a t i o n o f deep h i g h - p r e s s u r e r e s e r v o i r s have been v e r y l i m i t e d and t h e f i r s t y e a r s o f a v a i l a b i l i t y o f a r t i f i c i a l h i g h - q u a l i t y proppants s u f f e r e d f r o m s h o r t a g e i n s u p p l y and e l e v a t e d p r i c e s o f t h e new p r o ducts, t h e e a r l y 1980's became t h e golden y e a r s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g due t o performance o f numerous t r e a t m e n t s on v a r i o u s s c a l e i n t h e economic a l framework o f a h i g h o i l p r i c e and a s t r o n g US $. The 1986 o i l p r i c e c r a s h c o m p l e t e l y r e v e r s e d o i l p r i c e e v o l u t i o n and s t i m u l a t i o n market p o t e n t i a l . I n c o n t r a s t t o t h e USA, many p a r t s o f Europe a r e so f a r i n terms o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n s u f f i c i e n t l y a t t e n d e d markets where a l a r g e p o t e n t i a l e x i s t s w i t h numerous p o s s i b i l i t i e s o f s t i m u l a t i o n , b u t c o m p l i c a t e d g e o l o g i c a l and eng i n e e r i n g r e s e r v o i r c o n d i t i o n s , many d i f f e r e n t m e n t a l i t i e s and a d m i n i s t r a t i v e i n t e r l o c k i n g r e q u i r e much more t i m e and a c t i v i t y , w i t h p a r t i c u l a r n e c e s s i t y bei n g t h e development o f a sound m a r k e t i n g s t r a t e g y which i n t e g r a t e s g e o l o g i c a l , r e s e r v o i r e n g i n e e r i n g , t e c h n i c a l and economical concepts. The main d i f f e r e n c e s o f t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g market between USA and Europe comprise abs o l u t e and r e l a t i v e f r e q u e n c y o f d r i l l i n g and s t i m u l a t i o n , s u c c e s s / f a i l u r e r a t i o and c o n s o r t i a 1 s t r u c t u r e , o p e r a t i n g m e n t a l i t y and t r e a t m e n t c o s t , i n t e g r a t i o n o f s t i m u l a t i o n i n t o t h e t o t a l w e l l concept, and proppant m a r k e t i n g s t r a t e gy. L a c k i n g c o n t i n u i t y i n m a r k e t i n g a t t e n t i o n , m i s s i n g u n d e r s t a n d i n g o f t h e mark e t requirements, and f a i l u r e t o f o r e s e e d e v e l o p i n g market t r e n d s has i n Europe i n t h e l a s t y e a r s r e p e a t e d l y r e s u l t e d i n s e r i o u s business l o s s e s f o r p r o p p a n t companies b o t h t o p r o p p a n t c o m p e t i t i o n and n a t u r a l sand. A r e a l market e v a l u a t i o n s a r e g i v e n f o r b o t h Western Europe and E a s t e r n Europe f o r t h e p e r i o d s 1977 - 1985, 1986 ( p a r t i a l l y i n c l u d i n g t h e f i r s t h a l f o f 1987), and 1987 ( c o m p r i s i n g m a i n l y t h e second h a l f ) - 1990 (and a l s o e x t e n d i n g f u r t h e r i n t o t h e f u t u r e ) , t h u s c o n t a i n i n g review, s t a t u s , f o r e c a s t and p e r s p e c t i v e s . Emphasis i s a l s o p u t on economical, commercial, p o l i t i c a l and i d e o l o g i c a l aspects o f h y d r a u l i c p r o p p a n t f r a c t u r i n g e s p e c i a l l y i n E a s t e r n Europe. I n Western Europe, t h e main t a r g e t s o f s t i m u l a t i o n b y h y d r a u l i c p r o p p a n t f r a c t u r i n g i n p a s t and f u t u r e a r e deep l o w - p e r m e a b i l i t y g a s - b e a r i n g R o t l i e g e n d (Lower Permian) and C a r b o n i f e r o u s sandstones i n Germany FRG and N e t h e r l a n d s onshore as w e l l as i n B r i t i s h and Dutch Southern N o r t h Sea o f f s h o r e , w i t h m a i n l y i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants b e i n g r e q u i r e d due t o h i g h e r c l o s u r e s t r e s s e s i n t h e e a s t e r n p a r t o f t h e p r o s p e c t i v e R o t l i e g e n d and C a r b o n i f e r o u s b e l t , whereas i n i t s w e s t e r n p a r t , l o w e r c l o s u r e s t r e s s e s a l s o p e r m i t t h e a p p l i c a t i o n o f n a t u r a l sand i n combinat i o n w i t h o r even i n replacement o f s y n t h e t i c proppants. The t a i l - i n f r a c t u r i n g s t r a t e g y c o m p r i s i n g pumping o f s m a l l e r q u a n t i t i e s o f i n t e r m e d i a t e - o r h i g h -
98 s t r e n g t h p r o p p a n t s in t h e second s t a g e o f t h e t r e a t m e n t f o l l o w i n g i n j e c t i o n o f l a r g e r amounts o f sand i n t h e f i r s t s t a g e o f t h e o p e r a t i o n seems t o be t h e m o s t s u i t a b l e t e c h n i c a l and e c o n o m i c a l s t i m u l a t i o n d e s i g n f o r many B r i t i s h S o u t h e r n N o r t h Sea R o t l i e g e n d r e s e r v o i r s , w i t h t h e a d v a n t a g e s b e i n g c u t t i n g c o s t and inc r e a s i n g e f f e c t i v i t y o f h y d r a u l i c f r a c t u r i n g b y m i n i m i z i n g p r o p p a n t expenses and m a x i m i z i n g c l o s u r e s t r e s s r e s i s t i v i t y i n t h e v i c i n i t y o f t h e w e l l b o r e , r e s p e c t i v e l y . The R o t l i e g e n d and C a r b o n i f e r o u s MHF o p e r a t i o n s a r e c a r r i e d o u t w i t h p r o p p a n t q u a n t i t i e s r a n g i n g f r o m 100 t u p t o 650 t i n t h e l a r g e s t t r e a t m e n t s . A p a r t f r o m R o t l i e g e n d and C a r b o n i f e r o u s , B u n t s a n d s t e i n ( L o w e r T r i a s s i c ) gas r e s e r v o i r s a l s o r e q u i r e h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n i n p a r t s o f t h e m e n t i o n e d a r e a s . S m a l l f r a c t u r i n g j o b s w i t h 20 - 70 t o f sand have in t h e p a s t been m a i n l y done i n s h a l l o w o i l - b e a r i n g C r e t a c e o u s s a n d s t o n e s i n German y FRG and N e t h e r l a n d s as w e l l as T e r t i a r y s a n d s t o n e s i n A u s t r i a , C z e c h o s l o v a k i a , Y u g o s l a v i a and H u n g a r y . I n t h e N o r w e g i a n and s u b o r d i n a t e l y a l s o B r i t i s h N o r t h e r n N o r t h Sea, u n s t a b l e o i l - b e a r i n g C r e t a c e o u s c h a l k s and J u r a s s i c s a n d s t o nes r e q u i r e g r a v e l p a c k i n g a n d / o r h y d r a u l i c f r a c t u r i n g w i t h i n t e r m e d i a t e s t r e n g t h l o w - d e n s i t y p r o p p a n t s a n d / o r sand. P a r t i c u l a r l y s i g n i f i c a n t a r e combin a t i o n s o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g w i t h b o t h c o n v e n t i o n a l and r e s i n - c o a t e d l o w - d e n s i t y i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s i n some s o f t f l o w i n g c h a l k s i n undercompacted and o v e r p r e s s u r e d r e s e r v o i r s . The d r a m a t i c a l e f f e c t o f t h e o i l p r i c e d e c l i n e i n l a t e 1 9 8 5 / e a r l y 1986 ( w h i c h had an e v e n more s e v e r e and t r a g i c a l i m p a c t due t o t h e c o m b i n a t i o n w i t h a s e r i o u s d r o p o f t h e US $ e x change r a t e ) was i n Western Europe even a c c e n t u a t e d b y an i n c r e a s e d c a n c e l l a t i o n r a t e o f p l a n n e d t r e a t m e n t s due t o s e v e r a l t e c h n i c a l r e a s o n s r e s u l t i n g f r o m a m i s m a t c h o f p r e d i c t i o n and p e n e t r a t i o n and a l s o due t o p a r t i a l l y l o n g d r i l l i n g times e x t e n d i n g over several p e r i o d s w i t h d i f f e r e n t general economical f r a mework. P r o p p a n t p r i c e e v o l u t i o n i n t h e l a s t f i v e y e a r s i n c l u d e s t h e 1985 p r o p p a n t p r i c e l o w e r i n g and t h e 1987 p r o p p a n t p r i c e i n c r e a s e . I n a d d i t i o n t o p r i c e r e d u c t i o n s and r i s e s e n a c t e d b y s u p p l y i n g companies, f l u c t u a t i o n s i n U S $ e x change r a t e p r o v o k e a l t e r a t i o n s o f c o m p e t i t i v e n e s s o f v a r i o u s p r o p p a n t t y p e s . As a consequence o f t h e 1985 p r o p p a n t p r i c e c u t and t h e 1986 o i l p r i c e d r o p , a much h i g h e r - q u a l i t y p r o p p a n t m a t e r i a l i s now a v a i l a b l e i n s i g n i f i c a n t l y l a r g e r q u a n t i t i e s a t a c o n s i d e r a b l y l o w e r p r i c e t h a n some y e a r s ago, b u t i s s e r i o u s l y l i m i t e d i n a p p l i c a t i o n due t o a p r e s e n t l y p o o r e c o n o m i c a l framework p a r t i c u l a r l y in Europe, whereas some y e a r s ago, a much l o w e r - q u a l i t y p r o p p a n t m a t e r i a l t h a t was o f f e r e d a t a m a r k e d l y h i g h e r p r i c e t h a n now was v i r t u a l l y u n r e s t r i c t e d i n d i s t r i b u t i o n due t o a s t r o n g demand e x c e e d i n g t h e p r e v i o u s c a p a c i t i e s o f s u p p l y . T h e r e f o r e t h e 1986 o i l p r i c e c o l l a p s e c o m p l e t e l y r e v e r s e d t h e s t i m u l a t i o n m a r k e t b y c h a n g i n g t h e s i t u a t i o n f r o m good t o e x c e l l e n t f e a s i b i l i t y o f hyd r a u l i c f r a c t u r i n g when o n l y i n s u f f i c i e n t amounts o f h i g h e r - q u a l i t y p r o p p a n t s were a v a i l a b l e a t h i g h p r i c e s t o p o o r a t t r a c t i v i t y o f h y d r a u l i c f r a c t u r i n g when more t h a n enough p r o p p a n t m a t e r i a l i s a v a i l a b l e a t l o w p r i c e s . The v a r i o u s p r i c e d r o p s and r i s e s f o r a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s had a l s o s e r i o u s i m p a c t s o n t h e i r c o m p e t i t i v e power w i t h r e s p e c t t o z i r c o n i a - s i l i c a t e p r o p p a n t s w h i c h k e p t a l m o s t a l l t n e t i m e t h e i r f o r m e r e l e v a t e d p r i c e p o s i t i o n due t o more e x p e n s i v e p r o d u c t i o n p r o c e s s , and t h a t a r e i n t e r m s o f r e a l p r i c e i n E u r o p e n o t i n f l u e n c e d b y v a r i a t i o n s o f t h e US $ exchange r a t e i n e i t h e r d i r e c t i o n . I n c o n t r a s t t o W e s t e r n Europe, t h e d i f f e r e n t e c o n o m i c a l s y s t e m in E a s t e r n E u r o p e l e a d s t o a more o r l e s s i n d e p e n d e n c y f r o m t h e o i l p r i c e s c e n a r i o and a l s o f r o m t h e US $ exchange r a t e s c e n a r i o w h i c h means t h a t b a s i c a l l y t h e d r i l l i n g and s t i m u l a t i o n programmes go ahead r e g a r d l e s s o f g e n e r a l e c o n o m i c a l c r i s e s i n W e s t e r n Europe and USA. I n E a s t e r n Europe, t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g h i g h l i g h t i s c u r r e n t l y Y u g o s l a v i a where MHF t r e a t m e n t s f o c u s on deep h o t g a s - and condens a t e - b e a r i n g T e r t i a r y s a n d s t o n e s and c o n g l o m e r a t e s , w i t h p r o p p a n t q u a n t i t i e s b e i n g needed u p t o 650 t p e r w e l l w h i c h a r e t h u s e x c e e d i n g p a r t i a l l y e v e n t h e l o t s pumped d u r i n g t h e l a r g e s t o p e r a t i o n s i n Western Europe. The MHF j o b s in Yug o s l a v i a a r e m a i n l y c a r r i e d o u t w i t h s m a l l e r amounts o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y p r o p p a n t s i n t h e i n i t i a l s t a g e and l a r g e r volumes o f h i g h - s t r e n g t h I n t h e o t h e r E a s t e r n European proppants i n t h e main stage o f t h e treatment. countries, so f a r m a i n l y s m a l l e r f r a c t u r e j o b s w i t h a l m o s t e x c l u s i v e l y sand as p r o p p i n g a g e n t s in q u a n t i t i e s u p t o 20 - 150 t have been c a r r i e d o u t . M a j o r s t i m u l a t i o n campaigns have a l s o been p e r f o r m e d i n T u r k e y . I n t e r m s o f e c o n o m i c a l
99 and f i n a n c i a l h a n d l i n g , t h e t r e a t m e n t s a r e i n E a s t e r n Europe o f t e n done by own s e r v i c e w i t h purchase o f equipment abroad, and h a r d c u r r e n c y payment i s enabled by f a v o u r a b l e e x p o r t volumes o r World Bank c r e d i t s . C o n s i d e r a b l e improvement o f t h e s t i m u l a t i o n p o t e n t i a l i n E a s t e r n Europe i s expected f r o m t h e p o l i t i c a l and economical r e o r g a n i z a t i o n under t h e g u i d e l i n e s o f r e c o n s t r u c t i o n ( p e r e s t r o i k a ) , a c c e l e r a t i o n ( u s k o r e n i e ) and t r a n s p a r e n c y ( g l a s n o s t ) i n USSR and sooner o r l a t e r a l s o s a t e l l i t e c o u n t r i e s , and f r o m t h e o b l i g a t i o n s o f f u l f i l l i n g t h e g o a l s o f o i l and gas p r o d u c t i o n as f i x e d i n t h e c u r r e n t f i v e y e a r p l a n . As o i l and gas a r e t h e m a j o r source o f h a r d c u r r e n c y f o r t h e USSR and because b o t h o i l p r i c e drop and US $ exchange r a t e c o l l a p s e r e s u l t i n l o w e r n e t r e t u r n s , t h e l o s ses have t o be compensated by s e l l i n g i n c r e a s i n g amounts o f hydrocarbons i n West e r n c o u n t r i e s . T h i s has been r e n d e r e d p o s s i b l e by meeting and o v e r s h o o t i n g o f t h e p l a n f o r e c a s t i n t h e l a s t y e a r s by achievement o f a h i s t o r i c a l r e c o r d l e v e l o f o i l p r o d u c t i o n . The p a r t i a l independence o f t h e USSR f r o m t h e w o r l d market and i t s e f f o r t s t o a m e l i o r a t e hydrocarbon s u p p l y s e c u r i t y a r e u n d e r l i n e d by t h e f a c t t h a t i n c o n t r a s t t o c o n s i d e r a b l e budget c u t t i n g i n t h e western hemisphere f o r d r i l l i n g and c o m p l e t i o n and r e s u l t i n g s e r i o u s l y l e s s e x p l o r a t i o n and development a c t i v i t y w i t h r e s p e c t t o o r i g i n a l p l a n s b e f o r e t h e 1986 o i l p r i c e crash, t h e USSR has i n 1986 even stepped up d r i l l i n g i n o r d e r t o r a p i d l y r e p l a c e m a t u r i n g o i l p r o d u c t i o n f r o m p r o g r e s s i v e l y d e p l e t e d f i e l d s . Because o f i t s r o l e as an overwhelming source o f h a r d currency, p e t r o l e u m development commands t o p p r i o r i t y i n t h e USSR a l s o a f t e r t h e 1986 o i l p r i c e drop, whereas a t t h e same t i m e i n t h e USA, p e t r o l e u m has f a l l e n t o t h e bottom o f t h e government a l p r i o r i t y l i s t . While o p e r a t o r s i n USA and Europe a r e r e l u c t a n t t o i n v e s t money i n new p r o j e c t s , t h e USSR has boosted i t s e f f o r t s t o s u s t a i n and expand o i l and gas p r o d u c t i o n and e x p o r t s . The l o g i c a l r e s u l t o f these enormous e f f o r t s r e g a r d l e s s o f w o r l d economical c o n s t e l l a t i o n s have been t h e o v e r p a s s i n g o f t h e p l a n goal f o r t h e annual o i l p r o d u c t i o n q u a n t i t y i n 1987 w i t h b r e a k i n g t h e e x p l o i t a t i o n r e c o r d s o pronouncedly t h a t o i l e x p o r t s reached a maximum. Conc e r n i n g m a r k e t i n g approach f o r s t i m u l a t i o n and o t h e r s e r v i c e s , t h e d i f f e r e n t l o g i s t i c a l and f i n a n c i a l s i t u a t i o n between Western and E a s t e r n Europe r e s u l t s i n a d i f f e r e n t s t r a t e g y o f t r e a t m e n t o p e r a t i o n . While i n Western Europe and var i o u s o t h e r p a r t s o f t h e w o r l d o i l and gas p r o d u c t i o n companies commonly ask est a b l i s h e d s e r v i c e companies o f m a i n l y N o r t h American provenance t o c a r r y o u t t h e whole s t i m u l a t i o n j o b s i n c l u d i n g s u p p l y o f a l l t h e mechanical and chemical a d d i t i v e s f o r t h e o p e r a t i o n , i n E a s t e r n Europe t h e p r o d u c t i o n companies o f t e n p r e f e r t o buy t h e i r own equipment, t o purchase most o r a l l o f t h e m a t e r i a l d i r e c t l y , and t o do t h e j o b s w i t h o u t t h e s u p p o r t o f Western s e r v i c e companies a f t e r p r o b a b l y a few i n t r o d u c t o r y , t r a i n i n g o r p i l o t t r e a t m e n t s . V a r i o u s s t i m u l a t i o n p r o j e c t s i n b o t h Western and E a s t e r n Europe i n t h e near f u t u r e a r e b r i e f l y o u t l i n e d . The main n e c e s s i t y f o r many European c o u n t r i e s i n t h e near f u t u r e i s t o i n c r e a s e domestic hydrocarbon p r o d u c t i o n i n o r d e r t o achieve some more i n d e pendency f r o m u n f a v o u r a b l e changes o f t h e economical and p o l i t i c a l s e t t i n g ( t h i s a p p l i e s p a r t i c u l a r l y f o r E a s t e r n Europe). A s a consequence o f t h e r e g i o n a l m a r k e t i n g assessment, t h e o v e r a l l c o n c l u s i o n can be made t h a t p r o v i d e d an a g a i n more s t a b l e o i l p r i c e a t a reasonable l e v e l , p a r t i a l l y a l s o a s t r o n g e r val u e of t h e US $ ( e s p e c i a l l y a p p l y i n g f o r Western Europe), and an adequate p r i c i n g o f proppants and s e r v i c e , b o t h Western and E a s t e r n Europe a r e growing s t i m u l a t i o n markets w i t h i n c r e a s i n g demand o f proppants f o r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i n c l u d i n g p r o m o t i o n o f t r e a t m e n t s i n m a r g i n a l r e s e r v o i r s i n t h e near f u t u r e . 2.1.
2.2.
Introduction
........................................................
Economical influences on hydraulic proppant fracturing
2.2.1.
..............
I n f l u e n c e s o f hydrocarbon p r i c e changes ..................... 2.2.1.1. O i l p r i c e e v o l u t i o n ............................... 2.2.1.1.1. 1973 and 1979 o i l p r i c e e x p l o s i o n s . . . . O i l p r i c e t r e n d changes . 2.2.1.1.1.1. 2.2.1.1.1.2. Nominal o i l p r i c e l e v e l and impact o f o i l p r i c e jumps ...................
106 107 107 108 108 108 109
100 2.2.1.1.2.
2.2.1.2.
............
p r i c e l e v e l and i m p a c t o f t h e c r a s h . . . . . . . . . . . . . . . 110 2.2.1.1.3. 1987 and 1988 o i l p r i c e weakness . . . . . . 110 2.2.1.1.4. C y c l i c i t i e s during course o f the o i l p r i c e e v o l u t i o n ....................... 111 2.2.1.1.5. Reasons and e f f e c t s o f t h e 1986 o i l p r i c e c r a s h ........................... 111 2.2.1.1.5.1. N e t b a c k p r i c i n g and mark e t share s t r a t e g y vs. f i x e d p r i c i n g and p r o d u c t i o n l i m i t a t i o n . . . . . . . . . 111 2.2.1.1.5.2. O i l p r i c e d e f e n s e and market c o m p a t i b i l i t y vs. c h a n g i n g p r i c i n g p o l i c y . 112 E f f e c t o f o i l p r i c e changes on h y d r o c a r b o n i n d u s t r y 1 1 2 2.2.1.2.1. D r i l l i n g r i g c o u n t .................... 113 2.2.1.2.1.1. 1986 and 1987 . . . . . . . . . . . 113 2.2.1.2.1.2. 1988 _ . . _ _ . . . . _ _ . _ . 114 ..___. 2.2.1.2.2. A r t i f i c i a l and n a t u r a l m a r k e t f o r c e s .. 115 2.2.1.2.2.1. Swing p r o d u c t i o n f o r m a r k e t i m b a l a n c e compensat i o n .................... 115 2.2.1.2.2.2. OPEC r e s i d u a l m a r k e t s h a r e and p r i c e e f f e c t s . 115 2.2.1.2.3. V a r i a b l e s d e t e r m i n i n g o i l p r i c e l e v e l . 116 2.2.1.2.3.1. Coupling o f o i l product i o n r a t e and p r i c e n i v e a u . . . . . . . . . . . . . . . . . . 116 2.2.1.2.3.2. Current ideal o i l price l e v e l . . . . . . . . . . . . . . . . . . . 117 2.2.1.2.3.3. O i l p r o d u c t i o n c o s t s and o i l s e c t o r dominance . . . . 117 2.2.1.2.3.4. O i l p o l i t i c a l t r i a n g l e USA - S a u d i a A r a b i a - I r a n . . 118 2.2.1.2.4. M a r k e t f o r c e s and oil p r i c e e v o l u t i o n . 118 1970 - 1979 . . . . . . . . . . . . . 118 2.2.1.2.4.1. 1979 - 1986 . . . . . . . . . . . . . 119 2.2.1.2.4.2. 2.2.1.2.4.3. Market r e g u l a t i o n vs.disO i l c o n s u m p t i o n and o u t p u t . 2.2.1.2.5.1. OECD demand v s t i o n and s u b s t i t u t i o n ... 120 2.2.1.2.5.2. OPEC and non-OPEC p r o d u c t i o n v s . demand . . . . . . . . . 120 2.2.1.2.6. O t h e r a s p e c t s ......................... 120 I m p a c t o f v a r y i n g U S $ exchange r a t e on o i l p r i c e 121 e v o l u t i o n ......................................... 2.2.1.3.1. I m p o r t p r i c e f o r c r u d e o i l i n Germany 122 FRG ................................... 2.2.1.3.2. OPEC o i l s a l e income . . . . . . . . . . . . . . . . . . 122 2.2.1.3.2.1. O i l p r i c e c o n v e n t i o n and ... d e f e n s e . . . . . . _ _ . . _ _ . .123 2.2.1.3.2.2. OPEC v s . n o oduc. . . . . 123 tion ...... ing 2.2.1.3.2.3. OPEC p r o d u c and o v e r h a n g c a p a c i t y . . . 124 2.2.1.3.3. C u r r e n t v s . c o n s t a n t US $ r a t e c a l c u l a t i o n .............
2.2.1.2.5.
2.2.1.3.
1986 o i l p r i c e c o l l a p s e
101 2.2.1.4.
2.2.1.5.
Possible future consequences of the 1986 oil price collapse .......................................... Declining reserves due to suspension of 2.2.1.4.1. exploration and development ........... Production decline ex2.2.1.4.1.1. ceeding reserve addition Oil demand vs. supply in 2.2.1.4.1.2. the next decades ........ Influence of political 2.2.1.4.1.3. systems and financial relationships ............. Regaining power of the OPEC ........... 2.2.1.4.2. 2.2.1.4.2.1. Evolution of world oil production and demand ... 2.2.1.4.2.2. Symmetrical boom-crashboom cycle .............. 2.2.1.4.2.3. Reserve life and oil demand increase ........... Netback pricing, swing 2.2.1.4.2.4. production and market share recapturing ....... 2.2.1.4.2.5. Oil reserve distribution and OPEC capacity utilization .................. Imbalance between oil re2.2.1.4.2.6. serves and consumption .. Increasing oil demand ex2.2.1.4.2.7. ceeding critical OPEC output rate ............. 2.2.1.4.3. Oil price instability and market uncertainty ................................ 2.2.1.4.3.1. Cooperation attempts between OPEC and non-OPEC .. 2.2.1.4.3.2. Market shares vs. price stability ............... 2.2.1.4.3.3. Downstream integration of oil-producing countries . 2.2.1.4.3.4. Pricing policy of list vs. flexible prices ..... 2.2.1.4.3.5. Possibilities of another oil price war ........... 2.2.1.4.3.6. Impact of a possible general economic recession 2.2.1.4.3.7. Oil price collapse as result of lacking international cooperation ...... 2.2.1.4.3.8. Different assessment of US $ 18 agreement price . Energy supply diversification effects . 2.2.1.4.4. Present consequences of the 1986 oil price drop for the stimulation market ............................ 2.2.1.5.1. Fracturing strategy of deep gas wells before and after the 1986 oil industry crisis ................................ 2.2.1.5.1.1. Fracturing potential before the 1986 oil price drop .................... Fracturing potential af2.2.1.5.1.2. ter the 1986 oil price crash ................... 2.2.1.5.2. Reduction of capital expenditure and organizatory streamlining in explora-
125 125 125 126 126 127 127 128 129 129 130 130 131 131 132 132 132 133 133 134 134 134 135 135 135 135 136
102
2.2.1.5.3.
t i o n and p r o d u c t i o n companies ......... 2.2.1.5.2.1. Development and completion activity ... 2.2.1.5.2.2. D r i l l i n g meterage 2.2.1.5.2.3. Staff layoff .... 2.2.1.5.2.4. Company merging . 2.2.1.5.2.5. Cash management . . . . . . . . . 2.2.1.5.2.6. Survival of independents 2.2.1.5.2.7. Responsibility transfer by c o n t r a c t i n g and t a k e o v e r .................... 2.2.1.5.2.8. Executive s k i l l requirement change . . . . . . . . . . . . . B u s i n e s s d e t e r i o r a t i o n f o r s e r v i c e comp a n i e s ................................ 2.2.1.5.3.1. S t i m u l a t i o n b o a t employ2.2.1.5.3.2.
2.2.1.5.4.
2.2.1.6.
Oil p r i c e 2.2.1.6.1. 2.2.1.6.2.
2.2.1.6.3.
Well s e r v i c e e x p e n d i t u r e s and t o t a l number o f j o b s 2.2.1.5.3.3. S e r v i c e c o s t p e r w e l l and share of recompletions .. Banking r e l a t i o n s h i p s and i n v e s t m e n t s t r a t e g i e s ............................ 2.2.1.5.4.1. Banking r e l a t i o n s h i p s 2.2.1.5.4.2. Investment s t r a t e g i e s . . . 2.2.1.5.4.3. Other a s p e c t s . . . . . . . . . . . e v o l u t i o n and g a s m a r k e t . . . . . . . . . . . . . . . . C h a r a c t e r i s t i c f e a t u r e s o f t h e g a s market and i t s i n d e p e n d e n c e from the o i l m a r k e t ................................ I n f l u e n c e o f the 1986 o i l p r i c e d r o p on g a s development d r i l l i n g i n North S e a . 2.2.1.6.2.1. Gas s u p p l y c o n t r a c t s and economical s a f e t y . . . . . . . 2.2.1.6.2.2. Continuity of gas-f i e l d development . . . . . . . . . . . . . 2.2.1.6.2.3. Government back-up and supply s e c u r i t y ......... 2.2.1.6.2.4. G a s - f i e l d development vs. g a s p r i c e and demand evolution .................. 2.2.1.6.2.5. Disturbance of o i development . . . . . 2.2.1.6.2.6. Gas i m p o r t and c o f i e l d development . . . . . . . Significance of the east-west energy t r a d e i n the European g a s m a r k e t . . . . . . 2.2.1.6.3.1. Evolution o gas market 2.2.1.6.3.2. Supply/dema 2.2.1.6.3.2.1. General aspects .. 2 . 2 . 1 . 6 . 3 . 2 . 2 . Nether2.2.1.6.3.2.3. 2.2.1.6.3.2.4. 2.2.1.6.3.2.5. 2.2.1.6.3.2.6. 2.2.1.6.3.2.7.
137
140 140 141 141 142
142 143 143 144 144 145 145 146 146 147 147 148 148 149 149
151
Great B r i t a i n . . 152 Norway . . . 152 USSR . . . . . 1 5 3 Supply s e c u r i t y . 154 Mutual b e n e f i t . . 154
103 2.2.1.6.3.3.
2.2.2.
2.3.
I n t e r n a t i o n a l cooperation and c o m p e t i t i v e c h a l l e n g e 155 2.2.1.6.3.4. Market shares and r e s e r v e d i s t r i b u t i o n ............ 155 2.2.1.6.3.5. Future e v o l u t i o n o f the N o r t h Sea gas p r o v i n c e . . 156 E v o l u t i o n a r y t r e n d s o f t h e w o r l d gas 2.2.1.6.4. 156 market ................................ 2.2.1.6.4.1. Problematic l i n k i n g o f o i l and gas p r i c e s ...... 157 2.2.1.6.4.2. Energy s u p p l y d i v e r s i f i c a t i o n concepts . . . . . . . . . 157 2.2.1.6.4.3. Economical and t e c h n i c a l t r e n d s .................. 157 O i l s u b s t i t u t i o n by gas . 158 2.2.1.6.4.4. 2.2.1.6.4.5. Unconventional gas exp l o i t a t i o n .............. 158 2.2.1.6.4.6. Gas m a r k e t i n g s t r a t e g i e s 159 159 I n f l u e n c e s o f proppant p r i c e changes ........................ 2.2.2.1. 1985 p r o p p a n t p r i c e l o w e r i n g ...................... 160 2.2.2.1.1. M a r k e t i n g c o n s i d e r a t i o n s f o r 1985 p r o p p a n t p r i c e l o w e r i n g ................... 160 S u p e r i m p o s i t i o n by t h e US $ exchange 2.2.2.1.2. 161 r a t e drop ............................. 2.2.2.1.3. Economical r e s u l t s o f 1985 proppant 161 p r i c e d e c l i n e ......................... 2.2.2.1:3.1. Reversal o f f r a c t u r i n g p o t e n t i a l and p r o p p a n t a v a i l a b i l i t y . . . . . . . . . . . . 162 2.2.2.1.3.2. Capital expenditure c u t t i n g and s t i m u l a t i o n j o b suspension . . . . . . . . . . . . . . 162 163 2.2.2.2. 1987 p r o p p a n t p r i c e i n c r e a s e ...................... Reasoning f o r renewed p r o p p a n t p r i c e 2.2.2.2.1. 163 i n c r e a s e .............................. 2.2.2.2.2. Market r e a c t i o n and economical consequences o f t h e 1987 p r o p p a n t p r i c e i n 164 crease ................................ 2.2.2.3. C o m p e t i t i o n o f v a r i o u s proppant types ............. 164 I n f l u e n c e o f t h e 1985 p r o p p a n t p r i c e 2.2.2.3.1. r e d u c t i o n ............................. 164 I n f l u e n c e o f t h e 1987 p r o p p a n t p r i c e 2.2.2.3.2. 165 i n c r e a s e ..............................
Stimulation market differences between USA and Europe ............... 2.3.1. General aspects ............................................. 2.3.2. Frequency o f d r i l l i n g and s t i m u l a t i o n ....................... 2.3.2.1. Percentages o f f r a c t u r i n g n e c e s s i t y f o r economical p r o d u c t i o n ........................................ 2.3.2.2. Treatment c o s t and s i g n i f i c a n c e o f f r a c t u r i n g i n w e l l c o m p l e t i o n ................................... 2.3.3. S u c c e s s / f a i l u r e r a t i o and c o n s o r t i a 1 s t r u c t u r e . . . . . . . . . . . . . . 2.3.4. O p e r a t i n g m e n t a l i t y and t r e a t m e n t c o s t ...................... 2.3.4.1. I n t e g r a t i o n o f s t i m u l a t i o n i n t o t h e t o t a l w e l l conc e p t .............................................. 2.3.4.2. Proppant p r i c e and t r e a t m e n t c o s t . . . . . . . . . . . . . . . . . 2.3.5. US $ exchange r a t e d e c l i n e .. .. . . . _ ... . .. .. . . . . .. .. .. . . .. . . .. 2.3.6. Proppant m a r k e t i n g s t r a t e g y ................................. 2.3.6.1. Lacking c o n t i n u i t y i n marketing a t t e n t i o n ... . .. .. . 2.3.6.2. Business shares o f t h e d i f f e r e n t p r o p p a n t companies 2.3.6.2.1. Skewed market share r a t i o o f m a j o r proppant s u p p l i e r s ....................
166 166 166 167 167 167 168 168 169 169 170 170 171
171
104 Impact o f s e r v i c e companies v s . own t r e a t m e n t management .................. 172 P r i c e changes and market t u r n i n g . . . . . . . . . . . . . . . . . . 172 2.3.6.2.2. 2.3.6.3.
2.4. Areal marketing assessment of hydraulic proppant fracturing . . . . . . . . . 173 2.4.0. 2.4.1.
2.4.2.
2.4.3.
General aspects ............................................. 173 Western Europe 1977 - 1985 .................................. 174 174 2.4.1.1. Germany FRG ....................................... 2.4.1.1.1. Deep gas f r a c t u r i n g ................... 174 2.4.1.1.1.1. R e s e r v o i r d e p t h and p r o p p a n t q u a n t i t i e s ......... 176 2.4.1.1.1.1.1. 1975-1978 176 2.4.1.1.1.1.2. 1978-1985 176 F r a c t u r e l e n g t h and s t i 2.4. 1 . 1 . 1 . 2 . m u l a t i o n r a t i o . . . . . . . . . . 177 Cost and temporal execu2.4 1.1.1.3. t i o n o f t r e a t m e n t s . . . . . . 177 2.4 1 . 1 . 1 . 4 . T e c h n i c a l and economical fracturing potential i n 1985-1989 . . . . . . . . . . . . . . . 178 2.4.1.1.2. Sha low o i l f r a c t u r i n g . . . . . . . . . . . . . . . . 178 N a t u r a l sand . . . . . . . . . . . . 178 2.4. 1.1.2.1. Intermediate-strength 2.4. 1.1.2.2. s y n t h e t i c p r o p p a n t s ..... 182 2.4.1.2. B r i t i s h Southern N o r t h Sea ........................ 182 2.4.1.2.1. General aspects ....................... 182 2.4.1.2.2. M o d e r a t e l y deep gas f r a c t u r i n g . . . . . . . . 183 2.4.1.2.2.1. C a r b o n i f e r o u s . . . . . . . . . . . 183 2.4.1.2.2.2. R o t l i e g e n d .............. 183 2.4.1.2.3. T a i l - i n f r a c t u r i n g p o l i c y . . . . . . . . . . . . . 186 2.4.1.2.3.1. Aims and performance o f p r o p p a n t t a i l - i n schedule 186 2.4.1.2.3.2. Principles o f t a i l - i n p r o p p a n t s t a g i n g . . . . . . . . 188 2.4.1.2.3.3. Proppant c o s t containment 188 2.4.1.2.3.4. Proppant m i x i n g and s e t t l i n g .................... 189 2.4.1.2.3.5. Proppant flowback preven189 t i o n .................... 2.4.1.2.3.6. T a i l - i n o p t i m i z a t i o n and e f f e c t i v i t y . . . . . . . . . . . . . 190 2.4.1.2.4. D i f f e r e n c e s between o f f s h o r e and on190 shore f r a c t u r i n g ...................... 2.4.1.2.5. M o d e r a t e l y deep o i l f r a c t u r i n g . . . . . . . . 192 192 Western Europe 1986 ......................................... 2.4.2.1. O i l p r i c e d e c l i n e ................................. 192 193 2.4.2.2. A r e a l f r a c t u r i n g a c t i v i t y ......................... 2.4.2.2.1. R o t l i e g e n d gas f r a c t u r i n g i n N o r t h Sea 193 2.4.2.2.2. Unemployment o f d r i l l i n g r i g s and s t i 194 m u l a t i o n b o a t s ........................ 2.4.2.3. I n c r e a s e d c a n c e l l a t i o n r a t e s o f j o b s .............. 194 2.4.2.3.1. R e s e r v o i r p r o p e r t i e s . . . . . . . . . . . . . . . . . . 194 196 2.4.2.3.2. Well t y p e ............................. 2.4.2.4. Proppant m a r k e t i n g a c t i v i t y ....................... 196 2.4.2.4.1. Business l o s s t o p r o p p a n t c o m p e t i t i o n . 197 2.4.2.4.1.1. Y u g o s l a v i a .............. 197 2.4.2.4.1.2. B r i t i s h Southern Northsea 197 2.4.2.4.2. Business loss t o n a t u r a l sand ......... 199 2.4.2.4.2.1. Y u g o s l a v i a .............. 199 2.4.2.4.2.2. Turkey . . . . . . . . . . . . . . . . . . 199 2.4.2.4.2.3. B r i t i s h Southern Northsea 199 201 E a s t e r n Europe u n t i l 1986 ...................................
105
2.4.4.
2.4.4.2.
2.4.4.3.
2.4.4.4. 2.4.4.5.
2.4.4.6.
2.4.5.
Y u g o s l a v i a and Hungary ............................ 201 USSR .............................................. 202 O t h e r E a s t e r n European c o u n t r i e s .................. 202 Southern Europe and N o r t h e r n A f r i c a ............... 203 E a s t e r n Europe .................................... 203 O i l p r i c e and US $ exchange r a t e .................. 204 2.4.4.1.1. E x p o r t o f USSR o i l and gas t o Western and E a s t e r n Europe .................... 204 Dependency o f Comecon c o u n t r i e s on USSR 2.4.4.1.2. o i l and gas ........................... 206 2.4.4.1.3. B a r t e r trade vs . hard currency sale o f 206 o i l and gas ........................... 2.4.4.1.4. R o l e o f hydrocarbon e x p o r t s i n t h e USSR 207 f o r e i g n t r a d e ......................... USSR o i l p r o d u c t i o n ................... 207 2.4.4.1.5. 2.4.4.1.5.1. M e e t i n g and o v e r s h o o t i n g o f p l a n f o r e c a s t ........ 208 2.4.4.1.5.2. Impact o f r e c o r d product i o n f o r o i l e x p o r t s .... 208 2.4.4.1.5.3. S i g n i f i c a n c e o f Western S i b e r i a share ........... 208 2.4.4.1.6. Currency c o n v e r t i b i l i t y and exchange 208 r a t e s ................................. P o l i t i c a l r e s t r i c t i o n s i n t h e p a s t ................ 209 2.4.4.2.1. USA p e t r o l e u m equipment s u p p l y embargo 1978-1987 ............................. 209 2.4.2.2.2. Status a f t e r l i q u i d a t i o n o f the f o r e i g n p o l i c y s a n c t i o n s ...................... 209 P r e s e n t s t i m u l a t i o n p o t e n t i a l and economical r e s t r u c t u r i n g ....................................... 210 2.4.4.3.1. Supply o f s y n t h e t i c h i g h - q u a l i t y p r o p 210 p a n t s ................................. 2.4.4.3.2. P o l i t i c a l and economical r e o r g a n i z a t i o n 210 2.4.4.3.3. Hydrocarbon p r o d u c t i o n g o a l s o f t h e f i v e - y e a r p l a n ........................ 210 2.4.4.3.4. Differences i n d r i l l i n g a c t i v i t y i n Western and E a s t e r n Europe a f t e r t h e 1986 o i l p r i c e c r a s h .................. 211 S e l f - s e r v i c i n g t r e a t m e n t p o l i c y ................... 211 Economical and f i n a n c i a l aspects .................. 212 2.4.4.5.1. World Bank c r e d i t s f o r Y u g o s l a v i a Hungary, Poland and Romania .............. 212 2.4.4.5.2. E x p o r t revenues i n Yugoslavia, Czechos l o v a k i a , Germany GDR and B u l g a r i a .... 212 2.4.4.5.3. Hard c u r r e n c y a c q u i s i t i o n o f USSR and 212 Germany GDR ........................... 2.4.4.5.4. P o l i t i c a l and economical r e o r g a n i z a t i o n 213 2.4.4.5.4.1. Restructuring, transparency, a c c e l e r a t i o n ( p e r e s t r o i k a , g l a s n o s t , usk o r e n i e ................. 213 2.4.4.5.4.2. Mutual b e n e f i t f o r i n t e r n a t i o n a l c o o p e r a t i o n .... 213 214 2.4.4.5.5. J o i n t v e n t u r e s ........................ 2.4.4.5.5.1. B a l t i c Sea .............. 214 2.4.4.5.5.2. Other USSR o f f s h o r e areas 214 R e s e r v o i r e n g i n e e r i n g and hydrocarbon s u p p l y asp e c t s ............................................. 215 215 2.4.4.6.1. Gas s u p p l y ............................ O i l s u p p l y ............................ 215 2.4.4.6.2. Europe 1987 - 1990 .................................. 216 B r i t i s h Southern N o r t h Sea ........................ 216
2.4.3.1. 2.4.3.2. 2.4.3.3. 2.4.3.4. Status i n 2.4.4.1.
Western 2.4.5.1.
.
106 2.4.5.1.1.
2.4.5.2. 2.4.5.3. 2.4.5.4.
R o l e o f m a j o r s t i m u l a t i o n campaigns i n t h e European gas development scene . . . . 2.4.5.1.2. Cancellation r i s k o f s t i m u l a t i o n jobs . 2 . 4 . 5 . 1 . 3 . S e l f - s u f f i c i e n c y o f U n i t e d Kingdom gas i n d u s t r y .............................. 2.4.5.1.4. S i g n i f i c a n c e o f gas development a c t i v i t y ......................... Dutch Southern N o r t h Sea . . . . . . . . . . . . . . . O t h e r areas ....................................... S i g n i f i c a n c e o f f r a c t u r i n g f o r i n c r e a s i n g proven
..................................
2.4.5.5.1.
2.4.6.
2.4.5.5.2. 2.4.5.5.3. E a s t e r n Europe 1987 - 1990 .................................. 2 . 4 . 6 . 1 . General aspects ................................... 2.4.6.2. Y u g o s l a v i a and Hungary ...................... 2.4.6.3. USSR .............................................. Hydraulic proppant f r a c t u r i n g p o t e n t i a l 2.4.6.3.1. I m p o r t vs. domestic m a n u f a c t u r i n g o f 2.4.6.3.2. 2.6.4.3.3. 2.6.4.3.4.
2.5.
t m a r k e t i n g s t r a t e g y .... D i r e c t p r o p p a n t s a l e s t o end u 2.4.5.5.1.1. Drawbacks o f s e r v i c e company s t o c k s . . . . . . . . . . . . . 2.4.5.5.1.2. Treatment s e l f p e r f o r m a n c e i n E a s t e r n Europe . . . . . . . L i q u i d a t i o n o f s e r v i c e company s t o c k s . Stand-by s u p p l y by p r o p p a n t companies .
217
219 221
221 222 222 223 223 224 224 225 225
h i g h - s t r e n g t h p r o p p a n t s ............... 225 Significance o f i n t e n s i f i c a t i o n o f o i l and gas e x p l o i t a t i o n . . . . . . . . . 226 P r i o r i t y o f p e t r o l e u m development vs. o i l p r i c e l e v e l ....................... 226
China ................................. . ,. O t h e r E a s t e r n European c o u n t r i e s . . . . . . . . . . . . . . . . . . Southern Europe and N o r t h e r n A f r i c a . . . . . . . . . . . . . . . Concluding assessment o f p r o p p a n t m a r k e t i n g p o t e n t i a l . . . . . . . . . . . . . . .
2.4.6.4. 2.4.6.5. 2.4.6.6.
216 217
226 227 227 228
2.1. Introduction H y d r a u l i c p r o p p a n t f r a c t u r i n g o f l o w - p e r m e a b i l i t y o i l - and g a s - b e a r i n g format i o n s i s p a r t i c u l a r l y i n Europe ( b u t a l s o i n many o t h e r p a r t s o f t h e w o r l d ) n o t o n l y i n f l u e n c e d by t e c h n i c a l and r e s e r v o i r e n g i n e e r i n g aspects, b u t a l s o s t r o n g l y depends on m a r k e t i n g , economical and p o l i t i c a l r e l a t i o n s h i p s . I n c o n t r a s t t o t h e USA, Europe i s n o t o n l y p o l i t i c a l l y a complex aggregate o f c o u n t r i e s speaki n g a s u i t e o f d i f f e r e n t languages and h a v i n g a v a r i e t y o f m e n t a l i t i e s , w i t h t h e m a j o r d i s t i n c t i o n c o n c e r n i n g economical s i t u a t i o n , s t a t u s o f t e c h n o l o g i c a l development, commercial c a p a c i t y , f i n a n c i a l r e l a t i o n s h i p s , a d m i n i s t r a t i v e p r o c e dure and f o r e i g n t r a d e a c t i v i t y h a v i n g t o be made between Western and E a s t e r n Europe, b u t a l s o i n c l u d e s a wide spectrum o f o i l and gas r e s e r v o i r t y p e s i n terms o f s t r a t i g r a p h i c a l , t e c t o n i c a l , s e d i m e n t o l o g i c a l , p e t r o p h y s i c a l and e n g i n e e r i n g c h a r a c t e r i z a t i o n . An e v a l u a t i o n o f t h e a r e a l h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g p o t e n t i a l f o r a p p l i c a t i o n of proppants f r o m b o t h t e c h n i c a l and m a r k e t i n g p o i n t o f view has t o i n t e g r a t e t h e g e n e r a l s i t u a t i o n by comparing t h e d i f f e r e n t cases and summarizing n o t o n l y s c i e n t i f i c , e n g i n e e r i n g and t e c h n o l o g i c a l , b u t a l s o economical and even i d e o l o g i c a l and m e n t a l i t y - r e l a t e d a s p e c t s . F o l l o w i n g an i n t r o d u c t i o n o f p r o p p a n t t y p e s and g r a i n s i z e s i n c l u d i n g assessment o f t h e most s i g n i f i c a n t aspects o f c o m p o s i t i o n and p r o p e r t i e s as a base f o r proppant s e l e c t i o n f o r v a r i o u s s p e c i f i c a p p l i c a t i o n s ( c h a p t e r l ) , t h e p r e s e n t d i s c u s s i o n g i v e s r e v i e w , s t a t u s and f o r e c a s t o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n Western and E a s t e r n Europe p r e d o m i n a n t l y from m a r k e t i n g and economical p o i n t s o f view, w i t h emphasis b e i n g p u t on s e v e r a l i m p o r t a n t f a c t o r s c o n t r o l l i n g t h e g e n e r a l f e a s i b i l i t y s i t u a t i o n of t h e o i l and gas i n d u s t r y . Comparative
107 e v a l u a t i o n w i t h USA, M i d d l e E a s t and o t h e r p a r t s o f t h e w o r l d i s f r e q u e n t l y p e r formed d u r i n g course o f t h e g e n e r a l and r e g i o n a l assessments. The account o f t e c h n i c a l and m a r k e t i n g aspects o f R o t l i e g e n d and C a r b o n i f e r o u s s t i m u l a t i o n i n Europe i n d i v i s i o n 3 r e p r e s e n t s an a r e a l case s t u d y and forms a t r a n s i t i o n f r o m t h e e c o n o m i c a l l y i n f l u e n c e d s e c t i o n s t o t h e p a r t s c o n c e n t r a t i n g on t e c h n i c a l and r e s e r v o i r e n g i n e e r i n g p o i n t s o f h y d r a u l i c proppant f r a c t u r i n g i n c h a p t e r 4. D i v i s i o n 5 p r e s e n t s a m a i n l y t e c h n i c a l overview and s u b o r d i n a t e l y a l s o marketi n g o u t l i n e o f g r a v e l packing. U n i t 6 summarizes f r a c t u r e and g r a v e l pack monit o r i n g . V a r i o u s p o s s i b i l i t i e s o f p r o m o t i o n and enhancement o f r e s e r v o i r s t i m u l a t i o n by h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i n c o n n e c t i o n w i t h a p p l i c a t i o n o f proppants a r e compiled i n c h a p t e r 4 t h a t i s f o c u s s i n g on t e c h n i c a l and r e s e r v o i r e n g i n e e r i n g aspects o f f o r m a t i o n t r e a t m e n t . A c o m p i l a t i o n o f an e x t e n s i v e b i b l i o g r a p h y w i t h a s u b j e c t key c l a s s i f i c a t i o n i n t o v a r i o u s s p e c i a l t o p i c s and t h e general r e f e r e n c e l i s t a r e g i v e n i n s e c t i o n 7 . A f t e r an i n t r o d u c t o r y o u t l i n e o f t h e main economical i n f l u e n c e s on h y d r a u l i c p r o p p a n t f r a c t u r i n g b e i n g changes o f hydrocarbon p r i c e s and p r o p p a n t p r i c e s and commenting on t h e m a j o r f l u c t u a t i o n s o f t h e o i l p r i c e i n e i t h e r d i r e c t i o n dur i n g t h e l a s t 15 y e a r s ( d i s c u s s e d i n c o m b i n a t i o n w i t h t h e f l u c t u a t i o n s o f t h e US $ exchange r a t e ) , d i f f e r e n c e s o f t h e s t i m u l a t i o n market between USA and Eur o p e a r e taken as a t o o l t o s k e t c h t h e s p e c i f i c r e a c t i o n s o f b o t h m a r k e t s t o t h e l a t e 1 9 8 5 / e a r l y 1986 o i l p r i c e drop and t h e r e b y induced m a j o r c r i s i s o f t h e hydrocarbon p r o d u c t i o n i n d u s t r y . A m a j o r p a r t o f t h e r e p o r t i s devoted t o an a r e a l m a r k e t i n g assessment o f h y d r a u l i c proppant f r a c t u r i n g i n Western and East e r n Europe, w i t h d i s t i n c t i o n b e i n g made between r e v i e w (1977 - 1985), s t a t u s ( 1 9 8 6 / e a r l y 1987) and f o r e c a s t ( l a t e 1987 - 1990 and a l s o f o l l o w i n g y e a r s ) f o r the i n d i v i d u a l sections o f the market.
2.2. Economical influences on hydraulic proppant fracturing D u r i n g t h e l a s t 10 y e a r s s i n c e h y d r a u l i c proppant f r a c t u r i n g (VEATCH 1983, VEATCH & M O S C H O V I D I S 1986) o f deep t i g h t r e s e r v o i r s s t a r t e d t o boom w i t h t h e i n v e n t i o n o f t h e f i r s t h i g h - s t r e n g t h proppants (COBS 1985, WESTERN PETROLEUM 1985) and t h e i r i n t r o d u c t i o n t o p e t r o l e u m i n d u s t r y i n l a t e 1976 (COOKE 1976, 1977; ATTEBERRY, TUCKER & RITZ 1979; HICKEY, BROWN & CRITTENDEN 1981; c f . sect i o n 1.2.), v a r i o u s e v e n t s i n t h e g e n e r a l w o r l d economy had c o n s i d e r a b l e i n f l u e n c e s on t h e hydrocarbon s t i m u l a t i o n m a r k e t and t h e p o s s i b i l i t i e s o f p r o p p a n t p r o m o t i o n . The most i m p o r t a n t f a c t o r s c o n t r o l l i n g t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g a c t i v i t y a r e changes o f hydrocarbon p r i c e s (BEUDELL 1986 a, 1986 b; GRIFFITHS 1986, HARBEN 1986, MENGES 1986, OEHME 1986, RUNGE 1986, V A R Z I 1986, MADER 1987) and changes o f p r o p p a n t p r i c e s .
A s b o t h p r i c e s a r e v a l i d w o r l d - w i d e i n US $ ( w i t h t h e e x c e p t i o n b e i n g o n l y a few p r o p p a n t t y p e s t h a t a r e s o l d f o r o t h e r c u r r e n c i e s as w e l l ) , t h e v a r i a t i o n s o f t h e exchange r a t e o f t h e US $ ( w i t h r e s p e c t t o o t h e r c o n v e r t i b l e g u i d e c u r r e n c i e s such as DM and t ) have a l s o an i m p o r t a n t superimposed s i g n i f i c a n c e on t h e European s t i m u l a t i o n market (comparable t o t h e i r impact on t h e r e l a t i v e o i l p r i c e e v o l u t i o n i n Europe). V a r i o u s impacts o f hydrocarbon p r i c e changes and p r o p p a n t p r i c e changes on h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l a r e d i s c u s s e d as f o l l o w s .
2.2.1. Influences o f hydrocarbon price changes The t e c h n o l o g i c a l e v o l u t i o n o f m a n u f a c t u r i n g o f v a r i o u s proppant t y p e s took p l a c e p a r a l l e l t o s e v e r a l m a j o r changes o f t h e hydrocarbon p r i c e s c e n a r i o . F o l l o w i n g a summary o f t h e o i l p r i c e e v o l u t i o n i n t h e l a s t decades, v a r i o u s a c t u a l consequences o f r i s i n g and f a l l i n g o i l p r i c e f o r t h e s t i m u l a t i o n market a r e d i s cussed p a r t i c u l a r l y i n view o f t h e tremendous impact o f t h e 1986 o i l p r i c e d r o p and t h e d e c l i n e o f t h e US $ exchange r a t e which reached i t s h i s t o r i c a l minimum l e v e l i n t h e f o u r t h q u a r t e r o f 1987 and k e p t b e i n g weak t h r o u g h o u t t h e f i r s t
108 p a r t o f 1988 ( t h e r e c o v e r y o f a b t . 20 - 25 % o f i t s v a l u e h a v i n g t a k e n p l a c e i n t h e second h a l f o f 1988 has a t t h e bottom o f t h e economical l i n e t o be c l a s s i f i e d as b e i n g s t i l l i n s u f f i c i e n t , m a i n l y due t o t h e ongoing u n r e l i a b i l i t y o f t h e US $ e v o l u t i o n and t h e r e s u l t i n g s t i l l r e t a r d e d r e a d i n e s s f o r i n v e s t m e n t s , and was even a l r e a d y p a r t i a l l y a g a i n compensated by a renewed d r o p a t t h e end o f 1988). An o u t l o o k i s a l s o g i v e n on p o s s i b l e f u t u r e consequences o f t h e 1986 o i l p r i c e c o l l a p s e . F i n a l l y , some comments a r e o f f e r e d on t h e impetus o f t h e o i l p r i c e e v o l u t i o n f o r t h e gas market,
2.2.1.1. Oil price evolution The o i l p r i c e e v o l u t i o n i n t h e l a s t 15 y e a r s was c h a r a c t e r i z e d by s e v e r a l booming and f a l l i n g e v e n t s which s e r i o u s l y a f f e c t e d hydrocarbon i n d u s t r y i n e i t h e r way. Changes o f t h e o i l p r i c e i n upwards o r downwards d i r e c t i o n were a l s o r e p e a t e d l y accentuated by s h i f t i n g l e v e l s o f t h e US $ exchange r a t e w i t h r e s p e c t t o o t h e r l e a d i n g w o r l d c u r r e n c i e s such as DM and t . The o i l p r i c e h i s t o r y o f t h e l a s t 20 y e a r s can be c l a s s i f i e d i n t o two main p e r i o d s c o m p r i s i n g t h e 1970's o i l p r i c e e x p l o s i o n s and t h e 1980's o i l p r i c e drop and c r a s h . The d i s c u s s i o n as f o l l o w s focusses on 1973 and 1979 o i l p r i c e e x p l o s i o n s , 1986 o i l p r i c e c o l l a p s e , and 1987 and 1988 o i l p r i c e weakness. Emphasis i s p a r t i c u l a r l y p u t on t h e r o l e o f t h e O r g a n i z a t i o n o f Petroleum E x p o r t i n g C o u n t r i e s (OPEC) on t h e p r o g r e s s i o n o f o i l p r i c e e v o l u t i o n . Some reasons and e f f e c t s o f t h e 1986 oil p r i c e c r a s h a r e a l s o o u t l i n e d , and comments a r e a l s o o f f e r e d on c y c l i c i t i e s d u r i n g course o f t h e o i l p r i c e e v o l u t i o n .
2.2.1.1.1. 1973 and 1979 oil price explosions The l a t e 1 9 8 5 / e a r l y 1986 o i l and gas i n d u s t r y c r i s i s i s t h e f i f t h a f t e r t h e Second World War ( e a r l i e r c r i s e s have been i n t h e m i d d l e 1950's, l a t e 1950's, e a r l y 1960's and i n t h e l a t e 1 9 6 0 ' s / e a r l y 1970's b e f o r e t h e f i r s t o i l p r i c e i n crease shock i n l a t e 1 9 7 3 / e a r l y 1974) and p r o b a b l y had t h e w o r s t e f f e c t o f a l l t h e h i t h e r t o c r i s e s , as t h e o i l and gas market was most s e v e r e l y h i t b y numer o u s o p e r a t i o n s becoming uneconomical almost o v e r n i g h t ( c f . a l s o LEE 1 9 8 2 ) . F o l l o w i n g t h e d r a s t i c a l o i l p r i c e i n c r e a s e i n two main s t e p s i n l a t e 1 9 7 3 / e a r l y 1974 (when t h e OPEC began t o f i x and t o c o n t r o l t h e o i l p r i c e ; B I L D 1987) and l a t e 1 9 7 9 / e a r l y 1980 (OEHME 1986, RUNGE 1986) up t o a l e v e l t h a t was d u r i n g extreme p e r i o d s more than 20 t i m e s o f t h a t i n t h e e a r l y 1970's, t h e p r i c e r a p i d l y c o l l a p s e d almost t o a n i v e a u t h a t e x i s t e d i n t h e t i m e s h o r t l y a f t e r t h e f i r s t o i l p r i c e shock ( a t l e a s t i f c a l c u l a t i n g n o t o n l y w i t h t h e a b s o l u t e f i g u r e , b u t a l s o i n c o r p o r a t i n g i n f l a t i o n and d i f f e r e n t exchange r a t e s between US $ and o t h e r guide c u r r e n c i e s i n t h e w o r l d market; c f . s e c t i o n 2 . 2 . 1 . 3 . ) . Some comments a r e g i v e n as f o l l o w s on o i l p r i c e t r e n d changes as w e l l as nominal o i l p r i c e l e v e l and impact o f o i l p r i c e jumps.
2.2.1.1.1.1. Oil price trend changes As a consequence o f t h e s t a b i l i z a t i o n o f t h e s i t u a t i o n a f t e r t h e second o i l p r i c e shock i n l a t e 1 9 7 9 / e a r l y 1980, t h e o i l p r i c e s l o w l y s t a r t e d to l e a v e i t s c e r t a i n l y much exaggerated l e v e l i n 1982 and approached more and more a reason a b l e n i v e a u u n t i l l a t e 1985 when t h e d r a s t i c a l drop was i n i t i a t e d t h r o u g h some p r e c u r s o r y f l u c t u a t i o n s and soon a f t e r t h e b e g i n n i n g o f 1986 a l m o s t w i t h i n a c o u p l e o f months u n t i l e a r l y 1986 r e v e r s e d more t h a n 12 y e a r s o i l p r i c e h i s t o r y ( w i t h t h e o i l p r i c e d e c l i n e becoming i n Europe a t t h e b o t t o m o f t h e l i n e even more s e r i o u s by t h e accompanying decrease o f t h e US $ exchange r a t e ; c f . sect i o n 2 . 2 . 1 . 3 . ) . O i l p r i c e e v o l u t i o n and t h e a s s o c i a t e d p e r i o d s o f c r i s i s and boom a r e summarized by A T T I G A ( 1 9 8 7 ) .
Many o f t h e a c t u a l problems a r e t h e r e s u l t of t h e o i l p r i c e e v o l u t i o n a f t e r t h e Second World War (SPATSCHEK 1987). I n t h e booming p e r i o d o f 1950 - 1972,
109 low o i l p r i c e s supported a r a p i d t o s a l t a t i n g increase o f o i l demand and prog r e s s i v e replacement o f o t h e r energy media, and i n the h i g h - p r i c e p e r i o d 1973 - 1985, t h e OPEC-dictated o i l p r i c e explosions enabled the extension o f e x p l o r a t i o n t o new f r o n t i e r s and t h e development o f energy-saving technologies. A t l e a s t p a r t i a l l y t h i s h i s t o r i c a l o i l p r i c e and energy supply c o n s t e l l a t i o n programmed the 1986 o i l p r i c e crash.
2.2.1.1.1.2. Nominal oil price level and impact of oil price jumps The h i s t o r i c a l o i l p r i c e e v o l u t i o n i n c l u d e s several major e x p l o s i o n a r y i n creases of t h e l e v e l from f o r m e r l y a b t . 1 - 5 US $ per b a r r e (BISHOP 1988) bef o r e 1973 t o ( p a r t i a l l y as a r e a c t i o n t o the I s r a e l i a n / E g y p t i n war) 10 - 15 US $ i n 1973/1974 (when the OPEC group s t a r t e d t o d i c t a t e the o i l p r i c e l e v e l by s t r o n g l y c o n t r o l l i n g p r o d u c t i o n q u a n t i t i e s and t h e i r p h y s i c a l d i s t r i b u t i o n as w e l l as by p o l i t i c a l t h r e a t e n i n g ) and then ( p a r t i a l l y t r i g g e r e d by t h e beginn i n g o f t h e I r a q i a n - I r a n i a n war) up t o 35 US $ in 1979/1980 ( w i t h a maximum up t o 40 US $ o r i n places such as t h e spot market even more than 45 US $ having been reached t e m p o r a r i l y i n 1980/1981). The consequence o f t h i s o i l p r i c e development was t h a t 1981 has seen the peak o f the world-wide hydrocarbon d r i l l i n g r i g count so f a r (GRIFFITHS 1986, HARBEN 1986, DOUGHERTY 1987; the delay of t h e climax o f r i g employment i n t h e North Sea t o 1984/1985 i s a s p e c i a l e f f e c t of r e t a r d e d approach t o t h i s expens i v e o f f s h o r e area; OILMAN 1985; c f . s e c t i o n 2.2.1.2.1.). The h i g h o i l p r i c e n i veau guaranteed t h e OPEC group an a d d i t i o n a l income o f i n t o t a l more than 5 B i l l . DM d u r i n g the l a s t 13 years (OEHME 1986; c f . s e c t i o n 2.2.1.3.2.). Aspects o f European o i l and gas supply a f t e r the f i r s t o i l p r i c e e x p l o s i o n are discussed by LOTTRINGHAUS & SCHMITT (1975).
2.2.1.1.2. 1986 oil price collapse IR l a t e 1981 and p a r t i c u l a r l y i n 1982, t h e o i l p r i c e s t a r t e d s l o w l y t o leave i t s c e r t a i n l y s e r i o u s l y exaggerated niveau (being p a r t i a l l y an e f f e c t o f a gener a l l y decreasing energy demand due t o t h e e f f o r t s p u t on conservation; KEG 1986) and g r a d u a l l y d e c l i n e d t o 25 - 30 US $ (MINERALULWIRTSCHAFT 1983, BEUDELL 1986 c ) . I n l a t e 1985, t h e decrease had p a r t i a l l y already proceeded t o r e a l p r i ces i n the range o f 20 - 25 US $ ( a l t h o u g h t h e o f f i c i a l OPEC p r i c e was s t i l l a t 28 - 30 US $; BECK 1987, ERDOEL-ERDGAS AKTUELL 1984, ERDUL UND KOHLE - ERDGAS PETROCHEMIE 1987 b ) , thereby announcing the b i g event which a r r i v e d s h o r t l y a f t e r the beginning o f 1986 when t h e p r i c e a t once f e l l down t o 10 - 15 US $ and reached t e m p o r a r i l y even l e v e l s o f 5 - 9 US $ (GRIFFITHS 1986, HARBEN 1986, KEG 1986, THOMAS 1986, ALGAR 1987 b, ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 C; MABRO 1987, 1988; SCHMIDT 1987, McCANN 1988). Some e f f e c t s o f t h e 1986 o i l p r i c e drop are o u t l i n e d as f o l l o w s , and comments are a l s o g i v e n on r e t u r n t o reasonable o i l p r i c e l e v e l and impact o f the crash.
2.2.1.1.2.1. Effects of the 1986 oil price drop T h i s event represented a s h o r t - t e r m r e d u c t i o n o f the o i l p r i c e t o l e s s than 50 % o f i t s value d u r i n g abt. h a l f a year o r a 60 - 70 % r e d u c t i o n o f the l a t e 1985 p r i c e (CROUSE 1987) and was a consequence o f t h e f a c t t h a t i n view o f pers i s t i n g oversupply and d e c l i n i n g market shares, t h e OPEC (ROBERTS 1986, 1987) renounced o f i t s q u a n t i t y and p r i c e c o n t r o l e f f e c t i v e end o f 1985 and thereby terminated a p e r i o d o f o i l p r i c e d i c t a t i o n t h a t l a s t e d more than 12 years s i n c e i t s beginning i n l a t e 1973, g i v i n g way t o a f r e e market system due t o the l a c k o f a surplus management concept f o r p r o d u c t i o n t o match o v e r a l l demand made up o f a c t u a l consumption and stock movements (BEUOELL 1986 c, A T T I G A 1987; c f . sect i o n s 2.2.1.2.2. and 2.2.1.2.4.). The o i l p r i c e o f e a r l y t o mid 1986 ranged i n
110 r e a l terms n e a r t h e low l e v e l s j u s t p r i o r t o t h e f i r s t o i l p r i c e e x p l o s i o n i n 1973 (THOMPSON & SINGLETON 1 9 8 8 ) . I n m i d t o l a t e 1986, t h e average o i l p r i c e ( t h e a c t u a l p r i c e k e p t f l u c t u a t i n g w i t h a range o f a few US $ upwards o r downwards) l e f t i t s minimum below U S $ 10 ( a n average v a l u e o f 6 - 9 US $ was h e l d i n J u l y 1986 a t t h e peak o f t h e c r i s i s ; McCANN 1988) and g r a d u a l l y approached a n i v e a u o f a b t . US $ 13 - 15 (CROUSE 1987). W i t h no agreement between OPEC and non-OPEC t o r e s t r a i n product i o n , t h e downward p r e s s u r e on t h e o i l p r i c e became i r r e s i s t i b l e , and f i n a l l y OPEC was f o r c e d t o f o r m a l l y abandon i t s market share p o l i c y and t o r e v e r t t o c u t t i n g back i n o u t p u t (McCANN 1988).
2.2.1.1.2.2. Return t o reasonable oil price level and impact o f the crash Near t h e end o f 1986, r e g a i n i n g consensus and agreement o f t h e OPEC under t h e f o r c e o f enormous l o s s e s o f income and t h e t h r e a t o f even some members f a c i n g b a n k r u p t c y e s t a b l i s h e d t h e average o i l p r i c e a t a b t . US $ 18 (BECK 1987, CROUSE 1987; ERDOL UND KOHLE - ERDGAS - PETROCHEMIE 1987 a, 1987 b; VIELVOYE 1987 a, 1987 b; c f . s e c t i o n 2.2.1.4.3.8.), b u t c o u l d n o t l i q u i d a t e numerous m i n o r v a r i a t i o n s i n p r e d o m i n a n t l y downwards d i r e c t i o n due t o c u r r e n t e s c a l a t i o n s i n t h e p o l i t i c a l c o n f l i c t s i t u a t i o n i n t h e M i d d l e E a s t as w e l l as temporal s l i g h t changes i n t h e w o r l d o i l demand and s u p p l y c o n s t e l l a t i o n . The i n s t a b i l i t y o f t h e o i l p r i c e i s a l s o caused by t h e disagreement i n t h e OPEC group on t h e s t a t u s o f t h e US $ 18 c o n v e n t i o n p r i c e l e v e l (OIL GAS JOURNAL 1987 c ) . W h i l e t h e key members i n t e n d t o h o l d t h e p r i c e a t t h i s n i v e a u i n o r d e r t o a l l o w t h e o i l t o r e g a i n some o f i t s l o s t market share, o t h e r a g g r e s s i v e count r i e s would l i k e t o use t h i s base as a s p r i n g b o a r d t o h i g h e r p r i c e s i n 1988 t o 1990 ( c f . s e c t i o n 2 . 2 . 1 . 4 . 3 . 8 . ) . B i g n a t i o n a l markets such as t h a t i n t h e USA more o r l e s s t r a c k e d t h e i n t e r n a t i o n a l o i l p r i c e e v o l u t i o n , w i t h t h e changes i n t h e USA h a v i n g been f r o m U S $ 24 - 27 i n January 1986 v i a US $ 13 i n t h e second q u a r t e r and US $ 8 - 9 i n August t o US $ 14 - 15 i n December 1986 and US $ 17 - 19 i n t h e second q u a r t e r o f 1987 (BECK & SMITH 1987, BECK & WILLIAMS 1987, OPL GAS JOURNAL 1987 g, WEST 1987). E v a l u a t i n g t h e e f f e c t o f an US $ 18 o i l p r i c e b e f o r e and a f t e r t h e 1986 c r a s h (KINNEY 1987) r e v e a l s t h a t w h i l e an US $ 18 l e v e l was f e a r e d by t h e hydrocarbon i n d u s t r y when t h e p r i c e was s t i l l a t US $ 25 - 30, a f t e r t h e drop down below US $ 10 a renewed c l i m b t o US $ 18 was t a ken as a s i g n o f market s t a b i l i z a t i o n and t r i g g e r e d s l i g h t t o moderate p i c k - u p o f a c t i v i t y w i t h reasonable improvement o f e x p l o r a t i o n and p r o d u c t i o n c a s h f l o w .
2.2.1.1.3. 1987 and 1988 oil price weakness Throughout 1987, t h e o i l p r i c e more o r l e s s remained i n t h e range o f US $ 15 - 22 w i t h t h e average b e i n g US $ 1 7 - 18. I n e a r l y 1988, however, some p o l i t i c a l and economical e f f e c t s r e l a t e d t o t h e s t i l l ongoing I r a n i a n - I r a q i a n war and t h e renewed weakness o f t h e US $ w i t h o u t any hope o f n e a r - f u t u r e r e a s o n a b l e improvement o f i t s exchange r a t e t o o t h e r l e a d i n g w o r l d c u r r e n c i e s such as DM and k t r i g g e r e d a n o t h e r a t l e a s t temporal d e c l i n e o f t h e r e a l o i l p r i c e t o an average l e v e l o f US $ 12 - 15 d e s p i t e t h e o f f i c i a l l i s t p r i c e r e m a i n i n g a t US $ 18 (AL-CHALABI 1987, ATTIGA 1987, VIELVOYE 1987 a, NARAGHl 1988; O I L DAILY 1988 a, 1988 e ) a t which t h e economics o f f i n d i n g and d e v e l o p i n g new r e s e r v e s i n non-OPEC p r o d u c i n g c o u n t r i e s a r e r a t h e r m a r g i n a l (NARAGHI 1 9 8 8 ) . S h o r t - t e r m f l u c t u a t i o n s ( d a i l y t o m o n t h l y ) o f o i l p r i c e may i n c l u d e p l u s o r minus U S $ 5 . I n m i d 1988, n e i t h e r t h e end o f t h e I r a n i a n - I r a q i a n war n o r t h e s l i g h t l y r e c o v e r i n g US $ exchange r a t e had a s t a b i l i z i n g impact on t h e o i l p r i c e which r e mained t o be weak and i n t h e l o n g e r range u n p r e d i c t a b l e i n terms o f i t s f u r t h e r e v o l u t i o n . I n l a t e 1988, an a g a i n s l i g h t l y f a l l i n g v a l u e o f t h e US $ a f t e r an i n t e r m i t t e n t r e c o v e r y and v a r i o u s p o l i t i c a l c o n s t e l l a t i o n s i n c l u d i n g predomi-
111 n a n t l y t h e t h r e a t o f another OPEC p r i c e war (WILLIAMS 1988) l e a d i n g i n d i s p e n s i b l y t o a r e p e t i t i o n o f t h e 1986 p r i c e c o l l a p s e w i t h p r o b a b l y even worse consequences had a downwards impact on t h e o i l p r i c e which averaged a t US $ 12 - 15 (OIL GAS JOURNAL 1988 d) and i n s h o r t p e r i o d s even ranqed a t US $ 10 - 13. Downwards p r e s s u r e n t h e US $ exchange r a t e and thus a l s o - t h e r e a l o i l p r i c e was y t h e p r e s i d e n t i a l e l e c t i o n i n t h e USA a t t h e end o f 1988 which also exerted gave a f o r e c a s t o f t h e economical p o l i c y t o be expected f o r t h e n e x t c o u p l e o f years.
2.2.1.1.4. Cyclicities during course of the oil price evolution Several cyc i c i t i e s have been o f key s i g n i f i c a n c e d u r i n g course o f t h e o i l p r i c e e v o l u t i o n and had d e c i s i v e impact on t h e p r o g r e s s i o n o f t h e s u i t e o f hydrocarbon p r i c e changes. O i l p r i c e h i s t o r y was i n summary c h a r a c t e r i z e d i n 1979 - 1981 by c y c l i c a l l y overheated o i l p r i c e s t r i g g e r i n g an overexpansion o f d r i l l i n g t h a t i n t u r n caused an o v e r b u i l d i n g o f c a p a c i t y and i n v e n t o r y (PERKINS 1988). The d e c l i n i n g a c t i v i t y due t o s l o w l y f a l l i n g o i l p r i c e l e d t o c y c l i c a l c o r r e c t i o n o f excess i n v e n t o r y i n 1982 - 1984, whereas t h e s c e n a r i o was d i s t u r bed i n 1984 - 1985 by a c y c l i c a l l y o v e r v a l u e d US $. I n 1986, c y c l i c a l c o r r e c t i o n o f o i l p r i c e s caused d r i l l i n g c o l l a p s e , and add i t i o n a l impact was g i v e n by b r i n g i n g t h e US $ back t o r e a l i s t i c v a l u e s and even below t h a t . 1987 was a t r a n s i t i o n a l y e a r a f t e r t h e b i g s l i d e and was char a c t e r i z e d by hope and o u t l o o k t o b e t t e r p o s s i b i l i t i e s , and t h e temporary agreement on a c o n v e n t i o n p r i c e ( c f . s e c t i o n 2.2.1.4.3.8) gave an a r t i f i c i a l s t a b i l i z a t i o n o f t h e s i t u a t i o n which many companies took advantage o f and c o n s o l i d a t e d . The 1988 renewed o i l p r i c e i n s t a b i l i t y and v o l a t i l i t y i s m a i n l y t h e r e s u l t o f t h e i n d e c i s i v e n e s s o f t h e OPEC as a whole c o n c e r n i n g p r i c e p o l i t i c s and l a c k i n g d i s c i p l i n e o f v a r i o u s member c o u n t r i e s t o observe a l l o c a t e d and agreed o u t p u t quotas (CROUSE 1988 b ) . M i d t o l a t e 1988 has p a r a l l e l s w i t h t h e g r e a t 1986 o i l p r i c e c r a s h even though netback p r i c i n g i s d i s c o n t i n u e d , w i t h t h e o n l y comf o r t b e i n g t h a t a f u l l r e r u n o f 1986 i s n o t expected (VIELVOYE 1988 b ) .
2.2.1.1.5. Reasons and effects of the 1986 oil price crash Reviewing t h e p o l i t i c a l and economical c o n s t e l l a t i o n s , t h e renouncement o f d e f i n i t e p r o d u c t i o n q u a n t i t i e s and f i x e d crude o i l p r i c e s i n l a t e 1985 and t h e replacement o f t h e f o r m e r l e a d i n g aspects by t h e new goal o f achievement o f a reasonable share o f w o r l d o i l p r o d u c t i o n by t h e OPEC c o n s o r t i u m had t h e r e s u l t t h a t t h e p r o d u c i n g c o u n t r i e s had t o c o n c e n t r a t e on t h e w o r l d market c o n c e r n i n g p r i c i n g and c o n t r a c t n e g o t i a t i n g (ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 c ) . The main i n f l u e n c i n g f a c t o r s a r e netback p r i c i n g and market share s t r a t e g y vs. f i xed p r i c i n g and p r o d u c t i o n l i m i t a t i o n , and o i l p r i c e defense and market compatib i l i t y v s . changing p r i c i n g p o l i c y .
2.2.1.1.5.1. Netback pricing and market share strategy vs. fixed pricing and production limitation The consumers u t i l i z e d t h e c o s t - e f f e c t i v e s u p p l y a t netback c o n d i t i o n s o r on t h e s p o t market (BECK & WILLIAMS 1987, OIL GAS JOURNAL 1987 f ) , w i t h t h e consequence b e i n g t h a t i n 1986, almost 90 % o f t h e w o r l d o i l t r a d e was done based on s p o t market c o n d i t i o n s . T h i s new s t r a t e g y o f netback c o n d i t i o n s f i n a l l y caused t h e 1986 o i l p r i c e c r a s h . I n l a t e 1986, t h e OPEC group a d j u s t e d themselves t o t h e new s i t u a t i o n b y r e n o u n c i n g f r o m m a r k e t share p o l i c y and r e t u r n i n g t o p r o d u c t i o n q u a n t i t y agreement and d i s c i p l i n e ( w i t h t h e l a t t e r aim h a v i n g been a t l e a s t intended, w h i l e achievement was hampered by c o n t i n u o u s v i o l a t i o n through some members o f t h e c a r t e l ) , and s t a b i l i z e d t h e o i l p r i c e a t a b t . US $ 18 by dec i d i n g t o p r e v e n t f u r t h e r e x t e n s i o n o f t h e p r i c e war, s t o p netback s a l e s by end o f 1986 and r e t u r n t o base p r i c e s and p r o d u c t i o n l i m i t a t i o n ( c f . s e c t i o n
112 2.2.1.4.3.8.). Thus t h e i n c r e a s i n g o i l market share s t r a t e g y o r c h e s t r a t e d by S a u d i - A r a b i a i n l a t e 1985 b r o u g h t about t h e most p r e c i p i t o u s d e c l i n e i n o i l p r i c e s i n t h e OPEC h i s t o r y w i t h i n a v e r y s h o r t t i m e p e r i o d (NARAGHI 1988). The m a j o r i m p l i c a t i o n o f t h i s e x e r c i s e i n c o n t r a s t t o p r e v a i l i n g c o n v e n t i o n a l wisdom was t h a t t h e r e e x i s t e d a s h o r t r u n p r i c e e l a s t i c i t y o f demand f o r OPEC o i l o f substant i a l magnitude and e f f e c t i v e n e s s . The immediate t a s k f a c e d by t h e OPEC was t o implement a m a r k e t - c l e a r i n g o i l p r i c e s t r a t e g y t h a t would balance c u r r e n t econom i c a l u t i l i t y o f member c o u n t r i e s , would e f f e c t i v e l y r e p r e s e n t t h e m a r g i n a l c o s t o f crude o i l p r o d u c t i o n by non-OPEC c o u n t r i e s , and would c u r t a i l s u b s t i t u t i o n o f t h e o t h e r competing f u e l s ( c f . s e c t i o n s 2.2.1.2.2. and 2 . 2 . 1 . 2 . 4 . ) .
2.2.1.1.5.2.
Oil price defense and market c m a t i b i l i t y
vs. changing pricing policy
I n t h e p a s t , OPEC had been a b l e t o defend t h e o i l p r i c e when i t s m a r k e t share had been more o r l e s s c o m p a t i b l e w i t h i t s needs and c o u l d be d i s t r i b u t e d among member c o u n t r i e s w i t h o u t c r e a t i n g f i n a n c i a l h a r d s h i p (AL-CHALABI 1987). I n t h e golden years, OPEC was a b l e t o m a i n t a i n i t s r o l e o f a swing p r o d u c e r t o defend a p r i c e f r o m w h i c h o t h e r s b e n e f i t e d i n terms o f g r e a t e r i n v e s t m e n t s i n o i l sources ( c f . s e c t i o n 2.2.1.2.2.1.). W i t h d e c l i n i n g market share, however, t h e p r i c e defense mechanism became more and more p r e c a r i o u s i n 1985. Saudi Arab i a , assuming t h e r o l e o f swing producer w i t h i n t h e OPEC by a b s o r b i n g t h e a d d i t i o n a l f a l l i n demand and a l l o w i n g t h e o t h e r members t o produce a t t h e i r f u l l q u o t a o r even exceed i t , was t e m p o r a r i l y a b l e t o s t i l l defend t h e p r i c e . Once demand dropped below t h e c r i t i c a l r a t e o f 2 M i o . b b l / d p r o d u c t i o n , Saudi A r a b i a had t o change i t s p o l i c y i n o r d e r t o have t h e p o s s i b i l i t y t o produce i t s f u l l q u o t a as f i x e d i n l a t e 1984. The m a r k e t - o r i e n t e d p r i c e mechanism o f Saudi Arab i a then l e d t o de f a c t o abandonment o f t h e f i x e d p r i c e system w h i c h c o u l d be defended o n l y by p r o d u c t i o n programming, t o more o r l e s s market-share p r i c i n g p o l i c y , which i n t u r n had tremendous impact on t h e o i l p r i c e l e v e l and caused the worldwide petroleum i n d u s t r y crash. TEWKSBURY (1977) performed a long-range f o r e c a s t o f o i l p r i c e e v o l u t i o n which concluded w i t h t h e p r e d i c t i o n o f a reasonable o i l p r i c e r e d u c t i o n i n 1985, w i t h t h i s p r o g n o s i s h a v i n g been a c t u a l l y met i n l a t e 1 9 8 5 / e a r l y 1986 a l most t e n y e a r s a f t e r t h e p r o g n o s i s was made and n e a r l y e x a c t l y as i t was p r e viewn. The f r e e f a l l o f o i l market p r i c e s and ensuing market chaos f r o m a more o r l e s s s t a b l e range o f U S $ 28 - 34 f o r many y e a r s up t o l a t e 1985 and t h e r a p i d i t y w i t h which nominal p r i c e s plunged t o US $ 6 - 9 i n e a r l y t o m i d 1986 i s t h e drawback o f t h e f o r m e r success o f t h e OPEC i n m a i n t a i n i n g h i g h o i l p r i c e s (McCANN 1988). The h i g h p r i c e s o f t h e l a t e 1970's and e a r l y 1980's c a l l e d f o r t h a r a p i d i n c r e a s e i n non-OPEC o i l p r o d u c t i o n which i n c r e a s i n g l y t h r e a t e n e d OPEC 's market share.
2.2.1.2.
Effects of o i l price changes on hydrocarbon industry
Summarizing t h e h i s t o r i c a l e v o l u t i o n , a f t e r two o i l p r i c e e x p l o s i o n s w h i c h generated an i n c r e a s e i n e x p l o r a t i o n and development a c t i v i t y up t o a l e v e l bei n g much h i g h e r t h a n e v e r b e f o r e s i n c e t h e Second World War, t h e slow and c o n t i nuous d e p r e s s i o n f r o m t h e c e r t a i n l y exaggerated peak t o more o r l e s s normal pos i t i o n s and somewhat below was suddenly a c c e l e r a t e d by a pronounced o i l p r i c e c r a s h t h a t t u r n e d t h e apparent o r a r t i f i c i a l undersupply problem i n t o a r e a l underdemand c r i s i s . While t h e hydrocarbon i n d u s t r y i n a l l p a r t s o f t h e w o r l d was more than happy t o a d j u s t t o t h e h i g h e r o i l p r i c e s by expansion o f t h e b u s i ness, t h e r e v e r s e a d a p t a t i o n t o t h e l o w e r p r i c e s by c u t t i n g back was v e r y p a i n f u l and h i t t h e whole economical system, e s p e c i a l l y because i t was a l s o more o r l e s s accompanied by t h e most s e r i o u s s l i d e - d o w n o f t h e US $ exchange r a t e dur i n g t h e l a s t 40 y e a r s .
113 The considerable consequences o f t h e o i l p r i c e crash on e x p l o r a t i o n and exp l o i t a t i o n a c t i v i t y have t o be understood i n l i g h t o f c o s t p e r b a r r e l being the most important economic d e c i s i o n t o o l (EYCK 1987). The o i l p r i c e c o l l a p s e i n i t i a t e d a whole s e r i e s o f fundamental changes i n the hydrocarbon i n d u s t r y , rang i n g from a c t u a l sharp c u t s o f income f o r governments and companies t o a very changed environment w i t h many u n c e r t a i n t i e s (HOLMES 1987). The o u t l i n e as f o l lows comments on d r i l l i n g r i g count as w e l l as a r t i f i c i a l and n a t u r a l market forces. Some v a r i a b l e s determining o i l p r i c e l e v e l , market f o r c e s and o i l p r i c e e v o l u t i o n , and o i l consumption and o u t p u t are a l s o i l l u s t r a t e d .
2.2.1.2.1.
Drilling rig
count
An overview o f the h i s t o r i c a l o i l p r i c e e v o l u t i o n i n c l u d i n g comments on p r i c e e l a s t i c i t y and p o s s i b i l i t i e s o f p r e d i c t i o n o f f u t u r e o i l p r i c e changes and c r e d i b i l i t y o f such f o r e c a s t s i s a l s o presented by DOUGHERTY (1987). The i m p a c t o f t h e upwards and downwards changes o f the o i l p r i c e d u r i n g the l a s t f i f teen years i s h i g h l i g h t e d by the USA count o f a c t i v e r i g s d r i l l i n g on hydrocarbon t a r g e t s ( t h a t can be considered as being the most p r e s c r i p t i v e i n d i c a t o r o f the i n d u s t r y ' s investment l e v e l ) .
2.2.1.2.1.1.
1986 and 1987
The annual average USA r i g count reached from a number below 1,500 b e f o r e 1973 i t s climax i n 1981 w i t h abt. 4,500 r i g s , w i t h t h i s f i g u r e being f i v e and a h a l f times h i g h e r than b o t h t e n years e a r l i e r and f i v e years l a t e r when i n 1986 a t o t a l world-wide number o f l e s s than 1,000 a c t i v e r i g s was recorded (MILLER & NEWLIN 1987, SCHUBERT 1987) comprising i n e a r l y 1986 a b t . 900 and i n mid 1986 a b t . 700. I n the N o r t h Sea r e p r e s e n t i n g the most important European petroleum p r o vince, the sharp i n c i s i o n caused by t h e 1986 o i l p r i c e crash i s r e f l e c t e d by a t o t a l count o f w e l l s d r i l l e d o f 260 i n 1984 and 1985 each vs. 180 i n 1986 and 200 i n 1987 (QUINLAN 1988). I n the whole o f f s h o r e Europe area comprising N o r t h Sea, I r i s h Sea, B a l t i c Sea and A d r i a t i c Sea, abt. 50 - 60 r i g s were a c t i v e a f t e r the o i l p r i c e crash i n 1986 and 1987, whereas p r i o r t o t h a t more than 90 r i g s were running (McNALLY 1987). A s i m i l a r i n d i c a t i o n i s g i v e n by t h e worldwide number o f seismical p a r t i e s a c t i v e i n hydrocarbon e x p l o r a t i o n which d e c l i ned from abt. 1,250 i n the golden y e a r o f 1981 t o a b t . 400 i n e a r l y 1987 (ERDUL UND KOHLE - ERDGAS - PETROCHEMIE 1987 b ) . I n the USA, abt. 12,700 o p e r a t o r s d r i l l e d i n 1982 a t l e a s t one w e l l on an o p e r a t o r - o f - r e c o r d basis, b u t i n 1987, t h i s number had dropped t o o n l y a b t . 5,700 operators, w i t h v i r t u a l l y almost a l l o f these 7,000 l o s t o p e r a t o r s having been independents (MOORE 1988 a; c f . sect i o n 2.2.1.5.2.). The t o t a l number o f w e l l s completed i n the USA d u r i n g 1986 was abt. 35,000 which was t h e lowest amount since 33,000 boreholes were f i n i s h ed i n 1974. With t h e c o n t i n u a t i o n o f low o i l p r i c e s , e x p l o r a t i o n and a p p r a i s a l d r i l l i n g l e v e l s i n t h e N o r t h Sea were slashed t o t h e bare minimum i n t h e second h a l f o f 1986 and the f i r s t h a l f o f 1987 ( c f . s e c t i o n s 2.2.1.5.2. and 2.4.2.), whereas i n t h e second h a l f o f 1987, a c t i v i t y p i c k e d up considerably, because companies learned t o l i v e w i t h t h e prospect o f a medium-term o i l p r i c e around US $ 18 ( c f . s e c t i o n 2.2.1.4.3.8.), and r e l a t i v e l y inexpensive r i g and t h e r e f o r e t o t a l o p e r a t i n g r a t e s s t a r t e d t o look a t t r a c t i v e (CAPEL 1988). D r i l l i n g a c t i v i t y i s p a r t i c u l a r l y s e n s i t i v e t o boom/bust c y c l e s (PERKINS 1988). D r i l l i n g i s i n h e r e n t l y c y c l i c a l because i t i s e x t r a o r d i n a r i l y c a p i t a l - i n t e n s i v e . The h i g h r i s k , l a r g e number o f d e c i s i o n makers and h i g h frequency o f reinvestment d e c i s i o n s add t o the v o l a t i 1 it y . C o n t r a s t i n g w i t h a l l o t h e r r e p o r t s , POPESCU & HODGSHON (1987) p u t forward t h a t e x p l o r a t o r y d r i l l i n g i n Western Europe as a whole was v i r t u a l l y unchanged i n 1986 from i t s 1985 l e v e l , and N o r t h Sea d r i l l i n g r e d u c t i o n i n 1986 was o n l y
114 a b t . 10 %, w i t h , however, t h e amount o f w e l l s h a v i n g been d r i l l e d i n t h e f i r s t q u a r t e r o f 1986 i n t h e B r i t i s h N o r t h Sea h a v i n g been h i g h e r t h a n i n any o t h e r q u a r t e r of U n i t e d Kingdom o f f s h o r e o i l and gas a c t i v i t y . T h i s r e l a t i o n s h i p f a l s i f i e s t h e o v e r a l l p i c t u r e , as many o f t h e i t e m s d r i l l e d i n t h e f i r s t q u a r t e r o f 1986 would have been suspended o r c a n c e l l e d i f i t would n o t have been t o o l a t e a t t h e moment when t h e o i l p r i c e r a p i d l y dropped. On t h e o t h e r hand, a s l i g h t t o moderate decrease i n d r i l l i n g frequency i n t h e B r i t i s h N o r t h Sea i n 1986 i s n o t n e c e s s a r i l y a d i r e c t consequence o f t h e o i l p r i c e c o l l a p s e , b u t has a l s o t o be understood i n l i g h t o f 1984 and 1985 h a v i n g s e t r e c o r d s o f U n i t e d Kingdom o f f s h o r e d r i l l i n g abundance (HOLBROOK 1987). Aspects o f hydrocarbon acq u i s i t i o n a c t i v i t y i n an e r a o f low o i l p r i c e s a r e a l s o d i s c u s s e d by WOODS & HOLTBERG ( 1 9 8 6 ) . The f l u c t u a t i o n s o f t h e d r i l l i n g r i g number as a consequence o f t h e economical f e a s i b i l i t y s c e n a r i o changes h i g h 1 i g h t t h a t t h e r e c o v e r a b l e hydrocarbon r e s e r v e s a r e a f f e c t e d by o i l and gas p r i c e s (GRUY, GARB & HERRON 1980; GARB, GRUY & HERRON 1981).
2 . 2 . 1 . 2 . 1 . 2 . 1988 I n 1988, o v e r a l l w o r l d d r i l l i n g i s r i s i n g a g a i n c o n s i d e r a b l y i n many areas and i s exceeding t h e l e v e l o f 1986 and p o s s i b l y even t h a t o f 1985 (WORLD O I L 1988 a ) . T o t a l number o f w e l l s d r i l l e d worldwide a r e a b t . 60,100 i n 1986, a b t . 57,100 i n 1987 and a b t . 60,900 i n 1988. By end o f 1987, t o t a l w o r l d o i l r e s e r ves have reached a b t . 800 B i l l . b b l , and t o t a l w o r l d gas r e s e r v e s have a t t a i n e d a b t . 3,860 T c f . I n t h e USA, o i l w e l l c o m p l e t i o n s decreased f r o m a b t . 41,000 in 1984 v i a a b t . 26,000 i n 1985 t o a b t . 17,000 i n 1986, and gas w e l l c o m p l e t i o n s d i m i n i s h e d f r o m a b t . 15,700 i n 1984 v i a 10,000 i n 1985 t o o n l y 6,500 i n 1986 (MOORE 1986, 1987 b ) . Reviewing o i l p r i c e h i s t o r y w i t h a p r i c e e s c a l a t i o n o f a b t . 40 % / y e a r on average o v e r a p e r i o d o f s i x y e a r s (1973 - 1979) r e v e a l s t h a t t h e impetus of t h e o r d e r o f magnitude i n c r e a s e i n p r i c e ( w h i c h was g r e a t l y i n excess o f t h e i n f l a t i o n r a t e ) caused reduced consumption t h r o u g h buyer c h o i c e , c o n s e r v a t i o n , s u b s t i t u t i o n , enhanced e f f i c i e n c y and c o n v e r s i o n t o o t h e r energy sources, and b r o u g h t on m a j o r i n c r e a s e s i n non-OPEC o i l p r o d u c t i o n c a p a c i t y . A f t e r t h e peak o f t h e c r a s h , t h e USA d r i l l i n g r i g c o u n t o n l y s l o w l y improved v i a a s l i g h t f u r t h e r d e c l i n e u n t i l e a r l y 1987 r e f l e c t i n g numerous p r o j e c t canc e l l a t i o n s and postponements t o a l i t t l e h i g h e r f i g u r e o f a b t . 800 - 950 i n m i d 1987 (BECK 1987), t h u s s t i l l s t a y i n g a t a l e v e l o f o n l y a b t . one f i f t h o f t h a t i n t h e golden y e a r s (AL-CHALABI 1987). I n t h e f i r s t h a l f o f 1988, an i n c r e a s e i n a c t i v e d r i l l i n g r i g s o f a b t . 20 % was recorded which c o u l d b r i n g t h e annual r i g c o u n t average by end o f 1988 t o 1,030 - 1,120, w i t h 1988 t h u s o v e r a l l app e a r i n g t o be a n o t h e r r e c o v e r y y e a r f o r t h e p e t r o l e u m i n d u s t r y as a c t i v i t y i n creases f o r t h e second s t r a i g h t y e a r s i n c e t h e 1986 o i l p r i c e c o l l a p s e (CROUSE 1988 a ) . I t t h u s t e m p o r a r i l y l o o k e d l i k e as i f t h e boundary o f 1,000 r i g s m i g h t p r o b a b l y f o r t h e f i r s t t i m e be exceeded a g a i n i n 1988 a f t e r t h e depressed y e a r s o f 1986 and 1987 when f o l l o w i n g a number o f almost 2,000 i n 1985 t h e r i g c o u n t f e l l f o r two y e a r s on average below 1,000. The mid t o l a t e 1988 o i l p r i c e dec l i n e f r o m U S $ 16 - 18 t o 12 - 15, however, r e s u l t e d i n t h e average m o n t h l y r i g c o u n t i n t h e second h a l f o f 1988 d r o p p i n g t o 920 - 940 (OIL DAILY 1988 f ) which most p r o b a b l y b r i n g s t h e annual average again down below 1,000. Counting w i t h a c u t o f f o f 45 days a c t i v i t y p e r i o d , t h e number o f a c t i v e r i g s was i n 1988 w i t h a b t . 1,530 a b t . 10 % h i g h e r t h a n i n 1987 when a b t . 1,390 r i g s were a c t i v e i n t h e USA, i n d i c a t i n g t h a t a f t e r s i x c o n s e c u t i v e y e a r s o f d e c l i n i n g number o f a v a i l a b l e r i g s , heavy r i g f l e e t a t t r i t i o n i n t h e USA i s p r o b a b l y coming t o an end (FITTS & CROWHURST 1988). R i g u t i l i z a t i o n w h i c h i s d e f i n e d as t h e r a t i o o f a c t i v e r i g s t o a v a i l a b l e r i g s was 56 % i n 1988 w i t h r e s p e c t t o 42 % i n 1987 which, however, i s s t i l l c o n s i d e r a b l y l o w e r t h a n t h e 67 % o f 1984 and l a g s markedly b e h i n d t h e average r a t e found d u r i n g t h e l a s t 35 y e a r s .
115
Artificial and natural market forces
2,2.1.2.2.
The u n y i e l d i n g downward p r e s s u r e o f t h i s c o m b i n a t i o n caused t h e e x p o r t volume o f t h e OPEC t o f a l l d r a m a t i c a l l y , f i n a l l y r e s u l t i n g i n c a p i t u l a t i o n o f t h e OPEC t o market f o r c e s i n l a t e 1985 w i t h abandonment o f p r o d u c t i o n r e s t r a i n t i n f a v o u r o f s e e k i n g i t s f a i r share i n t h e i n t e r n a t i o n a l o i l market (RANDOL 1986). Some aspects o f swing p r o d u c t i o n f o r market imbalance compensation as w e l l as OPEC r e s i d u a l market share and p r i c e e f f e c t s a r e o u t l i n e d as f o l l o w s .
2.2.1.2.2.1.
Swing production for market imbalance compensation
The main reason f o r t h e 1986 o i l p r i c e c r a s h was t h e d e c i s i o n o f Saudi Arab i a t o be no l o n g e r t h e swing p r o d u c e r i n t h e OPEC ( b y swinging, Saudi A r a b i a f o r a l o n g t i m e s u c c e s s f u l l y p r e v e n t e d t h e c o l l a p s e o f t h e o i l p r i c e due t o mark e t power; ERDUL UND KOHLE - ERDGAS - PETROCHEMIE 1987 c, OIL GAS JOURNAL 1987 f, VIELVOYE 1988 b ) , because t h e c o u n t r y was g e n u i n e l y f e d up w i t h i t s f o r m e r s e l f - c h o s e n r o l e o f b e i n g t h e swing producer (BECK & WILLIAMS 1987; c f . s e c t i o n 2.2.1.4.2.4.) and s i n g l e - h a n d e d l y p r o p p i n g up t h e sagging p r i c e s t r u c t u r e o f t h e a s s o c i a t i o n w h i l e o t h e r OPEC members showed no d i s c i p l i n e i n r e s t r a i n i n g o u t p u t and w h i l e t h e r e was no c o o p e r a t i o n f r o m non-OPEC o i l e x p o r t e r s t o h e l p m a i n t a i n a s t a b l e market. W h i l e t h e s e l f - r e s t r i c t i o n o f Saudi A r a b i a ' s p r o d u c t i o n t o a b t . 2.5 Mio. b b l / d i n 1985 s t i l l e f f e c t i v e l y compensated t h e excesses o f e x p l o i t a t i o n by o t h e r u n d i s c i p l i n e d OPEC members t h a t c o n t i n u e d t o v i o l a t e t h e i r quotas, an i n crease t o a b t . 5.0 Mio. b b l / d s i n c e t h e end o f 1985 (ERDUL UND KOHLE - ERDGAS PETROCHEMIE 1987 c ) when Saudi A r a b i a was f o r c e d t o pay more s e r i o u s a t t e n t i o n t o i t s own economical and f i n a n c i a l balance r a t h e r t h a n t o subvent b r o t h e r count r i e s t h a t a r e u n w i l l i n g t o r e s p e c t and accept t h e r u l e s and r e g u l a t i o n s , r e s p e c t i v e l y , i n i t i a t e d t h e f r e e f a l l o f t h e o i l p r i c e down t o a l e v e l which r e f l e c t s i t s n a t u r a l market v a l u e r a t h e r t h a n t h e a r t i f i c i a l l y l i f t e d p o s i t i o n t h a t was c h a r a c t e r i z e d by p o l i t i c a l t h r e a t e n i n g and d i c t a t i o n . W i t h Saudi Arab i a b e i n g u n w i l l i n g t o f u r t h e r p l a y t h e swing producer, no o t h e r OPEC n a t i o n i s l i k e l y t o be ready o r even a b l e t o f u l f i l l t h i s f u n c t i o n (PETROLEUM REVIEW 1987 a). I n o r d e r n o t t o t h r e a t t h e US $ 18 agreement p r i c e , Saudi A r a b i a n e v e r t h e l e s s c o n t i n u e d t o p l a y t h e r o l e o f swing producer i n 1987 when necessary (BECK 1987, CROUSE 1988 b ) . D u r i n g t h e l a s t t h r e e years, almost a l l t h e i n c r e a s e s and r e d u c t i o n s o f t o t a l OPEC o u t p u t w i t h i n a f l u c t u a t i o n marge o f 3 - 4 Mio. b b l / d were performed and backed-up by Saudi A r a b i a through swinging, w i t h Saudi Arab i a t h e r e f o r e h a v i n g a p i v o t a l r o l e i n t h e w o r l d o i l m a r k e t (THOMPSON & SINGLETON 1988). Comments on o i l p r i c e and demand i n t e r a c t i o n s a r e a l s o g i v e n by LUMSDEN ( 1 9 8 8 ) .
2.2.1.2.2.2.
OPEC residual market share and price effects
I n terms o f market share, v a r i o u s o i l p r i c e e x p l o s i o n s and t h e b i g o i l p r i c e c r a s h have a l l d r a s t i c a l l y changed t h e p a r t i c i p a t i o n o f t h e OPEC group i n t h e t o t a l w o r l d o i l m a r k e t (BILD 1987). The OPEC group s t a r t e d t o f i x and t o cont r o l t h e o i l p r i c e and t h u s t o c l a i m a market share i n 1973/1974. A f t e r t h e second o i l p r i c e shock i n 1979/1980, t h e slow downwards o r i e n t a t i o n o f t h e o i l p r i c e f o r c e d t h e OPEC b i t t o b i t t o concede on p r i c e , w h i l e a t t h e same t i m e t h e market share d r a s t i c a l l y f e l l and was h a l v e d i n t h e p e r i o d 1981 t o 1985. R e a l i s t i c a l l y , t h e OPEC group always had o n l y t h e r e s i d u a l m a r k e t share (PETROLEUM REVIEW 1988 c ) i n s u p p l y i n g those q u a n t i t i e s necessary f o r d e l i v e r y which t h e customer c o u n t r i e s c o u l d n o t e s t a b l i s h by domestic p r o d u c t i o n o r i m p o r t f r o m somewhere e l s e . Increasing
exploitation
of
own r e s e r v e s and c o n s i d e r a b l e e f f e c t s o f energy
116 c o n s e r v a t i o n were t h e main reasons f o r t h e h a l v i n g o f t h e r e s i d u a l market share o f t h e OPEC. As market share i s a t t h e b o t t o m o f t h e l i n e always c a l c u l a t e d i n q u a n t i t y t i m e s p r i c e , f o l l o w i n g t h e o f f i c i a l renouncement o f a f i x e d p r i c e i n l a t e 1985 and t h e g i v e - u p o f p r o d u c t i o n l i m i t a t i o n quotas, t h e tremendous p r i c e f a l l i n combination w i t h s l i g h t l y increasing q u a n t i t i e s could n o t reverse t h e downwards o r i e n t a t i o n o f t h e o i l s a l e revenues f o r t h e OPEC group i n 1986, and o n l y t h e r e t u r n t o f i x e d p r i c e s and l i m i t e d p r o d u c t i o n q u a n t i t i e s i n l a t e 1986/ e a r l y 1987 was a b l e t o t r i g g e r a g a i n a moderate i n c r e a s e i n m a r k e t share and r e venues. The n e t e f f e c t o f t h e 1986 o i l p r i c e c r a s h was a 16 % i n c r e a s e o f t h e OPEC o i l p r o d u c t i o n t o reach a b t . 33 % share o f w o r l d o i l o u t p u t (ERDUL-ERDGASKOHLE NACHRICHTEN 1987 e ) . The 1986 o i l p r i c e c r a s h has a much b i g g e r impact on company upstream spendi n g t h a t upon a c t u a l s u p p l y (PETROLEUM REVIEW 1987 a ) . I t has t u r n e d o u t t h a t non-OPEC p r o d u c t i o n c o s t s were l o w e r than was g e n e r a l l y assumed, and a f u r t h e r b u f f e r a g a i n s t more d r a s t i c i n d i g e n o u s p r o d u c t i o n l o s s e s were those o u t p u t t a x regimes i n p l a c e . I f a company i s p a y i n g a r e l a t i v e l y h i g h m a r g i n a l t a x r a t e , o i l p r i c e d e c l i n e s can i n i t i a l l y impact more s e v e r e l y on t h e t r e a s u r y o f t h e government imposing t h a t t a x t h a n upon t h e company i t s e l f . S i m u l t a n e o u s l y , t h e p r i c e f a l l engendered a s t r o n g upstream e f f i c i e n c y d r i v e a l o n g w i t h s h a r p l y f a l l i n g o i l f i e l d equipment and s e r v i c e c o s t s .
2 . 2 . 1 . 2 . 3 . Variables determining oil price level Some v a r i a b l e s d e t e r m i n i n g o i l p r i c e l e v e l a r e d i s c u s s e d as f o l l o w s a l o n g t h e l i n e s o f c o u p l i n g o f o i l p r o d u c t i o n r a t e and p r i c e , c u r r e n t i d e a l o i l p r i c e l e v e l , o i l p r o d u c t i o n c o s t s and o i l s e c t o r dominance, and o i l p o l i t i c a l t r i a n g l e USA - Saudi A r a b i a - I r a n .
2.2.1.2.3.1. Coupling o f oil production rate and price niveau O i l p r i c e e v o l u t i o n a l s o has shown t h a t t h e p r i m a r y d e t e r m i n a n t o f c r u d e o i l p r i c e i s t h e volume o f crude o i l o f f e r e d f o r s a l e , w i t h r e l a t i v e l y small percentage changes i n volume c a u s i n g r e l a t i v e l y l a r g e percentage changes i n p r i c e ( f o r example, a l o w e r i n g o f t h e q u a n t i t y o f f e r e d f o r purchase b y a b t . 4 % o f t h e w o r l d consumption i n l a t e 1 9 8 6 / e a r l y 1987 l i f t e d t h e p r i c e by a b t . 40 % from 12 t o 18 US $; DOUGHERTY 1987, NARAGHI 1988, OIL DAILY 1988 e ) . C a l c u l a t i n g w i t h t h e o f f i c i a l OPEC l i s t p r i c e o f 29 US $ / b b l which was v a l i d s t i l l i n l a t e 1985, t h e c r a s h i n 1986 b r o u g h t t h e p r i c e down t o a b t . 25 - 30 % o f t h e f o r m e r v a l u e a t t h e peak o f t h e c r i s i s and a b t . 40 % on average, whereas t h e r e g a i n i n g consensus o f t h e group and r e t u r n t o p r o d u c t i o n l i m i t a t i o n s moved t h e p r i c e back t o a b t . 60 % o f i t s e a r l i e r l e v e l w i t h o u t , however, r e e s t a b l i s h i n g c o n f i d e n c e i n i t s s t a b i l i t y t h a t has been l o s t d u r i n g t h e p r i c e c o l l a p s e . I n l a t e 1987, o i l p r i c e was a b t . 25 % h i g h e r t h a n i n l a t e 1986 (CROUSE 1988 b ) . A t t h e b o t t o m o f t h e l i n e , t h e o i l p r i c e i s even e s t a b l i s h e d by t h e r a t e sel e c t e d by t h e m a j o r producer o f t h e w o r l d , w i t h t h i s p o s i t i o n h a v i n g changed f r o m t h e USA t o Saudi A r a b i a i n t h e 1960's. Examining t h e OPEC e f f o r t s t o come t o an agreement on p r o d u c t i o n r a t e s and p r i c e s , and j u d g i n g t h e r o l e o f Saudi A r a b i a f r o m t h e i r i n t e r e s t i n b o t h s e c u r i n g n a t i o n a l income and s t r e n g t h e n i n g t h e i r market l e a d e r s h i p , t h e 1986 o i l p r i c e upheaval cannot be i n t e r p r e t e d as r e f l e c t i n g t h e c o l l a p s e o f t h e OPEC, b u t r a t h e r was t h e l o g i c a l u n f o l d i n g o f Saudi A r a b i a ' s p l a n t o b r i n g o i l p r i c e down t o a s u s t a i n a b l e l e v e l . The e f f e c t o f Saudi A r a b i a ' s i n c r e a s i n g o i l market share s t r a t e g y was plummeting o f t h e average marker o i l p r i c e f r o m US $ 28 - 30 t o US $ 1 2 - 15, and d u r i n g t h e t i m e i n t e r v a l o f f r e e market f o r c e s , a 1 % i n c r e a s e i n g l o b a l o i l s u p p l y r e s u l t e d i n a 12 % d e c l i n e i n average o i l p r i c e (NARAGHI 1988). O t h e r economical aspects o f o i l p r i c e and i t s changes i n p a s t and f u t u r e a r e d i s c u s s e d by AL-BLEHED (1987), OOUGHERTY & AL-BLEHED (1987) and LUMSOEN (1988), and t h e c o n s t e l l a t i o n s w i t h i n t h e OPEC group t h a t l e d t o t h e o i l p r i c e c r a s h a r e e v a l u a t e d by RANDOL ( 1 9 8 6 ) .
117 SAFER (1977) comments on c h a l l e n g e s and o p p o r t u n i t i e s o f t h e w o r l d o i l market, p r e d i c t i o n o f o i l p r i c e e v o l u t i o n , and r o l e o f t h e OPEC group ( c f . a l s o TEWKSBURY 1977). WEFA (1988) analyzes boom o r b u s t o f t h e o i l market i n c l u d i n g upand downswings, and MACKAY CONSULTANTS (1988) p r e s e n t a p r e d i c t i o n o f t h e o i l and gas i n d u s t r y t r e n d s i n t h e n e x t y e a r s . KONZELMANN (1976) even assesses o i l as b e i n g t h e f a t e o f mankind. The g l o b a l o i l p r i c e i s p r e d o m i n a n t l y s e t by c o m p e t i t i o n among t h e w o r l d ' s o i l - e x p o r t i n g c o u n t r i e s and n o t by economists o f t h e o i l - i m p o r t i n g n a t i o n s (IVANHOE 1988). O i l consumed i n t e r n a l l y w i t h i n any p r o d u c i n g c o u n t r y s e l l s a t t h e i n t e r n a t i o n a l p r i c e s e t by t h e e x p o r t e r s . Non-OPEC p r o d u c t i o n i n c r e a s e s s i n c e t h e f i r s t o i l p r i c e e x p l o s i o n i n 1973 t h a t a r e used i n t e r n a l l y w i t h i n any n a t i o n a r e n o t a v a i l a b l e f o r e x p o r t and consequently t h i s o i l does n o t a f f e c t t h e i n t e r n a t i o n a l p r i c e , w i t h t h e c r i t i c a l volume t h u s o n l y b e i n g t h e n e t amount t h a t i s a v a i l a b l e f o r e x p o r t which i s u s u a l l y l e s s than 25 % o f t h e t o t a l p r o d u c t i o n o f non-OPEC c o u n t r i e s .
2.2.1.2.3.2. Current ideal oil price level The i d e a l o i l p r i c e l e v e l f o r a t l e a s t t h e OPEC group i s a t t h e moment a b t . US $ 18, because i t i s low enough t o spur demand b u t n o t s u f f i c i e n t l y h i g h t o head o f f d e c l i n e s i n non-OPEC p r o d u c t i o n (OIL GAS JOURNAL 1988 e ) . US $ 18 means a l e v e l o f demand f o r OPEC o i l t h a t y i e l d s optimum revenues f o r OPEC memb e r s t h u s s a t i s f y i n g OPEC needs w h i l e t a k i n g i n t o account t h e p r i c e e l a s t i c i t y o f supplyldemand. W h i l e US $ 18 i s h i g h l y d e s i r a b l e , US $ 15 would be t h e absol u t e minimum t o l e r a b l e f o r t h e OPEC group. A t US $ 18, demand f o r OPEC o i l i s a b t . 19.5 Mio. b b l / d i n 1987/1988. I f o i l p r i c e s d e c l i n e , OPEC's revenue s h o r t f a l l i n c r e a s e s g e o m e t r i c a l l y , w i t h t h e gap expanding a t i t s f a s t e s t r a t e once p r i c e s d r o p under US $ 15. I f o i l p r i c e s were t o remain i n t h e US $ 17 - 22 r a n ge, OPEC c o u l d b a s i c a l l y g e n e r a t e t h e same amount of revenues i f i t a d j u s t s i t s p r o d u c t i o n a c c o r d i n g l y , whereas f o r p r i c e s o f more t h a n US $ 22, OPEC would a g a i n be worse o f f . T h e r e f o r e i t i s expected t h a t i n l i g h t o f own i n t e r e s t , t h e OPEC group w i l l be a t t e m p t i n g t o s t a b i l i z e t h e o i l p r i c e a t a b t . US $ 18 t h e r e b y c o n f i r m i n g t h e l a t e 1986 agreement ( c f . s e c t i o n s 2.2.1.1.2. and 2.2.1.4.3.8.) by r e s t r i c t i n g p r o d u c t i o n t o a b t . 19.5 Mio. b b l / d . US $ 18 can be regarded as t h e maximum d e s i r a b l e p r i c e f o r t h e OPEC a t t h e moment, because a t t h i s l e v e l even denominated i n weakening d o l l a r s , some new p r o d u c t i o n c o n t i n u e s t o come on stream o u t s i d e OPEC, and w i t h h i g h e r p r i c e s , n a t u r a l gas and c o a l b e g i n t o r e p l a c e o i l (OIL GAS JOURNAL 1987 n ) .
2.2.1.2.3.3. Oil production costs and oil sector dominance I n view o f t h e enormous d i f f e r e n c e i n p r o d u c t i o n c o s t s o f 0.05 - 0.2 US $ / b b l i n t h e OPEC area and 5 - 15 US $ i n many o f t h e non-OPEC c o u n t r i e s , i t would have been a s i m p l e o p e r a t i o n f o r OPEC t o have d r i v e n down t h e p r i c e t o a p o i n t l o w e r t h a n non-OPEC l o n g - r u n m a r g i n a l c o s t l e v e l s (McCANN 1988). F o r var i o u s reasons, however, t h i s was never a v i a b l e s o l u t i o n f o r OPEC. Such a p r i c e d e c l i n e would be p o l i t i c a l l y unacceptable t o those OPEC members l a c k i n g l a r g e r e s e r v e s and would l e a d t o i n c r e a s i n g d i v i s i o n s between c o u n t r i e s w i t h m a j o r and m i n o r r e s e r v e s , w i t h l o w - r e s e r v e OPEC members t h u s b e i n g e f f e c t i v e l y e x c l u ded f r o m p a r t i c i p a t i n g i n a l o n g - t e r m s t r a t e g i c p r i c e d e c l i n e . Another reason i s t h e dominance o f t h e o i l s e c t o r t o OPEC member economies and e x p o r t revenues as opposed t o t h e r e l a t i v e l y m i n o r p a r t p l a y e d by t h e o i l s e c t o r i n t h e t o t a l g r o s s n a t i o n a l p r o d u c t o f many b i g g e r non-OPEC p r o d u c e r s (such as G r e a t B r i t a i n , Norway and M e x i c o ) . F i n a l l y , OPEC o i l p r o d u c t i o n i s r e l a t i v e l y p r i c e - i n e l a s t i c e s p e c i a l l y i n t h e s h o r t term, w i t h t h e d e s i r e d nonOPEC response t o any p r i c e f a l l b e i n g lagged, whereas t h e s h o r t - t e r m c o s t s t o
118
the c a r t e l would be f e l t immediately. T h u s the r e s t o r a t i o n of market s t a b i l i t y i n l a t e 1986 by p l a c i n g t h e US $ 18 agreement p r i c e ( c f . s e c t i o n s 2 . 2 . 1 . 1 . 2 . and 2 . 2 . 1 . 4 . 3 . 8 . ) a t more o r l e s s a midpoint between the p r i c e h i g h s of t h e e a r l y 1980's and the lows of e a r l y t o mid 1986 has t u r n e d o u t t o be t h e o v e r a l l optimum s o l u t i o n .
2.2.1.2.3.4.Oilpolitical triangle USA
-
Saudi Arabia
-
Iran
I n terms of p o l i t i c a l r e l a t i o n s h i p s , t h e o i l p r i c e g y r a t i o n s of the 1970's, t h e o i l p r i c e c o l l a p s e of 1986, and t h e subsequent o i l p r i c e r e c o v e r y a r e the r e s u l t of t h e i n t e r a c t i o n s w i t h i n a t r i a n g u l a r r e l a t i o n s h i p w i t h the USA a t the t o p and Saudi Arabia and I r a n forming the base of t h e t r i a n g l e (PETROLEUM REVIEW 1987 a ) . I t a p p e a r s from o i l p r i c e h i s t o r y t h a t whenever USA and e i t h e r Saudi Arabia o r I r a n a g r e e on a c e r t a i n p o l i c y towards o i l p r i c e , t h e t h i r d p a r t y has very l i t t l e i n f l u e n c e . In 1973, the Arabian o i l embargo l e d by Saudi Arab i a was undermined by the USA which voiced i t s t o t a l o p p o s i t i o n and a c t i v e l y encouraged I r a n and o t h e r non-Arabian c o u n t r i e s t o t a k e advantage of the s i t u a t i o n and t o overproduce. In t h e l a t e 1970's, USA s u p p o r t e d Saudi Arabia t o f l o o d the o i l market i n o r d e r t o p r e v e n t o i l p r i c e s from r i s i n g even h i g h e r . Throughout the 1980's, t h e USA was keen t o keep o i l p r i c e s a s low a s p o s s i b l e i n o r d e r t o reduce e n e r g y c o s t s and promote growth, which c o i n c i d e d w i t h the i n t e r e s t of Saudi Arabia t o s t i m u l a t e o i l demand t h a t had been badly a f f e c t e d by the p r i c e h i k e s of t h e 1 9 7 0 ' s . The l e v e l of o i l p r i c e s and t h e i n f l u e n c e which t h e p o l i t i c a l t r i a n g l e USA - Saudi Arabia - I r a n has upon them w i l l p l a y an i m p o r t a n t r o l e i n f u t u r e e n e r g y s e c u r i t y . I n a d d i t i o n , w i t h o u t t h e O P E C , o i l p r i c e s would d e c l i n e t o l e v e l s t h a t would encourage g r e a t e r dependence on Gulf s u p p l i e s , t h e r e b y p o s s i b l y harming long-term energy s e c u r i t y of the OECD.
2.2.1.2.4. Market forces and oil price evolution The o i l p r i c e e v o l u t i o n d u r i n g t h e l a s t 20 y e a r s was predominantly d r i v e n by market f o r c e s (PETROLEUM REVIEW 1988 c ) . Four p e r i o d s can be d i s t i n g u i s h e d : 1970 - 1973 ( l e a d i n g t o t h e f i r s t o i l p r i c e i n c r e a s e ) , 1974 - 1979 ( g u i d i n g t o the second o i l p r i c e u p l i f t ) , 1979 - 1985 ( g i v i n g r i s e t o t h e 1986 o i l p r i c e plunge) and the c u r r e n t s i t u a t i o n 1986 - 1989 and beyond. Comments a r e a l s o o f f e r e d on market r e g u l a t i o n vs. d i s e q u i l i b r i u m compensation.
2.2.1.2.4.1. 1970
-
1979
Between 1970 and 1973, world o i l demand i n c r e a s e d by o v e r 8 Mio. b b l / d mainl y due t o t h e d e c l i n i n g r e a l o i l p r i c e s of the p r e v i o u s two decades (PETROLEUM REVIEW 1988 c ) . As non-OPEC o i l s u p p l y i n c r e a s e d o n l y m a r g i n a l l y , demand f o r OPEC o i l i n c r e a s e d s h a r p l y by a l m o s t 7 . 5 M i o . b b l / d . T h e r e f o r e the p r o d u c t i o n l e v e l came very c l o s e t o OPEC o u t p u t c a p a c i t y and i f t h e s i t u a t i o n had c o n t i nued, demand would have by f a r exceeded s u p p l y . T h u s market f o r c e s s e t the s t a g e f o r the f i r s t o i l p r i c e i n c r e a s e i n l a t e 1973, w i t h OPEC's r o l e having been i m p o r t a n t i n t i m i n g , b u t p o l i t i c a l f a c t o r s such a s the A r a b - I s r a e l war and t h e magnitude of p r i c e i n c r e a s e s having been c o n s i d e r a b l y more s i g n i f i c a n t . I n t h e second p e r i o d , demand f o r o i l d e s p i t e the now r e a s o n a b l y h i g h e r p r i c e continued t o increase. While non-OPEC o i l s u p p l y began t o r i s e , demand f o r OPEC o i l was c l o s e t o i t s p r o d u c t i o n c a p a c i t y , w i t h thus the o i l p r i c e e x p l o d i n g a g a i n i n l a t e 1979 i n o r d e r t o c r e a t e a brake of the e x c e s s i v e l y p r o g r e s s i n g demand. The I r a n i a n r e v o l u t i o n and the I r a n i a n - I r a q i a n war were i m p o r t a n t f a c t o r s , n o t so much t h a t t h e y reduced the o i l flow from the r e g i o n , b u t because t h e y c r e a t e d demand f o r p r e c a u t i o n a r y and s p e c u l a t i v e s t o c k , w i t h a g a i n market fundamentals p l a y i n g a
119 c r u c i a l r o l e i n t h e two sharp p r i c e i n c r e a s e s ( c f . s e c t i o n 2 . 2 . 1 . 1 . 1 . ) .
2.2.1.2.4.2.
1979 - 1986
The 1986 o i l p r i c e c o l l a p s e was a l s o caused by market f o r c e s . The sharp f a l l i n o i l demand between 1979 and 1985 and t h e even g r e a t e r i n c r e a s e i n non-OPEC o i l s u p p l i e s r e s u l t e d i n a w o r l d r e q u i r e m e n t f o r OPEC o i l a t a l e v e l t h a t was t o o small f o r t h e o r g a n i z a t i o n t o s u s t a i n (PETROLEUM REVIEW 1988 c ) . The dec l i n e o f more t h a n 13 Mio. b b l / d i n OPEC o i l demand l e d t o an OPEC o i l product i o n o f l e s s than 16 Mio. b b l / d i n 1985 and a r e c o r d minimum o f 13.4 Mio. b b l / d i n 1987 (compared w i t h t h e maximum o f 31 Mio. b b l / d i n 1973; ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 d ) . The l o n g - t e r m exposure t o t h e two o i l p r i c e e x p l o s i o n s has been a s i g n i f i c a n t advance by t h e i n d u s t r i a l i z e d c o u n t r i e s i n energy conservat i o n measures on t h e one hand ( r e p r e s e n t i n g a macroeconomical response t o t h e o i l shocks) and a s i g n i f i c a n t improvement i n energy e f f i c i e n c y ( b e i n g a m i c r o economical response t o t h e p r i c e r i s e s ; McCANN 1988). As market f o r e c a s t s i n m i d t o l a t e 1985 i n d i c a t e d f u r t h e r d e t e r i o r a t i o n i n demand f o r OPEC o i l i n case o f absence o f m a j o r steps, t h e o n l y o p t i o n was t o f l o a t o i l p r i c e s i n o r d e r t o s t o p o r even r e v e r s e t h e d e c l i n i n g t r e n d (PETROLEUM R E V I E W 1988 c ) . The impact o f t h e 1986 o i l p r i c e plunge was t h u s l o g i c a l l y a t u r n a r o u n d o f t h e o i l market. Demand i n c r e a s e d r e l a t i v e l y s h a r p l y by 1 . 8 Mio. b b l / d between 1985 and 1987, and t h e r i s e i n non-OPEC s u p p l y was more o r l e s s t e r m i n a t e d . I t was t h i s t u r n a r o u n d i n t h e market t h a t was i n s t r u m e n t a l i n OPEC once a g a i n emb a r k i n g on a r e g u l a t i o n o f t h e o i l market. Thus w h i l e OPEC p l a y e d a r e g u l a t i n g r o l e , market fundamentals s e t t h e stage f o r t h e two o i l p r i c e i n c r e a s e s o f 1973/1974 and 1979/1980 ( c f . s e c t i o n 2.2.1.1.1.) as w e l l as f o r t h e 1986 p r i c e c o l l a p s e ( c f . s e c t i o n 2.2.1.1.2.). The r o l e o f OPEC was i m p o r t a n t i n t h e two p r i c e i n c r e a s e s i n r e l a t i o n t o t h e magnitude o f t h e r i s e and o f f i c i a l i z i n g i t , b u t i t s r o l e was even more s i g n i f i c a n t between 1982 and 1985 when i t managed t o p r e v e n t a p r i c e c r a s h by r e d u c i n g o u t p u t .
2.2.1.2.4.3.
Market regulation vs . di sequ i 1 ibr ium compensat ion
H i s t o r i c a l l y , t h e o i l market has been r e g u l a t e d by m a j o r o i l companies u n t i l 1970 and by OPEC a f t e r w a r d s (PETROLEUM REVIEW 1988 c ) . The system o f r i g i d o i l p r i c i n g , however, w i l l have d i f f i c u l t i e s i n s u r v i v i n g because o f t h e s t r u c t u r a l changes t h a t have t a k e n p l a c e i n o i l t r a d i n g . The f i r s t r i g i d i t y i n t h e OPEC p r i c i n g system arose because w i t h g i v e n demand, i t i s n o t p o s s i b l e t o f i x b o t h s u p p l y and p r i c e s , b u t i t c o u l d o n l y happen by c o i n c i d e n c e i f t h e market i s i n balance a t a g i v e n p r i c e . I f adjustments t o t h e system a r e n o t made, however, d i s e q u i l i b r i u m emerges. One p o s s i b i l i t y i s t o have a swing producer t o a d j u s t i t s o u t p u t t o balance t h e market, b u t u n l e s s o v e r a l l s u p p l y and p r i c e s a r e f r e q u e n t l y a d j u s t e d , t h e swing p r o d u c e r w i l l l o s e i t s market share.
I n t h e l a s t decade, fundamental changes i n t h e system o f o i l t r a d i n g have f o r c e d o i l p r i c e s t o become more market r e s p o n s i v e . The s e r i e s o f reasons i n c l u des i n c r e a s e i n s p o t t r a n s a c t i o n s , emergence o f o i l f u t u r e and f o r w a r d markets, i n c r e a s i n g i n t e g r a t i o n o f t h e downstream s e c t o r i n t o t h e o i l segment o f t h e p r o d u c i n g c o u n t r i e s ; appearance o f new m a r k e t i n g p r a c t i c e s such as netback p r i c i n g , p r o c e s s i n g d e a l s and b a r t e r t r a d e ; i n c r e a s i n g number o f p a r t i c i p a n t s i n t h e o i l market, and f a l l i n t h e OPEC share i n g l o b a l o i l s u p p l i e s .
2.2.1.2.5.
Oil consumption and output
Aspects o f o i l consumption and o u t p u t a r e discussed a l o n g t h e l i n e s o f OECD demand vs. c o n s e r v a t i o n and s u b s t i t u t i o n as w e l l as OPEC and non-OPEC product i o n vs. demand as f o l l o w s .
120
2.2.1.2.5,l.OECD demand vs . conservation and s u b s t i t u t i o n O i l and t o t a l energy consumption p e r u n i t o f gross n a t i o n a l p r o d u c t dropped i n most o f t h e O r g a n i z a t i o n f o r Economic C o o p e r a t i o n and Development (OECD) c o u n t r i e s d u r i n g t h e 1973 - 1985 i n t e r v a l , b u t s i n c e t h e o i l p r i c e c o l l a p s e i n 1986, t h i s t r e n d has r e v e r s e d (NARAGHI 1988). Due t o c o n s e r v a t i o n measures and f u e l s u b s t i t u t i o n e f f e c t s b r o u g h t about by t h e 1973/1974 and 1979/1980 p r i c e h i kes, t h e OECD c o u n t r i e s used 38 % l e s s o i l p e r u n i t o f o u t p u t i n 1985 t h a n i n 1973. I n 1986, o i l consumption l e v e l s i n a l l r e g i o n s o f t h e w o r l d have i n c r e a sed f o r t h e f i r s t t i m e s i n c e 1981 as a consequence o f now much cheaper o i l bei n g a v a i l a b l e , and g l o b a l o i l consumption i n 1986 has reached t h e same n i v e a u as i n 1976. L e v e l l i n g - o f f a n d / o r s l i g h t l y i n c r e a s i n g o i l f u e l share o f p r i m a r y energy consumption i n 1986 t o o k p l a c e on a w o r l d - w i d e b a s i s . The consequence o f t h e r e t u r n t o a more e l e v a t e d l i s t p r i c e o f US $ 18 i n l a t e 1986 means an a b t . 40 % income i n c r e a s e w i t h r e s p e c t t o a US $ 12 s c e n a r i o r e p r e s e n t i n g t h e average l e v e l d u r i n g t h e peak o f t h e c r i s i s . I n 1986, f o r t h e f i r s t t i m e s i n c e 1978, t h e share o f o i l i n p r i m a r y energy consumption r e v e r s e d a d e c l i n i n g p a t t e r n , and i n t h e f r e e market economies ( p a r t i c u l a r l y i n t h e USA), t h e i n c r e a s i n g share o f apparent o i l consumption i n 1986 was accompanied by d e c r e a s i n g shares f o r gas and c o a l consumption. I f c o n t i n u e d i n t h e f u t u r e , these t r e n d s c o u l d s i g n a l p o s s i b l e r e v e r s a l o f consumer b e h a v i o u r and conservat i o n measures and p r o v i d e a b a s i s f o r t h e c l a s s i c a l debate o v e r t h e e v e r - i l l u s i v e e q u i l i b r i u m p r i c e o f crude o i l .
2.2.1.2.5.2.OPEC and non-OPEC product ion vs . demand A l t h o u g h f r e q u e n t l y e x h i b i t i n g no consensus and d i s c i p l i n e c o n c e r n i n g p r i c e and p r o d u c t i o n q u a n t i t i e s , t h e OPEC group has e x t r a o r d i n a r i l y w e l l h e l d t o g e t h e r s i n c e i n 1981/1982 t h e f i r s t l o s s e s i n revenues o c c u r r e d due t o b e g i n n i n g o f d e c l i n i n g demand and f a l l i n g p r i c e s (HOLMES 1987). Because i n l a t e 1985 as a consequence o f l o w e r demand and h i g h e r non-OPEC p r o d u c t i o n t h e a b s o l u t e l e v e l o f o i l e x p o r t by t h e OPEC had f a l l e n down t o h a l f t h e amount o f 1980/1981, t h e OPEC t r i e d t o g e t again more market share by r e n o u n c i n g f r o m f i x e d p r i c e and p r o d u c t i o n quotas which provoked t h e 1986 o i l p r i c e c r a s h t h a t had t h r e e f o l d r e s u l t s . The f i r s t e f f e c t was OPEC's l o s s o f revenue w i t h v e r y much l o w e r p r i c e s , because t h e income i n 1985 was o n l y s t i l l h a l f o f t h a t i n 1980/1981, and t h e revenue i n 1986 was a g a i n o n l y h a l f o f t h a t i n 1985. N e v e r t h e l e s s , t h e OPEC group r e t a i n e d i t s cohesion and r e - e s t a b l i s h e d p r i c e t a r g e t and volume d i s cipline. The second main r e s u l t o f t h e 1986 o i l p r i c e c o l l a p s e was t h e c u r t a i l m e n t o f non-OPEC p r o d u c t i o n . The 1986 o u t p u t l e v e l s were a b t . 0 . 8 M i o . b b l / d l e s s t h a n t h e y m i g h t have been expected a t p r i c e s r e m a i n i n g a t 1985 n i v e a u . The b u l k o f t h i s l o s s was i n t h e USA where p r o d u c t i o n may have dropped as much as 0.5 Mio. b b l / d . The t h i r d i m p o r t a n t e f f e c t o f t h e 1986 o i l p r i c e s l i d e was t h e i n f l u e n c e o f l o w e r oil p r i c e s on demand. World demand i n c r e a s e d by a b t . 0 . 8 Mio. b b l / d o v e r what i t m i g h t have been i f p r i c e s remained a t a b t . US $ 28. Some o f t h i s i n c r e a s e d demand i n 1986 was s u b s t a n t i a l s t o c k b u i l d e s p e c i a l l y in Europe, and some was due t o s u b s t i t u t i o n o f o t h e r energy sources by l o w - p r i c e d f u e l o i l p a r t i c u l a r l y i n t h e USA, t h u s u n d e r l i n i n g t h a t o i l demand a t t h e m a r g i n i s s t r o n g l y i n f l u e n c e d by o i l p r i c e l e v e l .
2.2.1.2.6. Other aspects E f f e c t s o f t h e 1986 c r i s i s on o i l C Y , DANCY & TAYLOR (1987) and KELKAR e v a l u a t e d by ROBERTS (1984), HILL BELL & HUBBARD (1984), ZWICKY (1984)
and gas i n d u s t r y a r e a l s o d i s c u s s e d by DAN(1987), and o t h e r o i l p r i c e aspects a r e (1987) and POWERS & STEVENSON ( 1 9 8 7 ) . CAMPand DOUGHERTY (1987) i n v e s t i g a t e p o s s i b i l i -
121 t i e s o f reasonably r e l i a b l e p r e d i c t i o n o f the d i r e c t i o n s o f o i l p r i c e evolut i o n , and KELLY (1987) assesses t h e q u e s t i o n o f o i l p r i c e s t a b i l i t y and c o n s o l i d a t i o n . VEATCH (1986) e v a l u a t e s economical aspects o f h y d r a u l i c p r o p p a n t f r a c t u r i n g . Aspects o f t h e r o l e o f t h e OPEC i n w o r l d o i l market and p r i c e e v o l u t i o n a r e a1 so d i s c u s s e d by GATELY (1984), and AL-CHALABI (1987) and ATTIGA (1987) summarize o i l p r i c e h i s t o r y and a s s o c i a t e d p e r i o d s o f boom and c r i s i s p a r t i c u l a r l y from the viewpoint o f the organization o f Arabian petroleum exporting c o u n t r i e s (OAPEC). P o l i t i c a l p r o s p e c t s w i t h i n t h e A r a b i a n area a r e i l l u s t r a t e d by SHARABI (1988), and t h e r e l a t i o n s h i p between p o l i t i c a l i n f l u e n c e s and o i l / gas p r i c e e v o l u t i o n i s sketched by GLASNER (1985). FRANSSEN (1988) comments on p o l i t i c a l , s t r a t e g i c a l and r e s o u r c e i m p l i c a t i o n s o f o i l s u p p l y dimensions, and LUMSDEN (1988) e v a l u a t e s o i l p r i c e and demand i n t e r a c t i o n s . MORSE (1988) i l l u s t r a t e s t h e impact on markets o f non-OPEC o i l s u p p l i e s . T A R I K I (1985) i l l u m i n a t e s t h e s i t u a t i o n o f g l o b a l o i l markets o f b e i n g under s t r e s s . As a consequence of t h e d i m i n i s h i n g revenues f r o m o i l s a l e , more and more OPEC members a r e pushing i n t o downstream v e n t u r e s i n o r d e r t o e s t a b l i s h i n t e g r a t e d p r o d u c t i o n (WILLIAMS 1988; c f . s e c t i o n 2.2.1.4.3.3.). The downstream moves a r e t h e most apparent r e i n t e g r a t i o n e f f o r t s and a r e a n a t u r a l r e s u l t o f t h e i r c a p a c i t y s u r p l u s , c o m p e t i t i o n i n t h e o i l market, and t h e d e s i r e o f t h e produc i n q c o u n t r i e s f o r more assured o u t l e t s q i v e n t h i s market share c o m p e t i t i o n (PETROLEUM REVIEW 1987 a ) . Downstream i n t e g F a t i o n o f OPEC s t a t e s as a c o m p e l i i n g c o m p e t i t i v e edge i s a l s o emphasized b y PETROLEUM REVIEW ( 987 a) and STAUFER (1988).
2.2.1.3. Impact o f varying US $ exchange rate on oil price evolution The low o i l D r i c e w h i c h was i n 1986 t e m o r a r i l v c o n s i d e r a b Y below US $ 10 and a l s o t h r o u g h o u t 1987 and e a r l y 1988 n e v e r exceeded an average v a l u e o f ' U S $ 18 was o u t s i d e t h e USA and o t h e r d o l l a r - c u r r e n c y c o u n t r i e s even accentuated by t h e accompanying weakness o f t h e US $ ( c f . a l s o s e c t i o n 2.3.5.). The exchange r a t e o f t h e US $ w i t h r e s p e c t t o o t h e r w o r l d g u i d e c u r r e n c i e s such as DM and L s t a r t e d t o d e c l i n e i n m i d t o l a t e 1985, reached i n t h e f o u r t h q u a r t e r o f 1987 i t s h i s t o r i c a l minimum l e v e l a f t e r d i m i n i s h i n g i t s v a l u e by 40 - 50 % w i t h i n two years, k e p t b e i n g low t h r o u g h o u t t h e f i r s t p a r t o f 1988, and o n l y i n t h e second h a l f o f 1988 experienced a s l i g h t r e c o v e r y o f a b t . 20 - 25 % o f i t s val u e t h a t , however, may be o f o n l y temporary n a t u r e , as a l r e a d y i n d i c a t e d by an a l r e a d y renewed s l i g h t d r o p a f t e r t h e p r e s i d e n t i a l e l e c t i o n i n t h e USA a t t h e end o f 1988). Another m a j o r i n f l u e n c i n g f a c t o r i s t h e i n f l a t i o n which had t h e consequence t h a t t h e n e t r e t u r n f r o m hydrocarbon s a l e i n Europe was i n m i d 1986 o n l y a b t . 40 % o f t h a t on average i n 1985, and t h a t an o i l p r i c e i n nominal terms o f 10 15 US $ i n 1986 i s i n r e a l terms e q u i v a l e n t t o a b t . 5 US $ i n 197211973. R e l a t i v e t o t h e DM, t h e v a l u e o f t h e US $ was h a l v e d f r o m 3.20 OM i n e a r l y 1985 t o 1.60 DM (and t e m p o r a r i l y even below) i n l a t e 1987 (THOMPSON & SINGLETON 1988) which i n terms o f o i l p r i c e means t h a t an US $ 27 b a r r e l p r i c e i n e a r l y 1985 was e q u i v a l e n t t o DM 86, whereas US $ 19 i n l a t e 1987 corresponded t o o n l y DM 31. Thus t h e w o r l d o i l p r i c e was o n l y 30 % l o w e r i n US $ b u t 65 % l o w e r i n DM, r e s u l t i n g i n t h e f a c t t h a t on t h e i n t e r n a t i o n a l market, a b a r r e l o f o i l was more t h a n 50 % cheaper i n l a t e 1987 t h a n i n e a r l y 1985, w i t h o v e r h a l f o f t h i s decrease b e i n g d i r e c t l y a t t r i b u t a b l e t o t h e d e p r e c i a t e d US $. The e f f e c t s o f t h e sharp 1986 o i l p r i c e d r o p have been d r a m a t i c t o even t r a u m a t i c , l e a d i n g f o r p r o d u c t i o n , s e r v i c e and s u p p l y companies t o a b a t t l e f o r s u r v i v a l o f t h e f i t t e s t , w i t h many small companies h a v i n g been f a c e d w i t h g o i n g o u t o f business and v a r i o u s l a r g e r companies h a v i n g t r i e d t o escape t h i s f a t e by merging and mutual c o r p o r a t e t a k e o v e r ( c f . s e c t i o n 2.2.1.5.2.4.) f o r easier s u r v i v a l u n t i l b e t t e r t i m e s come back (GRIFFITHS 1986, MEISTER & CADY 1986). US $ revenues t o OPEC have c o n s i d e r a b l y d e c l i n e d b o t h i n r e a l and nominal terms
122 due t o t h e d e p r e c i a t i n g U S $ on f o r e i g n exchange m a r k e t s d u r i n g t h e l a s t y e a r s (McCANN 1988). Thus OPEC was squeezed f r o m two s i d e s r e c e n t l y , w i t h o n t h e one hand l o w e r o i l p r i c e s h a v i n g r e d u c e d t h e v a l u e o f o i l e x p o r t r e v e n u e s in n o m i n a l terms, whereas on t h e o t h e r hand, t h e d e p r e c i a t i n g US $ a g a i n s t a b a s k e t o f o t h e r OECD-member c u r r e n c i e s l e d t o a d e c l i n e o f p u r c h a s i n g power o f t h e nomin a l r e v e n u e s i n r e a l t e r m s . The v a r i o u s r e l a t i o n s h i p s and i m p a c t s a r e i l l u s t r a t e d as f o l l o w s b y o u t l i n i n g some examples c o m p r i s i n g i m p o r t p r i c e f o r c r u d e o i l i n Germany FRG, OPEC o i l s a l e income, and c u r r e n t v s . c o n s t a n t US $ r a t e c a l c u lation. I m p a c t s o f c h a n g i n g US $ r a t e on t h e w o r l d o i l m a r k e t a r e a l s o d i s c u s sed b y WEFA (1988).
2 . 2 . 1 . 3 . 1 . Import price for crude oil in Germany FRG As a consequence o f o i l p r i c e d r o p and US $ r a t e c o l l a p s e , t h e i m p o r t p r i c e c r u d e o i l a t t h e Germany FRG b o u n d a r y w h i c h r e a c h e d i t s h i s t o r i c a l maximum i n M a r c h 1985 w i t h 697 DM/t and had an a v e r a g e o f 622 DM/t i n 1985 f e l l v i a 493 O M / t i n J a n u a r y 1986 down t o t h e minimum o f 120 DM/t i n J u l y 1986 (REINICKE, TRENEL, HERBST, KEMPGEN & LINZ 1988) and 178 D M / t i n A u g u s t 1986 (EROOL-ERDGASKOHLE NACHRICHTEN 1987 C , ERDUL UND KOHLE - ERDGAS - PETROCHEMIE 1987 b ) and p r o b a b l y a t l e a s t t e m p o r a r i l y e v e n f u r t h e r i n l a t e 1 9 8 7 / e a r l y 1988 when a n o t h e r s l i g h t r e a l o i l p r i c e d e c l i n e c o i n c i d e d w i t h an e x t r e m e weakness o f t h e U S $ . The minimum o f 178 O M / t i n A u g u s t 1986 r e p r e s e n t s a l e v e l t h a t f o r t h e l a s t t i m e had been a c h i e v e d a t t h e end o f 1973 b e f o r e t h e o n s e t o f t h e OPEC-induced p r i c e explosions. of
The i m p o r t p r i c e o f c r u d e o i l as w e l l as t h e r e v e n u e s o f d o m e s t i c c r u d e o i l t h u s were i n 1986 and 1987 more t h a n 60 % l o w e r t h a n i n 1985, w i t h t h e i m p r o v e ment o f t h e oil p r i c e l e v e l f r o m 1986 t o 1987 h a v i n g been d u r i n g m o s t o f t h e t i m e more o r l e s s compensated b y a f u r t h e r weakening o f t h e US $ (EROOL UND KOHLE - ERDGAS - PETROCHEMIE 1988). I n 1987, t h e i m p o r t p r i c e f o r c r u d e o i l i n c r e a sed t o a b t . 220 - 260 DM/t (ERDUL-ERDGAS-KOHLE 1988 a ) t h r o u g h o u t t h e f i r s t h a l f o f t h e y e a r due t o t h e i n c r e a s e o f t h e a v e r a g e o i l p r i c e f r o m 8 - 1 2 US $ i n m i d 1986 t o 18 US $ i n l a t e 1986 and e a r l y 1987 ( c f . s e c t i o n s 2 . 2 . 1 . 1 . 2 . and 2 . 2 . 1 . 4 . 3 . 8 . ) , whereas t h e US $ k e p t b e i n g weak and had n o s t r e n g t h e n i n g i n f 1uence. As a consequence o f t h e d i m i n i s h i n g s t r e n g t h o f t h e US $ i n l a t e r 1987, t h e r e v e n u e f o r d o m e s t i c c r u d e o i l i n Germany FRG f e l l f r o m 200 - 220 DM/t o n aver a g e i n 1986 and 1987 t o 140 D M / t i n l a t e 1 9 8 7 / e a r l y 1988. I n a s i m i l a r way, t h e n e t o i l b i l l o f Germany FRG was more t h a n h a l v e d f r o m 64.5 B i l l . DM i n 1985 t o 30.8 B i l l . DM i n 1986 i n s p i t e o f i n c r e a s e d n e t o i l i m p o r t s (ERDO’L-ERDGASKOHLE NACHRICHTEN 1987 c ) . I m p a c t s o f US $ weakness o n o i l p r i c e and e x p l o r a t i o n / d e v e l o p m e n t a c t i v i t y a r e a l s o s k e t c h e d b y QUINLAN (1988).
2.2.1.3.2.
OPEC oil sale income
As a consequence o f b o t h e n e r g y s a v i n g campaigns and o i l p r i c e d r o p , t h e t o t a l income o f t h e OPEC g r o u p f r o m s a l e o f t h e i r o i l f e l l down f r o m t h e # , 1 : ~ m u m o f a b t . 280 B i l l . U S $ i n lp80/1981 t o a b t . 134 B i l l . U S $ i n 1985 and t o o n l y a b t . 75 B i l l . US $ i n 1986 (A’CGAR 1987 b; ERDOL-ERDGAS-KOHLE NACHRICHTEN 1987 b , 1987 c; ERDOL UND KOHLE - ERDGAS - PETROCHEMIE 1987 d, HOLMES 1987). I n t e r m s o f o i l volume, t h e d e c l i n i n g power o f t h e o i l m a r k e t f o l l o w i n g t h e second o i l p r i c e e x p l o s i o n i n 1979 and t h e w o r l d p e t r o l e u m r e c e s s i o n w h i c h c u l m i n a t e d i n t h e 1986 o i l p r i c e c r a s h t r i g g e r e d a s t e a d y r e d u c t i o n o f t h e OPEC o i l e x p o r t q u a n t i t y f r o m 11 B i l l . b b l i n 1979 t o o n l y 5 B i l l . b b l i n 1986 when t h e c a p a c i t y o f o t h e r c o u n t r i e s had r e a c h e d such a l e v e l t h a t USSR/China e x p o r t e d 2 B i l l . b b l and o t h e r non-OPEC n a t i o n s e x p o r t e d 3 B i l l . b b l o f o i l , t h e r e b y c u t t i n g o f f c o n s i d e r a b l e b u s i n e s s f r o m t h e OPEC, and t h e r e s t was compensated b y e n e r g y c o n v e r s i o n and c o n s e r v a t i o n (IVANHOE 1988).
123 The combined l o s s o f the OPEC group i n 1986 as a consequence o f t h e o i l p r i c e c o l l a p s e exceeds 50 B i l l . US $ (AL-CHALABI 1987, McCANN 1988), and i n 1987, the OPEC o i l revenues have been again o n l y a b t . one q u a r t e r o f t h e peak l e v e l o f 280 B i l l . US $ reached i n 1980/1981. The establishment o f t h e US $ 18 agreement p r i c e i n l a t e 1986 ( c f . s e c t i o n s 2.2.1.1.2. and 2.2.1.4.3.8.) brought the 1987 OPEC o i l s a l e revenue back t o a b t . 97 B i l l . US $ which i s a b t . t h e midp o i n t o f the l e v e l s o f 1985 and 1986 (McCANN 1988). Some aspects o f o i l p r i c e convention and defense, OPEC vs. non-OPEC production, and OPEC p r o d u c t i o n c e i l i n g and overhang c a p a c i t y are i l l u s t r a t e d as f o l l o w s .
2.2.1.3.2.1.
Oil price convention and defense
I n l a t e 1986, t h e s i t u a t i o n was g r a d u a l l y s t a b i l i z i n g again t o an o i l p r i c e niveau o f a b t . 18 US $ on average ( b e i n g t h e r e s u l t o f a r e o r i e n t a t i o n o f t h e OPEC group towards a consensus f o r c e d by t h e d r a s t i c a l c o l l a p s e o f t h e i r i n come; IBG 1986; ROBERTS 1986, 1987; ALGAR 1987 b, NARAGHI 1988; c f . s e c t i o n s 2.2.1.1.2. and 2.2.1.4.3.8.) which was s t i l l more o r less v a l i d i n mid 1987 ( w i t h i n t h e range o f 15 - 22 US $ according t o s h o r t - t e r m f l u c t u a t i o n s o f t h e r e a l value), w i t h again, however, t h i s nominal- terms l e v e l being as a consequence o f US $ weakness and i n f l a t i o n i n r e a l terms e q u i v a l e n t t o o n l y a b t . 8 10 US $ i n 1972/1973 (MENGES 1986, OEHME 1986, RUNGE 1986). The 1986 revenues o f t h e OPEC group amounted o n l y t o t h e i r 1973 l e v e l w i t h i n f l a t i o n assumed t o be j u s t 6 % a n n u a l l y i n t h e i n t e r i m p e r i o d (OIL GAS JOURNAL 1987 f ) . These sever a l abrupt p r i c e changes d u r i n g the l a s t 15 years have turned r e p e a t e d l y o i l and general w o r l d economics o u t o f e q u i l i b r i u m (SCHAEFER & POWERS 1984, OEHME 1986). The d a i l y OPEC s a l e volume f e l l from 31 Mio. bbl i n 1979 b e f o r e t h e second o i l p r i c e e x p l o s i o n v i a 19 Mio. bbl i n 1982 t o 16 Mio. bbl and l e s s i n 1985 p r i o r t o the major o i l p r i c e crash (AL-CHARABI 1987; ERDUL UND KOHLE - ERDGAS - PETROCHEMIE 1987 c ) . The OPEC p r i c e defence p o l i c y by s e t t i n g p r i c e s a t c e r t a i n l e v e l s below which no member c o u n t r y s o l d i t s o i l and by l e a v i n g the p r o d u c t i o n niveau t o be r e g u l a t e d by the market had t h e o n l y drawback t h a t t h e p r i c e s were s e t a t such h i g h l e v e l s t h a t the r e s u l t s became s e l f - d e f e a t i n g by d r a s t i c a l drop o f demand f o r o i l i n general and i n c r e a s i n g s u p p l i e s o f o i l o u t s i d e t h e OPEC, r e s u l t i n g i n a d e c l i n e o f the OPEC w o r l d o i l supply share from abt. 70 % i n 1980/1981 t o l e s s than 40 % i n 1985/1986 (AL-CHARABI 1987). Between 1975 and 1985, some 10 Mio. b b l / d o f a d d i t i o n a l supplies from the non-OPEC producing r e g i o n were added t o non-Communist output, and some 7 Mio. b b l / d were l o s t i n non-Communist w o r l d demand as a consequence o f energy conservation and f u e l s u b s t i t u t i o n .
2.2.1.3.2.2.
OPEC
vs . non-OPEC product ion
The tremendous l o s s i n OPEC o i l revenues as a r e s u l t o f s l o w l y f a l l i n g p r i ces i n the p e r i o d 1981 - 1985 and the o i l p r i c e drop i n 1986 i s h i g h l i g h t e d by comparing OPEC and non-OPEC p r o d u c t i o n (PETROLEUM REVIEW 1988 a ) . The e v o l u t i o n o f OPEC t o non-OPEC (non-Communist world) o u t p u t passed from 30 - 31 t o 14 - 15 Mio b b l / d i n 1973 through 1977 v i a 31 t o 17.5 i n 1979, 22.5 t o 18.8 i n 1981 and 18.9 t o 20.1 i n 1981 t o 15.4 t o 23.1 i n 1985 (McCANN 1988), w i t h thus t h e r e l a t i o n s h i p having been f o r t h e f i r s t time already reversed i n 1981 and having reached a s i g n i f i c a n t d i s e q u i l i b r i u m f o r the OPEC i n 1985. OPEC share i n w o r l d energy dropped from one t h i r d i n 1973 t o abt. 10 % i n 1987 (SCANLON 1988). Thus by m a i n t a i n i n g h i g h p r i c e s , OPEC has a t the bottom o f t h e l i n e p r i c e d i t s e l f o u t o f the market and was c o n t i n u o u s l y l o s i n g market share (PETROLEUM R E V I E W 1988 c ) . I n 1987 and 1988, as a consequence o f i n c r e a s i n g demand f o r the reason o f l o wer p r i c e s , OPEC p r o d u c t i o n climbed again t o a b t . 18 - 20 Mio. b b l / d (OIL GAS JOURNAL 1988 d, VIELVOYE 1988 b ) a f t e r having reached a r e c o r d minimum o f 13.4 Mio. b b l / d i n e a r l y 1987 (ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 d) and having
124 amounted t o 14 - 18 Mio. b b l / d i n t h e p e r i o d 1983 - 1986 (ERDOEL-ERDGAS AKTUELL 1984). On t h e o t h e r hand, non-OPEC o u t p u t i n 1986 t h r o u g h 1988 s t a y e d more o r l e s s c o n s t a n t a t 22.5 - 22.6 Mio. b b l / d , w i t h t h e s t a g n a n t s i t u a t i o n even a l r e a dy s l i g h t l y p o i n t i n g downwards (McCANN 1988). Thus w h i l e i n 1973 OPEC and nonOPEC counted f o r 67.5 % and 32.5 % o f t o t a l non-Communist w o r l d o i l p r o d u c t i o n , r e s p e c t i v e l y , t h e r a t i o had c o m p l e t e l y t u r n e d around i n 1986 w i t h non-OPEC acc o u n t i n g f o r o v e r h a l f o f t o t a l non-Communist w o r l d o i l p r o d u c t i o n w i t h 55.2 % and OPEC d e l i v e r i n g t h e balance o r t h e r e s i d u e o f 44.8 %.
2.2.1.3.2.3. OPEC product i o n c e i 1ins and overhang capacity The average agreed p r o d u c t i o n c e i l i n g o f t h e OPEC group i n t h e l a s t y e a r s was i n t h e range o f 15 - 1 7 Mio. b b l / d , w i t h t h e r e s t o f t h e a c t u a l t o t a l o u t p u t o f up t o 3 Mio. b b l / d b e i n g c o n t r i b u t e d by u n c o n t r o l l e d and v i o l a t e d o v e r p r o d u c t i o n o f some member c o u n t r i e s which f o r v a r i o u s reasons d i d n o t observe t h e i r quotas (PETROLEUM R E V I E W 1987 a ) . The average p r o d u c t i o n c e i l i n g o f a b t . 16.5 - 1 7 . 0 Mio. b b l / d i s more o r l e s s matching domestic o i l consumption i n t h e USA (BECK 1987) which can o n l y be covered by a b t . 50 % by own p r o d u c t i o n , w i t h t h e second h a l f o f i t h a v i n g t o be i m p o r t e d (CROUSE 1987). The a b t . 8.5 Mio. b b l / d i m p o r t and domestic p r o d u c t i o n volume o f t h e USA, r e s p e c t i v e l y , again i s e q u i v a l e n t t o a b t . two t h i r d s o f t o t a l USSR domestic o u t p u t ( c f . s e c t i o n 2 . 4 . 4 . 1 . ) . The USA i s t h e b i g g e s t o i l i m p o r t e r o f t h e w o r l d , whereas t h e USSR i s t h e l a r g e s t producer and t h e g r e a t e s t non-OPEC e x p o r t e r (PETROLEUM R E V I E W 1987 a ) . The c u r r e n t overhang p r o d u c t i o n c a p a c i t y o f t h e OPEC i s i l l u s t r a t e d by t h e f a c t t h a t an annual o u t p u t p o t e n t i a l o f 1 . 5 B i l l . t i s p o s s i b l e , b u t a t p r e s e n t o n l y 800 - 900 Mio t a r e produced p e r y e a r (LUBBEN & LEINER 1988). Annual OPEC o u t p u t r o s e f r o m a b t . 440 Mio. t i n 1960 c o n t i n u o u s l y up t o 1 . 5 B i l l . t i n 1973, w i t h t h i s l e v e l h a v i n g been k e p t more o r l e s s c o n s t a n t u n t i l 1979 when a gradual s l i d e s t a r t e d t h a t b r o u g h t annual OPEC p r o d u c t i o n down t o 800 M i o . t i n t h e l a s t y e a r s . On t h e o t h e r hand, non-OPEC o u t p u t was a b t . 600 Mio. t i n 1966 and s i n c e t h a t y e a r i n c r e a s e d up t o 2 B i l l . t which was reached some y e a r s ago, w i t h t h i s l e v e l h a v i n g so f a r been k e p t c o n s t a n t o r a l r e a d y s l i g h t l y d e c l i n i n g , because i n c o n t r a s t t o t h e OPEC, t h e non-OPEC c o u n t r i e s do n o t have any o v e r hang c a p a c i t y , b u t i n s t e a d of t h i s have a l r e a d y reached t h e maximum l i m i t and t h e i n e v i t a b l e d e c l i n e b e g i n s as a consequence of f a s t e r consumption t h a n r e s e r ve a d d i t i o n b y new d i s c o v e r i e s . T o t a l w o r l d annual o i l p r o d u c t i o n i n t h e l a s t y e a r s amounted t o 2.8 - 3.0 B i l l . t (ERDOEL-ERDGAS AKTUELL 1982, ERDTJL- ERDGASKOHLE NACHRICHTEN 1987 e ) . OPEC annual o i l e x p o r t i n 1974 t h r o u g h 1979 reached 1.45 B i l l . t, and due t o t h e p r i c e e x p l o s i o n s , t h e income was doubled f r o m 91 B i l l . US $ i n 1974 t o 199 B i l l . US $ i n 1979 (ERDOEL-ERDGAS AKTUELL 1980 a ) .
2.2.1.3.3. Current vs. constant US f r a t e c a l c u l a t i o n YU (1988) g i v e s an o u t l i n e o f t h e impact o f US $ exchange r a t e v a r i a t i o n s on o i l p r i c e e v o l u t i o n by examining t h e development o f o i l p r i c e s as p a i d i n l o c a l c u r r e n c i e s . The r i s e and f a l l o f o i l p r i c e s in r e c e n t y e a r s has been d r a m a t i c a l enough based on t h e US $, b u t w i t h t h e f l u c t u a t i n g exchange r a t e o f t h e w o r l d t r a d e base c u r r e n c y , o i l p r i c e a l t e r a t i o n s a r e even more s t a r t l i n g when convert e d i n t o t h e l o c a l c u r r e n c i e s as p a i d by t h e o i l i m p o r t i n g c o u n t r i e s . From l a t e 1980 t o e a r l y 1985, t h e average exchange r a t e o f t h e US $ i n c r e a s e d by a l m o s t 60 % . As a r e s u l t o f t h i s , a l t h o u g h t h e w o r l d crude o i l p r i c e i n t h e same per i o d d e c l i n e d f r o m i t s peak o f US $ 45 o r more t o US $ 30 ( r e p r e s e n t i n g a d r o p o f 33 % i n c u r r e n t d o l l a r s o r 47 % i n 1980 c o n s t a n t d o l l a r s ) , t h e average p r i c e drop m a r g i n c a l c u l a t e d i n t h e n e t i m p o r t i n g c o u n t r i e s 1980 l o c a l c u r r e n c i e s was c o n s i s t e n t l y much s m a l l e r t h a n t h e former, and i n 1984, t h e r e was even a r i s e . I n a d d i t i o n , i n Western Europe which i s t h e w o r l d ' s l a r g e s t o i l i m p o r t i n g mark e t w i t h a 38 % share o f t o t a l w o r l d o i l i m p o r t s , t h e average p r i c e o f import e d o i l i n terms of l o c a l 1980 c u r r e n c i e s even r o s e f r o m e a r l y 1983 and reached
125 i t s c l i m a x by l a t e 1984 when i t was 67 % h i g h e r t h a n t h a t o f t h e USA. Consequently, t h e e f f e c t o f p r i c e drop i n US $ d u r i n g t h i s p e r i o d was subs t a n t i a l l y weakened i n s t i m u l a t i n g w o r l d o i l demand and c o n t a i n i n g h i g h - c o s t o i l p r o d u c t i o n , energy c o n s e r v a t i o n and o i l s u b s t i t u t i o n . Since e a r l y 1985, t h e average US $ exchange r a t e dropped t o a l e v e l i n l a t e 1986 b e i n g 28 % below t h a t o f e a r l y 1985. T h i s d e c l i n e made t h e margin o f t h e p r i c e f a l l i n c o n s t a n t l o c a l currencies o f o i l - i m p o r t i n g countries l a r g e r than t h a t c a l c u l a t e d i n curr e n t and c o n s t a n t US $ and narrowed t h e gap between them. I n l a t e 1986, t h e act u a l w o r l d o i l p r i c e was 67 % l o w e r than i t s peak i n l a t e 1980. I n 1987, t h e weakening o f t h e US $ continued, w i t h t h e US $ 18 o i l p r i c e i n l a t e 1986 s u f f e r i n g a f u r t h e r d r o p by 20 % due t o exchange r a t e f l u c t u a t i o n s i n 1987. A f t e r r e a c h i n g i t s h i s t o r i c a l minimum i n l a t e 1987 f o l l o w i n g a v a l u e d i m i n u t i o n o f a b t . 40 - 50 % w i t h i n two y e a r s , t h e US $ exchange r a t e s l i g h t l y improved i n m i d 1988 by a b t . 20 - 25 % which b r o u g h t t h e s t i l l v a l i d o f f i c i a l US $ 18 o i l mark e r p r i c e a l i t t l e back t o i t s r e a l i t y a l s o i n terms o f o t h e r c u r r e n c i e s . I n l a t e 1988, however, a n o t h e r downwards t r e n d had a l r e a d y a g a i n p a r t i a l l y compens a t e d t h e b e n e f i t o f t h e m i d 1988 r e c o v e r y o f t h e US $, and a l s o t h e o i l p r i c e was on a renewed downwards movement c o n s i d e r a b l y below t h e s t i l l v a l i d o f f i c i a l marker p r i c e .
2.2.1.4. Possible future consequences of the 1986 oil price collapse The most i m p o r t a n t f u t u r e consequences o f t h e 1986 o i l p r i c e c o l l a p s e ( c f . s e c t i o n 2.2.1.1.2.) and t h e u n c e r t a i n t y i n hydrocarbon i n d u s t r y which i t c r e a t e d ( c f . a l s o s e c t i o n 2.4.2.) and t h a t s t i l l had c o n s i d e r a b l e impact on p l a n n i n g and o p e r a t i n g d u r i n g 1987 and 1988 a r e d e c l i n i n g r e s e r v e s due t o suspens i o n and c a n c e l l a t i o n o f e x p l o r a t i o n and development, r e g a i n i n g power o f t h e OPEC, o i l p r i c e i n s t a b i l i t y and m a r k e t u n c e r t a i n t y , and energy s u p p l y d i v e r s i f i cation effects.
2.2.1.4.1. Declining reserves due to suspension of exploration and development The g r e a t e s t danger o f t h e 1986 m a j o r o i l p r i c e drop and t h e r e s u l t i n g uncert a i n t y and f r i g h t e n i n g o f t h e m a r k e t i s p a r t i c u l a r l y suspension o f much o f t h e e x p l o r a t i o n and a p p r a i s a l d r i l l i n g a c t i v i t y as w e l l as s t i m u l a t i o n o p e r a t i o n s ( c f . a l s o s e c t i o n s 2.4.2. and 2 . 3 . ) i n many p a r t s o f t h e w o r l d . A l t h o u g h t h e o i l p r i c e c o l l a p s e w i l l n o t depress N o r t h Sea p r o d u c t i o n r a t e s i n t h e s h o r t t e r m due t o t h e r e l a t i v e l y low o p e r a t i n g c o s t o f most f i e l d s p ? r t i c u l a r l y a t f u l l u t i l i z a t i o n o f c a p a c i t y , i t w i l l r e s u l t i n postponement and r e s t r u c t u r i n g o f new p r o j e c t s , because a t t h e c u r r e n t o i l p r i c e l e v e l , a l l o f t h e d i s c o v e r i e s coming up f o r development a r e e i t h e r j u s t m a r g i n a l l y economical o r even n o t economical a t a l l a t l e a s t i n t h e B r i t i s h s e c t o r (BEUDELL 1986 c ) . Emphasis i n t h e t r a n s i t i o n a l stage w i l l t h e r e f o r e be on r e t a r d e d development i n c l u d i n g p a r t i a l e x p l o i t a t i o n o f r e s e r v o i r s t h r o u g h fewer w e l l s . Some aspects o f p r o d u c t i o n dec l i n e exceeding r e s e r v e a d d i t i o n , o i l demand vs. s u p p l y i n t h e n e x t decades as w e l l as i n f l u e n c e o f p o l i t i c a l systems and f i n a n c i a l r e l a t i o n s h i p s a r e d i s c u s sed as f o l l o w s .
2.2.1.4.1.1. Production decline exceeding reserve addition C o n s i d e r a b l e e f f e c t s a r e e s p e c i a l l y v i s i b l e i n t h e USA where e x p l o r a t i o n d r i l l i n g has dropped c a t a s t r o p h i c a l l y a f t e r t h e 1986 o i l p r i c e c r a s h (HOLMES 1987; c f . s e c t i o n 2 . 2 . 1 . 5 . 2 . ) . The USA has always o p e r a t e d on a low r e s e r v e bas i s which has been p o s s i b l e i n r e c e n t y e a r s because o f a h i g h l e v e l o f e x p l o r a t i o n a c t i v i t y t h a t has tended more o r l e s s t o r e p l a c e t h e produced o i l . W i t h
126 s u c h a l o w amount o f a c t i v i t y as i n 1986 ( c f . s e c t i o n 2 . 4 . 2 . ) and w i t h t h e sehowever, i t w i l l be v e r e d e n t i n c o n f i d e n c e in t h e i n d u s t r y i n t h a t c o u n t r y , d i f f i c u l t t o regenerate e x p l o r a t i o n a c t i v i t y a t a s u f f i c i e n t l y high l e v e l t o s t e m p r o d u c t i o n d e c l i n e t h a t has been p r e d i c t e d and i s i n e v i t a b l e , b u t h a s been a c c e l e r a t e d b y s e v e r a l y e a r s b y t h e 1986 o i l p r i c e c o l l a p s e . Assuming a s l i g h t t o m o d e r a t e o i l p r i c e r i s e , many non-OPEC c o u n t r i e s w i l l seek t o e n c o u r a g e o i l e x p l o r a t i o n and p r o d u c t i o n f o r r e a s o n s o f s u p p l y s e c u r i t y , payment b a l a n c e , f o r e i g n exchange, and l o c a l employment a l m o s t i r r e s p e c t i v e o f p r i c e l e v e l . N e v e r t h e l e s s , t h e a d d i t i o n a l o i l w h i c h c a n b e e x p e c t e d i s l i k e l y t o b e more t h a n o u t w e i g h e d b y t h e r e d u c t i o n o f o u t p u t i n USA and G r e a t B r i t a i n (PETROLEUM ENGINEER 1988 c ) where p r o d u c t i o n h a s a l r e a d y peaked and i s i n e v i t a b l y g o i n g down. T h i s w i l l r e s u l t i n g r o w i n g dependence on i m p o r t e d oil i n a number o f k e y i n d u s t r i a l c o u n t r i e s . The l o w o i l p r i c e i n 1986 p a r t i c u l a r l y i l l u s t r a t e d t h a t c a p i t a l e x p e n d i t u r e o n o i l p r o j e c t s i s s e n s i t i v e t o a d e q u a t e c a s h f l o w and p r i c e p r o s p e c t s . The s i g n i f i c a n c e o f a d d i t i o n a l e x p l o r a t i o n and a p p r a i s a l f o r p r e v e n t i o n o f a n o t h e r o i l c r i s i s i s a l s o u n d e r l i n e d b y LORBACH ( 1 9 8 8 b ) , and t h e d i s c r e p a n c y between much o i l and n o t enough e x p l o r a t i o n i s i l l u m i n a t e d b y STAHMER ( 1 9 7 9 ) .
2.2.1.4.1.2. Oil demand vs. supply in the next decades As o i l w i l l d e f i n i t e l y r e m a i n t o b e t h e l e a d i n g p r i m a r y e n e r g y s o u r c e w i t h i n t h e n e x t y e a r s ( p r o b a b l y e v e n beyond t h e y e a r 2000) d e s p i t e a s l o w d e c l i n e i n r e l a t i v e m a r k e t s h a r e due t o s u b s t i t u t i o n b y gas and n u c l e a r power (LINDEN 1985, FEHRINGER 1986, OEHME 1986, SCHMIDT 1 9 8 7 ) , n e g l e c t a n c e o f e x t e n s i o n o f e x i s t i n g c a p a c i t i e s due t o e c o n o m i c a l f o r c e s combined w i t h l a c k o f e n c o u r a g e ment and m i s s i n g o p t i m i s t i c o u t l o o k , and t h e r e s u l t i n g c u r t a i l m e n t o f e x p l o r a t i o n and d e v e l o p m e n t a c t i v i t i e s f o r a c o u p l e o f y e a r s ( c f . a l s o HOLBROOK 1987, RENSBURG & MALIK 1987, SCHAEFER & POWERS 1987, WOODS 1987) c o u l d mean t h a t w i t h i n t h e n e a r f u t u r e when more and more c u r r e n t l y p r o d u c t i v e w e l l s and f i e l d s become e x h a u s t e d , n o t enough new r e s e r v e s m i g h t be i m m e d i a t e l y a v a i l a b l e f o r e x p l o i t a t i o n and t h e g l o b a l e n e r g y s u p p l y w i l l d e c r e a s e ( a n a n a l y s i s o f s u p p l y and demand f o r o i l and gas and a s t u d y o f t h e c h a l l e n g e s o f t h e i n d u s t r y i s p e r f o r m e d b y SWEENEY & WYANT 1980, SCHLESINGER 1982, ECK 1983, H I L L 1983, PICKENS 1983 and CECIL 1 9 8 7 ) . A s p e c t s o f o i l p r i c e c r i s e s and s u p p l y s e c u r i t y a r e a l s o d i s c u s s e d b y BAUER ( 1 9 8 1 ) , and a p r o g n o s i s o f w o r l d e n e r g y demand i s g i v e n b y WESTERHAUSEN ( 1 9 8 0 ) . T o t a l w o r l d e n e r g y demand, however, w i l l s i g n i f i c a n t l y i n c r e a s e u n t i l t h e y e a r 2000 p r e d o m i n a n t l y due t o r i s i n g c o n s u m p t i o n i n t h e d e v e l o p i n g c o u n t r i e s b y a d j u s t m e n t o f l i v i n g s t a n d a r d and i n d u s t r i a l g r o w t h (HULTIN 1983, LINDEN 1985, DOUGHERTY & AL-BLEHED 1987, OSMAN 1987, SCHMIDT 1 9 8 7 ) , whereas i n t h e h i g h l y i n d u s t r i a l i z e d c o u n t r i e s , demand w i l l o n l y more o r l e s s f l u c t u a t e a r o u n d i t s p r e s e n t l e v e l due t o s t i l l i n c r e a s i n g e f f e c t i v i t y o f e n e r g y c o n s e r v a t i o n , p r o c e s s r a t i o n a l i z a t i o n , o i l s u b s t i t u t i o n c h i e f l y b y g a s and o n l y s l i g h t i n d u s t r i a l g r o w t h t o s t a g n a t i o n o r e v e n some s h r i n k a g e due t o p r o g r e s s i v e m a r k e t sat u r a t i o n i n many s e c t o r s and b r a n c h e s ( I B G 1 9 8 6 ) .
2.2.1.4.1.3.Influence o f political systems and financial relationships U n l e s s t h e g e n e r a l demand w i l l be r e d u c e d b y a m a j o r e c o n o m i c a l r e c e s s i o n i n t h e n e a r f u t u r e (OFFSHORE O I L INTERNATIONAL 1 9 8 8 ) , t h e d e l a y e d e x p l o r a t i o n o f new r e s e r v e s ( c f . a l s o s e c t i o n 2 . 2 . 1 . 5 . 2 . ) c o u l d r e s u l t i n an e s t a b l i s h e d s h o r t a g e i n t h e coming y e a r s . P r o g n o s i s i s g i v e n t h a t f o l l o w i n g t h e 1986 o i l p r i c e o i l p r o d u c t i o n i n t h e B r i t i s h N o r t h Sea w i l l r e crash ( c f . s e c t i o n 2.2.1.1.2.), m a i n s t e a d y f o r a b o u t a n o t h e r y e a r and b e g i n f a l l i n g a f t e r t h a t u n l e s s d e l a y e d p r o j e c t s a r e r e v i v e d , whereas i n t h e N o r w e g i a n N o r t h Sea, new f i e l d s a r e a b l e t o b o o s t p r o d u c t i o n u n l e s s t h e government c o n t i n u e s t o l i m i t e x p o r t s ( O I L GAS
127 JOURNAL 1986). O i l o u t p u t d e c l i n e i n t h e USA w i l l g a i n speed i n 1987 due t o mor i b u n d d r i l l i n g i n 1986, whereas t h e USSR by c o n t r a s t stepped up d r i l l i n g i n 1986 i n i t s e f f o r t s t o r a p i d l y r e p l a c e m a t u r i n g p r o d u c t i o n i n p r o g r e s s i v e l y dep l e t e d f i e l d s ( c f . s e c t i o n 2.4.4.). R i s k , s t o p p i n g r u l e s and economical v i a b i l i t y o f e x p l o r a t i o n d r i l l i n g programs a r e d i s c u s s e d by R A N K I N (1982), and JAMISON (1984) analyzes f u t u r e p e t r o l e u m e x p l o r a t i o n i n t h e USA. NEAL & REED (1981) comment on t h e i m p a c t o f t a x e s on e x p l o r a t i o n economics and t h u s f e a s i b i l i t y o f d i s c o v e r y o f a d d i t i o n a l hydrocarbon r e s e r v e s . Economical aspects o f t h e low o i l p r i c e s c e n a r i o a r e a l s o i n v e s t i g a t e d by FLATH (1987). GLASNER (1985) d i s c u s s e s t h e dependency o f o i l p r i c e e v o l u t i o n f r o m p o l i t i c a l i n f l u e n c e s . The consequences o f t h e o i l p r i c e h i s t o r y on demand and s u p p l y a r e summarized by YU ( 1 9 8 8 ) . The l o w o i l p r i c e i n t h e f i r s t t w e n t y - f i v e post-Second World War y e a r s had t r i g g e r e d t h e r a p i d growth o f w o r l d o i l demand and t h e c o n v e r s i o n o f w o r l d energy consumption s t r u c t u r e f r o m c o a l t o petroleum. The d r a s t i c a l esc a l a t i o n o f t h e w o r l d o i l p r i c e d u r i n g t h e 1970's and e a r l y 1980's ( c f . s e c t i o n 2.2.1.1.1.) had b r o u g h t about a d e c l i n e o f demand and t h e p r e s e n t o i l g l u t . Unf o r t u n a t e l y i n t h e m i d 1970's and e a r l y 1980's, almost t h e whole w o r l d i g n o r e d t h e impact which t h e d r a s t i c a l e s c a l a t i o n o f o i l p r i c e s had upon energy demand and supply, and f a i l e d t o f o r e s e e t h e subsequent o i l g l u t and p r i c e f a l l . Theref o r e t h e meaning o f t h e p r e s e n t low l e v e l o f o i l p r i c e s i n t h e f u t u r e i n terms o f demand and s u p p l y and t h e p r i c e o f o i l i s a problem which c a l l s f o r c l o s e watch and s e r i o u s study.
2.2.1.4.2. Regaining power o f the OPEC The e v o l u t i o n as sketched above w i l l a t l e a s t i n n some areas sooner o r l a t e r enable t h e O r g a n i z a t i o n o f Petroleum E x p o r t i n g C o u n t r i e s (OPEC) t o a c q u i r e again a c o n s i d e r a b l y l a r g e r o i l m a r k e t share t h a n be o r e ( w h i c h i s t h e n i g h t mare o f a l l non-OPEC c o u n t r i e s ) , w i t h t h e f i r s t s l i g h t renewed market e x t e n s i o n a l r e a d y h a v i n g been achieved i n l a t e 1 9 8 6 / e a r l y 1987 (OFFSHORE ENGINEER 1986 d, ALGAR 1987 b, ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 b, OIL DAILY 1988 a ) , w i t h market f a c t o r s moving i n OPEC's f a v o u r as demand r i s e s . I n 1987/1988, w o r l d o i l r e q u i r e m e n t has a l r e a d y i n c r e a s e d by a b t . 1 . 8 Mio. b b l / d s i n c e 1985, r e s u l t i n g i n a 1.6 Mio. b b l / d o r 10 % i n c r e a s e i n demand f o r OPEC o i l (PETROLEUM REVIEW 1988 c ) . W i t h t h e assumption o f 1.5 % annual i n c r e a s e i n w o r l d o i l r e q u i r e m e n t and non-OPEC o i l s u p p l y r e m a i n i n g more o r l e s s a t t h e c u r r e n t l e v e l , t h e demand f o r OPEC o i l w i l l remain below 20 Mio. b b l / d i n t h e near f u t u r e , b u t i n t h e second h a l f o f t h e 1990's, i t w i l l r e a c h c l o s e t o 27 M i o . b b l / d which i s near t h e maximum OPEC p r o d u c t i o n c a p a c i t y , w i t h t h i s l o n g - t e r m t r e n d , however, n o t r e s p e c t i n g sharp s h o r t - t e r m f l u c t u a t i o n s i n demand. H i s t o r i c a l l y , demand f o r OPEC o i l has f l u c t u a t e d by more than 1 Mio. b b l / d on e i t h e r s i d e o f t h e average annual demand, and p a r t i c u l a r l y these f l u c t u a t i o n s have c r e a t e d problems f o r o i l market s t a b i l i t y . The r e g a i n i n g power o f t h e OPEC i n t h e f u t u r e i s d i s c u s s e d as f o l l o w s along t h e l i n e s o f e v o l u t i o n o f w o r l d o i l p r o d u c t i o n and demand, symmetrical boom-crash-boom c y c l e , r e s e r v e l i f e and o i l demand i n c r e a s e ; netback p r i c i n g , swing p r o d u c t i o n and market share r e c a p t u r i n g ; o i l r e s e r v e d i s t r i b u t i o n and OPEC c a p a c i t y u t i l i z a t i o n , imbal a n c e between o i l r e s e r v e s and consumption, and i n c r e a s i n g o i l demand exceeding c r i t i c a l OPEC o u t p u t r a t e .
2.2.1.4.2.1. Evolution o f world oil production and demand The dependency o f t h e w o r l d f r o m OPEC o i l i n t h e f u t u r e i s a l s o i l l u s t r a t e d by p r o d u c t i o n c a p a c i t y and r e s e r v o i r d u r a t i o n i n OPEC and non-OPEC groups (LOBBEN & LEINER 1988). OPEC annual p r o d u c t i o n r o s e f r o m 440 Mio. t i n 1960 c o n t i n u o u s l y t o 1 . 5 B i l l . t i n 1973 ( r e p r e s e n t i n g t h e f u l l o u t p u t c a p a c i t y o f t h e a s s o c i a t i o n ) which was k e p t u n t i l 1979, b u t l a t e r d e c l i n i n g demand b r o u g h t t h e annual OPEC p r o d u c t i o n down t o 800 - 900 Mio. t i n t h e l a s t y e a r s . I n c o n t r a s t
128 t o t h i s , non-OPEC a n n u a l o u t p u t c l i m b e d f r o m 600 M i o . t i n 1966 u p t o 2 B i l l . t i n r e c e n t y e a r s w h i c h means r e a c h i n g o f t h e u p p e r l i m i t o f t h e w i t h d r a w a l p o t e n t i a l w i t h n o o v e r h a n g c a p a c i t y b e i n g l e f t , whereas t h e OPEC g r o u p has a t t h e moment a b t . 35 - 45 % o v e r h a n g p r o d u c t i o n p o t e n t i a l ( c f . s e c t i o n 2 . 2 . 1 . 3 . 2 . 3 . ) . As w o r l d o i l r e s e r v e s a r e c u r r e n t l y e s t i m a t e d as b e i n g a b t . 120 B i l l . t o r 1,750 B i l l . b b l ( w i t h t h e s e r e s e r v e s b e i n g d i s t r i b u t e d b y 59 % t o OPEC, 27 % t o non-OPEC non-communist and 14 % t o communist c o u n t r i e s ; PETROLEUM R E V I E W 1987 a ) , based on p r e s e n t p r o d u c t i o n l e v e l s o f 2 B i l l . t o f t h e non-OPEC g r o u p and 900 M i o . t o f t h e OPEC c a r t e l , t h e t o t a l w o r l d r e s e r v e s w o u l d r e a c h a b t . 42 y e a r s i f c a l c u l a t e d w i t h t o t a l w o r l d w i t h d r a w a l o f 2.9 B i l l . t (ERDOEL-ERDGAS AKTUELL 1982, ERDDL-ERDGAS- KOHLE NACHRICHTEN 1987 e ) . The peak o f w o r l d o i l p r o d u c t i o n has been r e a c h e d i n 1979 and 1980 w i t h a b t . 3.1 - 3.2 B i l l . t (EROOEL-EROGAS AKTUELL 1 9 8 1 a ) . The b o u n d a r y o f 3 B i l l . t a n n u a l w o r l d o i l p r o d u c t i o n was exceeded f o r t h e f i r s t t i m e i n 1977, w i t h i n t h i s y e a r t h r e e c o u n t r i e s c o n t r i b u t i n g h a l f o f t h e d e l i v e r y i n c l u d i n g USSR w i t h a b t . 550 M i o . t ( c f . secUSA w i t h a b t . 460 M i o . t and Saudi A r a b i a w i t h a b t . 450 M i o . t t i o n 2.4.4.), (EROOEL-EROGAS AKTUELL 1978 b ) . C u r r e n t w o r l d d a i l y o i l o u t p u t i s a b t . 53.6 - 5 5 . 1 M i o . b b l , o f w h i c h OPEC a s s o c i a t i o n c o n t r i b u t e s 15.7 - 1 7 . 5 , non-OPEC non-communist g r o u p 2 2 . 4 - 22.7, and communist b l o c k 1 5 . 0 - 1 7 . 5 . M i o . b b l (BECK 1 9 8 7 ) . D a i l y p r o d u c t i o n o f t h e USSR w i t h 1 2 . 1 - 12.6 M i o . b b l ( c f . s e c t i o n 2 . 4 . 4 . ) c o n t r a s t s s h a r p l y w i t h t h a t o f t h e USA o f 8 . 3 - 9 . 1 M i o . b b l and t h e t o t a l demand o f t h e USA o f 1 6 . 2 - 1 6 . 5 M i o . b b l , w i t h i m p o r t t h u s a c c o u n t i n g f o r a b t . 50 % o f t h e USA o i l demand (MOORE ( 1 9 8 8 d ) . I f e v a l u a t i n g s e p a r a t e l y w i t h OPEC and non-OPEC p r o d u c t i o n quantities, non-OPEC r e s e r v e s w o u l d l a s t o n l y s t i l l 15 y e a r s , whereas t h e OPEC p o t e n t i a l i s s t i l l s p a n n i n g a b t . 100 y e a r s . Due t o d e c l i n i n g o u t p u t i n t h e nonOPEC w o r l d , demand o f OPEC o i l c a n be e x p e c t e d t o r i s e t o a b t . 1 . 3 B i l l . t i n t h e m i d t o l a t e 1990's w h i c h w o u l d m o s t p r o b a b l y have t h e consequence o f an o i l p r i c e i n c r e a s e t o a b t . US $ 20 - 30. E s t i m a t i o n s o f f u t u r e o i l demand a r e a l s o g i v e n b y LORBACH ( 1 9 8 8 a ) , and t e c h n i c a l and p o l i t i c a l a s p e c t s o f p r e v e n t i o n o f a n o t h e r o i l p r i c e c r i s i s a r e d i s c u s s e d b y LORBACH ( 1 9 8 8 b ) . W o r l d o i l r e s e r v e s and demand a r e e v a l u a t e d b y LORBACH ( 1 9 8 0 ) and WESTERHAUSEN ( 1 9 8 0 ) . S I N O R ( 1 9 8 6 ) i n v e s t i g a t e s l o n g - t e r m c y c l e s and p r e d i c t i o n o f p e t r o l e u m demand.
2.2.1.4.2.2.
Symnetr ical boom-crash-boom cycle
When t h e r e f o r e a t some t i m e i n t h e f u t u r e t h e w o r l d i s a g a i n f a c i n g a n a t u r a l o r a r t i f i c i a l o i l s h o r t a g e (MINERALOLWIRTSCHAFT 1983) o r a s p e c i a l demand/ s u p p l y s i t u a t i o n i s a t l e a s t a p p a r e n t l y g e n e r a t e d due t o t h e e f f e c t i v i t y o f t h e n a t u r a l m a r k e t mechanisms i n c o m b i n a t i o n w i t h d i r e c t e d changes o r i n d e p e n d e n t l y f r o m e x t e r n a l d i s t u r b a n c e s (OEHME 1 9 8 6 ) , p r i c e s c o u l d be e x p e c t e d t o r i s e a g a i n d r a m a t i c a l l y i n a s i m i l a r way as a l r e a d y happened l e s s t h e n f i f t e e n y e a r s ago, w i t h t h e n a f u l l s y m m e t r i c a l c y c l e o f o i l p r i c e e v o l u t i o n c o m p r i s i n g boom, c r a s h and renewed b u r s t h a v i n g been c o m p l e t e d (GRIFFITHS 1 9 8 6 ) . I n t e r m s o f market forces, i n t h e n e x t few y e a r s a l r e a d y pressure i s expected t o b u i l d up on o i l p r i c e s (PETROLEUM REVIEW 1988 c ) . Temporarily a t l e a s t balancing o f t h e market by o u t p u t adjustment can prev e n t a n o t h e r o i l p r i c e c o l l a p s e . The f i r s t h a l f o f t h e 1990's s h o u l d w i t n e s s a more o r l e s s s t a b l e o i l m a r k e t w i t h o u t , however, r e t u r n i n g t o f i x e d and r i g i d p r i c e s . Towards t h e second h a l f o f t h e 199O's, t i g h t e n i n g o f s u p p l y and demand b a l a n c e s a r e p r e d i c t e d t o c u l m i n a t e i n an upwards p r e s s u r e on o i l p r i c e s . W h i l e t h e a v e r a g e a n n u a l demand f o r OPEC o i l s t i l l r e m a i n s w i t h i n t h e p r o d u c t i o n capac i t y , t h e s h o r t - t e r m demand f l u c t u a t i o n s w i l l i n e v i t a b l y r e s u l t i n s h o r t a g e s w h i c h may l e a d t o a n o t h e r v e r y s h a r p i n c r e a s e i n o i l p r i c e .
129
2.2.1.4.2.3. Reserve life and oil demand increase I n view o f t h e d r a s t i c a l changes o f t h e OPEC o i l e x p o r t volume f r o m 11 B i l l . b b l i n 1979 t o o n l y 5 B i l l . b b l i n 1986, t h e c r i t i c a l l e v e l a t which t h e OPEC can be expected t o be back i n t h e l e a d i n g p o s i t i o n f o r g l o b a l o i l p r i c i n g i s a b t . 7.3 B i l l . b b l p e r y e a r w h i c h corresponds t o a b t . 20 Mio. b b l / d , because a t t h i s volume OPEC i s supposed t o be a b l e t o m a i n t a i n p r i c e d i s c i p l i n e w i t h i n i t s r a n k s . I n m i d t o l a t e 1988, OPEC p r o d u c t i o n was s t i l l a b t . 15 % below t h i s c r i t i c a l e x p o r t l e v e l , and s t i l l s l i g h t l y d i m i n i s h i n g demand does a t t h e moment n o t p r o v i d e any t h r e a t t h a t t h e m a r k e t c o u l d t u r n around soon a g a i n . As i t can be e s t i m a t e d , however, t h a t none o f t h e non-OPEC c o u n t r i e s e x c e p t o f Norway and Mexico (and c e r t a i n l y USSR and China) have o i l r e s e r v e s l a s t i n g beyond t h e y e a r 2000 and i n l i g h t o f t h e d i m i n i s h i n g g l o b a l o i l d i s c o v e r y r a t e s i n c e a l r e a d y 1965, i t i s o n l y a q u e s t i o n o f t i m e when OPEC w i l l be a g a i n i n a s t r o n g p o s i t i o n , because t h i s area o f t h e w o r l d i s t h e o n l y one where s t i l l new g i a n t o i l f i e l d s a r e discovered, whereas i n non-OPEC n a t i o n s new d i s c o v e r i e s a r e no l o n g e r keeping up w i t h p r o d u c t i o n . The overhang o f o i l o v e r s u p p l y a t t h e moment i s h i g h l i g h t e d by t h e f a c t t h a t due t o quota agreements w i t h i n t h e group i n o r d e r t o s t a b i l i z e and defend t h e o i l p r i c e l e v e l , OPEC i s l i m i t e d t o produce a b t . 15 - 18 Mio. b b l / d w h i c h i s a b t . h a l f o f t h e a c t u a l o u t p u t c a p a c i t y (AL-CHALABI 1987) and a b t . h a l f o f t h e r e a l p r o d u c t i o n volume i n 1973 - 1979 ( 2 8 - 32 Mio. b b l / d ; HOLMES 1987). W i t h d e c l i n i n g o u t p u t i n non-OPEC c o u n t r i e s w i t h i n t h e n e x t years, t h e annual i n crease i n demand f o r OPEC o i l i s e s t i m a t e d t o be a b t . 1 Mio. b b l / d i n t h e near f u t u r e (HOLMES 1987). The c u r r e n t o i l p r i c e l e v e l does no l o n g e r s u f f i c i e n t l y c o v e r t h e i n c r e a s i n g e x p l o r a t i o n and p r o d u c t i o n c o s t s , and t h e tendency w i l l be t h a t t h e o i l p r i c e w i l l r i s e t h e f a s t e r , t h e l o n g e r t h e p r e s e n t unequal expense s i t u a t i o n i s l a s t i n g (OEHME 1986). P r e d i c t i o n s o f f u t u r e o i l p r i c e e v o l u t i o n a r e a l s o made by SEKERA (1986) and THOMPSON & SINGLETON ( 1 9 8 8 ) . G e o p o l i t i c a l i m p l i c a t i o n s o f M i d d l e E a s t o i l a r e d i s c u s s e d by KEPLINGER (1986), RANDOL (1986), ALGAR (1987 b ) , MIDDLETON (1987) and ROBERTS (1987), and p o s s i b i l i t i e s o f r e l i a b l e f o r e c a s t o f f u t u r e o i l p r i c e e v a l u a t i o n a r e assessed by DOUGHERTY ( 1 9 8 7 ) . P o l i t i c a l aspects o f energy p o l i c y a r e summarized by BARRETT (1986), SCOTT (1986) and FRANSSEN (1988), and CETRON (1986) o u t 1 i n e s i n t e r n a t i o n a l p o l i t i c a l r i s k s and energy p r o j e c t investments. The r o l e o f t h e OPEC group i n t h e w o r l d o i l market i s a l s o h i g h 1 i g h t e d by SAFER (1977) and TEWKSBURY ( 1 9 7 7 ) .
2.2.1.4.2.4. Netback p r ic ins, swing product ion and market share recapturing RANDOL (1986) d e s c r i b e s some r e a s o n i n g b e h i n d t h e l a t e 1985 d e c i s i o n o f Saud i - A r a b i a t o be no l o n g e r t h e swing producer i n t h e OPEC (AL-CHALABI 1987, BECK & WILLIAMS 1987; c f . s e c t i o n 2.2.1.2.2.1.), because t h e c o u n t r y became f e d up w i t h i t s s e l f - c h o s e n r o l e o f s i n g l e - h a n d e d l y p r o p p i n g up t h e sagging p r i c e s t r u c t u r e o f t h e OPEC, w h i l e o t h e r OPEC members show no d i s c i p l i n e i n r e s t r a i n i n g o u t p u t and w h i l e t h e r e was no c o o p e r a t i o n f r o m t h e non-OPEC o i l e x p o r t e r s t o h e l p m a i n t a i n a s t a b l e market. Netback p r i c i n g i n 1986 has t u r n e d o u t t o be a v e r y e f f e c t i v e OPEC i n s t r u m e n t t o r e c a p t u r e o i l market shares on a volume base f r o m non-OPEC producers which had been l o s t i n e a r l i e r y e a r s and t o r e bound f r o m r e c o r d low market shares i n 1985, b u t on t h e o t h e r hand, t h e f l o o d o f OPEC netback o i l overburdened an a l r e a d y s o f t o i l market and f o r c e d t h e o i l p r i c e t o tumble below US $ 10 (BECK & WILLIAMS 1987; c f . s e c t i o n 2.2.1.1.2.). The s u c c e s s f u l r e c a p t u r i n g o f m a r k e t shares f o r t h e OPEC i s h i g h l i g h t e d by t h e f a c t t h a t t h e o i l i m p o r t s i n Western Europe i n 1986 i n c r e a s e d by a b t . 10 % f o l l o w i n g a 5 % d r o p i n 1985, w i t h most o f t h i s d i f f e r e n c e coming from OPEC count r i e s (CROLL 1987).
130 The 1986 o i l p r i c e c r a s h t h a t was p r o v o k e d b y t h e OPEC d e c i s i o n t o abandon production l i m i t a t i o n i n favour o f seeking t h e f a i r share o f t h e i n t e r n a t i o n a l o i l m a r k e t l e a d s t o r e t a r d a t i o n o f f u r t h e r d e v e l o p m e n t on non-OPEC s u p p l i e s as a consequence o f s h a r p e x p l o r a t i o n and p r o d u c t i o n e x p e n d i t u r e c u t t i n g i n r e sponse t o t h e e a r n i n g s i m p a c t o f p l u m m e t i n g o i l p r i c e s . The OPEC a l s o s p e c u l a t e s on t h e r e l a t i o n s h i p t h a t t h e l o w e r t h e o i l p r i c e i n s h o r t - t e r m , t h e s t r o n g e r t h e f o r c e s p u s h i n g i t u p a g a i n in t h e l o n g - t e r m ( c f . a l s o OEHME 1 9 8 6 ) , b r i n g i n g t h e g r o u p t h e n q u i c k l y back t o t h e l e a d i n g r o l e i n t h e w o r l d e n e r g y m a r k e t . A s p e c t s o f o i l s u p p l y and demand as w e l l as p r i c e e v o l u t i o n f r o m t h e s t a n d p o i n t o f t h e o r g a n i z a t i o n o f A r a b i a n p e t r o l e u m e x p o r t i n g c o u n t r i e s (OAPEC) a r e summarized b y ATTIGA ( 1 9 8 7 ) .
2.2.1.4.2.5.Oil reserve distribution and OPEC capacity utilization Over 90 % o f w o r l d o i l r e s e r v e s a r e s i t u a t e d i n OPEC, USA, USSR and European Economic Community (EEC; NARAGHI 1 9 8 8 ) . W h i l e a c c o u n t i n g f o r i n e x c e s s o f 60 % o f t h e o i l demand o f t h e f r e e w o r l d , USA and EEC have t h e h i g h e s t o i l r e s e r v e d e p l e t i o n r a t e s a l o n g w i t h t h e l o w e s t p r o v e n o i l r e s e r v e s . Assuming t h a t OPEC i s t h e p r i c e s e t t e r , non-OPEC p r o d u c e r s a r e p r i c e t a k e r s and s u p p o r t e r s , and n o major m a t e r i a l l y adverse p o l i t i c a l upheavals occur around t h e P e r s i a n G u l f , i t c a n be s u r m i s e d t h a t t h e a n n u a l w e i g h t e d a v e r a g e c r u d e o i l p r i c e w i l l m o s t p r o b a b l y s t a y in t h e US $ 18 - 20 r a n g e u n t i l 1992 - 1994 when t h e OPEC c a p a c i t y u t i l i z a t i o n r a t e w i l l have r i s e n f r o m i t s c u r r e n t 65 - 67 % t o t h e h i s t o r i c a l l y s i g n i f i c a n t 80 - 85 % l e v e l . Consequent t o t h e 1992 - 1994 p e r i o d when OPEC i s e x p e c t e d t o become t h e maj o r s u p p l i e r o f c r u d e o i l a g a i n , t h e o i l p r i c e w i l l e x p e r i e n c e w i t h more dramat i c annual i n c r e a s e s i n r e a l t e r m s . Non-OPEC o i l p r o d u c t i o n i s e x p e c t e d t o r e a c h i t s summit i n 1988 t o 1990 and t h e n i s c o n s i d e r e d t o d e c l i n e s l o w l y (PETROLEUW R E V I E W 1988 a ) . OPEC p r o d u c e s a b t . 40 % o f t h e w o r l d s o u r c e s , and a b t . 85 % o f t h e OPEC r e s e r v e s a r e s i t u a t e d i n t h e G u l f r e g i o n ( c f . a l s o SCANLON 1988) where p r o d u c t i o n c o s t s p e r b a r r e l a r e 0 . 0 5 - 0 . 2 U S $ i n c o n t r a s t t o 5 - 15 US $ i n many p a r t s o f t h e r e s t o f t h e w o r l d (McCANN 1 9 8 8 ) . As a consei t c a n be p r e d i c t e d t h a t t h e w h o l e w o r l d quence o f a l l t h e s e r e l a t i o n s h i p s , w i l l depend upon t h e G u l f OPEC c o u n t r i e s c o n c e r n i n g o i l s u p p l y sometime a f t e r 1995, w i t h OPEC r e v i v a l b e i n g i n i t i a t e d b y c h e a p e r o i l c u r r e n t l y d i s c o u r a g i n g e n e r g y e f f i c i e n c y and f r o n t i e r e x p l o r a t i o n .
2.2.1.4.2.6. Imbalance between oil reserves and consumption The c o n t i n u i n g dependence o f t h e w o r l d o i l m a r k e t on t h e OPEC i s u n d e r l i n e d b y t h e c o n s i d e r a b l e i m b a l a n c e between r e s e r v e s and c o n s u m p t i o n ( O I L GAS JOURNAL 1987 m ) . The OECD c o u n t r i e s r e p r e s e n t i n g t h e non-communist i n d u s t r i a l w o r l d a c c o u n t f o r more t h a n 57 % o f t o t a l w o r l d c o n s u m p t i o n b u t h o l d l i t t l e more t h a n 6 % o f w o r l d o i l r e s e r v e s , whereas t h e OPEC g r o u p r e p r e s e n t s l i t t l e more t h a n 5 % o f t o t a l c o n s u m p t i o n b u t has more t h a n 75 % o f t o t a l r e c o v e r a b l e r e s e r v e s , w i t h 63 % o f t h e t o t a l r e s e r v e s b e i n g c o n c e n t r a t e d i n t h e P e r s i a n G u l f r e g i o n . O i l p r o d u c t i o n r a t e s a r e much more e v e n l y d i s t r i b u t e d t h a n r e s e r v e s , w i t h t h e OECD g r o u p a c c o u n t i n g f o r n e a r l y 26 % o f t o t a l o u t p u t , OPEC 32 %, communist b l o c k 28 % and d e v e l o p i n g c o u n t r i e s 1 4 % . R e s e r v e l i v e s a t t h e b o t t o m o f t h e l i n e c o m p r i s e f o r t h e OECD a b t . 11 y e a r s , OPEC t o t a l communist b l o c k a b t . 14 y e a r s , d e v e l o p i n g c o u n t r i e s a b t . 27 y e a r s , a b t . 104 y e a r s and P e r s i a n G u l f OPEC members a b t . 1 3 1 y e a r s . These r e l a t i o n s h i p s r e f l e c t t h a t d e s p i t e i t s c u r r e n t i n a b i l i t y t o d e f e n d an US $ 18 r e f e r e n c e p r i c e , t h e OPEC g r o u p w i l l r e m a i n i n c o n t r o l o f t h e o i l m a r k e t f o r as l o n g as t h e non-OPEC w o r l d needs i t s o i l . Temporary a p p a r e n t abandonment o f t h e p r i c e t a r g e t b y t h e OPEC seems a t l e a s t p a r t i a l l y t o be d e s i g n e d t o g u a r a n t e e t h a t t h e r e q u i r e m e n t o f OPEC o i l p e r s i s t s and grows.
131
2.2.1.4.2.7, Increasing oil demand exceeding critical OPEC output r a t e THOMPSON & SINGLETON (1988) s k e t c h a v e r y p e s s i m i s t i c worst-case s c e n a r i o and p r e d i c t c o n s i d e r a b l e i n c r e a s e o f t h e o i l p r i c e a l r e a d y w i t h i n t h e n e x t few y e a r s . A s a consequence o f l o w e r o i l p r i c e s a f t e r t h e crash, w o r l d p e t r o l e u m consumption i n c r e a s e d by 2 . 5 % f r o m 1985 t o 1986 f o l l o w i n g f o u r y e a r s o f almost no growth, and s i m i l a r o r even l a r g e r i n c r e a s e r a t e s a r e expected f o r 1987 and 1988. T h e r e f o r e t h e demand f o r OPEC o i l w i l l soon exceed t h e 20 Mio. b b l / d bound a r y and can be expected t o r i s e t o 25 - 30 Mio. b b l / d i n some y e a r s . T h i s demand l e v e l would b r i n g OPEC i n t o a v e r y s t r o n g p o s i t i o n t o f o r c e r e a l p r i c e i n creases, w i t h t h e model o f THOMPSON & SINGLETON (1988) p r e d i c t i n g t h a t t h e o i l p r i c e i s g o i n g t o r i s e t o t h e 1980/1981 n i v e a u i n r e a l terms and i s h o l d i n g t h a t l e v e l t h r o u g h 1994/1995. The h i g h n i v e a u o f r e a l o i l p r i c e w i l l p e r s i s t l o n g e r i n t h e e a r l y 1990's t h a n i n t h e e a r l y 1980's p r i m a r i l y because OPEC's p r i c e - m a k i n g power w i l l be more h e a v i l y c o n c e n t r a t e d i n t h e P e r s i a n G u l f r e g i o n and USA consumers w i l l n o t have access t o l o w - c o s t c a p i t a l t o make energy conservation investments. GUSTAJTIS (1985) comments a l r e a d y b e f o r e t h e 1986 o i l p r i c e c r a s h and l o o k i n g o u t f r o m t h e 1985 o i l p r i c e l e v e l t h a t t h e unprecedented r i s e o f w o r l d o i l p r i c e i n t h e l a s t f i f t e e n y e a r s has r e s u l t e d i n a massive i n v e s t m e n t boom i n o i l and gas e x p l o r a t i o n and development. Much o f t h e i n v e s t m e n t made s i n c e 1980 i s now viewed as an enormous business m i s t a k e which leads t o a d e c l i n e o f i n vestment c a p i t a l . A s e x p e c t a t i o n s t h a t a b t . 70 % o f p r o d u c t i o n r e q u i r e d i n t h e n e x t decade w i l l come f r o m r e s e r v e s y e t u n d i s c o v e r e d w i l l p r o v e t o be u n a t t a i n a b l e u n l e s s investments a r e made, f a i l u r e t o do so w i l l p r e c i p i t a t e another o i l c r i s i s by c a t a p u l t i n g t h e p r i c e c o n s i d e r a b l y upwards.
2.2.1.4.3. Oil price instability and market uncertainty A f t e r t h e peak o f t h e 1986 c r i s i s , t h e hydrocarbon market began t o r e c o v e r w i t h t h e s u p p o r t o f an again s l o w l y r i s i n g o i l p r i c e , b u t i n l a t e 1 9 8 6 / e a r l y 1987 f i r s t some l i t t l e s t e p s f o r w a r d were o f t e n a l t e r n a t i n g w i t h o t h e r a l m o s t compensating s t e p s i n backwards d i r e c t i o n which gave no s e c u r i t y f o r p l a n n i n g of c a p i t a l e x p e n d i t u r e , second so f a r an adequate o i l p r i c e l e v e l w i t h s u f f i c i e n t c e r t a i n t y o f s t a b i l i z a t i o n which would enable t o r e t u r n t o an a c t i v i t y a t l e a s t more o r l e s s comparable t o t h a t b e f o r e t h e onset o f t h e p r i c e f a l l has n o t y e t been reached, and t h i r d t h e US $ exchange r a t e ( t h a t s t a r t e d t o d e c l i n e i n l a t e 1985 and reached i t s p r e l i m i n a r y minimum l e v e l i n e a r l y t o m i d 1986) k e p t b e i n g v e r y low s t i l l i n m i d 1987, w i t h o n l y m i n o r p e r s p e c t i v e s o f an i n crease t o a reasonable l e v e l w i t h i n t h e n e x t few months ( t h u s by m i d 1987 an again h i g h e r a b s o l u t e o i l p r i c e which i s almost a t a n i v e a u s i m i l a r t o t h a t i n m i d 1985 i s i n Europe r e l a t i v e l y even much lower than i n those days due t o t h e now much weaker US $ than two y e a r s ago and t h e p r o g r e s s i v e i n f l a t i o n s i n c e t h a t time). I n l a t e 1987, a s l i g h t l y s t a b i l i z i n g o i l p r i c e was compensated by f u r t h e r f a l l i n g o f t h e US $ exchange r a t e down t o i t s a b s o l u t e minimum s i n c e t h e end o f t h e Second World War, and i n e a r l y 1988, a s l i g h t l y r e v i v i n g US $ which stopped i t s d e c l i n e and a t l e a s t s t a y e d more o r l e s s c o n s t a n t c o i n c i d e d w i t h another l i t t l e decrease o f t h e average r e a l o i l p r i c e ( O I L D A I L Y 1988 a; c f . s e c t i o n 2.2.1.3.). A t t h e end o f 1988, t h e p r e s i d e n t i a l e l e c t i o n i n t h e USA c o n f i r m i n g t h e g e n e r a l economical g u i d e l i n e s f o r t h e coming y e a r s r e s u l t e d i n renewed downwards p r e s s u r e on t h e US $ exchange r a t e . T h i s c o n s t e l l a t i o n o f a n o t h e r downwards tendency, a l t h o u g h c o m p r i s i n g o n l y d e c l i n e o f t h e r e a l o i l p r i c e o f a few US $ p e r b a r r e l , c r e a t e d a g a i n much unc e r t a i n t y i n t h e hydrocarbon e x p l o r a t i o n and p r o d u c t i o n i n d u s t r y and caused n o t
132 o n l y managements t o be even more c a u t i o u s w i t h l o n g - t e r m d e v e l o p m e n t and i n v e s t ment d e c i s i o n s as w e l l as p e r s o n n e l p l a n n i n g , b u t had a l s o p o l i t i c a l consequenc e s o n t h e h i g h e s t l e v e l r e l a t e d t o t h e o i l scene. The d i s c u s s i o n as f o l l o w s c o n c e n t r a t e s on c o o p e r a t i o n a t t e m p t s between OPEC and non-OPEC, m a r k e t s h a r e s v s . p r i c e s t a b i l i t y , downstream i n t e g r a t i o n o f o i l - p r o d u c i n g c o u n t r i e s , p r i c i n g p o l i c y o f l i s t v s . f l e x i b l e p r i c e s , p o s s i b i l i t i e s o f a n o t h e r o i l p r i c e war, i m p a c t o f a p o s s i b l e g e n e r a l e c o n o m i c a l r e c e s s i o n , o i l p r i c e c o l l a p s e as r e s u l t o f l a c k i n g i n t e r n a t i o n a l c o o p e r a t i o n , and d i f f e r e n t assessment o f t h e U S $ 1 8 agreement p r i c e .
2.2.1.4.3.1. Cooperation at terrpts between OPEC and non-OPEC F o r t h e f i r s t t i m e i n t h e i r h i s t o r y , t h e OPEC i s s u e d a s e r i o u s a l a r m f o r p r o t e c t i o n o f t h e o i l p r i c e and p r e v e n t i o n o f i t s f u r t h e r downwards o r i e n t a t i o n n o t o n l y w i t h i n t h e group, b u t addressed t h e i n v i t a t i o n f o r a c r i t i c a l session a l s o t o v a r i o u s non-OPEC o i l p r o d u c i n g c o u n t r i e s (I4cCANN 1988, O I L DAILY 1988 a ) . LUKMAN ( 1 9 8 7 ) emphasizes t h a t c o o p e r a t i o n between OPEC and non-OPEC count r i e s i s t h e k e y t o m a r k e t s t a b i l i t y ( c f . a l s o O I L GAS JOURNAL 1988 g ) , and ALCHALABI ( 1 9 8 7 ) e v e n c o n c l u d e s t h a t t r i l a t e r a l c o o p e r a t i o n between cotisuming nations, OPEC c o u n t r i e s and o t h e r o i l - p r o d u c e r s i s n e c e s s a r y t o d e f e n d t h e o i l p r i c e w h i c h i s backed u p b y t h e f a c t t h a t i n t h e l o n g r u n , non-OPEC o i l - p r o d u c i n g c o u n t r i e s c o u l d even s u f f e r more t h a n OPEC f r o m any p r i c e c o l l a p s e g i v e n t h e v e r y h i g h c o s t o f r e p l a c e m e n t o f consumed o i l t h r o u g h e x p e n s i v e d i s c o v e r i e s o f new f i e l d s o r enhanced d e v e l o p m e n t o f o l d f i e l d s . U n c e r t a i n t i e s a b o u t f u t u r e w o r l d o i l p r i c e s a r e a l s o d i s c u s s e d b y KILGORE ( 1 9 8 2 ) . I n t e r n a t i o n a l c o o p e r a t i o n between o i l p r o d u c e r s and consumers a i m i n g on m a r k e t r e g u l a t i o n i s e x p e c t e d t o be a v e r y p o s i t i v e and c o n s t r u c t i v e s t e p i n t h e d i r e c t i o n o f p r e v e n t i n g f u r t h e r s h a r p o i l p r i c e changes (PETROLEUM REVIEW 1988 c ) .
2.2.1.4.3.2. Market shares vs. price stability P r i c e s t a b i l i t y w h i c h i s a t t h e moment d i f f i c u l t t o a c h i e v e b y p o l i t i c a l a r rangement and m a r k e t i n s t r u m e n t s c a n i n t h e n e a r f u t u r e be i m p r o v e d i n c a s e o f a g e n e r a l e c o n o m i c a l r e c e s s i o n w h i c h l o w e r s demand f o r o i l (OFFSHORE O I L INTERNATIONAL 1 9 8 8 ) . The r e q u e s t f o r m a r k e t s t a b i l i t y as a base f o r i n t e r n a t i o n a l o i l p r i c i n g and d e c i s i o n - m a k i n g i s r e v i e w e d by STORY ( 1 9 8 6 ) , and DAVID & CARSON ( 1 9 8 3 ) comment on e n e r g y f i n a n c i n g i n t i m e s o f u n c e r t a i n t y . ROSBACO ( 1 9 8 3 ) g i ves an o v e r v i e w on i n v e s t m e n t e v a l u a t i o n s i n i n f l a t i o n a r y and u n s t a b l e scenar i o s . Aspects o f w o r l d energy markets coping w i t h i n s t a b i l i t y a r e a l s o compiled b y ROWSE ( 1 9 8 7 ) . F u r t h e r o i l p r i c e u n c o n s o l i d a t i o n i n a d d i t i o n t o abandonment o f production r e s t r a i n t i n favour o f seeking f a i r share i n t h e i n t e r n a t i o n a l o i l m a r k e t a r e n e t b a c k p r i c i n g d e a l s (RANDOL 1986; c f . s e c t i o n 2 . 2 . 1 . 4 . 2 . 4 . ) w h i c h remove t h e i n c e n t i v e f o r o i l companies t o s h a r p l y c u r t a i l l i f t i n g s f r o m t h e OPEC b y i n s u l a t i n g them f r o m m a r k e t r e a l i t y . OPEC s t r a t e g y f o r p r i c i n g d e c i s i o n s i s one o f t h e m o s t i m p o r t a n t t o p i c s f o r t h e g l o b a l e n e r g y m a r k e t s , w i t h r a t i o n a l i t y and s u s t a i n a b i l i t y o f OPEC imposed p r i c e s , p r o s p e c t f o r c o n t i n u e d s t a b i l i t y o f OPEC a s a c o h e s i v e body, and o u t l o o k f o r o i l p r i c e s i n t h e f u t u r e a t t r a c t i n g g r e a t e s t a t t e n t i o n (NARAGHI 1 9 8 8 ) .
2.2.1.4.3.3. Downstream integration o f oi 1-producing countries I n m i d 1988, t h e c o n t i n u i n g f a i l u r e o f OPEC t o s u s t a i n o f f i c i a l s e l l i n g p r i c e s i n t h e f a c e o f c o m p e t i t i o n b y non-OPEC e x p o r t e r s as w e l l as i t s own i n a b i l i t y t o r e i n p r o d u c t i o n has s p a r k e d new emphasis on OPEC r e i n t e g r a t i o n (WILLIAMS 1 9 8 8 ) . S i n c e l a t e 1987, o i l m a r k e t s have r e m a i n e d m o s t l y d e p r e s s e d , OPEC d i v i s i veness has grown worse, and o v e r t u r e s b y a g r o u p o f non-OPEC e x p o r t e r s t o s h a r e o u t p u t r e s t r a i n t and t h u s buoy o i l p r i c e s have so f a r f a i l e d . More and more OPEC members a r e t h e r e f o r e p u s h i n g i n t o downstream v e n t u r e s i n o r d e r t o become i n t e g r a t e d p r o d u c e r s . The downstream a c t i v i t i e s o f l a r g e - r e s e r v e p r o d u c e r s a r e
133 i n t e n d e d t o i n c r e a s e market share and d i v e r s i f y r i s k , b u t t h e r e s u l t c o u l d be c o n t i n u e d low o i l p r i c e s , as these producers t r y t o keep demand h i g h (WORLD OIL 1988 a ) . I n a d d i t i o n , t h e market share o f t h e l a r g e producers comes a t t h e expense o f s m a l l e r p r o d u c e r s t h a t do n o t have downstream access ( c f . s e c t i o n 2.2.1.5.2.) w h i c h w i l l have a d i v i s i v e e f f e c t on OPEC u n i t y and w i l l o f f e r l i t t l e hope t h a t t h e c a r t e l w i l l be a b l e t o improve c o o p e r a t i o n and d i s c i p l i n e among i t s members. The focus on d i v e r s i f i c a t i o n , t h e f o r e i g n downstream push, a p o s s i b l e mass i v e i n t e r v e n t i o n i n f u t u r e markets r e c e n t l y , and t h e s p u r n i n g o f non-OPEC o v e r t u r e s on p r o d u c t i o n r e s t r a i n t p o i n t t o t h e l i k e l i h o o d t h a t m a r k e t - r e l a t e d p r i c i n g w i l l c o n t i n u e t o spread and t h a t key members i n OPEC w i l l no l o n g e r t o l e r a t e l o s s o f m a r k e t share t o non-OPEC e x p o r t e r s i n o r d e r t o b o l s t e r p r i c e s (WILLIAMS 1988). Longer-term r e i n t e g r a t i o n by some OPEC members and non-OPEC export e r s c o u l d mean depressed o i l p r i c e s , depressed margins f o r r e f i n e r s and market e r s , and a c c e l e r a t i n g o i l i m p o r t s i n n e t consuming n a t i o n s t h r o u g h t h e 1990's. Much o f t h e impetus f o r OPEC r e i n t e g r a t i o n comes f r o m t h e t h r e a t t o t h e market share o f t h e o r g a n i z a t i o n by r i s i n g p r o d u c t i o n f r o m non-OPEC e x p o r t e r s .
2.2.1.4.3.4. Pricing policy o f list vs. flexible prices I n c r e a s i n g non-OPEC o i l s u p p l y and l a g g i n g demand growth i s g o i n g t o r e i n OPEC-s a b i l i t y t o r a i s e o i l p r i c e s u n t i l t h e y e a r 2000, because h i s t o r i c a l l y , OPEC has been a b l e t o j a c k up o i l p r i c e s o n l y when t h e group's p r o d u c t i o n was a t 80 % o r more o f i t s t o t a l c a p a c i t y (WILLIAMS 1988). C o m p e t i t i o n between OPEC and non-OPEC p r o d u c e r s f o r market share i s expected t o p e r s i s t a t l e a s t t h r o u g h 1995. F i x e d p r i c i n g has proven unworkable w i t h o u t s a c r i f i c e s t h a t key members such as Saudi A r a b i a a r e no l o n g e r w i l l i n g t o make ( c f . s e c t i o n 2 . 2 . 1 . 2 . 2 . 1 . ) . Thus i n o r d e r t o m a i n t a i n market share, OPEC p r o b a b l y must adopt a f l e x i b l e p r i c i n g p o l i c y and e v e n t u a l l y abandon o f f i c i a l l i s t p r i c i n g . The group c o u l d t h e n t a k e advantage o f immediate i n c r e a s e s i n s p o t p r i c e s and a v o i d t h e r o l e o f swing pro,ducer. A b t . 85 % o f t h e p r o j e c t e d e x p o r t volume s h o u l d be s o l d under c o n t r a c t and a b t . 15 % s h o u l d be b r o u g h t i n t o t h e s p o t m a r k e t f o r more t h a n t h e n e g o t i a t e d p r i c e s i n t h e c o n t r a c t . Such an approach would n o t d u p l i c a t e t h e n e t b a c k - p r i c e - r e l a t e d market c r a s h o f 1986 ( c f . s e c t i o n 2.2.1.1.2.) because t h e r e would be adherence t o quotas vs. t h e q u o t a - b r e a k i n g e f f o r t t o r e g a i n market share i n terms o f volume, and m a r g i n a l OPEC o i l would s e l l a t a premium i n s t e a d o f a t a d i s c o u n t . Spot s a l e s a l s o would be a b u f f e r a g a i n s t supply/demand imbalances, because s p o t buyers a r e t h e f i r s t t o c u t purchases i n a s o f t market and t o pay a premium i n a t i g h t market.
2.2.1.4.3.5. Possibilities o f another oil price war The downstream push, however, may be a n o t h e r s i g n t h a t key OPEC p r o d u c e r s may be g i r d i n g f o r a n o t h e r p r i c e war (WILLIAMS 1988). Even i n t h e s c e n a r i o f o r f l e x i b l e p r i c i n g , t h e OPEC members must s t r i c t l y adhere t o quotas, because o t h e r w i s e t h e o i l p r i c e w i l l c o l l a p s e i n an o v e r s u p p l i e d m a r k e t no m a t t e r how t h e s u p p l y i s made a v a i l a b l e . U n t i l t h e main o i l e x p o r t i n g c o u n t r i e s g a i n enough share i n t h e downstream market t o f e e l c o m f o r t a b l e about t h e i r i n t e g r a t e d and o v e r a l l income, t h e y w i l l keep t h e o i l p r i c e u n s t a b l e i n o r d e r t o d i s courage development o f r e s e r v e s elsewhere. Saudi A r a b i a seems t o be p r e p a r e d t o teach o t h e r o i l - e x p o r t i n g c o u n t r i e s once more t h e l e s s o n t h a t f a i l u r e t o - c o o p e r a t e i n r e s t r a i n i n g p r o d u c t i o n w i l l have d i s a s t r o u s e f f e c t s on everyone s i n come, w i t h a s h o r t p r i c e war b e i n g expected t o encourage c o o p e r a t i o n and l i a b i l i t y . Continuous OPEC o v e r p r o d u c t i o n i n l a t e 1987 and a l m o s t t h r o u g h o u t 1988 was p r o g r e s s i v e l y undermining t h e o i l p r i c e ( c f . s e c t i o n 2.2.1.1.3.), and i n c o m b i n a t i o n w i t h t h e p r o l o n g e d g e n e r a l weakness o f t h e US $, t h e o i l p r i c e may again f a l t e r if t h e surge of OPEC o u t p u t c o n t i n u e s (BECK & WILLIAMS 1987).
134
On t h e o t h e r hand, however, t h e s i t u a t i o n i n m i d t o l a t e 1988 i s somewhat d i f f e r e n t f r o m t h a t i n 1986 because i n o r d e r t o move o i l i n t h e e a r l i e r m a r k e t s h a r e b a t t l e , t h e OPEC c o u n t r i e s had t o g u a r a n t e e t h e i r c u s t o m e r s - r e f i n i n g marg i n s w i t h n e t b a c k s a l e s agreements ( O I L GAS JOURNAL 1988 c ; c f . s e c t i o n 2.2.1.4.2.4.). I n 1988, however, t h e r e f i n i n g m a r k e t s a r e s t r o n g due t o m a r g i n w i d e n i n g b y t h e o i l p r i c e d r o p , and t h e r e f o r e renewed s u b s t a n t i a l o v e r p r o d u c t i o n and m a r k e t f l o o d i n g w o u l d n o t s e r v e t h e a p p a r e n t e c o n o m i c a l i n t e r e s t s o f any o i l - e x p o r t i n g c o u n t r y . A s u i t a b l e b r a k e f o r a l o n g e r and d e e p e r o i l p r i c e slump and a r e a s o n a b l e w a r n i n g f o r t h e OPEC t o s t o p l a p s e o f d i s c i p l i n e i s t h e memory of t h e m o s t d e v a s t a t i n g a s p e c t o f t h e 1986 o i l p r i c e c o l l a p s e w h i c h was t h e f e a r t h a t t h e o i l p r i c e c o u l d r e m a i n b e l o w US $ 10 f o r a l o n g e r t i m e , i f n o t i n economical terms meaning almost i n d e f i n i t e l y .
2.2.1.4.3.6.Imact o f a possible general economical recession I n a d d i t i o n t o i n s t a b i l i t y g e n e r a t e d w i t h i n t h e OPEC g r o u p , a m a j o r e x t e r n a l f a c t o r i n t h e assemblage o f consumers i s t h e f e a r o f r e c e s s i o n w h i c h has become an i m p o r t a n t w i l d c a r d i n o i l m a r k e t f o r e c a s t s ( O I L GAS JOURNAL 1987 e ) . S l o w e r e c o n o m i c a l g r o w t h r e d u c e s demand f o r p e t r o l e u m and i t s p r o d u c t s and t h e r e f o r e c a l l s o n OPEC o i l . A r e c e s s i o n w o u l d s e r i o u s l y c o m p l i c a t e e f f o r t s t o s t a b i l i z e and r e s t o r e o i l p r i c e s , w i t h p r o s p e c t s o f r e c e s s i o n t h u s p o s i n g a g r o w i n g t h r e a t t o t h e o i l m a r k e t . The weakness o f t h e U S $ ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) is one o f t h e s t r o n g e s t s i g n s o f u n f a v o u r a b l e e c o n o m i c a l framework and i s e x p e c t e d t o p r e c i p i t a t e a l s o s o o n e r o r l a t e r a c o l l a p s e o f t h e o i l p r i c e . On t h e o t h e r hand, t h e f e a r o f a n o t h e r o i l m a r k e t c r a s h w h i c h a t t h e b o t t o m o f t h e l i n e w o u l d s e r i o u s l y h u r t a l l t h e p a r t i c i p a n t s i n t h e game has an o f f s e t t i n g i n f l u e n c e w h i c h h o l d s a s s o c i a t i o n s o f r i v a l i z i n g members d e s p i t e l a c k i n g c o n s e n sus s t i l l t o g e t h e r ( O I L GAS JOURNAL 1987 f ) . O i l p r i c e v o l a t i l i t y a l s o l e a d s t o company management r e s t r u c t u r i n g w i t h c h a n g i n g emphasis f r o m t e c h n i c a l s k i l l t o f i n a n c i a l e x p e r i e n c e and u n d e r s t a n d i n g of e x e c u t i v e s ( O I L GAS JOURNAL 1987 4).
2.2.1.4.3.7.Oil price collapse as result o f lacking international cooperat ion A n o t h e r p o s s i b l e o i l p r i c e c o l l a p s e as t h e l o g i c a l c o n t i n u a t i o n o f t h e r e a l p r i c e d e c l i n e b y a b t . U S $ 5 f r o m a b t . U S $ 18.50 i n A p r i l 1988 t o a b t . US $ 1 3 . 7 5 i n September 1988 c a n o c c u r i f p r o d u c e r s c o n t i n u e t o f a i l u r e t o c o o p e r a t e ( O I L GAS JOURNAL 1988 e ) . The r e c e n t downward p r e s s u r e on p r i c e s m a i n l y stems from f e a r s t h a t overproduction w i l l l e a d t o a sharp increase i n stocks by year end. The o i l p r i c e d r o p o f more t h a n US $ 4 i n J u l y 1988 was p r o v o k e d b y t h e r e f u s a l o f Saudi A r a b i a and K u w a i t t o a c c e p t p r o d u c t i o n c u t o f f e r s made b y nonOPEC c o u n t r i e s , t h e r e b y f o r c i n g OPEC t o f o c u s on i n t e r n a l p r o b l e m s such as o u t p u t d i s c i p l i n e . Many o f t h e u n d e r l y i n g f a c t o r s t h a t b r o u g h t a b o u t t h e 1986 deb a c l e have reemerged i n 1988, and s o l v i n g t h e p e r p e t u a l l a c k o f q u o t a d i s c i p l i n e among OPEC member n a t i o n s r e m a i n s c e n t r a l t o t h e f u t u r e h e a l t h o f t h e hyd r o c a r b o n i n d u s t r y (WORLD OIL 1988 a ) . The non-OPEC p r o p o s a l o f a 5 % p r o d u c t i o n c u t b a c k f o r b o t h OPEC and non-OPEC c o u n t r i e s was r e j e c t e d b y some OPEC k e y members because of t h e i r v i e w t h a t demand w o u l d b e more s t i m u l a t e d w i t h o i l p r i ces b e l o w U S $ 18, whereas any p r o d u c t i o n r e s t r i c t i o n s c o u l d r a i s e t h e p r i c e above U S $ 18 (CROUSE 1988 a, PIcCANN 1988, O I L GAS JOURNAL 1988 9 ) .
2.2.1.4.3.8.Different assessment o f the US
$
18 agreement price
I n a d d i t i o n , t h e G u l f members o f t h e OPEC g r o u p p u t f o r w a r d t h a t OPEC h a d done enough t o c u r b e x c e s s s u p p l i e s u n i l a t e r a l l y s i n c e OPEC o u t p u t was down a b t . 40 % f r o m 1979 l e v e l s , w h i l e non-OPEC c o u n t r i e s were p r o d u c i n g a t c a p a c i t y (CROUSE 1988 a ) . F u r t h e r i n s t a b i l i t y i s c r e a t e d b y d i f f e r e n t meanings o f t h e OPEC members on t h e US $ 18 agreement p r i c e ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . 2 . ) , with
135 t h e Gulf Cooperation C o u n c i l (GCC) h a v i n g a f a i r l y o p t i m i s t i c and r e a l i s t i c und e r s t a n d i n g o f demand and n o t b e i n g anxious t o see p r i c e s s u s t a i n e d above US $ 18 f o r t h e reason o f f e a r t h a t r i s i n g p r i c e s w i l l depress demand, whereas among OPEC members o u t s i d e t h e G u l f r e g i o n , US $ 18 i s seen more as t h e minimum accept a b l e p r i c e ( O I L GAS JOURNAL 1988 9 ) . D e s p i t e t h e u n s u c c e s s f u l a t t e m p t i n e a r l y t o m i d 1988, f u t u r e c o o p e r a t i o n between OPEC and non-OPEC i s t h e r e f o r e more r a t h e r than l e s s l i k e l y t o be a v a l u a b l e p o l i c y o p t i o n f o r c o n s o l i d a t i n g t h e w o r l d o i l market (McCANN 1988). Aspects o f o i l p r i c e i n s t a b i l i t y a r e a l s o d i s cussed by CROUSE (1987) and O I L GAS JOURNAL (1987 n ) .
2.2.1.4.4. Energy supp 1 y diversification effects Another i n t e r n a t i o n a l e f f e c t i s t h a t a t o i l p r i c e s below U S $ 15, t h e w o r l d tends t o have t o o few energy o p t i o n s f o r t h e l o n g term, whereas a t p r i c e s above US $ 30, t h e r e a r e t o o many o f them, w i t h an optimum o i l p r i c e a t t h e b o t t o m o f t h e l i n e t h u s b e i n g i n t h e range o f US $ 15 - 30 (OFFSHORE ENGINEER 1986 f ) . W i t h i n t h e frame o f an o i l p r i c e o f US $ 15 - 30, a balance e x i s t s between t h e use o f l o w - c o s t o i l and higher-expense e n e r g i e s i n c l u d i n g h i g h e r - c o s t o i l . On t h e o t h e r hand, a world-wide s u r v e y o f undeveloped m a r g i n a l o f f s h o r e f i e l d s r e v e a l s t h a t even w i t h an o i l p r i c e o f US $ 15 and i n l i g h t o f e x i s t i n g t e c h n o l o gy and f i s c a l regimes, t h e development o f small o f f s h o r e o i l f i e l d s around t h e w o r l d w i l l n o t s t o p e n t i r e l y , b u t f o r a m e l i o r a t i n g i t s pace, e i t h e r o i l p r i ces must r i s e above US $ 20 o r governments must improve f i s c a l c o n d i t i o n s t o p r o v i d e a b e t t e r i n c e n t i v e t o t h e i n d u s t r y . Some o f t h e m a r g i n a l f i e l d s c u r r e n t l y h e l d by m a j o r o p e r a t o r s b u t c o n s i d e r e d t o be u n i n t e r e s t i n g by them c o u l d be a t t r a c t i v e t o i m a g i n a t i v e independent o i l companies once t h e y a r e r e l i n q u i s h e d .
2.2.1.5. Present consequences of the 1986 oil price drop for the stimulation market The most i m p o r t a n t p r e s e n t consequences o f t h e 1986 o i l p r i c e drop f o r t h e European s t i m u l a t i o n m a r k e t a r e i l l u s t r a t e d a l o n g t h e l i n e s o f f r a c t u r i n g s t r a t e g y o f deep gas w e l l s b e f o r e and a f t e r t h e 1986 o i l i n d u s t r y c r i s i s , r e d u c t i o n o f c a p i t a l e x p e n d i t u r e and o r g a n i z a t o r y s t r e a m l i n i n g o f e x p l o r a t i o n and product i o n companies, business d e t e r i o r a t i o n f o r s e r v i c e companies, and banking r e l a t i o n s h i p s and i n v e s t m e n t s t r a t e g i e s .
2.2.1.5.1. Fracturing strategy o f deep gas wells before and after the 1986 oil industry crisis Abt. 15 y e a r s ago when t h e o i l p r i c e approached i t s f i r s t b i g i n c r e a s e s (RUNGE 1986) t h e r e b y a m e l i o r a t i n g t h e f e a s i b i l i t y o f many h i t h e r t o m a r g i n a l o r even uneconomical p r o j e c t s , many deep gas w e l l s i n Western Europe und USA were ready f o r MHF s t i m u l a t i o n , b u t c o u l d n o t be t r e a t e d , because s u i t a b l e s y n t h e t i c i n t e r mediate- and h i g h - s t r e n g t h proppants t h a t c o u l d w i t h s t a n d t h e h i g h c l o s u r e s t r e s s e s i n these depths were n o t y e t developed, and o n l y n a t u r a l sand e x i s t e d which crushed under those c o n d i t i o n s (TUNN 1971; ATTEBERRY, TUCKER & RITZ 1979; COOKE & GIDLEY 1979; HICKEY, BROWN & CRITTENDEN 1981; KOHLHAAS 1982, CLARK 1983, B R I M 1986). Some aspects o f t h e f r a c t u r i n g p o t e n t i a l b e f o r e and a f t e r t h e 1986 o i l p r i c e c r a s h a r e o u t l i n e d as f o l l o w s .
2.2.1.5.1.1. Fracturing potential before the 1986 oil price drop Many e c o n o m i c a l l y v e r y a t t r a c t i v e f r a c t u r e j o b s c o u l d n o t be c a r r i e d o u t many y e a r s ago due t o l a c k o f s u i t a b l e p r o p p i n g m a t e r i a l s i n t h e e a r l y 1970's. I n t h e golden y e a r s o f h i g h o i l p r i c e s i n t h e l a t e 1970's and e a r l y 1980's ( i n t h e l a t t e r p e r i o d i n Europe even accentuated by a h i g h US $ exchange r a t e ) ,
136 s u p p l y o f i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants c o u l d as a consequence o f i n s u f f i c i e n t m a n u f a c t u r i n g c a p a c i t i e s i n r e l a t i o n s h i p t o t h e growing m a r k e t due t o wrong o r n o t enough o p t i m i s t i c f o r e c a s t and assessment o f t h e r e q u i r e m e n t s t e m p o r a r i l y n o t keep pace w i t h t h e p a r t i a l l y enormous demand. Thus as a r e s u l t o f p r e f e r e n t i a l s e r v i c e o f t h e USA market, p a r t i c u l a r l y i n Western Europe numerous h i g h - r a n k l a r g e - s c a l e f r a c t u r e j o b s had t o be delayed u n t i l t h e r e q u i r e d q u a n t i t i e s o f h i g h - q u a l i t y proppants were a v a i l a b l e . Expens i v e w a i t i n g t i m e s o f r i g s i n those days j u s t i f i e d i n some cases even payment o f a i r f r e i g h t f o r f a s t d e l i v e r y o f intermediate- o r high-strength proppants f r o m t h e p l a n t s i n t h e USA t o t h e w e l l s i t e i n Europe o r b u y i n g a c o n s i d e r a b l y more expensive p r o p p a n t t y p e t h a n scheduled f r o m l o c a l c o m p e t i t i o n i n o r d e r t o g e t t h e j o b r a p i d l y done and make p r o f i t f r o m q u i c k p a y - o u t i n t h e s c e n a r i o o f h i g h o i l p r i c e and h i g h US $ exchange r a t e ( c f . s e c t i o n 2.2.2.3.). Much promot i o n o f t h e s t i m u l a t i o n market, however, was hampered by t h e u n d e r s u p p l y o f b o t h European and w o r l d markets w i t h proppants i n t h e p e r i o d 1976 - 1984 comp r i s i n g t h e t i m e f r o m i n v e n t i o n o f i n t e r m e d i a t e - and h i g h - s t r e n g t h s y n t h e t i c proppants t o b e g i n n i n g o f more s e r i o u s o i l p r i c e d e c l i n e and p r i o r t o t h e 1985 p r o p p a n t p r i c e changes i n downwards d i r e c t i o n along w i t h i m p r o v i n g s u p p l y capac i t y ( c f . section 2.2.2.1.).
2.2.1.5.1.2. Fracturing potential after the 1986 oil price crash A f t e r t h e m a j o r o i l p r i c e d e c l i n e i n e a r l y 1986 (GRIFFITHS 1986, RUNGE 1986), t h e s i t u a t i o n has reversed, w i t h a g a i n numerous deep gas w e l l s t h a t a r e w a i t i n g on t r e a t m e n t (and which p a r t i a l l y have o r i g i n a l l y been d r i l l e d i n o r d e r t o be f r a c t u r e d soon a f t e r w a r d s as a r e s u l t o f p r e d i c t i o n o f t i g h t r e s e r v o i r f a c i e s i n t h e l o w - p e r m e a b i l i t y zone e x p l o r e d by u n c o n v e n t i o n a l gas a p p r a i s a l i n t h e b e l t o f m a r g i n a l pay zone development, w i t h t h e s t i m u l a t i o n c o s t a l r e a d y hav i n g been added t o t h e d r i l l i n g c o s t i n t h e economical f e a s i b i 1 it y assessment b e f o r e spudding o f t h e w e l l s ) n o t b e i n g a b l e t o be f r a c t u r e d , b u t t h i s t i m e f o r economical i n s t e a d o f t e c h n i c a l reasons. I n c o n t r a s t t o t h e e a r l i e r y e a r s , h i g h - q u a l i t y proppants a r e now a v a i l a b l e i n almost oversupply, w i t h t h u s b e i n g no l i m i t a t i o n o f q u a n t i t i e s t o be d e l i v e r e d s t r a i g h t a f t e r o r d e r e n t r y , and proppants b e i n g a l s o o f f e r e d f o r n e a r l y dumping p r i c e s w i t h r e s p e c t t o t h e f o r mer s i t u a t i o n . The low o i l p r i c e ( w h i c h has i n Europe even a more s e r i o u s impact due t o t h e accompanying weakness o f t h e US $ ) , however, now does h a r d l y g i v e any economic a l j u s t i f i c a t i o n f o r c a r r y i n g - o u t o f many j o b s p r i o r t o a reasonable r e s t a b i l i z a t i o n o f t h e hydrocarbon p r i c e s c e n a r i o a t an a c c e p t a b l e l e v e l ( w h i c h would be most s u i t a b l y r e i n f o r c e d by a r i s i n g v a l u e o f t h e U S $ ) t o be adequate t o t h e investments t o be made by e x e c u t i n g s t i m u l a t i o n ( t h i s s i t u a t i o n i s i n p a r t s o f Europe s t i l l c u r r e n t ) . S i m i l a r d i f f i c u l t i e s o f economical f e a s i b i l i t y a t l o w e r o i l p r i c e s as a p p l y i n g f o r h y d r a u l i c proppant f r a c t u r i n g a r e a l s o t o u c h i n g enhanced o i l r e c o v e r y p r o j e c t s (BRASHEAR, BECKER & BIGLARBIGI 1988). I n a d d i t i o n , t h e most i m p o r t a n t f a c t o r c h a r a c t e r i z i n g t h e hydrocarbon e x p l o r a t i o n and development scene a f t e r t h e o i l p r i c e c r a s h i n 1986 i s l a c k i n g c o n f i d e n c e f o r f u t u r e i n v e s t m e n t o f money d e s p i t e e x i s t i n g hope o r even o p t i m i s m (KINNEY 1987), and t h i s c r i t e r i o n i s t h e most i m p o r t a n t reason why t h e c a p i t a l e x p e n d i t u r e p r o gram s l a s h e d down and o n l y s l o w l y p i c k s up again. The impact o f t h e c o n f i d e n c e l o s s i s so s t r o n g t h a t i t can be expected t h a t even i f t h e o i l market rebounds and e n t e r s a n o t h e r boom phase s t i l l t h i s c e n t u r y , t h e l e s s o n s o f t h e o i l m a r k e t shocks i n t h e 1970's and 1980's may change f o r e v e r t h e g u i d e l i n e s f o r c a p i t a l i n v e s t m e n t and i n d u s t r y management (OIL GAS JOURNAL 1987 9 ) .
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Reduction o f capital expenditure and organizatory streamlining in exploration and product ion c m a n i e s
2.2.1.5.2.
The r e v i s i o n o f budgets f o r d r i l l i n g , c o m p l e t i o n and e x p l o i t a t i o n by t h e hydrocarbon e x p l o r a t i o n and p r o d u c t i o n companies has n o t o n l y a f f e c t e d t h e i r own c a p i t a l e x p e n d i t u r e , f i n a n c i a l f l e x i b i l i t y and c a s h - f l o w s t r u c t u r e , b u t a l s o had s i g n i f i c a n t impact on s e r v i c e company business, banking r e l a t i o n s h i p s and i n v e s t m e n t s t r a t e g i e s . Economical reassessment o f t h e N o r t h Sea o i l - and gasf i e l d development p l a n s a f t e r t h e 1986 o i l p r i c e d r o p ( c f . s e c t i o n s 2.2.1.1.2. and 2.4.2.) has r e s u l t e d i n r e d u c t i o n o f c a p i t a l e x p e n d i t u r e on c u r r e n t development p r o j e c t s , d e f e r r i n g o f investments i n new f i e l d development p r o j e c t s , p r e f e r e n c e g i v e n t o B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas a c q u i s i t i o n and t o s m a l l - s c a l e developments u s i n g subsea c o m p l e t i o n s f o r t y i n g - i n o f s a t e l l i t e f i e l d s t o e x i s t i n g p r o d u c t i o n p l a t f o r m s , severe c u t s i n planned e x p l o r a t i o n p r o grams o f a b t . 25 - 40 % (KEG 1986, OIL GAS JOURNAL 1986, ALGAR 1987 b, HOLBROOK 1987, SCHRUDER 1987) w h i c h w i l l a f f e c t f u t u r e a v a i l a b i l i t y o f f i e l d s f o r development, and r e d u c t i o n o f o p e r a t i n g c o s t s f o r e x i s t i n g p r o d u c i n g f i e l d s (BEUDELL 1986 b, OFFSHORE ENGINEER 1986 a, THOMAS 1986). T h i s i s more o r l e s s r e f l e c t i n g t h e decreasing e a r n i n g s and revenues by a b t . 34 % and 24 %, r e s p e c t i v e l y , f o r t h e group o f m a j o r o i l and gas companies f o l l o w i n g t h e 1986 o i l p r i c e c r a s h (BECK & SMITH 1987). The d i s c u s s i o n as f o l l o w s focusses on development and c o m p l e t i o n a c t i v i t y , d r i l l i n g meterage, s t a f f l a y o f f , company merging, cash management, s u r v i v a l o f independents, r e s p o n s i b i l i t y t r a n s f e r by c o n t r a c t i n g and takeover, and e x e c u t i v e s k i l l r e q u i r e m e n t change.
2.2.1.5.2.1.Development and completion activity The 1986 o i l p r i c e c o l l a p s e l e d t o a 15 % d e c l i n e i n w o r l d w i d e e x p l o r a t i o n d r i l l i n g a c t i v i t y which r e s u l t e d i n l o s s o f r e s e r v e replacement o f a b t . 2.5 - 3.0 B i l l . b b l o i l (POPESCU & HODGSHON 1987). T h i s was t h e c u l m i n a t i o n o f a t r e n d which s t a r t e d i n 1982 when t h e z e n i t h was passed and once t h e n a d i r i n 1986 was reached, w o r l d w i d e e x p l o r a t i o n d r i l l i n g had decreased by a l m o s t 30 % s i n c e t h e golden y e a r s . I n t h e USA, t h e e f f e c t was even more pronounced and comp r i s e d a 60 % r e d u c t i o n i n e x p l o r a t i o n d r i l l i n g s i n c e t h e peak i n 1981 ( c f . sect i o n 2.2.1.2.1.). Subsea development o f s a t e l l i t e f i e l d s forms a v e r i t a b l e complementary s e t t o m a j o r t r a d i t i o n a l developments w i t h p l a t f o r m and p i p e l i n e , because i t enabl e s small f i e l d s t o s t a r t p r o d u c i n g on t h e b a s i s o f e x i s t i n g p r o c e s s i n g c a p a c i t i e s and c a p a b i l i t i e s t h a t have become a v a i l a b l e because o f t h e d e c l i n e o f r e a sonably nearby f i e l d s (VALENTIN, VALAIS & CHAMPLON 1988). BARTON (1988) e v a l u a t e s p o s s i b i l i t i e s o f making s m a l l e r f i e l d s v i a b l e , and HARPER (1988) i n v e s t i g a t e s d e v e l o p i n g c o s t - e f f e c t i v e t e c h n o l o g y f o r t h e f u t u r e . COX (1984) d i s c u s s e s economical aspects o f m a r g i n a l f i e l d development i n t h e N o r t h Sea. Adjustments o f o i l f i e l d a c t i v i t i e s t o low revenues a r e a l s o i l l u s t r a t e d by R E I N I C K E , TRENEL, HERBST, KEMPGEN & LINZ (1988). Another p o i n t i l l u m i n a t i n g t h e s e v e r i t y o f t h e i m p a c t o f t h e 1986 o i l p r i c e c r a s h i s t h e f a c t t h a t f o r t h e f i r s t t i m e s i n c e d r i l l i n g began almost 25 y e a r s ago, o i l p r o d u c t i o n d e c l i n e d i n t h e B r i t i s h N o r t h Sea s e c t o r i n 1986 (McNALLY 1987). I n t h e USA, o i l p r o d u c t i o n was 9 % l o w e r i n 1987 t h a n i n 1985, l a r g e l y because o f s h u t t i n g - i n o f s t r i p p e r w e l l s (THOMPSON & SINGLETON 1988).
2.2.1.5.2.2. Drill in9 meterage Germany FRG i s an e x c e l l e n t example how d r i l l i n g meterage f l u c t u a t e s as a consequence o f o i l p r i c e e x p l o s i o n s and c o l l a p s e s . I n Germany FRG, d r i l l i n g i n 1986 was l e s s t h a n 50 % o f t h e meterage o f 1985, and i n 1987 and 1988 f u r t h e r decreases took p l a c e (ERDUL UND KOHLE - ERDGAS - PETROCHEMIE 1988) which can be
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e x p l a i n e d by p r o p e r p l a n n i n g of the whole y e a r i n c l u d i n g c a n c e l l a t i o n of such m a r g i n a l l y f e a s i b l e items t h a t i n 1986 have a l r e a d y been spudded o r were ready t o s t a r t when t h e o i l p r i c e dropped. The e v o l u t i o n of t o t a l d r i l l e d meterage i n Germany FRG d u r i n g t h e l a s t 15 y e a r s i s pronouncedly r e f l e c t i n g o i l p r i c e h i s t o r y (SCHRODER 1980, 1987, 1 9 8 8 ) . From t h e low i n 1973 with 1 3 6 . 4 km t o t a l o i l and gas d r i l l i n g , a c o n t i n u o u s i n c r e a s e took p l a c e t o 1977 and 1978 when 243.4 and 264.9 k m were d r i l l e d , r e s p e c t i v e l y . I n 1979, however, the d r i l l i n g meter a g e growth s t o p p e d , s t a g n a t i o n o c c u r r e d , and even a s l i g h t s h o r t r e v e r s e e v o l u t i o n was i n i t i a t e d , l e a d i n g from 253.7 k m i n 1979 t o 236.1 k m i n 1980. As a consequence of the second o i l p r i c e e x p l o s i o n i n 1979/1980, d r i l l i n g met e r a g e d r a s t i c a l l y i n c r e a s e d i n the golden y e a r s of hydrocarbon a c q u i s i t i o n and reached 262.1 km i n 1 9 8 1 and the r e c o r d o f 334.6 km i n 1982. I n 1983 and 1984, beginning d e c l i n e of o i l p r i c e s provoked a f a l l - b a c k from the 1982 summit t o more o r l e s s t h e niveau of 1981, w i t h i n t o t a l 252.2 km and 265.6 k m having been d r i l l e d i n 1983 and 1984, r e s p e c t i v e l y . A meterage of 246.6 k m i n 1985 was a l r e a d y t h e i n d i c a t i o n of an a c c e l e r a t e d s l i d e down which was f o l l o w e d by t h e complete c o l l a p s e i n 1986 when o n l y 176.9 km were d r i l l e d , and due t o numerous c a n c e l l a t i o n s n o t having been immediately e f f e c t i v e i n 1986 b u t f u l l y broke through i n 1987, t h e r e s u l t of 1987 was w i t h 118.9 km even c o n s i d e r a b l y l e s s than i n 1986, and 1988 i s n o t coming o u t b e t t e r . In 1986 and f o l l o w i n g y e a r s , c o n t i n u i n g o i l p r i c e i n s t a b i l i t y encouraged o n l y a few very s t r o n g American companies t o i n c r e a s e a g a i n c a p i t a l e x p e n d i t u r e s i g n i f i c a n t l y (MOORE 1988 d ) , w h i l e most of t h e c o r p o r a t i o n s were c o n t i n u i n g t o c u t back i n a l l branches of i n v e s t m e n t . I n t h e USA, t o t a l o i l company l o s s e s i n 1986 and 1987 a s a r e s u l t of t h e o i l p r i c e c r a s h amounted t o more than 50 B i l l . US $ ( A L - C H A L A B I
1987).
2.2.1.5.2.3.Staff layoff F u r t h e r consequences of a d j u s t m e n t t o t h e r e v i s e d s i t u a t i o n and company s t r e a m l i n i n g were c u t - b a c k s on g e n e r a l overheads and s t a f f ( B E U D E L L 1986 c ) which r e s u l t e d i n s h r i n k a g e o r c l o s u r e of r e g i o n a l o f f i c e s o r even b u s i n e s s l i q u i d a t i o n , g i v i n g r i s e t o s e v e r a l t e r r i b l e world-wide l a y - o f f a c t i o n s a f f e c t i n g c o n s i d e r a b l e s h a r e s of g e o s c i e n t i s t s , e n g i n e e r s and a d m i n i s t r a t i o n people ( A L GAR 1987 b e s t i m a t e s thar. world-wide a b t . a q u a r t e r of a m i l l i o n hydrocarbon i n d u s t r y j o b s d i s a p p e a r e d due t o the 1986 o i l p r i c e c r a s h ) . The o i l p r i c e drop i s t h e main reason why predominantly i n USA, G r e a t B r i t a i n and Germany FRG thousands of g e o l o g i s t s and g e o p h y s i c i s t s a r e unemployed and c a n n o t f i n d a q u a l i f i e d j o b a t a l l , and a l s o r e s e r v o i r e n g i n e e r s have run i n t o s e r i o u s problems concerning working p l a c e and j o b s e c u r i t y . As a consequence of high l a y o f f r a t e s , some of t h e o i l and g a s c a p i t a l s even p a r t i a l l y t u r n e d i n t o g h o s t - l i k e c i t i e s a s a r e s u l t of empty o f f i c e towers and abandoned l i v i n g d i s t r i c t s due t o mass exodus of people having become unemployed. One of the w o r l d ' s l a r g e s t o i l and g a s e x p l o r a t i o n and p r o d u c t i o n c o r p o r a t i o n s a d j u s t e d i t s s t a f f s t e p w i s e t o the d e c l i n i n g market s i n c e t h e s l i g h t b e g i n n i n g s of t h e r e c e s s i o n by end o f 1981 and u p t o now reduced i t s manpower by a b t . 45 %. Most of the o i l and g a s companies i n the USA l a i d o f f a b t . 25 - 30 % of t h e i r employees i n 1986 (THOMPSON & SINGLETON 1 9 8 8 ) . I n e a r l y 1988, most compan i e s had r e s t r u c t u r e d and were o p e r a t i n g w i t h minimum personnel r e q u i r e d t o perform b u s i n e s s ( C R O U S E 1988 b ) . Some companies, however, even s t a r t e d a n o t h e r l a y o f f round of a b t . 10 % of t h e i r s t a f f i n 1988 ( M O O R E 1988 d ) . J o b r o t a t i o n and l o s s a f f e c t e d a l l l e v e l s o f company employment from l o w e s t workers t o h i g h e s t e x e c u t i v e s , w i t h t o p management r e s t r u c t u r i n g aiming on b e t t e r economic a l guidance of t h e c o r p o r a t i o n which was c a r r i e d o u t by changing emphasis from t e c h n i c a l s k i l l t o f i n a n c i a l e x p e r i e n c e and u n d e r s t a n d i n g of e x e c u t i v e s (OIL GAS JOURNAL 1987 9 ) . In Germany F R G , a b t . 15 % s t a f f r e d u c t i o n s i n c e the o i l p r i c e c r a s h was
rea-
139 ched by end o f 1987 m a i n l y as a consequence o f renouncement o f r e p l a c i n g r e t i r i n g o r l e a v i n g employees, b u t p a r t i a l l y a l s o due t o l a y o f f (ERDUL UND KOHLE ERDGAS - PETROCHEMIE 1988). The renewed o i l p r i c e v o l a t i l i t y i n m i d t o l a t e 1988 which b r o u g h t t h e p r i c e l e v e l down t o 12 - 15 US $ i n r e a l terms and c r e a t e d a s i t u a t i o n a t t h e s w e l l o f a n o t h e r p o s s i b l e c r a s h l e d s e v e r a l companies t o p l a n f u r t h e r manpower r e d u c t i o n by 25 - 30 % u n t i l 1992 which has t o be c a r r i e d o u t p r e f e r e n t i a l l y by e a r l y r e t i r e m e n t , b u t some l a y o f f u n d o u b t e d l y w i l l n o t be a b l e t o be avoided. SCHOLL (1986) comments on f u t u r e manpower r e q u i r e m e n t s i n a d r a m a t i c a l l y changing p e t r o l e u m i n d u s t r y .
2.2.1.5.2.4. Company merging The d r a m a t i c a l o i l p r i c e d r o p i n 1986 had p a r t i c u l a r l y severe consequences f o r s m a l l e r p r o d u c t i o n , s e r v i c e and s u p p l y companies, many o f which were conf r o n t e d w i t h t h e f a t e o f h a v i n g t o go o u t o f business due t o b a n k r u p t c y sooner o r l a t e r (SCOTTISH PETROLEUM ANNUAL 1987) o r b e i n g taken o v e r by o r merging w i t h l a r g e r c o r p o r a t i o n s , i n c l u d i n g a l s o c o n s i d e r a b l e dismemberment o f independents (BEUDELL 1986 c, 1986 d; MEISTER & CADY 1986). The s u i t e of consequences o f t h e economical c r i s i s a r e l o s s o f j o b s , l y i n g i d l e o f c a p i t a l assets, o c c u r rence o f b a n k r u p t c i e s and absence o f i n c e n t i v e i n t h e f u t u r e (SCOTTISH PETROLEUM ANNUAL 1987). S e r v i c e companies c u t s t a f f by 30 % on average t o a d j u s t t o t h e c o n s i d e r a b l y d i m i n i s h e d demand o f w e l l o p e r a t i o n s and t r e a t m e n t s (ERDOL UND KOHLE - ERDGAS - PETROCHEMIE 1988). F o r l a r g e r p r o d u c t i o n companies, t h e c r i s i s was a m a t t e r o f s t r e a m l i n i n g by swing and roundabouts i n c l u d i n g d i v e r s i f i c a t i o n o f cash f l o w o u t s i d e t h e N o r t h Sea r e g i o n and even i n t o o t h e r i n t e g r a t e d a c t i v i t i e s such as downstream f a c i l i t i e s ( w i t h p a r t i a l l y a d d i t i o n a l b e n e f i t coming f r o m s c a l i n g down c a p a c i t i e s i n m a n u f a c t u r i n g p l a n t s , d i s p o s i n g o f excess p l a n t s and equipment, and w r i t i n g down i n v e n t o r i e s t o e s t i m a t e d n e t r e a l i z a b l e value; MILLER & NEWLIN 1987), b u t f o r s m a l l e r independents w i t h o u t downstream p o s s i b i l i t i e s , a t once a b t . 50 % o r even t w o - t h i r d s o f t h e income was wiped o u t (GRIFFITHS 1986). Aspects o f v e r t i c a l i n t e g r a t i o n o f up- and downstream branches as w e l l as c o r p o r a t e t a k e o v e r f o r i n v e s t m e n t s e c u r i t y and r e s e r v e access a r e o u t l i n e d as f o l l o w s .
2.2.1.5.2.4.1. Vertical integration
of
UP-
and downstream branches
V e r t i c a l i n t e g r a t i o n o f up- and downstream branches o f i n t e r n a t i o n a l compan i e s was t h e o n l y chance t o s t a y s t r o n g d u r i n g t h e peak o f t h e c r i s i s (CROUSE 1 9 8 7 ) . L a r g e r companies a l s o had t o r a t i o n a l i z e and r e o r g a n i z e , p a r t i a l l y by means o f merging w i t h o t h e r s , w i t h r e p e a t e d m a j o r and m i n o r c o r p o r a t e t a k e o v e r s and f u s i o n s h i g h l i g h t i n g t h e r e s t r u c t u r i n g o f t h e hydrocarbon i n d u s t r y (MEISTER & CADY 1986, MILLER & NEWLIN 1987) f o r l o n g - t e r m s u r v i v a l (HASSELL 1986). Compan i e s w i t h b i g d e b t l o a d s have h a r d l y any p o s s i b i l i t y t o f e n d o f f t a k e o v e r b i d s (OIL GAS JOURNAL 1987 4 ) . Company r e s t r u c t u r i n g i s a l s o g u i d e d by t h i n k i n g o f f i n a n c i a l r a t h e r t h a n v e r t i c a l i n t e g r a t i o n , w i t h j o i n t v e n t u r e s and a c q u i s i t i o n s t a k i n g p l a c e across i n d u s t r i a l and g e o g r a p h i c a l boundaries. P r o f i t cent e r s may develop h o r i z o n t a l l y r a t h e r than v e r t i c a l l y . Aspects o f company a c q u i s i t i o n and merging a r e a l s o d i s c u s s e d by SIMOES ( 1 9 8 5 ) .
2.2.1.5.2.4.2.Corporate takeover f o r
investment security and reserve access
The r e a s o n i n g b e h i n d takeover a c t i o n s i s t h a t a t low o i l p r i c e s , b i g product i o n companies p r e f e r t o o b t a i n r e s e r v e s by b u y i n g t h e i r c o m p e t i t o r s r a t h e r t h a n by i n v e s t i n g h e a v i l y i n e x p l o r a t i o n e f f o r t s (OIL DAILY 1988 c ) . Aspects o f company merging and t a k e o v e r a r e a l s o mentioned by SCHUBERT ( 1 9 8 7 ) . I n some o u t -
140 s t a n d i n g examples, even mega-merging was done, w i t h t h e managements o f t h e p r e v i o u s y i n d e p e n d e n t companies h a v i n g c o m p l e t e l y changed t h e way t o l o o k a t one a n o t h e r f r o m c o m p e t i t o r s and a r c h r i v a l s t o p a r t n e r s (MILLER & NEWLIN 1987, SCHUBERT 1987) w h i c h a r e good example cases o f how t o s u r v i v e and p r o s p e r in a d e p r e s s e d m a r k e t . I n 1987, companies c o n t i n u e d t o c u t c o s t s and i n c r e a s e e f f i c i e n c y b y f o c u s s i n g on c o r e b u s i n e s s e s and s e l l i n g o r w r i t i n g down m a r g i n a l b r a n c h e s o r a s s e t s t h a t do no l o n g e r meet t h e r e q u i r e m e n t o f more s t r i n g e n t r a t e o f r e t u r n (BECK & S M I T H 1 9 8 7 ) . L e v e r a g e d b u y o u t s i n t h e p e t r o l e u m i n d u s t r y a r e a l s o i l l u s t r a t e d b y PAULUS (1988). Various aspects o f t h e r e s t r u c t u r i n g i n t h e o i l patch are o u t l i n e d by MODESITT ( 1 9 8 6 ) and EDGAR ( 1 9 8 7 ) , and p o i n t s o f f i n a n c i n g m e r g e r s and a c q u i s i t i o n s a r e commented b y PETRI€ ( 1 9 8 3 ) and SOBOTKA ( 1 9 8 6 ) . BENTLEY ( 1 9 8 8 ) e v a l u a t e s company s t r a t e g i e s i n a c h a n g i n g m a r k e t , and ANDERSON ( 1 9 8 8 ) r e v i e w s t h e r e o r i e n t a t i o n o f t h e o i l and gas i n d u s t r y . Examples o f s i g n i f i c a n t c o r p o r a t e d e a l s i n c l u d i n g t a k e o v e r s and a c q u i s i t i o n s a r e a l s o r e p o r t e d b y CAPEL ( 1 9 8 8 ) , w i t h i n t h e N o r t h Sea i n t e r c o m p a n y d e a l i n g g o i n g t o i n c r e a s e and l e a d i n g t o t h e end r e s u l t o f d e c r e a s i n g number o f s m a l l e r companies.
2.2.1.5.2.5.
Cash management
I n d e p e n d e n t p r o d u c e r s a r e u n d e r i n c r e a s i n g p r e s s u r e t o seek o u t e v e r y a v a i l a b l e d o l l a r o f i n t e r n a l l y generated cashflow from otherwise i d l e assets o r prem a t u r e f i n a n c i a l o b l i g a t i o n s (CHAN 1985 a, 1985 b ) . Many p e t r o l e u m companies c a n b e t t e r manage t h e i r e x i s t i n g o p e r a t i o n s t o i m p r o v e t h e i r p e r f o r m a n c e s t h r o u g h c a s h management, u p d a t e d c o l l e c t i o n p o l i c i e s , c a s h p r o c e s s i n g and d i s bursements, short-term investments, and more a c c u r a t e p l a n n i n g . N e t income i s n o t a l w a y s i n d i c a t i v e o f an i n d e p e n d e n t ' s v i a b i l i t y because a c t u a l c a s h f l o w may b e a r no r e l a t i o n t o e i t h e r n e t income o r t h e t r a d i t i o n a l d e f i n i t i o n o f c a s h f l o w . O p e r a t i o n a l c a s h f l o w a d j u s t s t h e t r a d i t i o n a l c a s h f l o w f o r changes in w o r k i n g c a p i t a l a c c o u n t s such as i n v e n t o r y , a c c o u n t s r e c e i v a b l e and p a y a b l e , and o t h e r s i n o r d e r t o d e r i v e n e t c a s h income, and n o n - o p e r a t i o n a l c a s h f l o w b r e a k s a p a r t t h e e x t e r n a l s o u r c e s and uses o f f u n d s . The m o s t f u n d a m e n t a l c o n c e p t i n v o l v e d in t h e p e t r o l e u m i n d u s t r y t o d a y i s obt a i n i n g s u f f i c i e n t c a s h t o s a t i s f y d e b t o b l i g a t i o n s , d r i l l i n g commitments, and n e c e s s a r y c a p i t a l e x p e n d i t u r e s , and s t i l l t o be in a p o s i t i o n t o p a r t i c i p a t e i n sound e c o n o m i c a l a c q u i s i t i o n s and d e v e l o p m e n t p r o j e c t s when o p p o r t u n i t i e s a r i s e . An i n c r e a s i n g l y i m p o r t a n t a s p e c t t o c o m p e t i n g in t h e e c o n o m i c a l e n v i r o n ment s u r r o u n d i n g t o d a y ' s p e t r o l e u m i n d u s t r y i s t h e q u a l i t y o f c a s h management t h a t an i n d e p e n d e n t p r o d u c e r u s e s . E f f e c t i v e c a s h management systems p r o v i d e e f f i c i e n t methods t o g a t h e r and d i s b u r s e cash, f o r e c a s t w o r k i n g c a p i t a l needs o r cash surpluses, i m p l e m e n t s h o r t - t e r m i n v e s t m e n t and b o r r o i q i n g p o l i c i e s , e s t a b l i s h r e p o r t i n g and c o n t r o l l i n g p r o c e d u r e s , and d i r e c t b a n k i n g r e l a t i o n s h i p s .
2.2.1.5.2.6.
Survival of independents
D u r i n g t h e p r o l o n g e d slump o f t h e o i l and g a s i n d u s t r y f o l l o w i n g t h e 1986 o i l p r i c e c r a s h , t h e w o r s t damage h a s been s u f f e r e d b y s e r v i c e companies and i n dependent o p e r a t o r s (MOORE 1988 a ) . W h i l e i n 1982 i n t h e USA a b t . 12,700 o p e r a t o r s d r i l l e d a t l e a s t one w e l l on an o p e r a t o r - o f - r e c o r d b a s i s , t h i s number d r o p ped i n 1987 t o o n l y a b t . 5,700 o p e r a t o r s , w i t h v i r t u a l l y a l m o s t a l l o f t h e 7,000 l o s t o p e r a t o r s h a v i n g been i n d e p e n d e n t s . Those i n d e p e n d e n t s s t i l l e x i s t i n g are t r y i n g t o survive, w i t h outside f i n a n c i a l c a p i t a l being s t i l l the l i f e l i n e o f t h e independent, b u t m o s t o f i t has been u s e d up and c o m p e t i t i o n f o r what remains i s f i e r c e . Therefore independents a r e l o o k i n g f o r l e s s exposure o f t h e i r c a p i t a l and f o r q u i c k e r p a y o u t s , w i t h many o f them r e t u r n i n g i n t o e x i s t i n g f i e l d s and t r y i n g t o b o o s t p r o d u c t i o n b y r e e n t e r i n g o l d w e l l s . T h i s i s a m a j o r r e a s o n why i n t h e USA i n 1987 r e c o m p l e t i o n s f o r t h e f i r s t t i m e s i n c e t h e 1980's outnumbered new e x p l o i t a t i o n i n s t a l l a t i o n s . I n d e p e n d e n t s w h i c h have s u r -
141 v i v e d t h e 1986 o i l p r i c e c r a s h f o l l o w s t r a t e g i e s c a l l i n g f o r r e d u c t i o n i n d e b t and o p e r a t i n g c o s t s , s a l e o f m a r g i n a l leases, and c a u t i o u s approach t o c a p i t a l spending (WEST 1987). MANCHESTER (1983) o u t l i n e s t h e a p p l i c a t i o n o f p r o j e c t f i nancing p r i n c i p l e s t o independents, and WYMAN (1978) g i v e s an overview o f t h e r o l e o f s m a l l independent producers i n t h e r a p i d l y changing o i l and gas i n d u s try.
I n 1987, t h e s l i g h t r e c o v e r y o f o i l p r i c e s enabled t h e independent o p e r a t o r s i n t h e USA t o r e t u r n t o p r o f i t a b i l i t y a f t e r c o n s i d e r a b l e l o s s e s i n 1986 (WILLIAMS 1988 b ) . The absence o f h e f t y writedowns o f o i l and gas p r o p e r t i e s which axed e a r n i n g s i n 1986 h e l p e d t o renew p r o f i t a b i l i t y f o r t h e independents i n 1987. Only a c o u p l e o f them s t i l l s u f f e r e d f r o m l o s s e s compared w i t h more than h a l f o f them i n 1986, a l t h o u g h company performance a l s o i n 1987 and 1988 f l u c t u a t e s w i t h o i l and gas p r i c e v a r i a t i o n s . Large and small independents a l i k e r e t u r n e d t o p r o f i t a b i l i t y i n 1987 a f t e r t a k i n g charges a g a i n s t e a r n i n g s i n 1986 t o w r i t e down o i l and gas book v a l u e s . The independents a l s o f o l l o w e d t h e l e a d o f t h e m a j o r s i n sending more upstream i n v e s t m e n t d o l l a r s overseas as o i l p r i ces c r a t e r e d and s l i g h t l y l e s s as o i l p r i c e s rebounded, w i t h t h e i r e a r n i n g s and operational r e s u l t s tending t o r e f l e c t the trend.
2.2.1.5.2.7. Responsibi 1 i ty transfer by contracting and takeover B i g companies w i t h s u f f i c i e n t p o s s i b i l i t i e s o f d i v e r s i f i c a t i o n a r e p a r t i a l l y i n a p o s i t i o n t o p i c k up a s s e t s f o r s a l e w o r l d - w i d e i n c l u d i n g p r o d u c t i o n as w e l l as acreage f o r f u t u r e e x p l o r a t i o n . The low o i l p r i c e s a r e a l s o d i s s i p a t i n g t h e c u r r e n t m a r k e t premiums o f weaker companies t h u s s e t t i n g t h e stage f o r f u r t h e r r a t i o n a l i z a t i o n o f t h e independent s e c t o r (BATT 1983, RAMSAY 1983, ELTING 1987), because s o f a r companies t h a t have n o t been expected t o s u r v i v e sooner o r l a t e r have had premiums i n t h e i r share p r i c e s due t o e x p e c t a t i o n s o f i n v e s t o r s t h a t t h e y w i l l be t h e s u b j e c t o f a t a k e o v e r b i d (ALGAR 1987 b ) . Cost sav i n g s and s t r e a m l i n i n g campaigns have a l s o c o n s i d e r a b l e impact on p r o j e c t management o r c o n t r a c t i n g o f s m a l l - t o medium-sized companies which show i n c r e a s i n g i n t e r e s t i n t h e scope o f t r a n s f e r r i n g r e s p o n s i b i l i t i e s and r i s k s f o r t h e p r o v i s i o n and o p e r a t i o n o f p r o d u c t i o n f a c i l i t i e s t o t h e l a r g e i n t e r n a t i o n a l s e r v i c e companies w i t h f i r s t c l a s s c r e d i t r a t i n g s and s t r o n g b a l a n c e s h e e t s . CHAN (1985) emphasizes t h e n a t u r e o f o i l - f i e l d c a s h f l o w as t h e l i f e b l o o d o f t h e independents f o r t h e i r s u r v i v a l , and THOMPSON (1987) i l l u s t r a t e s t h e impact o f t h e new t a x law on i n t e r n a l cash f l o w g e n e r a t i o n .
2.2.1.5.2.8. Executive skill requirement change When o i l p r i c e s boomed i n t h e l a t e 1970's and e a r l y 1980's, almost e v e r y bus i n e s s d e c i s i o n i n energy i n d u s t r y and i n v e s t m e n t b a n k i n g community ( r e g a r d l e s s o f b e i n g p o s i t i v e o r n e g a t i v e and good o r bad, r e s p e c t i v e l y ) t u r n e d o u t t o be a f i n a n c i a l success s i m p l y because t h e o i l p r i c e c o n t i n u e d t o r i s e (MOORE 1988 a ) . S u r v i v a l a f t e r t h e 1986 o i l p r i c e crash, however, r e q u i r e s much more c a r e f u l a t t e n t i o n on much l e s s c a p i t a l , and more i m p o r t a n t l y , i t i s necessary t o ant i c i p a t e f u t u r e changes and t o r e a c t and adapt t o them q u i c k l y and e f f i c i e n t l y . T h e r e f o r e t h e c o n t i n u i n g o i l p r i c e v o l a t i l i t y has a l s o s e r i o u s impact on r e s t r u c t u r i n g o f t h e company managements, w i t h t o p l e v e l r e o r g a n i z a t i o n b e i n g char a c t e r i z e d by a change f r o m emphasis on t e c h n i c a l s k i l l t o f i n a n c i a l e x p e r i e n c e and economical u n d e r s t a n d i n g (OIL GAS JOURNAL 1987 9 ) . W h i l e i n t h e e a r l y 1980's d u r i n g t h e d r i l l i n g and c o m p l e t i o n boom a t t i m e s o f h i g h o i l p r i c e and s t r o n g US $ m a i n l y e n g i n e e r s and g e o l o g i s t s r a n t h e businesses and main d e c i s i o n p o t e n t i a l o f t h e c o r p o r a t i o n e x e c u t i v e s was on t e c h n i c a l aspects o f hydrocarbon r e s e r v e s and ways t o u n l o c k them under a more o r l e s s p r e d i c t a b l e p r i c e regime, t h e 1986 o i l p r i c e c r a s h has c o m p l e t e l y r e v e r s e d t h i s c o n s t e l l a t i o n . I n a c o m p e t i t i v e environment o f low o i l p r i c e and weak US $, f i n a n c i a l e x p e r t s and economical s p e c i a l i s t s a r e now c o n s t i t u t i n g t h e boards o f many c o r p o r a t i o n s , be-
142 cause managing c a s h f l o w and e c o n o m i c a l r i s k i s now t h e c h i e f t a s k o f p e t r o l e u m company e x e c u t i v e s i n a m i l i e u o f h i g h l y u n p r e d i c t a b l e p r i c e e v o l u t i o n s .
2 . 2 . 1 . 5 . 3 . Business deterioration for service companies F o r s e r v i c e and s u p p l y companies, l e s s b u s i n e s s a v a i l a b l e means i n c r e a s i n g competition, t h o u g h c u t - t h r o a t c o m p e t i t o r s c a n n o t s u r v i v e f o r l o n g , and f r o m t h e o p e r a t o r ' s p o i n t o f view; t h e c h e a p e s t t e n d e r i s n o t n e c e s s a r i l y t h e b e s t s o l u t i o n i n t h e l o n g r u n , because e x p e r t i s e i s p a r t o f t h e s e r v i c e o f f e r e d and s c r e w i n g - u p o f j o b s i s much more c o s t l y t h a n a p p r o v i n g a t o n c e a more e x p e n s i v e and more sound p r o p o s a l (GRIFFITHS 1 9 8 6 ) . Some a s p e c t s o f s t i m u l a t i o n b o a t emp l o y m e n t , w e l l s e r v i c e e x p e n d i t u r e s and t o t a l number o f t r e a t m e n t s , and s e r v i c e c o s t p e r w e l l and s h a r e o f r e c o m p l e t i o n s a r e d i s c u s s e d as f o l l o w s .
2 . 2 . 1 . 5 . 3 . 1 . Stimulation boat employment C o m p e t i t i o n was a c c e n t u a t e d a t l e a s t d u r i n g t h e peak of t h e c r i s i s i n 1986 and e a r l y 1987 b y t h e c o e x i s t e n c e o f t h r e e s e r v i c e v e s s e l s of a l m o s t e q u a l p e r formance c a p a b i l i t y ( c f . s e c t i o n 3 . 8 . 1 . 2 . ) w h i c h had t o s h a r e a s h r i n k i n g o r s t a g n a n t N o r t h Sea s t i m u l a t i o n m a r k e t (FOX 1985, OILMAN 1985 b ) , w i t h a l l o f t h e t h r e e h i g h - l e v e l t e c h n o l o g y t r e a t m e n t b o a t s h a v i n g been p u r p o s e - b u i l t v e s s e l s r a t h e r t h a n r e p r e s e n t i n g c o n v e r t e d and a d a p t e d s u p p l y s h i p s (COOPER & MARSHALL 1984, OFFSHORE ENGINEER 1984; c f . s e c t i o n 3 . 7 . 1 . 1 . ) . The a d v a n t a g e s o f b o a t o p e r a t i o n s o v e r j o b s f r o m s k i d s mounted on t h e p l a t f o r m a r e r e d u c e d r i g up/down t i m e , f e w e r e q u i p m e n t l i f t s and moves, o n s i t e b l e n d i n g , and t h e p o s s i b i l i t y o f pumping m u l t i p l e j o b s f r o m one s h i p l o a d of f l u i d s and p r o p p a n t s (BRANNON, NETTERS & GRIMMER 1 9 8 7 ) . Only t h e i n t r o d u c t i o n o f s t i m u l a t i o n v e s s e l s w i t h l a r g e p r o p p a n t and f l u i d s t o r a g e c a p a c i t y has a l l o w e d t o p e r f o r m MHF t r e a t m e n t s on N o r t h Sea r e s e r v o i r s w h i c h have been f o r m e r l y i m p o s s i b l e due t o l i m i t e d space a v a i l a b l e on t h e p l a t f o r m s . On t h e o t h e r hand, d i s a d v a n t a g e s o f v e s s e l t r e a t m e n t s v s . s k i d j o b s a r e p a r t i a l l y t i m e - and c o s t - e x p e n s i v e t r a v e l t o / f r o m l o c a t i o n , and bad w e a t h e r c a n cause an o p e r a t i o n t o be a b o r t e d o r d e l a y e d . A s p e c t s o f N o r t h Sea s t i m u l a t i o n l o g i s t i c s and r e q u i r e m e n t s a r e o u t l i n e d b y MACADAM ( 1 9 8 4 ) . The s o l u t i o n w h i c h emerged i n t h e N o r t h Sea i n 1986/1987 f r o m t h e t r e a t m e n t f l e e t o v e r c a p a c i t y d i lemma as a consequence o f underdemand, a g g r e s s i v e t o e x a g g e r a t e d c o m p e t i t i o n , e s t a b l i s h m e n t o r r e p u t a t i o n and e x p e r i e n c e , and v a r i o u s r e l a t i o n s h i p s was a t l e a s t t e m p o r a r i l y t h a t due t o p r o p o s a l s u b m i s s i o n and a p p r o v a l u n d e r t h e inf l u e n c e o f s e r v i c e companies and o p e r a t o r s , one b o a t c a r r i e d o u t t h e b u l k o f t h e p r o p p a n t f r a c t u r i n g work, t h e second v e s s e l t o o k c a r e o f t h e m a i n amount o f t h e a c i d f r a c t u r i n g j o b s , and t h e t h i r d b o a t s p e n t much o f t h e t i m e unemployed i n t h e h a r b o u r . R e s u l t i n g f r o m t h e d e c r e a s i n g amounts o f o r d e r s and t h e c o n c l u s i o n o f b u s i n e s s a t c o n s i d e r a b l y l o w e r r a t e s due t o s t r o n g c o m p e t i t i o n w h i c h was sometimes e v e n n e a r t h e b r e a k - e v e n p o i n t , a l s o s e r v i c e companies had t o adj u s t t o t h e new s i t u a t i o n b y p e r f o r m a n c e o f a d a p t i v e s h r i n k a g e w h i c h was i n e v i t a b l y l i n k e d w i t h o f f i c e r e o r g a n i z a t i o n and d i s m i s s a l o f numerous p e o p l e ( t h e same a l s o a p p l i e s f o r d r i l l i n g c o n t r a c t o r s ) . S e r v i c e company s t r a t e g i e s f o r s u r v i v a l a r e a l s o e v a l u a t e d b y CLARK ( 1 9 8 6 ) .
2 . 2 . 1 . 5 . 3 . 2 . Well service expenditures and total number o f jobs The amount o f b u s i n e s s d e c l i n e f o r t h e s e r v i c e companies due t o t h e 1986 o i l p r i c e c r a s h i s h i g h l i g h t e d f o r t h e USA m a r k e t b y MOORE ( 1 9 8 8 b ) . W e l l s e r v i c e e x p e n d i t u r e s b y USA o p e r a t o r s d r o p p e d f r o m t h e 1 9 8 1 h i g h o f more t h a n 4 B i l l . US $ v i a a b t . 3 . 5 B i l l . US $ i n 1984 and a b t . 2 . 5 B i l l . US $ i n 1985 t o a b t . 1 . 5 B i l l . US $ i n 1986 w h i c h r e p r e s e n t s o n l y one t h i r d o f t h e l e v e l i n t h e peak y e a r of 1 9 8 1 o r a 65 % v a l u e d r o p . P i c k i n g u p o f b u s i n e s s i n l a t e 1986 b r o u g h t t h e w e l l s e r v i c e e x p e n d i t u r e l e v e l i n 1987 back t o 2 . 3 B i l l . US $. I n t e r m s o f
143 executed s e r v i c e jobs, t h e d e c l i n e was f r o m 565,000 j o b s i n 1981 t o 480,000 oper a t i o n s i n 1985 and 390,000 t r e a t m e n t s i n 1986, and t h e improvement o f t h e economical f e a s i b i l i t y scene i n 1987 l i f t e d t h e number a g a i n t o 415,000 j o b s . The r e l a t i o n s h i p s between t o t a l number o f o p e r a t i o n s and c u m u l a t i v e e x p e n d i t u r e s emphasizes t h a t a t low o i l p r i c e s , f i r s t of a l l c o n s i d e r a b l y cheaper w e l l s e r v i c e j o b s a r e c a r r i e d out, and t h e number o f j o b s decreases o n l y i n t h e second l i n e .
2.2.1.5.3.3. Service cost per well and share o f recompletions The t r e n d as d i s c u s s e d above i s f u r t h e r i l l u s t r a t e d by t h e e v o l u t i o n o f w e l l s e r v i c e c o s t on a p e r - w e l l b a s i s f r o m a b t . 5,100 US $ i n 1981 v i a a b t . 3,900 US $ i n 1983, a b t . 3,200 US $ i n 1984, a b t . 2,750 US $ i n 1985 and a b t . 1,800 US $ i n 1986 t o a b t . 2,350 US $ i n 1987 and a b t . 2,500 US $ i n 1988 (MOORE 1986, 1987 b ) . A t t h e b o t t o m o f t h e l i n e , 1986 was t h e w o r s t y e a r f o r t h e s e r v i c e i n d u s t r y s i n c e e x p e n d i t u r e s peaked i n 1981, w i t h a c t i v i t y h a v i n g dropped by more than 50 % a f t e r t h e o i l p r i c e c r a s h . A f t e r a c o n s i s t e n t c l i m b o f a b t . 1 - 4 X a n n u a l l y i n t h e t o t a l USA w e l l p o p u l a t i o n o v e r t h e l a s t decade, t o t a l borehole number decreased f o r t h e f i r s t t i m e again i n 1986, because w e l l abandonments outpaced new c o m p l e t i o n s . The d i s t u r b e d m a r k e t i s h i g h l i g h t e d by t h e f a c t t h a t o p e r a t o r s spent n e a r l y as much money f o r w e l l s e r v i c i n g i n Texas a l o n e d u r i n g 1981 as t h e y d i d i n t h e whole USA i n 1986. Completions outpaced r e c o m p l e t i o n s i n 1986 by a r a t i o o f o n l y 7 t o 5 compar e d w i t h r a t i o s o f 2 t o 1 i n 1985 and 3 t o 1 between 1982 and 1984. A h i g h e r percentage o f r e c o m p l e t i o n s i s an obvious i n d i c a t i o n t h a t o p e r a t o r s a r e more w i l l i n g t o recomplete o l d e r w e l l s w i t h proven r e s e r v e s and p r o d u c t i o n c a p a b i l i t i e s than r i s k money d r i l l i n g f o r unproven o i l and gas r e s e r v e s ( t h e same g u i d e l i n e a p p l i e s f o r company a c q u i s i t i o n s and takeovers, because i t i s s a f e r t o buy known f i e l d s w i t h proven r e s e r v e s than t o i n v e s t money f o r e x p l o r a t i o n and app r a i s a l d r i l l i n g w h i c h c o u l d p a r t i a l l y r e s u l t i n f a i l u r e s and a l m o s t complete l o s s o f c a p i t a l e x p e n d i t u r e ) . S t i m u l a t i o n j o b s i n t h e USA d e c l i n e d by 25 % f r o m a b t . 28,800 i n 1985 t o a b t . 21,500 i n 1986.
2.2.1.5.4. Banking relationships and investment strategies F o l l o w i n g i l l u s t r a t i o n o f t h e s i t u a t i o n o f e x p l o r a t i o n and p r o d u c t i o n as w e l l as s e r v i c e companies, t h e e v a l u a t i o n o f p r e s e n t consequences o f t h e 1986 o i l p r i c e d r o p i s concluded by i l l u m i n a t i o n o f banking r e l a t i o n s h i p s , i n v e s t ment s t r a t e g i e s and o t h e r aspects.
2.2.1.5.4.1. Banking relationships C o n s i d e r a b l e s t r e s s was a l s o p u t on t h e banking s e c t o r by t h e 1986 o i l p r i c e c o l l a p s e as a consequence o f cash-poor s m a l l e r p r o d u c t i o n and s e r v i c e companies seeking postponement o f t h e i r p r i n c i p a l and i n t e r e s t payments on l o a n s and c r e d i t f a c i l i t i e s f o r e i t h e r p r o j e c t s undertaken o r a c q u i s i t i o n s made e a r l i e r (BEUDELL 1986 c ) . Companies r e a c h i n g b a n k r u p t c y s t a t u s throws t h e problem on t h e banks whether t o f o r e c l o s e o r n o t i n view o f t h e u n d e r l y i n g a s s e t s h a v i n g been h a l v e d when p u t up f o r s a l e , w i t h p a r t i a l l y even t h e s i t u a t i o n o c c u r r i n g t h a t t h e stockmarket quote i s more i m p o r t a n t t o e i t h e r bank o r company purchaser than t h e u n d e r l y i n g a s s e t s (ALGAR 1987 b ) . O t h e r c a s u a l t i e s w i l l be among those companies owned by conglomerates, banks and f i n a n c i a l i n s t i t u t i o n s which w i l l seek t o w i t h d r a w f r o m N o r t h Sea e x p l o r a t i o n and p r o d u c t i o n . STEWART (1986) comments on secured o i l and gas l e n d i n g i n b a n k r u p t c i e s , and M I N T Z (1981) e v a l u a t e s s p e c i f i c r e q u i r e m e n t s and a l t e r n a t i v e s o f reserve-based f i n a n c i n g . PELLEY & CONSTABLE (1982) i n v e s t i g a t e r e s e r v o i r r i s k s and economical assumptions w i t h r e s p e c t t o bank l e n d i n g . R e s u l t i n g f r o m t h e severe d e v a l u a t i o n o f o i l , banks a r e e x p e c t e d n o t t o
lend
144 readily
anymore
f o r new o i l - f i e l d d e v e l o p m e n t s and a l s o p l a c e s t r i c t c o v e n a n t s whereas on t h e o t h e r hand gas p r o j e c t s a r e s t r o n g l y s u p p o r t e d ( c f . s e c t i o n 2 . 2 . 1 . 6 . 2 . ) . T h i s c o u l d f o r c e m a j o r o p e r a t o r s t o d e f e r o i l p r o j e c t s , because t h e i r f i n a n c i a l l y weaker p a r t i c i p a n t s may have no c a s h o r i n s u f f i c i e n t c o l l a t e r a l t o p a y t h e i r s h a r e o f new o r i n c r e m e n t a l d e v e l o p m e n t c o s t s . Thus a t t h e b o t t o m o f t h e l i n e , t h e o i l p r i c e crash i n i t i a t e d a s u i t e o f domino-type e f f e c t s i n t h e economical scene o f t h e h y d r o c a r b o n i n d u s t r y . Perhaps t h e g r e a t e s t damage was done t o f u t u r e h y d r o c a r b o n p r o d u c t i o n r a t e s and t o c o n f i d e n c e , because h a v i n g l e a r n e d t h e p a i n f u l l e s s o n o f t h e sudden and u n e x p e c t e d l y s e r i o u s 1986 o i l p r i c e c r a s h , company managements w i l l p r o b a b l y n e v e r a g a i n a u t o m a t i c a l l y assume s t e a d y o r i n c r e a s i n g o i l p r i c e s and t h e r e f o r e y e a r s may p a s s b e f o r e i n v e s t m e n t r e g a i n s momentum (ALGAR 1987 b ) . W h i l e r e t a r d i n g e x p l o r a t i o n and new d e v e l o p m e n t s w h i c h a r e no l o n g e r e c o n o m i c a l l y f e a s i b l e , however, t h e t u m b l i n g o i l p r i c e s h a v e g i ven enormous i n c e n t i v e t o i n n o v a t i v e t e c h n o l o g y w h i c h c a n f u r t h e r h e l p i n o v e r a l l c o s t c u t t i n g . The n e c e s s i t y o f e f f e c t i v e e v a l u a t i o n o f l o a n s t o r e d u c e r i s k i s emphasized b y McPHAIL ( 1 9 8 7 ) , and ROSBACD ( 1 9 8 3 ) comments o n i n v e s t m e n t e v a l u a t i o n s i n i n f l a t i o n a r y and u n s t a b l e s c e n a r i o s . PELLEY ( 1 9 8 2 ) d i s c u s s e s r e l a t i o n s h i p s between b a n k i n g and e n e r g y .
on any c o r p o r a t e b o r r o w i n g s e c u r e d on l o w - p r i c e d o i l (BEUDELL 1986 c ) ,
2.2.1.5.4.2. Investment strategies D u r i n g t h e l a s t decade, t h e p e t r o l e u m i n d u s t r y i n t h e f r e e w o r l d s p e n t ann u a l l y more t h a n 100 B i l l . US $ f o r o i l and gas e x p l o r a t i o n and p r o d u c t i o n (OEHM E 1 9 8 6 ) . A c c o r d i n g t o such h i g h expenses, i t i s r e a s o n a b l e t o p o s t u l a t e t h a t t h e s h a r e o f s t i m u l a t i o n s h o u l d be i n c r e a s e d as much as p o s s i b l e i n o r d e r t o a c h i e v e maximum p r o f i t f r o m t h e i n v e s t m e n t s . I n t h e B r i t i s h s e c t o r o f t h e N o r t h Sea, o v e r 50 B i l l . t were i n v e s t e d i n t o h y d r o c a r b o n e x p l o r a t i o n and p r o d u c t i o n s i n c e t h e d i s c o v e r y o f t h e f i r s t c o m m e r c i a l gas f i e l d i n 1965 (OFFSHORE ENGINEER 1986 d; c f . s e c t i o n s 2.2.1.6.2. and 3 . 3 . ) . As a consequence o f t h e h i g h s i g n i f i c a n c e o f t h e R o t l i e g e n d gas i n t h e S o u t h e r n N o r t h Sea, t h e h i t h e r t o e x penses i n c l u d e d a l r e a d y c o n s i d e r a b l e s h a r e s f o r s t i m u l a t i o n , and t h e amount of investments f o r h y d r a u l i c proppant f r a c t u r i n g i s even g o i n g t o i n c r e a s e d u r i n g t h e m a j o r R o t l i e g e n d gas d e v e l o p m e n t campaigns i n 1987 - 1990 and beyond ( c f . sections 2.4.5.1. and 3 . 1 . 2 . ) . E s t i m a t i o n c a n be made t h a t t h e a n n u a l s t i m u l a t i o n m a r k e t p o t e n t i a l i n t h e N o r t h Sea i s a b t . 20 M i o . L (OILMAN 1985 b; c f . section 3.8.1.2.).
I n 1987, t h e p r o m i s i n g o u t l o o k f o r h i g h e r a v e r a g e o i l p r i c e s as compared w i t h t h o s e o f 1986 r e s u l t e d i n l e s s s e v e r e w o r l d - w i d e c a p i t a l c u t t i n g c o m p r i s i n g o n l y a b t . 10 - 20 % i n c o n t r a s t t o 25 - 40 % i n 1986 w i t h r e s p e c t t o t h e o r i g i n a l b u d g e t i n g made i n l a t e 1985, t h e r e b y s l o w i n g down t h e s l i d e i n h y d r o c a r b o n e x p l o r a t i o n and d e v e l o p m e n t (BEUDELL 1987 a; some a s p e c t s o f p i c k i n g - u p o f b u s i n e s s i n 1987 a r e a l s o o u t l i n e d b y QUINLAN 1 9 8 8 ) . I m p o r t a n t i n f l u e n c e s a r e t h e b e n e f i t o f t h e o i l and gas i n d u s t r y f r o m t h e s h a r p r e d u c t i o n i n c o s t o f d r i l l i n g , w o r k o v e r , c o m p l e t i o n and s t i m u l a t i o n s e r v i c e s , w i t h l o w e r r a t e s b e i n g a good r e a s o n f o r b u d g e t d e t e r m i n a t i o n i n t h e l i g h t o f s t a b i l i z a t i o n o f t h e cap i t a l expenditure level.
2.2.1.5.4.3.
Other aspects
A l t h o u g h t h i s s i t u a t i o n was a g a i n s l i g h t l y u n d e r m i n e d b y a n o t h e r m i n o r dec l i n e o f t h e r e a l o i l p r i c e i n e a r l y 1988 as a consequence o f t h e p o l i t i c a l unc e r t a i n t i e s i n t h e M i d d l e E a s t a r e a ( w h i c h was a c c e p t e d t o be so s e r i o u s t h a t t h e OPEC a l a r m e d a l s o n o n - g r o u p c o u n t r i e s t o p a r t i c i p a t e i n a c r i t i c a l s e s s i o n l o o k i n g f o r j o i n t e f f o r t s t o s t o p t h e downwards o r i e n t a t i o n o f t h e o i l p r i c e ; O I L DAILY 1988 a; c f . s e c t i o n 2 . 2 . 1 . 4 . 3 . ) , t h e g e n e r a l r i s i n g t r e n d o f t h e hyd r o c a r b o n e x p l o r a t i o n and p r o d u c t i o n i n d u s t r y r e c o v e r i n g f r o m t h e 1986 c r i s i s i s g o i n g on, as a l s o r e f l e c t e d b y d o u b l i n g o f t h e number o f r i g s b e i n g a c t i v e i n t h e N o r t h Sea w i t h r e s p e c t t o 1987 ( O I L DAILY 1988 b ) f o l l o w i n g t h e i r h a l -
145 v i n g f r o m e a r l y t o l a t e 1986 when numerous unemployed r i g s were s t a c k e d a l o n g t h e c o a s t l i n e (SCOTTISH PETROLEUM ANNUAL 1987; c f . s e c t i o n 2 . 4 . 2 . ) . The most s i g n i f i c a n t i n f l u e n c e on r e t a r d e d p i c k - u p o f a c t i v i t y d e s p i t e more o r l e s s s t a b i l i z i n g o i l p r i c e l e v e l c o n s i d e r a b l y h i g h e r t h a t a t t h e peak o f t h e c r i s i s i s l a c k i n g c o n f i d e n c e i n f u t u r e i n v e s t m e n t o f money d e s p i t e s u f f i c i e n t hope o r even optimism (KINNEY 1987). Managements s i m p l y do n o t r e l y on t h e s i t u a t i o n , and h e s i t a t i o n w i t h postponement i f any doubt can be found i s c h a r a c t e r i s t i c f o r t h e scenery. Challenges t o i n d u s t r y v i a b i l i t y f r o m t h e v i e w p o i n t o f t h e i n vestment community a r e analyzed by MAXWELL (1986). The t r a d i t i o n a l areas o f h y d r a u l i c proppant f r a c t u r i n g i n t h e w o r l d a r e p r e d o m i n a n t l y t h e USA and p a r t s o f Western Europe ( i n c l u d i n g m a i n l y t h e B r i t i s h and Dutch Southern N o r t h Sea as w e l l as N e t h e r l a n d s and Germany FRG onshore). E a s t e r n Europe and N o r t h e r n A f r i c a a r e d e v e l o p i n g areas where so f a r i n s u f f i c i e n t m a r k e t i n g a c t i v i t y was made by p r o p p a n t and p a r t i a l l y a l s o s e r v i c e compan i e s , w i t h m a j o r r e g i o n s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n E a s t e r n Europe bei n g Yugoslavia, Hungary and Turkey. I n view o f p r o p p a n t m a r k e t i n g and s a l e s , t h e 1986 o i l p r i c e d e c l i n e had d i f f e r e n t consequences i n USA and Europe ( c f . c h a p t e r 2 . 3 . ) , and an i m p o r t a n t a d d i t i o n a l f e a t u r e e x c l u s i v e l y a f f e c t i n g t h e European market was t h e US $ exchange r a t e drop ( c f . s e c t i o n 2.2.1.1.3.).
2.2.1.6. Oil price evolution and gas market O i l and gas a r e d i f f e r e n t hydrocarbon phases which o f t e n o c c u r t o g e t h e r ( a t l e a s t more o r l e s s gas i s d i s s o l v e d w i t h i n t h e o i l , o r i n some cases, t h e o i l r e s e r v o i r has a gas cap when f r e e gas has e x s o l v e d f r o m t h e o i l ) , b u t a l s o a r e frequently d i s t r i b u t e d i n d i f f e r e n t r e s e r v o i r s comprising separate storeys o f t h e g e o l o g i c a l column. I n a d d i t i o n t o t h e s p l i t n a t u r a l occurrences, t h e i n d u s t r i a l a p p l i c a t i o n and economical s i g n i f i c a n c e o f o i l and gas a r e i n many r e s p e c t s d i f f e r e n t and t h u s t h e market s t r u c t u r e and p o t e n t i a l o f b o t h h y d r o c a r bon phases i s n o t s i m p l y comparable. These r e l a t i o n s h i p s have an i m p o r t a n t i m p a c t on t h e r e a c t i o n and p o s i t i o n o f t h e gas market a b s o l u t e l y as w e l l as r e l a t i v e l y t o t h a t o f t h e o i l market i n view o f t h e changes o f t h e o i l p r i c e l e v e l i n e i t h e r d i r e c t i o n i n t h e p a s t . Aspects o f c h a r a c t e r i s t i c f e a t u r e s o f t h e gas market and i t s independence f r o m t h e o i l market, i n f l u e n c e o f t h e 1986 o i l p r i ce drop on gas development d r i l l i n g i n t h e N o r t h Sea, s i g n i f i c a n c e o f t h e e a s t west energy t r a d e i n t h e European gas market, and e v o l u t i o n a r y t r e n d s o f t h e w o r l d gas market a r e e v a l u a t e d as f o l l o w s .
The o u t l i n e o f v a r i o u s economical and t e c h n i c a l f a c t o r s o f t h e gas m a r k e t i n c l u d i n g f u t u r e t r e n d s i s p r e s e n t e d i n some more d e t a i l because o f t h e s i g n i f i c a n t r o l e which s t i m u l a t i o n p l a y s i n enhancement o f gas r e s e r v e s and t h e i r p r o d u c i b i l i t y . H y d r a u l i c p r o p p a n t f r a c t u r i n g i s a l r e a d y now much more abundant i n gas r e s e r v o i r s t h a n i n o i l pay zones b o t h i n Europe and i n N o r t h America, and t h i s r a t i o i s expected t o s h i f t even more towards gas i n t h e coming decades, i n c o n t r a s t t o t h e p a s t when emphasis has been almost everywhere i n t h e w o r l d on t h e o i l s e c t o r b e f o r e t h e i n c r e a s i n g r e c o g n i t i o n o f t h e economical v a l u e o f gas s t a r t e d some f i f t e e n y e a r s ago ( a l m o s t c o i n c i d e n t w i t h t h e f i r s t p o l i t i c a l a v a i l a b i l i t y t h r e a t and o i l p r i c e jump) and a c c e l e r a t e d a t t h e t i m e when s y n t h e t i c h i g h - and i n t e r m e d i a t e - s t r e n g t h proppants f o r deep w e l l s t i m u l a t i o n were i n t r o duced t o t h e hydrocarbon i n d u s t r y .
2.2.1.6.1. Characteristic features o f the gas market and its independence from the oil market As a consequence o f i t s e n v i r o n m e n t a l l y l e s s hazardous n a t u r e , gas which went a t t h e b o t t o m o f t h e l i n e l a t e r on stream f o r e x p l o i t a t i o n and l a r g e - s c a l e i n d u s t r i a l and urban a p p l i c a t i o n t h a n o i l , has a much b e t t e r f u t u r e i n comparison t o o i l a t l e a s t i n Europe due t o i n c r e a s i n g replacement o f o i l by gas f o r a1 1 t y p e s o f h e a t i n g and combustion systems. Hydrocarbon e x p l o r a t i o n and deve-
146 lopment d r i l l i n g i n a n t l y f o c u s on y e a r 2000 o r e v e n 1987 a, BAND 1987,
n t h e n e x t y e a r s p a r t i c u l a r l y i n t h e N o r t h Sea w i l l p r e d o m i s e v e r a l r i c h gas p r o v i n c e s w h i c h w i l l s e c u r e s u p p l y u n t i l t h e beyond (FISHMAN 1986; GREGORY 1986, 1987; THOMAS 1986, ALGAR POTTER 1987; c f . s e c t i o n 2 . 4 . 5 . and 3 . 1 . ) .
A l t h o u g h o i l and g a s p r i c e s a r e l i n k e d i n some r e s p e c t (LINDEN 1 9 8 5 ) , t h e gas m a r k e t h a s a c e r t a i n i n d e p e n d e n c e and c o n t i n u e s t o e v o l v e r a p i d l y , whereas t h e o i l m a r k e t i s c h a r a c t e r i z e d b y a more o r l e s s s t a g n a n t n a t u r e due t o s t i l l p e r s i s t i n g aims on i n c r e a s i n g r e p l a c e m e n t o f o i l by gas and o t h e r a l t e r n a t i v e s i n o r d e r t o g e t r i d o f t h e p o l i t i c a l dependence f r o m t h e OPEC g r o u p and t o r e a c h t h e p o i n t where demand can be s a t i s f i e d i n l a r g e amounts f r o m t h e N o r t h Sea and o t h e r European o i l p r o v i n c e s . N a t u r a l gas has i n t h e p a s t and f u t u r e y e a r s a l s o an i m p o r t a n t r o l e i n t h e e n e r g y t r a d e between E a s t e r n and W e s t e r n Eur o p e (O'ANDREA 1987) and t h u s c o n t r i b u t e s t o t h e p r o g r e s s i v e a p p r o a c h o f t h e two p o l i t i c a l complexes w i t h d i f f e r e n t e c o n o m i c a l and s o c i a l s t r u c t u r e . A s p e c t s o f gas demand and p r i c i n g a r e o u t l i n e d b y FRANSSEN ( 1 9 8 2 ) and J I M I S O N ( 1 9 8 6 ) . HOLTBERG, WOODS & ASHBY ( 1 9 8 7 ) c o n c l u d e t h a t n a t u r a l gas w i l l p l a y a m a j o r r o l e i n a h i g h l y c o m p e t i t i v e e n e r g y m i x w e l l i n t o t h e n e x t c e n t u r y . DAHLE ( 1 9 8 8 ) ill u s t r a t e s s i m i l a r i t i e s and d i f f e r e n c e s o f gas and o i l , and GADONNIEUX (1988) o u t l i n e s gas c h a l l e n g e s i n a c o m p e t i t i v e m a r k e t . BRIERLEY ( 1 9 8 8 ) summarizes t h e s i g n i f i c a n c e o f gas as a g l o b a l e n e r g y s o u r c e . TECLAW ( 1 9 8 8 ) c h a r a c t e r i z e s n a t u r a l gas as a s t r a t e g i c f u e l f o r t h e 1 9 9 0 ' s . W o r l d t o t a l a n n u a l gas p r o d u c t i o n i s i n t h e r a n g e o f 1,500 - 2,000 B i l l . m3 s i n c e s e v e r a l y e a r s (ERDOEL-ERDGAS AKTUELL 1 9 8 1 b ) . Because gas i s l i k e l y t o r e m a i n a premium f u e l i n t h e f o r e s e e a b l e f u t u r e , demand s h o u l d c o n t i n u e t o i n c r e a s e o v e r t h e n e x t decade o r more d e s p i t e a s h o r t t e r m e r o s i o n o f c o m p e t i t i v e n e s s f o l l o w i n g t h e 1986 o i l p r i c e c o l l a p s e (FISHMAN 1986).
2.2.1.6.2. I n f l u e n c e o f the 1986 o i l p r i c e drop on gas development d r i l l i n g i n the North Sea I n t e r m s o f t h e s c h e d u l e d m a j o r d e v e l o p m e n t campaigns o f R o t l i e g e n d gas f i e l d s i n t h e B r i t i s h S o u t h e r n N o r t h Sea ( c f . s e c t i o n s 2 . 4 . 3 . and 3 . 1 . ) , t h e 1986 o i l p r i c e d r o p had o n l y m i n o r i m p e t u s on t h e p r o g r e s s o f p r e p a r a t i o n and a c q u i s i t i o n as a consequence o f t h e r e l a t i v e independence o f t h e g a s m a r k e t f r o m t h e o i l m a r k e t due t o l o n g - t e r m p l a n n i n g and a r r a n g e m e n t s , w i t h m o s t o f t h e gas h a v i n g a l r e a d y been s o l d p r i o r t o p r o d u c t i o n d r i l l i n g and s t i m u l a t i o n . The i n f l u e n c e s o f t h e 1986 o i l p r i c e c o l l a p s e o n d e v e l o p m e n t d r i l l i n g p a r t i c u l a r l y i n t h e B r i t i s h N o r t h Sea a r e i l l u s t r a t e d a l o n g t h e l i n e s o f gas s u p p l y c o n t r a c t s and e c o n o m i c a l s a f e t y , c o n t i n u i t y o f p l a n n e d and a p p r o v e d g a s - f i e l d d e v e l o p m e n t , government back-up and s u p p l y s e c u r i t y , g a s - f i e l d d e v e l o p m e n t v s . gas p r i c e and demand e v o l u t i o n , d i s t u r b a n c e o f o i l - f i e l d development, and gas i m p o r t and c o n d e n s a t e f i e l d d e v e l o p m e n t .
2.2.1.6.2.1. Gas supply contracts and economical s a f e t y Gas s u p p l y c o n t r a c t s a r e l o n g - t e r m r a t h e r t h a n s p o t - m a r k e t - r e l a t e d l i k e o i l demand and s u p p l y , and i n c l u d e s p e c i f i e d p r i c e s , minimum o f f t a k e q u a n t i t i e s , take-or-pay c l a u s e s (YURKEWICZ 1 9 8 6 ) , and s t r i c t f o r m u l a s f o r p r i c e a d j u s t ments, w i t h a l l t h e s e measures b e i n g n e c e s s a r y t o s a f e g u a r d t h e l a r g e i n v e s t ment i n f u l l y d e d i c a t e d f i e l d f a c i l i t i e s and i n f r a s t r u c t u r e o f e x p o r t p i p e l i n e and i m p o r t t e r m i n a l needed t o g e t t h e gas t o t h e p a r t i c u l a r m a r k e t t o w h i c h i t has been c o n t r a c t e d (THOMAS 1 9 8 6 ) . Thus lo^ o i l p r i c e s have l i t t l e d i r e c t i m p a c t o n B r i t i s h S o u t h e r n N o r t h Sea g a s - f i e l d development, i n d e p e n d e n t f r o m t h e d u r a t i o n o f t h e p e r i o d o f cheap o i l and w i t h o u t any c o n n e c t i o n t o t h e a d a p t a t i o n o f o i l - f i e l d d e v e l o p m e n t . W h i l e m o s t o f t h e c o n t r a c t s r e m a i n unchanged, o t h e r n e g o t i a t i o n s w i l l s u f f e r f r o m a d e c l i n e i n p r i c e s u p t o a b t . 30 % as a consequence o f some agreement l i n k a g e t o t h e now l o w e r g a s / o i l p r i c e r a t i o (BEU-
147 DELL 1986 b ) , w i t h t h e o v e r a l l e f f e c t , however, b e i n g n o t h i n g l i k e as severe as t h a t e x p e r i e n c e d by o i l p r o d u c t i o n companies who had t o contend w i t h a p r i c e slump o f some 60 % . The o n l y c r o s s l i n k i n g element i s t h e f a c t t h a t most o f t h e companies w i t h drawing gas i n t h e B r i t i s h Southern N o r t h Sea a r e a l s o m a j o r o i l producers and some o f t h e economy measures b e i n g t a k e n i n t h e wake o f low o i l p r i c e s a r e a f f e c t i n g budgets n o t j u s t f o r t h e o i l areas, b u t t o some e x t e n t a l s o f o r gas exp l o r a t i o n and development (BEUDELL 1986 c, FISHMAN 1986). I n many areas, howe v e r , t h e r e i s a c l e a r s h i f t o f a c t i v i t y w i t h p r o m o t i o n o f gas p r o j e c t s a t t h e expense o f o i l p l a n s which a r e c u t back o r suspended (BEUOELL 1986 b ) . On t h e o t h e r hand and i n c o n t r a s t t o Europe, t h e gas market i n t h e USA a l s o s u f f e r e d c o n s i d e r a b l y f r o m t h e o i l p r i c e c r a s h . Gas s u p p l y and m a r k e t i n g t r e n d s a r e a l s o o u t l i n e d by BERGMANN (1982).
2.2.1.6.2.2. Continuity o f planned and approved gas-f ield development Gas development d r i l l i n g i n t h e B r i t i s h Southern N o r t h Sea ( c h i e f l y i n t h e t i g h t R o t l i e g e n d gas b e l t ) , b u t t o m i n o r amounts a l s o i n N e t h e r l a n d s and Germany FRG i n R o t l i e g e n d and C a r b o n i f e r o u s , i s t h e r e f o r e more o r l e s s independent f r o m t h e o i l p r i c e s c e n a r i o , because s u p p l y has t o be p r o v i d e d i n o r d e r t o f u l f i l l t h e n e g o t i a t e d and concluded l o n g - t e r m d e l i v e r y c o n t r a c t s ( c f . s e c t i o n s 2.4.5. and 3 . 1 . ) . As t h e gas market i s s t i l l booming due t o c o n t i n u o u s l y i n c r e a s i n g replacement o f o i l by gas p a r t i c u l a r l y i n urban h e a t i n g , development o f t h e t i g h t R o t l i e g e n d gas b e l t has a good f u t u r e a t l e a s t f o r t h e n e x t f i v e y e a r s when t h e m a j o r d r i l l i n g and f r a c t u r i n g campaigns a r e g o i n g t o be c a r r i e d o u t ( c f . s e c t i o n s 2.4.1.2. and 2.4.5.1.). I n a d d i t i o n , development o f t h e p l a n ned f i e l d s f o r w h i c h t h e gas had been a l r e a d y s o l d by c o n t r a c t has been approved by t h e government and i s backed-up a g a i n s t t h e danger o f gas i m p o r t s f r o m abroad. Proppant demand i s a l s o more o r l e s s f o r e s e e a b l e , because e x p l o r a t i o n and a p p r a i s a l d r i l l i n g has a l r e a d y shown t h a t i n many f i e l d s almost a l l t h e development w e l l s w i l l have t o be f r a c t u r e d (OILMAN 1987 a ) , and t h e c l o s u r e s t r e s s i n t h e r e s e r v o i r depth as w e l l as t h e l o n g - t e r m f i e l d p r o d u c t i o n p l a n n i n g o f up t o 20 - 30 y e a r s l e a v e s no o t h e r p o s s i b i l i t y f o r g o i n g on t h e s a f e s i d e than t o p r o p t h e whole f r a c t u r e w i t h i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s and n o t t o use n a t u r a l sand a t a l l . T h e r e f o r e t h e huge d r i l l i n g and f r a c t u r i n g campaigns i n t h e Southern N o r t h Sea R o t l i e g e n d gas b e l t f r o m 1987 onwards u n t i l t h e e a r l y 1990's (FISHMAN 1986; GREGORY 1986, 1987; OFFSHORE ENGINEER 1986 a, OILMAN 1987 a) comprise t h e b u l k o f t h e European business f o r t h e proppant companies i n t h e n e x t y e a r s and c o u l d even r e p r e s e n t one o f t h e m a j o r h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n a c t i o n s which o i l and gas i n d u s t r y has e x p e r i e n c e d so f a r . The s i g n i f i c a n c e o f n a t u r a l gas i n t h e N o r t h Sea i s u n d e r l i n e d by a s t r o n g i n c r e a s e i n p r o d u c t i o n i n 1987 f o l l o w i n g t h e g e n e r a l p i c k i n g - u p o f a c t i v i t y a f t e r t h e 1986 o i l p r i c e c r a s h (QUINLAN 1986). The development campaigns i n 1987 - 1990 and beyond a r e p r o b a b l y t h e g r e a t e s t a c t i v i t y s i n c e t h e i n i t i a l boom t h a t f o l l o w e d gas d i s c o v e r y i n t h e B r i t i s h Southern N o r t h Sea i n t h e e a r l y 1960's (VIELVOYE 1988 a) and a r e c e r t a i n l y t h e l a r g e s t Western European gas f i e l d development p r o j e c t i n t h e l a s t t e n y e a r s (McNALLY 1987).
2.2.1.6.2.3.
Government back-up and supply security
T h i s upsurge o f gas development has been t r i g g e r e d by t h e need o f B r i t i s h Gas t o r e p l a c e s u p p l i e s f r o m t h e F r i g g f i e l d (MURE 1987) which s t a r t e d t o p r o duce i n 1977 (ERDOEL-ERDGAS AKTUELL 1976 b) and w i l l be d e p l e t e d by 1992/1993 ( i n i t i a l r e c o v e r a b l e r e s e r v e s o f F r i g g have been 250 B i l l . m3 o r 10 T c f , o f ERDOELw h i c h a b t . h a l f each i s on B r i t i s h and Norwegian N o r t h Sea t e r r i t o r y ; EROGAS AKTUELL 1976 b ) . B r i t i s h Gas has o r i g i n a l l y planned t o r e p l a c e t h e F r i g g
148 s u p p l i e s w i t h gas f r o m t h e S l e i p n e r f i e l d i n t h e Norwegian N o r t h Sea, b u t t h e B r i t i s h government vetoed t h e i m p o r t c o n t r a c t i n 1985 ( a b t . 20 % o f t h e t o t a l U n i t e d Kingdom gas s u p p l y i s coming f r o m t h e F r i g g f i e l d ; PETROLEUM R E V I E W 1988 d ) . I n a d d i t i o n t o replacement, gas demand has c o n s i d e r a b l y i n c r e a s e d d u r i n g t h e l a t e 1970's and e a r l y 1980's. T h e r e f o r e s t i l l b e f o r e t h e 1986 o i l p r i c e c r a s h huge development p l a n s f o r R o t l i e g e n d gas f i e l d s have been made and s t a r t e d t o c o n f i r m by a p p r a i s a l d r i l l i n g which i s t h e base f o r c o n c l u s i o n o f gas del i v e r y c o n t r a c t s t h a t have been s i g n e d w i t h B r i t i s h Gas, w i t h t h e f i r s t f i e l d s s t a r t i n g s u p p l y t o B r i t i s h Gas i n autumn 1988 (VIELVOYE 1987 c, 1987 d) and t h e o t h e r s s u c c e s s i v e l y f o l l o w i n g t h r o u g h 1990 and beyond (CAPEL 1988, OILMAN 1987 a). S t r o n g governmental s u p p o r t o f gas development i n t h e B r i t i s h Southern N o r t h Sea g i v e s t h e o p e r a t i n g companies s u f f i c i e n t s a f e t y a l s o a f t e r t h e 1986 o i l p r i c e c o l l a p s e t h a t a l l t h e p r o j e c t s can be f i n i s h e d e c o n o m i c a l l y f e a s i b l y . There i s o n l y l i t t l e concern t h a t B r i t i s h Gas c o u l d s t o p t h e gas development boom by s t a r t i n g t o c o n t r a c t gas f r o m t h e Norwegian S l e i p n e r and T r o l l (GRAY 1987) f i e l d s f o r i m p o r t f r o m t h e m i d 1990's (Norway has o p e n l y s t a t e d t h a t i t needs t o g e t i n t o t h e U n i t e d Kingdom gas market; OILMAN 1987 a ) , because a l though h a v i n g c o n v e r t e d r e c e n t l y t o a p r i v a t e company, B r i t i s h Gas s t i l l r e mains t h e m o n o p o l i s t i c gas buyer i n Great B r i t a i n and needs p e r m i s s i o n t o i m p o r t n a t u r a l gas f r o m t h e government which p r e f e r s t o back-up domestic compan i e s and t o keep o u t i m p o r t s , w i t h n a t i o n a l hydrocarbon o p e r a t o r s a l s o t h e i r s e l ves m a i n t a i n i n g p r e s s u r e on t h e government t o s t i c k t o t h i s a t t i t u d e (VIELVOYE 1988 a ) . The T r o l l f i e l d has a b t . 45 T c f (1,125 B i l l . m3) r e c o v e r a b l e gas r e s e r ves which make i t t h e t e n t h l a r g e s t gas f i e l d i n t h e w o r l d (McNALLY 1 9 8 7 ) . The planned p u t t i n g on stream o f t h e gas f i e l d complex Vulcan, V a l i a n t , Vanguard, Ravenspurn South ( c f . f i g . 6 ) and C l e e t o n as t h e f i r s t group o f newly developed Rotliegend f i e l d s ( w i t h the exception o f Cleeton having a l l required consider a b l e amounts o f h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n ) s t i l l i n f a l l 1988 has been s u c c e s s f u l l y achieved (PETROLEUM R E V I E W 1988 d, VIELVOYE 1988 c ) .
2.2.1.6.2.4. Gas-f ield development vs. gas price and demand evolution The development h i s t o r y o f t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas f i e l d b e l t i s an e x c e l l e n t example how gas p r i c e c o n t r o l s a c q u i s i t i o n a c t i v i t y (PETROLEUM R E V I E W 1988 d ) . A f t e r t h e i n i t i a l phase o f i n t e n s e e x p l o r a t i o n and development i n t h e l a t e 1960's and e a r l y 1970's, a downturn o c c u r r e d u n t i l t h e b e g i n n i n g o f t h e 1980's which was t r i g g e r e d by market f o r c e s . By t h a t time, B r i t i s h Gas had secured s u f f i c i e n t s u p p l i e s f r o m t h e o r i g i n a l l a r g e f i e l d s t o meet t h e i r r e q u i r e m e n t s and t h e p r i c e t h e y were o f f e r i n g f o r a d d i t i o n a l r e s e r v e s d i d n o t warrant f u r t h e r exploration. During t h i s period, the operators turned t h e i r a t t e n t i o n t o t h e N o r t h e r n N o r t h Sea and t h e development o f o i l r e s e r v e s . Throughout t h e 1970's, however, demand f o r gas t r e b l e d , and by t h e end o f t h e decade, B r i t i s h Gas had t o l o o k f o r new s u p p l i e s and o f f e r e d p r i c e s t h a t made i t a t t r a c t i v e f o r t h e o p e r a t o r s t o r e t u r n t o t h e Southern N o r t h Sea and t o r e sume e x p l o r a t i o n which i n i t i a t e d t h e gas f i e l d d r i l l i n g , c o m p l e t i o n and s t i m u l a t i o n boom o f t h e 1980's t h a t w i l l e x t e n d a t l e a s t t o t h e e a r l y 1990's.
2.2.1.6.2.5.
Disturbance of oi 1 -field development
I n c o n t r a s t t o t h e gas market s i t u a t i o n which i s c h a r a c t e r i z e d by more o r l e s s s e c u r i t y , t h e 1986 o i l p r i c e c o l l a p s e had t e r r i b l e consequences f o r t h e o i l market, w i t h f i e l d s s t i l l r e m a i n i n g t o be developed h a v i n g become hopeless uneconomical a f t e r t h e 1986 o i l p r i c e drop and i n m i d t o l a t e 1987 i n a s l i g h t l y more s t a b i l i z e d s i t u a t i o n s t i l l r e m a i n i n g m a r g i n a l e x c e p t o f l a r g e r f i e l d s . S m a l l e r s a t e l l i t e s can be made more f e a s i b l e o n l y by widespread use o f subsea c o m p l e t i o n s w i t h p r o d u c t i o n processed a t e x i s t i n g p l a t f o r m s a t t h e b i g s p o t s nearby (THOMAS 1986; aspects of development o f small o i l and gas f i e l d d i s c o v e -
149 r i e s i n t h e N o r t h Sea a r e i l l u s t r a t e d by MORRISON & JOLLIFFEE 1982 and VOSPER & STEVENS 1982; c f . s e c t i o n 2.2.1.5.2.1.). Thus t h e a c t i v i t y a f t e r t h e 1986 o i l p r i c e c r a s h focussed on spur e x p l o i t a t i o n e s p e c i a l l y f o r f i e l d s b e i n g c l o s e t o e x i s t i n g i n f r a s t r u c t u r e s w i t h spare c a p a c i t i e s and cheap t y i n g - i n o f s a t e l l i t e s (ALGAR 1987 b ) . On t h e o t h e r hand, many new o i l f i e l d developments a r e s t i l l e c o n o m i c a l l y a t t r a c t i v e a t an o i l p r i c e o f a b t . US $ 15, a l t h o u g h a h i g h e r o i l p r i c e l e v e l and p a r t i c u l a r l y a b e t t e r back-up by a s t r o n g e r US $ exchange r a t e would c e r t a i n l y g i v e a much b e t t e r i n c e n t i v e .
2.2.1.6.2.6.
Gas import and condensate-f i e l d development
The 1986 o i l p r i c e c o l l a p s e c o u l d indeed have a p o s i t i v e impact on t h e a c t i v i t y i n t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t due t o t h e delayed development o f gas condensate f i e l d s i n t h e N o r t h e r n N o r t h Sea (FISHMAN 1986 a, GREGORY 1987) which s u f f e r f r o m t h e problem o f l a r g e i n c r e m e n t a l c a p i t a l i n v e s t ment b e i n g r e q u i r e d t o c y c l e t h e gas and o f d e f e r r a l o f income f r o m s e l l i n g t h e gas t h r o u g h o u t t h e c y c l i n g p e r i o d (ALGAR 1987 b ) . Gas condensate f i e l d development, i f t a k i n g p l a c e a t a l l , i s t h e r e f o r e r e s t r i c t e d t o those patches w i t h exc e p t i o n a l l y h i g h l i q u i d c o n t e n t , and even those w i l l need some t y p e o f enhanced o i l r e c o v e r y i n v e s t m e n t i n c e n t i v e t o be e c o n o m i c a l l y f e a s i b l e . The e x c l u s i o n o f Norwegian gas so f a r f r o m i m p o r t i n t o t h e U n i t e d Kingdom (KNOTT 1986) a l s o a f t e r t h e p r i v a t i z a t i o n o f B r i t i s h Gas which s t i l l a c t s as m o n o p o l i s t i c gas buyer, i s t r i g g e r i n g c o n s i d e r a b l e f u r t h e r u p g r a d i n g o f t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t (GREGORY 1986), which means a g a i n f u r t h e r promot i o n o f the s i g n i f i c a n c e o f hydraulic proppant f r a c t u r i n g f o r increasing acquis i t i o n o f gas f o r c o v e r i n g r i s i n g domestic demand i n G r e a t B r i t a i n w i t h o u t t h e s u p p o r t o f c o n t r a c t i n g gas abroad, b u t e x c l u s i v e l y f r o m indigeneous sources. As i t i s e v i d e n t t h a t B r i t i s h Gas has t o buy s i g n i f i c a n t new q u a n t i t i e s f o r s e c u r i n g s u p p l y o f t h e f o r e c a s t e d demand u n t i l t h e n e x t c e n t u r y , t h e r e i s a good market f o r t h e gas l i b e r a t e d f r o m R o t l i e g e n d f i e l d s i n t h e B r i t i s h Sout h e r n N o r t h Sea, and t h e economical impetus i s g i v e n p a r t i c u l a r l y by t h e f a c t t h a t i f t h e demand o f B r i t i s h Gas cannot be s a t i s f i e d by s u p p l y from t h e U n i t e d Kingdom N o r t h Sea s e c t o r , B r i t i s h Gas would have t o c o n t r a c t s u b s t a n t i a l v o l u mes f r o m Norway a l r e a d y i n t h e n e x t y e a r s (GREGORY 1987). B r i t i s h Gas i t s e l f , however, has a s t r o n g i n t e r e s t n o t t o d i s c o u r a g e domestic e x p l o r a t i o n and development by b u y i n g abroad e a r l i e r t h a n a b s o l u t e l y necessary, b u t a t t h e b o t t o m o f t h e l i n e has t o meet t h e demand o f i t s customers. T h e r e f o r e t h e c o n s t e l l a t i o n o f t h e B r i t i s h gas market i s c o n s i d e r e d t o be t h e d r i v i n g f o r c e f o r e x p l o r a t i o n and p r o d u c t i o n companies t o develop as many gas f i e l d s as p o s s i b l e t h o r o u g h l y which i n d i s p e n s i b l y i n c o r p o r a t e s l a r g e - s c a l e h y d r a u l i c p r o p p a n t f r a c t u r i n g campaigns f o r l i b e r a t i o n o f a d d i t i o n a l amounts o f gas.
2.2.1.6.3. Significance o f the east-west energy trade i n the European gas market The importance o f gas development d r i l l i n g i n c l u d i n g s t i m u l a t i o n i n t h e N o r t h Sea and o t h e r hydrocarbon p r o v i n c e s i n Europe i n p a s t and f u t u r e has t o be understood w i t h t h e background o f t h e s i g n i f i c a n c e o f t h e e a s t - w e s t energy t r a d e i n t h e European gas market (D'ANDREA 1987). The development o f t h e econom i c a l p a r t n e r s h i p between E a s t e r n and Western Europe was t h e l o g i c a l consequence o f r e s t r u c t u r i n g o f t h e energy consumption p r o f i l e d u r i n g t h e l a s t decades which was t o c o n s i d e r a b l e e x t e n t i n f l u e n c e d by t h e o i l p r i c e e v o l u t i o n . Some comments a r e o f f e r e d as f o l l o w s on e v o l u t i o n o f t h e European gas market s u p p l y and demand s i t u a t i o n , and i n t e r n a t i o n a l c o o p e r a t i o n and c o m p e t i t i v e c h a l l e n g e s , and f u t u r e e v o l u t i o n o f t h e N o r t h Sea gas p r o v i n c e .
150
2 . 2 . 1 . 6 . 3 . 1 . Evolution o f the European gas market During c o u r s e of more than 100 y e a r s of n a t u r a l g a s a c q u i s i t i o n and e x p l o i t a t i o n h i s t o r y , 1987 was t h e y e a r of f i r s t exceeding 100,000 B i l l . m3 ( 4 , 0 0 0 T c f ) world Sas r e s e r v e s ( E R D U L U N D K O H L E - ERDGAS - PETROCHEMIE 1987 d ) . So f a r a b t . 150,000 B i l l . m3 (6,000T c f ) have been d i s c o v e r e d , of which h i t h e r t o a b t . one q u a r t e r has been produced a n d / o r consumed. The b i g g e s t g a s p r o d u c e r s of t h e world a r e i n descending o r d e r USSR, USA, Canada, N e t h e r l a n d s , A l g e r i a , G r e a t B r i t a i n , Romania. I n d o n e s i a , Mexico and Norway, which t o g e t h e r have more than 80 % of the t o t a l world g a s p r o d u c t i o n . S i n c e 1950, world gas p r o d u c t i o n has i n c r e a s e d more than t e n t i m e s . Germany FRG i s t h e second l a r g e s t n a t u r a l gas i m p o r t e r of t h e world ( a f t e r J a p a n ) w i t h a t o t a l import q u a n t i t y of 40 B i l l . m3 i n 1985 which d e r i v e d by 33 % from N e t h e r l a n d s , 24 % from USSR and 1 3 % from Norway and Denmark, w i t h t h e remaining 30 % coming from domestic p r o d u c t i o n ( E R D U L U N D KOHLE - ERDGAS PETROCHEMIE 1986) of R o t l i e g e n d , B u n t s a n d s t e i n , Z e c h s t e i n and C a r b o n i f e r o u s f i e l d s i n the Northwest German g a s - f i e l d b e l t . The d i v e r s i f i c a t i o n of c o n t r a c t e d g a s among v a r i o u s s o u r c e s of d e l i v e r y s e c u r e s s u p p l y , d i m i n i s h e s o r even e l i m i n a t e s p o l i t i c a l dependency, and i n c r e a s e s f l e x i b i l i t y a n d c o m p e t i t i o n ( E R D OEL-ERDGAS 1 9 7 8 ) . The f o l l o w i n g o u t l i n e f o c u s s e s on h i s t o r i c a l changes of energy consumption spectrum a s well a s p o l i t i c a l and economical i n f l u e n c e s on m u l t i l a t e r a l energy t r a d e .
2 . 2 . 1 . 6 . 3 . 1 . 1 . Historical changes o f energy consumption spectrum I n t h e 1 9 5 0 ' s , c o a l and t o a l e s s e r amount o i l and w a t e r power c o n t r i b u t e d o v e r 95 % of primary e n e r g y consumption, w i t h t h e s h a r e of n a t u r a l g a s being a l most n o n - e x i s t e n t ( O ' A N D R E A 1 9 8 7 ) . S i g n i f i c a n t changes in e n e r g y consumption p a t t e r n s t a r t e d t o develop i n the 1960's and 1970'5, and the p r o c e s s of r e o r g a n i z a t i o n has s t a b i l i z e d i n t h e l a s t y e a r s , a s a number of c o u n t r i e s had a c h i e ved a r e a s o n a b l e b a l a n c e i n energy supply d i v e r s i f i c a t i o n . I m p o r t a n t g a s d i s c o v e r i e s i n Western Europe and USSR i n t h e l a t e 1950's and e a r l y 1960's provided v a r i o u s c o u n t r i e s the o p p o r t u n i t y and d e s i r a b i l i t y of e s t a b l i s h i n g the n e c e s s a r y i n f r a s t r u c t u r e f o r t h e use of n a t u r a l g a s . F a c t o r s i n f l u e n c i n g development and growth of g a s markets i n Europe and world a r e s e c u r i t y of e n e r g y s u p p l i e s and d i v e r s i f i c a t i o n , h i g h e r e f f i c i e n c y of n a t u r a l gas compared t o o t h e r f u e l s , a v a i l a b i l i t y of n a t u r a l gas a t r e l a t i v e l y low c o s t s t o consumers, long-term cont r a c t u a l r e l a t i o n s h i p i n s a l e and purchase of g a s s u p p l i e s , c a p a b i l i t y f o r t r a n s p o r t of n a t u r a l gas o v e r long d i s t a n c e s by l a r g e - d i a m e t e r p i p e l i n e s and sea-going v e s s e l s , development of technology i n e x p l o r a t i o n and p r o d u c t i o n of o f f s h o r e r e s o u r c e s , promotion of i n t e r - and i n t r a r e g i o n a l g a s t r a d e , and i n c r e a sed p u b l i c awareness of t h e r o l e of n a t u r a l g a s f o r an improved environment.
2 . 2 . 1 . 6 . 3 . 1 . 2 . Pol it ical and economical
influences on multilateral energy trade
S i g n i f i c a n c e and i n f l u e n c e of t h e above mentioned f a c t o r s have v a r i e d i n the d i f f e r e n t European gas markets due t o e x i s t e n c e of d i f f e r e n t economical s y s tems; p r e v a i l i n g p o l i t i c a l , economical and f i s c a l r e l a t i o n s h i p s ; and e x p o s u r e of g a s markets t o competing s o u r c e s of energy ( D ' A N D R E A 1 9 8 7 ) . I n Western E u r o p e , s e c u r i t y of e n e r g y supply and d i v e r s i f i c a t i o n achieved a high d e g r e e o f importance f o l l o w i n g t h e f i r s t major i n c i s i o n i n t o o i l p r i c e h i s t o r y by the 1973/74 o i l p r i c e e x p l o s i o n ( c f . s e c t i o n 2 . 2 . 1 . 1 . 1 . ) when governments implement e d v a r i o u s measures t o reduce t h e i r r e s p e c t i v e dependence on o i l which a s a t o p p r i o r i t y item i n c l u d e d an i n c r e a s e d usage of n a t u r a l g a s . I n l i g h t of being no l o n g e r c o n s t r a i n e d by p h y s i c a l l i m i t a t i o n s f o r moving gas s u p p l i e s t o mark e t s , n a t u r a l g a s q u i c k l y r e c e i v e d renewed r e c o g n i t i o n a s a v i a b l e e n e r g y a l t e r n a t i v e t o economies which had p l a c e d primary r e l i a n c e on coal and n u c l e a r power t o a l l e v i a t e t h e i r energy problems.
151 T h i s s i t u a t i o n a l s o caused renewed i n t e r e s t and growth i n t h e o v e r a l l p e r s p e c t i v e o f energy t r a d e w i t h i n Europe as w e l l as i n t e r - and i n t r a r e g i o n a l dependency on gas t r a d e . I n c r e a s e d interdependence on gas t r a d e f r o m sources w i t h i n and o u t s i d e Europe caused governments, o f t e n t o t h e c o n s t e r n a t i o n o f p r i v a t e commercial e n t e r p r i s e s , t o be d i r e c t l y i n v o l v e d i n n e g o t i a t i n g gas t r a d e agreements, as p o l i t i c a l and economical c o n s i d e r a t i o n s achieved a new l e v e l o f s t r a t e g i c a l importance i n b i l a t e r a l and m u l t i l a t e r a l energy t r a d e r e l a t i o n s . Some aspects o f t h e energy p o l i c y i n t h e European community a r e summarized by CLEUT I N X (1986), and c h a r a c t e r i z a t i o n s o f t h e European gas market a r e performed by BERGMANN (1988) and N O R V I K ( 1 9 8 8 ) . D'ANDREA (1979) comments on t h e p r o s p e c t s o f n a t u r a l gas s u p p l i e s and markets i n Europe and t h e i r i n f l u e n c e on t h e USA. L I E SEN (1977) analyzes p r e s e n t and f u t u r e s i g n i f i c a n c e o f n a t u r a l gas f o r Western Europe.
2.2.1.5.3.2. Supply/demand situation The gas r e s e r v e s o f Western Europe a r e much g r e a t e r t h a n i t s o i l r e s e r v e s w i t h r e g a r d t o t h e i r energy c o n t e n t , and i t s gas p r o d u c t i o n covers n e a r l y t h r e e q u a r t e r s o f i t s own needs as opposed t o o n l y one t h i r d f o r crude o i l (VALENTIN, VALAIS & CHAMPLON 1988). Western European gas r e s e r v e s a r e t h u s an i m p o r t a n t a s s e t i n t h e s e c u r i t y o f i t s energy s u p p l i e s . The most s t r i k i n g aspect o f European gas r e s o u r c e s i s t h e o f f s h o r e c o n c e n t r a t i o n which i s more pronounced than t h a t o f any o t h e r c o n t i n e n t , w i t h more than 64 % o f t h e proven r e s e r v e s and p r o b a b l y more t h a n 75 % o f t h e u l t i m a t e r e s o u r c e s b e i n g s i t u a t e d o f f s h o r e , and w i t h t h e N o r t h Sea h a v i n g t h e g r e a t e s t share due t o Norway, N e t h e r l a n d s and U n i t e d Kingdom t h e r e h a v i n g more t h a n 85 % o f t h e o v e r a l l r e s e r v e s . A f t e r d i s c u s s i o n o f some g e n e r a l aspects, c o u n t r y - s p e c i f i c comments on s u p p l y and demand a r e o f f e r e d f o r N e t h e r l a n d s , G r e a t B r i t a i n , Norway and USSR. Some p o i n t s o f s u p p l y s e c u r i t y and mutual b e n e f i t a r e a l s o touched.
2.2.1.6.3.2.1. General aspects T o t a l n a t u r a l gas consumption i n Europe i n 1985 amounted t o 925 B i l l . m3 (37 T c f ) o f which 27 % and 73 % were t a k e n i n Western and E a s t e r n Europe, r e s p e c t i v e l y , w i t h t h e USSR h a v i n g an 85 % share o f t h e E a s t e r n European q u a n t i t y (D'ANDREA 1987). Source o f gas s u p p l i e s f o r t h i s consumption l e v e l o r i g i n a t e d essent i a l l y f r o m w i t h i n Europe i t s e l f , w i t h o n l y 2.5 % h a v i n g been i m p o r t e d f r o m A l g e r i a and L i b y a . I n Western Europe, gas consumption was c o n s i d e r a b l y i n c r e a s i n g f r o m 1970 t o 1979, t h e n d e c l i n i n g i n t h e y e a r s 1980 - 1982 r e f l e c t i n g a g e n e r a l decrease i n energy demand, and s i n c e 1983 r i s i n g a g a i n t o reach a l e v e l h i g h e r t h a n e v e r b e f o r e i n t h e l a s t y e a r s (KEG 1986). W h i l e s e v e r a l s i g n i f i c a n t p r o d u c e r s such as Germany FRG, I t a l y , France and Romania c o u l d n o t a f f o r d t o e x p o r t gas due t o domestic energy demand exceeding i n t e r n a l s u p p l y and even r e q u i r i n g c o n s i d e r a b l e i m p o r t , s u f f i c i e n t q u a n t i t i e s i n N e t h e r l a n d s , Norway and USSR enabled t h e l a t t e r c o u n t r i e s t o e x p o r t l a r g e volumes o f gas, w i t h i n a d d i t i o n t o t h a t o n l y Denmark s u p p l y i n g small amounts t o f o r e i g n c o u n t r i e s . G r e a t B r i t a i n i s more o r l e s s s e l f - s u f f i c i e n t w i t h n e i t h e r s i g n i f i c a n t import nor considerable export i f the q u a n t i t i e s from the F r i g g f i e l d ( w h i c h i s c r o s s e d by t h e boundary between t h e B r i t i s h and Norwegian sect o r s o f t h e N o r t h Sea and t h u s r e p r e s e n t s a s p e c i a l s i t u a t i o n ) a r e n e g l e c t e d .
2.2.1.6.3.2.2. Netherlands The N e t h e r l a n d s have h i t h e r t o been t h e p r i n c i p a l gas e x p o r t i n g c o u n t r y t o Western European markets because o f e a r l y d i s c o v e r y and development o f s i z e a b l e r e s e r v e s i n t h e g i a n t Groningen f i e l d (JENSEN 1988; c f . f i g . 6 and s e c t i o n 3 . 3 . ) and o t h e r s m a l l e r patches p r o d u c i n g f r o m p a r t i a l l y t i g h t R o t l i e g e n d where
152 a l s o some s t i m u l a t i o n by h y d r a u l i c f r a c t u r i n g was necessary t o p u t t h e f a c i l i t i e s on stream, b u t r e c e n t e v a l u a t i o n s o f gas r e s e r v e s and domestic m a r k e t r e quirements i n d i c a t e t h a t e x p o r t s f r o m t h e N e t h e r l a n d s w i l l d e c l i n e i n t h e e a r l y 1990's as p a r t o f t h e l o n g - t e r m s t r a t e g y o f t h e N e t h e r l a n d s t o r e t a i n n a t u r a l gas as t h e p r i m a r y source o f energy f o r domestic s u p p l y r e q u i r e m e n t s . T h i s conc e p t , however, c o u l d a l s o be a s i g n i f i c a n t i n c e n t i v e t o push development o f f u r t h e r R o t l i e g e n d gas f i e l d s i n t h e Dutch N o r t h Sea t o compensate t h e d e c l i n i n g o u t p u t f r o m Groningen and t h e o t h e r e x i s t i n g f i e l d s , which i n c l u d e s a reasonable c o n t r i b u t i o n o f hydraulic proppant f r a c t u r i n g f o r the a c q u i s i t i o n o f l a r g e new r e s e r v e s i n t h e t i g h t f a c i e s b e l t o f t h e R o t l i e g e n d .
2.2.1.6.3.2.3.
Great B r i t a i n
Great B r i t a i n which s o f a r has e x p o r t e d gas o n l y i n v e r y l i m i t e d amounts a l s o needs i n c r e a s i n g amounts o f i t s indigeneous gas p r o d u c t i o n f o r domestic demand, b u t w h i l e i t t e m p o r a r i l y seemed t h a t t h e U n i t e d Kingdom would be f o r c e d t o c o n t r a c t some q u a n t i t i e s t o be i m p o r t e d f r o m Norway, a t l e a s t i n t e r m i t t e n t governmental d e c i s i o n t o renounce f r o m i m p o r t and t o expand t h e indigeneous f a c i l i t i e s has c o n s i d e r a b l y promoted t h e development p o t e n t i a l i n t h e B r i t i s h Sout h e r n N o r t h Sea as w e l l as t h e p r o s p e c t s o f measuring m a r k e t p e n e t r a t i o n o f nat u r a l gas i n t h e U n i t e d Kingdom (KNOTT 1986, OILMAN 1987 a ) . A t t h e moment, 20 - 30 % o f t h e Great B r i t a i n domestic gas s u p p l y i s s t i l l coming f r o m t h e F r i g g f i e l d (DUFONO & LAFFRONT 1978, GREGORY 1987, MURE 1987) which i s c u t by t h e b o r d e r l i n e between U n i t e d Kingdom and Norwegian N o r t h Sea s e c t o r s , b u t i n t h e e a r l y 199O's, d e c l i n i n g d e l i v e r y f r o m F r i g g w i l l be compensated by i n c r e a s i n g o u t p u t f r o m Southern N o r t h Sea t i g h t R o t l i e g e n d f i e l d s f o l l o w i n g s t i m u l a t i o n o f t h e r e s e r v o i r s by h y d r a u l i c p r o p p a n t f r a c t u r i n g and c o m p l e t i o n d u r i n g course o f t h e c u r r e n t and coming m a j o r development campaigns ( c f . s e c t i o n s 2.4.5.1.). T h e r e f o r e t h e U n i t e d Kingdom can be expected t o m a i n t a i n s e l f - s u f f i c i e n c y o f o i l and gas s u p p l y beyond t h e y e a r 2000 (PETROLEUM R E V I E W 1988 b) which i n terms o f gas i s p r i m a r i l y a s c e r t a i n e d by i n c r e a s i n g development o f m a r g i n a l f i e l d s and i n c r e a s i n g r e c o v e r y f r o m e x i s t i n g f i e l d s c h i e f l y t h r o u g h h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n (KNOTT 1985). LEWIS-DAVIES (1988) i n v e s t i g a t e s t h e f u t u r e gas s u p p l y and demand o f t h e U n i t e d Kingdom, and WHITTING (1979) o u t l i n e s t h e B r i t i s h n a t u r a l gas s u p p l y and demand s t r a t e g y . JOHNSON (1978) p r e s e n t s an economical and f i n a n c i a l a n a l y s i s o f t h e B r i t i s h N o r t h Sea s e c t o r r e l a t i n g gross n a t i o n a l p r o d u c t t o i n v e s t m e n t s t r a t e g y , and BAYLEY (1984) e v a l u a t e s market p r o s p e c t s and t h e gas balance o f t h e U n i t e d Kingdom. I n terms o f d i s t r i b u t i o n o f R o t l i e g e n d gas r e s e r v e s , a b t . 20 T c f (500 B i l l . m3) had been c u m u l a t i v e l y produced f r o m t h e Southern N o r t h Sea t h r o u g h a b t . 20 y e a r s s i n c e o f f t a k e s t a r t e d i n 1967 (PETROLEUM R E V I E W 1988 d ) . Average d a i l y o f f s h o r e p r o d u c t i o n i n t h e B r i t i s h s e c t o r i s a b t . t w i c e t h a t o f t h e Dutch sect o r . The v a s t Groningen f i e l d onshore Netherlands, however, on t h e o t h e r hand produces a l o n e almost as much as t h e e n t i r e U n i t e d Kingdom c o n t i n e n t a l s h e l f . According t o t h e l a r g e r e s e r v e s , t h e B r i t i s h Southern N o r t h Sea s e c t o r c o n t a i n s enough gas t o s a t i s f y t h e demand o f B r i t i s h Gas as w e l l as t o l o o k i n t o d i r e c t sales t o i n d u s t r i a l users.
2.2.1.4.3.2.4.
Norway
Norway has no domestic markets f o r gas and t h e r e f o r e i s a i m i n g on p l a y i n g an i n c r e a s i n g r o l e i n s a t i s f y i n g f u t u r e Western European gas r e q u i r e m e n t s e s p e c i a l l y f o l l o w i n g development o f s e v e r a l l a r g e f i e l d s such as S l e i p n e r and T r o l l (GRAY 1987) i n t h e l a s t y e a r s and i n t h e n e a r f u t u r e (KNOTT 1986). Norway has s o f a r d i v i d e d i t s gas e x p o r t s between U n i t e d Kingdom and c o n t i n e n t a l Europe (JENSEN 1988). The development o f t h e g i a n t gas f i e l d s T r o l l and S l e i p n e r i n t h e coming y e a r s f a c e s Norway w i t h t h e f a c t t h a t i t s r e s e r v e a d d i t i o n s i n n a t u -
153 r a l gas p a r t i c u l a r l y i n t h e more remote N o r t h e r n N o r t h Sea a r e i n excess o f any e a r l y market requirements, and t h e r e f o r e Norway has a l r e a d y o p e n l y s t a t e d t h a t i t needs t o g e t i n t o t h e U n i t e d Kingdom gas market w i t h e x p o r t s (OILMAN 1987 a) a f t e r t e r m i n a t i o n o f e x p l o i t a t i o n o f t h e F r i g g f i e l d ( c r o s s i n g t h e boundary b e t ween t h e B r i t i s h and Norwegian s e c t o r s o f t h e N o r t h Sea) i n a s e m i - j o i n t vent u r e . T r o l l i s t h e t e n t h l a r g e s t gas f i e l d o f t h e w o r l d due t o r e c o v e r a b l e r e serves o f 45 T c f (1,125 B i l l . m3), and t h e l a r g e amounts o f gas enable Norway t o become t h e w o r l d ' s t h i r d b i g g e s t e x p o r t e r o f n a t u r a l gas a f t e r .USSR and N e t h e r l a n d s (McNALLY 1987). The f u t u r e o f Norwegian gas i s analyzed by AGER-HANSSEN (1982), JOHNSEN (1982) and JACOBSEN (1988), and t h e g e n e r a l s i t u a t i o n o f Norway b e i n g a c o u n t r y w i t h an energy s u r p l u s i n t h e European energy market i s i l l u s t r a t e d by FROGNES (1981, 1983). Aspects o f Norway's p o s i t i o n i n t h e o i l market and i t s r e l a t i o n s t o t h e OPEC a r e o u t l i n e d by DAHLE (1987), GODEC & ALLARD (1987), NORENG (1987), SOLHEIM (1987) and UDGAARD (1987). A l g e r i a which p i o n e e r e d l i q u i d n a t u r a l gas e x p o r t s t o Europe f r o m i t s g i a n t Hassi R'Mel f i e l d l o s t some m a r k e t p o s i t i o n dur i n g t h e e x t e n s i v e p r i c e debates f o l l o w i n g t h e second o i l p r i c e shock (JENSEN 1988).
2.2.1.5.3.2.5.
USSR
The dominant gas e x p o r t i n g c o u n t r y i n whole Europe i s t h e USSR w h i c h h o l d s c u r r e n t l y a b t . 43 % o f t h e w o r l d ' s proven gas r e s e r v e s and r a n k s f i r s t i n t h e w o r l d gas p r o d u c t i o n (D'ANDREA 1987; ERDOL UND KOHLE - ERDGAS - PETROCHEMIE 1986, 1987 d; JENSEN 1988, O I L GAS JOURNAL 1988 f ) , and i s t h e l a r g e s t o i l and gas producer o f t h e w o r l d (CHADWICK, LONG & NISSANKE 1988). The USSR t o t a l gas r e s e r v e s reached a b t . 1,500 T c f (37,500 B i l l . m3) by end o f 1987 (FUEG 1987) i n comparison w i t h a b t . 3,860 T c f (96,500 B i l l . m3) worldwide (WORLD OIL 1988 a ) . Having s u f f i c i e n t p o t e n t i a l and b e i n g f a c e d w i t h u n c e r t a i n t i e s o f t h e f u t u r e p r o s p e c t s o f o i l p r o d u c t i o n b e i n g adequate t o meet domestic and e x p o r t n e c e s s i t i e s , t h e USSR has g i v e n n a t u r a l gas a h i g h p r i o r i t y i n i t s o v e r a l l energy p l a n n i n g a c t i v i t i e s ( c f . s e c t i o n 2.4.4.). N a t u r a l gas has p r o v i d e d a reasonable c h o i c e f o r t h e USSR o f d i v e r s i f y i n g i t s energy s u p p l i e s and i n c o n t r i b u t i n g an i m p o r t a n t share i n h a r d c u r r e n c y e a r n i n g s e s p e c i a l l y w i t h t h e r e c e n t d e c l i n e i n exporting other fuels. The USSR accounts t o g e t h e r w i t h I r a n and Q a t a r f o r n e a r l y 80 % o f t h e w o r l d ' s e x p o r t a b l e gas s u r p l u s e s (JENSEN 1988), and s i x o t h e r c o u n t r i e s i n c l u d i n g Norway, A l g e r i a , N i g e r i a , M a l a y s i a and Abu Dhabi have ample gas r e s e r v e s t o s u p p o r t any f o r e s e e a b l e w o r l d - s c a l e e x p o r t p r o j e c t . The Ob r e g i o n i n Western S i beria/USSR has t h e l a r g e s t c o n c e n t r a t i o n o f gas r e s e r v e s i n t h e w o r l d . I t i n c l u des w i t h t h e s u p e r g i a n t s Urengoi ( p r o d u c i n g sweet gas f r o m Upper Cretaceous Cenomanian r e s e r v o i r s i n 1,100 - 3,100 m depth, and t h e J u r a s s i c i n 4,500 m depth i s o i l - b e a r i n g i n some f i e l d s ) and Yamburg t h e w o r l d ' s l a r g e s t and t h i r d l a r g e s t gas f i e l d s which a r e connected by t h e Yamal p i p e l i n e system o f o v e r 5,500 km l e n g t h and 1,420 mm ( 5 6 i n ) d i a m e t e r t o Western Europe. O t h e r s u p e r g i a n t gas f i e l d s which a r e p a r t i a l l y s t i l l i n t h e development stage a r e Astrakhan, Bovanenko, Karachaganak, Medvezhye and Sovietabad (FUEG 1988; c f . t a b . 9 ) . Urengoi accounted i n 1987 f o r 42 % o f t h e t o t a l gas p r o d u c t i o n i n t h e USSR and had i n i t i a l r e s e r v e s more t h a n double t h a n those proven f o r t h e e n t i r e N o r t h Sea, and Yamburg a l s o i s b e l i e v e d t o have g r e a t e r p r o d u c t i o n p o t e n t i a l t h a n a l l N o r t h Sea f i e l d s found t o d a t e (OIL GAS JOURNAL 1988 f ) . Western S i b e r i a i s scheduled t o r e a c h a b t . 70 % o f t h e t o t a l USSR gas o u t p u t i n 1990, and t h e s u p e r g i a n t Urengoi which came on stream i n 1978 (OIL GAS JOURNAL 1987 0 ) and has a l r e a d y c u m u l a t i v e l y produced o v e r 40 T c f (1,000 B i l l . m3) o f gas i s n o t expected t o dec l i n e u n t i l t h e l a t e 1990's (RIGASSI 1986). Western S i b e r i a c o n t a i n s w i t h a b t . 60 - 70 % t h e b u l k o f t h e USSR gas r e s e r ves (FISCHER 1981). T o t a l USSR gas p r o d u c t i o n reached t h e r e c o r d l e v e l o f 25.6 T c f (640 B i l l . m3) i n 1987 a f t e r a r a p i d e v o l u t i o n f r o m 10.2 T c f (255 B i l l . m3)
154 i n 1975 v i a 1 5 . 4 T c f ( 3 8 5 B i l l . m3) in 1980 and 22.7 T c f ( 5 6 8 B i l l . m3) in 1985, and i s e x p e c t e d t o i n c r e a s e u n t i l t h e y e a r 2000 up t o 45 T c f (1,125 B i l l . m3; O I L GAS JOURNAL 1987 0). T h e r e f o r e b y 1989, gas w i l l p r o b a b l y f o r t h e f i r s t t i m e r e p r e s e n t a l a r g e r p e r c e n t a g e o f t h e USSR f u e l b a l a n c e t h a n o i l . W e s t e r n S i b e r i a ' s s h a r e i n t h e t o t a l USSR qas o u t p u t i n c r e a s e d f r o m 1.3 T c f ( 3 2 . 5 B i l l . m3) i n 1975 v i a 5 . 6 T c f ( 1 4 0 B i i l . m 3 ) ' i n 9 8 0 and 1 3 . 1 T c f ( 3 2 8 s i l l . m3) i n 1985 t o 1 6 . 1 T c f ( 4 0 3 B i l l . m3) i n 1987 w h i c h r e p r e s e n t s a b t . 63 % o f t o t a l USSR gas p r o d u c t i o n and i s e x p e c t e d t o r i s e t o a b t . 35.3 T c f ( 8 8 3 B i l l . m3) in 2000 w h i c h t h e n w o u l d c o r r e s p o n d t o o v e r 78 % o f t h e t o t a l USSR o u t p u t . The magn i t u d e o f t h e S o v i e t e x p o r t a b l e gas s u r p l u s e s and t h e r e l a t i v e l y f a v o u r a b l e e c o n o m i c s o f t h e i r d e l i v e r y has r e n d e r e d i m p o r t p o l i c y o f t h e c u s t o m e r s r a t h e r t h a n a v a i l a b l e r e s e r v e s and p r o d u c t p r i c i n q he l i m i t i n g f a c t o r d e t e r m i n i n g t h e m a r k e t s h a r e w h i c h S o v i e t gas c a n c a r v e o u t n E u r o p e . The r e s e r v e s a r e s o l a r g e t h a t t h e USSR c o u l d more t h a n d o u b l e i t s gas e x p o r t s t o H e s t e r n E u r o p e w i t h i n t e n y e a r s (STERN 1 9 8 6 ) . A c c o r d i n g t o t h e e c o n o m i c a l n e c e s s i t y o f t h e USSR t o e a r n h a r d c u r r e n c y f r o m c h i e f l y h y d r o c a r b o n s a l e ( c f . s e c t i o n 2 . 4 . 4 . ) , t h e r e i s no r e a s o n o f f e a r i n g c o m m e r c i a l o r s e c u r i t y r i s k s o f i m p o r t i n g oil and gas f r o m t h e USSR w h i c h i s a s o l i d t r a d e p a r t n e r and d e l i v e r s r e l i a b l y and has no i n t e r e s t t o e x e r c i s e economic l e v e r a g e f o r p o l i t i c a l c o n c e s s i o n s . Gas d e l i v e r i e s t o l l e s t e r n Europe have c o n s e q u e n t l y r e a c h e d h i t h e r t o r e c o r d l e v e l s in 1986 and 1987, and p l a n s a r e made f o r s u p p l y a l s o t o o t h e r c o u n t r i e s t h a n t h o s e a l r e a d y c o n t r a c t i n g g a s f r o m t h e USSR (GORST 1 9 8 8 ) .
2.2.1.6.3.2.6.
Supply security
Up t o t h e y e a r 2000, a b t . 60 % o f t h e t o t a l European gas c o n s u m p t i o n c a n be c e r t a i n l y s u p p l i e d f r o m s o u r c e s w i t h i n Europe, whereas l a t e r a l s o v a r i o u s quant i t i e s may d e r i v e f r o m o t h e r c o u n t r i e s (KEG 1 9 8 6 ) . I n a d d i t i o n , t h e gas s u p p l y n e t w o r k s t r u c t u r e and t h e r e l a t e d s e c u r i t y measures g u a r a n t e e t h e c o m p e n s a t i o n o f d e l i v e r y i n t e r r u p t i o n s such as p o s s i b l e embargo s i t u a t i o n s e n a c t e d b y one s u p p l i e r b y f l e x i b i l i t y t o s h i f t t o o t h e r s o u r c e s and t o s w i t c h q u a n t i t i e s w i t h i n t h e s y s t e m b y i n c r e a s i n g gas a v a i l a b i l i t y on s p e c i f i c demand. The s u p p l y saf e t y i s m a i n l y given by t h e broad d i v e r s i f i c a t i o n o f sources o f imported volumes w i t h o u t e x c e s s i v e dependency on one i n d i v i d u a l s u p p l i e r , r e a s o n a b l e s e c u r i t y measures f o r t h e c a s e o f u n f o r e s e e n d e l i v e r y i n t e r r u p t i o n s s u c h as r e s e r v e c a p a c i t i e s f o r d o m e s t i c p r o d u c t i o n and u n d e r g r o u n d s t o r a g e o f peak c o m p e n s a t i o n volumes, and b o u n d a r y - c r o s s i n g c o o p e r a t i o n between t h e n a t i o n a l gas i n d u s t r i e s ( c f . a l s o LIESEN 1 9 8 5 ) . The e n e r g y s c e n a r i o h i s t o r y i n W e s t e r n E u r o p e c a n be summarized a s r e f l e c t i n g i n c r e a s i n g r e l i a n c e o n n a t u r a l gas (D'ANDREA 1 9 8 3 ) . Asp e c t s o f t h e USSR n a t u r a l gas i n d u s t r y a r e a l s o d i s c u s s e d b y FISCHER ( 1 9 8 1 ) .
2.2.1.6.3.2.7.
Mutual benefit
Thus t h e o i l p r i c e e x p l o s i o n s and r e l a t e d e n e r g y shocks o f t h e 1 9 7 0 ' s have had a p r o f o u n d i m p a c t on t h e e n e r s y economy o f Europe, because g o v e r n m e n t s i m p l e m e n t e d p o l i c i e s t o r e d u c e t h e i r dependence on oil i m p o r t s , i n c r e a s e t h e i r dom e s t i c e n e r g y p r o d u c t i o n , p r o m o t e i n t e r f u e l s u b s t i t u t i o n and i n c r e a s e e n e r ~ i ye f f i c i e n c y t h r o u g h c o n s e r v a t i o n and t e c h n i c a l i n n o v a t i o n s (D'ANDREA 1 9 8 1 ) . A:; i m p o r t a n t component o f t h i s t r a n s f o r m a t i o n in t h e European e n e r g y economy was t h e e x p a n s i o n o f t h e e a s t - w e s t t r a d e . I n c r e a s e d e n e r g y i m p o r t s f r o m t h e e a s t have a l l o w e d i n d u s t r i a l i z e d m a r k e t economies i n t h e w e s t t o d i v e r s i f y t h e i r s o u r c e s o f e n e r g y s u p p l y and p r o v i d e b u s i n e s s o p p o r t u n i t i e s , w h i l e i n c r e a s e d r e v e n u e s f r o m e n e r g y e x p o r t s have p e r m i t t e d c e n t r a l l y p l a n n e d economies i n t h e e a s t t o i m p o r t goods and s e r v i c e s t h a t a r e e s s e n t i a l f o r t h e u p g r a d i n g o f t h e i r i n d u s t r i a l technologization.
155
2.2.1.6.3.3. International cooperat ion
and competitive challenges
Some f u t u r e c h a l l e n g e s t o t h e European gas i n d u s t r y a r e a l s o o u t l i n e d by L I E SEN ( 1 9 8 5 ) . Gas i s bought and s o l d on an i n t e r n a t i o n a l market where s u b s t a n t i a l and o f t e n fundamental d i f f e r e n c e s p e r s i s t among v a r i o u s c o u n t r i e s , and t h e c h a l lenges faced d i f f e r c o r r e s p o n d i n g l y . While i n some r e g i o n s , t h e development o f t h e gas i n d u s t r y i s e s s e n t i a l l y complete and t h e s t r a t e g i c o b j e c t i v e s a r e concerned w i t h f u t u r e s u p p l y f o r t h e l o n g t e r m o r f o r t h e c o n s o l i d a t i o n o f m a r k e t p o s i t i o n s i n an i n c r e a s i n g l y c o m p e t i t i v e environment, t h e c o s t - e f f e c t i v e i n s t a l l a t i o n o f a gas t r a n s m i s s i o n and d i s t r i b u t i o n system i s t h e t a r g e t i n o t h e r areas, and p r o b a b l y i n c e r t a i n c o u n t r i e s , n a t u r a l gas w i l l never be o f m a j o r i m p o r t a n c e f o r a v a r i e t y o f reasons. The n e c e s s i t y t o a d j u s t t h e e x i s t i n g c a p a c i t y t o meet a s m a l l e r o v e r a l l energy demand i s t h e most s e r i o u s c h a l l e n g e f a c e d on o t h e r energy s e c t o r s , b u t does h a r d l y a p p l y t o t h e gas i n d u s t r y which e n j o y s c o n t i n u i t y due t o t e c h n i c a l and s t r u c t u r a l f a c t o r s . The c h i e f r e q u i r e m e n t i s t o m a i n t a i n and promote a c o n s t r u c t i v e a t t i t u d e t o wards gas on t h e p a r t o f consumers and governments. The m a j o r t e c h n o l o g i c a l advantages o f gas a r e t h e l a c k o f any severe impact on t h e environment and t h e easy c o n t r o l . E x t e r n a l source d i v e r s i f i c a t i o n c o u p l e d w i t h a m a j o r indigeneous s u p p l y base i s t h e key s t r a t e g y f o r energy d e l i v e r y s e c u r i t y , which i s backed up by t h e proven w o r l d - w i d e n a t u r a l gas r e s e r v e s exceeding c u r r e n t w o r l d gas consumption by a f a c t o r o f o v e r 50. One o f t h e main arguments f o r n a t u r a l gas i n Europe i s t h e r e d u c t i o n i n u n i l a t e r a l i m p o r t dependence and t h e i n c r e a s e d sec u r i t y o f s u p p l i e s achieved by t h e i n t e g r a t i o n o f gas i n t h e o v e r a l l energy market. Aspects o f government p o l i c y and i n t e r n a t i o n a l gas t r a d e i n t h e USA a r e examined by P R I C E (1986), and LICHTBLAU (1986) comments on p e t r o l e u m i m p o r t and exp o r t p o l i c i e s i n t h e USA. KALISCH (1988) g i v e s a gas energy s u p p l y o u t l o o k unt i l t h e y e a r 2010 f o r t h e USA under d i f f e r e n t o i l and gas p r i c e s c e n a r i o s . NAYLOR & FERRIES (1988) summarize g a s . p r i c e and m a r k e t i n g e v o l u t i o n i n t h e USA. O t h e r r e l a t i o n s h i p s i n t h e n a t u r a l gas i n d u s t r y a r e i l l u s t r a t e d by MAY & MATHEWS ( 1 9 8 3 ) . SCHLESINGER (1982) p r e s e n t s an o u t l o o k o f gas energy demand and c o m p e t i t i v e n e s s i n t h e USA, and o t h e r aspects o f t h e USA gas m a r k e t a r e d i s c u s sed by ABBOTT (1982), BUSH 1982), K I N G (1982), MEANS (1982) and HOCHHEISER ( 1 9 8 8 ) . KUMAR (1988) comments on r e s t r u c t u r i n g o f n a t u r a l gas markets by r e g u l a t i o n and l e g i s l a t i o n . CUNNINGHAM (1982) emphasizes t h e r o l e o f market competit i o n f o r shaping o f f u t u r e gas p r i c e s . TEK (1979) c h a r a c t e r i z e s t h e c h a l l e n g e o f worldwide n a t u r a l gas matching s u p p l y - l i m i t e d markets and m a r k e t - l i m i t e d supp l i e s . RIETMAN & KID0 (1977) document improvement o f u l t i m a t e r e c o v e r y o f n a t u r a l gas by r e a l i s t i c gas p r i c e s .
2.2.1.6.3.4.Market shares and reserves distribution W h i l e proven w o r l d w i d e gas r e s e r v e s compare f a v o u r a b l y t o those o f o i l , many gas d i s c o v e r i e s a r e remote from consuming markets and t h u s c o s t l y t o commercial i z e (JENSEN 1988). Less than h a l f o f t h e w o r l d ' s t o t a l gas r e s e r v e s a r e dedicat e d t o s p e c i f i c l o c a l o r e x p o r t m a r k e t o u t l e t s , and an e q u a l l y l a r g e q u a n t i t y o f gas may a l s o be c l a s s i f i e d as e x p o r t a b l e s u r p l u s b e i n g l a r g e enough t o supp o r t an e x p o r t p r o j e c t , b u t h a v i n g no market o u t l e t y e t a v a i l a b l e . Since new exp o r t commitments a r e n o t growing as r a p i d l y as gas d i s c o v e r i e s i n m a j o r e x p o r t r e g i o n s a r e i n c r e a s i n g , t h e e x p o r t a b l e s u r p l u s c a t e g o r y i s s t e a d i l y becoming l a r g e r which a l s o p r o g r e s s i v e l y enhances o v e r a l l s u p p l y s e c u r i t y . On w o r l d - w i d e scale, o i l consumption d e c l i n e d b y 3.5 % between 1977 and 1986, whereas gas consumption i n c r e a s e d by 29 %, w i t h t h e g a i n s t h a t n a t u r a l gas has made i n t h e w o r l d energy markets, however, h a v i n g been r e g i o n a l l y v e r y uneven. I n t h e U n i t e d S t a t e s where gas has h i s t o r i c a l l y h e l d a h i g h share
of
energy
156 m a r k e t s , demand has d e c l i n e d i n a b s o l u t e t e r m s o v e r t h e m e n t i o n e d p e r i o d , wher e a s i t s usage in Japan has more t h a n t r i p l e d as a consequence o f g r o w t h f r o m a s m a l l b a s e . By f a r t h e l a r g e s t i n c r e m e n t o f e x p a n s i o n has t a k e n p l a c e in t h e USSR where an a g g r e s s i v e gas u t i l i z a t i o n p r o g r a m has e s t a b l i s h e d one o f t h e w o r l d ' s m o s t g a s - i n t e n s i v e economies ( c f . s e c t i o n 2 . 4 . 4 . ) . G r o w t h o f gas cons u m p t i o n i n E u r o p e has been c o m p a r a t i v e l y r a p i d a t an e a r l i e r p e r i o d , b u t f o r many p a r t s o f Europe, g a s has now a c h i e v e d more o r l e s s a l e v e l o f m a r k e t matur i t y and i s n o t s i g n i f i c a n t l y i n c r e a s i n g i t s s h a r e o f e n e r g y m a r k e t s . The r e d u c e d g r o w t h p r o s p e c t s f o r gas demand i n Europe and N o r t h A m e r i c a w h i c h have dampened t h e e x p e c t a t i o n s f o r w o r l d g a s t r a d e r e l a t i v e t o w h a t was a n t i c i p a t e d a decade ago a r e p a r t i a l l y t h e r e s u l t o f e n e r g y c o n s e r v a t i o n and i n d u s t r i a l r e structuring.
2.2.1.6.3.5. Future evolution o f the North Sea gas province The m o s t i m p o r t a n t g a s p r o v i n c e i n Western Europe i s t h e N o r t h Sea where c u r r e n t l y Norway, N e t h e r l a n d s and U n i t e d Kingdom have more t h a n 85 % o f t h e o v e r a l l r e s e r v e s (VALENTIN, VALAIS & CHAMPLON 1 9 8 8 ) . The n e a r e r and f a r t h e r f u t u r e w i l l be c h a r a c t e r i z e d b y g r a d u a l l y d i m i n i s h i n g p r o d u c t i o n s o u r c e s o f e a s i l y a c c e s s i b l e and r e t r i e v a b l e gas. The d e c l i n e o f t h e p r e s e n t l y p r o d u c i n g f i e l d s as w e l l as t h o s e w h i c h a r e d e v e l o p e d a t t h e moment c a n o n l y be compensated in t h e f u t u r e b y e x p l o r a t i o n and a c q u i s i t i o n o f new g e n e r a t i o n s o f f i e l d s i n i n c r e a s i n g w a t e r d e p t h , more r e m o t e g e o g r a p h i c a l l o c a t i o n and more u n f a v o u r a b l e c l i m a t i c c o n d i t i o n s . F u t u r e p r o j e c t s c o n c e r n i n g new g e n e r a t i o n s o f f i e l d s i n W e s t e r n Europe and e s p e c i a l l y i n t h e N o r t h Sea w i l l r e q u i r e l a r g e r and l a r g e r i n v e s t ments t o g e t h e r w i t h t e c h n o l o g i c a l d e v e l o p m e n t s w h i c h w i l l be f a r ahead o f t h e t e c h n i q u e s t h a t have a l r e a d y been m a s t e r e d , w i t h t h i s b e i n g t h e p r i c e t h a t West e r n Europe w i l l have t o p a y i n o r d e r t o l i m i t t h e i n c r e a s e i n i t s dependency on i m p o r t e d gas s u p p l i e s . The f r o n t i e r gas f i e l d s (DELACOUR 1986) w h i c h have t o be d e v e l o p e d a r e a t t h e l i m i t o r e v e n beyond o f c o m m e r c i a l f e a s i b i l i t y g i v e n t h e p r e s e n t s t a t e o f t e c h n o l o g y and e c o n o m i c a l c o n d i t i o n s ( c f . a l s o VALAIS 1987).
2.2.1.6.4. Evolutionary trends o f the world gas market LINDEN ( 1 9 8 5 ) o u t l i n e s some e v o l u t i o n a r y t r e n d s o f t h e w o r l d gas m a r k e t b y commenting on t h e r o l e o f gas i n a c h a n g i n g t e c h n o l o g i c a l and c o m p e t i t i v e e n v i ronment. Gas c u r r e n t l y c o v e r s a b t . 20 % o f t h e t o t a l w o r l d e n e r g y demand ( L I E SEN 1 9 8 5 ) , b u t t h i s r a t e i s c o n s i d e r e d t o grow s u b s t a n t i a l l y d u r i n g t h e n e x t decades (LINDEN 1985) w h i c h a r e e x p e c t e d t o be c h a r a c t e r i z e d by a c c e l e r a t i n g t e c h n o l o g i c a l change, g r o w i n g i n t e r f u e l c o m p e t i t i o n and c o n t i n u i n g s t r u c t u r a l changes i n t h e i n d u s t r i a l i z e d economies, w i t h gas more and more s u b s t i t u t i n g and r e p l a c i n g o i l in many b r a n c h e s o f a p p l i c a t i o n . Because r a d i c a l changes o f t h e w o r l d ' s f u t u r e e n e r g y p e r c e p t i o n have o c c u r r e d o v e r t h e r e l a t i v e l y s h o r t span o f the l a s t twenty years, the r e l i a b i l i t y o f a f o r e c a s t f o r the next twenty y e a r s i s m a i n l y d e p e n d i n g o n f u t u r e r a t e o f e n e r g y demand g r o w t h , adequacy o f w o r l d o i l and gas s u p p l i e s , and o u t l o o k f o r w o r l d o i l p r i c e s t a b i l i t y . BRIERLEY ( 1 9 8 8 ) emphasizes t h e s i g n i f i c a n c e o f g a s as a g l o b a l e n e r g y s o u r ce, and GADONNIEUX ( 1 9 8 8 ) i l l u s t r a t e s gas c h a l l e n g e s i n a c o m p e t i t i v e e n v i r o n ment. GUNDERMANN ( 1 9 8 2 ) c h a r a c t e r i z e s n a t u r a l gas i n t h e e n e r g y m a r k e t o f t h e f u t u r e , and ERDOEL-ERDGAS ( 1 9 7 8 ) o u t l i n e s d e v e l o p m e n t o f p r o d u c t i o n and d i s t r i b u t i o n as w e l l as s u p p l y s e c u r i t y o f n a t u r a l gas u p t o t h e y e a r 2000 and beyond. The summary as f o l l o w s c o n c e n t r a t e s on p r o b l e m a t i c l i n k i n g o f o i l and gas prices, e n e r g y s u p p l y d i v e r s i f i c a t i o n c o n c e p t s , e c o n o m i c a l and t e c h n i c a l t r e n d s , o i l s u b s t i t u t i o n b y gas, u n c o n v e n t i o n a l gas e x p l o i t a t i o n , and g a s m a r keting strategies.
157
2.2.1.6.4.1. Problematic linking of oil and gas prices One of t h e m a j o r problems o f t h e p r e s e n t energy m a r k e t i s t h e a r t i f i c i a l l i n k i n g of t h e p r i c e s t r u c t u r e f o r gas and o i l (LINDEN 1985). T y i n g gas t o o i l g e o l o g i c a l l y , e c o n o m i c a l l y and c o n c e p t u a l l y l e a d s t o l o o k i n g a t i t s p r o s p e c t s w i t h t u n n e l v i s i o n , r e s u l t i n g i n f a i l u r e t o pursue a g g r e s s i v e gas s u p p l y and u t i l i z a t i o n s t r a t e g i e s . Independent f r o m t h e o i l p r i c e e v o l u t i o n , t h e v a l u e o f gas v a r i e s w i d e l y depending on where i t i s s o l d , a t what r a t e i t i s taken and a t what degree o f r e l i a b i l i t y i t i s p r o v i d e d . Because gas has an e s s e n t i a l , v e r y c a p i t a l - i n t e n s i v e , g e o g r a p h i c a l l y c o n s t r a i n e d and t h e r e f o r e q u i t e approp r i a t e l y r e g u l a t e d u t i l i t y f u n c t i o n t o p r o v i d e r e l i a b l e and a f f o r d a b l e energy s e r v i c e s t o r e s i d e n t i a l , commercial and i n d u s t r i a l customers t h a t have l i t t l e o r no f u e l s w i t c h i n q c a p a b i l i t y , t h e r e i s no immutable and fundamental r a t i o o n c e r n i ng nale f o r l i n k i n g the p r i c e . o f g a s - w i t h t h a t o f residual f u e l o i l . supply, t h e obvious abundance o f n a t u r a l gas o f b o t h c o n v e n t i o n a l and unconvent i o n a l t y p e r a i s e s t h e q u e s t i o n o f t h e a p p r o p r i a t e use o f a v a i l a b l e f i n a n c i a l and i n t e l l e c t u a l r e s o u r c e s t o i n c r e a s e f u t u r e gas s u p p l y s e c u r i t y .
2.2.1.6.4.2.Energy
supp 1y
diversification concepts
The f u t u r e s u p p l y d i v e r s i f i c a t i o n concept should be based on t h e e a s t - c o s t energy s e r v i c e s t r a t e g y which moves n a t u r a l gas as t h e cheapest, c l e a n e s t and h i g h e s t e f f e c t i v e energy t y p e i n t o a v e r y f a v o u r a b l e p o s i t i o n (LINDEN 1985). A l though t h e r e a r e n a t i o n a l s e c u r i t y , monetary and b r o a d e r g e o p o l i t i c a l and geos t r a t e g i c a l c o n s i d e r a t i o n s t h a t r e q u i r e f l e x i b i l i t y i n terms o f energy s u p p l y spectrum, t h e q u e s t i o n i s whether any o f those p o l i t i c a l and economical conc e p t s a r e based on m i s t a k e n n o t i o n s o f t h e p o t e n t i a l o f domestic resources, t h e i n e v i t a b i l i t y o f shortages, t h e i n h e r e n t u n r e l i a b i l i t y o f f o r e i g n sources, and t h e n a t i o n a l s e c u r i t y b e n e f i t s o f a u t a r k y . Adequate and p r o p e r l y balanced e n e r gy r e s e a r c h programs a r e c l e a r l y an e s s e n t i a l component o f sound l e a s t - c o s t s t r a t e g i e s f o r p r o v i d i n g energy s e r v i c e s which n o t o n l y have t o i n c l u d e s o l u t i o n s t o d e f i n a b l e problems, b u t a l s o i n s u r a n c e f o r a reasonable range o f cont i n g e n c i e s and o p p o r t u n i t i e s f o r i n n o v a t i o n and n o n - t r a d i t i o n a l approaches. HOLTBERG, WOODS & ASHBY (1987) o u t l i n e t h a t n a t u r a l gas w i l l pay a m a j o r r o l e i n a h i g h l y c o m p e t i t i v e energy mix w e l l i n t o t h e n e x t c e n t u r y . New s u p p l y t e c h n o l o g i e s , p a r t i c u l a r l y advanced techniques f o r p r o d u c t i o n f r o m u n c o n v e n t i o n a l gas resources, w i l l enhance t h e accentuated p o s i t i o n o f gas i n i n t e r f u e l c o m p e t i t i o n and assure t h e r e l i a b i l t y o f l o n g - t e r m s u p p l i e s .
2.2.1.6.4.3. Economical and technical trends Concerning p e r c e i v a b l e economical and t e c h n i c a l t r e n d s , t h e o v e r r i d i n g f a c t o r i s t h e l a r g e i n c r e a s e i n c o s t and r i s k o f f i n a n c i n g m a j o r energy p r o j e c t s (LINDEN 1985). The u n l i k e l i n e s s o f s i g n i f i c a n t m i t i g a t i o n o f t h e u n d e r l y i n g causes b e i n g a b n o r m a l l y h i g h i n t e r e s t r a t e s , u n c o n t r o l l a b l e s l i p p a g e o f c o n s t r u c t i o n schedules, l a r g e c o s t o v e r r u n s and g e n e r a l v o l a t i l i t y o f t h e energy mark e t , has d r a s t i c a l l y s h i f t e d t h e r e l a t i v e a t t r a c t i v e n e s s of a l t e r n a t i v e energy s u p p l y and u t i l i z a t i o n investments. M a j o r o i l and gas e x p l o r a t i o n p r o j e c t s i n remote l o c a t i o n s and h o s t i l e environments have become l e s s a t t r a c t i v e , whereas development o f even m a r g i n a l hydrocarbon resources i n l o w - r i s k m i l i e u and cons e r v a t i o n have become more a t t r a c t i v e . New e x p l o r a t i o n , c o m p l e t i o n and product i o n t e c h n o l o g i e s expanding t h e a v a i l a b l e p r o s p e c t s a r e t h e guarantee t h a t maj o r commercial o i l and gas d i s c o v e r i e s w i l l c o n t i n u e w e l l beyond t h e y e a r 2000. The tendency o f making m a r g i n a l sources o f gas e c o n o m i c a l l y v i a b l e w i l l be enhanced by t h e growing r e c o g n i t i o n o f t h e t r u e f o r m v a l u e o f gas r e l a t i v e t o o i l , c o a l and c e n t r a l l y generated e l e c t r i c i t y w i t h t h e advent o f new and improved end-use t e c h n o l o g i e s . I n l i g h t o f s e c u r i n g supply, some o f t h e f a s t e s t - m o v i n g energy t e c h n o l o g i e s i n t h e n e x t decades w i l l be more a c c u r a t e and e f f e c t i v e e x p l o r a t i o n concepts and d r i l l i n g d i a g n o s t i c s , r e m o t e l y o p e r a t e d and c o n t r o l l e d
158 deep o n s h o r e and d e e p - w a t e r o f f s h o r e d r i l l i n g , d e e p - w a t e r c o m p l e t i o n and g a s - f i red cogeneration. As t h e r e i s p r e s e n t l y no r e a s o n t o assume any m a j o r upwards o i l p r i c e move i n r e a l t e r m s b e f o r e t h e y e a r 2000, t h e f u t u r e o f g a s is c h a r a c t e r i z e d b y cont i n u e d t o u g h b u r n e r t i p c o m p e t i t i o n f r o m f u e l o i l in s t a t i o n a r y h e a t e n e r g y app l i c a t i o n s where t h e s u p e r i o r c o m b u s t i o n c h a r a c t e r i s t i c s and n o n - p o l l u t i n g n a t u r e o f gas do n o t o f f e r a s u b s t a n t i a l c o m p e t i t i v e a d v a n t a g e . Even w i t h o u t o i l c o m p e t i t i o n , t h e p r i c e o f gas i n l o w f o r m v a l u e a p p l i c a t i o n s w o u l d be l i m i t e d b y c o a l and e l e c t r i c i t y c o m p e t i t i o n , p a r t i c u l a r l y due t o t h e r e l a t i v e l y h i g h e f f i c i e n c i e s and l o w i n v e s t m e n t c o s t s o f t h e l a t t e r . The r a p i d i n t r o d u c t i o n o f more e f f i c i e n t and c o s t - e f f e c t i v e gas u t i l i z a t i o n t e c h n o l o g i e s and t h e s t a b i l i z a t i o n o f n a t u r a l gas p r i c e s have now stemmed t h e g r o w i n g e l e c t r i c a l i n r o a d s i n t o t h e t r a d i t i o n a l gas m a r k e t s and have opened many new m a r k e t s where g a s i s o f superior competitiveness.
2 . 2 . 1 . 6 . 4 . 4 . Oil substitution by gas I n a r e a s w i t h a b u n d a n t gas r e s o u r c e s , gas w i l l c o n t i n u e t o s u b s t i t u t e o i l i n v a r i o u s a p p l i c a t i o n s , t h e r e b y f u r t h e r r e d u c i n g p r e s s u r e o n w o r l d o i l demand and thus extending t h e period o f r e l a t i v e o i l p r i c e s t a b i l i t y . I n areas w i t h o u t suff i c i e n t d o m e s t i c gas r e s o u r c e s b u t w i t h i n r e a c h o f e c o n o m i c a l l y c o m p e t i t i v e e x p o r t p r o j e c t s , gas c o n s u m p t i o n w i l l a l s o r e m a i n a t h i g h l e v e l s o r e v e n grow (LINDEN 1 9 8 5 ) . C o n c e r n i n g i m p o r t , t h e s o u r c e s o f gas a r e h i g h l y d i v e r s i f i e d and have so f a r p r o v e n t o be more r e l i a b l e t h a n t h o s e o f o i l , w i t h f u r t h e r i n c r e a s e o f e n e r g y s e c u r i t y b e i n g p r o v i d e d b y i n c r e a s i n g d i v e r s i f i c a t i o n o f f u e l u s e and o n g o i n g s u b s t i t u t i o n o f o i l b y gas, t h e r e b y i n c r e a s i n g t h e o i l e x p o r t p o t e n t i a l . As w o r l d - w i d e gas e x p l o r a t i o n i s s t i l l i n a r e l a t i v e l y e a r l y s t a g e , there i s a h i g h p r o b a b i l i t y t h a t b o t h p r o v e n r e s e r v e s and r e m a i n i n g r e c o v e r a b l e r e s o u r c e s w i l l s u b s t a n t i a l l y r i s e o v e r t h e n e x t decades, p a r t i c u l a r I y w i t h p r o g r e s s i v e e x p l o r a t i o n and e x p l o i t a t i o n o f u n c o n v e n t i o n a l g a s . A g a s e n e r g y s u p p l y o u t l o o k u n t i l t h e y e a r 2010 i s a l s o g i v e n by KALISCH ( 1 9 8 8 ) , and JENSEN ( 1 9 8 8 ) comments on w o r l d g a s s u p p l i e s , m a r k e t s and t r a d e . A s p e c t s o f a r e a l g a s p o t e n t i a l a r e a l s o p r o v i d e d b y SHARMA, PATIL, KAMATH & GOOBOLE ( 1 9 8 8 ) .
2.2.1.6.4.5.
Unconvent i ona 1 gas exp loi tat ion
The d e f i n i t i o n o f u n c o n v e n t i o n a l gas i s f l e x i b l e , because t h e b o u n d a r y b e t ween w h a t i s e c o n o m i c a l l y and t e c h n o l o g i c a l l y p o s s i b l e o r n o t has a l r e a d y s h i f t e d s e v e r a l t i m e s q u i t e r a p i d l y d u r i n g c o u r s e o f gas a c q u i s i t i o n and w i t h d r a w a l t e c h n o l o g y e v o l u t i o n (LINDEN 1 9 8 5 ) . C o p r o d u c t i o n o f i m m o b i l e g a s i n s o l i d h y d r a t e s and gas d i s s o l v e d i n g e o p r e s s u r e d b r i n e s ( c f . s e c t i o n 4 . 4 . 2 . ) b y new t e c h n i ques l o o k s e c o n o m i c a l l y p r o m i s i n g e v e n a t t o d a y ' s m a r k e t p r i c e s . The e c o n o m i c a l l y and s t r a t e g i c a l l y m o s t i m p o r t a n t u n c o n v e n t i o n a l n a t u r a l gas r e s o u r c e s u n t i l t h e y e a r 2000 a r e t i g h t gas sands ( c f . s e c t i o n 4 . 4 . 1 . ) , s h a l e gas ( c f . s e c t i o n 4 . 4 . 4 . ) and deep u n m i n e a b l e c o a l seam gas ( c f . s e c t i o n 4 . 4 . 3 . ; HAAS, BRASHEAR & MORRA 1985; KUUSKRAA & HAAS 1 9 8 8 ) , w i t h a l l o f t h e s e r e s e r v o i r s r e q u i r i n g h y d r a u l i c proppant f r a c t u r i n g i n smaller o r l a r g e r scale i n o r d e r t o ensure feas i b l e p r o d u c t i o n r a t e s ( c f . a l s o s e c t i o n 4 . 4 . ) . As t h e s e t h r e e s o u r c e s t o g e t h e r y i e l d a l r e a d y s u b s t a n t i a l amounts o f gas f o r commercial p r o d u c t i o n i n t h e USA which i s by f a r t h e most mature gas-producing r e g i o n o f t h e world, t h e t r a n s i t i o n f r o m u n c o n v e n t i o n a l t o c o n v e n t i o n a l gas r e s o u r c e s t a k e s a l r e a d y p l a c e , and i n t h e n e a r f u t u r e , huge m a r g i n a l gas r e s o u r c e s o f t h e m e n t i o n e d t y p e s w i l l come i n t o p l a y i n o t h e r c o u n t r i e s as w e l l . I f massive h y d r a u l i c proppant f r a c t u r i n g i s p r u d e n t l y a p p l i e d , a t o t a l o f more t h a n 600 T c f (15,000 B i l l . m3) o f t i g h t gas r e s e r v e s a r e w a i t i n g o n a c q u i s i t i o n i n t h e USA, w i t h u p t o 4 - 8 T c f (100 - 200 B i l l . m3) p e r y e a r b e i n g a b l e t o be accessed b y 1990 ( P A I , G A R B I S & HALL 1 9 8 3 ) .
159 The economical and s t r a t e g i c a l importance o f adding t o n a t u r a l gas r e s o u r c e s t h a t can be r e c o v e r e d a t c o m p e t i t i v e c o s t s i s obvious (LINDEN 1985). I t would t a k e t h e c o n s t r u c t i o n o f s y n t h e t i c f u e l p l a n t s a t g r e a t expense t o produce subs t i t u t e gas f r o m c o a l , o r r e p l a c e l i q u i d s f r o m o i l s h a l e o r c o a l , t o y i e l d as much u s e f u l o u t p u t o v e r t h e i r economical l i f e t i m e s as t h e c o n v e r s i o n o f a d d i t i o n a l m a r g i n a l n a t u r a l gas r e s o u r c e s i n t o f e a s i b l e r e s e r v e s . F u r t h e r improvements i n deep e x p l o r a t i o n and d r i l l i n g t e c h n o l o g y w i l l p e r m i t e x t e n s i o n o f t h e e f f e c t i v i t y g a i n s t o new deep gas d i s c o v e r i e s . A t l e a s t i n t h e USA, however, t h e e a r l i e s t l a r g e increment o f new s u p p l y i s c l e a r l y t h a t o b t a i n a b l e through improvement i n s t i m u l a t i o n t e c h n i q u e s o f b o t h s h a l l o w and deep t i g h t r e s e r v o i r s which w i l l make r e c o v e r y o f a g r e a t e r p o r t i o n o f these r e s o u r c e s p r o f i t a b l e a t c u r r e n t and p r o j e c t e d m a r k e t - c l e a r i n g p r i c e s , w i t h a g a i n h y d r a u l i c p r o p p a n t f r a c t u r i n g p l a y i n g a key r o l e i n t h i s s c e n a r i o .
2.2.1.6.4.6.
Gas market ins strategies
Concerning gas m a r k e t i n g s t r a t e g y , l e a s t - c o s t s e r v i c e o p t i o n s t o energy consumers a r e key elements o f t h e concept which has t o be designed t o achieve i t s e s s e n t i a l o b j e c t i v e s independent f r o m o i l p r i c e e v o l u t i o n and c o m p e t i t i o n o f gas w i t h o t h e r energy t y p e s (LINDEN 1985). A s t r a t e g y f l e x i b l e enough t o cope w i t h t h e u n c e r t a i n t i e s o f t h e w o r l d energy market must emphasize v a l u e o f s e r v i c e o v e r volume o f s e r v i c e , which i n t u r n demands c o n t i n u e d r a p i d development and c o m m e r c i a l i z a t i o n o f h i g h - e f f i c i e n c y end-use t e c h n o l o g i e s opening a wide spectrum f o r gas p e n e t r a t i o n . These t e c h n o l o g i e s a l s o have t o o f f e r a t t r a c t i v e i n v e s t m e n t o p p o r t u n i t i e s w i t h s h o r t payback times t o p r e s e n t and p r o s p e c t i v e gas u s e r s o r o u t s i d e i n v e s t o r s , even i f t h a t means temporary o r even permanent s h r i n k a g e o f s a l e s volume. Any downside e f f e c t on gas demand caused by t h e i n t r o d u c t i o n o f more e f f i c i e n t end-use t e c h n o l o g i e s w i l l be o f f s e t by t h e i n c r e a sed p r o b a b i l i t y o f keeping e x i s t i n g customers i n s t e a d o f l o s i n g them t o competit i v e energy s e r v i c e o p t i o n s , and t h e enhanced a b i l i t y t o f u r t h e r p e n e t r a t e i e x i s t n g markets w i t h r e l a t i v e l y cheap gas t h a t has been conserved, as w e l l as t o c a p t u r e new markets. Gas s u p p l y and m a r k e t i n g t r e n d s a r e a l s o s t u d i e d by BERGMANN ( 1 9 8 2 ) . T h e r e f o r e a m a r k e t i n g s t r a t e g y based on more e f f i c i e n t end-use i s a l s o a conc e p t f o r m a i n t a i n i n g gas i n d u s t r y p r o f i t a b i l i t y . C o s t - e f f e c t i v e end-use e f f i c i e n c y improvements i n c r e a s e r a t h e r t h a n decrease t h e m a r g i n between d e l i v e r e d c o s t o f gas and what i t i s w o r t h t o end users, because h i g h e r f i x e d c o s t s p e r u n i t s a l e a s s o c i a t e d w i t h l o w e r volumes w i l l be more t h a n compensated by t h e c o m b i n a t i o n o f l o w e r average a c q u i s i t i o n c o s t r e s u l t i n g f r o m s l o w e r d e p l e t i o n o f r e l a t i v e l y low m a r g i n a l c o s t sources o f supply, and h i g h e r break-even b u r n e r t i p p r i c e o r f o r m v a l u e coming f r o m u s i n g l e s s gas t o p r o v i d e t h e same o r even h i g h e r - q u a l i t y energy s e r v i c e i n improved equipment whose h i g h e r f i r s t c o s t i s p a i d back q u i c k l y . The e x i s t i n g f i x e d i n v e s t m e n t can t h e r e f o r e be j u s t i f i e d on t h e b a s i s o f h i g h e r net-added v a l u e o f t h e energy s e r v i c e s p r o v i d e d r a t h e r than m e r e l y r e l a t e d t o t h e volume o f gas s o l d . The c o m p e t i t i v e p o s i t i o n o f gas i s f u r t h e r improved by p r o g r e s s i v e c o n s e r v a t i o n and c o g e n e r a t i o n by c o s t - e f f e c t i v e e f f i c i e n c y ameliorations. M a r k e t i n g and economical aspects o f gas sources and s u p p l i e s a r e a l s o d i s c u s sed by AL-FARISS (1988), JENSEN (1988), KALISCH (1988), NAYLOR & FERRIES (1988) and SALAMA ( 1 9 8 8 ) . Comments on t h e f u t u r e market f o r n a t u r a l gas and i t s e f f e c t on e x p l o r a t i o n a r e a l s o g i v e n by GRUB (1984). MAY & WHITE (1981) i n v e s t i g a t e problems and p r o s p e c t s o f n a t u r a l gas p r i c e r e g u l a t i o n .
2.2.2. Influences o f proppant price changes A p a r t f r o m o i l p r i c e e v o l u t i o n ( c f . s e c t i o n 2.2.1.1.) and development o f t h e US $ exchange r a t e ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) , v a r i a t i o n s i n proppant p r i c e a l s o have s i g n i f i c a n t i n f l u e n c e on t h e economical f e a s i b i l i t y o f h y d r a u l i c f r a c t u -
160 r i n g i n t h e a c t u a l o i l and gas economical s c e n a r i o and f i n a n c i a l c o n s t e l l a t i o n , and t h u s have i m p o r t a n t impact on t h e m a r k e t i n g p o t e n t i a l o f r e s e r v o i r s t i m u l a t i o n treatments . Changes o f p r o p p a n t p r i c e l e v e l tqok a l s o p l a c e almost p a r a l l e l t o m a j o r changes i n t h e hydrocarbon p r i c e n i v e a u and i n a l a t e r stage a l s o t o t h e US $ v a l u e f l u c t u a t i o n s . I n t h e l a t e 1970's and t h e e a r l y 1980.5, l a r g e p r o p p a n t demand which t e m p o r a r i l y even c o u l d n o t be met by s u p p l y k e p t p r o p p a n t p r i c e s h i g h , w i t h t h e s t r o n g t o p a r t i a l l y even e x c e s s i v e demand h a v i n g been n o t o n l y a consequence o f t h e f a v o u r a b l e g e n e r a l economical s i t u a t i o n d u r i n g t h e g o l d e n age o f h y d r a u l i c f r a c t u r i n g , b u t r e s u l t i n g a l s o f r o m t h e s a t i s f a c t i o n o f t h e consumers t h a t a f t e r a l o n g t i m e o f f r u s t r a t i o n , e x p e c t a t i o n and hope now f i n a l l y s u i t a b l e proppants t h a t c o u l d w i t h s t a n d t h e h i g h c l o s u r e s t r e s s e s i n deep t i g h t r e s e r v o i r s were a v a i l a b l e and t h u s a l l o w e d t o c a r r y o u t o p e r a t i o n s which f o r m e r l y had t o be suspended due t o l a c k o f adequate proppants t h a t would have r e s i s t e d t o c r u s h i n g and t h u s c o l l a p s e o f t h e h y d r a u l i c a l l y - g e n e r a t e d f r a c t u r e i n g r e a t e r depth. W h i l e no changes o f p r o p p a n t p r i c e s were necessary i n t h e g o l den age o f h y d r a u l i c f r a c t u r i n g , t h e o i l p r i c e d e c l i n e b e g i n n i n g i n t h e e a r l y 1980's c r e a t e d another s i t u a t i o n . Proppant p r i c e e v o l u t i o n i n t h e l a s t f i v e y e a r s i n c l u d e s t h e 1985 p r o p p a n t p r i c e l o w e r i n g and t h e 1987 p r o p p a n t p r i c e i n c r e a s e . Some s p e c i a l impacts o f p r o p p a n t p r i c e changes on c o m p e t i t i o n o f v a r i o u s proppant t y p e s a r e a l s o i l l u strated.
2.2.2.1. 1985 proppant price lower ins The e x p e c t a t i o n s f r o m and t h e consequences o f t h e 1985 p r o p p a n t p r i c e 1oweri n g a r e e x p l a i n e d along t h e l i n e s o f m a r k e t i n g c o n s i d e r a t i o n s and econom c a l r e price s u l t s . Aspects o f s u p e r i m p o s i t i o n and a c c e n t u a t i o n o f t h e 1985 proppan l o w e r i n g by t h e U S $ exchange r a t e drop a r e a l s o o u t l i n e d .
2.2.2.1.1. Marketing considerations f o r 1985 proppant price lowering The main m a r k e t i n g c o n s i d e r a t i o n s f o r t h e 1985 p r o p p a n t p r i c e l o w e r ng were p r o m o t i o n o f h y d r a u l i c f r a c t u r i n g a c t i v i t y as w e l l as c a p t u r e o f m a r k e t share f r o m o r even e l i m i n a t i o n o f C o m p e t i t i o n . When t h e hydrocarbon p r i c e s s t a r t e d t o d e c l i n e s l o w l y a c o u p l e o f y e a r s ago ( b e i n g t h e p r e c e d i n g s t e p s o f t h e anyway unexpected b i g d r o p t h a t took p l a c e i n e a r l y 1986, b u t b e i n g p a r t i a l l y a l s o t h e consequence o f t h e i n c r e a s e o f t h e US $ exchange r a t e w i t h r e s p e c t t o o t h e r w o r l d guide c u r r e n c i e s such as DM and t i n t h e e a r l y 1980's s i n c e i t s f i r s t minimum l e v e l i n t h e l a t e 1970's). c o m p e t i t i o n o f t h e p r o p p a n t companies manuf a c t u r i n g alumina o x i d e and s i i i c a t e ' proppants as a r e s u l t o f d i m i n i s h i n g b o t h a b s o l u t e and r e l a t i v e m a r k e t shares, and customer demand r e q u i r e d enhanced qual i t y , b e t t e r a v a i l a b i l i t y and l o w e r p r i c e s o f t h e v a r i o u s t y p e s o f p r o p p a n t s o f f e r e d a t t h e hydrocarbon s t i m u l a t i o n market. Another i d e a was t o i n c r e a s e t h e c o m p e t i t i v e d i f f e r e n c e t o z i r c o n i a - s i l i c a t e proppants t h a t a r e s o l d i n c u r r e n c y o t h e r than U S $ and which moved i n t o a worse f e a s i b i l i t y p o s i t i o n by l o o s i n g governmental financial s u p p o r t f o r p r o d u c t i o n and m a r k e t i n g ( c f . s e c t i o n 2.2.2.3.). Thus i n c o m b i n a t i o n w i t h t h e i n s t a l l a t i o n o f new m a n u f a c t u r i n g c a p a c i t i e s by c o n s t r u c t i o n o f new p l a n t s and e x t e n s i o n o f e x i s t i n g f a c i l i t i e s i n o r d e r t o be a b l e t o f u l f i l l t h e expected demand when l o w e r i n g t h e p r i c e , t h e t r a d i t i o n a l m a r k e t i n g concepts were r e v i s e d and some p a r t i e s i n i t i a t e d s e v e r a l p u l s a t o r y s t e p s o f f a l l i n g p r i c e s f o r alumina o x i d e and s i l i c a t e p r o p p a n t s t o f i n a l l y a b t . h a l f i t s o r i g i n a l l e v e l i n m i d t o l a t e 1985, which i n c l u d i n g i n f l a t i o n means even almost r e d u c t i o n t o a b t . a q u a r t e r o f t h e f o r m e r p r i c e , whereas t h e p r i c e f o r z i r c o n i a - s i l i c a t e p r o p p a n t s was k e p t more o r l e s s a t i t s e l e v a t e d po-
161 s i t i o n o r even i n c r e a s e d due t o t h e t e r m i n a t i o n o f governmental support.
protection
and
2.2.2.1.2. Superimpos tion by the US S exchange rate drop F o r t h e European customers, t h e r e d u c t i o n o f p r i c e s f o r alumina o x i d e and s i l i c a t e proppants was e s p e c i a l l y i n 1986 and e a r l y t o m i d 1987 even a c c e n t u a t e d by t h e d e c l i n e o f t h e US $ exchange r a t e w i t h r e s p e c t t o s t r o n g European g u i d e c u r r e n c i e s such as DM and L t h a t a f t e r a c h i e v i n g i t s peak n i v e a u i n t h e v e r y e a r l y 1980's s l o w l y s t a r t e d t o go down i n t h e e a r l y t o m i d 1980's and t h e n accel e r a t e d t o f a l l f r o m l a t e 1985 onwards t o r e a c h i n l a t e 1 9 8 6 / e a r l y 1987 f o r t h e second t i m e i n t h e f i n a n c e p o l i t i c a l h i s t o r y a f t e r t h e Second World War i t s r e l a t i v e minimum l e v e l ( a n d l a t e r i n t h e f o u r t h q u a r t e r o f 1987 even a c h i e v i n g i t s h i s t o r i c a l a b s o l u t e minimum n i v e a u ) . The f a v o u r a b l e impact o f t h e US $ exchange r a t e d r o p on t h e p r o p p a n t p r i c e f o r t h e European customers b e i n g hydrocarbon p r o d u c t i o n companies a t t h e b o t t o m o f t h e l i n e , however, was more than compensated by i t s d i s a s t r o u s consequence on t h e o i l p r i c e w h i c h r e p r e s e n t s a f u r t h e r d e v a l u a t i o n and t h u s i n c r e a s i n g d e t e r i o r a t i o n o f t h e g e n e r a l economical s i t u a t i o n c o m p l e t e l y independent f r o m t h e o i l p r i c e c r a s h i t s e l f ( c f . s e c t i o n 2.2.1.3.). Given t h e s c e n a r i o o f h i g h o i l p r i c e s ( i n c o m b i n a t i o n w i t h a h i g h US $ exchange r a t e ) i n t h e e a r l y 1980's, t h i s d r a s t i c a l r e d u c t i o n o f p r o p p a n t p r i c e s ( f o r Europe f a v o u r a b l y combined w i t h t h e drop o f t h e US $ exchange r a t e ) c o u l d r e a l l y have t r i g g e r e d t h e b e g i n n i n g o f t h e g r e a t e s t h y d r a u l i c f r a c t u r i n g camp a i g n which o i l and gas i n d u s t r y had e v e r seen, because p a r t i c u l a r l y i n deep t i g h t gas r e s e r v o i r s t i m u l a t i o n , t h e p r o p p a n t c o s t may r e a c h up t o two t h i r d s o f t h e t o t a l expenses o f t h e t r e a t m e n t and an a b t . 50 - 75 % r e d u c t i o n o f p r o p p a n t c o s t ( o r more i f i n f l a t i o n and d e c l i n e o f US $ v a l u e a r e p r o p e r l y i n c l u ded) would have been q u i t e an a m e l i o r a t i o n o f t h e economical f e a s i b i l i t y o f t h e whole j o b and would have enabled t h e e x e c u t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n such f o r m a t i o n s o f more o r l e s s m a r g i n a l t y p e where i t s a p p l i c a t i o n was f o r m e r l y f a r f r o m b e i n g commercial ( f o r example t h e c u m u l a t i v e c o s t o f a MHF operat i o n i n a R o t l i e g e n d w e l l i n t h e S o h l i n g e n field/Germany FRG i n 1982 where a b t . 550 t o f p r o p p a n t s had been pumped amounted up t o 6 Mio. DM; JOHN 1983, KLOSE & KRUMER 1983, LEICHT 1985; c f . s e c t i o n s 2.4.1.1. and 3 . 3 . ) . Many p r o p p a n t end u s e r s t h e r e f o r e h e a r t i l y welcomed t h e r e d u c t i o n o f p r i c e s f o r alumina o x i d e and s i l i c a t e proppants as a v e r y s u i t a b l e o p p o r t u n i t y t o cons i d e r now more w e l l s f o r h y d r a u l i c p r o p p a n t f r a c t u r i n g t h a n e v e r b e f o r e , because i n t h e p a s t t h e h i g h e r p r o p p a n t c o s t rendered many h i g h - p o t e n t i a l i t e m s m a r g i n a l l y economical and p a r t i c u l a r l y f r i g h t e n e d s m a l l e r o p e r a t i n g companies w i t h l i m i t e d f i n a n c i a l p o s s i b i l i t i e s , as a consequence o f t h e f a c t t h a t f o r European customers and t h e i r way o f understanding, f r a c t u r i n g s t i m u l a t i o n o f a w e l l i s o f t e n n o t p a r t o f t h e whole i n v e s t m e n t f o r d r i l l i n g and t e s t i n g , b u t i s something a d d i t i o n a l and o p t i o n a l ( c f . s e c t i o n 2 . 3 . 4 . ) . The economical r e s u l t o f t h e p r o p p a n t p r i c e r e d u c t i o n s , however, was n o t m e e t i n g t h e e x p e c t a t i o n s due t o t h e d i s t u r b i n g e f f e c t o f t h e o i l p r i c e c o l l a p s e which t o o k p l a c e c o m p l e t e l y independent f r o m b o t h US $ exchange r a t e d r o p and p r o p p a n t p r i c e decrease,
2.2.2.1.3.
Economical results of 1985 proppant price decline
U n f o r t u n a t e l y , however, s h o r t l y a f t e r t h e enormous r e d u c t i o n o f t h e p r i c e s f o r alumina o x i d e and s i l i c a t e proppants enacted by some m a r k e t - l e a d e r - t y p e p r o p p a n t companies came i n t o f o r c e i n m i d t o l a t e 1985, t h e o i l and gas market was c o n s i d e r a b l y d i s t u r b e d i n e a r l y 1986 by becoming f l o o d e d w i t h v a s t q u a n t i t i e s o f o i l d e r i v i n g f r o m u n c o o r d i n a t e d and v i o l a t e d o v e r p r o d u c t i o n o f r i v a l i z i n g OPEC c o u n t r i e s ( t h e o i l o v e r s u p p l y was p a r t i a l l y a l s o t h e consequence o f t h e I r a n i a n - I r a q i a n war and of t h e desperate a t t e m p t s o f Saudi A r a b i a t o g e t t h e o t h e r OPEC c o u n t r i e s under c o n t r o l and t o reason, u n i t y and r e l i a b i l i t y ;
162 c f . s e c t i o n 2.2.1.1.4. and 2.2.1.5.) and r e s u l t i n g i n such a t e r r i b l e l o w e r i n g o f t h e o i l p r i c e t h a t f o r most o f t h e hydrocarbon p r o d u c t i o n companies s e r i o u s c u t t i n g back o f t h e c a p i t a l e x p e n d i t u r e budget i n c l u d i n g suspension o f almost a l l t h e s t i m u l a t i o n j o b s was t h e o n l y answer which was e c o n o m i c a l l y sound as an acceptable s o l u t i o n . Coupled w i t h a d r a s t i c a l f a l l o f t h e v a l u e o f t h e US $ which s t a r t e d t o go down t o r e a c h f o r t h e second t i m e i n t h e post-Second-World-War h i s t o r y a l o w e r exchange r a t e ( f o r example i n r e l a t i o n t o DM and t ) t h a n e v e r b e f o r e , o i l consumers c o u l d make d o u b l e p r o f i t o f t h e o v e r s u p p l y and p r i c e c o l l a p s e s i t u a t i o n by b u y i n g a l m o s t u n l i m i t e d q u a n t i t i e s f o r cheap p r i c e s , b u t t h e p r o d u c t i o n companies were s e r i o u s l y h i t i n a t w o - f o l d manner, w i t h a few s m a l l e r operat i o n s b e i n g now r e l a t i v e l y ( w i t h r e s p e c t t o cash b a c k f l o w ) much more e x p e n s i v e and t h e r e f o r e much l e s s e c o n o m i c a l l y f e a s i b l e t h a n s e v e r a l l a r g e r j o b s b e f o r e . Some aspects o f r e v e r s a l o f f r a c t u r i n g p o t e n t i a l and p r o p p a n t a v a i l a b i l i t y as w e l l as c a p i t a l e x p e n d i t u r e c u t t i n g and s t i m u l a t i o n j o b suspension a r e o u t l i n e d as f o l l o w s .
2.2.2.1.3.1. Reversal o f fracturing potential and proppant availability The p a r t i c u l a r t r a g e d y f o r p r o p p a n t and s e r v i c e companies i s t h a t d u r i n g t h e golden y e a r s o f hydrocarbon d r i l l i n g and s t i m u l a t i o n o f t h e l a t e 1970's and e a r l y 1980's, abundant f r a c t u r e j o b s were c a r r i e d o u t a t h i g h p r o p p a n t and s e r v i c e p r i c e s (backed-up by h i g h o i l p r i c e s and s t r o n g US $ g u a r a n t e i n g f a s t p a y - o u t o f i n v e s t m e n t f o r t h e hydrocarbon p r o d u c t i o n companies), whereas s i n c e t h e maj o r o i l p r i c e d e c l i n e i n e a r l y 1986 u n t i l a t l e a s t e a r l y t o m i d 1987, a n o t h e r s i t u a t i o n has a r r i v e d . F o l l o w i n g p r o p p a n t p r i c e r e d u c t i o n , o i l p r i c e c r a s h ( c f . section 2.2.1.1.2.) and US $ exchange r a t e drop ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) , propp a n t s and - due t o i n e v i t a b l e a d j u s t m e n t o f t h e s e r v i c e companies ( c f . s e c t i o n - s e r v i c e a r e cheaper t h a n e v e r b e f o r e and p r o p p a n t s a r e nowadays 2.2.1.5.3.) r e a d i l y a v a i l a b l e i n almost u n l i m i t e d q u a n t i t y and even enhanced q u a l i t y w i t h r e s p e c t t o t h a t a c o u p l e o f y e a r s ago when p r o p p a n t demand was so l a r g e t h a t t h e m a n u f a c t u r i n g companies had t o produce a t maximum c a p a c i t y , and no t i m e was l e f t f o r q u a l i t y improvement and e f f o r t s were o n l y made t o i n c r e a s e t h e o u t p u t given a constant q u a l i t y .
A s a consequence o f t h e 1985 p r o p p a n t p r i c e c u t s and t h e 1986 o i l p r i c e drop, a much h i g h e r - q u a l i t y p r o p p a n t m a t e r i a l i s now a v a i l a b l e i n s i g n i f i c a n t l y l a r g e r q u a n t i t i e s a t a c o n s i d e r a b l y l o w e r p r i c e t h a n some y e a r s ago, b u t i s ser i o u s l y l i m i t e d i n a p p l i c a t i o n due t o a p r e s e n t l y p o o r economical framework p a r t i c u l a r l y i n Europe, whereas some y e a r s ago, a much l o w e r - q u a l i t y p r o p p a n t mater i a l t h a t was o f f e r e d a t a markedly h i g h e r p r i c e t h a n now was v i r t u a l l y u n r e s t r i c t e d i n d i s t r i b u t i o n due t o a s t r o n g demand exceeding t h e p r e v i o u s c a p a c i t i e s o f s u p p l y . T h e r e f o r e t h e 1986 o i l p r i c e c o l l a p s e c o m p l e t e l y r e v e r s e d t h e s t i m u l a t i o n m a r k e t by changing t h e s i t u a t i o n f r o m good t o e x c e l l e n t f e a s i b i l i t y o f h y d r a u l i c f r a c t u r i n g when o n l y i n s u f f i c i e n t amounts o f h i g h - q u a l i t y p r o p p a n t s were a v a i l a b l e a t h i g h p r i c e s t o p o o r a t t r a c t i v i t y o f h y d r a u l i c f r a c t u r i n g when more t h a n enough p r o p p a n t m a t e r i a l i s a v a i l a b l e a t l o w p r i c e s .
2.2.2.1.3.2. Capita 1 expenditure cut t ins and stimulation job suspension I n 1986 and p a r t i a l l y even s t i l l i n e a r l y 1987, t h e bad g e n e r a l economical s i t u a t i o n o f an o i l p r i c e approaching almost t h e l e v e l around o r even below t h e f i r s t c l a s s i c a l p r i c e shock a b t . 15 y e a r s ago ( a t l e a s t i f p r o g r e s s i o n o f i n f l a t i o n s i n c e t h a t t i m e and t h e p r e s e n t low U S $ v a l u e a r e i n c o r p o r a t e d i n t o t h e c a l c u l a t i o n ; c f . s e c t i o n 2.2.1.1.2.) d u r i n g t h e w o r s t phase and b e i n g i n Europe even accentuated by t h e d e c l i n e of t h e US $ exchange r a t e ( w i t h a l l t h e e f f e c t s
163 t o g e t h e r t r i g g e r i n g i n e a r l y 1986 w i t h i n o n l y a few weeks a r e c u r r e n t i n c i s i o n i n t o t h e o i l p r i c e e v o l u t i o n t o a l e v e l f a r back i n post-Second-World-War hist o r y ) , however, c o u l d even g i v e n t h e mentioned proppant- (and a l s o s e r v i c e - ) r e l a t e d f a c t s n o t o n l y n o t j u s t i f y an expansion o f t h e s t i m u l a t i o n a c t i v i t y as an answer t o t h e c o n s i d e r a b l y lowered proppant and s e r v i c e c o s t , b u t a l s o even n o t p r o v i d e any c o n v i n c i n g arguments f o r a t l e a s t m a i n t a i n i n g t h e p r e v i o u s l e v e l o f f r a c t u r e t r e a t m e n t s , w i t h most o f t h e hydrocarbon p r o d u c t i o n companies t h u s c u t t i n g down t h e i r budget o f c a p i t a l e x p e n d i t u r e by a t l e a s t 25 % (OEHME 1986) and p o s t p o n i n g almost a l l t h e i r s t i m u l a t i o n a c t i v i t y f o r a t l e a s t one o r two y e a r s . The g e n e r a l s i t u a t i o n i s o n l y r e a s o n a b l y a m e l i o r a t i n g i n t h e m a j o r c e n t r e s o f a c t i v i t y s i n c e a b t . m i d 1987, whereas i n o t h e r common and m a r g i n a l areas, much o f t h e p r o m i s i n g p i c k i n g - u p was i n l a t e 1987 a g a i n s e r i o u s l y d i s t u r b e d by t h e c o l l a p s e o f t h e US $ exchange r a t e down t o a l e v e l which had never been r e a ched b e f o r e ( c f . s e c t i o n 2.2.1.3.), and i n e a r l y t o m i d 1988 by an a g a i n l i t t l e s l i d e o f t h e o i l p r i c e which a l t h o u g h b e i n g almost n e g l i g i b l e i n i t s range, nev e r t h e l e s s h i t a v e r y s e n s i b l e i n d u s t r y t h a t i n t e r p r e t e d t h e f l u c t u a t i o n beyond t h e average v a r i a t i o n s as a market s i g n a l and became again c o n s i d e r a b l y uncert a i n c o n c e r n i n g f u r t h e r i n v e s t m e n t d e c i s i o n s . The s e r i o s i t y o f t h e ongoing c r i s i s c o n s t e l l a t i o n i s h i g h l i g h t e d by t h e a l a r m which t h e OPEC group addressed a l s o t o v a r i o u s non-members f o r a desperate t r i a l t o s t o p f u r t h e r downwards o r i e n t a t i o n o f t h e o i l p r i c e ( c f . s e c t i o n 2.2.1.4.3.).
2.2.2.2.
1987
proppant price increase
As t h e 1985 p r o p p a n t p r i c e l o w e r i n g had n o t t h e expected f a v o u r a b l e impact on t h e hydrocarbon e x p l o r a t i o n and p r o d u c t i o n i n d u s t r y by t r i g g e r i n g a renewed boom o f s t i m u l a t i o n a c t i v i t y as a consequence o f market d e t e r i o r a t i o n by t h e 1986 o i l p r i c e crash, t h e p r o p p a n t s u p p l i e r s were a l r e a d y a n x i o u s l y w a i t i n g f o r t h e e a r l i e s t p o s s i b i l i t y t o r a i s e t h e p r i c e again i n o r d e r t o b o t h compensate t h e business l o s s e s d u r i n g t h e c r i s i s i n i t i a t e d by o i l p r i c e c o l l a p s e and US $ exchange r a t e d r o p and provoked s t i m u l a t i o n l e v e l d e c l i n e , and i n c r e a s e t h e i r p r o f i t i n a t i m e o f renewed p i c k i n g - u p o f t r e a t m e n t a c t i v i t y and r i s i n g p r o p p a n t consumption. The o u t l i n e as f o l l o w s i n c l u d e s comments on t h e r e a s o n i n g f o r renewed p r o p p a n t p r i c e i n c r e a s e s , and m a r k e t r e a c t i o n and economical consequences o f t h e 1987 p r o p p a n t p r i c e i n c r e a s e .
2.2.2.2.1.
Reasoning f o r renewed proppant price increases
Independent f r o m a l l economical i n f l u e n c e s , t h e f u t u r e i s c h a r a c t e r zed by need and urgency f o r u n c o n v e n t i o n a l gas (KUUSKRAA 1980) w h i c h a u t o m a t i c a l y p r o mises a t l e a s t some booming o f h y d r a u l i c proppant f r a c t u r i n g . E f f e c t s o price and technology on gas r e s o u r c e s i n l o w - p e r m e a b i l i t y r e s e r v o i r s a r e a l s o d i s c u s sed by BAKER ( 1 9 8 1 ) . SCHAEFER & POWERS (1984) comment on t h e imDact o f r a p i d l y changjng economical environment on p e t r o l e u m e x p l o r a t i o n and' a c q u i s i t i o n . E f f e c t s o f i n c r e a s i n g gas p r i c e s on known gas r e s e r v e s a r e o u t l i n e d by H I C K S & RAHMAN ( 1 9 8 4 ) . Concerning p r o p p a n t p r i c e e v o l u t i o n , p r e p a r a t i o n o f t h e s u p p l y companies f o r t h e coming b i g demand o f m a t e r i a l d u r i n g course o f t h e R o t l i e g e n d gas f i e l d development campaigns i n t h e B r i t i s h Southern N o r t h Sea i n 1987 - 1990 i n Europe ( c f . s e c t i o n s 2.2.1.6.2., 2.4.1.2. and 2.4.5.1.) as w e l l as t h e a d a p t a t i o n t o t h e w o r l d - w i d e p i c k i n g - u p of h y d r a u l i c proppant f r a c t u r i n g i n c l u d e d more o r l e s s c a r e f u l l y planned and t i m e d a d j u s t m e n t o f t h e p r o p p a n t p r i c e s t o t h e expect e d m a j o r businesses and p r o f i t s . I n o r d e r t o o p t i m i z e t h e i r n e t r e t u r n d u r i n g t h i s p r o b a b l y h i s t o r i c a l h i g h l i g h t o f h y d r a u l i c r e s e r v o i r s t i m u l a t i o n i n Europe as w e l l as m a x i m i z i n g t h e payback w i t h p r o g r e s s i v e l y i m p r o v i n g b u s i n e s s perspect i v e s ( t h e r e b y a l s o p a r t i a l l y compensating t h e income l o s s e s generated by t h e 1985 proppant p r i c e l o w e r i n g w h i c h had n o t t h e expected impact on s a l e s volumes due t o t h e s h o r t l y a f t e r w a r d s f o l l o w i n g 1986 o i l p r i c e c r a s h ) , some p a r t i e s i n
164 m i d t o l a t e 1987 r e v e r s e d t h e i r 1985 p r i c e d r o p p o l i c y and i n c r e a s e d t h e p r i c e s again s i g n i f i c a n t l y , w i t h o u t , however, r e a c h i n g t h e l e v e l h e l d i n 1985 b e f o r e the i n i t i a t i o n o f the p r i c e f a l l .
2.2.2.2. Market reaction and economical consequences o f the 1987 price increase W h i l e some s l i g h t t o moderate i n c r e a s e i n p r o p p a n t p r i c e would c e r t a i n l y have been more than j u s t i f i e d i n o r d e r t o l e t a l s o proppant and s e r v i c e compan i e s s u r v i v e and r e c o n s o l i d a t e , t h e market has r e a c t e d w i t h s e r i o u s b u s i n e s s l o s s e s f o r some p a r t i e s which showed l i m i t e d f l e x i b i l i t y i n p r i c i n g t o m a j o r customers and t r i e d t o p l a y t h e c a r d o f i n c r e a s e d p r i c e s even f o r c o n t r a c t s where b i d d i n g was made a c c o r d i n g t o t h e e a r l i e r p r i c i n g c o n c e p t d u r i n g t h e t r a n s i t i o n a l phase between o l d and new l i s t p r i c e s , and a few m a j o r d e a l s went t o c o m p e t i t o r s thus p u n i s h i n g and d i s a d v a n t a g i n g some o f t h e i n i t i a t o r s o f t h e p r i c e r i s e which d i d n o t want t o a d j u s t t o t h e demand o f t h e market o r s i m p l y d i d n o t understand t h e European market and i t s s p e c i a l f e a t u r e s ( c f . a l s o sect i o n 2.4.2.4.1.). view o f t h e expected demand o f a n n u a l l y between 5 and 10 m i l l i o n l b s . - 4,600 t ) o f i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s f o r t h e p e r i o d o f t h r e e o r f o u r y e a r s i n Europe ( c o m p r i s i n g c h i e f l y q u a n t i t i e s needed f o r R o t l i e g e n d f r a c t u r i n g i n t h e B r i t i s h Southern N o r t h Sea), even a p r i c e i n c r e a s e o f a c o u p l e of c e n t s r e s u l t s i n case o f more o r l e s s c o o r d i n a t i o n w i t h t h e c o m p e t i t i o n i n considerable amelioration o f the desolate t o c r i t i c a l f i n a n c i a l s i t u a t i o n o f the proppant companies which more o r l e s s made s e r i o u s l o s s e s d u r i n g t h e 1986 c r i s i s . The l a t e 1987 p r o p p a n t p r i c e i n c r e a s e was subsequently i n e a r l y t o m i d 1988 from t h e view o f t h e European customers even accentuated by an a g a i n s l i g h t l y s t a b i l i z i n g US $ exchange r a t e . In
(‘2,300
2 . 2 . 2 . 3 . Comet i t ion o f various proppant types The v a r i o u s p r i c e drops and r i s e s f o r alumina o x i d e and s i l i c a t e proppants had a l s o s e r i o u s impacts on t h e i r c o m p e t i t i v e power w i t h r e s p e c t t o z i r c o n i a - s i l i c a t e proppants which k e p t almost a l l t h e t i m e t h e i r f o r m e r e l e v a t e d p r i c e pos i t i o n due t o more expensive p r o d u c t i o n process, and which a r e manufactured i n Europe and s o l d f o r c u r r e n c i e s o t h e r than US $, w i t h t h u s t h e i r p r i c i n g i n Europe and o t h e r n o n - d o l l a r markets o f t h e w o r l d n o t b e i n g i n f l u e n c e d by v a r i a t i o n s o f t h e US $ exchange r a t e i n e i t h e r d i r e c t i o n . The f o l l o w i n g o u t l i n e f o cusses on some aspects o f consequences o f t h e 1985 p r i c e r e d u c t i o n and t h e 1987 p r i c e increase f o r the competitiveness o f the d i f f e r e n t proppant types.
2.2.2.3.1. Influence o f the 1985 proppant price reduction While b e f o r e t h e f i r s t p r o p p a n t p r i c e r e d u c t i o n i n m i d 1985 c o m p e t i t i o n had t o be made through performance and s e r v i c e a t a l e v e l o f more o r l e s s comparabl e p r i c e s f o r a l l t h e d i f f e r e n t p r o d u c t s , w i t h o u t anyone b e i n g s i g n i f i c a n t l y cheaper t o a l l o w a p p l i c a t i o n of t h e p r i c e i n s t r u m e n t as a p o w e r f u l m a r k e t i n g and s a l e s t o o l , t h e p r i c e drops f o r alumina o x i d e and s i l i c a t e proppants have moved them i n t o a v e r y advantageous p r i c e s i t u a t i o n w i t h r e s p e c t t o z i r c o n i a - s i l i c a t e proppants.
As a consequence o f t h e f a c t t h a t due t o t h e p r i c e c u t s , t h e c o s t f o r a l u m i na o x i d e and s i l i c a t e proppants was f i n a l l y o n l y between two t h i r d s and h a l f t h e p r i c e f o r z i r c o n i a - s i l i c a t e proppants, t h e l a t t e r p r o d u c t g o t almost unmark e t a b l e i n Europe and l o s t c o n s i d e r a b l e shares. T h i s e f f e c t was even accentuat e d by t h e s t a r t i n g d e c l i n e o f t h e US $ exchange r a t e which made alumina s i l i c a t e and o x i d e proppants i n Europe a g a i n cheaper, and by t e r m i n a t i o n o f government a l f i n a n c i a l s u p p o r t f o r m a n u f a c t u r i n g and d i s t r i b u t i o n o f z i r c o n i a - s i l i c a t e
165 proppants t h a t rendered t h e i r p r i c i n g even more tough w i t h l e a v i n g even l e s s room f o r downwards adjustments t h a n b e f o r e . D u r i n g t h e t i m e s i n c e m i d 1985 and p a r t i c u l a r l y a f t e r t h e o i l p r i c e c r a s h i n e a r l y 1986, a l m o s t no customer was w i l l i n g t o pay such a h i g h premium f o r z i r c o n i a - s i l i c a t e p r o p p a n t s and t h u s t h e i r a p p l i c a t i o n was i n Europe v i r t u a l l y l i m i t e d t o a few t a i l - i n o p e r a t i o n s i n o r d e r t o make b e n e f i t o f t h e p o s s i b i l i t y t o p e r f o r m gamma-ray f r a c t u r e h e i g h t m o n i t o r i n g by l o g g i n g t h e n a t u r a l r a d i o a c t i v i t y o f t h e z i r c o n i a - s i l i c a t e proppants ( c f . s e c t i o n 6 . 2 . 1 . 2 . 1 . ) . T h e r e f o r e t h e companies m a n u f a c t u r i n g and s e l l i n g alumina o x i d e and s i l i c a t e proppants had a b i g advantage f r o m t h e i r p r i c e drop by n e a r l y c o m p l e t e l y e l i m i n a t i n g a s e r i o u s c o m p e t i t o r w i t h whom t h e y had t o share t h e m a r k e t i n f o r m e r years, b u t who now p a r t i c u l a r l y a f t e r t h e o i l p r i c e c o l l a p s e when c o s t c o n t a i n ment advanced t o a t o p - p r i o r i t y i t e m i n most o f t h e o p e r a t i n g companies had a l most no arguments l e f t t o convince c l i e n t s t o t a k e i t s p r o d u c t . The c o m p e t i t i v e d i f f e r e n c e between alumina o x i d e and s i l i c a t e proppants and z i r c o n i a - s i l i c a t e proppants was f o r European customers i n 1986 and e a r l y 1987 even i n c r e a s e d by t h e f a l l i n g US $ exchange r a t e down t o i t s r e l a t i v e minimum i n post-Second World War h i s t o r y , t h e r e b y p r o v i d i n g a n o t h e r proppant p r i c e c u t beyond t h e i n fluence o f the s u p p l i e r o f the m a t e r i a l .
2.2.2.3.2. Influence of the 1987 proppant price increase The l a t e 1987 p r o p p a n t p r i c e i n c r e a s e , however, was a l r e a d y a s t e p i n backwards d i r e c t i o n . A l t h o u g h even a f t e r t h e r e v e r s e p r i c e a d j u s t m e n t i n o r d e r t o a m e l i o r a t e t h e n e t r e t u r n s i t u a t i o n , t h e d i f f e r e n c e i n c o s t between alumina o x i d e and s i l i c a t e p r o p p a n t s and z i r c o n i a - s i l i c a t e p r o p p a n t s i s s t i l l s u f f i c i e n t t o remain a p o w e r f u l m a r k e t i n g t o o l , t h e danger i s t h e r e t h a t a n o t h e r p r i c e r i s e f o r alumina o x i d e and s i l i c a t e proppants c o u l d t u r n t h e m a r k e t i n t o another d i r e c t i o n . The producers o f z i r c o n i a - s i l i c a t e proppants, however, have l i t t l e i f any p o s s i b i l i t i e s o f a d j u s t m e n t by l o w e r i n g t h e i r p r i c e s , because t h e process o f making f u s e d proppants w i t h r e s p e c t t o s i n t e r e d ones i s c o n s i d e r a b l y more expensive and t h e European p r o p p a n t company p r o d u c i n g z i r c o n i a - s i l i c a t e proppants c o u l d n o t b e n e f i t f r o m t h e a u t o m a t i c a l a d d i t i o n a l p r i c e decrease t h a t was p r o v i d e d by t h e d e c l i n e o f t h e US $ exchange r a t e . A c c o r d i n g t o t h e s i t u a t i o n i n e a r l y t o m i d 1988 when a s l i g h t l y a g a i n s t a b i l i z i n g US $ exchange r a t e performed a n o t h e r m i n o r p r i c e r i s e f o r alumina o x i d e and s i l i c a t e proppants independent f r o m any i n t e n t i o n s o f t h e i r supp l i e r s , t h e p r o d u c e r s o f t h e l a t t e r m a t e r i a l s have t o be warned t h a t a n o t h e r company-induced p r i c e i n c r e a s e would b r i n g t h e z i r c o n i a - s i l i c a t e p r o p p a n t s back i n t o t h e i r c o m p e t i t i v e p o s i t i o n w h i c h t h e y had p r i o r t o t h e m i d 1985 p r o p p a n t p r i c e drops and t h e n t h e expected improvement o f n e t r e t u r n s f o r alumina o x i d e and s i l i c a t e proppants c o u l d be more t h a n compensated by c o n s i d e r a b l e b u s i n e s s l o s s e s t o z i r c o n i a - s i l i c a t e p r o p p a n t s ( c f . a l s o s e c t i o n 2.3.6.3.). T h i s danger i s p a r t i c u l a r l y s e r i o u s because a f u r t h e r r e c o v e r i n g US $ exchange r a t e c o u l d p r o v i d e an a u t o m a t i c a l p r i c e r i s e f o r alumina o x i d e and s i l i c a t e p r o p p a n t s a t t h e b o t t o m o f t h e l i n e f o r t h e European customers, w i t h t h e producers o f t h e l a t t e r m a t e r i a l s h a v i n g no i n f l u e n c e on an i n c r e a s i n g v a l u e o f t h e US $ e x c e p t o f once a g a i n l o w e r i n g t h e p r o p p a n t p r i c e i n Europe which then, however, would be c e r t a i n l y a t t h e expense o f c r e d i b i l i t y , c o n f i d e n c e and r e l i a b i l i t y t h a t t h e y r e c e i v e f r o m t h e i r customers. W h i l e t h e r e c o v e r i n g and s t r e n g t h e n i n g US $ i n m i d t o l a t e 1988 provoked a l ready a n o t h e r s t e p towards a p o s s i b l e renewed t u r n a r o u n d o f t h e p r o p p a n t mark e t , t h e a g a i n d e c l i n i n g power o f t h e US $ a f t e r t h e p r e s i d e n t i a l e l e c t i o n i n t h e USA a t t h e end o f 1988 r e l e a s e d a t l e a s t t e m p o r a r i l y t h i s p r e s s u r e and p r e vented t h e s u p p l i e r s o f alumina s i l i c a t e and o x i d e proppants f r o m r e g r e t t i n g t h e 1987 p r o p p a n t p r i c e i n c r e a s e . A l t h o u g h t h e f o r e c a s t o f t h e e v o l u t i o n o f t h e US $ exchange r a t e i n t h e v e r y n e a r f u t u r e does n o t i n c l u d e any s i g n i f i c a n t son t o f e a r t h e achievement o f such a c o n s t e l l a t i o n as sketched above, t h e whole i n -
166 t e r n a t i o n a l m u l t i l a t e r a l p o l i t i c a l and economical game c o n t a i n s t o o much u n c e r t a i n t i e s t h a t any p a r t y can f e e l t h e i r p o s i t i o n t o be s a f e , and changes a g a i n s t t h e p r o g n o s i s a r e always a b l e t o a r r i v e .
2.3. Stimulation market differences between USA and Europe W i t h r e s p e c t t o t h e USA market, t h e e a r l y t o m i d 1986 c r i s i s ( i n p a r t s o f t h e area s t i l l l a s t i n g t o l a t e 1986 and even e a r l y 1987 o r l o n g e r ) t h a t was i n i t i a t e d by t h e s e r i o u s o i l p r i c e d e c l i n e h i t t h e ( m a i n l y Western) European s t i m u l a t i o n market much s t r o n g e r , r e s u l t e d i n much more c u t - b a c k o f t h e a c t i v i t y and took much more t i m e t o r e c o v e r and t o r e a d j u s t t o an a g a i n b e t t e r s i t u a t i o n ( t h e l a t t e r phase i s s t i l l n o t t e r m i n a t e d and i s s l o w l y moving a l o n g t h e r i s i n g branch) due t o s e v e r a l reasons. The d i f f e r e n c e s o u t l i n e d as f o l l o w s i n c l u d e f r e quency o f d r i l l i n g r i g s and s t i m u l a t i o n , s u c c e s s / f a i l u r e r a t i o and c o n s o r t i a 1 s t r u c t u r e , o p e r a t i n g m e n t a l i t y and t r e a t m e n t c o s t , US $ exchange r a t e d e c l i n e , and proppant m a r k e t i n g s t r a t e g y .
2.3.1. General aspects M A R T I N S (1987) p r e s e n t s a summarized comparison o f t h e s t i m u l a t i o n markets o f t h e t i g h t gas b a s i n s i n t h e USA and t h e Rotliegend/Carboniferous t i g h t gas b e l t i n t h e Southern N o r t h Sea t o i l l u s t r a t e g e n e r a l aspects o f t h e d i f f e r e n t s e t t i n g o f t h e h y d r a u l i c proppant f r a c t u r i n g markets i n Europe and USA. Absolut e l y h i g h e r w e l l and f r a c t u r e c o s t i n Europe a r e t h e reason f o r a r e l a t i v e l y l o wer f r a c t u r e / w e l l expense r a t i o o f a b t . 10 - 20 % i n Europe w i t h r e s p e c t t o a b t . 40 % i n t h e USA. Well numbers p e r f i e l d r e a c h i n t h e Southern N o r t h Sea o n l y 4 - 40 i n c o n t r a s t t o 40 - 1,000 w e l l s p e r f i e l d i n t i g h t gas b a s i n s i n t h e USA. The l o w e r f r a c t u r e / w e l l c o s t r a t i o r e l a t i v e l y promotes s t i m u l a t i o n i n Europe as an a d d i t i o n a l s t e p of t e s t i n g o r even an i n t e g r a t e d p a r t o f t h e whole d r i l l i n g and c o m p l e t i o n o p e r a t i o n , w h i l e t h e h i g h e r w e l l q u a n t i t y i n t h e USA g i v e s r i s e t o an a b s o l u t e l y h i g h e r f r a c t u r e a c t i v i t y i n t h e l a t t e r area. F l u i d e f f i c i e n c y i s i n Europe l e s s t h a n in t h e USA, whereas gas f l o w r a t e s a r e b e t t e r i n t h e f o r m e r area, b u t a r e o f t e n l i n k e d w i t h s e r i o u s non-Darcy f l o w which i s c o n s i d e r e d t o be u n i m p o r t a n t i n t h e l a t t e r r e g i o n . S e r v i c e b o a t c a p a c i t y i s o f t e n t h e l i m i t i n g f a c t o r o f t h e f r a c t u r e t r e a t m e n t s i n t h e Southern N o r t h Sea where t h e l o c a t i o n s a r e up t o s e v e r a l hundreds o f k i l o m e t r e s away from t h e shore l i n e ( c f . s e c t i o n 3 . 8 . 1 . 2 . ) , i n c o n t r a s t t o many l a n d t i g h t gas f i e l d s i n t h e USA where p l e n t y o f m a t e r i a l and equipment can be i n s t a l l e d w i t h o u t p h y s i c a l l i m i t a t i o n o f s u r f a c e f a c i l i t i e s ( c f . s e c t i o n 1.1.1.4.). ODD (1982) c h a r a c t e r i s t i c s t h e economics o f o i l and gas p r o d u c t i o n i n N o r t h America as a benchmark f o r Europe, and MORRISON & JOLLIFFEE (1982) p r e s e n t an o u t l i n e o f t h e N o r t h Sea t a x and economical system.
2.3.2. Frequency of drilling and stimulation Much more d r i l l i n g r i g s a r e o p e r a t i n g i n t h e USA w i t h r e s p e c t t o Europe, and thus as a consequence o f t h e a b s o l u t e l y c o n s i d e r a b l y l a r g e r market a l s o a r e l a t i v e l y h i g h e r p o t e n t i a l o f s t i m u l a t i o n i s p r e s e n t i n t h e USA. The d i f f e r e n c e i n r i g c o u n t a l s o remained d u r i n g t h e peak o f t h e c r i s i s , w i t h t h e d i f f e r e n c e i n proppant demand a l s o r e l a t i v e l y s t a y i n g independent f r o m t h e a b s o l u t e change o f t h e s a l e s l e v e l i n b o t h areas. Some comments a r e o f f e r e d as f o l l o w s on p e r c e n t a ges o f f r a c t u r i n g n e c e s s i t y f o r economical p r o d u c t i o n as w e l l as t r e a t m e n t c o s t and s i g n i f i c a n c e o f f r a c t u r i n g i n w e l l c o m p l e t i o n .
167
2.3.2.1. Percentages of fracturing necessity for economical production The a b s o l u t e as w e l l as t h e r e l a t i v e frequency o f s t i m u l a t i o n j o b s i s much h i g h e r i n t h e USA t h a n i n Europe b o t h b e f o r e t h e l a t e 1 9 8 5 / e a r l y 1986 o i l p r i c e d e c l i n e and a f t e r t h e c o l l a p s e o f t h e hydrocarbon p r o d u c t i o n i n d u s t r y due t o det e r i o r a t i o n o f t h e g e n e r a l economical s i t u a t i o n . I n t h e USA, a b t . 35 - 40 % o f a l l t h e hydrocarbon w e l l s d r i l l e d i n t h e l a s t decade have been f r a c t u r e d i n o r d e r t o achieve commercial p r o d u c t i o n , w i t h a b t . 25 - 30 % o f t h e t o t a l USA r e serves h a v i n g been made e c o n o m i c a l l y p r o d u c i b l e by h y d r a u l i c s t i m u l a t i o n (VEATCH 1983, OIL GAS JOURNAL 1984), whereas i n Europe, t h e percentage i s gener a l l y much l o w e r and even i n h i g h - p o t e n t i a l areas o f s t i m u l a t i o n o f t e n does n o t exceed 10 - 20 %, i f r e a c h i n g these shares a t a l l . Percentages o f f r a c t u r i n g n e c e s s i t y o f 30 - 50 % o r even more ( i n extreme cases up t o 100 % ) a p p l y i n Europe o n l y f o r w e l l - d e f i n e d patches o f some p l a y s such as a s u i t e o f t i g h t R o t l i e g e n d gas f i e l d s i n t h e Southern N o r t h Sea ( c f . 2.4.1.2., 2.4.5.1. and 3.3.), w i t h t h e o v e r a l l p o t e n t i a l s e c t i o n s 2.2.1.6.2., i n t h e whole f a c i e s b e l t , however, b e i n g c o n s i d e r a b l y l o w e r . I n a d d i t i o n t o t h e r e l a t i v e d i f f e r e n c e i n f r a c t u r e p o t e n t i a l , as a consequence o f much more w e l l s b e i n g d r i l l e d i n USA w i t h r e s p e c t t o Europe, a l s o an a b s o l u t e l y l a r g e r amount o f w e l l s i s s t i m u l a t e d a c c o r d i n g t o t h e mentioned shares. The h i g h e r frequency o f j o b s i n t h e USA r e s u l t s i n more o f t e n use o f t h e equipment, and t h u s s e r v i c e charges a r e a l s o much lower w i t h r e s p e c t t o Europe. S t i m u l a t i o n l o g i s t i c s and r e q u i r e m e n t s i n t h e N o r t h Sea i n Europe a r e o u t l i n e d by MACADAM ( 1 9 8 4 ) .
2.3.2.2. Treatment cost and significance o f fracturing in well completion I n t h e USA, t h e percentage o f w e l l s b e i n g f r a c t u r e d has c o n s i d e r a b l y i n c r e a sed i n t h e l a s t t h r e e years, because t r e a t m e n t c o s t has decreased due t o p r i c e c u t s by s e r v i c e and proppant companies, w i t h cheaper j o b s f i t t i n g p a r t i c u l a r l y b e t t e r i n t o r e v i s e d budgets o f independents (BATT 1983, RAMSEY 1983, ELTING 1987) which o n l y have a r a t h e r f i x e d c e r t a i n amount o f money t o be s p e n t f o r s t i m u l a t i o n i n t h e i r c a p i t a l e x p e n d i t u r e p l a n n i n g p e r i o d and f o r which cash management has t o p p r i o r i t y ( c f . s e c t i o n 2.2.1.5.2.5.). CHAN (1985) s t r e s s e s t h e n a t u r e o f o i l - f i e l d c a s h f l o w as t h e independents- l i f e b l o o d f o r s u r v i v a l , w i t h t h e independent producers b e i n g under i n c r e a s i n g p r e s s u r e t o seek o u t e v e r y a v a i l a b l e d o l l a r o f i n t e r n a l l y generated c a s h f l o w f r o m o t h e r w i s e i d l e a s s e t s o r premature f i n a n c i a l o b l i g a t i o n s . As a consequence o f t h e e v e r i n c r e a s i n g development o f l o w - p e r m e a b i l i t y o i l and gas r e s e r v o i r s , h y d r a u l i c p r o p p a n t f r a c t u r i n g has i n t h e USA a l r e a d y become one o f t h e most i m p o r t a n t aspects o f w e l l c o m p l e t i o n (WATERS 1980; ROBINSON, HOLDITCH & WHITEHEAD 1986), whereas i n Europe, i t i s s t i l l a l o n g way t o go t o t h i s stage, and p r e s e n t l y t h e e v o l u t i o n o f s t i m u l a t i o n i s i n Europe i n many areas s t i l l a t t h e l e v e l o f performance o n l y i n case o f e x c e p t i o n a l and i n d i s p e n s i b l e circumstances, w i t h t h e o n l y e x c e p t i o n p r o b a b l y b e i n g t h e t h o r o u g h l y planned R o t l i e g e n d development campaigns i n t h e N o r t h Sea i n 1987 - 1990 and beyond w h i c h a r e based on t h e c o n c e p t t h a t e n c o u n t e r i n g t h e expected t i g h t f a c i e s c o u l d mean f r a c t u r i n g almost e v e r y w e l l i n t h e s u i t e o f p r o d u c t i o n d r i l l i n g .
2.3.3. Success/failure ratio and consortia1 structure I n t h e USA, i t i s a l r e a d y s i n c e a l o n g t i m e w e l l understood t h a t because t h e search f o r o i l and gas becomes more d i f f i c u l t w i t h i n c r e a s i n g p r o s p e c t i o n and e x p l o r a t i o n m a t u r i t y o f more and more b a s i n s where t h e b i g and obvious d e p o s i t s have been found a l r e a d y much e a r l i e r , and nowadays and i n t h e f u t u r e s m a l l e r c o n v e n t i o n a l as w e l l as i n n o v a t i v e d e p o s i t s have t o be d i s c o v e r e d which r e q u i r e
168 more and more s o p h i s t i c a t e d s t r a t e g i e s and t e c h n i q u e s and where t h e s u c c e s s / f a i l u r e r a t i o may be lower, i t i s i n c r e a s i n g l y i m p o r t a n t t o f r a c t u r e as many e x i s t i n g w e l l s as p o s s i b l e t o compensate t h e d e c l i n i n g impact o f e x p l o r a t i o n d r i l l i n g on s u p p l y o f a d d i t i o n a l r e s e r v e s by t h e impetus o f s t i m u l a t i o n (KNOTT 1 9 8 6 ) . H y d r a u l i c p r o p p a n t f r a c t u r i n g w i l l p l a y an e v e r i n c r e a s i n g r o l e i n maint a i n i n g t h e l e v e l o f o i l and gas p r o d u c t i o n i n t h e USA as h i g h as p o s s i b l e (WATERS 1980), and t h i s c o n c l u s i o n a l s o needs u n d e r s t a n d i n g i n Europe and o t h e r areas where i t i s n o t y e t p a r t o f common sense i n hydrocarbon r e s e r v o i r complet i o n and e x p l o i t a t i o n . I n Europe, t h e s t r u c t u r e o f t h e c o n s o r t i a r e q u i r e s agreement between t h e p a r t n e r s , w i t h f r a c t u r e j o b s as w e l l as whole d r i l l i n g p r o j e c t s b e i n g o f t e n ser i o u s l y delayed i f one o f t h e two t o f o u r o r even more p a r t i e s sees no need f o r i n v e s t m e n t a t t h e moment, and t h e s t i l l h i g h e r p r i c e s o f m a t e r i a l s and operat i o n s w i t h r e s p e c t t o t h e USA have i n Europe so f a r n o t had a s i m i l a r t r i g g e r i n g and p r o m o t i n g e f f e c t , and i f t h e r e had been one, i t was k i l l e d by t h e o i l p r i c e d r o p (GRIFFITHS 1986, RUNGE 1986; c f . s e c t i o n 2.2.1.1.) and t h e d e c l i n e o f t h e US $ v a l u e ( c f . s e c t i o n 2.2.1.3.; a l t h o u g h a t l e a s t some N o r t h Sea f i e l d s a r e s t i l l a b l e t o c o v e r o p e r a t i n g c o s t and r o y a l t y payments i n case o f an even s t i l l l o w e r o i l p r i c e ; BEUDELL 1986 a ) . T h r e a t e n i n g u n w i l l i n g p a r t n e r s w i t h s o l e - r i s k o p e r a t i o n s o n l y works i n e x p l o r a t i o n , b u t cannot be a p p l i e d i n development d r i l l i n g and s t i m u l a t i o n . F i n a n c i n g p r o j e c t s v i a t u r n k e y c o n t r a c t o r s i s o u t l i n e d by MARPLE & RULEY (1987).
2.3.4. Operating mentality and treatment cost The o p e r a t i n g m e n t a l i t y and p h i l o s o p h y i n t h e USA i s much more d i r e c t and dec i s i v e t h a n i n Europe, b e i n g an e f f e c t o f t h e g e n e r a l l y s t r a i g h t f o r w a r d American t h i n k i n g i n c o n t r a s t t o t h e h e s i t a t i n g and w e l l - r e a s o n i n g European m e n t a l i t y . I n Europe, r e p e a t e d r e v i e w o f t h e p l a n s , renewed economical f e a s i b i l i t y s t u d i e s , more c o m p l i c a t e d h i e r a r c h i c a l p e r m i s s i o n procedures w i t h o f t e n t o o many and p a r t i a l l y r i v a l i z i n g d e c i d i n g f u n c t i o n s , another m e n t a l i t y where sometimes s u b j e c t i v e i n t e r e s t s can s e r i o u s l y i n f l u e n c e o b j e c t i v e purposes and can hamper p r o j e c t a p p r o v a l s , and c e r t a i n l y sometimes a l s o l a c k i n g encouragement f o r unpop u l a r approaches and m i s s i n g readyness t o c o n s i d e r more r i s k o f t e n l e a d t o a l t e r a t i o n , suspension o r even t o t a l c a n c e l l a t i o n o f planned jobs, whereas i n t h e USA a c t i o n i s made a c c o r d i n g t o t h e r u l e " g e t i t done as soon as p o s s i b l e " , and a l s o a h i g h e r r i s k i s o f t e n accepted (NEWENDORP 1983 o u t l i n e s a s t r a t e g y f o r i m p l e m e n t i n g r i s k a n a l y s i s ) i n c o n t r a s t t o Europe where e v e r y t h i n g s h o u l d i d e a l l y be on t h e most s a f e s i d e . E v a l u a t i o n s o f economical r i s k f o r N o r t h Sea o f f s h o r e f i e l d development a r e c a r r i e d o u t by SULLIVAN (1982) and TWEEDIE & KENNY ( 1 9 8 2 ) . Some aspects of i n t e g r a t i o n o f s t i m u l a t i o n i n t o t h e t o t a l w e l l concept, and p r o p p a n t p r i c e and t r e a t m e n t c o s t a r e o u t l i n e d as f o l l o w s .
2.3.4.1. Integration o f stimulation into the total well concept The f u n d a m e n t a l l y d i f f e r e n t s t i m u l a t i o n approach between Europe and USA i s p a r t i a l l y a l s o a consequence of t h e American concept and u n d e r s t a n d i n g t h a t t h e f r a c t u r i n g t r e a t m e n t i s p a r t o f t h e d r i l l i n g and t e s t i n g budget and i s a u t o m a t i c a l l y i n c l u d e d t h e r e i n , whereas a c c o r d i n g t o t h e t r a d i t i o n a l European t h i n k i n g , h y d r a u l i c p r o p p a n t f r a c t u r i n g was f o r l o n g t i m e s c o n s i d e r e d t o be something add i t i o n a l and o p t i o n a l and t h e r e f o r e once d e c i s i o n was t o be made, t h e r e was a l ways c o n s i d e r a b l e r e a s o n i n g whether t h e a d d i t i o n a l i n v e s t m e n t s h o u l d be made o r n o t . Thus a t a t i m e a f t e r t h e o i l p r i c e c r a s h when even w e l l - l o g g i n g r u n s were s u b j e c t t o s i g n i f i c a n t s t r e a m l i n i n g , c a s i n g schedules f o r d r i l l i n g were changed i n o r d e r t o save money, r e s e r v o i r c o r i n g was reduced t o t h e b a r e minimum necess a r y f o r p e t r o p h y s i c a l c a l i b r a t i o n o f l o g i n t e r p r e t a t i o n o r was even a l m o s t t o t a l l y e l i m i n a t e d , and many expensive s t i m u l a t i o n j o b s were e i t h e r suspended and postponed o r c o m p l e t e l y c a n c e l l e d . The American way o f u n d e r s t a n d i n g t h e whole package as an i n t e g r a t e d o b j e c t i s o n l y r e c e n t l y spreading i n Europe as w e l l ,
169 m a i n l y i n c o n n e c t i o n w i t h t h e huge development d r i l l i n g campaigns i n t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d b e l t ( c f . s e c t i o n s 2.2.1.6.2., 2.4.1.2., 2.4.5.1. and 3.3.) where any o t h e r s t r a t e g y o f gas a c q u i s i t i o n would be condemned t o f a i l u r e . The n e c e s s i t y o f a more r i s k y o p e r a t i n g m e n t a l i t y i s p a r t i c u l a r l y i n Germany FRG u n d e r l i n e d by t h e phenomena t h a t t h e l a s t c o u p l e o f b i g g e r gas s t r i k e s i n t h e C a r b o n i f e r o u s by e x p l o r a t i o n w e l l s d a t e s back more t h a n f i v e y e a r s (SCHRUDER 1985) and o n l y a few s m a l l e r d i s c o v e r i e s were made i n 1986/1987 (SCHRUDER 1987), which i s i n marked c o n t r a s t t o t h e f a c t t h a t most o f t h e s e v e r a l dozens o f MHF j o b s h a v i n g been c a r r i e d o u t i n Germany FRG so f a r have been performed i n t h e C a r b o n i f e r o u s . D i s c o v e r y r a t e s i n t h e R o t l i e g e n d have been c o n s i d e r a b l y h i g h e r t h a n i n t h e C a r b o n i f e r o u s i n Germany FRG i n t h e l a s t y e a r s due t o a s t r o n g and pushed gas e x p l o r a t i o n and a p p r a i s a l d r i l l i n g programme even a f t e r t h e 1986 o i l p r i c e crash, b u t an even h i g h e r amount o f new f i e l d s o r b l o c k s c o u l d have been proven t o c o n t a i n e c o n o m i c a l l y a c c e s s i b l e gas r e s e r v e s i f hyd r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n would have had an adequate p o s i t i o n as compared t o t h e emphasis on d r i l l i n g .
2.3.4.2. Proppant price and treatment cost The t o t a l p r o p p a n t p r i c e i n Europe i s a t t h e bottom o f t h e l i n e a l m o s t t i m e s 1 . 5 o r even t w i c e t h e p r i c e i n t h e USA as a consequence o f sea f r e i g h t as w e l l as a d d i t i o n a l h a n d l i n g and warehousing surcharges. The n e a r l y doubled p r i c e makes many o p e r a t i o n s i n Europe ( w i t h r e s p e c t t o j o b s i n t h e USA) much l e s s econom i c a l l y f e a s i b l e ( i f a t a l l ) and t h u s i s one o f t h e main reasons f o r t h e absolut e l y much l o w e r s t i m u l a t i o n a c t i v i t y ( i n view o f h y d r a u l i c p r o p p a n t f r a c t u r i n g , because p r o p p a n t c o s t i s o f t e n t h e l a r g e s t share o f t h e t o t a l t r e a t m e n t expenses) a l r e a d y i n good t i m e s and t h u s e s p e c i a l l y i n t h e 1986 c r i s i s and t h e f o l l o w i n g y e a r s . I n c o n t r a s t t o up t o 100 % c o s t d i f f e r e n c e between Europe and cent r a l USA, a l t e r a t i o n s o f expenses up t o 20 - 30 % can a l s o o c c u r f o r r e g i o n a l p o s i t i o n s w i t h i n t h e USA w i t h r e s p e c t t o t h e m i d d l e p o r t i o n o f t h a t area as a r e s u l t o f l o g i s t i c a l and e n v i r o n m e n t a l d i f f e r e n c e s .
2.3.5. US
$
exchange rate decline
The c o s t f o r m a t e r i a l and o p e r a t i o n s i n Europe was r e l a t i v e l y cheaper f r o m l a t e 1985 onwards w i t h r e s p e c t t o t h e USA a l s o as a consequence o f t h e d e c l i n i n g US $ exchange r a t e ( w i t h r e s p e c t t o DM and L ) t h a t superimposed i n Europe t h e p r i c e r e d u c t i o n s o f t h e s u p p l y and s e r v i c e companies. A t t h e same time, however, t h e d e c r e a s i n g v a l u e o f t h e US $ d e t e r i o r a t e d p r o g r e s s i v e l y t h e g e n e r a l economical s i t u a t i o n by g i v i n g a l o w e r n e t r e t u r n f r o m t h e hydrocarbon s a l e s ( t h u s i n Europe i n d u c i n g an even s t r o n g e r r e d u c t i o n o f t h e o i l p r i c e t h a n e f f e c t i v e i n t h e USA; c f . s e c t i o n 2.2.1.3.). T h i s f i n a n c i a l p o l i t i c a l c o n s t e l l a t i o n had an i m p o r t a n t a d d i t i o n a l impact on t h e s t i m u l a t i o n m a r k e t i n Europe and o t h e r n o n - d o l l a r - c u r r e n c y areas i n c o n t r a s t t o i t s i n s i g n i f i c a n c e i n t h e USA. The cheaper p r i c e t o be p a i d f o r goods and s e r v i c e r e s u l t i n g f r o m p r i c e c u t s t o g e t h e r w i t h US $ d e v a l u a t i o n t h e r e f o r e d i d n o t l e a d t o an i n c r e a s e o f i n v e s t ments i n Europe, because t h e n e g a t i v e e f f e c t o f t h e weak US $ on t h e a l r e a d y u n f a v o u r a b l y low o i l p r i c e d i d no l o n g e r p e r m i t t o make i n v e s t m e n t s w i t h a s u f f i c i e n t l y h i g h d i s c o u n t e d cash f l o w r a t e even under s i m p l e s t t e c h n i c a l c o n d i t i o n s . Thus i n many p r e d o m i n a n t l y Western European companies an almost t o t a l f r e e z e o f t h e budget o f c a p i t a l e x p e n d i t u r e (above c e r t a i n i n e v i t a b l e o b l i g a t i o n s s e t by t h e m i n i n g a u t h o r i t y when l i c e n s i n g t h e concessions and e x c e p t o f such i t e m s where c o n t r a c t s had a l r e a d y been signed) was t h e g e n e r a l s t r a t e g y t o s u r v i v e u n t i l a t i m e w i t h h i g h e r o i l p r i c e and s t r o n g e r US $ came s l o w l y back by s l i g h t s t a b i l i z a t i o n f r o m l a t e 1986 onwards, a l t h o u g h h a v i n g s t i l l i n m i d 1988 n o t y e t reached a l e v e l t h a t a l l o w s renewed s a f e p l a n n i n g o f s i g n i f i c a n t l y h i g h e r i n v e s t m e n t s than d u r i n g t h e peak o f t h e c r i s i s and s h o r t l y a f t e r i t .
170
2.3.6. Proppant marketing strategy A s t h e main p r o p p a n t producers have b o t h head o f f i c e s and m a n u f a c t u r i n g p l a n t s i n t h e USA, t h i s area ( i n c l u d i n g Canada) i s much b e t t e r covered by mark e t i n g and s a l e s p r o m o t i o n a c t i v i t y and i s developed s i n c e many y e a r s , whereas many p a r t s o f Europe were v i r t u a l l y unattended u n t i l r e c e n t l y . Sales t o Europe were i n t h e l a s t y e a r s p r e d o m i n a n t l y made upon s p o t - l i k e r e q u e s t by t h e market, w i t h t h e h i g h l i g h t s b e i n g t h e MHF t r e a t m e n t s i n Germany FRG; B r i t i s h , D u t c h and Norwegian N o r t h Sea; and Yugoslavia, and o r d e r s were o b t a i n e d by i n d i v i d u a l proppant companies i n some cases by pure chance and l u c k (sometimes i n c l u d i n g unexpected backdoor approaches t h r o u g h s u p p l y companies which g o t t h e business, b u t were u n a b l e t o d e l i v e r t h e r e q u e s t e d q u a n t i t i e s f r o m t h e i r own s o u r c e s ) , b u t n o t as a r e s u l t o f s t e p w i s e groundwork by sound m a r k e t i n g and s a l e s promot i o n concepts based on u n d e r s t a n d i n g o f g e o l o g i c a l , e n g i n e e r i n g and economical r e 1 a t ion s h ips .
The much b e t t e r communication and t r a n s p a r e n c y o f t h e l a t e s t t e c h n i c a l news c o n c e r n i n g a p p l i c a t i o n o f p r o p p a n t types and t h e c o n s u l t i n g by t h e s a l e s r e p r e s e n t a t i v e s i n t h e USA r e i n f o r c e s t h e p o t e n t i a l o f h y d r a u l i c f r a c t u r i n g i n t h i s area and t h u s h e l p s p r o d u c t i o n and s e r v i c e companies t o d e c i d e i n a d i r e c t i o n f a v o u r a b l e f o r t h e p r o p p a n t company i n a much more e f f e c t i v e way t h a n i n Europe. Given comparable s t a n d a r d o f m a r k e t i n g management, an adequate p e n e t r a t i o n i s a l s o i n Europe undoubtedly p o s s i b l e and i s o n l y a q u e s t i o n o f manpower and t i m e . The f o l l o w i n g d i s c u s s i o n summarizes aspects o f l a c k i n g c o n t i n u i t y i n marketing attention, b u s i n e s s shares o f t h e d i f f e r e n t p r o p p a n t companies, and p r i c e changes and m a r k e t t u r n i n g .
2.3.6.1. Lacking c o n t i n u i t y i n marketing a t t e n t i o n The h i t h e r t o u n s a t i s f y i n g p r o p p a n t s a l e s l e v e l i n Europe i s t h e r e f o r e t h e l o g i c a l and i n e v i t a b l e consequence o f l a c k i n g c o n t i n u i t y and c o n c e p t o f m a r k e t i n g and s a l e s s t r a t e g y i n t h a t area. I n c o n t r a s t t o market t r a n s p a r e n c y and i l l u s t r a t i o n , c o n f u s i o n and embarassment was c r e a t e d by v a r i o u s p r o p p a n t companies by n o t o n l y n e g l e c t i n g t h e i r m a r k e t i n g a c t i v i t y , b u t i n a d d i t i o n t o t h a t chang i n g names o f p r o p p a n t p r o d u c t s and companies ( i n some cases r e p e a t e d l y w i t h i n a c o u p l e o f y e a r s ) , a l t e r n a t i o n o f l i m i t e d p e r i o d s o f t i m e when l o c a l t e c h n i c a l s a l e s r e p r e s e n t a t i v e s were a c t i n g w i t h phases when nobody e x c e p t t h e head o f f i c e i n t h e USA was r e s p o n s i b l e , employment o f m a r k e t i n g managers w i t h soon f o l l o w i n g premature t e r m i n a t i o n w i t h o u t l o g i c a l and u n d e r s t a n d a b l e r e a s o n i n g and w i t h o u t r e s p e c t i n g t h e r e q u i r e m e n t s o f t h e European market, and f i n a l l y chang i n g proppant p r i c e s r e p e a t e d l y i n downwards and upwards d i r e c t i o n . A s a consequence o f a l l these shortcomings, Europe i s i n many p a r t s s t i l l nowadays a p o o r l y understood p r o p p a n t m a r k e t . Reviewing t h e p a s t and c u r r e n t m a r k e t i n g a c t i v i t y o f p r o p p a n t companies i n Europe, c o n c l u s i o n has t o be made t h a t t h e g e n e r a l l y u n s a t i s f y i n g p a s t and p r e s e n t s a l e s l e v e l i s t h e l o g i c a l and i n e v i t a b l e r e s u l t o f s e r i o u s n e g l e c t a n c e and exaggerated u n d e r e v a l u a t i o n o f t h e p o t e n t i a l o f a market where numerous poss i b i l i t i e s o f s t i m u l a t i o n e x i s t , b u t c o m p l i c a t e d r e s e r v o i r c o n d i t i o n s , many d i f f e r e n t m e n t a l i t i e s and a d m i n i s t r a t i v e i n t e r l o c k i n g r e q u i r e much more t i m e and a c t i v i t y , c o u p l e d w i t h p a t i e n c e , t o achieve t h e goal o f t h e m a r k e t i n g s t r a t e g y by h a v i n g t h e o p e r a t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g j o b s . T h i s t a r g e t , howe v e r , can even today w i t h good i n t e r c o n t i n e n t a l o r a l and w r i t t e n communication i n s c i e n t i f i c , t e c h n i c a l and commercial aspects n o t be achieved w i t h o u t a f u l l t i m e l o c a l m a r k e t i n g management r e p r e s e n t a t i o n w i t h i n Europe. Release o f t h e p r o p p a n t m a r k e t i n g p o t e n t i a l i n Europe and i t s c o n v e r s i o n i n t o o r d e r s can t h e r e f o r e a t t h e moment o n l y be achieved by i n v e s t m e n t i n t o t h e market which, however, s h o u l d i n f a c t h a v i n g been done a l r e a d y much e a r l i e r .
171 Again i t has t o be emphasized t h a t c o n t i n u i t y o f work s h o u l d be achieved by a l o n g e r assignment o f m a r k e t i n g personnel r a t h e r than h i r i n g and f i r i n g p o l i c y w i t h premature personnel t e r m i n a t i o n w i t h o u t understandable reasons s i m p l y because e x e c u t i v e s a r e n o t w i l l i n g t o w a i t s u f f i c i e n t l y l o n g t o observe t h e mark e t r e a c t i o n a f t e r a reasonable l e a n t i m e . The market needs i n f a c t q u i t e some t i m e i n o r d e r t o response and t o r e f l e c t t h e r e s u l t s o f t h e market attendance e f f o r t s i n v i s i b l y i n c r e a s i n g s a l e s l e v e l s independent f r o m seasonal f l u c t u a t i o n s and v a r i a t i o n s a c c o r d i n g t o t h e g e n e r a l economical framework.
2.3.6.2.
Business shares o f the d i f f e r e n t proppant companies
Aspects o f business shares o f t h e d i f f e r e n t proppant companies a r e e v a l u a t e d a l o n g t h e l i n e s o f skewed market share r a t i o o f m a j o r proppant s u p p l i e r s as w e l l as impact o f s e r v i c e companies vs. own t r e a t m e n t management as f o l l o w s .
2.3.6.2.1.
Skewed market share r a t i o o f major proppant suppliers
As t h e s e r v i c e companies b e i n g t h e main proppant customers do n o t d i v i d e t h e p r o p p a n t business by 50 : 50 between t h e m a j o r s u p p l i e r s ( a s i m i l a r r e l a t i o n s h i p a p p l i e s f o r t h e w e l l - l o g g i n g business p r o v i d e d by t h e hydrocarbon product i o n companies), b u t almost always generate an asymmetrical d i s t r i b u t i o n w i t h s l i g h t f a v o u r i z a t i o n o f one s u p p l i e r w i t h r e s p e c t t o i t s c o m p e t i t i o n , t h e i n d i v i d u a l p r o p p a n t companies should be i n s p i r e d t o undertake e f f o r t s t o skew t h e r a t i o towards 60 : 40 o r even 70 : 30 t o t h e i r b e n e f i t and a t t h e expense o f t h e i r c o m p e t i t i o n . W i t h t h e background o f i d e n t i c a l p r i c e s and comparable p e r formance o f t h e v a r i o u s c o r r e l a t i n g p r o p p a n t types, t h i s goal can o n l y be a c h i e ved b y t h e e x p e r i e n c e and knowledge o f t h e t e c h n i c a l and m a r k e t i n g r e p r e s e n t a t i ve, w i t h g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g e x p e r i e n c e b e i n g e s s e n t i a l t o obt a i n c r e d i b i l i t y and c o n f i d e n c e and b e i n g regarded by t h e customers n o t s i m p l y as a s a l e s agent t h a t t r i e s t o promote p r o d u c t no m a t t e r how and i r r e s p e c t i v e o f p r a c t i c a l demand, b u t as a t e c h n i c a l and s c i e n t i f i c e x p e r t n o t o n l y f o r t h e c o m m e r c i a l l y o f f e r e d p r o p p a n t types, b u t a l s o f o r h y d r a u l i c f r a c t u r i n g i n gener a l and t h e r e q u i r e m e n t s o f t h e s p e c i f i c r e s e r v o i r s . A d d i t i o n a l m a r k e t shares can be o b t a i n e d by t r y i n g t o c o n v i n c e s e r v i c e compan i e s and o p e r a t o r s t o i n c r e a s i n g amounts o f t h e b e n e f i t s o f r e p l a c i n g n a t u r a l sand by ceramic p r o p p a n t s . T a l k i n g t o a l l t h e o p e r a t o r s i s t h e o n l y way t o make s u r e t h a t a l l t h e r e l e v a n t i n f o r m a t i o n i s p r o p e r l y f e d t o t h e end u s e r and i s n o t o b l i t e r a t e d o r drowned i n t h e b u l k p r o p o s a l o f t h e s e r v i c e company. The skew o f t h e p r o p p a n t market f r o m 50 : 50 towards 60 : 40 o r even 70 : 30 was achieved by one o f t h e m a j o r proppant s u p p l i e r s i n Europe f o r q u i t e some t i m e by good c o o p e r a t i o n and c o n f i d e n t b u s i n e s s r e l a t i o n s h i p w i t h one s e r v i c e company which a c c o r d i n g t o i t s c o m p e t i t i v e n e s s i n terms o f p r i c e and p e r f o r mance won most o f t h e p r o p p a n t f r a c t u r i n g c o n t r a c t s i n t h e N o r t h Sea d u r i n g t h e l a s t few years, w i t h t h e p r o p p a n t company t h e r e b y h a v i n g had an a c c i d e n t a l p r e f e r e n t i a l o u t l e t i n t o t h e N o r t h Sea market ( w h i c h i s t h e l a r g e s t share o f t h e European f r a c t u r i n g scene) d e s p i t e g e n e r a l l y i n s u f f i c i e n t m a r k e t i n g a t t e n t i o n . F o l l o w i n g t h e l a t e 1987 p r o p p a n t p r i c e increase, however, t h i s advantage was l o s t t o t h e c o m p e t i t i o n as a consequence o f p l a y i n g t h e p r i c e c a r d t o o h i g h due t o l i m i t e d u n d e r s t a n d i n g o f t h e m a r k e t requirements ( c f . s e c t i o n 2 . 4 . 2 . 4 . 1 . ) . T h i s example most s i g n i f i c a n t l y h i g h l i g h t s t h e need o f improved m a r k e t i n g and s a l e s s t r a t e g i e s by e x p e r i e n c e d l o c a l r e p r e s e n t a t i v e s who a r e a l s o f a m i l i a r w i t h t h e p a r t i c u l a r needs o f t h e European m a r k e t and t h e m e n t a l i t y o f t h e people w o r k i n g t h e r e .
172
2.3.6.2.2. Impact o f service companies
vs. own treatment management
The f o r m e r l y s t r o n g t o d o m i n a n t i m p a c t o f t h e s e r v i c e companies o n s t i m u l a t i o n d e s i g n and c h i e f l y p r o p p a n t s e l e c t i o n i s d e c l i n i n g . The r e a s o n i s t h a t o n t h e one hand, W e s t e r n European companies have i n i n c r e a s i n g amounts t h e i r own e x p e r t s who p l a y s i m u l a t i o n s i n t h e c o m p u t e r w i t h i n - h o u s e p r o g r a m s and make a d e s i g n i n c l u d i n g p r o p p a n t c h o i c e , and t h e n ask t h e s e r v i c e company t o e i t h e r mod i f y and i m p r o v e t h e c o n c e p t o r e v e n a l m o s t d i r e c t l y e x e c u t e t h e p l a n n e d t r e a t ment. On t h e o t h e r hand, E a s t e r n European companies have f r e q u e n t l y own e q u i p ment and c a r r y o u t many j o b s ( p a r t i c u l a r l y s m a l l e r ones) t h e m s e l v e s , w i t h e v e n t h e s e r v i c e companies b e i n g p r e p a r e d t o a c c e p t d i r e c t p r o p p a n t p u r c h a s e b y t h e E a s t e r n European o p e r a t o r s and t r y i n g t o f o c u s t h e i r b u s i n e s s o n f l u i d s , a d d i t i ves and o t h e r c h e m i c a l s . The E a s t e r n European m e n t a l i t y i s q u i t e d i f f e r e n t f r o m t h e W e s t e r n European and A m e r i c a n way o f t h i n k i n g . F r i e n d s h i p has a v e r y h i g h r a n k a l s o i n b u s i n e s s l i f e . Once a company r e p r e s e n t a t i v e i s acknowledged t o b e a t e c h n i c a l e x p e r t and i s r e g a r d e d as a p e r s o n a l f r i e n d , t h e E a s t e r n European s p e c i a l i s t s w i l l a l m o s t a l w a y s a d d r e s s t h e m s e l v e s t o h i m and a l s o g i v e h i m t h e i r b u s i n e s s , w i t h i n some c a s e s even n o t t a l k i n g t o c o m p e t i t i o n a t a l l r e g a r d l e s s o f any p r i c i n g s t r u c t u r e .
2.3.6.3. Price changes and market turning S i n c e t h e 1985 p r i c e d r o p s f o r a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s w h i c h moved t h e more e x p e n s i v e z i r c o n i a - s i l i c a t e p r o p p a n t s i n t o an a l m o s t unmarket a b l e s i t u a t i o n , t h e m a r k e t i n E u r o p e was v i r t u a l l y o n l y s h a r e d b y t h e m a j o r p r o d u c e r s o f a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s . The r a t i o o f 60 : 40 o r 70 : 30 w h i c h may be a c h i e v e d b y one o f t h e m a j o r companies w i t h r e s p e c t t o t h e i r c o m p e t i t i o n , however, i s o n l y v a l i d as l o n g as t h e p r i c e d i f f e r e n c e b e t ween a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s o n t h e one hand and z i r c o n i a - s i l i c a t e p r o p p a n t s o n t h e o t h e r hand i s s u f f i c i e n t t o p r e v e n t r e a s o n a b l e m a r k e t p e n e t r a t i o n o f the l a t t e r product. T h e r e f o r e a g a i n a w a r n i n g has t o be i s s u e d t o t h e m a n u f a c t u r e r s o f a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s t h a t f o l l o w i n g t h e l a t e 1987 p r i c e r i s e and t h e r e c o v e r i n g US $ exchange r a t e i n e a r l y 1988, a n o t h e r company-induced p r i c e i n c r e a se c o u l d b r i n g t h e z i r c o n i a - s i l i c a t e p r o p p a n t s back i n t o t h e i r f o r m e r c o m p e t i t i ve p o s i t i o n w h i c h w o u l d c e r t a i n l y d e s t r o y much o f t h e above s k e t c h e d m a r k e t i n g c o n c e p t s , w i t h t h e p r o b a b i l i t y b e i n g v e r y h i g h t h a t i n t h i s case, t h e z i r c o n i a s i l i c a t e p r o p p a n t s w o u l d a g a i n a c h i e v e a t l e a s t 20 - 30 % m a r k e t s h a r e as compar e d t o t h e i r a l m o s t i n s i g n i f i c a n c e a t t h e moment c o m p r i s i n g n o t more t h a n 5 - 10 % o f t h e w h o l e p r o p p a n t b u s i n e s s ( c f . s e c t i o n 2 . 2 . 2 . 3 . 2 . ) . The w a r n i n g n o t t o p e r f o r m a n o t h e r a r t i f i c i a l p r i c e r i s e has t o be examined p a r t i c u l a r l y w i t h r e s p e c t t o p o s s i b l e a u t o m a t i c a l p r i c e i n c r e a s e s f o r t h e European c u s t o m e r s b y a f u r t h e r r e g a i n i n g US $ exchange r a t e t o w h i c h t h e p r o p p a n t companies have a b s o l u t e l y no i n f l u e n c e a t a l l .
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2.4. Areal marketing assessment o f hydraulic proppant f r a c t u r i n g H y d r a u l i c p r o p p a n t f r a c t u r i n g a c t i v i t y i n Europe i s o f d i f f e r e n t s i g n i f i c a n c e i n t h e w e s t e r n and e a s t e r n p a r t o f t h e area due t o r e s e r v o i r e n g i n e e r i n g , t e c h n o l o g i c a l and economical reasons. A r e a l m a r k e t i n g e v a l u a t i o n s a r e g i v e n as f o l l o w s f o r b o t h Western Europe and E a s t e r n Europe f o r t h e p e r i o d s 1977 - 1985, 1986 ( i n c l u d i n g p a r t i a l l y t h e f i r s t h a l f o f 1987), and 1987 ( c o m p r i s i n g m a i n l y t h e second h a l f ) - 1990 ( a n d p a r t i a l l y even beyond), t h u s c o n t a i n i n g review, s t a t u s , f o r e c a s t and p e r s p e c t i v e s . The f i r s t p e r i o d comprises t h e range b e f o r e t h e m a j o r 1986 o i l p r i c e c r a s h and covers t h e t i m e f r o m t h e o n s e t o f a v a i l a b i l i t y o f s y n t h e t i c h i g h - q u a l i t y p r o p p a n t s t h r o u g h t h e golden y e a r s o f hydrocarbon s t i m u l a t i o n t o t h e market c o l l a p s e . The second i n t e r v a l i n c l u d e s t h e immediate o i l p r i c e d r o p range and t h e p e r i o d o f reduced and/or r e t a r d e d a c t i v i t y s h o r t l y a f t e r t h e m a r k e t t u r n i n g . The t h i r d s e c t i o n f i n a l l y comprises t h e b e g i n n i n g r e c o v e r y f r o m t h e o i l p r i c e c r a s h and t h e f o l l o w i n g y e a r s which a r e more t h a n e v e r b e f o r e c h a r a c t e r i z e d by r e a l i s t i c p r o j e c t assessment w i t h o u t exaggerated optimism, b u t on t h e o t h e r hand c e r t a i n l y a l s o n o t based on e x c e s s i v e p e s s i mism. As a r e s u l t o f t h e d i f f i c u l t y t o prognose and t o p l a n i n e x a c t t i m e f r a mes due t o many t e c h n i c a l and economical u n c e r t a i n t i e s , t h e l a t t e r p e r i o d i s p a r t i a l l y even f a r t h e r e x t e n d i n g i n t o t h e f u t u r e as a consequence o f some del a y s and postponements as w e l l as appearance o f f o l l o w - u p p r o j e c t s due t o favour a b l e impact o f e a r l i e r j o b s and campaigns.
2.4.0. General aspects Emphasis i s i n t h e p r e s e n t r e v i e w , s t a t u s r e p o r t and p r e d i c t i o n a l s o p u t on economical, commercial, p o l i t i c a l and even i d e o l o g i c a l aspects of h y d r a u l i c p r o p p a n t f r a c t u r i n g e s p e c i a l l y f o r E a s t e r n Europe. As f a r as more or l e s s r e g i o n a l l y o r t e m p o r a l l y l i n k e d w i t h h y d r a u l i c p r o p p a n t f r a c t u r i n g , comments a r e a l s o o f f e r e d on g r a v e l p a c k i n g w h i c h i n terms o f p r o p p a n t consumption has gener a l l y o n l y a small share o f t h e t o t a l p r o p p a n t q u a n t i t y d e l i v e r e d t o h y d r o c a r bon i n d u s t r y and i s i n many areas w i t h g e o l o g i c a l l y o l d e r and more l i t h i f i e d sandstones even almost o f n e g l i g i b l e n a t u r e i n comparison t o h y d r a u l i c f r a c t u r i n g , whereas o t h e r r e g i o n s w i t h g e o l o g i c a l l y younger and l e s s c o n s o l i d a t e d sands and c h a l k s a r e c h a r a c t e r i z e d by p r o p p a n t u t i l i z a t i o n p r e d o m i n a n t l y f o r gravel packing w i t h o n l y a subordinate q u a n t i t y going t o hydraulic f r a c t u r i n g ( c f . c h a p t e r 5 ) . References on g e n e r a l and s p e c i f i c , r e g i o n a l and s t r a t i g r a p h i c a l , r e s e r v o i r e n g i n e e r i n g and t e c h n i c a l aspects o f h y d r a u l i c p r o p p a n t f r a c t u r i n g a r e compiled i n c h a p t e r 7, and f u r t h e r m a r k e t i n g d e t a i l s a r e c o l l e c t e d i n MADER ( 1 9 8 7 ) . An overview map o f t h e c e n t r e s o f a c t i v i t y i n Europe i s p r e s e n t e d i n f i g . 2, and a summary of c h a r a c t e r i s t i c f e a t u r e s i s e x h i b i t e d i n t a b . 5. The European p r o p p a n t market can be s p l i t i n t o c e n t r e s o f a c t i v i t y o f mass i v e h y d r a u l i c f r a c t u r i n g (MHF), s m a l l e r - s c a l e f r a c t u r i n g and g r a v e l p a c k i n g ( c f . f i g . 2 ) . Massive h y d r a u l i c f r a c t u r i n g a c t i v i t y i n p a s t and f u t u r e concent r a t e s on B r i t i s h and Dutch Southern N o r t h Sea, N e t h e r l a n d s onshore, Germany FRG onshore, and Yugoslavia. S m a l l e r - s c a l e f r a c t u r i n g t a k e s p l a c e i n t h e areas o f MHF e x e c u t i o n and i n a d d i t i o n t o these i n t h e B r i t i s h and Norwegian N o r t h e r n N o r t h Sea, Germany GDR, Poland, S o v i e t Union, Czechoslovakia, Hungary, A u s t r i a , France, I t a l y , B u l g a r i a , Romania, Turkey, T u n i s i a and A l g e r i a . Gravel p a c k i n g focusses on Norwegian and B r i t i s h N o r t h e r n N o r t h Sea, Germany FRG, A u s t r i a , Czec h o s l o v a k i a , I t a l i a n and Yugoslavian A d r i a t i c Sea, and S o v i e t Union ( c f . f i g s . 4 - 8 ) . The N o r t h Sea i s by f a r t h e area w i t h h i g h e s t s t i m u l a t i o n p o t e n t i a l i n p a s t and f u t u r e and i s a mature o i l and gas p r o v i n c e , w i t h c u r r e n t developments r e p r e s e n t i n g a l r e a d y t h e t h i r d g e n e r a t i o n (THOMAS 1986).
174
2.4.1. Western Europe 1977
-
1985
When t h e f i r s t i n t e r m e d i a t e - and h i g h - s t r e n g t h s y n t h e t i c proppants were i n t r o d u c e d i n t o t h e s t i m u l a t i o n m a r k e t a b t . 10 y e a r s ago (DOWELL SCHLUMBERGER 1985, WESTERN PETROLEUM 1985; c f . s e c t i o n 1 . 2 . 4 . ) , e s p e c i a l l y i n t h e USA and p a r t s o f Western Europe many w e l l s were w a i t i n g on h y d r a u l i c f r a c t u r i n g t h a t i n d i s p e n s i b l y r e q u i r e d s y n t h e t i c h i g h - q u a l i t y proppants which c o u l d w i t h s t a n d h i g h c l o s u r e s t r e s s e s i n deep t i g h t gas r e s e r v o i r s ( c f . s e c t i o n 2 . 2 . 2 . 1 . 3 . ) . As b e f o r e t h e i n v e n t i o n o f adequate proppants t h e t r e a t m e n t s c o u l d f r e q u e n t l y n o t be c a r r i e d o u t due t o t h e f a c t t h a t sand and o t h e r t r a d i t i o n a l m a t e r i a l s were n o t s t a b l e enough t o p r e v e n t c r u s h i n g , t h e appearance o f i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants ( t h e f i r s t commercial f i e l d a p p l i c a t i o n o f s i n t e r e d baux i t e f o r p r o p p i n g o f s t i m u l a t e d f o r m a t i o n s took p l a c e i n 1976; COOKE 1976, 1977; ATTEBERRY, TUCKER & R I T Z 1979; HICKEY, BROWN & CRITTENDEN 1981) i n i t i a t e d a boom o f h y d r a u l i c f r a c t u r i n g which became t h e golden y e a r s o f s t i m u l a t i o n i n t h e l a t e 1970‘s and p a r t i c u l a r l y t h e e a r l y 1980‘s when t h e economical f e a s i b i l i t y o f t h e t r e a t m e n t s was even enhanced by f u r t h e r r i s i n g o i l p r i c e s and espec i a l l y i n t h e l a t t e r p e r i o d i n Europe backed- up by a h i g h US $ exchange r a t e ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) . The most i m p o r t a n t areas o f f r a c t u r i n g performance a r e Germany FRG and B r i t i s h Southern N o r t h Sea ( c f . f i g . 6 as w e l l as t a b s . 6 and 7) which a r e d i s c u s s e d i n t h e s e c t i o n s below.
2.4.1.1. Germany FRG I n Western Europe, t h e main t a r g e t s o f MHF s t i m u l a t i o n i n t h e p e r i o d 1977 1985 have been deep t i g h t R o t l i e g e n d ( c f . c h a p t e r 3 ) and C a r b o n i f e r o u s gas-beari n g sandstones i n Germany FRG onshore, N e t h e r l a n d s onshore and o f f s h o r e , and B r i t i s h Southern N o r t h Sea. I n some cases, f r a c t u r e o p e r a t i o n s have a l s o been performed i n B u n t s a n d s t e i n g a s - b e a r i n g sandstones i n t h e mentioned areas ( s e d i m e n t o l o g i c a l and d i a g e n e t i c a l aspects o f B u n t s a n d s t e i n and R o t l i e g e n d d e p o s i t s as a base f o r r e s e r v o i r m o d e l l i n g a r e d i s c u s s e d i n PLEIN 1978, LUTZNER 1981, R O BINSON 1981; DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS 1982; GLENNIE 1983, 1986; LEE 1984; MADER 1981, 1982, 1983, 1984, 1985, 1988 b; MADER & TEYSSEN 1985, MADER & YARDLEY 1985; BIFANI, GEORGE & LEVER 1987; GRALLA 1988, MADER & CHATALOV 1988, M A R T I N & EVANS 1988 and RICHARDSON, SANGREE & SNEIDER 1988; c f . s e c t i o n 3 . 2 . 1 . 1 . ) . Some aspects o f deep gas and s h a l l o w o i l f r a c t u r i n g a r e o u t l i n e d as f o l l o w s .
2.4.1.1.1.
Deep gas f r a c t u r i n g
The most i m p o r t a n t f i e l d s where MHF s t i m u l a t i o n t r e a t m e n t s o f R o t l i e g e n d and C a r b o n i f e r o u s deep t i g h t gas r e s e r v o i r s have been c a r r i e d o u t i n Germany FRG a r e i n t h e E a s t Hannover area ( f i e l d names a r e p r e s e n t e d i n a l p h a b e t i c a l o r d e r i n each r e g i o n ) Hamwiede and W i t t o r f i n t h e C a r b o n i f e r o u s and E b s t o r f , Munsterl a g e r , Preyersmuhle-Hastedt and S o h l i n g e n i n t h e R o t l i e g e n d ; i n t h e South Hannov e r area A l f e l d - E l z e i n t h e R o t l i e g e n d and H i l d e s h e i m e r Wald i n t h e C a r b o n i f e rous; i n t h e South Oldenburg area Cappeln, Goldenstedt, Syke, Uchte, Varnhorn and Wietingsmoor i n t h e C a r b o n i f e r o u s ( t h e R o t l i e g e n d i s i n t h e l a t t e r area n o t p r e s e n t o r n o t develuped i n r e s e r v o i r f a c i e s ) ; i n t h e West Emsland area Rutenbrock i n t h e R o t l i e g e n d and Emlichheim, Fehndorf, Frenswegen and Rutenbrock i n t h e C a r b o n i f e r o u s (accompanied by Coevorden on t h e i m m e d i a t e l y n e i g h b o u r i n g Dutch t e r r i t o r y on t h e o t h e r s i d e o f t h e s t a t e boundary); and i n t h e E a s t F r i e s l a n d area Hamburger Sand, Leer and Leybucht i n t h e R o t l i e g e n d and Uphuser Meer i n t h e C a r b o n i f e r o u s ( c f . maps and e x p l a n a t i o n s i n BRINKMANN 1982, JOHN 1983; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985; SCHR’CIDER 1986; c f . f i g . 4 and t a b . 6 ) . Deep gas f r a c t u r i n g i n R o t l i e g e n d and C a r b o n i f e r o u s t i g h t sandstones i n Germany FRG i s i l l u s t r a t e d a l o n g t h e l i n e s o f r e s e r v o i r d e p t h and p r o p p a n t q u a n t i t i e s , f r a c t u r e l e n g t h and s t i m u l a t i o n r a t i o , and c o s t and temporal e x e c u t i o n o f ’ t r e a t m e n t s as f o l l o w s . The t e c h n i c a l and economical f r a c t u r i n g p o t e n t i a l i n t h e p e r i o d 1985 - 1989 i s a l s o b r i e f l y o u t l i n e d .
175
Hydraulic Proppant Fracturing and Gravel Packing in Europe CENTRES OF ACTIVITY I N PAST AND FUTURE I
m a s s i v e hydraulic fracturing (MHF)
= smaller-scale fracturing
A
=
gravel packing
0
Moscow Kaliningrade
Smolensk
Charkov
0
500 krn
Fig. 2
Overview map o f p a s t , p r e s e n t and f u t u r e h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g p o t e n t i a l i n Western and E a s t e r n Europe. D i s t i n c t i o n c a n b e made i n t h e p r o p p a n t s t i m u l a t i o n m a r k e t between m a s s i v e h y d r a u l i c f r a c t u r i n g , s m a l l e r s c a l e f r a c t u r i n g , and g r a v e l p a c k i n g .
176
2 .4.1.1.1.1.
Reservoir depth and proppant quantities
Comments on r e s e r v o i r depth and p r o p p a n t q u a n t i t i e s i n h y d r a u l i c f r a c t u r i n g t r e a t m e n t s i n Germany FRG a r e g i v e n f o r t h e p e r i o d s 1975 - 1978 when h i g h s t r e n g t h s y n t h e t i c proppants were i n t r o d u c e d t o t h e hydrocarbon s t i m u l a t i o n mark e t , and 1978 - 1985 c o m p r i s i n g t h e golden y e a r s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n b o t h Europe and USA.
2.4.1.1.1.1.1.
1975
-
1978
The demand/supply s i t u a t i o n i n t h e European gas market l e d t o an e x t e n s i v e e f f o r t t o develop t i g h t gas r e s e r v o i r s i n Germany FRG s t a r t i n g i n 1977 (BRINKMANN 1982) w i t h some p r e c u r s o r t r e a t m e n t s b e g i n n i n g i n 1975. Massive h y d r a u l i c f r a c t u r i n g i n Germany FRG was f i r s t a p p l i e d i n 1975 i n t h e w e l l A l f e l d - E l z e Z 4 ( R o t l i e g e n d ) and 1976 i n t h e w e l l H i l d e s h e i m e r Wald Z 1 ( C a r b o n i f e r o u s ) where as a consequence o f s t i l l u n a v a i l a b i l i t y o f h i g h - s t r e n g t h p r o p p a n t s a b t . 80 150 t o f n a t u r a l sand have been i n s e r t e d i n t o t h e c r a c k s (ERDOEL-ERDGAS AKTUELL 1975, 1976 a ) . S m a l l e r - s c a l e sand f r a c t u r i n g has a l r e a d y been c a r r i e d o u t e a r l i e r , w i t h success, however, h a v i n g been l i m i t e d by r a p i d c l o s i n g o f t h e c r a c k s due t o sand c r u s h i n g i n t h e h i g h - p r e s s u r e regime o f t h e deep gas r e s e r v o i r s (TUNN 1971). I n 1978, b o t h s i n t e r e d b a u x i t e and z i r c o n i a - s i l i c a t e p r o p p a n t s ( c f . s e c t i o n 1 . 3 . ) were f i r s t used i n massive h y d r a u l i c f r a c t u r i n g o p e r a t i o n s i n Germany FRG when a b t . 70 t o f s i n t e r e d b a u x i t e and a b t . 140 t o f z i r c o n i a - s i l i c a t e p r o p p a n t s were pumped i n t o two d i f f e r e n t C a r b o n i f e r o u s r e s e r v o i r s t o r e y s i n t h e w e l l Hamwiede Z 2 (ERDOEL-ERDGAS AKTUELL 1978 a ) . The f i r s t s u c c e s s f u l a p p l i c a t i o n o f h i g h - s t r e n g t h p r o p p a n t s opened s u i t a b l e p e r s p e c t i v e s f o r t h e economicall y f e a s i b l e a c q u i s i t i o n o f R o t l i e g e n d and C a r b o n i f e r o u s t i g h t gas r e s e r v e s , and a l t h o u g h i n t h e t r a n s i t i o n a l phase o c c a s i o n a l l y s t i l l n a t u r a l sand was used ( w e l l Cappeln Z 4 a was t r e a t e d w i t h 250 t o f sand i n l a t e 1978; ERDOEL-ERDGAS AKTUELL 1979 a ) , s y n t h e t i c h i g h - s t r e n g t h proppants were a b u n d a n t l y t o e x c l u s i v e l y u t i l i z e d i n progressively increasing quantities.
2 . 4 . 1 . 1 . 1 . 1 . 2 . 1978
-
1985
The b i g g e s t t r e a t m e n t s and t h e l a r g e s t number o f massive h y d r a u l i c f r a c t u r i n g j o b s i n Europe d u r i n g t h e e a r l i e r p a r t o f t h e p e r i o d 1977 - 1985 have been c a r r i e d o u t i n Germany FRG (BRINKMANN, FUHRBERG & SCHUBER 1980; BRINKMANN, KRUMER & R E I N I C K E 1980; BRINKMANN 1982, GUNDERMANN 1982, SCHWARZ & SCHUBER 1982; JOHN 1983, 1987; KLOSE & KRUMER 1983, BLEAKLEY 1984, RIECKMANN & PUSCH 1984, LEICHT 1985; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985; SOMMER 1987, 1988) where t h e R o t l i e g e n d ( c f . c h a p t e r 3) and C a r b o n i f e r o u s r e s e r v o i r r o c k s ( c f . f i g . 3 ) a r e i n depths between 3,000 and 6,000 m ( e q u i v a l e n t t o between 9,000 and 18,000 f t ) w i t h b o t t o m h o l e temperatures between 120 and 180 OC (PHILIPP & R E I N I C K E 1982, JOHN 1983, KLOSE & KRUMER 1983) and t h u s i n almost a l l t h e cases t h e whole t r e a t m e n t had t o be made w i t h i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p pants, and o n l y i n a few e x c e p t i o n s a l s o sand had been used ( c f . s e c t i o n 3.3.2.). D e t a i l s o f R o t l i e g e n d and C a r b o n i f e r o u s f r a c t u r e o p e r a t i o n s a r e g i v e n by KLOSE & KRUMER (1983); R E I N I C K E , BRINKMANN, SCHWARZ & HUENI (1985) and SOMMER (1987), and aspects o f i n d i v i d u a l massive h y d r a u l i c f r a c t u r e o p e r a t i o n s a r e a l s o r e p o r t e d by ERDOEL-ERDGAS AKTUELL (1977, 1980 b ) and SCHRUDER ( 1 9 8 0 ) . As such h i g h r e s e r v o i r depths o f p r o d u c t i v e sandstones and c o n s e q u e n t l y a l s o c l o s u r e s t r e s s e s a r e nowhere e l s e reached i n t h e R o t l i e g e n d and C a r b o n i f e r o u s g a s - f i e l d b e l t f r o m G r e a t B r i t a i n t o Poland, and a l s o a s i m i l a r c o n c e n t r a t i o n o f p r o s p e c t i v e patches w i t h h y d r a u l i c f r a c t u r i n g p o t e n t i a l i s n o t f o u n d e l s e where, Germany FRG was f o r many y e a r s t h e b i g g e s t European m a r k e t f o r i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s . Several dozens o f l a r g e - s c a l e f r a c t u r e j o b s
177 have been c a r r i e d o u t d u r i n g t h i s time i n Northwest Germany s i n c e 1976/1977 ( a couple o f operations p e r y e a r ) i n t h e East Hannover, South Oldenburg, West Emsl a n d and East F r i e s l a n d areas (ALBERTSEN 1985) between the bounding gas f i e l d s Groningen/The Netherlands (STKUBLE & MILIUS 1970, ROSSUM 1975, BEEK & TROOST 1978) i n the west and Salzwedel/Germany GDR i n t h e e a s t (JOHN 1983; c f . f i g . 4) where t h e t r e a t e d w e l l s occupy a broad seam r e p r e s e n t i n g a zone o f h i g h f r a c t u r i n g p o t e n t i a l a l s o f o r f u r t h e r w e l l s i n t h e f u t u r e . Proppant q u a n t i t i e s consumed i n t h e MHF treatments exceeded i n most cases 100 t, w i t h i n a t l e a s t one t h i r d o f the o p e r a t i o n s comprising more than 200 t and i n t h e b i g g e s t j o b s even r a n g i n g between 400 and 650 t.
2.4.1.1.1.2. Fracture length and stimulation ratio F r a c t u r e l e n g t h ( a s evaluated w i t h a c t u a l treatment data) v a r i e d between 100 and 550 m, and propped f r a c t u r e h e i g h t ranged w i t h i n the l i m i t s o f 10 m ( t h i n Carboniferous sandstones) and 115 m ( t h i c k Rotliegend sandstones; KLOSE & KRUMER 1983; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985). Various w e l l s have been f r a c t u r e d i n several v e r t i c a l l y successive horizons t h a t are separated by t h i c k mudstone i n t e r v a l s from each other, and have been f r a c t u r e d i n d i v i d u a l l y and then commingled f o r pressure b u i l d - u p a n a l y s i s and p a r t i a l l y a l s o p r o d u c t i o n (REINICKE, BRINKMANN, SCHWARZ & HUENI 1985; w i t h commingling here i m p l y i n g i n t e r h o r i z o n communication o n l y through the w e l l b o r e and n o t v i a t h e f r a c t u r e i n these multilayer/multifracture systems; BENNETT, REYNOLDS & RAGHAVAN 1982 a, 1982 b; c f . s e c t i o n s 4.2.1.1. and 4.10.1.). While most o f the f r a c t u r i n g operat i o n s have been g r e a t successes r e s u l t i n g i n s t i m u l a t i o n r a t i o s o f 8 - 10, o t h e r treatments have been l e s s e f f e c t i v e and o n l y gave r i s e t o improvement r a t i o s o f 3 - 6, and a l s o several jobs have turned o u t t o be f a i l u r e s m a i n l y due t o h i g h water s a t u r a t i o n ( c f . s e c t i o n 3.12.1.) and s o f t r e s e r v o i r rocks (BRINKMANN 1982), w i t h n e q a t i v e items i n c l u d i n q b o t h premature screenout t e r m i n a t i o n and compiete f a i l u r e , and b o r d e r l i n e ca;es have a l s o come o u t (SLUSSER & R I E C K MANN 1976).
2.4.1.1.1.3. Cost and
temoral execution o f treatments
T o t a l f r a c t u r i n g expenses i n t h e deep t i g h t formations reached up t o 6 Mio. DM o r 2.5 Mio. US $ p e r j o b (JOHN 1983, KLOSE & KRUMER 1983, BLEAKLEY 1984, LEICHT 1985). The b i g Sohlingen R o t l i e g e n d gas f i e l d has become the h i g h l i g h t o f h y d r a u l i c proppant f r a c t u r i n g i n Germany FRG p a r t i c u l a r l y due t o s i z e and success o f t h e executed jobs, b u t comparably voluminous and b e n e f i c i a l t r e a t ments have a l s o been performed i n many o t h e r f i e l d s . Late 1985 has seen some o f t h e l a r g e s t operations t h a t were ever done i n Europe which, however, have s o f a r been a l s o the l a s t j o b s i n Germany FRG. As a consequence o f t h e s e r i o u s o i l p r i c e d e c l i n e i n e a r l y 1986 ( c f . s e c t i o n 2.2.1.1.2.; t h a t had an even worse e f f e c t due t o t h e accompanying US $ exchange r a t e drop; c f . s e c t i o n 2.2.1.3.), no h y d r a u l i c proppant f r a c t u r i n g o p e r a t i o n was performed i n Germany FRG i n 1986 a t a l l , and the same subsequently a p p l i e d f o r 1987 and 1988 when i n s p i t e o f t h e s l i g h t t o moderate market recovery o n l y a few small- t o m i n i - s c a l e treatments were c a r r i e d o u t i n a time o f u n c e r t a i n t y o f o i l p r i c e e v o l u t i o n coupled w i t h extremely weak and u n r e l i a b l e US $ ( c f . s e c t i o n 2.2.1.1.3.). The t e r m i n a t i o n o f t h e MHF a c t i v i t y i n Germany FRG i s the consequence o f ser i o u s l y c u t t i n g back o f the d r i l l i n g budget ( c f . s e c t i o n 2.2.1.5.2.2.) t h a t comp r i s e s i n 1987 and 1988 probably n o t more than one t h i r d and/or one h a l f , r e s p e c t i v e l y , o f b o t h number o f items and t o t a l meterage o f t h e niveau achieved i n 1985 b e f o r e t h e beginning o f the o i l p r i c e drop. The a c t i v i t y l e v e l i n 1986 i s somewhat a r t i f i c i a l and d i s t u r b e d by v a r i o u s influences, as a f t e r t h e w o r s t f i n a l stage o f t h e o i l p r i c e f a l l i n e a r l y 1986, a l l the companies t r i e d t o del e t e o r s t r e a m l i n e as much o b j e c t s as possible, b u t t h a t was no l o n g e r a b l e f o r such items where t h e d r i l l i n g c o n t r a c t had already been awarded, t h e d r i l l i n g s i t e had been prepared o r the w e l l had even already been spudded. Containment
178 o f c a p i t a l e x p e n d i t u r e e s p e c i a l l y a f f e c t s predominantly development d r i l l i n g and c o m p l e t i o n , whereas e x p l o r a t i o n and a p p r a i s a l d r i l l i n g c o n t i n u e d t o r u n a t c o m p a r a t i v e l y h i g h e r l e v e l s i n 1986 and e a r l y 1987, b u t w e r e i n l a t e 1987 and 1988 a l s o p r o g r e s s i v e l y r e d u c e d t o a minimum as a consequence o f p e r s i s t i n g oil p r i c e and US $ weakness. A p p r a i s a l and d e v e l o p m e n t d r i l l i n g i s c u r r e n t l y o n l y p e r f o r m e d a t a l e v e l t h a t i s s u f f i c i e n t t o s e c u r e gas d e l i v e r y c o n t r a c t o b l i g a t i o n s i n t h e n e a r f u t u r e , and t o i n c r e a s e p r o v e n r e s e r v e s i n o r d e r t o a l l o w more p r o f i t a b l e gas m a r k e t i n g and t h u s b u s i n e s s c o n s o l i d a t i o n .
2.4.1.1.1.4. Technical and economical fracturing potential in 1985 - 1989 W h i l e a t l e a s t some m a r g i n a l MHF c a n d i d a t e s a r e s t i l l a v a i l a b l e , t h e o n l y p e r s p e c t i v e f o r a g a i n a r e a s o n a b l e s t i m u l a t i o n a c t i v i t y i n Germany FRG compar a b l e t o t h a t p r i o r t o 1986 w o u l d be an o i l p r i c e o f a t l e a s t US $ 20 w i t h a s u f f i c i e n t l y s t a b l e a v e r a g e v a l u e and an a g a i n c o n s i d e r a b l y s t r o n g e r US $ t h a n a t t h e moment, w i t h t h e b o u n d a r y p r o b a b l y b e i n g an exchange r a t e o f a t l e a s t 2 DM. U n f o r t u n a t e l y , however, e v e n i n m i d t o l a t e 1988 t h e r e i s n o t y e t any hope f o r e i t h e r a m e l i o r a t i o n o f t h e e c o n o m i c a l f e a s i b i l i t y o f renewed MHF t r e a t m e n t s i n Germany FRG, and t h e r e i s d e s p i t e a s l i g h t improvement o f t h e US $ exchange r a t e e v o l u t i o n i n t h e l a s t t h i r d o f 1988 a l m o s t n o hope t h a t t h e b o u n d a r y o f 2 DM i s g o i n g t o be c r o s s e d i n t h e f o r e s e e a b l e f u t u r e , because a t t h e e n d o f 1988 t h e US $ was a l r e a d y s l i d i n g down a g a i n ( c f . s e c t i o n 2.2.1.3.). T h e r e f o r e s i n c e t h e l a s t j o b s h a v i n g been c a r r i e d o u t i n l a t e 1985, n o mass i v e h y d r a u l i c f r a c t u r i n g t r e a t m e n t has been p e r f o r m e d f o r more t h a n t h r e e y e a r s i n t h e p e r i o d 1986 - 1988, and a c c o r d i n g t o t h e c u r r e n t l y s t i l l d e p r e s s e d o i l p r i c e and US $ exchange r a t e s i t u a t i o n , i t l o o k s l i k e as i f a l s o i n 1989 n o s t i m u l a t i o n w i l l t a k e p l a c e . W h i l e i n m o s t o f t h e MHF t r e a t m e n t s c a r r i e d o u t s o f a r i n Germany FRG c o n v e n t i o n a l w a t e r - o r o i l - b a s e d g e l s o f v a r i o u s v i s c o s i t i e s have been u s e d as f r a c t u r i n g f l u i d s , r e c e n t l y a l s o CO2-foam has been t a k e n as a g e n t t r a n s p o r t i n g and p l a c i n g t h e p r o p p a n t s b e i n g a p i l o t a p p l i c a t i o n i n Eur o p e (SLATTER, RUCKER & C R I S P 1986; SOMMER 1987, 1988; c f . s e c t i o n 3 . 1 1 . ) . Labor a t o r y t e s t i n g has r e v e a l e d t h a t t h e C a r b o n i f e r o u s r e s e r v o i r r o c k s e x h i b i t a h i g h d e g r e e o f p l a s t i c i t y (SLUSSER & RIECKMANN 1976; c f . s e c t i o n 4 . 2 . 3 . 2 . 3 . ) .
2.4.1.1.2. Shallow oil fracturing W h i l e s h a l l o w o i l f r a c t u r i n g has been h i t h e r t o p e r f o r m e d i n v a r i o u s European c o u n t r i e s a l m o s t e x c l u s i v e l y w i t h n a t u r a l sand as p r o p p i n g a g e n t , some r e s e r v o i r s e x i s t where i n t e r m s o f a d e q u a t e c l o s u r e s t r e s s r e s i s t i v i t y i n t e r m e d i a t e strength synthetic proppants are required.
2.4.1.1.2.1. Natural sand From t h e s u i t e o f s m a l l f r a c t u r e j o b s c a r r i e d o u t w i t h a b t . 20 - 70 t o f sand, s e v e r a l campaigns i n C r e t a c e o u s s h a l l o w s a n d s t o n e o i l r e s e r v o i r s i n t h e Emsland a r e a i n t h e b o u n d a r y b e l t between N e t h e r l a n d s and Germany FRG d e s e r v e p a r t i c u l a r mentioning. I n t h e T e r t i a r y o f t h e Vienna B a s i n i n A u s t r i a , sometimes even o c c a s i o n a l l y v e r y s m a l l t r e a t m e n t s i z e s o f a b t . 6 - 12 t c a n b e t h e optimum (DOLAK & PERNTHANER 1976, MURER 1981) and a r e s u c c e s s f u l b o t h i n new w e l l s and i n o l d w e l l s p r o d u c i n g u n d e r l o w f o r m a t i o n s t r e s s and w i t h good i s o l a t i o n a g a i n s t n e i g h b o u r i n g i n t e r v a l s . I n t h e Lower C r e t a c e o u s B e n t h e i m e r Sands t e i n ( V a l a n g i n i a n ) i n t h e Emsland a r e a , numerous f r a c t u r e t r e a t m e n t s as w e l l as v a r y i n g amounts o f g r a v e l pack o p e r a t i o n s have been c a r r i e d o u t o n b o t h s i d e s o f t h e s t a t e b o u n d a r y i n s e v e r a l f i e l d s c o m p r i s i n g m a i n l y R u h l e r m o o r and G e o r g s d o r f on t h e German s i d e and Schoonebeek on t h e D u t c h s i d e (GESEMANN & BADURA 1976; f o r l o c a t i o n maps and f i e l d c r o s s s e c t i o n s c f . BOIGK 1980; c f . fig.
179
Fig. 3
Schematical geological cross-section (without scale) through the gas-bearing reservoir column in Northwest Europe (Germany FRG and adjoining areas). Tight gas sandstones are predominantly Rotliegend and Carboniferous, whereas the Buntsandstein requires only in some regions artificial transmissibility enhancement by stimulation. While Carboniferous, Rotliegend and Buntsandstein pays contain exclusively sweet gas (in the Rotliegend, apart from combustible methane occasionally also inert nitrogen occurs in larger shares), the Zechstein carbonates include sour gas with considerable amounts of the aggressive and toxic hydrogen sulphide. In many cases, the individual storeys of the reservoir pile are characterized by different gas-water-contact levels. Rotliegend anticlines and horst-graben-structures are especially favourably developed below Zechstein salt diapirs, because the thick salt plugs resulted in retarded heat flux and lower reservoir temperature than in the surroundings of the salt domes. The Carboniferous contains not only sandstone reservoirs, but also bears numerous coal seams where the gas for all the four pay horizons originated during course of coalification and subsequently migrated through the cover strata along faults and fissures into the various traps. The thickest reservoirs with the best petrophysical development in the Rotliegend occur in the lower and upper parts of the section, whereas the middle portion of the sequence is in most cases characterized by low-permeability sandstones which require massive hydraulic fracturing stimulation in order to achieve economically feasible gas production rates. The tight Carboniferous needs in most cases fracturing treatments for allowing profitable gas withdrawal, while in the Buntsandstein, hydraulic stimulation has only occasionally to be performed in some areas of the red bed basin. Legend : 1 = gas-bearing sandstone, 2 = water-containing sandstone, 3 = coal seam, 4 = carbonates, 5 = mudstones and marls; 6 = Zechstein salt layers, cushions and plugs; 7 = gas-water-contact, 8 = tight gas-bearing sandstone reservoirs with various hydraulic proppant fracturing potential, 9 = unconformity.
180
. H 100 km
Overview map of centres of hydraulic proppant fracturing and gravel packing in past, present and future in Western and Eastern Europe. F o r more details in Germany FRG, Netherlands and North Sea cf. figs. 4 and 6, and for general outline cf. also fig. 2.
181
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GREAT BRITAIN GERMANY FRG
182 5 and t a b . 6 ) . Sand f r a c t u r i n g was a l s o common s i n c e t h e e a r l y days o f o i l e x p l o r a t i o n and p r o d u c t i o n s h o r t l y a f t e r t h e Second World War in many p a r t s o f t h e oil f i e l d b e l t i n N o r t h w e s t Germany and t o s u b o r d i n a t e amounts i n t h e l a s t 20 y e a r s a l s o i n Southern Germany, w i t h some r e p r e s e n t a t i v e f i e l d s b e i n g i n N o r t h w e s t Germany Adorf, Emlichheim, Meerdorf and Scheerhorn, and i n Southern Germany Hauerz, Monchsrot and P f u l l e n d o r f - O s t r a c h . To m i n o r amounts, sand f r a c t u r i n g was a l s o done i n T e r t i a r y sandstones i n t h e Vienna B a s i n i n A u s t r i a ( f o r example i n t h e f i e l d s Matzen, H o c h l e i t e n , Neusiedel, Stockerau, T r a t n a c h and Z i s t e r s d o r f ; DOLAK & PERNTHANER 1976, MURER 1981) as w e l l as i n t h e Po V a l l e y and t h e A d r i a t i c Sea i n I t a l y ( c f . f i g . 5 and t a b . 8 ) .
2.4.1.1.2.2.
Intermediat e-st rengt h synt het i c proppant s
The low c l o s u r e s t r e s s e s i n t h e s h a l l o w r e s e r v o i r depth o f a b t . 700 - 2,500 m (2,100 - 7,500 f t ) d i d so f a r n o t r e q u i r e t h e a p p l i c a t i o n o f i n t e r m e d i a t e s t r e n g t h s y n t h e t i c proppants i n o i l - w e l l f r a c t u r i n g i n Germany FRG and o t h e r c o u n t r i e s . The few deeper w e l l s i n Germany FRG where sand would n o t have been an adequate p r o p p i n g agent were suspended f o r t h e t i m e b e i n g due t o i n s u f f i c i e n t r e s e r v e s t o e c o n o m i c a l l y j u s t i f y s t i m u l a t i o n w i t h expensive p r o p p a n t s i n t h e p e r i o d o f b o t h h i g h o i l and p r o p p a n t p r i c e s i n t h e e a r l y 1980's. A f t e r the d e c l i n e o f b o t h o i l and p r o p p a n t p r i c e s , however, t h e j o b s a r e i n t h e l a t e 1980's s t i l l f a r f r o m b e i n g f e a s i b l e , even in l i g h t o f Only s m a l l q u a n t i t i e s o f sand o r p r o p p a n t s b e i n g needed i n o i l - w e l l f r a c t u r i n g t h a t do n o t have such a l a r g e share o f t h e t o t a l c o s t o f t h e s t i m u l a t i o n t r e a t m e n t in comparison t o deep gas w e l l s .
2.4.1.2. B r i t i s h Southern North Sea I n t h e B r i t i s h Southern N o r t h Sea, R o t l i e g e n d and p a r t i a l l y a l s o C a r b o n i f e r o u s r e s e r v o i r r o c k depth i s much s h a l l o w e r and c l o s u r e s t r e s s c o n s e q u e n t l y i s c o n s i d e r a b l y l o w e r t h a n i n Germany FRG and N e t h e r l a n d s (ALLISON 1986; c f . sect i o n 3 . 3 . ) , and t h u s many f r a c t u r e t r e a t m e n t s can be c a r r i e d o u t a t l e a s t p a r t i a l l y s t i l l w i t h sand i n c o n t r a s t t o t h e o t h e r area where o p e r a t i o n s w i t h a l most e x c l u s i v e l y i n t e r m e d i a t e - and/or h i g h - s t r e n g t h proppants a r e i n e v i t a b l e f o r s u c c e s s f u l l o n g - t e r m achievement o f t h e goal o f t h e s t i m u l a t i o n j o b . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l aspects, v a r i o u s p o i n t s o f m o d e r a t e l y deep gas f r a c t u r i n g , t a i l - i n f r a c t u r i n g p o l i c y , d i f f e r e n c e s between o f f s h o r e and onshore f r a c t u r i n g , and m o d e r a t e l y deep o i l f r a c t u r i n g a r e o u t l i n e d .
2.4.1.2.1. General aspects Due t o a t i m e l a g i n e x p l o r a t i o n and development d r i l l i n g and c o m p l e t i o n w i t h r e s p e c t t o t h e onshore areas, h y d r a u l i c p r o p p a n t f r a c t u r i n g i n t h e B r i t i s h and Dutch Southern N o r t h Sea p r e d o m i n a n t l y took p l a c e i n t h e l a t e r p a r t o f t h e golden s t i m u l a t i o n p e r i o d ( c f . s e c t i o n 3 . 3 . ) , w i t h t h i s b e i n g one o f t h e r e a sons why e s p e c i a l l y t h e B r i t i s h Southern N o r t h Sea i s s t i l l a v e r y p r o l i f e r i c area f o r s t i m u l a t i o n i n t h e coming y e a r s d e s p i t e t h e d e t e r i o r a t e d g e n e r a l econom i c a l s i t u a t i o n as a consequence o f much l o w e r o i l p r i c e s t h a n b e f o r e (SCHUNE I C H 1983, STEPHENS & MARTINS 1985; c f . s e c t i o n 2.2.1.1.2.) and t h e a d d i t i o n a l impact o f t h e c o n s i d e r a b l y low U S $ exchange r a t e s i n c e a b t . e a r l y 1986 ( c f . section 2.2.1.3.). An o u t l i n e o f t h e p e t r o l e u m p o t e n t i a l o f t h e N o r t h Sea i s a l s o g i v e n by HOYLE (1983), and NYSTAD (1981) p r o v i d e s and economical a n a l y s i s o f t h e N o r t h Sea o i l and gas r e g i o n . Another m a j o r reason f o r t h e i n d i s p e n s i b l e o p e r a t i o n o f some l a r g e s t i m u l a t i o n campaigns i n t h e coming y e a r s a r e a l r e a d y s i g n e d gas d e l i v e r y c o n t r a c t s ( t h a t guarantee t h e s a l e o f t h e gas which s t i l l has t o be a c q u i r e d by f i e l d de-
183 velopment; OILMAN 1987 a; c f . s e c t i o n 2.2.1.6.2.) which n e c e s s i t a t e t o f u l f i l l t h e e n t e r e d commitments almost r e g a r d l e s s o f t h e economical and p o l i t i c a l framework ( c e r t a i n l y backed-up, however, by a t l e a s t p a r t i a l l y s t i l l good p r i c e s hav i n g been f i x e d i n t h e agreements, as w e l l as by government p r o t e c t i o n t o keep gas i m p o r t s o f c o n s i d e r a t i o n s ; c f . s e c t i o n 2.2.1.6.3.). The Southern N o r t h Sea i s t h e t r a d i t i o n a l c e n t r e o f gas p r o d u c t i o n i n Great B r i t a i n and has h i s t o r i c a l l y c o n t r i b u t e d up t o a b t . 90 % o f t h e t o t a l gas s u p p l y i n t h e U n i t e d Kingdom, and i t i s e x p e c t e d t h a t t h e Southern N o r t h Sea w i l l c o n t i n u e t o p l a y i t s domin a n t r o l e u n t i l t h e end o f t h e c e n t u r y , because i t s t i l l h o l d s o v e r 50 % of a l l U n i t e d Kingdom proven and p r o b a b l e gas r e s e r v e s . The a c c e n t u a t e d p o s i t i o n o f t h e B r i t i s h Southern N o r t h Sea i n terms o f s t i m u l a t i o n p o t e n t i a l and hydrocarbon p r o s p e c t i v i t y w i t h i n t h e whole N o r t h Sea i n comparison t o t h e s e c t o r s o f o t h e r c o u n t r i e s i s h i g h l i g h t e d by t h e predominant share o f t h i s area i n t h e r e c o r d d r i l l i n g r i g c o u n t i n 1984/1985 (OILMAN 1985). Economical e v a l u a t i o n s o f t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t a r e a l s o g i v e n by FISHMAN (1986), GREGORY (1986), THOMAS (1986), ALGAR (1987 a ) , BAND (1987) and POTTER (1987). Successful s t i m u l a t i o n p r a c t i c e s i n t h e Dutch Southern N o r t h Sea a r e summarized by COULTER & P U R V I S (1980). Development d e t a i l s f o r b o t h o f f s h o r e p r o v i n c e s u n t i l t h e y e a r 2000 a r e g i v e n by OFFSHORE ENGINEER (1985 c, 1986 c ) . A p a r t f r o m p r o d u c t i o n w e l l s , some f r a c t u r e j o b s have i n t h i s area a l s o been c a r r i e d o u t i n underground gas s t o r a g e w e l l s ( c f . HOLLIS 1982).
2.4.1.2.2. Moderately deep gas fracturing Proppant q u a n t i t i e s between 100 and 250 t a r e most common f o r h y d r a u l i c f r a c t u r i n g o f R o t l i e g e n d g a s - b e a r i n g sandstones i n t h e B r i t i s h Southern N o r t h Sea, w i t h l a r g e r j o b s r e q u i r i n g 250 - 400 t o f p r o p p i n g m a t e r i a l b e i n g r a r e except i o n s . Some aspects o f h y d r a u l i c proppant f r a c t u r i n g i n C a r b o n i f e r o u s and R o t l i e g e n d a r e o u t l i n e d as f o l l o w s .
2.4.1.2.2.1. Carboniferous The C a r b o n i f e r o u s has so f a r i n t h e B r i t i s h Southern N o r t h Sea o n l y i n a few cases been encountered gas-bearing, as most o f t h e deeper r e s e r v o i r s a r e below t h e g a s - w a t e r - c o n t a c t which i s s i m i l a r l y as f o r m a t i o n d e p t h r i s i n g f r o m Germany FRG i n t h e E a s t t o t h e B r i t i s h Southern N o r t h Sea i n t h e West (some more Carbon i f e r o u s d i s c o v e r i e s a r e expected d u r i n g p r o g r e s s i n g e x p l o r a t i o n ; OILMAN 1987 e, VIELVOYE 1987 b, CAPEL 1988, PETROLEUM REVIEW 1988 d ) . I n a d d i t i o n , t h e Carb o n i f e r o u s u n d e r l y i n g t h e R o t l i e g e n d ( c f . f i g . 3) has i n many cases been almost t o t a l l y i g n o r e d so f a r , because t h e m a j o r i t y o f e x p l o r a t i o n and development w e l l s have been d r i l l e d t o a t a r g e t depth j u s t p e n e t r a t i n g t h e t o p o f t h e Carbon i f e r o u s (OILMAN 1987 e ) as a consequence o f t h e c o m p a r a t i v e l y p o o r r e s e r v o i r f a c i e s and t h e h i g h g a s - w a t e r - c o n t a c t i n t h i s s t o r e y . The B u n t s a n d s t e i n has i n some f i e l d s i n t h e B r i t i s h and e s p e c i a l l y t h e Dutch Southern N o r t h Sea a l s o r e p e a t e d l y been f r a c t u r e d .
2.4.1.2.2.2. Rot 1 iegend R e s e r v o i r d e p t h o f t h e R o t l i e g e n d sandstones i s i n most cases a b t . 8,000 9,000 f t ( c o r r e s p o n d i n g t o a b t . 2,700 - 3,000 m), and o n l y i n some patches a b t . 6,000 f t ( a b t . 2,000 m) f o r t h e s h a l l o w e r range and a b t . 10,000 f t ( a b t . 3,300 in) f o r t h e deeper range ( c f . f i g s . 3 and 6 ) . Summaries o f N o r t h Sea w e l l s t i m u l a t i o n a r e g i v e n by LAFLEUR & JOHNSON (1973) and MARTINS & HARPER ( 1 9 8 5 ) . The most i m p o r t a n t f i e l d s where MHF s t i m u l a t i o n o p e r a t i o n s o f t i g h t gas r e s e r v o i r s have a l r e a d y been c a r r i e d o u t and/or a r e s t i l l g o i n g t o be p e r f o r m e d a r e i n t h e B r i t i s h N o r t h Sea i n t h e R o t l i e g e n d ( f i e l d names a r e s t a t e d i n a l p h a b e t i c a l o r d e r ) Amethyst, I n d e f a t i g a b l e , Leman Bank, Ravenspurn, Rough, Thames, V a l i a n t ,
184 Tab. 5
Centres of past and future activity of hydraulic proppant fracturing in Europe z
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Fig. 7
Various possibilities of proppant distribution types within hydraulic fractures exist including tail-in, lead-in, banking and pillaring of proppants in order to optimize flow conditions and closure stress resistivity. Legend : 1 = reservoir sandstone surrounding the hydraulic fracture, 2 = bounding mudstones, 3 = fine-grained proppants (40/60 o r 50/70), 4 = medium-grained proppants (20/40), 5 = coarse-grained proppants (16/20, 12/20 o r 8/12), 5 = boundary of the hydraulic fracture wing, 7 = perforations. Schematically, no scale.
185
Proppant distribution types within hydraulic fractures
Fig. 7
I
186 Vanguard, Vulcan and West Sole, and i n t h e B u n t s a n d s t e i n Esmond, Forbes and Gordon ( c f . f i g s . 6 and 8 as w e l l as t a b . 7 ) . I n t h e Dutch N o r t h Sea, emphasis o f t h e a c t i v i t y i s i n t h e R o t l i e g e n d i n Ameland, K-12, L-8, L-10 and P-2, whereas B u n t s a n d s t e i n s t i m u l a t i o n focusses on P-6 and P-8 ( c f . f i g s . 6 and 8 as w e l l as tab. 8 ) . W h i l e i n t h e p a s t f r a c t u r e j o b s have been p r e d o m i n a n t l y done w i t h e i t h e r o n l y sand o r e x c l u s i v e l y i n t e r m e d i a t e - t o h i g h - s t r e n g t h proppants, i n t h e l a s t y e a r s t h e concept o f t a i l i n g - i n (HOWARD & FAST 1970 a, HOWELL & THOMAS 1978; HICKEY, BROWN & CRITTENDEN 1981) has been more and more a p p l i e d , w i t h t h i s met h o d commonly i n c l u d i n g f i r s t pumping o f 100 - 200 t o f sand o f 20/40 g r a i n s i z e f o l l o w e d by a t a i l - i n o f 25 - 75 t o f i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s o f 20/40 o r 16/20 g r a i n s i z e . S u b o r d i n a t e a p p l i c a t i o n s a r e t h r e e - f o l d d i v i d e d j o b s w i t h s u c c e s s i v e pumping o f 20/40, 16/20 and f i n a l l y 12/20 g r a i n s i z e o f p r o p p i n g m a t e r i a l , o r i n r a r e r cases a l s o successions o f i n j e c t i o n o f 40/60, 20/40 and 16/20 o r 12/20 p r o p p a n t g r a i n s i z e s .
2 . 4 . 1 . 2 . 3 . Tail-in fracturing policy T a i l i n g - i n can comprise sequences o f d i f f e r e n t p r o p p a n t t y p e s and/or g r a i n s i z e s , w i t h t h e b e s t r e s u l t s b e i n g achieved when v a r y i n g b o t h parameters (compar a t i v e e f f e c t i v i t y s t u d i e s o f v a r i o u s p r o p p a n t successions and m i x t u r e s a r e c a r r i e d o u t by McOANIEL & WILLINGHAM 1978; HICKEY, BROWN & CRITTENDEN 1981). The o u t l i n e as f o l l o w s i n c l u d e s aims and performance o f p r o p p a n t t a i l - i n schedules, p r i n c i p l e s o f t a i l - i n proppant s t a g i n g , proppant c o s t containment ( b e i n g t h e m a j o r t a i l - i n advantage), p r o p p a n t m i x i n g and s e t t l i n g ( b e i n g t h e most s e r i o u s t a i l - i n drawbacks), p r o p p a n t flowback p r e v e n t i o n , and t a i l - i n o p t i m i z a t i o n and e f f e c t i v i t y . The i l l u s t r a t i o n o f t h e proppant t a i l - i n p o l i c y i s i n c l u d e d i n t o t h i s c h a p t e r due t o t h e predominant a p p l i c a t i o n o f t a i l - i n o p e r a t i o n s i n R o t l i e gend f r a c t u r e s t i m u l a t i o n o f t i g h t gas r e s e r v o i r sands i n t h e B r i t i s h Southern N o r t h Sea.
2.4.1.2.3.1.
Aims and performance o f proppant tail-in schedules
T a i l i n g - i n u s u a l l y i n c l u d e s succession o f d i s c r e t e q u a n t i t i e s t h a t a r e n o t i n t e r m i x e d ( c f . f i g . 7 ) ; b l e n d i n g o f m a t e r i a l s (such as f o r m e r l y done w i t h sand and g l a s s beads o r s i n t e r e d b a u x i t e ; c f . s e c t i o n 1.4.11.1.1.) i s nowadays no l o n g e r c o n s i d e r e d , as a m i x t u r e o f d i f f e r e n t t y p e s and/or g r a i n s i z e s o f p r o p p a n t s l e a d s t o e q u a l i z a t i o n and homogenization and p a r t i a l l y a l s o compensation o f f a v o u r a b l e p r o p e r t i e s o f t h e i n d i v i d u a l components (McOANIEL & WILLINGHAM 1978 and KOHLHAAS 1982 r e p o r t t h a t m i x t u r e s o f sand and g l a s s beads have a t h i g h s t r e s s n o t any c o n d u c t i v i t y more t h a n t h a t o f sand a l o n e o r have even l e s s depending on m i x t u r e r a t i o , g r a i n s i z e and s t r e s s regime; c f . c h a p t e r 1 . 4 . 1 1 . ) . The use o f h i g h e r - q u a l i t y p r o p p a n t t y p e s and/or l a r g e r p r o p p a n t g r a i n s i z e s f o r t a i l - i n s p r o v i d e s maximum c o n d u c t i v i t y near t h e w e l l b o r e where i t i s needed most u r g e n t l y (CALLANAN, CIPOLLA & LEWIS 1983). D e t e r m i n a t i o n when t o u i e a t a i l - i n i n s t e a d o f pumping a l l h i g h - q u a l i t y p r o p p a n t i s a f u n c t i o n o f i r i c , w i e n t a l i n c r e a s e in hydrocarbon p r o d u c t i o n vs. added p r o p p a n t c o s t . A s p e c i a l a p p l i c a t i o n o f t a i l - i n schedules i s i n s e r t i n g r a d i o a c t i v e l y - c o a t e d proppants i n t h e t e r m i n a l stage f o r placement near t h e w e l l b o r e i n o r d e r t o e n a b l e gamma-ray f r a c t u r e height monitoring o f n a t u r a l o r a r t i f i c i a l proppant r a d i o a c t i v i t y (HART, ENGI & MORRIS 1983; c f . s e c t i o n 6 . 2 . 1 . 2 . ) . A p a r t f r o m proppants, a l s o modern water-based f r a c t u r i n g f l u i d s can be t a i l o r e d t o meet v i s c o s i t y s p e c i f i c a t i o n s i n a wide range o f temperature a p p l i c a t i o n s by p r o p e r use o f p r i m a r y and secondary g e l l i n g agents (ATTEBERRY, TUCKER & RITZ 1979; P A I , G A R B I S & HALL 1983; LEE & OANESHY 1985; c f . s e c t i o n 4 . 3 . 4 . ) , and s i m i l a r l y , t a i l o r e d - p u l s e l o a d i n g can be used f o r m u l t i p l e e x p l o s i v e f r a c t u SCHMIDT, COOPER, WALLING & NORTHROP 1979; r i n g o f b o r e h o l e s (WARPINSKI,
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Tab. 6
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SCHMIDT, W A R P I N S K I & COOPER 1980; SWIFT & 1986; McHUGH & KEOUGH 1982; KEOUGH, CURRAN & a l s o s e c t i o n s 4.2.2. and 4 . 8 . 8 . 6 . ) . CHARLES, t a i l o r i n g o f proppants i n o r d e r t o o b t a i n t h e
KUSUBOV McHUGH HUDOCK, desired
1980, 1981; CUOERMAN 1982, 1984; STOLLER 1985; c f . U O I C K & HALL (1983) r e p o r t conductivity ratio.
2.4.1.2.3.2.Principles o f tail-in proppant staging The most common t a i l - i n schedule i s pumping a l a r g e amount o f 20/40 sand o r i n t e r m e d i a t e - s t r e n g t h p r o p p a n t f o l l o w e d by a t e r m i n a l l i t t l e q u a n t i t y o f 16/20 o r even 12/20 i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p p a n t c o m p r i s i n g a b t . 10 - 40 % o f t h e t o t a l p r o p p a n t q u a n t i t y (PAI, GARBIS & HALL 1983; GARBIS, BROWN & MAUR I T Z 1985). The t a i l - i n amount o f p r o p p a n t s s h o u l d be i n terms o f c r a c k l e n g t h i d e a l l y a b t . 100 f t i n s m a l l f r a c t u r e s , w h i l e i n l a r g e r c r a c k s , t h e t a i l - i n p r o p p a n t l e n g t h should a t l e a s t equal a c t u a l f r a c t u r e h e i g h t . On t h e o t h e r hand, a l s o t h e r e v e r s e arrangement r e p r e s e n t i n g l e a d - i n s t r a t e g y i s p o s s i b l e , w i t h f i r s t pumping o f a small amount o f 20/40 o r even f i n e r sand o r i n t e r m e d i a t e - s t r e n g t h p r o p p a n t as t h e head o f t h e t r e a t m e n t and t h e n f o l l o w i n g w i t h t h e main body o f 20/40 o r 16/20 i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s . T h i s c o n f i g u r a t i o n can be even completed by a l i t t l e f i n a l t a i l - i n o f 1 6 / 2 0 o r 12/20 i n t e r m e d i a t e o r high-strength proppants. W h i l e 20/40 and 40/70 sand a r e most commonly used as l e a d - i n proppant, S I N CLAIR (1980) d e s c r i b e s s u c c e s s f u l a p p l i c a t i o n o f 100 mesh sand f r o m M i l l c r e e k / Oklahoma which i s more f r e q u e n t l y used as f l u i d - l o s s a d d i t i v e and r e t a i n s a l a r g e percentage o f i t s p e r m e a b i l i t y under l o a d due t o i t s many s u p p o r t p o i n t s a l l o w e d by t h e f i n e g r a i n s i z e ( c f . s e c t i o n s 4.8.8.3.1.2. and 4 . 8 . 8 . 3 . 2 . 3 . ) . Asp e c t s o f t a i l - i n p r o p p a n t s c h e d u l i n g o f v a r i o u s b o t h c o a r s e r t a i l o r f i n e r head g r a i n s i z e assemblages and successions a r e a l s o d i s c u s s e d by D A V I S (1975), KUNOERT & SMINK (1979), LINOLEY & McGHEE (1983), McMECHAN & CONWAY (1983), PA1 & G A R B I S (1983 b) and GARBIS, BROWN & MAURITZ ( 1 9 8 5 ) . S p e c i a l d i v i d e d t r e a t m e n t s o f l e a d - i n o r t a i l - i n c h a r a c t e r comprise pumping o f f i r s t smaller o r l a r g e r q u a n t i t i e s o f l i g h t intermediate- o r high-strength proppants and second l a r g e r o r s m a l l e r q u a n t i t i e s o f heavy i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p p a n t s . T h i s concept aims on e n a b l i n g a s a f e r l e a d - i n by t h e l i g h t p r o p p a n t which has b e t t e r suspension p r o p e r t i e s i n t h e f l u i d and decreases t h e r i s k o f e a r l y s c r e e n o u t b e f o r e reasonable amounts o f p r o p p a n t s c o u l d be p l a c e d i n t h e d i s t a l segment o f t h e f r a c t u r e , and on a c h i e v i n g a s t r o n g e r f r a c t u r e s u p p o r t by a h i g h e r - q u a l i t y m a t e r i a l i n t h e p r o x i m a l s e c t i o n o f t h e c r a c k where t h e h i g h e s t c l o s u r e s t r e s s i s e f f e c t i v e and screenout f a i l u r e i n t h e l a t e r stage o f t h e o p e r a t i o n would n o t have such a t e r r i b l e e f f e c t , because a t l e a s t a l r e a d y t h e l i g h t p r o p p a n t has been s u c c e s s f u l l y i n s e r t e d ( t h i s s p e c i a l combined f r a c t u r i n g s t r a t e g y has been a p p l i e d w i t h g r e a t b e n e f i t i n Yugoslavia; c f . section 2.4.3.1.).
2.4.1.2.3.3.Proppant cost containment The main purpose o f t h e t a i l - i n s t r a t e g y i s t o c u t down s t i m u l a t i o n c o s t by i n s e r t i n g an expensive s y n t h e t i c p r o p p a n t o n l y i n t h e immediate v i c i n i t y o f t h e w e l l b o r e where c l o s u r e s t r e s s has i t s maximum p a r t i c u l a r l y upon h i g h drawdown o f r e s e r v o i r p r e s s u r e i n t h e stage o f advanced d e p l e t i o n a f t e r many y e a r s o f p r o d u c t i o n h i s t o r y (AGARWAL 1980), whereas i n those p a r t s o f t h e f r a c t u r e wings t h a t a r e f a r t h e r away f r o m t h e immediate s u r r o u n d i n g s o f t h e borehole, cheap n a t u r a l sand i s o f t e n s t i l l s u f f i c i e n t t o r e s i s t t o t h e c o m p a r a t i v e l y low c l o sure s t r e s s e s i n t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d r e s e r v o i r s i n compar i s o n t o those i n Germany FRG and N e t h e r l a n d s where d e p t h and t h u s p r e s s u r e o f most o f t h e g a s - b e a r i n g f o r m a t i o n s a r e s l i g h t l y t o c o n s i d e r a b l y h i g h e r and t h u s i n t e r m e d i a t e - o r h i g h - s t r e n g t h proppants a r e i n most cases i n d i s p e n s i b l e f o r t h e whole o p e r a t i o n ( c f . s e c t i o n 3 . 3 . ) . M i n i m i z a t i o n o f h y d r a u l i c f r a c t u r i n g c o s t s can be p a r t i c u l a r l y achieved by s e l e c t i v e a p p l i c a t i o n o f d i f f e r e n t p r o p -
189 p a n t t y p e s as t a i l - i n l o t s as a consequence o f d i f f e r e n t p r i c e s o f n a t u r a l sand, i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants (PA1 & G A R B I S 1983; c f . s e c t i o n 1.3.). D u r i n g t i m e s o f h i g h proppant p r i c e s , ceramic proppants were a t l e a s t t h r e e t o f o u r t i m e s as expensive as sand ( i n t h e e a r l y days o f h i g h - s t r e n g t h p r o p p a n t i n t r o d u c t i o n and a p p l i c a t i o n , l i m i t a t i o n o f p r o d u c t i o n c a p a c i t i e s r e s u l t e d i n a p r i c e d i f f e r e n c e o f s i n t e r e d b a u x i t e and sand up t o 15 t i m e s o r even more) and now i n a p e r i o d o f low p r o p p a n t p r i c e s , ceramic proppants a r e s t i l l two t o f o u r t i m e s as expensive as sand ( c f . s e c t i o n 2.2.2.). T h e r e f o r e t h e t a i l - i n p h i l o sophy i n t h e r e l a t i v e l y s h a l l o w R o t l i e g e n d r e s e r v o i r s i n t h e B r i t i s h Southern N o r t h Sea makes s u r e t h a t i n t h e most c r i t i c a l s e c t i o n around t h e w e l l b o r e where c l o s u r e o f t h e f r a c t u r e would d e t e r i o r a t e t h e e f f e c t o f t h e whole operat i o n ( c f . s e c t i o n 4.12.4.2.), a p r o p p a n t adequate t o a l l p o s s i b l e ranges o f c l o s u r e s t r e s s o r i g i n a t i n g d u r i n g l o n g p r o d u c t i o n h i s t o r y guarantees t h a t t h e d r a i nage p a t h i s k e p t open i n t h e zone which i s w i t h advanced d e p l e t i o n and d e c l i n e o f the r e s e r v o i r pressure subjected t o the highest closure stresses ( e f f e c t s o f p r o d u c i n g t i m e and drawdown a r e d i s c u s s e d by AGARWAL 1980).
2.4.1.2.3.4. Proppant mixing and settling On t h e o t h e r hand, t h e i n s e r t i o n o f sand i n t h e more d i s t a n t t i p s o f t h e f r a c t u r e wings i s s u f f i c i e n t t o w i t h s t a n d t o t h e c o m p a r a t i v e l y low c l o s u r e s t r e s s e s and as a consequence o f t h e volume s p l i t t i n g between sand and i n t e r m e d i a t e - s t r e n g t h proppants, c o n s i d e r a b l e amounts o f s t i m u l a t i o n c o s t can be saved e s p e c i a l l y i n l a r g e f r a c t u r i n g campaigns a f f e c t i n g up t o s e v e r a l dozens o f w e l l s i n b i g t i g h t gas f i e l d s . The s e l e c t i o n o f c o a r s e r g r a i n s i z e s i n t h e n e a r e r v i c i n i t y o f t h e w e l l b o r e i n comparison t o f i n e r g r a i n s i z e s i n t h e f r a c t u r e wing s e c t i o n s b e i n g f a r t h e r away f r o m t h e b o r e h o l e i s a l s o i n d e p e n d e n t l y f r o m p r o p p a n t t y p e a i m i n g on enhancement o f c o n d u c t i v i t y between f r a c t u r e and f o r m a t i o n ( c f . s e c t i o n 4 . 3 . 5 . ) i n t h e zone o f g r e a t e s t f l o w r a t e a t t h e f r a c t u r e mouth. The m a j o r drawbacks o f t h e t a i l - i n p o l i c y , however, a r e t h e p o s s i b i l i t y o f d r a s t i c a l p e r m e a b i l i t y r e d u c t i o n i n t h e t r a n s i t i o n a l zone between d i f f e r e n t g r a i n s i z e s i n t h e p r o p p a n t package due t o a t l e a s t some m i x i n g ( c f . s e c t i o n 1.4.11.) w i t h p l u g g i n g o f t h e pores between c o a r s e r g r a i n s by f i n e r g r a i n s , and t h e - a c c o r d i n g t o Stokes' law - a b t . f o u r t i m e s f a s t e r s e t t l i n g v e l o c i t y o f a b t . t w i c e l a r g e r g r a i n s which may l e a d t o c r e a t i o n o f asymmetrical wedges o f c o a r s e r m a t e r i a l near t h e b o r e h o l e ( c f . s e c t i o n 4.2.2.6.) t h a t a r e n o t i n c o n t i nuous c o n t a c t w i t h t h e r e s e r v o i r and t h u s a homogeneous s t r i p - l i k e d i s t r i b u t i o n o f t h e t a i l around t h e w e l l b o r e i s n o t achieved ( a s p e c t s o f s e t t l i n g v e l o c i t i e s o f proppants a r e d i s c u s s e d by LOCKETT & AL-HABBOOBY (1973), M I R Z A & RICHARDSON (1979), DUNAND & SOUCEMARIANADIN 1985, KIRKBY & ROCKEFELLER 1985, ROODHART 1985; c f . s e c t i o n 4 . 1 2 . 2 . ) . Another disadvantage o f t a i l - i n p r o p p a n t s i z e d i s t r i b u t i o n w i t h i n t h e f r a c t u r e d r e s e r v o i r i s t h a t an a c c i d e n t a l unwanted i n t e r m i x i n g o f v a r i o u s s i z e s and shapes o f p r o p p a n t s ( c f . s e c t i o n 1.4.11.) w i t h i n t h e t r a n s i t i o n zone o f s e v e r a l pumping stages would l e a d t o i n c r e a s i n g p a c k i n g e f f i c i e n c y o f t h e g r a i n s dlid p a r t i a l l y a l s o enhanced i n f l o w o f p r o p p a n t p a r t i c l e s i n t o t h e w e l l b o r e (CLARK 1983).
2.4.1.2.3.5. Proppant f lowback prevent ion T a i l i n g - i n o f c o a r s e r proppants i n t o t h e p r o x i m a l p a r t o f t h e f r a c t u r e near t h e w e l l b o r e i s a l s o an e f f e c t i v e means f o r proppant flowback p r e v e n t i o n , because t h e c o a r s e r proppant s l u g a c t s l i k e a g r a v e l pack (CROW 1977; c f . s e c t i o n I f t h e g r a i n s i z e d i f f e r e n c e between t a i l - i n l o t and main proppant 4.12.3.2.). q u a n t i t y has been p r o p e r l y s e l e c t e d , t h e t a i l - i n segment b r i d g e s moving s m a l l e r
190 p r o p p a n t g r a i n s a t t h e i n t e r f a c e between b o t h p r o p p a n t s t a g e s and s t o p s f u r t h e r m i g r a t i o n , t h e r e b y s t a b i l i z i n g t h e p r o p p a n t p o p u l a t i o n i n p l a c e and s e c u r i n g p e r s i s t e n c e o f h i g h f r a c t u r e c o n d u c t i v i t y . The t a i l - i n o f c o a r s e r g r a i n s r e t a i n i n g t h e s m a l l e r p a r t i c l e s i n t h e more d i s t a l p o r t i o n s o f t h e c r a c k i s compar a b l e t o s t o p p i n g movement o f f o r m a t i o n sand b y p e r f o r a t i o n t u n n e l p l u g g i n g w i t h g r a v e l i n m e c h a n i c a l sand c o n t r o l ( c f . s e c t i o n 5 . 8 . 8 . ) .
2.4.1.2.3.6. Tail-in optimization and effectivity In l i g h t o f t h e above d i s c u s s e d drawbacks o f t h e t a i l - i n t r e a t m e n t t e c h n i que, i n v i e w o f t h e b o u n d a r y s t r e s s r e s i s t i v i t y o f sand and f o r t h e p u r p o s e o f a b s o l u t e l y g o i n g on t h e s a f e s i d e t o have t h e s e c u r i t y t h a t t h e p r o p p i n g mater i a l w i l l n o t c o l l a p s e even when s u b j e c t e d t o t h e h i g h e s t p o s s i b l e r e s e r v o i r p r e s s u r e drawdown and t h u s c l o s u r e s t r e s s i n c r e a s e d u r i n g l o n g - t e r m h y d r o c a r b o n p r o d u c t i o n ( c f . s e c t i o n 4 . 1 2 . 4 . ) f r o m t h e o f f s h o r e f i e l d s where any remedy oper a t i o n causes s i g n i f i c a n t l y h i g h e r expenses t h a n i n o n s h o r e s e t t i n g s , several companies f i n a l l y d e c i d e d t o p e r f o r m i n t h e 1987 - 1990 f r a c t u r i n g campaigns i n t h e B r i t i s h S o u t h e r n N o r t h Sea o n l y a t a i l - i n c o n c e p t c o n c e r n i n g g r a i n s i z e s o f i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s ( m a i n l y s t a r t i n g w i t h a l i t t l e volume o f a b t . 25,000 - 100,000 l b s . ( a b t . 12 - 50 t ) o f 20140 f o l l o w e d b y t h e m a i n q u a n t i t y o f a b t . 300,000 - 400,000 l b s . ( a b t . 150 - 200 t ) o f 16/20, w i t h i n some c a s e s e v e n a l i t t l e f i n a l t a i l o f 12/20 m a t e r i a l b e i n g p o s s i b l e ) , b u t n o t t o u s e sand a t a l l as a p r o p p i n g a g e n t . Other intermediate-strength proppant treatment schedules a r e l e a d i n g - i n abt. 25 - 80 t ( a b t . 50,000 - 160,000 l b s ) o f 2 0 / 4 0 t h a t i s f o l l o w e d b y a b t . 100 200 t (200,000 - 400,000 l b s ) o f 16/20 o r i n some c a s e s a l s o 20/40, w i t h t h e j o b b e i n g t e r m i n a t e d b y t a i l i n g - i n a b t . 25 - 50 t (50,000 - 100,000 l b s ) o f 1 6 / 2 0 o r even 1 2 / 2 0 m a t e r i a l . A p a r t f r o m g e t t i n g h i g h e r c o n d u c t i v i t i e s o f t h e f r a c t u r e s propped w i t h i n t e r m e d i a t e - s t r e n g t h m a t e r i a l w i t h r e s p e c t t o c r a c k s i n f i l l e d w i t h sand, t h i s d e s i g n s c h e d u l e a l s o g i v e s t h e g u a r a n t e e o f s a t i s f a c t o r y f r a c t u r e p e r f o r m a n c e t h r o u g h o u t f i e l d l i f e u n a f f e c t e d f r o m any changes o f p r e s s u r e r e g i m e and t h u s t h e i n v e s t m e n t w i l l p a y o f f v e r y w e l l . C o n c e r n i n g t a i l - i n e f f e c t i v i t y , LI & ZHU ( 1 9 8 6 ) r e p o r t p o o r p e r f o r m a n c e in m o d e r a t e - t o h i g h - d e p t h o i l r e s e r v o i r s in C h i n a where h i g h - s t r e n g t h b a u x i t e t a i l - i n s f o l l o w i n g pumping o f sand have n o t p r o v i d e d h i g h e r c o n d u c t i v i t i e s t h a n u n i f o r m f r a c t u r e packages. The q u a n t i t a t i v e s h a r e o f t h e t a i l w i t h r e s p e c t t o t h e t o t a l p r o p p a n t l o a d s h o u l d n o t b e s e l e c t e d t o o low, because t h i s c o u l d cause s t r a t i f i c a t i o n a n d / o r m i x t u r e o f p r o p p a n t s a t t h e w e l l b o r e w h i c h a r e enhanced b y p e r m e a b i l i t y and f r a c t u r e g r a d i e n t v a r i a t i o n s (GARBIS, BROWN & MAURITZ 1 9 8 5 ) . A s p e c t s o f R o t l i e g e n d gas r e s e r v o i r f r a c t u r i n g i n t h e B r i t i s h Sout h e r n N o r t h Sea a r e a l s o d i s c u s s e d b y ACHARYA & K I M ( 1 9 8 7 ) , and COULTER & PURV I S ( 1 9 8 0 ) p r e s e n t e x p e r i e n c e o f R o t l i e g e n d and B u n t s a n d s t e i n f r a c t u r i n g i n p a r t i a l l y w a t e r - s e n s i t i v e r e s e r v o i r s i n t h e D u t c h S o u t h e r n N o r t h Sea.
2.4.1.2.4. Differences between offshore and onshore fracturing Due t o t h e d i f f e r e n t l o g i s t i c a l s i t u a t i o n o f o n s h o r e and o f f s h o r e p r o j e c t s ( c f . a l s o s e c t i o n 3 . 8 . ) and r e s u l t i n g f r o m p r e v i o u s l y more o r l e s s f i x e d f i e l d d e v e l o p m e n t p l a n s i n c l u d i n g commitment o f s e l l i n g t h e gas p r i o r t o t h e s t a r t o f production d r i l l i n g ( c f . s e c t i o n 2.2.1.6.2.), t h e o i l p r i c e d r o p i n e a r l y 1986 ( i n Europe s u p e r i m p o s e d b y t h e d e c l i n e o f t h e US $ exchange r a t e w h i c h r e a c h e d a minimum a l m o s t a t t h e same t i m e ; c f . s e c t i o n 2 . 2 . 1 . 3 . ) has o n l y l e a d t o a more o r l e s s c u t back o f t h e l e v e l o f s t i m u l a t i o n a c t i v i t y ( w h i c h was f o r t h e s e r v i c e and p r o p p a n t companies n e g a t i v e l y a c c e n t u a t e d by t h e c o i n c i d e n c e w i t h h i g h e r c a n c e l l a t i o n r a t e s f o r t e c h n i c a l r e a s o n s ; c f . s e c t i o n 2.4.2.2.) i n c l u d i n g postponement o f o n l y l i m i t e d amounts o f p r o j e c t s , b u t has i n c o n t r a s t t o t h e s i t u a t i o n i n p a r t s of o n s h o r e Europe ( c f . s e c t i o n 2 . 4 . 1 . 1 . 1 . ) not given r i s e t o t o t a l s u s p e n s i o n and t h u s a l s o q u i t e some j o b s have been c a r r i e d o u t i n
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192 1986 i n t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t ( i n c o n t r a s t t o Germany FRG where n o t h i n g took p l a c e ) . 1987 has seen a l r e a d y a g a i n some m a j o r campaigns and p a r t i c u l a r l y 1988 met t h e schedule t o become t h e b e g i n n i n g o f a pronounced boom o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n t h i s area ( w h i c h extends u n t i l 1989 and 1990 o r even l a t e r ) , although c o n t i n u i n g o i l p r i c e and US $ weakness caused i n some f i e l d s q u i t e some d e l a y o f t h e o r i g i n a l p l a n n i n g and i n some cases even i n c l u d e d schedule m o d i f i c a t i o n s f o r t h e purpose o f c o s t containment. F u r t h e r aspects o f d i f f e r e n t f r a c t u r e p h i l o s o p h y and s t r a t e g y i n o f f s h o r e and onshore areas a r e d i s c u s s e d i n sect i o n 3.8. Some d i f f e r e n c e s between o f f s h o r e and onshore f r a c t u r i n g s t i m u l a t i o n t r e a t m e n t s a r e a l s o o u t l i n e d by ROODHART, K U I P E R & D A V I E S ( 1 9 8 6 ) .
2.4.1.2.5. Moderately deep oil fracturing I n a d d i t i o n t o m o d e r a t e l y deep gas f r a c t u r i n g i n t h e B r i t i s h Southern N o r t h Sea, so f a r a l s o some m o d e r a t e l y deep o i l f r a c t u r i n g i n t h e B r i t i s h N o r t h e r n N o r t h Sea has been c a r r i e d o u t i n a few f i e l d s (DESPAX, CHARLEZ, CLINKEMAILLE & ECONOMIDES 1987). The t r e a t m e n t s have been performed i n t h e J u r a s s i c B r e n t Sand group (JOHNSON & KROL 1984) and have been executed e i t h e r w i t h sand o r w i t h i n t e r m e d i a t e - o r h i g h - s t r e n g t h alumina o x i d e and s i l i c a t e p r o p p a n t s . W h i l e h i t h e r t o m a i n l y i n d i v i d u a l w e l l s have been s t i m u l a t e d due t o a c t u a l demand, s e v e r a l s m a l l e r o r l a r g e r o p e r a t i o n campaigns c o n s i s t i n g o f a s u i t e o f j o b s a r e schedul e d f o r t h e coming y e a r s . The T e r t i a r y Eocene sandstones which have r e c e n t l y been proven t o be another m a j o r o i l - b e a r i n g r e s e r v o i r s t o r e y (BEUDELL 1987 b, CAPEL 1988) a r e expected to r a t h e r r e q u i r e g r a v e l p a c k i n g t h a n h y d r a u l i c p r o p p a n t f r a c t u r i n g . Some o f t h e hydrocarbon f i e l d s i n t h e N o r t h e r n N o r t h Sea a r e c o n s i d e r a b l y o v e r p r e s s u r e d (LINDBERG & FERTL 1980). The most i m p o r t a n t f i e l d s where m o d e r a t e l y deep o i l f r a c t u r i n g t r e a t m e n t s have a l r e a d y been c a r r i e d o u t and/or a r e s t i l l g o i n g t o be performed a r e ( i n a l p h a b e t i c a l o r d e r ) Brae, Heather, H u t t o n , Montrose, T a r t a n and T h i s t l e ( c f . f i g . 8 and t a b . 7 ) .
2.4.2. Western Europe 1986 The c o n s i d e r a b l y decreased l e v e l o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n Western Europe i n 1986 (and p a r t i a l l y s t i l l i n t h e f i r s t h a l f o f 1987) i s n o t o n l y a consequence o f t h e u n e x p e c t e d l y s t r o n g o i l p r i c e drop ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) and t h e accompanying superimposing impact o f t h e severe US $ exchange r a t e dec l i n e ( c f . s e c t i o n 2.2.1.3.), b u t a l s o r e s u l t s f r o m t h e c o i n c i d e n c e o f t h e deter i o r a t i o n o f t h e g e n e r a l economical s i t u a t i o n w i t h i n c r e a s e d C a n c e l l a t i o n r a t e s o f s t i m u l a t i o n o p e r a t i o n s due t o t e c h n i c a l reasons, and i s a l s o t h e e f f e c t o f t h e h i t h e r t o m a r k e t i n g s t r a t e g y and a c t i v i t y o f t h e p r o p p a n t companies. The f o l l o w i n g summary i n c l u d e s o i l p r i c e d e c l i n e , a r e a l f r a c t u r i n g a c t i v i t y , i n c r e a s e d c a n c e l l a t i o n r a t e s o f j o b s , and p r o p p a n t m a r k e t i n g a c t i v i t y .
2.4.2.1. Oil price decline The o i l p r i c e dropped s e r i o u s l y i n l a t e 1985 and e s p e c i a l l y e a r l y 1986 (RUNGE 1986; c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) , w i t h t h e e f f e c t i n Europe b e i n g even more ser i o u s due t o t h e f a l l o f t h e US $ exchange r a t e which a c c e l e r a t e d i n l a t e 1985 and reached i t s minimum i n e a r l y 1986 t h r o u g h m i d 1987, and improved n o t s u f f i c i e n t l y s i g n i f i c a n t t o regain confidence f o r l a r g e r investments u n t i l mid t o l a t e 1988 ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) . The r e s u l t o f t h e marked changes o f t h e gener a l economical framework, e s p e c i a l l y a t t h e peak o f t h e c r i s i s i n e a r l y t o m i d 1986 when t h e f r e s h o i l p r i c e c r a s h was h u r t i n g t h e whole hydrocarbon i n d u s t r y s e v e r e l y , was t h a t almost a l l t h e o i l and gas p r o d u c t i o n companies i n Western Europe s e v e r e l y r e v i s e d t h e i r budgets of c a p i t a l e x p e n d i t u r e , and c u t down i n -
193 by d r a s t i c a l l y r e d u c i n g vestments by a t a b t . 25 - 40 % ( c f . s e c t i o n 2.2.1.5.2.) o r even t o t a l l y f r e e z i n g t h e d r i l l i n g and s t i m u l a t i o n programmes c h i e f l y w i t h i n t h e N o r t h Sea (BEUDELL 1986 c, GRIFFITHS 1986, HARBEN 1986, OEHME 1986, VIELVOYE 1986, ALGAR 1987 b; MABRO 1987, 1988). Summarizing f o r Western Europe as a whole, however, i n d i c a t e s t h a t e x p l o r a t o r y d r i l l i n g was v i r t u a l l y unchanged i n 1986 f r o m i t s 1985 l e v e l , t h e r e b y r e f l e c t i n g p r i m a r i l y t h e h i g h e r i n t e r e s t i n onshore e x p l o r a t i o n where much l e s s s t i m u l a t i o n p o t e n t i a l e x i s t s (POPESCU & HODGSHON 1987). The s t r e a m l i n i n g o f companies and t h e i r c a p i t a l e x p e n d i t u r e a l s o a f f e c t e d gen e r a l overheads and personnel, g i v i n g r i s e t o s e v e r a l m a j o r l a y - o f f campaigns i n hydrocarbon p r o d u c t i o n and s e r v i c e i n d u s t r y a f f e c t i n g w o r l d - w i d e p r o b a b l y a l most a q u a r t e r o f a m i l l i o n employees and p a r t i a l l y c a u s i n g mass p e o p l e exodus f r o m o i l c a p i t a l s t h u s l e a v i n g t h e r e some g h o s t - l i k e abandoned q u a r t e r s ( c f . s e c t i o n 2 . 2 . 1 . 5 . 2 . ) . N e v e r t h e l e s s , t h e a c t i v i t y o f some m a j o r p r o d u c t i o n compan i e s t h a t have a b e t t e r o v e r a l l p o s i t i o n , a r e backed-up by b e i n g owned o r shar e - h o l d e d by o t h e r p r o f i t a b l e businesses, have market p r o t e c t i o n o r f i n a n c i a l s u p p o r t by t h e government and a r e t h u s almost o f s e m i - n a t i o n a l n a t u r e , o r a r e committed t o f u l f i l l t h e i r gas d e l i v e r y c o n t r a c t s which had been concluded p r i o r t o t h e c r i s i s ( c f . s e c t i o n 2.2.1.6.2.) and t h u s a r e f o r c e d t o go ahead r e g a r d l e s s o f t h e economical s i t u a t i o n , t o g e t h e r w i t h a c o u p l e o f j o b s f r o m small e r companies s t i l l m a i n t a i n e d a p r i n c i p a l l y reasonable l e v e l o f market potent i a l i n view o f h y d r a u l i c p r o p p a n t f r a c t u r i n g .
2.4.2.2. Areal fracturing activity The a r e a l h y d r a u l i c p r o p p a n t f r a c t u r i n g a c t i v i t y i n Western Europe i n 1986 i s c h a r a c t e r i z e d a l o n g t h e l i n e s o f R o t l i e g e n d gas f r a c t u r i n g i n t h e N o r t h Sea as w e l l as unemployment o f d r i l l i n g r i g s and s t i m u l a t i o n b o a t s as f o l l o w s .
2.4.2.2.1. Rotliegend gas fracturing i n the North Sea H y d r a u l i c proppant f r a c t u r i n g i n Western Europe was i n 1986 almost e x c l u s i v e l y r e s t r i c t e d t o t h e R o t l i e g e n d gas f i e l d s i n t h e B r i t i s h and Dutch Southern N o r t h Sea where i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y alumina o x i d e and s i l i c a t e proppants were used e i t h e r f o r t h e f u l l t r e a t m e n t o r as t a i l - i n t e r m i n a l stages ( c f . s e c t i o n 2.4.1.2.3.) f o l l o w i n g a m a j o r e a r l y phase o f sand pumping. The few e x c e p t i o n s were Cretaceous o i l - b e a r i n g sandstones i n t h e D u t c h Southern N o r t h Sea where sand was a p p l i e d , and Cretaceous o i l - b e a r i n g c h a l k i n t h e Norwegian N o r t h e r n N o r t h Sea where b o t h c o n v e n t i o n a l and r e s i n - c o a t e d i n t e r m e d i a t e s t r e n g t h l o w - d e n s i t y ceramic proppants were t a k e n (HARTLEY & BOSMA 1985; TANSOE, K I N G & HOLMAN 1986) f o r combined h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i n s o f t f l o w i n g undercompacted and o v e r p r e s s u r e d c h a l k s (ATTARD, MATHES & MOWER 1988; c f . s e c t i o n 4.5.4.3.). Some t i g h t Mesozoic sands i n t h e N o r t h e r n N o r t h Sea were a l s o s t i m u l a t e d (DESPAX, CHARLEZ, CLINKEMAILLE & ECONOMIDES 1987). Onshore Germany FRG and N e t h e r l a n d s , n o t h i n g took p l a c e b o t h i n deep g a s - w e l l f r a c t u r i n g w i t h s y n t h e t i c proppants and s h a l l o w o i l - w e l l f r a c t u r i n g w i t h n a t u r a l sand ( c f . s e c t i o n 2 . 4 . 1 . 1 . 1 . ) . The B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t has i t s own development concept and due t o predominant s a l e o f t h e gas p r i o r t o development d r i l l i n g and h y d r a u l i c p r o p p a n t f r a c t u r i n g ( c f . s e c t i o n 2.2.1.6.2.), the province kept i t s a c t i v i t y more o r l e s s u n a f f e c t e d by t h e 1986 o i l p r i c e drop and was s t r a i g h t f o r w a r d c o n t i n u i n g i t s approach t o t h e m a j o r development campaigns i n 1987 - 1990 (economical aspects a r e a l s o summarized by FISHMAN 1986, GREGORY 1986, OFFSHORE ENGINEER 1986 a, THOMAS 1986, ALGAR 1987 a, BAND 1987, POTTER 1987). T h i s i s r e f l e c t e d by t h e f a c t t h a t i n t h e f i r s t q u a r t e r o f 1986, i n t o t a l more w e l l s had been d r i l l e d i n t h e B r i t i s h N o r t h Sea than i n any o t h e r quart e r i n t h e h i s t o r y o f U n i t e d Kingdom o f f s h o r e o i l and gas a c t i v i t y , and f o r 1986 as a whole, o n l y s l i g h t l y fewer w e l l s have been d r i l l e d t h a n t h e average
194 number h a v i n g been b r o u g h t down i n t h e p e r i o d 1983 - 1985 (POPESCU & HODGSHON 1 9 8 7 ) . I n t h e D u t c h S o u t h e r n N o r t h Sea, p r o j e c t s were s h i f t e d f r o m o i l s p o t s t o gas p a t c h e s a l o n g w i t h g e n e r a l b u d g e t c u t t i n g as a consequence o f t h e o i l p r i c e c o l l a p s e (BEUDELL 1986 b ) .
2.4.2.2.2.
Unemployment of drilling rigs and stimulation boats
The c o n s i d e r a b l y r e d u c e d d r i l l i n g and s t i m u l a t i o n a c t i v i t y in t h e N o r t h Sea (GRIFFITHS 1986, VIELVOYE 1986) was b e s t e x p r e s s e d in t h e second h a l f o f 1986 b y numerous unemployed j a c k - u p p l a t f o r m s h a v i n g been l i n e d u p l i k e a p e a r l c h a i n a l o n g t h e B r i t i s h and N o r w e g i a n c o a s t s ( p a r t i c u l a r l y between N e w c a s t l e and Aberdeen on t h e B r i t i s h s i d e and between S t a v a n g e r and B e r g e n on t h e Norweg i a n s i d e ; c f . a l s o SCOTTISH PETROLEUM ANNUAL 1 9 8 7 ) . An i n d i r e c t i n d i c a t o r was t h e employment o f t h e s t i m u l a t i o n b o a t s i n t h e N o r t h Sea where a c c o r d i n g t o t h e f a c t t h a t t h r e e s e r v i c e v e s s e l s (FOX 1985, OILMAN 1985 b ) o f p u r p o s e - b u i l t nat u r e (COOPER & MARSHALL 1984, OFFSHORE ENGINEER 1984) h a d t o s h a r e a s t a g n a n t o r s h r i n k i n g f r a c t u r i n g m a r k e t , some b o a t s k e p t w a i t i n g f o r j o b s i n t h e h a r b o u r o f t h e i r o f f s h o r e base up t o s e v e r a l months t i m e ( c f . s e c t i o n s 3 . 7 . 1 . 1 . and 3.8.1.2.). The t o t a l number o f a c t i v e r i g s in t h e N o r t h Sea d r o p p e d by a l m o s t 30 % dur i n g 1986, w i t h more t h a n 40 p l a t f o r m s b e i n g w i t h o u t j o b i n l a t e 1986 i n cont r a s t t o o n l y a c o u p l e o f them one y e a r e a r l i e r (OEHME 1 9 8 6 ) . Up t o a b t . 70 r i g s have been s t a c k e d o f f N o r t h w e s t Europe i n t h e w o r s t p e r i o d o f 1986 and e a r l y 1987, and l e s s t h a n 40 r i g s r e m a i n e d w o r k i n g i n t h a t p e r i o d (McNALLY 1987, VIELVOYE 1987 b ) . I n t h e second h a l f o f 1987, r e c o v e r y o f t h e N o r t h Sea m a r k e t r e s u l t e d in r i s e o f t h e a c t i v e d r i l l i n g r i g number t o a b t . 65 and f a l l o f t h e q u a n t i t y o f s t a c k e d r i g s t o a b t . 40.
2.4.2.3.
Increased cancellation rates
A c c o r d i n g t o Murphy's law, t h e c r i s i s t h a t was t r i g g e r e d b y t h e o i l p r i c e dec l i n e was f o r t h e European h y d r a u l i c p r o p p a n t f r a c t u r i n g m a r k e t i n g segment e v e n n e g a t i v e l y a c c e n t u a t e d , as t h e r e d u c e d amount o f d r i l l i n g and s t i m u l a t i o n a c t i v i t y in m a i n l y t h e N o r t h Sea d u r i n g t h e peak o f t h e c r i s i s c o i n c i d e d w i t h an unu s u a l l y increased r a t e o f c a n c e l l a t i o n s o f planned f r a c t u r e jobs ( w i t h r e s p e c t t o t h e e x p e c t a t i o n s ) i n e a r l y t o l a t e 1986. M o s t o f t h e r e a s o n s f o r t h e i n c r e a sed c a n c e l l a t i o n r a t e s r e l a t e t o r e s e r v o i r p r o p e r t i e s and w e l l t y p e .
2.4.2.3.1.
Reservoir properties
The r e a s o n s f o r s t i m u l a t i o n j o b c a n c e l l a t i o n a g a i n s t e x p e c t a t i o n and p l a n n i n g a r e m u l t i p l e and d e p e n d i n g on a c t u a l w e l l r e s u l t s . The f i r s t p o s s i b i l i t y i s t h a t no gas was f o u n d i n a w a t e r - b e a r i n g r e s e r v o i r i n e x p l o r a t i o n w e l l s when t h e t a r g e t f o r m a t i o n was e n c o u n t e r e d deeper t h a n p r o g n o s e d o r t h e g a s - w a t e r - c o n t a c t was m e t in a h i g h e r p o s i t i o n t h a n p r e d i c t e d . The second example i s t h a t t h e p r o v e n r e s e r v o i r i s t o o t i g h t t o a l l o w e c o n o m i c a l gas w i t h d r a w a l e v e n a f t e r a h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t ( t o o l o w p r e - f r a c t u r i n g gas o f f t a k e r a t e o r c o m p l e t e l y t i g h t ) in c o n t r a s t t o t h e a s s u m p t i o n o f p r e s e n c e o f l o w p e r m e a b i l i t i e s i n a f a v o u r a b l e range f o r a c o m m e r c i a l i t y o f p r o d u c t i o n a f t e r f r a c t u r i n g . The t h i r d cause i s t h a t t h e r e s e r v o i r has u n e x p e c t e d l y good p e r m e a b i l i t y and t h u s a f r a c t u r i n g o p e r a t i o n i s n o t n e c e s s a r y , t h e r e b y c o n t r a d i c t i n g t h e e x p e c t a t i o n o f p o o r r e s e r v o i r c o n d i t i o n s t h a t w o u l d have r e q u i r e d s t i m u l a t i o n . The f o u r t h e f f e c t i s t h a t t h e h y d r a u l i c t r e a t m e n t i s suspended and t h e w e l l i s s h u t - i n t o w a i t f o r a b e t t e r g e n e r a l e c o n o m i c a l s i t u a t i o n as an answer t o t h e change o f framework c o n d i t i o n s d u r i n g d r i l l i n g o f d e e p e r w e l l s w h i c h w e r e o f t e n spudded w i t h t h e e x p e c t a t i o n o f t h e n e c e s s i t y o f l a t e r f r a c t u r i n g a t a t i m e when t h e o i l p r i c e was s t i l l p r o m i s i n g , b u t w h i c h f i n a l l y e n c o u n t e r e d t h e t a r g e t f o r m a t i o n a t a t i m e when t h e o i l p r i c e l e v e l t o g e t h e r w i t h t h e US $ v a l u e
195
Centres of hydraulic proppant fracturing and gravel packing in the North Sea massive hydraulic f r a c t u r i n g l M H F l smaller-scale f r a c t u r i n g gravel packing
M100 km
. *
Fig. 8
196 had so f a r c o l l a p s e d t h a t the o r i g i n a l p l a n s could no l o n g e r be m a i n t a i n e d and d e c i s i o n was made t o postpone t h e t r e a t m e n t . All the mentioned r e a s o n s have r e p e a t e d l y r e s u l t e d i n c a n c e l l a t i o n s of hyd r a u l i c proppant f r a c t u r i n g t r e a t m e n t s t h a t had been scheduled and d e s i g n e d bef o r e , w i t h some of t h e w i t h d r a w a l s having been come i n t o f o r c e a l m o s t i n t h e l a s t minute of j o b p r e p a r a t i o n predominantly i n 1986 and p a r t i a l l y a l s o i n t h e f i r s t h a l f of 1987. For t h e i n d i v i d u a l proppant and s e r v i c e companies, a l l t h e s e e f f e c t s a d d i t i o n a l l y i n t e r f e r e d w i t h l o s s of some b u s i n e s s t o c o m p e t i t i o n which, however, i s a l s o t a k i n g p l a c e i n good t i m e s , and w i t h g e t t i n g some o r d e r s l a t e r than e x p e c t e d o r even n o t a t a l l due t o i n t e r m i t t e n t s t o c k l i q u i d a t i o n by t h e s e r v i c e companies e n a b l i n g them t o c a r r y o u t s e v e r a l o p e r a t i o n s of v a r i o u s s i z e w i t h o u t i n between t a k i n g new proppant m a t e r i a l ( c f . s e c t i o n 2 . 4 . 5 . 5 . 2 . ) . In the poor economical p e r i o d , a l l t h e s e f a c t o r s c o n t r i b u t e t o h i t t h e i n d u s t r y very s e r i o u s l y . A n e g a t i v e consequence d i r e c t l y r e l a t e d t o t h e s t r e a m l i n i n g of the budgets i n t h e c r i s i s s i t u a t i o n ( c f . s e c t i o n 2 . 2 . 1 . 5 . 2 . ) was replacement of e x p e n s i v e s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h propp a n t s by cheap n a t u r a l sand i n t h e s t i m u l a t i o n d e s i g n wherever p o s s i b l e according t o r e s e r v o i r c o n d i t i o n s , i n c o n t r a s t t o sometimes generous a p p l i c a t i o n of a h i g h e r - q u a l i t y proppant t y p e even i f n o t i n d i s p e n s i b l y r e q u i r e d f o r t h e whole j o b i n t h e golden y e a r s of h y d r a u l i c f r a c t u r i n g .
2.4.2.3.2.
Well type
The high r a t e of c a n c e l l a t i o n s i n e a r l y t o l a t e 1986 was mainly caused by t h e predominance of e x p l o r a t i o n and a p p r a i s a l w e l l s where g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g r i s k i s o f t e n very h i g h . For ( a t l e a s t the second h a l f o f ) 1987 and e s p e c i a l l y f o r 1988, the c a n c e l l a t i o n r a t e can be e x p e c t e d t o have been much lower, a s i n t h e s e y e a r s , p r o d u c t i o n (development) d r i l l i n g was p r e v a i l i n g where the r i s k i s c o n s i d e r a b l y lower, because the s t r u c t u r e s and t h e i r c o n t e n t s a r e more o r l e s s a l r e a d y known. I n t h e f i r s t h a l f of 1987, t h e c a n c e l l a t i o n r a t e s were a l r e a d y lower than in 1986 a l s o f o r the reason of reduced o r i g i n a l p l a n n i n g of t r e a t m e n t s i n l i g h t of t h e poor economical framework, w i t h t h e d e l e t i o n s of t h e i t e m s p r i o r t o t h e i r appearance on t h e program l i s t of w e l l s t o be spudded mainly a f f e c t i n g h i g h e r - r i s k p r o s p e c t s which c o u l d e a s i l y be s u b j e c t e d a l s o t o l a t e r c a n c e l l a t i o n ( i n 1986 s h o r t l y a f t e r the w o r s t s t a g e of t h e c r i s i s was r e a c h e d , i t was o f t e n t o o l a t e t o s t o p and prevent such items t o be d r i l l e d ) .
2.4.2.4.
Proppant marketing a c t i v i t y
I n view o f t h e t o t a l marketing and s a l e s l e v e l of p r o p p a n t s i n Europe, ano t h e r reason f o r t h e c o m p a r a t i v e l y poor r e s u l t i n 1986 w i t h r e s p e c t t o the exp e c t a t i o n s i s t h e h i t h e r t o underdevelopment of E a s t e r n Europe, Southern Europe and Northern A f r i c a a s a consequence of t h e n e g l e c t a n c e of t h e s e markets i n the p a s t . I f having been b e t t e r a t t e n d e d , h y d r a u l i c f r a c t u r i n g campaigns would be now i n a much more advanced s t a g e of p r e p a r a t i o n in E a s t e r n Europe, Southern E u rope and Northern A f r i c a and could have p a r t i a l l y even a l r e a d y i n 1986 and f o l lowing y e a r s r e s u l t e d i n a d d i t i o n a l b u s i n e s s f o r p a r t i c u l a r l y t h e p r o p p a n t comp a n i e s ( t h a t supply i n a d d i t i o n t o t h e e s t a b l i s h e d s e r v i c e companies which u s u a l l y o p e r a t e the j o b s a s c o n t r a c t o r s a l s o t o t h o s e hydrocarbon p r o d u c t i o n companies t h a t c a r r y o u t s m a l l e r o r l a r g e r j o b s themselves w i t h o u t i n t e r c a l a t i o n o r i n t e r v e n t i o n of s e r v i c e companies; c f . s e c t i o n 2 . 4 . 4 . 4 . and 2 . 4 . 5 . 5 . 1 . ) t h a t would have been a t the bottom of t h e l i n e a very welcome compensation of the q u a n t i t i e s l o s t i n Western Europe due t o the r e a s o n s d i s c u s s e d above.
The i n s u f f i c i e n t marketing a t t e n t i o n by t h e proppant companies d u r i n g t h e l a s t y e a r s i s h i g h l i g h t e d by the f a c t t h a t even in h i g h - p o t e n t i a l a r e a s such a s G r e a t B r i t a i n and N e t h e r l a n d s , t h e r e have s t i l l r e c e n t l y been e n c o u n t e r e d some o i l and g a s companies where t h e r e s p o n s i b l e r e s e r v o i r e n g i n e e r s had never heard
197 b e f o r e of t h e p r o d u c t s u i t e and t h e l o g i s t i c a l o r g a n i z a t i o n o f t h e m a j o r p r o p p a n t s u p p l i e r s , a l t h o u g h these companies a l s o do have w e l l s t h a t r e q u i r e f r a c t u r i n g w i t h i n t e r m e d i a t e - s t r e n g t h proppants i f adequate l a y o u t i s performed. On t h e o t h e r hand, t h e p r o p p a n t companies so f a r know v e r y l i t t l e o r a l m o s t not h i n g about t h e demand o f s e v e r a l p o t e n t i a l Western and E a s t e r n European count r i e s . Some examples o f business l o s s t o proppant c o m p e t i t i o n and n a t u r a l sand a r e mentioned as f o l l o w s .
2.4.2.4.1. Business loss to proppant competition Business l o s s t o p r o p p a n t c o m p e t i t i o n o c c u r r e d r e p e a t e d l y on l a r g e - s c a l e o r d e r b a s i s i n t h e l a s t y e a r s i n Y u g o s l a v i a and i n t h e B r i t i s h Southern N o r t h Sea. Some p a r t i c u l a r l y s p e c t a c u l a r examples where m a j o r chances o f business conc l u s i o n have been l o s t due t o v a r i o u s reasons a r e i l l u s t r a t e d as f o l l o w s . W h i l e some o f t h e cases a r e t h e consequence o f e x c e s s i v e p r i c e p o k e r i n g o r s i m p l e neg l e c t a n c e o f t h e market i n e a r l i e r times, o t h e r i t e m s a r e t h e r e s u l t o f e l i m i n a t i o n o f t h e n a t u r a l market f o r c e s by s h o r t - c i r c u i t i n g due t o i n t r a c o r p o r a t i o n r e 1a t i o n s h i p s .
2.4.2.4.1.1. Yugoslavia F o l l o w i n g a s m a l l - s c a l e p r e c u r s o r t r e a t m e n t i n Yugoslavia ( c f . s e c t i o n 2.4.2.4.2.1.) which was v e r y s u c c e s s f u l and opened t h e way f o r a b i g MHF camp a i g n i n t h a t c o u n t r y , t h e i n s u f f i c i e n t m a r k e t i n g a t t e n t i o n i n E a s t e r n Europe by t h e m a j o r p r o p p a n t companies was s e r i o u s l y punished by enormous business l o s s f o r some o f them i n l a t e 1985 when a f t e r an u n s u c c e s s f u l approach o f var i o u s proppant companies by t h e Yugoslavians f o r a q u o t a t i o n , i n t o t a l 5,400 t o f proppants were purchased i n almost one l o t f r o m s u p p l y companies w h i c h was scheduled t o be enough f o r t h e 10 - 12 planned j o b s o f t h e MHF s t i m u l a t i o n camp a i g n t h r o u g h o u t a c o u p l e o f years, and s e v e r a l m a j o r proppant companies were o n l y awaking by t h i s e v e n t and t o o l a t e r e a l i z i n g t h a t a l s o a t l e a s t p a r t s o f E a s t e r n Europe have c o n s i d e r a b l e m a r k e t i n g p o t e n t i a l . W h i l e some o f t h e m a j o r p r o p p a n t companies c o u l d s t i l l s l i p i n t o t h e deal by an unexpected backdoor approach due t o t h e l i m i t e d d e l i v e r y c a p a c i t y o f one o f t h e c o m p e t i t o r s , o t h e r p a r t i e s had a b s o l u t e l y no p a r t i c i p a t i o n i n t h i s a c t i o n which p r o b a b l y was t h e h i s t o r i c a l l y b i g g e s t proppant deal i n one movement i n whole Europe.
2.4.2.4.1.2. British Southern North Sea Some case s t u d i e s o f p r o p p a n t business l o s s t o c o m p e t i t i o n i n t h e B r i t i s h Southern N o r t h Sea a r e i l l u s t r a t e d a l o n g t h e l i n e s o f p r i c e i n s i s t e n c e and l a c k i n g market u n d e r s t a n d i n g as w e l l as i n t e r n a l company s t r u c t u r e s and s h o r t - c u t deals.
2.4.2.4.1.2.1. Price insistence and lacking market understanding ’ Another s e r i o u s business l o s s t o p r o p p a n t c o m p e t i t i o n happened d u r i n g t h e p r e p a r a t i o n o f one o f t h e b i g h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n campaigns i n t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d t i g h t gas p r o v i n c e . W h i l e b i d d i n g o f t h e s e r v i c e companies t o t h e o p e r a t o r was made i n e a r l y t o m i d 1987, t h e p r i c e s f o r alumina o x i d e and s i l i c a t e proppants were r a i s e d e f f e c t i v e l a t e 1987 by t h e supp l i e r s ( c f . s e c t i o n 2.2.2.2.) who f o r a l o n g t i m e had a l r e a d y w a i t e d f o r a good o p p o r t u n i t y t o i n c r e a s e t h e p r i c e a f t e r t h e u n f o r t u n a t e and subsequently r e g r e t W h i l e f o r many y e a r s t e d p r i c e l o w e r i n g i n l a t e 1985 ( c f . s e c t i o n 2.2.2.1.). one s e r v i c e company p e r f o r m i n g most o f t h e s t i m u l a t i o n work i n t h e N o r t h Sea had a p r e f e r e n t i a l r e l a t i o n s h i p t o one m a j o r proppant s u p p l i e r , w i t h even f r e q u e n t l y t a k i n g t h e i r m a t e r i a l a t once w i t h o u t even t a l k i n g t o c o m p e t i t i o n a t a l l , t h e business n e g o t i a t i o n s f o r t h e b i g 1988 campaign changed t h e s i t u a t i o n
198 completely. Becoming u p s e t a b o u t t h e i n t e n t i o n o f i t s f o r m e r s t a n d a r d s u p p l i e r t o c h a r g e t h e new h i g h e r p r o p p a n t p r i c e a l r e a d y f o r t h e d e a l t h a t was o b t a i n e d b y t h e s e r v i c e company based on o f f e r i n g t h e o l d l o w e r p r i c e , t h e s e r v i c e company made an agreement w i t h t h e c o m p e t i t i o n w h i c h was w i l l i n g t o l i s t e n and t o u n d e r s t a n d t h e s p e c i a l r e l a t i o n s h i p s and precommitments and t h e r e f o r e g u a r a n t e e d s t i l l t h e o l d p r o p p a n t p r i c e f o r t h e w h o l e campaign c o m p r i s i n g s e v e r a l t h o u s a n d s o f t o n s o f material, t h e s e r v i c e company changed i t s s u p p l i e r o b v i o u s l y n o t o n l y f o r t h i s one d e a l , b u t was c e r t a i n l y l o o k i n g i n t o l o n g e r - t e r m f r u i t f u l c o o p e r a t i o n f o r m u t u a l b e n e f i t s i m i l a r t o t h e p r a c t i c e done w i t h t h e c o m p e t i t o r b e f o r e . Thus as a consequence o f p l a y i n g t h e p r i c e c a r d t o o h i g h due t o l a c k i n g u n d e r s t a n d i n g o f t h e m a r k e t r e q u i r e m e n t s and t h e f u t u r e p o t e n t i a l , t h e p r o p p a n t o u t l e t i n t o t h e N o r t h Sea was a l m o s t c o m p l e t e l y t a k e n away f r o m t h e f o r m e r supp l i e r and g i v e n t o i t s c o m p e t i t o r , r e s u l t i n g i n s e r i o u s b u s i n e s s l o s s and n e a r l y t o t a l l y r e v e r s i n g s e v e r a l y e a r s o f p u r c h a s e s t r a t e g y o f t h e s e r v i c e company c a r r y i n g o u t most o f t h e h y d r a u l i c proppant s t i m u l a t i o n work. These examples i l l u s t r a t e t h a t i n s u f f i c i e n t u n d e r s t a n d i n g o f t h e European s t i m u l a t i o n m a r k e t by r e m o t e h a n d l i n g and l a c k i n g l o c a l knowledge c a n c o s t much more money i n t e r m s o f l o s t b u s i n e s s t h a t w o u l d be t h e expenses f o r an a d e q u a t e r e g i o n a l r e p r e s e n t a t i o n . A p r o p e r c o v e r a g e o f t h e w h o l e European a r e a w o u l d comp r i s e t h e i m p l e m e n t a t i o n o f g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g c o n c e p t s i n t o t h e p r o p p a n t m a r k e t i n g s t r a t e g y and t h e o p e r a t i o n o u t o f a permanent European marketing representation (the significance o f inclusion o f geological factors i n t o h y d r a u l i c p r o p p a n t f r a c t u r e t r e a t m e n t d e s i g n i s a l s o emphasized by BRITT & LARSEN 1 9 8 6 ) .
2.4.2.4.1.2.2.
I n t e r n a l company s t r u c t u r e s and short-cut deals
Company and c o r p o r a t i o n i n t e r n a l s t r u c t u r e s , however, c a n i n some c a s e s ser i o u s l y d i s t u r b m a r k e t f o r c e s and c a n r e s u l t i n b u s i n e s s a p p r o v a l f o r c e r t a i n proppant s u p p l i e r s o n l y thanks t o i n t r a - m u l t i n a t i o n a l o r g a n i z a t i o n r e l a t i o n s h i p s . A n example o f such a c t i o n a g a i n s t m a r k e t i n g e f f o r t s o f c o m p e t i t o r s and s h o r t - c i r c u i t i n g o f b u s i n e s s c o n c l u s i o n w i t h r e n d e r i n g a l l t h e r e a l and open m a r k e t i n g a c t i v i t i e s o f o t h e r s u p p l i e r s and n a t u r a l m a r k e t f o r c e s a p r i o r i u s e l e s s and a w a s t e o f t i m e and money happened a l s o in t h e p r e p a r a t i o n o f t h e 1988 p r o p p a n t f r a c t u r i n g campaigns i n t h e B r i t i s h S o u t h e r n N o r t h Sea. A l t h o u g h hav i n g n o t t h e b e s t r e p u t a t i o n t o t h e s e r v i c e company e x e c u t i n g t h e j o b s as a r e s u l t of i n a d e q u a t e b u s i n e s s b e h a v i o u r and e x a g g e r a t e d p r i c e p o k e r i n g , one o f t h e m a j o r p r o p p a n t companies g o t t h e c o n t r a c t t o s u p p l y m a t e r i a l f o r a l a r g e campaign o n l y b y i n t e r v e n t i o n o f t h e o p e r a t o r w h i c h had f o r m e r l y t o g e t h e r w i t h t h e p r o p p a n t m a n u f a c t u r e r been u n d e r t h e head o f a b i g m u l t i n a t i o n a l c o r p o r a t i o n , and e v e n a f t e r d i v o r c e as a consequence o f company s p l i t t i n g and s a l e dur i n g c o u r s e o f s t r e a m l i n i n g measures, a s h o r t - c u t agreement w i t h o u t i n v o l v e m e n t o f t h e s e r v i c e company g u a r a n t e e d t h e o p e r a t o r a b e t t e r p r i c e f o r t h e m a t e r i a l and gave t h e p r o p p a n t s u p p l i e r t h e b i g d e a l . This business i n f a c t represented the f i r s t major action o f d i r e c t proppant s a l e and p u r c h a s e , r e s p e c t i v e l y , f r o m s u p p l i e r t o o p e r a t o r w i t h o u t i n t e g r a t i o n and i n v o l v e m e n t o f t h e s e r v i c e company. I t i s t y p i c a l f o r such t y p e o f m a r k e t i n g f o r c e i n v a l i d a t i o n t h a t a t t h e bottom o f t h e l i n e , o p p o r t u n i s t i c behaviour w i t h s e c u r i n g o f f o r m e r i n - h o u s e r e l a t i o n s h i p s f o r t h e mere sake o f b u s i n e s s c o n c l u s i o n and money-making e n a b l e d t h e p r o p p a n t m a n u f a c t u r e r t o a g r e e w i t h o u t h e s i t a t i o n t o a p r i c e f o r t h e w h o l e p r o p p a n t l o t w h i c h i t had e a r l i e r r e p e a t e d l y r e j e c t e d f r o m t a k i n g i n t o c o n s i d e r a t i o n when d i r e c t l y t a l k i n g t o t h e e x e c u t i n g s e r v i c e company. C o n c l u s i o n has t h e r e f o r e t o be drawn t h a t i n some cases, m a r k e t i n g e f f o r t s o n a sound e c o n o m i c a l and t e c h n i c a l base c a n be c o m p l e t e l y s p o i l e d b y o p p o r t u n i s m and a d v a n t a g e e x p l o i t a t i o n due t o i n t r a - m u l t i n a t i o n a l c o r p o r a t i o n r e l a t i o n -
199 s h i p s and d e a l s a r e made a l l o f a sudden which c o m p l e t e l y d i v e r g e f r o m o r cont r a d i c t t h e commonly e x h i b i t e d m a r k e t i n g p o l i c y o f t h e p r o p p a n t m a n u f a c t u r e r . The o u t l i n e d examples i l l u s t r a t e t h a t business l o s s t o p r o p p a n t c o m p e t i t i o n has v e r y d i v e r s e aspects which comprise n o t o n l y t e c h n i c a l and economical quest i o n s , b u t i n c l u d e a l s o p o l i t i c a l and o r g a n i z a t o r y p o i n t s t h a t sometimes even i n c o r p o r a t e complete e l i m i n a t i o n o f t h e market f o r c e s , w i t h i n such cases b e i n g a t t h e bottom o f t h e l i n e no d i f f e r e n c e between s a t i s f a c t o r y and engaged marketi n g a t t e n t i o n on t h e one hand and i n s u f f i c i e n t r e c o g n i t i o n and n e g l e c t a n c e o f t h e market on t h e o t h e r hand.
2.4.2.4.2. Business loss to natural sand I n a d d i t i o n t o business l o s s t o p r o p p a n t c o m p e t i t i o n , s e v e r a l d e a l s o f p a r t i a l l y c o n s i d e r a b l e q u a n t i t i e s o f s y n t h e t i c proppants f e l l through because o f l o s s t o n a t u r a l sand f o r v a r i o u s reasons. Some examples f r o m Yugoslavia, Turkey and B r i t i s h Southern N o r t h Sea a r e o u t l i n e d as f o l l o w s . The reasons f o r t h e missed o p p o r t u n i t i e s have been s i m i l a r l y as i n case o f business l o s s t o p r o p p a n t c o m p e t i t i o n p r e d o m i n a n t l y n e g l e c t a n c e o f t h e market and e x c e s s i v e p r i c e pokering.
2.4.2.4.2.1. Yugoslavia The undersupply o f t h e European market w i t h m a t e r i a l f o r many y e a r s as a r e s u l t o f wrong p l a n n i n g , u n r e a l i s t i c approach, i n s u f f i c i e n t a t t e n t i o n and l a c k i n g u n d e r s t a n d i n g had i t s consequences s t i l l i n 1985 b e f o r e t h e p r o p p a n t p r i c e r e d u c t i o n when n e i t h e r p r o d u c t i o n n o r s e r v i c e company were a b l e t o o b t a i n i n t e r m e d i a t e - s t r e n g t h proppants f o r a h y d r a u l i c f r a c t u r i n g j o b i n Y u g o s l a v i a ( w h i c h f o r American p r o p p a n t companies was so f a r a w h i t e p a t c h on t h e s t i m u l a t i o n map) t h a t p r e c u r s e d t h e MHF campaign i n t h i s c o u n t r y , and as a r e s u l t o f t h e u n a v a i l a b i l i t y o f t h e d e s i r e d p r o d u c t i n t i m e t h a t was s e l e c t e d as b e i n g s u i table, coupled w i t h only l i m i t e d p o s s i b i l i t i e s o f r a p i d d e l i v e r y o f l o c a l proppants, t h e designed f r a c t u r e l e n g t h had t o be reduced and t h e c r a c k had t o be i n f i l l e d w i t h sand f o l l o w e d by a t a i l - i n o f h i g h - s t r e n g t h proppants r a t h e r t h a n b e i n g c o m p l e t e l y plugged w i t h i n t e r m e d i a t e - s t r e n g t h proppants as would have been a p p r o p r i a t e i n t h e g i v e n c l o s u r e s t r e s s regime (ECONOMIDES, CIKES, PFURTER, U D I C K & URODA 1986).
2.4.2.4.2.2. Turkey Another example o f l o o s i n g b u s i n e s s as a consequence o f l a c k i n g m a r k e t i n g act i v i t y was t h e purchase o f tremendous amounts o f cheap sand by l o c a l o p e r a t o r s f o r a m a j o r f r a c t u r i n g campaign i n Turkey where i n t e r m e d i a t e - s t r e n g t h proppants would have been t e c h n i c a l l y r e q u i r e d a t l e a s t f o r t a i l - i n o p e r a t i o n s . W i t h t h e background o f a huge sand i n v e n t o r y i n t h e warehouse, however, n e i t h e r p r o p p a n t n o r s e r v i c e companies had any argument t o c o n v i n c e t h e p r o d u c t i o n company i n Turkey t o change t h e j o b design, and a l e v e l o f t e c h n i c a l l y sound u n d e r s t a n d i n g was o n l y reached when t h e sand p i l e had f i n a l l y been c o m p l e t e l y c l e a r e d by pumpi n g downhole. A p p r o p r i a t e m a r k e t i n g a t t e n t i o n o f t h e area would c e r t a i n l y have p r e v e n t e d t h e e x c e s s i v e purchase o f sand by t h e l o c a l o p e r a t o r and would have opened t h e door f o r c o n s i d e r a b l e i n t e r m e d i a t e - s t r e n g t h p r o p p a n t b u s i n e s s i n stead o f t h e sand.
2.4.2.4.2.3. British Southern North Sea L a r g e - s c a l e b u s i n e s s l o s s o f p r o p p a n t s u p p l i e r s t o n a t u r a l sand a l s o o c c u r r e d i n t h e B r i t i s h Southern N o r t h Sea d u r i n g course o f R o t l i e g e n d gas f i e l d development. A l t h o u g h t h e o p e r a t o r i n t h e example under d i s c u s s i o n c o n v e n t i o n a l l y used n a t u r a l sand as t h e main q u a n t i t y o f t h e f r a c t u r i n g t r e a t m e n t due t o s t i l l
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201 s u f f i c i e n t l y s h a l l o w r e s e r v o i r d e p t h (a1 though a l r e a d y near t h e boundary range of sand s t a b i l i t y ) and i n s e r t e d o n l y a c o m p a r a t i v e l y small volume o f i n t e r m e d i a t e - s t r e n g t h s y n t h e t i c proppants as t a i l - i n f o r t h e purpose o f p r o v i d i n g maximum c o n d u c t i v i t y and c l o s u r e s t r e s s r e s i s t i v i t y i n t h e s u r r o u n d i n g s o f t h e b o r e hole, replacement o f n a t u r a l sand by l o w - d e n s i t y alumina s i l i c a t e p r o p p a n t s was i n s e r i o u s d i s c u s s i o n s t a g e and was a l s o h e a v i l y promoted by t h e c o n t r a c t e d s e r v i c e company which r e c o g n i z e d t h i s o p p o r t u n i t y a l s o as a chance o f f u r t h e r sharpening o f p r o f i l e and a p p l i c a t i o n spectrum. The deal c o m p r i s i n g a t l e a s t 4 Mio. l b s (2,000 t ) o f m a t e r i a l , however, f i n a l l y f e l l t h r o u g h and n a t u r a l sand was p r e f e r r e d , because t h e p r o p p a n t s u p p l i e r c o u l d n o t be convinced t h a t f o r such a m a j o r p i l o t campaign o f u t i l i z i n g l o w - d e n s i t y alumina s i l i c a t e proppants i n t h e whole t r e a t m e n t t h r o u g h o u t t h e f i e l d t o be developed and t o renounce c o m p l e t e l y f r o m sand ( p r o g r e s s i v e replacement o f n a t u r a l sand by s y n t h e t i c p r o p p a n t s i s one of t h e m a j o r m a r k e t i n g g u i d e l i n e s i n t h e c u r r e n t p r o p p a n t scene; c f . sect i o n 2 . 2 . 2 . 3 . ) , a n o t h e r r e d u c t i o n o f t h e p r i c e d i f f e r e n c e t o n a t u r a l sand would be necessary t o o b t a i n t h e approval f r o m t h e o p e r a t o r . As a consequence o f l a c k i n g u n d e r s t a n d i n g o f t h e s i t u a t i o n and m i s s i n g o u t l o o k c a p a b i l i t y , t h e proppant m a n u f a c t u r e r i n s i s t e d on t h e s t a n d a r d p r i c e f o r the intermediate-strength l o w - d e n s i t y alumina s i l i c a t e p r o p p a n t s even i n view o f t h e l a r g e q u a n t i t y t o be f i r m l y o r d e r e d as one l o t and d e s p i t e t h e cooperat i o n o f f e r e d by t h e s e r v i c e company i n marking up t h e m a t e r i a l c o n s i d e r a b l y l e s s t h a n u s u a l t o h e l p b r i d g i n g t h e p r i c e gap c l o s e t o n a t u r a l sand. A t t h e b o t t o m o f t h e l i n e , t h e proppant s u p p l i e r t h u s p r e f e r r e d t o s e l l o n l y t h e t a i l i n q u a n t i t y ( w h i c h amounted t o a b t . one f i f t h o f t h e volume under d i s c u s s i o n f o r t h e whole j o b s ) a t t h e u s u a l p r i c e r a t h e r t h a n t o d e l i v e r a c o n s i d e r a b l y l a r g e r l o t f o r a s l i g h t l y reduced p r i c e . T h i s example i l l u s t r a t e s t h a t t h e d i f f e r e n c e o f even a c o u p l e o f c e n t s p e r pound i n proppant p r i c e can be s u f f i c i e n t t o k i l l m a j o r p o s s i b i l i t i e s o f p r o p p a n t p r o m o t i o n i n t o new segments o f a p p l i c a t i o n i f l a c k i n g encouragement, u n d e r s t a n d i n g o f t h e t e c h n i c a l n e c e s s i t y , and f o r e c a s t o f s t i m u l a t i n g f o l l o w - u p a c t i v i t y a r e compensated by r i g o r o u s i n s i s t i n g on s t a n d a r d p r i c e .
2.4.3. Eastern Europe u n t i l 1986 Almost near t h e end o f t h e p e r i o d 1977 - 1985 which r e p r e s e n t s t h e golden age o f h y d r a u l i c p r o p p a n t f r a c t u r i n g a t l e a s t i n Germany FRG ( i n t h e B r i t i s h Southern N o r t h Sea, t h e t i m e 1988 - 1992 m i g h t become t h e peak o f t h e a c t i v i t y ) i n Western Europe, MHF s t i m u l a t i o n s t a r t e d t o t a k e a l s o p l a c e i n p a r t s o f East e r n Europe. Several p a r t i a l l y e x t r e m e l y l a r g e f r a c t u r e j o b s have been c a r r i e d o u t i n Y u g o s l a v i a i n 1985 and p a r t i c u l a r l y 1986 (ECONOMIDES, C I K E S , PFURTER, U D l C K & URODA 1986), and w i t h o u t t h e s e r i o u s d r o p o f t h e o i l p r i c e i n e a r l y 1986, i t would have been even more. Comments a r e o f f e r e d as f o l l o w s on Yugoslav i a and Hungary, USSR, o t h e r E a s t e r n European c o u n t r i e s , and Southern Europe and N o r t h e r n A f r i c a .
2.4.3.1. Yugoslavia and Hungary R e s e r v o i r c o n d i t i o n s i n Y u g o s l a v i a (and a l s o i n a d j o i n i n g Hungary) a r e q u i t e c o m p l i c a t e d due t o u n u s u a l l y h i g h temperatures o f 180 - 240 OC i n 3,000 - 4,000 m d e p t h ( c o r r e s p o n d i n g t o 9,000 - 12,000 f t ) which cause problems w i t h t h e f r a c t u r i n g f l u i d s ( c f . s e c t i o n s 4.3.4.4. and 4.7.4.2.) and make t h e pumping o f heav i e r proppants v e r y d i f f i c u l t . I n a d d i t i o n , t h e r e s e r v o i r f l u i d s c o n t a i n H2S and CO2 which a r e a g g r e s s i v e t o t h e equipment. The gas- and condensate-bearing f o r m a t i o n (ECONOMIDES, C I K E S , PFURTER, U D I C K & URODA 1986) c o n s i s t s o f T e r t i a r y conglomerates, sandstones and l i m e s t o n e b r e c c i a s . One o f t h e main aims o f hyd r a u l i c f r a c t u r i n g i s t o a v o i d e x c e s s i v e r e t r o g r a d e condensation .(CASTELIJNS 1981, KENYON & BEHLE 1987; DANESH, HENDERSON, K R I N I S & PEDEN 1988). The f i e l d s belong t o t h e Molve-Kal i n o v a c - S t a r i Gradac complex i n t h e Pannonian B a s i n (DRAG I C E V I C , M A R I N O V I C , STANKOVIC, BATUSIC & NIKOLIC 1983; I B R A H I M P A S I C 1983; R I -
202
G A S S I 1985, 1986; BAUK, SECEN, BABIC & SACER 198b; ENERGIEWIRTSCHAFT 1986 b, GORST 1987) o n t h e Y u g o s l a v i a n s i d e and t o t h e B a r c s - I n k e - M a k o g r o u p on t h e Hung a r i a n s i d e ( c f . f i g . 5 and t a b s . 8 - 9 ) . The f r a c t u r e j o b s i n Y u g o s l a v i a have been c a r r i e d o u t i n a t w o - s t e p manner w i t h pumping o f 100 - 200 t o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y p r o p p a n t s i n t h e e a r l y s t a g e and 200 - 450 t o f i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y o r h i g h s t r e n g t h p r o p p a n t s i n t h e l a t e s t a g e , t h u s b e i n g a s p e c i a l t y p e o f t a i l - i n and/ o r l e a d - i n f r a c t u r i n g s t r a t e g y (HICKEY, BROWN & CRITTENDEN 1981; c f . s e c t i o n 2.4.1.2.2.). T o t a l p r o p p a n t q u a n t i t i e s p e r t r e a t m e n t r a n g e d between 300 and 650 t and t h u s t h e o p e r a t i o n s i n Y u g o s l a v i a i n c l u d e d some o f t h e b i g g e s t MHF j o b s w h i c h have e v e r been c a r r i e d o u t i n Europe ( o n l y i n Germany FRG have s o f a r been t r e a t m e n t s o f c o m p a r a b l e m a g n i t u d e p e r f o r m e d ; c f . s e c t i o n 2 . 4 . 1 . 1 . 1 . ) . The d e s i g n o f t h e j o b s i n Hungary i s s m a l l e r due t o o t h e r r e s e r v o i r c h a r a c t e r i s t i c s , t e c h n i c a l c i r c u m s t a n c e s and e c o n o m i c a l c o n d i t i o n s w i t h r e s p e c t t o t h o s e i n Yugoslavia.
2 . 4 . 3 . 2 . USSR The f i r s t h y d r a u l i c p r o p p a n t f r a c t u r i n g o p e r a t i o n s were a l r e a d y c a r r i e d o u t i n t h e USSR i n 1952. The t o t a l a n n u a l f r a c t u r i n g a c t i v i t y i n t h e USSR r e a c h e d i t s peak i n t h e y e a r s 1958 - 1962 w i t h u p t o 1,500 o p e r a t i o n s p e r y e a r , and l a t e r d e c l i n e d t o a p l a t e a u - l i k e l e v e l w i t h more o r l e s s 100 t r e a t m e n t s p e r y e a r . The c e n t r e s o f h i t h e r t o h y d r a u l i c f r a c t u r e o p e r a t i o n p e r f o r m a n c e i n t h e USSR a r e t h e K r a s n o d a r , V o l g o - U r a l , T a t a r i a ( m a i n l y t h e Romashkinskoe and T u m a i s i f i e l d s ) , B a s h k i r i a , K u j b y s h e v , Turkmenia, D a g i s t a n and S i b e r i a ( m a i n l y t h e Sal i m s k i and S u r g u t s k o e f i e l d s ) a r e a s . H y d r a u l i c f r a c t u r e s t i m u l a t i o n has been w a t e r - i n j e c t i o n and w a t e r - d i s p o s a l w e l l s , b u t m a i n l y done i n o i l - p r o d u c t i o n , s u b o r d i n a t e l y a l s o i n d e g a s i f i c a t i o n w e l l s o f c o a l - b e a r i n g s t r a t a and g e o l o g i c a l i n v e s t i g a t i o n w e l l s . MHF t r e a t m e n t s have so f a r n o t been done i n t h e USSR, and a l s o s m a l l - s c a l e f r a c t u r i n g d i d n o t p l a y a m a j o r r o l e i n w e l l c o m p l e t i o n , because p r e f e r e n c e was g i v e n t o d r i l l i n g and f a s t e x p l o i t a t i o n o f e a s i l y a c c e s s i b l e and r e c o v e r a b l e r e s e r v e s . The f u l l s i g n i f i c a n c e o f s t i m u l a t i o n was o n l y l a t e l y b e t t e r r e a l i z e d when o i l p r o d u c t i o n l a g g e d b e h i n d t h e g o a l s o f t h e f i v e y e a r p l a n ( c f . s e c t i o n 2.4.4.1.5.) and t h e b e g i n n i n g o f t h e m a j o r w o r l d o i l p r i c e c r i s i s t o g e t h e r w i t h a d i m i n i s h i n g s t r e n g t h o f t h e US $ gave t h e i m p e t u s f o r s e r i o u s l o o k i n g i n t o p o s s i b i l i t i e s o f enhanced e x p l o i t a t i o n o f e x i s t i n g c a p a c i t i e s (EBEL 1986, ORASIANU 1987; CHADWICK, LONG & NISSANKE 1 9 8 8 ) . Up t o now, o n l y n a t u r a l sand i n q u a n t i t i e s o f up t o 130 t, b u t i n m o s t c a s e s o n l y 20 - 50 t has been u s e d as p r o p p i n g a g e n t i n h y d r a u l i c f r a c t u r i n g t r e a t ments i n t h e USSR. S y n t h e t i c h i g h - q u a l i t y p r o p p a n t s have e i t h e r n o t been n e c e s s a r y i n s h a l l o w r e s e r v o i r s , o r deep p a y zones had s o f a r l i t t l e p r i o r i t y f o r s t i m u l a t i o n . A l l t h e h i t h e r t o p e r f o r m e d j o b s have been c a r r i e d o u t i n own s e r v i c e u s i n g h i g h - p r e s s u r e pumps o f A m e r i c a n p r o v e n a n c e w h i c h were b o u g h t more t h a n 10 y e a r s ago s t i l l b e f o r e t h e embargo e n a c t e d b y P r e s i d e n t C a r t e r i n 1 9 7 8 when American s e r v i c e companies e v e n had o f f i c e s i n Moscow, as w e l l as w i t h dom e s t i c and Romanian e q u i p m e n t . The h y d r a u l i c f r a c t u r i n g t e c h n o l o g y i s w e l l u n d e r s t o o d and a p p r e c i a t e d i n t h e USSR f r o m where i n f a c t a l s o some o f t h e f i r s t i n s p i r a t i o n s and c o n c e p t s d e r i v e d and p i o n e e r i n g work had been done when t h e s t i m u l a t i o n method was b o r n a l m o s t 40 y e a r s ago (CLARK 1948, WILSEY & BEARDEN 1954, KHRISTIANOVICH & ZHELTOV 1955; c f . s e c t i o n 1 . 2 . ) .
2 . 4 . 3 . 3 . Other Eastern European c o u n t r i e s In m o s t o f t h e o t h e r E a s t e r n European c o u n t r i e s , s i m i l a r l y as i n t h e USSR s o f a r o n l y s m a l l e r f r a c t u r i n g t r e a t m e n t s have been done w i t h a b t . 20 - 150 t o f sand as p r o p p i n g a g e n t ( e x a m p l e s c o m p r i s e m a i n l y Hungary, P o l a n d , Y u g o s l a v i a and Germany GDR, and s u b o r d i n a t e l y a l s o C z e c h o s l o v a k i a and Romania, whereas i n B u l g a r i a and A l b a n i a , n o t h i n g has been done up t o n o w ) . I n t e r m e d i a t e - t o h i g h -
203 s t r e n g t h proppants have h i t h e r t o o n l y been a p p l i e d i n v e r y l i m i t e d q u a n t i t i e s f o r a t a i l - i n i n Hungary (coming f r o m v e r y r e s t r i c t e d domestic m a n u f a c t u r i n g as b y - p r o d u c t i n alumina i n d u s t r y ; c f . s e c t i o n 1 . 3 . 1 . 2 . ) . I n view o f t h e apparent d e l a y i n development o f r e s e r v o i r s t i m u l a t i o n i n East e r n Europe, i t has t o be understood t h a t b e f o r e t h e 1986 o i l p r i c e crash, f i r s t t h e p r i o r i t y o f domestic o i l and gas a c q u i s i t i o n was on d r i l l i n g and comp l e t i o n o f e a s i l y a c c e s s i b l e pay zones which caused no m a j o r t r e a t m e n t problems and n e c e s s i t i e s , and second t h e USSR d e l i v e r e d s u f f i c i e n t amounts o f o i l and gas on b a r t e r - t r a d e base t o i t s sate1 1 it e s , t h e r e b y s e c u r i n g s u p p l y and t h u s n o t t r i g g e r i n g any s e r i o u s need o f i n c r e a s i n g domestic hydrocarbon o f f t a k e s i n t h e o t h e r c o u n t r i e s o f t h e s o c i a l i s t i c b l o c k . The 1986 o i l p r i c e c o l l a p s e coupl e d w i t h t h e U S $ exchange r a t e drop, however, a b r u p t l y changed t h e whole s i t u a t i o n by a f f e c t i n g t h e t o t a l i n t e r w o v e n f a b r i c between t h e group members on b i l a t e r a l s c a l e w i t h r e s p e c t t o t h e USSR as w e l l as on m u l t i l a t e r a l s c a l e among each o f t h e s a t e l l i t e s and f r o m each o f them t o t h e USSR.
2.4.3.4. Southern Europe and Northern A f r i c a L a r g e r f r a c t u r i n g campaigns i n c l u d i n g c o o p e r a t i o n w i t h Western s e r v i c e comp a n i e s have been made i n 1986 and e a r l i e r y e a r s i n Turkey where again a l m o s t exc l u s i v e l y sand was used as p r o p p i n g m a t e r i a l . I n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y proppants would have been most s u i t a b l e a t l e a s t as a t a i l - i n f o r a number o f o p e r a t i o n s , b u t t h e e a r l y purchase o f v a s t amounts o f sand i n h i b i t e d t h e a p p l i c a t i o n o f t h e b e t t e r , b u t a l s o more expensive m a t e r i a l i n t h e h i t h e r t o p e r f o r med campaigns ( c f . s e c t i o n 2.4.2.4.2.). I n N o r t h e r n A f r i c a , h y d r a u l i c proppant f r a c t u r i n g o f l o w - p e r m e a b i l i t y sandstones has so f a r m a i n l y been c a r r i e d o u t i n A l g e r i a and i n s u b o r d i n a t e amounts a l s o i n L i b y a , Egypt, Sudan, Oman, T u n i s i a , Z a i r e , G u l f o f Guinea and Congo, w i t h i n most cases o n l y sand h a v i n g been pumped, b u t i n t e r m e d i a t e - o r h i g h s t r e n g t h proppants a l s o a l r e a d y r e p e a t e d l y having been a p p l i e d . S t i m u l a t i o n i n t h e f a r t h e r M i d d l e East h i t h e r t o m a i n l y i n c l u d e d I n d i a and P a k i s t a n .
2 . 4 . 4 . Status i n Eastern Europe I n c o n t r a s t t o Western Europe, t h e d i f f e r e n t economical system i n E a s t e r n Europe r e s u l t s i n a more o r l e s s independency f r o m t h e o i l p r i c e s c e n a r i o and p a r t i a l l y a l s o f r o m t h e US $ exchange r a t e s c e n a r i o which means t h a t b a s i c a l l y t h e d r i l l i n g and s t i m u l a t i o n programmes s h o u l d have gone ahead a l s o d u r i n g 1986 t h r o u g h 1988 w i t h o u t m a j o r changes. I n view o f market p e n e t r a t i o n f o r h y d r a u l i c proppant f r a c t u r i n g , however, t h e E a s t e r n European market ( w i t h t h e e x c e p t i o n b e i n g Y u g o s l a v i a and p a r t i a l l y a l s o Hungary; c f . s e c t i o n s 2.4.3.1. and 2.4.6.2.) i s s e r i o u s l y underdeveloped o r p a r t i a l l y s t i l l v i r t u a l l y unattended by Western s e r v i c e and p r o p p a n t companies due t o n e g l e c t a n c e o f m a r k e t i n g a c t i v i t y i n t h e p a s t when b a s i c a l l y o n l y t h e b i g and u r g e n t demand i n USA and p a r t s o f Western Europe was s a t i s f i e d by m a r k e t i n g c o n s u l t a n c y and a d v i c i n g as w e l l as by d e l i v e r y o f m a t e r i a l . Due t o t h e h i t h e r t o p r e f e r e n t i a l b o t h e x p l o r a t i o n and p r o d u c t i o n d r i l l i n g as w e l l as c o n v e n t i o n a l c o m p l e t i o n and t e s t i n g w i t h o n l y l i t t l e s t i m u l a t i o n a c t i v i ty, t h e E a s t e r n European market i s nowadays s t i l l i n an immature t o submature stage o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and p a r t i a l l y a l s o g r a v e l p a c k i n g i n cont r a s t t o Western Europe w h i c h i s a l r e a d y mature s i n c e many y e a r s , and t o t h e p a r t i a l l y supermature s i t u a t i o n o f t h e USA e s p e c i a l l y d u r i n g t h e golden age o f s t i m u l a t i o n i n t h e l a t e 1970's and e a r l y 1980's. As a consequence o f t h e i n t e r l o c k i n g r e l a t i o n s h i p between f o r e i g n t r a d e o f f i c e s and more o r l e s s n a t i o n a l hydrocarbon p r o d u c t i o n companies as w e l l as v a r i o u s f i n a n c i a l c o m p l i c a t i o n s , negot i a t i o n and c o n c l u s i o n o f business c o n t r a c t s and a p p l i c a t i o n o f t e c h n o l o g y and m a t e r i a l t a k e much more t i m e i n E a s t e r n Europe t h a n i n Western Europe. Comments
204 a r e o f f e r e d as f o l l o w s on o i l p r i c e and U S $ exchange r a t e , p o l i t i c a l r e s t r i c t i o n s i n t h e p a s t , p r e s e n t s t i m u l a t i o n p o t e n t i a l and e c o n o m i c a l r e s t r u c t u r i n g , s e l f - s e r v i c i n g t r e a t m e n t p o l i c y , e c o n o m i c a l and f i n a n c i a l a s p e c t s , and r e s e r v o i r e n g i n e e r i n g and gas s u p p l y a s p e c t s .
2.4.4.1. Oil p r i c e and US $ exchange r a t e The E a s t e r n European c o u n t r i e s ( e x c e p t o f t h e USSR) a r e i n t h e p r e s e n t s i t u a t i o n o f low o i l prices, i n c r e a s i n g devaluation o f t h e i r c u r r e n c i e s w i t h r e s p e c t t o U S $, DM and t , and p r o g r e s s i v e i n f l a t i o n more o r l e s s s e v e r e l y f o r c e d t o i n c r e a s e t h e i r d o m e s t i c o i l and gas p r o d u c t i o n b y a l l means, because t h e USSR has a f t e r t h e 1986 o i l p r i c e c r a s h ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) and t h e more o r l e s s accompanying U S $ exchange r a t e c o l l a p s e ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) c u t back t h e q u a n t i t i e s o f h y d r o c a r b o n s u p p l y t o i t s s a t e l l i t e s (ERDOEL-ERDGAS AKTUELL 1985 d, R I G A S S I 1985) due t o i n c r e a s i n g own demand as a s o u r c e o f h a r d c u r r e n c y b y s a l e t o Western c o u n t r i e s . The o u t l i n e as f o l l o w s p r e s e n t s some a s p e c t s o f e x p o r t o f USSR o i l and gas t o W e s t e r n and E a s t e r n Europe, dependency o f Comecon c o u n t r i e s on USSR oil and gas, b a r t e r t r a d e v s . h a r d c u r r e n c y s a l e o f o i l and gas, r o l e o f h y d r o c a r b o n e x p o r t s i n t h e USSR f o r e i g n t r a d e , USSR oil p r o d u c t i o n , and c u r r e n c y c o n v e r t i b i l i t y and exchange r a t e s .
2.4.4.1.1. Export o f USSR o i l and gas t o Western and Eastern Europe The gas coming f r o m t h e g i a n t S i b e r i a n gas f i e l d U r e n g o i and o t h e r s i s t o l a r g e amounts b r o u g h t i n t r a n s i t t h r o u g h t h e USSR s a t e l l i t e s t a t e s t o W e s t e r n Europe where i t i s s o l d t o v a r i o u s consumers a g a i n s t h i g h p r i c e s i n h a r d c u r r e n c y t h a t were f i x e d b y d e l i v e r y c o n t r a c t s p a r t i a l l y s t i l l d u r i n g t h e t i m e o f h i g h o i l and gas p r i c e s and t h a t a r e now c e r t a i n l y k e p t v a l i d b y t h e USSR and a r e backed-up b y r e l i a b l e d e l i v e r y , and o n l y b a l a n c e d and c o r r e s p o n d i n g q u a n t i t i e s o f t h e gas a r e d i s t r i b u t e d in t h e E a s t e r n European s a t e l l i t e s o f t h e USSR (FEHRINGER 1 9 8 6 ) . The USSR 1986 g a s s a l e s t o W e s t e r n E u r o p e amounted t o 1 . 3 9 T c f ( a b t . 35 B i l l . m3), whereas e x p o r t t o t h e s o c i a l i s t i c c o u n t r i e s r e a c h e d s t i l l 1 . 4 3 T c f ( a b t . 36 B i l l . m3), b u t was a l r e a d y on a marked d e c l i n e ( O I L GAS JOURNAL 1987 0 ) . A n o t h e r r e a s o n f o r t h e o i l s u p p l y s h o r t a g e o f t h e USSR was d e c l i n i n g own o i l p r o d u c t i o n i n t h e y e a r s 1982 - 1985 (RIGASSI 1 9 8 5 ) , w i t h t h e c u m u l a t i v e o f f t a k e even f a l l i n g behind t h e g o a l s s e t i n t h e f i v e - y e a r p l a n . I n 1986 when t h e o i l p r i c e c r a s h t o o k p l a c e , o i l p r o d u c t i o n i n t h e USSR s t a r t e d t o i n c r e a s e a g a i n as a consequence o f t h e e f f e c t i v i t y o f t h e r e o r g a n i z a t i o n p o l i t i c s ( p e r e s t r o i k a ; GORBATCHEV 1987; c f . s e c t i o n 2 . 4 . 4 . 3 . 2 . ) and r e a c h e d 1987 a r e c o r d l e v e l , b u t because l a r g e r q u a n t i t i e s were a s s o c i a t e d w i t h l o w e r p r i c e s , t h e r e s u l t a t t h e b o t t o m o f t h e l i n e r e m a i n s t h e same, and i n o r d e r t o i n c r e a s e h a r d c u r r e n c y income, o i l and gas e x p o r t have t o r i s e f u r t h e r . E x p o r t o f o i l and g a s a c c o u n t f o r a b t . 60 - 8 0 % o f c o n v e r t i b l e c u r r e n c y e a r n i n g o f t h e USSR and t h e r e f o r e have a n o v e r w h e l m i n g i m p o r t a n c e i n f o r e i g n t r a d e (STERN 1986, AL-CHALABI 1987, GORST 1 9 8 8 ) . T h i s r e q u i r e m e n t :Jf t h e USSR t o g e t h a r d c u r r e n c y f r o m h y d r o c a r b o n s a l e and t h e l a r g e gas r e s e r v e s w h i c h a r e s u f f i c i e n t f o r e v e n d o u b l i n g t h e s u p p l y q u a n t i t i e s t o Western E u r o p e in t h e n e x t y e a r s a r e t h e reason t h a t t h e r e i s no b a s i s o f f e a r i n g commercial o r s e c u r i t y r i s k s o f i m p o r t i n g gas o r o i l f r o m t h e USSR w h i c h has n o i n t e r e s t t o e x e r c i s e economic l e v e r a g e f o r p o l i t i c a l c o n c e s s i o n s . The USSR e x p o r t e d i n 1987 a b t . 12 % and a b t . 3 1 % o f i t s t o t a l gas and o i l p r o d u c t i o n volume, r e s p e c t i v e l y ( O I L GAS JOURNAL 1988 f ) , w i t h gas and o i l r e p r e s e n t i n g a b t . 1 0 % and a b t . 33 % o f t h e t o t a l v a l u e o f t h e USSR e x p o r t s , r e s p e c t i v e l y . T o t a l USSR o i l e x p o r t r e a ched a r e c o r d h i g h i n 1987 w i t h 3 . 7 M i o . b b l / d ( w i t h t o t a l p r o d u c t i o n a m o u n t i n g t o 1 2 . 3 - 1 2 . 5 M i o . b b l / d ) , and i t s s i g n i f i c a n c e f o r t h e s t a t e b u d g e t i s r e f l e c t e d b y t h e f a c t t h a t e v e n i n 1985 when t h e t o t a l o u t p u t f e l l t o t h e u n p r e c e d e n t e d l o w o f 1 1 . 9 M i o . b b l / d , t h e e x p o r t volume a c h i e v e d 3 . 4 Mio. b b l / d w h i c h i s o n l y s l i g h t l y l e s s t h a n t h a t o f t h e r e c o r d y e a r o f 1987 ( O I L GAS JOURNAL 1987
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206 The 1986 o i l p r i c e c r a s h , lume b y t h e c o n s i d e r a b l y o i l and gas s h a r e i n o v e r a l l 4 9 . 4 % i n 1985 and 43.7 % i n 0).
2.4.4.1.2.
however, more t h a n compensated t h e i n c r e a s i n g vod r o p p i n g p r i c e . A f t e r r i s i n g t o 51.6 % i n 1984, t h e v a l u e o f USSR e x p o r t s t o a l l c o u n t r i e s s l i d e d t o 1986.
Dependency o f Comecon countries on USSR
oil and gas
A t t h e b o t t o m o f t h e l i n e , a l l t h e Comecon ( R a t f u r g e g e n s e i t i g e W i r t s c h a f t s h i l f e , RGW) c o u n t r i e s a r e d e p e n d i n g t o l a r g e r o r s m a l l e r d e g r e e on t h e USSR i n t e r m s o f b o t h o i l and g a s s u p p l y (FISCHER 1978), because due t o s h o r t a g e o r l a c k o f h a r d c u r r e n c y , t h e y c a n n o t a f f o r d t o b u y o i l and gas a t t h e w o r l d market, and w i t h t h e e x c e p t i o n o f Romania, a l l t h e s o c i a l i s t i c s a t e l l i t e s o f t h e USSR c a n n o t c o v e r t h e i r w h o l e own demand b y d o m e s t i c p r o d u c t i o n . I n a d d i t i o n , p i p e l i n e i n f r a s t r u c t u r e does n o t a l l o w t h e s m a l l e r Comecon c o u n t r i e s t o r e nounce f r o m t h e s t e a d y d i r e c t s u p p l y f r o m t h e USSR and t o a c c e s s o t h e r s o u r c e s . D u r i n g t h e l a s t 20 y e a r s , t h e USSR has s u f f i c i e n t l y d e l i v e r e d o i l and gas t o t h e s a t i s f a c t i o n o f t h e Comecon c o u n t r i e s and even sometimes s u p p l i e d them w i t h l a r g e r q u a n t i t i e s a t t h e expense o f c u t t i n g e x p o r t s t o t h e West, and o n l y t h e 1986 o i l p r i c e c r a s h and t h e accompanying US $ weakness have c r e a t e d a n o t h e r s i tuation.
A l l t h e a f o r e m e n t i o n e d t e c h n i c a l , e c o n o m i c a l and p o l i t i c a l r e l a t i o n s h i p s , e s p e c i a l l y t h e l o w e r o i l p r i c e and t h e weaker U S $ w i t h r e s p e c t t o e a r l i e r y e a r s , h i g h l i g h t a g a i n t h e f o r c e l a s t i n g on t h e USSR and p a r t i a l l y a l s o t h e o t h e r East e r n European c o u n t r i e s t o i n c r e a s e t h e i r d o m e s t i c o i l and g a s p r o d u c t i o n b y a l l p o s s i b l e means (OILMAN 1987 c ) , w i t h s t i m u l a t i o n o f r e s e r v o i r s i n e x i s t i n g f i e l d s b e i n g one o f t h e t o p - p r i o r i t y i t e m s . P a r t i c u l a r e x c e p t i o n s f r o m t h e a f o r e m e n t i o n e d s c e n a r i o a r e C z e c h o s l o v a k i a where c o u n t e r t r a d e w i t h t h e USSR i s s t i l l on a m u t u a l l y s u f f i c i e n t l e v e l , and i n some e x t e n t a l s o t h e GDR w h i c h p r o f i t s f r o m s p e c i a l t r a d e a r r a n g e m e n t s w i t h t h e FRG and f r o m p a r t i c u l a r l y c l o s e r e l a t i o n s h i p s t o t h e USSR, and t a k e s whenever p o s s i b l e a d v a n t a g e o f i t s g e o g r a p h i c a l p o s i t i o n a t t h e b o u n d a r y o f t h e two b i g p o l i t i c a l b l o c k s .
2.4.4.1.3.
Barter trade vs. hard currency sale o f oil and gas
O i l and gas a r e t h e m a j o r s o u r c e o f h a r d c u r r e n c y f o r t h e USSR (GRIFFITHS 1 9 8 6 ) , and as b o t h o i l p r i c e d r o p and US $ exchange r a t e c o l l a p s e r e s u l t i n l o wer n e t r e t u r n s ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) , t h e l o s s e s have t o be compensated b y s e l l i n g i n c r e a s i n g amounts o f h y d r o c a r b o n s i n Western c o u n t r i e s w h i c h t o g e t h e r w i t h an i n c r e a s e d d o m e s t i c demand b y e n l a r g i n g t h e i n d u s t r y i n t h e USSR a r e m i s s i n g f o r d e l i v e r y i n t o t h e s o c i a l i s t i c s a t e l l i t e s t a t e s on b a r t e r - t r a d e b a s i s . I n a d d i t i o n , many o f t h e E a s t e r n European c o u n t r i e s v i r t u a l l y f a c e b a n k r u p t c y and a r e no l o n g e r a b l e t o p u r c h a s e even f r o m t h e USSR w h i c h in t h e p r e s e n t gener a l e c o n o m i c a l s c e n a r i o i n t u r n i s no l o n g e r w i l l i n g t o s u p p l y f r e e o f c h a r g e o r even o n c r e d i t . I n s t e a d o f b a r t e r i n g w i t h t h e USSR, t h e s a t e l l i t e c o u n t r i e s a r e t h e i r s e l v e s more and more f o r c e d t o g e t a c c e s s t o h a r d c u r r e n c y by s e l l i n g w h a t e v e r p o s s i b l e i n t h e W e s t e r n w o r l d , and t h e USSR does t h e same w i t h p r o d u c t s f o r m e r l y d e l i v e r e d on exchange b a s i s t o i t s g r o u p members. On t h e o t h e r hand, however, t h e USSR has t o m a i n t a i n o i l e x p o r t s t o i t s s a t e l l i t e c o u n t r i e s a t a r e a s o n a b l e l e v e l f o r b o t h p o l i t i c a l and e c o n o m i c a l r e a s o n s , because t h i s e n a b l e s t h e USSR t o o b t a i n p r o d u c t s on b a r t e r l e v e l t h a t i t o t h e r w i s e w o u l d have t o p u r c h a s e f o r US $ on t h e w o r l d m a r k e t , and p e r m i t s t h e s a t e l l i t e s t o a v o i d a f u r t h e r inc r e a s e o f t h e i r a l r e a d y tremendous d e b t i n h a r d c u r r e n c y (RIGASSI 1 9 8 5 ) . The m o s t p r o n o u n c e d example o f b a r t e r i n g i s t h e p u r c h a s e o f Romanian d r i l l i n g and c o m p l e t i o n e q u i p m e n t w h i c h i s p a i d b y o i l and gas d e l i v e r i e s ( O I L GAS JOURNAL 1987 1, R I G A S S I 1 9 8 6 ) . The s i g n i f i c a n c e o f t h i s b a r t e r t r a d e o f e q u i p ment v s . h y d r o c a r b o n s i s u n d e r l i n e d b y t h e f a c t t h a t i n t h e p e r i o d 1 9 8 1 - 1987,
207 Romania accounted f o r 43 - 50 % o f t h e USSR's t o t a l i m p o r t s o f o i l and gas f i e l d equipment which comprise c h i e f l y heavy d r i l l i n g r i g s , b u t a l s o substant i a l amounts o f c o m p l e t i o n and s t i m u l a t i o n equipment. The steady i n c r e a s e o f i m p o r t l e v e l f r o m Romania i n t h e l a s t y e a r s went p a r a l l e l t o t h e d e c l i n i n g o i l p r i c e t h a t had t h e consequence o f more and more c u t t i n g equipment i m p o r t q u a n t i t i e s f r o m Western Europe and Japan ( t h o s e f r o m t h e USA were i n d e p e n d e n t l y s l a s h e d by t h e r e s t r i c t i o n s on s t r a t e g i c s a l e s ; c f . s e c t i o n 2.4.4.2.) by r a t e s O f 50 - 80 %.
2.4.4.1.4.
Role o f hydrocarbon exports i n the USSR f o r e i g n trade
The s i g n i f i c a n t r o l e o f o i l e x p o r t s i n t h e o v e r a l l USSR balance o f payments i s a l s o i l l u s t r a t e d by CHAOWICK, LONG & NISSANKE (1988), whereas GORST (1985) o u t l i n e s t h e p e r s i s t e n t s h o r t a g e o f o i l i n t h e E a s t e r n European s a t e l l i t e count r i e s due t o i n s u f f i c i e n t d e l i v e r y volumes coming f r o m t h e USSR. The s i t u a t i o n o f p a s t and p r e s e n t domestic p r o d u c t i o n , e x p o r t p o t e n t i a l and i m p o r t r e q u i r e ments i n USSR and o t h e r E a s t e r n European c o u n t r i e s i s summarized by OILMAN (1987 c ) and ORASIANU ( 1 9 8 7 ) . The 1986 o i l p r i c e c r a s h was t h e reason t h a t f o r t h e f i r s t t i m e s i n c e 40 years, t h e v a l u e o f t h e t o t a l f o r e i g n t r a d e o f t h e USSR s l a s h e d by a b t . 10 %, w i t h most o f t h e drop stemming f r o m lower o i l and gas p r i c e s ( O I L GAS JOURNAL 1987 h, 1987 1 ) . A t t h e bottom o f t h e l i n e , however, i n 1986 reduced e x p o r t s t o Western Europe and o t h e r w o r l d market c o u n t r i e s accounted f o r t h e e n t i r e dec l i n e i n t h e USSR f o r e i g n t r a d e value, whereas t h e USSR t r a d e w i t h t h e s o c i a l i s t i c s a t e l l i t e s c o n t i n u e d i t s steady i n c r e a s e i n 1986. The reason f o r t h i s cons t e l l a t i o n i s t h e t r a n s i t i o n a l phase f r o m h i g h t o low o i l p r i c e s which was o f d i f f e r e n t d u r a t i o n i n Western and E a s t e r n Europe. W h i l e e x p o r t s t o Western Europe ( m a i n l y t o Germany FRG) were r a p i d l y reduced because t h e low o i l p r i c e a f t e r t h e c r a s h was a t once v a l i d t h e r e , agreements w i t h i t s s o c i a l i s t i c s a t e l l i t e s t o base t h e t r a d e on average w o r l d p r i c e s d u r i n g t h e p r e v i o u s f i v e y e a r s ( c o r r e s p o n d i n g t o p l a n p e r i o d s ) r a t h e r than c u r r e n t w o r l d p r i c e s enabled t h e USSR t o make s t i l l c o m p a r a t i v e l y more p r o f i t f r o m e i t h e r s e l l i n g o r b a r t e r i n g w i t h o t h e r E a s t e r n European c o u n t r i e s i n a p r o l o n g e d t r a n s i t i o n a l phase which, however, as a consequence o f p e r s i s t i n g r e l a t i v e l y low o i l p r i c e s u n t i l t h e end o f 1988 and beyond, i s c e r t a i n l y soon coming t o an end.
2.4.4.1.5.
USSR o i l production
I n 1987, t h e annual p l a n q u a n t i t y of o i l p r o d u c t i o n was f o r t h e f i r s t t i m e s i n c e 9 y e a r s n o t o n l y met, b u t a l s o exceeded (RIGASSI 1986, 1987), and b e n e f i t i n g f r o m t h i s , o i l e x p o r t volume r o s e t o a r e c o r d l e v e l , b u t on t h e o t h e r hand, lower o i l p r i c e , i n c r e a s i n g e x p l o r a t i o n c o s t s and weaker US $ have i n t h e meantime eroded t h e p r o f i t a b i l i t y o f energy t r a d i n g (GORST 1988). C a l c u l a t i n g on g l o b a l l e v e l , 1987 has b r o u g h t t h e USSR t o t h e second p l a c e a f t e r Saudi Arab i a i n t h e w o r l d league o f o i l e x p o r t i n g c o u n t r i e s . D e s p i t e gloomy f o r e c a s t s f o l l o w i n g a two-year d e c l i n e i n o u t p u t i n 1984 and 1985 and t h e r e o r g a n i z a t i o n phase i n 1986, o v e r s h o o t i n g o f t h e t a r g e t q u a n t i t y f o r 1987 was e s t a b l i s h e d so w e l l t h a t t h e lower range o f t h e 1990 p r o d u c t i o n goal has a l r e a d y been almost reached. T h i s again u n d e r l i n e s t h e n e c e s s i t y o f s t a b i l i z a t i o n o f t h i s l e v e l and even expansion o f c a p a c i t y i n t h e n e x t y e a r s which c e r t a i n l y has t o i n c l u d e a reason a b l e amount o f s t i m u l a t i o n . The s u c c e s s f u l achievement o f t h e r e c o r d e x p l o i t a t i o n l e v e l a l s o h i g h l i g h t s t h e e f f e c t i v i t y o f t h e p o l i t i c a l and economical r e o r g a n i z a t i o n ( p e r e s t r o i k a ; GORBATCHEV 1987; c f . s e c t i o n 2 . 4 . 4 . 3 . ) which l e d t o maj o r improvements a l r e a d y i n a s h o r t p e r i o d o f t i m e . The e v o l u t i o n o f o i l product i o n i n t h e USSR d u r i n g t h e l a s t decade i s summarized i n a d i s c u s s i o n o f meeti n g and o v e r s h o o t i n g o f p l a n f o r e c a s t , impact o f r e c o r d p r o d u c t i o n f o r o i l exp o r t s , and s i g n i f i c a n c e o f Western S i b e r i a share.
208
2.4.4.1.5.1.Meeting and overshooting of plan forecast O i l p r o d u c t i o n l e v e l o f t h e USSR i n t h e l a s t y e a r s peaked a t 12.4 M i o . b b l / d i n l a t e 1982 and then g r a d u a l l y decreased v i a 12.3 i n 1983 and 12.2 i n 1984 t o 11.9 i n 1985, b u t then as a consequence o f t h e e f f e c t i v i t y o f economical and t e c h n i c a l r e o r g a n i z a t i o n ( p e r e s t r o i k a ; GORBATCHEV 1987; c f . s e c t i o n 2.4.4.3.) s t a r t e d t o r i s e v i a 12.3 i n 1986 and 1 2 . 3 - 12.5 i n 1987 t o t h e r e c o r d o f 12.4 - 1 2 . 8 i n 1988, w i t h t h e p l a n f o r e c a s t o f 12.8 i n 1990 t h u s b e i n g a l r e a d y touched ( R I G A S S I 1986, 1 9 8 7 ) . T h i s o v e r s h o o t i n g o f t h e p l a n t a r g e t t e r m i n a t e d an almost f i f t e e n y e a r p e r i o d o f annual f a l l i n g s h o r t o f t h e p l a n g o a l , opens t h e way t o f u r t h e r economical and t e c h n i c a l improvement o f o i l e x p l o i t a t i o n and supply, and u n d e r l i n e s t h e e f f e c t i v i t y o f t h e r e s t r u c t u r i n g concept. I t i s p l a n ned t o f u r t h e r i n c r e a s e d a i l y o i l p r o d u c t i o n t o a b t . 13.0 - 13.5 M i o . b b l b y t h e y e a r 2000 ( O I L GAS JOURNAL 1987 0 ) and due t o t h e r e c o r d p r o d u c t i o n o f 1986 - 1988, i t i s almost w i t h o u t doubt t h a t t h e p l a n goal o f 12.8 M i o . b b l f o r 1990 w i l l be met.
2.4.4.1.5.2. Impact of record production for oil exports The r e c o r d annual o i l p r o d u c t i o n o f 624.2 Mio. t i n 1987 (ERDOL-ERDGAS-KOHLE NACHRICHTEN 1988 b) enabled t h e USSR t o become t h e most i m p o r t a n t o i l s u p p l i e r f o r t h e European Economic Community (EEC) group i n 1987 and t o i n c r e a s e i t s mark e t share w i t h r e s p e c t t o 1986 c o n s i d e r a b l y a t t h e expense o f t h a t o f t h e OPEC and p a r t i c u l a r l y Saudi A r a b i a (ERDOL-ERDGAS-KOHLE NACHRICHTEN 1988 a ) . W h i l e i n 1986 t h e EEC n e t o i l i m p o r t s amounted t o 355.5 M i o . t o f which 33.4 M i o . t o r 9.4 % d e r i v e d f r o m t h e USSR, 71.3 M i o . t o r 20.1 % came f r o m Saudi A r a b i a and 246.1 Mio. t o r 69.2 % were d e l i v e r e d by t h e OPEC i n t o t a l , t h e t r e n d was comp l e t e l y r e v e r s e d i n 1987 when t h e 346.9 Mio. t o f n e t o i l i m p o r t s were covered by 41.1 Mio. t o r 11.8 % by t h e USSR, 39.3 Mio. t o r 11.3 % by Saudi A r a b i a , and 212.8 M i o . t o r 61.3 % by t h e OPEC i n t o t a l .
2.4.4.1.5.3. Significance of Western Siberia share Western S i b e r i a c u r r e n t l y accounts f o r more than 60 % o f t h e t o t a l USSR o i l o u t p u t . The s u p e r g i a n t o i l f i e l d Samotlor i s t h e second l a r g e s t i n t h e w o r l d and has a l r e a d y produced n e a r l y 20 B i l l . b b l o f o i l ( R I G A S S I 1986). O t h e r West e r n S i b e r i a n s u p e r g i a n t s a r e Fedorovo and Mamontovo ( O I L GAS JOURNAL 1987 0 ) . Western S i b e r i a p r o v i d e d a b t . 30 % o f t h e USSR o i l o u t p u t i n 1975, and s i n c e then t h e share o f t h e area p r o g r e s s i v e l y i n c r e a s e d v i a 52 % i n 1980 and 6 1 % i n 1985 t o 64 % i n 1987, w i t h e s t i m a t i o n s p r e d i c t i n g a f u r t h e r r i s e t o a b t . 75 % i n 2000. Assessments document t h a t p r o b a b l y o n l y some 30 % o f t h e u l t i m a t e geol o g i c a l o i l r e s o u r c e s have been found so f a r i n t h e USSR, and c u r r e n t proven and h i g h l y p r o b a b l e r e c o v e r a b l e r e s e r v e s r e a c h a b t . 100 B i l l . b b l ( e x c l u d i n g add i t i o n a l r e s e r v e s t h a t can be produced by enhanced o i l r e c o v e r y t e c h n i q u e s ) , w i t h two t h i r d s o f a l l t h e known r e s e r v e s b e i n g c o n c e n t r a t e d i n Western S i b e r i a (RIGASSI 1987).
2.4.4.1.6. Currency convertibility and exchange rates The USSR has a s p e c i a l s i t u a t i o n i n t h e w o r l d economical system, as t h e agreed o r f i x e d exchange r a t e s between t h e n o n - c o n v e r t i b l e r o u b l e and t h e conv e r t i b l e US $, DM and t remain more o r l e s s c o n s t a n t d e s p i t e any m i n o r o r m a j o r a l t e r a t i o n s o f exchange r a t e s among Western c u r r e n c i e s and o t h e r E a s t e r n European c u r r e n c i e s . I n a d d i t i o n , t h e USSR i s t h e o n l y E a s t e r n European c o u n t r y which can buy goods i n almost u n l i m i t e d q u a n t i t y and w i t h o u t u n f a v o u r a b l e r e s t r i c t i o n s o f t i m i n g a g a i n s t h a r d c u r r e n c y t h a t can be p a i d w i t h o u t any p r o blems once Western m a t e r i a l , equipment o r t e c h n o l o g y i s u r g e n t l y needed. The
209 o n l y c o m p l i c a t i o n i s t h e e a r l y budget f i x a t i o n i n t h e f i v e - y e a r p l a n s which makes m a j o r changes and s h i f t s d u r i n g r u n n i n g p l a n p e r i o d s d i f f i c u l t u n l e s s supp o r t e d by e x c e p t i o n a l unforeseen s i t u a t i o n s and backed-up on a h i g h p o l i t i c a l and t e c h n i c a l l e v e l .
2.4.4.2. Political restrictions in the past The impact o f p o l i t i c a l r e s t r i c t i o n s i n t h e p a s t i s c h a r a c t e r i z e d by d i s c u s s i n g t h e USA p e t r o l e u m equipment s u p p l y embargo 1978 - 1987 and t h e s t a t u s a f t e r l i q u i d a t i o n o f the f o r e i g n trade sanctions.
2.4.4.2.1. USA petroleum equipment supply embargo 1978
-
1987
The m a r k e t i n g and s a l e s p o t e n t i a l f o r Western s e r v i c e and p r o p p a n t companies i n t h e USSR was u n t i l r e c e n t l y s e r i o u s l y d i s t u r b e d by t h e f o r e i g n t r a d e p o l i c y s a n c t i o n s enacted by P r e s i d e n t C a r t e r i n 1978 and l a t e r i s s u e d i n even more ser i o u s f o r m i n l a t e 1 9 7 9 / e a r l y 1980 and l a t e 1 9 8 1 / e a r l y 1982 ( O I L GAS JOURNAL 1987 p) w h i c h p r a c t i c a l l y n e a r l y c o m p l e t e l y stopped e x p o r t s o f p e t r o l e u m e q u i p ment f r o m t h e USA t o t h e USSR f o r almost t e n y e a r s . The s a n c t i o n s have o n l y been removed by P r e s i d e n t Reagan i n e a r l y 1987 t o a d j u s t t o t h e f a c t t h a t USA companies no l o n g e r c l e a r l y dominate t h e w o r l d market i n o i l - f i e l d equipment, b u t comparable m a t e r i a l c o u l d now be purchased by t h e USSR f r o m a l l around t h e w o r l d and e s p e c i a l l y f r o m Europe and Japan (COTTRILL 1987, ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 a, LANDERS 1987, OIL GAS JOURNAL 1987 i ) , w i t h a t t h e b o t t o m o f t h e l i n e t h e USA h a v i n g l o s t tremendous business o p p o r t u n i t i e s t o competit o r s i n o t h e r p a r t s o f t h e w o r l d r a t h e r than a c h i e v i n g t h e goal o f t h e embargo t o h u r t t h e USSR by h i n d e r i n g t h e growth o f i t s hydrocarbon i n d u s t r y . Due t o t h e widespread f o r e i g n a v a i l a b i l i t y o f p r o d u c t s o f comparable technol o g y l e v e l p a r t i c u l a r l y i n Europe and Japan, t h e American s a n c t i o n s on hydrocarbon equipment and t e c h n o l o g y had t o t a l l y l o s t t h e i r impact, and i n a d d i t i o n t o t h e t e c h n i c a l nonsense, t h e y had a s i g n i f i c a n t d e b i l i t a t i n g e f f e c t on t h e American p e t r o l e u m i n d u s t r y by r e s u l t i n g i n a b t . 2 B i l l . US $ business l o s s dur i n g t h e t i m e o f t h e embargo (OIL GAS JOURNAL 1987 i ) . Trade s a n c t i o n s , however, s t i l l a p p l y f o r computer technology, advanced s e i s m i c p r o d u c t s , and h i g h p r e s s u r e equipment i n c l u d i n g s t i m u l a t i o n pumps (RIGASSI 1987).
2.4.4.2.2. Status after liquidation of foreign policy sanctions As t h e e a r l y 1987 o i l f i e l d s a n c t i o n s l i f t , however, took p l a c e a f t e r c o n c l u s i o n o f t h e o r g a n i z a t i o n o f t h e new f i v e - y e a r p l a n (1986 - 1990), t h e renewed m a r k e t i n g approach t o t h e USSR by f o r e i g n companies may a p a r t f r o m some except i o n s t a k e more t i m e t h a n u s u a l l y expected, b u t t h i s i s no reason f o r any del a y , because t h e p r e p a r a t i o n s and n e g o t i a t i o n s should p r e f e r a b l y be concluded e a r l y enough p r i o r t o t h e f i n a l d e c i s i o n s on t h e budget and t h e d i s t r i b u t i o n o f expenses f o r t h e n e x t f i v e - y e a r p l a n , t h u s r e s u l t i n g f o r t h e USA a t t h e b o t t o m of t h e l i n e i n a n o t h e r s e r i o u s business loss t o European and Japanese competit i o n . T h e r e f o r e t h e u l t i m a t e r e l a x a t i o n o f t h e r e s t r i c t i o n s may have a r r i v e d t o o l a t e f o r many American companies ( O I L GAS JOURNAL 1987 p ) , and r e g a i n i n g o f l o s t market shares i s even c o m p l i c a t e d by t h e u n r e l i a b i l i t y o f t h e USA f o r e i g n p o l i c y which a t any t i m e c o u l d r e v e r t back t o s a n c t i o n s , and t h e e f f o r t s o f t h e USSR t o become s e l f - s u f f i c i e n t i n o i l and gas t e c h n o l o g y r e q u i r e m e n t s . The p r e s e n t l y tremendous demand o f a p p l i c a t i o n o f s t i m u l a t i o n as a v i a b l e t o o l o f i n c r e a s i n g o i l and gas p r o d u c t i o n i n t h e USSR became v e r y p r o m i n e n t dur i n g t h e i n t e r n a t i o n a l o i l and gas e x h i b i t i o n i n Moscow i n October 1987 ( N e f tegaz; COTTRILL 1987) which was t h e f i r s t m a j o r stage o f u n l i m i t e d c o n v e r s a t i o n on a l l aspects a f t e r l i f t i n g o f t h e American o i l - f i e l d embargo and which was c o n s e q u e n t l y used by t h e S o v i e t s t o express t h e i r enormous i n t e r e s t i n coope-
210 r a t i o n w i t h W e s t e r n companies on t h e base o f m u t u a l b e n e f i t . P a r t i c u l a r i n t e r e s t o f A m e r i c a n and W e s t e r n European companies aimed o n t h e o f f s h o r e m a r k e t i n t h e USSR w h i c h has a b t . one f i f t h o f t h e w o r l d ' s e n t i r e c o n t i n e n t a l s h e l v e s , b u t has so f a r n o t v e r y much d e v e l o p e d o f f s h o r e a c t i v i t y (COTTRILL 1987; c f . a l s o section 2.4.4.5.5.).
2.4.4.3. Present stimulation potential
and economical restructuring
H y d r a u l i c p r o p p a r i t f r a c t u r i n g in E a s t e r n Europe ( w i t h t h e e x c e p t i o n o f JUGOSLAWIA) i n c l u d e d s o f a r m a i n l y s m a l l - t o m e d i u m - s i z e j o b s t h a t w e r e c a r r i e d o u t w i t h n a t u r a l sand as a p r o p p i n g a g e n t and w i t h p r e d o m i n a n t l y R u s s i a n o r Roman i a n e q u i p m e n t i n own s e r v i c e ( s u c h j o b s have p a r t i c u l a r l y been done i n Hung a r y , P o l a n d , Germany GDR and t h e USSR, and s u b o r d i n a t e l y a l s o in C z e c h o s l o v a k i a and Romania, b u t have so f a r n o t been p e r f o r m e d i n B u l g a r i a and A l b a n i a ) . The o v e r v i e w as f o l l o w s i l l u m i n a t e s some a s p e c t s o f s u p p l y o f s y n t h e t i c h i g h q u a l i t y p r o p p a n t s , p o l i t i c a l and e c o n o m i c a l r e o r g a n i z a t i o n , and h y d r o c a r b o n p r o d u c t i o n g o a l s o f t h e f i v e - y e a r p l a n . D i f f e r e n c e s i n d r i l l i n g a c t i v i t y i n West e r n and E a s t e r n E u r o p e a f t e r t h e 1986 o i l p r i c e c r a s h a r e a l s o o u t l i n e d .
2.4.4.3.1. Supply o f synthetic high-quality proppants D o m e s t i c p r o d u c t i o n o f s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s i s h i t h e r t o l i m i t e d t o m a n u f a c t u r i n g o f v e r y s m a l l q u a n t i t i e s i n H u n g a r y as a b y - p r o d u c t i n aluminum i n d u s t r y ( c f . a l s o s e c t i o n 1 . 3 . 1 . 2 . ) . As a consequence o f t h e u r g e n t need o f t h e USSR t o o b t a i n h a r d c u r r e n c y b y p e t r o l e u m s a l e i n t h e West and t h e n e c e s s i t y o f i n c r e a s i n g d o m e s t i c s u p p l y o f o i l and gas i n t h e sat e l l i t e c o u n t r i e s o f t h e USSR, s t i m u l a t i o n b y b o t h a c i d i z i n g and h y d r a u l i c f r a c t u r i n g is c o n s i d e r e d t o have a g r e a t p o t e n t i a l i n E a s t e r n E u r o p e i n t h e coming y e a r s , w i t h s u p p l y o f s y n t h e t i c p r o p p a n t s and e q u i p m e n t a n d / o r s e r v i c e f r o m West e r n c o u n t r i e s ( c f . a l s o s e c t i o n s 2 . 4 . 5 . 5 . 3 . and 2 . 4 . 6 . 3 . 2 . ) b e i n g i n d i s p e n s i b l e f o r a c h i e v i n g t h e g o a l o f a c c e l e r a t i o n o f own h y d r o c a r b o n p r o d u c t i o n i n t h e E a s t e r n European s t a t e s .
2.4.4.3.2. Pol it ica 1 and economica 1 reorganization Acceleration, r e s t r u c t u r i n g and t r a n s p a r e n c y a r e t h e c u r r e n t t o p g u i d e l i n e s of t h e USSR b o t h d o m e s t i c and f o r e i g n p o l i c y ( u s k o r e n i e , p e r e s t r o i k a and g l a s n o s t ; GORBATCHEV 1987) w h i c h c e r t a i n l y w i l l be s o o n e r o r l a t e r t r a n s f e r r e d a l s o t o the s a t e l l i t e countries, w i t h the only question being the actual timing o f t h e i d e o l o g i c a l t r a n s f e r . Due t o t h e i r v a l i d i t y i n a l l b r a n c h e s o f t h e economic a l and p o l i t i c a l systems, t h e g u i d e s l o g a n s o f a c c e l e r a t i o n , r e s t r u c t u r i n g and t r a n s p a r e n c y t h u s a l s o a p p l y f o r o i l and gas i n d u s t r y d e v e l o p m e n t , w i t h enhancement o f h y d r o c a r b o n p r o d u c t i o n h a v i n g p r e s e n t l y p a r t i c u l a r l y h i g h p r i o r i t y . W h i l e r e s t r u c t u r i n g i s t h e m a j o r g o a l o f improvement o f t h e p o l i t i c a l system, s p e e d i n g - u p i s m o s t i m p o r t a n t t o enhance t h e e f f e c t i v i t y o f t h e o i l and gas i n d u s t r y t o e n a b l e f a s t e r access t o known p o t e n t i a l b y b o t h i n c r e a s i n g r e s e r v e s and a c c e l e r a t i n g o f f t a k e , and t r a n s p a r e n c y i s a t o p i t e m f o r a m e l i o r a t i o n o f t h e t e c h n o l o g i c a l e v o l u t i o n b y g u a r a n t e i n g b e t t e r d o m e s t i c and i n t e r n a t i o n a l c o m m u n i c a t i o n and exchange and t h u s f a c i l i t a t i n g u n d e r s t a n d i n g .
2.4.4.3.3. Hydrocarbon production goals of the five-year plan A c c o r d i n g t o t h e new USSR f i v e - y e a r p l a n (1986 - 1 9 9 0 ) , o i l c o n s u m p t i o n s h o u l d be d e c r e a s e d and o i l i s t o be r e p l a c e d in i n c r e a s i n g amounts b y g a s w h i c h means i n t e n s i f i c a t i o n of n a t u r a l as p r o d u c t i o n t h a t i s c e r t a i n l y i n c l u d i n g q u i t e some s t i m u l a t i o n (ERDOEL-FRDGAS AKTUELL 1985 b, ERDOL-ERDGAS-KOHLE NACHRICHTEN 1986 a ) . F u r t h e r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i m p e t u s
211 can be expected f r o m t h e r e c e n t u n s a t i s f y i n g o i l p r o d u c t i o n which lagged b e h i n d t h e commitments i n t h e l a s t f i v e - y e a r p l a n (1980 - 1985; EBEL 1986, ERDUL-EROGAS-KOHLE NACHRICHTEN 1986 a; c f . a l s o s e c t i o n 2.4.4.1.5.), and t h e r e f o r e somet h i n g has t o be done t o compensate t h e b a c k l o g by c o n t i n u o u s l y m e e t i n g t h e goal a t t h e end o f t h e c u r r e n t f i v e - y e a r p l a n by i n c r e a s i n g o u t p u t f r o m e x i s t i n g f i e l d s i n a d d i t i o n t o new d i s c o v e r i e s and a p p r a i s a l s . Another i n d i c a t o r o f t h e enormous s t i m u l a t i o n p o t e n t i a l i s t h e r e t a r d a t i o n o f f i e l d development which i s h i g h l i g h t e d by t h e f a c t t h a t a l t h o u g h t h e USSR i s one o f t h e o l d e s t o i l - p r o d u c i n g c o u n t r i e s i n t h e w o r l d , o v e r 70 % o f t h e c u r r e n t o u t p u t comes f r o m o n l y 5 % o f t h e d e p o s i t s (ORASIANU 1987). I n 1987, f o r t h e f i r s t t i m e s i n c e 9 y e a r s t h e planned annual o i l p r o d u c t i o n volume had been f u l f i l l e d and even exceeded (RIGASSI 1986, 1987) and a r e c o r d i n e x p l o i t a t i o n was set, w i t h now t h e emphasis b e i n g on s t a b i l i z a t i o n o f t h i s l e v e l so t h a t c o n s i d e r a b l e q u a n t i t i e s can be d i v e r t e d f o r e x p o r t t o Western Europe (OIL GAS JOURNAL 1987 1, ORASIANU 1987, GORST 1988). I n 1987 and 1988, t h e r e c o r d o i l p r o d u c t i o n reached 1 2 . 4 - 12.8 Mio b b l / d (FUEG 1988; c f . s e c t i o n 2 . 4 . 4 . 1 . 5 . ) . CHADWICK, LONG & NISSANKE (1988) g i v e an overview o f USSR o i l exp o r t s and a s s o c i a t e d t r a d e adjustments, r e f i n i n g c o n s t r a i n t s and market behav i o u r , as w e l l as s u b s t i t u t i o n o f o i l by gas as a consequence o f backlogs i n o i l e x p l o r a t i o n and e x p l o i t a t i o n . Past, p r e s e n t and f u t u r e o i l p r o d u c t i o n v o l u mes and p o t e n t i a l s o f t h e USSR a r e summarized by ORASIANU ( 1 9 8 7 ) .
Differences in drilling activity in Western and Eastern Europe after the 1986 oil price crash
2.4.4.3.4.
The p a r t i a l independence o f t h e USSR f r o m t h e w o r l d market and i t s e f f o r t s t o a m e l i o r a t e hydrocarbon s u p p l y s e c u r i t y a r e u n d e r l i n e d by t h e f a c t t h a t i n c o n t r a s t t o c o n s i d e r a b l e budget c u t t i n g i n t h e western hemisphere f o r d r i l l i n g and c o m p l e t i o n and r e s u l t i n g s e r i o u s l y l e s s e x p l o r a t i o n and development a c t i v i t y w i t h r e s p e c t t o o r i g i n a l p l a n s b e f o r e t h e 1986 o i l p r i c e crash, t h e USSR has i n 1986 even stepped up d r i l l i n g i n o r d e r t o r a p i d l y r e p l a c e m a t u r i n g o i l p r o d u c t i o n f r o m p r o g r e s s i v e l y d e p l e t e d f i e l d s (OIL GAS JOURNAL 1986). Because o f i t s r o l e as an overwhelming source o r h a r d c u r r e n c y , p e t r o l e u m development commands t o p p r i o r i t y i n t h e USSR a l s o a f t e r t h e 1986 o i l p r i c e drop, whereas a t t h e same t i m e i n t h e USA, p e t r o l e u m has f a l l e n t o t h e b o t t o m o f t h e government a l p r i o r i t y l i s t (KINNEY 1987). W h i l e o p e r a t o r s i n USA and Western Europe a r e r e l u c t a n t t o i n v e s t money i n new p r o j e c t s , t h e USSR has boosted i t s e f f o r t s t o s u s t a i n and expand o i l and gas p r o d u c t i o n and e x p o r t s . The l o g i c a l r e s u l t o f these enormous e f f o r t s r e g a r d l e s s o f economical c o n s t e l l a t i o n s have been t h e overpassing o f t h e p l a n goal f o r t h e annual o i l p r o d u c t i o n q u a n t i t y i n 1987 w i t h b r e a k i n g t h e e x p l o i t a t i o n r e c o r d so pronouncedly t h a t o i l e x p o r t s reached a maximum (RIGASSI 1987, GORST 1988). I n o r d e r t o m a i n t a i n o r even i n c r e a s e t h i s f a v o u r a b l e c o n s t e l l a t i o n , i t i s planned t o exceed t h e e x p l o r a t i o n and app r a i s a l d r i l l i n g budget volume o f t h e p e r i o d 1981 - 1985 by a b t . 40 % i n 1986 1990 (RIGASSI 1986).
2.4.4.4.
Self-servicing treatment pol icy
Concerning m a r k e t i n g approach f o r s t i m u l a t i o n and o t h e r s e r v i c e s , t h e d i f f e r e n t l o g i s t i c a l and f i n a n c i a l s i t u a t i o n between Western and E a s t e r n Europe r e s u l t s i n a d i f f e r e n t s t r a t e g y o f t r e a t m e n t o p e r a t i o n . W h i l e i n Western Europe and v a r i o u s o t h e r p a r t s o f t h e w o r l d o i l and gas p r o d u c t i o n companies commonly ask e s t a b l i s h e d s e r v i c e companies o f m a i n l y N o r t h American provenance t o c a r r y o u t t h e whole s t i m u l a t i o n j o b s i n c l u d i n g s u p p l y o f a l l t h e mechanical and chem i c a l a d d i t i v e s f o r t h e o p e r a t i o n ( c f . a l s o PRAST 1980), i n E a s t e r n Europe t h e p r o d u c t i o n companies o f t e n p r e f e r t o buy t h e i r own equipment, t o purchase most o r a l l o f t h e m a t e r i a l d i r e c t l y and t o do t h e j o b s w i t h o u t t h e s u p p o r t o f Wes t e r n s e r v i c e companies a f t e r p r o b a b l y a few i n t r o d u c t o r y , t r a i n i n g o r p i l o t t r e a t m e n t s ( e x c e p t i o n s o f t h i s a t t i t u d e a r e up t o now t h e MHF o p e r a t i o n s i n Yu-
212 g o s l a v i a and t o a l e s s e r amount a l s o i n Hungary on t h e o t h e r s i d e o f boundary).
the
state
While t h e s e l f - s e r v i c i n g s t r a t e g y s o f a r m a i n l y i n c l u d e d performance o f small e r f r a c t u r i n g j o b s w i t h p r e d o m i n a n t l y Russian o r Romanian equipment, several E a s t e r n European c o u n t r i e s ( p a r t i c u l a r l y Yugoslavia, s u b o r d i n a t e l y a l s o Poland and USSR) a r e i n t e n d i n g t o c a r r y o u t a l s o b i g g e r t r e a t m e n t s t h e i r s e l v e s w i t h h i g h - p r e s s u r e equipment which t h e y a r e g o i n g t o buy i n t h e west. The b u s i n e s s o f Western s e r v i c e and s u p p l y companies i n E a s t e r n Europe i s t h e r e f o r e i n many areas r e s t r i c t e d t o d e l i v e r y o f proppants, f l u i d s , a d d i t i v e s and o t h e r chemic a l s , w i t h o n l y i n some r e g i o n s a l s o i n c l u d i n g i n t r o d u c t o r y s e r v i c e , training, s u p e r v i s i o n and a d v i c e .
2.4.4.5. Economical and financial aspects I n terms o f economical systems and f i n a n c i a l p o s s i b i l i t i e s , d i f f e r e n c e s have t o be made between t h e i n d i v i d u a l E a s t e r n European c o u n t r i e s . Comments a r e o f f e r e d on w o r l d bank c r e d i t s f o r Yugoslavia, Hungary, Poland and Romania; e x p o r t revenues i n Yugoslavia, Czechoslovakia, Germany GDR and B u l g a r i a ; h a r d c u r r e n c y a c q u i s i t i o n o f USSR and Germany GDR, p o l i t i c a l and economical r e o r g a n i z a t i o n and j o i n t v e n t u r e s .
2.4.4.5.1. World Bank credits for
Yugoslavia, Hungary, Poland and Romania The f i r s t group o f E a s t e r n European c o u n t r i e s i n c l u d e s Y u g o s l a v i a , Hungary, Poland and Romania which a r e members of t h e I n t e r n a t i o n a l Monetary Fund and thus a r e a b l e t o o b t a i n c r e d i t s f r o m t h e World Bank i n US $ ( c f . a l s o R I G A S S I 1985, 1986; SAUNDERS & S H I R A Z I 1986) f o r expansion o f domestic hydrocarbon p r o d u c t i o n by d r i l l i n g , t e s t i n g and s t i m u l a t i o n . World Bank P e t r o l e u m P r o j e c t s i n c l u d i n g c o o p e r a t i o n w i t h Western s e r v i c e and s u p p l y companies a r e a l r e a d y r u n n i n g i n Y u g o s l a v i a and Hungary ( c f . s e c t i o n s 2.4.3.1. and 2.4.6.2.) and a r e c o o r d i n a t e d by t h e n a t i o n a l f o r e i g n t r a d e o f f i c e s . Poland and Romania a r e a t t h e moment p r e p a r i n g a p p l i c a t i o n s f o r World Bank sponsorship o r a r e a l r e a d y i n t h e stage o f n e g o t i a t i o n on amount and t i m i n g o f f u n d i n g .
2.4.4.5.2. Export revenues in Yugoslavia, Czechoslovakia, Germany GDR and Bulgaria O t h e r E a s t e r n European c o u n t r i e s c o m p r i s i n g m a i n l y Yugoslavia, Czechoslovak i a , Germany GDR and B u l g a r i a have s u f f i c i e n t income o f h a r d c u r r e n c y f r o m h i g h e x p o r t volumes o f v a r i o u s goods and s e r v i c e s t o be independent f r o m World Bank C r e d i t s and t o be a b l e t o f i n a n c e p e t r o l e u m d r i l l i n g and s t i m u l a t i o n themselves. The most pronounced example o f t h i s c a t e g o r y so f a r was t h e MHF campaign i n Y u g o s l a v i a in 1986 ( c f . s e c t i o n 2.4.3.1.) w h i c h was performed w i t h o u t any s u p p o r t by t h e World Bank and w h i c h i s c o n t i n u e d in 198711988 i n a d d i t i o n t o an independent World-Bank-sponsored a c t i o n b e i n g p a r t o f a l a r g e r p e t r o l e u m p r o ject.
2.4.4.5.3. Hard currency acquisition o f USSR and Germany GDR Hard c u r r e n c y f o r payment o f m a t e r i a l , equipment and s e r v i c e which c a n n o t be purchased i n E a s t e r n Europe and which a r e i n d i s p e n s i b l e f o r t h e decided technol o g i c a l p r o g r e s s and t h e achievement o f t h e s e t g o a l s o f economical p l a n n i n g i s p r i n c i p a l l y no problem f o r USSR and Germany GDR. The USSR i s p r o f i t i n g f r o m l a r ge incomes f r o m e x p o r t o f p r e d o m i n a n t l y gas and o i l ( t h e r o l e o f n a t u r a l gas i n t h e e a s t - w e s t energy t r a d e i n Europe i s o u t l i n e d by D'ANDREA 1987; c f . a l s o sec-
213 t i o n 2.2.1.6.3.). Other USSR c o n v e r t i b l e money sources a r e i n t e r n a t i o n a l l a n d and sea c a r g o and passenger s e r v i c e and e x p o r t o f v a r i o u s o t h e r goods, and t h e GDR i s making c o n s i d e r a b l e advantages f r o m t h e p r e f e r e n t i a l t r a d e w i t h t h e FRG a t s p e c i a l b i l a t e r a l c o n d i t i o n s as w e l l as t a k i n g f e e s f r o m t r a n s i t t r a v e l , obl i g a t o r y exchange a t an expensive a r t i f i c i a l r a t e f r o m f o r e i g n c i t i z e n s coming t o t h e c o u n t r y as v i s i t o r s as w e l l as p a r t i c i p a n t s i n conferences, f a i r s and exh i b i t i o n s . To a l e s s e r amount, almost t h e same a p p l i e s f o r Romania and B u l g a r i a which a r e b o t h w o r k i n g as d r i l l i n g c o n t r a c t o r s abroad, w i t h Romania a l s o e x t e n s i v e l y r e b u i l d i n g Western o i l - f i e l d equipment and s e l l i n g i t w i t h i n E a s t e r n Europe f o r c e r t a i n l y dumping p r i c e s , b u t f o r e x c l u s i v e l y h a r d c u r r e n c y o r a l s o b a r t e r t r a d e o f goods which i t o t h e r w i s e would have t o purchase f o r much money on t h e open w o r l d market.
2.4.4.5.4. Pol it ica 1 and economical reorganizat ion The p o l i t i c a l and economical r e o r g a n i z a t i o n which i s assessed t o be t h e second r e v o l u t i o n i n t h e USSR (GORBATCHEV 1987) i s b r i e f l y c h a r a c t e r i z e d a l o n g t h e g u i d e l i n e s o f r e s t r u c t u r i n g , t r a n s p a r e n c y and a c c e l e r a t i o n ( p e r e s t r o i k a , g l a s n o s t and u s k o r e n i e ) , and t h e mutual b e n e f i t f o r i n t e r n a t i o n a l c o o p e r a t i o n i s also stressed.
2.4.4.5.4.1. Restructuring, transparency, acceleration
(perestroika, glasnost, uskorenie) The p o l i t i c a l and economical r e o r g a n i z a t i o n ( p e r e s t r o i k a ; GORBATCHEV 1987) i n t h e USSR g i v e s a l s o i n c r e a s i n g freedom and r e s p o n s i b i l i t y t o e x p l o r a t i o n and p r o d u c t i o n companies and f i e l d o p e r a t i o n s . While f o r m e r l y a l l t h e s u b s i d i a r i e s were f u l l y dependant on t h e m i n i s t r i e s o f o i l and gas i n d u s t r y c o n c e r n i n g budget i n g , a p p r o v a l , n e g o t i a t i o n s w i t h f o r e i g n companies and p u r c h a s i n g d e c i s i o n s , many o f them a r e now a l r e a d y more o r l e s s independent and have t h e i r own f i n a n c i a l budget w h i c h t h e y can spend a c c o r d i n g t o t h e i r r e q u i r e m e n t s w i t h o u t t h e nec e s s a r y back-up by t h e m i n i s t r i e s , and even n o t e v e r y t h i n g has t o go n e c e s s a r i l y t h r o u g h t h e f o r e i g n t r a d e o f f i c e s which h i t h e r t o were almost e x c l u s i v e l y r e s p o n s i b l e f o r c o n d u c t i o n o f n e g o t i a t i o n s and c o n c l u s i o n o f c o n t r a c t s f o r i m p o r t and e x p o r t (OILMAN 1987 b ) . The economical r e s t r u c t u r i n g r e q u i r i n g i n d i v i d u a l e n t e r p r i s e s t o be s e l f - f i nancing i s expected t o be a s u i t a b l e motor f o r energy c o n s e r v a t i o n and r e s e r v e m a x i m i z a t i o n i n o r d e r t o a v o i d c l o s u r e o f companies f a i l i n g t o show p r o f i t a b l e o p e r a t i o n (GORST 1988). The main g u i d e l i n e s f o r economical r e s t r u c t u r i n g and adm i n i s t r a t i v e r e o r g a n i z a t i o n a r e i n s t i t u t i o n o f c o s t a c c o u n t i n g as w e l l as p r o f i t and l o s s c r i t e r i a i n d e t e r m i n i n g e f f i c i e n c y o f d i f f e r e n t i n d u s t r y branches (OIL GAS JOURNAL 1987 0 ) . T h i s new s t r u c t u r e c h a r a c t e r i z e d by t r a n s p a r e n c y ( g l a s n o s t ) g i v e s much more f l e x i b i l i t y f o r problem a n a l y s i s and t r e a t m e n t cont r a c t c o n c l u s i o n and t h e r e f o r e i s one o f t h e ways t h a t w i l l v e r y much a l l o w t o expand t h e volume o f w e l l s t i m u l a t i o n i n t h e USSR. D i r e c t n e g o t i a t i o n s w i t h West e r n s u p p l y and s e r v i c e companies as w e l l as improved cash f l o w based on own budgets a r e s i g n i f i c a n t elements o f a c c e l e r a t i o n ( u s k o r e n i e ) o f t h e procedures which a l l o w much f a s t e r business c o n c l u s i o n than h i t h e r t o ( O I L GAS JOURNAL 1987 P).
2.4.4.5.4.2. Mutua 1 benefit for i nternat iona 1 cooperat ion Temporally matching w i t h t h e l i f t i n g o f t h e former USA s a n c t i o n s a g a i n s t the USSR o i l and gas i n d u s t r y , t h e p r e s e n t r e o r g a n i z a t i o n ( p e r e s t r o i k a ) does n o t o n l y a f f e c t p o l i t i c a l r e l a t i o n s h i p s , b u t a l s o t h e economical p a t t e r n by s i m p l i f y i n g e a r l i e r i n t e r w o v e n r e l a t i o n s h i p s , s h o r t - c i r c u i t i n g o f procedures, and gen e r a t i o n o f d i r e c t communication and a p p r o v a l . The r e s u l t s o f t h e governmental r e b u i l d i n g o f t h e s t a t e s t r u c t u r e f o r t h e hydrocarbon i n d u s t r y a r e mutual bene-
214
f i t f o r i n t e r n a t i o n a l c o o p e r a t i o n and j o i n t e f f o r t s t o m a s t e r a s i t u a t i o n w h i c h i s c r i t i c a l f o r e i t h e r s i d e and where o n l y combined a c t i o n s w i t h t h e a i m o f a c h i e v i n g t h e optimum g o a l o f t h e o b j e c t i v e s c a n g i v e r i s e t o t h e e x p e c t e d s u c c e s s and o v e r a l l improvement o f t h e i n d i g e n e o u s h y d r o c a r b o n s u p p l y s i t u a t i o n as w e l l as i n t e r n a t i o n a l c a s h f l o w and h e a l t h y n e s s o f s e r v i c e and s u p p l y i n d u s t r y (some a s p e c t s o f t h e e c o n o m i c a l r e s t r u c t u r i n g i n t h e USSR a r e a l s o d i s c u s s e d i n O I L GAS JOURNAL 1987 p ) .
2.4.4.5.5.
J o i n t ventures
J o i n t v e n t u r e s a r e o f p a r t i c u l a r i n t e r e s t f o r t h e USSR f o r p r o s p e c t i o n and a c q u i s i t i o n o f h y d r o c a r b o n r e s e r v e s i n o f f s h o r e a r e a s . Comments a r e o f f e r e d as f o l l o w s on B a l t i c Sea and o t h e r USSR o f f s h o r e a r e a s .
2.4.4.5.5.1.
B a l t i c Sea
The s p e c i a l s i t u a t i o n o f t h e USSR i s a l s o v a l i d f o r t h e j o i n t v e n t u r e e x p l o r a t i o n and e x p l o i t a t i o n o f t h e B a l t i c Sea b y USSR, P o l a n d and Germany GDR ( O I L GAS JOURNAL 1987 b ) , w h i c h i s t h e o n l y c a s e w i t h i n w h o l e E a s t e r n E u r o p e where even c o o p e r a t i o n w i t h and c o o r d i n a t i o n b y a f o r e i g n t r a d e o f f i c e i s n o t n e c e s s a r i l y included i n t o the procedure o f p r a c t i c a l l y almost o n l y d i r e c t n e g o t i a t i o n s between t h e j o i n t v e n t u r e h y d r o c a r b o n p r o d u c t i o n company and W e s t e r n s u p p l y and s e r v i c e companies. 25 w e l l s have been d r i l l e d i n t h e B a l t i c Sea s i n c e 1 9 8 1 and s e v e r a l p r o m i s i n g h y d r o c a r b o n a c c u m u l a t i o n s have been f o u n d (FUEG 1 9 8 8 ) . C o n c l u s i o n c a n t h u s be made t h a t more o r l e s s a l l t h e E a s t e r n European c o u n t r i e s have p o s s i b i l i t i e s o f f i n a n c i n g h y d r a u l i c p r o p p a n t f r a c t u r i n g done w i t h W e s t e r n mater i a l , e q u i p m e n t and s e r v i c e , and t h e o n l y t a s k i s t o a p p r o a c h t h e s e m a r k e t s w i t h a sound s t r a t e g i c a l c o n c e p t and s u f f i c i e n t p a t i e n c e f o r t h e l o n g n e c e s s a r y p r e p a r a t i o n o f b u s i n e s s c o n c l u s i o n and j o b e x e c u t i o n , and w i t h t h e m e n t a l i t y o f u n r e s t r i c t e d c o o p e r a t i o n f o r m u t u a l b e n e f i t w i t h o u t any p r e j u d g e m e n t s . Regardless o f t h e i r d i f f e r e n t hard currency p o s s i b i l i t i e s , a l l t h e Eastern European companies a r e a l w a y s l o o k i n g i n t o t h e m a t t e r o f c o u n t e r o r b a r t e r t r a d e t o enhance t h e i r f i n a n c i a l power b y s a v i n g c o n v e r t i b l e money f o r o t h e r p u r p o s e s . As many E a s t e r n European companies c a n d e l i v e r v a r i o u s raw m a t e r i a l s and goods v e r y cheap due t o low-expense l a b o u r , exchange t r a d e w i t h s u i t a b l e mat e r i a l s i s c e r t a i n l y a l s o o f i n t e r e s t t o Western s u p p l i e r s , p a r t i c u l a r l y because an agreement on c o u n t e r o r b a r t e r t r a d e i s o f t e n t h e f a s t e s t way o f b u s i ness c o n c l u s i o n i n Eastern Europe.
2.4.4.5.5.2.
Other USSR o f f s h o r e areas
The e c o n o m i c a l and p o l i t i c a l r e o r g a n i z a t i o n a l s o c r e a t e d some new b u s i n e s s p o s s i b i l i t i e s f o r f o r e i g n companies i n t h e USSR b y e n t e r i n g j o i n v e n t u r e s where f o r e i g n p a r t i c i p a n t s c a n h o l d u p t o 49 % s h a r e and have t o r e p a t r i a t e p r o f i t (FUEG 1 9 8 8 ) . Such j o i n t v e n t u r e s have t h e i r own i n d e p e n d e n t b u d g e t , t h e i r s e l f financing, and a r e n o t s u b j e c t t o t h e n a t i o n a l p l a n (RIGASSI 1 9 8 7 ) . The j o i n t v e n t u r e a l s o has p r i o r i t y i n USSR a l l o c a t i o n o f f i n a n c i a l and c o n s t r u c t i o n r e s o u r c e s . S e v e r a l Western European and A m e r i c a n companies a r e a l r e a d y i n advanc e d n e g o t i a t i o n s t a g e w i t h S o v i e t m i n i s t r i e s and f i r m s t o b r i n g such j o i n t vent u r e s i n t o l i f e . A j o i n t v e n t u r e c o u l d a l s o emerge as s o l u t i o n of t h e demarcat i o n d i s p u t e between USSR and Norway c o n c e r n i n g t h e B a r e n t s Sea (RIGASSI 1 9 8 6 ) . The s p e c i a l i n t e r e s t o f t h e USSR i s c o n c l u d i n g j o i n t v e n t u r e s f o r e x p l o r a t i o n and d e v e l o p m e n t o f r e m o t e o f f s h o r e a r e a s (RIGASSI 1 9 8 7 ) . The j o i n t v e n t u r e s gen e r a l l y s h o u l d have t h e a b i l i t y t o e a r n h a r d c u r r e n c y t h r o u g h e x p o r t s (OIL GAS JOURNAL 1987 p ) .
215
2.4.4.6. Reservoir engineering and hydrocarbon supply aspects D i f f e r e n t n e c e s s i t i e s and r e q u i r e m e n t s e x i s t f o r gas and o i l s u p p l y i n t h e as a consequence o f r e s e r v o i r c h a r a c t e r i s t i c s and r e s e r v e a v a i l a b i l i t i e s . While gas d e l i v e r y i n l a r g e amounts exceeding t h e a c t u a l demand i s no problem a t t h e moment and i n t h e f u t u r e , c o n s i d e r a b l e improvement o f o i l s u p p l y i s ess e n t i a l t o enable s a t i s f a c t o r y i n d u s t r i a l and urban d e l i v e r y as w e l l as t o f u l f i l l the goals o f the five-year plan.
USSR
2.4.4.6.1. Gas supply The reason why MHF o p e r a t i o n s on gas r e s e r v o i r s have so f a r n o t been c a r r i e d o u t i n t h e USSR, and a l s o s m a l l e r - s c a l e f r a c t u r i n g t r e a t m e n t s were f r e q u e n t l y performed i n i n d i v i d u a l w e l l s upon s p e c i a l occasions and n o t i n l a r g e r planned campaigns a f f e c t i n g whole f i e l d s o r even groups o f patches, i s t h e presence o f huge amounts o f gas i n moderate- t o h i g h - p e r m e a b i l i t y sandstone r e s e r v o i r s p a r t i c u l a r l y i n S i b e r i a where s t i m u l a t i o n i s n o t necessary, because s i g n i f i c a n t i n creases o f t h e a l r e a d y p r i m a r i l y h i g h p r o d u c t i o n r a t e s cannot be achieved. I n c o n t r a s t t o r e s e r v o i r t r e a t m e n t s w h i c h a r e t h e dominant c h a l l e n g e s i n many West e r n European gas p r o v i n c e s , s u r f a c e measures caused by p e r m a f r o s t - r e l a t e d p r o blems a r e t h e m a j o r t a s k i n t h e S i b e r i a n gas d i s t r i c t . The e a s i l y a c c e s s i b l e r e serves i n t h e S i b e r i a n g i a n t gas f i e l d s which can be produced cheaply a r e t h e main base f o r t h e l a r g e gas q u a n t i t i e s which t h e USSR e x p o r t s t o Western Europe f o r t h e sake o f r e c e i v i n g h a r d c u r r e n c y ( c f . s e c t i o n 2 . 2 . 1 . 6 . 3 . ) , and t h e p o t e n t i a l f o r s a t i s f y i n g b o t h f o r e i g n and domestic demand seems t o be almost u n l i m i t e d a t l e a s t d u r i n g t h e n e x t decades, e s p e c i a l l y a f t e r a l o n g l a r g e - d i a m e t e r p i p e l i n e has been b u i l t and i s a l r e a d y under s a f e o p e r a t i o n w i t h o u t any d i s t u r b a n ces s i n c e s e v e r a l y e a r s . T h e r e f o r e i n c o n t r a s t t o M i d d l e Europe t h e r e i s a t t h e moment and p r o b a b l y a l s o i n t h e n e x t few y e a r s no need f o r abundant MHF s t i m u l a t i o n s on gas r e s e r v o i r s i n t h e USSR, because t h e enormous amounts o f gas which can be r e c o v e r e d w i t h o u t treatment are s u f f i c i e n t t o f u l f i l l the i n t e r n a t i o n a l d e l i v e r y o b l i g a t i o n s a l l o w i n g even s t i l l e x t e n s i o n s o f t h e commitments i n case t h e Western European market i s f u r t h e r growing and demanding and t o secure indigeneous s u p p l y w i t h a g a i n s u f f i c i e n t f r e e c a p a c i t y t o meet p o t e n t i a l l y r i s i n g r e q u i r e m e n t by domestic i n d u s t r i a l enlargement. Given a comparable s i t u a t i o n o f t h e N o r t h Sea R o t l i e g e n d t i g h t gas f i e l d b e l t i n t h e USSR, t h e r e i s no doubt t h a t i n such a case a l s o s i m i l a r l a r g e - s c a l e h y d r a u l i c f r a c t u r i n g would t a k e p l a c e t h e r e , because t h e t e c h n o l o g i c a l u n d e r s t a n d i n g i s w e l l developed and no problems e x i s t c o n c e r n i n g f i n a n c i a l c a p a b i l i t y f o r purchases o f equipment, m a t e r i a l s and/or s e r v i c e i n t h e w o r l d market.
2.4.4.6.2. Oil supply O i l e x p l o i t a t i o n , however, needs i n c o n t r a s t t o gas p r o d u c t i o n much i n t e n s i f i c a t i o n p a r t i c u l a r l y i n t h e coming y e a r s (EBEL 1986, ORASIANU 1987), because a l s o t h e USSR was h i t by t h e 1986 o i l p r i c e c r a s h ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) and t h e r e c e n t US $ exchange r a t e c o l l a p s e ( c f . s e c t i o n 2.2.1.3.) and t h u s has t o compensate p r o f i t l o s s e s f r o m s a l e o f o i l on t h e w o r l d market which were n o t foreseen and p l a n n e d and t h u s t h e e q u i v a l e n t s o f money and m a t e r i a l a r e l a c k i n g i n t h e budgets ( c f . s e c t i o n 2 . 4 . 4 . 1 . 5 . ) . While most o f t h e h i t h e r t o performed h y d r a u l i c p r o p p a n t f r a c t u r i n g i n t h e USSR a l r e a d y focussed on o i l p r o d u c t i o n and o i l - f i e l d w a t e r i n j e c t i o n w e l l s and comprised t o m i n o r amounts a l s o unminea b l e c o a l seam d e g a s i f i c a t i o n b o r e h o l e s , s t i l l c o n s i d e r a b l y more o i l r e s e r v o i r s have t o be t r e a t e d i n t h e near f u t u r e f o r t h e purpose o f m e e t i n g t h e g o a l s s e t by t h e new f i v e - y e a r p l a n i n o r d e r t o secure domestic supply, d e l i v e r y t o s a t e l l i t e c o u n t r i e s by b a r t e r t r a d e , and s a l e i n t h e w o r l d market f o r h a r d c u r r e n c y a t a s u f f i c i e n t l y high level.
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2.4.5. Western Europe 1987 - 1990 A f t e r t h e slow r e a d j u s t m e n t and r e s t a b i l i z a t i o n o f t h e o i l p r i c e l e v e l f o l l o w i n g t h e m a j o r d e c l i n e i n e a r l y 1986, w i t h , however, almost no chance b e i n g seen t h a t i t would a g a i n r e a c h t h e n i v e a u o f t h e g o l d e n l a t e 1970‘s and p a r t i c u l a r l y e a r l y 1980‘s, h y d r a u l i c p r o p p a n t f r a c t u r i n g i n klestern Europe has a l r e a d y i n t h e f i r s t h a l f o f 1987 more o r l e s s r e a s o n a b l y resumed ( t h i s b e i n g a t i m e l a g t o t h e USA where business s t a r t e d t o s l o w l y p i c k up a g a i n a l r e a d y i n t h e l a s t t h i r d o f 1986) and good e x p e c t a t i o n s e x i s t f o r p r e d o m i n a n t l y 1988 and 1989. I n l a t e r 1987, t h e d e c l i n e i n worldwide e x p l o r a t i o n d r i l l i n g f i n a l l y passed i t s n a d i r which was reached i n m i d 1986 and was more o r l e s s k e p t u n t i l e a r l y t o m i d 1987 and t h a t marked t h e c u l m i n a t i o n o f a p r o g r e s s i v e s l i d e - d o w n f r o m t h e z e n i t h i n 1981/1982 (POPESCU & HODGSHON 1987). The r e a d a p t a t i o n o f t h e o i l p r i c e t o a h i g h e r l e v e l than d u r i n g t h e peak o f t h e 1986 c r i s i s (VARZI 1986), however, i s i n Europe a t t h e b o t t o m o f t h e l i n e r e t a r d e d due t o t h e s t i l l low US $ exchange r a t e w h i c h k e p t f l u c t u a t i n g around i t s p r e l i m i n a r y r e l a t i v e minimum t h r o u g h o u t m i d t o l a t e 1986 and even reached i n t h e f o u r t h q u a r t e r o f 1987 i t s h i s t o r i c a l a b s o l u t e minimum l e v e l ( w i t h i n s p i t e o f t h e temporary s l i g h t t o moderate i m p r o v a l o f t h e US $ s t r e n g t h i n m i d t o l a t e 1988 even now no p e r s p e c t i v e s b e i n g seen f o r a renewed r e a s o n a b l e i n crease o f t h e US $ v a l u e w i t h i n t h e p r e d i c t a b l e near f u t u r e e s p e c i a l l y i n view o f t h e renewed downwards p r e s s u r e on t h e US $ v a l u e a t t h e end o f 1988; c f . sect i o n 2 . 2 . 1 . 3 . ) . A g e n e r a l f o r e c a s t o f o i l p r i c e e v o l u t i o n and US $ r a t e development r e p r e s e n t i n g t h e b a s i c framework o f hydrocarbon e x p l o r a t i o n and e x p l o i t a t i o n i s g i v e n by WEFA (1988), and MACKAY CONSULTANTS (1988) p r o v i d e a p r e d i c t i o n o f t h e o i l and gas i n d u s t r y t r e n d s . I n Western Europe, an i n c r e a s i n g number o f gas f i e l d s i s developed o r p l a n ned t o be developed i n t h e n e x t y e a r s i n which h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n i s e s s e n t i a l t o o b t a i n economical p r o d u c t i o n r a t e s , and i n many cases, f e a s i b l e development r e q u i r e s w e l l p r o d u c t i o n m a x i m i z a t i o n by e f f e c t i v e f r a c t u r e t r e a t m e n t s w i t h optimum p r o p p a n t performance (OFFSHORE ENGINEER 1985 c , 1986 c; ROODHART, KUIPER & O A V I E S 1986). The most i m p o r t a n t p r o j e c t s i n t h e B r i t i s h and Dutch Southern N o r t h Sea as w e l l as o t h e r areas a r e b r i e f l y o u t l i n e d as f o l l o w s . A s h o r t assessment o f t h e s i g n i f i c a n c e o f h y d r a u l i c f r a c t u r i n g f o r i n c r e a s i n g proven gas r e s e r v e s i s a l s o given, and some comments a r e o f f e r e d on t h e impact o f p r o p p a n t m a r k e t i n g s t r a t e g y on t h e s t i m u l a t i o n scenery.
2.4.5.1. B r i t i s h Southern North Sea The o u t s t a n d i n g s i g n i f i c a n c e o f t h e B r i t i s h Southern N o r t h Sea t i g h t gas r e serves and t h e i r a c q u i s i t i o n i n t h e n e x t y e a r s a r e i l l u s t r a t e d by d i s c u s s i n g as f o l l o w s r o l e o f t h e m a j o r s t i m u l a t i o n campaigns i n t h e European gas development scene, c a n c e l l a t i o n r i s k o f s t i m u l a t i o n j o b s , s e l f - s u f f i c i e n c y o f t h e U n i t e d Kingdom gas i n d u s t r y , and s i g n i f i c a n c e o f gas development a c t i v i t y .
2.4.5.1.1. Role o f major s t i m u l a t i o n campaigns i n the European gas development scene The main reason f o r t h e coming b i g h y d r a u l i c proppant f r a c t u r i n g campaigns p r e f e r e n t i a l l y i n t h e B r i t i s h Southern N o r t h Sea a r e d e f i n i t e l y planned f i e l d developments i n t h e l o w - p e r m e a b i l i t y zone o f t h e R o t l i e g e n d gas b e l t (ALLISON 1986; GREGORY 1986, 1987) which a r e r e i n f o r c e d by t h e f a c t t h a t t h e gas has a l ready been s o l d by means o f commitment o f s u p p l y c o n t r a c t s ( c f . s e c t i o n s 2 . 2 . 1 . 6 . 2 . and 2 . 2 . 1 . 6 . 3 . 2 . ) and i s one o f t h e m a j o r p r i m a r y energy r e s o u r c e s d u r i n g t h e n e x t decades even beyond t h e y e a r 2000 (LUBBEN 1982, LINDEN 1985, SCHINDEWOLF 1985, STEINMANN 1985, FEHRINGER 1986, BAND 1987, OILMAN 1987 a ) , and domestic a c t i v i t y i s a l s o s t r o n g l y backed-up by t h e government w h i c h wants
217 t o keep i m p o r t s o f f o r e i g n gas n o t b e i n g taken i n t o c o n s i d e r a t i o n f o r a t l e a s t t h e near f u t u r e . The R o t l i e g e n d development campaigns i n t h e B r i t i s h Southern N o r t h Sea i n 1987 - 1990 and even l a t e r (GREGORY 1986, 1987; OFFSHORE ENGINEER 1986 a, THOMAS 1986, FOX 1987, POTTER 1987, QUINLAN 1988; c f . s e c t i o n 3.3.) a r e p r o b a b l y t h e o n l y s u i t e o f j o b s and/or p o t e n t i a l area where s i m i l a r l y as b e i n g a l r e a d y common sense i n t h e USA, h y d r a u l i c p r o p p a n t f r a c t u r i n g has become one o f t h e most i m p o r t a n t aspects o f w e l l c o m p l e t i o n (ROBINSON, HOLDITCH & WHITEHEAD 1986; c f . s e c t i o n 2 . 3 . 4 . ) . A p a r t f r o m some patches o f t h e Norwegian N o r t h e r n N o r t h Sea where o i l - b e a r i n g Upper Cretaceous c h a l k i n d i s p e n s i b l y r e q u i r e s p r o p p a n t f r a c t u r i n g , R o t l i e gend gas development i n t h e B r i t i s h Southern N o r t h Sea i s t h e m a j o r share o f t h e European h y d r a u l i c s t i m u l a t i o n a c t i v i t y i n e a r l y 1987 and w i l l p r o b a b l y more o r l e s s keep t h i s r o l e t h r o u g h o u t t h e n e x t y e a r s ( c f . a l s o s e c t i o n 2 . 4 . 1 . 2 . ) . The R o t l i e g e n d campaign i s t h e l a r g e s t gas development p r o j e c t i n t h e N o r t h Sea i n t h e l a s t t e n y e a r s (McNALLY 1987), and t h e planned p u t t i n g on stream o f t h e f i r s t group o f f i e l d s c o m p r i s i n g Vulcan, V a l i a n t , Vanguard, Ravenspurn South and C l e e t o n i n l a t e 1988 has been achieved a c c o r d i n g t o schedule The i n a u g u r a t i o n o f t h i s (PETROLEUM R E V I E W 1988 d; cf,. s e c t i o n 2.2.1.6.9.). f i e l d complex i n autumn 1988 matches w i t h a b t . 50 r i g s d r i l l i n g i n t h e B r i t i s h Southern N o r t h Sea which r e p r e s e n t s t h e h i g h e s t l e v e l o f a c t i v i t y s i n c e s e v e r a l y e a r s (PETROLEUM R E V I E W 1988 d) and a g a i n u n d e r l i n e s t h e s i g n i f i c a n c e o f t h e R o t l i e g e n d b e l t f o r gas s u p p l y o f t h e U n i t e d Kingdom.
2.4.5.1.2. Cancellation risk of stimulation jobs Another reason f o r an o p t i m i s t i c e x p e c t a t i o n of t h e h y d r a u l i c f r a c t u r i n g scen e r y i s r e s u l t i n g f r o m t h e l a r g e development p l a n s and i n c l u d e s t h e s h i f t f r o m m a i n l y e x p l o r a t i o n d r i l l i n g i n 1986 t o p r e d o m i n a n t l y a p p r a i s a l and development d r i l l i n g i n 1987 t h r o u g h 1990 and beyond. E x p l o r a t i o n d r i l l i n g i s o f t e n l i n k e d w i t h c o n s i d e r a b l e g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g r i s k as c o n f i r m e d by t h e h i g h c a n c e l l a t i o n r a t e s ( h a v i n g been above t h e e x p e c t a t i o n l e v e l j u s t a t t h e peak o f t h e c r i s i s , t h e r e b y a c c e n t u a t i n g i t i n a Murphy-law-type manner; c f . s e c t i o n 2.4.2.2.). On t h e o t h e r hand, a p p r a i s a l and p r o d u c t i o n d r i l l i n g w h i c h i s s t a r t i n g i n t h e second h a l f o f 1987 and l a s t i n g u n t i l 1989 o r even l o n g e r has much l o w e r r i s k , because t h e s t r u c t u r e s and t h e i r c o n t e n t s a r e a l r e a d y more o r l e s s known, w i t h much lower c a n c e l l a t i o n r a t e s t h u s b e i n g a b l e t o be expect e d ( b u t n o t guaranteed due t o always p r e s e n t g e o l o g i c a l u n c e r t a i n t i e s as a consequence o f n a t u r a l v a r i a b i l i t y even beyond s o p h i s t i c a t e d human and c o m p u t e r i zed m o d e l l i n g and i n t e r p r e t a t i o n ) . From a p p r a i s a l and e a r l y development d r i l l i n g , e x p e c t a t i o n can be made w i t h s u f f i c i e n t c o n f i d e n c e t h a t most o f t h e i f i n some f i e l d s n o t a l l o f them, w i l l have t o be s t i m u l a t e d by hydrauwells, l i c p r o p p a n t f r a c t u r i n g i n o r d e r t o e s t a b l i s h e c o n o m i c a l l y f e a s i b l e gas f l o w r a t e s f r o m t h e t i g h t sandstones (OILMAN 1987 a; c f . s e c t i o n s 3.4.2. and 3 . 8 . ) . The c e n t r e s o f MHF s t i m u l a t i o n o f t i g h t R o t l i e g e n d gas r e s e r v o i r s a r e I n d e f a t i g a b l e , Ravenspurn, Rough, Thames, V a l i a n t , Vanguard and Vulcan ( c f . f i g . 6 and t a b . 7 ) . An o u t l i n e o f t h e p e t r o l e u m e x p l o r a t i o n , development and s t i m u l a t i o n p o t e n t i a l of t h e N o r t h Sea i s a l s o g i v e n by HOYLE (1983), and NYSTAD (1981) p r o v i d e s an economical a n a l y s i s o f t h e N o r t h Sea o i l and gas r e g i o n .
2.4.5.1.3. Self-sufficiency of United Kingdom gas industry The s i g n i f i c a n c e o f t h e B r i t i s h Southern N o r t h Sea i s h i g h l i g h t e d by t h e has f a c t s t h a t i t i s t h e t r a d i t i o n a l c e n t r e o f U n i t e d Kingdom gas p r o d u c t i o n , h i s t o r i c a l l y c o n t r i b u t e d up t o 90 % o f G r e a t B r i t a i n gas d e l i v e r y , s t i l l h o l d s o v e r 50 % of t h e t o t a l B r i t i s h proven and p r o b a b l e gas r e s e r v e s and needs e x t e n s i o n o f d e l i v e r y c a p a c i t y i n o r d e r t o be a b l e t o meet w i n t e r r e q u i r e m e n t s o f d a i l y up t o 10 B i l l . f t 3 ( c f . s e c t i o n 2 . 2 . 1 . 6 . 3 . ) . The c i r c u m s t a n c e t h a t B r i t i s h Gas as t h e n a t i o n a l d i s t r i b u t o r i s needing s i g n i f i c a n t new s u p p l i e s o f gas
218 i n t h e 1990's i s t h e m a j o r c h a l l e n g e f o r t h e o p e r a t i n g companies t o d e v e l o p R o t l i e g e n d f i e l d s i n t h e B r i t i s h S o u t h e r n N o r t h Sea ( c f . s e c t i o n 2 . 2 . 1 . 6 . 2 . ) , because t h e y have an i m p o r t a n t a d v a n t a g e o v e r t h e N o r w e g i a n c o m p e t i t i o n i n t h a t t h e U n i t e d Kingdom t a x s y s t e m i s f a r more f a v o u r a b l e t h a n t h e h a r s h N o r w e g i a n t a x r e g i m e , and t h a t d e v e l o p m e n t and p r o d u c t i o n c o s t s a r e i n t h e S o u t h e r n N o r t h Sea c o n s i d e r a b l y c h e a p e r t h a n i n t h e N o r t h e r n N o r t h Sea. I n c l u d i n g abundant l a r g e - s c a l e h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n , s u f f i c i e n t amounts of gas c a n be l i b e r a t e d f r o m t h e R o t l i e g e n d f i e l d b e l t t o meet t h e demand o f b o t h B r i t i s h Gas and d i r e c t i n d u s t r i a l u s e r s (PETROLEUM R E V I E W 1988 d ) . By e n d o f 1987, t h e S o u t h e r n N o r t h Sea R o t l i e g e n d f i e l d b e l t had c u m u l a t i v e l y p r o d u c e d a b t . 8 0 T c f (2,000 B i l l . m3) o f gas s i n c e t h e b e g i n n i n g o f o f f t a k e i n 1967. Average d a i l y w i t h d r a w a l and number o f f i e l d s b e i n g d e v e l o p e d i n t h e B r i t i s h s e c t o r i s t w i c e as h i g h as i n t h e D u t c h s e c t o r . On t h e o t h e r hand, t h e v a s t G r o n i n g e n f i e l d o n s h o r e N e t h e r l a n d s p r o d u c e s a l o n e a l m o s t as much as t h e e n t i r e U n i t e d Kingdom c o n t i n e n t a l s h e l f . A d d i t i o n a l i n s p i r a t i o n and f e e d - b a c k a r e t h e improvement o f t h e p r o s p e c t s o f o b t a i n i n g bank f i n a n c e f o r gas developments, as a t l e a s t some banks w a n t t o i n c r e a s e l o a n s t h a t a r e backed b y gas w i t h i t s more s t a b l e p r i c e t h a n b y t h e u n r e l i a b l e o i l (FISHMAN 1986) where much o f t h e e a r l i e r i n v e s t m e n t s a n d c r e d i t s have been d e v a l i d a t e d ( c f . s e c t i o n 2 . 2 . 1 . 5 . 4 . ) . A l t h o u g h t h e a r e a i s m a t u r e and p r o d u c t i o n from t h e major f i e l d s i s i n d e c l i n e , t h e r e i s s t i l l a s u i t e o f small e r a c c u m u l a t i o n s w h i c h c a n be t i e d i n t o t h e e x i s t i n g i n f r a s t r u c t u r e t o m e e t and t o compensate t h e p r o j e c t e d U n i t e d Kingdom gas s u p p l y s h o r t f a l l i n t h e n e x t decade (GREGORY 1 9 8 6 ) .
2.4.5.1.4.Significance o f gas development activity The B r i t i s h S o u t h e r n N o r t h Sea i s e x p e c t e d t o be t h e m o s t a c t i v e European a r e a f o r gas f i e l d d e v e l o p m e n t d u r i n g t h e n e x t f e w y e a r s (KNOTT 1 9 8 6 ) , p a r t i c u l a r l y due t o s o f a r c a n c e l l a t i o n o f t h e i d e a of c o n t r a c t i n g gas f r o m new Norweg i a n f i e l d s such as S l e i p n e r and T r o l l i n o r d e r t o compensate t h e d e c l i n i n g del i v e r y f r o m F r i g g ( w h i c h i s c r o s s e d b y t h e b o u n d a r y between U n i t e d Kingdom and N o r w e g i a n N o r t h Sea s e c t o r s ) and d e c i s i o n t o r e p l a c e t h e q u a n t i t i e s b y i n c r e a s i n g o f f t a k e f r o m t i g h t R o t l i e g e n d gas f i e l d s ( c f . s e c t i o n 2 . 2 . 1 . 6 . 2 . ) . The f i e l d d e v e l o p m e n t campaigns n e c e s s a r y t o d e l i v e r t h e gas q u a n t i t i e s t h a t have t o r e p l a c e t h e d e c l i n i n g and l a t e r c e a s i n g s u p p l y f r o m F r i g g and t o s a t i s f y t h e i n c r e a s e d demand have t h e d e a d l i n e t o f e e d f i r s t g a s volumes i n t o t h e p i p e l i n e n e t w o r k o f B r i t i s h Gas b y autumn 1988 ( w h i c h has i n t h e meantime been s u c c e s s f u l l y a c h i e v e d ) and s u b s e q u e n t l y p r o g r e s s i v e l y t h r o u g h 1990 and beyond (OILMAN 1987 a, CAPEL 1988, VIELVOYE 1988 a ) . The c o n s i d e r a b l e d r i l l i n g , c o m p l e t i o n and s t i m u l a t i o n campaigns t a k i n g p l a c e s i n c e m i d 1987 r e p r e s e n t t h e m o s t p r o n o u n c e d a c t i v i t y i n t h e B r i t i s h S o u t h e r n N o r t h Sea s i n c e t h e i n i t i a l boom t h a t f o l l o w e d gas d i s c o v e r y i n t h e e a r l y 1960's (VIELVOYE 1988 a ) and mark a m a j o r m i l e s t o n e a l m o s t m a t c h i n g w i t h t h e 2 5 y e a r j u b i l e e o f d e v e l o p m e n t h i s t o r y of gas f i e l d s i n t h e B r i t i s h S o u t h e r r i N o r t h Sea (OILMAN 1987 a ) . A s u i t e o f d e v e l o p m e n t p r o p o s a l s have a l r e a d y beer1 ;::prov e d b y t h e government i n t i m e t o f i t i n t o t h e gas d e l i v e r y s c h e d u l e , m o s t o f t h e gas has been s o l d t o B r i t i s h Gas w h i c h i s s t i l l a f t e r i t s p r i v a t i z a t i o n t h e m o n o p o l i s t i c gas b u y e r i n t h e U n i t e d Kingdom ( c f . s e c t i o n 2 . 2 . 1 . 6 . 2 . ) , and ano t h e r g a l a x y o f f i e l d s i s w a i t i n g on a c c e p t a n c e o f d e v e l o p m e n t p l a n s and c o n t r a c t i o n o f gas (OILMAN 1987 a ) . A l t h o u g h G r e a t B r i t a i n i s a r e l a t i v e l y young h y d r o c a r b o n p r o d u c e r and s u p p l i e r because N o r t h Sea e x p l o r a t i o n and e x p l o i t a t i o n s t a r t e d o n l y some 20 y e a r s ago, i t i s a l r e a d y s e l f - s u f f i c i e n t i n o i l and gas s u p p l y s i n c e many y e a r s , and t h e l a r g e - s c a l e a c q u i s i t i o n o f f u r t h e r g a s r e serves i n c l u d i n g s i g n i f i c a n t c o n t r i b u t i o n s o f hydraulic proppant f r a c t u r i n g s t i m u l a t i o n o f t i g h t R o t l i e g e n d gas h e l p s t o s e c u r e t h e U n i t e d Kingdom s e l f - s u f f i c i e n c y o f gas a t l e a s t u n t i l t h e y e a r 2000 (KNOTT 1985, 1 9 8 6 ) .
219
2.4.5.2.
Dutch Southern North Sea
I n comparison t o t h e B r i t i s h Southern N o r t h Sea, t h e gas p r o s p e c t i v i t y o f t h e R o t l i e g e n d i n t h e D u t c h N o r t h Sea i s s l i g h t l y t o c o n s i d e r a b l y l o w e r and thus a l s o t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l i s n o t so pronounced. Reasonable a c t i v i t y , however, i s a l s o g o i n g t o t a k e p l a c e i n t h e Dutch Southern N o r t h Sea as a consequence o f s h i f t i n g p r o j e c t s f r o m t h e o i l s p o t s t o t h e gas patches f o l l o w i n g t h e 1986 o i l p r i c e crash, w i t h t h e main a t t r a c t i v i t y g i v e n t o i n v e s t m e n t and p r o f i t l e v e l s by t h e s h a l l o w e r w a t e r d e p t h o f t h e Dutch s e c t o r and t h e g e n e r a l p r o x i m i t y t o e x i s t i n g p i p e l i n e s and p r o c e s s i n g f a c i l i t i e s , comb i n e d w i t h a reasonable t a x regime (BEUOELL 1986 b ) . R o t l i e g e n d h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n i n t h e Dutch Southern N o r t h Sea has been a l r e a d y c a r r i e d o u t and/or i s s t i l l g o i n g t o be performed m a i n l y i n t h e f i e l d s K-12, L-8, L-10 and P-2, and B u n t s a n d s t e i n t r e a t m e n t s focussed i n t h e p a s t and/or c o n c e n t r a t e i n t h e f u t u r e on t h e f i e l d s P-6 and P-8 ( c f . f i g . 6 and tab. 8 ) . Some aspects o f R o t l i e g e n d gas f i e l d s i n t h e Dutch Southern N o r t h Sea a r e d i s c u s s e d by HAAK & ELEWAUT ( 1 9 8 8 ) . I n t h e German N o r t h Sea, so f a r no commercial gas o r o i l d i s c o v e r y has been made, and due t o t h e sequence o f e x p l o r a t i o n w e l l s i n c l u d i n g ( a t l e a s t i n view o f economical f e a s i b i l i t y i n terms o f t h e o f f s h o r e s i t u a t i o n ) so f a r o n l y f a i l u r e s because o f i n s u f f i c i e n t s t r u c t u r a l r e s e r v e s o r t o o h i g h n i t r o g e n c o n t e n t o f t h e gas ( c f . s e c t i o n 3.3.4.; w i t h t h e o n l y e x c e p t i o n b e i n g two l i t t l e i n s h o r e f i e l d s ) , o n l y v e r y l i t t l e gas p r o s p e c t i v i t y ( i f a t a l l ) remains i n t h i s area and c e r t a i n l y t h e r e i s a t t h e moment no f r a c t u r i n g p o t e n t i a l u n l e s s a f u t u r e exp l o r a t i o n w e l l has a more f a v o u r a b l e r e s u l t . C a r b o n i f e r o u s f r a c t u r i n g onshore as w e l l as o f f s h o r e i n t h e B r i t i s h and D u t c h Southern N o r t h Sea w i l l c e r t a i n l y be i n a b e t t e r p o s i t i o n i f gas p r o d u c t i o n w i t h h i g h w a t e r c u t (ARENS & BUHNEL 1985) can be p u t on a more economical base, t h e r e b y a l l o w i n g a l s o t o s t i m u l a t e C a r b o n i f e r o u s gas r e s e r v o i r s w i t h h i g h e r w a t e r s a t u r a t i o n i n t h e n e a r f u t u r e O i l and gas f i e l d development d e t a i l s f o r B r i t i s h , ( c f . section 2.4.1.2.2.1.). Dutch and Norwegian N o r t h Sea u n t i l t h e y e a r 2000 a r e g i v e n by OFFSHORE E N G I NEER (1986 c ) .
The coming decades a r e a l s o i n Europe c h a r a c t e r i z e d by t h e need and urgency f o r u n c o n v e n t i o n a l gas (KUUSKRAA 1980) w h i c h i s u n d e r l i n e d by t h e t i g h t R o t l i e gend f r a c t u r i n g campaigns. Economical aspects o f t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t and t h e development o f v a r i o u s m a j o r f i e l d s i n t h i s p r o v i n ce i n t h e n e a r f u t u r e a r e a l s o discussed by FISHMAN (1986), GREGORY (1986, 1987), THOMAS (1986), ALGAR (1987 a ) , BAND (1987) and POTTER (1987; c f . sect i o n s 2.2.1.6.2., 2.2.1.6.3.2. and 2 . 4 . 1 . 2 . ) . An overview o f developed and sched u l e d o i l and gas f i e l d s i n B r i t i s h and Norwegian N o r t h Sea i s g i v e n i n OFFSHORE ENGINEER (1985 C, 1986 C) and SCOTTISH PETROLEUM ANNUAL ( 1 9 8 7 ) .
2.4.5.3.
Other areas
A p a r t f r o m R o t l i e g e n d gas f i e l d development i n t h e B r i t i s h Southern N o r t h Sea t h a t w i l l i n terms o f proppant a p p l i c a t i o n be e i t h e r m a i n l y done w i t h 20/40 sand f o l l o w e d by a t a i l - i n o f 16/20 i n t e r m e d i a t e - s t r e n g t h proppants ( o f a b t . 10 - 30 % o f t h e sand q u a n t i t y ) i n case o f s h a l l o w e r r e s e r v o i r s ( a b t . 6,000 8,000 f t ) o r i n case o f deeper r e s e r v o i r s ( a b t . 8,000 - 10,000 f t ) e x c l u s i v e l y w i t h i n t e r m e d i a t e - s t r e n g t h proppants of f i r s t 20/40 ( 5 - 20 % o f t h e whole l o t ) and then second 16/20 m a t e r i a l , and which w i l l comprise by f a r t h e l a r g e s t amount o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n Europe i n t h e v e r y near f u t u r e , s t i m u l a t i o n a c t i v i t y i n ( m a i n l y t h e second h a l f o f ) 1987 and 1988 ( p r o b a b l y s t i l l i n c l u d i n g t h e f o l l o w i n g c o u p l e o f y e a r s ) w i l l a l s o f o c u s on ( c l a s s i f i e d inforrnat i o n i s c o m p i l e d i n t a b s . 6 - 9, and l o c a l i t i e s and r e g i o n s a r e i l l u s t r a t e d i n f i g s . 4 - 8)
* some R o t l i e g e n d and C a r b o n i f e r o u s sandstone gas w e l l s i n Germany FRG (BRINK-
220 MANN, FUHRBERG & SCHOBER 1980; BRINKMANN, KRUMER & R E I N I C K E 1980; BRINKMANN 1982, SCHWARZ & SCHUBER 1982; JOHN 1983, 1987; LEICHT 1985; REINICKE, B R I N K MANN, SCHWARZ & HUENI 1985; c f . f i g . 4 ) w h i c h w i l l be t r e a t e d w i t h i n t e r m e d i a t e - o r h i g h - s t r e n g t h h i g h - d e n s i t y p r o p p a n t s i n a r e s e r v o i r d e p t h between 3,000 and 6,000 m (9,000 - 18,000 f t ; c f . s e c t i o n 2 . 4 . 1 . 1 . ) ,
*
p a r t s o f some R o t l i e g e n d gas f i e l d s and Cretaceous sandstone o i l f i e l d s i n t h e D u t c h S o u t h e r n N o r t h Sea ( c f . f i g . 6 ) w h i c h w i l l r e q u i r e sand a n d / o r i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y c e r a m i c p r o p p a n t s i n a b t . 6,000 - 10,000 f t (2,000 - 3,300 in) r e s e r v o i r d e p t h ( c f . s e c t i o n 3 . 3 . 3 . ) ,
*
some Upper T r i a s s i c o i l - b e a r i n g sandstones i n Germany FRG t h a t r e q u i r e i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s i n a b t . 1,500 - 2,500 m (4,500 - 7,500 f t ) r e s e r v o i r d e p t h ( c f . s e c t i o n 2.4.1.1.2.2. and f i g . 5 ) ,
*
some Mesozoic and Cenozoic o i l - and g a s - b e a r i n g sandstones i n Y o r k s h i r e and M i d l a n d s onshore G r e a t B r i t a i n where sand o r i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s w i l l be a p p l i e d i n a r e s e r v o i r d e p t h o f 5,000 - 9,000 f t (1,500 - 3,000 m),
*
p a r t s o f some J u r a s s i c sandstone ( a n d l o c a l l y a l s o c o n g l o m e r a t e ) o i l f i e l d s i n t h e B r i t i s h and t o s u b o r d i n a t e amounts a l s o Norwegian N o r t h e r n N o r t h Sea where sand a n d / o r i n t e r m e d i a t e - t o h i g h - s t r e n g t h p r o p p a n t s w i l l be a p p l i e d ( B r e n t Sand group; c f . JOHNSON & KROL 1984; DESPAX, CHARLEZ, CLINKEMAILLE & ECONOMIDES 1987) i n a r e s e r v o i r d e p t h o f 10,000 - 14,000 f t (3,300 - 4,600 m; c f . f i g . 8),
*
some J u r a s s i c and T e r t i a r y o i l f i e l d s i n t h e B r i t i s h N o r t h e r n N o r t h Sea where i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p p a n t s a r e r e q u i r e d i n 9,000 - 11,000 f t (3,000 - 3,300 m ) r e s e r v o i r d e p t h ( c f . f i g . 8 ) ,
*
some C r e t a c e o u s c h a l k o i l f i e l d s i n t h e Norwegian N o r t h e r n N o r t h Sea (HARTLEY & BOSMA 1985; TANSOE, K I N G & HOLMAN 1986; ATTARD, MATHES & MOWER 1988; c f . section 4.5.4.3.) where c o n v e n t i o n a l and r e s i n - c o a t e d i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y c e r a m i c p r o p p a n t s a r e needed i n a c o m b i n a t i o n o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i n a b t . 2,500 - 3,300 m (8,000 - 10,000 f t ) r e s e r v o i r depth ( c f . f i g . 8 ) ,
*
some T e r t i a r y gas- and c o n d e n s a t e - b e a r i n g sandstones and c o n g l o m e r a t e s i n YuPFURTER, U D I C K & URODA 1986; c f . f i g . 5 ) where g o s l a v i a (ECONOMIDES, C I K E S , t h e requested m a t e r i a l i s i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y ceramic proppants i n t h e e a r l y s t a g e and h i g h - s t r e n g t h p r o p p a n t s i n t h e l a t e s t a g e o f t h e oper a t i o n s i n a r e s e r v o i r d e p t h o f a b t . 3,000 - 4,000 m (9,000 - 12,000 f t ) ,
*
p a r t s o f some gas f i e l d s i n T u r k e y w h i c h w i l l be done w i t h sand p o t e n t i a l l y f o l l o w e d by a t a i l - i n o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y c e r a m i c p r o p p a n t i n a r e s e r v o i r d e p t h o f a b t . 1,500 - 2,500 m (4,500 - 7,500 f t ; c f . f i g . 2 ) ,
*
some T e r t i a r y sandstone gas f i e l d s i n S i c i l y / I t a l y ( c f . f i g . 5 ) where i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y p r o p p a n t s w i l l be needed i n a b t . 3,500 - 5,000 m (10,500 - 15,000 f t ) r e s e r v o i r depth, and
*
p r o b a b l y some T e r t i a r y sandstone and c a r b o n a t e w e l l s i n Hungary, S o u t h e r n F r a n c e and N o r t h e r n A f r i c a w h i c h a l s o r e q u i r e i n t e r m e d i a t e - s t r e n g t h h i g h - d e n s i t y p r o p p a n t s i n a r e s e r v o i r d e p t h o f a b t . 1,500 - 4,500 m (4,500 - 13,500 ft).
*
The o i l f r a c t u r i n g w i t h sand i n Cretaceous s h a l l o w f i e l d s i n Germany FRG and N e t h e r l a n d s i s a l s o s c h e d u l e d t o go ahead d u r i n g t h e coming y e a r s , w i t h demand o f a b t . 20 - 70 t o f sand p e r w e l l i n a r e s e r v o i r d e p t h o f a b t . 700 2,500 m (2,100 - 7,500 f t ) .
221
2.4.5.4. Significance o f f r a c t u r i n s f o r increasing proven gas reserves R o t l i e g e n d f r a c t u r i n g i n Germany FRG has p a r t i c u l a r l y t o be assessed i n view o f t h e e s t i m a t i o n o f JOHN (1983) t h a t proven gas r e s e r v e s ( i n l a t e 1982) were 120 B i l l . m3, b u t t h e r e m a i n i n g s t i m u l a t i o n p o t e n t i a l was a t t h a t t i m e a t l e a s t i n t h e range o f 90 - 120 B i l l . m3, i m p l y i n g t h a t consequent s u c c e s s f u l hydraul i c f r a c t u r i n g can - i n case o f v e r i f i c a t i o n o f a l l t h e assumptions - n e a r l y REIdouble t h e h i t h e r t o proven R o t l i e g e n d gas r e s e r v e s ( c f . s e c t i o n 3 . 1 . 2 . ) . NICKE, BRINKMANN, SCHWARZ & HUENI (1985) a l s o come t o t h e c o n c l u s i o n t h a t a b t . 100 B i l l . m3 o f gas a r e o n l y a b l e t o be accessed by l a r g e - s c a l e h y d r a u l i c p r o p p a n t f r a c t u r i n g o f t h e t i g h t pay f o r m a t i o n s , w i t h t h e t o t a l h i t h e r t o proven gas r e s e r v e s amounting t o a b t . 300 B i l l . m3 ( w i t h t h i s q u a n t i t y , however, i n c l u d i n g a l s o t h e Z e c h s t e i n carbonate s o u r gas r e s e r v e s ) and a n o t h e r 300 B i l l . m3 b e i n g p r e d i c t e d f o r f u t u r e d i s c o v e r i e s , w i t h t h u s one t h i r d o f t h i s q u a n t i t y r e a c h i n g economical p r o d u c t i o n o n l y through MHF s t i m u l a t i o n t r e a t m e n t s ( t h i s matches i n percentage w i t h t h e s i t u a t i o n i n t h e USA where i t i s e s t i m a t e d t h a t 30 - 35 % o f t h e r e c o v e r a b l e hydrocarbon r e s e r v e s may be a t t r i b u t e d t o t h e a p p l i c a t i o n o f h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n ; WATERS 1980). O f f s h o r e h y d r a u l i c p r o p p a n t f r a c t u r i n g which i s so f a r l i m i t e d t o r i g - , p l a t form- and boat-based s t i m u l a t i o n j o b s w i l l be promoted d u r i n g t h e n e x t y e a r s by t h e t e c h n o l o g i c a l i n n o v a t i o n o f a subsea s t i m u l a t i o n vessel t h a t i s equipped w i t h a r i s e r and t h a t i s a b l e t o c a r r y o u t o p e r a t i o n s independent f r o m w e l l heads r e a c h i n g t h e w a t e r surface, t h u s opening t h e p o s s i b i l i t y o f f r a c t u r i n g a l s o w e l l s w i t h subsea c o m p l e t i o n i n t h e N o r t h Sea (STENA 1986). Some operat i o n s have a l s o a l r e a d y been c a r r i e d o u t f r o m a semisubmersible (HOOD & POWELL 1982).
2.4.5.5.
Impact o f proppant marketing strategy
I n terms o f m a r k e t i n g s t r a t e g y and i t s impact on proppant s a l e s l e v e l s , comments a r e o f f e r e d on d i r e c t p r o p p a n t s a l e s t o end users, l i q u i d a t i o n o f s e r v i c e company s t o c k s , and stand-by s u p p l y by proppant companies.
2.4.5.5.1.
D i r e c t proppant sales t o end users
The h y d r a u l i c f r a c t u r i n g scenery and p a r t i c u l a r l y p r o p p a n t c h o i c e can t o some e x t e n t a l s o be i n f l u e n c e d by t h e m a r k e t i n g s t r a t e g y o f t h e p r o p p a n t comp a n i e s . W h i l e t r a d i t i o n a l l y s a l e s were almost e x c l u s i v e l y made t o s e r v i c e comp a n i e s as i n t e r m e d i a t e u s e r s which t h e n i n c o r p o r a t e d t h e proppants i n t o t h e whole s t i m u l a t i o n o p e r a t i o n and o f f e r e d t h e i r c l i e n t s , t h e p r o d u c t i o n companies r e p r e s e n t i n g t h e end users, t h e complete j o b package as an i n t e g r a t e d s e t ( i n c l u d i n g a s t a n d a r d b u s i n e s s mark-up range o f t h e proppants), t h e r a p i d r e v i s i o n o f many budget i t e m s f o r d r i l l i n g and f r a c t u r i n g a f t e r t h e 1986 o i l p r i c e d r o p has opened t h e door f o r a c o n s i d e r a b l e change a l s o i n t h i s m a t t e r . Some aspects o f drawbacks o f s e r v i c e company s t o c k s and t r e a t m e n t s e l f - p e r f o r m a n c e i n East e r n Europe a r e o u t l i n e d as f o l l o w s .
2.4.5.5.1.1.
Drawbacks o f service conpany stocks
As a consequence o f p a r t i a l l y e x c e s s i v e s t o c k s on which s e r v i c e companies k e p t s i t t i n g f o r q u i t e some t i m e a f t e r o p t i m i s t i c a l l y o r d e r i n g enough m a t e r i a l t o be a b l e t o c a r r y o u t two t o t h r e e j o b s o v e r n i g h t i n good t i m e s o f t h e f r a c t u r i n g scenery and then p a i n f u l l y awaking when a f t e r t h e 1986 o i l p r i c e c r a s h t h e expected programmes were suspended o r c a n c e l l e d by t h e o p e r a t o r s , w i t h t h e purchased m a t e r i a l b l o c k i n g t h e s e r v i c e company warehouses and r e m a i n i n g i n t h e i r books, t h e m e n t a l i t y o f b o t h s e r v i c e companies and p r o p p a n t s u p p l i e r s i s
222 a t l e a s t i n p a r t s o f t h e European s t i m u l a t i o n m a r k e t c h a n g i n g t o w a r d s d i r e c t p u r c h a s e o f t h e p r o p p a n t s b y t h e e n d c u s t o m e r s , t h e p r o d u c t i o n companies. T h i s c o n s t e l l a t i o n w o u l d c e r t a i n l y g i v e much more m a r k e t t r a n s p a r e n c y t o t h e p r o p p a n t companies t h a n b e f o r e , because i t w o u l d b e now i m m e d i a t e l y e v i d e n t who i s t h e end u s e r and how much m a t e r i a l he needs f o r w h i c h t y p e o f j o b . The s e r v i c e company w o u l d g e t r i d o f t h e r i s k o f s i t t i n g on t h e p r o p p a n t s once an oper a t i o n i s suspended o r c a n c e l l e d , and w o u l d compensate t h e h i t h e r t o m a r k - u p o f t h e p r o p p a n t p r i c e b y pumping s u r c h a r g e s p e r volume u n i t , t h u s n o t s i g n i f i c a n t l y l o s i n g money a t t h e b o t t o m o f t h e l i n e and e a r n i n g p r o f i t a l m o s t e x c l u s i v e l y t h r o u g h s e r v i c e and own c h e m i c a l s . The p o s s i b i l i t y o f d i r e c t p r o p p a n t p u r c h a s e f r o m t h e m a n u f a c t u r e r s i n s t e a d o f t a k i n g t h e m a t e r i a l t h r o u g h t h e s e r v i c e compan i e s i s a l s o m e n t i o n e d b y CUTLER, E N N I S S , JONES & CARROLL ( 1 9 8 3 ) .
2.4.5.5.1.2.
Treatment s e l f -performance i n Eastern Europe
The c o n c e p t o f d i r e c t s a l e s w o u l d b e p a r t i c u l a r l y v a l i d f o r E a s t e r n E u r o p e where n o t o n l y t h e w h o l e e c o n o m i c a l s t r u c t u r e i s q u i t e d i f f e r e n t f r o m t h a t i n Western Europe and USA, b u t many o p e r a t i n g companies have own pumping e q u i p m e n t a t l e a s t f o r some j o b s and t h e i n f l u e n c e o f s e r v i c e companies i s i n some c a s e s e v e n l i m i t e d t o s a l e o f c h e m i c a l s . I n W e s t e r n Europe, m a j o r companies p e r f o r m i n g l a r g e r f r a c t u r i n g campaigns i n 1987 - 1990 may w i s h t o s e r i o u s l y l o o k i n t o p o s s i b i l i t i e s o f d i r e c t purchase o f proppants ( c f . a l s o s e c t i o n 2.4.2.4.1.2.2.) w h i c h c a n be c a r r i e d o u t e v e n i n agreement w i t h t h e s e r v i c e companies. Concerning the s t i l l highly competitive situation, i t i s c e r t a i n l y worthwhile that b o t h p a r t i e s t a l k t o g e t h e r on a l l t h e s e a s p e c t s i n o r d e r t o f i n d a s o l u t i o n w h i c h i s m o s t b e n e f i c i a l f o r b o t h o f them.
2.4.5.5.2.
L i q u i d a t i o n o f s e r v i c e company stocks
A t e m p o r a r y drawback f o r t h e p r o p p a n t s u p p l i e r s i n t h e t r a n s i t i o n a l phase o f p r o b a b l y a t l e a s t p a r t i a l l y c h a n g i n g m a r k e t i n g p o l i c y and p u r c h a s e s t r a t e g y as w e l l as p a r a l l e l t o b u s i n e s s p i c k - u p a l o n g w i t h r e c o v e r y f r o m t h e peak o f t h e o i l p r i c e c r a s h i s t h e l i q u i d a t i o n o f t h e i n v e n t o r i e s b y t h e s e r v i c e companies. D u r i n g t h e g o l d e n y e a r s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g , t h e s e r v i c e companies c o u l d make much money and w i n c o n t r a c t s a g a i n s t c o m p e t i t o r s b y b e i n g a b l e t o move t o w a r d s l o c a t i o n i m m e d i a t e l y a f t e r o r d e r e n t r y b y c a l l w i t h o u t h a v i n g s t i l l t o g e t access t o m a t e r i a l . The common p o l i c y i n t h e s e d a y s was t o k e e p p r o p p a n t s , f l u i d s and o t h e r c h e m i c a l s i n such amounts i n t h e warehouse and on t h e b o a t t h a t t w o o r even t h r e e s t a n d a r d j o b s c o u l d b e c a r r i e d o u t o v e r n i g h t w i t h o u t any r e s t o c k i n g .
H a v i n g s i t on t h i s r e s e r v e i n v e n t o r y f o r q u i t e some t i m e a f t e r t h e sudden o i l p r i c e d r o p , t h e p i c k i n g u p o f t h e a c t i v i t y i n l a t e 1986 and t h r o u g h o u t 1987 was t a k e n b y t h e s e r v i c e companies as a welcomed o c c a s i o n t o l i q u i d a t e t h e i r e x c e s s i v e s t o c k s n o t o n l y i n t h e o f f s h o r e bases, b u t a l s o i n warehouses a t s e v e r a l p l a c e s on t h e c o n t i n e n t , and t o g e t t h e i n v e n t o r i e s o u t o f t h e i r b o o k s . T h i s c o u l d be done because t h e e x p e r i e n c e has shown d u r i n g t h e l a s t y e a r s t h a t m o s t o f t h e o p e r a t i o n s do n o t have t o be c a r r i e d o u t a t once, o r d e c i s i o n and a p p r o v a l a r e made e a r l y enough t o a l l o w p r o p e r p r e p a r a t i o n i n t e r m s o f m a t e r i a l a c q u i s i t i o n and s t i m u l a t i o n v e s s e l l o a d i n g , and warehouses o f many s u p p l i e r s i n s t r a t e g i c a l l y s u i t a b l e l o c a t i o n s i n Europe a l l o w s h i p p i n g o f m a t e r i a l t o a r r i v e a t t h e b o a t base w i t h i n a c o u p l e o f days. T h a t i s why t h e p r o p p a n t companies d i d n o t f e e l t h e i m p r o v i n g b u s i n e s s s i t u a t i o n i m m e d i a t e l y b y a jump of t h e sal e s l e v e l , because b y l i q u i d a t i n g t h e i r s t o c k s , t h e s e r v i c e companies c a r r i e d o u t s e v e r a l j o b s f r o m t h e i r own s u p p l y ( w i t h e v e n some s h o r t - c i r c u i t m a t e r i a l move between c o m p e t i n g s e r v i c e companies h a v i n g o c c u r r e d f o r t h e p u r p o s e o f c h e a p e r b u y i n g and f a s t e r i n v e n t o r y l i q u i d a t i o n ) b e f o r e upon c l e a r i n g o f t h e i r warehouse p u r c h a s i n g new m a t e r i a l f o r t h e f o l l o w i n g o p e r a t i o n .
223
2.4.5.5.3.
Stand-by supply by proppant companies
As t h e p r o d u c t i o n companies as u l t i m a t e customers, however, would c e r t a i n l y n o t r i s k t o r e p e a t t h e u n p l e a s a n t e x p e r i e n c e o f t h e s e r v i c e companies t o p i l e up m a t e r i a l which i s then n o t used up f o r some t i m e once t h e p r o p p a n t market t u r n s a t l e a s t p a r t i a l l y t o d i r e c t purchase by t h e end users, t h e m a r k e t i n g and s a l e s p o l i c y o f t h e p r o p p a n t s u p p l y companies would have t o f o c u s more and more on s h o r t - t e r m stand-by, w i t h o r d e r s coming i n almost i n t h e l a s t m i n u t e and t h e l a r g e s t c o m p e t i t i v e advantage b e i n g w i t h t h e proppant company which can d e l i v e r t h e f a s t e s t and always i n s u f f i c i e n t q u a n t i t y . The f r e q u e n t o r d e r i n g f o r d e l i v e r y almost o v e r n i g h t i n d i s p e n s i b l y r e q u i r e s t h e presence o f l o c a l s t o c k s near a c e n t r a l p o r t i n Europe f r o m which b o t h N o r t h Sea and onshore areas can be supp l i e d v e r y f a s t , w i t h those companies t h a t h i t h e r t o r e l i e d on t i m e l y o r d e r s e a r l y enough t o b r i n g i n t h e m a t e r i a l f r o m t h e USA h a v i n g then c e r t a i n l y a b i g d i s advantage which i n t h e w o r s t case can c u t them c o m p l e t e l y o f f f r o m b u s i n e s s . Stock management improvement has t o aim on enhancing t h e p o s s i b i l i t i e s o f reasonable f o r e c a s t i n g o f coming s t i m u l a t i o n j o b s and t h e i r m a t e r i a l demand i n terms o f p r o p p a n t type, g r a i n s i z e and q u a n t i t y . T h i s task needs c o n t i n u o u s mark e t survey w i t h c l o s e a t t e n t i o n t o t h e o r i g i n a l sources and d e f i n i t e l y r e q u i r e s a r e g i o n a l European c o n t a c t o f f i c e as w e l l as f r e q u e n t t r a v e l l i n g f o r v i s i t i n g and i n t e r v i e w i n g s t i m u l a t i o n s p e c i a l i s t s and commercial r e p r e s e n t a t i v e s i n p r o d u c t i o n and s e r v i c e companies i n o r d e r t o r e c e i v e t h e most a c t u a l i n f o r m a t i o n f o r t h e p l a n n i n g o f t h e p h y s i c a l d i s t r i b u t i o n o f t h e proppant q u a n t i t i e s . W h i l e d i r e c t s a l e s o f proppants t o p r o d u c t i o n companies as end u s e r s have been performed a l r e a d y i n a few cases i n E a s t e r n Europe ( c f . s e c t i o n 2.4.4.4.), t h i s s t r a t e g y would be more o r l e s s a m a r k e t i n g i n n o v a t i o n f o r Western Europe ( e x c e p t o f e a r l i e r l i m i t e d d e a l s w i t h some p r o d u c t i o n companies which have t h e i r own in-house s e r v i c e branch f o r s m a l l e r j o b s ) , b u t f o r t h e reasons d i s c u s sed above, t h i s change o f t h e m a r k e t i n g t r e n d would be more than s e l f - u n d e r s t a n d i n g and c o u l d c o n s i d e r a b l y h e l p t o secure business f o r a l l t h e p a r t i c i p a t i n g p a r t i e s by o f f e r i n g c o m p e t i t i v e s o l u t i o n s f o r t h e p r o d u c t i o n company by i n t e n s i f y i n g t h e p r i c e b i d d i n g o f t h e s e r v i c e companies t o g e t t h e j o b which, once p r o p p a n t p r i c e mark-up i s s m a l l e r than i t used t o be, w i l l have t o c h i e f l y o r even a l m o s t e x c l u s i v e l y c o n c e n t r a t e on s e r v i c e r a t e s and t h u s e f f i c i e n c y o f work, and f e a s i b i l i t y and r e l i a b i l i t y o f equipment.
2.4.6. Eastern Europe 1987
-
1990
I n c o n t r a s t t o Western Europe where t h e expected boom o f s t i m u l a t i o n i n t h e p e r i o d 1987 - 1990 i n t h e N o r t h Sea i s p a r t i a l l y t h e r e s u l t o f t h e r e c o v e r y o f t h e hydrocarbon i n d u s t r y a f t e r t h e m a j o r breakdown due t o t h e o i l p r i c e drop i n e a r l y 1986, and p a r t i a l l y was anyway scheduled i n t h e frame of l o n g - t e r m o i l and e s p e c i a l l y g a s - f i e l d development p l a n s t h a t were c o n t r o l l e d by t h e e x t r a o r d i n a r y o f f s h o r e l o g i s t i c a l s i t u a t i o n ( c f . s e c t i o n 3.8.1.), t h e more o r l e s s i n dependence o f t h e E a s t e r n European market f r o m o i l p r i c e t r a g e d i e s and US $ exchange r a t e c r i s e s up t o a c e r t a i n degree ( c f . s e c t i o n 2 . 4 . 4 . ) i s t h e reason f o r t h e absence o f a s p e c i a l b o o s t i n t h i s area d u r i n g t h e n e x t y e a r s . E a s t e r n Europe i s c h a r a c t e r i z e d i n t h e v e r y near f u t u r e by c o n t i n u a t i o n o f t h e e x i s t i n g and a l r e a d y f i r m l y p l a n n e d campaigns and p r o j e c t s as w e l l as by performance o f a d d i t i o n a l j o b s as a consequence o f s u c c e s s f u l p r o m o t i o n by b o t h e x t e r n a l mark e t i n g a c t i v i t y and i n t e r n a l economical f o r c e t o a m e l i o r a t e p r o d u c t i o n f o r enhanced domestic s u p p l y p a r t i c u l a r l y i n case o f renewed severe w i n t e r s compar a b l e t o t h a t i n e a r l y 1987, a l t h o u g h t h e r i s k o f t e c h n i c a l d e l a y and administ r a t i v e postponement o f o p e r a t i o n s i s i n p a r t s o f E a s t e r n Europe much g r e a t e r than i n Western Europe.
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2.4.6.1. General aspects The main n e c e s s i t y i n t h e near f u t u r e f o r many European c o u n t r i e s , b u t p a r t i c u l a r l y f o r E a s t e r n Europe, i s t o i n c r e a s e b o t h r e l a t i v e and a b s o l u t e amount o f domestic hydrocarbon p r o d u c t i o n i n o r d e r t o s a t i s f y t h e i n c r e a s i n g urban and i n d u s t r i a l demand ( w h i c h t h e severe w i n t e r 1986/87 has d r a s t i c a l l y u n d e r l i n e d t h a t had p a r t i c u l a r l y t e r r i b l e e f f e c t s o f energy s h o r t a g e i n Romania and was i n f a c t t h e w o r s t w i n t e r s i n c e a b t . 40 y e a r s ; R I G A S S I 1985, 1986) and t o a c h i e v e some more independence f r o m u n f a v o u r a b l e changes o f t h e p o l i t i c a l and economic a l framework. As e x p l o r a t i o n d r i l l i n g has i n p a r t s o f t h e areas a l r e a d y r e a ched such a mature stage t h a t s p e c t a c u l a r new d i s c o v e r i e s seem t o be n o t v e r y l i k e l y and t h e p o s s i b l e i n c r e a s e o f p r o d u c i n g f i e l d s by new e x p l o r a t i o n h i t s m i g h t be compensated by d e c l i n i n g o u t p u t f r o m o l d e r p r o g r e s s i v e l y d e p l e t e d f i e l d s , t h e o n l y r e a l l y s i g n i f i c a n t impact i s seen i n m a j o r s t i m u l a t i o n camp a i g n s i n e x i s t i n g f i e l d s and h i t h e r t o m a r g i n a l r e s e r v o i r s . Unless a comprehensive agreement on l i m i t a t i o n o f s t r a t e g i c a l c o n v e n t i o n a l and n u c l e a r arms and a ban o f t h e t h r e a t o f a space war i s reached between USA and USSR r e a s o n a b l y soon, t h e urgency o f t h e E a s t e r n European c o u n t r i e s t o amel i o r a t e t h e i r own hydrocarbon s u p p l y i s c o n s i d e r e d t o become even more s e r i o u s w i t h i n t h e n e x t few y e a r s ( c f . a l s o R I G A S S I 1985), t h u s a l s o i n c r e a s i n g t h e nec e s s i t y o f s t i m u l a t i o n i n o r d e r t o achieve as much as p o s s i b l e p o l i t i c a l a u t a r k y and power. Many areas o f a p p l i c a t i o n o f h y d r a u l i c f r a c t u r i n g can be expected, because so f a r i n most o f t h e c o u n t r i e s d r i l l i n g and c o n v e n t i o n a l complet i o n had a h i g h e r p r i o r i t y . Some o f t h e most e v i d e n t and p a r t i a l l y a l r e a d y conc l u d e d p r o j e c t s i n Y u g o s l a v i a and Hungary, USSR, o t h e r E a s t e r n European count r i e s , and Southern Europe and N o r t h e r n A f r i c a a r e b r i e f l y o u t l i n e d as f o l l o w s , and comments a r e a l s o o f f e r e d on China.
2.4.6.2. Yugoslavia and Hungary Y u g o s l a v i a p r o b a b l y remains t o be t h e most i m p o r t a n t area i n terms o f hydraul i c p r o p p a n t s t i m u l a t i o n i n E a s t e r n Europe because o f t h e c o n t i n u a t i o n o f t h e MHF campaign ( c f . s e c t i o n 2.4.3.1.; r e i n f o r c e d by t h e f a c t t h a t i n v e s t m e n t f o r t h e proppants f o r a l l t h e planned j o b s has a l r e a d y been made, c f . s e c t i o n 2.4.2.4.1.1., and t h e w e l l s which o r i g i n a l l y have been d r i l l e d f o r t h e purpose t o be subsequently f r a c t u r e d , a r e p r e s e n t l y s h u t - i n and a r e w a i t i n g on f r a c t u r i n g t o be a b l e t o be produced) t h a t was p a r t i a l l y suspended and r e s c h e d u l e d f o l l o w i n g d e t e r i o r a t i o n o f t h e g e n e r a l n a t i o n a l economical s i t u a t i o n a f t e r t h e o i l p r i c e d e c l i n e i n e a r l y 1986. The b e s t p r o m o t i o n o f a r a p i d e x e c u t i o n o f t h e r e s t o f t h e MHF p r o j e c t i s t h e p a r t i a l l y e x c e l l e n t r e s u l t o f t h e f i r s t p a r t o f t h e l a r g e - s c a l e t r e a t m e n t campaign i n 1986 which gave a f a s t p a y - o u t o f t h e t o t a l s t i m u l a t i o n expenses. A p a r t f r o m t h e MHF j o b s b e i n g f i n a n c e d by t h e Yugoslavians themselves w i t h o u t any e x t e r n a l h e l p , t h e independent World Bank Petroleum P r o j e c t ( c f . SAUNDERS & S H I R A Z I 1988) i n c l u d e s a s u i t e o f s m a l l e r f r a c t u r e j o b s where a l s o b o t h i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y and h i g h - s t r e n g t h proppants a r e g o i n g t o be used. V a r i o u s r e s e r v o i r s a r e a l s o w a i t i n g on s m a l l - t o medium-scale t r e a t m e n t w i t h sand p o t e n t i a l l y f o l l o w e d by a t a i l - i n o f i n t e r m e d i a t e - o r h i g h - s t r e n g t h proppants, w i t h t h e l a t t e r j o b s , however, p r e f e r e n t i a l l y b e i n g o p e r a t e d by t h e Yugoslavian companies w i t h own equipment. The scheduled purchase o f h i g h - p r e s sure equipment aims on i n c r e a s i n g independence e n a b l i n g t h e Yugoslavians t o c a r r y o u t a l s o l a r g e r f r a c t u r e j o b s w i t h o u t t h e s u p p o r t o f Western s e r v i c e companies. H y d r a u l i c p r o p p a n t f r a c t u r i n g i s a l s o planned i n Hungary i n those p a r t s o f t h e f i e l d zone w h i c h e x t e n d f r o m Y u g o s l a v i a across t h e s t a t e boundary as w e l l as i n o t h e r r e g i o n s i n Southwestern and E a s t e r n Hungary, w i t h most o f t h e t r e a t ments b e i n g i n c l u d e d i n t h e c u r r e n t World Bank P e t r o l e u m P r o j e c t and b e i n g done w i t h sand and/or i n t e r m e d i a t e - t o h i g h - s t r e n g t h p r o p p a n t s .
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2.4.6.3.
USSR
Other a c t i v i t y i n Eastern Europe i s so f a r d i f f i c u l t t o p r e d i c t . C e r t a i n l y t r a d i t i o n a l sand f r a c t u r i n g w i l l continue i n the USSR w i t h own high-pressure equipment o f American provenance which has been bought s t i l l b e f o r e t h e sanct i o n s enacted by P r e s i d e n t C a r t e r i n 1978 t h a t have i n the meantime f i n a l l y been l i f t e d by P r e s i d e n t Reagan i n e a r l y 1987 (ERDUL-ERDGAS-KOHLE NACHRICHTEN 1987 a, LANDERS 1987; c f . s e c t i o n 2.4.4.2.) came i n t o f o r c e . Some aspects o f h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l , i m p o r t vs. domestic manufacturing o f h i g h - s t r e n g t h proppants, s i g n i f i c a n c e o f i n t e n s i f i c a t i o n o f o i l and gas e x p l o i t a t i o n , and p r i o r i t y o f petroleum development vs. o i l p r i c e l e v e l are discussed as f o l l o w s .
2.4.6.3.1.
Hydraulic proppant f r a c t u r i n g p o t e n t i a l
H y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l o f p a r t i a l l y even massive s i z e i n the n e x t few years i s p a r t i c u l a r l y seen i n l o w - p e r m e a b i l i t y gas-bearing sandstones i n t h e o l d gas f i e l d s Shebelinskoe/Ukraina (3,000 - 4,000 m r e s e r v o i r depth; c f . tab. 9), Stavropolskoe/Northern Caucasus (2,000 - 3,000 m depth), Saratovskoe (2,000 m depth) and Krasnodarskoe (3,000 - 4,000 m depth). I n the new gas f i e l d s Orenburgskoe (3,000 - 4,000 m depth) and Karatchaganakskoe o r Borodinskoe (4,000 - 5,000 m depth), c o n t r a c t s w i t h N o r t h American s e r v i c e compan i e s are a l r e a d y concluded f o r c a r r y i n g o u t f r a c t u r i n g treatments w i t h intermed i a t e - s t r e n g t h proppants s t i l l i n 1988 o r 1989. P o t e n t i a l f o r l a r g e r - o r small e r - s c a l e h y d r a u l i c f r a c t u r i n g i s a l s o p r e s e n t i n the o l d o i l f i e l d Sachalinskoe (2,000 - 3,000 depth) and the new o i l and gas f i e l d s Samotlorskoe/Siberia (2,000 m depth; t h i s i s a whole complex o f f i e l d s w i t h many p o t e n t i a l s t i m u l a t i o n candidates), and I r k u t s k s k o e (4,000 m depth). Other p r o l i f i c o i l and gas f i e l d s where h y d r a u l i c proppant f r a c t u r i n g w i l l become i m p o r t a n t i n the near f u t u r e are s i t u a t e d i n Astrachan a t t h e n o r t h e r n border o f the Caspian Sea, i n the area n o r t h o f the Black Sea, and i n the surroundings o f I r k u t s k and Novosib i r s k . P a r t i c u l a r problems p r e s e n t l y e x i s t i n sour gas e x p l o r a t i o n and product i o n ( e s p e c i a l l y i n Orenburgskoe and Astrahanskoe/Lower Volga).
2.4.6.3.2. Import vs. domestic manufacturing o f h igh-s t rengt h proppant s The USSR has a l a r g e s u i t e o f o i l and gas r e s e r v o i r s i n v a r i o u s g e o l o g i c a l formations and i n d i f f e r e n t depth ranges and as t h e r e i s h i t h e r t o e i t h e r no poss i b i l i t y o r no i n t e n t i o n o f domestic manufacturing, i t i s o n l y a q u e s t i o n o f time (and o f p r i o r i t y i n the governmental planning) when i n t e r m e d i a t e - and/or h i g h - s t r e n g t h proppants w i l l be purchased from Western s u p p l i e r s f o r a p p l i c a t i o n i n p r e f e r e n t i a l l y deep t i g h t o i l and gas w e l l s i n t h e USSR. Although according t o t h e general s t a t e o f the i n d u s t r y , i t has t o be assumed t h a t i f r e q u i r e d t h e USSR c e r t a i n l y has p o s s i b i l i t i e s o f domestic product i o n o f s y n t h e t i c proppants i n s u f f i c i e n t q u a n t i t y , i t i s almost c e r t a i n t h a t a t l e a s t d u r i n g the n e x t few years, man-made proppants w i l l have t o be imported i n t o t h e USSR f o r t h e s t i m u l a t i o n treatments i n t h a t country, and o n l y l a t e r t h e r e might be an o p t i o n f o r t h e Russians t o proceed t o domestic f a b r i c a t i o n which, however, m i g h t be a more expensive approach i n c l u d i n g q u i t e some r e j e c t s and s u b - s p e c i f i c a t i o n m a t e r i a l b e f o r e the advanced experience o f the t r a d i t i o n a l proppant producers can be met and proppants o f comparable q u a l i t y can be made i n s u f f i c i e n t q u a n t i t y f o r widespread and u n r e s t r i c t e d a p p l i c a t i o n accord i n g t o the requirements ( c f . a l s o s e c t i o n 2.4.4.3.1.).
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2.4.6.3.3. Sign if i cance of intensif icat ion o f oil and gas exploitation I n t e n s i f i c a t i o n o f oil and gas e x p l o i t a t i o n has c u r r e n t l y a v e r y h i g h p r i o r i t y i n t h e g e n e r a l s t a t e i n d u s t r i a l p l a n n i n g (KINNEY 1987, ORASIANU 1 9 8 7 ) , and t h e new f i v e - y e a r p l a n ( 1 9 8 6 - 1990) c o n t a i n s t h e o b l i g a t i o n t o d e c r e a s e o i l c o n s u m p t i o n b y p r o g r e s s i v e l y s u b s t i t u t i n g o i l b y gas, w h i c h r e s u l t s i n t h e n e c e s s i t y o f i n c r e a s i n g n a t u r a l gas p r o d u c t i o n t h a t i n d i s p e n s i b l y i n c l u d e s q u i t e some s t i m u l a t i o n (ERDOEL-ERDGAS AKTUELL 1985 b, EBEL 1986, ERDUL-ERDGAS-KOHLE NACHRICHTEN 1986 a ) . The e c o n o m i c a l i m p e t u s o f i n c r e a s i n g o i l p r o d u c t i o n b y a l l p o s s i b l e means i s t h e o v e r w h e l m i n g i m p o r t a n c e of o i l e x p o r t s f o r f o r e i g n t r a d e b y b e i n g t o g e t h e r w i t h g a s t h e s o u r c e o f some 6 0 % o f c o n v e r t i b l e c u r r e n c y e a r n i n g (STERN 1986; c f . s e c t i o n s 2 . 4 . 4 . 1 . 1 . and 2 . 4 . 4 . 1 . 4 . ) . W h i l e p a r t i c u l a r l y o i l p r o d u c t i o n has s t e a d i l y d e c l i n e d s i n c e 1984, t h e e c o n o m i c a l r e q u i r e m e n t o f i n c r e a s i n g p r o d u c t i o n a f t e r t h e 1986 o i l p r i c e c r a s h has c o n s i d e r a b l y a c c e l e r a t e d e x p l o i t a t i o n c u l m i n a t i n g so f a r b y m i d 1988 i n a h i t h e r t o r e c o r d o u t p u t , w i t h t h e p r o g r e s s i v e improvement o f o i l p r o d u c t i o n s i n c e t h e 1986 o i l p r i c e c o l l a p s e h a v i n g e n a b l e d t h e USSR t o e x p o r t b o t h i n 1987 and 1988 h i g h e r q u a n t i t i e s o f o i l t h a n e v e r b e f o r e , and a l s o gas p r o d u c t i o n has r e a c h e d i t s so f a r c l i m a x l e v e l i n e a r l y 1988 ( O I L GAS JOURNAL 1988 b; c f . s e c t i o n 2 . 4 . 4 . 1 . 5 . ) .
2.4.6.3.4. P r i o r i t y o f petroleum development vs. oil price level T h i s i s t h e r e s u l t o f p e t r o l e u m d e v e l o p m e n t commanding t o p p r i o r i t y i n t h e USSR s i n c e s e v e r a l y e a r s d e s p i t e t h e i n t e r v e n i n g 1986 o i l p r i c e c r a s h , w i t h t h e USSR even a f t e r t h e 1986 o i l p r i c e d r o p h a v i n g s t i l l a g a i n b o o s t e d i t s e f f o r t s t o s u s t a i n and expand o i l and gas p r o d u c t i o n and e x p o r t s , w h i l e o n t h e o t h e r hand a t t h e same t i m e i n t h e USA, p e t r o l e u m has f a l l e n t o t h e b o t t o m o f t h e g o v e r n m e n t a l p r i o r i t y l i s t and l a c k i n g c o n f i d e n c e i n f u t u r e i n v e s t m e n t o f c a p i t a l l e a d s t o postponement o f any p r o j e c t s i f o n l y s l i g h t d o u b t s a r e c a s t on t h e i r i r r e v o c a b l e e c o n o m i c a l f e a s i b i l i t y i n v a r i o u s s c e n a r i o s (KINNEY 1 9 8 7 ) . The p e r s i s t i n g t o p p r i o r i t y o f p e t r o l e u m d e v e l o p m e n t and t h e s i g n i f i c a n c e o f i n c r e a s i n g o i l and gas p r o d u c t i o n i s a l s o u n d e r l i n e d b y t h e f a c t t h a t t h e o i l p r i c e c r a s h i n 1986 was t h e r e a s o n t h a t f o r t h e f i r s t t i m e s i n c e t h e Second W o r l d War, t h e t o t a l f o r e i g n t r a d e v a l u e o f t h USSR s l a s h e d b y a b t . 10 % i n 1986, w i t h m o s t o f t h e d r o p stemming f r o m l o w e r o i l and gas p r i c e s ( O I L GAS JOURNAL 1987 h; c f . s e c t i o n 2 . 4 . 4 . 1 . ) .
2.4.6.4. China I n a s i m i l a r way as f o r t h e USSR, an i n c r e a s i n g f r a c t u r i n g p o t e n t i a l c a n be e x p e c t e d i n China, because t h e new C h i n e s e f i v e - y e a r p l a n ( 1 9 8 6 - 1990) a l s o i n c l u d e s t h e commitment o f s i g n i f i c a n t l y i n c r e a s i n g d o m e s t i c o i l and g a s p r o d u c t i o n (REID 1981, EBEL 1986, ENERGIEWIRTSCHAFT 1986 a, WONG 1 9 8 7 ) , and w h i l e c e r t a i n l y much a d d i t i o n a l c a p a c i t y i s e x p e c t e d t o come f r o m s u c c e s s f u l o f f s h o r e e x p l o r a t i o n , economical e x p l o i t a t i o n w i l l be i n d i s p e n s i b l y l i n k e d w i t h h y d r a u l i c proppant f r a c t u r i n g ( a s p e c t s o f f r a c t u r i n g s t i m u l a t i o n i n China a r e discussed b y L I & ZHU 1986 and PARKER, ADAMS & LIANKUI 1 9 8 6 ) . The m a r k e t p e r s p e c t i v e i s i n c r e a s i n g , because a r e s t r u c t u r i n g campaign c o m p a r a b l e t o t h a t i n t h e USSR ( c f . s e c t i o n 2 . 4 . 4 . 5 . 4 . ) i s a l s o t a k i n g p l a c e i n C h i n a and i s t o s t r e a m l i n e and d e c e n t r a l i z e o i l and g a s i n d u s t r y a d m i n i s t r a t i o n and o p e r a t i o n s (KHIN 1 9 8 8 ) . R e o r g a n i z a t i o n o f energy i n d u s t r y , r e l a x a t i o n o f o f f s h o r e c o n t r a c t terms, cons i d e r a b l e o f f s h o r e d i s c o v e r i e s , and e x t e n s i v e o f f s h o r e and o n s h o r e d e v e l o p m e n t s a r e t h e p r i n c i p a l h i g h l i g h t s o f a c t i v i t y i n C h i n a i n t h e f i r s t h a l f o f 1988, and i n 1987, d r i l l i n g i n c r e a s e d f o r t h e e i g h t h c o n s e c u t i v e y e a r , m o v i n g C h i n a i n t o t h e t h i r d r a n k a f t e r USA and USSR i n d r i l l i n g . Some t i m e ago, C h i n a e v e n p r e s e n t e d i t s own h i g h - s t r e n g t h p r o p p a n t s w h i c h dur i n g t e s t s i n Western r e s e a r c h l a b o r a t o r i e s have been p r o v e n t o be o f good qua-
227 l i t y ( c f . s e c t i o n 1 . 3 . 1 . 2 . ) , b u t l a t e r n o t v e r y much was heard about a p p l i c a t i o n and w h i l e q u i t e some domestic pumping o f t h e m a t e r i a l can be assumed, t h e Chinese proppants so f a r were never i n s e r t e d i n t o a f r a c t u r e i n f o r e i g n count r i e s . So f a r b o t h n a t u r a l sand and h i g h - s t r e n g t h b a u x i t e proppants have a l r e a dy been pumped i n h y d r a u l i c f r a c t u r i n g t r e a t m e n t s i n China, w i t h t o t a l p r o p p a n t q u a n t i t i e s n o t exceeding 120 t i n many o p e r a t i o n s ( L I & ZHU 1986). F i e l d exper i e n c e has shown t h a t i n v a r i o u s cases, f r a c t u r e s propped w i t h l o w e r c o n c e n t r a t i o n s o f b a u x i t e have b e t t e r c o n d u c t i v i t i e s than c r a c k s plugged w i t h h i g h e r sat u r a t i o n s o f sand. The use o f h i g h - s t r e n g t h proppants as t a i l - i n a f t e r t h e i n j e c t i o n o f sand as main proppant l o a d g i v e s l i t t l e s u p e r i o r i t y i n g e t t i n g b e t t e r f r a c t u r e c o n d u c t i v i t y , w i t h an u n i f o r m p r o p p a n t package t h e r e f o r e p r o v i d i n g t h e optimum r e s u l t .
2.4.6.5.
Other Eastern European countries
Germany GDR which has l i m i t e d domestic s u p p l y by gas p r o d u c t i o n f r o m R o t l i e gend sandstones t h a t i s n o t m e e t i n g t h e c o n s i d e r a b l e demand ( c f . s e c t i o n 3.10.2.2.) and i n l i g h t o f p r o b a b l y no l o n g e r r e c e i v i n g s u f f i c i e n t d e l i v e r y f r o m t h e USSR i s assessed t o be sooner o r l a t e r f o r c e d t o enhance o f f t a k e by l a r g e r - s c a l e h y d r a u l i c proppant s t i m u l a t i o n than done so f a r . Several o i l - and gas-bearing f o r m a t i o n s i n s m a l l e r o r l a r g e r f i e l d s which would r e q u i r e s t i m u l a t i o n f o r achievement o f adequate p r o d u c t i o n r a t e s a r e a l s o p r e s e n t onshore and o f f s h o r e i n Poland, Romania ( c f . ORASINAU 1986) and B u l g a r i a and a r e e x p e c t e d t o come t o c o n c l u s i o n and j o b e x e c u t i o n w i t h i n t h e n e x t few y e a r s . H y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l can be r e a s o n a b l y assumed i n e v e r y E a s t e r n European c o u n t r y , b u t t h e o p e r a t i o n s a r e expected t o t a k e i n some s t a t e s such as Czechoslovakia, B u l g a r i a and p a r t i c u l a r l y A l b a n i a ( w h i c h has a spec i a l o r g a n i z a t i o n o f f o r e i g n a f f a i r p o l i c y ) s t i l l some t i m e o f p r e p a r a t i o n f o r v a r i o u s circumstances o f b o t h t e c h n i c a l and a d m i n i s t r a t i v e n a t u r e . The d i s c o v e r i e s i n t h e B a l t i c Sea which have so f a r been made m a i n l y i n i t s e a s t e r n p a r t a r e h i t h e r t o n o t commercial and development would p r e s e n t m a j o r e n v i r o n m e n t a l hazards (OIL GAS JOURNAL 1987 b ) , w h i l e i n t h e p r o m i s i n g western p a r t r e c e n t l y t h e g e o l o g i c a l model c o l l a p s e d , and s t r u c t u r a l r e i n t e r p r e t a t i o n and r e t h i n k i n g i s necessary i n o r d e r t o upgrade t h i s p a r t o f t h e o i l and gas p r o v i n c e .
2.4.6.6.
Southern Europe and Northern A f r i c a
I n c o n n e c t i o n w i t h t h e f r a c t u r i n g p o t e n t i a l i n t h e E a s t e r n European complex, a l s o some h i g h l i g h t s f r o m Southern Europe and N o r t h e r n A f r i c a deserve a t t e n t i o n . Turkey s h o u l d be p r i m a r i l y mentioned i n t h i s c o n n e c t i o n as a n o t h e r area o f c o n s i d e r a b l e s t i m u l a t i o n p o t e n t i a l where f o l l o w i n g t h e s u c c e s s f u l h y d r a u l i c p r o p p a n t f r a c t u r i n g campaigns i n 1986 and e a r l i e r y e a r s q u i t e some t r e a t m e n t s t o be c a r r i e d o u t w i t h sand and/or i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y p r o p p a n t s i n s e v e r a l gas f i e l d s a r e scheduled f o r 1987 and succeeding y e a r s ( c f . s e c t i o n 2.4.2.4.2.2.). O t h e r p o t e n t i a l areas o f h y d r a u l i c p r o p p a n t f r a c t u r i n g w i t h sand and/or i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s a r e A l g e r i a , L i b y a , Oman, G u l f o f Guinea (Gabon, Cameroun and N i g e r i a ) , Angola, Congo and Z a i r e i n N o r t h e r n A f r i c a as w e l l as I n d i a and P a k i s t a n i n t h e f a r t h e r M i d d l e E a s t . I n t h e F a r East, syst e m a t i c a l f r a c t u r i n g i s n o t c a r r i e d o u t a t t h e moment, and o n l y o c c a s i o n a l l y f r a c t u r i n g j o b s a r e performed w i t h p r e d o m i n a n t l y n a t u r a l sand. I n t h e M i d d l e East, t h e overwhelming share o f carbonates i n t h e s u i t e o f r e s e r v o i r s i s t h e reason why a l m o s t e x c l u s i v e l y a c i d f r a c t u r i n g i s done i n t h i s area. I n South America, p r o p p a n t f r a c t u r i n g focusses on B r a z i l and A r g e n t i n a , whereas Venezuel a i s c h a r a c t e r i z e d by a c i d f r a c t u r i n g . Proppant f r a c t u r i n g has f o r m e r l y a l s o been c a r r i e d o u t i n Peru. While i n most o f t h e mentioned c o u n t r i e s f r a c t u r e s t i m u l a t i o n i s c h i e f l y made w i t h n a t u r a l sand, e s p e c i a l l y i n B r a z i l and A l g e r i a var i o u s types o f s y n t h e t i c proppants a r e i n c o r p o r a t e d i n t o t h e p r o j e c t s o f w e l l
228 treatment
2 . 5 . Concluding
assessment o f proppant marketing potential
As a consequence o f t h e r e g i o n a l m a r k e t i n g assessment c o m p r i s i n g r e v i e w , s t a t u s and f o r e c a s t , and i n c o m b i n a t i o n w i t h an o u t l i n e o f t e c h n o l o g i c a l perspect i v e s o f enhancement o f t h e p o t e n t i a l a p p l i c a t i o n s and t h u s e x t e n s i o n o f t h e market ( c h a p t e r 4 ) , c o n c l u s i o n can be made t h a t p r o v i d e d an a g a i n more s t a b l e o i l p r i c e a t a reasonable l e v e l ( c f . a l s o ERDUL-ERDGAS-KOHLE NACHRICHTEN 1986 b ) , p a r t i a l l y a l s o a more f a v o u r a b l e h i g h e r US $ exchange r a t e ( e s p e c i a l l y i n Western Europe) and an adequate p r i c i n g of proppants and s e r v i c e based on h i g h t e c h n i c a l q u a l i t y , b o t h Western and E a s t e r n Europe a r e growing s t i m u l a t i o n mark e t s w i t h i n c r e a s i n g demand o f i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s f o r hyd r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i n c l u d i n g p r o m o t i o n o f t r e a t m e n t s i n marg i n a l r e s e r v o i r s i n t h e near f u t u r e ( a s t a b l e h i g h e r o i l p r i c e i s c o n s i d e r e d t o b e n e f i t a l l p r o d u c i n g c o u n t r i e s i n c l u d i n g OPEC members, because d u r i n g t h e 1986 c r i s i s , even Saudi A r a b i a has suspended some o f i t s f u t u r e c a p i t a l e x p e n d i t u r e programmes; GRIFFITHS 1986). C a r e f u l m a r k e t i n g approach w i t h s t r a t e g i e s i n t e g r a t i n g geological, r e s e r v o i r e n g i n e e r i n g , t e c h n i c a l and economical concepts a r e e s s e n t i a l f o r d e v e l o p i n g o f new a r e a l markets where so f a r s t i m u l a t i o n has n o t had such a s i g n i f i c a n c e as i n p a r t s o f Western Europe o r i n t h e USA. Proppant m a r k e t i n g f o r h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i s i n Europe n o t so much depending on s c i e n t i f i c , t e c h n i c a l and e n g i n e e r i n g aspects o f p r o p p a n t c h o i c e f o r t h e s p e c i f i c a p p l i c a t i o n , b u t r e q u i r e s i n comparison w i t h t h e USA much more d i v e r s i f i c a t i o n due t o d i f f e r e n t economical, f i n a n c i a l and mental i t y - d e r i v e d aspects i n t h e spectrum o f c o u n t r i e s speaking n o t o n l y a s u i t e o f d i f f e r e n t languages, b u t h a v i n g a l s o more o r l e s s d i f f e r e n t s t r a t e g i c a l p l a n n i n g and p r a c t i c a l e x e c u t i o n o f hydrocarbon e x p l o i t a t i o n . The main t a s k o f t e c h n i c a l l y o r i e n t e d m a r k e t i n g concepts f o r a p p l i c a t i o n o f p r o p p a n t s i n t h e n e a r f u t u r e i s p r o m o t i o n a l o n g t h e l i n e s o f t e c h n o l o g i c a l p o s s i b i l i t i e s o f enhancement o f economical f e a s i b i l i t y o f m a r g i n a l o i l - and g a s - b e a r i n g f o r m a t i o n s ( c h a p t e r 4 ) which w i l l have t o c o n t r i b u t e i n i n c r e a s i n g amounts t o t h e o v e r a l l product i o n w i t h continuing decline o f the supply from conventional r e s e r v o i r s .
229
3 . R o t l i e g e n d 3.0.Summary
a n d
S t i m u l a t i o n
a s s o c i a t e d
t a b l e
i n o f
E u r o p e c o n t e n t s
The R o t l i e g e n d (Lower Permian) c o n t i n e n t a l r e d bed r e s e r v o i r complex c o n t a i n i n g l a r g e amounts o f gas i n B r i t i s h and Dutch N o r t h Sea as w e l l as N e t h e r l a n d s and Germany FRG onshore has t h e g r e a t e s t h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l o f a l l European h y d r o c a r b o n - b e a r i n g f o r m a t i o n s . Numerous MHF t r e a t m e n t s i n t i g h t R o t l i e g e n d gas sandstone pay h o r i z o n s have a l r e a d y been c a r r i e d o u t d u r i n g t h e l a s t t e n y e a r s and w i l l be performed i n t h e n e a r f u t u r e , w i t h p r o p p a n t q u a n t i t i e s o f up t o 650 t p e r w e l l b e i n g i n j e c t e d i n t o t h e h y d r a u l i c a l l y induced c r a c k s d u r i n g s t i m u l a t i o n j o b s w h i c h c o s t up t o s e v e r a l Mio. US $. The economic a l s i g n i f i c a n c e o f R o t l i e g e n d p r o p p a n t f r a c t u r i n g i s u n d e r l i n e d by t h e f a c t t h a t a b t . 100 B i l l . m3 ( 4 T c f ) and 500 B i l l . m3 (20 T c f ) gas r e s e r v e s i n Germany FRG and N o r t h Sea, r e s p e c t i v e l y , can o n l y be a c q u i s i t e d and f e a s i b l y produced by l a r g e - s c a l e s t i m u l a t i o n o f t h e t i g h t pay zones. S u c c e s s f u l jumbo t r e a t ments a r e a b l e t o v e r i f y and c o n f i r m up t o 500 Mio. m3 (20 B c f ) o f a d d i t i o n a l gas r e s e r v e s , and t h i s f a s t a m o r t i z a t i o n i s t h e most pronounced j u s t i f i c a t i o n f o r t h e performance o f s u p e r g i a n t s t i m u l a t i o n o p e r a t i o n s i n R o t l i e g e n d and a l s o C a r b o n i f e r o u s t i g h t p r o s p e c t i v e i n t e r v a l s . The h y d r a u l i c p r o p p a n t f r a c t u r i n g pot e n t i a l o f R o t l i e g e n d and a l s o C a r b o n i f e r o u s r e s e r v o i r s , t o m i n o r amounts a l s o B u n t s a n d s t e i n (Lower T r i a s s i c ) pay zones, i s i n f l u e n c e d by v a r i o u s g e o l o g i c a l and p e t r o p h y s i c a l f a c t o r s g i v i n g r i s e t o a zoned o r g a n i z a t i o n o f r e s e r v o i r qual i t y and t h u s a l s o s t i m u l a t i o n n e c e s s i t y . D i s t i n c t i o n can be made between p r i m a r y p e t r o p h y s i c a l z o n a t i o n a c c o r d i n g t o r e s e r v o i r r o c k p e r m e a b i l i t y , secondary geochemical z o n a t i o n due t o gas c o m p o s i t i o n , and t e r t i a r y p o l i t i c a l z o n a t i o n o f R o t l i e g e n d gas r e s e r v o i r s w i t h f r a c t u r i n g p o t e n t i a l d i f f e r e n c e s as a consequence o f o r g a n i z a t i o n and dynamics o f t h e economical and a d m i n i s t r a t i v e systems i n t h e t e r r i t o r i e s where t h e gas f i e l d s a r e l o c a t e d . C o n s i d e r a b l e shares o f i n e r t n i t r o g e n accompanying t h e c o m b u s t i b l e methane i s t h e m a j o r geochemical drawback o f r e s e r v o i r p r o s p e c t i v i t y and t h u s a l s o h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l i n s e v e r a l areas o f t h e R o t l i e g e n d B a s i n c o m p r i s i n g p a r t i c u l a r l y German N o r t h Sea and Germany GDR onshore. The R o t l i e g e n d i n Germany FRG i s s t r a t i g r a p h i c a l l y d i v i d e d i n t o t h r e e pay s t o r e y s w i t h d i f f e r e n t p r i m a r y p e t r o p h y s i c a l pot e n t i a l a n d / o r secondary r e s e r v o i r e n g i n e e r i n g r e q u i r e m e n t o f h y d r a u l i c p r o p p a n t f r a c t u r i n g w i t h i n t h e Weser-Elbe p r o v i n c e o r E a s t Hannover f a c i e s zone (Wechselfolge f a c i e s b e l t ) . The Schneverdingen-Formation c o n t a i n i n g t h e Schnev e r d i n g e n - S a n d s t e i n a t t h e base o f t h e R o t l i e g e n d succession i n Germany FRG i s e q u i v a l e n t t o t h e Lower S l o c h t e r e n - F o r m a t i o n i n N e t h e r l a n d s and Dutch N o r t h Sea, and t h e Emden-Formation i n c l u d i n g t h e H a u p t s a n d s t e i n i n t h e m i d d l e s e c t i o n o f t h e R o t l i e g e n d p r o f i l e i n Germany FRG c o r r e l a t e s w i t h t h e Upper S l o c h t e r e n F o r m a t i o n i n N e t h e r l a n d s and D u t c h N o r t h Sea. The Hannover-Formation c o n t a i n i n g t h e Wustrow-Sandstein and v a r i o u s o t h e r Wechselfolge sandstones a t t h e t o p o f t h e R o t l i e g e n d s e r i e s i n Germany FRG corresponds w i t h t h e Ten Boer-Formation i n N e t h e r l a n d s and Dutch N o r t h Sea. The a e o l i a n dune Schneverdingen-Sandstein ( a b t . 20 - 50 m i l l i d a r c y ) i n t h e l o w e r p a r t and t h e f l u v i a l Wustrow-Sandstein ( a b t . 5 - 10 m i l l i d a r c y ) i n t h e upper s e c t i o n o f t h e R o t l i e g e n d sequence o f t h e Weser-Elbe f a c i e s p r o v i n c e i n Germany FRG a r e g e n e r a l l y moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s n o t r e q u i r i n g s t i m u l a t i o n , whereas t h e a e o l i a n and p l a y a sand f l a t H a u p t s a n d s t e i n (below 1 m i l l i d a r c y down t o a b t . 0.5 - 1 m i c r o d a r c y ) i n t h e m i d d l e p o r t i o n i s m a i n l y a l o w - p e r m e a b i l i t y t o t i g h t r e s e r v o i r and R o t l i e g e n d MHF t r e a t m e n t s a r e u s u a l l y c a r r i e d o u t i n t h i s h o r i z o n . I n t h e B r i t i s h N o r t h Sea, d i s t i n c t i o n i s made between two l a t e r a l l y i n t e r t o n g u i n g u n i t s i n t h e R o t l i e g e n d s t r a t i g r a p h i c a l complex b e i n g t h e Leman Sandstone F o r m a t i o n w h i c h c o n s i s t s o f c l e a n a e o l i a n and f l u v i a l sandstones a t t h e s o u t h e r n and w e s t e r n bas i n margins, and t h e i n t e r d i g i t a t i n g S i l v e r p i t F o r m a t i o n i n t h e b a s i n c e n t r e t h a t i s b u i l t up o f m a r g i n a l a e o l i a n / p l a y a and l a c u s t r i n e sandstones, s i l t s t o nes, mudstones and e v a p o r i t e s . The R o t l i e g e n d r e s e r v o i r complexes i n t h e Mid-European B a s i n ( S o u t h e r n Permian B a s i n ) a r e m a i n l y c o n c e n t r a t e d t o t h e a e o l i a n f a c i e s b e l t s e p a r a t i n g t h e m a r g i n a l a l l u v i a l - f a n c h a i n and b r a i d e d r i v e r p l a i n
230
from the central playa sand flat and desert lake area. While cross-stratified aeolian dune sands and horizontal-laminated dry sheet sands represent excellent reservoirs usually not necessitating permeability improvement by hydraulic proppant fracturing, tight facies units originate by increasing humidity in the depositional area of a damp to wet playa sand and mud flat in the Wechselfolge facies belt, with the finer-grained and poorer-sorted intercalations chiefly deteriorating vertical permeability. The main primary-depositional features of permeability destruction that are secondarily requiring enhancement of transmissibility by hydraulic proppant fracturing are adhesion of sand and mud during transitional stages between dry, damp and wet playa and floodplain surfaces as well as sedimentation of thin mud drapes as a consequence of interdune and overbank sandflat flooding by fluvial invasion and atmospherical precipitation. Reservoir quality is influenced by facies development and changes as a consequence of palaeoenvironmental, palaeoclimatological and palaeotectonical history of the Rotliegend Basin. In some areas, the facies complex succession also reflects an evolution of fluvial style including progressive expansion of the alluvial braided-river system within the depositional basin as the marginal fan chains and the associated aeolian dune belt shrink in extension or retreat further towards the border of the sedimentary area as a consequence of basin enlargement by stepwise onlap on the fringing highlands with successive infilling of the trough. Progressive diminution of the degree of braiding of the river systems as the erosional highlands in the provenance area are denudated, decreasing palaeoslope gradient, declining amount and grain size of clastic material delivered from the source region, and ceasing transport capacity of the channel network give rise to enlargement of interstream overbank flats where upon desiccation and reasonable persistance of quiet aquatic sedimentary conditions migration of aeolian dunes and accumulation of aeolian sheet sands from the detritus deflated from emerged sand and gravel bars in the shallow to moderately deep braided river channels is becoming more and more widespread, or reversely is diminishing in importance in case of successively increasing frequency and extension of alluvial inundations by flood pulses overtopping the channel banks at high stage in combination with high ground water level as a consequence of higher stream discharge and increasing precipitation rates once climatological conditions are becoming damper and wetter. Apart from the primary depositional facies zonation controlling together with the diagenetical history predominantly of clay minerals the distribution of petrophysical properties, an anisotropical arrangement of hydraulic proppant fracturing potential i s also created by the gas composition within the Rotliegend reservoir rocks. This secondary geochemical zonation originates from the admixture of inert nitrogen to the combustible hydrocarbons in some areas of the Rotliegend Basin, with the nitrogen content exceeding a certain boundary level no longer permitting commercial exploitation of the gas accumulations. The impact of the gas composition on fracturing potential can be modified or even compensated by economical and administrative aspects of a tertiary political zonation based on domestic hydrocarbon demand vs. supply and financial capacity. The continental-inland arid aeolian and fluvial Rotliegend is underlain by the Carboniferous terrestrial and marginalmarine humid fluvial and deltaic grey bed facies belt containing coal seams, with this formation having acted both as source of the gas during coalification and as collector by trapping considerable amounts of gas in channel and overbank sandstones. Above the Rotliegend and separated by the Zechstein marine evaporite cycle sequence including mudstones, carbonates, sulphates and salt, the Buntsandstein continental-inland arid fluvial, aeolian and lacustrine red bed complex has also captured gas having migrated from the Carboniferous coal seams through faults, joints and fissures into the fluvial sheet-sand reservoirs. In both Rotliegend and Buntsandstein, the primary depositional facies zonation was in various parts of the basin unfavourably overprinted by secondary tectonical deformation and subsidence. The high-quality reservoir belt in the southern part o f the area is still at the surface or within shallow depth and therefore is not able to retain gas, whereas the moderate- to poor-quality pay zone seam in the northern part has been buried in several thousands of metres depth where structural traps are well sealed and allowed the accumulation of large gas fields. This combination of poor reservoir facies buried in suitable depth for
231 hydrocarbon accumulation and t r a p p i n g i n c o n t r a s t t o t h e good pay f a c i e s occurr i n g i n s h a l l o w d e p t h o r even a t t h e s u r f a c e where gas c o n c e n t r a t i o n i s n o t poss i b l e due t o l a c k o f s e a l i n g and t r a p p i n g u n d e r l i n e s t h e s i g n i f i c a n c e o f t h e R o t l i e g e n d and B u n t s a n d s t e i n d i s t a l - m e d i a l t i g h t r e s e r v o i r f a c i e s b e l t which c r o s s e s t h e n o r t h e r n p a r t o f M i d d l e Europe and t h a t r e p r e s e n t s t h e most p o t e n t i a l and i m p o r t a n t b e l t o f h y d r a u l i c p r o p p a n t f r a c t u r i n g . R o t l i e g e n d gas accumul a t i o n s i n Germany FRG a r e f r e q u e n t l y f o u n d beneath Z e c h s t e i n s a l t p l u g s where t h e l o w e r t o t a l geopressure and t h e r e t a r d e d h e a t f l u x w i t h l o w e r r e s e r v o i r temp e r a t u r e o f t e n p r e s e r v e d b e t t e r p o r o s i t i e s t h a n i n t h e r e g i o n s between t h e s a l t domes. A l l t h e f o u r d i f f e r e n t sedimentary g e o l o g i c a l f o r m a t i o n s have been l a i d down i n t e m p o r a l l y s e p a r a t e d e p o s i t i o n a l b a s i n s which h o r i z o n t a l l y occupied more o r l e s s t h e same g e o g r a p h i c a l area, r e s u l t i n g i n t h e i r v e r t i c a l superimpos i t i o n i n t h e s t r a t i g r a p h i c a l column d u r i n g course o f b u r i a l h i s t o r y . L a t e r a l l y , t h e v a r i o u s i n d i v i d u a l b a s i n s a r e n o t homogeneous, b u t i n c o n t r a s t t o p a r t i a l l y good c o r r e l a t i o n a l o n g s t r i k e s i g n i f i c a n t f a c i e s d i f f e r e n c e s o c c u r p e r p e n d i c u l a r l y t o s t r i k e , and a l s o p a r a l l e l t o t h e b a s i n margins, f r e q u e n t l y cons i d e r a b l e d i v e r s i f i c a t i o n o f d e p o s i t i o n a l environment l e a d i n g t o complex j u x t a p o s i t i o n o f d i f f e r e n t g e n e t i c a l sedimentary bodies i s encountered. These r e l a t i o n s h i p s g i v e r i s e t o v e r t i c a l and h o r i z o n t a l z o n a t i o n o f d i f f e r e n t r e s e r v o i r complexes and t o g e t h e r w i t h e f f e c t s o f hydrocarbon g e n e r a t i o n , m i g r a t i o n and acc u m u l a t i o n a l s o t o segregated arrangement o f n a t u r a l gas d e p o s i t s w i t h i n t h e heterogeneous and a n i s o t r o p i c pay zone networks a c c o r d i n g t o t h e i n t e r n a l geomet r i c a l o r g a n i z a t i o n o f t h e l a t t e r . The d i s c u s s i o n o f t e c h n i c a l and m a r k e t i n g asp e c t s o f R o t l i e g e n d s t i m u l a t i o n i n Europe i n c l u d e s h o r i z o n t a l and v e r t i c a l d i s t r i b u t i o n o f main f i e l d s , h i s t o r i c a l and a r e a l development o f p r o d u c t i o n and f r a c t u r i n g , p e r m e a b i l i t y p r e d i c t i o n and e f f e c t i v e p e r m e a b i l i t y , p e r m e a b i l i t y det e r i o r a t i o n and c l a y m i n e r a l d i s t r i b u t i o n , minimum p r e - f r a c t u r i n g gas product i o n r a t e and poroperm p r o p e r t i e s , s t i m u l a t i o n c o s t and f r a c t u r i n g p o l i c y , p h i losophy o f o f f - and onshore e x p l o r a t i o n w e l l f r a c t u r i n g , i m p l i c a t i o n s o f f r a c t u r i n g p h i l o s o p h y on p r o p p a n t s e l e c t i o n , h o r i z o n t a l and v e r t i c a l d i f f e r e n c e s o f f r a c t u r i n g p o t e n t i a l , w a t e r s e n s i t i v i t y and o t h e r aspects. The e x p e r i e n c e o f MHF s t i m u l a t i o n o f b o t h R o t l i e g e n d and C a r b o n i f e r o u s sandstones i n t h e l a s t t e n y e a r s has shown t h a t u n l e s s a c e r t a i n minimum p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e i s p r e s e n t , no economical p r o d u c e r can be achieved by c a r r y i n g o u t a hyd r a u l i c f r a c t u r i n g o p e r a t i o n . D u r i n g course o f f i e l d development campaigns, d i s t i n c t i o n can be made between s p o t f r a c t u r i n g c o m p r i s i n g s e q u e n t i a l l y d r i l l i n g , t e s t i n g and s t i m u l a t i o n o f one w e l l a f t e r t h e o t h e r , and b a t c h f r a c t u r i n g meani n g f i r s t d r i l l i n g o f a l l t h e w e l l s one a f t e r t h e o t h e r and t h e n second i n ano t h e r round p e r f o r a t i n g , s t i m u l a t i n g and p u t t i n g on stream o f t h e whole sequence o f w e l l s s u c c e s s i v e l y . S e r v i c e b o a t c a p a c i t y i s o f t e n t h e l i m i t i n g f a c t o r f o r s t i m u l a t i o n i n t h e Southern N o r t h Sea. Some p a r t s o f t h e R o t l i e g e n d pay s e r i e s a r e e x t r e m e l y w a t e r - s e n s i t i v e as a r e s u l t o f t h e c l a y m i n e r a l c o n s t e l l a t i o n , w i t h foamed o i l - b a s e d f r a c t u r i n g f l u i d s b e i n g s u i t a b l e c a r r i e r s f o r t r e a t ment o f such r e s e r v o i r s . Carbon d i o x i d e o r n i t r o g e n can be used as foaming o r e n e r g i z i n g agents i n such f l u i d s , and s u c c e s s f u l s t i m u l a t i o n can a l s o be p e r f o r med w i t h u t i l i z a t i o n o f c r o s s l i n k e d a c i d systems and low-pH methanol systems. C o n c l u s i o n i s made t h a t t h e case s t u d y o f R o t l i e g e n d t i g h t gas r e s e r v o i r s r e f l e c t s t h e complex problems o f t e r r e s t r i a l l o w - p e r m e a b i l i t y pay zones and h e l p s t o understand t h e s u i t a b l e approaches o f t i g h t gas r e s e r v e s which w i l l c o n t r i b u t e t o t h e t o t a l w o r l d energy s u p p l y i n i n c r e a s i n g amounts d u r i n g t h e coming y e a r s . I m p o r t a n t sedimentary s t r u c t u r e s o f f l u v i a l , a e o l i a n and l a c u s t r i n e depos i t s i n R o t l i e g e n d and B u n t s a n d s t e i n i l l u s t r a t i n g d i s t r i b u t i o n and o r i g i n o f r e s e r v o i r h e t e r o g e n e i t i e s and n a t u r a l f r a c t u r e s as w e l l as i l l u m i n a t i n g t h e s i g n i f i c a n c e o f t h e s e f e a t u r e s f o r p r o p a g a t i o n o f h y d r a u l i c a l l y induced f r a c t u r e s and f u n c t i o n as f l o w b a r r i e r s o r p r e f e r e n t i a l c u r r e n t p a t h s a r e f i g u r e d i n e l e ven p h o t o g r a p h i c p l a t e s t h a t a r e accompanied by d e t a i l e d e x p l a n a t i o n s w h i c h can be e v a l u a t e d independent f r o m t h e main t e x t . 3.1.
I n t r o d u c t i o n ........................................................ 3.1.1. General aspects 3.1.2. Economical aspects .......................................... 3.1.2.1. R o t l i e g e n d f r a c t u r i n g p o l i c y and u n d e r s t a n d i n g ....
.............................................
236 236 237 237
232 3.1.2.2.
3.1.3.
3.2.
F r a c t u r i n g j o b s i z e s and reserve a c q u i s i t i o n potent i a l .............................................. 3.1.2.2.1. S t i m u l a t i o n treatment volumes and expenses ................................ 3.1.2.2.2. Probably a c c e s s i b l e gas reserves ...... F a c t o r s i n f l u e n c i n g the f r a c t u r i n g p o t e n t i a l o f R o t l i e g e n d gas r e s e r v o i r s ..............................................
238 238 238 240
Distribution o f main f i e l d s ......................................... 240 3.2.1. V e r t i c a l d i s t r i b u t i o n ....................................... 241 3.2.1.1. D e p o s i t i o n a l environment .......................... 241 R e s e r v o i r q u a l i t y and sedimentary f a 3.2.1.1.1. cies 244 3.2.1.1.2. P a l a e o c l i m a t o l o g i c a l e v o l u t i o n ........ 244 3.2.1.1.3. Pal a e o t e c t o n i c a l and p a l aeoenvironment a l e v o l u t i o n ......................... 244 245 3.2.1.1.4. Other aspects ......................... 3.2.1.2. T e c t o n i c a l subsidence and gas accumulation 245 3.2.1.2.1. Sedimentary vs t e c t o n i c a l b a s i n zona245 t i o n .................................. 3.2.1.2.2. Gas-water-contact l e v e l ............... 246 246 3.2.1.3. S t r a t i g r a p h y ...................................... 3.2.1.3.1. Germany FRG ........................... 246 247 3.2.1.3.2. Netherlands ........................... 3.2.1.3.3. B r i t i s h Southern N o r t h Sea 247 3.2.1.4. Gas composition ................................... 247 248 3.2.2. H o r i z o n t a l d i s t r i b u t i o n ..................................... 3.2.2.1. Germany FRG ....................................... 248 248 3.2.2.2. B r i t i s h and Dutch N o r t h Sea ....................... 3.2.2.3. Germany GDR and Poland ............................ 249 3.2.3. Other aspects ............................................... 249
..................................
.
........
............
3.3.
H i s t o r i c a l development o f production and fracturing ................. 249 250 3.3.1. B r i t i s h N o r t h Sea ........................................... 3.3.1.1. Treatment s i z e and proppant q u a n t i t y 250 3.3.1.2. O f f s h o r e f r a c t u r i n g technology and s t i m u l a t i o n b o a t 250 3.3.1.3. S i g n i f i c a n c e o f N o r t h Sea r e s e r v e f o r Great B r i t a i n 250 3.3.2. Germany FRG 251 251 3.3.2.1. Evolution o f gas-field discovery 3.3.2.2. Treatment s i z e and proppant q u a n t i t y .............. 251 252 3.3.3. Dutch N o r t h Sea ............................................. 252 3.3.4. German N o r t h Sea ............................................ 3.3.5. Other areas ................................................. 252
..............
................................................. ..................
3.4.
P e m a b i l i t y prediction and e f f e c t i v e permeability .................. 253 3.4.1. T i g h t r e s e r v o i r p e r m e a b i l i t y range .......................... 253 3.4.1.1. Sedimentological and d i a g e n e t i c a l e v o l u t i o n o f t i g h t gas sands ................................... 254 254 3.4.1.1.1. B l a n k e t sandstones .................... 3.4.1.1.2. L e n t i c u l a r sandstones ................. 254 3.4.1.1.3. P o r o s i t y and p e r m e a b i l i t y r e l a t i o n s h i p s 255 3.4.1.1.4. R e s e r v o i r pressure and f o r m a t i o n damage 255 3.4.1.2. Microdarcy pay zone p e r m e a b i l i t y 255 3.4.1.2.1. Absolute p e r m e a b i l i t y l e v e l s 256 3.4.1.2.2. P e r m e a b i l i t y c r e a t i o n ................. 256 257 3.4.1.3. P e r m e a b i l i t y s t r a t i g r a p h y ......................... 3.4.1.4. Gas and f l u i d f l o w i n t i g h t rock m a t r i x ........... 257 3.4.2. P e r m e a b i l i t y d e t e r m i n a t i o n and p r e d i c t i o n ................... 257 3.4.2.1. P e r m e a b i l i t y d e t e r m i n a t i o n ........................ 258 258 3.4.2.2. P e r m e a b i l i t y p r e d i c t i o n ........................... 3.4.2.3. P e r m e a b i l i t y v a r i a t i o n s ........................... 258 3.4.3. Temporary damage and r e s t o r a t i o n o f p e r m e a b i l i t y ............ 259
.................. ..........
233
3.5.
permeability deterioration and clay mineral distribution 3.5.1.
3.5.2.
3.6.
Minimum pre-fracturing gas production rate and poropenn properties 3.6.1.
3.6.2.
3.7.
3.7.1.
3.8.2.
3.9.
..............................
3.9.2.
3.9.3.
......... 273
Exploration strategy and stimulation testing ................ 273 Proppant cost containment and tail-in policy ...... 273 3.9.1.1. 3.9.1.2. Proper interpretation of reservoir capacity ....... 274 Field experience and development concepts ................... 274 3.9.2.1. Advantages of intermediate-strength proppant insertion .............................................. 274 3.9.2.2. Retardation of Carboniferous strikes .............. 275 Appraisal drilling strategy concepts and experience ......... 275
3.10. Horizontal and vertical differences of fracturing potential 3.10.1.
.......... 270
Philosophy of offshore exploration well fracturing .......... 270 3.8.1.1. Well evaluation and fracture design time .......... 271 Service boat capacity limit ....................... 271 3.8.1.2. Philosophy of onshore exploration well fracturing ........... 272 3.8.2.1. Job suspension vs . cost writing-off ............... 272 Changing aims of hydraulic fracturing ............. 272 3.8.2.2.
Implications of fracturing philosophy on proppant selection
3.9.1.
267
Fracturing policy ........................................... 268 Impact of fracturing boats on stimulation strategy 268 3.7.1.1. Reservoir depth and stimulation expenses .......... 268 3.7.1.2. Pricing scenario ............................................ 269 Fracturing treatment sequence strategy ...................... 269 Spot fracturing ................................... 269 3.7.3.1. 3.7.3.2. Batch fracturing .................................. 270
Philosophy of off- and onshore exploration well fracturing 3.8.1.
.. 266
Pre-fracturing economical feasibility estimation ............ 266 3.6.1.1. Minimum fracture flow capacity and gas offtake rate 266 Minimum reservoir properties for profitable stimu3.6.1.2. lation ............................................ 267 3.6.1.3. Stimulation rentability classification ............ 267 Differences between exploration and development wells ....... 267
Stimulation cost and fracturing policy
3.7.2. 3.7.3.
3.8.
............ 259
Clay mineralogy and morphology .............................. 260 Clay morphology ................................... 260 3.5.1.1. 3.5.1.2. Clay mineralogy ................................... 260 Detrital and authigenic clay minerals ............. 261 3.5.1.3. 3.5.1.4. Other aspects ..................................... 261 Permeability deterioration .................................. 262 Clay mineral dispersion and water sensitivity ..... 262 3.5.2.1. 3.5.2.1.1. Clay mineral dispersion control ....... 262 Clay mineral constellation and permea3.5.2.1.2. bility damage ......................... 263 3.5.2.1.3. Water- vs . oil-based stimulation fluids 263 3.5.2.2. Clay mineral stabilization ........................ 264 Low- and high-pH solutions ............ 264 3.5.2.2.1. 3.5.2.2.2. Methanol prepad for illite preconditioning .................................. 264 3.5.2.3. Clay mineral distribution and log interpretation .. 264 3.5.2.3.1. Porosity evaluation ................... 265 3.5.2.3.2. Deep filtrate invasion in tight sandstones ................................ 265 Negative orientation of permeability changes ...... 265 3.5.2.4. 3.5.2.5. Capillary pressure and permeability relationships . 265
......... 276
Permeability ................................................ 3.10.1.1. Depositional environment and grain size distribu-
276
234 t i o n .............................................. P e r m e a b i l i t y s t r a t i g r a p h y ......................... Sedimentary f a c i e s and p r o p p a n t f r a c t u r i n g p o t e n t i a l 3.10.1.3.1. Aeolian sediments ..................... 3.10.1.3.2. F l u v i a l and a l l u v i a l - f a n d e p o s i t s 3.10.1.3.3. L a c u s t r i n e sediments and c a l c r e t e palaeosols ............................ 3.10.1.3.4. Lateral continuity o f reservoir u n i t s . Gas composition ............................................. 3.10.2.1. N i t r o g e n c o n t e n t .................................. 3.10.2.2. Impact o f economical systems on g a s - f i e l d o p e r a t i o n 3.10.1.2. 3.10.1.3.
.....
3.10.2.
277 277 278 278 278 279 279 280 280 281
................................................... 281 281 General aspects ............................................. 3.11.1.1. Water s e n s i t i v i t y d i s t r i b u t i o n i n t h e R o t l i e g e n d .. 282 3.11.1.2. S i g n i f i c a n c e o f expandable c l a y m i n e r a l s .......... 282 Foam-fracturing s t i m u l a t i o n ................................. 283 3.11.2.1. Proppant t r a n s p o r t c a p a c i t y and foam s t a b i l i t y .... 283 3.11.2.1.1. Impact o f proppant a d d i t i o n on foam s t a b i l i t y and f l u i d v i s c o s i t y ......... 284 3.11.2.1.2. Rheological c h a r a c t e r i s t i c s o f foam systems ............................... 284 3.11.2.1.3. W a l l - b u i l d i n g o f g e l - b e a r i n g foams . . . . 285 3.11.2.2. Carbon d i o x i d e vs . n i t r o g e n as foaming o r e n e r g i 285 z i n g agent ........................................ 3.11.2.2.1. F l u i d v i s c o s i t y and c o m p a t i b i l i t y 285 3.11.2.2.2. F l u i d s o l u b i l i t y and proppant t r a n s p o r t 286 3.11.2.2.3. Foam d e n s i t y and r e c o v e r a b i l i t y ....... 286 3.11.2.2.4. Rheological n a t u r e and t r a n s i t i o n o f foams and emulsions ................... 286 3.11.2.3. Flow behaviour and foam v i s c o s i t y ................. 287 3.11.2.4. F l u i d recovery and w e l l cleanup ................... 287 3.11.2.4.1. F r a c t u r i n g f l u i d r e t e n t i o n ............ 288 3.11.2.4.2. Water b l o c k i n g ........................ 288 289 3.11.2.5. Other aspects ..................................... C r o s s l i n k e d a c i d systems .................................... 289 289 Methanol systems ............................................ L i q u i d carbon d i o x i d e proppant f r a c t u r i n g ................... 290 3.11.5.1. F l u i d v i s c o s i t y , foam d e n s i t y and s o l u t i o n pH-level 290 291 3.11.5.2. Proppant c o n c e n t r a t i o n s ........................... 291 3.11.5.3. Staged f r a c t u r i n g treatments ...................... 3.11.5.4. Formation damage e l i m i n a t i o n ...................... 291 3.11.5.5. Pumping r a t e and bottom h o l e temperature .......... 292
3.11. Water s e n s i t i v i t y 3.11.1.
3.11.2.
.....
3.11.3. 3.11.4. 3.11.5.
3.12. Other aspects 3.12.1. 3.12.2. 3.12.3.
.......................................................
Water s a t u r a t i o n ............................................ Gas demand .................................................. R e s e r v o i r performance .......................................
292 292 293 293
3.13. Sedimentary s t r u c t u r e s o f f l u v i a l , a e o l i a n and l a c u s t r i n e d e p o s i t s i n R o t l i e g e n d and Buntsandstein i l l u s t r a t i n g d i s t r i b u t i o n o f r e s e r v o i r h e t e r o g e n e i t i e s and n a t u r a l f r a c t u r e s ............................... 3.13.0. I n t r o d u c t i o n ............................................... 3.13.1. Angular u n c o n f o r m i t i e s and basement f o l d i n g ................ 3.13.1.1. General aspects ................................. 3.13.1.2. Explanations o f i l l u s t r a t i o n s on p l a t e I ........ 3.13.1.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. 3.13.2. C r o s s - s t r a t i f i c a t i o n i n a e o l i a n dune sands and f l u v i a l chann e l bar d e p o s i t s ........................................... 3.13.2.1. General aspects ................................. 3.13.2.2. Explanations of i l l u s t r a t i o n s on p l a t e I1 .......
294 294 295 296 296 298 299 299 299
235 3.13.2.3.
3.13.3.
3.13.4.
3.13.5.
Geological formations and outcrop l o c a l i t i e s o f presented examples Gravel agglomerations and mud drapes i n a l l u v i a l - f a n brecc i a s and f l u v i a l channel sandstones 3.13.3.1. General aspects ................................. 3.13.3.2. Explanations o f i l l u s t r a t i o n s on p l a t e I11 3.13.3.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. N a t u r a l f r a c t u r e s and sedimentary h e t e r o g e n e i t i e s i n f l u v i a l channel and f l o o d p l a i n sandstones and mudstones 3.13.4.1. General aspects 3.13.4.2. Explanations o f i l l u s t r a t i o n s on p l a t e I V ....... 3.13.4.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. G r a i n - s i z e d i s t r i b u t i o n changes and e r o s i o n a l surfaces i n b r a i d e d - r i v e r channel sandstones and conglomerates ......... 3.13.5.1. General aspects ................................. 3.13.5.2. Explanations o f i l l u s t r a t i o n s on p l a t e V 3.13.5.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. Synsedimentary d e s i c c a t i o n f r a c t u r i n g i n l a c u s t r i n e and f l u v i a l f l o o d p l a i n mudstones and n a t u r a l propping o f cracks ... 3.13.6.1. General aspects ................................. 3.13.6.2. Explanations o f i l l u s t r a t i o n s on p l a t e VI 3.13.6.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. L a c u s t r i n e and f l o o d p l a i n mud drapes i n f l u v i a l channel and a l l u v i a l p l a y a sandstones .................................. 3.13.7.1. General aspects ................................. 3.13.7.2. Explanations o f i l l u s t r a t i o n s on p l a t e VII ...... 3.13.7.3. Geological f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f presented examples .............................. N a t u r a l f r a c t u r e s and sedimentary h e t e r o g e n e i t i e s i n crosss t r a t i f i e d a e o l i a n dune and sheet sands as w e l l as f l u v i a l channel sands .............................................. 3.13.8.1. General aspects ................................. 3.13.8.2. Explanations o f i l l u s t r a t i o n s on p l a t e VIII ..... 3.13.8.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. Sedimentary and deformational f e a t u r e s o f l a c u s t r i n e mud drapes i n f l u v i a l channel, overbank and a l l u v i a l p l a y a sandstones ..................................................... 3.13.9.1. General aspects ................................. 3.13.9.2. Explanations o f i l l u s t r a t i o n s on p l a t e IX ....... 3.13.9.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. Root tubes and carbonate c o n c r e t i o n s i n c a l c r e t e palaeosols 3.13.10.1. General aspects ................................. 3.13.10.2. Explanations o f i l l u s t r a t i o n s on p l a t e X 3.13.10.3. Geological formations and outcrop l o c a l i t i e s o f presented examples .............................. N a t u r a l f r a c t u r e o r i g i n and d i s t r i b u t i o n i n v a r i o u s r e s e r v o i r rocks ................................................. 3.13.11.1. General aspects ................................. 3.13.11.2. Explanations o f i l l u s t r a t i o n s on p l a t e XI ....... 3.13.11.3. Geological formations and outcrop l o c a l i t i e s o f presented examples ..............................
.............................. 302 ........................ 302 302 ...... 302
............ 305 ................................. 305
........
3.13.6.
.......
3.13.7.
3.13.8.
3.13.9.
3.13.10.
........
3.13.11.
304
305 306
308 308 308 310 310 311 311 312 314 314 314 317 318 318 318 321 321 321 322 324 324 325 325 326 328 328 328 330
236
3.1. Introduction H y d r a u l i c proppant f r a c t u r i n g i n Western and Eastern Europe i s p r e s e n t l y i n the s t a t u s o f r e t h i n k i n g and r e c o n s t r u c t i o n . I n Western Europe, t h e major recess i o n o f s t i m u l a t i o n o f o i l - and gas-bearing formations provoked by t h e l a t e 1985/early 1986 o i l p r i c e drop ( c f . s e c t i o n 2.2.1.2.; n e g a t i v e l y accentuated by the accompanying US $ exchange r a t e f a l l which was p a r t i a l l y r e s p o n s i b l e f o r the p r o l o n g a t i o n o f t h e unfavourable general economical c o n s t e l l a t i o n u n t i l l a t e 1987 when the US $ exchange r a t e reached i t s h i s t o r i c a l minimum l e v e l ; c f . s e c t i o n 2.2.1.3.) f o l l o w i n g the golden y e a r s o f h y d r a u l i c proppant f r a c t u r i n g i n the l a t e 1 9 7 0 ' s l e a r l y 1980's (VEATCH 1983, GRIFFITHS 1986, OEHME 1986, RUNGE 1986, VEATCH & M O S C H O V I D I S 1986, VIELVOYE 1986, MADER 1987) has p r o v i d e d t h e nec e s s i t y o f r e v i e w i n g and/or r e s c h e d u l i n g and p a r t i a l l y a l s o r e d e s i g n i n g t h e MHF treatments i n predominantly deep t i g h t Rot1 iegend and Carboniferous gas-bearing sandstones i n Germany FRG and Netherlands onshore as w e l l as B r i t i s h and Dutch Southern N o r t h Sea o f f s h o r e . I n Eastern Europe, the i n c r e a s i n g urgency f o r hard currency coming from exp o r t o f o i l and gas, the r i s i n g demand t o a m e l i o r a t e domestic hydrocarbon supply and t h e p o l i t i c a l r e o r g a n i z a t i o n under the guide slogans o f a c c e l e r a t i o n (uskorenie; GORBATCHEV 1987), r e o r g a n i z a t i o n and r e c o n s t r u c t i o n ( p e r e s t r o i k a ) and transparency ( g l a s n o s t ) , i n combination w i t h the more o r l e s s independency from t h e general w o r l d economical s i t u a t i o n t h a t i s d i c t a t e d by o i l p r i c e scenar i o and US $ value scenario ( c f . s e c t i o n 2.4.4.), w i l l a l r e a d y i n the near f u t u r e r e s u l t i n focussing much more on s t i m u l a t i o n o f o i l - and gas-bearing format i o n s than i n the p a s t when d r i l l i n g and f a s t completion o f e a s i e r a c c e s s i b l e hydrocarbon reserves had h i g h e r p r i o r i t y . Much promotion o f t h e h y d r a u l i c proppant f r a c t u r i n g and g r a v e l packing scener y i n Western and Eastern Europe i n the coming years, however, has t o i n c l u d e sound g e o l o g i c a l , t e c h n i c a l and r e s e r v o i r engineering concepts o f development o f i n c r e a s i n g amounts o f h i t h e r t o marginal r e s e r v o i r s , w i t h p r o d u c t i o n from these t a r g e t s becoming i n the i n d i v i d u a l c o u n t r i e s soon more o r l e s s i m p o r t a n t c o n t r i b u t i o n s f o r t h e compensation o f t h e n a t u r a l d e c l i n e o f t h e o u t p u t from many p r o g r e s s i v e l y d e p l e t e d conventional r e s e r v o i r s . R o t l i e g e n d (Lower Permian) and a l s o Carboniferous, t o subordinate amounts a l s o Buntsandstein (Lower T r i a s s i c ) t i g h t gas sandstone s t i m u l a t i o n i n Western Europe r e p r e s e n t the h i g h l i g h t s o f h y d r a u l i c proppant f r a c t u r i n g i n t h i s area b o t h i n p a s t and f u t u r e . A f t e r d i s c u s s i o n o f some general and economical aspects, f a c t o r s i n f l u e n c i n g t h e f r a c t u r i n g p o t e n t i a l o f R o t l i e g e n d gas r e s e r v o i r s are o u t l i n e d .
3.1.1. General aspects F o l l o w i n g t h e preceding o u t l i n e o f proppant s e l e c t i o n as a consequence o f r e s e r v o i r c o n d i t i o n s and proppant p r o p e r t i e s ( c h a p t e r 1) and t h e summary o f mark e t i n g and economical aspects o f h y d r a u l i c f r a c t u r i n g and g r a v e l packing i n West e r n and Eastern Europe (MADER 1987; chapter Z ) , the p r e s e n t r e p o r t emphasizes v a r i o u s g e o l o g i c a l , t e c h n i c a l and r e s e r v o i r e n g i n e e r i n g p o s s i b i l i t i e s o f enhanc i n g R o t l i e g e n d s t i m u l a t i o n p o t e n t i a l which according t o my view and understand i n g w i l l become i n c r e a s i n g l y s i g n i f i c a n t f o r a c h i e v i n g the g o a l s o f ameliorat e d supply-demand-relationships independent from any p o l i t i c a l t h r e a t e n i n g i n the n e x t years. As deep t i g h t gas-bearing R o t l i e g e n d and Carboniferous sandstones w i l l remain t o be t h e major t a r g e t s o f massive h y d r a u l i c f r a c t u r i n g (MHF) operations i n Western Europe i n the near f u t u r e , v a r i o u s f a c t o r s i n f l u e n c i n g the f r a c t u r i n g p o t e n t i a l o f these p a r t i c u l a r r e s e r v o i r s a r e i l l u s t r a t e d ( c f . f i g . 9). While many i m p o r t a n t t i g h t gas sands i n v a r i o u s basins i n t h e USA have a l r e a dy been e x t e n s i v e l y described ( a r e c e n t summary volume has been compiled by SPENCER & MAST 1986), a g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g s y n t h e s i s o f Rotl i e g e n d and Upper Carboniferous which are t h e most i m p o r t a n t t i g h t gas sand f o r -
237 mations i n Europe has h i t h e r t o n o t been presented. The purpose o f t h i s o u t l i n e i s t o p r o v i d e an i n t e g r a t e d overview o f g e o l o g i c a l , t e c h n i c a l and economical aspects o f R o t l i e g e n d and Carboniferous t i g h t gas sands i n N o r t h Sea and a d j o i n i n g onshore areas i n Europe where the predominant amount o f massive h y d r a u l i c proppant f r a c t u r i n g i n p a s t and f u t u r e i s concentrated. The present s e c t i o n i s a g e o l o g i c a l and engineering case study forming t h e t r a n s i t i o n between t h e economically i n f l u e n c e d d i v i s i o n s and the t e c h n i c a l l y focussed p a r t s o f t h e review and a c t u a l summary o f h y d r a u l i c proppant f r a c t u r i n g . The d i s c u s s i o n i n chapter 4 represents a s t a t u s r e p o r t o f h y d r a u l i c stimul a t i o n technology and emphasizes numerous methods f o r enhancing the h y d r a u l i c f r a c t u r i n g p o t e n t i a l i n Europe and o t h e r p a r t s o f t h e world. Chapter 5 g i v e s an account o f gravel packing as the second a p p l i c a t i o n o f n a t u r a l sand and synthet i c proppants i n o i l and gas i n d u s t r y , as w e l l as an overview o f o t h e r sand cont r o l techniques. U n i t 6 summarizes h y d r a u l i c proppant f r a c t u r i n g and g r a v e l packing m o n i t o r i n g . The e x t e n s i v e l i t e r a t u r e i s i n c l u d e d i n t o a general r e f e rence l i s t on h y d r a u l i c proppant f r a c t u r i n g and g r a v e l packing and i s a l s o processed i n t o s u b j e c t key s e c t i o n s i n a b i b l i o g r a p h y ( c h a p t e r 7 ) . Technical overviews o f t h e s t a t e o f t h e a r t o f h y d r a u l i c proppant f r a c t u r i n g and summary exper i e n c e r e p o r t s are g i v e n by HOWARD & FAST (1970 a), WATERS (1980), SCHOLS (1983), VEATCH (1983), ECONOMIDES (1986), VEATCH & MOSCHOVIDIS (1986), ECONOMIDES & NOLTE (1987) and GIOLEY (1988).
3.1.2. Economical aspects Several dozens o f MHF operations i n R o t l i e g e n d and Carboniferous gas-bearing sandstones have a l r e a d y been c a r r i e d o u t onshore and o f f s h o r e i n t h e Southern Permian Basin ( N o r t h Sea and Northwest Europe onshore; ZIEGLER 1982) i n t h e p a s t u n t i l 1986, and almost up t o hundred treatments o r even more are scheduled f o r t h e c u r r e n t and coming major o f f s h o r e ( B r i t i s h Southern N o r t h Sea) development campaigns (1987 - 1990; c f . s e c t i o n 2.4.1.). I n p a r t s o f t h e area, a l s o t h e Buntsandstein has s u b o r d i n a t e l y been subjected t o s t i m u l a t i o n o p e r a t i o n s ( m a i n l y i n the B r i t i s h and Dutch Southern N o r t h Sea) and w i l l a l s o have t o be f r a c t u r e d i n some more patches i n the near f u t u r e . Some comments a r e o f f e r e d as f o l l o w s on R o t l i e g e n d f r a c t u r i n g p o l i c y and understanding as w e l l as f r a c t u r i n g j o b s i z e s and reserve a c q u i s i t i o n p o t e n t i a l .
3.1.2.1. Rot 1 iegend fractur ins pol icy and understanding I n view o f t h e c u r r e n t and coming major a c t i v i t y i n 1987 - 1990
( c f . secand 2.4.5.1.), t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d b e l t t i o n s 2.2.1.6.2. ( c f . f i g . 10) i s probably t h e o n l y area i n Europe where almost according t o t h e a l r e a d y common sense i n t h e USA, h y d r a u l i c proppant f r a c t u r i n g i s one o f t h e most i m p o r t a n t aspects o f w e l l completion (ROBINSON, HOLDITCH & WHITEHEAD 1986; c f . s e c t i o n 2.3.), because t h e development d r i l l i n g campaigns have been planned w i t h the c e r t a i n t y t h a t i n case o f encountering the R o t l i e g e n d r e s e r v o i r i n t h e p r e d i c t e d o r expected t i g h t f a c i e s , almost every w e l l would have t o be f r a c t u r e d i n t h e p o o r e s t f a c i e s patches and every second o r t h i r d w e l l would require hydraulic stimulation i n other sections o f the low-permeability facies belt. I n a d d i t i o n , as a consequence o f long-term development scheduling and product i o n planning, t h e B r i t i s h Southern N o r t h Sea R o t l i e g e n d gas b e l t i s more o r l e s s independent from t h e f l u c t u a t i o n s o f o i l p r i c e and general economical f r a mework ( c f . s e c t i o n 2.2.1.6.2. and 2.2.1.6.3.2.), w i t h the a c t i v i t y having been o n l y s u b o r d i n a t e l y t o n e g l i g i b l y a f f e c t e d by the 1986 o i l p r i c e drop and having so f a r s t r a i g h t f o r w a r d proceeded t o t h e beginning o f t h e major development campaigns i n 1987 - 1990 (economical aspects o f t h i s R o t l i e g e n d gas p r o v i n c e a r e a l s o summarized by FISHMAN 1986, GREGORY 1986, THOMAS 1986, ALGAR 1987, BAND 1987 and POTTER 1987; c f . s e c t i o n 2.4.1.2.).
238
3.1.2.2. Fracturing job sizes and reserve acquisition potential The importance o f t h e R o t l i e g e n d and C a r b o n i f e r o u s r e s e r v o i r s i n terms o f b o t h g e o l o g i c a l - e n g i n e e r i n g and m a r k e t i n g aspects o f s t i m u l a t i o n as w e l l as t h e c o n s i d e r a b l e expenses o f MHF t r e a t m e n t s r e q u i r e an o u t l i n e o f a s u i t e o f f a c t o r s t h a t are p o s i t i v e l y o r negatively i n f l u e n c i n g the f r a c t u r i n g p o t e n t i a l o f R o t l i e g e n d and s u b o r d i n a t e l y a l s o C a r b o n i f e r o u s gas w e l l s . Some aspects o f s t i m u l a t i o n t r e a t m e n t volumes and and expenses as w e l l as p r o b a b l y a c c e s s i b l e gas r e s e r v e s a r e d i s c u s s e d as f o l l o w s .
3.1.2.2.1. Stimulation treatment volumes and expenses F o r example, t h e t o t a l c o s t o f t h e o p e r a t i o n i n a R o t l i e g e n d w e l l i n t h e Sohl i n g e n field/Germany FRG i n l a t e 1982 where a b t . 550 t ( 1 . 2 Mio. l b s ) o f p r o p p a n t s have been pumped i n t o t h e f r a c t u r e amounted t o a b t . 6 Mio. DM ( 2 . 5 Mio. US $ a c c o r d i n g t o t h e exchange r a t e a t t h a t t i m e ) which was e c o n o m i c a l l y j u s t i f i e d due t o a c q u i s i t i o n o f a b t . 500 Mio. m3 ( 2 0 B c f ) gas r e s e r v e s by t h e succ e s s f u l jumbo t r e a t m e n t (JOHN 1983, KLOSE & KRUMER 1983, BLEAKLEY 1984; c f . sect i o n s 2.4.1.1. and 3.3.2.2.). The numerous t r e a t m e n t s h a v i n g been c a r r i e d o u t w i t h p r o p p a n t q u a n t i t i e s o f a b t . 100 - 400 t i n t h e p e r i o d 1977 - 1985 i n Germany FRG a l l have amounted t o a b t . 1 - 2 Mio. US $ p e r j o b ( s e v e r a l m i l l i o n DM each w i t h r e s p e c t t o v a r y i n g US $ exchange r a t e ) , and t h e s t i m u l a t i o n operat i o n s i n t h e B r i t i s h Southern N o r t h Sea can be assessed t o have been even more expensive and have reached a c o u p l e o f m i l l i o n US $ each, w i t h some d i s c o u n t p r o b a b l y h a v i n g been achieved i n case o f whole f i e l d campaigns w i t h c o n t i n u o u s employment o f t h e s t i m u l a t i o n b o a t ( c f . s e c t i o n 3.7.1.1.). V a r i o u s aspects o f MHF s t i m u l a t i o n s c a r r i e d o u t i n t h e l a s t y e a r s i n R o t l i e gend and C a r b o n i f e r o u s sandstones a r e r e p o r t e d by TUNN (1971); BRINKMANN, FUHRBERG & SCHUBER (1980) ; BRINKMANN, KRUMER & R E I N I C K E (1980) ; BRINKMANN (1982), GUNDERMANN (1982), SCHWARZ & SCHUBER (1982), JOHN (1983, 1987), KLOSE & KRUMER (1983), BLEAKLEY (1984), RIECKMANN & PUSCH (1984), LEICHT (1985) ; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI (1985); STEPHENS & MARTINS (1985), ACHARYA & K I M (1987), SOMMER (1987, 1988) and TAN, McGOWEN, LEE & SOLIMAN (1988).
3.1.2.2.2. Probably accessible gas reserves The s i g n i f i c a n c e o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i s u n d e r l i n e d by t h e e s t i mate t h a t i n Germany FRG, t h e r e m a i n i n g p o t e n t i a l o f R o t l i e g e n d and C a r b o n i f e r o u s t i g h t gas sands i s a b t . 100 B i l l . m3 ( 4 T c f ; t h i s i s one t h i r d o f t h e exp e c t e d f u t u r e d i s c o v e r i e s ) which can o n l y be produced e c o n o m i c a l l y i n case o f i n v o l v e m e n t o f MHF s t i m u l a t i o n t r e a t m e n t s (JOHN 1983; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985). The a c t u a l proven gas r e s e r v e s a r e a b t . 250 B i l l . m3 ( 1 0 T c f ) , o f which a t l e a s t 10 % can o n l y be e c o n o m i c a l l y a c q u i s i t e d and withdrawn b y massive h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n (SCHRUDER 1988). ACHARYA & K I M (1987) e s t i m a t e t h e t o t a l proven and p r o b a b l e R o t l i e g e n d gas r e s e r v e s i n N o r t h Sea, N e t h e r l a n d s and Germany FRG t o be i n t h e range o f a b t . 85 T c f (2,125 B i l l . m3), and FISHMAN (1986) expects an a d d i t i o n a l q u a n t i t y o f a b t . 20 T c f (500 B i l l . m3) t o be a b l e t o be o b t a i n e d by development ( i n c l u d i n g s t i m u l a t i o n ) dur i n g t h e n e x t y e a r s . MARTIN & EVANS (1988) o u t l i n e t h a t t h e R o t l i e g e n d a e o l i a n dune sand r e s e r v o i r s i n t h e B r i t i s h Southern N o r t h Sea a r e e s t i m a t e d t o c o n t a i n r e c o v e r a b l e r e s e r v e s o f o v e r 800 B i l l . m3 ( 3 2 T c f ) i n f i e l d s i n p r o d u c t i o n and under development. I t can be e s t i m a t e d t h a t a t l e a s t a q u a r t e r o f t h i s amount can o n l y be e c o n o m i c a l l y l i b e r a t e d by h y d r a u l i c f r a c t u r i n g e i t h e r i n p a s t o r f u ture.
239 Rotliegend a n d C a r b o n i f e r o u s g a s - b e a r l n g s a n d s t o n e h y d r a u l i c p r o p p a n t f r a c t u r i n g i n N o r t h w e s t e r n Europe Geofogrcol block m o d e / scheme w i t h o u t scale /
Fig. 9 (above) and Fig. 10 (below)
I
Centres of massive hydraulic proppant fracturing in Rotliegend (01, Carboniferous ( 0 ) and Buntsandstein ( 0 )in North Sea and onshore Middle Europe
240
3 . 1 . 3 . Factors influencing the fracturing potential of Rot 1 iegend gas reservoirs Deep t i g h t R o t l i e g e n d and Carboniferous sandstone gas r e s e r v o i r s ( c f . f i g . 11) have been t h e major t a r g e t s o f t h e MHF treatments i n Great B r i t a i n ( o f f shore), i n t h e Netherlands (onshore and o f f s h o r e ) and p a r t i c u l a r l y i n Germany FRG (onshore) d u r i n g t h e l a s t t e n years ( w i t h t h e q u a n t i t i e s o f pumped proppants r a n g i n g between 100 and 650 t p e r w e l l ) and w i l l c o n t i n u e t o p l a y t h e l e a d i n g r o l e s o f h y d r a u l i c proppant f r a c t u r i n g i n Western Europe ( e s p e c i a l l y i n the B r i t i s h Southern N o r t h Sea) d u r i n g t h e n e x t years i n terms o f b o t h frequenc y and s i z e o f t h e j o b s (and thus a l s o proppant consumption; MADER 1987, 1988
b) .
A f t e r an i n t r o d u c t i o n by sketching some general and economical aspects and commenting on d i s t r i b u t i o n o f main f i e l d s and h i s t o r i c a l development o f product i o n and f r a c t u r i n g , the d i s c u s s i o n as f o l l o w s i n c l u d e s questions o f p e r m e a b i l i t y p r e d i c t i o n and e f f e c t i v e p e r m e a b i l i t y , p e r m e a b i l i t y d e t e r i o r a t i o n and c l a y mineralogy, water s a t u r a t i o n , gas demand, minimum p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e s and poroperm p r o p e r t i e s , r e s e r v o i r performance, s t i m u l a t i o n c o s t and f r a c t u r i n g p o l i c y , philosophy o f o f f s h o r e and onshore e x p l o r a t i o n w e l l f r a c t u r i n g , i m p l i c a t i o n s o f f r a c t u r i n g philosophy on proppant s e l e c t i o n , a p p r a i s a l d r i l l i n g s t r a t e g y concepts and experience, h o r i z o n t a l and v e r t i c a l d i f f e r e n c e s o f f r a c t u r i n g p o t e n t i a l ( i n c l u d i n g p e r m e a b i l i t y and gas composition aspects), and wat e r s e n s i t i v i t y ( v a r i o u s references on b o t h general and s p e c i a l t o p i c s from numerous f i e l d s o f a p p l i c a t i o n are compiled i n chapter 7 ) . Sedimentological and d i a g e n e t i c a l aspects o f Buntsandstein and R o t l i e g e n d dep o s i t s as a base f o r r e s e r v o i r m o d e l l i n g are discussed by PLEIN (1978), LUTZNER (1981), ROBINSON (1981) ; DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS (1982) ; GLENNIE (1983, 1986), LEE (1984), MADER (1981, 1982, 1983, 1984 a, 1985 a, 1985 b, 1985 c, 1988 b), MADER & TEYSSEN (1985), MADER & YARDLEY (1985); BIFANI, GEORGE & LEVER (1987) ; GRALLA (1988), MADER & CHATALOV (1988), MARTIN & EVANS (1988) and RICHARDSON, SANGREE & SNEIOER (1988). An o u t l i n e o f t h e d e p o s i t i o n a l environment o f the Upper Carboniferous i s g i v e n i n OILMAN (1987 e ) . I m p o r t a n t sedimentary s t r u c t u r e s o f f l u v i a l , a e o l i a n and l a c u s t r i n e d e p o s i t s i n R o t l i e gend and Buntsandstein i l l u s t r a t i n g d i s t r i b u t i o n o f r e s e r v o i r h e t e r o g e n e i t i e s and being s i g n i f i c a n t f o r f l u i d f l o w and propagation o f h y d r a u l i c a l l y induced f r a c t u r e s are compiled i n photographic p l a t e s and a r e equipped w i t h l o n g e r exp l a n a t i o n s t h a t can be evaluated a l s o independent from the main t e x t ( c f . sect i o n 3.13.).
3.2. Distribution of main fields The Mid-European R o t l i e g e n d Basin (Southern Permian Basin; ZIEGLER 1982) and the gas-prospective c o n t i n e n t a l - i n l a n d a r i d aeolian, f l u v i a l , a l l u v i a l - f a n , lac u s t r i n e and palaeosol r e d bed f a c i e s b e l t along i t s southern margin reaches from England through B r i t i s h and Dutch Southern N o r t h Sea, Netherlands, Germany FRG and Germany GDR t o Poland and almost t o the Russian boundary i n Pomerania (PLEIN 1978, GLENNIE 1983, MADER 1985 a ) . The R o t l i e g e n d i s u n d e r l a i n by t h e Carboniferous t e r r e s t r i a l and marginal-marine humid f l u v i a l and d e l t a i c g r e y bed f a c i e s b e l t c o n t a i n i n g coal seams, and o v e r l a i n by t h e Zechstein marine evap o r i t e c y c l e sequence i n c l u d i n g mudstones, carbonates, sulphates and s a l t , and t h e Buntsandstein c o n t i n e n t a l - i n l a n d a r i d aeolian, f l u v i a l , l a c u s t r i n e and palaeosol r e d bed f a c i e s b e l t ( c f . f i g . 11).
A l l these f o u r d i f f e r e n t sedimentary g e o l o g i c a l formations have been l a i d down i n t e m p o r a l l y separate d e p o s i t i o n a l basins which h o r i z o n t a l l y occupied more o r l e s s the same geographical area, w i t h the more o r l e s s s p a t i a l c o i n c i dence r e s u l t i n g i n t h e i r v e r t i c a l superimposition i n the s t r a t i g r a p h i c a l column
241 d u r i n g course o f b u r i a l h i s t o r y . L a t e r a l l y , the various i n d i v i d u a l basins are n o t homogeneous, b u t i n c o n t r a s t t o p a r t i a l l y good c o r r e l a t i o n along s t r i k e , s i g n i f i c a n t f a c i e s d i f f e r e n c e s occur p e r p e n d i c u l a r l y t o s t r i k e , and a l s o p a r a l l e l t o the basin margins, f r e q u e n t l y considerable d i v e r s i f i c a t i o n o f d e p o s i t i o n a l environment l e a d i n g t o a complex j u x t a p o s i t i o n o f d i f f e r e n t g e n e t i c a l sedimentary bodies i s encountered. These r e l a t i o n s h i p s g i v e r i s e t o v e r t i c a l and hor i z o n t a l zonation o f d i f f e r e n t r e s e r v o i r complexes and together w i t h e f f e c t s o f hydrocarbon generation, m i g r a t i o n and accumulation a l s o t o segregated arrangement o f n a t u r a l gas deposits w i t h i n the heterogeneous and a n i s o t r o p i c pay zone networks according t o the i n t e r n a l geometrical o r g a n i z a t i o n o f the l a t t e r . The discontinuous arrangement o f gas f i e l d s i n t h e sedimentary f a c i e s b e l t as a consequence o f t e c t o n i c a l o v e r p r i n t i n g and t r a p formation a l s o has s i g n i f i c a n t imp a c t on a r e a l d i s t r i b u t i o n o f proppant s t i m u l a t i o n requirement. V e r t i c a l and hor i z o n t a l d i s t r i b u t i o n o f gas f i e l d s and h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l are discussed i n more d e t a i l as f o l l o w s .
3.2.1. Vertical distribution W i t h i n the Rotliegend trend, gas f i e l d s have been mainly discovered i n the B r i t i s h Southern N o r t h Sea, Dutch North Sea, Netherlands onshore, Germany FRG onshore and Western Poland. Only minor amounts o f accumulations have been found i n Germany GDR and Eastern Poland ( c f . f i g s . 5 and 1 0 ) . The Carboniferous grey coal-bearing sandstone and mudstone succession u n d e r l i e s the Rotliegend i n the whole area o f d i s t r i b u t i o n of s u i t a b l e Permian r e s e r v o i r s ( c f . f i g . 11) and has acted both as source formation o f the gas d u r i n g c o a l i f i c a t i o n o f the various t h i n n e r and t h i c k e r seams ( m a t u r i t y aspects o f the Upper Carboniferous are d i s cussed by KETTEL 1981), and as c o l l e c t i n g horizons by t r a p p i n g considerable amounts o f the generated gas i n Upper Carboniferous f l u v i a l and d e l t a i c sandstones. Rotliegend and Carboniferous are i n many cases separated by a pronounced angular unconformity ( c f . f i g s . 3 and 11 as w e l l as p l a t e I i n s e c t i o n 3.13.1.). Due t o m i g r a t i o n along non-sealing normal f a u l t s and through f i s s u r e s and j o i n t s i n the geological column, the gas which was l i b e r a t e d from Upper Carboniferous coal seams n o t o n l y accumulated i n Upper Carboniferous f l u v i a l sandstones o f f r e q u e n t l y ribbon- and l e n t i c u l a r type and o n l y s u b o r d i n a t e l y b l a n k e t nature, and R o t l i e g e n d a e o l i a n and f l u v i a l sandstones o f i n many case b l a n k e t char a c t e r and o n l y t o minor amounts a l s o ribbon- and l e n t i c u l a r type, b u t i n some places a l s o reached Lower T r i a s s i c Buntsandstein f l u v i a l sheet-sand r e s e r v o i r s o f p r e v a i l i n g b l a n k e t geometry. Aspects o f d e p o s i t i o n a l environment, t e c t o n i c a l subsidence and gas accumulation, s t r a t i g r a p h y , and gas composition o f the s u i t e o f r e s e r v o i r storeys are o u t l i n e d as f o l l o w s ( i l l u s t r a t i o n s o f sedimentary s t r u c t u r e s of Rotliegend and Buntsandstein f a c i e s associations are assembled on photographic p l a t e s which together w i t h d e t a i l e d explanations are compiled i n s e c t i o n 3.13. and i n MADER 1985 a, 1985 b ) .
3.2.1.1. Depositional environment The r e s e r v o i r complexes i n t h e R o t l i e g e n d are mainly associated t o the aeol i a n f a c i e s b e l t ( c f . p l a t e 11) between the marginal a l l u v i a l - f a n ( c f . p l a t e 111) and f l u v i a l g r a v e l l y sandstone seam ( c f . p l a t e s I V and V) and the c e n t r a l damp t o wet playa and d e s e r t l a k e mudstone body ( c f . p l a t e s V I and V I I ) c o n t a i n i n g s a l t i n t e r c a l a t i o n s (Wechselfolge f a c i e s and marginal f a c i e s ; GRALLA 1988). Some aspects o f r e s e r v o i r q u a l i t y and sedimentary facies, p a l a e o c l i m a t o l o g i c a l e v o l u t i o n , and p a l a e o t e c t o n i c a l and palaeoenvironmental e v o l u t i o n are b r i e f l y o u t l i n e d as f o l l o w s .
242
Fig. 11 S c h e m a t i c a l g e o l o g i c a l c r o s s - s e c t i o n t h r o u g h t h e gas-bearing r e s e r v o i r column i n Northwest Germany and s u r r o u n d i n g areas. Three t i g h t gas sandstone s t o r e y s can be d i s t i n g u i s h e d c o m p r i s i n g C a r b o n i f e r o u s , R o t l i e g e n d and B u n t s a n d s t e i n . The Z e c h s t e i n carbonates c o n t a i n s o u r gas w i t h c o n s i d e r a b l e shares o f t h e agg r e s s i v e and t o x i c hydrogen s u l p h i d e , whereas t h e R o t l i e g e n d i n c l u d e s sweet gas t h a t o n l y c o n s i s t s o f c o m b u s t i b l e methane and sometimes a l s o o f i n e r t n i t r o g e n . All t h e gas d e r i v e s f r o m c o a l i f i c a t i o n o f C a r b o n i f e r o u s c o a l seams and has m i g r a t e d t h r o u g h t h e cover h o r i z o n sequence a l o n g f a u l t s and f i s s u r e s . C a r b o n i f e r o u s and R o t l i e g e n d t i g h t gas sandstones a r e i n many cases c a n d i d a t e s f o r large-scale h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n , whereas t h e Buntsands t e i n r e q u i r e s o n l y o c c a s i o n a l l y s y n t h e t i c p e r m e a b i l i t y improvement. The gas-water-contacts i n t h e d i f f e r e n t reservoir l e v e l s are frequently d i f f e r e n t , with t h e r e b y each o f t h e u n i t s r e p r e s e n t i n g a s e p a r a t e pay s t o r e y . Legend : 1 = gas-bearing sandstones, 2 = w a t e r - c o n t a i n i n g sandstones, 3 = c o a l seams, 4 = carbonates, 5 = mudstones and m a r l s , 6 = Z e c h s t e i n s a l t , 7 = Muschelk a l k s a l t , 8 = h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l , 9 = unconformity.
Fig. 12
=distribution
types within hydraulic fractures
Homogeneous
Proppant banking
M u l t i p l e proppant
proppant
w i t h open f l o w
banking w i t h open
Multiple proppant banking w l t h o u t o p e n
distribution
flow channel
flow channel
f l o w channel
I
243
Fig. I1
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3.2.1.1.1. Reservoir quality and sedimentary facies The b e s t r e s e r v o i r s a r e found i n l a r g e - s c a l e cross-bedded a e o l i a n dune sands (MADER & YARDLEY 1985; c f . p l a t e II/l-6), whereas p o o r e r f a c i e s c o n d i t i o n s a r e l i n k e d w i t h s m a l l - s c a l e cross-bedded a e o l i a n dune sands and h o r i z o n t a l - l a m i n a t e d a e o l i a n s h e e t sands ( c f . p l a t e VIII/l-5) which f r e q u e n t l y c o n t a i n i n t e r c a l a t i o n s o f h o r i z o n t a l - s t r a t i f i e d damp t o wet p l a y a sandstones and mudstones (MADER 1983 a; c f . p l a t e IX) t h a t d e t e r i o r a t e p r e d o m i n a n t l y v e r t i c a l p e r m e a b i l i t y ( c f . a l s o s e c t i o n 3.10.1.1.). S p e c i a l t r a p s o f f a v o u r a b l e R o t l i e g e n d r e s e r v o i r f a c i e s a s s o c i a t i o n s a r e synsedimentary t e c t o n i c a l f e a t u r e s o f c h i e f l y graben nat u r e where a c c e l e r a t e d subsidence w i t h r e s p e c t t o t h e s u r r o u n d i n g p l a t f o r m s and a d d i t i o n a l p r o t e c t i o n f r o m e r o s i o n p e r m i t t e d accumulation o f p a r t i c u l a r l y t h i c k and sometimes a l s o e s p e c i a l l y p u r e a e o l i a n dune sands ( t h e most s i g n i f i c a n t synd e p o s i t i o n a l R o t l i e g e n d graben f e a t u r e i s t h e Schneverdingen-Graben; DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS 1982; HEDEMANN, MASCHEK, PAULUS & PLElN 1984; GRALLA 1988). C h a r a c t e r i z a t i o n o f c o n t i n e n t a l d e p o s i t i o n a l f a c i e s t y p e s f o r hydrocarbon p r o d u c t i o n a r e g i v e n by RICHARDSON, SANGREE & SNEIDER (1986).
3.2.1.1.2. Palaeoclimatological evolution Throughout t h e R o t l i e g e n d sequence, a p a l a e o c l i m a t o l o g i c a l e v o l u t i o n l e a d i n g f r o m d r i e r c o n d i t i o n s i n t h e l o w e r p a r t where m a i n l y t h i c k cross-bedded a e o l i a n dune sands ( c f . p l a t e II/l-6) formed t o w e t t e r c o n d i t i o n s i n t h e upper p a r t where more and more s m a l l - s c a l e c r o s s - s t r a t i f i e d a e o l i a n dune sands and h o r i z o n t a l - l a m i n a t e d a e o l i a n sheet sands ( c f . p l a t e VIII/l-5) and damp t o wet a e o l i a n , adhesion and a q u a t i c p l a y a sediments ( c f . p l a t e s V I and VII) o r i g i n a t e d t o g e t h e r w i t h i n c r e a s i n g amounts o f l a c u s t r i n e mudstones and f l u v i a l sandstones ( c f . p l a t e IV) t h a t were l a i d down i n l a r g e s h a l l o w p l a y a l a k e s and b r a i d e d s m a l l d r a i n a g e channels i n t e r s e c t i n g t h e sand and mud f l a t as w e l l s as sheetf l o o d i n u n d a t i o n s o f t h e c o n t i n e n t a l p l a i n . I n some areas, t h e s u c c e s s i o n o f f a c i e s complexes a l s o r e f l e c t s an e v o l u t i o n o f f l u v i a l s t y l e (MADER 1983 b, 1984 a, 1985 a, 1985 b; MADER & CHATALOV 1988) i n c l u d i n g p r o g r e s s i v e expansion o f t h e a l l u v i a l b r a i d e d - r i v e r system w i t h i n t h e d e p o s i t i o n a l b a s i n as t h e m a r g i n a l f a n c h a i n s and t h e a s s o c i a t e d a e o l i a n dune b e l t s h r i n k i n e x t e n s i o n o r r e t r e a t f u r t h e r towards t h e b o r d e r o f t h e sedimentary area as a consequence o f b a s i n enlargement by s t e p w i s e o n l a p on t h e f r i n g i n g h i g h l a n d s w i t h s u c c e s s i v e i n f i l l i n g o f the trough.
3.2.1.1.3. Palaeotectonical and palaeoenvironmental evolution P r o g r e s s i v e d i m i n u t i o n o f t h e degree o f b r a i d i n g o f t h e r i v e r systems as t h e e r o s i o n a l h i g h l a n d s i n t h e provenance a r e a a r e denudated, p a l a e o s l o p e g r a d i e n t decreases, amount and g r a i n s i z e o f c l a s t i c m a t e r i a l d e l i v e r e d f r o m t h e source r e g i o n d e c l i n e s , and t r a n s p o r t c a p a c i t y o f t h e channel network ceases, g i v e s r i s e t o enlargement o f i n t e r s t r e a m overbank f l a t s where upon d e s i c c a t i o n and reasonable p e r s i s t e n c e o f q u i e t a q u a t i c sedimentary c o n d i t i o n s m i g r a t i o n o f aeol i a n dunes ( c f . p l a t e II/l-6) and accumulation o f a e o l i a n s h e e t sands ( c f . p l a t e VIII/1-5) f r o m t h e d e t r i t u s d e f l a t e d f r o m emerged sand and g r a v e l b a r s i n t h e s h a l l o w t o m o d e r a t e l y deep b r a i d e d r i v e r channels i s becoming more and more widespread (MADER 1983 b, 1984 a, 1985 a, 1985 b ) , o r r e v e r s e l y i s d i m i n i s h i n g i n importance i n case o f s u c c e s s i v e l y i n c r e a s i n g f r e q u e n c y and e x t e n s i o n o f a l l u v i a l i n u n d a t i o n s by f l o o d p u l s e s o v e r t o p p i n g t h e channel banks a t h i g h stage i n c o m b i n a t i o n w i t h h i g h ground w a t e r l e v e l as a consequence o f . h i g h e r stream d i s c h a r g e and i n c r e a s i n g p r e c i p i t a t i o n r a t e s once c l i m a t o l o g i c a l c o n d i t i o n s a r e becoming damper and w e t t e r .
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3.2.1.1.4. Other aspects Aspects o f a e o l i a n ( c f . p l a t e s I 1 and V I I I ) , f l u v i a l ( c f . p l a t e s I 1 1 - V and V I I ) and l a c u s t r i n e ( c f . p l a t e s V I , V I I and I X ) d e p o s i t i o n a l environments i n t h e R o t l i e g e n d r e s e r v o i r complex, t h e s p a t i a l and temporal i n t e r t o n g u i n g o f t h e d i f f e r e n t sedimentary m i l i e u s , t h e f a c t o r s c o n t r o l l i n g sand body geometry and d i s t r i b u t i o n , and t h e consequences f o r gas accumulation a r e discussed by GLENN I E , MUDD & NAGTEGAAL (1978); PLEIN (1978); DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS (1982); GLENNIE (1983), LEE (1984), MADER (1983, 1985, 1988 b ) , MADER & YARDLEY (1985), GRALLA (1988), HAAK & ELEWAUT (1988), MARTIN & EVANS (1988) and RICHARDSON, SANGER & SNEIDER (1988). The Carboniferous t i g h t gas sands o r i g i n a t e d as channel bars and drapes i n shallow b r a i d e d r i v e r s and sheet f l o o d s and have predominantly s h e e t - l i k e geometry (BRINKMANN 1982). The Buntsandstein gas r e s e r v o i r sands m a i n l y r e p r e s e n t a l s o stream bars and sheets ( c f . p l a t e I V ) having been l a i d down i n watercourses o f shallow b r a i d e d r i v e r systems o r by sheet floods, b u t o c c a s i o n a l l y a l s o a e o l i a n dune and i n t e r d u n e sheet sands ( c f . p l a t e V I I I / l - 5 ) occur (MAOER 1985, MADER & YARDLEY 1985). I n a l l t h r e e formations, d e p o s i t i o n a l environment and d i a g e n e t i c a l h i s t o r y i n f l u e n c e r e s e r v o i r rock c h a r a c t e r i s t i c s and thus t r e a t ment o p t i o n s .
3.2.1.2. Tectonical subsidence and gas accumulation I n terms of d i s t r i b u t i o n of p o t e n t i a l and n e c e s s i t y o f h y d r a u l i c proppant f r a c t u r i n g f o r economical gas production, t h e primary zonation o f sedimentary f a c i e s b e l t s i n b o t h R o t l i e g e n d and Buntsandstein i n Middle Europe was unfavour a b l y o v e r p r i n t e d by secondary t e c t o n i c a l deformation and t e r t i a r y hydrocarbon accumulation. Aspects o f sedimentary vs. t e c t o n i c a l b a s i n zonation and gaswater-contact l e v e l are b r i e f l y o u t l i n e d as f o l l o w s .
3.2.1.2.1. Sedimentary vs. tectonical zonation The marginal and proximal d e p o s i t i o n a l f a c i e s seams i n t h e southern p a r t s o f the R o t l i e g e n d and Buntsandstein basins where coarse f l u v i a l and a e o l i a n sands ( c f . p l a t e s I 1 and V I I I ) r e p r e s e n t h i g h - q u a l i t y r e s e r v o i r s (marginal f a c i e s ; GRALLA 1988) a r e nowadays e i t h e r outcropping a t the surface o r b u r i e d i n s h a l low depth w i t h o u t s u f f i c i e n t l y i s o l a t e d s t r u c t u r a l , s t r a t i g r a p h i c a l and/or f a c i e s t r a p s t o r e t a i n gas. The more d i s t a l and c e n t r a l f a c i e s realms f u r t h e r t o the north, however, which are moderate- t o l o w - q u a l i t y pay zones c o n t a i n i n g o n l y s m a l l e r amounts o f a e o l i a n cross-bedded dune and h o r i z o n t a l - s t r a t i f i e d sheet sands ( c f . p l a t e V I I I ) , b u t i n c l u d i n g m a i n l y f i n e - g r a i n e d f l u v i a l channel damp sand ( c f . p l a t e I V ) and overbank mud d e p o s i t s ( c f . p l a t e s I I I / 1 - 2 and I X ) , t o wet p l a y a sand and mud sediments ( c f . p l a t e s V I and V I I ; Wechselfolge f a c i e s ; GRALLA 1988), and playa-lake mud and s a l t ( b a s i n f a c i e s ; GRALLA 1988), have undergone subsidence t o such a depth where t h e t h i c k s e a l i n g overburden a l lows t h e accumulation o f huge amounts o f gas t h a t o n l y have t o be l i b e r a t e d from the poor r e s e r v o i r s , and t e c t o n i c a l o v e r p r i n t i n g has generated s t r u c t u r a l t r a p s o f m a i n l y a n t i c 1 i n a l and horst/graben n a t u r e ( f l o w e r s t r u c t u r e s representi n g i n v e r t e d graben f i l l i n g s t h a t are common i n t h e Mesozoic o i l - b e a r i n g sequence o f J u r a s s i c and Cretaceous a r e n o t developed i n the Palaeozoic gas-beari n g p a r t o f t h e g e o l o g i c a l column). T h i s combination o f poor r e s e r v o i r f a c i e s b u r i e d i n s u i t a b l e depth f o r hydrocarbon accumulation and t r a p p i n g i n c o n t r a s t o f the good pay f a c i e s o c c u r r i n g i n shallow depth o r even a t t h e surface where gas c o n c e n t r a t i o n i s n o t p o s s i b l e due t o l a c k o f t r a p p i n g and s e a l i n g u n d e r l i n e s t h e s i g n i f i c a n c e o f t h e R o t l i e gend and Buntsandstein d i s t a l - m e d i a l t i g h t r e s e r v o i r f a c i e s b e l t which crosses t h e n o r t h e r n p a r t o f M i d d l e Europe (MADER 1985 a) and which represents t h e most p o t e n t i a l and i m p o r t a n t b e l t o f h y d r a u l i c proppant f r a c t u r i n g i n t h e near f u -
246 t u r e . I f t h e sedimentary and t e c t o n i c a l z o n a t i o n s would be o f i n v e r s e n a t u r e , c e r t a i n l y n o t v e r y much l a r g e - s c a l e h y d r a u l i c f r a c t u r i n g would be necessary f o r access o f t h e gas r e s e r v e s , b u t a c c o r d i n g t o t h e s i t u a t i o n developed i n R o t l i e gend and Buntsandstein, t h e most s u i t a b l e c o n d i t i o n s f o r a c c u m u l a t i o n and r e t a i n m e n t o f gas a r e c o n c e n t r a t e d i n t h e r e l a t i v e l y p o o r e r o r even a l m o s t w o r s t r e s e r v o i r r o c k s where o n l y l a r g e - s c a l e expensive s t i m u l a t i o n j o b s a l l o w economic a l l y feasible exploitation. The most i m p o r t a n t aspect o f s a l t d i a p i r i s m t e c t o n i c s o r h a l o k i n e t i c s i s t h e e f f e c t o f r e t a r d e d d i a g e n e s i s i n t h e R o t l i e g e n d sandstones below t h e s a l t p l u g s as a consequence o f t h e c o o l i n g e f f e c t o f t h e t h i c k s a l t mass ( t e m p e r a t u r e s i n R o t l i e g e n d r e s e r v o i r s below s a l t d i a p i r s a r e on average 12 - 15 OC l o w e r t h a n those i n R o t l i e g e n d pays between t h e s a l t domes; FRRBER 1984).
3.2.1.2.2. Gas-water-contact level An a d d i t i o n a l c o m p l i c a t i o n i n R o t l i e g e n d sequences where t h i c k n e s s and comp l e t e n e s s o f t h e sequence has been enhanced by synsedimentary graben t e c t o n i c s i s subsidence o f t h e pay zone column p a r t i a l l y below t h e g a s - w a t e r - c o n t a c t . T h e r e f o r e i n many cases i n Germany FRG t h e l o w e r s e c t i o n ( t h e SchneverdingenSandstein; c f . s e c t i o n s 3.10.1. and 3.2.1.3.1.) o f the Rotliegend succession c o n t a i n i n g t h e b e s t r e s e r v o i r s i s e i t h e r p a r t i a l l y o r i n some cases even complet e l y below t h e gas-water-contact, whereas t h e t i g h t m i d d l e s e c t i o n ( t h e Haupts a n d s t e i n ) i s i n most cases above t h e gas-water-contact. W h i l e c e r t a i n l y much more p r i m a r y p r o d u c t i o n f r o m t h e good l o w e r r e s e r v o i r s t o r e y would t a k e p l a c e i n case o f a more f a v o u r a b l e g a s - w a t e r - l e v e l , subsidence o f t h e b e s t pay zone i n t e r v a l p a r t i a l l y below t h e g a s - w a t e r - c o n t a c t i s a n o t h e r c r i t e r i o n g i v i n g r i s e t o n e c e s s i t y o f l a r g e h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n s i n t h e poor m i d d l e p o r t i o n o f the r e s e r v o i r p i l e . W h i l e i n some occasions a t l e a s t reasonable p o r t i o n s o f t h e SchneverdingenS a n d s t e i n o r even t h e whole R o t l i e g e n d sequence i n c l u d i n g l o n g e r s e c t i o n s of t h e u n d e r l y i n g C a r b o n i f e r o u s p r o f i l e a r e above t h e g a s - w a t e r - c o n t a c t s i n Germany FRG, t h e C a r b o n i f e r o u s i n t h e B r i t i s h Southern N o r t h Sea i s i n t h e overwhelmi n g p a r t o f s t r u c t u r e s below t h e g a s - w a t e r - l e v e l which i n a d d i t i o n t o t h e t i g h t r e s e r v o i r f a c i e s i s a n o t h e r i m p o r t a n t drawback o f t h e gas p r o s p e c t i v i t y o f t h i s g e o l o g i c a l s t o r e y ( c f . a l s o s e c t i o n 3.8.1.).
3.2.1.3. Stratigraphy Concerning R o t l i e g e n d s t r a t i g r a p h y i n M i d d l e Europe ( S o u t h e r n Permian Basin; ZIEGLER 1982), d i s t i n c t i o n has t o be made between Germany FRG, N e t h e r l a n d s and B r i t i s h Southern N o r t h Sea.
3.2.1.3.1. Germany FRG The R o t l i e g e n d s e c t i o n i n Germany FRG i s p a r t i c u l a r l y w e l l - d e v e l o p e d and can be c l a s s i f i e d i n t o v a r i o u s f o r m a t i o n s a c c o r d i n g t o r e c e n t s t r a t i g r a p h i c a l i n v e s t i g a t i o n s and c o r r e l a t i o n s (HEDEMANN, MASCHEK, PAULUS & PLEIN 1984; GRALLA 1988). The most i m p o r t a n t f a c i e s p r o v i n c e i s t h e Weser-Elbe p r o v i n c e (GRALLA 1988) o r E a s t Hannover p r o v i n c e (HEDEMANN, MASCHEK, PAULUS & PLEIN 1984) where most o f t h e MHF s t i m u l a t i o n s o f R o t l i e g e n d t i g h t gas sands have so f a r t a k e n p l a c e ( c f . f i g . 4) and which i s t h e c e n t r e o f R o t l i e g e n d e x p l o r a t i o n and product i o n . I n t h i s f a c i e s p r o v i n c e , t h r e e s t o r e y s can be d i s t i n g u i s h e d i n t h e s t r a t i g r a p h i c a l column. The l o w e r l e v e l comprises t h e Schneverdingen-Formation w h i c h r e f l e c t s t h e i n i t i a l stage o f b a s i n i n f i l l i n g w i t h c e n t r a l subsidence and i s o n l y p r e s e n t i n a s u i t e o f s m a l l e r and l a r g e r grabens. The m i d d l e s e c t i o n i n c l u des t h e Emden-Formation which r e p r e s e n t s t h e main stage o f b a s i n i n f i l l i n g w i t h a r e a l coverage o f s e d i m e n t a t i o n . The upper p o r t i o n comprises t h e Hannover-Forma-
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tion or Wechselfolge which devotes its origin to the late stage of basin infilling when large expansion of the depositional area to the south enabled the subsequent extensive Zechstein transgression. The main reservoir rocks are the Schneverdingen-Sandstein in the Schneverdingen-Formation at the base of the Rotliegend profile, the Slochteren-Hauptsandstein or simply Hauptsandstein in the middle section of the Rotliegend sequence, and the Wustrow-Sandstein (GRALLA, NIEBERDING & SOBOTT 1988) representing the petrophysically best developed one of numerous thin sandstones in the Hannover-Formation or Wechselfolge at the top of the Rotliegend succession. In the East Friesland province, the Hauptsandstein is very reduced, the Schneverdingen-Formation has so far not been encountered by drilling, and the main pay zone in this region are several Wechselfolge sandstones.
3.2.1.3.2. Nether lands In terms of correlation with the Netherlands adjoining Germany FRG in the west and due to the offshore extension of the Rotliegend basin in the subsurface also connecting the stratigraphy from Germany FRG to Dutch and British North Sea, the Schneverdingen-Formation containing the Schneverdingen-Sandstein in Germany FRG is equivalent to the Lower Slochteren-Formation in Netherlands and westwards adjoining areas. The Emden-Formation including the Hauptsandstein in Germany FRG is correlating with the Upper Slochteren-Formation in Netherlands and North Sea, and the Hannover-Formation containing the Wustrow-Sandstein and other Wechselfolge sandstones in Germany FRG can be connected with the Ten Boer-Formation further westwards.
3.2.1.3.3. British Southern North Sea In the British Southern North Sea, the Rotliegend section is not subdivided in vertical direction, and distinction is made between two laterally intertonguing units being the Leman Sandstone Formation which consists of clean aeolian and fluvial sandstones in the southern and western basin margins, and the interdigitating Silverpit Formation in the basin centre that is built up of marginal aeolian/playa and lacustrine sandstones, siltstones, mudstones and evaporites (MARTIN & EVANS 1988). Other aspects of Rotliegend stratigraphy in the British Southern North Sea are discussed by GRAY (1975), MARIE (1975), VEEN (1975) and GAGE (1980).
3.2.1.4. Gas conposition In contrast to the formation of sweet gas deposits in Carboniferous, Rotliegend and Buntsandstein consisting only of combustible methane (unless inert nitrogen is associated with the hydrocarbons in greater shares; cf. section 3.10.2.), immigration of coal-seam-deriving gas into the Zechstein carbonates was accompanied by mixing with additional amounts of gas that were generated within the Zechstein and which include also the poisonous and corrosive H2S in smaller or larger amounts, thus particularly in the Hengstlage-Sagermeer complex in the South Oldenburg area (cf. fig. 4) in Germany FRG (but in subordinate amounts also in Poland) creating complications of industrial exploitation due to the necessity of removal of H2S by expensive sour gas purification in large special plants. Hydraulic proppant fracturing has so far never been carried out in the Zechstein carbonates in Middle Europe where stimulation is restricted to matrix acidizing and rarely also acid fracturing (if necessary at all), with all types of reservoir treatment often being hampered and complicated by high H2S-concentrations in the formation gas. Sour gas purification comprising removal of the aggressive and poisonous H2S is also a great problem for exploitation of many gas-bearing carbonate formations in the USSR.
248 The o n l y drawback i n the R o t l i e g e n d sweet gas r e s e r v o i r b e l t and s t o r e y comp l e x i s the a s s o c i a t i o n o f n i t r o g e n w i t h t h e hydrocarbons i n p a r t s o f t h e Lower Permian pay seam, w i t h t h e n i t r o g e n c o n t e n t reaching from q u a n t i t a t i v e l y n e g l i g i b l e admixtures v i a reasonable shares which r e q u i r e removal i n gas p u r i f i c a t i o n p l a n t s t o e x t r a o r d i n a r i l y h i g h amounts which no l o n g e r a l l o w economically f e a s i b l e hydrocarbon p r o d u c t i o n ( c f . s e c t i o n 3.10.2.). Helium, q u i c k s i l v e r and higher-homology hydrocarbons a r e o n l y associated i n t r a c e s t o t h e methane i n R o t l i e g e n d r e s e r v o i r s which do n o t r e q u i r e a t t e n t i o n concerning gas withdrawal, processing and transmission.
3.2.2.
Horizontal distribution
I n terms o f h o r i z o n t a l d i s t r i b u t i o n o f h y d r a u l i c proppant f r a c t u r i n g potent i a l o f R o t l i e g e n d and Carboniferous t i g h t gas sandstones, d i s t i n c t i o n can be made between Germany FRG, B r i t i s h and Dutch N o r t h Sea, and Germany GDR and Poland. While i n Germany FRG most o f t h e operations have a l r e a d y taken p l a c e dur i n g the l a s t t e n years and c u r r e n t t o n e a r - f u t u r e a c t i v i t y i s almost zero as a consequence o f the o i l p r i c e crash and t h e US $ exchange r a t e weakness, t h e predominant amount o f s t i m u l a t i o n work i n t h e B r i t i s h and Dutch N o r t h Sea i s c a r r i e d o u t a t t h e p r e s e n t time and w i t h i n the n e x t few years, although a l s o a l r e a dy considerable treatment campaigns have been undertaken i n t h e p a s t ( c f . sect i o n s 2.2.1.6.2. and 2.4.1.). I n Germany GDR and Poland, so f a r o n l y l i t t l e s t i m u l a t i o n has been performed, b u t t h e near f u t u r e w i l l be c h a r a c t e r i z e d by i n c r e a s i n g and a c c e l e r a t i n g n e c e s s i t y t o u t i l i z e h y d r a u l i c proppant f r a c t u r i n g f o r i n c r e a s i n g domestic reserves and supply. H o r i z o n t a l d i s t r i b u t i o n o f gas f i e l d s and h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l i n Germany FRG, B r i t i s h and Dutch N o r t h Sea, and Germany GDR and Poland i s o u t l i n e d i n more d e t a i l as f o l 1ows .
3.2.2.1.
Germany FRG
T y p i c a l places o f R o t l i e g e n d and Carboniferous MHF e x e c u t i o n i n Germany FRG i n the gas f i e l d b e l t between t h e bounding patches o f Groningen/The Netherlands (STXUBLE & MILIUS 1970, ROSSUM 1975, BEEK & TROOST 1978) i n t h e west and Wustrow (Germany FRG)/Salzwedel (Germany GDR) i n the e a s t (PHILIPP & R E I N I C K E 1982, JOHN 1983) have been so f a r Sohlingen (ERDOEL-ERDGAS AKTUELL 1985 c ) and s a t e l l i t e s i n t h e area o f t h e Schneverdingen-Graben (DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS 1982; HEDEMANN, MASCHEK, PAULUS & PLEIN 1984; GRALLA 1988) i n the R o t l i e g e n d o f the East Hannover area o r Weser-Elbe p r o v i n c e (where t h e h i t h e r t o l a r g e s t h y d r a u l i c proppant f r a c t u r i n g o p e r a t i o n s have been c a r r i e d out; KLOSE & KRUMER 1983, BLEAKLEY 1984, LEICHT 1985; c f . f i g s . 4 and 1 0 ) . Other R o t l i e g e n d items o f considerable MHF s i g n i f i c a n c e are Leer, Leybucht and Hamburger Sand i n the East F r i e s l a n d area and f a c i e s province, and A l f e l d Elze i n the South Hannover area and f a c i e s province. The h i g h l i g h t s o f Carbonif e r o u s h y d r a u l i c proppant f r a c t u r i n g a r e Uphuser Meer i n t h e East F r i e s l a n d area (OIETZEL & HANTELMANN 1985); Cappeln, Goldenstedt, Syke, Uchte, Varnhorn and Mietingsmoor as w e l l as Hamwiede i n t h e South Oldenburg and East Hannover areas, r e s p e c t i v e l y ( f o r l o c a t i o n maps c f . JOHN 1983; REINICKE, BRINKMANN, SCHWARZ & HUENI 1985; SCHRUDER 1986, 1987); and Emlichheim, Fehndorf and Frenswegen i n the West Emsland area (SOMMER 1987, 1988; c f . f i g s . 4 and 10 and tab. 6 ) , w i t h Coevorden a d j o i n i n g i n t h e Netherlands s e c t o r on t h e o t h e r s i d e o f t h e s t a t e boundary.
3.2.2.2.
B r i t i s h and Dutch North Sea
The a c t i v i t y i n the B r i t i s h Southern N o r t h Sea concentrates on the R o t l i e gend f i e l d complex around the Sole P i t area (comprising m a i n l y the r e g i o n o f t h e Amethyst, I n d e f a t i g a b l e , Leman Bank (VEEN 1975; GLENNIE, MUDD & NAGTEGAAL
249 1978; AMOCO 1988), Ravenspurn, Rough, Thames, V a l i a n t , Vanguard, Vulcan and West Sole f i e l d s (LAFLEUR & JOHNSON 1973; OFFSHORE ENGINEER 1985 c, 1986 a, 1986 c; WORLD OIL 1985, ALLISON 1986, BRITISH PETROLEUM 1986, FISHMAN 1986, GLENNIE 1986; GREGORY 1986, 1987; THOMAS 1986, ACHARYA & K I M 1987, FOX 1987, OILMAN 1987 a, POTTER 1987, MARTIN & EVANS 1988) o f f s h o r e Great Yarmouth b e t ween Teesside i n Great B r i t a i n and Den Helder i n the Netherlands ( f o r l o c a t i o n maps c f . ALLISON 1986, OILFIELD PUBLICATIONS 1986, QUINLAN 1988; c f . f i g s . 6 and 10 as w e l l as tabs. 7 - 8 ) . I n the Dutch North Sea, the main places o f Rotl i e g e n d h y d r a u l i c proppant f r a c t u r i n g s t i m u l a t i o n are the K-12, L-8, L-10 and P-2 f i e l d s . While most o f the hydrocarbon accumulations c o n t a i n gas, the A r g y l l f i e l d a t the margin o f the c e n t r a l N o r t h Sea graben i s n e a r l y the o n l y p l a c e where the R o t l i e g e n d sandstones c o n t a i n o i l (BIFANI, GEORGE & LEVER 1987).
3.2.2.3. Germany GDR and Poland I n Germany GDR, so f a r o n l y the Salzwedel f i e l d a t the FRG (Wustrow)/GDR boundary ( c f . f i g . 4) and some small s a t e l l i t e s around the b i g patch are product i v e , because the r e s e r v o i r c o n t a i n s f u r t h e r t o the e a s t ( f o r example near Berl i n ) almost e x c l u s i v e l y i n e r t n i t r o g e n ( c f . a l s o s e c t i o n 3.10.2.). I n Poland, again sweet combustible gas has accumulated i n R o t l i e g e n d sandstones, w i t h the most important f i e l d s being i n the Fore-Sudetic Monocline Bogdaj-Uciechow near Ostrbw Wielkopolski; Zalecze, Zuchlbw and Wierzchowice; i n the area south o f Poznan Grodzisk, Ujazd, Bukowiec, Buk and Kleka; and i n the Pomeranian area around Szczecin Miedzyzdroje ( c f . f i g . 5 and tab. 9 ) . H y d r a u l i c proppant f r a c t u r i n g has h i t h e r t o been c a r r i e d o u t i n the R o t l i e g e n d i n Germany GDR and Poland o n l y o c c a s i o n a l l y and on s m a l l e r s c a l e i n c o n t r a s t t o abundant MHF a p p l i c a t i o n i n Germany FRG.
3.2.3. Other aspects The R o t l i e g e n d i s i n Middle Europe already since several years the most pros p e c t i v e t a r g e t h o r i z o n o f gas e x p l o r a t i o n (SCHRUDER 1985) and w i l l keep t h i s p o s i t i o n a l s o a t l e a s t i n the near f u t u r e . The Carboniferous w i l l be o f progress i v e l y i n c r e a s i n g importance, b u t i s expected t o always l a g behind the R o t l i e gend i n s i g n i f i c a n c e , w i t h the main reason being the h i g h e r water s a t u r a t i o n w i t h i n the p e t r o p h y s i c a l l y poorer r e s e r v o i r s which are f r e q u e n t l y s i t u a t e d c l o s e t o o r even below the gas-water-contact ( c f . s e c t i o n 3.8.1.). Laboratory t e s t i n g has shown t h a t the Carboniferous r e s e r v o i r rocks e x h i b i t a h i g h degree o f p l a s t i c i t y (SLUSSER & RIECKMANN 1976; c f . s e c t i o n 4.2.3.2.3.). R o t l i e g e n d gas f i e l d s i n Germany FRG are f r e q u e n t l y found beneath Zechstein s a l t p l u g s where the lower t o t a l geopressure and the r e t a r d e d heat f l u x o f t e n preserved b e t t e r p o r o s i t i e s than i n the r e g i o n s between t h e s a l t domes (FARBER 1984; c f . s e c t i o n 3.2.1.2.1.). This geological constellation i s i n contrast t o the c o n d i t i o n s i n the USA where except o f overhang d r i l l i n g , no hydrocarbon prod u c t i o n i s encountered under s a l t d i a p i r s , and the p e n e t r a t i o n o f s a l t i n great e r thicknesses p o i n t i n g t o a s a l t p l u g i s even u s u a l l y the s i g n t o abandon the we1 1 (BLEAKLEY 1984).
3.3. Historical development o f production and fracturing The beginning o f R o t l i e g e n d g a s e x p l o r a t i o n , p r o d u c t i o n and s t i m u l a t i o n i n Europe o n l y dates back some 25 years (McNALLY 1988). I n 1959, the g i a n t Groningen f i e l d i n t h e Netherlands ( t h e e a s t e r n t i p o f which extends through t h e Ems e s t u a r y on Germany FRG t e r r i t o r y ) was discovered ( b y e x p l o r a t i o n w e l l Slochter e n 1; ACHARYA & K I M 1987), and i n 1963 i t s l a r g e s i z e was recognized from app r a i s a l d r i l l i n g (SCHOONBEEK 1976), w i t h Groningen belonging t o the t o p t e n gas f i e l d s i n the w o r l d and c o n t a i n i n g abt. 2,500 B i l l . m3 (100 T c f ) reserves. The Groningen gas s t r i k e e x a c t l y 20 years ago ( o n l y some 10 years a f t e r the indus-
250 t r i a l i n t r o d u c t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g ; c f . s e c t i o n 1 . 2 . ) was t h e b r e a k t h r o u g h f o r t h e b e g i n n i n g o f i n t e n s e e x p l o r a t i o n i n B r i t i s h N o r t h Sea, Germany FRG, D u t c h N o r t h Sea, German N o r t h Sea and o t h e r a r e a s .
3.3.1. British North Sea The d i s c o v e r y o f t h e g i a n t R o t l i e g e n d gas f i e l d G r o n i n g e n (STKUBLE & M I L I U S 1970, ROSSUM 1975, BEEK & TROOST 1978) f o c u s s e d s u b s e q u e n t l y t h e a t t e n t i o n o f e x p l o r a t i o n o n b o t h o n s h o r e and o f f s h o r e p a r t s o f t h e b e l t o f f a v o u r a b l e R o t l i e gend r e s e r v o i r f a c i e s i n t h e S o u t h e r n Permian B a s i n ( f o r p a l a e o g e o g r a p h i c a l maps c f . ZIEGLER 1982, GLENNIE 1 9 8 6 ) . Some a s p e c t s o f t r e a t m e n t s i z e and p r o p p a n t q u a n t i t y , o f f s h o r e f r a c t u r i n g t e c h n o l o g y and s t i m u l a t i o n b o a t s , and s i g n i f i c a n c e o f N o r t h Sea r e s e r v e s f o r G r e a t B r i t a i n a r e d i s c u s s e d as f o l l o w s .
3.3.1.1. Treatment size and proppant quantity I n 1965, West S o l e became t h e f i r s t c o m m e r c i a l R o t l i e g e n d gas d i s c o v e r y i n t h e B r i t i s h S o u t h e r n N o r t h Sea ( o n l y one y e a r a f t e r t h e g e n e r a l s t a r t o f N o r t h Sea o i l and gas d r i l l i n g ) and soon l a t e r a l s o t h e t a r g e t o f f i r s t o f f s h o r e h y d r a u l i c f r a c t u r i n g t r e a t m e n t s i n t h e R o t l i e g e n d (COOPER & MARSHALL 1984, BRITISH PETROLEUM 1986) w h i c h i n t h o s e days were o f c o m p a r a t i v e l y s m a l l - t o m i n i a t u r e - s i z e and were c a r r i e d o u t w i t h a b t . 50,000 - 200,000 l b s . ( 2 5 - 100 t ) o f n a t u r a l sand ( a b t . 10 y e a r s o f p a r t i a l l y even more b e f o r e t h e i n v e n t i o n and m a r k e t i n t r o d u c t i o n o f s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h p r o p p a n t s ) . O t h e r 19651'1966 B r i t i s h S o u t h e r n N o r t h Sea R o t l i e g e n d gas s t r i k e s w e r e Leman, H e w e t t , I n d e f a t i g a b l e and V i k i n g . F o r comparison, t h e f i r s t c o m m e r c i a l N o r t h Sea o i l d i s c o v e r y was o n l y made i n 1969 ( t h u s f o u r y e a r s a f t e r t h e f i r s t m a j o r g a s s t r i k e ; THOMAS 1 9 8 6 ) . W i t h t i m e , t r e a t m e n t t e c h n o l o g y e v o l v e d p r o g r e s s i v e l y t o m a s s i v e h y d r a u l i c f r a c t u r i n g w i t h q u a n t i t i e s o f a b t . 100 - 500 t (200,000 - 1 M i o . l b s ) o f n a t u r a l sand a n d / o r i n t e r m e d i a t e - s t r e n g t h s y n t h e t i c p r o p p a n t s w h i c h s t a r t e d t o be c a r r i e d o u t i n a second m a j o r s e r i e s o f campaigns i n t h e l a t e 1 9 8 0 ' s a l m o s t 20 y e a r s a f t e r t h e o r i g i n a l d i s c o v e r y o f R o t l i e g e n d gas p r o s p e c t i v i t y i n t h e N o r t h Sea and t h e o n s e t o f p r o p p a n t s t i m u l a t i o n f o r t h e amel i o r a t i o n o f gas l i b e r a t i o n ( c f . s e c t i o n 2 . 2 . 1 . 6 . 2 . ) .
3.3.1.2. Offshore fracturing technology and stimulation boats H y d r a u l i c p r o p p a n t f r a c t u r i n g i n t h e S o u t h e r n N o r t h Sea e v o l v e d f r o m f i r s t m a j o r t r e a t m e n t s i n t h e m i d 1 9 6 0 ' s v i a numerous j o b s w i t h s k i d - i n s t a l l e d , platform-mounted e q u i p m e n t between 1965 and 1980 and f i r s t s h i p o p e r a t i o n s i n 1980 b y s u p p l y v e s s e l s t h a t were c o n v e r t e d i n t o s t i m u l a t i o n b o a t s t o t r e a t m e n t s c a r r i e d o u t b y p u r p o s e - b u i l t f r a c t u r i n g v e s s e l s f r o m 1984 onwards (COOPER & MARSHALL 1984), w i t h now t h r e e c o m p a r a b l e h i g h - l e v e l t e c h n o l o g y s t i m u l a t i o n b o a t s b e i n g a v a i l a b l e i n t h e N o r t h Sea (FOX 1985, OILMAN 1985 b; c f . a l s o s e c t i o n 3 . 8 . 1 . 2 . ) . The i n t r o d u c t i o n o f h i g h - c a p a c i t y f r a c t u r i n g s h i p s a l l o w e d t o d r a s t i c a l l y i n c r e a s e t r e a t m e n t s i z e s i n t e r m s o f f l u i d and p r o p p a n t volume and o n l y t h a n k s t o t h e s t i m u l a t i o n b o a t s i t was p o s s i b l y t o a c h i e v e t h e s t a g e of >sive h y d r a u l i c p r o p p a n t f r a c t u r i n g i n t h e h o s t i l e N o r t h Sea o f f s h o r e e n v i r o n m e n t . Asp e c t s o f h y d r a u l i c f r a c t u r i n g i n p a r t i a l l y w a t e r - s e n s i t i v e R o t l i e g e n d and B u n t s a n d s t e i n gas r e s e r v o i r s a r e a l s o p r e s e n t e d b y COULTER & PURVIS ( 1 9 8 0 ) .
..,"
3.3.1.3. Significance o f North Sea reserves for Great Britain The S o u t h e r n N o r t h Sea i s t h e t r a d i t i o n a l c e n t r e o f gas p r o d u c t i o n i n G r e a t B r i t a i n and has h i s t o r i c a l l y c o n t r i b u t e d n e a r l y 90 % o f t h e t o t a l gas d e l i v e r y i n t h e U n i t e d Kingdom, and t h e r e i s e v e r y r e a s o n t o b e l i e v e t h a t t h e S o u t h e r n N o r t h Sea w i l l keep i t s d o m i n a n t r o l e u n t i l t h e y e a r 2000, as i t s t i l l c o n t a i n s more t h a n 50 % o f t h e t o t a l p r o v e n and p r o b a b l e gas r e s e r v e s i n t h e U n i t e d K i n g -
251 dom (FISHMAN 1986; c f . a l s o s e c t i o n s 2.2.1.6.2. and 2 . 2 . 1 . 6 . 3 . 2 . ) . I n t h e some t w e n t y y e a r s f r o m t h e 1965 s t a r t o f B r i t i s h o f f s h o r e gas d r i l l i n g , s t i m u l a t i o n and e x p l o i t a t i o n , o v e r 50 B i l l . L have been i n v e s t e d i n t o hydrocarbon e x p l o r a t i o n and p r o d u c t i o n i n t h e U n i t e d Kingdom s e c t o r o f t h e N o r t h Sea (OFFSHORE ENGINEER 1986 d ) . As a consequence o f t h e s i g n i f i c a n c e o f R o t l i e g e n d gas a c q u i s i t i o n i n c l u d i n g c o n s i d e r a b l e shares o f h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n , G r e a t B r i t a i n advanced u n t i l 1985 t o t h e w o r l d ' s f o u r t h b i g g e s t gas producer and s i m u l t a n e o u s l y a l s o achieved t h e rank o f b e i n g t h e w o r l d ' s f i f t h b i g g e s t o i l p r o ducer. The s t i m u l a t i o n market o f t h e N o r t h Sea can be e s t i m a t e d t o have an annual p o t e n t i a l o f a b t . 20 Mio. L (OILMAN 1985 b; c f . s e c t i o n 2 . 2 . 1 . 5 . 3 . ) .
3.3.2. Germany FRG The development of Rotliegend and C a r b o n i f e r o u s h y d r a u l i c p r o p p a n t f r a c t u r i n g i n Germany FRG i s summarized i n an account o f e v o l u t i o n o f g a s - f i e l d d i s c o v e r y as w e l l as t r e a t m e n t s i z e and p r o p p a n t q u a n t i t y .
3.3.2.1. Evolution o f gas-field discovery F o l l o w i n g t h e d i s c o v e r y o f t h e g i a n t Groningen f i e l d i n t h e N e t h e r l a n d s i n 1959 and t h e f i r s t R o t l i e g e n d gas f i e l d s i n t h e B r i t i s h Southern N o r t h Sea i n 1965 (West Sole f i e l d and s a t e l l i t e s ; c f . s e c t i o n 3.3.1.), onshore M i d d l e Europe i n 1966 t h e gas f i e l d Wustrow/Salzwedel a t t h e FRGIGDR-boundary ( c f . f i g . 4) was found which formed t h e t h i r d m i l e s t o n e i n R o t l i e g e n d gas access. The b r e a k t h r o u g h i n Germany FRG was achieved w i t h t h e s t r i k e o f Schmarbeck i n 1971 which opened a whole new g a s - f i e l d p r o v i n c e i n t h e East Hannover area, w i t h i n subsequent y e a r s a l a r g e s u i t e o f w e l l s h a v i n g been d r i l l e d and t h e new d i s c o v e r i e s c u l m i n a t i n g so f a r i n 1980 when one o f t h e most i m p o r t a n t and g r e a t e s t f i e l d s was found i n S o h l i n g e n which c o n f i r m e d v a r i o u s s t r a t e g i c a l concepts o f e x p l o r a t i o n c o n c e r n i n g t e c t o n i c s , s e d i m e n t a t i o n and d i a g e n e t i c a l h i s t o r y (BRADEL & DRAXLER 1982, PHILIPP & R E I N I C K E 1982) i n c l u d i n g t h e p l a y o f synsedimentary Rot1 iegend graben t e c t o n i c s (DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS 1982; HEDEMANN, MASCHEK, PAULUS & PLEIN 1984; GRALLA 1988) t h a t subsequently gave r i s e t o an e n t i r e b e l t o f new gas s t r i k e s around t h e new i m p o r t a n t d i s c o v e ry spot. I n t h e l a s t decade, a whole s u i t e o f s a t e l l i t e gas f i e l d s have been found around t h e b i g S o h l i n g e n p a t c h p r e d o m i n a n t l y i n a system o f synsedimentary g r a bens ( o f which t h e Schneverdingen graben i s t h e most i m p o r t a n t one) where e x c e l l e n t s t o r a g e c a p a b i l i t i e s e x i s t i n t h e Schneverdingen-Sandstein, and i n t h e cov e r s t r a t a p i l e where c h i e f l y t h e l a t e r a l l y u n l i m i t e d H a u p t s a n d s t e i n c o n t a i n s enormous q u a n t i t i e s o f gas i n p r i m a r i l y t i g h t f a c i e s t h a t i s i n d i s p e n s i b l y nec e s s i t a t i n g h y d r a u l i c p r o p p a n t f r a c t u r i n g on l a r g e r s c a l e f o r economical gas lib e r a t i o n . 1987 - 1990 f i n a l l y i s g o i n g t o see a m a j o r campaign o f development o f a whole s u i t e o f t i g h t R o t l i e g e n d gas f i e l d s i n t h e B r i t i s h Southern N o r t h Sea which a c c o r d i n g t o t h e q u a n t i t i e s o f accumulated gas and t h e d i f f i c u l t i e s t o r e c o v e r i t i n an e c o n o m i c a l l y most f e a s i b l e way w i l l become one o f t h e h i g h l i g h t s i n R o t l i e g e n d gas a c q u i s i t i o n , h y d r a u l i c proppant f r a c t u r i n g and e x p l o i t a t i o n ( c f . section 2.2.1.6.2.).
3.3.2.2. Treatment size and proppant quantity H y d r a u l i c p r o p p a n t f r a c t u r i n g o f R o t l i e g e n d gas r e s e r v o i r s i n Germany FRG a l s o s t a r t e d soon a f t e r t h e f i r s t d i s c o v e r i e s o f l o w - p e r m e a b i l i t y pay zones ( w i t h i n t h e e a r l y y e a r s due t o l a c k o f s u i t a b l e m a t e r i a l a l s o m a i n l y s m a l l e r o p e r a t i o n s h a v i n g been c a r r i e d o u t w i t h a b t . 25 - 100 t (50,000 - 200,000 l b s ) o f n a t u r a l sand; c f . TUNN 1971) and was p a r t i c u l a r l y pronounced i n t h e l a s t t e n y e a r s s i n c e t h e b e g i n n i n g o f a v a i l a b i l i t y o f s y n t h e t i c i n t e r m e d i a t e - and h i g h s t r e n g t h proppants which were f i r s t a p p l i e d i n Germany FRG i n 1978 (ERDOEL-ERD-
252
GAS AKTUELL 1978 a ) , w i t h t h e h i t h e r t o h i g h l i g h t b e i n g t h e s o f a r b i g g e s t oper a t i o n i n t h e w e l l Sohlingen Z 4 i n 1982 where t h e a b t . 550 t (1.2 Mio. l b s ) o f b a u x i t e p r o p p a n t s i n j e c t e d were t e m p o r a r i l y t o g e t h e r w i t h t h e f i n a l p r o p p a n t pumping r a t e o f a b t . 9,000 l b s . / m i n . even w o r l d r e c o r d i n MHF e x e c u t i o n w i t h h i g h - s t r e n g t h proppants, w i t h t h e r e s u l t o f t h e s u c c e s s f u l jumbo j o b h a v i n g been t h e approval o f a b t . 500 Mio. m3 ( a b t . 20 B c f ) o f gas r e s e r v e s (KLOSE & KRUMER 1983, BLEAKLEY 1984). F o r comparison, b i g t r e a t m e n t s h a v i n g been c a r r i e d o u t i n t h e USA approached pumping o f a b t . 3 - 5 Mio. l b s o f sand (WATERS 1980), and 1987 has seen t h e w o r l d r e c o r d sand q u a n t i t y o f 3,150 t o r 6.3 Mio. l b s h a v i n g been pumped i n t o a gas w e l l i n South Texas/USA (CONSTIEN, BRANNON & BANNISTER 1988; P I T T S 1988; c f . section 1.1.1.4.) which a l s o p a i d o f f v e r y q u i c k l y due t o c o n f i r m a t i o n o f enormous a d d i t i o n a l gas r e s e r v e s . Massive h y d r a u l i c f r a c t u r i n g w i t h p r o p p a n t q u a n t i t i e s o f 100 - 650 t has been f r e q u e n t l y c a r r i e d o u t i n R o t l i e g e n d and Carb o n i f e r o u s deep t i g h t gas r e s e r v o i r s i n t h e p e r i o d 1977 - 1985 ( c f . s e c t i o n 2.4.1.1.).
3.3.3. Dutch North Sea In t h e N e t h e r l a n d s , o f f s h o r e e x p l o r a t i o n a l s o was t h e l o g i c a l consequence o f t h e d i s c o v e r y o f t h e g i a n t Groningen f i e l d onshore and w i t h i n t h e f o l l o w i n g years, a l s o i m p o r t a n t R o t l i e g e n d gas f i e l d s were found e s p e c i a l l y i n t h e K - , Land P - b l o c k s o f t h e Dutch Southern N o r t h Sea ( f o r g e o g r a p h i c a l d i s t r i b u t i o n c f . OILFIELD PUBLICATIONS 1986, QUINLAN 1988; c f . f i g . 1 0 ) . H y d r a u l i c p r o p p a n t f r a c t u r i n g has a l r e a d y been c a r r i e d o u t i n reasonable amounts i n b o t h B r i t i s h and Dutch Southern N o r t h Sea, b u t i t s c l i m a x w i l l be reached i n 1987 - 1990 when t h e m a j o r development campaigns o f R o t l i e g e n d gas f i e l d s i n t h e B r i t i s h Southe r n N o r t h Sea ( f o r economical aspects c f . a l s o BEUDELL 1986 b, FISHMAN 1986; GREGORY 1986, 1987; THOMAS 1986, ALGAR 1987 a, BAND 1987, POTTER 1987) r e q u i r e o u t s t a n d i n g s t i m u l a t i o n a c t i v i t y which m i g h t be much more than a l l t h e t r e a t ments performed h i t h e r t o i n t h e whole R o t l i e g e n d B a s i n t o g e t h e r . Successful s t i m u l a t i o n p r a c t i c e s i n t h e D u t c h Southern N o r t h Sea a r e summarized by COULTER & PURVIS (1980).
3.3.4. German North Sea U n f o r t u n a t e l y so f a r no commercial R o t l i e g e n d gas s t r i k e has been made i n t h e German N o r t h Sea ( i f n e g l e c t i n g t h e two small f i e l d s Emshorn and Leybucht which a r e o n l y a few km away f r o m t h e c o a s t l i n e and have t o be c l a s s i f i e d as i n shore r a t h e r than o f f s h o r e l o c a l i t i e s ; OFFSHORE ENGINEER 1986 b ) . Some gas accum u l a t i o n s have been found t h a t c o u l d be e x p l o i t e d e c o n o m i c a l l y f e a s i b l e i f t h e y would be s i t u a t e d onshore, b u t t h e a d d i t i o n a l t r a n s p o r t expenses f o r up t o 300 km f r o m t h e o f f s h o r e l o c a t i o n t o t h e onshore gas t e r m i n a l c a n c e l s t h e p r o j e c t due t o p r o f i t a b i l i t y reasons. O t h e r R o t l i e g e n d r e s e r v o i r s c o n t a i n t o o h i g h n i t r o g e n shares ( u p t o 60 % and more) i n t h e gas t o be c o m m e r c i a l l y a t t r a c t i v e (PLEIN 1975, SCHUNEICH 1981).
3.3.5. Other areas The areas o f t h e Danish N o r t h Sea as w e l l as o f t h e German, P o l i s h and Russ i a n B a l t i c Sea c o n t a i n o n l y reduced R o t l i e g e n d s e c t i o n s i n u n f a v o u r a b l e f a c i e s development i n c l u d i n g o n l y n e g l i g i b l e amounts o f sands w i t h r e s e r v o i r q u a l i t y , o r a r e a l r e a d y o u t s i d e o f t h e Permian d e p o s i t i o n a l b a s i n on t h e Fennoscandian S h i e l d . The I r i s h o r C e l t i c Sea has s o f a r a l s o n o t d e l i v e r e d economical gas acc u m u l a t i o n s i n t h e R o t l i e g e n d i n c o n t r a s t t o some f e a s i b l e d e p o s i t s i n t h e Lower T r i a s s i c B u n t s a n d s t e i n - f a c i e s r e d beds (such as t h e Morecambe f i e l d ) . I n Germany GDR onshore, a p a r t f r o m Salzwedel ( c f . f i g . 4 ) and i t s s m a l l s a t e l l i t e s eastwards no o t h e r b i g R o t l i e g e n d gas f i e l d has been encountered s o f a r w i t h
253 reasonable shares o f c o m b u s t i b l e methane ( i n t h e B e r l i n r e g i o n , t h e r e s e r v o i r c o n t a i n s almost o n l y i n e r t n i t r o g e n ) , whereas i n Poland onshore, commercial R o t 1 iegend gas d i s c o v e r i e s were made p a r t i c u l a r l y around Ostrbw Wiel kopol s k i s o u t h o f Poznah and n o r t h o f Wroclaw as w e l l as around Szczecin ( c f . f i g . 5 and s e c t i o n 3.10.2.1.). R e s e r v o i r subsidence due t o framework compaction by pore c o l g i b l e i n t h e R o t l i e g e n d sandstones and comprises a b t . 10 y e a r s i n c o n t r a s t t o s i g n i f i c a n t subsidence i n Cretaceous c h a l k ges up t o s e v e r a l metres (SCHOONBEEK 1976; c f . a l s o s e c t i o n 4.5
apse i s n e g l i 25 cm i n 20 - 40 pays which r a n 4.1.4.).
3.4. Permeability prediction and effective permeab lity W i t h i n t h e l o w - p e r m e a b i l i t y zone o f t h e g a s - b e a r i n g R o t l i e g e n d sandstone b e l t i n t h e B r i t i s h and Dutch Southern N o r t h Sea (and i n l e s s pronounced f o r m a l s o i n Germany FRG and t h e N e t h e r l a n d s onshore), p r e d i c t i o n o f t h e n e c e s s i t y o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i s o f t e n v e r y d i f f i c u l t , because l i t t l e changes i n p e r m e a b i l i t y i n e i t h e r d i r e c t i o n can a l r e a d y r e s u l t i n e x e c u t i o n o r canc e l l a t i o n o f a s t i m u l a t i o n job, w i t h t h i s r e l a t i o n s h i p meaning t h a t o u t o f an o f f s h o r e d r i l l i n g campaign which comprises s e v e r a l dozens o f w e l l s ( a c o u p l e o f such a c t i o n s a r e scheduled i n t h e B r i t i s h Southern N o r t h Sea i n t h e coming y e a r s ) where h a l f o f t h e amount o f producers i s planned t o have t o be f r a c t u red, f o r m a t i o n e v a l u a t i o n by l o g g i n g and t e s t i n g can r e s u l t i n t h e n e c e s s i t y t o switch t o hydraulic proppant f r a c t u r i n g o f e i t h e r only a quarter o r less o r even almost t h r e e q u a r t e r s o r more o f t h e t o t a l q u a n t i t y o f t h e w e l l s , depend i n g upon degree and o r i e n t a t i o n o f t h e mismatch o f p r e d i c t i o n and a c q u i s i t i o n o f p e r m e a b i l i t y i n d i r e c t i o n o f b e t t e r o r p o o r e r f a c i e s . Some aspects o f t i g h t r e s e r v o i r p e r m e a b i l i t y range, p e r m e a b i l i t y d e t e r m i n a t i o n and p r e d i c t i o n , and temporary damage and r e s t o r a t i o n o f p e r m e a b i l i t y a r e b r i e f l y sketched as f o l lows.
3.4.1. Tight reservoir permeability range T i g h t gas sandstone r e s e r v o i r s a r e t h e m a j o r source o f f u t u r e gas p r o d u c t i o n and a r e p r e d i c t e d t o achieve a share o f a b t . 50 % by t h e y e a r 2020 i n comparison t o t h e i r c o n t r i b u t i o n o f a b t . 5 - 10 % i n t h e 1980's i n t h e USA (BAKER 1981). The maximum r e c o v e r a b l e gas r e s e r v e i n t h e t i g h t sand r e s o u r c e i s o v e r 600 T c f (15,000 B i l l . m3; NORTHROP & FROHNE 1988; c f . s e c t i o n 4 . 4 . 1 . ) . Permeabil i t y d i s t r i b u t i o n t r e n d s a r e e x t r e m e l y complex and no reasonable and r e l i a b l e p r o g n o s i s i s p o s s i b l e , w i t h t h e customary method o f e x p l o r a t i o n and a p p r a i s a l b e i n g p a t t e r n d r i l l i n g on whole t r e n d s which have been found by s e i s m i c e v a l u a tion. I n c o n t r a s t t o c o n t i n u o u s o r b l a n k e t r e s e r v o i r s , l e n t i c u l a r pay zones must be c o n t a c t e d by d r i l l i n g w e l l s on a spacing more t y p i c a l o f o i l - f i e l d than conv e n t i o n a l g a s - f i e l d development, and communication f r a c t u r i n g between t h e i s o l a t e d sand b o d i e s t o connect them t o t h e i r neighbours and t o t h e b o r e h o l e has t o be a p p l i e d f r e q u e n t l y ( c f . s e c t i o n 4 . 1 0 . 2 . ) . F i e l d boundaries a r e s e t o n l y p a r t l y by s t r u c t u r e and p o r o s i t y , b u t t o l a r g e amounts by p e r m e a b i l i t y , t h i c k ness and economics, w i t h i n t e r s p e r s e d h i g h e r - p e r m e a b i l i t y areas o c c u r r i n g as sweet s p o t s . The f o l l o w i n g o u t l i n e sketches some s p e c i a l f e a t u r e s o f R o t l i e g e n d and C a r b o n i f e r o u s i n Europe by commenting on m i c r o d a r c y pay zone p e r m e a b i l i t y , s t r a t i g r a p h i c a l d i s t r i b u t i o n o f p e r m e a b i l i t y , and gas and f l u i d f l o w i n t i g h t r o c k m a t r i x . The d i s c u s s i o n i s i n t r o d u c e d by an overview o f t h e g e n e r a l sediment o l o g i c a l and d i a g e n e t i c a l e v o l u t i o n o f t i g h t gas sands.
254
3.4.1.1. Sedimentological and diagenetical evolution o f tight gas sands T i g h t gas sands a r e c h a r a c t e r i z e d by an i n - s i t u p e r m e a b i l i t y t o gas o f 0 . 1 md o r l e s s and can be d i f f e r e n t i a t e d i n t o b l a n k e t and l e n t i c u l a r sandstones (SPENCER 1 9 8 3 ) . While b l a n k e t sandstones a r e p r e d o m i n a n t l y o f m a r i n e o r m a r g i n a l - m a r i n e genesis, l e n t i c u l a r sandstones m a i n l y o r i g i n a t e d i n f l u v i a l and d e l t a i c environments. A t h i r d c a t e g o r y o f t i g h t r e s e r v o i r s comprises c h a l k ( c f . section 4.5.4.). Sandstones a r e t i g h t as a consequence o f p r i m a r y f i n e g r a i n s i z e o r due t o secondary i n t e n s e cementation, w i t h t h e pores b e i n g p o o r l y conn e c t e d by s m a l l t h r o a t s and c a p i l l a r i e s ( c f . s e c t i o n 3 . 5 . 1 . 1 . ) . D i s c r e t e gas-wat e r c o n t a c t s a r e absent i n l e n t i c u l a r and some o f t h e b l a n k e t sandstone r e s e r v o i r s , and most o f t h e gas occurs i n s t r a t i g r a p h i c a l p e r m e a b i l i t y t r a p s . Low por o s i t y , h i g h c l a y c o n t e n t and h i g h g r a i n d e n s i t i e s o f t i g h t gas sands r e n d e r w e l l l o g a n a l y s i s d i f f i c u l t ( c f . s e c t i o n 3 . 5 . 2 . 3 . ) , and many t i g h t gas b a s i n s a r e e i t h e r o v e r p r e s s u r e d o r underpressured ( c f . s e c t i o n 4 . 4 . 2 . ) . I n c o n t r a s t t o c o n v e n t i o n a l pay complexes, t i g h t gas sands do n o t e x h i b i t a c o n s i s t e n t r e l a t i o n s h i p between p o r o s i t y and p e r m e a b i l i t y . Some aspects o f b l a n k e t and l e n t i c u l a r sandstones, p o r o s i t y and p e r m e a b i l i t y r e l a t i o n s h i p s , and r e s e r v o i r p r e s s u r e and f o r m a t i o n damage a r e summarized as f o l l o w s .
3.4.1.1.1. Blanket sandstones B l a n k e t sandstones i n e n g i n e e r i n g sense have r e l a t i v e l y b e t t e r h o r i z o n t a l c o n t i n u i t y than l e n t i c u l a r r e s e r v o i r s and respond t o h y d r a u l i c f r a c t u r i n g u s u a l l y i n a somewhat p r e d i c t a b l e manner. I n c r e a s i n g t h e p r o p p a n t volume r e s u l t s gen e r a l l y i n i n c r e a s i n g w e l l p r o d u c t i v i t y up t o c e r t a i n l i m i t s . Some examples o f b l a n k e t sandstones i n t h e Rocky Mountains t i g h t gas b a s i n s i n USA i n c l u d e t h e Lower Cretaceous "J" Sandstone i n t h e Denver-Julesburg Basin, t h e Upper C r e t a ceous Almond F o r m a t i o n and F r o n t i e r F o r m a t i o n i n t h e G r e a t e r Green R i v e r Basin, and t h e Upper Cretaceous Corcoran and C o z z e t t e Sandstones i n t h e Piceance Creek B a s i n . A l t h o u g h b e i n g c o n s i d e r a b l y more e x t e n s i v e i n h o r i z o n t a l d i r e c t i o n t h a n l e n t i c u l a r r e s e r v o i r s , a l s o b l a n k e t sandstones may e x h i b i t r e a s o n a b l e l a t e r a l v a r i a b i l i t y (LORENZ 1983, NORTHROP & FROHNE 1988). A summary of b l a n k e t t i g h t gas sands i n Texas i s g i v e n by FINLEY & O'SHEA (1983) and FINLEY ( 1 9 8 6 ) .
3.4.1.1.2. Lenticular sandstones L e n t i c u l a r r e s e r v o i r s a r e f l u v i a l sandstones which a r e v e r y d i s c o n t i n u o u s and e x h i b i t many i n t e r n a l p e r m e a b i l i t y v a r i a t i o n s ( c f . s e c t i o n 4 . 2 . 4 . 2 . ) . Geomet r y and dimensions o f t h e r e s e r v o i r u n i t s a r e d i f f i c u l t t o p r e d i c t , and t h e r e f o r e t h e response o f l e n t i c u l a r sandstones t o h y d r a u l i c f r a c t u r i n g i s v e r y e r r a t i c and g e n e r a l l y t h e s t i m u l a t i o n r e s u l t s a r e p o o r e r than u s u a l l y p o s s i b l e i n massive b l a n k e t sandstones. Some examples o f l e n t i c u l a r sandstones i n t h e Rocky Mountains t i g h t gas b a s i n s i n USA i n c l u d e f l u v i a l sandstones i n t h e Upper C r e t a ceous Mesaverde group and T e r t i a r y sandstones i n t h e San Juan, U i n t a , Piceance Creek and G r e a t e r Green R i v e r B a s i n s . The economical s e v e r i t y o f sandstone l e n t i c u l a r i t y depends on l e n s geometry, w i t h broad l e n s e s b e i n g n e a r l y as commerc i a l l y a t t r a c t i v e as b l a n k e t sands (KUUSKRAA, BRASHEAR, ELKINS & MORRA 1979). Economical f e a s i b i l i t y o f l e n t i c u l a r r e s e r v o i r s can be c o n s i d e r a b l y improved by f r a c t u r e p e n e t r a t i o n through l e n s e s which a r e n o t d i r e c t l y i n c o n t a c t w i t h t h e w e l l by communication f r a c t u r i n g ( c f . s e c t i o n 4 . 1 0 . 2 . 1 . ) . Chalks a r e marine m i c r o c r y s t a l l i n e l i m e s t o n e s o f b l a n k e t c h a r a c t e r and a r e i n t h e Rocky Mountains t i g h t gas b a s i n s r e p r e s e n t e d by t h e Upper Cretaceous N i o b r a r a and Greenhorn f o r m a t i o n s which c o n t a i n a l s o b l a n k e t t i g h t gas sands ( a s p e c t s of c h a l k s t i m u l a t i o n a r e c o m p i l e d i n s e c t i o n 4 . 5 . 4 . ) .
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3.4.1.1.3. Porosity and permeability relationships T i g h t gas r e s e r v o i r s e x h i b i t two v a r i e t i e s o f p o r o s i t y - p e r m e a b i l i t y r e l a t i o n s h i p s . The f i r s t t y p e comprises r o c k s w i t h h i g h p o r o s i t y b u t low p e r m e a b i l i t y owing t o t h e s m a l l s i z e o f pores and t h r o a t s which a r e u s u a l l y w e l l i n t e r c o n n e c t e d . T h i s t y p e i s r e p r e s e n t e d by c h a l k ( c f . s e c t i o n 4.5.4.) and some marine b l a n k e t sandstones. The second v a r i e t y i n c l u d e s l o w - p o r o s i t y sandstones where s i g n i f i c a n t p o r o s i t y amounts a r e c r e a t e d by p o s t - d e p o s i t i o n a l dissolution o f framework g r a i n s and/or m a t r i x cements, w i t h secondary p o r o s i t y (SCHMIDT & McDONALD 1979) m a i n l y e x h i b i t e d as s m a l l i s o l a t e d pores o r microvugs. R e l i c t g r a i n s and p a r t i c l e s r e s u l t i n g f r o m i n c o m p l e t e d i s s o l u t i o n do a l s o o c c u r . The second t y p e o f p o r o s i t y - p e r m e a b i l i t y r e l a t i o n s h i p s i s m a i n l y r e p r e s e n t e d by f l u v i a l l e n t i c u l a r and marine b l a n k e t sandstones. Secondary microvugs and r e s i d u a l s o f p r i m a r y p o r o s i t y a r e s c a t t e r e d t h r o u g h t h e r e s e r v o i r and a r e connected by s m a l l r i b b o n - o r s h e e t - l i k e t o r t u o u s c a p i l l a r i e s . The s m a l l s i z e o f t h e c a p i l l a r i e s r e s t r i c t s gas f l o w and causes g e n e r a l l y h i g h c a p i l l a r y p r e s s u r e s which can p r o voke l i q u i d i m b i b i t i o n d u r i n g d r i l l i n g and f r a c t u r i n g t h a t i n t u r n can t r i g g e r f o r m a t i o n damage. The r e s e r v o i r s a r e v e r y s t r e s s - s e n s i t i v e due t o c l o s i n g o f s m a l l c a p i l l a r i e s o r c r a c k s when c o n f i n i n g p r e s s u r e i s a p p l i e d . Many o f t h e microvugs i n v e r y t i g h t rocks are p a r t i a l l y o r completely f i l l e d w i t h authigenic c l a y minerals ( c f . s e c t i o n 3.5.1.1.). N o n - p o r e - f i l l i n g c l a y s a r e a l s o p r e s e n t as d e t r i t a l s h a l e c l a s t s and as a l t e r a t i o n p r o d u c t s o f v o l c a n i c and o t h e r r o c k fragments (MADER 1981, 1982, 1983 c ) . B l a n k e t sandstone r e s e r v o i r s a r e i n some cases h i g h e r i n percentage o f q u a r t z and t e n d t o become t i g h t a t s l i g h t l y g r e a t e r depths t h a n l e n t i c u l a r sandstones. W h i l e p a r t i a l l y p r i m a r y p o r o s i t y i s p r e s e r ved, c o n s i d e r a b l e amounts o f secondary p o r o s i t y a r e a l s o developed i n many t i g h t gas sands (AL-SHAIEB & WALKER 1986; LAW, POLLASTRO & K E I G H I N 1986). Diagen e t i c c o m p l e x i t y i s f r e q u e n t l y a f u n c t i o n o f d e p o s i t i o n a l environment as w e l l as b u r i a l and thermal h i s t o r y o f t h e b a s i n .
3.4.1.1.4. Reservoir pressure and formation damage R e s e r v o i r p r e s s u r e i s dependent on o r g a n i c r i c h n e s s , palaeotemperature, p r e sent-day temperature, a b i l i t y o f o r g a n i c m a t t e r t o s t i l l generate gas, and occ u r r e n c e o f l a t e r a l and v e r t i c a l s e a l i n g beds. The main mechanism f o r o v e r p r e s s u r i n g i s a c t i v e thermal gas g e n e r a t i o n i n source beds a s s o c i a t e d w i t h low-perm e a b i l i t y r e s e r v o i r s . As a consequence o f t h e i r p o r e and c a p i l l a r y c h a r a c t e r , t i g h t gas r e s e r v o i r s have a h i g h p o t e n t i a l of f o r m a t i o n damage. The t y p e s o f p o s s i b l e f o r m a t i o n damage i n c l u d e movement o f secondary c l a y s c a u s i n g p l u g g i n g o f pore throats, s w e l l i n g o f c l a y minerals, increasing water s a t u r a t i o n w i t h consequent r e d u c t i o n o f r e l a t i v e gas p e r m e a b i l i t y , l e a v i n g o f f r a c t u r i n g g e l compounds i n t h e pay zone, and p r e c i p i t a t i o n o f m i n e r a l s and compounds caused by chemical a d d i t i v e s d u r i n g a c i d i z i n g and h y d r a u l i c proppant f r a c t u r i n g . O t h e r g e o l o g i c a l and e n g i n e e r i n g aspects o f t i g h t gas sands a r e summarized by K E I G H I N (1979), R I C E & SHURR (1980), BROWN & CRAFTON (1981), GAUTIER & R I C E (1981), WALLS (1981); FINLEY, GARRETT, HAN, LIN, S A U C I E R & TYLER (1983); FINLEY & O'SHEA (1983); FINLEY (1984, 1986); FINLEY, DUTTON, LIN & S A U C I E R (1985); JENKINS (1985), AL-SHAIEB & WALKER (1986); BROWN, SMAGACA & HAEFECE (1986); CAW, POLLASTRO & K E I G H I N (1986); LAUGHREY & HARPER (19861, MOSLOW & TILLMAN (1986); PITMAN, ANDERS, FOUCH & NICHOLS (1986); WEIMER, SONNENBERG & YOUNG (1986) and DUTTON & FINLEY ( 1 9 8 8 ) .
3.4.1.2. Microdarcy pay zone permeability T i g h t r e s e r v o i r s such as o c c u r r i n g i n t h e European R o t l i e g e n d and Carbonif e r o u s and i n o t h e r f o r m a t i o n s have p e r m e a b i l i t i e s i n t h e m i c r o d a r c y range up t o a b t . 1 m i l l i d a r c y (ATTEBERRY, TUCKER & RITZ 1979; KLOSE & KRTIMER 1983, BLEAK-
256 LEY 1984, ALBERTSEN 1985; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985). Some asp e c t s o f a b s o l u t e p e r m e a b i l i t y l e v e l s and p e r m e a b i l i t y c r e a t i o n a r e i l l u s t r a t e d as f o l l o w s .
3.4.1.2.1. Absolute permeability levels A c c o r d i n a t o t h e d e f i n i t i o n by t h e F e d e r a l Energy Reserve Commission (FERC) o f t h e USA, t i g h t gas r e s e r v o i r s have a m a t r i x p e r m e a b i l i t y l e s s t h a n 0 . 1 mD (HUNT, RAYMOND, HASKETT & P I R I E 1981; HAAS, BRASHEAR & MORRA 1985; P A I , G A R B I S & HALL 1985; KUUSKRAA & HAAS 1988). I n such t i g h t r e s e r v o i r s , drawdown o f f l o wing p r e s s u r e i s p a r t i c u l a r l y pronounced d u r i n g p r o d u c t i o n l i f e and t h u s a wide range o f c l o s u r e s t r e s s e s i s p l a c e d on t h e proppants e s p e c i a l l y when f i e l d exp l o i t a t i o n c o v e r s up t o s e v e r a l decades (CLARK 1983). R o t l i e g e n d and C a r b o n i f e r o u s g a s - b e a r i n g sandstones i n Germany FRG e x h i b i t up t o 20 % and 15 % t o t a l p o r o s i t y i n extreme cases, r e s p e c t i v e l y , b u t c l a y m i n e r a l s and w a t e r s a t u r a t i o n c o n s i d e r a b l y reduce e f f e c t i v e p o r o s i t y and t h u s a v a i l a b l e p e r m e a b i l i t y f o r gas f l o w i s o n l y 10 - 50 m i c r o d a r c y ( i n comparison t o Western and E a s t e r n USA t i g h t gas sands w i t h 3 - 9 % p o r o s i t y and 1 - 50 m i c r o d a r c y p e r m e a b i l i t y ; GIDLEY, MUTTI, NIERODE & KEHN 1977; ELKINS 1980, JONES & OWENS 1980; McKETTA, K O Z I A R & COOK 1980; HUNT, RAYMOND, HASKETT & P I R I E 1981; WALLS 1981; JOHN 1983, 1987; SPENCER 1983, ECONOMIDES 1987; SATTLER, RAIBLE, GALL & GILL 1988; SOEDER & CHOWDIAH 1988) o r 1 - 100 m i c r o d a r c y (BRINKMANN 1982; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985). A comparison o f c o n v e n t i o n a l and l o w - p e r m e a b i l i t y gas r e s e r v o i r s i s g i v e n by GAUTIER & R I C E (1981), and a r e v i e w o f l o w - p e r m e a b i l i t y gas sands i s p r o v i d e d by KAZEMI ( 1 9 8 3 ) .
3.4.1.2.2. Permeability creation The m i c r o d a r c y p e r m e a b i l i t y of many t i g h t gas sandstones i s caused by cons i d e r a b l e r e d u c t i o n o f p r i m a r y p o r o s i t y up t o remnant s l o t pores between a d j o i n i n g a u t h i g e n i c q u a r t z overgrowths on d e t r i t a l sand g r a i n s (SOEDER & CHOWDIAH 1988), and much o f t h e pore volume i s o f secondary n a t u r e (sensu SCHMIDT & McDONALD 1979), a l t h o u g h f l o w p a t h s between t h e secondary pores o f t e n c o n s i s t o f n a r row s l o t p o r e s (RANDOLPH, SOEDER & CHOWDIAH 1984). I n some r e s e r v o i r s , t h e deg r e e o f p o r e a l t e r a t i o n and hence t h e degree o f t i g h t n e s s v a r i e s d i r e c t l y as a f u n c t i o n o f d e p t h (SOEDER & CHOWDIAH 1988), and t r a n s i t i o n a l stages between h i g h - p e r m e a b i l i t y sandstones ( u p t o 10 md o r more) c o n t a i n i n g o n l y s l i g h t l y a l t e r e d p r i m a r y p o r o s i t y as t h e one end member and l o w - p e r m e a b i l i t y sands (down t o 1 m i c r o d a r c y o r l e s s ) w i t h h i g h l y - a l t e r e d p r i m a r y p o r o s i t y c h a r a c t e r i z e d by narrow s l o t pores as t h e o t h e r end member o c c u r w i t h i n t h e g e o l o g i c a l column w i t h p r o g r e s s i v e b u r i a l depth and i n r e l a t i o n t o thermal h i s t o r y . F l u o r e s c e n t p e t r o g r a p h y and mercury p o r o s i m e t r y a r e u s e f u l techniques f o r r a p i d l y and i n e x p e n s i v e l y assessing t h e q u a l i t y o f a t i g h t sandstone r e s e r v o i r and can q u i c k l y r e v e a l which zones c o n t a i n highly-permeable, g r a i n - s u p p o r t e d p r i m a r y p o r o s i t y , which i n t e r v a l s a r e c h a r a c t e r i z e d by t i g h t , p r o d u c t i v e s l o t p o r e / s o l u t i o n p o r e morphology, and which s e c t i o n s i n c l u d e a h i g h l y - a 1 t e r e d low-permeabil ity p o r e structure. L o w - p e r m e a b i l i t y gas sandstones can o n l y produce gas f r o m t h e r o c k m a t r i x a t economical r a t e s i f f l o w t o t h e w e l l b o r e i s a i d e d by n a t u r a l o r h y d r a u l i c a l l y induced f r a c t u r e systems o r a c o m b i n a t i o n o f b o t h s e t s o f c r a c k s , b u t r e g a r d l e s s o f t h i s , gas p r o d u c t i o n r a t e i s s t i l l governed by t h e r a t e of gas f l o w f r o m t h e m a t r i x i n t o t h e f r a c t u r e s which depends on d e t a i l e d m a t r i x p o r e s t r u c t u r e and d i s t r i b u t i o n o f w a t e r c o n t a i n e d t h e r e i n (WARD & MORROW 1985).
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3.4.1.3. Permeability stratigraphy E f f e c t i v e p e r m e a b i l i t y under r e s e r v o i r c o n d i t i o n s a l s o v a r i e s w i t h s t r a t i g r a phy o f the R o t l i e g e n d ( c f . a l s o s e c t i o n 3.10.1.2.), w i t h t h e SchneverdingenSandstein i n t h e lower p a r t o f the R o t l i e g e n d succession i n the Weser-Elbe f a c i e s province i n Germany FRG ( f o r R o t l i e g e n d s t r a t i g r a p h y c f . s e c t i o n 3.2.1.3.) being t h e b e s t s e c t i o n w i t h abt. 20 - 50 m i l l i d a r c y (according t o p r o d u c t i o n t e s t s and core measurements), and the Wustrow-Sandstein i n the upper p a r t being a l s o reasonable w i t h up t o abt. 5 - 10 m i l l i d a r c y , whereas the Hauptsandstein i n the middle p a r t i s o f t e n very poor w i t h i n some cases n o t exceeding 1 m i l l i darcy, b u t f r e q u e n t l y having much l e s s p e r m e a b i l i t y down t o abt. 0.5 - 1 m i c r o darcy (KLOSE & KRUMER 1983; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985). As economical gas p r o d u c t i o n w i t h o u t f r a c t u r e treatment r e q u i r e s e f f e c t i v e r e s e r v o i r p e r m e a b i l i t y o f a t l e a s t 0.5 - 1 m i l l i d a r c y (LEICHT 1985), the middle s e c t i o n o f the R o t l i e g e n d pay column ( t h e Hauptsandstein) i s the major t a r g e t f o r MHF operations i n Germany FRG. Some rocks, however, are t o o t i g h t t o produce a t comm e r c i a l r a t e s even i n case o f presence o f abundant n a t u r a l f r a c t u r e s and/or f o l l o w i n g h y d r a u l i c proppant f r a c t u r i n g (SNEIDER, KING, HAWKES & D A V I S 1981; c f . s e c t i o n 3.6.1.2.).
3.4.1.4. Gas and fluid flow in tight rock matrix The behaviour o f t i g h t gas sandstone p e r m e a b i l i t y i n response t o changing c o n f i n i n g pressure can be e x p l a i n e d q u a l i t a t i v e l y by t h e complex and t o r t u o u s pore s t r u c t u r e which r e s u l t s from e x t e n s i v e compaction and diagenesis (WALLS 1981). Narrow s l o t - l i k e apertures e x i s t between pores, w i t h these t h i n cracks p r o v i d i n g the major c o n n e c t i v i t y as they a l l o w f l u i d t o move when t h e rock i s under e f f e c t i v e pressure c o n d i t i o n s , b u t they are e a s i l y c l o s e d by i n c r e a s i n g overburden pressure. A t h i g h e r c l o s u r e s t r e s s , the c o n t r i b u t i o n o f the narrow s l o t - l i k e apertures t o f l o w i s almost e l i m i n a t e d and p e r m e a b i l i t y i s p r o v i d e d by t h e rounder c r o s s - s e c t i o n channels. The e f f e c t o f p a r t i a l s a t u r a t i o n on gas p e r m e a b i l i t y i s a l s o l a r g e l y c o n t r o l l e d by pore s t r u c t u r e . Water tends t o c o l l e c t i n small pores and cracks due t o c a p i l l a r y f o r c e s and thereby prevents these channels from t r a n s m i t t i n g gas unless the pressure g r a d i e n t i s s u f f i c i e n t l y l a r g e t o cause water displacement which may be p a r t i a l l y r e s p o n s i b l e f o r the f a i l u r e o f some h y d r a u l i c f r a c t u r i n g operations t o improve w e l l performance. The s t i m u l a t i o n f l u i d i t s e l f invades the f o r m a t i o n t o some e x t e n t thereby creat i n g a zone o f almost zero gas p e r m e a b i l i t y on each s i d e o f t h e f r a c t u r e , w i t h t h i s same waterblock phenomenon being on a much l a r g e r s c a l e probably the t r a p p i n g mechanism f o r huge volumes o f gas i n some deep basins (MASTERS 1979; c f . s e c t i o n 3.11.2.4.2.). Gas i n p l a c e can be estimated f o r h y d r a u l i c a l l y f r a c t u r e d w e l l s i n t i g h t gas r e s e r v o i r s w i t h i n a margin o f e r r o r acceptable f o r engineering purposes i f the pressure data i s c o l l e c t e d a f t e r pseudo-radial f l o w regimes (SULLIVAN, POSTON & PIPER 1988).
3.4.2. Permeability determination and predict ion E f f e c t i v e p e r m e a b i l i t i e s are determined by s t a t i s t i c a l e v a l u a t i o n o f l a b o r a t o r y core measurements and recomputation w i t h a c o r r e c t i o n f a c t o r t o r e s e r v o i r c o n d i t i o n s (JONES & OWENS 1980, LEICHT 1985). An important c o m p l i c a t i o n i s perm e a b i l i t y a n i s o t r o p y and h e t e r o g e n e i t y i n t i g h t formations (ROSE 1982). Some comments are o f f e r e d as f o l l o w s on determination, p r e d i c t i o n and v a r i a t i o n s o f permeabi 1 ity.
258
3.4.2.1. Permeabi 1 ity determination I n terms o f p r o p e r d e t e r m i n a t i o n , c a u t i o n has t o be e x e r c i s e d f o r n o t f a l s i f y i n g t h e t r u e p e r m e a b i l i t y by u n s u i t a b l e l a b o r a t o r y h a n d l i n g t e c h n i q u e s (FREEMAN & BUSH 1983, SOEDER & DOHERTY 1983, CHOWDIAH 1986, LUFFEL & HOWARD 1 9 8 7 ) . Some aspects o f r e l a t i v e p e r m e a b i l i t y d e t e r m i n a t i o n a r e d i s c u s s e d by DELSHAD, MacALLISTER, POPE & ROUSE (1985); WARD & MORROW (1985), FIROOZABADI & A Z I Z (1986) ; G R A V I E R , LEMOUZY, BARROUX & ABED (1986) ; HONARPOUR, KOEDERITZ & HARVEY (1986); PARMESWEAR & MAEREFAT (1986); WATSON, K E R I G , RICHMOND & TAO (1986) ; C I V A N & DONALDSON (1987); DONALDSON, ALAM & C I V A N (1987); MOHANTY & MILLER (1988) and WATSON & ERTEKIN ( 1 9 8 8 ) . MILLER & RAMEY (1985) e v a l u a t e t h e e f f e c t s o f temperature and c o n s o l i d a t i o n on r e l a t i v e p e r m e a b i l i t y , and ALI, AL-MARHOUN, ABU-KHAMSIN & CELIK (1987) comment on t h e e f f e c t o f overburden p r e s s u r e on r e 1 a t ive permeabi 1 it y .
AGGOUR & MALIK (1987) analyze t h e e f f e c t o f c y c l i c f o r m a t i o n p r e s s u r e changes on p e r m e a b i l i t y , and WATSON (1988) i n v e s t i g a t e s t h e impact o f s t e e p temper a t u r e g r a d i e n t s on r e l a t i v e p e r m e a b i l i t y . BAKER (1988) comments on three-phase r e l a t i v e p e r m e a b i l i t y c o r r e l a t i o n s . G R I (1988) p r e s e n t s examples o f p e r m e a b i l i t y a l t e r a t i o n s as a consequence o f s t r e s s changes. WARD & MORROW (1985) o u t l i n e t h a t d i f f e r e n c e s between r e l a t i v e p e r m e a b i l i t i e s a c c o r d i n g t o overburden p r e s s u r e i n c r e a s e w i t h d e c r e a s i n g p o r o s i t y and i n c r e a s i n g s e n s i t i v i t y o f permeabil i t y t o c o n f i n i n g p r e s s u r e . L o w - p e r m e a b i l i t y gas sandstones have h i g h s u r f a c e area p e r u n i t volume, and s u r f a c e a d s o r p t i o n and c a p i l l a r y condensation c o n t r i b u t e t o r e t e n t i o n o f s i g n i f i c a n t amounts o f w a t e r a t e x t r e m e l y h i g h c a p i l l a r y p r e s s u r e s . BYRNES, SAMPATH & RANDOLPH (1979) i n v e s t i g a t e t h e e f f e c t o f p r e s s u r e and w a t e r s a t u r a t i o n on t i g h t gas sandstone p e r m e a b i l i t y . BERGOSH, LORD & OWEN (1987) i n t r o d u c e t h e overburden m u l t i d i r e c t i o n a l p e r m e a b i l i t y t e c h n i q u e . E f f e c t i v e p e r m e a b i l i t i e s o f R o t l i e g e n d and C a r b o n i f e r o u s sandstones a r e a l s o s t u d i e d by RIECKMANN ( 1 9 7 1 ) . GOGGIN, CHANDLER, KOCUREK & LAKE (1986) o u t l i n e p e r m e a b i l i t y p a t t e r n s o f a e o l i a n d e p o s i t s . V e r t i c a l p e r m e a b i l i t y can be r e l a t e d t o a known d i s t r i b u t i o n o f l a t e r a l p e r m e a b i l i t y by a method o f s h a l e management i n f i e l d - s c a l e models i n c l u d i n g t h e g e n e r a t i o n o f a p a t t e r n o f d e t e r m i n i s t i c a l l y and s t o c h a s t i c a l l y d i s t r i b u t e d s h a l e s (HALDORSEN & LAKE 1984).
3.4.2.2. Permeabi 1i ty predict ion In c o n t r a s t t o h i g h - p e r m e a b i l i t y r e s e r v o i r s where moderate d i f f e r e n c e s of pet r o p h y s i c a l p r o p e r t i e s between e x p e c t a t i o n and r e s u l t have o f t e n almost no consequences f o r c o m p l e t i o n and t e s t i n g , p r o d u c t i o n p l a n n i n g i s f r e q u e n t l y s e r i o u s l y a f f e c t e d by o n l y l i t t l e d e v i a t i o n s f r o m t h e p r o g n o s i s in l o w - p e r m e a b i l i t y pays. While p o r o s i t y mapping may be s t i l l more o r l e s s r e l i a b l e i n sandstone r e s e r v o i r s even i n t h e v i c i n i t y o f t h e boundary between sand-dominated and mud-dom i n a t e d f a c i e s b e l t s , p e r m e a b i l i t y c o n t o u r i n g i s o f t e n more d i f f i c u l t as a consequence o f much more f a c t o r s w i t h s t r o n g e r v a r i a b i l i t y and t h u s l e s s c e r t a i n p o s s i b i l i t i e s o f p r e - w e l l assessment i n f l u e n c i n g p e r m e a b i l i t y w i t h r e s p e c t t o p o r o s i t y , t h e r e b y c a u s i n g more s u r p r i s e s a f t e r p e n e t r a t i o n o f t h e r e s e r v o i r f o r ma t i on by t h e d r i 1 1 ing b i t.
3.4.2.3. Permeabi 1i ty variations P e r m e a b i l i t y changes i n t i g h t gas r e s e r v o i r s can o f t e n have d r a m a t i c e f f e c t s on r e c o v e r a b l e r e s e r v e s . KUUSKRAA, BRASHEAR, ELKINS & MORRA (1979) o u t l i n e an example where a p e r m e a b i l i t y o f 0.01 md and a c o n s t a n t f r a c t u r e d e s i g n r e s u l t s i n a r e c o v e r y e f f i c i e n c y o f 68 % o f t h e gas i n place, w i t h p e r m e a b i l i t y changes o f one o r d e r t o magnitude i n upwards and downwards d i r e c t i o n t o 0.1 and 0.001 md l e a d i n g t o r e c o v e r y e f f i c i e n c i e s o f 80 % and 25 % o f t h e gas i n p l a c e , r e spec t i v e l y .
259 A p a r t from p e t r o p h y s i c a l e f f e c t s r e l a t e d t o d i a g e n e t i c a l c l a y m i n e r a l a u t h i genesis and t r a n s f o r m a t i o n , p e r m e a b i l i t y i s i n f l u e n c e d by s e d i m e n t o l o g i c a l and p e t r o g r a p h i c a l c o m p l i c a t i o n s p a r t i c u l a r l y i n conglomerates where v a r y i n g d i s t r i b u t i o n o f g r a v e l c l a s t s o f d i f f e r e n t s i z e , frequency, shape and c o m p o s i t i o n i n t r o d u c e f u r t h e r a n i s o t r o p y and inhomogeneity f a c t o r s i n t o t h e r e s e r v o i r r o c k beyond t h e c o n t r o l e x e r t e d by sand g r a i n s and mud p a r t i c l e s and t h e i r o r i g i n a l nat u r e and secondary a l t e r a t i o n ( c f . p l a t e s 111/5-8, V/1-4, V I / 4 and V I I / 5 i n sect i o n 3.13.).
3.4.3. Temporary damage and restoration o f Permeability I n - s i t u gas p e r m e a b i l i t y i n t i g h t sandstone r e s e r v o i r s i s o f t e n t e n t o more than thousand times l e s s than i n d i c a t e d by r o u t i n e t e s t s owing t o t h e combined e f f e c t s o f overburden pressure, reduced gas s l i p p a g e , and presence o f connate w a t e r (JONES & OWENS 1980). I n v a s i o n o f aqueous d r i l l i n g o r f r a c t u r i n g f l u i d f i l t r a t e has l i t t l e permanent i n f l u e n c e on r o c k p e r m e a b i l i t y , b u t clean-up times may be e x t e n s i v e because o f t h e low l e v e l o f f o r m a t i o n p e r m e a b i l i t y (TANN I C H 1975), a l t h o u g h p e r m e a b i l i t y p r e s e r v a t i o n i n t h e invaded r e s e r v o i r zone can be a s s i s t e d by m i n i m i z a t i o n o f i n f i l t r a t i o n (JONES & OWENS 1980). The chemic a l c o m p o s i t i o n o f mud f i l t r a t e s o r f r a c t u r i n g f l u i d s i s o r d i n a r i l y o f secondar y importance i n p r e v e n t i n g p e r m e a b i l i t y impairment. E f f e c t s o f s t r e s s , gas s l i p p a g e and w a t e r a r e c o r r e l a t i n g w i t h p e r m e a b i l i t y , b u t n o t d i r e c t l y w i t h c l a y c o n t e n t , w i t h t i g h t r e s e r v o i r s e x p e r i e n c i n g l a r g e e f f e c t s w i t h b o t h low and h i g h c l a y c o n t e n t s . Most o f t h e r e s e r v o i r f o r m a t i o n s r e g a i n up t o 70 - 80 % o f t h e i n i t i a l l y undamaged p e r m e a b i l i t y o r o r i g i n a l p r o d u c t i v i t y a f t e r b e i n g s u b j e c t e d t o t r e a t ment f l u i d s (DERBY & SMITH 1979). I n t i g h t gas pays, p e r m e a b i l i t y r e d u c t i o n due t o f r a c t u r i n g f l u i d i n v a s i o n does n o t a p p r e c i a b l y a f f e c t p r o d u c t i v i t y due t o t h e i n s i g n i f i c a n t p r e s s u r e d r o p a c r o s s t h e invaded zone as compared t o t h e p r e s s u r e drop f r o m r e s e r v o i r t o w e l l b o r e (PARROT & LONG 1979). R e s t o r a t i o n o f gas p e r m e a b i l i t y by removing h i g h f l u i d s a t u r a t i o n s a d j a c e n t t o t h e f r a c t u r e f a c e can a l s o be achieved by gas c y c l i n g (BRANAGAN, COTNER & GETTMAN 1981; ZAHNER & CRAFTON 1985). R e l i a b l e r e s e r v o i r p e r m e a b i l i t y d e t e r m i n a t i o n i s e s s e n t i a l f o r p r e v e n t i n g c o s t l y d e s i g n o f MHF t r e a t m e n t s (JONES & OWENS 1980). POOLLEN (1957), HOLDITCH (1979) and SOLIMAN & HUNT (1983) a l s o assume temporary permeab i l i t y damage, whereas o t h e r s have p u t f o r w a r d arguments i n favour- o f permanent p r o d u c t i v i t y damage.
3.5. Permeability deterioration and clay mineral distribution An i m p o r t a n t mechanism o f p e r m e a b i l i t y d e t e r i o r a t i o n which a p p l i e s f o r b o t h R o t l i e g e n d and C a r b o n i f e r o u s sandstones i s t h e d i a g e n e t i c a l t r a n s f o r m a t i o n o f more o r l e s s u n s t a b l e framework g r a i n s ( m a i n l y r o c k fragments and f e l d s p a r s ) i n t o aggregates o f c l a y m i n e r a l s which o f t e n have t h e c r y s t a l morphology o f r o l l s and b o o k l e t s , p a r t i a l l y i n c o m b i n a t i o n w i t h t h e a d d i t i o n a l a u t h i g e n i c growth o f such m i n e r a l s on t h e s u r f a c e o f framework g r a i n s o r i n t h e p o r e space. A l t h o u g h t h e replacement o f i s o m e t r i c a l f a b r i c g r a i n s by c l a y m i n e r a l packages r e s u l t s i n o r i g i n o f secondary i n t e r - c l a y m i n e r a l m i c r o p o r o s i t y (SCHMIDT & McDONALD 1979) w i t h i n t h e i n t e r w o v e n networks, p l u g g i n g o f t h e f o r mer i n t e r - f a b r i c g r a i n pore space by neoforrned c l a y m i n e r a l and i r o n o x i d e agg r e g a t e s (MADER 1981, 1982, 1983 c ) , as w e l l as o r g a n i z a t i o n o f t h e much small e r and p a r t i a l l y n o t p r o p e r l y i n t e r c o n n e c t e d i n t e r s t i t i a l s i n t h e p o r e network g i v e r i s e t o much worse f l u x o f w a t e r and gas through t h e r e s e r v o i r than b e f o r e c l a y mineral diagenesis. f n the Carboniferous, t h i s i n t e r - c l a y mineral microporosity o f t e n leads a l s o t o a h i g h e r w a t e r s a t u r a t i o n (due t o i n c r e a s i n g amounts o f connate w a t e r ) t h a n t h a t expected f r o m p o r o s i t y d e t e r m i n a t i o n and gas shows, w h i c h i s one o f t h e main problems o f p r o p e r p e t r o p h y s i c a l w e l l l o g i n t e r p r e t a t i o n i n t h i s s t o r e y o f
260 the gas-prospective geological column a n d based on t h a t decision on running casing, perforating, t e s t i n g and probably a l s o f r a c t u r i n g , or not t o do anything b u t plug and abandon the well. Formation damage by permeability d e t e r i o r a t i o n increases with f r a c t u r i n g f l u i d pH and time of exposure (SIMON & COON 1976, ZAHNER & CRAFTON 1985). Reduct i o n of pH-value can be e f f e c t i v e l y performed by addition of low amounts of methanol to a crosslinked stimulation gel (HARP 1980). Formation damage by increasing r e l a t i v e water permeability and water imbibition ( c f . section 4 . 5 . 4 . 5 . ) i s becoming worse the longer the r e s e r v o i r i s exposed t o the treatment f l u i d s as well as the l a r g e r the f r a c t u r e area i s subjected t o f l u i d invasion, with t h u s frequently longer cracks being more s e r i o u s l y affected t h a n s h o r t e r ones (ZAHNER & CRAFTON 1985). Aspects of clay mineralogy and morphology as well as permeability d e t e r i o r a t i o n a r e discussed as follows.
3.5.1. Clay mineralogy and morphology Different aspects of clay mineralogy and morphology as a f f e c t i n g r e s e r v o i r sandstone porosity and permeability a s we1 1 as supporting o r i n h i b i t i n g hydrocarbon flow a r e i l l u s t r a t e d as follows. An o u t l i n e i s a l s o given on the d i f f e rences between d e t r i t a l and authigenic clay minerals.
3.5.1.1. Clay morphology Three main morphological types of dispersed clay minerals in sandstone r e s e r voirs can be distinguished : d i s c r e t e , pore-lining intergrown a n d pore-bridging aggregated p a r t i c l e s (NEASHAM 1977 a ) . Discrete p a r t i c l e s are s c a t t e r e d throughout the pore system and do not form intergrown c r y s t a l frameworks on pore walls or within i n t e r g r a n u l a r pores. They have a more o r l e s s random arrangement with respect t o one another a n d a f f e c t petrophysical properties mainly by reducing intergranular pore volume and behaving as migrating f i n e s in the pore system. Pore-lining p a r t i c l e s a r e attached t o pore walls t o form a r e l a t i v e l y c o n t i nuous thin clay mineral coating. The c r y s t a l s are oriented e i t h e r p a r a l l e l o r perpendicular to the pore wall surface. Pore-bridging aggregates extend f a r i n t o o r completely across pore t h r o a t s t o c r e a t e the bridging e f f e c t by extensive development of intergrown and/or i n tertwined clay minerals within the pore system, creating both microporosity and tortuous fluid-flow pathways. Low-permeability r e s e r v o i r rocks frequently cont a i n pore-bridging clay mineral types. Distinction should be made between pores which are l a r g e spaces surrounded by rock p a r t i c l e s , a n d t h r o a t s which a r e the narrow p a r t s between two adjacent p a r t i c l e s ( G A O , SHEN & TU 1986).
3.5.1.2.
Clay mineralogy
Formation clays have a profound reducing e f f e c t on matrix permeability ( K U KAL, BIDDISON, HILL, MONSON & SIMONS 1983; McLEOD 1984). D i f f e r e n t clay miner a l s p r e c i p i t a t e as d i f f e r e n t morphological forms. Kaolinite i s deposited as d i s c r e t e booklets of clay c r y s t a l s s c a t t e r e d throughout the pore system, a n d since k a o l i n i t e more commonly occupies intergranular space t h a n i t coats or l i nes pore w a l l s , i t i s susceptible t o migration when conditions of f l u i d flow p r e v a i l . M i g r a t i n g clays adversely a f f e c t permeability by clogging or p a r t i a l l y bridging and blocking pore t h r o a t s . The most commonly observed form of diagenet i c clays i s as pore coatings o r l i n i n g s . Chlorite, i l l i t e a n d smectite a r e a l l deposited as r e l a t i v e l y continuous l i n i n g s on m a t r i x grain surfaces. Smectite and i l l i t e tend t o grow outward from the grain surface i n t o the pore space, res u l t i n g in microporosity and d r a s t i c a l l y reduced permeability. Smectite swells w h e n i n contact w i t h water foreign t o the r e s e r v o i r , thereby rendering permeabil i t y reduction during the stimulation treatment a serious problem.
261
3.5.1.3. Detrital and authigenic clay minerals D i s t i n c t i o n can be made between d e t r i t a l c l a y m i n e r a l s having been incorporat e d i n t o the sand d u r i n g d e p o s i t i o n w i t h i n the sedimentary b a s i n below the base l e v e l o f erosion, and a u t h i g e n i c c l a y m i n e r a l s which o r i g i n a t e d a f t e r deposit i o n o f the sand through e a r l y palaeosol f o r m a t i o n (MADER 1983 c, 1984 b), earl y t o advanced ground-water-table i n f l u e n c e , advanced b u r i a l diagenesis o r even l a t e renewed t e c t o n i c a l u p l i f t and weathering impact. D e t r i t a l c l a y s which are o r i g i n a l l y i n s e r t e d i n t o the r e s e r v o i r a t the time o f d e p o s i t i o n a r e o f t e n l a i d down as h o r i z o n t a l o r s u b h o r i z o n t a l laminae o r l e n t i c l e s w i t h i n a sandstone which a c t as d i r e c t i o n a l p e r m e a b i l i t y b a r r i e r s l i m i t i n g v e r t i c a l p e r m e a b i l i t y g r e a t l y w h i l e having i n most cases l i t t l e e f f e c t on h o r i z o n t a l p e r m e a b i l i t y (unless t h e c l a y drapes are p a r a l l e l t o c r o s s - s t r a t i f i c a t i o n s t r u c t u r e s where they extend o b l i q u e l y through the r e s e r v o i r and can a l s o c o n s i d e r a b l y d i s t u r b o r i n h i b i t l a t e r a l f l o w ) . The o r i g i n o f a u t h i g e n i c c l a y s means f o r m a t i o n o f secondary p o r o s i t y (SCHMIDT & McDONALD 1979) a t the expense o f p r i m a r y p o r o s i t y . Primary p o r o s i t y r e d u c t i o n c o n t r i b u t e s h e a v i l y t o the h i g h c a p i l l a r y pressure o f the rock and r e s u l t s i n h i g h water s a t u r a t i o n and low r e l a t i v e gas p e r m e a b i l i t y . Aspects of c l a y mineralogy and morphology as w e l l as d e t e r m i n a t i o n o f volume, type and d i s t r i b u t i o n o f c l a y m i n e r a l s from w i r e l i n e l o g g i n g records a r e summarized by FERTL & CHILINGAR (1988). R e l a t i o n ships o f R o t l i e g e n d p o r o s i t y and p e r m e a b i l i t y are discussed by FERTL & CAVANAUGH (1973).
3.5.1.4. Other aspects Morpholopy and d i s t r i b u t i o n o f c l a y m i n e r a l s i n o i l - and gas-bearing pay f o r mations ( p a r t i a l l y based on scanning e l e c t r o n microscope a n a l y s i s , energy d i s p e r s i v e spectrometry, image processing and x-ray d i f f r a c t i o n a n a l y s i s ) a r e a l s o s t u d i e d by TIMUR, HEMPKINS & WEINBRANDT (1971) ; GAIDA, ROHL & ZIMMERLE (1973) ; K I E K E & HARTMAN (1973), STALDER (1973), DULLIEN & DHAWAN (1974); HOLUB, MALY, NOEL & WEINBRANDT (1974); THOMAS, CRDWE & SIMPSON (1976); NEASHAM (1977 b), WILSON & PITTMAN (1977), THOMAS (1978), WARDLAW & CASSAN (1978), K E I G H I N (1979), PITTMAN (1979), PITTMAN & THOMAS (1979), COULTER & P U R V I S (198D), SCHRANK & HUNT (1981); SNEIDER, KING, HAWKES & D A V I S (1981); WALLS (1981); CROCKER, DONALDSON & MARCHIN (1983); KUKAL, BIDDISON, HILL, MONSON & SIMONS (1983); HARR I S , HAYNES & EGGER (1984); LEE (1984), ORTIZ & McLANE (1984), RUZYLA (1984), BACKFISCH & HELING (1985); GARBIS, BROWN & MAURITZ (1985); McLANE & EDGINGTON (1985); GAD, SHEN & TU (1986); LAUGHREY & HARPER (1986); SCHAIBLE, AKPAN & AYOUB (1986); G I E S (1987) and KRUEGER (1988). Pore s t r u c t u r e s and a u t h i g e n i c c l a y m i n e r a l s i n R o t l i e g e n d and Carboniferous gas r e s e r v o i r s are s t u d i e d by DRONG (1979), CDULTER & PURVIS (1980), JOHN (1983, 1987), DEURER & MENZ (1984), DIETZEL & HANTELMANN (1985); i n Western USA t i g h t gas sands by WALLS (1981) and SOEDER & RANDOLPH (1987), and i n o t h e r f o r mations by VITTHAL, GUPTA & SHARMA (1987). WILSON (1981) reviews the o r i g i n o f c l a y s c o n t r o l l i n g p e r m e a b i l i t y i n t i g h t gas sands. RUHOVETS & FERTL (1982) present volumes, types and d i s t r i b u t i o n o f c l a y m i n e r a l s i n r e s e r v o i r r o c k s on the base o f w e l l - l o g e v a l u a t i o n . An assessment o f f o r m a t i o n p r o p e r t i e s from stimul a t i o n treatments f o r Appalachian t i g h t gas sands i s performed by CHARLES, HUDOCK, UDICK & HALL (1983) and BRANAGAN, CIPOLLA, LEE & WILMER (1985). SPENCER (1983), FINLEY (1985) ; WARPINSKI, BRANAGAN, SATTLER, LORENZ, NORTHROP, MANN & FROHNE (1985) and JOHN (1987) discuss some g e o l o g i c a l and p e t r o p h y s i c a l aspects o f t i g h t gas r e s e r v o i r s . OSTENSEN (1983) ; RANDOLPH, SOEDER & CHDWDIAH (1984) and SOEDER & RANDOLPH (1987) summarize t h e knowledge on p o r o s i t y and p e r m e a b i l i t y of t i g h t sands. An overview o f the d i a g e n e t i c a l e v o l u t i o n o f Buntsandstein r e d beds i s compiled by MADER (1981, 1982, 1983 c ) .
262 Experimental p e t r o p h y s i c a l m o d e l l i n g o f t i g h t gas sands i s c a r r i e d o u t by SATTLER, HECKES & CLARK ( 1 9 8 4 ) . CHOWDIAH & SOEDER (1988) p r e s e n t a comparison sandstones. DUTTON & FINLEY o f pore geometry i n h i g h - and l o w - p e r m e a b i l i t y (1988) and FRACASSO, DUTTON & FINLEY (1988) g i v e a comprehensive s e d i m e n t o l o g i c a l and p e t r o g r a p h i c a l e v a l u a t i o n o f a t y p i c a l t i g h t gas sandstone f o r m a t i o n . O A V I E S & ALMON (1981) s t r e s s t h e r o l e o f d i a g e n e s i s i n s u c c e s s f u l f o r m a t i o n i n t e r p r e t a t i o n , w e l l p r o d u c t i o n , r e s e r v o i r s t i m u l a t i o n and enhanced r e c o v e r y .
3.5.2. Permeabi 1 i ty deterioration LEONE & SCOTT (1987) summarize t h e mechanisms by which r o c k - f l u i d i n t e r a c t i o n s can l e a d t o p e r m e a b i l i t y damage by m i g r a t i o n o f f i n e s . D i s t i n c t i o n has t o be made between mechanical f i n e s m i g r a t i o n , w a t e r and/or b r i n e s e n s i t i v i t y , and geochemical t r a n s f o r m a t i o n . P e r m e a b i l i t y l o s s by mechanical f i n e s m i g r a t i o n occ u r s when l o o s e l y a t t a c h e d p a r t i c l e s a r e m o b i l i z e d due t o d r a g f o r c e s e x e r t e d by t h e f l o w i n g f l u i d s and subsequently c o l l e c t i n p o r e t h r o a t s i n s u f f i c i e n t q u a n t i t i e s t o cause f l o w p a t h p l u g g i n g . Water and/or c l a y s e n s i t i v i t y l e a d s t o s w e l l i n g o f c l a y minerals, w i t h the p a r t i c l e s i n the swollen s t a t e r e q u i r i n g more space than n o r m a l l y and e a s i e r t e n d i n g t o r e l e a s i n g f i n e s . Geochemical t r a n s f o r m a t i o n s comprise d i s s o l u t i o n , p r e c i p i t a t i o n and chemical r e a c t i o n when f o r e i g n f l u i d s a r e i n j e c t e d i n t o t h e r e s e r v o i r such as d u r i n g s t i m u l a t i o n o r enhanced r e c o v e r y . Some aspects o f c l a y m i n e r a l d i s p e r s i o n and w a t e r s e n s i t i v i t y , c l a y m i n e r a l s t a b i l i z a t i o n , c l a y m i n e r a l d i s t r i b u t i o n and l o g i n t e r p r e t a t i o n , n e g a t i v e o r i e n t a t i o n o f p e r m e a b i l i t y changes, and c a p i l l a r y p r e s s u r e and permeab i l i t y r e l a t i o n s h i p s a r e d i s c u s s e d as f o l l o w s .
3.5.2.1. Clay mineral dispersion and water sensitivity The most i m p o r t a n t process o f p e r m e a b i l i t y d e t e r i o r a t i o n i s c l a y m i n e r a l d i s p e r s i o n and m i g r a t i o n i n sandstone r e s e r v o i r s (GRAY & REX 1965). D i s t i n c t i o n can be made between n a t u r a l p r i m a r y - d e t r i t a l provenance o r s e c o n d a r y - a u t h i g e n i c o r i g i n o f c l a y m i n e r a l c r y s t a l s which a r e m o b i l i z e d by e x t e r n a l i n f l u e n c e s coming f r o m d r i l l i n g , c o m p l e t i o n , t e s t i n g and t r e a t i n g mechanisms, and t e r t i a r y a r t i f i c i a l d e r i v a t i o n , w i t h t h e p a r t i c l e s b e i n g s y n t h e t i c a l l y i n s e r t e d i n t o and d i s t r i b u t e d w i t h i n t h e r e s e r v o i r by v a r i o u s a c q u i s i t i o n and s t i m u l a t i o n p r o c e s ses. The o u t l i n e as f o l l o w s focusses on aspects o f c l a y m i n e r a l d i s p e r s i o n cont r o l as w e l l as c l a y m i n e r a l c o n s t e l l a t i o n and p e r m e a b i l i t y damage. Comments a r e a l s o o f f e r e d on w a t e r - vs. o i l - b a s e d s t i m u l a t i o n f l u i d s f o r t h e c o n t r o l o f water-sensi t i v e reservoirs.
3.5.2.1.1. Clay mineral dispersion control Clay m i n e r a l d i s p e r s i o n w i t h i n p r e d o m i n a n t l y w a t e r - s e n s i t i v e r e s e r v o i r s can t a k e p l a c e by h y d r a t i o n o f exchangeable c a t i o n s and p a r t i c l e s u r f a c e s , r e p u l s i o n o f i n t e r a c t i n g c l o u d s o f exchangeable c a t i o n s , d e s o r p t i o n o r chemical remoVal o f sorbed b i n d i n g agents, n e u t r a l i z a t i o n o f p o s i t i v e charges on p a r t i c l e edges, mechanical shear, and thermal (Brownian) motions (HOLCOMB 1986). O p t i o n s f o r c l a y c o n t r o l i n damage-potential r e s e r v o i r s a r e l e a v i n g c l a y s alone, r e n d e r c l a y s e i t h e r t e m p o r a r i l y o r permanently i n a c t i v e , remove c l a y s by d i s s o l u t i o n , o r t a k e o f f w a t e r f r o m h y d r a t i n g o r m o b i l i z i n g f o r c e s . KC1 o r NH4C1 b r i n e s s h o u l d be used f o r temporary c l a y c o n t r o l o n l y . Organic c a t i o n i c polymers a r e t h e most w i d e l y used c l a y - s t a b i l i z i n g additives t h a t f o r m monomolecular f i l m s on c l a y s u r f a c e s by c a t i o n exchange, w i t h t h i s f i l m i n g a l l o w i n g more permanent c l a y c o n t r o l due t o p a r t i c l e b r i d g i n g o v e r m u l t i p l e c a t i o n i c s i t e s a l o n g polymer c h a i n s . Longer l a s t i n g s t i m u l a t i o n r e s u l t s can a l s o be p r o v i d e d by anhydrous low-pH methanol prepads o r breakdown t r e a t m e n t i n c l u d i n g f i n e s - s u s p e n d i n g s u r f a c t a n t and c l a y c o n t r o l a d d i t i v e t o leak o f f i n the near-wellbore r e g i o n t o p a r t i a l l y s o l u b i l i z e , shrink, stabilize
263 and suspend r e l e a s e d f i n e s as w e l l as reduce w a t e r b l o c k ( c f . s e c t i o n I n any case, p r e v e n t i n g da3.11.2.4.2.) o r i m b i b i t i o n ( c f . s e c t i o n 4.5.4.5.). mage t o a r e s e r v o i r f o r m a t i o n i s always e a s i e r t h a n a t t e m p t i n g t o c o r r e c t damage a l r e a d y caused.
3.5.2.1.2. Clay mineral constellation and permeability damage The p o r e space o f R o t l i e g e n d sandstones i s o f t e n dominated by f i b r o u s i l l i t e composed o f needles and l e a v e s w h i c h s t a r t r a d i a l l y f r o m t h e h e m a t i t e and c l a y p e l l i c l e s and f o r m t h i c k c o a t i n g s around t h e framework g r a i n s i n a more o u t wards p o s i t i o n than t h e e a r l i e r s u r f a c e drapes (JOHN 1983, 1987; DIETZEL & HANTELMANN 1985). Mechanical s t r e s s causes i l l i t e needles t o break i n t o p i e c e s , y i e l d i n g t o g e t h e r w i t h s i l t - s i z e d g r a i n s a l a r g e p o t e n t i a l o f p e r m e a b i l i t y damage by p l u g g i n g w i t h f i n e s . As a consequence o f temperature, d e p t h and b u r i a l h i s t o r y , most o f t h e a u t h i g e n i c c l a y m i n e r a l s have been t r a n s f o r m e d t o i l l i t e i n l a t e r d i a g e n e t i c a l stages, w i t h t h e t o t a l c o n t e n t o f d i s p e r s e d c l a y m i n e r a l s v a r y i n g i n l a t e r a l and v e r t i c a l d i r e c t i o n . P a r t i c u l a r l y s e r i o u s f o r p e r m e a b i l i ty d e s t r u c t i o n i s the concentration o f i l l i t e f i b r e s i n the pore throats ( l e a v i n g t h e c e n t r e s o f l a r g e pores e i t h e r open o r c l o s i n g them a l s o w i t h a micropor o u s f a b r i c ) and t h u s r e a s o n a b l y p r e s e r v e p o r o s i t y , b u t o b s t r u c t o r p l u g t h e c o n n e c t i o n s between a d j o i n i n g pores, t h e r e b y d r a s t i c a l l y r e d u c i n g t h e i n t e r c o n nectedness o f t h e i n t e r s t i t i a l s and t h u s a l s o f l u i d f l u x (JOHN 1983, 1987). E f f e c t s o f p o r e - b l o c k i n g mechanisms on f o r m a t i o n damage a r e a l s o r e p o r t e d by WOJTANOWICZ, KRILOV & LANGLINAIS (1987), and MUECKE (1978), GAUTIER & R I C E (1981), SHARMA & YORTSOS (1986) and PRIISHOLM, NIELSEN & HASLUNO (1987) comment on f i n e s m i g r a t i o n , b l o c k i n g and c l a y s w e l l i n g as main r e s e r v o i r d e t e r i o r a t i o n mechanisms. POTTER & DIBBLE (1983) p r e s e n t f o r m a t i o n damage by c o l l o i d p l u g g i n g . R e s t r i c t e d p o r e c o n n e c t i o n s a r e a l s o i n d i c a t e d by h i g h c a p i l l a r y p r e s s u res i n r e l a t i o n t o t o t a l p o r o s i t y . V e r t i c a l permeability i s p a r t i a l l y very l i m i ted, r e s u l t i n g i n d i r e c t i o n a l p e r m e a b i l i t y a n i s o t r o p y up t o 80 % ( c f . s e c t i o n 4 . 2 . 4 . 2 . ) . PALLATT, WILSON & McHARDY (1984) d i s c u s s t h e r e l a t i o n s h i p between p e r m e a b i l i t y and morphology o f d i a g e n e t i c c l a y m i n e r a l s . Aspects o f m i g r a t i o n o f f i n e s and c l a y m i n e r a l s i n t h e m a t r i x o f p a r t i a l l y w a t e r - s e n s i t i v e sandstones a r e a l s o d i s c u s s e d by GABRIEL & INAMDAR (1983); SHARMA, YORTSOS & HANDY (1985) and SHARMA & YORTSOS ( 1 9 8 6 ) . COULTER & PURVIS (1980) a n a l y z e c l a y miner a l s i n t h e R o t l i e g e n d o f t h e Dutch N o r t h Sea f o r t h e purpose o f f r a c t u r e f l u i d c o m p a t i b i l i t y determination.
3.5.2.1.3. Water- vs . oi 1 -based stimulation fluids Although i l l i t e s are n o t s w e l l i n g clays, they e x h i b i t a c e r t a i n water sensit i v i t y . The p o r e space i s i n f i l l e d w i t h connate w a t e r and i n i t i a l gas s a t u r a t i o n , w i t h t h e i l l i t e needles and l e a v e s b e i n g enveloped by connate w a t e r (DEURER & MENZ 1984, DIETZEL & HANTELMANN 1985). D u r i n g l o w - r a t e gas f l o w , t h e conn a t e w a t e r does n o t move and t h e r i g i d i l l i t e c r y s t a l s a r e s h e l t e r e d a g a i n s t mec h a n i c a l f o r c e s . F l u s h i n g o f t h e p o r e space w i t h a water-based f l u i d , however, t r a n s m i t s t h e c u r r e n t movement t o t h e connate w a t e r and t h u s o n t o t h e i l l i t e needles, r e s u l t i n g i n b r e a k i n g - o f f o f t h e r i g i d i l l i t e s t r u c t u r e s and t r a n s p o r t i n g t h e d e b r i s w i t h t h e f l u s h i n g f l u i d t h r o u g h t h e p o r e space. I n a d d i t i o n t o d i s i n t e g r a t i o n o f t h e m a t r i x framework compoundness, p l u g g i n g o f p o r e t h r o a t s by t h e i l l i t e p i e c e s may o c c u r . U s i n g an o i l - b a s e d f l u i d , t h e connate w a t e r s a t u r a t i o n may be decreased t o t h e i r r e d u c i b l e minimum, b u t t h e w a t e r envelopes s t i l l p r o t e c t t h e c l a y c r y s t a l s a g a i n s t t h e c u r r e n t movement i n t h e n o n - w e t t i n g f l u s h i n g f l u i d , and due t o t h e d i s c r e t e i n t e r f a c e , no a x i a l f o r c e s a r e t r a n s m i t t e d t h r o u g h t h e i r r e d u c i b l e w a t e r s a t u r a t i o n o n t o t h e i l l i t e needles. T h i s c l a y m i n e r a l o g i c a l c o n s t e l l a t i o n i s t h e reason f o r t h e w a t e r s e n s i t i v i t y o f t h e R o t l i e g e n d r e s e r v o i r s i n sev e r a l patches of t h e b a s i n (OIETZEL & HANTELMANN 1985; SOMMER 1987, 1988), whe-
264 r e a s i n o t h e r spots, t h e f o r m a t i o n i s non-water s e n s i t i v e and common water-based f l u i d s can be a p p l i e d (KLOSE & KRUMER 1983; c f . a l s o s e c t i o n 3 . 1 1 . ) . Water s e n s i t i v i t y o f R o t l i e g e n d and B u n t s a n d s t e i n gas r e s e r v o i r s due t o c l a y m i n e r a l s i s a l s o r e p o r t e d by COULTER & P U R V I S ( 1 9 8 0 ) .
3.5.2.2. Clay mineral stabilization Clay s t a b i l i z i n g agents a c t by permanently h o l d i n g c l a y p a r t i c l e s t o g e t h e r and p r e v e n t i n g t h e i r movement o r d i s p e r s i o n and subsequent p a r t i c l e p l u g g i n g (DOWELL SCHLUMBERGER 1988 d ) . They a r e used t o i n h i b i t p e r m e a b i l i t y damage due t o f o r e i g n w a t e r i n t r u s i o n i n w a t e r - s e n s i t i v e r e s e r v o i r s . C l a y s t a b i l i z e r s can be a p p l i e d a f t e r m a t r i x a c i d i z i n g p r i o r t o g r a v e l p a c k i n g o r as a spearhead f o r h y d r a u l i c p r o p p a n t f r a c t u r i n g w i t h water-based f l u i d s . C l a y m i n e r a l s t a b i l i z a t i o n i n w a t e r - s e n s i t i v e r e s e r v o i r s can u s u a l l y be b e s t achieved by 2 % KC1 s o l u t i o n s which a r e used as a s t a n d a r d s a f e t y p r e c a u t i o n i n t h e prepad and d i s p l a cement stages o f h y d r a u l i c f r a c t u r i n g t r e a t m e n t s s i n c e many y e a r s (WATERS 1980; c f . s e c t i o n 1.4.10.4.2.). KC1-bearing water-based f l u i d s can a l s o be used f o r s t i m u l a t i o n i f t h e p r o p e r pH-value i s chosen and c a r e f u l l y k e p t c o n s t a n t d u r i n g t h e t r e a t m e n t (ORTIZ & McLANE 1984). Some comments a r e o f f e r e d as f o l l o w s on low- and high-pH s o l u t i o n s as w e l l as methanol prepad f o r i l l i t e p r e c o n d i t i o n ing.
3.5.2.2.1. Low- and high-pH solutions Low-pH ( a b t . 4) KC1-bearing water-based f l u i d s a r e v e r y e f f e c t i v e i n d i r t y sandstones by removing some o f t h e a u t h i g e n i c c l a y s t r u c t u r e s t h a t c l o g p o r e t h r o a t s , t h e r e b y r e d u c i n g t h e number o f s i t e s f o r p o t e n t i a l w a t e r b l o c k i n g w h i l e l e a v i n g t h e d e t r i t a l cementing c l a y s i n t a c t . Higher-pH ( a b t . 7 ) s o l u t i o n s , however, a r e c o n s i d e r a b l y damaging, because a l t h o u g h n o t c a u s i n g s w e l l i n g o r m i g r a t i o n o f c l a y p a r t i c l e s , t h e more o r l e s s n e u t r a l f l u i d does n o t have t h e c a p a c i t y t o clean-up c l a y - f i l l e d p o r e spaces and may i n f a c t be t r a p p e d i n t h e e x t r e mely small c a v i t i e s o f t h e m i c r o p o r o s i t y network. O t h e r aspects o f c l a y s t a b i l i z a t i o n w i t h KC1 s o l u t i o n s a r e d i s c u s s e d by BLACK & HOWER (1965); McLAUGHLIN, ELPHINGSTONE & HALL (1976); BLACK (1978); BLACK, RIPLEY, BEECROFT & PAMPLIN (1979) and YOUNG & McLAUGHLIN (1979; c f . s e c t i o n 1.4.10.4.2.).
3.5.2.2.2. Methanol prepad f o r i 1 1 i te precondi t ioning Another method o f c l a y m i n e r a l s t a b i l i z a t i o n i s p r e c o n d i t i o n i n g o f t h e f r a c t u r e w a l l s u r f a c e s w i t h a methanol prepad c o n t a i n i n g a l s o a c e t i c a c i d and s u r f a c t a n t a d d i t i v e s which r e a c t s w i t h f i l m y i l l i t e b r i d g i n g t h e p o r e t h r o a t s i n such a way t h a t t h e i l l i t e r o o t s s t i l l c l i n g t o t h e g r a i n s u r f a c e s , t h e r e b y h e l p i n g t o m a i n t a i n m a t r i x competence, whereas t h e p o r e space i s l e f t u n o b s t r u c t e d a f t e r f l u s h i n g (ORTIZ & McLANE 1984). T h i s p r e c o n d i t i o n i n g r e s u l t s i n e n l a r g i n g o f p o r e i n t e r s t i t i a l s and r e d u c i n g o f s u r f a c e t e n s i o n o f t h e i n v a d i n g wat e r , and t h u s a f o l l o w i n g water-based t r e a t m e n t f l u i d i s l e s s l i k e l y t o be t r a p ped by t h e m i c r o p o r o s i t y o f t h e a u t h i g e n i c i l l i t e webs. Aspects o f c l a y p r o t e c t i o n chemicals i n s t i m u l a t i o n f l u i d s a r e a l s o d i s c u s s e d by CALLAWAY, O R T I Z & HOLCOMB ( 1 9 8 2 ) . F o r m a t i o n damage d u r i n g w a t e r f l o o d i n g and water-based f r a c t u r i n g i n h i g h - c l a y - c o n t e n t r e s e r v o i r s i s e v a l u a t e d by O R T I Z & McLANE (1984) and LEONE & SCOTT (1987), and C I V A N & KNAPP (1987) comment on e f f e c t s o f c l a y swell i n g and f i n e s m i g r a t i o n on f o r m a t i o n p e r m e a b i l i t y .
3.5.2.3. Clay mineral distribution and log interpretation Problems o f l o g i n t e r p r e t a t i o n of t i q h t qas sands a r e assessed by RUHOVETS & FERTL (1982); KUKAL, BIDDISON, HILL, MONSON & SIMONS (1983); MONSON & SIMONS (1983), KUKAL (1984); BUSCH, SOBKOWICH, NEILL, SINHA & FERTL (1985); KUKAL &
265
SIMONS (1985), RAIBLE & GALL (1985), TSAY & FANG (1986) and KUCHUK ( 1 9 8 7 ) . The i n t e r p r e t a t i o n o f w i r e l i n e l o g s i n l o w - p o r o s i t y and - p e r m e a b i l i t y t i g h t gas sandstones i s c o m p l i c a t e d by time-dependent r e s i s t i v i t y changes due t o deep f i l t r a t e i n v a s i o n (KUUSKRAA & HAAS 1988). C o r r e c t i d e n t i f i c a t i o n o f d r i l l i n g f l u i d - f l u s h e d i n t e r v a l s t h a t have h i g h gas s a t u r a t i o n which t r a d i t i o n a l l o g eval u a t i o n would r e a d as w a t e r - b e a r i n g , a c c u r a t e p r e d i c t i o n o f i r r e d u c i b l e w a t e r s a t u r a t i o n , and p e r m e a b i l i t y c a l c u l a t i o n can be performed w i t h an i n t e g r a t e d l o g a n a l y s i s system. Some aspects o f p o r o s i t y e v a l u a t i o n and deep f i l t r a t e i n v a s i o n i n t i g h t gas sands a r e i l l u s t r a t e d as f o l l o w s .
3.5.2.3.1. Porosity evaluation P o r o s i t y i n t e r p r e t a t i o n i n t i g h t gas sands f r o m geophysical w e l l l o g s i s comp l i c a t e d by s e v e r a l g e o l o g i c a l l y - d e p e n d e n t f a c t o r s i n c l u d i n g m a t r i x v a r i a t i o n s , i n c o m p l e t e f l u i d i n v a s i o n , complex r e s e r v o i r l i t h o l o g i e s , and f o r m a t i o n c l a y s (KUKAL, BIDDISON, HILL, MONSON & SIMONS 1983). S p e c i a l c o m p l i c a t i o n s a r e format i o n c l a y q u a n t i f i c a t i o n , w a t e r r e s i s t i v i t y i n t e r p r e t a t i o n and w a t e r s a t u r a t i o n e v a l u a t i o n . The g e o l o g i c a l l y dependent assessment problems a r e r e l a t e d t o t h e complex d e p o s i t i o n a l , d i a g e n e t i c a l and s t r u c t u r a l h i s t o r i e s o f t h e t i g h t gas sands. O t h e r d i f f i c u l t i e s o f t i g h t gas sand l o g i n t e r p r e t a t i o n a r e d i s c u s s e d by KUKAL (1981) and ROSEPILER ( 1 9 8 1 ) .
3.5.2.3.2. Deep filtrate invasion in tight sandstones L o w - p e r m e a b i l i t y l o w - p o r o s i t y r o c k s a r e g e n e r a l l y deeply invaded by mud f i l t r a t e d u r i n g t h e process o f d r i l l i n g a w e l l , w h i l e more porous and permeable f o r m a t i o n s have p r i n c i p a l l y s h a l l o w e r i n v a s i o n (KUKAL, B I D D I S O N , HILL, MONSON & SIMONS 1983). The reason f o r t h i s i s t h a t i n more porous r o c k s which have more space a v a i l a b l e f o r l i q u i d s , a l e s s e r d e p t h o f f i l t r a t e i n v a s i o n occurs b e f o r e a mudcake b u i l d s up on t h e b o r e h o l e w a l l w h i c h i s e s s e n t i a l l y h a l t i n g t h e i n v a i n l e s s porous rocks, t h e same amount o f f i l t r a t e s i o n process. Conversely, w i l l e x t e n d f u r t h e r i n t o t h e f o r m a t i o n . Most t i g h t gas sands a r e compacted and t h e s o n i c compaction f a c t o r can be d i s r e g a r d e d , w i t h s h a l l o w o r underpressured t i g h t gas sands b e i n g r a r e e x c e p t i o n s .
3.5.2.4. Negative orientation of permeability changes P e r m e a b i l i t y v a r i a t i o n s i n sandstones d u r i n g course o f s t i m u l a t i o n , a c i d i z i n g , i n j e c t i o n and p r o d u c t i o n have almost always a n e g a t i v e o r i e n t a t i o n r e s u l t i n g i n d e t e r i o r a t i o n as a consequence o f t h e combined e f f e c t s o f s w e l l i n g and r e t e n t i o n o f c l a y p a r t i c l e s (CIVAN & KNAPP 1987). P o s i t i v e o r i e n t a t i o n s o f p e r m e a b i l i t y t r a n s f o r m a t i o n g i v i n g r i s e t o enhancement o f t r a n s m i s s i b i l i t y a r e r a t h e r r a r e e x c e p t i o n s . Water-based substances such as b r i n e and s u r f a c t a n t s a l t e r l o c a l e q u i l i b r i u m c o n d i t i o n s between t h e porous s o l i d m a t r i x and f l u i d s cont a i n e d i n i t s p o r e spaces, g i v i n g r i s e t o s w e l l i n g and e n l a r g i n g o f w a t e r - s e n s i t i v e c l a y m i n e r a l s . B e i n g subsequently u n a b l e t o f i t t o t h e i r o r i g i n a l p o s i t i o n s , c l a y p a r t i c l e s a l o n g t h e p o r e s u r f a c e a r e c a p t u r e d by t h e shear f o r c e o f t h e f l o w i n g f l u i d and c a r r i e d downstream, and d u r i n g movement t h r o u g h t h e p o r e system, some of t h e c l a y f l a k e s a r e i m m o b i l i z e d by a b s o r p t i o n on p o r e w a l l s , deep-bed f i l t r a t i o n o r p l u g g i n g o f i n t e r s t i t i a l s and p o r e t h r o a t s .
3.5.2.5. Capi 1 l a w pressure and permeabi 1 i ty relationships AMMER, SAWYER & DROPHIN (1984) and ERTEKIN, K I N G & SCHWERER (1986) comment on p r o d u c t i o n and f l o w mechanisms i n t i g h t gas r e s e r v o i r s . HALE (1986) analyzes w e l l p r o d u c t i o n h i s t o r i e s i n t i g h t gas sands, and KABIR & HASAN (1986) e v a l u a t e p r e f r a c t u r e t e s t i n g i n t i g h t gas r e s e r v o i r s . WAROLAW & TAYLOR (1976), WALLS & AMAEFULE (1985) and WARD & MORROW (1985) r e p o r t on c a p i l l a r y p r e s s u r e , p o r o s i t y
266 and p e r m e a b i l i t y r e l a t i o n s h i p s i n t i g h t gas sands. C a p i l l a r y p r e s s u r e s can be q u i t e s i g n i f i c a n t e s p e c i a l l y i n l o w - p e r m e a b i l i t y low-pressure r e s e r v o i r s and c o u l d c o n s t i t u t e a d d i t i o n a l b a r r i e r s t o f l o w (AMAEFULE & MASUO 1984). THOMAS & WARD ( 1 9 7 2 ) and WALLS, NUR & BOURBIE (1982) emphasize t h e e f f e c t o f overburden p r e s s u r e and w a t e r s a t u r a t i o n on gas p e r m e a b i l i t y , and JENNINGS, CARROLL & R A I B LE (1981) s t r e s s t h e r e l a t i o n s h i p s o f p e r m e a b i l i t y and c o n f i n i n g p r e s s u r e . NOMAN & ARCHER (1987) d i s c u s s t h e e f f e c t o f p o r e s t r u c t u r e , o n non-Darcy gas f l o w i n some l o w - p e r m e a b i l i t y r o c k s . MAZZULLO & MAZZULLO (1985) i n v e s t i g a t e t h e c l a y m i n e r a l o g y o f some l o w - p e r m e a b i l i t y r e s e r v o i r s . O t h e r aspects o f t i g h t gas sands a r e e v a l u a t e d by JONES & OWENS (1980) and HALE, FIRTH, HANSEN, MURPHY- & WOODCOX (1987).
3.6. Minimum pre-fracturing gas production rate and poroperm properties The e x p e r i e n c e o f MHF s t i m u l a t i o n o f b o t h R o t l i e g e n d and C a r b o n i f e r o u s sandstones i n t h e l a s t t e n y e a r s has shown t h a t u n l e s s a c e r t a i n minimum p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e i s p r e s e n t , no economical producer can be achieved by c a r r y i n g o u t a f r a c t u r e j o b e s p e c i a l l y i n t h e p r e s e n t t i m e o f lowered hydrocarbon p r i c e s . The consequence o f t h i s r e l a t i o n s h i p i s t h a t deep t i g h t R o t l i e gend and C a r b o n i f e r o u s gas w e l l s which would have been a t once f r a c t u r e d some y e a r s ago when l o g - and c o r e - d e r i v e d p e t r o p h y s i c a l i n t e r p r e t a t i o n and f o r m a t i o n t e s t e v a l u a t i o n o f t h e l o w - p e r m e a b i l i t y r e s e r v o i r r o c k s were s t i l l f a r f r o m t h e i r p r e s e n t advanced e x p e r i e n c e and when m a j o r l e s s o n s o f success and f a i l u r e i n c o m p l e t i o n and s t i m u l a t i o n had s t i l l t o be l e a r n t , a r e nowadays c a r e f u l l y e v a l u a t e d i n terms o f e x i s t i n g p r e - f r a c t u r i n g and expected p o s t - f r a c t u r i n g p r o duction rates. I n case o f doubt, i t i s f r e q u e n t l y p r e f e r r e d t o s h u t - i n t h e w e l l s o r t o p e r form a g a i n l o n g e r p r o d u c t i o n t e s t s and p r e s s u r e b u i l d - u p measurements r a t h e r t h a n t o c a r r y o u t t h e s t i m u l a t i o n j o b s s t r a i g h t away, o r even t o c o l l e c t f u r t h e r i n f o r m a t i o n on r e s e r v o i r c o m p o s i t i o n and b e h a v i o u r by p e r f o r m i n g a s m a l l s c a l e o r m i n i a t u r e ( o r even m i c r o - ) t r e a t m e n t p r i o r t o a l a t e r f u l l - s c a l e o r even jumbo j o b . A more advanced r e s u l t o f t h i s changing c o n c e p t i s t h a t even w e l l s which f o r m e r l y were d r i l l e d w i t h t h e f i r m pre-spud knowledge t h a t f r a c t u r i n g would be i n e v i t a b l e , a r e nowadays o f t e n postponed i n o r d e r t o a w a i t a b e t t e r g e n e r a l hydrocarbon p r i c e s c e n a r i o (and i n Europe a l s o a s t r o n g e r US $ e x change r a t e s c e n a r i o ) o r a r e even f o r t h e t i m e b e i n g t o t a l l y removed f r o m t h e d r i l l i n g budget d e s p i t e t h e i r p a r t i a l l y h i g h rank i n a p p r a i s a l d r i l l i n g s t r a t e gy s t u d i e s . Some aspects o f p r e - f r a c t u r i n g economical f e a s i b i l i t y e s t i m a t i o n and d i f f e r e n c e s between e x p l o r a t i o n and development w e l l s a r e o u t l i n e d as f o l lows.
3.6.1. Pre- f ract ur i ng economical feas i bi 1 i ty estimation P r e - f r a c t u r i n g economical f e a s i b i l i t y e s t i m a t i o n can be made a l o n g t h e l i n e s o f minimum f r a c t u r e f l o w c a p a c i t y and gas o f f t a k e r a t e , minimum r e s e r v o i r p r o p e r t i e s , and s t i m u l a t i o n r e n t a b i l i t y c l a s s i f i c a t i o n .
3.6.1.1. Minimum fracture flow capacity and gas offtake rate Reasonable e s t i m a t i o n o f economical f e a s i b i l i t y can be made i n t h e R o t l i e gend o f Germany FRG w i t h t h e p r o d u c t o f p e r m e a b i l i t y t i m e s n e t t h i c k n e s s where t h e l o w e r boundary i s a b t . 2 - 5 mdm ( f o r example a r e s e r v o i r o f 0 . 1 md permeab i l i t y and 20 - 50 m n e t t h i c k n e s s ; DIETZEL & ERNST 1987; minimum f r a c t u r e f l o w c a p a c i t i e s i n o t h e r f o r m a t i o n s a r e i n d i c a t e d by ATTEBERRY, TUCKER & R I T Z 1979), and w i t h t h e p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e which has t o be a t l e a s t 800 1,500 m3/h a c c o r d i n g t o h i t h e r t o s t i m u l a t i o n and gas w i t h d r a w a l e x p e r i e n c e , w i t h these values a p p l y i n g a t l e a s t under t h e c o n d i t i o n s o f t h e p r e s e n t economi-
267 c a l s c e n a r i o . KLOSE & KRUMER (1983) determined f o r t h e w e l l Sohlingen Z 4 which was f r a c t u r e d i n 1982 up t o 1 mdm and a b t . 600 m3/h, and LEICHT (1985) c o n c l u des t h a t t h e r e s u l t o f a f r a c t u r e t r e a t m e n t i s o n l y economical i n case o f a p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e o f a t l e a s t s e v e r a l hundreds o f m3/h. I f e i t h e r v a l u e were s m a l l e r , commercial p r o d u c t i o n cannot be expected even a f t e r an expensive MHF t r e a t m e n t .
3.6.1.2. Minimum reservoir properties for profitable stimulation DIETZEL & ERNST (1987) a l s o comment on minimum r e s e r v o i r p r o p e r t i e s r e q u i r e d f o r p r o f i t a b l e stimulations. R e n t a b i l i t y o f withdrawal from low-permeability gas r e s e r v o i r s i s m a i n l y determined by t h e increment o f p r o d u c t i o n o b t a i n a b l e by h y d r a u l i c p r o p p a n t f r a c t u r i n g as w e l l as by t h e i n i t i a l f l o w c a p a c i t y o f t h e u n t r e a t e d w e l l . A minimum v a l u e o f t h e i n t e r n a l r a t e o f r e t u r n o f 15 % b e f o r e taxes i s c o n s i d e r e d as p r o f i t a b i l i t y boundary. The l i m i t f o r n o n - s t i m u l a t e d w e l l s i s c h a r a c t e r i z e d by t h e d i m e n s i o n l e s s e q u i v a l e n t r a d i u s rD = 1 w h i c h spec i f i e s t h e minimum c o m b i n a t i o n o f n e t h e i g h t and p e r m e a b i l i t y r e q u i r e d f o r an e c o n o m i c a l l y f e a s i b l e p r o d u c t i o n w i t h o u t s t i m u l a t i o n . The range between r D = 1 and r D = 400 i n a n e t h e i g h t vs. p e r m e a b i l i t y p l o t covers t h e minimum parameter combination f o r a f r a c t u r e d well, w i t h s t i m u l a t i o n w i t h i n t h i s i n t e r v a l being necessary and a l s o p r o f i t a b l e i n case o f s u f f i c i e n t f r a c t u r e c o n d u c t i v i t y . Below r D = 400, e c o n o m i c a l l y f e a s i b l e p r o d u c t i o n i s n o t p o s s i b l e even i f a t e c h n i c a l l y s u c c e s s f u l and p e r f e c t l y designed h y d r a u l i c f r a c t u r e s t i m u l a t i o n t r e a t ment was performed.
3.6.1.3. Stimulation rentability classification Experience o f p a s t h y d r a u l i c p r o p p a n t f r a c t u r i n g o p e r a t i o n s a l l o w s t o c a r r y o u t a q u i c k - l o o k c l a s s i f i c a t i o n o f w e l l s based on t e s t r e s u l t s and l o g e v a l u a t i o n i n t o wells rentable without stimulation, wells possibly p r o f i t a b l e a f t e r MHF t r e a t m e n t , and w e l l s n o t e c o n o m i c a l l y f e a s i b l e i n any case. Once a s u i t a b l e model which i s composed a c c o r d i n g t o r e s e r v o i r and economical boundary c o n d i t i o n s has been e s t a b l i s h e d f o r a s p e c i f i c r e g i o n , t h e n e t h e i g h t vs. p e r m e a b i l i t y c u r v e can be d i r e c t l y used f o r t h e e x p l o r a t i o n r i s k a n a l y s i s (DIETZEL & ERNST 1987). Boundary c o n d i t i o n s and f e a s i b i l i t y r e q u i r e m e n t s o f gas r e c o v e r y f r o m deep l a y e r e d t i g h t f o r m a t i o n s a r e a l s o analyzed by JOHN ( 1 9 8 7 ) .
3.6.2. Differences between exploration and development wells D i s t i n c t i o n has f u r t h e r t o be made between e x p l o r a t i o n w e l l s where a l o w e r p o s t - f r a c t u r i n g gas p r o d u c t i o n r a t e would be s t i l l economical due t o t h e p o s s i b i l i t y o f w r i t i n g o f f t h e d r i l l i n g c o s t , whereas a development w e l l r e q u i r e s a h i g h e r p o s t - f r a c t u r i n g gas p r o d u c t i o n r a t e , as t h e d r i l l i n g expenses cannot be w r i t t e n o f f ( c f . s e c t i o n 3 . 8 . 2 . 1 . ) . The e x p e r i e n c e d u r i n g t h e l a s t t e n y e a r s has shown t h a t a f i v e - t o s i x - t i m e s ( i n extreme cases a l s o f o u r - t o t e n - t i m e s ) i n c r e a s e o f t h e p r e - f r a c t u r i n g gas p r o d u c t i o n r a t e a f t e r t h e s t i m u l a t i o n operat i o n i s t h e upper l i m i t o f e x p e c t a t i o n , w i t h t h i s r e l a t i o n s h i p s e t t i n g t h e bound a r y o f hope a c c o r d i n g t o t h e f l o w c a p a c i t y o f t h e u n t r e a t e d r e s e r v o i r . I n a d d i tion, t h e h i g h e s t p r o d u c t i o n r a t e i s achieved immediately a f t e r s t i m u l a t i o n and clean-up and l a t e r p r o g r e s s i v e l y d e c l i n e s w i t h i n c r e a s i n g o f f t a k e . The degree o f o r i g i n a l p r o d u c t i o n r a t e improvement as w e l l as t h e amount o f subsequent cap a c i t y d i m i n u t i o n a r e d e c i s i v e f o r t h e e v a l u a t i o n o f t e c h n i c a l and economical success o f t h e o p e r a t i o n (ALBERTSEN 1985).
3.7. Stimulation cost and fractur ins pol icy The c o n s i d e r a b l e t o i n some cases even almost exaggerated c o s t containment o f o i l and gas p r o d u c t i o n companies f o l l o w i n g t h e m a j o r o i l p r i c e d e c l i n e i n
268 1986 ( c f . s e c t i o n 2 . 2 . 1 . 5 . ) and t h e worsening accompanying US $ exchange r a t e drop ( c f . s e c t i o n 2.2.1.3.; w i t h the l i m i t a t i o n s o f c a p i t a l expenditure p a r t i a l l y even now s t i l l n o t h a v i n g been l i f t e d again) a l s o i n f l u e n c e s t h e p h i l o s o p h y o f h y d r a u l i c p r o p p a n t f r a c t u r i n g o f o f f s h o r e and onshore e x p l o r a t i o n w e l l s ( p a r t i c u l a r l y i n t h e B r i t i s h Southern N o r t h Sea) which underwent c o n s i d e r a b l e r e v i s i o n a c c o r d i n g t o t h e changed g e n e r a l economical framework. E s t i m a t i n g t h e c o s t of a h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t t o be i n t h e range o f 5 - 20 % (onshore) o r 10 - 40 % ( o f f s h o r e ) o f t h e t o t a l d r i l l i n g expenses (depending on r e s e r v o i r depth, thickness o f the hydrocarbon-bearing section, s t i m u l a t i o n j o b d e s i g n and t r e a t m e n t performance), i n f o r m e r y e a r s an a p p a r e n t l y d r y t i g h t h o l e was much more o f t e n s t i l l s u b j e c t e d t o a f i n a l t r i a l t o c o n v e r t i t i n t o an economical producer by means o f s t i m u l a t i o n b e f o r e p l u g g i n g and abandoning an expensive o f f s h o r e w e l l and p r o b a b l y b u r y i n g hopes i n s m a l l e r s p o t s o r l a r g e r patches o f t h e f a c i e s b e l t o r hydrocarbon p l a y ( f o r comp a r i s o n , a f r a c t u r i n g t r e a t m e n t can account f o r 5 - 50 % o f t h e t o t a l w e l l c o s t i n t h e USA; P A I , G A R B I S & HALL 1983; ROBINSON, HOLDITCH & LEE 1983; VEATCH 1983; GARBIS, BROWN & M A U R I T Z 1985; ROBINSON, HOLDITCH & WHITEHEAD 1986; c f . s e c t i o n 2 . 3 . ) . The f o l l o w i n g d i s c u s s i o n focusses on aspects o f f r a c t u r i n g p o l i cy, p r i c i n g s c e n a r i o and f r a c t u r i n g t r e a t m e n t sequence s t r a t e g y .
3.7.1. Fracturing policy P a r t i c u l a r l y s i n c e t h e o i l p r i c e s e r i o u s l y dropped i n e a r l y 1986 ( c f . sect i o n 2 . 2 . 1 . 1 . 2 . ) and a t t h e same t i m e t h e US $ exchange r a t e c o l l a p s e d ( c f . sect i o n 2.2.1.3.), however, e x p l o r a t i o n w e l l s a r e e s p e c i a l l y o f f s h o r e f r a c t u r e d o n l y under e x c e p t i o n a l circumstances and a r e much more f r e q u e n t l y t h a n d u r i n g e a r l i e r y e a r s l e f t w i t h o u t complete d e t a i l e d i n v e s t i g a t i o n . The more comprehens i v e assessment i s postponed t o a l a t e r e x p l o r a t i o n o r a p p r a i s a l w e l l i n o t h e r p a r t s o f t h e s t r u c t u r e o r even t o t h e development campaign when t h e f e a s i b i l i t y i s improved by f i r s t d r i l l i n g n o t o n l y one w e l l f r o m one p l a t f o r m l o c a t i o n , b u t d r i l l i n g a s u i t e o f deviated w e l l s i n a s t a r - o r s p i d e r - l i k e geometrical f a b r i c f r o m one r i g s i t e t h a t l a t e r becomes t h e p l a c e o f t h e p r o d u c t i o n p l a t f o r m . Some aspects o f impact o f f r a c t u r i n g b o a t s on s t i m u l a t i o n s t r a t e g y as w e l l as r e s e r v o i r depth and s t i m u l a t i o n expenses a r e discussed as f o l l o w s .
3.7.1.1. Impact o f fracturing boats on stimulation strategy Secondly, a l s o t h e b a s i c f i x e d c o s t f o r t h e s t i m u l a t i o n vessel w h i c h m i g h t be on c a l l f o r t h e whole s u i t e of w e l l s i s l o w e r w i t h r e s p e c t t o t h a t i f o r d e r i n g t h e b o a t t o come f o r o n l y one f r a c t u r i n g j o b i n an e x p l o r a t i o n w e l l . There a r e no o p e r a t i o n a l r e s t r i c t i o n s f o r a f r a c t u r i n g s e r v i c e vessel b e i n g on c a l l o r even on standby, because c u r r e n t l y t h r e e s t i m u l a t i o n b o a t s a r e s h a r i n g t h e s t i l l v e r y c o m p e t i t i v e and l i m i t e d h y d r a u l i c proppant t r e a t m e n t market i n t h e 3.3.1.2. and 3 . 8 . 1 . 2 . ) , w i t h t h u s N o r t h Sea (FOX 1985; c f . s e c t i o n s 2.2.1.5.3., w a i t i n g time f o r boat a v a i l a b i l i t y being almost n o t e x i s t i n g . This l a t t e r r e v i sed s t i m u l a t i o n s t r a t e g y has been o v e r l a i n i n 1986 by t h e superimposing e f f e c t o f i n c r e a s e d c a n c e l l a t i o n r a t e s o f f r a c t u r i n g t r e a t m e n t s due t o t e c h n i c a l r e a sons which even d e t e r i o r a t e d t h e a l r e a d y poor g e n e r a l s e t t i n g (MADER 1987; c f . section 2 . 4 . 2 . 3 . ) .
3.7.1.2. Reservoir depth and stimulation expenses The d i f f e r e n t r e l a t i o n s h i p s between d r i l l i n g and s t i m u l a t i o n c o s t between onshore Germany FRG and N e t h e r l a n d s and o f f s h o r e B r i t i s h and Dutch Southern N o r t h Sea r e s u l t f r o m t h e changing r e s e r v o i r d e p t h t h a t g i v e s r i s e t o r e l a t i v e l y l o wer d r i l l i n g expenses i n t h e N o r t h Sea t h a n i n Western Europe onshore i f o n l y comparing r e s e r v o i r d e p t h and n e t c o s t t o r e a c h t h e t a r g e t . On t h e o t h e r hand, o f f s h o r e d r i l l i n g i s i n t o t a l g e n e r a l l y much more money-consuming t h a n onshore
269 d r i l l i n g as a consequence o f c o n s i d e r a b l y h i g h e r f i x e d gross r i g r a t e s . The h i g h c o s t f o r h y d r a u l i c proppant f r a c t u r i n g i s t h e consequence o f t h i c k pay zones w h i c h r e q u i r e l a r g e q u a n t i t i e s o f proppants f o r t h e t r e a t m e n t s . These r e s e r v o i r and t e c h n i c a l e n g i n e e r i n g d i f f e r e n c e s c o n t r i b u t e t o t h e economical f e a s i b i l i t y o f t h e s t i m u l a t i o n o p e r a t i o n s and t h u s i n e v i t a b l y i n f l u e n c e t h e f r a c t u r i n g philosophy.
3.7.2. Pricing scenario P a r t i c u l a r l y i n t h e 1986 c r i s i s b u t s t i l l l a s t i n g a t t h e moment, t h e o f f shore d r i l l i n g c o s t s c e n a r i o i s c o n s i d e r a b l y d i s t u r b e d due t o o n l y dumping p r i ces t h a t have t o be p a i d f o r d r i l l i n g a w e l l as a consequence o f many unemployed r gs w a i t i n g f o r j o b s , and a l s o t h e s t i m u l a t i o n expenses a r e l o w e r t h a n bef o r e as a r e s u l t o f s e r i o u s c o m p e t i t i o n between t h e s e r v i c e companies i n o r d e r t o ge as much o p e r a t i o n s as p o s s i b l e t o keep more o r l e s s a c t i v e t h e i n t o t a l t h r e e f r a c t u r i n g v e s s e l s p r e s e n t l y w o r k i n g i n t h e N o r t h Sea ( c f . s e c t i o n 2.2.1 5 . 3 . ) .
As b o t h d r i l l i n g and s t i m u l a t i o n c o s t have remarkably dropped, t h e r e l a t i o n s h i p between b o t h can be expected t o have more o r l e s s remained t h e same. I n t h e second h a l f o f 1987, extreme p r i c e c u t t i n g has no l o n g e r been necessary, as t h e p r o d u c t i o n companies c o n v e r t e d f r o m exaggerated c o s t containment t o more a p p r e c i a t i o n o f maintenance o f a h i g h t e c h n i c a l q u a l i t y l e v e l o f t h e work c a r r i e d o u t by t h e s e r v i c e companies and have a r r i v e d a t t h e u n d e r s t a n d i n g t h a t a f a i r p r i c i n g o f t h e s e r v i c e companies has t o be accepted i n o r d e r t o l e t them a l s o s u r v i v e , improve t r e a t m e n t q u a l i t y and r e l i a b i l i t y , and g i v e some r e n t a b i l i t y t o a l l t h e t h r e e p u r p o s e - b u i l t s t i m u l a t i o n b o a t s (COOPER & MARSHALL 1984, OFFSHORE ENGINEER 1984) which have t o share t h e N o r t h Sea market (FOX 1985, OILMAN 1985 b; c f . s e c t i o n 3.8.1.2.). W i t h a l i t t l e more c o n s o l i d a t e d o i l p r i c e s i t u a t i o n i n t h e second h a l f o f 1987 and an i n c r e a s e d h y d r a u l i c p r o p p a n t f r a c t u r i n g and p a r t i c u l a r l y a c i d f r a c t u r i n g a c t i v i t y ( w h i c h i s t h e most p r o f i t a b l e s t i m u l a t i o n work f o r t h e s e r v i c e companies), t h e r e i s a l s o no l o n g e r an economic a l base o r r e q u i r e m e n t f o r extreme p r i c e dumping o f t h e s t i m u l a t i o n j o b s .
3.7.3. Fracturing treatment sequence strategy D u r i n g course o f f i e l d development campaigns, two d i f f e r e n t f r a c t u r e t r e a t ment sequence s t r a t e g i e s have t o be d i s t i n g u i s h e d c o m p r i s i n g s p o t and b a t c h f r a c t u r i n g which a r e c h a r a c t e r i z e d i n more d e t a i l as f o l l o w s .
3.7.3.1. Spot fracturing Spot f r a c t u r i n g comprises s e q u e n t i a l d r i l l i n g , t e s t i n g and s t i m u l a t i o n o f one w e l l a f t e r t h e o t h e r w i t h complete t e r m i n a t i o n o f work a t one l o c a t i o n bef o r e moving t o t h e n e x t , t h e r e b y a l l o w i n g t o i n c o r p o r a t e a l l t h e i n f o r m a t i o n and e x p e r i e n c e f r o m e a r l i e r o p e r a t i o n s i n t o d e s i g n and e x e c u t i o n o f l a t e r t r e a t ments. B a t c h f r a c t u r i n g means f i r s t d r i l l i n g o f a l l t h e w e l l s one a f t e r t h e o t h e r and t h e n second i n a n o t h e r round p e r f o r a t i n g , s t i m u l a t i n g and p u t t i n g on stream o f t h e whole sequence o f w e l l s s u c c e s s i v e l y . W h i l e some companies p r e f e r t h e f i r s t o r t h e second s t r a t e g i c a l concept t h r o u g h o u t t h e f i e l d development campaign, i t may be e c o n o m i c a l l y more f e a s i b l e t o s t a r t w i t h s p o t f r a c t u r i n g unt i l a reasonable l e v e l o f e x p e r i e n c e has been g a i n e d and t h e r e s e r v o i r and i t s b e h a v i o u r a f t e r s t i m u l a t i o n i s f a i r l y w e l l understood, and t h e n l a t e r s w i t c h o v e r t o b a t c h f r a c t u r i n g , i n o r d e r t o have t h e s t i m u l a t i o n v e s s e l c o n t i n u o u s l y o p e r a t e d and t h u s h a v i n g l o w e r g e n e r a l t r e a t m e n t expenses by a v o i d i n g t i m e - and money-consuming j o u r n e y s o f t h e b o a t f r o m base t o l o c a t i o n and back.
270
3.7.3.2.
Batch fracturing
B a t c h o p e r a t i o n i s t h u s a more mature f r a c t u r i n g s t r a t e g y t h a t s h o u l d be f a voured once reasonable knowledge o f r e s e r v o i r b e h a v i o u r and s a t i s f a c t o r y exper i e n c e o f t r e a t m e n t performance have been accumulated. Batch e x e c u t i o n o f j o b s i s p a r t i c u l a r l y s u i t a b l e f o r l a r g e development campaigns where s u f f i c i e n t i n f o r m a t i o n has a l r e a d y been g a i n e d d u r i n g a p p r a i s a l w e l l e v a l u a t i o n and e a r l y p r o d u c t i o n d r i l l i n g assessment, whereas s p o t f r a c t u r i n g i s i n a d d i t i o n t o i t s submature stage i n l a r g e f i e l d s t h a t a r e t o be developed t h e most e f f e c t i v e s t r a t e gy i n e x p l o r a t i o n and a p p r a i s a l stage as w e l l as f o r development o f s m a l l f i e l d s w i t h o n l y a c o u p l e o f b o r e h o l e s . Batch c o m p l e t i o n aspects o f g r a v e l packi n g i n o i l w e l l s a r e d i s c u s s e d by HENG ( 1 9 8 7 ) . The l a r g e f i e l d campaigns o f h y d r a u l i c proppant f r a c t u r i n g o f R o t l i e g e n d t i g h t gas sands i n t h e B r i t i s h Southern N o r t h Sea i n t h e p e r i o d 1987 - 1990 a r e based on b o t h concepts, and s e l e c t i o n o f t h e s t i m u l a t i o n s t r a t e g y i s made acc o r d i n g t o f i e l d development m a t u r i t y stage by d r i l l i n g and c o m p l e t i o n . W h i l e some campaigns a r e c a r r i e d o u t a l o n g t h e s p o t g u i d e l i n e , o t h e r a c t i o n s f o l l o w the batch strategy.
3.8. Philosophy o f o f f - and onshore exploration well fracturing I n terms o f s t r a t e g i c a l p l a n n i n g and o p e r a t i o n a l e x e c u t i o n , d i s t i n c t i o n has t o be made between o f f s h o r e and onshore e x p l o r a t i o n w e l l f r a c t u r i n g c o n c e r n i n g l o g i s t i c a l approach and economical arrangement o f j o b sequences and f i e l d deve1opmen t s .
3.8.1. Philosophy of offshore exploration well fracturing I n c o n t r a s t t o Germany FRG and N e t h e r l a n d s onshore where t h e C a r b o n i f e r o u s has been f r e q u e n t l y found g a s - b e a r i n g and has o f t e n been f r a c t u r e d d u r i n g t h e l a s t t e n y e a r s ( c f . f i g . l o ) , t h e C a r b o n i f e r o u s i n t h e B r i t i s h Southern N o r t h Sea has been so f a r met i n most s t r u c t u r e s w a t e r - b e a r i n g ( a s a consequence o f i n comi t s g r e a t e r r e s e r v o i r d e p t h o f 11,000 - 13,000 f t o r 3,700 - 4,400 m, p a r i s o n t o t h e g a s - w a t e r - c o n t a c t which i s o f t e n q u i t e f u r t h e r up i n t h e s t r a t i g r a p h i c a l s e c t i o n ) and o n l y i n t h e l a s t y e a r s e x p l o r a t i o n succeeded i n encount e r i n g more and more g a s - b e a r i n g patches w i t h f a v o u r a b l e c l o s u r e h e i g h t and s u i t a b l e p e t r o p h y s i c a l p r o p e r t i e s b e i n g above a l l w a t e r s a t u r a t i o n . The common s t r a t e g y i n t h e C a r b o n i f e r o u s a t t h e moment i s n o t t o i m m e d i a t e l y f r a c t u r e exp l o r a t i o n w e l l s because t i m e i s even i n t h e p r e s e n t l o w - p r i c e s c e n a r i o s t i l l t o o expensive t o l e t t h e r i g w a i t f o r t h e s t i m u l a t i o n t r e a t m e n t i n c l u d i n g i t s p r e p a r a t i o n . C a r b o n i f e r o u s development, however, c o u l d be c o n s i d e r a b l y enhanced by a c o u p l e o f e x p e r i m e n t a l f r a c t u r e j o b s i n a few s e l e c t e d w e l l s i n o r d e r t o g e t a b e t t e r f e e l i n g f o r t h e p o t e n t i a l o f t h e r e s e r v o i r and t o improve product i o n p l a n n i n g . T h i s has p a r t i c u l a r l y been demonstrated by t h e d i f f i c u l t i e s t o g e t t h e p e t r o p h y s i c a l e v a l u a t i o n o f t h e C a r b o n i f e r o u s t o a reasonable approach t o t h e r e a l i t y , w i t h t h e main problem b e i n g t h e w a t e r s a t u r a t i o n o f t h e t i g h t C a r b o n i f e r o u s r e s e r v o i r sandstones. The 1986 o i l p r i c e c r i s i s has t r a n s f e r r e d t h i s p o l i c y o f h e s i t a t i o n o f f r a c t u r i n g o f e x p l o r a t i o n w e l l s a f t e r a d r i l l stem t e s t a l s o t o R o t l i e g e n d items, w i t h again, however, t h e e x e c u t i o n o f a few s e l e c t e d h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s i n e v e r y f i e l d b e i n g e s s e n t i a l t o back-up t h e d e c i s i o n o f commercial i t y by g e t t i n g an u n d e r s t a n d i n g o f t h e r e a l c a p a c i t y o f t h e r e s e r v o i r . A spec i a l a p p l i c a t i o n o f hydraulic proppant f r a c t u r i n g i n the Rotliegend o f the B r i t i s h Southern N o r t h Sea was s t i m u l a t i o n o f r e s e r v o i r s i n some w e l l s o f underground gas s t o r a g e f i e l d s i n t h e l a s t y e a r s . Some comments a r e o f f e r e d as f o l lows on w e l l e v a l u a t i o n and f r a c t u r e d e s i g n time, and s e r v i c e b o a t c a p a c i t y limit.
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3.8.1.1. Well evaluation and fracture design time An i m p o r t a n t aspect o f e x p l o r a t i o n w e l l f r a c t u r i n g r e g a r d l e s s o f an o f f s h o r e o r onshore s i t u a t i o n i s t h e t i m e t h a t i s needed f o r p r o p e r w e l l e v a l u a t i o n and f r a c t u r e d e s i g n ( c f . B R I M 1986). A l t h o u g h computer programs e x i s t which can p r o v i d e f a s t e s t i m a t i o n o f t h e optimum s o l u t i o n i n terms o f c h o i c e o f p r o p p a n t t y p e and g r a i n s i z e (CHENG 1984; COBB 1985, 1986; c f . s e c t i o n 1 . 4 . 1 2 . ) , designi n g o f t h e whole t r e a t m e n t may need t o o much t i m e t o l e t t h e r i g w a i t , and i f t o o many r a p i d guesses a r e done, t h e r i s k o f u n s a t i s f a c t o r y performance o r even f a i l u r e o f t h e s t i m u l a t i o n o p e r a t i o n i s increased. F r a c t u r i n g o f l a t e r development w e l l s has t h e b i g advantage t h a t r e s e r v o i r parameters a r e a l r e a d y more o r l e s s known f r o m e a r l i e r e x p l o r a t i o n and a p p r a i s a l w e l l s ( a l t h o u g h many s u r p r i ses s t i l l may o c c u r ) , b u t t h e m a j o r drawback o f renouncing t o s t i m u l a t e an exp l o r a t i o n w e l l i s always t h e danger t h a t based on i n s u f f i c i e n t r e s e r v o i r t e s t i n g w i t h o u t f r a c t u r i n g , d e c i s i o n c o u l d be made t h a t e x p l o i t a t i o n o f t h e hydrocarbon c o n t e n t o f t h e encountered s t r u c t u r e i s uncommercial and t h u s t h e p a t c h never sees development i n t o a f i e l d , w i t h t h e r e b y p r o b a b l y c o n s i d e r a b l e gas r e serves b e i n g l o s t t h a t would have more than p a i d o f f t h e i n i t i a l s t i m u l a t i o n operation. While many w e l l s w i t h i n a f i e l d which i s s u b j e c t e d t o a l a r g e r h y d r a u l i c f r a c t u r i n g campaign can be s u c c e s s f u l l y and p r o f i t a b l y s t i m u l a t e d f o l l o w i n g o n l y one o r a few base p r o p o s a l s and t e s t jobs, i n o t h e r cases more o r l e s s i n d i v i d u a l i z e d f r a c t u r i n g t r e a t m e n t d e s i g n a c c o r d i n g t o l o c a l circumstances i n t h e b o r e h o l e s i s r e q u i r e d (MACK & MASON 1985) and much e x p e r i m e n t a l f r a c t u r i n g i n c l u d i n g submature j o b s and a l s o f a i l u r e s a r e t h e o n l y way t o o p t i m i z e t r e a t ment d e s i g n i n t h e f i e l d campaigns and t o achieve t h e b e s t p o s s i b l e o p e r a t i o n l a y o u t f o r t h e p a r t i c u l a r r e s e r v o i r complex.
3.8.1.2. Service boat capacity limit Service boat capacity i s o f t e n the l i m i t i n g f a c t o r f o r s t i m u l a t i o n t r e a t ments i n t h e Southern N o r t h Sea (MARTINS 1987). I n t h e e a r l y y e a r s o f European o f f s h o r e p r o p p a n t f r a c t u r i n g , o n l y platform-mounted equipment was a v a i l a b l e , w i t h t r e a t m e n t q u a n t i t i e s h a v i n g been q u i t e l i m i t e d (LOUVIERE 1987). I n t h e e a r l y 1980's, t h e o p e r a t i o n s g o t l a r g e r when t h e f i r s t s e l f - c o n t a i n e d s e r v i c e vess e l s were developed t h a t a t t h e b e g i n n i n g r e p r e s e n t e d c o n v e r t e d s u p p l y boats (COOPER & MARSHALL 1984; c f . s e c t i o n 3.3.1.2.). The s e r v i c e s h i p s i n c l u d e pumpi n g equipment, f l u i d and p r o p p a n t storage, j o b c o n t r o l and m o n i t o r i n g , and qual i t y supervision laboratory. Stimulation vessels provide g r e a t l y increased vers a t i l i t y i n r a t e s , volumes and designs o f t h e treatments, whereas p l a t f o r m s t i m u l a t i o n s were l i m i t e d b o t h i n r a t e and volume due t o space c o n s t r a i n t s (SNOW & HOUGH 1988). I n j e c t i o n r a t e s a r e o f t e n l i m i t e d by w e l l c o n f i g u r a t i o n and f r i c t i o n l o s s , w i t h t h e main i n f l u e n c i n g f a c t o r s b e i n g r a t e , horsepower and government r e g u l a t i o n s . I n t h e m a i n l y h i g h l y - d e v i a t e d t o even o c c a s i o n a l l y almost h o r i z o n t a l w e l l s , t h e l o n g e r b o r e h o l e s l i m i t r a t e and p r e s s u r e o f f r a c t u r i n g b y s e r v i c e boats. The t h r e e p u r p o s e - b u i l t s t i m u l a t i o n v e s s e l s (COOPER & MARSHALL 1984, OFFSHORE ENGINEER 1984; c f . s e c t i o n 2.2.1.5.3.) c u r r e n t l y operating i n the North Sea a r e a l l comparable i n c a p a c i t y o f j o b performance such as pumping power and m a t e r i a l s t o r a g e f a c i l i t i e s (FOX 1985, OILMAN 1985 b; c f . s e c t i o n 3 . 7 . 1 . 1 . ) , w i t h t h e r e f o r e b o a t employment h a v i n g been a c r i t i c a l c o m p e t i t i v e f a c t o r d u r i n g t h e w o r s t t i m e of t h e c r i s i s f o l l o w i n g t h e 1986 o i l p r i c e c r a s h ( c f . s e c t i o n 2.2.1.3.2.2.). N o r t h Sea s t i m u l a t i o n l o g i s t i c s and r e q u i r e m e n t s a r e o u t l i n e d by MACADAM ( 1 9 8 4 ) .
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3.8.2. Philosophy of onshore exploration well fracturing A c c o r d i n g t o t h e poor g e n e r a l economical s i t u a t i o n a f t e r t h e m a j o r o i l p r i c e drop i n e a r l y 1986 ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) and t h e p a r a l l e l U S $ exchange r a t e f a l l ( c f . s e c t i o n 2 . 2 . 1 . 3 . ) , a l s o t h e onshore R o t l i e g e n d e x p l o r a t i o n and a p p r a i s a l s t r a t e g y concept i n Germany FRG (and p a r t i a l l y a l s o i n t h e N e t h e r l a n d s ) has been c o n s i d e r a b l y changed. Some aspects o f j o b suspension vs. c o s t w r i t i n g - o f f and changing aims o f h y d r a u l i c f r a c t u r i n g a r e b r i e f l y i l l u s t r a t e d as f o l l o w s .
3.8.2.1. Job suspension vs. cost writing-off I n c o n t r a s t t o f o r m e r y e a r s where e x p l o r a t i o n and a p p r a i s a l w e l l s had been d r i l l e d a l s o on such t a r g e t s t h a t were known b e f o r e t o r e q u i r e expensive l a r g e s c a l e f r a c t u r i n g i f c o n t a i n i n g gas, such p r o s p e c t s a r e p r e s e n t l y postponed and o n l y items a r e d r i l l e d where reasonable p r o d u c t i o n r a t e s w i t h o u t s t i m u l a t i o n c o u l d be expected ( i n c o n t r a s t t o t h i s onshore p o l i c y , t h e development campaigns o f t i g h t gas f i e l d s o f f s h o r e a r e q u i t e another s t o r y ) . Thus a t t h e moment f r a c t u r e j o b s i n onshore e x p l o r a t i o n w e l l s a r e o n l y coming up randomly and by chance when t h e p r e d i c t e d b e t t e r f a c i e s s i t u a t i o n was n o t encountered a c c o r d i n g l y , and when s u f f i c i e n t q u a n t i t i e s o f gas a r e found t h a t c o u l d o n l y be p r o duced a t economical r a t e s a f t e r s t i m u l a t i o n a l s o i n t h e r e v i s e d g e n e r a l f i n a n c i a l framework. As a consequence o f p r e s e n t l y s u f f i c i e n t c a p a c i t y , one o f t h e f i r s t d r i l l i n g program t r i m m i n g and c a n c e l l a t i o n a c t i o n s a f t e r t h e 1986 o i l p r i c e c o l l a p s e a f f e c t e d development w e l l s i n onshore t i g h t gas f i e l d s , and a l s o e x p l o r a t i o n and a p p r a i s a l w e l l s i n r e s e r v o i r patches where MHF s t i m u l a t i o n would be u n d o u b t e d l y i n d i s p e n s i b l y f o r p r o p e r pay zone e v a l u a t i o n have been i n many cases a t l e a s t suspended and postponed, i f n o t p a r t i a l l y f o r t h e t i m e b e i n g c o m p l e t e l y d e l e t e d ( c f . s e c t i o n s 2.2.1.5. and 2 . 2 . 2 . 1 . 3 . ) . T h i s a d j u s t m e n t t o changed economical f e a s i b i l i t y framework c o n d i t i o n s i s a l s o r e f l e c t e d by t h e f a c t t h a t i n Germany FRG t h e l a s t MHF s t i m u l a t i o n operat i o n s on R o t l i e g e n d and C a r b o n i f e r o u s t i g h t gas r e s e r v o i r s have been c a r r i e d o u t i n 1985 as a c u l m i n a t i o n o f t h e p e r i o d 1977 - 1985 when s e v e r a l dozens o f w e l l s have been f r a c t u r e d i n t o t a l . A c c o r d i n g t o suspension and c a n c e l l a t i o n o f d r i l l i n g and s t i m u l a t i o n a c t i v i t y a f t e r t h e 1986 o i l p r i c e c r a s h , however, n o t a s i n g l e MHF j o b has been c a r r i e d o u t i n t h e p e r i o d 1986 - 1988, and t h e expect a t i o n s f o r 1989 a r e a l s o n o t v e r y f a v o u r a b l e , w i t h o n l y l i t t l e hope r e m a i n i n g t h a t t h e f o r m e r h y d r a u l i c p r o p p a n t f r a c t u r i n g a c q u i s i t i o n o f R o t l i e g e n d and Carb o n i f e r o u s t i g h t gas w i l l be resumed w i t h i n t h e n e x t few y e a r s ( c f . a l s o sect i o n s 2.4.1.1. and 2 . 4 . 2 . ) . Apart from the o v e r a l l f e a s i b i l i t y , e x p l o r a t i o n w e l l s o f t e n c o n f r o n t w i t h t h e n e c e s s i t y o f f a s t d e c i s i o n o f e i t h e r a t once p l u g g i n g and abandoning t i g h t r e s e r v o i r s , w r i t i n g o f f t h e w e l l as a f a i l u r e and p r o b a b l y f o r g e t t h e e x p e c t e d r e s e r v e s i n some spots, o r t r y i n g t o g e t s t i l l a p o s i t i v e r e s u l t by h y d r a u l i c p r o p p a n t f r a c t u r i n g (and t h u s t o c o n f i r m g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g concept and model), whereas development w e l l s cannot be w r i t t e n - o f f i n case o f negative r e s u l t ( c f . section 3.6.2.). Thus g i v e n t h e f a c t o f m e e t i n g a p r e f r a c t u r i n g g a s - f l o w r a t e t h a t i s s o f a r n o t commercial b u t above a c e r t a i n m i n i mum l e v e l which p e r m i t s t o e x p e c t a good p r o d u c i n g c a p a c i t y a f t e r s t i m u l a t i o n , t h e w e l l i s sooner o r l a t e r f r a c t u r e d .
3.8.2.2. Changing aims of hydraulic fracturing The g e n e r a l f r a c t u r i n g p o l i c y i n Western Europe has a l s o changed d u r i n g t h e l a s t y e a r s . I n t h e p a s t , s t i m u l a t i o n o f R o t l i e g e n d and C a r b o n i f e r o u s sandstones was done as much as p o s s i b l e i n o r d e r t o maximize t h e proven gas r e s e r v e s . Nowadays t h e main purpose o f s t i m u l a t i o n i s t o i n c r e a s e t h e g e n e r a l p o t e n t i a l o f t h e area and t o double t h e l i f e t i m e of t h e f i e l d s , w i t h demand o f i n c r e a s i n g t h e proven gas r e s e r v e s b e i n g no l o n g e r s o v i o l e n t T y c h a l l e n g i n g than b e f o r e ,
273 because most o f t h e h i t h e r t o encountered gas has a l r e a d y been s o l d t h r o u g h l o n g - t e r m s u p p l y c o n t r a c t s and a l s o t h e market f o r s a l e o f gas i s a t l e a s t i n p a r t s o f t h e area (and p r o b a b l y o n l y t e m p o r a r i l y ) approaching s a t u r a t i o n due t o o v e r s u p p l y f r o m v a r i o u s sources and l e s s q u i c k growth and t h u s more s l o w l y r i s i n g demand t h a n expected a few y e a r s ago. Unless gas r e q u i r e m e n t w i l l a g a i n become h i g h e r i n t h e n e x t y e a r s due t o a g a i n s l i g h t l y a c c e l e r a t i n g urban and i n d u s t r i a l growth, t h e l a t t e r tendency o f t h e gas m a r k e t c o u l d a l s o s e r i o u s l y a f f e c t t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g market. Some d i f f e r e n c e s between o f f shore and onshore h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n t r e a t m e n t s a r e a l s o o u t l i n e d by ROODHART, K U I P E R & D A V I E S ( 1 9 8 6 ) .
3.9. Imlications o f fracturing philosophy on proppant selection I n a d d i t i o n , i f a h y d r a u l i c f r a c t u r i n g j o b iii such e x p l o r a t i o n w e l l s i s c a r r i e d o u t a t a l l , i t i s g e n e r a l l y o n l y done f o r d e l i v e r y o f i n f o r m a t i o n on which an e x t e n s i o n d r i l l i n g and f i e l d development concept c o u l d be based on, and t h e b o r e h o l e i s o f t e n plugged back and abandoned a f t e r temporary p o s t - f r a c t u r i n g t e s t p r o d u c t i o n t h a t has t h e o n l y task t o determine t h e r e a l w e l l c a p a c i t y . T h i s statement i s v a l i d u n l e s s t h e b o r e h o l e has an e x t r a o r d i n a r i l y f a v o u r a b l e r e s u l t which may l e a d t o s t r a i g h t c o m p l e t i o n f o r b e i n g l a t e r p u t a t once on stream when a c q u i s i t i o n o f t h e p a t c h by p i p e l i n e o r o t h e r hydrocarbon t r a n s p o r t f a c i l i t i e s i s made, o r t h e development campaign i s c a r r i e d o u t w i t h i n s t a l l a t i o n o f a p r o d u c t i o n and g a t h e r i n g p l a t f o r m ) . As a consequence o f no p r e v i o u s i n f o r m a t i o n i n t h e f i e l d when t h e f i r s t exp l o r a t i o n w e l l i s d r i l l e d , t h e r e i s a l s o o f t e n s t i l l t h e r i s k t h a t t h e product i o n r a t e i s even uncommercial a f t e r t h e f r a c t u r i n g t r e a t m e n t and thus t h e boreh o l e would have t o be d i s c a r d e d due t o economical reasons, i n c o n t r a s t t o development w e l l s where t h e r i s k i s much l o w e r and t h e b o r e h o l e s a r e l o o k i n g t o wards a l o n g p r o d u c t i o n h i s t o r y f o r p r o b a b l y s e v e r a l decades. The v a r i o u s p o s s i b i l i t i e s o f f r a c t u r i n g p h i l o s o p h y a r e summarized as f o l l o w s by commenting on exp l o r a t i o n s t r a t e g y and s t i m u l a t i o n t e s t i n g , f i e l d e x p e r i e n c e and development concepts, and a p p r a i s a l d r i l l i n g s t r a t e g y concepts and e x p e r i e n c e .
3.9.1. Exploration strategy and stimulation testing Concerning p r o p p a n t marketing, many o p e r a t o r s u n f o r t u n a t e l y b e l i e v e t h a t s h o r t - t e r m t e s t p e r i o d s f o r t h e goal o f a n a l y s i s o f r e a l w e l l c a p a c i t y o n l y r e q u i r e p r o p p i n g o f t h e f r a c t u r e s w i t h cheap sand t h a t can more o r l e s s reasonabl y w i t h s t a n d h i g h e r p r e s s u r e s f o r a c o u p l e o f days t o weeks i n t e m p o r a r i l y metas t a b l e c o n d i t i o n s , i n s t e a d o f u s i n g expensive i n t e r m e d i a t e - t o h i g h - s t r e n g t h proppants t h a t would be adequate t o t h e c l o s u r e s t r e s s i n t h e pay f o r m a t i o n and c o u l d a l s o r e s i s t t o i n c r e a s i n g c l o s u r e s t r e s s e s w i t h drawdown o f t h e r e s e r v o i r p r e s s u r e upon d e p l e t i o n a f t e r l o n g - t e r m gas p r o d u c t i o n d u r i n g up t o 20 y e a r s o r more. Some aspects o f p r o p p a n t c o s t containment and t a i l - i n p o l i c y as w e l l as p r o p e r i n t e r p r e t a t i o n o f r e s e r v o i r c a p a c i t y a r e i l l u s t r a t e d as f o l l o w s .
3.9.1.1. Proppant cost containment and tail-in policy E x c e p t i o n s o f t h i s c o s t - c o n t a i n m e n t p o l i c y a r e i n some cases m i n o r t a i l - i n q u a n t i t i e s o f s y n t h e t i c h i g h - q u a l i t y proppants i n t h e t e r m i n a l stages o f t h e treatments i n order t o create b e t t e r support i n the v i c i n i t y o f the borehole (HOWARD & FAST 1970 a; HICKEY, BROWN & CRITTENDEN 1981; c f . a l s o s e c t i o n s 2.4.1.2. and 4 . 2 . 2 . ) . The t a i l - i n s t r a t e g y a s c e r t a i n s p r o p e r p l u g g i n g o f t h e i n duced c r a c k by h i g h - q u a l i t y proppants a t l e a s t i n t h e immediate s u r r o u n d i n g s o f t h e w e l l b o r e where f l o w v e l o c i t y and p r e s s u r e drawdown reaches t h e h i g h e s t values, and t h e r e f o r e s h o r t - t e r m t e s t p r o d u c t i o n may almost approach r e a l r e s e r v o i r c a p a c i t y i f adequate proppant t a i l - i n i s made. I t would, however, c e r t a i n l y p r o v i d e a much b e t t e r assessment o f pay c a p a b i l i t y i n terms o f l o n g - t e r m p r o -
274 d u c t i v i t y i f a f u l l - s c a l e f r a c t u r i n g treatment w i t h corresponding proppant d i s t r i b u t i o n w i t h i n t h e c r a c k w o u l d be c a r r i e d o u t . A p a r t f r o m proppants, a l s o f r a c t u r i n g f l u i d s can be t a i l o r e d t o meet v i s c o s i t y s p e c i f i c a t i o n s i n a wide range o f temperature a p p l i c a t i o n s by p r o p e r use o f p r i m a r y and s e c o n d a r y g e l l i n g a g e n t s ( A T T E B E R R Y , TUCKER & R I T Z 1979; L E E & DANESHY 1985; c f . s e c t i o n 4 . 3 . 4 . 3 . ) , and s i m i l a r l y , t a i l o r e d - p u l s e l o a d i n g c a n b e u s e d f o r m u l t i p l e f r a c t u r i n g o f b o r e h o l e s (SWIFT & KUSUBOV 1981, CUDERMAN 1982; c f . section 4.2.2.1.3.3.). I n a d d i t i o n t o t a i l - i n setting, s e l e c t i v e proppant placement w i t h i n d i f f e r e n t successive p a r t s o f t h e f r a c t u r e wing can a l s o be a c h i e v e d b y d e n s i t y c o n t r o l foam a p p l i c a t i o n (BARBER & T H E M I G 1985; c f . s e c t i o n 4.2.2.3.3.3.) and m u l t i p l e - f l u i d t r e a t m e n t ( L E E & DANESHY 1986).
3.9.1.2. Proper interpretation o f reservoir capacity The m a i n drawback o f a m o n e y - s a v i n g f r a c t u r i n g l a y o u t i n e x p l o r a t i o n and app r a i s a l w e l l s w i t h pumping o n l y sand o r r e s t r i c t i n g s y n t h e t i c p r o p p a n t s t o a i s s i m i l a r t o t h e danger o f t o l i t t l e t e r m i n a l t a i l - i n ( c f . s e c t i o n 2.4.1.2.3.) t a l l y r e n o u n c i n g o f t h e s t i m u l a t i o n o p e r a t i o n . F a i l u r e o f t h e n a t u r a l sand t o withstand the closure stress s u f f i c i e n t l y i n i t s metastable nature i n t h e highpressure regime d u r i n g t h e r e l a t i v e l y s h o r t t e s t i n g p e r i o d , impact o f t h e worse c o n d u c t i v i t y o f sand w i t h r e s p e c t t o s y n t h e t i c h i g h - q u a l i t y p r o p p a n t s ( w h i c h i s f a l s i f y i n g o r mimicking the r e a l r e s e r v o i r e x p l o i t a b i l i t y ) , improper f r a c t u r i n g and/or p r o d u c t i o n r e s u l t s o r o t h e r t e c h n i c a l c o m p l i c a t i o n s can e a s i l y l e a d t o m i s i n t e r p r e t a t i o n o f r e s e r v o i r c a p a c i t y and p o t e n t i a l , and based on s u c h i n a d e q u a t e i n f o r m a t i o n , assessment c o u l d b e made t h a t t h e d e p o s i t i s u n c o m m e r c i a l and t h u s t h e s t r i k e w o u l d n e v e r see i t s d e v e l o p m e n t i n t o a f i e l d . A l t h o u g h t h e a p p l i c a t i o n o f h i g h - q u a l i t y p r o p p a n t s i n e x p l o r a t i o n and a p p r a i s a l w e l l f r a c t u r i n g i s f a r from b e i n g a g u a r a n t e e f o r f a v o u r a b l e r e s u l t s due t o many g e o l o g i c a l and e n g i n e e r i n g f a c t o r s c o n t r i b u t i n g t o o v e r a l l r e s e r v o i r p e r formance, t h e p o s s i b i l i t y t h a t a p o o r f o r m a t i o n w i l l p r o d u c e a t a p p r e c i a b l y h i g h r a t e s a f t e r s t i m u l a t i o n c a n be m a x i m i z e d b y c h o o s i n g t h e optimum p r o p p i n g m a t e r i a l . Much money c a n b e l o s t i f based o n f a l s e o r i n s u f f i c i e n t i n f o r m a t i o n d e c i s i o n i s made t h a t t h e p a t c h i s u n c o m m e r c i a l , and a f o r t u n e c a n be g a i n e d i n case o f reasonable r e s u l t s from a p r o p e r l y designed t r e a t m e n t .
3.9.2. Field experience and development concepts I n c o n t r a s t t o e x p l o r a t i o n and a p p r a i s a l where i n many t o i n some a r e a s e v e n m o s t c a s e s e x c l u s i v e l y sand o r a l i t t l e t a i l - i n o f i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s f o l l o w i n g a m a i n l o a d o f sand a r e i n j e c t e d i n t o t h e f r a c t u r e f o r t h e r e a son o f s h o r t - t e r m c o s t c o n t a i n m e n t ( a n d in many i n s t a n c e s u n f o r t u n a t e l y a t t h e expense o f l o n g - t e r m r e s e r v o i r b e h a v i o u r i n f o r m a t i o n ) , d e c i s i o n s f o r t h e m a j o r d e v e l o p m e n t campaigns i n 1987 - 1990 ( c f . s e c t i o n s 2 . 2 . 1 . 6 . 2 . and 2 . 4 . 1 . 2 . ) have been made b y s e v e r a l companies t o go on t h e a b s o l u t e l y s a f e s i d e i n v i e w o f l o n g - t e r m p r o d u c t i o n h i s t o r y and t o choose e x c l u s i v e l y i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s and n o t t o s e l e c t any q u a n t i t i e s o f s a n d f o r t h e d e s i g n o f t h e h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s . Some comments a r e o f f e r e d as f o l l o w s on a d v a n t a g e s o f i n t e r m e d i a t e - s t r e n g t h p r o p p a n t i n s e r t i o n and r e t a r d a t i o n o f C a r b o n i f e r o u s strikes.
3.9.2.1. Advantages o f intermediate-strength proppant insertion Concerning
a l l t h e p o s s i b l e r e s e r v o i r p r e s s u r e changes d u r i n g r e p e a t e d s h u t and p r o g r e s s i v e d e p l e t i o n dur i n g long f i e l d l i f e ( c f . section 4.12.4.2.), the a d d i t i o n a l investment f o r the p r o p p a n t w i t h h i c j h e r c o n d u c t i v i t y and s t r e s s r e s i s t i v i t y p e r f o r m a n c e i s c e r t a i n l y p a y i n g o f f v e r y w e l l and i n v i e w o f i m p r o v e d gas p r o d u c t i o n r a t e s a l s o r e a s o -
in and open-up o f w e l l s ( c f . s e c t i o n 4 . 1 2 . 4 . 3 . )
275 n a b l y f a s t . I n terms o f g e t t i n g adequate f i e l d experience, some companies even a v o i d p r o p p i n g o f f r a c t u r e s i n e x p l o r a t i o n w e l l s w i t h cheap sand and p r e f e r t h e h i g h e r i n v e s t m e n t f o r t a k i n g i n t e r m e d i a t e - s t r e n g t h proppants i n o r d e r t o c a r r y o u t a sound f i e l d experiment which g i v e s t h e c o n f i r m a t i o n t h a t t h e d e s i g n o f t h e o p e r a t i o n s i n t h e l a t e r development stage i s t e c h n i c a l l y p o s s i b l e t o be c a r r i e d o u t w i t h o u t m a j o r drawbacks and t h e r e s u l t i s s a t i s f a c t o r y .
3.9.2.2. Retardation o f Carboniferous strikes The g e o l o g i c a l and e n g i n e e r i n g disadvantage o f t h e p r e s e n t e x p l o r a t i o n w e l l f r a c t u r i n g p h i l o s o p h y i s h i g h l i g h t e d by t h e f a c t t h a t ( w i t h o n l y a few m i n o r exc e p t i o n s ) t h e l a s t c o u p l e o f m a j o r gas s t r i k e s i n t h e Upper C a r b o n i f e r o u s i n Germany FRG dates back more than f i v e y e a r s (SCHRUDER 1985), a l t h o u g h most o f t h e h i t h e r t o accomplished s e v e r a l dozens o f MHF s t i m u l a t i o n s i n Germany FRG have been c a r r i e d o u t i n t h e Upper C a r b o n i f e r o u s and p l e n t y o f e x p l o r a t i o n w e l l s have been d r i l l e d i n t h e meantime. S t r a i g h t p l u g g i n g and abandoning o f exp l o r a t i o n w e l l s o f a p p a r e n t l y f a i l u r e n a t u r e , however, i s n o t c o n s i d e r e d t o be an adequate s t r a t e g y f o r i n c r e a s i n g t i g h t gas r e s e r v e s and b e t t e r u n d e r s t a n d i n g o f t h e complex r e s e r v o i r o r g a n i z a t i o n and a r c h i t e c t u r e , and h y d r a u l i c p r o p p a n t f r a c t u r i n g f o r f i n a l c o n f i r m a t i o n o f s u i t a b i l i t y and p r o f i t a b i l i t y o f w e l l comp l e t i o n i s regarded t o be e s s e n t i a l p a r t i c u l a r l y i n t h e c o m p l i c a t e d C a r b o n i f e r o u s pay assemblages. T h i s r e l a t i o n s h i p u n d e r l i n e s t h e n e c e s s i t y o f a more agg r e s s i v e f r a c t u r i n g p o l i c y t o t r y t o g e t some e x t e n s i o n o f t h i s p r o d u c i n g h o r i zon ( t h i s c o n c l u s i o n i s v a l i d f o r Germany FRG as w e l l as s u r r o u n d i n g c o u n t r i e s i n b o t h onshore and o f f s h o r e a r e a s ) .
3.9.3. Appraisal drilling strategy concepts and experience The f o r m e r e x p e c t a t i o n and e s t i m a t i o n o f t h e p o t e n t i a l o f t i g h t R o t l i e g e n d and C a r b o n i f e r o u s sandstones i n terms o f h y d r a u l i c proppant f r a c t u r i n g have been c o n s i d e r a b l y r e v i s e d d u r i n g t h e l a s t y e a r s p a r t i c u l a r l y i n Germany FRG. W h i l e p r e v i o u s l y many l o w - p e r m e a b i l i t y r e s e r v o i r s t h a t a r e w o r t h f r a c t u r i n g t o become economical producers were p r e d i c t e d i n b o t h f o r m a t i o n s , i n the R o t l i e gend e i t h e r v e r y good o r e x t r e m e l y p o o r p e r m e a b i l i t i e s have been f r e q u e n t l y enc o u n t e r e d which e i t h e r d i d n o t r e q u i r e s t i m u l a t i o n o r l e a v e no o t h e r p o s s i b i l i t y than t o p l u g t h e w e l l . The C a r b o n i f e r o u s has o f t e n been proven t o be e i t h e r c o m p l e t e l y t i g h t o r t o be watered. A d j u s t i n g t o t h e new economical s i t u a t i o n which a c c o r d i n g t o a l l t h e p r e s e n t f o r e c a s t s w i l l n o t a g a i n l e a d t o a f e a s i b i l i t y s e t t i n g comparable t o t h a t dur i n g t h e e a r l y 1980's w i t h i n t h e n e x t few years, t h e gas a p p r a i s a l d r i l l i n g s t r a t e g y has a l s o been changed i n v a r i o u s e x t e n t i n many o f f s h o r e and onshore p a r t s o f Western Europe. I n c o n t r a s t t o t h e golden p e r i o d i n t h e e a r l y 1980's when d u r i n g a t i m e o f p a r t i a l l y e x c e s s i v e l y h i g h o i l p r i c e and o v e r r e a s o n a b l y s t r o n g US $, b i g p l a n n i n g was made f o r unconventional gas e x p l o r a t i o n and app r a i s a l i n such f a c i e s b e l t s o f e s p e c i a l l y t h e R o t l i e g e n d B a s i n (Southern Permian Basin; ZIEGLER 1982) where h y d r a u l i c p r o p p a n t f r a c t u r i n g c o u l d a l r e a d y be i n c l u d e d i n t o t h e economical assessment o f t h e w e l l p r i o r t o i t s spudding, these h i g h - r i s k and - c o s t i t e m s have now been i n many cases s l i g h t l y t o consider a b l y postponed i n b o t h d r i l l i n g schedules and a p p r a i s a l s t r a t e g y s t u d i e s , and n e a r - f u t u r e e x p l o r a t i o n c o n c e n t r a t e s on l e s s c r i t i c a l areas i n f a c i e s seams w i t h b a s i c a l l y higher permeability. T h i s a p p l i e s p a r t i c u l a r l y f o r onshore gas e x p l o r a t i o n i n t h e R o t l i e g e n d i n Germany FRG where t h e p r e s e n t f o c u s i s on a c q u i s i t i o n o f p e t r o p h y s i c a l l y w e l l in developed i n t e r v a l s o f t h e Schneverdingen-Sandstein ( c f . s e c t i o n 3.2.1.3.) synsedimentary graben systems which do n o t r e q u i r e f r a c t u r i n g s t i m u l a t i o n and t h a t p e r m i t t o w i t h d r a w l a r g e q u a n t i t i e s o f gas o v e r l o n g e r p e r i o d s o f t i m e bef o r e upon d e p l e t i o n o f t h e h i g h - p e r m e a b i l i t y Schneverdingen-Sandstein t h e o v e r l y i n g l o w - p e r m e a b i l i t y c o n t i n u o u s l y - e x t e n d i n g Hauptsandstein has t o be prepared
276 f o r p r o d u c t i o n by l a r g e - s c a l e h y d r a u l i c proppant f r a c t u r i n g . The o f f s h o r e p l a n n i n g , however, can p a r t i a l l y n o t r e a c t so q u i c k l y , because c o n t r a c t i n g o f p l a t forms f o r development d r i l l i n g and p r i o r t o t h a t s e l l i n g o f t h e proven and p a r t s o f t h e unproven gas r e s e r v e s r e s u l t i n t h e n e c e s s i t y o f keeping more o r l e s s t h e e s t a b l i s h e d a p p r a i s a l d r i l l i n g s t r a t e g y concept almost i r r e s p e c t i v e o f economical d e t e r i o r a t i o n s i n t h e meantime( c f . s e c t i o n 2.2.1.6.2.).
3.10. Horizontal and vertical difference o f fracturing potential The f r a c t u r i n g p o t e n t i a l i n t h e R o t l i e g e n d B a s i n (Southern Permian Basin; ZIEGLER 1982) i s n o t homogeneous and i s o t r o p i c i n n e i t h e r h o r i z o n t a l n o r v e r t i c a l d i r e c t i o n i n t h e g a s - b e a r i n g r e s e r v o i r column. H o r i z o n t a l l y , t h e d i s t r i b u t i o n o f pay zone b e l t s s u i t a b l e f o r h y d r a u l i c proppant f r a c t u r i n g i s c o n t r o l l e d by t h e sedimentary f a c i e s and t h u s r e s e r v o i r q u a l i t y z o n a t i o n w i t h i n t h e d e p o s i t i o n a l area, w i t h t h e seam where s t i m u l a t i o n i s e s s e n t i a l f o r economical gas p r o d u c t i o n b e i n g c o n c e n t r a t e d t o t h e boundary between t h e m a r g i n a l a e o l i a n dune f i e l d ( c f . p l a t e s I 1 and V I I I i n s e c t i o n 3.13.) and b r a i d e d f l u v i a l wadi systems ( c f . p l a t e s I 1 1 and I V ) and t h e c e n t r a l p l a y a mud f l a t and d e s e r t l a k e ( c f . p l a t e s V I , V I I and I X ) w i t h i n t h e main d e p o s i t i o n a l b a s i n ( W e c h s e l f o l g e f a c i e s a s s o c i a t i o n ; GRALLA 1988; c f . a l s o s e c t i o n 3 . 2 . 1 . 1 . ) . The succession o f several r e s e r v o i r storeys i n the gas-bearing formation p i l e ( f o r s t r a t i g r a p h i c a l s u b d i v i s i o n o f t h e R o t l i e g e n d s e r i e s c f . s e c t i o n 3 . 2 . 1 . 3 . ) r e s u l t s i n superi m p o s i t i o n o f s i m i l a r sedimentary and p e t r o p h y s i c a l f a c i e s zones, o r i n case o f s h i f t i n g d e p o s i t i o n a l f a c i e s b e l t s i n t h e main t r o u g h o r c o n c e n t r a t i o n o f some sediments i n subbasins o r s y n d e p o s i t i o n a l t e c t o n i c a l f e a t u r e s such as grabens g i v e s r i s e t o v e r t i c a l s t a c k i n g o f d i f f e r e n t f a c i e s and r e s e r v o i r q u a l i t y zones and t h u s p e t r o p h y s i c a l l y v a r i a b l e sediments. D i s t i n c t i o n can be made between p r i m a r y p e t r o p h y s i c a l z o n a t i o n a c c o r d i n g t o p e r m e a b i l i t y o f t h e r e s e r v o i r r o c k s , secondary geochemical z o n a t i o n due t o gas c o m p o s i t i o n , and t e r t i a r y p o l i t i c a l z o n a t i o n o f R o t l i e g e n d gas r e s e r v o i r s w i t h f r a c t u r i n g p o t e n t i a l as a consequence o f t h e economical and a d m i n i s t r a t i v e systems i n t h e t e r r i t o r y where t h e gas f i e l d s a r e l o c a t e d . Comments a r e o f f e r e d as f o l l o w s on p e r m e a b i l i t y which e x h i b i t s m a i n l y a v e r t i c a l z o n a t i o n . and on gas c o m p o s i t i o n which i n c l u d e s p r e d o m i n a n t l y h o r i z o n t a l v a r i a t i o n s .
3.10.1. Permeabi 1i ty The R o t l i e g e n d i n Germany FRG i s s t r a t i g r a p h i c a l l y d i v i d e d i n t o t h r e e r e s e r v o i r s t o r e y s (BRADEL & DRAXLER 1982, PHILIPP & R E I N I C K E 1982, KLOSE & KRUMER 1983; HEDEMANN, MASCHEK, PAULUS & PLEIN 1984; GRALLA 1988; c f . s e c t i o n 3.2.1.3.) w i t h d i f f e r e n t p r i m a r y p e t r o p h y s i c a l p o t e n t i a l and/or secondary r e s e r v o i r e n g i n e e r i n g r e q u i r e m e n t f o r h y d r a u l i c p r o p p a n t f r a c t u r i n g . The c o m p o s i t i o n o f t h e c o n t i n e n t a l succession o f t h r e e d i f f e r e n t palaeoenvironmental and p e t r o p h y s i c a l complexes o f s t r a t i g r a p h i c a l importance i s p a r t i c u l a r l y w e l l - d e v e l o p e d i n t h e Weser-Elbe f a c i e s p r o v i n c e (GRALLA 1988) o r E a s t Hannover a r e a (HEDEMANN, MASCHEK, PAULUS & PLEIN 1984). The l o w e r s e c t i o n (Schneverdingen-Sandstein w i t h i n t h e Schneverdingen-Format i o n i n Germany FRG o r Lower S l o c h t e r e n - F o r m a t i o n i n the N e t h e r l a n d s and Dutch N o r t h Sea) and t h e upper p a r t (Wustrow-Sandstein as t h e most p o t e n t i a l Wechself o l g e pay zone w i t h i n t h e Hannover-Formation i n Germany FRG o r Ten Boer-Format i o n i n t h e N e t h e r l a n d s and Dutch N o r t h Sea; GRALLA, N I E B E R D I N G & SOBOTT 1988) a r e g e n e r a l l y moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s n o t r e q u i r i n g s t i m u l a t i o n , whereas t h e m i d d l e p o r t i o n ( H a u p t s a n d s t e i n w i t h i n t h e Emden-Formation i n Germany FRG o r Upper S l o c h t e r e n - F o r m a t i o n i n t h e N e t h e r l a n d s and D u t c h N o r t h Sea; Slochteren-Hauptsandstein) i s m a i n l y a l o w - p e r m e a b i l i t y t o t i g h t r e s e r v o i r and u s u a l l y r e q u i r e s p r o p p a n t f r a c t u r i n g ( i n t h e l a t t e r h o r i z o n , most o f t h e NHF o p e r a t i o n s on R o t l i e g e n d have been c a r r i e d o u t , p a r t i c u l a r l y i n t h e S o h l i n yen f i e l d ; KLOSE & KRDMER 1983). Some p o i n t s o f d e p o s i t i o n a l environment and
277 g r a i n s i z e d i s t r i b u t i o n g i v i n g r i s e t o t h i s permeability s t r a t i g r a p h y are d i s cussed as f o l l o w s . R e l a t i o n s h i p s between sedimentary f a c i e s and h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l are also o u t l i n e d .
3.10.1.1. Depositional environment and grain size distribution The d i f f e r e n t n e c e s s i t y o f h y d r a u l i c proppant f r a c t u r i n g m a i n l y r e l a t e s t o t h e sedimentary and p e t r o p h y s i c a l f a c i e s o f t h e i n d i v i d u a l s t o r e y s o f t h e gasb e a r i n g R o t l i e g e n d column ( t h e d i s c u s s i o n as f o l l o w s i s based on t h e Weser-Elbe o r E a s t Hannover f a c i e s p r o v i n c e i n Germany FRG where t h e b u l k o f R o t l i e g e n d exp l o r a t i o n , development and f r a c t u r e s t i m u l a t i o n o c c u r s ) . The a e o l i a n Schneverdingen-Sandstein ( c o n s i s t i n g f r e q u e n t l y o f cross-bedded sands h a v i n g been produced by m i g r a t i o n o f l a r g e and h i g h dunes; c f . p l a t e II/1-6 i n s e c t i o n 3.13.) and t h e f l u v i a l Wustrow-Sandstein ( h a v i n g been d e p o s i t e d by s m a l l s h a l l o w b r a i ded streams o r sheet f l o o d s ; c f . p l a t e s I 1 1 and I V ) u s u a l l y have s u f f i c i e n t p r i mary and/or secondary p o r o s i t y and t h u s a l s o p e r m e a b i l i t y , whereas t h e m a i n l y d r y t o wet i n t e r d u n e p l a y a sand f l a t ( w i t h i n many cases o n l y s u b o r d i n a t e amounts o f a e o l i a n dunes, b u t abundant d r y a e o l i a n , damp p l a y a and wet l a c u s t r i n e sheet sands; c f . p l a t e s V I I , VIII/1-5 and I X ) H a u p t s a n d s t e i n has f r e q u e n t l y poor p e r m e a b i l i t y due t o abundant p a r t i c i p a t i o n o f adhesion o f sand and mud i n sedimentary a c c r e t i o n , p r i m a r y o r secondary c l a y i n f i l t r a t i o n d u r i n g d e p o s i t i o n and/or e a r l y diagenesis, and worse g r a i n s i z e d i s t r i b u t i o n . D e p o s i t i o n a l environments and d i a g e n e t i c a l h i s t o r y o f R o t l i e g e n d and Bunts a n d s t e i n sediments as a base f o r r e s e r v o i r m o d e l l i n g a r e d i s c u s s e d by PLEIN (1978), LOTZNER (1981), ROBINSON (1981); DRONG, PLEIN, SANNEMANN, SCHUEPBACH & ZIMDARS (1982);GLENNIE (1983, 1986), LEE (1984), MADER (1981, 1982, 1983, 1984, 1985, 1988 a ) , MADER & TEYSSEN (1985),MADER & YARDLEY (1985);BIFANI, GEORGE & LEVER (1987); GRALLA (1988), MADER & CHATALOV (1988), MARTIN & EVANS (1988) and RICHARDSON, SANGREE & SNEIDER (1988; c f . a l s o s e c t i o n 3.2.1.1.). I n some p a r t s o f a few f i e l d s , however, a l s o t h e H a u p t s a n d s t e i n can be enc o u n t e r e d i n a f a c i e s o f s u f f i c i e n t q u a l i t y f o r b e i n g a commercial producer w i t h o u t s t i m u l a t i o n ( i n case o f l a r g e r c o n t r i b u t i o n s o f cross-bedded a e o l i a n dune sands ( c f . p l a t e II/l-6)which r e p r e s e n t t h e h i g h e s t r e s e r v o i r q u a l i t y i n t h e R o t l i e g e n d ) , o r t h e Schneverdingen-Sandstein and t h e Wustrow-Sandstein m i g h t be o n l y economical pay f o r m a t i o n s a f t e r h y d r a u l i c p r o p p a n t f r a c t u r i n g ( i f t h e f o r m e r c o n t a i n s more a l l u v i a l - f a n b r e c c i a s ( c f . p l a t e 111/5-8)b u i l t up o f v o l c a n i c r o c k c l a s t s i n s t e a d o f a e o l i a n dune sands as a consequence o f s h i f t i n g shares and r a t e s o f i n t e r t o n g u i n g o f b o t h f a c i e s a s s o c i a t i o n s i n t h e m a r g i n a l p o r t i o n s o f graben systems c l o s e r t o t h e boundary f a u l t s , o r i f b o t h a r e more t i g h t l y cemented i n t h e v i c i n i t y o f s y n d e p o s i t i o n a l graben f a u l t s , above e x t r a o r d i n a r i l y t h i c k volcanic lava f l o w p i l e s , o r i n the water-bearing aureole o f t h e gas f i e l d s ) .
3.10.1.2.
Permeabi 1 i ty st rat iqraphy
E f f e c t i v e p e r m e a b i l i t i e s under r e s e r v o i r c o n d i t i o n s ( a s determined f r o m p r o d u c t i o n t e s t s and c o r e measurements) a r e i n t h e Sohlingen f i e l d i n t h e WeserElbe o r E a s t Hannover f a c i e s p r o v i n c e i n t h e Schneverdingen-Sandstein 20 - 50 m i l l i d a r c y , i n t h e Wustrow-Sandstein and o t h e r Wechselfolge sandstones up t o 10 m i l l i d a r c y and i n t h e H a u p t s a n d s t e i n i n t h e b e s t cases 0.5 - 1 m i l l i d a r c y , b u t f r e q u e n t l y much l o w e r p e r m e a b i l i t y values down t o o n l y 0.5 m i c r o d a r c y o c c u r (KLOSE & KRUMER 1983; f o r R o t l i e g e n d s t r a t i g r a p h y c f . s e c t i o n 3.2.1.3.). T h i s permeability d i s t r i b u t i o n i n v e r t i c a l d i r e c t i o n c l e a r l y r e f l e c t s the major s i g n i f i c a n c e o f t h e H a u p t s a n d s t e i n as t h e t a r g e t o f MHF o p e r a t i o n s , whereas t h e o t h e r R o t l i e g e n d sandstones can u s u a l l y be p u t on stream w i t h o u t s t i m u l a t i o n ( t h e minimum p e r m e a b i l i t y f o r economical gas p r o d u c t i o n w i t h o u t t r e a t m e n t i s a b t . 0.5 - 1 m i l l i d a r c y ; LEICHT 1985; c f . a l s o s e c t i o n s 3.4.1.3. and 3.6.1.2.).
278
3.10.1.3.
Sedimentary facies and proppant fracturing potential
I n Rotliegend a n d Buntsandstein continental red bed sequences in Middle E u rope, p o t e n t i a l and/or n eces s i t y of hydraulic proppant f r a c t u r i n g stim ula tion w i t h the aim o f increasing recoverable reserves a n d / o r offta ke r a t e s of natural gas a r e influenced by the sedimentary f a c i e s of the re se rvoir rocks as well as the l a t e r a l c o n t in u i t y of the sand bodies. Distinc tion can ge ne ra lly be made between a e o l i a n , f l u v i a l and a l l u v i a l - f a n , and l a c u s t r i n e de posits ( c h a r a c t e r i s t i c s of continental depositional f a c i e s types f o r hydrocarbon production a r e g i ven by RICHARDSON, SANGREE & SNEIDER 1986). Some aspects of c a l c r e t e palaeosols are a l s o mentioned, and an o u t l i n e i s a l s o given on l a t e r a l c o n t i n u i t y of r e s e r v o i r u n i t s . Detailed sedimentological c h a r a c t e r i z a t i o n s and i n t e r p r e t a t i o n s of a e o l i a n , f l u v i a l and l a c u s t r i n e deposits as well as palaeosols in Rotliegend and Buntsandstein ( i n cl u d i n g numerous colour photographs) a r e contained i n MADER (1985 a , 1985 b ) , and a s e l e c t i o n of s t r u c t u r e s a re a l s o presented in plat e s I - XI together w i t h longer explanations t h a t a re p a r t i a l l y independent from the t e x t ( c f . s ect i o n 3.13.).
3.10.1.3.1.
Aeolian sediments
Aeolian dune sands w i t h well-developed c r o s s - s t r a t i f i c a t i o n (MADER 1985 a , 1985 b; MADER & YARDLEY 1985; c f . p l a t e 1 1 / 1 4 ) and weak t o moderate dia ge ne ti-
cal cementation ar e the r es er v o i r rocks w i t h the highest permeability i n R o t l i e gend a n d Buntsandstein and usually allow production o f gas a t s u f f i c i e n t l y high r a t e s without n e ces s i t at i n g f r a c t u r i n g treatment. While la rge -sc a le c r o s s - s t r a t i f i e d dune sands form extensive sand bodies in both horizontal and v e r t i c a l d i r e c t i o n , small-scale cross-bedded dune sands ( c f . p l a t e VIII/1-5) a re not only lim i t e d in thickness, b u t may a l s o be r e s t r i c t e d in l a t e r a l c o n t i n u i t y and communication between the s u b u n i t s . Small-scale c r o s s - s t r a t i f i e d aeolian dune sands a re fre que ntly v e r t i c a l l y interbedded and l a t e r a l l y intertonguing w i t h horizontal-laminated aeolian she e t sands deposited on dry s u b s t r a t e s (MADER & YARDLEY 1985) as well a s playa a n d playa-lake s a n d and mud l a i d down on damp a n d wet ground w i t h varying influe nces of d e f l a t i o n and adhesion of aeolian sand (MADER 1983 a , 1985 b; c f . p l a t e s VIII a n d I X ) . Damp t o wet s u r f ace conditions give r i s e to i n t e r c a l a t i o n of mud laminae and l a y e r s i n t o the sand complexes ( c f . p l a t e III/1-2) which d e t e r i o r a t e r e s e r v o i r communication, and a l s o lead t o i n s e r t i o n of fine r-gra ine d sand which has negative impact on permeability. I n c o n t r a s t t o la rge -sc a le cross-bedded dune sands which usually d o n o t r eq u i re hydraulic f r a c t u r i n g stim ula tion, small-scale c r o s s - s t r a t i f i e d dune sands in combination w i t h horizontal-laminated dry aeolian s h eet , damp playa and wet playa-lake sediments (MADER 1983 a , 1985 b; MADER & YARDLEY 1985) f r eq u en t l y n e c e s s i t a t e r e s e r v o i r treatment by hyd r a u l i c proppant f r a c t u r i n g i n order t o increase recoverable reserves a n d t o a l low t o produce the gas a t economically f e a s i b l e r a t e s . The s i g n i f i c a n c e of aeolian dune sands ( c f . p l a t e 11/1-6) as the highest-permeability gas r e s e r v o i r s i n the Rotliegend of the North Sea i s a l s o outline d by HAAK & ELEIJAUT (1988), MARTIN & EVANS (1988) and RICHARDSON, S A N G R E E & SNEIDER (1988). Sedimentary and petrophysical f e a t u r e s of aeolian sandstone r e s e r v o i r s a re a l s o i l l u s t r a t e d by MADER & YARDLEY (1985),N U R M I (1985) a n d GOGGIN, CHANDLER, K O C U R E K & LAKE (1986).
3.10.1.3.2. Fluvial
and alluvial-fan deposits
F l u v i a l ( c f . p l a t e IV/3-8)a n d a l l u v i a l - f a n ( c f . p l a t e 111/5-8) de posits a r e s i m i l a r l y as aeolian dune sands ( c f . p l a t e 11/1-6) i n many cases good t o moder a t e primary r es er v o i r rocks, b u t while the poroperm prope rtie s of dry aeolian s h e e t , damp playa and wet playa-lake sediments a re already primarily d e t e r i o r a -
279 t e d by i n s e r t i o n o f mud and f i n e sand, p e r m e a b i l i t y o f f l u v i a l and a l l u v i a l - f a n d e p o s i t s s u f f e r s i n many cases f r o m secondary d i a g e n e t i c a l d e s t r u c t i o n . W e l l s o r t e d f l u v i a l sandstones ( c f . p l a t e I V / 3 - 8 ) and conglomerates ( c f . p l a t e V/1-4) o r i g i n a t i n g i n s h a l l o w t o m o d e r a t e l y deep b r a i d e d r i v e r channels o r p a r t i a l l y even u n c o n f i n e d sheet f l o o d s (MADER 1983 b, 1984 a, 1985 c; MADER & TEYSSEN 1985, MADER & CHATALOV 1988) have s u f f i c i e n t l y coarse g r a i n s i z e t o possess good p r i m a r y p o r o s i t y and p e r m e a b i l i t y . I n t e n s i v e q u a r t z a u t h i g e n e s i s d u r i n g course o f t h e d i a g e n e t i c a l e v o l u t i o n (MADER 1981, 1982, 1983 c ) , however, g i v e s r i s e t o f r e q u e n t l y t i g h t cementation o f t h e m a t r i x which l e a d s t o r e q u i r e m e n t of. h y d r a u l i c proppant f r a c t u r i n g f o r l i b e r a t i o n o f t r a p p e d gas a t e c o n o m i c a l l y feasible rates. P o o r l y - s o r t e d a l l u v i a l - f a n b r e c c i a s , conglomerates and sandstones ( c f . p l a t e s 111/5-8, V I / 4 and V I I / 5 ) have a l r e a d y p r i m a r i l y worse poroperm p r o p e r t i e s due t o d i s p e r s i o n o f mud and f i n e sand i n t h e m a t r i x as w e l l as o f t e n i r r e g u l a r shape o f sand and g r a v e l g r a i n s and d i s c o n t i n u o u s d i s t r i b u t i o n o f g r a v e l clasts, and t h e r e f o r e r e p r e s e n t a l r e a d y p r i m a r i l y l e s s s u i t a b l e pay r o c k s . Secondary d i a g e n e t i c a l l i t h i f i c a t i o n i s an i m p o r t a n t c o n t r i b u t i n g f a c t o r f o r f u r t h e r d e t e r i o r a t i o n o f r e s e r v o i r c h a r a c t e r i s t i c s . I f such submature t o immature a l l u v i a l - f a n d e p o s i t s a r e gas-bearing, h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n o f f r e q u e n t l y l a r g e s c a l e i s t h e o n l y way t o achieve p r o d u c t i o n a t e c o n o m i c a l l y f e a s i b l e r a t e s , b u t i n some cases, r e s e r v o i r c o n d i t i o n s a r e so poor and/or l a t e r a l pay c o n t i n u i t y i s n o t s u f f i c i e n t t o achieve reasonable gas o f f t a k e r a t e s a t all.
3.10.1.3.3. Lacustrine sediments and calcrete palaeosols L a c u s t r i n e sediments a r e m a i n l y mudstones and f i n e sandstones (MADER 1983 a; cf. plates III/1-2, V I , V I I / 1 - 6 and I X ) which i n R o t l i e g e n d and B u n t s a n d s t e i n f o r m r a t h e r s e a l i n g h o r i z o n s and l a y e r s i n s t e a d o f r e p r e s e n t i n g r e s e r v o i r r o c k s themselves. I f f i n e - g r a i n e d homogeneous m o n o l i t h i c o r heterogeneous p o l y l i t h i c l a c u s t r i n e sediments happen t o c o n t a i n gas, economical p r o d u c t i o n r a t e s f r o m these t i g h t r e s e r v o i r s can o n l y be achieved by h y d r a u l i c p r o p p a n t f r a c t u r i n g s t i m u l a t i o n . R e s e r v o i r h e t e r o g e n e i t y i s m a i n l y c r e a t e d by a l t e r n a t i n g d r y , damp and w e t c o n d i t i o n s a t t h e s u r f a c e o f temporary l a k e s o f p l a y a t y p e w i t h i n t e r t o n g u i n g and succession o f dune and wind r i p p l e m i g r a t i o n , adhesion o f d e f l a t e d a e o l i a n sand, a q u a t i c r e d e p o s i t i o n o f windblown sand, and s e d i m e n t a t i o n o f f i n e sand and mud i n t h e s h a l l o w p l a y a l a k e . C a l c r e t e p a l a e o s o l f o r m a t i o n (MADER 1984; c f . p l a t e s X and X I / 8 ) by pedogen i c o v e r p r i n t i n g o f v a r i o u s types o f p r i m a r y sediments u s u a l l y comprises a u t h i genesis o f c l a y m i n e r a l s and i r o n o x i d e s i n t h e p o r e space (MADER 1981, 1982, 1983 c ) t o g e t h e r w i t h growth o f carbonate c o n c r e t i o n s e i t h e r as i s o l a t e d nodul e s o r as tubes f o l l o w i n g f o s s i l i z e d r o o t tubes ( c f . p l a t e X/1-5), w i t h i n mat u r e stages o f c a l i c h e f o r m a t i o n a g g r e g a t i o n o f i s o l a t e d c o n c r e t i o n s t o c o n t i nuous c r u s t s t a k i n g p l a c e (MADER 1985). O r i g i n o f c a l c r e t e p a l a e o s o l s may t r a n s f o r m even h i g h - q u a l i t y r e s e r v o i r h o r i z o n s i n t o l o w - q u a l i t y pay zones o r even t i g h t b a r r i e r layers.
3.10.1.3.4. Lateral continuity o f reservoir units L a t e r a l c o n t i n u i t y o f r e s e r v o i r u n i t s i s an i m p o r t a n t f a c t o r c o n t r o l l i n g economical success o f t h e s t i m u l a t i o n o p e r a t i o n s . H y d r a u l i c proppant f r a c t u r i n g o f c o n t i n u o u s pay zone sheets p r o v i d e s optimum r e s u l t s and h e l p s t o decrease w e l l number i n t h e f i e l d by a l l o w i n g t o i n c r e a s e t h e d r a i n a g e area o f t h e i n d i v i d u a l o f f t a k e p o i n t s . F r a c t u r i n g s t i m u l a t i o n o f l e n t i c u l a r formations ( c f . p l a t e s I I / 7 - 8 and V/3-4) where t h e d i f f e r e n t r e s e r v o i r u n i t s and s u b u n i t s have l i m i t e d h o r i z o n t a l e x t e n s i o n and s e v e r a l l a t e r a l l y a d j o i n i n g pay zone elements must n o t be connected, b u t i n many cases a r e separated by mudstone i n t e r v a l s , however, r e q u i r e s p r o p e r f e a s i b i l i t y m o d e l l i n g and c a l c u l a t i o n p r i o r t o j o b e x e c u t i o n ,
280 because l a t e r a l d i s c o n t i n u i t y o f g a s - b e a r i n g sedimentary f a c i e s b o d i e s can r e s u l t i n access o f o n l y l i m i t e d r e s e r v e s and/or p r o d u c t i o n a t o n l y r e s t r i c t e d r a t e s which soon d i m i n i s h a f t e r i n i t i a l peak o f f t a k e . Drainage o f l e n t i c u l a r pay zones a l s o f r e q u e n t l y r e q u i r e s c l o s e r w e l l spac i n g w i t h respect t o production from continuous r e s e r v o i r s which i n t u r n a f f e c t s t h e d r i l l i n g c o s t budget, and t o g e t h e r w i t h t h e l a r g e r number o f s t i m u l a t i o n c a n d i d a t e s c a r e f u l assessment has t o be made whether t h e e f f o r t s pay o f f o r n o t . A p o s s i b l e s o l u t i o n i s c o n n e c t i o n o f i s o l a t e d sand l e n s e s w i t h neighbour i n g s e p a r a t e sand bodies and w i t h t h e w e l l b o r e through a l a r g e - s c a l e c r a c k by communication f r a c t u r i n g ( c f . s e c t i o n 4.10.2.1.).
3.10.2. Gas composition I n a d d i t i o n t o the primary petrophysical c o n t r o l o f hydraulic proppant f r a c t u r i n g p o t e n t i a l i n t h e g a s - b e a r i n g R o t l i e g e n d sandstones i n m a i n l y v e r t i c a l d i r e c t i o n , a secondary economical i n f l u e n c e i n p r e d o m i n a n t l y h o r i z o n t a l d i r e c t i o n i s g e o c h e m i c a l l y e x e r t e d by t h e gas c o m p o s i t i o n which i n c l u d e s l a r g e r amounts o f i n e r t n i t r o g e n a p a r t f r o m c o m b u s t i b l e methane i n v a r i o u s p a r t s o f t h e gas f i e l d p r o v i n c e s (PHILIPP & R E I N I C K E 1982) t h a t cannot be used, b u t has t o be sep a r a t e d and removed i n p u r i f i c a t i o n p l a n t s . The overview as f o l l o w s c o n c e n t r a t e s on n i t r o g e n c o n t e n t and impact o f economical systems on g a s - f i e l d operation.
3.10.2.1. Nitrogen content L a t e r a l l y , t h e n i t r o g e n c o n t e n t i n c r e a s e s f r o m Sohlingen and Munster i n t h e west ( a b t . 20 % o r l e s s ) t o Wustrow/Salzwedel i n t h e e a s t (60 - 90 % ) and f i n a l l y i n t h e B e r l i n area almost 100 % n i t r o g e n i s encountered ( c f . f i g . 4 ) . F a r t h e r t o t h e e a s t , t h e n i t r o g e n c o n t e n t decreases again t o a b t . 60 - 80 % near t h e western boundary o f Poland and t h e n f u r t h e r along t h e R o t l i e g e n d f a c i e s b e l t towards t h e e a s t e r n b a s i n m a r g i n a t t h e Pomeranian Swell t o a b t . 20 % . O t h e r areas o f h i g h n i t r o g e n c o n t e n t ( u p t o more than 60 % ) i n c l u d e t h e German N o r t h Sea (OFFSHORE ENGINEER 1986 b; c f . s e c t i o n 3 . 3 . 4 . ) , whereas o n l y v e r y low n i t r o g e n shares occur i n Dutch and B r i t i s h N o r t h Sea R o t l i e g e n d f i e l d s (SCHRUDER & SCHUNEICH 1986) as w e l l as i n N e t h e r l a n d s onshore ( a b t . 15 % i n G r o n i n gen; ROSSUM 1 9 7 5 ) . To m i n o r amounts ( p a r t i a l l y a b t . 2 - 3 % w i t h i n 100 m), t h e n i t r o g e n c o n t e n t i s a l s o v e r t i c a l l y i n c r e a s i n g w i t h d e p t h i n t h e western p a r t o f t h e R o t l i e g e n d f a c i e s b e l t f r o m c e n t r a l N o r t h e r n Germany FRG t o Salzwedel/Wustrow a t t h e GDRFRG-boundary, whereas i n Poland around Ostrbw W i e l k o p o l s k i and Szczecin, i n c r e a s i n g r e s e r v o i r depth sometimes p r o v i d e s l o w e r n i t r o g e n c o n t e n t s ( c f . f i g . 5 ) . T h e r e f o r e t h e whole l a t e r a l d i s t r i b u t i o n of n i t r o g e n c o n t e n t s i n t h e R o t l i e gend gas i s p r o b a b l y n o t a s i m p l e b i s y m m e t r i c a l arrangement, b u t i t r a t h e r seems t h a t Ostrow W i e l k o p o l s k i and s u r r o u n d i n g f i e l d s i n Poland a l r e a d y belong t o another m a j o r gas p r o v i n c e , w i t h b o t h h o r i z o n t a l and v e r t i c a l d i s t r i b u t i o n o f c o m b u s t i b l e methane i n r e l a t i o n t o i n e r t n i t r o g e n b e i n g i n f l u e n c e d by t h e s t r a t i g r a p h i c a l and s e d i m e n t o l o g i c a l c o m p o s i t i o n o f t h e g e o l o g i c a l r e c o r d and i t s p a l a e o t e c t o n i c a l and palaeogeochemical e v o l u t i o n as w e l l as i t s diagenet i c a l and gas m a t u r a t i o n h i s t o r y .
Traces o f h e l i u m and q u i c k s i l v e r f o l l o w t h e n i t r o g e n t r e n d s i n normal and i n verse o r i e n t a t i o n , r e s p e c t i v e l y . The n i t r o g e n d e r i v e s f r o m metamorphosis o f d e e p l y b u r i e d sediments f a r below R o t l i e g e n d and C a r b o n i f e r o u s and has r i s e n a l o n g m a j o r f a u l t s (PHILIPP & R E I N I C K E 1982; aspects o f f o r m a t i o n o f R o t l i e g e n d gas d e p o s i t s a r e a l s o d i s c u s s e d by HETZER & KATZUNG 1985).
281
3.10.2.2.
Impact o f economical systems on gas-field operation
Concerning h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l , o p e r a t i o n s i n t h e west e r n p a r t o f t h e E a s t Hannover area ( e s p e c i a l l y Sohlingen) where t h e n i t r o g e n c o n t e n t i s c o m p a r a t i v e l y low a r e c e r t a i n l y v e r y f e a s i b l e , whereas around Wust r o w i n t h e e a s t e r n p a r t on Germany FRG t e r r i t o r y , i t would be h a r d l y p o s s i b l e t o e c o n o m i c a l l y j u s t i f y an expensive MHF t r e a t m e n t w i t h i n a r e s e r v o i r c o n t a i n i n g p r e d o m i n a n t l y t o almost e x c l u s i v e l y n i t r o g e n and o n l y s u b o r d i n a t e shares of hydrocarbons (and i n a d d i t i o n t o t h a t h a v i n g i n t h e meantime a r r i v e d a t an advanced stage o f gas e x p l o i t a t i o n and r e s e r v o i r p r e s s u r e d e p l e t i o n a f t e r more t h a n 20 y e a r s o f p r o d u c t i o n ) . The Wustrow/Salzwedel complex ( c f . f i g . 4 ) which i s d i v i d e d by t h e FRG/GDRboundary i n t o two halves, however, i s an e x c e l l e n t example o f t h e e f f e c t i v i t y o f a t e r t i a r y p o l i t i c a l z o n a t i o n o f h y d r a u l i c proppant f r a c t u r i n g p o t e n t i a l . On t h e Germany GDR s i d e i n t h e Salzwedel s u b s t r u c t u r e , t h e s i t u a t i o n i s q u i t e d i f f e r e n t f r o m t h a t i n Wustrow on t h e Germany FRG s i d e . I n c o n t r a s t t o t h e l a r g e b e l t o f R o t l i e g e n d gas f i e l d s i n Germany FRG e s p e c i a l l y i n t h e E a s t Hannover area, w i t h S o h l i n g e n r e p r e s e n t i n g t h e c e n t r e o f t h e r i c h e s t p r o v i n c e and t h e Munster-Schmarbeck complex b e i n g a n o t h e r pronounced spot, Salzwedel i s t h e o n l y b i g R o t l i e g e n d gas f i e l d on GDR t e r r i t o r y and t h e s m a l l s a t e l l i t e s s u r r o u n d i n g t h e l a r g e p a t c h can d e l i v e r o n l y r e l a t i v e l y m i n o r a d d i t i o n a l q u a n t i t i e s . T h e r e f o r e any e f f o r t t o i n c r e a s e domestic gas s u p p l y i n Germany GDR has t o t a k e under c o n s i d e r a t i o n l a r g e - s c a l e f r a c t u r i n g o f t h e R o t l i e g e n d r e s e r v o i r r o c k s i n t h e Salzwedel f i e l d r e g a r d l e s s o f gas composition, economical p a y o u t and p a r t i a l l y a l s o stage o f gas w i t h d r a w a l , as i n case o f own s e r v i c e f o r t h e s t i m u l a t i o n o p e r a t i o n s , f i n a n c i n g can be made i n domestic c u r r e n c y which i s n o n - c o n v e r t i b l e , whereas purchase o f gas abroad has t o be made on t h e b a s i s o f b a r t e r t r a d e c o m p r i s i n g e x p o r t o f goods f r o m Germany GDR t h a t a r e t h e n l a c k i n g t h e r e , o r even by i n v e s t m e n t o f r a r e h a r d c u r r e n c y .
3.11. Water sensitivity Some p a r t s o f t h e R o t l i e g e n d pay s e r i e s a r e e x t r e m e l y w a t e r - s e n s i t i v e as determined by f l o w t e s t s on v a r i e d f l u i d damage o f b o t h m a t r i x and f r a c t u r e cond u c t i v i t y (JOHN 1983, DIETZEL & HANTELMANN 1985). S h u t - i n and clean-up under r e s e r v o i r c o n d i t i o n s r e s u l t s i n r e g a i n e d i n i t i a l p e r m e a b i l i t y and f r a c t u r e conduct i v i t y , w i t h much s u p e r i o r r e c o v e r y b e i n g achieved by foamed i n comparison t o unfoamed f l u i d v e r s i o n s . Foamed o i l - b a s e d f r a c t u r i n g f l u i d s a r e s u i t a b l e c a r r i e r s f o r t r e a t m e n t o f such r e s e r v o i r s . A p p l i c a t i o n o f water-based f r a c t u r i n g f l u i d s l e a d s t o complete l o s s o f c r a c k c o n d u c t i v i t y and severe p r o p p a n t embedment. The f o l l o w i n g o u t l i n e focusses on g e n e r a l aspects, f e a t u r e s o f foam f r a c t u r i n g s t i m u l a t i o n , c h a r a c t e r i s t i c s o f c r o s s l i n k e d a c i d systems, methanol systems, and l i q u i d carbon d i o x i d e p r o p p a n t f r a c t u r i n g . The d i f f e r e n t t r e a t m e n t techniques f o r w a t e r - s e n s i t i v e r e s e r v o i r s a r e d i s c u s s e d i n more d e t a i l w i t h i n t h e c h a p t e r on R o t l i e g e n d and C a r b o n i f e r o u s s t i m u l a t i o n i n Europe because o f t h e i r s i g n i f i c a n c e i n s u c c e s s f u l l y approaching these pay f o r m a t i o n s which o f t e n s u f f e r f r o m c o n s i d e r a b l e w a t e r s e n s i t i v i t y and t h e r e f o r e a r e f r e q u e n t l y n o t accessible t o conventional f r a c t u r i n g operations.
3.11.1. General aspects The g e n e r a l i n t r o d u c t i o n s t a r t s w i t h commenting on w a t e r s e n s i t i v i t y d i s t r i b u t i o n i n t h e R o t l i e g e n d and t h e n h i g h l i g h t s t h e s i g n i f i c a n c e o f expandable c l a y m i n e r a l s as one o f t h e more common reasons f o r w a t e r s e n s i t i v i t y o f var i o u s r e s e r v o i r rocks.
282
3.11.1.1. Water sensitivity distribution in the Rot liegend The water s e n s i t i v i t y of Rotliegend and p a r t i a l l y a l s o Carboniferous sandstones i s mainly due t o incorporation of water envelopes around authigenic i l l i t e needles a n d leaves in the pore space i n t o the c urre nt movement in case o f f l u s h i n g with a water-based f l u i d , r e s u l t i n g i n d estruc tion a n d breakage of i l l i t e c r y s t a l s which leads t o d i s i n t e g r a t i o n of matrix compound, framework c olla pse a n d plugging of pore t h r o a t s by i l l i t e fragments ( D I E T Z E L & HANTELMANN 1985). Using an oil-based f l u i d f o r f l u s h i n g , the connate water s a t u r a t i o n i s decreased t o the i r r e d u c i b l e minimum, b u t due t o the d i s c r e t e i n t e r f a c e between nonwetting oil-based f l u i d a n d wetting water p e l l i c l e s of the i l l i t e c r y s t a l s , the c la y mineral needles and leaves do not s u f f e r a xia l forc e s and thus the rock fa b r i c remains i n i t s o r i g i n al s t a b l e s i t u a t i o n ( c f . a l s o se c tion 3 . 5 . ) .
I n c o n t r a s t t o wat er - s en s i t i v e Rotliegend s e c t i o n s , the re a r e a l s o patches where the s t r a t i g r a p h i c a l succession i s n o t water s e n s i t i v e ( K L O S E & KRUMER 1983), i n d i c a t i n g t h a t d i f f er en ces i n clay-mineralogy and diagenesis probably r e l a t e d t o d i f f e r e n ces in burial h i s t o r y e x i s t in l a t e r a l d i r e c t i o n between var io u s p a r t s of the Rotliegend b as i n , with t h i s primary mineralogical zonation i n an i r r e p l a r a n d l o cal l y - i n f l u en ced manner giving r i s e t o a secondary d i v i sion i n t o w a t e r- s en s i t i v e and non-water-sensitive p a r t s of the formation t h a t induce a t e r t i a r y d i s t i n c t i o n of treatment modes w i t h main emphasis on the med i u m on which f r a c t u r i n g f l u i d s a r e based. Many low-permeability sandstone formations a r e highly s e n s i t i v e t o damage from increased water s a t u r a t i o n (SLUSSER & RIECKMANN 1976). Such zones a re char a c t e r i z e d by high i r r e d u c i b l e water s a t u r a t i o n caused by the small pore s i z e d i s t r i b u t i o n i n the rock, and by high r et en t i on and slow re le a se of invaded wat e r above the minimum i r r e d u c i b l e value because of the e f f e c t of high c a p i l l a r y pressure. This may r e s u l t in excessively long cleanup times a f t e r well completio n or f r a c t u r e s t i mu l at i o n e s p e c i a l l y i f la rge volumes of aqueous f l u i d s a r e l o s t i n t o the zone.
3.11.1.2. Significance of expandable clay minerals Water s e n s i t i v i t y of r e s e r v o i r sandstones i s c ha ra c te riz e d by a s h a r p loss of formation permeability upon co n t act with water t h a t i s s u b s t a n t i a l l y l e s s s a l i n e than connate water (ECONOMIDES 1986). The most frequent reason of water s e n s i t i v i t y i s the presence of expandable clay minerals such as montmorillonite which swell when touching water, thereby pinching off pores a n d blocking flow channels (SMITH, PAVLICH & SLOVINSKY 1964). Water s e n s i t i v i t y can a l s o occur in sandstones containing only non-expandable cl ays such as i l l i t e o r k a o l i n i t e a n d may be caused by changes i n pH a n d s a l i n i t y (MUNGAN 1965). Permeability reductio n i s mainly provoked by blocking of the pore passages by dispersed c la y miner a l s , cement p a r t i c l e s a n d o t h er f i n e s . Dispersed c la y minerals a r e pre se nt a s d i s c r e t e , pore-lining and pore-bridging c l a y s , w i t h the d i f f e r e n t c la y morpholog ie s s i g n i f i c a n t l y a f f e c t i n g sandstone p o r o s ity a n d permeability (NEASHAM 1977, PITTMAN & THOFIAS 1978). Many sandstones contain l ar g e amounts of very small, loose s o l i d p a r t i c l e s t h a t vary widely i n s i z e and chemical composition, with p a r t i c l e w e t t a b i l i t y and s u r f a c e / i n t e r f a c i a l f o r ces s t r o n g l y influencing p a r t i c l e mobility when m u l t i - f l u i d phases a r e p r es en t (MUECKE 1978). Thus water s e n s i t i v i t y i s a very comp lic a t e d phenomenon n o t only caused by swelling of expandable c l a y s , b u t dispe rs i o n , migration and redeposition of f i n e p a r t i c l e s of various s i z e s a n d chemic a l compositions cr eat i n g the damage. As brines a r e not nearly so damaging t o sandstone as f r e s h water i s , 1 - 3 % KC1 solutions a r e normally used a s the base l i q u i d i n f r a c t u r i n g f l u i d s (ECONOMIDES 1986). Potassium ions s t a b i l i z e c la y s a g a i n st water invasion a n d consequently prevent swelling, and a l s o help n e u t r a l i z i n g the negative charges on the cl ay p a r t i c l e surfa c e which supports to control d i sp e r s i o n tendencies.
283 A p a r t f r o m s w e l l i n g and m i g r a t i n g c l a y s , f o r m a t i o n s e n s i t i v i t y problems i n c l u d e e m u l s i f y i n g tendencies w i t h f o r m a t i o n f l u i d s , c a p i l l a r y b l o c k s and s k i n damage caused by f l u i d r e s i d u e (DEYSARKAR, DAWSON & ACHARYA 1987). Water s e n s i t i v i t y e f f e c t s o f c l a y m i n e r a l s i n p e t r o l e u m r e s e r v o i r sands a r e a l s o d i s c u s s e d by DODD, CONLEY & BARNES (1955), HEWITT (1963) and GAUTIER & R I C E ( 1 9 8 1 ) .
3.11.2. Foam-fracturing stimulation W a t e r - s e n s i t i v e c l a y - b e a r i n g p a r t s o f t h e R o t l i e g e n d and C a r b o n i f e r o u s r e s e r v o i r column can a l s o be s u c c e s s f u l l y approached w i t h t h e t e c h n i q u e o f carbon d i o x i d e foam f r a c t u r i n g ( d e t a i l s on t h e s u c c e s s f u l European p i l o t t r e a t m e n t i n t h e Fehndorf f i e l d i n t h e Emsland arealGermany FRG a r e g i v e n by SOMMER 1987, 1988; f u r t h e r aspects on carbon d i o x i d e foam f r a c t u r i n g a r e d i s c u s s e d i n SLATTER, RUCKER & C R I S P 1986; q u e s t i o n s o f dynamic f l u i d l o s s i n foam f r a c t u r i n g a r e t r e a t e d by K I N G 1977 and H A R R I S 1982; and aspects o f occurrence and a c q u i s i t i o n o f carbon d i o x i d e a r e compiled i n TRIELOFF 1983). Foam i s a l o w - v i s c o s i t y , p e r f e c t - s u p p o r t medium which p r o v i d e s more r a p i d and complete t r e a t m e n t f l u i d r e c o v e r y , h e l p s t o c l e a n t h e w e l l w i t h o u t swabbing, and reduces f o r m a t i o n damage by m i n i m i z i n g t h e q u a n t i t y o f aqueous f l u i d which e n t e r s t h e f o r m a t i o n (PHILLIPS, COUCHMAN & WILKE 1987). Foam i s a p a r t i c u l a r l y e x c e l l e n t medium i n r e s e r v o i r s w i t h low b o t t o m h o l e p r e s s u r e s such as some s h a l l o w gas sands (BALLARD 1977). Foam has been used s i n c e more t h a n 20 y e a r s i n o i l and gas p r o d u c t i o n i n a l l phases o f r e c o v e r y i n c l u d i n g d r i l l i n g , a c i d i z i n g and f r a c t u r i n g s t i m u l a t i o n , secondary and t e r t i a r y r e c o v e r y , d i v e r t i n g d u r i n g workover, and g r a v e l p a c k i n g (HARMS, SMITH, K I N G & POSEY 1988). The main f a v o u r a b l e mechanical p r o p e r t i e s o f foam a r e low h y d r o s t a t i c head, a b i l i t y t o s u p p o r t f i n e s , low l e a k o f f r a t e s , and s h e a r - t h i n n i n g r h e o l o g i c a l behaviour, which i n c o m b i n a t i o n w i t h low l i q u i d cont e n t and e n e r g i z e d f l u i d r e t u r n f a c i l i t a t e t h e use o f foam i n w a t e r - s e n s i t i v e f o r m a t i o n s i n l o w - p r e s s u r e gas zone c o m p l e t i o n and l o w - p e r m e a b i l i t y r e s e r v o i r t r e a t m e n t . Aspects o f p r o p p a n t t r a n s p o r t c a p a c i t y and foam s t a b i l i t y , carbon d i o x i d e vs. n i t r o g e n as foaming o r e n e r g i z i n g agent, f l o w b e h a v i o u r and foam v i s c o s i t y , and f l u i d r e c o v e r y and w e l l cleanup a r e discussed as f o l l o w s .
3.11.2.1. Proppant transport capacity and foam stability Foam f r a c t u r i n g i s c a r r i e d o u t i n hydrocarbon r e s e r v o i r s t i m u l a t i o n s i n c e more t h a n 15 y e a r s (HOLCOMB, CALLAWAY & CURRY 1980, 1981; HOLCOMB 1983). Foams used f o r c a r r y i n g s o l i d p a r t i c l e s a r e a c t u a l l y three-phase systems, w i t h t h e proppants g r a i n s b e i n g suspended i n t h e g a s - l i q u i d boundary b y s u r f a c e t e n s i o n . P a r t i c l e s a t t a c h e d t o foam bubbles p r e v e n t bubble coalescence and t h e r e f o r e enhance foam s t a b i l i t y . Foam bubbles a r e s t a b i l i z e d a t r e s e r v o i r temperature and p r e s s u r e c o n d i t i o n s by s u r f a c e t e n s i o n due t o s u r f a c t a n t e f f e c t , v i s c o s i t y due t o g e l s t a b i l i z e r e f f e c t , and bubble d r a i n a g e c o n t r o l . Foam breakage d u r i n g w e l l cleanup i s achieved by foam expansion and bubble coalescence as p r e s s u r e d e c l i n e s , t u r n i n g t h e foam i n t o a m i s t o f l i q u i d d r o p l e t s i n gas which has low v i s c o s i t y and can no l o n g e r p r o v i d e p r o p p a n t s u p p o r t f o r t h e t h i r d phase b e i n g t h e proppants. T h i s expansion/coalescence t r e n d dominoes f r o m t h e f r a c t u r e r e g i o n through t h e p e r f o r a t i o n s i n t o t h e w e l l b o r e . As t h e m i s t begins t o c l i m b up t h e b o r e h o l e t u b u l a r s , t u r b u l e n c e and compress i o n due t o back p r e s s u r e f r o m t h e choke assembly can t r i g g e r foam r e g e n e r a t i o n which, i f t a k i n g p l a c e a f t e r foam b r e a k i n g and l e a v i n g p r o p p a n t b e h i n d i n t h e f r a c t u r e can be an advantage i n m a i n t a i n i n g simultaneous r e c o v e r y o f b o t h gas and l i q u i d . U s i n g t o o much foaming agents, s t a b i l i z e r o r l i q u i d g e l l i n g agent can cause o v e r s t a b i l i t y o f t h e foam and m i g h t n o t a l l o w foam expansion and b u b b l e coalescence t o t a k e p l a c e , t h e r e b y i n h i b i t i n g foam breakage and t h u s c a u s i n g foam blockage and/or p r o p p a n t flowback. On t h e o t h e r hand, n o t enough
284 foaming agent can g i v e r i s e t o premature foam i n s t a b i l i t y t h a t can l e a d t o j o b screenout o r h i g h foam i n e f f i c i e n c y e s p e c i a l l y i n terms o f p r o p p a n t c a r r i a g e . The d i s c u s s i o n as f o l l o w s focusses on impact o f p r o p p a n t a d d i t i o n on foam s t a b i l i t y and f l u i d v i s c o s i t y , r h e o l o g i c a l c h a r a c t e r i s t i c s o f foam systems, and w a l l - b u i l d i n g o f g e l - b e a r i n g foams.
3.11.2.1.1. Imact of proppant addition
on foam stability and fluid viscosity
The p r i m a r y l i m i t a t i o n o f t h e foam f r a c t u r i n g process i s t h e amount o f p r o p p a n t s t h a t can be p h y s i c a l l y p l a c e d i n t o t h e foam (JENNINGS & DARDEN 1 9 7 9 ) . I f 2 l b s / g a l sand c o n c e n t r a t i o n i s d e s i r e d i n an 80 % q u a l i t y foam, 10 l b s / g a l o f sand have t o be added t o t h e 20 % aqueous phase p r i o r t o d i l u t i o n w i t h n i t r o g e n gas. Foam i s g e n e r a l l y a p e r f e c t p r o p p a n t t r a n s p o r t medium w i t h slow s e t t l i n g r a t e s , w i t h t h e p r o p p a n t e s s e n t i a l l y n o t s e t t l i n g as t h e foam moves down t h e f r a c t u r e d u r i n g t h e t i m e o f t r e a t m e n t (BLAUER & KOHLHAAS 1 9 7 4 ) . W h i l e c o n v e n t i o n a l foam s t i m u l a t i o n o p e r a t i o n s have been f r e q u e n t l y i n c o n s i s t e n t i n p l a c i n g h i g h p r o p p a n t c o n c e n t r a t i o n s and o f t e n reached a p r e s s u r e l i m i t a t i o n w h i c h p r e m a t u r e l y t e r m i n a t e d t h e j o b , t h e c o n s t a n t i n t e r n a l phase t e c h n i q u s has overcome p r e v i o u s p r o p p a n t s a t u r a t i o n boundaries and has a l l o w e d o p e r a t i o n s t o be pumped w i t h reduced p r e s s u r e f l u c t u a t i o n s (HARRIS, KLEBENOW & KUNDERT 1988). Convent i o n a l foam t r e a t m e n t s on e i t h e r carbon d i o x i d e o r n i t r o g e n base when pumped a t h i g h r a t e s down s m a l l - d i a m e t e r t u b i n g f r e q u e n t l y s u f f e r f r o m c o n s i d e r a b l e i n crease o f o p e r a t i o n a l p r e s s u r e as p r o p p a n t i s added u n t i l t h e maximum a l l o w a b l e p r e s s u r e i s reached as a consequence o f h i g h f r i c t i o n p r e s s u r e s . While p r o p p a n t a d d i t i o n causes no m a j o r changs i n v i s c o u s c h a r a c t e r o f nonfoamed f r a c t u r i n g f l u i d s , t h e two-phase s t r u c t u r e d foam f l u i d c o n s i s t i n g o f gaseous i n t e r n a l phase and l i q u i d e x t e r n a l phase i s c o n s i d e r a b l y d i s t u r b e d and p r o p p a n t a d d i t i o n t o h i g h - q u a l i t y foams g i v e s r i s e t o a s i g n i f i c a n t v i s c o s i t y i n c r e a s e . I n o r d e r t o m a i n t a i n a c o n s t a n t - v i s c o s i t y f r a c t u r i n g f l u i d , t h e bal a n c e between i n t e r n a l phase vs. e x t e r n a l phase must be k e p t c o n s t a n t . The conc e p t of c o n s t a n t i n t e r n a l phase d e s i g n keeps b o t h downstream f l o w r a t e and v i s c o s i t y c o n s t a n t by a d d i t i o n o f s o l i d proppant w h i l e m a i n t a i n i n g a c o n s t a n t liq u i d r a t e b u t d e c r e a s i n g gas f l o w r a t e s u f f i c i e n t l y t o equal a b s o l u t e s o l i d f l o w r a t e . In c o n v e n t i o n a l foam t r e a t m e n t s , p r o p p a n t i s added t o t h e l i q u i d phase by b l e n d i n g , and t h e s l u r r y passes then through h i g h - p r e s s u r e pumps t o t h e w e l l h e a d where n i t r o g e n o r carbon d i o x i d e a r e added t h u s d i l u t i n g p r o p p a n t c o n c e n t r a t i o n by s e v e r a l f o l d . By u s i n g c o n s t a n t i n t e r n a l phase design, however, n i t r o g e n o r carbon d i o x i d e shares a r e decreased as proppant s a t u r a t i o n i s i n creased, and t h e r e f o r e t h e d i l u t i o n e f f e c t o f t h e gas i s l e s s and h i g h e r downh o l e p r o p p a n t c o n c e n t r a t i o n s may be achieved. Pumping p r e s s u r e u s u a l l y r i s e s t o a maximum by t h e end of t h e pad stage, b u t then s t e a d i l y decreases as added p r o p p a n t i n c r e a s e s t h e h y d r o s t a t i c w e i g h t o f t h e f l u i d column.
3.11.2.1.2. Rheological characteristics of foam systems D e s i r a b l e p r o p e r t i e s o f foam f r a c t u r i n g f l u i d s a r e h i g h p r o p p a n t t r a n s p o r t c a p a b i l i t y , low h y d r o s t a t i c head, low p r e s s u r e drops due t o f r i c t i o n , and no r e d u c t i o n of f r a c t u r e c o n d u c t i v i t y due t o f l u i d i n g r e d i e n t s (BLAUER & KOHLHAAS 1974, CALJIEZEL & NILES 1 9 8 7 ) . Foams have e x t r e m e l y good r h e o l o g i c a l c h a r a c t e r i s t i c s and f l u i d l o s s performance under a f a i r l y wide range o f c o n d i t i o n s (VEATCH & I;lOSCHOVIDIS 1986). Carbon-dioxide-foam f r a c t u r i n g f l u i d s a l s o p r o v i d e a l o n g s u s t a i n e d s o l u t i o n - g a s d r i v e due t o h i g h s o l u b i l i t y of carbon d i o x i d e w i t h r e s e r v o i r f l u i d s (DEYSARKAR, DAWSON & ACIiARYA 1987). Carbon d i o x i d e can be comm i n g l e d w i t h c o n v e n t i o n a l f r a c t u r i n g f l u i d s as an e n e r g i z i n g component o f subord i n a t e q u a n t i t a t i v e share o r used as t h e gaseous phase in f o r m u l a t i n g foams (GARBIS ?I TAYLOR 1985).
285 Energized o r foam systems s h o u l d be c o n s i d e r e d when t h e pay zone i s underp r e s s u r e d and f r a c t u r i n g f l u i d r e c o v e r y i s q u e s t i o n a b l e , i n case t h e f o r m a t i o n i s s e n s i t i v e t o c o n t a c t by f o r e i g n f l u i d s , and i f a l o w - v i s c o s i t y p e r f e c t - s u p p o r t f l u i d i s d e s i r e d t o m i n i m i z e c r a c k h e i g h t growth. NORTON & HOFFMAN (1982) r e p o r t t h a t c o n v e n t i o n a l water- o r o i l - b a s e d f r a c t u r i n g t r e a t m e n t s may o u t p e r f o r m foam s t i m u l a t i o n o p e r a t i o n s o n l y because o f t h e placement o f l a r g e r amounts o f proppants by t h e f o r m e r f l u i d s i'n t h e f r a c t u r e d f o r m a t i o n . BLAUER, PHILLIPS & C R A I G (1988) comment on r h e o l o g i c a l and p h y s i c a l d i f f e r e n c e s between foam and emulsion f r a c t u r i n g f l u i d s .
3.11.2.1.3. Wall-bui lding of gel-bear ing foams The types o f foams which a r e used i n many h y d r a u l i c f r a c t u r i n g t r e a t m e n t s c o n t a i n g e l i n t h e l i q u i d phase and a r e w a l l - b u i l d i n g f l u i d s (HARRIS 1982) which a r e c o n s t r u c t i n g g e l l a y e r s on t h e f r a c t u r e faces t h a t c o n t r o l f l u i d l e a k o f f . Foams c o n t a i n i n g no g e l i n t h e l i q u i d a r e n o t - w a l l - b u i l d i n g f l u i d s . Foams c o n t a i n i n g polymers b u i l d i n g f r a c t u r e w a l l drapes i n c l u d e t h e p o t e n t i a l o f damage o f o r i g i n a l m a t r i x p e r m e a b i l i t y . Since e x c e s s i v e f l u i d l e a k o f f i s one pot e n t i a l cause o f premature j o b t e r m i n a t i o n and a l s o foam f r a c t u r i n g t r e a t m e n t s o c c a s i o n a l l y screen o u t , adequate knowledge o f f l u i d l o s s c o e f f i c i e n t s i s essent i a l f o r p r o p e r s u p e r v i s i o n o f foam f r a c t u r i n g jobs, t o o .
3.11.2.2. Carbon dioxide vs. nitrogen as foaming or energizing agent H i g h - q u a l i t y foams were developed i n t o a v i a b l e f r a c t u r i n g s t i m u l a t i o n t o o l f o r o i l and gas w e l l s i n t h e 1970's (HARRIS, KLEBENOW & KUNDERT 1988). Foam f l u i d s t y p i c a l l y c o n t a i n i n g a b t . 65 - 80 % n i t r o g e n gas by volume a r e c h a r a c t e r i z e d by low damage t o s e n s i t i v e f o r m a t i o n s and r a p i d r e c o v e r y o f t r e a t m e n t f l u i d . Proppant c o n c e n t r a t i o n s i n n i t r o g e n foams c o u l d be r i s e n f r o m 2 - 3 l b s / g a l a t t h e b e g i n n i n g t o 5 - 7 l b s / g a l i n t h e e a r l y 1980's due t o b l e n d e r and pump d e s i g n improvement. H i g h - q u a l i t y carbon d i o x i d e foam f l u i d s were i n t r o duced i n 1982. Carbon d i o x i d e had been used i n shares o f 5 - 50 % s i n c e t h e 1950's t o p r o v i d e g a s - a s s i s t e d f l u i d r e c o v e r y . The use o f t y p i c a l l y 65 - 80 % carbon d i o x i d e caused a s i g n i f i c a n t b o o s t i n v i s c o s i t y f o r c a r r y i n g p r o p p a n t i n t o t h e f o r m a t i o n . The f o l l o w i n g d i s c u s s i o n focusses on f l u i d v i s c o s i t y and c o m p a t i b i l i t y , f l u i d s o l u b i l i t y and p r o p p a n t t r a n s p o r t , foam d e n s i t y and r e c o v e r a b i l i t y , and r h e o l o g i c a l n a t u r e and t r a n s i t i o n s o f foams and emulsions.
3.11.2.2.1. Fluid viscosity and cornpat i bi 1 i ty I n a d d i t i o n t o t h e o t h e r advantages o f n i t r o g e n foam f l u i d s , carbon d i o x i d e foam p r o v i d e s b e t t e r c o m p a t i b i l i t y w i t h f o r m a t i o n f l u i d s and lowered i n t e r f a c i a l t e n s i o n , and i t s h i g h e r d e n s i t y g i v e s g r e a t e r foam s t a b i l i t y and a l l o w s gen e r a l l y lower pumping p r e s s u r e s d u r i n g j o b e x e c u t i o n (HARRIS, KLEBENOW & KUNDERT 1988). The g r e a t e ' r s o l u b i l i t y o f carbon d i o x i d e i n f o r m a t i o n f l u i d s p r o v i des gas a s s i s t t o r e s e r v o i r s h a v i n g lower p r e s s u r e s t h u s enhancing f l u i d recovery. As f o r example 70 % carbon d i o x i d e foam t r e a t m e n t s can be pumped down cas i n g a t s i g n i f i c a n t l y l o w e r o p e r a t i o n a l p r e s s u r e s than those needed f o r s i m i l a r n i t r o g e n foam j o b s , t h e a p p l i c a t i o n o f carbon d i o x i d e foams a l l o w s t r e a t m e n t s t o be pumped i n t o deeper and h o t t e r r e s e r v o i r s than i s p r a c t i c a l w i t h n i t r o g e n foams. Simple foams c o n t a i n i n g water, n i t r o g e n and s u r f a c t a n t a r e v e r y u n s t a b l e , because t h e l i q u i d around t h e bubbles i s t h i n and d r a i n s away e a s i l y (ECONOMIDES 1986). I n c r e a s i n g foam s t a b i l i t y can be performed w i t h polymer a d d i t i o n (WENDORFF & AINLEY 1981), c r o s s l i n k i n g o f t h e polymer i n t h e aqueous phase (WATKINS, WENDORFF & AINLEY 1983) which a l s o improves foam r h e o l o g y and f l u i d l o s s c o n t r o l , and c o m p o s i t i o n a l a l t e r i n g o f t h e foam by exchanging n i t r o g e n a g a i n s t
286 c a r b o n d i o x i d e i n o r d e r t o c r e a t e a d e n s e r foam and c o n s e q u e n t l y l o w e r s u r f a c e t r e a t i n g p r e s s u r e s . M e t h a n o l n a y be u s e d i n h i g h c o n c e n t r a t i o n s in t h e aqueous phase t o make t h e foam much l e s s damaging and even e a s i e r t o c l e a n u p f r o m wat e r - s e n s i t i v e f o r m a t i o n s (ECONOMIDES 1 9 8 6 ) .
3.11.2.2.2. Fluid solubility and proppant transport C o n c e r n i n g a p p l i c a t i o n b o t h as m a j o r ( f o a m ) o r m i n o r ( e n e r g i z e r ) component, c a r b o n d i o x i d e o f f e r s s e v e r a l a d v a n t a g e s o v e r n i t r o g e n , because i t i s more sol u b l e i n t r e a t i n g f l u i d s and t h u s l o n g e r s h u t - i n t i m e s may be t o l e r a t e d w i t h o u t e x c e s s i v e l o s s o f gas e n e r g y , and t h e h i g h e r d e n s i t y p r o v i d e s i n c r e a s e d h y d r o s t a t i c p r e s s u r e w h i c h may r e s u l t i n l o w e r s u r f a c e t r e a t i n g p r e s s u r e s , p r o v i d i n g t h e a d v a n t a g e i s n o t o f f s e t b y h i c j h e r t u b u l a r f r i c t i o n p r e s s u r e s (HARRIS, KLEBENOW & KUNDERT 1 9 8 8 ) . The h i g h e r d e n s i t y o f c a r b o n d i o x i d e a l s o has a p o s i t i v e i m p a c t on p r o p p a n t t r a n s p o r t c a p a c i t y . I n s h a l l o w l o w - t e m p e r a t u r e w e l l s , longer s h u t - i n times ?re r e q u i r e d w i t h carbon d i o x i d e than w i t h n i t r o g e n i n o r d e r t o a l l o w i t t o h e a t u p s u f f i c i e n t l y t o v a p o r i z e . Many deep w e l l s have been t r e a t e d w i t h c a r b o n d i o x i d e foams s i m p l y because t h e p r e s s u r e l i m i t a t i o n s o f t h e t u b u l a r s w o u l d n o t a l l o w t o c a r r y o u t n i t r o g e n foam t r e a t m e n t s . In c o n t r a s t t o i t s t e c h n i c a l advantages, however, an e c o n o m i c a l and l o g i s t i c a l drawback o f c a r b o n d i o x i d e i s t h e f a c t t h a t n i t r o g e n i s u s u a l l y s u p p l i e d and pumped b y t h e s e r v i c e companies p e r f o r m i n g t h e s t i m u l a t i o n t r e a t m e n t s , whereas c a r b o n d i o x i d e i s n o r m a l l y d e l i v e r e d b y an o u t s i d e c o n t r a c t o r (GARBIS & TAYLOR 1 9 8 5 ) .
3.11.2.2.3, Foam density and recoverability Foaming a d d i t i v e s g i v e b e t t e r r e c o v e r a b i l i t y w i t h c a r b o n d i o x i d e t h a n w i t h n i t r o g e n (NIELL, DOBBS, PRUITT & CRAWFORD 1 9 6 4 ) . C o m p a r a t i v e e v a l u a t i o n o f t h e e f f e c t s o f c a r b o n d i o x i d e and n i t r o g e n as e n e r g i z i n g ( c o m m i n g l e d s y s t e m s ) and f o a m i n g a g e n t s a r e a l s o u n d e r t a k e n b y H A R R I S (1982, 1985, 1 9 8 7 ) ; H A R R I S , REIDENBACH & CHISHOLM ( 1 9 8 6 ) ; REIDENBACH, H A R R I S , LEE & LORD ( 1 9 8 6 ) and YOST, OVERBEY, WILKINS & LOCKE ( 1 9 8 8 ) . DOSCHER, OYEKAN & EL-ARAB1 ( 1 9 8 3 ) and WARD ( 1 9 8 4 ) compare t h e p e r f o r m a n c e o f n i t r o g e n v s . c a r b o n d i o x i d e f o r o i l d i s p l a c e m e n t i n h i g h l y - i n c l i n e d r e s e r v o i r s . A n o t h e r a p p l i c a t i o n i s e n e r g i z e d f r a c t u r i n g w i t h 50 % c a r b o n d i o x i d e f o r i m p r o v e d h y d r o c a r b o n r e c o v e r y (BLACK & LANGSFORD 1 9 8 1 ) . A l t h o u g h n i t r o g e n i s more e x p e n s i v e t h a n c a r b o n d i o x i d e , i t i s o f t e n p r e f e r r e d as g a s i f y i n g a g e n t o f f r a c t u r i n g f l u i d s i n o r d e r t o enhance f l u i d r e t u r n d u r i n g c l i a n u p a s a consequence o f i t s b e t t e r c o n t r o l l i n g c h a r a c t e r i s t i c s o f t h e pH o f t h e f r a c t u r i n g f l u i d (AHMED, SCHATZ, HOLLAND, JONES & GREENFIELD 1 9 8 2 ) .
3.11.2.2.4. Rheolosical nature and
transition of foams and emulsions
Foam and e m u l s i o n f r a c t u r i n g f l u i d s a r e u s e d when i t i s d e s i r a b l e t o r e s t r i c t t h e volume o f e i t h e r w a t e r o r s o l i d g e l l i n g m a t e r i a l s pumped i n t o a f o r m a t i o n because o f t h e i r h i g h a p p a r e n t v i s c o s i t y , l o w f l u i d l o s s and good p r o p p a n t t r a n s p o r t c a p a c i t y t h a t c a n be a c h i e v e d w i t h t h e m i x t u r e o f two i m m i s c i b l e f l u i d s arid a s i n a l l q u a n t i t y o f s u r f a c t a n t (BLAUER, PHILLIPS & C R A I G 1 9 8 8 ) . The mechanics r e s p o n s i b l e f o r t h e s e r h e o l o g i c a l p r o p e r t i e s a r e e s s e n t i a l l y t h e same f o r b o t h foams and e m u l s i o n s . Foam c o n s i s t s o f d i s c o n t i n u o u s gas b u b b l e s suspended i n a l i q u i d , whereas an e m u l s i o n i s composed o f s e p a r a t e d r o p l e t s o f l i q u i d suspended i n a second i m m i s c i b l e l i q u i d . The d e s i r e d f l u i d p r o p e r t i e s a r e a c h i e ved when t h e volume o f t h e d i s c o n t i n u o u s phase i s g r e a t e r t h a n a b t . 52 % when t h e i n d i v i d u a l d r o p l e t s o r bubbles b e g i n t o i n t e r f e r e w i t h each o t h e r d u r i n g f l o w which gives r i s e t o i n c r e a s i n g v i s c o s i t y . D i s t i n c t i o n between foam and e m u l s i o n i s v e r y s i m p l e i f t h e a v e r a g e f l u i d pressure i s l e s s than the c r i t i c a l pressure o f both l i q u i d s w i t h i n t h e f l u i d , w i t h foam o r e m u l s i o n b e i n g p r e s e n t if t h e t e m p e r a t u r e i s g r e a t e r o r l e s s t h a n
287 t h e bubble p o i n t , r e s p e c t i v e l y . W i t h approach o f average f l u i d p r e s s u r e and temp e r a t u r e t o t h e c r i t i c a l p o i n t o f t h e d i s c o n t i n u o u s phase, however, d i s t i n c t i o n o f foam and emulsion becomes b l u r r e d , and foam can be c o n v e r t e d i n t o emulsion o r v i c e versa w i t h o u t s u b s t a n t i a l changes i n r h e o l o g i c a l p r o p e r t i e s . Above c r i t i c a l pressure, d i s c r i m i n a t i o n between foam and emulsion i s o n l y p o s s i b l e by t h e change o f shape o f t h e d i s c o n t i n u o u s phase d r o p l e t o r b u b b l e f r o m round g l o b u l e s t o f a c e t t e d polyhedrons a t t h e passage f r o m emulsion t o foam. L a b o r a t o r y i n v e s t i g a t i o n s o f t h e t r a n s i t i o n have r e v e a l e d t h a t carbon d i o x i d e w a t e r m i x t u r e s w i l l t y p i c a l l y appear as emulsions d u r i n g f r a c t u r i n g o p e r a t i o n s w i t h no passage t o foam, whereas n i t r o g e n i s commonly developed as foam. B o t h mater i a l s , however, behave as gases d u r i n g w e l l cleanup.
3.11.2.3. Flow behaviour and foam viscosity D u r i n g f r a c t u r i n g t r e a t m e n t s , foam e x i s t s i n an unsteady s t a t e due t o cons t a n t changes i n temperature, p r e s s u r e and shear r a t e s which a f f e c t f l o w behav i o u r p r o p e r t i e s and may d r a s t i c a l l y a l t e r foam h y d r a u l i c b e h a v i o u r (CAWIEZEL & NILES 1987). Foam f l o w i s dynamic because v i s c o s i t y , d e n s i t y and f l o w r a t e v a r y c o n t i n u o u s l y w i t h changing temperature, p r e s s u r e and shear r a t e . Foam as a mixt u r e o f gas and l i q u i d i s a v e r y complex f l u i d t h a t can d i s p l a y many c h a r a c t e r i s t i c s depending on how t h e foam was generated and m a i n t a i n e d p r i o r t o and dur i n g c h a r a c t e r i z a t i o n . As foam q u a l i t y r i s e s , apparent v i s c o s i t y i n c r e a s e s expon e n t i a l l y and t h e foam becomes more p s e u d o p l a s t i c . Pressure r i s e s s i g n i f i c a n t l y i n c r e a s e v i s c o s i t y o f foam f l u i d s i n t h e low-shear r a t e range, whereas t h e e f f e c t i s l e s s s i g n i f i c a n t i n h i g h e r - s h e a r r a t e ranges. I n c r e a s e s i n temperature c o n s i d e r a b l y d i m i n u i s h apparent v i s c o s i t y o f t h e foam f l u i d s up t o a c r i t i c a l temperature a f t e r w h i c h l i t t l e change o c c u r s . DEYSARKAR, DAWSON & ACHARYA (1987) o u t l i n e t h a t h i g h e r v i s c o s i t i e s o f CO2l a d e n f r a c t u r i n g f l u i d s can be achieved by a p p l i c a t i o n o f a l c o h o l - b a s e d c a r r i e r s which c o n s e q u e n t l y have b e t t e r p r o p p a n t t r a n s p o r t c h a r a c t e r i s t i c s . The h i g h v i s c o s i t y o f such combined and c r o s s l i n k e d f l u i d s ensures e f f i c i e n t p r o p p a n t placement i n t h e f r a c t u r e as w e l l as m i n i m i z a t i o n o f f l u i d l o s s t o t h e f o r m a t i o n . R h e o l o g i c a l aspects o f aqueous s t i m u l a t i o n foams a r e o u t l i n e d by HOLCOMB, CALLAWAY & CURRY (1980 b ) .
3.11.2.4. Fluid recovery and well cleanup I n l o w - p e r m e a b i l i t y f o r m a t i o n s , l e a k o f f c o e f f i c i e n t s f o r some foams can be l o w e r than those o f c r o s s l i n k e d aqueous f r a c t u r i n g f l u i d s . The s e t t l i n g r a t e o f proppants i n foamed c r o s s l i n k e d g e l i s almost two o r d e r s o f magnitude h i g h e r than i n foamed l i n e a r ( u n c r o s s l i n k e d ) g e l (CRAIGHEAD, H O S S A I N I & FREEMAN 1985). Foam i s p a r t i c u l a r l y advantageous i n low-pressure f o r m a t i o n s where o n l y 1 i m i t e d r e s e r v o i r energy i s a v a i l a b l e t o c l e a n up a w e l l a f t e r t e r m i n a t i o n o f t h e hyd r a u l i c f r a c t u r i n g t r e a t m e n t . The disadvantage o f t h e r e s t r i c t i o n t o achieve h i g h p r o p p a n t c o n c e n t r a t i o n s i n t h e f i n a l g a s / l i q u i d m i x t u r e has been r e c e n t l y somewhat a l l e v i a t e d by t e c h n o l o g i c a l i n n o v a t i o n s (VEATCH & M O S C H O V I D I S 1986; Another a l t e r n a t i v e a r e o i l foams which have v e r y low c f . sectiori 4.3.4.1.4.). f l u i d l e a k o f f r a t e s t h a t reduce t h e need t o use f u r t h e r f l u i d - l o s s a d d i t i v e s exc e p t f o r n a t u r a l l y f r a c t u r e d r e s e r v o i r s (DRISCOLL, BOWEN & ROBERTS 1980; NORTON & HOFFMAN 1982).
Energized f l u i d s c o n t a i n i n g carbon d i o x i d e and/or n i t r o g e n and n o t r e p r e s e n t i n g foams, b u t c o n t a i n i n g a h i g h e r l i q u i d share, c l e a n up more r a p i d l y and more e f f i c i e n t l y t h a n non-energized f l u i d s (NALL, CAMPBELL & BONEY 1983). U s i n g energized f l u i d s , t h e t o t a l amount o f l o a d w a t e r t o be r e c o v e r e d i s reduced w i t h o u t s a c r i f i c i n g t h e i n c r e a s e d proppant c o n c e n t r a t i o n s which may be t r a n s p o r t e d by g e l l e d f l u i d s . The a d d i t i o n a l c o s t i n c u r r e d by e n e r g i z i n g f l u i d s i s a small compensation o f t h e advantage o f i n c r e a s e d l o a d w a t e r r e c o v e r y e f f i c i e n c y and t h e reduced amount o f l i q u i d p l a c e d i n t h e f o r m a t i o n . V a r i o u s aspects o f
2 88 f r a c t u r i n g f l u i d r e t e n t i o n and w a t e r b l o c k i n g a r e o u t l i n e d a s f o l l o w s
3.11.2.4.1.
F r a c t u r i n g f l u i d r e t e n t ion
Energized f l u i d s a r e a l s o s u i t a b l e s o l u t i o n s f o r s u c c e s s f u l s t i m u l a t i o n of f o r m a t i o n s t h a t a r e c h a r a c t e r i z e d by poor load r e c o v e r y due t o high f l u i d r e t e n t i o n a s a consequence of abnormally high c a p i l l a r y f o r c e s , u n d e r s a t u r a t e d reserv o i r and p r e s e n c e of m i g r a t i n g c l a y p a r t i c l e s (PA1 & GARBIS 1 9 8 3 ) . The a d d i t i o n of carbon d i o x i d e o r n i t r o g e n g a s p r o v i d e s energy t o enhance f l u i d r e c o v e r y and d e c r e a s e s the volume of l i q u i d t o be r e c o v e r e d . Foam has a b u i l t - i n g a s l i f t advantage when t h e well i s opened a f t e r f r a c t u r i n g which i s p a r t i c u l a r l y d e s i r a b l e i n low-pressure w e l l s (JENNINGS & DARDEN 1 9 7 9 ) . N i t h l a r g e volumes of f l u i d used, t h e n i t r o g e n i n t h e l e a d i n g edge of the t r e a t m e n t may d i s s i p a t e i n t o n a t u r a l f r a c t u r e s and t h e f u l l advantage of t h e gas a s s i s t may n o t be e v i Gent .
As t h e f u l l p o t e n t i a l of g a s w e l l s could n o t be a t t a i n e d i f t h e aqueous t r e a t i n g f l u i d i s n o t r e c o v e r e d , e i t h e r w a t e r r e c o v e r y a i d s have t o be u t i l i z e d o r non-aqueous media have t o be chosen f o r the s t i m u l a t i o n o p e r a t i o n (McMECHAN & CONWAY 1 9 8 3 ) . F l u i d recovery a i d a d d i t i v e s i n c r e a s e t h e c o n t a c t a n g l e which reduces c a p i l l a r y p r e s s u r e and thus d i m i n i s h e s t h e amount of aqueous f l u i d which imbibes i n t o t h e r e s e r v o i r r o c k . F r a c t u r e flow c a p a c i t y i n low-permeabilit y r e s e r v o i r s a l s o needs t o be s i g n i f i c a n t l y h i g h e r f o r p r o p e r r e t u r n of stimul a t i o n f l u i d than r e q u i r e d f o r optimum gas p r o d u c t i o n . Apparent high f l u i d l o s s and f l u i d r e t e n t i o n a r e s i g n i f i c a n t c o n t r i b u t o r s t o s c r e e n o u t f a i l u r e r i s k and t o poor p o s t - f r a c t u r i n g hydrocarbon p r o d u c t i o n .
3.11.2.4.2. Water b l o c k i n g Poor gas p r o d u c t i o n from t i g h t r e s e r v o i r r o c k s f o l l o w i n g a w a t e r - b a s e d f r a c t u r i n g t r e a t m e n t i s o f t e n a t t r i b u t e d t o w a t e r b l o c k i n g of f r a c t u r e a n d / o r format i o n ( H O L D I T C H 1979, ABRAMS & VINEGAR 1985; c f . a l s o s e c t i o n 4 . 8 . 8 . 3 . 1 . 5 . ) . \.lat e r b l o c k i n g o c c u r s i f the drawdown p r e s s u r e g r a d i e n t i n the f o r m a t i o n n e a r the f r a c t u r e f a c e does n o t exceed the rock c a p i l l a r y p r e s s u r e s u f f i c i e n t l y f o r g a s f l o w . blater b l o c k i n g i s g e n e r a l l y n o t a s e r i o u s problem i n most t i g h t pay zones because drawdown p r e s s u r e and w a t e r n o b i l i t y a r e u s u a l l y high enough f o r e f f i c i e n t d i s p l a c e m e n t o f f r a c t u r e f l u i d from the f o r m a t i o n , b u t may develop i f res e r v o i r p r e s s u r e g r a d i e n t s in t h e n e a r f r a c t u r e f a c e r e g i o n a r e low o r i f f l u i d m o b i l i t i e s a r e c o n s i d e r a b l y reduced by f o r m a t i o n damage. T h e b e s t s o l u t i o n f o r minimizing o r even a v o i d i n g of w a t e r b l o c k i n g i s u t i l i z a t i o n of o n l y p a r t i a l l y aqueous o r even rion-aqueous s t i m u l a t i o n media such a s foam, e n e r g i z e d f l u i d s , n i t r o g e n o r carbon d i o x i d e . Other p o s s i b i l i t i e s of reducing w a t e r b l o c k i n g and b r i n e i m b i b i t i o n i s d i m i n i s h i n g w a t e r - w e t t i n g of t h e rock s u r f a c e by an a l c o h o l / s u r f a c t a n t package which l e a d s t o s u f f i c i e n t lowering of the c a p i l l a r y pressure and i n c r e a s i n g w a t e r m o b i l i t y d u r i n g g a s d i s p l a c e m e n t due to s u r f a c e tension reduction. F r a c t u r i n g f l u i d r e t e n t i o n i s o f t e n caused by d e t r i t a l o r a u t h i g e n i c c l a y m i n e r a l s . b u t can a l s o be the r e s u l t of abnormally high c a p i l l a r y f o r c e s , unders a t u r a t e d c o n d i t i o n of the f o r m a t i o n , and p r e s e n c e of m i g r a t i n g c l a y p a r t i c l e s (PA1 & GARBIS 1983 b ) . F l u i d r e t e n t i o n problems can be s i g n i f i c a n t l y reduced by employing s a n d s t o n e w a t e r - w e t t i n g s u r f a c t a n t s i n c o n j u n c t i o n w i t h l a r g e volumes of n i t r o g e n o r carbon d i o x i d e g a s . The a p p l i c a t i o n of u l t r a low s u r f a c e t e n s i o n f l u o r o s u r f a c t a n t s improves l o a d recovery f i g u r e s , and the a d d i t i o n of g a s p r o v i d e s energy t o enhance f l u i d r e c o v e r y and d e c r e a s e s t h e volume of l i q u i d req u i r e d t o s t i m u l a t e , thus r e d u c i n g t h e amount of l i q u i d t o be r e c o v e r e d .
289
3.11.2.5. Other aspects V a r i o u s aspects o f foam f r a c t u r i n g w i t h carbon d i o x i d e , n i t r o g e n and o t h e r media a r e d i s c u s s e d by NEILL, DOBBS, PRUITT & CRAWFORD (1964); BALLARD (1977), K I N G (1977, 1983, 1985); HOLCDMB, CALLAWAY & CURRY (1980); BLACK & LANGSFORD (1981), WENDORFF & AINLEY (1981), AINLEY & CHARLES (1982), LILLIES (1982), NORTON & HOFFMAN (1982), SHEA & BUCHER (1982), BULLEN & LILLIES (1983); CRAIGHEAD, H O S S A I N I & WATKINS (1983); GRUNDMANN 8 LORD (1983); HARRIS, BAILEY & EVERTZ (1983); H A R R I S , WARNOCK & K I N G (1983); WATKINS, WENDORFF & AINLEY (1983); WENDORFF & EARL (1983); HARRIS, HAYNES & EGGER (1984); HARRIS, BAILEY, EVERTZ & REEVES (1984); HARRIS & REIDENBACH (1984), ORTIZ & McLANE (1984), WARD (1984), BARBER & THEMIG (1985); CRAIGHEAO, HOSSAINI & FREEMAN (1985); GARBIS & TAYLOR (1985), MCLANE & EDGINGTON (1985); WARNOCK, HARRIS & K I N G (1985); FREEMAN, B I L DEN & H O S S A I N I (1986); MACK & BAUMGARTNER (1986); REIDENBACH, H A R R I S , LEE & LORD (1986); CAWIEZEL & NILES (1987); DEYSARKAR, DAWSON & ACHARYA (1987) and PHILLIPS, COUCHMAN & WILKE ( 1 9 8 7 ) . Aspects o f w a t e r - s e n s i t i v e hydrophobic o i l - b e a r i n g f o r m a t i o n s a r e d i s c u s s e d by KOHLHAAS (1982), and w a t e r s e n s i t i v i t y o f sandstones i s e v a l u a t e d by WALDORF (1965), VELEY (1979), KHILAR & FOGLER (1983), O R T I Z & McLANE (1984) and KHILAR, FOGLER & AHLUWALIA ( 1 9 8 3 ) . Another method i s c o m b i n a t i o n o f f r a c t u r i n g f l u i d i n s e r t i o n w i t h HC1 pumping i n w a t e r - s e n s i t i v e r e s e r v o i r s (GWINN & McMANUS 1977).
3.11.3. Crosslinked acid systems Another p o s s i b i l i t y o f t r e a t i n g w a t e r - s e n s i t i v e r e s e r v o i r s a r e h i g h - s t r e n g t h c r o s s l i n k e d a c i d systems w i t h e l e v a t e d temperature s t a b i l i t y (COULTER, H A R R I S & KLEBENOW 1980; PABLEY & HOLCOMB 1980, 1981, 1982; NORMAN, CONWAY & WILSON 1981; PABLEY, EWING & CALLAWAY 1982; SMITH, DAWSON & SCOGGINS 1 9 8 3 ) . The low pH o f t h e a c i d g e l systems m i n i m i z e s c l a y damage by i n h i b i t i o n o f s w e l l i n g , w i t h c r o s s l i n k e d a c i d systems t h e r e f o r e r e p r e s e n t i n g a reasonable c o m p e t i t i o n t o foam and e n e r g i z e d f r a c t u r i n g f l u i d s e s p e c i a l l y i n l i g h t o f t h e good p r o p p a n t t r a n s p o r t c a p a c i t y . C r o s s l i n k e d a c i d f l u i d s a r e a l s o t h e optimum means f o r comb i n e d a c i d and p r o p p a n t f r a c t u r i n g ( c f . s e c t i o n 4 . 8 . 9 . ) . Both c o n v e n t i o n a l and delayed c r o s s l i n k i n g o f t h e a c i d a l l o w s c o n s i d e r a b l e p o s s i b i l i t i e s o f r e d u c i n g t u b u l a r f r i c t i o n p r e s s u r e . F l u i d l o s s c o n t r o l matching o r s u r p a s s i n g t h a t a c h i e ved by water-based c r o s s l i n k e d f l u i d s i n s u r e s deep p e n e t r a t i o n o f l i v e a c i d , and an e x t r e m e l y r e t a r d e d r e a c t i o n r a t e p e r m i t s i n many cases t h e c r o s s l i n k e d a c i d t o be p l a c e d i n t o t h e f o r m a t i o n w i t h l i t t l e o r no decrease i n a c i d s t r e n g t h ( c f . a l s o s e c t i o n s 4.3.4.3. and 4 . 5 . 1 . 2 . ) .
3.11.4. Methanol systems I n a d d i t i o n t o foam and e n e r g i z e d f l u i d f r a c t u r i n g and c r o s s l i n k e d a c i d systems, w a t e r - s e n s i t i v e r e s e r v o i r s can be s u c c e s s f u l l y approached w i t h low-pH methanol f r a c t u r i n g f l u i d s (CALLAWAY, ORTIZ & HOLCOMB 1982; O R T I Z & McLANE 1984, CREMA & ALM 1985, PICLANE & EDGINGTON 1985). The low-pH methanol systems t e n d t o dissolve and t o remove a u t h i g e n i c c l a y s i n p w e spaces w i t h o u t a f f e c t i n g t h e d e t r i t a l c l a y s w h i c h a r e g e n e r a l l y t h e cementing m a t e r i a l s , t h e r e b y g r e a t l y i m p r o v i n g communication between f o r m a t i o n and propped f r a c t u r e . High-pH water-based f l u i d s , a l t h o u g h t h e y can be a d j u s t e d w i t h a d d i t i v e s t o c o n t r o l c l a y s w e l l i n g and m i g r a t i o n , do n o t have t h e a b i l i t y t o remove t h e a u t h i g e n i c c l a y s c l u t t e r i n g t h e p o r e t h r o a t s , and a r e a l s o c a u s i n g severe w a t e r b l o c k i n g as evidenced by r a p i d l y d e c l i n i n g p r o d u c t i o n curves, w h i l e low-pH methanol i s a l s o b e n e f i c i a l i n removing suspected w a t e r b l o c k s i n t h e immediate w e l l b o r e v i c i n i t y by l o w e r i n g s u r f a c e t e n s i o n o f t h e connate water, and can even be used as p e r f o r a t i n g and breakdown f l u i d . Methanol i n c o m b i n a t i o n w i t h foam p r o v i d e s t h e most d i v e r s i f i e d s u i t e o f advantages c o m p r i s i n g improvement o f f r a c t u r e f a c e p e r m e a b i l i t y , c o n t r o l o f v e r t i -
290 c a l f r a c t u r e growth, removal o f n a t u r a l w a t e r b l o c k s and more e f f i c i e n t l o a d r e c o v e r y . Low-pH methanol can a l s o be an e f f e c t i v e enhancement o f c o n v e n t i o n a l wat e r - b a s e d systems used f o r s t i m u l a t i o n o f l o w - p e r m e a b i l i t y r e s e r v o i r s by a p p l i c a t i o n as a prepad ahead o f s t a n d a r d high-pH water-based t r e a t m e n t s . Methanol tends t o s t a b i l i z e t h e f r a c t u r i n g g e l by r e t a r d i n g v i s c o s i t y d e g r a d a t i o n o f g e l s o l u t i o n s and i s used i n s t i m u l a t i o n s t o b o t h a c c e l e r a t e and i n c r e a s e t r e a t m e n t f l u i d r e c o v e r y (SATTLER, RAIBLE, GALL & GILL 1988). F i e l d examples o f h y d r a u l i c p r o p p a n t f r a c t u r i n g w i t h methanol systems i n w a t e r - s e n s i t i v e and/or n a t u r a l l y f r a c t u r e d t i g h t gas sandstones a r e r e p o r t e d by CRAIGHEAD, H O S S A I N I & WATSON (1983) and BLASIO, BAILEY & BERGTHOLD ( 1 9 8 5 ) . A l c o h o l a d d i t i v e s a r e a l s o a b l e t o speed up a c i d r e c o v e r y and cleanup i n sandstone m a t r i x a c i d i z i n g (McLEOD & CDULTER 1966; McLEOD, McGINTY & SMITH 1966).
3.11.5. Liquid carbon dioxide proppant fracturing Carbon d i o x i d e has been used as an a d d i t i v e i n h y d r a u l i c f r a c t u r i n g f l u i d s t o a i d t h e r e c o v e r y o f s t i m u l a t i o n f l u i d s s i n c e t h e e a r l y 1960's (BLACK & LANGSFORD 1982; HARRIS, HAYNES & EGGER 1984). The p r i m a r y chemical advantage o f add i n g carbon d i o x i d e t o a t r e a t i n g f l u i d comes from i t s s o l u b i l i t y i n b o t h aqueous f l u i d s and o i l . D u r i n g i n j e c t i o n , l i q u i d carbon d i o x i d e i s mixed w i t h an aqueous t r e a t i n g f l u i d and pumped i n t o t h e b o r e h o l e under h i g h p r e s s u r e when carbon d i o x i d e i s p a r t i a l l y s o l u b l e i n t h e s t i m u l a t i o n f l u i d as w e l l as i n f o r m a t i o n f l u i d s . F o l l o w i n g i n j e c t i o n , w e l l h e a d p r e s s u r e i s lowered, and carbon d i o x i d e begins t o come o u t o f s o l u t i o n , f o r m i n g a s o l u t i o n - g a s d r i v e f o r r e c o v e r y o f t r e a t i n g and f o r m a t i o n f l u i d s . Secondary b e n e f i t s o f carbon d i o x i d e d e r i v i n g f r o m i t s s o l u b i l i t y comprise l o w e r i n g o f i n t e r f a c i a l t e n s i o n o f c a r b o n - d i o x i d e - s a t u r a t e d aqueous f l u i d s t o a l e v e l s i m i l a r t o t h a t achieved w i t h many s u r f a c t a n t s which i s i m p o r t a n t i n r e d u c i n g c a p i l l a r y f o r c e s t h a t can impede p r o d u c t i o n o f t r e a t m e n t f l u i d s imbibed by t h e pores i n t h e f o r m a t i o n . L i q u i d carbon d i o x i d e p r o p p a n t f r a c t u r i n g was i n t r o duced t o t h e i n d u s t r y i n 1981 (LANCASTER 1 9 8 6 ) . A f t e r some g e n e r a l comments on f l u i d v i s c o s i t y , foam d e n s i t y and s o l u t i o n p H - l e v e l , aspects o f p r o p p a n t concent r a t i o n s , staged f r a c t u r i n g t r e a t m e n t s , f o r m a t i o n damage e l i m i n a t i o n , and pumpi n g r a t e and b o t t o m h o l e temperature a r e d i s c u s s e d as f o l l o w s .
3.11.5.1. Fluid viscosity, foam density and solution pH-level Water which i s s a t u r a t e d w i t h carbon d i o x i d e forms c a r b o n i c a c i d t h a t accordi n g t o t h e low pH i s most s u i t a b l e f o r p r o t e c t i o n o f c l a y - r i c h f o r m a t i o n s , w i t h a pH o f a b t . 3 . 5 s t i l l n o t b e i n g s o low t h a t d i s s o l u t i o n o f i r o n f r o m i r o n - b e a r i n g m i n e r a l s and subsequent p r e c i p i t a t i o n becomes a problem. Concerning i n c r e a s i n g o f t h e percentage o f carbon d i o x i d e i n t h e s t i m u l a t i o n f l u i d , l i q u i d c a r bon d i o x i d e has a l o w e r v i s c o s i t y than water, whereas a f r a c t u r i n g f l u i d must have enough v i s c o s i t y t o t r a n s p o r t and p l a c e proppants. The use o f a two-phase s t r u c t u r e d f l u i d w i t h a h i g h i n t e r n a l phase r a t i o such as 70 % q u a l i t y foam p r o v i d e s an e x c e l l e n t way o f u s i n g h i g h carbon d i o x i d e percentages. Another import a n t p r o p e r t y o f f l u i d s e n e r g i z e d w i t h carbon d i o x i d e i s t h e h i g h d e n s i t y o f t h e foam i n t h e range o f 7 - 8 lbm/gal (840 - 1,080 kg/m3) which g i v e s a hydros t a t i c head a p p r o x i m a t e l y t h e same as w a t e r i t s e l f and a l l o w s carbon d i o x i d e foam f l u i d s t o be pumped i n t o deep b o r e h o l e s w i t h o u t e x c e s s i v e w e l l h e a d p r e s s u r e s . Carbon d i o x i d e i s i n j e c t e d as a l i q u i d a t t h e s u r f a c e , b u t c o n v e r t s t o a h i g h - d e n s i t y gas when heated b y t h e f o r m a t i o n downhole. Proppant i s added t o t h e aqueous phase o f t h e f l u i d p r i o r t o commingling w i t h t h e l i q u i d carbon d i oxide.
291
3.11.5.2.
Proppant concentrations
Proppant c o n c e n t r a t i o n s i n t h e aqueous p o r t i o n o f t h e foam system have t o be q u i t e high, w i t h f o r example pumping o f a 70 % q u a l i t y carbon d i o x i d e t r e a t i n g f l u i d c o n t a i n i n g 3 lbm/gal (360 kg/m3) proppants downhole a t 10 b b l / m i n ( 1 . 6 m3/min) r e q u i r i n g t h a t t h e aqueous p o r t i o n c a r r i e s 10 lbm/gal (1,200 kg/m3) p r o p p a n t s a t 3 b b l / m i n ( 0 . 5 m3/min) p r i o r t o commingling w i t h t h e carbon d i o x i d e . V i s c o s i f y i n g agents a r e o f t e n added t o a i d p r o p p a n t t r a n s p o r t through b l e n d i n g and pumping equipment a t t h e s u r f a c e as w e l l as t o s t a b i l i z e f l u i d s t r u c t u r e and c o n t r o l f l u i d l e a k o f f downhole. A p o s s i b l e disadvantage o f carbon d i o x i d e foam f r a c t u r i n g i s t h e t i m e r e q u i r e d t o remove a l l t h e i n j e c t e d carbon d i o x i d e (HARRIS, HAYNES & EGGER 1984). Small amounts o f carbon d i o x i d e (such as 5 m o l % ) i n t h e produced n a t u r a l gas may cause c o r r o s i o n problems o r gas purchase o r s e l l i n g d i f f i c u l t i e s i f t h e c o n t r a c t s e t s a c e i l i n g on t h e carbon d i o x i d e c o n t e n t o f t h e c o m b u s t i b l e gas ( t h e r e i s , however, gas-permeation membrane technology a v a i l a b l e f o r r e d u c i n g t h e carbon d i o x i d e c o n t e n t o f t h e n a t u r a l gas recovered; RUSSELL & COADY 1982).
3.11.5.3.
Staged f r a c t u r i n g treatments
Comparative e v a l u a t i o n o f d i f f e r e n t t r e a t m e n t t e c h n i q u e s c o m p r i s i n g staged f r a c t u r i n g w i t h c r o s s l i n k e d g e l s and p e r f o r a t i o n b a l l s e a l e r s , s e p a r a t e c r o s s l i n k e d - g e l s t i m u l a t i o n w i t h sand p l u g s t o achieve zone i s o l a t i o n , and 70 % qual i t y carbon d i o x i d e foam f r a c t u r i n g r e v e a l s t h a t each t e c h n i q u e has advantages and drawbacks (HARRIS, HAYNES & EGGER 1984). Staged f r a c t u r i n g j o b s a r e t h e l e a s t expensive ones s i n c e m u l t i p l e i n t e r v a l s can be t r e a t e d w i t h a s i n g l e horsepower charge, b u t p o s t - f r a c t u r e p r o d u c t i o n l o g s o f t e n i n d i c a t e t h a t t h e s t i m u l a t i o n l i q u i d was n o t d i v e r t e d t o a l l t h e i n t e n d e d zones. Gel f r a c t u r i n g j o b s u s i n g sand p l u g s g e n e r a l l y r e s u l t i n t h e c r a c k r e a c h i n g t h e d e s i r e d zone by r a r e f a i l u r e o f t h e sand blockages, w i t h these o p e r a t i o n s u s u a l l y g i v i n g r i s e t o h i g h e r - r a t e \4ellS s i n c e a l l t h e zones a r e s t i m u l a t e d . On t h e o t h e r hand, g e l f r a c t u r i n g j o b s a r e a l s o t h e most expensive complet i o n s s i n c e s e t t i n g and t e s t i n g o f t h e sand p l u g a l o n g w i t h s e p a r a t e s t i m u l a t i o n o p e r a t i o n horsepower charges a r e added t o t h e t o t a l c o s t . Carbon d i o x i d e foam f r a c t u r i n g t r e a t m e n t s reduce t h e amount of i n j e c t e d w a t e r s i g n i f i c a n t l y , s i n c e 70 % o f t h e f l u i d i s carbon d i o x i d e and o n l y 30 % i s w a t e r . Carbon d i o x i d e foam f r a c t u r i n g i s most s u i t a b l e f o r s i n g l e - z o n e t r e a t m e n t , because a mult i p l e - i n t e r v a l c o m p l e t i o n w i t h carbon d i o x i d e foam r e q u i r e s pumping o f a sand p l u g w h i c h would a l l o w a d d i t i o n a l w a t e r e n t r y i n t o t h e zone and d e f e a t t h e main purpose f o r d o i n g a carbon d i o x i d e foam s t i m u l a t i o n j o b . Staged f r a c t u r i n g w i t h carbon d i o x i d e foam and b a l l s e a l e r s may s u f f e r f r o m l a c k o f zone coverage.
3.11.5.4.
Format i o n damage el iminat i o n
H y d r a u l i c p r o p p a n t f r a c t u r i n g w i t h l i q u i d carbon d i o x i d e as t r a n s p o r t i n g agent e l i m i n a t e s most o f t h e f o r m a t i o n damage n o r m a l l y a s s o c i a t e d w i t h s t i m u l a t i o n f l u i d s and enables v e r y r a p i d cleanup (LILLIES & K I N G 1982), because a t s t a b i l i z e d r e s e r v o i r temperature and pressure, t h e l i q u i d carbon d i o x i d e v a p o r i zes i n a gas w e l l o r v a p o r i z e s and i s p a r t i a l l y d i s s o l v e d i n t h e r e s e r v o i r f l u i d s i n an o i l w e l l . L i q u i d carbon d i o x i d e can be t r a n s p o r t e d a t t h e s u r f a c e a t temperatures between -20 and -40 OF which a r e non-cryogenic, and t h u s i t can be pumped w i t h c o n v e n t i o n a l equipment and i n j e c t e d d i r e c t l y i n t o t h e w e l l as a l i q u i d . Carbon d i o x i d e i s v e r y s o l u b l e i n crude o i l and g r e a t l y reduces o i l v i s c o s i t y , t h e r e b y a i d i n g i n o i l r e c o v e r y (~.lcMECtIAN& CONWAY 1983, however, r e p o r t an unusual example where l i q u i d carbon d i o x i d e d i d n o t t h i n t h e o i l - b a s e d d r i l l i n g mud, b u t i n f a c t i n c r e a s e d i t s v i s c o s i t y t o t h e p o i n t t h a t i t would no l o n g e r f l o w ) . L i q u i d carbon d i o x i d e can a l s o be g e l l e d i n o r d e r t o improve i t s v i s c o s i t y . The carbon d i o x i d e b o t t o m h o l e temperature i s o f p r i m e importance, be-
292 cause t h e f r a c t u r e must be c o o l e d below t h e c r i t i c a l temperature o f 88 OF f o r t h e carbon d i o x i d e t o e x i s t as a l i q u i d and t o generate s u f f i c i e n t c r a c k w i d t h t o a l l o w p r o p p a n t s t o be i n j e c t e d .
3.11.5.5.
Pumping r a t e and bottom-hole temperature
A h i g h pump r a t e i s t h e most e f f e c t i v e parameter i n G e n e r a t i n g low bottomh o l e temperatures and e x t e n d i n g e f f e c t i v e l i q u i d p e n e t r a t i o n i n t h e f r a c t u r e . F r a c t u r i n g down t u b i n g r a t h e r t h a n c a s i n g and i n s u l a t i n g t h e annulus w i t h o i l r a t h e r t h a n w a t e r a r e a l s o v e r y e f f e c t i v e i n l o w e r i n g b o t t o m - h o l e temperature, whereas w e l l depth, b o t t o m - h o l e s t a t i c temperature and c a r b o n - d i o x i d e - p a d volume have a l e s s e r e f f e c t on b o t t o m - h o l e t r e a t i n g temperature. F r a c t u r e l e a k o f f w i t h carbon d i o x i d e i s p r i m a r i l y c o n t r o l l e d by v i s c o s i t y and expansion when c a r bon d i o x i d e e x i s t s i n i t s l i q u i d phase and by v a p o r i z a t i o n expansion when c a r bon d i o x i d e i s p r e s e n t i n i t s gaseous phase. Carbon d i o x i d e has been used s i n c e t h e e a r l y 1960's i n f r a c t u r e s t i m u l a t i o n o f o i l and gas w e l l s . F i r s t carbon d i o x i d e was pumped w i t h o i l - o r water-based t r e a t i n g f l u i d s i n r a t i o s s u f f i c i e n t t o gas l i f t t h e l i q u i d back t o t h e s u r f a c e a f t e r t h e j o b . F u r t h e r development l e d t o h i g h e r ' carbon d i o x i d e r a t i o s where l i q u i d carbon d i o x i d e i s e f f e c t i v e l y p a r t o f t h e f r a c t u r e - g e n e r a t i n g l i q u i d ( c o m p r i s i n g up t o 50 % ) , w i t h t h e p r o p p a n t pumped i n o i l , w a t e r o r methanol. I m p o r t a n t improvements were l i q u i d volume r e d u c t i o n and ample gas energy s u p p l i e d t o recover the l i q u i d from the formation. I n j e c t i n g proppant d i r e c t l y i n t o the l i q u i d carbon d i o x i d e and u s i n g t h e l a t t e r as t h e s o l e c a r r i e r f l u i d e l i m i n a t e s t h e r e s i d u a l l i q u i d l e f t i n t h e pay and p r o v i d e s a f r a c t u r i n g f l u i d r e c o v e r y niechanisro t h a t i s n o t dependent upon r e s e r v o i r p r e s s u r e . Aspects o f l i q u i d carbon d i o x i d e f r a c t u r i n g a r e a l s o d i s c u s s e d by GREENHORN & L I ( 1 9 8 5 ) .
3.12. Other aspects I n a d d i t i o n t o t h e p o i n t s d i s c u s s e d i n t h e above s e c t i o n s , o t h e r aspects such as w a t e r s a t u r a t i o n , gas demand and r e s e r v o i r performance a r e i n f l u e n c i n g t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l o f R o t l i e g e n d and C a r b o n i f e r o u s gas r e s e r v o i r s which a r e b r i e f l y o u t l i n e d as f o l l o w s .
3.12.1. Water s a t u r a t i o n F u r t h e r problems o f c o m p l e t i o n and p r o d u c t i o n i n o f f s h o r e and onshore R o t l i e gend (and p a r t i a l l y a l s o C a r b o n i f e r o u s ) gas f i e l d s which a f f e c t t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l a r e i n c r e a s i n g w a t e r s a t u r a t i o n w i t h d e c l i r i i n g mac r o - o r m i c r o - p o r o s i t i e s between sand g r a i n s o r c l a y m i n e r a l p a r t i c l e s , respectively, t h a t may c r e a t e a b e t t e r r e l a t i v e p e r m e a b i l i t y f o r w a t e r than f o r gas. Another drawback i s a h i g h g a s - w a t e r - c o n t a c t which bears t h e danger o f f r a c t u r i n g i n t o t h e w a t e r - b e a r i n g s e c t i o n of t h e r e s e r v o i r column u n l e s s t h e c r a c k p r o p a g a t i o n can be s u c c e s s f u l l y c o n t a i n e d i n v e r t i c a l d i r e c t i o n p a r t i c u l a r l y i n s h a l l o w s t r u c t u r e s w i t h low c l o s u r e h e i g h t (SIMONSON, ABOU-SAYED & CLIFTON 1978; MARPINSKI, SCHMIDT & NORTHROP 1980; TEUFEL & CLARK 1981, EEKELEN 1982; W A R P I N S K I , CLARK, SCHMIDT & HUDDLE 1982). C o m p l i c a t i o n s a r e a l s o imposed by t h e n e c e s s i t y o f f r a c t u r i n 9 o n l y i n some t h i n n e r sands h i g h e r up i n t h e g r o s s pay i n t e r v a l where s t i m u l a t i o n has t o be postponed u n t i l d e p l e t i o n o f t h i c k e r sands i n t h e l o w e r p a r t s o f t h e r e s e r v o i r column t h a t can be produced w i t h o u t t r e a t ment i s reached ( w i t h these t h i n sandstones i n most cases n o t j u s t i f y i n g t h e d r i l l i n g o f s e p a r a t e w e l l s , b u t o n l y a l l o w i n g t o w a i t u n t i l successive r e t r e a t towards t h e t o p o f t h e g a s - b e a r i n g s e c t i o n a r r i v e s a t t h e upper s t o r e y s f o l l o w i n g d e p l e t i o n , p l u g g i n g and abandonment o f t h e l o w e r l a y e r s ) .
293
3.12.2. Gas demand I n c o n t r a s t t o t h e B r i t i s h Southern N o r t h Sea where t h e huge R o t l i e g e n d f r a c t u r i n g campaigns i n 1987 - 1990 a r e t h e consequence o f c o n c l u s i o n o f numerous gas d e l i v e r y c o n t r a c t s i n t h e l a s t y e a r s (and p a r t i a l l y s t i l l i n t h e v e r y near f u t u r e ) which now have t o be f u l f i l l e d even r e g a r d l e s s o f t h e p r e s e n t l y n o t v e r y f a v o u r a b l e g e n e r a l economical s i t u a t i o n and even more gas i s needed i n t h e near f u t u r e ( c f . s e c t i o n 2.2.1.6.2.), onshore i n t h e N e t h e r l a n d s and i n Germany FRG, h y d r a u l i c p r o p p a n t f r a c t u r i n g o f R o t l i e g e n d gas w e l l s i s f o r some compan i e s a t t h e moment i n a d d i t i o n t o t h e d e t e r i o r a t e d economical f e a s i b i l i t y s i t u a t i o n a l s o n o t a t t r a c t i v e due t o i n s u f f i c i e n t p o s s i b i l i t i e s o f p u r i f y i n g , c o n d i t i o n i n g and m a r k e t i n g o f a l l t h e gas, a l r e a d y enough e x i s t i n g c a p a c i t i e s t o meet t h e p r e s e n t l y agreed gas s u p p l y commitments, and some s t a g n a t i o n t o even s a t u r a t i o n o f t h e gas market where i n c o n t r a s t t o e a r l i e r e x p e c t a t i o n s and f o r e c a s t s , t h e growth t a k e s p l a c e l e s s q u i c k l y o r has even a l r e a d y a t l e a s t temporar i l y declined. The p r e s e n t l y r e t a r d e d demand p a r t i c u l a r l y i n Germany FRG i s t h e r e s u l t o f t h e gas p r o d u c t i o n , m a r k e t i n g and p u r c h a s i n g p o l i c y o f t h e l a s t y e a r s . W i t h a domestic s u p p l y o f a b t . 30 - 50 % o f t h e t o t a l demand, Germany FRG has been f o r ced s i n c e many y e a r s t o c o n t r a c t c o n s i d e r a b l e gas volumes f r o m f o r e i g n count r i e s ( t h e s i t u a t i o n i n t h e gas market i s much b e t t e r t h a n i n t h e o i l m a r k e t where o n l y a b t . 5 % d e r i v e s f r o m i n l a n d p r o d u c t i o n and t h e r e s t has t o be bought and i m p o r t e d ) . W i t h r e g a r d t o c o n s i d e r a b l e domestic gas r e s e r v e s , t h e pol i c y d u r i n g t h e l a s t y e a r s was n o t t o e x c e s s i v e l y e x p l o i t t h e own resources, b u t produce o n l y a reasonable amount and l e a v e t h e r e s t o f t h e gas i n t h e ground as a bank f o r t h e near f u t u r e . T h e r e f o r e c o n s i d e r a b l e q u a n t i t i e s o f gas have been c o n t r a c t e d f r o m abroad, w i t h t h e spectrum o f d i v e r s i f i c a t i o n a i m i n g on p o l i t i c a l independence and s u p p l y s a f e t y i n c l u d i n g Netherlands, Norway, Denmark, S o v i e t Union and N o r t h A f r i c a ( c f . s e c t i o n 2.2.1.6.3.1.). E x p l o r a t i o n and a p p r a i s a l d r i l l i n g as w e l l as h y d r a u r i c f r a c t u r i n g , however, i s c o n t i n u i n g t o be v e r y s i g n i f i c a n t f o r p r o p e r assessment o f t h e p o t e n t i a l r e s e r v e s and success i v e t r a n s f o r m a t i o n o f more and more p r o b a b l e r e s e r v e s i n t o proven ones, i n o r d e r t o a l l o w f o r b e t t e r e v a l u a t i o n o f domestic gas a v a i l a b i l i t y and t h u s more r e l i a b l e planning o f f u t u r e supply s e c u r i t y .
3.12.3. Reservoir performance Concerning r e s e r v o i r performance, gas o f f t a k e f r o m R o t l i e g e n d sandstones i s f r e q u e n t l y a f f e c t e d by sudden w a t e r breakthrough on h i g h - p e r m e a b i l i t y s t r e a k s , p a r t i a l l y a l s o sand c o n t r o l problems, and superimposed e f f e c t s o f simultaneous gas w i t h d r a w a l f r o m s e v e r a l sandstones w i t h d i f f e r e n t p e r m e a b i l i t y c a p a c i t i e s , r e s u l t i n g i n d i f f e r e n t i a t e d p r o d u c t i o n p r e s s u r e and w a t e r h i s t o r y (JOHN 1983). E x p l o i t a t i o n - d e p e n d e n t changes o f thermal s t r e s s , a g g r e s s i v e b r i n e s , cement c h a n n e l l i n g b e h i n d p i p e s i n d e v i a t e d w e l l s and reasonable p r e s s u r e d i f f e r e n c e s between d e p l e t e d gas-bearing and a d j o i n i n g w a t e r - c o n t a i n i n g s e c t i o n s cause p r o blems t o i d e n t i f y w a t e r f l o w b e h i n d t h e c a s i n g . Non-producing t i g h t sandstones a r e c h a r a c t e r i z e d by sudden p r e s s u r e d e c l i n e , d i m i n i s h i n g r a t e s , and e x t r e m e l y long pressure build-up periods w i t h o u t regaining i n i t i a l r e s e r v o i r pressure a f t e r w i t h d r a w a l o f o n l y small amounts o f gas. Maximum drawdown has t o be a v o i ded r e g a r d i n g confinement p r e s s u r e and p o r e - c o l l a p s e c o n d i t i o n s . I n such c o m p l i c a t e d patches o f t h e R o t l i e g e n d r e s e r v o i r complex, h y d r a u l i c p r o p p a n t f r a c t u r i n g has t o be v e r y c a r e f u l l y c o n s i d e r e d f o r n o t k i l l i n g a r e a s o n a b l y gas-produc i n g w e l l by s u p p o r t i n g w a t e r breakthroughs.
294
3.13. Sedimentary structures of fluvial, aeolian and lacustrine deposits in Rotliegend and Buntsandstein illustrating distribution o f reservoir heterogeneities and natural fractures D i s t r i b u t i o n o f r e s e r v o i r h e t e r o g e n e i t i e s and n a t u r a l f r a c t u r e s in o i l - and g a s - b e a r i n g p r o s p e c t i v e i n t e r v a l s i n v a r i o u s g e o l o g i c a l f o r m a t i o n s have c o n s i d e r a b l e i m p a c t o n n e c e s s i t y o f h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n o f t h e p a y zones, p r o p a g a t i o n modes o f t h e i n d u c e d c r a c k s , and d i s t r i b u t i o n o f p o r o s i t y and p e r m e a b i l i t y c o n t r o l l i n g p r i m a r y a c c u m u l a t i o n and s e c o n d a r y l i b e r a t i o n o f h y d r o c a r bons. V a r i o u s s e d i m e n t a r y s t r u c t u r e s o f f l u v i a l , a e o l i a n and l a c u s t r i n e depos i t s i n R o t l i e g e n d and B u n t s a n d s t e i n a r e c o m p i l e d i n t h i s s e c t i o n o n 11 p h o t o g r a p h i c p l a t e s f o r t h e purpose o f i l l u s t r a t i o n o f d i s t r i b u t i o n p a t t e r n s o f r e s e r v o i r h e t e r o g e n e i t i e s and n a t u r a l f r a c t u r e s . The numerous f i g u r e s on t h e p h o t o g r a p h i c p l a t e s t o g e t h e r w i t h d e t a i l e d e x p l a n a t i o n s t h a t c a n be e v a l u a t e d a l s o i n d e p e n d e n t f r o m t h e m a i n t e x t a r e assembled a t t h e e n d o f t h e t o p i c a l c h a p t e r o n R o t l i e g e n d r e s e r v o i r c h a r a c t e r i s t i c s and h y d r a u l i c p r o p p a n t f r a c t u r i n g a s p e c t s in o r d e r t o i l l u s t r a t e t h e c a s e s t u d y o f t e r r e s t r i a l r e d b e d s t i m u l a t i o n , t o p r o v i d e t r a n s p a r e n c y o f t h e s e d i m e n t a r y and t e c t o n i c a l f e a t u r e s c o n t r o l l i n g h y d r a u l i c f r a c t u r i n g p o t e n t i a l and n e c e s s i t y d i s c u s s e d i n t h e m a i n t e x t , and t o f o r m a t r a n s i t i o n f r o m t h e c a s e s t u d y i n c l u d i n g e c o n o m i c a l and mark e t i n g a s p e c t s t o t h e g e n e r a l t e c h n i c a l o v e r v i e w s and summaries o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n t h e f o l l o w i n g c h a p t e r s . The sequence o f 11 p h o t o g r a p h i c p l a t e s w i t h i n t o t a l 88 f i g u r e s a l s o r e p r e s e n t s an a d d i t i o n a l s o u r c e o f f i g u r e d and w r i t t e n i n f o r m a t i o n o n d e p o s i t i o n a l e n v i r o n m e n t a l e v o l u t i o n o f f l u v i a l , a e o l i a n and l a c u s t r i n e s e d i m e n t s as w e l l as c a l c r e t e p a l a e o s o l s i n R o t l i e g e n d and B u n t s a n d s t e i n i n c o m b i n a t i o n w i t h t h e ill u s t r a t i o n base o f 204 and 117 f i g u r e s o n 20 c o l o u r and 16 b l a c k - a n d - w h i t e phot o g r a p h i c p l a t e s , r e s p e c t i v e l y , i n MADER (1985 a ) , and 262 f i g u r e s on 32 b l a c k a n d - w h i t e p h o t o g r a p h i c p l a t e s i n MADER (1985 b ) . F o l l o w i n g a s h o r t g e n e r a l i n t r o d u c t i o n o f the s i g n i f i c a n c e of t h e i l l u s t r a t e d f e a t u r e s f o r r e s e r v o i r evaluat i o n and u n d e r s t a n d i n g as w e l l as assessment o f f r a c t u r i n g r e q u i r e m e n t and p r e d i c t i o n o f e v o l u t i o n o f c r a c k m o r p h o l o g y and g e o m e t r y , t h e e x p l a n a t i o n s t o t h e p h o t o g r a p h i c p l a t e s f o c u s on a n g u l a r u n c o n f o r m i t i e s and basement f o l d i n g , in a e o l i a n dune sands and f l u v i a l c h a n n e l b a r d e p o s i t s , cross-stratification g r a v e l a g g l o m e r a t i o n s and mud d r a p e s i n a l l u v i a l - f a n b r e c c i a s and f l u v i a l chann e l s a n d s t o n e s , n a t u r a l f r a c t u r e s and s e d i m e n t a r y h e t e r o g e n e i t i e s i n f l u v i a l c h a n n e l and f l o o d p l a i n s a n d s t o n e s and mudstones, g r a i n - s i z e d i s t r i b u t i o n changes and e r o s i o n a l s u r f a c e s i n b r a i d e d - r i v e r c h a n n e l s a n d s t o n e s and c o n g l o m e r a t e s , s y n s e d i m e n t a r y d e s i c c a t i o n f r a c t u r i n g i n l a c u s t r i n e and f l u v i a l f l o o d p l a i n mudstones and n a t u r a l p r o p p i n g o f c r a c k s , l a c u s t r i n e and f l o o d p l a i n mud d r a p e s i n f l u v i a l c h a n n e l and a l l u v i a l p l a y a s a n d s t o n e s , n a t u r a l f r a c t u r e s and sediment a r y h e t e r o g e n e i t i e s i n c r o s s - s t r a t i f i e d a e o l i a n dune and s h e e t sands as w e l l as f l u v i a l c h a n n e l sands, s e d i m e n t a r y and d e f o r m a t i o n a l f e a t u r e s of l a c u s t r i n e mud d r a p e s i n f l u v i a l c h a n n e l , o v e r b a n k and a l l u v i a l p l a y a s a n d s t o n e s , r o o t t u b e s and c a r b o n a t e c o n c r e t i o n s i n c a l c r e t e p a l a e o s o l s , and n a t u r a l f r a c t u r e o r i g i n and d i s t r i b u t i o n in v a r i o u s r e s e r v o i r r o c k s .
3.13.0.
Introduction
The e l e v e n p h o t o g r a p h i c p l a t e s assembled t o g e t h e r w i t h d e t a i l e d e x p l a n a t i o n s i n t h i s s e c t i o n i l l u s t r a t e v a r i o u s aspects o f o r i g i n , d i s t r i b u t i o n , propagat i o n , and p l u g g i n g o f n a t u r a l f r a c t u r e s as w e l l as f o r m a t i o n , m o r p h o l o g i c a l f e a t u r e s and f u n c t i o n o f f l o w b a r r i e r s and f r a c t u r e p r o p a g a t i o n o b s t a c l e s o f r e s e r v o i r h e t e r o g e n e i t i e s i n p r e d o m i n a n t l y R o t l i e g e n d and B u n t s a n d s t e i n in s e v e r a l a r e a s o f c o n t i n e n t a l r e d bed b a s i n s i n E u r o p e . P l a t e I i n s e c t i o n 3.13.1.
f o c u s s e s on t h e i m p a c t o f
angular
unconformities
295 and basement f o l d i n g i n c l u d i n g subsequent e r o s i o n a l d e g r a d a t i o n on d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s and h e t e r o g e n e i t i e s i n t h e r e s e r v o i r complex. P l a t e I 1 i n s e c t i o n 3.13.2. c h a r a c t e r i z e s c r o s s - s t r a t i f i c a t i o n p a t t e r n s i n a e o l i a n dune sands and f l u v i a l channel b a r d e p o s i t s as i n t e r n a l s t r u c t u r e s o f t h e r e s e r v o i r r o c k types w i t h t h e b e s t p e t r o p h y s i c a l p r o p e r t i e s i n R o t l i e g e n d and Buntsands t e i n , and sketches a l s o t h e d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s p a r a l l e l t o bedd i n g p l a n e s . P l a t e 111 i n s e c t i o n 3.13.3. examines d i s t r i b u t i o n o f g r a v e l agglom e r a t i o n s and mud drapes i n a l l u v i a l - f a n b r e c c i a s and f l u v i a l channel sandstones and t h e s i g n i f i c a n c e o f t h e g r a i n - s i z e a n i s o t r o p i e s f o r r e s e r v o i r heterogeanalyzes n e i t y and n a t u r a l f r a c t u r e p r o p a g a t i o n . P l a t e I V i n s e c t i o n 3.13.4. d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s and sedimentary h e t e r o g e n e i t i e s due t o g r a i n s i z e t r e n d s and bedding s t r u c t u r e s i n f l u v i a l channel and f l o o d p l a i n d e p o s i t s o f m o d e r a t e l y - b r a i d e d r i v e r systems and t h e i r impact on n a t u r a l f r a c t u r e e x t e n c o n c e n t r a t e s on changes o f s i o n and d e f l e c t i o n . P l a t e V i n s e c t i o n 3.13.5. g r a i n - s i z e d i s t r i b u t i o n i n f l u v i a l channel d e p o s i t s o f sandy and c o n g l o m e r a t i c b r a i d e d - r i v e r systems which r e p r e s e n t a l s o i m p o r t a n t pay i n t e r v a l f a c i e s types i n R o t l i e g e n d and Buntsandstein, and e v a l u a t e s t h e consequences o f t h e g r a i n s i z e p a t t e r n s and a l t e r a t i o n s f o r r e s e r v o i r c o n t i n u i t y and h e t e r o g e n e i t y . P l a t e V I i n s e c t i o n 3.13.6. p r e s e n t s synsedimentary d e s i c c a t i o n s h r i n k a g e f r a c t u r i n g i n l a c u s t r i n e and f l u v i a l f l o o d p l a i n muddy d e p o s i t s as a source o f p r i m a r y n a t u r a l c r a c k s i n r e s e r v o i r r o c k s and a d j o i n i n g b a r r i e r s as w e l l as an example o f n a t u r a l p r o p p i n g o f f r a c t u r e s w i t h h i g h - c o n d u c t i v i t y sandy m a t e r i a l . P l a t e V I I i n s e c t i o n 3.13.7. discusses d i s t r i b u t i o n o f l a c u s t r i n e mud drapes and f l u v i a l f l o o d p l a i n mud l a y e r s i n f l u v i a l channel and a l l u v i a l p l a y a sandstones and t h e i r i m p a c t on r e s e r v o i r h e t e r o g e n e i t y by c o m p r i s i n g f l u i d f l o w b a r r i e r s and o b s t a c l e s o f f r a c t u r e p r o p a g a t i o n l e a d i n g t o d e f l e c t i o n , stoppage and a b s o r p t i o n o f e x t e n d i n g c r a c k s . P l a t e V I I I i n s e c t i o n 3.13.8. analyzes d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s i n h o r i z o n t a l - l a m i n a t e d and c r o s s - s t r a t i f i e d a e o l i a n dune and sheet sands as w e l l as f l u v i a l channel sands and s i g n i f i c a n c e o f s e d i mentary bedding s t r u c t u r e s f o r r e s e r v o i r h e t e r o g e n e i t y i n c l u d i n g c h i e f l y hydraul i c communication and c o n t i n u i t y o r i n t e r r u p t i o n o f f r a c t u r e p r o p a g a t i o n . P l a t e I X i n s e c t i o n 3.13.9. focusses on d i s t r i b u t i o n o f sedimentary and d e f o r m a t i o n a l f e a t u r e s o f l a c u s t r i n e mud drapes and l e n s e s i n f l u v i a l channel and overbank sandstones as w e l l as sandy p l a y a d e p o s i t s and examines t h e i r impact on propagat i o n , i n t e r r u p t i o n , d e f l e c t i o n and a b s o r p t i o n o f n a t u r a l and induced f r a c t u r e s i n sandstones as w e l l as t h e i r s i g n i f i c a n c e f o r r e s e r v o i r h e t e r o g e n e i t y and ani s o t r o p y i n c l u d i n g above a l l h y d r a u l i c communication problems around t h e f l o w b a r r i e r s . P l a t e X i n s e c t i o n 3.13.10. i n v e s t i g a t e s t h e importance o f r o o t t u bes and carbonate c o n c r e t i o n s i n c a l c r e t e p a l a e o s o l s f o r d e t e r i o r a t i o n o f p r i mary d e p o s i t i o n a l r e s e r v o i r p r o p e r t i e s by secondary pedogenic o v e r p r i n t i n g . P l a t e X I i n s e c t i o n 3.13.11. g i v e s an overview o f n a t u r a l f r a c t u r i n g due t o s e d i mentary d e s i c c a t i o n shrinkage, p l a n t r o o t p r o p a g a t i o n i n p a l a e o s o l s , v o l c a n i c c o o l i n g c o n t r a c t i o n and t e c t o n i c a l e x t e n s i o n , and d i s t r i b u t i o n o f n a t u r a l j o i n t s i n s e l e c t e d r o c k monuments. The f o l l o w i n g s e c t i o n s c o n t a i n i n g t h e p h o t o g r a p h i c p l a t e s a r e each o r g a n i z e d i n a t h r e e - f o l d manner. The d i v i s i o n s b e g i n w i t h a g e n e r a l i n t r o d u c t o r y o v e r view o f t h e t o p i c s i l l u s t r a t e d i n t h e f i g u r e s and d i s c u s s e d i n t h e explanat i o n s , w i t h t h e general comments a l s o summarizing t h e s i g n i f i c a n c e o f t h e p r e sented s t r u c t u r e s f o r p r o p a g a t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r e s . The main p a r t o f t h e s e c t i o n s c o n s i s t s o f t h e d e t a i l e d e x p l a n a t i o n s o f t h e f i g u r e s assemb l e d on t h e p h o t o g r a p h i c p l a t e s . The f i n a l d i v i s i o n p r o v i d e s s h o r t i n f o r m a t i o n on g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples i n o r d e r t o enable t h e i n t e r e s t e d r e a d e r t o p l u n g e i n t o f u r t h e r d e t a i l s and t o examine t h e p r o f i l e s i n t h e quoted r o c k monuments and q u a r r i e s i n t h e f i e l d .
3.13.1.
Angular unconformities and basement folding
The s t r u c t u r e s f i g u r e d on p l a t e I analyze t h e impact o f a n g u l a r unconformit i e s and basement f o l d i n g i n c l u d i n g subsequent e r o s i o n a l d e g r a d a t i o n on d i s t r i -
296 b u t i o n o f n a t u r a l f r a c t u r e s and h e t e r o g e n e i t i e s i n t h e r e s e r v o i r complex. F o l l o w i n g d i s c u s s i o n o f some general aspects, e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e I a r e g i v e n , and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e sented examples a r e quoted.
3.13.1.1. General aspects Changes o f n a t u r a l f r a c t u r e frequency and s i z e d i s t r i b u t i o n as w e l l as d i r e c t i o n a l o r i e n t a t i o n a t s t r a t i g r a p h i c a l and t e c t o n i c a l u n c o n f o r m i t i e s r e p r e s e n t special f e a t u r e s o f r e s e r v o i r h e t e r o g e n e i t y and a n i s o t r o p y ( c f . s e c t i o n 4 . 2 . 4 . 1 . ) i n f l u e n c i n g p r o p a g a t i o n o f h y d r a u l i c a l l y induced f r a c t u r e s . A n g u l a r u n c o n f o r m i t i e s w i t h changes o f b o t h d i r e c t i o n and a n g l e o f d i p o f t h e sediment a r y bedding p l a n e s and t h e s t r a t i g r a p h i c a l l a y e r s have an i m p o r t a n t impact on e x t e n s i o n o f h y d r a u l i c f r a c t u r e s by b e i n g a b l e t o c o n s i d e r a b l y d e f l e c t and r e f r a c t p r o p a g a t i n g c r a c k s ( c f . s e c t i o n 4 . 2 . 3 . 4 . ) . Angular u n c o n f o r m i t i e s a r e t h e r e s u l t o f compressional f o l d i n g o f an o l d e r g e o l o g i c a l s t o r e y d u r i n g orogenes i s , subsequent denudation o f t h e c r e a t e d r e l i e f , and l a t e r c o v e r i n g o f t h e degraded remnant o f t h e morphology by sediments o f a younger i n t e r v a l o f t h e geol o g i c a l column. Unconformities also f r e q u e n t l y separate geological formations w i t h d i f f e r e n t l i t h o l o g y , g r a i n s i z e , s t a g e of d i a g e n e t i c a l e v o l u t i o n ( i n c l u d i n g mechanical compaction and chemical c e m e n t a t i o n ) and t h u s a l s o r o c k p r o p e r t i e s , w h i c h c r e a t e s an a d d i t i o n a l e f f e c t on t h e p r o p a g a t i o n b e h a v i o u r o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s c r o s s i n g these b o u n d a r i e s . W h i l e most o f t h e p r e s e n t e d examples a r e t e c t o n i c a l and s t r a t i g r a p h i c a l u n c o n f o r m i t i e s , sedimentary i n h o m o g e n e i t i e s such as wedging s t r a t a i n t e r r e s t r i a l sandstones and p a t c h y massive r e e f s i n bedded marine carbonates generate comparable e f f e c t s o f m a t e r i a l changes w i t h i n t h e lit h o l o g i c a l column.
3.13.1.2. Explanations o f illustrations on plate I 1 - 6 : Angular u n c o n f o r m i t y between Lower T r i a s s i c B u n t s a n d s t e i n t e r r e s t r i a l r e d beds and Devonian carbonates ( 1 - Z ) , sandstones ( 3 ) and q u a r t z i t e s ( 4 - 6 ) . The s e p a r a t i o n o f two d i f f e r e n t g e o l o g i c a l f o r m a t i o n s w i t h d i f f e r e n t d e p o s i t i o n a l environment and d i f f e r e n t stage o f p o s t s e d i m e n t a r y h i s t o r y (comp r i s i n g mechanical framework compaction and chemical l i t h i f i c a t i o n by cementat i o n ) r e p r e s e n t s marked boundaries i n t h e l i t h o l o g i c a l r e c o r d which t e s t i f y t o V a r i s c a n o r o g e n e s i s and subsequent e r o s i o n a l d e g r a d a t i o n o f t h e f o l d e d and upl i f t e d mountain range i n t o a s u i t e of h i l l o c k s on t h e p r e - T r i a s s i c c o n t i n e n t s u r f a c e t h a t were p r o g r e s s i v e l y covered d u r i n g d e p o s i t i o n o f B u n t s a n d s t e i n f l u v i a l and a e o l i a n sands. The n a t u r a l f r a c t u r e systems i n b o t h g e o l o g i c a l u n i t s a r e a l s o o f d i f f e r e n t t y p e and p a t t e r n . W h i l e i n t h e Devonian basement many c l o sely-spaced v e r t i c a l j o i n t s and a l s o h o r i z o n t a l bedding p l a n e c r a c k s a r e t h e r e s u l t of compressional t e c t o n i c a l d e f o r m a t i o n d u r i n g f o l d i n g as w e l l as normal and i n v e r s e f a u l t i n g , and p a r t i a l l y a l s o o r i g i n a t e d due t o p r e s s u r e r e l e a s e dur i n g subsequent u p l i f t i n g and e r o s i o n a l exhumation ( c f . s e c t i o n 4 . 8 . 8 . 1 . ) , the B u n t s a n d s t e i n cover s t r a t a c o n t a i n a s m a l l e r amount o f more w i d e l y - s p a c e d v e r t i c a l j o i n t s and h o r i z o n t a l bedding p l a n e openings which a r e t h e consequence o f b u r i a l compaction and l i t h i f i c a t i o n s t r e s s as w e l l as e x t e n s i o n a l t e c t o n i c a l def o r m a t i o n d u r i n g normal f a u l t i n g . The main d i f f e r e n c e s between b o t h n a t u r a l f r a c t u r e systems a r e a n g u l a r o r i e n t a t i o n , spacing and s i z e o f t h e v e r t i c a l j o i n t s and h o r i z o n t a l bedding p l a n e cracks, w i t h t h i s change o f g e o m e t r i c a l r e l a t i o n s h i p s b e i n g a b l e t o n o t o n l y cause a change i n d i r e c t i o n o f h y d r a u l i c f r a c t u r e propagation, b u t probably also r e q u i r i n g higher pressures f o r crossing o f t h e a n i s o t r o p y p l a n e s and f u r t h e r p e n e t r a t i o n o f t h e induced c r a c k s which m i g h t be m i s t a k e n f o r i n d i c a t o r s o f screenout f i n i s h i n g o f t h e o p e r a t i o n a t t h e s u r f a c e as judged f r o m f r a c t u r e p r e s s u r e m o n i t o r i n g ( c f . s e c t i o n s 4 . 2 . 3 . 2 . 4 . and 4 . 2 . 3 . 5 . ) . The v a r i o u s systems o f n a t u r a l c r a c k s o f d i f f e r e n t l e n g t h and w i d t h a r e n o t o n l y a b l e t o s t o p and/or d e f l e c t e x t e n d i n g h y d r a u l i c f r a c t u r e s ,
297
Plate I
298 b u t c a n a l s o g i v e r i s e t o b r a n c h i n g and s t r a n d i n g o f t h e a r t i f i c i a l l y - i n d u c e d c r a c k s ( c f . s e c t i o n 4.2.3.4.) as w e l l as b a l l o o n i n g and p r o l o n g a t i o n o f n a t u r a l f r a c t u r e s by f o l l o w i n g t h e p r e - e x i s t i n g j o i n t f a b r i c .
7 : S i m i l a r l y as s t r a t i g r a p h i c a l and t e c t o n i c a l u n c o n f o r m i t i e s , s e d i m e n t a r y wedges o f i n t e r c a l a t e d beds c r e a t e changes o f m a t e r i a l p r o p e r t i e s and o r i e n t a t i o n o f geometrical f a b r i c w i t h i n l i t h o l o g i c a l complexes (cf. section 4.2.4.3.). The m o s t common i n t e r c a l a t e d wedges and l e n s e s i n c o n t i n e n t a l r e d bed sequences such as R o t l i e g e n d and B u n t s a n d s t e i n a r e f l u v i a l c h a n n e l i n f i l l i n g s w h i c h l a t e r a l l y d i e o u t i n muddy a n d / o r sandy a l l u v i a l o v e r b a n k p l a i n dep o s i t s o r a e o l i a n dune sands. The e r o s i o n a l f e a t u r e s a r e t h e r e s u l t o f f l u v i a l s t r e a m c o u r s e s c u t t i n g t h r o u g h a l l u v i a l f l o o d p l a i n s and a e o l i a n dune f i e l d s as a consequence o f l a t e r a l m i g r a t i o n and v e r t i c a l i n c i s i o n , and i n f i l l i n g o c c u r r e d b y s u b s e q u e n t p l u g g i n g o f t h e w a t e r c o u r s e s w i t h w a n i n g f l o w e n e r g y upon abandonment o f t h e d r a i n a g e c h a n n e l s , w i t h g r a i n s i z e o f t h e s e d i m e n t s r e f l e c t ing c u r r e n t v e l o c i t y and w a t e r d e p t h i n t h e a g g r a d i n g s t r e a m . F l u v i a l c h a n n e l p l u g s o f sandy a n d / o r c o n g l o m e r a t i c c o m p o s i t i o n a r e a l s o i m p o r t a n t l e n t i c u l a r h y d r o c a r b o n r e s e r v o i r s w h i c h i n c a s e of c o n s i d e r a b l e p e r m e a b i l i t y c o n t r a s t b e t ween i s o l a t e d c h a n n e l and s u r r o u n d i n g and s e p a r a t i n g f l o o d p l a i n o r i n t e r d u n e dep o s i t s have t o be c o n n e c t e d t o e a c h o t h e r and t o t h e w e l l b o r e b y c o m m u n i c a t i o n f r a c t u r i n g ( c f . section 4.10.2.1.). 8 : Patchy massive carbonate r e e f s i n t e r c a l a t e d i n t o w e l l - s t r a t i f i e d c a r b o n a t e beds r e p r e s e n t n o d e - l i k e i n h o m o g e n e i t i e s where b e d d i n g i s a l m o s t a b s e n t and t h e i n t e r n a l g e o m e t r i c a l o r g a n i z a t i o n i s n e a r l y i s o t r o p i c o r c h a o t i c i n cont r a s t t o t h e o r g a n i z e d l a y e r i n g and j o i n t p a t t e r n i n t h e a n i s o t r o p i c bedded c a r bonate formation. Reefs o r i g i n a t e i n shallow marine water by c o n c e n t r a t i o n o f o r g a n i c components and a c t i v e g r o w t h o f b i o g e n i c c a r b o n a t e s i n i s o l a t e d s p o t s i n c o n t r a s t t o passive chemical and/or organogenic p r e c i p i t a t i o n o f carbonate mud o r sand i n t h e d e p r e s s i o n s between t h e g r o w i n g nodes. The dome shape o r i g i nates p a r t i a l l y already p r i m a r i l y , b u t i s secondarily accentuated by d i f f e r e n t i a l c o m p a c t i o n o f t h e r i g i d node and t h e l e s s c o m p e t e n t l a y e r e d c a r b o n a t e s . Such m a s s i v e i n t e r s p e r s e d s p o t s a r e n o t o n l y i n t e r r u p t i n g t h e n a t u r a l f r a c t u r e system and c a n g e n e r a t e f l o w b a r r i e r s w i t h i n t h e u n s t i m u l a t e d r e s e r v o i r , b u t are a l s o centres o f disturbance o f hydraulic f r a c t u r e propagation which can even t r i g g e r stoppage o f f u r t h e r e x t e n s i o n o f t h e induced cracks, because t h e i s o t r o p i c t o c h a o t i c nodes t e n d t o n e i t h e r d e f l e c t n o r r e f r a c t , but rather t o a b s o r b p r o p a g a t i n g h y d r a u l i c f r a c t u r e s and c a n o n l y be c r o s s e d w i t h h i g h e r p r e s sures t h a t d u r i n g o p e r a t i o n a t t h e s u r f a c e m i g h t mimick screenout t e r m i n a t i o n o f t h e j o b ( c f . s e c t i o n s 4 . 2 . 3 . 2 . 4 . and 4 . 2 . 3 . 5 . ) .
3 . 1 3 . 1 . 3 . Geological format ions and outcrop localities of presented examples 1 : M i d d l e Buntsandstein o v e r l y i n g M i d d l e Devonian carbonates i n t h e N o r t h e a s t e r n H o l y C r o s s M o u n t a i n s ( P o l a n d ) . D i a m e t e r o f f i g u r e a b t . 15 m. Q u a r r y a t t h e s o u t h e r n s i d e o f t h e S w i s l i n a v a l l e y s o u t h o f DoJy O p a c i e between S t a r a c h o w i c e and O s t r o w i e c S w i c t o k r z y s k i ( t o p o g r a p h i c a l map 1 : 100 000, s h e e t Bodz e n t y n , Pas 45, SJup 32, r 44 700, h 8 4 8 0 0 ) . 2 : M i d d l e B u n t s a n d s t e i n c o v e r i n g M i d d l e D e v o n i a n c a r b o n a t e s i n t h e W e s t e r n H o l y C r o s s M o u n t a i n s ( P o l a n d ) . Diamet e r o f f i g u r e a b t . 8 m. Q u a r r y a t t h e n o r t h e r n s i d e o f B o b r z a v a l l e y n o r t h o f Zagnahsk-ZacheJmie between K i e l c e and Skarzysko-Kamienna. 3 : New Red ( P e r m i a n R o t l i e g e n d a n d / o r T r i a s s i c ) c o n g l o m e r a t e s and s a n d s t o n e s o v e r l y i n g O l d Red ( D e v o n i a n ) s a n d s t o n e s . D i a m e t e r o f f i g u r e a b t . 1 . 2 m. Shore s e c t i o n i n t h e K i l k e n n y Bay a t P o r t i s h e a d n e a r B r i s t o l ( E n g l a n d ) . 4 - 6 : M i d d l e B u n t s a n d s t e i n o v e r l y i n g f o l d e d and e r o d e d Lower D e v o n i a n q u a r t z i t e s i n t h e S o u t h e r n S a a r a r e a (Germany FRG). 4 : D i a m e t e r o f f i g u r e a b t . 1 . 2 m. Rocks a t t h e n o r t h e a s t e r n s i d e o f t h e Saar v a l l e y n o r t h e a s t o f S t . G a n g o l f e a s t o f D r e i s b a c h ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t 6505 M e r z i g , r 4 1 480, h 83 4 5 0 ) . 5 : D i a m e t e r o f f i g u r e a b t . 15 m. Q u a r r y a t t h e S a l z m u h l e s o u t h w e s t o f D r e i s b a c h ( s h e e t 6505 M e r z i g , r 40 300, h 8 3 0 0 0 ) . 6 : D i a m e t e r o f f i g u r e a b t . 1.1 m. Rock T e u f e l s s c h o r n -
299 s t e i n a t t h e e a s t e r n s i d e o f t h e E i s e n k o p f west o f Saarholzbach ( s h e e t 6405 Freudenburg, r 43 440, h 86 760). 7 : O t t e r Sandstones (Lower T r i a s s i c ) o f South Devon ( E n g l a n d ) . Diameter o f f i g . a b t . 2 m. Rocks a t t h e shore between Smallstones P o i n t and Crab Ledge between B u d l e i g h S a l t e r t o n and Sidmouth ( s h e e t SY 08/18 Sidmouth, between r 09 200, h 83 840 and r 09 600, h 84 780). 8 : T e r t i a r y o f t h e Mainz Basin. Diameter o f f i g u r e a b t . 11 m. Q u a r r y s o u t h e a s t o f Budenheim n o r t h w e s t o f Mainz ( t o p o g r a p h i c a l map 1 : 100 000, sheet C 6314 Mainz, r 41 700, h 43 200). F o r f u r t h e r d e t a i l s o f t h e f i g u r e d s e c t i o n s c f . MADER (1985 a, 1985 b ) .
3.13.2. Cross-stratification in aeolian dune sands and f luvial channel bar deposits The s t r u c t u r e s f i g u r e d on p l a t e I 1 i n t r o d u c e c r o s s - s t r a t i f i c a t i o n p a t t e r n s i n a e o l i a n dune sands and f l u v i a l channel b a r d e p o s i t s which a r e t h e p e t r o p h y s i c a l l y best-developed r e s e r v o i r s i n R o t l i e g e n d and B u n t s a n d s t e i n ( c f . s e c t i o n 3.2.1.1.), and i l l u s t r a t e t h e d i s t r i b u t i o n o f n a t u r a l f r a c t u r e d p a r a l l e l t o bedd i n g planes o f v a r i o u s a n g u l a r o r i e n t a t i o n through t h e r e s e r v o i r . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l aspects, e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e I 1 a r e given, and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e quoted.
3.13.2.1.
General aspects
Cross-bedding i n a e o l i a n and f l u v i a l sandstones i s an i m p o r t a n t i n t e r n a l s t r u c t u r e n o t o n l y g e n e r a t i n g r e s e r v o i r h e t e r o g e n e i t y by d i f f e r e n e s o f g r a i n s i z e between t h e i n c l i n e d l a y e r s , b u t a l s o t r i g g e r i n g i n some cases t h e o r i g i n o f s p e c i a l g e o m e t r i c a l arrangements o f n a t u r a l c r a c k s p a r a l l e l t o t h e o b l i q u e l a m i n a t i o n . L a r g e - s c a l e cross-bedded a e o l i a n dune sands a r e t h e b e s t gas r e s e r v o i r s i n t h e R o t l i e g e n d o f M i d d l e Europe and i n case o f s u f f i c i e n t p e r m e a b i l i t y do n o t r e q u i r e s t i m u l a t i o n f o r economical p r o d u c t i o n , whereas s m a l l - s c a l e c r o s s - l a m i n a t e d and h o r i z o n t a l - s t r a t i f i e d a e o l i a n dune and sheet sands i n combin a t i o n w i t h damp and wet p l a y a sediments and c o n t a i n i n g numerous t h i n l a y e r s o f d i f f e r e n t g r a i n s i z e which g i v e r i s e t o f l o w i n h i b i t i o n and p e r m e a b i l i t y d e t e r i o r a t i o n can o n l y be f e a s i b l y e x p l o i t e d f o l l o w i n g massive h y d r a u l i c proppant f r a c t u r i n g ( c f . s e c t i o n s 3.2.1.1. and 3.10.1.). While many r e s e r v o i r s composed o f t h i c k c r o s s - s t r a t i f i c a t i o n s e t s a r e h i g h - q u a l i t y pay zones due t o medium o r coarse g r a i n s i z e and l a r g e h o r i z o n t a l e x t e n s i o n o f t h e sand bodies, r e s e r v o i r communication i s o f t e n n e g a t i v e l y i n f l u e n c e d by f i n e r - g r a i n e d drapes on some i n c l i n e d bedding p l a n e s . L a r g e - s c a l e t a b u l a r c r o s s - s t r a t i f i c a t i o n g e n e r a l l y t e s t i f i e s t o downwind o r downcurrent m i g r a t i o n o f a e o l i a n dunes and f l u v i a l sand b a r s w i t h more o r l e s s s t r a i g h t c r e s t , whereas trough-shaped forms g i v e evidence o f sinuous c r e s t s o f t h e d e p o s i t i o n a l sand b o d i e s . The o r i g i n o f h i g h s e t s r e f l e c t s s t r o n g winds o r c u r r e n t s , abundant s u p p l y o f sand, m o d e r a t e l y deep channels o f t h e b r a i d e d r i v e r system o r i n f i l l i n g o f depressions on t h e landscape by windblown sand, and s t a b l e sedimentary and e r o s i o n a l c o n d i t i o n s o v e r some t i m e . The d o m i n a n t l y o b l i que i n t e r n a l f a b r i c a l s o has i t s impact on t h e d i r e c t i o n o f p r o p a g a t i o n o f hyd r a u l i c a l l y - i n d u c e d f r a c t u r e s which c o u l d be a t t e n u a t e d and/or d e f l e c t e d i f t h e c r o s s - s t r a t i f i c a t i o n planes a r e accentuated by opening o f b e d d i n g - p a r a l l e l n a t u r a l c r a c k s i n more l i t h i f i e d sandstones, whereas t h e l a m i n a t i o n developed i n more l o o s e sands has i n some cases o n l y n e g l i g i b l e impact on p e n e t r a t i o n o f hydraulic fractures.
3.13.2.2.
Explanations of illustrations on plate I 1
1 - 3 : M o d e r a t e l y - t o i n t e n s e l y - l i t h i f i e d a e o l i a n ( 1 - 2 ) and f l u v i a l ( 3 ) sandstones w i t h pronounced t a b u l a r cross-bedding a r e c h a r a c t e r i z e d by w e l l - d e -
300 v e l o p e d b e d d i n g - p a r a l l e l n a t u r a l c r a c k s o f n a r r o w e r o r w i d e r o p e n i n g . Such f o r mations e x h i b i t r e s e r v o i r h e t e r o g e n e i t y i n terms o f o r i e n t a t i o n o f d e p o s i t i o n a l l a m i n a t i o n as w e l l as i n l i g h t o f d i r e c t i o n a l d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s . Propagation o f h y d r a u l i c a l l y induced f r a c t u r e s i s considerably i n f l u e n c e d b y t h e v a r i o u s g e o m e t r i c a l c o n s t e l l a t i o n s o f t h e n a t u r a l c r a c k s y s t e m composed o f v e r t i c a l j o i n t s , h o r i z o n t a l b e d d i n g p l a n e f r a c t u r e s and o b l i q u e c r o s s - s t r a t i large-scale f i c a t i o n c r a c k s . In c a s e o f s u f f i c i e n t m a t r i x p e r m e a b i l i t y , c r o s s - b e d d e d a e o l i a n dune sands a r e t h e t o p q u a l i t y o f gas r e s e r v o i r s i n t h e R o t l i e g e n d i n M i d d l e Europe and do n o t r e q u i r e s t i m u l a t i o n f o r a c h i e v e m e n t o f e c o n o m i c a l o f f t a k e r a t e s ( c f . s e c t i o n s 3 . 2 . 1 . 1 . and 3 . 1 0 . 1 . ) , w i t h n a t u r a l f r a c t u r e p e r m e a b i l i t y even i m p r o v i n g p r o d u c t i v i t y o f t h e h o r i z o n s g i v e n a s u f f i c i e n t d i s t a n c e f r o m t h e g a s - w a t e r - c o n t a c t . I f such t h i c k a e o l i a n dune sands, however, a r e i n t e n s e l y l i t h i f i e d and t h u s m a t r i x p e r m e a b i l i t y i s l a r g e l y d e s t r o yed, f e a s i b l e p r o d u c t i o n can o n l y be o b t a i n e d a f t e r m a s s i v e h y d r a u l i c p r o p p a n t f r a c t u r i n g w i t h c o n n e c t i o n o f t i g h t m a t r i x and n a t u r a l c r a c k s y s t e m b y a l o n g synthetic fracture t o the wellbore.
4 : Trough-shaped l a r g e - s c a l e c r o s s - s t r a t i f i c a t i o n s e t s i n a e o l i a n o r f l u v i a l s a n d s t o n e s t e s t i f y t o m i g r a t i o n o f s i n u o u s - c r e s t e d w i n d - d r i v e n dunes o r wat e r - m o v e d sand b a r s a t t h e b o t t o m o f s h a l l o w t o m o d e r a t e l y deep b r a i d e d r i v e r c h a n n e l s . The c u r v a t u r e o f t h e c r o s s - s t r a t i f i c a t i o n laminae presents another c o m p l i c a t i o n o f r e s e r v o i r h e t e r o g e n e i t y e s p e c i a l l y i n terms o f rock mechanical b e h a v i o u r o f p r o p a g a t i n g h y d r a u l i c a l l y i n d u c e d f r a c t u r e s . The t r o u g h - s h a p e d u n i t s have t h e e f f e c t o f d i v e r g i n g o r c o n v e r g i n g p r o p a g a t i n g f r a c t u r e b u n d l e s and t h u s s p l i t t i n g o r m e r g i n g o f m a j o r f r a c t u r e b r a n c h e s ( c f . section
4.2.3.4.).
5 - 6 : W h i l e t h e g r a i n - s i z e d i s t r i b u t i o n i n many l a r g e - s c a l e c r o s s - s t r a t i f i c a t i o n s e t s in f l u v i a l and a e o l i a n s a n d s t o n e s i s more o r l e s s c o n s t a n t and the sediments a r e w e l l - s o r t e d w i t h l i t t l e shares o f f i n e r f r a c t i o n s , o t h e r u n i t s d i s p l a y c o n s i d e r a b l e i n t e r n a l h e t e r o g e n e i t i e s w h i c h a r e d e v e l o p e d as t h i n f i n e - s a n d y a n d / o r muddy d r a p e s on c r o s s - b e d d i n g p l a n e s i n f l u v i a l s a n d s t o n e s ( 5 ) o r f i n e r - g r a i n e d w i n d r i p p l e l a m i n a e i n t e r f e r i n g and i n t e r t o n g u i n g w i t h c o a r s e r - g r a i n e d g r a i n f l o w and g r a i n f a l l l a m i n a e a t t h e base o f a e o l i a n dunes ( 6 ) . Such g r a i n - s i z e v a r i a t i o n s p r i m a r i l y a f f e c t r e s e r v o i r f l o w p a t t e r n s and t r i g g e r i n t e r r u p t i o n o f p a y zone c o n t i n u i t y and c o m m u n i c a t i o n , b u t s e c o n d a r i l y a l s o c r e a t e elements i n f l u e n c i n g t h e d i r e c t i o n o f propagation o f h y d r a u l i c a l l y i n d u c e d f r a c t u r e s . An abundance o f such f i n e r - g r a i n e d l a m i n a e a l s o c o n s i d e r a b l y deteriorates reservoir permeability i n spite o f high porosity i n the coarser l a y e r s , and t h e r e f o r e a e o l i a n and p l a y a s e d i m e n t s h a v i n g o r i g i n a t e d u n d e r f r e q u e n t l y f l u c t u a t i n g c o n d i t i o n s w i t h changing moistness o f t h e surface a r e the t o p candidates f o r massive h y d r a u l i c proppant f r a c t u r i n g i n o r d e r t o enable econ o m i c a l l y f e a s i b l e gas p r o d u c t i o n ( c f . s e c t i o n 3 . 1 0 . 1 . ) .
7 - 8 : I s o l a t e d l e n t i c u l a r f l u v i a l sand b o d i e s o f p r i m a r y - d e p o s i t i o n a l (7) o r secondary-erosional (8) o r i g i n represent separated r e s e r v o i r u n i t s which are o n l y v e r y l i m i t e d o r even n o t a t a l l i n c o n t a c t w i t h o t h e r p a y zone u n i t s . The c o m p o s i t i o n o f r e s e r v o i r s o f l e n t i c u l a r o r c h a n n e l i z e d sand b o d i e s n e c e s s i t a t e s c a r e f u l e c o n o m i c a l e v a l u a t i o n w h e t h e r b o r e h o l e s and h y d r a u l i c f r a c t u r e s c a n g e t access t o s u f f i c i e n t h y d r o c a r b o n volumes t o j u s t i f y t h e enormous c a p i t a l expend i t u r e by reasonably q u i c k payout o f t h e o p e r a t i o n , o r whether t h e drainage r e g i o n o f w e l l and f r a c t u r e i s t o o s m a l l t o a c h i e v e a m o r t i z a t i o n o f t h e i n v e s t ment f r o m a d d i t i o n a l h y d r o c a r b o n o f f t a k e . Fundamental d i s t i n c t i o n has t h u s t o be made between l e n t i c u l a r and s h e e t r e s e r v o i r s t o r e y s f o r e c o n o m i c a l l y b a c k ing-up t h e investment f o r h y d r a u l i c f r a c t u r e s t i m u l a t i o n by a c q u i s i t i n g s u f f i c i e n t newly c r e a t e d i n t e r n a l r e s e r v o i r surface i n o r d e r t o e x p l o i t t h e expected enhanced h y d r o c a r b o n r a t e s . I s o l a t e d l e n t i c u l a r sand b o d i e s t h a t a r e s e p a r a t e d b y b l a n k e t - t y p e b a r r i e r s a l s o have t o be c o n n e c t e d t o e a c h o t h e r and t o t h e w e l l b o r e by c o m m u n i c a t i o n f r a c t u r i n g ( c f . s e c t i o n 4 . 1 0 . 2 . 1 . ) i n o r d e r t o a l l o w economically f e a s i b l e hydrocarbon o f f t a k e r a t e s i n case o f presence o f s u f f i c i e n t reserves.
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3.13.2.3. Geological formations and outcrop localities of presented examples 1 : Middle Buntsandstein o f the Vitosha Mountains ( B u l g a r i a ) . Diameter o f f i g u r e a b t . 4 m. Road s e c t i o n a t t h e n o r t h e r n s i d e o f t h e r i v e r v a l l e y e a s t o f Bosnek s o u t h e a s t o f P e r n i k n e a r S o f i a ( c f . MADER & CHATALOV 1 9 8 8 ) . 2 : M i d d l e Buntsandstein o f t h e Western H o l y Cross Mountains (Poland). Diameter o f f i g u r e a b t . 4 m. Q u a r r y a t t h e e a s t e r n s i d e o f t h e B o b r z a v a l l e y n o r t h o f Ciosowa b e t ween K i e l c e and K o n s k i e . 3 : M i d d l e B u n t s a n d s t e i n o f W e s t e r n E i f e l (Germany FRG). D i a m e t e r o f f i g u r e a b t . 4 m. Road s e c t i o n s o u t h e a s t o f t h e Immenhof s o u t h e a s t o f R o t h ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t 5705 G e r o l s t e i n , r 46 000, h 67 000). 4 : Upper R o t l i e g e n d of t h e Saar-Nahe S y n c l i n e (Germany FRG). Diamet e r o f f i g u r e a b t . 12 m. Q u a r r y a t t h e n o r t h w e s t e r n s i d e o f t h e Nahe v a l l e y n o r t h o f Bad K r e u z n a c h ( t o p o g r a p h i c a l map 1 : 100 000, s h e e t C 6310 Bad K r e u z nach, r 1 8 700, h 25 0 0 0 ) . 5 : M i d d l e B u n t s a n d s t e i n o f U p p e r F r a n c o n i a / N o r t h e a s t e r n B a v a r i a (Germany FRG). D i a m e t e r o f f i g u r e a b t . 2.5 m. Sand p i t a t t h e n o r t h e r n s i d e o f t h e Hasenberg n e a r M o s t h o l z s o u t h w e s t o f H a i g ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t 5733 Kronach, r 47 740, h 7 1 230. 6 : M i d d l e B u n t s a n d s t e i n o f t h e S o u t h e r n Saar a r e a (Germany FRG). D i a m e t e r o f f i g u r e a b t . 0 . 2 5 m. Quarry a t the eastern side o f the Seffersbach v a l l e y south o f B r o t d o r f northe z s t o f M e r z i g ( s h e e t 6506 Reimsbach, r 49 000, h 8 0 3 8 0 ) . 7 : M i d d l e B u n t s a n d s t e i n o f t h e N o r t h e r n Saar a r e a (Germany FRG). D i a m e t e r o f f i g u r e a b t . 1 . 5 m. Sand p i t a t t h e e a s t e r n s i d e o f t h e L i e s c h e r B e r g s o u t h e a s t o f W a s s e r l i e s c h w e s t o f K o n z e r b r u c k ( s h e e t 6305 S a a r b u r g , r 39 360, h 06 9 3 0 ) . 8 : M i d d l e B u n t s a n d s t e i n of Upper F r a n c o n i a / N o r t h e a s t e r n B a v a r i a (Germany FRG). D i a m e t e r o f f i g u r e a b t . 0.4 m. Sand p i t a t t h e e a s t e r n s i d e o f t h e K r o n a c h e r B e r g a t t h e n o r t h e a s t e r n s i d e o f t h e LeRbach v a l l e y e a s t o f WeiBenbrunn ( s h e e t 5734 W a l l e n f e l s , between r 53 960, h 62 730 and r 53 860, h 63 0 0 0 ) . F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a, 1985 b ) .
3.13.3. Gravel agglomerations and mud drapes in a1 luvial-fan breccias and f luvial channel sandstones The s t r u c t u r e s f i g u r e d on p l a t e 111 i l l u m i n a t e t h e d i s t r i b u t i o n o f g r a v e l agg l o m e r a t i o n s and mud d r a p e s i n a l l u v i a l - f a n b r e c c i a s and f l u v i a l c h a n n e l sandstones, and o u t l i n e t h e s i g n i f i c a n c e of t h e g r a i n - s i z e a n i s o t r o p i e s f o r r e s e r v o i r h e t e r o g e n e i t y and n a t u r a l f r a c t u r e p r o p a g a t i o n . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l a s p e c t s , e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s o n p l a t e 111 a r e g i v e n , and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e quoted.
3.13.3.1. General aspects T y p i c a l e l e m e n t s o f r e s e r v o i r h e t e r o g e n e i t y i n t e r r e s t r i a l f l u v i a l and a l l u v i a l - f a n c h a n n e l s e d i m e n t sequences a r e t h i n mud d r a p e s ( 1 - 2 ) , d e f o r m e d sand b a l l s and n o d u l e s ( 3 - 4 ) , and t r a i n s and p o c k e t s o f g r a v e l - s i z e d c l a s t s ( 5 - 8 ) . A l l these f e a t u r e s a r e i n t e r r u p t i n g t h e c o n t i n u a t i o n o f sandstone h o r i zons b y c h a n g i n g r o c k m e c h a n i c a l p r o p e r t i e s on s m a l l - s c a l e and a l s o i n t r o d u c i n g b a r r i e r s t o f l o w w i t h i n t h e p a y zone. Mudstone i n t e r c a l a t i o n s o f l a t e r a l l y l i m i t e d e x t e n s i o n a r e t h e m o s t s e a l i n g f e a t u r e s , whereas g r a v e l c o n c e n t r a t i o n s may e v e n i n c r e a s e r e s e r v o i r p e r m e a b i l i t y . The f i g u r e d examples a r e w i d e s p r e a d s t r u c t u r e s i n v a r i o u s p a r t s o f R o t l i e g e n d and B u n t s a n d s t e i n i n M i d d l e E u r o p e .
3.13.3.2. Explanations o f illustrations on plate I11 1 - 2 : T h i n s h e e t - l i k e o r l e n t i c u l a r mud d r a p e s i n t e r r u p t i n g f l u v i a l chann e l s a n d s t o n e s and c o n g l o m e r a t e s o r i g i n a t e b y r a p i d l y f l u c t u a t i n g f l o w c o n d i t i o n s d u r i n g c o u r s e o f a g g r a d a t i o n o f s h a l l o w t o m o d e r a t e l y deep b r a i d e d r i v e r
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304 c h a n n e l s o r even u n c o n f i n e d s h e e t f l o o d s i n u n d a t i n g o v e r b a n k f l a t s . D i v e r s i o n o f c u r r e n t s b y d e f l e c t i o n a r o u n d l a r g e sand b a r s , c u t t i n g - o f f o f some w a t e r c o u r ses f r o m t h e a c t i v e s y s t e m and s h e l t e r i n g o f p o o l s in i n t e r - o r i n t r a b a r d e p r e s i n t h e d r a i n a g e p a t h s e n a b l e s c o v e r i n g o f sandy s i o n s d u r i n g l o w water,,level b e d f o r m s w i t h t h i n mudstone d r a p e s i n q u i e t w a t e r a l l o w i n g s u s p e n s i o n s e t t l i n g . Subsequent r e s u m i n g o f h i g h - e n e r g y f l u v i a l c h a n n e l d e p o s i t i o n w i t h r e t u r n i n g f l o o d s t a g e r e s u l t s i n b u r i a l o f t h e mud d r a p e s b y s a n d s t o n e b l a n k e t s and l e n ses t h e r e b y i n c o r p o r a t i n g t h e p r o o f s o f t e m p o r a r y q u i e t c o n d i t i o n s i n t o t h e dep o s i t i o n a l record ( c f . section 4.2.4.1.1.). The s h a r p b o u n d a r i e s o f s a n d s t o n e s and mudstones a r e t h e r e a s o n f o r p r o n o u n c e d r o c k m e c h a n i c a l p r o p e r t y c o n t r a s t s w h i c h g i v e r i s e t o a t t e n u a t i o n and d e f l e c t i o n o f h y d r a u l i c a l l y p r o p a g a t i n g f r a c t u r e s e s p e c i a l l y i n case o f g r e a t e r t h i c k n e s s o f t h e mudstone i n t e r v e n t i o n s ( c f . a l s o p l a t e s V I I / 5 - 6 and I X ) .
3 - 4 : Deformed s a n d s t o n e b a l l s and n o d u l e s w i t h i n mudstone sequences o r b e l o w t h e d e p o s i t i o n a l o r e r o s i o n a l b o u n d a r y between t h i c k e r s a n d s t o n e and muds t o n e l a y e r s r e f l e c t r a p i d l o a d i n g o f s t i l l w e t and p l a s t i c a l l y r e a c t i v e muds t o n e s b y s a n d s t o n e s o f d i f f e r e n t t h i c k n e s s . W h i l e in some c a s e s such as l o o s e g r a i n f a b r i c o f o v e r l y i n g s a n d s t o n e s and i n t e r n a l o v e r p r e s s u r e o f t h e s t i l l u n c o n s o l i d a t e d mudstones, t h e r e a c t i o n t o l o a d i n g i s d i a p i r i c a l i n t r u s i o n o f mud into t h e o v e r l y i n g sand ( c f . P l a t e I X / 7 ) , a more common consequence i s d o w n s i n k i n g o f sand i n t o t h e mud whereby o r i g i n a l l y c o n t i n u o u s l a y e r s a r e d i s s e c t e d i n t o i s o l a t e d b a l l s and n o d u l e s and a r e c o n s i d e r a b l y deformed b y c o n v o l u t i o n o f i n t e r n a l s t r a t i f i c a t i o n . Such b a l l and p i l l o w s t r u c t u r e s u n d e r l i n e t h e r a p i d changes o f s e d i m e n t a r y c o n d i t i o n s in t h e a l l u v i a l f l o o d p l a i n w i t h o n l y i n s u f f i c i e n t t i m e o f s t a b i l i z a t i o n and d e w a t e r i n g o f mudstones h a v i n g been l a i d down i n l a k e s and ponds due t o q u i c k s u b s e q u e n t l o a d i n g b y s a n d s t o n e b o d i e s o r i g i n a t i n g f r o m l a t e r a l c h a n n e l m i g r a t i o n and s h e e t - f l o o d i n u n d a t i o n o f t h e o v e r b a n k p l a i n . E s p e c i a l l y b i g n o d u l e s and b a l l s a r e p r o m i n e n t c e n t r e s o f r e s e r v o i r h e t e r o g e n e i t y w i t h p r o n o u n c e d i m p a c t on p r o p a g a t i n g h y d r a u l i c f r a c t u r e s w h i c h in case o f e n c o u n t e r i n g such l a r g e d i s t u r b i n g b o d i e s c a n n o t o n l y be a t t e n u a t e d o r d e f l e c t e d , b u t t h e i r p e n e t r a t i o n c a n a l s o be c o m p l e t e l y s t o p p e d .
5 - 8 : G r a v e l c o n c e n t r a t i o n s i n a l l u v i a l - f a n and f l u v i a l c h a n n e l s a n d s t o n e s a r e r e s e r v o i r h e t e r o g e n e i t i e s w h i c h a r e p a r t i a l l y i n c r e a s i n g p a y zone q u a l i t y by i n t r o d u c i n g h i g h e r p e r m e a b i l i t y , b u t on t h e o t h e r hand a l s o t r i g g e r inhomog e n e i t y o f f l o w as w e l l as changes i n r o c k m e c h a n i c a l p r o p e r t i e s . As h y d r a u l i c f r a c t u r i n g in c o n g l o m e r a t e s r e q u i r e s p a r t i c u l a r l y s o p h i s t i c a t e d s u p e r v i s i o n and c a r e f u l planning f o r successfully propping cracks crossing gravel c l a s t s o f various size, c o n g l o m e r a t e s and g r a v e l - b e a r i n g s a n d s t o n e s w i t h i r r e g u l a r d i s t r i b u t i o n o f c l a s t s may be t h e m o s t c o m p l i c a t e d p a y zones to p r o p e r l y t r e a t b y hydraulic stimulation. Traversing o f the various gravel concentrations requires f r e q u e n t l y h i g h e r pressures than necessary f o r c r o s s i n g c l e a n sandstones which may a t t h e s u r f a c e p a r t i a l l y be i n t e r p r e t e d as i n d i c a t i n g a p p r o a c h i n g o f s c r e e n o u t t e r m i n a t i o n o f t h e j o b ( c f . sections 4.2.3.2.4. and 4 . 2 . 3 . 5 . ) . This i s e s p e c i a l l y v a l i d i n l i g h t o f t h e d i f f i c u l t l a t e r a l and v e r t i c a l m o d e l l i n g o f i s o l a t e d g r a v e l e n r i c h m e n t s i n p o c k e t s , l e n s e s , c h a n n e l s and s c o u r h o l e s , in c o n t r a s t t o r e a s o n a b l e p r e d i c t i o n and assessment o f g r a v e l d i s t r i b u t i o n in continuous conglomerate horizons.
3.13.3.3. Geological formations and outcrop localities o f presented examles 1 : Upper B u n t s a n d s t e i n o f t h e V i t o s h a M o u n t a i n s ( B u l g a r i a ) . D i a m e t e r o f f i g u r e a b t . 3 m. Road s e c t i o n a t t h e n o r t h e r n s i d e o f t h e r i v e r v a l l e y e a s t o f Bosnek s o u t h e a s t o f P e r n i k n e a r S o f i a ( c f . MADER & CHATALOV 1988). 2 : M i d d l e B u n t s a n d s t e i n o f Upper F r a n c o n i a / N o r t h e a s t e r n B a v a r i a (Germany FRG). D i a m e t e r o f f i g u r e a b t . 1.2 m. Rocks a t t h e w e s t e r n base o f t h e w a l l o f t h e P l a s s e n b u r g n e a r Kulmbach ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t 5834 Kulmbach, r 6 1 700, h 52 600). 3 : Upper C a r b o n i f e r o u s o f Wales ( G r e a t B r i t a i n ) . D i a m e t e r o f f i g u r e a b t . 1 m. Rocks a t t h e s h o r e a r o u n d Pembroke. 4 : Upper B u n t s a n d s t e i n o f t h e
305 N o r t h e r n S o l l i n g (Germany FRG). Diameter of f i g u r e a b t . 1.5 m. Q u a r r y a t t h e western s i d e o f t h e H e i d b r i n k a t t h e s o u t h e a s t e r n s i d e o f t h e Beverbach v a l l e y southwest o f Arholzen ( s h e e t 4123 Stadtoldendorf, r 38 760, h 47 490). 5 - 8 : R o t l i e g e n d (Lower Permian; New Red Sandstone) o f South Devon ( E n g l a n d ) . Diameter o f f i g u r e s a b t . 1.5 m each. 5 + 7 : Rocks a t t h e shore a t Watcombe Head south o f Maidencombe between Shaldon and Torquay ( t o p o g r a p h i c a l map 1 : 50 000, sheet 202 Torbay and South Dartmoor, r 92 720, h 67 6 0 0 ) . 6 : Rocks a t t h e shore a t Mackerel Cove n o r t h o f Maidencombe between Shaldon and Torquay ( s h e e t 202 Torbay and South Dartmoor, r 92 920, h 69 100). 8 : Rocks a t t h e shore a t Watcombe Head south o f Maidencombe between Shaldon and Torquay ( s h e e t 202 Torbay and South Dartmoor, r 92 840, h 67 440). F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a, 1985 b ) .
3.13.4. Natural fractures and sedimentary het erogene i t ies in f luvial channel and floodplain sandstones and mudstones The f e a t u r e s f i g u r e d on p l a t e I V c h a r a c t e r i z e t h e d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s and sedimentary h e t e r o g e n e i t i e s due t o g r a i n - s i z e t r e n d s and bedding s t r u c t u r e s i n f l u v i a l channel and f l o o d p l a i n d e p o s i t s o f m o d e r a t e l y - b r a i d e d r i v e r systems. F o l l o w i n g d i s c u s s i o n o f g e n e r a l aspects, e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e I V a r e given, and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e quoted.
3.13.4.1. General aspects N a t u r a l j o i n t i n g and a r t i f i c i a l e x p l o s i v e f r a c t u r i n g i n t e r r e s t r i a l r e d sandstones o f t h e B u n t s a n d s t e i n i l l u s t r a t e t h e f a b r i c o f c r a c k s i n r e s e r v o i r r o c k s w i t h f r a c t u r e p o r o s i t y . W h i l e h o r i z o n t a l - s t r a t i f i e d and cross-bedded f l u v i a l sandstones ( 4 - 7) which a r e t h e most widespread l i t h o f a c i e s t y p e i n t h e Bunts a n d s t e i n g e n e r a l l y c o n t a i n w e l l - d e v e l o p e d p l a n e t o s u b h o r i z o n t a l l a y e r boundar y c r a c k s and t o s u b o r d i n a t e amounts a l s o v e r t i c a l j o i n t s , e x p l o s i v e f r a c t u r i n g by q u a r r y b l a s t i n g c r e a t e s s t a r - s h a p e d p a t t e r n s o f m u l t i p l e r a d i a l f r a c t u r e systems ( 1 ) i n c l u d i n g c e n t r a l r u b b l i z a t i o n o f t h e r o c k m a t r i x a t t h e c o r e o f t h e f i r e d charge. Reworked f l o o d p l a i n mud c l a s t s i n f l u v i a l channel sandstones a r e n o t o n l y capable o f a b s o r p t i o n and a t t e n u a t i o n o f p r o p a g a t i n g h y d r a u l i c f r a c t u r e s and a l s o l i m i t t h e e x t e n s i o n o f n a t u r a l c r a c k s , b u t a l s o t e n d t o d e f l e c t t h e network o f r a d i a l f r a c t u r e s generated by e x p l o s i o n .
3.13.4.2. Explanations o f illustrations on plate IV 1 : E x p l o s i v e f r a c t u r i n g o f c l a s t i c r e s e r v o i r s i n o i l and gas w e l l s ( c f . s e c t i o n s 4.7.3. and 4.8.9.6.) can be s t u d i e d i n s u r f a c e sandstone q u a r r i e s where b l a s t i n g i s c a r r i e d o u t t o s u p p o r t mechanical b u i l d i n g stone p r o d u c t i o n . D e t o n a t i o n o f t h e e x p l o s i v e charge g i v e s r i s e t o r a d i a l p r o p a g a t i o n o f m u l t i p l e f r a c t u r e s i n a sun- o r s t a r - l i k e network w i t h v e r y c l o s e spacing o f t h e i n d i v i dual c r a c k s , b u t on t h e o t h e r hand l i m i t e d l a t e r a l e x t e n s i o n o f t h e s e p a r a t e f r a c t u r e s . C o n d u c t i v i t y i n t h e a x i a l p a r t i s d e t e r i o r a t e d by r u b b l i z a t i o n o f t h e r o c k m a t r i x around t h e d e t o n a t e d charge, and t h e d r a i n a g e area i s r e s t r i c t e d as a consequence o f t h e s h o r t r e a c h o f t h e c r a c k s i n t o t h e r e s e r v o i r away f r o m t h e c e n t r e . A c c o r d i n g t o t h e comparison w i t h s u r f a c e q u a r r y b l a s t i n g , conv e n t i o n a l e x p l o s i v e f r a c t u r i n g i s most s u i t a b l e f o r b r e a k i n g through f o r m a t i o n damage b e l t s s u r r o u n d i n g t h e borehole, whereas f o r d r a i n a g e o f l a r g e r p a r t s o f t h e pay zone, e i t h e r c o n t r o l l e d e x p l o s i v e f r a c t u r i n g g i v i n g r i s e t o fewer and l o n g e r f r a c t u r e s has t o be c a r r i e d o u t , o r h y d r a u l i c f r a c t u r i n g l e a d i n g t o openi n g o f o n l y one o r a few e x t e n s i v e c r a c k s has t o be chosen.
2 : Large mud c l a s t s o r i g i n a t e by r e w o r k i n g o f f i n e - g r a i n e d overbank p l a i n sediments d u r i n g c o u r s e o f sidewards displacement o f t h e p o s i t i o n o f m a j o r and m i n o r d r a i n a g e channels which i n c i s e i n t o t h e i r substratum, c u t o u t l a r g e mud
306 c l a s t s from the o l d e r f l o o d b a s i n d e p o s i t s , t r a n s p o r t the m o b i l i z e d b l o c k s f o r a s h o r t d i s t a n c e by t r a c t i o n on the f l o o r of the w a t e r c o u r s e and then c o v e r t h e i n t r a f o r m a t i o n a l b o u l d e r s w i t h sand a n d / o r g r a v e l upon waning of c u r r e n t v e l o c i t y and thus d i m i n i s h i n g sediment c a r r y i n g c a p a c i t y . The mud c l a s t s a r e c e n t r e s of r e s e r v o i r inhomogeneity both i n terms of p e r m e a b i l i t y and rock p r o p e r t i e s and t h e r e f o r e have a l s o d e c i s i v e impact on development o f n a t u r a l f r a c t u r e s y s tems a s well a s p r o p a g a t i o n of h y d r a u l i c a l l y induced c r a c k s . Reworked mud c l a s t s of s m a l l e r o r l a r g e r s i z e a r e very common e l e m e n t s i n f l u v i a l s a n d s t o n e s u c c e s s i o n s and thus have t o be e x p e c t e d i n v a r i a b l e q u a n t i t y i n numerous hydrocarbon-bearing c o n t i n e n t a l s u c c e s s i o n s ( c f . a l s o p l a t e s V/4, V I I / l and X/8).
3 - 8 : The most prominent open c r a c k s i n more o r l e s s w e l l - s t r a t i f i e d cont i n e n t a l r e d s a n d s t o n e s a r e p l a n e t o s u b h o r i z o n t a l bedding b o u n d a r i e s i n massive o r p l a t e y r o c k s . P e r f e c t l y h o r i z o n t a l - l a m i n a t e d s a n d s t o n e s ( 4 - 5 ) break i n t o a t a b u l a r p i l e of numerous s l i c e s of d i f f e r e n t t h i c k n e s s which a r e s e p a r a t e d by i n t e r c a l a t e d mudstone l a y e r s . Such t a b u l a r u n i t s w i t h s l i c e - l i k e o r g a n i z a t i o n of s a n d s t o n e beds and s t r a t i f i c a t i o n p l a n e c r a c k s a r e q u i t e homogeneous systems w i t h l i t t l e i r r e g u l a r i t i e s and t h e r e f o r e p r o p a g a t i n g h y d r a u l i c f r a c t u r e s e n c o u n t e r a l m o s t always n a t u r a l c r a c k e l e m e n t s of comparable o r i e n t a t i o n , w i t h t h u s h y d r a u l i c f r a c t u r e p e n e t r a t i o n being g r e a t l y enhanced by o n l y s u b o r d i n a t e d e f l e c t i o n and r e s i s t a n c e t o e x t e n s i o n being e x e r t e d by the n a t u r a l f r a c t u r e system. C r o s s - s t r a t i f i e d s a n d s t o n e s ( 6 ) mainly d i s p l a y s e l e c t e d c r o s s beds by open n a t u r a l c r a c k s , whereas the e r o s i o n a l b o u n d a r i e s do n o t show u p so p r o minent and a r e sometimes even o b l i t e r a t e d . I n more massive t o c r u d e l y h o r i z o n t a l - b e d d e d s a n d s t o n e s ( 3 , 7 - 8 ) , t h e most obvious e l e m e n t s of t h e l i t h o l o g i c a l r e c o r d g i v i n g r i s e t o opening of s t r a t i f i c a t i o n c r a c k s a r e t h i n mudstone d r a p e s of l a t e r a l l y c o n t i n u o u s o r r e s t r i c t e d d i s t r i b u t i o n which o r i g i n a t e d a s c o v e r s of sand b a r s d u r i n g waning-flow s t a g e s a t the end of main f l o o d c y c l e s o r i n low-water phases s e p a r a t i n g s e v e r a l h i g h - d i s c h a r g e e v e n t s i n s h a l l o w t o moderat e l y deep b r a i d e d r i v e r c h a n n e l s o r s h e e t f l o o d overbank i n u n d a t i o n s . The d i s t r i b u t i o n of n a t u r a l f r a c t u r e s i n c r o s s - s t r a t i f i e d o r c r u d e l y h o r i z o n t a l - b e d d e d s a n d s t o n e s c o n t a i n i n g e r o s i o n a l b o u n d a r i e s of i r r e g u l a r o r i e n t a t i o n r e p r e s e n t s moderate t o c o n s i d e r a b l e inhomogeneity and a n i s o t r o p y which r e s u l t s in major def l e c t i o n , a t t e n u a t i o n and r e s i s t a n c e t o f u r t h e r p e n e t r a t i o n of p r o p a g a t i n g hyd r a u l i c f r a c t u r e s ( c f . s e c t i o n 4 . 2 . 3 . 4 . ) . The d i f f i c u l t i e s of f r a c t u r e e x t e n s i o n a r e r e f l e c t e d by pressure i n c r e a s e s d u r i n g the t r e a t m e n t which may i n some c a s e s even be m i s i n t e r p r e t e d a s i n d i c a t i n g premature s c r e e n o u t t e r m i n a t i o n of the j o b ( c f . s e c t i o n s 4 . 2 . 3 . 2 . 4 . and 4 . 2 . 3 . 5 . ) .
3.13.4.3. Geological formations and outcrop localities o f presented examples 1 + 3 : Upper B u n t s a n d s t e i n of t h e Black F o r e s t (Germany FRG). Diameter of f i g u r e s a b t . 1 . 5 m e a c h . Quarry i n D i e t e r s w e i l e r between F r e u d e n s t a d t and Horb. 2 : Upper B u n t s a n d s t e i n of the Hessian Depression (Germany FRG). Diameter of f i g u r e a b t . 3 m. Quarry a t t h e e a s t e r n s i d e of the Elbe v a l l e y n o r t h of Olmuhle n o r t h n o r t h e a s t of Zuschen ( t o p o g r a p h i c a l map 1 25 000, s h e e t 4821 F r i t z l a r , r 16 240, h 7 1 7 2 0 ) . 4 - 5 : Middle B u n t s a n d s t e i n of t h e N o r t h e a s t e r n Holy C r o s s Mountains ( P o l a n d ) . Diameter of f i g u r e s a b t . 1 . 5 m and 1 m, r e s p e c t i v e l y . Quarry east of Stryczowice between Wajnibw and Ostrowiec S w i e t o k r z y s k i ( t o p o g r a p h i c a l map 1 : 100 000, sheet Bodzentyn, pas 45, s l u p 32, r 50 400, h 74 7 0 0 ) . 6 : Middle B u n t s a n d s t e i n of the Western Holy Cross Mountains ( P o l a n d ) . Diameter of f i g u r e a b t . 2 m . Q u a r r y s o u t h of Tumlin between K i e l c e and Konskie. 7 : Upper B u n t s a n d s t e i n of the Northern S o l l i n g (Germany F R G ) . Diameter of f i g u r e a b t . 2 m . Quarry a t t h e s o u t h w e s t e r n s i d e of t h e Sundern e a s t of Negenborn n e a r S t a d t o l d e n d o r f ( s h e e t 4123 S t a d t o l d e n d o r f , r 41 370, h 50 9 6 0 ) . 8 : Cambrian Neksa Sandstone Formation of Bornholm (Denmark). Diameter of f i g u r e a b t . 1 . 5 m . Quarry n e a r B o d i l s k e r . For f u r t h e r d e t a i l s of the s e c t i o n s f i g u r e d c f . MADER (1985 a , 1985 b ) .
307
308
Grain-size distribution changes and erosional surfaces in braided-river channel sandstones and conglomerates
3.13.5.
The p a t t e r n s f i g u r e d on p l a t e V document changes o f g r a i n - s i z e d i s t r i b u t i o n as w e l l as morphology and r e l i e f o f e r o s i o n a l s u r f a c e s i n f l u v i a l channel depos i t s o f sandy and c o n g l o m e r a t i c b r a i d e d - r i v e r systems and t h e i r impact on r e s e r v o i r c o n t i n u i t y and h e t e r o g e n e i t y . F o l l o w i n g d i s c u s s i o n o f g e n e r a l aspects, exp l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e V a r e given, and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e quoted.
3.13.5.1. General aspects A p a r t f r o m s m a l l - s c a l e mudstone i n t e r c a l a t i o n s i n t o sandstone beds ( c f . p l a t e III/l - 2, IV/4 - 8 and IX), prominent examples o f r e s e r v o i r h e t e r o g e n e i t y a r e s m a l l - s c a l e sandstone i n t e r v e n t i o n s i n t o conglomerate beds ( 1 - 2), chann e l i z e d bodies o f i n t e n s e l y - l i t h i f i e d o r c o a r s e r - c l a s t i c sandstones i n t h e r e s e r v o i r complex ( 3 - 4; c f . a l s o p l a t e II/7 - 8 ) , e r o s i o n a l boundaries o f i r r e g u l a r shape between beds o r d i f f e r e n t l i t h o l o g y ( 5 - 8; c f . a l s o p l a t e s II/7 and IV/3 + 6 ) and i s o l a t e d g r a v e l t r a i n s i n sandstones (7). Such s m a l l - s c a l e r e s e r v o i r h e t e r o g e n e i t i e s a r e n o t o n l y i n h i b i t i n g and d e v i a t i n g f l o w o f p o r e sol u t i o n s t h r o u g h t h e sedimentary column, b u t a l s o i n c o r p o r a t e changes o f mechanic a l p r o p e r t i e s t h e r e b y i n f l u e n c i n g t h e p r o p a g a t i o n o f h y d r a u l i c a l l y induced f r a c t u r e s which s u f f e r f r o m a t t e n u a t i o n and d e f l e c t i o n when c r o s s i n g t h e boundar i e s between a d j o i n i n g r o c k t y p e s o f d i f f e r e n t c o m p o s i t i o n ( c f . s e c t i o n s 4.2.3.2.4. and 4.2.4.1.). S m a l l - s c a l e r e s e r v o i r h e t e r o g e n e i t i e s can e a s i l y be m o d e l l e d i n o u t c r o p s e c t i o n s , b u t t h e r e c o n s t r u c t i o n o f reasons f o r a n i s o t r o p i c mechanical b e h a v i o r f r o m l i m i t e d subsurface i n f o r m a t i o n i s q u i t e d i f f i c u l t . The p r e s e n t e d examples show t h a t w i t h i n t h e d i s c u s s e d t e r r e s t r i a l environment, s m a l l - s c a l e r e s e r v o i r h e t e r o g e n e i t i e s comparable t o t h e f i g u r e d t y p e s have t o be expected anywhere and have t o be accounted f o r by u n d e r s t a n d i n g t h e pay f o r m a t i o n as a complex mosaic o f v a r i o u s elements.
3.13.5.2. Explanations o f illustrations on plate V 1 - 2 : T h i n sandstone i n t e r b e d d i n g s i n conglomerate sequences r e f l e c t f l u c t u a t i n g c o n d i t i o n s i n sha'llow t o m o d e r a t e l y deep b r a i d e d r i v e r channels where gravel b a r s m i g r a t e downstream under t h e i n f l u e n c e o f s t r o n g c u r r e n t s . Temporary c u t o f f o f some watercourses f r o m t h e a c t i v e system o r d e f l e c t i o n o f t h e main w a t e r l o a d around l a r g e g r a v e l b a r s c r e a t e s q u i e t e r c o n d i t i o n s i n p a r t s o f t h e r u n o f f p a t h s and enables s u p e r i m p o s i t i o n o f l i t t l e sand b a r s o r sheets on i n a c t i v a t e d g r a v e l b a r s . L i m i t e d l a t e r a l e x t e n t o f t h e environment s u i t a b l e f o r waning-flow d e p o s i t i o n r e s u l t s i n p r e d o m i n a n t l y l e n t i c u l a r charact e r o f t h e sandstone i n t e r v e n t i o n s i n conglomerate s e c t i o n s . Resuming o f h i g h energy s e d i m e n t a t i o n covers t h e sandstone drapes w i t h conglomerate sheets above a boundary o f d e p o s i t i o n a l o r e r o s i o n a l n a t u r e . Concerning p r o p a g a t i o n o f hyd r a u l i c f r a c t u r e s , sandstone i n t e r c a l a t i o n s i n conglomerate sequences a r e poss i b l e h o r i z o n s of a c c e l e r a t e d f l u i d loss ( c f . s e c t i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.) due t o b e t t e r f i l t e r i n g p r o p e r t i e s than t h e f r e q u e n t l y p o o r e r s o r t e d conglomerates, and o f p r e f e r e n t i a l p r o p p a n t b r i d g i n g due t o d e c r e a s i n g f r a c t u r e w i d t h i n t h e o f t e n s t i l l weakly c o n s o l i d a t e d sandstone drapes, t h e r e b y i n c r e a s i n g t h e s c r e e n o u t r i s k ( c f . s e c t i o n s 4.2.2.2.3. and 6.2.4.2.1.). 3 - 4 : Channelized bodies of more i n t e n s e l y l i t h i f i e d sandstone (3) o r i n t r a f o r m a t i o n a l r e w o r k i n g conglomerate ( 4 ) i n t h i c k f l u v i a l stream b a r sandstones r e p r e s e n t i n t e r c a l a t i o n s o f i r r e g u l a r shape and d i f f e r e n t mechanical p r o p e r t i e s i n t o an o t h e r w i s e more o r l e s s homogeneous succession. Carbonate cement (3) and reworked c l a s t s o f p a l a e o s o l s (4) as w e l l as c a n n i b a l i z e d mud fragments ( c f . p l a t e s IV/2 and VII/l) a r e t h e main elements i n these examples w h i c h a r e q u i t e f r e q u e n t i n R o t l i e g e n d and B u n t s a n d s t e i n r e d bed columns and t h e r e f o r e
309
Plate V
310 have t o be e x p e c t e d t o o c c u r a l s o i n r e s e r v o i r r o c k s t h a t a r e s u b j e c t e d t o hyd r a u l i c p r o p p a n t f r a c t u r i n g . The l i t h o f a c i e s e l e m e n t s g i v e e v i d e n c e o f e r o s i o n a l d e s t r u c t i o n o f p a r t s o f t h e e a r l i e r g e n e r a t e d d e p o s i t s and r e w o r k i n g o f i s o l a t e d c l a s t s i n s u b s e q u e n t s e d i m e n t s ( c f . a l s o p l a t e X / 7 ) . Such i n t e r c a l a t i o n s have an i m p o r t a n t i m p a c t on p r o p a g a t i o n o f h y d r a u l i c f r a c t u r e s w h i c h a r e r e q u i r i n g h i g h e r p r e s s u r e t o c r o s s t h e m a s s i v e i n t e r v e n t i o n s and m i g h t a l s o be def l e c t e d o r a t t e n u a t e d ( c f . s e c t i o n s 4 . 2 . 3 . 4 . and 4 . 2 . 4 . 1 . ) .
5 - 8 : I r r e g u l a r e r o s i o n a l b o u n d a r i e s between d e p o s i t s o f d i f f e r e n t l i t h o l o g i c a l c o m p o s i t i o n document h i g h - e n e r g y e v e n t s d u r i n g f o r m a t i o n o f t h e r o c k s w h i c h r e p e a t e d l y t e r m i n a t e d phases o f r a t h e r u n d i s t u r b e d a g g r a d a t i o n o f sediment a r y s u b e n v i r o n m e n t s . The r e l i e f w i t h p o s i t i v e r e l i c t i c p i l l a r s and n e g a t i v e s c o u r e d h o l e s i s t h e e x p r e s s i o n o f t h e dynamics o f c u r r e n t s f l o w i n g a l o n g t h e b o t t o m o f s h a l l o w t o m o d e r a t e l y deep b r a i d e d r i v e r c h a n n e l s a t h i g h f l o o d s t a ges when e r o s i o n a l i n c i s i o n i n t o t h e s u b s t r a t u m i s p a r t i c u l a r l y p o w e r f u l . The i r r e g u l a r shape o f t h e e r o s i o n a l b o u n d a r i e s t o g e t h e r w i t h t h e c h a n g i n g l i t h o l o g y o f t h e r o c k s a t b o t h s i d e s o f t h e b o r d e r r e p r e s e n t marked d i f f e r e n c e s o f m a t e r i a l p r o p e r t i e s w h i c h g i v e r i s e t o c o n s i d e r a b l e i n f l u e n c e on t h e p r o p a g a tion o f hydraulic fractures crossing such interbeddings (cf. section 4.2.3.4.).
3.13.5.3. Geological formations and outcrop localities of presented examples 1 : M i d d l e B u n t s a n d s t e i n o f t h e N o r t h e r n E i f e l (Germany FRG). D i a m e t e r o f f i g u r e a b t . 1 . 5 m. Rock monument a t t h e E i s e r n e s K r e u z a t t h e r o a d t o Gemund northwest o f K a l l ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t 5405 M e c h e r n i c h , r 39 440, h 01 0 2 0 ) . 2 : T e r t i a r y ( O l i g o c e n e ) i n t h e s u r r o u n d i n g s o f M a r s e i l l e ( F r a n c e ) . D i a m e t e r o f f i g u r e a b t . 1 . 2 m. Road s e c t i o n a r o u n d t h e h a r b o u r o f M a r s e i l l e . 3 - 5 : O t t e r Sandstones ( L o w e r T r i a s s i c ) o f S o u t h Devon ( E n g l a n d ) . 3 : D i a m e t e r o f f i g u r e a b t . 2.5 m. Rocks a t t h e s h o r e between Crab Ledge and B l a c k Head between B u d l e i g h S a l t e r t o n and Sidmouth ( s h e e t SY 0 8 / 1 8 Sidmouth, between r 0 8 580, h 82 620 and r 09 200, h 83 8 4 0 ) . 4 : D i a m e t e r o f f i g u r e a b t . 1 . 5 m. Rocks a t t h e s h o r e between O t t e r t o n Ledge and Danger P o i n t e a s t o f Budl e i g h S a l t e r t o n ( s h e e t SY 0 8 / 1 8 Sidmouth, between r 07 950, h 8 1 980 and r 0 8 080, h 82 100). 5 : D i a m e t e r o f f i g u r e a b t . 0.7 rn. Rocks a t t h e s h o r e a t Ladrarn Bay between Ladram Rock and S m a l l s t o n e s P o i n t e a s t o f O t t e r t o n ( s h e e t SY 0 8 / 1 8 Sidmouth, r 09 680, h 85 1 5 0 ) . 6 : M i d d l e B u n t s a n d s t e i n o f t h e N o r t h e r n Saar a r e a (Germany FRG). D i a m e t e r o f f i g u r e a b t . 0 . 8 rn. Rocks a t t h e e a s t e r n s i d e o f t h e h i l l s o u t h w e s t o f t h e j u n c t i o n o f r i v e r s Saar and M o s e l l e w e s t o f K o n z e r b r u c k ( s h e e t 6305 S a a r b u r g , r 39 680, h 06 8 0 0 ) . 7 : O t t e r Sandstones ( L o w e r T r i a s s i c ) o f S o u t h Devon ( E n g l a n d ) . D i a m e t e r o f f i g u r e a b t . 0 . 8 m. Rocks a t t h e s h o r e between Danger P o i n t and B r a n d y Head e a s t o f B u d l e i g h S a l t e r t o n ( s h e e t SY 0 8 / 1 8 Sidmouth, r 0 8 080, h 82 100 and r 0 8 980, h 83 1 6 0 ) . 8 : C a r b o n i f e r o u s F o u n t a i n F o r m a t i o n o f t h e Rocky M o u n t a i n s F r o n t H i l l Range i n C o l o r a d o (USA). D i a m e t e r o f f i g u r e a b t . 1 m. Rocks a t t h e H o r s e t o o t h r e s e r v o i r n e a r F o r t C o l l i n s n o r t h o f D e n v e r . F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a , 1985 b ) .
3.13.6. Synsedimentary desiccation fracturing in lacustrine and
f luvial floodplain mudstones and natural propping of cracks
The f e a t u r e s p r e s e n t e d on p l a t e V I document s y n s e d i m e n t a r y d e s i c c a t i o n s h r i n kage f r a c t u r i n g i n l a c u s t r i n e ( p l a y a - l a k e ) and f l u v i a l f l o o d p l a i n ( o v e r b a n k l a k e ) muddy d e p o s i t s as a s o u r c e o f p r i m a r y n a t u r a l c r a c k s i n r e s e r v o i r r o c k s and a d j o i n i n g b a r r i e r s as w e l l as an example o f n a t u r a l p r o p p i n g o f f r a c t u r e s w i t h h i g h - c o n d u c t i v i t y m a t e r i a l . F o l l o w i n g discussion o f general aspects, explan a t i o n s o f t h e i l l u s t r a t i o n s o n p l a t e V I a r e g i v e n , and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e q u o t e d .
311
3.13.6.1. General aspects Mud and sand c r a c k s a r e a s p e c i a l t y p e o f n a t u r a l f r a c t u r e s which o r i g i n a t e a l r e a d y d u r i n g f o r m a t i o n o f sedimentary r o c k s by s h r i n k a g e o f cohesive d e p o s i t s as a consequence o f s u b a e r i a l d e s i c c a t i o n o r subaquatic s y n e r e s i s . P r e f e r e n t i a l l y muddy d e p o s i t s , b u t a l s o sandy sediments c o n t a i n i n g d i s p e r s e d mud p a r t i c l e s c r a c k upon d r y i n g and become i n t e r s e c t e d by p o l y g o n a l d e s i c c a t i o n f r a c t u r e s which s t a y open f o r t h e t i m e o f s u b a e r i a l o r subaquatic exposure. C o n t i n u i n g agg r a d a t i o n o f t h e d e p o s i t i o n a l s u b m i l i e u s l a t e r , however, l e a d s i n most cases t o n a t u r a l p r o p p i n g o f t h e p o l y g o n a l synsedimentary s h r i n k a g e f r a c t u r e systems w i t h p r e d o m i n a n t l y sand, b u t s u b o r d i n a t e l y a l s o g r a v e l . S i m i l a r l y as s y n t h e t i c p r o p p i n g o f n a t u r a l j o i n t s ( c f . s e c t i o n 4.8.8.6.) and h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s , n a t u r a l p r o p p i n g o f d e p o s i t i o n a l s h r i n k a g e c r a c k s takes almost a l ways p l a c e by p l u g g i n g w i t h c o a r s e r g r a i n s i z e s than t h e s u r r o u n d i n g r o c k mat r i x , thereby p r i m a r i l y also c r e a t i n g a considerable c o n d u c t i v i t y c o n t r a s t ( c f . s e c t i o n s 4.5.1.1.2. and 4.6.1.) which enables t h e n a t u r a l l y propped s h r i n k a g e c r a c k s t o become p r e f e r e n t i a l f l o w channels f o r p o r e s o l u t i o n s d u r i n g e a r l y d i a genes is . I n c o n t r a s t t o p r o p p a n t packages i n h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s , however, p r o g r e s s i v e b u r i a l d i a g e n e s i s i n t h e sedimentary column c o n t a i n i n g t h e s y n g e n e t i c n a t u r a l c r a c k i n f i l l i n g s o f t e n l e a d s t o enhanced cementation o f t h e c o a r s e r m a t e r i a l p l u g g i n g t h e s h r i n k a g e c r a c k s as a consequence o f t h e i r b e t t e r t r a n s m i s s i b i l i t y f o r p o r e s o l u t i o n s and t h e r e f o r e d e s t r u c t i o n o f p o r o s i t y and p e r m e a b i l i t y , and thus i n advanced p o s t s e d i m e n t a r y h i s t o r y , t h e r o l e o f t h e i n f i l l e d n a t u r a l c r a c k systems i s c o n v e r t e d f r o m d r a i n a g e p a t h s t o f l o w b a r r i e r s . Another e f f e c t comparable t o h y d r a u l i c f r a c t u r e p r o p p i n g i s t h e i n h i b i t i o n o f c l o s u r e o f t h e c r a c k by t h e p l u g g i n g sand o r g r a v e l . A l t h o u g h t h e r e i s no p r e s s u r e necessary t o keep t h e s h r i n k a g e c r a c k s open s h o r t l y a f t e r f o r m a t i o n due t o c o n t r a c t i o n o f t h e d e p o s i t i o n a l m a t e r i a l upon t r a n s f o r m a t i o n f r o m wet v i a damp t o d r y s t a t e , t h e p r o p p i n g w i t h sand o r g r a v e l p r e v e n t s c l o s u r e d u r i n g l a t e r bur i a l d i a g e n e s i s by compressive l a t e r a l f o r c e s . The n a t u r a l s h r i n k a g e c r a c k systems a r e p a l a e o e n v i r o n m e n t a l l y s i g n i f i c a n t i n d i c a t o r s o f i n t e r r u p t i o n o f s e d i m e n t a t i o n and s u b a e r i a l o r s h a l l o w subaquatic exposure o f d e p o s i t i o n a l s u r f a c e s where t h e c r a c k s a r e formed. A s most o f t h e p o l y g o n a l n a t u r a l f r a c t u r e systems o r i g i n a t e by s u b a e r i a l d e s i c c a t i o n , these sedimentary s t r u c t u r e s a r e a d e c i s i v e i n d i c a t o r o f a t l e a s t temporary t e r r e s t r i a l c o n d i t i o n s d u r i n g f o r m a t i o n o f t h e s t r a t a and t h e r e f o r e a l l o w a l s o some c o n c l u s i o n s c o n c e r n i n g n o t o n l y t h e palaeoenvironmental s i t u a t i o n , b u t a l s o t h e g e o t e c t o n i c a l p o s i t i o n o f t h e beds.
3.13.6.2. Explanations o f illustrations on plate VI 1 - 3 : D e s i c c a t i o n s h r i n k a g e c r a c k s i n r e c e n t muddy sediments
s t a y open due t o s e p a r a t i o n o f t h e f r a c t u r e w a l l s as a consequence o f m a t e r i a l c o n t r a c t i o n and e x t e n s i o n a l c r a c k i n g . Several f r a c t u r e systems f o r m i n g a p o l y g o n a l network o r i g i n a t e i n v a r i o u s cases, w i t h f r a c t u r e w i d t h d e c r e a s i n g w i t h s u c c e s s i v e l y higher order o f the crack patterns. Fracture width o f the oldest, f i r s t - o r d e r c r a c k network i s m a i n l y a f u n c t i o n o f i n t e n s i t y o f d r y i n g and t h i c k n e s s o f t h e mud l a y e r , w i t h t h e t h i c k e r t h e mud bed, t h e deeper t h e wedge-shaped s t r a i g h t , c u r v e d o r i r r e g u l a r l y sinuous d e s i c c a t i o n c r a c k s can r e a c h downwards i n t o t h e f i n e - g r a i n e d d e p o s i t s and t h e r e f o r e t h e b r o a d e r t h e opening a t t h e s u r f a c e . Downwards f r a c t u r e p r o p a g a t i o n i n mud s t r a t a u s u a l l y i s t e r m i n a t e d a t t h e i n t e r f a c e t o t h e u n d e r l y i n g sand bed, b u t i n some cases even c o n t i n u e s i n t o t h e sand l a y e r , a l t h o u g h i n l i m i t e d w i d t h and depth. L a t e r a l p r o p a g a t i o n o f s h r i n k a g e c r a c k s i s r e s t r i c t e d due t o t h e o r i g i n o f p o l y g o n a l p a t t e r n s as a r e s u l t o f numerous s i m u l t a n e o u s l y i n i t i a t i n g c r a c k s which meet t o f o r m t h e c h a r a c t e r i s t i c network, g i v i n g r i s e t o a d e n d r i t i c system o f m u l t i p l e f r a c t u r e s o f d i f f e r e n t o r d e r . The t y p i c a l p e n t a - and/or hexagonal s h r i n k a g e c r a c k p a t t e r n o r i g i n a t e s i n b o t h t h i c k ( 1 - 2 ) and t h i n ( 3 ) mud l a y e r s . I n f i l l i n g o f t h e open c r a c k s
312 w i t h sand d u r i n g subsequently c o n t i n u i n g s e d i m e n t a t i o n f o l l o w i n g t h e d e s i c c a t i o n e v e n t t e m p o r a r i l y i n t e r r u p t i n g d e p o s i t i o n props t h e f r a c t u r e s w i t h c o a r s e r m a t e r i a l , b u t d u r i n g compaction, t h e mud l a y e r s a r e c o n s i d e r a b l y s e t t l i n g espec i a l l y i f h a v i n g g r e a t e r t h i c k n e s s and t h e r e f o r e t h e sand wedges i n f i l l i n g t h e n a t u r a l f r a c t u r e s a r e deformed i n a p l a s t i c manner. 4 - 7 : Plugging o f n a t u r a l shrinkage cracks during continuing deposition r e p r e s e n t s n a t u r a l p r o p p i n g o f synsedimentary f r a c t u r e s ( c f . a l s o p l a t e XI/7) w i t h p r e d o m i n a n t l y c o a r s e r m a t e r i a l t h a n t h e s u r r o u n d i n g r o c k m a t r i x (5 - 6 ) , b u t s u b o r d i n a t e l y a l s o f i n e r g r a i n s i z e s than t h a t o f t h e h o s t r o c k ( 4 ) . D e s i c c a t i o n f r a c t u r e s i n mud and sand a r e u s u a l l y i n f i l l e d by f i n e t o c o a r s e sand ( 6 - 7 ) , b u t i n some cases a l s o w i t h f i n e g r a v e l (5), t h e r e b y c r e a t i n g an o r i g i n a l l y c o n s i d e r a b l e p e r m e a b i l i t y c o n t r a s t which g i v e s r i s e t o a s i g n i f i c a n t c o n d u c t i v i t y d i f f e r e n c e f o r p o r e s o l u t i o n s i n e a r l y d i a g e n e s i s . T h i s cond u c t i v i t y c o n t r a s t i s t h e reason f o r t h e f r e q u e n t b l e a c h i n g o f b o t h sand packages i n t h e n a t u r a l f r a c t u r e systems ( 6 ) as w e l l as s u r r o u n d i n g h o s t r o c k mat r i x . Advanced b u r i a l d i a g e n e s i s , however, l e a d s t o p r e f e r e n t i a l p e r m e a b i l i t y d e s t r u c t i o n o f t h e sand wedges i n t e r s e c t i n g t h e f i n e r sediments by f r e q u e n t l y more i n t e n s e cementation i n t h e l a r g e r p o r e spaces o f t h e n a t u r a l l y propped f r a c t u r e . I n some cases, s y n d e p o s i t i o n a l o r e a r l y p o s t s e d i m e n t a r y mechanical i n f i l t r a t i o n and m i g r a t i o n o f c l a y m i n e r a l s i n t h e p o r e space i s a l s o a s i g n i f i c a n t f a c t o r o f c o n d u c t i v i t y impairment o f t h e n a t u r a l c r a c k p l u g s s i m i l a r l y as happening t o p r o p p a n t packages i n h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s by r e d i s t r i b u t i o n o f f r a c t u r i n g f l u i d r e s i d u e s d e r i v i n g f r o m chemical b r e a k i n g and thermal d e g r a d a t i o n as w e l l as f i l t e r cake a c c u m u l a t i o n on f r a c t u r e w a l l s .
8 : Cooling shrinkage cracks are a special f e a t u r e o f syngenetic f r a c t u r e s and r e s u l t f r o m a l t e r n a t i n g h e a t i n g and c o o l i n g o f sedimentary b l o c k s i n c o r p o r a t e d i n t o v o l c a n i c t u f f s and l a v a a t t h e boundary between v o l c a n i c e r u p t i o n p i p e and d e p o s i t i o n a l bed column. L i t h i f i e d sediments b o r d e r i n g t h e w a l l o f t h e v o l c a n i c p i p e become d e t r a c t e d by t h e high-energy h o t gas and f l u i d stream and a r e e i t h e r e j e c t e d t o g e t h e r w i t h t e p h r a p a r t i c l e s o r a r e i n t e g r a t e d i n t o t h e mat e r i a l remaining i n the pipe. While e j e c t e d pieces g e n e r a l l y are f i r s t n o t suff i c i e n t l y heated up and second t o o q u i c k l y c o o l e d down f o r e n a b l i n g g e n e r a t i o n o f s h r i n k a g e cracks, b l o c k s implemented i n t o t h e l a v a o r t u f f s i n t h e p i p e a r e s l o w l y heated up t o almost m e l t i n g temperatures which r e s u l t s i n expansion o f t h e m a t r i x , and l a t e r w i t h c r y s t a l l i z a t i o n o f t h e v o l c a n i c r o c k s c o o l s l o w l y t o m o d e r a t e l y r a p i d l y down t o f i n a l l y geothermic temperature i n c l u d i n g c o n t r a c t i o n o f t h e m a t e r i a l , and d u r i n g t h i s process, p o l y g o n a l j o i n t p a t t e r n s c o n s i s t i n g o f systems o f more o r l e s s v e r t i c a l c r a c k s o r i g i n a t e b o t h i n v o l c a n i c l a v a ( p l a t e XI/5 - 6 ) and d e p o s i t i o n a l x e n o l i t h s . I n c o n t r a s t t o sedimentary mud l a y e r s where d e s i c c a t i o n c r a c k s f o r m by s h r i n k a g e under s u b a e r i a l e x p o s i t i o n c h i e f l y as a consequence o f d r y i n g by i n s o l a t i o n , and f r a c t u r e w i d t h and p o l y gon diameter a r e m a i n l y a f u n c t i o n o f d e s i c c a t i o n i n t e n s i t y and bed t h i c k n e s s , t h e parameters o f c o o l i n g j o i n t systems i n c l u d i n g m a i n l y polygon d i a m e t e r a r e p r i m a r i l y c o n t r o l l e d by m a t e r i a l p r o p e r t i e s such as chemical c o m p o s i t i o n o f l a v a o r sedimentary rock, and o n l y s e c o n d a r i l y a l s o by u n i t t h i c k n e s s . W h i l e gen e r a l l y q u i t e small c o o l i n g s h r i n k a g e c r a c k s f o r m i n sedimentary sandstone xenol i t h s as a r e s u l t o f t h e i r h i g h s i l i c a c o n t e n t and a l s o as an e f f e c t o f t h e small s i z e o f t h e i n c o r p o r a t e d b l o c k s w i t h r e s p e c t t o t h e l a r g e volume o f t h e s u r r o u n d i n g v o l c a n i c r o c k m a t r i x , l a r g e r f r a c t u r e systems o r i g i n a t e i n b a s a l t i c l a v a which i s c h a r a c t e r i z e d by low s i l i c a shares.
3.13.6.3. Geological format ions and
outcrop localities of presented examples
1 - 3 : Recent i n u n d a t i o n s o f sand p l a i n s w i t h d e p o s i t i o n o f muddy l a y e r s i n q u i e t w a t e r upon waning o f t h e f l o o d s and p r o g r e s s i v e emergence o f t h e f i n e g r a i n e d sediments. 1 - 2 : Diameter o f f i g u r e s a b t . 1 . 5 m and 0.8 m, r e s p e c t i v e l y . Q u a r r y a t t h e e a s t e r n s i d e o f t h e R o t e r Main v a l l e y s o u t h e a s t o f U n t e r s t e i nach between B a y r e u t h and Wei denberg/Northeastern B a v a r i a (Germany FRG) .
313
Plate V I
314 3 : Diameter o f f i g u r e a b t . 1 m. Weser f l o o d p l a i n near H o l z m i n d e n / S o l l i n g (Germany FRG). 4 - 6 : New Red Sandstone ( C a r b o n i f e r o u s t o Permian ( R o t l i e g e n d ) and T r i a s s i c ) o f South Devon ( E n g l a n d ) . 4 : Diameter o f f i g u r e a b t . 0.8 m. Rocks a t t h e shore between Mackerel Cove and Labrador Bay between Torquay and Shaldon ( t o p o g r a p h i c a l map 1 : 50 000, sheet 202 Torbay and South Dartmoor, between r 92 900, h 69 100 and r 93 560, h 70 820. 5 - 6 : Diameter o f f i g u r e s a b t . 0.5 m and 0 . 6 m, r e s p e c t i v e l y . Rocks a t t h e shore near H o l l i c o m b e between Paignt o n and Torquay ( s h e e t 202 Torbay and South Oartmoor, r 89 950, h 62 300). 7 : M i d d l e B u n t s a n d s t e i n o f t h e Western H o l y Cross Mountains ( P o l a n d ) . Diameter o f f i g u r e a b t . 0.4 m. Q u a r r y a t t h e Sosnowica h i l l west o f Kajetanbw between K i e l c e and Zagnahsk. 8 : T e r t i a r y v o l c a n i c p i p e f i l l i n g c o n t a i n i n g Buntsands t e i n x e n o l i t h s in Western E i f e l (Germany FRG). Diameter o f f i g u r e a b t . 0 . 8 m. Q u a r r y a t t h e Arensberg n o r t h e a s t o f Z i l s d o r f ( t o p o g r a p h i c a l map 1 : 25 000, sheet 5706 H i l l e s h e i m , r 52 120, h 72 720). F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a, 1985 b ) .
3.13.7. Lacustrine and floodplain mud drapes in
fluvial channel and alluvial playa sandstones
The f e a t u r e s p r e s e n t e d on p l a t e V I I document t h e d i s t r i b u t i o n o f l a c u s t r i n e mud drapes and f l u v i a l f l o o d p l a i n mud l a y e r s in f l u v i a l channel and a l l u v i a l p l a y a sandstones and t h e i r impact on r e s e r v o i r h e t e r o g e n e i t y and p r o p a g a t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r e s . F o l l o w i n g d i s c u s s i o n o f g e n e r a l aspects, e x p l a n a t i o n s of t h e i l l u s t r a t i o n s on p l a t e \I11 a r e given, and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e quoted.
3.13.7.1. General aspects V a r i o u s t y p e s o f r e s e r v o i r h e t e r o g e n e i t y a r e c r e a t e d by reworked i n t r a f o r m a t i o n a l mud c l a s t s ( l ) , p a t c h i l y d i s t r i b u t e d carbonate cement ( Z ) , t h i n mud d r a pes i n t e r c a l a t e d i n t o sandstones and conglomerates ( 5 - 6 as w e l l as p l a t e s III/l- 2, I V / 4 - 8 and I X ) , b i o t u r b a t i o n by s e d i m e n t - f e e d i n g organisms ( 7 ) a n d c a l c r e t e p a l a e o s o l f o r m a t i o n ( 8 as w e l l as p l a t e V). I n a d d i t i o n t o i r r e g u l a r e r o s i o n a l boundaries ( c f . p l a t e s I / 7 , I V / 6 - 8 and V/3 - 8 ) , c h a n n e l i z e d i n t e r v e n t i o n s o f o t h e r d e p o s i t types ( c f . p l a t e s I / 7 , I I / 7 - 8, V/3 - 4 and V111/7), and i n t e r b e d d i n g o f i s o l a t e d g r a v e l t r a i n s ( c f . p l a t e V/7), t h e mentioned exampl e s o f r e s e r v o i r h e t e r o g e n e i t y a r e g e n e r a l l y d e t e r i o r a t i n g pay zone communicat i o n , and because o f t h e s m a l l e r d r a i n a g e area w i t h worse hydrocarbon f l o w cond i t i o n s , such inhomogeneous sediments f r e q u e n t l y r e q u i r e h y d r a u l i c p r o p p a n t f r a c t u r i n g i n o r d e r t o achieve economical p r o d u c t i o n ( c f . s e c t i o n s 4 . 2 . 3 . 4 . and 4.2.4.1.). The f i g u r e d cases a r e common f e a t u r e s i n t e r r e s t r i a l r e d bed sequences o f R o t l i e g e n d and B u n t s a n d s t e i n i n M i d d l e Europe and have t o be expected t o o c c u r i n v a r i o u s o t h e r s i m i l a r successions o f a e o l i a n and f l u v i a l o r i g i n w i t h abund a n t l y f l u c t u a t i n g sedimentary c o n d i t i o n s . R e s e r v o i r h e t e r o g e n e i t i e s do n o t o n l y have a n e g a t i v e impact on pay zone communication, b u t a l s o tend :a i n f l u e n c e t h e p r o p a g a t i o n o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s by a t t e n u a t i n g and d e f l e c t i n g t h e p e n e t r a t i n g c r a c k s when c r o s s i n g t h e boundaries o f l i t h o f a c i e s types w i t h d i f f e r e n t r o c k mechanical p r o p e r t i e s .
3.13.7.2. Explanations o f illustrations on plate VII 1 : Channelized i n t e r c a l a t i o n s o f c o a r s e r l i g h t sandstones c o n t a i n i n g t r a i n s o f i n t r a f o r m a t i o n a l l y reworked mud c l a s t s i n t o f i n e r dark sandstones a r e common f e a t u r e s i n b r a i d e d - r i v e r sediments which o r i g i n a t e d i n s h a l l o w t o m o d e r a t e l y deep watercourses o f an i n t e r w o v e n stream network where c u t o f f o f f l o w p a t h s and changing w a t e r l e v e l f r e q u e n t l y p r o v i d e d c o n s i d e r a b l e c u r r e n t v e l o c i t y f l u c t u a t i o n s and p u l s a t i o n s . The secondary w h i t e c o l o u r o f t h e o r i g i n a l l y a l s o
315
PlateI ' UII
316 r e d c o a r s e r s a n d s t o n e s i s t h e c l e a r e s t e x p r e s s i o n o f h i g h e r p e r m e a b i l i t y and therefore p r e f e r e n t i a l pore water f l u x during course o f the diagenetical evolut i o n . B l e a c h i n g o f t h e c o a r s e r l a y e r s , however, i s o f t e n a s s o c i a t e d w i t h more i n t e n s e c e m e n t a t i o n , and t h e r e f o r e t h e p r i m a r i l y s u p e r i o r r e s e r v o i r q u a l i t y due t o h i g h e r p e r m e a b i l i t y i s in many c a s e s s e c o n d a r i l y r e v e r s e d and f i n e r - g r a i n e d s a n d s t o n e s may a c t u a l l y have b e t t e r f l o w c o n d i t i o n s . C o m p l e t e l y i s o l a t e d c h a n n e l i z e d sand b o d i e s o f f a v o u r a b l e p e t r o p h y s i c a l p r o p e r t i e s c a n be l a t e r a l l y connected t o comparable l i t h o f a c i e s u n i t s by communication f r a c t u r i n g ( c f . s e c t i o n 4.10.2.1.) i n order t o a l i g n various separated hydrocarbon r e s e r v o i r s e c t i o n s t o one l o n g d r a i n a g e p a t h and t o a l l o w e c o n o m i c a l l y f e a s i b l e w i t h d r a w a l t h r o u g h t h e we1 1b o r e .
2 : P a t c h e s and s p o t s o f c a r b o n a t e cement c o n c e n t r a t i o n a r e h i g h l i g h t e d b y l i g h t c o l o u r i n t h e u s u a l l y d a r k s a n d s t o n e s w i t h o u t c a r b o n a t e cement. The i r r e g u l a r d i s t r i b u t i o n o f t h e p o r e cement i n a more o r l e s s homogeneous d e t r i t a l mat r i x i s a l s o a prominent element o f r e s e r v o i r heterogeneity. P r e d i c t i o n o f c a r b o n a t e cement d i s t r i b u t i o n i s f r e q u e n t l y q u i t e c o m p l i c a t e d , because c a r b o n a t e i s i n a d d i t i o n t o i t s p r i m a r y c o n c e n t r a t i o n s u b j e c t t o abundant m o b i l i z a t i o n and r e p r e c i p i t a t i o n d u r i n g c o u r s e o f t h e w h o l e d i a g e n e t i c a l e v o l u t i o n o f t h e r e s e r v o i r r o c k s . C a r b o n a t e cement c h i e f l y i n t r o d u c e s f l o w b a r r i e r s i n t o t h e o t h e r w i s e c o m m u n i c a t i n g s e d i m e n t s ( c f . s e c t i o n 4 . 2 . 4 . 1 . ) , whereas d e f l e c t i o n o f p r o p a g a t i n g h y d r a u l i c f r a c t u r e s i s l e s s p r e v a l e n t as a consequence o f o n l y m i n o r changes in m e c h a n i c a l p r o p e r t i e s ( c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) . 3 : T h i n f l u v i a l channel sandstones i n t e r c a l a t e d i n t o t h i c k e r f l u v i a l overbank mudstones a r e i s o l a t e d s h e e t r e s e r v o i r s w h i c h o n l y c o n t a i n a t t r a c t i v e h y d r o c a r b o n r e s e r v e s i n c a s e o f s u f f i c i e n t l a t e r a l e x t e n s i o n o f t h e sand b o d i e s . I f m a t r i x and n a t u r a l c r a c k p e r m e a b i l i t y a r e n o t a l l o w i n g e c o n o m i c a l p r o d u c tion, s t i m u l a t i o n b y s m a l l - s c a l e o p e r a t i o n s w i t h v e r t i c a l f r a c t u r e g r o w t h cont a i n m e n t ( c f . s e c t i o n 4 . 2 . 2 . ) i s a s u i t a b l e p o s s i b i l i t y o f enhancement o f p r o s p e c t i v i t y . V e r t i c a l f r a c t u r e c o n t a i n m e n t in such t h i n r e s e r v o i r s i s no m a j o r p r o b l e m in c a s e of s u f f i c i e n t t h i c k n e s s o f t h e u n d e r - and o v e r l y i n g mudstone s e a l s , whereas t h i n n e r mudstone b a r r i e r s b e a r t h e d a n g e r o f u n c o n t r o l l e d f r a c t u r e breakthrough t o neighbouring sandstone h o r i z o n s e s p e c i a l l y i f t h e c r i t i c a l p r e s s u r e and i n j e c t i o n r a t e i s exceeded. 4 : I n f i n e - g r a i n e d r i p p l e - l a m i n a t e d l a c u s t r i n e p l a y a sandstones which were l a i d down in v e r y s h a l l o w w a t e r c o u r s e s t o a l m o s t u n c o n f i n e d s h e e t f l o o d s w i t h g e n t l e f l o w c o n d i t i o n s i n a l a r g e i n l a n d playa, n a t u r a l f r a c t u r e s are accentuat e d b y b l e a c h i n g seams a l o n g t h e v e r t i c a l j o i n t s w h i c h t e s t i f y t o t h e s u p e r i o r p e r m e a b i l i t y o f t h e normal c r a c k s w i t h r e s p e c t t o t h e m a t r i x o f t h e c l a s t i c r e s e r v o i r . P r e f e r e n t i a l f l o w o f pore s o l u t i o n s along the n a t u r a l f r a c t u r e s a l s o r e s u l t e d i n removal o f t h e r e d c o l o u r i n t h e v i c i n i t y o f t h e cracks, w i t h thus t h e b l e a c h e d b e l t s i l l u s t r a t i n g t h e d i s t r i b u t i o n o f m a j o r f l o w p a t h s in t h e s e d i m e n t a r y c o l u m n . The l o n g t h i n n a t u r a l f r a c t u r e s u n d e r l i n e d b y t h e d e c o l o u r a t i o n seams a r e a l s o i n s t r u c t i v e m o d e l s o f t h e n e t w o r k o f h y d r a u l i c f r a c t u r e s surrounded by a b e l t o f f l u i d i n v a s i o n i n t o the m a t r i x o f the r e s e r v o i r rock.
5 - 6 : T h i n mudstone d r a p e s o f s h e e t - l i k e o r l e n t i c u l a r g e o m e t r y i n t e r c a l a t e d i n t o t h i c k e r c o n g l o m e r a t e s and s a n d s t o n e s i l l u s t r a t e r a p i d l y f l u c t u a t i n g s e d i m e n t a r y c o n d i t i o n s in s h a l l o w t o m o d e r a t e l y deep b r a i d e d - r i v e r c h a n n e l s f r o m h i g h - e n e r g y c u r r e n t s w i t h h i g h v e l o c i t y and c o n s i d e r a b l e e r o s i o n a l and d e p o s i t i o n a l p o t e n t i a l t o l o w - e n e r g y f l o w o r even q u i e t w a t e r w i t h p o s s i b i l i t i e s o f s u s p e n s i o n s e t t l i n g o f mud i n p r o t e c t e d p a t c h e s o f w a t e r c o u r s e s and l a k e s . The s h a r p b o u n d a r i e s between t h e d e p o s i t i o n a l u n i t s u n d e r l i n e t h e q u i c k changes o f p a l a e o e n v i r o n m e n t a l c o n d i t i o n s and a l s o document p r o n o u n c e d d i f f e r e n ces in r o c k m e c h a n i c a l p r o p e r t i e s w h i c h have s i g n i f i c a n t i n f l u e n c e on p r o p a g a t i n g h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s . C r o s s i n g o f such l i t h o l o g y c o n t r a s t s b y p e n e t r a t i n g h y d r a u l i c f r a c t u r e s i s i n e v i t a b l y l i n k e d w i t h a t t e n u a t i o n and def l e c t i o n o f t h e s y n t h e t i c a l l y i n d u c e d c r a c k s , and h i g h e r p r e s s u r e s n e c e s s a r y t o p e r m i t f u r t h e r f r a c t u r e e x t e n s i o n beyond t h e o b s t a c l e s c a n a t t h e s u r f a c e i n e x t r e m e c a s e s e v e n m i m i c k submature s c r e e n o u t t e r m i n a t i o n o f t h e j o b ( c f . sec-
317 t i o n s 4.2.2.2.3.
and 6 . 2 . 4 . 2 . 1 . ) .
7 : A f t e r d e p o s i t i o n , g r a i n - s i z e d i s t r i b u t i o n and p e r m e a b i l i t y o f f l u v i a l sandstones and conglomerates can be m o d i f i e d and d e t e r i o r a t e d i n terms o f commun i c a t i o n and f l o w p o s s i b i l i t i e s by sediment-feeding organisms which d i g h o r i z o n t a l and v e r t i c a l burrows and tubes i n t o t h e d e p o s i t s exposed a t t h e b o t t o m o f t h e s h a l l o w w a t e r b o d i e s . B i o g e n i c r e w o r k i n g o f t h e sediment m a t r i x by d w e l l i n g worms r e s u l t s i n g r a i n - s i z e d i m i n u t i o n , more t i g h t e r p a c k i n g o f t h e framework of t h e g r a i n s , and i n s e r t i o n o f f i n e - g r a i n e d m a t e r i a l , w i t h a l l these e f f e c t s h a v i n g n e g a t i v e impact on r e s e r v o i r q u a l i t y . As no m a j o r changes i n r o c k mechan i c a l p r o p e r t i e s a r e i n t r o d u c e d , b i o t u r b a t i o n o f f l u v i a l sandstones has gen e r a l l y h a r d l y any i n f l u e n c e on p r o p a g a t i o n o f h y d r a u l i c f r a c t u r e s , b u t has i m p o r t a n t consequences f o r r e s e r v o i r communication and i n case o f abundant d i s t r i b u t i o n can be t h e reason o f r e q u i r e m e n t o f h y d r a u l i c p r o p p a n t s t i m u l a t i o n . 8 : C a l c r e t e pedogenesis w i t h g r o w t h o f n o d u l a r carbonate c o n c r e t i o n s p r e d o m i n a n t l y i n overbank mudstones, b u t t o s u b o r d i n a t e amounts a l s o i n f l u v i a l channel sandstones and r a r e l y even i n a e o l i a n dune sands, i s a common f e a t u r e o r p o s t s e d i m e n t a r y t r a n s f o r m a t i o n i n c o n t i n e n t a l sequences w h i c h o r i g i n a t e d und e r s e m i - a r i d p a l a e o c l i m a t o l o g i c a l c o n d i t i o n s . The d i f f e r e n t t y p e s o f d i s t r i b u t i o n o f pedogenic carbonates which c h i e f l y comprise autochthonous c o n c e n t r a t i o n o f g l a e b u l e s ( c f . a l s o p l a t e X/4 and 6 ) and a g g l o m e r a t i o n o f reworked fragments o f carbonate c o n c r e t i o n s i n b r e c c i a s ( c f . p l a t e V/3 - 4) r e p r e s e n t s consider a b l e r e s e r v o i r h e t e r o g e n e i t y i n p a r t s o f t h e s e r i e s . As a consequence o f t h e predominant c o n c e n t r a t i o n o f pedogenic carbonate c o n c r e t i o n s i n overbank mudstones, c a l c r e t e p a l a e o s o l f o r m a t i o n has f r e q u e n t l y o n l y m i n o r impact on r e s e r v o i r communication, b u t i s o f c o n s i d e r a b l e s i g n i f i c a n c e f o r h y d r a u l i c f r a c t u r e propag a t i o n due t o i n t r o d u c t i o n o f changes o f r o c k mechanical p r o p e r t i e s l e a d i n g t o a t t e n u a t i o n and d e f l e c t i o n o f p e n e t r a t i n g c r a c k s ( c f . s e c t i o n s 4.2.3.2.4. and 4.2.3.4.).
3.13.7.3. Geological formations and outcrop localities of presented examples 1 : O l d Red Sandstone (Devonian) o f Wales ( G r e a t B r i t a i n ) . Diameter of f i gure a b t . 2.2 m. Rocks a t t h e shore around M i l f o r d Haven. 2 : C a r b o n i f e r o u s F o u n t a i n F o r m a t i o n o f t h e Rocky Mountains F r o n t H i l l Range i n Colorado (USA). Diameter o f f i g u r e a b t . 2.8 m. Rocks a t t h e shore o f t h e H o r s e t o o t h r e s e r v o i r near F o r t C o l l i n s n o r t h o f Denver. 3 : M i d d l e B u n t s a n d s t e i n o f t h e N o r t h e a s t e r n H o l y Cross Mountains ( P o l a n d ) . Oiameter o f f i g u r e a b t . 2.4 m. Rocks and sect i o n s a t t h e e a s t e r n s i d e o f t h e Pokrzywinska v a l l e y between Wieloborowice and Pokrzywnica ( t o p o g r a p h i c a l map 1 : 100 000, sheet Bodzentyn, pas 45, s l u p 32, between r 41 400, h 80 500 and r 40 800, h 8 1 000). 4 - 5 : Upper R o t l i e g e n d o f t h e Saar-Nahe S y n c l i n e (Germany FRG). 4 : Diameter o f f i g u r e a b t . 2 m. Rocks a t t h e Kahlenberg s o u t h o f Nackenheim ( s h e e t C 6314 Mainz, r 52 800, h 30 600). 5 : Diameter o f f i g u r e a b t . 1.5 m. Rocks a t t h e western s i d e o f t h e Burgberg i n t h e T r o l l b a c h v a l l e y e a s t n o r t h e a s t o f Burglayen ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t 6013 Bingen, r 19 600, h 33 370). 6 : Upper B u n t s a n d s t e i n o f t h e N o r t h e r n S o l l i n g (Germany FRG). Diameter o f f i g u r e a b t . 0.6 m. Q u a r r y a t t h e southwest e r n s i d e o f t h e Sundern e a s t o f Negenborn near S t a d t o l d e n d o r f ( s h e e t 4123 S t a d t o l d e n d o r f , r 41 370, h 50 9 6 0 ) . 7 : Lower Permian ( R o t l i e g e n d ) t o Upper C a r b o n i f e r o u s (New Red Sandstone) o f South Devon (England). Diameter o f f i g u r e a b t . 1 m. Rocks a t t h e shore near S a l t e r n Cove between P a i g n t o n and Broadsands ( t o p o g r a p h i c a l map 1 : 50 000, sheet 202 Torbay and South Dartmoor, r 89 450, h 58 950). 8 : M i d d l e T r i a s s i c o f t h e LodGve B a s i n ( F r a n c e ) . Diameter of f i g u r e a b t . 1.2 m. Rocks a t t h e w e s t e r n s i d e o f t h e h i l l n o r t h o f La L i e u d e between Brenas and Octon southwest o f Lodeve. F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MAOER (1985 a, 1985 b ) .
318
3.13.8. Natural fractures and sedimentary heterogeneities in
cross-stratified aeolian dune and sheet sands as well as fluvial channel sands The f e a t u r e s p r e s e n t e d on p l a t e V I I I document t h e d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s i n c r o s s - s t r a t i f i e d and h o r i z o n t a l - l a m i n a t e d a e o l i a n dune and s h e e t sands as w e l l as f l u v i a l c h a n n e l sands and t h e s i g n i f i c a n c e o f t h e s e d i m e n t a r y bedding s t r u c t u r e s f o r r e s e r v o i r heterogeneity. F o l l o w i n g discussion o f general a s p e c t s , e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e V I I I a r e g i v e n , and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e q u o t e d .
3.13.8.1. General aspects The d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s i n t e r r e s t r i a l s e d i m e n t s o f R o t l i e g e n d and B u n t s a n d s t e i n i n M i d d l e E u r o p e depends p r i m a r i l y o n r o c k t y p e and s e c o n d a r i l y a l s o on d e p o s i t i o n a l f a c i e s ( c f . s e c t i o n 4.8.8.1.). While f l u v i a l sandstones a r e f r e q u e n t l y i n t e n s e l y l i t h i f i e d and t h e r e f o r e a r e a l s o o f t e n p r o m i n e n t l y nat u r a l l y f r a c t u r e d , a e o l i a n s a n d s t o n e s a r e i n many c a s e s o n l y w e a k l y c o n s o l i d a t e d and do n o t e x h i b i t w e l l r e c o g n i z a b l e n a t u r a l c r a c k s . L e n g t h and s p a c i n g o f n a t u r a l f r a c t u r e s i n f l u v i a l sandstones i s a l s o a f u n c t i o n o f t h i c k n e s s o f t h e s e d i m e n t s . C o n s i d e r a b l e r e s e r v o i r h e t e r o g e n e i t y i s i n t r o d u c e d b y s m a l l - o r medium-scale i n t e r b e d d i n g o f f l u v i a l channel sandstones w i t h a l l u v i a l overbank mudstones o r a e o l i a n dune and s h e e t sands. A d d i t i o n a l c o m p l i c a t i o n s o f t h e geom e t r i c a l framework as w e l l as f u r t h e r p o s s i b i l i t i e s o f a t t e n u a t i o n and d e f l e c t i o n o f propagating h y d r a u l i c f r a c t u r e s a r e created by i r r e g u l a r morphology o f f l u v i a l s a n d s t o n e s due t o e r o s i v e c h a n n e l f o r m s , d e p o s i t i o n a l l e n t i c u l a r o u t l i nes and d e f o r m a t i o n a l n o d u l a r shapes.
3.13.8.2. Explanations of illustrations on plate VIII 1 - 2 : Small-scale i n t e r b e d d i n g o f i n t e n s e l y - l i t h i f i e d f l u v i a l sandstones e x h i b i t i n g w e l l - d e v e l o p e d n a t u r a l f r a c t u r i n g and w e a k l y - c o n s o l i d a t e d aeolian s a n d s t o n e s showing a l m o s t n o n a t u r a l c r a c k i n g a r e t h e m o s t w i d e s p r e a d f a c i e s a s s o c i a t i o n i n t h e M i d d l e Buntsandstein i n e x t e n s i v e p a r t s o f t h e Mid-European B a s i n . Such a l t e r n a t i n g sequences o f d i f f e r e n t m a t r i x and f r a c t u r e p e r m e a b i l i t y r e p r e s e n t q u i t e h e t e r o g e n e o u s r e s e r v o i r s b o t h i n t e r m s o f f l o w i n h i b i t i o n and hydraulic fracture modification ( c f . s e c t i o n 4.2.3.4.). A d d i t i o n a l var i a b i l i t i e s a r e t r i g g e r e d b y d i f f e r e n t t h i c k n e s s and s p a c i n g o f t h e i n d i v i d u a l l a y e r s . Comparable i n t e r b e d d e d s u c c e s s i o n s a r e among t h e t o p c a n d i d a t e s f o r mass i v e h y d r a u l i c p r o p p a n t f r a c t u r e s t i m u l a t i o n o f p r e d o m i n a n t l y R o t l i e g e n d gas r e s e r v o i r s i n o r d e r t o c o n n e c t t h e v a r i o u s i n d i v i d u a l t h i n l a y e r s o f h i g h p a y qual i t y a c r o s s t h e s e p a r a t i n g t i g h t i n t e r v a l s b y a l a r g e v e r t i c a l f r a c t u r e as w e l l as t o e x t e n d t h e d r a i n a g e h o r i z o n t a l l y f a r o u t i n t o t h e p a y zone away f r o m t h e b o r e h o l e ( v e r t i c a l and l a t e r a l c o m m u n i c a t i o n f r a c t u r i n g ; c f . section 4.10.). The p r e s e n t e d examples c a n s e r v e as m o d e l s f o r o t h e r t y p e s o f i n t e r b e d d i n g o f f a c i e s e l e m e n t s o f d i f f e r e n t r e s e r v o i r q u a l i t y such as d r y , damp and w e t p l a y a sediments which a r e a l s o prominent candidates f o r l a r g e - s c a l e h y d r a u l i c propp a n t f r a c t u r i n g t r e a t m e n t s i n o r d e r t o o b t a i n e c o n o m i c a l l y f e a s i b l e gas p r o d u c t i o n r a t e s ( c f . s e c t i o n s 3 . 2 . 1 . 1 . and 3 . 1 0 . 1 . ) . 3 : S m a l l - s c a l e i n t e r b e d d i n g o f l i g h t f l u v i a l c h a n n e l o r s h e e t f l o o d sands t o n e s and d a r k a l l u v i a l o v e r b a n k p l a i n mudstones a l s o r e p r e s e n t a c o n s i d e r a b l y h e t e r o g e n e o u s p i l e o f c o n t i n e n t a l s e d i m e n t s . The f l u v i a l s a n d s t o n e s o r i g i n a t e d b y m i g r a t i o n o f b a r s and r i p p l e s a l o n g t h e f l o o r o f s h a l l o w t o m o d e r a t e l y deep r i v e r c o u r s e s o r a c r o s s i n t e r c h a n n e l f l a t s i n u n d a t e d b y s h e e t f l o o d s and a r e sep a r a t e d b y a l l u v i a l t o p s t r a t u m mudstones w h i c h were l a i d down i n s h a l l o w l a k e s and ponds i n t h e o v e r b a n k f l a t between t h e s t r e a m s . R e s e r v o i r h e t e r o g e n e i t y i s c r e a t e d b y d i f f e r e n t t h i c k n e s s and s p a c i n g o f s a n d s t o n e s and mudstones as w e l l as r e s t r i c t i o n o f n a t u r a l f r a c t u r e s t o t h e s a n d s t o n e s where s p a c i n g , s i z e and
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320 opening o f t h e c r a c k s depends on t h i c k n e s s and l i t h i f i c a t i o n o f t h e sandstones. Such sequences a l s o serve as models f o r i l l u s t r a t i o n o f t h e e f f e c t i v i t y o f l a r g e v e r t i c a l f r a c t u r e s f o r v e r t i c a l c o n n e c t i o n and commingling o f t h e i n d i v i dual l a y e r s w i t h h i g h r e s e r v o i r q u a l i t y across t h e s e p a r a t i n g t i g h t b a r r i e r s and t h e r e f o r e a l l o w i n g o f j o i n t p r o d u c t i o n o f a l o n g sequence i n s t e a d o f d r a i n i n g o n l y a l i m i t e d i n t e r v a l ( c f . s e c t i o n 4.10.). I n t e r b e d d e d sequences o f sandstones o f d i f f e r e n t o r i g i n and r e s e r v o i r q u a l i t y as w e l l as sandstones and mudstones a r e q u i t e common i n R o t l i e g e n d and B u n t s a n d s t e i n o f M i d d l e Europe and t h e r e f o r e have t o be expected t o o c c u r a l s o i n many g a s - b e a r i n g s e c t i o n s o f t h e g e o l o g i c a l column.
4 - 5 : I s o l a t e d s m a l l - s c a l e a e o l i a n dune and sheet sands w i t h i n t h i c k e r succ e s s i o n s o f a l l u v i a l - f a n o r f l u v i a l sandstones and conglomerates a r e high-qual i t y r e s e r v o i r s o f l i m i t e d t h i c k n e s s and e x t e n s i o n . As c l e a n a e o l i a n sands a r e t h e b e s t gas r e s e r v o i r s i n R o t l i e g e n d and B u n t s a n d s t e i n i n M i d d l e Europe ( c f . s e c t i o n 3.2.1.1.), empllasis o f e x p l o r a t i o n and development o f these f o r m a t i o n s has t o be p u t on a c q u i s i t i o n o f t h i c k a e o l i a n dune sands ( c f . s e c t i o n s II/1 - 6 ) i n t h e g a s - b e a r i n g zone. U n f o r t u n a t e l y t h e i n t e r t o n g u i n g o f a e o l i a n dunes and f l u v i a l b r a i d e d - r i v e r channels o r a l l u v i a l - f a n cones, l o b e s and r i v e r p l a i n s takes p r e d o m i n a n t l y p l a c e i n more s o u t h e r n f a c i e s zones c l o s e r t o t h e m a r g i n o f t h e b a s i n w h i c h a r e nowadays near t h e s u r f a c e o r i n s h a l l o w d e p t h and t h u s do n o t c o n t a i n gas accumulations ( c f . s e c t i o n 3.2.1.2.). T h i n i s o l a t e d aeol i a n dune and sheet sands i n t e r s p e r s e d w i t h i n t h i c k e r sequences o f p o o r e r r e s e r v o i r q u a l i t y a r e a l s o t y p i c a l f o r Wechselfolge sandstones i n t h e upper p a r t o f t h e g a s - b e a r i n g R o t l i e g e n d s e c t i o n i n N o r t h w e s t Germany FRG ( c f . s e c t i o n s 3.2.1.3. and 3.10.1.)where t h e y achieve s u f f i c i e n t t h i c k n e s s t o be economicall y productive w i t h o u t proppant f r a c t u r e s t i m u l a t i o n . 6 - 7 : Channelized f l u v i a l sandstone b o d i e s i n t e r c a l a t e d i n t o a l l u v i a l o v e r bank mudstones ( c f . a l s o p l a t e s I/7, II/7 - 8, III/4 and V/3 - 4) a r e documenti n g l i m i t e d l a t e r a l extension o f h i g h - q u a l i t y r e s e r v o i r f a c i e s w i t h i n continent a l sequences which can be v e r y i m p o r t a n t f o r p l a n n i n g o f w e l l spacing and s t i m u l a t i o n p a t t e r n s . I n c o n t r a s t t o c o n t i n u o u s s h e e t - t y p e sediments, lenticular d e p o s i t s i m p l y r e s t r i c t e d r e s e r v o i r communication b o t h p r i o r t o and a f t e r hyd r a u l i c f r a c t u r e s t i m u l a t i o n ( c f . s e c t i o n 3.4.1.1.), because w i t h i n a p i l e o f i s o l a t e d sandstone l e n s e s d i s p e r s e d w i t h i n mudstone successions, a l a r g e v e r t i c a l f r a c t u r e cannot connect a l l o f them as a consequence o f l i m i t e d d r a i n a g e path extension w i t h respect t o the three-dimensional d i s t r i b u t i o n o f the i s o l a t e d r e s e r v o i r u n i t s . An improved s o l u t i o n c o u l d be a l t e r e d - s t r e s s network f r a c t u r i n g ( c f . s e c t i o n 4.9.4.1.3.)o r m u l t i p l e f r a c t u r e p r o p a g a t i o n i n a c r o s s - o r s t a r - l i k e p a t t e r n ( c f . s e c t i o n 4 . 2 . 2 . ) i n o r d e r t o c r e a t e s e v e r a l d r a i n a g e avenues which i n t o t a l i n t e r s e c t a l a r g e r number o f i s o l a t e d sand b o d i e s and conn e c t them t o t h e w e l l b o r e . Such pay zone columns r e q u i r e c a r e f u l economical eval u a t i o n w i t h r e s p e c t t o t h e p o s s i b i l i t y o f achievement o f s u f f i c i e n t p r o d u c t i o n r a t e s and access t o enough r e s e r v e s t o j u s t i f y expensive t r e a t m e n t s . The i r r e g u l a r morphology o f t h e e r o s i o n a l boundaries o f f l u v i a l channels as w e l l as t h e d e p o s i t i o n a l shape o f o t h e r l e n t i c u l a r sand b o d i e s p r o v i d e s p o s s i b i l i t i e s o f d i vergence o r convergence o f p r o p a g a t i n g h y d r a u l i c f r a c t u r e s , w i t h s p l i t t i n g and merging o f bundles b e i n g e f f e c t s a d d i t i o n a l t o a t t e n u a t i o n and d e f l e c t i o n ( c f . s e c t i o n 4.2.3.4.). 8 : I n t e r b e d d e d f l u v i a l channel sandstones and a l l u v i a l overbank mudstones a r e t y p i c a l f o r c o n t i n e n t a l b r a i d p l a i n sequences and a r e c h a r a c t e r i z e d n o t o n l y by changes o f r e s e r v o i r m a t r i x q u a l i t y , b u t a l s o o f n a t u r a l f r a c t u r e d i s t r i b u t i o n . W h i l e sandstones c o n t a i n w e l l - d e v e l o p e d n a t u r a l c r a c k s e s p e c i a l l y i f more i n t e n s e l y l i t h i f i e d , mudstones a r e t i g h t b a r r i e r s s e a l i n g and s e p a r a t i n g t h e i n d i v i d u a l sandstone l a y e r s . Sharp boundaries between sandstones and mudstones as a consequence o f r a p i d l y changing environmental c o n d i t i o n s f r o m h i g h l y - a g i t a t e d f l o w i n g w a t e r t o q u i e t s t a n d i n g w a t e r i n f l u v i a l channels, overbank s h e e t f l o o d s and i n t e r s t r e a m watercourses and l a k e s a r e a l s o marked boundaries o f d i f f e r e n t r o c k mechanical p r o p e r t i e s i n f l u e n c i n g p r o p a g a t i o n o f h y d r a u l i c a l l y - i n d u c e d fractures.
32 1
3.13.8.3. Geological formations and outcrop local it ies of presented examples 1 - 2 : M i d d l e Buntsandstein o f t h e P f a l z e r Wald a t the western margin o f the Rhinegraben v a l l e y (Germany FRG). Diameter o f f i g u r e s abt. 4 m each. Road s e c t i o n a t the n o r t h e r n s i d e o f the Eckbach v a l l e y south o f Neuleiningen ( t o p o g r a p h i c a l map 1 : 25 000, sheet 6414 Grunstadt-West). 3 : M i d d l e Buntsandstein o f t h e I s k a r v a l l e y ( B u l g a r i a ) . Diameter o f f i g u r e abt. 8 m. Rocks a t t h e east e r n s i d e o f t h e I s k a r v a l l e y e a s t o f Zerovo n o r t h o f S o f i a ( c f . MADER & CHATALOV 1988). 4 - 5 : Lower Permian (Rotliegend) t o Upper Carboniferous (New Red Sandstone) i n South Devon (England). Diameter o f f i g u r e s a b t . 3 m each. Rocks at Roundham Head e a s t o f Paignton south o f Torbay ( t o p o g r a p h i c a l map 1 : 50 000, sheet 202 Torbay and South Dartmoor, r 89 800, h 60 000). 6 : Rotl i e g e n d (Lower Permian) o f the I s k a r v a l l e y ( B u l g a r i a ) . Diameter o f f i g u r e a b t . 1.5 m. Rocks a t the road a t the western s i d e o f the I s k a r v a l l e y between Novi I s k a r and Rebrovo n o r t h o f S o f i a . 7 : M i d d l e Buntsandstein o f the Northeastern Holy Cross Mountains (Poland). Diameter o f f i g u r e abt. 2 m. Q u a r r y a t t h e southwestern s i d e o f t h e h i l l e a s t o f J a r u g i between Stryczowice and Czerwona Gbra near Ostrowiec Swietokrzyski ( t o p o g r a p h i c a l map 1 : 100 000, sheet Bodzentyn, pas 45, szup 32, r 52 700, h 74 200). 8 : Upper Buntsandstein o f t h e I s k a r v a l l e y ( B u l g a r i a ) . Diameter o f f i g u r e a b t . 5 m. Rocks a t t h e western s i d e o f the I s k a r v a l l e y west and northwest o f Zerovo n o r t h o f S o f i a ( c f . MADER & CHATALOV 1988). F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a, 1985 b )
.
3.13.9. Sedimentary and deformational features of lacustrine mud
drapes in f luvial channel, overbank and alluvial playa sandstone The p a t t e r n s f i g u r e d on p l a t e I X document the d i s t r i b u t i o n o f l a c u s t r i n e mud drapes and lenses i n f l u v i a l channel and overbank sandstone sediments as w e l l as sandy p l a y a deposits, and t h e i r impact on propagation o f n a t u r a l and hydraul i c f r a c t u r e s i n sandstones as w e l l as t h e i r s i g n i f i c a n c e f o r r e s e r v o i r heterog e n e i t y and a n i s o t r o p y . F o l l o w i n g d i s c u s s i o n o f general aspects, e x p l a n a t i o n s o f the i l l u s t r a t i o n s on p l a t e I X are given, and g e o l o g i c a l formations and o u t crop l o c a l i t i e s o f the presented examples are quoted.
3.13.9.1. General aspects Small-scale mudstone i n t e r c a l a t i o n s i n t o sandstone beds are prominent exampl e s o f r e s e r v o i r heterogeneity r e s u l t i n g i n f l o w i n h i b i t i o n and r e s t r i c t i o n i n several d i r e c t i o n s and scales of t h e pay zone complex. P r i m a r y - d e p o s i t i o n a l t h i n mud drapes o f s i n g u l a r , double o r m u l t i p l e n a t u r e as w e l l as secondary-eros i o n a l mud remnants and/or deformational mud lumps and domes i n t e r r u p t the continuous sandstone package and r e p r e s e n t f l o w b a r r i e r s f o r hydrocarbons and wat e r as w e l l as inhomogeneities f o r h y d r a u l i c f r a c t u r e propagation ( c f . s e c t i o n 4.2.3.4. and 4.2.4.1.). Primary-sedimentary mud drapes are i m p o r t a n t t o o l s f o r palaeoenvironmental m o d e l l i n g of the r e s e r v o i r complex by i n d i c a t i n g the f u l l spectrum o f p o s s i b i l i t i e s o f hydrodynamic c o n d i t i o n s being developed. Mud l a y e r s and lenses s e p a r a t i n g f l u v i a l channel sandstones ( c f . a l s o p l a t e s I I I / 1 - 2, I V / 4 t 8 and V I I / 5 - 6 ) g e n e r a l l y i n d i c a t e f l u c t u a t i n g water l e v e l d u r i n g course o f p u l s a t o r y f l o o d s i n t e r r u p t e d by waning phases and low-water stages i n the shallow t o moderately-deep b r a i d e d - r i v e r system, and r e f l e c t temp o r a r y q u i e t c o n d i t i o n s a l l o w i n g mud s e t t l i n g between p e r i o d s o f high-energy c u r r e n t s and m i g r a t i o n o f sand and g r a v e l bars downstream. Sharp lower and upper boundaries o f mud drapes t o a d j o i n i n g sandstone beds t e s t i f y t o r a p i d l y changing energy l e v e l o f d e p o s i t i o n w i t h c u t t i n g o f f o f some
322 p a r t s o f t h e c h a n n e l n e t w o r k f r o m t h e a c t i v e p a t t e r n and p r o t e c t i o n o f some p o o l s and ponds i n i n t e r b a r and i n t r a b a r d e p r e s s i o n s b y d e f l e c t i o n o f c u r r e n t s a r o u n d t h e sand b o d i e s . I n t e r c a l a t i o n o f sand s t r e a k s i n t o t h e mud l a y e r s and lenses g i v e s evidence o f c l o s e coexistence o f d e p o s i t i o n a l subenvironments w i t h d i f f e r e n t t r a n s p o r t c a p a c i t y and m a t e r i a l s u p p l y , w i t h h i g h e r - e n e r g y p u l s e s o f f l o o d w a t e r d e l i v e r i n g m i n o r amounts o f sand a l s o t o s i t e s o f mud s e t t l i n g i n q u i e t water.
3.13.9.2. Explanations o f illustrations on plate IX 1, 4, 5 : T h i n n e r o r t h i c k e r mud d r a p e s w i t h o r w i t h o u t sand i n t e r c a l a t i o n s s e p a r a t i n g m a s s i v e o r w e l l - s t r a t i f i e d s a n d s t o n e complexes o f f l u v i a l c h a n n e l o r i g i n r e p r e s e n t i n t e r m i t t e n t w a n i n g - f l o w d e p o s i t s and t e s t i f y t o t e m p o r a l l y and s p a t i a l l y c h a n g i n g s e d i m e n t a r y c o n d i t i o n s . C o n c e r n i n g s t i m u l a t i o n o f such r e s e r v o i r s , mud d r a p e s o f d i f f e r e n t t h i c k n e s s a c t as b a r r i e r s ( c f . s e c t i o n 4.2.4.1.) and c a n p r o v i d e some c o n t a i n m e n t o f f r a c t u r e g r o w t h i n t h i n r e s e r v o i r s p r o v i d e d t r e a t m e n t i s p e r f o r m e d a t l o w p r e s s u r e r a t e s and m o d e r a t e cumulat i v e f l u i d volumes ( c f . s e c t i o n 4 . 2 . 2 . 3 . ) . R e s e r v o i r s u b d i v i s i o n b y mud d r a p e s i n t o several subunits therefore gives r i s e t o f r a c t u r e narrowing crossing the i n t e r c a l a t e d b a r r i e r s o r e v e n f r a c t u r e s p l i t t i n g i n c a s e o f c o n t a i n m e n t o f separ a t e f r a c t u r e b r a n c h e s w i t h i n t h e i n d i v i d u a l s t o r e y s and s u b s t o r e y s ( c f . s e c t i o n 4.2.3.4.). I n t e r m s o f r o c k mechanics, mud d r a p e s d e f l e c t p r o p a g a t i n g h y d r a u l i c f r a c t u r e s b y c h a n g i n g d i r e c t i o n and w i d t h o f t h e p e n e t r a t i n g c r a c k , w i t h t h i c k e r mud l a y e r s e v e n b e i n g a b l e t o a l m o s t c o m p l e t e l y s t o p f r a c t u r e e x t e n s i o n i n t h e w o r s t c a s e s ( c f . s e c t i o n 4 . 2 . 4 . 3 . ) . I n c o n t r a s t t o r e s e r v o i r det e r i o r a t i o n b y f r e q u e n t l y more i n t e n s e d i a g e n e t i c a l c e m e n t a t i o n o f t h e sands t o n e a r o u n d t h e i n t e r b e d d e d mudstone and a l s o d e c r e a s i n g g r a i n s i z e o f t h e s a n d s t o n e when a p p r o a c h i n g t h e mudstone, t h e t h i n mud d r a p e s p r o v i d e e x c e l l e n t t o o l s f o r d e t a i l e d s u b d i v i s i o n o f t h e p a y zone column i n t o s u b u n i t s and p r o p e r i d e n t i f i c a t i o n o f t h e i n d i v i d u a l l a y e r s i n gamma-ray l o g s f o r d e p t h and s t r a t i g r a p h i c a l c o n t r o l a t l e a s t i n c a s e o f l a t e r a l l y c o n t i n u o u s e x t e n s i o n o f t h e mud d r a p e s beyond s i n g l e o r d o u b l e w e l l s p a c i n g . 2 - 3 : Mud d r a p e s b e i n g i n t e r c a l a t e d i n t o sand sequences do n o t have t o be d e v e l o p e d as e v e n s h e e t s o r l e n s e s o f c o n s t a n t t h i c k n e s s , b u t c a n a l s o e x h i b i t i r r e g u l a r g e o m e t r y as a consequence o f c o v e r i n g p r i m a r y d e p o s i t i o n a l o r second a r y e r o s i o n a l r e l i e f s h a v i n g been g e n e r a t e d i n t h e u n d e r l y i n g s a n d s t o n e p r i o r t o mud bed a g g r a d a t i o n , o r as a r e s u l t o f s e c o n d a r y e r o s i o n a l m o d i f i c a t i o n o f t h e mudstone a f t e r i t s d e p o s i t i o n . W h i l e e r o s i o n a l s c u l p t u r i n g o f o r i g i n a l l y c o n t i n u o u s mud l a y e r s b r e a k s them i n t o i s o l a t e d lumps and r e m n a n t s w h i c h have o n l y l i t t l e i m p a c t on r e s e r v o i r c o m m u n i c a t i o n and f r a c t u r e p r o p a g a t i o n due t o t h e i r l i m i t e d l a t e r a l e x t e n s i o n and a t t h e b o t t o m o f t h e l i n e d i s c r e t e e r o s i o n a l r e l i c t s o f mudstones do n o t d i s t u r b f l u i d f l o w and f r a c t u r e p e n e t r a t i o n , c o v e r i n g o f p r i m a r y s e d i m e n t a r y o r s e c o n d a r y e r o s i o n a l r e l i e f s b y mud d r a p e s w h i c h a r e a d j u s t i n g t o t h e e a r l i e r m o r p h o l o g y and o t h e r w i s e f o r m c o n t i n u o u s beds s i m i l a r l y as t h o s e o v e r l y i n g d e p o s i t i o n a l s a n d s t o n e s u r f a c e s g i v e s r i s e t o c o m p a r a b l e p e r s i s t e n t mud d r a p e s t h a t a r e e f f e c t i v e e l e m e n t s o f r e s e r v o i r s u b d i v i s i o n and f r a c t u r e p e n e t r a t i o n d e t e r i o r a t i o n . 6 : I s o l a t e d sand l o a d s t r u c t u r e s w i t h i n a mudstone bed s e p a r a t i n g t h i c k e r c h a n n e l s a n d s t o n e s r e f l e c t p l a s t i c d e f o r m a t i o n o f sand as a consequence o f subs i d e n c e i n t o n o t y e t c o n s o l i d a t e d mudstones w h i c h had k e p t w a t e r - s a t u r a t e d and s o f t d u r i n g s h o r t i n t e r r u p t i o n s o f high-energy f l u v i a l sedimentation. I n cont r a s t t o l o n g e r b r e a k s o f d e p o s i t i o n w i t h d e s i c c a t i o n o f mud f l a t s and o r i g i n o f polygonal shrinkage cracks, s h o r t episodes o f q u i e t water c o n d i t i o n s t h a t a r e soon a g a i n r e p l a c e d b y a g i t a t e d c u r r e n t s e n a b l e p l a s t i c r e a c t i o n o f t h e muds t o n e once l o a d e d a g a i n b y sand w h i c h s i n k s i n t o t h e s o f t mudstones and d e f o r m s there t o discrete load balls.
7 : A n o t h e r form of p l a s t i c r e a c t i o n i s r e l a t e d t o a c t i v i t y o f t h e mudstones t h e i r s e l v e s . W h i l e l o a d i n g of s t i l l w a t e r - s a t u r a t e d mudstones b y t h i n n e r sand-
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IX
324 stone beds u s u a l l y r e s u l t s i n downsinking o f t h e sand i n t o t h e mud, t h e r e v e r s e i s developed f o r t h i c k e r sandstone l a y e r s t o p p i n g s t i l l u n c o n s o l i d a t e d mudstone u n i t s . Upon l o a d i n g , t h e p l a s t i c mud i n t r u d e s l i k e d i a p i r s i n t o t h e l o o s e l y packed, f r e s h l y d e p o s i t e d f l u v i a l channel sand i n a s i m i l a r p a t t e r n as s a l t p l u g s and domes ( c f . f i g s . 3 and 11) a r e formed f o r t h e reason o f s t r e s s equal i z a t i o n . B o t h downsinking o f sand i n t o mud and upwards i n j e c t i o n o f mud i n t o sand do n o t c o m p l e t e l y i n t e r r u p t t h e l a t e r a l e x t e n s i o n o f t h e mud l a y e r s , w i t h some t h i n remnants almost r e m a i n i n g everywhere, b u t t h i n n i n g o f mud beds a t some patches i n o r d e r t o compensate t h i c k e n i n g a t o t h e r s p o t s g i v e s r i s e t o weak e n i n g b a r r i e r c h a r a c t e r o f t h e mud drapes and t h u s p r o v i d e s e a s i e r f r a c t u r e p r o p a g a t i o n w i t h l e s s growth c o n t r o l . D i a p i r i c i n t r u s i o n o f u n s t a b l e mud i n t o t h e o v e r l y i n g f r e s h l y d e p o s i t e d and p r o b a b l y s t i l l w e t f l u v i a l channel o r s h e e t f l o o d sand documents t h e range o f t h e spectrum o f a l l u v i a l sedimentary and def o r m a t i o n a l processes and i s i n marked c o n t r a s t t o reworked c a n n i b a l i z e d mud c l a s t s t h a t a r e c a r v e d o u t o f a l m o s t c o m p l e t e l y c o n s o l i d a t e d mud t h a t can even f o r m e r o s i o n a l remnants w i t h overhanging o r oversteepened e r o s i o n a l b o u n d a r i e s ( c f . p l a t e s IV/2 and V I I / l ) .
8 : Mud drapes s e p a r a t i n g sand h o r i z o n s can be o f s i n g u l a r o r d o u b l e t o mult i p l e t y p e a c c o r d i n g t o d e p o s i t i o n a l c o n d i t i o n s and t h e i r d i s t r i b u t i o n i n l a t e r a l d i r e c t i o n . Mud seams a r e s p l i t up i n t o s e v e r a l branches by i n t e r c a l a t i o n o f i s o l a t e d l e n t i c u l a r sand bodies and condense t o a p p a r e n t l y c o n t i n u o u s c o v e r s i n p l a c e s where i n t e r v e n i n g sand l e n s e s a r e absent. The l a t e r a l c o e x i s t e n c e o f b o t h cases u n d e r l i n e s t h e c o m p l e x i t y o f r e s e r v o i r c o m p o s i t i o n and h i g h l i g h t s t h e i n f l u e n c e o f even s m a l l - s c a l e r e s e r v o i r h e t e r o g e n e i t i e s on f r a c t u r e propagat i o n and p e n e t r a t i o n ( c f . s e c t i o n s 4 . 2 . 4 . 1 . ) .
3 . 1 3 . 9 . 3 . Geological formations and outcrop localities o f presented examples 1 : M i d d l e B u n t s a n d s t e i n o f t h e N o r t h e a s t e r n H o l y Cross Mountains ( P o l a n d ) . Diameter o f f i g u r e a b t . 1 . 2 m. Q u a r r y a t t h e southwestern s i d e o f t h e h i l l e a s t o f J a r u g i between S t r y c z o w i c e and Czerwona Gbra n e a r O s t r o w i e c S w i e t o k r z y s k i ( t o p o g r a p h i c a l map 1 : 100 000, sheet Bodzentyn, pas 45, s l u p 32, r 52 700, h 74 2 0 0 ) . 2 - 6 : O t t e r Sandstones (Lower T r i a s s i c ) i n t h e New Red Sandstone o f South Devon (England). Diameter o f f i g u r e s a b t . 1 m each. 2, 5, 6 : Rocks a t t h e shore between Smallstones P o i n t and Crab Ledge between B u d l e i g h S a l t e r t o n and Sidmouth ( t o p o g r a p h i c a l map 1 : 25 000, sheet SY 08/18 Sidmouth, between r 09 200, h 83 840 and r 09 600, h 84 780). 3 : Diameter o f f i g u r e a b t . 0 . 7 m. Rocks a t t h e shore between Hern Rock and B i g P i c k e t Rock e a s t n o r t h e a s t o f O t t e r t o n and southwest o f Sidmouth ( s h e e t SY 08/18 Sidmouth, between r 09 800, h 85 260 and r 10 320, h 85 780). 4 : Diameter o f f i g . a b t . 1 . 5 m. Rocks a t t h e shore n e a r Windgate west o f Sidmouth ( s h e e t SY 08/18 Sidmouth, r 10 720, h 86 450). 7 - 8 : M i d d l e B u n t s a n d s t e i n o f Upper F r a n c o n i a / N o r t h e a s t e r n B a v a r i a (Germany FRG). Diameter o f f i g u r e s a b t . 0 . 9 m and 0.6 m, r e s p e c t i v e l y . Sand p i t a t t h e n o r t h w e s t e r n s i d e o f t h e Kammberg s o u t h o f t h e B i r k a c h e r Graben west o f Gundelsdorf ( s h e e t 5733 Kronach, 7 : r 52 100, h 71 390; 8 : r 51 960, h 71 360). F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a, 1985 b ) .
3 . 1 3 . 1 0 . Root tubes and carbonate
concretions in calcrete palaeosols The f e a t u r e s f i g u r e d on p l a t e X document t h e s i g n i f i c a n c e o f r o o t tubes and carbonate c o n c r e t i o n s i n c a l c r e t e p a l a e o s o l s f o r d e t e r i o r a t i o n o f p r i m a r y depos i t i o n a l r e s e r v o i r p r o p e r t i e s by secondary pedogenic o v e r p r i n t i n g . F o l l o w i n g d i s c u s s i o n o f g e n e r a l aspects, e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e X a r e given, and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d exampl e s a r e quoted.
325
3.13.10.1.
General aspects
Calcrete palaeosol formation in terrestrial red beds by growth of carbonate nodules (cf. MADER 1984), rooting of plants and conversion of parts of the red colour of the sediments into blue and violet by hematite authigenesis in the pore space in addition to the primary hematite pellicles surrounding the detrital grains (cf. MADER 1983 c) is an abundant type o f early diagenetic transformation of continental deposits. Neoformation of clay minerals in the pore space, pedoturbation by plant roots and concentration of additional iron oxides give rise to diminution of porosity and permeability, with therefore pedogenic overprinting resulting in deterioration of reservoir quality and increasing the potential or even necessity of hydraulic fracture stimulation in order to achieve economically feasible gas production rates. Calcrete palaeosol formation is quite comnon in parts of Rotliegend and especially Buntsandstein in Middle Europe, although predominantly occurring in the more marginal facies zones in more southern parts of the basin which are nowadays at the surface or in shallow depth and do not contain gas accumulations (cf. section 3.2.1.2.). As calcrete palaeosols are widespread in comparable terrestrial sandstone and mudstone sequences throughout the world (cf. MADER 1985 a, 1985 b) which frequently include gas enrichments, the understanding of the pedogenic transformation as an important process of reservoir deterioration assists proper planning and design of stimulation treatments.
3.13.10.2.
Explanations of illustrations on plate
X
1 - 3 : Vertical carbonate root tubes intersecting horizontal-stratified, cross-bedded or massive red sandstones represent the lower part of thick palaeosol profiles where only deeply reaching roots of plants are present, but the other characteristical transformations such as clay mineral authigenesis and hematite neoformation giving rise to the typical blue-violet colour have not taken place (cf. MADER 1983 c). The fossilized roots are emphasized by infilling and surrounding with carbonate as a consequence of the concentration gradient exerted by the organic matter after deposition. Additional carbonate precipitations occur as sheets lining bedding plane cracks and dispersed spherical nodules in the rock matrix. The abundance of root tubes and concretions in some horizons introduces considerable reservoir heterogeneity with respect to clean red sandstones and also increases the resistance of the sediments against propagation of hydraulically induced fractures which require higher pressure (cf. section 6.2.4.2.1.) to cross the zone containing sometimes a nearly chaotic arrangement of carbonate enrichments in a sandy matrix. Additional complications in terms of reservoir communication and fracture penetration are sometimes generated by interbedding of the root tube horizons with sandstones containing abundant carbonate cement ( 3 ) and partially representing reworking horizons of palaeosols including numerous tiny fragments deriving from erosional destruction of pedogenic carbonate concretions in a sandy-carbonatic matrix (cf. plate V/3 - 4 ) . In terms of drainage of migrating solutions, the root tube channel networks and assemblages in calcrete palaeosols have comparable morphology and function to rows of perforation tunnels that are infilled with packing gravel in mechanical sand control (cf. section 5.6.1.). 4 + 6 : In contrast to carbonate enrichments in red sandstones which represent reservoir quality deterioration, carbonate concentrations in fluvial overbank or lacustrine playa mudstones are only altering the rock mechanical properties of the sequence and have therefore pronounced impact on propagation of hydraulically induced cracks. In some cases, carbonate is filling a polyedrical network of natural cracks which originated during pedogenesis as a consequence of pedoturbation and subaerial exposition (6), thereby delineating a system of natural fractures which otherwise would have completely healed and vanished in the mudstone matrix. Sandstone beds which are intercalated into or are over-
326 l y i n g t h e p e d o g e n i c a l l y t r a n s f o r m e d mudstones o x i d e removal a f t e r d e p o s i t i o n due t o c i r c u w e l l as due t o p r e f e r e n t i a l p o r e w a t e r f l u x a The s i g n i f i c a n c e f o r a t t e n u a t i o n and d e f l e c t u r e s i s c o n t r o l l e d by amount and g e o m e t r i c a l c r e t i o n s i n as w e l l as t h i c k n e s s o f t h e t h i c k n e s s o f i n t e r c a l a t e d sandstone beds.
a r e f r e q u e n t l y bleached by i r o n a t i o n o f a g g r e s s i v e s o i l f l u i d s as ong t h e h i g h - p e r m e a b i l i t y l a y e r s . i o n o f propagating h y d r a u l i c f r a c o r i e n t a t i o n o f t h e c a r b o n a t e conp a l a e o s o l p r o f i l e o r f r e q u e n c y and
5 : Large r o o t tubes o f h o r i z o n t a l , v e r t i c a l and o b l i q u e d i r e c t i o n a l o r i e n t a t i o n i n t h e p a l a e o s o l w i t h r e s p e c t t o bedding p l a n e s and pedogenic base l e v e l a r e t h e most p r o m i n e n t p r o d u c t s o f s o i l f o r m a t i o n and document t h e g r o w t h o f p l a n t s i n p l a c e . L i m i t e d amounts o f r o o t tubes i n r e d sandstones w i t h o u t a l t e r a t i o n o f sediment c o l o u r t o b l u e - v i o l e t u n d e r l i n e t h e p o s i t i o n i n t h e l o w e r p a r t o f t h e p a l a e o s o l and t e s t i f y a l s o t o r e s t r i c t e d palaeoenvironmental and pal a e o e c o l o g i c a l c o n d i t i o n s which d i d n o t f a v o u r t h e widespread c o l o n i z a t i o n o f t h e c o n t i n e n t a l p l a i n w i t h p l a n t s . T h i c k l o n g r o o t tubes a r e v e r y p r o m i n e n t e l e ments o f r o c k mechanical p r o p e r t y h e t e r o g e n e i t y which a r e a b l e t o a t t e n u a t e and t o d e f l e c t p r o p a g a t i n g h y d r a u l i c a l l y induced f r a c t u r e s , whereas t h e i r e f f e c t on r e s e r v o i r q u a l i t y d e t e r i o r a t i o n and t h u s impact on i n c r e a s i n g s t i m u l a t i o n p o t e n t i a l and n e c e s s i t y a r e n e g l i g i b l e i f t h e i n d i v i d u a l r o o t tubes a r e spaced s u f f i c i e n t l y f a r away f r o m t h e i r n e i g h b o u r s .
7 + 8 : Flow b a r r i e r s and r o c k mechanical p r o p e r t y d i s c o n t i n u i t i e s a r e i n t r o duced i n sandstone sequences b y i n t e r c a l a t i o n o f c a r b o n a t e b r e c c i a s ( c f . a l s o p l a t e s V/3 - 4) r e p r e s e n t i n g r e w o r k i n g h o r i z o n s o f c a l c r e t e p a l a e o s o l s o v e r l y i n g degraded remnants o f pedogenic p r o f i l e s which have been c u t down by e r o s i o n ( 7 ) . R e s e r v o i r inhomogeneity i s a l s o t r i g g e r e d by l a y e r s o f reworked mud c l a s t s d e r i v i n g f r o m i n t r a f o r m a t i o n a l r e m o b i l i z a t i o n by e r o s i v e c u r r e n t s ( c f . a l s o p l a t e s I V / 2 and V I I / l ) , w i t h p r i m a r i l y c o n t i n u o u s mudstone beds h a v i n g been s e c o n d a r i l y c u t i n t o p i e c e s and i n t e g r a t e d i n t o f l u v i a l channel sands ( 8 ) . The t y p i c a l carbonate r e w o r k i n g b r e c c i a s a r e i n some f o r m a t i o n s t h e o n l y p r e served i n d i c a t i o n s o f c a l c r e t e p a l a e o s o l o r i g i n i f p o s t s e d i m e n t a r y e r o s i o n a l condensation o f t h e sequence was so s t r o n g t h a t many overbank mudstones and c a l C r e t e p a l a e o s o l s have been c o m p l e t e l y removed f r o m t h e d e p o s i t i o n a l r e c o r d and o n l y reworked fragments have been p r e s e r v e d i n f l u v i a l channel sandstones and conglomerates as g h o s t - l i k e remnants o f f o r m e r l y p r e s e n t f a c i e s elements t h a t have undergone complete d e s t r u c t i o n , and a s i m i l a r s i g n i f i c a n c e a p p l i e s f o r t h e i s o l a t e d mud c l a s t s ( c f . MADER 1984).
3.13.10.3. Geological format ions and outcrop localities of presented examples 1 - 3 : O t t e r Sandstones (Lower T r i a s s i c ) o f South Oevon ( E n g l a n d ) . Diameter o f f i g u r e s a b t . 4 m each. Rocks a t t h e shore between Crab Ledge and B l a c k Head between B u d l e i g h S a l t e r t o n and Sidmouth ( t o p o g r a p h i c a l map 1 : 25 000, s h e e t SY 08/18 Sidmouth, between r 08 580, h 82 620 and r 09 200, h 83 8 4 0 ) . 4 : M i d d l e T r i a s s i c o f t h e Lodeve B a s i n ( F r a n c e ) . Diameter o f f i g u r e a b t . 3.5 m. 5 : O t t e r Sandstones (Lower T r i a s s i c ) of South Devon (England). Diameter o f f i g u r e a b t . 1 . 5 m. Rocks a t t h e shore between Hern Rock and B i g P i c k e t Rock between B u d l e i g h S a l t e r t o n and Sidmouth ( s h e e t SY 08/18 Sidmouth, r 10 240, h 85 7 0 0 ) . 6 : M i d d l e T r i a s s i c o f t h e Lodeve B a s i n ( F r a n c e ) . Diameter o f f i g u r e a b t . 1 . 2 m. Rocks a t t h e e a s t e r n s i d e o f t h e h i l l near Le G a s i e l a s n o r t h o f La Lieude southwest o f Octon ( t o p o g r a p h i c a l map 1 : 50 000, sheet 989 Lodeve, r 675 310, h 148 800). 7 : O t t e r Sandstones ( M i d d l e T r i a s s i c ) o f South Devon ( E n g l a n d ) . Diameter o f f i g u r e a b t . 2.5 m. Rocks a t t h e shore between Crab Ledge and B l a c k Head between B u d l e i g h S a l t e r t o n and Sidmouth ( t o p o g r a p h i c a l map 1 : 25 000, sheet SY 08/18 Sidmouth, between r 08 580, h 82 620 and r 09 200, h 83 8 4 0 ) . 8 : O l d Red Sandstone o f N o r t h e a s t N o r t h u m b e r s h i r e (England). Diameter o f f i g u r e a b t . 2 m. Rocks a t t h e shore a t Cove Harbour n e a r Cockb u r n s p a t h . F o r f u r t h e r d e t a i l s o f t h e s e c t i o n s f i g u r e d c f . MADER (1985 a, 1985 b).
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3.13.11. Natural fracture origin and distribution in various reservoir rocks The f e a t u r e s f i q u r e d on p l a t e X I document n a t u r 1 f r a c t u r i n g due t o sedime t a r y d e s i c c a t i o n shrinkage, p l a n t r o o t p r o p a g a t i o n i n p a l a e o s o l s , v o l c a n i c coo l i n a c o n t r a c t i o n and t e c t o n i c a l e x t e n s i o n . and d i s t r i b u t i o n o f n a t u r a l . i o i n S i n ;elected r o c k monuments. F o l l o w i n g d i s c u s 6 i o n o f g e n e r a l aspects, e x p l a n a t i o n s o f t h e i l l u s t r a t i o n s on p l a t e X I a r e g i v e n , and g e o l o g i c a l f o r m a t i o n s and o u t c r o p l o c a l i t i e s o f t h e p r e s e n t e d examples a r e quoted.
3.13.11.1. General aspects J o i n t s and f a u l t s a r e t h e main elements o f t h e n a t u r a l f r a c t u r e system i n var i o u s r o c k t y p e s o f sedimentary and v o l c a n i c o r i g i n . The d i f f e r e n t elements t h a t a r e developed i n t h e i n d i v i d u a l r o c k s f o r m a g e o m e t r i c a l f a b r i c i n terms o f frequency, s i z e and d i r e c t i o n a l o r i e n t a t i o n which i n f l u e n c e s t h e p r o p a g a t i o n o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s t h r o u g h r e s e r v o i r f o r m a t i o n s i n t h e subsurf a c e . Outcrop s t u d i e s e n a b l e v i s u a l i z a t i o n o f t h e c o m p l e x i t y o f pay zone compos i t i o n b o t h i n terms o f d e p o s i t i o n a l s t r u c t u r e s and d e f o r m a t i o n a l f e a t u r e s such as c r a c k s .
3.13.11.2. Explanations of illustrations on plate XI 1 - 2 : B u n t s a n d s t e i n and R o t l i e g e n d r o c k monuments i n t h e landscape o f E i f e l and P f a l z as w e l l as o t h e r areas a r e t h e r e s u l t o f e r o s i o n a l s c u l p t u r i n g o f sandstone f o r m a t i o n s due t o t h e i r m a t r i x cementation and j o i n t f a b r i c . V e r t i c a l w a l l s a r e t h e consequence o f t h e predominance o f normal c r a c k s i n t e r s e c t i n g t h e r o c k s which d e r i v e f r o m s i m p l e e x t e n s i o n a l t e c t o n i c s . I n t e r b e d d i n g o f l a y e r s o f weaker and s t r o n g e r l i t h i f i c a t i o n ( w h i c h p a r t i a l l y c o r r e l a t e w i t h d e p o s i t i o n a l f a c i e s ) g i v e s r i s e t o o r i g i n o f t a b l e - o r mushroom-shaped r o c k monuments w h i c h are very prominent expressions o f the d i r e c t i o n a l s i g n i f i c a n c e of the n a t u r a l fractures. 3 : S l i d i n g f a u l t s of p a r t i a l l y s y n g e n e t i c o r i g i n i n v o l c a n i c t u f f beds ill u s t r a t e c o m p l i c a t e d morphology n o t o n l y i n c l u d i n g s t r a i g h t even planes, b u t a l s o c o m p r i s i n g c u r v e d and i r r e g u l a r s u r f a c e s . The p r o p a g a t i o n o f t h e f a u l t s i s s i m i l a r l y as t h a t o f h y d r a u l i c f r a c t u r e s c o n t r o l l e d by m a t e r i a l p r o p e r t y d i f f e rences between a d j o i n i n g l a y e r s o f t h e r o c k column, w i t h l i t h o l o g i c a l c o n t r a s t s between n e i g h b o u r i n g beds r e s u l t i n g i n d e f l e c t i o n and m o d i f i c a t i o n o f t h e penet r a t i n g c r a c k ( c f . s e c t i o n s 4.2.3.4. and 4 . 2 . 3 . 2 . 4 . ) . W h i l e some p a r t s o f t h e f a u l t are i n f i l l e d w i t h fine-grained sediment d e b r i s which i s moved a l o n g and/or across t h e f a u l t plane, o t h e r s e c t i o n s s t a y open. The d i f f e r e n t s t a t u s o f n a t u r a l p l u g g i n g o r p r o p p i n g o f f a u l t s g i v e s evidence o f complex c o n d u c t i v i t y d i s t r i b u t i o n o f h y d r a u l i c f r a c t u r e s p r o p a g a t i n g t h r o u g h heterogeneous r o c k piles. 4 : N a t u r a l c r a c k s w i t h o r i g i n a l wide opening i n sandstones and/or carbonat e s a r e i n many cases s e c o n d a r i l y i n f i l l e d w i t h l o o s e sediment d e b r i s r e s u l t i n g f r o m d e s t r u c t i o n o f g r a i n b i n d i n g a l o n g movement p l a n e s . A l t h o u g h such p l u g g i n g o r p r o p p i n g has i n many cases t h e adverse e f f e c t o f c o n s i d e r a b l y d e s t r o y i n g t h e p e r m e a b i l i t y o f t h e open f l o w channels i n t h e n a t u r a l cracks, the i n f i l l i n g s w i t h l o o s e sand can remain p r e f e r e n t i a l d r a i n a g e p a t h s i f n o t t o o much l i t h i f i e d i n l a t e r d i a g e n e t i c a l e v o l u t i o n and if b e i n g s u f f i c i e n t l y c o a r s e - g r a i n e d . The p r e s e n t example r e f l e c t s t h e u n d e r l i n i n g o f t h e weaker cohesion and p a r t i a l l y a l s o more f a v o u r a b l e g r a i n - s i z e d i s t r i b u t i o n by t h e c o n c e n t r a t i o n o f n e s t burrows o f r e c e n t s o l i t a r y bees w i t h i n t h e c r a c k p l u g g i n g i n c o n t r a s t t o t h e i r r a r i t y t o absence i n t h e s u r r o u n d i n g intensely-cemented m a t r i x ( c f . MADER 1980, 1984 c, 1985 d ) . I n terms o f c o n d u c t i v i t y , t h e d i g g i n g o f t u n n e l s and h o l e s
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330 i n t o t h e s l i g h t l y cohesive sand i n f i l l i n g t h e crack by t h e s o l i t a r y bees r e p r e sents organogenic a m e l i o r a t i o n of d r a i n a g e c o n d i t i o n s by i n c r e a s i n g p o r o s i t y and p e r m e a b i l i t y beyond t h e n a t u r a l boundaries. A s i m i l a r e f f e c t can be a c h i e ved i n p r o p p i n g o f h y d r a u l i c f r a c t u r e s by i n t e r m i x i n g s o l u b l e o r degradable l a r g e g r a i n s i n t o t h e p r o p p a n t package ( c f . s e c t i o n s 4.2.2.4. and 4 . 3 . 3 . 2 . ) , w i t h t h u s a s u i t e o f open patches o r i g i n a t i n g by d i s s o l u t i o n o r decomposition o f t h e u n s t a b l e a d d i t i v e s a f t e r placement o f t h e proppants i n t h e c r a c k .
5 - 6 : V o l c a n i c l a v a s a r e c h a r a c t e r i z e d by p o l y g o n a l columnar j o i n t p a t t e r n o r i g i n a t i n g f r o m c o o l i n g o f t h e f l o w s and p i p e i n f i l l i n g s and s h r i n k a g e c r a c k i n g o f t h e m a t r i x upon c o n t r a c t i o n d u r i n g s o l i d i f i c a t i o n . A l t h o u g h t h e n a t u r a l c o o l i n g f r a c t u r e system o f v o l c a n i c r o c k s seems t o be a v e r y homogeneous c r a c k network, h e t e r o g e n e i t y i s i n t r o d u c e d by v a r y i n g column d i a m e t e r as a consequence o f f l o w t h i c k n e s s , c u r r e n t z o n a t i o n , p i p e d i a m e t e r and morphology o f p i p e w a l l . W h i l e t h e columnar j o i n t s a r e almost p a r a l l e l t o each o t h e r i n e x t e n s i v e l a v a f l o w s of l i m i t e d t h i c k n e s s , p i p e i n f i l l i n g s a r e c h a r a c t e r i z e d by a r a d i a l - s y m m e t r i c a l o r i e n t a t i o n o f d i v e r g i n g o r c o n v e r g i n g columns, w i t h t h e i n d i v i d u a l columns a l l d e v e l o p i n g normal t o t h e c o o l i n g s u r f a c e . I n t h i c k e r l a v a f l o w s , t h e l o w e r and upper p a r t a r e h i g h l i g h t e d by l a r g e r column d i a m e t e r ( 5 ) , whereas the m i d d l e s e c t i o n e x h i b i t s s m a l l e r column d i a m e t e r accompanied by some c u r v a t u r e s i n p a r t s o f t h e c r a c k system ( 6 ) . Columnar j o i n t i n g o f v o l c a n i c l a vas as a consequence o f c o o l i n g s h r i n k a g e i s one o f t h e most n a r r o w l y - s p a c e d and more r e g u l a r l y developed n a t u r a l c r a c k systems and would be a v e r y s u i t a b l e s u p p o r t f o r h y d r a u l i c f r a c t u r e s and t h e i r e f f e c t i v i t y once t h e v o l c a n i c r o c k s are happening t o be gas-bearing.
7 : I n f i l l i n g o f d e s i c c a t i o n c r a c k s i n f i n e sandstones and mudstones by medium t o coarse sand r e p r e s e n t s p l u g g i n g o f t h e n a t u r a l f r a c t u r e s w i t h c o a r s e r g r a i n s i z e s t h a n those b u i l d i n g up t h e s u r r o u n d i n g h o s t r o c k ( c f . a l s o p l a t e V I / 5 - 7 ) . The g r a i n s i z e d i f f e r e n c e between t h e c o a r s e r c r a c k i n f i l l i n g and t h e f i n e r f o r m a t i o n m a t r i x c r e a t e s s i m i l a r l y as i n s y n t h e t i c p r o p p i n g o f hydraul i c f r a c t u r e s a c o n d u c t i v i t y c o n t r a s t ( c f . s e c t i o n s 4.5.1.1.2. and 4 . 6 . 1 . ) which g i v e s r i s e t o predominant d r a i n a g e o f p o r e s o l u t i o n s t h r o u g h t h e n a t u r a l l y propped d e s i c c a t i o n c r a c k d u r i n g course o f e a r l y d i a g e n e s i s and b u r i a l . I n l a t e r stages o f p o s t s e d i m e n t a r y e v o l u t i o n w i t h advanced subsidence, preferent i a l cementation o f t h e most permeable elements f r e q u e n t l y t u r n s t h e s i g n i f i cance o f t h e n a t u r a l f r a c t u r e p l u g s f r o m f l o w channels t o c o m m u n i c a t i o n - i n h i b i t ing barriers. 8 : F o s s i l i z e d v e r t i c a l r o o t tubes i n c a l c r e t e p a l a e o s o l s i n t h e Upper B u n t s a n d s t e i n a r e another element o r r e s e r v o i r h e t e r o g e n e i t y i n t e r r e s t r i a l r e d bed sandstones ( c f . a l s o p l a t e X / 1 - 5 ) . A l t h o u g h t h e s u r r o u n d i n g f o r m a t i o n rock m a t r i x has t o a l a r g e e x t e n t n o t y e t been m o d i f i e d i n submature p a l a e o sols, the concentration o f blue t o v i o l e t spots t e s t i f i e s t o i r o n oxide authigen e s i s t o g e t h e r w i t h c l a y m i n e r a l neoformation ( c f . MAOER 1983 c ) , and t h e pedot u r b a t i o n i n and around t h e r o o t tubes a l s o g i v e s r i s e t o t h e i r i n f i l l i n g w i t h l e s s permeable m a t e r i a l . T h e r e f o r e f o s s i l i z e d r o o t tubes i n submature p a l a e o s o l s a r e pronounced f l o w b a r r i e r s and i n case o f o c c u r r i n g as l a r g e bundles, t h e y can even a c t as elements d e f l e c t i n g and/or a t t e n u a t i n g m i g r a t i n g h y d r a u l i c f r a c t u r e s . P r o p a g a t i o n p r e s s u r e o f r o o t s as a consequence o f p l a n t growth and enlargement o f r o o t tubes i n l a t e r a l and v e r t i c a l d i r e c t i o n g i v e s r i s e t o spec i a l types o f n a t u r a l f r a c t u r e s i n t h e p a l a e o s o l h o r i z o n s . Concerning d r a i n a g e o f m i g r a t i n g s o l u t i o n s , t h e r o o t tube channel systems have comparable morphol o g y and f u n c t i o n t o rows o f p e r f o r a t i o n t u n n e l s t h a t a r e i n f i l l e d w i t h p a c k i n g g r a v e l i n mechanical sand c o n t r o l ( c f . s e c t i o n 5.6.1.).
3.13.11.3. Geological formations and outcrop localities of presented examples
331
r 08 620, h 51 630). 2 : Middle Buntsandstein of the Northern Saar area (Germany F R G ) . A l t f e l s rock monument a t the southern s i d e of the Pinschbach valley southeast of Kastel ( s h e e t 6405 Freudenburg, r 41 430, h 91 780). 3 : Quaternary volcanic t u f f s of Western Eifel (Germany FRG). Diameter of f i g . a b t . 4 m. Quarry a t the northern s i d e of the Goldberg northwest of Ormont south of Hallschlag (topographical map 1 : 100 000, sheet C 5902 Prum, r 32 900, h 77 900). 4 : Middle J u r a s s i c of the Deister Foreland (Germany F R G ) . Diameter of f i g u r e a b t . 0.6 m. Quarry a t the southern s i d e of the Stemmer Berg northeast of Stemmen near Hannover (topographical map 1 : 25 000, sheet 3623 Gehrden, r 36 040, h 01 850). 5 - 6 : Quaternary b a s a l t i c lavas of Western Eifel (Germany F R G ) . Diameter of f i g u r e s abt. 8 m each. 5 : Quarry a t the eastern s i d e of the Ruderbusch north of Oberbettingen ( s h e e t 5705 Gerolstein, r 44 500, h 73 000). 6 : Quarry a t the southern s i d e of the Ruderbusch north of Oberbettingen ( s h e e t 5705 Gerolstein, r 44 680, h 73 160). 7 : Old Red Sandstone (Devonian) of Wales (Great B r i t a i n ) . Diameter of f i g u r e a b t . 0.6 m. Rocks a t the shore around Milford Haven. 8 : Middle Buntsandstein of the Southern Saar area (Germany F R G ) . Diameter of f i g u r e a b t . 4 m. Quarry between S t . Ingbert and Neunkirchen. For f u r t h e r d e t a i l s of the sections figured c f . MAOER (1985 a, 1985 b). The New Red Sandstone (Permian Rotliegend and T r i a s s i c Buntsandstein) of South Devon (England) represents a p a r t i c u l a r l y pronounced example of d i v e r s i f i cation of intertonguing of d i f f e r e n t depositional environments in time and space during course of the evolution of f l u v i a l s t y l e ( c f . tab. 1 0 ) . The New Red Sandstone incorporates an e x t r a o r d i n a r i l y well-developed red bed succession with almost continuous environmental t r a n s i t i o n between Permian Rotliegend and T r i a s s i c Buntsandstein. General d i s t i n c t i o n can be made between f l u v i a l braidplain, f l u v i a l floodplain and playa lake, a l l u v i a l - f a n , aeolian and palaeosol f a c i e s associations which have d i f f e r e n t d i s t r i b u t i o n and significance in the various s t r a t i g r a p h i c u n i t s . Aeolian sandstones form a broader o r narrower b e l t in f r o n t of the a l l u v i a l - f a n chain seaming the erosional source area and prograde o r r e t r e a t as a consequence of extension o r contraction of the marginal fan lobe row o r the axial t o d i s t a l braidplain. The New Red Sandstone comprising Permian Rotliegend and T r i a s s i c Buntsands t e i n c o n s i s t s of three magnacycles t h a t in turn c o n s i s t of various megacycles and submegacycles. The f i r s t magnacycle comprising the Lower Permian originated in very a r i d t o a r i d climate and was characterized by predominantly ephemeral flow. Alluvial-fan complexes fringing the borders of the highlands were very widespread. With the exception of r i v e r s reworking aeolian dunes, a f l u v i a l braidplain and floodplain system was not developed. The dunes formed moderately t o very extensive aeolian sand seas, and c a l c r e t e palaeosols could not be generated as a consequence of the dry and hot climate. The second magnacycle includes the Upper Permian which originated in a r i d t o semi-arid climate under condit i o n s of more perennial flow. I n c o n t r a s t t o the Lower Permian, no a l l u v i a l - f a n chains existed any more, b u t a f l u v i a l braidplain and floodplain t o playa lake system was developed. Dunes were r e s t r i c t e d t o small to moderately extensive aeolian bedform f i e l d s . Moderate amounts of c a l c r e t e palaeosols formed predominantly in subaqueous o r ground milieu, and Brockelbank reworking breccias of c a l c r e t e palaeosols are unknown. The t h i r d magnacycle comprising the Lower T r i a s s i c Buntsandstein was l a i d down in a r i d and semi-arid to a r i d climate u n der conditions of more perennial than ephemeral flow. An extensive f l u v i a l braidplain and floodplain t o playa lake system was developed, whereas a l l u v i a l fan lobes were no longer b u i l t . Moderately extensive aeolian dune f i e l d s e x i s ted in the desiccated p a r t s o f the f l u v i a l braidplain. Large amounts of c a l Crete palaeosols of mainly subaerial type originated under favourable c l i m a t i c conditions, a n d numerous Brockelbank reworking breccias t e s t i f y t o abundant des t r u c t i o n of autochthonous pedogenic carbonates by migrating r i v e r channels and redeposition of the fragments t o c h a r a c t e r i s t i c reworking horizons ( c f . MADER 1985 b).
STRATIGRAPHY AND SEDIMENTOLOGY SS T
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Tab. 10
Stratigraphy and sedimentology of the New Red Sandstone (Permian Rotliegend and Triassic Buntsandstein) in South Devon (England). Stratigraphic subdivision and lithofacies log are not in scale to the thickness of the sequences. The New Red Sandstone in South Devon is an extraordinarily well-developed example of continental red bed facies complexes in Rotliegend and Buntsandstein and is characterized by an intertonguing of different sedimentary environments in time and space.
333
4 . H y d r a u l i c 4.0.Summary
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H y d r a u l i c p r o p p a n t f r a c t u r i n g i n Europe has so f a r been m a i n l y performed i n deep t i g h t g a s - b e a r i n g R o t l i e g e n d and C a r b o n i f e r o u s sandstones i n Germany FRG and N e t h e r l a n d s onshore as w e l l as i n t h e B r i t i s h and Dutch Southern N o r t h Sea o f f s h o r e . O t h e r h i t h e r t o a p p l i c a t i o n s o f h y d r a u l i c f r a c t u r i n g and g r a v e l packi n g comprise p r e d o m i n a n t l y o i l - b e a r i n g Cretaceous c h a l k s and J u r a s s i c sandstones i n t h e Norwegian and B r i t i s h N o r t h e r n N o r t h Sea, Cretaceous sandstones i n Germany FRG and Netherlands, and T e r t i a r y sandstones i n Yugoslavia, Hungary, A u s t r i a , Czechoslovakia and I t a l y . P o s s i b i l i t i e s o f f u r t h e r p r o m o t i o n o f hydraul i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n Europe i n c l u d i n g approach o f var i o u s m a r g i n a l r e s e r v o i r s i n o r d e r t o achieve a d d i t i o n a l c o n t r i b u t i o n s o f hydrocarbon o u t p u t f o r compensation o f d e c l i n i n g p r o d u c t i o n f r o m c o n v e n t i o n a l depos i t s i n t h e n e a r f u t u r e a r e o u t l i n e d by d i s c u s s i n g v a r i o u s t e c h n o l o g i c a l ways o f enhancing t h e s t i m u l a t i o n p o t e n t i a l i n Europe. The d i s c u s s i o n o f v a r i o u s t e c h n o l o g i c a l aspects i n c l u d e s containment of f r a c t u r e p r o p a g a t i o n i n t h i n sandstones, p r o p p a n t s a t u r a t i o n and c o n d u c t i v i t y improvement, t a r g e t s o f unconvent i o n a l gas s t i m u l a t i o n , h y d r a u l i c p r o p p a n t f r a c t u r i n g o f carbonate rocks, h i g h p e r m e a b i l i t y r e s e r v o i r f r a c t u r i n g , geothermal f r a c t u r i n g , m i n i f r a c t u r i n g and other small-scale f r a c t u r i n g , o i l - r e s e r v o i r fracturing, communication f r a c t u r i n g , f r a c t u r i n g o f d e v i a t e d w e l l s , f r a c t u r e damage, and a g g r e s s i v e f r a c t u r i n g d e s i g n . Containment o f f r a c t u r e p r o p a g a t i o n i n t h i n sheet sandstones w i t h i n sandwich-type mudstone-sandstone-interbeddings w i l l become i n c r e a s i n g l y import a n t f o r t h e s t i m u l a t i o n o f o i l - and gas-bearing sandstones i n v a r i o u s p a r t s o f Western and E a s t e r n Europe. I m p o r t a n t aspects o f containment o f f r a c t u r e propag a t i o n a r e s i n g u l a r vs. m u l t i p l e f r a c t u r i n g and d i v e r t i n g techniques, f r a c t u r e morphology and s t r e s s s t a t e , simultaneous e x t e n s i o n o f m u l t i p l e f r a c t u r e s i n v e r t i c a l and h o r i z o n t a l w e l l s , t r e a t m e n t parameter adjustment, l i m i t e d - e n t r y fracturing, i n - s i t u s t r e s s c o n t r a s t and d i s t r i b u t i o n , and s e l e c t i v e p r o p p a n t placement by d i v e r t i n g and f l u i d d e n s i t y c o n t r o l . F r a c t u r e p r o p a g a t i o n i s char a c t e r i z e d along the l i n e s o f i n - s i t u s t r e s s v a r i a t i o n s w i t h r e s e r v o i r l i t h o l o gy, pay e l a s t i c i t y vs. p l a s t i c i t y , impact o f weakness p l a n e s and i n t e r f a c e s , shear s l i p p a g e and t e n s i l e f a i l u r e , i n f l u e n c e o f g e o l o g i c a l d i s c o n t i n u i t i e s , and r e s e r v o i r h e t e r o g e n e i t i y and a n i s o t r o p y . Proppant s a t u r a t i o n enhancement o f f r a c t u r i n g f l u i d s by s u c c e s s i v e achievement o f f i n a l l y u l t r a - h i g h c o n c e n t r a t i o n s a l l o w s t o i n c r e a s e f r a c t u r e w i d t h and t o o p t i m i z e t a i l - i n p o l i c i e s by u s i n g d i f f e r e n t p r o p p a n t t y p e s and g r a i n s i z e s i n v a r i o u s q u a n t i t a t i v e r e l a t i o n s h i p s i n t h e sequence o f s t e p s o f t h e t r e a t m e n t . Proppant s a t u r a t i o n and conduct i v i t y improvement i s sketched a l o n g t h e p o i n t s o f p r o p p a n t embedment and c r u s h i n g vs. f r a c t u r e w i d t h and number o f p r o p p a n t l a y e r s , p r o p p a n t p i l l a r i n g and banking, f r a c t u r e w i d t h vs. p r o p p a n t b r i d g i n g , proppant t r a n s p o r t and f r a c t u r e damage, f l u i d breakdown and r e s i d u e c o n c e n t r a t i o n , f l u i d v i s c o s i t y and c a r r i a g e a b i l i t y , impact o f c o n v e n t i o n a l and delayed f l u i d c r o s s l i n k i n g , f l u i d h e a t capac i t y and r e s e r v o i r temperature, and f l u i d l e a k o f f containment. U l t r a h i g h f r a c t u r e c o n d u c t i v i t y can be achieved by e x t r a o r d i n a r i l y l a r g e p r o p p a n t g r a i n s i z e s as w e l l as by m i x i n g o f round and a n g u l a r p r o p p a n t g r a i n s . The main t a r g e t s o f u n c o n v e n t i o n a l gas s t i m u l a t i o n a r e t i g h t gas sandstones, geopressured r e s e r v o i r s c o n t a i n i n g methane t h a t i s d i s s o l v e d i n b r i n e , c o a l seams, s h a l e s and high-temperature r e s e r v o i r rocks. Hydraulic proppant f r a c t u r i n g o f n o t o n l y sandstones, b u t a l s o o f carbonate r o c k s i s a m a j o r f i e l d o f p o s s i b l e s t i m u l a t i o n a c t i v i t y improvement i n Europe where t h e p o l i c y of enhancing t h e c o n d u c t i v i t y c o n t r a s t between f r a c t u r e and f o r m a t i o n r e g a r d l e s s o f r e s e r v o i r l i t h o l o g y i s s t i l l much l e s s common t h a n i n t h e USA, and t h e same a p p l i e s f o r high-permeab i l i t y pay f r a c t u r i n g where t h e l a r g e s u i t e o f d i f f e r e n t p r o p p a n t t y p e s and g r a i n s i z e s a l l o w s a wide spectrum o f v a r i a t i o n s o f t h e c o n d u c t i v i t y g r a d i e n t between f r a c t u r e and f o r m a t i o n i r r e s p e c t i v e o f t h e p e r m e a b i l i t y o f t h e l a t t e r . H y d r a u l i c p r o p p a n t f r a c t u r i n g o f carbonate r o c k s i s o u t l i n e d by commenting on enhancement o f d r a i n a g e p a t h and c o n d u c t i v i t y c o n t r a s t , and advantages and draw-
334 backs o f f r a c t u r e p r o p p i n g . Combined p r o p p a n t and a c i d f r a c t u r i n g i s i l l u m i n a t e d by f o c u s s i n g on c o n d u c t i v i t y improvement by a c i d o p t i m i z a t i o n , enllanc_.,ient o f a c i d p e n e t r a t i o n d i s t a n c e , open vs. propped f r a c t u r e s , and a c i o f l u i d - l o s s c o n t r o l . Chalk i s a s p e c i a l t y p e o f carbonate r e s e r v o i r where sometimes combinat i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i s t h e o n l y p o s s i b i l i t y o f e c o n o m i c a l l y f e a s i b l e s t i m u l a t i o n f o r p r o f i t a b l e o i l and gas w i t h d r a w a l . P a r t i c u l a r c h a l k f r a c t u r i n g t e c h n i q u e s a r e t i p -:reenout, i n t e r f a c e , whisper and d e n d r i t i c f r a c t u r i n g . I n a d d i t i o n t o p r o p p a n t f r a c t u r i n g , t h e p r o d u c t i v i t y o f some sandstone r e s e r v o i r s can be a l s o s u c c e s s f u l l y i n c r e a s e d by h y d r o f l u o r i c a c i d f r a c t u r i n g and/or m a t r i x t r e a t m e n t s , p a r t i a l l y i n combinAtion w i t h convent i o n a l f r a c t u r i n g i f proppants t h a t a r e n o t a t t a c k e d by t h e h y d r o f l u o r i c a c i d a r e s e l e c t e d . I m p o r t a n t aspects o f sandstone a c i d i z i n g a r e a c i d r e a c t i o n , a c i d p e n e t r a t i o n d i s t a n c e , a c i d t y p e s and d i s s o l u t i o n p o s s i b i l i t i e s , and a c i d i z i n g t a r g e t s . S i m i l a r l y as f o r carbonate f r a c t u r i n g , t h e most i m p o r t a n t goal o f moder a t e - t o h i g h - p e r m e a b i l i t y sandstone h y d r a u l i c s t i m u l a t i o n i s achievement o f s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r m a t r i x and propped f r a c t u r e . Geothermal f r a c t u r i n g i s a s p e c i a l a p p l i c a t i o n o f p r o p p a n t s t i m u l a t i o n where a l s o communication f r a c t u r i n g f r o m one source w e l l t o another t a r g e t w e l l i n l i n e w i t h the geostress f i e l d provides considerable p o s s i b i l i t i e s o f f u t u r e prom o t i o n o f h y d r a u l i c t r e a t m e n t . A s a consequence o f e x t r e m e l y h i g h r e s e r v o i r temp e r a t u r e s , w a t e r i s t h e most a b u n d a n t l y used f r a c t u r i n g f l u i d i n geothermal s t i m u l a t i o n . Comments a r e a l s o o f f e r e d on e x p l o s i v e non-proppant v s . h y d r a u l i c proppant f r a c t u r i n g , stem-induced e x p l o s i v e s t i m u l a t i o n , n u c l e a r f r a c t u r i n g , and c r a c k system enlargement by h e a t e x t r a c t i o n . M i n i f r a c t u r i n g (mHF) and r e l a ted small-scale f r a c t u r i n g w i t h l i m i t e d q u a n t i t i e s o f propping m a t e r i a l s plugg i n g s h o r t f r a c t u r e s i n t h e v i c i n i t y o f t h e w e l l b o r e i s an i m p o r t a n t t o o l f o r t h e stepwise approach o f many m a r g i n a l r e s e r v o i r s , w i t h some o f t h e main a p p l i c a t i o n s b e i n g sandwich-type sandstone sheets, u n c o n s o l i d a t e d f l a s e r y sandstones, s o f t c h a l k s , carbonate s h e l l l a y e r s and i s o l a t e d sandstone l e n s e s and chann e l s i n mudstone sequences, l o w - p e r m e a b i l i t y patches and s t r i n g s i n high-permeab i l i t y r e s e r v o i r s o f any l i t h o l o g y , and a l s o t h e c o n v e n t i o n a l way o f b r e a k i n g t h r o u g h f o r m a t i o n damage around t h e b o r e h o l e . D i s t i n c t i o n has t o be made b e t ween m i n i f r a c t u r i n g , m i c r o f r a c t u r i n g and MHF s t i m u l a t i o n , w i t h a t r a n s i t i o n a l i t e m between t h e l a t t e r b e i n g t h e mini-massive f r a c t u r e . O t h e r t a r g e t s o f m i n i f r a c t u r i n g a r e a r t i f i c i a l enhancement o f n a t u r a l f r a c t u r e s , i n c r e a s e o f t h e s u r f a c e around t h e w e l l b o r e f o r uptake o f i n j e c t i o n f l u i d s and gases, c o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r e s , r e f r a c t u r i n g o f w e l l s where o l d e r c r a c k s have c o l l a p s e d due t o c l o s u r e by c r u s h i n g o f t h e f o r m e r l y i n s e r t e d sand, combin a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g , and c u t t i n g s t i m u l a t i o n c o s t o f MHF oper a t i o n s by a mHF t r e a t m e n t b e i n g t h e f i r s t small s t e p b e f o r e t h e second b i g one ( t h e MHF j o b ) , w i t h t h e l a t t e r b e i n g no l o n g e r necessary i n case o f a s u f f i c i e n t r e s u l t o f t h e f o r m e r . The o u t l i n e i n c l u d e s d i s c u s s i o n o f p r e - f r a c t u r e p r e s s u r e breakdown t e s t and f r a c t u r e parameter d e t e r m i n a t i o n , c a l i b r a t i o n exper i m e n t s and i n - s i t u s t r e s s d e t e r m i n a t i o n , p r o p p a n t vs. non-proppant m i n i f r a c t u r i n g , f o r m a t i o n damage around t h e w e l l b o r e , and mHF vs. MHF. H o r i z o n t a l w e l l d r i l l i n g and s t i m u l a t i o n i s sketched a l o n g t h e l i n e s o f improved n a t u r a l f r a c t u r e i n t e r s e c t i o n , d i s t a n c e f r o m f l u i d c o n t a c t s , r e d u c t i o n o f coning, s e l e c t i o n o f d r a i n a g e and i n j e c t i o n d i r e c t i o n , s i n g l e - vs. m u l t i p l e - l a y e r r e s e r v o i r s , pay t h i c k n e s s and p e r m e a b i l i t y . One o f t h e most common combinations o f h o r i z o n t a l w e l l s and h y d r a u l i c f r a c t u r i n g i s m u l t i p l e v e r t i c a l f r a c t u r i n g o f h i g h l y - i n c l i ned b o r e h o l e s . Uptake c a p a c i t y enhancement o f o i l - f i e l d w a t e r - i n j e c t i o n w e l l s has t o d i f f e r e n t i a t e between c o n t r o l l e d and a c c i d e n t a l f r a c t u r i n g . Combination o f n a t u r a l and a r t i f i c i a l f r a c t u r e s i s i l l u s t r a t e d a l o n g t h e p o i n t s o f f r a c t u r e i n t e r f e r e n c e and s u p e r i m p o s i t i o n , i n t e r f r a c t u r e commmunication, f r a c t u r e d i v e r gence, temporary n a t u r a l f r a c t u r e damage, d e n d r i t i c f r a c t u r i n g , and p r o p p a n t plugging o f n a t u r a l f r a c t u r e s . Refracturing o r late-stage o r i g i n a l f r a c t u r i n g o f o l d w e l l s i n advanced d e p l e t i o n h i s t o r y i s c h a r a c t e r i z e d a l o n g t h e i t e m s o f c a n d i d a t e w e l l choice, m o b i l i z a t i o n o f a d d i t i o n a l hydrocarbon r e s e r v e s , p r o p p a n t s e l e c t i o n , f r a d t u r e r e o p e n i n g and p r o p p a n t placement d u r i n g r e f r a c t u r i n g , and c o m b i n a t i o n o f h y d r a u l i c and e x p l o s i v e f r a c t u r i n g . Combination o f a c i d and p r o p p a n t f r a c t u r i n g i s c h a r a c t e r i z e d by p r o p p a n t d i s s o l u t i o n , and t r e a t m e n t schedule and a c i d improvement. F r a c t u r e l e n g t h and h e i g h t o p t i m i z a t i o n a r e s k e t -
335 ched a l o n g t h e l i n e s r e s e r v o i r p e r m e a b i l i t y and w e l l s p a c i n g vs. f r a c t u r e l e n g t h , f r a c t u r e c o n d u c t i v i t y vs. f r a c t u r e l e n g t h , f r a c t u r e f l o w c a p a c i t y vs. p r o p p a n t d i s t r i b u t i o n , and n e t p r e s e n t v a l u e o p t i m i z a t i o n . S p e c i a l a p p l i c a t i o n s o f h y d r a u l i c s t i m u l a t i o n i n o i l r e s e r v o i r s i s c o m b i n a t i o n o f h o r i z o n t a l and v e r t i c a l f r a c t u r i n g i n s t e a m - d r i v e o p e r a t i o n s where f o r m a t i o n h e a t i n g t r i g g e r s changing g e o s t r e s s regime by thermal pay expansion. Steam-drive e f f e c t s on f r a c t u r e p r o p a g a t i o n a r e i l l u s t r a t e d by commenting on s t r e s s i n v e r s i o n , c y c l i c f r a c t u r e c o l l a p s e and opening, f r a c t u r e c r e a t i o n and management, and changes o f f r a c t u r e azimuth o r i e n t a t i o n . O t h e r p o s s i b i l i t i e s o f changing s t r e s s d i r e c t i o n a r e a l t e r e d s t r e s s f r a c t u r i n g , and r e s e r v o i r temperature and f r a c t u r e g r a d i e n t r e d u c t i o n d u r i n g c o l d w a t e r i n j e c t i o n i n c o n v e n t i o n a l w a t e r - f l o o d p r o j e c t s . Comm u n i c a t i o n f r a c t u r i n g has p a r t i c u l a r s i g n i f i c a n c e i n a p p r a i s a l d r i l l i n g t o cont a i n d i r e c t i o n a l s i d e t r a c k i n g expenses by c o n n e c t i n g t h e b o r e h o l e w i t h prospect i v e r e s e r v o i r f a c i e s i n case o f marked basement topography and r a p i d l y chang i n g f a c i e s b e l t s , w i t h c a u t i o n h a v i n g t o be e x e r c i s e d t o a v o i d f r a c t u r e i n t e r f e r e n c e between n e i g h b o u r i n g w e l l s . I n t i g h t gas sandstones o f l e n t i c u l a r i n s t e a d o f b l a n k e t type, c o n n e c t i o n o f t h e s e p a r a t e sand bodies w i t h t h e i r n e i g h bours and t o t h e w e l l b o r e by communication f r a c t u r i n g i s t h e o n l y p o s s i b i l i t y f o r s u c c e s s f u l and f e a s i b l e a c q u i s i t i o n o f gas r e s e r v e s i n many e c o n o m i c a l l y m a r g i n a l cases. The p a r t i c u l a r advantages o f h i g h - q u a l i t y s y n t h e t i c proppants f o r enhancing t h e p r o d u c t i v i t y o f s h a l l o w o i l r e s e r v o i r s even below t h e c r i t i c a l c l o s u r e s t r e s s f o r n a t u r a l sand a r e based on t h e much b e t t e r c o n d u c t i v i t y and t h u s f r a c t u r e f l o w c a p a c i t y p r o v i d e d by t h e premium m a t e r i a l . The main drawbacks o f f r a c t u r e p r o p a g a t i o n and o r i e n t a t i o n i n d e v i a t e d w e l l s a r e mismatch o f w e l l b o r e a x i s and i n - s i t u p r i n c i p a l s t r e s s d i r e c t i o n , and l i m i t e d communication i n t e r v a l between f r a c t u r e p l a n e and w e l l b o r e s e c t i o n . O t h e r e f f e c t s i n d e v i a t e d - w e l l f r a c t u r i n g a r e p r o p p a n t s t r a t i f i c a t i o n and f l u i d l a y e r i n g , and hydrocarbon p r o d u c t i o n and f l o w regimes. I n terms o f f r a c t u r e and b o r e h o l e geometry, t h e s i t u a t i o n o f h i g h l y - d e v i a t e d w e l l s c r o s s i n g g e n t l y i n c l i n e d pays i s compar a b l e t o t h a t o f v e r t i c a l boreholes t r a v e r s i n g steeply dipping r e s e r v o i r s . Fract u r e damage a f t e r t h e placement o f t h e proppants i n t h e c r a c k can o c c u r by s e t t l i n g , flowback and c r u s h i n g o f proppants i n case o f t o o r a p i d p r e s s u r e drawdown d u r i n g c l e a n - u p and e a r l y p r o d u c t i o n . Proppant s e t t l i n g aspects a r e d i s c u s s e d a l o n g t h e l i n e s o f p o s t - f r a c t u r e p r e s s u r e d e c l i n e , p r o p p a n t t r a n s p o r t and banki n g , c a r r i e r f l u i d v i s c o s i t y , s l u r r y c o n s i s t e n c y and p r o p p a n t c l u s t e r i n g . Propp a n t flowback i s sketched by commenting on f r a c t u r e c l o s u r e and p r o p p a n t t r a p p i n g , p r o p p a n t flowback c o n t r o l and d i m i n u t i o n , e f f e c t s o f f r a c t u r i n g f l u i d and c a p i l l a r y p r e s s u r e on w e l l cleanup, p r o p p a n t g r a i n s i z e , and r e s i n - c o a t e d p r o p p a n t t a i l - i n . Proppant c r u s h i n g i s o u t l i n e d by i l l u m i n a t i n g p r o p p a n t shock l o a d i n g , p r e s s u r e drawdown and hydrocarbon p r o d u c t i o n r a t e s , p r o d u c t i o n c y c l i n g by repeated w e l l shut-in, and c o m b i n a t i o n o f proppant embedment and c r u s h i n g . I n view o f t h e n e c e s s i t y t o i n c r e a s e t h e r e c o v e r a b l e r e s e r v e s o f p a r t i c u l a r l y gas i n b o t h Western and E a s t e r n Europe i n t h e coming years, recommendation i s made t h a t an a g g r e s s i v e f r a c t u r i n g d e s i g n which approaches t h e boundaries o f t e c h n i c a l p o s s i b i l i t i e s and i n e v i t a b l y i n c l u d e s t h e r i s k o f some f a i l u r e s s h o u l d be c a r r i e d o u t , i n o r d e r t o c o n t i n u e p r o g r e s s and n o t l a g b e h i n d b y s t i c k i n g t o c o n v e n t i o n a l o r c o n s e r v a t i v e schemes, w i t h o u t , however, p r o v o k i n g a s c r e e n o u t i n e v e r y t r e a t m e n t . C o n c l u s i o n i s made t h a t p r o v i d e d an a g a i n b e t t e r g e n e r a l economical framework o f a h i g h e r o i l p r i c e and p a r t i a l l y a l s o a s t r o n g e r US $, b o t h Western and E a s t e r n Europe w i l l be growing s t i m u l a t i o n markets w i t h i n c r e a s i n g demand o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i n c l u d i n g promot i o n o f t r e a t m e n t s i n m a r g i n a l r e s e r v o i r s i n t h e near f u t u r e . 4.1.
356 I n t r o d u c t i o n ........................................................ 4.1.1. P o s s i b i l i t i e s o f expanding t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g market i n t h e near f u t u r e ................................... 356 4.1.2. O u t l i n e o f t h e t e c h n i c a l r e v i e w and s t a t u s r e p o r t ........... 356
4.2.
Containment o f f r a c t u r e propagation i n t h i n sandstones .............. 357 4.2.1. A r e a l d i s t r i b u t i o n of p o t e n t i a l r e s e r v o i r s .................. 358 4.2.2. T e c h n i c a l aspects o f f r a c t u r i n g o p e r a t i o n ................... 358 4.2.2.1. S i n g u l a r v s . m u l t i p l e f r a c t u r i n g and d i v e r t i n g
336 techniques 4.2.2.1.1.
4.2.2.2.
4.2.2.3.
........................................
359 D i v e r t i n g a g e n t p e r f o r m a n c e _ . _ . _ _ _359 _.._ 4.2.2.1.1.1. Advantaqes and drawbacks o f b a l l - s e a l e r s . . . . . . . . . 360 I n j e c t i o n r a t e vs. t r e a t 4.2.2. 1.1.2. ment s t a g i n g . . . . . . . . . . . . 360 F l o a t i n g and s i n k i n g b a l l 4.2.2 1.1.3. b a l l s e a l e r s . . . . . . . . . . . . 361 T r e a t m e n t sequence i n 4.2.2 1.1.4. mu1 t i s t a g e r e s e r v o i r s . . . 361 I n s e r t i o n and r e m o v a l o f 4.2.2 1.1.5. b l o c k i n g m a t e r i a l s . . . . . . 361 Wax-based o e l l e t s . . . . . . . 362 4.2.2 1 . 1 . 6 . 4.2.2.1.2. D i v e r i n g a g e n t s e l e c t i o n . . . . . . . . . . . . . 362 4.2.2.1.3. M u l t i p l e f r a c t u r i n g a s p e c t s . . . . . . . . . . 362 4.2.2..1.3.1. F r a c t u r e . m o r p h o l o g y . . . . 363 4.2.2.1.3.2. F r a c t u r e number and s t r e s s s t a t e .......... . 364 4.2.2.1.3.3. T a i l o r e d p u l s e l o a d i n g . 364 4.2.2.1.3.3.1. Intermed ate stra n r a t e s . . . 364 4.2.2.1.3.3.2. Multiple radial c r a c k s . . . 365 4.2.2.1.3.3.3. Other a s p e c t s . . 365 4.2.2.1.3.4. C o n t r o l l e d vs. convention a l e x p l o s i v e f r a c t u r i n g 366 4.2.2.1.3.5. Simultaneous propagation o f m u l t i p l e f r a c t u r e s . . . 367 4.2.2.1.3.6. O t h e r t e c h n i q u e s and e f f e c t s . . . . . . . . . . . . . . . . . . . 368 L i m i t e d - e n t r y f r a c t u r i n g t e c h n i q u e . . . . . . . . . . . . . . . . 368 4.2.2.2.1. Simultaneous f l u i d e n t r y i n t o m u l t i p l e 369 zones ................................. 4.2.2.2.2. P r e s s u r e d r o p a c r o s s t h e p e r f o r a t i o n s . 369 4.2.2.2.2.1. F l u i d volume and p e r f o r a t i o n number . . . . . . . . . . . . . 370 4.2.2.2.2.2. S h e a r d e g r a d a t i o n and f l u i d v i s c o s i t y . . . . . . . . . 370 4.2.2.2.3. I n f l u e n c e o f s c r e e n o u t s . . . . . . . . . . . . . . . 370 4.2.2.2.4. F l u i d d i s t r i b u t i o n and p e r f o r a t i o n a r 371 rangement ............................. F r a c t u r e containment by treatment parameter a d j u s t 371 ment .............................................. Factors influencing fracture contain4.2.2.3.1 ment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 4.2.2.3.1.1. Fracture orientation changes . . . . . . . . . . . . . . . . . 372 4.2.2.3.1.2. F r a c t u r e geometry v a r i a t i o n s ................... 373 4.2.2.3.1.3. C o n f i n i n g s t r e s s c o n t r a s t 373 4.2.2.3.1.4. R e s e r v o i r d e p l e t i o n and s t r e s s c o n t r a s t m a g n i t u d e 373 4.2.2.3.1.5. P o s i t i v e and n e g a t i v e f r a c t u r e c o n t a i n m e n t . . . . 374 4.2.2.3.1.6. Fracture growth terminat i o n a t i n t e r f a c e s . . . . . . 374 374 I n j e c t i o n r a t e s ....................... 4.2.2.3.2 4.2.2.3.2.1. D e c r e a s i n g pumping r a t e and t o t a l j o b s i z e . . . . . . 375
337 4.2.2.3.2.2.
4.2.3.
Pressure growth and screenout t e r m i n a t i o n ... 375 F r a c t u r e h e i g h t and f l u i d 4.2.2.3.2.3. d i s t r i b u t i o n ........_... 376 Tubular enlargement and 4.2.2.3.2.4. horseDower d i m i n u t i o n ... 376 377 4.2.2.3.3. F l u i d p r o p e r t i e s ...................... 4.2.2.3.3.1. Proppant d e n s i t y and f l u i d v i s c o s i t y ......... 377 4.2.2.3.3.2. Foam and enerqized f l u i d s 377 4.2.2.3.3.3. Densi t y - c o n t r o l l e d f 1 u i d s 378 4.2.2.3.4. Propped f r a c t u r e h e i g h t vs. r e s e r v o i r thickness ............................. 378 4.2.2.3.5. P e r f o r a t i o n i n t e r v a l s e l e c t i o n ........ 378 4.2.2.4. F r a c t u r e containment by buoyant and s e t t l i n g d i v e r t e r s and o t h e r b l o c k i n g a d d i t i v e s ................. 379 4.2.2.4.1. General aspects ....................... 379 4.2.2.4.2. Buoyant d i v e r t e r s ..................... 379 4.2.2.4.3. S e t t l i n g d i v e r t e r s . . . . . . . . . . . . . . . . . . . . 380 4.2.2.4.4. F l u i d v i s c o s i t y and d i v e r t e r d e n s i t y .. 380 4.2.2.4.5. Other b l o c k i n g a d d i t i v e s .............. 381 382 4.2.2.4.6. Pressure d i v e r s i o n .................... 4.2.2.5. M u l t i p l e - z o n e f r a c t u r e m i g r a t i o n . . . . . . . . . . . . . . . . . . 382 382 4.2.2.5.1. General aspects ....................... 4.2.2.5.2. Continuous survey and m o d e l l i n g ....... 382 4.2.2.5.3. V e r t i c a l f r a c t u r a b i l i t y d i f f e r e n c e s ... 383 4.2.2.5.3.1. F r a c t u r e o r i e n t a t i o n and movement ................ 383 4.2.2.5.3.2. I n - s i t u stress differences w i t h f a c i e s type . . . . 384 4.2.2.6. S e l e c t i v e proppant placement ...................... 384 4.2.2.6.1. Accidental proppant f a l l o u t and banking 385 4.2.2.6.2. C o n t r o l l e d proppant placement by f l u i d d e n s i t y r e g u l a t i o n .................... 385 4.2.2.6.3. Proppant s t r a t i f i c a t i o n and zonation . . 386 4.2.2.6.4. F l u i d d e n s i t y c o n t r o l and proppant d i s t r i b u t i o n ............................. 386 4.2.2.7. Special phenomena i n coal seams ................... 387 387 4.2.2.8. Other aspects ..................................... F r a c t u r e propagation ........................................ 388 388 4.2.3.1. Terminology and nomenclature ...................... 4.2.3.2. F a c t o r s c o n t r o l l i n g f r a c t u r e propagation .......... 389 4.2.3.2.1. I n - s i t u s t r e s s c o n t r a s t and o t h e r i n f l u e n c i n g f a c t o r s ..................... 389 4.2.3.2.2. H o r i z o n t a l s t r e s s d i f f e r e n c e s and f r a c t u r e geometry ......................... 390 4.2.3.2.2.1. Stress v a r i a t i o n s w i t h l i t h o l o g y ............... 390 4.2.3.2.2.2. Natural fracture d i s t r i b u t i o n i n v a r i o u s rocks . 391 4.2.3.2.2.3. I n - s i t u s t r e s s c o n t r a s t s 391 4.2.3.2.2.4. F r a c t u r e i n s t a b i l i t y and movement ................ 391 4.2.3.2.3. R e s e r v o i r e l a s t i c i t y vs. p l a s t i c i t y ... 392 4.2.3.2.3.1. Distribution of plasticit y zones i n the r e s e r v o i r 392 4.2.3.2.3.2. P l a s t i c i t y vs. n o n - l i n e a r e l a s t i c i t y .............. 393 4.2.3.2.3.3. P l a s t i c i t y e f f e c t s on hyd r a u l i c f r a c t u r i n g ...... 393 4.2.3.2.4. F r a c t u r e propagation across weakness planes ................................ 394
338 P r e m a t u r e s c r e e n o u t t e r m i n a t i o n . . . . . . . 394 4.2.3.2.5.1. Gel d e g r a d a t i o n . . . . . . 395 4.2.3.2.5.2. Insufficient fracture width ....... 395 4.2.3.2.5.3. I m p a c t of 100 mes l e a d - i n s t a g e s .......... 396 4.2.3.2.6. S t r e s s d i f f e r e n c e s and c o n t a i n m e n t 396 4.2.3.2.6.1. F l u i d breakthrough vs . i n j e c t i o n pressure .. 396 4.2.3.2.6.2. P r o p p a n t b r i d g i n g vs . barrier stability 397 4.2.3.2.6.3. O t h e r a s p e c t s . . . . . . . . . . . 397 4.2.3.3. Shear s l i p p a g e and t e n s i l e f a i l u r e . . . . . . . . . 397 4.2.3.3.1. J o i n t d e f o r m a t i o n p r i n c i p l e s . . . . . . . . . . 397 4.2.3.3.2. I n f l u e n c e o f f l u i d v i s c o s i t y . . . . . . . . . . 398 4.2.3.4. Influence o f geological discontinuities ....... 398 4.2.3.4.1. F r a c t u r e b r a n c h i n g .................... 399 4.2.3.4.1.1. F r a c t u r e s t r a n d i n g and m e a n d e r i n g . . . . . . . . . . . . . . 399 4.2.3.4.1.2. Factors influencing crack b r a n c h i n g . . . . . . . . . . . . . . . 399 4.2.3.4.2. I m p a c t o f f a u l t s and j o i n t s . . . . . . . . . . . 400 4.2.3.4.3. I n t e r a c t i o n between h y d r a u l i c f r a c t u r e s and g e o l o g i c a l d i s c o n t i n u i t i e s . . . . . . . . 400 4.2.3.4.4. Shear s l i p p a g e vs . t e n s i l e f a i l u r e .... 400 4.2.3.4.5. Consequences f o r f r a c t u r e m o r p h o l o g y . . 4 0 1 4.2.3.5. F r a c t u r e p r o p a g a t i o n and p r e s s u r e e v o l u t i o n . . . . . . . 401 4.2.3.6. F i e l d e x p e r i e n c e o f r e q u i r e d s e a l t h i c k n e s s . . . . . . . 402 R e s e r v o i r h e t e r o g e n e i t y and a n i s o t r o p y ...................... 402 ....... 402 4 . 2 . 4 . 1 . S e d i m e n t o l o q i c a l a s p e c t s .............. 4.2.4.1.1. - I n t e r - and i n t r a - s a n d b o d y g e o m e t r y ... 403 4.2.4.1.1.1. Sand body c o u p i i n g and s e p a r a t i o n . . . . . . . . . . . . . . 403 4.2.4.1.1.2. Reservoir heterogeneity and t r e a t m e n t s i z e . . . . . . 403 4.2.4.1.1.3. R o l e o f mud d r a p e s . . . . . . 404 4.2.4.1.2. M a t r i x and f r a c t u r e a n i s o t r o p y . . . . . . . . 404 4.2.4.1.2.1. Blanket vs . l e n t i c u l a r sand b o d i e s . . . . . . . . . . . . . 404 4.2.4.1.2.2. I n t e r f e r e n c e and p u l s e t e s t i n g . . . . . . . . . . . . . . . . . 405 4.2.4.1.2.3. T r a c e r a n a l y s i s . . . . . . . . . 405 4.2.4.1.2.4. Impact o f t h e n a t u r a l f r a c t u r e s y s t e m . . . . . . . . . 405 4.2.4.1.2.5. O t h e r a s p e c t s . . . . . . . . . . . 406 406 P e t r o p h y s i c a l a s p e c t s ............................. 4.2.4.2 4.2.4.2.1. P e r m e a b i l i t y a n i s o t r o p y . . . . . . . . . . . . . . . 407 4.2.4.2.2. I n t e r s e c t i o n o f n a t u r a l f r a c t u r e s y s t e m 407 408 Rock m e c h a n i c a l a s p e c t s ........................... 4.2.4.3 4.2.4.3.1. F r a c t u r e extension across m a t e r i a l d i s 408 c o n t i n u i t i e s .......................... 4.2.4.3.1.1. Fracture deflection a t b o u n d i n g p l a n e s . . . . . . . . . 408 4.2.4.3.1.2. Material property d i f f e r e n c e s .................. 409 4.2.4.3.1.3. I n t e r f a c e s h e a r s t r e n g t h 409 409 4.2.4.3.2. O t h e r f e a t u r e s ........................ C o m b i n a t i o n o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g . . . . . . 410 E c o n o m i c a l s i g n i f i c a n c e ..................................... 411 4.2.3.2.5.
4.2.4.
4.2.5. 4.2.6.
4.3.
Proppant saturation and conductivity improvement .................... 4.3.1.
General aspects
.............................................
412 412
339
4.3.2.
4.3.3.
4.3.4.
Types o f p r o p p a n t c o n c e n t r a t i o n ................... 412 S i g n i f i c a n c e o f h i g h proppant s a t u r a t i o n . . . . . . . . . . 412 o f p r o p p a n t t y p e .................................. 413 Proppant d e n s i t y and c o n c e n t r a t i o n ................ 413 S i g n i f i c a n c e o f i n t e r m e d i a t e - s t r e n g t h proppants . . . 413 B e n e f i t s o f h i g h sand c o n c e n t r a t i o n p r i o r t o synt h e t i c p r o p p a n t i n v e n t i o n ......................... 414 414 Proppant embedment and f r a c t u r e w i d t h ....................... M u l t i l a y e r f r a c t u r e p r o p p i n g ...................... 415 4.3.3.1. 4.3.3.1.1. General aspects ....................... 415 4.3.3.1.2. Crushing and embedment r e d u c t i o n ...... 415 4.3.3.1.2.1. General aspects ......... 416 4.3.3.1.2.2. S o f t sands .............. 416 4.3.3.1.2.3. Hard sandstones ......... 417 4.3.3.1.3. C o n d u c t i v i t y enhancement .............. 417 418 Proppant p i l l a r i n g ................................ 4.3.3.2. 4.3.3.2.1. Open f l o w channels between d i s c r e t e 418 p r o p p a n t patches ...................... 4.3.3.2.2. D i s c o n t i n u o u s p r o p p a n t d i s t r i b u t i o n ... 419 4.3.3.2.3. F l u i d d e n s i t y c o n t r o l and s e l e c t i v e 419 p r o p p a n t l o a d ......................... F r a c t u r e w i d t h vs . non-Darcy f l o w ................. 420 4.3.3.3. 4.3.3.3.1. Proppant t y p e and g r a i n s i z e . . . . . . . . . . 420 4.3.3.3.2. Shearing o f polymer f l u i d s ............ 421 4.3.3.3.3. H i g h l y - d e v i a t e d w e l l s ................. 421 4.3.3.3.4. Secondary f r a c t u r e n a r r o w i n g .......... 422 F r a c t u r e w i d t h vs . p r o p p a n t b r i d g i n g .............. 422 4.3.3.4. 4.3.3.4.1. Minimum f r a c t u r e w i d t h f o r p r o p p a n t i n f i l l i n g ............................... 422 4.3.3.4.2. F r i c t i o n p r e s s u r e drop . . . . . . . . . . . . . . . . 422 Proppant c o n c e n t r a t i o n schedules .................. 423 4.3.3.5. 4.3.3.5.1. Ramping vs . s t a i r - s t e p p e d schedules ... 423 4.3.3.5.2. M i c r o p r o c e s s o r c o n t r o l and automation . 423 4.3.3.5.3. B l e n d i n g equipment t y p e vs . t r e a t m e n t s i z e .................................. 424 4.3.3.5.3.1. Continuous-mix f r a c t u r i n g 424 4.3.3.5.3.2. Batch-mix f r a c t u r i n g .... 424 425 Proppant t r a n s p o r t and f r a c t u r e damage ...................... F l u i d breakdown and r e s i d u e c o n c e n t r a t i o n ......... 425 4.3.4.1. 4.3.4.1.1. General aspects ....................... 426 426 4.3.4.1.2. F l u i d breakdown ....................... 4.3.4.1.3. Residue c o n c e n t r a t i o n ................. 427 4.3.4.1.3.1. F i l t e r cake t h i c k n e s s ... 427 4.3.4.1.3.2. Polymer and b r e a k e r conc e n t r a t i o n .............. 428 4.3.4.1.3.3. Residues p l u g g i n g m a t r i x and p r o p p a n t p e r m e a b i l i t y 429 4.3.4.1.4. Gel f i l t e r cake on f r a c t u r e w a l l ...... 429 4.3.4.1.4.1. Gel f i l t e r cake t h i c k n e s s vs . proppant g r a i n diarnet e r ..................... 429 4.3.4.1.4.2. Proppant c o n c e n t r a t i o n vs . f r a c t u r e c o n d u c t i v i t y 430 4.3.4.1.5. P o s s i b i l i t i e s of f r a c t u r i n g f l u i d i m provement ............................. 430 4.3.4.1.5.1. U t i l i z a t i o n o f foam . . . . . 430 4.3.4.1.5.2. F l u i d e f f i c i e n c y vs . r e sidue deposition 431 4.3.4.2. F l u i d v i s c o s i t y and t r a n s p o r t a b i l i t y ............. 431 4.3.4.2.1. Proppant c o n c e n t r a t i o n and v e r t i c a l 432 f r a c t u r e growth ....................... 4.3.4.2.1.1. H i g h - v i s c o s i t y f l u i d s ... 432
4.3.1.1. 4.3.1.2. S e l e c t i o nI 4.3.2.1. 4.3.2.2. 4.3.2.3.
........
340
4.3.4.2.2.
4.3.4.2.3.
4.3.4.3.
Impact o f 4.3.4.3.1.
4.3.4.3.2.
4.3.4.3.3.
4.3.4.3.4.
4.3.4.2.1.2. L o w - v i s c o s i t y f l u i d s .... 432 4.3.4.2.1.3. Shearing o f g e l l e d f l u i d s 433 4.3.4.2.1.4. O t h e r aspects . . . . . . . . . . . 433 Proppant d e n s i t y l f l u i d v i s c o s i t y r e l a t i o n s h i p and f l u i d volume p e r r e s e r v o i r t h i c k n e s s u n i t ........................ 433 4.3.4.2.2.1. Friction loss i n tubular goods . . . . . . . . . . . . . . . . . . . 433 4.3.4.2.2.2. Gel d e g r a d a t i o n . . . . . . . . . 434 4.3.4.2.2.3. F l u i d viscosity modificat i o n .................... 434 4.3.4.2.2.4. F r a c t u r e h e i g h t vs. w i d t h 434 Overpressure c a l i b r a t e d s t i m u l a t i o n de435 s i g n .................................. 4.3.4.2.3.1. Conventional underestimat i o n o f n e t f r a c t u r e prop a g a t i o n p r e s s u r e . . . . . . . 435 4.3.4.2.3.2. Reasons f o r e l e v a t e d n e t f r a c t u r e propagation p r e s s u r e s . . . . . . . . . . . . . . . 435 4.3.4.2.3.3. Scale dependency o f appar e n t f r a c t u r e toughness . 436 4.3.4.2.3.4. H i g h e r p r o p p a n t volumes and c o n c e n t r a t i o n s pumped w i t h overpressure c a l i b r a t e d f r a c t u r e d e s i g n .. 436 437 delayed c r o s s l i n k i n g .................... C r o s s l i n k i n g t y p e s ............_....._.437 4.3.4.3.1.1. General b e n e f i t o f c r o s s l i n k i n g . . . . . . . . . . . . . . . . . 437 4.3.4.3.1.2. Conventional and delayed c r o s s l i n k i n g ............ 438 4.3.4.3.1.3. V i s c o s i t y r e l a t i o n s h i p s . 438 4.3.4.3.1.4. Continuous v s . b a t c h m i x 439 i n g ..................... 4.3.4.3.1.5. Delayed c r o s s l i n k e d f r a c t u r i n g f l u i d s vs. d u a l v i s c o s i t y gravel packing f l u i d s . . . . . . . . . . . . . . . . . . 439 4.3.4.3.1.6. M i x i n g o f delayed and r a p i d c r o s s l i n k i n g agents . 440 C r o s s l i n k i n g s i g n i f i c a n c e and p e r f o r 440 mance ................................. 4.3.4.3.2.1. F r i c t i o n pressure lower440 i n g ..................... 4.3.4.3.2.2. Screenout r a t e r e d u c t i o n 441 4.3.4.3.2.3. F r i c t i o n p r e s s u r e vs. f r a c t u r e h e i g h t growth . . . 441 4.3.4.3.2.4. F l u i d v i s c o s i t y and p r o p p a n t suspension improvement .................... 442 4.3.4.3.2.5. F r a c t u r e c l o s u r e and b r e a k e r a c t i o n . . . . . . . . . . 442 Improvement o f p r o p p a n t placement . . . . . 443 4.3.4.3.3.1. Proppant t r a n s p o r t capac i t y and f l u i d - l o s s a d d i t i v e s . . . . . . . . . . . . . . . . . . . 443 4.3.4.3.3.2. F r a c t u r e p e n e t r a t i o n vs. f l u i d c r o s s l i n k i n g ...... 444 Enhanced f l u i d s t a b i l i t y and t r a n s p o r t 444 c a p a c i t y .............................. 4.3.4.3.4.1. A c i d g e l l i n g and c r o s s -
341 l i n k i n g ................. 444 Rheological d i f f e r e n c e s o f f r a c t u r i n g f l u i d stage 445 4.3.4.3.4.3. Impact o f shear d u r i n g m i x i n g and pumping ...... 446 4.3.4.4. F l u i d heat c a p a c i t y and r e s e r v o i r temperature 446 446 4.3.4.4.1. General aspects ....................... 4.3.4.4.1.1. Temperature s e n s i t i v i t y of fracturing f l u i d visc o s i t y .................. 447 4.3.4.4.1.2. Temperature d i f f e r e n c e between f o r m a t i o n and 447 stimulation f l u i d s 4.3.4.4.2. Heat blockage and f o r m a t i o n c o o l i n g ... 447 4.3.4.4.2.1. F l u i d rheology and proppant g r a i n s i z e ......... 447 4.3.4.4.2.2. Enlargement o f pre-pad and pad s i z e ............ 448 4.3.4.4.2.3. Changing temperature and f l u i d v i s c o s i t y ......... 449 4.3.4.4.3. Primary and secondary g e l l a n t s ........ 449 4.3.4.4.4. F r a c t u r i n g f l u i d v i s c o s i t y improvement 450 450 4.3.4.5. F l u i d v i s c o e l a s t i c i t y ............................. 4.3.4.6. F l u i d l e a k o f f containment ......................... 451 4.3.4.6.0. Types and impact o f f l u i d l o s s ........ 451 4.3.4.6.1. P o s s i b i l i t i e s o f f l u i d l o s s c o n t r o l ... 452 4.3.4.6.1.1. General aspects ......... 452 4.3.4.6.1.2. Drawbacks o f h i g h f l u i d l e a k o f f r a t e s ........... 452 4.3.4.6.1.3. S o l i d p a r t i c l e s i z e vs. pore t h r o a t diameter .... 453 4.3.4.6.1.4. F l u i d uptake s a t u r a t i o n by pre-pad and pad stages 453 4.3.4.6.1.5. F l u i d v i s c o s i t y vs. l e a k o f f ..................... 454 4.3.4.6.2. N a t u r a l f r a c t u r e s and p a r t i c u l a t e addi454 t i v e s ................................. 4.3.4.6.2.1. Liquid fluid-loss additi455 ves ..................... 4.3.4.6.2.2. Solid fluid-loss additi455 ves ..................... 4.3.4.6.2.3. Foams and r e s i n s ........ 457 4.3.4.6.2.4. Impact o f f r a c t u r e geomet r y and morphology ...... 457 Mushroom-type f r a c t u r e s and f l u i d e f f i 4.3.4.6.3. 458 ciency ................................ 4.3.4.6.4. Combined f l u i d - l o s s c o n t r o l a d d i t i v e 458 and gel breaker ....................... 4.3.4.6.4.1. Proppant p e r m e a b i l i t y r e tainment due t o secondary f i l t e r cake d i s s o l u t i o n . 458 4.3.4.6.4.2. Polyester concentration and f i l t e r cake resorp459 t i o n .................... 4.3.4.6.4.3. Degradable p o l y e s t e r vs. 100 mesh sand ........... 459 U l t r a - h i g h f r a c t u r e c o n d u c t i v i t y achievement ................ 460 4.3.5.1. E x t r a o r d i n a r i l y l a r g e proppant g r a i n s i z e s ........ 460 460 4.3.5.1.1. Advantages ............................ 461 4.3.5.1.2. Drawbacks ............................. 4.3.5.2. M i x i n g o f round and angular proppant g r a i n s ....... 461 Uniform vs. changing f r a c t u r e c o n d u c t i v i t y .................. 462 4.3.4.3.4.2.
.....
......
4.3.5.
4.3.6.
342 4.4.
Targets o f unconventional gas stimulation ........................... 4.4.1.
4.4.2
4.4.3
Tiqht qas sandstones ........................................ 4.4.1.7. T i g h t g a s s a n d body morphology and s i g n i f i c a n c e o f h y d r a u l i c f r a c t u r i n g .............................. 4.4.1.2. D i s t r i b u t i o n of important t i g h t gas sand formations and b a s i n s ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1.3. I n t e r r e l a t i o n s h i p o f t i g h t g a s s a n d s and c o a l seams 4.4.1.4. O v e r h y d r o s t a t i c r e s e r v o i r p r e s s u r e ................ G e o p r e s s u r e d r e s e r v o i r s . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2.1. Geotectonical s e t t i n g ... .. ... 4.4.2.2. Poroperm p r o p e r t i e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2.3. Ball-out treatments 4 . 4 . 2 . 4 . Other s o u r c e Coal seams . . . . . _ . . . _ . 4.4.3.1. General a s p e c t s . . . 4.4.3.1.1. I m p o r t a n c e o f the c l e a t s y s t e m f o r g a s
464 464 465 465 466 466 467 467 468 468 468
469 S i g n i f i c a n c e of hydraulic f r a c t u r i n g in deep unmineable c o a l seams f o r g a s exp l o i t a t i o n ...................... 469 Aims and p u r p o s e s o f c o a l f r a c t u r i n g . . 470 4.4.3.1.3. 4 . 4 . 3 . 1 . 4 . D i s t r i b u t i o n of important coal b a s i n s and g a s reserves ...................... 470 4 . 4 . 3 . 2 . C l e a t geometry .... 471 4 . 4 . 3 . 3 . S p e c i a l mechanical 471 4 . 4 . 3 . 3 . 1 . T r e a t m e n t p r e s s u r e s and c o a l c h i p 472 4.4.3.3.2. Impact o f h y d r a u l i c f r a c t u r e g e o m e t r y . 472 4.4.3.4. F r a c t u r i n g s t i m u l a t i o n t y p e s o f c o a l seams . . . . . . . . 473 4.4.3.4.1. Fluid viscosity .... 473 4.4.3.4.1.1. Gel- vs. w a t e r - b a s e d s t i m u l a t i o n . . . . . . . . . . . . . . . . 473 4.4.3.4.1.2. P r o p p a n t s e t t l i n g and r o l l i n g in water fractu474 r i n g .................... Mu1 t i p l e i n t e r v a l f r a c t u r i n g . . . . . . . . . . 474 4.4.3.4.2 4.4.3.4.2.1. Reservoir depth of coal seams . . . . . . . . . . . . . . . . . . . 474 4.4.3.4.2.2. L i m i t e d - e n t r y s t i m u l a t i o n 474 F r a c t u r i n g s t r a t e g i e s . . . . . . . . . . . . . . . . . 475 4.4.3.4.3 4.4.3.4.3.1. G e n e r a l a s p e c t s _ . . . . . . . . 475 4.4.3.4.3.2. Single vertical fractures intersecting several t h i n c o a l seams . . . . . . . . . . . . . . 475 4.4.3.4.3.3. Complex m u l t i p l e f r a c t u res c o n t a i n e d i n a s i n g l e t h i c k c o a l seam . . . . . . . . . 476 4.4.3.4.3.4. Injection rates, fluid l o s s and t r e a t m e n t p r e s sure .................... 476 F r a c t u r e p r o p a g a t i o n d i r e c t i o n and 4.4.3.4.4 t r e a t m e n t pressure i n c r e a s e . . . . . . . . . . . 477 4.4.3.5. F r a c t u r e c o n d u c t i v i t y m a x i m i z a t i o n . . . . . . . . . . . . . . . . 477 4.4.3.6. Vertical hydraulic f r a c t u r i n g vs. horizontal d r i l ling .............................................. 478 4.4.3.7. R e s e r v o i r p r e s s u r e ................................ 478 4.4.3.8. R e l a t i o n s h i p s between c o a l seam and bounding s t r a t a 478 4.4.3.8.1. S a n d s t o n e s ............................ 479 479 4.4.3.8.2. Shales ................................ 4.4.3.8.3. F r a c t u r e h e i g h t c o n t a i n m e n t and e l e v a t e d t r e a t i n o< ,o r e s s u r e s . . . . . . . . . . . . . . . . 479 Shales ...................................................... 480 4.4.3.1.2.
4.4.4,
463 463
343 R e s e r v o i r p r o p e r t i e s and p r o d u c t i o n mechanisms . . . . 480 4.4.4.1.1. Gas d r a i n a g e through n a t u r a l f r a c t u r e s 480 4.4.4.1.2. S i g n i f i c a n c e f o r gas i n d u s t r y and r e serve p o t e n t i a l ....................... 481 4.4.4.1.3. Impact o f h o r i z o n t a l d r i l l i n g and hy481 d r a u l i c t r e a t i n g ...................... Non-proppant f r a c t u r i n g ........................... 482 4.4.4.2. 4.4.4.2.1. N i t r o g e n f r a c t u r i n g . . . . . . . . . . . . . . . . . . . 482 4.4.4.2.1.1. Advantages o f n i t r o g e n gas f r a c t u r i n g . . . . . . . . . . 482 4.4.4.2.1.2. W e l l b o r e v i c i n i t y damage removal and r e s e r v o i r breakdown . . . . . . . . . . . . . . . 482 4.4.4.2.2. Carbon d i o x i d e and foam f r a c t u r i n g .... 483 4.4.4.2.2.1. E f f e c t i v i t y o f carbon d i o x i d e vs. n i t r o g e n t r e a t 483 i n g ..................... 4.4.4.2.2.2. Foam f r a c t u r i n g and t a i l o r e d p u l s e l o a d i n g . . . . . 483 4.4.4.2.2.3. Auto-propping o f c r a c k s c r e a t e d by gas f r a c t u r i n g 483 484 Proppant f r a c t u r i n g ............................... 4.4.4.3. 4.4.4.3.1. Proppant placement and f l u i d r e t r i e v a l 484 4.4.4.3.1.1. Comparative s u p e r i o r i t y o f proppant f r a c t u r i n g . . 484 4.4.4.3.1.2. Combination o f gas and foam proppant f r a c t u r i n g 485 4.4.4.3.2. V e r t i c a l vs. h o r i z o n t a l h o l e d r i l l i n g 485 and f r a c t u r i n g ........................ E f f e c t i v i t y o f proppant v s . n i t r o g e n f r a c t u r i n g ... 486 4.4.4.4. 486 4.4.4.4.1. General aspects ....................... 4.4.4.4.2. L i q u i d r e t e n t i o n and w e l l cleanup . . . . . 487 4.4.4.4.3. F e a s i b i l i t y o f MHF t r e a t m e n t s . . . . . . . . . 487 4.4.4.4.3.1. R e s e r v o i r p r e s s u r e . . . . . . 487 4.4.4.4.3.2. Proppant c o n c e n t r a t i o n . . 488 S p e c i a l f e a t u r e s o f s h a l e qas r e s e r v o i r s .......... 488 4.4.4.5. 4.4.4.6. E x p l o s i v e f r a c t u r i n g .............................. 489 489 High-temperature r e s e r v o i r s ................................. 4.4.5.1. F l u i d s t a b i l i t y adjustment t o e l e v a t e d pay temperat u r e .............................................. 489 4.4.5.2. Heat t r a n s f e r i n f r a c t u r e s and temperature s t a b i l i t y o f s t i m u l a t i o n f l u i d s .......................... 490 4.4.5.3. F r a c t u r i n g f l u i d r e q u i r e m e n t i n h o t environment ... 490 4.4.5.4. F r a c t u r i n g f l u i d r e q u i r e m e n t i n c o l d environment . . 491 Gas condensate r e s e r v o i r s ................................... 491 4.4.4.1.
4.4.5
4.4.6
4.5.
Hydraulic proppant fracturing o f carbonate rocks
4.5.1.
....................
E x t e n s i o n o f d r a i n a g e p a t h and c o n d u c t i v i t y c o n t r a s t . . . . . . . . 4.5.1.1. Advantages and drawbacks o f f r a c t u r e p r o p p i n g ..... 4.5.1.1.1. M a t r i x a c i d i z i n g vs. f r a c t u r i n g . . . . . . . 4.5.1.1.1.1. Removal o f f o r m a t i o n damage .................... 4.5.1.1.1.2. Auto-propping o f rugged fracture walls after etching . . . . . . . . . . . . . . . . . . . 4.5.1.1.1.3. F r a c t u r e c o n d u c t i v i t y vs. fracture length ......... 4.5.1.1.1.4. Double f r a c t u r i n g i n r e servoirs with hairline cracks . . . . . . . . . . . . . . . . . . 4.5.1.1.2. C o n d u c t i v i t y c o n t r a s t between r e s e r v o i r m a t r i x and propped f r a c t u r e . . . . . .
492 492 493 493 493 494 495 495 496
344 4.5.1.1.2.1.
4.5.2. 4.5.3. 4.5.4.
Fracture flow capacity c o n t r a s t ................ 496 4.5.1.1.2.2. P e r m e a b i l i t y and conduct i v i t y c o n t r a s t . . . . . . . . . 496 4.5.1.1.3. Carbonate s c a l e f o r m a t i o n and f l u i d 496 l o s s .................................. 4.5.1.1.3.1. Scale p r e c i p i t a t i o n p r e v e n t i o n ................. 497 4.5.1.1.3.2. C o r r o s i o n i n h i b i t i o n . . . . 497 4.5.1.1.4. I r o n s e q u e s t e r i n g and complexing . . . . . . 498 4.5.1.1.4.1. Sources o f i r o n i n a c i d i z i n g and f r a c t u r i n g . . . . . 498 4.5.1.1.4.2. Compatibility o f iron-seq u e s t e r i n g and complexing agents . . . . . . . . . . . . . . . . . . 498 4.5.1.2. C o n d u c t i v i t y improvement by a c i d o p t i m i z a t i o n . . . . . 499 499 4.5.1.2.1. A c i d s t r e n g t h ......................... 4.5.1.2.2. A c i d f l u i d - l o s s c o n t r o l ............... 499 4.5.1.2.3. A c i d c r o s s l i n k i n g ..................... 500 4.5.1.2.4. A c i d g e l l i n g .......................... 500 4.5.1.2.5. A c i d h e a t i n g .......................... 501 4.5.1.3. Enhancement o f a c i d p e n e t r a t i o n d i s t a n c e . . . . . . . . . . 501 4.5.1.3.1. A c i d i n j e c t i o n and r e a c t i o n . . . . . . . . . . . 501 4.5.1.3.1.1. A c i d c h a n n e l i z a t i o n . . . . . 501 4.5.1.3.1.2. F l u i d property modifica502 t i o n .................... 4.5.1.3.2. A c i d r e t a r d a t i o n ...................... 502 4.5.1.3.2.1. S l o w l y r e a c t i n g a c i d s and s o l i d a d d i t i v e s . . . . . . . . . 503 4.5.1.3.2.2. A c i d e m u l s i f i c a t i o n . . . . . 503 4.5.1.3.2.3. A c i d g e l l i n g and f i l t e r cakes . . . . . . . . . . . . . . . . . . . 504 4.5.1.3.2.4. A c i d c o n c e n t r a t i o n and f r i c t i o n l o s s . . . . . . . . . . . 504 4.5.1.4. A c i d foaming .............. .................. 504 . . . . . . . . . . . . . . . . . . 505 4.5.1.5. Open vs . propped f r a c t u r e s 4.5.1.5.1. Drawbacks o f a u t o - p r o p p i n g o f a c i d i z e d 505 f r a c t u r e w a l l s ........................ 4.5.1.5.2. Significance o f proppant i n f i l l i n g o f 506 t h e f r a c t u r e .......................... 506 4.5.1.6. A c i d f l u i d - l o s s c o n t r o l ........................... 4.5.1.6.1. A c i d r e a c t i o n s d u r i n g f r a c t u r i n g . . . . . . 506 4.5.1.6.2. A l t e r n a t i n g a c i d and g e l l e d w a t e r s t a ges ................................... 507 4.5.1.6.3. Wormhole growth and a c i d f l u i d l o s s ... 507 4.5.1.6.3.1. Wormhole p e n e t r a t i o n and a c i d c h a n n e l l i n g . . . . . . . . 508 4.5.1.6.3.2. F l u i d invasion pattern and f r a c t u r e c o n d u c t i v i t y development . . . . . . . . . . . . . 508 P o s s i b i l i t i e s f o r improving a c i d f l u i d 4.5.1.6.4. 508 l o s s c o n t r o l .......................... Combination o f n a t u r a l and a r t i f i c i a l f r a c t u r e s . . . . . . . . . . . . . 509 509 P o s s i b i l i t i e s o f a p p l i c a t i o n i n Europe ...................... . 509 Chalk s t i m u l a t i o n ...................................... 4.5.4.1. P e t r o p h y s i c a l p r o p e r t i e s and p r o d u c t i v e n e s s . . . . . . . 510 510 4.5.4.1.1. General aspects ....................... 511 4.5.4.1.2. S t r a t i g r a p h y .......................... 4.5.4.1.2.1. Hod, Tor and E k o f i s k f o r mations . . . . . . . . . . . . . . . . . 511 4.5.4.1.2.2. F r a c t u r e s t r a t i g r a p h y ... 511 4.5.4.1.3. Petrography and p e t r o p h y s l c s . . . . . . . . . . 512
345 4.5.4.1.3.1.
4.5.4.2.
4.5.4.3.
4.5.4.4.
4.5.4.5.
Autochthonous and a l l o c h thonous c h a l k ........... 512 4.5.4.1.3.2. F r a c t u r e c l o s u r e ........ 512 4.5.4.1.4. N a t u r a l f r a c t u r e t y p e s ................ 512 4.5.4.1.4.1. Healed and t e c t o n i c f r a c t u r e s ................... 513 4.5.4.1.4.2. Stylolite-associated f r a c t u r e s ............... 513 4.5.4.1.4.3. Chalk s t i m u l a t i o n t a r g e t s 513 P r i m a r y c h a l k p r o p e r t i e s and s t i m u l a 4.5.4.1.5. t i o n response ......................... 514 P r i m a r y c h a l k p r o p e r t i e s and secondary 4.5.4.1.6. 514 f i e l d subsidence ...................... 4.5.4.1.6.1. R e s e r v o i r compaction and f i e l d subsidence ........ 515 4.5.4.1.6.2. Permi an sandstone vs . Cretaceous c h a l k s u b s i dence ................... 515 4.5.4.1.6.3. Subsidence r a t e s i n d i f f e r e n t c h a l k f i e l d s ..... 516 Combination o f a c i d and p r o p p a n t f r a c t u r i n g ....... 516 Viscous f i n g e r i n g a c i d i z i n g and worm4.5.4.2.1. 516 h o l e growth ........................... 4.5.4.2.1.1. Whisper f r a c t u r i n g ...... 517 4.5.4.2.1.2. Viscosity contrast requir e d f o r f i n g e r i n g ....... 517 4.5.4.2.1.3. Density d i f f e r e n c e r e q u i r e d f o r f i n g e r i n g ....... 518 4.5.4.2.2. F r a c t u r e p r o p a g a t i o n c o n t r o l .......... 518 4.5.4.2.2.1. S h o r t wide f r a c t u r e s and h i g h p r o p p a n t s a t u r a t i o n 518 4.5.4.2.2.2. Impact o f s t i m u l a t i o n b o a t s on a c i d vs . p r o p p a n t f r a c t u r i n g ......... 519 Comparative e f f e c t i v i t y o f p r o p p a n t and 4.5.4.2.3. 519 a c i d f r a c t u r i n g ....................... 4.5.4.2.3.1. Longevity o f created f r a c t u r e ................ 519 4.5.4.2.3.2. F o r m a t i o n breakdown and c l o s e d f r a c t u r e a c i d i z i n g 519 Combination o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g ........................................... 520 520 4.5.4.3.1. General aspects ....................... 4.5.4.3.2. Up-and-under s t i m u l a t i o n vs . combined h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g 521 4.5.4.3.3. Design and purpose o f c h a l k g r a v e l p a c k i n g ............................... 521 4.5.4.3.4. W e l l b o r e c l e a n o u t e f f e c t i v i t y ......... 522 S p e c i a l c h a l k f r a c t u r i n g techniques ............... 522 4.5.4.4.1. T i p s c r e e n o u t f r a c t u r i n g .............. 522 4.5.4.4.2. I n t e r f a c e f r a c t u r i n g .................. 522 4.5.4.4.3. D e n d r i t i c f r a c t u r i n g .................. 523 4.5.4.4.3.1. General aspects ......... 523 4.5.4.4.3.2. Combination o f d e n d r i t i c n a t u r a l and h y d r a u l i c f r a c t u r i n g .............. 523 4.5.4.4.3.3. R e f r a c t u r i n g aspects .... 524 4.5.4.4.4. O t h e r aspects o f c h a l k f r a c t u r i n g ..... 524 Water i m b i b i t i o n and o i l r e c o v e r y ................. 525 4.5.4.5.1. Water f l o o d i n g ........................ 525 4.5.4.5.2. Water f r a c t u r i n g ...................... 526 526 4.5.4.5.3. Foam f r a c t u r i n g .......................
346 4.5.5.
4.5.4.6. H o r i z o n t a l w e l l d r i l l i n g .......................... Sandstone a c i d i z i n g ...................... 4.5.5.1. Acid reaction .................. 4.5.5.1.1. Partial matrix disintegration . 4.5.5.1.2. S e p a r a t i o n o f c o n n a t e w a t e r and p r e f l u s h .............................. 4.5.5.2. Acidizing targets ...... 4.5.5.2.1. C l a y and i r o 4.5.5.2.2. Carbonate m i 4.5.5.3. Acid penetration distanc 4.5.5.3.1. Type o f f o r m a t i o n damage 4.5.5.3.2. Acid concent 4.5.5.3.3. Low- and h i g h - p H a c i d s . . . . 4.5.5.3.4. Water b l o c k i n g ........................ 4.5.5.4. A c i d t y p e s and d i s s o l u t i o n p o s s i b i l i t i e s . . . . . . . . . . 4.5.5.4.1. G e n e r a l a s p e c t s ....................... 4.5.5.4.2. C l a y m i n e r a l type vs. a c i d i z i n g t r e a t ment ... .... ..... 4.5.5.4.3. Delayed r e a c t i o n and i n - s i t u a c i d g e n e r a t i o n ............................ 4.5.5.4.4. Selective acid stimulation o f injection w e l l s .................................
526
528
530 530 530 531 532 532
4.6. High-permeability reservoir fracturing .............................. 4.6.1.
4.6.2. 4.6.3.
4.6.4.
4.7.
533 533 Fracture-formation conductivity contrast ..... 4.6.1.1. R e s e r v o i r t y p e s .................... 533 4.6.1.2. F r a c t u r e f l o w c a p a c i t y ............................ 534 P r o p p a n t g r a i n s i z e and c o n c e n t r a t i o n . . . . . . . . . . . . . . . . . 534 534 P r o p p a n t / f l u i d s l u r r y m i x i n g t y p e s .......................... 534 4.6.3.1. B a t c h - m i x p r e p a r a t i o n ............................. 4.6.3.2. C o n t i n u o u s - m i x b l e n d i n g ........................... 535 4.6.3.3. S p e c i a l d e s i g n f o r w a t e r f l o o d o p e r a t i o n s . . . . . . . . . . 535 F r a c t u r e d e s i g n c h a r a c t e r i s t i c s ........................ 4.6.4.1. F r a c t u r e w i d t h and l e n g t h ......................... 536 4.6.4.2. H i g h p r o p p a n t c o n c e n t r a t i o n s and g r a i n s i z e . . . . . . . 536
Geothermal fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1. 4.7.2.
4.7.3.
537 Geological aspects o f geothermal r e s e r v o i r s 537 4.7.1.1. D i s t r i b u t i o n o f geothermal r e s e r v o 537 4.7.1.2. Permeability evolution ........... 537 Geothermal r e s e r v o i r d i s t r i b u t i o n and e x p l o i t a t i o n 538 4.7.2.1. G e o g r a p h i c a l d i s t r i b u t i o n o f European geo r e s e r v o i r s .............................. 538 4.7.2.2. Worldwide geothermal energy e x p l o i 539 E x p l o s i v e and n o n - p r o p p a n t f r a c t u r i n g ....................... 539 4.7.3.1. G e n e r a l a s p e c t s ................................... 539 4.7.3.1.1. F o r m a t i o n damage removal i n v a r i o u s r e s e r v o i r types ......................... 540 4.7.3.1.2. S h o r t f r a c t u r e e x t e n s i o n and r e s e . rock r u b b l i z a t i o n ............... 540 4.7.3.2. C o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g and r e c e n t vations .... . 541 4.7.3.3. Stem-induced . . . . . . . . . . . . . . . . . 541 4.7.3.3.1. Controlled o r i g i n o f limited multiple f r a c t u r e system ....................... 542 4.7.3.3.2. V e r t i c a l f r a c t u r e o r i e n t a t i o n regardless o f geostress s t a t e .... 542 4.7.3.3.3. L i m i t a t i o n s and p o s s i b i l i t i e vement ................................ 542 4.7.3.3.4. Proppant i n s e r t i o n i n t o explosion-gener a t e d f r a c t u r e s ....................... 543 4.7.3.4. Nuclear f r a c t u r i n g . . . . ..... 543
347
4.7.4.
4.7.5.
4.7.6. 4.7.7.
4.8.
4.7.3.4.1. P r i n c i p l e s ............................ 4.7.3.4.2. A p p l i c a t i o n s .......................... 4.7.3.5. Water f r a c t u r i n g .................................. Proppant f r a c t u r i n g ......................................... 4.7.4.1. Proppant aspects .................................. 4.7.4.1.1. High-temperature p r o p p a n t performance . 4.7.4.1.2. F i e l d e x p e r i e n c e ...................... 4.7.4.2. F l u i d aspects ..................................... 4.7.4.2.1. Fluid viscosity sensitivity ........... 4.7.4.2.2. Fluid viscosity stabilization ......... 4.7.4.3. Treatment aspects ................................. Communication f r a c t u r i n g .................................... 4.7.5.1. Connection o f i n j e c t i o n and p r o d u c t i o n w e l l s ...... 4.7.5.2. Connection o f w e l l s and l a r g e - s c a l e n a t u r a l f r a c t u r e s ............................................... F r a c t u r e system enlargement by h e a t e x t r a c t i o n . . . . . . . . . . . . . . Shear s t i m u l a t i o n vs . t e n s i l e f r a c t u r i n g .................... 4.7.7.1. O r i g i n o f shear s l i p p a g e .......................... 4.7.7.2. D i l a t i o n b e h a v i o u r ................................ 4.7.7.3. J o i n t d e f o r m a t i o n spectrum ........................ 4.7.7.4. D e n d r i t i c f r a c t u r i n g t r e a t m e n t s ...................
543 544 544 545 545 545 546 546 546 547 547 547 548 548 548 549 549 550 550 551
M i n i f r a c t u r i n g and other small-scale f r a c t u r i n g ..................... 552 4.8.1. M i n i f r a c t u r e . m i c r o f r a c t u r e and MHF d e f i n i t i o n and p r i n c i p l e s 552 4.8.1.1. M i n i f r a c t u r e and m i c r o f r a c t u r e .................... 552 4.8.1.2. Massive f r a c t u r e .................................. 553 4.8.1.2.1. Proppant q u a n t i t i e s and f r a c t u r e wing l e n g t h s ............................... 553 4.8.1.2.2. A p p l i c a t i o n s i n t i g h t gas sand s t i m u l a t i o n .................................. 553 4.8.1.3. M i n i - m a s s i v e f r a c t u r e ............................. 554 4.8.1.3.1. D i l a t a n c y o f h y d r a u l i c f r a c t u r i n g ..... 554 4.8.1.3.2. Design o f mini-massive f r a c t u r i n g ..... 554 4.8.2. P r e - f r a c t u r e p r e s s u r e breakdown t e s t and f r a c t u r e parameter d e t e r m i n a t i o n ............................................... 555 4.8.2.1. M i n i f r a c t u r e c a l i b r a t i o n experiments .............. 555 4.8.2.1.1. D i m i n u t i o n o f screenout r a t e s . . . . . . . . . 555 4.8.2.1.2. F l u i d - l o s s c o e f f i c i e n t d e t e r m i n a t i o n .. 556 4.8.2.2. M i c r o f r a c t u r e breakdown procedures ................ 556 4.8.2.3. Pressure a n a l y s i s and d e s i g n i n p u t d a t a c o l l e c t i o n 557 4.8.2.3.1. M i n i f r a c t u r e s t e s t and e v a l u a t i o n t y p e s 557 4.8.2.3.1.1. S t e p - r a t e t e s t .......... 557 4.8.2.3.1.2. Pump-in/flow-back t e s t .. 558 4.8.2.3.1.3. Pump-in/shut-in t e s t . . . . 559 4.8.2.3.2. I n - s i t u s t r e s s d e t e r m i n a t i o n . . . . . . . . . . 559 4.8.2.3.3. C l o s u r e s t r e s s vs . f l u i d t y p e ......... 560 4.8.2.3.4. Premature screenout t e r m i n a t i o n r e c o g n i t i o n ................................ 560 4.8.2.3.5. F l u i d s and mechanics t e s t i n g . . . . . . . . . . 561 4.8.2.4. S i z e and s i g n i f i c a n c e o f m i n i f r a c t u r i n g . . . . . . . . . . . 561 4.8.2.4.1. General aspects ....................... 561 4.8.2.4.2. Proppant q u a n t i t i e s and f r a c t u r e wing l e n g t h s ............................... 562 4.8.2.5. Proppant vs . non-proppant m i n i f r a c t u r i n g . . . . . . . . . . 562 4.8.2.5.1. E f f e c t i v i t y d i f f e r e n c e . . . . . . . . . . . . . . . . 563 4.8.2.5.2. Comparative assessment . . . . . . . . . . . . . . . . 563 4.8.2.5.3. S h o r t p r e c i s e propped m i n i f r a c t u r e s ... 564 4.8.2.5.3.1. Proppant c o n c e n t r a t i o n and d i s t r i b u t i o n . . . . . . . . 564 4.8.2.5.3.2. ImDact o f batch-mix f r a c t u r i n g . . . . . . . . . . . . . . . . . . 564 4.8.2.6. M o d i f i e d m i c r o f r a c t u r i n g and m i n i f r a c t u r i n g a n a l y -
348
4.8.3.
4.8.4.
4.8.5.
4.8.6.
s i s f o r o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e d e s i g n . . . 564 4.8.2.6.1. L a y o u t and p u r p o s e o f c o n v e n t i o n a l m i c r o f r a c t u r e s and m i n i f r a c t u r e s . . . . . . . . 565 4.8.2.6.2. M o d i f i e d m i c r o f r a c t u r e s and m i n i f r a c 565 t u r e p r o c e d u r e s ....................... 4.8.2.6.2.1. Reopening, p r o p a g a t i o n , s h u t - i n and f l o w b a c k c y c l e s ..................... 566 4.8.2.6.2.2. Global vs. l o c a l p r e s s u r e d e c l i n e match . . . . . . . . . . . 566 D e s i g n p a r a m e t e r s f o r optimum m i c r o 4.8.2.6.3. and m i n i f r a c t u r e .. ................ 567 4.8.2.6.3.1. Test d r a t e s and v i s c o s i t i e s . . . . . . . . . . . . . . . . 567 4.8.2.6.3.2. S h u t - i n t i m e and f l o w b a c k 567 r a t e .................... F o r m a t i o n damage a r o u n d t h e w e l l b o r e . . . . . . . . . . . . . . 568 4.8.3.1. O r i g i n o f f o r m a t i o n damage ........................ 4.8.3.1.1. Formation exposure d u r i n g w e l l opera568 t i o n .................................. 4.8.3.1.2. F l u i d l o s s i n t o t h e r e s e r v o i r . . . . . . . . . 569 4.8.3.2. Types o f f o r m a t i o n damage ......................... 569 4.8.3.3. F o r m a t i o n damage b r e a k t h r o u g h b y m i n i f r a c t u r i n g . . . 570 4.8.3.3.1. Small p r o p p a n t v o l s f o r plugging of . . . . . . . . . . . . . . . . . . 570 short cracks ..... 4.8.3.3.2. Damage h a l o b r e a k a by s h o r t high-cond u c t i v i t y f r a c t u r e s . . . . . . . . . . . . . . . . . . . 570 . . 571 4.8.3.4. F o r m a t i o n damage removal b y a c i d i z i n g . . . . . . . F i e l d e x p e r i m e n t s and mHF v s . MHF ........................... 571 4.8.4.1. F r a c t u r i n g expenses v s . e f f e c t i v i t y . . . . . . . . . . . . . . . 571 4.8.4.2. F l u i d l e a k o f f v s . w a t e r b l o c k i n g . . . . . . . . . . . . . . . . . . 572 572 4.8.4.3. O t h e r a s p e c t s ..................................... P i l o t s t i m u l a t i o n o f m a r g i n a l r e s e r v o i r s .................... 573 4.8.5.1. A r e a l d i s t r i b u t i o n o f p o t e n t i a l r e s e r v o i r s . . . . . . . . 573 4.8.5.2. A p p l i c a t i o n o f resin-coated proppants i n unstable p a y zones ......................................... 573 4.8.5.2.1. P r o p p a n t wedge s t a b i l i z a t i o n and embed574 ment p r e v e n t i o n ....................... 4.8.5.2.2. T a i l - i n propping f o r flowback i n h i b i tion ........ . . . . . . . . . . . . . . . 574 . . . . . . . . . . . . . . . 574 4.8.5.3. T i p screenout f r a c t u r i n g , , . . . . . . . . . . . . . 574 4.8.5.3.1. P r i n c i p l e s .. 4.8.5.3.2. A p p l i c a t i o n s .......................... 575 4.8.5.4. D i r t y s a n d s t o n e s t i m u l a t i o n ....................... 575 H o r i z o n t a l w e l l o r d r a i n h o l e d r i l l i n g and f r a c t u r i n g . . . . . . . . 575 576 4.8.6.1. H o r i z o n t a l h o l e d r i l l i n g .......................... 4.8.6.1.0. H i s t o r i c a l develop atus o f . . . . . . . . . . 576 horizontal wells . 4.8.6.1.1. M a i n t a r g e t s o f h o r i z o n t a l w e l l s . . . . . . 576 4.8.6.1.2. H o r i z o n t a l w e l l s v s . d r a i n h o l e s . . . . . . . 577 4.8.6.1.2.1. Radius f o r b o r e h o l e t r a j e c t o r y t u r n i n g . . . . . . . . . 577 4.8.6.1.2.2. Drainhole d r i l l i n g applic a t i o n s . . . . . . . . . . . . . . . . . 578 4.8.6.1.3. H o r i z o n t a l w e l l s vs. extended-reach 578 d r i l l i n g .............................. 4.8.6.1.3.1. Horizontal displacement and t o t a l a c c e s s i b l e a r e a 578 4.8.6.1.3.2. Extended- v s . l i m i t e d r e a c h d r i l l i n g . . . . . . . . . . 578 4.8.6.1.4. I n v e r t e d d r i l l i n g ..................... 579 4.8.6.1.5. S i g n i f i c a n c e f o r r e s e r v o i r d r a i n a g e . . . 579
349 4.8.6.1.5.1. 4.8.6.1.5.2. 4.8.6.1.5.3. 4.8.6.1.5.4. 4.8.6.1.5.5. 4.8.6.1.5.6. 4.8.6.1.5.7.
Communication p a t h b e t ween we1 1 b o r e t r a j e c t o r y and r e s e r v o i r . . . . . . . . . . . Improved n a t u r a l f r a c t u r e i n t e r s e c t i o n ............ Choke e f f e c t on v e r t i c a l fractures ............... D i s t a n c e f r o m f l u i d contacts ................... Reduction o f coning . . . . . S e l e c t i o n o f d r a i n a g e and injection direction ..... R e s e r v o i r geometry and n a t u r a l crack d i s t r i b u t i o n .................... Wellbore i n s t a b i l i t y . . . . h o l e c o m p l e t i o n and s t i m u l a -
580 580 581 581 582
582 582 4.8.6.1.5.8. 4.8.6.1 6. H o r i z o n t a l t i o n .................................. 583 4.8.6.1 7 . O t h e r aspects ......................... 583 584 4.8.6.2. H o r i z o n a1 h o l e f r a c t u r i n g ........................ 4.8.6.2 1. H o r i z o n t a l h o l e d r i l l i n q v s . f r a c t u r i n q 584 4.8.6.2.1.1. A l t e r n a t i c e o r combinat i o n .................... 584 4.8.6.2.1.2. T i g h t gas r e s e r v o i r d r a i 585 nage .................... 4.8.6.2.1.3. I d e n t i c a l exposed s u r f a ces i n c o a l seams ....... 585 4.8.6.2.1.4. S i n g l e - vs. m u l t i p l e l a y e r r e s e r v o i r s . . . . . . . . 586 4.8.6.2.1.5. Orientation t o natural f r a c t u r e s and p e r m e a b i l i t y a n i s o t r o p i e s ......... 587 Horizontal hole d r i l l i n g i n combination 4.8.6.2.2 w i t h f r a c t u r i n g ....................... 587 4.8.6.2.2.1. R e s e r v o i r t h i c k n e s s and p e r m e a b i l i t y . . . . . . . . . . . . 587 4.8.6.2.2.2. Wellbore angle buildup technology and s l a n t h o l e 588 P r e p e r f o r a t e d l i n e r c o m p l e t i o n and 4.8.6.2.3 p r o p p a n t requirements . . . . . . . . . . . . . . . . . 588 Mu1 t i p l e v e r t i c a l f r a c t u r i n g o f 4.8.6.2.4 h i g h l y i n c l i n e d b o r e h o l e s ............. 588 4.8.6.2.4.1. Sets o f p a r a l l e l f r a c t u r e p l a n e s . . . . . . . . . . . . . . . . . . 589 4.8.6.2.4.2. Natural f r a c t u r e i n t e r s e c t i o n . . . . . . . . . . . . . . . . . 590 4.8.6.2.4.3. M u l t i p l e - vs. l i m i t e d - e n t r y f r a c t u r i n g . . . . . . . . . . 590 4.8.6.2.4.4. F r a c t u r e morphology and o r i e n t a t i o n . . . . . . . . . . . . . 590 4.8.6.2.4.5. Influence o f i n j e c t i o n r a t e s . . . . . . . . . . . . . . . . . . . 591 Uptake c a p a c i t y enhancement o f i n j e c t i o n w e l l s . . . . . . . . . . . . . . 592 4.8.7.1. O i l - f i e l d w a t e r - i n j e c t i o n w e l l s . . . . . . . . . . . . . . . . . . . 592 4.8.7.1.1. C o n t r o l 1ed r a c t u r i n g ................. 592 4.8.7.1.1.1 General aspects . . . . . . . . . 592 4.8.7.1.1.2 Proppant s i z e and r e s e r v o i r p r e s s u r e . . . . . . . . . . . 593 Decreasing i n - s i t u s t r e s s 4.8.7.1.1.3 by c o l d w a t e r i n j e c t i o n . 593 4.8.7.1.1.4 Sweep e f f i c i e n c y vs. i n j e c t i v i t y . . . . . . . . . . . . . . . 594 ~
4.8.7.
579
350 Accidental f r a c t u r i n g . . 4 . 8 . 7 . 1 . 2 . 1 . Fluid brea sweep e f f i c i e n c y . . . . . . . . 595 4 . 8 . 7 . 1 . 2 . 2 . Open cracks above f r a c ture pressure a n d nonproppant hydraulic stimul a t i o n . . . . . . . . . . . . . . . . . . 595 4 . 8 . 7 . 1 . 2 . 3 . Other aspects . . . . . . . 4 . 8 . 7 . 2 . Waste-liquid disposal wells . . . . . . . . . . . . . . . . . . . 4 . 8 . 7 . 2 . 1 . Safe removal of waste l i q u i d s by underground disposal ....................... 596 4 . 8 . 7 . 2 . 2 . P r o p p a n t v s . non-proppant disposal f r a c t u r i n g ............................ 596 Combination of n a t u r a l and a r t i f i c i a l f r a c t u r e s . . 4 . 8 . 8 . 1 . F r act u r e i n t er f er en ce a n d superimposition . . . . . . . . . 597 4 . 8 . 8 . 1 . 1 . Di s t r i b u t i o n of n a t u r a l f r a c t u r e s . . . . . 598 4 . 8 . 8 . 1 . 1 . 1 . S t r e s s diffe re nc e s a n d depositional environment 598 4 . 8 . 8 . 1 . 1 . 2 . Reservoir permeability and communication . . . . . . . 598 4 . 8 . 8 . 1 . 2 . Generation of natural f r 4.8.8.1.2.1. Impact of r e s e r v o i r heterogenei t i e s . . . 4 . 8 . 8 . 1 . 2 . 2 . Fra c ture s thickness 4 . 8 . 8 . 1 . 2 . 3 . Fra c ture m a n d s t r e s s s e n s i t i v i t y . . 599 4.8.8.1.3. I n t er act i o n between natural and hydraul i c f r a c t u r e s ...................... 4 . 8 . 8 . 1 . 3 . 1 . General aspects . . . . . . 4 . 8 . 8 . 1 . 3 . 2 . O rie nta tion of na tura l a n d hydraulic f r a c t u r e s . 601 4 . 8 . 8 . 1 . 3 . 3 . Relationship between a n c i e n t a n d actual s t r e s s system . . . . . . . . . . . . . . 602 4 . 8 . 8 . 1 . 4 . I n t e r f r a c t u r e communication 4 . 8 . 8 . 1 . 4 . 1 . Permeabil 4.8.8.1.4.2. Stress se n siti 4 . 8 . 8 . 2 . F r act u r e divergence . . . . . . . . . . . . . . . 4 . 8 . 8 . 3 . Temporary natural f r a c t u r e damage . 4 . 8 . 8 . 3 . 1 . I n t e r a c t i o n s between na a n d s t i mu l ation f l u i d s 4.8.8.3.1.1. Natural c 4 . 8 . 8 . 3 . 1 . 2 . Dual f l u i d leakoff . . . . . . 605 . 606 4 . 8 . 8 . 3 . 1 . 3 . Ca pilla ry forc e s s 607 4.8.8.3.1.4. Impact of breaker 4 . 8 . 8 . 3 . 1 . 5 . Water blocking . . . . . . . . . . 607 607 4 . 8 . 8 . 3 . 2 . Fluid l o s s ............................ 4 . 8 . 8 . 3 . 2 . 1 . Breakdown f r a c t u r i n g p r o cedures . . . . . . . . . 608 4 . 8 . 8 . 3 . 2 . 2 . Ca pilla ry pressure a n d surfa c e tension . . . . . . . . . 609 4 . 8 . 8 . 3 . 2 . 3 . Dual leakoff behaviour . . 609 4 . 8 . 8 . 3 . 3 . Limited f r a c t u r e interconnectedness . . . 612 4 . 8 . 8 . 3 . 4 . Elevated treatment pressure . . . . . . . . . . . 612 4 . 8 . 8 . 3 . 5 . S t r e s s - s e n s i t i v i t y behaviour a n d asper i t y shearing . . . . . . . . . . . . . . . . . . . . . . . . 613 4 . 8 . 8 . 4 . Dendritic f r a c t u r i n g technique . . . . . . . . . . . . . . . . . . . 614 4 . 8 . 8 . 4 . 1 . Connection of natural a n d hydrau i c . . . . . . 614 f r a c t u r e s ...................... 4 . 8 . 8 . 4 . 1 . 1 . Diverting sta ge s . . . . . . . 614 4 . 8 . 8 . 4 . 1 . 2 . Reverse-f 1 ow p u l se stimu4.8.7.1.2.
4.8.8.
351
4.8.9.
4.8.10.
4.8.11.
l a t i o n . . . . . . . . . . . . . . . . . . 615 4.8.8.4.2. Shear s l i p p a g e vs . t e n s i l e f a i l u r e . . . . 615 616 4.8.8.5. T a i l o r e d p u l s e l o a d i n g ............................ 4.8.8.6. Proppant p l u g g i n g o f n a t u r a l f r a c t u r e s . . . . . . . . . . . . 616 4.8.8.6.1. P r o d u c t i o n a c c e l e r a t i o n by p r o p p i n g o f 616 n a t u r a l c r a c k s ........................ F r e s h r e s e r v o i r volume a c q u i s i t i o n and 4.8.8.6.2. r e c o v e r y enhancement by a r t i f i c i a l 617 f r a c t u r i n g ............................ 4.8.8.6.3. N a t u r a l p r o p p i n g o f sedimentary s h r i n 618 kage c r a c k s ........................... 4.8.8.7. O t h e r aspects ..................................... 618 619 L a t e - s t a g e r e f r a c t u r i n g o f o l d w e l l s ........................ 4.8.9.0. General r e f r a c t u r i n g s i g n i f i c a n c e . . . . . . . . . . . . . . . . . 619 4.8.9.1. R e f r a c t u r i n g c a n d i d a t e w e l l c h o i c e . . . . . . . . . . . . . . . . 619 4.8.9.1.1. Well performance a f t e r o r i g i n a l s t i m u 620 l a t i o n t r e a t m e n t ...................... 4.8.9.1.1.1. General aspects . . . . . . . . . 620 4.8.9.1.1.2. Success vs . f a i l u r e o f o r i g i n a l f r a c t u r e j o b ... 621 4.8.9.1.2. Comparable approach o f r e f r a c t u r i n g c a n d i d a t e s e l e c t i o n and m i n i f r a c t u r e c a l i b r a t i o n t e s t e v a l u a t i o n . . . . . . . . . . . 621 4.8.9.2. A r e a l d i s t r i b u t i o n o f p o t e n t i a l r e s e r v o i r s ........ 622 4.8.9.3. M o b i l i z a t i o n o f a d d i t i o n a l hydrocarbon r e s e r v e s . . . 622 4.8.9.3.1. A m e l i o r a t e d f e a s i b i l i t y due t o w e l l a m o r t i z a t i o n .......................... 622 4.8.9.3.2. P r o d u c t i o n d e c l i n e o f f s e t t i n g . . . . . . . . . 623 4.8.9.3.3. E a s i e r t e c h n i c a l t r e a t m e n t performance by presence o f weakness p l a n e . . . . . . . . . 623 4.8.9.3.4. Recovery enhancement independent f r o m r e s e r v o i r d e p l e t i o n stage . . . . . . . . . . . . . 624 4.8.9.3.5. Reservoir types s u i t a b l e f o r r e f r a c t u r i n g .................................. 624 Proppant s e l e c t i o n f o r r e f r a c t u r i n g ............... 624 4.8.9.4 4.8.9.4.1. S y n t h e t i c proppants vs . n a t u r a l sand . . 625 4.8.9.4.2. R e s e r v o i r p r e s s u r e vs . p r o p p a n t concent r a t i o n ............................... 625 F r a c t u r e r e o p e n i n g and proppant placement d u r i n g 4.8.9.5 626 r e f r a c t u r i n g ...................................... 4.8.9.5.1. I n c r e a s i n g f r a c t u r e w i d t h and p r o p p a n t 626 c o n c e n t r a t i o n ......................... 4.8.9.5.1.1. Arrangement o f p r i m a r y and secondary p r o p p a n t 626 l o a d .................... 4.8.9.5.1.2. Refracturing treatment 627 s i z e .................... 4.8.9.5.1.3. P r e s s u r e and s t r e s s d i s t r i b u t i o n . . . . . . . . . . . . . . . 627 4.8.9.5.1.4. P e r f o r a t e d i n t e r v a l conf i g u r a t i o n . . . . . . . . . . . . . . 627 4.8.9.5.2. P o s s i b i l i t i e s o f r e i n j e c t i o n complicat i o n s ................................. 627 4.8.9.6 . Combination o f h y d r a u l i c and e x p l o s i v e f r a c t u r i n g . 628 Combination o f a c i d and p r o p p a n t f r a c t u r i n g . . . . . . . . . . . . . . . . . 628 4.8.10.1. Proppant d i s s o l u t i o n .............................. 629 4.8.10.2. Treatment schedule and a c i d improvement . . . . . . . . . . . 629 4.8.10.2.1. J o i n t o r separated o p e r a t i o n . . . . . . . . . . 629 4.8.10.2.2. A c i d g e l l i n g and c r o s s l i n k i n g . . . . . . . . . 630 4.8.10.2.3. Succession o f stages . . . . . . . . . . . . . . . . . . 630 4.8.10.3. S p e c i a l a p p l i c a t i o n s .............................. 630 F r a c t u r e l e n g t h and h e i g h t o p t i m i z a t i o n ..................... 631
352 4 . 8 . 1 1 . 1 . R e s e r v o i r p e r m e a b i l i t y and w e l l s p a c i n g v s . f r a c ture l e n g t h ....................................... 631 4 . 8 . 1 1 . 1 . 1 . Long and s h o r t f r a c t u r e s i n low- and h i g h - p e r m e a b i l i t y r e s e r v o i r s . . . . . . . . . . 631 4.8.11.1.1.1. Reservoir permeability and f r a c t u r e l e n g t h . . . . 632 4.8.11.1.1.2. N e a r - w e l l b o r e damage zone b y p a s s . . . . . . . . . . . . 632 4.8.11.1.1.3. Fracture flow capacity and l e n g t h o p t i m i z a t i o n 632 4 . 8 . 1 1 . 1 . 2 . R e s e r v o i r and f r a c t u r e p a r a m e t e r s vs. economical a s p e c t s ........... 4.8.11.1.2.1. Fracture penetr n e t pay t h i c k n e s s .. Other a s p e c t s . . . . . . . . . . 6 3 3 4.8.11.1.2.2. 4 . 8 . 1 1 . 1 . 3 . P r e s s u r e d e c l i n e and f l u i d f l o w p a t t e r n 634 4 . 8 . 1 1 . 1 . 4 . F r a c t u r e e x t e n s i o n v s . r e s e r v o i r s h a p e 634 4 . 8 . 1 1 . 2 . F r a c t u r e c o n d u c t i v i t y vs. f r a c t u r e l e n g t h . . . . . . . . . 635 4 . 8 . 1 1 . 2 . 1 . Impact o f p r o p p a n t t y p e . . . . . 4 . 8 . 1 1 . 2 . 2 . F r a c t u r e f l o w c a p a c i t y o p t i m i z a t i o n . . . 636 4 . 8 . 1 1 . 2 . 3 . F r a c t u r e p r o d u c t i v i t y and f l u i d f l o w p a t t e r n ............................... 636 4 . 8 . 1 1 . 2 . 4 . A c c e l e r a t i o n v s . u l t i m a t e r e c o v e r y enenhancement ........................... 637 4 . 8 . 1 1 . 2 . 5 . C r e a t e d , propped and e f f e c t i v e f r a c t u r e 637 l e n g t h ................................ 4 . 8 . 1 1 . 3 . F r a c t u r e f l o w c a p a c i t y vs. p r o p p a n t d i s t r i b u t i o n . . 637 4 . 8 . 1 1 . 4 . Net p r e s e n t v a l u e o p t i m i z a t i o n .................... 638 4 . 8 . 1 1 . 5 . F i e l d examples .................................... 639 .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639 4.8.11.6, Fracture height .
4.9. Oil-reservoir fracturing . . . . . . . . . . . . . . 4.9.1. 4.9.2. 4.9.3. 4.9.4.
General a s p e c t s . . . . . . . . . . . . . . . 4.9.1.1. Gas- vs. o i l - r e s e r v o i 4.9.1.2. Proppant a p p l i c a t i o n Conductivity c o n t r a s t ............. ............. 4.9.2.1. P r o p p a n t t y p e ........... ............. 4.9.2.2. Proppant g r a i n s i z e ....... Economical f e a s i b i l i t y . . . . . . . . . . . . . . 4.9.3.1. Synthetic proppants v s . n a t 4.9.3.2. F r a c t u r e d e s i g n and p l u g g i n g ...................... S t e a m - d r i v e e f f e c t s on f r a c t u r e p r o p a g a t i o n . . . . . . . . . . . . . . . . . 4 . 9 . 4 . 1 . Changing g e o s t r e s s regime by t h e r m a l r e s e r v o i r e x pansion ........................................... 4.9.4.1.1. S t r e s s i n v e r s i o n due t o a n i s s e r v o i r expansion . . . . . . . . . . 4.9.4.1.2. C y c l i c f r a c t u r e c o l l a p s e and 4.9.4.1.2.1. Water f l a s h i n g . . . . . . . . . . 4.9.4.1.2.2. Thermal r e s e r v o i r expand p l a s t i c i t y chan4.9.4.1.2.3.
4.9.4.1.3.
643 643 644
645
. . . . . . . . . . . . . . . . . . 646
Steam i n j e c t i v i t y improvement by r e s e r v o i r f r a c turing .................. 4.9.4.1.2.4. I n t e r w e l l communication and f r a c t u r e i n t e r f e r e n c e 4.9.4.1.2.5. Gravity drainage through fractures ............... 4.9.4.1.2.6. Other aspects ........... Other p o s s i b i l i t i e s o f c h a n g i n g s t r e s s d i r e c t i o n ............................. 4.9.4.1.3.1. Altered-stress fracturing
646 646 647 647 641 647
353 S t r u c t u r a l c o n t r o l vs . s t r e s s c o n t r o l .......... 4.9.4.2. Combination o f h o r i z o n t a l and v e r t i c a l f r a c t u r i n g . 4.9.4.2.1. F r a c t u r e c r e a t i o n ..................... 4.9.4.2.1.1. Thermal r e s e r v o i r expans i o n and i n c r e a s i n g c l o s u r e s t r e s s ............. 4.9.4.2.1.2. H o r i z o n t a l f r a c t u r e s and h o r i z o n t a l vs . v e r t i c a l w e l l s ................... 4.9.4.2.1.3. M u l t i p l e h o r i z o n t a l and v e r t i c a l f r a c t u r e propag a t i o n .................. 4.9.4.2.1.4. Combination o f h y d r a u l i c and e x p l o s i v e f r a c t u r i n g 4.9.4.2.2. F r a c t u r e management ................... 4.9.4.2.3. F r a c t u r e d e t e r m i n a t i o n ................ 4.9.4.2.3.1. F r a c t u r e g r a d i e n t vs . overburden s t r e s s g r a d i e n t ................... 4.9.4.2.3.2. R e s e r v o i r expansion and changing i n - s i t u s t r e s s . 4.9.4.3. Changes o f f r a c t u r e azimuth o r i e n t a t i o n ........... 4.9.4.3.1. On-trend and o f f - t r e n d w e l l p a t t e r n s .. 4.9.4.3.2. Staggered steam i n j e c t i o n i n t o o f f t r e n d w e l l p a i r s ...................... 4.9.4.3.3. Mechanisms o f f r a c t u r e azimuth o r i e n t a t i o n changes .......................... C o l d w a t e r i n j e c t i o n e f f e c t s on h y d r a u l i c f r a c t u r i n g ........ 4.9.5.1. I n - s i t u s t r e s s and f r a c t u r e p r e s s u r e r e d u c t i o n .... 4.9.5.2. R e s e r v o i r temperature and f r a c t u r e g r a d i e n t reduct i o n .............................................. 4.9.5.3. F o r m a t i o n p a r t i n g and s t r e s s r e d u c t i o n a t f r a c t u r e t i p ............................................... 4.9.5.4. P r i m a r y and secondary f r a c t u r e s ................... 4.9.5.5. R e s e r v o i r pressure, i n j e c t i v i t y and f r a c t u r e l e n g t h 4.9.4.1.3.2.
4.9.5.
4.10. Comnunication fracturing ............................................ 4.10.1. C o n v e n t i o n a l s t o r e y - w i s e f r a c t u r i n g ......................... 4.10.1.1. Repeated s t e p w i s e f r a c t u r i n g i n R o t l i e g e n d and Carb o n i f e r o u s o f Germany FRG ......................... 4.10.1.2. Commingling o f s e p a r a t e l a y e r s a f t e r i n d i v i d u a l f r a c t u r i n g ........................................ 4.10.1.3. M u l t i l a y e r r e s e r v o i r s w i t h and w i t h o u t c r o s s f l o w .. 4.10.1.4. Connection o f s e p a r a t e l a y e r s by j o i n t f r a c t u r i n g . 4.10.2. A p p l i c a t i o n in p r o d u c t i o n d r i l l i n g .......................... 4.10.2.1. Connection o f i s o l a t e d sand bodies ................ 4.10.2.1.1. Magnitude o f i n - s i t u s t r e s s c o n t r a s t .. 4.10.2.1.2. F r a c t u r e azimuth vs l e n s a x i s o r i e n t a t i o n .................................. 4.10.2.1.3. Sand body s i z e and spacing ............ 4.10.2.2. Connection o f s e p a r a t e d r e s e r v o i r s t o r e y s ......... 4.10.3. I n f l u e n c e o f p e r f o r a t i o n arrangement ........................ 4.10.3.1. P e r f o r a t i o n o r i e n t a t i o n and s t r e s s d i r e c t i o n . . . . . . 4.10.3.2. Angular d i s t r i b u t i o n o f p e r f o r a t i o n s .............. 4.10.3.3. P e r f o r a t i o n breakdown ............................. 4.10.4. A p p l i c a t i o n i n a p p r a i s a l d r i l l i n g ........................... 4.10.5. F r a c t u r e i n t e r f e r e n c e ....................................... 4.10.5.1. O i l and gas r e s e r v o i r s ............................ 4.10.5.1.1. F r a c t u r e l e n g t h and w e l l d r a i n a g e r a d i u s .................................. 4.10.5.1.2. F r a c t u r e o r i e n t a t i o n p r e d i c t i o n and i n -
.
650 651 651 651 651 652 653 653 654 654 655 655 656 656 656 657 657 658 658 659 659 659 660 660 660 661 661 662 662 662 663 663 663 664 664 665 665 665 666 666 667
354
4.10.5.2.
t e r f e r e n c e avoidance L e n t i c u l a r t i g h t gas Steam-drive o p e r a t i o n . . . . . . . . . . . . . . . . . 667 Geothermal r e s e r v o i r s ............................. 668
4.10.5.1.3. 4.10.5.1.4.
4.11. Fracturing o f deviated wells
.
......
.....
4.11.1. Fracture propagation a .................... 4.11.1.1. Wellbore a x i s n o t p a r a l l e l t o i n - s i t u p r i n c i p a l
s t r e s s d i r e c t i o n .................................. L i m i t e d communication i n t e r v a l between f r a c t u r e p l a n e and w e l l b o r e s e c t i o n ........................ 4 . 1 1 . 1 . 2 . 1 . Drawbacks i n o p e r a t i o n and performance 4.11.1.2.1.1. Wellbore i n s t a b i l i t y . . . . 4 . 1 1 . 1 . 2 . 1 . 2 . Fracture height determin a t i o n by w e l l l o g g i n g .. 4 . 1 1 . 1 . 2 . 2 . Comparable s i t u a t i o n i n v e r t i c a l w e l l s traversing steeply dipping reservoirs . 4 . 1 1 . 2 . Proppant s t r a t i f i c a t i o n and f l u i d l a y e r i n g . . . . . .... 4 . 1 1 . 3 . Hydrocarbon p r o d u c t i o n and f l o w regimes ..................... 4 . 1 1 . 3 . 1 . R e d u c t i o n o f l o n g - t e r m w e l l p r o d u c t i v i t y by chang i n g f l o w p a t t e r n .......................... ...... 4 . 1 1 . 3 . 1 . 1 . Flow i n t e r v a l types i n v e r t i c a l e l l s . 4 . 1 1 . 3 . 1 . 2 . Flow i n t e r v a l t y p e s i n h o r i z o n t a w e l l s 4 . 1 1 . 3 . 2 . Non-Darcy f l o w and s k i n f a c t o r _............ . . . . . . 4 . 1 1 . 4 . Improvements o f f r a c t u r e e f f e c t i v i t y i n d e v i a t e d w e l l s . . . . . .
4.11.1.2.
668 668 669 669 670 670 670 671 671 671 671 672 672 672 673
. . . . . . 673 4 . 1 2 . Fracture damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . 1 2 . 1 . General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673 4 . 1 2 . 1 . 1 . F r a c t u r e c o n d u c t i v i t y vs. f o r m a t i o n permeabi ity . . . . . . 674 damaae ..................................... 4.12:l. 1.1. C o n t r a s t magnitude .... 4.12.1.1.2. Relative significance .......... 4 . 1 2 . 1 . 1 . 3 . Breaker t y p e and c o n c e n t r a t i o n . . . . . . . . 675 4 . 1 2 . 1 . 2 . Proppant c r u s h i n g and embedment vs. c l o s u r e s t r e s s 675 4 . 1 2 . 2 . Proppant s e t t l i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675 4 . 1 2 . 2 . 1 . P o s t - f r a c t u r i n g p r e s s u r e d e c l i n e . . . . . . . . . . . . . . . . . . 676 4 . 1 2 . 2 . 2 . Proppant t r a n s p o r t and banking . . . . . . . . 676 4 . 1 2 . 2 . 2 . 1 . Proppant s e t t l i n g ..................... 677 4 . 1 2 . 2 . 2 . 1 . 1 . Clustered proppant trans-
p o r t and h i n d e r e d s e t t l i n g .................... 677 4 . 1 2 . 2 . 2 . 1 . 2 . F o r c e d f r a c t u r e c l o s u r e . 678 4 . 1 2 . 2 . 2 . 1 . 3 . W e l l b o r e v i c i n i t y and t a i l - i n . . . . . . . . . . . . . . . . . 678 4 . 1 2 . 2 . 2 . 1 . 4 . O t h e r aspects ........... 6 7 9 4 . 1 2 . 2 . 2 . 2 . Proppant banking and f o r m a t i o n g r a i n s i z e d i s t r i b u t i o n ..................... 679 4 . 1 2 . 2 . 2 . 2 . 1 . F i n i n g - and c o a r s e n i n g upwards g r a i n - s i z e sequences ................. 679 4 . 1 2 . 2 . 2 . 2 . 2 . Hydrocarbon d i s t r i b u t i o n i n granulometrical cyclothems . . . . . . . . . . . . . . . . . . . 680 4 . 1 2 . 2 . 2 . 3 . F r a c t u r e c l o s u r e and f l u i d v i s c o s i t y . . 680 .. 680 4 . 1 2 . 2 . 2 . 4 . O t h e r aspects ....................... 4 . 1 2 . 2 . 3 . C a r r i e r f l u i d v i s c o s i t y ........................ . . 681 4 . 1 2 . 2 . 3 . 1 . C r o s s l i n k i n g types . . . . . . . . . . . . . . . . . . . 6 8 1 4 . 1 2 . 2 . 3 . 2 . Proppant t r a n s p o r t c a p a c i t y . . . . . . . . . . 681 4 . 1 2 . 2 . 3 . 3 . F l u i d r h e o l o g y s t a b i l i t y . . . . . . . . . . . . . 682 4 . 1 2 . 2 . 4 . Pad and c a r r i e r f l u i d volumes . . . . . . . . . . . . . . . . . . . . 682 4 . 1 2 . 2 . 5 . S l u r r y c o n s i s t e n c y and p r o p p a n t c l u s t e r i n g . . . . . .. 682 4 . 1 2 . 2 . 5 . 1 . Proppant c l u s t e r i n g and f l u i d v i s c o s t Y
355 683 v a r i a t i o n ............................. S l u r r y s e d i m e n t a t i o n and p r o p p a n t c l u s t e r f o r m a t i o n ......................... 683 P r o p p a n t f l o w b a c k ........................................... 683 684 4 . 1 2 . 3 . 1 . W e l l c l e a n u D ......... ............................. 4 . 1 2 . 3 . 1 . 1 . F r a c t u r e c o s u r e and p r o p p a n t t r a p p i n g 684 4 . 1 2 . 3 . 1 . 1 1. P r e s s u r e d e p l e t i o n and f l u i d l e a k o f f ........... 684 4.12.3.1.1 2 . Pressure d e c l i n e a s s i s tance by c o n t r o l l e d f l u i d p r o d u c t i o n . . . . . . . . . . . . . . 685 4 . 1 2 . 3 . 1 . 2 . P r o p p a n t f owback c o n t r o l and d i m i n u t i o n ..... ............................ 685 4.12.3.1.2.1. F l u i d f l o w v e l o c i t y and p r e s s u r e d r o p ........... 685 4.12.3.1.2.2. F l u i d v i s c o s i t y and w e l l c l e a n u p ................. 686 4 . 1 2 . 3 . 1 . 3 . E f f e c t s . o f f r a c t u r i n g f l u i d and c a p i l l a r y p r e s s u r e o n w e l l c l e a n u p ......... 686 4.12.3.1.3.1. Dimensionless f r a c t u r e c o n d u c t i v i t y . . . . . . . . . . . . 687 4.12.3.1.3.2. C a p i l l a r y p r e s s u r e . . . . . . 687 687 4.12.3.2. P r o p p a n t g r a i n s i z e ............................... 4 . 1 2 . 3 . 3 . R e s i n - c o a t e d p r o p p a n t t a i l - i n ..................... 688 4.12.3.3.1. F r a c t u r i n g o p e r a t i o n f a i l u r e by propp a n t f l o w b a c k ......................... 688 4.12.3.3.2. Package s t a b i l i t y improvement b y r e s i n 689 c o a t e d p r o p p a n t s ...................... 4.12.3.3.2.1. P r o p p a n t f l o w b a c k c o n t r o l 689 4 . 1 2 . 3 . 3 . 2 . 2 . T a i l - i n l e n g t h and a d d i t i o n a l gravel packing s t a b i l i z a t i o n . . . . . . . . . . . 689 4.12.3.4. Foam and e n e r g i z e d f r a c t u r i n g f l u i d s .............. 690 691 4 . 1 2 . 3 . 5 . S h u t - i n t i m e a d j u s t m e n t ........................... 691 P r o p p a n t c r u s h i n g ........................................... 691 4 . 1 2 . 4 . 1 . P r o p p a n t shock l o a d i n g ............................ 4 . 1 2 . 4 . 1 . 1 . B o t t o m h o l e f l o w i n g p r e s s u r e and choke 691 s i z e .................................. 4 . 1 2 . 4 . 1 . 2 . S t r e s s c o r r o s i o n c r a c k i n g . . . . . . . . . . . . . 692 4.12.4.2. P r e s s u r e drawdown and h y d r o c a r b o n p r o d u c t i o n r a t e s 692 4.12.4.2.1. Careful slow s t a r t i n g o f w e l l product i o n a f t e r hydraulic fracturing o r gra692 v e l p a c k i n g ........................... 4 . 1 2 . 4 . 2 . 2 . E f f e c t o f c r u s h i n g on c o n d u c t i v i t y o f 693 sand and b a u x i t e ...................... 4.12.4.2.3. F r a c t u r e c o n d u c t i v i t y degradation vs . r e s i d u a l p r e s e r v a t i o n . . . . . . . . . . . . . . . . . 694 4.12.4.2.4. P r o p p a n t c r u s h i n g and c l o s u r e s t r e s s r e s i s t i v i t y i n t e r v a l s . . . . . . . . . . . . . . . . . 694 4.12.4.3. P r o d u c t i o n c y c l i n g b y r e p e a t e d w e l l s h u t - i n . . . . . . . 694 4.12.4.3.1. Irrecoverable fracture conductivity di695 m i n u t i o n .............................. 4 . 1 2 . 4 . 3 . 2 . C o m b i n a t i o n o f p r o p p a n t embedment and 695 c r u s h i n g .............................. 4 . 1 2 . 4 . 3 . 3 . I m p a c t o f s h u t - i n t i m e . . . . . . . . . . . . . . . . 696 F l u i d a s p e c t s ............................................... 696 4.12.2.5.2.
4.12.3.
4.12.4.
4.12.5.
4.13. Aggressive f r a c t u r i n g design ........................................ 4.13.1. P r o g r e s s i n s t e a d o f r o u t i n e ................................. 4.13.2. R i s k y a p p r o a c h o f m a r g i n a l r e s e r v o i r s ....................... 4.14. C o n c l u s i o n ..........................................................
696 697 697 698
356
4.1. Introduction As a consequence o f t h e p o i n t s d i s c u s s e d i n t h e p r e c e d i n g e c o n o m i c a l l y - o r i e n ted chapters, the r e l a t i v e l y retarded s i t u a t i o n i n the hydraulic proppant f r a c t u r i n g and g r a v e l p a c k i n g scenery i n Europe and o t h e r i n t e r n a t i o n a l markets ( w i t h r e s p e c t t o t h e USA where a c t i v i t y i s g e n e r a l l y on a h i g h e r l e v e l ) a f t e r t h e o i l p r i c e drop i n 1986 ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ; p a r t i a l l y l a s t i n g u n t i l 1987 and s t i l l n o t f u l l y r e c o v e r e d i n 1988 i n some areas, a l s o as a consequence o f i t s a c c e n t u a t i o n by t h e accompanying and f o l l o w i n g US $ exchange r a t e c o l l a p se up t o i t s h i s t o r i c a l minimum l e v e l ; c f . s e c t i o n 2 . 2 . 1 . 3 . ) s h o u l d be t a k e n by b o t h s e r v i c e and p r o p p a n t companies as a c h a l l e n g i n g o p p o r t u n i t y t o u n d e r s t a n d t h e l e s s o n and n o t t o r e p e a t t h e m i s t a k e s o f t h e p a s t , b u t t o l a y now p a t i e n t l y t h e necessary groundwork i n h i t h e r t o i n s u f f i c i e n t l y a t t e n d e d markets f o r n e a r f u t u r e development o r even booming by h e l p i n g t o i n t r o d u c e and t o promote var i o u s t e c h n o l o g i c a l p o s s i b i l i t i e s ( i n c o r p o r a t e d i n t o sound m a r k e t i n g concepts) o f enhancing t h e g e n e r a l s t i m u l a t i o n p o t e n t i a l and t h u s a t t h e b o t t o m o f t h e l i n e o i l and gas s u p p l y . A f t e r b r i e f l y s k e t c h i n g and c h a r a c t e r i z i n g p o s s i b i l i t i e s o f expanding t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g market i n t h e near f u t u r e , an o u t l i n e o f t h e t e c h n i c a l r e v i e w i s g i v e n .
4.1.1. Possibilities o f expanding the hydraulic proppant fracturing market i n the near future A c c o r d i n g t o my g e o l o g i c a l , r e s e r v o i r e n g i n e e r i n g , t e c h n i c a l and m a r k e t i n g e x p e r i e n c e , a s u i t e o f more o r l e s s u n c o n v e n t i o n a l methods o f performance o r i m p r o v a l o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g e x i s t i n economicall y and t e c h n i c a l l y more o r l e s s mature s t a t u s prepared f o r e x e c u t i o n w h i c h p a r t i a l l y a r e a l r e a d y more o r l e s s f a m i l i a r i n t h e USA, b u t have s t i l l n o t reached common sense o r widespread a p p l i c a t i o n i n Europe and p r o b a b l y a l s o o t h e r i n t e r n a t i o n a l m a r k e t s . O t h e r methods a r e s t i l l i n t h e s t a t u s o f t e c h n o l o g i c a l prepar a t i o n , enhancement and p e r f e c t i v a t i o n and need i n t r o d u c t i o n t o t h e m a r k e t by p i l o t o p e r a t i o n s s u p p o r t e d by t r i g g e r i n g a t t r a c t i v e i n n o v a t i v e o f f e r s . Both groups o f techniques c o u l d i n c l u d e v e r y soon some q u i t e s u i t a b l e concepts o f s t i m u l a t i o n o f h i t h e r t o m a r g i n a l r e s e r v o i r s when i n c r e a s i n g demand and d e c l i n i n g s u p p l y i n t h e near f u t u r e i s a l s o i n Europe and o t h e r areas o u t s i d e o f t h e USA more and more a c c e n t u a t i n g t h e f o r c e o f enhancement o f domestic p r o d u c t i o n w i t h i n t h e i n d i v i d u a l c o u n t r i e s ( t h i s a p p l i e s e s p e c i a l l y f o r E a s t e r n Europe i n l i g h t o f t h e p r e s e n t economical, p o l i t i c a l and f i n a n c i a l framework; c f . s e c t i o n
2.4.4.).
Many o f t h e concepts and t e c h n o l o g i e s d i s c u s s e d in t h i s c h a p t e r , however, s t i l l r e q u i r e some homework b e f o r e t h e i r widespread f i e l d u t i l i z a t i o n i n var i o u s r e s e r v o i r types, and success i n t h e p a r t i c u l a r o p e r a t i o n s needs improved u n d e r s t a n d i n g , s k i l l e d s e l e c t i o n and c a r e f u l i m p l e m e n t a t i o n . The main s t r a t e g i e s o f market expansion aim on p r o g r e s s i v e replacement o f n a t u r a l sand by synt h e t i c p r o p p a n t s i n a p p l i c a t i o n s near t h e boundary o f p h y s i c a l and chemical s t a b i l i t y o f sand and f o r t h e reason o f a c h i e v i n g h i g h e r c o n d u c t i v i t y , and on hyd r a u l i c s t i m u l a t i o n o f troublesome r e s e r v o i r s f o r which a s u i t a b l e and successf u l concept has n o t y e t been found i n t h e p a s t , b u t can be o f f e r e d when u t i l i z i n g a l l t h e b e n e f i t s which a r e p r o v i d e d by t h e v a r i o u s t y p e s o f man-made h i g h qua1 it y p r o p p a n t s .
4.1.2. Outline of the technical review and status report A few p a r t i c u l a r l y i m p o r t a n t t e c h n o l o g i c a l aspects o f n e a r - f u t u r e enhancement o f o i l and gas p r o d u c t i o n by approaching m a r g i n a l r e s e r v o i r s by h y d r a u l i c p r o p p a n t f r a c t u r i n g o r by c a r r y i n g o u t u n c o n v e n t i o n a l s t i m u l a t i o n j o b s i n Europe and o t h e r i n t e r n a t i o n a l markets ( a s f a r as r e c o g n i z e d f r o m my view) comprise containment o f f r a c t u r e p r o p a g a t i o n i n t h i n sandstones, p r o p p a n t s a t u r a t i o n
357 and c o n d u c t i v i t y improvement, t a r g e t s o f unconventional gas s t i m u l a t i o n , hydraul i c proppant f r a c t u r i n g o f carbonate rocks, h i g h - p e r m e a b i l i t y r e s e r v o i r f r a c t u r i n g , geothermal f r a c t u r i n g , m i n i f r a c t u r i n g and o t h e r s m a l l - s c a l e f r a c t u r i n g , o i l - r e s e r v o i r f r a c t u r i n g , and communication f r a c t u r i n g . These methods and techniques are proposed f o r c o n s i d e r a t i o n by the a r e a l s p e c i a l i s t s and are b r i e f l y presented i n summarizing o u t l i n e and commented as f o l l o w s i n c l u d i n g d i s c u s s i o n o f the most s i g n i f i c a n t l i t e r a t u r e (numerous references on general and s p e c i f i c t e c h n i c a l and r e s e r v o i r engineering aspects are compiled i n b o t h main l i s t and s u b j e c t key c l a s s i f i c a t i o n i n chapter 7;-aspects o f p r o d u c t i o n geology and assoc i a t e d r e s e r v o i r engineering are summarized by DICKEY 1981, DIKKERS 1985 and NORTH 1985). Factors i n f l u e n c i n g o r t r i g g e r i n g f r a c t u r e damage are a l s o reported, and as a general c o n c l u s i o n an aggressive f r a c t u r i n g design i s recommended i n o r d e r n o t t o g e t stuck w i t h c o n s e r v a t i v e and conventional techniques, b u t t o m a i n t a i n progress by c a r r y i n g o u t unconventional and c h a l l e n g i n g i n n o v a t i v e methods desp i t e o f i n c l u d i n g some r i s k o f drawbacks o r even i n t e r m i t t e n t f a i l u r e s i n marginal a p p l i c a t i o n s o r p i l o t operations. The account does n o t aim on any completeness, b u t focusses on s u b j e c t s which due t o my own assessment according t o my experience i n the market need p r e f e r e n t i a l a t t e n t i o n d u r i n g t h e coming years f o r improving e s p e c i a l l y the European s t i m u l a t i o n p o t e n t i a l and t o enhance the f r a c t u r i n g a c t i v i t y i n t h i s scene.
4.2. Containment o f fracture propagation i n t h i n sandstones While f r a c t u r e operations w i t h i n t h i c k sandstone r e s e r v o i r columns s u f f i c i e n t l y f a r away from gas-water-contact and bounding mudstones are p a r t i c u l a r l y t a k i n g p l a c e i n g r e a t e r depth where danger o f breakthrough t o t h e ground-water s t o r e y i s no t o p i c f o r environmental d i s c u s s i o n and the t h r e a t o f connection t o water-bearing sandstones w i t h i n o r below t h e pay column i s a l s o g e n e r a l l y no i t e m o f worrying, treatment o f t h i n sandstone r e s e r v o i r s w i t h i n sandwich-type a l t e r n a t i n g sandstone-mudstone successions w i t h p a r t i a l l y d i f f e r e n t f l u i d cont e n t o f t h e v a r i o u s sandstone horizons c r e a t e s problems o f containment o f f r a c t u r e propagation i n v e r t i c a l d i r e c t i o n i n o r d e r n o t t o achieve unwanted d e t e r i o r a t i o n s o f t h e e f f e c t o f the f r a c t u r e jobs (DANESHY 1978; SIMONSON, ABOU-SAYED & CLIFTON 1978; ROSEPILER 1979; WARPINSKI, SCHMIDT & NORTHROP 1980; PRATS 1981, TEUFEL & CLARK 1981, EEKELEN 1982; WARPINSKI, CLARK, SCHMIDT E HUDDLE 1982; SETTAR1 1983, WARPINSKI & TEUFEL 1984). Precise c o n t r o l o f f r a c t u r e h e i g h t growth and p r e v e n t i o n o f p e n e t r a t i o n o f t h e crack through the bounding b a r r i e r s i s e s s e n t i a l f o r a v o i d i n g unwanted e f f e c t s such as water o r gas coning and breakthrough a f t e r t h e treatment and thus d e t e r i o r a t i o n o f the r e s u l t o f the operation, and f o r securing o p t i m i z a t i o n o f the area d r a i n e d by the f r a c t u r e (BEN NACEUR & TOUBOUL 1987). Undesirable zones o f f r a c t u r e e x t e n s i o n n o t o n l y comprise gas caps and water bottoms o f o i l r e s e r v o i r s , b u t g e n e r a l l y i n c l u d e a l l unproductive l a y e r s a d j o i n i n g t o the p r o s p e c t i ve i n t e r v a l , w i t h f r a c t u r e containment aiming on maximization o f crack propagat i o n w i t h i n the h o r i z o n o f i n t e r e s t and m i n i m i z a t i o n o f excessive upwards and downwards growth. The i n t r u s i o n o f a h y d r a u l i c f r a c t u r e from the pay zone i n t o the horizons l y i n g above and below i s a s e r i o u s concern i n crack design, because i f the h y d r a u l i c f r a c t u r e i s n o t contained w i t h i n the producing f o r m a t i o n and progagates i n b o t h v e r t i c a l and l a t e r a l d i r e c t i o n s , f a i l u r e o f t h e s t i m u l a t i o n treatment can occur due t o s u b s t a n t i a l l o s s o f f l u i d and proppants which are used up and wasted f o r s t i m u l a t i n g the unproductive l a y e r s a d j o i n i n g the r e s e r v o i r (NGUYEN & LARSON 1983, ACHARYA & K I M 1987). F i e l d experience has shown t h a t the most common cause of poor f r a c t u r e treatment response i s excessive f r a c t u r e h e i g h t growth (HOLDITCH, ROBINSON & WHITEHEAD 1986), because o u t - o f c o n t r o l f r a c t u r i n g r e s u l t s i n s h o r t e r f r a c t u r e l e n g t h s than d e s i r e d (GREGORCZYK, PAULS, HOLTMYER, CHISHOLM & VENDITTO 1984). Massive
hydraulic
proppant
fracturing
has
been h i s t o r i c a l l y developed by
358
t r i a l a n d e r r o r and i t s r e s u l t s a r e uncertain in many s i t u a t i o n s (TEUFEL & CLARK 1981; c f . s ect i o n s 1 . 1 . 1 . and 4 . 8 . 1 . 2 . ) . While some of these la rge -sc a le s t im u l a t i o n e f f o r t s have been s u cces s f u l , o t h e r s have turned o u t t o be extremel y disappointing f a i l u r e s mainly f o r the reason of lack of improved understandi n g of f r a c t u r e propagation and containment. The economical s i g n i f i c a n c e of f r a c t u r e extension control i s t h a t i f the crack i s not confined within the p r o ducing sandstone a n d propagates in b o t h v e r t i c a l and l a t e r a l d i r e c t i o n s in a n e l l i p t i c a l shape, then t h er e i s an e f f e c t i v e wasting of expensive stim ula tion f l u i d s a n d proppants which a r e consumed t o f r a c t u r e a n d t o support barren horizons next t o the r es er v o i r complex, r es p ect i v ely. The o u t l i n e a s follows concent r a t e s on a r e a l d i s t r i b u t i o n of p o t en t i al r e s e r v o i r s , technical aspects of f r a c t u r i n g o p e r a t i o n, f r a c t u r e propagation, r es ervoir heterogeneity and anisotropy ( in c l u d i n g sedimentological, petrophysical and rock mechanical a s p e c t s ) , combination of hydraulic f r a c t u r i n g a n d gravel packing, a n d economical s i g n i f i c a n c e .
4.2.1. Areal distribution of potential reservoirs T h i n s i n g l e o r mu l t i p l e, bundle- or sandwich-type sandstone horizons within mudstone sequences ( c f . p l a t e s IV/4 - 8 , VII/1 - 6, V I I I and I X ) with d i f f e r e n t thicknesses of both sandstone l a y e r s and mudstone i n t e r v a l s v e r t i c a l l y separat i n g adjoining sandstone s h eet s o f l a t e r a l l y more or l e s s continuous type a n d subordinately a l s o overlapping or juxtaposed lenses of various degree of i n t e r connectedness a r e in Europe p a r t i c u l a r l y present i n the upper p a r t of the Rotliegend gas-bearing sandstone succession (Hannover-Formation or Ten Boer-Formatio n sandstones; Wechselfolge f a c i e s as s o ci at ion; BRADEL & DRAXLER 1982, PHIL I P P & R E I N I C K E 1982, KLOSE & KROMER 1983; HEDEMANN, MASCHEK, PAULUS & P L E I N 1984; GRALLA 1988; c f . s ect i o n 3 . 2 . 1 . 3 . ) in the B r i t i s h and Dutch Southern North Sea a s well as i n Netherlands and Germany FRG onshore, Germany GDR and Poland; i n the J u r a s s i c o i l - b ear i n g sandstone succession i n the B r i t i s h a n d Norweg i a n Northern North Sea, in some Palaeozoic and Mesozoic oil- and gas-bearing s e c t i o n s in the B a l t i c Sea offshore Poland a n d USSR, in Mesozoic a n d Cenozoic o i l - a n d gas-bearing sandstones onshore a n d offshore (Black Sea) in Bulgaria, i n Cretaceous a n d p a r t i a l l y a l s o J u r a s s i c shallow oil-be a ring sandstones a n d carbonates in the northwestern p a r t of Germany FRG ( p a r t i a l l y extending across the boundary t o Netherlands), in T e r t i a r y t o T r i a s s i c moderately-deep o i l - b e a r ing sandstones i n the southern p a r t of Germany FRG, and in T e r t i a r y o i l - and gas-bearing sandstones in the Vienna Basin i n Austria ( c f . KREUTZER 1985, 1986; B R K U E R 1987) and adjoining Czechoslovakia ( c f . THON 1985) a n d t o minor amounts a l s o i n the A d r i a t i c Basin in I t a l y and Yugoslavia offshore ( c f . DRAGICEVIC, MARINOVIC, STANKOVIC, BATUSIC & N I K O L I C 1983; IBRAHIMPASIC 1983) a s well a s onshore i n Hungary a n d Yugoslavia ( c f . t ab s . 5 - 9 ) . Some unstable Cretaceous shallow gas-bearing sandstones of limited thickness a re a l s o present i n Mozambique i n A f r i c a .
4.2.2. Technical aspects o f fracturing operation While in p a r t s of the mentioned formations as well as i n othe r comparable re s e r v o i r s in various p a r t s of the w o r l d already f r a c t u r i n g jobs w i t h lim ite d vert i c a l propagation ( a n d i n many cases consequently a l s o r e s t r i c t e d horizontal extension, r e s u l t i n g i n consumption of only low to moderate q u a n t i t i e s of p r o p ping m a t e r i a l s ) have been s u cces s f u l l y c a r r i e d out, much optimization could s t i l l be done by enhancing p r o p p a n t choice in terms of both type a n d gra in s i z e , amelioration of t a i l - i n o p er at i o n s , accentuation of the permeability cont r a s t between r e s e r v o i r sandstone a n d proppant package by s h i f t i n g t o l a r g e r p r o p p a n t grain s i z e s a n d varying the r e l a t i v e q u a n t i t i e s of d i f f e r e n t m a t e r i a l s in stepwise treatments, a n d improvement of f r a c t u r e width by pumping of higher proppant concentrations per f l u i d u n i t . Aspects of singula r vs. m ultiple f r a c t u ring and d i v e r t i ng techniques, limited-entry f r a c t u r i n g technique, f r a c t u r e containment i n layered r e s e r v o i r s by treatment parameter adjustment a s well as by buoyant a n d s e t t l i n g d i v e r t e r s a n d blocking a d d i t i v e s , multiple-zone f r a c t u r e
359 m i g r a t i o n concept, s e l e c t i v e p r o p p a n t placement, and s p e c i a l phenomena seams a r e d i s c u s s e d as f o l l o w s .
in
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4.2.2.1. Singular vs. multiple fracturing and diverting techniques H y d r a u l i c p r o p p a n t f r a c t u r i n g can be performed i n e i t h e r s i n g u l a r o r m u l t i p l e manner depending on r e s e r v o i r p i l e c o m p o s i t i o n and c o m p l e t i o n t y p e i n c l u d i n g commingling o f s e v e r a l separate l a y e r s o r n o t . S i n g u l a r s t o r e y - w i s e f r a c t u r i n g w i t h f i r s t s t i m u l a t i o n o f t h e l o w e r zone and f o l l o w i n g i t s i s o l a t i o n and upwards movement second t r e a t i n g o f an upper h o r i z o n ( c f . s e c t i o n s 2.4.1.1., 3.3. and 4.10.1.) has t o be c a r r i e d o u t i f t h e s p e c i f i c r e s e r v o i r c o n d i t i o n s r e q u i r e d i f f e r e n t f r a c t u r i n g f l u i d types (HOEL 1988) o r d i f f e r e n t p r o p p a n t types t o r e n d e r t h e i n d i v i d u a l j o b s t e c h n i c a l l y s u c c e s s f u l and/or e c o n o m i c a l l y p r o f i t a b l e . I f t h e same f r a c t u r i n g f l u i d and t h e same p r o p p a n t t y p e can be a p p l i e d f o r s t i m u l a t i o n o f s e v e r a l v e r t i c a l l y successive l e v e l s o f t h e p r o s p e c t i v e complex, m u l t i p l e f r a c t u r i n g r e p r e s e n t i n g j o i n t t r e a t i n g o f a l l t h e zones i s t h e adequate s o l u t i o n p r o v i d e d t h e i n - s i t u s t r e s s e s a r e i n a comparable range t o a l l o w more o r l e s s simultaneous opening o f t h e f r a c t u r e s i n a l l t h e t a r g e t e d sections. CLARK (1987) compares t u b i n g vs. c a s i n g as t r e a t i n g medium and e v a l u a t e s casing preparation f o r l a t e r f r a c t u r i n g stimulation. I f the operation i s p e r f o r med through t h e casing, t h e w e l l must be k i l l e d a f t e r f r a c t u r i n g f l u i d cleanup i n o r d e r t o i n s t a l l t h e t u b i n g (WEBSTER 1977), w i t h w e l l k i l l i n g always i n c l u d i n g c o n s i d e r a b l e r i s k o f crack damage (SLUSSER & RIECKMANN 1976). An a l t e r n a t i v e t o w e l l k i l l i n g f o r t u b i n g i n s t a l l a t i o n i s t h e r e f o r e f r a c t u r i n g down t u b i n g and/or annulus a t reduced i n j e c t i o n r a t e s and w i t h h i g h f r i c t i o n p r e s s u r e . The most i m p o r t a n t q u e s t i o n s a r e d i v e r t i n g agent performance and s e l e c t i o n as w e l l as m u l t i p l e f r a c t u r i n g aspects.
4.2.2.1.1. Diverting agent performance I n terms o f o p e r a t i o n s , t h e f r a c t u r e s can be c a r r i e d o u t s e p a r a t e l y w i t h i n e v e r y i n d i v i d u a l sandstone h o r i z o n o f t h e sandwich-type i n t e r b e d d i n g by s e a l i n g w i t h b r i d g e p l u g and packer ( c f . s e c t i o n s 2.4.1.1., 3.3. and 4 . 1 0 . 1 ) , o r pumping may i n t h e e a r l y stage o f t h e t r e a t m e n t i n c l u d e t h e whole s e c t i o n o f s e v e r a l v e r t i c a l l y successive sandstone l a y e r s ( w i t h t h e i n j e c t i o n p o i n t s b e i n g f i x e d by t h e p e r f o r a t e d i n t e r v a l s ) , and i n t h e l a t e stage o f t h e j o b , b a l l s a r e dropped o r b a f f l e s a r e s e t t o c e r t a i n l e v e l s t o c o n t r o l f r a c t u r e confinement t o d i s t i n c t r e s e r v o i r segments and t o s e p a r a t e t h e i n d i v i d u a l p r o d u c t i o n h o r i z o n s by c l o s i n g c e r t a i n p e r f o r a t e d i n t e r v a l s ( t h i s method has so f a r p a r t i c u l a r l y been a p p l i e d f o r f r a c t u r i n g o f w a t e r i n j e c t i o n w e l l s ; CLARK 1968; BALE 1984; c f . sect i o n 4 . 8 . 7 . ) . I n many cases, b a l l s e a l e r d r o p p i n g a t a r a t e o f one b a l l p e r b a r r e l o f f l u i d s a t i s f a c t o r i l y h e l p s t o o b t a i n u n i f o r m r e s e r v o i r breakdown (BLASiO, BAILEY & BERGTHOLD 1985), and n o r m a l l y 50 % excess b a l l s e a l e r s a r e spaced e v e n l y i n t h e f l u i d t o achieve a b a l l o u t and t o ensure t h a t a l l t h e p e r f o r a t i o n s a r e opened (PA1 & G A R B I S 1983 b ) . B a l l s e a l e r s can be used i n b o t h p r o p p a n t and a c i d f r a c t u r i n g i n o r d e r t o d i v e r t t h e t r e a t m e n t t o t h e n e x t zone between each stage (WEBSTER 1 9 7 7 ) . ROBINSON, HOLDITCH & LEE (1983) recommend a p p l i c a t i o n o f b a l l s e a l e r s t w i c e as much as p e r f o r a t i o n h o l e s a r e s h o t . GWINN & McMANUS (1977) r e p o r t m u l t i p l e - z o n e comp l e t i o n s u s i n g b a l l s and b a f f l e s where up t o f i v e h o r i z o n s can be s t i m u l a t e d i n one c o n t i n u o u s o p e r a t i o n . U t i l i z i n g t h e b a l l and b a f f l e technique, t h e l o w e r most zone i s p e r f o r a t e d and f r a c t u r e d , and i s t h e n a l l o w e d t o f l o w back immediat e l y . The n e x t zone i s p e r f o r a t e d as soon as flowback p e r m i t s e n t r y o f t h e p e r f o r a t i n g s t r i p i n t o t h e w e l l . A f t e r t h i s zone i s p e r f o r a t e d , t h e p r o p e r l y - s i z e d b a l l i s pumped down w i t h breakdown a c i d and a l l o w e d t o s e a t on t h e b a f f l e r i n g below t h e zone t o be n e x t s t i m u l a t e d . Each successive zone i s t r e a t e d i n a s i m i -
360 l a r manner u s i n g l a r g e r - d i a m e t e r f r a c t u r i n g b a l l s and b a f f l e s . The immediate flowback a f t e r each stage i s g e n e r a l l y s t r o n g enough t o u n s e a t and produce back the f r a c t u r i n g b a l l s . The c o n v e n t i o n a l o r m o d i f i e d l i m i t e d - e n t r y t e c h n i q u e ( c f . s e c t i o n 4 . 2 . 2 . 2 . ) using b a l l sealers o f f e r s the best p o s s i b i l i t i e s o f d i v e r t i n g s t i m u l a t i o n t r e a t ments o f new w e l l c o m p l e t i o n s (HARRISON 1971). S i n g l e - s t a g e m u l t i p l e - z o n e s t i m u l a t i o n i s a l s o d i s c u s s e d by EASON ( 1 9 8 5 ) . S e l e c t i v e f r a c t u r i n g w i t h s e a l i n g b a l l s as d i v e r t i n g m a t e r i a l s i s a d j u s t i n g t h e r e q u i r e m e n t o f zones w i t h d i f f e r e n t f r a c t u r e p r e s s u r e c a p a c i t y and i n c r e a s i n g t h e coverage o f propped f r a c t u r e w i t h i n a l o n g p e r f o r a t i o n i n t e r v a l ( L I & ZHU 1986). The d i s c u s s i o n as f o l l o w s focusses on advantages and drawbacks o f b a l l s e a l e r s , i n j e c t i o n r a t e vs. t r e a t ment s t a g i n g , f l o a t i n g and s i n k i n g b a l l s e a l e r s , t r e a t m e n t sequence i n m u l t i l a y e r r e s e r v o i r s , and i n s e r t i o n and removal o f b l o c k i n g m a t e r i a l s . Some comments a r e a l s o o f f e r e d on wax-based p e l l e t s .
4.2.2.1.1.1. Advantages and drawbacks o f ball sealers M u l t i s t a g e f r a c t u r i n g w i t h d r o p p i n g b a l l d i v e r t e r s and s e a l e r s between i n d i v i d u a l stages i s e s p e c i a l l y s u i t a b l e f o r l o n g c o m p l e t i o n i n t e r v a l s w i t h c o n s i d e r a b l e d i f f e r e n c e s between gross and n e t t h i c k n e s s r e s u l t i n g i n v a r i o u s t h i n t a r g e t h o r i z o n s i n t e r s p e r s e d w i t h i n a t h i c k r e s e r v o i r s t o r e y complex (PEARCE 1979). I n many cases, however, d i v e r t i n g w i t h b a l l s e a l e r s i s n o t e f f e c t i v e due t o washing-out o f p e r f o r a t i o n s w i t h proppants such t h a t b a l l s do n o t s u f f i c i e n t l y seal o f f pressure, and a l s o as a consequence o f f r e q u e n t c o v e r i n g o f t h e pay zones w i t h p r o p p a n t s a l r e a d y b e f o r e b a l l d i v e r t e r s can be s e t . Staged t r e a t m e n t s u s i n g b a l l s e a l e r s as w e l l as temporary sand, g e l o r mechan i c a l p l u g s a r e a l s o i m p r a c t i c a l i f o n l y t h i n mudstone i n t e r v a l s s e p a r a t e t h e successive sandstone s t o r e y s and i n case o f h i g h p r e s s u r e d i f f e r e n c e s between t h e i n d i v i d u a l t a r g e t zones (BUNOY 1981). On t h e o t h e r hand, s e a l i n g b a l l s a r e p a r t i c u l a r l y e f f e c t i v e f o r complete p l u g g i n g o f f o f t h i n r e s e r v o i r l a y e r s separ a t e d by p r e f e r e n t i a l l y t h i c k e r b a r r i e r s . A t l e a s t 200 f t o f r a t h o l e c o n t a i n i n g c l e a n f l u i d i s r e q u i r e d f o r p e r f o r m i n g a s u c c e s s f u l m u l t i s t a g e d i v e r s i o n . I n add i t i o n , u n l e s s a r e s e r v o i r i s e x t r e m e l y homogeneous, t h e p r o b a b i l i t y o f f r a c t u r i n g s e v e r a l pay zones t o g e t h e r which a r e separated by mudstone breaks i s r e mote due t o s t r e s s d i f f e r e n c e s between t h e v a r i o u s l a y e r s (PEARCE 1979; c f . sect i o n s 6 . 2 . 5 . and 6 . 3 . 6 . ) .
4.2.2.1.1.2. Injection rate vs. treatment staging I n j e c t i o n r a t e a f f e c t s f r a c t u r e h e i g h t and f l u i d d i s t r i b u t i o n and i s v e r y c r i t i c a l t o t h e o v e r a l l success o f t h e t r e a t m e n t (PAI, GARBIS & HALL 1983). Wher e i n t e r v a l y i e l d o f t u b i n g and/or c a s i n g l i m i t s i n j e c t i o n r a t e , t h e o p e r a t i o n s h o u l d be staged u s i n g b a l l s e a l e r s . A t s u i t a b l e pumping r a t e s , n a t u r a l pressur e d i v e r s i o n o f t h e t r e a t i n g f l u i d can a l s o be achieved (NALL, CAMPBELL & BONEY 1983). M u l t i p l e - s t a g e j o b s can a l s o be performed w i t h a m o d i f i e d l i m i t e d - e n t r y b a f f l e and b a l l arrangement, w i t h d r o p p i n g o f b a l l s e a l e r s c l o s i n g t h e b a f f l e s above t h e lower f o r m a t i o n and i s o l a t i n g t h e upper f o r m a t i o n f o r s t i m u l a t i o n , and subsequent opening of t h e w e l l p e r m i t t i n g t h e b a l l s e a l e r s t o f l o w o u t , t h e r e b y g u a r a n t e i n g f l u i d c l e a n u p f r o m b o t h t r e a t e d zones s i m u l t a n e o u s l y (BLACK, RIPLEY, BEECROFT & PAMPLIN 1979). ERBSTOESSER (1980) comments on improved b a l l s e a l e r d i v e r s i o n . W h i l e s e t t l i n g heavy b a l l s e a l e r s r e q u i r e h i g h e r i n j e c t i o n r a t e s f o r s a t i s f a c t o r y performance, buoyant b a l l s e a l e r s a r e o p e r a t i o n a l l y d i f f i c u l t i n h i g h l y - i n c l i n e d boreholes, because t h e y t e n d t o become s t u c k downhole and f l o w back a t l a t e r t i m e s i n t h e l i f e o f t h e w e l l t h e r e b y c a u s i n g problems w i t h t h e s u r f a c e equipment (SNOW & HOUGH 1988).
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4.2.2.1.1.3. Floating and sinking ball sealers S i m i l a r l y as buoyant and s e t t l i n g d i v e r t i n g m a t e r i a l s f o r f r a c t u r e t o p and bottom p l u g g i n g and growth l i m i t a t i o n ( c f . s e c t i o n 4.2.2.4.), f l o a t i n g and s i n k i n g b a l l s e a l e r s can be a p p l i e d f o r f r a c t u r i n g t r e a t m e n t d i v e r s i o n (McLEOD 1984). S i n k i n g b a l l s e a l e r s u s u a l l y r e q u i r e 200 X excess and a h i g h pumping r a t e . F l o a t i n g o r n e u t r a l - d e n s i t y b a l l s e a l e r s have a s p e c i f i c g r a v i t y m a t c h i n g t h a t o f t h e f l u i d , and t h e r e f o r e b e t t e r b a l l a c t i o n t a k e s p l a c e as compared t o s i n k i n g s e a l e r s . On t h e o t h e r hand, s u r f a c e equipment has t o be m o d i f i e d t o c a t c h t h e f l o a t i n g b a l l s e a l e r s d u r i n g flowback (GABRIEL & ERBSTOESSER 1984). I n terms o f a c i d i z i n g , b a l l s e a l e r s a r e r e s t r i c t e d i n use and s h o u l d n o t be app l i e d i n l o n g i n t e r v a l s w i t h h i g h p e r f o r a t i o n d e n s i t y , l o w - r a t e t r e a t m e n t s and g r a v e l - p a c k e d w e l l s (McLEOD 1984). An a l t e r n a t i v e t o b a l l s e a l e r s a r e r e t r i e v a b l e c o i l e d - t u b i n g i n f l a t a b l e packers and b r i d g e p l u g s which can be used f o r s e l e c t i v e s t i m u l a t i o n t r e a t m e n t s by i s o l a t i n g a zone f r o m a f r a c t u r i n g j o b u t i l i z i n g e i t h e r a packer t o s t i m u l a t e a l o w e r zone o r a p l u g t o t r e a t an upper zone (WALSH & HOLDER 1988).
4.2.2.1.1.4.
Treatment sequence in mu1 t i layer reservoirs
W h i l e i n most cases s e q u e n t i a l s t o r e y - w i s e f r a c t u r i n g w i t h a s s i s t a n c e o f b a l l s e a l e r s has t o be c a r r i e d o u t f r o m b o t t o m t o t o p o f t h e borehole, o c c a s i o n a l l y an i n v e r t e d procedure w i t h s t e p w i s e s t i m u l a t i o n f r o m t o p t o b o t t o m can be performed i f t h e d i f f e r e n t h o r i z o n s t o be t r e a t e d a r e c h a r a c t e r i z e d by d i f f e r e n t r e s e r v o i r d e p l e t i o n stage (SNOW & HOUGH 1988). The l e s s d e p l e t e d zones a r e p r e f e r e n t i a l l y stimulated f i r s t . I f the l e s s depleted l a y e r i s lower i n the prospective section, i t i s t r e a t e d and a p l u g i s s e t across those p e r f o r a t i o n s w h i l e t h e upper segment i s s h o t and s t i m u l a t e d . I f t h e l e s s d e p l e t e d zone i s h i g h e r i n t h e sequence, i t i s p e r f o r a t e d and t r e a t e d f i r s t , and once t h e l o w e r i n t e r v a l i s opened and s t i m u l a t e d , b a l l s e a l e r s a r e pumped t o d i v e r t away f r o m the p r e v i o u s l y t r e a t e d horizon.
4.2.2.1.1.5. Insertion and removal o f blocking materials O t h e r t e c h n i q u e s f o r simultaneous f r a c t u r i n g o f s e v e r a l l a y e r s i n b o r e h o l e s w i t h l o n g hydrocarbon-bearing s e c t i o n s i n c l u d e d i v e r s i o n o f t h e f l u i d t o t h e d i f f e r e n t zones where t r e a t i n g packers a r e used o r s o l i d chemicals as d i v e r t i n g agents a r e added t o t h e f l u i d s . Simultaneous i n j e c t i o n i n t o s e v e r a l s e l e c t e d pay zone l a y e r s can a l s o be performed by p i n - p o i n t sand f r a c t u r i n g (MURPHY & JUCH 1960; l i m i t e d - e n t r y f r a c t u r i n g ; c f . s e c t i o n 4 . 2 . 2 . 2 . ) . Extremely viscous g e l s can be a p p l i e d a l o n e and can be t a i l o r e d t o be almost s o l i d a t b o t t o m h o l e c o n d i t i o n s and y e t w i l l r e v e r t t o a l o w - v i s c o s i t y f l u i d a f t e r a c o n t r o l l e d per i o d o f t i m e ( c f . s e c t i o n 4 . 3 . 4 . ) . I n case o f o p e r a t i o n t h r o u g h p e r f o r a t e d cas i n g , h i g h - s t r e n g t h rubber-covered p l a s t i c b a l l s a r e used t o s e a l t h e p e r f o r a t i o n s t a k i n g f l u i d and d i v e r t t h e f l u i d t o o t h e r p e r f o r a t i o n s (WATERS 1980). I n terms o f d i v e r t i n g agents, one m a j o r d i f f i c u l t y encountered w i t h any b l o c k i n g m a t e r i a l i s removing i t f r o m t h e permeable zone a f t e r t r e a t m e n t (HARRISON 1971). The p e r f e c t b l o c k i n g m a t e r i a l i s one t h a t l a s t s l o n g enough t o d i v e r t f l u i d d u r i n g an o p e r a t i o n and then becomes i n e f f e c t i v e w i t h o u t h a v i n g t o be d i s s o l v e d by c o n t a c t w i t h o t h e r agents o r by i n t e r n a l breakage ( t h e same app l i e s f o r f l u i d - l o s s additives; c f . section 4.3.4.6.4.). The optimum i s a d i v e r t i n g agent t h a t s t a y s i n t a c t o n l y f o r t h e d u r a t i o n o f t h e t r e a t m e n t , w i t h a s o l i d m a t e r i a l degrading i m m e d i a t e l y b e i n g most d e s i r a b l e , whereas d i v e r t i n g agents depending on v i s c o s i t y a l o n e do n o t appear t o o f f e r p o s i t i v e b l o c k i n g because o f t h e v a r i a t i o n and i n p r e d i c t a b i l i t y o f g e l s and emulsions. HUCKABEE (1988) r e p o r t s u t i l i z a t i o n o f r o c k s a l t as d i v e r t i n g agent f o r a c i d s t i m u l a t i o n s i n carbonate r e s e r v o i r s , w i t h t h e w a t e r - s o l u b l e r o c k s a l t p a r t i c l e s b e i n g one o f t h e most s u i t a b l e d i v e r t i n g m a t e r i a l s due t o t h e i r subsequent u n h i n d e r e d vanishment once t h e f o r m a t i o n s t a r t s t o f l o w o r p e r f o r a t i o n washing
362 i s c a r r i e d o u t ( r o c k s a l t has a l s o e x c e l l e n t c h a r a c t e r i s t i c s as f l u i d - l o s s a d d i t i v e f o r t e m p o r a r y h a i r l i n e c r a c k p l u g g i n g f o l l o w e d b y d i s s o l u t i o n and t h u s e l i m i n a t i o n o f any b l o c k i n g e f f e c t s ; c f . s e c t i o n s 1 . 4 . 1 1 . 2 . and 4 . 8 . 8 . 3 . ) .
4.2.2.1.1.6.Wax-based pel lets An e f f i c i e n t d i v e r t e r u s e d s e v e r a l decades ago a r e wax-based p e l l e t s w h i c h i m p r o v e m u l t i p l e f r a c t u r e p a t t e r n s i n s i n g l e o p e r a t i o n and p e r m i t b e t t e r cont r o l o f a c i d i z i n g ( O I L GAS JOURNAL 1965) so t h a t i t w o r k s o v e r t h e e n t i r e f a c e o f t h e exposed w e l l b o r e . The wax-based beads f u n c t i o n as t e m p o r a r y s e a l i n g a g e n t s w h i c h p l u g any o p e n i n g i n t h e w e l l b o r e t h r o u g h w h i c h f l u i d s p a s s . Wax-based p e l l e t s b l o c k h y d r a u l i c f r a c t u r e s as soon as t h e y a r e c r e a t e d and propped, t h e r e b y p e r m i t t i n g t o s t i m u l a t e w i t h i n t h e r e s e r v o i r complex s e v e r a l o t h e r t i mes d u r i n g t h e same o p e r a t i o n . I n a c i d i z i n g , wax-based beads p l u g any g e n e r a t e d holes, thereby preventing the a c i d from being drained o f f i n t o the formation a t a s i n g l e p o i n t and f o r c i n g i t t o a t t a c k o t h e r a r e a s o f t h e b o r e h o l e as w e l l .
4.2.2.1.2. Diverting agent selection S e l e c t i o n o f d i v e r t i n g t e c h n i q u e and t r e a t m e n t method f o r s i m u l t a n e o u s m u l t i p l e f r a c t u r i n g o f s e v e r a l sand s t r i n g e r s w i t h i n one i n t e r v a l depends on t h e s e a l i n g n a t u r e o f s e p a r a t i n g mudstones (PARKER, ADAMS & LIANKUI 1 9 8 6 ) . I f t h e e n c l o s i n g mudstones a c t as b a r r i e r s t o v e r t i c a l c r a c k g r o w t h , l i m i t e d - e n t r y p e r f o r a t i n g and f r a c t u r i n g t e c h n i q u e s a r e s u i t a b l e f o r j o i n t s t i m u l a t i o n o f a b u n d l e of sand s t r i n g e r s . I f t h e b o u n d i n g mudstone, however, does n o t f o r m a b a r r i e r t o upward c r a c k e x t e n s i o n , f r a c t u r e h e i g h t i s g o v e r n e d b y pumping r a t e and j o b volume ( c f . s e c t i o n 4 . 2 . 2 . 3 . ) , and c l u s t e r p e r f o r a t i o n s i n a c e n t r a l a r e a combined w i t h h i g h - r a t e l a r g e - q u a n t i t y t r e a t m e n t s may be b e s t f o r s y n c h r o nous s t i m u l a t i o n o f s e v e r a l sand s t r i n g e r s . C l u s t e r p e r f o r a t i o n f r a c t u r i n g may r e q u i r e a d d i t i o n a l p e r f o r a t i o n s a f t e r t h e s t i m u l a t i o n o p e r a t i o n i n o r d e r t o commingle production from the v a r i o u s horizons ( c f . s e c t i o n s 4.10.1.1. and 4.10.3.). W h i l e t r a d i t i o n a l l y p e r f o r a t i o n breakdown p r i o r t o s t i m u l a t i o n t r e a t m e n t s was p e r f o r m e d b y b a l l and c u p s e a l e r s , t e m p o r a r y b l o c k i n g a g e n t s , o t h e r d i v e r t i n g means (HARRISON 1 9 7 1 g i v e s a h i s t o r i c a l r e v i e w o f d i v e r t i n g a g e n t s ) , and h i g h f l u i d i n j e c t i o n r a t e s , SCOTT ( 1 9 8 6 ) p r e s e n t s p r e c i s i o n p e r f o r a t i o n b r e a k down b y u s i n g a h o o k - w a l l s t r a d d l e p a c k e r t h a t i s o p e r a t e d w i t h m i n i m a l p r e s s u r e and pumping r a t e t o h e l p p r e v e n t c r a c k e x t e n s i o n o r p r o l o n g a t i o n i n t o unw a n t e d gas- o r w a t e r - b e a r i n g zones o r h o r i z o n s . M u l t i p l e f r a c t u r i n g w i t h i n one w e l l can i n c l u d e c r e a t i o n o f a s i n g l e f r a c t u r e each i n e v e r y p a y h o r i z o n o r c a n c o m p r i s e f o r m a t i o n o f s e v e r a l c r a c k s w i t h i n one r e s e r v o i r l e v e l . I n t h e l a t t e r case, t h e m u l t i p l e s y s t e m c o n s i s t s o f p r i m a r y and a u x i l i a r y f r a c t u r e s w i t h u s u a l l y f o u r - t o e i g h t - w i n g r a d i a l f r a c t u r e s (CHOO & WU 1987; c f . a l s o s e c t i o n s 4.2.2.1.3. and 4 . 7 . 3 . ) . I n c h a l k f r a c t u r i n g , c r o s s l i n k e d p o l y m e r s a r e o c c a s i o n a l l y u t i l i z e d as d i v e r t i n g a g e n t s ( H I L L , CLEMENT & WARD 1977; c f . s e c t i o n 4.5.4.).
4.2.2.1.3.Multiple fracturing aspects A very important question o f multiple fracturing operations i s formation breakdown and a s c e r t a i n m e n t t h a t a l l t h e p e r f o r a t i o n s w i l l a c c e p t f l u i d , w h i c h i s b e s t e n s u r e d b y b r e a k i n g down each zone s e p a r a t e l y (MARPLE, RULEY, WORLEY & FREEMAN 1 9 8 7 ) . F r a c t u r i n g o f e a c h zone i n d e p e n d e n t l y enhances t h e q u a l i t y o f t h e s t i m u l a t i o n t r e a t m e n t w i t h i n e a c h l a y e r and d e c r e a s e s t h e p o s s i b i l i t y o f s c r e e n o u t f a i l u r e . Comments a r e o f f e r e d on f r a c t u r e m o r p h o l o g y , f r a c t u r e number and s t r e s s s t a t e , t a i l o r e d p u l s e l o a d i n g , c o n t r o l l e d v s . c o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g , s i m u l t a n e o u s p r o p a g a t i o n o f m u l t i p l e f r a c t u r e s , and o t h e r t e c h n i aues and e f f e c t s .
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4.2.2.1.3.1. Fracture morphology Mineback experiments by p h y s i c a l e x c a v a t i o n o f h y d r a u l i c f r a c t u r e s and observ a t i o n o f t h e i r r e l a t i o n s h i p s t o geological d i s c o n t i n u i t i e s reveal t h a t the w i d e l y h e l d assumption o f i d e a l smooth p l a n a r s i n g u l a r p a r a l l e l f r a c t u r e s o f cons t a n t h e i g h t i s p r o b a b l y u n t e n a b l e i n many r e s e r v o i r s (WARPINSKI & TEUFEL 1984). M u l t i p l e s t r a n d i n g by o f f s e t t i n g ( c f . p l a t e I V / 6 ) , c r a c k meandering ( c f . p l a t e X I / 3 ) , en-echelon f r a c t u r i n g , l a r g e - s c a l e s u r f a c e roughness and waviness, and o v e r l a p p i n g s t r a n d i n g a r e common m o r p h o l o g i c a l f e a t u r e s o f f r a c t u r e s . Some comments a r e o f f e r e d as f o l l o w s on f r a c t u r e s t r a n d i n g and o f f s e t t i n g as w e l l as f r a c t u r e roughness, waviness and w i d t h v a r i a t i o n .
4.2.2.1.3.1.1.Fracture stranding and offsetting M u l t i p l e s t r a n d i n g i s predominant i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s , w i t h hyd r a u l i c f r a c t u r e s b e i n g s i g n i f i c a n t l y o f f s e t when t h e y c r o s s t h e j o i n t s (WARPINS K I , SCHMIDT & NORTHROP 1980; WARPINSKI 1983 a; c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) . Three t y p e s of i n t e r a c t i o n s between h y d r a u l i c f r a c t u r e s and p r e e x i s t i n g j o i n t s occur. The h y d r a u l i c f r a c t u r e s e i t h e r c r o s s t h e j o i n t , a r e a r r e s t e d by opening and d i l a t i n g t h e c r a c k ( c f . p l a t e s V I / l - 7 and X I / 3 + 7 ) as i n d i c a t e d by f l u i d f l o w along t h e j o i n t , o r a r e stopped by shear s l i p p a g e o f t h e c r a c k w i t h no d i l a t i o n and f l u i d f l o w a l o n g t h e j o i n t ( c f . a l s o s e c t i o n 4 . 2 . 3 . 4 . ) . Roughness o f f r a c t u r e s u r f a c e depends on r o c k t y p e and d i m i n i s h e s w i t h i n c r e a s i n g c o n f i n i n g s t r e s s (BIOT, MEDLIN & MASSE 1984). The e f f e c t o f v a r i a b i l i t y o f f r a c t u r e shape, p a t h and s u r f a c e f e a t u r e s i s an i n c r e a s e i n p r e s s u r e d r o p a l o n g t h e l e n g t h o f t h e crack compared w i t h t h a t o f t h e i d e a l case (WARPINSKI 1983 a ) , w i t h t h e main elements p r o v o k i n g l a r g e r p r e s s u r e d r o p b e i n g t o r t u o s i t y o f f r a c t u r e path, m u l t i p l e c r a c k s t r a n d s , sharp t u r n s o r c o r n e r s o f t h e f l o w p a t h o r i g i n a t i n g a t i n t e r s e c t i o n s o f n a t u r a l j o i n t s and h y d r a u l i c f r a c t u r e s , and r o c k p r o p e r t y v a r i a t i o n s . F r i c t i o n f a c t o r s a s s o c i a t e d w i t h f l o w i n hydraul i c f r a c t u r e s a r e f r e q u e n t l y 2 - 3 times g r e a t e r than p r e d i c t e d by v i s c o u s theory as a r e s u l t o f s u r f a c e roughness, c r a c k o f f s e t t i n g and m u l t i p l e s t r a n d i n g , w i t h t h e l a t t e r e f f e c t o c c u r r i n g r e g a r d l e s s o f r o c k t y p e and s t r e s s . E x t e n s i v e h y d r a u l i c f r a c t u r e c o n t o r t i n g t a k e s p l a c e a t n a t u r a l j o i n t s and i n c l u s i o n s (WARP I N S K I 1983 a; c f . p l a t e I V / 1 - 2 ) .
4.2.2.1.3.1.2. Fracture roughness, waviness and width variation Other f e a t u r e s a r e f r a c t u r e waviness ( c f . p l a t e X I / 3 ) and w i d t h v a r i a b i l i t y . O f f s e t s o f t h e h y d r a u l i c c r a c k happen a t e v e r y p o i n t where a n a t u r a l j o i n t i s i n t e r s e c t e d . F r a c t u r e w i d t h v a r i a t i o n o r l a r g e - s c a l e f r a c t u r e roughness have pronounced e f f e c t s on c r a c k shape, p a t h and s u r f a c e f e a t u r e s , w i t h t h e main f e a t u r e b e i n g i n c r e a s i n g p r e s s u r e drop along t h e l e n g t h o f t h e f r a c t u r e as compar e d t o t h e i d e a l case. Sharp t u r n s such as a t f r a c t u r e o f f s e t s ( c f . p l a t e X I / 3 ) can have s i g n i f i c a n t e f f e c t s on p r e s s u r e l o s s e s i n high-Reynolds number t r e a t ments c h a r a c t e r i z e d by h i g h f l o w r a t e , low v i s c o s i t y o r small h e i g h t .
SMITH, ROSENBERG & BOWEN (1982) document s m a l l - s c a l e d e v i a t i o n s o f i d e a l l y s t r a i g h t v e r t i c a l f r a c t u r e wings on b o t h s i d e s o f t h e w e l l b o r e as r e v e a l e d by o p t i c a l t e l e v i s i o n ( c f . s e c t i o n 6.2.1.9.3.). Such d e v i a t i o n s i n c l u d e s l i g h t jogs, b i f u r c a t i o n s and s p a l l i n g o f t h e f r a c t u r e edges, and a r e presumably r e l a t e d t o t h e dynamic n a t u r e o f t h e breakdown phenomenon which i s p r o b a b l y i n c l u ding f r a c t u r e i n i t i a t i o n a t m u l t i p l e p o i n t s i n the wellbore.
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4.2.2.1.3.2.Fracture number and stress state In a r e s e r v o i r complex c o n s i s t i n g o f v a r i o u s s a n d s t o n e s t o r e y s s e p a r a t e d b y mudstone b a r r i e r s , s i n g l e f r a c t u r e s a r e c r e a t e d i n c a s e o f p r e s e n c e o f a s i n g l e g r a d i e n t s h i f t , whereas m u l t i p l e f r a c t u r e s w i t h i n d i v i d u a l c r a c k s c o n t a i n e d i n e a c h l a y e r a r e g e n e r a t e d i n c a s e o f s e v e r a l d i s t i n c t g r a d i e n t s h i f t s b e i n g devel o p e d (BUNDY 1 9 8 1 ) . H i g h e r t r e a t m e n t p r e s s u r e s t h a n u s u a l c a n be caused b y v i s cous f l o w t h r o u g h a complex m u l t i p l e f r a c t u r e s y s t e m i n s t e a d o f a s i n g u l a r p l a n a r f r a c t u r e (MEDLIN & FITCH 1983, WARPINSKI & TEUFEL 1984) as w e l l as b y a l a y e r o f r e l a t i v e l y small cracks around a l a r g e h y d r a u l i c f r a c t u r e t h a t t r i g g e r s s i g n i f i c a n t i n c r e a s e o f a p p a r e n t f r a c t u r e e n e r g y w i t h c r a c k g r o w t h and thus i n c r e a s e o f o p e r a t i o n a l p r e s s u r e s (SHLYAPOBERSKY 1985; c f . s e c t i o n 4.3.4.2.3.). E f f e c t s o f s t r e s s s t a t e on h y d r a u l i c f r a c t u r e s a r e a l s o d i s c u s s e d b y ZOBACK, ZOBACK & SEEBURGER ( 1 9 8 1 ) . When a f r a c t u r e p e n e t r a t e s f r o m a l o w - s t r e s s zone i n t o a h i g h e r - s t r e s s zone, a c r a c k g e o m e t r y w i t h l a r g e r w i d t h s i s c r e a t e d i n t h e l o w - s t r e s s zone (SMITH, ROSENBERG & BOWEN 1 9 8 2 ) . F o r a f r a c t u r e i n a homogeneous f o r m a t i o n , t h e w i d t h / p r e s s u r e s l o p e i s g r e a t e r f o r p o i n t s n e a r t h e c e n t e r o f t h e c r a c k . M o d u l u s var i a t i o n s and b a r r i e r c h a r a c t e r s o f r o c k l a y e r s a d j o i n i n g t h e r e s e r v o i r c a n t r i g g e r f r a c t u r e w i d t h v a r i a t i o n s . O p t i c a l t e l e v i s i o n i n s p e c t i o n i n some examples r e v e a l s a v e r y w i d e f r a c t u r e i n t h e u p p e r p a r t o f t h e p a y i n t e r v a l and a n a r r o w e r c r a c k t h r o u g h t h e s h a l e s and l o w e r p a y s e c t i o n ( c f . s e c t i o n 6 . 2 . 1 . 9 . 3 . ) . Sharp f r a c t u r e c l o s u r e s o v e r t h e b o t t o m o f t h e r e s e r v o i r a r e a t l e a s t p a r t i a l l y t h e consequence o f a h i g h - m o d u l u s l a y e r b e l o w . I n a d d i t i o n t o t h e s e r e l a t i o n ships, actual crack widths over the r e s e r v o i r i n t e r v a l are probably smaller t h a n t h e measured w i d t h s due t o s D a l l i n q and e r o s i o n o f f r a c t u r e e d q e s . A s o e c t s o f f r a c t u r e w i d t h a r e a l s o d i s c u s s e d b y PERKINS & KERN ( 1 9 6 1 ) and iEDL1N &'MASSE ( 1 9 8 4 ) .
4.2.2.1.3.3.Tailored pulse loading M u l t i p l e f r a c t u r e generation by t a i l o r e d pulse l o a d i n g i s a combination o f t h e e f f e c t s o f h y d r a u l i c f r a c t u r i n g and e x p l o s i v e w e l l s h o o t i n g (SWIFT & KUSUBOV 1981; c f . s e c t i o n 4 . 7 . 3 . ) . T a i l o r e d p u l s e l o a d i n g i s p a r t i c u l a r l y s u i t a b l e f o r n a t u r a l l y f r a c t u r e d r e s e r v o i r s ( c f . section 4.8.8.5.). I d e a l l y , t h e optimum s t i m u l a t i o n t e c h n i q u e i s t o p r o d u c e maximum s u r f a c e a r e a f o r t h e l e a s t amount o f e n e r g y expended. C o n s i d e r i n g t h e l i m i t a t i o n s o f h y d r a u l i c f r a c t u r i n g and e x p l o s i v e s h o o t i n g , i t i s a p p a r e n t t h a t a t e c h n i q u e f o r i n i t i a t i n g and s u s t a i n i n g m u l t i p l e f r a c t u r e s i n a w e l l b o r e w i t h m i n i m a l damage i m p a r t e d t o t h e b o r e h o l e w a l l p r o v i d e s an a t t r a c t i v e means f o r m a x i m i z i n g p e r m e a b l e c o m m u n i c a t i o n o f a w e l l t o t h e s u r r o u n d i n g m e d i a . Some p o i n t s o f i n t e r m e d i a t e s t r a i n r a t e s and m u l t i p l e r a d i a l c r a c k s a r e t o u c h e d as f o l l o w s .
4.2.2.1.3.3.1. Intermediate strain rates T a i l o r e d p u l s e l o a d i n g a t i n t e r m e d i a t e s t r a i n r a t e s can c r e a t e m u l t i p l e f r a c t u r e s w i t h o u t damaging t h e w e l l b o r e r e g i o n (WARPINSKI, SCHMIDT, COOPER, WALLING & NORTHROP 1979; SCHMIDT, WARPINSKI & COOPER 1 9 8 0 ) . The d e g r e e o f m u l t i p l e f r a c t u r i n g i s q u i t e s e n s i t i v e t o b o r e h o l e c o n d i t i o n s as w e l l as t o l o a d i n g r a t e and appears t o be s t r o n g l y i n f l u e n c e d b y t r a n s i e n t p o r e - p r e s s u r e e f f e c t s . F r a c t u r e threshold-breakdown pressure increases p r o p o r t i o n a t e l y w i t h l o a d i n g r a t e , w i t h t h i s dependence b e i n g a t t r i b u t e d t o t r a n s i e n t w a t e r i n t r u s i o n a r o u n d t h e b o r e h o l e . M u l t i p l e f r a c t u r i n g b y t a i l o r e d p u l s e l o a d i n g c o u p l e s d e f o r m a t i o n and f a i l u r e c h a r a c t e r o f t h e r o c k t o f l u i d d i f f u s i o n through t h e permeable r o c k m a t r i x and f l u i d f l o w i n t h e c r a c k s (SWIFT & KUSUBOV 1 9 8 1 ) . T a i l o r e d p u l s e l o a d i n g i s an i n t e r m e d i a t e s t r a i n r a t e r o c k f r a c t u r i n g p r o c e s s g e n e r a t i n g m u l t i p l e c r a c k n e t w o r k s w h i c h have a h i g h e r p r o b a b i l i t y o f i n t e r s e c t i n g f a r - f i e l d n a t u r a l f r a c t u r e s t h a n c r a c k s c r e a t e d b y e x p l o s i v e o r con-
365 v e n t i o n a l h y d r a u l i c f r a c t u r i n g (STOLLER 1985; c f . s e c t i o n 4 . 7 . 3 . ) . As a consequence o f t h e c o n t r o l by t h e i n - s i t u s t r e s s s t a t e o f t h e f o r m a t i o n , a s i n g u l a r h y d r a u l i c f r a c t u r e tends t o p a r a l l e l t h e dominant t r e n d o f t h e e x i s t i n g n a t u r a l f r a c t u r e system, t h e r e b y r e d u c i n g t h e p o s s i b i l i t y o f i t s i n t e r s e c t i o n ( c f . sect i o n 4 . 8 . 8 . 1 . ) . C o u p l i n g o f t h e h y d r a u l i c f r a c t u r e i n t o t h e n a t u r a l c r a c k and j o i n t system i s t h u s dependent upon spacing o f t h e m i n o r o r o r t h o g o n a l f r a c t u r e t r e n d and e x t e n t t o which t h e l a t t e r i s connected t o t h e dominant n a t u r a l f r a c t u r e trend ( c f . section 4.2.4.3.). The m e r i t s o f a m u l t i p l e f r a c t u r e network depend upon c r e a t i o n o f c r a c k s o f s u f f i c i e n t l e n g t h t o s u b s t a n t i a l l y i n f l u e n c e p r o d u c t i o n and l o n g - t e r m maintenanc e o f c r e a t e d f r a c t u r e c o n d u c t i v i t y . Mechanisms which can m a i n t a i n c o n d u c t i v i t y o f a m u l t i p l e c r a c k network d e s p i t e t h e l a c k o f i n s e r t e d p r o p p a n t s a r e channell i n g and gouging on t h e f r a c t u r e surfaces, and t r a n s l a t i o n o f one f r a c t u r e s u r f a c e r e l a t i v e t o t h e o t h e r when n o t o r i e n t e d i n t h e p r i n c i p a l s t r e s s d i r e c t i o n s , w i t h s h e a r i n g s t r e s s i n f l u e n c e on o f f - a x i s c r a c k s b e i n g v e r y i m p o r t a n t . I f f r a c t u r e c o n d u c t i v i t y can t h u s be m a i n t a i n e d even under h i g h c l o s u r e s t r e s ses, p r o d u c t i v i t y enhancement r e s u l t s through a s y n e r g i s t i c c o m b i n a t i o n o f i n creases o f e f f e c t i v e r e s e r v o i r p e r m e a b i l i t y and e f f e c t i v e w e l l b o r e r a d i u s .
4.2.2.1.3.3.2.Multiple radial cracks ZUBER, LEE & GATENS (1987) o u t l i n e t h e e f f e c t i v i t y o f m u l t i p l e r a d i a l f r a c t u r e s c r e a t e d by t a i l o r e d p u l s e l o a d i n g i n Devonian s h a l e gas r e s e r v o i r s ( c f . sect i o n 4.4.4.6.). The c o n t r o l l e d d e t o n a t i o n o f e x p l o s i v e s i n t h e w e l l b o r e p r e f e r e n t i a l l y by i n s e r t i o n o f t h e charge i n t o n a t u r a l c r a c k s o r i n t o a h y d r a u l i c a l l y - i n d u c e d m i n i f r a c t u r e generates a t a i l o r e d p u l s e ( c f . s e c t i o n 4.8.9.6.). The main r e q u i r e m e n t s and b e n e f i t s o f t h i s t e c h n i q u e a r e t h a t peak r a d i a l s t r e s s o f t h e p u l s e i s below f l o w s t r e s s and above t e n s i l e s t r e n g t h o f t h e r e s e r v o i r rock, i n i t i a l l o a d i n g r a t e i s l a r g e enough t o t r i g g e r t h e o r i g i n o f f r a c t u r e s , and d u r a t i o n o f p u l s e and generated gases f r o m t h e e x p l o s i o n a r e s u f f i c i e n t t o extend the cracks f o r s i g n i f i c a n t distances. M u l t i p l e r a d i a l f r a c t u r i n g created by t a i l o r e d p u l s e l o a d i n g has t u r n e d o u t t o be e c o n o m i c a l l y much more a t t r a c t i v e than standard explosive f r a c t u r i n g ( c f . section 4.7.3.), b u t i s o f i n f e r i o r s i g n i f i c a n c e compared t o h y d r a u l i c f r a c t u r i n g which i s a l s o i n t h e Devon i a n s h a l e t h e most e f f e c t i v e s t i m u l a t i o n t e c h n i q u e ( c f . s e c t i o n 4 . 4 . 4 . 6 . ) . Caut i o n has t o be e x e r c i s e d c o n c e r n i n g o r i e n t a t i o n o f induced f r a c t u r e s w i t h r e s p e c t t o t h e n a t u r a l c r a c k system w h i c h c r e a t e s an a n i s o t r o p i c a l p e r m e a b i l i t y d i s t r i b u t i o n ( c f . s e c t i o n 4.2.4.2.) i n t h e s h a l e r e s e r v o i r s and r e s u l t s i n t h e r e l a t i o n s h i p t h a t a w e l l w i t h an induced f r a c t u r e p e r p e n d i c u l a r t o t h e n a t u r a l c r a c k t r e n d p e r f o r m s much more f a v o u r a b l y t h a n a w e l l w i t h an induced f r a c t u r e para1 l e l t o t h e n a t u r a l c r a c k t r e n d . T a i l o r e d pulse loading i s a l s o a successful technique f o r m i c r o f r a c t u r i n g breakdown f o r m i n i m i z a t i o n o f n a t u r a l l y f r a c t u r e d r e s e r v o i r damage (BRANAGAN & WILMER 1988; c f . s e c t i o n 4 . 8 . 2 . 2 . ) . T a i l o r e d p u l s e e x p l o s i v e breakdown procedur e s e n t a i l t h e use o f a r e l a t i v e l y slow b u r n i n g p r o p e l l a n t designed t o generate a m u l t i p l e s e t o f f r a c t u r e s r a d i a t i n g l i k e spokes f r o m t h e p e r f o r a t e d w e l l b o r e . I n o r d e r t o c r e a t e t h e c o r r e c t environment f o r t h e t a i l o r e d p u l s e e x p l o s i v e t o p r o p e r l y i n i t i a t e m u l t i p l e f r a c t u r e s , a f l u i d w i t h a d e n s i t y about t h a t o f wat e r b u t c o n s i d e r a b l y i n excess o f most a v a i l a b l e gases i s r e q u i r e d as a tamping medium i n t h e casing, w i t h l i q u i d carbon d i o x i d e b e i n g an i d e a l w e l l b o r e tampi n g f l u i d . T a i l o r e d p u l s e l o a d i n g i s a l s o a s u i t a b l e technology f o r c o m b i n a t i o n o f h y d r a u l i c and e x p l o s i v e f r a c t u r i n g ( c f . s e c t i o n 4 . 8 . 9 . 6 . ) .
4.2.2.1.3.3.3.Other aspects A s u i t e o f f a c t o r s c o n t r o l s o r i g i n o f m u l t i p l e c r a c k s by t a i l o r e d p u l s e l o a d i n g (SWIFT & KUSUBOV 1981). T h r e s h o l d breakdown p r e s s u r e i n c r e a s e s p r o p o r t i o n a t e l y w i t h l o a d i n g r a t e , and t h e o n s e t l o a d i n g r a t e f o r m u l t i p l e f r a c t u r i n g de-
366 c r e a s e s w i t h i n c r e a s i n g w a t e r c o n t e n t . Crack number i n c r e a s e s w i t h l o a d i n g r a t e above t h e o n s e t l o a d i n g r a t e . I f f l u i d i n t r u s i o n i s p r e v e n t e d , o n l y a f e w f r a c t u r e s o c c u r r e g a r d l e s s o f l o a d i n g r a t e and c o n f i n e m e n t c o n d i t i o n s . The m u l t i p l e f r a c t u r i n g process a t i n t e r m e d i a t e l o a d i n g r a t e s i s governed b y f l u i d d i f f u s i o n through t h e porous s o l i d coupled t o t h e s t r u c t u r a l deformation o f the s o l i d . The mechanism r e s p o n s i b l e f o r m u l t i p l e f r a c t u r i n g a p p e a r s t o b e t h e e f f e c t o f t r a n s i e n t pore pressure buildup i n the near v i c i n i t y o f the wellbore r e s u l t i n g f r o m t h e d i f f u s i v e i n t r u s i o n o f t h e b o r e h o l e f l u i d . B r i t t l e f r a c t u r i n g f o r und e r s t a n d i n g o f h i g h - e n e r g y g a s s t i m u l a t i o n i s d i s c u s s e d b y KRUGMAN ( 1 9 8 6 ) . Aspects o f m u l t i p l e f r a c t u r i n g by t a i l o r e d pulse loading, p r o p e l l a n t s o r o t h e r t e c h n i q u e s a r e a l s o d i s c u s s e d b y WARPINSKI, SCHMIDT, COOPER, WALLING & NORTHROP ( 1 9 7 9 ) ; SCHMIDT, WARPINSKI & COOPER ( 1 9 8 0 ) ; SWIFT & KUSUBOV (1980, 1 9 8 1 ) ; AHMED, SCHATZ, HOLLAND, JONES & GREENFIELD ( 1 9 8 2 ) ; CUDERMAN ( 1 9 8 2 ) ; McHUGH & KEOUGH ( 1 9 8 2 ) , HINN ( 1 9 8 3 ) ; KEOUGH, CURRAN & McHUGH ( 1 9 8 4 ) ; CUDERMAN & NORTHROP ( 1 9 8 4 ) ; FOURNEY, HOLLOWAY & S I M H A ( 1 9 8 4 ) ; HANSON, SCHMIDT, COOLEY & SCHATZ ( 1 9 8 4 ) ; AHMED, NEWBERRY & CANNON ( 1 9 8 5 ) ; STOLLER ( 1 9 8 5 ) and WATSON, BENSON & F I L L 0 ( 1 9 8 6 ) . STRUBHAR, FITCH & GLENN ( 1 9 7 4 ) o u t l i n e m u l t i p l e v e r t i c a l f r a c t u r i n g i n h i g h l y - d e v i a t e d w e l l b o r e s as a means t o enhance d r a i n a g e e f f e c t i v i t y o f i n c l i n e d boreholes ( c f . section 4.8.6.).
4 . 2 . 2 . 1 . 3 . 4 . Control led vs . conventional explosive fracturing In c o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g , f i r i n g o f a c h a r g e i n t h e w e l l b o r e gen e r a l l y c r e a t e s a l a r g e number o f f r a c t u r e s o f s h o r t e x t e n t (SWIFT & KUSUBOV 1 9 8 1 ) . The r a t e o f p r e s s u r i z a t i o n i s v e r y h i g h w h i c h c a u s e s c o m p r e s s i v e s t r e s ses i n t h e w e l l b o r e v i c i n i t y t o be much h i g h e r t h a n t h e i n - s i t u s t r e s s s t a t e . T h i s s t r e s s e n v i r o n m e n t c a n cause c o m p a c t i o n o r p u l v e r i z a t i o n o f a f i n i t e zone a r o u n d t h e w e l l b o r e t o such a d e g r e e t h a t p e r m e a b i l i t y i s s i g n i f i c a n t l y d e c r e a sed (SMITH, BASS & TYLER 1 9 7 8 ) . Thus e v e n t h o u g h t h e i n d u c e d m u l t i p l e f r a c t u r e s may i n t e r s e c t w i t h many o f t h e n a t u r a l p r e e x i s t i n g c r a c k s ( c f . p l a t e I V / l ) , t h e damage i m p a r t e d t o t h e w e l l b o r e w a l l b y c o m p a c t i o n o r p u l v e r i z a t i o n h i n d e r s p e r meable communication t o t h e surrounding media. Decoupling t h e e x p l o s i v e charge f r o m t h e w a l l c a n l o w e r t h e a p p l i e d s t r e s s and l e s s e n t h e amount o f damage t o t h e w a l l r e g i o n , b u t t h e s h o r t d u r a t i o n o f t h e e x p l o s i v e i m p u l s e does n o t a l l o w f r a c t u r e g r o w t h t o l o n g d i s t a n c e s (SWIFT & KUSUBOV 1981; c f . s e c t i o n 4 . 7 . 3 . ) . T a i l o r e d p u l s e loading i s a p o s s i b i l i t y t o c r e a t e m u l t i p l e f r a c t u r e s o f maximum l e n g t h b y c o n t r o l l i n g t h e i n c r e a s e r a t e and t h e r e f o r e p r e v e n t i n g f o r m a t i o n o f a p r e s s u r e s t r e s s cage w h i c h i s common i n s t a n d a r d e x p l o s i v e f r a c t u r i n g (FOURNEY, HOLLOWAY & SIMHA 1 9 8 4 ) . When d e t o n a t i n g o r d i n a r y e x p l o s i v e s , t h e p r e s s u r e r i s e r a t e s w i t h i n t h e b o r e h o l e a r e t o o r a p i d and c r e a t e a s t r e s s cage b y p l a s t i c deformation o f t h e r o c k i n t h e w e l l b o r e v i c i n i t y . A f t e r t h e passage o f o u t g o i n g s t r e s s waves, t h e e x p l o s i v e l o a d i n g r e s u l t s i n a c o m p r e s s i v e r e s i d u a l s t r e s s w h i c h p r e v e n t s t h e h i g h - p r e s s u r e gases f r o m e n t e r i n g t h e n e w l y - f o r m e d f r a c t u r e s , t h e r e b y l e a d i n g t o much s h o r t e r f r a c t u r e s t h a n d e s i r e d and a l s o i n h i b i t i n g t h e f r e e f l o w o f gas o r o i l back i n t o t h e w e l l b o r e . T a i l o r e d p u l s e l o a d i n g b y p r o p e l l a n t s (SCHMIDT, WARPINSKI & COOPER 1980) o r stemming (FOURNEY, BARKER & HOLLOWAY 1981; FOURNEY, HOLLOWAY & SIMHA 1984; YOUNG, BARKER & CLARK 1 9 8 4 ) , however, c a n s u c c e s s f u l l y c r e a t e s u f f i c i e n t l y l o n g m u l t i p l e f r a c t u r e s . W h i l e p r o p e l l a n t a p p l i c a t i o n i s q u i t e e x p e n s i v e and t h e r a t e o f d e f l a g r a t i o n o f p r o p e l l a n t s depends s t r o n g l y o n c o n f i n e m e n t and t h e r e f o r e o n p r e s s u r e , steminduced f r a c t u r i n g works w i t h highly-decoupled conventional e x p l o s i v e charges t h a t a r e l o c a t e d a t t h e b o t t o m o f a b o r e h o l e t h a t has been d r i l l e d t o a d e p t h beyond t h e pay zone. The w e l l i s c l o s e d o f f w i t h a p a c k e r o r p e d a l b a s k e t l o c a t e d a t t h e t o p o f t h e p a y zone, and s e v e r a l m e t r e s o f c r u s h e d g r a v e l a r e u s e d t o e n s u r e t h a t t h e stemming column w i l l b r i d g e i n t h e h o l e when t h e e x p l o s i v e d e t o n a t e s . Upon f i r i n g , a i r shock and d e t o n a t i o n p r o d u c t s p r o p a g a t e upwards i n t h e b o r e h o l e and when t h e y r e a c h t h e stemming, shock r e f l e c t i n g c r e a t e s a sever a l - f o l d p r e s s u r e i n c r e a s e o f f a v o u r a b l e m a g n i t u d e and r a t e . The absence o f
367 s t r e s s cage and f i n e p a r t i c l e s p l u g g i n g t h e c r a c k s w i t h r e s p e c t t o a chargearea s i t u a t i o n p r e v e n t s q u i c k a r r e s t o f t h e generated m u l t i p l e r a d i a l f r a c t u r e system which i s d r i v e n f o r l o n g e r d i s t a n c e s by t h e i n c r e a s e d p u l s e d u r a t i o n . O t h e r aspects o f stem-induced t a i l o r e d - p u l s e f r a c t u r i n g a r e d i s c u s s e d i n sect i o n 4.7.3.3.
4.2.2.1.3.5.Simu 1 taneous propagat ion o f mu 1 t iple fractures Concerning simultaneous p r o p a g a t i o n o f m u l t i p l e f r a c t u r e s , d i s t i n c t i o n has t o be made between v e r t i c a l and h o r i z o n t a l w e l l s from which t h e v a r i o u s c r a c k s emanate e i t h e r i n t o s e p a r a t e l a y e r s o f t h e p r o s p e c t i v e i n t e r v a l and b e i n g cont a i n e d more o r l e s s w i t h i n each s t o r e y , o r i n t o t h e same l a y e r l i k e a sequence o f s l i c e s and b e i n g as a whole s e r i e s c o n f i n e d w i t h i n t h i s h o r i z o n .
4.2.2.1.3.5.1. Vertical wells BEN NACEUR & ROEGIERS (1988) comment on m u l t i p l e f r a c t u r e p r o p a g a t i o n s i m u l t a n e o u s l y f r o m one v e r t i c a l w e l l b o r e i n t o d i f f e r e n t v e r t i c a l l y successive p r o d u c t i v e l a y e r s of t h e r e s e r v o i r complex. Simultaneous growth o f f r a c t u r e s can occur w i t h v e r t i c a l o r h o r i z o n t a l c r a c k s e x t e n d i n g f r o m d i f f e r e n t p e r f o r a t i o n s e t s . I n t h e f i r s t case, t h e f r a c t u r e s a r e c o l l i n e a r , whereas t h e y a r e a p p r o x i mated by p a r a l l e l c r a c k s i n t h e second one, a l t h o u g h t h e f r a c t u r e s may a c t u a l l y tend t o move away f r o m each o t h e r . Due t o e l a s t i c i n t e r a c t i o n , t h e w i d t h s f o r two v e r t i c a l f r a c t u r e s f o r a g i v e n p r e s s u r e a r e l a r g e r than t h a t f o r a s i n g l e c r a c k . M u l t i p l e v e r t i c a l f r a c t u r e s a r e a l s o more e l l i p t i c a l , w i t h maximum w i d t h o f t h e upper f r a c t u r e b e i n g reached above t h e c e n t e r o f t h e c r a c k . Since w i d t h i s p r o p o r t i o n a l t o f r a c t u r e l e n g t h ( c f . s e c t i o n 4.3.3.3.), t h e r e i s an asymptot i c tendency o f w i d t h r a t i o , and a l s o merging o f f r a c t u r e s o c c u r s . F o r v e r t i c a l c r a c k s o f d i f f e r e n t s i z e s , t h i s e f f e c t depends on t h e r e l a t i v e s i z e s o f t h e fractures, w i t h m o d i f i c a t i o n o f crack w i d t h p r o f i l e being l a r g e r f o r a f r a c t u r e t h a t i s i n f l u e n c e d by a l a r g e r c r a c k s i z e .
4.2.2.1.3.5.2. Horizontal wells H o r i z o n t a l w e l l s can be e f f e c t i v e l y s t i m u l a t e d by c r e a t i o n o f a s e r i e s o f p a r a l l e l v e r t i c a l f r a c t u r e s t o connect t h e d i f f e r e n t s t r a t a ( c f . a l s o s e c t i o n 4.8.6.2.2.). I n t e r a c t i o n between s e v e r a l c r a c k s r e s u l t s i n s m a l l e r f r a c t u r e w i d t h s . D u r i n g a f i r s t phase, o n l y one f r a c t u r e r e p r e s e n t i n g t h e c r a c k w i t h t h e lower c l o s u r e s t r e s s can propagate u n t i l a boundary s t r e s s l e v e l i s reached. The second f r a c t u r e s t a r t s p r o p a g a t i n g w i t h t h e same p r e s s u r e as t h e f i r s t one, and t h e p r e s s u r e tends a s y m p t o t i c a l l y t o t h e lower c u r v e . The i n j e c t i o n r a t e i n t h e f i r s t c r a c k i s a t t h e b e g i n n i n g c o n s t a n t and then s t a r t s d e c l i n i n g r a p i d l y t o one-half value. I n t e r a c t i o n o f m u l t i p l e v e r t i c a l f r a c t u r e s i n both v e r t i c a l and h o r i z o n t a l w e l l s r e s u l t s i n s h o r t e r e x t e n s i o n o f t h e c r a c k s w i t h r e s p e c t t o l e n g t h w h i c h c o u l d be achieved by s i n g u l a r f r a c t u r e s . M o d e l l i n g simultaneous p r o p a g a t i o n o f a s e r i e s o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s t h u s r e v e a l s t h a t t h e e l a s t i c i n t e r a c t i o n between c o l l i n e a r c r a c k s g i v e s r i s e t o l a r g e r f r a c t u r e w i d t h s o r l o w e r apparent s t i f f n e s s e s , whereas f o r p a r a l l e l cracks, t h e r e s u l t s a r e r e v e r s e , and t h e r e f o r e i n d i v i d u a l f r a c t u r e s a r e i n v a r i a b l y s h o r t e r i n t h e f i r s t case. I n a s e r i e s o f f r a c t u r e s o r i g i n a t i n g f r o m d i f f e r e n t i n t e r v a l s , t h e v a r i o u s i n d i v i d u a l c r a c k s grow i n d e p e n d e n t l y f r o m each o t h e r and even a r e n o t knowing t h e e x i s t e n c e o f t h e i r neighbours. Q u a l i t a t i v e e s t i m a t e s o f f r a c t u r e geometries can be made c o r r e s p o n d i n g t o i n c r e m e n t a l i n j e c t i o n pressures, and t h e knowledge o f s t r e s s d i s t r i b u t i o n s p r e v a i l i n g w i t h i n t h e p e r f o r a t i o n i n t e r v a l s i s e s s e n t i a l f o r an a c c u r a t e p r e d i c t i o n o f f r a c t u r e sizes. YOST,
OVERBEY,
WILKINS & LOCKE (1988) d e s c r i b e simultaneous p r o p a g a t i o n o f
368 m u l t i p l e h y d r a u l i c f r a c t u r e s i n s h a l e gas r e s e r v o i r s t r a v e r s e d by a h o r i z o n t a l b o r e h o l e . M u l t i p l e h y d r a u l i c c r a c k s can be induced f r o m a h o r i z o n t a l b o r e h o l e d u r i n g a s i n g l e pumping event, b u t f r a c t u r e p r o p a g a t i o n i s s t r o n g l y i n f l u e n c e d by t h e n a t u r a l c r a c k system. Low i n j e c t i o n r a t e s d u r i n g a h y d r a u l i c f r a c t u r i n g o p e r a t i o n a l l o w e x t e n s i o n o f n a t u r a l j o i n t s w i t h a low-angle r e l a t i o n s h i p t o t h e p r i n c i p a l s t r e s s o r i e n t a t i o n , whereas h i g h e r i n j e c t i o n r a t e s g e n e r a l l y r e s u l t i n i n d u c i n g f r a c t u r e s c o n t r o l l e d by t h e s t r e s s f i e l d . WANNER, CHESNUT & COX (1979) p o i n t o u t t h a t i n t h i c k gross r e s e r v o i r s e c t i o n s where numerous t h i n sands a r e d i s t r i b u t e d which f o r m o n l y a f r a c t i o n o f t h e t o t a l n e t pay i n d i c a t e d by l o g s , t h e key t o s u c c e s s f u l e x p l o i t a t i o n o f these t h i n sands l i e s i n t h e use o f r e l a t i v e l y s m a l l d i s c r e t e f r a c t u r e t r e a t m e n t s f o r each sand r a t h e r t h a n mass i v e f r a c t u r i n g o f a t h i c k gross s e c t i o n ( c f . a l s o KNUTSON & WARD 1977).
4.2.2.1.3.6.
Other techniques and e f f e c t s
FREDRICKSON & BROADDUS (1976) and BARBER & THEMIG (1985) i n t r o d u c e t h e conc e p t o f s e l e c t i v e placement o f f l u i d s i n a f r a c t u r e by c o n t r o l l i n g d e n s i t y and v i s c o s i t y ( c f . section 4.2.2.6.4.), w i t h separate loading o f the i n d i v i d u a l stages w i t h d i f f e r e n t p r o p p a n t t y p e s and/or g r a i n s i z e s r e p r e s e n t i n g a d i v i d e d t r e a t m e n t schedule and r e s u l t i n g i n p l a c i n g o f d i s t i n c t h o r i z o n t a l segments o f t h e whole p r o p p a n t q u a n t i t y b e i n g i d e a l l y head a t t h e f r a c t u r e t i p , main body i n t h e m i d d l e s e c t i o n o f t h e wedge, and t a i l a t t h e c r a c k mouth, o r v e r t i c a l l y d i s c r e t e l o t s a t top, m i d d l e and b o t t o m o f t h e f r a c t u r e ( c f . a l s o s e c t i o n s 2 . 4 . 1 . 2 . 3 . and 4 . 3 . 3 . 2 . ) . HANNAH, HARRINGTON & ANDERSON (1977) d e s c r i b e p o s s i b i l i t i e s o f l o c a t i o n o f v a r i o u s f l u i d schedule segments i n t h e f r a c t u r e a f t e r pumping w h i c h p r o v i d e s u s e f u l i n f o r m a t i o n f o r p l a n n i n g o f g e l l i n g agent and b r e a k e r l o a d i n g t i m e t a b l e s as w e l l as p r o p p a n t placement. Case s t u d i e s o f i n t e r a c t i o n between g e o l o g i c a l c o n d i t i o n s , t r e a t m e n t charact e r i s t i c s and p r o d u c t i o n a r e r e p o r t e d by McLENNAN, R O E G I E R S & MARX ( 1 9 8 3 ) . A c o n t i n u o u s m u l t i s t a g e f r a c t u r i n g t e c h n i q u e i s o u t l i n e d by WEBSTER, G O I N S & BERR Y ( 1 9 7 0 ) . MURPHY & JUCH (1960) p r e s e n t p i n - p o i n t sand f r a c t u r i n g as a method o f simultaneous i n j e c t i o n i n t o s e l e c t e d sands. O t h e r f e a t u r e s o f m u l t i p l e - f r a c t u r e d w e l l s a r e o u t l i n e d by WILLIAMS, N I E T O , GRAHAM & LEIBACH (1973); CHOO & WU (1987); SMITH, MILLER & HAGA (1987) and SULLIVAN, LEE & HOLDITCH (1987; c f . also section 2.4.1.2.). KULAR, CHHINA & BEST (1988) r e p o r t m u l t i p l e h y d r a u l i c f r a c t u r e p r o p a g a t i o n i n o i l sands. AL-TAMIMI & EL-MZEIN (1987) comment on s t i m u l a t i o n techniques o f t i g h t m u l t i l a y e r e d carbonates. Aspects o f s i n g l e - s t a g e m u l t i p l e - z o n e s t i m u l a t i o n a r e d i s c u s s e d by EASON ( 1 9 8 5 ) . Design o f f r a c t u r i n g t r e a t m e n t s i n m u l t i l a y e r e d f o r m a t i o n s i s i n v e s t i g a t e d by BEN NACEUR & ROEGIERS (1988).
4.2.2.2.
L imi ted-en t r y f rac t ur-i ng technique
M u l t i p l e f r a c t u r i n g i s commonly p e r m i t t e d by t h e l i m i t e d - e n t r y s t i m u l a t i o n technique where t h e number o f p e r f o r a t i o n s i n a c o m p l e t i o n i n t e r v a l i s l i m i t e d f o r p r o m o t i o n o f development o f p e r f o r a t i o n f r i c t i o n p r e s s u r e d u r i n g a s t i m u l a t i o n o p e r a t i o n (LAGRONE & RASMUSSEN 1963, HOWARD & FAST 1970, SMALL 1985, CRAMER 1 9 8 7 ) . The l i m i t e d - e n t r y t e c h n i q u e comprises a c c o m p l i s h i n g o f t r e a t i n g sever a l i n t e r v a l s i n a t h i c k pay s e c t i o n w i t h a s i n g l e - s t a g e o p e r a t i o n by l i m i t i n g number o f p e r f o r a t i o n s and p r o v i d i n g an i n j e c t i o n r a t e h i g h enough t o assure t h a t a l l t h e p e r f o r a t i o n s w i l l be t a k i n g f l u i d (LAGRONE & RASMUSSEN 1963). The pumping r a t e i s chosen such t h a t t h e f r i c t i o n a l p r e s s u r e drop across t h e p e r f o r a t i o n s w i l l i n c r e a s e p r e s s u r e i n t h e w e l l b o r e t o a v a l u e above t h e f o r m a t i o n breakdown p r e s s u r e o f each zone p e r f o r a t e d , w i t h t h i s concept b e i n g an e f f e c t i v e and t h u s w i d e l y a p p l i e d d i v e r t i n g t e c h n i q u e . L i m i t e d - e n t r y f r a c t u r i n g has o r i g i n a l l y been i n t r o d u c e d t o t h e l i t e r a t u r e as p i n - p o i n t sand f r a c t u r i n g (MURPHY & JUCH 1960). Limited-entry
perforation
d e s i g n may be d e p i c t e d as t h e a b i l i t y o f a s e t o f
369 p e r f o r a t i o n s t o pass a f i n i t e volume o f l i q u i d a t a g i v e n d i f f e r e n t i a l p r e s s u r e (EASON 1 9 8 5 ) . Each p e r f o r a t i o n a c t s as a choke d i v e r t i n g f l u i d t o each s u c c e s s i ve one. W i t h i n c r e a s i n g i n j e c t i o n r a t e , a l l p e r f o r a t i o n s w i l l e v e n t u a l l y t a k e up f l u i d a t maximum c a p a c i t y i f a s u f f i c i e n t p r e s s u r e d i f f e r e n t i a l i s h o l d across t h e p e r f o r a t i o n s . The f o l l o w i n g overview p r e s e n t s some aspects o f s i m u l taneous f l u i d e n t r y i n t o m u l t i p l e zones, p r e s s u r e d r o p across t h e p e r f o r a t i o n s , i n f l u e n c e o f screenouts, and f l u i d d i s t r i b u t i o n and p e r f o r a t i o n arrangement.
4.2.2.2.1. Simultaneous fluid entry into multiple zones The c h o k i n g e f f e c t r e s u l t i n g f r o m t h e l i m i t e d - e n t r y o p e r a t i o n c r e a t e s excess p r e s s u r e i n t h e casing, a l l o w i n g t h e simultaneous e n t r y o f f r a c t u r i n g f l u i d i n t o m u l t i p l e zones o f v a r y i n g i n - s i t u s t r e s s s t a t e s . Having been r e s t r i c t e d t o a c i d f r a c t u r i n g and s m a l l - s c a l e p r o p p a n t f r a c t u r i n g t r e a t m e n t s i n e a r l i e r years, l i m i t e d - e n t r y techniques have r e c e n t l y a l s o been i n t r o d u c e d t o MHF s t i m u l a t i o n s (CRAMER 1987). P a r t i c u l a r a p p l i c a t i o n s i n c l u d e r e s e r v o i r s w i t h two poros i t y zones w h i c h a r e s e p a r a t e d by a t i g h t b a r r i e r (BEGNAUD & CLAIBORNE 1985) where h y d r a u l i c communication between t h e two pay i n t e r v a l s o c c u r s r o u t i n e l y , a l t h o u g h proppant remains i s o l a t e d i n t h e p e r f o r a t e d i n t e r v a l due t o an appar e n t w i d t h r e d u c t i o n i n t h e p o r t i o n o f t h e f r a c t u r e p e n e t r a t i n g t h e bounding i nt e r b e d . When b o t h zones a r e t r e a t e d s e p a r a t e l y , t h e a d j a c e n t u n p e r f o r a t e d pay zone a c t s as a permeable f l u i d s i n k and t h u s proppant c o n c e n t r a t i o n w i t h a s s o c i a t e d s c r e e n o u t r i s k s w i l l o c c u r . I n case o f simultaneous s t i m u l a t i o n w i t h l i m i t e d e n t r y techniques, however, t h i s l e a k o f f source w i l l be e l i m i n a t e d and l e s s t o t a l f r a c t u r i n g f l u i d w i l l be needed t o d i s p l a c e a g i v e n volume o f p r o p p a n t . I n t e r z o n a l r a t e d i s t r i b u t i o n i s d i c t a t e d by c o u p l i n g o f p e r f o r a t i o n and f o r m a t i o n f r a c t u r e e x t e n s i o n p r e s s u r e c h a r a c t e r i s t i c s (CRAMER 1987). M a t e r i a l changes a t t h e p e r f o r a t i o n s and i n t h e f r a c t u r e s can cause i n j e c t i o n r a t e and thus t r e a t ment volume r e d i s t r i b u t i o n among zones which l e a v e some s e c t i o n s o f t h e r e s e r v o i r complex u n d e r s t i m u l a t e d and i n c e r t a i n circumstances can c o n t r i b u t e t o a w e l l screenout. L i m i t e d - e n t r y p e r f o r a t i o n enables m u l t i p l e - z o n e c o m p l e t i o n and h e l p s t o c o n t r o l e x c e s s i v e f r a c t u r e h e i g h t growth (HOOVER & ADAMS 1988).
4.2.2.2.2. Pressure drop across the perforations L i m i t e d - e n t r y f r a c t u r i n g d e s i g n f r e q u e n t l y r e q u i r e s t h e use o f a minimum p e r f o r a t i o n p r e s s u r e drop (BUNDY 1981). Many s t i m u l a t i o n j o b s designed t o t r e a t l a r g e i n t e r v a l s f a i l because t h e f r a c t u r i n g f l u i d i s n o t p r o p e r l y d i s t r i b u t e d o v e r t h e e n t i r e completed i n t e r v a l and when a p a r t i c u l a r zone o f t h e completed s e c t i o n does n o t r e c e i v e enough f l u i d , i t screens o u t e a r l y i n t h e j o b , because t h e necessary f r a c t u r e w i d t h has n o t been c r e a t e d ( c f . s e c t i o n s 4.2.3.2.5. and 6.2.4.2.1.). The minimum p r e s s u r e d r o p across t h e p e r f o r a t i o n s f o r c e s t h e f r a c t u r i n g f l u i d i n t o a l l the perforations. O t h e r l i m i t e d - e n t r y t r e a t m e n t s f a i l because c r a c k d e s i g n assumes t h a t a s i n g l e f r a c t u r e w i l l be generated across t h e e n t i r e completed i n t e r v a l , whereas i n r e a l i t y a m u l t i f r a c t u r e system o r i g i n a t e s c o n s i s t i n g o f independent c r a c k s w i t h i n the i n d i v i d u a l horizons i f s u f f i c i e n t l y t h i c k b a r r i e r s separate the storeys, w i t h each f r a c t u r e h a v i n g t o r e c e i v e enough f l u i d i n o r d e r t o p r e v e n t p r e mature s c r e e n o u t t e r m i n a t i o n o f t h e j o b ( c f . s e c t i o n 4.2.2.2.3.). The more i n d e pendent c r a c k s o r i g i n a t e across t h e completed segment, t h e more a c c u r a t e f l u i d d i s t r i b u t i o n has t o be f o r t h e purpose o f a v o i d i n g screenout f a i l u r e . Some comments on f l u i d volume and p e r f o r a t i o n number as w e l l as shear d e g r a d a t i o n and f l u i d v i s c o s i t y a r e o f f e r e d as f o l l o w s .
370
4.2.2.2.2.1.Fluid volume and perforation number The a m o u n t of f l u i d t h a t i s required f o r each zone i s determined by i t s h e i g h t , and the required number of p er f o r at i o ns f o r each inte rva l can be calcul a t e d once the optimum pressure drop across the pe rfora tions has been e s t a b l i shed. Maximizing pressure drop ensures t h a t f l u i d i s dive rte d t o a l l the perfor a t i o n s . Pressure d r o p across the p er f o r at i ons i s constant throughout the comp le t e d i n t e r v a l o n l y when formation t r e a t i n g pressure (being f r a c t u r e gra die nt times depth) i s constant across the s e c t i o n . I n case of v a r y i n g f r a c t u r e grad i e n t s , d i s t r i b u t i o n and number of p er f o r at i o ns have t o be adjusted i n orde r to compensate d i f f e r e n t t r e a t i n g pressures in various zones. Minimum pe rfora tion pressure drops have t o be used i f the f r a c t u r i n g r a t e i s lim ite d by tubing a n d pressure r e s t r i c t i o n s . P r ef r act u r i n g i n j e c t i v i t y t e s t i n g and b a l l o u t a re pe rformed t o a s c e r t a i n t h a t a l l the p er f o r at i o n s a r e broken down and w ill take f l u i d .
4.2.2.2.2.2.Shear degradation and fluid viscosity As shear degradation of f r a c t u r i n g f l u i d s ( c f . se c tions 4.3.3.3. and 4 . 3 . 4 . 2 . ) usually takes p r e f e r e n t i a l l y place a t the p e r f o r a t i o n s , highly limited-entry f r a c t u r e treatments a r e ch ar act er iz e d by lower f l u i d v i s c o s i t i e s which provide deeper proppant p en et r at i o n within a long crack with l e s s height growth, a n d thus connect more n et pay t o the wellbore ( H O E L 1988). Low-viscosit y f r a c t u r i n g f l u i d s having undergone shear a t the pe rfora tions in limited-ent r y c o n s t e l l a t i o n s , however, a r e not able t o generate s u f f i c i e n t width t o t r a n s p o r t l a r g e r amounts of proppants. On the othe r h a n d , i f a higher v i s c o s i t y f l u i d were pumped in an attempt t o car r y l ar g e p r o p p a n t q u a n t i t i e s , l a r g e r f r a c tu r e widths could be obtained which a r e causing more height growth o u t of zone and l e s s p r o p p a n t p en et r at i o n of the cen t r al re se rvoir se c tion. The g r e a t e s t ben e f i t of l i m i t e d - en t r y f r a c t u r i n g i s thus improved proppant pe ne tra tion which compensates f o r the f a c t t h a t t h i s goal i s achieved a t the expense of high propp a n t concentration.
4.2.2.2.3. Influence o f screenouts Designing e f f e c t i v e limited-entry treatments requires a blending of engineering d i s c i p l i n e s i n a s y n t h es i s of conventional f r a c t u r e modelling w i t h knowlege of the pressure l o s s c h a r a c t e r i s t i c s a t the pe rfora tions. While screenouts can be d e s i r a b l e i n c e r t a i n s i t u a t i o n s ( c f . SMITH, M I L L E R & HAGA 1987), they have to be s t r i c t l y avoided i n limited-entry a p p l i c a t i o n s , because the d i f f e r e n t i a l pressure a t the offending zone wi l l decrease and cause a r e d i s t r i b u t i o n of treatment volume i n t o t h e o t h er horizons, with the r e s u l t a n t r a t e loss a c c e le ra tin g the screenout process i n t h a t i n t er v al (CRAMER 1987). The remaining t r e a t ment volume w i l l then be d i s t r i b u t e d among the open horizons, a n d the p a d volumes i n these zones may be i n s u f f i c i e n t t o develop f r a c t u r e area and e s t a b l i s h leakoff control f o r the additional l o a d , w i t h t o t a l treatment screenout being able t o occur. A l l horizons need t o be f a i r l y sim ila r i n leakoff c h a r a c t e r i s t i c s f o r the l i m i t ed - en t r y technique t o be v i a ble . I f one inte rva l h a s r e l a t i v e l y h i g h leakoff c h a r a c t e r i s t i c s , the overall treatment staging needed t o prevent a screenout may leave the o t h er horizons inadequately stim ula te d, with the possible occurrence of d i l u t e d proppant co n centra tions. Other aspects of candidate screening ar e high s t r e s s diffe re nc e s between the individual s t o r e y s o f the r e s e r v o i r complex ( c f . se c tions 4 . 2 . 2 . 5 . 3 . a n d 4 . 2 . 3 . 2 . 2 . ) which lead t o g r e a t d i f f er en ces i n proppant e n t r y per pe rfora tion between the zones. I n a large-magnitude treatment, a runaway condition may occur i n which i n j e c t i o n ceases i n the h i g h er - stre sse d horizon due t o the i n a b i l i t y t o m a i n t a i n s u f f i c i e n t pressure drop across pe rfora tions. Even in completion i n t e r v a l s c h a r a c t er i zed by small d i f f er en ces i n instantaneous shut-in pre ssure , p e r f o r a t i o n erosion wi l l ev en t u al l y outrun the a b i l i t y t o compensate the inc re a sing t r e a t i n g r a t e . Screenout f a i l u r e s ( c f . s e c tions 4 . 2 . 3 . 2 . 5 . and 6 . 2 . 4 . 2 . 1 . )
371 a r e f r e q u e n t l y provoked by t h e s p i k e s i n s t a i r s t e p p r o p p a n t schedules, w i t h smooth ramping schedules b r i n g i n g i n h i g h e r proppant c o n c e n t r a t i o n s i n t o t h e f r a c t u r e w i t h l e s s screenout r i s k ( c f . s e c t i o n 4 . 3 . 3 . 5 . ) .
4.2.2.2.4.
F l u i d d i s t r i b u t i o n and p e r f o r a t i o n arrangement
L i m i t e d e n t r y p e r m i t s t h e f r a c t u r e t o be p l a c e d i n a l l d e s i r a b l e p a r t s o f a r e s e r v o i r and g i v e s maximum c o n t r o l o f s t i m u l a t i o n f l u i d s a t each f r a c t u r e p o i n t (AHMED, NEWBERRY & CANNON 1985). I n essence, 1 i m i t e d e n t r y p r o v i d e s f o r equal d i s t r i b u t i o n o f t r e a t i n g f l u i d s through a l l p e r f o r a t i o n s by l i m i t i n g numb e r and s i z e o f p e r f o r a t i o n s , and by c o n t r o l l i n g t h e d i f f e r e n t i a l p r e s s u r e across t h e p e r f o r a t i o n s . C o r r e c t l y a p p l i e d under t h e p r o p e r w e l l c o n d i t i o n s , lim i t e d - e n t r y can be e x t r e m e l y e f f e c t i v e . Aspects o f p s e u d o l i m i t e d - e n t r y f r a c t u r i n g a r e d i s c u s s e d by MATHIAS ( 1 9 7 0 ) . A p p l i c a t i o n o f b a l l s e a l e r s and d i v e r t e r s r e p r e s e n t s m o d i f i e d l i m i t e d e n t r y t e c h n i q u e (BLACK, RIPLEY, BEECROFT & PAMPLIN 1979; P A I , GARBIS & HALL 1983; c f . s e c t i o n s 4 . 2 . 2 . 1 . 1 . and 4 . 2 . 2 . 4 . ) . L i m i t e d - e n t r y t e c h n i q u e o f t e n a s c e r t a i n s t h a t t h e number o f open p e r f o r a t i o n s matches t r e a t i n g r a t e which can be achieved (CONWAY & H A R R I S 1982). I f t o o many p e r f o r a t i o n s e x i s t o v e r a l a r g e i n t e r v a l i n a m u l t i p l e - e n t r y design, then t o o many o f t h e p e r f o r a t i o n s may serve o n l y as f l u i d t h i e f zones which can cause t o o s h o r t f r a c t u r e l e n g t h due t o p r e s s u r e d i s s i p a t i o n o r can even provoke premature s c r e e n o u t t e r m i n a t i o n . On t h e o t h e r hand, i f o n l y a few o f t h e p e r f o r a t i o n s t h a t have been s h o t a r e open, then h i g h t r e a t i n g p r e s s u r e s may r e s u l t which i n t u r n can g i v e r i s e t o screenout f a i l u r e o r non-optimum p r o p p a n t p l a c e ment. L i m i t e d - e n t r y p e r f o r a t i n g i s o f t e n combined w i t h s e p a r a t e a c i d b a l l - o u t i n o r d e r t o open t h e p e r f o r a t i o n s (WEBSTER 1977). Another a p p l i c a t i o n o f l i m i t e d - e n t r y techniques i n a manner comparable t o t h a t i n h y d r a u l i c p r o p p a n t f r a c t u r i n g i s i n s t e a m - f l o o d enhanced o i l r e c o v e r y t o c o n t a i n steam o v e r r i d e and t o m i n i m i z e t h e o r i g i n o f o i l pockets o r b l i n d s p o t s from i n s u f f i c i e n t v e r t i c a l conformance o f t h e i n j e c t e d steam (HUANG & HIGHT 1986; c f . s e c t i o n 4 . 9 . 4 . ) . Limited-entry fracturing i s a s i g n i f i c a n t means o f v e r t i c a l f r a c t u r e growth r e s t r i c t i o n due t o t h e g e n e r a l tendency o f t h e c r a c k t o propagate upwards as a consequence o f decreasing c l o s u r e s t r e s s ( i n - s i t u s t r e s s v a r i e s w i t h depth; CLIFTON & ABOU-SAYED 1983) w i t h d e c l i n i n g overburden, w i t h f r a c t u r e h e i g h t c o n t r o l t h u s commonly b e i n g performed by o n l y p e r f o r a t i n g t h e l o w e r p o r t i o n o f t h e i n t e r v a l t o be t r e a t e d (LAMBERT, DOLAN & GALLUS 1983; P A I , G A R B I S & HALL 1983). Aspects o f l i m i t e d - e n t r y f r a c t u r i n g techn i q u e a r e a l s o d i s c u s s e d by ODEH (1976), I C H A R A & RAGHAVAN (1981), CHANCELLOR & JOHNSON (1988) and ZHOU, 9 1 & X I A O (1988).
4.2.2.3. Fracture containment by treatment parameter adjustment The dominant f a c t o r f o r a r r e s t i n g o r a t l e a s t r e s t r i c t i n g v e r t i c a l growth o f f r a c t u r e s i s t h e i n - s i t u minimum h o r i z o n t a l s t r e s s c o n t r a s t between pay zone and b a r r i e r (WARPINSKI, SCHMIDT & NORTHROP 1980; TEUFEL & CLARK 1981, NGUYEN & LARSON 1983; c f . s e c t i o n s 4.2.2.5.3. and 4 . 2 . 3 . 2 . 2 . ) . I f shear s t r e n g t h o f t h e i n t e r f a c e i s small r e l a t i v e t o t e n s i l e s t r e n g t h and minimum h o r i z o n t a l compress i v e s t r e s s o f t h e boundary l a y e r , v e r t i c a l f r a c t u r e s may t u r n i n t o i n t e r f a c i a l f r a c t u r e s ( c f . s e c t i o n 4.5.4.4.2.) and t h u s containment i s achieved (TEUFEL & CLARK 1981), a l t h o u g h t h i s e f f e c t may be o n l y temporary, because a f t e r t h e f r a c t u r e i s l a t e r a l l y d i s p l a c e d a t t h e i n t e r f a c e , i t can s t i l l grow i n t o t h e boundi n g l a y e r i f t h e f r i c t i o n a l p r o p e r t i e s o f t h e i n t e r f a c e change (HANSON, SHAFFER & ANDERSON 1980). Shales a r e no a b s o l u t e b a r r i e r s t o f r a c t u r e m i g r a t i o n , b u t a r e o n l y e f f i c i e n t i n c o n t a i n i n g c r a c k s i f a c e r t a i n c r i t i c a l s t r e s s c o n t r a s t i s n o t underpassed (AHMED 1988; c f . s e c t i o n s 4.2.2.5.3. and 4 . 2 . 3 . 2 . 2 . ) . Weakly-bonded i n t e r f a c e s a t s h a l l o w d e p t h may s t o p f r a c t u r e growth (DANESHY 1978), and a l s o
372 s t r e s s d i s c o n t i n u i t i e s such as f a u l t s may t e r m i n a t e f r a c t u r e p r o p a g a t i o n (ROEG I E R S & WILES 1981; ROEGIERS, McLENNAN & MURPHY 1982; McLENNAN, ROEGIERS & MARX 1983; c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) . C a l c u l a t i o n o f v e r t i c a l f r a c t u r e containment i n l a y e r e d f o r m a t i o n s i s e v a l u a t e d by FUNG, VIJAYAKUMAR & CORMACK ( 1 9 8 7 ) . F o l l o w i n g d i s c u s s i o n o f some f a c t o r s i n f l u e n c i n g f r a c t u r e containment, emphasis i s p u t on i n j e c t i o n r a t e s , f l u i d p r o p e r t i e s , propped f r a c t u r e h e i g h t vs. r e s e r v o i r t h i c k n e s s , and p e r f o r a t i o n i n t e r v a l s e l e c t i o n .
4 . 2 . 2 . 3 . 1 . Factors influencing fracture containment C o n t r a s t i n m a t e r i a l p r o p e r t i e s and p o r e p r e s s u r e g e n e r a l l y has o n l y secondaon f r a c t u r e containment, w i t h e l a s t i c modulus b e i n g more i n f l u e n t i a l than t h e o t h e r m a t e r i a l p r o p e r t i e s , b u t b e i n g o n l y a b l e t o reduce t h e w i d t h o f t h e o u t - o f - z o n e f r a c t u r e segment and n o t t o s t o p i t s ungoverned h e i g h t propagat i o n (EEKELEN 1982, NGUYEN & LARSON 1983). I f t h e f r a c t u r i n g p r e s s u r e i s h i g h enough, t h e c r a c k can propagate across any b a r r i e r (HUNT, RAYMOND, HASKETT & P I R I E 1981; c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) , w i t h t h e r e f o r e p r e s s u r e c o n t r o l p l a y i n g a c h i e f r o l e in f r a c t u r e c o n t a i n m e n t a t t e m p t s . H i g h i n j e c t i o n r a t e s a r e necessary t o c a r r y t h e proppant i n t h e f r a c t u r i n g g e l and t o reduce t h e p r o b a b i l i t y o f a premature s c r e e n o u t d u r i n g s t i m u l a t i o n (PA1 & G A R B I S 1983; c f . section 6.2.4.2.1.). ry e f f e c t s
I n order t o contain a f r a c t u r e w i t h i n the formation o f i n t e r e s t o r t o minimize i t s v e r t i c a l h e i g h t growth, t h e excess p r e s s u r e w i t h i n t h e f r a c t u r e must be m i n i m i z e d (HOLDITCH, ROBINSON, WHITEHEAD & ELY 1988). The excess p r e s s u r e i s a l most e n t i r e l y dependent on t h e f r i c t i o n p r e s s u r e drop down t h e f r a c t u r e ( c f . s e c t i o n 4.3.4.2.2.) which i n t u r n i s a f u n c t i o n o f g e l v i s c o s i t y , i n j e c t i o n r a t e , and f r i c t i o n c o e f f i c i e n t between f l u i d and f r a c t u r e w a l l , w i t h g e l v i s c o s i t y and f r i c t i o n c o e f f i c i e n t h a v i n g t h e g r e a t e s t i n f l u e n c e on f r i c t i o n p r e s sure drop ( c f . s e c t i o n 4 . 3 . 4 . 2 . ) . The o u t l i n e as f o l l o w s i l l u s t r a t e s f r a c t u r e o r i e n t a t i o n changes, f r a c t u r e geometry v a r i a t i o n s , c o n f i n i n g s t r e s s c o n t r a s t , r e s e r v o i r d e p l e t i o n and s t r e s s c o n t r a s t magnitude, p o s i t i v e and n e g a t i v e f r a c t u r e containment, and f r a c t u r e growth t e r m i n a t i o n a t i n t e r f a c e s .
4 . 2 . 2 . 3 . 1 . 1 . Fracture orientation changes S p e c i a l i n f l u e n c e s o f screenouts a r e changes o f f r a c t u r e o r i e n t a t i o n f r o m hor i z o n t a l t o v e r t i c a l i n s h a l l o w r e s e r v o i r s t o r e y s (SCHRAUFNAGEL & LAMBERT 1988). M u l t i p l e c o a l seam f r a c t u r i n g i n a s t r e s s l e v e l where h o r i z o n t a l r a t h e r than v e r t i c a l f r a c t u r e s a r e more l i k e l y t o o r i g i n a t e can be performed by t h e lim i t e d - e n t r y s t i m u l a t i o n t e c h n i q u e which p r o v i d e s f o r m u l t i p l e h o r i z o n t a l l y i n d u ced f r a c t u r e s and/or complex f r a c t u r e s w i t h b o t h h o r i z o n t a l and i n c l i n e d components ( a s p e c t s o f composed f r a c t u r e s a r e a l s o d i s c u s s e d i n s e c t i o n s 4.4.3.4.3. and 4 . 9 . 4 . 2 . ) . P e r f o r a t i o n s serve as p r e s s u r e chokes t h a t c o n t r o l t h e r a t e o f f l u i d i n j e c t i o n i n t o each t a r g e t e d zone. The l i m i t e d - e n t r y t e c h n i q u e r e q u i r e s maintenance o f a p r e d e t e r m i n e d w e l l b o r e p r e s s u r e t h r o u g h o u t t h e o p e r a t i o n . P e r f o r a t i o n d i a m e t e r grows due t o p r o p p a n t a b r a s i o n which causes p r e s s u r e reduct i o n s . S i g n i f i c a n t p r e s s u r e drops can i n d i c a t e screenout o f s e v e r a l p e r f o r a t i o n s and i n i t i a t i o n o f a v e r t i c a l f r a c t u r e f o l l o w i n g t e r m i n a t i o n o f propagat i o n o f the e a r l i e r h o r i z o n t a l crack, Synthetic a l t e r e d - s t r e s s f r a c t u r i n g i n s t e a d o f n a t u r a l c r a c k o r i e n t a t i o n changes p r o f i t s f r o m a r t i f i c i a l l y - i n d u c e d c o n d i t i o n s f o r a1 t e r a t i o n o f f r a c t u r e p r o p a g a t i o n d i r e c t i o n ( c f . s e c t i o n 4.9.4.1.3.1.). When t h e c r i t i c a l t r e a t i n g p r e s s u r e i s reached and t h e f r a c t u r e b r e a k s o u t o f zone, t h e s t r e s s c o n c e n t r a t i o n i s h i g h e s t i n t h e near w e l l b o r e area (ROSE 1988). T h e r e f o r e u n c o n t r o l l e d f r a c t u r e h e i g h t growth can o n l y o c c u r a t t h e crack t i p where no p r o p p a n t s l u r r y i s p r e s e n t , w i t h t h i s c o n d i t i o n t y p i c a l l y r e s u l t i n g i n a sudden l o s s o f pad volume and t i p screenout. Experimental modell i n g o f the stress gradient a f f e c t i n g the height o f v e r t i c a l hydraulic f r a c t u -
373 r e s i s c a r r i e d o u t by THIERCELIN & LEMANCZYK (1983).
4.2.2.3.1.2.
Fracture geometry v a r i a t i o n s
F r a c t u r e geometry can a l s o be i n f l u e n c e d by t r e a t m e n t parameters such as f l u i d v i s c o s i t y and d e n s i t y , p r o p p a n t c o n c e n t r a t i o n , pumping r a t e , and p e r f o r a t i o n placement (NGUYEN & LARSON 1983). By v a r y i n g f l u i d p r o p e r t i e s , p r o p p a n t sat u r a t i o n and/or i n j e c t i o n r a t e , f r i c t i o n a l p r e s s u r e drop a l o n g t h e f r a c t u r e f a ces can be k e p t below t h e minimum h o r i z o n t a l s t r e s s d i f f e r e n c e between c o n f i n i n g zone and pay h o r i z o n . D e t e c t i o n o f t h e p o s s i b l e o n s e t o f u n d e s i r a b l e growth of f r a c t u r e h e i g h t i s p o s s i b l e f r o m t r e a t i n g p r e s s u r e s which may a l l o w t o u n d e r t a k e c o r r e c t i v e a c t i o n s d u r i n g t h e o p e r a t i o n (NOLTE & SMITH 1 9 8 1 ) . Exc e s s i v e p r e s s u r e caused by t h e f l u i d f l o w i n g down t h e f r a c t u r e can overcome t h e confining stresses i n adjacent rock layers, r e s u l t i n g i n excessive f r a c t u r e h e i g h t growth as w e l l as u n c o n t r o l l e d h o r i z o n t a l c r a c k p r o p a g a t i o n (HOLDITCH, ROBINSON & WHITEHEAD 1986).
4.2.2.3.1.3.
Confining s t r e s s contrast
The main f a c t o r s c o n t r o l l i n g v e r t i c a l h e i g h t growth o f h y d r a u l i c f r a c t u r e s a r e c o n f i n i n g s t r e s s c o n t r a s t , e l a s t i c s t i f f n e s s c o n t r a s t , f r a c t u r e toughness, and i n f l u e n c e o f s e t t l e d proppant bank o r buoyant p r o p p a n t f o r downward o r upward c r a c k e x t e n s i o n (SETTARI 1985). C o n f i n i n g s t r e s s c o n t r a s t i s t h e s t r o n g e s t containment mechanism, whereas s t i f f n e s s c o n t r a s t cannot c o m p l e t e l y a r r e s t h e i g h t growth. F r a c t u r e toughness i s i m p o r t a n t o n l y f o r c r a c k s c r e a t e d w i t h l o w - v i s c o s i t y f l u i d s o r a f t e r f l u i d breakdown. A p r o p p a n t bank a t t h e f r a c t u r e b o t t o m o r buoyant proppants c o n c e n t r a t e d a t t h e c r a c k t o p can be an e f f e c t i v e b a r r i e r t o v e r t i c a l p r o p a g a t i o n ( c f . s e c t i o n 4.2.2.4.), and l o c a l temperature changes a t t h e f r a c t u r e t i p a r e n o t l i k e l y t o i n c r e a s e h e i g h t growth r a t e ser i o u s l y ( a summary o f f r a c t u r e containment mechanisms i s g i v e n by SNEDDON & LOWENGRUB 1969). I n - s i t u s t r e s s c o n t r a s t s between sandstones and. mudstones ( c f . s e c t i o n 4.2.3.2.2.1.) can i n some cases be s u f f i c i e n t t o c o n t a i n t h e f r a c t u r e s i f t h e y a r e l a r g e enough ( c f . s e c t i o n s 4.2.2.5.3. and 4 . 2 . 3 . 2 . 2 . ) . Smaller i n - s i t u s t r e s s c o n t r a s t s can e f f e c t i v e l y l i m i t f r a c t u r e p r o p a g a t i o n i f a s s i s t e d by h i g h e r p r e s s u r e drops i n v e r t i c a l d i r e c t i o n due t o small c r a c k w i d t h o r i n e f f i c i e n c i e s a s s o c i a t e d w i t h f r a c t u r e e x t e n s i o n across m u l t i p l e complex beds cont a i n i n g s e p a r a t i n g mudstone drapes where c r a c k e x t e n s i o n energy i s p r o g r e s s i v e l y d i m i n i s h e d (WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LOREN2 & THORNE 1988). F r a c t u r e containment by t r e a t m e n t parameter adjustment i s g e n e r a l l y known as c o n t r o l l e d process zone f r a c t u r i n g i n c l u d i n g o p t i m i z a t i o n o f i n j e c t i o n r a t e s , pressure, f l u i d p r o p e r t i e s and o t h e r f a c t o r s i n o r d e r t o t a i l o r t h e c r a c k (AHMED, STRAWN, WILSON & SCHATZ 1983). E f f e c t s o f s t r e s s s t a t e on h y d r a u l i c f r a c t u r e s a r e a l s o i l l u s t r a t e d by ZOBACK, ZOBACK & SEEBURGER ( 1 9 8 1 ) .
4.2.2.3.1.4.
Reservoir d e p l e t i o n and s t r e s s contrast magnitude
D e p l e t i o n o f r e s e r v o i r p o r e p r e s s u r e w i t h p r o g r e s s i v e hydrocarbon e x p l o r a t i o n l e a d s t o i n c r e a s i n g f r a c t u r e g r a d i e n t between pay zone and a d j a c e n t s h a l e s and t h e r e f o r e c r e a t e s a more e f f e c t i v e boundary f o r c r a c k p r o p a g a t i o n (HUEBINGER, WEBSTER, CHISHOLM, VENDITTO & HUNT 1988). W i t h a h i g h e r c o n f i n i n g s t r e s s contrast, f r a c t u r e s can be b e t t e r r e s t r i c t e d t o t h e c o r e o f t h e p r o s p e c t i v e i n t e r v a l , and t h e r e s u l t i n g r e d u c t i o n o f c r a c k h e i g h t a l l o w s t o s u b s t a n t i a l l y decrease b o t h t o t a l proppant q u a n t i t y and p r o p p a n t c o n c e n t r a t i o n (case s t u d i e s r e veal t h a t p r o p p a n t volume can be d i m i n i s h e d by up t o 40 % ) . T h i s i s t h e reason why r e f r a c t u r i n g ( o r a l t e r n a t i v e l y l a t e - s t a g e i n f i l l - w e l l s t i m u l a t i o n ; c f . sect i o n 4.8.9.) i s f r e q u e n t l y c o m p a r a t i v e l y cheaper t h a n o r i g i n a l o r e a r l y - s t a g e t r e a t m e n t s and i s o f t e n s t i l l economical even w i t h d e c l i n i n g p r o d u c t i o n and f a l -
374 l i n g o i l price.
4.2.2.3.1.5.Posit ive and negative fracture conta i nment D i s t i n c t i o n has t o be made between p o s i t i v e and n e g a t i v e f r a c t u r e c o n t a i n ment c o n d i t i o n s (WARPINSKI, CLARK, SCHMIDT & HUDDLE 1 9 8 2 ) . I n f o r m a t i o n s w i t h p o s i t i v e containment conditions, f r a c t u r e growth out-of-zone i s not anticipat e d , and l o n g d e e p l y p e n e t r a t i n g f r a c t u r e s can be e f f e c t i v e l y u s e d t o d e v e l o p t h e r e s o u r c e and have a h i g h e c o n o m i c a l p a y - o f f . I n reservoirs w i t h negative c o n t a i n m e n t c o n d i t i o n s , however, e x c e s s i v e f r a c t u r e h e i g h t g r o w t h c a n o n l y be prevented by renouncing from large-scale treatments which would waste f l u i d s and p r o p p a n t s i n a d j o i n i n g n o n - p r o s p e c t i v e f o r m a t i o n s , and b y l i m i t a t i o n t o s m a l l - s c a l e o p e r a t i o n s r e s u l t i n g i n o n l y l o w e r w e l l p r o d u c t i v i t y improvement, because t h e o n l y o t h e r a l t e r n a t i v e i s t o r e j e c t w e l l and r e s e r v o i r h o r i z o n as unfeasible.
4.2.2.3.1.6. Fracture growth termination at interfaces F r a c t u r e c o n t a i n m e n t may be a c h i e v e d b y t e r m i n a t i o n o f f r a c t u r e g r o w t h a t a d i s c r e t e , weak o r unbounded i n t e r f a c e (ANDERSON 1979, TEUFEL 1979, TEUFEL & CLARK 1981) o r r e s t r i c t i o n i n t h e b o u n d i n g l a y e r t o w h i c h t h e c r a c k p r o p a g a t e s and where e x t e n s i v e g r o w t h does n o t t a k e p l a c e (WARPINSKI, CLARK, SCHMIDT & HUDDLE 1982; c f . s e c t i o n 4 . 2 . 3 . ) . I f t h e f r a c t u r e p e n e t r a t e s i n t o a r e g i o n hav i n g an a d v e r s e s t r e s s g r a d i e n t , c o n t i n u e d p r o p a g a t i o n r e s u l t s i n h i g h e r s t r e s ses on t h e f r a c t u r e t h u s s e l f - l i m i t i n g g r o w t h (SIMONSON, ABOU-SAYED & CLIFTON 1 9 7 8 ) . A n o t h e r mechanism o f f r a c t u r e r e s t r i c t i o n i s p r o p a g a t i o n i n t o a h i g h e r modulus r e g i o n where t h e d e c r e a s e d w i d t h r e s u l t s i n i n c r e a s e d p r e s s u r e d r o p i n the crack which might i n h i b i t extensive growth i n t o t h a t r e g i o n r e l a t i v e t o t h e l o w e r modulus zone. H y d r a u l i c f r a c t u r i n g c a n be done t h r o u g h t u b i n g i n c a s e o f smaller treatments i n order t o avoid k i l l i n g o f the w e l l a f t e r termination o f t h e s t i m u l a t i o n j o b and because t u b i n g s t r e n g t h i s much g r e a t e r t h a n c a s i n g s t r e n g t h (WEIMER, SONNENBERG & YOUNG 1 9 8 6 ) .
4.2.2.3.2. Injection rates Containment o f f r a c t u r e s i n t h i n sandstones i s i n p r a c t i c e g e n e r a l l y reached by r e d u c e d pumping r a t e s when i n a s t a b l e s y s t e m t h e c r a c k i s j u s t r u n n i n g a l o n g t h e s a n d s t o n e l a y e r and i s c o n d u c t e d b y t h e b o u n d i n g t i g h t r o c k h o r i z o n ( f o l l o w i n g the l e a s t resistance f o r i t s propagation), w i t h t h e mudstone s e a l forming a t i g h t b a r r i e r t h a t i s l i m i t i n g v e r t i c a l f r a c t u r e extension unless the c r i t i c a l p r e s s u r e s t r e s s o f t h e b o u n d i n g mudstones t o c o n f i n e t h e c r a c k i s e x ceeded ( c f . a l s o CONWAY, McGOWEN, GUNDERSON & K I N G 1985; BEN NACEUR & TOUBOUL 1 9 8 7 ) . A d v e r s e c o n t a i n m e n t c o n d i t i o n s b e i n g s m a l l s t r e s s c o n t r a s t s between adj o i n i n g l a y e r s a r e overcome b y f r a c t u r e h e i g h t c o n t r o l w i t h s m a l l i n j e c t i o n r a t e s and/or low f l u i d v i s c o s i t i e s i n o r d e r t o a v o i d exceeding t h e c r i t i c a l press u r e t h a t w o u l d l e a d t o p r o p a g a t i o n o u t o f zone e s p e c i a l l y d u r i n g f r a c t u r e i n i tiation. A l t h o u g h i n j e c t i n g a t a h i g h e r r a t e y i e l d s a more e f f e c t i v e t r e a t m e n t , i t i s o f t e n n e c e s s a r y t o keep t h e o p e r a t i o n a l p r e s s u r e s b e l o w f o r m a t i o n c r i t i c a l p r e s s u r e d u r i n g t h e j o b i n o r d e r t o m a x i m i z e l a t e r a l g r o w t h and t o c o n t r o l v e r t i c a l p r o p a g a t i o n o f t h e f r a c t u r e (MENG & BROWN 1 9 8 7 ) . F o r d e t e r m i n a t i o n o f t h e m a x i mum a l l o w a b l e pumping r a t e , p i p e f r i c t i o n p r e s s u r e , shear degradation o f t h e f r a c t u r i n g f l u i d , and s u r f a c e t r e a t i n g p r e s s u r e have a l s o t o be c o n s i d e r e d . The d i s c u s s i o n as f o l l o w s f o c u s s e s o n d e c r e a s i n g pumping r a t e and t o t a l j o b s i z e , p r e s s u r e g r o w t h and s c r e e n o u t t e r m i n a t i o n , f r a c t u r e h e i g h t and f l u i d d i s t r i b u t i o n , and t u b u l a r e n l a r g e m e n t and horsepower d i m i n u t i o n .
375
4.2.2.3.2.1. Decreasing pumping rate and total job size Another p o s s i b i l i t y t o m i n i m i z e v e r t i c a l f r a c t u r e growth i s t o l i m i t t h e s i ze of t h e s t i m u l a t i o n t r e a t m e n t along w i t h r e s t r i c t e d pumping r a t e s (BARBER & THEMIG 1985). F r a c t u r e containment c o n t r o l by decreasing pumping r a t e and f l u i d v i s c o s i t y l e a d s i n many cases a u t o m a t i c a l l y t o a s m a l l e r t o t a l j o b s i z e , as t h e reduced r a t e causes i n c r e a s i n g f l u i d l o s s which i s l i m i t i n g c u m u l a t i v e f l u i d q u a n t i t y by i n c r e a s i n g s c r e e n o u t r i s k . An a l t e r n a t i v e c o m b i n a t i o n i s i n c r e a s i n g f l u i d d e n s i t y , and l o w e r i n g pumping r a t e s o r r e d u c i n g f l u i d p r e s s u r e (SIMONSON, ABOU-SAYED & CLIFTON 1978; K O Z I K , BAILEY & HOLDITCH 1979). Decreasing pumping r a t e s and n e v e r t h e l e s s m a i n t a i n i n g adequate proppant placement t h e r e b y r e d u c i n g t h e number o f uneconomical s t i m u l a t i o n j o b s can be b e s t achieved by u t i l i z i n g a l o w e r - d e n s i t y proppant, w i t h t h e r e f o r e i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y a l u m i na s i l i c a t e proppants b e i n g o f c o n s i d e r a b l e s i g n i f i c a n c e (CUTLER, JONES, SWANSON & CARROLL 1981) f o r a s s i s t i n g v e r t i c a l f r a c t u r e containment i n t h i n r e s e r v o i r s . I n some s p e c i a l cases, pay c o n d i t i o n s and s t a b i l i t y o f t h e boundaries can n e c e s s i t a t e t o reduce t r e a t m e n t s i z e and pumping r a t e s so s i g n i f i c a n t l y t o achieve t h e goal o f t h e crack s t a y i n g i n zone t h a t proppant f r a c t u r i n g i s below t h e minimum economical f e a s i b i l i t y volume and cannot compete w i t h a c i d f r a c t u r i n g and m a t r i x a c i d i z i n g i n carbonates (HUCKABEE 1988). The v i s c o s i t y range o f e x i s t i n g f r a c t u r i n g f l u i d s makes m i n i m i z i n g f l u i d v i s c o s i t y a much more e f f e c t i v e method o f c o n t r o l l i n g p r e s s u r e than l o w e r i n g pumpi n g r a t e (NOLTE 1982). Decreasing o f s p e c i f i c proppant g r a v i t y has t h e advantage t h a t i d e n t i c a l p r o p p a n t t r a n s p o r t t o t h a t c u r r e n t l y a c h i e v a b l e can t a k e p l a ce i n l o w e r - v i s c o s i t y f l u i d s , o r a l t e r n a t i v e l y h i g h e r p r o p p a n t volumes can be pumped i n a g i v e n amount o f a h i g h - v i s c o s i t y f r a c t u r i n g f l u i d (CUTLER, ENNISS, JONES & SWANSON 1985; t h e same a p p l i e s f o r t r a n s p o r t and s e t t l i n g o f g r a v e l i n sand c o n t r o l c a r r i e r f l u i d s ; c f . s e c t i o n 5 . 3 . 3 . ) . Another p o s s i b i l i t y o f adaptat i o n i s a p p l i c a t i o n o f h i g h - s t r e n g t h s y n t h e t i c proppants a t l o w e r c o n c e n t r a t i o n s and i n narrower f r a c t u r e s i n s t e a d o f h i g h e r n a t u r a l sand s a t u r a t i o n s i n b r o a d e r c r a c k s , because narrower f r a c t u r e s a r e s l o w e r p r o p a g a t i n g i n v e r t i c a l d i r e c t i o n and can t h e r e f o r e be b e t t e r c o n t r o l l e d than w i d e r c r a c k s ( L I & ZHU 1986). T h i s procedure i s backed up by t h e f i e l d e x p e r i e n c e t h a t i n many cases, f r a c t u r e s propped w i t h l o w e r c o n c e n t r a t i o n s o f b a u x i t e have b e t t e r c o n d u c t i v i t i e s t h a n c r a c k s propped w i t h h i g h e r s a t u r a t i o n s o f sand. Boundary zones s t o p p i n g f r a c t u r e p r o p a g a t i o n and i n h i b i t i n g f u r t h e r c r a c k e x t e n s i o n can be r e g i o n s o f i n c r e a s e d f o r m a t i o n s t r e s s , w i t h t h e h i g h - s t r e s s b a r r i e r s h a r p l y c o n t r a s t i n g w i t h t h e l o w - s t r e s s r e s e r v o i r , o r can be r e p r e s e n t e d by t h e p o i n t a t which f l u i d l e a k o f f becomes equal t o pumping r a t e i n permeable columns and t h u s a l s o f u r t h e r f r a c t u r e p r o p a g a t i o n i s n o t p o s s i b l e (HUNT, R A Y MOND, HASKETT & P I R I E 1987). D i f f i c u l t i e s i n c r a c k containment by t r e a t i n g p a r a meter a d j u s t m e n t can o c c u r i n d e p l e t e d r e s e r v o i r s where f a i r l y severe v e r t i c a l growth and/or l e a k o f f can e a s i l y o c c u r (BEGNAUD & CLAIBORNE 1985). E f f e c t s o f t r e a t m e n t parameters on f r a c t u r e geometry a r e a l s o discussed by DANESHY (1971 b) .
4.2.2.3.2.2. Pressure growth and screenout termination Pressure g r o w t h c o m p r i s i n g a b n o r m a l l y l a r g e i n c r e a s e s i n t r e a t i n g p r e s s u r e i s m a i n l y caused by f r a c t u r e b r a n c h i n g and i s a f u n c t i o n o f pumping r a t e and f l u i d v i s c o s i t y (MEDLIN & FITCH 1983). T h i n f r a c t u r i n g f l u i d s pumped a t h i g h r a t e s generate smooth p r e s s u r e c u r v e s which r i s e p a r a b o l i c a l l y w i t h t i m e t o f i r s t o r d e r , whereas t h i c k g e l s pumped a t low r a t e s c r e a t e e r r a t i c p r e s s u r e c u r v e s t h a t t e n d t o r i s e more o r l e s s l i n e a r l y w i t h t i m e and show f r e q u e n t l y sharp i r r e g u l a r i t i e s w i t h a b r u p t i n c r e a s e s and d e c l i n e s . The b e g i n n i n g o f screenout appears as a sudden sharp r i s e i n t r e a t i n g p r e s s u r e on t h e r e c o r d r e g a r d l e s s o f f l u i d t y p e and pumping r a t e ( c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) . The s c r e e n o u t i s a s s o c i a t e d w i t h t h e amount o f p r e s s u r e i n c r e a s e r a t h e r
than
376 t h e magnitude o f t h e p r e s s u r e i t s e l f . Whi e p r e s s u r e growth c o u l d be avoided o r g r e a t l y reduced by pumping t h i n f r a c t u r i n g f l u i d s a t low r a t e s , t h i s approach i s i m p r a c t i c a l due t o l a c k i n g p r o p p a n t r a n s p o r t c a p a b i l i t y . T h i n f l u i d s must be pumped a t h i g h r a t e s t o c a r r y proppants a t p r a c t i c a l c o n c e n t r a t i o n s , w i t h these h i g h r a t e s then i n t u r n a g a i n n c r e a s i n g s c r e e n o u t r i s k . A two-stage t r e a t m e n t i s t h e oDtimum s o l u t i o n o f these Droblems. The f i r s t staqe comDrises pumping o f a t h i n f l u i d a t low r a t e s w i t h o u t proppants i n o r d e r t o g e n e r a t e a l a r g e f r a c t u r e . A f t e r cleanup, t h e second stage i n c l u d e s pumping o f a c o n v e n t i o n a l t h i c k g e l t r e a t m e n t w i t h h i g h p r o p p a n t s a t u r a t i o n which s i m p l y reopens t h e e x i s t i n g f r a c t u r e and t h u s a p r e s s u r e i n c r e a s e i s n o t expected.
4.2.2.3.2.3. Fracture height and fluid distribution I n j e c t i o n r a t e a f f e c t s f r a c t u r e h e i g h t and f l u i d d i s t r i b u t i o n and i s v e r y c r i t i c a l t o t h e o v e r a l l success o f t h e t r e a t m e n t (PAI, GARBIS & HALL 1983). Where i n t e r v a l y i e l d o f t u b i n g and/or c a s i n g l i m i t s i n j e c t i o n r a t e , t h e operat i o n s h o u l d be staged u s i n g b a l l s e a l e r s . I t c o u l d be f a v o u r a b l e f o r t h e j o b procedure t o s t a r t a t h i g h i n j e c t i o n r a t e s and low p r o p p a n t c o n c e n t r a t i o n s and t h e n l a t e r t a p e r i n g i n j e c t i o n r a t e and i n c r e a s i n g p r o p p a n t s a t u r a t i o n (GIDLEY, M U T T I , NIERODE, KEHN & MUECKE 1979; MATTHEWS, MILLER & SCHLOTTMAN 1 9 8 2 ) . The high i n i t i a l i n j e c t i o n r a t e allows f o r the m a j o r i t y o f the u l t i m a t e f r a c t u r e h e i g h t t o be c r e a t e d w h i l e pad and l o w - p r o p p a n t - c o n c e n t r a t i o n s l u r r i e s a r e pumped, t h e r e b y e s t a b l i s h i n g l e a k o f f c o n t r o l o v e r t h e e n t i r e f r a c t u r e h e i g h t w h i l e t h e pad i s b e i n g i n s e r t e d and t h u s r e d u c i n g s c r e e n o u t r i s k (PAI, G A R B I S & HALL 1 9 8 3 ) . D i m i n i s h i n g i n j e c t i o n r a t e d u r i n g l a t e r t r e a t m e n t stages decreases t h e chances o f f u r t h e r f r a c t u r e h e i g h t growth and a l s o p r e v e n t s equipment o v e r l o a d d u r i n g h i g h p r o p p a n t c o n c e n t r a t i o n pumping. On t h e o t h e r hand, however, h i g h i n j e c t i o n r a t e s a r e necessary t o c a r r y t h e proppants i n t h e g e l and t o reduce t h e p r o b a b i l i t y o f a premature s c r e e n o u t d u r i n g f r a c t u r i n g (PA1 & G A R B I S 1983), w i t h t h u s a sound compromise f o r s a t i s f y i n g a l l t h e purposes h a v i n g t o be found.
AHMED, STRAWN, SCHMIDT, O'SHEA & VEGHTE (1983) comment on s t i m u l a t i n g t i g h t sands i n t h e presence o f weak s t r e s s b a r r i e r s ( c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) by modif i e d h y d r a u l i c f r a c t u r i n g t e c h n i q u e s . The e f f e c t o f bounding f o r m a t i o n s on v e r t i c a l p r o p a g a t i o n o f a h y d r a u l i c a l l y - i n d u c e d f r a c t u r e can be i g n o r e d f o r s h o r t cracks, b u t i s v e r y p r o m i n e n t and o f c o n s i d e r a b l e i n f l u e n c e f o r l o n g f r a c t u r e s (KOZIK, BAILEY & HOLDITCH 1979; c f . s e c t i o n 4 . 8 . 1 1 . ) . The t i m e l a g between f r a c t u r e t i p a r r i v a l ( w i d t h response) and f l u i d a r r i v a l ( p r e s s u r e response) p r o v i des an e s t i m a t e o f f l u i d l a g (WARPINSKI 1983 a ) .
4.2.2.3.2.4. Tubular enlargement and horsepower diminution Decreasing i n j e c t i o n r a t e s and c o n s e q u e n t l y d i m i n i s h i n g h y d r a u l i c horsepower r e q u i r e d f o r t h e e x e c u t i o n o f t h e t r e a t m e n t can a l s o be achieved by a h i g h e r s t r e n g t h c a s i n g t h r o u g h which a l a r g e r t u b i n g can be r u n (GARBIS, BROWN & MAUR I T Z 1 9 8 5 ) . R e d u c t i o n o f h y d r a u l i c horsepower by t h e l a r g e r t u b i n g s h o u l d o f f s e t t h e e x t r a expenses f o r t h e l a r g e r t u b u l a r goods ( c f . s e c t i o n 4 . 3 . 4 . 2 . 2 . 1 . ) . A p a r t f r o m s u c c e s s f u l s t i m u l a t i o n o f a t h i c k r e s e r v o i r zone i n a c o s t - e f f e c t i v e manner, another b e n e f i t o f t h e l a r g e r t u b i n g / c a s i n g program i s l o w e r s u r f a c e t r e a t i n g p r e s s u r e d u r i n g t h e f r a c t u r i n g o p e r a t i o n w h i c h r e s u l t s i n s a f e r ons i t e conditions. F r a c t u r e h e i g h t containment can a l s o be achieved by c r e a t i n g an a r t i f i c i a l b a r r i e r w i t h a d d i t i v e s (NGUYEN & LARSON 1983), w i t h c a u t i o n , however, h a v i n g t o be e x e r c i s e d by p r e v e n t i n g t h e c r a c k t o grow t h r o u g h t h e b a r r i e r (LUISKUTTY, TOMUTSA & PALMER 1986) by exceeding t h e boundary s t r e s s beyond t h e f r a c t u r i n g g r a d i e n t o f t h e a d j o i n i n g beds w i t h h i g h e r e l a s t i c i t y moduls. F i e l d e x p e r i e n c e o f the s i g n i f i c a n c e o f natural b a r r i e r s t o contain v e r t i c a l f r a c t u r e height growth i s r e p o r t e d by HOLDITCH, ROBINSON, WHITEHEAD & ELY ( 1 9 8 7 ) . L a b o r a t o r y e x -
377 p e r i m e n t s on f r a c t u r e p r o p a g a t i o n and l i m i t i n g e f f e c t s a r e c a r r i e d o u t by MEOLIN & MASSE ( 1 9 8 4 ) .
4.2.2.3.3. Fluid properties F r a c t u r i n g f l u i d p r o p e r t i e s a l s o have an i n f l u e n c e on p r o p p a n t t r a n s p o r t and v a r i a b l e c r a c k h e i g h t b e h a v i o u r (MORALES & ABOU-SAYED 1985, BEN NACEUR & TOUBOUL 1987). V e r t i c a l f r a c t u r e m i g r a t i o n o u t of c o n t r o l occurs when l a r g e r f l u i d i n j e c t i o n r a t e s and/or h i g h e r - v i s c o s i t y f l u i d s a r e used ( L i & ZHU 1986). Crack a r r e s t o r r e t a r d a t i o n o f f r a c t u r e p r o p a g a t i o n can be achieved by a l t e r i n g t h e v i s c o u s p r o p e r t i e s o f t h e i n j e c t e d f l u i d w i t h i n p a r t s o f t h e c r a c k (CLEARY 1980, SETTARI 1985). W h i l e h e a t t r a n s f e r e f f e c t s near t h e f r a c t u r e t i p w i l l gen e r a l l y i n c r e a s e l e a k o f f due t o f l u i d d e g r a d a t i o n and w i l l n o t s e l e c t i v e l y a f f e c t t h e p r o p a g a t i o n ( c f . s e c t i o n 4.3.4.4.), a much g r e a t e r p o t e n t i a l i s p r o v i ded by t h e i n j e c t i o n o f l i g h t o r heavy p r o p p a n t s (BRAUNLICH 1967, NGUYEN & LARSON 1983). SMITH, MILLER & HAGA (1987) propose a comparable t i p s c r e e n o u t t e c h n i q u e f o r s o f t f o r m a t i o n s i n o r d e r t o i n c r e a s e t h e f r a c t u r e opening a t t h e w e l l b o r e ( c f . s e c t i o n 4.5.4.4.1.). Some comments on proppant d e n s i t y and f l u i d v i s c o s i t y , foam and e n e r g i z e d f l u i d s , and d e n s i t y - c o n t r o l l e d f l u i d s a r e o f f e r e d as follows.
4.2.2.3.3.1. Proppant density and fluid viscosity Heavy p a r t i c l e s f a l l i n g a t t h e b o t t o m o f t h e f r a c t u r e i n c r e a s e package v i s c o s i t y , o r e q u i v a l e n t l y v e r t i c a l t r a n s m i s s i b i l i t y i s decreased by presence o f conc e n t r a t e d packages. S e l e c t e d p r o p p i n g agents t h a t have a d e n s i t y l o w e r t h a n t h e t r e a t i n g f l u i d w i l l r i s e t o t h e f r a c t u r e t o p through buoyancy and e v e n t u a l l y decrease c r a c k p r o p a g a t i o n (BEN NACEUR & TOUBOUL 1987; c f . s e c t i o n 4 . 2 . 2 . 4 . ) . When b o t h upward and downward growth o f t h e f r a c t u r e has t o be c o n t r o l l e d , t h e combined use o f heavy s e t t l i n g and l i g h t buoyant p a r t i c l e s i s r e q u i r e d . The char a c t e r i s t i c s o f t h e s t r u c t u r e o f such b i - d i s p e r s e s l u r r i e s as w e l l as s e t t l i n g and r i s i n g r a t e d i f f e r s i g n i f i c a n t l y f r o m those a p p l y i n g f o r mono-disperse systems due t o t h e presence o f s t r e a m i n g columns i n t h e s l u r r y (BATCHELOR & RENSBURG 1986, BEN NACEUR & TOUBOUL 1987). V e r t i c a l p r o p a g a t i o n o f t h e f r a c t u r e can be l i m i t e d by r e d u c i n g f l u i d p r e s s u r e which i s much b e t t e r achieved by m i n i m i z i n g f l u i d v i s c o s i t y t h a n by l o w e r i n g pumping r a t e s (SIMONSON, ABOU-SAYEO & CLIFTON 1978). Lower f l u i d v i s c o s i t i e s a r e p o s s i b l e t o be a p p l i e d i n c o n n e c t i o n w i t h l i g h t e r proppants, w i t h a l most i d e n t i c a l t r a n s p o r t c o n d i t i o n s b e i n g p r o v i d e d f o r l i g h t p r o p p a n t s by lowv i s c o s i t y f l u i d s and f o r heavy p r o p p a n t s by h i g h - v i s c o s i t y f l u i d s ( t h e same app l i e s f o r placement o f g r a v e l by s a n d - c o n t r o l c a r r i e r f l u i d s ; c f . section 5 . 3 . 3 . ) . The p o s s i b i l i t y o f r e d u c i n g f l u i d p r e s s u r e by f l u i d v i s c o s i t y f o r f r a c t u r e containment by v a r i a t i o n o f p r o p p a n t w e i g h t i s t h e g r e a t e s t advantage o f l o w - d e n s i t y i n t e r m e d i a t e - s t r e n g t h proppants (CUTLER, ENNISS, JONES & CARROLL 1983).
4.2.2.3.3.2. Foam and energized fluids M i n i m i z a t i o n o f f r a c t u r e h e i g h t growth can a l s o be achieved by a p p l i c a t i o n o f l o w - v i s c o s i t y p e r f e c t - s u p p o r t f l u i d s such as systems w i t h carbon d i o x i d e o r n i t r o g e n as m a j o r (foam) o r m i n o r ( e n e r g i z e r ) component (GARBIS & TAYLOR 1985). E n e r g i z e d and foam systems g i v e enhanced f l u i d r e c o v e r y and t h e r e f o r e b e t t e r c o n t r o l o f f r a c t u r e p r o p a g a t i o n and p r o p p a n t placement. The main advantages o f t h e use o f carbon d i o x i d e i n f r a c t u r e s t i m u l a t i o n t r e a t m e n t s a r e f a s t e r and i n creased l o a d r e c o v e r y , b u f f e r i n g o f aqueous f r a c t u r i n g f l u i d s t o a pH o f a b t . 3.5 t h e r e b y c o n t r o l l i n g c l a y - s w e l l i n g i n w a t e r - s e n s i t i v e r e s e r v o i r s , and r e p l a cement o f some o f t h e w a t e r i n t h e o p e r a t i o n , t h u s r e d u c i n g t h e l i q u i d volume t h a t has t o be r e c o v e r e d ( c f . a l s o s e c t i o n 3 . 1 1 . 2 . ) . Carbon d i o x i d e o r n i t r o g e n
378 foam systems c a n a l s o be u s e d f o r s e l e c t i v e p r o p p a n t p l a c e m e n t (BARBER & 1985; c f . s e c t i o n 4 . 4 . 4 . 2 . ) .
THEMIG
4.2.2.3.3.3, Densi ty-control led fluids MISAK, ATTEBERRY, VENDITTO & FREDRICKSON ( 1 9 7 8 a, 1978 b ) p r e s e n t d e n s i t y c o n t r o l l e d f r a c t u r i n g i n c l u d i n g i n s e r t i o n o f a h i g h e r - d e n s i t y pad f o l l o w e d b y l o w e r - d e n s i t y p r o p p a n t - c a r r y i n g t r e a t i n g f l u i d s . The h i g h e r - d e n s i t y p a d o c c u p i e s t h e l o w e r p o r t i o n o f t h e c r a c k and a c t s as a c u s h i o n f o r t h e f o l l o w i n g l o w e r - d e n s i t y p r o p p a n t - t r a n s p o r t i n g f l u i d s w h i c h s l i d e above t h e heavy p a d and move t h e p r o p p a n t o n l y i n t o t h e u p p e r p a r t o f t h e f r a c t u r e . As t h e l o w e r s e c t i o n o f t h e c r a c k r e c e i v e s no p r o p p a n t due t o s e t t l i n g i n h i b i t i o n a t t h e i n t e r f a c e o f t h e two f l u i d systems w i t h d i f f e r e n t s p e c i f i c g r a v i t y , f r a c t u r e c l o s u r e w i t h f a l l i n g p r e s s u r e l e a v e s o n l y t h e u p p e r segment o f t h e c r a c k p r o p p e d ( c f . a l s o section 4.2.2.6.4.). The p u r p o s e o f t h i s s p e c i a l t r e a t m e n t i s t o e x c l u d e f r a c t u r e g r o w t h i n t o t h e l o w e r w a t e r - p r o d u c i n g i n t e r v a l o f t h e g a s - b e a r i n g p a y f o r m a t i o n due t o i t s c l o se p o s i t i o n t o t h e g a s - w a t e r - c o n t a c t i n a d o w n s t r u c t u r a l s i t u a t i o n , o r i f c r e a t i o n o f t h e c r a c k c a n n o t be p r e v e n t e d , t h e d e n s i t y - c o n t r o l l e d f r a c t u r i n g f l u i d sequence aims on e x c l u s i o n o f p r o p p i n g o f t h e l o w e r s e c t i o n o f t h e c r a c k , t h e r e b y p r o v o k i n g i t s c l o s u r e and r e h e a l i n g a f t e r t e r m i n a t i o n o f t h e j o b and t h u s g u a r a n t e i n g t h a t gas e x p l o i t a t i o n f r o m t h e p r o p p e d u p p e r s e c t i o n o f t h e f r a c t u r e w i l l n o t be d i s t u r b e d b y w a t e r p r o d u c t i o n f r o m t h e l o w e r i n t e r v a l . Comments on v e r t i c a l f r a c t u r e p r o p a g a t i o n c o n t a i n m e n t by c o n t r o l l i n g s t i m u l a t i o n f l u i d d e n s i t y as w e l l as pumping r a t e s and p r e s s u r e a r e a l s o g i v e n b y KOZIK, BAILEY & HOLDITCH ( 1 9 7 9 ) .
4.2.2.3.4. Propped fracture height vs. reservoir thickness D i s t i n c t i o n has g e n e r a l l y t o be made between p r o p p e d f r a c t u r e h e i g h t and r e s e r v o i r thickness, w i t h v e r t i c a l propagation o f t h e crack o u t of c o n t r o l i n t h i c k f o r m a t i o n s and s m a l l - s i z e j o b s b e i n g much l e s s s e r i o u s t h a n t h a t o c c u r r i n g i n l a r g e - s i z e t r e a t m e n t s i n t h i n p a y zones ( L I & ZHU 1 9 8 6 ) . F r a c t u r e h e i g h t p r o p a g a t e s v e r t i c a l l y o u t o f i n t e r v a l when l a r g e r i n j e c t i o n r a t e s and h i g h e r - v i s c o s i t y f l u i d s a r e used. When p a y t h i c k n e s s does n o t e x c e e d 30 % o f t h e p r o p p e d f r a c t u r e h e i g h t , t h e s t i m u l a t i o n o p e r a t i o n i s n o t g o i n g t o have an a c t u a l response, and t h e r e f o r e u n d e r s t a n d i n g o f t h e d i s t r i b u t i o n o f i n - s i t u s t r e s s i s i m p o r t a n t f o r c o n t r o l l i n g f r a c t u r e h e i g h t and e n h a n c i n g s t i m u l a t i o n result. D i f f e r e n c e s i n l i t h o l o g i c a l f o r m a t i o n p r o p e r t i e s have d e c i s i v e i m p a c t o n d i f ferences i n f r a c t u r e pressure capacity ( n e t pressure i n the f r a c t u r e ) i n d i f f e r e n t r e s e r v o i r s w h i c h r e p r e s e n t d i f f e r e n c e s i n minimum i n - s i t u s t r e s s between b o t h t r e a t e d zone and b a r r i e r l a y e r . S e l e c t i o n o f t h e r i g h t p e r f o r a t i o n i n t e r v a l c a n advance t o t h e k e y e l e m e n t o f f r a c t u r e h e i g h t c o n t a i n m e n t u n d e r c e r t a i n fracture-pressure capacities according t o the d i s t r i b u t i o n o f i n - s i t u stress i n t h e zones.
4.2.2.3.5. Perf orat ion interval select ion P e r f o r a t i o n i n t e r v a l choice i s also important f o r l i m i t i n g v e r t i c a l f r a c t u r e g r o w t h , w i t h p e r f o r a t i n g o n l y t h e l o w e r p o r t i o n o f t h e s e c t i o n t o be t r e a t e d g u a r a n t e i n g upwards d e c l i n i n g h y d r a u l i c p r e s s u r e t r a n s m i s s i o n , because t h e f r a c t u r e s a r e g e n e r a l l y p r o p a g a t i n g upwards i n t o t h e d i r e c t i o n o f d e c r e a s i n g o v e r b u r d e n s t r e s s (LAMBERT, DOLAN & GALLUS 1983; PAI, GARBIS & HALL 1983; c f . s e c t i o n 4.2.2.5.3.1.). On t h e o t h e r hand, HUCKABEE ( 1 9 8 8 ) p r e s e n t s an e x a m p l e o f f r a c t u r e e x t e n s i o n p o t e n t i a l t o w a r d t h e b o t t o m o f t h e r e s e r v o i r complex, w i t h s t i m u l a t i o n operations consequently being i n i t i a t e d i n the upper p o r t i o n o f t h e
379 r e s e r v o i r . P r e f e r e n t i a l downward g r o w t h o f h y d r a u l i c f r a c t u r e s i s a l s o r e p o r t e d by WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE 1988).
4.2.2.4. Fracture containment by buoyant and settling diverters and other blocking additives A p a r t f r o m t r e a t m e n t parameter adjustment, v e r t i c a l f r a c t u r e containment can be achieved by t h e a p p l i c a t i o n of buoyant and s e t t l i n g d i v e r t e r s and o t h e r b l o c k i n g a d d i t i v e s i n o r d e r t o l i m i t o r s t o p v e r t i c a l h e i g h t growth o f t h e p r o p a g a t i n g f r a c t u r e . Some p o i n t s o f f l u i d v i s c o s i t y and d i v e r t e r d e n s i t y as w e l l as p r e s s u r e d i v e r s i o n a r e a l s o i l l u s t r a t e d as f o l l o w s .
4.2.2.4.1. General aspects When a f r a c t u r e i s i n i t i a t e d i n a p r o d u c t i v e i n t e r v a l , i t w i l l grow i n a v e r t i c a l r a d i a l p a t t e r n u n t i l i t encounters r e s i s t a n c e t o t h i s growth mode. Such boundaries t o p r o p a g a t i o n a r e n o r m a l l y b a r r i e r r o c k s o r s t r a t a t h a t c o n t a i n h i g h e r s t r e s s and n o r m a l l y a r e tougher and l e s s permeable than t h e p r o d u c t i v e r o c k ( c f . s e c t i o n s 4.2.2.5.3. and 4 . 2 . 3 . 2 . 2 . ) . I f such b a r r i e r s e x i s t above and below t h e p r o s p e c t i v e i n t e r v a l , v e r t i c a l growth o f t h e crack w i l l be r e s t r a i n e d a t these boundaries and t h e f r a c t u r e w i l l t h e n c o n t i n u e t o grow outward i n t o t h e hydrocarbon-bearing f o r m a t i o n , t h e r e b y changing i t s shape f r o m i d e a l l y c i r c u l a r t o r e a l l y e l l i p t i c a l . These c o n d i t i o n s p r o v i d e e f f i c i e n t use o f f r a c t u r i n g f l u i d i n c r e a t i n g a deeply p e n e t r a t i n g f r a c t u r e . I f t h e b a r r i e r r o c k s a r e t o o weak t o w i t h s t a n d t h e p r e s s u r e r e q u i r e d t o propagate t h e f r a c t u r e , t h e bound a r i e s w i l l r u p t u r e and v e r t i c a l growth w i l l c o n t i n u e i n downwards and/or upwards d i r e c t i o n , and s t i m u l a t i o n e f f i c i e n c y i s v e r y poor, because much o f t h e f r a c t u r e area i s o u t s i d e o f t h e e c o n o m i c a l l y a t t r a c t i v e i n t e r v a l and l a t e r a l pen e t r a t i o n w i t h i n t h e hydrocarbon-bearing s e c t i o n i s s e v e r e l y l i m i t e d due t o waste i n o v e r - and/or u n d e r l y i n g b a r r e n zones. I n a d d i t i o n t o b a r r e n r o c k s above and below, an o i l - b e a r i n g s e c t i o n f o r examp l e m i g h t c o n t a i n w a t e r i n t h e b o t t o m o f t h e zone and/or a gas cap a t i t s t o p . O i l p r o d u c t i o n c o u l d be c o n s i d e r a b l y d e t e r i o r a t e d i n case o f f r a c t u r e b r e a k t h r o u g h f r o m t h e o i l - b e a r i n g p a r t t o t h e w a t e r - and/or g a s - c o n t a i n i n g segments o f t h e sequence. As o f f t a k e i n c r e a s e s a r e d i r e c t l y r e l a t e d t o f r a c t u r e area i n o r c r a c k p e n e t r a t i o n o f t h e p o t e n t i a l i n t e r v a l , containment o f t h e f r a c t u r e i n t h e p r o d u c t i v e zone n o t o n l y i n c r e a s e s f r a c t u r i n g e f f i c i e n c y , b u t a l s o p r o v i d e s g r e a t e r e x p l o i t a t i o n i n c r e a s e s (DOWELL SCHLUMBERGER 1987). I n s t e a d o f r e d u c i n g pumping r a t e s and f l u i d v i s c o s i t y which a l s o l e a d t o c u t back o f t h e o v e r a l l s i z e o f t h e t r e a t m e n t and t h u s d i m i n u i s h f r a c t u r e p e n e t r a t i o n i n t h e r e s e r v o i r , c r a c k containment can be achieved by c r e a t i n g a r t i f i c i a l upper and l o w e r p r o p a g a t i o n b a r r i e r s by i n s e r t i n g l i g h t buoyant and heavy s e t t l i n g proppants a c t i n g as d i v e r t e r s and c o n t r o l l i n g upwards and downwards e x t e n s i o n o f t h e crack, r e s p e c t i v e l y . The whole system a c t s as an e f f e c t i v e synthet i c b r a c k e t o f t o p and bottom o f t h e i n t e r v a l t o be f r a c t u r e d ( b r a c k e t s t i m u l a t i o n ; DOWELL SCHLUMBERGER 1986, 1987).
4.2.2.4.2. Buoyant diverters F r a c t u r e h e i g h t containment towards t h e t o p o f t h e f o r m a t i o n can be achieved by c r e a t i n g an a r t i f i c i a l b a r r i e r w i t h a l i g h t s o l i d a d d i t i v e (NGUYEN & LARSON 1983). A n o n r e a c t i v e buoyant d i v e r t e r i s used t o c o n t r o l upward p r o p a g a t i o n o f t h e f r a c t u r e d u r i n g t h e t r e a t m e n t which o f f e r s an u n i q u e s o l u t i o n o f p r e v e n t i n g e x c e s s i v e c r a c k h e i g h t growth independent o f f o r m a t i o n p r o p e r t i e s and g e o l o g i c a l s t r e s s environment. The buoyant d i v e r t e r i s pumped as a s l u r r y b e f o r e t h e p r o p p a n t stages o f a f r a c t u r e t r e a t m e n t and accumulates i n t h e uppermost p o r t i o n o f t h e newly c r e a t e d crack, f o r m i n g a compact wedge-shaped l o w - p e r m e a b i l i -
380
ty f l o w block. S i n c e t h i s a r t i f i c i a l b a r r i e r does n o t a l l o w f l u i d p r e s s u r e t o r e a c h t h e upp e r b o u n d i n g l a y e r , t r e a t i n g p r e s s u r e c a n be i n c r e a s e d w i t h o u t t h e r i s k o f hav i n g upward f r a c t u r e p r o p a g a t i o n . The p a r t i c l e s o f t h e b u o y a n t d i v e r t e r m u s t b e s m a l l enough s o t h a t o n l y n e g l i g i b l e m e c h a n i c a l damage o c c u r s d u r i n g t h e pumpi n g operation, and m u s t be l a r g e enough so t h a t t h e d i v e r t e r w i l l n o t be c a r r i e d i n t o t h e p r o p p a n t bed b y p r o d u c t i o n f l u i d s and r e d u c e t h e a v a i l a b l e f r a c t u re c o n d u c t i v i t y by invasion o f the i n t e r p r o p p a n t p o r o s i t y ( c f . s e c t i o n 1.4.11.2.1.). The r i s i n g r a t e o f t h e d i v e r t e r d e c r e a s e s w i t h i n c r e a s i n g c a r r i e r f l u i d v i s c o s i t y . I n o r d e r t o be e f f e c t i v e , t h e b u o y a n t d i v e r t e r has t o b e i n t r o duced i n t h e p r e p a d s t a g e f o r r a p i d d e p o s i t i o n o f t h e m a j o r p o r t i o n o f t h e b l o c k i n g a d d i t i v e n e a r t h e w e l l b o r e , as f r a c t u r e h e i g h t i s g r e a t e s t i n t h e immed i a t e s u r r o u n d i n g s o f t h e b o r e h o l e , because t h e h y d r a u l i c p r e s s u r e i n t h e v e r y v i c i n i t y o f t h e w e l l b o r e i s g r e a t e r t h a n a t any o t h e r p o i n t a l o n g t h e l e n g t h o f the fracture. GABRIEL & ERBSTOESSER ( 1 9 8 4 ) i n t r o d u c e b u o y a n t b a l l s e a l e r s t h a t p r o v i d e i m proved diversion possibilities (ERBSTOESSER 1980; cf. also section 4.2.2.1.1.3.). L i g h t w e i g h t b u o y a n t p r o p p a n t s i n a p r e p a d m i g r a t e upwards t h e r e b y c r e a t i n g a h i g h - p r e s s u r e g r a d i e n t i n t h e f r a c t u r e w h i c h c o n s e q u e n t l y encour a g e s l a t e r a l f r a c t u r e p e n e t r a t i o n (McLENNAN, R O E G I E R S & MARX 1 9 8 3 ) . A s p e c t s o f f r a c t u r e p a c k i n g w i t h l i g h t w e i g h t a d d i t i v e s w h i c h f l o a t and c r e a t e a p e r m e a b i l i t y b a r r i e r and t h e r e b y r e d u c e upwards c r a c k m i g r a t i o n a r e a l s o d i s c u s s e d b y AHMED, STRAWN, WILSON & SCHATZ ( 1 9 8 3 ) .
4.2.2.4.3. Sett 1ins diverters downward g r o w t h o f t h e f r a c t u r e can be l i m i t e d b y heavy s o l i d adInversely, d i t i v e s w h i c h f a l l o u t of t h e f r a c t u r i n g f l u i d and p l u g t h e l o w e r t i p o f t h e crack wing, t h e r e b y r e t a r d i n g o r t e r m i n a t i n g i t s p r o p a g a t i o n . The c o m b i n a t i o n o f l i g h t b u o y a n t d i v e r t e r s a t t h e t o p and heavy s e t t l i n g d i v e r t e r s a t t h e b o t tom o f t h e f r a c t u r e a c t s as a d u a l b r a c k e t i n g d e v i c e w h i c h e f f e c t i v e l y c o n t r o l s v e r t i c a l h e i g h t e x t e n s i o n o f t h e f r a c t u r e . The s i m u l t a n e o u s a p p l i c a t i o n o f b o t h components i s p o s s i b l e by d e n s i t y - c o n t r o l f l u i d s ( c f . s e c t i o n 4 . 2 . 2 . 3 . 3 . 3 . ) . Lim i t i n g f r a c t u r e h e i g h t t o w i t h i n o n l y t h e p a y zone r e s u l t s i n d i v e r s i o n f r o m b a r r e n zones, b o t t o m w a t e r o r gas cap, t h e r e b y g i v i n g r i s e t o i m p r o v e d c r a c k e f f i c i e n c y , deeper p e n e t r a t i n g f r a c t u r e s w i t h i n t h e t a r g e t horizons, improved p r o p p a n t d i s t r i b u t i o n and t h u s c o n s i s t e n t l y b e t t e r s t i m u l a t i o n r e s u l t s (DOWELL SCHLUMBERGER 1986, 1 9 8 7 ) . The method w o r k s p a r t i c u l a r l y w e l l if b e i n g combined w i t h f l u i d - v i s c o s i t y c o n t r o l . A p a r t f r o m d i f f e r e n t t y p e s o f s y n t h e t i c heavy p r o p p a n t s and o t h e r mater i a l s , a l s o 100 mesh sand has been s u c c e s s f u l l y u s e d as s e t t l i n g d i v e r t e r f o r p l u g g i n g o f t h e l o w e r f r a c t u r e t i p i n o r d e r t o l i m i t downwards f r a c t u r e e x t e n s i o n (HODGES & PAOLI 1982; c f . s e c t i o n s 1 . 4 . 1 1 . 2 . 1 . and 4 . 8 . 8 . 3 . 2 . 3 . ) . Similarl y as b u o y a n t and s e t t l i n g d i v e r t e r and p r o p p i n g m a t e r i a l s i n t h e h y d r a u l i c f r a c t u r e , f l o a t i n g and s i n k i n g b a l l s e a l e r s c a n be a p p l i e d f o r s t i m u l a t i o n t r e a t m e n t d i v e r s i o n i n t h e b o r e h o l e (McLEOD 1984; c f . s e c t i o n 4 . 2 . 2 . 1 . 1 . 3 . ) .
4.2.2.4.4. Fluid viscosity and diverter density F i e l d a p p l i c a t i o n examples o f upwards and downwards l i m i t a t i o n o f f r a c t u r e h e i g h t g r o w t h b y l i g h t b u o y a n t and heavy s e t t l i n g p r o p p a n t s a r e r e p o r t e d b y NOLTE ( 1 9 8 8 b ) . B r i d g i n g o f t h e l i g h t b u o y a n t m a t e r i a l a t t h e e n t r y o f t h e muds t o n e b a r r i e r and p e r f o r m i n g a t i g h t f l o w b l o c k i s a c h i e v e d b y a m i x t u r e o f s i l i c a f l o u r as w e l l as 100 mesh, 20/40 mesh and 1 0 / 2 0 mesh sand, r e p r e s e n t i n g a c o m b i n a t i o n where v i r t u a l l y n o p e r m e a b i l i t y i s l e f t i f i t i s p r o p e r l y s e t i n p l a c e ( c f . s e c t i o n 1.4.11.2.1.4.). As s u b s e q u e n t f l u i d c a n n o t p e n e t r a t e t h e f l o w o b s t a c l e , s l u r r y d e h y d r a t i o n caused b y h e i g h t g r o w t h i s e l i m i n a t e d , and
381 v e r t i c a l f r a c t u r e e x t e n s i o n i s a l s o a r r e s t e d because t h e p l u g does n o t a l l o w t h e a c t i v e f l u i d p r e s s u r e i n t h e main p o r t i o n o f t h e c r a c k t o r e a c h t h e h e i g h t extremities. Improvement o f t h e process i s achieved by low f l u i d l o s s and low v i s c o s i t y of t h e pad i n o r d e r t o m i n i m i z e v e r t i c a l f r a c t u r e p r o p a g a t i o n a l r e a d y b e f o r e t h e buoyant m a t e r i a l f o r m i n g t h e f l o w b l o c k i s p l a c e d . The subsequent p l u g - c a r r y i n g stage s h o u l d have r e l a t i v e l y h i g h v i s c o s i t y w i t h an i n c r e a s e d i n j e c t i o n r a t e t o enhance c r a c k h e i g h t growth a g a i n f o r t h e purpose o f a m e l i o r a t i n g v e r t i c a l f l o w and f a c i l i t a t i n g placement and b r i d g i n g o f t h e l i g h t f l o w b l o c k mater i a l i n t h e e n t r a n c e o f t h e bounding mudstones. The f o l l o w i n g p r o p p a n t - t r a n s p o r t i n g f l u i d s s h o u l d have minimum v i s c o s i t y needed t o move t h e p a r t i c l e s and should be i n s e r t e d a t l o w e r pumping r a t e s i n o r d e r t o reduce subsequent pressur e s , w i t h p r e s s u r e government b e i n g r e q u i r e d t o c o n t r o l f r a c t u r e w i d t h i n t h e flow b a r r i e r region w i t h o u t unseating the plug. The i n d i c a t e d changes i n f l u i d v i s c o s i t y do n o t o n l y m i n i m i z e u l t i m a t e f r a c t u r e h e i g h t , b u t a l s o enhance f l u i d displacement down and a l o n g t h e c r a c k through m o b i l i t y analogy. The h i g h e r v i s c o s i t y o f t h e p l u g - c a r r y i n g stage w i t h r e s p e c t t o t h e pad p r o v i d e s a f a v o u r a b l e m o b i l i t y c o n d i t i o n f o r d i s p l a c i n g t h e pad down t h e f r a c t u r e w i t h o u t b r e a k t h r o u g h o f f l o w - b l o c k i n g m a t e r i a l . The lower v i s c o s i t y o f t h e p r o p p a n t - t r a n s p o r t i n g f l u i d r e l a t i v e t o t h e p l u g stage g i v e s an u n f a v o u r a b l e m o b i l i t y c o n d i t i o n f o r d i s p l a c i n g t h e buoyant d i v e r t e r down t h e c r a c k and enhances p r o p p a n t b r e a k t h r o u g h which maximizes t h e amount o f f l o w b l o c k m a t e r i a l b e i n g d i s p l a c e d v e r t i c a l l y i n t o t h e s h a l e s and m i n i m i z e s t h e q u a n t i t y o f i t moved towards t h e l a t e r a l f r a c t u r e t i p . SETTARI (1985) a l s o conc l u d e s t h a t a proppant bank a t t h e f r a c t u r e bottom o r buoyant proppants concent r a t e d a t t h e f r a c t u r e t o p can p r o v i d e e f f e c t i v e b a r r i e r s t o v e r t i c a l c r a c k p r o pagation.
4.2.2.4.5. Other blocking additives A p a r t f r o m t h e c o n c e p t o f buoyant d i v e r t e r s (NGUYEN & LARSON 1983), v a r i o u s o t h e r a d d i t i v e s and methods have been proposed i n o r d e r t o l i m i t v e r t i c a l f r a c t u r e e x t e n s i o n and t o keep t h e c r a c k w i t h i n t h e zone o f i n t e r e s t . HANSON & LYONS (1964) p r e s e n t c r e a t i o n o f a semipermeable proppant s l u g i n o r d e r t o c l o se o f f t h e p a t h s o f l e a s t r e s i s t a n c e i n t h e f r a c t u r e by a l t e r n a t i n g i n c r e a s i n g and d e c r e a s i n g i n j e c t i o n r a t e a n d / o r proppant c o n c e n t r a t i o n r e p e a t e d l y . The b l o c k i n g concept o f HANSON (1964) i s based on i n c l u d i n g p l a s t i c a l l y deformable s o l i d s i n t o t h e f r a c t u r i n g f l u i d which have d e n s i t i e s equal t o o r d i f f e r e n t f r o m those o f t h e c a r r i e r . PRATER (1968) suggests simultaneous a c t i o n o f two o r more f l u i d s o f d i f f e r e n t d e n s i t i e s along w i t h a p r o p p a n t t h a t matches t h e d e n s i t y o f one f l u i d , w i t h d e n s i t y c h o i c e depending on whether downward o r upward f r a c t u r e p r o p a g a t i o n i s t o be c o n t r o l l e d . Downward c r a c k growth can a l s o be r e s t r i c t e d by p r o p e r l y p l a c i n g a heavy proppant i n t o t h e b o t t o m o f a newly c r e a t e d f r a c t u r e which l e a d s t o subsequent d i v e r s i o n o f t h e f l o w o f t h e f r a c t u r i n g f l u i d upward and l a t e r a l l y (BRAUNLICH 1967). 100 mesh sand can a l s o be s u c c e s s f u l l y used as b r i d g i n g agent t o c o n t r o l t h e downward c r a c k p r o p a g a t i o n (HOOGES & PAOLI 1982; c f . s e c t i o n s 1 . 4 . 1 1 . 2 . 1 . and 4 . 8 . 8 . 3 . 2 . 3 . ) . NOLTE (1982) r e p o r t s achievement of f r a c t u r e h e i g h t c o n t r o l by pumping a m i x t u r e o f v a r i o u s sand g r a i n s i z e s i n a h i g h - v i s c o s i t y f l u i d immed i a t e l y a f t e r t h e prepad b e f o r e t h e p r o p p a n t - l a d e n s l u r r y i s i n t r o d u c e d i n t o t h e crack, w i t h t h e sand m i x t u r e b e i n g designed t o b r i d g e i n t h e v e r t i c a l t i p s o f t h e f r a c t u r e t o f o r m a f l o w b l o c k and d i s c o u r a g e t h e c r a c k f r o m growing f u r t h e r v e r t i c a l l y . Aspects o f a r t i f i c i a l b a r r i e r s f o r f r a c t u r e h e i g h t g r o w t h l i m i t a t i o n a r e a l s o d i s c u s s e d by LI & ZHU (1986). DOERLER & PROUVOST (1987) comment on t h e performance o f d i v e r t i n g agents and m o d e l l i n g o f r e s u l t a n t zone i n j e c t i vity.
382
4.2.2.4.6. Pressure diversion Another p o s s i b i l i t y o f f r a c t u r i n g t r e a t m e n t d i v e r s i o n comprises e x p l o i t a t i o n o f t h e p r e s s u r e growth phenomenon which i n c l u d e s a b n o r m a l l y l a r g e i n c r e a s e s i n o p e r a t i n g p r e s s u r e and i s a f u n c t i o n o f pumping r a t e and f l u i d v i s c o s i t y (MEDLIN & FITCH 1983). T h i n s t i m u l a t i o n f l u i d s pumped a t h i g h r a t e s c r e a t e smooth p r e s s u r e curves t h a t r i s e p a r a b o l i c a l l y w i t h t i m e t o f i r s t o r d e r , whereas t h i c k g e l s pumped a t low r a t e s r e s u l t i n e r r a t i c p r e s s u r e curves which t e n d t o r i s e more o r l e s s l i n e a r l y w i t h t i m e and show f r e q u e n t l y sharp i r r e g u l a r i t i e s w i t h a b r u p t i n c r e a s e s and d e c l i n e s . The p r e s s u r e i n c r e a s e l e a d i n g i n most cases t o s c r e e n o u t ( c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) can be used as a method o f f l u i d d i v e r s i o n , because t h e p r e s s u r e d i v e r s i o n scheme d i r e c t s t h e l a r g e s t f r a c t i o n s o f t h e t r e a t m e n t t o t h e b e t t e r r e s e r v o i r zones. The p r e s s u r e d i v e r s i o n t e c h n i q u e p r o v i d e s a means o f f r a c t u r i n g v e r y l o n g int e r v a l s c o n t a i n i n g many p r o d u c t i v e zones i n a s i n g l e t r e a t m e n t . A f t e r p e r f o r a t i o n and breakdown o f each zone by b a l l s e a l e r s ( c f . s e c t i o n 4 . 2 . 2 . 1 . 1 . ) , fluid i s d i v e r t e d f r o m h o r i z o n t o h o r i z o n d u r i n g t h e main s t i m u l a t i o n o p e r a t i o n i n accordance w i t h t h e n a t u r a l i n c r e a s e s i n t r e a t i n g p r e s s u r e . I n i t i a l l y , c r a c k p r o p a g a t i o n b e g i n s i n t h e zone w i t h t h e l o w e s t f r a c t u r e g r a d i e n t . A f t e r some i n c r e ment o f p r e s s u r e i n c r e a s e i n t h i s l e v e l , f l u i d d i v e r s i o n b e g i n s t o t h e i n t e r v a l o f n e x t h i g h e s t f r a c t u r e g r a d i e n t , w i t h t h i s d i v e r t i n g process c o n t i n u i n g u n t i l a l l s t o r e y s r e c e i v e f l u i d i n p r o p o r t i o n t o t h e i r f r a c t u r e g r a d i e n t s . Under i d e a l c o n d i t i o n s , t h e j o b c o u l d be c o n t i n u e d u n t i l f i n a l s c r e e n o u t s t a r t s a t maximum o p e r a t i o n a l p r e s s u r e and each zone c o n t a i n s t h e l a r g e s t p r a c t i c a l f r a c t u r e . Proppant t r a n s p o r t problems, however, m i g h t p r e v e n t t h i s i d e a l c o n d i t i o n f r o m b e i n g achieved. Proppant f i l l u p i n t h e w e l l b o r e has t o be a v o i d e d by u s i n g a crosslinked gel w i t h near-perfect t r a n s p o r t properties (cf. section 4.3.4.3.). Another p o t e n t i a l problem i n proppant b r i d g i n g o f f r a c t u r e s b e f o r e t h e i r w i d t h becomes g r e a t enough t o a l l o w p r o p p a n t movement.
4.2.2.5. Multiple-zone fracture migration AHMED, NEWBERRY & CANNON (1985) i n t r o d u c e t h e f r a c t u r e m i g r a t i o n c o n c e p t as a technique f o r o p t i m i z a t i o n o f multiple-zone v e r t i c a l hydraulic s t i m u l a t i o n t r e a t m e n t s . A f t e r o u t l i n i n g o f some general aspects and commenting on c o n t i nuous survey and m o d e l l i n g , examples o f a p p l i c a t i o n i n r e s e r v o i r complexes w i t h v e r t i c a l f r a c t u r a b i l i t y differences are i l l u s t r a t e d .
4.2.2.5.1. General aspects Conventional methods o f d e s i g n i n g s t i m u l a t i o n o f w e l l s w i t h m u l t i p l e pay zo nes i n c l u d e l i m i t e d e n t r y , s t a g i n g ( w i t h t h e use o f d i v e r t i n g agents and b a l s e a l e r s ) and p a c k i n g - o f f s t o r e y s ( i n c l u d i n g b r i d g e p l u g s and sand plugbacks and a r e based on t h e assumption t h a t a l l h o r i z o n s open up and t h e f r a c t u r e s a1 propagate i n a s i m i l a r f a s h i o n (AHMED, SCHATZ, HOLLAND, JONES & GREENFIELD 1982; c f . s e c t i o n 4 . 2 . 2 . 1 . ) . A l l t h e p r e v i o u s m u l t i p l e - z o n e t r e a t m e n t t e c h n i ques, however, s u f f e r f r o m l a c k o f c a p a b i l i t y o f q u a n t i t a t i v e l y p r e d i c t i n g t h e c r e a t e d f r a c t u r e c h a r a c t e r i s t i c s i n each i n t e r v a l , m a i n l y as a consequence o f m i s s i n g t e c h n o l o g y f o r p r e d i c t i o n o f f r a c t u r e m i g r a t i o n (CLIFTON & ABOU-SAYED 1981; CLEARY, KAVVADAS & LAM 1983; AHMED, THOMPSON, KELKAR, VEGHTE & HATHAWAY 1984), and t h e u n a v a i l a b i l i t y o f r e l i a b l e and c o n t i n u o u s f r a c t u r e g r a d i e n t p r e s sure and mechanical p r o p e r t i e s d a t a has h i t h e r t o h i n d e r e d t h e commercial development o f a f r a c t u r e m i g r a t i o n p r e d i c t i v e t o o l ( c f . a l s o s e c t i o n 6 . 2 . 5 . ) .
4.2.2.5.2. Continuous survey and modelling The mu1 t i p l e - z o n e f r a c t u r e m i g r a t i o n model (AHMED, NEWBERRY & CANNON 1985) p r o v i d e s a c o n t i n u o u s survey a l o n g t h e p r o s p e c t i v e column. P r o p a g a t i o n o f t h e
383 c r a c k commences w i t h t h e i n t e r v a l s a v e r a g i n g t h e l e a s t f r a c t u r e g r a d i e n t pressur e , and o t h e r h o r i z o n s p r e f e r e n t i a l l y open up and propagate as t h e w e l l b o r e pumping p r e s s u r e i n c r e a s e s . P o s t s t i m u l a t i o n p r o d u c t i o n h i s t o r y surveys o f t e n r e veal t h a t s m a l l e r f r a c t u r e l e n g t h s a r e t y p i c a l l y a s s o c i a t e d w i t h pay zones e x h i b i t i n g t h e h i g h e s t f r a c t u r e g r a d i e n t pressure, l e a d i n g t o i n e f f i c i e n t deplet i o n , whereas l o w - s t r e s s e d i n t e r v a l s g i v e r i s e t o h i g h c r a c k c o n d u c t i v i t y which i s o f t e n more t h a n i s r e q u i r e d t o d r a i n t h e s u r r o u n d i n g r e s e r v o i r , and t r e a t ments i n i t i a t e d i n s t r u c t u r a l l y deep l o w - s t r e s s e d s t o r e y s may e a s i l y m i g r a t e i n t o a nearby a q u i f e r . H y d r a u l i c f r a c t u r e p r o p a g a t i o n f r o m one o r s e v e r a l p o i n t s depends on comparison o f f r a c t u r e g r a d i e n t p r e s s u r e and r o c k mechanical p r o p e r t i e s between source and s u r r o u n d i n g p o i n t s . As t h e c r a c k grows, t h e hydros t a t i c f l u i d g r a d i e n t begins t o a f f e c t t h e p r e s s u r e a t e v e r y p o i n t on t h e f r a c t u r e t i p which i n t u r n a f f e c t s f r a c t u r e p r o p a g a t i o n .
I f p r e s s u r e p o t e n t i a l w i t h i n t h e c r a c k t i p s and s t r e s s p o t e n t i a l w i t h i n t h e s u r r o u n d i n g r o c k can be d e f i n e d , p r e d i c t i o n of f r a c t u r e p r o p a g a t i o n w i t h i n c r e a s i n g t r e a t m e n t volume and p r e s s u r e w i t h i n t h e crack i s p o s s i b l e . F r a c t u r e propag a t i o n a t v a r i o u s m u l t i p l e s t o r e y s can t h e r e f o r e be c o n t r o l l e d by maneuvering t h e p r e s s u r e a t i n t e r v a l face, by s e l e c t i v e l y p o s i t i o n i n g t h e p e r f o r a t i o n s , and by u s i n g necessary s t a g i n g and p a c k o f f r e q u i r e m e n t s . The m u l t i p l e - z o n e f r a c t u r i n g p r o g n o s i s t e c h n i q u e (AHMED, NEWBERRY & CANNON 1985) combines s o n i c wavef o r m a n a l y s i s w i t h b u l k volume e v a l u a t i o n and l i n e a r f r a c t u r e mechanics w i t h t r e a t m e n t f l u i d d e n s i t y i n o r d e r t o y i e l d an a c c u r a t e f r a c t u r e g r a d i e n t p r e s s u r e d i s t r i b u t i o n i n and around t h e hydrocarbon-bearing i n t e r v a l s and t o p r o duce an e x a c t p r e d i c t i o n o f crack m i g r a t i o n b e h a v i o u r i n r e l a t i o n t o bottomhole treatment pressure ( c f . also section 6.2.5.). Fracture height migrates a t the w e l l b o r e w i t h i n c r e a s e s i n t r e a t m e n t p r e s s u r e above t h e average f r a c t u r e g r a d i e n t p r e s s u r e o f t h e p e r f o r a t e d zone. F r a c t u r e p r e s s u r e g r a d i e n t i s a f u n c t i o n p r i m a r i l y o f overburden s t r e s s g r a d i e n t , pore p r e s s u r e g r a d i e n t , and r a t i o o f h o r i z o n t a l t o v e r t i c a l s t r e s s (EATON 1969).
4.2.2.5.3. Vertical f racturabi 1 ity differences I f d i f f e r e n c e s i n f r a c t u r a b i l i t y e x i s t between s e v e r a l zones, such as f o r example i f t h e l o w e r and m i d d l e s e c t i o n s open up s i m u l t a n e o u s l y whereas t h e upp e r l e v e l breaks down a t a h i g h e r p r e s s u r e and t h u s l a g s behind, and t h e upper l a y e r has upwards f r a c t u r e m i g r a t i o n tendency, t h e m i d d l e i n t e r v a l has s t r o n g b a r r i e r s a t b o t h s i d e s , and t h e l o w e r h o r i z o n e x h i b i t s downward f r a c t u r e m i g r a t i o n t r e n d , adjustments o f p e r f o r a t i o n arrangement have t o be c a r r i e d o u t i f a l l t h e t h r e e zones a r e t o be s t i m u l a t e d s i m u l t a n e o u s l y . I n c r e a s i n g p e r f o r a t i o n s i z e and d e n s i t y i n t h e upper l e v e l has t h e consequence t h a t i t undergoes one l e s s n e t p r e s s u r e step, and p l a c i n g p e r f o r a t i o n s on t h e b o t t o m p a r t o f t h e t o p l a y e r and on t h e upper p a r t o f t h e base h o r i z o n i n h i b i t s f r a c t u r e g r o w t h away from the hydrocarbon-bearing i n t e r v a l . i f s u f f i c i e n t p r e s s u r e d i f f e r e n c e s cannot be c r e a t e d by p l a c i n g fewer p e r f o r a t i o n s i n t h e t o p h o r i z o n and i n c r e m e n t a l p e r f o r a t i o n s i n t h e b o t t o m s e c t i o n , then each p o r t i o n s h o u l d be t r e a t e d s e p a r a t e l y i n o r d e r t o c r e a t e s u f f i c i e n t l y l o n g f r a c t u r e s i n b o t h zones. Excessive f r a c t u r e p r e s s u r e s have t o be avoided, because t h i s may l e a d t o f l u i d l o s s i n t o boundary l i t h o l o g i e s between t h e pay sand i n t e r v a l s which can t r i g g e r premature screenout f a i l u r e o f t h e j o b ( c f . section 6.2.4.2.1.). Some comments on f r a c t u r e o r i e n t a t i o n and movement as w e l l as i n - s i t u s t r e s s d i f f e r e n c e s w i t h f a c i e s t y p e a r e o f f e r e d as f o l l o w s .
4.2.2.5.3.1. Fracture orientation and movement The m u l t i p l e - z o n e f r a c t u r e m i g r a t i o n concept ( c f . s e c t i o n 6 . 2 . 5 . ) o n l y app l i e s t o v e r t i c a l h y d r a u l i c s t i m u l a t i o n . H o r i z o n t a l h y d r a u l i c f r a c t u r i n g cannot be modelled, b u t a b s o l u t e values o f f r a c t u r e g r a d i e n t p r e s s u r e s p e r m i t t o d i s c e r n i f a pay zone i s amenable t o c r e a t i o n o f h o r i z o n t a l f r a c t u r e s . The t r a n s -
384 v e r s e l y e l a s t i c a l model used t o c a l c u l a t e i n - s i t u s t r e s s f r o m e l a s t i c a l p r o p e r t i e s does n o t i n c l u d e any l o c a l t e c t o n i c a l e f f e c t s . A l t h o u g h most t e c t o n i c a l e f f e c t s between s e v e r a l i n t e r v a l s cancel o u t d u r i n g computation, s c a l i n g by d i r e c t measurement t h r o u g h m i n i f r a c t u r i n g t r e a t m e n t s i s recommended i n o r d e r t o improve t h e m u l t i p l e - z o n e c r a c k m i g r a t i o n m o d e l l i n g . H y d r a u l i c f r a c t u r e s u s u a l l y move upwards due t o i n c r e a s i n g overburden s t r e s ses w i t h depth, a l t h o u g h i n many i n s t a n c e s , t h e c r a c k s have shown a preponderance o f downwards p r o p a g a t i o n ( c f . a l s o s e c t i o n 4.2.2.3.5.). F r a c t u r e h e i g h t i s g e n e r a l l y r e l a t e d t o competence, t h i c k n e s s and hardness o f t h e b a r r i e r and i s a l s o i n f l u e n c e d by t h e cement bond (PAI, G A R B I S & HALL 1983). B L I N T E N & A Z I Z (1985) comment on s t i m u l a t i o n o f v e r y l o n g gross i n t e r v a l s .
4.2.2.5.3.2.
I n - s i t u s t r e s s d i f f e r e n c e s w i t h f a c i e s type
The reason f o r v e r t i c a l f r a c t u r a b i l i t y d i f f e r e n c e s w i t h some r e s e r v o i r zones opening up e a r l i e r than o t h e r s and r e s t r i c t i o n s o f c o n n e c t i v i t y o f superimposed pay s e c t i o n s through i n t e r v e n i n g b a r r i e r h o r i z o n s a r e d i f f e r e n c e s o f i n - s i t u s t r e s s e s w i t h d e p o s i t i o n a l environment and l i t h o l o g y (NORTHROP 1988; c f . sect i o n 4.2.2.3.1.3. and 4.2.3.2.2.). Large c o n t r a s t s o f i n - s i t u s t r e s s magnitude between r e s e r v o i r and b a r r e n r o c k s i m p l y a s t r o n g tendency f o r f r a c t u r e c o n t a i n ment and v e r y h i g h p r e s s u r e s a r e r e q u i r e d t o break t h r o u g h n o n - r e s e r v o i r l i t h o l o g i e s . W h i l e t h e h o r i z o n t a l s t r e s s e s i n mudstones a r e e s s e n t i a l l y i s o t r o p i c and on average s l i g h t l y below t h e overburden value, h o r i z o n t a l s t r e s s e s i n sandstones and s i l t s t o n e s a r e a n i s o t r o p i c and have a f a i r l y c o n s i s t e n t o r i e n t a t i o n r e f l e c t i n g a c l o c k w i s e r o t a t i o n o f t h e maximum h o r i z o n t a l s t r e s s d i r e c t i o n w i t h depth as a consequence o f s t r e s s e s a s s o c i a t e d w i t h t h e l a r g e t o p o g r a p h i c r e l i e f superimposed on t h e r e g i o n a l s t r e s s f i e l d o f t h e b a s i n (CLARK 1983; c f . a l s o s e c t i o n 4.2.3.2.2.1.). As a r e s u l t o f d i f f e r e n t i n - s i t u s t r e s s e s i n d i f f e r e n t l i t h o l o g i e s , hydraul i c f r a c t u r e s do n o t e a s i l y break o u t o f a l o w - s t r e s s sandstone, t r a v e r s e t h e h i g h - s t r e s s c o n f i n i n g r o c k s and i n t e r s e c t a n o t h e r sandstone (NORTHROP 1988). T h e r e f o r e i n t e r s e c t i o n , p r o p p i n g and d r a i n a g e o f a l a r g e number o f remote and i s o l a t e d r e s e r v o i r u n i t s such as sandstone lenses and r i b b o n s which a r e n o t conn e c t e d t o t h e w e l l b o r e a r e n o t f e a s i b l e i n t h e presence o f l a r g e s t r e s s cont r a s t s , b u t w i l l o n l y pay o f f i n case o f s m a l l s t r e s s c o n t r a s t s . G R I (1988) r e p o r t s dense carbonates a l t e r n a t i n g w i t h t i g h t gas sandstones, w i t h t h e carbonat e s b e i n g e f f i c i e n t b a r r i e r s t o v e r t i c a l f r a c t u r e growth due t o t h e i r h i g h e r i n - s i t u stress.
4 . 2 . 2 . 6 . S e l e c t i v e proppan t p lacernent BARBER & THEMIG (1985) p r e s e n t v a r i o u s p o s s i b i l i t i e s o f o p t i m i z a t i o n o f t h e r e s u l t s o f f r a c t u r i n g t r e a t m e n t s by e f f e c t i v e l y l i m i t i n g downward f r a c t u r e growth i n t o t h e w a t e r - b e a r i n g s e c t i o n o f t h e r e s e r v o i r . Many d e p o s i t i o n a l sand bodies a r e c h a r a c t e r i z e d by f i n i n g - u p w a r d s g r a i n s i z e d i s t r i b u t i o n w i t h i n t h e environmental cyclothems ( c f . p l a t e s III/l - 2, IV/5, V/1 - 4, VII/3 and I X / 1 - 4 ) which r e f l e c t d e c r e a s i n g energy d u r i n g course o f a g g r a d a t i o n o f t h e m i l i e u f e a t u r e s ( s u c h as f o r example f l u v i a l channel and overbank f a c i e s sequences) t h a t g i v e s r i s e t o d e c r e a s i n g p e r m e a b i l i t y f r o m b o t t o m t o t o p o f t h e s e d i mentary package. D i a g e n e t i c a l cementation and m i n e r a l a u t h i g e n e s i s o f t e n p r e f e r s t h e f i n e r - g r a i n e d upper p o r t i o n s o f t h e pay which c o n s e q u e n t l y become q u i t e t i g h t i n c o n t r a s t t o the coarser-grained lower p a r t s o f the r e s e r v o i r t h a t a r e l e f t w i t h b e t t e r p e r m e a b i l i t i e s due t o weaker p o s t d e p o s i t i o n a l l i t h i f i c a t i o n . Subsequent f l u i d s t r a t i f i c a t i o n a f t e r hydrocarbon i m m i g r a t i o n f r e q u e n t l y r e s u l t s i n an o i l - w a t e r - c o n t a c t c r o s s i n g t h e d e p o s i t i o n a l sand body and separ a t i n g o i l i n t h e upper l o w - p e r m e a b i l i t y s e c t i o n f r o m w a t e r i n t h e l o w e r h i g h p e r m e a b i l i t y s t o r e y o f t h e p r o s p e c t i v e column.
385
I n case o f l a c k o f competent i n t r a f o r m a t i o n a l b a r r i e r s t o p r e v e n t downward growth o f h y d r a u l i c f r a c t u r e s o u t o f t h e upper t i g h t o i l - p r o d u c i n g i n t e r v a l i n t o the lower permeable water-producing section, attempts have t o be made t o des i g n the s t i m u l a t i o n treatment f o r m i n i m i z a t i o n o f downwards crack propagation i n o r d e r t o maximize e f f e c t i v e s t i m u l a t i o n o f the upper t i g h t r e s e r v o i r zone. This can be performed by l i m i t i n g pumping r a t e s and c o n t r o l l i n g j o b s i z e , and by s e l e c t i v e proppant placement by d e n s i t y - c o n t r o l foam f r a c t u r i n g . I n an unbounded pay, h i g h pumping r a t e s w i l l g e n e r a l l y promote f r a c t u r e h e i g h t growth, because the f r i c t i o n pressure c r e a t e d w i t h i n the f r a c t u r e from t h e w e l l b o r e t o the f r a c t u r e t i p w i l l r i s e w i t h i n c r e a s i n g r a t e (EEKELEN 1982). Therefore l o w - r a t e s t i m u l a t i o n operations o f r e s t r i c t e d s i z e are a s i g n i f i c a n t means o f l i m i t i n g v e r t i c a l f r a c t u r e extension, provided the pumping r a t e s are n o t t o o low, because t h i s would provoke premature screenout t e r m i n a t i o n o f the j o b ( c f . s e c t i o n s 4.2.2.2.3., 4.2.3.2.5. and 6.2.4.2.1.). The use o f lowv i s c o s i t y f l u i d s i s a l s o e f f e c t i v e i n r e s t r i c t i n g v e r t i c a l f r a c t u r e propagat i o n , b u t a t the same time the proppant t r a n s p o r t c a p a b i l i t i e s are severely a f f e c t e d ( c f . s e c t i o n 4.2.2.3.3.). Aspects o f a c c i d e n t a l proppant f a l l o u t and banking, c o n t r o l l e d placement by f l u i d d e n s i t y r e g u l a t i o n , proppant s t r a t i f i c a t i o n and zonation, and f l u i d d e n s i t y c o n t r o l and proppant d i s t r i b u t i o n are d i s cussed as f o l l o w s .
4.2.2.6.1. Accidental proppant fallout and banking I n s u f f i c i e n t c a r r i e r a b i l i t y o f l o w - v i s c o s i t y f l u i d s may r e s u l t i n proppant f a l l o u t d u r i n g pumping and b e f o r e c l o s u r e takes place, g i v i n g r i s e t o proppant banking i n the lower p a r t o f the f r a c t u r e d segment o f the r e s e r v o i r (BARBER & THEMIG 1985). While a c c i d e n t a l proppant banking a t the bottom o f the crack as a consequence o f proppant s e t t l i n g i n the s t i m u l a t i o n f l u i d p r i o r t o c l o s u r e o f the f r a c t u r e i s one type o f s e l e c t i v e proppant placement and may be d e s i r a b l e i n coarsening-upwards sedimentary sand bodies t h a t are f u l l y o i l - b e a r i n g i n o r der t o prop e f f e c t i v e l y the lower p o r t i o n o f the h y d r a u l i c f r a c t u r e i n t e r s e c t i n g the t i g h t lower i n t e r v a l o f the r e s e r v o i r i n case o f absence o f an o i l - w a t e r - c o n t a c t w i t h i n the pay zone, i t i s d e t r i m e n t a l t o the r e s u l t o f s t i m u l a t i o n treatments i n fining-upwards sequences i f proppant banking p r o v i d e s p r e f e r e n t i a l support o f the f r a c t u r e w a l l s i n t h e lower water-bearing s e c t i o n o f r e s e r voirs with f l u i d stratification. Therefore the b e n e f i t o f proppant banking depends on sedimentological compos i t i o n and/or d i a g e n e t i c a l h i s t o r y o f the p r o s p e c t i v e column, and the d e p o s i t i o n a l environment g i v i n g r i s e t o the s p e c i f i c t r e n d o f g r a i n - s i z e d i s t r i b u t i o n dec i d e s together w i t h f l u i d immigration and displacement h i s t o r y whether proppant banking i n h y d r a u l i c f r a c t u r e s w i l l be favourable o r d e t r i m e n t a l f o r the r e s u l t o f the s t i m u l a t i o n j o b and the enhancement o f o i l p r o d u c t i o n (aspects o f proppant banking are a l s o discussed i n s e c t i o n s 4.3.3.2. and 4.12.2.2.).
4.2.2.6.2. Control led proppant placement by
fluid density regulation
F l u i d d e n s i t y c o n t r o l g i v i n g r i s e t o s e l e c t i v e proppant placement i s a v i a b l e a l t e r n a t i v e t o more conventional types o f h y d r a u l i c s t i m u l a t i o n i f f r a c t u r e h e i g h t propagation cannot be s u f f i c i e n t l y c o n t r o l l e d by pumping r a t e , j o b s i z e and f l u i d v i s c o s i t y (BARBER & THEMIG 1985). F l u i d s w i t h c e r t a i n d e n s i t y d i f f e r e n c e s w i t h r e s p e c t t o s p e c i f i c g r a v i t y tend t o o v e r r i d e o r u n d e r r i d e a preceding f l u i d i n the f r a c t u r e depending on whether i t i s l i g h t e r o r heavier, r e s p e c t i v e l y (FREDRICKSON & BRDADDUS 1976). F l u i d v i s c o s i t i e s a l s o have an e f f e c t i n c o n t r o l l i n g d e n s i t y separation, w i t h the h i g h e r the d e n s i t y and v i s c o s i t y d i f f e r e n c e s between both f l u i d s , the more pronounced the d e n s i t y separat i o n e f f e c t ( c f . s e c t i o n 4.2.2.3.3. and 4.2.2.6.4.).
386 Proper v i s c o s i t y modelling o f both successive f l u i d s i s a c r i t i c a l p a r t o f t h e o p e r a t i o n (BARBER & THEMIG 1 9 8 5 ) . I f t h e v i s c o s i t y o f t h e p r e f l u s h i s t o o low, i t w i l l be swept away f r o m t h e w e l l b o r e a r e a and q u i t e p o s s i b l y r e m a i n ahead o f t h e f r a c t u r i n g f l u i d . The p r e f l u s h m u s t have enough v i s c o s i t y t o p r o v i d e some r e s i s t a n c e t o movement w i t h i n t h e c r a c k s t o a l l o w t h e f r a c t u r i n g f l u i d t o o v e r r i d e the p r e f l u s h i n o r d e r t o achieve the d e s i r e d d e n s i t y e f f e c t . I f t h e p r e f l u s h i s t o o v i s c o u s , however, i t becomes i m m o b i l e t o some e x t e n t , and i n s t e a d o f o v e r r i d i n g , v i s c o u s f i n g e r i n g ( c f . s e c t i o n 4 . 5 . 4 . 2 . 1 . ) takes p l a c e i n w h i c h t h e f r a c t u r i n g f l u i d moves t h r o u g h t h e p r e f l u s h i n s e v e r a l p a t h s o r channels i n s t e a d o f t h e d e s i r e d s i n g l e continuous f l u i d f r o n t .
4.2.2.6.3.
Proppant st rat if icat ion and zonation
F l u i d d e n s i t y c o n t r o l a l l o w s s e l e c t i v e placement o f t h e proppants i n t h e upp e r o r lower p o r t i o n o f t h e f r a c t u r e i n t h e cases o f o v e r r i d i n g o r u n d e r r i d i n g r e g a r d l e s s o f where t h e l o w e r o r u p p e r bounds o f h e i g h t g r o w t h may be, r e s p e c t i v e l y ( c f . s e c t i o n 4.2.2.3.3. and 4 . 2 . 2 . 6 . 4 . ) . The u s e o f foam i n t h i s p r o c e s s p r o v i d e s e x c e l l e n t p r o p p a n t t r a n s p o r t and t h u s e l i m i n a t e s m o s t o f t h e c o n c e r n s a b o u t p r o p p a n t s e t t l i n g t o p r e m a t u r e u n c o n t r o l l e d banks d u r i n g pumping and bef o r e f r a c t u r e c l o s u r e , The u s e o f n i t r o g e n i n foam g i v e s t h e d e n s i t y d i f f e r e n c e w i t h p o t a s s i u m c h l o r i d e w a t e r and w o u l d e l i m i n a t e t h e need f o r u s i n g w e i g h t e d s a l t w a t e r b r i n e s o l u t i o n s as i s u s u a l l y r e q u i r e d i n p e r f o r m i n g d e n s i t y c o n t r o l treatments. BARBER & T H E M I G ( 1 9 8 5 ) document s i g n i f i c a n t improvement o f f r a c t u r e s t i m u l a t i o n s i n f l u i d - s t r a t i f i e d f i n i n g - u p w a r d s sand-body r e s e r v o i r s b y f l u i d d e n s i t y c o n t r o l , w i t h o v e r r i d i n g p e r m i t t i n g s e l e c t i v e proppant placement i n t h e upper t i g h t o i l - b e a r i n g p o r t i o n o f t h e p a y zone w h i l e t h e l o w e r segment o f t h e c r a c k plane i n t e r s e c t i n g t h e lower permeable water-bearing i n t e r v a l o f t h e prospect i v e column i s l e f t u n p r o p p e d and t h u s w a t e r p r o d u c t i o n c a n be m i n i m i z e d ( c f . a l s o MISAK, ATTEBERRY, VENDITTO & FREDRICKSON 1978 b ) b y a l l o w i n g t h e l o w e r p o r t i o n o f t h e f r a c t u r e t o r e h e a l once c l o s u r e s t r e s s becomes e f f e c t i v e a f t e r t h e end o f t h e h y d r a u l i c t r e a t m e n t . S e l e c t i v e p r o p p a n t p l a c e m e n t i n t h e u p p e r r e s e r v o i r p o r t i o n b y o v e r r i d i n g c r e a t e d b y pumping o f a h i g h - d e n s i t y p a d f o l l o w e d b y lower-density proppant-bearing f l u i d s allows t o i n h i b i t connection t o the lower w a t e r - b e a r i n g s e c t i o n o f t h e p r o s p e c t i v e i n t e r v a l (MISAK, ATTEBERRY, VENDITTO & FREDRICKSON 1978 b ) .
4.2.2.6.4.Fluid density control and proppant distribution I n a s i m i l a r way as a p p l y i n g f o r l a y e r i n g o f p r o p p a n t - s u p p o r t e d and u n p l u g ged s e c t i o n s o f t h e r e s e r v o i r , a l s o h o r i z o n t a l s t r a t i f i c a t i o n o f d i f f e r e n t p r o p p a n t t y p e s c a n be a c h i e v e d b y i n s e r t i n g two s u c c e s s i v e p r o p p a n t - l a d e n f l u i d s o f d i f f e r e n t v i s c o s i t y i n t o t h e f r a c t u r e , r e s u l t i n g i n placement o f a proppant o f d i f f e r e n t type and/or g r a i n s i z e i n t h e upper p o r t i o n o f t h e f r a c t u r e w i t h r e s p e c t t o t h e l o w e r segment o f t h e c r a c k , t h e r e b y p e r m i t t i n g s e l e c t i v e o p t i m i z a t i o n o f t h e f r a c t u r e - f o r m a t i o n c o n d u c t i v i t y c o n t r a s t i n r e s e r v o i r s w i t h pronounc e d g r a i n s i z e d i f f e r e n c e s between v e r t i c a l l y s u c c e s s i v e u n i t s o r s t o r e y s ( c f . also section 4.2.2.3.3.). F l u i d d e n s i t y c o n t r o l , however, c a n o n l y g i v e r i s e t o d i f f e r e n t v e r t i c a l i n t e r v a l s w i t h i n t h e f r a c t u r e and v e r t i c a l p r o p p a n t z o n i n g , whereas p l a c e m e n t o f h o r i z o n t a l l y d i f f e r i n g proppant types and/or g r a i n s i z e s r e s u l t i n g i n l a t e r a l coexistence o f proppant p i l l a r s ( c f . section 4.3.3.2.) o f d i f f e r e n t composition has t o be a c h i e v e d by pumping a l t e r n a t i n g l o t s o f f l u i d s w i t h more o r l e s s s i m i l a r v i s c o s i t y t o a v o i d o v e r - o r u n d e r r u n n i n g , and b e i n g s a t u r a t e d w i t h o r w i t h o u t p r o p p a n t s i f c r e a t i o n o f d i s c r e t e p r o p p a n t p i l l a r s s e p a r a t e d b y open c o lumns i s d e s i r e d , o r c o n t a i n i n g s u c c e s s i v e l o t s o f p r o p p a n t s o f d i f f e r e n t t y p e and/or g r a i n s i z e i n case h o r i z o n t a l zoning o f proppants i n t h e f r a c t u r e w i t h t a i l a n d / o r head b e i n g o f d i f f e r e n t c o m p o s i t i o n t h a n t h e m a i n body i s w a n t e d .
387 S e l e c t i v e p r o p p a n t placement, however, has t o t a k e i n t o c o n s i d e r a t i o n t h a t v a r y i n g c o n d u c t i v i t y i n t h e f r a c t u r e f r o m w e l l b o r e t o t i p can s i g n i f i c a n t l y a f f e c t p r o d u c t i o n r a t e s (BENNETT, ROSATO & REYNOLDS 1981) e i t h e r i n p o s i t i v e o r n e g a t i v e d i r e c t i o n depending on g e o m e t r i c a l arrangement o f t y p e and g r a i n s i z e o f proppants and e f f e c t o f p o s s i b l e v o i d spaces between t h e i n d i v i d u a l d i s c r e t e proppant l o t s . I t i s t h e r e f o r e v e r y i m p o r t a n t t o determine p r o p p a n t d i s t r i b u t i o n schedules i n such a way t h a t achievement o f t h e a p p r o p r i a t e c o n d u c t i v i t y p r o f i l e i n t h e f r a c t u r e i s assured (VEATCH & M O S C H O V I D I S 1986).
4.2.2.7.
Special phenomena i n coal seams
I n c o a l seam gas r e s e r v o i r s , h i g h t r e a t m e n t p r e s s u r e s may l e a d t o u n c o n f i n e d v e r t i c a l f r a c t u r e growth i n case o f absence o f s u f f i c i e n t s t r e s s c o n t r a s t s i n t h e f o r m a t i o n s bounding t h e c o a l o r f o r t h i n c o a l seams (JONES, BELL, MORALES & SCHRAUFNAGEL 1987). Modulus c o n t r a s t s may p l a y an i n d i r e c t , b u t i m p o r t a n t r o l e i n a f f e c t i n g f r a c t u r e containment. F r a c t u r e - t i p p l u g g i n g w i t h c o a l f i n e s p r e vents u n c o n t a i n e d v e r t i c a l growth o f f r a c t u r e s i n c o a l seams by q u i c k l y s e a l i n g off o f narrow c r a c k s c r e a t e d i n high-modulus bounding f o r m a t i o n s ( c f . s e c t i o n 4.4.3.3.). Coal seams i n t e r b e d d e d w i t h sandstones and shales can a c t as t h i e f zones f o r f r a c t u r i n g s l u r r y when h y d r a u l i c s t i m u l a t i o n o f a sandstone i s attempted, because t h e minimum h o r i z o n t a l s t r e s s w i t h i n t h e c o a l i s l e s s t h a n w i t h i n sandstones and s h a l e s (DESPAX, CHARLEZ, CLINKEMAILLE & ECONOMIDES 1987). As c o a l a l s o has a low e l a s t i c modulus, c o n n e c t i o n o f t h e seam w i t h t h e p e r f o r a t i o n s would l e a d t o p r e f e r e n t i a l f r a c t u r i n g o f t h e c o a l r a t h e r t h a n t h e sandstone, t h u s s e t t i n g t h e proppant o u t o f t h e pay zone and r e s u l t i n g i n f a i l u r e o f t h e s t i m u l a t i o n job ( c f . section 4.4.3.4.). T h e r e f o r e containment a n a l y s i s i n such complex i n t e r b e d d e d r e s e r v o i r sequences has t o i n c l u d e i n - s i t u s t r e s s measurements by m i n i f r a c t u r e s i n a l l r e l e v a n t l a y e r s c o m p r i s i n g sandstone, s h a l e and c o a l . I n o r d e r f o r a f r a c t u r e t o s t a y c o n t a i n e d w i t h i n t h e f o r m a t i o n o f i n t e r e s t o r i n an e f f o r t t o m i n i m i z e t h e v e r t i c a l h e i g h t growth, t h e e x c e s s i v e p r e s s u r e w i t h i n t h e c r a c k has t o be m i n i m i zed. Excess p r e s s u r e i s almost e n t i r e l y dependant on f r i c t i o n p r e s s u r e d r o p down t h e f r a c t u r e which i n t u r n i s a f u n c t i o n o f g e l v i s c o s i t y , i n j e c t i o n r a t e , and f r i c t i o n c o e f f i c i e n t between f l u i d and f r a c t u r e w a l l (HOLDITCH, ROBINSON, WHITEHEAD & ELY 1987).
4.2.2.8.
Other aspects
H y d r a u l i c p r o p p a n t f r a c t u r i n g o f t h i n s h e e t - t y p e o i l - b e a r i n g sandstones i s a l s o r e p o r t e d by KOHLHAAS ( 1 9 8 2 ) . KELKAR, THOMPSON, HOLLAND, STRAWN, VEGHTE & HATHAWAY (1985) comment on h y d r a u l i c c r a c k h e i g h t e x t e n s i o n c o n t r o l u s i n g p e r m e a b i l i t y b a r r i e r s and d e t e r m i n a t i o n o f g e o l o g i c a l and e n g i n e e r i n g parameters f o r f r a c t u r e design. Another i m p o r t a n t parameter a f f e c t i n g h e i g h t growth and p r o p a g a t i o n o f hyd r a u l i c a l l y induced c r a c k s i s f r a c t u r e toughness (THIERCELIN, JEFFREY & BEN NACEUR 1987; WANG & CLIFTON 1989). I n f l u e n c e s on f r a c t u r e toughness a r e e x e r t e d by l a r g e - s c a l e h e t e r o g e n e i t i e s , c o n f i n i n g pkessure and p o r e p r e s s u r e . Bounding l a y e r s w i t h h i g h e r c r a c k toughness t h a n t h a t o f t h e pay i n t e r v a l can a c t t o e f f e c t i v e ? y c o n t a i n a h y d r a u l i c f r a c t u r e between them, and a pay zone w i t h h i g h e r crack toughness than bounding l a y e r s can l e a d t o f r a c t u r e g r o w t h i n t h e l i m i t i n g beds a t t h e expense o f p r o p a g a t i o n i n t h e r e s e r v o i r ( c f . a l s o s e c t i o n 4.3.4.2.3.). Aspects o f f r a c t u r i n g i n l a y e r e d r e s e r v o i r s where more o r l e s s containment o f c r a c k p r o p a g a t i o n i s r e q u i r e d a r e a l s o d i s c u s s e d by CAMACHO, RAGHAVAN & REY-
388 NOLDS ( 1 9 8 4 ) ; SCHULTE ( 1 9 8 4 ) ; BENNETT, CAMACHO, REYNOLDS & RAGHAVAN ( 1 9 8 5 ) ; BEN NACEUR & TOUBOUL ( 1 9 8 7 ) and BOUTECA ( 1 9 8 7 ) . ABOU-SAYED, S I N H A & CLIFTON ( 1 9 8 4 ) and ABOU-SAYED, CLIFTON, DOUGHERTY & MORALES ( 1 9 8 4 ) comment o n t h e i n f l u e n c e o f i n - s i t u r e s e r v o i r c o n d i t i o n s on t h e g e o m e t r y o f h y d r a u l i c f r a c t u r e s . A q u a n t i t a t i v e a n a l y s i s o f f a c t o r s i n f l u e n c i n g v e r t i c a l and l a t e r a l c r a c k g r o w t h i s p e r f o r m e d b y SETTARI ( 1 9 8 5 ) . P o s s i b i l i t i e s o f f r a c t u r e p e n e t r a t i o n t h r o u g h an i n t e r f a c e a r e e v a l u a t e d b y BIOT, MEDLIN & MASSE (1983; c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) .
4.2.3, Fracture propagat ion While i n t h i c k sandstones t h a t a r e o n l y p e r f o r a t e d along p a r t s o f t h e i r t h i c k n e s s a c c o r d i n g t o t h e l i m i t e d - e n t r y t e c h n i q u e (LAGRONE & RASMUSSEN 1963, HOWARD & FAST 1970, SMALL 1985, CRAMER 1987) t h e f r a c t u r e p r o p a g a t e s r a d i a l l y t h r o u g h t h e w h o l e r e s e r v o i r t h i c k n e s s t o t h e b o u n d i n g mudstones and t h e n c o n t i nues a l o n g t h e s e a l i n g h o r i z o n s i n l a t e r a l d i r e c t i o n , t h i n s a n d s t o n e s a r e o f t e n p e r f o r a t e d a l o n g a l m o s t t h e w h o l e s t r a t i g r a p h i c a l i n t e r v a l and t h e f r a c t u r e i s thus n e a r l y instantaneously propagating i n l a t e r a l d i r e c t i o n along the g u i d i n g b a r r i e r . P a r t i c u l a r l y i n t h i n sandstones, however, t h e d a n g e r o f u n c o n t r o l l e d v e r t i c a l c r a c k g r o w t h and b r e a k t h r o u g h t o a d j o i n i n g beds becomes g r e a t e r w i t h i n c r e a s i n g f r a c t u r e l e n g t h d e p e n d i n g on d i f f e r e n c e s i n s t a b i l i t y b e h a v i u r and s t r e s s s t a t u s o f t h e s u p e r i m p o s e d f o r m a t i o n s (KLOSE & KRUMER 1983) and a l s o o n t h e h i g h e r p r e s s u r e s t h a t ' have' t o be a p p l i e d t o ' c r e a t e l o n g e r f r a c t u r e s beyond a c e r t a i n boundary l e n g t h . When t h e s t r e s s i n t e n s i t y f a c t o r a t t h e t i p r e a c h e s i t s c r i t i c a l v a ue, t h e h y d r a u l i c f r a c t u r e becomes u n s t a b l e and b e g i n s t o move t o w a r d s t h e d r e c t i o n where t h e s t r e s s i n t e n s i t y f a c t o r has t h e h i g h e s t v a l u e (LU & YEN 1985 . A f t e r racture r e f e r r i n q t o t e r m i n o l o q y and n o m e n c l a t u r e , v a r i o u s f a c t o r s c o n t r o l l i n q p r o p a g a t i o n a r e b r i e f l y r e v i e w e d as f o l l o w s . An o u t l i n e o f t h e t w o d i f f e r e n t p r o c e s s e s o f s h e a r s l i p p a g e and t e n s i l e f a i l u r e o f r o c k s c r e a t i n g open j o i n t s i s g i v e n , t h e i n f l u e n c e o f g e o l o g i c a l d i s c o n t i n u i t i e s i s s k e t c h e d , comments a r e o f f e r e d on f r a c t u r e p r o p a g a t i o n and p r e s s u r e e v o l u t i o n , and a f i e l d example o f r e q u i r e d seal thickness f o r successful containment o f a large-scale f r a c t u r e i s reported.
4.2.3.1.
Terminology and nomenclature
C o n c e r n i n g t e r m i n o l o g y and n o m e n c l a t u r e , t h e p r e s s u r e n e c e s s a r y t o p r o p a g a t e a f r a c t u r e i s t h e minimum c r a c k e x t e n s i o n p r e s s u r e w h i c h i s r e q u i r e d a t t h e t i p t o keep t h e f r a c t u r e open (WHITEHEAD, HUNT & HOLDITCH 1 9 8 7 ) . D u r i n g i n j e c t i o n o f f l u i d , t h e p r e s s u r e a t t h e w e l l b o r e i s t h e e x t e n s i o n p r e s s u r e p l u s any e x c e s s p r e s s u r e i n t h e f r a c t u r e p l u s p r e s s u r e l o s s e s due t o f r i c t i o n t h r o u g h t h e p e r f o r a t i o n s . A f t e r shut-down o f pumping, t h e i n s t a n t a n e o u s s h u t - i n p r e s s u r e a t t h e w e l l b o r e i s e q u a l t o t h e l e a s t p r i n c i p a l h o r i z o n t a l s t r e s s p l u s any e x c e s s p r e s s u r e i n t h e f r a c t u r e . As t h e e x c e s s p r e s s u r e d e c r e a s e s due t o f l u i d l e a k o f f , t h e crack w i l l c l o s e a t the i n - s i t u s t r e s s which i s t h e r e s u l t a n t pressur e . The m o s t common synonyms f o r i n - s i t u s t r e s s a r e l e a s t p r i n c i p a l h o r i z o n t a l stress ( c f . section 1.2.1.1.), minimum p r i n c i p a l s t r e s s , f r a c t u r e c l o s u r e pressure, closure stress, l e a s t c o m p r e s s i v e s t r e s s and f r a c t u r e g r a d i e n t . The r e s u l t a n t p r e s s u r e can be d e t e r m i n e d b y p u m p - i n / f l o w - b a c k t e s t s , m i n i f r a c t u r i n g and i n - s i t u s t r e s s t e s t s ( c f . s e c t i o n 4 . 8 . 2 . ) , w i t h a l l o f t h e s e t e s t s b e i n g a b l e t o be p e r f o r m e d i n b o t h open and p e r f o r a t e d c a s e d h o l e . F r a c t u r e p r o p a g a t i o n c o n c e r n i s a l s o a m a t t e r o f t r e a t m e n t s c a l e and r e s e r v o i r t h i c k n e s s , w i t h v e r t i c a l f r a c t u r e g r o w t h i n a t h i c k f o r m a t i o n and a s m a l l s i z e j o b n o t b e i n g so s e r i o u s as t h a t o c c u r r i n g i n a l a r g e - s i z e j o b i n a t h i n f o r m a t i o n ( L I & ZHU 1 9 8 6 ) .
389
4.2.3.2.
Factors c o n t r o l l i n g f r a c t u r e propagation
Most f r a c t u r e s are o r i e n t e d more o r l e s s i n a v e r t i c a l plane and propagate outward i n opposite d i r e c t i o n s from a w e l l b o r e (VEATCH 1983). The f r a c t u r e s are t y p i c a l l y extending e i t h e r i n a r a d i a l f a s h i o n t o form penny-shaped cracks, o r propagate predominantly i n l a t e r a l d i r e c t i o n along n a t u r a l f o r m a t i o n i n t e r f a c e s and l e s s f r e q u e n t l y i n v e r t i c a l d i r e c t i o n across bedding planes. Local s t r e s s f i e l d s and v a r i a t i o n s i n stresses between adjacent r e s e r v o i r s have dominant e f f e c t s i n c o n t r o l l i n g f r a c t u r e o r i e n t a t i o n and v e r t i c a l growth tendency, whereas r e g i o n a l stresses may a f f e c t the azimuthal t r e n d o f the crack. The p r i n c i p a l sources o f f r a c t u r e containment are h o r i z o n t a l s t r e s s v a r i a t i o n s w i t h depth and p l a s t i c i t y o f bounding shale l a y e r s (MEDLIN & FITCH 1983). The o u t l i n e as f o l lows comments on i n - s i t u s t r e s s c o n t r a s t and o t h e r i n f l u e n c i n g f a c t o r s , horizont a l s t r e s s d i f f e r e n c e s and f r a c t u r e geometry, r e s e r v o i r e l a s t i c i t y and p l a s t i c i ty, f r a c t u r e propagation across weakness planes, premature screenout terminat i o n , and s t r e s s d i f f e r e n c e s and containment.
4.2.3.2.1.
I n - s i t u stress contrast and other influencing f a c t o r s
F r a c t u r e propagation i s m a i n l y c o n t r o l l e d by i n - s i t u s t r e s s c o n t r a s t s b e t ween the i n d i v i d u a l rock l a y e r s (WHITEHEAD, HUNT & HOLDITCH 1987), w i t h commonl y r e s e r v o i r sandstones and b a r r i e r mudstones being l o w - s t r e s s and h i g h - s t r e s s rocks, r e s p e c t i v e l y (NOLTE 1988 a, 1988 b; c f . a l s o s e c t i o n s 4.2.2.3.1.3. and 4.2.3.2.2.). Accurate p r e d i c t i o n o f the magnitude o f v e r t i c a l f r a c t u r e growth i s imperative f o r proper design o f f r a c t u r e length, w i d t h and h e i g h t . Apart from f r a c t u r e propagation upwards o r downwards o u t o f t h e zone o f i n t e r e s t , f u r t h e r c o m p l i c a t i o n s and drawbacks can be m u l t i p l e f r a c t u r e closures, extreme perf o r a t i o n damage and communication w i t h o t h e r sets o f p e r f o r a t i o n s through t h e crack o r behind the p i p e . F r a c t u r e containment depends m a i n l y on i n - s i t u s t r e s s c o n t r a s t s between f o r m a t i o n and a d j o i n i n g b a r r i e r s (SIMONSON, ABOU-SAYED & CLIFTON 1978; EEKELEN 1982, MENG & BROWN 1987), b u t a l s o on rock s t i f f n e s s and payzone thickness (BOUTECA 1987). Thus the e f f i c i e n c y o f a s t r e s s b a r r i e r has t o be considered t o g e t h e r w i t h e l a s t i c modulus o f the rock and r e s e r v o i r t h i c k ness. F u r t h e r parameters a f f e c t i n g v e r t i c a l f r a c t u r e propagation are i n - s i t u s t r e s s g r a d i e n t ( c f . s e c t i o n 4.2.2.3.1.), e l a s t i c o r p l a s t i c r e s e r v o i r rock beh a v i o u r ( c f . s e c t i o n 4.2.3.2.3.), toughness and d u c t i l i t y d i s c o n t i n u i t i e s ( c f . s e c t i o n 4.2.4.3.), n a t u r a l f r a c t u r e s ( c f . s e c t i o n 4.8.8.), leakoff o f fractur i n g f l u i d s ( c f . s e c t i o n 4.3.4.6.) and h e a t - t r a n s f e r p r o p e r t i e s ( c f . s e c t i o n 4.3.4.4.), as w e l l as v a r i a t i o n s o f i n - s i t u stresses e x i s t i n g i n d i f f e r e n t l a y e r s o f t h e g e o l o g i c a l column ( c l o s u r e s t r e s s p r o f i l e ) , r e l a t i v e bed t h i c k ness o f the formations i n the v i c i n i t y o f the f r a c t u r e , bonding between t h e rocks, f l u i d pressure g r a d i e n t s i n the f r a c t u r e , and pore pressure v a r i a t i o n s from one r e s e r v o i r s e c t i o n t o t h e o t h e r (VEATCH 1983). When a p r o s p e c t i v e zone c o n t a i n s e x c e l l e n t b a r r i e r s t o f r a c t u r e growth and w e l l engineered s t i m u l a t i o n treatments a r e performed, e x p e c t a t i o n can be made t o achieve e f f e c t i v e crack lengths t h a t a r e reasonably c l o s e t o the designed f r a c t u r e l e n g t h (ROBINSON, HOLDITCH ti LEE 1983). Other f a c t o r s i n f l u e n c i n g v e r t i c a l and l a t e r a l f r a c t u r e growth i n a d d i t i o n t o i n - s i t u c o n d i t i o n s are r h e o l o g i c a l p r o p e r t i e s and f l o w r a t e s o f the s t i m u l a t i o n f l u i d , proppant placement and temperature p r o f i l e s . The most e f f e c t i v e mechanism o f f r a c t u r e containment r e s u l t s from s u i t a b l e combination o f v i s c o s i t y and i n j e c t i o n r a t e o f the f r a c t u r i n g f l u i d and s t a g i n g o f heavy proppant (BEN NACEUR & TOUBOUL 1987). Aspects o f h o r i z o n t a l s t r e s s d i f f e r e n c e s , reservoir e l a s t i c i t y vs. p l a s t i c i t y , f r a c t u r e propagation across weakness planes, and premature screenout t e r m i n a t i o n are o u t l i n e d as f o l l o w s .
390
4.2.3.2.2.
Horizontal stress differences and fracture geometry
Containment o f t h e f r a c t u r e i n v e r t i c a l d i r e c t i o n by t h e o v e r - and u n d e r l y i n g t i g h t beds i s o n l y guaranteed i f t h e h o r i z o n t a l s t r e s s e s below and above t h e t a r g e t h o r i z o n a r e s t i l l d u r i n g e x e c u t i o n o f t h e j o b and s u p e r i m p o s i t i o n by t h e t r e a t m e n t p r e s s u r e g r e a t e r than i n t h e r e s e r v o i r t o be s t i m u l a t e d , w i t h o t h e r w i s e t h e p o s s i b i l i t y o f u n d e s i r e d c r a c k p r o p a g a t i o n i n penny shape e x i s t i n g (LEICHT 1985). The bounding mudstones a r e capable o f s t o p p i n g f r a c t u r e p r o p a g a t i o n i n t h e sandstone and t o conduct f u r t h e r c r a c k e x t e n s i o n a l o n g t h e i n t e r f a c e ( c f . section 4.2.2.3.5.) between b o t h d i f f e r e n t l i t h o f a c i e s o n l y i f t h e r e a r e a l s o reasonable d i f f e r e n c e s i n t h e e l a s t i c i t y modul (KLOSE & KROMER 1983 r e p o r t f o r R o t l i e g e n d mudstones an a b t . seven t i m e s g r e a t e r e l a s t i c i t y mod u l than i n t h e u n d e r l y i n g sandstone t h a t was f r a c t u r e d ; c f . s e c t i o n 3 . 3 . 2 . ) . R e s e r v o i r s bounded by weak b a r r i e r s which have n o t enough s t r e s s t o c o n f i n e an induced f r a c t u r e i n t h e pay zone a t h i g h e r s t r e s s e s can o n l y be t r e a t e d by small e r jobs i n v o l v i n g lower pressures t h a t are s t i l l guaranteing f r a c t u r e terminat i o n a t weak o r unbonded i n t e r f a c e s a t low c o n f i n i n g s t r e s s e s (AHMED, STRAWN, WILSON & SCHATZ 1983; c f . s e c t i o n 4 . 2 . 3 . 2 . 4 . ) .
I n case o f s u i t a b l e p r e s s u r e l e v e l s , h y d r a u l i c f r a c t u r e s can grow v e r t i c a l l y as much as 20 f t even when v e r y low i n j e c t i o n r a t e s and t o t a l pumped volumes a r e used (WHITEHEAD, HUNT & HOLDITCH 1987). Given i n s u f f i c i e n t s t r e s s c o n t r a s t between t h e n e t pay zones and t h e r o c k l a y e r s above and below ( c f . s e c t i o n 4 . 2 . 2 . 3 . 1 . 3 . and 4 . 2 . 3 . 2 . 1 . ) can be a g r e a t problem t o achieve t h e goal o f c r e a t i n g l o n g c o n d u c t i v e c r a c k s due t o e x c e s s i v e v e r t i c a l h e i g h t growth as a consequence o f l a c k i n g f r a c t u r e containment (HOLDITCH, ROBINSON, WHITEHEAD & ELY 1987). The e f f e c t o f t h e b o r e h o l e t o t h e s t r e s s i n t e n s i t y f a c t o r a t t h e f r a c t u r e t i p i s n e g l i g i b l e when c r a c k l e n g t h i s l a r g e r than t h e r a d i u s o f t h e w e l l which i s t h e case i n almost a l l t h e h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s ( B O W I E 1956, YEW & L I 1987). Some aspects o f s t r e s s v a r i a t i o n s w i t h l i t h o l o g y , n a t u r a l f r a c t u r e d i s t r i b u t i o n i n v a r i o u s rocks, i n - s i t u s t r e s s c o n t r a s t s , and f r a c t u r e i n s t a b i l i t y and movement a r e d i s c u s s e d as f o l l o w s .
4.2.3.2.2.1.
Stress variations with 1 ithology
H o r i z o n t a l s t r e s s d i f f e r e n c e s between t h e i n d i v i d u a l l a y e r s o f t h e g e o l o g i c a l column a r e i n r e s e r v o i r s e c t i o n s p a r t i c u l a r l y pronounced between sandstones, carbonates and mudstones (BRITT & LARSEN 1986). As carbonates have h i g h e r c l o s u r e s t r e s s than sandstones i n comparable r e s e r v o i r depth, s e l e c t i v e f r a c t u r i n g o f a carbonate h o r i z o n w i t h i n an i n t e r b e d d e d sequence has t o i n c l u d e p r o p e r s h u t - o f f o f t h e sandstones above and below t h e carbonate, because o t h e r w i s e once communication f r o m t h e carbonate t o t h e sandstone would be e s t a b l i s h e d , o n l y n e g l i g i b l e f r a c t u r e e x t e n s i o n would o c c u r i n t h e carbonate zone, and v i r t u a l l y maximum c r a c k l e n g t h which c o u l d be expected i n t h e carbonate i n t e r v a l would be t h e l e n g t h a l r e a d y achieved a t t h e t i m e o f communication w i t h t h e sandstone bed . On t h e o t h e r hand, dense carbonates a l t e r n a t i n g w i t h t i g h t gas sandstones a r e e f f e c t i v e b a r r i e r s t o v e r t i c a l f r a c t u r e growth due t o t h e i r c o n s i d e r a b l y h i g h e r i n - s i t u s t r e s s (GRI 1988). A p a r t f r o m l i t h o l o g y , s t r e s s l e v e l i s p a r t i a l l y a l s o depending on d e p o s i t i o n a l environment (WARPINSKI, BRANAGAN & WILMER 1985; c f . a l s o s e c t i o n s 4.2.2.3.1.3. and 4.2.3.2.1.). Shales and impermeable carbonates have h i g h e r h o r i z o n t a l s t r e s s e s than i n t e r bedded permeable sandstones p a r t i c u l a r l y a f t e r f o r m a t i o n p r e s s u r e drawdown i n t h e l a t t e r (LABUDOVIC 1981). I n - s i t u s t r e s s can be determined by b o t h open-hole and cased-hole s t r e s s t e s t i n g . I n t e r p r e t a t i o n d i f f i c u l t i e s may a r i s e f r o m f r a c t u r e p r o p a g a t i o n o u t o f t h e zone o f i n t e r e s t , m u l t i p l e f r a c t u r e c l o s u r e , f r a c t u r e growth around t h e packer, extreme p e r f o r a t i o n damage, communication w i t h o t h e r s e t s o f p e r f o r a t i o n s t h r o u g h t h e f r a c t u r e o r b e h i n d p i p e , o r equipment m a l f u n c t i o n s r e q u i r i n g s u r f a c e r a t h e r than downhole s h u t - i n s . YEW & C H I O U (1983) d i s c u s s e f f e c t s o f i n - s i t u s t r e s s e s and l a y e r p r o p e r t i e s on h y d r a u l i c f r a c t u r e containment, and EEKELEN (1981) e v a l u a t e s t h e r e l a t i o n s h i p between ho-
391 r i z o n t a l s t r e s s and depth i n sedimentary b a s i n s .
4.2.3.2.2.2. Natural fracture distribution in various rocks S i m i l a r l y as a p p l y i n g f o r h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s , s i z e and d i s t r i b u t i o n o f n a t u r a l c r a c k s w i t h i n t h e r e s e r v o i r complex depends on d i f f e r e n c e s o f i n - s i t u s t r e s s e s i n t h e i n d i v i d u a l l i t h o l o g i e s , w i t h main d i s t i n c t i o n b e i n g made between l o w - s t r e s s h i g h - p e r m e a b i l i t y sandstones and h i g h - s t r e s s low-permeab i l i t y mudstones (NORTHROP 1988; c f . s e c t i o n 4 . 8 . 8 . 1 . ) . F r a c t u r e s i n t e r s e c t i n g sandstone b o d i e s t e r m i n a t e a t mudstone c o n t a c t s b o t h a t i n t e r - and i n t r a r e s e r v o i r boundaries o r l i t h o l o g i c a l d i s c o n t i n u i t i e s . T h e r e f o r e t h e n a t u r a l f r a c t u r e system i s heterogeneously d i s t r i b u t e d through non-communicating l a y e r s (BRANAGAN, LEE, CIPOLLA & WILMER 1988). The d e n s i t y o f n a t u r a l f r a c t u r e s i s i n v e r s e l y p r o p o r t i o n a l t o t h i c k n e s s , w i t h t h e t h i n n e s t l a y e r b e i n g t h e most c o n d u c t i v e due t o narrow j o i n t spacing, b u t on t h e o t h e r hand n o t c o n t a i n i n g t h e b u l k o f p r o d u c i b l e gas which i s concent r a t e d i n t h i c k e r beds t h a t a r e c o m p a r a t i v e l y l e s s c o n d u c t i v e as a consequence o f wide c r a c k spacing. I n t h e absence o f m a t e r i a l p r o p e r t y d i f f e r e n c e s , t h e m i nimum h o r i z o n t a l s t r e s s d i f f e r e n c e s between pay zone and a d j a c e n t l a y e r s cont r o l t h e degree o f v e r t i c a l m i g r a t i o n o f a h y d r a u l i c f r a c t u r e (BEGNAUD & CLAIBORNE 1985). LERCHE & NARR (1984) e s t i m a t e subsurface c r a c k d e n s i t y i n c o r e and d i s c u s s e f f e c t s o f v a r i a b l e f r a c t u r e spacing, and YEW & LODDE (1983) analyze p r o p a g a t i o n o f h y d r a u l i c a l l y - i n d u c e d c r a c k s i n a l a y e r e d medium.
4.2.3.2.2.3. In-situ stress contrasts Concerning t h e e f f e c t o f i n - s i t u s t r e s s c o n t r a s t s on h y d r a u l i c f r a c t u r e s ( c f . section 4.2.2.3.1.), h i g h - s t r e s s r e g i o n s above and below t h e c r a c k e d zone r e s u l t i n a n e a r l y rectangular-shaped f r a c t u r e f o r l o w - v i s c o s i t y low-flow-rate t e s t s , whereas h i g h e r v i s c o s i t i e s and f l o w r a t e s t h a t induce h i g h e r f r a c t u r e p r e s s u r e s r e s u l t i n g r e a t e r h e i g h t s and a l e s s r e c t a n g u l a r shape (WARPINSKI 1983 a ) . The p r e s s u r e near t h e t i p o f t h e c r a c k i s t h e p o r e p r e s s u r e and n o t t h e minimum i n - s i t u s t r e s s v a l u e . B I O T , MASSE & MEDLIN (1986) o u t l i n e t h a t f r a c t u r e p r o p a g a t i o n i s i n most r e s e r v o i r s c o n t r o l l e d m a i n l y by t h e e l a s t i c a l e n e r gy r e q u i r e d t o f o r c e a p a r t t h e c r a c k f a c e s and n o t by s u r f a c e energy e f f e c t s a t t h e t i p . S t r e s s c o n t r a s t s i n t h e o r d e r o f 300 - 400 p s i a r e g e n e r a l l y s u f f i c i e n t t o r e s t r i c t f r a c t u r e growth i n t o t h e h i g h e r s t r e s s r e g i o n (AHMED, STRAWN, WILSON & SCHATZ 1983), whereas underpassing o f t h e minimum c r i t i c a l s t r e s s cont r a s t has t h e e f f e c t t h a t t h e a d j o i n i n g l a y e r s a r e no a b s o l u t e b a r r i e r s t o f r a c t u r e growth (AHMED 1988). M a t e r i a l p r o p e r t y i n t e r f a c e s a l o n e have l i t t l e e f f e c t on containment o f hyd r a u l i c f r a c t u r e s ( c f . s e c t i o n 4.2.3.2.4.), and i n - s i t u s t r e s s e s have t h e domin a n t i n f l u e n c e on c r a c k b e h a v i o u r and d i r e c t i o n o f f r a c t u r e growth (TEUFEL & CLARK 1981). Crack containment i s most l i k e l y t o o c c u r as a r e s u l t o f an i n crease o f minimum h o r i z o n t a l compressive s t r e s s which takes p l a c e a t t h e t r a n s i t i o n f r o m h i g h shear modulus i n t o low shear modulus r o c k s . I n h i b i t i o n o f v e r t i c a l f r a c t u r e growth i s a l s o t r i g g e r e d by weak i n t e r f a c i a l shear s t r e n g t h o f t h e d i f f e r e n t l a y e r s . LU & YEW (1985) d i s c u s s t h e s i g n i f i c a n c e o f s t r e s s i n t e n s i t y f a c t o r a t t h e f r a c t u r e t i p s which depends n o t o n l y on t h e r e l a t i v e moduli o f t h e l a y e r s b u t a l s o on t h e r e l a t i v e p o s i t i o n o f t h e c r a c k s t o t h e l a y e r i n t e r f a c e as w e l l as t h e i r r e l a t i v e s i z e .
4.2.3.2.2.4. Frac t ur e i ns t ab i 1 i t y and movement I n h y d r a u l i c f r a c t u r i n g design, two c r i t e r i a a p p l y i n many cases c o m p r i s i n g i n s t a b i l i t y o f t h e c r a c k which begins t o move when t h e s t r e s s i n t e n s i t y f a c t o r a t t h e f r a c t u r e t i p reaches t h e c r i t i c a l v a l u e which i s regarded as a m a t e r i a l
392 p r o p e r t y o f t h e medium, o r m i g r a t i o n o f t h e c r a c k i n t o t h e d i r e c t i o n a t which t h e s t r e s s i n t e n s i t y f a c t o r has t h e h i g h e s t v a l u e (LU & YEW 1985). Upward o r downward movement o f t h e f r a c t u r e can be decided m e r e l y by comparing t h e r e l a t i v e magnitude o f t h e c a l c u l a t e d s t r e s s i n t e n s i t y f a c t o r a t t h e c r a c k t i p s . Mat e r i a l i n t e r f a c e s o r d i s c o n t i n u i t i e s have v e r y small e f f e c t s on t h e s t r e s s i n t e n s i t y f a c t o r when t h e f r a c t u r e i s s i t u a t e d f a r f r o m t h e bounding p l a n e s . The r e l a t i v e magnitude o f t h e s t r e s s i n t e n s i t y f a c t o r a t t h e c r a c k t i p s can be used t o i n d i c a t e t h e d i r e c t i o n o f f r a c t u r e movement. F r a c t u r e p r e s s u r e g r a d i e n t i s a f u n c t i o n o f overburden s t r e s s g r a d i e n t , p o r e p r e s s u r e g r a d i e n t , and r a t i o o f hor i z o n t a l t o v e r t i c a l s t r e s s (EATON 1969).
4.2.3.2.3. Reservoir elasticity vs. plasticity Fracture propagation modelling i s u n r e a l i s t i c i f r e s e r v o i r rocks are t r e a t e d as i d e a l l y e l a s t i c m a t e r i a l s as done by a l l o f t h e commonly used t h e o r i e s , and o n l y becomes reasonable i f p l a s t i c i t y and s u r f a c e energy o f t h e f o r m a t i o n s a r e i n c l u d e d i n t h e approach (MEDLIN & MASSE 1986). F r a c t u r e o p e r a t i o n s a r e f r e q u e n t l y c a r r i e d o u t i n b r i t t l e sandstone o r l i m e s t o n e i n t e r v a l s t h a t a r e bounded by s o f t s h a l e s which a r e l i k e l y t o be v e r y p l a s t i c . As c r a c k p r o p a g a t i o n i n p l a s t i c mudstones would r e q u i r e much h i g h e r p r o p a g a t i n g p r e s s u r e s t h a n i n o t h e r l a y e r s , i t can be i n f e r r e d t h a t a f r a c t u r e i n i t i a t e d i n a b r i t t l e l a y e r w i t h p l a s t i c s h a l e boundaries would t e n d t o be c o n f i n e d t h e r e by p r o p a g a t i n g p r e s s u r e c o n s i d e r a t i o n s r e g a r d l e s s o f o t h e r e f f e c t s . Based on t h e e l a s t i c i t y concept, s u r f a c e energy and shear modulus d i f f e r e n c e s c o u l d a l s o l e a d t o f r a c t u r e c o n f i nement i n a l a y e r between d i s s i m i l a r m a t e r i a l s (BIOT, MEDLIN & MASSE 1 9 8 3 ) . Rock f a i l u r e i n h y d r a u l i c f r a c t u r i n g o c c u r s a t t h e l e a d i n g edge o f t h e i n d u ced c r a c k (SLUSSER & RIECKMANN 1976). I f e l a s t i c d e f o r m a t i o n and b r i t t l e f a i l u r e t a k e place, f r a c t u r e e x t e n s i o n r e q u i r e s e s s e n t i a l l y no i n c r e a s e i n p r e s s u r e a t t h e c r a c k t i p s . W i t h i n c r e a s i n g r o c k p l a s t i c i t y , t h e f o r m a t i o n tends t o b a l l o o n b e f o r e b r e a k i n g a t t h e f r a c t u r e t i p , t h u s r e q u i r i n g h i g h e r p r e s s u r e s t o ext e n d t h e c r a c k . The i n c r e a s e i n a p p l i e d p r e s s u r e causes an i n c r e a s e i n f r a c t u r e w i d t h and by m a t e r i a l b a l a n c e c o n s i d e r a t i o n s a s h o r t e r c r a c k l e n g t h . An i n crease i n i n j e c t i o n p r e s s u r e a t a c o n s t a n t r a t e t o g e t h e r w i t h an i n c r e a s e i n i n stantaneous s h u t - i n p r e s s u r e d u r i n g t h e t r e a t m e n t may i n d i c a t e t h a t t h e format i o n i s behaving i n a p l a s t i c manner. Comments a r e o f f e r e d as f o l l o w s on d i s t r i b u t i o n o f p l a s t i c i t y zones i n t h e r e s e r v o i r , p l a s t i c i t y vs. n o n - l i n e a r e l a s t i c i t y , and p l a s t i c i t y e f f e c t s on h y d r a u l i c f r a c t u r i n g .
4.2.3.2.3.1. Distribution o f plasticity zones in the reservoir Zones o f p l a s t i c b e h a v i o u r a r e o f t e n heterogeneously d i s t r i b u t e d i n t h i c k r e s e r v o i r i n t e r v a l s (MEDLIN & MASSE 1986). W h i l e many p r o s p e c t i v e r o c k s behave l i k e b r i t t l e m a t e r i a l s even a t t h e h i g h e s t c o n f i n i n g s t r e s s e s o f p r a c t i c a l i n t e r e s t , o t h e r pay f o r m a t i o n s a r e b r i t t l e a t low c o n f i n i n g s t r e s s , b u t p l a s t i c t o some degree a t h i g h c o n f i n i n g s t r e s s e s w h i c h a r e t y p i c a l o f deeper w e l l s . Res u l t s o f t e n s i l e s t r e s s / s t r a i n i n v e s t i g a t i o n s show t h a t p l a s t i c i t y e f f e c t s i n s h a l e s a r e l i k e l y t o be more i m p o r t a n t i n c o n t r o l l i n g f r a c t u r e h e i g h t t h a n o t h e r f a c t o r s i n c l u d i n g f a r - f i e l d s t r e s s (MEDLIN & MASSE 1986). F o r m a t i o n p l a s t i c i t y i s o f c o n s i d e r a b l e p r a c t i c a l i n t e r e s t , as t h e most i m p o r t a n t f a c t o r s cont r o l l i n g f r a c t u r e containment a r e p l a s t i c i t y o f bounding s h a l e l a y e r s and h o r i zontal s t r e s s v a r i a t i o n s w i t h d e p t h (MEDLIN & FITCH 1983; c f . s e c t i o n 4 . 2 . 2 . 3 . 1 . ) . D u c t i l i t y i n r e s e r v o i r r o c k s may a l s o l e a d t o a b n o r m a l l y h i g h t r e a t i n g p r e s s u r e s i n l a r g e o p e r a t i o n s . L a b o r a t o r y experiments a r e c a r r i e d o u t by MEDLIN & MASSE (1984), and o t h e r aspects o f f r a c t u r e p r o p a g a t i o n a r e c o n t r i b u t e d by B I O T , MASSE & MEOLIN ( 1 9 8 6 ) . P l a s t i c i t y depends on t h e r e l a t i v e magnitudes o f s u r f a c e energy and energy d i s s i p a t e d i n p l a s t i c d e f o r m a t i o n o f t h e r e s e r v o i r r o c k (MEDLIN & MASSE 1986). P l a s t i c i t y g e n e r a l l y shows up i n f r a c t u r i n g r e c o r d s as abnormal t r e a t i n g pressu-
393 r e s and r e s u l t s i n c r a c k s which a r e s h o r t e r and w i d e r than normal f o r p u r e l y b r i t t l e b e h a v i o u r . As energy s p e n t i n p l a s t i c d e f o r m a t i o n ahead o f t h e f r a c t u r e t i p i n c r e a s e s i n p r o p o r t i o n t o c r a c k l e n g t h , p l a s t i c i t y e f f e c t s which m i g h t be n e g l i g i b l e i n small t r e a t m e n t s c o u l d become s i g n i f i c a n t i n v e r y l a r g e o r massive h y d r a u l i c f r a c t u r i n g t r e a t m e n t s . P l a s t i c zones, however, u s u a l l y do n o t comp r i s e whole r e s e r v o i r l a y e r s and s t o r e y s , b u t a r e c h a r a c t e r i z e d by n o n - u n i f o r m p a t c h y d i s t r i b u t i o n w i t h i n t h e p r o s p e c t i v e f o r m a t i o n . P l a s t i c i t y must g e n e r a l l y be more o r l e s s c o n t i n u o u s l y d i s t r i b u t e d t h r o u g h o u t an i n t e r v a l t o be f r a c t u r e d t o have s i g n i f i c a n t e f f e c t on s t i m u l a t i o n r e s u l t s , because p l a s t i c zones o f lim i t e d e x t e n t w i t h i n such h o r i z o n s a r e expected t o have o n l y l i t t l e e f f e c t s i n c e f r a c t u r e s s i m p l y propagate around them. P l a s t i c i t y i n s h a l e s p l a y s an i m p o r t a n t r o l e i n c o n t r o l l i n g f r a c t u r e h e i g h t . F r a c t u r e t r e a t m e n t s a r e f r e q u e n t l y c a r r i e d o u t i n b r i t t l e sandstone o r carbonat e i n t e r v a l s bounded by s o f t s h a l e s which a r e l i k e l y t o be v e r y p l a s t i c . F r a c t u r e s i n i t i a t e d i n b r i t t l e sandstones o r carbonates w i t h p l a s t i c s h a l e boundaries t e n d t o be c o n f i n e d t h e r e by p r o p a g a t i n g p r e s s u r e c o n s i d e r a t i o n s r e g a r d l e s s o f other effects.
4.2.3.2.3.2. Plasticity vs. non-linear elasticity T y p i c a l r e s e r v o i r r o c k s may d i s p l a y a wide v a r i e t y o f b e h a v i o u r . Many behave l i k e b r i t t l e m a t e r i a l s even a t t h e h i g h e s t c o n f i n i n g s t r e s s e s o f p r a c t i c a l i n t e r e s t , w h i l e o t h e r s a r e b r i t t l e a t low c o n f i n i n g s t r e s s b u t p l a s t i c t o some deg r e e a t h i g h e r c o n f i n i n g s t r e s s e s t y p i c a l o f deeper w e l l s . A few pay zones show sound degree of p l a s t i c i t y even a t modest c o n f i n i n g s t r e s s . D i s t i n c t i o n has t o be made between p l a s t i c i t y and n o n - l i n e a r e l a s t i c i t y which can be done by h y s t e r e s i s curves t h a t d e f i n e t h e amount o f r e c o v e r a b l e s t r a i n . H y s t e r e s i s r e s u l t s show t h a t s t r a i n beyond a w e l l - d e f i n e d y i e l d p o i n t u s u a l l y r e p r e s e n t s p l a s t i c d e f o r m a t i o n . Gradual bending o v e r t h e s t r e s s - s t r a i n c u r v e i s g e n e r a l l y due t o n o n - l i n e a r e l a s t i c d e f o r m a t i o n w i t h r e c o v e r a b l e s t r a i n . Most o f t h e r o c k mater i a l s showing s i g n i f i c a n t bending o f t h e s t r e s s - s t r a i n c u r v e s d i s p l a y consider a b l e creep a t f i x e d e l o n g a t i o n . L i m i t i n g v e r t i c a l f r a c t u r e h e i g h t s t r i c t l y t o t h e pay zone l e a d s t o more e f f i c i e n t and more p r o d u c t i v e c r a c k s which a l s o o f t e n e x t e n d deeper i n t o t h e f o r m a t i o n . B o t h upwards and downwards growth o f t h e f r a c t u r e can be r e a s o n a b l y cont r o l l e d i n o r d e r t o channel t h e c r a c k as designed i n t h e d i r e c t i o n o f t h e pay zone (DDWELL SCHLUMBERGER 1986). Shales w i t h f r a c t u r e g r a d i e n t s g r e a t e r than 1.0 p s i / f t do n o t behave e l a s t i c a l l y and processes such as c r e e p and p o s s i b l y f r a c t u r i n g a r e t h e dominant mechanisms c o n t r o l l i n g t h e s t r e s s s t a t e (WARPINSKI, BRANAGAN & WILMER 1985). I f sandstones and s i l t s t o n e s h a v i n g much l o w e r s t r e s ses w i t h f r a c t u r e g r a d i e n t s o f 0.8 - 0.9 p s i / f t a r e interbedded, c r a c k c o n t a i n ment can be e x p e c t e d due t o i n - s i t u s t r e s s c o n t r a s t s .
4.2.3.2.3.3. Plasticity effects on hydraulic f ractur ins H i g h degrees o f p l a s t i c i t y o f f o r m a t i o n r o c k s have t h r e e main e f f e c t s on hyd r a u l i c f r a c t u r i n g (SLUSSER & RIECKMANN 1976) c o m p r i s i n g c r e a t i o n o f w i d e r c r a c k s t h a n p r e d i c t e d b y c o n v e n t i o n a l designs based on b r i t t l e r o c k f a i l u r e , est a b l i s h m e n t o f s h o r t e r f r a c t u r e l e n g t h , and r e q u i r i n g i n c r e a s i n g s u r f a c e t r e a t i n g p r e s s u r e a t c o n s t a n t i n j e c t i o n r a t e d u r i n g t h e s t i m u l a t i o n j o b . Wider c r a c k s a r e a b e n e f i c i a l r e s u l t , because t h e y i n c r e a s e f r a c t u r e c o n d u c t i v i t y esp e c i a l l y i n t h e c r i t i c a l area near t h e w e l l b o r e , p a r t i c u l a r l y by r e d u c i n g t h e magnitude o f f r a c t u r e damage by p r o p p a n t c r u s h i n g and embedment ( c f . s e c t i o n 4.3.3.1.1.). The e x t r a w i d t h o b t a i n e d i n more p l a s t i c f o r m a t i o n s can be v e r y i m p o r t a n t i n m a i n t a i n i n g l o n g - t e r m p r o d u c t i o n improvements w h i c h o t h e r w i s e c o u l d be l o s t by f r a c t u r e c l o s u r e . On
t h e o t h e r hand, s h o r t e r f r a c t u r e l e n g t h i s a d e t r i m e n t a l e f f e c t o f r e s e r -
394 v o i r p l a s t i c i t y . P r o d u c t i v i t y improvements o b t a i n e d b y f r a c t u r i n g i n c r e a s e gener a l l y w i t h p r o p p e d c r a c k l e n g t h p r o v i d e d t h e c r a c k has a d e q u a t e c o n d u c t i v i t y . A s h o r t e r f r a c t u r e l e n g t h f o r a g i v e n s i z e s t i m u l a t i o n o p e r a t i o n , however, may r e s u l t i n s i g n i f i c a n t l y l e s s p r o d u c t i o n improvement and c o u l d t u r n t h e a n t i c i p a t e d s u c c e s s f u l j o b i n t o an u n e x p e c t e d e c o n o m i c a l f a i l u r e . I n c r e a s i n g t r e a t i n g p r e s s u r e s a r e a l s o an u n f a v o u r a b l e drawback, because h i g h e r b o t t o m - h o l e i n j e c t i o n p r e s s u r e s r e q u i r e d f o r f r a c t u r i n g o f t h e more p l a s t i c f o r m a t i o n s w i l l a l s o r e q u i r e c o r r e s p o n d i n g l y h i g h e r s u r f a c e i n j e c t i o n p r e s s u r e . As a r e s u l t , i n j e c t i o n r a t e s and p r e s s u r e s f o r deep t i g h t gas f r a c t u r i n g may be l i m i t e d b y t h e saf e t y p r e s s u r e r a t i n g s on w e l l h e a d and t u b u l a r goods.
4.2.3.2.4.
Fracture propagation across weakness planes
DANESHY ( 1 9 7 4 ) i n v e s t i g a t e s h y d r a u l i c f r a c t u r e p r o p a g a t i o n i n t h e p r e s e n c e o f weakness p l a n e s . L a r g e m a t e r i a l f l a w s a r e u n a b l e t o d r a s t i c a l l y change c r a c k orientation. Even i f t h e f r a c t u r e does n o t e n c i r c l e f o r m a t i o n d e f e c t s and o v e r r i d e s t h e i r p o s s i b l e e f f e c t , t h e i n f l u e n c e o f m a t e r i a l f l a w s i s l o c a l and i n s i g n i f i c a n t on t h e r e s u l t s o f t h e s t i m u l a t i o n t r e a t m e n t . D i s t i n c t i o n has t o be made between o p e n i n g , s l i d i n g and t e a r i n g mode o f f r a c t u r e p r o p a g a t i o n (IRWIN 1 9 5 7 ) . H y d r a u l i c f r a c t u r e s a r e u s u a l l y i n d u c e d b y t h e o p e n i n g mode, p o s s e s s a volume and a r e c r e a t e d b y normal s t r e s s e s and m a t e r i a l t e n s i l e f a i l u r e , whereas s l i d i n g - and t e a r i n g - m o d e f r a c t u r e s o r i g i n a t e b y s h e a r s t r e s s e s and have n o v o lume. S m a l l - and medium-scale open o r c l o s e d m a t e r i a l f l a w s c a n b e e a s i l y c r o s sed by p r o p a g a t i n g h y d r a u l i c f r a c t u r e s , w i t h t h e a d v a n t a g e o f weakness e x i s t e n ce being a r e d u c t i o n i n e f f e c t i v e rock surface energy t h a t consequently decreases t h e e n e r g y n e c e s s a r y f o r c r a c k e x t e n s i o n . A l t h o u g h t h e h y d r a u l i c f r a c t u r e c a n n o t d i r e c t l y c r o s s n a t u r a l j o i n t s due t o m a t e r i a l d i s c o n t i n u i t y , i t c a n e n c i r c l e t h e f l a w and t h u s m a i n t a i n i t s c o n t i n u i t y i f i t s d i m e n s i o n s a r e much l a r g e r t h a n t h a t o f t h e d e f e c t . I f t h e p l a n e o f weakness i s so l a r g e t h a t i t c a n n o t be e n c i r c l e d b y t h e h y d r a u l i c f r a c t u r e , the c r a c k may i n t e r s e c t t h e f l a w and m a i n t a i n i t s o r i g i n a l d i r e c t i o n , o r t h e h y d r a u l i c f r a c t u r e may r e o r i e n t i t s e l f and p r o p a g a t e a l o n g t h e weakness p l a n e . Once t h e h y d r a u l i c c r a c k has e s t a b l i s h e d i t s o r i e n t a t i o n , i t i s u n l i k e l y f o r i t t o u n d e r g o d r a s t i c a l r e o r i e n t a t i o n , because i t t a k e s l e s s f l u i d p r e s s u r e t o c o n t i nue t h e f r a c t u r e i n t h e same p l a n e t h a n t o s t a r t a new one i n a s u b s t a n t i a l l y d i f f e r e n t direction. I n severely naturally fractured reservoirs w i t h very high i t i s d i f f i c u l t t o predict hydraulic crack propagation j o i n t permeabilities, due t o t h e m u t u a l i n t e r f e r e n c e o f t h e many f l a w s , b u t t h i s r e l a t i o n s h i p does n o t s e r i o u s l y j e o p a r d i z e t h e v a l i d i t y o f h y d r a u l i c f r a c t u r e e x t e n s i o n modelling, since intensively naturally jointed formations are highly productive witho u t r e q u i r i n g h y d r a u l i c s t i m u l a t i o n f o r h y d r o c a r b o n o f f t a k e enhancement (DANESHY 1974; c f . s e c t i o n 4 . 8 . 8 . ) . AHMED, STRAWN, SCHMIDT, O'SHEA & VEGHTE ( 1 9 8 3 ) comment on s t i m u l a t i n g t i g h t sands i n t h e p r e s e n c e o f weak s t r e s s b a r r i e r s b y m o d i f i e d h y d r a u l i c f r a c t u r i n g t e c h n i q u e s . Weak i n t e r f a c e s and i n - s i t u s t r e s s d i f f e r e n c e s a r e t h e m o s t l i k e l y f a c t o r s t o c o n t a i n a p r o p a g a t i n g h y d r a u l i c f r a c t u r e (WARPINSKI, CLARK, SCHMIDT & HUDDLE 1982; c f . s e c t i o n s 4.2.2.3.1. and 4 . 2 . 2 . 5 . 3 . ) . W h i l e weak i n t e r f a c e s a r e p r o b a b l y e f f e c t i v e o n l y a t shallow depths, i n - s i t u s t r e s s d i f f e r e n c e s a r e Differenthe predominant c o n t r o l i n g r e a t e r depths ( c f . s e c t i o n 4.2.3.2.2.3.). ces i n e l a s t i c m o d u l i between s u c c e s s i v e l a y e r s may r e s u l t i n s t r e s s d i f f e r e n ces t h a t e i t h e r enhance o r d e t e r i o r a t e c r a c k g r o w t h i n t o t h e b o u n d i n g n o n - r e s e r v o i r s t r a t a . F r a c t u r e c o n t a i n m e n t may o f t e n o c c u r a t m a t e r i a l p r o p e r t y i n t e r f a c e s because t h e b o u n d a r y l a y e r i s a h i g h e r s t r e s s r e g i o n .
4.2.3.2.5.
Premature screenout termination
Premature screenout t e r m i n a t i o n o f t h e h y d r a u l i c s t i m u l a t i o n j o b i n t e r r u p t s f r a c t u r e p r o p a g a t i o n a t an u n e x p e c t e d and n o t d e s i g n e d p o i n t and i s g e n e r a l l y
395 an unwanted drawback by b e i n g an e a r l y f a i l u r e n o t c o r r e s p o n d i n g t o t h e t r e a t ment p l a n . Screenout o r i g i n a t e s by f r a c t u r i n g m a t e r i a l becoming s t u c k i n t h e c r a c k by p r o p p a n t b r i d g i n g and/or f l u i d s t i c k i n g due t o e x c e s s i v e c a r r i e r v i s c o s i t y o r s l u r r y d e h y d r a t i o n . F r a c t u r e blockage by proppant b r i d g i n g t a k e s p l a c e i f c r a c k w i d t h i s s m a l l e r than a t l e a s t 2 . 5 t o 3 t i m e s t h e proppant g r a i n diameFluid stickt e r (BEN NACEUR 1987; c f . s e c t i o n s 1.4.11.2.1. and 4.8.8.3.2.3.). i n g o r i g i n a t e s f r o m development o f f r i c t i o n between p a r t i c l e s and f r a c t u r e w a l l s i f t h e proppant s l u r r y i s s u b j e c t e d t o s u f f i c i e n t f l u i d l o s s t o a l l o w t h e suspended p r o p p a n t g r a i n s t o come i n t o c o n t a c t , w i t h t h e r e s u l t a n t s o l i d mass o f c l u s t e r e d proppants becoming immobile and f o r m i n g an o b s t a c l e i n t h e c r a c k . A c c e l e r a t e d s l u r r y d e h y d r a t i o n by leakage i n t o n a t u r a l f r a c t u r e s and f i s s u r e s i s a v e r y s e r i o u s reason l e a d i n g t o s c r e e n o u t f a i l u r e ( c f . s e c t i o n s 4.2.2.2.3., 4.2.3.5.3. and 6 . 2 . 4 . 2 . 1 . ) . The o u t l i n e as f o l l o w s focusses on aspects o f g e l degradation, i n s u f f i c i e n t f r a c t u r e w i d t h , and impact o f 100 mesh sand l e a d - i n stages.
4.2.3 2.5.1. Gel degradation The s t r u c t u r e o f most f r a c t u r i n g g e l s tends t o degrade and l o s e i t s proppant t r a n s p o r t and suspension a b i l i t y t o some degree w i t h b o t h i n c r e a s i n g temperatur e and t i m e (POULSEN & LEE 1984). A l t h o u g h some g e l s r e q u i r e e l e v a t e d temperatur e s t o a t t a i n t h e i r maximum degree o f c r o s s l i n k i n g , these f l u i d s a l s o degrade w i t h s u f f i c i e n t temperature and t i m e ( c f . s e c t i o n 4.3.4.). Since f l u i d temperat u r e and r e s i d e n c e t i m e i n t h e f r a c t u r e b o t h i n c r e a s e w i t h d i s t a n c e f r o m t h e wellbore, i t f o l l o w s t h a t the c l o s e r the f l u i d i s t o the crack t i p , the less p r o p p a n t - c a r r y i n g a b i l i t y i t has. T h e r e f o r e f r e q u e n t l y p r o p p a n t b r i d g i n g and b l o c k i n g t a k e s p l a c e a t t h e f r a c t u r e t i p p r o v o k i n g premature o r submature screenout t e r m i n a t i o n o f t h e j-o b (. c f . a l s o s e c t i o n s 4.2.2.2.3.. 4.2.3 5.3. and 6.2.4.2.1.).
4.2.3.2.5.2. Insufficient fracture width O t h e r causes i n c l u d e i n s u f f i c i e n t f l u i d volume and v i s c o s i t y mode l i n g as w e l l as u n s a t i s f a c t o r y f r a c t u r e opening, w i t h t h e c r e a t e d w i d t h n o t r e a c h i n g t h e designed v a l u e . F r a c t u r e s p r o p a g a t i n g f r o m l o w - s t r e s s sandstone r e s e r v o i r s i n t o t h e a d j o i n i n g h i g h - s t r e s s mudstone boundaries s u f f e r f r o m t h e f a c t t h a t t h e r e l a t i v e c r a c k w i d t h i s v e r y narrow i n t h e mudstone s e c t i o n and t h e r e f o r e t o p and b o t t o m screenout can o c c u r . On t h e o t h e r hand, p r o p e r f r a c t u r e t r e a t ment p l a n n i n g and o p e r a t i o n can implement s o p h i s t i c a t e d t e c h n i q u e s f o r v e r t i c a l growth containment and l a t e r a l e x t e n s i o n l i m i t i n g by crack blockage w i t h l i g h t buoyant proppants a t t h e t o p o r heavy s e t t l i n g proppants a t t h e b o t t o m ( c f . sect i o n 4.2.2.4.) and t i p p l u g g i n g ( c f . s e c t i o n 4.8.4.3.), respectively, representi n g a r t i f i c i a l and c o n t r o l l e d s c r e e n o u t i n t e r r u p t i o n s o f f r a c t u r e p r o p a g a t i o n i n a determined d i r e c t i o n . Premature a c c i d e n t a l s c r e e n o u t t e r m i n a t i o n i s a l s o caused by f a i l u r e t o use a p r o p e r l y v i s c o s i f i e d base f l u i d w i t h adequate l e a k o f f and foam s t a b i l i t y c h a r a c t e r i s t i c s e s p e c i a l l y i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s (SATTLER, RAIBLE, GALL & GILL 1988). I n case o f a p r o p e r l y executed s t i m u l a t i o n j o b , unexpected s c r e e n o u t t e r m i n a t i o n g i v e s an i n d i c a t i o n o f t h e optimum t r e a t m e n t s i z e f o r t h e i n d i v i d u a l r e s e r v o i r and i s t h e r e f o r e a p o s i t i v e i n f o r m a t i o n b e i n g a r e a l i s t i c i n d i c a t o r o f nat u r a l pay zone geometry l i m i t a t i o n s and t h e i r impact on h y d r a u l i c p e r m e a b i l i t y enhancement. U n f o r t u n a t e l y , however, t h e u s e f u l aspect o f screenouts i s o f t e n o b l i t e r a t e d by j o b f a i l u r e due t o poor o p e r a t i o n a l c o n t r o l o r a p p l i c a t i o n o f i n s u f f i c i e n t l y c l e a n f l u i d s , w i t h these drawbacks b e i n g the i n s e r t i o n p o i n t o f maj o r e f f o r t s o f i m p r o v i n g s t i m u l a t i o n performance.
396
4.2.3.2.5.3. Impact of 100 mesh sand lead-in stages I n composed t r e a t m e n t s c o n s i s t i n g o f 100 mesh l e a d - i n stage and 20/40 mesh main l o t o f proppants, r a d i c a l i n c r e a s e s i n p r e s s u r e s l o p e t e s t i f y t o b e g i n n i n g o f b r i d g i n g i n t h e reduced w i d t h p o r t i o n o f t h e f r a c t u r e i n t h e l o w e r and upper b a r r i e r s (BEGNAUD & CLAIBORNE 1 9 8 5 ) . B r i d g i n g i s due t o r e d u c t i o n i n w i d t h r e l a t e d t o t h e d i f f e r e n c e i n r o c k p r o p e r t i e s and h o r i z o n t a l s t r e s s e s between pay zone and b a r r i e r s , w i t h n a t u r a l j o i n t s w i t h i n t h e pay zone t e n d i n g t o s i g n i f i c a n t l y reduce f r a c t u r e w i d t h w i t h i n t h e r e s e r v o i r due t o l e a k o f f . B r i d g i n g leads t o c o n t r o l l e d h e i g h t growth w i t h c r a c k e x t e n s i o n o c c u r r i n g w i t h a concurr e n t i n c r e a s e i n n e t p r e s s u r e w h i c h i s c o n c e p t u a l l y due t o t h e i n c r e a s e i n f r a c t u r e p r o p a g a t i o n p r e s s u r e along t h e c r a c k f a c e . When the s m a l l spacer pad o f c r o s s l i n k e d g e l r u n ahead o f t h e 100 mesh stage l e a k e d o f f , t h e 100 mesh stage dehydrates a t t h e f r a c t u r e t i p t h e r e b y p r o v o k i n g a screenout ( a s p e c t s o f 100 mesh sand l e a d - i n stages a r e a l s o d i s c u s s e d i n sect i o n s 1.4.11.2.1. and 4 . 8 . 8 . 3 . 2 . 3 . ) . Premature s c r e e n o u t t e r m i n a t i o n can be avoided by moving t h e 100 mesh stage f u r t h e r back i n t h e pad. Screenout f a i l u r e can a l s o be due t o opening o f secondary f r a c t u r e s ( c f . a l s o NOLTE & SMITH 1981; c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) . The e f f e c t o f a l l e g e d b r i d g i n g d i m i n i s h e s p r o p p a n t conc e n t r a t i o n i n b a r r i e r and r e s e r v o i r t h u s s e v e r e l y r e d u c i n g e f f e c t i v e c o n d u c t i v i t y t h r o u g h t h e b a r r i e r i f indeed any m i g h t e x i s t (BEGNAUD & CLAIBORNE 1985).
4.2.3.2.6. Stress differences and containment F r a c t u r e p r o p a g a t i o n under s t r e s s containment i s c h a r a c t e r i z e d by two asympt o t i c r e g i o n s (THIERCELIN, BEN NACEUR & LEMANCZYK 1 9 8 5 ) . A t t h e b e g i n n i n g o f p r o p a g a t i o n , c r a c k shape e v o l u t i o n i s c l o s e t o r a d i a l e x t e n s i o n and t h e f r a c t u r e i s n o t c o n t a i n e d . A f t e r a g i v e n time, good c r a c k containment i s achieved by decreasing h e i g h t growth and i n t u r n i n c r e a s i n g l e n g t h e x t e n s i o n , e s p e c i a l l y when f r a c t u r e h e i g h t i s almost c o n s t a n t f o r 50 % o f c r a c k l e n g t h . The t i p o f t h e e x t e n d i n g f r a c t u r e i s f r e q u e n t l y o f e l l i p t i c a l shape a c c o r d i n g t o t h e l e a d i n g edge concept ( S E T T A R I & CLEARY 1986). F r a c t u r e h e i g h t near t h e w e l l b o r e i s o f t e n somewhat s m a l l e r than t h a t f a r t h e r away f r o m t h e borehole, which i s r e l a t e d t o t h e s t r e s s c o n c e n t r a t i o n near t h e w e l l b o r e t h a t i s always a c t i v e near t h e v e r t i c a l end o f t h e c r a c k (THIERCELIN, BEN NACEUR & LEMANCZYK 1985). H y d r a u l i c f r a c t u r e s u s u a l l y t e n d t o move upwards due t o i n c r e a s i n g o v e r b u r den s t r e s s w i t h d e p t h t h a t l i m i t s downwards crack p r o p a g a t i o n , a l t h o u g h i n many i n s t a n c e s , f r a c t u r e s have shown a preponderance o f downwards growth (LAMBERT, DOLAN & GALLUS 1983; P A I , G A R B I S & HALL 1983). F r a c t u r e h e i g h t i s g e n e r a l l y r e l a t e d t o competence, t h i c k n e s s and hardness o f t h e mudstone b a r r i e r , and i s a l s o i n f l u e n c e d by t h e n a t u r e o f t h e cement bond. Some comments a r e o f f e r e d as f o l l o w s on f l u i d b r e a k t h r o u g h v s . i n j e c t i o n p r e s s u r e and p r o p p a n t b r i d g i n g vs. barrier stability.
4.2.3.2.6.1. Fluid breakthrough vs. injection pressure BEGNAUD & CLAIBORNE (1985) r e p o r t examples o f f l u i d b r e a k t h r o u g h f r o m one hor i z o n through t h e bounding seal i n t o another r e s e r v o i r s t o r e y as a consequence o f h i g h i n j e c t i o n p r e s s u r e s d u r i n g f r a c t u r i n g o p e r a t i o n . F l u i d communication e x i s t e d d u r i n g course o f t h e s t i m u l a t i o n j o b , b u t n o t a f t e r t h e f r a c t u r i n g j o b due t o l a c k o f proppant s u p p o r t o f t h e p o r t i o n o f t h e crack r u n n i n g t h r o u g h t h e s e p a r a t i n g b a r r i e r . The b r e a k t h r o u g h can be c o n f i r m e d by p o s t - f r a c t u r e gammar a y h e i g h t m o n i t o r i n g ( c f . s e c t i o n 6 . 2 . 1 . 3 . ) which r e v e a l s t h a t v a r y i n g amounts o f proppants r e f l e c t i n g l e s s e r r a d i a t i o n i n t e n s i t y l e v e l s were t r a n s p o r t e d through t h e b a r r i e r i n t o t h e o t h e r pay zone l e v e l . The e f f e c t o f a l l e g e d b r i d g i n g d i m i n i s h e s proppant c o n c e n t r a t i o n i n b a r r i e r and r e s e r v o i r , t h e r e b y s e r i o u s l y r e d u c i n g e f f e c t i v e c o n d u c t i v i t y through t h e b a r r i e r .
397
4.2.3.2.6.2. Proppant bridging vs. barrier stability I n terms o f p r o p p a n t b r i d g i n g vs. b a r r i e r s t a b i l i t y , some bounding s e a l s a r e o n l y m a r g i n a l l y e f f e c t i v e i n f r a c t u r e h e i g h t containment u n l e s s p r o p p a n t b r i d g i n g p r e v e n t s c r a c k p r o p a g a t i o n p r e s s u r e f r o m r e a c h i n g t h e f r a c t u r e t i p (BEGNAUD & CLAIBORNE 1985). I n case o f complete b a r r i e r breakthrough, p r o p p a n t b r i d g i n g m i g h t a l s o p r e v e n t c r a c k growth i n t o t h e lower s t r e s s pay zone on t h e o p p o s i t e s i d e o f t h e boundary s e a l . E a r l y p r o p p a n t b r i d g i n g improves containment and helps t o prevent f r a c t u r e propagation through the b a r r i e r i n t o the adjacent res e r v o i r s t o r e y . I f t h e c r a c k has n o t c o m p l e t e l y grown through t h e b a r r i e r a t t h e t i m e t h e p r o p p a n t i s i n t r o d u c e d , then b r i d g i n g i n t h e reduced w i d t h may p r e v e n t f r a c t u r e p r o p a g a t i o n p r e s s u r e f r o m r e a c h i n g t h e c r a c k e d edge i n t h e bound a r y seal t h u s i n h i b i t i n g e x t e n s i o n i n t o t h e n e i g h b o u r i n g p r o s p e c t i v e i n t e r v a l . Regardless o f t h e e f f e c t i v i t y o f p r o p p a n t b r i d g i n g , i t seems t h a t two a d j o i n i n g pay i n t e r v a l s separated by a b a r r i e r can u s u a l l y n o t be e f f e c t i v e l y propped w i t h a f r a c t u r e i n i t i a t i n g f r o m o n l y one o f them.
4.2.3.2.6.3. Other aspects Experimental approaches o f f r a c t u r e p r o p a g a t i o n m o d e l l i n g a r e a l s o c a r r i e d o u t by TEUFEL & CLARK (1981); HANSON, ANDERSON, SHAFFER & THORSON (1982); WARP I N S K I , CLARK, SCHMIDT & HUDDLE (1982); AHMED, WILSON & STRAWN (1983); PAPADDPOULOS, NARENDRAN & CLEARY (1983) and MEDLIN & MASSE (1984). F r a c t u r e h e i g h t o r w i d t h depending on l e n g t h p r o p a g a t i o n models a r e presented, compared and/or d i s cussed by PALMER & CARROLL (1982, 1983); CLEARY, KECK & MEAR (1983); ABOUSAYED, CLIFTON, DOUGHERTY & MORALES (1984); PALMER & LUISKUTTY (1985) and CROCKETT, OKUSU & CLEARY (1986). Aspects o f f r a c t u r e p r o p a g a t i o n a r e a l s o d i s cussed by BARENBLATT (1962), DUNLAP (1963), HOWARD & FAST (1970 b ) ; ABE, MURA & KEER (1976) ; DANESHY (1976), CLEARY (1978), T R A V I S (1981) ; CLEARY, NARENDRAN, SETTARI & WONG (1982); BARREE (1983), SETTARI (1985); THIERCELIN, BEN NACEUR & LEMANCZYK (1985) and EL RABAA ( 1 9 8 7 ) . RODRIGUEZ, HORNE & CINCD-LEY (1984) r e p o r t p a r t i a l l y penetrating fractures.
4.2.3.3. Shear slippage and tensile failure F r a c t u r e p r o p a g a t i o n and opening o f j o i n t s i s c o n t r o l l e d by two d i f f e r e n t r o c k mechanical processes c o m p r i s i n g shear s l i p p a g e and t e n s i l e f a i l u r e (MURPHY & FEHLER 1986; c f . a l s o s e c t i o n s 4.2.3.4.4. and 6 . 2 . 2 . 3 . ) . I n t e r a c t i v e c o u p l i n g o f f l u i d dynamics w i t h r o c k s t r e s s e s and d e f o r m a t i o n s r e v e a l s two t y p e s o f s t i m u l a t i o n p a t t e r n s . I n t h e f i r s t case, low f r i c t i o n a l r e s i s t a n c e t o shear s l i p page and l a r g e maximum d i l a t a n c y t o shear s l i p p a g e g i v e s r i s e t o c r e a t i o n o f o n l y a s i n g l e j o i n t which i s i n agreement w i t h t h e c o n v e n t i o n a l h y d r a u l i c f r a c t u r i n g t h e o r y t h a t a s i n g l e c r a c k opens a t a p r e s s u r e equal t o t h e minimum e a r t h s t r e s s (DANESHY 1973 a ) . I n t h e second case c o r r e s p o n d i n g t o h i g h shear r e s i s t a n c e o r s m a l l d i l a t a n c y , however, m u l t i p l y j o i n t s t i m u l a t i o n o c c u r s . Shear s l i p p a g e a l o n g t h e c r a c k s i s accompanied by s h e a r - s t r e s s drops, and t h e i n t e r a c t i o n o f these s t r e s s drops w i t h t h e a c t i n g e a r t h s t r e s s e s r e s u l t s i n opening o f j o i n t s more p e r p e n d i c u l a r t o t h e maximum s t r e s s , t h u s f o r m i n g a dend r i t i c o r branched p a t t e r n (MURPHY & FEHLER 1986; c f . s e c t i o n 4 . 8 . 7 . 3 . ) . Comments on j o i n t d e f o r m a t i o n p r i n c i p l e s and i n f l u e n c e o f f l u i d v i s c o s i t y a r e o f f e r e d as f o l l o w s .
4.2.3.3.1. Joint deformation principles J o i n t opening o r d i l a t i o n b e h a v i o u r f o r shear s l i p p a g e and s e p a r a t i o n by t e n s i l e f a i l u r e i s q u i t e d i f f e r e n t . With r i s i n g f l u i d pressure, d i l a t i o n i s f i r s t small and s i m p l y r e s u l t s f r o m d e c r e a s i n g e f f e c t i v e c l o s u r e s t r e s s . A t h i g h e r pressures, shear s l i p p a g e o f j o i n t s u r f a c e s occurs u n t i l one l a r g e roughness as-
398 p e r i t y l i e s a t o p another and f u r t h e r slippage would a l l o w the l a r g e s t a s p e r i t y to s l i d e over a n d down the o t h e r , with t h i s being a n a t u r a l l i m i t t o shear d i l a t i o n which i s maximally i n the o r d er of a f r a c t i o n of a m illim e te r (BARTON, BANDIS & BAKHTAR 1985). Further increasing j o i n t pressure leads t o se pa ra tion of the crack w a l l s , w i t h d i l a t i o n being t y p i c a l l y in the range of tens of m i l l i m e t e r s (PERKINS & K E R N 1961, DANESHY 1973 a ) , thereby being many times t h a t of shear d i l a t i o n .
I n a n actual hydraulic f r a c t u r i n g operation, the e n t i r e spectrum of j o i n t deformation comprising shear slippage and t e n s i l e separation occurs ( M U R P H Y & FEHLER 1986). Near the i n j ect i o n well where the flow passage area i s l i m i t e d , f l u i d v e l o c i t i e s and pressure g r ad i en t s ar e l a rge a n d t e n s i l e f a i l u r e of the rock gives r i s e t o crack s ep ar at i o n . Near the t i p s of the j o i n t s f a r away from the i n j e c t i o n w e l l , however, pressures a n d v e l o c i t i e s of the f l u i d a re much reduced, a n d shear slippage takes place.
4.2.3.3.2. Influence of fluid viscosity D i s t i n c t i o n has t o be made between low- a n d high-viscosity f l u i d s . High-visc o s i t y f l u i d s a r e t y p i c a l l y used i n hydrocarbon r e s e r v o i r stim ula tion a n d a re in j e c t e d a t high r a t e s . Therefore j o i n t s e pa ra tion i s dominant, a n d i f only a few n a t u r a l cracks ar e present a s f r eq u en t l y happens t o be the case i n petroleum-bearing formations, actual f r a c t u r i n g with c re a tion of new j o i n t planes by t e n s i l e f a i l u r e of i n t a c t rock occurs. Geothermal h o t dry rock r e s e r v o i r s , howe v e r , a r e o f t e n i n t en s el y n a t u r a l l y j o i n t ed , a n d high downhole temperatures render most of the v i s co s i f y i n g agents of f r a c t u r i n g f l u i d s use le ss. Thus water i s used i n geothermal f r a c t u r i n g and t h i s low-viscosity f l u i d gives r i s e t o dominat i n g shear slippage by moving and separation of already pre -e xisting cracks without opening of new f r a c t u r e s by actual rupturing of the rock ( c f . se c tion 4.7.6.).
4.2.3.4. Inf hence of geological discontinuities The widely held assumption t h a t the hydraulic f r a c t u r e s a re ideal planar fe a tu r e s of usually constant height i s probably untenable i n many r e s e r v o i r s because of geological d i s c o n t i n u i t i e s (WARPINSKI & TEUFEL 1984). Geological d i s c o n t i n u i t i e s such a s j o i n t s ( c f . p l a t e s I / l - 6 , IV/4 - 8 , VII/3 - 4, VIII/1 - 2 and XI/4 - 6), f a u l t s ( c f . p l a t e XI/3), bedding planes ( c f . p l a t e s 11, I I I / 1 - 2, IV, V I I , V I I I a n d IX) a n d s t r e s s c o n t r a s t s a re u b i q u i t o u s f e a t u r e s whose e f f e c t on propagation of hydraulic f r a c t u r e s depends on a n c i l l a r y treatment and such r e s e r v o i r parameters as operating pressure, i n - s i t u s t r e s s e s , o r i e n t a t i o n of d i s c o n t i n u i t i e s , a n d permeability. Geological d i s c o n t i n u i t i e s influence the overall geometry a n d e f f e c t i v e n e s s of the hydraulic f r a c t u r e by stopping v e r t i c a l propagation due t o containing action of i n - s i t u s t r e s s d i s t r i b u t i o n , material property v a r i a t i o n s , bounding shale p l a s t i c i t y , a n d cohesionless o r unbonded i n t e r f a c e s ( c f . a l s o ANDERSON 1981); a r r e s t i n g l a t e r a l propagation across f a u l t s ( c f . p l a t e XI/3) and sand lens boundaries ( c f . p l a t e s II/7 - 8, V I I / l , V I I I / 7 a n d IX/8) where s t r e s s e s may i n c r e a se , reducing t o t a l length by f l u i d le a koff, and f a c i l i t a t i n g o r i g i n of m u l t i p l e p a r a l l e l - f r a c t u r e systems, hindering p r o p p a n t t r a n s p o r t a n d placement because of non-planarity of the s i n g u l ar f r a c t u r e or multiple crack system, a n d inducing additional height growth from higher t r e a t i n g pre ssure s. The result may range from n e g l i g i b l e t o cat as t r o phic depending on the values of t h e a n c i l l a r y parameters. The most important aspects a r e f r a c t u r e branching, impact of f a u l t s and j o i n t s , i n t e r a c t i o n between hydraulic f r a c t u r e s a n d geological d i s c o n t i n u i t i e s , shear slippage vs. t e n s i l e f a i l u r e , and consequences f o r f r a c tu r e morphology which ar e s h o r t l y i l l u s t r a t e d as follows.
399
4.2.3.4.1. Fracture branching P h y s i c a l e x c a v a t i o n o f r o c k s i n o r d e r t o observe t h e f r a c t u r e d i r e c t l y by m i neback (TEUFEL & WARPINSKI 1983, WARPINSKI & TEUFEL 1984) documents t h a t t h e i n - s i t u s t r e s s d i s t r i b u t i o n has t h e o v e r r i d i n g i n f l u e n c e on h y d r a u l i c f r a c t u r e p r o p a g a t i o n and containment ( c f . a l s o s e c t i o n s 4..2.2.3.1. and 4.2.3.2.2.). F r a c t u r e containment f r o m a h i g h - s t r e s s r e g i o n i s a f i r s t - o r d e r e f f e c t , whereas i n t e r f a c e s , modulus o r s t r e n g t h changes, f l u i d p r e s s u r e g r a d i e n t s , and most bedding planes have o n l y second-order e f f e c t s on h e i g h t growth. A f t e r a s h o r t c h a r a c t e r i z a t i o n o f f r a c t u r e s t r a n d i n g and meandering, some f a c t o r s i n f l u e n c i n g crack branching are o u t l i n e d .
4.2.3.4.1.1. Fracture stranding and meandering The h y d r a u l i c f r a c t u r e s i n t h e mineback s e c t i o n s a r e f r e q u e n t l y observed t o d i v e r g e c o n s i d e r a b l y f r o m t h e usual p i c t u r e o f a p l a n a r f e a t u r e , w i t h m u l t i p l e s t r a n d i n g , f r a c t u r e meandering ( c f . p l a t e XI/3), sharp t u r n s and c o r n e r s , and l a r g e - s c a l e roughness and i r r e g u l a r morphology o f f r a c t u r e s u r f a c e s b e i n g common occurrences. I n t h e v i c i n i t y o f g e o l o g i c a l d i s c o n t i n u i t i e s , there i s f r e q u e n t l y n o t h i n g t h a t can be c a l l e d a p l a n a r f r a c t u r e , and t h e c r a c k complex can be b e t t e r d e s c r i b e d as a zone o f m u l t i p l e f r a c t u r i n g which i n extreme cases a t t a i n s 15 - 30 f t (5 - 10 m) w i d t h . F r a c t u r e b r a n c h i n g t h e r e f o r e i s a t l e a s t i n some r e s e r v o i r s a common phenomenon. Mineback v i s u a l i z a t i o n o f n a t u r a l and hyd r a u l i c a l l y - i n d u c e d f r a c t u r e s i s a l s o c a r r i e d o u t by LAYNE & BYRER (1986). Pressure growth c o m p r i s i n g a b n o r m a l l y l a r g e i n c r e a s e s i n t r e a t i n g p r e s s u r e l e a d s t o premature s c r e e n o u t t e r m i n a t i o n ( c f . s e c t i o n s 4.2.2.2.3., 4.2.3.2.5. and 6.2.4.2.1.) and t h e r e f o r e has adverse i n f l u e n c e s on f r a c t u r e e f f e c t i v e n e s s (MEDLIN & FITCH 1983). The most i m p o r t a n t reason f o r p r e s s u r e growth i s f r a c t u r e b r a n c h i n g a t r e s e r v o i r h e t e r o g e n e i t i e s o r n a t u r a l cracks, w i t h t h e m o s t prominent response t o p r e s s u r e growth b e i n g a l a r g e i n c r e a s e i n e f f e c t i v e f r a c t u r e w i d t h . F r a c t u r e b r a n c h i n g i s c o n s i d e r a b l y a l t e r i n g t h e normal mechanics o f c r a c k p r o p a g a t i o n . F r a c t u r e growth no l o n g e r c o n s i s t s o f a s i m p l e opening o f t h e c r a c k f a c e s a g a i n s t t h e minimum e a r t h s t r e s s , b u t s l i p m o t i o n i s i n t r o d u c e d a l o n g many o f t h e branch faces which generates f r i c t i o n t h a t i n c r e a s e s t h e energy needed t o s u s t a i n c r a c k growth.
4.2.3.4.1.2. Factors influencing crack branching As t h e a d d i t i o n a l energy must be s u p p l i e d by a r i s e i n f l u i d p r e s s u r e w i t h i n t h e branched c r a c k network, f r a c t u r e b r a n c h i n g leads t o a n a t u r a l i n c r e a s e i n t r e a t i n g p r e s s u r e . Once t h e i n i t i a l s i n g u l a r h y d r a u l i c f r a c t u r e has branched r e p e a t e d l y t o f o r m a l a r g e network o f small f r a c t u r e s , a t some d i s t a n c e f r o m t h e w e l l b o r e t h e d e n s i t y o f branched c r a c k s i s l a r g e enough t o approach a more o r l e s s homogeneous system when viewed on a l a r g e r s c a l e . Under such c o n d i t i o n s , f r a c t u r e e x t e n s i o n v i r t u a l l y ceases because o f t h e e x c e s s i v e f r i c t i o n a s s o c i a t e d w i t h t h e many branch crack faces, w i t h c o n t i n u e d f l u i d i n j e c t i o n then r e s u l t i n g i n b a l l o o n i n g o f t h e main f r a c t u r e w i t h a severe p r e s s u r e i n c r e a s e . F r a c t u r e b r a n c h i n g i s a f u n c t i o n o f pumping r a t e , w i t h low i n j e c t i o n r a t e s m i n i m i z i n g o r even t o t a l l y suppressing c r a c k s p l i t t i n g . Extremely h i g h p r o p u l s i o n r a t e s such as o c c u r r i n g d u r i n g e x p l o s i v e f r a c t u r i n g ( c f . s e c t i o n 4.7.3.) on t h e o t h e r hand g i v e r i s e t o immediate p r o p a g a t i o n o f a complex network o f branched s m a l l c r a c k s and may c o m p l e t e l y i n h i b i t f o r m a t i o n o f a s i n g u l a r main f r a c t u r e stem. Pressure growth produces o n l y l i t t l e v e r t i c a l f r a c t u r e e x t e n s i o n and i n d i c a t e s t h a t crack h e i g h t seems t o be c o n t r o l l e d more by p e r f o r a t e d i n t e r v a l l e n g t h t h a n by p r e s s u r e b e h a v i o u r . F r a c t u r e s g e n e r a l l y appear t o remain w e l l contained w i t h i n perforated i n t e r v a l s i n s p i t e o f l a r g e pressure increases.
400
4.2.3.4.2. Impact of faults and joints I n n a t u r a l l y f r a c t u r e d r e g i o n s , m u l t i p l e s t r a n d i n g o f t h e c r a c k s i s predominant, w i t h s i n g u l a r c r a c k s o n l y seldom h a v i n g been observed. F r a c t u r e s a r e u s u a l l y o f f s e t a t j o i n t s where sometimes two o r t h r e e f r a c t u r e branches a r e i n i t i a t e d when c r o s s i n g t h e n a t u r a l c r a c k s and becoming o f f s e t . Some o f t h e f r a c t u r e strands d i e o u t i n a s h o r t distance, w h i l e others p e r s i s t f o r long distances and d i v i d e and coalesce many times a l o n g t h e f r a c t u r e l e n g t h . E v o l u t i o n , scale and t o t a l e f f e c t o f t h e m u l t i p l e f r a c t u r e zone depends t o a l a r g e degree on t h e a n c i l l a r y parameters. F a u l t s a l s o e x h i b i t a s i g n i f i c a n t i n f l u e n c e on f r a c t u r e geometry and morphol o g y . F r a c t u r e s o f t e n t e r m i n a t e near f a u l t s , and i n case t h e y propagate across t h e f a u l t , t h e i r o r i e n t a t i o n s f r e q u e n t y change, which can be i n v a r i a b l y a t t r i b u t e d t o a change i n s t r e s s e s across t h e f a u l t and n o t t o t h e f a u l t p l a n e i t s e l f . Concerning t h e impact o f bedding o r p a r t i n g planes, two e f f e c t s a c t i n c o n c e r t . There i s s u f f i c i e n t f r i c t i o n on t h e p a r t i n g p l a n e t o a l l o w t h e f r a c t u r e t o r e i n i t i a t e on t h e o t h e r s i d e , b u t n o t enough t r a n s m i t t e d s t r e s s t o a l l o w i t t o propagate e a s i l y i n t h e h i g h e r s t r e s s l a y e r above. Thus t h e two f a c t o r s o f bedd i n g p l a n e and s t r e s s c o n t r a s t t o g e t h e r p r o v i d e an e x c e l l e n t containment f e a t u re. A l t h o u g h g e o l o g i c a l d i s c o n t i n u i t i e s may c o n s i d e r a b l y a l t e r f r a c t u r e geomet r y , t h e i r e f f e c t on proppant t r a n s p o r t may even be more severe. F o r c i n g sand through such f e a t u r e s as narrow m u l t i s t r a n d f r a c t u r e s o r o f f s e t s w i l l be much more d i f f i c u l t than t h r o u g h a w i d e r s i n g u l a r h y d r a u l i c f r a c t u r e . The consequence w i l l c e r t a i n l y be h i g h e r f r a c t u r i n g p r e s s u r e s and q u i t e o f t e n s c r e e n o u t can o c c u r . When l o c a l s m a l l screenouts s t a r t t o t a k e p l a c e i n some branches o f t h e c r a c k system, t h e i n c r e a s i n g frequency o f these e v e n t s can u l t i m a t e l y p r o voke a complete screenout o f t h e h y d r a u l i c f r a c t u r e complex.
4.2.3.4.3. Interact ion between hydraulic fractures and geological discontinuities Three t y p e s o f i n t e r a c t i o n s between h y d r a u l i c f r a c t u r e s and p r e - e x i s t i n g j o i n t s as a r e p r e s e n t a t i v e o f g e o l o g i c a l d i s c o n t i n u i t i e s have been r e c o g n i z e d i n l a b o r a t o r y e x p e r i m e n t s . The h y d r a u l i c f r a c t u r e s e i t h e r c r o s s t h e j o i n t , a r e a r r e s t e d by opening and d i l a t i n g t h e j o i n t as i n d i c a t e d by f l u i d f l o w a l o n g t h e crack, o r a r e stopped by shear s l i p p a g e o f t h e j o i n t w i t h no d i l a t i o n and f l u i d f l o w along t h e c r a c k . The d i f f e r e n t types o f i n t e r a c t i o n s a r e a f u n c t i o n o f t h e h o r i z o n t a l d i f f e r e n t i a l s t r e s s and t h e a n g l e o f approach o f t h e h y d r a u l i c f r a c t u r e t o t h e p r e - e x i s t i n g j o i n t . The h y d r a u l i c f r a c t u r e s c r o s s t h e p r e - e x i s t i n g j o i n t s o n l y a t h i g h h o r i z o n t a l d i f f e r e n t i a l s t r e s s and a t angles o f approach of 60 degrees o r g r e a t e r , do n o t c r o s s t h e j o i n t s a t low h o r i z o n t a l d i f f e r e n t i a l s t r e s s and low angles o f approach p r i m a r i l y because t h e f l u i d p r e s s u r e i n t h e h y d r a u l i c f r a c t u r e i s s u f f i c i e n t t o open and d i v e r t f l u i d a l o n g t h e j o i n t s , and a r e a r r e s t e d by shear s l i p p a g e o f t h e p r e - e x i s t i n g j o i n t w i t h no opening and d i v e r t i n g o f f l u i d a l o n g t h e j o i n t o n l y a t h i g h d i f f e r e n t i a l s t r e s s and low angles o f approach ( c f . a l s o BLANTON 1 9 8 2 ) .
4.2.3.4.4. Shear slippage vs. tensile failure A complete a n a l y s i s o f t h e i n f l u e n c e o f g e o l o g i c a l d i s c o n t i n u i t i e s on f r a c t u r e p r o p a g a t i o n i s d i f f i c u l t because o f a h i e r a r c h y o f mechanisms a v a i l a b l e t o p e r t u r b t h e f r a c t u r e (WARPINSKI & TEUFEL 1984). C o n s i d e r i n g o n l y l e a k o f f , t h e i n i t i a l loss mechanism i s a r e s u l t o f p e r m e a b i l i t y o f t h e j o i n t s , f a u l t s o r bedd i n g p l a n e s . W i t h i n c r e a s i n g f l u i d p r e s s u r e i n t h e n a t u r a l c r a c k s , shear s l i p page may occur, which may i n c r e a s e p e r m e a b i l i t y and g i v e r i s e t o a s e l f - e n h a n c i n g process, and when f l u i d p r e s s u r e becomes g r e a t enough t o d i l a t e t h e
401 j o i n t s , t h e r e s u l t w i l l p r o b a b l y be c a t a s t r o p h i c l e a k o f f . The s i t u a t i o n f o r mult i p l e zones o f f r a c t u r i n g i s even more complex. Good j o i n t p e r m e a b i l i t y may l e a d t o s u f f i c i e n t f l u i d and p r e s s u r e i n t h e j o i n t t o a l l o w f l a w s t o a c t i v a t e and f o r m a m u l t i s t r a n d system. I n a d d i t i o n , shear s l i p p a g e - i f o c c u r r i n g - may enhance t h e p e r m e a b i l i t y , a l l o w more f l u i d t o e n t e r t h e s u b s i d i a r y s t r a n d s and t r i g g e r the o r i g i n o f a wider i n t e r v a l o f f r a c t u r i n g . I f t h e p r e s s u r e becomes l a r g e enough t o d i l a t e t h e j o i n t s , an e x t r e m e l y wide zone o f f r a c t u r i n g i s p o s s i b l e , and a d d i t i o n a l l y these e f f e c t s may o c c u r as t h e f r a c t u r e i n t e r s e c t s t h e d i s c o n t i n u i t y , o r a t much l a t e r t i m e s as f l u i d l e a k s o f f i n t o t h e j o i n t system. Thus t h e a c t u a l r e s u l t s s t r o n g l y depend on t h e a n c i l l a r y parameters of c r a c k p e r m e a b i l i t y , f r i c t i o n a l p r o p e r t i e s , i n - s i t u s t r e s s e s , j o i n t spacing and o r i e n t a t i o n w i t h r e s p e c t t o t h e s t r e s s f i e l d , t r e a t m e n t p r e s s u r e and f l u i d v i s c o s i t y . I n a l l cases where t h e f r a c t u r e t i p i s b l u n t e d by a j o i n t , t h e n a t u r a l c r a c k w i l l d i l a t e because t h e t r e a t m e n t p r e s s u r e i s g r e a t e r than t h e maximum s t r e s s , w i t h t h i s p r o b a b l y b e i n g t h e main reason f o r t h e o r i g i n o f a zone o f m u l t i p l e f r a c t u r i n g .
4.2.3.4.5. Consequences for fracture morphology G e o l o g i c a l d i s c o n t i n u i t i e s can t h e r e f o r e be t h e cause o f many adverse e f f e c t s d u r i n g a f r a c t u r e t r e a t m e n t . When t h e d i f f e r e n c e s between i n - s i t u s t r e s ses a r e small, g e o l o g i c a l d i s c o n t i n u i t i e s t h a t a r e f a v o u r a b l y o r i e n t e d t o b l u n t t h e c r a c k w i l l a l s o p r o b a b l y be d i l a t e d , g i v i n g r i s e t o a w i d e zone o f f r a c t u r i n g w i t h many p a r a l l e l s t r a n d s p r o p a g a t i n g s i d e by s i d e . When t h e d i f f e r e n c e s between i n - s i t u s t r e s s e s a r e l a r g e , j o i n t d i l a t i o n i s n o t expected, b u t shear s l i p p a g e o v e r an e x t e n s i v e area i s p o s s i b l e , l e a d i n g t o enhanced l e a k o f f , which i n a d d i t i o n may a l s o be s u f f i c i e n t t o cause some f u r t h e r secondary f r a c t u r i n g by a c t i v a t i n g f l a w s i t e s a l o n g t h e c r a c k . The presence o f h y d r a u l i c f r a c t u r e s s t a b i l i z e s t h e j o i n t s i n such a way t h a t once a f r a c t u r e crosses a j o i n t , d i l a t i o n o r shear s l i p p a g e becomes more d i f f i c u l t even though t h e i n c r e a s e d p o r e p r e s s u r e r e s u l t i n g f r o m l e a k o f f may be q u i t e l a r g e . Thus f r a c t u r e b r a n c h i n g i s p r o b a b l y a c r a c k - t i p phenomenon, and j o i n t s a l o n g t h e c r a c k l e n g t h a r e expected t o s t a r t opening up when some t h r e s h o l d p r e s s u r e i s reached a f t e r t h e f r a c t u r e has passed. These e f f e c t s more o r l e s s a p p l y f o r b o t h v e r t i c a l c r a c k systems and h o r i z o n t a l p l a n e s o f weakness.
4.2.3.5. Fracture propagat ion and pressure evolution F r a c t u r e p r e s s u r e m o n i t o r i n g and i n t e r p r e t a t i o n i s a v a l u a b l e means f o r det e r m i n a t i o n o f c r a c k p r o p a g a t i o n i n p r e d o m i n a n t l y q u a l i t a t i v e manner (NOLTE 1988 a, 1988 b ) . The most i m p o r t a n t t o o l i s t h e l o g - l o g p l o t o f n e t p r e s s u r e vs. t i m e o r p r e f e r a b l y vs. i n j e c t e d volume i f pumping r a t e v a r i e s . R e s t r i c t e d h e i g h t growth and u n l i m i t e d l a t e r a l p r o p a g a t i o n a r e r e f l e c t e d by a s t r a i g h t l i n e w i t h a small p o s i t i v e s l o p e i n t h i s c r o s s p l o t . N e a r l y c o n s t a n t p r e s s u r e a t c o n s t a n t i n j e c t i o n c o n d i t i o n s t e s t i f i e s t o a c c e l e r a t e d f l u i d l o s s t o opening nat u r a l f i s s u r e s o r s t a b l e h e i g h t growth a t moderate r a t e s . I n c r e a s i n g p r e s s u r e on a p p r o x i m a t e l y u n i t s l o p e r e f l e c t s r e s t r i c t e d f r a c t u r e e x t e n s i o n , w i t h t h e lim i t a t i o n o f c r a c k p e n e t r a t i o n b e i n g caused by p r o p p a n t b r i d g i n g o r e x c e s s i v e f l u i d v i s c o s i t y due t o s l u r r y d e h y d r a t i o n , w i t h b o t h e f f e c t s l e a d i n g u l t i m a t e l y t o premature s c r e e n o u t t e r m i n a t i o n o f t h e o p e r a t i o n ( c f . s e c t i o n s 4.2.2.2.3., 4.2.3.2.5. and 6.2.4.2.1.). Decreasing p r e s s u r e w i t h o u t changing i n j e c t i o n cond i t i o n s i n d i c a t e s u n s t a b l e h e i g h t growth o f runaway n a t u r e r e s u l t i n g f r o m v e r t i c a l f r a c t u r e e x t e n s i o n i n t o a n o t h e r l o w - s t r e s s zone a f t e r c r o s s i n g o f t h e h i g h stress b a r r i e r interval. F r a c t u r e p r e s s u r e m o d e l l i n g i s p a r t i c u l a r l y s i g n i f i c a n t f o r h e i g h t growth analysis through formations w i t h v a r i a b l e h o r i z o n t a l stress i n d i f f e r e n t stor e y s . H o r i z o n t a l s t r e s s e s i n mudstones a r e g e n e r a l l y h i g h e r t h a n i n sandstones.
402 As f r a c t u r e s t i f f n e s s i s i n v e r s e l y p r o p o r t i o n a l t o i n i t i a l zone h e i g h t ( w h i c h i n i d e a l c a s e s r e p r e s e n t s g r o s s t h i c k n e s s o f t h e r e s e r v o i r i n t e r v a l t o be f r a c t u r e d ) and n e t p r e s s u r e i s a l s o a p p r o x i m a t e l y dependent i n t h e same manner, n e t p r e s s u r e i s h i g h e r i n s m a l l e r h o r i z o n s t h a n t h a t i n l a r g e r s e c t i o n s . The r e s u l t a n t height o f the smaller i n t e r v a l i s generally greater than t h a t o f the l a r g e r s e c t i o n , whereas t h e a v e r a g e w i d t h o f t h e s m a l l e r zone i s u s u a l l y l e s s t h a n t h a t o f t h e l a r g e r segment. As a consequence o f n e g l i g i b l e f l u i d loss i n boundi n g mudstones, f r a c t u r e p e n e t r a t i o n f o r s m a l l e r sandstone i n t e r v a l s i s p r i n c i p a l l y g r e a t e r t h a n t h a t f o r l o n g e r r e s e r v o i r zones w i t h t h e same volume o f i n jected f l u i d .
4.2.3.6. Field experience o f required seal thickness I n t e r m s o f n e c e s s a r y o r r e q u i r e d minimum s e a l t h i c k n e s s t o a c h i e v e s a f e f r a c t u r e c o n t a i n m e n t , i n t h e MHF o p e r a t i o n i n t h e R o t l i e g e n d t i g h t gas s a n d s t o n e s i n t h e w e l l S o h l i n g e n Z 4 where a b t . 550 t o f b a u x i t e p r o p p a n t s were i n j e c t e d i n t o a f r a c t u r e w i t h a b t . 115 m ( 3 8 0 f t ) h e i g h t and a b t . 250 m ( 8 0 0 f t ) length, t h e 1 0 m ( 3 0 f t . ) t h i c k b o u n d i n g mudstone above and t h e 40 m ( 1 3 0 f t ) t h i c k f a n g l o m e r a t e b e l o w t h e f r a c t u r e d sandstone, r e s p e c t i v e l y , have p r o v e n t o have been s u f f i c i e n t f o r c o n t a i n i n g and l o n g i t u d i n a l l y c o n d u c t i n g e v e n such a l a r g e - s c a l e h y d r a u l i c f r a c t u r e (KLOSE & KROMER 1983, BLEAKLEY 1984; c f . a l s o s e c t i o n s 2.4.1.1. and 3 . 3 . 2 . ) .
4.2.4. Reservoir heterogeneity and anisotropy F r a c t u r e p r o p a g a t i o n i s g e n e r a l l y d i s t u r b e d b y l a t e r a l and v e r t i c a l r e s e r v o i r h e t e r o g e n e i t y and a n i s o t r o p y . W h i l e i n t e r n a l d i s c o n t i n u i t i e s o f s a n d s t o n e complexes such as b o u n d a r i e s between i n d i v i d u a l s e p a r a t e d o r amalgamated sandlenses s t o n e b o d i e s l i k e c h a n n e l s ( c f . p l a t e s I / 7 , V/3 - 4, V I I / 1 and V111/6), ( c f . p l a t e s I I / 7 - 8 ) and b a r s ( c f . p l a t e III/l- 2 ) , o n l a p ( c f . p l a t e I / 1 - 6 ) and o f f l a p f e a t u r e s , e r o s i o n a l c o n t a c t s and u n c o n f o r m i t i e s may b e o f o n l y m i n o r s i g n i f i c a n c e f o r t h e e x t e n s i o n of l a r g e f r a c t u r e s i n t h i c k sandstone s e r i e s , t h e y a r e q u i t e s e r i o u s l y a f f e c t i n g f l u i d f l o w (WEBER 1982) as w e l l as f r a c t u r e propagation i n t h i n sheet sandstone l a y e r s . R e s e r v o i r geometry m o d e l l i n g t h e r e f o r e has t o e v a l u a t e p r i o r t o t h e t r e a t m e n t w h e t h e r t h e t a r g e t s h e e t s a n d s t o n e i s more o r l e s s homogeneous and i s o t r o p i c , o r w h e t h e r i t i s o n l y an a p p a r e n t s h e e t body c o n s i s t i n g o f more o r l e s s d i s t i n c t e l e m e n t s t h a t a r e o n l y o v e r l a p p i n g o r m e r g i n g t o f o r m a s e m i c o n t i n u o u s d r a p e ( c f . MADER & YARDLEY 1 9 8 5 ) . I n o r d e r t o i m p r o v e h y d r a u l i c f r a c t u r i n g t e c h n o l o g y and t r e a t m e n t e f f i c i e n c y , i t i s e s s e n t i a l t o b e t t e r u n d e r s t a n d t h e o p e r a t i o n p r o c e s s e s and t h u s g e t enhanced knowledge o f g e o m e t r y and f a c i e s d i s t r i b u t i o n o f t h e r e s e r v o i r when hyd r a u l i c f r a c t u r i n g i s c a r r i e d o u t (HOLDITCH, ROBINSON & WHITEHEAD 1987; HOLDITCH, ROBINSON, WHITEHEAD & ELY 1 9 8 7 ) . V a r i o u s s e d i m e n t o l o g i c a l , p e t r o p h y s i c a l and r o c k m e c h a n i c a l a s p e c t s a r e d i s c u s s e d as f o l l o w s , and numerous d e p o s i t i o n a l and t e c t o n i c a l f e a t u r e s a r e i l l u s t r a t e d on p l a t e s I - X I ( c f . s e c t i o n 3 . 1 3 . ) .
4.2.4.1. Sedimentological aspects Depending on how p r o n o u n c e d t h e i n t e r u n i t b o u n d a r i e s a r e d e v e l o p e d , t h e f r a c t u r e s may be c o n t a i n e d w i t h i n i n d i v i d u a l d e p o s i t i o n a l sequences o r s a n d s t o n e b o d i e s and p r o p a g a t i o n a c r o s s t h e b o u n d a r y may be o n l y p o s s i b l e i n c a s e of a p p l i c a t i o n o f h i g h e r p r e s s u r e s and pumping r a t e s w h i c h a u t o m a t i c a l l y i n c r e a s e s t h e r i s k o f p e r f o r a t i n g a l s o t h e s e a l i n g mudstone h o r i z o n . I n t r a s h e e t h e t e r o g e n e i t i e s ( c f . p l a t e s III/l- 2, I V / 4 - 8 ) , V / 1 - 2, V I I / 3 - 6 and I X / 1 - 8 ) a r e a l s o i m p o r t a n t f o r t h e c o m m u n i c a t i o n between s e v e r a l w e l l s i n t e r m s o f w a t e r f l o o d i n g , c h e m i c a l i n j e c t i o n o r steam d r i v e , and i n c a s e o f c l e a r s e p a r a t i o n o f i s o l a t e d s a n d s t o n e b o d i e s ( c f . p l a t e s I I / 7 - 8, I V / 4 - 5, V/4 and V I I / l ) , p r o b a b l y i n t e r b a r o r transchannel communication f r a c t u r i n g ( c f . a l s o s e c t i o n
403 4.10.) is stances.
a s u i t a b l e s o l u t i o n , i f t e c h n i c a l l y p o s s i b l e under t h e g i v e n c i r c u m -
The presence o f mud l a y e r s w i t h s i g n i f i c a n t parameter and s t r e s s c o n t r a s t may cause problems such as s c r e e n o u t c o n d i t i o n s as t h e pad volume i s spent f o r f r a c t u r e i n i t i a t i o n i n weak s e c t i o n s and s u b d i v i s i o n o f t h e pay i n t e r v a l conc e r n i n g l o n g - t e r m p o s t - f r a c t u r e r e s e r v o i r performance p r e d i c t i o n as a consequence o f d i f f e r e n c e s i n c l o s u r e c h a r a c t e r i s t i c s between sandstones and mudstones (JOHN 1983). Aspects o f i n t e r - and i n t r a - s a n d body geometry as w e l l as m a t r i x and f r a c t u r e a n i s o t r o p y a r e b r i e f l y sketched as f o l l o w s .
4.2.4.1.1.
I n t e r - and intra-sand body geometry
Many s e d i m e n t o l o g i c a l mechanisms and f e a t u r e s o f sand b o d i e s ( c f . MADER 1985 a, 1985 b ) have n o t o n l y more o r l e s s pronounced i n f l u e n c e s on f l u i d f l o w i n r e s e r v o i r s (WEBER 1982), b u t have a l s o s i g n i f i c a n t impact on f r a c t u r e propagat i o n . I m p o r t a n t sandstone body aspects f o r optimum f r a c t u r e p l a n n i n g r e s p e c t i n g p o s s i b i l i t i e s o f d e f l e c t i o n o f s m a l l e r c r a c k s i n t h i n n e r sandstones by i n t e r n a l a n i s o t r o p i e s a r e p r i m a r y - d e p o s i t i o n a l d i s t r i b u t i o n and secondary-erosional r e w o r k i n g and p r e s e r v a t i o n o f sand as w e l l as s e p a r a t i o n and comparison o f sheett y p e and non-sheet t y p e sands such as l e n t i c u l a r ( c f . p l a t e I I / 7 - 8) and chann e l i z e d ( c f . p l a t e s I / 7 , V/3 - 4, V I I / l and V I I I / 6 ) u n i t s i n terms o f i n t e r and i n t r a - s a n d body geometry. Sheet sands a r e c h a r a c t e r i z e d b y a c o n t i n u o u s d i s t r i b u t i o n i n c o n s t a n t o r v a r i a b l e t h i c k n e s s across l a r g e r areas, whereas nonsheet sands a r e l i m i t e d t o s m a l l e r patches w i t h more r a p i d l y changing t h i c k ness. Some aspects o f sand body c o u p l i n g and s e p a r a t i o n , r e s e r v o i r h e t e r o g e n e i t y and t r e a t m e n t s i z e , and r o l e o f mud drapes a r e i l l u s t r a t e d as f o l l o w s .
4.2.4.1.1.1.
Sand body coupling and separation
The most s i g n i f i c a n t f e a t u r e s o f i n t e r - s a n d body geometry f a b r i c s a r e v e r t i amalgamation t o m u l t i s t o r e y superimposed complexes ( c f . p l a t e s I/7, I I / 1 - 5, III/l,I V / 7 - 8, V / 1 - 5, V I I / 1 - 2 and X I / 1 - 2 ) , h o r i z o n t a l o v e r l a p p i n g t o m u l t i p h a s e drapes, and v e r t i c a l and/or h o r i z o n t a l s e p a r a t i o n o f i s o l a t e d sand b o d i e s b y muddy i n t e r v e n t i o n s ( c f . p l a t e s I I I / Z , I V / 4 - 5, V I I / 3 - 6, V I I I / 8 and I X / 1 - 8 ) and f i n e r - g r a i n e d sandy i n t e r c a l a t i o n s . I m p o r t a n t c h a r a c t e r i s t i c s o f i n t r a - s a n d body o r g a n i z a t i o n a r e uniphase c y c l i c a l c o m p o s i t i o n ( c f . p l a t e s I I I / 1 - 2, I V / 4 - 5 and V I / 5 - 8 ) , m u l t i p h a s e m i c r o c y c l i c a l s t r u c t u r e and i n t e r b e d d i n g o f d i v i d i n g mudstone l a y e r s and l e n s e s ( c f . p l a t e s IV/8, V / 1 - 4, V I I / 5 - 6 and I X / 1 - 8 ) . cal
W h i l e b a r r i e r s f o r r e s e r v o i r communication and f r a c t u r e p r o p a g a t i o n i n sheet sands almost e x c l u s i v e l y d e r i v e f r o m i n t r a - s a n d body aspects, c o n s i d e r a b l e cont r o l o f r e s e r v o i r d i s t r i b u t i o n and t h u s f r a c t u r e e f f e c t i v i t y i n non-sheet sands i s e x e r t e d by i n t e r - s a n d body f e a t u r e s which a r e accentuated by t h e i n t r a - s a n d body geometry. On a l a r g e r s c a l e , many sedimentary p i l e s a r e heterogeneous due t o the i n t e r f o r m a t i o n a l s t r u c t u r e comprising i d e a l l y a n t i c l i n e s w i t h primarycondensed o r s e c o n d a r y - f a u l t e d sequences, and i n t r a f o r m a t i o n a l c o m p o s i t i o n o f a s u i t e o f sand b o d i e s w i t h v e r t i c a l s e p a r a t i o n and l a t e r a l i s o l a t i o n by l i m i t e d ex t e n s i o n .
4.2.4.1.1.2.
Reservoir heterogeneity and treatment s i z e
W h i l e such elements o f r e s e r v o i r h e t e r o g e n e i t y as s t a t e d above may be o n l y o f s u b o r d i n a t e s i g n i f i c a n c e f o r t h e MHF s t i m u l a t i o n ( c f . s e c t i o n 1.1.1.) o f t h i c k sandstones where i n t r a - and i n t e r - s a n d body b a r r i e r s o f f l u i d f l o w a r e crossed by f r a c t u r i n g due t o t h e h i g h p r e s s u r e s a p p l i e d , t h e aforementioned e l e ments o f pay a n i s o t r o p y c o u l d be d e c i s i v e f o r success o r f a i l u r e o f mHF t r e a t ments ( c f . s e c t i o n 4.8.1.) where p r e s s u r e s and volumes a r e much l o w e r and t h u s
404 t h e p r o p a g a t i n g f r a c t u r e c o u l d be d e f l e c t e d , s p l i t i n t o s e v e r a l branches o r even a l r e a d y be c o m p l e t e l y stopped by m i n o r l i t h o f a c i e s f e a t u r e s as enumerated above.
4.2.4.1.1.3. Role o f mud drapes P o i n t - b a r sand bodies o r i g i n a t i n g i n meandering-channel r i v e r systems probabl y r e p r e s e n t t h e r e s e r v o i r t y p e w i t h s t r o n g e s t i n t e r n a l h e t e r o g e n e i t y i n terms o f sedimentary s t r u c t u r e s and p e t r o p h y s i c a l p r o p e r t i e s . Comparative e v a l u a t i o n o f meandering-channel p o i n t b a r s and b r a i d e d - r i v e r mid-channel b a r s (XUE 1986) r e v e a l s a much l o w e r o i l d i s p l a c e m e n t e f f i c i e n c y i n t h e former. Semicommunicat i o n i n t h e p o i n t - b a r sand body i s m a i n l y due t o mud drapes on l a t e r a l a c c r e t i o n s u r f a c e s which d i v i d e t h e r e s e r v o i r i n t o o b l i q u e s l i c e - s h a p e d s u b u n i t s . The development o f c h u t e b a r s on t o p o f p o i n t b a r s p r o v i d e s f u r t h e r complicat i o n . The s i g n i f i c a n c e o f i n t r a - and i n t e r - s a n d b o d y mud drapes ( c f . p l a t e s I I I / 1 - 2, I V / 4 - 8 , V I I / 5 - 6 and IX/1 - 8 ) i n gas r e s e r v o i r s i s a l s o emphasized by HAAK & ELEWAUT (1988). The o c c a s i o n a l presence o f mud drapes on t o p o f sands causes d i s t u r b a n c e s i n v e r t i c a l f l o w paths, w i t h t h e s e v e r i t y o f d i s t u r bance depending on t h e number o f mud drapes and t h e i r h o r i z o n t a l e x t e n t w h i c h i s expressed i n terms o f t o r t u o s i t y . Aspects o f s m a l l - s c a l e h o r i z o n t a l and v e r t i c a l f l o w b a r r i e r s and h e t e r o g e n e i t i e s i n r e s e r v o i r r o c k s on t h e s c a l e o f mud drapes a r e a l s o d i s c u s s e d by WEBER, PUSCH & MOLLER (1987) and SILVA, NIKO, BERGH & S A N C E V I C (1988).
4.2.4.1.2. Matrix and fracture anisotropy Aspects o f s t i m u l a t i o n and e f f e c t i v e p e r m e a b i l i t y d i s t r i b u t i o n i n heterogeneous r e s e r v o i r s a r e a l s o d i s c u s s e d by OTERO, SAEZ & RUSINEK (1987). The s i g n i f i c a n c e o f r e s e r v o i r a n i s o t r o p y f o r t h e e v a l u a t i o n o f n a t u r a l f r a c t u r e systems and t h e performance o f s t i m u l a t i o n i n n a t u r a l l y c r a c k e d r e s e r v o i r s i s summarized by NELSON (1986), and f u r t h e r p o i n t s o f n a t u r a l f r a c t u r e system a n i s o t r o p y a r e i l l u s t r a t e d by BRANAGAN, CIPOLLA, LEE & YAN (1987). The d i s c u s s i o n a s f o l lows p r e s e n t s aspects o f b l a n k e t v s . l e n t i c u l a r sand bodies, i n t e r f e r e n c e and p u l s e t e s t i n g , t r a c e r a n a l y s i s , and impact o f t h e n a t u r a l f r a c t u r e system.
4.2.4.1.2.1. Blanket vs. lenticular sand bodies HODGES & KNUTSON (1981) comment on channel c o n t i n u i t y and d i r e c t i v i t y and a s s o c i a t e d f l o w a n i s o t r o p y e f f e c t s i n t i g h t gas sands. Continuous b l a n k e t sands p r o v i d e a more e x t e n s i v e d r a i n a g e area t h a n narrow lens-shaped sand bodies, and i n l e n t i c u l a r sands ( c f . p l a t e s I I / 7 - 8 ) , t h e e f f e c t i v e n e s s o f f r a c t u r e t r e a t ments i s r e l a t e d t o shape and s i z e o f t h e sand body, f r a c t u r e o r i e n t a t i o n and w e l l b o r e p o s i t i o n i n t h e sand body ( c f . a l s o GAUTIER & R I C E 1981). MEEHAN, HORNE & A Z I Z (1988) o u t l i n e e f f e c t s o f r e s e r v o i r h e t e r o g e n e i t y and f r a c t u r e a z i muth on o p t i m i z a t i o n o f f r a c t u r e l e n g t h and w e l l spacing. I n c o m p l i c a t e d h e t e r o geneous non-continuous r e s e r v o i r s , t h e c o m b i n a t i o n o f w i d e l y v a r y i n g p e r m e a b i l i t i e s and d i f f e r e n t l e n s s i z e s seem t o c o n t r o l w e l l response p a r t i a l l y even more than s t i m u l a t i o n t r e a t m e n t s (CHARLES, HUDOCK, U O I C K & HALL 1983).
L e n t i c u l a r non-continuous o r semi-communicated f o r m a t i o n s a r e c h a r a c t e r i z e d by EVANS & CARROLL (1981), PETERSON & KOHOUT (1983), LORENZ (1985); SATTLER, RAIBLE & GALL (1985); XUE (1986); BRANAGAN, CIPOLLA, LEE & YAN (1987), EVANS & UBANI (1987) and UBANI & RAY (1988). Blanket-geometry t i g h t gas sands and t h e i r l a t e r a l v a r i a b i l i t y a r e reviewed by FINLEY & O'SHEA (1983), LORENZ (1983), FINLEY (1986) and GELLAND, TAYLOR, TESSMAN & LARNER (1986). TYLER & FINLEY (1988) comment on r e s e r v o i r a r c h i t e c t u r e i n heterogeneous pay complexes. MARTIN, EVANS & RAPER (1988) document p r e d i c t i o n and m o d e l l i n g o f sand-body c o n n e c t i v i t y . ANDERSON (1982) u t i l i z e s r e p e a t f o r m a t i o n t e s t e r and d i p m e t e r f o r d e f i n i t i o n o f r e s e r v o i r h e t e r o g e n e i t y and a n i s o t r o p y .
405
4.2.4.1.2.2.
I n t e r f e r e n c e and pulse t e s t ins
BENSON & L A 1 ( 1 9 8 6 ) analyze i n t e r f e r e n c e d a t a i n a h i g h l y heterogeneous n a t u r a l l y f r a c t u r e d geothermal r e s e r v o i r . WILSON ( 1 9 5 6 ) p r e s e n t s some aspects o f s a n d - o i l f r a c t u r i n g i n l o n g heterogeneous sand s e c t i o n s . BENNETT, ROSATO & REYNOLDS ( 1 9 8 1 ) and DUSSAN & SHARMA ( 1 9 8 7 ) d i s c u s s v a r i o u s p o i n t s and f e a t u r e s o f f r a c t u r e h e t e r o g e n e i t y and a n i s o t r o p y . I m p r o v i n g performance p r e d i c t i o n by more a c c u r a t e h e t e r o g e n e i t y assessment i s i l l u m i n a t e d by JENSEN & CURRIE ( 1 9 8 8 ) , and t h e i n f l u e n c e o f c r i t i c a l h e t e r o g e n e i t i e s on p r o d u c t i o n performance o f d i f f e r e n t r e s e r v o i r u n i t s i s i l l u s t r a t e d by HONARPOUR, TOMUTSA, CARROLL, SZPAKIEWICZ, SHARMA & T I L L M A N (1988). H o r i z o n t a l and v e r t i c a l p u l s e and i n t e r f e r e n c e t e s t i n g i n m u l t i l a y e r e d r e s e r v o i r s w i t h s p e c i a l emphasis on t h e r o l e o f non-comm u n i c a t i n g l a y e r s i s i n v e s t i g a t e d by BANDYOPADHYAY (1981), CHU & RAGHAVAN (1981); H U T F I L Z , COCKERHAM & McINTOSH ( 1 9 8 1 ) ; DAKE ( 1 9 8 2 ) , CHEN & LESCARBOURA ( 1 9 8 3 ) , PRATS ( 1 9 8 3 ) , OGBE & BRIGHAM ( 1 9 8 4 a, 1984 b ) , SMITH & BROWN ( 1 9 8 4 ) , ROEST ( 1 9 8 6 ) , WIJESINGHE & KECECIOGLU (1986 b ) ; BRANAGAN, L E E & WILMER (1988); GRABOWSKI, LOCKE, SARADJI, F E I L E R & YOST ( 1 9 8 8 ) ; OPDAL & KOSSACK ( 1 9 8 8 ) and SAMPSON & MARCOU ( 1 9 8 8 ) . NAJURIETA, ROBLES & EDWARDS ( 1 9 8 6 ) p r e s e n t i n t e r f e r e n ce t e s t i n g f o r p r e d i c t i o n o f e a r l y w a t e r breakthrough i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s , and SANDAL & RAMEY ( 1 9 7 8 ) comment on i n t e r f e r e n c e t e s t i n g w i t h w e l l b o r e s t o r a g e and s k i n e f f e c t a t t h e p r o d u c t i o n w e l l . SWIFT & BROWN ( 1 9 7 6 ) r e view i n t e r f e r e n c e t e s t i n g f o r r e s e r v o i r d e f i n i t i o n . O t h e r aspects o f i n t e r f e r e n ce t e s t i n g i n n a t u r a l l y and h y d r a u l i c a l l y f r a c t u r e d w e l l s a r e d i s c u s s e d by LESCARBOURA, WALTHER & WILSON ( 1 9 7 5 ) and RAMEY ( 1 9 7 5 b ) . KAMAL ( 1 9 8 2 ) and KAMAL & HEGEMAN ( 1 9 8 8 ) r e v i e w mu1 t i p l e - w e l l i n t e r f e r e n c e and p u l s e t e s t i n g i n homogeneous i s o t r o p i c r e s e r v o i r s , homogeneous r e s e r v o i r s w i t h v e r t i c a l l y f r a c t u r e d w e l l s , n a t u r a l l y f r a c t u r e d and m u l t i l a y e r e d r e s e r v o i r s w i t h d o u b l e - p o r o s i t y behaviour, and heterogeneous r e s e r v o i r s . STEWART, GUPTA & WESTAWAY ( 1 9 8 4 ) i n t e r p r e t i n t e r f e r e n c e t e s t s i n r e s e r v o i r s w i t h s e a l i n g and p a r t i a l l y communicating f a u l t s . L a c k i n g i n t e r f e r e n c e may be due t o w a t e r b l o c k e f f e c t s around t h e o b s e r v a t i o n w e l l s o r i t may i n d i c a t e t h a t t h e r e i s no o r poor c o n n e c t i o n between t h e n a t u r a l f r a c t u r e s i n t e r s e c t i n g i n d i v i d u a l w e l l s e i t h e r because o f s h a l e breaks o r l i m i t e d n a t u r e o f f r a c t u r e i n t e r c o n n e c t e d n e s s (WARPINSKI, BRANAGAN, SATTLER, LORENZ, NORTHROP, MANN & FROHNE 1 9 8 5 ) .
4.2.4.1.2.3.
Tracer a n a l y s i s
ROBINSON, TESTER & BROWN ( 1 9 8 4 ) d i s c u s s r e s e r v o i r a n a l y s i s and s i z i n g by i n e r t and c h e m i c a l l y r e a c t i n g t r a c e r s . WAGNER ( 1 9 7 7 ) r e v i e w s t r a c e r a p p l i c a t i o n f o r d i a g n o s i n g i n t e r w e l l r e s e r v o i r h e t e r o g e n e i t i e s . D'HOOGE, SHEELY & W I L L I A M S ( 1 9 8 7 ) comment on i n t e r w e l l t r a c e r a n a l y s i s o f m u l t i p a y d i s c o n t i n u o u s r e s e r v o i r s w i t h up t o f o u r t r a c e r s f o r m o n i t o r i n g o f d i f f e r e n t i a l f l u i d movement, and o t h e r aspects o f t r a c e r and p r e s s u r e t e s t a n a l y s i s f o r c h a r a c t e r i z a t i o n o f a r e a l l y heterogeneous r e s e r v o i r s a r e covered by MISHRA, BRIGHAM & ORR ( 1 9 8 8 ) . McLENNAN, ROEGIERS & MARX (1983) r e p o r t p o i n t s c o n c e r n i n g i n t e r a c t i o n between g e o l o g i c a l c o n d i t i o n s , t r e a t m e n t c h a r a c t e r i s t i c s and p r o d u c t i o n . T A R I Q , ICHARA & AYESTARAN ( 1 9 8 5 ) and ICHARA & CANNON ( 1 9 8 7 ) i n v e s t i g a t e t h e e f f e c t o f p e r f o r a t i o n l e n g t h , d e n s i t y and phasing a n g l e v a r i a t i o n s on t h e performance o f h e t e r o geneous r e s e r v o i r s t h a t a r e c h a r a c t e r i z e d by s h a l e - l a m i n a t i o n and cross-bedd i n g . L I G H T ( 1 9 8 3 ) o u t l i n e s t h e e f f e c t of l a t e r a l a n i s o t r o p y on f l o o d p a t t e r n dimensions and o r i e n t a t i o n . O t h e r aspects o f t r a c e r t e s t i n g i n heterogeneous f o r m a t i o n s a r e d i s c u s s e d by SEETHARAM & DEANS ( 1 9 8 6 ) .
4.2.4.1.2.4.
Inpact o f the n a t u r a l f r a c t u r e system
GRINGARTEN ( 1 9 8 8 ) analyzes w e l l t e s t s i n f i s s u r e d and m u l t i l a y e r e d r e s e r v o i r s w i t h h i g h p e r m e a b i l i t y c o n t r a s t s between i n d i v i d u a l beds. J A L A L I - Y A Z D I &
406
ERSHAGHI ( 1 9 8 7 ) comment o n h e t e r o g e n e o u s n a t u r a l l y f r a c t u r e d r e s e r v o i r s . BLASINGAME & LEE ( 1 9 8 6 ) r e p o r t on some homogeneous r e s e r v o i r s . H e t e r o g e n e i t y a s p e c t s o f m u l t i - l a y e r e d r e s e r v o i r s a r e d i s c u s s e d b y EHLIG-ECONOMIDES & JOSEPH ( 1 9 8 5 ) ; TANG, L I U & J I A N G ( 1 9 8 6 ) and HATZIGNATIOU, OGBE, DEHGHANI & ECONOMIDES ( 1 9 8 7 ) . CLEARY ( 1 9 7 8 b ) g i v e s an o u t l i n e o f p r i m a r y f a c t o r s g o v e r n i n g h y d r a u l i c f r a c t u r e s i n h e t e r o g e n e o u s s t r a t i f i e d f o r m a t i o n s . SMITH, SAWYER & ESPOSITO ( 1 9 8 1 ) e x p l a i n t h e e f f e c t o f a n i s o t r o p y and v e r t i c a l f r a c t u r e p e n e t r a t i o n o n p r o d u c t i o n d e c l i n e c u r v e s f o r l o w - p e r m e a b i l i t y gas r e s e r v o i r s . G e o l o g i c a l and p e t r o p h y s i c a l f a c t o r s c o n t r o l l i n g e x c e s s p r e s s u r e s i n i s o l a t e d sand l e n s e s w i t h i n s h a l e s t h a t a r e c h a r a c t e r i z e d b y l i m i t e d r e s e r v o i r c o n n e c t i o n a r e e v a l u a t e d b y OZKAYA (1987). NORTHROP ( 1 9 8 8 ) p r o v i d e s a d e t a i l e d c h a r a c t e r i z a t i o n o f a h e t e r o g e n e o u s and anisotropic n a t u r a l l y f r a c t u r e d r e s e r v o i r t h a t i s subjected t o hydraulic f r a c t u r i n g s t i m u l a t i o n . A s y n e r g i s t i c approach i n c l u d i n g v a r i o u s techniques i s necess a r y t o a c c u r a t e l y d e s c r i b e t h e p a y s e c t i o n g e o m e t r y . The d e p o s i t i o n a l e n v i r o n ment c o n t r o l s r e s e r v o i r m o r p h o l o g y , s i z e , i n t e r n a l s t r u c t u r e and n a t u r a l f r a c t u r i n g . W h i l e i n m a r i n e b l a n k e t s a n d s t o n e s t h e a z i m u t h o f t h e f r a c t u r e p l a n e has no i n f l u e n c e , t h e a n g l e between f r a c t u r e p l a n e and c h a n n e l d i r e c t i o n i s v e r y i m p o r t a n t i n f l u v i a l l e n t i c u l a r o r r i b b o n s a n d s t o n e s and c a n r e s u l t i n w e l l o v e r an o r d e r o f m a g n i t u d e d i f f e r e n c e i n t h e a r e a o f i n t e r s e c t e d p a y ( c f . a l s o sect i o n 4.10.2.).
4.2.4.1.2.5.
Other aspects
McELROY ( 1 9 8 2 ) comments o n t h e e f f e c t o f l a t e r a l a n i s o t r o p y o n f l o o d p a t t e r n d i m e n s i o n s and o r i e n t a t i o n . ABBASZADEH-DEHGHANI & BRIGHAM ( 1 9 8 4 ) employ w e l l t o - w e l l t r a c e r f l o w f o r d e t e r m i n a t i o n o f p a y h e t e r o g e n e i t y . ZUBER, LEE & GATENS ( 1 9 8 7 ) document p e r m e a b i l i t y a n i s o t r o p y i n D e v o n i a n s h a l e gas r e s e r v o i r s w h i c h i s c r e a t e d b y t h e n a t u r a l f r a c t u r e system, w i t h t h i s t y p e o f p r o s p e c t i v e i n t e r val heterogeneity causing hydraulic o r explosive f r a c t u r e s ( c f . section 4.4.4.) r u n n i n g p e r p e n d i c u l a r l y t o t h e n a t u r a l f r a c t u r e t r e n d b e i n g more e f f i c i e n t t h a n those being p a r a l l e l t o i t . Reservoir heterogeneity by n a t u r a l f r a c t u r e anisot r o p y i s s i g n i f i c a n t l y r e f l e c t e d b y t h e absence o f p r e s s u r e i n t e r f e r e n c e b e t ween c l o s e l y - s p a c e d n e i g h b o u r i n g w e l l s (LORENZ & FINLEY 1 9 8 7 ) .
I n t e r m s o f m a t r i x h e t e r o g e n e i t y , t h e success o f f r a c t u r i n g t r e a t m e n t depends on d i s t i n c t i o n o f t h e m a j o r r e s e r v o i r r o c k t y p e s and s e l e c t i v e i n v e s t i g a t i o n o f f l u i d c o m p a t i b i l i t y i n o r d e r t o e l i m i n a t e f a i l u r e c a u s e s due t o e x c e s s i ve f l u i d s e n s i t i v i t y (SIMON, KAUL & CULBERTSON 1 9 7 7 ) . The d e g r e e o f f l u i d s e n s i t i v i t y i s g e n e r a l l y r e l a t e d t o r o c k t y p e , p e r m e a b i l i t y and p o r o s i t y , w i t h t h e h i g h e r - p e r m e a b i l i t y c l e a n e r sandstones b e i n g r e l a t i v e l y i n s e n s i t i v e t o t h e t r e a t m e n t f l u i d used. N a t u r a l l y f r a c t u r e d r e s e r v o i r s can be r e g a r d e d as d o u b l e p o r o s i t y m e d i a c o n s i s t i n g o f i s o t r o p i c h e t e r o g e n e o u s p r i m a r y r o c k m a t r i x and a n i s o t r o p i c h e t e r o g e n e o u s s e c o n d a r y f r a c t u r e s y s t e m (NAKORNTHAP & EVANS 1 9 8 6 ) . O t h e r f e a t u r e s o f r e s e r v o i r h e t e r o g e n e i t y a r e d i s c u s s e d b y KAMAL ( 1 9 7 7 ) , LAMBERT ( 1 9 8 1 ) ; ARYA, HEWETT, LARSON & LAKE ( 1 9 8 5 ) and LEE & OKUYIGA ( 1 9 8 6 ) . H I R A S A K I , MORROW & WILLHITE ( 1 9 8 4 ) e s t i m a t e r e s e r v o i r h e t e r o g e n e i t y f r o m w a t e r f l o o d performance.
4.2.4.2.
Petrophysical aspects
As a consequence o f t h e d i f f e r e n t t y p e s and s c a l e s o f s e d i m e n t o l o g i c a l d i s c o n t i n u i t i e s and a n i s o t r o p i e s i n r e s e r v o i r r o c k s , many p a y f o r m a t i o n s a l s o e x h i b i t h e t e r o g e n e o u s d i s t r i b u t i o n of p e t r o p h y s i c a l p a r a m e t e r s , w i t h p e r m e a b i l i t y b e i n g most a f f e c t e d by d e p o s i t i o n a l inhomogeneities. P e r m e a b i l i t y a n i s o t r o p y i n v e r t i c a l and/or h o r i z o n t a l d i r e c t i o n i s the r e s u l t o f predominantly g r a i n s i z e d i f f e r e n c e s r e f l e c t i n g e n e r g y f l u c t u a t i o n s d u r i n g s e d i m e n t a t i o n o f t h e sand i n v a r i o u s e n v i r o n m e n t s w i t h d i f f e r e n t l e v e l s and m a i n t e n a n c e o f e n e r g y i n t i m e and space. A p a r t f r o m i t s d e p o s i t i o n a l provenance, p e r m e a b i l i t y h e t e r o g e n e i t i e s
407 can a l s o o r i g i n a t e d u r i n g d i a g e n e t i c a l h i s t o r y by d i f f e r e n t i a l m o b i l i z a t i o n and/or p r e c i p i t a t i o n o f framework g r a i n s and/or cements. Aspects o f p e r m e a b i l i t y a n i s o t r o p y as w e l l as i n t e r s e c t i o n o f n a t u r a l f r a c t u r e systems a r e b r i e f l y c h a r a c t e r i z e d as f o l l o w s .
4.2.4.2.1. Permeabi 1ity anisotropy P e t r o p h y s i c a l r e s e r v o i r h e t e r o g e n e i t i e s c o m p r i s i n g p e r m e a b i l i t y and c o n d u c t i v i t y a n i s o t r o p y can be t h r e e - d i m e n s i o n a l l y diagnosed and m o n i t o r e d by i n t e r w e l l t r a c e r o r i n t e r f e r e n c e t e s t i n g (WAGNER & BACKER 1 9 7 4 , WAGNER 1 9 7 7 , ABBASZADEHDEHGHANI & BRIGHAM 1982, HAGOORT 1982; OHNO, NANBA & HORNE 1985; H E I S L E R 1986; ONURI, REYNOLDS & RAGHAVAN 1 9 8 6 ) . D e t e r m i n a t i o n o f p e t r o p h y s i c a l r e s e r v o i r h e t e r o g e n e i t i e s and r e s u l t i n g f l o w p a t t e r n a n i s o t r o p y has an i m p o r t a n t impact n o t o n l y f o r f r a c t u r e p r o p a g a t i o n m o d e l l i n g , b u t a l s o f o r o p t i m i z a t i o n o f sweep e f f i c i e n c y o f w a t e r - , steam- and c h e m i c a l - f l o o d i n g enhanced o i l r e c o v e r y (OHNO, NANBA & HORNE 1 9 8 5 ) . I n t e r f e r e n c e and t r a c e r t e s t i n g should n o t be regarded as b e i n g c o m p e t i t i v e techniques, b u t b o t h methods s h o u l d be a p p l i e d i n a complement a r y sense i n o r d e r t o achieve o p t i m i z a t i o n o f d e t e r m i n a t i o n o f i n t e r n a l r e s e r v o i r s t r u c t u r e (MISHRA & RAMEY 1985, BRIGHAM & ABBASZADEH-DEHGHANI 1986). JOHNMARTIN & COOPER ( 1 9 8 4 ) and STALKUP & EVANS ( 1 9 8 6 ) g i v e an SON & KROL ( 1 9 8 4 ) , overview o f p e r m e a b i l i t y v a r i a t i o n s i n d i f f e r e n t d e p o s i t i o n a l environments. EVANS (1988) documents t h e i n f l u e n c e of common sedimentary s t r u c t u r e s and diagen e s i s on p e r m e a b i l i t y h e t e r o g e n e i t y and a n i s o t r o p y i n s e l e c t e d sandstones.
REVIERE & WU ( 1 9 8 6 ) o u t l i n e p o r o s i t y s t r i n g e r i z a t i o n g i v i n g r i s e t o extremee r r a t i c pay c o n t i n u i t y t h a t h e a v i l y d e t e r i o r a t e s t h e e f f e c t i v i t y o f w a t e r f l o o d i n g . Aspects o f heterogeneous r e s e r v o i r c o m p o s i t i o n a r e a l s o d i s c u s s e d by KNUTSON, HOOGES & RIGHTER ( 1 9 8 1 ) ; BEGG, CARTER & ORANFIELD ( 1 9 8 7 ) ; RAVENNA, GALL I , MATHIEU, MONTADERT & RUDXIBWICZ ( 1 9 8 7 ) ; SHARMA, HONARPOUR, SZPAKIEWICZ & SCHATZINGER ( 1 9 8 7 ) and TYLER ( 1 9 8 7 ) . ROSE ( 1 9 8 2 , 1 9 8 3 ) , B A L L & P I T T S ( 1 9 8 3 ) and PEACEMAN ( 1 9 8 3 ) comment on p e r m e a b i l i t y a n i s o t r o p y i n t i g h t f o r m a t i o n s which i s m a i n l y based on t h e f a c t t h a t v e r t i c a l p e r m e a b i l i t y i s s m a l l e r than h o r i z o n t a l p e r m e a b i l i t y (GIGER, R E I S S & JOURDAN 1984; GIGER 1985, 1 9 8 7 ; JOSH1 1986; KARCHER, GIGER & COMBE 1 9 8 6 ; KOSSACK & OPDAL ( 1 9 8 8 ) ; KUCHUK, BRICE, THAMBYNAYAGAM, SHERRARD & GOODE 1 9 8 8 ) . LEUNG ( 1 9 8 6 ) ; TAHA, H I L L & SEPEHRNOORI ( 1 9 8 6 ) and HOUSEWORTH ( 1 9 8 8 ) r e p o r t v a r i a b l e d i r e c t i o n a l p e r m e a b i l i t i e s i n a n i s o t r o p i c and h e t e rogeneous r e s e r v o i r s . ly
TANG, HENG & J I A N G ( 1 9 8 6 ) p r e s e n t c o m p l i c a t i o n s o f w a t e r f l o o d e f f e c t i v i t y i n heterogeneous sand-body arrangements w i t h pronounced i n t e r - and i n t r a - l a y e r p e r m e a b i l i t y d i f f e r e n c e s . HORIE, FIROOZABADI & ISHIMOTO ( 1 9 8 8 ) o u t l i n e c a p i l l a r y c o n t i n u i t y i n f r a c t u r e d r e s e r v o i r s . LEWIS ( 1 9 8 8 ) documents a h i e r a r c h i c a l c l a s s i f i c a t i o n o f p e r m e a b i l i t y h e t e r o g e n e i t y c o m p r i s i n g seven o r d e r s and emphasizes t h e need o f l i n k i n g o f o u t c r o p and c o r e s t u d i e s f o r more a c c u r a t e three-dimens i o n a l p r e d i c t i o n o f i n t e r - w e 1 1 h e t e r o g e n e i t y , w i t h t h e s p e c i a l r e q u i r e m e n t bei n g q u a n t i f i c a t i o n o f r e s e r v o i r h e t e r o g e n e i t y developed i n d i f f e r e n t d e p o s i t i o n a l environments. JONES ( 1 9 8 7 ) u t i l i z e s t h e non-Darcy f l o w c o e f f i c i e n t f o r r e s e r v o i r h e t e r o g e n e i t y c h a r a c t e r i z a t i o n . MARTIN & COOPER ( 1 9 8 4 ) document permeab i l i t y b a r r i e r d i s t r i b u t i o n m o d e l l i n g i n s e d i m e n t o l o g i c a l l y complex r e s e r v o i r s .
4.2.4.2.2. Intersection o f natural fracture systems LORENZ, BRANAGAN, WARPINSKI & SATTLER ( 1 9 8 6 ) d e s c r i b e a system c o n s i s t i n g o f s u b p a r a l l e l u n i d i r e c t i o n a l n a t u r a l f r a c t u r e s o r i g i n a t i n g under c o n d i t i o n s o f h i g h p o r e p r e s s u r e s and r e l a t i v e l y low d i f f e r e n t i a l h o r i z o n t a l s t r e s s ( c f . a l s o s e c t i o n 4 . 8 . 8 . 1 . ) . P e r m e a b i l i t y a n i s o t r o p y i s m a i n l y c r e a t e d by t h e heterogeneous f r a c t u r e system a l l o w i n g o n l y m a r g i n a l f r a c t u r e i n t e r - and cross-connect i o n s , w i t h t h u s o r t h o g o n a l p e r m e a b i l i t y b e i n g a b t . 1 - 2 o r d e r s o f magnitude lower than p r i m a r y - d i r e c t i o n p e r m e a b i l i t i e s . A l t h o u g h f r a c t u r e s o f any s c a l e and opening may i n c r e a s e gross r e s e r v o i r p e r m e a b i l i t y by more than an o r d e r o f
408 magnitude o v e r m a t r i x values, t h e enhanced pay d r a i n a g e w i l l be h i g h l y a n i s o t r o p i c due t o t h e heterogeneous f r a c t u r e geometry f a b r i c . Aspects o f r e s e r v o i r het e r o g e n e i t y r e l a t e d t o p e r m e a b i l i t y a n i s o t r o p y a r e a l s o o u t l i n e d by WARPINSKI, BRANAGAN, SATTLER, LORENZ, NORTHROP & FROHNE (1985) and LORENZ & FINLEY (1987), w i t h emphasis b e i n g p u t on d e p o s i t i o n a l environment c o n t r o l l i n g t h e c h a r a c t e r i s t i c s o f t h e m a t r i x rock and sedimentary r e s e r v o i r h e t e r o g e n e i t y as w e l l as f r a c t u r e p e r m e a b i l i t y system.
4.2.4.3. Rock mechanical aspects I n homogeneous f o r m a t i o n s where no s i g n i f i c a n t s t r e s s c o n t r a s t s can be assumed between r e s e r v o i r zones s e l e c t e d on t h e b a s i s o f p e r m e a b i l i t y v a r i a t i o n s , f r a c t u r e growth i s r a d i a l and t h e f r a c t u r e i s always p r o p a g a t i n g i n t o t h e l a y e r s a d j o i n i n g t h e pay zone, w i t h p e n e t r a t i o n depth b e i n g determined by d i f f e rences i n s t i f f n e s s and h o r i z o n t a l i n - s i t u s t r e s s between r e s e r v o i r and n e i g h b o u r i n g beds ( c f . s e c t i o n s 4.2.2.3.1.3., 4.2.2.5.3. and 4.2.3.2.2.; particularl y i n c h a l k f o r m a t i o n s ; EEKELEN 1982, HARTLEY & BOSMA 1985). A b e t t e r f r a c t u r e containment i n such homogeneous and p o o r l y s t a b i l i z e d f o r m a t i o n s can o n l y be achieved d u r i n g t h e advanced stage o f d e p l e t i o n when d i f f e r e n t r e s e r v o i r p r e s sure regimes may have developed i n t h e v a r i o u s i n d i v i d u a l h o r i z o n s and t h u s c r e a t e d a secondary s t r e s s z o n a t i o n i n a p r i m a r i l y almost i s o t r o p i c f o r m a t i o n in terms o f s t r e s s d i s t r i b u t i o n and f r a c t u r e growth. I n - s i t u s t r e s s d i f f e r e n c e s a r e more e f f e c t i v e f o r c r a c k containment than s t i f f n e s s c o n t r a s t s (CLEARY 1979, CLIFTON & ABOU-SAYED 1979; WARPINSKI, SCHMIDT & NORTHROP 1980). R e s e r v o i r h e t e r o g e n e i t y can c o n s i d e r a b l y i n f l u e n c e t h e success o f h y d r a u l i c p r o p p a n t f r a c t u r i n g (WATERS 1980 r e p o r t s good r e s u l t s i n c o n t i n u o u s pay h o r i z o n s , b u t l e s s f a v o u r a b l e performance i n d i s c o n t i n u o u s o r l e n t i c u l a r f o r m a t i o n s ) . V a r i o u s p o i n t s o f f r a c t u r e e x t e n s i o n across m a t e r i a l d i s c o n t i n u i t i e s and o t h e r f e a t u r e s a r e o u t l i n e d as f o l l o w s .
4.2.4.3.1. Fracture extension across material discont Inuit ies RUMMEL, HEUSER, MULLER & OTTO (1986, 1987) document by f r a c t u r e p r o p a g a t i o n m o d e l l i n g based on l a b o r a t o r y experiments t h a t m a t e r i a l inhomogeneities, a n i s o t r o p y and d i s c o n t i n u i t i e s have c o n s i d e r a b l e impact on d i r e c t i o n and v e l o c i t y o f c r a c k e x t e n s i o n ( t h e same a p p l i e s f o r p r e - e x i s t i n g n a t u r a l f r a c t u r e s ; BLANTON 1982; w i t h f r a c t u r e c o n n e c t i v i t y h a v i n g an i m p o r t a n t i n f l u e n c e ; WIJESINGHE 1985, WIJESINGHE & KECECIOGLU 1986; c f . a l s o s e c t i o n 4 . 8 . 7 . ) . An o u t l i n e i s g i ven on f r a c t u r e d e f l e c t i o n a t bounding planes, m a t e r i a l p r o p e r t y d i f f e r e n c e s , and i n t e r f a c e shear s t r e n g t h as f o l l o w s .
4.2.4.3.1.1. Fracture deflection at bounding planes L i t h o l o g i c a l d i s c o n t i n u i t i e s can slow down o r speed up f r a c t u r e growth o r can change t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n by d e f l e c t i o n , depending on t h e e l a s t i c a l and f r a c t u r e - m e c h a n i c a l c o n t r a s t o f t h e r o c k u n i t s on b o t h s i d e s o f t h e d i s c o n t i n u i t y . H i g h e l a s t i c i t y moduli and s t r e s s i n h o m o g e n e i t i e s i n f r o n t o f the propagating f r a c t u r e r e s u l t i n lowering o f the stress i n t e n s i t y a t the c r a c k t i p and t h e r e f o r e slow down f r a c t u r e growth. D i s c o n t i n u i t i e s which are i n c l i n e d w i t h respect t o the d i r e c t i o n o f o r i g i n a l f r a c t u r e propagation lead t o d e f l e c t i o n o f the crack i n t o another d i r e c t i o n (JEFFREY, VANDAMME & ROEGIERS 1987 comment on mechanical i n t e r a c t i o n s i n b r a n ched o r s u b p a r a l l e l h y d r a u l i c f r a c t u r e s ) . The o r i e n t a t i o n o f r a d i a l l y e x t e n d i n g c r a c k s i s h i g h l y s e n s i b l e a g a i n s t changes o f t h e o r i e n t a t i o n o f p r i n c i p a l s t r e s s d i r e c t i o n s , w i t h t h e f r a c t u r e always p r o p a g a t i n g i n t h e p l a n e w h i c h i s p e r p e n d i c u l a r t o t h e l e a s t main h o r i z o n t a l s t r e s s . I n case o f c h a r a c t e r i s t i c a l d i f f e r e n c e s i n m a t e r i a l p r o p e r t i e s between a d j o i n i n g r o c k types, i t was poss i b l e t o e x t e n d t h e c r a c k across t h e boundary p l a n e by a p p l i c a t i o n o f h i g h - v i s -
409 c o s i t y f r a c t u r i n g f l u i d s and/or h i g h i n j e c t i o n r a t e s .
4.2.4.3.1.2.
M a t e r i a l property d i f f e r e n c e s
The r o c k mechanical r e s u l t s show t h a t containment o f f r a c t u r e p r o p a g a t i o n by boundary h o r i z o n s w i t h s i g n i f i c a n t l y d i f f e r e n t m a t e r i a l p r o p e r t i e s can be succ e s s f u l l y achieved i n case o f c o n t r o l l e d pumping r a t e s and a d j u s t e d t r e a t m e n t design. T h i s c o n c l u s i o n f r o m t h e e x p e r i m e n t a l i n v e s t i g a t i o n s i s c o n f i r m e d by t h e examples o f f i e l d a p p l i c a t i o n ( c f . s e c t i o n 4.2.2.3.) where t h e goal o f cont a i n i n g f r a c t u r e e x t e n s i o n t o t h i n r e s e r v o i r h o r i z o n s has been s u c c e s s f u l l y r e a ched ( c f . KOHLHAAS 1982; CONWAY, McGOWEN, GUNDERSON & K I N G 1985). BIOT, MEDLIN & MASSE (1983) p o i n t o u t t h a t based on t h e e l a s t i c i t y concept, s u r f a c e energy and shear modulus d i f f e r e n c e s c o u l d a l s o l e a d t o f r a c t u r e confinement i n a l a y e r between d i s s i m i l a r m a t e r i a l s . I n c o r p o r a t i n g p l a s t i c i t y and s u r f a c e energy i n t o f r a c t u r e p r o p a g a t i o n m o d e l l i n g , MEDLIN & MASSE (1986) conclude t h a t p l a s t i c i t y e f f e c t s i n s h a l e s a r e more i m p o r t a n t i n c o n t r o l l i n g f r a c t u r e h e i g h t than any o t h e r phenomena ( c f . s e c t i o n 4 . 2 . 3 . 2 . 3 . ) . A f r a c t u r e generated i n a b r i t t l e sandstone o r l i m e s t o n e l a y e r bounded by p l a s t i c shales i s l i k e l y t o be c o n t a i ned because o f p r o p a g a t i n g p r e s s u r e d i f f e r e n c e s between p l a s t i c and b r i t t l e hor i z o n s ( c f . a l s o s e c t i o n s 4.2.2.5.3. and 4 . 2 . 3 . 2 . 2 . ) . F r a c t u r e s can propagate a c r o s s r o c k i n t e r f a c e s w i t h o u t s l i p because o f f r i c t i o n (TEUFEL 1979) i f t h e p r e s s u r e between t h e r o c k s exceeds a v a l u e approximat e l y equal t o t h e i r t e n s i l e s t r e n g t h . I n terms o f v e r t i c a l f r a c t u r e s , t h i s imp l i e s t h a t s l i p does n o t o c c u r a t t o p o r b o t t o m o f t h e c r a c k even f o r p e r f e c t l y f l a t and unbounded beds, i f t h e e f f e c t i v e v e r t i c a l p r e s s u r e ( o v e r b u r d e n minus p o r e p r e s s u r e ) exceeds t h e t e n s i l e s t r e n g t h o f t h e r o c k . F o r normal g r a d i e n t s , s l i p does t h u s n o t occur i n competent r o c k s i n depths below a b t . 1,750 f t . o r i f t h e overburden p r e s s u r e exceeds t h e p o r e p r e s s u r e by more than t h e t e n s i l e s t r e n g t h , whereas i n s h a l l o w e r depth, s l i p may t a k e p l a c e and r e s u l t i n decreasing pressure t o extend a f r a c t u r e .
4.2.4.3.1.3.
I n t e r f a c e shear strength
TEUFEL & CLARK (1981) document t h a t f r a c t u r e g r o w t h i n l a y e r e d r o c k i s cont r o l l e d by d i f f e r e n c e s i n mechanical p r o p e r t i e s o f t h e f o r m a t i o n s on e i t h e r s i d e o f t h e i n t e r f a c e between two l i t h o l o g i e s o f d i f f e r e n t s t r e s s l e v e l , changes i n h o r i z o n t a l s t r e s s s t a t e across t h e i n t e r f a c e , and shear s t r e n g t h o f t h e i n t e r f a c e . Containment can o c c u r whenever shear s t r e n g t h o f t h e l a y e r i n t e r f a c e i s s u f f i c i e n t l y weak r e l a t i v e t o t e n s i l e s t r e n g t h and minimum h o r i z o n t a l comp r e s s i v e s t r e s s o f t h e bounding l a y e r , w i t h t h e f r a c t u r e then more e a s i l y becoming an i n t e r f a c i a l f r a c t u r e than e x t e n d i n g across t h e i n t e r f a c e i n t o t h e boundi n g l a y e r ( c f . s e c t i o n 4 . 5 . 4 . 4 . 2 . ) . A low shear s t r e n g t h i n t e r f a c e can o c c u r a t low overburden s t r e s s e s due t o t h e f r i c t i o n a l e f f e c t o f t h e a p p l i e d s t r e s s on t h e i n t e r f a c e , p r o v i d e d t h a t t h e overburden s t r e s s i s s u f f i c i e n t l y g r e a t e r than t h e h o r i z o n t a l s t r e s s e s t o produce a v e r t i c a l h y d r a u l i c f r a c t u r e . Differences i n e l a s t i c a l properties o f the formations i n a d i l i t h o l o g i c a l l a y e r e d r o c k sequence i s t h e dominant f a c t o r i n d e t e r m i n i n g h o r i z o n t a l s t r e s s s t a t e w i t h i n i n d i v i d u a l l a y e r s , w i t h a p o s s i b l e compressional i n c r e a s e i n h o r i z o n t a l s t r e s s g o i n g f r o m h i g h shear modulus t o low shear modulus l a y e r s . Cont a i n m e n t and p r o p a g a t i o n o f f r a c t u r e s a r e g e n e r a l l y two competing mechanisms a t l i t h o l o g i c a l boundaries s e p a r a t i n g r o c k s w i t h d i f f e r e n t s t r e s s l e v e l .
4.2.4.3.2.
Other features
Rock mechanical e-xperiments on f r a c t u r e p r o p a g a t i o n i n l a y e r e d r o c k s a r e a l s o c a r r i e d o u t by TEUFEL & CLARK (1981), BLANTON (1982) and SHAFFER, THORPE, INGRAFFEA & HEUSE ( 1 9 8 4 ) . LUISKUTTY, TOMUTSA & PALMER (1986) p r e s e n t a semiana-
410 l y t i c a l model o f h y d r a u l i c c r a c k g r o w t h t h r o u g h weak b a r r i e r s . EL RABAA ( 1 9 8 7 ) r e p o r t s f e a t u r e s o f h y d r a u l i c f r a c t u r e p r o p a g a t i o n i n t h e p r e s e n c e o f s t r e s s var i a t i o n s . MOSCHOVIDIS ( 1 9 8 7 ) comments o n m i n i f r a c t u r i n g t r e a t m e n t s c a r r i e d o u t a t t h e i n t e r f a c e o f two f o r m a t i o n s , and BIOT, MEDLIN & MASSE ( 1 9 8 6 ) e v a l u a t e c r a c k p e n e t r a t i o n t h r o u g h an i n t e r f a c e . LAM & CLEARY ( 1 9 8 4 ) o u t l i n e s l i p p a g e and r e i n i t i a t i o n o f h y d r a u l i c f r a c t u r e s a t f r i c t i o n a l i n t e r f a c e s . JOHN ( 1 9 8 7 ) r e p o r t s a s p e c t s o f g a s r e c o v e r y f r o m deep t i g h t f o r m a t i o n s i n l i g h t o f n o n l i n e a r e l a s t i c a l d e f o r m a t i o n c h a r a c t e r i s t i c s . THIERCELIN, JEFFREY & BEN NACEUR ( 1 9 8 7 ) and WANG & CLIFTON ( 1 9 8 9 ) o u t l i n e t h e i n f l u e n c e o f f r a c t u r e t o u g h n e s s o n crack geometry. Rock m e c h a n i c a l a s p e c t s o f N o r t h Sea r e s e r v o i r s a r e e v a l u a t e d b y HOLT, INGSOY & MIKKELSEN ( 1 9 8 7 ) . EVANS ( 1 9 8 3 ) and EVANS & HOLZHAUSEN ( 1 9 8 3 ) comment o n d e v e l o p m e n t o f s h a l l o w h y d r a u l i c f r a c t u r e s as examined t h r o u g h t h e s u r f a c e def o r m a t i o n f i e l d . LABUDOVIC ( 1 9 8 4 ) r e v i e w s t h e e f f e c t o f P o i s s o n ' s r a t i o o n f r a c t u r e h e i g h t . LERCHE & NARR ( 1 9 8 6 ) d i s c u s s p o s s i b i l i t i e s o f e s t i m a t i n g s u b s u r f a c e c r a c k d e n s i t y f r o m c o r e e v a l u a t i o n s . M e c h a n i c a l a s p e c t s of f r a c t u r e e x t e n s i o n a r e a l s o o u t l i n e d b y HARRISON, McGUIRE & KIESCHNICK ( 1 9 5 4 ) ; HUBBERT & WILL I S (19571, HOWARD & FAST ( 1 9 7 0 a, 1970 b ) and DANESHY ( 1 9 7 6 ) .
4.2.5.
Combination of hydraulic fracturing and gravel packing
W i t h i n many s h a l l o w s a n d s t o n e s and c a r b o n a t e s h e l l l a y e r s , the question o f f r a c t u r e c o n t a i n m e n t i s accompanied b y t h e p r o b l e m o f i n s u f f i c i e n t r e s e r v o i r rock s t a b i l i t y t h a t requires o f t e n gravel packing i n a d d i t i o n t o hydraulic s t i mulation o r a p p l i c a t i o n o f resin-coated proppants ( c f . section 1.2.6.) i n order t o c r e a t e a s t a b l e m a s s i v e m u l t i l a y e r d r a i n a g e wedge w i t h i n t h e l o o s e f o r m a t i o n by c e m e n t a t i o n o f t h e p r o p p a n t g r a i n s a t t h e i r c o n t a c t s due t o m e l t i n g and g l u e i n g o f t h e r e s i n f i l m s c o a t i n g t h e p r o p p a n t p a r t i c l e s and t o m i n i m i z e o r e v e n a v o i d embedment o f p r o p p a n t s i n t h e s o f t f o r m a t i o n ( c f . s e c t i o n s 4 . 5 . 4 . 3 . and 5 . 3 . 3 . ) . Many c o n v e n t i o n a l f r a c t u r i n g j o b s c a r r i e d o u t so f a r i n such h o r i z o n s f a i l e d because o f t o o much p r o p p a n t embedment i n t h e s o f t r e s e r v o i r s ( c f . sect i o n 4 . 3 . 3 . ) , f r a c t u r e c l o s u r e due t o p l a s t i c m o b i l i t y o f p o o r l y c o n s o l i d a t e d o r o v e r p r e s s u r e d , i n t e r c a l a t e d o r a d j o i n i n g mudstones ( c f . p l a t e I X / 7 ) , o r sand c o n t r o l p r o b l e m s i n t h e w e l l s . The c o m b i n a t i o n o f p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g i s p a r t i c u l a r l y i n s o f t sands and c h a l k s a v e r y s u i t a b l e means f o r a v o i d i n g p r o p p a n t and f o r m a t i o n f l o w b a c k i n t o t h e w e l l b o r e a f t e r h y d r a u l i c s t i m u l a t i o n o f t h e r e s e r v o i r ( c f . s e c t i o n 4 . 5 . 4 . 3 . and 4 . 1 2 . 3 . 3 . ) . Combined h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g t r e a t m e n t s o f u n c o n s o l i d a t e d p a y zones a r e u s u a l l y p e r f o r m e d i n d e p t h r a n g e s u p t o 5,000 f t (1,700 m), b u t i n some c a s e s a l s o f o r m a t i o n s i n g r e a t e r d e p t h a r e t r e a t e d i n t h i s manner. Once r e s i n - c o a t e d m a t e r i a l s become n e c e s s a r y f o r h y d r a u l i c f r a c t u r i n g a n d / o r g r a v e l p a c k i n g (SINCLAIR & GRAHAM 1977, 1978; CONSTIEN & MAYER 1978, SAUNDERS & McKENZIE 1979, KANAT 1980; UNDERDOWN, DAY & SPARLIN 1980; POPE, WILES & PIERCE 1 9 8 7 ) , p r o p p a n t p r i c e i s a l w a y s a c r i t i c a l f a c t o r due t o t h e more e x p e n s i v e mat e r i a l , and m a r k e t i n g a c t i v i t y h a s t o t a k e n o t i c e o f t h e need o f s p e c i a l i n t r o d u c t o r y o f f e r s f o r promoting p i l o t treatments. Main t a r g e t s f o r t h e approJch i n t h i s c o n t e x t a r e m a r g i n a l c h a l k s , s h e l l - d e b r i s and s a n d - s t r e a k interLL.:,:iions i n t o mudstones i n many o f t h e a f o r e m e n t i o n e d g e o l o g i c a l f o r m a t i o n s and g e o g r a p h i c a l a r e a s ( c f . s e c t i o n s 4 . 2 . 1 . and 5 . 5 . 4 . ) t h a t a r e m e c h a n i c a l l y o r c h e m i c a l l y i n s u f f i c i e n t l y s t a b l e t o a l l o w c o n v e n t i o n a l s t i m u l a t i o n , and l o o s e p u r e o r muddy sands a l t e r n a t i n g w i t h mudstones i n a s a n d w i c h - t y p e manner. A s p e c t s o f c o m b i n a t i o n o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a r e a l s o d i s c u s s e d b y POOLLEN & MALONE ( 1 9 5 9 ) , SPARLIN & COPELAND ( 1 9 7 2 ) and SOLUM (1986; c f . sect i o n s 4 . 5 . 4 . 3 . and 5 . 3 . 3 . ) .
411
4.2.6. Economical significance While many s p e c i f i c t e c h n o l o g i c a l q u e s t i o n s o f t h e most s u i t a b l e methods and m a t e r i a l s have s t i l l t o be s o l v e d by f i e l d and l a b o r a t o r y s t u d i e s , I am r e a l i z i n g i n h y d r a u l i c proppant f r a c t u r i n g ( p a r t i a l l y i n c o m b i n a t i o n w i t h g r a v e l p a c k i n g ) o f r e s e r v o i r s o f l i m i t e d t h i c k n e s s and changing s t a b i l i t y c o n s i d e r a b l e p o t e n t i a l o f e x t e n s i o n o f b o t h r e s e r v o i r s t i m u l a t i o n and hydrocarbon e x p l o i t a t i o n markets i n b o t h Western and E a s t e r n Europe a c c o r d i n g t o my e x p e r i e n c e , as most o f these h o r i z o n s become i n c r e a s i n g l y i m p o r t a n t d u r i n g t h e n e x t y e a r s when p r o d u c t i o n f r o m o t h e r pay zones w h i c h a r e easy t o handle i n view o f r e s e r v o i r and t e c h n i c a l e n g i n e e r i n g i s d e c l i n i n g more and more, and t h e mentioned h o r i zons where p r o d u c t i o n was c a r r i e d o u t so f a r i n reduced r a t e s p a r t i a l l y f a r below t h e r e a l c a p a c i t y o r was even c o m p l e t e l y suspended due t o t h e d i s c u s s e d problems w i l l have t o c o n t r i b u t e i n i n c r e a s i n g amounts i n o r d e r t o d e l a y and slow down t h e g e n e r a l d e c l i n e o f annual c u m u l a t i v e domestic o i l (and p a r t i a l l y a l s o gas) p r o d u c t i o n i n t h e i n d i v i d u a l c o u n t r i e s .
A s i m i l a r conclusion w i l l apply i n the nearer o r f a r t h e r f u t u r e also t o the m a j o r R o t l i e g e n d and C a r b o n i f e r o u s g a s - b e a r i n g sandstones when new d i s c o v e r i e s by e x p l o r a t i o n r e a c h t h e stage t o be no l o n g e r a b l e t o keep pace w i t h t h e s t i l l i n c r e a s i n g demand o f gas, and then t h e m i s s i n g q u a n t i t i e s have t o d e r i v e a l s o i n these m a j o r f o r m a t i o n s i n g r e a t e r amounts f r o m s t i m u l a t i o n o f h i t h e r t o unconv e n t i o n a l t i g h t r e s e r v o i r s ( t h i s b e i n g more o r l e s s a c o n t i n u a t i o n o f t h e i n some areas p r e s e n t l y suspended e x p l o r a t i o n and a p p r a i s a l d r i l l i n g s t r a t e g y concept). I n such b i g o p e r a t i o n s i n b o t h Europe and USA where t h e MHF expenses can acc o u n t f o r up t o h a l f o f t h e t o t a l w e l l c o s t s , t h e importance o f f r a c t u r e s t i m u l a t i o n can even be equal t o o r g r e a t e r t h a n t h a t o f development d r i l l i n g f o r i n c r e a s i n g r e c o v e r a b l e r e s e r v e s and a h i g h degree o f r e s o l u t i o n i n f r a c t u r e des i g n i s e c o n o m i c a l l y v e r y v i a b l e (VEATCH 1983).
412
4.3. Proppant s a t u r a t i o n and c o n d u c t i v i t y improvement Advances i n t e c h n o l o g y and e q u i p m e n t a l l o w e d i n r e c e n t y e a r s an i m p o r t a n t breakthrough i n f r a c t u r i n g treatment e f f e c t i v i t y by successively increasing proppant concentration per f l u i d u n i t . While conventional f r a c t u r i n g stimulat i o n a l s o includes progressive stepwise increase o f proppant s a t u r a t i o n during c o u r s e o f t h e j o b , t h e d e c i s i v e i n n o v a t i o n i n t h e l a s t y e a r s was t h e a c h i e v e ment o f u l t r a - h i g h p r o p p a n t c o n c e n t r a t i o n s i n t h e f i n a l s t a g e s o f t h e o p e r a t i o n . I n r e c e n t y e a r s , t h e t r e n d i n h y d r a u l i c f r a c t u r i n g i s t o employ h i g h e r i n j e c t i o n r a t e s , l o n g e r pumping t i m e , h i g h e r p r o p p a n t c o n c e n t r a t i o n s , and more e f f i c i e n t f l u i d s (SHAH 1 9 8 4 ) . A f t e r d i s c u s s i o n o f some g e n e r a l a s p e c t s , comments a r e g i v e n as f o l l o w s on s e l e c t i o n o f p r o p p a n t t y p e , p r o p p a n t embedment and f r a c t u r e w i d t h , p r o p p a n t t r a n s p o r t and f r a c t u r e damage, u l t r a - h i g h f r a c t u r e conduct i v i t y a c h i e v e m e n t , and u n i f o r m v s . c h a n g i n g f r a c t u r e c o n d u c t i v i t y .
4.3.1. General aspects H i s t o r i c a l l y , s t i m u l a t i o n d e s i g n has r e p e a t e d l y changed a c c o r d i n g t o p r o gress i n the technological e v o l u t i o n o f hydraulic f r a c t u r i n g . I n e a r l i e r years, p r o p p a n t c o n c e n t r a t i o n s u p t o a b t . 1 . 5 l b s / g a l were used. W i t h i m p r o v i n g e q u i p ment c a p a c i t y , s t e p w i s e advance was made v i a s a t u r a t i o n s o f a b t . 3 - 5 l b s / g a l u n t i l r e c e n t l y i t has become p o s s i b l e t o h a n d l e e v e n u l t r a - h i g h c o n c e n t r a t i o n s u p t o 15 - 20 l b s / g a l o r e v e n more, and a l s o t h e p r e s e n t t e c h n i c a l b a r r i e r s a r e c h a l l e n g e d (GREGORCZYK, PAULS, HOLTMYER, VENDITTO & CHISHOLM 1984; MARPLE, RULEY, WORLEY & FREEMAN 1987; s i m i l a r improvements have been a c h i e v e d f o r p a c k i n g g r a v e l c o n c e n t r a t i o n i n m e c h a n i c a l sand c o n t r o l ; c f . s e c t i o n 5 . 3 . 1 . ) . The w o r l d r e c o r d o f t h e h i g h e s t sand s a t u r a t i o n i n a h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t i s 26 l b s / g a l w h i c h was pumped i n Caddo P a r i s h i n L o u i s i a n a / U S A i n 1981 (FRACFAX 1988 k ) . F o l l o w i n g d e f i n i t i o n o f t h e d i f f e r e n t t y p e s o f p r o p p a n t concentration, the s i g n i f i c a n c e o f h i g h proppant s a t u r a t i o n i s outlined.
4.3.1.1. Types o f proppant concentration Two t y p e s o f p r o p p a n t s a t u r a t i o n have t o b e d i s t i n g u i s h e d i n u l t r a - h i g h sand c o n c e n t r a t i o n t r e a t m e n t s (USC j o b s ; SHAH, SMITH & OONALDSON 1 9 8 3 ) . Sand concent r a t i o n on t h e s u r f a c e i s e x p r e s s e d i n pounds o f sand added t o a g a l l o n o f c l e a n f l u i d ( l b s / g a l ) , whereas sand s a t u r a t i o n w i t h i n t h e c r a c k i s r e f e r r e d t o as pounds o f sand p e r s q u a r e f o o t o f f r a c t u r e a r e a ( l b s / f t 2 ) . I n most o f t h e c o n v e n t i o n a l t r e a t m e n t s c o n d u c t e d w i t h p r o p p a n t - s u s p e n d i n g f l u i d s , s u r f a c e sand c o n c e n t r a t i o n s v a r y from 0.5 t o 4 l b s / g a l which t r a n s l a t e s t o l e s s than 2 lbs/ft2 (10 kg/m2) i n t h e f r a c t u r e downhole, whereas u l t r a - h i g h s a t u r a t i o n s o f 20 l b s / g a l a t t h e s u r f a c e r e s u l t i n p l a c e m e n t o f 7 l b s / f t 2 ( 3 5 kg/m2) downhole. The c u r r e n t i n d u s t r y s t a n d a r d r a n g e s more o r l e s s between 8 and 15 l b s / g a l f o r m o s t p a r t s of t h e t r e a t m e n t , and e x t r a o r d i n a r i l y h i g h s a t u r a t i o n s o f 15 - 25 l b s / g a l a r e o c c a s i o n a l l y r e a c h e d in t h e t e r m i n a l s t a g e s o f t h e j o b s .
4 . 3 . 1 . 2 . Significance o f h i g h proppant s a t u r a t i o n The method o f p r o g r e s s i v e l y r i s i n g p r o p p a n t c o n c e n t r a t i o n d u r i n g t h e j o b w i t h f i n a l l y r e a c h i n g u l t r a - h i g h s a t u r a t i o n s (COULTER & WELLS 1972, DOMSELAAR & V I S S E R 1974, SCHOLS & VISSER 1974, WATERS 1980; SHAH, SMITH & DONALDSON 1983; GREGORCZYK, PAULS, HOLTMYER, VENDITTO & CHISHOLM 1984; DUNANO & SOUCEMARIANADIN 1985; PAULS, VENDITTO, CHISHOLM, HOLTMYER & GREGORCZYK 1985) i n t h e t e r m i n a l p a r t o f t h e t r e a t m e n t a l l o w s b e t t e r i n f i l l i n g o f t h e f r a c t u r e s and t h e r e f o r e p e r m i t s a l s o t o i n c r e a s e c r a c k w i d t h t h u s i m p r o v i n g f r a c t u r e f l o w c a p a c i t y , and t o optimize t a i l - i n p o l i c i e s ( c f . s e c t i o n 2.4.1.2.3.) by using d i f f e r e n t propp a n t t y p e s and g r a i n s i z e s i n t h e sequence o f s t e p s o f t h e p r o c e d u r e . I m p r o v a l o f t h e r e s u l t o f t h e s t i m u l a t i o n j o b by i n c r e a s i n g p r o p p a n t s a t u r a t i o n s i s cons i d e r e d t o be one o f t h e m o s t i m p o r t a n t t o o l s o f p r o m o t i o n and e c o n o m i c a l l y f e a -
413 s i b l e o p e r a t i o n o f h y d r a u l i c f r a c t u r i n g o f deep t i g h t R o t l i e g e n d and C a r b o n i f e r o u s gas w e l l s i n Europe w i t h i n t h e n e x t y e a r s ( c f . s e c t i o n s 2.4.1. and 3 . 3 . ) . The more complete f r a c t u r e f i l l u p by h i g h p r o p p a n t c o n c e n t r a t i o n s l e a d s t o great e r s u s t a i n e d f r a c t u r e f l o w c a p a c i t y and h i g h e r proppant c r u s h i n g r e s i s t a n c e ( c f . s e c t i o n 4.12.4.) which u l t i m a t e l y g i v e r i s e t o a p o t e n t i a l l y h i g h e r s u s t a i ned p r o d u c t i o n i n c r e a s e a f t e r t h e s t i m u l a t i o n (ILSENG & CORTEZ 1985).
4.3.2. Selection of proppant type U l t r a - h i g h proppant concentrations i n the f l u i d together w i t h increasing f r a c t u r e w i d t h sometimes even a l l o w t h e a p p l i c a t i o n o f a l i g h t e r and t h u s cheap e r p r o p p a n t ( c f . s e c t i o n 1 . 3 . ) , as some c r u s h i n g o f g r a i n s i s c o n s i d e r e d t o be n e g l i g i b l e and t o be compensated by t h e h i g h p r o p p a n t s a t u r a t i o n where t h e i n d i v i d u a l g r a i n s a r e much b e t t e r s u p p o r t i n g each o t h e r i n a w i d e r f r a c t u r e and c l o s u r e s t r e s s i s d i s t r i b u t e d on more g r a i n s p e r area u n i t t h a n i n case o f l o w e r p r o p p a n t c o n c e n t r a t i o n s and n a r r o w e r f r a c t u r e s . U l t r a - h i g h p r o p p a n t s a t u r a t i o n i s a p a r t i c u l a r l y s u i t a b l e method f o r enhancing f r a c t u r e f l o w c a p a c i t y and d i m i n i s h i n g e f f e c t s o f p r o p p a n t embedment w i t h i n s o f t f o r m a t i o n s . Proppant d e n s i t y and c o n c e n t r a t i o n , s i g n i f i c a n c e o f i n t e r m e d i a t e - s t r e n g t h proppants, and benef i t s o f h i g h sand c o n c e n t r a t i o n s p r i o r t o s y n t h e t i c p r o p p a n t i n v e n t i o n a r e d i s cussed as f o l l o w s .
4.3.2.1. Proppant density and concentration The a p p l i c a t i o n o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y p r o p p a n t s i s e s p e c i a l l y a t t r a c t i v e , as w i t h l o w e r s p e c i f i c g r a v i t y and t h u s b u l k w e i g h t , more p r o p p i n g m a t e r i a l i n terms o f volume c o u l d be pumped p e r f l u i d u n i t than i n case o f heav i e r proppants ( c f . s e c t i o n s 1.4.7. and 1 . 4 . 1 2 . 5 . ) . I f c l o s u r e s t r e s s i s t o o h i g h t o a l l o w t h e usage o f i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y proppants, t h e h i g h - w e i g h t p r o p p a n t t y p e w i t h t h e l o w e s t d e n s i t y would c e r t a i n l y have an advantage o f c h o i c e i f i t s p r i c e i s n o t t o o h i g h and thus beyond c o n s i d e r a t i o n ( c f . s e c t i o n s 1.3. and 2 . 2 . 2 . 3 . ) . P r a c t i c a l e x p e r i e n c e has shown so f a r t h a t sand c o n c e n t r a t i o n s up t o 16 l b s / g a l f l u i d can be handled w i t h o u t any problems and sand s a t u r a t i o n s o f 8 - 12 l b s / g a l a r e f a i r l y common i n modern t r e a t m e n t s . Concerning i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants, c o n c e n t r a t i o n s h i g h e r than 6 - 8 l b s / g a l a r e u s u a l l y n o t needed because o f t h e b e t t e r f l o w c a p a c i t y o f t h e h i g h - q u a l i t y s y n t h e t i c proppants and t h e n e g l i g i b i l i t y t o even absence o f embedment i n deeper format i o n s where man-made p r o p p a n t s a r e r e q u i r e d i n c o n t r a s t t o s e r i o u s embedment problems i n s h a l l o w r e s e r v o i r s where n a t u r a l sand i s s t i l l f r e q u e n t l y pumped as a m a t t e r o f c o s t containment. R i s i n g proppant c o n c e n t r a t i o n , however, always has t o t a k e i n t o account t h e p r i n c i p a l l y i n c r e a s i n g r i s k o f s c r e e n o u t f a i l u r e by e a s i e r b r i d g i n g and s t i c k i n g o f t h e s a t u r a t e d s l u r r y i n t h e c r a c k as a consequence o f i n c r e a s i n g f r i c t i o n ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) .
4.3.2.2. Sign if icance o f i ntermediate-st rength proppant s I n t e r m e d i a t e - s t r e n g t h proppants can be a p p l i e d up t o c l o s u r e s t r e s s e s o f 12,000 - 13,000 p s i w i t h s a t i s f a c t o r y performance ( c f . s e c t i o n 1 . 3 . 2 . ) . A s most r e s e r v o i r s do n o t e x h i b i t such h i g h pressures, t h e r e i s n o t v e r y much r e q u i r e ment f o r h i g h - s t r e n g t h p r o p p a n t s such as s i n t e r e d b a u x i t e t o j u s t i f y t h e c o s t premium. A much b e t t e r s o l u t i o n f o r improvement o f f r a c t u r e c o n d u c t i v i t y i s t h u s i n c r e a s i n g proppant c o n c e n t r a t i o n i n s t e a d o f s e l e c t i n g h i g h - s t r e n g t h proppants f o r i n t e r m e d i a t e - s t r e n g t h ones. P r o g r e s s i v e proppant s a t u r a t i o n g i v e s r i s e t o a g r a d u a l c o n d u c t i v i t y i n c r e a s e and l e t s t h e f r a c t u r e respond l i k e hav i n g a h i g h e r c o n d u c t i v i t y w i t h l e s s t o t a l proppant m a t e r i a l . Economical and t e c h n i c a l improvement i n c l u d e two o p t i o n s c o m p r i s i n g
applica-
414 t i o n o f e i t h e r l e s s proppants t o g e t t h e same c o n d u c t i v i t y o r u s i n g t h e same amount o f proppants t o achieve a b e t t e r r e s u l t when comparing t h e performance o f two d i f f e r e n t proppant q u a l i t y t y p e s . I n h i g h e r c o n c e n t r a t i o n s , even a p r o p p a n t t y p e o f i n f e r i o r q u a l i t y can p e r f o r m b e t t e r than a s u p e r i o r p r o p p a n t t y p e i n low s a t u r a t i o n s , and t h e r e f o r e most o f t h e hydrocarbon-bearing p r o s p e c t i v e i n t e r v a l s can be s a t i s f a c t o r i l y f r a c t u r e d w i t h i n t e r m e d i a t e - s t r e n g t h low- o r h i g h - d e n s i t y proppants, w i t h t h e l i g h t alumina s i l i c a t e m a t e r i a l h a v i n g t h e g r e a t e s t p o t e n t i a l due t o i t s s p e c i f i c g r a v i t y comparable t o sand b u t much h i g h e r c l o s u r e s t r e s s r e s i s t i v i t y which i s even i n c r e a s e d i n m u l t i l a y e r packages ( c f . s e c t i o n 1 . 3 . 2 . ) .
B e n e f i t s o f high sand concentration P r i o r t o s y n t h e t i c Proppant i n v e n t i o n 4.3.2.3.
P r i o r t o i n v e n t i o n and market i n t r o d u c t i o n o f i n t e r m e d i a t e - and h i g h s t r e n g t h s y n t h e t i c proppants f o r deep gas r e s e r v o i r s t i m u l a t i o n ( c f . s e c t i o n 1 . 2 . ) , reasonable r e s i s t i v i t y o f t h e p r o p p a n t package a g a i n s t h i g h c l o s u r e s t r e s s e s has been achieved by pumping h i g h t o e x c e s s i v e c o n c e n t r a t i o n s o f n a t u r a l sand (HOLDITCH & ELY 1973). The advantage o f sand m u l t i l a y e r i n g w i t h r e s p e c t t o p a r t i a l o r complete m o n o l a y e r i n g has proven t o be q u i t e s i g n i f i c a n t a t l e a s t t o l i m i t e d c l o s u r e s t r e s s l e v e l s o f a b t . 6,000 p s i , and t h e r e f o r e h i g h c o n c e n t r a t i o n s o f n a t u r a l sand have t e m p o r a r i l y been even a c o s t a l t e r n a t i v e t o s i n t e r e d b a u x i t e (CALLANAN, CIPOLLA & LEWIS 1983). On t h e o t h e r hand, however, pumping r a t e s and f l u i d v i s c o s i t i e s necessary t o i n j e c t h i g h sand s a t u r a t i o n s a r e sometimes d e t r i m e n t a l by c a u s i n g a l a r g e p r e s s u r e drop a l o n g t h e f r a c t u r e and p r o m o t i n g e x c e s s i v e h e i g h t growth i n t o u n p r o d u c t i v e zones (NOLTE 1982; c f . section 4.2.2.3.). Aspects o f h i g h proppant c o n c e n t r a t i o n s and l a r g e t o t a l p r o p p a n t l o a d s i n h i g h - and low-temperature a p p l i c a t i o n s a r e d i s c u s s e d by SANDY, W I G G I N S & VENDITTO (1986), and f u r t h e r examples o f h i g h p r o p p a n t s a t u r a t i o n s i n h y d r a u l i c f r a c t u r i n g a r e c o n t r i b u t e d by SEIDEL & STAHL ( 1 9 7 2 ) , HOLDITCH & ELY (1973), CROSS & SHELDON (1977) ; COULTER, MATTHEWS, SEGLEM & SMITH (1980) ; PETZET (1982), WATK I N S (1982); H A R R I S , REIDENBACH & CHISHOLM (1986) and ROSE ( 1 9 8 8 ) . DEHLINGER, BROWNE & BUNDRANT (1954) comment on optimum sand c o n c e n t r a t i o n s i n w e l l t r e a t ments. Proppant p a c k i n g f e a t u r e s o f h y d r a u l i c f r a c t u r e s a r e a l s o e v a l u a t e d by ALDERMAN & WENDORFF ( 1 9 6 9 ) . I n terms o f proppant t y p e and s a t u r a t i o n , f i e l d e x p e r i e n c e i n moderate- t o h i g h - d e p t h o i l r e s e r v o i r s i n China has shown t h a t i n v a r i o u s cases, f r a c t u r e s propped w i t h l o w e r c o n c e n t r a t i o n s o f b a u x i t e have b e t t e r c o n d u c t i v i t i e s t h a n c r a c k s propped w i t h h i g h e r s a t u r a t i o n s o f sand ( L I & ZHU 1 9 8 6 ) .
4.3.3. Proppant embedment and f r a c t u r e width Proppant c o n c e n t r a t i o n t o g e t h e r w i t h f r a c t u r e w i d t h is an e x t r e m e l y import a n t f r a c t u r e d e s i g n parameter i n t r e a t i n g b o t h h a r d and s o f t r o c k s . P a r t i a l mon o l a y e r ( s i n g l e l a y e r ) p r o p p i n g (DARIN & H U I T T 1960) t h e o r e t i c a l l y p r o v i d e s maximum f r a c t u r e f l o w c a p a c i t y , b u t i s n o t p r a c t i c a l due t o embedment and c r u s h i n g i n s o f t and h a r d f o r m a t i o n s , r e s p e c t i v e l y (CLARK 1983). Thus under most cond i t i o n s , p a r t i a l monolayer p r o p p i n g i s n o t s u i t a b l e , because i t i s v e r y d i f f i c u l t t o achieve t h e d e s i r e d placement geometry (HOLDITCH 1984). Aspects o f m u l t i l a y e r f r a c t u r e propping, p r o p p a n t p i l l a r i n g , f r a c t u r e w i d t h v s . non-Darcy f l o w , f r a c t u r e w i d t h vs. p r o p p a n t b r i d g i n g , and p r o p p a n t c o n c e n t r a t i o n schedul e s a r e o u t l i n e d as f o l l o w s .
415
4.3.3.1.
Mult i l a y e r f r a c t u r e propping
M u l t i p l e l a y e r f r a c t u r e p r o p p i n g i s b e n e f i c i a l i n s o f t r o c k s because o n l y t h e o u t e r p r o p p a n t l a y e r s a r e s u b j e c t e d t o embedment, and i n h a r d f o r m a t i o n s , t h e i n c r e a s e d number of p a r t i c l e s r e s u l t s i n a g r e a t e r amount o f c o n t a c t p o i n t s t h a t a c t s t o m i n i m i z e f r a c t u r e d e t e r i o r a t i o n due t o c l o s u r e - s t r e s s induced propp a n t c r u s h i n g ( c f . s e c t i o n s 1.2.1.1. and 4.12.4.). W h i l e embedment improves r e t e n t i o n o f p r o p p a n t p e r m e a b i l i t y a t h i g h e r c l o s u r e s t r e s s e s because t h e s t r e s s i s spread o v e r a l a r g e r p o r t i o n o f t h e p a r t i c l e , i t a l s o a l l o w s p a r t i a l f r a c t u r e c l o s u r e t h e r e b y r e d u c i n g c r a c k w i d t h and c o n d u c t i v i t y (MONTGOMERY & STEANSON 1985). Embedment depends on d i a m e t e r and number o f proppant g r a i n s p e r u n i t a r e a o f t h e f r a c t u r e which matches p r o p p a n t c o n c e n t r a t i o n and i s a l s o v e r y sens i t i v e t o f o r m a t i o n hardness (HOWARD & FAST 1970). Proppant embedment i s o f t e n combined w i t h f i n e s m i g r a t i o n (SHAH, SMITH & DONALDSON 1983). F o r m a t i o n s t r e n g t h which i s a c o n t r o l l i n g f a c t o r on embedment can be determined by t h e mec h a n i c a l p r o p e r t i e s l o g (ANDERSON e t a l . 1975; c f . s e c t i o n s 6.2.1.10.7. and
6.3.6.).
4.3.3.1.1.
General aspects
Aspects o f embedment o r engulfment o f h i g h - s t r e n g t h proppants i n t o low-perm e a b i l i t y r e s e r v o i r r o c k s a r e d i s c u s s e d by VOLK, RAIBLE, CARROLL & SPEARS (1981), and HUITT & McGLOTHLIN (1958) and McGLOTHLIN & HUITT (1966) comment on r e l a t i o n o f f o r m a t i o n r o c k s t r e n g t h t o p r o p p i n g agent s t r e n g t h . ROODHART, K U I PER & D A V I E S (1986) o u t l i n e p r o p p a n t r o c k impairment. F r a c t u r e w i d t h i s d i r e c t l y p r o p o r t i o n a l t o g e l v i s c o s i t y and i n j e c t i o n r a t e o f t h e s t i m u l a t i o n f l u i d (BARBY & BARBEE 1987), and i n c r e a s i n g p r o p p a n t s a t u r a t i o n g i v e s r i s e t o h i g h e r g e l v i s c o s i t i e s t h e r e b y t r i g g e r i n g t h e o r i g i n o f w i d e r f r a c t u r e s . O p e r a t i n g cond i t i o n s which cause h i g h p r e s s u r e drop a l o n g t h e f r a c t u r e such as h i g h i n j e c t i o n r a t e s and v i s c o u s f l u i d s r e s u l t i n r e l a t i v e l y wide cracks, whereas convers e l y t r e a t m e n t parameters c a u s i n g low p r e s s u r e drop a l o n g t h e f r a c t u r e such as low i n j e c t i o n r a t e s and t h i n f l u i d s g i v e r i s e t o r e l a t i v e l y narrow c r a c k s (PERK I N S & KERN 1961, BARBY & BARBEE 1987; c f . s e c t i o n 4.2.2.3.). Pay zone t h i c k ness has no e f f e c t on p r e s s u r e o r w i d t h o f h o r i z o n t a l f r a c t u r e s , w h i l e p r e s s u r e drop a l o n g r e s t r i c t e d v e r t i c a l f r a c t u r e s i s l a r g e f o r t h i n zones b u t s m a l l f o r t h i c k i n t e r v a l s . On t h e o t h e r hand, c r a c k w i d t h a t t h e w e l l b o r e i s n e a r l y i n d e pendent o f v e r t i c a l f r a c t u r e h e i g h t ( c f . s e c t i o n 6.2.2.2.). R e s e r v o i r d e p t h has g e n e r a l l y l i t t l e e f f e c t on f r a c t u r e w i d t h e x c e p t o f v a r y i n g r o c k p r o p e r t i e s t o which f r a c t u r e w i d t h i s n o t p a r t i c u l a r l y s e n s i t i v e . High p r o p p a n t c o n c e n t r a t i o n s i n o r d e r t o a v o i d embedment a r e a l s o recommenf o r geopressured r e s e r v o i r s (ROBINSON, HOLDITCH & LEE 1983; c f . s e c t i o n 4.4.2.). I n h a r d r e s e r v o i r r o c k s o f c o n t i n u o u s sand body d i s t r i b u t i o n , p r o p p a n t embedment does n o t occur, b u t impairment o f c o n d u c t i v i t y i s caused b y r e s e r v o i r f o r m a t i o n c r e e p a t t h e f r a c t u r e w a l l . I n h a r d sandstone pay complexes where i s o l a t e d sandstone l e n s e s a r e separated by mudstones ( c f . p l a t e s II/7 - 8, 111/4, V I I / 3 + 6, V I I I / 7 - 8 and IX/l), proppant embedment i n t o i n t e r v e n i n g s h a l e s may be a problem. A much l a r g e r f r a c t u r e w i d t h than expected i n p r o p p a n t embedment t e s t i n g i n d i c a t e s occurrence o f p r o p p a n t s t a c k i n g (VOLK, RAIBLE, CARROLL & SPEARS 1981). The most i m p o r t a n t aims o f m u l t i l a y e r f r a c t u r e p r o p p i n g a r e c r u s h i n g and embedment r e d u c t i o n as w e l l as c o n d u c t i v i t y improvement which a r e i l l u s t r a t e d below, w i t h emphasis b e i n g p u t on t h e d i f f e r e n t p r o p e r t i e s o f s o f t and h a r d r e s e r v o i r r o c k s and t h e i r d i f f e r e n t r e a c t i o n t o mono- and m u l t i l a y e r f r a c t u r e propping. ded
4.3.3.1.2.
Crushing and embedment reduction
W h i l e some t i g h t r e s e r v o i r s w i t h p e r m e a b i l i t i e s o f a b t . 0.001 - 0.05 md can s t i l l be s u f f i c i e n t l y d r a i n e d w i t h crushed proppants o r sand because t h e f r a g ments s t i l l p r o v i d e s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t ( c f . s e c t i o n s 4.6.1. and
416 4 . 1 2 . 4 . ) , s e r i o u s c o n d u c t i v i t y impairment i s generated i n t h e p r o p p a n t package o f f r a c t u r e s i n moderate- t o h i g h - p e r m e a b i l i t y f o r m a t i o n s by c r u s h i n g and embedment (HOLDITCH & LEE 1 9 7 9 ) . A b e t t e r d e s i g n s o l u t i o n aiming on h i g h e r p r o p p a n t c o n c e n t r a t i o n p e r u n i t area o f t h e f r a c t u r e w a l l i s t h e r e f o r e a m u l t i p l e - l a y e r pack c o n f i g u r a t i o n o f p r o p p a n t p a r t i c l e s c o n s e q u e n t l y i n f i l l i n g a w i d e r f r a c t u r e . A f t e r d i s c u s s i o n o f some g e n e r a l aspects, s p e c i a l a p p l i c a t i o n s i n s o f t sands and h a r d sandstones a r e o u t l i n e d .
4.3.3.1.2.1.
General aspects
F r a c t u r e p o r o s i t y decreases as l o a d i s a p p l i e d because p r o p p a n t g r a i n s c r u s h and embed i n t o t h e r o c k m a t r i x (WENDORFF 1982). W h i l e i n competent r o c k s , p r o p p a n t p o r o s i t y i s u s u a l l y reduced by c r u s h i n g , p r o p p a n t embedment c o u p l e d w i t h r 3 - 7 p a n t c r u s h i n g r e s u l t s i n v e r y low f r a c t u r e p o r o s i t y i n s o f t f o r m a t i o n s . I n i i i o > t propped c r a c k s , however, b o t h embedment and crushing occur ( c f . section 4 . 1 2 . 4 . 3 . 2 . ) and t h e r e s u l t i n g p o r o s i t y i s a measure o f t h e combined e f f e c t . High p r o p p a n t c o n c e n t r a t i o n s p l u g g i n g w i d e r f r a c t u r e s a r e b e n e f i c i a l i n s o f t r o c k s because o n l y t h e o u t e r p r o p p a n t l a y e r s a r e s u b j e c t e d t o embedment and t h e i n n e r l a y e r s remain u n a f f e c t e d , and i n h a r d r o c k s due t o a g r e a t e r number o f c o n t a c t p o i n t s o f t h e i n c r e a s e d amount o f p a r t i c l e s m i n i m i z i n g f r a c t u r e f l o w cap a c i t y d e t e r i o r a t i o n by g r a i n c r u s h i n g i n h i g h e r c l o s u r e s t r e s s regimes which i s l i m i t e d t o p a r t s o f t h e o u t e r l a y e r s . C o n d u c t i v i t y o f f r a c t u r e s i n f i l l e d by p r o p p a n t m u l t i l a y e r s i s much l e s s depending on r e s e r v o i r r o c k hardness t h a n t h a t o f those plugged by p r o p p a n t monolayers o r even p a r t i a l monolayers (DARIN & H U I T T 1960, COOKE 1973). Wide f r a c t u r e s and h i g h p r o p p a n t c o n c e n t r a t i o n s r e s u l t i n g i n a m u l t i l a y e r package a r e a l s o t h e b e s t s o l u t i o n f o r s t i m u l a t i o n o f s o f t t i g h t c h a l k f o r m a t i o n s i n t h e N o r t h Sea (HARTLEY & BOSMA 1985; TANSOE, K I N G & HOLMAN 1986; c f . s e c t i o n 4 . 5 . 4 . ) . A p a r t f r o m monolayers s u f f e r i n g f r o m embedment and c r u s h i n g i n s o f t and h a r d r o c k s , r e s p e c t i v e l y , o t h e r f a c t o r s t e n d i n g t o reduce f r a c t u r e c o n d u c t i v i t y a r e g e l r e s i d u e and f i n e s r e s u l t i n g f r o m i n c o m p l e t e c l e a n - u p o f b r o k e n c r o s s l i n k e d s t i m u l a t i o n f l u i d s ( c f . s e c t i o n 4 . 3 . 4 . 1 . ) and improper s i e v i n g and s c r e e n i n g o f the p r o p p i n g agent, r e s p e c t i v e l y (CLARK 1 9 8 3 ) . Rocks w i t h low embedment p r e s sure a r e s o f t , w i t h proppants t e n d i n g t o embed i n these t y p e o f f o r m a t i o n s which r e s u l t s i n a h e a l i n g e f f e c t o f t h e f r a c t u r e , and r o c k s w i t h h i g h embedment p r e s s u r e a r e h a r d and n o t s u s c e p t i b l e t o p r o p p a n t embedment, b u t r a t h e r s u b j e c t e d t o p r o p p a n t c r u s h i n g . These two d i f f e r e n t f o r m a t i o n t y p e s a r e charact e r i z e d i n more d e t a i l as f o l l o w s .
4.3.3.1.2.2. Soft sands M u l t i l a y e r h i g h - c o n d u c t i v i t y p r o p p i n g o f f r a c t u r e s even p e r m i t s h y d r a u l i c s t i m u l a t i o n o f u n c o n s o l i d a t e d d i r t y sands ( c f . s e c t i o n 4 . 8 . 5 . 4 . ) which cannot be t r e a t e d w i t h c o n v e n t i o n a l t e c h n i q u e s due t o i n s t a n t a n e o u s t o t a l f r a c t u r e c l o s u r e as a consequence o f complete embedment o r e n g u l f m e n t o f p r o p p a n t monol a y e r s i n t o t h e s o f t r o c k m a t r i x (LAMBERT, DOLAN & GALLUS 1 9 8 3 ) . Heterogeneous l o o s e sands w i t h c o n s i d e r a b l e movement o f f o r m a t i o n f i n e s i m p a i r i n g hydrocarbon f l o w can be s u c c e s s f u l l y s t i m u l a t e d by r e d u c t i o n o f f l u i d f l o w v e l o c i t y a t and w i t h i n t h e r o c k f a c e t h r o u g h e f f e c t i v e w e l l b o r e enlargement by f o r m a t i o n f r a c t u r i n g w i t h m a x i m i z i n g p r o p p a n t s a t u r a t i o n and m i n i m i z i n g p r o p p a n t g r a i n s i z e . H i g h e r c a r r i e r f l u i d v i s c o s i t y p r o v i d e s e x c e l l e n t p r o p p a n t suspension and p e r m i t s t o use v e r y h i g h p r o p p a n t c o n c e n t r a t i o n s w i t h r e l a t i v e l y small pad v o l u mes. Proppant s a t u r a t i o n m a x i m i z a t i o n can a l s o be o b t a i n e d t h r o u g h l i m i t i n g f r a c t u r e growth by o n l y p e r f o r a t i n g t h e lower p o r t i o n o f t h e i n t e r v a l t o be t r e a t e d ( a s t h e f r a c t u r e i s p r o p a g a t i n g i n upwards d i r e c t i o n due t o d e c r e a s i n g overburden and c l o s u r e s t r e s s ; c f . s e c t i o n 4 . 2 . 2 . 3 . 1 . ) . Proppant packages i n f i l l i n g w i d e r f r a c t u r e s a r e l e s s s u b j e c t e d t o p l u g g i n g by f i n e s m i g r a t i o n and propp a n t embedment (SHAH, SMITH & DONALDSON 1983).
417 100 mesh o r 70/140 mesh g r a i n s i z e as a l e a d - i n proppant and f l u i d - l o s s agent a l l o w s h i g h - c o n d u c t i v i t y t i p packing o f secondary h a i r l i n e f r a c t u r e s which do n o t accept l a r g e r proppant g r a i n s ( c f . s e c t i o n s 1.4.11.2.1. and 4.8.8.3.2.3.). Flow c a p a c i t y o p t i m i z a t i o n near the w e l l b o r e can be achieved w i t h 8/16 o r 10/20 mesh t a i l - i n f o l l o w i n g conventional pumping o f 20/40 mesh proppants ( c f . s e c t i o n 2.4.1.2.3.). The m u l t i l a y e r proppant package w i t h h i g h proppant c o n c e n t r a t i o n p e r f r a c t u r e w a l l area u n i t guarantees t h a t a high-cond u c t i v i t y flow channel s t i l l remains open a f t e r considerable proppant embedment i n t o the weak f o r m a t i o n m a t r i x and i s the o n l y p o s s i b i l i t y o f s t i m u l a t i n g s o f t unconsolidated sands. I n case o f proper a p p l i c a t i o n , h y d r a u l i c m u l t i l a y e r proppant f r a c t u r i n g i s much s u p e r i o r t o gravel packing i n such types o f r e s e r v o i r s , because g r a v e l packing cannot s u f f i c i e n t l y minimize f o r m a t i o n f l u i d v e l o c i t i e s t o stop m i g r a t i o n o f f i n e p a r t i c l e s w i t h i n more d i s t a l s e c t i o n s o f the r e s e r v o i r and o n l y p r o v i d e s i n h i b i t i o n o f f i n e s movement i n the proximal pay zone seam around the borehole ( c f . s e c t i o n 5.3.5.). The general p o s s i b i l i t y o f hyd r a u l i c f r a c t u r i n g o f s o f t unconsolidated sands i s f r e q u e n t l y h i g h l i g h t e d by considerable losses o f s o l i d s - l a d e n d r i l l i n g mud ( c f . s e c t i o n 5.11.2.) when pen e t r a t i n g such horizons (LAMBERT, DOLAN & GALLUS 1983). S o f t formations r e q u i r e a large-diameter proppant which r e s i s t s t o embedment i n t o the r e s e r v o i r m a t r i x , and medium- t o h i g h - p e r m e a b i l i t y pay zones need a l a r g e - s i z e proppant i n o r d e r t o p r o v i d e adequate f l o w c a p a c i t y (CLARK 1983).
4.3.3.1.2.3. Hard sandstones Crushing r e d u c t i o n independent from proppant c o n c e n t r a t i o n can a l s o be achieved by replacement o f n a t u r a l sand by i n t e r m e d i a t e - o r h i g h - s t r e n g t h proppants i n hard sandstone r e s e r v o i r s i n b o t h deep and shallow w e l l s (PEARCE 1983; c f . sections 2.3.6. and 2.4.2.4.). Proppant monolayers i n hard sandstones n o t perm i t t i n g embedment s u f f e r from c r u s h i n g p r i m a r i l y by p o i n t - t o - p o i n t l o a d i n g o f i n d i v i d u a l sand g r a i n s which are t o o b r i t t l e t o deform. I n c r e a s i n g sand concent r a t i o n i n shallow pay zones where pumping has t o be done w i t h low i n j e c t i o n r a t e s ( c f . s e c t i o n 4.2.2.3.) bears t h e danger o f premature screenout t e r m i n a t i o n , b u t r e d u c t i o n o f c r u s h i n g can be e f f e c t i v e l y performed by u t i l i z i n g a h i g h e r s t r e n g t h proppant p a r t i c u l a r l y when proppant placement i s c a r r i e d o u t i n p i l l a r s o r columns w h i l e t h e f r a c t u r e i s h o l d open by h y d r a u l i c pressure (PUGH & SEGLEM 1977, PEARCE 1983; c f . s e c t i o n 4.3.3.2.). H i g h - q u a l i t y proppants i n mult i l a y e r packages together w i t h increased c o n c e n t r a t i o n are thus e f f e c t i v e methods o f t r e a t i n g b o t h very s o f t and very hard r e s e r v o i r rocks regardless o f t h e i r s t r u c t u r a l depth. Hard rocks u s u a l l y have h i g h c l o s u r e stresses associated w i t h them and theref o r e r e q u i r e a h i g h compressive s t r e n g t h proppant (CLARK 1983; c f . s e c t i o n 1.2.1.1.). Since embedment i s n o t the major problem i n these formations, a s m a l l - s i z e d proppant can be u t i l i z e d which a l s o a l l o w s an increased pack concent r a t i o n t h a t tends t o c o u n t e r a c t the h i g h c l o s u r e stresses a l s o i n l e s s wide f r a c t u r e s , because hard sandstones are u s u a l l y l o w - p e r m e a b i l i t y rocks which do n o t r e q u i r e l a r g e f l o w c a p a c i t i e s w i t h i n t h e cracks and are b e s t t r e a t e d w i t h long narrow highly-packed f r a c t u r e s ( c f . a l s o VOLK, RAIBLE, CARROLL & SPEARS 1981). The assumption o f n e g l i g i b l e embedment and proppant p e r m e a b i l i t y independent o f proppant s a t u r a t i o n i s o n l y v a l i d f o r proppant m u l t i l a y e r s and hard r e s e r v o i r rocks (CALLANAN, CIPOLLA & LEWIS 1983).
4.3.3.1.3. Conductivity enhancement As f r a c t u r e c o n d u c t i v i t y i s d e f i n e d as crack w i d t h times proppant package p e r m e a b i l i t y , wider f r a c t u r e s i n f i l l e d by proppant m u l t i l a y e r s are a s i g n i f i cant means of i n c r e a s i n g crack f l o w c a p a c i t y beyond the t e c h n i c a l and economic a l boundaries o f f r a c t u r e l e n g t h improvement (COOKE 1973). Proppant m u l t i -
418 l a y e r s o r i g i n a t e when s t i m u l a t i o n f l u i d v i s c o s i t y i s low enough t o a l l o w t h e proppants t o s e t t l e t o t h e b o t t o m o f t h e v e r t i c a l c r a c k d u r i n g f l u i d i n j e c t i o n , as w e l l as when f r a c t u r i n g f l u i d v i s c o s i t y i s h i g h enough t o suspend t h e p r o p p a n t i n case o f s u f f i c i e n t l y h i g h proppant c o n c e n t r a t i o n s and s a t i s f a c t o r i l y wide f r a c t u r e s ( c f . s e c t i o n 4 . 3 . 4 . 2 . ) . S p e c i a l a p p l i c a t i o n s comprise h i g h p r o p p a n t s a t u r a t i o n s i n v e r y s m a l l s t i m u l a t i o n j o b s which a r e p l a c e d c l o s e t o t h e w e l l b o r e i n o r d e r t o overcome format i o n damage o r t o s u c c e s s f u l l y t r e a t u n c o n s o l i d a t e d f o r m a t i o n s w i t h h i g h p r e s s u r e drawdown ( i n t h e l a t t e r case, t h e i n s e r t i o n o f r e s i n - c o a t e d p r o p p a n t s has a l s o proven t o be u s e f u l ; c f . s e c t i o n 4 . 1 2 . 3 . 3 . ) . S h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s which a r e c h a r a c t e r i z e d by m u l t i l a y e r p r o p p i n g a r e a common means o f s t i m u l a t i n g m o d e r a t e - p e r m e a b i l i t y sandstone o i l r e s e r v o i r s f o r b o t h o i l p r o d u c t i o n and w a t e r i n j e c t i o n ( c f . s e c t i o n s 4.5.1. and 4.6.), w i t h t h e necessary h i g h cond u c t i v i t y b e i n g achieved by b o t h h i g h c o n c e n t r a t i o n and g r a i n s i z e o f t h e p r o p p a n t s (BRITT 1985, B R I T T & BENNETT 1985, BRITT & LARSEN 1986), and have a l s o s i g n i f i c a n t a p p l i c a t i o n i n c h a l k o i l pay zones ( c f . s e c t i o n s 4.3.5. and 4.5.4.). Aspects o f c o n d u c t i v i t y c o n t r a s t enhancement by m u l t i l a y e r p r o p p i n g o f w i d e r f r a c t u r e s a r e a l s o d i s c u s s e d by MARPLE, RULEY, WORLEY & FREEMAN ( 1 9 8 7 ) , w i t h t h e p a r t i c u l a r advantages o f h i g h e r p r o p p a n t c o n c e n t r a t i o n s w i t h i n t h e f r a c t u r e b e i n g more complete c r a c k f i l l u p , h i g h e r f r a c t u r e f l o w c a p a c i t y , l e s s proppant c r u s h i n g and improved proppant pack t o l e r a n c e t o f i n e s . High p r o p p a n t s a t u r a t i o n s r e q u i r e s e l e c t i o n o f a f r a c t u r i n g f l u i d which w i l l have maximum v i s c o s i t y and proppant t r a n s p o r t p r o p e r t i e s once o u t s i d e t h e p e r f o r a t i o n s ( c f . s e c t i o n 4 . 3 . 4 . 2 . ) , and i t i s a l s o i m p e r a t i v e t h a t f l u i d l o s s w i t h i n t h e f r a c t u r e i s h e l d a t a minimum ( c f . s e c t i o n 4 . 3 . 4 . 6 . ) .
4.3.3.2. Proppant p i 1laring Another concept o f f r a c t u r e f l o w c a p a c i t y improvement i s p r o p p a n t p i l l a r i n g which comprises p r o p p a n t i n t r o d u c t i o n i n t o t h e crack i n such a way t h a t no cont i n u o u s s h e e t - t y p e wedge i s formed, b u t open f l o w channels remain between d i s c r e t e patches o r p i l l a r s which p r o v i d e even h i g h e r c o n d u c t i v i t y t h a n t h e h i g h p e r m e a b i l i t y p r o p p a n t package (TINSLEY & WILLIAMS 1975; FAST, HOLMAN & COLVIN 1977; c f . f i g s . 7 and 1 2 ) i f t h e i r subsequent c l o s u r e due t o h i g h r e s e r v o i r s t r e s s can be p r e v e n t e d . Some aspects o f open f l o w channels between d i s c r e t e p r o p p a n t patches, d i s c o n t i n u o u s p r o p p a n t d i s t r i b u t i o n , and f l u i d d e n s i t y cont r o l and s e l e c t i v e p r o p p a n t l o a d a r e summarized as f o l l o w s .
4.3.3.2.1.
Open flow channels between d i s c r e t e proppant patches
The p r o p p a n t p i l l a r s have t o be spaced s u f f i c i e n t l y c l o s e t o each o t h e r i n o r d e r t o a v o i d f r a c t u r e d e t e r i o r a t i o n by p a r t i a l c l o s u r e o f open channels b e t ween t h e s u p p o r t i n g p r o p p a n t patches (TINSLEY & WILLIAMS 1 9 7 5 ) . I n c o m b i n a t i o n w i t h m u l t i l a y e r p r o p p i n g , t h e t e c h n i q u e o f p i l l a r p r o p p i n g ( c f . f i g s . 7 and 12) c e r t a i n l y i s a v e r y e f f e c t i v e means t o maximize f r a c t u r e f l o w c a p a c i t y i f p r o p e r l y designed and performed. Proppant p i l l a r i n g has t h e advantage o f c r e a t i n g l e s s d i f f i c u l t i e s w i t h s c r e e n o u t f a i l u r e s due t o a l t e r n a t i n g pumping o f f l u i d s l u g s w i t h and w i t h o u t p r o p p a n t l o a d ( c f . s e c t i o n s 4 . 2 . 3 . 2 . 5 . and 6 . 2 . 4 . 2 . 1 . ) . FAST, HOLMAN & COLVIN (1977) b e l i e v e t h a t proppant p i l l a r i n g g i v e s r e s u l t s a t l e a s t comparable - i f n o t s u p e r i o r - t o those o f t o t a l l y p r o p p a n t packed f r a c t u r e s , because t h e open c r a c k p o r t i o n s have an enormous f l o w c a p a c i t y w i t h r e s p e c t t o t h e p r o p p a n t bed o r pack (TINSLEY & WILLIAMS 1 9 7 5 ) . Proppant p i l l a r i n g can under these c i r c u m s t a n c e s o f t e c h n i c a l l y a c c e p t a b l e performance advance t o an i m p o r t a n t means o f s t i m u l a t i o n expense containment by l o w e r i n g o f p r o p p a n t c o s t due t o r e q u i r e m e n t of s m a l l e r t o t a l p r o p p a n t q u a n t i t i e s than would be necessary f o r c o n v e n t i o n a l c o n t i n u o u s p l u g g i n g o f t h e c r a c k .
419 P a r t i a l m o n o l a y e r i n g o f p r o p p a n t s has been more s u c c e s s f u l i n t r e a t m e n t s where i n s o l u b l e p a r t i c l e s a r e i n j e c t e d a l o n g w i t h t h e p r o p p a n t and t h e m i x t u r e i s d e p o s i t e d as a m u l t i l a y e r (SINCLAIR 1980). Subsequent d i s s o l u t i o n o f these a d d i t i o n a l p a r t i c l e s t h e o r e t i c a l l y l e a v e s o n l y t h e p r o p p a n t monolayer, b u t r e s u l t s f r o m these t r e a t m e n t s can a l s o be e x p l a i n e d by o t h e r mechanisms, such as t h a t a m u l t i l a y e r e d proppant package i s p l a c e d w i t h an open f r a c t u r e channel l e f t above i t . Such a g e o m e t r i c a l arrangement i n h o r i z o n t a l d i v i s i o n o f p r o p p a n t package and open f l o w channel r e p r e s e n t s p r o p p a n t banking ( c f . s e c t i o n 4.12.2.2.), whereas t h e c o r r e s p o n d i n g v e r t i c a l d i s t r i b u t i o n matches p r o p p a n t p i l l a r i n g ( c f . f i g s . 7 and 1 2 ) . Proppant banking can be t e c h n i c a l l y achieved by g r a d u a l l y r e d u c i n g t h e pumping r a t e near t h e end o f t h e t r e a t m e n t i n o r d e r t o screen t h e b o r e h o l e o u t and l e a v e t h e f r a c t u r e w e l l - p r o p p e d n e a r t h e mouth and c l o s e t o t h e b o t t o m w i t h an open h i g h - c o n d u c t i v i t y channel towards t h e t o p (CAMPBELL, HANOLD, SINCLAIR & VETTER 1981), whereas p r o p p a n t p i l l a r i n g i s t r i g gered by pumping a l t e r n a t i n g l o t s o f f l u i d w i t h and w i t h o u t p r o p p a n t l o a d ( c f . also section 4.2.2.6.).
4.3.3.2.2. Discontinuous proppant distribution The performance o f n o n - u n i f o r m p r o p p a n t p a c k i n g w i t h open f l o w channels b e t ween i s o l a t e d p r o p p a n t i s l a n d s i s u n d e r l i n e d by s t i l l f a v o u r a b l e r e s u l t s i n many o l d w e l l s where f o r m e r l y t e c h n i c a l l y poor t r e a t m e n t s had been c a r r i e d o u t and c o n t i n u o u s proppant placement c o u l d n o t be achieved (TINSLEY & WILLIAMS 1975; c f . s e c t i o n 4.8.9.). The t r a n s i t i o n f r o m c o n t i n u o u s t o d i s c r e t e p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e can be c o n t r o l l e d by proppant c o n c e n t r a t i o n . As p r o p p a n t s e t t l i n g i s a s s o c i a t e d w i t h c o n v e c t i o n c e l l s ( c f . s e c t i o n 4.12.2.), h i g h e r proppant s a t u r a t i o n s f l a t t e n and dampen o u t t h e c o n v e c t i o n c e l l s i n t h e f l u i d i f s u f f i c i e n t c o n c e n t r a t i o n i s reached w i t h o n l y l i t t l e amounts o f low-sat u r a t e d f l u i d l e f t i n between. T r a n s i t i o n a l stages between u n i f o r m and n o n - u n i f o r m p r o p p a n t d i s t r i b u t i o n a r e proppant banks a t t h e lower s i d e o f t h e f r a c t u r e and i s o l a t e d p r o p p a n t p a t ches i n t h e upper p a r t o f t h e c r a c k w i t h open f l o w channels i n between, w i t h some p r o p p a n t p i l l a r s g o i n g up f r o m t h e p r o p p a n t bank zone i n t o t h e p r o p p a n t s p o t zone. I n some cases, a l s o h o r i z o n t a l proppant banking w i t h e n r i c h m e n t o f proppants t o a l a y e r i n t h e l o w e r p a r t and an open f l o w channel i n t h e upper s e c t i o n o f t h e f r a c t u r e i s a p r o f i t a b l e s o l u t i o n , because upon banking t h e propp a n t s s e t t l e i n t h e c r e a t e d w i d t h i n s t e a d o f t h e f r a c t u r e c l o s i n g on a l e s s conc e n t r a t e d and narrower pack as p l a c e d by p e r f e c t - s u p p o r t f l u i d s (HDEL 1988; c f . f i g s . 7 and 1 2 ) .
4.3.3.2.3. Fluid density control and selective proppant load Proppant banking can a l s o be r e a l i z e d by f l u i d d e n s i t y c o n t r o l w i t h o v e r - o r u n d e r r i d i n g o f two successive f l u i d s o f d i f f e r e n t s p e c i f i c g r a v i t y and v i s c o s i r e s u l t i n g i n s e l e c t i v e propt y (BARBER & THEMIG 1985; c f . s e c t i o n 4.2.2.6.), p a n t placement i n bank-type d i s t r i b u t i o n i n t h e upper o r l o w e r i n t e r v a l o f t h e f r a c t u r e i n case o f pumping f l u i d s w i t h and w i t h o u t p r o p p a n t load, o r i n s t r a t i f i c a t i o n o f d i f f e r e n t proppant t y p e s and/or g r a i n s i z e s i n t h e upper and l o wer r e s e r v o i r segments i n case o f i n j e c t i n g two f l u i d s charged w i t h d i f f e r e n t p r o p p a n t loads, r e s p e c t i v e l y ( c f . s e c t i o n 4 . 2 . 2 . 4 . ) . MALLINGER, R I X E & HOWARD (1964) comment on p r o p p i n g agent spacers i n o r d e r t o i n c r e a s e w e l l p r o d u c t i v i ty. The importance o f s e l e c t i v e p r o p p a n t placement w i t h i n t h e f r a c t u r e i s h i g h l i g h t e d by t h e f a c t t h a t c r a c k c o n d u c t i v i t y v a r y i n g f r o m w e l l b o r e t o t i p ( c f . s e c t i o n 4.3.6.) can s i g n i f i c a n t l y a f f e c t p r o d u c t i o n r a t e s (BENNETT, ROSATO & REYNOLDS 1 9 8 1 ) . I t i s t h e r e f o r e d e c i s i v e t o determine p r o p p a n t placement and s c h e d u l i n g programs f o r a t r e a t m e n t d e s i g n i n o r d e r t o assure t h a t t h e a p p r o p r i a t e d i s t r i b u t i o n o f c o n d u c t i v i t y i n t h e f r a c t u r e i s achieved (VEATCH & MOSCHOVI-
420
D I S 1986). Aspects o f p r o p p a n t p i l l a r i n g a r e a l s o d i s c u s s e d by PEARCE ( 1 9 8 3 ) . G e n e r a t i o n o f non-continuous p r o p p a n t p i l l a r s by u s i n g spacer f l u i d s between proppant slugs requires t h a t the proppant-carrying f l u i d i s capable o f p e r f e c t l y t r a n s p o r t i n g t h e p r o p p a n t s d u r i n g d u r a t i o n o f pumping, t h e spacer volumes f i n g e r t h r o u g h t h e p r o p p a n t - l a d e n f l u i d t o l e a v e channels t h r o u g h o u t t h e f r a c t u r e , and t h e volume o f p r o p p a n t - l a d e n f l u i d and spacer f l u i d s l u g s a r e such t h a t t h e p i l l a r s a r e c l o s e enough t o g e t h e r t o a s c e r t a i n almost complete c l o s u r e o f t h e v o i d channels when t h e w e l l i s p l a c e d on p r o d u c t i o n (PUGH, McDANIEL & SEGLEM 1977). The main economical b e n e f i t o f pumping a l t e r n a t i n g volumes o f v i s cous p r o p p a n t - l a d e n f l u i d and t h i n spacer f l u i d i s s i g n i f i c a n t r e d u c t i o n o f t r e a t m e n t c o s t s due t o l o w e r p r o p p a n t q u a n t i t i e s w i t h r e s p e c t t o c o n t i n u o u s fracture i n f i l l i n g .
4.3.3.3. Fracture width vs. non-Darcy flow Another advantage o f m u l t i l a y e r p r o p p i n g o f f r a c t u r e s i n comparison t o monol a y e r o r even p a r t i a l monolayer s u p p o r t i s t h a t t h e w i d e r t h e f r a c t u r e and t h e l a r g e r t h e p r o p p a n t g r a i n s , t h e b e t t e r t h e p r e s e r v a t i o n o f Darcy f l o w . I n n a r row f r a c t u r e s propped by monolayers o r p a r t i a l monolayers c o n s i s t i n g o f s m a l l p r o p p a n t g r a i n s , t h e v e l o c i t y o f t h e f l u i d f l o w i n g i n t h e f r a c t u r e may i n c r e a s e so much t h a t t u r b u l e n c e s o r i n e r t i a l e f f e c t s o r i g i n a t e w h i c h g r e a t l y i n c r e a s e t h e f l o w r e s i s t a n c e o f t h e f r a c t u r e under c o n d i t i o n s o f non-Darcy f l o w (COOKE 1973, COOKE & GIDLEY 1979; c f . s e c t i o n 4.11.3.2.) and t h u s reduce gas w e l l p r o d u c t i v i t y f o r l o w - c o n d u c t i v i t y f r a c t u r e s by c r e a t i o n o f a d d i t i o n a l p r e s s u r e l o s ses i n t h e c r a c k which decrease t h e f l o w i n g bottomhole p r e s s u r e r e q u i r e d t o w i t h d r a w a t a d e s i r e d f l o w r a t e i f t h e w e l l i s produced a g a i n s t a c o n s t a n t back p r e s s u r e . The i n e r t i a l f o r c e s a r e caused by t h e t o r t u o s i t y o f t h e f l o w p a t h as w e l l as by a c c e l e r a t i o n and d e c e l e r a t i o n o f f l u i d as i t f l o w s t h r o u g h t h e p o r e t h r o a t s i n a porous medium. I n e r t i a l gas f l o w i n a h y d r a u l i c f r a c t u r e i s s u b s t a n t i a l l y a f f e c t i n g t h e shape o f t h e p r e s s u r e b u i l d u p c u r v e (HOLDITCH & MORSE 1976) s i n c e i n e r t i a l f l o w e f f e c t s do n o t always q u i c k l y d i s s i p a t e when t h e w e l l i s s h u t i n . F r a c t u r e cond u c t i v i t y can be c o n s i d e r a b l y a f f e c t e d by t h e d e v i a t i o n f r o m Darcy f l o w a t h i g h c u r r e n t r a t e s (COOKE 1973), w i t h t h e a d d i t i o n a l p r e s s u r e drop p r o v o k i n g a reduct i o n o f e f f e c t i v e f r a c t u r e c o n d u c t i v i t y by more than a f a c t o r o f 3 (DAVIES & K U I P E R 1988; c f . s e c t i o n 1 . 4 . 1 0 . 4 . ) . Under such c o n d i t i o n s , Forchheimer's equat i o n r a t h e r t h a n Darcy's law has t o be used f o r c o r r e c t p r e s s u r e g r a d i e n t e s t i m a t i o n near t h e w e l l b o r e and c o n s e q u e n t l y f l o w b e h a v i o u r o f t h e h y d r a u l i c a l l y f r a c t u r e d w e l l f o r h i g h - v e l o c i t y environments (EZEUDEMBAH & DRANCHUK 1982, PEREZ & KELKAR 1988; PURSELL, HOLDITCH & BLAKELEY 1 9 8 8 ) . Some aspects o f p r o p p a n t t y p e and g r a i n s i z e , s h e a r i n g o f polymer f l u i d s , h i g h l y - d e v i a t e d w e l l s and secondary f r a c t u r e n a r r o w i n g a r e i l l u s t r a t e d as f o l l o w s .
4.3.3.3.1.
Proppant type and g r a i n s i z e
Turbulences l e a d i n g t o non-Darcy f l o w a r e a l s o c r e a t e d by s m a l l - t o m i c r o s c a l e h e t e r o g e n e i t i e s such as crushed g r a i n s i n t h e p r o p p a n t package ( c f . sect i o n 1.4.10.), w i t h crushed l a r g e r g r a i n s b e i n g more heterogeneous t h a n i n t a c t packs o f i n i t i a l l y s m a l l g r a i n s . Gas and l o w - v i s c o s i t y o i l a r e most s i g n i f i c a n t l y a f f e c t e d i n l i n e a r f l o w by t u r b u l e n c e s a t h i g h c u r r e n t r a t e s . I n case o f n a r row f r a c t u r e s , non-Darcy f l o w e f f e c t s a r e s i g n i f i c a n t and n e g l i g i b l e i n f r a c t u r e and f o r m a t i o n , r e s p e c t i v e l y (WATTENBERGER & RAMEY 1969; R E I N I C K E , B R I N K MANN, SCHWARZ & HUENI 1 9 8 5 ) . Non-Oarcy f l o w i n h i b i t i o n o f hydrocarbon p r o d u c t i o n i s t h e more s e r i o u s , t h e poorer i s proppant c o n d u c t i v i t y , w i t h h i g h - q u a l i t y synthetic proppants o f l a r g e r g r a i n s i z e g i v i n g r i s e t o much l e s s non-Darcy f l o w d i s t u r b a n c e t h a n n a t u r a l sand (CALLANAN, CIPOLLA & LEWIS 1983; c f . s e c t i o n 1 . 4 . 1 0 . ) . B o t h p r o p p a n t t y p e
42 1 and g r a i n s i z e a r e independently and j o i n t l y a f f e c t i n g f r a c t u r e c o n d u c t i v i t y damage by non-Darcy flow, w i t h h i g h e r proppant q u a l i t y and l a r g e r proppant g r a i n s i z e decreasing the heterogeneity e f f e c t s o f non-Darcy f l o w considerably. Conversely, any proppant-pack c o n d u c t i v i t y damage increases the non-Darcy f l o w f a c t o r , w i t h g r e a t e r increases being recorded f o r h i g h e r degrees o f damage (DAVIES & KUIPER 1988). PURSELL, HOLDITCH & BLAKELEY (1988), however, conclude from i n e r t i a l f l o w s t u d i e s t h a t the non-Darcy f l o w c o e f f i c i e n t depends on i n i t i a l proppant mesh s i z e and i s independent o f proppant type, w i t h g r a i n s i z e d i s t r i b u t i o n being the most i m p o r t a n t c o n t r o l l i n g f a c t o r . P e r m e a b i l i t i e s c a l c u l a t e d from the Forchheimer equation e x h i b i t the same t r e n d o f decreasing p e r m e a b i l i t y w i t h i n c r e a s i n g pore pressure as t h e Klinkenberg r e l a t i o n s h i p f o r Darcy f l o w . Non-Darcy c u r r e n t i n porous media i s c o n t r o l l e d by pore s t r u c t u r e (NOMAN & ARCHER 1987), w i t h i n e r t i a l f l o w e f f e c t s being m a i n l y i n f l u e n c e d by c u r v a t u r e o f streamlines, expansion and c o n t r a c t i o n losses, t h r o a t t o pore c o o r d i n a t i o n number, heterogen e i t i e s and s u r f a c e roughness (PURSELL, HOLDITCH & BLAKELEY 1988). The i n e r t i a l f l o w c o e f f i c i e n t increases w i t h r i s i n g pore pressure. Other aspects o f non-Darc y f l o w i n proppant packages are discussed by EVANS & EVANS (1986).
4.3.3.3.2. Shearing of polymer fluids Uncomplexed polymer s o l u t i o n s are f r e q u e n t l y i n t u r b u l e n t o r t r a n s i t i o n a l f l o w when c r o s s l i n k i n g r e a c t i o n s take p l a c e (GARDNER & EIKERTS 1982). The l e v e l o f turbulence o r w a l l shear s t r e s s can be expected t o have s i g n i f i c a n t e f f e c t upon apparent g e l v i s c o s i t y . Polymer systems are a f f e c t e d by h i g h shear d u r i n g the c r o s s l i n k i n g r e a c t i o n as w e l l as by l e v e l and d u r a t i o n o f pumping. The a b i l i t y o f a g e l t o g a i n v i s c o s i t y a f t e r a p e r i o d o f h i g h shear f l o w i s s t r o n g l y a f f e c t e d by l e v e l and d u r a t i o n o f h i g h shear f l o w . Proppant a d d i t i o n t o a g e l increases s l u r r y v i s c o s i t y a t a constant value over t h a t p r e d i c t e d f o r s i m i l a r proppant c o n c e n t r a t i o n s i n Newtonian f l u i d s . Approximation o f apparent v i s c o s i t y of a g e l as i t e n t e r s the f r a c t u r e a t the w e l l b o r e r e q u i r e s e s t i m a t i o n o f the combined e f f e c t s of shear and proppant on gel v i s c o s i t y . The e f f e c t s o f temp e r a t u r e and o f l o n g p e r i o d s o f low shear r a t e on gel v i s c o s i t y a f t e r m i x i n g and f l o w i n g a t h i g h r a t e s are needed t o b e t t e r c h a r a c t e r i z e g e l s w h i l e being i n the fracture. E f f e c t s o f pore s t r u c t u r e on non-Darcy gas f l o w are r e p o r t e d by NOMAN & ARCHER (1987), and o t h e r aspects o f non-Darcy gas f l o w are discussed by RIECKMANN (1970), WONG (1970), JONES (1972, 1987), GEERTSMA (1974), RAMEY (1975), HOLDITCH & MORSE (1976); JONES, BLOUNT & GLAZE (1976); HOLDITCH & LEE (1979), GUPPY (1980); GUPPY, CINCO-LEY & RAMEY (1980); K A D I & BROWN (1980); ALLAM, C R I C H LOW & SOLIMAN (1981); K A D I & BOUTEMY (1981), UBANI & EVANS (1982); CALLANAN, C I POLLA & LEWIS (1983); EVANS, HUDSON & GREENLEE (1985); NOMAN, SHRIMANKER & ARCHER (1985) ; EVANS & EVANS (1986), NGUYEN (1986); SALDANA, BRIGHAM & RAMEY (1986); SCHMIDT, CAUDLE & MILLER (1986); LEE, LOGAN & TEK (1987); MALONEY, GALL & RAIBLE (1987) ; FRAIM & LEE (1988) and PEREZ & KELKAR (1988). Non-Darcy f l o w i s p a r t i c u l a r l y o r i g i n a t i n g i n h i g h l y - d e v i a t e d w e l l s ( c f . s e c t i o n 4.11.3.2.) and i n case o f secondary f r a c t u r e narrowing which are b r i e f l y o u t l i n e d as f o l lows.
4.3.3.3.3. High 1y-deviat ed we1 1 s Non-Darcy f l o w i s a p a r t i c u l a r problem i n h i g h l y - d e v i a t e d w e l l s (MARTINS 1987). Flow has b o t h viscous and i n e r t i a l components. Consequences o f non-Darcy f l o w are l a r g e r e d u c t i o n i n e f f e c t i v e f r a c t u r e c o n d u c t i v i t y (up t o 5 - 20 times lower than normal), and w e l l performance i s very s e n s i t i v e t o poor c r a c k / w e l l bore communication as a consequence o f l i m i t e d p e r f o r a t i o n i n t e r v a l s i n h i g h l y i n c l i n e d boreholes. F r a c t u r e f l o w c a p a c i t y of a h y d r a u l i c a l l y s t i m u l a t e d gas w e l l can be s i g n i f i c a n t l y reduced by non-Darcy gas f l o w a t h i g h v e l o c i t y (HOL-
422 D I T C H & MORSE 1976). Excess p r e s s u r e g r a d i e n t s can be caused by e i t h e r t u r b u l e n c e o r i n e r t i a l r e s i s t a n c e o r a c o m b i n a t i o n o f t h e two, and non-Darcy gas f l o w can a l s o g i v e r i s e t o i n c r e a s i n g c l o s u r e s t r e s s which provokes r e d u c t i o n o f f r a c t u r e permeability ( c f . also section 4.11.3.2.).
4.3.3.3.4. Secondary fracture narrowing Non-Darcy
flow
can
a l s o o r i g i n a t e i n p r i m a r i l y s u f f i c i e n t l y wide f r a c t u r e s the fracture w a l l s ( c f . s e c t i o n s 1.4.10.3.5. and 4.3.4.1.4.) as t h e f l u i d f i l t r a t e l e a k s o f f i n t o t h e r e s e r v o i r f o r m a t i o n and subsequent embedment o f t h e o u t e r p r o p p a n t l a y e r s o f a m u l t i l a y e r package i n t o t h e s o f t f i l t e r cake o c c u r s (ROODHART, K U I PER & D A V I E S 1986; McDANIEL 1987, 1988; MUCH & PENNY 1987, PARKER & McDANIEL 1987, McDANIEL & PARKER 1988). A p a r t f r o m decreasing e f f e c t i v e f r a c t u r e w i d t h , an a d d i t i o n a l e f f e c t t r i g g e r i n g non-Oarcy f l o w i s p a r t i a l p l u g g i n g o f t h e p o r e space o f t h e p r o p p a n t package by f i n e s d e r i v i n g f r o m f o r m a t i o n c o l l a p s e , p r o p p a n t c r u s h i n g and p r e c i p i t a t i o n f r o m f l u i d s (RDODHART, KUIPER & D A V I E S 1986; c f . s e c t i o n 4 . 1 2 . 4 . ) . W i t h d e c r e a s i n g Darcy p e r m e a b i l i t y due t o t h e damaging e f f e c t s , t h e non-Darcy f l o w o r Beta f a c t o r i s c o r r e s p o n d i n g l y i n c r e a s i n g . i f t h e y a r e s e c o n d a r i l y narrowed by g e l f i l t e r cake d e p o s i t i o n on
COOKE (1973) was t h e f i r s t t o q u a n t i f y t h e e f f e c t s o f non-Darcy f l o w i n p r o p p a n t packages under s t r e s s . When no t u r b u l e n c e i s p r e s e n t , t h e Beta f a c t o r i s z e r o and s i m p l e Darcy c o n d i t i o n s f o r l a m i n a r f l o w e x i s t .
4.3.3.4. Fracture width vs. proppant bridging I n o r d e r t o accept proppants, f r a c t u r e w i d t h must be a t l e a s t i n t h e range o f 2.5 t i m e s t h e p r o p p a n t d i a m e t e r , w i t h t h i s r e q u i r e m e n t b e i n g f u l f i l l e d by most o f t h e h y d r a u l i c a l l y induced f r a c t u r e s f o r s t a n d a r d p r o p p a n t g r a i n s i z e s ( 1 6 / 2 0 and 20/40), w h i l e 100 mesh m i c r o p r o p p a n t f i t s i n t o h a i r l i n e c r a c k s and f i s s u r e s (THOMPSON 1977; c f . s e c t i o n s 1.4.11.2.1. and 4 . 8 . 8 . 3 . 2 . 3 . ) . POULSEN & SOLIMAN (1987) o u t l i n e t h a t t h e f r a c t u r e has t o be s u f f i c i e n t l y wide t o p r e v e n t b r i d g i n g o f proppants between t h e c r a c k w a l l s which would r e s u l t i n s c r e e n o u t f a i l u r e o f t h e j o b . Some aspects o f minimum f r a c t u r e w i d t h f o r p r o p p a n t i n f i l l i n g and f r i c t i o n p r e s s u r e drop a r e o u t l i n e d as f o l l o w s .
4.3.3.4.1. Minimum fracture width for proppant infilling F o r t h e s e l e c t e d c r e a t e d f r a c t u r e l e n g t h ( w h i c h has t o be a b t . 10 % l o n g e r than t h e d e s i r e d propped c r a c k l e n g t h in o r d e r t o m i n i m i z e t h e p o s s i b i l t y o f a screenout a t the f r a c t u r e t i p ; c f . section 4.8.11.), crack w i d t h a t the desired propped f r a c t u r e l e n g t h a t t h e t i m e o f j o b c o m p l e t i o n has t o exceed 2 5 t i m e s t h e diameter o f t h e p r o p p a n t p a r t i c l e s f o r a l l o w i n g unhindered passage o f t h e proppants t h r o u g h o u t t h e f r a c t u r e ( c f . s e c t i o n s 1 . 4 . 1 1 . 2 . 1 . and 4 . 8 . 8 . 3 . 1 . 2 . ) . Crack w i d t h has a l s o t o be enough f o r a t t a i n i n g t h e r e q u i r e d f r a c t u r e c o n d u c t i v i t y a t t h e w e l l b o r e u s i n g a reasonable p r o p p a n t c o n c e n t r a t i o n i n t h e s l u r r y . The amount o f chosen p r o p p a n t p e r area o f f r a c t u r e f a c e d i v i d e d by t h e maximum a l l o w e d p r o p p a n t s a t u r a t i o n i n t h e s l u r r y e s t a b l i s h e s t h e l o w e r bound o f t h e f r a c t u r e w i d t h w h i c h i n terms o f geometry c a l c u l a t i o n s l e a d s t o a l o w e r bound i n c r a c k l e n g t h ( c f . s e c t i o n 4 . 8 . 1 1 . ) . Aspects o f s i n g l e - and m u l t i p l e - p a r t i c l e b r i d g i n g i n f r a c t u r e s a r e a l s o d i s c u s s e d by LOEPPKE, GLOWKA & WRIGHT ( 1 9 8 8 ) .
4.3.3.4.2. Friction pressure drop F r a c t u r e w i d t h i s e s s e n t i a l l y c o n t r o l l e d by f r i c t i o n a l p r e s s u r e drop i n t h e c r a c k (PERKINS & KERN 1961). O p e r a t i n g c o n d i t i o n s t h a t l e a d t o h i g h p r e s s u r e d r o p r e s u l t i n r e l a t i v e l y wide cracks, whereas s m a l l p r e s s u r e drops produce t h i n c r a c k s . Proppant c o n c e n t r a t i o n changes a r e sometimes necessary d u r i n g
423 course o f s t i m u l a t i o n treatments i n o r d e r t o accommodate proppant b r i d g i n g o r i f bottomhole t r e a t i n g pressure i s lower than c l o s i n g pressure (GREENFIELD & AHMED 1983; c f . s e c t i o n s 4.3.3.5. and 6.2.4.3.). D i m i n i s h i n g proppant s a t u r a t i o n t o avoid b r i d g i n g i s a l s o an important means o f screenout f a i l u r e prevention, because lower proppant c o n c e n t r a t i o n p e r m i t s the f l u i d f l o w r a t e t o increase and thus keeps the f r a c t u r e open t o r e c e i v e f u r t h e r proppants ( c f . s e c t i o n s 4.2.3.2.5. and 6.2.4.2.1.).
4.3.3.5, Proppant concent rat ion schedules Achievement o f a continuous f r a c t u r e c o n d u c t i v i t y d i s t r i b u t i o n r e q u i r e s succ e s s i v e l y changing proppant s a t u r a t i o n throughout the treatment (POULSEN & SOLIMAN 1987) which can be performed by c u r r e n t l y a v a i l a b l e computer-controlled blenders. D i s t i n c t i o n can be made between s t a i r - s t e p p e d and ramping proppant s t a g i n g schedules (CHURCH & PETERS 1987, HANDKE 1987). Some comments on microprocessor c o n t r o l and automation as w e l l as on b l e n d i n g equipment type vs. treatment s i z e are a l s o o f f e r e d .
4.3.3.5.1. Ramping vs . stai r-stepped schedules I n s t a i r - s t e p arrangements, the proppant c o n c e n t r a t i o n increment between each treatment stage i s l a r g e and abrupt and i s n o t r e s t r i c t e d t o being cons t a n t and f l u e n t throughout the f r a c t u r e operation, every stage o f the assemblage t y p i c a l l y has a d i f f e r e n t s l u r r y volume, and the general shape o f the system i s c u r v i l i n e a r w i t h time. I n ramping organizations, the proppant s a t u r a t i o n increment between each j o b stage i s u s u a l l y small and continuous and i s r i g i d l y maintained a t a constant value throughout the f r a c t u r i n g job, every stage o f the schedule has an equal t o t a l s l u r r y volume except o f the f i r s t and l a s t s t a ges which have t o t a l f l u i d volumes each e x a c t l y 50 % the s i z e o f the o t h e r ramp stages, and the general shape o f the a s s o c i a t i o n i s l i n e a r i n c r e a s i n g o r decreas i n g w i t h time (CHURCH & PETERS 1987, HANDKE 1987, POULSEN & SOLIMAN 1987). Ramping proppant schedules have the advantage t h a t by c o n t i n u a l l y i n c r e a s i n g proppant apportionment r a t e s i n small increments throughout t h e treatment, more c o n t r o l over pressure c o n s i d e r a t i o n s can be achieved, and the e f f e c t s o f excess i v e f l u i d v i s c o s i t i e s i n the f r a c t u r e can be e l i m i n a t e d (CHURCH & PETERS 1987; c f . s e c t i o n 4.3.3.3.). The spikes i n s t a i r s t e p schedules are f r e q u e n t l y the reason o f premature screenout f a i l u r e o f the j o b ( c f . s e c t i o n s 4.2.3.2.5. and 6.2.4.2.1.), w i t h smooth ramping schedules b r i n g i n g i n much h i g h e r proppant c o n c e n t r a t i o n s w i t h l e s s screenout r i s k (HOOVER & ADAMS 1988).
4.3.3.5.2. Microprocessor control and automation The d i f f e r e n t step changes i n proppant s a t u r a t i o n between stages o f a f r a c t u r i n g o p e r a t i o n can be performed a u t o m a t i c a l l y and q u i c k l y w i t h a blender r e t r o f i t t e d w i t h a m i c r o p r o c e s s o r - c o n t r o l l e d proppant d e l i v e r y system, w i t h the prob a b i l i t y o f overshooting a new proppant c o n c e n t r a t i o n s e t p o i n t a l s o being drast i c a l l y reduced. The c a p a b i l i t i e s o f f i e l d - p r o v e n f r a c t u r i n g blenders are o n l y enhanced and n o t r e s t r i c t e d once r e t r o f i t t e d w i t h a microprocessor-supervised proppant d e l i v e r y system. Mixed proppant s a t u r a t i o n schedules c o n s i s t i n g o f an i n i t i a l s t a i r - s t e p p a t t e r n f o l l o w e d by a t a i l - i n o f ramping f e a t u r e a r e a l s o p o s s i b l e and have i n f i e l d t e s t s been proven t o r u n very smooth w i t h o u t s e t p o i n t overshooting (HANDKE 1987, POULSEN & SOLIMAN 1987). Proppant concentrat i o n can be metered w i t h r a d i o a c t i v e densimeters (MARPLE, RULEY, WORLEY & FREEMAN 1987). Ramp-controlled blenders w i t h automatic proppant c o n t r o l systems can be d i r e c t l y implemented i n new equipment and can be r e t r o f i t t e d i n o l d e r equipment. The a b i l i t y of computer-interfaced b l e n d i n g equipment f a c i l i t a t e s h i g h proppant c o n c e n t r a t i o n s pumped i n s h o r t stages a t h i g h i n j e c t i o n r a t e s (CLEARY 1988, HOOVER & ADAMS 1988) as w e l l as h i g h s a t u r a t i o n s o f l a r g e r proppant s i z e s
424 i n s h o r t e r f l u i d stages ( c f . s e c t i o n 6 . 2 . 4 . 3 . ) . Pumping h i g h p r o p p a n t c o n c e n t r a t i o n s downhole can l e a d t o excess of p r o p p a n t t r a n s p o r t c a p a c i t y o f t h e b l e n d e r (PAI, GARBIS & HALL 1983). I n such cases, t h e j o b can be d i v i d e d i n t o t h r e e banks w i t h t h r e e b l e n d e r s i n a m u l t i p l e bank system, w i t h a l l t h e b l e n d e r s pumping i n p a r a l l e l . Standby b l e n d e r s s h o u l d be r i g ged up i n l i n e and s h o u l d be c l o s e t o a p r o p p a n t source so t h a t minimum changeo v e r t i m e i s r e q u i r e d and t h e j o b may c o n t i n u e as p e r schedule i n case o f b l e n d e r f a i l u r e . Microprocessor-controlled b l e n d e r s a l l o w i n g i n t e r a c t i o n f r o m t h e computer van and a u t o m a t i c p r o p p a n t a d d i t i o n t h r o u g h c o m p u t e r - i n t e r f a c e d d e n s i tometers a r e a l s o d e s c r i b e d by HOOVER & ADAMS ( 1 9 8 8 ) .
4.3.3.5.3. Blending equipment type vs. treatment size Conventional c o n t i n u o u s - m i x b l e n d i n g equipment g r e a t l y f a c i l i t a t e s h a n d l i n g o f l a r g e volumes o f f l u i d and p r o p p a n t and enable i m p l e m e n t a t i o n o f massive hyd r a u l i c f r a c t u r i n g treatments i n low-permeability reservoirs, b u t i s p o o r l y s u i t e d t o p r e p a r e and d i s p l a c e small p r e c i s e volumes o f s l u r r y w i t h t h e aim o f achievement o f h i g h p r o p p a n t c o n c e n t r a t i o n s i n wide f r a c t u r e s i n moderate- t o h i g h - p e r m e a b i l i t y pay zones (CRAMER & SONGER 1988; c f . s e c t i o n s 4.5.4.4.1. and 4.6.1.). Some aspects o f continuous-mix and batch-mix f r a c t u r i n g a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.3.3.5.3.1. Continuous-mix fracturing Conventional continuous-mix t r e a t m e n t s s u f f e r f r e q u e n t l y f r o m e x c e s s i v e f r a c t u r e growth as a consequence o f l a r g e s l u r r y volumes used, w i t h t h e n o n l y a s m a l l p o r t i o n o f t h e f r a c t u r e c o n t r i b u t i n g t o w e l l b o r e t r a n s m i s s i b i l i t y and t h e danger o f a c c e l e r a t e d w a t e r breakthrough, h i n d e r e d v e r t i c a l and l a t e r a l sweep e f f i c i e n c y i n w a t e r f l o o d e d pay zones, and c h a n n e l l i n g i n t o v e r t i c a l l y a d j a c e n t r e s e r v o i r s e x i s t i n g (CRAMER & SONGER 1988; c f . s e c t i o n 4 . 2 . 2 . 3 . ) . The r e l a t i v e l y modest p r o p p a n t c o n c e n t r a t i o n s of c o n v e n t i o n a l c o n t i n u o u s - m i x o p e r a t i o n s l e a d t o c o r r e s p o n d i n g s m a l l c r a c k w i d t h w h i c h i m p l i e s low f r a c t u r e c o n d u c t i v i t y t h a t i s f u r t h e r d e t e r i o r a t e d by g e l r e s i d u e and o t h e r p r o p p a n t pack damage p r o blems ( c f . s e c t i o n 4 . 3 . 4 . 1 . ) . Conventional f r a c t u r i n g equipment i s p o o r l y s u i t e d t o mix and d i s c h a r g e s m a l l p r e c i s e volumes o f s l u r r y w i t h h i g h p r o p p a n t sat u r a t i o n . Non-uniform p r o p p a n t d i s t r i b u t i o n occurs i n t h e o f t e n - u s e d high-capac i t y volumer b l e n d e r t u b apparatus, i r r e g u l a r p r o p p a n t a d d i t i o n takes p l a c e due t o f l u c t u a t i n g t r e a t m e n t r a t e , and inhomogeneous p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e can r e s u l t i n l o w - c o n d u c t i v i t y b o t t l e n e c k s and j o b s c r e e n o u t due t o proppant s l u g g i n g ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) . On t h e o t h e r hand, continuous-mix t e c h n o l o g y p r o v i d e s a d d i t i o n a l f l e x i b i l i t y t o f r a c t u r i n g t r e a t m e n t s e s p e c i a l l y i n case o f v e r y l a r g e j o b s (CONSTIEN, BRANNON & BANNISTER 1988). F l u i d v i s c o s i t y can be r a i s e d o r lowered t o accommodate apparent v i s c o s i t y i n c r e a s e s caused by p r o p p a n t and r a p i d changes i n p r o p p a n t c o n c e n t r a t i o n and polymer s a t u r a t i o n can be a d j u s t e d i n response t o downhole p r e s s u r e changes, w i t h t h i s f l e x i b i l i t y economizing m a t e r i a l s and i n c r e a s i n g j o b c o n t r o l ( c f . section 6.2.4.3.). F r a c t u r i n g f l u i d s are normally prepared u s i n g a c o m b i n a t i o n o f b a t c h and continuous-mix processes. B a t c h m i x i n g cons i s t s o f f o r m u l a t i n g s e v e r a l components o f t h e f r a c t u r i n g f l u i d i n l a r g e t a n k s p r i o r t o t h e t r e a t m e n t , and once t h e o p e r a t i o n s t a r t s , t h e r e m a i n i n g components such as c r o s s l i n k i n g agents, degradants and proppants a r e c o n t i n u o u s l y metered i n t o t h e f l u i d stream t o produce t h e f i n a l f l u i d c o m p o s i t i o n .
4.3.3.5.3.2. Batch-mix fracturing I n batch-mix f r a c t u r i n g , h i g h p r o p p a n t c o n c e n t r a t i o n s can be p r e - b l e n d e d w i t h g e l l e d l i q u i d i n t h e i n v e n t o r y compartments and s l u r r y d e n s i t y can be v e r i -
425 f i e d before t h e s t a r t o f the treatment. T h i s assures placement o f an u n i f o r m proppant d i s t r i b u t i o n i n the f r a c t u r e , because proppant s a t u r a t i o n s a t the s u r f a c e w i l l n o t be a f f e c t e d by o p e r a t i o n r a t e f l u c t u a t i o n s (CRAMER & SONGER 1988). Therefore incremental improvements i n w e l l b o r e t r a n s m i s s i b i l i t y can i n many cases be achieved due t o c r e a t i o n o f s i g n i f i c a n t c o n d u c t i v i t y c o n t r a s t b e t ween r e s e r v o i r and f r a c t u r e . Small p r e c i s e s l u r r y volumes w i t h h i g h proppant concentrations are p a r t i c u l a r l y s u i t a b l e f o r moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s ( c f . s e c t i o n s 4.5.1.1. and 4.6.1.) and s u c c e s s f u l l y l i m i t v e r t i c a l and l a t e r a l f r a c t u r e extension, a v o i d n e g a t i v e consequences o f i n t e r z o n a l communicat i o n and w a t e r f l o o d sweep i n e f f i c i e n c y , and s i g n i f i c a n t l y reduce screenout p r o blems by achievement o f u n i f o r m proppant d i s t r i b u t i o n and the use o f a h i g h padlslurry ratio. I n most batch-mix f r a c t u r i n g operations, a l a r g e percentage o f the treatment volume c o n s i s t s o f pad f l u i d (up t o 65 - 80 % ) i n s u r i n g l e a k o f f c o n t r o l and s u f f i c i e n t f r a c t u r e w i d t h f o r passage o f the concentrated proppant s l u r r y ( c f . sect i o n 4.3.4.6.1.). Case s t u d i e s i n d i c a t e t h a t even i f coarse h i g h e r - p e r m e a b i l i t y proppants (8/12 o r 12/20 mesh) are used, longer narrowly propped f r a c t u r e s created by conventional continuous-mix b l e n d i n g are l e s s p r o d u c t i v e and l e s s conduct i v e than s h o r t e r wider propped cracks generated by batch-mix f r a c t u r i n g . Batch m i x i n g a l s o has t o be c a r r i e d o u t f o r continuous pumping o f h i g h - v i s c o s i t y c r o s s l i n k e d f r a c t u r i n g g e l s (FREE, FREDERICK & THOMPSON 1975). Aspects o f batch-mix and continuous-mix f r a c t u r i n g are a l s o discussed by EVANS & WILLIAMS (1985) and OLSEN & DEBONIS (1988; c f . s e c t i o n 4.3.4.3.1.4.).
4.3.4. Proppant transport and fracture damage I n o r d e r t o o b t a i n m u l t i p l e proppant layers, i t i s necessary t o choose a f r a c t u r i n g f l u i d which provides s u f f i c i e n t v i s c o s i t y t o c r e a t e wide f r a c t u r e s and t o c a r r y the amount o f proppant necessary t o pack the l a r g e crack. Select i o n o f a f r a c t u r i n g f l u i d i s n o t o n l y a t e c h n i c a l l y , b u t a l s o an economically c r i t i c a l p a r t o f the s t i m u l a t i o n treatment design, because f l u i d and a d d i t i v e s (proppants) may t o g e t h e r r e p r e s e n t more than 50 % o f the t o t a l c o s t o f the oper a t i o n (HOLDITCH 1984). The s e l e c t e d f l u i d should have h i g h enough v i s c o s i t y t o t r a n s p o r t h i g h proppant c o n c e n t r a t i o n s through the p i p e w i t h o u t s e t t l i n g ( c f . s e c t i o n 4.12.2.) and y e t avoid excessive p i p e f r i c t i o n pressure, and once the proppant-laden f l u i d i s i n the f r a c t u r e , s u f f i c i e n t v i s c o s i t y o f the f l u i d i s r e q u i r e d t o keep the proppants i n t o t a l suspension i n o r d e r t o a v o i d s e t t l i n g and banking which would d e t e r i o r a t e c o n d u c t i v i t y d i s t r i b u t i o n i n the crack (SHAH, SMITH & DONALDSON 1983). The i d e a l f r a c t u r i n g f l u i d keeps the proppant p a r t i c l e s completely i n suspens i o n d u r i n g b o t h pumping o p e r a t i o n and crack c l o s u r e sequence, thereby minimiz i n g proppant s e t t l i n g (CLARK 1983). The f o l l o w i n g account g i v e s a b r i e f summar y o f aspects o f breakdown and r e s i d u e concentration, v i s c o s i t y and t r a n s p o r t a b i l i t y , impact o f delaying, heat c a p a c i t y and r e s e r v o i r temperature, v i s c o e l a s ticity, and l e a k o f f containment o f p r e f e r e n t i a l l y c r o s s l i n k e d fracturing f 1u i d s .
4.3.4.1.
F l u i d breakdown and residue concentration
H y d r a u l i c f r a c t u r i n g mechanics c o n s i s t s o f transmission o f energy from s u r f a ce pumps v i a a s t i m u l a t i o n g e l t o the f o r m a t i o n t o be t r e a t e d (PAI, GARBIS & HALL 1983). I m p o r t a n t parameters f o r treatment f l u i d s e l e c t i o n a r e s u f f i c i e n t v i s c o s i t y t o t r a n s p o r t the proppants downhole, f l u i d - l o s s a d d i t i v e s , low tubul a r f r i c t i o n pressure, f r a c t u r e and f o r m a t i o n f l u i d c o m p a t i b i l i t y , non-damaging nature, ease o f recovery a f t e r the job, reasonable p r i c e and o p e r a t i o n a l safety.
426
4.3.4.1.1. General aspects The h y d r a u l i c f r a c t u r i n g process t y p i c a l l y c o n s i s t s o f b l e n d i n g s p e c i a l chem i c a l s t o generate t h e a p p r o p r i a t e s t i m u l a t i o n f l u i d , m i x i n g i t w i t h proppants, and pumping t h e s l u r r y i n t o t h e pay zone a t s u f f i c i e n t l y h i g h r a t e s and pressur e s such t h a t t h e f l u i d h y d r a u l i c a l l y wedges and e x t e n d s a f r a c t u r e w i t h i n t h e f o r m a t i o n where t h e p r o p p a n t s a r e c a r r i e d up t o t h e extreme t i p s o f t h e c r a c k , t h e r e b y c r e a t i n g a d r a i n a g e avenue o r superhighway f o r o i l and gas t o f l o w e a s i l y f r o m d i s t a l p o r t i o n s o f t h e r e s e r v o i r i n t o t h e b o r e h o l e (VEATCH 1983; c f . s e c t i o n 1 . 2 . 1 . 1 . ) . F l u i d p r o p e r t i e s s t r o n g l y govern f r a c t u r e p r o p a g a t i o n behav i o u r as w e l l as d i s t r i b u t i o n and placement o f p r o p p a n t s . R a p i d f l u i d l e a k o f f g i v e s r i s e t o low e f f i c i e n c y i n h y d r a u l i c wedging and e x t e n d i n g o f t h e f r a c t u r e , and may a l s o l e a v e an u n d e s i r a b l e c o n c e n t r a t i o n o f p a r t i c u l a t e r e s i d u e i n t h e crack t h a t c o u l d reduce c o n d u c t i v i t y ( c f . s e c t i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.). E f f e c t i v e f l u i d v i s c o s i t y c o n t r o l s i n t e r n a l f r a c t u r i n g pressure and p r o p p a n t t r a n s p o r t c h a r a c t e r i s t i c s . D e s i r a b l e f e a t u r e s o f s t i m u l a t i o n f l u i d s f o r most h y d r a u l i c t r e a t m e n t s i n c l u d e low f l u i d l o s s i n o r d e r t o o b t a i n t h e designed p e n e t r a t i o n w i t h minimum f l u i d volumes, s u f f i c i e n t e f f e c t i v e v i s c o s i t y t o c r e a t e t h e necessary f r a c t u r e w i d t h as w e l l as t o t r a n s p o r t and d i s t r i b u t e t h e proppants i n t h e f r a c t u r e as r e q u i r e d , no e x c e s s i v e f r i c t i o n i n t h e crack, good temperature s t a b i l i t y f o r the p a r t i c u l a r r e s e r v o i r t o be t r e a t e d , good shear s t a b i l i t y , minimal damaging e f f e c t s on f o r m a t i o n p e r m e a b i l i t y , minimal p l u g g i n g e f f e c t s on f r a c t u r e conductivity, low f r i c t i o n - l o s s b e h a v i o u r i n t h e p i p e , w e t t a b i l i t y t o l e a v e f o r m a t i o n and p r o p p a n t package water-wet (DERBY & SMITH 1979; GARBIS, BROWN & MAURITZ 1985), low pH t o p r e v e n t c l a y s w e l l i n g , minimum i n s o l u b l e r e s i d u e l e a v i n g , good p o s t - t r e a t m e n t b r e a k i n g c h a r a c t e r i s t i c s and s h o r t b r e a k i n g time, good p o s t - o p e r a t i o n c l e a n - u p and flowback behaviour, and low c o s t . W h i l e some f r a c t u r i n g f l u i d s e x h i b i t a Newtonian f l o w b e h a v i o u r , o t h e r s a r e non-Newtonian b u t essent i a l l y behave as power- aw f l u i d s , and t h e c r o s s l i n k e d polymers commonly used f o r MHF t r e a t m e n t s a r e ver complex non-power-law f l u i d s . Many Newtonian f l u i d s degrade w i t h time, and e l e v a t e d temperature and/or shear a c c e l e r a t e degradation. T r a n s p o r t o f proppants n h i g h e r s a t u r a t i o n s by t h e c a r r i e r f l u i d s i s accomp l i s h e d by i n c r e a s i n g t h e v i s c o s i t y by adding w a t e r - s o l u b l e polymers t o t h e wat e r phase and c r o s s l i n k i n q them w i t h a metal i o n t h e r e b y f o r m i n q c h e l a t e complexes ' (COOKE 1975; AHMED, J\BOU-SAYED & JONES 1979; ALMOND & BLANo 1984, K I M & ' LOSACANO 1985, ACHARYA 1987; c f . s e c t i o n 4 . 3 . 4 . 3 . ) . The f o l l o w i n g d i s c u s s i o n f o cusses on some aspects o f f l u i d breakdown, r e s i d u e c o n c e n t r a t i o n , g e l f i l t e r cake on f r a c t u r e w a l l , and p o s s i b i l i t i e s o f f r a c t u r i n g f l u i d improvement.
4.3.4.1.2.
Fluid breakdown
V i s c o s i t y and thus t r a n s p o r t c a p a c i t y o f t h e c r o s s l i n k e d f l u i d v a r i e s w i t h polymer and c r o s s l i n k e r t y p e and c o n c e n t r a t i o n , f l u i d temperature, and method o f f l u i d placement (VEATCH 1983, ALMOND & BLAND 1 9 8 4 ) . Upon c o m p l e t i o n o f t h e t r e a t m e n t , removal o f t h e g e l l e d f l u i d t o a l l o w i n c r e a s e d p r o d u c t i o n r a t e i s achieved by breakdown o f t h e h i g h - v i s c o s i t y c r o s s l i n k e d f l u i d s t o l o w - v i s c o s i t y s o l u t i o n s t h r o u g h thermal d e g r a d a t i o n o f t h e polymer a t r e s e r v o i r temperature and/or a d d i t i o n o f chemical b r e a k e r s ( c f . s e c t i o n s 4.3.4.2.2. and 4 . 1 2 . 2 . 3 . ) . Some o f t h e broken g e l s , however, l e a v e u n s o l u b l e r e s i d u e s i n f r a c t u r e and f o r m a t i o n which may cause c o n s i d e r a b l e f l o w impairment due t o s i g n i f i c a n t r e d u c t i o n o f p r o p p a n t p e r m e a b i l i t y o r c r a c k c o n d u c t i v i t y by b l o c k i n g o r r e s t r i c t i n g p o r e spaces. The most s u i t a b l e f r a c t u r i n g c a r r i e r f l u i d s a r e t h u s polymers which a f t e r c o m p l e t i o n o f t h e t r e a t m e n t degrade t h e r m a l l y o r break c h e m i c a l l y w i t h o u t l e a v i n g any r e s i d u e . Experimental work c a r r i e d o u t w i t h sand has shown t h a t sand p e r m e a b i l i t y i s c o n t i n u o u s l y decreased w i t h i n c r e a s i n g c l o s u r e s t r e s s ( c f . s e c t i o n 1 . 2 . 2 . ) , b u t
427 beyond 4,500 p s i t h e p e r m e a b i l i t y change i s i n c o n s i s t e n t w i t h polymer c o n c e n t r a t i o n and c l o s u r e s t r e s s v a r i a t i o n s , w i t h t h i s d i v e r g e n t b e h a v i o u r p o s s i b l y b e i n g r e l a t e d t o t h e b i l i n e a r i t y o f sand p o r o s i t y w i t h r i s i n g c l o s u r e s t r e s s . A l t e r n a t i v e s t o c r o s s l i n k e d f l u i d s a r e d i s c u s s e d by CLARK, HALVACI, GHAELI & PARKS (1985), and ROODHART & D A V I E S (1987) r e p o r t on t h e r e v i v a l o f polymer emulsions as f r a c t u r i n g f l u i d s . While thermal f l u i d breakdown may be s a t i s f a c t o r y i n h i g h - t e m p e r a t u r e r e s e r v o i r s , thermal d e g r a d a t i o n o f f r a c t u r i n g f l u i d s i s i n s u f f i c i e n t i n low-temperat u r e r e s e r v o i r s where o n l y chemical b r e a k e r s can decompose t h e f l u i d s .
4.3.4.1.3. Residue concentration F r a c t u r e d e s i g n w i t h h i g h e r p r o p p a n t s a t u r a t i o n s t h u s has t o i n c l u d e assessment o f f r a c t u r e c o n d u c t i v i t y damage by c r o s s l i n k e d g e l r e s i d u e which v a r i e s w i t h c o n c e n t r a t i o n o f t h e v i s c o s i t y - i n c r e a s i n g polymer vs. i n c r e m e n t a l c o n d u c t i v i t y b e n e f i t and p r o t e c t i o n a g a i n s t embedment o r c r u s h i n g f r o m h i g h e r proppant s a t u r a t i o n s i n f l u i d and f r a c t u r e w h i c h r e q u i r e t h e a p p l i c a t i o n o f more v i s c o u s f r a c t u r i n g f l u i d s t h a t a r e capable o f t r a n s p o r t i n g l a r g e r p r o p p a n t q u a n t i t i e s p e r u n i t volume (COOKE 1975). Some p o i n t s o f f i l t e r cake t h i c k n e s s , polymer and b r e a k e r c o n c e n t r a t i o n , and r e s i d u e s p l u g g i n g m a t r i x and p r o p p a n t p e r m e a b i l i t y a r e i l l u s t r a t e d as , f o l l o w s .
4.3.4.1.3.1. Filter cake thickness F l u i d l e a k o f f f r o m t h e induced f r a c t u r e i n t o r e s e r v o i r m a t r i x and n a t u r a l c r a c k s has t o be m i n i m i z e d i n o r d e r t o a v o i d premature s c r e e n o u t t e r m i n a t i o n o f t h e h y d r a u l i c s t i m u l a t i o n o p e r a t i o n ( c f . s e c t i o n s 4.2.3.2.5. and 6.2.4.2.1.) and t o p r e v e n t c r a c k damage by d e t e r i o r a t i n g p r o p p a n t p e r m e a b i l i t y . F r a c t u r e c o n d u c t i v i t y damage i s caused by l o c a l c o n c e n t r a t i o n s o f proppants and f l u i d remnants as a consequence o f proppant s e t t l i n g by g r a v i t y , f o r m a t i o n and e r o s i o n of a f i l t e r cake c o n t a i n i n g polymer r e s i d u e s and/or f l u i d - l o s s a d d i t i v e s on t h e c r a c k w a l l s , and i n t e r f e r e n c e o f t h e f r a c t u r e f a c e s w i t h p r o p p a n t t r a n s p o r t a l o n g t h e c r a c k . Some comments a r e o f f e r e d as f o l l o w s on r e s e r v o i r permeab i l i t y and f l u i d t y p e as w e l l as f l u i d p r e s s u r e and b r e a k i n g .
4.3.4.1.3.1.1. Reservoir permeability and fluid type The f i l t e r cake i s t h i c k e r i n h i g h - p e r m e a b i l i t y r e s e r v o i r i n t e r v a l s where f l u i d l e a k o f f occurs a t h i g h e r r a t e s , t h e r e b y i n c r e a s i n g t h e l o c a l c o n c e n t r a t i o n of r e s i d u e - f o r m i n g components o p p o s i t e permeable zones, whereas t i g h t pay sequences a r e c h a r a c t e r i z e d by l o w e r f l u i d escape r a t e s i n t o t h e f o r m a t i o n ( c f . s e c t i o n s 1.4.10.3.5. and 4.3.4.6.4.). Polymer remnants, however, may h e l p t o m i n i m i z e s u r f a c e a r e a e f f e c t s o f f l u i d - l o s s a d d i t i v e s w h i c h o t h e r w i s e have a s t r o n g i n f l u e n c e on f r a c t u r e p e r m e a b i l i t y . Enveloping o f f l u i d - l e a k o f f agents by polymer r e s i d u e may m i n i m i z e f r a c t u r e c o n d u c t i v i t y damage by p r e v e n t i n g movement o f f i n e p a r t i c l e s through pores t o p l u g narrow openings and by d e c r e a s i n g the surface area a f f e c t i n g f l o w resistance. C r o s s l i n k e d f l u i d s l e a v e more r e s i d u e p l u g g i n g t h e p o r o s i t y o f t h e p r o p p a n t package than polymer emulsion f l u i d s (DAVIES & KUIPER 1988). W h i l e polymer emuls i o n f l u i d s r e s u l t i n a p r o p p a n t package t h a t i s v i r t u a l l y r e s i d u e - f r e e , c r o s s l i n k e d f l u i d s produce a p r o p p a n t package c o n t a i n i n g a l o t o f f i b r o u s m a t e r i a l between t h e g r a i n s which a r e t h e n g l u e d t o g e t h e r .
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4.3.4.1.3.1.2. Fluid pressure and breaking High-pressure f r a c t u r i n g f l u i d s l e a k i n g away i n t o f o r m a t i o n m a t r i x and n a t u r a l c r a c k s provoke g e n e r a t i o n o f a f i l t e r cake c o n s i s t i n g m a i n l y o f polymer and f l u i d - l o s s a d d i t i v e s ( c f . s e c t i o n s 1.4.10.3.5. and 4 . 3 . 4 . 6 . 4 . ) . Filter-cake t h i c k n e s s i s determined by t y p e o f s t i m u l a t i o n f l u i d , f o r m a t i o n c h a r a c t e r i s t i c s , f r a c t u r e - t o - r e s e r v o i r pressure, and e r o s i o n a l e f f e c t s caused by s l u r r y bei n g pumped a l o n g t h e c r a c k f a c e s . F i l t e r cakes c r e a t e d by c r o s s l i n k e d f l u i d s w i t h d i e s e l as f l u i d - l o s s a d d i t i v e can almost c o m p l e t e l y p l u g narrow induced f r a c t u r e s , whereas polymer emulsion f l u i d s do n o t g i v e s i g n i f i c a n t f i l t e r - c a k e buildup. Proppant p e r m e a b i l i t y damage by c r o s s l i n k e d f l u i d s s u b s t a n t i a l l y depends on f i l t e r - c a k e b u i l d u p and i s m i n i m i z e d by use o f an e f f e c t i v e v i s c o s i t y b r e a k e r which i s p a r t i c u l a r l y necessary i n low-temperature s h a l l o w f o r m a t i o n s . W h i l e an inadequate b r e a k e r l e a d s t o v i r t u a l l y complete loss o f proppant-package conduct i v i t y , a g g r e s s i v e b r e a k e r schedules on t h e o t h e r hand can p r o v i d e h i g h r e t e n t i o n f a c t o r s o v e r 80 %, b u t can a l s o r e s u l t i n e x c e s s i v e p r o p p a n t s e t t l i n g i n t h e f r a c t u r e b e f o r e c l o s u r e , w i t h t h u s a p r o p e r balance b e i n g r e q u i r e d ( c f . sect i o n 4 . 1 2 . 1 . 1 . ) . Polymer emulsion f l u i d s a r e advantageous because c o n d u c t i v i t y r e c o v e r y i s l e s s s e n s i t i v e t o b r e a k e r e f f i c i e n c y , whereas s u r f a c t a n t s t h a t a r e sometimes used i n these f l u i d s g e n e r a t e o i l - w e t n e s s o f t h e p r o p p a n t package, w i t h r e t a i n e d e f f e c t i v e p e r m e a b i l i t y t o w a t e r b e i n g much l o w e r i n such cases.
4.3.4.1.3.2. Polymer and breaker concentration D r a s t i c a l r e d u c t i o n s o f f r a c t u r e c o n d u c t i v i t y p a r t i c u l a r l y o c c u r i n case o f low p r o p p a n t and h i g h f l u i d r e s i d u e s a t u r a t i o n s . I n terms o f accumulations o f h i g h c o n c e n t r a t i o n s o f f i n e s c a u s i n g p l u g g i n g o f t h e p r o p p a n t package, e f f e c t s o f column l e n g t h a r e i m p o r t a n t ( P Y E & SMITH 1973) and polymer remnants may even g i v e r i s e t o more s e r i o u s d e t e r i o r a t i o n o f f r a c t u r e c o n d u c t i v i t y t h a n f l u i d loss a d d i t i v e s ( c f . section 1.4.10.3.). Factors determining crack c o n d u c t i v i t y r e d u c t i o n by i n f l u e n c e s o f s t i m u l a t i o n f l u i d s a r e p r o p p a n t c o n c e n t r a t i o n i n t h e c a r r i e r medium, amount o f r e s i d u e i n t h e f l u i d , p r o p p a n t p o r o s i t y and q u a n t i t y share o f r e s i d u e r e t a i n e d i n t h e f r a c t u r e as t r e a t m e n t f l u i d l e a k s o f f (COOKE 1975). The amount o f f l u i d remnants depends on polymer c o n c e n t r a t i o n as w e l l as t y p e and s a t u r a t i o n o f t h e b r e a k e r c u t t i n g t h e c r o s s l i n k e d g e l i n t o s i m p l e r cons t i t u e n t s which f i n a l l y achieve a v i s c o s i t y i n t h e range o f t h a t o f w a t e r . L o c a l v a r i a t i o n s i n p r o p p a n t and r e s i d u e s a t u r a t i o n s i n a c r a c k can e x p l a i n why f r a c t u r i n g t r e a t m e n t s can be s u c c e s s f u l u s i n g s t i m u l a t i o n f l u i d s t h a t d r a s t i c a l l y decrease c r a c k c o n d u c t i v i t y a t normal s a t u r a t i o n o f i n j e c t e d p r o p p a n t s . E f f e c t s o f t r e a t m e n t f l u i d v e l o c i t y on f l u i d - l o s s agent performance a r e i n v e s t i g a t e d by HALL & DOLLARHIDE ( 1 9 6 4 ) . GALL & RAIBLE (1985) r e p o r t on m o l e c u l a r s i z e s t u d i e s o f degraded f r a c t u r i n g f l u i d polymers. I n case o f problems w i t h t h e o r i g i n a l b r e a k e r c o n c e n t r a t i o n and o p e r a t i o n a l i m p o s s i b i l i t y t o w a i t f o r complete thermal breakage, r e m e d i a l b r e a k e r t r e a t m e n t s by i n j e c t i n g b r e a k e r sol u t i o n s i n f i l l i n g t h e f r a c t u r e p o r o s i t y w i t h excess can be c a r r i e d o u t i n o r d e r t o ensure f r a c t u r e c o n d u c t i v i t y (WARPINSKI, BRANAGAN, SATTLER, LORENZ, NORTHROP, MANN & FROHNE 1985; c f . s e c t i o n 4.12.2.3.). Another aspect o f m i n i m i z i n g r e s i d u e c o n c e n t r a t i o n and f r a c t u r e c o n d u c t i v i t y damage i s i r o n c o n t r o l which a p p l i e s f o r b o t h a c i d and p r o p p a n t s t i m u l a t i o n s (DILL 1983). Many hydrocarbon-producing zones c o n t a i n i r o n compounds i n b o t h m a t r i x and f l u i d s which can g e n e r a t e i n c o m p a t i b i l i t i e s d u r i n g t r e a t m e n t s , because i r o n i n s o l u t i o n may p r e c i p i t a t e and d e p o s i t i n t h e f o r m a t i o n o r t h e fracture, thus r e d u c i n g f l o w c a p a c i t y . I r o n t h e r e f o r e has t o be m a i n t a i n e d i n s o l u t i o n u n t i l r e c o v e r y o f t h e s t i m u l a t i o n f l u i d s , because d i s s o l v e d i r o n has no blockage p o t e n t i a l and p r o d u c t i o n d e l i v e r a b i l i t y i s a t i t s maximum, i n c o n t r a s t t o i r o n p r e c i p i t a t i o n as c r y s t a l l i n e f i n e s o r g e l a t i n o u s mass which s e r i o u s l y hinders f l o w capacity ( c f . section 4.5.1.1.4.).
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4.3.4.1.3.3. Residues plugging matrix and proppant permeabi 1 i ty R e s u l t s o f MHF t r e a t m e n t s i n t i g h t gas sands v a r y f r o m e x t r e m e l y s u c c e s s f u l t o v e r y d i s a p p o i n t i n g f a i l u r e s (AHMEO, ABOU-SAYED & JONES 1979), w i t h t h e l a t t e r f a i l i n g t o c r e a t e l a r g e f r a c t u r e s o f a p p r o p r i a t e c o n d u c t i v i t y i n t h e zone o f h i g h gas s a t u r a t i o n . Achievement o f t h e necessary propped c r a c k geometry i s m a i n l y i m p a i r e d b y f r a c t u r e d e v i a t i o n f r o m t h e gas-bearing zone, p r o d u c t i o n damage due t o s t i m u l a t i o n f l u i d i n t e r a c t i o n w i t h t h e h o s t r o c k m a t r i x , and f r a c t u r e c o n d u c t i v i t y damage due t o t r e a t m e n t f l u i d i n t e r a c t i o n w i t h t h e p r o p p a n t package ( c f . s e c t i o n 4 . 1 2 . ) . F r a c t u r i n g f l u i d s e l e c t i o n f o r an e f f e c t i v e MHF j o b depends n o t o n l y upon f l u i d e f f e c t i v e n e s s i n f r a c t u r e c r e a t i o n and p r o p p a n t t r a n s p o r t , b u t a l s o on m i n i m i z a t i o n o f damage t o f o r m a t i o n p e r m e a b i l i t y and fracture conductivity. Loss o f s t i m u l a t i o n f l u i d t o t h e f o r m a t i o n a d j a c e n t t o t h e f r a c t u r e f a c e c r e a t e s a zone o f immobile f r a c t u r i n g l i q u i d and reduces gas f l o w t h r o u g h t h e f o r m a t i o n f a c e due t o d i m i n u t i o n o f r e l a t i v e gas p e r m e a b i l i t y , and i n a d d i t i o n t o t h a t , s t i m u l a t i o n f l u i d s l e a v e b e h i n d s o l i d and l i q u i d r e s i d u e s . R e d u c t i o n o f m a t r i x p e r m e a b i l i t y by f r a c t u r i n g f l u i d i s provoked by s w e l l i n g o f c l a y miner a l s and subsequent s o f t e n i n g o f t h e f r a c t u r e f a c e which l e a d s t o p r o p p a n t embedment ( c f . s e c t i o n 4.3.3.). Comparative e v a l u a t i o n o f d i f f e r e n t t r e a t m e n t f l u i d s has r e v e a l e d t h a t foam i s t h e most f a v o u r a b l e and s u c c e s s f u l f r a c t u r i n g medium ( c f . s e c t i o n 3.11.2.) and t h a t d i f f e r e n t s t i m u l a t i o n f l u i d s cause var i o u s amounts and i n t e n s i t i e s o f damage and i n t e r a c t d i f f e r e n t l y w i t h s p e c i f i c l i t h o l o g i e s by l e a v i n g d i f f e r e n t s o l i d and l i q u i d r e s i d u e s . T h e r e f o r e r e d u c t i o n o f f r a c t u r e c o n d u c t i v i t y depends on t h e c o m b i n a t i o n o f t r e a t m e n t f l u i d and r e s e r v o i r f o r m a t i o n and t h e i r mutual c o m p a t i b i l i t y . D i m i n u t i o n o f f r a c t u r e c o n d u c t i v i t y w i t h s t r e s s i s p r i n c i p a l l y due t o propp a n t p a c k i n g d e n s i t y . C o n d u c t i v i t y damage o f f r a c t u r e s plugged w i t h f u l l monol a y e r s o r m u l t i l a y e r s i s l e s s pronounced than t h a t o f p a r t i a l p r o p p a n t monol a y e r s . The most i m p o r t a n t reasons f o r f r a c t u r e c o n d u c t i v i t y d i m i n u t i o n a t i n s i t u c o n d i t i o n s a r e p r o p p a n t packing, p r o p p a n t embedment due t o f r a c t u r e f a c e softening, proppant crushing, residues from s t i m u l a t i o n f l u i d s , release o f f i nes f r o m f r a c t u r e face, and c l a y f l o c c u l a t i o n around p r o p p a n t embedment s i t e s ( c f . s e c t i o n 1.4.10.).
4.3.4.1.4. Gel filter cake on fracture wall Gel f i l t e r cakes a c c u m u l a t i n g on t h e f r a c t u r e w a l l s when f l u i d l e a k s o f f i n t o t h e r e s e r v o i r f o r m a t i o n have an i m p o r t a n t d e t e r i o r a t i n g impact on f r a c t u r e c o n d u c t i v i t y u n l e s s r e a s o n a b l y compensated by h i g h e r proppant c o n c e n t r a t i o n s i n Some asf i l l i n g w i d e r c r a c k s (PARKER & McOANIEL 1987; c f . s e c t i o n 1.4.10.3.5.). p e c t s o f g e l f i l t e r cake t h i c k n e s s vs. proppant g r a i n diameter and p r o p p a n t conc e n t r a t i o n v s . f r a c t u r e c o n d u c t i v i t y a r e o u t l i n e d as f o l l o w s .
4.3.4.1.4.1. Gel filter cake thickness vs. proppant grain diameter Gel f i l t e r cake t h i c k n e s s i s i n many cases c l o s e t o average p r o p p a n t g r a i n d i a m e t e r f o r 20/40 mesh p a r t i c l e s (PARKER & McDANIEL 1987). T h i s r e l a t i o n s h i p c r e a t e s a p o t e n t i a l t o f i l l t h e p r o p p a n t p o r o s i t y w i t h f i l t e r cake, and t o l o o s e t h e f l o w a l o n g t h e w a l l o f b o t h f r a c t u r e f a c e s and f o r a depth o f one o r more p r o p p a n t g r a i n s on each s i d e o f t h e c r a c k . As proppants embed i n t o t h e f i l t e r cake, t h e y may d i s p l a c e p a r t s o f t h i s t h i c k g e l p e l l i c l e towards t h e c e n t e r o f t h e p r o p p a n t package w i t h i n t h e f r a c t u r e , t h e r e b y s e r i o u s l y d e s t r o y i n g f r a c t u r e c o n d u c t i v i t y u n l e s s m u l t i l a y e r p r o p p i n g o f t h e c r a c k i s achieved by h i g h proppant concentrations.
430
4.3.4.1.4.2. Proppant concentration vs. fracture conductivity Experimental r e s u l t s show t h a t d o u b l i n g p r o p p a n t c o n c e n t r a t i o n f r o m 1 t o 2 l b s / f t 2 ( 5 t o 10 kg/m2) g i v e s r i s e t o a f i v e - t o t e n - f o l d i n c r e a s e o f f r a c t u r e conductivity, l e a d i n g t o c o n c l u s i o n t h a t i n case o f presence o f g e l f i l t e r cakes, proppant s a t u r a t i o n s o f 1 l b / f t 2 may n o t be s u f f i c i e n t f o r adequate product i o n s t i m u l a t i o n even a t low c l o s u r e s t r e s s l e v e l s . I n s o f t r e s e r v o i r r o c k s , p r o p p a n t c o n c e n t r a t i o n s o f 3 - 4 l b s / f t 2 (15 - 20 kg/m2) a r e recommended t o compensate p r o p p a n t embedment i n t o t h e f o r m a t i o n (GARBIS, BROWN & MAURITZ 1985; McDANlEL & PARKER 1988). The same a p p l i e s i n u n s t a b l e c o l l a p s i n g c h a l k r e s e r v o i r s where p r o p p a n t s a t u r a t i o n s h o u l d be a t l e a s t 2 lbs/m2 i n o r d e r t o a v o i d embedment d e t e r i o r a t i o n o f f r a c t u r e c o n d u c t i v i t y (SIMON, COULTER, K I N G & HOLMAN 1982; c f . s e c t i o n 4 . 5 . 4 . ) . McDANIEL & PARKER (1988) even conclude t h a t g e l f i l t e r cake damage i s n o t u n i v e r s a l l y r e p r e s e n t e d w i t h o n l y d a t a a t 2 l b s / f t 2 . As p e r m e a b i l i t y o f a propped f r a c t u r e i s n o t c o n s t a n t i n t h e presence o f g e l f i l t e r cakes, d e s i g n s i m u l a t o r s have t o a l l o w f o r i n p u t o f c o n d u c t i v i t y d a t a b e i n g a f u n c t i o n o f p r o p p a n t conc e n t r a t i o n . Aspects o f proppant s a t u r a t i o n and c o n d u c t i v i t y i n t h e presence o f g e l f i l t e r cakes a r e a l s o d i s c u s s e d by ROODHART, K U I P E R & D A V I E S ( 1 9 8 6 ) ; PARKER & McDANIEL (1987), PENNY (1987) and McDANIEL ( 1 9 8 8 ) . S i m i l a r l y as g e l f i l t e r cakes on f o r m a t i o n s u r f a c e s a t f r a c t u r e w a l l s d e t e r i o r a t e c r a c k c o n d u c t i v i t y by p r o p p a n t embedment, g e l s u r f a c e f i l t r a t i o n f r o m polymer s o l u t i o n s d u r i n g g r a v e l p a c k i n g c r e a t e s a l o w - p e r m e a b i l i t y drape on t h e r e s e r v o i r f a c e which upon r a p i d b u i l d u p dominates system b e h a v i o u r (TORREST & MARCANO 1986) and i n h i b i t s f u r t h e r f l u i d l e a k o f f i n t o t h e pay m a t r i x , t h e r e b y h a v i n g d e l e t e r i o u s e f f e c t s on g r a v e l placement and thus q u a l i t y and performance o f mechanical sand c o n t r o l ( c f . s e c t i o n s 1.4.10.3.5.3. and 5 . 3 . 1 . 1 . 2 . 2 . ) .
4.3.4.1.5. Possibilities o f fracturing fluid improvement Improvement o f f r a c t u r e c o n d u c t i v i t y i n terms o f f l u i d c o m p o s i t i o n can be c h i e f l y made by s e l e c t i o n o f c o m p l e t e l y degradable f l u i d - l o s s a d d i t i v e s w h i c h a r e s t a b l e a t h i g h temperatures and a r e decomposed w i t h o u t reasonable r e s i d u e ( c f . s e c t i o n s 4.3.4.6.4. and 4.8.8.3.1.), and o f d e l a y e d - a c t i v i t y b r e a k e r s which s t a y dormant u n t i l t h e end o f t h e f r a c t u r i n g j o b and t h e n q u i c k l y s p l i t t h e c r o s s l i n k e d g e l i n t o s m a l l fragments w i t h a g a i n n o t l e a v i n g any s i g n i f i c a n t r e s i d u e . T h i s i s p a r t i c u l a r l y i m p o r t a n t due t o t h e f a c t t h a t many o f t h e p r e s e n t l y applied breakers a c t very s h o r t l y a f t e r t h e i r a d d i t i o n t o the f l u i d stream and 60 - 90 % o f t h e o r i g i n a l f r a c t u r e c o n d u c t i v i t y i s l o s t a f t e r p l a c e ment o f t h e p r o p p a n t package by r e s i d u e p l u g g i n g ( c f . a l s o s e c t i o n 1 . 4 . 1 0 . ) . Some aspects o f foam u t i l i z a t i o n and f l u i d e f f i c i e n c y vs. r e s i d u e d e p o s i t i o n a r e o u t l i n e d as f o l l o w s .
4.3.4.1.5.1. Utilization of foam The a p p l i c a t i o n o f foam f r a c t u r i n g f l u i d s ( c f . s e c t i o n 3 . 1 1 . 2 . ) enables r e t a i n m e n t o f up t o 90 % o f t h e o r i g i n a l p r o p p a n t package c o n d u c t i v i t y due t o t h e b e t t e r c l e a n - u p c h a r a c t e r i s t i c s o f t h e foam which does n o t r e q u i r e b r e a k i n g , b u t f i r s t foams a r e l i m i t e d i n a p p l i c a t i o n due t o t h e i r temperature s e n s i t i v i t y and second h i g h foam q u a l i t y o f a b t . 75 % l e a v e s o n l y 25 % l i q u i d phase f o r t h e c a r r i a g e o f proppants, r e s u l t i n g i n o n l y l o w e r p r o p p a n t c o n c e n t r a t i o n s t o be p l a c e d downhole. T h i s e f f e c t , however, i s compensated by t h e b e t t e r f r a c t u r e c o n d u c t i v i t y r e t a i n m e n t w i t h r e s p e c t t o h i g h p r o p p a n t s a t u r a t i o n s p l a c e d by c r o s s l i n k e d g e l s ( c f . s e c t i o n 4.3.4.3.) where l a t e r much c o n d u c t i v i t y i s dest r o y e d by r e s i d u e p l u g g i n g as w e l l as by b e t t e r c l e a n - u p p r o p e r t i e s w h i c h a r e p a r t i c u l a r l y s i g n i f i c a n t i n low-pressure f o r m a t i o n s where r e s e r v o i r energy a v a i -
431 l a b l e f o r f l u i d flowback a f t e r t h e end o f t h e s t i m u l a t i o n t r e a t m e n t i s l i m i t e d . I n l o w - p e r m e a b i l i t y f o r m a t i o n s , l e a k o f f c o e f f i c i e n t s f o r some foams can be l o wer than those o f c r o s s l i n k e d aqueous f r a c t u r i n g f l u i d s (VEATCH & M O S C H O V I D I S 1986).
4.3.4.1.5.2. Fluid efficiency vs. residue deposition I n a d d i t i o n t o impairment o f f r a c t u r e c o n d u c t i v i t y by p l u g g i n g w i t h r e s i d u e formed upon breakage o r d e g r a d a t i o n o f g e l l e d f l u i d s , p e r m e a b i l i t y o f t h e propp a n t package can be d e t e r i o r a t e d by c a p t u r e and e n t r a i n m e n t o f f i n e p a r t i c l e s d e r i v i n g f r o m p r o p p a n t c r u s h i n g (CARROLL & BAKER 1979; c f . s e c t i o n 4.12.4.). The p r i n c i p a l mechanisms t h a t cause d e p o s i t i o n d u r i n g t r a n s p o r t o f p a r t i c l e s w i t h i n a porous medium a r e sedimentation, d i r e c t i n t e r c e p t i o n , and s u r f a c e a t t r a c t i v e p r o p e r t i e s o f p a r t i c l e s and medium. P l u g g i n g o f a propped f r a c t u r e c o r responds t o removal o f suspended p a r t i c l e s by c o n t a c t between t h e p o r e w a l l s and t h e g r a i n s when t h e pores a r e g e n e r a l l y l a r g e r than t h e p a r t i c l e s i n suspension. R e s i d u e - f o r m i n g m a t e r i a l caught i n t h e pores upon r e l e a s e o f t h e p a r t i n g p r e s s u r e reduces p o r e d i a m e t e r and p a r t i a l l y b l o c k s t h e i n t e r c o n n e c t i n g channels ( c f . s e c t i o n s 4.3.4.1.3.). Problems encountered i n h y d r a u l i c p r o p p a n t f r a c t u r i n g o f deep r e s e r v o i r s a r e o f t e n c r e a t e d by h i g h b o t t o m h o l e p r e s s u r e s and temperatures, small d i a m e t e r t u b i n g , and t u b i n g c o n t r a c t i o n d u r i n g s t i m u l a t i o n (CONWAY & H A R R I S 1982). The h i g h o p e r a t i n g p r e s s u r e s o f t e n n e c e s s i t a t e t h e use o f f l u i d s w i t h minimal pumpi n g f r i c t i o n pressures. I n order t o minimize c o n t r a c t i o n i n long tubing s t r i n g s w i t h s m a l l diameter, t r e a t i n g f l u i d s must o f t e n be p r e h e a t e d ( c f . s e c t i o n 4.5.1.). w i t h t h e h i g h i n i t i a l f l u i d temperatures c o n s i d e r a b l y a l t e r i n g f l u i d p r o p e r t i e s downhole. F l u i d e f f i c i e n c y i n most h y d r a u l i c f r a c t u r i n g o p e r a t i o n s v a r i e s f r o m 30 - 90 %, w i t h maximum percentage r e f l e c t i n g minimum l e a k o f f i n t o the formation which i s desirable f o r successful execution o f the job. I t i s h i g h l y d e s i r a b l e t o use f l u i d s which can p r o v i d e adequate v i s c o s i t y f o r propp a n t t r a n s p o r t , b u t y e t a r e c o n t r o l l a b l e so t h a t n e i t h e r e x c e s s i v e f r a c t u r e h e i g h t n o r e x c e s s i v e f r a c t u r e w i d t h do n o t o c c u r .
4.3.4.2. Fluid viscosity and transport ability Concerning p r o p p a n t t r a n s p o r t e f f e c t i v i t y , l o w - v i s c o s i t y f l u i d s (10 - 20 c p ) r e s u l t i n s e t t l i n g o f proppants t o t h e b o t t o m o f t h e f r a c t u r e , and bed h e i g h t can be i r r e g u l a r when p a r t i c l e s o f d i f f e r e n t s i z e a r e used d u r i n g t h e t r e a t ment. L o w - v i s c o s i t y c a r r i e r f l u i d s a r e t h e r e f o r e o n l y s u i t a b l e f o r p r o p p i n g o f s h o r t e r f r a c t u r e s , w i t h u n d e s i r e d s e g r e g a t i o n e f f e c t s h a v i n g t o be t a k e n i n t o c o n s i d e r a t i o n i n case o f t r a n s p o r t o f proppants f a r t h e r away f r o m t h e b o r e h o l e . M e d i u m - v i s c o s i t y f l u i d s ( 5 0 - 200 cp) p r o v i d e good b u t n o t p e r f e c t t r a n s p o r t , w i t h p r o p p a n t beds s t i l l p o s s i b l y b e i n g i r r e g u l a r . M e d i u m - v i s c o s i t y f l u i d s can c a r r y 20/40 o r s m a l l e r proppants f o r l o n g e r d i s t a n c e s i n more e x t e n s i v e f r a c t u r e s , b u t l a r g e r g r a i n s i z e s w i l l s e t t l e more r a p i d l y . H i g h - v i s c o s i t y f l u i d s ( o v e r 200 c p ) can c a r r y a l l s i z e s o f proppants w i t h o n l y minimal s e t t l i n g f a r away f r o m t h e w e l l b o r e w i t h i n l o n g cracks, and t h e r e s u l t i n g f r a c t u r e i s u s u a l l y v e r y u n i f o r m (HOLDITCH 1984). H i g h f l u i d v i s c o s i t y reduces f r i c t i o n w h i l e moving t h r o u g h t h e p i p e (SHAH 1984 and SHAH & LEE 1986 comment on f r i c t i o n p r e s s u r e s o f p r o p p a n t - l a d e n hydraul i c fracturing fluids; c f . s e c t i o n 4.3.4.2.2.1.), permits t r a n s p o r t o f h i g h p r o p p a n t s a t u r a t i o n s and c r e a t e s w i d e r f r a c t u r e s t h e r e b y making i t e a s i e r t o p l a c e t h e p r o p p i n g agent (WATERS 1980). The f o l l o w i n g d i s c u s s i o n focusses on p r o p p a n t c o n c e n t r a t i o n and v e r t i c a l f r a c t u r e growth as w e l l as on p r o p p a n t dens i t y l f l u i d v i s c o s i t y r e l a t i o n s h i p and f l u i d volume p e r r e s e r v o i r t h i c k n e s s u n i t . An o u t l i n e i s a l s o g i v e n o f t h e concept o f o v e r p r e s s u r e c a l i b r a t e d stimul a t i o n design.
432
4.3.4.2.1. Proppant concent rat ion and vertical fracture growth The s i g n i f i c a n c e o f p r o p p a n t c o n c e n t r a t i o n f o r v e r t i c a l f r a c t u r e growth i s i l l u s t r a t e d a l o n g t h e l i n e s o f h i g h - and l o w - v i s c o s i t y s t i m u l a t i o n f l u i d s as w e l l as s h e a r i n g o f g e l l e d f l u i d s .
4.3.4.2.1.1. High-vi scos i ty f 1 uids H i g h - v i s c o s i t y f l u i d s c r e a t e w i d e r f r a c t u r e s and a r e b e t t e r f o r t r a n s p o r t i n g p r o p p i n g agents, b u t a r e a l s o e x e r t i n g h i g h e r t r e a t i n g p r e s s u r e s which have a p o t e n t i a l f o r u n d e s i r e d v e r t i c a l h e i g h t growth (MENG & BROWN 1987; c f . s e c t i o n 4.2.2.3.3.). Proppant c o n c e n t r a t i o n and pad volume determine t h e f i n a l propped f r a c t u r e p e n e t r a t i o n and c o n d u c t i v i t y . I n s u f f i c i e n t pad volume o f t e n r e s u l t s i n premature s c r e e n o u t due t o e a r l y pad d e p l e t i o n ( c f . s e c t i o n s 4.2.3.2.5. and 6.2.4.2.1.), and s h o r t propped p e n e t r a t i o n i s n o t uncommon when pumping excess pad volume. I t i s t h e r e f o r e necessary t o determine t h e optimum pad s i z e and p r o p p a n t schedule i n o r d e r t o ensure t h a t t h e proppants w i l l be t r a n s p o r t e d t o t h e f r a c t u r e t i p a t t h e end o f t h e i n j e c t i o n process (NOLTE 1984, MENG & BROWN 1987). G e n e r a l l y h i g h - v i s c o s i t y f l u i d s p r o v i d e improved r e s u l t s f r o m h y d r a u l i c f r a c t u r i n g t r e a t m e n t s (ROSENE & SHUMAKER 1971).
4.3.4.2.1.2. Low-viscosity fluids L o w - v i s c o s i t y f l u i d s i n j e c t e d a t low pumping r a t e s i n t r e a t m e n t s o f r e s t r i c t e d s i z e a r e an e f f e c t i v e means o f c o n t r o l l i n g v e r t i c a l g r o w t h o f f r a c t u r e height, b u t s u f f e r from l i m i t e d proppant t r a n s p o r t c a p a b i l i t i e s , w i t h proppant banking o c c u r r i n g as a consequence o f p r o p p a n t s e t t l i n g d u r i n g f l u i d i n s e r t i o n i n t o t h e f r a c t u r e and b e f o r e c l o s u r e o f t h e c r a c k (BARBER & THEMIG 1985; c f . s e c t i o n 4.12.2.2.). C o o r d i n a t e d r a t h e r than random and u n d e s i r e d p r o p p a n t banki n g o r s t r a t i f i c a t i o n as w e l l as s e l e c t i v e placement can be achieved by f l u i d d e n s i t y c o n t r o l i n c l u d i n g pumping o f two f l u i d s o f d i f f e r e n t s p e c i f i c g r a v i t y and v i s c o s i t y which o v e r r i d e o r u n d e r r i d e each o t h e r and which can be charged w i t h d i f f e r e n t proppant loads f o r d i s t r i b u t i o n o f solid-supporting p a r t i c l e s i n t h e upper and/or l o w e r segment o f t h e f r a c t u r e as h o r i z o n t a l l a y e r s ( c f . sect i o n s 4.2.2.4. and 4 . 3 . 3 . 2 . 3 . ) .
4.3.4.2.1.3. Shearing o f gelled fluids Viscous g e l l e d w a t e r systems f e a t u r e moderate polymer l o a d i n g s and low r e s i due on b r e a k i n g (HANNAH & MATSON 1977). Shearing o f g e l l e d f l u i d s r e s u l t s i n r e d u c t i o n o f v i s c o s i t y and t h u s d i m i n u t i o n o f p r o p p a n t - c a r r y i n g c a p a c i t y (ROLL, V i s c o s i t y decrease o f HIMES, EWERT & DOERKSEN 1986; c f . s e c t i o n 4 . 3 . 3 . 3 . 2 . ) . f l u i d s by s h e a r i n g i s h i g h l y u n d e s i r a b l e , because i t l e a d s t o p r o p p a n t f a l l o u t d u r i n g pumping and placement w h i c h g i v e s r i s e t o premature s c r e e n o u t f a i l u r e by p r o p p a n t b r i d g i n g ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) i n pump, c a s i n g o r t u b i n g , screen, c r o s s o v e r t o o l , p e r f o r a t i o n s and o t h e r d e v i c e s . Proppant concent r a t i o n i n t h e c a r r i e r f l u i d has o n l y l i t t l e e f f e c t on shear d e g r a d a t i o n o f t h e g e l . F r a c t u r e h e i g h t and w i d t h i s g e n e r a l l y g r e a t e r w i t h v i s c o u s f l u i d s ( O S BORNE, McLEOD & SCHROEDER 1981). I n deep h i g h - p r e s s u r e w e l l s , t h e t u b u l a r goods o f t e n l i m i t i n j e c t i o n r a t e , and shear r a t e s a r e l o w e r i n these f r a c t u r i n g t r e a t ments s o t h a t apparent v i s c o s i t y o f f l u i d f l o w i n g i n these c r a c k s i s h i g h e r . Shearing o f g e l l e d f l u i d s i s p r e d o m i n a n t l y o c c u r r i n g a t t h e p e r f o r a t i o n s i n l i m i t e d - e n t r y c o n s t e l l a t i o n s (HOEL 1988; c f . s e c t i o n 4 . 2 . 2 . 2 . ) . Crosslinked f l u i d s h a v i n g undergone shear e x h i b i t l o w e r v i s c o s i t y which r e n d e r s them u n a b l e t o c a r r y and p l a c e h i g h e r p r o p p a n t c o n c e n t r a t i o n s , b u t a l l o w s deeper p r o p p a n t p e n e t r a t i o n w i t h l e s s h e i g h t growth t h a t r e s u l t s i n c o n n e c t i o n o f more n e t pay t o t h e w e l l b o r e (HOEL 1988). Ifa h i g h e r - v i s c o s i t y f l u i d were pumped i n an a t -
433 tempt t o t r a n s p o r t l a r g e volumes and t o p l a c e h i g h s a t u r a t i o n s o f proppant, broader f r a c t u r e s c o u l d be obtained which cause more h e i g h t growth o u t o f zone and l e s s proppant p e n e t r a t i o n w i t h i n the core o f the p r o s p e c t i v e i n t e r v a l . Case s t u d i e s reveal t h a t a t l e a s t f o r some r e s e r v o i r s , deep proppant p e n e t r a t i o n a t the expense o f h i g h proppant c o n c e n t r a t i o n i s the b e t t e r s o l u t i o n o f drainage maximization.
4.3.4.2.1.4. Other aspects High apparent v i s c o s i t i e s l e a d t o h i g h pressure drops i n t h e f r a c t u r e which can cause premature shut-downs because o f wellhead pressure l i m i t s , o r e l s e barr i e r shales surrounding t h e sandstone f o r m a t i o n are p a r t e d and the f l u i d f r a c t u r e s o u t o f zone. Premature shut-downs g i v e r i s e t o s h o r t e r cracks than expected, lower f i n a l i n j e c t e d proppant concentrations and poorer f r a c t u r e conductivities, and s t i m u l a t i n g out-of-zone leads t o h i g h e r b u t s h o r t e r cracks, w i t h i n b o t h cases the expected p r o d u c t i v i t y being lower than t h a t d e s i r e d . HANNAH, HARRINGTON & ANDERSON (1977) document t h a t the temperature o f f l u i d s f l o w i n g i n a f r a c t u r e i s near t h e f o r m a t i o n temperature over 50 - 75 % o f the crack l e n g t h . The apparent v i s c o s i t y a t t h e l e a d i n g edge o f t h e f r a c t u r e i s the most i m p o r t a n t one s i n c e t h a t i s where the g r e a t e s t treatment f l u i d pressure drop takes place. I t i s a l s o i m p o r t a n t t h a t the apparent v i s c o s i t i e s o f the f i nal f l u i d stages are n o t t o o h i g h under near-wellbore cool-down c o n d i t i o n s , as t h i s c o u l d cause u n d e s i r a b l e f r a c t u r e h e i g h t extension i n t o barren shales o r nonproductive sand sections.
4.3.4.2.2. Proppant density/f luid viscosity relationship and fluid volume oer reservoir thickness unit The g r e a t e s t advantage o f decreasing s p e c i f i c proppant g r a v i t y i s the f a c t t h a t l i g h t e r proppants can be t r a n s p o r t e d by l o w e r - v i s c o s i t y f l u i d s a t i d e n t i c a l c o n d i t i o n s as compared t o heavier proppants and h i g h e r - v i s c o s i t y f l u i d s , w i t h pumping o f l o w e r - v i s c o s i t y f l u i d s being a b l e t o be c a r r i e d o u t a t lower pressure (CUTLER, ENNISS, JONES & CARROLL 1983; c f . s e c t i o n s 1.3.2. and 4.2.2.3.3.). V e r t i c a l propagation o f the crack can be l i m i t e d by reducing f r a c t u r i n g f l u i d pressure, and m i n i m i z i n g f l u i d v i s c o s i t y by i n s e r t i o n o f l i g h t e r proppants i s a much more e f f e c t i v e method o f c o n t r o l l i n g pressure than l o w e r i n g the pumping r a t e (SIMONSON, ABOU-SAYED & CLIFTON 1978; NOLTE 1982; c f . s e c t i o n 4.2.2.3.2.). A l t e r n a t i v e l y , h i g h e r proppant volumes can be pumped i n a g i v e n amount o f h i g h - v i s c o s i t y f r a c t u r i n g f l u i d . F l u i d volumes per r e s e r v o i r thickness u n i t a l s o have an i m p o r t a n t i n f l u e n c e on proppant c o n c e n t r a t i o n placed w i t h i n the f r a c t u r e and thus post-treatment w e l l performance (NALL, CAMPBELL & BONEY 1983). F i e l d experience has shown t h a t the lowest s k i n e f f e c t s , g r e a t e s t c a l c u l a t e d f r a c t u r e p e r m e a b i l i t i e s , and h i g h e r decreased absolute open-flow p o t e n t i a l t e s t s were o b t a i n e d when volumes o f 2,000 g a l / f t n e t r e s e r v o i r thickness were pumped. I n terms o f h i g h e r f l u i d quantities, t h e key t o the observed p r o d u c t i v i t y increase i s the a b i l i t y t o p l a c e more proppants i n h i g h e r s a t u r a t i o n i n the f r a c t u r e w i t h the l a r g e r f l u i d amounts, b u t economics have t o be considered on a w e l l - t o - w e l l b a s i s t o d e t e r mine the b e s t p o s s i b l e treatment volume f o r the money spent. The d i s c u s s i o n as f o l l o w s focusses on f r i c t i o n l o s s i n t u b u l a r goods, gel degradation, f l u i d v i s c o s i t y m o d i f i c a t i o n , and f r a c t u r e h e i g h t vs. w i d t h .
4.3.4.2.2.1. Friction loss in tubular goods Tubing and c a s i n g design a l s o have a l a r g e i n f l u e n c e on treatment r a t e s and pressures which i n t u r n i n f l u e n c e proppant c o n c e n t r a t i o n t h a t t h e f o r m a t i o n can accept (PA1 & GARBIS 1983 b ) . F r i c t i o n c a l c u l a t i o n s f o r the f l u i d i n the p i p e
434 and across t h e p e r f o r a t i o n s need t o account f o r t h e i n c r e a s e d v i s c o s i t y and dens i t y due t o h i g h p r o p p a n t s a t u r a t i o n s (SHAH, S M I T H & OONALOSON 1983). Because f r i c t i o n l o s s across t h e p e r f o r a t i o n s can become e x c e s s i v e as f l u i d d e n s i t y r i ses, i t i s n o t uncommon t o i n c r e a s e t h e p e r f o r a t i o n number by 50 o r 100 % i n an u l t r a - h i g h p r o p p a n t c o n c e n t r a t i o n j o b . D u r i n g course o f t h e o p e r a t i o n , t h e act u a l bottomhole t r e a t i n g p r e s s u r e may be g r a d u a l l y r i s i n g b e i n g a p o s s i b l e i n d i c a t i o n o f a screenout, whereas t h e s u r f a c e p r e s s u r e i s d r o p p i n g due t o t h e l a r g e increases i n f l u i d densities.
4.3.4.2.2.2.Gel degradation The s t r u c t u r e o f most f r a c t u r i n g g e l s tends t o degrade and l o s e i t s p r o p p a n t t r a n s p o r t a b i l i t y t o some degree w i t h b o t h i n c r e a s i n g temperature and t i m e (POULSEN & LEE 1984). A l t h o u g h some g e l s r e q u i r e e l e v a t e d temperatures t o a t t a i n t h e i r maximum degree o f c r o s s l i n k i n g , these f l u i d s a l s o degrade w i t h s u f f i c i e n t temperature and t i m e ( c f . s e c t i o n 4 . 3 . 4 . 3 . ) . Since temperature and p r e sence t i m e o f t h e f l u i d i n t h e f r a c t u r e b o t h i n c r e a s e w i t h d i s t a n c e f r o m t h e wellbore, i t f o l l o w s t h a t the c l o s e r the f l u i d i s t o the crack t i p , the less p r o p p a n t - c a r r y i n g a b i l i t y i t has, w i t h t h e r e f o r e a t t h e f r a c t u r e t i p most e a s i l y proppant b r i d g i n g and b l o c k i n g o c c u r r i n g which provokes premature o r submat u r e s t i m u l a t i o n j o b t e r m i n a t i o n by s c r e e n o u t .
4.3.4.2.2.3. Fluid viscosity modification When m o n i t o r i n g n e t p r e s s u r e e v o l u t i o n d u r i n g course o f t h e f r a c t u r i n g t r e a t ment, t h e a b i l i t y t o a l t e r f l u i d v i s c o s i t y can improve s t i m u l a t i o n e f f e c t i v e ness (OLSEN & DEBONIS 1988). F l u i d v i s c o s i t y and pumping r a t e a f f e c t f r a c t u r e geometry almost e q u a l l y when a l t e r e d a t t h e same magnitude, b u t i n most cases, i t i s o p e r a t i o n a l l y e a s i e r and more c o s t e f f i c i e n t t o t r i p l e f l u i d v i s c o s i t y than t o t r i p l e pumping r a t e . T h e r e f o r e m o d i f y i n g f l u i d r h e o l o g y seems t o be a more v i a b l e means o f a l t e r i n g f r a c t u r e geometry i n a r e a l - t i m e mode t h a n pumpi n g r a t e , a l t h o u g h b o t h can be used c o n c u r r e n t l y ( c f . s e c t i o n 6 . 2 . 4 . 3 . ) . I f n e t f r a c t u r i n g p r e s s u r e exceeds c r i t i c a l f o r m a t i o n p r e s s u r e o r c a p a c i t y o f t h e l a t t e r , e i t h e r e x c e s s i v e f l u i d l e a k o f f o r u n c o n t r o l l e d f r a c t u r e h e i g h t g r o w t h occ u r which can be b o t h d e t r i m e n t a l t o f r a c t u r i n g e f f e c t i v e n e s s ( c f . s e c t i o n 4.2.2.3.). I f f o r m a t i o n p r e s s u r e c a p a c i t y f o r a g i v e n r e s e r v o i r i s known, s t i m u l a t i o n f l u i d v i s c o s i t y can be m o d i f i e d d u r i n g t h e t r e a t m e n t t o p r e v e n t n e t p r e s s u r e f r o m exceeding f o r m a t i o n p r e s s u r e c a p a c i t y .
4.3.4.2.2.4.Fracture height vs. width Pad o r o t h e r non-proppant stages a r e p r o p a g a t i n g t h e h y d r a u l i c a l l y - i n d u c e d f r a c t u r e . As t h i s i s t h e f i r s t f l u i d exposed t o new rock i n t h e f r a c t u r e , pad stage o f t e n s u f f e r s most s e v e r e l y f r o m f l u i d - l o s s c o n d i t i o n s . E x c e s s i v e pad f l u i d l o s s g i v e s r i s e t o almost complete f l u i d l e a k o f f i n t o t h e f o r m a t i o n t h u s b r i n g i n g p r o p p a n t - b e a r i n g f l u i d n e a r t h e f r a c t u r e f a c e . As p r o p p a n t - b e a r i n g f l u i d s do n o t propagate h y d r a u l i c cracks, f r a c t u r e p e n e t r a t i o n i s r e s t r i c t e d and b a l l o o n s i n w i d t h due t o t h e l a r g e d i f f e r e n c e between i n j e c t i o n r a t e and l e a k o f f r a t e ( c f . section 6.2.4.2.1.). A t t h i s p o i n t , t h e r e i s no need f o r f l u i d v i s c o s i t y t o m a i n t a i n f r a c t u r e w i d t h , and t h u s polymer l o a d i n g c o u l d be reduced t o t h e minimum v i s c o s i t y r e q u i r e d f o r p r o p p a n t t r a n s p o r t which d i m i n i shes t h e amount o f polymer used, decreases c o n d u c t i v i t y damage i n t h i s c o n f i n e d f r a c t u r e area, and a l s o slows down p r e s s u r e i n c r e a s e . A s u i t a b l e improvement i s c o n s i d e r a b l e enlargement o f t h e pad f l u i d stage o f t h e purpose o f b o t h l e a k o f f c o n t r o l and r e s e r v o i r c o o l i n g ( c f . s e c t i o n 4 . 3 . 4 . 4 . 2 . 2 . ) . On t h e o t h e r hand, sudden unexpected and u n a v o i d a b l e occurrence o f e x c e s s i v e f r a c t u r e h e i g h t can d r a m a t i c a l l y decrease c r a c k w i d t h which i n t u r n can cause p r o p p a n t b r i d g i n g i n t h e f r a c t u r e l e a d i n g t o r a p i d p r e s s u r e i n c r e a s e and prema-
435 t u r e t r e a t m e n t t e r m i n a t i o n by s c r e e n o u t ( c f . s e c t i o n 4 . 2 . 2 . 2 . 3 . ) . The w o r s t case i s t h e c o m b i n a t i o n o f h i g h f l u i d l o s s and s e r i o u s w i d t h r e d u c t i o n caused by u n c o n t r o l l e d h e i g h t growth which can provoke proppant b r i d g i n g f a i r l y c l o s e t o t h e w e l l b o r e t h a t can r e s u l t i n s c r e e n o u t f a i l u r e a t a v e r y e a r l y stage o f t h e t r e a t m e n t . V i s c o s i t y i n c r e a s e o f t h e f l u i d c o u l d compensate f o r t h e w i d t h l o s s f r o m h e i g h t growth and h e l p p r e v e n t i n g submature s c r e e n o u t f a i l u r e ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) . F i e l d e x p e r i e n c e r e v e a l s t h a t ramped p o l y mer l o a d i n g j o b s have much h i g h e r p r e s s u r e f a l l o f f r a t e s a f t e r shutdown t h a n c o n v e n t i o n a l c o n s t a n t polymer l o a d i n g o p e r a t i o n s .
4.3.4.2.3. Overpressure cal ibrated stimulation design Another method o f enhancing t r e a t m e n t s i z e and t h u s p r o p p a n t q u a n t i t y and c o n c e n t r a t i o n i s o v e r p r e s s u r e c a l i b r a t e d s t i m u l a t i o n d e s i g n (SHLYAPOBERSKY, WONG & WALHAUG 1988) where t h e n e t c r a c k p r e s s u r e measured d u r i n g m i n i - o r m i c r o f r a c t u r i n g j o b s (SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1988; c f . sect i o n 4.8.2.6.) s h o r t l y a f t e r s h u t - i n ( o v e r p r e s s u r e ) i s used t o determine appar e n t f r a c t u r e toughness o f t h e f o r m a t i o n which i s then taken as an i n p u t i n a r e s e r v o i r s i m u l a t o r t o d e s i g n h y d r a u l i c f r a c t u r e s t i m u l a t i o n volume and propp a n t i n j e c t i o n schedule. The o v e r p r e s s u r e i s d e f i n e d as t h e d i f f e r e n c e between t h e c o r r e c t e d i n s t a n t a n e o u s s h u t - i n p r e s s u r e and t h e minimum i n - s i t u s t r e s s . The o u t l i n e as f o l l o w s focusses on c o n v e n t i o n a l u n d e r e s t i m a t i o n o f n e t f r a c t u r e p r o p a g a t i o n pressure, reasons f o r e l e v a t e d n e t f r a c t u r e p r o p a g a t i o n p r e s s u r e s , s c a l e dependency o f apparent f r a c t u r e toughness, and h i g h e r p r o p p a n t volumes and c o n c e n t r a t i o n s pumped w i t h o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e d e s i g n .
4.3.4.2.3.1. Conventional underestimation of net fracture propagat ion pressure W i t h c o n v e n t i o n a l models, n e t f r a c t u r e p r o p a g a t i o n p r e s s u r e ( d e f i n e d as f r a c t u r e p r o p a g a t i o n p r e s s u r e minus minimum i n - s i t u s t r e s s o r c l o s u r e s t r e s s ) i s a l ways underestimated, w i t h these models t h u s p r e d i c t i n g a narrower and l o n g e r propped c r a c k t h a n a c t u a l l y i s b e i n g c r e a t e d i n t h e f i e l d , and t h e r e f o r e t h e des i g n i s n o t o p t i m i z e d t o produce t h e d e s i r e d f r a c t u r e geometry and maximum c r a c k c o n d u c t i v i t y t h r o u g h h i g h p r o p p a n t c o n c e n t r a t i o n s (SHLYAPOBERSKY, WONG & WALHAUG 1988). Apparent m o d e l l i n g o f w i d e r c r a c k s t h a n t e c h n i c a l l y p o s s i b l e as a consequence o f u n d e r e s t i m a t i o n o f n e t f r a c t u r e p r o p a g a t i o n p r e s s u r e can a l s o l e a d t o an unwanted a g g r e s s i v e t r e a t m e n t t h a t has a h i g h p o t e n t i a l o f submature screenout. O p t i m i z a t i o n of t h e h y d r a u l i c s t i m u l a t i o n o p e r a t i o n t h u s r e q u i r e s c a l i b r a t i o n o f t h e model w i t h t h e f i e l d n e t c r a c k p r o p a g a t i o n p r e s s u r e which r e s u l t s i n s h o r t e r p r e d i c t e d l e n g t h t h a t i s more c o n s i s t e n t w i t h t h e p r o d u c t i v e response o f t h e w e l l . Overpressure c a l i b r a t e d d e s i g n enables t o p e r f o r m more aggress i v e t r e a t m e n t s w i t h s m a l l e r pad and l a r g e r amount and s a t u r a t i o n o f p r o p p a n t s t h a n w i t h c o n v e n t i o n a l models and p e r m i t s t o s i g n i f i c a n t l y d i m i n i s h t h e r i s k o f premature s c r e e n o u t t e r m i n a t i o n ( c f . s e c t i o n s 4.2.2.2.3., 4.2.3.2.5. and 6.2.4.2.1.). The necessary d a t a f o r o v e r p r e s s u r e c a l i b r a t e d s t i m u l a t i o n d e s i g n a r e measured d u r i n g m i n i - and m i c r o f r a c t u r e f i e l d t e s t s (SHLYAPOBERSKY 1988; SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1988; c f . s e c t i o n 4 . 8 . 2 . 6 . ) .
4.3.4.2.3.2. Reasons for elevated
net fracture propagat ion pressures H i g h e r n e t f r a c t u r e p r o p a g a t i o n p r e s s u r e s as usual can be c r e a t e d by h i g h e r than expected f r i c t i o n a l p r e s s u r e s d e v e l o p i n g i n p e r f o r a t i o n s , near t h e w e l l bore r e g i o n and i n t h e c r a c k e s p e c i a l l y near i t s t i p (SHLYAPOBERSKY, WONG & WALHAUG 1988; c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) . The enhanced f r i c t i o n c o u l d be caused by i n -
436 creased s l u r r y v i s c o s i t y due t o l e a k o f f and p r o p p a n t c o n c e n t r a t i o n , fracture s u r f a c e roughness, t u r b u l e n t f r a c t u r e f l o w under non-Darcy c o n d i t i o n s ( c f . sect i o n 4 . 3 . 3 . 3 . ) , and m a j o r c u r r e n t c o n s t r i c t i o n s a l o n g t h e c r a c k caused by o r t h o gonal j o i n t s such as n a t u r a l f r a c t u r e s . Other reasons f o r e l e v a t e d n e t c r a c k p r o p a g a t i o n p r e s s u r e s a r e h i g h e r f r a c t u r e toughness than expected due t o a scale-dependent c r a c k l a y e r a t t h e f r a c t u r e t i p , and presence o f a f l u i d - n o n p e n e t r a t i n g region a t the l a t t e r p o i n t . C r e a t i o n o f a s e t o f m u l t i p l e p a r a l l e l f r a c t u r e s ( c f . s e c t i o n 4.8.6.2.4.) ins t e a d o f a s i n g l e dominant c r a c k i s u n l i k e l y t o be a reason f o r h i g h e r n e t f r a c t u r e p r o p a g a t i o n pressures, as many s u c c e s s f u l t r e a t m e n t s have been pumped w i t h moderate n e t c r a c k p r o p a g a t i o n p r e s s u r e s w i t h h i g h p r o p p a n t s a t u r a t i o n s and a v i s c o u s s l u r r y . D u r i n g f l u i d i n j e c t i o n , b o t h f r i c t i o n a l and f r a c t u r e toughness e f f e c t s a r e p r e s e n t and d i f f i c u l t t o separate, whereas s h o r t l y a f t e r s h u t - i n when t h e c r a c k remains open and n e a r l y ceases t o propagate w i t h f l u i d r e d i s t r i b u t i o n and p r e s s u r e i n s t a b i l i z a t i o n , t h e f r i c t i o n a l e f f e c t s a r e minimized, and t h e n e t f r a c t u r e p r e s s u r e a t t h i s moment r e p r e s e n t i n g t h e o v e r p r e s s u r e can t h e n be p r i m a r i l y a t t r i b u t e d t o apparent f r a c t u r e toughness (SHLYAPOBERSKY 1987).
4.3.4.2.3.3.
Scale dependency o f apparent f r a c t u r e toughness
Successive f i e l d f r a c t u r i n g t e s t s conducted i n t h e same f o r m a t i o n w i t h i d e n t i c a l r a t e and f l u i d v i s c o s i t y r e v e a l e d o c c a s i o n a l l y t h a t t h e i n s t a n t a n e o u s s h u t - i n p r e s s u r e i n c r e a s e s w i t h r i s i n g i n j e c t i o n volumes, whereas c l o s u r e s t r e s s remains n e a r l y unchanged (SHLYAPOBERSKY, WONG & WALHAUG 1988). As p r o p p a n t b r i d g i n g cannot o c c u r due t o proppant absence i n t h e m i n i - o r m i c r o f r a c t u r e t e s t s , the increase o f instantaneous shut-in pressure o r overpressure w i t h i n j e c t i o n volumes may be a m a n i f e s t a t i o n o f t h e s c a l e dependency o f a p p a r e n t f r a c t u r e toughness (SHLYAPOBERSKY 1987). I n o r d e r t o p r e d i c t r e a l i s t i c c r a c k dimensions, h y d r a u l i c f r a c t u r e d e s i g n t h e r e f o r e has t o i n c l u d e apparent c r a c k toughness o f t h e r o c k i n a d d i t i o n t o t h e o t h e r i n t e r a c t i n g processes o f v i s c o u s f l u i d f l o w , e l a s t i c r o c k d e f o r m a t i o n , and f l u i d l o s s through f r a c t u r e w a l l s . The energy a n a l y s i s o f h y d r a u l i c s t i m u l a t i o n model (SHLYAPOBERSKY 1985) i n c o r p o r a t e s t h e e f f e c t o f f r a c t u r e toughness f o r u n c o n t a i n e d ( c i r c u l a r o r r a d i a l ) and c o n t a i n e d ( r e c t a n g u l a r o r l i n e a r ) f r a c t u r e s . The model c o n s i d e r s t h e f l u i d - f i l l e d h y d r a u l i c c r a c k as an open thermodynamic system and t h e q u a s i - e q u i l i b r i u m p r o p a g a t i o n o f t h e f r a c t u r e as t h e e v o l u t i o n o f t h e system. An a p p r o x i mate s o l u t i o n f o r such a system i s o b t a i n e d by m i n i m i z i n g t o t a l energy d i s s i p a t i o n r a t e as r e q u i r e d by t h e u n i v e r s a l e v o l u t i o n a r y c r i t e r i o n . Energy d i s s i p a t i o n t a k e s p l a c e by c r e a t i o n o f t h e new f r a c t u r e s u r f a c e , c r a c k - o p e n i n g by def o r m i n g t h e s u r r o u n d i n g r e s e r v o i r , and v i s c o u s d i s s i p a t i o n i n f l u i d .
4.3.4.2.3.4. Higher proppant volumes and concentrations pumped w i t h overpressure c a l i b r a t e d f r a c t u r e design F i e l d examples o f comparative assessment o f Overpressure c a l i b r a t e d d e s i g n and c o n v e n t i o n a l m o d e l l i n g show t h a t i m p l e m e n t a t i o n o f t h e o v e r p r e s s u r e concept can more than double t h e t r e a t m e n t volume by p e r m i t t i n g c r e a t i o n o f w i d e r f r a c t u r e s t h a t accept h i g h e r p r o p p a n t c o n c e n t r a t i o n s and l a r g e r t o t a l p r o p p a n t quant i t i e s (SHLYAPOBERSKY, WONG & WALHAUG 1988). Even w i t h some chosen degree o f conservatism, t h e r e s u l t i n g o v e r p r e s s u r e c a l i b r a t e d t r e a t m e n t s a r e s t i l l much more a g g r e s s i v e i n b o t h s l u r r y volume and amount o f p r o p p a n t i n j e c t e d t h a n conv e n t i o n a l d e s i g n s . Thus o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e m o d e l l i n g l e a d s t o v e r y e f f e c t i v e s t i m u l a t i o n o p e r a t i o n s w i t h much l a r g e r f l u i d volumes and c o n s i d e r a b l y more a g g r e s s i v e p r o p p a n t i n j e c t i o n schedules than c o n v e n t i o n a l l y d e s i g ned. The i n c r e a s e d j o b s i z e s a r e t h e c o n f i r m a t i o n t h a t t h e s t a n d a r d models und e r e s t i m a t e n e t f r a c t u r e p r o p a g a t i o n p r e s s u r e . The t e c h n i c a l and economical ben e f i t o f o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e d e s i g n i s t h e p o s s i b i l i t y o f more t h a n d o u b l i n g t o t a l t r e a t m e n t volume and h a l v i n g pad s i z e w i t h r e s p e c t t o con-
437 v e n t i o n a l approaches. Overpressure c a l i b r a t i o n can a l s o be used t o o p t i m i z e t h e aggressiveness o f t h e p r o p p a n t schedule ( c f . s e c t i o n 4.13.) and t o reduce s c r e e n o u t r i s k accordi n g t o f i e l d o b s e r v a t i o n s and sound e n g i n e e r i n g judgement. A l a r g e r o v e r p r e s s u r e g e n e r a l l y r e s u l t s i n a w i d e r f r a c t u r e t h a t r e q u i r e s a g r e a t e r t r e a t m e n t volume t o produce a d e s i r e d economical f r a c t u r e l e n g t h and a l l o w s a more aggress i v e p r o p p a n t schedule t o o p t i m i z e t r e a t m e n t p r o d u c t i v i t y , whereas an o v e r p r e s s u r e s m a l l e r t h a n t h a t measured i n t h e f i e l d can be used i n t h e d e s i g n t o cont r o l t h e aggressiveness o f t h e p r o p p a n t schedule and t o reduce t h e r i s k o f p r e mature s c r e e n o u t ( c f . s e c t i o n 6.2.4.2.1.). The u t i l i z a t i o n o f o v e r p r e s s u r e i n s t e a d o f n e t f r a c t u r e p r o p a g a t i o n p r e s s u r e as a c a l i b r a t i o n parameter f o r hyd r a u l i c f r a c t u r e s t i m u l a t i o n d e s i g n t h u s p r o v i d e s c o n s i d e r a b l e advantages p a r t i c u l a r l y i n new f i e l d s where e x p e r i e n c e and u n c e r t a i n t i e s w i t h t h e model d a t a have n o t been f u l l y e s t a b l i s h e d , and i s an e x c e l l e n t t o o l f o r m i n i m i z a t i o n o f f r i c t i o n a l uncertainties, u n r e l i a b i l i t i e s i n using treatment pressure t o pred i c t f r a c t u r e growth, and r i s k o f premature screenout f a i l u r e . Aspects o f o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e d e s i g n a r e a l s o d i s c u s s e d by HUCKABEE ( 1 9 8 8 ) .
4.3.4.3. Impact of delayed crosslinking The impact o f delayed c r o s s l i n k i n g on proppant c o n c e n t r a t i o n i s predominantl y t h e reduced d e t e r i o r a t i o n o f p r o p p a n t s a t u r a t i o n by premature m a t e r i a l f a l l o u t . A f t e r some i n t r o d u c t o r y comments on c r o s s l i n k i n g types, s i g n i f i c a n c e and performance, improvement o f p r o p p a n t placement by r e t a r d e d c r o s s l i n k i n g as w e l l as enhanced f l u i d s t a b i l i t y and t r a n s p o r t c a p a c i t y i s o u t l i n e d .
4.3.4.3.1. Cross1 inking types C r o s s l i n k i n g o f f r a c t u r i n g f l u i d s w i t h some t y p e o f t r a n s i t i o n m e t a l l i c compound such as b o r a t e o r t i t a n a t e g i v e s r i s e t o f o r m a t i o n o f c h e l a t e complexes which have c o n s i d e r a b l y enhanced p r o p p a n t t r a n s p o r t and temperature s t a b i l i t y c h a r a c t e r i s t i c s than c o n v e n t i o n a l f r a c t u r i n g f l u i d s (GRONE, BAUMGARTNER & WOODROOF 1983; VEATCH 1983, WALSER 1985; KRAMER, PRUDHOMME, NORMAN & SANDY 1987), and t h a t a l s o a i d s i n f l u i d - l o s s c o n t r o l and combats p o t e n t i a l s c r e e n o u t f a i l u r e (NALL, CAMPBELL & BONEY 1983). The c r o s s l i n k i n g r e a c t i o n r a t e o f t h e organ o m e t a l l i c c h e l a t e w i t h d e r i v a t i z e d c e l l u l o s e o r guar i s i n f l u e n c e d by a combin a t i o n o f pH and temperature (HARRIS, HARMS & NORMAN 1987). The f o l l o w i n g o v e r view c o n t a i n s some comments on g e n e r a l b e n e f i t o f c r o s s l i n k i n g as w e l l as conv e n t i o n a l and delayed c r o s s l i n k i n g . Aspects o f v i s c o s i t y r e l a t i o n s h i p s , c o n t i nuous vs. b a t c h m i x i n g , delayed c r o s s l i n k e d f r a c t u r i n g f l u i d s vs. d u a l - v i s c o s i t y g r a v e l p a c k i n g f l u i d s , and m i x i n g o f delayed and r a p i d c r o s s l i n k i n g agents are also i l l u s t r a t e d .
4.3.4.3.1.1. General benefit o f crosslinking C r o s s l i n k i n g a l s o i n c r e a s e s f l u i d v i s c o s i t y and t h u s p r o p p a n t t r a n s p o r t capac i t y (GRONE, BAUMGARTNER & WOODROOF 1983). The i n t r o d u c t i o n o f c r o s s l i n k e d f r a c t u r i n g g e l s i n t h e l a t e 1960's has i n many ways been r e v o l u t i o n a r y , because t h e i r unique advantage i s t h e v e r y h i g h v i s c o s i t y y i e l d f o r modest polymer s a t u r a t i o n s w h i c h makes them i d e a l f o r p l a c i n g h i g h e r proppant c o n c e n t r a t i o n s due t o i n h e r e n t h i g h e f f i c i e n c y , a b i l i t y t o c r e a t e wide f r a c t u r e s , e x c e l l e n t propp a n t t r a n s p o r t c h a r a c t e r i s t i c s (HANNAH, HARRINGTON & POTTER 1980; PA1 & GARBIS 1983 b ) and e x t r e m e l y l o w f l u i d l o s s (NORTON & HOFFMAN 1982). As t h e y employ l o wer polymer c o n c e n t r a t i o n s , c r o s s l i n k e d f l u i d s t e n d t o break c l e a n e r and p r o duce l e s s damaging r e s i d u e than u n c r o s s l i n k e d f l u i d s . Conventional c r o s s l i n k e d f l u i d systems a r e s t a b l e up t o 275 OF, b u t w i t h enhanced temperature s t a b i l i t y m i x t u r e s , s u c c e s s f u l f r a c t u r e t r e a t m e n t s have been c a r r i e d o u t a t temperatures up t o 540 OF (280 oC; WILLIAMS, WOODROOF & BOX 1982; c f . s e c t i o n 4 . 7 . 4 . 2 . ) . O i f -
438 f e r e n t l i g a n d s can be u t i l i z e d f o r f a s t e r o r slower c r o s s l i n k i n g o f t h e f l u i d . Delayed c r o s s l i n k i n g systems a r e f o r m u l a t e d t o a c t i v a t e a f t e r t h e f l u i d has been pumped down t h e t u b u l a r s and through t h e p e r f o r a t i o n s and has been one o f t h e r e c e n t s i g n i f i c a n t improvements i n f r a c t u r i n g f l u i d t e c h n o l o g y (VEATCH & M O S C H O V I D I S 1986). The o n l y disadvantage o f c r o s s l i n k e d f r a c t u r i n g f l u i d s i s t h e i r u n s a t i s f a c t o r y f l u i d c o m p a t i b i l i t y (NORTON & HOFFMAN 1982), because many j o b s u s i n g c r o s s l i n k e d g e l s r e s u l t i n moderate t o severe w a t e r blockage problems ( c f . s e c t i o n 3.11.2.4.2.). While a d d i t i o n o f n i t r o g e n t o the crosslinked gel helps recoveri n g t h e t r e a t i n g f l u i d , t h e w a t e r blockage problems s t i l l o c c u r d e s p i t e p r e sence o f n i t r o g e n and use o f s u r f a c t a n t s . Water blockage, however, a l s o o c c u r s w i t h a p p l i c a t i o n o f n o n - c r o s s l i n k e d water-based g e l s . An unusual s p e c i a l a p p l i c a t i o n o f c r o s s l i n k i n g i s c o n n e c t i o n o f an a c r y l i c r e s i n and a l i q u i d epoxy r e s i n f o r d e s t r u c t i o n o f r e s e r v o i r p e r m e a b i l i t y by p l u g g i n g o f t h i e f zones (KNAPP & WELBDURN 1978; ZAITOUN, KOHLER & G U E R R I N I 1989; c f . s e c t i o n 5 . 9 . 1 . 7 . ) .
4.3.4.3.1.2. Conventional and delayed crosslinking Three types o f c r o s s l i n k e r systems have t o be d i s t i n g u i s h e d (ZIGRYE, OSBORNE & WESTBROOK 1 9 8 6 ) . E a r l y t i m e systems b e g i n c r o s s l i n k i n g i m m e d i a t e l y when i n j e c t e d i n t o t h e base g e l . Delayed c r o s s l i n k e r systems s t a r t t o a c t some t i m e a f t e r i n j e c t i o n i n t o t h e base g e l , w i t h t h e r e a c t i o n b e i n g a c t i v a t e d by t e m p e r a t u r e . The c o n t r o l l e d e a r l y t i m e c r o s s l i n k e r system i s a c o m b i n a t i o n o f t h e two o t h e r systems, and when i t i s i n j e c t e d i n t o t h e base g e l , some c r o s s l i n k i n g o c c u r s i m m e d i a t e l y f o l l o w e d by a m a j o r c r o s s l i n k i n g r e a c t i o n i n i t i a t e d a t h i g h e r temperat u r e o f a b t . 100 OF ( 3 8 oC). The p r i m a r y advantage o f d e l a y i n g t h e c r o s s l i n k i n g r e a c t i o n i s t h a t v i s c o s i t y would n o t i n c r e a s e u n t i l s h o r t l y b e f o r e t h e f r a c t u r i n g f l u i d reaches t h e b o t t o m o f t h e w e l l b o r e and e n t e r s t h e f r a c t u r e . D e l a y i n g can be achieved by u s i n g an i n h e r e n t l y delayed c r o s s l i n k e r (WALSER & OTT 1985) o r adding a d e l a y i n g agent t o a f a s t - r e a c t i n g c r o s s l i n k e r (RUMMO 1982). This delay i n c r o s s l i n k i n g minimizes f r i c t i o n pressure i n the i n j e c t i o n t u b i n g as w e l l as d e g r a d a t i o n o f c r o s s l i n k e d f r a c t u r i n g f l u i d caused by h i g h shear i n t h e t u b i n g and t h u s d i m i n i s h e s h y d r a u l i c horsepower r e q u i r e d f o r t h e j o b (GARBIS, BROWN & MAURITZ 1985) and p e r m i t s h i g h e r i n j e c t i o n r a t e s (ECONOMIDES 1986). The delayed c r o s s l i n k e d g e l forms when t h e f l u i d i s f a r down t h e t u b i n g s t r i n g a f t e r t h e h i g h - s h e a r p o r t i o n o f t h e t r e a t m e n t (HARRIS, HARMS & NORMAN 1987). The small immediate v i s c o s i t y i n c r e a s e generated by t h e c o n t r o l l e d e a r l y t i m e c r o s s l i n k e r system i s s u f f i c i e n t t o i n s u r e p r o p p a n t t r a n s p o r t as t h e f l u i d f l o w s i n t o t h e f r a c t u r e , b u t i t does n o t s i g n i f i c a n t l y i n c r e a s e t u b i n g f r i c t i o n p r e s s u r e . HODGE & BARANET (1987) e v a l u a t e f i e l d methods t o d e t e r m i n e c r o s s l i n k i n g time o f f r a c t u r i n g f l u i d s .
4.3.4.3.1.3.Viscosity relationships The v i s c o s i t y o f delayed c r o s s l i n k e d f r a c t u r i n g f l u i d s i s a f u n c t i o n o f f l o w geometry, c r i t i c a l s t r e s s and t i m e as w e l l as shear and thermal h i s t o r i e s ( C A MERON, GARDNER & VEATCH 1988). The s t r e s s - t r a n s f o r m a t i o n p r o p e r t y o f d e l a y e d c r o s s l i n k e d g e l s i s t h e r e s u l t o f g e l breaking-down and m i g r a t i n g i n t o t h e lowv i s c o s i t y f l u i d a t t h e w a l l s , and p o s s i b l y shear-induced f l o c c u l a t i o n and second a r y f l o w s . I n l o w - s t r e s s s t a t e where t h e g e l shows v e r y low apparent v i s c o s i t y because of w a l l s l i p , p e r f e c t p r o p p a n t t r a n s p o r t may be p o s s i b l e s i n c e t h e b u l k f l u i d i s c o m p l e t e l y g e l l e d . Delayed c r o s s l i n k e d g e l s a r e g e n e r a l l y c h a r a c t e r i zed by non-homogeneous f l o w (CAMERON 1988). D e l a y i n g c r o s s l i n k i n g u n t i l a f t e r exposure t o h i g h shear i n c r e a s e s g e l v i s c o s i t y (PRUDHOMME, ELLIS, CONSTIEN & KNOLL 1988). The r h e o l o g y o f c r o s s l i n k e d g e l s i s determined by t h e s t r u c t u r e o f t h e g e l network which i n t u r n i s c o n t r o l l e d by c r o s s l i n k e r r e a c t i o n k i n e t i c s and f l o w c o n d i t i o n s d u r i n g c r o s s l i n k i n g . I n f a s t c r o s s l i n k i n g systems where t h e c r o s s l i n k i n g process i s r a p i d enough t h a t t h e g e l s t r u c t u r e i s e s t a b l i s h e d du-
439 ring tion ture rent
m i x i n g o r d u r i n g exposure t o h i g h shear r a t e s , e v e r y d i f f e r e n t c o n f i g u r a o f t u b i n g geometry and pumping r a t e produces a d i f f e r e n t g e l network s t r u c and d i f f e r e n t f l u i d r h e o l o g y , and t h e same g e l c h e m i s t r y generates d i f f e v i s c o s i t i e s depending upon magnitude and d u r a t i o n o f shear.
D e l a y i n g o f g e l f o r m a t i o n u n t i l a t i m e when o n l y low l e v e l s o f shear a r e exp e r i e n c e d by t h e f l u i d ( f o r example a f t e r t h e f l u i d has t r a v e l l e d t h r o u g h t h e w e l l b o r e ) a l l o w s a more h i g h l y s t r u c t u r e d g e l t o o r i g i n a t e t h e r e b y r e s u l t i n g i n h i g h e r v i s c o s i t y . The r a t e a t which t h e g e l i s c h e m i c a l l y formed can be c o n t r o l l e d by t h e a d d i t i o n o f c h e l a t e s which slow t h e r e a c t i o n between t h e polymer mol e c u l e s and t h e t r a n s i t i o n - m e t a l i o n (KRAMER, PRUDHOMME, NORMAN & SANDY 1987). The v i s c o s i t y o f t h e delayed system i s s e v e r a l t i m e s g r e a t e r than t h a t o f t h e f a s t l i n k i n g g e l and m a i n t a i n s a h i g h e r v i s c o s i t y a t e l e v a t e d temperature, t h u s d e m o n s t r a t i n g t h a t h i g h e r g e l v i s c o s i t y can be o b t a i n e d f o r a g i v e n c h e m i s t r y i f g e l f o r m a t i o n occurs a f t e r exposure t o h i g h shear r a t e s .
4.3.4.3.1.4. Continuous vs. batch mixing Delayed c r o s s l i n k e d g e l v i s c o s i t y i s i n f l u e n c e d by shear r a t e and t h e degree o f base g e l h y d r a t i o n (HARRIS, HARMS & NORMAN 1987). Chemical f a c t o r s i n f l u e n c i n g v i s c o s i t y development i n c l u d e base g e l c o n c e n t r a t i o n , c r o s s l i n k i n g agent s a t u r a t i o n , f l u i d pH and i o n i c s t r e n g t h . H y d r a t i o n t i m e and pH r e q u i r e m e n t s may d i f f e r f o r s p e c i f i c p o l y m e r - c r o s s l i n k e r p a i r s . F o r many years, h y d r a u l i c f r a c t u r i n g o p e r a t i o n s have used p r e g e l l e d o r batch-mixed v i s c o s i f i e r s as t h e s t a n d a r d base f l u i d f o r a p p l i c a t i o n o f c r o s s l i n k e d g e l l e d f r a c t u r i n g f l u i d s . Continuous o r o n - t h e - f l y m i x i n g o f f l u i d s , however, i s more economical, b u t i t i s essent i a l t h a t base g e l polymer and a d d i t i v e s a r e d e l i v e r e d t o an aqueous stream so t h a t r a p i d u n i f o r m g e l a t i o n occurs, and g e l a t i o n must be s u f f i c i e n t l y complete p r i o r t o c r o s s l i n k i n g agent r e a c t i o n t o g i v e f u l l v i s c o s i t y development. The key elements i n p r o d u c i n g c o n t i n u o u s l y mixed f l u i d s h a v i n g performance equal t o batch-mixed f l u i d s i n c l u d e e f f i c i e n t g e l d e l i v e r y , p r o p e r m i x i n g e n e r gy l e v e l , pH c o n t r o l and u t i l i z a t i o n o f a delayed onset c r o s s l i n k e r . The most r a p i d base g e l h y d r a t i o n i s a t t a i n e d w i t h h i g h m i x i n g energy, low pH and m i n i mum s a l t c o n c e n t r a t i o n . N o r m a l l y t u b i n g t r a n s i t t i m e i s l o n g enough and format i o n temperature s u f f i c i e n t l y h i g h t o a l l o w polymer h y d r a t i o n and t o i n i t i a t e the c r o s s l i n k i n g reaction, b u t shallow w e l l s w i t h very s h o r t tubing t r a n s i t t i m e and r e l a t i v e l y low f o r m a t i o n temperature may r e q u i r e i n c r e a s e d p r e c i s i o n i n use o f d e l a y e d c r o s s l i n k e d g e l f l u i d s . H i g h e r c r o s s l i n k e r c o n c e n t r a t i o n s a r e r e q u i r e d f o r q u i c k e r a c h i e v i n g h i g h e r c r o s s l i n k e d g e l v i s c o s i t y a t low temperat u r e s , whereas l o w e r c r o s s l i n k e r s a t u r a t i o n s should be used a t e l e v a t e d temperatures. The most obvious a p p l i c a t i o n o f delayed c r o s s l i n k i n g agents a r e j o b s where i t i s d e s i r a b l e t o reduce polymer l o a d i n g and y e t o b t a i n comparable v i s c o s i t i e s t o those achieved w i t h c o n v e n t i o n a l systems a t h i g h e r polymer l o a d i n g s (CONWAY & HARRIS 1982) as w e l l as b e t t e r f l u i d s t a b i l i t y (ECONOMIDES 1986). The p o s s i b i l i t y o f delayed c r o s s l i n k e d f l u i d s t o t r a n s p o r t h i g h e r proppant c o n c e n t r a t i o n s enables performance of t r e a t m e n t s w i t h s m a l l e r f l u i d volumes. Pumping o f h i g h v i s c o s i t y crosslinked f r a c t u r i n g gels generally requires batch-mixing p r i o r t o t h e j o b (FREE, FREDERICK & THOMPSON 1975; c f . a l s o s e c t i o n 4 . 3 . 7 . 5 . 3 . ) .
4.3.4.3.1.5. Delayed cross1 inked f ractur ins fluids vs. dual-viscosity gravel packing fluids Delayed c r o s s l i n k i n g o f f r a c t u r i n g f l u i d s can t o some e x t e n t be compared w i t h dual c a r r i e r v i s c o s i t y m o d e l l i n g o f g r a v e l p a c k i n g f l u i d s ( c f . s e c t i o n 5.3.2.2.), a l t h o u g h t h e v i s c o s i t y change t a k e s p l a c e i n r e v e r s e d i r e c t i o n i n t h e l a t t e r system. W h i l e delayed c r o s s l i n k e d f r a c t u r i n g f l u i d s have l o w e r v i s c o s i t y a t t h e s u r f a c e and on t h e i r way down t h e t u b u l a r s i n o r d e r t o d i m i n i s h
440 f r i c t i o n p r e s s u r e and achieve h i g h e r v i s c o s i t y p r i o r t o e n t r a n c e i n t o t h e f r a c t u r e d r e s e r v o i r i n o r d e r t o be a b l e t o suspend and t r a n s p o r t h i g h p r o p p a n t conc e n t r a t i o n s as w e l l as t o l i m i t f l u i d l o s s , d u a l - v i s c o s i t y g r a v e l pack f l u i d s have h i g h e r v i s c o s i t y a t t h e s u r f a c e f o r a m e l i o r a t i n g m i x i n g and pumping o f t h e s l u r r y and l o w e r v i s c o s i t y downhole i n o r d e r t o f a c i l i t a t e t h e necessary h i g h f l u i d - l o s s r a t e s t o achieve a t i g h t pack.
4.3.4.3.1.6. Mixing of delayed and rapid crosslinking agents S p e c i a l a p p l i c a t i o n s a r e m i x i n g o f delayed and r a p i d c r o s s l i n k i n g agents ( Z I GRYE, OSBORNE & WESTBROOK 1986; HARRIS, HARMS & NORMAN 1987). As delayed c r o s s l i n k i n g agents may n o t produce s u f f i c i e n t c r o s s l i n k e d g e l v i s c o s i t y near t h e p e r f o r a t i o n s t o t r a n s p o r t proppant, adding o f a r a p i d c r o s s l i n k e r ensures t h a t t h e f l u i d i n o r near t h e w e l l b o r e has a l s o c r o s s l i n k e d g e l v i s c o s i t y . A n o t h e r s p e c i a l t y p e o f u t i l i z a t i o n i s i n - s i t u c r o s s l i n k i n g and subsequent g e l a t i o n o f w a t e r - s o l u b l e polymer s o l u t i o n s i n t h e v i c i n i t y o f t h e w e l l b o r e i n o r d e r t o decrease r e l a t i v e p e r m e a b i l i t y t o w a t e r and t o i n h i b i t w a t e r c o n i n g and channell i n g through n a t u r a l f r a c t u r e s a t h i g h - p e r m e a b i l i t y s t r e a k s i n heterogeneous r e s e r v o i r s (MODY, McKITRICK & SHAHSAVARI 1988). Aspects o f delayed c r o s s l i n k e d f l u i d systems a r e a l s o r e p o r t e d by CONWAY & H A R R I S (1382), RUMMO (1982), BARANET & AINLEY (1385), L I & ZHU (1986); CAMERON, GARONER & VEATCH (1988); CONSTIEN, BRANNON & BANNISTER (1988); HOLDITCH, ROBINSON, ELY & R A H I M (1988) and ROSE ( 1 9 8 8 ) . R h e o l o g i c a l f e a t u r e s o f c r o s s l i n k e d f r a c t u r i n g f l u i d s a r e a l s o d i s c u s s e d by PRUDHOMME, ELLIS, CONSTEIN & KNOLL (1988).
4.3.4.3.2. Cross1 inking sign if icance and performance C r o s s l i n k e d f l u i d s develop h i g h v i s c o s i t i e s t h a t e x h i b i t s e v e r a l advantages over n o n - c r o s s l i n k e d systems b e i n g e x t r e m e l y good f l u i d l e a k o f f c o n t r o l , y i e l d o f w i d e r f r a c t u r e s t h a t accept h i g h e r p r o p p a n t c o n c e n t r a t i o n s , p e r f e c t p r o p p a n t t r a n s p o r t p r o p e r t i e s , and b e t t e r temperature s t a b i l i t y (PA1 & GARBIS 1983 b ) . F i e l d e x p e r i e n c e has shown t h a t c o n t r o l l e d e a r l y t i m e c r o s s l i n k e r systems gener a l l y p e r f o r m much b e t t e r t h a n delayed c r o s s l i n k e r systems e s p e c i a l l y when t h e b o t t o m h o l e f l o w i n g temperature o f t h e f r a c t u r i n g f l u i d i s n o t h i g h enough t o i n i t i a t e t h e c r o s s l i n k i n g r e a c t i o n b e f o r e t h e f l u i d reaches t h e f r a c t u r e , which would g i v e r i s e t o p r o p p a n t d r o p - o u t r e s u l t i n g i n p r e s s u r e i n c r e a s e and screeno u t f a i l u r e o f the s t i m u l a t i o n job. T h e r e f o r e e a r l y t i m e v i s c o s i t y and r e t a i n e d v i s c o s i t y a r e e q u a l l y i m p o r t a n t t o i n s u r e success o f a f r a c t u r i n g t r e a t m e n t . Delayed c r o s s l i n k i n g c o n s i d e r a b l y enhances f l u i d v i s c o s i t y and enables t o i n c r e a s e p r o p p a n t c o n c e n t r a t i o n up t o 20 l b s / g a l f l u i d a t t h e s u r f a c e ( t r a n s l a t i n g t o up t o 7 l b s / f t 2 o r 35 kg/m2 i n the fracture; SHAH, SMITH & DONALDSON 1983; c f . s e c t i o n 4 . 3 . 1 . 1 . ) o r even more o v e r l o n g pumping t i m e s (GREGORCZYK, PAULS, HOLTMYER, CHISHOLM & VENOITTO 1984). Aspects o f f r i c t i o n p r e s s u r e l o w e r i n g , screenout r a t e r e d u c t i o n , f r i c t i o n p r e s s u r e v s . f r a c t u r e h e i g h t growth, f l u i d v i s c o s i t y and p r o p p a n t suspens i o n improvement, and f r a c t u r e c l o s u r e and b r e a k e r a c t i o n a r e b r i e f l y d i s c u s s e d as f o l l o w s .
4.3.4.3.2.1.Friction pressure lower ins Employing d e l a y e d c r o s s l i n k e r systems a l s o a v o i d s e x c e s s i v e f r i c t i o n p r e s s u r e i n t h e t u b u l a r goods which enables r e d u c t i o n o f t h e h y d r a u l i c horsepower r e q u i r e m e n t s f o r t h e f r a c t u r i n g t r e a t m e n t and thus p e r m i t s t o l o w e r t h e operat i o n a l c o s t s (SHAH & WATTERS 1386). C o r r e l a t i o n s o f f r i c t i o n p r e s s u r e s o f del a y e d c r o s s 1 i n k e d f r a c t u r i n g f l u i d s f r o m l a b o r a t o r y measurements, y a r d t e s t s o r f i e l d a n n u l a r f l o w s i t u a t i o n s , however, may n o t be a p p l i c a b l e t o f r i c t i o n loss
441 p r e d i c t i o n f o r f l o w down a l o n g v e r t i c a l t u b i n g s t r i n g (LORD & McGOWEN 1986). I n p a r t i c u l a r , c o r r e l a t i o n o f l a b o r a t o r y f r i c t i o n d i m i n u t i o n measurements made w i t h s t i m u l a t i o n g e l s w i t h o r w i t h o u t proppants o v e r p r e d i c t s f r i c t i o n l o s s o f these f l u i d s when t h e y a r e i n j e c t e d down f i e l d t u b i n g s t r i n g s . Delayed c r o s s l i n ked f r a c t u r i n g f l u i d systems g e n e r a l l y e x h i b i t e x c e l l e n t v i s c o u s p r o p e r t i e s i n t h e crack, good temperature s t a b i l i t y , and p r a c t i c a l l y no i n c r e a s e i n t u r b u l e n t f r i c t i o n p r e s s u r e o v e r base g e l (SHAH & WATTERS 1986). The v i s c o s i t y o b t a i n e d f r o m c r o s s l i n k e d f l u i d s i s p r o g r e s s i v e l y reduced by t i m e and i n t e n s i t y o f shear t o w h i c h t h e f l u i d i s exposed, and p r e h e a t i n g o f f l u i d s f o r t h e purpose o f t u b i n g c o n t r a c t i o n m i n i m i z a t i o n i n c r e a s e s c r o s s l i n k i n g r a t e and shear d e g r a d a t i o n o f f l u i d v i s c o s i t y (McMECHAN & CONWAY 1983). Delayed crosslinked f l u i d s w i t h c o n t r o l l a b l e c r o s s l i n k i n g time are n o t subjected t o t h e h i g h shear c o n d i t i o n s f o u n d i n t h e t u b u l a r goods, and t h e r h e o l o g y i s much more i n s e n s i t i v e t o t r e a t i n g f l u i d temperature (CONWAY & H A R R I S 1982). The use o f temperature-delayed c r o s s l i n k e d f l u i d s r e s u l t s i n l o w e r and more c o n s i s t e n t f r i c t i o n pressures w i t h respect t o conventional organometallic c r o s s l i n k e r s and base l i n e a r - g e l l e d f l u i d s , and an e x c e l l e n t f r i c t i o n p r e s s u r e t o l e rance and compensation range even l e a d s t o absence o r n e g l i g i b i l i t y o f f r i c t i o n p r e s s u r e r i s e upon a d d i t i o n o f proppants up t o c o n s i d e r a b l y h i g h c o n c e n t r a t i o n s (WALSER 1985).
4.3.4.3.2.2.
Screenout r a t e reduct i o n
The much improved p r o p p a n t t r a n s p o r t c o n d i t i o n s a r e u n d e r l i n e d by s i g n i f i c a n t l y reduced screenout r a t e s (SHAH & WATTERS 1986). H i g h e r average p r o p p a n t s a t u r a t i o n s a l l o w more proppant t o be p l a c e d downhole w i t h l e s s f l u i d which opens way f o r t o h y d r a u l i c horsepower d i m i n u t i o n , t r e a t m e n t f l u i d r e q u i r e m e n t s r e d u c t i o n , and polymer l o a d i n g l o w e r i n g , w i t h a l l o f these e f f e c t s g i v i n g r i s e t o s i g n i f i c a n t s a v i n g s o f s t i m u l a t i o n expenses. As a consequence o f v e r y e f f e c t i v e p r o p p a n t suspension, p r o p p a n t t r a n s p o r t c h a r a c t e r i s t i c s a r e somewhat i n d e pendent o f depth, b o t t o m h o l e s t a t i c temperature and pumping t i m e . HOLDITCH, ROBINSON & ELY (1988) comment on t h e e f f e c t o f v i s c o u s f l u i d p r o p e r t i e s on excess f r i c t i o n p r e s s u r e s d u r i n g h y d r a u l i c f r a c t u r i n g t r e a t m e n t s . WOODROOF & ANDERSON (1977) d i s c u s s f r i c t i o n r e d u c e r s i n a c i d s t i m u l a t i o n s and t h e p o s s i b i l i t y o f p r o v o k i n g f o r m a t i o n damage. Aspects o f f r i c t i o n l o s s d i m i n u t i o n a r e a l s o d i s c u s sed by OUSTERHOUT & HALL (1961); PRUITT, SIMMONS, NEILL & CRAWFORD (1965) and SUTTON ( 1 9 7 6 ) .
4.3.4.3.2.3.
F r i c t i o n pressure vs . f r a c t u r e height growth
Higher s l u r r y v i s c o s i t i e s r e s u l t i n g from high proppant concentrations w i l l s u b s t a n t i a l l y i n c r e a s e t h e f r i c t i o n p r e s s u r e i n t h e f r a c t u r e t o t h e p o i n t where i t may c o m p l e t e l y dominate t h e excess p r e s s u r e e s p e c i a l l y when narrower c r a c k s a r e b e i n g c r e a t e d (HOLDITCH, ROBINSON, ELY & R A H I M 1988). As a r e s u l t o f h i g h e r f r i c t i o n p r e s s u r e s i n t h e f r a c t u r e , u n c o n t a i n e d h e i g h t growth can o c c u r even when t h e excess p r e s s u r e i s r a p i d l y r i s i n g . I n c r e a s i n g p r e s s u r e and t h u s s t r e s s d i s t r i b u t i o n i n a r e s e r v o i r away f r o m t h e w e l l b o r e due t o p a r t i a l d e p l e t i o n g i ves way t o i n c r e a s i n g s t r e s s a t t h e c r a c k t i p as t h e f r a c t u r e propagates. Oel a y e d c r o s s l i n k e d f l u i d systems e x h i b i t v e r y complex non-Newtonian b e h a v i o u r due t o i n s i g n i f i c a n c e o f shear d e g r a d a t i o n , r e t a i n m e n t o f much h i g h e r v i s c o s i ty, and t r a n s p o r t o f much h i g h e r p r o p p a n t c o n c e n t r a t i o n s than w i t h c o n v e n t i o n a l c r o s s l i n k e d g e l s . The v i s c o s i t i e s o f delayed c r o s s l i n k e d f l u i d systems a r e cons t a n t l y changing w i t h t i m e and temperature w i t h i n t h e f r a c t u r e ( c f . s e c t i o n 4.3.4.3.1.2.) and t h e f r i c t i o n p r o p e r t i e s a r e s u b s t a n t i a l l y h i g h e r due t o great e r proppant s a t u r a t i o n s which i s i n c o n t r a s t t o c o n v e n t i o n a l c r o s s l i n k e d f l u i d s where b y t h e t i m e t h e y r e a c h t h e f r a c t u r e , f l u i d v i s c o s i t y has a l r e a d y been c o n s i d e r a b l y reduced by shear d e g r a d a t i o n and does n o t change much d u r i n g the treatment.
442
As the downhole v i s c o s i t i e s of delayed crosslinked f r a c t u r i n g f l u i d s a r e much l a r g e r t h a n those of conventionally crosslinked ones, they can c a r r y much higher proppant s a t u r a t i o n s which leads t o again increasing v i s c o s i t y , and because proppant concentrations a r e v ar i ab l e within the f r a c t u r e in most cases ( c f . a l s o s e c t i o n 4 . 3 . 6 . ) , s l u r r y v i s c o s i t i e s and f r i c t i o n pressure gra die nts a r e a l s o v a r i a b le in the crack. Another influence on f r i c t i o n p r o p e r t i e s of the f r a c t u r i n g gel i s wall s l i p . I n case of higher proppant s a t u r a t i o n s pumped i n t o narrow c r a c k s, s l u r r y v i s c o s i t y and r e s u l t i n g f r i c t i o n pressure inc re a se considerably and a r e ser i o u s l y a f f e c t i n g the excess pressure. Under such c onditions, f r a c t u r e height i s no longer being contained, b u t height i s growing a t a much f a s t e r r a t e t h a n length towards the end of the treatment, w i t h the inc re a se in height extension being caused by the r a p i d increase in excess pressure which i s dominated by the f r i c t i o n in the system.
4.3.4.3.2.4.
F l u i d v i s c o s i t y and proppant suspension improvement
Since the inception of massive hydraulic f r a c t u r i n g ( c f . se c tion 1 . 1 . 1 . ) where the l a r g e cracks ar e supported by multiple proppant l a y e r s , v i s c o s i t i e s s i g n i f i c a n t l y g r e a t e r t h a n t h a t of water a re required f o r the stim ula tion f l u i d s ( O L S E N & D E B O N I S 1988). As p r o p p a n t s e t t l i n g ve loc ity i s inve rse ly proportional t o c a r r i e r f l u i d v i s c o s i t y , the more viscous the f r a c t u r i n g f l u i d , the f a r t h e r the p r o p p a n t can be transported i n t o the crack without s e t t l i n g o u t of the c a r r i e r ( c f . s ect i o n 4 . 1 2 . 2 . ) . Viscous f l u i d s need more energy f o r moving through a f r a c t u r e than t h i n f l u i d s , and the a dditiona l energy i n form of pressure i s applied a g a i n s t the crack wal l s , thereby causing the f r a c t u r e t o widen which enables placement of more proppants within the crack i n order t o obt a i n the d e s i r e d multiple l a y e r s . While too low f l u i d v i s c o s i t y i s n o t benefic i a l , a l s o too h i g h v i s c o s i t y can be d et rim e nta l. Increased f l u i d v i s c o s i t y tends t o increase the pressure in the f r a c t u r e which i s r e l a t e d t o crack h e i g h t , with increasing f l u i d v i s c o s i t y and f r a c t u r e pressure increasing the danger of uncontrolled height propagation ( c f . se c tion 4 . 2 . 2 . 3 . ) . I f f r a c t u r e height g r e a t l y exceeds prospective inte rva l thic kne ss, the stimul a t i o n treatment l o s es e f f e c t i v e n e s s by wasting f r a c t u r i n g f l u i d a n d proppant upon unproductive rock. P r o p p a n t embedment in gel f i l t e r cakes i s a s i g n i f i c a n t c o n t r i b u t i o n t o proppant conductivity damage, with higher gel loadings causing g r e a t e r c o n d u c t i v i t y d e t e r i o r a t i o n than lower gel s a t u r a t i o n s ( c f . se c tions 1 . 4 . 1 0 . 3 . 5 . a n d 4 . 3 . 4 . 1 . 4 . ) . I t i s t h er ef o re necessary t o e s t a b l i s h a balance between too l i t t l e a n d too much v i s c o s i t y t o optimize proppant placement in the crack, w i t h the design goal being minimum v i s c o s i t y a n d f l u i d - l o s s control f o r accurate proppant i n s t a l l a t i o n . F l u i d s which ar e conventionally cr o s s l i n k ed a t the surfa c e lose a percentage of the i n i t i a l p r o p p a n t t r a n s p o r t capacity when being forced t h r o u g h a lim ite d number of p e r f o r at i o n s a t high i n j e c t i o n r a t e s p a r t i c u l a r l y during limited-ent r y f r a c t u r i n g (EASON 1985; c f . s ect i o n 4 . 2 . 2 . 2 . ) . Delayed c rosslinking taking place i n the formation a f t e r passage through the pe rfora tions keeps f l u i d degrad a t i o n t o a minimum, a n d preserves proppant suspension and move p r o p e r t i e s .
4.3.4.3.2.5.
Fracture
closure
and breaker a c t i o n
I n low-temperature r e s e r v o i r s , the use of a mixture of delayed c r o s s l i n k e r and rapid c r o s s l i n k e r reduces the v i s c o s i t y increase a t e le va te d temperatures obtained w i t h a delayed c r o s s l i n k e r only (ALMOND & GARVIN 1985; c f . se c tion 4 . 3 . 4 . 3 . 1 . 6 . ) . Optimization of the polymer-crosslinker i n t e r a c t i o n can diminuish the amount of base polymer required t o achieve a s p e c i f i c v i s c o s i t y and thus considerably decrease the c o s t of g e l l i n g . Rheological diffe re nc e s between various types of cr o s s l i n k ed f r a c t u r i n g f l u i d s a r e a l s o inve stiga te d by GARDNER & EIKERTS (1982, 1983), C R A I G I E (1983) ; LESCARBOURA, SIFFERMAN & WAHL (1984) ; ROGERS, VEATCH & NOLTE (1984); GUILLOT & D U N A N D (1985), K N O L L (1985) and SHAH &
443
WATTERS ( 1 9 8 6 ) . F i e l d e x p e r i e n c e w i t h delayed c r o s s l i n k e d g e l s i s r e p o r t e d by PEARSON, LYNCH, SCHMIDT & McCASLIN ( 1 9 8 8 ) . D e s p i t e c o n s i d e r a b l e p r o g r e s s made i n r e c e n t years, r h e o l o g i c a l c h a r a c t e r i z a t i o n o f c r o s s l i n k e d f r a c t u r i n g f l u i d s remains a d i f f i c u l t and e l u s i v e c h a l l e n g e (VEATCH & M O S C H O V I D I S 1986). I n case o f l a r g e t r e a t m e n t s and e f f i c i e n t f l u i d s , f r a c t u r e c l o s u r e would n o t occur f o r s e v e r a l hours which would a l l o w ample t i m e f o r complete p r o p p a n t s e t t l i n g i n water-based low- t o m o d e r a t e - v i s c o s i t y g e l s (HANNAH, HARRINGTON & POTTER 1980). C r o s s l i n k e d f r a c t u r i n g g e l s , however, can r e t a i n t h e a b i l i t y t o supp o r t proppants f o r s e v e r a l hours a t e l e v a t e d temperatures u n l e s s t h e i r h i g h e r v i s c o s i t y i s degraded t o some c r i t i c a l p o i n t due t o t h e a c t i o n o f b r e a k e r s o r a b s o l u t e thermal d e g r a d a t i o n . O x i d i z i n g b r e a k i n g agents a f f e c t b o t h p r o p p a n t s u p p o r t c h a r a c t e r i s t i c s and f l u i d loss p r o p e r t i e s o f c r o s s l i n k e d g e l s . I f break e r a c t i o n s i g n i f i c a n t l y reduces t h e t i m e d u r i n g which p e r f e c t proppant s u p p o r t i s p r e s e n t and f r a c t u r e c l o s u r e i s r e t a r d e d , then i m p e r f e c t p r o p p a n t d i s t r i b u t i o n w i l l r e s u l t and t h e o v e r a l l performance may be no b e t t e r t h a n i f a s i m p l e g e l were used ( c f . s e c t i o n 4.12.1.1.). On t h e o t h e r hand, i f b r e a k e r a c t i o n s i g n i f i c a n t l y i n c r e a s e s f l u i d l o s s dur i n g c l o s u r e , then f r a c t u r e c l o s u r e t i m e may be reduced and may a s s i s t i n t r a p p i n g proppants between t h e c r a c k w a l l s i n a f a v o u r a b l e d i s t r i b u t i o n . I n some cases, t h e a d d i t i o n o f b r e a k e r s d e s t r o y s w a l l l e a k o f f c o n t r o l and t r a n s f o r m s i t i n t o v i s c o s i t y c o n t r o l . Base g e l v i s c o s i t y d i c t a t e s performance o f c r o s s l i n k e d and p a r t i a l l y broken g e l s p e r f o r m s l i g h t l y p o o r e r t h a n would be expecfluids, t e d due t o t h e i r base g e l v i s c o s i t y (CONWAY & HARRIS 1982).
4.3.4.3.3.
Improvement o f proppant placement
FREEMAN, EILDEN & H O S S A I N I (1986) document t h e s u p e r i o r i t y o f delayed foamed c r o s s l i n k e d g e l s which a l l o w t h e placement o f much h i g h e r p r o p p a n t c o n c e n t r a t i o n s i n t h e f r a c t u r e t h a n w i l l be p o s s i b l e t o i n s e r t w i t h foamed l i n e a r g e l s as a consequence of extended fluid-loss properties. The enhanced c h a r a c t e r i s t i c s o f foamed delayed c r o s s l i n k e d g e l s even a l l o w r e d u c t i o n i n foam q u a l i t y i n c o n s i d e r a b l e magnitude w i t h o u t d e t r i m e n t a l e f f e c t s on p r o p p a n t t r a n s p o r t c a p a b i l i t y . The d i s c u s s i o n as f o l l o w s i n c l u d e s some aspects o f p r o p p a n t t r a n s p o r t c a p a c i t y and f l u i d - l o s s a d d i t i v e s as w e l l as f r a c t u r e p e n e t r a t i o n vs. f l u i d c r o s s l i n k i n g .
4.3.4.3.3.1.
Proppant transport capacity and f l u i d - loss a d d i t i v e
Comparative assessment has shown t h a t s o l i d f l u i d - l o s s m a t e r i a l s which have t o be i n c o r p o r a t e d w i t h foamed l i n e a r g e l s and r e s u l t i n p r o d u c t i v i t y i m p a i r ment due t o t h e damage caused by these a d d i t i v e s a r e n o t necessary t o be app l i e d w i t h delayed c r o s s l i n k e d foams because o f t h e h i g h e r v i s c o s i t y o f t h e l a t t e r (FREEMAN, BILDEN & H O S S A I N I 1986). The g r e a t e r v i s c o s i t y o f t h e foamed c r o s s l i n k e d g e l a l s o reduces p r o p p a n t f a l l r a t e up t o almost z e r o p r o p p a n t s e t t l i n g i n i d e a l cases, t h e r e b y n o t o n l y i m p r o v i n g proppant s a t u r a t i o n which can be s u c c e s s f u l l y placed, b u t a l s o p e r m i t t i n g a delayed s t a r t o f l o a d r e c o v e r y a f t e r t h e t r e a t m e n t t h a t c o n s i d e r a b l y decreases t h e r i s k o f proppant flowback by a l l o w i n g s u f f i c i e n t f r a c t u r e c l o s u r e r e s t r a i n i n g t h e p r o p p a n t package ( c f . s e c t i o n s 4.12.2.3. and 4 . 1 2 . 3 . 1 . 2 . ) . R h e o l o g i c a l aspects o f foamed c r o s s l i n k e d f r a c t u r i n g f l u i d s a r e a l s o d i s c u s s e d by REIDENBACH, HARRIS, LEE & LORD (1983); WATKINS, WENDORFF & AINLEY (1983); H A R R I S & REIDENBACH (1984); ROYCE, BECK & RICKARDS (1984) and CRAIGHEAD, H O S S A I N I & FREEMAN ( 1 9 8 5 ) . C r o s s l i n k e d f r a c t u r i n g f l u i d s i n c r e a s e f r i c t i o n drop t h r o u g h t u b i n g w h i c h i s f u r t h e r i n c r e a s e d b y t h e presence o f proppants due t o a b r a s i o n (PAI, GARBIS & HALL 1983). Employment o f delayed c r o s s l i n k e r s which p r e v e n t c r o s s l i n k i n g o f f l u i d u n t i l i t reaches t u b i n g b o t t o m lowers t h e t r e a t i n g p r e s s u r e and enables t o save c o n s i d e r a b l e expenses o f horsepower, as w e l l as t o make t h e e n t i r e ope-
444 r a t i o n s a f e r and more e f f i c i e n t by n o t o v e r l o a d i n g t h e equipment ( c f . s e c t i o n 4.3.4.3.1.). While c r o s s l i n k i n g o f f r a c t u r i n g f l u i d s g e n e r a l l y aids i n l e a k o f f c o n t r o l and combats p o t e n t i a l s c r e e n o u t f a i l u r e , proppant d i s t r i b u t i o n i n a n o n - c r o s s l i n k e d g e l l e d f l u i d i s more e f f e c t i v e when i n c r e a s e d r a t e s a r e used t o promote p r o p p a n t suspension because o f i n c r e a s e d f l u i d v e l o c i t y .
4.3.4.3.3.2. Fracture penetration vs. fluid crosslinking F i e l d e x p e r i e n c e has shown t h a t c r o s s l i n k e d g e l l e d 2 % KC1 w a t e r i s more e v e n l y d i s t r i b u t i n g t h e proppants t h r o u g h o u t t h e f r a c t u r e , whereas t h e nonc r o s s l i n k e d g e l l e d 2 % KC1 w a t e r l e a v e s a l a r g e r amount o f p r o p p a n t s n e a r t h e w e l l b o r e , w i t h t h i s e f f e c t b e i n g t h e consequence o f t h e b e t t e r t r a n s p o r t i n g char a c t e r i s t i c s o f t h e c r o s s l i n k e d system. N o n - c r o s s l i n k e d g e l l e d f l u i d s p r o v i d e a deeper p e n e t r a t i o n o f t h e propped f r a c t u r e due t o narrower c r a c k w i d t h w i t h r e s p e c t t o c r o s s l i n k e d f l u i d s , b u t on t h e o t h e r hand, t h e more u n i f o r m l y packed f r a c t u r e c r e a t e d by c r o s s l i n k e d f l u i d s i s c o n s i d e r e d t o be more i m p o r t a n t i n s u s t a i n i n g p r o d u c t i o n r a t e s o v e r t h e l i f e o f these w e l l s than t h e l e s s u n i f o r m deeper p e n e t r a t i n g f r a c t u r e s generated by n o n - c r o s s l i n k e d systems. Delayed c r o s s l i n k e d g e l s a l l o w h i g h e r pumping r a t e s as w e l l as t r a n s p o r t and placement o f h i g h e r p r o p p a n t s a t u r a t i o n s w i t h c o n s i d e r a b l y l o w e r r a t e s o f s c r e e n o u t f a i l u r e (KUUSKRAA & HAAS 1988 r e p o r t examples o f s u c c e s s f u l pumping o f sand c o n c e n t r a t i o n s up t o 12 l b s / g a l i n t o r e s e r v o i r s where s a t u r a t i o n s up t o 3 l b s / g a l i n conventional f l u i d s l e d formerly t o screenout t e r m i n a t i o n ) . Other measures t o a v o i d screenout f i n i s h i n g a r e d e s i g n i n g o f optimum l a r g e pad v o l u mes ( c f . s e c t i o n s 4 . 3 . 4 . 6 . 1 . 4 . ) .
4.3.4.3.4,Enhanced fluid stability and transport capacity The main advantages o f h i g h - v i s c o s i t y c r o s s l i n k e d f r a c t u r i n g f l u i d s as compared t o t h e c h a r a c t e r i s t i c s o f n o n - c r o s s l i n k e d f l u i d s a r e e x t r e m e l y good l e a k o f f c o n t r o l , y i e l d o f w i d e r f r a c t u r e s which accept h i g h e r p r o p p a n t c o n c e n t r a t i o n s , p e r f e c t proppant t r a n s p o r t p r o p e r t i e s , and enhanced temperature s t a b i l i t y (PA1 & G A R B I S 1983). A p a r t f r o m n e u t r a l f l u i d s , a l s o a c i d s can be g e l l e d and c r o s s l i n k e d ( c f . a l s o s e c t i o n 4 . 5 . 1 . 2 . ) which c o n s i d e r a b l y broadens t h e a p p l i c a t i o n spectrum o f these a g g r e s s i v e s t i m u l a t i o n f l u i d s . The o u t l i n e as f o l l o w s p r e s e n t s some aspects o f a c i d g e l l i n g and c r o s s l i n k i n g , r h e o l o g i c a l d i f f e r e n c e s o f f r a c t u r i n g f l u i d stages, and impact o f shear d u r i n g m i x i n g and pumping.
4.3.4.3.4.1. Acid gelling and crosslinking A p a r t f r o m more o r l e s s n e u t r a l w a t e r - o r polymer-based f l u i d s , a l s o a c i d can be g e l l e d and c r o s s l i n k e d i n b o t h c o n v e n t i o n a l and delayed f o r m (PABLEY & HOLCOMB 1980, 1981, 1982; CHURCH, QUISENBERRY & FOX 1981; PABLEY, EWING & CALLAWAY 1982; McLANE & SCOTT 1983; SMITH, DAWSON & SCOGGINS 1983; JOHNSON, FOX, BURNS & O'MARA 1988; FOX, O'MARA, BURNS & JOHNSON 1989). Some aspects o f v i s c o s i t y and temperature s t a b i l i t y as w e l l as l o w - c o n c e n t r a t i o n a c i d as f r a c t u r i n g base f l u i d a r e d i s c u s s e d as f o l l o w s .
4.3.4.3.4.1.1.Viscosity and temperature stability As c r o s s l i n k e d a c i d systems a r e c h a r a c t e r i z e d by h i g h s t r e n g t h , e l e v a t e d temp e r a t u r e s t a b i l i t y and c o n s i d e r a b l e p r o p p a n t t r a n s p o r t c a p a c i t y , t h e y a r e an optimum t o o l f o r c o m b i n a t i o n o f a c i d and proppant f r a c t u r i n g ( c f . s e c t i o n s 4.5.4.2. and 4 . 8 . 1 0 . ) and t h e r e f o r e compete w i t h polymer- o r water-based more o r l e s s n e u t r a l c r o s s l i n k e d f l u i d s , foam and e n e r g i z e d f l u i d s , and e m u l s i f i e d a c i d s . Delayed c r o s s l i n k e d a c i d systems have v e r y h i g h r e d u c t i o n i n t u b u l a r f r i c t i o n p r e s s u r e and l e a v e no i n s o l u b l e r e s i d u e , because once t h e f l u i d i s i n
445 t h e f r a c t u r e and t h e a c i d i s spent, t h e polymer i n v e r t s f r o m m i l d l y c a t i o n i c t o a n i o n i c thus becoming a b r i n e - s o l u b l e polymer which does n o t p r e c i p i t a t e even when becoming c o m p l e t e l y n e u t r a l i z e d i n t h e presence o f c a l c i u m i o n s . The low pH o f a c i d g e l systems m i n i m i z e s c l a y damage o f t h e f o r m a t i o n and thus also permits a p p l i c a t i o n o f crosslinked a c i d f l u i d s t o water-sensitive sandstone r e s e r v o i r s . V i s c o s i t y s t a b i l i t y reaches i n a c i d s t r e n g t h s up t o 28 % h y d r o c h l o r i c a c i d c o n c e n t r a t i o n and up t o 220 OF temperature. F l u i d l o s s cont r o l matching o r s u r p a s s i n g t h a t achieved by c r o s s l i n k e d water-based f l u i d s ensures deep p e n e t r a t i o n o f l i v e a c i d , and an e x t r e m e l y r e t a r d e d r e a c t i o n r a t e a l lows i n many cases t h e c r o s s l i n k e d a c i d t o be p l a c e d i n t o t h e f o r m a t i o n w i t h l i t t l e o r no decrease i n a c i d s t r e n g t h ( c f . a l s o s e c t i o n s 3.11.3. and 4.5.1.2.). F i e l d a p p l i c a t i o n o f h i g h - s t r e n g t h c r o s s l i n k e d a c i d g e l systems i s d i s c u s s e d by COULTER, H A R R I S & KLEBENOW (1980); PABLEY, EWING & CALLAWAY (1982); PABLEY & HOLCOMB (1982), McLANE & SCOTT (1983) and PA1 & G A R B I S (1983 b ) . MOLON & FOX (1983) and BAILEY & WICKHAM (1984) o u t l i n e enhancement o f f r a c t u r e p e n e t r a t i o n and p r o d u c t i v i t y by a c i d g e l l i n g . Temperature s t a b i l i t y o f a c i d g e l l i n g p o l y mers i s i n v e s t i g a t e d by NORMAN, CONWAY & WILSON (1981) and FOX, O'MARA, BURNS & JOHNSON ( 1 9 8 9 ) . I n a comparable way as f o r f r a c t u r i n g f l u i d s , extended thermal s t a b i l i t y o f a c i d can be achieved f o r a c i d i z i n g u t i l i z a t i o n , w i t h b u f f e r regul a t i o n o f h y d r o f l u o r i c a c i d p e r m i t t i n g sandstone a c i d i z i n g up t o 550 OF (280 oC) r e s e r v o i r temperature (SCHEUERMAN 1988; c f . s e c t i o n s 4 . 4 . 5 . 1 . and 4 . 5 . 5 . ) .
4.3.4.3.4.1.2. Low-concentration acid as fracturing base fluid COULTER, H A R R I S & KLEBENOW (1980) p r e s e n t t h e advantages o f u s i n g low-concent r a t i o n h y d r o c h l o r i c a c i d as a base f l u i d f o r h y d r a u l i c f r a c t u r i n g o f some sandstone f o r m a t i o n s . A g e l system w i t h l o w - r e s i d u e c r o s s l i n k e d polymer w i t h 3 - 5 % h y d r o c h l o r i c a c i d has s u p e r i o r f l u i d - r o c k c o m p a t i b i l i t y c h a r a c t e r i s t i c s and e l i m i n a t e s t h e e a r l i e r problems o f r a p i d d e g r a d a t i o n o f most g e l l i n g agents w i t h h y d r o c h l o r i c a c i d . A c i d g e l l i n g reduces a c i d r e d u c t i o n r a t e and t h e r e f o r e i n c r e a s e s a c i d p e n e t r a t i o n d i s t a n c e (BAILEY & WICKHAM 1984). O t h e r methods o f d e c r e a s i n g a c i d r e a c t i o n r a t e and t h u s enhancing a c i d p e n e t r a t i o n d i s t a n c e a r e a d d i t i o n o f chemical r e t a r d i n g agents and a c i d e m u l s i f i c a t i o n i n o i l , as w e l l as a p p l i c a t i o n o f g e l l e d w a t e r p r e f l u s h e s and u t i l i z a t i o n o f a l t e r n a t i n g stages of crosslinked polymer f l u i d s and a c i d ( c f . s e c t i o n s 4 . 5 . 1 . 3 . 2 . 2 . and 4 . 5 . 5 . 3 . ) . Aspects o f r e t a r d a t i o n o f r e a c t i o n r a t e by a c i d g e l l i n g a r e a l s o d i s cussed by NIERODE & KRUK (1973); CROWE, MARTIN & MICHAELIS (1981) and JOHNSON, FOX, BURNS & O'MARA ( 1 9 8 8 ) .
4.3.4.3.4.2. Rheological differences o f fracturing fluid stages H y d r a u l i c f r a c t u r i n g t r e a t m e n t s u s u a l l y c o n s i s t o f successive pumping o f f o u r f l u i d stages w i t h d i f f e r e n t r h e o l o g i c a l p r o p e r t i e s c o m p r i s i n g pre-pad, pad, p r o p p a n t - l a d e n c a r r i e r and displacement (LEE & OANESHY 1985). R h e o l o g i c a l p r o p e r t i e s and p r e d o m i n a n t l y v i s c o s i t y o f each f l u i d stage can change s i g n i f i c a n t l y due t o temperature changes i n t h e f r a c t u r e and t h e t i m e d u r a t i o n o f expos u r e t o each temperature. Shearing i n a h i g h - s t r e s s environment can d e s t r o y a g e l a f t e r c r o s s l i n k i n g (GARDNER & EIKERTS 1982, 1983; SHAH & WATTERS 1984; ROYCE, BECK & RICKARDS 1984), whereas h i g h l e v e l s of shear p r i o r t o c r o s s l i n k i n g have l i t t l e e f f e c t on o v e r a l l performance. Overshearing l e a d s t o b r e a k i n g o f t h e polymer c h a i n t h e r e b y decreasing t h e average m o l e c u l a r w e i g h t which u l t i m a t e l y means l o s s o f polymer f u n c t i o n a l i t y (HASHEMI, CAOTHIEN & R O V I G 1 9 8 8 ) .
446
4.3.4.3.4.3. Impact o f shear during mixing and pumping The p r o p e r t i e s o f c r o s s l i n k e d p o l y m e r f l u i d s i n h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s depend o n i n t e n s i t y and d u r a t i o n o f s h e a r t o w h i c h t h e f l u i d was s u b j e c t e d w h i l e m i x e d and pumped f r o m t h e s u r f a c e i n t o t h e f r a c t u r e (SHAH & WATTERS 1986), and c o n s e q u e n t l y f r a c t u r e g e o m e t r y and p r o p p a n t d i s t r i b u t i o n a r e a l s o a f f e c t e d . I n c r e a s e d p r e s s u r e d r o p i n t u r b u l e n t f l o w i s an i n d i c a t i o n o f c r o s s l i n k i n g r e a c t i o n o c c u r r i n g a t a f a s t e r r a t e t h a n s h e a r d e g r a d a t i o n . Time- and shear-dependent behaviour o f c r o s s l i n k e d polymer f l u i d s i s a l s o i n v e s t i g a t e d b y ROGER, VEATCH & NOLTE ( 1 9 8 1 ) and C R A I G I E ( 1 9 8 3 ) . W i t h r e s p e c t t o p o l y m e r emuls i o n f l u i d s , c r o s s l i n k e d g e l l e d w a t e r - b a s e d f l u i d s cause l e s s f o r m a t i o n damage, c a n c a r r y h i g h e r p r o p p a n t c o n c e n t r a t i o n s , a r e l e s s f l a m m a b l e and l e s s expens i v e , and have a much f a s t e r l o a d r e c o v e r y t i m e (ROBERTS 1981; c f . s e c t i o n 4 . 3 . 4 . 2 . ) . T e m p e r a t u r e i s g e n e r a l l y a c t i v a t i n g t h e c r o s s l i n k i n g mechanism (GARDNER & EIKERTS 1983; c f . s e c t i o n 4 . 3 . 4 . 3 . 1 . ) . Viscosity loss o f the c a r r i e r s t a g e due t o t i m e and t e m p e r a t u r e c a n g i v e r i s e t o i n a b i l i t y o f f u r t h e r suspendi n g t h e proppants, w i t h s t a b i l i z a t i o n o f t h i s f l u i d stage being a b l e by replacement b y a t w o - s t a g e g e l l e d f l u i d where t h e second g e l l i n g a g e n t i s a c t i v a t e d a t higher temperature ( c f . a l s o s e c t i o n 4.3.4.4.3.). As c r o s s l i n k e d f r a c t u r i n g f l u i d s a r e s h e a r - h i s t o r y d e p e n d e n t ( c f . s e c t i o n 4.3.4.2.1.), t h e y do n o t p o s s e s s u n i q u e r h e o l o g i c a l c h a r a c t e r i s t i c s (CONWAY & H A R R I S 1 9 8 2 ) . The r h e o l o g i c a l p r o p e r t i e s a r e a d d i t i o n a l l y i n f l u e n c e d b y t h e var i a b l e c o n d i t i o n s e n c o u n t e r e d d u r i n g t h e t r e a t m e n t such as t u b i n g s i z e , pumping r a t e , s u r f a c e f l u i d t e m p e r a t u r e and p i p e r e s i d e n c e t i m e .
4.3.4.4. Fluid heat capacity and reservoir temperature Heat t r a n s f e r d u r i n g f r a c t u r e p r o p a g a t i o n takes p l a c e by c o n v e c t i o n along t h e f r a c t u r e combined w i t h c o n d u c t i o n and c o n v e c t i o n i n t h e r e s e r v o i r (BEN NACEUR & STEPHENSON 1 9 8 5 ) . H e a t t r a n s f e r mechanisms p l a y a c r u c i a l r o l e i n deep w e l l f r a c t u r i n g e s p e c i a l l y when f l u i d s w i t h h i g h t e m p e r a t u r e dependency l i k e c r o s s l i n k e d p o l y m e r s o r foams a r e u s e d . I n a d d i t i o n t o c o n d u c t i o n and c o n v e c t i o n , a l s o d i f f u s i o n b y t h e r m a l exchanges and m e c h a n i c a l l e a k o f f t a k e p l a c e i n r e s e r v o i r r o c k s a d j a c e n t t o t h e f r a c t u r e . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l a s p e c t s , an o v e r v i e w i s g i v e n o n h e a t b l o c k a g e and f o r m a t i o n c o o l i n g , p r i m a r y and s e c o n d a r y g e l l a n t s , and f r a c t u r i n g f l u i d v i s c o s i t y improvement.
4.3.4.4.1. General aspects The i n f l u e n c e o f h e a t t r a n s f e r i s v e r y s i g n i f i c a n t f o r p r o p p a n t t r a n s p o r t p r o p e r t i e s due t o v i s c o s i t y v a r i a t i o n s , as w e l l as f o r f r a c t u r i n g p r e s s u r e anal y s i s . A t low f l u i d e f f i c i e n c y , f l u i d temperature i n t h e crack remains c l o s e t o i n j e c t i o n t e m p e r a t u r e , whereas w i t h i n c r e a s i n g e f f i c i e n c y , t h e temperature a l o n g t h e f r a c t u r e t e n d s e x p o n e n t i a l l y t o t h e r e s e r v o i r t e m p e r a t u r e (VEATCH 1 9 8 3 ) . A s p e c t s o f h e a t t r a n s f e r i n h y d r a u l i c f r a c t u r i n g and f l u i d t e m p e r a t u r e p r o f i l e s i n t h e c r a c k d u r i n g t h e t r e a t m e n t a r e a l s o d i s c u s s e d b y WHEELER ( 1 9 6 9 ) , WHITSITT & DYSART ( 1 9 7 0 ) , SINCLAIR ( 1 9 7 1 ) ; CLARK, CLOUD, WAREMBOURG & YUNKO ( 1 9 7 8 ) ; HARRINGTON, HANNAH & BEIRUTE ( 1 9 7 8 ) ; CROCKETT, V O & CLEARY ( 1 9 8 4 ) and POULSEN & LEE (1984; c f . a l s o s e c t i o n 4 . 4 . 5 . 1 . ) . V a r i a t i o n a l t e m p e r a t u r e a n a l y s i s i n h y d r a u l i c f r a c t u r i n g i s c a r r i e d o u t b y BIOT, MASSE & MEDLIN ( 1 9 8 4 ) . MOODY ( 1 9 8 8 ) comments on t h e r m a l d i s p l a c e m e n t i n f r a c t u r e d f o r m a t i o n s . The o u t l i n e as f o l l o w s f o c u s s e s on t e m p e r a t u r e s e n s i t i v i t y o f f r a c t u r i n g f l u i d v i s c o s i t y as w e l l as t e m p e r a t u r e d i f f e r e n c e between f o r m a t i o n and s t i m u l a t i o n f l u i d s .
447
4.3.4.4.1.1. Temperature sensitivity
of fracturing fluid viscosity
R e s e r v o i r temperature and h e a t t r a n s f e r e f f e c t s have an i m p o r t a n t impact on f r a c t u r i n g f l u i d v i s c o s i t y and t h u s a l s o p r o p p a n t t r a n s p o r t and suspension capab i l i t y (SINCLAIR 1971), p a r t i c u l a r l y i n w e l l s w i t h bottomhole temperatures beyond 200 OF. F r a c t u r i n g f l u i d v i s c o s i t y i s h i g h l y t e m p e r a t u r e - s e n s i t i v e . Gel deg r a d a t i o n a t h i g h temperature t a k e s p r e d o m i n a n t l y p l a c e by h y d r o l y s i s and o x i d a t i o n (CONWAY & HARRIS 1982). When t h e pH i s h e l d i n t h e optimum range, t h e r a t e o f polymer d e g r a d a t i o n can be slowed down by a d d i t i o n of temperature s t a b i l i zers, b u t l i t t l e e f f e c t on i n i t i a l apparent v i s c o s i t y a t temperature i s a c h i e ved, and t h e r a t e o f v i s c o s i t y l o s s f o r a c r o s s l i n k e d f l u i d w i l l be no l e s s than t h a t o f an e q u a l l y s t a b i l i z e d base g e l . F l u i d shear h i s t o r y i s t h e m a j o r f a c t o r d i c t a t i n g v i s c o s i t y enhancement a t h i g h temperatures. C r o s s l i n k i n g t i m e o f h i g h - t e m p e r a t u r e c r o s s l i n k i n g agents c o n t r o l s t h e a b i l i t y o f t h e l a t t e r t o p r o v i d e v i s c o s i t y enhancement i n t h e f r a c t u r e .
4.3.4.4.1.2. Temperature difference between
format ion and stimulation fluids
The temperature d i f f e r e n c e between t h e c o o l i n j e c t e d s t i m u l a t i o n f l u i d and the hot r e s e r v o i r rock a t the f r a c t u r e w a l l s ( c f . also section 6.2.1.1.) causes h e a t f l o w f r o m t h e pay zone t o t h e c r a c k . Both c o n d u c t i o n and c o n v e c t i o n o f h e a t t a k e s p l a c e i n v e r t i c a l f r a c t u r e s (DYSART & WHITSITT 1967, W H I T S I T T & DYSART 1970, PEACEMAN 1976). Heat blockage i s c r e a t e d by c o o l t r a n s p i r i n g c a r r i e r f l u i d s l e a k i n g o u t o f t h e c r a c k i n t o t h e f o r m a t i o n , because a l e a k i n g - o f f f l u i d moving i n a d i r e c t i o n o p p o s i t e t o t h a t o f t h e c o n d u c t i v e h e a t t r a n s f e r tends t o i n s u l a t e o r b l o c k t h e f l o w o f h e a t t o t h e f l u i d r e m a i n i n g i n t h e f r a c t u r e . Heat f l u x i n c l u d e s t h e p r i m a r y e f f e c t s o f c o n v e c t i o n w i t h i n t h e crack, h e a t conduct i o n w i t h i n t h e f l u i d , and h e a t exchange between f l u i d and r e s e r v o i r , and t h e secondary e f f e c t s o f h e a t c o n d u c t i o n and h e a t g e n e r a t i o n by energy d i s s i p a t i o n i n t h e f l u i d (CLIFTON & WANG 1988).
4.3.4.4.2. Heat blockage and formation cooling Conventional water-based g e l s which a r e v e r y e f f e c t i v e c o o l a n t s a r e t y p i f i e d by low apparent v i s c o s i t i e s and h i g h l e a k o f f r a t e s , w i t h t h u s f l u i d e f f i c i e n c y b e i n g low, g e n e r a t e d f r a c t u r e s b e i n g narrow and h e a t blockage b e i n g v e r y import a n t , because w a t e r has a much h i g h e r h e a t c a p a c i t y than o i l . Some aspects o f f l u i d r h e o l o g y and proppant g r a i n s i z e , enlargement o f pre-pad and pad s i z e , and changing temperature and f l u i d v i s c o s i t y a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.3.4.4.2.1. Fluid rheology and proppant grain size Since h i g h temperatures have d e t r i m e n t a l e f f e c t s on polymers, a d d i t i v e s and a c i d s , p r e c o o l i n g o f h o t f o r m a t i o n s w i t h w a t e r o r b r i n e ahead o f f r a c t u r i n g treatments are considerably improving the effectiveness o f the j o b ( c f . section 4.3.4.4.2.2.). Viscous water-based g e l s a r e non-Newtonian f l u i d s t h a t a r e occas i o n a l l y even v i s c o e l a s t i c a l (PAI, G A R B I S & HALL 1983; BEN NACEUR & STEPHENSON 1985). Viscous water-based g e l s e x h i b i t h i g h apparent v i s c o s i t y a t low o r moder a t e shear r a t e s , w i t h these r h e o l o g i c a l p r o p e r t i e s p r o v i d i n g low t u b u l a r f r i c t i o n l o s s , h i g h f r a c t u r e f r i c t i o n d i m i n u t i o n , low f l u i d l e a k o f f and good p r o p p a n t - c a r r y i n g a b i l i t y , w i t h v i s c o u s water-based g e l s t h u s g e n e r a t i n g wide f r a c t u r e s and a l l o w i n g u t i l i z a t i o n o f coarse proppants f o r p l u g g i n g o f t h e cracks, and a t t h e same t i m e h a v i n g t h e h i g h h e a t c a p a c i t y o f w a t e r . Viscous w a t e r - i n o i l d i s p e r s i o n s w i t h v e r y v i s c o u s o i l s p e r m i t t h e r o u t i n e use o f coarse 8/12 mesh proppants i n o i l - w e l l s t i m u l a t i o n and h e l p t o c r e a t e wide f r a c t u r e s (MATTHEWS 1969; c f . s e c t i o n 4 . 3 . 5 . ) . Heat blockage by s u i t a b l e s t i m u l a t i o n f l u i d s i s
448 an e f f e c t i v e means t o improve c r a c k performance, because when h e a t blockage occ u r s , f r a c t u r e s a r e w i d e r i n t h e e a r l y stages o f t h e t r e a t m e n t and have g r e a t e r volumes, and t h e need f o r f l u i d - l o s s a d d i t i v e s t h a t c o u l d damage r e s e r v o i r f o r m a t i o n and p r o p p a n t package i s reduced o r even c o m p l e t e l y e l i m i n a t e d . Wider f r a c t u r e s which a r e f a v o u r e d by e f f e c t i v e h e a t blockage a l l o w t h e use o f c o a r s e r proppants and p e r m i t t o s t a r t p r o p p a n t i n j e c t i o n much e a r l i e r t h a n usual d u r i n g course o f t h e s t i m u l a t i o n t r e a t m e n t , l e a d i n g t o a b e t t e r p r o p p a n t f i l l - u p o f t h e f r a c t u r e and e n a b l i n g a b e t t e r p r o p p a n t schedule f o r p l u g g i n g o f t h e c r a c k . S u b s t a n t i a l a m e l i o r a t i o n s o f f r a c t u r e geometry and d e s i g n a r e t h e consequence o f t h e importance o f h e a t blockage, g i v i n g r i s e t o i n c r e a s i n g p r o p p a n t volumes, e a r l i e r b e g i n n i n g o f p r o p p a n t i n s e r t i o n i n t h e j o b program, reduct i o n o r t o t a l e l i m i n a t i o n o f f l u i d - l o s s a d d i t i v e s , and e x t e n s i o n o f t h e upper temperature l i m i t o f t h e f r a c t u r i n g f l u i d . The o v e r a l l s i g n i f i c a n c e o f e f f e c t i v e h e a t blockage o f t h e c a r r i e r i s t h e c r e a t i o n o f w i d e r f r a c t u r e s and t h e p u s s i b i l i t y o f p l u g g i n g o f t h e c r a c k s w i t h h i g h e r p r o p p a n t c o n c e n t r a t i o n s and c o a r s e r p r o p p a n t g r a i n s i z e s , t h e r e b y m a x i m i z i n g f r a c t u r e c o n d u c t i v i t y ( c f . sect i o n 4 . 3 . 5 . ) . Aspects o f impact o f f o r m a t i o n temperature on h y d r a u l i c f r a c t u r i n g d e s i g n and h e a t t r a n s f e r i n c r a c k s a r e a l s o d i s c u s s e d by HILL & WAHL (1968) and WHEELER (1969; c f . s e c t i o n 4 . 4 . 5 . 1 . ) .
4.3.4.4.2.2. Enlargement of pre-pad and pad size F o r m a t i o n c o o l i n g by a pre-pad o f a b t . 100,000 g a l o f n o n - c r o s s l i n k e d p o l y mer f l u i d has t h e purpose o f slowing-down v i s c o s i t y l o s s i n t h e f r a c t u r i n g f l u i d and has t h e a d d i t i o n a l e f f e c t o f e s t a b l i s h i n g a f i l t e r cake on t h e c r a c k w a l l i n o r d e r t o m i n i m i z e f r a c t u r i n g f l u i d l e a k o f f which m i g h t r e s u l t i n premat u r e screenout t e r m i n a t i o n (PA1 1982; P A I , G A R B I S & HALL 1983). D u r i n g pre-pad pumping, r a t e s s h o u l d be maximized i n o r d e r t o c r e a t e a l r e a d y t h e t o t a l f r a c t u r e h e i g h t w i t h t h e pre-pad and pad f l u i d s i n s t e a d o f t h e expensive polymer c a r r i e r f l u i d . F o r m a t i o n c o o l i n g by pumping o f l a r g e f l u i d volumes i s p a r t i c u l a r l y s i g n i f i c a n t i n h i g h - t e m p e r a t u r e r e s e r v o i r s t o guarantee extended s t a b i l i t y o f c r o s s l i n k e d f r a c t u r i n g f l u i d s (GRONE, BAUMGARTNER & WOODROOF 1983; c f . section 4.7.4.2.). PA1 & G A R B I S (1983 b ) p r e s e n t a pre-pad system c o n s i s t i n g o f g e l l e d weak hyd r o c h l o r i c a c i d and carbon d i o x i d e c o n t a i n i n g s u r f a c t a n t s and c l a y s t a b i l i z e r i n o r d e r t o c o o l down t h e f o r m a t i o n t o m i n i m i z e v i s c o s i t y l o s s i n p r o p p a n t - l a den f l u i d , c o n d i t i o n t h e r e s e r v o i r by c r e a t i n g a low-pH environment t o m i n i m i z e c l a y s w e l l i n g and s l o u g h i n g , e s t a b l i s h a f i l t e r cake on t h e f r a c t u r e f a c e and m i n i m i z e l e a k o f f o f c r o s s l i n k e d f l u i d s t h u s r e d u c i n g s c r e e n o u t tendency, and decrease c l e a n - u p t i m e by carbon d i o x i d e a d d i t i o n . Aspects o f cooldown pad pumpi n g a r e a l s o d i s c u s s e d by SNOW & HOUGH ( 1 9 8 8 ) . The f o l l o w i n g o v e r v i e w c h a r a c t e r i z e s r e s e r v o i r c o o l i n g and f l u i d s a t u r a t i o n as t h e two main purposes o f pad enlargement and comments on some m u l t i p l e pad schedules.
4.3.4.4.2.2.1.Reservoir cooling and fluid saturation S i m i l a r enlargement o f pre-pad and pad stage s i z e s as a p p l y i n g f o r r e s e r v o i r c o o l i n g can be performed f o r f l u i d s a t u r a t i o n o f t h e pay zone complex and d i m i n u t i o n o f f l u i d l o s s . Proper s i z i n g o f t h e pad pumped ahead o f t h e p r o p p a n t c a r r i e r f l u i d i s v e r y i m p o r t a n t i n o r d e r t o keep t h e p r o p p a n t s l u r r y away f r o m t h e f r a c t u r e t i p r e g i o n and t h u s a v o i d i n g t i p screenout f a i l u r e (LEE 1987; c f . sect i o n s 4 . 2 . 3 . 2 . 5 . and 6 . 2 . 4 . 2 . 1 . ) . W h i l e continuous-mix b l e n d i n g f r a c t u r i n g oper a t e s w i t h lower p r o p p a n t c o n c e n t r a t i o n s , batch-mix f r a c t u r i n g a i m i n g on p l a c e ment o f h i g h e r proppant s a t u r a t i o n s commonly i s c a r r i e d o u t w i t h l a r g e p e r c e n t a ges o f t r e a t m e n t volume c o n s i s t i n g o f pad f l u i d ( u p t o 60 - 80 % o f t h e whole j o b ) i n s u r i n g l e a k o f f c o n t r o l and s u f f i c i e n t f r a c t u r e w i d t h f o r passage o f t h e c o n c e n t r a t e d p r o p p a n t s l u r r y (CRAMER & SONGER 1988; TAN, McGOWEN, LEE & SOLIMAN 1988; c f . s e c t i o n s 4.3.3.5.3. and 4 . 3 . 4 . 3 . 1 . 4 . ) .
449
4.3.4.4.2.2.2. Mu1 t iple pad schedules WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE (1988) r e p o r t a spec i a l case where t h r e e d i f f e r e n t pads were s u c c e s s i v e l y pumped i n c l u d i n g an i n i t i a l foamed-water/breaker spearhead, a m i d d l e normal foam pad, and a t e r m i n a l pad w i t h 100 mesh sand f o r f l u i d l e a k o f f c o n t r o l ( c f . s e c t i o n s 1.4.11.2.1. and 4.8.8.3.2.3.) p r i o r t o t h e p r o p p a n t - l a d e n f l u i d stages. HOLOITCH, ROBINSON, WHITEHEAD & ELY (1988) p r e s e n t an example o f v e r y low f l u i d e f f i c i e n c y o f o n l y 20 % which has t o be compensated by a l a r g e pre-pad and a 50 % v i s c o u s pad, because w i t h o u t s u c c e s s f u l l y c o n t r o l l i n g t h e l a r g e amount o f l e a k o f f , l a r g e hyd r a u l i c p r o p p a n t f r a c t u r e j o b s c a n n o t be pumped due t o r a p i d d e h y d r a t i o n screeno u t t e r m i n a t i o n ( c f . s e c t i o n s 4.2.2.2.3. and 6.2.4.2.1.). I n a d d i t i o n t o consid e r a b l y e n l a r g e d pre-pad and pad, e x t r a volumes o f l i q u i d and s o l i d f l u i d - l o s s a d d i t i v e s have t o be used f o r l e a k o f f m i n i m i z a t i o n ( c f . s e c t i o n s 4.3.4.6. and 4.8.8.3.).
4.3.4.4.2.3. Changing temperature and fluid viscosity As a consequence o f temperature d i f f e r e n c e s between s u r f a c e and f o r m a t i o n , s i g n i f i c a n t r e s e r v o i r and s t i m u l a t i o n f l u i d cooldown o c c u r s a c r o s s a p o r t i o n o f t h e f r a c t u r e (OLSEN & DEBONIS 1988). As many t r e a t m e n t s a r e designed f o r a worst-case temperature w i t h a t most two f l u i d s o f d i f f e r e n t v i s c o s i t y t o compens a t e f o r f l u i d temperature v a r i a n c e , t h e r e s u l t i s a f r a c t u r i n g f l u i d w i t h v a r y i n g v i s c o s i t y t h r o u g h o u t t h e c r a c k . O n - s i t e m i c r o p r o c e s s o r c o n t r o l ( c f . sect i o n s 4.3.3.5.3. and 6.2.4.3.) and a c c u r a t e continuous-mix g e l l i n g equipment a l lows t o v a r y g e l l o a d i n g i n o r d e r t o compensate f o r t i m e and temperature e f f e c t s t h u s y i e l d i n g a s t i m u l a t i o n f l u i d w i t h a more c o n s i s t e n t downhole v i s c o s i t y w h i l e u s i n g l e s s g e l l i n g agent. Most f r a c t u r i n g f l u i d s undergo c o n s i d e r a b l e temperature changes d u r i n g a s t i m u l a t i o n o p e r a t i o n due t o e l e v a t e d r e s e r v o i r temperatures, and v i s c o s i t y a l s o changes, because a l l modern v i s c o s i f i e d f l u i d s a r e non-Newtonian i n n a t u r e where v i s c o s i t y i s n o t c o n s t a n t b u t i s a f u n c t i o n o f shear r a t e . D e t e r m i n a t i o n o f f l u i d r h e o l o g y t h r o u g h o u t t h e c r a c k t h u s r e q u i r e s knowledge o f t h e temperat u r e p r o f i l e i n t h e f r a c t u r e . I n l a r g e t r e a t m e n t s , temperature and t h u s f l u i d v i s c o s i t y v a r i a t i o n s c o u l d be compensated t o a l i m i t e d e x t e n t by b a t c h m i x i n g o f s e v e r a l d i f f e r e n t polymer l o a d i n g s . Continuous m i x i n g , however, a l l o w s accur a t e t a i l o r i n g o f polymer l o a d i n g t o meet t h e t i m e and temperature c o n d i t i o n s t o which i t w i l l u l t i m a t e l y be exposed ( c f . s e c t i o n 4.3.3.5.3.). Ramped v i s c o s i t y procedures by c o n t i n u o u s m i x i n g b e n e f i t f r o m c o s t s a v i n g by reduced polymer l o a d i n g and improved f r a c t u r e c o n d u c t i v i t y by l e s s p r o p p a n t package damage due t o d i m i n i s h e d polymer l o a d i n g s , w i t h t h e l o w e r g e l c o n c e n t r a t i o n a l s o r e q u i r i n g l e s s b r e a k e r near t h e w e l l b o r e . Ramped polymer l o a d i n g p r o cedures, however, can o n l y compensate t h e maximum t i m e and temperature which a f l u i d experiences, w i t h c o n s t a n t v i s c o s i t y b e i n g achieved o n l y u n t i l n e a r t h e end o f t h e o p e r a t i o n . H i g h - t e m p e r a t u r e r e s e r v o i r f r a c t u r i n g f r e q u e n t l y r e q u i r e s v i s c o s i t y s t a b i l i z a t i o n of t h e s t i m u l a t i o n f l u i d (THOMAS & ROOT 1979; c f . sect i o n 4.7.4.2.).
4.3.4.4.3. Primary and secondary gellants PAULS, VENOITTO, CHISHOLM, HOLTMYER & GREGORCZYK (1985) comment on m i x i n g problems o f f l u i d s and proppants when h i g h sand c o n c e n t r a t i o n s s h o u l d be p l a c e d i n t o high-temperature reservoirs. Oil-based f r a c t u r i n g f l u i d s are t r a d i t i o n a l l y g e l l e d by b a t c h m i x i n g a t t h e s u r f a c e ( c f . s e c t i o n 4.3.3.5.3.), w i t h a l l the g e l l i n g agent and a c t i v a t o r r e q u i r e d b e i n g added p r i o r t o commencing t h e t r e a t ment. I n case o f h i g h - t e m p e r a t u r e f o r m a t i o n s up t o 260 OF (130 oC), t h e down-
450 h o l e v i s c o s i t y w h i c h c o n v e n t i o n a l l y m i x e d f l u i d s p r o v i d e i s much l e s s t h a n necessary, b u t i f more p r i m a r y g e l l a n t and a c t i v a t o r a r e added b y s u r f a c e b a t c h m i x i n g , t h e h i g h v i s c o s i t y o f t h e s e f l u i d s r e n d e r s them d i f f i c u l t b o t h t o h a n d l e w i t h s u r f a c e e q u i p m e n t and t o m i x w i t h s o l i d m a t e r i a l s such as f l u i d l o s s a d d i t i v e s and p r o p p a n t s , e s p e c i a l l y c o n c e r n i n g h i g h c o n c e n t r a t i o n s o f t h e latter. T h e r e f o r e s o l i d s e c o n d a r y g e l l i n g a g e n t s may be added t o t h e base g e l a f t e r passage o f t h e b l e n d e r on t h e f l y t o p r o d u c e a f l u i d w h i c h i s n o t t o o v i s c o u s t o be h a n d l e d a t t h e s u r f a c e and y e t p r o v i d e s t h e r e q u i r e d v i s c o s i t y i n t h e f r a c t u r e downhole, w i t h t h i s h i g h - t e m p e r a t u r e s e c o n d a r y g e l l a n t c o n s i d e r a b l y e x tending the usable range o f g e l l e d o i l - b a s e f l u i d s t o higher f r a c t u r e temperatur e s and l o n g e r pumping t i m e s , and a l s o a l l o w i n g t o s t i m u l a t e h i g h - t e m p e r a t u r e r e s e r v o i r s w i t h h i g h p r o p p a n t s a t u r a t i o n s . T h i s d u a l a d d i t i o n o f p r i m a r y and sec o n d a r y g e l l i n g a g e n t s r e p r e s e n t s a s e q u e n t i a l c o m b i n a t i o n o f b a t c h - and c o n t i nuous-mix f r a c t u r i n g and l i n k s t h e a d v a n t a g e s o f b o t h s e p a r a t e methods i n t o one j o i n t process.
4.3.4.4.4. Fracturing f l u i d viscosity improvement S i m u l t a n e o u s a p p l i c a t i o n o f p r i m a r y and s e c o n d a r y g e l l i n g a g e n t s r e p r e s e n t i n g s e q u e n t i a l l y combined b a t c h - and c o n t i n u o u s - m i x s t i m u l a t i o n c o m p r i s e s t h e f l e x i b i l i t y o f f o r m u l a t i n g a f l u i d t o meet b o t h s u r f a c e and downhole c o n d i t i o n s . O t h e r a s p e c t s o f i m p r o v i n g downhole f r a c t u r i n g f l u i d v i s c o s i t y w i t h o u t e x c e s s i v e p r o b l e m s o f s u r f a c e m i x i n g by a d d i t i o n o f s e c o n d a r y g e l l a n t s a n d / o r a c t i v a t o r s t o a base g e l downstream o f t h e b l e n d e r a r e d i s c u s s e d b y BURNHAM, HARRIS & McOANIEL ( 1 9 7 8 ) . LOUVIERE ( 1 9 8 7 ) o u t l i n e s t h a t g e l p r e p a r a t i o n c a n be done b y b a t c h m i x i n g o r s e m i - c o n t i n u o u s o p e r a t i o n , w i t h t h e l a t t e r t e c h n i q u e h a v i n g n o l i m i t a t i o n o n c e r t a i n s t o r a g e c a p a c i t y and j o b s i z e b e i n g a b l e t o be v a r i e d during the treatment. F r a c t u r i n g f l u i d recovery i s essential f o r successful s t i m u l a t i o n operations and depends t o l a r g e amounts on g e l volume added t o t h e base f l u i d ( P A I , GARBIS & HALL 1 9 8 3 ) . The a d d i t i o n o f g e l b r e a k e r s and l o w s u r f a c e t e n s i o n a g e n t s may c o n s i d e r a b l y f a c i l i t a t e c l e a n - u p , and t h e a d d i t i o n o f an e n e r g i z e r such as n i t r o g e n o r c a r b o n d i o x i d e t o t h e t r e a t m e n t t a i l may i n c r e a s e t h e p o s s i b i l i t y o f an e a r l y h y d r o c a r b o n show. R e a s o n a b l e c o s t r e d u c t i o n s c a n be a c h i e v e d b y t a p e r i n g p o l y m e r c o n c e n t r a t i o n o f t h e f l u i d system. As g e l w e i g h t i s r e d u c e d , j o b c o s t s d e c r e a s e due t o d i m i n i s h i n g p o l y m e r expenses, l e s s h o r s e p o w e r n e c e s s a r y a s a r e s u l t o f l o w e r f r i c t i o n p r e s s u r e , and f a s t e r w e l l c l e a n - u p as a c o n s e quence o f l e s s p o l y m e r h a v i n g been pumped downhole (SCHLOTTMAN, MILLER & LUEDERS 1 9 8 1 ) .
4.3.4.5. Fluid viscoelasticity ACHARYA ( 1 9 8 7 ) i l l u s t r a t e s t h e s i g n i f i c a n c e o f v i s c o e l a s t i c i t y o f c r o s s l i n ked f r a c t u r i n g f l u i d s f o r proppant t r a n s p o r t . Concerning proppant placement along the fracture, p r o p p a n t movement v e l o c i t y i s a f u n c t i o n o f f r a c t u r i n g f l u i d r h e o l o g y and p r o p p a n t p a r t i c l e c h a r a c t e r i s t i c s such as s i z e and d e n s i t y . Crosslinked g e l s can respond t o deformation by viscous f l o w which d i s s i p a t e s e n e r g y and b y e l a s t i c d i s p l a c e m e n t w h i c h s t o r e s f r e e e n e r g y . When t h e R e y n o l d s number o f t h e p a r t i c l e s i s g r e a t e r t h a n two, p r o p p a n t s e t t l i n g r a t e c a n b e i n f l u e n c e d b y v i s c o u s as w e l l as e l a s t i c c h a r a c t e r i s t i c s o f t h e f r a c t u r i n g f l u i d s . The n e t w o r k s t r u c t u r e o f t h e c r o s s l i n k e d g e l r e s t r i c t s p r o p p a n t s e t t l i n g , and d i s r u p t i o n o f t h e n e t w o r k s t r u c t u r e b y s h e a r , t e m p e r a t u r e o r c h e m i c a l breakup o n l y a l l o w s t h e proppant t o f a l l through t h e g e l .
A s p e c t s o f p r o p p a n t t r a n s p o r t a r e a l s o d i s c u s s e d b y NOVOTNY ( 1 9 7 7 ) , CLARK & QUADIR ( 1 9 8 1 ) ; SIEVERT, WAHL, CLARK & HARKIN ( 1 9 8 1 ) ; AHMED ( 1 9 8 4 ) ; CLARK, HALVACI, GHAELI & PARKS ( 1 9 8 5 ) ; MORALES & ABOU-SAYED ( 1 9 8 5 ) and LEE & DANESHY
45 1 ( 1 9 8 6 ) . E f f e c t s o f pumping equipment on sand-laden s l u r r i e s a r e covered by ROLL, HIMES, EWERT & DOERKSEN ( 1 9 8 6 ) . HANDKE (1987) r e v i e w s q u a l i t y and v e r s a t i l i t y improvement o f f r a c t u r i n g o p e r a t i o n s by sand c o n c e n t r a t i o n c o n t r o l . Comments on g r a v e l p a c k i n g f l u i d v i s c o e l a s t i c i t y a r e g i v e n by HUDSON & MARTIN (1988; c f . s e c t i o n 5 . 3 . ) .
4.3.4.6. Flu id leakoff containment F l u i d l o s s p l a y s a s i g n i f i c a n t r o l e by b e i n g one o f t h e more dominant f a c t o r s c o n t r o l l i n g h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t s (VEATCH & MOSCHOVIDIS 1986, McGOWEN & McDANIEL 1988) and b e i n g a b l e t o have as much e f f e c t on c r a c k p e n e t r a t i o n as v e r t i c a l f r a c t u r e h e i g h t growth has (VEATCH 1983). The volume o f f l u i d l e a k o f f determines f l u i d e f f i c i e n c y as w e l l as a f f e c t s t r e a t m e n t s i z e and p r o p p a n t s c h e d u l i n g . The r a t e of f l u i d l o s s a l s o impacts f r a c t u r e c l o s u r e t i m e and t h u s f i n a l proppant d i s t r i b u t i o n w i t h i n t h e c r a c k . W h i l e m i n i m i z a t i o n o f f l u i d l e a k o f f i s d e s i r a b l e d u r i n g t h e pumping o p e r a t i o n , i t i s u n d e s i r a b l e t o c o n t r o l f l u i d l o s s t o t h e p o i n t o f e x c e s s i v e p r o p p a n t s e t t l i n g due t o v e r y l o n g f r a c t u r e c l o s u r e t i m e ( c f . s e c t i o n 4.12.2.2.), w i t h thus a sound o p t i m i z a t i o n h a v i n g t o be performed by m i n i f r a c t u r e c a l i b r a t i o n t e s t s ( c f . s e c t i o n 4 . 8 . 1 . ) . F l u i d l e a k o f f i s g e n e r a l l y a f u n c t i o n o f r e s e r v o i r p r o p e r t i e s and f i l t e r cake (WILLIAMS 1970).
4.3.4.6.0. Types and inpact of fluid loss F l u i d l o s s comprises leakage o f t h e s t i m u l a t i o n f l u i d o u t o f t h e main f r a c t u r e i n t o formation m a t r i x porosity, natural h a i r l i n e cracks o r macrofractures, j o i n t s and f a u l t s (ECONOMIDES 1986). H a i r l i n e c r a c k s a r e t h i e f f r a c t u r e s which s t e a l f l u i d f r o m t h e main f r a c t u r e and c o n s e q u e n t l y l i m i t i t s e x t e n s i o n ( c f . s e c t i o n s 4.8.8.3.1.2. and 4 . 8 . 8 . 3 . 2 . 3 . ) . F l u i d l e a k o f f b e h a v i o u r depends on a number o f f a c t o r s i n c l u d i n g t y p e and q u a n t i t y o f f l u i d l o s s a d d i t i v e and g e l l i n g agent, p r e s s u r e d i f f e r e n t i a l between f r a c t u r e and f o r m a t i o n , r e s e r v o i r p e r m e a b i l i t y and p o r o s i t y as w e l l as f l u i d f l o w and c o m p r e s s i b i l i t y b e h a v i o u r , f r a c t u r i n g f l u i d and f l u i d f i l t r a t e v i s c o s i t y and temperature behaviour, r e s e r v o i r temperature, and c h a r a c t e r o f t h e n a t u r a l f r a c t u r e system i n t e r s e c t i n g t h e pay zone m a t r i x . Other parameters c o n t r o l l i n g f l u i d e f f i c i e n c y a r e f l u i d p r o p e r t i e s and i n j e c t i o n r a t e (WAREMBOURG, KLINGENSMITH, HODGES & ERDLE 1985). F l u i d l e a k o f f models and e v a l u a t i o n s a r e a l s o p r e s e n t e d by SETTARI (1985, 1988); SETT A R I , KRY & YEE (1988) and WARPINSKI (1988 a ) . Experimental m o d e l l i n g o f f l u i d l o s s has shown t h a t p r e s s u r e drop i s i n i t i a l as i f no a d d i t i v e were p r e s e n t (WILLIAMS 1970). T h i s b e g i n n i n g stage i s f o l l o w e d by an i n t e r m e d i a t e stage where a f i l t e r cake s t a r t s t o f o r m and l e a k o f f v e l o c i t y i s l o w e r because some p r e s s u r e d r o p o c c u r s across t h e drape. The f i n a l stage comprises achievement o f a steady s t a t e i n which t h e cake has c o n s t a n t t h i c k ness and f l u i d l e a k o f f v e l o c i t y i s c o n s t a n t . Flow v e l o c i t y t h r o u g h t h e steadys t a t e drape i s depending on f l o w v e l o c i t y across t h e r o c k s u r f a c e , f l u i d and add i t i v e p r o p e r t i e s , and m a t r i x p o r e s i z e . T o t a l f l u i d l o s s g e n e r a l l y c o n s i s t s o f s p u r t l o s s and f l u i d - l o s s o r w a l l - b u i l d i n g c o e f f i c i e n t . S p u r t l o s s p e r t a i n s t o t h e i n s t a n t a n e o u s l o s s when f l u i d i s f i r s t exposed t o t h e f r a c t u r e face, whereas f l u i d - l o s s o r w a l l - b u i l d i n g c o e f f i c i e n t r e p r e s e n t s l o n g - t e r m b e h a v i o u r o v e r t h e d u r a t i o n of exposure. The o u t l i n e as f o l l o w s d i s c u s s e s p o s s i b i l i t i e s o f f l u i d - l o s s c o n t r o l , n a t u r a l f r a c t u r e s and p a r t i c u l a t e a d d i t i v e s , mushroom-type f r a c t u r e s and f l u i d e f f i c i e n c y , and combined f l u i d - l o s s c o n t r o l a d d i t i v e and g e l b r e a k e r . MEDLIN & MASSE (1986 b) analyze f l u i d - l e a k o f f mechanisms f o r g e l s and suspensions. l y t o t a l l y d i s s i p a t e d across t h e f o r m a t i o n f a c e and f l u i d l e a k o f f occurs
452
4.3.4.6.1. Possibi 1 it ies of f luid-loss control The i m p a c t o f f l u i d l e a k o f f on t h e r e s u l t o f t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g treatment i s u n d e r l i n e d by t h e p o s s i b i l i t y t o achieve s i g n i f i c a n t l y longer f r a c t u r e s w i t h f l u i d s w i t h l o w e r f l u i d - l o s s c o e f f i c i e n t s . F l u i d l e a k o f f may b e c o n t r o l l e d w i t h s o l i d o r l i q u i d additives. While s o l i d p a r t i c u l a t e f l u i d - l o s s a d d i t i v e s may be v e r y e f f e c t i v e f o r m o d e r a t e - t o h i g h - p e r m e a b i l i t y f o r m a t i o n s b u t a r e much l e s s e f f e c t i v e i n t i g h t l o w - p e r m e a b i l i t y r e s e r v o i r s , l i q u i d f l u i d l o s s a d d i t i v e s such as h y d r o c a r b o n phases c a n r e d u c e f l u i d l e a k o f f s i g n i f i c a n t l y i n t i g h t f o r m a t i o n s b u t have l i t t l e o r n o e f f e c t i n m o d e r a t e - t o h i g h - p e r m e a b i l i t y rocks ( c f . s e c t i o n 4.3.4.6.2.). Liquid additives mainly r e s t r i c t f l u i d l o s s i n t o t h e m a t r i x b y v i s c o s i t y and c o m p r e s s i b i l i t y d i f f e r e n c e s o f r e s e r v o i r and s t i m u l a t i o n f l u i d s , whereas p a r t i c u l a t e a d d i t i v e s p r i n c i p a l l y l i m i t f l u i d l e a k o f f i n t o n a t u r a l f r a c t u r e s (McMECHAN & CONWAY 1983) b y p l u g g i n g o f c r a c k s and b u i l d i n g o f f i l t e r c a k e s on t h e w a l l .
4.3.4.6.1.1.General aspects P o l y m e r and c r o s s l i n k e r t y p e s have a p r o n o u n c e d e f f e c t o n f l u i d l e a k o f f behav i o u r (McGOWEN & McDANIEL 1 9 8 8 ) . Lower r e s i d u e p o l y m e r s e x h i b i t h i g h e r l e v e l s o f f l u i d l o s s , whereas c r o s s l i n k e r t y p e i n f l u e n c e s c r o s s l i n k e d f l u i d l e a k o f f . A d d i t i o n o f p r o p p a n t s o f t e n seems t o have n o a l t e r i n g i m p a c t o n f l u i d - l o s s behav i o u r o f c r o s s l i n k e d g e l s , and a l s o t h e f i l t e r c a k e s d e p o s i t e d o n t h e f r a c t u r e w a l l s do n o t e x h i b i t any s i g n s o f e r o s i o n w h i c h may have r e s u l t e d f r o m i n c l u s i o n o f proppant t o the flow. Pumping r a t e o b v i o u s l y compensates f o r f l u i d l e a k o f f i n t o t h e f o r m a t i o n i f h i g h enough and f e a s i b l e (VEATCH 1986), b u t does u s u a l l y n o t compete e c o n o m i c a l l y w i t h f l u i d - l o s s a d d i t i v e s t o overcome l e a k o f f . H i g h i n j e c t i o n r a t e s o f s u i t a b l e f l u i d s c a n o f f s e t f l u i d l o s s and e n s u r e adequate p r o p p a n t t r a n s p o r t w i t h i n t h e f r a c t u r e (NORTON & HOFFMAN 1 9 8 2 ) . I n l o w l e a k o f f s i t u a t i o n s , optimum r a t e s a r e g e n e r a l l y d o m i n a t e d b y f r i c t i o n l o s s b e h a v i o u r and h y d r a u l i c h o r s e p o w e r c o s t s , whereas i n h i g h l e a k o f f c a s e s where volume r e q u i r e m e n t s a r e a f f e c t e d b y pumping r a t e s , f r a c t u r i n g m a t e r i a l expenses e n t e r t h e p i c t u r e . D i s t i n c t i o n c a n be made between s t o r a g e - d o m i n a t e d o r l o w f l u i d - l o s s f r a c t u r i n g , and f l u i d - l e a k o f f - d o m i n a t e d o r h i g h f l u i d - l o s s f r a c t u r i n g (SMITH 1 9 8 1 ) . Drawbacks o f h i g h f l u i d l e a k o f f r a t e s as w e l l as a s p e c t s o f s o l i d p a r t i c l e s i z e v s . p o r e t h r o a t d i a m e t e r , f l u i d u p t a k e s a t u r a t i o n o f t h e f o r m a t i o n by p r e - p a d and p a d s t a g e s , and f l u i d v i s c o s i t y v s . l e a k o f f a r e d i s c u s s e d as f o l l o w s .
4.3.4.6.1.2.Drawbacks o f high fluid leakoff rates Fluid
l e a k o f f during hydraulic f r a c t u r i n g leads t o increasing f l u i d v i s c o s i the c r a c k as a r e s u l t o f volume d i m i n u t i o n o f t h e t o t a l s l u r r y w h i c h may be d e s i r a b l e up t o a c e r t a i n p o i n t (because o t h e r w i s e f l u i d v i s c o s i t y decreases b y s h e a r and t e m p e r a t u r e d e g r a d a t i o n and c a n r e a c h a p o i n t where s a t i s f a c t o r y p r o p p a n t t r a n s p o r t and s u s p e n s i o n c a p a c i t y i s no l o n g e r g u a r a n t e e d ) , but a t the same t i m e e x c e s s i v e f l u i d l o s s c o n s i d e r a b l y i n c r e a s e s t h e d a n g e r o f p r e m a t u r e s c r e e n o u t f a i l u r e b y g e l t h i c k e n i n g as w e l l as p r o p p a n t b r i d g i n g and p l u g g i n g and t h u s f l o w s t u c k i n t h e f r a c t u r e (HARRIS & PENNY 1985; PEARSDN, ABOU-SAYED & NGUYEN 1985; VEATCH & MOSCHOVIDIS 1986, SMITH 1987; c f . s e c t i o n s 4 . 2 . 3 . 2 . 5 . and 6.2.4.2.1.). I n a d d i t i o n , h i g h f l u i d l e a k o f f can g i v e r i s e t o severe c u r t a i l ment o f p r o d u c t i o n due t o f o r m a t i o n m a t r i x damage, a d v e r s e r e s e r v o i r f l u i d i n t e r a c t i o n s o r s i m p l y a l t e r e d f r a c t u r e g e o m e t r y (PENNY, CONWAY & LEE 1 9 8 4 ) . As f l u i d l o s s c a n e x c e e d 70 % o f t h e i n j e c t e d volume i f n o t c o n t r o l l e d p r o p e r l y , an o v e r w h e l m i n g amount o f f l u i d c a n b e n e c e s s a r y t o a c h i e v e a d e s i r e d f r a c t u r e g e o m e t r y i n a MHF t r e a t m e n t and t h u s f l u i d e f f i c i e n c y c a n g o v e r n t h e e c o n o m i c a l success o f t h e s t i m u l a t i o n o p e r a t i o n . t y due t o d e h y d r a t i o n and c o n s e q u e n t l y i n c r e a s i n g p r o p p a n t c o n c e n t r a t i o n i n
453 L e a k o f f r a t e s o f unbroken f r a c t u r i n g f l u i d s a r e g e n e r a l l y h i g h e r t h a n those o f broken ones because w a t e r i s squeezed out, b u t polymer p a r t i c l e s cannot b u i l d a f i l t e r cake, and t h e r e f o r e o n l y w a t e r l e a k s o f f . W h i l e unbroken f l u i d cannot f o r m f i l t e r cakes a t t h e f r a c t u r e w a l l , broken p a r t i c l e s can m i g r a t e and c o v e r t h e c r a c k boundary w i t h a f i l t e r cake which i s then l i m i t i n g f u r t h e r l o s s by r a t e r e d u c t i o n . Absence o f a f i l t e r cake p r i o r t o f l u i d b r e a k i n g as w e l l as m a t r i x s a t u r a t i o n w i t h w a t e r o r b r i n e a f t e r i n i t i a l l e a k o f f a r e t h e reasons f o r much h i g h e r f l u i d l o s s i n t h e e a r l y stage p r i o r t o b r e a k i n g t h a n i n t h e l a t e stage a f t e r b r e a k i n g . BENNY & FORD (1985) emphasize t h e i n f l u e n c e o f downhole c o n d i t i o n s on t h e l e a k o f f p r o p e r t i e s o f f r a c t u r i n g f l u i d s . Another e f f e c t o f f l u i d l o s s i n t o t h e n e a r - f r a c t u r e p o r e space can be f l u c t u a t i o n s i n c l o s u r e s t r e s s r e p r e s e n t i n g b a c k s t r e s s (WARPINSKI, BRANAGAN, SATTLER, CORENZ, NORTHROP, MANN & FROHNE 1985).
4.3.4.6.1.3.
S o l i d p a r t i c l e s i z e vs. pore t h r o a t diameter
Small-diameter s o l i d s d i s p e r s e d i n f r a c t u r i n g f l u i d s have s u f f i c i e n t concent r a t i o n and s i z e range t o a c t as f l u i d - l o s s a d d i t i v e s i n f o r m a t i o n s w i t h s m a l l p o r e - s i z e d i s t r i b u t i o n s (CARROLL & BAKER 1979). B l e e d - o f f t o t h e f o r m a t i o n f r o m t h e s t i m u l a t i o n f l u i d prepared w i t h such m a t e r i a l s i n c l u d e s a h i g h p r o b a b i l i t y o f some damage t o r e s e r v o i r p e r m e a b i l i t y . B l e e d - o f f t o t h e f o r m a t i o n f r o m a t r e a t m e n t f l u i d c o n t a i n i n g d i s p e r s e d s o l i d s and r e s i d u e - f o r m i n g m a t e r i a l s g i v e s r i s e t o b u i l d i n g o f a f i l t e r cake upon t h e f o r m a t i o n f a c e b e f o r e t h e p a r t i n g p r e s s u r e i s r e l e a s e d , a f t e r which t h e proppants a d j a c e n t t o t h e f o r m a t i o n f a c e s e t t l e i n t o t h e r a t h e r s o f t f i l t e r cake and come t o r e s t a t t h e f o r m a t i o n f a c e . Depending upon f i l t e r cake t h i c k n e s s , t h e v o i d volume o f t h e p r o p p a n t immediatel y a d j a c e n t t o t h e f r a c t u r e w a l l c o u l d be c o m p l e t e l y f i l l e d . Experimental r e s e a r c h r e v e a l s t h a t t h e p o s s i b i l i t y o f p l u g g i n g a propped f r a c t u r e i s c o n t r o l l e d by p a r t i c l e s i z e d i s t r i b u t i o n i n r e l a t i o n t o p o r e s i z e d i s t r i b u t i o n . Residue-forming m a t e r i a l i n t h e g e l s generate a p a r t i c l e s i z e d i s t r i b u t i o n t h a t even i n low c o n c e n t r a t i o n s w i l l s e v e r e l y damage f r a c t u r e conduct i v i t y . Proppants s t a r t c r u s h i n g o r a t l e a s t r e l e a s i n g f i n e s i m m e d i a t e l y upon a p p l i c a t i o n o f overburden pressure, and f i n e s adhere t o t h e proppants which upon l i b e r a t i o n i n a f r a c t u r i n g f l u i d may a c t as a f l u i d - l o s s a d d i t i v e . P a r t i c l e s i z e d i s t r i b u t i o n s used t o g e n e r a t e f r a c t u r i n g f l u i d s i n w a t e r may be s u f f i c i e n t l y h i g h enough t o c o n t r i b u t e t o b o t h f o r m a t i o n and f r a c t u r e p l u g g i n g .
4.3.4.6.1.4.
F l u i d uptake s a t u r a t i o n by pre-pad and pad stages
The pre-pad and/or t h e pad stages o f a h y d r a u l i c f r a c t u r i n g t r e a t m e n t a r e o f t e n s a c r i f i c e d f o r f l u i d l o s s i n t o t h e r e s e r v o i r i n o r d e r t o have a l r e a d y achieved a s a t u r a t i o n l e v e l i n f l u i d u p t a k e by t h e f o r m a t i o n once t h e c a r r i e r stage w i t h t h e suspended proppants i s i n s e r t e d i n t o t h e f r a c t u r e (SMITH 1987). F o r m a t i o n f l u i d l e a k o f f r a t e s can be e s t i m a t e d f r o m p r e s s u r e d e c l i n e a n a l y s i s f o l l o w i n g i n j e c t i o n by m i n i f r a c t u r e c a l i b r a t i o n t e s t (NOLTE 1979, NOLTE & SMITH 1981) and f r o m changes o f f r a c t u r i n g g r a d i e n t d u r i n g pumping based on measurements o f i n c r e a s i n g i n s t a n t a n e o u s s h u t - i n p r e s s u r e (NIERODE 1983). Concerning f l u i d - l o s s m o d e l l i n g by l a b o r a t o r y experiments, dynamic t e s t s can y i e l d q u i t e d i f f e r e n t r e s u l t s than s t a t i c runs, and shear r a t e and h i s t o r y have s i g n i f i c a n t e f f e c t s (GULBIS 1983, ROODHART 1983; ZIGRYE, WHITFILL & SIEVERT 1983; PENNY, CONWAY & LEE 1984; H A R R I S & PENNY 1985, FORD & PENNY 1988). PEARSON, LYNCH, SCHMIDT & McCASLIN (1988) o u t l i n e improvement o f f l u i d - l e a k o f f cont r o l by s u c c e s s i v e l y i n c r e a s i n g t h e pad stage volume d u r i n g course o f an e v o l u t i o n o f f r a c t u r i n g t e c h n o l o g y . I n c r e a s i n g pad q u a n t i t y a l s o aims on s c r e e n o u t r i s k m i n i m i z a t i o n , because e a r l y pad d e p l e t i o n due t o h i g h f l u i d - l o s s r a t e s i n creases t h e danger o f premature f a i l u r e o f t h e j o b . T h i s a p p l i e s p a r t i c u l a r l y f o r b r i d g e o u t c o n d i t i o n s o c c u r r i n g when t h e movement o f p r o p p a n t g r a i n s a t some p o s i t i o n i n t h e f r a c t u r e i s r e s t r i c t e d by i n s u f f i c i e n t f r a c t u r e w i d t h .
454 I n e n e r g i z e d f r a c t u r i n g t r e a t m e n t s , t h e pre-pad aims on e s t a b l i s h m e n t o f a f i l t e r cake on t h e f r a c t u r e f a c e i n o r d e r t o m i n i m i z e l e a k o f f o f c r o s s l i n k e d c a r r i e r f l u i d , t h e r e b y r e d u c i n g t h e tendency o f premature s c r e e n o u t f a i l u r e ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) , as w e l l as c o o l i n g down t h e f o r m a t i o n t o decrease v i s c o s i t y l o s s o f t h e p r o p p a n t - l a d e n f l u i d , c o n d i t i o n t h e r e s e r v o i r t o m i n i m i z e c l a y s w e l l i n g and sloughing, and r e d u c i n g c l e a n - u p t i m e by a d d i t i o n o f carbon d i o x i d e o r n i t r o g e n gas (PA1 & G A R B I S 1983; c f . s e c t i o n 3 . 1 1 . 2 . 2 . ) .
4.3.4.6.1.5.
F l u i d viscosity vs. leakoff
Most v i s c o s i f y i n g agents used i n f r a c t u r i n g f l u i d s a l s o p l a y an i m p o r t a n t r o l e i n c o n t r o l l i n g f l u i d l o s s (OLSEN & DEBONIS 1988). Reducing v i s c o s i t y i n t h e l a t t e r p o r t i o n s o f a t r e a t m e n t t h u s c o u l d i n c r e a s e f l u i d l e a k o f f , and i f needed, o t h e r m a t e r i a l s c o u l d be added t o t h e f l u i d t o m a i n t a i n an adequate l o s s r a t e and y e t n o t a f f e c t v i s c o s i t y . I f f l u i d l o s s f r o m a w a l l - b u i l d i n g v i s c o s i f i e r i s a f u n c t i o n of f l u i d amount t h a t has l e a k e d o f f across a g i v e n r o c k face, then t h e f i l t e r cake should always be t h i c k e s t near t h e w e l l b o r e where exposure t i m e has been l o n g e s t and t a p e r down towards t h e f r a c t u r e t i p where expos u r e t i m e i s s h o r t e s t . Concerning p r o p p a n t embedment i n g e l f i l t e r cakes t r i g g e r i n g c o n d u c t i v i t y d e t e r i o r a t i o n ( c f . s e c t i o n s 1.4.10.3.5. and 4 . 3 . 4 . 1 ) , t h e dang e r o f f r a c t u r e performance damage p a r t i c u l a r l y near t h e w e l l b o r e a l s o j u s t i f i e s t o reduce t h e amount o f f l u i d - l o s s m a t e r i a l i n t h e l a t t e r p o r t i o n o f t h e t r e a t m e n t . F l u i d - l e a k o f f c o n t r o l s h o u l d be p r e f e r e n t i a l l y done i n t h e pad stage o f t h e o p e r a t i o n t o m a i n t a i n a more even d i s t r i b u t i o n o f f i l t e r cake. The aim o f m i n i m i z i n g f l u i d l o s s i n h y d r a u l i c f r a c t u r i n g f o r t h e purpose o f o p t i m i z i n g p r o p p a n t t r a n s p o r t and p r e v e n t i o n o f screenout f a i l u r e due t o p r o p p a n t b r i d g i n g and/or s l u r r y d e h y d r a t i o n ( c f . s e c t i o n s 4 . 2 . 2 . 2 . 3 . , 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) i s i n c o n t r a s t t o t h e goal o f h i g h f l u i d l e a k o f f i n g r a v e l packi n g f o r t h e b e n e f i t o f adequate p a c k i n g d e n s i t y and p r o p e r p l u g g i n g o f t h e p e r f o r a t i o n tunnels ( c f . section 5.3.1.1.).
4.3.4.6.2.
Nat ura 1 fractures and particulate additives
F l u i d l e a k o f f i s i n an o v e r r i d i n g amount i n f l u e n c e d by n a t u r a l f r a c t u r e s and/or h i g h - p e r m e a b i l i t y s t r e a k s o f t h e s t i m u l a t e d r e s e r v o i r . N a t u r a l c r a c k s and f i s s u r e s p r o v i d e an a c c e l e r a t e d f l u i d d e p l e t i o n mechanism w i t h i n t h e propag a t i n g c r a c k analogous t o a p r e s s u r e r e g u l a t o r and a r e p r i n c i p a l l y u n d e s i r a b l e , because u n c o n t r o l l e d f l u i d l o s s l e a d s t o s l u r r y d e h y d r a t i o n and p r o p p a n t b r i d g i n g which g i v e r i s e t o s c r e e n o u t f a i l u r e o f t h e t r e a t m e n t (NOLTE 1988 b; c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) . Once t h e h i g h l e a k o f f r a t e s i n these weakness elements a r e curbed, f l u i d l o s s t o t h e l o w - p e r m e a b i l i t y m a t r i x i s s t r o n g l y governed by t h e shear r a t e w i t h i n t h e f r a c t u r e , p a r t i c u l a r l y i n h i g h - r a t e j o b s i n t h e e a r l y p a r t o f t h e pad where crack w i d t h s a r e r e l a t i v e l y s m a l l , w h i c h can l e a d t o changing l e a k o f f r a t e s d u r i n g t h e t r e a t m e n t . The c h a r a c t e r i s t i c s o f a p a r t i c u l a r f o r m a t i o n d i c t a t e t h e s e l e c t i o n o f f l u i d - l o s s a d d i t i v e s t h a t a r e r e q u i r e d t o achieve t h e necessary l e a k o f f cont r o l . P a r t i c u l a t e a d d i t i v e s such as 100 mesh sand, s i l i c a f l o u r , c a l c i u m carbonate o r o i l - s o l u b l e r e s i n grains are essential f o r containing l e a k o f f t o highp e r m e a b i l i t y r e s e r v o i r s t r e a k s (above a b t . 5 md p e r m e a b i l i t y ) and n a t u r a l f r a c t u r e s , whereas l i q u i d hydrocarbon a d d i t i v e s f u n c t i o n w e l l i n l o w - p e r m e a b i l i t y m a t r i x c c n t r o l (PENNY, CONWAY & LEE 1984; SMITH 1 9 8 7 ) . Hydrocarbon phases mixed w i t h s i l i c a f l o u r o r o t h e r f i n e - g r a i n e d p a r t i c u l a t e s and a s u r f a c t a n t can a l s o s i g n i f i c a n t l y reduce f l u i d l o s s (GULBIS 1983, ROODHART 1983; ZIGRYE, WHITFILL & SIEVERT 1983; PENNY, CONWAY & LEE 1984; H A R R I S & PENNY 1985, FORD & PENNY 1988). An overview o f f r a c t u r i n g f l u i d a d d i t i v e s i s g i v e n by H A R R I S ( 1 9 8 8 ) . WILLIAMSON & ALLENSON (1989) i n t r o d u c e a nondamaging p a r t i c u l a t e f l u i d - l o s s a d d i t i v e . The d i s c u s s i o n as f o l l o w s focusses on l i q u i d and s o l i d f l u i d - l e a k o f f
455 agents, foams and r e s i n s , and i m p a c t o f f r a c t u r e geometry and morphology.
4.3.4.6.2.1. Liquid f luid-loss additives I n c r o s s l i n k e d g e l systems, d i e s e l o i l p r o v i d e s b e t t e r f l u i d - l o s s c o n t r o l t h a n p a r t i c u l a t e f l u i d - l o s s a d d i t i v e s , whereas i n n o n - c r o s s l i n k e d l i n e a r g e l systems, p a r t i c u l a t e a d d i t i v e s p r o v i d e l o w e r l e a k o f f than d i e s e l o i l (OSBORNE, McLEOD & SCHROEDER 1981). The reason f o r t h i s d i a m e t r i c a l b e h a v i o u r and t h e sup e r i o r i t y o f the diesel o i l i n crosslinked gels i s t h a t the l a t t e r concentrates i n a t h i n l a y e r a l o n g t h e f r a c t u r e w a l l s as f i l t r a t e i s squeezed i n t o t h e format i o n pores, r e s e m b l i n g a f i b r o u s mat composed o f many polymer entanglements, w i t h t h e d i e s e l o i l d i s p e r s e d i n t o s m a l l d r o p l e t s by s u i t a b l e s u r f a c t a n t s f i l l i n g i n t h e i n t e r s t i c e s between these entanglements. CANTU & BOYD (1988) i n t r o duce a c o m b i n a t i o n o f f l u i d - l o s s c o n t r o l a d d i t i v e and g e l b r e a k e r . McDRNIEL, H O W & BRRRlNGER (1976) i n t r o d u c e a new g e n e r a t i o n o f s o l i d - f r e e f r a c t u r i n g f l u i d s which do n o t need s o l i d a d d i t i v e s f o r l e a k o f f c o n t r o l . I n cont r a s t t o c o n v e n t i o n a l s o l i d - l a d e n f l u i d s where polymers i m p a r t v i s c o s i t y and f l u i d - l o s s additives help t o l i m i t f l u i d leakoff, solid-free fracturing f l u i d s m i n i m i z e p e r m e a b i l i t y damage t o b o t h f o r m a t i o n and packed c r a c k . These s o l i d f r e e s t i m u l a t i o n f l u i d s a r e a p p l i c a b l e i n h i g h - and low-temperature r e s e r v o i r s . The h i g h - v i s c o s i t y water-based f l u i d s a r e m a i n l y used as spearheads i n a c i d i z i n g t r e a t m e n t s and as f r a c t u r i n g f l u i d s f o r proppant placement.
4.3.4.6.2.2. Sol id f luid-loss additives The a p p l i c a t i o n o f s o l i d f l u i d - l o s s a d d i t i v e s as t h e head o f t h e p r o p p a n t s l u r r y has t o be done w i t h c a u t i o n i n o r d e r n o t t o provoke i n t e r m i x i n g o f p r o p p a n t s and f i n e sand which would d r a s t i c a l l y d e t e r i o r a t e t h e f l o w c a p a c i t y o f t h e f r a c t u r e ( c f . s e c t i o n 1.4.11.2.1.). I n many n a t u r a l l y j o i n t e d r e s e r v o i r s , a d d i t i o n a l 100 mesh sand i n a c o n c e n t r a t i o n o f up t o 2 l b s / g a l f l u i d i s s u f f i c i e n t t o reduce l e a k o f f enough t o p r e v e n t p r o p p a n t screenout (COOPER, NELSON & SCHOPPER 1977; SCHUMACHER & DICKERMAN 1986; c f . s e c t i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.). The b e s t e f f e c t i s achieved when t h e n a t u r a l f r a c t u r e s and f i s s u r e s a r e opened by m a x i m i z i n g n e t p r e s s u r e and s e a l e d w i t h 100 mesh sand b e f o r e t h e p r o p p a n t i s placed, w i t h c a u t i o n , however, h a v i n g t o be undertaken t h a t crack tip does n o t screen o u t p r e m a t u r e l y (NOLTE 1988 b; c f . s e c t i o n 4 . 3 . 4 . 6 . 3 . ) . The d i s c u s s i o n a s f o l l o w s focusses on comparative e v a l u a t i o n o f var i o u s agents, 100 mesh sand i n s e r t i o n f o r dual l e a k o f f c o n t r o l , a c c e l e r a t e d l e a k o f f and screenout, and f i n e sand vs. b a u x i t e .
4.3.4.6.2.2.1. Comparative evaluation of various agents F i e l d examples o f a p p l i c a t i o n o f 100 mesh sand and s i l i c a f l o u r as f l u i d l o s s c o n t r o l a d d i t i v e s a r e a l s o r e p o r t e d by PEARSON, LYNCH, SCHMIDT & McCASLIN (1988) and SATTLER, RAIBLE, GALL & GILL (1988; c f . a l s o s e c t i o n 1 . 4 . 1 1 . 2 . 1 . 1 . ) . PENNY (1982) and WILLIAMSON & ALLENSON (1989) o u t l i n e non-damaging f l u i d - l o s s a d d i t i v e s . CRAWFORD (1983) p r e s e n t s p r o p p a n t schedules and f l u i d l e a k o f f c a l c u l a t i o n s . S I N H A (1976) comments on s t a t i c and dynamic f l u i d - l o s s c o n d i t i o n s . HALL & HOUK (1983) i n v e s t i g a t e f l u i d - l o s s c o n t r o l i n n a t u r a l l y f r a c t u r e d format i o n s . ZIGRYE, WHITFILL & SIEVERT (1983) e v a l u a t e f l u i d l e a k o f f containment d i f f e r e n c e s o f c r o s s l i n k e d and l i n e a r f r a c t u r i n g f l u i d s . DOLAK & PERNTHANER (1976) propose cement as an a l t e r n a t i v e t o 100 mesh sand f o r p l u g g i n g o f h a i r l i n e f i s s u r e s , w i t h pumping o f a b t . 100 sacks o f cement p r i o r t o t h e main f r a c t u r e f l u i d stages s u f f i c i e n t l y c l o s i n g t h e h a i r l i n e c r a c k s i n s m a l l - s c a l e sand f r a c t u r i n g t r e a t m e n t s . Cement, however, i s permanentl y d e s t r o y i n g h a i r l i n e p e r m e a b i l i t y and does n o t a l l o w t h e t i n y f i s s u r e s t o l a t e r c o n t r i b u t e t o p r o d u c t i o n , whereas 100 mesh sand i s h i g h l y - p e r m e a b l e and i s
456 the i d e a l combination o f both hairline-crack-plugging agent ( c f . s e c t i o n s 4 . 8 . 8 . 3 . 1 . 2 . and 4 . 8 . 8 . 3 . 2 . 3 . ) .
and
hydrocarbon-draining
4.3.4.6.2.2.2. 100 mesh sand insertion for dual leakoff control Dual l e a k o f f i s a common phenomenon i n h y d r a u l i c f r a c t u r i n g o f t i g h t gas r e s e r v o i r s (WARPINSKI 1988 a; c f . s e c t i o n s 4.8.8.3.1.2. and 4 . 8 . 8 . 3 . 2 . 3 . ) which i s c h a r a c t e r i z e d by a s i g n i f i c a n t l y i n c r e a s e d ( u p t o 50 t i m e s ) f l u i d l e a k o f f occ u r r i n g f r o m a t h r e s h o l d p r e s s u r e above f o r m a t i o n c l o s u r e p r e s s u r e onwards. The i n c r e a s e d l e a k o f f r e s u l t s i n r a p i d f l u i d d r a i n a g e and d e h y d r a t i o n , and i s a p r o b a b l e cause o f e a r l y screenouts u n l e s s r e s t r i c t e d by u s i n g 100 mesh sand b o t h as f l u i d - l o s s a d d i t i v e and as m i c r o p r o p p a n t i n f i l l i n g t h e h a i r l i n e c r a c k s and p l u g g i n g them open i n o r d e r t o guarantee t h a t t h e y w i l l keep s u f f i c i e n t conduct i v i t y t o e n a b l e gas f l o w d u r i n g subsequent e x p l o i t a t i o n . A r e l a t i v e l y f l a t p r e s s u r e c u r v e d u r i n g i n j e c t i o n i s a c h a r a c t e r i s t i c f e a t u r e o f dual l e a k o f f beh a v i o u r . Low c o n c e n t r a t i o n s o f 100 mesh sand t h a t a r e added t o b o t h pad and p r o p p a n t - l a d e n stages t h r o u g h o u t t h e t r e a t m e n t and w h i c h have t o be k e p t l i m i t e d i n o r d e r t o n o t s e r i o u s l y d e s t r o y i n g proppant c o n d u c t i v i t y , however, can o n l y r e a s o n a b l y r e t a r d a c c e l e r a t e d f l u i d l o s s and a r e by no means a b l e t o s t o p i t e n t i r e l y , w i t h l e a k o f f r a t e s s t i l l b e i n g v e r y h i g h (WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE 1988). On t h e o t h e r hand, an e x c l u s i v e a d d i t i o n o f 100 mesh sand d u r i n g t h e pad stage may n o t p e r m i t t o b r i n g i n and t o d e p o s i t enough m a t e r i a l t o p e r f e c t l y s e a l t h e h a i r l i n e c r a c k s such t h a t t h e y a r e n o t c o n t r i b u t i n g t o f l u i d l e a k o f f d u r i n g t h e f o l l o w i n g p r o p p a n t - l a d e n stages. The a c c e l e r a t e d secondary l o s s may s t a r t a t 50 t i m e s t h e b a s e - l e v e l l e a k o f f r a t e and can be reduced t o 20 t i m e s s h o r t l y a f t e r 100 mesh sand e n t e r s t h e p e r f o r a t i o n s and t o 5 t i m e s by t r e a t m e n t end p r o v i d e d c o n t i n u o u s i n s e r t i o n o f small amounts o f 100 mesh sand, w h i c h i s c e r t a i n l y a s i g n i f i c a n t r e t a r d a t i o n o f f l u i d escape i n t o t h e r e s e r v o i r t h r o u g h t h i e f h a i r l i n e c r a c k s ( c f . a l s o s e c t i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.). I n ord e r t o accept p r o p p i n g agents, f r a c t u r e w i d t h must be i n t h e range o f 2 . 5 t i m e s the proppant diameter, w i t h t h i s c o n d i t i o n being f u l f i l l e d f o r h y d r a u l i c a l l y i n duced f r a c t u r e s by s t a n d a r d p r o p p a n t g r a i n s i z e s (16/20 and 20/40), w h i l e 100 mesh m i c r o p r o p p a n t s f i t i n t o h a i r l i n e c r a c k s and f i s s u r e s (THOMPSON 1977; c f . section 1.4.11.2.1.).
4.3.4.6.2.2.3.Accelerated leakoff and screenout I n n a t u r a l l y f r a c t u r e d r e s e r v o i r s w i t h dual l e a k o f f b e h a v i o u r , s o l i d f l u i d l o s s a d d i t i v e s a r e i n d i s p e n s i b l e , because o t h e r w i s e e a r l y screenouts i m p a i r t h e t r e a t m e n t s b y r e s u l t i n g i n s h o r t h y d r a u l i c f r a c t u r e s , h i g h p r e s s u r e s and n a t u r a l c r a c k damage. I f c o n t r o l o f a c c e l e r a t e d o r secondary l e a k o f f i s o n l y p a r t i a l due t o necessary l i m i t a t i o n o f 100 mesh sand c o n c e n t r a t i o n f o r t h e purpose o f p r o p p a n t c o n d u c t i v i t y p r e s e r v a t i o n , c o n s i d e r a b l e f l u i d l o s s s t i l l o c c u r s and s c r e e n o u t i s imminent a t s h u t - i n as r e v e a l e d by h i s t o r y match r e s u l t s . I n such cases, o n l y r e s t r i c t e d amounts and l i m i t e d s a t u r a t i o n s o f proppants can be i n j e c t e d i n t o t h e r e s e r v o i r u n l e s s a more e f f i c i e n t means o f f l u i d l e a k o f f cont r o l can be discovered, because on t h e o t h e r hand a d d i t i v e s t h a t p l u g t h e n a t u r a l f r a c t u r e s and a r e n o t r e a d i l y produced back a r e unacceptable s i n c e r e a s o n a b l e w i t h d r a w a l r a t e s a r e o n l y achieved t h r o u g h t h e n a t u r a l f r a c t u r e system. A c c e l e r a t e d l e a k o f f m o d e l l i n g p r o v i d e s a l s o a p l a u s i b l e damage mechanism f o r foams when l i t t l e g e l and s m a l l l i q u i d volumes a r e i n j e c t e d (WARPINSKI 1988 a; WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE 1988; c f . s e c t i o n s I f h i g h f l u i d - l o s s r a t e s o c c u r due t o opening 4.8.8.3.1.2. and 4 . 8 . 8 . 3 . 2 . 3 . ) . o f n a t u r a l f r a c t u r e s w i t h i n t h e r e s e r v o i r r o c k s , l e a k o f f o f t h e t o t a l foam system i n t o t h e c r a c k s has t o be expected. D u r i n g p o s t - f r a c t u r e cleanup, t h e n a t u r a l c r a c k s c l o s e and p r o d u c t i o n o f t h e l i q u i d s t r a p p e d w i t h i n t h e j o i n t s would be n e a r l y i m p o s s i b l e , w i t h g e l and l i q u i d s w i t h i n t h e n a t u r a l f r a c t u r e s t h e n
457 c o n s i d e r a b l y damaging r e s e r v o i r p e r m e a b i l i t y . Thus b r e a k e r pre-pads s h o u l d be used i n o r d e r t o f a c i l i t a t e cleanup o f any g e l s t h a t a r e i n j e c t e d p a s t t h e 100 mesh sand b r i d g e s w i t h i n t h e h a i r l i n e cracks, b u t t h e main c o n t r o l on damage i s t h e e f f o r t t o m i n i m i z e l e a k o f f volumes, w i t h s t i m u l a t i o n and p r o d u c t i o n r e s u l t s suggesting t h a t t h i s i s a successful strategy.
4.3.4.6.2.2.4. Fine sand vs. bauxite F i n e s o l i d p a r t i c l e s o f sand and b a u x i t e a r e e f f e c t i v e l e a k o f f a d d i t i v e s i f t h e s t r e n g t h c o m p a t i b i l i t y between f i n e p a r t i c l e s and proppants i s c o n s i d e r e d t o a v o i d damage caused by l o w - s t r e n g t h g r a i n s ( L I & ZHU 1986). The amount o f f i n e f l u i d - l o s s p a r t i c l e s s h o u l d g e n e r a l l y n o t exceed 10 % o f t h e t o t a l w e i g h t o f t h e proppants because damage o f t h e propped zone by t h e f i n e s o l i d s i n c r e a ses s h a r p l y w i t h r i s i n g amount o f f i n e s . The damage c r e a t e d by h i g h - s t r e n g t h b a u x i t e i s l e s s t h a n t h a t generated by l o w - s t r e n g t h sand. HUDSON, COFFEY, SAUER & TEOT (1982) d i s c u s s n o n - s o l i d f l u i d - l o s s c o n t r o l agents and t h e i r p e r f o r mance.
4.3.4.6.2.3. Foams and resins O i l foams a r e a reasonable a l t e r n a t i v e f o r f l u i d - l o s s c o n t r o l i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s (NORTON & HOFFMAN 1982). F l u i d l e a k o f f r a t e s o f o i l foams a r e v e r y low w h i c h reduce t h e need t o use o t h e r f l u i d - l o s s a d d i t i v e s e x c e p t f o r n a t u r a l l y f r a c t u r e d pay zones where some a d d i t i v e s f u r t h e r s u p p o r t f l u i d l e a k o f f c o n t r o l . Aspects o f o i l foam f r a c t u r i n g a r e a l s o d i s c u s s e d by DRISCOLL, BOWEN & ROBERTS (1980) and GAYELOS & H A R R I S (1980). O i l - s o l u b l e r e s i n s a r e s u i t a b l e f l u i d - l o s s a d d i t i v e s when t h e y a r e p r o p e r l y s i z e d and have a h i g h enough s o f t e n i n g p o i n t e n a b l i n g them t o b r i d g e p o r e spaces, because t h e y can subseq u e n t l y be d i s s o l v e d i n t h e produced hydrocarbons and thus do n o t provoke any f o r m a t i o n o r proppant pack damage (ECONOMIDES 1986). O i l - i n - w a t e r emulsions cont a i n i n g l i t t l e amounts o f d i e s e l produce two-phase f l o w upon f l u i d l o s s w h i c h d r a s t i c a l l y reduces l e a k o f f .
4.3.4.6.2.4. Inpact o f fracture geometry and morphology G e o l o g i c a l d i s c o n t i n u i t i e s may c o n s i d e r a b l y a l t e r f r a c t u r e geometry ( c f . sect i o n 4.2.3.4.) and t h u s t h e i r e f f e c t on p r o p p a n t t r a n s p o r t and f l u i d l e a k o f f may be s e r i o u s (WARPINSKI & TEUFEL 1984). F o r c i n g proppants t h r o u g h such f e a t u r e s as narrow m u l t i s t r a n d f r a c t u r e s o r o f f s e t s i s much more d i f f i c u l t t h a n through a s i n g l e h y d r a u l i c crack, w i t h t h e consequence b e i n g h i g h e r f r a c t u r i n g p r e s s u r e and f r e q u e n t occurrence o f screenouts. I f l o c a l s m a l l screenouts t a k e p l a c e i n s e v e r a l branches o f a m u l t i p l e f r a c t u r e system, l i t t l e a d d i t i o n a l c r a c k growth i s s t i l l p o s s i b l e u n t i l t h e abundance o f these e v e n t s becomes t o o l a r g e and u l t i m a t e l y provokes a complete screenout o f t h e h y d r a u l i c f r a c t u r e system ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) . T h e r e f o r e f l u i d l e a k o f f c o e f f i c i e n t s may change d u r i n g course o f a t r e a t m e n t as a c o m p l i c a t e d c r a c k p a t t e r n develops. Successful h y d r a u l i c p r o p p a n t f r a c t u r i n g a l s o r e q u i r e s c a r e f u l e v a l u a t i o n o f t h e main r e s e r v o i r r o c k t y p e s i n heterogeneous f o r m a t i o n s , w i t h t h e c l a s s i f i c a t i o n o f f l u i d c o m p a t i b i l i t y a i m i n g on e l i m i n a t i o n o f p o s s i b l e f a i l u r e reasons due t o e x c e s s i v e f l u i d s e n s i t i v i t y o f some l a y e r s o f t h e pay s t o r e y complex ( S I MON, KAUL & CULBERTSON 1977) w i t h l e a k o f f c h a r a c t e r i s t i c s a g a i n b e i n g one o f t h e most i m p o r t a n t parameters. A summary o f f r a c t u r i n g f l u i d l o s s i s g i v e n by POULSEN ( 1 9 8 8 ) .
458
4.3.4.6.3.
Mushroom-type f r a c t u r e s and f l u i d e f f i c i e n c y
A s p e c i a l f l u i d - l o s s t r e a t m e n t i s n e c e s s a r y in c a s e o f mushroom-type f r a c t u r e s c o n s i s t i n g o f a v e r t i c a l c r a c k topped b y a h o r i z o n t a l f r a c t u r e l i k e a cap i n t r a n s i t i o n a l d e p t h r a n g e s o r when i n j e c t i o n r a t e s e x c e e d t h e s t r e s s r e q u i r e d t o l i f t t h e o v e r b u r d e n (FITZ-PATRICK, KARR & O'SHEA 1985; c f . s e c t i o n s 1 . 2 . 8 . 4 . , 4 . 9 . 4 . 2 . and 6 . 2 . 2 . 4 . ) . F l u i d - l o s s r e p l a c e m e n t t o t h e h o r i z o n t a l f r a c t u r e component and m a x i m i z a t i o n o f t h e e x t e n s i o n o f t h e v e r t i c a l f r a c t u r e compon e n t c a n be a c h i e v e d b y i n c r e a s i n g t h e pad s i z e t o a l m o s t 100 % and a d d i t i o n o f a j u n k sand s t a g e d e s i g n e d t o b r i d g e o f f t h e e n t r a n c e o f t h e n a r r o w h o r i z o n t a l fracture, thereby f o r c i n g the t r a i l i n g proppant t o stay i n the v e r t i c a l f r a c t u r e . The p a d i s g e n e r a l l y a more o r l e s s s a c r i f i c i a l f l u i d q u a n t i t y w h i c h m i g h t l e a k o f f and t h u s s a t u r a t e t h e p r i m a r y and s e c o n d a r y p o r o s i t y o f t h e r e s e r v o i r formation i n order t o prevent excessive loss o f c a r r i e r f l u i d which w o u l d p r o v o k e p r e m a t u r e s c r e e n o u t f a i l u r e (SMITH 1987; c f . s e c t i o n s 4 . 2 . 3 . 2 . 5 . and 6 . 2 . 4 . 2 . 1 . ) , and a l s o s e r v e s f o r c r e a t i o n o f s u f f i c i e n t f r a c t u r e w i d t h ( c f . section 4.3.4.4.1.2.). As f l u i d l o s s i s p a r t i c u l a r l y h i g h a t t h e c r a c k t i p , the p r o p p a n t - l a d e n s l u r r y c a t c h e s u p w i t h t h e f r a c t u r e t i p where i t i d e a l l y s h o u l d Useful inhave a r r i v e d a t t h e end o f t h e t r e a t m e n t ( c f . s e c t i o n 4 . 3 . 4 . 6 . 2 . 2 . ) . f o r m a t i o n a b o u t f l u i d l o s s i s g i v e n b y t h e p r e s s u r e d e c l i n e p a t t e r n ( c f . sect i o n 6.2.4.2.).
F l u i d e f f i c i e n c y i s t h e f r a c t i o n o f t h e i n j e c t e d volume t h a t i s i n t h e c r a c k a t t h e t i m e i n j e c t i o n s t o p s , and t h e r e m a i n i n g f r a c t i o n i s l o s t t o t h e f o r m a t i o n b y f l u i d l o s s (NOLTE 1984 a ) . Because f l u i d e f f i c i e n c y c a n b e d e t e r m i n e d f o r a s t i m u l a t i o n t r e a t m e n t i n d e p e n d e n t o f any f r a c t u r e model, t h e p r o s p e c t a r i ses t h a t t h e e f f i c i e n c y f o r a c a l i b r a t i o n o p e r a t i o n s h o u l d be u s e d t o d e s i g n a s u b s e q u e n t s t i m u l a t i o n j o b i n c l u d i n g pad s i z e and optimum p r o p p a n t s c h e d u l e comp l e t e l y w i t h o u t t h e need f o r a model and such a s s o c i a t e d p a r a m e t e r s as h e i g h t , l o s s c o e f f i c i e n t and v i s c o s i t y . T h i s w o u l d be e s p e c i a l l y d e s i r a b l e f o r t r e a t ments i n new p r o s p e c t a r e a s where l i t t l e i f any i n f o r m a t i o n may b e a v a i l a b l e t o d e t e r m i n e w h a t model o r a s s o c i a t e d p a r a m e t e r s a r e a p p r o p r i a t e . F l u i d e f f i c i e n c y c a n be d e t e r m i n e d f r o m p r e s s u r e - d e c l i n e a n a l y s i s f o r a m i n i f r a c t u r e c a l i b r a t i o n treatment performed b e f o r e t h e actual s t i m u l a t i o n j o b which provides i n f e r r e d v a l u e s o f f l u i d - l o s s c o e f f i c i e n t and f r a c t u r e w i d t h and o e n e t r a t i o n .
4.3.4.6.4.
Combined f l u i d - l o s s c o n t r o l a d d i t i v e and g e l breaker
CANTU & BOYD ( 1 9 8 8 ) p r e s e n t a n o r g a n i c a c i d c o n d e n s a t i o n p r o d u c t b e i n g a p o l y e s t e r as a combined f l u i d - l o s s c o n t r o l a d d i t i v e and g e l b r e a k e r f o r f r a c t u r i n g f l u i d s . The o r g a n i c e s t e r exposes i n c r e a s i n g a c i d f u n c t i o n a l g r o u p s when d e g r a d e d t o s o l u b l e monomers a t t e m p e r a t u r e s above 150 OF f o l l o w i n g t h e f r a c t u r i n g s t i m u l a t i o n t r e a t m e n t , and t h e r e f o r e t h e d e g r a d e d a d d i t i v e imposes o n l y m i n i m a l f o r m a t i o n damage. Some a s p e c t s o f p r o p p a n t p e r m e a b i l i t y r e t a i n m e n t due t o s e c o n d a r y f i l t e r cake d i s s o l u t i o n , p o l y e s t e r c o n c e n t r a t i o n and f i l t e r c a k e r e s o r p t i o n , and d e g r a d a b l e p o l y e s t e r v s . 100 mesh sand a r e d i s c u s s e d as f o l l o w s .
4.3.4.6.4.1. Proppant permeabi 1i t y retainment due t o secondary f i l t e r cake d i s s o l u t i o n The h i g h - a c i d - c o n t e n t d e g r a d a t i o n p r o d u c t b r e a k s t h e l i n e a r a n d / o r c r o s s l i n k e d g e l u s e d i n t h e f r a c t u r i n g f l u i d and e f f e c t i v e l y c l e a n s t h e p r o p p a n t p a c k age w h i c h s i g n i f i c a n t l y i n c r e a s e s f r a c t u r e c o n d u c t i v i t y , w i t h t h e p o l y m e r b r e a k down b e i n g m a i n l y a consequence o f d e c r e a s i n g pH caused b y t h e d e g r a d e d p a r t s o f t h e o r g a n i c a c i d p a r t i c u l a t e s and t a k i n g p l a c e b y backbone c l e a v a g e (TYSSEE & VETTER 1 9 8 1 ) . T h e r e f o r e s e c o n d a r y f i l t e r c a k e d i s s o l u t i o n r e i n s t a l l s p r i m a r y proppant permeability which thus i s r e t a i n e d i n the l a t e r stage o f t h e t r e a t m e n t i n c o n t r a s t t o c o n v e n t i o n a l o p e r a t i o n s where g e l f i l t e r c a k e p r e c i p i t a t i o n d e s t r o y s p r o p p a n t pack p e r m e a b i l i t y b y embedment. Replacement o f t h e common s i l i c a f l o u r as f l u i d - l o s s a d d i t i v e b y t h e p o l y e s t e r c a n r e s u l t i n more t h a n
459 200 % i n c r e a s e i n f r a c t u r e c o n d u c t i v i t y and p e r m e a b i l i t y (CANTU & BOYD 1988) and i n l e a k o f f c o n t r o l performance as good as o r b e t t e r than w i t h s i l i c a f l o u r i n c r o s s l i n k e d and l i n e a r g e l s as w e l l as n i t r o g e n foams. The p a r t i c u l a t e p o l y e s t e r i s c o n c e n t r a t e d and r e t a i n e d i n t h e g e l f i l t e r cake as t h e f l u i d s l e a k s o f f i n t o t h e f o r m a t i o n , w i t h f i l t e r cake cleanup t h u s b e i n g much more e f f i c i e n t w i t h a combined f l u i d - l o s s c o n t r o l a d d i t i v e and g e l breaker, because c o n v e n t i o n a l w a t e r - s o l u b l e g e l b r e a k e r s l e a k o f f i n t o t h e r e s e r v o i r m a t r i x more e a s i l y . The s i g n i f i c a n c e o f f l u i d - l o s s c o n t r o l i s u n d e r l i ned by t h e f a c t t h a t f r a c t u r i n g f l u i d s must have a minimum l e a k o f f w h i l e t r a n s p o r t i n g proppants i n o r d e r t o achieve an e f f e c t i v e t r e a t m e n t . F l u i d s c o n t a i n i n g p o l y e s t e r a d d i t i v e s can r e t a i n a l l o f t h e i n i t i a l proppant p e r m e a b i l i t y , whereas t h e s i l i c a f l o u r g e l b r e a k e r system may o n l y l e a v e 10 - 15 % o f o r i g i n a l p r o p p a n t p e r m e a b i l i t y even w i t h excess b r e a k e r c o n c e n t r a t i o n i n t h e s l u r r y volume ( c f . s e c t i o n s 1.4.10. and 4 . 1 2 . 1 . 1 . ) . This i n d i c a t e s t h a t i n o r d e r t o e f f e c t i v e l y break r e s i d u a l g e l s i n f r a c t u r e systems, i t i s i m p o r t a n t t o p l a c e t h e g e l b r e a k e r s p r o p e r l y i n t h e f i l t e r cake. C o n v e n t i o n a l s i l i c a f l o u r / g e l b r e a k e r systems can o n l y approach t h e e f f e c t i v i t y o f t h e newly i n t r o d u c e d p o l y e s t e r a d d i t i v e i f a b t . 5 % d i e s e l i s added t o t h e m i x t u r e , b u t i n any case t h e p o l y e s t e r i s s t i l l s l i g h t l y b e t t e r p e r f o r m i n g . A d d i t i o n o f 5 % d i e s e l t o t h e p o l y e s t e r m i x t u r e , however, d r a s t i c a l l y reduces and d e l a y s a c t i v i t y and e f f e c t i v i t y o f t h e l a t t e r .
4.3.4.6.4.2.
Polyester concentration and filter cake resorption
The p o l y e s t e r c o n c e n t r a t i o n i n t h e f i l t e r cake i s a l s o i m p o r t a n t i n g e l b r e a k i n g as evidenced by i n c r e a s i n g proppant c o n d u c t i v i t y w i t h h i g h e r p o l y e s t e r s a t u r a t i o n (CANTU & BOYD 1988). The h i g h e r t h e p o l y e s t e r c o n c e n t r a t i o n , t h e more a c i d i s r e l e a s e d as t h e p a r t i c l e s degrade t o s o l u b l e monomers, and w i t h dec r e a s i n g pH o f t h e f i l t e r cake, polymer breakdown l e a d i n g t o f i l t e r cake r e s o r p t i o n begins. The f l u i d b u f f e r i n g c a p a c i t y a l s o determines t h e r a t e a t w h i c h g e l b r e a k i n g t a k e s p l a c e . I f t h e f l u i d s u r r o u n d i n g t h e p o l y e s t e r p a r t i c u l a t e s has a h i g h b u f f e r i n g c a p a c i t y , t h e pH change w i l l be l i m i t e d due t o consumption o f t h e s o l u b l e a c i d by t h e b u f f e r and t h u s f i l t e r cake cleanup i s s l o w e r . Proper f l u i d c o n d i t i o n i n g and optimum p o l y e s t e r c o n c e n t r a t i o n g i v e s r i s e t o complete cleanup o f t h e f i l t e r cake by b r e a k i n g i n t o s o l u b l e monomers, and no f i l t e r cake l a y e r i s l e f t i n t o which p r o p p a n t embedment c o u l d t a k e p l a c e . T h i s removal o f t h e f i l t e r cake i n an advanced s t a g e o f t h e t r e a t m e n t when i t has s u c c e s s f u l l y f u l f i l l e d i t s t a s k t o l i m i t f l u i d l o s s and i s no l o n g e r needed i s a v e r y e f f e c t i v e mechanism t o l i q u i d a t e t h e f o r m a t i o n damage e f f e c t which t h e f i l t e r cake had provoked so f a r and t o l e a v e t h e f r a c t u r e w a l l s c o m p l e t e l y uni m p a i r e d t h a t a l s o does n o t c r e a t e any r e m a i n i n g damage o f t h e p r o p p a n t package and t h e r e f o r e guarantees maximum f r a c t u r e c o n d u c t i v i t y ( c f . a l s o s e c t i o n 1.4.10.3.5.4.).
4.3.4.6.4.3. Degradable polyester vs. 100 mesh sand Gel b r e a k i n g and f i l t e r cake cleanup by t h e p o l y e s t e r a r e a f u n c t i o n o f temp e r a t u r e , c o n c e n t r a t i o n and f l u i d pH. F l u i d - l o s s c o n t r o l by n o n - i o n i c degrad a b l e p l a t e l e t a d d i t i v e s i s d e s c r i b e d by NORTHCUTT, ROBERTSON & HANNAH ( 1 9 8 8 ) . Such degradable f l u i d - l e a k o f f a d d i t i v e s which l a t e r a r e c o m p l e t e l y d i s s o l v e d o r d i s i n t e g r a t e d and removed c o u l d a l s o be a good approach f o r d u a l f l u i d - l o s s control i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s (WARPINSKI 1988 a; c f . s e c t i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.) because i n c o n t r a s t t o 100 mesh sand, t h e y do n o t decrease p r o p p a n t c o n d u c t i v i t y ( c f . s e c t i o n 1.4.11.2.1.). On t h e o t h e r hand, however, absence o f m i c r o p r o p p a n t s p l u g g i n g t h e h a i r l i n e c r a c k s c o u l d r e s u l t i n t h e i r c l o s u r e w i t h d e c l i n i n g p r e s s u r e and t h u s no l o n g e r s a t i s f a c t o r y c o n t r i b u t i o n t o gas p r o d u c t i o n .
460 T h e r e f o r e i t i s d e s i r a b l e i n deep and h i g h - p r e s s u r e r e s e r v o i r s t o i n s e r t 100 mesh m i c r o p r o p p a n t s i n t o t h e n a t u r a l j o i n t l e t s i n o r d e r t o s e c u r e f l o w o f gas t h r o u g h t h e f i s s u r e s i n t o t h e b o r e h o l e , w i t h an improvement i n p a y zones w i t h h i g h c l o s u r e s t r e s s b e i n g a p p l i c a t i o n o f 100 mesh i n t e r m e d i a t e - t o h i g h s t r e n g t h s y n t h e t i c p r o p p a n t s once t h i s g r a i n s i z e becomes c o m m e r c i a l l y a v a i An a t t r a c l a b l e (NORTHCUTT, ROBERTSON & HANNAH 1988; c f . s e c t i o n 4 . 8 . 8 . 3 . 2 . 3 . ) . t i v e i n t e r m e d i a t e s o l u t i o n w o u l d be r e s i n - c o a t e d 100 mesh sand w h i c h s i m i l a r l y as c o n v e n t i o n a l 100 mesh sand has a b t . 5 - 7 d a r c y p e r m e a b i l i t y a t a l m o s t any c l o s u r e s t r e s s , w i t h a g g r e g a t i o n o f r e s i n f i l m s l o c k i n g t h e g r a i n package i n p l a c e i f m i c r o p r o p p a n t f l o w b a c k f r o m h a i r l i n e c r a c k s i s u n a c c e p t a b l e (FRACFAX 1988 f ) . VARGAS & OHLSON ( 1 9 8 6 ) d i s c u s s s u i t a b l e b r e a k e r s f o r g r a v e l - p a c k completion fluids.
4.3.5. Ultra-high fracture conductivity achievement U l t r a - h i g h f r a c t u r e c o n d u c t i v i t i e s c a n be a c h i e v e d w i t h a c o m b i n a t i o n o f v e r y h i g h p r o p p a n t s a t u r a t i o n s and e x t r a l a r g e p r o p p a n t g r a i n s i z e s (BARBY & BARBEE 1987) o r b y m i x i n g o f a n g u l a r and r o u n d p r o p p a n t g r a i n s (LARSEN & SMITH 1985).
4.3.5.1. Extraordinarily large proppant grain size The h i g h e s t p o s s i b l e c o n d u c t i v i t y i s p a r t i c u l a r l y e s s e n t i a l t o be r e a c h e d i n high-permeability r e s e r v o i r s ( c f . s e c t i o n 4.6.1.). Large proppant diameters o f 8 / 1 2 o r 6 / 1 0 r e s u l t i n n e g l i g i b l e embedment e v e n i n h a r d , w e l l - c e m e n t e d r e s e r v o i r s . The d a n g e r o f w e l l b o r e s c r e e n o u t f a i l u r e i n such j o b s i s p a r t i c u l a r l y h i g h i f f r a c t u r e w i d t h i s n o t l a r g e enough t o a c c e p t t h e h i g h c o n c e n t r a t i o n s and l a r g e s i z e s o f p r o p p a n t s , and i n o r d e r t o p e r f o r m a smooth t r e a t m e n t , o f t e n h i g h e r pumping r a t e s a r e r e q u i r e d , w i t h t h e n c a u t i o n h a v i n g t o be e x e r c i s e d t h a t e x c e s s i v e f r a c t u r e h e i g h t g r o w t h does n o t o c c u r ( c f . s e c t i o n s 4 . 2 . 3 . 2 . 5 . and 6 . 2 . 4 . 2 . 1 . ) . O t h e r o p e r a t i o n a l drawbacks a r e t h a t t h e consequences o f mechan i c a l p r o b l e m s such as e q u i p m e n t f a i l u r e o r e n g i n e e r i n g e r r o r s i n t h e f r a c t u r e d e s i g n a r e u s u a l l y u n f o r g i v i n g and i r r e v e r s i b l e i n t h e s e j o b s r e q u i r i n g a h i g h l e v e l and s t a n d a r d (BARBY & BARBEE 1 9 8 7 ) . The g a i n i n f l u i d d e n s i t y w i t h i n c r e a s e d p r o p p a n t c o n c e n t r a t i o n s h o u l d cause a s i g n i f i c a n t n e t r e d u c t i o n i n pumping p r e s s u r e , w i t h t h e absence o f p r e s s u r e decrease thus e a r l y i n d i c a t i , n g a premature screenout. I n terms o f proppant f l o w back, t h e l a r g e r t h e p r o p p a n t g r a i n s i z e , t h e more s t a b l e i s t h e package i n t h e f r a c t u r e and p r o p p a n t back p r o d u c t i o n i s r e d u c e d u n l e s s c o a r s e a b r a s i v e p r o p p a n t s e r o d e t h e f r a c t u r e f a c e a t t h e w e l l b o r e upon e n t r y o r l a r g e - v o l u m e a c i d i z i n g a n n i h i l a t e s t h e f r a c t u r e mouth. Some a d v a n t a g e s and drawbacks o f u l t r a h i g h f r a c t u r e c o n d u c t i v i t y achievement w i t h e x t r a o r d i n a r i l y l a r g e proppant g r a i n s i z e s a r e summarized as f o l l o w s .
4.3.5.1.1. Advantages I n terms o f c o n d u c t i v i t y maximization, 6/10 i n t e r m e d i a t e - s t r e n g t h proppants a t 1 l b / f t 2 ( 5 kg/m2; c f . s e c t i o n 4 . 3 . 1 . 1 . ) c o n c e n t r a t i o n d i s p l a y g r e a t e r c o n d u c t i v i t y t h a n 8 / 1 2 p a r t i c l e s a t 2 l b / f t 2 ( 1 0 kg/m2) s a t u r a t i o n , w i t h t h u s o n l y h a l f t h e volume o f 6 / 1 0 p r o p p a n t s b e i n g needed t o a c h i e v e more c o n d u c t i v i t y t h a n w i t h 8 / 1 2 p r o p p a n t s (BARBY & BARBEE 1 9 8 7 ) . The l o w e r c o n c e n t r a t i o n o f 6 / 1 0 p r o p p a n t s w o u l d be e a s i e r o n t h e pumping e q u i p m e n t , l e a v e l e s s p r o p p a n t i n t h e f r a c t u r e mouth n e a r t h e w e l l b o r e t h a t c o u l d be s u b j e c t e d t o f l o w b a c k , and d e l i ver the equivalent production r a t e a t lower costs. A general g u i d e l i n e f o r f r a c t u r e design i s t h a t i f a f o r m a t i o n can accept h i g h e r s a t u r a t i o n s o f s m a l l e r proppants, i t i s a l s o p o s s i b l e t o place lower concentrations o f l a r g e r propp a n t s . Examples o f f i e l d a p p l i c a t i o n o f 8 / 1 2 mesh p r o p p a n t s i n o i l r e s e r v o i r s
461 where b o t h l o n g l o w - c o n d u c t i v i t y and s h o r t h i g h - c o n d u c t i v i t y been c r e a t e d a r e d e s c r i b e d by BRITT & LARSEN (1986).
fractures
have
F r a c t u r e f l o w c a p a c i t y and t h u s p r o p p a n t c o n d u c t i v i t y l a y o u t s h o u l d be done based on t h e a p p l i c a t i o n o f a c o n d u c t i v i t y c o r r e c t i o n f a c t o r o f 0.5 i n o r d e r t o account f o r t h e performance d i f f e r e n c e o f proppants i n l a b o r a t o r y and f i e l d (POULSEN & SOLIMAN 1987; c f . s e c t i o n 1.4.10.3.). Improvement o f f r a c t u r e f l o w c a p a c i t y can be achieved by i n c r e a s i n g t h e maximum a l l o w a b l e p r o p p a n t c o n c e n t r a t i o n and/or by choosing a h i g h e r c o n d u c t i v i t y p r o p p a n t t y p e and/or g r a i n s i z e . F a i l u r e o f r e c o g n i t i o n o f t h e d i f f e r e n c e s between apparent and e f f e c t i v e propp a n t c o n d u c t i v i t y r e s u l t i n underdesign o f t h e f r a c t u r i n g t r e a t m e n t . L a r g e r g r a i n s i z e s o f h i g h e r - q u a l i t y p r o p p a n t t y p e s p r o v i d e g r e a t e r f r a c t u r e f l o w capac i t y t h a t would n o t be as a f f e c t e d by f o r m a t i o n f i n e s which a r e r e l e a s e d as s m a l l e r l o w e r - q u a l i t y proppants a r e c r u s h i n g due t o i n c r e a s i n g c l o s u r e p r e s s u r e (CROSS & SHELDON 1977).
4.3.5.1.2.
Drawbacks
Drawbacks o f e x t r a o r d i n a r i l y l a r g e p r o p p a n t g r a i n s i z e s a r e c r u s h i n g and embedment ( c f . s e c t i o n 4.3.5.1.). Closure stress a f f e c t s f i r s t coarser grains o f a g i v e n t y p e o f proppants due t o t h e l a r g e r s u r f a c e o f a t t a c k and t h u s c r u s h i n g o f l a r g e r p a r t i c l e s b e g i n s e a r l i e r as compared t o s m a l l e r g r a i n s , and embedment i n s o f t f o r m a t i o n s a l s o s t a r t s w i t h c o a r s e r p a r t i c l e s due t o t h e i r l o w e r q u a n t i t y and l e s s dense package w i t h r e s p e c t t o f i n e r g r a i n s . N e t p r e s e n t v a l u e ( b e i n g d e f i n e d as e x t r a hydrocarbon revenue by s t i m u l a t i o n minus t r e a t m e n t c o s t ; c f . s e c t i o n s 1.4.12.1. and 4.8.11.4.) r i s e s w i t h increasing proppant g r a i n s i z e , p r o p p a n t c o n c e n t r a t i o n and f r a c t u r e h a l f - l e n g t h . L a r g e r p r o p p a n t g r a i n s i z e , however, r e q u i r e s m u l t i l a y e r p r o p p i n g i n o r d e r t o s u f f i c i e n t l y decrease d e t e r i o r a t i o n by embedment. I n narrower f r a c t u r e s , a b e t t e r package p e r m e a b i l i t y can be achieved w i t h s m a l l e r p r o p p a n t g r a i n s . O t h e r d i f f i c u l t i e s w i t h l a r g e - d i a m e t e r p r o p p a n t g r a i n s a r e p a r t i c l e b r i d g i n g i n p e r f o r a t i o n s and f r a c t u r e r e s u l t i n g i n s c r e e n o u t f a i l u r e o f t h e j o b ( c f . s e c t i o n s 5.6. and 6 . 2 . 4 . 2 . 1 . ) . NIEMEYER & REINART (1986) document comparative j o b d e s i g n e v a l u a t i o n which e x h i b i t s s i g n i f i c a n t l y h i g h e r s c r e e n o u t r a t e s f o r 12/20 mesh w i t h r e s p e c t t o 20/40 mesh proppants under t h e same w e l l c o n d i t i o n s . An o p e r a t i o n a l drawback can o c c u r i n s o f t u n c o n s o l i d a t e d sands, because t a i l s o f 8/16 o r 10/20 mesh proppants f o l l o w i n g pumping o f 20/40 mesh m a t e r i a l m i g h t t o l a r g e amounts remain w i t h i n t h e w e l l b o r e . The s i t u a t i o n o f coarse t a i l - i n l o t s b e i n g l e f t i n t h e c a s i n g can be avoided by i n c r e a s i n g t h e share o f t h e t a i l i n t h e whole t r e a t m e n t q u a n t i t y (LAMBERT, DOLAN E GALLUS 1983; c f . sect i o n 2.4.1.2.3.).
4.3.5.2.
Mixing o f round and angular proppant grains
M a x i m i z a t i o n o f f o r m a t i o n - f r a c t u r e c o n d u c t i v i t y c o n t r a s t and f r a c t u r e f l o w c a p a c i t y can f u r t h e r be done by m i x i n g o f round and a n g u l a r p r o p p a n t o r sand g r a i n s , because t h e i n c r e a s e d v o i d space o f a r o u n d - a n g u l a r g r a i n b l e n d enhances t h e performance o f t h e c o m b i n a t i o n w i t h r e s p e c t t o t h a t o f t h e i n g r e d i e n t s a l o n e (LARSEN & SMITH 1985). L a b o r a t o r y experiments on 20/40 sand have r e v e a l e d t h a t o f a l l t h e m i x t u r e s t e s t e d , a 50 : 50 round-angular b l e n d has on t h e average t h e h i g h e s t r e l a t i v e f r a c t u r e c o n d u c t i v i t y t h r o u g h o u t a l l c l o s u r e s t r e s s l e v e l s up t o 6,000 p s i ( c f . s e c t i o n 1.4.11.3.). Therefore angular proppant g r a i n s i n t e r s p e r s e d i n t o round g r a i n p o p u l a t i o n s would i n c r e a s e s t a b i l i t y o f t h e package and improve t h e c o n d u c t i v i t y o f t h e proppant wedge w i t h i n t h e f r a c ture. O t h e r p o s s i b i l i t i e s o f maximizing f r a c t u r e c o n d u c t i v i t y a r e p r o p p a n t banking and p r o p p a n t p i l l a r i n g i n c l u d i n g c r e a t i o n o f h o r i z o n t a l and v e r t i c a l open f l o w channels within the fracture, r e s p e c t i v e l y ( c f . s e c t i o n s 4.2.2.4. and
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4.3.2.2.).
PAULS, VENDITTO, CHISHOLM, HOLTMYER & GREGORCZYK ( 1 9 8 5 ) i l l u s t r a t e t h e b e n e f i c i a l e f f e c t o f a c o m b i n a t i o n o f m a x i m i z i n g f r a c t u r e l e n g t h and p r o p p a n t c o n c e n t r a t i o n . The l o n g c r a c k s c o n t a c t a d d i t i o n a l r e s e r v e s and t h e h i g h proppant s a t u r a t i o n s n o t o n l y produce h i g h l y - c o n d u c t i v e f r a c t u r e s , b u t a l s o s e t t l e d bank h e i g h t s t h a t a r e c a p a b l e o f i n f i l l i n g t h e c r a c k b o t h b e l o w and through the r e s e r v o i r .
4.3.6.
Uniform vs. changing fracture conductivity
SOLIMAN ( 1 9 8 6 b ) e v a l u a t e s t h e p e r f o r m a n c e e f f e c t i v i t y o f u n i f o r m and chang i n g c o n d u c t i v i t y f r a c t u r e s and c o n c l u d e s t h a t a c h a n g i n g c o n d u c t i v i t y c r a c k behaves i n many r e s p e c t s as an u n i f o r m and a v e r a g e c o n d u c t i v i t y f r a c t u r e . The app a r e n t c o n d u c t i v i t y o f t h e w h o l e f r a c t u r e depends upon c o n d u c t i v i t y d i s t r i b u t i o n and n o t j u s t a v e r a g e c o n d u c t i v i t y . The r e a s o n why d e c r e a s i n g and u n i f o r m c o n d u c t i v i t y f r a c t u r e s a r e a l m o s t e q u i v a l e n t i n e f f e c t i s t h a t t h e amount o f f l u i d passing through a section o f the crack increases nearer t o the wellbore, and t h e r e f o r e i n c a s e o f u n i f o r m f r a c t u r e c o n d u c t i v i t y , the pressure gradient a l o n g t h e c r a c k i n c r e a s e s as i t n e a r s t h e w e l l b o r e . F u r t h e r away f r o m t h e w e l l bore, t h i s pressure g r a d i e n t can be so small t h a t decreasing f r a c t u r e c o n d u c t i v i t y w o u l d n o t a p p r e c i a b l y i n c r e a s e t o t a l p r e s s u r e d r o p a c r o s s t h e c r a c k . Unl e s s f r a c t u r e c o n d u c t i v i t y i s c h a n g i n g v e r y q u i c k l y , a f r a c t u r e may a p p e a r t o be u n i f o r m , and w e l l t e s t a n a l y s i s may n e v e r d e t e c t c h a n g i n g c o n d u c t i v i t y i n side a crack. Thus a n u n i f o r m h i g h c o n d u c t i v i t y i n s i d e t h e f r a c t u r e may n o t b e a l w a y s nec e s s a r y . U s u a l l y c r a c k c o n d u c t i v i t y n e a r t h e w e l l b o r e i s h i g h , and t h e p r e s s u r e g r a d i e n t i n s i d e t h e f r a c t u r e i s much l o w e r t h a n i t s g r a d i e n t i n t h e f o r m a t i o n . I f t h i s c r a c k c o n d u c t i v i t y n e a r t h e w e l l b o r e i s c o n s i d e r e d t o be s u f f i c i e n t t o p r o d u c e a p a r t i c u l a r f o r m a t i o n , t h e n i t i s n o t i n d i s p e n s i b l e t o m a i n t a i n such a c o n d u c t i v i t y t h r o u g h o u t t h e c r a c k . As f l o w r a t e t h r o u g h t h e f r a c t u r e d e c r e a s e s w i t h d i s t a n c e away f r o m t h e w e l l , i t i s s u f f i c i e n t t o m a i n t a i n o n l y t h e l o w pressure g r a d i e n t . T h i s constant pressure g r a d i e n t i s achieved through decreasi n g crack conductivity, w i t h therefore instead o f using a constant c o n d u c t i v i t y f r a c t u r e design, a p p l i c a t i o n o f c o n s t a n t p r e s s u r e g r a d i e n t b e i n g more e f f i c i e n t . T h i s has s i g n i f i c a n t i m p a c t on p r o p p a n t q u a n t i t y and t h u s s i z e and expenses o f t h e s t i m u l a t i o n t r e a t m e n t , as a c o n s t a n t p r e s s u r e g r a d i e n t f r a c t u r e may r e q u i r e l e s s t h a n h a l f o f t h e p r o p p a n t volume needed f o r a c o n s t a n t c o n d u c t i v i t y c r a c k . A s p e c t s o f c h a n g i n g f r a c t u r e c o n d u c t i v i t y and n o n - u n i f o r m d i s t r i b u t i o n a r e a l s o d i s c u s s e d b y GUPPY, CINCO-LEY & RAMEY ( 1 9 8 0 ) and BENNETT, ROSATO, REYNOLDS & RAGHAVAN ( 1 9 8 1 ) .
463
4.4. Targets o f unconventional gas stimulation T a r g e t s o f u n c o n v e n t i o n a l gas s t i m u l a t i o n comprise c h i e f l y t i g h t gas sandstone r e s e r v o i r s ( c f . s e c t i o n 3 . 4 . 1 . ) which a r e a l r e a d y e x p l o i t e d i n s e v e r a l p a r t s o f t h e w o r l d t o v a r i o u s degree, w i t h t h e c u r r e n t c e n t r e s b e i n g USA and Western Europe. I n a d d i t i o n t o l o w - p e r m e a b i l i t y c l a s t i c g a s - b e a r i n g sequences, some o t h e r m a r g i n a l t a r g e t s f o r s t i m u l a t i o n w i t h p a r t i c u l a r emphasis on hydraul i c p r o p p a n t f r a c t u r i n g i n t h e coming y e a r s i n c l u d e geopressured r e s e r v o i r s , c o a l seams, shales, h i g h - t e m p e r a t u r e r e s e r v o i r s , and gas condensate r e s e r v o i r s . Emphasis i s p u t on these v a r i o u s sources o f u n c o n v e n t i o n a l gas because o f t h e i r a l r e a d y c u r r e n t importance i n p a r t s o f t h e w o r l d (LINDEN 1985) and t h e i r i n c r e a s i n g s i g n i f i c a n c e i n t h e energy s u p p l y spectrum u n t i l t h e y e a r 2000, w i t h t h e share o f u n c o n v e n t i o n a l gas i n t h e w o r l d gas d e l i v e r y b e i n g a b l e t o be f o r e c a s t e d as r a n g i n g i n t h e o r d e r o f 10 - 30 % (LIESEN 1985, LINDEN 1985) o r as b e i n g a b t . 30 % and 50 % by t h e y e a r s 2000 and 2020, r e s p e c t i v e l y (BAKER 1981). An overview o f u n c o n v e n t i o n a l n a t u r a l gas i n t h e USA i s g i v e n by DOE (1984) and KUUSKRAA & HAAS ( 1 9 8 8 ) . The low p e r m e a b i l i t y o f u n c o n v e n t i o n a l r e s e r v o i r s imposes severe r e s t r i c t i o n s on t h e a b i l i t y o f t h e gas t o m i g r a t e a p p r e c i a b l e d i s t a n c e s (LAW, POLLASTRO & K E I G H I N 1986). T h e r e f o r e g e o l o g i c a l f a c t o r s such as temporal r e l a t i o n s h i p o f hydrocarbon g e n e r a t i o n and m i g r a t i o n w i t h r e s p e c t t o development o f s t r u c t u r a l and s t r a t i g r a p h i c a l t r a p s a r e n o t as i m p o r t a n t as i n c o n v e n t i o n a l r e s e r v o i r s . The more i m p o r t a n t g e o l o g i c a l c h a r a c t e r i s t i c s o f u n c o n v e n t i o n a l r e s e r v o i r s a r e source rock q u a l i t y and q u a n t i t y , o r g a n i c m a t u r a t i o n , thermal h i s t o ry, and d i s t r i b u t i o n and n a t u r e o f p o r o s i t y and p e r m e a b i l i t y ( c f . a l s o SPENCER 1985; c f . s e c t i o n 3 . 4 . 1 . ) . Unconventional gas p r o s p e c t s a r e m a i n l y c h a r a c t e r i z e d by m a j o r g e o l o g i c a l and e n g i n e e r i n g c h a l l e n g e s t h a t need t o be f a c e d i n t r a n s l a t i n g these p o t e n t i a l r e s o u r c e s i n t o e c o n o m i c a l l y proven r e s e r v e s (KUUSKRAA & HAAS 1988). Improved advances i n geosciences, s t r a t e g i c a l w e l l placement, and enhanced h y d r a u l i c p r o p p a n t f r a c t u r i n g technology a r e r e q u i r e d t o overcome t h e n a t u r a l g e o l o g i c a l d i s c o n t i n u i t i e s and p e r m e a b i l i t y b a r r i e r s impeding t h e a b i l i t y o f t r a d i t i o n a l i n f i l l d r i l l i n g t o r e c o v e r t h e u n c o n v e n t i o n a l gas r e s o u r c e s . The f o l l o w i n g d i s c u s s i o n i n c l u d e s h y d r a u l i c proppant f r a c t u r i n g aspects o f t i g h t gas sandstones, geopressured r e s e r v o i r s , c o a l seams, s h a l e s and h i g h - t e m p e r a t u r e r e s e r v o i r s . Some comments a r e a l s o o f f e r e d on gas condensate r e s e r v o i r s .
4.4.1.
Tight gas sandstones
T i g h t gas sandstones a r e t h e most p r o m i n e n t unconventional gas r e s e r v o i r s as w e l l as t h e l a r g e s t and most w i d e l y r e c o g n i z e d o f t h e l o w - p e r m e a b i l i t y n a t u r a l gas r e s o u r c e s (KUUSKRAA & HAAS 1988) and a r e a l r e a d y t h e f o c u s o f e x p l o r a t i o n and p r o d u c t i o n s i n c e some t i m e . T i g h t gas sandstones have p r o v i d e d b o t h i n USA and Europe a l r e a d y f r e q u e n t l y t h e stage f o r e x e c u t i o n o f s p e c t a c u l a r l a r g e - s c a l e h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t s ( c f . s e c t i o n 3.4.1.) w h i c h repeatedl y s e t w o r l d r e c o r d s i n terms o f f l u i d and proppant q u a n t i t y pumped as w e l l as o p e r a t i o n expenses ( c f . s e c t i o n s 1 . 1 . 1 . 4 . and 2 . 4 . 1 . 1 . ) . The l o w - p e r m e a b i l i t y ( l e s s t h a n 0 . 1 md) m a t r i x can o n l y l i b e r a t e t h e entrapped gas i n s u f f i c i e n t amounts i f i n t e r s e c t e d by a d e e p l y p e n e t r a t i n g l o n g h y d r a u l i c f r a c t u r e t h a t crosses as much r o c k m a t r i x as p o s s i b l e i n o r d e r t o maximize gas c o l l e c t i o n a l o n g t h e h i g h - c o n d u c t i v i t y pathway o f t h e propped f r a c t u r e ( c f . s e c t i o n 4 . 8 . 1 1 . ) . Large t r e a t m e n t volumes, p r o p e r l y p l a c e d and c o n t r o l l e d f r a c t u r e s , and s a t i s f a c t o r y s u p p o r t o f t h e c r a c k by p r e s s u r e - r e s i s t a n t p r o p p a n t s a r e essent i a l f o r a c h i e v i n g e c o n o m i c a l l y a t t r a c t i v e r a t e s o f gas p r o d u c t i o n . The o u t l i n e as f o l l o w s c o n c e n t r a t e s on t i g h t gas sand body morphology and s i g n i f i c a n c e o f h y d r a u l i c f r a c t u r i n g , d i s t r i b u t i o n o f i m p o r t a n t t i g h t gas sand f o r m a t i o n s and basins, i n t e r r e l a t i o n s h i p o f t i g h t gas sands and c o a l seams, and o v e r h y d r o s t a t i c r e s e r v o i r pressure.
464
4.4.1.1. Tight gas sand body morphology and significance of hydraulic fracturing T i g h t gas sands f o r m blanket-shaped o r l e n t i c u l a r b o d i e s ( c f . s e c t i o n 3.4.1.), w i t h b o t h t y p e s r e q u i r i n g h y d r a u l i c g e n e r a t i o n o f r i b b o n - and wedgel i k e h i g h - p e r m e a b i l i t y passageways t h r o u g h which t h e gas can f l o w f r o m t h e t i g h t m a t r i x t o t h e w e l l b o r e (SPENCER 1983, DOE 1984). L e n t i c u l a r sand b o d i e s ( c f . p l a t e s I I / 7 - 8, 111/4, V/4 and V I I / 3 ) n e c e s s i t a t e i n a d d i t i o n l i n k i n g o f v a r i o u s i s o l a t e d sand l e n s e s t o g e t h e r along an e x t e n s i v e f r a c t u r e i n o r d e r t o s u s t a i n s u f f i c i e n t p r o d u c t i o n o v e r t h e economical l i f e o f t h e p r o j e c t ( c f . sect i o n 4.10.2.1.). L o c a t i o n o f d i s c r e t e sand lenses can be performed by 3-D r e f l e c t i o n s e i s m i c s e c t i o n e v a l u a t i o n , v e r t i c a l s e i s m i c b o r e h o l e p r o f i l i n g and c r o s s - w e l l b o r e s e i s m i c c h e c k i n g ( c f . s e c t i o n 6 . 2 . 2 . 1 . 1 . ) . O t h e r heterogeneous g e o l o g i c a l p r o p e r t i e s such as u n c o n f o r m i t i e s and d i s c o n t i n u i t i e s ( c f . p l a t e I ) f u r t h e r c o m p l i c a t e t h e a c c e s s i b i l i t y o f n a t u r a l gas r e s o u r c e s i n t i g h t sandstones which as a consequence o f t h e i r d e f i n i t i o n r e p r e s e n t an a r b i t r a r y d e l i n e a t i o n o f a n a t u r a l g e o l o g i c a l c o n t i n u o u s p e r m e a b i l i t y spectrum o f t h e r e s e r v o i r r o c k s . Aspects o f t i g h t gas sandstone r e s e r v o i r s a r e a l s o d i s c u s s e d i n s e c t i o n s 1.1.1., 2.4.1. and 3.4. An overview o f t h e p o t e n t i a l o f t i g h t gas sands i s g i ven by HAAS, BRASHEAR & MORRA ( 1 9 8 5 ) . Massive h y d r a u l i c f r a c t u r i n g has a l r e a d y i n t h e e a r l y 1970's been i d e n t i f i e d as a p r o m i s i n g a l t e r n a t i v e t e c h n o l o g y f o r economical development o f t i g h t gas r e s e r v o i r s (KUUSKRAA, BRASHEAR, ELKINS & MORRA 1979). T e c h n o l o g i c a l improvements which have t h e l a r g e s t impact on u l t i m a t e r e c o v e r y and n e t p r e s e n t v a l u e i n c l u d e improved a b i l i t y t o d i f f e r e n t i a t e and c h a r a c t e r i z e t h e t i g h t pay zones, c a p a c i t y t o s t i m u l a t e m u l t i p l e r e s e r v o i r s f o r a common w e l l b o r e ( c f . s e c t i o n 4.2.2.1.3.), improved p r e d i c t a b i l i t y o f f r a c t u r e performance, i n c r e a s e d e f f e c t i v e l y propped f r a c t u r e l e n g t h ( c f . s e c t i o n 4 . 8 . 1 1 . ) , a b i l i t y f o r f r a c t u r e s t o i n t e r s e c t lenses n o t d i r e c t l y i n contact w i t h the w e l l ( c f . section 4.10.2.1.), and optimizing field development c o m p r i s i n g w e l l spacing ( c f . s e c t i o n 4 . 1 0 . 5 . 1 . ) , w e l l p a t t e r n and f r a c t u r e s i z e .
4.4.1.2. Distribution of important tight gas sand formation and basins The t i g h t gas sand b a s i n s i n t h e USA a r e expected t o have more than 400 T c f (10,000 B i l l . m3) c u m u l a t i v e r e c o v e r a b l e r e s e r v e s (OFFICE OF TECHNOLOGY ASSESSMENT 1985). Some o f t h e most i m p o r t a n t t i g h t gas sand b a s i n s a r e N o r t h e r n G r e a t P l a i n s / W i l l i s t o n B a s i n (138 T c f o r 3,450 B i l l . m3 gas i n p l a c e ) , G r e a t e r Green R i v e r BasinlWyoming and Colorado (135 T c f o r 3,375 B i l l . m3 GIP), Piceance Creek Basin/Utah and Colorado ( 4 9 T c f o r 1,225 B i l l . m3 GIP), Wind R i v e r B a s i n / Wyoming (34 T c f o r 850 B i l l . m3 GIP), C o t t o n V a l l e y Basin/Texas and L o u i s i a n a ( 2 2 T c f o r 550 B i l l . m3 GIP), U i n t a Basin/Utah and Colorado ( 2 1 T c f o r 525 B i l l . m3 GIP), Edwards L i n e Basin/Texas ( 1 4 T c f o r 350 B i l l . m3 GIP), Denver Bas i n / C o l o r a d o , Nebraska and Wyoming ( 1 3 T c f o r 325 B i l l . m3 GIP), Val Verde Basin/Texas ( 5 T c f o r 125 B i l l . m3 GIP), and San Juan Basin/New Mexico and C o l o r a do ( 3 T c f o r 75 B i l l . m3 G I P ; KUUSKRAA & HAAS 1988). The maximum t e c h n i c a l l y r e c o v e r a b l e gas r e s e r v e i n t h e t i g h t gas sandstone r e s o u r c e i s o v e r 600 T c f o r 15,000 B i l l . m3 (NORTHROP & FROHNE 1988). Some o f t h e most s i g n i f i c a n t t i g h t gas sands i n t h e USA a r e i n t h e Pennsylvan i a n t h e Atoka F o r m a t i o n i n New Mexico, and t h e C l e v e l a n d and Morrow F o r m a t i o n s i n t h e Anadarko Basin/Oklahoma and Texas. The Upper J u r a s s i c i n c l u d e s t h e Cott o n V a l l e y Sandstone ( S c h u l e r F o r m a t i o n ) i n t h e E a s t Texas BasinfTexas and L o u i siana, and t h e Lower Cretaceous c o n t a i n s t h e Muddy "J" and t h e Code11 Sandstones i n t h e Denver-Julesburg Basin/Colorado, t h e T r a v i s Peak (Hosston) F o r m a t i o n i n t h e E a s t Texas Basin/Texas and L o u i s i a n a , and the Dakota Sandstone i n t h e San Juan Basin/New Mexico. The Upper Cretaceous comprises t h e Corcoran, Cozzett e and R o l l i n s Sandstones i n t h e Mesaverde Group i n t h e Piceance Creek Basin/Co-
465 l o r a d o and G r e a t e r Green R i v e r BasinlWyoming, Utah and Colorado. The Upper Cretaceous Mesaverde Group c o n s i s t s o f a l o w e r m a r g i n a l marine s e c t i o n w i t h m o s t l y b l a n k e t r e s e r v o i r s (Corcoran, C o z z e t t e and R o l l i n s Sandstones) and an upper f l u v i a l t o d e l t a i c and p a l u d a l p a r t w i t h p r e d o m i n a n t l y l e n t i c u l a r pay zones (JOHNSON & N U C C I O 1986). F u r t h e r Upper Cretaceous r e s e r v o i r s a r e t h e N i o b r a r a and Greenhorn Format i o n s i n Rocky Mountains and G r e a t P l a i n s area/Colorado, Kansas and Nebraska; t h e Olmos F o r m a t i o n i n t h e M a v e r i c k Basin/Texas, t h e F r o n t i e r and Almond Format i o n s i n t h e G r e a t e r Green R i v e r Basin/Wyoming, U t a h and Colorado; and t h e Tuscher F o r m a t i o n i n t h e E a s t e r n U i n t a Basin/Utah. The Lower T e r t i a r y f i n a l l y bears t h e Wasatch and Green R i v e r Formations i n Piceance Creek B a s i n / C o l o r a d o and E a s t e r n U i n t a Basin/Utah ( c o m p i l e d f r o m SPENCER & MAST 1986). The most i m p o r t a n t t i g h t gas sands i n Europe a r e R o t l i e g e n d and Upper C a r b o n i f e r o u s i n N o r t h Sea and onshore N e t h e r l a n d s and Germany FRG ( c f . s e c t i o n 2.4.1. and chapt e r 3).
4.4.1.3. Interrelationship o f tight gas sands and coal seams T i g h t gas sands a r e f r e q u e n t l y i n t e r r e l a t e d w i t h c o a l seams r e p r e s e n t i n g t h e p r i m a r y sources and r e s e r v o i r s o f n a t u r a l gas which p a r t i a l l y l a t e r m i g r a t e d and accumulated s e c o n d a r i l y i n sandstone pays (RIGHTMIRE & CHOATE 1986). The most prominent examples i n c l u d e t h e San Juan and Piceance Creek Basins i n t h e Cretaceous o f t h e USA, and t h e Upper C a r b o n i f e r o u s i n N o r t h Sea and a d j o i n i n g onshore areas i n Europe ( c f . s e c t i o n 3.2.1.). Mass balances r e v e a l t h a t o n l y s m a l l p o r t i o n s o f t h e l a r g e volumes o f gas generated d u r i n g c o a l i f i c a t i o n remained w i t h i n t h e c o a l seams themselves, w i t h l a r g e amounts o f a d d i t i o n a l gas hav i n g e m i g r a t e d i n t o a d j a c e n t sandstone r e s e r v o i r s o r escaped t o t h e atmosphere. As a consequence o f c o n t i n u i t y o f d e p o s i t i o n a l environment, t i g h t gas sand comp l e x e s a r e r e a d y r e c i p i e n t s f o r t h e l a r g e volumes o f excess c o a l - b e d gas.
4.4.1.4. Overhydrostatic reservoir pressure Many Upper Cretaceous and Lower T e r t i a r y t i g h t gas sands i n Colorado, U t a h and Wyoming i n USA a r e o v e r p r e s s u r e d (LAW, POLLASTRO & K E I G H I N 1986) as a consequence o f gas accumulating a t r a t e s g r e a t e r than t h a t i t i s d e p l e t e d and l o s t (LAW & DICKINSON 1985). D u r i n g thermal g e n e r a t i o n o f gas, d e w a t e r i n g i s a c c e l e r a t e d and w a t e r s a t u r a t i o n i s decreased t o i r r e d u c i b l e l e v e l s which i s immobil e , t h e r e b y p r o d u c i n g a r e l a t i v e l y c l o s e d h y d r o l o g i c a l system i n which t r a n s p o r t a t i o n o f d i s s o l v e d m a t e r i a l o u t o f t h e system i s impeded and s i g n i f i c a n t enhancement o f p e r m e a b i l i t y and p o r o s i t y i s p r e c l u d e d . As processes l e a d i n g t o amel i o r a t i o n o f p e r m e a b i l i t y and p o r o s i t y a r e i n e f f e c t i v e , o t h e r p e r m e a b i l i t y - and p o r o s i t y - r e d u c i n g processes c o n t i n u e , t h u s r e s u l t i n g i n a v e r y l o w - p e r m e a b i l i t y r o c k sequence i n which r a t e s o f gas accumulation exceed r a t e s o f gas d e p l e t i o n . The gas accumulations a r e c h a r a c t e r i z e d by t h e presence o f u p - d i p w a t e r - b e a r i n g r e s e r v o i r s and down-dip g a s - c o n t a i n i n g pays, and t h e t o p o f t h e overpressur e d g a s - b e a r i n g r e s e r v o i r s c u t s across s t r u c t u r a l and s t r a t i g r a p h i c a l boundar i e s and i s n o t a s s o c i a t e d w i t h any p a r t i c u l a r l i t h o l o g i c a l u n i t . I n t h e deeper p a r t s o f t h e b a s i n , t h e r e l a t i v e l y c l o s e d n a t u r e o f t h e system imposes severe r e s t r i c t i o n s on t h e a b i l i t y o f t h e gas t o m i g r a t e a p p r e c i a b l e d i s t a n c e s f r o m t h e i n t e r b e d d e d source r o c k s . The t o p o f o v e r p r e s s u r i n g i n t h e g e o l o g i c a l column almost matches w i t h t h e t r a n s f o r m a t i o n o f s m e c t i t e i n t o i l l i t e and i s s i t u a t e d near t h e zone o f c o e x i s t e n c e o f c h l o r i t e - s m e c t i t e i n r e g u l a r i n t e r s t r a t i f i c a t i o n (LAW, POLLASTRO & K E I G H I N 1986; c f . s e c t i o n 3.5.1.). O t h e r aspects o f o v e r p r e s s u r i n g i n t i g h t gas sands a r e r e p o r t e d by LAW, SPENCER & BOSTICK (1980); LAW (1984) and SPENCER (1984). Together w i t h o t h e r pay zone t y p e s and gas d i s t r i b u t i o n types, o v e r p r e s s u r e d t i g h t gas sands b e l o n g t o t h e group o f geopressured r e s e r v o i r s which a r e discussed i n t h e s e c t i o n below.
466
4.4.2.
Geopressured reservoirs
A p a r t f r o m t h i n n e r o r t h i c k e r , s h e e t - t y p e o r l e n t i c u l a r t i g h t gas s a n d s t o nes, a n o t h e r m a j o r c h a l l e n g e o f n e c e s s a r y a p p r o a c h i n t h e n e x t y e a r s i s seen i n g e o p r e s s u r e d r e s e r v o i r s t i m u l a t i o n where gas i s d i s s o l v e d i n o v e r p r e s s u r e d b r i nes o r may a l s o o c c u r as f r e e phase (TUCKER 1979, WEEKS 1 9 7 9 ) . G e o p r e s s u r e d r e s e r v o i r s are c h a r a c t e r i z e d by abnormally h i g h pore f l u i d pressures exceeding t h e h y d r o s t a t i c p r e s s u r e r e q u i r e d t o r a i s e a f l u i d column f r o m t h e s u b s u r f a c e f o r m a t i o n t o t h e s u r f a c e (DUTTA 1 9 8 6 ) . O v e r p r e s s u r i n g o c c u r s w i t h i n i s o l a t e d s a n d s t o n e s t h a t a r e encased i n t o t h i c k e r mudstone sequences. Thus t h e f l u i d i n t h e s a t u r a t e d b u r i e d s e d i m e n t s i s t r a p p e d w i t h i n t h e p o r e spaces and p r e s s u r e changes f r o m h y d r o s t a t i c t o l i t h o s t a t i c d u r i n g p r o g r e s s i v e b u r i a l (JONES 1969; SWANSON, BERNARD & OSOBA 1 9 8 6 ) . As a consequence o f h a v i n g p r e s s u r e s g r e a t e r than normal, geopressured r e s e r v o i r s a r e capable o f p r o d u c t i o n w i t h o u t needing pumps o r o t h e r e q u i p m e n t t o b r i n g gas and w a t e r t o t h e s u r f a c e , i n c o n t r a s t t o h y d r o p r e s s u r e d r e s e r v o i r s w i t h a v e r a g e p r e s s u r e w h i c h c o n t a i n a v a s t amount o f gas t h a t m i g h t be r e c o v e r a b l e b u t w i l l n o t f l o w n a t u r a l l y t o t h e s u r f a c e (GRI 1988 b ) . R e s e r v o i r e n g i n e e r i n g a s p e c t s of g e o p r e s s u r e d f o r m a t i o n s a r e a l s o d i s c u s s e d b y T I M K O & FERTL ( 1 9 7 1 ) , ISOKRARI & KNAPP ( 1 9 7 6 ) ; KNAPP, ISOKRARI, GARG & PRITCHETT ( 1 9 7 7 ) ; LAW & SPENCER ( 1 9 8 1 ) , McPEEK ( 1 9 8 1 ) , QUITZAU & BASSIOUNI ( 1 9 8 1 ) , SPENCER & LAW ( 1 9 8 1 ) ; DOHERTY, RANDOLPH, ROGERS & POONAWALA ( 1 9 8 2 ) ; QUONG, OWEN & LOCKE ( 1 9 8 2 ) ; ROBINSON, HOLDITCH & LEE ( 1 9 8 3 ) ; ANDERSON, PETERSON & PARIS1 (1984); LAW ( 1 9 8 4 ) , MCMULLAN & BASSIOUNI ( 1 9 8 4 ) , ENGINEER (19851, LAW & OICKINSON ( 1 9 8 5 ) , SOOKPRASONG & ELBEL ( 1 9 8 5 ) , BEGLAND & WHITEHEAD ( 1 9 8 6 ) ; LAW; POLLASTRO & KEIGHIN ( 1 9 8 6 ) ; PRASAD ( 1 9 8 6 ) ; WERES, JUN & TSAO ( 1 9 8 6 ) ; CHARPENTIER, LAW & PRENSKY ( 1 9 8 7 ) ; IRELAND & ROBINSON ( 1 9 8 7 ) , POSTON & CHEN ( 1 9 8 7 ) , PRASAD ( 1 9 8 7 ) and FERTL & LEACH ( 1 9 8 8 ) . Case s t u d i e s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n g e o p r e s s u r e d r e s e r v o i r s a r e c a r r i e d o u t b y DOSCHER & A Z A R I ( 1 9 8 0 ) and ROBINSON, HOLDITCH & LEE ( 1 9 8 3 ) . A r e v i e w o f t e c h n o l o g y and economics o f methane p r o d u c t i o n f r o m g e o p r e s s u r e d a q u i f e r s i s g i v e n b y DOSCHER, OSBORNE, RHEE, COX & KUUSKRAA ( 1 9 7 9 ) , and DOSCHER & A Z A R I ( 1 9 8 0 ) a n a l y z e t h e e f f e c t o f c r i t i c a l gas s a t u r a t i o n on methane p r o d u c t i v i t y . GEER & COOK ( 1 9 7 8 ) d i s c u s s c r e a t i o n , augment a t i o n and e x p l o i t a t i o n o f h y d r o c a r b o n p o r e - v o l u m e s a t u r a t i o n i n g e o p r e s s u r e d a q u i f e r s , and MATTHEWS ( 1 9 8 1 ) r e v i e w s p o s s i b i l i t i e s o f e n h a n c i n g gas p r o d u c t i o n f r o m g e o p r e s s u r e d a q u i f e r s . Some comments o n g e o t e c t o n i c a l s e t t i n g , p o r o p e r m p r o p e r t i e s , b a l l - o u t t r e a t m e n t s and o t h e r s o u r c e s o f deep gas a r e o f f e r e d as f01 1 ows .
4.4.2.1.
Geotectonical setting
G e o p r e s s u r e d gas a c c u m u l a t i o n s a r e o f t e n d e e p - b a s i n o r b a s i n - c e n t e r e d r e s e r v o i r and d e p o s i t t y p e s w h i c h a r e commonly v e r y l a r g e , occupy t h e d e e p e r p a r t s of s e d i m e n t a r y and s t r u c t u r a l t r o u g h s , a r e f r e q u e n t l y downdip f r o m w a t e r - b e a r i n g r o c k s and c o n s i s t n e a r l y a l w a y s o f l o w - p e r m e a b i l i t y s a n d s t o n e s (CHARPENT I E R , LAW & PRENSKY 1 9 8 7 ) . The abnormal p r e s s u r e s a r e caused b y a c t i v e gas gener a t i o n i n t h e younger g e o l o g i c a l h i s t o r y , w i t h t h e poor p e r m e a b i l i t y o f t h e sandstone pays n o t a l l o w i n g t h e pressure t o e q u i l i b r a t e t o h y d r o s t a t i c l e v e l s (MEISSNER 1981; LAW, SPENCER & BOSTICK 1980; LAW & DICKINSON 1 9 8 5 ) . The o c c u r r e n c e o f t h e u n c o n v e n t i o n a l g e o p r e s s u r e d gas d e p o s i t s i s n o t c o m p l e t e l y c o n t r o l l e d by s t r u c t u r e and s t r a t i g r a p h y , b u t t h e more i m p o r t a n t a s p e c t s o f t h e s e a c c u m u l a t i o n s a r e t e m p e r a t u r e , t h e r m a l m a t u r i t y , o r g a n i c r i c h n e s s , gas c o m p r e s s i b i l i t y , w a t e r s a t u r a t i o n and e s p e c i a l l y p e r m e a b i l i t y (LAW 1984; CHARPENTIER, PRENSKY & LAW 1 9 8 7 ) . I n t h e USA, g e o p r e s s u r e d a q u i f e r s c o n t a i n i n g n a t u r a l g a s o c c u r p a r t i c u l a r l y i n t h e G u l f C o a s t a r e a i n Texas and L o u i s i a n a (RANDOLPH 1 9 7 7 ) .
467
4.4.2.2.
Poroperm p r o p e r t i e s
Geopressured gas r e s e r v o i r s a r e o f t e n c h a r a c t e r i z e d by h i g h secondary p o r o s i t i e s (BERG & HABECK 1982). The i n f l u e n c e o f i n c r e a s i n g f o r m a t i o n temperature w i t h b u r i a l causes t h e t r a n s f o r m a t i o n o f s m e c t i t e t o i l l i t e whereby w a t e r i s r e l e a s e d t h a t causes i n c r e a s i n g p o r e p r e s s u r e due t o i t s entrapment ( c f . s e c t i o n 3 . 5 . 1 . ) . Aquathermal p r e s s u r i n g o r i g i n a t e s f r o m thermal expansion o f i n t e r s t i t i a l w a t e r w i t h d e p t h o f b u r i a l . D u r i n g normal b u r i a l , d e w a t e r i n g o f t h e s e d i mentary s e c t i o n i s unequal as a consequence o f r e s t r i c t e d d r a i n a g e i n geopressur e d r e s e r v o i r s t h a t a r e surrounded by impervious p e r m e a b i l i t y b a r r i e r s . Generat i o n o f o i l and gas f r o m o r g a n i c m a t e r i a l f u r t h e r i n c r e a s e s t h e f l u i d volume. Secondary p o r o s i t y i s c r e a t e d by d i s s o l u t i o n o f m i n e r a l cements by t h e f l o w o f w a t e r f r o m h i g h - p r e s s u r e zones t o low-pressure patches. The p r e s s u r e response i n geopressured gas r e s e r v o i r s i s a d e q u a t e l y e x p l a i n e d i n a changing p o r e comp r e s s i b i l i t y s c e n a r i o (POSTON & CHEN 1987).
I n a b n o r m a l l y p r e s s u r e d w e l l s , p r o d u c t i o n r a t e and f l o w i n g t u b i n g p r e s s u r e d e c l i n e r a p i d l y f o l l o w i n g h y d r a u l i c f r a c t u r e s t i m u l a t i o n t r e a t m e n t s (SOOKPRASONG & ELBEL 1985). G e n e r a l l y t h e p r o d u c t i o n r a t e d e c l i n e depends on many f a c t o r s such as f o r m a t i o n p e r m e a b i l i t y , f r a c t u r e l e n g t h , f r a c t u r e c o n d u c t i v i t y , r e s e r v o i r p r e s s u r e and r e s e r v o i r s i z e . I n deep h i g h - p r e s s u r e l o w - p e r m e a b i l i t y w e l l s , c l o s u r e s t r e s s can p l a y an i m p o r t a n t r o l e on t h e r a t e o f d e c l i n e i n p r o d u c t i o n when l a r g e r a p i d p r e s s u r e drawdown i s r e q u i r e d f o r economical o f f t a k e r a t e s ( c f . s e c t i o n s 1.2.1.1. and 4 . 1 2 . 4 . ) . The economical s i g n i f i c a n c e o f r e c o v e r y o f gas d i s s o l v e d i n geopressured b r i n e s i s a l s o s t r e s s e d by LINDEN ( 1 9 8 5 ) . P r o d u c t i o n o f t h e gas d i s s o l v e d a t bubble p o i n t p r e s s u r e can be s t i m u l a t e d by b o t h f r a c t u r i n g and a c i d i z i n g a i m i n g on c r e a t i o n o f a h i g h - p e r m e a b i l i t y zone around t h e w e l l b o r e (DOSCHER & A Z A R I 1980). Excess p r e s s u r e s o f i s o l a t e d sand l e n s e s w i t h i n s h a l e s r e p r e s e n t i n g l i m i t a t i o n s o f r e s e r v o i r c o n n e c t i o n a r e cont r o l l e d by g e o l o g i c a l and p e t r o p h y s i c a l f a c t o r s (OZKAYA 1987). Flowback and cleanup o f f r a c t u r i n g f l u i d s i n geopressured r e s e r v o i r s i s g e n e r a l l y n o t a p r o blem due t o t h e a b n o r m a l l y h i g h p o r e p r e s s u r e s (ROBINSON, HOLDITCH & LEE 1983). BROWNE & PARKER (1986) r e p o r t subnormal l y p r e s s u r e d and f r a c t u r e d r e s e r v o i r s , and McLENNAN, ROEGIERS & MARX (1983) p r e s e n t underpressured s i l t s t o n e and s h a l e sequences. I n a d d i t i o n t o deep h i g h - p r e s s u r e d h a r d sandstones, a l s o s h a l l o w low-pressured s o f t sandstones may be t i g h t and c o n t a i n a p p r e c i a b l e gas r e s e r v e s w i t h i n t h e l o w - p e r m e a b i l i t y m a t r i x t h a t has t o be l i b e r a t e d by hydraul i c f r a c t u r i n g (RICE & SHURR 1978; NYDEGGER, R I C E & BROWN 1979; BROWN & CRAFTON 1981, GAUTIER & R I C E 1981; KUKAL, BIDDISON, HILL, MONSON & SIMMONS 1983). Some t i g h t gas sandstones a r e c o n s i d e r a b l y overpressured (McMECHAN & CONWAY 1983, BRANAGAN & WILMER 1985). Combined geopressured and geothermal a q u i f e r s w i t h l o w - s a l i n i t y w a t e r a t abnormal p r e s s u r e s and e l e v a t e d temperature c o n t a i n i n g d i s s o l v e d n a t u r a l gas a l s o o c c u r (KNAPP, I S O K R A R I , GARG & PRITCHETT 1977).
4.4.2.3.
Ball -out treatments
Many l o w - p e r m e a b i l i t y geopressured r e s e r v o i r s a r e r a t h e r s o f t r o c k s and should be b a l l e d o u t p r i o r t o p r o d u c t i o n t e s t i n g and f r a c t u r e t r e a t i n g o f t h e p r o s p e c t i v e i n t e r v a l , because f o r m a t i o n damage can o c c u r i f l a r g e p r e s s u r e g r a d i e n t s a r e imposed (ROBINSON, LEE & HOLDITCH 1983). B a l l - o u t t r e a t m e n t s u s u a l l y change t h e s k i n f r o m a p o s i t i v e v a l u e caused by d r i l l i n g mud and cement damage t o a s l i g h t l y n e g a t i v e v a l u e due t o c r e a t i n g m i n o r f r a c t u r e s around t h e w e l l bore, w i t h t h i s amount o f s t i m u l a t i o n a l l o w i n g t o produce a w e l l a t moderate f l o w r a t e s f o r s h o r t p e r i o d s o f t i m e w i t h o u t imposing l a r g e p r e s s u r e g r a d i e n t s around t h e w e l l b o r e . I n a d d i t i o n , d u r i n g p r e - f r a c t u r i n g t e s t i n g , c e r t a i n t y has t o be made t h a t a l l t h e n e t gas pay i s c o n t r i b u t i n g p r o d u c t i o n t o t h e w e l l . As a consequence o f t h e geopressure, i t i s expected t h a t n o t a l l t h e p e r f o r a t i o n h o l e s w i l l be i n e f f e c t i v e communication w i t h t h e f o r m a t i o n , and t h e r e f o r e b a l l o u t t r e a t m e n t s a r e necessary t o open up a l l o f t h e p e r f o r a t i o n s so t h a t a more a c c u r a t e p r e - f r a c t u r i n g f l o w t e s t and p r e s s u r e b u i l d u p t e s t can be performed.
468 A s p e c t s o f l i m i t e d e n t r y and a l s o d i s c u s s e d b y WEEKS ( 1 9 7 9 ) .
4.4.2.4.
i n i t i a l b a l l o u t i n geopressured r e s e r v o i r s are
Other sources o f deep gas
O t h e r s o u r c e s o f deep gas c a n be c l a s s i f i e d i n t o two c a t e g o r i e s c o m p r i s i n g a b i o g e n i c and s u b d u c t e d gas (DOE 1 9 8 4 ) . A b i o g e n i c gas i s s p e c u l a t e d t o have been f o r m e d b y n o n - o r g a n i c means i n t h e m a n t l e d u r i n g t h e p r i m o r d i a l c r e a t i o n o f t h e e a r t h , and d u r i n g c o u r s e o f g e o l o g i c a l t i m e , i t s l o w l y seeped t o s h a l l o wer d e p t h s . S u b d u c t e d gas i s o f o r g a n i c o r i g i n , b u t i n s t e a d o f b e i n g formed f r o m l a y e r s o f compacted s e d i m e n t s , t h e gas i s c r e a t e d when m a r i n e o r g a n i s m s and v e g e t a t i o n a r e pushed deep i n t o t h e e a r t h b y t e c t o n i c a l c r u s t a l movements. A c o m b i n a t i o n o f c h e m i c a l a c t i o n , magmatic h e a t , p r e s s u r e and t i m e c a u s e d t h e gas t o be g e n e r a t e d w h i c h l a t e r e i t h e r m i g r a t e d t o t h e s u r f a c e o r a c c u m u l a t e d b e n e a t h caps o f i m p e r v i o u s r o c k . As b o t h a b i o g e n i c and s u b d u c t e d gas r e s e r v o i r s a r e s t i l l f a r f r o m r e a l i s t i c access and a r e p a r t i a l l y s t i l l s e m i h y p o t h e t i c a l i t e m s , n o comments c a n be made c o n c e r n i n g h y d r a u l i c f r a c t u r i n g p o t e n t i a l o f these types o f hydrocarbon accumulations.
4.4.3. Coal seams Methane f r o m c o a l seams has been p r o d u c e d o n v a r i o u s s c a l e s i n c e t h e e a r l y 19OO's, b u t has o n l y i n t h e l a s t decades r e c e i v e d l a r g e r a t t e n t i o n (RIGHTMIRE & CHOATE 1 9 8 6 ) . Methane f o u n d i n c o a l d e p o s i t s was once r e g a r d e d as a s a f e t y haz a r d t o m i n e r s and was f l a r e d and v e n t e d t o t h e atmosphere, b u t i s t o d a y r e g a r ded as a p o t e n t i a l and v a l u a b l e e n e r g y s o u r c e (KUUSKRAA & MEYER 1980, DOE 1984, KUUSKRAA & HAAS 1 9 8 8 ) . I n a d d i t i o n t o i t s p r o v e n a n c e f r o m s e c o n d a r y e n r i c h ments, u n c o n v e n t i o n a l gas i s a l s o coming i n i n c r e a s i n g amounts d i r e c t l y f r o m t h e i r p r i m a r y p l a c e s o f g e n e r a t i o n r e p r e s e n t e d b y t h e c o a l seams ( w h i c h a r e b o t h s o u r c e and r e s e r v o i r r o c k s ) where s i g n i f i c a n t q u a n t i t i e s a r e t r a p p e d i n nat u r a l m i c r o c r a c k s (sometimes t r a g i c a l l y a c c e n t u a t e d b y e x p l o s i o n s o f gas w h i c h i s r e l e a s e d d u r i n g s u b s u r f a c e m i n i n g i n t h e g a l l e r i e s ) and has been p r e v e n t e d from migration. The l a r g e volumes o f gas w h i c h a r e s t i l l c o n t a i n e d w i t h i n t h e c o a l seams and have n o t y e t e m i g r a t e d i n t o i n t e r r e l a t e d t i g h t s a n d s t o n e s a r e a c q u i s i t e d b y c o a l f r a c t u r i n g i n v a r i o u s s c a l e (TREVITS, HANSON & WARD 1982; LASSITER & HOLLINGSWORTH 1984, SARDA 1984, SECCOMBE & SAKASHITA 1985; BOYER, STUBBS & SCHWERER 1986; JONES 1986; JONES, BELL & MORALES 1986; LAYNE & BYRER 1986; LOGAN, SECCOMBE & JONES 1986; MILITZER & SCHWERER 1986, PERLMAN 1986; HANSON, NIELSEN, SORRELS, BOYER & SCHRAUFNAGEL 1987; SCHRAUFNAGEL 1987; REEVES, WALLACE & BEAVERS 1987; ELY, HOLDITCH & CARTER 1988; HOLDITCH, ELY, SEMMELBECK & CARTER 1988; JEU, LOGAN & McBANE 1988; KELAFANT, BOYER & ZUBER 1988; LOGAN, ERWIN & McBANE 1988; ZUBER, KUUSKRAA & SAWYER 1 9 8 8 ) . The j o i n t s i g n i f i c a n c e o f c o a l seams as s o u r c e s and r e s e r v o i r s o f n a t u r a l gas and t h e i r i n t e r r e l a t i o n s h i p s w i t h t i g h t gas sands a r e emphasized b y RIGHTMIRE & CHOATE (1986; c f . s e c t i o n 4 . 4 . 1 . 3 . ) A f t e r i l l u s t r a t i o n o f some g e n e r a l a s p e c t s , t h e o u t l i n e as f o l l o w s i n cludes r e l a t i o n s h i p s o f c l e a t geometry, s p e c i a l mechanical coal p r o p e r t i e s , f r a c t u r i n g s t i m u l a t i o n o f c o a l seams, f r a c t u r e c o n d u c t i v i t y m a x i m i z a t i o n , v e r t i c a l h y d r a u l i c f r a c t u r i n g v s . h o r i z o n t a l d r i l l i n g , r e s e r v o i r p r e s s u r e , and r e l a t i o n s h i p s between c o a l seam and b o u n d i n g s t r a t a .
4.4.3.1.
General aspects
The methane a s s o c i a t e d w i t h c o a l seams was f o r m e d as a b y - p r o d u c t o f t h e nat u r a l b i o g e n i c and t h e r m a l m a t u r a t i o n p r o c e s s o f c o a l and e x i s t s b o t h as a d s o r bed gas on t h e i n t e r n a l s u r f a c e s o f t h e m i c r o p o r o u s c o a l s t r u c t u r e and as f r e e gas w i t h i n t h e p o r e s and i n t h e n a t u r a l f r a c t u r e and c l e a t s y s t e m o f t h e c o a l bed (KUUSKRAA & HAAS 1 9 8 8 ) , a n d / o r as d i s s o l v e d gas i n g r o u n d w a t e r w i t h i n t h e
469 c o a l bed (RIGHTMIRE & CHOATE 1986) and t h u s i s n o t s i m p l y compressed w i t h i n t h e p o r e spaces o f t h e r e s e r v o i r (JEU, LOGAN & McBANE 1988). Gas generated i n excess o f t h a t which can be adsorbed on t h e c o a l s u r f a c e s i s i n i t i a l l y p r e s e n t as f r e e gas w i t h i n t h e p o r o s i t y o f t h e c o a l , most n o t i c e a b l y w i t h i n t h e f r a c t u r e p o r o s i t y . T h i s gas i s a v a i l a b l e f o r s o l u t i o n i n ground w a t e r moving t h r o u g h t h e bed and m i g r a t i o n under hydrodynamic c o n d i t i o n s , o r i t may be r e t a i n e d i n t h e c o a l bed as t r a p p e d f r e e gas under p r o p e r h y d r o g e o l o g i c c o n d i t i o n s . P r o d u c t i o n o f methane f r o m c o a l seams i n v o l v e s removing o f w a t e r f r o m t h e c o a l c l e a t system, l o w e r i n g r e s e r v o i r p r e s s u r e s t o r e f l e c t gas d e s o r p t i o n , and i n i t i a t i n g t h e subsequent f l o w o f gas through t h e r e s e r v o i r i n t o a p r o d u c i n g w e l l b o r e . F o l l o w i n g d i s c u s s i o n o f t h e importance o f t h e c l e a t system f o r gas d r a i n a g e f r o m c o a l seams, an account i s g i v e n on s i g n i f i c a n c e o f h y d r a u l i c f r a c t u r i n g i n deep unmineable c o a l seams f o r gas e x p l o i t a t i o n , aims and purposes o f c o a l f r a c t u r i n g , and d i s t r i b u t i o n o f i m p o r t a n t c o a l b a s i n s and gas r e s e r v e s .
4.4.3.1.1. Importance of the cleat system for gas drainage from coal seams The mechanism o f gas p r o d u c t i o n f r o m c o a l i s s u b s t a n t i a l l y d i f f e r e n t f r o m t h a t i n c o n v e n t i o n a l r e s e r v o i r s (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988). There i s i n most cases l i t t l e o r no f r e e gas p r e s e n t i n t h e c o a l , b u t most o f t h e p o r e space i n t h e c l e a t system i s w a t e r - s a t u r a t e d and most o f t h e gas i s adsorbed on t h e c o a l s u r f a c e . I n o r d e r t o a l l o w gas w i t h d r a wal, t h e p r e s s u r e i n t h e c l e a t system must be reduced t o cause t h e gas t o des o r b f r o m t h e c o a l s u r f a c e t o t h e c l e a t system and t o d i f f u s e t h r o u g h t h e c o a l m a t r i x . Normally, s i g n i f i c a n t volumes o f w a t e r must be produced f o r l o w e r i n g t h e p r e s s u r e i n t h e c l e a t system so t h a t gas d e s o r p t i o n can b e g i n . W h i l e f r a c t u r e i n t e r s e c t i o n o f v e r y h i g h - p e r m e a b i l i t y zones would be b e n e f i c i a l t o gas o f f t a k e i n sandstones and carbonates, gas e x p l o i t a t i o n i n c o a l seams becomes d i f f i c u l t due t o t h e l a r g e volumes o f w a t e r t h a t must be removed, and t h e r e f o r e c o a l f r a c t u r i n g does n o t n e c e s s a r i l y aim on c r o s s i n g o f t h e s e c t i o n s w i t h h i g h e s t c l e a t p e r m e a b i l i t y (ZUBER, REEVES, JONES & SCHRAUFNAGEL 1988). As gas desorpt i o n i s t h e p r i m a r y source o f p r o d u c t i o n , t h e gas f l o w r a t e f r o m a c o a l seam may i n c r e a s e w i t h time, and i t i s n o t uncommon f o r maximum gas f l o w r a t e s t o occ u r months o r y e a r s a f t e r s t a r t i n g e x p l o i t a t i o n . D e s o r p t i o n i s a l s o a s i g n i f i c a n t mechanism f o r gas l i b e r a t i o n i n s h a l e s ( c f . s e c t i o n 4 . 4 . 4 . 1 . ) . Economical gas w i t h d r a w a l f r o m c o a l seams r e q u i r e s e x i s t e n c e o f an e x t e n s i v e c l e a t system i n o r d e r t o p r o v i d e t h e necessary p e r m e a b i l i t y , a h i g h enough gas c o n t e n t t o c o n s t i t u t e a r e s o u r c e t h a t i s w o r t h developing, and c o n n e c t i o n o f t h e c l e a t system t o t h e w e l l b o r e . The p r i m a r y r e s e r v o i r p r o p e r t i e s which cont r o l gas p r o d u c t i o n f r o m c o a l seams a r e adsorbed gas c o n t e n t , d e s o r p t i o n behav i o u r c h a r a c t e r i s t i c s , e f f e c t i v e d r a i n a g e area s i z e , r e s e r v o i r pressure, and p e r m e a b i l i t y volume and w a t e r s a t u r a t i o n o f t h e c l e a t system. F o l l o w i n g i l l u s t r a t i o n o f some aspects o f t h e s i g n i f i c a n c e o f h y d r a u l i c f r a c t u r i n g i n deep unmineable c o a l seams f o r gas e x p l o i t a t i o n , t h e d i s t r i b u t i o n o f i m p o r t a n t c o a l bas i n s and gas r e s e r v e s i s sketched.
4.4.3.1.2. Significance of hydraulic fracturing in deep unmineable coal seams for gas exploitation W h i l e h y d r a u l i c f r a c t u r i n g has f o r m e r l y been used as a p u r e s a f e t y procedure i n o r d e r t o a c c e l e r a t e methane removal f r o m c o a l seams ahead o f underground m i nes which then was f l a r e d and vented, d u r i n g t h e l a s t decade a growing awareness o f t h e v a s t economical and i n d u s t r i a l p o t e n t i a l o f h y d r a u l i c f r a c t u r i n g f o r methane e x p l o i t a t i o n f r o m c o a l seams has been e s t a b l i s h e d (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988). Coal-bed r e s e r v o i r s a r e low-press u r e l o w - f l o w producers because t h e y a r e a t a p p r o x i m a t e l y h y d r o s t a t i c p r e s s u r e
470 and once t h e f r e e gas i s d e p l e t e d , p r o d u c t i o n i s c o n t r o l l e d by methane d e s o r p t i o n and d i f f u s i o n t o the f r a c t u r e s . The r e l a t i v e p e r m e a b i l i t y t o g a s and w a t e r a r e c r i t i c a l t o i n i t i a t i n g g a s w i t h d r a w a l . Where the c o a l seams a r e w a t e r - s a t u r a t e d , the w a t e r c o n t e n t must be reduced i n o r d e r t o p e r m i t d e s o r p t i o n and flow of the gas t o t h e w e l l b o r e , because high w a t e r s a t u r a t i o n i s s e r i o u s l y l i m i t i n g gas f l o w . S t r a t i g r a p h i c a l l y , coal i s o f t e n d i s t r i b u t e d i n r e l a t i v e l y t h i n seams o v e r l a r g e i n t e r v a l s which means t h a t f o r economical r e c o v e r y , s p e c i a l product i o n p r a c t i c e s and m u l t i p l e c o m p l e t i o n s from the same w e l l b o r e a r e r e q u i r e d . S p e c i a l emphasis i n t h i s c o n n e c t i o n i s p u t on a c q u i s i t i o n of deep unmineable coal seams by d r i l l i n g and subsequent f r a c t u r i n g , whereas f u r t h e r e x p l o i t a t i o n of c o a l seams a s s o c i a t e d w i t h an a c t i v e underground coal mine i s of l e s s economical s i g n i f i c a n c e . Hydraulic f r a c t u r i n g of d e g a s i f i c a t i o n w e l l s of c o a l - b e a r ing s t r a t a has i n the p a s t a l r e a d y r e p e a t e d l y been c a r r i e d o u t i n the USSR. An a l t e r n a t i v e o r complementary t e c h n i q u e f o r a c q u i s i t i o n of i n - s i t u coal-seam g a s i s h o r i z o n t a l o r l a t e r a l d r i l l i n g (TRACY 1988; c f . s e c t i o n s 4 . 4 . 3 . 6 . and 4 . 8 . 6 . 2 . 1 . ) . LINDEN (1985) emphasizes t h e importance of deep unmineable c o a l seam gas f o r the improvement of the energy a c q u i s i t i o n spectrum i n the n e x t dec a d e s ( c f . s e c t i o n 2 . 2 . 1 . 6 . 4 . ) . Coalbed methane c u r r e n t l y r e p r e s e n t s t h e f a s t e s t growing and p o t e n t i a l l y t h e economically most a t t r a c t i v e of the low-permeab i l i t y gas r e s o u r c e s (KUUSKRAA & HAAS 1988).
4.4.3.1.3.
Aims and purposes o f coal f r a c t u r i n g
H y d r a u l i c f r a c t u r i n g i n c o a l a t t e m p t s t o bypass n e a r - w e l l b o r e damage ( c f . s e c t i o n 4 . 8 . 3 . ) , s t i m u l a t e p r o d u c t i o n and a c c e l e r a t e d e w a t e r i n g , d i s t r i b u t e the p r e s s u r e drawdown t o reduce f i n e s p r o d u c t i o n ( c f . s e c t i o n 4 . 2 . 2 . 7 . ) , and e f f e c t i v e l y connect the w e l l b o r e t o the r e s e r v o i r (JEU, LOGAN & McBANE 1 9 8 8 ) . The primary purpose of coal f r a c t u r i n g i s t o bypass any f o r m a t i o n damage t h a t may be p r e s e n t from d r i l l i n g mud o r cement i n v a s i o n , because f o r m a t i o n damage s h a r p l y r e d u c e s t h e e x t e n t of pressure drawdown t h a t r e a c h e s i n t o t h e r e s e r v o i r , t h e r e b y slowing the dewatering p r o c e s s and d e l a y i n g methane l i b e r a t i o n . Removing o r bypassing of f o r m a t i o n damage e n a b l e s e a r l y g a s p r o d u c t i o n and h i g h e r overall exploitation. Deeply p e n e t r a t i n g h y d r a u l i c f r a c t u r e s can s t i m u l a t e h i g h e r p r o d u c t i v i t y i n l o w e r - p e r m e a b i l i t y c o a l s a s i n o t h e r t i g h t f o r m a t i o n s ( c f . s e c t i o n 4 . 8 . 1 1 . ) . Bec a u s e of the need t o q u i c k l y drop r e s e r v o i r p r e s s u r e t h r o u g h o u t the pay and the b e n e f i t of a c c e l e r a t i n g t h e dewatering p r o c e s s , high f r a c t u r e c o n d u c t i v i t y i s paramount e s p e c i a l l y i n the moderate p e r m e a b i l i t y of s h a l l o w c o a l s . F r a c t u r e s t i m u l a t i o n s p r o v i d e b e t t e r a c c e s s t o the c o a l r e s e r v o i r than can be o b t a i n e d by p e r f o r a t i n g a l o n e . While open-hole c o m p l e t i o n s should a d e q u a t e l y a c c e s s t h e i n d i v i d u a l c o a l l a y e r s , p e r f o r a t i n g of c a s e d w e l l s may n o t s u f f i c i e n t l y a c c e s s the a l t e r n a t i n g s t r u c t u r e of t h i n l a y e r s of c l e a t e d c o a l and n o n - c l e a t e d c a r b o naceous s h a l e . H y d r a u l i c f r a c t u r i n g can h e l p t o r e a c h beyond t h e w e l l b o r e t o c o n t a c t c o a l d e p o s i t s t h a t t h e b o r e h o l e may n o t a c t u a l l y p e n e t r a t e ( c f . s e c t i o n 4.10.2.1.).
4.4.3.1.4. D i s t r i b u t i o n o f i m o r t a n t coal basins and gas reserve The coal b a s i n s i n the USA a r e e x p e c t e d t o have more than 400 Tcf o r 10,000 B i l l . m3 r e c o v e r a b l e r e s e r v e s ( B Y R E R , MROZ & COVATCH 1984; OFFICE OF TECHNOLOGY ASSESSMENT 1985, RIGHTMIRE & CHOATE 1986) o r even up t o 800 Tcf o r 20,000 B i l l . m3 p r o d u c t i o n p o t e n t i a l ( L A Y N E & BYRER 1 9 8 8 ) . E s t i m a t e s of r e s o u r c e p o t e n t i a l in v a r i o u s coal b a s i n s and a s s e s s m e n t s of r e s e r v e r e c o v e r a b i l i t y a r e a l s o c a r r i e d o u t by CHOATE, JURICH & SAULNIER ( 1 9 8 1 ) ; CHOATE, LENT & RIGHTMIRE ( 1 9 8 2 ) and ADAMS e t a l . ( 1 9 8 2 ) . Some of the most i m p o r t a n t coalbed methane b a s i n s i n the USA a r e P i c e a n c e Creek Basin/Utah and Colorado ( 8 4 Tcf o r 2,100 B i l l . m3 gas i n p l a c e ) , San Juan Basin/Colorado and New Mexico (80 Tcf o r 2,000 B i l l . m3 GIP), Northern Appalachian Basin ( 6 1 Tcf o r 1,525 B i l l . m3 G I P ) , C e n t r a l Appala-
471 c h i a n B a s i n ( 4 8 T c f o r 1,200 B i l l . m3 GIP), Powder R i v e r Basin/Montana and Wyoming ( 3 9 T c f o r 975 B i l l . m3 GIP), G r e a t e r Green R i v e r Basin/Wyoming and Color a d o ( 3 1 T c f o r 775 B i l l . m3 GIP), I l l i n o i s B a s i n ( 2 1 T c f o r 525 B i l l . m3 GIP), W a r r i o r Basin/Alabama ( 2 0 T c f o r 500 B i l l . m3 GIP), Raton Mesa B a s i n ( 1 8 T c f o r 450 B i l l . m3 GIP), Arkoma Basin/Oklahoma and Arkansas ( 4 T c f o r 100 B i l l . m3 GIP), Wind R i v e r Basin/Wyoming ( 2 T c f o r 50 B i l l . m3 GIP), and U i n t a Basin/Utah and Colorado ( 1 T c f o r 25 B i l l . m3 G I P ; KUUSKRAA & HAAS 1988). The g r e a t e s t c u r r e n t a c t i v i t y i s i n t h e W a r r i o r B a s i n and i n t h e San Juan Bas i n . I n t h e USA, a s s o c i a t e methane f r o m c o a l seams i s a l r e a d y now t h e p r i m a r y source o f n a t u r a l gas f o r Alabama and i s r a p i d l y becoming a m a j o r source o f nat u r a l gas i n t h e San Juan B a s i n i n New Mexico and Colorado (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988). Aspects o f c l e a t geometry, s p e c i a l mec h a n i c a l c o a l p r o p e r t i e s , f r a c t u r e s t i m u l a t i o n types o f c o a l seams, f r a c t u r e c o n d u c t i v i t y m a x i m i z a t i o n , v e r t i c a l h y d r a u l i c f r a c t u r i n g vs. h o r i z o n t a l d r i l l i n g , r e s e r v o i r pressure, and r e l a t i o n s h i p s between c o a l seams and bounding s t r a t a a r e o u t l i n e d as f o l l o w s .
4.4.3.2. Cleat geometry Most o f t h e r e t a i n e d gas w i t h i n t h e c o a l seams ( w h i c h i s much l e s s than t h e t o t a l amount generated d u r i n g c o a l i f i c a t i o n due t o c o n s i d e r a b l e e m i g r a t i o n i n t o n e a r e r o r f a r t h e r h i g h - p e r m e a b i l i t y r e s e r v o i r s ) i s t r a p p e d by a b s o r p t i o n on t h e s u r f a c e o f m i c r o p o r e s . S m a l l e r amounts a r e a l s o enclosed as f r e e gas w i t h i n f r a c t u r e s o r pores and as d i s s o l v e d gas i n f o r m a t i o n w a t e r . The c l e a t system which p r o v i d e s t h e p r i m a r y f l o w channel and t h u s r e p r e s e n t s c o a l p e r m e a b i l i t y u s u a l l y c o n s i s t s o f two s e t s o f c l o s e l y - s p a c e d v e r t i c a l n a t u r a l f r a c t u r e s which a r e o r i e n t e d o r t h o g o n a l t o each o t h e r . The p r i m a r y s e t o f n a t u r a l f r a c t u r e s a r e f a c e c l e a t s , and a l e s s d e f i n e d second s e t o f p e r p e n d i c u l a r f r a c t u r e s a r e b u t t c l e a t s (DECKER & SECCOMBE 1986; LOGAN, SCHWOEBEL & HORNER 1987). The f a c e c l e a t s a r e c o n t i n u o u s and c r o s s c u t t i n g , whereas t h e b u t t c l e a t s t e r m i n a t e a t t h e f a c e c l e a t s . Coal c l e a t s f o r m g e n e r a l l y i n response t o l o c a l s t r u c t u r a l f o r ces, a l t h o u g h t h e process o f c o a l i f i c a t i o n and o t h e r v a r i a b l e s a l s o i n f l u e n c e c l e a t d e n s i t y (McFALL, WICKS, KELSON, SEDWICK & BRANDENBURG 1987; c f . a l s o sect i o n 4.2.2.7.). Coal gas r e s e r v o i r s t y p i c a l l y have v e r y low m a t r i x p e r m e a b i l i t y , w i t h t h e gas b e i n g desorbed f r o m t h e m a t r i x t o t h e w e l l b o r e t h r o u g h a system of c l e a t s . I n a d d i t i o n t o these n a t u r a l f r a c t u r e s and f i s s u r e s , c o a l seam r e s e r v o i r s u s u a l l y r e q u i r e h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n t o achieve economical p r o d u c t i o n as a consequence o f t h e i r t i g h t n a t u r e . H y d r a u l i c a l l y - i n d u c e d c r a c k s commonly p r o pagate p a r a l l e l t o t h e maximum s t r e s s o r f a c e c l e a t d i r e c t i o n o f t h e c o a l seam and t h e r e f o r e may n o t n e c e s s a r i l y a d e q u a t e l y access an a n i s o t r o p i c r e s e r v o i r . S i g n i f i c a n t improvement can be made by a c o m b i n a t i o n o f a h o r i z o n t a l b o r e h o l e p l a c e d p e r p e n d i c u l a r l y t o t h e maximum p e r m e a b i l i t y d i r e c t i o n (LOGAN, SCHWOEBEL & HORNER 1987; PETZET 1988) and a h y d r a u l i c f r a c t u r e ( c f . s e c t i o n s 4.4.3.6. and 4.8.6.2.1.). C l e a t w i d t h and spacing c o n t r o l c o a l p e r m e a b i l i t y and as i n - s i t u s t r e s s i n c r e a s e s w i t h b u r i a l depth, t h e deeper t h e c o a l , t h e l o w e r t h e permeabil i t y (JEU, LOGAN & McBANE 1988). Assessment o f gas r e s e r v e s and p r o d u c t i o n p o t e n t i a l i n d e e p l y b u r i e d unmineab l e c o a l seams i s made by DOE (1983 b ) ; BYRER, MOROZ & COVATOH (1984); R I G H T M I R E , EDDY & K I R R (1984); DECKER & SECCOMBE (1986) and McFALL, WICKS, KELSON, SEDWICK & BRANDENBURG ( 1 9 8 7 ) . Comments on t h e c o a l - b e d d e g a s i f i c a t i o n process a r e o f f e r e d by ANCELL, LAMBERT & JOHNSON (1980) and GRAY ( 1 9 8 7 ) .
4.4.3.3. Special mechanical coal properties Some s p e c i a l mechanical p r o p e r t i e s o f c o a l seams t h a t a r e s i g n i f i c a n t f o r des i g n and e x e c u t i o n o f s t i m u l a t i o n o p e r a t i o n s a r e d i s c u s s e d a l o n g the l i n e s o f
472 treatment pressures geometry as f o l l o w s .
and
c o a l c h i p p i n g as w e l l as impact o f h y d r a u l i c f r a c t u r e
4.4.3.3.1. Treatment pressures and coal chipping Treatment p r e s s u r e s observed d u r i n g h y d r a u l i c f r a c t u r i n g o f c o a l seams a r e o f t e n d i f f e r e n t f r o m those o f c o n v e n t i o n a l f o r m a t i o n s (JONES, BELL, MORALES & SCHRAUFNAGEL 1987; c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) . A f t e r an e a r l y steep p r e s s u r e r i s e , o v e r a l l o p e r a t i o n p r e s s u r e s o f t e n exceed t h e l i t h o s t a t i c p r e s s u r e , b u t produce s u r p r i s i n g l y l i t t l e v e r t i c a l f r a c t u r e e x t e n s i o n . The a c c u m u l a t i o n o f c o a l c h i p s a t t h e w e l l b o r e and p a r t i a l p l u g g i n g o f an advancing c r a c k t i p by c o a l f i n e s c a r r i e d ahead o f o r w i t h i n t h e f l u i d o p e r a t i o n pad a r e t h e r e s p o n s i b l e mechanisms which account f o r t h e anomalous t r e a t m e n t p r e s s u r e d i s t r i b u t i o n ( c f . sect i o n 4 . 2 . 2 . 7 . ) . As a consequence o f narrow c l e a t spacing, c o a l tends t o break i n t o small cleat-bounded fragments a l r e a d y a t moderate mechanical s t r e s s . I n add i t i o n , t h e p a r t i c u l a r p r o p e r t i e s l e a d t o mechanical d e s t r u c t i o n o f c o a l around t h e w e l l b o r e w i t h time, w i t h t h e l o o s e c o a l c a u s i n g blockage a t t h e w e l l b o r e and p a r t i a l s c r e e n o u t a t t h e f r a c t u r e t i p . The e f f e c t s can be p a r t i a l l y o v e r come by j e t t i n g t h e c o a l w i t h h i g h - p r e s s u r e w a t e r p r i o r t o f r a c t u r i n g i n o r d e r t o c r e a t e a c a v i t y around t h e b o r e h o l e and t h e r e b y remove any s k i n damage i n d u ced by l o o s e c o a l fragments and t o reduce breakdown o r f r a c t u r e i n i t i a t i o n p r e s s u r e s (REEVES, WALLACE & BEAVERS 1987). The low v a l u e o f Young's modulus i n c o a l seams ( c f . s e c t i o n 4.4.3.8.3.) creat e s v e r y wide h y d r a u l i c f r a c t u r e s as w e l l as v e r y complex m u l t i p l e f r a c t u r e systems p r o p a g a t i n g i n m u l t i p l e d i r e c t i o n s (HOLDITCH, ELY, SEMMELBECK, CARTER, H I N KEL & JEFFREY 1988). The complex h y d r a u l i c f r a c t u r e b e h a v i o u r i s a consequence o f t h e presence o f t h e c l e a t system, t h e complex s t r u c t u r e o f t h e c o a l m a t r i x , and t h e s t r a t i g r a p h i c a l r e l a t i o n s h i p of t h e c o a l seams w i t h r e s p e c t t o t h e s u r r o u n d i n g sediments (JEU, LOGAN & McBANE 1988; c f . s e c t i o n 4 . 4 . 3 . 8 . ) . T h i s gener a t e s c o n s i d e r a b l e d i f f i c u l t i e s t o c r e a t e and t o p r o p l o n g h y d r a u l i c f r a c t u r e s . T h e r e f o r e coal-seam r e s e r v o i r s w i t h p o o r l y developed c l e a t systems may n o t be c a n d i d a t e s f o r development, because t h e y may n o t be capable o f s u p p l y i n g enough gas t o t h e r e l a t i v e l y s h o r t h y d r a u l i c f r a c t u r e . V a r i a t i o n s o f s t r e s s l e v e l i n c o a l seams may be t h e consequence o f f a u l t s c r o s s i n g t h e gas f i e l d which l o c a l l y cause weakening o f t h e c o a l m a t r i x so t h a t i t w i l l r e a d i l y f a i l and spa11 dur i n g the f r a c t u r e treatment, w i t h spalled coal chips b l o c k i n g the crack t o g e t h e r w i t h h i g h c l o s u r e s t r e s s e s r e s u l t i n g i n e l e v a t e d t r e a t i n g p r e s s u r e s (ZUBER, REEVES, JONES & SCHRAUFNAGEL 1988; c f . s e c t i o n 4 . 2 . 2 . 7 . ) . There i s a l s o a r e l a t i o n s h i p between b o r e h o l e s t a b i l i t y and c o a l f r i a b i l i t y (LOGAN 1 9 8 8 ) .
4.4.3.3.2.
Imact
o f hydraulic fracture geometry
R e d u c t i o n o f t h e amount o f c o a l f i n e s produced i s a p o t e n t i a l b e n e f i t o f hyd r a u l i c f r a c t u r i n g (JEU, LOGAN & McBANE 1988). A h i g h l y - c o n d u c t i v e f r a c t u r e can d i s t r i b u t e t h e p r e s s u r e drop a l o n g t h e c r a c k i n s t e a d o f c o n c e n t r a t i n g i t i n t h e immediate w e l l b o r e area. Thus t h e p r o p e n s i t y o f g e n e r a t i n g c o a l f i n e s s h o u l d be reduced, w i t h u t i l i z a t i o n o f l a r g e r proppant g r a i n s i z e s b e i n g a b l e t o a s s i s t maintenance o f f r a c t u r e c o n d u c t i v i t y by a l l o w i n g t h e s m a l l e r c o a l f i n e s t o pass t h r o u g h t h e p r o p p a n t package. Another problem o f p r o p p a n t f r a c t u r i n g i n c o a l seams i s p r o d u c t i o n o f c o a l g r i t , p r o p p a n t and w a t e r t o g e t h e r w i t h t h e gas. The a p p l i c a t i o n o f r e s i n - c o a t e d proppants i n t h e t a i l - i n p o r t i o n o f t h e f r a c t u r e ( c f . s e c t i o n 4 . 1 2 . 3 . 3 . ) g i v e s s u f f i c i e n t s t a b i l i z a t i o n t o p r e v e n t p r o p p a n t movement and flowback (FRACFAX 1988 e ) . The optimum s i z e o f f r a c t u r e h a l f - l e n g t h depends on w e l l spacing, c o a l seam p r o p e r t i e s and s t i m u l a t i o n c o s t s ( c f . s e c t i o n 4.8.11.), w i t h t h e h i g h e s t p e r f o r mance b e i n g achieved by v e r y h i g h c o n d u c t i v i t y and l o n g e x t e n s i o n o f t h e f r a c t u r e s (JONES, BELL, MORALES & SCHRAUFNAGEL 1987). When t r e a t m e n t p r e s s u r e exceeds l i t h o s t a t i c s t r e s s , h o r i z o n t a l f r a c t u r e s a r e i n i t i a t e d which may p o s s i b l y be e f -
473 f e c t i v e i n c o n t a c t i n g h i g h e r p e r m e a b i l i t y j o i n t s i n t h e f o r m a t i o n t h a t may n o t n o r m a l l y be accessed by v e r t i c a l f r a c t u r e s . Because o f t h e i r r a d i a l growth, however, h o r i z o n t a l f r a c t u r e s a r e c o n s i d e r a b l y s h o r t e r t h a n c o n f i n e d v e r t i c a l f r a c t u r e s f o r t h e same t r e a t m e n t volumes and t h e r e f o r e w i l l n o t be as e f f e c t i v e i n c o n s i s t e n t l y i n c r e a s i n g p r o d u c t i o n as l o n g h i g h - c o n d u c t i v i t y v e r t i c a l f r a c t u res.
4.4.3.4, Fracturing stimulation types o f coal seams Design and e x e c u t i o n o f s t i m u l a t i o n o p e r a t i o n s i n c o a l seams i s n o t s t r a i g h t forward, b u t h i g h i n j e c t i o n pressures, complex f r a c t u r e systems, screenouts, and p r o d u c t i o n o f proppants and c o a l f i n e s a r e t y p i c a l problems (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988). H y d r a u l i c p r o p p a n t f r a c t u r i n g i s n o r m a l l y i n e v i t a b l e f o r c o a l seam gas e x p l o i t a t i o n . The t y p e o f s t i m u l a t i o n t r e a t m e n t depends on depth, t h i c k n e s s and s t r a t i g r a p h y o f t h e c o a l seam. Some d i f f e r e n t f r a c t u r i n g o p e r a t i o n t y p e s o f c o a l seams a r e b r i e f l y sketched as f o l lows along t h e l i n e s o f f l u i d v i s c o s i t y , m u l t i p l e i n t e r v a l f r a c t u r i n g , f r a c t u r i n g s t r a t e g i e s , and f r a c t u r e p r o p a g a t i o n d i r e c t i o n and t r e a t m e n t p r e s s u r e i n crease.
4.4.3.4.1. Fluid viscosity ZUBER, KUUSKRAA & SAWYER (1988) compare d i f f e r e n t h y d r a u l i c f r a c t u r i n g t y p e s and w e l l spacings f o r economical r e c o v e r y o f coalbed methane. E v a l u a t i o n o f wat e r , foam and g e l f r a c t u r i n g r e v e a l s t h a t t h e b e s t r e s u l t s a r e achieved w i t h l a r g e - s c a l e g e l s t i m u l a t i o n t r e a t m e n t s , w i t h t h e economical b e n e f i t o f t h e c o s t l i e r gel-based f r a c t u r i n g o p e r a t i o n s more than compensating f o r t h e r e l a t i v e l y h i g h t r e a t m e n t expenses. Well s p a c i n g and r e s e r v o i r s t i m u l a t i o n e f f e c t i v e n e s s a r e t h u s t h e two key parameters d i c t a t i n g economical r e t u r n s f r o m c o a l b e d methane w e l l s and have a d r a m a t i c e f f e c t on b o t h t i m i n g o f peak p r o d u c t i o n r a t e s and u l t i m a t e w e l l r e c o v e r y (KUUSKRAA & HAAS 1988). Some comments a r e o f f e r e d on g e l - v s . water-based s t i m u l a t i o n as w e l l as p r o p p a n t s e t t l i n g and r o l l i n g i n wat e r f r a c t u r i n g as f o l l o w s .
4.4.3.4.1.1. Gel- vs. water-based stimulation HANSON (1988) f i n d s o u t t h a t l o w - v i s c o s i t y f l u i d s such as w a t e r can e f f e c t i v e l y induce f r a c t u r e h e i g h t growth through s e v e r a l c o a l seams v i a e n t r y a t t h e bottom o f t h e i n t e r v a l , and h i g h - v i s c o s i t y f l u i d s such as c r o s s - l i n k e d g e l s p r o mote a measure o f f r a c t u r e h e i g h t containment and l a t e r a l e x t e n s i o n i n l a y e r e d media. An o p t i m a l t r e a t m e n t f o r many c o a l seams c o n s i s t s o f a l a r g e w a t e r p r e pad f o r t h e purpose o f h e i g h t growth f o l l o w e d by a c r o s s l i n k e d g e l pad f o r t h e reason o f f r a c t u r e w i d e n i n g and e x t e n s i o n f r o m t h e w e l l b o r e . S l u r r y stages c a l l f o r c r o s s l i n k e d g e l t o e f f e c t i v e l y c a r r y t h e proppant and f o r c o a r s e - g r a i n e d proppants t o enhance f r a c t u r e c o n d u c t i v i t y . Casing s l o t t i n g t h r o u g h c o a l can r e duce h i g h t r e a t m e n t p e s s u r e i n d i n p a r t m i n i m i z e f r a c t u r e - i n l e t c o n s t r i c t i o n s . REEVES, WALLACE & EAVERS 1.987) p e r f o r m a comparative assessment o f d i f f e r e n t s t i m u l a t i o n tec n i q u e s i n terms o f c o s t and p r o d u c t i o n which i n d i c a t e s t h a t w e l l s f r a c t u r e d w i t h wlrte and foam-water t r e a t m e n t s produce a t h i g h e r wat e r and gas r a t e s t h a n those f r a c t u r e d w i t h e i t h e r g e l o r foam. I n a d d i t i o n , f o a m - w a t e r - s t i m u l a t e d w e l l s e x h i b i t h i g h f l u i d l o s s d u r i n g t h e f i r s t two months o f p r o d u c t i o n , whereas w a t e r - f r a c t u r e d w e l l s produce more w a t e r a f t e r t h e second month. T h e r e f o r e w a t e r f r a c t u r i n g appears t o be t h e most e f f e c t i v e t r e a t ment i n terms o f b o t h expenses and o f f t a k e . KUUSKRAA & HAAS (1988) f i n d o u t t h a t gel-based s t i m u l a t i o n s a r e s u p e r i o r t o water-based t r e a t m e n t s , because t h e y achieve peak p r o d u c t i o n r a t e s a t e a r l i e r times and n e a r l y double t h e peak r a t e s achieved by water-based j o b s . The i n c r e a s e i n gas r e c o v e r y due t o t h e h i g h e r f r a c t u r e c o n d u c t i v i t y and l e n g t h o f t h e gel-based s t i m u l a t i o n more than
474 compensates f o r t h e a d d i t i o n a l c o s t o f t h e s e t r e a t m e n t s
4.4.3.4.1.2.Proppant settling and rolling in water fracturing I n some e x c e p t i o n a l l y w e l l - c l e a t e d and t h u s h i g h - p e r m e a b i l i t y c o a l seams, ver y s m a l l - t o medium-size f r a c t u r e s t i m u l a t i o n s u s i n g water w i t h o n l y v e r y low p r o p p a n t c o n c e n t r a t i o n s c a n a l r e a d y be c o n s i d e r a b l y e f f e c t i v e (ZUBER, REEVES, JONES & SCHRAUFNAGEL 1 9 8 8 ) . The p o o r p r o p p a n t t r a n s p o r t c h a r a c t e r i s t i c s o f f r e s h water r e s u l t i n proppant s e t t l i n g near the wellbore during s l u r r y i n j e c t i o n , w i t h p r o p p a n t s t h e n b e i n g r o l l e d t o t h e head o f t h e bank a c r o s s i t s t o p d u r i n g f l u s h i n o r d e r t o i n c r e a s e propped f r a c t u r e l e n g t h . Although proppant transport i s considerably poorer w i t h water than w i t h h i g h - v i s c o s i t y f l u i d s , t h e b a n k i n g phenomena c a u s e d by p r o p p a n t s e t t l i n g c a n p o t e n t i a l l y r e s u l t i n a h i g h - c o n d u c t i v i t y f r a c t u r e w i t h p r o p p a n t s i n t h e b o t t o m and an open c h a n n e l i n t h e u p p e r s e c t i o n o f t h e c r a c k c r o s s i n g t h e c o a l seam ( c f . LASSITER & HOLLINGSWORTH 1984; c f . s e c t i o n 4 . 3 . 3 . 2 . ) . Water f r a c t u r i n g i s a l s o o f c o n s i d e r a b l e s i g n i f i c a n c e i n c h a l k sequences ( c f . s e c t i o n 4 . 5 . 4 . 5 . 2 . ) and g e o t h e r m a l r e s e r v o i r s ( c f . section 4.7.3.5.).
4.4.3.4.2.Multiple interval fracturing SCHRAUFNAGEL & LAMBERT ( 1 9 8 8 ) s t a t e t h a t h y d r a u l i c p r o p p a n t f r a c t u r i n g c a n be s u c c e s s i v e l y c a r r i e d o u t i n v a r i o u s s u p e r i m p o s e d c o a l seams and i s a c c o m p l i shed t h r o u g h t h e u s e o f b a f f l e s whereby a b a l l i s d r o p p e d down t h e c a s i n g t o s e a l o f f a zone f o l l o w i n g i t s t r e a t m e n t , w i t h m u l t i p l e r e p e t i t i o n o f t h i s p r o c e d u r e i n upwards d i r e c t i o n a l o n g t h e p r o s p e c t i v e s e c t i o n i n t h e b o r e h o l e r e s u l t i n g i n u p h o l e f r a c t u r i n g one l e v e l a f t e r t h e o t h e r i n s e q u e n t i a l o r d e r ( c f . sect i o n 4.2.2.1.1.). A n o t h e r p o s s i b i l i t y o f m u l t i p l e c o a l seam f r a c t u r i n g i s t h e l i m i t e d - e n t r y technique which r e q u i r e s maintenance o f a predetermined w e l l b o r e p r e s s u r e t h r o u g h o u t t h e o p e r a t i o n ( c f . s e c t i o n 4 . 2 . 2 . 2 . ) . Some a s p e c t s o f r e s e r v o i r d e p t h o f c o a l seams and l i m i t e d - e n t r y s t i m u l a t i o n a r e i l l u s t r a t e d as f o l 1ows.
4.4.3.4.2.1.Reservoir depth of coal seams I n s h a l l o w e r r e s e r v o i r s t o r e y s where h o r i z o n t a l r a t h e r t h a n v e r t i c a l f r a c t u r e s a r e more l i k e l y t o f o r m ( c f . s e c t i o n 1.2.8.), t h e s t i m u l a t i o n t e c h n i q u e must p r o v i d e f o r m u l t i p l e h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s o f h o r i z o n t a l o r i e n t a t i o n a n d / o r complex f r a c t u r e s w i t h b o t h h o r i z o n t a l and i n c l i n e d components (SCHRAUFNAGEL & LAMBERT 1 9 8 8 ) . Thus i n many c a s e s d e e p e r r e s e r v o i r s r e q u i r e c o a l seam p e n e t r a t i o n b y h i g h l y i n c l i n e d t o h o r i z o n t a l b o r e h o l e s f r o m w h i c h m u l t i p l e v e r t i c a l f r a c t u r e s emanate ( c f . section 4.8.6.2.4.), whereas i n s h a l l o w e r p a y zones, t h e c o a l seams a r e a c q u i s i t e d b y v e r t i c a l b o r e h o l e s f r o m w h i c h m u l t i p l e o b l i q u e d r a i n h o l e s may r a d i a t e o f f ( c f . s e c t i o n 4 . 8 . 6 . 1 . 2 . ) t h a t a r e i n t e r s e c t e d b y m u l t i p l e h;.-i:c:ntal f r a c t u r e s i n c a s e o f s i n g l e t h i c k e r c o a l seams, o r a r e t r a v e r s e d b y h i g h i y - d e v i a t e d t o h o r i z o n t a l boreholes from which o n l y s i n g u l a r h o r i z o n t a l h y d r a u l i c c r a c k s c a n be p r o p a g a t e d i n t o t h e c o a l m a t r i x f o r t h e o c c a s i o n o f g r o u p s o f t h i n n e r c o a l seams.
4.4.3.4.2.2.Limited-entry stimulation The l i m i t e d - e n t r y s t i m u l a t i o n t e c h n i q u e a p p l i e s f o r s i m u l t a n e o u s t r e a t m e n t o f m u l t i p l e zones b y e n g i n e e r i n g p l a c e m e n t , s i z e and number o f c a s i n g p e r f o r a t i o n s w h i c h s e r v e as p r e s s u r e chokes t h a t c o n t r o l t h e r a t e o f f l u i d i n j e c t i o n i n t o e a c h t a r g e t e d zone (SCHRAUFNAGEL & LAMBERT 1988; c f . s e c t i o n 4 . 2 . 2 . 2 . ) . P r o p p a n t a b r a s i o n l e a d s t o p e r f o r a t i o n d i a m e t e r g r o w t h and c o n s e q u e n t l y p r e s -
475 s u r e r e d u c t i o n . S i g n i f i c a n t p r e s s u r e drops i n some t r e a t m e n t stages can be caused by s c r e e n o u t o f v a r i o u s p e r f o r a t i o n s and i n i t i a t i o n o f a v e r t i c a l f r a c t u r e f o l l o w i n g termination o f propagation o f a horizontal crack ( c f . sections and 4 . 9 . 4 . 2 . ) . L i m i t e d e n t r y has t u r n e d o u t t o be a . v i a b l e approach t o 1.2.8.4. s t i m u l a t i n g a s e r i e s o f c o a l seams even a t s h a l l o w depths where h o r i z o n t a l f r a c t u r e s a r e a n t i c i p a t e d , and a t g r e a t e r depths where t h e p r i m a r y c r a c k o r i e n t a t i o n l i k e l y would be v e r t i c a l , t h e concept may a l s o p r o v e v i a b l e i n c o m p l e t i n g m u l t i p l e seams w i t h i n i n d i v i d u a l c o a l groups.
4.4.3.4.3. Fracturing strategies Four b a s i c f r a c t u r i n g s t r a t e g y s c e n a r i o s f o r h y d r a u l i c s t i m u l a t i o n o f c o a l seams (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988) i n c l u d i n g h o r i z o n t a l f r a c t u r e s i n a s i n g u l a r s h a l l o w c o a l seam, s i n g l e p l a n a r v e r t i c a l f r a c t u r e i n a s e r i e s o f t h i n c o a l seams, m u l t i p l e complex f r a c t u r e system i n a s i n g l e t h i c k c o a l seam, and i n i t i a l f r a c t u r e confinement w i t h i n a s i n g l e c o a l seam p a s s i n g l a t e r t o v e r t i c a l c r a c k p r o p a g a t i o n i n t o t h e bounding l a y e r s . A f t e r d i s c u s s i o n o f some g e n e r a l aspects, an overview i s g i v e n on s i n g l e v e r t i c a l f r a c t u r e s i n t e r s e c t i n g s e v e r a l t h i n c o a l seams and complex m u l t i p l e f r a c t u r e s c o n t a i n e d i n a s i n g l e t h i c k c o a l seam which a r e t h e most f r e q u e n t l y developed cases of c o a l f r a c t u r i n g . Some comments a r e a l s o o f f e r e d on i n j e c t i o n r a t e s , f l u i d l o s s and t r e a t m e n t p r e s s u r e p a r t i c u l a r l y f o r g e n e r a t i o n o f complex T-shaped f r a c t u r e s .
4.4.3.4.3.1. General aspects Concerning t h e g e n e r a l s t r a t e g y a p p l i c a b l e t o any c o a l seam, t h e main purpose o f h y d r a u l i c f r a c t u r i n g i s t o i n t e r c o n n e c t t h e c l e a t system o f t h e c o a l t o t h e b o r e h o l e . W i t h o u t a w e l l - d e v e l o p e d c l e a t system, c r e a t i o n and p r o p p i n g o f a l o n g f r a c t u r e i n c o a l seams i s u s u a l l y n o t p r o f i t a b l e and commercial a t c u r r e n t gas p r i c e s (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988). Bottomh o l e p r e s s u r e d u r i n g p r o d u c t i o n s h o u l d be m i n i m i z e d t o a c c e l e r a t e d e s o r p t i o n o f n a t u r a l gas. A wide c o n d u c t i v e propped f r a c t u r e system t h a t can w i t h s t a n d t h e maximum c l o s u r e s t r e s s must be c r e a t e d t o m i n i m i z e t h e p r e s s u r e drop down t h e f r a c t u r e . As a consequence o f h i g h t r e a t i n g p r e s s u r e s ( c f . s e c t i o n 4.4.3.8.3.) and complex n a t u r e o f t h e f r a c t u r e systems, r e a l - t i m e changes d u r i n g t h e operat i o n a r e t h e r u l e r a t h e r than t h e e x c e p t i o n ( c f . s e c t i o n 6 . 2 . 4 . 3 . ) . Cutting of p r o p p a n t i n s e r t i o n and r e i n i t i a t i n g t h e pad o r changing pad volume based upon t r e a t i n g p r e s s u r e s may be r e q u i r e d t o improve t h e r e s u l t s . i n s h a l l o w r e s e r v o i r depth, i n d i v i d u a l h o r i z o n t a l f r a c t u r e s w i t h i n t h e separ a t e c o a l seams have t o be c r e a t e d by u s i n g e i t h e r l i m i t e d - e n t r y methods ( c f . s e c t i o n 4.2.2.2.) o r mechanical d i v e r s i o n ( c f . s e c t i o n 4 . 2 . 2 . 1 . ) . L i n e a r f l u i d s w i t h a moderate pad volume s h o u l d be a p p l i e d and t h e bottomhole t r e a t i n g p r e s s u r e s h o u l d be i n excess o f 1 p s i / f t . Complex m u l t i p l e f r a c t u r e systems may be generated i f bottomhole p r e s s u r e s i n c r e a s e s u b s t a n t i a l l y d u r i n g t h e j o b .
4.4.3.4.3.2. Single vertical fractures intersecting several thin coal seams Concerning s i n g l e v e r t i c a l f r a c t u r e s i n t e r s e c t i n g a s e r i e s o f t h i n c o a l seams, t h i s s c e n a r i o i s analogous t o v e r t i c a l c r a c k s p e n e t r a t i n g l a y e r e d c l a s t i c and/or c a r b o n a t e r e s e r v o i r s (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1988). Well-developed c l e a t systems i n c o a l seams may provoke h i g h e r f l u i d l e a k o f f than i n c o n v e n t i o n a l pay zones. A common p r a c t i c e o f s t i m u l a t i o n o f t h i n c o a l seam complexes i s t o p e r f o r a t e i n a c l a s t i c zone a d j a c e n t t o t h e c o a l and t o f r a c t u r e t h e c o a l by a l l o w i n g t h e c r a c k t o grow v e r t i c a l l y , w i t h t h i s technique u s u a l l y r e s u l t i n g i n lower operating pressures than i f t r e a t i n g t h e c o a l e x c l u s i v e l y . Excess p r e s s u r e s a r e moderate .and d e c l i n e t h r o u g h o u t most
476 o f t h e j o b as t h e f r a c t u r e c o n t i n u e s t o grow v e r t i c a l l y . F r a c t u r e p r o p a g a t i o n g r a d i e n t s s h o u l d be l e s s t h a n 1 p s i / f t , and pad volumes o f a b t . 30 - 35 % a r e a p p r o p r i a t e . V i s c o u s s h e a r - s t a b l e d e l a y e d c r o s s l i n k e d f l u i d s o r foams s h o u l d be u s e d t o p r o v i d e a d e q u a t e p r o p p a n t t r a n s p o r t and t o m i n i m i z e p r o p p a n t s e t t l i n g .
4.4.3.4.3.3. C w l e x mu1 t iple fractures
contained in a single thick coal seam
The s c e n a r i o o f complex h y d r a u l i c f r a c t u r e s c o n t a i n e d w i t h i n a s i n g l e t h i c k c o a l seam i s u n i q u e t o c o a l gas r e s e r v o i r s and i s w e l l documented b y m i n e b a c k s . High t r e a t i n g pressures occur, w i t h t h e excess pressure i n t h e f r a c t u r e i n c r e a s i n g r a p i d l y when pumping b e g i n s and r e m a i n i n g h i g h d u r i n g t h e j o b , as a consequence o f c r e a t i o n o f m u l t i p l e v e r t i c a l o r T-shaped f r a c t u r e s ( o r m u s h r o o m - l i k e c r a c k s ; c f . s e c t i o n s 1 . 2 . 8 . 4 . and 4 . 9 . 4 . 2 . ) n e a r t h e w e l l b o r e (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1 9 8 8 ) . The t o r t u o u s p a t h f o r f l u i d f l o w gen e r a t e d b y t h e s e m u l t i p l e f r a c t u r e s r e s u l t s i n a zone where t h e p r e s s u r e d r o p i s q u i t e l a r g e . O t h e r e x p l a n a t i o n s f o r h i g h t r e a t i n g p r e s s u r e s a r e s l i p zones f o r m e d i n h i g h l y - c l e a t e d a r e a s i m m e d i a t e l y ahead o f t h e f r a c t u r e t i p . Due t o s t r e s s c o n c e n t r a t i o n s n e a r t h e f r a c t u r e t i p , s l i p c a n o c c u r t h a t t e n d s t o a b s o r b e n e r g y w h i c h o t h e r w i s e w o u l d be u s e d t o p r o p a g a t e t h e c r a c k . As a consequence o f t h e h i g h f l u i d l e a k o f f t h a t c a n t a k e p l a c e i n c l e a t e d c o a l , backstresses can increase d u r i n g t h e j o b which a l s o r a i s e s i n j e c t i o n p r e s s u r e through p o r o e l a s t i c e f f e c t s ( c f . section 4.4.3.8.3.). I f l a r g e volumes o f c o a l f i n e s a r e g e n e r a t e d , t h e y c a n c o n c e n t r a t e a t t h e f r a c t u r e t i p and p l u g i t t h e r e by i n h i b i t i n g f u r t h e r p r o p a g a t i o n and i n c r e a s i n g t h e o p e r a t i n g p r e s s u r e ( c f . s e c t i o n s 4.2.2.7. and 4 . 4 . 3 . 3 . ) . Coal f i n e s may a l s o r e m a i n e n t r a i n e d i n t h e f l u i d and t h u s cause i n c r e a s i n g a p p a r e n t v i s c o s i t y w h i c h a l s o may g i v e r i s e t o e l e v a t e d i n j e c t i o n p r e s s u r e (JONES e t a l . 1 9 8 7 ) . S e v e r a l p a r a l l e l f r a c t u r e s a l s o have c o n s i d e r a b l e i m p a c t on t r e a t i n g p r e s s u r e and c r a c k w i d t h (JEFFREY, VANDAMME, HINKEL & HORNER 1 9 8 7 ) .
4.4.3.4.3.4. Injection rates, fluid loss and treatment pressure F o r r e a l i z a t i o n o f t h e s t r a t e g y o f complex h y d r a u l i c f r a c t u r e s c o n t a i n e d w i t h i n a s i n g l e t h i c k c o a l seam, h i g h i n j e c t i o n r a t e s have t o be u s e d d u r i n g t h e s t i m u l a t i o n t r e a t m e n t t o combat h i g h f l u i d loss (HOLDITCH, ELY, SEMMELBECK, CARTER, HINKEL & JEFFREY 1 9 8 8 ) . I n a d d i t i o n t o h i g h i n j e c t i o n r a t e s , h i g h - v i s c o s i t y s h e a r - s t a b l e d e l a y e d c r o s s l i n k e d f l u i d s ( c f . s e c t i o n 4 . 3 . 4 . 3 . ) and b r i d ging f l u i d - l o s s a d d i t i v e s ( c f . section 4.3.4.6.) are required t o o f f s e t h i g h l e a k o f f . Because t h e c o a l has l o w Young modulus and h i g h f o r m a t i o n c o m p r e s s i b i l i t y and as a consequence o f t h e p r e s e n c e o f complex c r a c k systems, hydraulic f r a c t u r e s s e l d o m p e n e t r a t e more t h a n a f e w h u n d r e d f e e t away f r o m t h e w e l l b o r e . As e x c e s s p r e s s u r e r i s e s due t o complex f r a c t u r e g e o m e t r y ( c f . s e c t i o n 4.4.3.8.3.), a v e r t i c a l component may be i n i t i a t e d i n t o t h e b o u n d a r y l a y e r s a t a weakness p o i n t a t t h e b o r d e r i n t e r f a c e . I f t h i s happens, f l u i d e s c a p i n g t o t h e b o u n d a r y l a y e r c o u l d cause t h e f r a c t u r e s i n t h e c o a l seam t o d e c r e a s e i n w i d t h w h i c h c o u l d l e a d t o s c r e e n o u t i f h i g h p r o p p a n t c o n c e n t r a t i o n s a r e pumped when v e r t i c a l e x c e s s p r o p a g a t i o n s t a r t s (cf. sections 4.2.3.5.3. and 6.2.4.2.1.). I f the sharp drop i n t r e a t i n g pressure i n d i c a t i n g h e i g h t growth o u t o f zone o c c u r s n e a r t h e end o f t h e j o b , i n c r e a s i n g pumping r a t e may p r o v i d e the necessary f r a c t u r e w i d t h r e q u i r e d t o f i n i s h the job. I f t h e pressure drop, however, t a k e s p l a c e e a r l i e r i n t h e o p e r a t i o n , r e i n i t i a t i n g t h e p a d and pumping o f f l u i d w i t h o u t p r o p p a n t u n t i l s u f f i c i e n t f r a c t u r e w i d t h i s o b t a i n e d may be n e c e s s a r y p r i o r t o c o n t i n u i n g t h e p r o p p a n t s t a g e s o f t h e t r e a t m e n t . I f t h e pad i s reinitiated, a l a r g e enough p a d volume m u s t be pumped t o d e v e l o p a f r a c t u r e t h a t i s s u f f i c i e n t l y w i d e t o a c c e p t p r o p p a n t b e f o r e s t a r t i n g back w i t h p r o p p a n t insertion.
477
4.4.3.4.4. Fracture propagation direction
and treatment pressure increase
Treatment p r e s s u r e i n c r e a s e s o c c u r r i n g s h o r t l y a f t e r p r o p p a n t s e n t e r t h e c o a l seam r e f l e c t f l o w r e s t r i c t i o n due t o n e a r - w e l l b o r e p r o p p a n t blockage i n the fracture, and subsequently s h o r t p e r i o d s o f p r e s s u r e d e c l i n e t a k e p l a c e as t h e proppants a r e f l u s h e d away f r o m t h e w e l l b o r e (JONES 1987, 1988). S i m u l a t i o n r u n s r e v e a l t h a t t h e f r a c t u r e blockage i n c o a l seams f r e q u e n t l y c o n t a i n s b o t h proppants and c o a l c h i p s ( c f . s e c t i o n s 4.2.2.7. and 4.4.3.3.). Fracturing with water-based f l u i d s r e s u l t s i n s h o r t e r propped f r a c t u r e l e n g t h s , whereas h i g h e r v i s c o s i t y f r a c t u r i n g media such as l i n e a r g e l and n i t r o g e n foam t h a t have a l s o b e t t e r p r o p p a n t t r a n s p o r t c h a r a c t e r i s t i c s than w a t e r l e a d t o much l o n g e r propped c r a c k s . H y d r a u l i c f r a c t u r e s i n c o a l seams u s u a l l y propagate p a r a l l e l t o t h e maximum s t r e s s o r f a c e - c l e a t d i r e c t i o n ( c f . s e c t i o n 4.4.3.2.) and t h e r e f o r e may n o t adeq u a t e l y access an a n i s o t r o p i c r e s e r v o i r (LOGAN 1988). I n some cases, a h o r i z o n t a l b o r e h o l e p l a c e d p e r p e n d i c u l a r l y t o t h e maximum p e r m e a b i l i t y d i r e c t i o n may t h e r e f o r e be a more e f f e c t i v e s t i m u l a t i o n technique ( c f . s e c t i o n s 4.4.3.6. and 4.8.6.2.1.2.). During coal s t i m u l a t i o n treatments, f r a c t u r e s are h y d r a u l i c a l l y newly induced o r p r e - e x i s t i n g n a t u r a l f r a c t u r e s i n t h e c o a l seam a r e expanded (LAYNE & BYRER 1986). The complex n a t u r a l c r a c k system i n c o a l seams and s u r r o u n d i n g f o r m a t i o n may i n h i b i t t h e p r o d u c t i v i t y o f a s t i m u l a t i o n j o b by d i v e r t i n g f l u i d p a t h s away f r o m t h e pay zone f r a c t u r e channel. A l l c o a l d e p o s i t s c o n t a i n methane, a l t h o u g h t h e c o n c e n t r a t i o n s v a r y f r o m seam t o seam and sometimes even change w i t h i n t h e same seam (DOE 1984). U n l i k e o i l and gas w e l l s where w a t e r h e l p s t o push t h e hydrocarbons t o t h e s u r f a c e , t h e w a t e r i n c o a l seams h i n d e r s t h e gas f r o m moving t o t h e w e l l s and t h u s f i r s t t h e w a t e r has t o be pumped o u t b e f o r e the gas can be d r a i n e d o f f ( c f . s e c t i o n 4.4.3.1.). I n a d d i t i o n , n a t u r a l p r e s s u r e s a r e r e l a t i v e l y low i n a c o a l seam, f o r c i n g t h e gas t o be e i t h e r used o n - s i t e o r compressed i n o r d e r t o enable f l o w through a l o n g e r - d i s t a n c e p i p e l i n e . Coal seams can i n p l a c e s be o v e r p r e s s u r e d and w a t e r - s a t u r a t e d and can possess h i g h s t r u c t u r a l l y enhanced p e r m e a b i l i t y (McBANE & JEU 1988). A t t e m p t s t o produce n a t u r a l gas f r o m c o a l seams u s u a l l y enc o u n t e r s i g n i f i c a n t volumes o f w a t e r which must be removed i n o r d e r t o reduce r e s e r v o i r p r e s s u r e and hence a l l o w d e s o r p t i o n o f methane f r o m t h e c o a l (WAY 1988). Deeper c o a l seams have o v e r a l l l o w e r p e r m e a b i l i t i e s t h a n s h a l l o w e r ones, b u t a r e n e v e r t h e l e s s d e s i r a b l e and e c o n o m i c a l l y a t t r a c t i v e t a r g e t s because o f t h e i r h i g h e r rank and h i g h e r gas c o n t e n t (McKEE, BUMB & KOENIG 1986; SCHRAUFNAGEL & LAMBERT 1988). A comparative e v a l u a t i o n o f h y d r a u l i c f r a c t u r i n g t e c h n i ques f o r c o a l seams i s c a r r i e d o u t by LOGAN, ERWIN & McBANE ( 1 9 8 8 ) .
4.4.3.5.
Fracture conductivity maximization
I n o r d e r t o maximize f r a c t u r e c o n d u c t i v i t y i n c o a l seam s t i m u l a t i o n , p a r t i a l l y coarse 10/20 mesh proppants have t o be a p p l i e d ( c f . a l s o s e c t i o n 4.3.5.) and t h e t o t a l p r o p p a n t q u a n t i t y has t o be i n c r e a s e d o v e r s t a n d a r d designs (JONES, BELL, MORALES & SCHRAUFNAGEL 1987; REEVES, WALLACE & BEAVERS 1987; ZUBER, REEVES, JONES & SCHRAUFNAGEL 1988). The use o f l o w - v i s c o s i t y banking f l u i d s w i l l a l l o w proppants t o s e t t l e t o t h e b o t t o m o f t h e f r a c t u r e and t o f o r m a p r o p p a n t bank which b r i d g e s t h e e n t i r e w i d t h o f t h e b a l l o o n e d f r a c t u r e ( c f . s e c t i o n s 4.3.3.2. and 4.12.2.2.). Banking f l u i d s may a l l o w an u l t r a - h i g h - c o n d u c t i v i t y open f r a c t u r e t o form above t h e p r o p p a n t bank (LASSITER & HOLLINGSWORTH 1984). As a consequence o f i n e f f i c i e n t p r o p p a n t t r a n s p o r t , t h e proppant-banking approach u s i n g w a t e r r e s u l t s i n r e l a t i v e l y s h o r t propped f r a c t u r e l e n g t h which i s s a c r i f i c e d f o r h i g h e r c o n d u c t i v i t y (JONES, BELL, MORALES & SCHRAUFNAGEL 1987). ted
T h i n c o a l seams a r e n o t amenable t o v e r t i c a l d r i l l i n g and t h u s h i g h l y - d e v i a t o h o r i z o n t a l h o l e s have t o f o l l o w t h e c o a l seam more o r l e s s a l o n g i t s d i p
418 o r s t r i k e i n o r d e r t o f i r s t r e n d e r s t i m u l a t i o n more e f f e c t i v e and second t o p r o duce more gas (DUE 1984; c f . s e c t i o n 4 . 8 . 6 . 2 . 1 . ) . S p e c i a l t e c h n i q u e s f o r econom i c a l a c q u i s i t i o n a l s o i n c l u d e b i r d f o o t d r i l l i n g f r o m one c e n t r a l v e r t i c a l s t e m well r a d i a l l y i n t o various directions.
4.4.3.6. Vertical hydraulic fracturing vs. horizontal dr i 1 ling A p a r t f r o m p r e d o m i n a n t l y v e r t i c a l h y d r a u l i c f r a c t u r i n g as a consequence o f t h e m a i n l y g r e a t e r d e p t h o f c o a l seams, h o r i z o n t a l d r i l l i n g i s a n o t h e r s u i t a b l e t e c h n i q u e f o r gas d r a i n a g e f r o m c o a l r e s e r v o i r s , w i t h h o r i z o n t a l b o r e h o l e s f o r c o a l - s e a m d e g a s i f i c a t i o n h a v i n g been i m p l e m e n t e d f o r t h e f i r s t t i m e i n t h e USA more t h a n 30 y e a r s ago (ERTEKIN, SUNG & SCHWERER 1986). The m a i n a d v a n t a g e o f h o r i z o n t a l boreholes i s t h a t t h e d i r e c t i o n o f w e l l b o r e t r a j e c t o r y can be cont r o l l e d with respect t o the principal permeability directions o f the coal, whereas v e r t i c a l h y d r a u l i c f r a c t u r e s have random o r i e n t a t i o n t o t h e c l e a t s y s tem w h i c h i s d e t e r m i n e d b y t h e g e o t e c t o n i c a l s t r e s s s t a t e ( c f . s e c t i o n 4.8.6.2.1.5.). Coal seam t h i c k n e s s has a more p r o n o u n c e d e f f e c t o n a v e r t i c a l f r a c t u r e t h a n on a h o r i z o n t a l b o r e h o l e , because f r a c t u r e s u r f a c e a r e a i n c r e a s e s w i t h c o a l seam t h i c k n e s s i n c o n t r a s t t o t h e e x p o s e d s u r f a c e a r e a o f a h o r i z o n t a l b o r e h o l e w h i c h o n l y i n c r e a s e s w i t h l e n g t h o f t h e w e l l . Based on i d e n t i c a l e x p o s e d s u r f a c e a r e a s , v e r t i c a l f r a c t u r e s and h o r i z o n t a l w e l l s c r e a t e h i g h e r gas p r o d u c t i o n rates in early and l a t e e x p l o i t a t i o n , respectively (cf. also section 4 . 8 . 6 . 2 . 1 . ) . T h e r e f o r e t h e optimum s o l u t i o n f o r c o a l seam d r a i n a g e c o u l d be hor i z o n t a l boreholes from which m u l t i p l e v e r t i c a l f r a c t u r e s a r e propagating, w i t h t h i s c o m b i n a t i o n a c c e s s i n g t h e l a r g e s t p o s s i b l e c o a l seam s u r f a c e a r e a ( c f . a l s o section 4.8.6.2.4.). Horizontal w e l l s i n combination w i t h hydraulic f r a c t u r i n g a r e a l s o s i g n i f i c a n t i n s t r u m e n t s f o r gas d r a i n a g e f r o m s h a l e s ( c f . s e c t i o n 4.4.4.3.2.).
4.4.3.7. Reservoir pressure C o a l seam r e s e r v o i r s i n v a r i o u s b a s i n s a r e g e n e r a l l y c h a r a c t e r i z e d b y o v e r KELSON, p r e s s u r i n g , u n d e r p r e s s u r i n g o r n o r m a l p r e s s u r e r e g i m e (McFALL, WICKS, SEDWICK & BRANDENBURG 1 9 8 7 ) . O v e r p r e s s u r e d a r e a s a r e b a s i n r e g i o n s where gas gen e r a t i o n w i t h i n t h e c o a l d e p o s i t s may s t i l l be a c t i v e , where t h e c o a l s have been u p l i f t e d , o r where t h e o v e r b u r d e n h a s been e r o d e d . U n d e r p r e s s u r e d a r e a s a r e b a s i n r e g i o n s where t h e c o a l s a r e n o l o n g e r a c t i v e l y g e n e r a t i n g gas, where t h e v o i d space may have i n c r e a s e d due t o n a t u r a l f r a c t u r i n g o r c o a l d e s o r p t i o n , o r where t h e gas may have e s c a p e d from t h e c o a l d e p o s i t s t o t h e o v e r - and u n d e r l y i n g rock s t r a t a . Normally pressured areas are b a s i n regions i n which mobile w a t e r and r e s i d u a l gas c o e x i s t w i t h i n t h e c l e a t and n a t u r a l f r a c t u r e s y s t e m o f t h e c o a l d e p o s i t s . In t h e l a t t e r a r e a s , i t i s assumed t h a t an e q u i l i b r i u m h y d r o s t a t i c c o n d i t i o n has been r e a c h e d a f t e r w a t e r has i m b i b e d and r e s a t u r a t e d t h e c o a l seam.
4.4.3.8. Relationships between coal seams and bounding strata F r a c t u r e d e s i g n i n c o a l seams m u s t r e f l e c t t h e l i t h o l o g y o f t h e b o u n d i n g s t r a t a , because t h i s a f f e c t s f r a c t u r e h e i g h t g r o w t h and c r a c k g e o m e t r y (JEU, LOGAN & McBANE 1 9 8 8 ) . U i s t i n c t i o n has m a i n l y t o be made between s a n d s t o n e s and s h a l e s as l i t h o l o g i e s n e x t t o t h e c o a l seam. Some mechanisms o f f r a c t u r e h e i g h t c o n t a i n m e n t and e l e v a t e d t r e a t i n g p r e s s u r e s a r e a l s o i l l u s t r a t e d .
479
4.4.3.8.1. Sandstones Where c o a l seams a r e a d j a c e n t t o sandstone, h y d r a u l i c f r a c t u r e growth occurs i n b o t h sandstone and c o a l , and s t i m u l a t i o n p r e s s u r e s a r e l e s s than 1.1 p s i / f t . L i n e a r f r a c t u r i n g f l u i d s can be used e f f e c t i v e l y t o p l a c e proppants i n t h e c o a l seam i f t h e sandstone o v e r l i e s t h e c o a l (JEU, LOGAN & McBANE 1988). I f t h e sandstone i s s i t u a t e d beneath t h e c o a l seam, i n many cases s u c c e s s f u l s t i m u l a t i o n can be achieved by p e r f o r a t i n g t h e sandstone i n t e r v a l and p r o p a g a t i n g t h e hyd r a u l i c c r a c k f r o m t h e sandstone upwards i n t o t h e c o a l seam. T h i s upwards f r a c t u r i n g t e c h n i q u e has t h e advantage t h a t t h e g u i d i n g sandstone p e r m i t s a l o n g e r f r a c t u r e e x t e n s i o n deeper i n t o t h e r e s e r v o i r than would be p o s s i b l e w i t h i n t h e c o a l seam a l o n e and a l s o enables i n s e r t i o n o f l a r g e r q u a n t i t i e s and h i g h e r conc e n t r a t i o n s o f proppants which a t l e a s t i n p a r t s o f t h e c r e a t e d c r a c k r e a c h a l s o t h e c o a l seam ( s i m i l a r u p h o l e f r a c t u r i n g i s performed i n c h a l k sequences, w i t h t h e c r a c k b e i n g i n i t i a t e d i n a more competent h o r i z o n u n d e r l y i n g s o f t and c o l l a p s i n g s t o r e y s where most o f t h e hydrocarbon r e s e r v e s a r e c o n t a i n e d ; c f . section 4.5.4.3.2.).
4.4.3.8.2. Shales Much h i g h e r t r e a t i n g p r e s s u r e s o c c u r when c o a l i s bounded by shale, w i t h t h e f r a c t u r e s b e i n g p r i m a r i l y c o n t a i n e d t o t h e c o a l and t h e h i g h o p e r a t i o n a l p r e s s u r e s h a v i n g r e s u l t e d i n p a r t f r o m a complex f r a c t u r e geometry (JEU, LOGAN & McBANE 1988). H i g h - s t r e n g t h c a s i n g i s u s e f u l i n accommodating t h e h i g h e r t r e a t i n g p r e s s u r e s and a l l o w s l a r g e r j o b s t o be pumped where many o p e r a t i o n s had t o be t e r m i n a t e d p r e m a t u r e l y when t h e b u r s t s t r e n g t h o f t h e t u b u l a r s was reached. H i g h e r i n j e c t i o n r a t e s and s o l i d f l u i d - l o s s m a t e r i a l c r e a t e s u f f i c i e n t w i d t h i n t h e complex geometry and t h e r e b y i n c r e a s e t h e l i k e l i h o o d o f a s u c c e s s f u l t r e a t ment. I n many cases, t h e i n - s i t u s t r e s s i n sandstone i s l e s s than t h a t i n c o a l 4.2.3.2.2. which i n t u r n i s l e s s t h a n t h a t i n s h a l e ( c f . s e c t i o n s 4.2.2.3.1., and 4 . 2 . 4 . 3 . 1 . ) . I n c o a l seams bounded by shales, c o n f i n e d f r a c t u r e h e i g h t r e s u l t s i n h i g h e r t r e a t i n g p r e s s u r e s which causes a complex T-shaped f r a c t u r e geom e t r y t h a t has b o t h v e r t i c a l and h o r i z o n t a l components (JEFFREY, VANDAMME & ROEG I E R S 1987; c f . s e c t i o n s 1 . 2 . 8 . 4 . and 4 . 9 . 4 . 2 . ) . I n terms o f p r o p a g a t i o n , t h e r a d i u s o f t h e h o r i z o n t a l f r a c t u r e q u i c k l y exceeds c o a l seam t h i c k n e s s and i s more narrow, whereas t h e v e r t i c a l c r a c k i s u s u a l l y v e r y wide due t o i t s c o n f i n e ment (JEU, LOGAN & McBANE 1988). F r a c t u r e growth i n c o a l seams bounded by s h a l e i s u s u a l l y c o n t a i n e d w i t h i n t h e c o a l and a d j a c e n t s h a l e s even though p r e s s u r e s f a r exceed t h e i n - s i t u s t r e s s o f t h e s h a l e s . F r a c t u r i n g p r e s s u r e s f r e q u e n t l y a r e s i g n i f i c a n t l y h i g h e r t h a n 1.1 p s i / f t . Successful f r a c t u r e designs must ens u r e t h a t t h e complex T-shaped c r a c k w i l l have adequate w i d t h t o accommodate p r o p p a n t which i s achieved by i n c l u d i n g a g r e a t e r percentage o f pad ( c f . sect i o n 4.3.4.6.1.4.), h i g h e r i n j e c t i o n r a t e s , and use o f s o l i d f l u i d - l o s s agents such as 40/70 o r 100 mesh sand ( c f . s e c t i o n s 4.8.8.3.1.2. and 4 . 8 . 8 . 3 . 2 . 3 . ) .
4.4.3.8.3. Fracture height containment and elevated treating pressures P o t e n t i a l mechanisms f o r f r a c t u r e h e i g h t containment a r e p l a s t i c d e f o r m a t i o n o f shales, s l i p p a g e a t t h e c o a l / s h a l e i n t e r f a c e , t h e n e a r l y t e n f o l d c o n t r a s t i n Young's modulus between c o a l and t h a t o f s h a l e and sandstone, and l i m i t a t i o n o f f r a c t u r e h e i g h t g r o w t h by c r e a t i o n o f a h o r i z o n t a l f r a c t u r e due t o t h e h i g h t r e a t i n g p r e s s u r e . Magnitude and r a t e o f p r e s s u r e r i s e accompanying t h e s t a r t o f p r o p p a n t i n j e c t i o n l i m i t s c r a c k l e n g t h , w i t h t h e p r e s s u r e r i s e o f t e n reachi n g t h e b u r s t s t r e n g t h o f t h e c a s i n g and a p r e s s u r e o u t c a u s i n g a premature t e r m i n a t i o n o f t h e j o b . Accommodation o f t h e abnormally h i g h t r e a t i n g p r e s s u r e s i s r e q u i r e d t o be a b l e t o c r e a t e t h e v e r y l o n g h y d r a u l i c f r a c t u r e s t h a t a r e needed t o t a p deep l o w - p e r m e a b i l i t y c o a l s . H i g h o p e r a t i n g p r e s s u r e s g e n e r a l l y c r e a t e
480 s h o r t e r and w i d e r f r a c t u r e s , and complex c r a c k geometries cause t r a t i o n t o be v e r y i n e f f i c i e n t .
fracture
pene-
P o s s i b l e mechanisms f o r a b n o r m a l l y h i g h t r e a t i n g p r e s s u r e s a r e unusual f r a c t u r e h e i g h t containment, m u l t i p l e p a r a l l e l f r a c t u r e s ( c f . s e c t i o n 4 . 8 . 6 . 2 . 4 . ) , i n t e r a c t i o n r e s u l t i n g f r o m l e a k o f f i n f l a t i o n o f c o a l c l e a t s (sometimes r e f e r r e d t o as p o r o e l a s t i c o r b a c k s t r e s s e f f e c t s ) , c o a l f i n e s i n c r e a s i n g f l u i d v i s c o s i ty, c o a l f i n e s s c r e e n i n g o f f t h e f r a c t u r e t i p r e g i o n due t o c o a l f a i l u r e , p l a s t i c c o a l d e f o r m a t i o n a t t h e w e l l b o r e , p a r t i a l blockage o f t h e f r a c t u r e a t t h e b o r e h o l e due t o c o a l f a i l u r e ( c f . s e c t i o n s 4.2.2.7. and 4.4.3.3.), and i n c r e a ses i n e f f e c t i v e f r a c t u r e toughness due t o f r a c t u r e s p l i t t i n g n e a r t h e t i p r e gion.
4.4.4. Shales I n a d d i t i o n t o c o a l seam t r e a t m e n t and t i g h t sandstone s t i m u l a t i o n , s i g n i f i c a n t c o n t r i b u t i o n s o f u n c o n v e n t i o n a l gas a r e f u r t h e r d e r i v i n g f r o m s h a l e f r a c t u r i n g (SCHRIDER, OVERBEY, KOMAR & P A S I N I 1977; SCHRIDER & WISE 1980; SWARTZ & AHME0 1982; CHEN, ALAM, VOZNIAK & BLANTON 1984). Shales have a v e r y low m a t r i x p e r m e a b i l i t y ( 0 . 1 - 10 m i c r o d a r c y ) and t h e r e f o r e an e x t e n s i v e n a t u r a l o r synthet i c f r a c t u r e system i s r e q u i r e d i n t h e r e s e r v o i r t o move economical q u a n t i t i e s o f gas t o a w e l l b o r e (SOEDER 1 9 8 8 ) . Shales a r e r o c k s made up o f c l a y - t o s i l t s i z e d p a r t i c l e s independent f r o m p e t r o g r a p h i c a l c o m p o s i t i o n t h a t a r e l a m i n a t e d o r f i s s i l e o r both, w i t h t h e term s h a l e t h u s n o t i m p l y i n g t h a t t h e r o c k has a s i g n i f i c a n t c o n t e n t o f c l a y m i n e r a l s (HENSEL 1 9 8 8 ) . F o l l o w i n g an o u t l i n e o f r e s e r v o i r p r o p e r t i e s and p r o d u c t i o n mechanisms, aspects o f non-proppant f r a c t u r i n g , p r o p p a n t f r a c t u r i n g , e f f e c t i v i t y o f p r o p p a n t vs. n i t r o g e n f r a c t u r i n g , spec i a l f e a t u r e s o f s h a l e gas r e s e r v o i r s and e x p l o s i v e f r a c t u r i n g a r e d i s c u s s e d .
4.4.4.1. Reservoir properties and production mechanisms Shale gas r e s o u r c e s i n c l u d e f r e e gas i n t h e n a t u r a l f r a c t u r e and p o r o s i t y system as w e l l as adsorbed gas on t h e s u r f a c e o f t h e o r g a n i c kerogen (KUUSKRAA & HAAS 1988). The gas adsorbed on o r g a n i c m a t t e r desorbs o n l y s l o w l y i n response t o d e c r e a s i n g r e s e r v o i r p r e s s u r e . The gas r e s i d i n g i n t h e n a t u r a l f r a c t u r e s i s i n dynamical b a l a n c e w i t h t h e adsorbed gas i n t h e m a t r i x o f t h e a d j a c e n t w a l l r o c k . When a w e l l encounters t h e g a s - f i l l e d f r a c t u r e s o r i s j o i n e d t o t h e n a t u r a l c r a c k s by h y d r a u l i c s t i m u l a t i o n , gas moves r a p i d l y f r o m t h e f r a c t u r e system i n t o t h e w e l l b o r e . D u r i n g d e c l i n e o f f l u s h p r o d u c t i o n , t h e f r a c t u r e d gas s h a l e r e a c t s as a c o n v e n t i o n a l porous r e s e r v o i r . The f o l l o w i n g o u t l i n e focusses on gas d r a i n a g e t h r o u g h n a t u r a l f r a c t u r e s , s i g n i f i c a n c e f o r gas i n d u s t r y and r e s e r v e p o t e n t i a l , and impact o f h o r i z o n t a l d r i l l i n g and h y d r a u l i c f r a c t u r i n g .
4.4.4.1.1. Gas drainage through natural fractures Shale w e l l s g e n e r a l l y e x h i b i t a f a i r l y r a p i d i n i t i a l d e c l i n e c u r v e as gas i s d r a i n e d f r o m t h e n a t u r a l f r a c t u r e system, f o l l o w e d by a slow g r a d u a l d e c l i n e as gas f r o m t h e m a t r i x moves t o t h e c r a c k s (KUUSKRAA & HAAS 1988). Gas movement t h r o u g h t h e l o w - p e r m e a b i l i t y s h a l e m a t r i x i s so slow t h a t o n l y a f t e r s e v e r a l y e a r s t h e volume o f m a t r i x gas moving i n t o t h e f r a c t u r e s e q u a l s t h e volume o f gas f l o w i n g f r o m t h e c r a c k s i n t o t h e w e l l b o r e . P r i o r t o a t t a i n i n g t h i s steadys t a t e flow, t h e w i t h d r a w a l d i m i n u t i o n c u r v e shows a steep drop, b u t soon a f t e r t h e s t e a d y - s t a t e f l o w f r o m m a t r i x v i a f r a c t u r e t o w e l l b o r e has been reached, t h e p r o d u c t i o n d e c l i n e c u r v e f l a t t e n s almost t o t h e h o r i z o n t a l and shows o n l y s l i g h t d e c l i n e d u r i n g t h e n e x t s e v e r a l decades. Gas d r a i n a g e t h r o u g h n a t u r a l f r a c t u r e s i s a l s o a s i g n i f i c a n t mechanism f o r e x p l o i t a t i o n o f c o a l seams ( c f . section 4.4.3.1.). An i n c r e a s e i n t h e amount o f s u r f a c e area connected t o t h e
borehole
by
hy-
481 d r a u l i c f r a c t u r e s causes more o f t h e adsorbed gas t o be r e l e a s e d and produced over the e n t i r e l i f e o f the w e l l (MERCER, PRATT & YOST 1988). KAMATH & GODBOLE (1985) comment on h o t b r i n e s t i m u l a t i o n f o r gas p r o d u c t i o n from n a t u r a l gas hyd r a t e s . OZKAYA (1986) analyzes n a t u r a l h y d r a u l i c f r a c t u r i n g o f shales d u r i n g sedimentation. An assessment o f the p o t e n t i a l o f shale gas r e s e r v o i r s i n c l u d i n g o u t l i n e o f e f f e c t i v e p r o d u c t i o n techniques i s g i v e n by KUUSKRAA, WICKS, SAWYER, ESPOSITO & KOMAR (1983); McBANE & THOMPSON (1984), MASON (1987) and VANORSDALE (1987).
4.4.4.1.2. Significance for gas industry and reserve potential Gas from shale r e s e r v o i r s a c t u a l l y gave b i r t h t o the n a t u r a l gas i n d u s t r y i n the USA when the f i r s t commercial gas w e l l was d r i l l e d i n t o Devonian shales i n Canadaway Creek near Fredonia/New York i n 1821 ( t h u s preceding the f i r s t o i l w e l l d r i l l e d i n 1859 near T i t u s v i l l e / P e n n s y l v a n i a by 38 years) and heralded a new energy source (DOE 1984, WITT 1986) which was a l r e a d y abundantly e x p l o i t e d more than 100 years ago. The Devonian shales o f the Appalachian, I l l i n o i s and Michigan basins i n the USA c o n t a i n i n f a c t one o f the l a r g e s t worldwide concent r a t i o n o f o r g a n i c a l l y r i c h carbon and gas i n p l a c e (KUUSKRAA & HAAS 1988) and are estimated t o c o n t a i n more than 100 T c f o r 2,500 B i l l . m3 recoverable resources (OFFICE OF TECHNOLOGY ASSESSMENT 1985). The c u r r e n t development a c t i v i t y by d r i l l i n g and s t i m u l a t i o n i s concentrated i n West V i r g i n i a , Kentucky and Ohio. DOE (1983 a); GATENS, LEE & R A H I M (1985); KOZIAR (1985); KUUSKRAA, SEDWICK & YOST (1985); LEE & GATENS (1985); HAZLETT, LEE, NARAHARA & GATENS (1986); KEMPER, FRANKIE, MOODY & JOHNSTON (1986); WITT (1986); GATENS, LEE, LANE, WATSON, STANLEY & LANCASTER (1987) and GUIDRY, GRAHAM, LANCASTER & CURTIS (1987) present reviews on e v a l u a t i o n , completion and t e s t i n g o f the p r o d u c t i o n p o t e n t i a l o f Devonian shales. LINDEN (1985) u n d e r l i n e s the r o l e which shale gas p l a y s i n the gas supply d i v e r s i f i c a t i o n concepts o f the near f u t u r e ( c f . s e c t i o n 2.2.1.6.4.). A systematical approach f o r economical development o f shale gas resources i s documented by AMERI, AMINIAN, MILLER, D O R I C I C H & YOST (1985). Problems o f l o g i n t e r p r e t a t i o n i n shale gas r e s e r v o i r s are summarized by CAMPBELL & TRUMAN (1986); McBANE, CAMPBELL & TRUMAN (1986); G R I (1987 b ) and VANORSDALE (1987), w i t h t r a d i t i o n a l l o g e v a l u a t i o n having d i f f i c u l t i e s i n d i s t i n g u i s h i n g between apparent and e f f e c t i v e p o r o s i t y (KUUSKRAA & HAAS 1988).
4.4.4.1.3. Impact o f horizontal drilling and hydraulic treating S i m i l a r l y as a p p l y i n g f o r coal seams, t h e gas p r o d u c t i o n p o t e n t i a l o f shales can be g r e a t l y improved by a combination o f h o r i z o n t a l w e l l d r i l l i n g and mult i p l e h y d r a u l i c f r a c t u r i n g (YOST, OVERBEY, SALAMY, OKOYE & SARADJI 1987; ZUBER, LEE & GATENS 1987; c f . s e c t i o n s 4.8.6.2.1. and 4.8.6.2.4.). Directional d r i l l i n g o f a h o r i z o n t a l w e l l r e c t a n g u l a r l y c r o s s i n g the n a t u r a l cracks f o l l o w e d by m u l t i p l e v e r t i c a l h y d r a u l i c f r a c t u r e s t i m u l a t i o n o r t h o g o n a l l y t o the borehole t r a j e c t o r y i s the optimum means t o increase the shale surface area i n c o n t a c t w i t h the borehole i n o r d e r t o r e l e a s e more o f the adsorbed gas (HORTON, MERCER & SAWYER 1982; SALAMY, SARADJI, OKOYE, MERCER & YOST 1987). Aspects o f i n t e r f e r e n c e o f n a t u r a l and h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s i n shales are a l s o discussed by P A S I N I & OVERBEY (1969) and BLANTON (1982). P o r o s i t y and p e r m e a b i l i t y o f gas shales are evaluated by SOEDER (1988). Aspects o f shale w e l l s t i m u l a t i o n by h y d r a u l i c f r a c t u r i n g are o u t l i n e d by FOSTER (1975), TILLMAN (1983); OUTTON, BEMIS, JOHNSON & WHITE (1984); SHAW, GATENS, LANCASTER, LEE, AVARY & TERRY (1986); LANCASTER, GUIDRY, GRAHAM, CURTIS, SHAW & BLAKE (1987); KUUSKRAA, KELAFANT & HORNER (1988); LAYNE & SIRIWARDANE (1988) and NEAR I N G & STARTZMAN (1988). Shale r e f r a c t u r i n g i s o u t l i n e d by GUIDRY, GRAHAM, CURT I S , SHAW & BLAKE (1987). D i s t i n c t i o n has t o be made between non-proppant and proppant h y d r a u l i c f r a c t u r i n g as w e l l as e x p l o s i v e f r a c t u r i n g .
482
4.4.4.2. Non-proppant fracturing The m o s t i m p o r t a n t n o n - p r o p p a n t f r a c t u r i n g t e c h n i q u e s i n s h a l e gas r e s e r v o i r s a r e n i t r o g e n f r a c t u r i n g , c a r b o n d i o x i d e f r a c t u r i n g and foam f r a c t u r i n g w h i c h a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.4.4.2.1. Nitrogen fracturing When pumped a t s u f f i c i e n t r a t e s and p r e s s u r e s , gaseous n i t r o g e n a l o n e i s s u c c e s s f u l as a f r a c t u r i n g medium f o r s h a l e e v e n w i t h o u t p r o p p a n t s and may e v e n o u t p e r f o r m o t h e r t r e a t m e n t t y p e s (ABEL 1982, ABEL e t a l . 1 9 8 3 ) . The a d v a n t a g e s o f n i t r o g e n a r e non-damaging c h a r a c t e r i s t i c s , minimum s h u t - i n t i m e , and minimum n o m i n a l t r e a t m e n t c o s t s . L i q u i d s can c a u s e c l a y m i g r a t i o n and s w e l i i n g i n t h e formation which reduces p e r m e a b i l i t y (water s e n s i t i v i t y ; c f . s e c t i o n 3.11.2.), and t h e r e f o r e l i q u i d c o n t e n t o f f r a c t u r i n g media i s e x t r e m e l y c r u c i a l . I n cont r a s t t o f l u i d s , n i t r o g e n i s i n e r t , r e l a t i v e l y i n s o l u b l e and c o m p r e s s i b l e , with t h e s e p r o p e r t i e s p r e v e n t i n g any damage t o s h a l e f o r m a t i o n s . A f t e r an o u t l i n e o f a d v a n t a g e s o f n i t r o g e n gas f r a c t u r i n g , some comments a r e o f f e r e d on w e l l b o r e v i c i n i t y damage r e m o v a l and r e s e r v o i r breakdown.
4.4.4.2.1.1. Advantages o f nitrogen gas fracturing C o n c e r n i n g a d v a n t a g e s o f n i t r o g e n s t i m u l a t i o n , no f l u i d l o s s a d d i t i v e s a r e fractures n e c e s s a r y i n n i t r o g e n gas t r e a t m e n t s . S i n c e n i t r o g e n gas i s m o b i l e , t h a t v i s c o u s l i q u i d s o r s o l i d s c o u l d n o t p e n e t r a t e c a n be c o n n e c t e d f o r optimum d e l i v e r a b i l i t y . The s h u t - i n t i m e i s a l s o c r u c i a l , because t h e l o n g e r t h e liq u i d s r e m a i n i n t h e w e l l , t h e more s e v e r e f o r m a t i o n damage p r o v o k e d e s p e c i a l l y i n s h a l e s . F r a c t u r i n g t r e a t m e n t s u s i n g o n l y gaseous n i t r o g e n u n d e r p r e s s u r e t o c r e a t e c r a c k s a r e g e n e r a l l y l e s s c o s t l y t h a n o t h e r h y d r a u l i c methods and s i n c e no l i q u i d s a r e a p p l i e d , l i q u i d r e t e n t i o n and removal p r o b l e m s do n o t o c c u r (BRANDENBURG 1 9 8 8 ) . Because t h e s e o p e r a t i o n s a r e c o n v e n t i o n a l l y p e r f o r m e d w i t h o u t p r o p p a n t s , s p a l l i n g o r f r a c t u r e f a c e i r r e g u l a r i t i e s a r e r e l i e d upon f o r maintaining fracture conductivity.
4.4.4.2.1.2. We1 lbore vicinity damage removal and reservoir breakdown N i t r o g e n f r a c t u r i n g t r e a t m e n t s i n s h a l e gas r e s e r v o i r s m a i n l y remove n e a r w e l l b o r e damage ( s i m i l a r l y as i n c o a l - s e a m s t i m u l a t i o n ; c f . s e c t i o n 4 . 4 . 3 . 1 . 3 . ) and e s t a b l i s h c o m m u n i c a t i o n w i t h p r e v i o u s l y n o n - p r o d u c i n g p e r f o r a t i o n s and t h e f o r m a t i o n (ELY 1988, LANCASTER 1 9 8 8 ) . As n i t r o g e n f r a c t u r i n g v i r t u a l l y e l i m i n a tes t h e clean-up problems a s s o c i a t e d w i t h l i q u i d systems ( c f . sections 3 . 1 1 . 2 . 2 . ) , p r o d u c t i o n i s n o t l o s t and c o s t l y swabbing u n i t s t o r e c o v e r f l u i d s a r e n o t needed. Low i n i t i a l c o s t f o r n i t r o g e n f r a c t u r i n g g u a r a n t e e s r a p i d p a y back and r e n d e r s i t e v e n e c o n o m i c a l t o r e p e a t t h e s t i m u l a t i o n t r e a t m e n t a t a l a t e r s t a g e . N i t r o g e n f r a c t u r i n g o f gas s h a l e s w i t h p a r t i a l p r o p p a n t i n s e r t i o n i s o u t l i n e d by GOTTSCHLING, ROYCE & SCHUCK ( 1 9 8 5 ) , and K I N G (1982, 1983) documents h y d r a u l i c p r o p p a n t s t i m u l a t i o n w i t h l i q u i d c a r b o n d i o x i d e . An e c o n o m i c a l a n a l y s i s o f foam f r a c t u r i n g i n s h a l e s (sometimes i n c l u d i n g p r o p p a n t a d d i t i o n ) i s p e r f o r m e d b y LIEBENTHAL, KOMAR, RIEKE & SKILLERN ( 1 9 7 9 ) and STRANG & NORTON ( 1 9 8 3 ) . COURSEN ( 1 9 8 3 ) d i s c u s s e s t h e e f f e c t o f e x p l o s i v e s t i m u l a t i o n w i t h v a r y i n g c h a r g e t y p e s on gas p r o d u c t i v i t y o f s h a l e s . N i t r o g e n i s a l s o a s u i t a b l e medium f o r p e r f o r m i n g m i c r o f r a c t u r i n g breakdown treatments ( c f . s e c t i o n 4.8.2.2.) i n o r d e r t o minimize n a t u r a l f r a c t u r e system damage i n s h a l e s and t i g h t s a n d s t o n e s (BRANAGAN & WILMER 1 9 8 8 ) . S e v e r a l t e c h n i ques c a n be a p p l i e d i n c l u d i n g h y d r a u l i c n i t r o g e n f r a c t u r i n g u s i n g s u r f a c e pump t r u c k s , p e r f o r a t i n g w i t h an u n d e r b a l a n c e d n i t r o g e n column, h i g h - p r e s s u r e n i t r o -
483 gen impulse perforating/microfracturing combination, n i t r o g e n impulse i n j e c t i o n u s i n g a t u b i n g pump-out p l u g , and t a i l o r e d p u l s e e x p l o s i v e f r a c t u r i n g .
4.4.4.2.2. Carbon dioxide and foam fracturing A p a r t f r o m s t r a i g h t n i t r o g e n f r a c t u r i n g , o t h e r non-proppant h y d r a u l i c f r a c t u r i n g techniques commonly a p p l i e d i n s h a l e gas r e s e r v o i r s a r e carbon d i o x i d e f r a c t u r i n g and foam f r a c t u r i n g , w i t h t h e l a t t e r method p a r t i c u l a r l y f r e q u e n t l y a l s o b e i n g combined w i t h proppant i n s e r t i o n i n o r d e r t o improve t h e r e s u l t o f t h e j o b beyond s i m p l e communication between b o r e h o l e and r e s e r v o i r as w e l l as f o r m a t i o n damage removal around t h e w e l l b o r e ( c f . s e c t i o n 4 . 8 . 3 . ) . Some aspects o f e f f e c t i v i t y o f carbon d i o x i d e vs. n i t r o g e n t r e a t i n g , foam s t i m u l a t i o n and t a i l o r e d p u l s e l o a d i n g , and a u t o - p r o p p i n g o f c r a c k s c r e a t e d by gas f r a c t u r i n g a r e d i s c u s s e d as f o l l o w s .
4.4.4.2.2.1. Effectivity of carbon dioxide vs. nitrogen treating YOST, OVERBEY, WILKINS & LOCKE (1988) compare t h e e f f e c t i v i t y o f d i f f e r e n t h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s i n a h o r i z o n t a l b o r e h o l e t r a v e r s i n g s h a l e gas r e s e r v o i r s w i t h e x t e n s i v e n a t u r a l j o i n t i n g . Carbon d i o x i d e j o b s t u r n o u t t o be more t h a n f o u r t i m e s b e t t e r than n i t r o g e n f r a c t u r i n g i n i t i a l l y and t o l a s t t w i c e as l o n g p r i o r t o c l o s u r e o f t h e non-propped c r a c k s . The d e n s i t y c o n t r a s t between n i t r o g e n and carbon d i o x i d e may have caused t h e p r o p a g a t i o n o f more and w i d e r f r a c t u r e s w i t h carbon d i o x i d e r e s u l t i n g i n h i g h improvement r a t i o s . A l though s t r e s s r a t i o s i n d i c a t e t h a t t h e area under s t u d y i s n e a r l y t e c t o n i c a l l y r e l a x e d , proppants a r e s t i l l r e q u i r e d t o m a i n t a i n permeable f l o w p a t h s f o r more than s e v e r a l weeks.
4.4.4.2.2.2. Foam fracturing and tailored pulse loading I n a d d i t i o n t o n i t r o g e n and carbon d i o x i d e f r a c t u r i n g , o t h e r non-proppant s t i m u l a t i o n methods h i t h e r t o a p p l i e d i n s h a l e gas r e s e r v o i r s i n c l u d e foam f r a c t u r i n g and t a i l o r e d p u l s e l o a d i n g , w i t h t h e l a t t e r t e c h n i q u e c o m p r i s i n g i g n i t i o n o f s l o w - b u r n i n g p r o p e l l a n t s i n t h e b o r e h o l e ( D O E 1984; c f . s e c t i o n s 4.2.2.1.3.3., 4.7.3., 4.8.8.5. and 4 . 8 . 9 . 6 . ) . The f o r c e o f t h e b u r n p l u s t h e produced combustion gases crack t h e r o c k i n s e v e r a l d i r e c t i o n s a t once i n s t e a d o f t h e u s u a l s i n g l e - d i r e c t i o n f r a c t u r e s c r e a t e d by w a t e r o r foam. C o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g has a l s o h i s t o r i c a l l y been a p p l i e d i n s h a l e gas r e s e r v o i r s and r e s u l t s i n communication o f t h e b o r e h o l e w i t h t h e pay zone and i n a r e l a t i v e l y small degree o f s t i m u l a t i o n because t h e c r e a t e d f r a c t u r e s a r e m u l t i d i r e c t i o n a l , unpropped and o f l i m i t e d p e n e t r a t i o n (BRANDENBURG 1988; c f . s e c t i o n 4.4.4.6.).
4.4.4.2.2.3. Auto-propping of cracks created by gas fracturing Experience w i t h non-proppant n i t r o g e n f r a c t u r i n g t r e a t m e n t s has shown t h a t a u t o - p r o p p i n g o f t h e c r a c k s by gas e r o s i o n g e n e r a t i n g an e t c h e d s u r f a c e i n t h e f o r m a t i o n a t t h e f r a c t u r e w a l l , r a p i d flowback causing pay s p a l l i n g , o r some o t h e r mechanisms o f r e s e r v o i r d e f o r m a t i o n g i v i n g r i s e t o an a u t o - p r o p p i n g cond u c t i v e f l o w p a t h i s e x t r e m e l y r a r e , i f e x i s t i n g a t a l l (ELY 1988). C a l c u l a t i o n o f c u r r e n t r a t e s o f n i t r o g e n across t h e f r a c t u r e f a c e shows t h a t t h e p o s s i b i l i t y o f s i g n i f i c a n t e r o s i o n i s remote, and t h e f a c t t h a t c o n s i d e r a b l e amounts o f r e s e r v o i r m a t e r i a l a r e produced w i t h r a p i d flowback procedures does n o t p r o v e t h e i n s t a l l a t i o n o f a c o n d u c t i v e p a t h by s e l f - p r o p p i n g , b u t r a t h e r t e s t i f i e s t o f a i l u r e o f t h e s h a l e under t h e imposed s t r e s s . Auto-propping a l s o i s t h e mechanism t o keep open c r a c k s h a v i n g been generated by a c i d f r a c t u r i n g , b u t a l s o i n carbonate s t i m u l a t i o n , t h e much b e t t e r approach i s h y d r a u l i c p r o p p a n t t r e a t m e n t w i t h p r o p e r i n f i l l i n g and s u p p o r t o f t h e c r a c k ( c f . s e c t i o n 4 . 5 . 1 . 1 . 1 . 2 . ) .
484 T h e r e f o r e much s u p e r i o r r e s u l t s o f s h a l e gas s t i m u l a t i o n can be achieved by hyd r a u l i c p r o p p a n t f r a c t u r i n g . Aspects o f foam f r a c t u r i n g i n s h a l e gas r e s e r v o i r s a r e a l s o d i s c u s s e d by KOMAR & YOST (1979) and DRISCOLL, BOWEN & ROBERTS ( 1 9 8 0 ) .
A t t h e b e g i n n i n g o f h y d r a u l i c f r a c t u r i n g i n gas shales i n t h e e a r l y 1950's, o n l y w a t e r was used as t r e a t i n g f l u i d and y i e l d e d c o n s i d e r a b l y h i g h e r gas f l o w r a t e s t h a n e a r l i e r e x p l o s i v e s t i m u l a t i o n (WITT 1986). The l a t e r s w i t c h t o foam and l i q u e f i e d gas f r a c t u r i n g f u r t h e r i n c r e a s e d gas r e c o v e r y , decreased t h e t i m e required f o r the w e l l t o r e t u r n the f r a c t u r i n g f l u i d , l a r g e l y prevented water blockage i n t h e c r a c k s ( c f . s e c t i o n 3.11.2.4.2.), and p e r m i t t e d more c a r e f u l l y designed procedures f o r s t i m u l a t i n g m u l t i p l e zones i n one w e l l ( c f . s e c t i o n 4.2.2.1.3.).
4.4.4.3.
Proppant fractur ins
A l t h o u g h s t r a i g h t n i t r o g e n f r a c t u r i n g b o t h as s m a l l - s c a l e f o r m a t i o n and p e r f o r a t i o n breakdown t r e a t m e n t and as l a r g e - s c a l e s t i m u l a t i o n o p e r a t i o n a r e v e r y s u c c e s s f u l i n s h a l e gas r e s e r v o i r s p a r t i c u l a r l y due t o t h e i r e x t r e m e l y low c o s t and t e c h n i c a l s i m p l i c i t y , l o n g - t e r m s t i m u l a t i o n e f f e c t s can o n l y be achieved by p r o p p a n t f r a c t u r i n g i n d i f f e r e n t s c a l e s . Experience has shown, however, t h a t s h a l e gas r e s e r v o i r s a r e v e r y s e n s i t i v e pay zones which r e q u i r e p r o p e r d e s i g n o f t h e t r e a t m e n t s , w i t h emphasis b e i n g on achievement o f s u f f i c i e n t p r o p p a n t c o n c e n t r a t i o n i n t h e c r a c k ( c f . s e c t i o n 4.3.) and r a p i d cleanup o f t h e w e l l a f t e r t e r m i n a t i o n o f t h e j o b ( c f . s e c t i o n 3 . 1 1 . 2 . ) . Some aspects o f p r o p p a n t p l a cement and f l u i d r e t r i e v a l as w e l l as v e r t i c a l vs. h o r i z o n t a l h o l e d r i l l i n g and f r a c t u r i n g a r e d i s c u s s e d as f o l l o w s .
4.4.4.3.1. Proppant placement and f l u i d r e t r i e v a l LANCASTER, GUIDRY, GRAHAM, CURTIS, SHAW & BLAKE (1987) p r e s e n t p r o p p a n t f r a c t u r i n g o f s h a l e gas r e s e r v o i r s b o t h f o r o r i g i n a l t r e a t m e n t s and f o r l a t e r r e s t i m u l a t i o n . As p r e v i o u s e x p e r i e n c e has shown t h a t l a r g e h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t s a r e e f f e c t i v e means o f s h a l e s t i m u l a t i o n , the optimal design s t r a t e g y s h o u l d maximize p r o p p a n t placement and m i n i m i z e l i q u i d volumes. Good s o l u t i o n s a r e achieved w i t h n i t r o g e n foam f r a c t u r i n g f l u i d s c a r r y i n g h i g h p r o p p a n t c o n c e n t r a t i o n s w i t h a p p l i c a t i o n o f a p p r o p r i a t e chemical a d d i t i v e s t o ass i s t i n e f f i c i e n t cleanup o f l i q u i d s f o l l o w i n g t h e t r e a t m e n t ( c f . s e c t i o n 3.11.2.). Proppant f r a c t u r i n g has h i t h e r t o c o m p a r a t i v e l y r a r e l y been a p p l i e d i n s h a l e gas r e s e r v o i r s , b u t i s c o n s i d e r e d t o be o f p r o g r e s s i v e l y r i s i n g s i g n i f i cance i n t h e coming y e a r s . Aspects o f s h a l e proppant f r a c t u r i n g a r e a l s o d i s c u s sed by K I N G (1982, 1983) and GOTTSCHLING, ROYCE & SCHUCK ( 1 9 8 5 ) . Gas p r o d u c t i o n i n s h a l e r e s e r v o i r s i s f r e q u e n t l y a s s o c i a t e d w i t h i n t e r v a l s h a v i n g up t o 4 % h i g h e r p o r o s i t y and p e r m e a b i l i t y than t h e u s u a l s h a l e s e c t i o n , h a v i n g a s u b s t a n t i a l p o r t i o n o f t h e p o r o s i t y f i l l e d w i t h hydrocarbons ( t h e f r e e gas s a t u r a t i o n i s g r e a t e r than a b t . 2 % o f t h e b u l k volume) and h a v i n g a r e l a t i v e l y h i g h kerogen c o n t e n t ( g r e a t e r than 2 . 5 % ) . The f o l l o w i n g d i s c u s s i o n f o c u s ses on comparative s u p e r i o r i t y o f p r o p p a n t f r a c t u r i n g as w e l l as c o m b i n a t i o n o f gas and foam-proppant f r a c t u r i n g .
4 . 4 . 4 .3.1.1. Comarative s u p e r i o r i t y o f proppant f r a c t u r i n g ZUBER, LEE & GATENS (1987) compare t h e e f f e c t i v i t y o f d i f f e r e n t s t i m u l a t i o n t e c h n i q u e s i n Devonian s h a l e r e s e r v o i r s . P e r m e a b i l i t y a n i s o t r o p y i n t h e Devon i a n s h a l e i s p r e s e n t i n v a r i o u s degrees and i s a r e s u l t o f v a r i a t i o n o f n a t u r a l f r a c t u r e spacing, c o n d u c t i v i t y , geometry and o t h e r h e t e r o g e n e i t i e s . As a consequence o f a n i s o t r o p i c p e r m e a b i l i t y caused by n a t u r a l c r a c k t r e n d s , a w e l l w i t h an induced f r a c t u r e p e r p e n d i c u l a r l y t o t h e n a t u r a l j o i n t t r e n d w i l l p e r f o r m much more f a v o u r a b l y t h a n a w e l l w i t h an induced f r a c t u r e p a r a l l e l t o t h e
485 n a t u r a l crack t r e n d . Comparative assessment o f s i n g l e h y d r a u l i c f r a c t u r i n g , t a i lored-pulse s e l e c t i v e and c o n t r o l l e d e x p l o s i v e f r a c t u r i n g c r e a t i n g two o r more r a d i a l f r a c t u r e s being p e r p e n d i c u l a r t o each o t h e r ( c f . s e c t i o n s 4.2.2.1.3.3. and 4.7.3.3.), and standard e x p l o s i v e f r a c t u r i n g r e v e a l s a r a n k i n g o f e f f e c t i v i t y o f the d i f f e r e n t methods i n the mentioned sequence. H y d r a u l i c proppant f r a c t u r i n g t h e r e f o r e has t u r n e d o u t t o be the economically most a t t r a c t i v e s t i m u l a t i o n p r i n c i p l e a l s o i n Devonian shale gas r e s e r v o i r s . Other treatment methods comprise nitrogen-foam (KOMAR, YOST & SINCLAIR 1980) o r pure n i t r o g e n gas f r a c t u r i n g w i t h o r w i t h o u t proppant i n s e r t i o n (FREEMAN, ABEL, K I M & HEINRICH 1981; c f . s e c t i o n 4.4.4.2.1.). As a consequence o f t h e experience w i t h more advanced techniques i n t h e l a s t years, the h i s t o r i c a l l y t r a d i t i o n a l standard e x p l o s i v e s t i m u l a t i o n by borehole shooting w i t h 80 % g e l a t i n a t e d n i t r o g l y c e r i n e ( c f . s e c t i o n 4.4.4.6.) has been p r o g r e s s i v e l y replaced by s i n g l e h y d r a u l i c and m u l t i p l e r a d i a l t a i l o r e d - p u l s e f r a c t u r i n g . OVERBEY, YOST & WILKINS (1988) r e p o r t nitrogen-foam proppant f r a c t u r i n g . Other aspects o f shale r e s e r v o i r s t i m u l a t i o n are discussed by KOMAR (1978) and HORTON (1982). OZKAYA (1984) o u t l i n e s computer s i m u l a t i o n o f hydraul i c f r a c t u r i n g i n shales.
4.4.4.3.1.2. Combination of gas and foam-proppant fracturing LAYNE & SIRIWARDANE (1988) f i n d o u t from f i e l d experience t h a t t h e most e f f e c t i v e f r a c t u r e design may c o n s i s t o f a h y b r i d treatment w i t h a carbon d i o x i d e pad and h i g h - q u a l i t y foam as proppant t r a n s p o r t f l u i d which prevents screenout and f o r m a t i o n damage w h i l e m a i n t a i n i n g p o s t - s t i m u l a t i o n f r a c t u r e c o n d u c t i v i t y ( c f . s e c t i o n 3.11.2.2.). Carbon d i o x i d e may be the p r e f e r r e d base f l u i d f o r many shale formations, whereas n i t r o g e n foam has o f t e n h i g h e r proppant t r a n s p o r t c a p a c i t y than carbon d i o x i d e foam. F l u i d entrance i n t o enlarged n a t u r a l f r a c t u r e s d u r i n g course o f the s t i m u l a t i o n treatment takes p l a c e as annulus loss and l a r g e i n j e c t i o n f l o w i n t o d i s c r e t e f r a c t u r e s . L e a k o f f i s d e f i n e d as a small amount o f f l u i d t h a t does n o t p e n e t r a t e o r s i g n i f i c a n t l y deform the r e s e r v o i r , whereas l a r g e i n j e c t i o n f l o w represents a s i g n i f i c a n t r a t e o f f l u i d penet r a t i o n t h a t i s capable o f c a r r y i n g a proppant and i n f l a t i n g e x i s t i n g f r a c t u r e s t o enhance r e s e r v o i r p e r m e a b i l i t y . YOST, OVERBEY, WILKINS & LOCKE (1988) conclude t h a t t h e b e s t r e s u l t s o f hyd r a u l i c s t i m u l a t i o n o f a h o r i z o n t a l borehole through shale gas r e s e r v o i r s which are w a t e r - s e n s i t i v e would be a carbon d i o x i d e pneumatic proppant t r a n s p o r t f r a c t u r i n g process t h a t moves s u f f i c i e n t amounts o f proppants t o m a i n t a i n crack cond u c t i v i t y w i t h o u t a water base f l u i d (KING 1983, LEINAN & H A R R I S 1983, SINAL & CANCASTER 1987; MERCER, PRATT & YOST 1988) o r a mixed treatment c o n s i s t i n g o f a carbon-dioxide pad f o l l o w e d by proppant-laden n i t r o g e n foam where t h e l i q u i d phase c o n t a i n s water and methanol ( c f . s e c t i o n 3.11.2.2.). As s t r a i g h t carbon d i o x i d e f r a c t u r i n g operations had the h i g h e s t i n i t i a l improvement r a t i o , u l t i m a t e l y a carbon dioxide-based treatment may be p r e f e r r e d . N i t r o g e n foam can a l s o be a p p l i e d i n massive h y d r a u l i c proppant f r a c t u r i n g j o b s w i t h placement o f proppant c o n c e n t r a t i o n s up t o 6 l b s / g a l a t downhole conditions, w i t h l i q u i d volume m i n i m i z a t i o n and proppant t r a n s p o r t maximization being the o n l y p o s s i b i l i t y t o achieve a reasonable r e l a t i o n s h i p between treatment c o s t and e f f e c t i n the shale gas r e s e r v o i r s (GATENS 1988). Aspects o f proppant-laden n i t r o g e n foam f r a c t u r i n g are a1 so discussed by LANCASTER (1988).
4.4.4.3.2. Vertical vs. horizontal hole drilling and fracturing I n terms o f w e l l type, case s t u d i e s i n shale gas r e s e r v o i r s r e v e a l e d t h a t f o u r h o r i z o n t a l u n s t i m u l a t e d boreholes can produce 1.5 times as much gas as s i x teen v e r t i c a l u n t r e a t e d w e l l s i n b o t h i n f i l l d r i l l i n g and v i r g i n r e s e r v o i r development cases (MERCER, PRATT & YOST 1988). The e f f e c t o f f o u r u n s t i m u l a t e d h o r i z o n t a l boreholes can be achieved by o n l y two h o r i z o n t a l w e l l s i f they are hy-
486 d r a u l i c a l l y f r a c t u r e d . On t h e o t h e r hand, BRANDENBURG ( 1 9 8 8 ) c o n c l u d e s t h a t i n a number o f i n s t a n c e s , h y d r a u l i c f r a c t u r i n g f r o m v e r t i c a l w e l l s a p p e a r s t o o f f e r t h e b e s t o p p o r t u n i t y t o e x t r a c t gas r e s e r v e s f r o m s h a l e s i n an e c o n o m i c a l l y e f f i c i e n t manner. Horizontal wells crossing multiple natural fractures i n the reservoir are req u i r e d i n l o w - p e r m e a b i l i t y f o r m a t i o n s s u c h as s h a l e s i n o r d e r t o i n c r e a s e n a t u r a l gas r e c o v e r y and t o r e d u c e t h e r i s k o f d r i l l i n g a d r y h o l e (LAYNE & S I R I W A R DANE 1 9 8 8 ) . Pumping o f h i g h - p r e s s u r e f l u i d down t h r o u g h t u b i n g and a n n u l u s g i ves r i s e t o e n l a r g e m e n t o f numerous n a t u r a l j o i n t s i n t h e s h a l e sequence i n t e r sected by the horizontal w e l l ( c f . section 4.8.6.2.). Fracture containment i n v e r t i c a l d i r e c t i o n promotes e x t e n s i v e f l u i d p e n e t r a t i o n throughout t h e f r a c t u r e n e t w o r k and e n a b l e s i n d u c t i o n o f h i g h l y e l o n g a t e d e q u i v a l e n t c r a c k s t h r o u g h more t h a n a s i n g l e n a t u r a l j o i n t o r i e n t a t i o n ( a n e q u i v a l e n t f r a c t u r e h a s t h e same c r a c k volume e x t e n d e d i n t o t h e r e s e r v o i r and does n o t f o l l o w a s i n g l e d i r e c t i o n o f maximum p r i n c i p a l s t r e s s ) . I n p a y zones w i t h numerous f r a c t u r e i n t e r sections, l o w a n g l e s o f i n d u c e d c r a c k approach, and l o w r a t i o s o f maximum and minimum h o r i z o n t a l s t r e s s e s , i n t e r c o n n e c t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r e s w i t h m u t u a l f l u i d p e n e t r a t i o n has a v e r y h i g h p o t e n t i a l o f o c c u r r e n c e (BLANTON 1 9 8 6 ) . I n t e r s e c t i o n and p e n e t r a t i o n o f f l u i d s i n a n a t u r a l f r a c t u r e t e n d s t o i m pede p r o p a g a t i o n i n t h e same o r i e n t a t i o n and may d i v e r t t h e f l u i d back t o t h e wellbore. Horizontal w e l l s i n combination w i t h hydraulic f r a c t u r i n g are a l s o significant instruments f o r gas d r a i n a g e f r o m c o a l seams ( c f . s e c t i o n 4.4.3.6.).
4.4.4.4.Effectivity of proppant vs. nitrogen fracturing P r o p p a n t - l a d e n t r e a t m e n t s a r e g e n e r a l l y p e r f o r m e d w i t h a foam o r g e l l e d wat e r c a r r i e r t o p a r t t h e f o r m a t i o n and t r a n s p o r t p r o p p a n t t o s u p p o r t t h e c r e a t e d f r a c t u r e and t o h o l d i t open (BRANDENBURG 1 9 8 8 ) . A l t h o u g h many s h a l e s a r e n o t p a r t i c u l a r l y s e n s i t i v e t o f l u i d s , c a p i l l a r y a c t i o n may h o l d w a t e r i n s m a l l p o r e s , t h e r e b y r e d u c i n g t h e i r p e r m e a b i l i t y t o gas. As r e s e r v o i r p r e s s u r e s i n many s h a l e s a r e i n s u f f i c i e n t t o overcome t h e s e c a p i l l a r y f o r c e s and t o remove t h e wat e r , foam i s o f t e n p r e f e r r e d because i t r e d u c e s t h e l i q u i d volume o f t h e t r e a t ment ( c f . s e c t i o n 3 . 1 1 . 2 . ) . H y d r a u l i c p r o p p a n t f r a c t u r i n g o f s h a l e gas r e s e r v o i r s i n v a r i o u s s c a l e s f r e q u e n t l y had t h e s u r p r i s i n g r e s u l t t h a t p e r f o r m a n c e o f t h e w e l l s a f t e r t h e s t i m u l a t i o n j o b s was n o t a p p r e c i a b l y b e t t e r t h a n t h a t a f t e r s t r a i g h t n i t r o g e n f r a c t u r e t r e a t m e n t s w h i c h d i d l i t t l e more t h a n e s t a b l i s h c o m m u n i c a t i o n between b o r e h o l e and r e s e r v o i r . I n a d d i t i o n , w e l l p e r f o r m a n c e a f t e r s m a l l n i t r o g e n b r e a k down j o b s was o f t e n s i m i l a r t o t h a t seen a f t e r f u l l - s c a l e n i t r o g e n s t i m u l a t i o n s (SHAW, GATENS, LANCASTER, LEE, AVARY & TERRY 1986; LANCASTER, G U I O R Y , GRAHAM, CURTIS, SHAW & BLAKE 1 9 8 7 ) . U n s a t i s f a c t o r y s t i m u l a t i o n r e s u l t s f a l l i n g c o n s i d e r a b l y b e h i n d t h e e x p e c t a t i o n s a r e a l s o r e p o r t e d b y LANCASTER ( 1 9 8 8 ) . A f t e r d i s c u s s i o n o f some g e n e r a l a s p e c t s , a f e w p o i n t s o f l i q u i d r e t e n t i o n and w e l l c l e a n u p , and f e a s i b i l i t y o f MHF t r e a t m e n t s a r e i n v e s t i g a t e d as f o l l o w s .
4 . 4 . 4 . 4 . 1 . General aspects The gas r e s e r v o i r s a r e l o w - p r e s s u r e and l o w - p e r m e a b i l i t y l a y e r s w h i c h a r e d i s t r i b u t e d v e r t i c a l l y w i t h i n t h e s h a l e s e c t i o n , t h e r e b y m a k i n g c o m p l e t i o n s mec h a n i c a l l y d i f f i c u l t . S t r e s s c o n t r a s t s between p r o d u c t i v e i n t e r v a l s and p o t e n t i a l s t i m u l a t i o n b a r r i e r s a p p e a r t o be m i n i m a l ( c f . s e c t i o n s 4 . 2 . 2 . 3 . 1 . and 4.2.3.2.2.), w i t h t h u s v e r t i c a l f r a c t u r e g r o w t h c o n t a i n m e n t b e i n g an i m p o r t a n t p l a c e m e n t c o n s i d e r a t i o n . As a consequence o f l o w b o t t o m h o l e p r e s s u r e and l o w permeability, s h a l e gas r e s e r v o i r s s h o u l d g e n e r a l l y r e s p o n d v e r y w e l l t o t h e l a r g e increase i n permeable s u r f a c e area a v a i l a b l e f o r f l o w t h a t can be achieved w i t h p r o p e r l y c o n d u c t e d f r a c t u r e t r e a t m e n t s (ZUBER, LEE & GATENS 1987; ELY 1 9 8 8 ) . U n s a t i s f a c t o r y r e s u l t s o f many h i t h e r t o p e r f o r m e d h y d r a u l i c p r o p p a n t
487 f r a c t u r i n g t r e a t m e n t s i n s h a l e r e s e r v o i r s can be due t o usage o f i n s u f f i c i e n t b r e a k e r t o degrade t h e polymer v i s c o s i f i e r s i n t h e p r o p p a n t - c a r r y i n g f l u i d s ( c f . s e c t i o n s 4.12.1.1.), w i t h t h e r e s u l t i n g downhole v i s c o s i t y s e v e r e l y l o w e r i n g gas p e r m e a b i l i t y i n and around t h e c r e a t e d f r a c t u r e . O t h e r reasons a r e i n s u f f i c i e n t proppant c o n c e n t r a t i o n s h a v i n g been p l a c e d t o achieve t h e necessary f r a c t u r e c o n d u c t i v i t y , s t a n d a r d flowback procedures somet i m e s r e s u l t i n g i n l i t t l e o r no p r o p p a n t b e i n g p l a c e d across t h e p r o d u c i n g f o r mation, and slow l i q u i d cleanup due t o low bottomhole pressure, w i t h unbroken g e l o r g e l lumping b e i n g a b l e t o cause e i t h e r f a i l u r e t o s t i m u l a t e o r even damage s h a l e gas w e l l s . F i e l d e x p e r i e n c e has shown t h a t l i q u i d cleanup i n t h e lowp r e s s u r e s h a l e gas r e s e r v o i r s a f t e r l a r g e - s c a l e f r a c t u r i n g t r e a t m e n t s i s v e r y slow and w i l l p r o b a b l y never be complete (GATENS 1988), w i t h t h e r e t a i n e d liquids i n the f r a c t u r e therefore f o r a longer period o f time s e r i o u s l y reducing fracture conductivity.
4.4.4.4.2. Liquid retention and well cleanup Long cleanup p e r i o d s f o l l o w i n g h y d r a u l i c proppant f r a c t u r i n g t r e a t m e n t s i n s h a l e gas w e l l s a r e due t o a c o m b i n a t i o n o f l a r g e r e t a i n e d l i q u i d volumes, ins u f f i c i e n t l y broken polymer f l u i d s , and low bottomhole p r e s s u r e , w i t h foamed f l u i d s t h e r e f o r e b e i n g s u p e r i o r s o l u t i o n s o f t h e problem ( c f . s e c t i o n 3 . 1 1 . 2 . ) . Another answer t o t h e q u e s t i o n o f improvement o f performance a r e i n c r e a s e d p r o p p a n t c o n c e n t r a t i o n s t o be p l a c e d i n t h e f r a c t u r e ( c f . s e c t i o n 4.3.), w i t h the h i g h e r c o n d u c t i v i t y achieved by h i g h e r proppant s a t u r a t i o n s enhancing f l u i d cleanup r a t e s and c r e a t i n g more e f f i c i e n t f l o w p a t h s t o t h e w e l l b o r e (GATENS 1988). I n s u f f i c i e n t s t i m u l a t i o n e f f e c t s f o l l o w i n g proppant f r a c t u r i n g o f s m a l l o r l a r g e - s c a l e t y p e can a l s o be t h e r e s u l t o f u n c o n t a i n e d c r a c k h e i g h t growth which may have caused placement o f o n l y inadequate p r o p p a n t c o n c e n t r a t i o n s across t h e pay zone (LANCASTER 1987; c f . s e c t i o n 4.2.2.). R e d u c t i o n o f f l u i d r e t e n t i o n t i m e can a l s o be o b t a i n e d by i n t r o d u c i n g s u r f a c t a n t s which a r e f o r m u l a t e d so t h a t minimal l i q u i d a b s o r p t i o n occurs, t h e r e b y a l l o w i n g t o enhance flowback o f low s u r f a c e t e n s i o n f l u i d s . An accompanying meas u r e can be t r e a t i n g o f t h e proppants w i t h n o n - w e t t i n g s u r f a c t a n t s i n o r d e r t o enhance c o n d u c t i v i t y o f t h e p r o p p a n t package. Because o f t h e low p e r m e a b i l i t y o f t h e s h a l e gas r e s e r v o i r and i t s i n h e r e n t low l e a k o f f impact t o p r o p e r l y des i g n e d f r a c t u r i n g f l u i d s , c r a c k c l o s u r e may n o t occur f o r many hours i f f l u i d flowback i s n o t i n i t i a t e d by pumping o r v a l v e opening t o a r t i f i c i a l l y a c c e l e r a t e f r a c t u r e c l o s u r e o r i s n o t enhanced by e n e r g i z i n g components o r foams. As foam i s an e x c e l l e n t p r o p p a n t t r a n s p o r t medium, b u t n o t a p e r f e c t p r o p p a n t suspension f l u i d , t h e r e i s a p o s s i b i l i t y t h a t s i g n i f i c a n t p r o p p a n t s e t t l i n g o c c u r s u n l e s s t h e w e l l i s i m m e d i a t e l y f l o w e d back.
4.4.4.4.3. Feasibility o f MHF treatments GATENS (1988) r e v i e w s massive h y d r a u l i c p r o p p a n t f r a c t u r i n g i n s h a l e gas r e s e r v o i r s where t r e a t m e n t s have been c a r r i e d o u t w i t h up t o 1.6 M i o . l b s . (800 t ) o f sand i n o r d e r t o c r e a t e l o n g propped c r a c k s . Comparison o f p e r f o r mance o f MHF-stimulated w e l l s w i t h t h a t of o f f s e t w e l l s which were s h o t w i t h n i t r o g l y c e r i n e e x p l o s i v e s r e v e a l e d t h e s u r p r i s i n g f a c t t h a t t h e MHF-treated w e l l s a r e f r e q u e n t l y j u s t s l i g h t l y b e t t e r than t h e average s h o t w e l l s d e s p i t e t h e l a r g e s t i m u l a t i o n j o b s pumped. Some aspects o f r e s e r v o i r p r e s s u r e and p r o p p a n t c o n c e n t r a t i o n a r e d i s c u s s e d as f o l l o w s .
4.4.4.4.3.1. Reservoir pressure A p a r t i a l e x p l a n a t i o n o f t h i s paradoxon i s l i n k e d w i t h t h e much l o w e r r e s e r v o i r p r e s s u r e i n t h e MHF-treated w e l l s t h a t were s t i m u l a t e d s e v e r a l decades a f -
488 t e r t h e o t h e r w e l l s had been almost i n i t i a l l y s h o t w i t h n i t r o g l y c e r i n e a t a t i m e when r e s e r v o i r p r e s s u r e was c o n s i d e r a b l y h i g h e r , w i t h t h u s t h e MHF-treated w e l l s n o t h a v i n g t h e same amount o f r e s e r v o i r energy a v a i l a b l e t o h e l p b r i n g i n g t h e gas t o t h e b o r e h o l e ( c f . s e c t i o n 4 . 8 . 9 . ) . I f n o r m a l i z e d f o r p r e s s u r e , t h e MHF-stimulated w e l l s appear a l l t o be p e r f o r m i n g s i m i l a r l y t o t h e b e s t s h o t well, b u t i f normalized f o r r e s e r v o i r q u a l i t y (permeability times n e t t h i c k ness), t h e MHF-treated w e l l s a r e each c l e a r l y p e r f o r m i n g b e t t e r t h a n even t h e b e s t s h o t w e l l i n t h e area. N e v e r t h e l e s s i t i s s t i l l s u r p r i s i n g why t h e c o s t l y MHF o p e r a t i o n s do i n many cases n o t r e s u l t i n s i g n i f i c a n t l y b e t t e r performance t h a n r a t h e r s i m p l e w e l l s h o o t i n g and n i t r o g e n non-proppant f r a c t u r i n g , and t h e much h i g h e r i n v e s t m e n t i s o f t e n n o t compensated by a l s o a c o m p a r a t i v e l y b e t t e r pos t - s t imul a t ion produc t i on. Massive h y d r a u l i c f r a c t u r e e v a l u a t i o n by s i m u l a t i o n r e v e a l s t h a t i f t h e f r a c t u r e s were w e l l c o n t a i n e d , propped f r a c t u r e h a l f - l e n g t h s up t o 1,000 f t . (330 m) o r more c o u l d have been o b t a i n e d , and i f t h e c r a c k s were u n c o n f i n e d , h a l f - l e n g t h s o f 500 f t (165 m) o r more c o u l d have been achieved, w i t h t h u s l o n g propped f r a c t u r e s h a v i n g been c r e a t e d r e g a r d l e s s o f e x i s t e n c e o f good b a r r i e r s t o v e r t i c a l h e i g h t growth. I n t e r z o n e communication b e h i n d t h e p i p e i n d i c a t e s t h a t s i g n i f i c a n t v e r t i c a l h e i g h t growth has o c c u r r e d .
4.4.4.4.3.2. Proppant concentration Economical comparison o f s m a l l n i t r o g e n breakdown t r e a t m e n t s , s t r a i g h t n i t r o gen f r a c t u r i n g o p e r a t i o n s o f f u l l - s c a l e type, and l a r g e - s c a l e p r o p p a n t f r a c t u r i n g j o b s r e v e a l s t h a t i f p r o p e r l y designed and c a r r i e d o u t w i t h emphasis on h i g h p r o p p a n t c o n c e n t r a t i o n and r a p i d w e l l cleanup a f t e r t e r m i n a t i o n o f t h e s t i m u l a t i o n j o b , massive h y d r a u l i c f r a c t u r i n g i s t h e u l t i m a t e l y b e s t t r e a t m e n t t y p e i n t h e l o n g - t e r m range o f gas f i e l d e x p l o i t a t i o n . I t i s a l s o a t t r a c t i v e t o r e s t i m u l a t e a w e l l w h i c h had o r i g i n a l l y been t r e a t e d by s t r a i g h t n i t r o g e n f r a c t u r i n g by s m a l l - o r l a r g e - s c a l e p r o p p a n t foam f r a c t u r i n g (LANCASTER 1988; c f . a l s o s e c t i o n 4 . 8 . 9 . ) . Aspects o f massive h y d r a u l i c f r a c t u r i n g i n s h a l e gas r e s e r v o i r s a r e a l s o d i s c u s s e d by CREMEAN, McKETTA, OWENS & SMITH (1979) and GATENS, HOLGATE & LEE (1987). The s i g n i f i c a n c e o f p r o p p a n t c o n c e n t r a t i o n maximizat i o n and l i q u i d volume m i n i m i z a t i o n i n l a r g e c o a l and s h a l e f r a c t u r i n g t r e a t ments i s a l s o emphasized by KUUSKRAA & HAAS ( 1 9 8 8 ) .
4.4.4.5. Special features o f shale gas reservoirs The complex f l o w c o n d i t i o n s i n s h a l e gas r e s e r v o i r s can be p a r t i a l l y e x p l a i ned by b u b b l i n g and seeping o f condensate a n d / o r n a t u r a l gas l i q u i d s o u t o f nat u r a l c r a c k s a l o y g t h e b o r e h o l e w a l l and s u b l i m a t i n g i n t o gas, as r e v e a l e d by d i r e c t o b s e r v a t i o n o f t h i s process as w e l l as by r e c o r d i n g o f s t a i n e d f r a c t u r e p l a n e s by v i d e o camera survey (OVERBEY, YOST & YOST 1988; c f . s e c t i o n 6 . 2 . 1 . 9 . 3 . ) . T h i s phenomenon r a i s e s t h e p o t e n t i a l o f two-phase f l o w t h r o u g h t h e f r a c t u r e systems and can account f o r some unusual p r o d u c t i o n p a t t e r n s . Another n o t e w o r t h y e f f e c t i n s h a l e gas r e s e r v o i r s i s gas d r a i n a g e i n an e l l i p t i c a l p a t t e r n i n s t e a d o f a c i r c u l a r one common t o most c o n v e n t i o n a l w e l l s (DOE 1984), w i t h t h i s f e a t u r e i m p l y i n g t h a t s i g n i f i c a n t a d d i t i o n a l q u a n t i t i e s o f gas c o u l d be r e c o v e r e d by s p e c i a l i n f i l l d r i l l i n g . Other p a r t i c u l a r f e a t u r e s o f s h a l e gas r e s e r v o i r s a r e t h a t gas i s n o t immed i a t e l y f l o w i n g a f t e r p e r f o r a t i o n , and once gas c u r r e n t i s f i n a l l y e s t a b l i s h e d , t h e r e i s a l o n g p e r i o d o f w e l l b o r e s t o r a g e d u r i n g b o t h f l o w and s h u t - i n p e r i o d s (SCHETTLER, PARMLEY & LEE 1987; LANCASTER 1988). The problem o f r e t a r d e d i n i t i a t i o n o f gas c u r r e n t can be overcome by b r e a k i n g down t h e w e l l w i t h a s m a l l n i t r o g e n t r e a t m e n t ( c f . s e c t i o n 4 . 8 . 2 . 2 . ) , and e l i m i n a t i o n o f t h e w e l l b o r e s t o rage e f f e c t r e q u i r e s improved t e s t d e s i g n s i n c l u d i n g b o t h s u f f i c i e n t l y l o n g f l o w and s h u t - i n p e r i o d s . S m a l l - s c a l e n i t r o g e n o p e r a t i o n s have n o t o n l y t h e t a s k o f c r e a t i n g communication w i t h t h e f o r m a t i o n and e s t a b l i s h i n g a gas f l o w
489 rate, b u t are also c a r r i e d o u t i n order t o permit pressure buildup t e s t .
a
pre-stimulation
flow
and
4.4.4.6. Explosive fracturing E x p l o s i v e f r a c t u r i n g was t h e f i r s t s t i m u l a t i o n t e c h n i q u e i n s h a l e gas r e s e r v o i r s when t h i s r e s o u r c e s t a r t e d t o undergo more widespread development more than 50 y e a r s ago (WITT 1986). E x p l o s i v e f r a c t u r i n g was c a r r i e d o u t by detonat i n g g e l a t i n a t e d n i t r o g l y c e r i n e under a w a t e r o r g r a v e l tamp ( c f . s e c t i o n 4.7.3.). Massive s h o t s w i t h s e v e r a l t o n s o f e x p l o s i v e c o n s i d e r a b l y i n c r e a s e d t h e gas y i e l d , b u t a l s o o f t e n provoked subsequently a massive and p r o l o n g e d c l e a n o u t j o b because o f t h e l a r g e volume o f s h a l e r u b b l e t h a t g e n e r a l l y c o l l a p sed i n t o t h e w e l l b o r e . More than 90 % o f t h e w e l l s i n some f i e l d s were s t i m u l a t e d by e x p l o s i v e f r a c t u r i n g b e f o r e t h e i n v e n t i o n o f h y d r a u l i c f r a c t u r i n g i n t h e l a t e 1940's (RAY 1976). E x p l o s i v e s t i m u l a t i o n m a i n l y generated p e r m e a b i l i t y pathways t h a t l i n k e d t h e w e l l b o r e w i t h e x t e n s i v e n a t u r a l f r a c t u r e systems which d r a i n gas f r o m t h e m a t r i x , w i t h s i z e , e x t e n t and geometry o f t h e n a t u r a l f r a c t u r e system l a r g e l y g o v e r n i n g p r o d u c t i v i t y . W e l l s i n w h i c h s h o o t i n g f a i l e d t o i n t e r s e c t e x t e n s i v e n a t u r a l f r a c t u r e systems remained d r y h o l e s and were f a i l u res.
4.4.5. High- temperature reservoirs Another t a r g e t i s i n c r e a s i n g s t i m u l a t i o n o f v e r y h i g h - t e m p e r a t u r e t i g h t gas and condensate f o r m a t i o n s where s u c c e s s f u l j o b s have so f a r been r u n i n Europe m a i n l y i n Y u g o s l a v i a (ECONOMIDES, CIKES, PFURTER, UDICK & URODA 1986; c f . sect i o n 2.4.3.1.) and a s u i t e o f f u r t h e r w e l l s i s w a i t i n g on o p e r a t i o n . A f t e r d i s c u s s i o n o f some aspects o f f l u i d s t a b i l i t y a d j u s t m e n t t o e l e v a t e d pay temperat u r e , h e a t t r a n s f e r i n f r a c t u r e s and temperature s t a b i l i t y o f s t i m u l a t i o n f l u i d s , f r a c t u r i n g f l u i d r e q u i r e m e n t i n h o t and c o l d environment i s b r i e f l y o u t lined.
4.4.5.1. Fluid stability adjustment to elevated pay temperature The d e c i s i v e n e c e s s i t y o f approaching h i g h - t e m p e r a t u r e r e s e r v o i r s o f e i t h e r hydrocarbon o r geothermal w a t e r t y p e ( c f . s e c t i o n 4.7.) by h y d r a u l i c f r a c t u r i n g o p e r a t i o n s i s extended thermal s t a b i l i t y o f f r a c t u r i n g f l u i d s which can be r e a ched by b o t h d i r e c t and i n d i r e c t t r e a t m e n t s . D i r e c t ways o f improvement o f temperature r e s i s t i v i t y include c r o s s l i n k i n g f o r achieving higher v i s c o s i t y ( c f . s e c t i o n 4.3.4.3.) as w e l l as i n t e r m i x i n g o f a d d i t i v e s t h a t r e t a r d thermal breakage o f t h e polymers. I n d i r e c t p o s s i b i l i t i e s a r e f o r m a t i o n c o o l i n g by pumping l a r g e pre-pad and pad stages p r i o r t o i n s e r t i o n o f t h e c r o s s l i n k e d polymer c a r r i e r stage ( c f . s e c t i o n 4.3.4.6.1.4.), t h e r e b y m i m i c k i n g t o t h e l a t t e r phase a comparatively colder formation than present i n r e a l i t y . While t h i s heat blocki n g may work v e r y e f f e c t i v e l y i n n o t t o o h o t pay zones, some e x t r e m e l y h o t geothermal r e s e r v o i r s can o n l y be s t i m u l a t e d w i t h more o r l e s s w a t e r as f r a c t u r i n g f l u i d s ( c f . s e c t i o n 4.7.3.5.). On t h e o t h e r hand, h i g h - e f f i c i e n c y f r a c t u r i n g f l u i d s f o r l o w - t e m p e r a t u r e r e s e r v o i r r o c k s a r e r e p o r t e d by ALMOND & G A R V I N ( 1 9 8 5 ) . Experimental e v a l u a t i o n o f thermal s t a b i l i t y o f f r a c t u r i n g f l u i d s i s c a r r i e d o u t by LESCARBOURA, SIFFERMAN & WAHL ( 1 9 8 4 ) . V a r i o u s aspects o f f r a c t u r i n g f l u i d s e l e c t i o n and performance e x p e r i e n c e a r e d i s c u s s e d by BLASIO, BAILEY & BERGTHOLD (1985); SOLIMAN & HUNT (1985) and ROODHART & D A V I E S ( 1 9 8 7 ) . H i g h - q u a l i t y water-based f r a c t u r i n g f l u i d s a r e a l s o d e s c r i b e d by CHEN, CHEN & ZI ( 1 9 8 2 ) . H i g h - t e m p e r a t u r e e f f e c t s o f proppants, f l u i d s and equipment a r e a l s o s i g n i f i c a n t f o r geothermal r e s e r v o i r s t i m u l a t i o n ( c f . section 4.7.).
490
Heat transfer in fractures and temerature stability o f stimulation fluids
4.4.5.2.
Temperature e f f e c t s on s t i m u l a t i o n d e s i g n a r e a l s o d i s c u s s e d by W H I T S I T T & DYSART (1970) and McDANIEL (1986), and q u e s t i o n s o f h e a t t r a n s f e r i n f r a c t u r e s a r e examined by WHEELER (1969), W H I T S I T T & DYSART (197D), SINCLAIR (1971); CLARK, CLOUD, WAREMBOURG & YUNKD (1978); HARRINGTON, HANNAH & BEIRUTE (1978); CROCKETT, VO & CLEARY (1984) and POULSEN & LEE (1984; c f . s e c t i o n 4 . 3 . 4 . 4 . 1 . ) . The s i g n i f i c a n c e o f h i g h - e f f i c i e n c y f r a c t u r i n g f l u i d s f o r t h e s u c c e s s f u l t r e a t ment o f h i g h - t e m p e r a t u r e , l o w - p e r m e a b i l i t y r e s e r v o i r s i s o u t l i n e d by ELY, CHATTERJI, HOLTMYER & TINSLEY (1975); THOMAS & ELBEL (1979), ELBEL & THOMAS (1980), WHITE & DANIEL (1980), HSU & CONWAY (1981), WENDORFF & AINLEY (1981), HORTON (1982) ; BAUMGARTNER, PARKER, WILLIAMS & WOODROOF (1983); GRONE, BAUMGARTNER & WOODROOF (1983 a, 1983 b ) ; HARMS, GOSS & PAYNE (1984); POULSEN & LEE (1984) and WALSER & OTT ( 1 9 8 5 ) . Time-temperature s t a b i l i t y o f c r o s s l i n k e d s t i m u l a t i o n f l u i d s i s i n v e s t i g a t e d by CONWAY, PAULS & H A R R I S (1980) and H A R R I S & REIDENBACH ( 1 9 8 4 ) . I n c r e a s i n g h i g h - t e m p e r a t u r e performance o f f r a c t u r i n g f l u i d s i s m a i n l y achieved by reduc i n g t h e r a t e o f v i s c o s i t y l o s s w h i c h r e s u l t s i n more e f f e c t i v e p r o p p a n t t r a n s p o r t and a l l o w s t o r e p l a c e t h e flammable methanol (ELBEL & THOMAS 1 9 8 0 ) . The f l e x i b i l i t y o f f o r m u l a t i n g o i l - b a s e d f r a c t u r i n g f l u i d s t o meet b o t h s u r f a c e and downhole c o n d i t i o n s i n h i g h - t e m p e r a t u r e r e s e r v o i r s i n terms o f v i s c o s i t y and h a n d l i n g by a d d i t i o n o f secondary g e l l a n t s and/or a c t i v a t o r s t o a p r i m a r y base g e l on t h e f l y downstream o f t h e b l e n d e r i s i n v e s t i g a t e d by BURNHAM, H A R R I S & McDANIEL (1978) and PAULS, VENDITTO, HOLTMYER & GREGORCZYK ( 1 9 8 5 ) . The use o f v i s c o s i t y ' s t a b i l i z e r s i n h i g h - t e m p e r a t u r e f r a c t u r i n g i s e v a l u a t e d by THOMAS & ROOT ( 1 9 7 9 ) .
4.4.5.3. Fracturing fluid requirements in hot environment More and more hydrocarbon r e s e r v e s a r e c u r r e n t l y found i n deep h o t r e s e r v o i r s which demand u n i q u e s t i m u l a t i o n f l u i d c h a r a c t e r i s t i c s (HARMS, G O S S & PAYNE 1984). The i d e a l f r a c t u r i n g f l u i d f o r h i g h - t e m p e r a t u r e a p p l i c a t i o n s h o u l d c o n t a i n t h e l o w e s t polymer l o a d i n g c o n s i s t e n t w i t h s u f f i c i e n t v i s c o s i t y f o r p r o p p a n t t r a n s p o r t and minimal f l u i d l e a k o f f ( c f . s e c t i o n 4 . 3 . 4 . 6 . ) . The f l u i d a l s o has t o m a i n t a i n i t s v e l o c i t y and f l u i d - l o s s c o n t r o l under shear and p r o l o n ged exposure t o h i g h temperature i n o r d e r t o e x t e n d t h e f r a c t u r e and p l a c e t h e proppant, and f r i c t i o n p r e s s u r e l o s s e s s h o u l d be low f o r r e d u c i n g t u b i n g pressur e s and m i n i m i z i n g pumping energy ( c f . s e c t i o n 4 . 3 . 4 . 2 . 2 . ) . A u n i q u e combinat i o n o f heterogeneously s u b s t i t u t e d p o l y s a c c h a r i d e as a key component f o r g e l l i n g aqueous systems, s t a b i l i z e r s and delayed c r o s s l i n k i n g c h e m i c a l s p r o v i d e s t h e extended thermal s t a b i l i t y and low f r i c t i o n p r e s s u r e necessary f o r high-temp e r a t u r e MHF s t i m u l a t i o n w h i l e m i n i m i z i n g t h e p o s s i b i l i t y o f f o r m a t i o n damage (HARMS, GOSS & PAYNE 1984). GRONE, BAUMGARTNER & WOODROOF (1983) document t h e chemical e v o l u t i o n o f a h i g h - t e m p e r a t u r e f r a c t u r i n g f l u i d . Shear and thermal s t a b i l i t y a t h i g h bottomhol e temperatures i s p r o v i d e d by h i g h polymer c o n c e n t r a t i o n s and/or l a r g e f l u i d volumes which m a i n t a i n e f f e c t i v e f l u i d v i s c o s i t i e s i n t h e f r a c t u r e , b u t on t h e o t h e r hand, h i g h polymer s a t u r a t i o n s l e a d t o h i g h e r f r i c t i o n p r e s s u r e , h i g h e r t r e a t m e n t c o s t and h i g h broken g e l r e s i d u e l e v e l s , and l a r g e f l u i d volumes i n crease t h e c o s t o f t h e t r e a t m e n t ( c f . s e c t i o n 4 . 3 . 4 . 2 . 2 . ) . C r o s s l i n k e d f r a c t u r i n g f l u i d s have been developed f o r extended temperature s t a b i l i t y due t o f a v o u r a b l e v a r i a t i o n s o f polymer, b u f f e r , g e l s t a b i l i z e r and c r o s s l i n k e r ( c f . sect i o n 4 . 3 . 4 . 3 . ) . Pumping l a r g e volirmes o f f l u i d aim on c o o l i n g down t h e format i o n and can p a r t i a l l y be o p e r a t e d w i t h a d u a l - s t a g e polymer system ( c f . sect i o n 4 . 3 . 4 . 6 . 1 . 4 . ) . Gel s t a b i l i z e r s p r e v e n t o r r e t a r d t h e chemical d e g r a d a t i o n o f polymer s o l u t i o n s , b u t many o f them i n t e r f e r e w i t h t h e c r o s s l i n k i n g mechanism.
491 The c u r r e n t temperature r e c o r d s o f s u c c e s s f u l f r a c t u r i n g comprise b o t t o m h o l e v a l u e s o f 420 OF f o r a 15,000 f t deep gas w e l l , 540 OF (280 oC; WILLIAMS, WOODROOF & BOX 1982) f o r a 5,500 f t deep hydrothermal w e l l , and 600 OF f o r a 13,500 f t deep h o t d r y r o c k w e l l (FRACFAX 1988 a ) . Comments on h i g h - t e m p e r a t u r e d r i l l i n g mud and f l u i d - l o s s c o n t r o l a d d i t i v e s f o r geothermal r e s e r v o i r s and deep h e a t - s e n s i t i v e f o r m a t i o n s a r e g i v e n by HILSCHER & CLEMENTS (1982) and CARNEY, GUVEN & McGREW (1982), r e s p e c t i v e l y ( c f . a l s o s e c t i o n 4 . 7 . 4 . 2 . ) . Similarly as f r a c t u r i n g f l u i d s , thermal s t a b i l i t y e x t e n s i o n o f a c i d can be achieved, w i t h b u f f e r r e g u l a t i o n o f HF a c i d a l l o w i n g sandstone a c i d i z i n g up t o 550 OF (280 oC) r e s e r v o i r temperature (SCHEUERMAN 1988).
4.4.5.4. Fracturing requirements in cold environment I n c o n t r a s t t o e x t r e m e l y h i g h - t e m p e r a t u r e r e s e r v o i r s , h y d r a u l i c f r a c t u r e oper a t i o n s have a l s o t o be a d j u s t e d t o e x t r a o r d i n a r y low-temperature environments r e s u l t i n g f r o m c o l d weather i n A r c t i c l a t i t u d e s where c u r r e n t l y e x p l o r a t i o n i s b o o s t i n g . SCHUMACHER & DICKERMAN (1986) r e p o r t s u c c e s s f u l h y d r a u l i c p r o p p a n t s t i m u l a t i o n i n a n a t u r a l l y f r a c t u r e d r e s e r v o i r i n an a t m o s p h e r i c a l temperature below -50 OF where p r o p p a n t and a d d i t i v e s c o u l d o n l y be mixed on t h e f l y and r h e o l o g i c a l c o n d i t i o n s o f t h e f r a c t u r i n g f l u i d s were s t i l l adequate t o p l a c e up t o 4 l b s o f p r o p p a n t p e r g a l l o n o f c a r r i e r medium i n t o t h e h y d r a u l i c c r a c k i n t e r s e c t i ng t h e f o r m a t i on.
4.4.6. Gas condensate reservoirs Gas condensate r e s e r v o i r s r e q u i r e i n many cases s o p h i s t i c a t e d s t i m u l a t i o n t r e a t m e n t s p a r t i c u l a r l y i f b e i n g t i g h t and abnormally p r e s s u r e d . I f t h e r e t r o grade condensation p o i n t (DANESH, HENDERSON, K R I N I S & PEDEN 1988) i s v e r y c l o s e t o t h e i n i t i a l f o r m a t i o n pressure, condensate p r o d u c t i o n can l e a d t o r e t r o g r a d e f a l l o u t and i n c r e a s i n g w a t e r s a t u r a t i o n i n t h e v i c i n i t y o f t h e w e l l b o r e , and two-phase f l o w r e s u l t s i n r e d u c t i o n o f t h e r e l a t i v e p e r m e a b i l i t y t o gas and t h u s f l o w c a p a c i t y o f t h e w e l l (ENGINEER 1985). R e s e r v o i r p r e s s u r e can be p r i n c i p a l l y m a i n t a i n e d h i g h enough t o m i n i m i z e r e t r o g r a d e l i q u i d condensation by d r y gas i n j e c t i o n , b u t i n heterogeneous t i g h t r e s e r v o i r s , gas i n j e c t i v i t y i s t o o low and t h u s t h e d e s i r e d e f f e c t i s n o t achieved. I n such cases, h y d r a u l i c f r a c t u r i n g has been shown t o c r e a t e a c o n d u c t i v e p a t h w h i c h c o n s i d e r a b l y improves t h e d r a i n a g e p a t t e r n o f t h e r e s e r v o i r and t h e r e b y h e l p s t o keep t h e f l o w i n g p r e s s u r e above t h e r e t r o g r a d e condensation p o i n t . H y d r a u l i c f r a c t u r i n g f o r a v o i d i n g r e t r o g r a d e f a l l o u t (DANESH, HENDERSON, K R I N I S & PEDEN 1988; c f . sect i o n 2.4.3.1.) has been s u c c e s s f u l l y c a r r i e d o u t i n h i g h - t e m p e r a t u r e gas condens a t e w e l l s i n Y u g o s l a v i a (ECONOMIDES, C I K E S , PFURTER, UDICK & URODA 1986), and p o t e n t i a l i s a l s o seen i n o t h e r f i e l d s w i t h comparable c o n d i t i o n s . I n a d d i t i o n t o t h e aforementioned t y p e s o f u n c o n v e n t i o n a l gas d e p o s i t s , n a t u r a l gas may a l s o be t r a p p e d i n i c e - l i k e c r y s t a l s below t h e p e r m a f r o s t o r ben e a t h t h e s e a f l o o r , a l t h o u g h t h e p o t e n t i a l o f such c o n c e n t r a t i o n s i s s t i l l undetermined (DOE 1984). Such methane h y d r a t e s were f i r s t i d e n t i f i e d i n t h e 1930's when t h e y formed i n n a t u r a l gas p i p e l i n e s t h a t were b u i l t i n c o l d c l i m a t e s . Methane h y d r a t e s have i n t h e r e c e n t decades been proven t o o c c u r n a t u r a l l y i n b o t h onshore and o f f s h o r e d e p o s i t s and may i n cases even f o r m a n a t u r a l s e a l o v e r more c o n v e n t i o n a l g a s - b e a r i n g s t r a t a . Aspects o f n a t u r a l gas h y d r a t e s a r e a l s o d i s c u s s e d by GODBOLE ( 1 9 8 5 ) .
492
4.5. Hydraulic proppant f r a c t u r i n g o f carbonate rocks I n Europe and many o t h e r p a r t s o f t h e w o r l d , s o f a r p r e d o m i n a n t l y sandstone r e s e r v o i r s a r e s t i m u l a t e d by h y d r a u l i c p r o p p a n t f r a c t u r i n g . Carbonate format i o n s which a r e a l s o commonly t r e a t e d w i t h t h i s method and w i t h encouraging r e s u l t s i n t h e USA (RENSHAW & JOINES 1977, WILLIAMS 1977; M I S A K , ATTEBERRY, VENDITTO & FREDRICKSON 1978 b; ATTEBERRY, TUCKER & RITZ 1979; K O Z I K , BAILEY & HOLD I T C H 1979; ROSEPILER 1979; SCHLOTTMAN, MILLER & LUEDERS 1981; CLARK 1983, B A I LEY & WICKHAM 1984, EASON 1985, ILSENG & CORTEZ 1985, MACDONALD & FRANK 1985, BRITT & LARSEN 1986, CRAMER 1987; HUCKABEE 1987, 1988; HOOVER & ADAMS 1988; JOHNSON, FOX, BURNS & O'MARA 1988) a r e i n Europe s t i l l almost e x c l u s i v e l y subj e c t e d t o c o n v e n t i o n a l m a t r i x a c i d o r a c i d f r a c t u r i n g s t i m u l a t i o n . Massive hyd r a u l i c f r a c t u r i n g (MHF; c f . RANDOLPH 1974, MURPHY & CARNEY 1977, PA1 & GARBIS 1983; c f . s e c t i o n 1 . 1 . 1 . ) u s u a l l y stands f o r proppant f r a c t u r i n g and i s o f t e n d e f i n e d as c o m p r i s i n g c r a c k s o f a t l e a s t 1,500 f t (500 m) p e n e t r a t i o n i n each d i r e c t i o n f r o m t h e w e l l b o r e (FAST, HOLMAN & COVLIN 1977), whereas i n most cases no p r o p p i n g agents a r e i n s e r t e d i n massive a c i d f r a c t u r i n g (MAF; McDONALD 1983). A c i d t r e a t m e n t s can be p r i n c i p a l l y s p l i t i n t o m a t r i x a c i d i z i n g and a c i d f r a c t u r i n g (KING 1986). While m a t r i x a c i d i z i n g w i t h h y d r o c h l o r i c a c i d i n carbonates i s a l r e a d y common p r a c t i c e s i n c e more t h a n 60 y e a r s (LABRID 1975) and w i t h h y d r o f l u o r i c a c i d i n sandstones ( c f . s e c t i o n 4 . 5 . 5 . ) has been i n t r o d u c e d o v e r 30 y e a r s ago (SMITH & HENDRICKSON 1965), a c i d f r a c t u r i n g o r f r a c t u r e a c i d i z i n g i n c a r b o n a t e r o c k s has been f i r s t a p p l i e d more t h a n 20 y e a r s ago (HENDRICKSON & CAMERON 1968) and i s t h e r e f o r e a c o n s i d e r a b l y younger procedure than h y d r a u l i c p r o p p a n t f r a c t u r i n g w h i c h i s an e s t a b l i s h e d t e c h n i q u e s i n c e o v e r 40 y e a r s ( c f . s e c t i o n 1 . 2 . ) . BAILEY & WICKHAM (1984) i n v e s t i g a t e p r o p p a n t f r a c t u r i n g vs. f r a c t u r e a c i d i z i n g , and BEN NACEUR & ECONOMIDES (1988 b) e v a l u a t e t h e e f f e c t i v e n e s s o f a c i d f r a c t u r i n g . LO & DEAN (1988) model a c i d f r a c t u r i n g . D i s t i n c t i o n has t o be made between a c i d f r a c t u r i n g and f r a c t u r e a c i d i z i n g (BRISCOE 1978), w i t h a c i d f r a c t u r i n g i n c l u d i n g t h e p r i m a r y process o f opening o f t h e c r a c k and subsequently i m p r o v i n g i t s c o n d u c t i v i t y by a c i d a t t a c k , wher e a s f r a c t u r e a c i d i z i n g comprises c h i e f l y secondary a c i d i n j e c t i o n i n t o p r e - e x i s t i n g e i t h e r n a t u r a l j o i n t s o r a r t i f i c i a l l y - c r e a t e d f r a c t u r e s . Aspects o f e x t e n s i o n o f d r a i n a g e p a t h and c o n d u c t i v i t y c o n t r a s t , c o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r e s , and p o s s i b i l i t i e s o f a p p l i c a t i o n i n Europe a r e o u t l i n e d as f o l l o w s . P a r t i c u l a r emphasis i s g i v e n t o c h a l k s t i m u l a t i o n w h i c h i s a v e r y spec i a l t y p e o f carbonate r e s e r v o i r s w i t h c o m p l i c a t e d c o n d i t i o n s t h a t r e q u i r e ext e n s i v e and s o p h i s t i c a t e d t r e a t m e n t s . Comments on sandstone a c i d i z i n g a r e a l s o offered.
4 . 5 . 1 . Extension o f drainage path and conductivity contrast A l t h o u g h i n many cases t h e e c o n o m i c a l l y c o m p a r a t i v e l y cheap and t e c h n i c a l l y r e l a t i v e l y s i m p l e a c i d j o b s have e x c e l l e n t r e s u l t s i n carbonate r e s e r v o i r s t i m u l a t i o n and p a r t i c u l a r l y p a y - o u t o f t h e t r e a t m e n t i s v e r y f a s t due t o t h e low c o s t and a l s o a p p a r e n t l y many carbonate f o r m a t i o n s respond b e t t e r t o a c i d i z i n g than t o h y d r a u l i c p r o p p a n t f r a c t u r i n g (JENNINGS & DARDEN 1979), t h e r e a r e q u i t e some hydrocarbon-bearing c a r b o n a t e f o r m a t i o n s i n Europe and o t h e r p a r t s o f t h e w o r l d where h y d r a u l i c p r o p p a n t f r a c t u r i n g would be t h e b e s t s o l u t i o n i n view o f r e s e r v o i r e n g i n e e r i n g and would g i v e r i s e t o t h e h i g h e s t p r o d u c t i o n r a t e s o f a l l s t i m u l a t i o n t r e a t m e n t s t h a t c o u l d be a p p l i e d , The e v a l u a t i o n as f o l l o w s f o cusses on advantages and drawbacks o f f r a c t u r e p r o p p i n g , c o n d u c t i v i t y improvement by a c i d o p t i m i z a t i o n , enhancement of a c i d p e n e t r a t i o n d i s t a n c e , a c i d foami n g , open vs. propped f r a c t u r e s , and a c i d f l u i d - l o s s c o n t r o l . '
493
4.5.1.1. Advantages and drawbacks of fracture propping The main reason f o r t h e s u p e r i o r i t y o f proppant f r a c t u r i n g a l s o i n carbonate r o c k s i s t h e much l a r g e r e x t e n s i o n and t h e more c o n t i n u o u s shape o f t h e d r a i n a ge p a t h t h r o u g h t h e r e s e r v o i r i n case o f a propped f r a c t u r e as compared t o t h e e f f e c t of e n l a r g i n g n a t u r a l j o i n t s o r s o l v i n g o u t r e l a t i v e l y u n s t a b l e components o f t h e m a t r i x o f carbonate r o c k s by a c i d i z i n g . I n a d d i t i o n , t h e d r a i n a g e r a d i u s o f a propped f r a c t u r e i s c e r t a i n l y much l a r g e r than t h a t o f n a t u r a l j o i n t s accentuated by a c i d s o l u t i o n which i s more o r l e s s r e s t r i c t e d t o t h e c l o s e r v i c i n i t y o f the b o r e h o l e . While t h e advantages comprise m a i n l y g e n e r a t i o n o f a pronounced c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r m a t r i x and propped f r a c t u r e ( c f . a l s o s e c t i o n 4 . 6 . 1 . ) , t h e most i m p o r t a n t drawbacks o f i n s t a l l a t i n g propped f r a c t u r e s i n c a r bonate r o c k s a r e s c a l e f o r m a t i o n i n t h e proppant package and f l u i d l o s s ( c f . s e c t i o n 4 . 5 . 1 . 6 . ) i n e x t e n s i v e and narrowly-spaced n a t u r a l c r a c k systems. B A I LEY & WICKHAM (1984) r e p o r t a d e f i n i t e p r o d u c t i o n b e n e f i t by proppant f r a c t u r i n g o v e r a c i d f r a c t u r i n g i n carbonate o i l r e s e r v o i r s . The o u t l i n e as f o l l o w s focusses on m a t r i x a c i d i z i n g v s . f r a c t u r i n g , c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r m a t r i x and propped f r a c t u r e , carbonate s c a l e f o r m a t i o n and f l u i d l o s s , and i r o n s e q u e s t e r i n g and complexing.
4.5.1.1.1. Matrix acidizing vs. fracturing W h i l e m a t r i x a c i d i z i n g below f r a c t u r e p r e s s u r e i s s t i l l b e n e f i c i a l i n h i g h p e r m e a b i l i t y damaged carbonate f o r m a t i o n s ( o v e r 50 md), a c i d f r a c t u r i n g i s gener a l l y used i n l o w - p e r m e a b i l i t y carbonate r e s e r v o i r s ( l e s s than 10 md; i n cont r a s t t o sandstone p r o p p a n t f r a c t u r i n g where such v a l u e s comprise s t i l l moderat e - t o h i g h - p e r m e a b i l i t y t a r g e t s ) , because much more e f f e c t i v e s t i m u l a t i o n i s achieved w i t h l o n g a c i d - e t c h e d f r a c t u r e s (McLEOD 1984), and an even b e t t e r r e s u l t can be o b t a i n e d w i t h p r o p p a n t f r a c t u r i n g . The main purpose o f m a t r i x a c i d i z i n g i n sandstones and carbonates i s t o d i s s o l v e formation-damaging c l a y miner a l s and o t h e r f i n e s and t o r e s t o r e more o r l e s s o r i g i n a l p e r m e a b i l i t y (DOWELL SCHLUMBERGER 1988 d; c f . s e c t i o n 4 . 5 . 5 . 2 . ) by f l o w p a t t e r n t h r o u g h t h e n a t u r a l m a t r i x p e r m e a b i l i t y below f r a c t u r e p r e s s u r e . E x t r a o r d i n a r y h i g h amounts o f s o l u b l e m i n e r a l s can a l s o g i v e r i s e t o i n c r e a se o f secondary s o l u t i o n p e r m e a b i l i t y beyond i t s p r i m a r y l e v e l , w i t h c a u t i o n , however, h a v i n g t o be e x e r c i s e d t h a t e x c e s s i v e m a t r i x d i s s o l u t i o n does n o t l e a d t o rock framework c o l l a p s e and d e s t r u c t i o n o f t h e s t i m u l a t i o n e f f e c t ( c f . sect i o n 4.5.5.4.2.). A c i d f r a c t u r i n g i s a p p l i e d i n o r d e r t o improve low f o r m a t i o n p e r m e a b i l i t y i n sandstones and carbonates beyond i t s p r i m a r y amount by c r e a t i o n o f a c o n d u c t i v e c r a c k a t i n j e c t i o n p r e s s u r e s above f r a c t u r e g r a d i e n t . Flow p a t t e r n i s e s s e n t i a l l y through t h e h y d r a u l i c a l l y - g e n e r a t e d c r a c k , a l t h o u g h much o f t h e f l u i d does l e a k o f f i n t o t h e m a t r i x a l o n g t h e f r a c t u r e f a c e s . HUCKABEE (1988) p r e s e n t s a comparative e v a l u a t i o n o f t e c h n i c a l and economical e f f e c t i v i t y o f p r o p p a n t f r a c t u r i n g , m a t r i x f r a c t u r i n g , and s i n g l e - and m u l t i p l e - s t a g e mat r i x a c i d i z i n g f o r a case example. Some aspects o f removal o f f o r m a t i o n damage, a u t o - p r o p p i n g o f rugged f r a c t u r e w a l l s a f t e r e t c h i n g , f r a c t u r e c o n d u c t i v i t y vs. f r a c t u r e l e n g t h , and double f r a c t u r i n g i n r e s e r v o i r s w i t h h a i r l i n e c r a c k s a r e i l l u s t r a t e d as f o l l o w s .
4.5.1.1.1.1. Removal o f formation damage M a t r i x a c i d i z i n g f o r removal o f f o r m a t i o n damage around t h e b o r e h o l e i s o n l y s u c c e s s f u l i n s h a l l o w p e n e t r a t i o n depth. whereas deep s o l i d p e r m e a b i l i t y p l u g g i n g o f t h e r e s e r v o i r i n t h e w e l l b o r e v i c i n i t y has t o be c o r r e c t e d by c r e a t i n g a c o n d u c t i v e f r a c t u r e t h r o u g h t h e damage zone e i t h e r by a c i d o r by p r o p p a n t f r a c t u r i n g ( c f . s e c t i o n 4 . 8 . 3 . ) . On t h e o t h e r hand, t h e r e a r e p l e n t y o f cases where a c i d cannot d i s s o l v e t h e m a t e r i a l s c a u s i n g w e l l and f o r m a t i o n damage o r
494 where e v e n a c i d i t s e l f may cause damage upon c o n t a c t w i t h t h e r e s e r v o i r s ( c f . s e c t i o n 4 . 5 . 5 . 3 . ) . Under such c o n d i t i o n s , e i t h e r c h e m i c a l t r e a t m e n t w i t h nona c i d f l u i d s (BROADDUS 1988) o r - i n many i n s t a n c e s w i t h much b e t t e r r e s u l t - h y d r a u l i c p r o p p a n t f r a c t u r i n g a r e r e q u i r e d f o r damage removal o r c r e a t i o n o f a c o n v e c t i v e avenue f r o m t h e w e l l b o r e t o t h e undamaged v i r g i n a l p a y zone, r e s p e c t iv e l y . F o r m a t i o n damage r e m o v a l and p e r m e a b i l i t y r e s t o r a t i o n d u r i n g m a t r i x a c i d i z i n g i s a c h i e v e d b y e i t h e r d i s s o l v i n g t h e damaging m a t e r i a l i t s e l f o r p a r t s o f t h e r o c k i n w h i c h t h e damage e x i s t s (DOWELL SCHLUMBERGER 1988 d ) , w i t h t h e l a t t e r method b e i n g m o s t e f f e c t i v e i n c a r b o n a t e r e s e r v o i r s w i t h a c i d s o l u b i l i t i e s g r e a t e r t h a n 50 % . A l t h o u g h f r a c t u r i n g p r e s s u r e s h o u l d g e n e r a l l y n o t be e x c e e ded, i t may b e n e c e s s a r y t o c r e a t e an i n t e r m i t t e n t f r a c t u r e i n o r d e r t o open t h e p e r f o r a t i o n s a f t e r w h i c h p r e s s u r e c a n be r e d u c e d t o s u c h an e x t e n t t h a t t h e f r a c t u r e c l o s e s again, t h e r e b y r e i n s t a l l i n g m a t r i x f l o w p a t t e r n ( s i m i l a r l y as i n g r a v e l p a c k i n g ; c f . s e c t i o n s 5 . 6 . 4 . and 5 . 6 . 6 . ) . I n s a n d s t o n e a c i d i z i n g p e r f o r m e d w i t h a c o m b i n a t i o n o f h y d r o c h l o r i c a c i d and h y d r o f l u o r i c a c i d ( c f . sect i o n 4.5.5.), a s p e a r h e a d o f h y d r o c h l o r i c a c i d s h o u l d be i n j e c t e d t o r e a c t w i t h c a r b o n a t e s b e f o r e i n t r o d u c t i o n o f t h e HCl/HF m i x t u r e i f t h e r o c k c o n t a i n s a b t . 5 - 10 % c a r b o n a t e s . I n c a s e o f c a r b o n a t e c o n t e n t s above 20 %, hydrofluoric a c i d i s p r o b a b l y n o t needed a t a l l e x c e p t f o r g i v i n g e n t r y t h r o u g h t h e c l a y damage m a n t l e a r o u n d t h e b o r e h o l e . I n a d d i t i o n t o o r as an a l t e r n a t i v e o f l a r g e - s c a l e m a t r i x a c i d i z i n g t r e a t ments, s m a l l volume o p e r a t i o n s c a n b e c a r r i e d o u t b y s p o t t i n g a c i d a c r o s s t h e p e r f o r a t i o n s o r o p e n - h o l e i n t e r v a l and a l l o w i n g i t t o soak (HUCKABEE 1 9 8 8 ) . F u r t h e r a c i d volumes c a n be squeezed i n t o t h e r e s e r v o i r . These t e c h n i q u e s c a n a l s o be combined o r c a n a l t e r n a t e w i t h p r o p p a n t f r a c t u r i n g on v a r i o u s s c a l e ( c f . s e c t i o n s 4.5.1.6.2. and 4 . 5 . 4 . 2 . ) .
4.5.1.1.1.2. Autopropping of rugged f r a c t u r e walls a f t e r etching I n a c i d f r a c t u r i n g t r e a t m e n t s , p r o p p a n t s a r e i n many c a s e s n o t i n e v i t a b l y r e q u i r e d f o r p r o v i d i n g f r a c t u r e s u p p o r t and c o n d u c t i v i t y enhancement, because a c i d r e a c t i o n w i t h t h e c r a c k f a c e s o f c a r b o n a t e r e s e r v o i r s i s n o r m a l l y v e r y uneven, w i t h t h i s h e t e r o g e n e o u s e t c h i n g p a t t e r n c r e a t i n g r o u g h e n e d s u r f a c e s w i t h r e l i e f w h i c h p r e v e n t t h e f r a c t u r e f r o m f u l l y c l o s i n g (McLEOD 1984, OOWELL SCHLUMBERGER 1988 d ) . T h e r e f o r e a h i g h l y c o n d u c t i v e c r a c k may e x i s t a f t e r h y d r a u l i c p r e s s u r e s a r e r e l e a s e d , b u t enough a c i d s t r e n g t h and volume m u s t be used t o generate s u f f i c i e n t d i s s o l u t i o n f o r e s t a b l i s h m e n t o f t h e r e q u i r e d cond u c t i v i t y . On t h e o t h e r hand, t h e r u g g e d f r a c t u r e w a l l s do n o t keep open i n such a c o n t i n u o u s manner t h a t a l l t h e i n d i v i d u a l e t c h e d c h a n n e l s and h o l e s s t a y i n connection a f t e r f r a c t u r e closure, b u t f r e q u e n t l y c a v i t i e s a r e separated by p r o t r u d i n g p i l l a r s where a c i d a t t a c k was s l o w e r . T h i s p r i m a r y p e r m e a b i l i t y det e r i o r a t i o n i s f u r t h e r enhanced b y s e c o n d a r y c o l l a p s e and c r u s h i n g o f u n s t a b l e p i l l a r s and b r i d g e s once c l o s u r e s t r e s s i s a p p l i e d , and i r r e g u l a r and d i s c o n t i nuous auto-propping o f parts o f the fracture walls ( c f . also section 4.5.1.5.1.) w i t h broken carbonate r o c k f u r t h e r decreases crack c o n d u c t i v i t y . A l l t h e s e e f f e c t s c a n be a v o i d e d b y a d e q u a t e i n f i l l i n g o f t h e e t c h e d f r a c t u r e w i t h p r o p p a n t s w h i c h keeps t h e w h o l e space open and p r o v i d e s an i d e a l l y connect e d and c o n d u c t i v e f l o w c h a n n e l t o t h e b o r e h o l e .
A l t h o u g h f r a c t u r e f l o w c a p a c i t y r e s u l t i n g f r o m a c i d r e a c t i o n i s g e n e r a l l y ver y h i g h , i t may b e c o n s i d e r a b l y d e c r e a s e d when r o c k embedment s t r e n g t h a n d / o r r o c k s o l u b i l i t y a r e l o w o r c l o s u r e s t r e s s i s h i g h (NIERODE & KRUK 1 9 7 3 ) . The m a i n a d v a n t a g e o f a c i d f r a c t u r i n g i s deep c r a c k p e n e t r a t i o n i n t o c a r b o n a t e r e s e r v o i r s w i t h l o w - c o s t f l u i d s and a c i d r e p l a c i n g e x p e n s i v e p r o p p a n t s (HENDRICKSON & CAMERON 1 9 6 8 ) . A c i d f r a c t u r i n g i s t h e r e f o r e v e r y e c o n o m i c a l , because i n e x p e n s i v e s t i m u l a t i o n f l u i d s a r e u s e d t o c r e a t e t h e c r a c k a r e a and t o p r o p a g a t e i t t o f i n a l p e n e t r a t i o n i n c l u d i n g u n d e r t a k i n g measures a g a i n s t f l u i d l o s s , and t h e n much s m a l l e r q u a n t i t i e s o f a c i d a r e f o l l o w i n g i n t h e second s t a g e o f t h e
495 treatment t o e t c h the desired c o n d u c t i v i t y a t the r e q u i r e d depth o f penetration i n t h e r e s e r v o i r . As c o n v e n t i o n a l l y no proppants a r e a p p l i e d which c o u l d b r i d g e i n t h e c r a c k ( c f . s e c t i o n s 4.2.3.2.5. and 6 . 2 . 4 . 2 . 1 . ) , t h e r e i s no danger o f screenout f a i l u r e o f a c i d f r a c t u r i n g j o b .
4.5.1.1.1.3. Fracture conductivity vs. fracture length A comparative e v a l u a t i o n o f a c i d i z e d f r a c t u r e c o n d u c t i v i t y vs. a c i d concent r a t i o n p r o f i l e , amount o f r o c k d i s s o l v e d p e r u n i t area, c l o s u r e s t r e s s and f o r m a t i o n r o c k s t r e n g t h has shown t h a t s t i m u l a t i o n i s g e n e r a l l y l i m i t e d by a c i d i zed f r a c t u r e c o n d u c t i v i t y and n o t by a c i d i z e d c r a c k l e n g t h (NOVOTNY 1976). A c i d i z e d f r a c t u r e s c r e a t e d by h y d r o c h l o r i c a c i d have l i m i t e d l e n g t h b u t h i g h cond u c t i v i t y , whereas low f l u i d - l o s s a c i d emulsions ( c f . s e c t i o n 4.5.1.1.3.2.) gen e r a t e d l o n g e r a c i d i z e d c r a c k s than h y d r o c h l o r i c a c i d , because t h e a c i d emuls i o n r e a c t s a f u r t h e r d i s t a n c e down t h e f r a c t u r e and t h u s r e s u l t a n t c o n d u c t i v i t y i s l o w e r than t h a t c r e a t e d w i t h h y d r o c h l o r i c a c i d . When c l o s u r e s t r e s s e s on t h e c r a c k s a r e h i g h and f r a c t u r e c o n d u c t i v i t y l i m i t e d s t i m u l a t i o n , h y d r o c h l o r i c a c i d t r e a t m e n t s p r o v i d e s l i g h t l y b e t t e r r e s u l t s , whereas a t low c l o s u r e s t r e s ses and when a c i d i z e d f r a c t u r e l e n g t h l i m i t s s t i m u l a t i o n , t h e a c i d emulsion g i ves s u b s t a n t i a l l y improved t r e a t m e n t e f f e c t o v e r p l a i n h y d r o c h l o r i c a c i d .
4.5.1.1.1.4. Double fracturing in reservoir with hairline cracks A s u i t a b l e c o m b i n a t i o n o f h y d r a u l i c p r o p p a n t and a c i d f r a c t u r i n g ( c f . sect i o n 4.5.4.2.) i s double f r a c t u r i n g i n r e s e r v o i r s c o n t a i n i n g reasonable amounts o f carbonates t h a t can be d i s s o l v e d by a c i d (DOWELL SCHLUMBERGER 1988 c, 1988 d ) . Double f r a c t u r i n g c o n s i s t s o f a l t e r n a t i n g stages o f n o n - r e a c t i v e v i s c o u s pad f l u i d and a c i d ( c f . s e c t i o n 4 . 5 . 1 . 6 . 2 . ) . Many carbonate f o r m a t i o n s c o n t a i n numerous n a t u r a l h a i r l i n e c r a c k s and f i s s u r e s which a r e e x t r e m e l y narrow o r c l o sed and have l i t t l e e f f e c t on o v e r a l l p e r m e a b i l i t y , b u t once opened h y d r a u l i c a l l y o r e n l a r g e d by a c i d e t c h i n g , t h e y can cause e x c e s s i v e f l u i d l e a k o f f and become t h e dominant i n f l u e n c e on f l u i d l o s s ( s i m i l a r l y as a p p l y i n g f o r dual f l u i d l e a k o f f i n sandstone r e s e r v o i r s ; WARPINSKI 1988 a; c f . s e c t i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.). U s i n g i n t e r m i t t e n t stages o f pad f l u i d and a c i d , t h e acid-enl a r g e d h a i r l i n e c r a c k s a r e exposed t o n o n - r e a c t i v e pad f l u i d as t h e h y d r a u l i c f r a c t u r e p e n e t r a t e s outward i n t o t h e r e s e r v o i r which keeps t h e a c i d i n t h e main f r a c t u r e and a v o i d s a c i d l o s s by f u r t h e r opening o f t h e h a i r l i n e c r a c k s .
W h i l e normal f l u i d - l o s s agents do n o t r e s t r i c t f l o w i n t o t h e h a i r l i n e f r a c t u r e s , v i s c o u s pad stages a l t e r n a t i n g w i t h a c i d stages a r e p u t a g a i n s t and i n t o h a i r l i n e c r a c k s as soon as t h e y b e g i n t o comprise t h e main source o f l e a k o f f , t h e r e b y c o n t r o l l i n g and subsequently r e d u c i n g f l o w i n t o t h e i n t e r s e c t i n g e n l a r ged h a i r l i n e f r a c t u r e s which a l l o w s t h e a c i d t o r e a c h degree o f p e n e t r a t i o n and f o l d s o f i n c r e a s e as designed. Even more e f f e c t i v e f l u i d l e a k o f f c o n t r o l can be achieved by b r i d g i n g t h e h a i r l i n e c r a c k s w i t h 100 mesh sand t h a t i s suspended i n t h e pad f l u i d ( c f . a l s o MILLER & WAREMBOURG 1975; COULTER, CROWE, BARRETT & MILLER 1976; WARPINSKI 1988 a; c f . s e c t i o n s 4.8.8.3.1.2. and 4 . 8 . 8 . 3 . 2 . 3 . ) . An a l t e r n a t i v e s o l i d f l u i d - l o s s agent t o 100 mesh sand i s f i n e s a l t o f 30/60 and/ o r 80/120 mesh g r a i n s i z e which i s b e t t e r than s i l i c a because i t packs t o lower p e r m e a b i l i t i e s i n h a i r l i n e c r a c k s under p r e s s u r e and subsequently d i s s o l v e s a g a i n o u t o f t h e f r a c t u r e s , t h e r e b y a l l o w i n g them t o s t a y open and t o c o n t r i b u t e t o p r o d u c t i o n (SCHRIETER & SHAW 1978; c f . s e c t i o n s 1.4.11.2.1.2. and 5 . 1 1 . 4 . 2 . ) . S a l t a d d i t i v e a l s o a v o i d s pump and equipment a b r a s i o n caused by s i 1i c a .
496
4.5.1.1.2. Conductivity contrast between
reservoir matrix and propped fracture
The e x p l a n a t i o n f o r t h e optimum e f f e c t o f s t i m u l a t i o n by h y d r a u l i c p r o p p a n t f r a c t u r i n g o f b o t h sandstone and carbonate r e s e r v o i r s i s t h e p e r m e a b i l i t y cont r a s t t h a t i s c r e a t e d between t h e t i g h t m a t r i x o f t h e h y d r o c a r b o n - b e a r i n g r o c k s ( r e g a r d l e s s o f t h e i r l i t h o l o g i c a l c o m p o s i t i o n ) and t h e propped f r a c t u r e ( c f . section 4.6.1.). Depending on d e p t h and c l o s u r e s t r e s s , t h e p e r m e a b i l i t y cont r a s t can be i n f l u e n c e d by t h e p o s s i b i l i t y o f c h o i c e between v a r i o u s p r o p p a n t t y p e s which have d i f f e r e n t c o n d u c t i v i t i e s as w e l l as between d i f f e r e n t g r a i n s i zes ( c f . s e c t i o n 1 . 3 . ) t h a t i n d e p e n d e n t l y f r o m t y p e a l s o r e s u l t i n changing p e r m e a b i l i t i e s o f t h e p r o p p a n t package i n f i l l i n g t h e f r a c t u r e . Some aspects o f f r a c t u r e f l o w c a p a c i t y c o n t r a s t as w e l l as p e r m e a b i l i t y and c o n d u c t i v i t y cont r a s t a r e o u t l i n e d as f o l l o w s .
4.5.1.1.2.1. Fracture flow capacity contrast A f l o w c a p a c i t y c o n t r a s t o f a t l e a s t 100 between f o r m a t i o n and f r a c t u r e i s c o n s i d e r e d t o be e s s e n t i a l f o r t h e achievement o f adequate s t i m u l a t i o n r e s u l t s , and t h i s approach can o n l y be c o n t r o l l e d by s e l e c t i n g a s u i t a b l e h i g h - q u a l i t y p r o p p a n t (WATERS 1980). P A I , G A R B I S & HALL (1983) recommend f l o w c a p a c i t y cont r a s t r a t i o s between c r a c k and f o r m a t i o n o f 1,000 - 1,000,000, w i t h f r a c t u r e c o n d u c t i v i t y b e i n g a f u n c t i o n o f type, s i z e , s t r e n g t h , q u a l i t y , and d e n s i t y o f t h e proppant, as w e l l as f o r m a t i o n overburden p r e s s u r e ( c f . s e c t i o n s 1 . 3 . and 4 . 6 . 1 . ) . S t i m u l a t i o n o f carbonate r o c k s i n Europe and i n many o t h e r p a r t s o f t h e w o r l d has s o f a r been m a i n l y done by a c i d i z i n g due t o much l o w e r c o s t o f such t r e a t m e n t s w i t h r e s p e c t t o t h e expenses f o r h y d r a u l i c p r o p p a n t f r a c t u r i n g t h a t has t u r n e d o u t t o be v e r y s u c c e s s f u l i n many o p e r a t i o n s i n t h e USA, and as a consequence o f h i t h e r t o more o r l e s s s a t i s f a c t o r y t e c h n i c a l and economical r e s u l t s o f a c i d f r a c t u r i n g o r a c i d washing. E f f e c t s o f f l o w c a p a c i t y c o n t r a s t b e t ween f r a c t u r e and f o r m a t i o n on w e l l p r o d u c t i v i t y a r e a l s o d i s c u s s e d by FAST, FLICKINGER & HOWARD ( 1 9 6 1 ) .
4.5.1.1.2.2. Permeability and conductivity contrast The p e r m e a b i l i t y c o n t r a s t between f o r m a t i o n m a t r i x and propped f r a c t u r e i s p a r t i c u l a r l y h i g h i n t i g h t gas sandstones such as t h e R o t l i e g e n d - H a u p t s a n d s t e i n i n some f i e l d s i n Germany FRG where t h e r e s e r v o i r r o c k m a t r i x p e r m e a b i l i t y o f a b t . 10 m i c r o d a r c y c o n t r a s t s w i t h a p e r m e a b i l i t y o f t h e p r o p p a n t pack i n t h e f r a c t u r e o f a b t . 100 d a r c y (KLOSE & KRUMER 1983; c f . s e c t i o n s 3.4. and 3 . 5 . ) . MONTGOMERY & STEANSON (1985) l i s t e f f e c t i v e p e r m e a b i l i t i e s f o r n a t u r a l sand i n t h e range o f 35 - 80 darcy, f o r i n t e r m e d i a t e - s t r e n g t h proppants w i t h i n t h e lim i t s o f 80 - 900 darcy, and f a r h i g h - s t r e n g t h p r o p p a n t s w i t h i n t h e b o u n d a r i e s o f 65 - 1000 d a r c y (depending on g r a i n s i z e and m i n e r a l o g i c a l c o m p o s i t i o n of t h e proppants as w e l l as on c l o s u r e s t r e s s regime; c f . s e c t i o n 1 . 3 . ) . The e f f e c t i v e p e r m e a b i l i t y f o r 20/40 i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants which a r e t h e most f r e q u e n t l y a p p l i e d s y n t h e t i c m a t e r i a l s i s i n t h e range o f 180 - 260 darcy, and t h a t o f 20/40 n a t u r a l sand which i s s t i l l today t h e most common p r o p p i n g agent i s w i t h i n t h e l i m i t s o f 50 - 80 d a r c y . A b s o l u t e p e r m e a b i l i t y and i t s dependence on c o n f i n i n g pressure, p o r e p r e s s u r e and temperature a r e r e v i e w e d by GOBRAN, BRIGHAM & RANEY ( 1 9 8 1 ) .
4.5.1.1.3. Carbonate scale formation and fluid loss On t h e o t h e r hand, packages o f n a t u r a l sand o r s y n t h e t i c proppants i n f i l l i n g h y d r a u l i c f r a c t u r e s i n carbonate r o c k s ( p a r t i c u l a r l y c h a l k s ) s u f f e r f r o m second a r y p e r m e a b i l i t y d e t e r i o r a t i o n by s c a l e f o r m a t i o n due t o c a r b o n a t e c e m e n t a t i o n o f t h e p r o p p a n t a g g l o m e r a t i o n s . One o f t h e m a j o r drawbacks o f c a r b o n a t e f r a c t u -
497 r i n g w i t h r e s p e c t t o sandstone s t i m u l a t i o n i s the h i g h e r d e n s i t y and e f f e c t i v e ness o f n a t u r a l cracks i n carbonate r e s e r v o i r s i n comparison t o sandstone pays ( c f . s e c t i o n 4.5.2.) which g i v e r i s e t o more serious f l u i d l e a k o f f , thereby c r e a t i n g a h i g h e r r i s k o f premature screenout f a i l u r e o f the h y d r a u l i c proppant f r a c t u r i n g j o b (DEES & COULTER 1986; c f . s e c t i o n s 4.2.3.5.3. and 6.2.4.2.1.), and a l s o have a worse i n f l u e n c e on h y d r a u l i c crack propagation which i s subject e d t o v a r i o u s d e f l e c t i o n and r e f r a c t i o n events when c r o s s i n g the abundance o f p r e - e x i s t i n g j o i n t s i n the carbonate pay zone. Thus t h e n a t u r a l f r a c t u r e s are a c t i n g as pronounced f l u i d t h i e v e s which have a s e r i o u s impact on the e f f e c t i v i t y o f creation, propagation and p l u g g i n g o f the h y d r a u l i c f r a c t u r e ( c f . sect i o n s 4.8.8.3.1.2. and 4.8.8.3.2.3.). Some comments on s c a l e p r e c i p i t a t i o n prev e n t i o n and c o r r o s i o n i n h i b i t i o n are o f f e r e d as f o l l o w s .
4.5.1.1.3.1. Scale precipitation prevention VETTER, NILSSEN, LANKFORD & SHELTON (1988) i n v e s t i g a t e s c a l e i n h i b i t o r s f o r proppant and a c i d f r a c t u r i n g . The a d d i t i o n o f scale i n h i b i t o r s t o s t i m u l a t i o n f l u i d s i s essential f o r prevention o f precipitations during well o r reservoir operations a f t e r the f r a c t u r i n g treatment i s completed. I n terms o f chemical c o m p a t i b i l i t y , numerous s c a l e i n h i b i t o r s p r e v e n t the s t i m u l a t i o n f l u i d s from acq u i r i n g any g e l s t r e n g t h even a t very low preventor concentrations, and many i n h i b i t o r s even provoke r a p i d complete breakage o f c r o s s l i n k e d gel systems. Gel s t r e n g t h i n t h e presence o f s c a l e preventors can be p r o v i d e d by i n c r e a s i n g c r o s s l i n k e r concentration, w i t h some o f t h e excess c r o s s l i n k e r becoming s a c r i f i c i a l , b u t on t h e o t h e r hand, the g e l becomes extremely u n s t a b l e upon a p p l i c a t i o n o f mechanical shear as a consequence o f supersaturation, w i t h the p o s s i b i l i t y o f c r e a t i n g f o r m a t i o n damage. Chemical and p h y s i c a l i n c o m p a t i b i l i t i e s b e t ween s c a l e i n h i b i t o r s and s t i m u l a t i o n f l u i d s can become serious problems. Concerning h y d r a u l i c proppant f r a c t u r i n g , scale preventors can be added by d i s s o l u t i o n w i t h i n the aqueous phase o f the s t i m u l a t i o n f l u i d t o the l i q u i d pre-pad d u r i n g the treatment where they h o p e f u l l y do n o t g e t i n t o c o n t a c t w i t h the f r a c t u r i n g f l u i d i t s e l f , o r t o the proppant i n s o l i d form where they do n o t d i s s o l v e t o any major e x t e n t w i t h i n the s t i m u l a t i o n f l u i d . While some f r a c t u r i n g f l u i d s are r a t h e r s e n s i t i v e t o any s c a l e i n h i b i t o r a d d i t i o n , o t h e r s are n o t . Concerning a c i d f r a c t u r i n g , s c a l e i n h i b i t o r s are i n e f f e c t i v e a t low pH values, because the pH value o f produced f l u i d s a f t e r an a c i d s t i m u l a t i o n j o b may s t a y t o o low f o r e f f e c t i v e s c a l e prevention. Scale i n h i b i t o r s may n o t absorb p r o p e r l y on the i n t e r n a l surface area o f the r e s e r v o i r rock thus d r a s t i c a l l y r e ducing the l i f e t i m e o f a squeeze j o b d u r i n g o r p r i o r t o an a c i d j o b .
4.5.1 L 3 . 2 . Corrosion inhibition DAVIES, LIEVAART & NITTERS (1988) study e f f e c t s o f c o r r o s i o n i n h i b i t o r s and mutual s o l v e n t s on m a t r i x a c i d i z i n g treatments. F i l t e r - c a k e - f o r m i n g i n h i b i t o r s have a s t r o n g e r impact on h i g h - p e r m e a b i l i t y rocks, whereas the e f f e c t s o f adsorbing i n h i b i t o r s are g r e a t e r i n l o w - p e r m e a b i l i t y rocks. F i l t e r e d - o u t o r adsorbed preventors can be removed from the f o r m a t i o n by mutual s o l v e n t s which s o l u b i l i z e damaging i n h i b i t o r residues and p r e v e n t f o r m a t i o n p l u g g i n g d u r i n g a c i d i n j e c t i o n . E f f e c t s o f a c i d c o r r o s i o n preventors on m a t r i x s t i m u l a t i o n r e s u l t s are a l s o i n v e s t i g a t e d by WOODROOF, BAKER & JENKINS (1975); SMITH, DOLLARHIDE & BYTH (1978) and CROWE & MINOR (1982). I n h i b i t o r adsorption and f o r m a t i o n p l u g g i n g by p r e v e n t o r residues can have a major i n f l u e n c e on clean-up r a t e and u l t i mate e f f e c t i v e p e r m e a b i l i t y o f the t r e a t e d zone. Formation damage can a l s o be provoked by f r i c t i o n reducers i n a c i d s t i m u l a t i o n (OUSTERHOUT & HALL 1961; PRUITT, SIMMONS, NEILL & CRAWFORD 1965; SUTTON 1976, WOODROOF & ANDERSON 1977).
498
4.5.1.1.4. Iron sequestering and complexing Many c a r b o n a t e f o r m a t i o n s c o n t a i n i r o n o x i d e s and s e q u e s t e r i n g agen s m u s t Iron be u s e d w i t h a c i d t r e a t m e n t s in t h e s e f o r m a t i o n s (JENNINGS & DARDEN 1979) p r e c i p i t a t i o n has been r e c o g n i z e d as a s i g n i f i c a n t p r o b l e m i n a c i d i z i n g o p e r a t i o n s f o r o v e r 30 y e a r s . I r o n d i s s o l v e d b y a c i d f r o m r e s e r v o i r f o r m a t on o r b o r e h o l e c o m p l e t i o n r e m a i n s in s o l u t i o n u n t i l t h e a c i d i s s p e n t (SMITH, CROWE & NOLAN 1 9 6 8 ) . As pH o f t h e s p e n t a c i d b e g i n s t o r i s e , i r o n l o s e s i t s s o l u b i l i t y and p r e c i p i t a t e s , t h e r e b y s e r i o u s l y damaging t h e f l o w c h a n n e l s j u s t opened b y t h e a c i d r e a c t i o n i n t h e p a y zone complex. S e q u e s t e r i n g a g e n t s a c t b y complexi n g i r o n i o n s t o i n h i b i t p r e c i p i t a t i o n as h y d r o c h l o r i c a c i d spends. The p r e s e n c e o f i r o n compounds in a f o r m a t i o n i s a l s o i m p o r t a n t when a zone i s t r e a t e d b y hydraulic fracturing, s i n c e a c i d i s f r e q u e n t l y u s e d as a f o r m a t i o n breakdown f l u i d p r i o r t o s t i m u l a t i o n . A f t e r an o u t l i n e o f t h e m o s t i m p o r t a n t s o u r c e s o f i r o n in a c i d i z i n g and f r a c t u r i n g , p r o b l e m s o f c o m p a t i b i l i t y o f i r o n - s e q u e s t e r i n g and - c o m p l e x i n g a g e n t s a r e i l l u s t r a t e d .
4.5.1.1.4.1.Sources of iron in acidizing and fracturing A n a l y s i s o f i r o n problems i n f o r m a t i o n s n o t c o n t a i n i n g i r o n - b e a r i n g m i n e r a l s i n s u f f i c i e n t q u a n t i t y r e v e a l s t h a t t h e primary source o f i r o n i n a c i d t r e a t ments i s p i p e dope and m i l l s c a l e o f i r o n and r u s t in d r i l l i n g s t r i n g , casing and t u b i n g o f t h e w e l l (GOUGLER, HENDRICK & COULTER 1985; c f . s e c t i o n 5 . 1 1 . 5 . ) . P a r t i c u l a r l y s e r i o u s s o u r c e s a r e new t u b u l a r goods where i n e x t r e m e c a s e s , t h e m i l l s c a l e c o u l d n e a r l y spend t h e f i r s t p o r t i o n o f a c i d t h e r e b y c a u s i n g i t t o be i n e f f e c t i v e . O t h e r a s p e c t s o f i r o n p r e c i p i t a t i o n d u r i n g a c i d t r e a t m e n t s and i t s p r e v e n t i o n a r e examined b y SMITH, CROWE & NOLAN ( 1 9 6 8 ) ; D I L L & FREDETTE ( 1 9 8 3 ) , COULTER & GOUGLER ( 1 9 8 4 ) , CROWE ( 1 9 8 4 ) , GDANSKI & PEAVY ( 1 9 8 6 ) and SMOLARCHUK & D I L L ( 1 9 8 6 ) . HALL & D I L L ( 1 9 8 8 ) summarize c o m p a t i b i l i t i e s o f i r o n - c o n t r o l a d d i t i v e s f o r l i m e s t o n e and s a n d s t o n e a c i d i z i n g o f sweet and s o u r w e l l s . D e p o s i t s o f i r o n compounds i n t u b u l a r s and f o r m a t i o n d e t e r m i n e amount o f i r o n d i s s o l v e d b y t h e a c i d and t h u s s e v e r i t y o f i r o n p r o b l e m s . I r o n - s e q u e s t e r i n g a d d i t i v e s may p r e s e n t c o m p a t i b i l i t y d i f f i c u l t i e s and may l o s e e f f e c t i v e n e s s because o f d e g r a d a t i o n . The m a i n i r o n s o u r c e s a r e m i l l s c a l e i n new p i p e , i r o n - b e a r i n g m i n e r a l s i n s a n d s t o n e and c a r b o n a t e f o r m a t i o n s , and i r o n in r e c o v e r e d s p e n t a c i d s o l u t i o n s (SMITH, CROWE & NOLAN 1 9 6 8 ) . I n j e c t i o n w e l l s u s u a l l y c o n t a i n more i r o n o x i d e s c a l e s t h a n p r o d u c t i o n w e l l s . W h i l e h y d r o c h l o r i c a c i d i n c a r b o n a t e f o r m a t i o n s does n o t show c o m p a t i b i l i t y p r o b l e m s w i t h i r o n sequestering additives, considerable d i f f i c u l t i e s occur w i t h hydrochlorich y d r o f l u o r i c a c i d s o l u t i o n s i n sandstone f o r m a t i o n s ( c f . s e c t i o n 4 . 5 . 5 . ) . I n r e s e r v o i r r o c k s c o n t a i n i n g abundant d e t r i t a l and/or d i a g e n e t i c a l i r o n b e a r i n g m i n e r a l s such as s i d e r i t e , a n k e r i t e , h e m a t i t e , l i m o n i t e and p y r i t e , h y d r o c h l o r i c a c i d d i s s o l v e s t h e i r o n compounds w h i c h c a n r e p r e c i p i t a t e as t h e a c i d c o m p l e t e l y spends on t h e c a r b o n a t e p h a s e s . I r o n compound p r e c i p i t a t i o n t h e r e f o r e has t o b e p r e v e n t e d o r c o n t r o l l e d b y i r o n - s e q u e s t e r i n g o r - c o m p l e x i n g a g e n t s i n t h e a c i d (McLEOD 1 9 8 4 ) .
4.5.1.1.4.2.Compatibility of iron-
sequestering and -comlexing agents
SMITH, CROWE & NOLAN ( 1 9 6 8 ) p r e s e n t a c o m p a r a t i v e p e r f o r m a n c e e v a l u a t i o n o f v a r i o u s i r o n - s e q u e s t e r i n g o r - c o m p l e x i n g a g e n t s , w i t h a l l o f them, however, hav i n g t o b e c a u t i o u s l y a p p l i e d , as t h e r e i s p r i n c i p a l l y n o i r o n - c o n t r o l a d d i t i v e t h a t can economically p r e v e n t i r o n p r e c i p i t a t i o n w i t h o u t danger o f agent p r e c i p i t a t i o n i f i r o n i s n o t f o u n d downhole i n t h e e x p e c t e d q u a n t i t y , and i n some c a ses, t h e i r o n even keeps t h e s e q u e s t e r i n g a d d i t i v e i n s o l u t i o n i n s p e n t a c i d . A s i m i l a r e f f e c t as i r o n p r e c i p i t a t i o n f o l l o w i n g a c i d i z i n g i s i r o n c o l l o i d p l u g g i n g i n s a n d s t o n e m a t r i x (POTTER & D I B B L E 1 9 8 5 ) . F l o c c u l a t i o n a n d / o r c o a g u l a -
499 t i o n o f t h e f e r r i c h y d r o x i d e l e a d s t o f o r m a t i o n o f f i l t e r cake i n low-pH cases, whereas c o l l o i d and q u a r t z s u r f a c e i n t e r a c t i o n produces a more u n i f o r m accumulat i o n o f c o l l o i d a t h i g h e r pH. I n a d d i t i o n t o p r e v e n t i n g p l u g g i n g , s e l e c t i v e c o l l o i d f o u l i n g m i g h t be used as a m o b i l i t y b u f f e r i n r e s e r v o i r s w i t h heterogeneous p e r m e a b i l i t y d i s t r i b u t i o n . I r o n - c o n t a m i n a t e d a c i d i s a b l e t o promote p r e c i p i t a t i o n o f asphaltenes when a c i d i z i n g c e r t a i n o i l - b e a r i n g r e s e r v o i r s (NEWBERRY & BARKER 1985, JACOBS & THORN E 1986) even i n t h e presence o f moderate amounts o f a n t i - s l u d g i n g agents. T r a d i t i o n a l i r o n - c o n t r o l a d d i t i v e s a r e i n e f f e c t i v e as t h e y sequester i r o n a f t e r t h e a c i d i s spent. H i g h l e v e l s o f a n t i - s l u d g i n g a d d i t i v e s can sometimes f o r m r e verse emulsions as a consequence o f o r i g i n a l i r o n complexes w i t h n o n - e m u l s i f y i n g agents and a n t i - s l u d g i n g a d d i t i v e s . A c i d s t r e n g t h has s e r i o u s impact on t h e amount o f sludge and t h e i n a b i l i t y o f t y p i c a l a n t i - s l u d g i n g a d d i t i v e s t o cont r o l i t . The use o f an improved i r o n - s u p e r v i s i o n a d d i t i v e i n c o n j u n c t i o n w i t h a p p r o p r i a t e amounts o f a n t i - s l u d g i n g agent and c o r r o s i o n i n h i b i t o r can e l i m i n a t e asphaltene p r e c i p i t a t i o n i n m o d e r a t e - s t r e n g t h a c i d s when measures a r e t a ken t o keep t h e i r o n c o n t e n t i n t h e a c i d t o a minimum. T h e r e f o r e i t i s most i m p o r t a n t t o remove i r o n s c a l e f r o m a l l p i p i n g by back-washing t e c h n i q u e s .
4.5.1.2.
Conductivity improvement by acid optimization
A c i d f r a c t u r i n g i s performed by i n j e c t i o n o f a c i d w i t h a bottomhole t r e a t i n g p r e s s u r e exceeding f o r m a t i o n f r a c t u r e p r e s s u r e which l e a d s t o h y d r a u l i c opening o f t h e c r a c k and e t c h i n g o f t h e c r e a t e d f r a c t u r e f a c e s by t h e a c i d , t h e r e b y g i v i n g r i s e t o an open f l o w channel (ECONOMIDES 1987 b ) . The p o s s i b i l i t i e s o f a c i d improvement i n c l u d e a c i d s t r e n g t h and f l u i d - l o s s c o n t r o l , a c i d g e l l i n g and c r o s s l i n k i n g , and a c i d h e a t i n g .
4.5.1.2.1. Acid strength I n c r e a s i n g f r a c t u r e c o n d u c t i v i t y and l e n g t h i s t h e most i m p o r t a n t means o f improvement o f a c i d f r a c t u r i n g t r e a t m e n t s (NOVOTNY 1976) as w e l l as p r o p p a n t f r a c t u r i n g o p e r a t i o n s ( c f . s e c t i o n s 1 . 3 . , 1.4.10. and 4 . 8 . 1 1 . ) . T h i s can be achieved by i n c r e a s i n g t h e d i s s o l v i n g power o f t h e a c i d system. A c i d - e x t e r n a l emulsions c r e a t e l o n g a c i d i z e d f r a c t u r e s , b u t t h e y n o r m a l l y c o n t a i n o n l y onet h i r d a c i d and t h u s t h e r e s u l t a n t c r a c k c o n d u c t i v i t y i s low. P l a i n h y d r o c h l o r i c a c i d g i v e s r i s e t o o r i g i n o f v e r y c o n d u c t i v e f r a c t u r e s i n carbonates, b u t because o f i t s poor f l u i d - l o s s c h a r a c t e r i s t i c s , crack l e n g t h i s s h o r t and l i m i t s s t i m u l a t i on. A c i d emulsions p r o v i d e e x c e l l e n t improvements a f t e r t r e a t m e n t i n s h a l l o w r e s e r v o i r s where c l o s u r e s t r e s s e s a r e low and f r a c t u r e c o n d u c t i v i t y i s h i g h , as t h e crack s t a y s more o r l e s s open, whereas i n deeper r e s e r v o i r s , unpropped f r a c t u r e s a r e c o n s i d e r a b l y damaged by c l o s u r e and t h u s reasonable a m e l i o r a t i o n can be o n l y o b t a i n e d by s u p p o r t i n g t h e c r a c k s w i t h p r e f e r a b l y l a r g e proppants i n o r d e r t o g e t maximum c o n d u c t i v i t y o f t h e propped d r a i n a g e p a t h t h a t would s t i l l be s u f f i c i e n t l y h i g h e r t h a n a p a r t i a l l y c l o s e d u n f i l l e d f r a c t u r e . W h i l e a u t o p r o p p i n g o f f r a c t u r e w a l l s ( c f . s e c t i o n s 4.5.1.1.1.2. and 4.5.1.5.) can be s t i l l q u i t e e f f e c t i v e i n s h a l l o w pay depth, c r u s h i n g o f broken r o c k fragments c a u s i n g s e l f - p r o p p i n g i n deep r e s e r v o i r s i s a s i g n i f i c a n t f a c t o r o f c o n d u c t i v i t y damage i n f r a c t u r e s c r e a t e d by a c i d and n o t plugged by h i g h - q u a l i t y synthet i c proppants.
4.5.1.2.2.
Acid fluid-loss control
F l u i d l o s s can be c o n t r o l l e d by u s i n g a v i s c o u s i n e r t pad ahead o f p l a i n hyd r o c h l o r i c a c i d , b u t f i r s t h y d r o c h l o r i c a c i d i s denser than t h e pad f l u i d which would cause t h e h y d r o c h l o r i c a c i d t o f l o w a l o n g t h e bottom o f t h e f r a c t u r e
500 ( t h i s problem c o u l d be c o r r e c t e d by a p p l y i n g b a l a n c e - d e n s i t y f l u i d s ; FREDRICKSON & BROADOUS 1976; c f . s e c t i o n 4 . 2 . 2 . 6 . ) , and second t h e poor f l u i d - l o s s cont r o l o f h y d r o c h l o r i c a c i d a l l o w e d i t t o wormhole through t h e pad and d e s t r o y e f f e c t (NOVOTNY 1976). NIERODE & KRUK (1973) a l s o comment on a c i d f l u i d - l o s s a d d i t i v e s and a c i d i z e d f r a c t u r e c o n d u c t i v i t y . The use o f a l t e r n a t i n g stages o f a c i d and n o n - a c i d f l u i d ( c f . s e c t i o n 4.5.1.6.2.) containing particulate f l u i d - l o s s a d d i t i v e s can be e f f e c t i v e i n c o n t r o l l i n g f l u i d l e a k o f f t o t h e f o r m a t i o n (PARKER, AOAMS & LIANKUI 1986). Viscous a c i d a l l o w s b u l k d i s p l a c e m e n t o f f l u i d down t h e f r a c t u r e i n o r d e r t o promote e x p o s i t i o n o f t h e e n t i r e c r a c k t o a c i d , w i t h r e t a r d a t i o n o f a c i d r e a c t i o n r a t e b e i n g an a d d i t i o n a l b e n e f i t ( a s p e c t s o f a c i d f l u i d - l o s s c o n t r o l are also discussed i n section 4.5.1.6.). A c i d c o n c e n t r a t i o n s a r e determined more by f o r m a t i o n m i n e r a l o g y than by t h e p l u g g i n g s o l i d damaging t h e r e s e r v o i r (McLEOD 1984). V a r i o u s a c i d c o n c e n t r a t i o n s d i s s o l v e damage p a r t i c u l a r l y i f small amounts a r e c r i t i c a l l y p l a c e d around t h e p e r f o r a t i o n s , b u t l o w e r a c i d s a t u r a t i o n s reduce p r e c i p i t a t i o n p r o blems i n a c i d - s e n s i t i v e pay zones. Low h y d r o c h l o r i c a c i d c o n c e n t r a t i o n s ( 3 - 5 % ) a r e o c c a s i o n a l l y used f o r v a r i o u s d r i l l i n g and c o m p l e t i o n purposes (SIMON & UNDERWOOD 1977).
4.5.1.2.3. Ac id cross 1 ink i ng A c i d o p t i m i z a t i o n can be e f f e c t i v e l y achieved by g e l l i n g a n d / o r c r o s s l i n k i n g t h e a c i d (COULTER, HARRIS & KLEBENOW 1980; PABLEY & HOLCOMB 1980, 1981, 1982; CHURCH, QUISENBERRY & FOX 1981; PABLEY, EWING & CALLAWAY 1982; McLANE & SCOTT 1983; SMITH, DAWSON & SCOGGINS 1983; c f . s e c t i o n 4.3.4.3.4.1.). Crosslinked h i g h - s t r e n g t h a c i d systems a r e e x c e l l e n t f l u i d s f o r a c i d f r a c t u r i n g and a r e a l s o capable o f c a r r y i n g s u b s t a n t i a l amounts o f proppants, t h e r e b y r e p r e s e n t i n g t h e i d e a l medium f o r combined a c i d and p r o p p a n t f r a c t u r i n g ( c f . s e c t i o n 4 . 5 . 4 . 2 . ) . H i g h - s t r e n g t h c r o s s l i n k e d a c i d systems p r o v i d e c o n s i d e r a b l e r e d u c t i o n o f t u b u l a r f r i c t i o n pressure. F u r t h e r advantages a r e t h e a b i l i t y t o i n h i b i t c l a y s w e l l i n g i n w a t e r - s e n s i t i ve r e s e r v o i r s , and t h e r e f o r e t h e h i g h - s t r e n g t h c r o s s l i n k e d a c i d system can even compete w i t h foam f r a c t u r i n g f l u i d s and e m u l s i f i e d a c i d s a l s o because o f superi o r proppant t r a n s p o r t c a p a c i t y . F l u i d - l o s s c o n t r o l e q u a l l i n g o r exceeding t h a t achieved by c r o s s l i n k e d water-based f l u i d s ensure deep p e n e t r a t i o n o f l i v e a c i d , and an e x t r e m e l y r e t a r d e d r e a c t i o n r a t e a l l o w s i n many cases t h e c r o s s l i n ked a c i d t o be p l a c e d i n t o t h e f o r m a t i o n w i t h l i t t l e o r no decrease i n a c i d s t r e n g t h . C o n t r o l l e d b r e a k o u t e f f e c t i v e l y a l l o w s t h e a c i d t o spend as t h e f l u i d i s moving back toward t h e w e l l b o r e d u r i n g cleanup ( c f . a l s o s e c t i o n s 3.11.3. and 4 . 3 . 4 . 3 . ) .
4.5.1.2.4.
Acid gelling
G e l l i n g o f a c i d by polymers p r o v i d e s s t a b l e v i s c o s i t y and c o n t i n u e d s o l u b i l i t y o f t h e a c i d a t e l e v a t e d temperatures due t o r e s i s t a n c e t o thermal degradat i o n and h y d r o l y s i s o f t h e a c i d (JOHNSON, FOX, BURNS & O'MARA 1988; FOX, O'MARA, BURNS & JOHNSON 1989; c f . s e c t i o n 4 . 3 . 4 . 3 . 4 . 1 . ) . Some polymers a r e a l s o sol u b l e i n t h e c a l c i u m and magnesium b r i n e s produced by spending o f h y d r o c h l o r i c a c i d i n c a r b o n a t e f o r m a t i o n s . The v i s c o s i t y o f t h e b r i n e i s e s s e n t i a l l y t h e same as t h a t o f t h e o r i g i n a l g e l l e d a c i d which guarantees c o n t i n u e d r e s i s t a n c e t o f l u i d l o s s . A c i d r e t a r d a t i o n ( c f . s e c t i o n 4.5.1.3.2.) by p r o d u c i n g a s t a b l e v i s c o s i t y r e s t r i c t s c o n v e c t i o n i n t h e a c i d . Under dynamic c o n d i t i o n s where a c i d spending r a t e i s h e a v i l y i n f l u e n c e d by c u r r e n t regime, maintenance o f l a m i n a r f l o w i s i m p o r t a n t i n p r o l o n g a t i o n o f spending t i m e . Enhancement o f p r o d u c t i o n s t i m u l a t i o n r e s u l t i n g from m a t r i x o r f r a c t u r e a c i d i z i n g i s due t o improved a c i d p e n e t r a t i o n , a m e l i o r a t e d a c i d r e t a r d a t i o n and reduced f l u i d l o s s . I n some s p e c i a l a p p l i c a t i o n s , combinations o f
gelled
and
non-gelled
acids
501
a r e a p p l i e d , w i t h g e l l e d a c i d b e i n g i n s e r t e d f i r s t f o r t h e purpose o f f l u i d l e a k o f f c o n t r o l and l i v e a c i d p e n e t r a t i o n a m e l i o r a t i o n , and n o n - g e l l e d a c i d b e i n g pumped b e h i n d t h e g e l l e d a c i d t o promote f i n g e r i n g o f t h e t h i n f l u i d t h r o u g h t h e more v i s c o u s f l u i d ( c f . s e c t i o n 4.5.4.2.1.) and enhancement o f a d i f f e r e n t i a l e t c h and d i s s o l u t i o n p a t t e r n on t h e f r a c t u r e face. L i g h t l y g e l l i n g o f a c i d i s o c c a s i o n a l l y c a r r i e d o u t i n o r d e r t o enhance wormhole p e n e t r a t i o n ( c f . s e c t i o n 4.5.1.6.3.) and t o p r o v i d e s u f f i c i e n t r e s i d u a l v i s c o s i t y f o r f i n e s suspension and removal. As a consequence o f i t s i n c r e a s e d v i s c o s i t y , g e l l e d a c i d has c o n s i d e r a b l y reduced f l u i d l o s s and a l s o i n h i b i t s m i g r a t i o n o f i o n s which r e t a r d s r e a c t i o n r a t e . V i s c o s i t y i s r e t a i n e d a f t e r a c i d spending which a l lows i n s o l u b l e m a t e r i a l s t o remain suspended and t o be r e t u r n e d w i t h flowback.
4.5.1.2.5. Acid heating A c i d h e a t i n g a l s o i s a p o s s i b i l i t y t o improve a c i d p e n e t r a t i o n and e f f e c t i v i of r o c k d i s s o l u t i o n e s p e c i a l l y on c o o l e r f o r m a t i o n s , and t h e i n c r e a s e d a c i d temper a t u r e can s o f t e n o r g a n i c d e p o s i t s f r e q u e n t l y found on r e s e r v o i r f a c e s which h i n d e r t h e a c i d - t o - r o c k c o n t a c t . Heated a c i d can a l s o m i n i m i z e r e p r e c i p i t a t i o n o f s o l i d s i n o i l used by r a p i d thermal shock and can p r e v e n t e x c e s s i v e t u b i n g con t r a c t i o n . t y (HOCH, WALKER, FREDRICKSON & NORMAN 1986). Heated a c i d i n c r e a s e s t h e r a t e
4.5.1.3. Enhancement o f acid penetration distance When a pad o f f l u i d i s used ahead o f t h e a c i d t o achieve a l o n g w i d e f r a c t u r e , maximum p o s s i b l e p e n e t r a t i o n d i s t a n c e i s based on t h e assumption o f a c i d r e a c t i o n i n t h e f r a c t u r e geometry e x i s t i n g a t t h e end o f t h e pad, and t h e minimum p o s s i b l e a c i d p e n e t r a t i o n d i s t a n c e w i l l be t h e dynamic c r a c k l e n g t h c a l c u l a t e d assuming t h a t f l u i d l o s s i s c o n t r o l l e d by t h e v i s c o s i t y o f r e a c t e d a c i d , w i t h these two c o n d i t i o n s b e i n g r e a c t i o n r a t e l i m i t and f l u i d - l o s s boundary, respect i v e l y (WILLIAMS & NIERODE 1971). The a c t u a l p e n e t r a t i o n d i s t a n c e p r o b a b l y var i e s between t h e maximum and minimum d i s t a n c e d u r i n g a t r e a t m e n t , because a c i d r e a c t i o n produces wormholes l e a d i n g f r o m t h e f r a c t u r e i n t o t h e f o r m a t i o n ( c f . s e c t i o n 4.5.1.6.3.) which e l i m i n a t e f l u i d - l e a k o f f c o n t r o l by t h e pad. F o l l o w i n g an o u t l i n e o f a c i d i n j e c t i o n and r e a c t i o n , some aspects o f a c i d r e t a r d a t i o n a r e i11u s t r a t e d .
4.5.1.3.1. Acid injection and reaction The most i m p o r t a n t v a r i a b l e s a f f e c t i n g a c i d p e n e t r a t i o n d i s t a n c e a r e a c i d i n j e c t i o n r a t e , f r a c t u r e w i d t h c r e a t e d by t h e pad f l u i d , temperature and a c i d conc e n t r a t i o n . A c i d p e n e t r a t i o n and r e a c t i o n r a t e a r e c o n t r o l l e d by f l o w v e l o c i t y , e x i s t i n g damage, a c i d c o n c e n t r a t i o n and temperature, and t o t a l amount o f a c i d i n j e c t e d , as w e l l as by r e a c t i o n p r o d u c t s , f l u i d v i s c o s i t y , and area/volume r a t i o (BRANNON, NETTERS & GRIMMER 1987; DOWELL SCHLUMBERGER 1988 d ) . I n c r e a s i n g temperature l e a d s t o d i m i n i s h i n g a c i d v i s c o s i t y and an a s s o c i a t e i n c r e a s e i n t h e e f f e c t i v e m i x i n g c o e f f i c i e n t w h i c h a l t o g e t h e r g i v e r i s e t o r e d u c t i o n o f pen e t r a t i o n d i s t a n c e . Temperature c o n t r o l l i n g a c i d r e a c t i o n i s a f f e c t e d by a c i d i n j e c t i o n temperature as a m a j o r f a c t o r and by t h e h e a t l i b e r a t e d by t h e i n j e c t i o n i t s e l f as a m i n o r f a c t o r . As a c i d spends, r e a c t i o n r a t e decreases due t o reduced l i v e a c i d c o n c e n t r a t i o n and t h e e f f e c t o f d i s s o l v e d r e a c t i o n p r o d u c t s . Some aspects o f a c i d c h a n n e l i z a t i o n and f l u i d p r o p e r t y m o d i f i c a t i o n a r e i l l u s t r a t e d as f o l l o w s .
4.5.1.3.1.1. Acid channel izat ion A c i d f l o w r a t e can be i n c r e a s e d by h i g h e r i n j e c t i o n r a t e s o r by d e s i g n i n g a c h a n n e l i z e d t r e a t m e n t where t h e a c i d c o n t a c t s o n l y a f r a c t i o n o f t h e t o t a l
502 crack height, w i t h t h e necessary containment being achieved by using a viscous rapad f l u i d s o t h a t a c i d c h a n n e l s t h r o u g h t h e f l u i d ( c f . s e c t i o n 4 . 5 . 4 . 2 . 1 . ) t h e r t h a n u n i f o r m l y d i s p l a c i n g i t (GRAHAM, KERVER & MORGAN 1 9 6 5 ) . A c i d c h a n n e l i z a t i o n t o a b t . one f o u r t h o f t o t a l f r a c t u r e h e i g h t s i g n i f i c a n t l y i m p r o v e s a c i d p e n e t r a t i o n d i s t a n c e (WILLIAMS & NIERODE 1 9 7 1 ) . The i m p o r t a n c e o f p r e c e d i n g a c i d i n j e c t i o n w i t h a v i s c o u s f l u i d pad i s u n d e r l i n e d b y t h e i n c r e a s e o f f r a c t u r e w i d t h i n p r o p o r t i o n t o f l u i d v i s c o s i t y (GEERTSMA & DEKLERK 1969; c f . s e c t i o n 4.5.1.6.2.). I n c r e a s i n g a c i d s a t u r a t i o n enhances p e n e t r a t i o n d i s t a n c e s i n c e t h e more conc e n t r a t e d a c i d i s more v i s c o u s and has a l o w e r v a l u e o f t h e e f f e c t i v e m i x i n g c o e f f i c i e n t (WILLIAMS & N I E R O D E 1 9 7 1 ) , w i t h f u r t h e r i n c r e a s e o f p e n e t r a t i o n d i s t a n c e b e i n g a b l e t o be a c h i e v e d b y a d d i n g more m a t e r i a l s w h i c h c r e a t e h i g h e r a c i d v i s c o s i t y . Advantages o f h i g h a c i d c o n c e n t r a t i o n s a r e a l s o o u t l i n e d b y HARRIS, HENDRICKSON & COULTER ( 1 9 6 6 ) . SEVOUGIAN, SCHECHTER & SEPEHRNOORI ( 1 9 8 7 ) comment on o p t i m i z a t i o n o f v e r t i c a l a c i d f r a c t u r e s i n s t e a d y - s t a t e f l o w . On t h e o t h e r hand, r e d u c e d a c i d s t r e n g t h s g i v e r i s e t o s l o w e r r e a c t i o n r a t e s t h a t t r i g g e r a pH e n v i r o n m e n t w h i c h s t a b i l i z e s t h e s o l u t i o n s t h u s m i n i m i z i n g p r e c i p i t a t i o n o f s p e n t a c i d b y - p r o d u c t s (BRANNON, NETTERS & GRIMMER 1 9 8 7 ) . T h i s p a r t i c u l a r l y i n h i b i t s a c i d r e a c t i o n s l o u g h i n g w h i c h may l e a d t o p e r m e a b i l i t y d i m i n u t i o n v i a c l a y and f i n e s m i g r a t i o n and s u b s e q u e n t b l o c k a g e .
4.5.1.3.1.2. Fluid property modification A c i d p e n e t r a t i o n d i s t a n c e and r a t e c a n b e i n c r e a s e d b y a d d i t i o n o f 100 mesh sand i n v a r i o u s c o n c e n t r a t i o n s and f r a c t u r i n g t r e a t m e n t s t a g e s as a f l u i d - l o s s c o n t r o l a d d i t i v e i n case o f h i g h l e a k o f f (BAILEY & WICKHAM 1984; c f . s e c t i o n s 1 . 4 . 1 1 . 2 . 1 . 1 . and 4 . 8 . 8 . 3 . 2 . 3 . ) . L o n g e r p e r s i s t e n c e o f l i f e a c i d t h a t c a n move f u r t h e r outwards i n t o t h e formation along the f r a c t u r e i s a l s o achieved by r e d u c t i o n o r r e t a r d a t i o n o f a c i d r e a c t i o n r a t e b y c h e m i c a l r e t a r d i n g a g e n t s , ernuls i f i c a t i o n o f a c i d i n o i l , and a c i d g e l l i n g . F u r t h e r r e d u c t i o n of l e a k o f f and r e a c t i o n r a t e c a n b e o b t a i n e d b y v a r i o u s volumes o f g e l l e d w a t e r p r e f l u s h , w i t h t h i n n e r g e l T e d f l u i d s b e i n g d e s i g n e d as c o o l d o w n and l e a k o f f m o d i f i e r s , whereas t h i c k e r f l u i d s i n j e c t e d ahead o f t h e a c i d a r e u s e d as f l u i d l o s s m o d i f i e r s and p r e s e n t a t h i c k f l u i d t h r o u g h w h i c h t h e a c i d can f i n g e r ( c f . s e c t i o n 4 . 5 . 4 . 2 . 1 . ) . A n o t h e r method o f i n c r e a s i n g a c i d p e n e t r a t i o n i s using a l t e r n a t i n g stages o f c r o s s l i n k e d polymer f l u i d f o l l o w e d by a c i d ( c f . s e c t i o n 4 . 5 . 1 . 6 . 2 . ) . Reduction o f surface t e n s i o n by mutual s o l v e n t s and e l i m i n a t i o n o f w a t e r b l o c k i n g ( c f . s e c t i o n 3 . 1 1 . 2 . 4 . 2 . ) a l s o improves a c i d p e n e t r a t i o n and i n c r e a s e s c l e a n - u p (HARMS, SMITH, KING & POSEY 1 9 8 8 ) . ALMOND, BRADY & UNDERDOWN ( 1 9 8 8 ) d i s c u s s m o n i t o r i n g o f a c i d p e n e t r a t i o n d e p t h and s p e n d i n g r a t e by r e t u r n f l u i d a n a l y s i s .
4.5.1.3.2. Acid retardation A c i d e f f e c t i v i t y o p t i m i z a t i o n w i t h i n c r e a s i n g p e n e t r a t i o n d e p t h c a n a l s o be a c h i e v e d b y r e t a r d a t i o n o f r e a c t i o n r a t e and t h u s s p e n d i n g t i m e (DOWELL SCHLUMBERGER 1988 d ) . R e t a r d e d a c i d t r a v e l s f a r t h e r away f r o m b o r e h o l e and p e r f o r a t i o n s i n t o t h e f o r m a t i o n b e f o r e becoming s p e n t and i s t h u s a b l e t o p e r f o r m spec i a l t r e a t m e n t s such as c r e a t i o n o f a few l o n g l a r g e - d i a m e t e r c h a n n e l s , deep uniform m a t r i x invasion, deep p e n e t r a t i o n and e t c h i n g o f n a t u r a l h a i r 1 i n e c r a c k s , and g e n e r a t i o n and a t t a c k o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s . A c i d r e t a r d i n g a g e n t s d e p o s i t h y d r o p h o b i c f i l m s on c a r b o n a t e s u r f a c e s , o r a r e f o a m i n g a d d i t i v e s w h i c h s t a b i l i z e a c a r b o n d i o x i d e foam b a r r i e r on t h e r o c k s u r f a c e (CROWE, McGOWAN & BARANET 1 9 8 8 ) . D i s t i n c t i o n can be made between chemic a l r e t a r d e r s , e m u l s i o n s , foams, g e l l e d a c i d ( c f . s e c t i o n s 4 . 3 . 4 . 3 . 1 . 4 . and 4.5.1.2.3.) and o r g a n i c a c i d s . T e c h n i q u e s i n v o l v i n g i n j e c t i o n o f p a d f l u i d s t o c o o l t h e f r a c t u r e faces ( c f . s e c t i o n 4.3.4.6.1.4.) and t o d e p o s i t a p r o t e c t i v e
503 f i l t e r cake a r e a l s o employed. I n t h e absence o f f i l t e r cake development, v i s c o s i t y i m p a r t e d by a d d i t i o n o f g e l l i n g agents has l i t t l e e f f e c t on r e a c t i o n r a t e . E m u l s i f i c a t i o n o f a c i d reduces r e a c t i o n r a t e , w i t h t h e g r e a t e s t r e t a r d a t i o n p r o v i d e d by o i l - e x t e r n a l a c i d emulsions. The summary as f o l l o w s compiles some p o i n t s o f s l o w l y r e a c t i n g a c i d s and s o l i d a d d i t i v e s , a c i d e m u l s i f i c a t i o n , a c i d g e l l i n g and f i l t e r cakes, and a c i d c o n c e n t r a t i o n and f r i c t i o n l o s s .
4.5.1.3.2.1.
Slowly reacting acids and solid additives
R e t a r d a t i o n o f h y d r o c h l o r i c a c i d i s most e f f e c t i v e i n f r a c t u r e a c i d i z i n g and can be c a r r i e d o u t by adding a slower r e a c t i n g a c i d such as a c e t i c , f o r m i c o r propionic a c i d t o the hydrochloric acid. Other p o s s i b i l i t i e s o f r e t a r d a t i o n are u s i n g agents t o i n t r o d u c e a b a r r i e r on t h e s u r f a c e o f t h e r o c k p r e v e n t i n g n o r mal c o n t a c t w i t h t h e a c i d , a p p l y i n g a d d i t i v e s such as c a l c i u m c h l o r i d e t o gener a t e a common i o n e f f e c t , and i n c r e a s i n g a c i d c o n c e n t r a t i o n i n o r d e r t o p r o l o n g spending t i m e , W i t h r e s p e c t t o t h e s l o w l y r e a c t i n g a c e t i c , f o r m i c and p r o p i o n i c a c i d s (DILL & KEENEY 1978), deeper m a t r i x p e n e t r a t i o n i s o b t a i n e d by t h e f a s t e r r e a c t i n g h y d r o c h l o r i c a c i d , because c h a n n e l l i n g o r wormholing ( c f . s e c t i o n 4.5.1.6.3.) reduces t h e area/volume r a t i o t h e r e b y p r o l o n g i n g r e a c t i o n t i m e . O r g a n i c a c i d s a r e p a r t i c u l a r l y used t o t r e a t h i g h - t e m p e r a t u r e w e l l s (CROWE, McGOWAN & BARANET 1 9 8 8 ) . On t h e o t h e r hand, a c e t i c , f o r m i c and p r o p i o n i c a c i d s o b t a i n deeper p e n e t r a t i o n i n f r a c t u r e s t h a n h y d r o c h l o r i c acid, b u t l a r g e r volumes would be r e q u i r e d t o d i s s o l v e an e q u i v a l e n t amount o f r o c k and t h e h i g h e r c o s t may p r o h i b i t t h e i r e x t e n s i v e use. Formic and a c e t i c a c i d s a r e p a r t i c u l a r l y c h a r a c t e r i z e d by low c o r r o s i v e n e s s on m e t a l s and c o m p a t i b i l i t y w i t h o i l . I n c o r p o r a t i o n o f c a l c i u m c h l o r i d e i n t o h y d r o c h l o r i c a c i d r e t a r d s r e a c t i o n r a t e and p r o v i d e s h i g h e r v i s c o s i t y o f t h e s o l u t i o n once t h e a c i d i s spent which may a i d i n clean-up. Calcium c h l o r i d e a d d i t i o n f u r t h e r extends t h e i n h e r e n t l o n g e r spending t i m e o f t h e conc e n t r a t e d h y d r o c h l o r i c a c i d . H y d r o f l u o r i c a c i d r e t a r d a t i o n can a l s o be p e r f o r med b y s e q u e n t i a l HF l i b e r a t i o n (HDLDEN, PRIHODA & HALL 1981; c f . s e c t i o n 4.5.5.4.3.).
4.5.1.3.2.2.
Acid emulsification
E m u l s i f i c a t i o n i s one o f t h e e a r l i e s t methods used f o r a c i d r e t a r d a t i o n , w i t h b o t h o i l - e x t e r n a l and a c i d - e x t e r n a l emulsions h a v i n g been a p p l i e d (CROWE, McGOWAN & BARANET 1988). A c i d - i n - o i l emulsions a l s o have r e t a r d e d r e a c t i o n r a t e s , because t h e a c i d i n t h e emulsion i s t o some e x t e n t p r e v e n t e d f r o m completel y c o n t a c t i n g t h e r o c k s u r f a c e by presence o f an o i l f i l m p a r t i c u l a r l y i n emuls i o n s w i t h a t l e a s t 20 % o i l as t h e o u t e r phase. I n a d d i t i o n , t h e h i g h e r v i s c o s i t y o f t h e emulsion tends t o r e s t r i c t f l o w i n t o t h e s m a l l e r pores, t h u s m i n i m i z i n g c o n t a c t w i t h e x t r e m e l y h i g h area/volume r a t i o s . A c i d - i n - o i l emulsions p r o v i d e i n c r e a s e d c h a n n e l l i n g o r wormholing ( c f . s e c t i o n 4 . 5 . 1 . 6 . 3 . ) i n m a t r i x f l o w . S u r f a c t a n t s r e s u l t i n t h e presence o f o i l i n a hydrophobic o r w a t e r - r e p e l l e n t o i l - l i k e f i l m on t h e r o c k s u r f a c e which r e s t r i c t s a c i d - r o c k c o n t a c t and i s thus a l s o b e n e f i c i a l i n reducing a c i d r e a c t i o n r a t e leading t o extension o f spending t i m e and t r a v e l d i s t a n c e . Some o i l - e x t e r n a l a c i d emulsions a r e capable o f r e t a r d i n g r e a c t i o n r a t e by as much as 98 %, b u t on t h e o t h e r hand, such an extreme l e v e l o f a c i d r e t a r d a t i o n would l e a d t o inadequate e t c h i n g o f f r a c t u r e faces (BERGSTROM & MILLER 1975). F l u i d - l o s s m a t e r i a l s and g e l l i n g agents b e i n g a c i d t h i c k e n i n g a d d i t i v e s ( c f . s e c t i o n 4.3.4.3.4.1. and 4 . 5 . 1 . 2 . 3 . ) a l s o t e n d t o reduce t h e r e a c t i o n o f hydroc h l o r i c a c i d by f i l m development on r o c k s u r f a c e s and r e s t r i c t i o n o f d i f f u s i o n (CROWE, MARTIN & MICHAELIS 1981). DAVIS, MANCILLAS & MELNYK (1965) p r e s e n t a procedure u s i n g a d d i t i v e s i n a spearhead f l u i d ahead o f t h e a c i d j o b i n o r d e r t o c r e a t e new f r a c t u r e s . The s p e c i a l l y compounded a d d i t i v e s a r e o n l y s l o w l y d i s p e r s i b l e i n a c i d , and t h e i r presence on exposed f o r m a t i o n and f r a c t u r e s u r f a c e s
504 as a t h i n s e a l i n g f i l m i n h i b i t s and postpones a c i d r e a c t i o n u n t i l t h e e n t i r e t r e a t m e n t volume has e n t e r e d t h e r e s e r v o i r .
4.5.1.3.2.3.Acid gelling and filter cakes The use o f g e l l e d a c i d s has i n c r e a s e d due t o development o f t h i c k e n i n g agents h a v i n g g r e a t e r s t a b i l i t y i n h o t a c i d (CROWE, McGOWAN & BARANET 1988). A c i d g e l l i n g reduces r e a c t i o n r a t e by s l o w i n g c o n v e c t i o n and hydrogen i o n d i f f u s i o n , and f u r t h e r d i m i n u t i o n o f g e l l e d a c i d r e a c t i o n r a t e s can be a c h i e v e d by a d d i t i o n o f a foaming agent o r a c r o s s l i n k e r ( c f . s e c t i o n s 4.3.4.3.4.1. and 4.5.1.2.3.). A l t h o u g h i n c r e a s i n g a c i d v i s c o s i t y does n o t n e c e s s a r i l y d i m i n u i s h r e a c t i o n r a t e , g e l l i n g agents can r e t a r d a c i d r e a c t i o n by d e p o s i t i n g a f i l t e r cake on carbonate s u r f a c e s (GDANSKI & NORMAN 1983). On t h e o t h e r hand, n e i t h e r polymer n o r s u r f a c t a n t g e l l e d a c i d s e x h i b i t s i g n i f i c a n t r e t a r d a t i o n i n t h e absence o f c o n d i t i o n s which cause f i l t e r cake f o r m a t i o n , b u t i n many cases r e a c t i o n r a t e s a r e i n s t e a d a c c e l e r a t e d (CROWE, McGOWAN & BARANET 1988). Escape o f g e l l e d a c i d i n t o permeable carbonates can r e s u l t i n g e n e r a t i o n o f f i l t e r cakes p r o v i d e d t h a t l e a k o f f v e l o c i t y i s n o t s u f f i c i e n t t o cause wormhole development ( c f . s e c t i o n 4.5.1.6.3.): As t h e p r i m a r y s i t e o f a c i d f l u i d - l o s s i s f r o m t h e area around t h e wormhole t i p s r a t h e r t h a n i n t o t h e f r a c t u r e f a c e s , l i t t l e f i l t e r cake development o c c u r s on t h e c r a c k w a l l s and t h u s r e a c t i o n r a t e i s n o t r e t a r d e d i n case o f e x t e n s i v e wormholing. When u s i n g g e l l e d a c i d s , i t i s doubly i m p o r t a n t t o c o n t r o l a c i d f l u i d - l o s s i n order t o both h y d r a u l i c a l l y extend the fracture and a l l o w f i l t e r cake development ( c f . s e c t i o n s 4.3.4.3.1.4. and 4.5.1.2.3.).
4.5.1.3.2.4.
Acid concentration and friction loss
High a c i d c o n c e n t r a t i o n s have l o n g e r r e a c t i o n t i m e s than l o w e r s a t u r a t i o n s because t h e r e i s more a c i d t o r e a c t , t h e a d d i t i o n a l r e a c t i o n p r o d u c t s f u r t h e r r e t a r d r e a c t i o n r a t e s , and t h e e n l a r g e d f l o w p a t h w i t h reduced area/volume r a t i o p r o l o n g s spending t i m e and t r a v e l d i s t a n c e o f a h i g h - c o n c e n t r a t i o n a c i d . NIERODE & KRUK (1973) and MILLER & BERGSTROM (1975) p r e s e n t an a c i d e m u l s i o n system which overcomes t h e v i s c o s i t y and f r i c t i o n - l o s s problem a s s o c i a t e d w i t h many emulsion systems and p r o v i d e s b e t t e r l i v e a c i d p e n e t r a t i o n . A c i d f r a c t u r i n g s t i m u l a t i o n can be i n c r e a s e d by u s i n g e f f e c t i v e f l u i d - l o s s a d d i t i v e s i n hyd r o c h l o r i c a c i d or s i g n i f i c a n t l y i n c r e a s e a c i d v i s c o s i t y . W h i l e a c i d emulsions have a low f l u i d - l o s s r a t e and a r e r e t a r d e d , o i l - w e t t i n g s u r f a c t a n t s u s u a l l y g i ve no r e t a r d a t i o n a t t y p i c a l f i e l d i n s p e c t i o n r a t e s . Retarded h y d r o f l u o r i c a c i d r e a c t s slower w i t h c l a y m i n e r a l s and f e l d s p a r s and t h u s a l l o w s deeper p e n e t r a t i o n o f l i v e a c i d (GDANSKI & PEAVY 1986; c f . s e c t i o n 4 . 5 . 5 . 4 . ) . The r e a c t i o n r a t e o f a c i d on s o l i d carbonate s u r f a c e s i s a l s o a f f e c t e d by c o n f i n i n g p r e s s u r e (CROWE, McGOWAN & BARANET 1988), w i t h d i s s o l u t i o n r a t e s a t l o w e r p r e s s u r e s b e i n g much h i g h e r due t o s u r f a c e a g i t a t i o n by carbon d i o x i d e bubbles, b u t i n c r e a s i n g p r e s s u r e s r e d u c i n g r e a c t i o n r a t e . The impact o f v a r i o u s r e t a r d i n g agents on temperature s e n s i t i v i t y o f a c i d r e a c t i o n r a t e s i s v e r y d i f f e r e n t . Aspects o f a c i d r e t a r d a t i o n a r e a l s o d i s c u s s e d by KNOX, LASATER & DILL (1964), NIERODE & KRUK (1973), GDANSKI (1985) and CROWE, McGOWEN & BARANET (1988).
4.5.1.4. Acid foaming A c i d foaming a l s o has s i g n i f i c a n t impact on a c i d p e n e t r a t i o n d i s t a n c e and r a t e (FORD & ROBERTS 1982). I n c r e a s i n g foam q u a l i t y r e s u l t s i n d e c r e a s i n g a c i d pen e t r a t i o n d i s t a n c e , and t h e h i g h e r t h e foam q u a l i t y , t h e l o w e r t h e a c i d c o n t e n t o f t h e foam, w i t h t h e l e s s a c i d p r e s e n t i n t h e foam, t h e l o w e r t h e r o c k - d i s s o l v i n g power o f t h e foam. The w i d e r t h e f r a c t u r e , t h e l o n g e r i t t a k e s f o r hydro-
505 gen i o n s t o r e a c h t h e r o c k s u r f a c e and t h u s t h e a c i d t r a v e l s f a r t h e r down t h e c r a c k b e f o r e spending, w i t h t h i s r e l a t i o n s h i p a p p l y i n g f o r foamed a c i d s as we17 as f o r non-foamed a c i d s . I f pumping r a t e i s i n c r e a s e d and f r a c t u r e h e i g h t r e mains c o n s t a n t , t h e d i s t a n c e which foamed a c i d t r a v e l s down a f r a c t u r e b e f o r e spending i s i n c r e a s i n g . A c i d spending i n a c r a c k i s governed p r i m a r i l y by t h e mass t r a n s f e r o f t h e a c i d t o t h e f r a c t u r e w a l l . Channel e t c h i n g i s due t o d e n s i t y e f f e c t s and may be reduced by u s i n g a foam s t a b i l i z e r which i n c r e a s e s g e l l e d f l u i d viscosity. Foamed a c i d i s non-damaging and p r o v i d e s u n e x c e l l e d f l u i d - l e a k o f f c o n t r o l esp e c i a l l y when used i n a l t e r n a t i n g stages o f a c i d and i n e r t pad f l u i d s (SCHERUBEL & CROWE 1978; c f . s e c t i o n 4.5.1.6.2.), and i s p a r t i c u l a r l y b e n e f i c i a l i n low-pressure, l o w - p e r m e a b i l i t y , l i q u i d - s e n s i t i v e f o r m a t i o n s (FORD 1980; FORD, BURKLECA & SQUIRE 1980) as w e l l as i n h i g h l y s o l u b l e r e s e r v o i r s t h a t r e l e a s e l a r g e amounts o f f i n e s . As a c i d e n l a r g e s t h e f l o w p a t h s t h e r e b y c o n s t a n t l y i n c r e a s i n g l e a k o f f r a t e , t h e b e s t f l u i d l o s s c o n t r o l by s t a b l e a c i d foam i s a c h i e ved by pumping g e l l e d water-based pads ahead o f t h e a c i d foam i n o r d e r t o r e duce escape o f gas (SCHERUBEL & CROWE 1978). The use o f foamed a c i d i n f r a c t u r e a c i d i z i n g t r e a t m e n t s has g a i n e d widespread acceptance i n t h e l a s t y e a r s . Asp e c t s o f foamed a c i d s a r e a l s o d i s c u s s e d by HOLCOMB (1977), HOLCOMB & WILSON (1978), WILSON (1978), PETRYK & GARUK (1979), FORD (1981) and HARMS, SMITH, KING & POSEY (1988). Aspects o f o p t i m i z a t i o n and improvement o f a c i d r e a c t i o n i n m a t r i x and f r a c t u r e a c i d i z i n g a r e a l s o d i s c u s s e d by SUTTON & LASATER (1972); DOMSELAAR, SCHOLS & V I S S E R (1973); McCUNE, AULT & DUNLAP (1975) and COULTER, CROWE, BARRETT & MILLER (1976).
4.5.1.5. Open vs. propped fractures Proppant p l u g g i n g o f t h e a c i d i z e d f r a c t u r e i s c o n s i d e r e d t o be t h e most e f f e c t i v e way o f i m p r o v i n g c o n d u c t i v i t y i n a c i d f r a c t u r i n g t r e a t m e n t s o f deeper r e s e r v o i r s where c l o s u r e s t r e s s i s t o o h i g h t o a l l o w a c i d i z e d f r a c t u r e s t o s t a y open w i t h o n l y n e g l i g i b l e damage and t o p r o v i d e s u f f i c i e n t c o n d u c t i v i t y f o r l o n g - t e r m hydrocarbon p r o d u c t i o n . Some aspects o f drawbacks o f a u t o - p r o p p i n g o f a c i d i z e d f r a c t u r e w a l l s and s i g n i f i c a n c e o f p r o p p a n t i n f i l l i n g o f t h e f r a c t u r e a r e b r i e f l y d i s c u s s e d as f o l l o w s .
4.5.1.5.1. Drawbacks o f auto-propping of acidized fracture walls S e l f - p r o p p i n g by c r u s h i n g o f exposed p o r t i o n s o f t h e i r r e g u l a r f r a c t u r e w a l l c r e a t e d by a c i d l e a c h i n g o r b r i d g i n g o f broken fragments o f t h e r e s e r v o i r r o c k m a t r i x w i t h i n t h e c r a c k ( c f . a l s o s e c t i o n 4.5.1.1.1.2.) can be e f f e c t i v e , b u t might also r e s u l t i n serious f r a c t u r e c o n d u c t i v i t y d e t e r i o r a t i o n i f l a r g e amounts o f f i n e s a r e c r e a t e d upon squeezing and c r u s h i n g o f s o f t d u c t i l e and h a r d b r i t t l e carbonate f o r m a t i o n m a t r i x , r e s p e c t i v e l y . W h i l e smooth f r a c t u r e w a l l s t e n d t o e a s i e r p r i m a r y c l o s u r e once s u f f i c i e n t s t r e s s i s a c t i n g , rough c r a c k f a c e s e x h i b i t more r e s i s t a n c e t o c l o s u r e s t r e s s , and once exceeding t h e s t a b i l i t y boundary, i t i s p o s s i b l e t h a t b r e a k i n g of t h e p r o t r u s i o n s i n t o l a r g e r pieces leads t o auto-propping o f the crack a t l e a s t i n metastable nature, b u t i t i s much more p o s s i b l e t h a t c r u s h i n g o f these fragments i n t o f i n e s p l i n t e r s w i l l l a t e r r e s u l t i n secondary c l o s u r e o f t h e f r a c t u r e . The p o s s i b i l i t y o f c r u s h i n g o f p r o t r u s i o n s and subsequent a t l e a s t p a r t i a l well-banked f r a c t u r e closure i s e s p e c i a l l y given i n hard b r i t t l e fine-grained, and - j o i n t e d sedimentary and b i o c l a s t i c carbonates, whereas massive b i o l i t h i c carbonates c o n s t r u c t e d b y r e e f - b u i l d i n g organisms ( c f . p l a t e f/8) p r o v i d e more i r r e g u l a r i t i e s b o t h i n terms o f p r i m a r y c r a c k opening and secondary s e l e c t i v e f r a c t u r e e t c h i n g . P a r t i a l l y c l o s e d c r a c k s resemble p i l l a r p r o p p i n g ( c f . s e c t i o n 4 . 3 . 3 . 2 . ) , b u t i n c o n t r a s t t o i n f i l l i n g w i t h round s t a b l e s y n t h e t i c high-conduc-
506 t i v i t y p r o p p a n t s , s u p p o r t of t h e f r a c t u r e by i r r e g u l a r n a t u r a l c a r b o n a t e f r a g t h a t a r e s u b j e c t t o c o n s i d e r a b l e c r u s h i n g due t o t h e i r u n s u i t a b i l i t y t o w i t h s t a n d the c l o s u r e s t r e s s represents patchy flow o b s t r u c t i o n r a t h e r than amel i o r a t i o n , w i t h f r a c t u r e auto-propping by b r e a k i n g and c o l l a p s i n g c a r b o n a t e p a r t i c l e s t h e r e f o r e being e s p e c i a l l y i n g r e a t e r r e s e r v o i r d e p t h h i g h l y u n d e s i r a b l e . G e n e r a l l y , e t c h e d flow c h a n n e l s c r e a t e d by a c i d f r a c t u r i n g can have s i g n i f i c a n t l y lower c o n d u c t i v i t y than propped c r a c k s because of t h e much h i g h e r l i k e l i h o o d of the open unsupported f r a c t u r e t o c l o s e due t o f o r m a t i o n c o l l a p s e (ECONOMIDES 1987 b ) .
ments
4.5.1.5.2. Significance o f proppant i n f i l l i n g o f t h e fracture F r a c t u r e wall roughness t h e r e f o r e i s an i m p o r t a n t parameter f o r p r e s e r v a t i o n o r d e t e r i o r a t i o n of c r a c k c o n d u c t i v i t y c r e a t e d by a c i d t r e a t m e n t s o n l y i n s h a l low t o moderately-deep r e s e r v o i r s . Proppant plugging of the f r a c t u r e s h o u l d a t l e a s t be c o n s i d e r e d f o r t h e immediate w e l l b o r e s u r r o u n d i n g s , a s t h i s t a i l of the f r a c t u r e wedge i s most s u s c e p t i b l e t o p r e s s u r e drawdown ( c f . s e c t i o n s 2 . 4 . 1 . 2 . 3 . and 4 . 1 2 . 3 . 3 . ) . S e c u r i n g the r e s u l t of an a c i d f r a c t u r i n g j o b by app l y i n g a proppant of known c o n d u c t i v i t y f o r i n f i l l i n g of t h e c r a c k i s e s p e c i a l l y b e n e f i c i a l a s a consequence of t h e f a c t t h a t t h e c o n d u c t i v i t y d e l i v e r e d by an a c i d t r e a t m e n t c a n n o t be p r e d i c t e d w i t h c e r t a i n t y (WILLIAMS & NIERODE 1971) i n c o n t r a s t t o t h a t provided by a proppant package which can be more o r l e s s f o r e c a s t e d ( c f . s e c t i o n 1 . 4 . 1 0 . ) . C o n d u c t i v i t y o b t a i n e d from a c i d f r a c t u r i n g oper a t i o n s i s a f u n c t i o n of f o r m a t i o n t y p e , a c i d c o n c e n t r a t i o n , c o n t a c t time b e t ween a c i d and r o c k , and c l o s u r e stress and t h u s r e s e r v o i r d e p t h , whereas propp a n t c o n d u c t i v i t y i s ( w i t h i n the range of c l o s u r e s t r e s s r e s i s t i v i t y o f the i n d i v i d u a l proppant t y p e s ) independent from a l l t h e s e f a c t o r s and o n l y i s r e l a t e d t o t h e c o n f i g u r a t i o n of t h e proppant package. In h i g h - p e r m e a b i l i t y r e s e r v o i r s w i t h low f r a c t u r e c o n d u c t i v i t y , g a s breakthrough o c c u r s e a r l y i n time and n e a r t h e w e l l b o r e which r e d u c e s f r a c t u r e e f f e c t i v i t y f o r g a s p r o d u c t i o n (SOLIMAN & H U N T 1 9 8 6 ) . T h i s drawback can be overcome by d e s i g n i n g a h i g h e r c r a c k c o n d u c t i v i t y by implementing p r o p p a n t s of l a r g e r g r a i n s i z e and b e t t e r q u a l i t y ( c f . s e c t i o n s 4 . 3 . 5 . and 4 . 6 . 1 . ) . AL-TAMIMI & ELMZEIN (1987) comment on s t i m u l a t i o n t e c h n i q u e s of t i g h t m u l t i l a y e r e d carbonat e s . Another combination method r e l a t e d t o a c i d and proppant f r a c t u r i n g i s chemical f r a c t u r e - s q u e e z e t r e a t m e n t (TINSLEY, LASATER & KNOX 1 9 6 7 ) .
4.5.1.6.
Acid f l u i d - l o s s c o n t r o l
Acid f l u i d l o s s i s e x t r e m e l y d i f f i c u l t t o c o n t r o l and i s g e n e r a l l y c o n s i d e red t o be the major f a c t o r l i m i t i n g the e f f e c t i v e n e s s of a c i d f r a c t u r i n g t r e a t ments (CROWE, HUTCHINSON & TRITTIPO 1 9 8 7 ) . Chemical e r o s i o n of c r a c k f a c e s and development of wormholes a r e l a r g e l y r e s p o n s i b l e f o r t h e reduced e f f i c i e n c y of a c i d f r a c t u r i n g . The c r e a t i o n of a c i d wormholes i n c r e a s e s t h e e f f e c t i v e a r e a from which l e a k o f f o c c u r s thus r e d u c i n g the h y d r a u l i c e f f i c i e n c y of t h e a c i d . Once wormholes form, most a c i d f l u i d l o s s o r i g i n a t e s from these wormholes r a t h e r than p e n e t r a t i n g uniformly i n t o t h e f r a c t u r e f a c e ( c f . s e c t i o n s 4 . 5 . 1 . 2 . 1 . and 4 . 5 . 1 . 6 . 3 . ) . A f t e r an i n t r o d u c t o r y o u t l i n e of a c i d r e a c t i o n s d u r i n g f r a c t u r i n g , some comments a r e o f f e r e d on a l t e r n a t i n g a c i d and g e l l e d w a t e r s t a g e s , wormhole growth and a c i d f l u i d l o s s , and p o s s i b i l i t i e s f o r improving a c i d fluid-loss control.
4.5.1.6.1. Acid react ions during f r a c t u r ins Acid f r a c t u r i n g comprises a c i d i n j e c t i o n i n t o a c a r b o n a t e f o r m a t i o n a t press u r e s s u f f i c i e n t t o f r a c t u r e the r e s e r v o i r o r t o open e x i s t i n g n a t u r a l j o i n t s (CROWE, HUTCHINSON & TRITTIPO 1 9 8 7 ) . As a c i d flows along the f r a c t u r e , p o r t i o n s of the c r a c k f a c e s a r e d i s s o l v e d and eroded i n a non-uniform manner, and thus
507 c o n d u c t i v e channels a r e c r e a t e d which u s u a l l y remain f o l l o w i n g f r a c t u r e c l o sure. The e f f e c t i v e l e n g t h o f t h e c r a c k i s determined by t h e volume o f a c i d used, i t s r e a c t i o n r a t e , and t h e a c i d f l u i d - l o s s r a t e , and p r o d u c t i v i t y improvement i s l a r g e l y a f u n c t i o n o f t h e l e n g t h o f t h e e t c h e d f r a c t u r e . The maximum e f f e c t i v i t y d i s t a n c e o b t a i n a b l e by a c i d f r a c t u r i n g i s l i m i t e d by e i t h e r spending o r f l u i d l e a k o f f . I f t h e a c i d spends t o o r a p i d l y , t h e e t c h e d p o r t i o n of t h e c r a c k w i l l be r a t h e r s h o r t . I f a c i d f l u i d - l o s s c h a r a c t e r i s t i c s a r e poor, e x c e s s i v e l e a k o f f w i l l t e r m i n a t e f r a c t u r e growth. I n many a c i d f r a c t u r i n g t r e a t m e n t s , o p e r a t i n g p r e s s u r e c o n t i n u a l l y d e c l i n e s and e v e n t u a l l y f a l l s below a l e v e l r e q u i r e d t o propagate t h e c r a c k . I n some cases, t r e a t i n g p r e s s u r e never reaches a l e v e l s u f f i c i e n t t o i n i t i a t e a h y d r a u l i c f r a c t u r e , i n d i c a t i n g t h a t t h e a c i d i s expended i n a m a t r i x s t i m u l a t i o n o f t h e n e a r - w e l l b o r e r e g i o n . C o n t r o l o f f l u i d l o s s d u r i n g a c i d f r a c t u r i n g i n carbonate f o r m a t i o n s p r e s e n t s problems which a r e unique t o r e a c t i v e f l u i d s , because most f l u i d - l e a k o f f a d d i t i v e s and g e l l i n g agents used i n n o n r e a c t i v e aqueous f r a c t u r i n g f l u i d s a r e c h e m i c a l l y u n s t a b l e i n t h e a g g r e s s i v e a c i d . I n a d d i t i o n , a c i d f l o w a l o n g carbon a t e c r a c k s u r f a c e s produces c o n s t a n t chemical e r o s i o n , t h u s making i t d i f f i c u l t f o r w a l l - b u i l d i n g f l u i d s t o d e p o s i t an e f f e c t i v e f i l t e r cake. A c i d tends a l s o t o s e l e c t i v e l y e n l a r g e p o r e s and n a t u r a l f r a c t u r e s r e s u l t i n g i n wormhole c r e a t i o n and channel g e n e r a t i o n p e r p e n d i c u l a r l y t o t h e c r a c k face, w i t h these phenomena g r e a t l y i n c r e a s i n g t h e e f f e c t i v e s u r f a c e area f r o m which l e a k o f f occ u r s and g e n e r a l l y b e i n g b e l i e v e d t o a d v e r s e l y a f f e c t f l u i d l o s s .
4.5.1.6.2. Alternating acid and gelled water stages While a c i d - and t e m p e r a t u r e - r e s i s t a n t a d d i t i v e s a r e e i t h e r o f l i m i t e d chemic a l and/or p h y s i c a l a p p l i c a b i l i t y o r a r e i n many cases t o o expensive, a c i d l e a k o f f c o n t r o l can e a s i l y be performed by i n j e c t i o n o f a v i s c o u s n o n r e a c t i v e pad p r e c e d i n g t h e a c i d (DAVIS, MANCILLAS & MELNYK 1965; COULTER, CROWE, BARRETT & MILLER 1976; CROWE, HUTCHINSON & TRITTIPO 1988). T h i s t e c h n i q u e u s u a l l y employs a water-based f r a c t u r i n g f l u i d t o c o o l t h e f r a c t u r e ( c f . s e c t i o n 4.3.4.6.1.4.) and l a y down an impermeable f i l t e r cake on t h e crack f a c e . The f i l t e r cakes cov e r i n g f r a c t u r e w a l l s , however, a r e r e l a t i v e l y i n e f f e c t i v e f o r a c i d f l u i d - l o s s control, because t h e y a r e q u i c k l y p e n e t r a t e d by wormholes ( c f . section 4.5.1.6.3.) and t h e n t h e i r e f f e c t i s gone. While t h e use o f t h e pad p r o b a b l y p r o v i d e s o n l y l i m i t e d a c i d f l u i d - l e a k o f f s u p e r v i s i o n , i t does have o t h e r u s e f u l f u n c t i o n s i n c l u d i n g c o o l i n g o f t h e t u b u l a r goods through which a c i d f l o w s t h e r e by r e d u c i n g c o r r o s i o n , c o o l i n g t h e f r a c t u r e and i n c r e a s i n g c r a c k w i d t h r e s u l t i n g i n reduced a c i d r e a c t i o n r a t e and i n c r e a s e d l i v e a c i d p e n e t r a t i o n , and p r o m o t i n g a c i d f i n g e r i n g through i t s v i s c o s i t y t h u s r e d u c i n g t h e amount o f r e a c t i v e s u r f a c e area t o which t h e a c i d i s exposed and i m p r o v i n g f r a c t u r e l e n g t h and c o n d u c t i v i t y ( c f . a l s o s e c t i o n s 4.5.1.1.1.4. and 4.5.4.2.1.). Enhanced v a r i a t i o n s o f pad pumping a r e i n i t i a l f r a c t u r e c r e a t i o n by a g e l l e d pad a f t e r w h i c h a l t e r n a t i n g stages o f a c i d and a d d i t i o n a l pad a r e i n j e c t e d t h a t a r e designed t o e n t e r and seal t h e wormholes c r e a t e d by t h e p r e c e d i n g a c i d . By a l t e r n a t i n g a c i d and g e l stages, a c i d l e a k o f f i n t o wormholes and e n l a r g e d n a t u r a l f r a c t u r e s i s c o n t r o l l e d and t r e a t m e n t e f f i c i e n c y improved. F i n e p a r t i c u l a t e m a t e r i a l s such as 100 mesh sand a r e o f t e n added t o t h e pad stages t o a i d i n f l u i d - l o s s supervision ( c f . section 4.5.1.1.1.4.). A c i d f l u i d l e a k o f f can a l s o be reduced by a c i d g e l l i n g ( c f . s e c t i o n s 4.3.4.3.4.1. and 4 . 5 . 1 . 2 . 3 . ) .
4.5.1.6.3. Wormhole growth and acid fluid loss Wormhole growth i s generated by most o f t h e a c i d e n t e r i n g a few e n l a r g e d por e s . C r e a t i o n and b r a n c h i n g o f wormholes and enlargement o f n a t u r a l f r a c t u r e s has a m a j o r e f f e c t on a c i d f l u i d loss s i n c e i t i n c r e a s e s t h e e f f e c t i v e amount o f s u r f a c e area f r o m which a c i d f l u i d l e a k o f f occurs. Some aspects o f wormhole
508 p e n e t r a t i o n and acid channelling as well as f l u i d invasion pa tte rn a n d f r a c t u r e conductivity development ar e o u t l i n ed as follows.
4.5.1.6.3.1.Wormhole penetration and acid channelling Factors governing wormhole propagation ar e analogous t o those a f f e c t i n g a c id spending in a hydraulic f r a c t u r e with wormholes continuing t o grow i n length providing t h a t l i v e acid i s delivered t o the wormhole t i p . Increased temperatur e diminishes wormhole p en et r at i o n d i s t an ce, while wormhole depth i s lengthened by increased acid concentration a n d i n j ect i o n r a t e . Once wormholes reach the maximum spending-limited depth, i n j e c t i o n of additional acid mainly a c t s to e n l a r ge the diameter of e x i s t i n g wormholes. Wormholes grow ra pidly in length during e a r l y st a g e s of acid i n j e c t i o n due t o s e l e c t i v e acid channelling. I n many cas e s , no s i n g l e wormholes b u t r a t h e r a branched wormhole network i s developed. Thus leakoff of only a small q u an t i t y of acid g r e a t l y inc re a se s the amount of new s u r f a c e area from which f l u i d l o s s can occur, with t h i s being a major reason why acid f l u i d leakoff i s more d i f f i c u l t t o control t h a n escape of non-react i v e f l u i d s . Factors o t h er than f l u i d - l o s s v e loc ity influencing wormhole depth a r e temperature a n d acid concentration (DACCORD, TOUBOUL & LENORMAND 1987; HUNG, H I L L & SEPEHRNOORI 1987).
4.5.1.6.3.2.Fluid invasion pattern
and fracture conductivity development I n addition t o increasing acid f l u i d l o s s , wormholes a l s o a l t e r the pa tte rn of f l u i d invasion i n t o the f r a c t u r e f ace. Once developed, acid l u i d leakoff o r i g i n a t e s predominantly from e x i s t i n g wormholes with l i t t l e f l u i d oss a c t u a l ly occurring a t the f r a c t u r e f a c e . F i l t e r cakes deposited by non r e a c t i v e pad f l u i d s have i n many cases l i t t l e influence on subsequent acid f l u i d l o s s , because once wormholes p en et r at e the f i l t e r cake, they a re re sponsible f o r almost a l l acid f l u i d leakoff r eg ar d l es s of the presence of the remaining f i l t e r cake ( c f . s e c t i o n 4 . 5 . 1 . 2 . 1 . ) . On the o t h er hand, acid re a c tion r a t e i s re ta rde d by addition of f i l t e r a b l e s o l i d s t o the aci d , with the a dditive s depositing f i l t e r cakes which a c t as b a r r i e r s between acid a n d f r a c t u r e surfa c e . While t h i s method of acid r e t a r d a t i o n i s e f f e c t i v e under conditions of l i t t l e o r no wormhole development, the ex i s t en ce of deeply p en et r ating wormholes e lim ina te s most leakoff a t the f r a c t u r e face and prevents f i l t e r cakes from being formed (DACCORD, TOUBOUL & LENORMAND 1987; HUNG, H I L L & SEPEHRNOORI 1987). The e x i st e n c e of wormholes a l s o influences the development of f r a c t u r e cond u c t i v i t y . Since acid f r a c t u r i n g depends upon etching of f r a c t u r e fa c e s t o c re a t e flow channels, acid l o s t t o wormholes r e s u l t s i n l e s s d i s s o l u t i o n of rock from the crack face a n d thus lower co n d u ct i v ity. I t i s more be ne fic ia l t o expend a c i d i n e t c hi n g f r a c t u r e walls rather than in c r e a t i o n of wormholes perpend i c u l a r t o the hydraulic f r a c t u r e . Localized changes i n crack face permeability have a f a r smaller e f f e c t on p r o d u ct i v i t y t h a n does f r a c t u r e length o r conductivity.
4.5.1.6.4. Possibilities for improving acid fluid-loss control Both f l u i d - l o s s and wormhole development can be e f f e c t i v e l y c ontrolle d by i n j e c t i o n of m u l t i p l e s t ag es of acid and g elle d water i n an a l t e r n a t i n g manner ( c f . s e c t i o n s 4 . 5 . 1 . 6 . 2 . a n d 4 . 5 . 4 . 2 . ) . Gelled acids have found increased usage i n acid f r a c t u r i n g s i n ce development of more s t a b l e acid g e l l i n g agents which, however, reduce b o t h f l u i d leakoff and aci d re a c tion r a t e due to acid thickening by forming mi cel l ar networks within the acid s o l u t i o n . Viscosity loss a t e le v a t e d temperature i s n o t caused by g e l l i n g agent degradation, b u t occurs as the r e s u l t of d i s r u p t i o n of the mi cel l ar as soc ia tion network, a n d re a c tion products formed d u r i n g aci d spending a l s o d i s r u p t the m ic e lla r assemblage, thereby
509 c a u s i n g t h e spent the treatment.
a c i d t o r e v e r t t o a t h i n f l u i d which a i d s i n r e c o v e r y a f t e r
The wormhole problem i n a c i d f l u i d - l o s s c o n t r o l can be s o l v e d w i t h a g e l l e d a c i d h a v i n g u n i q u e r h e o l o g i c a l p r o p e r t i e s . T h i s g e l l e d a c i d system has a moderat e i n i t i a l v i s c o s i t y , b u t spending o f t h e a c i d d u r i n g escape produces a r i s e i n pH which i n i t i a t e s a process c r e a t i n g a v e r y l a r g e temporary i n c r e a s e i n e f f e c t i v e l e a k o f f v i s c o s i t y . F i n a l spending o f t h e a c i d t r i g g e r s a n o t h e r chemical r e a c t i o n which r e v e r s e s t h e temporary v i s c o s i t y i n c r e a s e , t h e r e b y a l l o w i n g t h e spent a c i d t o be e a s i l y r e c o v e r e d f r o m t h e f o r m a t i o n .
4.5.2. Combination of natural and artificial fractures Another means o f enhancement o f t h e e f f e c t i v i t y o f carbonate s t i m u l a t i o n i s t h e c o m b i n a t i o n o f n a t u r a l j o i n t i n g and a r t i f i c i a l h y d r a u l i c f r a c t u r i n g ( c f . a l s o s e c t i o n 4 . 8 . 8 . ) . As carbonates and sandy carbonates a r e i n many cases i n t e r s e c t e d by b e t t e r p e n e t r a t i n g and a l s o c l o s e r spaced n a t u r a l c r a c k s than sandstones ( c f . s e c t i o n 4.5.1.1.1.3.), t h e y a l s o s u f f e r i n a l a r g e r degree f r o m t h e f a c t t h a t n a t u r a l f r a c t u r e s b e i n g c r o s s e d by a h y d r a u l i c a l l y - i n d u c e d propagat i n g f r a c t u r e may d e f l e c t , a t t e n u a t e o r even t o t a l l y s t o p t h e g r o w t h o f t h e l a t ter (CONWAY, McGOWEN, GUNDERSON & KING 1985; c f . s e c t i o n s 4.2.2.3.1., 4.2.3.2.2. and 4.2.4.3.1.). As g e l l e d h y d r o c h l o r i c a c i d c o u l d h e l p p r e v e n t i n g f r a c t u r e d e f l e c t i o n and a r r e s t m e n t i f t h e f o r m a t i o n has s i g n i f i c a n t h y d r o c h l o r i c a c i d s o l u b i l i t y (GDANSKI & NORMAN 1983), t h e c o m b i n a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g i n carbonate r o c k s i s a l s o a v e r y e f f e c t i v e means o f s t i m u l a t i o n o f n a t u r a l l y - j o i n t e d f o r m a t i o n s , w i t h emphasis b e i n g i d e a l l y p u t on e n l a r gement o f e x i s t i n g c r a c k s p a r a l l e l t o t h e i r o r i e n t a t i o n r a t h e r than c r e a t i o n o f a new independent system o f f r a c t u r e s p e r p e n d i c u l a r t o t h e p r e - e x i s t i n g one ( c f . also section 4.8.8.).
4.5.3. Possibilities of application in Europe Various carbonate r e s e r v o i r s i n d i f f e r e n t geological formations are targets o f u n c o n v e n t i o n a l gas and o i l e x p l o r a t i o n and p r o d u c t i o n i n Europe as w e l l as o t h e r areas o f t h e w o r l d i n t h e near f u t u r e . While m a j o r f o r m a t i o n s such as t h e Z e c h s t e i n carbonate s o u r gas b e l t ( r e a c h i n g f r o m G r e a t B r i t a i n t o Poland and c o n t a i n i n g t h e l a r g e s t and most i m p o r t a n t r e s e r v o i r s i n Germany FRG) a r e o n l y i n e x c e p t i o n a l cases needing t r e a t m e n t s beyond m a t r i x a c i d i z i n g o r a c i d f r a c t u r i n g ( i f r e q u i r i n g s t i m u l a t i o n a t a l l ) , p a r t i c u l a r l y J u r a s s i c , Cretaceous and T e r t i a r y carbonate r o c k s i n t h e N o r t h e r n N o r t h Sea, onshore Germany FRG, Sout h e r n France, Yugoslavia, Hungary, I t a l y , A u s t r i a and t h e M i d d l e E a s t a r e p r o l i f e r o u s g o a l s f o r o p t i m i z a t i o n o f e x p l o i t a t i o n by h y d r a u l i c p r o p p a n t f r a c t u r i n g . The most e f f e c t i v e h e l p f o r l a y i n g d e c i s i v e groundwork i n t h i s approach, however, would be a r i s i n g o i l p r i c e and i n c r e a s i n g US $ v a l u e which w i l l consider a b l y i n c r e a s e t h e a t t r a c t i v i t y o f h y d r a u l i c proppant f r a c t u r i n g o f carbonate r o c k s i n Europe and o t h e r p a r t s o f t h e w o r l d , because i n t h e p r e s e n t unfavour a b l e g e n e r a l economical framework, h y d r a u l i c proppant f r a c t u r i n g o f c a r b o n a t e f o r m a t i o n s would be everywhere on t h e v e r y b o t t o m o f t h e r a n k i n g l i s t of p l a n ned i n v e s t m e n t s ( c f . s e c t i o n 2.2.1.5.). The most p r o m i s i n g c a r b o n a t e r e s e r v o i r t y p e f o r h y d r a u l i c p r o p p a n t f r a c t u r i n g (sometimes i n c o m b i n a t i o n w i t h g r a v e l p a c k i n g ) i s t h e c h a l k which i s c h a r a c t e r i z e d i n more d e t a i l i n t h e f o l l o w i n g section.
4.5.4. Chalk stimulation A s p e c i a l case o f c a r b o n a t e r e s e r v o i r s i s t h e Upper Cretaceous and p a r t i a l l y a l s o Lower T e r t i a r y c h a l k i n t h e N o r t h Sea and p a r t s o f onshore M i d d l e Europe as w e l l as i n t h e Rocky Mountain area i n Colorado, Nebraska and Kansas/USA.
510 T h i s h i g h - p o r o s i t y , b u t l o w - p e r m e a b i l i t y and l o w - s t a b i l i t y f o r m a t i o n c o n t a i n s c o n s i d e r a b l e o i l and o c c a s i o n a l l y a l s o gas r e s e r v e s . W h i l e t h e c h a l k i n some deeper f i e l d s i s more c o n s o l i d a t e d and s u f f i c i e n t l y n a t u r a l l y f r a c t u r e d t o have o n l y t o be t r e a t e d b y a c i d i z i n g , t h e c h a l k i n o t h e r s h a l l o w e r f i e l d s i s s o f t and f l o w i n g and i n t h e s e cases i n e v i t a b l y has t o be approached b y s o p h i s t i c a t e d b o t h h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g f o r t h e a c h i e v e m e n t o f economically f e a s i b l e production rates (various stimulation-related aspects o f c h a l k r e s e r v o i r s a r e d i s c u s s e d b y MANCILLAS, MATSON & Z I A R A 1976; Z I A R A , MANCILLAS & MATSON 1977; VLIS, DUNS & FERNANDEZ-LUQUE 1979; BARK & THOMAS 1980, NEWMAN 1981, BETZ 1982; SIMON, COULTER, K I N G & HOLMAN 1982; WATTS 1983, BREWSTER & DANGERFIELD 1984, PEETERS & HARTLEY 1984, GISTAU 1985, HARTLEY & BOSMA 1985; BREWSTER, DANGERFIELD & FARRELL 1986; JOHNSON & RHETT 1986; TANSOE, K I N G & HOLMAN 1986; SMITH, MILLER & HAGA 1987; FARRELL 1988, HOWES 1988, SNOW & HOUGH 1988). U n s t a b l e c h a l k i s p r o b a b l y t h e w o r s t r e s e r v o i r t y p e t o handle i n terms o f c o m p l e t i o n and s t i m u l a t i o n i n N o r t h Sea and o t h e r a r e a s . The m a i n o i l - p r o d u c i n g c h a l k f i e l d s i n t h e Norwegian and D a n i s h N o r t h Sea a r e E k o f i s k (BARK & THOMAS 1980, 1981; OFFSHORE ENGINEER 1985 b, PEKOT & GERSIB 1987, SNOW & HOUGH 1988), E l d f isk ( M I C H A U D 1 9 8 7 ) , B a l m o r a l , Hod ( NORBURY 1987), T o r (D 'HEUR 1 9 8 7 ) , Dan and V a l h a l l (TANSOE, K I N G & HOLMAN 1986; LEONARD & MUNNS 1987; ATTARD, MATHES & MOWER 1988; BISHLAWI & PENG 1988; RUDDY, ANDERSEN, PATTILLD & FOGED 1 9 8 8 ) , wher e a s o t h e r f i e l d s such as T r o l l and Tommeliten (D'HEUR & PEKOT 1987) s t i l l have t o be d e v e l o p e d . A s p e c t s o f p e t r o p h y s i c a l p r o p e r t i e s and p r o d u c t i v e n e s s , combin a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g , c o m b i n a t i o n o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g , s p e c i a l c h a l k f r a c t u r i n g t e c h n i q u e s , and w a t e r i m b i b i t i o n and o i l r e c o v e r y o f u n s t a b l e s o f t f l o w i n g c h a l k a r e d i s c u s s e d as f o l l o w s . Some comments a r e a l s o o f f e r e d on h o r i z o n t a l w e l l d r i l l i n g .
4.5.4.1. Petrophysical properties and productiveness The c h a l k i s t h e r e s e r v o i r r o c k where t h e f i r s t commercial o i l d i s c o v e r y i n t h e N o r t h Sea was made i n 1969 i n E k o f i s k i n t h e Norwegian N o r t h Sea, w i t h p r o d u c t i o n h a v i n g s t a r t e d i n 1971 ( t h u s f o u r y e a r s a f t e r t h e f i r s t gas s t r i k e and s i x y e a r s a f t e r t h e o n s e t o f o i l and gas e x p l o r a t i o n i n t h e N o r t h Sea; THOMAS 1986; c f . s e c t i o n 3 . 3 . 1 . ) . E k o f i s k i s a l s o t h e f i r s t g i a n t o i l f i e l d t h a t was d i s c o v e r e d i n Western Europe (BARK & THOMAS 1980, 1981; SNOW & HOUGH 1 9 8 8 ) . I n t h e l a s t y e a r s , a p a r t f r o m o i l p r o d u c t i o n and r e s e r v o i r s t i m u l a t i o n , t h e n a t u r e o f t h e c h a l k pay zones i n p a r t s o f t h e N o r t h Sea has been h i g h l i g h t e d b y tremendous f i e l d s u b s i d e n c e as a consequence o f r e s e r v o i r d r a i n a g e and d e p l e t i o n p a r t i c u l a r l y i n E k o f i s k (BLEAKLEY 1986, DIJK & KLOTERS 1987; THOMAS, D I X O N , EVANS & VIENOT 1987; BOADE, C H I N & SIEMERS 1988; JOHNSON, RHETT & SIEMERS 1988; MENG H I N I 1988, MES 1988, RENTSCH & MES 1988, SULAK & DANIELSEN 1988) w h i c h had t o be compensated b y a c o m p l i c a t e d p l a t f o r m j a c k - u p o p e r a t i o n t o r e n d e r t o e q u i l i b r i u m a v e r t i c a l d i f f e r e n c e o f more t h a n s i x m e t e r s (MANNESMANN REXROTH 1988, S M I T H 1988; SMITH, S M I T H & MONSHAUGEN 1 9 8 8 ) . Subsidence on an even l a r g e r s c a l e t h a n i n E k o f i s k has been d e t e c t e d i n t h e V a l h a l l c h a l k o i l f i e l d (OILMAN 1987 d; RUDDY, ANDERSON, PATTILLO, BISHLAWI & FOGED 1 9 8 8 ) . O i l - and g a s - f i e l d s u b s i dence p r o b l e m s a l s o o c c u r i n o t h e r c h a l k s , d i a t o m i t e 5 and l o o s e f i n e r - g r a i n e d sands (GEERTSMA 1973, WAAL & SMITS 1988; WENDEL, KUNKEL & SWANSON 1 9 8 8 ) . M O R I TA, WHITFILL, NYGAARD & BALE ( 1 9 8 8 ) d e s c r i b e d e t e r m i n a t i o n o f s u b s i d e n c e , r e s e r v o i r c o m p a c t i o n , and i n - s i t u s t r e s s i n d u c e d b y r e s e r v o i r d e p l e t i o n .
4.5.4.1.1. General aspects Chalk i s h i t h e r t o h y d r o c a r b o n - p r o d u c t i v e i n N o r t h Sea, M i d d l e E a s t , G u l f C o a s t and Rocky M o u n t a i n area/USA, and S c o t i a n S h e l f / C a n a d a (BLANTON 1981, HARTLEY & BOSMA 1 9 8 5 ) . Chalk i s a m a s s i v e , r a t h e r homogeneous, s o f t and e x t r e m e l y f i n e - g r a i n e d s e d i m e n t w h i c h has a d u c t i l e b e h a v i o u r , w i t h a i l t h e s e f e a t u r e s c a u s i n g o r t r i g g e r i n g p o r e c o l l a p s e and l o s s o f p e r m e a b i l i t y , embedment o f p r o p -
511 p a n t s ( c f . s e c t i o n 4.3.3.) and d e t e r i o r a t i o n o f c o n d u c t i v i t y , e v e n l y e t c h i n g by a c i d r e s u l t i n g i n low c r a c k c o n d u c t i v i t y when t h e p r e s s u r e i s r e l e a s e d and an unpropped f r a c t u r e c l o s e s , and l a c k o f s i g n i f i c a n t c r a c k b a r r i e r s l e a d i n g t o r a d i a l f r a c t u r e development w i t h o u t containment (KING & HOLMAN 1982). As a consequence o f t h e s o f t n e s s , any r i d g e s l e f t a f t e r a c i d i z a t i o n c o l l a p s e under t h e s t r e s s e s imposed by p r o d u c t i o n u n l e s s t h e y a r e m u l t i - l a y e r propped (BLANTON 1981; SIMON, COULTER, K I N G & HOLMAN 1982). Some r o c k mechanical c h a r a c t e r i s t i c s o f N o r t h Sea r e s e r v o i r s a r e a l s o i n v e s t i g a t e d by HOLT, INGSOY & MIKKELSEN ( 1 9 8 7 ) . Aspects o f s t r a t i g r a p h y , p e t r o g r a p h y and p e t r o p h y s i c s , and n a t u r a l f r a c t u r e types a r e b r i e f l y summarized as f o l l o w s . Comments a r e a l s o o f f e r e d on p r i mary c h a l k p r o p e r t i e s and s t i m u l a t i o n response as w e l l as secondary f i e l d s u b s i dence.
4.5.4.1.2. Stratigraphy N o r t h Sea c h a l k (HARPER & SHAW 1974) e x h i b i t s d i f f e r e n c e s i n p e t r o p h y s i c a l p r o p e r t i e s i n s t r a t i g r a p h i c a l succession (TANSOE, K I N G & HOLMAN 1986) by b e i n g a m u l t i s t o r e y r e s e r v o i r w i t h d i s t i n c t t h i c k n e s s , p o r o s i t y , p e r m e a b i l i t y and wat e r s a t u r a t i o n t r e n d s w i t h i n each l a y e r (BREWSTER, DANGERFIELD & FARRELL 1986). D i s t i n c t i o n can be made between Hod, T o r and E k o f i s k f o r m a t i o n s , and i n some f i e l d s , even a f r a c t u r e s t r a t i g r a p h y can be e s t a b l i s h e d .
4.5.4.1.2.1. Hod, Tor and Ekofisk formations The Tor F o r m a t i o n (upper p a r t o f t h e Upper Cretaceous M a a s t r i c h t i a n c h a l k sequence) i s a h i g h - p o r o s i t y ( 4 0 - 50 % ) n a t u r a l l y f r a c t u r e d c h a l k w i t h a m a t r i x p e r m e a b i l i t y o f a b t . 5 md, whereas t h e u n d e r l y i n g Hod F o r m a t i o n ( l o w e r p a r t o f t h e Upper Cretaceous M a a s t r i c h t i a n c h a l k s e r i e s ) i s much denser w i t h a b t . 30 - 35 % p o r o s i t y and p e r m e a b i l i t i e s between a b t . 2 md and a few m i c r o d a r c y . A t h i r d s t r a t i g r a p h i c a l l e v e l i s t h e Lower T e r t i a r y Danian c h a l k ( E k o f i s k Format i o n ; FARRELL 1988). While t h e Hod c h a l k a t t h e base i s r e l a t i v e l y s t a b l e and a l s o due t o i t s t i g h t e r f a c i e s i s o n l y s u b o r d i n a t e l y p r o d u c t i v e , and a l s o t h e E k o f i s k c h a l k a t t h e t o p i s a t h i n t i g h t l a y e r g e n e r a l l y a c t i n g as a h y d r a u l i c b a r r i e r , t h e Tor c h a l k i n t h e m i d d l e o f t h e r e s e r v o i r complex c o n t a i n s t h e m a j o r c h a l k o i l f i e l d s i n t h e N o r t h Sea, b u t i s o f t e n s e r i o u s l y u n s t a b l e and a t l e a s t i n t h e V a l h a l l f i e l d almost e v e r y w e l l needs more o r l e s s e x t e n s i v e t r e a t m e n t by b o t h h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g t o be a b l e t o be produced a t e c o n o m i c a l l y f e a s i b l e r a t e s ( c f . s e c t i o n 4.5.4.3.). I n the Ekofisk f i e l d , the c h a l k i s more s t a b l e and o n l y c o n v e n t i o n a l h y d r a u l i c f r a c t u r e s t i m u l a t i o n , b u t no g r a v e l pack c o n s o l i d a t i o n o f t h e r e s e r v o i r f o r m a t i o n has t o be c a r r i e d o u t (FARRELL 1988).
4.5.4.1.2.2. Fracture stratigraphy I n some f i e l d s , t h e c o r r e l a t i o n o f n a t u r a l crack t r e n d s and d i s t r i b u t i o n w i t h i n t h e r e s e r v o i r column r e s u l t s i n e s t a b l i s h m e n t o f a f r a c t u r e s t r a t i g r a phy. I n t h e E k o f i s k f i e l d , t h e T e r t i a r y E k o f i s k F o r m a t i o n i s m a i n l y c h a r a c t e r i zed by t e c t o n i c a l f r a c t u r e s o f c o n j u g a t e type, whereas t h e Cretaceous T o r Format i o n c o n t a i n s p r e d o m i n a n t l y s t y l o l i t e - a s s o c i a t e d c r a c k s which propagate between and t e r m i n a t e a t s t y l o l i t e s and f o r m anastomosing networks o f n e a r l y v e r t i c a l e x t e n s i o n f r a c t u r e s . The c o r r e l a t i o n between crack i n t e n s i t y and sedimentary f a c i e s o f t h e c h a l k g i v e s r i s e t o a p r e d i c t i v e f r a c t u r e model (FARRELL 1988).
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4.5.4.1.3.
Petrography and petrophysics
The Tor c h a l k c o n s i s t s o f c o c c o l i t h s and c a l c i t e c r y s t a l s and i s almost uncemented which a p a r t f r o m o v e r p r e s s u r i n g and undercompaction i s one o f t h e main reasons f o r i t s i n s t a b i l i t y . The low f o r m a t i o n s t r e n g t h i s a l s o due t o a v e r y low c o n c e n t r a t i o n o f connate w a t e r ( l e s s t h a n 5 X ) which has decreased t h e n o r mal e f f e c t o f p r e s s u r e s o l u t i o n a t g r a i n c o n t a c t s and t h u s c e m e n t a t i o n i s sever e l y r e t a r d e d . The h i g h p o r o s i t y o f t h e Tor c h a l k has a l s o been p r e s e r v e d by a v e r y low n e t c o n f i n i n g s t r e s s (overburden minus pore p r e s s u r e ) caused by a v e r y h i g h overpressure, r e s u l t i n g i n p r e v e n t i o n o f f o r m a t i o n p o r o s i t y c o l l a p s e because t h e f l u i d i n t h e pore-space i s a m a j o r s u p p o r t i n g element o f t h e overburden. While t h e m a t r i x p e r m e a b i l i t y o f t h e Tor c h a l k i s o n l y a b t . 5 md, p r e s s u r e b u i l d u p e v a l u a t i o n r e v e a l s p e r m e a b i l i t i e s up t o 100 md which a r e t h e e f f e c t o f an e x t e n s i v e n a t u r a l f r a c t u r e network. The s o f t c h a l k does n o t f a i l and move as i n d i v i d u a l f i n e p a r t i c l e s , b u t as a deformable p l a s t i c f l o w o f an u n i t mass ;, LANTON 1981, NEWMAN 1981; TANSOE, K I N G & HOLMAN 1986; ATTARD, MATHES & MOWER 1988). Some aspects o f autochthonous and a l l o c h t h o n o u s ch-alk as w e l l as f r a c t u r e c l o s u r e a r e d i s c u s s e d as f o l l o w s .
4.5.4.1.3.1.
Autochthonous and a1 lochthonous cha 1k
Chalk p o r o s i t y i s b o t h s t r a t i g r a p h i c a l l y and m i n e r a l o g i c a l l y c o n t r o l l e d (SNOW & HOUGH 1988). The c h a l k h o r i z o n s o f Hod, Tor and E k o f i s k f o r m a t i o n s cons i s t o f a l t e r n a t i n g l a y e r s o f a l l o c h t h o n o u s and autochthonous sediments. The autochthonous c h a l k s were d e p o s i t e d by g r a d u a l r a i n i n g down o f c a l c a r e o u s mater i a l o n t o t h e ocean f l o o r , whereas t h e a l l o c h t h o n o u s c h a l k s were l a i d down r a p i d l y by movements on graben f a c e s . Because o f t h e i r s l o w e r s e d i m e n t a t i o n r a t e , t h e autochthonous c h a l k s a r e harder, have more cementation and h i g h e r s i l i c a c o n t e n t , underwent more dewatering, have lower p o r o s i t y and p e r m e a b i l i t y , have l e s s f r e q u e n t n a t u r a l f r a c t u r i n g , and a g r e a t e r percentage o f t h e i r n a t u r a l f r a c t u r e s a r e cemented t h a n i n t h e a l l o c h t h o n o u s d e p o s i t s . These v a r i a t i o n s i n c h a r a c t e r i s t i c s can cause d r a m a t i c changes i n p o r o s i t i e s a c r o s s a f i e l d and w i t h i n a w e l l b o r e and a r e a l s o a t l e a s t p a r t i a l l y t h e source f o r d i f f e r e n t f r a c t u r i n g s t i m u l a t i o n r e q u i r e m e n t s i n v a r i o u s p l a c e s and s t o r e y s .
4.5.4.1.3.2.
Fracture closure
L a b o r a t o r y t e s t i n g proved t h e n a t u r a l f r a c t u r e s t o heal and e t c h e d open chann e l s t o c l o s e above 2,000 p s i and 800 p s i n e t c o n f i n i n g p r e s s u r e , r e s p e c t i v e l y . Wide f r a c t u r e s and h i g h p r o p p a n t c o n c e n t r a t i o n s i n t h e c r a c k s p r o v i d e s t a b l e f r a c t u r e f l o w c a p a c i t y when c o n f i n i n g p r e s s u r e i s i n c r e a s e d i n c o n t r a s t t o h e a l i n g o f unpacked f r a c t u r e s a t moderate c o n f i n i n g loads. E x c e s s i v e p r e s s u r e drawdown r e s u l t s i n f a i l u r e o f t h e f o r m a t i o n and c a t a s t r o p h i c loss o f p r o d u c t i o n i n c l u d i n g c o l l a p s e o f c r a c k s c r e a t e d by b o t h p r o p p a n t and a c i d f r a c t u r i n g . O'HEUR (1984) documents p o r o s i t y and hydrocarbon d i s t r i b u t i o n i n N o r t h Sea c h a l k r e s e r voi r s . Formations comparable t o t h e N o r t h Sea and Rocky Mountain c h a l k s i n terms o f p e t r o g r a p h i c a l c o m p o s i t i o n and hydrocarbon p r o d u c t i o n d i f f i c u l t i e s a r e diatomaceous e a r t h s and d i a t o m i t e s i n C a l i f o r n i a / U S A (STRUBHAR, MEDLIN, NAB1 & ANDREAN I 1984; STRICKLAND 1985; WENDEL, KUNKEL & SWANSON 1988).
4.5.4.1.4.
Natural f r a c t u r e types
The N o r t h Sea c h a l k r e s e r v o i r s a r e e x t e n s i v e l y n a t u r a l l y f r a c t u r e d . D i s t i n c t i o n can be made between healed, t e c t o n i c and s t y l o l i t e - a s s o c i a t e d f r a c t u r e s (BREWSTER, DANGERFIELD & FARRELL 1986; FARRELL 1988). The main c h a l k s t i m u l a t i o n targets are also o u t l i n e d .
513
4.5.4.1.4.1. Healed and tectonic fractures Healed f r a c t u r e s a r e t h e e a r l i e s t cracks, because t h e y a r e c r o s s - c u t by a l l o t h e r j o i n t c l a s s e s (BREWSTER, DANGERFIELD & FARRELL 1986; FARRELL 1988). Heal e d f r a c t u r e s a r e s i n g l e o r anastomosing networks and a r e i n f i l l e d w i t h f i n e mat e r i a l b e i n g h a r d e r t h a n t h e c h a l k m a t r i x . No v i s i b l e p o r o s i t y can be d e t e c t e d w i t h i n t h e h e a l e d f r a c t u r e s which p r e s e n t l y do n o t f o r m permeable f l o w chann e l s , b u t r a t h e r may i n h i b i t f l u i d f l o w by a c t i n g as m i c r o b a r r i e r s . The o v e r a l l geometry and t e x t u r e o f t h e h e a l e d f r a c t u r e s suggests t h e i r o r i g i n f r o m l o c a l b r i t t l e d e f o r m a t i o n o f p a r t i a l l y l i t h i f i e d sediment, and t h i s t y p e o f c r a c k s i s i n t i m a t e l y a s s o c i a t e d w i t h reworked c h a l k . T e c t o n i c f r a c t u r e s p r o v i d e t h e most e f f e c t i v e p e r m e a b i l i t y enhancement and a r e e s s e n t i a l l y h i g h - a n g l e normal f a u l t s commonly f o r m i n g p a r a l l e l s e t s o f conj u g a t e c r a c k s . I n such g r e g a r i o u s zones, one s e t o f f r a c t u r e s i s always domin a n t and may have an average c r a c k spacing as small as 10 - 15 cm. The f r a c t u r e s a r e u s u a l l y p l a n a r w i t h smooth s u r f a c e s , w i t h s l i c k e n s i d e s and secondary m i n e r a l i z a t i o n o f t h e c r a c k s b e i n g unusual. Communication o f r e s e r v o i r f l u i d s b e t ween m a t r i x and t e c t o n i c f r a c t u r e s i s c o n s i d e r e d t o be v e r y good. Displacement across t h e c r a c k s i s commonly i n t h e o r d e r o f a few cm. T e c t o n i c f r a c t u r e s devel o p e d under a n e a r v e r t i c a l maximum p r i n c i p a l s t r e s s .
4.5.4.1.4.2. Stylol ite-associated fractures S t y l o l i t e - a s s o c i a t e d f r a c t u r e s a l s o o r i g i n a t e d under a v e r t i c a l p r i n c i p a l s t r e s s system (NELSON 1981, WATTS 1983; BREWSTER, DANGERFIELD & FARRELL 1 9 8 6 ) . These c r a c k s f o r m a d j a c e n t t o s t y l o l i t e s and u s u a l l y p a r a l l e l t h e s t y l o l i t e columns. They a r e i n t e r p r e t e d as t e n s i o n gashes, a l t h o u g h a shear f r a c t u r e geomet r y i s o c c a s i o n a l l y seen. The c r a c k s t e r m i n a t e a t t h e s t y l o l i t e s u r f a c e s and nev e r c r o s s them. S t y l o l i t e - a s s o c i a t e d f r a c t u r e s may o c c u r i n d i v i d u a l l y i n areas w i t h low c r a c k i n t e n s i t i e s , b u t a r e u s u a l l y seen i n anastomosing networks where a f r a c t u r e s p a c i n g o f 5 cm i s common and 0.5 cm i s n o t unusual. I n d i v i d u a l c r a c k s do n o t e x t e n d f a r i n t o t h e c h a l k m a t r i x f r o m t h e s t y l o l i t e s . F r a c t u r e l e n g t h may r e a c h up t o 5 - 15 cm. S t y l o l i t e - a s s o c i a t e d c r a c k s a r e s t a t i s t i c a l l y near v e r t i c a l , b u t t h e y f o r m b e d d i n g - p a r a l l e l permeable zones a d j a c e n t t o i n d i v i d u a l s t y l o l i t e s . A d d i t i o n a l r e s e r v o i r p e r m e a b i l i t y i s i n some p a r t s o f t h e c h a l k p r o v i d e d by f r a c t u r e s a l o n g slump s u r f a c e s . O u t l i n e s o f t h e n a t u r a l f r a c t u r e system i n N o r t h Sea c h a l k r e s e r v o i r s a r e a l s o g i v e n by THOMAS, DIXON, EVANS & VIENOT (1987); SYLTE, HALLENBECK & THOMAS (1988) and JENSEN, BRESLING, RASMUSSEN, FOGED & PETERSEN ( 1 9 8 9 ) .
4.5.4.1-4.3.Chalk stimulation targets Besides m a t r i x p e r m e a b i l i t y and n e t pay t h i c k n e s s , w e l l p r o d u c t i v i t y i s h i g h 1988). Because t h e n a t u r a l c r a c k s a r e s h o r t and o c c u r f r e q u e n t l y , and due t o h i g h m a t r i x permeabil i t i e s , dual p o r o s i t y b e h a v i o u r i s n o t observed i n t r a n s i e n t t e s t s , b u t t h e nat u r a l f r a c t u r e s e s s e n t i a l l y behave as an i n c r e a s e d m a t r i x p e r m e a b i l i t y . Three s t i m u l a t i o n t a r g e t s i n c h a l k s o f t h e E k o f i s k area can be d i s t i n g u i s h e d : c h a l k s w i t h good m a t r i x p e r m e a b i l i t y and good n a t u r a l f r a c t u r i n g , c h a l k s w i t h good mat r i x p e r m e a b i l i t y b u t p o o r n a t u r a l f r a c t u r i n g , and c h a l k s w i t h low m a t r i x p e r m e a b i l i t y and poor n a t u r a l f r a c t u r i n g . N a t u r a l c r a c k occurrence i s b o t h l o c a l l y and r e g i o n a l l y c o n t r o l l e d . L o c a l l y t h e n a t u r a l j o i n t s appear t o r a d i a t e f r o m t h e c r e s t o f t h e s t r u c t u r e , whereas r e g i o n a l l y t h e y t r e n d NNE-SSW. Conductive n a t u r a l f r a c t u r e s a r e i n f l u e n c e d by i n - s i t u s t r e s s e s and may o c c u r i n a d i r e c t i o n p e r p e n d i c u l a r t o t h a t o f minimum s t r e s s . l y dependent on n a t u r a l f r a c t u r e occurrences (SNOW & HOUGH
514
4.5.4.1.5. Primary chalk properties and stimulation response The v a r i o u s n a t u r a l f r a c t u r e systems a r e e s p e c i a l l y w e l l - d e v e l o p e d i n h a r d c h a l k s t h a t a r e s u f f i c i e n t l y c o n s o l i d a t e d t o a l l o w t h e c r a c k s t o s t a y open. T h i s i s o n l y t h e c a s e i n some f i e l d s and s t r a t i g r a p h i c h o r i z o n s , whereas i n o t h e r p a t c h e s and s t o r e y s , t h e c h a l k i s s o f t and p o o r l y c o h e s i v e and t h u s t h e n a t u r a l f r a c t u r e s a r e i n most cases rehealed. U l t i m a t e recovery, p r o d u c i n g r a t e and d r a i n a g e e f f i c i e n c y f o r e a c h w e l l depend on t h e success o f c o n n e c t i n g t h e n a t u r a l c r a c k s w i t h t h e w e l l b o r e b y a h y d r a u l i c f r a c t u r e ( t h e same a p p l i e s f o r v e r y f i n e - g r a i n e d h a r d o r s o f t s a n d s t o n e s t o s i l t s t o n e s o f d i f f e r e n t m a t r i x coh e s i o n t y p e s w h i c h c a n i n t e r m s o f r o c k m e c h a n i c a l p r o p e r t i e s b e compared w i t h c h a l k ; PEARCE 1983; and t h e same i s t r u e f o r d i a t o m i t e s and d i a t o m a c e o u s e a r t h s ; STRUBHAR, MEDLIN, NAB1 & ANDREANI 1984; STRICKLAND 1985; WENDEL, KUNKEL & SWANSON 1 9 8 8 ) . The c h a l k i s a d i f f i c u l t s t i m u l a t i o n t a r g e t , because i t s m a s s i v e u n i f o r m nat u r e r e s u l t s i n s h o r t r a d i a l f r a c t u r e g r o w t h (SNOW & HOUGH 1 9 8 8 ) . Low h a r d n e s s and d u c t i l e b e h a v i o u r o f t h e c h a l k l e a d t o p o r e c o l l a p s e and c r e e p c a u s i n g e x c e s s i v e p r o p p a n t embedment ( c f . s e c t i o n 4 . 3 . 3 . ) . The homogeneous n a t u r e o f t h e c h a l k a l s o g i v e s r i s e t o e v e n l y e t c h e d c r a c k f a c e s d u r i n g a c i d i z i n g and a c i d fracturing. Low h a r d n e s s and l o w y i e l d s t r e n g t h o f t h e c h a l k c a n a l s o r e s u l t in h e a l i n g o f n a t u r a l c r a c k s and f a i l u r e o f any a c i d - e t c h e d g r o o v e s . The c h a l k s a l s o l o s e m e c h a n i c a l s t r e n g t h when c o n t a c t e d by f o r e i g n f l u i d s , t h u s c a u s i n g t r e a t m e n t s w i t h w a t e r - b a s e d f l u i d s t o a c t u a l l y damage i n s t e a d o f s t i m u l a t i n g the formation. P r o d u c t i v i t y d e c l i n e o f chalk r e s e r v o i r s f o l l o w i n g f r a c t u r i n g and/or a c i d i z i n g i s p a r t i a l l y due t o two-phase f l o w and c o n d e n s a t e d r o p o u t . The m a j o r i t y o f t h e o u t p u t l o s s i s a consequence o f t o t a l f a i l u r e and p l a s t i c f l o w o f t h e chalk, s t a b l e creep flow, crushing o f the f r a c t u r e face, b r i t t l e f a i l u r e , breaki n g o f f o f f i n e s on t h e c r a c k f a c e t h e r e b y c l o g g i n g u p t h e f r a c t u r e , and s l i p page a l o n g t h e j o i n t w a l l o f t h e c h a l k . As e f f e c t i v e s t r e s s e s i n c r e a s e , c h a l k u n d e r g o e s c r e e p f a i l u r e and p o r e c o l l a p s e , and as t h i s f a i l u r e o c c u r s o n t h e f r a c t u r e f a c e , t h e c r a c k l o s e s some m e c h a n i c a l s t r e n g t h and may c l o s e . F i n e s hav i n g been c h i p p e d o f f t h e c r a c k w a l l and h a v i n g f i l l e d up t h e c o n d u c t i v e chann e l a r e t h e most i m p o r t a n t reason f o r c o n d u c t i v i t y r e d u c t i o n .
4.5.4.1.6. Primary chalk properties and secondary field subsidence OEN, ENGELL-JENSEN & BARENDREGT ( 1 9 8 8 ) d e s c r i b e a c o m b i n a t i o n o f s m a l l - s c a l e f a u l t s and m i c r o f r a c t u r e s w i t h l a r g e - s c a l e f a u l t s i n a c r a c k s y s t e m e x t e n d i n g throughout t h e r e s e r v o i r which p r o v i d e s a communicating network o f h i g h l y p e r meable f l u i d c o n d u i t s t h a t r e s u l t i n h i g h w e l l p r o d u c t i v i t i e s and e x c e l l e n t f i e l d d r a i n a g e . The f a u l t and f r a c t u r e s y s t e m i s q u i t e w e l l s u p p o r t e d b y t h e c h a l k m a t r i x framework i n o r d e r t o s t a y open even i n c a s e o f c o n s i d e r a b l e r e n e wed c o m p a c t i o n o f t h e r e s e r v o i r due t o f i e l d s u b s i d e n c e (SULAK & DANIELSEN 1988), The l a t t e r p r o c e s s i s p r o v o k e d b y o i l d e p l e t i o n and phase changes due t o r e p l a c e m e n t o f t h e p o r e volume f o r m e r l y i n f i l l e d by o i l s u b s e q u e n t l y b y w a t e r w h i c h a l t e r s f l u i d v i s c o s i t y and t h u s r o c k c o h e s i o n , and t h e r e b y t r i g g e r s second a r y r e c o m p a c t i o n o f t h e m a t r i x w i t h o u t , however, s i g n i f i c a n t l y a f f e c t i n g f r a c t u r e p o r o s i t y and o i l p r o d u c t i v i t y . I n c o n t r a s t t o t h e r a t h e r n e g l i g i b l e i m p a c t reo f f i e l d s u b s i d e n c e on p e t r o p h y s i c a l p r o p e r t i e s and r e s e r v o i r e n g i n e e r i n g , c o m p a c t i o n o f t h e c h a l k as an a d j u s t m e n t t o a l t e r e d f l u i d phase d i s t r i b u t i o n has tremendous consequences f o r t e c h n i c a l f i e l d o p e r a t i o n and r e q u i r e s c o m p l i c a t e d jack-ups f o r e q u i l i z a t i o n o f s u b s i d e n c e ( c f . D I J K & KLOTERS 1987, MANNESMANN REXROTH 1988; S M I T H , SMITH & MONSHAUSEN 1 9 8 8 ) . The d i s c u s s i o n as f o l l o w s f o c u s s e s on r e s e r v o i r c o m p a c t i o n and f i e l d s u b s i d e n c e , P e r m i a n ( R o t l i e g e n d ) s a n d s t o n e v s . C r e t a c e o u s c h a l k s u b s i d e n c e , and s u b s i d e n c e r a t e s i n d i f f e r e n t
515 chalk f i e l d s .
4.5.4.1.6.1. Reservoir compaction and field subsidence Subsidence i s t h e gradual s e t t l i n g o f t h e l a n d mass o r ocean f l o o r o v e r o i l and gas r e s e r v o i r s o r a q u i f e r s where f l u i d removal causes p r e s s u r e r e d u c t i o n i n t h e p o r e space (BLEAKLEY 1986). Subsidence i s t r i g g e r e d by compaction o f t h e pay r o c k under p r e s s u r e f r o m t h e overburden when r e s e r v o i r p r e s s u r e i s r e l i e ved. E f f e c t i v e s t r e s s on t h e pay f o r m a t i o n i s equal t o overburden p r e s s u r e m i nus r e s e r v o i r p o r e p r e s s u r e . As t h e p o r e p r e s s u r e d e c l i n e s due t o o i l and gas p r o d u c t i o n , s t r e s s on t h e pay r o c k i n c r e a s e s and compaction o c c u r s depending on f o r m a t i o n c o m p r e s s i b i l i t y . F o r l a r g e a r e a l r e s e r v o i r e x t e n s i o n s , t h e overburden cannot be expected t o p r o v i d e s u p p o r t through a r c h i n g and b r i d g i n g o f g r a i n s because o f t h e d i s t a n c e s i n v o l v e d r e g a r d l e s s o f t h e n a t u r e o f t h e overburden r o c k . Subsidence t a k e s p l a c e i f t h e pay zone covers a l a r g e area, has g r e a t t h i c k n e s s and i s compressible, and a l a r g e p r e s s u r e drop i s p r e s e n t i n t h e p o r e space. Subsidence i s a l s o more l i k e l y t o o c c u r o v e r a s h a l l o w r e s e r v o i r than a deep one. I n o v e r p r e s s u r e d and u n d e r s a t u r a t e d c h a l k s , d e p l e t i o n o f r e s e r v o i r p r e s s u r e w i t h f i e l d development causes l a r g e changes i n n e t s t r e s s e s on t h e f o r m a t i o n which r e s u l t i n p l a s t i c d e f o r m a t i o n and compaction (RUDDY, ANDERSON, PATTILLO, BISHLAWI & FOGED 1988). The e f f e c t s o f r o c k compaction m a n i f e s t themselves i n two i m p o r t a n t ways b e i n g s i g n i f i c a n t c o n t r i b u t i o n t o r e s e r v o i r energy i n f o r m o f l i t h i c d r i v e , and p a r t i a l t r a n s f e r o f t h i s compaction through t h e overburden l e a d i n g t o m u d l i n e subsidence. The l i t h i c d r i v e process r e s u l t i n g f r o m r o c k comp a c t i o n p r o v i d e s a s i g n i f i c a n t p a r t o f t h e hydrocarbon r e c o v e r y f r o m overpressur e d and u n d e r s a t u r a t e d c h a l k s , w i t h r o c k c o m p r e s s i b i l i t y and i t s v a r i a t i o n w i t h p o r e p r e s s u r e b e i n g one o f t h e most i m p o r t a n t parameters i n r e s e r v o i r modell i n g . Aspects o f r e s e r v o i r compaction and f i e l d subsidence due t o p o r e c o l l a p s e i n c h a l k f o r m a t i o n s a r e a l s o d i s c u s s e d by SMITS, WAAL & KOOTEN ( 1 9 8 8 ) . BLANTON (1981) emphasizes t h e unusual d u c t i l e b e h a v i o u r o f t h e c h a l k which g e n e r a l l y h i n d e r s o r i n h i b i t s t h e f r a c t u r i n g process. The t r a n s i t i o n f r o m p o s i t i v e t o neg a t i v e d i l a t a n c y w i t h increasing c o n f i n i n g pressure r e f l e c t s a t r a n s i t i o n from m i c r o c r a c k i n g t o pore c o l l a p s e .
4.5.4.1.6.2.
Permian sandstone vs. Cretaceous chalk subsidence
While i n sandstones pay f o r m a t i o n compaction g e n e r a l l y r e s u l t s i n p e r m e a b i l i i n c h a l k s occ a s i o n a l l y c o n s t a n t l y r e m a i n i n g hydrocarbon p r o d u c t i o n r a t e s (SULAK & DANIELSEN 1988) r e f l e c t t h a t s i g n i f i c a n t p e r m e a b i l i t y d i m i n u t i o n d i d n o t o c c u r . On t h e o t h e r hand, t h e more s t a b l e framework o f sandstones i n h i b i t s m a j o r p o r e c o l l a p se which l e a d s t o o n l y m i n o r subsidence r a t e s (SCHOONBEEK 1976, BLEAKLEY 1986), whereas i n c h a l k s , t h e g r a i n f a b r i c i s much more s u b j e c t e d t o p o r e f a i l u r e and c l o s u r e t h a t g i v e s r i s e t o much h i g h e r compaction which can be up t o 10 - 20 t i mes g r e a t e r t h a n f o r sandstones. Water f l o o d i n g i n o i l and gas f i e l d s s u f f e r i n g f r o m subsidence does n o t o n l y aim on i n c r e a s i n g u l t i m a t e r e c o v e r y , b u t serves a dual r o l e i n c l u d i n g m a i n t a i n i n g r e s e r v o i r p r e s s u r e t o m i t i g a t e compaction. Wat e r f l o o d i n g i s t h e f i r s t c h o i c e o f pressure-maintenance system i n o i l f i e l d s , because gas i n j e c t i o n i n t o t h e r e s e r v o i r means d i r e c t l o s s o f revenue u n l e s s i n e r t n i t r o g e n i s u t i l i z e d i n s t e a d o f c o m b u s t i b l e methane. t y r e d u c t i o n as pores a r e c l o s e d and t h e r o c k becomes more dense,
The s p e c i a l mechanical b e h a v i o u r o f c h a l k r e s e r v o i r s w i t h f i e l d subsidence as a consequence o f compaction by p o r e c o l l a p s e i s i l l u s t r a t e d by a comparison between Cretaceous ( M a a s t r i c h t i a n ) c h a l k o i l f i e l d s and Permian ( R o t l i e g e n d ) sandstone gas f i e l d s (SCHOONBEEK 1976). W h i l e subsidence e q u a l i z a t i o n i n t h e c h a l k o i l f i e l d s E k o f i s k and V a l h a l l i n t h e Norwegian N o r t h Sea r e q u i r e s j a c k up o p e r a t i o n s up t o 6 m v e r t i c a l d i f f e r e n c e f o r compensation o f an annual s e t t l i n g r a t e o f a b t . 8 - 16 i n . ( 2 0 - 40 cm; BLEAKLEY 1986), R o t l i e g e n d sandstone
516 subsidence as a r e s u l t o f framework condensation due t o gas w i t h d r a w a l i n t h e Groningen f i e l d / N e t h e r l a n d s onshore i s i n t h e range o f 10 - 25 cm f o r a product i o n p e r i o d o f 20 - 40 y e a r s . T h i s i s t h e consequence o f t h e much more s t a b l e framework o f R o t l i e g e n d sandstones w i t h r e s p e c t t o Cretaceous c h a l k s (SCHOONBEEK 1 9 7 6 ) . O t h e r hydrocarbon f i e l d s w i t h s i g n i f i c a n t r e s e r v o i r subsidence o f a b t . 2 m ( 6 f t ) o r more a r e W i l m i n g t o n and Inglewood i n C a l i f o r n i a / U S A , Bachaquero and L a g u n i l l a s i n Venezuela and one f i e l d i n t h e USSR (BLEAKLEY 1986).
4.5.4.1.6.3. Subsidence rates in different chalk fields The E k o f i s k c h a l k f i e l d i s s u b s i d i n g a t a r a t e o f a b t . 20 - 40 cm/year
(8 16 i n . ) , and f r o m t h e annual r a t e , maximum t o t a l subsidence f r o m t h e o n s e t o f p r o d u c t i o n i n 1971 u n t i l t h e y e a r 2011 when t h e o p e r a t i n g l i c e n s e e x p i r e s has been c a l c u l a t e d and c o n s i d e r e d as a base f o r compensation o f compaction by t h e 1987 j a c k - u p o p e r a t i o n ( c f . DIJK & KLUTERS 1987, MANNESMANN REXROTH 1988; SMITH, SMITH & MONSHAUSEN 1988). W h i l e subsidence so f a r has ranged f r o m a b t . 8 i n . v i a 12 i n . on average t o 16 i n . p e r y e a r , i t i s expected t h a t as a consequence o f massive w a t e r f l o o d p r e s s u r e maintenance, f u t u r e subsidence w i l l be no more t h a n h a l f o f t h a t (PETROLEUM ENGINEER INTERNATIONAL 1987 a ) . The V a l h a l l c h a l k o i l f i e l d , however, i s even s l i g h t l y f a s t e r s u b s i d i n g t h a n E k o f i s k a t a r a t e o f more than 40 cm/year, w i t h p r e s e n t e s t i m a t e s b e i n g t h a t t h e maximum s i n kage o f V a l h a l l w i l l be between 3 and 5 m o v e r t h e n e x t t e n y e a r s (OILMAN 1987 a) i n s t e a d o f t h r o u g h o u t 20 - 40 y e a r s i n E k o f i s k .
4.5.4.2. Combination of acid and proppant fracturing Combination o f a c i d and p r o p p a n t f r a c t u r i n g i s a s u i t a b l e s t i m u l a t i o n t e c h n i que f o r c h a l k r e s e r v o i r s , because i t a l l o w s t o p r o f i t f r o m t h e advantages o f b o t h i n d i v i d u a l t r e a t m e n t p o s s i b i l i t i e s . The sequence o f combined a c i d and p r o p p a n t f r a c t u r i n g i n c l u d e s g e l i n j e c t i o n t o i n i t i a t e f r a c t u r e growth, a c i d i n s e r t i o n t o e t c h t h e f r a c t u r e w a l l s , again g e l pumping f o r r e o p e n i n g o f an even w i d e r f r a c t u r e due t o a c i d e t c h i n g , and f i n a l l y i n j e c t i o n o f h i g h p r o p p a n t concent r a t i o n s i n t o t h e c r a c k (SNOW & HOUGH 1988; c f . a l s o s e c t i o n 4 . 5 . 1 . 1 . 1 . 4 . ) . Asp e c t s r e q u i r i n g a t t e n t i o n d u r i n g j o i n t a c i d and p r o p p a n t f r a c t u r i n g a r e v i s c o u s f i n g e r i n g a c i d i z i n g and wormhole growth, w h i s p e r f r a c t u r i n g , and f r a c t u r e propag a t i o n c o n t r o l . The comparative e f f e c t i v i t y o f proppant and a c i d f r a c t u r i n g i n chalk s t i m u l a t i o n i s also discussed.
4.5.4.2.1.Viscous fingering acidizing and wormhole growth Enhancement o f c h a l k s t i m u l a t i o n can be achieved by a c o m b i n a t i o n o f m u l t i l a y e r p r o p p i n g o f s h o r t l a r g e f r a c t u r e s and v i s c o u s f i n g e r i n g a c i d i z i n g (HARTLEY & BOSMA 1985; D A V I E S , BOSMA & V R I E S 1987; K I N G & SCHER 1987; SMITH, MILLER & HAGA 1987), w i t h t h e l a t t e r t e c h n i q u e c r e a t i n g widely-spaced e t c h e d r i d g e s and t h u s an i r r e g u l a r f r a c t u r e w a l l (SHELL 1979). Viscous f i n g e r i n g a c i d i z i n g (FREORICKSON & 8ROADDUS 1976; O A V I E S , BOSMA & V R I E S 1987) i s t o e s t a b l i s h f r a c t u r e geometry w i t h a v i s c o u s c r o s s l i n k e d n o n - a c i d pad f o l l o w e d by l o w - v i s c o s i t y a c i d (PARKER, ADAMS & LIANKUI 1 9 8 6 ) . The l o w - v i s c o s i t y a c i d f i n g e r s t h r o u g h t h e v i s c o u s pad and c r e a t e s h i g h - c o n d u c t i v i t y channels i n t h e c r a c k , i n c o n t r a s t t o c o n v e n t i o n a l a c i d f r a c t u r i n g where t h e e n t i r e c r a c k f a c e is e t c h e d and c o n d u c t i v i t y depends on t h e d i f f e r e n t i a l e t c h i n g p a t t e r n s c r e a t e d by t h e n a t u r e o f t h e r o c k . Widely-spaced p e r f o r a t i o n s can h e l p t o g e t good s e p a r a t i o n o f t h e a c i d f i n g e r s o r channels as t h e y e n t e r t h e f r a c t u r e (PARKER, AOAMS & LIANKUI 1986). PEETERS & HARTLEY (1984) r e p o r t a c i d f i n g e r i n g across t h e c r a c k f a c e and f i l l i n g o f t h e r e s u l t i n g e t c h e d r i d g e s w i t h proppants, w i t h i n t h i s t e c h n i q u e some p a r t s o f t h e f r a c t u r e f a c e n o t b e i n g c o n t a c t e d by a c i d . O t h e r aspects o f v i s cous f i n g e r i n g a r e covered by DACCORD, NITTMAN & STANLEY (1986); LENORMAND ( 1 9 8 6 ) , SARMA (1986), FAYERS & NEWLEY (1987), HOMSY (1987), ALLEN & BOGER
517 (1988), KOSSACK (1988), EWING & RUSSELL (1988), GRADER & O'MEARA (1988), ODEH & COHEN (1988); EWING, RUSSELL & YOUNG (1989); KU, H I R S H (1989) and MILLER, MOISSIS & WHEELER (1989). The d i s c u s s i o n as f o l l o w s on whisper f r a c t u r i n g , and v i s c o s i t y c o n t r a s t and d e n s i t y d i f f e r e n c e f o r fingering.
& OPDAL & TAYLOR focusses required
4.5.4.2.1.1. Whisper fracturing Whisper f r a c t u r i n g i s a p s e u d o l i m i t e d - e n t r y treatment s p e c i a l l y designed f o r chalk r e s e r v o i r s (HARTLEY & BOSMA 1985). I n l i m i t e d - e n t r y l a y o u t , t h e p e r f o r a t i o n s are r e s t r i c t e d enough t o c r e a t e h i g h f r i c t i o n pressures i n t h e p e r f o r a t i o n s which behave l i k e backpressure valves ensuring t h a t h i g h pressures are app l i e d across a l l p e r f o r a t i o n s and t h a t breakdown o f the e n t i r e i n t e r v a l occurs (SNOW & HOUGH 1988; c f . s e c t i o n 4.2.2.2.). The h i g h f l u i d v e l o c i t i e s a t each p e r f o r a t i o n d u r i n g p r o d u c t i o n r e s u l t i n h i g h f r i c t i o n losses, r e d u c t i o n of w e l l p r o d u c t i v i t y , and p o t e n t i a l l y h i g h p e r f o r a t i o n e r o s i o n and subsequent casing weakening. I n a d d i t i o n , o n l y small a c i d volumes c o u l d be i n j e c t e d i n t o hydraul i c f r a c t u r e s b e f o r e they c l o s e up. I n p s e u d o l i m i t e d - e n t r y designs, more p e r f o r a t i o n s are used, the treatments are staged, and the phases are separated by d i v e r t e r s ( c f . s e c t i o n s 4.2.2.1.). I n whisper f r a c t u r i n g , the p e r f o r a t i o n s are spaced such t h a t viscous f i n g e r i n g occurs and uneven r i d g e s are etched by the a c i d . Another p a t t e r n i s t o p e r f o r a t e c l o s e t o t h e crack boundary t o increase f r a c t u r e p e n e t r a t i o n by generat i n g semi-radial i n s t e a d of r a d i a l crack growth. Whisper f r a c t u r i n g treatments can u t i l i z e b o t h a c i d and proppant. I f the chalk i s unstable, proppant can be i n j e c t e d f o l l o w i n g t h e a c i d . Large proppant volumes may be used t o support t h e e n t i r e etched crack, o r small proppant q u a n t i t i e s c o u l d be i n j e c t e d t o support f r a c t u r e c o n d u c t i v i t y i n o n l y the near-wellbore r e g i o n . F o r more competent chalks, a proppant j o b f o l l o w e d by an a c i d s l u g t o e s t a b l i s h h i g h near-wellbore conductivity i s also a suitable solution.
4.5.4.2.1.2. Viscosity contrast required f o r finger ins Promotion of viscous f i n g e r i n g has t o be ensured by e s t a b l i s h i n g a v i s c o s i t y c o n t r a s t o f a t l e a s t 200 - 300, w i t h the h i g h - v i s c o s i t y g e l a l s o reducing l e a k o f f and r e a c t i o n r a t e s by c r e a t i n g a w i d e r f r a c t u r e (SNOW & HDUGH 1988). A t l e a s t some o f the a c i d must have reduced l e a k o f f t o increase a c i d p e n e t r a t i o n e s p e c i a l l y i n t h e h i g h f l u i d - l o s s p o r t i o n s o f the chalk, w i t h reduced l e a k o f f and increased p e n e t r a t i o n being achieved by a c i d g e l l i n g ( c f . s e c t i o n s 4.3.4.3.4.1. and 4.5.1.2.2.). Aspects o f f r a c t u r e a c i d i z i n g treatment design are a l s o o u t l i n e d by BROADDUS, KNOX & FREDRICKSON (1975) and ANDERSON & FREDRICKSON (1987), and mechanical model 1 i n g o f wormhole growth i n carbonate a c i d f r a c t u r i n g i s performed by DACCORD, TOUBOUL & LENORMAND (1987) and HUNG, HILL & SEPEHRNOORI (1987). The wormhole growth phenomenon ( c f . s e c t i o n 4.5.1.6.3.) i n matrix acidizing o f carbonate formations i s analogous t o t h e viscous f i n g e r i n g e f f e c t i n waterf l o o d s t u d i e s (ARAKTINGI & ORR 1988, VOGE & CRAFTON 1988). As the c o n d u c t i v i t y o f acid-etched f r a c t u r e s i s s e n s i t i v e t o r e s e r v o i r d e p l e t i o n and any e l e v a t e d t o excessive pressure drawdown associated w i t h p r o d u c t i o n increases the e f f e c t i v e s t r e s s beyond the s t a b i l i t y t h r e s h o l d o f the chalk, propping o f the f r a c t u r e s i s i n e v i t a b l e . While s i n g l e - l a y e r propped cracks s u f f e r i n performance ser i o u s l y from embedment o f g r a i n s (SIMON, CDULTER, K I N G & HOLMAN 1982; HARTLEY & BOSMA 1985), m u l t i - l a y e r propping i s the optimum s o l u t i o n . Another technique i s closed f r a c t u r e a c i d i z i n g (FREDRICKSON 1987). I n some areas, harder and more permeable c h a l k s achieve economical p r o d u c t i o n r a t e s a l r e a d y w i t h a c i d f r a c t u r i n g , whereas s o f t e r and l e s s permeable chalks can o n l y be f e a s i b l y e x p l o i t e d if s t i m u l a t e d by proppant f r a c t u r i n g (ANDERSEN, HANSEN & FJELDGAARD 1988).
518
4.5.4.2.1.3.
Density difference required f o r f i n g e r i n g
F o r d e n s i t y d i f f e r e n c e s between a c i d and pad f l u i d o f l e s s t h a n 0 . 1 g/cm3, g r a v i t y has no e f f e c t on v i s c o u s f i n g e r i n g (SNOW & HOUGH 1 9 8 8 ) , whereas f o r dens i t y d i f f e r e n c e s g r e a t e r t h a n 0 . 2 g/cm3, g r a v i t y o v e r - o r u n d e r r u n n i n g c a n domin a t e t h e f l o w p a t t e r n . I n t h e r a n g e between 0 . 1 and 0 . 2 g/cm3 d e n s i t y d i f f e r e n ce, b o t h f i n g e r i n g and g r a v i t y s e g r e g a t i o n o c c u r , w i t h t h e s e l i m i t s b e i n g s l i g h t l y i n c r e a s e d i f t h e g e l i s c r o s s l i n k e d . Once f i n g e r i n g b e g i n s , i t c o n t i nues r e g a r d l e s s o f t h e v i s c o s i t y r a t i o . Uneven e t c h i n g d e v e l o p s i n homogeneous c h a l k s even w i t h o u t v i s c o s i t y d i f f e r e n c e s . F i n g e r i n g i s i n i t i a t e d by near-perf o r a t i o n e f f e c t s and t h e uneven f r a c t u r e f a c e , and once i n i t i a t e d , uneven e t c h i n g becomes a s e l f - p r o p a g a t i n g p r o c e s s , because as a c i d e t c h e s , t h e f r a c t u r e widens, thus causing t h e a c i d t o c o n t i n u e t o f l o w i n t h e w i d e r s e c t i o n o f t h e crack.
4.5.4.2.2.
Fracture propagation c o n t r o l
A c r i t i c a l factor i n chalk reservoirs i s the control o f fracture propagation i n v e r t i c a l d i r e c t i o n (EEKELEN 1982, McOONALD 1983, HARTLEY & BOSMA 1 9 8 5 ) . F r a c t u r e g r o w t h i n t o n e i g h b o u r i n g zones had r e p e a t e d l y o c c u r r e d i n c h a l k f o r m a t i o n s , b e i n g a consequence o f t h e s p e c i a l r e s e r v o i r r o c k p r o p e r t i e s of t h e c h a l k . The f r a c t u r e s n e a r l y a l w a y s e n t e r t h e l a y e r s a d j o i n i n g t o t h e p a y zone, and p e n e t r a t i o n d e p t h i s d e t e r m i n e d b y t h e d i f f e r e n c e s i n s t i f f n e s s and h o r i z o n t a l i n - s i t u s t r e s s between p a y zone and n e i g h b o u r i n g beds. F r a c t u r e g r o w t h i s r a d i a l i n c h a l k f o r m a t i o n s where n o s i g n i f i c a n t s t r e s s c o n t r a s t e x i s t s between r e s e r v o i r zones s e l e c t e d on t h e b a s i s o f p e r m e a b i l i t y v a r i a t i o n s i n an o t h e r w i s e r a t h e r homogeneous f o r m a t i o n . A l t h o u g h v e r t i c a l z o n a t i o n o f t h e c h a l k comp l e x based on r e s e r v o i r q u a l i t y ( a s c h i e f l y r e f l e c t e d b y p e r m e a b i l i t y and f r a m e work s t a b i l i t y ) may n o t be i m p o r t a n t f o r f r a c t u r e g r o w t h i n t h e i n i t i a l p r o d u c t i o n s t a g e , i t may become s i g n i f i c a n t w i t h d e p l e t i o n once d i f f e r e n t p r e s s u r e r e gimes a r e d e v e l o p e d i n t h e v a r i o u s s u p e r i m p o s e d pay column zones. Some a s p e c t s o f s h o r t w i d e f r a c t u r e s and h i g h p r o p p a n t s a t u r a t i o n as w e l l as i m p a c t o f s t i m u l a t i o n b o a t s on a c i d v s . p r o p p a n t f r a c t u r i n g a r e d i s c u s s e d as f o l l o w s .
4.5.4.2.2.1.
Short wide fractures and high proppant saturation
A p p r o p r i a t e s t i m u l a t i o n s o l u t i o n s a c c o r d i n g t o f i e l d and l a b o r a t o r y e v i d e n c e a r e s h o r t w i d e f r a c t u r e s t h a t a r e i n d u c e d h y d r a u l i c a l l y and a r e e n l a r g e d b y c r e a t i o n o f w i d e l y - s p a c e d e t c h e d r i d g e s b y a c i d i z i n g (SHELL 1979, HARTLEY & BOSMA 1985, KING & SCHER 1 9 8 7 ) . The r e s u l t i n g w i d e f r a c t u r e i s p r o p p e d b y a m u l t i l a y e r i n f i l l i n g . Another p o s s i b i l i t y i s combination o f h y d r a u l i c proppant f r a c t u r i n g and g r a v e l p a c k i n g w i t h b o t h c o n v e n t i o n a l and r e s i n - c o a t e d l o w - d e n s i t y i n t e r m e d i a t e - s t r e n g t h p r o p p a n t s where t h e a g g r e g a t i o n o f t h e r e s i n f i l m s a t g r a i n c o n t a c t s p r o v i d e s a s t a b l e wedge- and m a n t l e - l i k e package i n t h e s o f t r e s e r v o i r (TANSOE, K I N G & HOLMAN 1986; ATTARD, MATHES & MOWER 1988; c f . s e c t i o n 4.5.4.2.). The s p e c t r u m o f t r e a t i n g t e c h n i q u e s c a r r i e d o u t i n c h a l k r e s e r v o i r s a l s o includes whisper f r a c t u r i n g ( c f . section 4.5.4.2.1.1.). C h a l k f r a c t u r i n g p r o v i d e s f l o w avenues f r o m t h e c o l l a p s i n g m a t r i x t o t h e w e l l b o r e i f proppant c o n c e n t r a t i o n i s i n excess o f 2 l b s / f t 2 i n o r d e r t o a v o i d embedment i m p a i r m e n t (SIMON, COULTER, K I N G & HOLMAN 1 9 8 2 ) . O i l - b a s e d f l u i d s a r e s u g g e s t e d f o r s t i m u l a t i n g c h a l k i n t e r v a l s w i t h h i g h o i l s a t u r a t i o n , because t h e y do n o t c o n t r i b u t e t o c h a l k r e s e r v o i r s l o s i n g s t r e n g t h n e a r l y as much as w a t e r - b a s e d f l u i d s w h i c h p r o v o k e f l u i d phase and v i s c o s i t y changes t h a t u l t i m a t e l y c o u l d g i v e r i s e t o m a t r i x framework c o l l a p s e i n a s i m i l a r way as r e s e r v o i r d e p l e t i o n leads t o f i e l d subsidence.
519
4.5.4.2.2.2. Impact of stimulation boats on acid vs. proppant fracturing P r i o r t o i n v e n t i o n o f s t i m u l a t i o n boats, l e s s t r e a t m e n t v e r s a t i l i t y due t o space c o n s t r a i n t s on p l a t f o r m s r e s u l t e d i n c o n v e n t i o n a l a c i d s t i m u l a t i o n o f t h e c h a l k (SNOW & HOUGH 1 9 8 8 ) . A c i d t r e a t m e n t has t h e dual e f f e c t o f overcoming n e a r - w e l l b o r e damage and o f p u t t i n g t h e n a t u r a l c r a c k s i n d i r e c t f l u i d communic a t i o n w i t h t h e b o r e h o l e . Proppants were o r i g i n a l l y n o t i n s e r t e d , because i t was expected t h a t proppants would screen o u t o r f l o w back r a t h e r than s t i m u l a t e t h e n a t u r a l f r a c t u r e s . I n areas w i t h p o o r n a t u r a l j o i n t i n g , h y d r a u l i c f r a c t u r i n g o c c u r r e d d u r i n g a c i d i z i n g due t o pressure parting (cf. section 4.5.1.1.1.1.; s i m i l a r p r e s s u r e p a r t i n g t a k e s o c c a s i o n a l l y p l a c e i n g r a v e l packing; c f . s e c t i o n s 5.6.4. and 5 . 6 . 6 . ) .
4.5.4.2.3. Conparat ive effectivity o f proppant and acid fracturing Flow i n t h e near p e r f o r a t i o n r e g i o n i s e x t r e m e l y t u r b u l e n t and v e l o c i t y p r o f i l e s around t h e p e r f o r a t i o n make even a c i d f l o w and e t c h i n g u n l i k e l y i n t h i s area (SNOW & HOUGH 1988). Proppant f r a c t u r i n g i s l i k e l y t o r e s u l t i n g r e a t e r crack l e n g t h s t h a n a c i d f r a c t u r i n g f r o m p r o d u c t i v i t y s t a n d p o i n t . I n i t i a l c r a c k c o n d u c t i v i t i e s a r e comparable o r m o d e r a t e l y l o w e r i n proppant t r e a t m e n t s t h a n i n a c i d j o b s p r i m a r i l y due t o t h e p o t e n t i a l f o r a f i n e s problem. F i n e s a r e gener a t e d b o t h as p r o p p a n t i s f l o w e d i n t o t h e f r a c t u r e , upon e t c h i n g o f t h e f r a c t u r e face, and d u r i n g p r o d u c t i o n . Some aspects o f l o n g e v i t y o f c r e a t e d f r a c t u r e s as w e l l as f o r m a t i o n breakdown and c l o s e d f r a c t u r e a c i d i z i n g a r e i l l u s t r a t e d as f o l l o w s .
4.5.4.2.3.1. Longevity of created fracture Proppant t r e a t m e n t s , however, a r e n o t as e f f e c t i v e i n s t i m u l a t i n g n a t u r a l f r a c t u r e s as a c i d j o b s and may a c t u a l l y c l o g them up o r o t h e r w i s e damage them. As t h e p r i m a r y mechanisms c o n t r i b u t i n g t o t h e l o s s o f c o n d u c t i v i t y a r e t h e same f o r b o t h a c i d and p r o p p a n t o p e r a t i o n s , t h e l o n g e v i t y o f t h e c r e a t e d f r a c t u r e i s n o t g r e a t e r f o r p r o p p a n t j o b s e x c e p t i n case o f an u l t r a - h i g h p r o p p a n t concent r a t i o n where h i g h e r f r a c t u r e c o n d u c t i v i t i e s may be r e t a i n e d f o r a l o n g e r per i o d o f t i m e because t h e f i n e s problem w i l l n o t be as severe. As t h e w e l l s a r e deep, h o t and h i g h l y d e v i a t e d , and a t l e a s t moderate proppant s a t u r a t i o n s a r e r e q u i r e d , t h e p o s s i b i l i t y o f proppant d r o p o u t i n t h e w e l l b o r e i s h i g h . Once t h e proppant e n t e r s t h e f r a c t u r e , t h e r i s k o f screenout i s high, because t h e c r a c k may bend and t u r n , and s i n c e l e a k o f f t o n a t u r a l f r a c t u r e s may be h i g h . G r e a t e r f r a c t u r e l e n g t h s t h a n can be a t t a i n e d w i t h a c i d a r e o n l y r e q u i r e d i n t h e l o w - p e r m e a b i l i t y r e s e r v o i r p o r t i o n s , whereas optimum f r a c t u r e l e n g t h s i n t h e moderate- t o h i g h - p e r m e a b i l i t y l o w - n a t u r a l c r a c k areas a r e a l s o o b t a i n a b l e w i t h a c i d s . Thus a t t h e bottom o f t h e l i n e , a c i d f r a c t u r i n g may be s u p e r i o r i n moderate- t o h i g h - p e r m e a b i l i t y r e g i o n s , whereas p r o p p a n t f r a c t u r i n g i s p r e f e r e n t i a l i n l o w - p e r m e a b i l i t y areas o f t h e c h a l k o i l f i e l d s . Proppant s t i m u l a t i o n s h o u l d be c a r r i e d o u t w i t h moderate c o n c e n t r a t i o n and w i t h p o s s i b l y some a c i d ahead o f t h e p r o p p a n t l o t t o e t c h t h e n a t u r a l f r a c t u r e s depending on c h a l k char a c t e r i s t i c s . P s e u d o l i m i t e d - e n t r y a c i d f r a c t u r i n g ( w h i s p e r f r a c t u r i n g ; c f . sect i o n 4.5.4.2.1.1.) has t u r n e d o u t t o be t h e most s u i t a b l e s t i m u l a t i o n t e c h n i q u e f o r the chalk r e s e r v o i r s i n the Ekofisk f i e l d .
4.5.4.2.3.2. Formation breakdown and closed fracture acidizing H i g h - r a t e a c i d i n j e c t i o n l e a d s t o f o r m a t i o n breakdown i n a l l p a r t s o f t h e f i e l d i n c l u d i n g areas w i t h narrow s p a c i n g o f n a t u r a l c r a c k s (SNOW & HOUGH
520 1988). W h i l e some o f t h e breakdown i s caused by n a t u r a l j o i n t opening, hydraul i c f r a c t u r i n g due t o p r e s s u r e p a r t i n g accounts f o r much o f t h e breakdown i n many r e g i o n s ( c f . s e c t i o n s 4.5.1.1.1.1. and 4.5.4.2.2.). Increased i n j e c t i o n r a t e s have i n c r e a s e d a c i d p e n e t r a t i o n by c r e a t i n g h i g h e r p r e s s u r e s i n t h e induced f r a c t u r e s t h u s k e e p i n g them open and w i d e n i n g them, and b y l i m i t i n g l e a k o f f . The spectrum o f c h a l k a c i d s t i m u l a t i o n a l s o i n c l u d e s c l o s e d f r a c t u r e a c i d i z i n g c o m p r i s i n g opening o f t h e c r a c k by t h e pad f o l l o w e d by p e r m i t t i n g i t t o c l o s e and t o s t a y p a r t i a l l y open by a u t o - p r o p p i n g ( c f . s e c t i o n s 4.5.1.1.1.2. and 4 . 5 . 1 . 5 . ) , w i t h t h e n t h e a c i d b e i n g i n j e c t e d i n t o t h e p a r t i a l l y open c r a c k below f r a c t u r e p r e s s u r e and e t c h i n g much more s e l e c t i v e l y . There i s , however, t h e r i s k t h a t t h e a c i d may r e a c t q u i c k l y i n t h e n e a r - w e l l b o r e r e g i o n and equal i z e t h e uneven r i d g e s , t h e r e b y d e c r e a s i n g r a t h e r t h a n i n c r e a s i n g f r a c t u r e cond u c t i v i t y o r even c o n t r i b u t i n g t o c a s i n g f a i l u r e s . I n o r d e r t o a v o i d c a s i n g c o l l a p s e by l a r g e c a v i t i e s f o r m i n g i m m e d i a t e l y around t h e b o r e h o l e , i t i s necessar y t o maximize a c i d v e l o c i t y i n t h e n e a r - w e l l b o r e r e g i o n so as t o p l a c e t h e a c i d f a r o u t i n t o t h e f o r m a t i o n and m i n i m i z e e x c e s s i v e n e a r - w e l l b o r e e t c h i n g .
4.5.4.3. Combination of hydraulic fracturing and gravel packing S p e c i a l t e c h n i q u e s i n c l u d i n g c o m b i n a t i o n s o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g o r p r o p p a n t and a c i d f r a c t u r i n g as w e l l as v a r i o u s r e s e r v o i r engineer i n g p e c u l i a r i t i e s considerably enlarge the p o s s i b i l i t i e s o f hydraulic proppant f r a c t u r i n g o f carbonate r o c k s . The most s p e c t a c u l a r case o f a p p l i c a t i o n o f comb i n e d h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g i s t h e V a l h a l l f i e l d (ATTARD, MATHES & MOWER 1988) where t h e T o r F o r m a t i o n which c o n t a i n s a b t . 2/3 o f t h e o i l i n - p l a c e has e x t r e m e l y h i g h p o r o s i t y ( a b t . 40 - 50 % ) as a consequence o f t h e o v e r p r e s s u r e d n a t u r e o f t h e r e s e r v o i r . A f t e r d i s c u s s i o n o f some g e n e r a l asp e c t s , t h e o u t l i n e as f o l l o w s c o n c e n t r a t e s on up-and-under s t i m u l a t i o n vs. comb i n e d h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l packing, d e s i g n and purpose o f c h a l k g r a v e l packing, and w e l l b o r e c l e a n o u t e f f e c t i v i t y .
4.5.4.3.1.
General aspects
The i n a b i l i t y o f f o r m a t i o n f l u i d s t o escape as t h e c h a l k was b u r i e d r e s u l t e d i n l a c k o f compaction t h e r e b y p r e s e r v i n g h i g h e r p o r o s i t y t h a n i n o t h e r f i e l d s and s t r a t i g r a p h i c a l l e v e l s (ATTARD, MATHES & MOWER 1988). The h i g h p o r o s i t y i n c o m b i n a t i o n w i t h l a c k i n g c e m e n t a t i o n between t h e c h a l k g r a i n s g i v e s r i s e t o a v e r y weak m a t r i x which i s prone t o c o l l a p s e and t o p l a s t i c f l o w when s u b j e c t e d t o s t r e s s e s such as l a r g e p r e s s u r e drawdowns f r o m t h e pay zone i n t o t h e w e l l bore ( c f . s e c t i o n 4 . 5 . 4 . 1 . 3 . ) . I n c o n t r a s t t o t h e T o r Formation, t h e l o w e r Hod F o r m a t i o n c o n t a i n i n g t h e r e m a i n i n g 1 / 3 o f t h e o i l - i n - p l a c e i s a much s t r o n g e r and more t y p i c a l l i m e s t o n e r e s e r v o i r . D e s p i t e t h e h i g h p o r o s i t y , t h e p e r m e a b i l i t y o f t h e T o r F o r m a t i o n i n t h e V a l h a l l f i e l d i s r e l a t i v e l y low ( a b t . 1 - 10 md) due t o t h e v e r y small s i z e o f t h e c a l c i t e p a r t i c l e s composing t h e r o c k . N a t u r a l f r a c t u r e s i n c r e s t a l p l a c e s can i n c r e a s e p e r m e a b i l i t y up t o 100 md. The o n l y p o s s i b l e c o m p l e t i o n t e c h n i q u e f o r economical p r o d u c t i o n o f t h e T o r F o r m a t i o n i n t h e V a l h a l l f i e l d i s h y d r a u l i c p r o p p a n t f r a c t u r i n g f o l l o w e d by g r a v e l packing, w i t h b o t h t r e a t m e n t s b e i n g c a r r i e d o u t w i t h r e s i n - c o a t e d alumina s i l i c a t e proppants i n most stages ( o n l y t h e h y d r a u l i c f r a c t u r i n g j o b i s i n i t i a t e d w i t h c o n v e n t i o n a l alumina s i l i c a t e proppants f o r c o s t c o n t a i n m e n t purpos e s ) . As above c e r t a i n c o n f i n i n g s t r e s s e s t h e Tor F o r m a t i o n f a i l s by e n t e r i n g mass f l o w o r p l a s t i c f l o w mode i n s t e a d o f m i g r a t i o n o f i n d i v i d u a l p a r t i c l e s and because t h e presence of w a t e r r e s u l t s i n more r a p i d f a i l u r e o f t h e c h a l k a t l o wer n e t c o n f i n i n g pressures, wide f r a c t u r e s w i t h h i g h p r o p p a n t c o n c e n t r a t i o n s t h a t a r e aggregated t o g e t h e r by r e s i n f i l m bonding ( c f . s e c t i o n 1 . 2 . 6 . ) a r e i n d i s p e n s i b l e t o p r o v i d e s t a b l e f l o w c a p a c i t y a t h i g h drawdowns, and an a d d i t i o n a l g r a v e l pack m a n t l e guarantees s a t i s f a c t o r y and r e l i a b l e c o n s o l i d a t i o n o f t h e c h a l k r e s e r v o i r i n t h e immediate v i c i n i t y o f t h e w e l l b o r e .
52 1
4.5.4.3 2. up-and-under s t mulat i o n vs. combined hydraul c proppant f r a c t u r ng and gravel packing I n i t i a l l y developments were f r a c t u r e d up i n t o the Tor Formation through perf o r a t i o n s i n the Hod Formation, w i t h the reasoning o f t h i s up-and-under t e c h n i que having been t h a t most o f the pressure drop through the w e l l b o r e d u r i n g prod u c t i o n would be experienced by t h e more competent Hod Formation and the weaker Tor Formation would n o t f a i l and c o l l a p s e ( s i m i l a r uphole f r a c t u r i n g i s p e r f o r med o c c a s i o n a l l y i n c o a l - b e a r i n g s t r a t a , w i t h an u n d e r l y i n g sandstone g u i d i n g the crack i n t e r s e c t i n g the coal seam; c f . s e c t i o n 4.4.3.8.1.). High-rate wells, however, s u f f e r e d subsequently from massive f l o w o f proppants and c h a l k when drawn down below r e l a t i v e l y h i g h wellhead f l o w i n g pressures, and newly d r i l l e d w e l l s became more and more d i f f i c u l t t o complete w i t h Hod t o Tor f r a c t u r i n g as the r e s e r v o i r pressure d i f f e r e n t i a l between Tor and Hod formations increased owing t o f a s t e r d e p l e t i o n o f t h e more p r o d u c t i v e Tor Formation (ATTARD, MATHES & MOWER 1988). Therefore the combination o f f r a c t u r i n g and gravel packing was i n t r o d u c e d i n c l u d i n g a two-stage gravel pack c o n s i s t i n g o f fracture/prepack ( c f . s e c t i o n 5.6.7.) f o l l o w e d by an annulus pack w i t h screen and l i n e r ( c f . s e c t i o n High s h o t d e n s i t y and l a r g e average p e r f o r a t i o n diameter ( c f . s e c t i o n 5.10.1.). 5.6.1.) were s e l e c t e d t o p r o v i d e a l a r g e f l o w area f o r p r o d u c t i o n and thus m i n i mize the pressure drop across t h e gravel pack. The h y d r a u l i c proppant f r a c t u r i n g treatment i s performed t o c r e a t e a wide h i g h - c o n d u c t i v i t y crack through the Tor Formation w i t h as many o f the p e r f o r a t i o n s as p o s s i b l e packed w i t h proppants thereby a l s o a c t i n g as a prepack t o precede the a c t u a l g r a v e l pack j o b . The d e s i r e d f r a c t u r e c o n s t e l l a t i o n i s achieved by t i p screenout o p e r a t i o n ( c f . s e c t i o n 4.5.4.4.1.). Aggregation o f the proppant wedge by r e s i n bonding ( c f . s e c t i o n 1.2.6.) had t h e s p e c i a l a i m t o minimize proppant flowback d u r i n g w e l l cleanup and p r o d u c t i o n ( c f . s e c t i o n 4.12.3.3.).
4.5.4.3.3.
Design and purpose o f chalk gravel packing
The purpose of chalk gravel packing was t o prevent mass f l o w o f c h a l k i n t o the w e l l b o r e through p e r f o r a t i o n s l e f t u n f i l l e d a f t e r f r a c t u r i n g and t o r e s t r a i n t h e f r a c t u r e l p r e p a c k proppant p l u g s (ATTARD, MATHES & MOWER 1988). 20/40 mesh gravel g r a i n s i z e was chosen because t e s t i n g had revealed t h a t t h i s s i z e e f f e c t i v e l y r e s t r a i n s the f r a c t u r e d chalk w h i l e r e t a i n i n g s u b s t a n t i a l hydrocarbon f l o w c a p a c i t y . Gravel g r a i n s i z e design f o l l o w i n g the standard r u l e s ( c f . s e c t i o n 5.2.3.) would r e s u l t i n a p r o h i b i t i v e l y small gravel g r a i n s i z e f o r ens u r i n g stoppage o f the t i n y c h a l k p a r t i c l e s , which would be simply a waste o f e f f e c t because p l a s t i c c h a l k f l o w occurs as a cohesive mass r a t h e r than as single grains. As a consequence o f r e s e r v o i r depth, borehole d e v i a t i o n and n e c e s s i t y t o m i nimize b r i n e l o s s , a o n e - t r i p c i r c u l a t i o n s l u r r y g r a v e l pack was s e l e c t e d as app r o p r i a t e technique, and a t o t a l amount o f 3,000 - 4,000 l b s (1,300 - 1,800 kg) o f alumina s i l i c a t e gravel was pumped w i t h a maximum gravel c o n c e n t r a t i o n o f 7 . 5 l b s / g a l (900 kg/m3) i n o r d e r t o a v o i d gravel b r i d g i n g i n the small clearance between l i n e r and g r a v e l pack assembly ( c f . s e c t i o n 5.2.3.4.). According t o the h i g h r e s e r v o i r pressure and the water s e n s i t i v i t y o f the chalk, the g r a v e l pack had t o be c a r r i e d o u t under pressure u s i n g a snubbing u n i t and w i t h d i e s e l as t h e work f l u i d . Opening o f as much p e r f o r a t i o n s as p o s s i b l e p r i o r t o f r a c t u r i n g and g r a v e l packing was achieved by breakdown w i t h a c i d o r g e l l e d d i e s e l and b a l l s e a l e r s f o r d i v e r s i o n ( c f . s e c t i o n 4.2.2.1.1.).
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4.5 -4.3.4.We1 lbore cleanout effectivity The l i g h t alumina s i l i c a t e proppants ( c f . s e c t i o n 1 . 3 . 2 . 1 . ) were chosen i n s t e a d o f sand t o i n c r e a s e f r a c t u r e c o n d u c t i v i t y and i n s t e a d o f o t h e r h e a v i e r s y n t h e t i c proppants t o achieve an e f f e c t i v e b o r e h o l e c l e a n o u t p r i o r t o r u n n i n g t h e g r a v e l pack equipment (ATTARO, MATHES & MOWER 1988). W e l l b o r e c l e a n o u t w i t h p r o p p a n t removal can b e s t be performed w i t h more v i s c o u s f r a c t u r i n g g e l i n s t e a d o f t h i n d i e s e l . I n c r e a s i n g f l u i d v e l o c i t y and improvement o f b o r e h o l e c l e a n o u t e f f i c i e n c y was reached by r e v e r s e c i r c u l a t i o n d u r i n g g r a v e l p a c k i n g ( c f . sect i o n 5 . 3 . 2 . 3 . ) . R e p e r f o r a t i o n s and g r a v e l p a c k i n g under p r e s s u r e i n an a l r e a d y completed i n t e r v a l a l s o r e s u l t e d i n s a t i s f a c t o r y p r o d u c t i v i t i e s . As a consequence o f t h e s u c c e s s f u l performance o f these combined t r e a t m e n t s , t h e w e l l s a r e p r o d u c i n g a t t h e expected f l o w r a t e s w i t h no s i g n s o f b o r e h o l e damage and a t much l o w e r f l o w i n g w e l l h e a d p r e s s u r e than w i t h o u t s t i m u l a t i o n , r e s u l t i n g i n c o r respondingly higher withdrawal r a t e s .
4.5.4.4. Spec ia 1 cha 1 k fracturing techniques As t h e c h a l k i s a p e c u l i a r r e s e r v o i r r o c k w i t h s p e c i a l p r o p e r t i e s , a l s o hyd r a u l i c f r a c t u r i n g s t i m u l a t i o n has t o a p p l y unusual methods i n o r d e r t o successf u l l y t r e a t t h i s t y p e o f pay f o r m a t i o n . Some s p e c i a l c h a l k s t i m u l a t i o n t e c h n i ques such as t i p s c r e e n o u t f r a c t u r i n g , i n t e r f a c e f r a c t u r i n g , and d e n d r i t i c f r a c t u r i n g a r e o u t l i n e d as f o l l o w s ,
4.5.4.4.1. Tip screenout fracturing T i p s c r e e n o u t f r a c t u r i n g i s a s p e c i a l technique f o r c r e a t i n g s h o r t wide h i g h - c o n d u c t i v i t y f r a c t u r e s (SMITH, MILLER & HAGA 1987). The d e s i g n o f t h i s method i s based on i n t e n t i o n a l l y s c r e e n i n g o u t t h e t i p o f t h e f r a c t u r e w i t h sand and then c o n t i n u i n g t o pump s l u r r y i n o r d e r t o i n c r e a s e f r a c t u r e w i d t h and t o f i l l t h e c r a c k w i t h p r o p p a n t m u l t i l a y e r s t o o b t a i n t h e necessary h i g h c o n d u c t i v i t y ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) . The f l u i d - l e a k o f f c o e f f i c i e n t o f t h e f o r m a t i o n has t o be c a r e f u l l y determined by a m i n i f r a c t u r i n g c a l i b r a t i o n j o b ( c f . s e c t i o n 4 . 8 . 2 . ) b e f o r e t h e main t r e a t m e n t , because t h i s s p e c i a l t e c h n i q u e i n v o l v e s severe r i s k o f premature s c r e e n o u t ( c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) and f a i l u r e t o a c h i e ve t h e planned t i p screenout w i l l n o t y i e l d t h e d e s i r e d a m e l i o r a t i o n o f product i o n r a t e ( c f . also section 4.8.5.3.). W h i l e t i p s c r e e n o u t i s c h a r a c t e r i z e d by p l u g g i n g o f t h e f r a c t u r e f r o n t w i t h proppants o r s o l i d f l u i d - l o s s a d d i t i v e s , r a p i d f l u i d l e a k o f f o f p r o p p a n t - l a d e n stages and c o r r e s p o n d i n g i n c r e a s i n g p r o p p a n t c o n c e n t r a t i o n s i n t h e s l u r r y w i t h i n t h e f r a c t u r e g i v e r i s e t o d e h y d r a t i o n screenout (WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE 1988; c f . s e c t i o n s 4.2.3.5.3. and 6 . 2 . 4 . 2 . 1 . ) . T i p screenout f r a c t u r i n g i s e s p e c i a l l y s u i t a b l e f o r t h e c h a l k i n t h e V a l h a l l f i e l d which i s s o f t e r , more w a t e r - s e n s i t i v e , and more l i k e l y t o undergo p l a s t i c d e f o r m a t i o n t h a n t h e c h a l k i n t h e E k o f i s k f i e l d (SMITH, MILLER & HAGA 1987; ATTARO, MATHES & MOWER 1 9 8 8 ) . I n o r d e r t o o b t a i n h i g h p r o p p a n t s a t u r a t i o n s and t o h e l p r e s t r a i n t h e c h a l k f r o m f l o w i n g i n on t h e propped f r a c t u r e , an engineered s c r e e n o u t i s provoked, r e s u l t i n g i n h i g h e r p r e s s u r e s w i t h i n t h e f r a c t u r e w h i c h cause t h e crack t o widen. As t h e s c r e e n o u t c o n t i n u e s , t h e w i d e r f r a c t u r e i s packed f u l l o f proppant, t h e r e b y g i v i n g r i s e t o a l a r g e w e l l - p a c k e d c r a c k (SNOW & HOUGH 1988).
4.5.4.4.2. Interface fractur ins I n t e r f a c e f r a c t u r i n g i s a special technique o f c r e a t i n g hydraulic cracks near o r a t t h e i n t e r f a c e o f two f o r m a t i o n s w i t h d i f f e r e n t l e a k o f f c h a r a c t e r i s t i c s and mechanical p r o p e r t i e s f o r t h e purpose o f s t a b i l i z a t i o n o f N o r t h Sea c h a l k c o m p l e t i o n s (MOSCHOVIDIS 1 9 8 7 ) . The p r i n c i p a l o i l - b e a r i n g zone w h i c h i s
523 p o o r l y c o n s o l i d a t e d and c r e a t e s s o l i d s p r o d u c t i o n problems i s b r o u g h t i n commun i c a t i o n w i t h t h e u n d e r l y i n g more competent c h a l k zone t h a t i s a l s o o i l - b e a r i n g v i a a hydraulic f r a c t u r e i n order t o minimize i n f l o w o f formation f i n e s ( c f . a l s o s e c t i o n s 4.5.4.3. and 4.4.3.8.1.). Good t r e a t m e n t r e s u l t s w i t h high-conduct i v i t y f r a c t u r e s a r e achieved by a screenout towards t h e end o f t h e o p e r a t i o n . I n t h i s combined h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a p p l i c a t i o n , t h e volume c u r v e s i n t e r s e c t towards t h e end o f t h e treatment, b u t a c o n s i d e r a b l e sand volume has been pumped above t h e s i m u l a t e d crack c a p a c i t y , w i t h a p o s s i b i l i t y o f e x p l a n a t i o n o f t h i s r e l a t i o n s h i p b e i n g t h a t t h e s i m u l a t i o n s do n o t account f o r any b a l l o o n i n g of t h e f r a c t u r e due t o h i g h e r t r e a t i n g p r e s s u r e caused by t o t a l o r p a r t i a l c r a c k p r o p a g a t i o n a r r e s t . H i g h e r o p e r a t i n g p r e s s u r e s t h a n normal a r e observed towards t h e end o f t h e f r a c t u r i n g j o b s and a r e i n t e n d e d t o i n c r e a se c r a c k w i d t h . The i n t e r s e c t i o n p o i n t s o f g r a v e l pack and f r a c t u r e volume c u r ves determine t h e s l u r r y volume a t which a g l o b a l s c r e e n o u t occurs, w i t h c o r r e c t i o n s h a v i n g t o be made f o r t h e p o r o s i t y o f t h e p r o p p a n t package when dehydrat e d a t s c r e e n o u t c o n d i t i o n s , t h e a c c e s s i b i l i t y o f t h e f r a c t u r e volume t o t h e proppants, and i n c r e a s i n g c r a c k w i d t h due t o p l a s t i c d e f o r m a t i o n and n o n - l i n e a r i t y o f r e s e r v o i r behaviour.
4.5.4.4.3. Dendr i t ic f ractur ins Another s p e c i a l o p e r a t i o n a l procedure r e p r e s e n t i n g a c o m b i n a t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r i n g i s d e n d r i t i c f r a c t u r i n g where t h e t r e a t m e n t i s c o n f i ned t o a l r e a d y e x i s t i n g n a t u r a l c r a c k s by u s i n g l a r g e volumes o f l i g h t l y g e l l e d w a t e r pumped a t h i g h r a t e s and v e l o c i t i e s , t h e r e b y a v o i d i n g c r e a t i o n o f new f r a c t u r e s as more v i s c o u s f l u i d s would (LOPUS, SEIFERT & SCHEIN 1987; c f . sect i o n 4.8.8.4.). V a r i o u s stages o f t h e t r e a t m e n t i n c o r p o r a t e small amounts o f proppants i n o r d e r t o a i d i n a b r a s i o n o f e x i s t i n g f r a c t u r e s plugged by secondar y m i n e r a l p r e c i p i t a t i o n s as w e l l as f l u i d - l o s s c o n t r o l , o r b r i d g i n g agent t o cause s l i g h t changes i n d i f f e r e n t i a l p r e s s u r e w i t h i n a f r a c t u r e t o a s s i s t i n d i v e r t i n g i n t o other f r a c t u r e s ( c f . section 6.2.1.3.4.5.). After briefly outlin i n g some g e n e r a l aspects, t h e s p e c i a l c o n s t e l l a t i o n o f combined d e n d r i t i c n a t u r a l and h y d r a u l i c f r a c t u r i n g i s i l l u s t r a t e d . Some r e f r a c t u r i n g aspects a r e a l s o sketched.
4.5.4.4.3.1. General aspects O e n d r i t i c f r a c t u r i n g i s c a r r i e d o u t by i n j e c t i o n o f a t h i n water-based f l u i d a t h i g h r a t e s i n t o t h e f o r m a t i o n ( K I E L 1976, 1977, 1984), and pumping i s t h e n stopped and t h e w e l l i s a l l o w e d t o f l o w back, w i t h t h i s i n j e c t i o n / f l o w b a c k c y c l e b e i n g r e p e a t e d s e v e r a l t i m e s . The process causes i n - s i t u m a t e r i a l t o break o f f f r o m t h e f r a c t u r e f a c e and lodge i n s i d e o f t h e c r a c k t h e r e b y p r o p p i n g i t open (MURPHY & FEHLER 1986). D e n d r i t i c f r a c t u r i n g t h u s i n c l u d e s a u t o - p r o p p i n g o f t h e c r a c k w a l l s by f o r m a t i o n fragments and broken i n f i l l i n g o f h e a l e d j o i n t s ( c f . s e c t i o n s 4.5.1.1.1. and 4 . 5 . 1 . 5 . 1 . ) . D e n d r i t i c f r a c t u r i n g ( c f . a l s o s e c t i o n 4 . 8 . 8 . 4 . ) has been s u c c e s s f u l l y c a r r i e d o u t i n t h e A u s t i n Chalk (Texas/ USA) and i n t h e E k o f i s k c h a l k (Norwegian N o r t h Sea) as w e l l as i n v a r i o u s o t h e r n a t u r a l l y f r a c t u r e d carbonate r e s e r v o i r s .
4.5.4.4.3.2. Combinat ion o f dendr i t ic natural and hydraulic fracturing D e n d r i t i c h y d r a u l i c f r a c t u r i n g i s p a r t i c u l a r l y s u i t a b l e f o r c h a l k s which a r e c h a r a c t e r i z e d by an a l r e a d y d e n d r i t i c n a t u r a l f r a c t u r e system as a consequence o f t e c t o n i c a l f o r c e s such as subsidence, u p l i f t and i n t r u s i o n h a v i n g been app l i e d a t d i f f e r e n t t i m e s b o t h r e g i o n a l l y and l o c a l l y on an a l t e r e d s t r e s s f i e l d (HUBBARD & PIERSON 1986; c f . s e c t i o n 4 . 8 . 8 . 1 . ) . I n some c h a l k s , t h e d e n d r i t i c n a t u r a l f r a c t u r e system i s t h e hydrocarbon r e s e r v o i r w i t h l i t t l e o r no m a t r i x
524 p o r o s i t y c o n t r i b u t i o n ( c f . s e c t i o n 4 . 5 . 4 . 1 . 4 . ) . As t h e n a t u r a l c h a l k f r a c t u r e s a r e o f t e n o n l y p a r t i a l l y c o n d u c t i v e and can be plugged beyond r e p a i r by d r i l l i n g mud and cement, and can a l s o be i n h e r e n t l y f i l l e d w i t h c a l c i t e and s a l t , d e n d r i t i c hydraulic f r a c t u r i n g i s a s u i t a b l e s t i m u l a t i o n technique. D e n d r i t i c h y d r a u l i c f r a c t u r i n g u t i l i z e s l a r g e volumes o f l i g h t l y g e l l e d f r e s h w a t e r pumped down t h e c a s i n g a t h i g h v e l o c i t y ( c f . a l s o s e c t i o n 4 . 8 . 8 . 4 . ) . The t h i n f l u i d w i t h i t s h i g h l e a k o f f r a t e seeks f i n e h a i r l i n e c r a c k s r a t h e r t h a n propagat i n g and w i d e n i n g m a j o r f r a c t u r e s as more v i s c o u s f l u i d s would do. Small q u a n t i t i e s o f p r o p p a n t s a r e used as a b a n k - b r i d g i n g medium. Since t h e t h i n f l u i d i s n o t a p e r f e c t t r a n s p o r t system, t h e p r o p p a n t s a r e banked i n t h e f r a c t u r e s r e p r e s e n t i n g h i g h e r - v e l o c i t y streams u n t i l v e l o c i t y i s no l o n g e r adequate t o t r a n s p o r t t h e p r o p p a n t s . A t t h i s p o i n t , t h e p r o p p a n t s a c t t o c r e a t e a h i g h e r p r e s s u r e d i f f e r e n t i a l a l o n g exposed c r a c k f a c e s t h a t subseq u e n t l y p e r m i t s f l u i d and p r o p p a n t s t o be c a r r i e d i n t o o t h e r f r a c t u r e s as t h e y open. The process i s c o n t i n u e d u n t i l f u r t h e r d i v e r s i o n w i t h i n t h e f o r m a t i o n i s no l o n g e r p o s s i b l e w i t h proppants alone. A t t h i s stage, temporary d i v e r t i n g I t i s n o t unagents such as r o c k s a l t a r e i n t r o d u c e d ( c f . s e c t i o n 4 . 2 . 2 . 1 . 1 . ) . usual t h a t more t h a n one temporary p l u g i s r e q u i r e d f o r d i v e r s i o n , and s e v e r a l may be u t i l i z e d t h r o u g h o u t t h e t r e a t m e n t ( c f . s e c t i o n 6.2.1.3.4.5.). Since the purpose o f temporary p l u g s i s t o achieve d i v e r s i o n away f r o m t h e w e l l b o r e , r e l a t i v e l y small-volume p l u g s a r e u t i l i z e d , and t h e procedure i s r e p e a t e d i n stages u n t i l changes i n bottomhole p r e s s u r e i n d i c a t e e x c e s s i v e f r a c t u r e growth.
4.5.4.4.3.3. Ref ractur ing aspects Refracturing o f a conventionally stimulated well requires only s l i g h t modific a t i o n o f t h e procedure. Standard t r e a t m e n t s c a r r y l a r g e p r o p p a n t q u a n t i t i e s and a t t e m p t t o c r e a t e and pack f r a c t u r e s . Thus an e f f i c i e n t f i l l e r system i s p r o v i d e d f o r d e p o s i t i o n o f eroded f i n e s as w e l l as a c c u m u l a t i o n and r e d e p o s i t i o n o f s a l t and c a l c i t e . H y d r o c h l o r i c a c i d i s u t i l i z e d ahead o f each f r a c t u r i n g stage t o remove c a l c i t e p r e c i p i t a t i o n f r o m t h e e a r l i e r p r o p p a n t packages, thus a l l o w i n g t h e o r i g i n a l p r o p p a n t i n f i l l i n g o f t h e c r a c k t o g e t u n c o n s o l i d a t e d by d e s t r u c t i o n and d i s s o l u t i o n o f t h e secondary c a l c i t e cement and t o be t r a n s p o r t e d away f r o m t h e w e l l b o r e which r e q u i r e s somewhat l a r g e r h i g h - v e l o c i t y pads ( c f . s e c t i o n 4 . 8 . 9 . ) .
4.5.4.4.4. Other aspects of chalk fracturing N a t u r a l f r a c t u r e s i n t h e c h a l k o f t h e E k o f i s k f i e l d s a r e s h o r t , o c c u r most f r e q u e n t l y i n t h e reworked i n t e r v a l s , and a r e non-continuous f r o m t o p t o b o t t o m o f t h e s e c t i o n s (BREWSTER, DANGERFIELO & FARRELL 1986; FARRELL 1988, SNOW & HOUGH 1988). Because o f t h e i r d i s c o n t i n u o u s n a t u r e , h y d r a u l i c f r a c t u r e p e n e t r a t i o n i s o f t e n b e n e f i t i n g p r o d u c t i o n beyond a n t i c i p a t i o n by e n c o u n t e r i n g n a t u r a l c r a c k s . F l u i d l e a k o f f c o e f f i c i e n t s can be e x t r e m e l y h i g h because o f i n t e n s e nat u r a l j o i n t i n g , l a r g e c a p i l l a r y f o r c e s , and q u i c k a c i d - c h a l k r e a c t i o n r a t e s . I n duced f r a c t u r e growth i s g e n e r a l l y v e r t i c a l and r a d i a l , w i t h l o c a l changes i n s t r e s s e s n o t b e i n g l a r g e enough t o c o n f i n e f r a c t u r e h e i g h t , a l t h o u g h t h e c r a c k s t e n d t o grow commonly upwards due t o t h e i n - s i t u s t r e s s g r a d i e n t . D e v i a t i o n s f r o m t h i s i d e a l f r a c t u r e geometry occur f r e q u e n t l y due t o s e v e r a l reasons. As t h e w e l l s a r e h i g h l y i n c l i n e d up t o 60 degrees i n o r d e r t o access numerous l o c a t i o n s by extended-reach d r i l l i n g ( c f . s e c t i o n 4.8.6.1.3.) from the platforms, a t l e a s t i n t h e r e g i o n near t h e w e l l b o r e f r a c t u r e g r o w t h w i l l n o t be i d e a l , b u t w i l l e i t h e r r e s u l t i n m u l t i p l e f r a c t u r e s o r i n a c o n t i n u o u s o r en-echelon bendi n g o f t h e c r a c k t o v e r t i c a l p o s i t i o n (STRUBHAR, FITCH & GLENN 1975; SNOW & HOUGH 1988; c f . s e c t i o n 4 . 1 1 . 1 . ) .
525
4.5.4.5. Water imbibition and oi 1 recovery Water i m b i b i t i o n p l a y s an important r o l e i n o i l recovery from c h a l k format i o n s b o t h through water f l o o d i n g w i t h i n j e c t i o n i n one w e l l and p r o d u c t i o n from another through m a t r i x and n a t u r a l cracks, and water f r a c t u r i n g by pumping water down w i t h a pressure exceeding the r e s e r v o i r f r a c t u r e g r a d i e n t and t a k i n g o f f the f l u i d from the same w e l l . I n a water-wet r e s e r v o i r such as t h e chalk, i m b i b i t i o n i s the process by which o i l r e p r e s e n t i n g the non-wetting phase i s d i s p l a c e d by water being the w e t t i n g phase (OEN, ENGELL-JENSEN & BARENDREGT 1988). Transverse i m b i b i t i o n occurs i n f r a c t u r e d r e s e r v o i r s ( d o u b l e - p o r o s i t y rocks; BECKNER, ISHIMOTO, YAMAGUCHI, FIROOZABADI & A Z I Z 1987; BECKNER, FIROOZABAD1 & A Z I Z 1988). BECH, JENSEN & NIELSEN (1989) comment on g r a v i t y - i m b i b i t i o n and g r a v i ty-drainage processes. WENDEL, KUNKEL & SWANSON (1988) r e p o r t spontaneous water i m b i b i t i o n i n d i a t o m i t e r e s e r v o i r s which are rock-mechanically q u i t e comparable t o chalks. D i a t o m i t e s and diatomaceous e a r t h s can be successf u l l y s t i m u l a t e d w i t h h i g h proppant concentrations up t o 20 l b s / g a l (STRUBHAR, MEDLIN, NAB1 & ANDREANI 1984; STRICKLAND 1985). The o u t l i n e as f o l l o w s focusses on water f l o o d i n g and water f r a c t u r i n g . Some comments are a l s o o f f e r e d on foam fracturing.
4.5.4.5.1. Water flooding Concerning secondary o i l recovery, n a t u r a l l y f r a c t u r e d c h a l k r e s e r v o i r s w i l l produce under a displacement process d u r i n g w a t e r f l o o d i n g , i n c o n t r a s t t o sandstones which r e l y on a p i s t o n - l i k e a c t i o n o f the water (OFFSHORE ENGINEER 1985 b ) . I n the very l o w - p e r m e a b i l i t y chalk, the primary movement o f f l u i d s i s through the n a t u r a l cracks r a t h e r than the a c t u a l rock, a l l o w i n g o i l t o be d i s placed from the rock as the water i s p r e f e r e n t i a l l y absorbed. A p a r t from t h i s water i m b i b i t i o n ( c f . MATTAX & KYTE 1962, AGUILERA 1975, MENOUAR & KNAPP 1980, C H I E R I C I 1984, TORSAETER 1984, BALDWIN 1986, WU & PRUESS 1986; THOMAS, DIXON, EVANS & VIENOT 1987; OEN, ENGELL-JENSEN & BARENDREGT 1988), the i n j e c t e d water can move t o o f a s t through the f r a c t u r e system thus bypassing much o f the format i o n and q u i c k l y breaking through t o the w e l l b o r e s . These e f f e c t s a l s o have t o be taken under c o n s i d e r a t i o n when designing h y d r a u l i c proppant and a c i d f r a c t u r i n g s t i m u l a t i o n i n the c h a l k . I n the N o r t h Sea, the chalk i n the E k o f i s k f i e l d i s more s t a b l e and o i l recovery can be enhanced by w a t e r f l o o d i n g through the n a t u r a l crack system w i t h o u t n e c e s s a r i l y i n v o l v i n g h y d r a u l i c f r a c t u r i n g and g r a v e l packing, whereas the c h a l k i n the V a l h a l l f i e l d can o n l y be brought t o economically f e a s i b l e product i o n by the s p e c i a l technique o f combined h y d r a u l i c proppant f r a c t u r i n g and grav e l packing w i t h b o t h conventional and resin-coated low-density i n t e r m e d i a t e s t r e n g t h alumina s i l i c a t e proppants ( c f . s e c t i o n 4.5.4.3.). Water i m b i b i t i o n d i s p l a c i n g and e x p u l s i n g o i l as the dominant p r o d u c t i o n mechanism i n f r a c t u r e d c h a l k r e s e r v o i r s t h a t are subjected t o secondary water f l o o d i n g i s l a r g e l y depending on r e s e r v o i r w e t t a b i l i t y being the magnitude o f f l u i d - s u r f a c e i n t e r a c t i o n (BALDWIN 1986). Water i m b i b i t i o n can a l s o s i g n i f i c a n t l y be a f f e c t e d by the removal o f small amounts o f t i g h t l y bound organic coati n g s on t h e chalk surfaces. The h i g h e r the l e v e l o f o x i d a t i o n o f the most t i g h t l y bound m a t e r i a l , the g r e a t e r i s water i m b i b i t i o n . EHRLICH (1970) discusses the e f f e c t of temperature on w a t e r - o i l imbibition r e l a t i v e permeability. I n b o t h chalk and d i a t o m i t e r e s e r v o i r s , w a t e r f l o o d i n g i s e s s e n t i a l f o r b o t h o i l r e covery improvement and r e s e r v o i r pressure maintenance which i s h a l t i n g o i l f i e l d subsidence (WENDEL, KUNKEL & SWANSON 1988; c f . s e c t i o n 4.5.4.1.6.). Other aspects o f water i m b i b i t i o n i n c h a l k r e s e r v o i r s are discussed by SYLTE, HALLENBECK & THOMAS (1988).
526
4.5.4.5.2.Water fracturing Water i m b i b i t i o n t o g e t h e r w i t h c r e a t i o n o f a m i n i f r a c t u r e i s a l s o t h e reason why s m a l l - s c a l e w a t e r f r a c t u r i n g i n d e p l e t e d c h a l k o i l f i e l d s by pumping w a t e r downhole w i t h a p r e s s u r e exceeding f o r m a t i o n t e n s i l e s t r e s s and p r o d u c i n g i t back f r o m t h e same b o r e h o l e t o g e t h e r w i t h i n c r e m e n t a l o i l i s a c o m p a r a t i v e l y simple, cheap and e f f e c t i v e s t i m u l a t i o n t e c h n i q u e (JOURNAL PETROLEUM TECHNOLOGY 1987). Such s m a l l - s c a l e w a t e r f r a c t u r i n g can be p r i n c i p a l l y compared w i t h steam-soak h u f f and p u f f t r e a t m e n t s and b a s i c a l l y c l e a n s up t h e w e l l b o r e and i t s immediate v i c i n i t y , d i s s o l v e s and removes p a r t i c l e s t h a t a r e p l u g g i n g t h e f o r m a t i o n , breaks through n e a r - b o r e h o l e damage haloes and d i s p l a c e s t h e o i l . I n j e c t i o n o f w a t e r a t s u p e r c r i t i c a l p r e s s u r e s g i v e s r i s e t o opening o f e i t h e r s i n g u l a r o r m u l t i p l e f r a c t u r e s i n c l u d i n g a t l e a s t p a r t i a l enlargement o f p r e e x i s t i n g n a t u r a l c r a c k s , and p e n e t r a t i o n o f t h e w a t e r i n t o b o t h t h e a r t i f i c i a l f r a c t u r e and n a t u r a l c r a c k systems. The enlargement o f t h e r e s e r v o i r s u r f a c e acc e s s i b l e t o t h e i n j e c t e d w a t e r i s t h e reason f o r an a m e l i o r a t e d d r a i n a g e a r e a f o r o i l , w i t h t h e r e f o r e i n c r e a s e d amounts o f o i l f l o w i n g t o t h e w e l l once pumpi n g o f w a t e r i s stopped and b a c k - p r o d u c t i o n o f t h e i n j e c t e d f l u i d i n combination with o i l offtake starts. I n c o n t r a s t t o m a i n l y s m a l l - s c a l e w a t e r f r a c t u r i n g i n carbonates and p a r t i c u l a r l y c h a l k , l a r g e - s c a l e non-proppant w a t e r f r a c t u r i n g i s f r e q u e n t l y c a r r i e d o u t i n geothermal r e s e r v o i r s where o f t e n v a r i o u s f r a c t u r i n g f l u i d s a r e no l o n g e r s t a b l e due t o h i g h r e s e r v o i r temperature ( c f . s e c t i o n 4 . 7 . 3 . 5 . ) . Water f r a c t u r i n g i s a l s o o f c o n s i d e r a b l e s i g n i f i c a n c e i n c o a l seams ( c f . s e c t i o n 4.4.3.4.1.).
4.5.4.5.3. Foam fracturing A p a r t f r o m t h e N o r t h Sea and a d j o i n i n g onshore areas i n Europe, l a r g e b e l t s o f Upper Cretaceous c h a l k o c c u r i n t h e Rocky Mountain area i n Colorado, Nebraska and Kansas i n USA. A l t h o u g h o i l and gas have been produced f r o m c h a l k r e s e r v o i r s i n t h e Rocky Mountains/USA s i n c e t h e e a r l y 1900's (SCHOLLE 1977), o n l y i n t h e 1970's an upsurge o f e x p l o r a t i o n and development o f c h a l k gas i n t h e Upper Cretaceous N i o b r a r a and Greenhorn f o r m a t i o n s took p l a c e as a consequence o f t h e s u c c e s s f u l a c q u i s i t i o n and e x p l o i t a t i o n o f hydrocarbon r e s e r v e s i n Upper C r e t a ceous and l o w e r T e r t i a r y c h a l k s i n t h e N o r t h Sea i n Europe (POLLASTRO & SCHOLLE 1986). O t h e r f a c t o r s p r o m o t i n g development o f t h e c h a l k gas boom were i n c r e a s i n g gas p r i c e s , advanced e x p l o r a t i o n techniques f o r l o w - p e r m e a b i l i t y r e s e r v o i r s , and m a j o r improvements i n r e c o v e r y technology i n c l u d i n g above a l l hydraul i c proppant f r a c t u r i n g . Foam f r a c t u r i n g has proven t o be t h e most e f f e c t i v e s t i m u l a t i o n t e c h n i q u e i n N i o b r a r a F o r m a t i o n c h a l k s (ROHRET & JONES 1978, LOCKRIDGE & SCHOLLE 1978) because o f m i n i m i z i n g f l u i d volume and s i g n i f i c a n t l y r e d u c i n g f o r m a t i o n damage by a l l o w i n g r a p i d c l e a n - u p a f t e r t h e t r e a t m e n t . Foam f r a c t u r i n g f l u i d s a l s o c a r r y h i g h p r o p p a n t c o n c e n t r a t i o n s and p r e v e n t l a r g e p r e s s u r e drops due t o f r i c t i o n ( c f . s e c t i o n 3 . 1 1 . 2 . ) . The advantage o f a l c o h o l i n foamed methanol-water systems i s r a p i d and more complete cleanup due t o h i g h vapour p r e s s u r e , h i g h s o l u b i l i t y i n produced gas, h i g h m i s c i b i l i t y i n f o r m a t i o n waters, and low s u r f a c e t e n s i o n p r o p e r t i e s (ROHRET & JONES 1978; c f . s e c t i o n 3 . 1 1 . 4 . ) . The e v o l u t i o n o f f r a c t u r i n g s t i m u l a t i o n techniques i s summarized by BROWN, CRAFTON & GOLSON ( 1 9 8 2 ) . O i l - b a s e d foam f r a c t u r i n g i n c h a l k s i s d i s c u s s e d by BOWEN & DRISCOLL ( 1 9 8 0 ) . Aspects o f c h a l k f r a c t u r i n g by d i f f e r e n t methods and i n v a r i o u s format i o n s a r e a l s o d i s c u s s e d by WEBSTER (1977) and LUPPENS ( 1 9 8 0 ) .
4.5.4.6. Horizontal well drilling Comparative e v a l u a t i o n s o f h o r i z o n t a l w e l l s vs. v e r t i c a l f r a c t u r e s ( c f . a l s o s e c t i o n 4.8.6.2.1.) i n t h e N o r t h Sea c h a l k have shown t h a t a n o n - s t i m u l a t e d ho-
527 r i z o n t a l w e l l c o u l d y i e l d a p r o d u c t i v i t y equal t o a s u c c e s s f u l l y propped v e r t i c a l l y f r a c t u r e d d e v i a t e d w e l l , whereas i n s t a l l a t i o n c o s t o f t h e l a t t e r b o r e h o l e i s h i g h e r (ANDERSEN, HANSEN & FJELDGAARD 1988). While t h e p r o d u c t i v i t y i n d e x i m provement f o r a m a t r i x - a c i d i z e d h o r i z o n t a l w e l l compared w i t h an o p t i m a l l y f r a c tured v e r t i c a l w e l l i s o n l y marginal, a matrix-acidized h o r i z o n t a l w e l l could be b e n e f i c i a l i n m u l t i p h a s e f l u i d r e s e r v o i r s because b r e a k t h r o u g h o f gas and/or w a t e r i s delayed as a r e s u l t o f drawdown r e d u c t i o n .
4.5.5. Sandstone acidizing In c o n t r a s t t o carbonates w h i c h need proppant f r a c t u r i n g r a t h e r t h a n a c i d f r a c t u r i n g i n o r d e r t o achieve l a r g e r p r o d u c t i o n improvements, some sandstones can i n a d d i t i o n t o proppant f r a c t u r i n g a r r i v e a t s t i l l a m e l i o r a t e d o u t p u t by f r a c t u r i n g o r m a t r i x a c i d i z i n g w i t h h y d r o f l u o r i c a c i d o r HCl/HF-mixtures (ROSS, PIERSON & COULTER 1963; SMITH, ROSS & HENDRICKSON 1965; GATEWOOD, HALL, ROBERTS & LASATER 1970; WILLIAMS & WHITELEY 1971, HOLCOMB 1975; McCUNE, FOGLER, LUND, CUNNINGHAM & AULT 1975). Aspects o f sandstone a c i d i z i n g a r e a l s o d i s c u s s e d by FARLEY, MILLER & SCHOETTLE (1970); GIDLEY (1970, 1975), SUTTON & LASATER (1972), BURKHILL & PIERRE (1975), HALL (1975); McCUNE, AULT & DUNLAP (1975); WILLIAMS (1975); GIDLEY, RYAN & MAYHILL (1976); LUND, FOGLER & McCUNE (1976); ASELTINE & ELLENBERGER (1977); HILL, LINDSAY, SCHECHTER & SILBERBERG (1977); THOMAS & CROWE (1978 b ) , PACCALONI (1979); WILLIAMS, GIDLEY & SCHECHTER (1979); SHAUGHNESSY & KUNZE (1981), CROWE & M I N O R (1982), SCHECHTER (1982); McLEOD, LEDLOW & T I L L (1983); GIDLEY (1985), BURMAN & HALL (1986); SMITH, CLARK, HENDRICKSON & EGBOGAH (1986); TAHA, HILL & SEPEHRNOORI (1986, 1987) and BRANNON, NETTERS & GRIMMER (1987). REN & XIONG (1986) r e p o r t f a c t o r s i n f l u e n c i n g e f f e c t i v e a c i d p e n e t r a t i o n i n f r a c t u r e systems. Some aspects o f a c i d r e a c t i o n , a c i d i z i n g t a r g e t s , a c i d p e n e t r a t i o n d i s t a n c e , and a c i d types and d i s s o l u t i o n p o s s i b i l i t i e s a r e b r i e f l y i l l u s t r a t e d as f o l l o w s .
4.5.5.1. Acid reaction The commercial development o f a c i d i z i n g as a s t i m u l a t i o n t e c h n i q u e has become widespread a l r e a d y more than 60 y e a r s ago when h y d r o c h l o r i c a c i d t r e a t i n g o f carbonates was i n t r o d u c e d (LABRID 1975). Sandstone a c i d i z i n g was i n v e n t e d o v e r 30 y e a r s ago (SMITH & HENDRICKSON 1965) and i s based on t h e unique q u a l i t y o f h y d r o f l u o r i c a c i d t o a t t a c k s i l i c a and a l u m i n o - s i l i c a t e s . C l a y and f e l d s p a r d i s s o l u t i o n i s much f a s t e r than t h a t o f s i l i c a p r o b a b l y because o f t h e l a r g e r s u r f a c e area, and t a k e s p l a c e i n t h e f i r s t stage by u n i f o r m a l t e r a t i o n o f t h e c r y s t a l l a t t i c e and t h e n i n t h e second phase by p r o g r e s s i v e e x t r a c t i o n o f a l u m i num i n t h e f o r m o f f l u o r i n a t e d complexes. The main r e a c t i o n p r o d u c t o f s i l i c a s o l u b i l i z a t i o n i s f l u o r o s i l i c a c i d accompanied by a small amount o f c o l l o i d a l s i l i c a ( f o r thermodynamic and k i n e t i c aspects o f a c i d r e a c t i o n s w i t h v a r i o u s m i n e r a l s c f . LUND, FOGLER, McCUNE & AULT 1973; LABRIO 1975). Chemical p r e c i p i t a t i o n mechanisms a r e i m p o r t a n t f a c t o r s o f f o r m a t i o n damage d u r i n g s t i m u l a t i o n w i t h h y d r o f l u o r i c a c i d and r e q u i r e c a r e f u l e v a l u a t i o n (WALSH, LAKE & SCHECHTER 1982). A f t e r f l u s h i n g and r e d u c t i o n o f h y d r o f l u o r i c a c i d c o n c e n t r a t i o n can be used t o combat s i l i c a p r e c i p i t a t i o n i n sandstone m a t r i x a c i d i z i n g (SHAUGHNESSY & KUNZE 1980). Some aspects o f p a r t i a l m a t r i x d i s i n t e g r a t i o n as w e l l as separat i o n o f connate w a t e r and a c i d by p r e f l u s h a r e discussed as f o l l o w s .
4.5.5.1.1. Partial matrix disintegration The most i m p o r t a n t element o f a c i d r e a c t i o n w i t h sandstone i s t h e p a r t i a l d i s i n t e g r a t i o n o f t h e m a t r i x and t h e r e l e a s e o f f o r m a t i o n f i n e s i n t h e p o r e space (GIDLEY 1970). D i s s o l u t i o n o f m a t r i x cement and l o o s e n i n g o f f i n e p a r t i c l e s , however, a l s o i n c l u d e s t h e danger o f downstream f l o w o f m o b i l i z e d s o l i d fragments which l a t e r t e n d t o p l u g p o r e channels. E f f e c t i v e a c i d t r e a t m e n t o f sandstone f o r m a t i o n s should p r o v i d e a s u b s t a n t i a l i n c r e a s e i n p e r m e a b i l i t y , no
528 adverse changes i n f o r m a t i o n w e t t a b i l i t y , r a p i d cleanup f o l l o w i n g t h e j o b and minimum o p e r a t i o n a l c o s t . The most e f f i c i e n t t r e a t m e n t i s composed o f t h r e e s t a ges c o m p r i s i n g p r e f l u s h , a c i d and a f t e r f l u s h and i s conducted a t r a t e s t h a t w i l l n o t open f r a c t u r e s i n t h e f o r m a t i o n which d i d n o t a l r e a d y e x i s t b e f o r e a c i d i n s e r t i o n (GIDLEY, RYAN & MAYHILL 1976 found t h a t t h e p o o r e s t t r e a t m e n t response o c c u r r e d when a c i d was i n j e c t e d a t p r e s s u r e s exceeding f r a c t u r i n g p r e s sure).
4.5.5.1.2. Separation of connate water and acid by preflush The p r e f l u s h c o n s i s t i n g o f h y d r o c h l o r i c a c i d s h o u l d p r o v i d e a s e p a r a t i o n b e t ween connate w a t e r and h y d r o f l u o r i c a c i d i n o r d e r t o p r e v e n t f o r m a t i o n o f sodium f l u o r o s i l i c a t e , r e a c t w i t h t h e carbonates p r e s e n t i n t h e r e s e r v o i r r o c k t o a v o i d d i s s i p a t i o n o f t h e more expensive h y d r o f l u o r i c a c i d , and m a i n t a i n a low pH t o i n h i b i t d e p o s i t i o n o f c a l c i u m f l u o r i d e as p r o d u c t o f h y d r o f l u o r i c a c i d r e a c t i n g w i t h c a r b o n a t e s . Concerning amount o f a c i d , c a u t i o n has t o be e x e r c i sed, as e x c e s s i v e q u a n t i t i e s o f a c i d u l t i m a t e l y d i s s o l v e m a t r i x cements and des t r o y sandstone c o n s o l i d a t i o n , r e s u l t i n g i n recompaction by overburden s t r e s s t o a new and l o w e r p o r o s i t y and a g r e a t l y reduced p e r m e a b i l i t y . The b e t t e r t h e l i t h i f i c a t i o n o f t h e r e s e r v o i r r o c k and t h e more mature i t s s t a g e in b u r i a l d i a genesis, t h e l a r g e r t h e a c i d volume r e q u i r e d f o r e f f e c t i v e sandstone s t i m u l a tion.
4.5.5.2. Acidizing targets Concerning a c i d f l o w t h r o u g h sandstone m a t r i x , damaged f o r m a t i o n s a r e p a r t i a l l y plugged by c l a y and hence have l o w e r p o r o s i t y and h i g h e r c o n c e n t r a t i o n of d i s s o l u b l e m i n e r a l s (McCUNE, FOGLER, LUNO, CUNNINGHAM & AULT 1975). A l t h o u g h t h e t o t a l c o n c e n t r a t i o n o f d i s s o l u b l e p a r t i c l e s i s increased, much o f them m i g h t be bypassed because o f p o s s i b l e c h a n n e l l i n g , g i v i n g r i s e t o a n e t decrease o f m i n e r a l s a c c e s s i b l e t o a c i d , w i t h t h e o v e r a l l n e t e f f e c t p r o b a b l y b e i n g a decrease i n a c i d c a p a c i t y number and an i n c r e a s e i n r e q u i r e d a c i d volume o r c o n c e n t r a t i o n f o r r e s e r v o i r damage removal. S u i t a b l e t a r g e t s f o r a c i d t r e a t m e n t a r e p a r t i c u l a r l y m i n e r a l o g i c a l l y immature and submature sandstones which c o n t a i n c o m p a r a t i v e l y low amounts o f q u a r t z , b u t have h i g h e r shares o f det r i t a l f e l d s p a r s and u n s t a b l e r o c k fragments, as w e l l as c l a s t i c and d i a g e n e t i c c l a y m i n e r a l s and a u t h i g e n i c carbonate cement. Mature q u a r t z sandstones can o n l y be s u c c e s s f u l l y s t i m u l a t e d by a c i d i z i n g i f c o n t a i n i n g r e a s o n a b l e q u a n t i t i e s o f c a r b o n a t e cement. The f o l l o w i n g d i s c u s s i o n comments on c l a y and i r o n b e a r i n g m i n e r a l s as w e l l as on carbonate m i n e r a l s .
4.5.5.2.1. Clay and iron-bearing minerals L a r g e r amounts o f c l a y and i r o n m i n e r a l s a r e u s u a l l y u n d e s i r a b l e c h a r a c t e r i s t i c s i n sandstone a c i d i z i n g c a n d i d a t e s , because such r o c k c o m p o s i t i o n s a r e o f t e n s u s c e p t i b l e t o chemical ( c l a y damage) o r mechanical ( p a r t i c u l a t e p l u g g i n g caused by e n t r a i n e d s o l i d s i n t h e c o m p l e t i o n f l u i d ) impairment (BRANNON, NETTERS & GRIMMER 1987) and a l s o subsequent i r o n p r e c i p i t a t i o n problems ( c f . sect i o n 4 . 5 . 1 . 1 . 4 . ) . On t h e o t h e r hand, however, d i r t y sandstones w i t h e x c e s s i v e c o n t e n t s o f c l a y m i n e r a l s a r e h a r d l y s t i m u l a t i o n c a n d i d a t e s a t a l l because o f t o o p o o r r e s e r v o i r p r o p e r t i e s t h a t i n some cases even cannot be s u f f i c i e n t l y i m proved f o r achievement o f economical hydrocarbon p r o d u c t i o n r a t e s by n e i t h e r hyd r a u l i c f r a c t u r i n g n o r a c i d i z i n g . C a u t i o n has t o be e x e r c i s e d i n a c i d i z i n g deep h o t sandstone f o r m a t i o n s w i t h h y d r o c h l o r i c a c i d , because d e t e r i o r a t i o n o f p r o d u c t i v i t y m i g h t o c c u r (BARBY & BARBEE 1987). F i e l d s t u d i e s have shown t h a t i n some r e s e r v o i r s , h i g h i n j e c t i o n r a t e s o f w a t e r can have a b e t t e r f o r m a t i o n breakdown e f f e c t than a c i d t r e a t m e n t s . A c i d can cause m i g r a t i o n o f f i n e s and i r o n c o l l o i d s which may p l u g t h e p o r e t h r o a t s o f t h e r e s e r v o i r .
529
4.5.5.2.2.
Carbonate minerals
H i g h c o n t e n t s o f carbonate cement i n q u a r t z sandstones g i v e r i s e t o h i g h a c i d s o l u b i l i t y o f t h e r o c k . I f t h e cement share i s exceeding a c e r t a i n boundary percentage o f t o t a l r o c k volume, carbonate d i s s o l u t i o n can r e s u l t i n severe l o s s o f f o r m a t i o n s t r e n g t h f o l l o w i n g a c i d t r e a t m e n t and t h u s f o r m a t i o n m a t r i x c o l l a p s e . T h e r e f o r e i n many sandstone r e s e r v o i r s , h y d r a u l i c p r o p p a n t f r a c t u r i n g i s t h e s u p e r i o r s t i m u l a t i o n method and g i v e s i n most cases b e t t e r d r a i n a g e e f f e c t s w i t h l e a v i n g t h e m a t r i x f a b r i c i n t a c t i n comparison t o a c i d t r e a t m e n t s which n o t o n l y on t h e one hand c r e a t e pathways f o r f l u i d f l o w , b u t a l s o on t h e o t h e r hand weaken f o r m a t i o n cohesion up t o s u p e r c r i t i c a l l e v e l s where g r a i n f a b r i c c o l l a p s e d e t e r i o r a t e s t h e e f f e c t o f t h e s t i m u l a t i o n o p e r a t i o n (PEARCE 1979). A s u i t a b l e c o m b i n a t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and a c i d i z i n g would c e r t a i n l y be t h e optimum s o l u t i o n f o r improval o f b o t h m a t r i x and f r a c t u r e drainage, b u t i t r e q u i r e s c o n s i d e r a b l e e x p e r i e n c e t o g e t a s a t i s f a c t o r y f e e l i n g how much o f t h e m a t r i x can be d i s s o l v e d w i t h o u t r i s k i n g framework c o l l a p s e t h a t would screw up most of t h e success o f t h e s t i m u l a t i o n j o b .
4,5.5.3.
Acid penetration distance
A c i d p e n e t r a t i o n d i s t a n c e i n f r a c t u r e s i s t h e way t h e a c i d t r a v e l s b e f o r e spending t o some predetermined degree (ROBERTS & G U I N 1974) w h i c h i s i n most cases a b t . 10 % o f i t s o r i g i n a l c o n c e n t r a t i o n (NOVOTNY 1976). A c i d p e n e t r a t i o n d i s t a n c e i s e s s e n t i a l f o r e s t i m a t i n g t h e p r o d u c t i o n improvement o b t a i n a b l e by f r a c t u r e a c i d i z i n g . W h i l e a c i d p e n e t r a t i o n d i s t a n c e s i n carbonates a r e l i m i t e d by h i g h spending r a t i o s due t o r e a c t i o n o f t h e a c i d w i t h most, i f n o t a l l , o f t h e r e s e r v o i r m a t r i x , a c i d can m i g r a t e e x t r e m e l y f a r i n sandstones where commonly o n l y s u b o r d i n a t e shares o f t h e framework r e a c t w i t h t h e a c i d and spending i s t h u s c o n s i d e r a b l y s l o w e r i n sandstones w i t h r e s p e c t t o carbonates. Some asp e c t s o f t y p e o f f o r m a t i o n damage, a c i d c o n c e n t r a t i o n and r e a c t i o n r a t e , lowand high-pH acids, and w a t e r b l o c k i n g a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.5.5.3.1.
Type o f formation damage
Comparing t h e e f f i c i e n c y o f a c i d i z i n g t r e a t m e n t s o f undamaged, natural-damaged and mud-damaged sandstone f o r m a t i o n s , p r o d u c t i o n i n c r e a s e s a r e most s i g n i f i c a n t f o r i n t e r s t i t i a l a c i d i z i n g o f damaged f o r m a t i o n s (GATEWOOD, HALL, ROBERTS & LASATER 1970). I n r e s e r v o i r s w i t h d r i l l i n g - m u d damage r e s u l t i n g f r o m c l a y p a r t i c l e i n v a s i o n , volumes o f a c i d s u f f i c i e n t t o remove o n l y t h e c l a y c o n t a i n e d w i t h i n t h e narrow damage m a n t l e around t h e w e l l b o r e y i e l d t h e most economical p r o d u c t i o n i n c r e a s e s i f no n a t u r a l damage has o c c u r r e d as a consequence o f cont a c t i n g o f w a t e r - s e n s i t i v e c l a y s by mud f i l t r a t e . When t r e a t i n g f o r m a t i o n s w i t h n a t u r a l c l a y damage, t h e p r o d u c t i o n i n c r e a s e i s d i r e c t l y dependent upon t h e d i s t a n c e w h i c h l i v e h y d r o f l u o r i c a c i d can be pumped i n t o t h e r e s e r v o i r . Penetrat i o n d e p t h o f l i v e h y d r o f l u o r i c a c i d i s a f u n c t i o n o f c l a y c o n c e n t r a t i o n , format i o n temperature, i n i t i a l h y d r o f l u o r i c a c i d c o n c e n t r a t i o n , r a t e o f h y d r o f l u o r i c a c i d r e a c t i o n and pumping r a t e . I n c r e a s i n g f o r m a t i o n temperature and c l a y conc e n t r a t i o n b o t h decrease t h e p e n e t r a t i o n r a d i u s o f l i v e o r unspent a c i d .
4.5.5.3.2.
Acid concentration and reaction r a t e
Greater penetration depth o f l i v e a c i d leading t o higher production increase i s obtained by r i s i n g the i n i t i a l h y d r o f l u o r i c a c i d concentration, r e t a r d a t i o n o f t h e r e a c t i o n r a t e o f h y d r o f l u o r i c a c i d on silicates (cf. section 4.5.5.4.3.), and i n c r e a s i n g pumping r a t e . Enhancement o f s t i m u l a t i o n r e s u l t s can a l s o be achieved by a c i d h e a t i n g (WALKER, FREDRICKSON & NORMAN 1984; c f . s e c t i o n 4.5.1.2.4.). W h i l e an improvement o f s t i m u l a t i o n by i n c r e a s i n g a c i d volumes i s d e s i r a b l e and l i m i t a t i o n o f q u a n t i t i e s i s u s u a l l y g i v e n by economical
5 30
f a c t o r s of j o b f e a s i b i l i t y , a c i d c o n c e n t r a t i o n has t o be c a r e f u l l y s e l e c t e d a c c o r d i n g t o f o r m a t i o n c h a r a c t e r i s t i c s , because the h i g h e r r e a c t i o n r a t e of more c o n c e n t r a t e d a c i d a l s o g i v e s r i s e t o i n c r e a s e d s w e l l i n g , detachment and migrat i o n of s e n s i t i v e c l a y m i n e r a l s and thus d e t e r i o r a t i o n of p e r m e a b i l i t y by plugg i n g of pore t h r o a t s ( G I D L E Y 1 9 7 1 ) . B u f f e r r e g u l a t i o n of h y d r o f l u o r i c a c i d e n a b l e s e x t e n s i o n of thermal s t a b i l i t y and a p p l i c a t i o n f o r s a n d s t o n e a c i d i z i n g u p t o 550 OF (280 oC; SCHEUERMAN 1 9 8 8 ) .
4.5.5.3.3. Low- and high-PH acids I n terms of performance, d i s t i n c t i o n has a l s o t o be made between low-pH ( b e low 4 ) and high-pH ( 4 - 6 ) a c i d s t i m u l a t i o n systems (ABRAMS, LYBARGER, RICHARDSON & NEASHAM 1978; TEMPLETON, RICHARDSON, SCHEUERMAN & ABRAMS 1979; ABRAMS, SCHEUERMAN, TEMPLETON & RICHARDSON 1 9 8 3 ) . BRISCOE ( 1 9 7 8 ) , SCHERUBEL & CRONE ( 1 9 7 8 ) and FORD (1981 a , 1981 b ) i n t r o d u c e s t a b i l i z e d foamed a c i d which p r o v i des g r e a t e r f r a c t u r e p e n e t r a t i o n along w i t h i n c r e a s e d f r a c t u r e c o n d u c t i v i t y t h r o u g h o u t f r a c t u r e l e n g t h ( c f . s e c t i o n 4 . 5 . 1 . 4 . ) . Another improvement i s a s e q u e n t i a l h y d r o f l u o r i c a c i d system which has a r e t a r d i n g e f f e c t ( H A L L & ANDERSON 1977; H A L L , TINNEMEYER & UNDERWOOD 1981; c f . s e c t i o n 4 . 5 . 5 . 4 . 3 . ) . L E E (1985 b) d i s c u s s e s geometry d e t e r m i n a t i o n f o r m u l t i - s t a g e a c i d i z i n g t r e a t m e n t s w i t h o r w i t h o u t v i s c o u s p r e f l u s h , and C L A R K , WONG & MUNGAN (1982) i n t r o d u c e new a c i d systems f o r s a n d s t o n e s t i m u l a t i o n .
4 . 5 . 5 . 3 . 4 . Water blocking Sandstone m a t r i x a c i d i z i n g o f t e n s u f f e r s from very slow c l e a n - u p of s p e n t a c i d a s a r e s u l t o f w a t e r b l o c k i n g i n t h e c r i t i c a l m a t r i x s u r r o u n d i n g the w e l l bore (McLEOD & COULTER 1966; McLEOD, McGINTY & SMITH 1966; c f . s e c t i o n 3 . 1 1 . 2 . 4 . 2 . ) . Addition of a l c o h o l t o the a c i d can o f t e n p r e v e n t the w a t e r - b l o c k ing problem t h e r e b y speeding u p s p e n t a c i d c l e a n - u p . Although w a t e r b l o c k i n g can occur in c l e a n w e l l - c o n s o l i d a t e d s a n d s t o n e , i t i s more l i k e l y t o be p r e s e n t t o g e t h e r w i t h o t h e r forms o f d r a i n a g e such a s p a r t i c l e b l o c k s o r rock w e t t a b i l i t y changes. Water b l o c k i n g i s most s e v e r e i n heterogeneous s a n d s t o n e f o r m a t i o n s p a r t i c u l a r l y w h e n high- and l o w - p e r m e a b i l i t y l a y e r s a l t e r n a t e such a s i n many cross-bedded sediments ( c f . p l a t e I I / 5 - 6 ) . Alcohol a d d i t i v e s a r e s u c c e s s f u l i n water-block removal i n both m a t r i x a c i d i z i n g and h y d r a u l i c f r a c t u r i n g . T h e most c r i t i c a l f a c t o r s f o r s u c c e s s f u l s a n d s t o n e m a t r i x a c i d i z i n g a r e mud a c i d volume and t r e a t i n g pressure (GIDLEY, RYAN & MAYHILL 1 9 7 6 ) . The p o o r e s t j o b r e s p o n s e o c c u r s when a c i d i s i n j e c t e d a t p r e s s u r e s exceeding f o r m a t i o n f r a c ture g r a d i e n t , because the a c i d i s then o n l y c h a n n e l l i n g along t h e c r a c k and does n o t a d e q u a t e l y flow through the s a n d s t o n e m a t r i x .
4.5.5.4. Acid types and dissolution possibilities S e l e c t i o n of a c i d t y p e s f o r t r e a t m e n t of both c a r b o n a t e and s a n d s t o n e format i o n s depends on the composition of the contaminants which a r e plugging r e s e r v o i r p e r m e a b i l i t y i n c a s e of f o r m a t i o n damage removal n e c e s s i t y (McLEOD 1 9 8 4 ) . Following d i s c u s s i o n of some g e n e r a l a s p e c t s , an o u t l i n e i s given on c l a y miner a l type v s . a c i d i z i n g t r e a t m e n t a s well a s on d e l a y e d a c i d r e a c t i o n and i n - s i t u a c i d g e n e r a t i o n . Some comments a r e a l s o given on s e l e c t i v e a c i d s t i m u l a t i o n of i n j e c t i o n w e l l s .
4.5.5.4.1. General aspects H y d r o c h l o r i c a c i d does n o t d i s s o l v e p i p e dope, p a r a f f i n o r a s p h a l t e n e s , w i t h t r e a t m e n t of t h e s e s o l i d s o r plugging a g e n t s r e q u i r i n g an e f f e c t i v e o r g a n i c s o l v e n t . A c e t i c a c i d e f f e c t i v e l y d i s s o l v e s calcium c a r b o n a t e s c a l e , b u t does n o t
531 d i s s o l v e i r o n o x i d e s c a l e . Calcium s u l p h a t e can be c o n v e r t e d t o c a l c i u m carbon a t e o r c a l c i u m h y d r o x i d e by t r e a t m e n t w i t h potassium h y d r o x i d e o r sodium carbonate, and h y d r o c h l o r i c a c i d can then be used t o d i s s o l v e t h e c o n v e r t e d s c a l e . Calcium s u l p h a t e can a l s o be d i s s o l v e d i n one s t e p w i t h t h e sodium s a l t o f e t h y l e n e diamine t e t r a a c e t i c a c i d . H y d r o c h l o r i c a c i d does n o t d i s s o l v e f o r m a t i o n c l a y m i n e r a l s o r d r i l l i n g mud, w i t h h y d r o f l u o r i c a c i d h a v i n g t o be used t o d i s s o l v e these a l u m i n o s i l i c a t e s i n r o c k pores around t h e w e l l b o r e i n sandstones, whereas t h i s t y p e o f f o r m a t i o n damage can be r e p a i r e d i n carbonates a l s o by hyd r o c h l o r i c a c i d which e t c h e s p a r t s o f t h e carbonate r o c k m a t r i x i n s t e a d o f t h e damaging m a t e r i a l s t h a t a r e l e f t i n p l a c e , t h e r e b y a l s o i n c r e a s i n g p e r m e a b i l i ty. Mud a c i d c o n t a i n s h y d r o c h l o r i c and h y d r o f l u o r i c a c i d s t o g e t h e r w i t h a d d i t i ves f o r p r o p e r s i l i c a t e c o n t r o l , d e m u l s i f i c a t i o n , and s u r f a c e t e n s i o n c h a r a c t e r i s t i c s which a l l o w t h e a c i d t o r e s t o r e and i n c r e a s e p e r m e a b i l i t y near t h e w e l l b o r e . Mud a c i d can be used f o r m a t r i x a c i d i z i n g o f sandstone r e s e r v o i r s , spearheading o f h y d r a u l i c p r o p p a n t f r a c t u r i n g t r e a t m e n t s and squeeze cementing, l o o sening s t u c k d r i l l p i p e , and c l e a n i n g g r a v e l - p a c k i n g l i n e r s and screens. The p r i m a r y use o f h y d r o f l u o r i c a c i d t o g e t h e r w i t h i n t e r m i x e d h y d r o c h l o r i c a c i d i s i n sandstone m a t r i x t r e a t m e n t s t o remove c l a y - p a r t i c l e damage w i t h p e r m e a b i l i t y r e s t o r a t i o n o r t o i n c r e a s e o r i g i n a l p e r m e a b i l i t y o f c l a y - b e a r i n g sandstones. H i g h c o n c e n t r a t i o n s o f h y d r o c h l o r i c a c i d i n t h e presence o f h y d r o f l u o r i c a c i d may improve h y d r o f l u o r i c a c i d e f f i c i e n c y (LUND, FOGLER & McCUNE 1976) and speed up h y d r o f l u o r i c a c i d r e a c t i o n (McCUNE, FOGLER, LUND, CUNNINGHAM & AULT 1975). The use o f o r g a n i c a c i d s such as a c e t i c , f o r m i c and p r o p i o n i c a c i d s as s t i m u l a t i o n and p e r f o r a t i o n f l u i d s i n carbonate r e s e r v o i r s has i n c r e a s e d i n t h e l a s t years, because t h e y have i n h e r e n t l y r e t a r d e d r e a c t i o n r a t e s and a r e much e a s i e r t o i n h i b i t a g a i n s t c o r r o s i o n a t h i g h temperatures (300 - 400 OF) than h y d r o c h l o r i c a c i d (DOWELL SCHLUMBERGER 1988 d ) , whereas i n sandstones, t h e i r u t i l i z a t i o n does o n l y o c c a s i o n a l l y p r o v i d e t e c h n i c a l and economical advantages w i t h r e s p e c t t o mud a c i d . FRENIER (1989) p r e s e n t s o r g a n i c chemicals f o r i n h i b i t i o n o f a c i d i zing f l u i d s f o r high-temperature r e s e r v o i r s t i m u l a t i o n .
4.5.5.4.2,
Clay mineral type vs. acidizing treatment
Concerning d i f f e r e n t c l a y m i n e r a l s , c a u t i o n has t o be e x e r c i s e d t o s e l e c t the proper treatment version f o r d i f f e r e n t c l a y mineral types ( c f . section 3.5.1.). K a o l i n i t e forms s t a c k s o f hexagonal p l a t e s which a r e e a s i l y d i s p e r s e d and can m i g r a t e through r o c k pores b r i d g i n g i n s m a l l e r p o r e t h r o a t s , and t h e r e f o r e has t o be c o n t r o l l e d by c l a y s t a b i l i z e r s i n t h e f i n a l o v e r f l u s h a f t e r a hyd r o f l u o r i c a c i d job. l l l i t e i s a f i b r o u s c l a y m i n e r a l t h a t can be broken o f f o r moved b y h i g h f l o w v e l o c i t i e s o r surges c l o s e t o t h e w e l l b o r e ( c f . s e c t i o n 3.5.2.1.), and t h u s l o w a c i d i n j e c t i o n r a t e s and c o n t r o l l e d p r o d u c t i o n r a t e s a f t e r t h e t r e a t m e n t a r e e s s e n t i a l . C h l o r i t e i s u s u a l l y i r o n - r i c h and p a r t i a l l y sol u b l e i n hydrochloric acid, w i t h i r o n being extracted leaving a s i l i c a residue b e h i n d a c i d . T h e r e f o r e c h l o r i t e must be a c i d i z e d c a u t i o u s l y o r n o t a t a l l , w i t h p r e f e r e n c e t o be g i v e n t o s e l e c t i o n o f a c o m p l e t i o n procedure t h a t makes a c i d i z i n g unnecessary. S m e c t i t e can o n l y be s u c c e s s f u l l y a c i d i z e d i f i t i s s p a r s e l y d i s t r i b u t e d ( l e s s than 3 w t . % ) i n h i g h - p e r m e a b i l i t y f o r m a t i o n s , whereas i n r e s e r v o i r s w i t h h i g h e r s m e c t i t e c o n t e n t s ( o v e r 5 w t . % ) , any aqueous f l u i d i n c l u d i n g a c i d may cause g r e a t damage and w e l l s t i m u l a t i o n has t o be made w i t h p r o p p a n t f r a c t u r i n g . O t h e r s i g n i f i c a n t d i a g e n e t i c a l m i n e r a l s u s u a l l y comprise carbon a t e s which a r e s o l u b l e i n h y d r o c h l o r i c a c i d . Mud a c i d has t h e a b i l i t y t o d i s s o l v e c l a y m i n e r a l s and i s m a i n l y employed f o r removal o f f o r m a t i o n damage c o n s i s t i n g o f p l u g g i n g o f p o r e t h r o a t s by c l a y m i n e r a l s i n t h e w e l l b o r e v i c i n i t y (GIDLEY 1985). Gas w e l l s , o i l w e l l s and w a t e r i n j e c t i o n w e l l s each r e q u i r e d i f f e r e n t t r e a t m e n t s t o o b t a i n maximum response t o mud a c i d s t i m u l a t i o n . Gas w e l l s respond t o mud a c i d i z i n g most p r e d i c t a b l y w i t h u n i f o r m i n c r e a s e i n p r o d u c t i o n b u i l d u p w i t h i n c r e a s i n g a c i d volume, and w i t h
532 u n i f o r m i n c r e a s e i n s t i m u l a t i o n success w i t h i n c r e a s i n g p e r m e a b i l i t y . O i l w e l l s e x p e r i e n c e maximum i n response as a f u n c t i o n o f t h e amount o f a c i d used. T r e a t ment success does n o t appear t o i n c r e a s e u n i f o r m l y w i t h f o r m a t i o n p e r m e a b i l i t y , and t h e a f t e r f l u s h - t o - a c i d volume r a t i o does n o t a f f e c t b u i l d u p u n i f o r m l y . Res u l t s on o i l w e l l s seem t o be more s i t e - s p e c i f i c and t o be more i n f l u e n c e d by t h e n a t u r e o f t h e r e s e r v o i r f l u i d s . Water i n j e c t i o n w e l l s sometimes show t h e odd response o f h a v i n g t h e g r e a t e s t b u i l d u p t o t h e l e a s t a c i d j o b . Enhanced removal o f deep c l a y damage can be performed by i n - s i t u h y d r o f l u o r i c a c i d c r e a t i o n (HALL 1978) and s e l f - g e n e r a t i n g mud a c i d (TEMPLETON, RICHARDSON, KARNES & LYBARGER 1975).
4.5.5.4.3. Delayed acid reaction and in-situ acid generation D e l a y e d - a c t i o n a c i d systems a t t r i b u t e t h e i r success t o slow g e n e r a t i o n o f hyd r o f l u o r i c a c i d i n - s i t u f r o m h y d r o l y s i s o f methyl f o r m a t e w h i c h y i e l d s m e t h y l a l c o h o l and f o r m i c a c i d , and t h e l a t t e r then r e a c t s w i t h ammonium f l u o r i d e t o r e l e a s e h y d r o f l u o r i c a c i d , w i t h h y d r o f l u o r i c a c i d o r i g i n a t i n g downhole i n t h e r e s e r v o i r i n such d i s t a n c e s f r o m t h e b o r e h o l e t h a t mud a c i d w h i c h i s c o n v e n t i o n a l l y pumped n o r m a l l y does n o t r e a c h b e f o r e spending. I n o r d e r t o a c h i e v e o p t i mum r e s u l t , t h e w e l l s have t o be r e t u r n e d t o p r o d u c t i o n by g r a d u a l l y opening t h e choke o v e r a three-month p e r i o d and never a l l o w i n g an e x c e s s i v e drawdown. Another process o f i n - s i t u h y d r o f l u o r i c a c i d g e n e r a t i o n i s h y d r o l y z a t i o n o f f l u o b o r i c a c i d t o f o r m h y d r o x y f l u o b o r i c a c i d and h y d r o f l u o r i c a c i d (THOMAS & CROWE 1978 a) which p r o v i d e s a slow r e l e a s e source o f h y d r o f l u o r i c a c i d t h a t can p e n e t r a t e deeply b e f o r e spending, and i n a d d i t i o n , t h e h y d r o x y f l u o b o r i c a c i d t h a t i s c r e a t e d v e r y s l o w l y r e a c t s w i t h c l a y m i n e r a l s t o f o r m a non-swell i n g , n o n - d i s p e r s i n g p r o d u c t t h a t s t a b i l i z e s c l a y and even h o l d s f i n e s i l i c a p a r t i c l e s i n p l a c e ( c f . a l s o McBRIDE, RATHBONE & THOMAS 1979). On t h e o t h e r hand, f l u o b o r i c a c i d may spend a t v a r i o u s f o r m a t i o n temperatures a t a r a p i d r a t e almost s i m i l a r t o t h a t o f c o n v e n t i o n a l h y d r o f l u o r i c a c i d (KUNZE & SHAUGHNESSY 1980). A c i d r e t u r n experiments and analyses have r e v e a l e d t h a t excess h y d r o c h l o r i c a c i d i s almost c o m p l e t e l y consumed by t h e h y d r o f l u o r i c a c i d chemical process (GDANSKI & PEAVY 1986), w i t h t h e main mechanisms o f h y d r o c h l o r i c a c i d consumpt i o n b e i n g spending on carbonates, exchange spending f o r sodium on ion-exchangea b l e c l a y s such as mixed l a y e r s , and t h e h y d r o f l u o r i c a c i d spending process i t s e l f . Lower h y d r o f l u o r i c a c i d c o n c e n t r a t i o n s s h o u l d be used when a c i d i z i n g f o r mations c o n t a i n i n g more t h a n 15 % f e l d s p a r s , and h i g h e r h y d r o c h l o r i c a c i d s a t u r a t i o n s s h o u l d be a p p l i e d w i t h h y d r o f l u o r i c a c i d t o a l l o w maximum f o r m a t i o n d i s s o l u t i o n ( c f . a l s o BRANNON, NETTERS & GRIMMER 1986). PERTHUIS (1989) d i s c u s s e s weak o r s l o w l y g e n e r a t i n g h y d r o f l u o r i c a c i d s o l u t i o n s as a c h o i c e f o r sandstone acidizing.
4.5.5.4.4. Selective stimulation o f injection wells S e l e c t i v e a c i d i z i n g o f sandstone r e s e r v o i r s i n i n j e c t i o n w e l l s can d i s c r e t e l y s t i m u l a t e i n e f f e c t i v e i n t e r v a l s and can s i g n i f i c a n t l y improve i n j e c t i o n p r o f i l e s (ELLENBERGER & ASELTINE 1976).
533
4.6. High-permeability reservoir fracturing T r a d i t i o n a l l y h y d r a u l i c p r o p p a n t f r a c t u r i n g was i n Europe and o t h e r p a r t s o f t h e w o r l d almost e x c l u s i v e l y l i m i t e d t o t h e s t i m u l a t i o n o f l o w - p e r m e a b i l i t y , more o r l e s s t i g h t sandstone r e s e r v o i r s ( l e s s than a b t . 5 md r e a c h i n g down t o t h e m i c r o d a r c y range; c f . s e c t i o n 3 . 4 . 1 . ) . The p h i l o s o p h y o f t r e a t i n g a l s o high-permeabi 1 it y hydrocarbon-bearing sandstones ( a b t . 10 - 30 md o r more) which i s a l r e a d y common sense i n t h e USA s i n c e l o n g e r time, however, has so f a r n o t been adopted as g e n e r a l procedure i n Europe and o t h e r p a r t s o f t h e w o r l d , w i t h t h e main reasons b e i n g c o s t containment f o r s t i m u l a t i o n j o b s and a l s o comp a r a t i v e l y l o w e r n e c e s s i t y o f a p p l i c a t i o n due t o f r e q u e n t l y s u f f i c i e n t product i o n r a t e s a l s o w i t h o u t t r e a t m e n t i n many f o r m a t i o n s i n Europe. Moderate- and h i g h - p e r m e a b i l i t y r e s e r v o i r f r a c t u r i n g i s c o n s i d e r e d t o have i n Europe t h e great e s t impact on enhancement o f s t i m u l a t i o n o f o i l - b e a r i n g f o r m a t i o n s (KOHLHAAS 1982, McDONALD 1983; NORMAN, CIPOLLA & WEBB 1983; B R I T T 1985). The most import a n t aspects a r e f r a c t u r e - f o r m a t i o n c o n d u c t i v i t y c o n t r a s t , p r o p p a n t g r a i n s i z e and c o n c e n t r a t i o n , p r o p p a n t / f l u i d s l u r r y m i x i n g types, and f r a c t u r e d e s i g n char a c t e r i s t i c s w h i c h a r e b r i e f l y commented as f o l l o w s .
4.6.1. Fracture-formation conductivity contrast E x t e n s i o n o f t h e s t i m u l a t i o n p o t e n t i a l i n t h e near f u t u r e , however, i s c o n s i dered t o be a l s o i n Europe i n e v i t a b l y l i n k e d w i t h t h e a p p l i c a t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g a l s o i n moderate- t o h i g h - p e r m e a b i l i t y sandstone r e s e r v o i r s . High-permeability r e s e r v o i r s generally r e q u i r e high f r a c t u r e conductivit i e s b u t do n o t need deeply p e n e t r a t i n g cracks, whereas l o w - p e r m e a b i l i t y format i o n s r e q u i r e deeply p e n e t r a t i n g f r a c t u r e s b u t can t o l e r a t e l o w e r c r a c k conduct i v i t i e s (VEATCH 1983). While f r a c t u r e h a l f - l e n g t h requirements a r e t y p i c a l l y l e s s t h a n 1,000 f t (330 m) i n moderate- t o h i g h - p e r m e a b i l i t y pay zones, t i g h t gas sands may demand c r a c k h a l f - l e n g t h s up t o 4,500 f t (1,500 m; c f . s e c t i o n 4.8.11.). Some aspects o f r e s e r v o i r t y p e s and f r a c t u r e f l o w c a p a c i t y a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.6.1.1. Reservoir types S i m i l a r l y as a p p l y i n g f o r carbonate r e s e r v o i r f r a c t u r i n g , i n h i g h - p e r m e a b i l i t y sandstone s t i m u l a t i o n a g a i n t h e c r i t i c a l f a c t o r i s t h e c o n d u c t i v i t y c o n t r a s t between t h e propped c r a c k and t h e m a t r i x o f t h e f r a c t u r e d f o r m a t i o n , w i t h var i o u s types and g r a i n s i z e s o f proppants a l l o w i n g t o model t h e optimum permeabil i t y c o n t r a s t w i t h i n t h e frame o f depth and c l o s u r e s t r e s s ( c f . s e c t i o n 4.5.1.1.2.). Enhancement o f t h e c o n t r a s t between t h e f i n i t e - c o n d u c t i v i t y f r a c t u r e and t h e r e s e r v o i r can be done by i m p r o v i n g t y p e and/or g r a i n s i z e o f t h e p r o p p a n t i n s e r t e d f o r s u p p o r t o f t h e c r a c k . While i n v e r y t i g h t r e s e r v o i r s even a poor f r a c t u r e permeability provides already s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t (CALLANAN, CIPOLLA & LEWIS 1983), optimum proppant p r o p e r t i e s and d i s t r i b u t i o n s have t o be s e l e c t e d and i n s t a l l e d i n o r d e r t o r e s u l t i n s a t i s f a c t o r y c o n d u c t i v i t y c o n t r a s t s between f r a c t u r e and f o r m a t i o n i n moderate- t o high-permeabil i t y reservoirs. S p e c i a l a p p l i c a t i o n s a r e seen f o r i n s u f f i c i e n t l y c o n s o l i d a t e d sandstones and carbonate s h e l l l a y e r s t h a t f r e q u e n t l y i n a d d i t i o n t o proppant f r a c t u r i n g r e q u i r e s t a b i l i z a t i o n by g r a v e l p a c k i n g o r by usage o f r e s i n - c o a t e d p r o p p a n t s ( c f . s e c t i o n 1.2.6.) which f o r m a massive h a r d wedge i n t h e l o o s e r e s e r v o i r upon i n t e r l o c k i n g o f g r a i n s by g l u e i n g o f t h e r e s i n f i l m s a t t h e i r c o n t a c t s ( c f . a l s o s e c t i o n s 4.2.5. and 4.12.3.3.), because t h e h i g h p e r m e a b i l i t y o f t e n c r e a t e s a weak r o c k f a b r i c t h a t c o l l a p s e s w i t h d e c l i n i n g f l o w i n g p r e s s u r e o r i n c r e a s i n g w a t e r c u t o f t h e o i l o r i s even u n s t a b l e f r o m t h e v e r y b e g i n n i n g o f p r o d u c t i o n ( c f . s e c t i o n 5 . 4 . ) , w i t h t h e propped f r a c t u r e s t a y i n g open as a r i g i d wedge and p r o v i d i n g a pathway f o r t h e hydrocarbons t o t h e w e l l b o r e .
534
4.6.1.2. Fracture flow capacity The g e n e r a l r u l e o f t h e l a r g e r t h e p e r m e a b i l i t y c o n t r a s t between f o r m a t i o n and p r o p p a n t package i n t h e f r a c t u r e , t h e b e t t e r t h e f l o w c a p a c i t y and r a t e , e q u a l l y a p p l i e s t o s a n d s t o n e s and c a r b o n a t e s and i s one o f t h e s t r o n g e s t a r g u ments t o s u p p o r t h y d r a u l i c p r o p p a n t f r a c t u r i n g o f n o t o n l y sandstone, but also c a r b o n a t e r e s e r v o i r s o f b o t h l o w - and h i g h - p e r m e a b i l i t y t y p e s , because e v e n t h e b e s t n a t u r a l f o r m a t i o n p e r m e a b i l i t y i s s t i l l separated by a pronounced steep g r a d i e n t f r o m t h e e x t r e m e l y h i g h - p e r m e a b i l i t y p r o p p a n t package ( c f . s e c t i o n 4.5.1.1.2.). A c o n t r a s t o f a t l e a s t 100 between t h e f l o w c a p a c i t i e s o f f o r m a t i o n and f r a c t u r e a r e c o n s i d e r e d t o be e s s e n t i a l t o o b t a i n a d e q u a t e s t i m u l a t i o n r e s u l t s , and t h i s c a n o n l y be i n f l u e n c e d b y s e l e c t i n g a h i g h - q u a l i t y p r o p p a n t t o f i l l i n t h e f r a c t u r e (WATERS 1 9 8 0 ) , w i t h g r a i n s i z e i n a d d i t i o n t o a n d / o r i n combination w i t h type o f proppants being a very important t o o l f o r maximizing t h e c o n d u c t i v i t y c o n t r a s t between f r a c t u r e and f o r m a t i o n ( c f . s e c t i o n 4 . 3 . 5 . ) . H i g h - c o n d u c t i v i t y s h o r t f r a c t u r e s and t h e i r e c o n o m i c a l b e n e f i t i n m o d e r a t e - p e r m e a b i l i t y o i l r e s e r v o i r s a r e i l l u s t r a t e d b y BRITT (1985; c f . s e c t i o n 4 . 9 . 2 . ) . The n e c e s s i t y o f l a r g e r p r o p p a n t g r a i n s i z e s f o r e f f e c t i v e s t i m u l a t i o n o f mod e r a t e - t o h i g h - p e r m e a b i l i t y r e s e r v o i r s i s n o t o n l y t h e consequence o f t h e r e q u i r e m e n t t o p r o v i d e a d e q u a t e f l o w c a p a c i t y , b u t i s a l s o due t o t h e need t o r e s i s t embedment i n t o t h e f r e q u e n t l y s o f t t o p o o r l y c o n s o l i d a t e d f o r m a t i o n s (CLARK 1983; c f . s e c t i o n 4 . 9 . 2 . ) .
4.6.2. Proppant grain size and concentration U l t r a - h i g h f r a c t u r e p e r m e a b i l i t i e s maximizing the c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r and d r a i n a g e p a t h c a n t h u s be b e s t a c h i e v e d b y a c o m b i n a t i o n o f v e r y l a r g e propparit s a t u r a t i o n o f t h e f r a c t u r i n g f l u i d l e a d i n g t o high-concent r a t i o n m u l t i l a y e r p r o p p i n g o f t h e f r a c t u r e and e x t r a l a r g e p r o p p a n t g r a i n s i zes (BARBY & BARBEE 1987; c f . s e c t i o n s 4 . 3 . 4 . and 4 . 3 . 5 . ) . The g r e a t e r t h e c o n d u c t i v i t y c o n t r a s t , t h e h i g h e r t h e p r o d u c t i o n r a t e s , and c u m u l a t i v e q u a n t i t i e s w i l l be up t o l i m i t s d e f i n e d b y f r a c t u r e and r e s e r v o i r g e o m e t r i c a l and h y d r o d y namical p r o p e r t i e s . F r a c t u r e p e r m e a b i l i t y and t h u s c o n d u c t i v i t y c a n be enhanced b y t a i l - i n p r o p p a n t p l a c e m e n t ( c f . s e c t i o n s 2 . 4 . 1 . 2 . 3 . and 4 . 1 2 . 3 . 3 . ) . Ultra-high conductivity f r a c t u r e s t i m u l a t i o n by predominantly g r a i n - s i z e modelling o f s y n t h e t i c propp a n t s i s a l s o d i s c u s s e d by BARBY & BARBEE ( 1 9 8 7 ) . M o d e r a t e - t o h i g h - p e r m e a b i l i t y r e s e r v o i r s c a n o f t e n a l r e a d y be s u c c e s s f u l l y s t i m u l a t e d b y m i c r o f r a c t u r i n g o r m i n i f r a c t u r i n g (CRAMER & SONGER 1988; c f . s e c t i o n 4 . 8 . 1 . ) .
4.6.3. Proppant/fluid slurry mixing types M o d e r a t e - t o h i g h - p e r m e a b i l i t y r e s e r v o i r s a r e b e s t s t i m u l a t e d b y pumping o f s m a l l p r e c i s e f l u i d volumes c o n t a i n i n g h i g h p r o p p a n t c o n c e n t r a t i o n s (CRAMER & SONGER 1988) f o r c r e a t i o n o f s h o r t h i g h l y c o n d u c t i v e f r a c t u r e s . The f o l l o w i n g a c c o u n t p r e s e n t s a d i s c u s s i o n o f b a t c h - m i x p r e p a r a t i o n and c o n t i n u o u s - m i x b l e n d i n g . Comments a r e a l s o o f f e r e d on s p e c i a l d e s i g n f o r w a t e r f l o o d o p e r a t i o n s .
4.6.3.1. Batch-mix preparation While continuous-mix b l e n d i n g equipment g r e a t l y f a c i l i t a t e s h a n d l i n g o f l a r g e f l u i d and p r o p p a n t volumes and e n a b l e s i m p l e m e n t a t i o n and e x e c u t i o n o f massive h y d r a u l i c f r a c t u r i n g treatments i n t i g h t r e s e r v o i r s , i t i s p o o r l y s u i t e d t o p r e p a r e and d i s p l a c e s m a l l p r e c i s e s l u r r y volumes (CRAMER & SONGER 1988; c f . s e c t i o n 4.3.3.5.3.). Batch-mix f r a c t u r i n g comprising u t i l i z a t i o n o f pre-mixed f l u i d / p r o p p a n t s l u r r i e s i s a s u i t a b l e t e c h n i q u e t o i n c r e a s e w e l l b o r e t r a n s -
535 r n i s s i b i l i t y i n moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s due t o bypass o f t h e w e l l b o r e - v i c i n i t y damage zone ( c f . s e c t i o n 4 . 8 . 3 . ) and e x t e n s i o n o f t h e e f f e c t i ve b o r e h o l e r a d i u s . High proppant c o n c e n t r a t i o n s can be p r e - b l e n d e d w i t h g e l l e d l i q u i d i n t h e batch-mix i n v e n t o r y compartments and s l u r r y d e n s i t y can be v e r i f i e d b e f o r e t h e s t a r t o f t h e j o b . T h i s assures placement o f an u n i f o r m p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e , s i n c e p r o p p a n t s a t u r a t i o n s a t t h e s u r f a c e a r e n o t a f f e c t e d by t r e a t m e n t r a t e f l u c t u a t i o n s . I n c r e m e n t a l improvements i n w e l l b o r e t r a n s m i s s i b i l i t y can i n many cases be achieved by c r e a t i o n o f a s i g n i f i c a n t c o n d u c t i v i t y c o n t r a s t between r e s e r v o i r and f r a c t u r e . The use o f small p r e c i s e s l u r r y v o l u mes a l s o s u c c e s s f u l l y l i m i t s v e r t i c a l and l a t e r a l f r a c t u r e e x t e n s i o n , a v o i d s neg a t i v e consequences o f i n t e r z o n a l communication and w a t e r f l o o d sweep i n e f f i c i e n cy, and reduces s c r e e n o u t problems by achievement o f u n i f o r m p r o p p a n t d i s t r i b u t i o n and use o f a h i g h p a d l s l u r r y r a t i o .
4.6.3.2.
Continuous-mix blending
Conventional c o n t i n u o u s - m i x t r e a t m e n t s have i n h e r e n t l i m i t a t i o n s i n moderasection t e - t o h i g h - p e r m e a b i l i t y r e s e r v o i r s (CRAMER & SONGER 1988; c f . 4.3.3.5.3.). A s r e l a t i v e l y l a r g e s l u r r y volumes a r e o f t e n used, f r a c t u r e dimens i o n s can become e x t e n s i v e and t h i s can r e s u l t i n o n l y a s m a l l p o r t i o n o f t h e crack c o n t r i b u t i n g t o wellbore t r a n s m i s s i b i l i t y g i v i n g r i s e t o wastefulness, and can l e a d t o achievement o f e x c e s s i v e f r a c t u r e l e n g t h and h e i g h t e n a b l i n g acc e l e r a t e d w a t e r breakthrough, h i n d e r e d v e r t i c a l and l a t e r a l sweep e f f i c i e n c y i n w a t e r f l o o d e d r e s e r v o i r s , and c h a n n e l l i n g i n t o v e r t i c a l l y a d j a c e n t pay zones. Continuous-mix b l e n d i n g equipment g r e a t l y f a c i l i t a t e s h a n d l i n g o f l a r g e f l u i d and proppant volumes, b u t i s p o o r l y s u i t e d t o prepare and d i s p l a c e s m a l l p r e c i s e s l u r r y volumes. Non-uniform proppant d i s t r i b u t i o n and i r r e g u l a r p r o p p a n t a d d i t i o n due t o f l u c t u a t i n g t r e a t m e n t r a t e occurs i n t h e f r e q u e n t l y used h i g h c a p a c i t y / v o l u m e b l e n d e r t u b apparatus. I r r e g u l a r p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e can r e s u l t i n l o w - c o n d u c t i v i t y b o t t l e n e c k s and i n s c r e e n o u t f a i l u r e due t o proppant s l u g g i n g ( c f . s e c t i o n s 4.2.3.5.3. and 6 . 2 . 4 . 2 . 1 . ) . Conventional t r e a t m e n t s a l s o o f t e n u t i l i z e r e l a t i v e l y modest p r o p p a n t concent r a t i o n s and t o g e t h e r w i t h narrow f r a c t u r e w i d t h t h i s leads t o low c r a c k conduct i v i t y which i s f u r t h e r d e t e r i o r a t e d by g e l r e s i d u e and o t h e r p r o p p a n t package damage mechanisms. Depending on t h e v i s c o e l a s t i c c h a r a c t e r i s t i c s o f t h e s t i m u l a t i o n f l u i d and on c r a c k c l o s u r e time, proppant s e t t l i n g may o c c u r and enhance fracture c o n d u c t i v i t y i n the lower p o r t i o n s o f the crack ( c f . sections 4.3.3.2. and 4.12.2.2.) which can l e a d t o p r e f e r e n t i a l w a t e r p r o d u c t i o n i f a w a t e r l e g i s p r e s e n t o r may aggravate f l o o d w a t e r c y c l i n g i f a h i g h e r - p e r m e a b i l i t y zone i s l o c a t e d a t t h e f r a c t u r e base. Screenouts o f t e n o c c u r when p e r f o r m i n g convent i o n a l t r e a t m e n t s i n moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s due t o severe fluid-loss r a t e s d u r i n g t h e j o b , g i v i n g r i s e t o s u b s t a n t i a l pad f l u i d l e a k o f f and s l u r r y d e h y d r a t i o n a t t h e l e a d i n g edge o f t h e f r a c t u r e , w i t h t h e screenouts f u r t h e r c o n t r i b u t i n g t o t r e a t m e n t waste and c o s t i n e f f i c i e n c y ( c f . s e c t i o n s 4.2.3.5.3. and 6.2.4.2.1.).
4 . 6 . 3 . 3 . Special design for waterflood operations S p e c i a l c o n s i d e r a t i o n s c o n c e r n i n g f r a c t u r e l e n g t h and c o n d u c t i v i t y a p p l y t o w a t e r f l o o d e d moderate- t o h i g h - p e r m e a b i l i t y o i l r e s e r v o i r s ( c f . a l s o s e c t i o n 4 . 8 . 7 . 1 . ) . Premature i n j e c t i o n w a t e r breakthrough may r e s u l t when c o n d u c t i v e f r a c t u r e s i n excess o f 25 % o f t h e d r a i n a g e r a d i u s a r e c r e a t e d i n b o t h i n j e c t i o n and p r o d u c t i o n w e l l s (CRAMER & SONGER 1988). Water c y c l i n g may be exacerbat e d when a f r a c t u r e i s extended i n t o a h i g h - p e r m e a b i l i t y r o c k l a y e r a t t h e e x pense o f l o w e r - p e r m e a b i l i t y h o r i z o n s o r r e l a t e d l y when p r o p p a n t i s p l a c e d p r e f e r e n t i a l l y a d j a c e n t t o t h e h i g h e r - p e r m e a b i l i t y zone due t o s e t t l i n g ( c f . sect i o n s 4.2.2.4. and 4.12.2.2.2.). B o t h e f f e c t s decrease sweep e f f i c i e n c y o f t h e
536 f l o o d and n e c e s s i t a t e i n j e c t i o n o f s i g n i f i c a n t l y l a r g e r w a t e r volumes t o a c h i e ve a g i v e n o i l r e c o v e r y . I f r e s e r v o i r s a r e bounded by r o c k l a y e r s n o t o f f e r i n g s u f f i c i e n t r e s t r i c t i o n t o v e r t i c a l c r a c k h e i g h t growth, f r a c t u r e communication i n t o a q u i f e r s , gas caps o r o t h e r p r o d u c t i v e zones as a consequence o f a l a r g e s c a l e u n c o n t r o l l e d t r e a t m e n t may cause severe r e s e r v o i r d i s r u p t i o n .
4.6.4. Fracture design character i s t ics The p r i m a r y t r e a t m e n t o b j e c t i v e s i n moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s a r e t o bypass w e l l b o r e v i c i n i t y damage ( c f . s e c t i o n 4 . 8 . 3 . ) , e x t e n d e f f e c t i v e b o r e h o l e r a d i u s , maximize f r a c t u r e c o n d u c t i v i t y ( c f . s e c t i o n s 4.3.5., 4.5.1.1.2. and 4.6.1.), a v o i d e x c e s s i v e f r a c t u r e h e i g h t and l e n g t h ( c f . s e c t i o n 4.2.2.), and m i n i m i z e t r e a t m e n t c o s t (CRAMER & SONGER 1988). Pad f l u i d volume and p r o p e r t i e s should be s e l e c t e d t o enable p r o p p a n t e n t r y and o p t i m i z e p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e . Enough pad s h o u l d be pumped t o keep pace w i t h s p u r t l o s s f r o m t h e f r a c t u r e w h i l e b u i l d i n g a l o w - p e r m e a b i l i t y f i l t e r cake a t the crack w a l l s thus reducing f l u i d - l o s s r a t e . I n some h i g h - p e r m e a b i l i t y r e s e r v o i r s and w i t h c e r t a i n f l u i d systems, f i l t e r cake g e n e r a t i o n i s d i f f i c u l t t o achieve a t a l l and t h e whole f r a c t u r i n g f l u i d can l e a k o f f i n t o t h e f o r m a t i o n m a t r i x . T o t a l l o s s o f pad f l u i d g i v e s r i s e t o proppant b r i d g i n g near t h e f r a c t u r e t i p t h e r e b y p r o v o k i n g t i p s c r e e n o u t ( c f . s e c t i o n s 4.2.3.5.3. and 6 . 2 . 4 . 2 . 1 . ) , and l a c k o f f i l t e r cake g e n e r a t i o n would r e s u l t i n s l u r r y d e h y d r a t i o n and p r o p p a n t c o n c e n t r a t i n g - t y p e s c r e e n o u t i f s l u r r y s u r f a c e s a t u r a t i o n s a r e s u f f i c i e n t l y h i g h and s l u r r y r e s i d e n c e t i m e i s s a t i s f a c t o r i l y l o n g . Some aspects o f f r a c t u r e w i d t h and l e n g t h as w e l l as h i g h p r o p p a n t c o n c e n t r a t i o n and g r a i n s i z e a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.6.4.1. Fracture width and length Treatment i n j e c t i o n r a t e and t h e v i s c o u s c h a r a c t e r i s t i c s o f t h e pad combine w i t h pad f l u i d l o s s p r o p e r t i e s t o determine h y d r a u l i c f r a c t u r e w i d t h . I n o r d e r t o p r e v e n t t h e occurrence o f p r o p p a n t b r i d g i n g i n t h e f r a c t u r e , h y d r a u l i c w i d t h must be a t l e a s t 2.6 t i m e s g r e a t e r than maximum p r o p p a n t p a r t i c l e d i a m e t e r (CRAMER & SONGER 1988). As propped w i d t h i s p r o p o r t i o n a l t o h y d r a u l i c w i d t h , pad v i s c o s i t y , pad volume and i n j e c t i o n r a t e can be i n c r e a s e d t o widen t h e f r a c t u r e d w e l l beyond t h e minimum e n t r y w i d t h f o r t h e purpose o f i n c r e a s i n g c r a c k cond u c t i v i t y . Proppant s e l e c t i o n and s c h e d u l i n g u l t i m a t e l y determine t r e a t m e n t r e s u l t s , w i t h propped f r a c t u r e l e n g t h b e i n g on average up t o 25 % o f t h e d r a i n a g e r a d i u s . A c t u a l propped l e n g t h can exceed p r o j e c t e d l e n g t h due t o p o s t - i n j e c t i o n a f t e r f l o w and v a r i a b l e f l u i d l s l u r r y v e l o c i t y p r o f i l e s i n t h e c r a c k d u r i n g i n s e r t i o n . I n a d d i t i o n t o pad i n j e c t i o n techniques, f r a c t u r e c o n d u c t i v i t y can be max i m i z e d by i n c r e a s i n g s u r f a c e p r o p p a n t c o n c e n t r a t i o n i n t h e s l u r r y t h u s i n c r e a s i n g propped f r a c t u r e w i d t h , which can be t e c h n i c a l l y achieved by b a t c h m i x i n g r a t h e r t h a n continuous-mix b l e n d i n g w i t h o u t i n c r e a s i n g s c r e e n o u t r i s k ( c f . sect i o n s 4.3.3.5.3. and 4 . 6 . 3 . 1 . ) .
4.6.4.2.
High proppant concentration and g r a i n
size
F r a c t u r e c o n d u c t i v i t y can be f u r t h e r enhanced by n o t adding any f l u i d - l o s s m a t e r i a l d u r i n g t h e s l u r r y stages, because r e s i d u e development on t h e c r a c k w a l l s forms p r i m a r i l y a f t e r i n j e c t i o n c e s s a t i o n and i s d i r e c t l y a t t r i b u t a b l e t o t h e remnant c h a r a c t e r i s t i c s o f t h e s l u r r y (CRAMER & SONGER 1988). H i g h p r o p p a n t c o n c e n t r a t i o n s a l s o h a s t e n f r a c t u r e c l o s u r e which m i n i m i z e s s e t t l i n g and a f t e r f l o w e f f e c t s , and a l s o maximizes propped c r a c k h e i g h t i n cases where p r o p p a n t s e t t l i n g i s s i g n i f i c a n t . C o n d u c t i v i t y m a x i m i z a t i o n by coarse p r o p p a n t s ( s u c h as 8/12 and 12/20 mesh; c f . s e c t i o n 4 . 3 . 5 . ) has t o t a k e i n t o account t h a t u n l e s s p e r f o r a t i o n h o l e diameter i s a t l e a s t s i x t i m e s t h e average p r o p p a n t p a r t i c l e s i z e , proppant b r i d g i n g a t t h e p e r f o r a t i o n s l e a d s t o premature t r e a t m e n t f a i -
537 l u r e ( c f . s e c t i o n 5 . 6 . 1 . ) . The f r a c t u r e c o n d u c t i v i t y improvement b e n e f i t o f h i g h e r - q u a l i t y proppants has t o be e v a l u a t e d a g a i n s t t h e a n t i c i p a t e d increment a l p r o d u c t i o n r e t u r n i n o r d e r t o determine i f t h e use o f h i g h e r c o s t / h i g h e r performance r e l a t i o n s h i p i s w a r r a n t e d .
4.7. Geothermal fracturing A p a r t f r o m o i l and gas r e s e r v o i r s , h y d r a u l i c p r o p p a n t f r a c t u r i n g i s a l s o a s u i t a b l e means o f s t i m u l a t i o n o f f o r m a t i o n s b e a r i n g h o t w a t e r o r b e i n g h o t d r y rocks, w i t h t h e p o s s i b i l i t y o f e x p l o i t a t i o n o f geothermal h e a t . Some comments a r e o f f e r e d as f o l l o w s on g e o l o g i c a l aspects as w e l l as a r e a l d i s t r i b u t i o n and e x p l o i t a t i o n o f geothermal r e s e r v o i r s , e x p l o s i v e and non-proppant f r a c t u r i n g , proppant f r a c t u r i n g , communication f r a c t u r i n g , f r a c t u r e system enlargement by h e a t e x t r a c t i o n , and shear s t i m u l a t i o n vs. t e n s i l e f r a c t u r i n g .
4.7.1. Geological aspects of geothermal reservoirs P e r m e a b i l i t y and f l u i d f l o w i n h i g h - t e m p e r a t u r e geothermal systems i s predom i n a n t l y f r a c t u r e - c o n t r o l l e d (MOORE & NIELSON 1986). The f r a c t u r e system cons i s t s o f f a u l t s and j o i n t s . G e o l o g i c a l and geochemical t e c h n i q u e s can p r o v i d e v a l u a b l e i n s i g h t s i n t o b e h a v i o u r and e v o l u t i o n o f a c t i v e f r a c t u r e - c o n t r o l l e d geothermal r e s e r v o i r s . Some comments on d i s t r i b u t i o n o f geothermal r e s e r v o i r t y pes and p e r m e a b i l i t y e v o l u t i o n a r e o f f e r e d as f o l l o w s .
4.7.1.1. Distribution of geothermal reservoir types Many geothermal systems a r e h o s t e d by v o l c a n i c r o c k s e r i e s i n s t r u c t u r a l comp l e x e s o f c a l d e r a s and s t r a t o v o l c a n o e s , b u t sandstone and p l u t o n i c geothermal r e s e r v o i r s a l s o o c c u r . V a r i o u s methods can be a p p l i e d f o r e v a l u a t i o n o f permeab i l i t y d i s t r i b u t i o n and f l u i d f l o w p a t t e r n s i n d i f f e r e n t g e o l o g i c a l e n v i r o n ments. I n many v o l c a n i c geothermal f i e l d s , f l u i d f l o w i s p r i n c i p a l l y c o n f i n e d t o s t e e p l y d i p p i n g normal f a u l t s and s u b s i d i a r y f r a c t u r e s , a l t h o u g h i n t e r b e d d e d sandstone l e n s e s can a l s o a c t as s t r a t i g r a p h i c a q u i f e r s when i n t e r s e c t e d by t h e f a u l t s (HULEN & NIELSON 1982, 1983). T e c t o n i c a l doming can s u b j e c t t h e s t r a t a i n v a r i o u s p a r t s o f a g e o l o g i c a l p i l e t o d i f f e r e n t s t r e s s e s (MOORE & NIELSON 1986). I n t h e upper p o r t i o n s o f t h e dome, t h e r o c k s a r e under t e n s i o n , r e s u l t i n g i n normal f a u l t i n g and open j o i n t s a l o n g t h e c r e s t a l p a r t o f t h e s t r u c t u r e . The upper t e n s i o n a l zone i s s e p a r a t e d by t h e n e u t r a l p l a n e f r o m t h e l o w e r compressional zone where c o n j u g a t e shears a r e p r e s e n t and t h e j o i n t s a r e more o r l e s s c l o s e d . P r i m a r y e x p l o r a t i o n below t h e d e p t h o f t h e n e u t r a l p l a n e i s n o n - p r o d u c t i v e due t o l a c k o f p e r m e a b i l i t y and t h u s l a r g e - s c a l e h y d r a u l i c f r a c t u r i n g i s necessary t o make advantage o f t h e h i g h e r r e s e r v o i r temperature i n g r e a t e r depth.
4.7.1.2. Permeability evolution I t i s i m p o r t a n t i n geothermal e x p l o r a t i o n t o d i f f e r e n t i a t e between f o s s i l f l u i d c o n d u i t s and those which h o s t t h e p r e s e n t l y a c t i v e geothermal system. T r a ce element s i g n a t u r e s t u d i e s can be used t o d i s t i n g u i s h between f l u i d f l o w p a t h s u t i l i z e d by a n c i e n t geothermal systems and t h e g u i d e channels o f t h e modern system. Chemical and i s o t o p i c a l analyses o f f l u i d s can be a p p l i e d f o r assessment o f f l o w p a t t e r n s and r e s i d e n c e t i m e s . Lack o f m i x i n g o f s e v e r a l b r i n e types i n d i c a t e s r e s t r i c t i o n o f f l u i d f l o w t o d i s c r e t e f a u l t and f r a c t u r e zones i n a l o w - p e r m e a b i l i t y m a t r i x i n some r e s e r v o i r s . I n mountaineous t e r r a i n s , t h e se c o n d i t i o n s f a v o u r f l u i d f l o w d r i v e n by t o p o g r a p h i c a l l y c o n t r o l l e d h e a t d i f f e rences r a t h e r t h a n c o n v e c t i v e f l o w common i n more h i g h l y permeable geothermal f i e l d s (ADAMS, MOORE & FORSTER 1985). The importance o f t o p o g r a p h i c a l c o n t r o l
538 on t h e f l u i d f l o w p a t t e r n i s i n some cases i l l u s t r a t e d by t h e c o i n c i d e n c e o f t h e thermal and t o p o g r a p h i c a l d i v i d e s e p a r a t i n g areas o f d i f f e r e n t temperature and f l u i d t y p e . Vein m i n e r a l i z a t i o n and f l u i d i n c l u s i o n r e l a t i o n s h i p s i n sandstone g e o t h e r mal r e s e r v o i r s can document t h e t r a n s i t i o n f r o m m a t r i x t o f r a c t u r e p e r m e a b i l i t y d u r i n g course o f t h e s t r u c t u r a l e v o l u t i o n o f geothermal complexes (ELDERS 1979, MOORE & NIELSON 1986) where a l s o geothermal metamorphism o f sandstones (McDOWELL & ELDERS 1979) takes p l a c e . D i s t i n c t i o n can be made between an i n i t i a l phase w i t h s t r a t i f i e d porous f l o w o f thermal f l u i d s i n sandstones beneath a l a r g e l y impermeable mudstone and e v a p o r i t e caprock complex t h a t forms an a q u i t a r d t h r o u g h o u t t h e h i s t o r y o f t h e system, and an advanced stage w i t h upwards m i g r a t i o n o f f l u i d s o f d i f f e r e n t s a l i n i t y t o d i f f e r e n t depths a l o n g f r a c t u r e s , and then l a t e r a l l y i n p a r t i a l l y porous r o c k s a t s h a l l o w depths where f l u i d f l o w becomes i n c r e a s i n g l y f r a c t u r e - c o n t r o l l e d .
4.7.2. Geothermal reservoir distribution and exploitation Some p o i n t s o f geothermal r e s e r v o i r d i s t r i b u t i o n and e x p l o i t a t i o n a r e d i s c u s sed as f o l l o w s a l o n g t h e l i n e s o f g e o g r a p h i c a l d i s t r i b u t i o n o f European g e o t h e r mal r e s e r v o i r s and worldwide geothermal energy e x p l o i t a t i o n .
4.7.2.1.
Geographical distribution
o f European geo t he rma 1 reservo i r s
Geothermal sources i n c r y s t a l l i n e and sedimentary r o c k s t h a t a r e p a r t i a l l y a l r e a d y n a t u r a l l y f r a c t u r e d a r e i n Europe p r e d o m i n a n t l y p r e s e n t i n I c e l a n d ( K r a f l a ; STEFANSSON 1981, BODVARSSON e t a l . 1984; N e s j a v e l l i r ; BODVARSSON e t a l . 1986; S v a r t s e n g i ; GUDMUNDSSON & OLSEN 1985), I t a l y (Bagnore; ATKINSON, ‘TLn. T I , C O R S I & KUCUK 1980; L a r d e r e l l o ; ATKINSON e t a l . 1978), Southern France and Y u g o s l a v i a (JELIC 1982) and a r e p a r t i a l l y a l r e a d y c o m m e r c i a l l y used (RAMEY & GRINGARTEN 1975). P i l o t s t u d i e s a r e a l s o c a r r i e d o u t i n Germany FRG (ERDOEL-ERDGAS AKTUELL 1978 c, LEYDECKER 1981, M E I E R & ERNST 1981) and o t h e r c o u n t r i e s . O t h e r i n i t i a l p r o j e c t s a r e l o c a t e d i n England (PINE & LEDINGHAM 1 9 8 3 ) . An o v e r view o f t h e European geothermal d r i l l i n g e x p e r i e n c e i s g i v e n by BARON & UNGEMACH ( 1 9 8 1 ) , and aspects o f geothermal h e a t e x p l o i t a t i o n a r e d i s c u s s e d by ERNST & HUEBINGER ( 1 9 7 9 ) . D i s t i n c t i o n has t o be made between low- and h i g h - e n t h a l p y r e s e r v o i r s . Low-ent h a l p y r e s e r v o i r s a r e p r o l i f i c and dependable i n foredeep and i n t r a c r a t o n i c bas i n s wherever sedimentary m u l t i - a q u i f e r systems a r e developed ( s u c h as A q u i t a n i a n and P a r i s Basins/France, Po V a l l e y / I t a l y ; Wessex, East Y o r k s h i r e and Nort h e r n I r e l a n d B a s i n s / U n i t e d Kingdom, Southern Molasse A l p i n e Foredeep and Nort h e r n MunsterlandlGermany FRG, N e t h e r l a n d s and Denmark), low- t o h i g h - e n t h a l p y f o r m a t i o n s i n c o n t i n e n t a l r i f t v a l l e y s and areas w i t h young b u t e x t i n c t v o l c a nism (such as Limague and Rh6ne V a l l e y / F r a n c e , and Rhine Graben and Urach region/Germany FRG). H i g h - e n t h a l p y r e s e r v o i r s o c c u r in areas w i t h s u h r r c m t l y t o r e c e n t l y a c t i v e v o l c a n i s m and t e c t o n i c s w h i c h a r e p l a c e d a l o n g t h e MeuiLerranean b e l t , c o m p r i s i n g c h i e f l y t h e Tuscanian d i s t e n s i v e t e c t o n i c system, t h e Nort h e r n L a t i u m and C a m p a n i a n l I t a l y P l i o c e n e - Q u a t e r n a r y v o l c a n i s m t h a t i s p a r t i a l l y s t i l l a c t i v e a t l e a s t a t p o s t - v o l c a n i c stage, and t h e s u b d u c t i o n magmatism o f t h e A e o l i a n Arc/Greece. H i g h - e n t h a l p y geothermal r e s e r v o i r s a r e i n Europe r e s t r i c t e d t o C e n t r a l and Southern I t a l y and t o Greece (BARON & UNGEMACH 1981). I n terms o f energy c a r r i e r phase, d i s t i n c t i o n has t o be made between vapor-domin a t e d and water-dominated h i g h - t e m p e r a t u r e geothermal systems (GUDMUNDSSON & OLSEN 1 9 8 5 ) .
539
4.7.2.2. Worldwide geothermal energy exploitation D I P I P P O (1984) p r e s e n t s an overview o f worldwide geothermal power development, w i t h t h e r a n k i n g l i s t i n terms o f h e a t g e n e r a t i n g c a p a c i t y b e i n g USA, P h i l i p p i n e s , I t a l y , Japan, Mexico, New Zealand, E l Salvador, I c e l a n d , Indonesia, Kenya, S o v i e t Union, China, A z o r e s / P o r t u g a l , Turkey, Nicaragua, Guadeloupe/France, Guatemala, Costa R i c a and Greece, w i t h o t h e r c o u n t r i e s b e i n g o n l y o f subord i n a t e s i g n i f i c a n c e . Most o f t h e geothermal p l a n t s i n t h e USA a r e c o n c e n t r a t e d i n The Geysers f i e l d i n N o r t h e r n C a l i f o r n i a and I m p e r i a l V a l l e y i n Southern Cal i f o r n i a , w i t h o t h e r areas o f a c t i v i t y i n c l u d i n g s i t e s i n Hawaii, Idaho, Nevada, Oregon, Utah, New Mexico and Texas-Louisiana. I t a l y o p e r a t e s more i n d i v i dual geothermal power u n i t s t h a n any o t h e r c o u n t r y , a l t h o u g h l a g g i n g b e h i n d i n t o t a l c u m u l a t i v e c a p a c i t y . The most i m p o r t a n t geothermal f i e l d s i n I t a l y a r e i n a l p h a b e t i c o r d e r Bagnore, B e l l a v i s t a , Castelnuovo, Gabbro, Lagoni Rossi, La Lecc i a , L a r d e r e l l o , M o l i n e t t o , Monte Rotondo, Pianacce, Piancastagnaio, Radicondoli, San M a r t i n o , Sasso, Serrazzano, T r a v a l e , Tuscan and V a l l o n s o r d o (BARON & UNGEMACH 1981; C I G N I , GAUD10 & FABBRI 1981). The Wairakei f i e l d i n New Zealand i s a l r e a d y in s e r v i c e f o r more t h a n 25 y e a r s (STACEY & THAIN 1983). P r e s e n t s t a t e and f u t u r e p r o s p e c t s o f geothermal energy e x p l o i t a t i o n development a r e summarized by EDWARDS, CHILINGAR, R I E K E & FERTI- (1982) and CATALDI & SOMMARUGA ( 1 9 8 6 ) . I n t h e f a r t h e r M i d d l e East, geothermal energy i s e x p l o i t e d i n I n d i a . Hightemperature p r o p p a n t t e s t i n g f o r p a r t i c u l a r a p p l i c a t i o n i n geothermal f r a c t u r e s t i m u l a t i o n i s performed by SINCLAIR (1980). Aspects o f geothermal w e l l s t i m u l a t i o n a r e a l s o d i s c u s s e d by SINCLAIR, P I T T A R D & HANOLD (1980); GRANT, DONALDSON & BIXLEY (1982); MORRIS & SINCLAIR (1984) and WALKUP & HDRNE ( 1 9 8 5 ) . Experience r e p o r t s on geothermal f r a c t u r i n g a r e g i v e n by ERNST (1977); HUNSBEDT, KRUGER & LONDON (1977) ; MURPHY & TESTER (1979) ; WILLIAMS, ROWLEY, NEUDECKER & BRITTENHAM (1979) ; MORRIS, VERITY, SCHROEDER & GORANSDN (1980) ; CAMPBELL, HANOLD, SINCLAIR & VETTER (1981), and TESTER, MURPHY, GRIGSBY, POTTER & ROBINSON ( 1 9 8 6 ) .
4.7.3. Explosive and non-proppant fracturing E x p l o s i v e f r a c t u r i n g has i n f o r m e r decades been a b u n d a n t l y used as an a l t e r n a t i v e t o h y d r a u l i c f r a c t u r i n g i n b o t h hydrocarbon ( c f . s e c t i o n s 1 . 2 . 1 . and 4.4.4.) and geothermal r e s e r v o i r s t i m u l a t i o n , b u t due t o v a r i o u s reasons, i t s a p p l i c a t i o n has d e c l i n e d d u r i n g t h e l a s t years, m a i n l y as a consequence o f t h e p r o g r e s s i v e l y i m p r o v i n g e f f e c t i v i t y o f h y d r a u l i c f r a c t u r i n g . I n r e c e n t time, however, some r e v i v a l and r e n a i s s a n c e o f e x p l o s i v e f r a c t u r i n g can be observed i n t h e f i e l d o f enhancement o f o i l , gas and w a t e r p r o d u c t i o n , r e s u l t i n g f r o m some i n n o v a t i o n s which make advanced e x p l o s i v e f r a c t u r i n g c o m p e t i t i v e t o hydraul i c f r a c t u r i n g o r even a l l o w a c o m b i n a t i o n o f b o t h s t i m u l a t i o n t e c h n o l o g y branches ( c f . s e c t i o n 4 . 8 . 9 . 6 . ) . I n geothermal r e s e r v o i r s , s t i m u l a t i o n can g e n e r a l l y be performed by hydraul i c f r a c t u r i n g , e x p l o s i v e f r a c t u r i n g and thermal s t r e s s c r a c k i n g (HUNSBEDT, KRUGER & LONDON 1977). The aim o f geothermal r e s e r v o i r s t i m u l a t i o n i s t o i n c r e a s e v e r t i c a l and h o r i z o n t a l f o r m a t i o n p e r m e a b i l i t y i n o r d e r t o p r o v i d e adequate f l u i d c i r c u l a t i o n and t o expose new r o c k s u r f a c e t o a l l o w h e a t t r a n s f e r f r o m t h e h o t r o c k t o t h e f l u i d (HUNSBEDT, KRUGER & LONDON 1975). F o l l o w i n g t h e o u t l i n e o f some g e n e r a l aspects and a s h o r t r e v i e w o f c o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g and r e c e n t i n n o v a t i o n s , t h e stem-induced e x p l o s i v e f r a c t u r i n g t e c h n i q u e (YOUNG, BARKER & CLARK 1984) i s sketched which p r o v i d e s a reasonable means o f c o n t r o l l e d r e s e r v o i r s t i m u l a t i o n b o t h i n hydrocarbon and geothermal f i e l d s . Some comments on n u c l e a r and w a t e r f r a c t u r i n g a r e a l s o o f f e r e d .
4.7.3.1. General aspects Geothermal s t i m u l a t i o n has so f a r been m a i n l y done by c r e a t i o n of j o i n t s i n t i g h t f o r m a t i o n s by c o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g where a more o r l e s s mul-
540 t i p l e r a d i a l f r a c t u r e system ( c f . p l a t e I V / l ) o r i g i n a t e s around a d e t o n a t e d cyl i n d r i c a l c a v i t y ( i n c o n t r a s t t o a h y d r a u l i c a l l y - i n d u c e d f r a c t u r e which f r e q u e n t l y i s s i n g u l a r o r i n some cases a l s o c o n s i s t s o f a narrow b u n d l e o f m u l t i p l e c l o s e l y - s p a c e d c r a c k s i n a s i m i l a r way as n a t u r a l s t r u c t u r a l f a u l t s due t o i t s f o r m a t i o n p e r p e n d i c u l a r l y t o t h e l e a s t p r i n c i p a l s t r e s s ; SCHOLS 1983), b u t i n s e v e r a l cases s e l f - p r o p p i n g ( c f . s e c t i o n s 4.5.1.1.1.2. and 4 . 5 . 1 . 5 . 1 . ) o f t h e v a r i o u s c r a c k s has n o t proven t o be t h e optimum s o l u t i o n . I n a d d i t i o n , t h e p e r m e a b i l i t y around a detonated c a v i t y f o l l o w i n g e x p l o s i v e f r a c t u r i n g decreases e x p o n e n t i a l l y w i t h d i s t a n c e , w i t h t h e e f f e c t t h u s b e i n g h a r d l y h i g h e r than t h a t o b t a i n e d by h y d r a u l i c p r o p p a n t f r a c t u r i n g , i f n o t even i n f e r i o r i n r e s u l t . The s t i m u l a t i o n e f f e c t f r e q u e n t l y s u f f e r e d f r o m o n l y v e r y l i m i t e d p o s s i b i l i t i e s t o c o n t r o l q u a n t i t y and s i z e o f t h e p r o p a g a t i n g f r a c t u res, as w e l l as f r o m c o n s i d e r a b l e c o n d u c t i v i t y d e t e r i o r a t i o n b y e x c e s s i v e rubbl i z a t i o n o f t h e r e s e r v o i r r o c k i n t h e c e n t r a l p a r t o f t h e f r a c t u r e d zone around t h e b o r e h o l e . Some aspects o f f o r m a t i o n damage removal i n v a r i o u s r e s e r v o i r t y p e s as w e l l as s h o r t f r a c t u r e e x t e n s i o n and r e s e r v o i r r o c k r u b b l i z a t i o n a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.7.3.1.1. Formation damage removal in various reservoir types E x p l o s i v e f r a c t u r i n g by d e t o n a t i o n o f chemical o r n u c l e a r d e v i c e s i n open hol e s was b e f o r e t h e advent o f h y d r a u l i c p r o p p a n t f r a c t u r i n g a l s o a common s t i m u l a t i o n t e c h n i q u e o f sandstone o i l and gas r e s e r v o i r s , w i t h t h e e x p l o s i o n c r e a t i n g a r u b b l i z e d f o r m a t i o n and f r a c t u r e s r a d i a t i n g o u t f r o m t h e w e l l b o r e ( c f . p l a t e I V / l ) t h e r e b y i n c r e a s i n g i t s e f f e c t i v e d i a m e t e r ( c f . s e c t i o n s 1 . 2 . 1 . and 4 . 4 . 4 . ) . The main purpose o f s t a n d a r d e x p l o s i v e f r a c t u r i n g i s t o i n c r e a s e t h e s i z e o f t h e w e l l b o r e and t o remove any n e a r - b o r e h o l e f o r m a t i o n damage r e s u l t i n g f r o m m a i n l y d r i l l i n g and s u b o r d i n a t e l y a l s o c o m p l e t i o n (ZUBER, LEE & GATENS 1987; c f . s e c t i o n 4 . 8 . 3 . ) . O p t i m i z a t i o n was a l s o made by p l a c i n g e x p l o s i v e s i n t o n a t u r a l f r a c t u r e s i n t h e f o r m a t i o n , b u t a l l these a p p l i c a t i o n s had a c o n s i d e r a b l y i n f e r i o r e f f e c t i n comparison t o h y d r a u l i c p r o p p a n t f r a c t u r i n g (WATERS 1980). The l i m i t e d p r o d u c t i o n i n c r e a s e f r o m o i l and gas w e l l s s t i m u l a t e d by e x p l o s i v e f r a c t u r i n g c o u l d p r e f e r e n t i a l l y be a t t r i b u t e d t o t h e inadequate o r even l a c k i n g success o f s e l f - p r o p p i n g ( c f . s e c t i o n s 4.5.1.1.1.2. and 4 . 5 . 1 . 5 . 1 . ) of t h e c r e a t e d f r a c t u r e s which c o n s e q u e n t l y a l r e a d y p r e m a t u r e l y c l o s e d o r healed. Aspects o f e x p l o s i v e f r a c t u r i n g a r e a l s o d i s c u s s e d by HOWELL & HILLS (1970); R A MEY, KRUGER & RAGHAVAN (1973); CARTER & OLINGER (1976); GRADY, K I P P & SMITH (1979); SCHMIDT, WARPINSKI & COOPER (1980); McHUGH ( 1 9 8 3 ) ; YOUNG, BARKER & CLARK (1984); WATSON, BENSON & F I L L 0 (1985); BENNETT (1987) and BRANAGAN, LEE, CIPOLLA & WILMER ( 1 9 8 8 ) . WINKLE & MIGNOTTE (1963) o u t l i n e i m p l o s i v e shock t r e a t ment o f i n j e c t i o n w e l l s ,
4.7.3.1.2. Short fracture extension and reservoir rock rubblization E x p l o s i v e f r a c t u r i n g c r e a t e s a h i g h l y cracked and p a r t i a l l y r u b b l i z e d r e g i o n around t h e w e l l b o r e through p r o p a g a t i o n o f h i g h - e n e r g y s t r e s s waves (STOLLER 1985). The e x t r e m e l y h i g h p r e s s u r e s exceed t h e dynamic compressive s t r e n g t h o f t h e w e l l b o r e r o c k , w i t h t h e r e s u l t a n t permanent r o c k compaction p r o d u c i n g a ver y l o w - p e r m e a b i l i t y b a r r i e r a t t h e w e l l b o r e . Even w i t h o u t t h i s p e r m e a b i l i t y b a r r i e r , e f f e c t i v e e x p l o s i v e f r a c t u r i n g i s r e s t r i c t e d t o those f o r m a t i o n s w i t h a h i g h n a t u r a l c r a c k d e n s i t y because o f t h e r e l a t i v e l y s h o r t f r a c t u r e s c r e a t e d , w i t h t h e main s t i m u l a t i o n e f f e c t b e i n g i n t e r c o n n e c t i o n o f numerous i s o l a t e d nat u r a l c r a c k s by t h e r a d i a l l y p r o p a g a t i n g s h o r t f r a c t u r e s generated by t h e e x p l o s i v e t r e a t m e n t ( c f . p l a t e I V / l ) . I n terms o f o p e r a t i o n s , c o n v e n t i o n a l e x p l o s i v e f r a c t u r i n g i s l i m i t e d t o open h o l e s where t h e shock wave coming f r o m t h e detonat i o n o f t h e charge i n t h e w e l l b o r e has d i r e c t c i r c u m f e r e n t i a l access t o t h e r e -
541 s e r v o i r f o r m a t i o n w i t h o u t any r e s t r i c t i o n . S e l e c t i v e placement of charges i n l a r g e r n a t u r a l f r a c t u r e s o r p e r f o r a t i o n t u n n e l s i s a p o s s i b i l i t y t o e x t e n d more s o p h i s t i c a t e d e x p l o s i v e f r a c t u r i n g t o cased h o l e s .
BRANAGAN, LEE, CIPOLLA & WILMER (1988) use h i g h - p r e s s u r e n i t r o g e n gas and e x p l o s i v e f r a c t u r i n g i n t i g h t gas sandstones i n o r d e r t o e l i m i n a t e f o r e i g n liq u i d s from e n t e r i n g t h e r e s e r v o i r c r a c k system f o l l o w i n g p e r f o r a t i o n f o r t h e purpose of m i n i m i z a t i o n o f e x t e r n a l i n t e r v e n t i o n i n t h e n a t u r a l p r o d u c t i o n p r o cess.
4.7.3.2. Conventional explosive fracturing and recent innovation D u r i n g t h e l a s t decade, increasing m o d i f i c a t i o n o f conventional explosive f r a c t u r i n g techniques i n o r d e r t o develop c o s t - and p e r f o r m a n c e - c o m p e t i t i v e a l t e r n a t i v e s t o h i g h - e f f e c t i v i t y h y d r a u l i c s t i m u l a t i o n have l e d t o v a r i o u s innovat i o n s which m a i n l y f o c u s on p r o p e l l a n t - b a s e d charges f o r w e l l t r e a t m e n t (FOURNEY, BARKER & HOLLOWAY 1983; CUDERMAN & NORTHROP 1984; YOUNG, BARKER & CLARK 1984). Research e f f o r t s c o n c e n t r a t e d on development o f e x p l o s i v e - and p r o p e l lant-founded techniques designed t o m i n i m i z e w e l l b o r e damage which i s provoked by c o n v e n t i o n a l e x p l o s i v e s t i m u l a t i o n treatments, and t o maximize t h e r a d i a l d i s t a n c e t o w h i c h f r a c t u r e s m i g h t be propagated i n t o t h e f o r m a t i o n . The e x p l o s i v e - b a s e d techniques i n c l u d e methods f o r d i r e c t l y d i s p l a c i n g a sens i t i v e l i q u i d e x p l o s i v e i n t o a h y d r a u l i c f r a c t u r e system b e f o r e d e t o n a t i o n (SPENCER 1981) which l e a d s t o development o f s t a r - l i k e secondary c r a c k s r a d i a t i n g f r o m t h e main f r a c t u r e t h a t have a s i m i l a r i n c r e a s i n g d r a i n a g e area e f f e c t as a bundle o f low-angle t o h o r i z o n t a l d r a i n h o l e s emanating f r o m a main subvert i c a l o r i n c l i n e d w e l l b o r e , and i n t r o d u c t i o n o f a l o w - d e t o n a t i o n - v e l o c i t y e x p l o s i v e t h a t would be p l a c e d d i r e c t l y across t h e pay zone (WATSON, BENSON & F I L L 0 1985). I n s e r t i o n o f e x p l o s i v e charges i n t o h y d r a u l i c a l l y - c r e a t e d f r a c t u r e s and s e l e c t i v e f i r i n g r e p r e s e n t s a c o m b i n a t i o n o f e x p l o s i v e and h y d r a u l i c f r a c t u r i n g ( c f . s e c t i o n 4.8.9.4.), b u t i n a s i m i l a r way a l s o i n t r o d u c t i o n o f e x p l o s i v e charges i n t o w i d e r n a t u r a l j o i n t s i s p o s s i b l e . T h i s t e c h n i q u e has t h e advantage t h a t e i t h e r h y d r a u l i c f r a c t u r e o r n a t u r a l crack serve as a g u i d e channel, and t h e i n c r e a s i n g d i s t a n c e o f t h e d e t o n a t i o n p o i n t away f r o m t h e b o r e h o l e l e a v e s t h e immediate w e l l b o r e v i c i n i t y i n t a c t and a v o i d s f o r m a t i o n damage by r e s e r v o i r rock r u b b l i z a t i o n i n the region c l o s e s t t o the borehole. Contained e x p l o s i o n s and d e f l a g r a t i o n s r e p r e s e n t a new p e r s p e c t i v e o f w e l l s h o o t i n g (THOMAS & ROOT 1979). The e f f o r t s u s i n g p r o p e l l a n t s as chemical energy source have focussed on t a i l o r i n g p r o p e l l a n t b u r n r a t e and consequent p r e s s u r e / t i m e h i s t o r y so t h a t maximum w e l l b o r e f r a c t u r i n g and minimum w e l l b o r e damage a r e r e a l i z e d . Explosive-based techniques which a r e much cheaper and more r e l i a b l e i n f i e l d performance, however, may have broader a p p l i c a b i l i t y and may be p r e f e r a b l e o v e r any p r o p e l l a n t - b a s e d means. Out o f many concepts, t h e method o f stem-induced e x p l o s i v e f r a c t u r i n g (YOUNG, BARKER & CLARK 1984) has proven t o be one o f t h e most s u c c e s s f u l i n n o v a t i o n s .
4.7.3.3.
Stem- i nduced exp 10s ive f rac t ur ins
The e f f e c t i v i t y o f e x p l o s i v e f r a c t u r i n g can be g r e a t l y enhanced by t h e steminduced t e c h n i q u e (YOUNG, BARKER & CLARK 1984) i n v o l v i n g placement o f t h e p r i m a r y e x p l o s i v e charge i n an expendable r a t h o l e below t h e t a r g e t zone i n such a manner t h a t t h e h o r i z o n t o be f r a c t u r e d i s n o t d i r e c t l y s u b j e c t e d t o t h e h i g h detonation pressures o f the explosive i n c o n t r a s t t o conventional s t i m u l a t i o n where t h e e x p l o s i v e charge i s f i r e d immediately o p p o s i t e t o t h e t a r g e t i n t e r v a l . The h i g h - p r e s s u r e gases generated by t h e d e t o n a t i o n o f t h e e x p l o s i v e , however, a r e used t o f r a c t u r e t h e w e l l b o r e t h r o u g h t h e placement o f a s o l i d stem o f g r a v e l a t o r near t h e t o p o f t h e zone t o be t r e a t e d . W e l l b o r e f r a c t u r i n g i s i n i t i a t e d and l a r g e l y c o n t r o l l e d by t h e p r e s s u r e l t i m e h i s t o r y o f t h e e x p l o s i v e
542
gases a s they r e f l e c t off the s o l i d stem a f t e r moving u p through the open borehole s e c t i o n between the explosive charge and the gravel p l u g (FOURNEY, BARKER
& HOLLOWAY 1981; FOURNEY, HOLLOWAY & S I M H A 1984; YOUNG, BARKER & CLARK 1 9 8 4 ) .
Stem-induced explosive f r a c t u r i n g i s s u i t a b l e f o r treatment of only one i s o l a t e d t a r g e t h o r i z o n o r two d i s t i n c t and s ep a ra te pay zones, with the m ultiple la y e r st i m u l a t i on configuration designed t o f r a c t u r e the upper r e s e r v o i r sectio n by r e f l e c t i o n of the explosive gases o f f the s o l i d gravel stem a n d t o crack the lower pay level by i t s proximity t o the explosive charge. The discussion as follows concentrates on co n t r o l l ed o r i g i n of limited multiple f r a c t u r e system, v e r t i c a l f r a c t u r e o r i e n t a t i o n r ega rdle ss of ge ostre ss s t a t e , l i m i t a tio n s and p o s s i b i l i t i e s of improvement, and proppant i n s e r t i o n i n t o explosiongenerated f r a c t u r e s .
4.7.3.3.1.
C o n t r o l l e d o r i g i n o f l i m i t e d m u l t i p l e f r a c t u r e system
The m a i n advantage of the stem-induced explosive f r a c t u r i n g technique i s i t s c o n t r o l l e d performance with o r i g i n of only two s e t s of ra dia l cracks propagatin g i n roughly orthogonal d i r e c t i o n s as v isua liz e d by borehole t e l e v i s i o n a n d photography (YOUNG, BARKER & CLARK 1984; c f . se c tion 6 . 2 . 1 . 9 . 3 . ) . The f a c t t h a t m u l t i p l e f r a c t u r i n g by stem-induced explosive detonation i s so lim ite d s p a t i a l l y and does not exceed four orthogonal f r a c t u r e s even in c lose proximity t o the primary charge a t t e s t s t o the b en ef i ci al e f f e c t s of energy source decoupl ing a n d i s the main di f f er en ce t o conventional explosive f r a c t u r i n g where a whole bundle of closely-spaced or even almost merged multiple f r a c t u r e s in a frequentl y s t a r - l i k e p a t t e r n ( c f . p l a t e I V / 1 ) i s cr ea te d which together with rubbliz a t i o n v i r t u a l l y destroys the r e s e r v o i r f a b r i c in c o n t r a s t t o smooth i n t e r s e c t i o n of an i n t a c t formation matrix by d i s c r e t e perpendicular f r a c t u r e s . This s u p e r i o r i t y of the stem-induced explosive f r a c t u r i n g concept has been v e r i f i e d by comparative experiments with high detonation r a t e s and r e s u l t i n g h i g h loading r a t e s of n i t r o g l y c e r i n charges t h a t led t o o r i g i n of u p to ten vert i c a l cracks forming a mu l t i p l e bundle even i n slim boreholes, with f r a c t u r e a p e r t u r e and thus a l s o p en et r at i o n depth i n t o the formation being q u i t e lim ite d as opposed t o stem-induced treatments where good f r a c t u r e openings indic a te reasonable propagation i n t o the r e s e r v o i r .
4.7.3.3.2. V e r t i c a l f r a c t u r e o r i e n t a t i o n regardless o f geostress s t a t e The most s i g n i f i c a n t c h a r a c t e r i s t i c f e a t u r e of stem-induced explosive f r a c t u ring i s the d o m i n a n t v e r t i c a l o r i e n t a t i o n and e xte nt of the cracks re ga rdle ss of the i n - s i t u s t r e s s d i s t r i b u t i o n in the r e s e r v o i r column YOUNG, BARKER & CLARK 1984). The a b i l i t y t o develop dominant v e r t i c a l f r a c t u r ng a l s o in a n ins i t u s t r e s s s t a t e t h a t g en er al l y d i c t a t e s the o r i g i n of horizontal cracks ( f o r example i n hydraulic s t i mu l at i o n of shallow r e s e r v o i r s ) i s a s i g n i f i c a n t benef i t t h a t can be obtained w i t h the proper ap p lic a tion of explosive and/or propell a n t s t i m u l a t i o n techniques and i s one of the advantages w i t h re spe c t t o hydraul i c f r a c t u r i n g where no influence can be exerted on the i n - s i t u s t r e s s s t a t e a n d thus o r i e n t a t i o n of the cr ack s . This r e l a t i o n s h i p even allows the combinat i o n of h o r i z o n t a l hydraulic a n d v e r t i c a l explosive f r a c t u r e c re a tion giving r i s e t o orthogonal crack networking f o r s p ec ia l re se rvoir stim ula tion ( c f . sectio n 4 . 8 . 9 . 6 . ) .
4.7.3.3.3. L i m i t a t i o n s and p o s s i b i l i t i e s o f improvement The stem-induced explosive f r a c t u r i n g technique has thus been proven t o be q u i t e e f f e c t i v e i n modifying o r t a i l o r i n g the pressure/time h i s t o r i e s obtained from a l a r g e explosive charge (YOUNG, BARKER & CLARK 1 9 8 4 ) . The stem-induced a p -
543 proach o f f e r s t h e p o s s i b i l i t y o f f r a c t u r i n g w i t h o u t s i g n i f i c a n t w e l l b o r e damage a zone w i t h much more e x p l o s i v e gas energy than c o u l d be p l a c e d i n t h a t r e s e r v o i r l e v e l d i r e c t l y by e i t h e r decoupled e x p l o s i v e charges o r c o n t r o l l e d - b u r n r a t e p r o p e l l a n t s . The o n l y l i m i t a t i o n o f t h e stem-induced f r a c t u r i n g t e c h n i q u e i s t h e a v a i l a b i l i t y o f an expendable r a t h o l e f o r placement o f t h e p r i m a r y e x p l o s i v e charge. On t h e o t h e r hand, however, t h e m a j o r disadvantage o f any e x p l o s i v e f r a c t u r i n g method w i t h r e s p e c t t o h y d r a u l i c s t i m u l a t i o n i s t h e i n a b i l i t y t o s u p p o r t t h e c r a c k s w i t h proppants i n o r d e r t o p r e v e n t them f r o m subsequently c l o s i n g . The l a c k o f p l u g g i n g m a t e r i a l e x c e p t o f random f i n e f o r m a t i o n r u b b l e ( a u t o - p r o p ping; c f . s e c t i o n s 4.5.1.1.1.2. and 4.5.1.5.1.) gives r i s e t o rapid fracture f l o w d e t e r i o r a t i o n by p a r t i a l c l o s u r e o f t h e c r a c k upon p r e s s u r e drawdown dur i n g hydrocarbon o f f t a k e l e a d i n g t o r e s e r v o i r d e p l e t i o n . Another n e g a t i v e e f f e c t i s i n f i l l i n g o f p a r t s o f t h e c r a c k s w i t h r u b b l i z e d o r even p u l v e r i z e d r o c k d e b r i s o r i g i n a t i n g f r o m r e s e r v o i r framework d e s t r u c t i o n i n t h e immediate s u r r o u n d i n g s o f t h e d e t o n a t i o n p o i n t o f t h e e x p l o s i v e charge ( c f . p l a t e I V / l ) .
4.7.3.3.4. Proppant insert ion into exp 10sion-generated fractures T h e r e f o r e t h e most c h a l l e n g i n g t a s k f o r f u t u r e e x p l o s i v e f r a c t u r i n g o p t i m i z a t i o n would be t h e i n v e s t i g a t i o n o f p o s s i b i l i t i e s o f f i r s t opening a double s e t o f c r a c k s by e x p l o s i v e t r e a t m e n t and then second f o l l o w i n g w i t h a h y d r a u l i c oper a t i o n which i s t o e n l a r g e t h e a l r e a d y e x i s t i n g f r a c t u r e s , t o f l u s h o u t the f i n e d e b r i s f r o m t h e cracks, and t o i n f i l l them w i t h proppants i n o r d e r t o a v o i d t h e i r subsequent c l o s u r e ( s u c h a c o m b i n a t i o n o f e x p l o s i v e and h y d r a u l i c f r a c t u r i n g resembles i n f a c t an i n t e r l o c k i n g o f p r e e x i s t i n g n a t u r a l and l a t e r a r t i f i c i a l l y - c r e a t e d h y d r a u l i c f r a c t u r e s ; c f . s e c t i o n s 4.5.2. and 4 . 8 . 9 . 4 . ) , w i t h such a l i n k i n g , however, o n l y b e i n g a b l e t o be performed i n g e o s t r e s s r e gimes f a v o u r i n g t h e development o f v e r t i c a l h y d r a u l i c f r a c t u r e s due t o t h e e x c l u s i v e c r e a t i o n o f v e r t i c a l c r a c k s by e x p l o s i v e s t i m u l a t i o n . O t h e r aspects o f stem-induced t a i l o r e d - p u l s e f r a c t u r i n g a r e d i s c u s s e d i n s e c t i o n 4.2.2.1.3.2.
4.7.3.4. Nuclear fracturing A p a r t f r o m c o n v e n t i o n a l charges, e x p l o s i v e f r a c t u r i n g o f underground format i o n s can a l s o be c a r r i e d o u t by d e t o n a t i n g n u c l e a r charges (HOWARD & FAST 1970 a ) . While n u c l e a r f r a c t u r i n g has so f a r o n l y r a r e l y been a p p l i e d i n geothermal r e s e r v o i r s b u t would c e r t a i n l y r e p r e s e n t a s u i t a b l e t e c h n i q u e f o r l a r g e - s c a l e c r a c k i n g o f t i g h t rocks, n u c l e a r e x p l o s i o n s have more f r e q u e n t l y been u t i l i z e d i n o i l and gas e x p l o i t a t i o n . N u c l e a r d e t o n a t i o n s can be c o n t a i n e d i n t h e zone o f i n t e r e s t and a r e capable o f r u b b l i n g l a r g e r o c k volumes t h e r e b y c r e a t i n g mult i p l e f r a c t u r e s such as o r i g i n a t i n g a l s o d u r i n g c o n v e n t i o n a l o i l - w e l l s h o o t i n g ( c f . s e c t i o n 4.7.3.1.), b u t on a much l a r g e r s c a l e . Some comments on p r i n c i p l e s and a p p l i c a t i o n s a r e o f f e r e d as f o l l o w s .
4.7.3.4.1. Principles F r a c t u r e s r e s u l t i n g f r o m underground n u c l e a r e x p l o s i o n s d i f f e r f r o m those c r e a t e d by h y d r a u l i c s t i m u l a t i o n i n b o t h number o f f r a c t u r e s generated and penet r a t i o n d e p t h i n t o t h e m a t r i x around t h e w e l l b o r e (HOUARD & FAST 1970 a ) . N u c l e a r e x p l o s i o n s a r e expected t o generate a c a v i t y o f 100 - 300 f t ( 3 0 - 100 m) diameter, w i t h numerous f r a c t u r e s r a d i a t i n g f r o m i t t h u s f o r m i n g a high-permeab i l i t y zone and c o n s i d e r a b l y e n l a r g i n g t h e e f f e c t i v e w e l l b o r e r a d i u s . Upon d e t o n a t i o n o f t h e n u c l e a r device, a c a v i t y i s generated as a r e s u l t o f v a p o r i z a t i o n o f r o c k s and s a t u r a t i n g f l u i d s . The gases c o n t a i n e d w i t h i n t h e c a v i t y a r e i n i t i a l l y a t e x t r e m e l y h i g h p r e s s u r e and temperature. Compaction o f t h e r o c k i n t h e l o w e r hemisphere c o u p l e d w i t h upward and l a t e r a l r o c k movement r e s u l t s i n a s p h e r o i d c a v i t y . The gas p r e s s u r e t e m p o r a r i l y supports t h e overburden t h u s p r e -
544 s e r v i n g t h e c a v i t y shape. Subsequent h e a t l o s s e s , gas l e a k o f f t h r o u g h t h e c r a c k systcm, and vapor condensation reduce t h e p r e s s u r e u n t i l t h e f r a c t u r e d r o c k above t h e c a v i t y can no l o n g e r be supported. Rock c o l l a p s e i n t o t h e c a v i t y forms a complex chimney-rubble zone, w i t h most o f t h e m o l t e n m a t e r i a l and r a d i o a c t i v e f i s s i o n p r o d u c t s c o l l e c t i n g i n t h e b o t t o m o f t h e zones. Changes i n c a v i t y shape f r o m s p h e r i c a l may be due t o i m p l o s i o n o f c a v i t y w a l l s as w e l l as upl i f t and d r o p p i n g o f t h e c e i l i n g . I n areas where h y d r a u l i c f r a c t u r i n g i s e f f e c t i v e , n u c l e a r e x p l o s i o n s c a n n o t compete because o f h i g h c o s t s , b u t s u b s t a n t i a l volumes o f hydrocarbons a r e i n p l a c e i n r e s e r v o i r s t h a t do n o t y i e l d t o p r e s e n t h y d r a u l i c f r a c t u r i n g t e c h n i ques, and these pay zones may t h e r e f o r e by p o t e n t i a l s i t e s f o r n u c l e a r e x p l o s ive s t i m u l a t i on.
4.7.3.4.2. Am1 icat ions The h i t h e r t o main areas o f a p p l i c a t i o n o f n u c l e a r f r a c t u r i n g a r e New Mexico and Colorado/USA (HOWARD & FAST 1970 a) and E a s t e r n Siberia/USSR ( O I L GAS JOURNAL 1987 b, 1987 d ) . A f t e r l o w e r i n g t h e n u c l e a r charge i n t o t h e b o r e h o l e , t h e w e l l i s cemented t o t h e s u r f a c e , and o n l y i n such a h e r m e t i c a l l y s e a l e d e n v i r o n ment, t h e n u c l e a r charge i s f i r e d , w i t h no r a d i a t i o n b e i n g v e n t e d t o t h e atmosphere. N u c l e a r e x p l o s i o n s i n t h e USSR have r e c e n t l y prover) t o be a b l e t o i n t e r connect a number o f s m a l l s e p a r a t e o i l - b e a r i n g l e n s e s f r o m which o i l so f a r c o u l d n o t be r e c o v e r e d p r o f i t a b l y . The e x p l o s i o n r e s u l t s i n f r a c t u r i n g beyond t h e r e s e r v o i r boundaries and combines t h e m i n i d e p o s i t s i n t o a s i n g l e o i l p a t c h which p e r m i t s maximum r e c o v e r y , w i t h t h e n u c l e a r e x p l o s i o n b e i n g a s p e c i a l case o f communication f r a c t u r i n g t o l i n k i s o l a t e d pay u n i t s ( c f . s e c t i o n 4 . 1 0 . 2 . 1 . ) . E a r l i e r a t t e m p t s t o f r a c t u r e such f o r m a t i o n s w i t h c o n v e n t i o n a l e x p l o s i v e s were o n l y m a r g i n a l l y e f f e c t i v e . N u c l e a r b l a s t s a l s o have t u r n e d o u t t o be f e a s i b l e t o c r e a t e underground s t o r a g e r e s e r v o i r s i n s a l t f o r m a t i o n s . Aspects o f n u c l e a r f r a c t u r i n g i n v a r i o u s r e s e r v o i r t y p e s a r e a l s o discussed by ATKINSON & LEKAS (1963); COFFER, BRAY, KNUTSON & RAWSTON (1964) and BRAY, KNUTSON, WAHL & DEW (1965).
A p r o b a b l e f u r t h e r a p p l i c a t i o n f o r n u c l e a r f r a c t u r i n g a r e massive o i l s h a l e s i n Colorado/USA and s u r r o u n d i n g s which can be r e t o r t e d i n p l a c e (HOWARD & FAST 1970 a ) . The o i l i s removed i n l i q u i d o r gaseous f o r m f r o m t h e c a v i t y c r e a t e d by n u c l e a r e x p l o s i o n , t h e r e b y e l i m i n a t i n g t h e need t o mine t h e s h a l e r o c k and t o process i t through a r e t o r t t o e x t r a c t t h e hydrocarbons (HOWARD & FAST 1970 a).
4.7.3.5, Water fracturing A s a consequence o f h i g h r e s e r v o i r temperatures which o f t e n exceed t h e t h e r mal s t a b i l i t y boundary o f many t r e a t i n g f l u i d s , geothermal w e l l s t i m u l a t i o n i s f r e q u e n t l y c a r r i e d o u t by s i m p l e w a t e r f r a c t u r i n g w i t h o u t p r o p p a n t s . H y d r a u l i c f r a c t u r e s c o n n e c t i n g n a t u r a l c r a c k systems p r o v i d i n g c o n t a c t t o a f a u l t zone o r c r e a t i n g communication t o a n e i g h b o u r i n g w e l l a r e generated by i n j e c t i o n o f wat e r above f r a c t u r e p r e s s u r e . Since w a t e r has v e r y l i m i t e d p o s s i b i l i t i e s o f propp a n t t r a n s p o r t , such t r e a t m e n t s a r e commonly o p e r a t e d w i t h o u t p r o p p a n t i n s e r t i o n . S i m i l a r non-proppant f r a c t u r i n g i n c l u d e s n i t r o g e n s t i m u l a t i o n o f s h a l e s ( c f . s e c t i o n 4 . 4 . 4 . ) . W h i l e geothermal w a t e r f r a c t u r i n g comprises m a i n l y l a r g e scale jobs ( c f . section 4.7.4.1.2.), s m a l l - s c a l e w a t e r f r a c t u r i n g can successf u l l y enhance o i l p r o d u c t i o n f r o m carbonates and e s p e c i a l l y c h a l k by s u p p o r t i n g w a t e r i m b i b i t i o n ( c f . s e c t i o n 4 . 5 . 4 . 5 . 2 . ) . Water f r a c t u r i n g i s a l s o o f c o n s i d e r a b l e s i g n i f i c a n c e i n coal-seam s t i m u l a t i o n ( c f . s e c t i o n 4 . 4 . 3 . 4 . 1 . )
545
4.7.4.
Proppant f r a c t u r i n g
H y d r a u l i c f r a c t u r i n g s t i m u l a t i o n o f geothermal w e l l s p r e s e n t s c h a l l e n g i n g problems due t o f o r m a t i o n temperatures i n t h e range 300 - 600 OF which r e q u i r e c a r e f u l e v a l u a t i o n o f t h e b e h a v i o u r o f f l u i d s , proppants and equipment i n t h i s h o s t i l e h o t b r i n e environment (CAMPBELL, HANOLD, SINCLAIR & VETTER 1981; c f . I n o r d e r t o a v o i d p o s s i b l e damage t o t h e p r o d u c i n g h o r i z o n , s e c t i o n 4.4.5.). h i g h - t e m p e r a t u r e chemical c o m p a t i b i l i t y between i n - s i t u and i n s e r t e d m a t e r i a l s must be v e r i f i e d . The most s i g n i f i c a n t p o i n t i s t h a t i n geothermal w e l l s , t h e r e q u i r e d t e c h n i q u e s must be capable o f b r i n g i n g about t h e p r o d u c t i o n o f v e r y l a r g e amounts o f f l u i d s . T h i s n e c e s s i t y f o r h i g h f l o w r a t e s r e p r e s e n t s a s i g n i f i c a n t d e p a r t u r e f r o m c o n v e n t i o n a l p e t r o l e u m w e l l s t i m u l a t i o n and demands t h e c r e a t i o n o f v e r y h i g h n e a r - w e l l b o r e p e r m e a b i l i t y and/or f r a c t u r e s w i t h v e r y h i g h f l o w c o n d u c t i v i t y . H y d r a u l i c s t i m u l a t i o n t r e a t m e n t s i n geothermal f i e l d s may be c a r r i e d o u t i n f o r m a t i o n s which produce e i t h e r h o t w a t e r o r steam f r o m b o t h m a t r i x p e r m e a b i l i t y and f r o m n a t u r a l f r a c t u r e systems. I n t e r e s t i n g t a r g e t s f o r h y d r a u l i c f r a c t u r e s t i m u l a t i o n a r e geothermal w e l l s t h a t do n o t i n t e r s e c t nearby m a j o r n a t u r a l c r a c k systems, which can b e n e f i t f r o m t h e e s t a b l i s h m e n t o f h i g h - c o n d u c t i v i t y l i n e a r f l o w channels t o improve f l o w capacity from surrounding l o c a l i z e d regions o f low-permeability formation, which s u f f e r e d f r o m a r t i f i c i a l damage d u r i n g d r i l l i n g , c o m p l e t i o n o r workover o p e r a t i o n s i n c l u d i n g mud o r cement i n v a s i o n , and t h a t r e q u i r e p e r i o d i c a l remed i a l t r e a t m e n t as a r e s u l t o f f l u i d - p r o d u c t i o n - r e l a t e d damage. I f f r a c t u r e s t i m u l a t i o n can d i m i n u i s h o r e l i m i n a t e t h e need f o r new w e l l o r s i d e t r a c k d r i l l i n g i n t h e aforementioned s i t u a t i o n s , t h e p o t e n t i a l f o r i m p r o v i n g geothermal development economics and e x t e n d i n g t h e r e s o u r c e base i s c e r t a i n l y s u b s t a n t i a l . Some aspects o f proppants, f l u i d s and t r e a t m e n t s i n geothermal r e s e r v o i r s t i m u l a t i o n a r e b r i e f l y sketched as f o l l o w s .
4.7.4.1.
Proppant aspects
I n geothermal w e l l s , c l o s u r e s t r e s s s h o u l d i n most cases be much l o w e r than i n e q u i v a l e n t - d e p t h o i l and gas w e l l s because o f t h e s i n g l e - f l u i d system ( a l though i n some cases a two-phase system o c c u r s i f b o t h w a t e r and steam a r e p r e s e n t ) . C l o s u r e s t r e s s i s more o r l e s s i n t h e o r d e r o f t h e h y d r o s t a t i c g r a d i e n t which i s a b t . 0.5 p s i / f t and t h u s ranges f r o m 2,000 t o 6,000 p s i i n w e l l s f r o m 4,000 t o 12,000 f t i n d e p t h (SINCLAIR 1980). T h e r e f o r e i n most cases n a t u r a l sand c o u l d be a p p l i e d as p r o p p i n g agent ( c f . s e c t i o n s 1.2.2. and 1.3.2.) i f t h e h i g h - t e m p e r a t u r e environment i s n e g l e c t e d . According t o t h e h o t f o r m a t i o n w a t e r and i n o r d e r t o meet a l s o boundary c o n d i t i o n s i n deeper w e l l s , however, s i n t e r e d b a u x i t e i s c e r t a i n l y t h e s u p e r i o r m a t e r i a l f o r p r o p p i n g o f f r a c t u r e s i n geothermal w e l l s , because t h e r e i s a d e f i n i t e need f o r c h e m i c a l l y i n e r t proppants t h a t r e t a i n high permeability f o r long periods o f time i n the high-temperature r e s e r v o i r and t h u s t e m p e r a t u r e - s e n s i t i v e p r o p p a n t t y p e s have t o be d i s c a r d e d ( c f . s e c t i o n s 1.3.2.1. and 5 . 7 . 1 . ) . Some comments on h i g h - t e m p e r a t u r e p r o p p a n t performance and f i e l d e x p e r i e n c e a r e o f f e r e d as f o l l o w s .
4.7.4.1.1.
High-temperature proppant performance
High-temperature p r o p p a n t t e s t i n g under geothermal r e s e r v o i r c o n d i i o n s has shown t h a t s i n t e r e d b a u x i t e , r e s i n - c o a t e d s i n t e r e d b a u x i t e and r e s n-coated sand a r e t h e most s t a b l e m a t e r i a l s and a r e i n s o l u b l e up t o 500 OF ( 2 5 oC) and e i t h e r i n s o l u b l e o r o n l y s l i g h t l y s o l u b l e above 500 OF (SINCLAIR 1980). Convent i o n a l n a t u r a l sand which i s w i d e l y used i n o i l and gas i n d u s t r y i s n o t s t r o n g enough t o w i t h s t a n d t h e c o n d i t i o n s i n most geothermal w e l l s , because i t i s a f f e c t e d by h i g h - t e m p e r a t u r e w a t e r and b r i n e p a r t i c u l a r l y under s t r e s s ( c f . sect i o n 5 . 7 . 1 . ) . The main disadvantages o f sand a r e b r i t t l e f a i l u r e by p o i n t - t o p o i n t l o a d i n g , weakening by numerous m i c r o f r a c t u r e s and f a u l t s w i t h i n t h e g r a i n s , and s t r e s s c o r r o s i o n c r a c k i n g ( c f . s e c t i o n 1.4.10.3.2.) i n hot fluids
546 already a t low cl o s u r e s t r e s s e s , w i t h n a t u r a l sand the re fore being inadequate as p r o p p a n t i n geothermal r e s e r v o i r s . Glass beads a re n o t acceptable f o r geothermal well stim ula tion due t o t h e i r b r i t t l e f a i l u r e a l o n g w i t h s t r e s s corrosion cracking i n high-temperature brine ( c f . s e c t i o n 1 . 2 . 3 . 1 . ) . Resin-coated sand i s the most s u i t a b l e and probably a l s o the most c o s t - e f f e c t i v e p r o p p a n t f o r geothermal wells due t o i t s i n s e n s i t i v i t y t o temperature a n d l o a d , p r o t ect i v e coating of i n e r t r e s i n , and low de nsit y ( c f . s e c t i o n 1 . 2 . 6 . ) . Resin-coated s i n t e r e d bauxite i s the be st proppant ever t e st e d showing no temperature s e n s i t i v i t y and permeability decrease under load and w o u l d c e r t a i n l y be the f i r s t choice f o r geothermal r e s e r v o i r f r a c t u r i n g i f being l e s s expensive ( S I N C L A I R 1980; CAMPBELL, HANOLD, SINCLAIR & VETTER 1981). Although s l i g h t l y crushable, s i n t e r e d bauxite i s much stronge r than sand a n d e f f e c t i v e l y i n e r t in hot b r i n es a l s o w i t h o u t re sin p e l l i c l e s envelop i n g the g r a i n s .
4.7.4.1.2. Field experience Experience r ep o r t s of p r o p p a n t f r a c t u r i n g of geothermal r e s e r v o i r s a r e given by CAMPBELL, HANOLD, SINCLAIR & VETTER (1981) a n d MORRIS & SINCLAIR (1984). The h o t t e s t geothermal well i n the world which has been p r o p p a n t f r a c t u r e d so f a r i s Baca in New Mexico/USA where a high-conductivity crack h a s been generated by proppicg w i t h s i n t e r e d bauxite i n a hydrothermal well i n a depth o f a b t . 5,000 f t (1,700 m ) and a bottomhole temperature of 540 OF (280 oC; VERITY & MORRIS 1981; IIILLIAMS, MOODROOF & BOX 1982; MORRIS & SINCLAIR 1984). A l t h o u g h the s t i mulation treatment d i d not u l t i mat el y r e s u l t i n a commercial well because p r o d u c t i v i t y declined t o an uneconomical level a f t e r the f r a c t u r i n g job due t o rel a t i v e permeability reduction associated with two-phase flow e f f e c t s in the water-steam system t h a t r e s u l t s from r e s t r i c t e d inflow as a consequence of the t i g h t r e s e r v o i r matrix around the f r a c t u r e , the successful technical perforinance of the operation promises hydraulic f r a c t u r i n g of geothermal p a y zones t o be a v a l i d f u t u r e a l t e r n a t i v e t o s i d et r ack i n g o r new well d r i l l i n g . Aspects of proppant f r a c t u r i n g o f geothermal wells a r e a l s o t r e a t e d by CARDEN, NICHOLSON, PETTIT & ROWLEY (1983). Geothermal we11 s t i mula tion operations a r e a l s o evaluated by CAMPBELL, FIORRIS & VERITY (1981). Non-proppant geothermal f r a c t u r i n g of hot d r y rock r e s e r v o i r s w i t h c h i e f l y water as f r a c t u r i n g f l u i d ( c f . s ect i o n 4 . 7 . 3 . 5 . ) includes the l a r g e s t hydraulic stimulation operations ever c a r r i e d out i n terms of f l u i d volume. The c u r r e n t world record comprises a treatment in 13,500 f t re se rvoir depth with 600 OF b o t tonhole temperature where over 10 Mio. gal of water plus f r i c t i o n reducer a n d calcium carbonate have been i n j ect ed (FRACFAX 1988 b ) .
4.7.4.2. Fluid aspects I n terms of f l u i d s , h i g h - v i s co s i t y g el s which a re commonly applied in lowtemperature o i l a n d gas wells cannot be used in geothermal r e s e r v o i r s where low-viscosity f l u i d s have t o be the r u l e , because of the extremely high temperatu r e s t h a t degrade the g el l ed polymer f l u i d s , and a l s o due t o stim ula tion f l u i d c o s t . Some a s p e c t s of s e n s i t i v i t y and s t a b i l i z a t i o n of f l u i d v i s c o s i t y a r e o u t lin e d as follows.
4.7.4.2.1. Fluid viscosity sensitivity Water-soluble
polymers
are
the main v i s c o s i f i e r s f o r h y d r a u l c f r a c t u r i n g , i n viscosity . Delaying of adding small 11.4.). High-
b u t above 250 OF, almost a l l the polymer systems show a de c line (CAMPBELL, HANOLD, SINCLAIR & VETTER 1981; c f . se c tion 4.4.5. t h i s decline or degradation in p r o p er t i es can be performed by amounts of methanol t o the polymer-water s o lutions ( c f . se c tion 3
547 temperature f l u i d s t a b i l i z a t i o n can a l s o be achieved by adding an oxygen scaveng e r i n o r d e r t o m i n i m i z e polymer d e g r a d a t i o n by d i s s o l v e d oxygen. R a p i d d e c l i n e i n v i s c o s i t y o f t h e f l u i d once i n s e r t e d i n t o t h e f r a c t u r e would i n e v i t a b l y r e s u l t i n poor p r o p p a n t placement o r would even provoke premature s c r e e n o u t f a i l u r e ( c f . s e c t i o n s 4.2.3.2.5.and 6.2.4.2.1.) o f h i g h - t e m p e r a t u r e geothermal s t i m u l a t i o n t r e a t m e n t s . H i g h - v i s c o s i t y f r a c t u r i n g f l u i d s can o n l y be app i e d i n h i q h - t e m e r a t u r e qeothermal r e s e r v o i r s i f preceded by a l a r q e c o o l i n q w a t e r pre-pad t h a t give; r i s e t o h e a t blockage ( c f . s e c t i o n - 4 . 3 . 4 . 6 . i . 4 . ) .
4.7.4.2.2. Fluid viscosity stabilization HARMS. GOSS & PAYNE (1984) p r e s e n t a new q e n e r a t i o n o f f r a c t u r i n q f l u ds f o r u l t r a h i g h - t e m p e r a t u r e a p p l i c a t i o n where an un7que c o m b i n a t i o n o f h e t e r o g neousl y s u b s t i t u t e d p o l y s a c c h a r i d e polymer, s t a b i l i z e r s and delayed c r o s s l i n k i n g chem i c a l s p r o v i d e s t h e extended thermal s t a b i l i t y . An i n n o v a t i v e c r o s s l i n k i n g mechanism enhances t h e thermal p r o p e r t i e s o f t h e f l u i d , and c h e m i c a l l y c o n t r o l l e d c r o s s l i n k i n g r e a c t i o n a l l o w s t h e maintenance o f c r o s s l i n k i n g t i m e o v e r a wide range o f chemical and/or p h y s i c a l v a r i a b l e s such as temperature and polymer l o a d i n g . The c o n t r o l l e d v i s c o s i t y development g i v e s r i s e t o l o w e r f r i c t i o n p r e s sure and b e t t e r downhole s t a b i l i t y .
GRONE, BAUMGARTNER & WOODROOF (1983) document a c r o s s l i n k e d f r a c t u r i n g f l u i d system which has extended thermal s t a b i l i t y due t o f a v o u r a b l e v a r i a t i o n s o f polymer, b u f f e r , g e l s t a b i l i z e r and c r o s s l i n k e r and t h a t a l l o w e d s u c c e s s f u l propp a n t f r a c t u r i n g j o b s up t o 540 OF (280 oC) r e s e r v o i r temperature (WILLIAMS, WOODROOF & BOX 1982; c f . a l s o s e c t i o n 4.4.5.2.).
4.7.4.3. Treatment aspects Geothermal w e l l s t i m u l a t i o n u t i l i z e s b o t h l a r g e - s c a l e c o n v e n t i o n a l h y d r a u l i c f r a c t u r i n g o f deep l o w - p e r m e a b i l i t y zones and m i n i a t u r e f r a c t u r i n g t r e a t m e n t s o f s h a l l o w h i g h - p e r m e a b i l i t y r e s e r v o i r s (CAMPBELL, HANOLD, SINCLAIR & VETTER 1981). F l u i d f l o w w i t h i n t h e f r a c t u r e systems and between w e l l s can be m o n i t o r e d by t r a c e r r e t u r n a n a l y s i s and t r a c e r r e t e n t i o n m o d e l l i n g (TESTER, BIVENS & POTTER 1982; WALKUP & HORNE 1985) which i s p a r t i c u l a r l y u s e f u l f o r p r o p e r p l a n n i n g o f waste w a t e r r e i n j e c t i o n i n t o h i g h l y - f r a c t u r e d geothermal r e s e r v o i r s i n o r d e r n o t t o reduce t h e e n t h a l p y o f t h e produced w a t e r by s h o r t - c i r c u i t i n g w i t h i n t h e pay zone (PRUESS & BODVARSSON 1984), w i t h a correspondence e x i s t i n g b e t ween f a s t t r a c e r r e t u r n r a t e s and e n t h a l p y d e t e r i o r a t i o n upon r e i n j e c t i o n (HORNE 1982, WALKUP & HORNE 1985). Aspects o f h y d r a u l i c f r a c t u r i n g i n geothermal energy r e s e r v o i r s a r e a l s o d i s cussed by AAMODT e t a l . (1977), BODVARSSON & GAULKE (1986) and TESTER, MURPHY, GRIGSBY, POTTER & ROBINSON (1986). E f f e c t s o f non-proppant f r a c t u r i n g t r e a t ments a r e r e p o r t e d by MAHONEY, STUBBS, SCHWERER & DOBSCHA (1980). T e s t i n g o f geothermal and geopressured f o r m a t i o n s i s reviewed by McCOY, HARTSOCK & DOBSON (1980); DA S I E & HOANG (1985); LEAVER, SAGEEV & RAMEY (1986) and MARCOU & GUDMUNDSSON (1986). BODVARSSON, PRUESS & LIPPMANN (1986) summarize m o d e l l i n g o f geothermal systems i n c l u d i n g e v a l u a t i o n o f f r a c t u r e - d o m i n a t e d f l o w . I n j e c t i o n and r e c o v e r y e f f e c t s i n f r a c t u r e d geothermal r e s e r v o i r s a r e analyzed by PRUESS & BODVARSSON (1984) and BODVARSSON, PRUESS & SULLIVAN (1985).
4.7.5. Comnunication fracturing I n terms o f geothermal communication f r a c t u r i n g , d i s t i n c t i o n has t o be made between c o n n e c t i o n o f i n j e c t i o n and p r o d u c t i o n w e l l s on t h e one hand and connect i o n o f w e l l s and l a r g e - s c a l e n a t u r a l f r a c t u r e s on t h e o t h e r hand.
548
4 . 7 . 5 . 1 . Connection o f injection and production wells A p a r t f r o m c o n v e n t i o n a l p r o d u c t i o n o f h o t w a t e r f r o m a q u i f e r - t y p e geothermal r e s e r v o i r s w i t h o r w i t h o u t s t i m u l a t i o n , a s p e c i a l method i n geothermal energy e x p l o i t a t i o n i s t h e c r e a t i o n o f communication f r a c t u r e s ( c f . s e c t i o n 4 . 1 0 . ) b e t ween n e i g h b o u r i n g w e l l s and pumping o f c o l d w a t e r f r o m one b o r e h o l e t h r o u g h t h e c r a c k t o t h e o t h e r w e l l i n o r d e r t o h e a t i t up i n t h e h o t d r y r o c k . The d i r e c t i o n o f f r a c t u r e p r o p a g a t i o n can be e s t i m a t e d f r o m d e t e r m i n a t i o n o f t h e s t r e s s f i e l d by o r i e n t e d c o r e s o r i n - s i t u measurements by b r e a k o u t l o g g i n g ( c f . sect i o n 6 . 2 . 1 . 8 . ) , and t h e t a r g e t w e l l can then be d r i l l e d a c c o r d i n g l y ( p o s s i b i l i t i e s o f i n - s i t u s t r e s s d e t e r m i n a t i o n a r e o u t l i n e d by R O E G I E R S 1974; ABDU-SAYED, BRECHTEL & CLIFTON 1977; WARPINSKI, SCHMIDT & NORTHROP 1980; VOEGELE & JONES 1980, BLUMLING 1983, TEUFEL 1983; DANESHY, SLUSHER, CHISHOLM & MAGEE 1984; BLANTON & TEUFEL 1986). Enhancement o f f r a c t u r e p r o p a g a t i o n t h r o u g h t h e c e n t r e s o f t h e b o r e h o l e s can be achieved by o r i e n t i n g t h e p e r f o r a t i o n s t o t h e d i r e c t i o n s o f t h e s t r e s s f i e l d o f t h e r e s e r v o i r (AHMED, THOMPSON, KELKAR, VEGHTE & HATHAWAY 1984; JARED, SCOTT & EVANS 1984; SCOTT 1986; c f . s e c t i o n s 4.10.3. and 4.10.5.). As a l s o i n h y d r a u l i c f r a c t u r i n g o f c r y s t a l l i n e r o c k s and t i g h t sedimentary f o r m a t i o n s t h e s e l f - p r o p p i n g ( c f . s e c t i o n s 4.5.1.1.1.2. and 4 . 5 . 1 . 5 . 1 . ) o f some p i l o t f r a c t u r e s has n o t t u r n e d o u t t o be s a t i s f a c t o r i l y e f f e c t i v e and n a t u r a l c r a c k s o f b o t h sandstone and carbonate f o r m a t i o n s have a l s o n o t shown t o be s u f f i c i e n t pathways, t h e a p p l i c a t i o n o f proppants f o r s u p p o r t i n g t h e c r a c k s w i l l become more and more i m p o r t a n t w i t h t h e p r o g r e s s o f geothermal energy e x p l o i t a t i o n i n t h e near f u t u r e . F i e l d examples o f communication f r a c t u r i n g between two u e l l s i n geothermal r e s e r v o i r s a r e r e p o r t e d by ALBRIGHT & PEARSON ( 1 9 8 0 ) , PEARSON ( 1 9 8 1 ) ; BATCHELOR, B A R I A & HEARN (1983); SARDA (1984), TALEBI & CORNET (1985), BAME & FEHLER (1986) and ROBERTS & CRAMPIN ( 1 9 8 6 ) .
4.7.5.2. Connection o f wells and large-scale natural fractures A s p e c i a l a p p l i c a t i o n o f communication f r a c t u r i n g i n geothermal r e s e r v o i r s i s t o connect t h e w e l l s w i t h nearby m a j o r p r o d u c t i v e c r a c k s o r f a u l t s and t o achieve t h e d e s i r e d h i g h temperatures o f t h e produced f l u i d ( u p t o 270 OF o r mor e ; CAMPBELL, HANOLD, SINCLAIR & VETTER 1981). D e n d r i t i c f r a c t u r i n g t e c h n i q u e ( c f . a l s o s e c t i o n s 4 . 5 . 4 . 4 . 3 . and 4 . 8 . 8 . 4 . ) o f f e r s t h e b e s t chance o f i n t e r s e c t i n g m a j o r n a t u r a l c r a c k s , whereas a s i n g l e p l a n a r f r a c t u r e m i g h t o n l y p a r a l l e l and n o t c r o s s t h e p r i n c i p a l n a t u r a l c r a c k s . D e n d r i t i c f r a c t u r e s a r e caused by p u l s i n g t h e f o r m a t i o n w i t h r e v e r s e f l o w which provokes f o r m a t i o n s p a l l i n g and d i v e r s i o n o f t h e f r a c t u r e wings by downhole s t r e s s m o d i f i c a t i o n . The b e s t r e s u l t s a r e achieved i n n a t u r a l l y j o i n t e d f o r m a t i o n s where m a j o r and m i n o r f r a c t u r e systems a l r e a d y e x i s t b u t may n o t have f l o w c a p a b i l i t y . M u l t i p l e pumping p e r i o d s a r e used w i t h each stage o f d e n d r i t i c f r a c t u r i n g u t i l i z i n g a l o u - v i s c o s i t y f l u i d , sand s l u g s and s e v e r a l flow-back p e r i o d s , w i t h h i g h f l o w r a t e s and f r i c t i o n r e d u c t i o n b e i n g a p p l i e d i n these t r e a t m e n t s (CAMPBELL, HANOLD, S I N CLAIR & VETTER 1981; MURPHY & FEHLER 1986). An a l t e r n a t i v e t o t h e g e n e r a t i o n o f new f r a c t u r e s can be r e a c t i v a t i o n o f p r e - e x i s t i n g j o i n t s and f i s s u r e s i n some cases (NEIER & ERNST 1 9 8 1 ) .
4.7.6.
Fracture system enlargement
by
heat extraction
TESTER, MURPHY, GRIGSBY, POTTER & ROBINSON (1986) document h y d r a u l i c f r a c t u r i n g and subsequent i n c r e a s e o f a c c e s s i b l e r e s e r v o i r volume and c r a c k e d r o c k s u r f a c e area by energy e x t r a c t i o n o p e r a t i o n s which causes s u b s t a n t i a l thermal drawdown i n p o r t i o n s o f t h e f o r m a t i o n and r e s u l t s i n r e s e r v o i r c o n t r a c t i o n due t o c o o l i n g which i n c l u d e s opening o f s h r i n k a g e j o i n t s ( i n a comparable manner as t a k i n g p l a c e d u r i n g s o l i d i f i c a t i o n and c r y s t a l l i z a t i o n o f v o l c a n i c l a v a f l o w s and p i p e f i l l i n g s ; c f . p l a t e X I / 5 - 6 ) . Excessive r e s e r v o i r s t r e s s e s r e s u l t i n g from h e a t removal can be r e l e a s e d w i t h s t r e s s u n l o c k i n g t e c h n i q u e s c o m p r i s i n g
549 i n j e c t i o n a t e l e v a t e d p r e s s u r e s exceeding t h e i n - s i t u normal and f r i c t i o n a l s t r e s s e s w h i c h h o l d t h e f r a c t u r e f a c e s t o g e t h e r , t h u s g i v i n g r i s e t o shear s l i p page and m i c r o s e i s m i c i t y ( c f . s e c t i o n 6.2.2.3.2.). Large-scale heterogeneities such as s u p e r p o s i t i o n o f f l o w s i n m u l t i p l e j o i n t s undoubtedly e x e r t g r e a t i n f l u e n c e s on h e a t t r a n s f e r behaviour, s i n c e t h e s p a t i a l p o s i t i o n i n g o f these low-impedance c o n d u i t s e f f e c t i v e l y d e f i n e s t h e a c c e s s i b l e volume o f t h e r o c k . I n t h e absence o f thermal s t r e s s c r a c k i n g , r e s e r v o i r growth due t o h e a t e x t r a c t i o n i s r e a l l y a thermal c o n t r a c t i o n e f f e c t , w i t h t h e f r a c t u r e s expanding due t o s h r i n k a g e o f t h e r o c k s s u r r o u n d i n g t h e c r a c k s upon thermal drawdown. Overp r e s s u r i z a t i o n d u r i n g i n j e c t i o n can a c c e l e r a t e enlargement o f t h e f r a c t u r e system, and f u r t h e r s u p p o r t i s g i v e n by remedial p r e s s u r i z a t i o n l e a d i n g t o s t r e s s unlocking. BODVARSSON, PRUESS & O-SULLIVAN (1985) a l s o document t h e b e n e f i c i a l e f f e c t o f i n j e c t i o n f o r i n c r e a s i n g t h e l o n g - t e r m energy o u t p u t f r o m w e l l s because o f s u p p o r t o f h e a t e x t r a c t i o n f r o m t h e r e s e r v o i r r o c k s and maintenance o f h i g h f l o w r a t e s . T h i s r e l a t i o n s h i p i s v e r y h e l p f u l f o r geothermal o p e r a t i o n s where i n most h i g h - t e m p e r a t u r e f i e l d s b r i n e s have t o be r e i n j e c t e d as a consequence o f r e s e r v o i r p r e s s u r e maintenance, enhancement o f energy r e c o v e r y f r o m h o t d r y o r wet f o r m a t i o n rocks, and e n v i r o n m e n t a l s a f e t y o b l i g a t i o n s n o t p e r m i t t i n g s u r f a c e d i s p o s a l o f t h e b r i n e s . Aspects o f i n j e c t i o n response o f geothermal r e s e r v o i r s a r e a l s o d i s c u s s e d by LIPPMANN, TSANG & WITHERSPOON (1977); BODVARSSON & TSANG ( 1982) ; GUDMUNDSSON, HAUKSSON, THORHALLSSON, ALBERTSSON & THOROLFSSON (1984) and GUDMUNDSSON & OLSEN ( 1 9 8 5 ) . R e i n j e c t i o n has t o be done w i t h f l u i d s o f low gas s a t u r a t i o n , because t h e l o w e r t h e f l o w i n g gas c o n c e n t r a t i o n , the more b a c k f l o w o f f l u i d s f r o m t h e f r a c t u r e s i n t o t h e m a t r i x o f t h e r e s e r v o i r r o c k t a k e s p l a c e , t h e r e b y u l t i m a t e l y i n c r e a s i n g t h e r e c o v e r a b l e geothermal energy r e s e r v e s (0-SULLIVAN, BODVARSSON, PRUESS & BLAKELEY 1985; BODVARSSON & GAULKE 1986). Success m o n i t o r i n g o f r e s e r v o i r enlargement by h e a t e x t r a c t i o n can a l s o be performed by m u l t i p l e i n t e r f e r e n c e t e s t i n g o f w e l l s which d e l i n e a t e s r e s e r v o i r boundaries (EARLOUGHER & RAMEY 1973; LEAVER, SAGEEV & RAMEY 1986; c f . s e c t i o n 4.2.4.1.2.2.).
4.7.7. Shear stimulation vs. tensile fracturing F i e l d e x p e r i e n c e o f h y d r a u l i c t r e a t m e n t s o f j o i n t e d geothermal r e s e r v o i r s i n d i c a t e s shear s t i m u l a t i o n t o be t h e d e c i s i v e mechanism i n n a t u r a l l y f r a c t u r e d f o r m a t i o n s r a t h e r t h a n c o n v e n t i o n a l t e n s i l e f a i l u r e (MURPHY & FEHLER 1986; c f . s e c t i o n s 4.8.8.4.2. and 6.2.2.3.2.). A c o u p l e d r o c k mechanics and f l u i d f l o w mod e l shows t h a t shear s l i p p a g e a l o n g p r e - e x i s t i n g j o i n t s i s more e a s i l y induced t h a n c o n v e n t i o n a l t e n s i l e f a i l u r e , p a r t i c u l a r l y when t h e d i f f e r e n c e between m i nimum and maximum e a r t h s t r e s s e s i s l a r g e , t h e j o i n t s a r e o r i e n t e d a t angles between 30 and 60 degrees t o t h e p r i n c i p a l e a r t h s t r e s s e s , and a l o w - v i s c o s i t y f l u i d i s i n j e c t e d . Aspects o f o r i g i n o f shear s l i p p a g e , d i l a t i o n b e h a v i o u r , j o i n t d e f o r m a t i o n spectrum, and d e n d r i t i c f r a c t u r i n g t r e a t m e n t s a r e o u t l i n e d as follows.
4.7.7.1.
Origin o f shear slippage
Most r o c k masses c o n t a i n p r e - e x i s t i n g f r a c t u r e s o r j o i n t s (MURPHY & FEHLER 1986). I n j e c t i o n o f f l u i d s r e s u l t s i n r e d u c t i o n o f t h e e f f e c t i v e c l o s u r e s t r e s s . W i t h c o n t i n u i n g i n j e c t i o n , t h e f l u i d p r e s s u r e can a t t a i n a v a l u e h i g h enough t h a t t h e e f f e c t i v e c l o s u r e s t r e s s no l o n g e r p r o v i d e s s u f f i c i e n t f r i c t i o n i n o r d e r t o r e s i s t s h e a r i n g s t r e s s e s a c t i n g p a r a l l e l t o t h e j o i n t mode, and shear s l i p p a g e w i l l occur. S u f f i c i e n t shear s l i p p a g e l e a d s t o o v e r r i d i n g o f one s u r f a c e a s p e r i t y o v e r o r a t o p another, so t h a t even i f t h e p r e s s u r e i s suddenly reduced, j o i n t opening and p e r m e a b i l i t y a r e i r r e v e r s i b l y i n c r e a s i n g , r e s u l t i n g i n shear s t i m u l a t i o n . If
f l u i d i n j e c t i o n r a t e s a r e modest, shear s t i m u l a t i o n may g i v e r i s e t o s u f -
550 f i c i e n t permeability t h a t no f u r t h e r increase i n pressure i s a t t a i n a b l e . If, however, t h e f o r m a t i o n o f v o i d space b y s h e a r i n g i s i n s u f f i c i e n t t o accommodate t h e f l u i d volume i n j e c t e d i n t o t h e r o c k j o i n t s , t h e p r e s s u r e w i l l c o n t i n u e t o r i s e and e v e n t u a l l y a t t a i n a v a l u e e q u a l t o t h e e a r t h s t r e s s a c t i n g n o r m a l t o t h e j o i n t and c o n s e q u e n t l y t h e e f f e c t i v e c l o s u r e s t r e s s i s z e r o , w i t h t h e n t h e o p p o s i n g s u r f a c e s o f t h e r o c k t h a t meet a t t h e j o i n t s t a r t i n g t o p a r t . T h i s j o i n t s e p a r a t i o n does n o t i n v o l v e a c t u a l r u p t u r i n g o f t h e r o c k and t h u s t h e p r o c e s s s h o u l d n o t be c a l l e d f r a c t u r i n g . I f p r o p p a n t s , e i t h e r p u r p o s e l y i n j e c t e d w i t h the f l u i d o r a c c i d e n t a l l y broken o f f the j o i n t surfaces, a r e trapped w i t h i n t h e crack f o l l o w i n g shut-in, j o i n t s t i m u l a t i o n i s achieved by i r r e v e r s i b l e increase o f j o i n t opening.
4.7.7.2. Dilation behaviour The d i l a t i o n b e h a v i o u r f o r s l i p p a g e and s e p a r a t i o n o f j o i n t s i s q u i t e d i f f e r e n t (MURPHY & FEHLER 1 9 8 6 ) . W i t h i n c r e a s i n g f l u i d p r e s s u r e , d i l a t i o n i s f i r s t s m a l l and s i m p l y r e s u l t s f r o m t h e d e c r e a s e o f e f f e c t i v e c l o s u r e s t r e s s , but t h e n s h e a r s l i p p a g e ensues. L4ith c o n t i n u i n g j o i n t s l i p p a g e , t h e c r a c k s u r f a c e s a t t a i n a s t a t e i n w h i c h one l a r g e r o u g h n e s s a s p e r i t y l i e s a t o p a n o t h e r and f u r t h e r s l i p p a g e w o u l d a l l o w t h e l a r g e s t a s p e r i t y t o s l i d e o v e r and down t h e o t h e r , w i t h t h e maximum s h e a r d i l a t i o n t y p i c a l l y b e i n g i n t h e o r d e r t o a f r a c t i o n o f a m i l l i m e t e r (BARTON, BANDIS & BAKHTAR 1 9 8 5 ) . F u r t h e r i n c r e a s i n g j o i n t p r e s s u r e t h e n l e a d s t o s e p a r a t i o n w i t h o r i g i n o f an o p e n i n g o f t y p i c a l l y t e n s o f m i l l i m e t e r s (PERKINS & KERN 1961, DANESHY 1973 a ) . T h e r e f o r e t h e k e y t o u n d e r s t a n d i n g s t i m u l a t i o n l i e s i n a c o m b i n a t i o n o f r o c k m e c h a n i c s and f l u i d dynam i c s and i n v o l v e s b o t h s h e a r s l i p p a g e and t e n s i l e f a i l u r e (LOCKNER & BYERLEE and 6 . 2 . 2 . 1 . 2 . ) . 1977, MURPHY & FEHLER 1986; c f . s e c t i o n s 4.8.8.4.2. Thus w h i l e h y d r a u l i c f r a c t u r i n g has been c o n v e n t i o n a l l y c o n s i d e r e d t o comp r i s e a l m o s t e x c l u s i v e l y t e n s i l e f a i l u r e o f t h e r o c k and s e p a r a t i o n o f t h e c r a c k w a l l s , s h e a r s l i p p a g e s t i m u l a t i o n i s a l s o a v e r y i m p o r t a n t p r o c e s s (LOCKNER & BYERLEE 1977, HAST 1979; SOLBERG, LOCKNER & BYERLEE 1 9 8 0 ) , p a r t i c u l a r l y i n h o t d r y r o c k g e o t h e r m a l r e s e r v o i r s (MURPHY & FEHLER 1986; TESTER, IWRPHY, GRIGSBY, POTTER & ROSINSON 1 9 8 6 ) .
4.7,7.3. Joint deformation system I n an a c t u a l h y d r a u l i c f r a c t u r i n g o p e r a t i o n , t h e e n t i r e s p e c t r u m o f j o i n t def o r m a t i o n t a k e s p l a c e (MURPHY & FEHLER 1 9 8 6 ) . N e a r t h e i n j e c t i o n w e l l where t h e f l o w passage a r e a i s l i m i t e d , f l u i d v e l o c i t i e s and p r e s s u r e g r a d i e n t s a r e l a r g e and s e p a r a t i o n o c c u r s . N e a r t h e t i p s o f t h e f r a c t u r e s f a r away f r o m t h e i n j e c tion well, however, f l u i d v e l o c i t i e s and p r e s s u r e s a r e much r e d u c e d and s h e a r s t i m u l a t i o n t a k e s p l a c e . D i s t i n c t i o n has t o b e made between r o c k m e c h a n i c s and f l u i d d y n a n i c s i n h y d r o c a r b o n and g e o t h e r m a l r e s e r v o i r s . I n p e t r o l e u m - b e a r i n g formations, very viscous f r a c t u r i n g f l u i d s are normally i n j e c t e d a t h i g h rates, w i t h c o n s e q u e n t l y j o i n t s e p a r a t i o n b e i n g d o m i n a n t , and because i n many c a s e s o n l y a l i m i t e d amount o f n a t u r a l c r a c k s e x i s t s , a c t u a l j o i n t i n g o f t h e r o c k b y t e n s i l e f a i l u r e o c c u r s . I n g e o t h e r m a l r e s e r v o i r s , however, n a t u r a l j o i n t s o c c u r f r e q u e n t l y , and h i g h downhole t e m p e r a t u r e s r e n d e r m o s t v i s c o s i f y i n g a g e n t s u s e l e s s , w i t h t h e r e f o r e w a t e r b e i n g u s e d as f r a c t u r i n g f l u i d and t h i s l o w f l u i d v i s c o s i t y l e a d s t o a dominance o f s h e a r s t i m u l a t i o n ( c f . s e c t i o n 4 . 7 . 3 . 5 . ) . Microearthquake m o n i t o r i n g o f geothermal r e s e r v o i r s t i m u l a t i o n r e v e a l s shear s l i p p a g e a l o n g e x i s t i n g j o i n t s t o be t h e d o m i n a n t cause o f s e i s m i c i t y and a z o ne o f m u l t i p l e f r a c t u r e s o c c u r r i n g p r e d o m i n a n t l y b y s h e a r f a i l u r e , t h u s b e i n g i n c o n t r a s t t o the conventional hydraulic f r a c t u r i n g theory t h a t p r e d i c t s the p r o p a g a t i o n o f a s i n g l e c r a c k due t o t e n s i l e r o c k f a i l u r e a t a p r e s s u r e e q u a l t o t h e minimum e a r t h s t r e s s (MURPHY & FEHLER 1986; c f . s e c t i o n 6 . 2 . 2 . 3 . 2 . ) . Shear s t i m u l a t i o n m o d e l l i n g b y i n t e r a c t i v e c o u p l i n g o f f l u i d dynamics w i t h r o c k s t r e s s e s and d e f o r m a t i o n shows t h a t when t h e o r i e n t a t i o n s o f t h e p r e - e x i s t i n g
551 j o i n t s were r o t a t e d 30 degrees f r o m t h e p r i n c i p a l s t r e s s d i r e c t i o n s and a lowv i s c o s i t y f r a c t u r i n g f l u i d l i k e w a t e r i s used, two types o f s t i m u l a t i o n p a t t e r n occur. The f i r s t t y p e comprises c r e a t i o n o f o n l y a s i n g l e j o i n t i f f r i c t i o n a l r e s i s tance t o shear s l i p p a g e i s low and maximum d i l a t a n c y ( c f . s e c t i o n 4.8.1.3.1.) due t o shear i s l a r g e . The second t y p e i n c l u d e s m u l t i p l e j o i n t s t i m u l a t i o n c o r responding t o h i g h shear r e s i s t a n c e o r small d i l a t a n c y . Shear s l i p p a g e a l o n g t h e j o i n t s i s accompanied by s h e a r - s t r e s s drops, and t h e i n t e r a c t i o n o f these s t r e s s drops w i t h t h e a c t i n g e a r t h s t r e s s e s r e s u l t s i n opening o f j o i n t s more p e r p e n d i c u l a r t o t h e maximum s t r e s s , w i t h t h u s a d e n d r i t i c o r branched j o i n t p a t t e r n o r i g i n a t i n g . T h i s d e n d r i t i c s t i m u l a t i o n f a b r i c c o n s i s t i n g o f mu1 t i p l e branches has i m p o r t a n t i m p l i c a t i o n s i n r e s e r v o i r e n g i n e e r i n g , because volume d r a i n a g e i s more e f f i c i e n t than a r e a l d r a i n a g e .
4 7.7.4. Dendr i t i c f ract ur i n s treatments D e n d r i t i c f r a c t u r i n g t r e a t m e n t s have o r i g i n a l l y been i n t r o d u c e d t o t h e hydrocarbon e x p l o i t a t i o n i n d u s t r y and a r e u s u a l l y c a r r i e d o u t by r e p e a t e d l y f r a c t u r i n g a w e l l w i t h a p r o p p a n t - b e a r i n g f l u i d and then s h u t t i n g - i n and v e n t i n g i t (KIEL 1977; c f . s e c t i o n s 4.5.4.4.3. and 4 . 8 . 8 . 4 . ) . The f i r s t c y c l e o f p r e s s u r i z a t i o n r e s u l t s i n s p a l l i n g and s e l f - p r o p p i n g ( c f . s e c t i o n s 4.5.1.1.1.2. and 4.5.1.5.1.) o f t h e main f r a c t u r e , and i n subsequent c y c l e s , t h e proppant purpos e l y introduced i n the f r a c t u r i n g f l u i d bridges the spall-proppants, w i t h thus p r e s s u r e r i s i n g and l a t e r a l f r a c t u r e s b e i n g propagated p e r p e n d i c u l a r t o t h e main c r a c k . W h i l e t h e mechanism proposed by KIEL (1977) p o s s i b l y works when t h e p r i n c i p a l s t r e s s e s i n two d i r e c t i o n s a r e n e a r l y t h e same, t h e model o f MURPHY & FEHLER (1986) shows t h a t i n case o f c o n s i d e r a b l y d i f f e r i n g s t r e s s e s , p r e s s u r e r i s e i n t h e b l o c k e d main f r a c t u r e would s i m p l y r e s u l t i n f u r t h e r l i f t - o f f o f t h e l a r g e c r a c k , t h e r e b y overcoming t h e temporary blockage and a l l o w i n g t h e main f r a c t u r e t o c o n t i n u e i t s p r o p a g a t i o n w i t h o u t opening o f secondary l a t e r a l cracks. The
key t o d e n d r i t i c f r a c t u r i n g , however, i s shear which a l l o w s t h e necessa-
r y r e d u c t i o n o f t h e e a r t h s t r e s s p a r a l l e l t o t h e main f r a c t u r e i n o r d e r t o p e r m i t opening o f l a t e r a l j o i n t s ( c f . s e c t i o n 4 . 8 . 8 . 4 . 2 . ) . Dendritic fracturing
can t h u s o c c u r under c e r t a i n c o n d i t i o n s , b e i n g t h a t t h e m a j o r j o i n t s a r e n o t p a r a l l e l t o t h e p r i n c i p a l e a r t h s t r e s s e s and t h a t f l o w r a t e and f l u i d v i s c o s i t y w i t h i n t h e j o i n t s a r e low enough t h a t shear d i l a t i o n i s s t i l l s u f f i c i e n t t o transmit the f l u i d r a t e w i t h o u t excessive pressure gradients.
552
4.8. Minifracturing and other small-scale fracturing H y d r s u l i c p r o p p a n t f r a c t u r i n g c a r r i e d o u t i n Europe and o t h e r p a r t s o f t h e w o r l d so f a r m a i n l y comprised more o r l e s s f u l l - s c a l e l a r g e r o r s m a l l e r t r e a t ments w i t h r e s p e c t t o r e s e r v o i r s i z e and aimed on optimum s t i m u l a t i o n ( w i t h t h e achievement o f t h i s g o a l , however, a l s o i n c l u d i n g h i g h c o s t o f t h e o p e r a t i o n ) , w i t h i n most cases b o t h f r a c t u r e l e n g t h and f r a c t u r e h e i g h t h a v i n g been d e s i g ned near t h e upper boundaries o f t e c h n i c a l p o s s i b i l i t y o f performance as w e l l as economical f e a s i b i l i t y ( c f . s e c t i o n 4 . 8 . 1 1 . ) . M i n i a t u r e h y d r a u l i c p r o p p a n t f r a c t u r i n g and o t h e r s m a l l - s c a l e f r a c t u r i n g , however, can n o t o n l y o f t e n be a t e c h n i c a l a l t e r n a t i v e t o l a r g e - s c a l e s t i m u l a t i o n , b u t can i n many cases a l s o r e p r e s e n t e c o n o m i c a l l y s u i t a b l e approaches o f m a r g i n a l r e s e r v o i r s . V a r i o u s aspects o f m i n i f r a c t u r i n g and o t h e r s m a l l - s c a l e f r a c t u r i n g i n c l u d i n g p r e - t r e a t m e n t p r e s s u r e breakdown t e s t and f r a c t u r e parameter d e t e r m i n a t i o n , f o r m a t i o n damage around t h e w e l l b o r e , f i e l d experiments and mHF vs. MHF, p i l o t s t i m u l a t i o n o f m a r g i n a l w s e r v o i r s , h o r i z o n t a l w e l l o r d r a i n h o l e d r i l l i n g and f r a c t u r i n g , u p t a k e c a p a c i t y enhancement o f i n j e c t i o n w e l l s , c o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r e s , l a t e - s t a g e r e f r a c t u r i n g o f o l d w e l l s , and c o m b i n a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g and o t h e r aspects a r e b r i e f l y sketched as f o l lows. Comments a r e a l s o o f f e r e d on o p t i m i z a t i o n o f f r a c t u r e l e n g t h and h e i g h t which a r e some o f t h e key parameters f o r t r e a t m e n t d e s i g n . The d i s c u s s i o n i s i n troduced by a s h o r t o v e r v i e w o f c h a r a c t e r i s t i c f e a t u r e s and d e f i n i t i o n s o f minif r a c t u r e , m i c r o f r a c t u r e and MHF o p e r a t i o n s .
4.8.1. Minifracture, microfracture and MHF definitions and principles The account on m i n i f r a c t u r i n g and o t h e r s m a l l - s c a l e t r e a t m e n t s i s i n t r o d u c e d by b r i e f l y d e f i n i n g and o u t l i n i n g m i n i f r a c t u r e , m i c r o f r a c t u r e and massive f r a c t u r e o p e r a t i o n s w i t h c h a r a c t e r i z a t i o n o f t y p i c a l f e a t u r e s and comparative e f f e c t i v e n e s s . Some comments on mini-massive f r a c t u r e a r e a l s o o f f e r e d .
4.8.1.1. Minifracture and microfracture M i n i f r a c t u r e s a r e scaled-down t r e a t m e n t s which a r e pumped a t more o r l e s s t h e same f l u i d r a t e s p e r t i m e i n t e r v a l as t h e f u l l - s i z e b i g o p e r a t i o n (VEATCH 1983). M i n i f r a c t u r e s a r e i n most cases performed by i n j e c t i o n o f f l u i d a t r a t e s o f 12 - 15 b b l / m i n , and t h e o n l y d i f f e r e n c e t o l a r g e - s c a l e f r a c t u r i n g j o b s i s t h e l i m i t e d t o t a l pumped volume o f t h e m i n i f r a c t u r e which i s m a i n l y i n t h e r a n ge o f 5,000 - 40,000 g a l (PAI, GARBIS & HALL 1983). The f l u i d t a k e n f o r t h e m i n i f r a c t u r e c a l i b r a t i o n t r e a t m e n t ( c f . s e c t i o n 4 . 8 . 2 . 1 . ) i s u s u a l l y t h e same as a n t i c i p a t e d t o be a p p l i e d t h r o u g h o u t t h e m a j o r p o r t i o n o f t h e MHF o p e r a t i o n . F o r p r o p e r f o r m a t i o n t e s t i n g , t h e m i n i f r a c t u r i n g f l u i d has t o be p r o p p a n t - f r e e i n o r d e r t o a l l o w u n r e s t r i c t e d c l o s u r e o f t h e f r a c t u r e ( c f . s e c t i o n 4.8.2.5.). M i c r o f r a c t u r e s d i f f e r from m i n i f r a c t u r e s by c o m p r i s i n g r e d u c t i o n o f b o t h t o t a l volume and pumped f l u i d r a t e s w i t h r e s p e c t t o f u l l - s i z e o p e r a t i o n s . M i c r o f r a c t u r e s a r e t y p i c a l l y performed w i t h v e r y low f l u i d r a t e s down t o 1 g a l / m i n and w i t h v e r y low t o t a l f l u i d amounts up t o a b t . 500 - 1,000 g a l , and a r e small breakdown f r a c t u r e s c o m p r i s i n g h y d r a u l i c f o r m a t i o n breakdown a f t e r b u l l e t o r j e t p e r f o r a t i o n (BRANAGAN & WILMER 1988; c f . s e c t i o n 4 . 8 . 2 . 2 . ) . W h i l e p r o p p a n t i n s e r t i o n i n t o c r a c k s c r e a t e d by m i n i f r a c t u r i n g may be q u i t e common, m i c r o f r a c t u r i n g i s e x c l u s i v e l y done w i t h o u t p r o p p a n t s . M i n i f r a c t u r e s o r m i c r o f r a c t u r e s f o r s p e c i a l purposes may comprise volumes as low as 500 - 10,000 g a l o r 1 - 250 g a l , r e s p e c t i v e l y (WARPINSKI, BRANAGAN & WILMER 1985). Aspects o f m i c r o f r a c t u r i n g a r e a l s o discussed by EVANS & ENGELDER (1986), CRAMER & SONGER (1988), HUCKABEE (1988) ; SHLYAPOBERSKY, UALHAUG, SHEFFIELD & HUCKABEE (1988) and SHLYAPOBERSKY, WONG & WALHAUG ( 1 9 8 8 ) . M i n i f r a c t u r i n s f l u i d
553 q u a n t i t y i s o f t e n designed t o be approximately equal t o t h e pad volume o f t h e main treatment (SATTLER, KAIBLE, GALL & GILL 1988). P o i n t s o f m i n i - and microf r a c t u r i n g are a l s o i l l u s t r a t e d by McLENNAN & ROEGIERS (1982) and SHLYAPOBERSKY (1985). Another small-scale s t i m u l a t i o n treatment i s the b u c k e t f r a c t u r e where more o r l e s s a bucket o f n a t u r a l sand o r s y n t h e t i c proppants i s i n j e c t e d i n t o the very near surroundings o f t h e borehole ( c f . DETOURNAY, McLENNAN & ROEGIERS 1986; ROEGIERS 1987). CROW (1977) documents m i n i a t u r e massive f r a c t u r i n g as a t r a n s i t i o n a l o r i n t e r m e d i a t e s t i m u l a t i o n technique f o r l o w - p o r o s i t y and -permeab i l i t y zones ( c f . s e c t i o n 4.8.1.3.).
4.8.1.2. Massive fracture Massive f r a c t u r e s are c h a r a c t e r i z e d as f o l l o w s by proppant q u a n t i t i e s and f r a c t u r e wing l e n g t h s i n comparison t o m i n i f r a c t u r e s and m i c r o f r a c t u r e s . Some examples o f a p p l i c a t i o n o f massive h y d r a u l i c f r a c t u r i n g i n t i g h t gas sand stimul a t i o n are a l s o o u t l i n e d .
4.8.1.2.1. Proppant quantities and fracture wing lengths I n comparison t o t h e s i z e o f m i n i f r a c t u r e s and m i c r o f r a c t u r e s , massive hyd r a u l i c f r a c t u r i n g (MHF) treatments comprise f l u i d volumes i n the range o f 100,000 -1 Mio. gal o r even more (PAI, G A R B I S & HALL 1983; VEATCH 1983; up t o 3,000 m3; TEUFEL 1981). Proppant volumes from a b t . 100 t (200,000 l b s ) v i a f r e q u e n t l y 1,000 t (2,000,000 l b s ) up t o 2,500 t ( 5 Mio. l b s ) are pumped i n MHF ft operations where f r a c t u r e wings i n excess o f 1,000 f t (300 m) up t o 2,500 (800 m) i n most a p p l i c a t i o n s and i n extreme cases up t o 4,500 f t (1,500 m) l e n s t h and over 200,000 f t 2 area are c r e a t e d ( c f . s e c t i o n 1.1.1.4.). Gross f r a c t u r e h e i g h t determining crack volume and n e t f r a c t u r e h e i g h t marking t h e h e i g h t over which f l u i d l o s s w i l l occur are i n the ranges o f 100 - 1,000 f t (30 - 330 in) and 30 - 300 f t (10 - 100 m), r e s p e c t i v e l y , i n most o f the MHF treatments. MHF operations are g e n e r a l l y c a r r i e d o u t i n t i g h t gas sands w i t h l e s s than 0 . 1 md e f f e c t i v e r e s e r v o i r m a t r i x p e r m e a b i l i t y ( c f . s e c t i o n 3.4.1.) and have i n the USA so f a r been s u c c e s s f u l l y performed i n many regions o f the Rocky Mountains area.
4.8.1.2.2. Applications in tight gas sand stimulation Some o f the key a p p l i c a t i o n patches o f MHF treatments i n the USA comprise San Juan Basin (Dakota r e s e r v o i r ) o f Northwest New Mexico, Wattenberg f i e l d (Muddy " 5 " ) i n the Denver B a s i n o f Colorado, Wamsutter f i e l d (Mesa Verde) i n the Green R i v e r Basin i n Southern Wyoming, Moxa Arch ( F r o n t i e r ) i n Southwestern Wyoming, Cotton V a l l e y Sand and Lime i n East Texas, Cotton V a l l e y Formation i n Louisiana and Arkansas, Canyon Sand Formation o f West Texas, A u s t i n Chalk o f South and C e n t r a l Texas, and Morrow and Atoka Formations o f Southeast New MexiMassive h y d r a u l i c f r a c t u co (PAI, GARBIS & HALL 1983; c f . s e c t i o n 3.4.1.1.). r i n g i n t i g h t gas sands i n Europe i s c h i e f l y c a r r i e d o u t i n R o t l i e g e n d and Upper Carboniferous i n B r i t i s h and Dutch Southern N o r t h Sea as w e l l as Netherlands and Germany FRG onshore ( c f . s e c t i o n s 2.4.1. and 3.3.) and i n the T e r t i a r y o f Yugoslavia and Hungary ( c f . s e c t i o n 2.4.3.1.). While most t i g h t gas sands are deep, compacted and normal- t o overpressured, o c c a s i o n a l l y a l s o shallow o r underpressured t i g h t gas sands occur (RICE & SHURR 1978; NYDEGGER, R I C E & BROWN 1979; BROWN & CRAFTON 1981, GAUTIER & R I C E 1981; KUKAL, BIDDISON, HILL, MONSON & SIMONS 1983). D i s t i n c t i o n has a l s o t o be made between b l a n k e t - t y p e and l e n t i c u l a r t i g h t gas sands, w i t h the l a t t e r category o f l o w - p e r m e a b i l i t y r e s e r v o i r s b e i n g w i d e l y d i s t r i b u t e d i n t h e Greater Green R i ver, Piceance, Wind R i v e r and U i n t a basins (WARPINSKI, BRANAGAN, SATTLER, LORENZ, NORTHROP, MANN & FROHNE 1985). Other aspects o f t i g h t gas sand formations and basins a r e discussed i n s e c t i o n 3.4.1.
554
4.8.1.3. Mini -massive fracture M i n i - m a s s i v e s t i m u l a t i o n i s an i n t e r m e d i a t e o r t r a n s i t i o n a l t e c h n i q u e b e t ween massive t r e a t m e n t and m i n i f r a c t u r i n g which combines s e v e r a l methods commonl y used t o i n c r e a s e f r a c t u r i n g e f f i c i e n c y and t o improve r e s u l t s i n c l u d i n g cont r o l o f f l u i d l o s s i n t o h a i r l i n e f r a c t u r e s and m a t r i x p e r m e a b i l i t y , s u p e r v i s i o n o f f r a c t u r e h e i g h t , and achievement o f a d e e p l y - p e n e t r a t i n g and h i g h l y - p a c k e d f r a c t u r e (CROW 1 9 7 7 ) . F o l l o w i n g some comments on d i l a t a n c y o f h y d r a u l i c f r a c t u r i n g , t h e d e s i g n o f mini-massive f r a c t u r e s i s o u t l i n e d .
4.8.1.3.1. Dilatancy of hydraulic fracturing H y d r a u l i c f r a c t u r i n g o f underground f o r m a t i o n s i s c h a r a c t e r i z e d by d i l a t a n c y which comprises g e n e r a t i o n o f numerous t i n y c r a c k s i n c e r t a i n d i r e c t i o n s as r o c k approaches i t s b r e a k i n g p o i n t (CROW 1977). Subsurface r o c k s a r e t r i a x i a l l y loaded due t o t h e f o r c e e x e r t e d upon them by t h e o v e r l y i n g s t r a t a . T h i s v e r t i c a l f o r c e o r s t r e s s imposed on t h e r o c k causes a h o r i z o n t a l s t r e s s t o e x i s t w i t h i n t h e r o c k system. I f t h e r o c k were n o t c o n f i n e d w i t h i n t h e e a r t h , t h i s v e r t i c a l f o r c e would express i t s e l f by c a u s i n g t h e r o c k t o deform o r expand i t s l a t e r a l dimensions. When p r e s s u r e i s a p p l i e d f r o m w i t h i n such a system, t h e r o c k r u p t u r e s o r f r a c t u r e s i n a p l a n e p e r p e n d i c u l a r t o t h e l e a s t amount o f s t r e s s on t h e system, w i t h these s t r e s s e s c o n t r o l l i n g f r a c t u r e d i r e c t i o n and det e r m i n i n g whether t h e c r a c k p l a n e w i l l be h o r i z o n t a l , v e r t i c a l o r i n c l i n e d . D u r i n g h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n , numerous t i n y c r a c k s f o r m as t h e r o c k approaches i t s b r e a k i n g p o i n t due t o t h e a p p l i e d pressure, and d i l a t a n c y then c o n t i n u e s t o occur along t h e l e a d i n g edge o f t h e c r a c k as i t p r o g r e s s e s outward i n t o t h e f o r m a t i o n . D i l a t a n c y d u r i n g h y d r a u l i c f r a c t u r i n g i s t h e reason f o r premature screenout i n v e r y l o w - p e r m e a b i l i t y r e s e r v o i r s ( c f . s e c t i o n s 4.2.3.5.3. and 6 . 2 . 4 . 2 . 1 . ) , f a i l u r e of t h e f r a c t u r e t o p e n e t r a t e t h e designed l e n g t h ( c f . s e c t i o n 4 . 8 . 1 1 . ) , and f a i l u r e o f w e l l p r o d u c t i v i t y t o a c h i e v e p r e d i c t e d f o l d s o f i n c r e a s e . The e x p l a n a t i o n i s t h a t a p o r t i o n o f t h e s t i m u l a t i o n f l u i d l e a k s o f f i n t o t h e numerous t i n y c r a c k s and t h e encountered n a t u r a l f r a c t u r e s , w i t h t h e f l u i d h a v i n g been l o s t t o o r i g i n a l and a r t i f i c i a l h a i r l i n e f i s sures n o t b e i n g a v a i l a b l e f o r h y d r a u l i c f r a c t u r e e x t e n s i o n ( t h i s secondary f l u i d l e a k o f f i n t o n a t u r a l and s y n t h e t i c h a i r l i n e c r a c k s i s a l s o d i s c u s s e d by WARPINSKI 1988 a; c f . s e c t i o n 4 . 8 . 8 . 3 . 2 . 3 . ) .
4.8.1.3.2. Design of mini-massive fracturing Secondary f l u i d l o s s i n t o h a i r l i n e f i s s u r e s can be s a t i s f a c t o r i l y c o n t r o l l e d w i t h 100 mesh sand as a spearhead i n t h e h y d r a u l i c f r a c t u r i n g t r e a t m e n t (CROW 1977; c f . s e c t i o n s 1.4.11.2.1. and 4 . 8 . 8 . 3 . 1 . 2 . ) . Dropping o f proppants o u t o f t h e c r a c k p r i o r t o i t s c l o s u r e and f a l l o u t i n t u b i n g o r c a s i n g can be a v o i d e d by b l e e d i n g t h e w e l l i m m e d i a t e l y back a f t e r t h e t r e a t m e n t t o a l l o w t h e f r a c t u r e t o c l o s e and t o h o l d t h e p r o p p a n t s i n p l a c e ( c f . s e c t i o n 4.12.2.2.1.), and f u r t h e r improvement can be made b y u s i n g h i g h p r o p p a n t c o n c e n t r a t i o n f o r complete p a c k i n g o f t h e e n t i r e c r a c k ( c f . s e c t i o n 4.3.5.). T h e r e f o r e m i n i - m a s s i v e f r a c t u r i n g i s designed t o t i e t o g e t h e r s e v e r a l techniques t h a t have been used i n d i v i d u a l l y i n t h e p a s t t o i n c r e a s e c r a c k e f f i c i e n c y . The sequence o f elements i n m i n i - m a s s i v e t r e a t i n g i n c l u d e s i d e a l l y pumping o f a pad f l u i d c o n t a i n i n g 100 mesh sand i n o r d e r t o p r o p and t o c o n t r o l l e a k o f f i n t o s m a l l c r a c k s and n a t u r a l f r a c tures, i n j e c t i n g a s t i m u l a t i o n f l u i d l i m i t i n g l o s s i n t o m a t r i x permeability, u s i n g a p r o p p a n t s a t u r a t i o n as h i g h as o p e r a t i o n a l l y f e a s i b l e i n o r d e r t o f i l l t h e e n t i r e c r e a t e d f r a c t u r e h e i g h t , and b l e e d i n g t h e w e l l back as soon and as f a s t as p r a c t i c a l so t h a t t h e f r a c t u r e w i l l c l o s e and t r a p t h e p r o p p a n t w h i l e i t i s s t i l l suspended i n t h e s t i m u l a t i o n f l u i d ( c f . s e c t i o n 4 . 1 2 . 3 . 1 . ) .
555 F i e l d e x p e r i e n c e has r e v e a l e d t h a t t h e optimum s i z i n g o f m i n i - m a s s i v e f r a c t u r e j o b s i s a b t . 10,000 - 50,000 l b s o f 100 mesh sand, 50,000 - 100,000 l b s o f 20/40 proppants, f a c u l t a t i v e l y up t o 10,000 l b s o f 16/20 o r 101’20 proppants, and 20,000 - 50,000 g a l o f g e l l e d w a t e r o r o t h e r f r a c t u r i n g f l u i d . Thus t h e f l u i d volume i s w i t h i n t h e range o f c o n v e n t i o n a l m i n i f r a c t u r e s , whereas t h e p r o p p a n t q u a n t i t y i s a l r e a d y approaching t h e lower boundary o f massive s t i m u l a t i o n t r e a t m e n t s , and t h e b r i d g e f r o m mere c a l i b r a t i o n t e s t t o r e a l r e s e r v o i r i m provement i s b u i l t by i n c o r p o r a t i n g t h e 100 mesh sand stage which i s a m a j o r element o f performance a m e l i o r a t i o n .
4.8.2. Pre-fracturing pressure breakdown test and fracture parameter determination M i n i a t u r e h y d r a u l i c f r a c t u r i n g j o b s ( m i n i f r a c t u r e s , mHF; NOLTE 1979, SMITH 1981; CONWAY, McGOWEN, GUNDERSON & K I N G 1985) have h i t h e r t o been m a i n l y done as p r e - f r a c t u r i n g breakdown and/or i n j e c t i o n t e s t s i n o r d e r t o open t h e f o r m a t i o n w i t h o u t i n s e r t i o n o f proppants, w i t h t h e main purpose b e i n g t o o b t a i n d e f i n i t e f i g u r e s t h a t c o u l d r e p l a c e t h e p r e l i m i n a r y and p a r t i a l l y s t i l l q u e s t i o n a b l e assumptions o f f r a c t u r e g r a d i e n t , f r a c t u r e pressure, c l o s u r e s t r e s s ( c f . s e c t i o n s 1.2.1.1. and 1.3.2.1.), c l o s u r e time, f l u i d e f f i c i e n c y , e f f e c t i v e f l u i d l e a k o f f c o e f f i c i e n t , f r a c t u r e l e n g t h ( c f . s e c t i o n 4.8.11.), fracture height, fracture w i d t h , and o t h e r i m p o r t a n t d a t a i n t h e d e s i g n w i t h t h e f r a c t u r i n g s i m u l a t o r i n o r d e r t o c a r r y o u t f i n a l o p t i m i z a t i o n r u n s b e f o r e t h e b i g j o b i s executed (LOCKE & SAWYER 1975; ABOU-SAYEO, BRECHTEL & CLIFTON 1977; ZOBACK & POLLARD 1978; NOLTE 1979, 1982; SMITH 1981; VOEGELE, ABOU-SAYEO & JONES 1982; AHMED, KELKAR & SCHATZ 1983; VEATCH 1983, CUESTA & ELPHICK 1984; PEARSON, ABOU-SAYED & NGUYEN 1984; CONWAY, McGOWEN, GUNDERSON & K I N G 1985; ELPHICK 1985; LEE 1985, 1987; MARTINS & HARPER 1985; HOLOITCH, ROBINSON & WHITEHEAD 1986; ROBINSON, HOLDITCH & WHITEHEAD 1986; TANSOE, K I N G & HOLWAN 1986; MOSCHOVIDIS 1987; SMITH, MILLER & HAGA 1987; TAN, McGOWEN, LEE & SOLIMAN 1988). The determined parameters a r e t h e n used f o r e v a l u a t i o n o f presence o f n a t u r a l f r a c t u r e s , pad volume r e q u i r e d , b e s t f l u i d - l o s s a d d i t i v e s t o be used, and achievement o f optimum f r a c t u r i n g t r e a t m e n t d e s i g n (TAN, McGOWEN, LEE & SOLIMAN 1988). Some p o i n t s o f m i n i f r a c t u r e c a l i b r a t i o n experiments, m i c r o f r a c t u r e breakdown procedures, p r e s s u r e a n a l y s i s and d e s i g n i n p u t d a t a c o l l e c t i o n , s i z e and s i g n i f i c a n c e o f m i n i f r a c t u r i n g , p r o p p a n t vs. non-proppant m i n i f r a c t u r i n g , and m o d i f i e d m i c r o f r a c t u r i n g and m i n i f r a c t u r i n g a n a l y s i s f o r o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e d e s i g n a r e o u t l i n e d as f o l l o w s .
4.8.2.1. Minifracture calibration experiments M i n i f r a c t u r e s can t h e r e f o r e a l s o be regarded as c a l i b r a t i o n experiments f o r t h r e e - d i m e n s i o n a l f r a c t u r e s i m u l a t o r s (BRANAGAN, CIPOLLA, LEE & YAN 1987; HOLDITCH, ROBINSON, WHITEHEAD & ELY 1987) and i n f a c t r e p r e s e n t scaled-down t r i a l r u n s o f t h e main f r a c t u r e t r e a t m e n t (BARBY & BARBEE 1987) o r dummy stages p r i o r t o t h e a c t u a l f r a c t u r i n g o p e r a t i o n (OSBORNE, McLEOD & SCHROEDER 1981) and a r e i n f a c t d a t a - g a t h e r i n g j o b s (HOOVER & AOAMS 1988). The most i m p o r t a n t aims and purposes o f m i n i f r a c t u r e c a l i b r a t i o n experiments a r e d i m i n u t i o n o f s c r e e n o u t r a t e s and f l u i d - l o s s c o e f f i c i e n t d e t e r m i n a t i o n .
4.8.2.1.1. Diminution of screenout rates If p r o p e r l y performed, m i n i f r a c t u r e s have c o n s i d e r a b l e s i g n i f i c a n c e f o r c u t t i n g down s c r e e n o u t r a t e s i n t h e main s t i m u l a t i o n treatments, because many screenout reasons a r e based on i n s u f f i c i e n t knowledge o f f o r m a t i o n b e h a v i o u r and incorrect assumptions o f f l u i d l o s s ( c f . s e c t i o n s 4.2.3.5.3. and 6.2.4.2.1.). F l u i d l e a k o f f may be caused by occurrence o f n a t u r a l f r a c t u r e s , f a i l u r e o f f l u i d - l o s s a d d i t i v e and h i g h e r m a t r i x p e r m e a b i l i t y t h a n expected.
556 O t h e r common s c r e e n o u t reasons a r e f o r m a t i o n response o t h e r than a n t i c i p a t e d and l e s s t h a n adequate q u a l i t y c o n t r o l o f f r a c t u r i n g f l u i d s . I n a d d i t i o n t o b e t t e r information derived from m i n i f r a c t u r i n g experimental treatments, screenout r a t e s can be decreased by computer m o n i t o r i n g ( c f . s e c t i o n 6.2.4.3.) which o f t e n enables e a r l y s c r e e n o u t p r e d i c t i o n d u r i n g t h e j o b and i t s p r e v e n t i o n by ont h e - s p o t changing o f t h e pumping schedule. Improvements o f f l u i d s t o guarantee t h e i r s t a b i l i t y i n h i g h e r temperature f o r l o n g e r t i m e a l s o a s s i s t i n a v o i d i n g screenout f a i l u r e .
4.8.2.1.2.
Fluid-loss coefficient determination
One o f t h e most i m p o r t a n t f r a c t u r i n g t r e a t m e n t d e s i g n parameters t o be d e t e r mined by m i n i f r a c t u r e a n a l y s i s i s t h e f l u i d - l o s s c o e f f i c i e n t (TAN, McGOWEN, LEE & SOLIMAN 1988). Loss o f s t i m u l a t i o n f l u i d t o t h e f o r m a t i o n i s a dominant and decisive f a c t o r i n hydraulic f r a c t u r e propagation. Local geological v a r i a b i l i t y and presence o f n a t u r a l c r a c k s r e n d e r l a b o r a t o r y s i m u l a t i o n s ambiguous and r e q u i r e i n - s i t u f l u i d - l e a k o f f parameter d e t e r m i n a t i o n f r o m p r e s s u r e d e c l i n e f o l l o w i n g a s m a l l - s c a l e c a l i b r a t e d f r a c t u r i n g schedule. Since t h e aim o f mapping a m i n i f r a c t u r e by techniques such as a c o u s t i c e m i s s i o n i n t e r p r e t a t i o n ( c f . sect i o n 6.2.2.3.2.5.) i s t o look a t the f e a s i b i l i t y o f large-scale h y d r a u l i c f r a c t u r i n g t r e a t m e n t s , i t i s i m p o r t a n t t o measure t h e e v o l u t i o n o f downhole p r e s s u r e which can be c o r r e l a t e d w i t h l o c a l s t r e s s e s (SARDA, PERREAU & DEFLANDRE 1988; c f . s e c t i o n 6 . 2 . 4 . 2 . 4 . ) . A p p l i c a b i l i t y and r e l i a b i l i t y o f m i n i f r a c t u r i n g a n a l y s i s has i n t h e l a s t y e a r s been f a c i l i t a t e d s i g n i f i c a n t l y by i n t r o d u c t i o n o f computerized vans e q u i p ped w i t h s p e c i a l hardware t o more a c c u r a t e l y m o n i t o r p r e s s u r e d e c l i n e and development o f adequate s o f t w a r e ( c f . s e c t i o n 6 . 2 . 4 . 3 . ) . Aspects o f m i n i f r a c t u r e assessment a r e a l s o d i s c u s s e d by LEE (1986) and SOLIMAN ( 1 9 8 6 ) . An a d d i t i o n a l technique o f f o r m a t i o n e v a l u a t i o n i s p r e s s u r e d e c l i n e a n a l y s i s (NOLTE 1979, NOLTE & SMITH 1981) which g i v e s optimum r e s u l t s i n c o m b i n a t i o n w i t h m i n i f r a c t u r e i n t e r p r e t a t i o n . Some p o i n t s o f m i c r o f r a c t u r e breakdown procedures, p r e s s u r e anal y s i s and d e s i g n i n p u t d a t a c o l l e c t i o n , s i z e and s i g n i f i c a n c e o f m i n i f r a c t u r i n g , and p r o p p a n t v s . non-proppant m i n i f r a c t u r i n g a r e o u t l i n e d as f o l l o w s .
4.8.2.2.
Microfracture breakdown procedures
BRANAGAN & WILMER (1988) d i s c u s s a s e r i e s o f m i c r o f r a c t u r i n g breakdown p r o c e dures t o m i n i m i z e n a t u r a l f r a c t u r e damage i n j o i n t e d t i g h t gas sandstone r e s e r v o i r s . I f p e r f o r a t i n g a l o n e does n o t p r o v i d e s u f f i c i e n t p r o d u c t i o n o r when p e r f o r a t i o n s y i e l d r e s t r i c t e d , uneven o r damaged f l o w paths, m i c r o f r a c t u r i n g breakdown i s c a r r i e d o u t i n o r d e r t o m i t i g a t e these u n d e s i r a b l e e f f e c t s . M i c r o f r a c t u r i n g breakdown c r e a t e s small c r a c k s e x t e n d i n g f r o m t h e w e l l b o r e t h r o u g h t h e p e r f o r a t i o n s i n t o the formation.
The m i c r o f r a c t u r i n g process g e n e r a l l y c o n s i s t s o f pumping o f a l o w - v i s c o s i t y non-damaging f l u i d a t s u f f i c i e n t l y h i g h r a t e s t o i n i t i a t e and propagate a s h o r t c r a c k t h a t emanates f r o m t h e p e r f o r a t i o n s and extends beyond t h e damage zone. The use o f b a l l s e a l e r s o r o t h e r d i v e r t i n g agents ( c f . s e c t i o n 4.2.2.1.1.) dur i n g m i c r o f r a c t u r i n g p r o v i d e s adequate c o n t r o l o f f l u i d and f r a c t u r e placement. I n t h e course o f d r i l l i n g and c o m p l e t i n g a w e l l , numerous mechanisms can degrade t h e o r i g i n a l h i g h c o n d u c t i v i t y o f t h e n a t u r a l f r a c t u r e s ( c f . s e c t i o n 4.8.8.3.) such as i n v a s i o n o f d r i l l i n g mud ( c f . s e c t i o n 5.11.2.) t h a t may u l t i m a t e l y a c t as a p l u g , cements squeezed i n t o t h e f r a c t u r e , p e r f o r a t i o n s and n e a r - w e l l b o r e s t r e s s changes t h a t c o u l d t e n d t o c l o s e t h e f r a c t u r e , and i n t e r f a c i a l t e n s i o n a l f o r c e s g e n e r a l l y d e s c r i b e d by c a p i l l a r y p r e s s u r e o f invaded comp l e t i o n l i q u i d s which l i m i t f r a c t u r e f l o w c a p a c i t y . Non-aqueous
microfracturing
breakdown techniques t h a t aim on a v o i d i n g w a t e r
557 i n c l u d e hyb l o c k i n g i n t h e n a t u r a l f r a c t u r e system ( c f . s e c t i o n 3.11.2.4.2.) d r a u l i c n i t r o g e n f r a c t u r i n g u s i n g s u r f a c e pump t r u c k s , p e r f o r a t i n g w i t h an underbalanced n i t r o g e n column, h i g h - p r e s s u r e n i t r o g e n impulse p e r f o r a t i n g / m i c r o f r a c t u r i n g combination, n i t r o g e n impulse i n j e c t i o n u s i n g a t u b i n g pump-out p l u g , and t a i l o r e d p u l s e e x p l o s i v e f r a c t u r i n g ( c f . s e c t i o n 4.2.2.1.3.3.). Nitrogen a l o n e can be e f f e c t i v e l y employed i n m i c r o f r a c t u r i n g breakdown procedures (as w e l l as on l a r g e r s c a l e f o r h y d r a u l i c f r a c t u r i n g o f shales; c f . s e c t i o n 4.4.4.2.). P e r f o r a t i n g i n a d r y underbalanced n i t r o g e n w e l l b o r e column s u c c e s s i v e l y connects b o r e h o l e and h i g h l y c o n d u c t i v e n a t u r a l c r a c k s . N i t r o g e n impulse perforating/microfracturing combination using high-pressure n i t r o g e n t o simultaneously create f r a c t u r e s during p e r f o r a t i o n i s also successful i n achieving the d e s i r e d w e l l b o r e enhancement. N i t r o g e n impulse t e c h n i q u e i s a p r a c t i c a l remed i a l t r e a t m e n t o f n e a r - w e l l b o r e and n a t u r a l f r a c t u r e damage.
4.8.2.3. Pressure analysis and design input data collection The i n t r o d u c t i o n o f m i n i f r a c t u r e a n a l y s i s by p r e s s u r e m o d e l l i n g r e s u l t i n g i n crack parameter and geometry d e t e r m i n a t i o n i n 1979 (NOLTE 1979, NOLTE & SMITH 1981; c f . s e c t i o n 6.2.4.2.4.) was t h e second m a j o r b r e a k t h r o u g h i n h y d r a u l i c f r a c t u r i n g t e c h n o l o g y a p a r t f r o m t h e i n v e n t i o n o f s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants (SOLIMAN 1986). M i n i f r a c t u r e e v a l u a t i o n c o n s i s t s o f p e r f o r m i n g a s m a l l f r a c t u r i n g t r e a t m e n t w i t h l i t t l e o r no proppants a f t e r which t h e w e l l i s s h u t i n and p r e s s u r e d e c l i n e w i t h t i m e i s m o n i t o r e d . As t h e r a t e o f p r e s s u r e d e c l i n e w i t h t i m e depends on f o r m a t i o n , f l u i d and f r a c t u r e parameters, m i n i f r a c t u r e assessment a l l o w s f i e l d c a l c u l a t i o n o f i m p o r t a n t i n p u t d a t a f o r t h e d e s i g n o f t h e main t r e a t m e n t . The most i m p o r t a n t g o a l s o f m i n i f r a c t u r i n g a r e c r a c k d i a g n o s t i c s and containment p r e d i c t i o n (SATTLER, RAIBLE & GALL 1985). The d i s c u s s i o n as f o l l o w s focusses on m i n i f r a c t u r i n g t e s t and e v a l u a t i o n types, i n - s i t u s t r e s s d e t e r m i n a t i o n , c l o s u r e s t r e s s vs. f l u i d type, premature screeno u t t e r m i n a t i o n r e c o g n i t i o n , and f l u i d s and mechanics t e s t i n g .
4.8.2.3.1. Minifracturing test and evaluation types The m i n i f r a c t u r i n g procedure o f t e n i n c l u d e s c o n v e n t i o n a l s t e p r a t e t e s t , r e peated pump-in/flow-back t e s t s and p u m p - i n / s h u t - i n p r e s s u r e d e c l i n e t e s t (FELSENTHAL 1974; SMITH, ROSENBERG & BOWEN 1982; KIM, CHAMPION & COOPER 1985; WARP I N S K I , BRANAGAN, SATTLER, LORENZ, NORTHROP, MANN & FROHNE 1985; B R I T T & LARSEN 1986, ROSE 1988; TAN, McGOWEN, LEE & SOLIMAN 1988; WAREMBOURG 1988, WARPINSKI 1988 a ) . The i n d i v i d u a l elements o r stages o f m i n i f r a c t u r i n g a r e b r i e f l y charact e r i z e d as f o l l o w s .
4.8.2.3.1.1. Step-rate test The s t e p - r a t e t e s t i s used t o determine f r a c t u r e e x t e n s i o n p r e s s u r e and r a t e s , w i t h t h e i n j e c t e d f l u i d u s u a l l y b e i n g an i n c o m p r e s s i b l e Newtonian f l u i d such as w a t e r (TAN, McGOWEN, LEE & SOLIMAN 1988). The f r a c t u r e e x t e n s i o n pressur e o r p a r t i n g p r e s s u r e p r o v i d e s an a c c u r a t e i n d i c a t i o n o f p r e s s u r e r e q u i r e d t o e x t e n d an e x i s t i n g f r a c t u r e and t h u s determine t h e h y d r a u l i c horsepower needed t o p e r f o r m t h e a c t u a l t r e a t m e n t . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l aspects o f m u l t i p l e - s t e p - r a t e t e s t , t h e t w o - s t e p - r a t e t e s t r e p r e s e n t i n g an improved method i s introduced.
4.8.2.3.1.1.1. General aspects o f mu1 t iple-step-rate test During conventional multiple-step-rate tests, the f l u i d i s i n j e c t e d i n t o the f o r m a t i o n a t a v e r y low i n i t i a l r a t e which i s t h e n stepped up i n s m a l l i n c r e ments and m a i n t a i n e d a t each i n c r e m e n t f o r a c o n s t a n t t i m e span w h i c h i s s u f f i c i e n t f o r p r e s s u r e s t a b i l i z a t i o n (TAN, McGOWEN, LEE & SOLIMAN 1’988). The s t e p -
558 r a t e t e s t i s p a r t i c u l a r l y b e n e f i c i a l f o r d e t e r m i n a t i o n o f t h e maximum i n j e c t i o n r a t e t h a t c a n be pumped u n d e r m a t r i x c o n d i t i o n s . A s p e c t s o f s t e p - r a t e t e s t s a r e a l s o d i s c u s s e d b y FELSENTHAL ( 1 9 7 4 ) , ALLEN & BAZA ( 1 9 8 0 ) , BENNETT & CLARK ( 1 9 8 5 ) ; WAREMBOURG, KLINGENSMITH, HOOGES & ERDLE ( 1 9 8 5 ) ; PALMER & VEATCH ( 1 9 8 7 ) and SINGH, AGARWAL & KRASE ( 1 9 8 8 ) . S t e p - r a t e t e s t i n g i s t h e p r i m a r y m e t h o d t o d e f i n e t h e maximum s a f e i n j e c t i o n p r e s s u r e w i t h o u t f r a c t u r i n g t h e r e s e r v o i r rock, being t h e formation o r f r a c t u r e p a r t i n g pressure which i n i t i a t e s a crack i n an u n f r a c t u r e d w e l l and e x t e n d s an e x i s t i n g c r a c k i n a f r a c t u r e d w e l l .
4.8.2.3.1.1.2.
Two-step-rate t e s t
S I N G H & AGARWAL ( 1 9 8 8 ) p r e s e n t t h e t w o - s t e p - r a t e t e s t as an advanced p r o c e d u r e f o r d e t e r m i n i n g f o r m a t i o n p a r t i n g p r e s s u r e . Compared t o a c o n v e n t i o n a l s t e p r a t e t e s t w h i c h c o n s i s t s o f s e v e r a l ( u s u a l l y seven o r more) c o n s t a n t - r a t e i n j e c t i o n steps o f equal duration, t h e two-step-rate t e s t r e q u i r e s o n l y two cons t a n t - r a t e s t e p s d u r i n g w h i c h p r e s s u r e s a r e measured c o n t i n u o u s l y . The twos t e p - r a t e t e s t has p o t e n t i a l f o r a l a r g e - s c a l e u s e because o f t i m e and c o s t sav i n g s as compared t o c o n v e n t i o n a l s t e p - r a t e t e s t s . The t w o - s t e p - r a t e t e s t c a n be r u n i n a t l e a s t f o u r d i f f e r e n t modes. The f i r s t mode c o m p r i s e s s h u t - i n o f the i n j e c t i o n w e l l f o r t h e f i r s t step f o l l o w e d by i n j e c t i o n a t a constant r a t e f o r t h e second s t e p t o e x c e e d f o r m a t i o n o r f r a c t u r e p a r t i n g p r e s s u r e . The sec o n d mode i n c l u d e s t h e w e l l b e i n g o n s t a b i l i z e d l o w - r a t e i n j e c t i o n , w i t h t h e f i r s t s t e p c o n s i s t i n g o f t h e i n j e c t i o n r a t e b e i n g stepped up t o a c o n s t a n t val u e b e l o w f o r m a t i o n o r f r a c t u r e p a r t i n g p r e s s u r e , and o n l y t h e second s t e p e x c e e d i n g t h e l a t t e r v a l u e . The t h i r d mode r e p r e s e n t s r e d u c t i o n o f i n j e c t i o n r a t e t o a l o w e r c o n s t a n t r a t e f o r t h e f i r s t s t e p and i n c r e a s e t o a h i g h e r c o n s t a n t r a t e f o r t h e second s t e p . I n t h e f o u r t h mode, t h e w e l l i s s t a b i l i z e d a t s h u t - i n c o n d i t i o n s , w i t h t h e t e s t b e i n g composed o f two c o n s t a n t - r a t e i n j e c t i o n s t e p s w i t h progressively increasing rate. A l l t h e f o u r d i f f e r e n t modes a r e c h a r a c t e r i z e d b y t h e p r e s s u r e s a t t a i n e d f o r the e n t i r e d u r a t i o n o f t h e f i r s t s t e p b e i n g below f r a c t u r e o r f o r m a t i o n press u r e , and t h e i n j e c t i o n r a t e f o r t h e second s t e p b e i n g chosen such t h a t t h e i n j e c t i o n p r e s s u r e exceeds t h e f o r m a t i o n o r f r a c t u r e p a r t i n g p r e s s u r e d u r i n g t h i s s t e p . I t i s i m p o r t a n t t h a t i n j e c t i o n r a t e s a r e s t a b l e and c o n s t a n t b e f o r e and d u r i n g e a c h s t e p and t h e t i m e - r a t e - p r e s s u r e d a t a a r e r e c o r d e d a c c u r a t e l y o n a c o n t i n u o u s b a s i s d u r i n g t h e s t e p s . The r e s u l t s a r e a n a l y z e d b y m u l t i p l e - r a t e sup e r p o s i t i o n m e t h o d s . The t e s t s h o u l d be d e s i g n e d such t h a t t h e d u r a t i o n o f t h e f i r s t s t e p i s l o n g enough t o b e o u t o f w e l l b o r e s t o r a g e e f f e c t s o r a t l e a s t l o n g enough f o r t h e p r e s s u r e d a t a t o be o u t o f t h e e n t i r e w e l l b o r e s t o r a g e domin a t e d f l o w p e r i o d . The d u r a t i o n o f t h e f i r s t s t e p c a n a l s o be d e s i g n e d based on t h e time t o reach t h e r a d i a l / p s e u d o - r a d i a l f l o w regime ( c f . s e c t i o n 4.11.3.), o r f o r a f r a c t u r e d w e l l i t can be l i m i t e d t o t h e end o f t h e l i n e a r f l o w regime. I f t h e f i r s t s t e p c o n s i s t s o f a s h u t - i n o r a r e d u c e d r a t e i n j e c t i o n p e r i o d (comp r i s i n g t h e f i r s t o r t h i r d modes), e q u a l - t i m e s t e p s i z e c a n be u s e d f o r t h e f i r s t and second s t e p s . I f t h e i n j e c t i o n r a t e i s i n c r e a s e d f o r e a c h s u b s e q u e n t s t e p ( i n c l u d i n g t h e second o r f o u r t h modes), t h e f i r s t s t e p s h o u l d be t w i c e as l o n g as t h e second s t e p .
4.8.2.3.1.2.
Pump-in/flow-back t e s t
The p u m p - i n / f l o w - b a c k t e s t i s a p p l i e d t o determine t h e l e a s t p r i n c i p a l s t r e s s o r c l o s u r e p r e s s u r e o f t h e f o r m a t i o n and i s p e r f o r m e d b y i n j e c t i n g a volume o f f l u i d ( n o r m a l l y w a t e r ) a t f r a c t u r i n g r a t e s f o l l o w e d b y f l o w i n g back t h e w e l l a t a c o n s t a n t r a t e u n t i l c l o s u r e has been d e t e c t e d (TAN, McGOWEN, LEE & SOLIMAN 1 9 8 8 ) . C l o s u r e p r e s s u r e i s t h e f l u i d p r e s s u r e r e q u i r e d t o i n i t i a t e openi n g o f an e x i s t i n g f r a c t u r e , and i s e q u i v a l e n t t o and c o u n t e r a c t s s t r e s s i n t h e r o c k p e r p e n d i c u l a r t o t h e c r a c k p l a n e . C l o s u r e p r e s s u r e may be e q u a l o r l e s s t h a n breakdown p r e s s u r e r e q u i r e d t o i n i t i a t e an e x i s t i n g f r a c t u r e , b u t w i l l a l ways be l e s s t h a n t h e f r a c t u r e e x t e n s i o n p r e s s u r e . I f t h e f l o w b a c k i s w i t h i n
559 t h e c o r r e c t range, c l o s u r e p r e s s u r e i s i n d i c a t e d by t h e i n f l e c t i o n p o i n t o r change i n c u r v a t u r e f r o m concave t o convex on t h e p r e s s u r e vs. t i m e p l o t ( c f . s e c t i o n 6 . 2 . 4 . 2 . ) , w i t h t h e i n f l e c t i o n p o i n t b e i n g caused by t r a n s i t i o n f r o m linear t o r a d i a l f l o w as t h e f r a c t u r e c l o s e s ( c f . s e c t i o n 4 . 1 1 . 3 . ) . C l o s u r e p r e s sure can be c o n f i r m e d by t h e p r e s s u r e t h a t rebounds f r o m a s h u t - i n a f t e r t h e flowback p e r i o d , and r e p e t i t i o n o f t h e pump-in/flow-back t e s t u s i n g d i f f e r e n t flowback r a t e s g i v e s a more r e l i a b l e value. C l o s u r e p r e s s u r e a f t e r t h e main f r a c t u r i n g t r e a t m e n t i s i n most cases h i g h e r t h a n t h a t determined p r i o r t o t h e main j o b by a m i n i f r a c t u r i n g c a l i b r a t i o n t e s t . M i n i f r a c t u r i n g a n a l y s i s i s c h i e f l y performed by c o n v e n t i o n a l t y p e - c u r v e matching and energy-balance e q u a t i o n t e c h n i q u e (LEE 1987). M a i n t a i n i n g c o n s t a n t flowback r a t e i s c r i t i c a l t o c l o s u r e p r e s s u r e e v a l u a t i o n which t o g e t h e r w i t h f l u i d - l e a k o f f c o e f f i c i e n t belongs t o t h e most i m p o r t a n t v a r i a b l e s t o be o b t a i ned i n m i n i f r a c t u r i n g a n a l y s i s t h a t may w a r r a n t m o d i f i c a t i o n o f f l u i d volume, p r o p p a n t schedule, pumping r a t e , n e c e s s i t y o f a d d i t i v e s and/or r e q u i r e m e n t o f enhanced c l o s u r e o f t h e main t r e a t m e n t . M u l t i p l e i n j e c t i o n s w h i l e changing f l u i d p r o p e r t i e s and a d d i t i v e s enhance d a t a c o l l e c t i o n w h i l e p r o v i d i n g a means o f d e t e r m i n i n g whether o r n o t f l u i d - l o s s agents a r e d e s i r a b l e . I f f l u i d l e a k o f f i s r e l a t i v e l y low, i t may n o t be advantageous t o reduce i t f u r t h e r f o r p r o p p a n t s e t t l i n g c h a r a c t e r i s t i c s . I f t h e pad f l u i d w i t h o u t f l u i d - l o s s a d d i t i v e s i s n o t s u f f i c i e n t f o r f l u i d - l e a k o f f c o n t r o l , subsequent i n j e c t i o n s and d e c l i n e s i n c l u d i n g f l u i d - l o s s agents can be r u n u n t i l f l u i d l e a k o f f management i s a c c e p t a b l e .
4.8.2.3.1.3.Pump-in/shut-in test The p u m p - i n / s h u t - i n t e s t i s p r o b a b l y t h e most common m i n i f r a c t u r i n g t e s t used and comprises i n j e c t i o n o f t h e s t i m u l a t i o n f l u i d chosen f o r use i n t h e main t r e a t m e n t a t t h e designed r a t e f o r t h e b i g j o b f o r a s p e c i f i c time, w i t h then t h e w e l l b e i n g s h u t - i n f o r p r e s s u r e d e c l i n e m o n i t o r i n g (TAN, McGOWEN, LEE & SOLIMAN 1988). I n f o r m a t i o n s w i t h h i g h l e a k o f f o r f r a c t u r e growth i n t o h i g h e r - s t r e s s zones, c l o s u r e p r e s s u r e determined f r o m p r e s s u r e d e c l i n e d a t a may be more a p p r o p r i a t e t h a n t h a t measured f r o m pump-in/flow-back t e s t s . During shut-in, t h e p r e s s u r e d e c l i n e approaches a l i n e a r r e l a t i o n w i t h t h e square r o o t o f t i m e f r o m s h u t - i n , w i t h t h e i n f l e c t i o n o r change o f s l o p e f r o m t h e l i n e a r dec l i n e i n d i c a t i n g c l o s u r e . I d e n t i f i c a t i o n o f t h e c o r r e c t s t r a i g h t l i n e may be enhanced by u t i l i z a t i o n o f a d d i t i o n a l p l o t s . E s t i m a t i o n o f f r a c t u r i n g g r a d i e n t and i n j e c t i o n r a t e can a l s o be o b t a i n e d b y a c o m b i n a t i o n o f m i n i f r a c t u r e dummy t r e a t m e n t and b a l l - o u t o p e r a t i o n (OSBDRNE, McLEOD & SCHROEDER 1981; c f . s e c t i o n 4.4.2.3.). SHELLEY & McGOblEN (1986) document how pump-in t e s t c o r r e l a t i o n s can be used f o r proppant placement p r e d i c t i o n s . CASTILLO (1987) and WAREMBOURG (1988) o u t l i n e f r a c t u r e h e i g h t determinat i o n f r o m pump-in c a l i b r a t i o n t r e a t m e n t s .
4.8.2.3,2. In-situ stress determination A l l t h e pump-in methods have become t h e most p r e v a l e n t techniques f o r measur i n g i n - s i t u s t r e s s w i t h d e t e r m i n a t i o n o f b o t h magnitude and d i r e c t i o n (WARPINSK I , BRANAGAN & WILMER 1983). The minimum i n - s i t u s t r e s s can be c a l c u l a t e d f r o m t h e i n f l e c t i o n p o i n t s i n t h e observed p r e s s u r e p r o f i l e s b e f o r e and a f t e r f r a c t u r e c l o s u r e (ECONOMIDES 1987 b ) . O t h e r techniques f o r i n - s i t u s t r e s s determinat i o n a r e a n e l a s t i c a l s t r a i n r e c o v e r y o f o r i e n t e d c o r e s (TEUFEL 1982, 1983, 1985; BLANTON & TEUFEL 1983; c f . s e c t i o n 6.2.2.3.3.4.), a c o u s t i c w a v e t r a i n measurements (ARON & MURRAY 1978; MAO, SWEENEY, HANSON & COSTANTINO 1984; NEWBERRY, NELSON & AHMED 1985; c f . s e c t i o n s 6.2.1.6.1. and 6.2.1.10.4.), and mechanic a l p r o p e r t i e s l o g g i n g ( c f . s e c t i o n s 6.2.1.7.7. and 6 . 3 . 6 . ) . WOLFF, BREDEHOEFT, KEYS & SHUTER (1974) p e r f o r m t e c t o n i c a l s t r e s s d e t e r m i n a t i o n by m i n i f r a c t u r i n g . While
pressure
d e c l i n e m o n i t o r i n g and a n a l y s i s o f m i n i f r a c t u r i n g t r e a t m e n t s
560 was h i t h e r t o c a r r i e d o u t i n u n i f o r m r e s e r v o i r s , M O S C H O V I D I S ( 1 9 8 7 ) p r e s e n t s an i n t e r p r e t a t i o n n o f m i n i f r a c t u r e s i n i t i a t e d near o r a t t h e i n t e r f a c e o f two f o r m a t i o n s o f d i f f e r e n t l e a k o f f c h a r a c t e r i s t i c s and m e c h a n i c a l p r o p e r t i e s and penet r a t i n g both sedimentary l a y e r s ( c f . s e c t i o n 4.5.4.4.2.). D i s t i n c t i o n c a n be made between h i g h - e f f i c i e n c y ( s t o r a g e - d o m i n a t e d ) m i n i f r a c t u r e s where t h e c r a c k a r e a i s p r o p o r t i o n a l t o pumping t i m e , and l o w - e f f i c i e n c y ( l e a k o f f - d o m i n a t e d ) m i n i f r a c t u r e s where t h e c r a c k a r e a i s p r o p o r t i o n a l t o t h e s q u a r e r o o t o f pumping t i m e . F r a c t u r i n g t r e a t m e n t s i n i t i a t e d n e a r o r a t t h e i n t e r f a c e o f two f o r m a t i o n s have been a p p l i e d i n N o r t h Sea c h a l k o i l f i e l d s t o o b t a i n r e l a t i v e l y stable w e l l completions. S i m i l a r i n t e r f a c e - i n s e r t i n g f r a c t u r i n g i s s i g n i f i c a n t i n c o a l - s e a m s t i m u l a t i o n , w i t h an u n d e r l y i n g s a n d s t o n e g u i d i n g t h e c r a c k t h a t e x t e n d s upwards i n t o t h e c o a l seam ( c f . s e c t i o n 4 . 4 . 3 . 8 . 1 . ) . When h y d r a u l i c f r a c t u r i n g o p e r a t i o n s a r e c o n d u c t e d i n t h i c k r e s e r v o i r s e c t i o n s , i t i s common f o r t h e p e r f o r a t e d i n t e r v a l t o be l o n g b u t y e t s t i l l s i g n i f i c a n t l y s h o r t e r t h a n t h e d i s t a n c e between r e l i a b l e b a r r i e r s t o c r a c k p r o p a s a t i o n , o r t h e h e i g h t o f t h e c r e a t e d f r a c t u r e i s o f t h e same o r d e r as t h e p e r f o r a t e d s e c t i o n , w i t h c r a c k p r o p a g a t i o n b e i n g u n a f f e c t e d b y c o n f i n i n g s t r a t a (MARTINS & HARPER 1 9 8 5 ) . U n d e r such c i r c u m s t a n c e s , f r a c t u r e shape changes c o n t i n u o u s l y as i t e v o l v e s f r o m t h e p e r f o r a t e d i n t e r v a l , t h u s c o m p l i c a t i n g i n t e r p r e t a t i o n o f p r e s s u r e d e c l i n e d a t a measured d u r i n g p r e - s t i m u l a t i o n m i n i f r a c t u r i n g t e s t s . A s u i t a b l e a p p r o a c h i s t o model t h e f r a c t u r e as h a v i n g e v o l v e d as a ser i e s o f confocal ellipses.
4.8.2.3.3.Closure stress vs. fluid type As c l o s u r e s t r e s s d e t e r m i n a t i o n s w i t h a c i d may be q u i t e n e a r t o t h o s e o b t a i ned w i t h w a t e r , t h e u s e o f m o d e r a t e a c i d volumes ahead o f s h u t - i n p e r i o d s does n o t h i n d e r and i n d e e d a i d s c l o s u r e p r e s s u r e e v a l u a t i o n as a r e s u l t o f p e r f o r a t i o n c l e a n u p (BEGNAUD & CLAIBORNE 1985; c f . s e c t i o n 5 . 6 . 4 . 2 . ) . V e r y s m a l l v o l u mes pumped i n h i g h - r e s o l u t i o n methods l e a d t o a w i d e s c a t t e r i n measured m i n i mum s t r e s s e s and i n t r o d u c e v u l n e r a b i l i t y t o a n i s o t r o p i c c h a r a c t e r i s t i c s . I t i s t h e r e f o r e i m p e r a t i v e t o a p p l y s u f f i c i e n t l y l a r g e volumes f o r m i n i f r a c t u r e c a l i b r a t i o n t e s t i n g . A n o t h e r c o n s i d e r a t i o n when pumping m o d e r a t e volumes u t i l i z e d i n t h e p a y zones i s t h e p o s s i b i l i t y o f l e a k o f f - i n d u c e d b a c k s t r e s s i n c r e a s i n g c l o s u r e s t r e s s d e t e r m i n a t i o n . Comparison o f c l o s u r e s t r e s s e s o b t a i n e d f r o m m i c r o f r a c t u r e and m i n i f r a c t u r e c a l i b r a t i o n j o b s o f t e n shows good agreement w h i c h adds c r e d i b i l i t y t o t h e h y p o t h e s i s o f e a r l y b a r r i e r e f f e c t s . The i n f e r r e d l a t e o c c u r r e n c e o f b a r r i e r phenomena i n t h e r e d u c e d p o r e p r e s s u r e i n t e r v a l m i g h t b e a t t r i b u t e d t o l e s s g r o w t h i n t o t h e b a r r i e r e a r l y o n due t o l o w e r r e l a t i v e f r a c t u r e propagation pressure.
4.8.2.3.4. Premature screenout termination recognition An i m p o r t a n t a p p l i c a t i o n o f m i n i f r a c t u r i n g c a l i b r a t i o n t r e a t m e n t s i s a l s o r e c o g n i t i o n o f p o s s i b l e p r e m a t u r e s c r e e n o u t t e r m i n a t i o n o f t h e m a i n j o b and c o l l e c t i o n o f i n f o r m a t i o n t o p r e v e n t i t s o c c u r r e n c e o r t o r e s c h e d u l e t h e m a i n oper a t i o n a c c o r d i n g l y ( c f . s e c t i o n 6.2.4.2.3.). Another s i g n i f i c a n t u t i l i z a t i o n a r e i n - s i t u s t r e s s measurements, w i t h c r e a t i o n o f s m a l l h y d r a u l i c f r a c t u r e s bei n g t h e m o s t p r a c t i c a l method t o d e t e r m i n e t h e s t a t e o f s t r e s s a c t i n g a t g r e a t d e p t h s w i t h i n t h e e a r t h (GREENFIELD & AHMED 1 9 8 3 ) . The i n - s i t u s t r e s s measurement v a l u e s a r e n e c e s s a r y f o r c a l c u l a t i o n o f t h e b o t t o m h o l e t r e a t i n g p r e s s u r e r e q u i r e d t o c r e a t e and t o c o n t a i n a l a r g e - s c a l e h y d r a u l i c f r a c t u r e , w i t h t h e det e r m i n e d maximum b o t t o m h o l e t r e a t i n g p r e s s u r e s b e i n g i n d i s p e n s i b l y f o r a v o i d i n g f r a c t u r e p r o p a g a t i o n i n t o o v e r l y i n g and u n d e r l y i n g s t r a t a .
I n - s i t u s t r e s s d i r e c t i o n and m a g n i t u d e d e t e r m i n a t i o n i s a l s o e s s e n t i a l f o r p l a n n i n g and d e s i g n o f w e l l p a t t e r n and s p a c i n g i n o r d e r t o p r e v e n t f r a c t u r e i n t e r f e r e n c e ( c f . s e c t i o n 4.10.5.) o r even m a n i p u l a t i o n s o f t h e p r e s s u r e regime i n o r d e r t o change t h e p r e f e r e n t i a l d i r e c t i o n o f f r a c t u r e e x t e n s i o n ( c f . sec-
561 t i o n 4.9.4.1.). M i c r o h y d r a u l i c f r a c t u r i n g s t r e s s measurements are a l s o c a r r i e d o u t by BREDEHOEFT, WOLFF, KEYS & SHUTER (1976). F i e l d experience o f m i n i f r a c t u r i n g f o r parameter d e t e r m i n a t i o n i s a l s o r e p o r t e d by HODGES & PAOLI (1982), BEGNAUD & CLAIBORNE (1985), G R I (1988), NORTHROP (1988); HOLDITCH, ROBINSON, WHITEHEAD & ELY (1988); PEARSON, LYNCH, SCHMIDT & McCASLIN (1988) and WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE (1988).
4.8.2.3.5. Fluids and mechanics testing I n o r d e r t o be f u l l y r e l i a b l e , m i n i f r a c t u r e a n a l y s i s has t o consider a l s o the r h e o l o g i c a l p r o p e r t i e s o f the s t i m u l a t i o n f l u i d (LEE 1987). T h i s goal can be achieved by i n c o r p o r a t i o n o f an energy-balance equation i n t o m i n i f r a c t u r e e v a l u a t i o n , a l l o w i n g geometry p r e d i c t i o n from m i n i f r a c t u r e i n t e r p r e t a t i o n t o agree w e l l w i t h t h a t d e r i v i n g from f u l l - s c a l e f r a c t u r e design programs based on the same o r on s i m i l a r models. The s i g n i f i c a n c e o f m i n i f r a c t u r i n g and pressure d e c l i n e a n a l y s i s f o r proper f u l l - s c a l e j o b p l a n n i n g i s a l s o o u t l i n e d by CLEARY (1988). I n a d d i t i o n t o measuring d u r i n g the m i n i f r a c t u r e , c l o s u r e s t r e s s can be determined from an i n - s i t u s t r e s s t e s t and from the a c t u a l f u l l - s c a l e f r a c t u r i n g s t i m u l a t i o n (ROBINSON, HOLDITCH & WHITEHEAD 1986). I n - s i t u p e r m e a b i l i t y and i n i t i a l r e s e r v o i r pressure a r e o b t a i n e d from p r o d u c t i o n and pressure b u i l d - u p t e s t s , and minimum bottom h o l e f l o w i n g pressure can be estimated from f i e l d producing c o n d i t i o n s . With a l l the v a r i a b l e s obtained from m i n i f r a c t u r e and o t h e r sources, optimum crack l e n g t h can be determined by p r o j e c t i n g recovery e f f i c i e n cy as a f u n c t i o n o f f r a c t u r e l e n g t h and drainage area (HOLDITCH, JENNINGS & NEUSE 1978; ROBINSON, HOLDITCH & LEE 1983; c f . s e c t i o n 4.8.11.1. Aspects o f o p t i mum f r a c t u r e l e n g t h are a l s o discussed by HOLDITCH, JENNINGS, NEUSE & WYMAN (1978) and BENNETT, ROSATO & REYNOLDS (1981). M i n i f r a c t u r e c a l i b r a t i o n and checking treatments a l s o serve as a mechanical t e s t on f r a c t u r e l i n e , wellhead p r o t e c t o r , tubing, packer, and a l l o t h e r equipment being used, and g i v e s a l s o an idea o f average t r e a t i n g pressures (PAI, GARBIS & HALL 1983). M i n i f r a c t u r i n g jobs are conducted i n c o n j u n c t i o n w i t h r o u t i n e p e r f o r a t i o n breakdown operations ( c f . s e c t i o n 4.8.2.2.). Case s t u d i e s o f m i n i f r a c t u r i n g pressure t e s t s and measurements are a l s o r e p o r t e d by BUNDY (1981); KIM, CHAMPION & COOPER (1985); LEE (1987) and WARPINSKI & BRANAGAN (1988). SINGH, AGARWAL & KRASE (1988) document systematic design and a n a l y s i s o f stepr a t e t e s t s f o r d e t e r m i n a t i o n o f f o r m a t i o n p a r t i n g pressure.
4.8.2.4. Size and significance of minifracturing While the performance o f m i n i f r a c t u r e s f o r e v a l u a t i o n o f the above o u t l i n e d parameters w i t h o u t p l u g g i n g o f the crack by proppants i s a l r e a d y common sense i n p r e p a r a t i o n o f l a r g e - s c a l e s t i m u l a t i o n treatments s i n c e q u i t e some time f o r the reason o f d e t e r m i n a t i o n o f u n r e s t r i c t e d f r a c t u r e c l o s u r e i n pre-MHF c a l i b r a t i o n t e s t s (PAI, GARBIS & HALL 1983), propping o f the s h o r t cracks t h a t are created d u r i n g the pressure breakdown t e s t b e f o r e the main o p e r a t i o n w i t h the aim on s t i m u l a t i n g the f o r m a t i o n a l r e a d y w i t h the scaled-down p r e c u r s o r run, however, has o n l y i n the l a s t years become s l i g h t l y more common. A f t e r discuss i o n o f some general aspects, proppant q u a n t i t i e s and f r a c t u r e wing l e n g t h o f m i n i f r a c t u r i n g treatments i n comparison t o massive f r a c t u r i n g o p e r a t i o n s a r e illustrated.
4.8.2.4.1. General aspects Computer programs support the i n t e r p r e t a t i o n o f pressure d e c l i n e a f t e r breakdown-test m i n i f r a c t u r e s (HALLIBURTON 1985). The s i g n i f i c a n c e o f p r e - f r a c t u r e i n j e c t i o n surveys f o r a successful main s t i m u l a t i o n o p e r a t i o n i s a l s o s t r e s s e d by
562
BUNDY
( 1 9 8 1 ) , and LEE ( 1 9 8 7 ) comments o n e f f e c t s o f f l u i d r h e o l o g y on m i n i f r a c ture analysis.
WYMAN, HOLDITCH & RANDOLPH ( 1 9 7 9 ) r e p o r t p r o p p a n t m i n i f r a c t u r i n g f o r t h e p u r pose o f p r o d u c t i o n and b u i l d u p t e s t i n g i n o r d e r t o d e t e r m i n e r e s e r v o i r c a p a c i ty, w i t h t h e f i e l d experimental t r e a t m e n t i n a d d i t i o n t o t h i s g a t h e r i n g informat i o n o n t h e number o f p e r f o r a t i o n s a c t u a l l y a c c e p t i n g f l u i d f o r t h e r e a s o n o f s e l e c t i o n o f r a t e and p r e s s u r e t o b e used o n t h e l a r g e - s c a l e f r a c t u r i n g j o b .
4.8.2.4.2.
Proppant q u a n t i t i e s and f r a c t u r e wing lengths
M i n i a t u r e h y d r a u l i c proppant f r a c t u r i n g jobs are g e n e r a l l y performed w i t h a b t . 5 - 50 t o f p r o p p a n t s , i n e x c e p t i o n a l l y l a r g e c a s e s a l s o w i t h volumes u p t o 100 - 150 t ( d e p e n d i n g on t h i c k n e s s , l i t h o l o g y and h y d r o c a r b o n p o t e n t i a l o f the r e s e r v o i r ) t h a t are i n f i l l i n g f r a c t u r e s ranging i n length predominantly between a few m and s e v e r a l t e n s o f m and o n l y i n s u b o r d i n a t e c a s e s r e a c h i n g u p t o a b t . 100 m ( i n c o n t r a s t t o m a s s i v e h y d r a u l i c f r a c t u r e o p e r a t i o n s w h i c h i n c l u d e c r a c k l e n g t h s between a b t . 100 m and m o s t f r e q u e n t l y a b t . 500 m, b u t i n e x t r e m e c a s e s up t o more t h a n 1,500 m t h a t a r e p l u g g e d w i t h p r o p p a n t q u a n t i t i e s between a b t . 100 t and m o s t a b u n d a n t l y a b t . 800 t, b u t i n u l t r a - l a r g e jumbo j o b s u p t o more t h a n 2,500 t; c f . s e c t i o n s 1 . 1 . 1 . 4 . and 4 . 8 . 1 . 2 . ) . The r e l a t i o n s h i p s between r e s e r v o i r c a p a c i t y , f l u i d t y p e , d e p o s i t s i z e and m i n i f r a c t u r e d e s i g n a r e v e r y i m p o r t a n t ( c f . AGARWAL, CARTER & POLLOCK 1 9 7 9 ) , because a m i n i f r a c t u r i n g o r o t h e r s m a l l - s c a l e s t i m u l a t i o n j o b c a r r i e d o u t i n a h i g h - p o t e n t i a l deep t h i c k t i g h t g a s - b e a r i n g s a n d s t o n e c o u l d be c o n s i d e r a b l y l a r g e r i n view o f absolute s i z e than a f u l l - s c a l e conventional f r a c t u r i n g operat i o n i n a shallow t h i n moderate-permeability o i l - b e a r i n g sandstone.
4.8.2.5. Proppant vs. non-proppant m i n i f r a c t u r i n g The r e a s o n f o r h i t h e r t o m a i n l y c a r r y i n g o u t n o n - p r o p p a n t m i n i f r a c t u r i n g i s t o a v o i d h a v i n g an o b s t a c l e i n t h e c r a c k i n c a s e o f l a t e r p e r f o r m i n g l a r g e r s c a l e r e f r a c t u r i n g . R e o p e n i n g o f an unpropped c r a c k i s m e c h a n i c a l l y s i m p l e r , and g r e a t e r s a f e t y a g a i n s t p r e m a t u r e s c r e e n o u t t e r m i n a t i o n i s a c h i e v e d i f t h e m i n i f r a c t u r e does n o t have t o b e c l e a r e d b y r e m o v i n g o r d i s p l a c i n g t h e s m a l l s c a l e p r o p p a n t package w h i c h keeps i t open. On t h e o t h e r hand, however, r e f r a c t u r i n g o f a p r o p p a n t - f i l l e d c r a c k does o f t e n p e r f o r m q u i t e s m o o t h l y ( c f . s e c t i o n 4.8.9.5.), and p r o p p a n t m i n i f r a c t u r i n g has t h e d e c i s i v e a d v a n t a g e t o a l l o w p r o p e r assessment o f t h e r e s e r v o i r c a p a c i t y due t o a d e q u a t e s u p p o r t o f t h e c r a c k t h a t i s l i m i t e d t o t h e n e a r w e l l b o r e s u r r o u n d i n g s w h i c h i s t h e r e g i o n subj e c t e d t o t h e h i g h e s t c l o s u r e s t r e s s e s due t o maximum p r e s s u r e drawdown upon dep l e t i o n ( c f . s e c t i o n 3 . 9 . 1 . ) . The drawback o f enhanced s c r e e n o u t d a n g e r b y t h e proppant plug i n the small-scale crack which hinders r e e n t r y o f the f r a c t u r e c a n be e l i m i n a t e d b y c o n s i d e r a b l y i n c r e a s i n c j w i d t h o f t h e y o u n g e r c r a c k u p t o f o u r t i m e s i t s o r i g i n a l s i z e and t o f i l l i n t h e l a r g e r f r a c t u r e w i t h h i g h e r p r o p p a n t s a t u r a t i o n s ( S M I T H , MILLER & HAGA 1 9 8 7 ) . Another reason f o r non-proppant m i n i f r a c t u r i n g i s t o a v o i d proppant i n t e r f e r e n c e w i t h and d i s t o r t i o n o f t h e p r e s s u r e d e c l i n e c u r v e a f t e r s h u t - i n w h i c h c o u l d a f f e c t t h e c a l c u l a t i o n o f t h e f l u i d - l o s s c o e f f i c i e n t (WAREMBOURG, KLINGENSMITH, HODGES & ERDLE 1985) and c o u l d make good m a t c h i n g i m p o s s i b l e (TAN, McGOWEN, LEE & SOLIMAN 1 9 8 8 ) . T h e r e i s a l s o n o a s s u r a n c e t h a t t h e c a l c u l a t e d f l u i d volume i n s i d e t h e f r a c t u r e a f t e r pumping ( b a s e d o n c a l c u l a t e d f l u i d e f f i c i e n c y ) i s l a r g e r t h a n b u l k p r o p p a n t volume. F i e l d examples o f p r o p p a n t m i n i f r a c t u r i n g a r e r e p o r t e d b y TAN, McGOWEN, LEE & SOLIMAN ( 1 9 8 8 ) . P r o p p a n t m i n i f r a c t u r i n g and s u b s e q u e n t p r e s s u r e drawdown t e s t i n g c a n t h e r e f o r e a l s o s e r v e as f i e l d e x p e r i m e n t s i n v e s t i g a t i n g t h e s u i t a b i l i t y o f v a r i o u s p r o p p a n t t y p e s f o r p l u g g i n g o f m a s s i v e h y d r a u l i c f r a c t u r e s , because t h e s h o r t
563 m i n i f r a c t u r e s happen t o be s i t u a t e d i n t h e zone o f g r e a t e s t c l o s u r e s t r e s s sens i t i v i t y w i t h i n t h e p r e s s u r e d e p l e t i o n cone s u r r o u n d i n g t h e w e l l b o r e . The f o l l o w i n g o u t l i n e d i s c u s s e s some p o i n t s o f e f f e c t i v i t y d i f f e r e n c e , comparative assessment and s h o r t p r e c i s e propped m i n i f r a c t u r e s .
4.8.2.5.1. Effectivity difference HARPER, HAGAN & M A R T I N S (1985) e v a l u a t e some case s t u d i e s o f s m a l l - s c a l e p r o p p a n t and non-proppant f r a c t u r i n g t r e a t m e n t s . A l t h o u g h p r o p p a n t - f r e e hydraul i c s t i m u l a t i o n s do p r o v i d e some c o n d u c t i v i t y as r e f l e c t e d by p r o d u c t i v i t y i n creases of t h e w e l l s ( p a r t i c u l a r l y i f f l u i d - l o s s agents a r e added t o t h e f r a c t u r i n g f l u i d s , such as 100 mesh sand which has been proven t o have a c e r t a i n p r o p p i n g e f f e c t i t s e l f ; SINCLAIR 1980; CAMPBELL, HANOLD, SINCLAIR & VETTER 1981; WARPINSKI 1988 a; c f . s e c t i o n s 1.4.11.2.1. and 4.8.8.3.2.3.) and almost no e v i dence f o r a more r a p i d d e c l i n e i n w e l l c a p a b i l i t y a f t e r f r a c t u r i n g w i t h o u t propp a n t r e l a t i v e t o p r o p p a n t s t i m u l a t i o n c o u l d be observed i n t h e p r o d u c t i v i t y dec l i n e curves, t h e p r o p p a n t f r a c t u r i n g t r e a t m e n t s have an o v e r a l l markedly b e t t e r e f f e c t than t h e p r o p p a n t - f r e e s t i m u l a t i o n s . I n some cases, non-proppant f r a c t u r i n g can e l i m i n a t e s a n d - r e l a t e d o p e r a t i n g problems ( c f . s e c t i o n 5.3.5.2.), and a reason why f r a c t u r i n g w i t h o u t p r o p p a n t may g i v e r i s e t o a c o n d u c t i v e channel i s shear displacement o f a magnitude comp a r a b l e t o sandstone g r a i n d i a m e t e r between opposing crack f a c e s s o t h a t a mismatch r e s u l t s when t h e f r a c t u r e a t t e m p t s t o c l o s e ( c f . s e c t i o n s 4.7.7. and i n a s i m i l a r way as l a r g e b l o c k s l i d i n g and d i l a t i o n o f j o i n t s oc6.2.2.3.2.), c u r s o c c a s i o n a l l y i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s , g i v i n g r i s e t o some i n c r e a sed c r a c k roughness (TSANG & WITHERSPOON 1983).
4.8.2.5.2. C m a r a t i v e assessment R e l a t i v e l y s o f t r o c k s e x p e r i e n c e more shear displacements l e a d i n g t o i n c r e a s i n g f r a c t u r e c o n d u c t i v i t y t h a n s t i f f e r r o c k s , and h i g h s t r e s s d i f f e r e n c e s f a vour r e l a t i v e l y h i g h shear displacements (HARPER, HAGAN & MARTINS 1985). The d i s t a n c e f o r which p r o p a g a t i o n i n t h e w e l l b o r e p l a n e occurs r a t h e r than c u r v i n g i n t o a p r i n c i p a l p l a n e m i g h t be maximized by h i g h pumping r a t e s d u r i n g t h e f r a c t u r i n g t r e a t m e n t . The s m a l l - s c a l e shear displacements a r e l i k e l y t o a r i s e when c r a c k s i n i t i a t e f r o m w e l l b o r e s i n c l i n e d t o p r i n c i p a l s t r e s s planes w i t h m i n o r d e v i a t i o n s o f t h e b o r e h o l e f r o m t h e p r i n c i p a l s t r e s s planes b e i n g commonplace, because i t i s h i g h l y improbable t h a t a w e l l b o r e w i l l be d r i l l e d p r e c i s e l y i n a p r i n c i p a l s t r e s s p l a n e . PEMPER, FLECKER, McWHIRTER & OLIVER (1988) r e p o r t nonp r o p p a n t f r a c t u r i n g i n o r d e r t o i n c r e a s e n a t u r a l gas p r o d u c t i o n by a p p l y i n g f l u i d p r e s s u r e f o r enhancing communication between n a t u r a l c r a c k s . The e v a l u a t i o n o f e a r l i e r r e p o r t s on p r o p p a n t - f r e e f r a c t u r i n g t r e a t m e n t s (GHAURI 1980, LAMBERT & TREVIS 1980; MAHONEY, STUBBS, SCHWERER & OOBSCHA 1980; FREEMAN, ABEL & K I M 1981) l e a d s HARPER, HAGAN & MARTINS (1985) t o t h e c o n c l u s i o n t h a t a t l e a s t s h o r t - t e r m improvement o f w e l l p r o d u c t i v i t y i n low-permeabil i t y r e s e r v o i r s can be achieved by s t i m u l a t i o n w i t h o u t proppants, even i n w e l l s w i t h o u t s k i n damage o r w i t h o u t t h e p o t e n t i a l f o r access t o n a t u r a l f r a c t u r e s o r more permeable zones c l o s e t o t h e w e l l b o r e . Whenever comparisons a r e a v a i l a b l e , however, non-proppant f r a c t u r i n g has been c l e a r l y l e s s e f f e c t i v e than propped t r e a t m e n t s . Aspects o f unpropped m i n i f r a c t u r i n g a r e a l s o mentioned by BRANAGAN, CIPOLLA, LEE & WILMER (1985); BRITT & LARSEN (1986); BRANAGAN, CIPOLLA, LEE & YAN (1987); HOLDITCH, ROBINSON & WHITEHEAD (1987) and HOLDITCH, ROBINSON, WHITEHEAD & ELY ( 1 9 8 7 ) . O t h e r t y p e s o f non-proppant f r a c t u r i n g a r e n i t r o g e n s t i m u l a t i o n o f s h a l e s (ABEL 1982; c f . s e c t i o n 4.4.4.2.) and w a t e r f r a c t u r i n g o f geothermal ( c f . s e c t i o n 4.1.3.5.) and c h a l k ( c f . s e c t i o n 4 . 5 . 4 . 5 . 2 . ) r e s e r v o i r s as w e l l as w a s t e - l i q u i d d i s p o s a l w e l l s t i m u l a t i o n ( c f . s e c t i o n 4 . 8 . 7 . 2 . 2 . ) .
564
4.8.2.5.3.
Short p r e c i s e propped mini f rac tures
A special a p p l i c a t i o n o f proppant m i n i f r a c t u r i n g i s c r e a t i o n o f s h o r t prec i s e hydraulic cracks f o r d i s t i n c t i v e s t i m u l a t i o n o f s e n s i t i v e r e s e r v o i r s (SMITH 1981) a i m i n g on g e n e r a t i o n o f a propped f r a c t u r e o f t h e r e q u i r e d l e n g t h w i t h o u t r e g a r d t o t h e t o t a l h y d r a u l i c l e n g t h , and f o l l o w i n g t h a t on o p e r a t i o n o f t h e w e l l below f r a c t u r e c l o s u r e p r e s s u r e . Some aspects o f p r o p p a n t c o n c e n t r a t i o n and d i s t r i b u t i o n as w e l l as impact o f batch-mix f r a c t u r i n g a r e e v a l u a t e d as f o l l o w s .
4.8.2.5.3.1.
Proppant concent r a t i o n and d i s t r i but i o n
I n comparable s i t u a t i o n s , f o r m e r l y l o w - r a t e , l o w - v i s c o s i t y , low-proppant conc e n t r a t i o n approaches have been used t o d r i b b l e - i n a s h o r t f r a c t u r e , b u t t h e l a t t e r process had t h e disadvantages t h a t inadequate knowledge and p o o r p r e d i c t i v e c a p a b i l i t i e s c o n c e r n i n g p r o p p a n t s e t t l i n g made i t d i f f i c u l t t o d e s i g n f r a c t u r e length accurately, low-viscosity f l u i d r e s u l t e d i n a triangular-shaped p r o p p a n t d i s t r i b u t i o n , and low p r o p p a n t s a t u r a t i o n gave r i s e t o n o n - u n i f o r m and inadequate f r a c t u r e c o n d u c t i v i t y . The b a s i c concept o f c r e a t i o n o f s h o r t p r e c i s e h y d r a u l i c c r a c k s i s t o pump a r e l a t i v e l y l a r g e pad t h u s e n s u r i n g adequate w i d t h , and then t o s w i t c h d i r e c t l y t o a h e a v y - c o n c e n t r a t i o n p r o p p a n t s l u r r y . Maj o r problems a r e t h e r e q u i r e m e n t o f s m a l l e x a c t s l u r r y volumes and t h e e f f e c t o f a f t e r f l o w o f the f r a c t u r i n g f l u i d . A f t e r w e l l shut-in, the s l u r r y continues t o f l o w u n t i l t h e c r a c k c l o s e s t o l o c k t h e proppants i n p l a c e . The l e n g t h o f t h i s a f t e r f l o w p e r i o d depends on f l u i d - l o s s r a t e and p r o p p a n t s a t u r a t i o n .
4.8.2.5.3.2.
Impact o f batch-mix f r a c t u r i n g
O p e r a t i o n o f s h o r t p r e c i s e small-volume f r a c t u r i n g t r e a t m e n t s p l a c i n g h i g h p r o p p a n t c o n c e n t r a t i o n s i n c r a c k s c r o s s i n g moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s can be o p t i m i z e d by batch-mix f r a c t u r i n g (CRAMER & SONGER 1988; c f . sect i o n s 4.3.3.5.3. and 4 . 6 . 3 . 1 . ) c o m p r i s i n g use o f pre-mixed f l u i d / p r o p p a n t s l u r r i e s . H y d r a u l i c f r a c t u r e s i n moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s i n c r e a s e b o r e h o l e t r a n s m i s s i b i l i t y m a i n l y due t o bypass o f t h e w e l l b o r e - v i c i n i t y damage zone ( c f . s e c t i o n 4.8.3.) and e x t e n s i o n o f t h e e f f e c t i v e b o r e h o l e r a d i u s . O p t i mal performance i n pay zones i n t e r s e c t e d by c l o s e l y - s p a c e d w e l l s r e q u i r e s u t i l i z a t i o n o f small p r e c i s e f l u i d volumes c o n t a i n i n g h i g h p r o p p a n t s a t u r a t i o n s . As most continuous-mix equipment i s p o o r l y s u i t e d t o p r e p a r e and d i s p l a c e s m a l l p r e c i s e s l u r r y volumes, such t r e a t m e n t s a r e p r e f e r a b l y c a r r i e d o u t by b a t c h m i x i n g where h i g h p r o p p a n t c o n c e n t r a t i o n s can be p r e - b l e n d e d w i t h g e l l e d l i q u i d i n t h e batch-mix i n v e n t o r y compartments and s l u r r y d e n s i t y can be v e r i f i e d b e f o r e the s t a r t o f the job. Batch m i x i n g assures placement o f an u n i f o r m p r o p p a n t d i s t r i b u t i o n i n t h e f r a c t u r e s i n c e proppant s a t u r a t i o n s a t t h e s u r f a c e w i l l n o t be a f f e c t e d by t r e a t m e n t r a t e f l u c t u a t i o n s , and i n c r e m e n t a l improvements i n w e l l b o r e t r a n s m i s s i b i l i t y can be achieved i n many cases due t o c r e a t i o n o f a s i g n i f i c a n t conduct i v i t y c o n t r a s t between r e s e r v o i r and c r a c k . The use o f small p r e c i s e s l u r r y volumes a l s o s u c c e s s f u l l y l i m i t s v e r t i c a l and l a t e r a l f r a c t u r e e x t e n s i o n , a v o i d s n e g a t i v e consequences o f i n t e r z o n a l communication and w a t e r f l o o d sweep i n e f f i c i e n c y , and reduces screenout f a i l u r e problems due t o achievement o f u n i f o r m proppant d i s t r i b u t i o n and use o f a h i g h p a d l s l u r r y r a t i o .
4.8.2.6. Modified micro- and m i n i f r a c t u r i n g a n a l y s i s f o r overpressure c a l i b r a t e d f r a c t u r e design SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE (1988) propose m o d i f i e d m i n i and m i c r o f r a c t u r e t e s t s i n o r d e r t o determine t h e f r a c t u r i n g parameters r e q u i r e d f o r o v e r p r e s s u r e c a l i b r a t e d t r e a t m e n t design (SHLYAPOBERSKY, WONG & WALHAUG
565 1988; c f . s e c t i o n 4.3.4.2.3.). F r a c t u r e reopening p r e s s u r e i s d e f i n e d f r o m lowr a t e i n j e c t i o n t e s t s , and f r a c t u r e c l o s u r e p r e s s u r e i s d e f i n e d f r o m flowback and/or extended f a l l o f f t e s t s analyzed by g l o b a l p r e s s u r e match which r e s u l t s i n more o b j e c t i v e d e t e r m i n a t i o n o f minimum i n - s i t u s t r e s s and t o t a l l e a k o f f coe f f i c i e n t t h a n c o n v e n t i o n a l t e c h n i q u e s . The r e v i s e d model a l s o d e f i n e s c o r r e c t e d i n s t a n t a n e o u s s h u t - i n pressure, f r a c t u r e o v e r p r e s s u r e and c r e a t e d c r a c k d i mensions. The o u t l i n e as f o l l o w s focusses on l a y o u t and purpose o f c o n v e n t i o n a l m i c r o f r a c t u r e s and m i n i f r a c t u r e s , m o d i f i e d m i c r o f r a c t u r e and m i n i f r a c t u r e p r o c e dures, and d e s i g n parameters f o r optimum m i c r o - and m i n i f r a c t u r e t e s t s .
4.8.2.6.1. Layout and Purpose of conventional microfractures and minifractures Conventional m i c r o f r a c t u r e s a r e small h y d r a u l i c f r a c t u r i n g t r e a t m e n t s p e r f o r med i n a s e l e c t e d i n t e r v a l and a r e t y p i c a l l y used t o determine a minimum i n - s i t u stress p r o f i l e o r v e r t i c a l s t r e s s v a r i a t i o n f o r p r e d i c t i n g v e r t i c a l crack growth (SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1988). Conventional m i c r o f r a c t u r e t e s t s assume t h a t t h e c r e a t e d c r a c k i s small and i s n e a r l y c l o s e d a t t h e moment o f s h u t - i n . M i n i f r a c t u r e s a r e l a r g e r h y d r a u l i c f r a c t u r i n g t r e a t m e n t s and a r e used f o r d e t e r m i n i n g l e a k o f f c o e f f i c i e n t , r e p r e s e n t a t i v e v a l u e s o f gross f r a c t u r e i n t e r v a l , and c r e a t e d c r a c k dimensions o f l e n g t h and average w i d t h , w i t h these parameters b e i n g o b t a i n e d f r o m p r e s s u r e d e c l i n e a n a l y s i s a f t e r s h u t - i n ( c f . s e c t i o n 4 . 8 . 1 . ) . U n l i k e t h e m i c r o f r a c t u r e t e s t where t h e m i n i it is mum i n - s i t u s t r e s s i s o b t a i n e d f r o m t h e i n s t a n t a n e o u s s h u t - i n pressure, d e r i v e d i n t h e m i n i f r a c t u r e t e s t f r o m e i t h e r an extended p r e s s u r e f a l l o f f o r a c o n s t a n t - r a t e flowback. As f i e l d e x p e r i e n c e has r e v e a l e d t h a t t h e n e t f r a c t u r e p r e s s u r e i s f r e q u e n t l y much h i g h e r than t h a t p r e d i c t e d by c l a s s i c a l design, o v e r p r e s s u r e c a l i b r a t e d f r a c t u r e m o d e l l i n g (SHLYAPOBERSKY, WONG & WALHAUG 1988; c f . s e c t i o n 4.3.4.2.3.) i s i n t r o d u c e d and m o d i f i c a t i o n s o f m i c r o - and m i n i f r a c t u r e t e s t a n a l y s i s (SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1988) a r e p r o posed. O t h e r aspects o f t h e r e v i s e d m i c r o - and m i n i f r a c t u r i n g sequence a r e d i s cussed by HUCKABEE ( 1 9 8 8 ) . D i s c r e p a n c i e s between n e t f r a c t u r e p r e s s u r e s p r e d i c t e d by c o n v e n t i o n a l mod e l s and those measured i n t h e f i e l d a r e t h e consequence o f t h e assumption o f t h e c l a s s i c a l models t h a t t h e apparent f r a c t u r e toughness o f t h e f o r m a t i o n r o c k i s a m a t e r i a l c o n s t a n t , whereas i n r e a l i t y i t i s a scale-dependent parameter ( c f . section 4.3.4.2.3.3.). T h e r e f o r e apparent f r a c t u r e toughness must be d e t e r mined f r o m f i e l d measurements i n s t e a d o f l a b o r a t o r y e v a l u a t i o n s , and f r a c t u r e p r e s s u r e d e c l i n e a n a l y s i s has t o i n c l u d e t h e e f f e c t o f f i e l d apparent f r a c t u r e toughness (SHLYAPOBERSKY 1987).
4.8.2.6.2. Modified microfracture and mini fracture procedures The approach f o r measuring f r a c t u r e parameters as proposed by SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE (1988) i n v o l v e s a s e l e c t e d sequence o f reopening, p r o p a g a t i o n , s h u t - i n and flowback procedures which a r e s i m i l a r f o r b o t h m i c r o and m i n i f r a c t u r e t e s t s and d i f f e r o n l y i n t h e magnitude o f r a t e s and volumes pumped and t h e types o f f l u i d used. B e f o r e c r a c k i n i t i a t i o n , a f i l t r a t i o n t e s t can be performed i n o r d e r t o determine t h e m a t r i x l e a k o f f p r o p e r t i e s o f an unf r a c t u r e d f o r m a t i o n and t h e c o m p r e s s i b i l i t y o f t h e b o r e h o l e f l u i d by pumping o f w a t e r o r b r i n e a t a v e r y low r a t e f o r p r e s s u r e b u i l d u p w i t h o u t exceeding t h e exp e c t e d minimum i n - s i t u s t r e s s . The f i l t r a t i o n t e s t a l s o h e l p s t o i d e n t i f y an adequate r e o p e n i n g r a t e and t h e f r a c t u r e breakdown i n j e c t i o n r a t e i n permeable formations. D u r i n g t h e f i r s t i n j e c t i o n a t a r a t e s u f f i c i e n t t o c r e a t e a s i n g l e dominant f r a c t u r e , t h e f o r m a t i o n i s broken down f o r t h e purpose o f i n i t i a t i n g a s i n g l e dominant c r a c k f r o m t h e b o r e h o l e . I n l o w - p e r m e a b i l i t y pays, i n j e c t i o n r a t e i s n o r m a l l y much l a r g e r t h a n f i l t r a t i o n r a t e , whereas i n moderate- t o high-permea-
566 b i l i t y r e s e r v o i r s and n a t u r a l l y f r a c t u r e d sequences, i n j e c t i o n r a t e h a s t o be much g r e a t e r t h a n maximum l e a k o f f r a t e . F o l l o w i n g t h e breakdown, a s e r i e s o f r e opening, propagation, s h u t - i n and f l o w b a c k c y c l e s c a n b e c o n d u c t e d . Some a s p e c t s o f r e o p e n i n g , p r o p a g a t i o n , s h u t - i n and f l o w b a c k c y c l e s as w e l l as g l o b a l v s . l o c a l p r e s s u r e d e c l i n e match a r e d i s c u s s e d as f o l l o w s .
4.8.2.6.2.1.
Reopening, propagat ion, shut-in and f lowback cycles
D u r i n g c o u r s e o f r e o p e n i n g , p r o p a g a t i o n , s h u t - i n and f l o w b a c k c y c l e s , t h e f r a c t u r e i s p r o p a g a t e d t o a d e s i r e d s i z e , and s h u t - i n and f l o w b a c k p r o c e d u r e s a r e performed t o i d e n t i f y crack c l o s u r e pressure, overpressure, f l u i d - l o s s coeff i c i e n t , f r a c t u r e d i m e n s i o n s and o t h e r c r a c k - s p e c i f i c p a r a m e t e r s (SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1 9 8 8 ) . F r a c t u r e c l o s u r e and o p e n i n g p r e s s u r e s c a n measure t h e minimum i n - s i t u s t r e s s o n l y i f m o s t o f t h e c r e a t e d c r a c k s u r f a c e i s p e r p e n d i c u l a r t o t h e minimum s t r e s s component. As t h e h y d r a u l i c f r a c t u r e may be i n i t i a t e d a t some a n g l e t o t h e p l a n e p e r p e n d i c u l a r t o t h e f a r - f i e l d minimum i n - s i t u s t r e s s , and a f t e r p r o p a g a t i n g away f r o m t h e w e l l t h e c r a c k s u r f a c e w i l l be a l i g n e d w i t h t h i s d i r e c t i o n , t h e f r a c t u r e has t o b e p r o p a g a t e d f a r enough f r o m t h e w e l l t o m i n i m i z e t h e e f f e c t o f i n i t i a l c r a c k o r i e n t a t i o n a t t h e borehole. Because v i s c o u s f r i c t i o n a l p r e s s u r e s c a n b e v e r y h i g h e v e n f o r l o w - v i s c o s i t y f l u i d s pumped a t s m a l l r a t e s as a consequence o f s m a l l f r a c t u r e a p e r t u r e and amp l i f i c a t i o n o f t h e e f f e c t b y w a v i n e s s and r o u g h n e s s o f t h e r e a l c r a c k s u r f a c e , and s i n c e t h e p r e s s u r e measured a t t h e b o r e h o l e may d i f f e r s i g n i f i c a n t l y f r o m t h e a v e r a g e p r e s s u r e i n t h e f r a c t u r e , and c r a c k o p e n i n g and c l o s u r e p r e s s u r e det e r m i n e d f r o m r e o p e n i n g and f l o w b a c k p r o c e d u r e s may be v e r y s e n s i t i v e t o f l o w r a t e s , t h e f r a c t u r e s h o u l d be s i z e d l a r g e enough, b u t n o t t o o l a r g e t o r i s k b r e a k i n g i n t o a d j a c e n t t e s t i n t e r v a l s . The a c c u r a t e d e s i g n o f t h e i n j e c t e d volume f o r c r e a t i n g a c r a c k o f d e s i r e d d i m e n s i o n w h i c h i s c o n f i n e d i n t h e t e s t e d zone has t o a c c o u n t f o r t h e f i e l d - d e t e r m i n e d o v e r p r e s s u r e and t h e c o r r e s p o n d i n g a p p a r e n t f r a c t u r e t o u g h n e s s as w e l l a s f o r t h e l e a k o f f c o e f f i c i e n t .
4.8.2.5.2.2.
Global vs. local pressure decline match
I n l a t e r s t a g e s o f r e o p e n i n g , p r o p a g a t i o n , s h u t - i n and f l o w b a c k c y c l e s , t h e f r a c t u r e i s r e o p e n e d a t a c o n s t a n t pumping r a t e t o d e t e r m i n e c r a c k r e o p e n i n g p r e s s u r e . A f t e r t h e d e s i g n e d volume has been pumped, t h e w e l l i s s h u t i n a g a i n and e x t e n d e d s h u t - i n a n d / o r f l o w b a c k t e s t s a r e p e r f o r m e d t o d e t e r m i n e f r a c t u r e c l o s u r e p r e s s u r e (SHLYAPDBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1 9 8 8 ) . A more a c c u r a t e a n a l y s i s o f t h e c r a c k p r e s s u r e d e c l i n e c a n be p e r f o r m e d i f t h e t h e o r e t i c a l p r e s s u r e d e c r e a s e i s matched w i t h f i e l d p r e s s u r e d a t a f o r a t i m e i n t e r v a l ( g l o b a l match) r a t h e r than matching t h e pressure d e r i v a t i v e ( l o c a l match) a t one p o i n t o n l y . S e v e r a l r e o p e n i n g , p r o p a g a t i o n , s h u t - i n and f l o w b a c k c y c l e s s h o u l d be p e r f o r m e d a t d i f f e r e n t r e o p e n i n g and f l o w b a c k r a t e s i n o r d e r t o i d e n t i f y t h e bounds o f f r a c t u r e c l o s u r e and o p e n i n g p r e s s u r e s w h i c h a r e c o n s i d e r e d t o be t h e e s t i m a t e s o f minimum i n - s i t u s t r e s s . C o n s t a n t - r a t e f l o w b a c k p r o c e d u r e s a r e u s e d t o a c c e l e r a t e and a c c e n t u a t e f r a c t u r e c l o s u r e . The d i s t i n c t f l o w b a c k p r e s s u r e c h a r a c t e r i s due t o t h e i n c r e a s e o f f r i c t i o n a l p r e s s u r e i n t h e f r a c t u r e a n d / o r t h e d e c r e a s e o f c r a c k c o m p l i a n c e d u r i n g continuous f r a c t u r e aperture r e d u c t i o n before complete mechanical c l o s u r e occurs. Crack r e o p e n i n g a t a c o n s t a n t l o w r a t e may i d e n t i f y an u p p e r bound e s t i m a t e f o r t h e minimum i n - s i t u s t r e s s . As d u r i n g i n j e c t i o n i n t o t h e f r a c t u r e t h e measured b o t t o m - h o l e p r e s s u r e i s h i g h e r t h a n t h e p r e s s u r e i n t h e c r a c k because o f v i s c o u s f r i c t i o n a l l o s s e s , t h e f r a c t u r e r e o p e n i n g p r e s s u r e g i v e s an u p p e r l i m i t f o r t h e minimum i n - s i t u s t r e s s . A g l o b a l p r e s s u r e m a t c h a n a l y s i s o f t h e e x t e n ded f a l l o f f t e s t can a l s o be u s e d t o d e t e r m i n e minimum i n - s i t u s t r e s s o r f r a c t u r e c l o s u r e p r e s s u r e and t o t a l l e a k o f f c o e f f i c i e n t i n more objective and r e l i a b l e manner t h a n w i t h c o n v e n t i o n a l t e c h n i q u e s (SHLYAPOBERSKY 1 9 8 8 ) .
567
4.8.2.6.3. Design parameters for optimum micro- and minifracture Design o f m i c r o - and m i n i f r a c t u r e t e s t s i n v o l v e s a p p r o p r i a t e s e l e c t i o n o f i n jection fluid, i n j e c t i o n r a t e and volume, p r e s s u r e f a l l o f f t i m e a f t e r s h u t - i n , and flowback r a t e and t i m e (SHLYAPOBERSKY, WALHAUG, SHEFFIELD & HUCKABEE 1988). Some comments a r e o f f e r e d as f o l l o w s on t e s t f l u i d r a t e s and v i s c o s i t i e s as w e l l as s h u t - i n t i m e and flowback r a t e .
4.8.2.6.3.1. Test fluid rates and viscosities The m i c r o f r a c t u r e t e s t f l u i d s h o u l d i d e a l l y be a l o w - v i s c o s i t y p e n e t r a t i n g f l u i d which m i n i m i z e s n e a r - w e l l b o r e and f r a c t u r e f r i c t i o n a l e f f e c t s . M i n i f r a c t u r e t e s t s o f t e n use two o r more d i f f e r e n t f l u i d s , w i t h a f l u i d r e p r e s e n t a t i v e o f t h e f u l l - s c a l e t r e a t m e n t h a v i n g t o be used t o determine t h e l e a k o f f c o e f f i c i e n t p r o p e r l y which i s r e q u i r e d f o r d e s i g n c a l i b r a t i o n ( c f . s e c t i o n 4 . 8 . 2 . 1 . ) . I n j e c t i o n r a t e s f o r r e s e r v o i r breakdown and f r a c t u r e p r o p a g a t i o n must be adequate t o ensure c r e a t i o n and e x t e n s i o n o f a s i n g l e dominant c r a c k and t o gener a t e s u f f i c i e n t f r a c t u r e volume f o r subsequent extended s h u t - i n and f l o w b a c k . M i c r o f r a c t u r e t e s t s s h o u l d be pumped a t r a t e s as low as p r a c t i c a l i n o r d e r t o m i n i m i z e f r i c t i o n and t o propagate t h e f r a c t u r e i n a q u a s i - s t a t i c manner, w h i l e m i n i f r a c t u r e t e s t s s h o u l d be pumped a t t h e planned f u l l - s c a l e t r e a t m e n t r a t e s i n o r d e r t o e v a l u a t e p o t e n t i a l f r a c t u r e containment b a r r i e r s and t o determine Fracture t h e r a t e s e n s i t i v i t y o f t h e c r a c k o v e r p r e s s u r e ( c f . s e c t i o n 4.8.1.1.). reopenings s h o u l d be performed a t s e v e r a l low i n j e c t i o n r a t e s i n o r d e r t o m i n i mize t h e u n c e r t a i n t i e s o f f r i c t i o n i n p e r f o r a t i o n s and near t h e w e l l b o r e , as c r a c k r e o p e n i n g p r e s s u r e can be v e r y s e n s i t i v e t o t h e r e o p e n i n g r a t e because o f t h e pressure-dependent f r a c t u r e p e r m e a b i l i t y . The c o m b i n a t i o n o f i n j e c t i o n r a t e and volume must c r e a t e a c r a c k w i t h adequate f l u i d volume r e m a i n i n g i n t h e f r a c t u r e a t shutdown t o p r o v i d e a s t a b l e c o n s t a n t - r a t e flowback b e f o r e c r a c k c l o s u r e . The o b j e c t i v e o f a m i c r o f r a c t u r e t e s t i s t o generate a small r a d i a l c r a c k l i m i t e d t o t h e i n t e r v a l o f i n t e r e s t , whereas t h e o b j e c t i v e o f a m i n i f r a c t u r e i s t o provoke a c r a c k o f adequate dimens i o n s as t o t e s t a l l f r a c t u r e parameters t o be encountered i n t h e f u l l - s c a l e t r e a t m e n t , and t h e pumped volume should be enough t o propagate t h e f r a c t u r e t o t h e expected containment boundaries f o r t h e main j o b .
4.8.2.6.3.2. Shut-in time and flowback rate S h u t - i n t i m e between s t o p o f i n j e c t i o n and s t a r t o f flowback s h o u l d be l o n g enough t o a l l o w t h e p r e s s u r e i n s i d e t h e f r a c t u r e t o e q u a l i z e and i s i n p r a c t i c e o f t e n a t l e a s t 50 % o f t h e t o t a l i n j e c t i o n time, a l t h o u g h c o n s i d e r a b l e d e v i a t i o n s o f t h i s r u l e may be r e q u i r e d by s p e c i a l circumstances. Flowback r a t e y i e l d i n g t h e d e s i r e d p r e s s u r e c h a r a c t e r depends s t r o n g l y on t h e a c t u a l t e s t cond i t i o n s and may v a r y between 5 % and 20 % o f t h e i n j e c t i o n r a t e . Minimum and maximum flowback r a t e s a r e determined by l e a k o f f r a t e and volume o f f l u i d remaini n g i n t h e f r a c t u r e a t t h e b e g i n n i n g o f f l o w b a c k . Minimum flowback r a t e must be g r e a t e r t h a n l e a k o f f r a t e a t t h e s t a r t o f t h e flowback t o g e n e r a t e t h e c h a r a c t e r i s t i c i n c r e a s e i n p r e s s u r e d e c l i n e r a t e a t f r a c t u r e c l o s u r e . Maximum flowback r a t e must p r o v i d e a s t a b l e p r e s s u r e d e c l i n e b e f o r e c r a c k c l o s u r e which i s depend e n t on m a i n t a i n i n g adequate f r a c t u r e volume a t t h e s t a r t o f f l o w b a c k . O n - s i t e m o d i f i c a t i o n s a r e c r i t i c a l f o r s u c c e s s f u l t e s t i n g because c r a c k i m p e r f e c t i o n s may d o m i n a n t l y appear i n reopenings and flowbacks, and t h e proposed procedures o f f e r t h r e e independent ways o f e s t i m a t i n g minimum i n - s i t u s t r e s s f r o m f r a c t u r e reopening and c l o s u r e p r e s s u r e s .
568
4.8.3. Formation damage around the wellbore M i n i f r a c t u r i n g i n c l u d i n g i n s e r t i o n o f proppants t h u s g e n e r a l l y r e p r e s e n t s r e l a t i v e l i m i t a t i o n o f f r a c t u r e p r o p a g a t i o n t o t h e n e a r e r s u r r o u n d i n g s o f t h e bor e h o l e ( w i t h r e s p e c t t o f u l l - s c a l e t r e a t m e n t s ) and has t h e r e f o r e h i t h e r t o i n most cases been used o n l y t o overcome f o r m a t i o n damage around t h e w e l l b o r e by b r e a k i n g t h r o u g h t h e b e l t w i t h d e t e r i o r a t e d p e r m e a b i l i t y seaming t h e b o r e h o l e i n t o t h e v i r g i n a l r e s e r v o i r (some o f t h e p r o p p a n t m i n i f r a c t u r e s can be compared w i t h enhanced g r a v e l packs; D A V I E S & HAGELAARS 1981; c f . s e c t i o n 5 . 3 . 5 . ) , with t h i s s p e c i a l workover- o r r e p a i r - t y p e a p p l i c a t i o n b e i n g independent f r o m permeab i l i t y o f t h e v i r g i n a l f o r m a t i o n and t h u s h a v i n g been c a r r i e d o u t i n b o t h lowand h i g h - p e r m e a b i l i t y r e s e r v o i r s . F o l l o w i n g some remarks on o r i g i n and t y p e s o f f o r m a t i o n damage, b r e a k t h r o u g h methods by m i n i f r a c t u r i n g a r e b r i e f l y sketched. Comments a r e a l s o o f f e r e d on f o r m a t i o n damage removal by a c i d i z i n g .
4.8.3.1. Origin of formation damage R e s e r v o i r damage comprises any impairment o f p r o d u c t i v i t y o r i n j e c t i v i t y w i t h i n wellbore, perforations, formation adjacent t o the wellbore, o r f r a c t u r e s communicating w i t h t h e w e l l b o r e t h a t u l t i m a t e l y reduce t h e n a t u r a l c a p a c i t y t o produce o r i n j e c t f l u i d s o r gases (ALLEN & ROBERTS 1982, HOLCOMB 1986; KRUEGER 1986, 1988). While most o f t h e m i n i f r a c t u r i n g t r e a t m e n t s t o overcome f o r m a t i o n damage a r e c a r r i e d o u t i n t h e e a r l y stage o f w e l l l i f e a f t e r d r i l l i n g , cementi n g and c o m p l e t i o n , a f u t u r e tendency w i l l be t o focus a l s o on o l d e r w e l l s i n l a t e stage o f p r o d u c t i o n h i s t o r y i n o r d e r t o g e t s t i l l c o n s i d e r a b l e amounts o f hydrocarbon r e s e r v e s o u t which o t h e r w i s e would remain w i t h i n t h e pay zone ( c f . section 4.8.9.). One o f t h e most i m p o r t a n t f e a t u r e s o f f o r m a t i o n damage i s d i s p e r s i o n and m i g r a t i o n o f c l a y m i n e r a l s in sandstones (GRAY & REX 1965), w i t h t h e p a r t i c l e s b e i n g o f e i t h e r n a t u r a l p r i m a r y - d e t r i t a l provenance o r s e c o n d a r y - a u t h i g e n i c o r i g i n ( c f . section 3.5.1.) and b e i n g m o b i l i z e d by e x t e r n a l i n f l u e n c e s d e r i v i n g f r o m d r i l l i n g , c o m p l e t i o n , t e s t i n g and/or t r e a t i n g , o r b e i n g t e r t i a r y - a r t i f i c i a 1 and r e s u l t f r o m b e i n g s y n t h e t i c a l l y i n s e r t e d i n t o and d i s t r i b u t e d w i t h i n t h e r e s e r v o i r by v a r i o u s a c q u i s i t i o n and s t i m u l a t i o n processes. Some aspects o f f o r m a t i o n exposure d u r i n g w e l l o p e r a t i o n and f l u i d l o s s i n t o t h e r e s e r v o i r a r e i l l u s t r a t e d as f o l l o w s .
4.8.3.1.1. Formation exposure during well operation F o r m a t i o n damage comprises p h y s i c a l , chemical and/or b i o l o g i c a l r e d u c t i o n o f r e s e r v o i r p e r m e a b i l i t y i n t h e immediate s u r r o u n d i n g s o f t h e w e l l b o r e due t o var i o u s d i f f e r e n t d r i l l i n g and c o m p l e t i o n i n f l u e n c e s (GIDLEY 1970) a t any t i m e dur i n g l i f e o f t h e w e l l (SCHAIBLE, AKPAN & AYDUB 1986). G e n e r a l l y e v e r y w e l l oper a t i o n d i s t u r b s t h e n a t u r a l chemical, hydrodynamical and thermodynamical e q u i l i b r i u m and may thus p o t e n t i a l l y damage t h e f o r m a t i o n (ALEGRE & OOUGHERTY 1988). P e r m e a b i l i t y d e t e r i o r a t i o n around t h e p e r f o r a t i o n s can o c c u r as a consequence o f m a t r i x c r u s h i n g and compaction caused by t h e shaped charge o r by gun d e b r i s (WHITE, WALTER & DIEBOLD 1965). Any o p e r a t i o n such as d r i l l i n g , cementing, p e r f o r a t i n g , c o m p l e t i o n , s t i m u l a t i o n , g r a v e l packing, i n j e c t i o n and workover t h a t exposes t h e f o r m a t i o n t o an a l i e n f l u i d may r e s u l t i n adverse i n t e r a c t i o n s between w e l l b o r e f l u i d s and f o r m a t i o n f l u i d s and/or t h e f o r m a t i o n i t s e l f . S o l i d s suspended i n t h e w e l l b o r e f l u i d s may p l u g t h e f o r m a t i o n , and o r g a n i c and/or i n o r g a n i c chemical s p e c i e s may p r e c i p i t a t e o u t o f e i t h e r f l u i d group and reduce f o r m a t i o n p e r m e a b i l i t y (HOUCHIN & HUDSON 1986). Clay m i n e r a l s w i t h i n t h e r e s e r v o i r framework may i n response t o chemical and/or mechanical f o r c e s change l o c a t i o n and c o n f i g u r a t i o n and t h u s reduce f l u i d t r a n s m i s s i b i l i t y ( c f . s e c t i o n 3 . 5 . 2 . ) .
569
4.8.3.1.2.
F l u i d loss i n t o the reservoir
Loss o f c o m p l e t i o n f l u i d s , f i l t r a t e s o r p a r t i c l e s f r o m d r i l l i n g mud may cause c l a y s w e l l i n g (MONAGHAN, SALATHIEL, MORGAN & K A I S E R 1959), p a r t i c l e p l u g g i n g by d i s p e r s e d f o r m a t i o n f i n e s , p a r t i c l e i n v a s i o n (GLENN & SLUSSER 1957) o r adverse changes i n f l u i d s a t u r a t i o n , f l u i d v i s c o s i t y and f o r m a t i o n w e t t a b i l i t y . H i g h l y a l c a l i n e f i l t r a t e s o c c u r r i n g w i t h t h e cementing o p e r a t i o n may reduce e f f e c t i v e p e r m e a b i l i t y n e a r t h e w e l l b o r e , and p e r m e a b i l i t y d e t e r i o r a t i o n may a l s o o c c u r d u r i n g hydrocarbon p r o d u c t i o n such as p r e c i p i t a t i o n o f scales, a s p h a l t e nes and o t h e r i n s o l u b l e s as f o r m a t i o n p r e s s u r e and temperature a r e lowered. Many o f t h e adverse p e r m e a b i l i t y e f f e c t s c o r r e l a t e d i r e c t l y t o s w e l l i n g , dispers i o n and p l u g g i n g o f c l a y p a r t i c l e s i n t h e f i n e - g r a i n e d f r a c t i o n o f t h e r e s e r v o i r r o c k m a t r i x (GIDLEY 1970). Most o f t h e e f f e c t s c a u s i n g f o r m a t i o n damage a r e provoked by c l o g g i n g o f por e s w i t h f i n e s (RIESE, SCHECHTER & RIESE 1988). The u l t i m a t e n e t r e s u l t o f a l l t h e d i f f e r e n t t y p e s o f f o r m a t i o n damage i s r e s e r v o i r p e r m e a b i l i t y r e d u c t i o n . A p r o g r e s s r e p o r t on t h e a p p l i c a t i o n o f e x p e r t systems f o r t h e d i a g n o s i s o f format i o n damage problems i s g i v e n by ALEGRE & DOUGHERTY ( 1 9 8 8 ) . An overview o f f o r m a t i o n damage by a1 t e r a t i o n o f p h y s i c a l c h a r a c t e r i s t i c s o f r e s e r v o i r r o c k s i n n e a r - w e l l b o r e zones i s p r o v i d e d by MICHAJLOV (1987).
4.8.3.2.
Types o f formation damage
D i s t i n c t i o n can be made between mud damage and n a t u r a l damage o f r e s e r v o i r f o r m a t i o n s (GATEWOOD, HALL, ROBERTS & LASATER 1970). Mud damage o r i g i n a t e s by d r i l l i n g mud permeating t h e p o r e spaces o f t h e f o r m a t i o n a d j a c e n t t o t h e w e l l bore. The degree o f damage i s u s u a l l y dependent upon d r i l l i n g time, pay zone p e r m e a b i l i t y , mud type, and number o f t r i p s made d u r i n g t h e d r i l l i n g o p e r a t i o n , w i t h maximum p e n e t r a t i o n o f d r i l l i n g mud p a r t i c l e s i n t o t h e i n t e r s t i c e s o f t h e f o r m a t i o n b e i n g s h a l l o w . A d d i t i o n a l damage may r e s u l t when mud f i l t r a t e l e a k s i n t o t h e r e s e r v o i r and c o n t a c t s n a t u r a l l y o c c u r r i n g f o r m a t i o n c l a y s . I n pay zones w i t h h i g h percentages o f w a t e r - s e n s i t i v e c l a y s , f i l t r a t e damage c o u l d be as severe as damage caused by mud p a r t i c l e i n v a s i o n . N a t u r a l l y damaged r e s e r v o i r s e x h i b i t a r e d u c t i o n i n v i r g i n p e r m e a b i l i t y due t o s w e l l i n g o r m i g r a t i o n o f sens i t i v e c l a y s t o p l u g t h e f o r m a t i o n f l o w channels. T h i s can o c c u r a f t e r a w e l l has been exposed t o f r e s h water, b r i n e s , a c i d o r c o m p l e t i o n f l u i d s , or after mud f i l t r a t e l e a k o f f . Another t y p e o f damage i s w a t e r b l o c k i n g i n f r a c t u r e and f o r m a t i o n (HOLDITCH 1979, LIN & HUANG 1988; c f . s e c t i o n 3.11.2.4.2.). Aspects o f f o r m a t i o n damage a r e d i s c u s s e d by HOLDITCH (1979); McCORRISTON, DEMBY & PEASE (1981); POTTER & DIBBLE (1983); ALLEN, RILEY & STRASSNER (1984); AMAEFULE & MASUO (1984); AMAEFULE, PADILLA, MCCAFFERY & TEAL (1984); GOODE, BERRY & STACY (1984); MARX & RAHMAN (1984), POTTER (1984); SOMERTON, CHEN, SCHUH & YUEN (1984); TODD, SOMERVILLE & SCOTT (1984); TYLER, METZGER & TWYFORD (1984); DEURER, SAHR & UJMA (1985); MEYERS, SKILLMAN & HERRING (1985); HOLCOMB (1986), MORGENTHALER (1986) ; AMAEFULE, AJUFO, PETERSON & DURST (1987) ; BARKER (1987), LEONE & SCOTT (1987); PRIISHOLM, NIELSEN & HASLUND (1987); VITTHAL, GUPTA & SHARMA (1987) and WOJTANOWICZ, KRILOV & LANGLINAIS ( 1 9 8 7 ) . The r o l e o f p e t r o g r a p h i c a l analyses i n t h e d e s i g n o f non-damaging t r e a t m e n t programs i s reviewed by DERSEY ( 1 9 8 6 ) . K I N G & HOLMAN (1986) r e p o r t on hydrocarbon s o l v e n t s as an a l t e r n a t i v e t o a c i d f o r removing some f o r m a t i o n damage. MALY (1976) s t r e s s e s t h e i m portance o f paying close a t t e n t i o n t o the smallest j o b d e t a i l s f o r minimizing f o r m a t i o n damage, and COLLE (1978) i n t r o d u c e s a c o m p l e t i o n t e c h n i q u e f o r o v e r coming f o r m a t i o n damage. SARKAR & SHARMA (1988) r e v i e w f o r m a t i o n damage by f i nes m i g r a t i o n i n two-phase f l o w .
570
4.8.3.3.
Formation damage breakthrough by m i n i f r a c t u r i n g
M i n i f r a c t u r i n g i n o r d e r t o overcome f o r m a t i o n damage has f r e q u e n t l y shown v e r y p r o m i s i n g r e s u l t s and as a consequence o f t h e g a t h e r e d e x p e r i e n c e , r e c e n t l y a l s o some p r o p p a n t m i n i f r a c t u r e s were made as t h e f i r s t s t e p p r e c e d i n g an opt i o n a l f o l l o w i n g second expensive s t e p b e i n g t h e MHF t r e a t m e n t . Some p o i n t s o f small p r o p p a n t volumes f o r p l u g g i n g o f s h o r t c r a c k s as w e l l as damage h a l o breakage by s h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s a r e discussed as f o l l o w s .
4.8.3.3.1.
Small proppant volumes f o r plugging o f short cracks
F o r m a t i o n damage b r e a k t h r o u g h by m i n i f r a c t u r i n g r e q u i r e s o n l y v e r y s m a l l p r o p p a n t volumes i n o r d e r t o p l u g t h e s h o r t c r a c k s e x t e n d i n g f r o m t h e b o r e h o l e i n t o i t s immediate s u r r o u n d i n g s . As most o f t h e r e s e r v o i r d e t e r i o r a t i o n zones r e p r e s e n t r a d i a l m a n t l e s e n v e l o p i n g t h e w e l l b o r e and h a r d l y exceeding a c o u p l e o f d e c i m e t r e s t h i c k n e s s , s i n g u l a r o r m u l t i p l e f r a c t u r e s i n t h e o r d e r o f a few metres l e n g t h would be s u f f i c i e n t f o r o p t i m i z a t i o n o f t h e d r a i n a g e . C a u t i o n has t o be e x e r c i s e d c o n c e r n i n g s e l e c t i o n o f s u i t a b l e proppants, because t h e s m a l l s c a l e f r a c t u r e s i n t h e c l o s e s t v i c i n i t y o f t h e b o r e h o l e a r e s u b j e c t e d t o maximum p r e s s u r e drawdown upon d e p l e t i o n o f t h e r e s e r v o i r d u r i n g course o f f i e l d p r o d u c t i o n ( c f . s e c t i o n s 2 . 4 . 1 . 2 . 3 . and 4 . 1 2 . 4 . 2 . ) . As i t has been proven t h a t i n many cases a l r e a d y t h e m i n i f r a c t u r e c o u l d r e s u l t i n a r e a s o n a b l e i n c r e a s e o f f l o w r a t e which i s s u f f i c i e n t f o r c o n v e r t i n g a p r e v i o u s l y uncommercial w e l l i n t o an economical producer and t h u s t h e MHF t r e a t ment t h a t was scheduled t o f o l l o w was no l o n g e r necessary t o be c a r r i e d o u t , c o n s i d e r a b l e s t i m u l a t i o n c o s t c o u l d be saved i f m i n i f r a c t u r e s w i t h p r o p p a n t i n f i l l i n g o f t h e c r a c k s become a more common t o o l o f f o r m a t i o n e v a l u a t i o n p r i o r t o p l a n n i n g and d e c i s i o n o f e x e c u t i o n of expensive MHF t r e a t m e n t s . T e c h n i c a l performance o f s m a l l - s c a l e f r a c t u r i n g has n o t n e c e s s a r i l y t o be done e x c l u s i v e l y by h y d r a u l i c pumping and i n j e c t i o n , b u t methods have a l s o been developed dur i n g t h e y e a r s f o r f r a c t u r i n g by v i b r a t i o n (METZGER 1958). F i e l d examples o f s m a l l - s c a l e h y d r a u l i c s t i m u l a t i o n f o r b r e a k i n g through w e l l b o r e - s u r r o u n d i n g f o r m a t i o n damage a r e r e p o r t e d by GRIFFIN ( 1 9 8 5 ) .
4.8.3.3.2. Damage h a l o breakage by short high-conductivity f r a c t u r e s NIEMEYER & REINART (1985) document complex f o r m a t i o n damage which i s caused by i n t e r a c t i o n s between mud f i l t r a t e , cement f i l t r a t e and connate water, g i v i n g r i s e t o c a l c i u m carbonate and b a r i u m s u l p h a t e p r e c i p i t a t i o n , w a t e r b l o c k f r o m c o m p l e t i o n f l u i d s ( c f . s e c t i o n s 3.11.2.4.2. and 4 . 8 . 4 . 2 . ) , c l a y d i s p e r s i o n by a d r i l l i n g f l u i d a d d i t i v e and m o b i l e f o r m a t i o n f i n e s . The small c o n d u c t i v i t y cont r a s t a t t a i n a b l e in t h e m o d e r a t e - p e r m e a b i l i t y o i l r e s e r v o i r made i t uneconomic a l t o c r e a t e f r a c t u r e s any l o n g e r than necessary t o bypass n e a r - w e l l b o r e damage and e s t a b l i s h reasonable w i t h d r a w a l r a t e s by f l o w e f f i c i e n c y improvement. S h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s have t u r n e d o u t t o be a s a t i s f a c t o r y v i n s t o break through t h e f o r m a t i o n damage h a l o around t h e b o r e h o l e and t o access t h e r e s e r v e s i n t h e pay zone. Excessive f l u i d l e a k o f f i n t o t h e m o d e r a t e - p e r m e a b i l i t y m a t r i x i s compensated by pumping a l a r g e pad c o m p r i s i n g more t h a n h a l f o f t h e t o t a l j o b volume ( c f . s e c t i o n 4.3.4.6.1.4.) and a d d i t i o n o f s i l i c a f l o u r and 100 mesh sand ( c f . sect i o n s 1.4.11.2.1. and 4 . 8 . 8 . 3 . 2 . 3 . ) . M i n i f r a c t u r i n g w i t h r e d u c t i o n o f b o t h i n j e c t i o n r a t e and t o t a l q u a n t i t y o f t h e t r e a t m e n t p e r m i t s s u c c e s s f u l c r a c k h e i g h t l i m i t a t i o n and p r e v e n t s v e r t i c a l f r a c t u r e b r e a k t h r o u g h i n t o a d j o i n i n g o t h e r pay zones i n cases where s e a l t h i c k n e s s i s i n s u f f i c i e n t t o c o n t a i n a s t a n d a r d - s i z e j o b and f r a c t u r e e m i g r a t i o n i s c o n s i d e r e d t o be u n d e s i r a b l e due t o o t h e r f l u i d and p r e s s u r e regimes i n t h e o v e r l y i n g n e x t r e s e r v o i r s t o r e y . Asp e c t s o f w e l l s t i m u l a t i o n and s c a l e i n h i b i t o r s aiming on p r e v e n t i o n o f forma-
571 t i o n damage a r e d i s c u s s e d by VETTER & LANKFORD (1988), and PRASAD (1985) e v a l u a t e s p r o d u c t i v i t y improvement o f a damaged w e l l by f r a c t u r i n g .
4.8.3.4. Formation damage removal by matrix acidizing F o r m a t i o n damage removal has o f t e n been c a r r i e d o u t by m a t r i x a c i d i z i n g o r a c i d f r a c t u r i n g i n s t e a d o f p r o p p a n t f r a c t u r i n g f o r t h e reason o f c o s t c o n t a i n ment ( c f . s e c t i o n 4.5.1.1.1.). M a t r i x a c i d i z i n g , however, o n l y s u c c e s s f u l l y t a kes away f o r m a t i o n damage i n s h a l l o w depth around t h e borehole, whereas deep sol i d p e r m e a b i l i t y p l u g g i n g has t o be c o r r e c t e d by c r e a t i n g a c o n d u c t i v e f r a c t u r e through t h e damage zone by e i t h e r a c i d o r proppant f r a c t u r i n g (McLEOD 1984). On t h e o t h e r hand, f o r m a t i o n s can be e a s i l y damaged b y improper use o f h y d r o f l u o r i c a c i d , because spent h y d r o f l u o r i c a c i d w i l l p r e c i p i t a t e s i l i c a , c a l c i u m f l u o r i d e and o t h e r compounds, e s p e c i a l l y when n o t enough h y d r o c h l o r i c a c i d p r e f l u s h i s used t o remove c a l c i u m carbonate i n t h e r e s e r v o i r p r i o r t o h y d r o f l u o r i c a c i d pumping (WALSH, LAKE & SCHECHTER 1982; McLEOD 1984; c f . s e c t i o n 4 . 5 . 1 . 1 . ) . F l u i d b u f f e r s have t o be used t o i s o l a t e f o r m a t i o n f l u i d s f r o m a c i d i n o r d e r t o a v o i d u n d e s i r e d r e s e r v o i r damage.
4.8.4. Field experiments and mHF vs. flHF P a r t i c u l a r l y i n l i g h t o f t h e o i l p r i c e c r i s i s i n 1986 ( c f . s e c t i o n 2.2.1.1.2.) which i s i n p a r t s o f Europe and a l s o i n o t h e r s e c t i o n s o f t h e w o r l d s t i l l s t r o n g l y i n f l u e n c i n g t h e budget o f c a p i t a l e x p e n d i t u r e ( c f . s e c t i o n 2 . 2 . 1 . 5 . 2 . ) , much p r o m o t i o n o f s t i m u l a t i o n a c t i v i t y can be expected i n t h e n e x t y e a r s f r o m an abundant a p p l i c a t i o n o f cheap proppant m i n i f r a c t u r e s which c o u l d f r e q u e n t l y r e p l a c e expensive MHF t r e a t m e n t s o r a t l e a s t postpone t h e i r execut i o n w i t h o u t renouncing t o o much on t h e d e s i r e d e f f e c t o f a c c e l e r a t i o n o f p r o d u c t i o n and t h u s save a t l e a s t t h e c o s t f o r t h e d i f f e r e n c e i n p r o p p a n t q u a n t i t y which can comprise up t o a c o u p l e o f hundred t o n s o f a d d i t i o n a l m a t e r i a l ( f o r which t h e expenses may amount up t o 1 Mio. US $ ) f o r a MHF i n s t e a d o f a mHF. Some aspects o f f r a c t u r i n g expenses vs. e f f e c t i v i t y and f l u i d l e a k o f f vs. w a t e r b l o c k i n g a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.8.4.1. Fracturing expenses vs. effectivity A p a r t f r o m p r o p p a n t and f l u i d c o s t , t h e o t h e r expenses o f t e n remain more o r l e s s t h e same, because f o r o f f s h o r e j o b s t h e s t i m u l a t i o n vessel has t o go anyway on l o c a t i o n and f o r onshore t r e a t m e n t s a l s o most p a r t s o f t h e f r a c t u r i n g t r u c k assembly have t o be i n s t a l l e d ( c f . s e c t i o n 3 . 8 . ) . Proppant c o s t , however, i s t h e l a r g e s t share o f t h e whole o p e r a t i o n p a r t i c u l a r l y i n deep gas w e l l s ( c f . section 2.3.4.), and thus t h e a p p l i c a t i o n o f o n l y a b t . 10 - 25 % o f t h e propp a n t q u a n t i t y and a b t . 20 - 50 % o f t h e f l u i d volume necessary f o r a MHF j o b i n a mHF t r e a t m e n t i s r e a l l y a c h a l l e n g i n g a l t e r n a t i v e i f t h e l a t t e r o p e r a t i o n i s s u f f i c i e n t f o r achievement o f an economical p r o d u c t i o n r a t e f r o m t h e r e s e r v o i r a f t e r t h e o p e r a t i o n . The s i g n i f i c a n c e o f a m i n i - o r m i c r o f r a c t u r e t r e a t m e n t bef o r e t h e main j o b w i t h t h e o p t i o n o f p o s s i b l y t h e s m a l l - s c a l e o p e r a t i o n a l r e a d y g i v i n g a s a t i s f a c t o r y r e s u l t l e a d i n g t o suspension o r c a n c e l l a t i o n o f t h e expens i v e b i g j o b i s a l s o emphasized by BRANAGAN & WILMER ( 1 9 8 8 ) . H y d r a u l i c proppant m i n i f r a c t u r i n g i s s i g n i f i c a n t p r e d o m i n a n t l y f o r unconvent i o n a l r e s e r v o i r s where m i n i f r a c t u r e s can a l s o be regarded as f i e l d experiments t o g e t r e l i a b l e i n f o r m a t i o n upon which then e s p e c i a l l y designed s t i m u l a t i o n p r o j e c t s c o u l d be based on. The p o p u l a r i z a t i o n o f cheap m i n i f r a c t u r e s i s t h u s assumed t o be a v e r y i m p o r t a n t t o o l f o r p r o m o t i o n o f s t i m u l a t i o n by h y d r a u l i c propp a n t f r a c t u r i n g d u r i n g t h e n e x t years, because t h e economical f e a s i b i l i t y of s m a l l e r t r e a t m e n t s i s much b e t t e r than t h a t o f l a r g e r j o b s and much expenses can be saved i f t h e m i n i f r a c t u r e l e a d s a l r e a d y t o an a c c e p t a b l e s o l u t i o n i n terms o f i n c r e a s e o f p r o d u c t i o n r a t e a t l e a s t f o r a c e r t a i n t i m e .
572 An i n t e r e s t i n g example i s r e p o r t e d b y NORTHCUTT, ROBERTSON & HANNAH ( 1 9 8 8 ) f r o m an i n t e n d e d MHF s t i m u l a t i o n o f t i g h t gas r e s e r v o i r s . As a consequence o f u n d e r e s t i m a t i o n o f f l u i d l e a k o f f i n t h e n a t u r a l l y f r a c t u r e d p a y zone, t h e t r e a t ment s u f f e r e d f r o m v e r y e a r l y d e h y d r a t i o n s c r e e n o u t , b u t a l t h o u g h t e c h n i c a l l y b e i n g a f a i l u r e w i t h o n l y having achieved a j o b s i z e comparable t o t h a t o f a larger minifracture, i t was a c o n s i d e r a b l e commercial s u c c e s s b y s e v e r a l t i m e s m u l t i p l y i n g t h e p r e - f r a c t u r i n g gas f l o w r a t e , t h u s e m p h a s i z i n g t h a t t h e m a i n e f f e c t o f t h e o p e r a t i o n was a l r e a d y a c h i e v e d a f t e r b r e a k i n g t h r o u g h t h e damage zone i n t h e i m m e d i a t e b o r e h o l e v i c i n i t y w i t h a c o m p a r a t i v e l y s m a l l t o v e r y s m a l l t r e a t m e n t . The n e a r - w e l l b o r e f o r m a t i o n d e t e r i o r a t i o n r e s p o n s i b l e f o r f l o w i n h i b i t i o n without fracture job i s the r e s u l t o f fines migration, scaling, clos u r e o f n a t u r a l gas r e s e r v o i r f l o w p a t h s o r a c o m b i n a t i o n o f t h e s e f a c t o r s .
4.8.4.2.
Fluid l e a k o f f vs. water blocking
F l u i d l e a k o f f w h i c h i s v e r y s i g n i f i c a n t f o r p r o p e r d e s i g n o f t h e j o b depends o n f r a c t u r e f l u i d v i s c o s i t y , r e s e r v o i r r o c k p e r m e a b i l i t y , v i s c o s i t y and compress i b i l i t y o f t h e h y d r o c a r b o n s i n t h e p a y f o r m a t i o n , and t h e a b i l i t y o f t h e s t i m u l a t i o n f l u i d t o d e p o s i t a mud c a k e a t t h e b o r e h o l e w a l l , w i t h f l u i d l o s s b e i n g b e s t d e t e r m i n e d b y a m i n i f r a c t u r e o r p r e s s u r e breakdown t e s t ( c f . s e c t i o n 4 . 8 . 2 . 1 . 2 . ) . I f t o o much f r a c t u r i n g f l u i d f i l t r a t e l e a k s o f f d u r i n g t h e t r e a t ment i n t o t h e f o r m a t i o n m a t r i x and i s t h e r e f i x e d b y c a p i l l a r y and i n t e r f a c e f o r c e s , w a t e r s a t u r a t i o n c a n c o n s i d e r a b l y i n c r e a s e up t o r e s i d u a l gas c o n c e n t r a t i o n n e a r t h e f r a c t u r e w a l l , and e f f e c t i v e p e r m e a b i l i t y c a n be c o n s i d e r a b l y det e r i o r a t e d b y w a t e r b l o c k i n g i n h i b i t i n g i n i t i a t i o n o f gas f l o w e s p e c i a l l y i n t i g h t r e s e r v o i r s w i t h h i g h c a p i l l a r y p r e s s u r e (HOLDITCH 1979; c f . s e c t i o n 3.11.2.4.2.). M i t i g a t i o n o f w a t e r b l o c k i n g c a n b e a c h i e v e d by u s i n g a s o l v e n t i n t h e pad f l u i d s w i t h a s u r f a c t a n t i n t h e r e s t o f t h e f l u i d , thereby r e d u c i n g w a t e r b l o c k i n g i n t h e f r a c t u r e as w e l l as s i g n i f i c a n t l y e n h a n c i n g f r a c t u r i n g f l u i d r e c o v e r y and p r o d u c t i o n (PHILLIPS & WILSON 1 9 8 4 ) , because o t h e r w i s e c o n s i d e r a b l y h i g h excess pressures a r e necessary t o break through t h e w a t e r - b l o c k i n g f l o w i n h i b i t i o n (ROODHART, KUIPER & DAVIES 1 9 8 6 ) . R e s u l t s from pump-in/shut-in t e s t s ( c f . s e c t i o n 4.8.2.3.1.3.) can be used t o d e t e r m i n e t h e maximum recommended j o b s i z e and c r i t i c a l p r o p p a n t c o n c e n t r a t i o n s f o r l o w - p e r m e a b i l i t y r e s e r v o i r s where l e a k o f f i n t o n a t u r a l f r a c t u r e s o c c u r s (SHELLEY & McGOWEN 1986; TAN, McGOWEN, LEE & SOLIMAN 1988) i f t h e f o r m a t i o n has s u f f i c i e n t l y low m a t r i x p e r m e a b i l i t y t h a t n a t u r a l f r a c t u r e s predominantly cont r o l f l u i d loss.
4.8.4.3.
Other aspects
Stimulation data analysis o n - s i t e ( c f . section 6.2.4.2.4.) a f t e r the minif r a c t u r e i s a s u i t a b l e means o f e v a l u a t i o n o f t h e t r e a t m e n t and c a n h e l p t o dec i d e on n e c e s s i t y , d e s i g n and sequence o f f u r t h e r o p e r a t i o n s t a g e s (ELPHICK 1985, PALMER & VEATCH 1 9 8 7 ) . C l o s u r e s t r e s s c a n be checked b y s t e p - r a t e t e s t o r p u m p - i n / f l o w - b a c k t e s t ( c f . s e c t i o n 4 . 8 . 2 . ) , and f l u i d l e a k o f f i s g i v e n b y an u n p r o p p e d m i n i f r a c t u r e , w i t h c l o s u r e s t r e s s d e p e n d i n g on f o r m a t i o n p o r e p r e s s u r e w h i c h v a r i e s d u r i n g s t i m u l a t i o n due t o f l u i d l o s s ( c f . a l s o s e c t i o n 4.4.10.). An i m p o r t a n t a s p e c t f o r t h e p l a n n i n g o f mHF v s . MHF, as w e l l as optimum l e n g t h o f t h e f r a c t u r e wings independent from t h e s i z e o f t h e job, i s t h e f a c t t h a t e v e r y g i v e n f r a c t u r e c o n d u c t i v i t y has i t s a s s o c i a t e d optimum l e n g t h , w i t h e x c e s s i v e l y l o n g f r a c t u r e s t h e n h a v i n g no f u r t h e r e f f e c t e x c e p t o f c a u s i n g unnec e s s a r y a d d i t i o n a l c o s t (BAUMGARTNER 1983, LEICHT 1 9 8 5 ) . T h i s means t h a t f r a c t u r e l e n g t h i s u l t i m a t e l y a l s o d e p e n d i n g on p r o p p a n t s e l e c t i o n and t h e c o n d u c t i v i t y of t h e d e s i g n e d p r o p p i n g m a t e r i a l ( c f . s e c t i o n 4 . 8 . 1 1 . ) . A s p e c t s o f e x c e s s i ve f r a c t u r e l e n g t h a r e a l s o d i s c u s s e d b y CIPOLLA & LEE ( 1 9 8 7 ) .
573
4.8.5. Pilot stimulation of marginal reservoirs L i t t l e cheap f r a c t u r i n g j o b s a r e a l s o q u i t e a t t r a c t i v e as p i l o t p r o j e c t s f o r b o t h b e t t e r i n v e s t i g a t i o n and i n t r o d u c t o r y t r i a l t r e a t m e n t s ( w h i c h have t o be r e i n f o r c e d by t h e s e r v i c e and p r o p p a n t companies by s p e c i a l p r o m o t i o n p r i c i n g ) i n m a r g i n a l r e s e r v o i r s . The f o l l o w i n g d i s c u s s i o n g i v e s an o u t l i n e o f a r e a l d i s t r i b u t i o n o f p o t e n t i a l r e s e r v o i r s , a p p l i c a t i o n o f r e s i n - c o a t e d p r o p p a n t s i n uns t a b l e pay zones, t i p s c r e e n o u t f r a c t u r i n g , and d i r t y sandstone s t i m u l a t i o n .
4.8.5.1. Areal distribution o f potential reservoirs P o t e n t i a l r e s e r v o i r s o f m a r g i n a l t y p e which can be s u c c e s s f u l l y approached by m i n i f r a c t u r i n g as t h e f i r s t s t e p o f s t i m u l a t i o n t e c h n o l o g y a r e f o r example
* * *
* *
s i n g l e o r m u l t i p l e sandwich-type sandstone sheets and lenses, u n c o n s o l i d a t e d t o w e a k l y - l i t h i f i e d f i n e - g r a i n e d f l a s e r y o r m u d d y - s i l t y sandstones and s o f t u n s t a b l e c h a l k s ( w h i c h occur i n N o r t h Sea, G u l f Coast, Rocky Mountain r e g i o n and M i d d l e East; BLANTON 1986), i n d i v i d u a l o r grouped carbonate s h e l l ( b i o c l a s t i c ) l a y e r s i n mudstone sequences, l o w - p e r m e a b i l i t y sandstone b e l t s o r sheets h o r i z o n t a l l y o r v e r t i c a l l y separat i n g h i g h - p e r m e a b i l i t y zones, and i s o l a t e d s h o e - s t r i n g o r l e n t i c u l a r sandstone bodies w i t h i n mudstone formations.
I n many o f these a p p l i c a t i o n s , i n t e r m e d i a t e - s t r e n g t h l o w - d e n s i t y alumina s i l i c a t e proppants i n b o t h c o n v e n t i o n a l and r e s i n - c o a t e d f o r m (SINCLAIR & GRAHAM 1977, 1978; COSTEIN & MAYER 1978, SAUNDERS & McKENZIE 1979, KANAT 1980; UNDERDOWN; DAY & SPARLIN 1980; POPE, WILES & P I E R C E 1987; c f . s e c t i o n 1 . 2 . 6 . ) will become i n c r e a s i n g l y s i g n i f i c a n t d u r i n g t h e n e x t years, as many e a r l i e r t r i a l r u n s w i t h sand have n o t l e d t o p r o m i s i n g r e s u l t s and h e a v i e r proppants a r e n o t an adequate m a t e r i a l i n t h e r e l a t i v e l y s h a l l o w d e p t h i n most o f t h e cases due t o s e t t l i n g problems i n t h e c a r r i e r f l u i d s ( c f . s e c t i o n 4.12.2.) and t h e h i g h e r p r i c e o f t h e m a t e r i a l (ATTARD, MATHES & MOWER 1988; c f . s e c t i o n s 1.3.2. and 2.2.2.3.). S u i t a b l e t a r g e t f o r m a t i o n s f o r such p i l o t m i n i f r a c t u r i n g o f m a r g i n a l r e s e r v o i r s i n Europe a r e m a i n l y M i d d l e J u r a s s i c and Lower Cretaceous sandstones, Upp e r J u r a s s i c o r Lower Cretaceous carbonate s h e l l l a y e r s i n mudstone sequences, and Upper Cretaceous and p a r t i a l l y a l s o Lower T e r t i a r y c h a l k i n Germany FRG and t h e N e t h e r l a n d s ; J u r a s s i c sandstones and Upper Cretaceous t o Lower T e r t i a r y c h a l k i n p a r t s o f t h e Norwegian and B r i t i s h N o r t h e r n N o r t h Sea; and T e r t i a r y sandstones i n A u s t r i a ( c f . DOLAK & PERNTHANER 1976, KRULL 1984; KREUTZER 1985, 1986; BRKUER 1987), B u l g a r i a , Czechoslovakia ( c f . THON 1985), I t a l y , Hungary and Y u g o s l a v i a ( c f . D R A G I C E V I C , M A R I N O V I C , STANKOVIC, BATUSIC & NIKOLIC 1983). R e q u i r e d p r o p p a n t q u a n t i t i e s f o r such s m a l l - s c a l e j o b s may be as low as 6 - 12 t, b u t may a l s o comprise h i g h e r o r even a l s o l o w e r volumes i n case o f problems t o meet t h e average f i g u r e s (DOLAK & PERNTHANER 1 9 7 6 ) .
4.8.5.2. Appl icat ion o f res i n-coated proppants in unstable pay zones The main s i g n i f i c a n c e o f t h e a p p l i c a t i o n o f r e s i n - c o a t e d proppants i n a s u i t e o f t h e aforementioned cases i s t o p r o v i d e a s t a b l e aggregated package i n s o f t t o i n s u f f i c i e n t l y c o n s o l i d a t e d f o r m a t i o n s w i t h low embedment p r e s s u r e where conv e n t i o n a l proppants tend t o embed, t h e r e b y r e s u l t i n g i n a h e a l i n g e f f e c t t o occ u r i n the f r a c t u r e ( i n c o n t r a s t t o crushing o f grains i n a f r a c t u r e d hard l i t h i f i e d r o c k w i t h h i g h embedment pressure; CLARK 1983; c f . s e c t i o n 4 . 3 . 3 . ) . Some
574 a s p e c t s o f p r o p p a n t wedge s t a b i l i z a t i o n and embedment p r e v e n t i o n as w e l l as t a i l - i n p r o p p i n g f o r f l o w b a c k i n h i b i t i o n a r e d i s c u s s e d as f o l l o w s .
4.8.5.2.1. Proppant wedge stabi 1 izat ion and embedment prevent ion W h i l e i n many s o f t t o m o d e r a t e l y - c e m e n t e d f o r m a t i o n s , t h e c h o i c e o f a p r o p p a n t w i t h a l a r g e r d i a m e t e r i s a l r e a d y enough f o r s a t i s f a c t o r i l y l i m i t i n g p r o p p a n t embedment, o t h e r r o c k t y p e s l i k e t h e above enumerated ones have t o r e c e i v e an a r t i f i c i a l s t a b i l i z a t i o n a t l e a s t w i t h i n t h e f r a c t u r e i n f i l l wedge a n d / o r t h e g r a v e l pack m a n t l e b y s t i c k i n g t o g e t h e r o f r e s i n - c o a t e d g r a i n s ( c f . s e c t i o n 4.12.3.3.). Thermal s t a b i l i t y o f r e s i n - c o a t e d p r o p p a n t s and t h e i r c o n s o l i d a t i o n p r o p e r t i e s a r e i n v e s t i g a t e d b y RENSVOLD ( 1 9 8 3 ) . R e s i n c o a t i n g c a n be c a r r i e d o u t f o r any c o m m e r c i a l l y a v a i l a b l e p r o p p a n t b y a d d i n g a t h i n p e l l i c l e o f p h e n o l i c r e s i n t o t h e i n d i v i d u a l g r a i n s (POPE, WILES & P I E R C E 1 9 8 7 ) . When t e m p e r a t u r e s i n e x c e s s o f 140 OF and c l o s u r e s t r e s s a r e app l i e d t o the curable resin-coated proppants i n the r e s e r v o i r formation, the res i n s o f t e n s , s t a r t s t o c u r e and c h e m i c a l l y bonds t o g e t h e r t o g e n e r a t e a c o n s o l i d a t e d p e r m e a b l e package ( c f . s e c t i o n 1 . 2 . 6 . ) .
4.8.5.2.2. Tail-in propping for flowback inhibition C o n c e r n i n g t a i l - i n p r o p p i n g p o l i c y ( c f . s e c t i o n 2 . 4 . 1 . 2 . 3 . ) , a t l e a s t one t h i r d t o h a l f o f t h e amount o f t h e pumped p r o p p a n t s have t o be r e s i n - c o a t e d i n o r d e r t o g e t an aggregated r i g i d p l u g o f t h e f r a c t u r e which i s w i t h s t a n d i n g c o l lapse ( c f . s e c t i o n 4.12.3.3.), whereas t h e o u t e r p a r t s o f t h e c r a c k n e a r t h e t i p s may be i n f i l l e d w i t h c o n v e n t i o n a l p r o p p a n t s i f a m u l t i - l a y e r package i s g u a r a n t e e d . I n u n s t a b l e f o r m a t i o n s , however, any c o s t c o n t a i n m e n t a c h i e v e d b y t h e p a r t i a l r e p l a c e m e n t o f r e s i n - c o a t e d p r o p p a n t s b y c o n v e n t i o n a l ones c a n be more t h a n compensated by s e r i o u s c o n d u c t i v i t y d e t e r i o r a t i o n b y p r o p p a n t embedment and f r a c t u r e c l o s u r e ( c f . s e c t i o n 4 . 3 . 3 . ) . FRIEDMAN, SURLES & K I E K E ( 1 9 8 6 ) comment o n h i g h - t e m p e r a t u r e sand c o n s o l i d a t i o n . T a i l i n g - i n o f r e s i n - c o a t e d p r o p p a n t s i s a p a r t i c u l a r l y s u c c e s s f u l method f o r c o n t r o l l i n g p r o p p a n t f l o w b a c k n e a r t h e w e l l b o r e and t h u s a v o i d i n g f r a c t u r e damage ( c f . s e c t i o n 4 . 1 1 . 3 . ) by c r e a t i n g a s t a b l e c o n s o l i d a t e d wedge h o l d i n g t h e p r o p p a n t i n p l a c e i r r e s p e c t i v e o f f l o w r a t e s and v e l o c i t i e s o f t h e p r o d u c e d f l u i d s .
4.8.5.3.
Tip screenout fracturing
A n o t h e r t e c h n i q u e f o r t h e s t i m u l a t i o n o f weak, unstable formations i s t i p s c r e e n o u t f r a c t u r i n g (SMITH, MILLER & HAGA 1987; c f . s e c t on 4 . 5 . 4 . 4 . 1 . ) w h i c h p r i n c i p a l l y aims on c r e a t i n g s h o r t h i g h . - c o n d u c t i v i t y f r a c t u es in s o f t r e s e r v o i r r o c k s such as c h a l k s , d i a t o m a c e o u s e a r t h s and u n s t a b l e s a n d s . Some p r i n c i p l e s and a p p l i c a t i o n s o f t i p s c r e e n o u t f r a c t u r i n g a r e o u t l i n e d as f o l l o w s .
4.8.5.3.1. Principles The d e s i g n i s based on i n t e n t i o n a l l y s c r e e n i n g o u t t h e t i p o f t h e f r a c t u r e w i t h p r o p p a n t s and t h e n c o n t i n u i n g t o pump s l u r r y t o i n c r e a s e c r a c k w i d t h and t o pack t h e f r a c t u r e w i t h p r o p p a n t m u l t i l a y e r s i n o r d e r t o o b t a i n h i g h c o n d u c t i v i t y (SMITH, M I L L E R & HAGA 1 9 8 7 ) . Because t h i s i n v o l v e s s e v e r e r i s k o f premat u r e screenout ( c f . s e c t i o n 6.2.4.2.1.) and as f a i l u r e t o a c h i e v e t i p s c r e e n o u t w i l l not y i e l d the desired stimulation, i t i s indispensible t o perform special p r e f a c t u r e t e s t s i n o r d e r t o e x a c t l y determine design v a r i a b l e s ( p a r t i c u l a r l y f l u i d leakoff coefficient). T i p s c r e e n o u t f r a c t u r i n g c a n a l s o be p e r f o r m e d i n m u l t i f r a c t u r e systems w i t h f o r example p e r f o r a t i n g t h e l o w e r i n t e r v a l and c a r r y i n g o u t a t i p s c r e e n o u t
575 f r a c t u r i n g t r e a t m e n t t o c r e a t e a h i g h - c o n d u c t i v i t y c o n d u i t f r o m t h e upper zone t o t h e p e r f o r a t i o n s (SMITH, MILLER & HAGA 1987; i n t e r f a c e f r a c t u r i n g ; c f . sect i o n 4.5.4.4.2.). As widely-propped c r a c k s a r e necessary t o m a i n t a i n f r a c t u r e c o n d u c t i v i t y i n s o f t f o r m a t i o n s , c o n d u c t i v i t y l o s s r e s u l t i n g f r o m p r o p p a n t embedment i n t o weak u n s t a b l e r e s e r v o i r s would make t h e f l o w c o n d u i t f r o m t h e upp e r i n t e r v a l t o t h e p e r f o r a t i o n s i n e f f e c t i v e . Aspects o f p e r f o r a t i n g i n one zone and f r a c t u r i n g i n t o another h o r i z o n a r e a l s o d i s c u s s e d by HOLM (1975); AHMED, WILSON & STRAWN (1983) and ATTARD, MATHES & MOWER ( 1 9 8 8 ) .
4.8.5.3.2. Applications T i p screenout f r a c t u r i n g has t h e drawback o f s i g n i f i c a n t p r e s s u r e drop and l o s s o f p r o d u c t i v i t y through t h e crack which m i g h t o c c u r a t h i g h hydrocarbon o f f t a k e r a t e s (SMITH, MILLER & HAGA 1 9 8 7 ) . I f t i p screenout f r a c t u r i n g i s n o t s u c c e s s f u l l y executed, hydrocarbon p r o d u c t i o n r a t e s r e a c h a p o i n t where f u r t h e r w e l l h e a d p r e s s u r e r e d u c t i o n has v e r y l i t t l e o r no e f f e c t on w e l l performance. I n o r d e r t o e l i m i n a t e t h i s c h o k i n g e f f e c t , f r a c t u r e c o n d u c t i v i t y has t o be i n creased and/or d i s t a n c e between upper zone and p e r f o r a t i o n s has t o be decreased. I n c r e a s i n g f r a c t u r e c o n d u c t i v i t y can be achieved by r e d e s i g n i n g t h e operat i o n s such as t o r e f l e c t t r i p l e o r quadruple w i d t h i n s t e a d o f p r i m a r y doublew i d t h design p h i l o s o p h y , a l o n g w i t h i n c r e a s i n g maximum proppant c o n c e n t r a t i o n . I n a d d i t i o n , d e t r i m e n t a l p r e s s u r e drops through t h e c r a c k can be avoided by c a r r y i n g out a m u l t i f r a c t u r e completion instead o f a s i n g l e s t i m u l a t i o n job. M u l t i f r a c t u r e c o m p l e t i o n may be executed as separate t i p screenout o p e r a t i o n s i n d i s c r e t e l a y e r s which a r e l a t e r commingled ( c f . s e c t i o n 4.10.1.1.), o r as j o i n t m u l t i p l e f r a c t u r i n g i n c l u d i n g c r e a t i o n o f communication pathways between t h e v e r t i c a l l y superimposed r e s e r v o i r s t o r e y s i f necessary and d e s i r a b l e ( c f . sect i o n 4.10.1.2.). T i p screenout f r a c t u r i n g i s a p a r t i c u l a r l y s u i t a b l e technique i n w a t e r - s e n s i t i v e s o f t f o r m a t i o n s where a c i d i z i n g cannot be done ( c f . a l s o sect i o n 4.5.4.3.1.). M i n i f r a c t u r i n g i s a l s o c o n s i d e r e d t o be a s u i t a b l e p i l o t s t i m u l a t i o n means f o r t h e s u c c e s s f u l approach o f o t h e r u n c o n v e n t i o n a l t a r g e t s such as geopressur e d r e s e r v o i r s , c o a l seams, s h a l e s and h i g h - t e m p e r a t u r e r e s e r v o i r s ( c f . a l s o s e c t i o n 4.4.).
4.8.5.4. Dirty sandstone s t irnulat ion M i n i f r a c t u r i n g as p r e c u r s o r - o r e x p e r i m e n t a l - t y p e t r e a t m e n t i s a l s o a s u i t a b l e method f o r t h e approach o f d i r t y sandstones which comprise 80 % o f t h e w o r l d - s c l a s t i c p e t r o l e u m r e s e r v o i r s , b u t o n l y 15 % o f t h e w o r l d - s o i l f i e l d s (DAVIES 1980). W h i l e h y d r a u l i c p r o p p a n t f r a c t u r i n g o f c l e a n sandstones i s p r a c t i c e d s i n c e l o n g e r time, d i r t y r e s e r v o i r s t i m u l a t i o n has o n l y moved i n t o t h e f o cus o f t r e a t i n g schedules i n t h e l a s t decades. P a r t i c u l a r l y as a consequence o f t h e n e c e s s i t y o f i n c r e a s i n g o i l r e s e r v e s i n s p i t e o f p r o d u c t i o n exceeding a d d i t i o n a l d i s c o v e r i e s ( c f . s e c t i o n 2.2.1.5.2.), hydraulic proppant f r a c t u r i n g o f d i r t y sandstones w i l l become p r o g r e s s i v e l y more i m p o r t a n t i n t h e near f u t u r e . W i t h p r o p e r l y designed s t i m u l a t i o n procedures, 80 % o f t h e w o r l d ' s p r o s p e c t i v e sandstone p o t e n t i a l become h i g h l y p r o s p e c t i v e e x p l o r a t i o n and development t a r gets.
4.8.6. Horizontal well or drainhole drilling and fracturing I n a d d i t i o n t o c o n v e n t i o n a l v e r t i c a l d r i l l i n g and a l t h o u g h b e i n g q u i t e expens i v e as w e l l as t e c h n i c a l l y complicated, p o t e n t i a l o f enhanced hydrocarbon r e c o v e r y i s a l s o seen f o r h y d r a u l i c p r o p p a n t s m a l l e r - o r l a r g e r - s c a l e f r a c t u r i n g o f h o r i z o n t a l o r d r a i n h o l e w e l l s (GIGER 1983, STEENBOCK 1985, V I C A N E K 1985, JOSH1 1987) i n v a r i o u s r e s e r v o i r s o f p r e d o m i n a n t l y t h i n s h e e t - l i k e sandstone t y p e . The most common procedures f o r a c c e l e r a t i o n o f o i l p r o d u c t i o n a r e h y d r a u l i c
576 f r a c t u r i n g , h o r i z o n t a l d r i l l i n g and v e r t i c a l i n f i l l d r i l l i n g (PLAHN, STARTZMAN & WATTENBARGER 1987). V a r i o u s aspects o f h o r i z o n t a l h o l e d r i l l i n g and f r a c t u r i n g a r e b r i e f l y sketched as f o l l o w s .
4.8.6.1.Horizontal hole drilling H o r i z o n t a l o r d r a i n h o l e d r i l l i n g has a l r e a d y r e p e a t e d l y been performed i n Europe i n France (Lacq and C a s t e r a Lou f i e l d s ; WINNOCK & PONTALIER 1970; MONTIGNY, SORRIAUX, LOUIS & LESS1 1988), Germany FRG (Georgsdorf, Berkhopen and L e h r t e f i e l d s ; PREVEDEL 1985, 1987), I t a l y (Rospo Mare f i e l d ; G I G E R 1983, BEUDELL 1985 a, ERDOEL-ERDGAS AKTUELL 1985 e, R E I S S 1985, FOX 1986, OFFSHORE ENGINEER 1986; DUSSERT, SANTORO & SOUDET 1988), N e t h e r l a n d s (MURPHY 1988), England (HARDMAN 1986; BORNER, HALDORSEN, HARRISON, HOPWOOD & MACDONALD 1988; MACDONALD 1988; OIL GAS JOURNAL 1988 b, PETROLEUM REVIEW 1988 e ) , Denmark (WENANDE 1987; .'ERSEN, HANSEN & FJELDGAARO 1988), Norway (FANTOFT, KROGH & POLLEN 1988), I n d i a (KHIN 1988) and i n t h e USA (where h o r i z o n a l w e l l d r i l l i n g has been a p p l i e d f o r speeding-up o f oil p r o d u c t i o n s i n c e o v e r 40 years; STORMONT 1935, RANNEY 1941, LANDRUM & CRAWFORD 1955, ROEMERSHAUSEN & HAWKINS 1955, DIAMOND & OYLER 1980, MOORE 1980, STIEGLER 1982, GORODY 1984, D I C K I N S O N & D I C K I N S O N 1985, B O S I O 1986; D I C K I N S O N , ANDERSON & D I C K I N S O N 1986; L I , CHANDELLE & BRYCH 1986; WILKIRSON, SMITH, STAGG & WALTERS 1986; SHERRARD, B R I C E & MacDONALD 1986; OVERBEY, YOST & WILKINS 1988) as w e l l as i n t h e USSR (where t h e t e c h n o l o g y i s a l s o app l i e d s i n c e a l s o more than 40 years; B O R I S O V 1964, G R I G O R I A N 1969, BUTLER 1984, R E I S S 1985, ERDUL UND KOHLE - EROGAS - PETROCHEMIE 1987 e ) w i t h o u t m a j o r t e c h n i cal d i f f i c u l t i e s . #'
4.8.6.1.0. Historical development and status o f horizontal wells While e a r l y h o r i z o n t a l d r i l l i n g i n b o t h USA and USSR i n t h e 1950's t o e a r l y 1970's l e a d t o c o n c l u s i o n t h a t h o r i z o n t a l o r h i g h l y - d e v i a t e d w e l l s a r e t e c h n i c a l l y f e a s i b l e b u t e c o n o m i c a l l y d i s a p p o i n t i n g (AUSTIN, ROSE & SCHUH 1988), t h e development o f improved d i r e c t i o n a l d r i l l i n g methods r e s u l t i n g f r o m e x p e r i e n c e g a i n e d i n G u l f o f Mexico and N o r t h Sea i n t h e l a t e 1970's and 1980's t u r n e d t h e key and gave way t o economical a t t r a c t i v i t y o f h i g h l y - i n c l i n e d and h o r i z o n t a l b o r e h o l e s f o r p r o f i t a b l e r e s e r v o i r e x p l o i t a t i o n (REISS, JOUROAN & G I G E R 1984).
I n t h e l a s t decade, a b t . 150 i m p o r t a n t h o r i z o n t a l b o r e h o l e s have been d r i l l e d i n t h e w o r l d (PETZET 1988), and c u r r e n t l y a b t . 30 h o r i z o n t a l w e l l s a r e succ e s s f u l l y p r o d u c i n g o i l w o r l d w i d e a t e c o n o m i c a l l y f e a s i b l e r a t e s (JOSH1 1986 a, 1987 a ) . Some e s t i m a t e s even p l a c e t h e c u r r e n t number o f h o r i z o n t a l w e l l s b e t ween 300 and 600 (AUSTIN, ROSE & SCHUH 1988). H o r i z o n t a l d r a i n h o l e l e n g t h s up t o 2,000 m (6,000 f t ) i n r e s e r v o i r depth up t o 3,000 m (9,000 f t ) have been achieved (ARMESSEN, JOURDAN & M A R I O T T I 1 9 8 8 ) . D r i l l i n g f a i l u r e s o f h o r i z o n t a l w e l l s a r e c o m p a r a t i v e l y r a r e , p e n e t r a t i o n r a t e s approach those o f v e r t i c a l w e l l s , d r i l l i n g o f a c c u r a t e t r a j e c t o r i e s has proven t o be c o m p l e t e l y f e a s i b l e , and d r i l l i n g c o s t s a l s o a r e n o t s u b s t a n t i a l l y h i g h e r than those o f c o n v e n t i o n a l w e l l s i f compared on a base o f d r i l l i n g p r o g r e s s u n i t . F o l l o w i n g an o u t l i n e o f the main t a r g e t s , t h e d i s c u s s i o n c o n c e n t r a t e s on aspects o f d i s t i n c t i o n o f h o r i z o n t a l w e l l s and d r a i n h o l e s , h o r i z o n t a l b o r e h o l e s vs. extended-reach d r i l l i n g , i n v e r t e d d r i l l i n g , s i g n i f i c a n c e f o r r e s e r v o i r drainage, and h o r i z o n t a l h o l e comp l e t i o n and s t i m u l a t i o n .
4.8.6.1.1. Main targets o f horizontal wells The main t a r g e t s o f h o r i z o n t a l w e l l d r i l l i n g a r e t i g h t gas f o r m a t i o n s and heavy o i l r e s e r v o i r s which a r e p a r t i a l l y a l s o t r e a t e d by steam i n j e c t i o n f l o o d i n g (BUTLER & STEPHENS 1980, RIAL 1984, GOODE & THAMBYNAYAGAM 1985, HUANG & HIGHT 1986; c f . s e c t i o n 4 . 9 . 4 . ) , r e s e r v o i r s w i t h gas and w a t e r c o n i n g problems (CHAPERON 1986; KUSSACK, KLEPPE & AASEN 1987), n a t u r a l l y f r a c t u r e d pay zones
577 w i t h v e r t i c a l cracks (MERCER, PRATT & YOST 1988; c f . s e c t i o n 4.8.6.1.5.1.), t h i n p r o s p e c t i v e s e c t i o n s (JOSHI 1986 a, 1987 a; FANTOFT, KROGH & POLLEN 1988), and edge water o r gas d r i v e r e s e r v o i r s (BOSIO & G I A N N E S I N I 1988, PETZET 1988). Subordinate a p p l i c a t i o n s a r e coal-seam d e g a s i f i c a t i o n w e l l s (ERTEKIN, SUNG & SCHWERER 1986; KING, ERTEKIN & SCHWERER 1986; SUNG, ERTEKIN & SCHWERER 1986; LOGAN, SCHWOEBEL & HORNER 1987; c f . s e c t i o n 4.4.3.6.) and gas shale e x p l o i t a t i o n holes (SALAMY, SARADJI, OKOYE, MERCER & YOST 1987; YOST, OVERBEY, SALAMI, OKOYE & SARADJI 1987; c f . s e c t i o n 4.4.4.3.2.). I n o i l f i e l d s , h o r i z o n t a l w e l l s can be a l s o used t o c o n t r o l and t o d i m i n u i s h severe coning problems (REISS 1985), t o a c c e l e r a t e g r a v i t y - d r a i n a g e o i l product i o n (BEZAIRE & MARKIW 1979), t o enhance o f f t a k e from s o l u t i o n - g a s - d r i v e reserv o i r s (PLAHN, STARTZMAN & WATTENBARGER 1987), and t o reduce drawdown (MACDONALD 1988). H o r i z o n t a l d r i l l i n g a l s o improves o i l and gas p r o d u c t i o n from r e s e r v o i r s w i t h gas caps and bottom water (PETZET 1988; FANTOFT, KROGH & POLLEN 1988). Other candidates are pay zones w i t h sparsely d i s t r i b u t e d v e r t i c a l f r a c t u r e s t h a t a normal w e l l has l i t t l e chance o f i n t e r s e c t i n g (JOSHI 1986 a ) . S h o r t - r a d i u s l a t e r a l holes can serve as h y d r a u l i c f r a c t u r e replacement i n c h a l k and o t h e r n a t u r a l l y f r a c t u r e d formations. H o r i z o n t a l d r i l l i n g i s n o t o n l y s u i t a b l e f o r o i l and gas r e s e r v o i r e x p l o i t a t i o n , b u t a l s o p r o f i t a b l e f o r condensate pay zone d e p l e t i o n (HOWES 1988). Summaries o f h o r i z o n t a l w e l l d r i l l i n g are compiled by R E I S S , JOURDAN, GIGER & ARMESSEN (1984); JOURDAN, R E I S S & JOLY (1986) and JOSHI (1987 a), and a s t a t u s r e p o r t o f o p e r a t i o n a l aspects i s g i v e n by PETZET (1988). H o r i z o n t a l borehol e s aim on p r o d u c t i o n from plane o r p a r a l l e l f l o w w i t h u n i f o r m pressure drop i n c o n t r a s t t o v e r t i c a l w e l l s which operate under r a d i a l c i r c u l a r f l o w w i t h concent r a t e d pressure drop (HOWES 1988), and f o r achievement o f plane flow, b o t h h i g h angle and l o n g r e s e r v o i r i n t e r s e c t i o n p a t h are c r i t i c a l , w i t h h o r i z o n t a l h o l e t r a j e c t o r y through the pay zone being p r e f e r a b l y a t l e a s t 10 times the r e s e r v o i r thickness ( c f . s e c t i o n 4.11.3.).
4.8.6.1.2. Horizontal wells vs. drainholes D i s t i n c t i o n has t o be made between h o r i z o n t a l w e l l s and d r a i n h o l e s (JOSHI 1987 a ) . H o r i z o n t a l w e l l s are n o r m a l l y new boreholes reaching a l e n g t h o f the h o r i z o n t a l s e c t i o n o f 1,000 - 3,000 f t (300 - 1,000 m) and are d r i l l e d from the surface by s t a r t i n g o f f v e r t i c a l l y and then a t the k i c k - o f f p o i n t p r o g r e s s i v e l y i n c r e a s i n g i n c l i n a t i o n by b u i l d i n g - u p d e v i a t i o n angle up t o f i n a l l y subhorizont a l and h o r i z o n t a l . Drainholes o r l a t e r a l holes are s i d e t r a c k s from e x i s t i n g v e r t i c a l w e l l s and a t t a i n a l e n g t h o f 100 - 700 f t (30 - 250 m). E i t h e r s i n g l e o r m u l t i p l e d r a i n h o l e s can be d r i l l e d through a s i n g l e v e r t i c a l w e l l , w i t h mult i p l e d r a i n h o l e s emanating from the same o r from d i f f e r e n t e l e v a t i o n s o f the par e n t w e l l and generating a b i r d f o o t p a t t e r n . Drainholes r e p r e s e n t secondary high-angle t o h o r i z o n t a l holes which are d r i l l e d r a d i a l l y from p r i m a r y v e r t i c a l o r low-angle holes (TRACY 1988). The l a r g e r the diameter o f the h o s t v e r t i c a l w e l l , t h e more high-angle o r h o r i z o n t a l d r a i n h o l e s can be d r i l l e d o u t o f i t (PETZET 1988). Most o f t h e h o r i z o n t a l boreholes d r i l l e d so f a r have been i n s t a l l e d as d r a i n h o l e s o u t o f v e r t i c a l w e l l s t h a t have a l r e a d y been reasonably good producers (AUSTIN, ROSE & SCHUH 1988). F o l l o w i n g some comments on r a d i u s f o r borehole t r a j e c t o r y t u r n i n g , a p p l i c a t i o n s o f d r a i n h o l e d r i l l i n g are o u t l i n e d .
4.8.6.1.2.1. Radius for borehole trajectory turning C l a s s i f i c a t i o n o f h o r i z o n t a l w e l l and d r a i n h o l e d r i l l i n g techniques can be made i n t o f o u r broad c a t e g o r i e s depending on the r a d i u s r e q u i r e d t o t u r n the bor e h o l e t r a j e c t o r y from v e r t i c a l t o h o r i z o n t a l d i r e c t i o n o r the r a t e o f angle b u i l d i n g , thus d i s t i n g u i s h i n g u l t r a - s h o r t - , short-, medium- and l o n g - r a d i u s d r a i n h o l e s (EDLUND 1987, JOSHI 1987; AUSTIN, ROSE & SCHUH 1988; HOWES 1988, JURGENS 1988, LOGAN 1988, MAHONEY 1988, TRACY 1988, ZALESKI & SPATZ 1988). DICKIN-
578
SON, ANDERSON & DICKINSON ( 1 9 8 6 ) o u t l i n e m u l t i p l e r a d i a l d r i l l i n g . The primary d i f f e r e n c e between c o n v e n t i o n a l o r e x t e n d e d - r e a c h d i r e c t i o n a l d r i l l i n g and s h o r t e r - r a d i u s d r a i n h o l e t e c h n i q u e s i s b u i l d a n g l e , with a c o n s i d e r a b l y h i g h e r b u i l d a n g l e f o r s h o r t e r - r a d i u s d r a i n h o l e s r e s u l t i n g in l e s s b o r e h o l e t r a j e c t o r y and t h u s l e s s c o s t w i t h r e s p e c t t o c o n v e n t i o n a l d i r e c t i o n a l d r i l l i n g (LOGAN 1988).
4.8.6.1.2.2.Drainhole drilling applications D r a i n h o l e d r i l l i n g i s a promising technology enhancing o i l r e c o v e r y and improving p r o d u c t i o n e f f i c i e n c y and economics (TRACY 1 9 8 8 ) . I t s major a p p l i c a t i o n s i n o i l and g a s i n d u s t r y a r e i r r e g u l a r r e s e r v o i r s which can be reached w i t h o u t a d d i t i o n a l v e r t i c a l w e l l b o r e s , l i m i t a t i o n of i n v a s i o n of unwanted format i o n f l u i d s , p e n e t r a t i o n of v e r t i c a l f r a c t u r e s , and maximization of p r o d u c t i o n of l o w - p e r m e a b i l i t y and low-energy r e s e r v o i r s by i n c r e a s i n g f o r m a t i o n e x p o s u r e . D r a i n h o l e d r i l l i n g can a l s o be used i n mining i n d u s t r y t o i n c r e a s e r e c o v e r y e f f i c i e n c y and c o n t r o l i n s o l u t i o n mining, t o improve methane removal from coal p r i o r t o mining s t a r t - u p , and t o f a c i l i t a t e underground g a s i f i c a t i o n in c o a l deposi t s .
4.8.6.1.3.Horizontal wells vs. extended-reach drilling Another t y p e of h i g h l y - d e v i a t e d t o h o r i z o n t a l b o r e h o l e a p p l i c a t i o n i s e x t e n ded-reach d r i l l i n g which has the o b j e c t i v e t o c r e a t e a d r a i n a g e p o i n t i n the r e s e r v o i r a s f a r a s p o s s i b l e from t h e s u r f a c e o u t l e t (BOSIO & GIANNESINI 1 9 8 8 ) . Current technology a l l o w s t o d r i l l l a t e r a l d i s p l a c e m e n t s of the e x t e n d e d - r e a c h s e c t i o n s IJP t o 10 k m ( 6 mi) i n a t r u e v e r t i c a l t o t a l depth of a b t . 4 k m (HOWES 1988, OFFSHORE E N G I N E E R 1988) o r a b t . 32,000 f t l a t e r a l d i s p l a c e m e n t a t a t r u e v e r t i c a l d e p t h of 12,000 f t a t an i n c l i n a t i o n of 72 - 75 d e g r e e s ( P E T R O L E U M E N G I N E E R INTERNATIONAL 1988 a ) . Some comments a r e o f f e r e d on h o r i z o n t a l d i s p l a c e ment and t o t a l a c c e s s i b l e a r e a a s well a s extended- vs- l i m i t e d - r e a c h d r i l l i n g as follows.
4.8.6.1.3.1.Horizontal displacement and total accessible area The c u r r e n t world r e c o r d of s u c c e s s f u l e x t e n d e d - r e a c h d r i l l i n g i s a b t . 15,100 f t ( 5 , 0 0 0 m) l a t e r a l d i s p l a c e m e n t a t a true v e r t i c a l d e p t h of a b t . 8,000 f t ( 2 , 6 0 0 m ) a t an a n g l e of a b t . 70 d e g r e e s . The h i t h e r t o maximum achievement i n Germany FRG i s a b t . 2,100 m ( 6 , 3 0 0 f t ) l a t e r a l d i s p l a c e m e n t a t a t r u e v e r t i c a l depth of a b t . 1,400 m ( 4 , 2 0 0 f t ) a t a maximum a n g l e of 6 5 . 5 d e g r e e s ( G R E B E 1 9 8 1 ) . H o r i z o n t a l b o r e h o l e s e c t i o n s u p t o 3,000 f t (1,000 m ) i n e x t e n d e d - r e a c h w e l l s have been f r e q u e n t l y achieved (FERTL & N I C E 1 9 8 8 ) . Extended-reach d r i l l i n g i s g e n e r a l l y c a r r i e d o u t w i t h i n c l i n a t i o n a n g l e s b e t ween 60 and 90 d e g r e e s and u s u a l l y aims a t h o r i z o n t a l d i s p l a c e m e n t s i n e x c e s s of 2 m i l e s (HOWES 1 9 8 8 ) . The reason f o r e x t e n d e d - r e a c h d r i l l i n g i s t o a c c e s s remote t a r g e t s from a c e n t r a l p o i n t thus reducing t h e number of h i g h - c o s t p l a t forms o r subsea w e l l s t h a t a r e n e c e s s a r y f o r development of the f i e l d u s u a l l y u p t o 50 % o r i n extreme c a s e s even up t o 75 % . I f d r i l l i n g from a p l a t f o r m i s i n c r e a s e d from a maximum a n g l e of h o l e i n c l i n a t i o n from 40 t o 80 d e g r e e s , the t o t a l a c c e s s i b l e a r e a i s r i s i n g by a f a c t o r of 30 ( M O O R E 1987 a ) . Extendedr e a c h d r i l l i n g i n f i e l d a p p r a i s a l i s i l l u s t r a t e d by LOCKE, JOHNSON & JEWKES (1988).
4.8.6.1.3.2. Extended- vs. 1 imited-reach dri 1 1 ins Extended-reach d r i l l i n g i s c h a r a c t e r i z e d by two main b u i l d i n g c u r v e s t h a t are i n i t i a t e d by two k i c k - o f f p o i n t s and which a r e connected by a long s t r a i g h t
579 s e c t i o n between t h e k i c k - o f f marks, w i t h t h e upper b u i l d u p c u r v e b e i n g o f t e n s i t u a t e d f a i r l y h i g h i n t h e w e l l , and t h e l o w e r c o n n e c t i n g t h e o b l i q u e s t r a i g h t s e c t i o n w i t h t h e h o r i z o n t a l o r s u b v e r t i c a l l a n d i n g t r a c e depending on d e p l e t i o n and s t i m u l a t i o n requirements (JOURDAN, ARMESSEN & POUSSELET 1988). L i m i t e d reach d r i l l i n g i s i n c o n t r a s t t o t h i s c h a r a c t e r i z e d by o n l y one b u i l d u p c u r v e and a J - t y p e shape o f t h e b o r e h o l e t r a j e c t o r y , w i t h t h e s i n g l e k i c k - o f f p o i n t b e i n g s i t u a t e d a t a f a i r l y deep l e v e l . F o r b e t t e r r e s e r v o i r p e n e t r a t i o n geomet r y e s p e c i a l l y i n case o f v e r t i c a l h y d r a u l i c f r a c t u r i n g and g r a v e l packing, a S-shaped b o r e h o l e t r a j e c t o r y w i t h one o r two k i c k - o f f p o i n t s i s p r e f e r e n t i a l , w i t h t h i s shape b e i n g c r e a t e d by r e d u c i n g a g a i n t h e w e l l b o r e i n c l i n a t i o n p r i o r t o p r o p a g a t i o n t h r o u g h t h e pay complex (BRUIST, JEFFERIS & BOTTS 1983).
4.8.6,1.4*Inverted drilling A s p e c i a l t y p e o f h i g h - a n g l e t o h o r i z o n t a l d r i l l i n g i s i n v e r t e d d r i l l i n g and c o m p l e t i o n (PETZET 1988) which i n v o l v e s c r e a t i o n o f b o r e h o l e t r a j e c t o r i e s i n excess o f 90 degrees. I n v e r t e d w e l l s s t a r t v e r t i c a l l y and then g r a d u a l l y b u i l d up an i n c l i n a t i o n up t o 90 degrees which i s t h e n i n t h e t e r m i n a l phase exceeded up t o a b t . 100 - 110 degrees by t u r n i n g a g a i n upwards, t h u s p r o p a g a t i n g i n i n v e r s e d i r e c t i o n i n t h e f i n a l stage. I n v e r t e d d r i l l i n g may l e a v e a g r e a t e r d i s t a n c e o r s t a n d o f f between t h e g a s - o i l - c o n t a c t i n a r e s e r v o i r and t h e p e r f o r a t i o n s which extends l o n g - t e r m w e l l p r o d u c t i v i t y and p r a c t i c a l l y e l i m i n a t e s gas c h a n n e l l i n g . Examples o f i n v e r t e d d r i l l i n g w i t h t e r m i n a l angles i n excess o f 90 degrees a r e a l s o r e p o r t e d by GREENER, SAUVAGEAU & PASTERNACK (1988), and STEENBOCK & MOLLER-LINK (1987) p r e s e n t a l t e r n a t i v e d r a i n h o l e d r i l l i n g w i t h an a r t i c u l a t e d s p r i n g f o r i n v e r t e d d r i l l i n g up t o 110 degrees h o l e i n c l i n a t i o n . Examples o f i n v e r t e d d r i l l i n g w i t h f i n a l angles o f a b t . 100 degrees a r e a l s o documented by WESTERN OIL WORLD ( 1 9 8 8 ) .
4.8.6.1.5. Significance f o r reservoir drainage H o r i z o n t a l d r i l l i n g aims on r a d i c a l l y changing o f t h e f l o w c o n d i t i o n s i n t h e r e s e r v o i r by c r e a t i o n o f a p l a n a r f l o w p a t t e r n i n s t e a d o f t h e c o n v e n t i o n a l r a d i a l c i r c u l a r f l o w network ( B O S I O & G I A N N E S I N I 1988) which i s t h e c o n t r o l l i n g f a c t o r o f p r o d u c t i v i t y and/or r e c o v e r y improvement ( c f . s e c t i o n 4 . 1 1 . 3 . ) . The most fundamental parameter o f h o r i z o n t a l d r i l l i n g i s t h e r e f o r e t h e l e n g t h o f t h e d r a i n a g e s e c t i o n t r a v e r s i n g t h e r e s e r v o i r h o r i z o n t a l l y , o r more s p e c i f i c a l l y the r e l a t i v e length o f the drainage i n t e r v a l i n r e l a t i o n t o r e s e r v o i r t h i c k ness and d r a i n a g e area dimensions. Some comments a r e o f f e r e d as f o l l o w s on comm u n i c a t i o n p a t h between w e l l b o r e t r a j e c t o r y and r e s e r v o i r , improved n a t u r a l f r a c t u r e i n t e r s e c t i o n , choke e f f e c t on v e r t i c a l f r a c t u r e s , d i s t a n c e f r o m f l u i d c o n t a c t s , r e d u c t i o n o f coning, s e l e c t i o n o f drainage and i n j e c t i o n d i r e c t i o n , r e s e r v o i r geometry and n a t u r a l f r a c t u r e d i s t r i b u t i o n , and w e l l b o r e i n s t a b i l i t y .
4.8.6.1.5.1. Comnunicat ion path between wellbore trajectory and reservoir H i g h e r p r o d u c t i o n r a t e s and i n many cases a l s o h i g h e r u l t i m a t e r e c o v e r y r e s u l t s f r o m a l o n g communication p a t h between w e l l b o r e t r a j e c t o r y and r e s e r v o i r i n a h o r i z o n t a l b o r e h o l e i n c o n t r a s t t o a v e r t i c a l w e l l where t h e l e n g t h o f t h e pay f o r m a t i o n open t o t h e b o r e h o l e i s more o r l e s s l i m i t e d t o p r o s p e c t i v e zone t h i c k n e s s (PETZET 1988). I n t h i s connection, however, a t t e n t i o n has t o be p a i d t o the f a c t t h a t the r e s e r v o i r drainage e f f e c t o f h o r i z o n t a l boreholes t r a v e r sing s l i g h t l y - d i p p i n g formations is equivalent t o t h a t o f v e r t i c a l w e l l s crossi n g s t e e p l y - i n c l i n e d beds p r o v i d i n g t h e most s u i t a b l e c o n s t e l l a t i o n o f maximum communication p a t h o f w e l l b o r e t r a j e c t o r y and pay zone, whereas t h e i n v e r s e comb i n a t i o n o f h o r i z o n t a l b o r e h o l e s i n t e r s e c t i n g s t e e p l y - d i p p i n g s t r a t a and v e r t i c a l w e l l s p r o p a g a t i n g through g e n t l y - i n c l i n e d r e s e r v o i r s r e p r e s e n t i n g t h e w o r s t p o s s i b l e case o f s h o r t e s t c o n t a c t l e n g t h o f w e l l b o r e and pay i n t e r v a l ( c f . a l s o
580 section 4.11.1.2.2.). E x p l o i t a t i o n o f incremental reserves by h o r i z o n t a l boreholes i s f r e q u e n t l y t h e consequence o f r e d u c e d c o n i n g , i m p r o v e d p r e s s u r e m a i n t e n a n c e , and l a r g e r d r a i n a g e a r e a . Even i f t h e y do n o t r a i s e u l t i m a t e r e c o v e r y , h o r i z o n t a l w e l l s may i m p r o v e economics b y a l l o w i n g t h e b o r e h o l e s t o p r o d u c e a t r a t e s t h a t p a y o u t f a s t e r . The e f f e c t i v i t y and p r o f i t a b i l i t y o f h o r i z o n t a l w e l l s s t r o n g l y depends f r o m p e r m e a b i l i t y a n i s o t r o p y i n t h e r e s e r v o i r (JOSH1 1988, MUKHERJEE & ECONOMIDES 1988; c f . s e c t i o n 4 . 2 . 4 . 2 . ) .
4.8.6.1.5.2.Improved natural fracture intersection An i m p o r t a n t a d v a n t a g e o f h o r i z o n t a l w e l l s i s t h e i r much b e t t e r and more e f f e c t i v e i n t e r s e c t i o n o f systems o f v e r t i c a l n a t u r a l f r a c t u r e s i n t h e r e s e r v o i r (REISS 1985) w h i c h a l l o w s much g r e a t e r p r o d u c t i o n p r o f i t f r o m t h e j o i n t f a b r i c t h a n i n v e r t i c a l w e l l s . As a c c o r d i n g t o t h e r o c k s t r e s s d i s t r i b u t i o n , vertical o r h i g h l y - i n c l i n e d n a t u r a l f r a c t u r e s a r e p r e s e n t i n m o s t o f t h e o i l and gas r e s e r v o i r s w h i c h c a n be b e t t e r d e v e l o p e d w i t h h o r i z o n t a l w e l l d r i l l i n g , t h i s acq u i s i t i o n and c o m p l e t i o n t e c h n i q u e has c o n s i d e r a b l e s i g n i f i c a n c e f o r enhanced e x p l o i t a t i o n o f b o t h m a t r i x - and f r a c t u r e - p o r o s i t y p a y systems. Another advantage i s t h e l a r g e r spread o f t h e f o r m a t i o n i n t e r v a l i n t e r s e c t e d by t h e b o r e h o l e w h i c h p e r m i t s s e l e c t i v e p r o d u c t i o n o f head o r t a i l o f t h e penet r a t e d r e s e r v o i r segment i n a much more e f f i c i e n t way t h a n w o u l d b e p o s s i b l e f o r v e r t i c a l w e l l s . The p r e s e n c e o f open n a t u r a l f r a c t u r e s i n t h e r e s e r v o i r f o r m a t i o n i s u s u a l l y i n d i c a t e d b y s e v e r e mud l o s s e s up t o t o t a l l e a k o f f o f c i r c u l a t i o n d u r i n g t h e d r i l l i n g phase. I n t e r s e c t i o n o f numerous v e r t i c a l n a t u r a l f r a c t u r e s by t h e h o r i z o n t a l w e l l leads t o d r a s t i c a l increase o f w e l l p r o d u c t i v i ty, a l l o w i n g i n t u r n an o f f t a k e u n d e r v e r y l i m i t e d f l o w i n g p r e s s u r e w h i c h g i v e s r i s e t o o n l y g e n t l e deformation o f the oil-water-contact, with this effect b e i n g one o f t h e k e y s f o r a v e r y s a t i s f a c t o r y o i l sweep e f f i c i e n c y i n h o r i z o n t a l boreholes ( c f . a l s o section 4.8.6.2.4.2.).
4.8.6.1.5.3. Choke effect on vertical fractures The c o n t a c t between h o r i z o n t a l b o r e h o l e s and v e r t i c a l f r a c t u r e s r e s u l t s i n a choke b e i n g a s p e c i a l s k i n e f f e c t (MUKHERJEE & ECONOMIOES 1 9 8 8 ) . H y d r a u l i c a l l y f r a c t u r e d h o r i z o n t a l w e l l s c a n b e r e g a r d e d as choked v e r t i c a l c r a c k s because o f t h e l i m i t e d c o n t a c t between b o r e h o l e and f r a c t u r e ( c f . s e c t i o n 4 . 1 1 . 1 . ) . T h i s choke e f f e c t i s n e g l i g i b l e i n t h e i d e a l c o n s t e l l a t i o n o f v e r t i c a l f r a c t u r e plane o r i e n t a t i o n b e i n g p a r a l l e l t o h o r i z o n t a l borehole t r a j e c t o r y f o r t h e case o f a s i n g l e f r a c t u r e , whereas i t s maximum i s r e a c h e d f o r t h e g e o m e t r i c a l r e l a t i o n s h i p o f mu1 t i p l e v e r t i c a l c r a c k s i n t e r s e c t i n g t h e h o r i z o n t a l we1 1 o r t h o g o n a l l y t o t h e o r i e n t a t i o n o f i t s t r a j e c t o r y . On t h e o t h e r hand, m u l t i p l e f r a c t u r e s p r o p a g a t i n g p e r p e n d i c u l a r l y t o t h e h o r i z o n t a l borehole a i d i n the b e s t cover a g e o f t h e d r a i n a g e a r e a . The c h o k e - t y p e s k i n e f f e c t r e s u l t s i n an a d d i t i o n a l p r e s s u r e d r o p due t o t h e r a d i a l f l o w g e n e r a t e d a r o u n d t h e e n t r y p o i n t . As t h e s k i n e f f e c t i s e q u a l t o z e r o when t h e w e l l i s v e r t i c a l o r i n c l i n e d a l o n g t h e f r a c t u r e d i r e c t i o n , d e v i a t e d h o l e s s h o u l d be p r e f e r e n t i a l l y d r i l l e d such t h a t t h e y a r e t u r n e d v e r t i c a l l y downward f o r t h e i n t e r s e c t i o n o f t h e r e s e r v o i r i n order t o minimize contact s k i n e f f e c t ( c f . a l s o section 4.8.6.1.3.). In terms o f r e s e r v o i r e x p l o i t a t i o n , t h e performance o f a f r a c t u r e d h o r i z o n t a l w e l l w h i c h has been d r i l l e d a l o n g t h e c r a c k p l a n e c a n n o t be b e t t e r t h a n t h a t o f a f r a c t u r e d v e r t i c a l h o l e , whereas h o r i z o n t a l w e l l p r o d u c t i v i t y c a n s u b s t a n t i a l l y b e n e f i t v i a m u l t i p l e f r a c t u r e completion despite the occurring s k i n e f f e c t s i n t h e l a t t e r case i n comparison t o almost zero s k i n i n t h e former c o n s t e l l a t i o n .
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4.8.6.1.5.4. Distance from fluid contacts O t h e r advantages o f h o r i z o n t a l w e l l s i n c l u d e l o n g e r completed i n t e r v a l s r e s u l t i n g f r o m i n c r e a s e d d i r e c t c o n t a c t between b o r e h o l e and pay zone and t h e r e f o r e increased production r a t e s i n the longer wellbores, increased standoff f r o m f l u i d c o n t a c t s i n o i l r e s e r v o i r s o v e r l y i n g an a q u i f e r o r l o c a t e d under a gas cap i m p r o v i n g p r o d u c t i o n r a t e s w i t h o u t c a u s i n g o r w i t h a t l e a s t d e f e r r i n g coning, r e d u c t i o n o f drawdown f o r a g i v e n p r o d u c t i o n r a t e by t h e l o n g e r w e l l bore, f i e l d development w i t h fewer b o r e h o l e s because o f t h e l a r g e r d r a i n a g e area o f each w e l l , and a l l o w i n g development o f some f i e l d s where c o n v e n t i o n a l v e r t i c a l b o r e h o l e s would be uneconomical (SHERRARD, BRICE & MACDONALD 1986) o r t h e s u r f a c e a r e a i s u n a c c e s s i b l e due t o environmental reasons. The c r i t i c a l coning r a t e f o r a h o r i z o n t a l w e l l i s higher than t h a t f o r a corresponding v e r t i c a l b o r e h o l e (MACDONALD 1988). I f c o n i n g i s i n e v i t a b l e , h o r i z o n t a l w e l l s s t i l l p r o v i d e improved sweep e f f i c i e n c y , because t h e shape o f c o n v e n t i o n a l cones i n v e r t i c a l b o r e h o l e s r e s u l t s i n l e s s displacement o f hydrocarbons towards t h e producer t h a n t h e tent-shaped cone which i s c h a r a c t e r i s t i c f o r h o r i z o n t a l w e l l s . The r a t i o o f v e r t i c a l t o hor i z o n t a l permeability i s very important i n delaying water breakthrough i n h o r i z o n t a l b o r e h o l e s (GIGER 1986).
4.8.6.1.5.5. Reduction of coning Concerning r e d u c t i o n o f gas- and w a t e r - c o n i n g tendencies, h o r i z o n t a l w e l l s e x h i b i t b e t t e r performance t h a n v e r t i c a l boreholes, as i t r e q u i r e s much s m a l l e r p r e s s u r e drawdown t o produce o i l a t t h e same r a t e and because i t has almost a l i n e a r pressure gradient from the wellbore t o the drainage radius, r e s u l t i n g i n a steady gas d i p o r w a t e r r i s e o v e r t h e l o n g p r o d u c i n g h o r i z o n t a l - w e l l l e n g t h (SHERRARD, B R I C E & MACDONALD 1986). I n c o n t r a s t , i n a v e r t i c a l b o r e h o l e a l o g l i n e a r pressure gradient from wellbore t o drainage radius, along w i t h a conical f l o w a t a s i n g l e p o i n t , a c c e l e r a t e s c o n i n g problems. F u r t h e r p r o m i s i n g aspects a r e t h e a b i l i t y t o complete s e v e r a l t o v a r i o u s h i g h l y - d e v i a t e d o r h o r i z o n t a l w e l l s from a s i n g l e l o c a t i o n c o v e r i n g a l a r g e d r a i n a g e area which i s e s s e n t i a l i n e n v i r o n m e n t a l l y s e n s i t i v e o r h o s t i l e areas as w e l l as o f f s h o r e where p r e p a r a t i o n o f d r i l l i n g s i t e s i s a m a j o r expense (HUANG & HIGHT 1986). P a r t i c u l a r l y f e a s i b l e a p p l i c a t i o n s o f h o r i z o n t a l w e l l s a r e t h i n o i l zones which a r e sandwiched between gas cap and a q u i f e r where p r o d u c t i o n r a t e s i n v e r t i c a l b o r e h o l e s a r e s e v e r e l y l i m i t e d by gas and w a t e r c o n i n g (FANTOFT, KROGH & POLLEN 1988; GILHUUS 1988). The main advantages o f h o r i z o n t a l w e l l s i n such a s e t t i n g a r e l o w e r f l u i d v e l o c i t i e s n e a r t h e w e l l b o r e due t o l o n g e r p r o d u c i n g s e c t i o n and l o n g e r d i s t a n c e t o f l u i d c o n t a c t s , whereas a p o t e n t i a l disadvantage i s t h e i n a b i l i t y t o s h i f t t h e v e r t i c a l d e p t h o f t h e p r o d u c i n g i n t e r v a l t o compensate f o r movements o f t h e phase c o n t a c t s . H o r i z o n t a l b o r e h o l e s l o c a t e d c l o s e enough t o t h e g a s - o i l - c o n t a c t would have e a r l y gas breakthrough r a t h e r t h a n wat e r breakthrough, and f o l l o w i n g r e c o m p l e t i o n f o r gas, p r o d u c t i o n can c o n t i n u e w i t h l i t t l e o r no w a t e r c u t . The h o r i z o n t a l w e l l has t o p e n e t r a t e a l l p e r m e a b i l i t y b a r r i e r s i n o r d e r t o ensure h i g h l a y e r sweep e f f i c i e n c y and maximum o i l r e c o v e r y . While v e r t i c a l w e l l p r o d u c t i v i t y i s l i m i t e d by r e s e r v o i r t h i c k n e s s , h o r i z o n t a l w e l l e x p l o i t a b i l i t y can be improved by i n c r e a s i n g i t s l e n g t h (MACDONALD 1988). Water breakt h r o u g h can be delayed by p l a c i n g t h e s t a n d o f f o f t h e h o r i z o n t a l s e c t i o n above t h e o i l - w a t e r c o n t a c t as g r e a t as p o s s i b l e (GIGER 1986). Another a p p l i c a t i o n o f h o r i z o n t a l w e l l s o r drainholes i s connection o f v e r t i c a l shafts o r h o r i z o n t a l g a l l e r y systems t o improve t h e e f f e c t i v i t y o f o i l m i n i n g (BEST, CORDELL & HASTON 1987; ERDUL-ERDGAS-KOHLE 1988 b, MOORE 1988 c ) . T h i s t e c h n i q u e aims on g r a v i t y d r a i n a g e o f heavy o i l sands v i a h o r i z o n t a l w e l l s emanating f r o m t h e underground t u n n e l system.
582
4.8.6.1.5.6.Selection o f drainage and injection direction H o r i z o n t a l w e l l s a l s o a l l o w t o i n j e c t a n d / o r produce f l u i d s o r t h o g o n a l t o t h o s e from a v e r t i c a l b o r e h o l e , t h e r e b y p r o v i d i n g t h e p o t e n t i a l of improving sweep e f f i c i e n c y of a f l o o d and thus r e c o v e r y e f f i c i e n c y . The d e c i s i v e advant a g e of h o r i z o n t a l w e l l s with r e s p e c t t o v e r t i c a l b o r e h o l e s a n d / o r v e r t i c a l f r a c t u r e s ( c f . s e c t i o n 4 . 8 . 6 . 2 . 1 . ) i s t h e p o s s i b i l i t y of d r i l l i n g the h i g h l y - d e v i a t e d b o r e h o l e i n e v e r y d i r e c t i o n t h a t i s d e s i r a b l e from r e s e r v o i r d r a i n a g e p o i n t of view, i n c o n t r a s t t o the more o r l e s s predetermined d i r e c t i o n of hyd r a u l i c f r a c t u r e p r o p a g a t i o n ( a l s o a l t e r e d - s t r e s s f r a c t u r i n g does n o t a l l o w t o turn t h e c r a c k i n t o e v e r y p o s s i b l e d i r e c t i o n , b u t only p e r m i t s t o s h i f t t h e o r i e n t a t i o n of t h e f r a c t u r e i n s e v e r a l l i n e s more o r l e s s normally t o each o t h e r ; c f . s e c t i o n 4 . 9 . 4 . 1 . 3 . 1 . ) and the s t a n d a r d p e r p e n d i c u l a r p e n e t r a t i o n of vertical wells. I n j e c t i o n of steam from a h o r i z o n t a l b o r e h o l e e v e n t u a l l y p r o v i d e s a more homogeneous steam f r o n t and b e t t e r i n j e c t i v i t y i n d i c e s (RIAL 1984; SUNG, ERTEKIN & SCHWERER 1986; c f . s e c t i o n 4 . 9 . 4 . 2 . 1 . 2 . ) . H o r i z o n t a l w e l l s produce on a v e r a g e around f o u r t i m e s more than v e r t i c a l b o r e h o l e s , and a s they produce i n a wider a r e a , i t i s n o t o n l y t h e p r o d u c t i v i t y , b u t a l s o the t o t a l e x p l o i t a t i o n t h a t can be m u l t i p l i e d by a f a c t o r of a b t . f o u r ( D U D A & KUMAR 1988, HOWES 1 9 8 8 ) . A g r e a t e r p r o d u c t i o n i s achieved w i t h an i n c r e a s e d r e c o v e r y f a c t o r and wider d r a i n a g e a r e a . Comparative e v a l u a t i o n s of p r o d u c t i v i t y and f e a s i b i l i t y of h o r i z o n t a l v s . v e r t i c a l w e l l s are c a r r i e d o u t by HUANG & HIGHT ( 1 9 8 6 ) ; KOSSACK, KLEPPE & AASEN ( 1 9 8 7 ) ; DUDA & KUMAR (1988) and MUKHERJEE & ECONOMIDES ( 1 9 8 8 ) .
4.8.6.1.5.7.
Reservoir geometry and natural crack distribution
I n summary, t h e main f i e l d s of a p p l i c a t i o n of h o r i z o n t a l d r i l l i n g a r e t h u s pay zones w i t h v e r t i c a l f r a c t u r e s t h a t a r e s p a r s e l y d i s t r i b u t e d and c a n n o t be a c c i d e n t a l l y c r o s s e d by v e r t i c a l w e l l s o r v e r t i c a l h y d r a u l i c c r a c k s a l o n e , res e r v o i r s w i t h coning problems where a h o r i z o n t a l b o r e h o l e can be p o s i t i o n e d away from the f l u i d c o n t a c t and a l l o w s a lower drawdown of the unwanted f l u i d , in t h i n pay zones where t o o many v e r t i c a l w e l l s would be r e q u i r e d , i n edge wat e r o r g a s d r i v e r e s e r v o i r s where a h o r i z o n t a l b o r e h o l e combines the e f f e c t of a more e f f i c i e n t g r a v i t y d r a i n a g e and a wider d r a i n a g e a r e a , secondary o r t e r t i a r y r e c o v e r y p r o c e s s e s where h o r i z o n t a l w e l l s can improve i n j e c t i v i t y and a r e a l sweep e f f i c i e n c y , e v a l u a t i o n w e l l s a f t e r a d i s c o v e r y i s made i n o r d e r t o s t u d y t h e l a t e r a l e v o l u t i o n of t h e f a c i e s and t o p r o v i d e v a l u a b l e i n f o r m a t i o n b e f o r e t h e f i n a l development d e c i s i o n i s made, t o h e l p s o l v i n g environmental problems by r e d u c i n g the s p a c e occupied by d r i l l i n g s i t e s and p r o d u c t i o n l i n e s , and t o reduce t h e number of r e q u i r e d o f f s h o r e p l a t f o r m s (BOSIO & GIANNESINI 1988).
An i m p o r t a n t a p p l i c a t i o n o f both h o r i z o n t a l w e l l s and v e r t i c a l h y d r a u l i c f r a c t u r e s i s p e n e t r a t i o n of i s o l a t e d sand b o d i e s of l e n t i c u l a r geometry (MACDON A L D 1 9 8 8 ) . The a m e l i o r a t i o n of r e s e r v o i r c o n t i n u i t y w i t h i n t e r s e c t i o n of more sand b o d i e s p r o v i d e s a c c e s s t o l a r g e r hydrocarbon volumes ( c f . s e c t i o n 4.10.2.1.).
4.8.6.1.5.8. We1 lbore instabi 1 i ty Highly-deviated t o horizontal boreholes a r e subjected t o considerable problems o f w e l l b o r e i n s t a b i l i t y which provoke f r e q u e n t l y b o r e h o l e c o l l a p s e and f o r m a t i o n f r a c t u r i n g (FUH, WHITFILL & SCHUH 1 9 8 8 ) . Borehole c o l l a p s e pressure g r a d i e n t i n c r e a s e s w i t h h o l e a n g l e , whereas f r a c t u r e g r a d i e n t remains the same. As a consequence of t h i s r e l a t i o n s h i p , t h e working range between t h e two g r a d i e n t s i s narrower a t h i g h e r h o l e a n g l e s and thus w i t h i n c r e a s i n g d e v i a t i o n a n g l e of the b o r e h o l e , the p o t e n t i a l f o r unwanted a c c i d e n t a l f o r m a t i o n f r a c t u -
583 r i n g d u r i n g d r i l l i n g i n c r e a s e s . Optimum mud w e i g h t f o r w e l l b o r e s t a b i l i t y i n h i g h l y - i n c l i n e d t o h o r i z o n t a l h o l e s ranges between b o r e h o l e c o l l a p s e p r e s s u r e g r a d i e n t r e p r e s e n t i n g t h e l o w e r l i m i t and f o r m a t i o n f r a c t u r e g r a d i e n t b e i n g t h e upper boundary. Aspects o f i n s t a b i l i t y o f h i g h l y - d e v i a t e d t o h o r i z o n t a l w e l l s a r e a l s o d i s c u s s e d by BRADLEY (1979); HOTTMAN, SMITH & PURCELL (1979); AADN0Y & CHENEVERT ( 1 9 8 7 ) , H S I A O (1987), MAURY & SAUZAY (1987) and YEW & LI ( 1 9 8 7 ) .
4.8.6.1.6.Horizontal hole completion and stimulation R E I S S (1985) g i v e s an e x p e r i e n c e r e p o r t o f h o r i z o n t a l w e l l p r o d u c t i o n a f t e r f i v e y e a r s , and HUANG & H I G H T (1986) and PETZET (1987) e v a l u a t e s t e a m f l o o d p r o cesses i n h o r i z o n t a l b o r e h o l e s . A comparison o f e x p l o i t a b i l i t y o f h o r i z o n t a l and v e r t i c a l w e l l s i s performed by KOSSACK, KLEPPE & AASEN ( 1 9 8 7 ) . H y d r a u l i c f r a c t u r i n g o f h i g h l y - d e v i a t e d w e l l s i s i n v e s t i g a t e d by YEW & L I (1987), and g r a v e l p a c k i n g o f h o r i z o n t a l b o r e h o l e s i n analyzed by D I C K I N S O N & ANDERSON (1987) and SPREUX, GEORGES & L E S S I (1988; c f . a l s o s e c t i o n 5 . 8 . 3 . ) . J O S H I (1987 a) p r e sents a r e v i e w o f h o r i z o n t a l w e l l and d r a i n h o l e t e c h n o l o g y . Logging methods i n h o r i z o n t a l b o r e h o l e s a r e e v a l u a t e d by BRATOVICH, B E L L & KAAZ ( 1 9 7 7 ) ; BARON & W I T T R I S C H (1982), D A N I E L & FERTL (1984); JOLY, DORMIGNY, CATALA & PINSON (1985); R E I S S (1985); DICKINSON, ANDERSON & D I C K I N S O N (1986); ANDERSON & LATOS (1987), FERTL & HOTZ (1987), FERTL & MARTIN (1987); SPREUX, L O U I S & ROCCA (1987); FERTL & N I C E (1988); MONTIGNY, SORRIAUX, L O U I S & L E S S I (1988); MACEWEN (1988) and SPREUX, GEORGES & L E S S I (1988).
WEIRICH, Z A L E S K I & MULCAHY (1987) summarize p e r f o r a t i o n t e c h n i q u e s f o r h o r i z o n t a l b o r e h o l e s . P r o d u c t i v i t y o f h o r i z o n t a l w e l l s i s e v a l u a t e d by BABU & ODEH (1988), J O S H I (1988) and MUKHERJEE & ECONOMIDES (1988). H o r i z o n t a l w e l l complet i o n types comprise open hole, s l o t t e d o r p e r f o r a t e d l i n e r o r casing, e x t e r n a l c a s i n g packers w i t h t u b i n g - o p e r a t e d subs, cemented l i n e r o r casing, prepacked l i n e r , and screen w i t h e x t e r n a l g r a v e l p a c k i n g c o m p l e t i o n s (AUSTIN, ROSE & SCHUH 1988).
4.8.6.1.7. Other aspects O t h e r aspects o f h o r i z o n t a l w e l l s and d r a i n h o l e s a r e d i s c u s s e d by JOURDAN, R E I S S , BARON & W I T T R I S C H (1982); PUGH (1982); JOURDAN, R E I S S , BARON, GIGER & W I T T R I S C H (1983); GIGER, R E I S S & JOURDAN (1984); HYLAND (1984), JOURDAN & BARON (1984), GIGER (1985, 1987), GOODE & THAMBYNAYAGAM (1985), CLONTS & RAMEY (1986), J O S H I (1986); KARCHER, GIGER & COMBE (1986); SHERRARD, B R I C E & MACDONALD (1986); D I E T R I C H (1987), FINCHER (1987), H S I A D (1987); LOGAN, SCHWOEBEL & HORNER ( 1 9 8 7 ) ; MOORE (1987); PLAHN, STARTZMAN & WATTENBARGER (1987); SALAMY, SAR A D J I , OKOYE, MERCER & YOST ( 1 9 8 7 ) ; WEIRICH, Z A L E S K I & MULCAHY (1987); WILSON & S A B I N S (1987) ; YOST, OVERBEY, SALAMY, OKOYE & SARADJI (1987); FUH, W H I T F I L L & SCHUH (1988) ; GREENER, SAUVAGEAU & PASTERNACK (1988) ; KERR & BROUTHERS (1988), KUCHUK & GOODE (1988), OZKAN & RAGHAVAN (1988), PETZET (1988); REICEY, STAGE & V I D R I N E (1988) ; SHAW & SUTHERLAND (1988), STEWART & WILLIAMSON (1988), V R I E L I N K & HIPPMANN (1988) and WINTERS, L U C I A & JOHNSON ( 1 9 8 8 ) .
A s p e c t a c u l a r h i g h l y - d e v i a t e d t o h o r i z o n t a l d r i l l i n g o p e r a t i o n f o r t h e Grand Canyon w a t e r l i n e p r o j e c t i s p r e s e n t e d by LATTIMORE, CARDEN & FISCHER ( 1 9 8 7 ) . HERNANDEZ & CADE (1988) d i s c u s s s c r e e n i n g c r i t e r i a f o r h o r i z o n t a l w e l l s . A more s o p h i s t i c a t e d technique i s s h o r t - r a d i u s l a t e r a l d r i l l i n g where v a r i o u s s h o r t hor i z o n t a l r a d i a l h o l e s a r e made f r o m a v e r t i c a l w e l l b o r e ( D I C K I N S O N & D I C K I N S O N 1985, PENDLETON & RAMESH 1985; DICKINSON, ANDERSON & D I C K I N S O N 1986; PARSONS & FINCHER 1987). The b u i l d i n g - u p o f t h e h i g h i n c l i n a t i o n a n g l e up t o f i n a l l y h o r i z o n t a l p o s i t i o n can i n s h a l l o w depths be supported by s l a n t d r i l l i n g r i g s w i t h mast i n c l i n a t i o n s o f 30 - 45 degrees which a l l o w s t o access t a r g e t s t h a t c o u l d n o t be reached by c o n v e n t i o n a l d i r e c t i o n a l d r i l l i n g (LOXAM 1982, BLEAKLEY 1985, MOORE 1987 a, EDWARDS & STRELKOV 1988, HOWES 1987). REHM (1987) r e p o r t s h o r i z o n -
584 t a l d r i l l i n g i n s l i m h o l e s . COOPER & THRONCOSO (1988) g i v e an o v e r v i e w o f h o r i zontal w e l l completion technology.
4.8.6.2. Horizontal hole f rac tur ins Most o f t h e aforementioned r e s e r v o i r examples would a l s o be s u i t a b l e t a r g e t s f o r h o r i z o n t a l o r d r a i n h o l e d r i l l i n g i n c l u d i n g subsequent m i n i f r a c t u r i n g and o t h e r s m a l l e r - s c a l e h y d r a u l i c s t i m u l a t i o n . The c o m b i n a t i o n o f h o r i z o n t a l w e l l d r i l l i n g and v e r t i c a l o r h o r i z o n t a l f r a c t u r i n g i s c o n s i d e r e d t o be t h e o n l y a v a i l a b l e method t o enable e c o n o m i c a l l y f e a s i b l e p r o d u c t i o n r a t e s t o be a c h i e ved f r o m some t h i n sheet r e s e r v o i r s . Comments a r e o f f e r e d as f o l l o w s on h o r i z o n t a l h o l e d r i l l i n g vs. f r a c t u r i n g , h o r i z o n t a l h o l e d r i l l i n g i n c o m b i n a t i o n w i t h f r a c t u r i n g , p r e p e r f o r a t e d l i n e r c o m p l e t i o n and p r o p p a n t requirements, and mult i p l e v e r t i c a l f r a c t u r i n g o f h i g h l y - i n c l i n e d boreholes.
4.8.6.2.1. Horizontal hole drilling vs. fracturing H o r i z o n t a l w e l l s and d r a i n h o l e s can be m o d e l l e d as r e p r e s e n t i n g s t i m u l a t e d b o r e h o l e s where f r a c t u r e h e i g h t i s l i m i t e d by t h e w e l l b o r e d i a m e t e r (JOSH1 1987 a; c f . s e c t i o n 4 . 1 1 . 1 . ) . A p r o p e r l y designed h o r i z o n t a l b o r e h o l e would t h e r e f o r e be e q u i v a l e n t t o a v e r t i c a l w e l l w i t h a f u l l y p e n e t r a t i n g f r a c t u r e and i n f a c t r e p r e s e n t s a l o n g c o n t r o l l e d t u b e - l i k e i n s t e a d ' o f sheet-shaped v e r t i c a l c r a c k . A f t e r d i s c u s s i o n o f some g e n e r a l aspects o f a l t e r n a t i v e o r combination, t h e o u t l i n e as f o l l o w s c o n c e n t r a t e s on t i g h t gas r e s e r v o i r drainage, identical exposed s u r f a c e s i n c o a l seams, s i n g l e - vs. m u l t i p l e - l a y e r r e s e r v o i r s , and o r i e n t a t i o n t o n a t u r a l f r a c t u r e s and p e r m e a b i l i t y a n i s o t r o p i e s .
4.8.6.2.1.1.Alternative or combination W h i l e i n most h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n j o b s i t i s d i f f i c u l t t o obt a i n i n f i n i t e c r a c k c o n d u c t i v i t y which even decreases over time, a h o r i z o n t a l b o r e h o l e o f f e r s an a l m o s t permanent i n f i n i t e - c o n d u c t i v i t y f l u i d f l o w p a t h . I n a d d i t i o n , i n r e s e r v o i r s where t h e b o t t o m w a t e r o r t o p gas cap r e n d e r s f r a c t u r i n g d i f f i c u l t , a h o r i z o n t a l w e l l o f f e r s an a l t e r n a t i v e t o o b t a i n h i g h product i o n r a t e s w i t h o u t gas o r w a t e r coning. A h o r i z o n t a l w e l l t h e r e f o r e p r o v i d e s a v i a b l e c o m p l e t i o n o p t i o n and w i l l compete w i t h h y d r a u l i c f r a c t u r i n g i n t h e f u t u r e (JOSH1 1987 a ) . H o r i z o n t a l h o l e s can be approached as r e p r e s e n t i n g and behaving l i k e t h i n l o n g t u b e - l i k e i n f i n i t e - c o n d u c t i v i t y induced f r a c t u r e s (SNOW & HOUGH 1 9 8 8 ) . Hor i z o n t a l w e l l s p e r f o r m s a t i s f a c t o r i l y i n t h i n r e s e r v o i r s w i t h good v e r t i c a l p e r m e a b i l i t i e s , whereas i n t h i c k and/or l a y e r e d pays o r i n case o f low v e r t i c a l p e r m e a b i l i t y , s e v e r a l h o r i z o n t a l stems a r e r e q u i r e d . I n such occasions, a h i g h a n g l e w e l l o f more than 60 degrees i n c l i n a t i o n has t h e advantages o f b o t h expos i n g more b o r e h o l e l e n g t h t o t h e f o r m a t i o n and o f p e n e t r a t i n g s e v e r a l l a y e r s o f t h e r e s e r v o i r . I n case o f e f f e c t i v e s t i m u l a t i o n by h y d r a u l i c f r a c t u r i n g , there i s an i n c r e a s e d l i k e l i h o o d o f h i g h l y - d e v i a t e d w e l l s t o i n t e r s e c t m u l t i p l e h o r i zons. As t h e c o s t o f s t i m u l a t i o n i s u s u a l l y o n l y a f r a c t i o n o f t h e d r i l l i n g expenses ( c f . s e c t i o n s 2.3.4. and 3 . 7 . ) , h o r i z o n t a l o r d r a i n h o l e w e l l s s h o u l d n o t be regarded as an a l t e r n a t i v e t o h y d r a u l i c f r a c t u r i n g (VICANEK 1985), b u t emphasis s h o u l d be p u t on optimum r e s e r v o i r d e p l e t i o n by a c o m b i n a t i o n o f h o r i z o n t a l o r d r a i n h o l e d r i l l i n g and s t i m u l a t i o n e s p e c i a l l y f o r t h e enhancement o f p r o d u c t i o n f r o m l o w - p e r m e a b i l i t y pay f o r m a t i o n s . SUNG & ERTEKIN (1987) g i v e a performance comparison o f v e r t i c a l and h o r i z o n t a l f r a c t u r e s and h o r i z o n t a l b o r e h o l e s i n l o w - p e r m e a b i l i t y r e s e r v o i r s . Aspects o f l o w - p e r m e a b i l i t y r e s e r v o i r development w i t h h o r i z o n t a l w e l l s a r e a l s o d i s c u s s e d by GIGER ( 1 9 8 7 ) . DUDA & KUTSKA (1988) s t u d y gas p r o d u c t i o n f r o m t i g h t sandstone r e s e r v o i r s v i a h o r i z o n t a l b o r e h o l e s
585 by s i m u l a t i o n r u n s .
4.8.6.2.1.2. Tight gas reservoir drainage KRUYADIJK & NIKO (1988) d i s c u s s h o r i z o n t a l h o l e d r i l l i n g vs. massive hydraul i c f r a c t u r i n g f o r d r a i n i n g t i g h t n a t u r a l l y j o i n t e d gas r e s e r v o i r s . The breakeven p o i n t f o r a massive h y d r a u l i c f r a c t u r e i n t e r s e c t i n g a n a t u r a l l y j o i n t e d zone w i t h r e s p e c t t o a h o r i z o n t a l w e l l i s determined by t h e balance between e x t r a f r a c t u r e s u r f a c e and p r e s s u r e drop o v e r t h e crack w i n g i n t e r s e c t i n g t h e f r a c t u r e d i n t e r v a l . S h o r t c r a c k s r e q u i r e v e r y h i g h c o n d u c t i v i t i e s i n o r d e r t o be comp e t i t i v e w i t h h o r i z o n t a l h o l e s , and o p e r a t i o n a l disadvantages and l a c k i n f r a c t u r e d i r e c t i o n c o n t r o l a r e m a j o r concerns i n MHF s c e n a r i o s . As h o r i z o n t a l boreh o l e s can be o r i e n t e d a c c o r d i n g t o r e s e r v o i r n e c e s s i t y which t h e r e f o r e a l l o w s s e l e c t i o n o f a favourable d i r e c t i o n w i t h respect t o hydrocarbon-water-contact and/or v e r t i c a l f r a c t u r e p a t t e r n ( c f . s e c t i o n 4.8.6.1.5.), h o r i z o n t a l w e l l s are a r e a l a l t e r n a t i v e t o h y d r a u l i c f r a c t u r i n g i n t h e development o f t i g h t n a t u r a l l y j o i n t e d gas r e s e r v o i r s . I n view o f t h e l a r g e c o n t r a s t between f r a c t u r e d zone and r e s e r v o i r p e r m e a b i l i t y , w e l l b o r e l e n g t h i s o f no importance, and b o r e h o l e d e v i a t i o n angle i s s i g n i f i c a n t o n l y w i t h regard t o the wellbore surface i n t e r secting the f r a c t u r e d i n t e r v a l . Extended e f f e c t i v e f r a c t u r e l e n g t h by l i n k i n g o f n a t u r a l c r a c k s b y t h e hyd r a u l i c f r a c t u r e g i v e s r i s e t o p r o l o n g e d b i l i n e a r f l o w ( c f . s e c t i o n 4.11.3.), w i t h then f r a c t u r e d s e c t i o n c o n d u c t i v i t y h a v i n g o n l y a minimal e f f e c t on p r e s s u r e response d u r i n g t h i s f l o w p e r i o d . H o r i z o n t a l d r i l l i n g as s i d e t r a c k s o u t o f e x i s t i n g v e r t i c a l h o l e s can i n some cases a l s o be c o m p a r a t i v e l y more f e a s i b l e a t t h e b o t t o m o f t h e l i n e than r e m e d i a l g r a v e l p a c k i n g (MURPHY 1988). H o r i z o n t a l b o r e h o l e s o f optimum l e n g t h can o c c a s i o n a l l y produce a b t . t w i c e as much gas as a v e r t i c a l w e l l i n c o m b i n a t i o n w i t h a v e r t i c a l h y d r a u l i c f r a c t u r e i n b l a n k e t sandstones, whereas i n l e n t i c u l a r sands, t h e l a r g e r i n t e r s e c t i o n p l a n e o f t h e h y d r a u l i c f r a c t u r e may be more p r o f i t a b l e t h a n t h e l i m i t e d t r a j e c t o r y o f t h e hor i z o n t a l b o r e h o l e (DUDA & KUMAR 1988). I n t h e case o f an i d e a l i z e d v e r t i c a l i s o t r o p i c medium, h o r i z o n t a l w e l l s appear t o be p r e f e r a b l e t o v e r t i c a l w e l l s i n a l most a l l cases, whereas f o r reasonable v e r t i c a l a n i s o t r o p y and low p e r m e a b i l i ty, even an extended-reach h o r i z o n t a l w e l l does n o t o u t p e r f o r m a v e r t i c a l w e l l i n t e r s e c t e d by a v e r t i c a l h y d r a u l i c f r a c t u r e (MUKHERJEE & ECONOMIDES 1988).
4.8.6.2.1.3. Identical exposed surfaces in coal seams ERTEKIN, SUNG & SCHWERER (1986) and LOGAN (1988) d i s c u s s t h e comparative e f f e c t i v i t y o f v e r t i c a l h y d r a u l i c f r a c t u r e s and h o r i z o n t a l b o r e h o l e s i n c o a l seams which a r e c h a r a c t e r i z e d by a dual p o r o s i t y system c o n s i s t i n g o f m a t r i x por e s and n a t u r a l f r a c t u r e s c o m p r i s i n g f a c e and b u t t c l e a t s . W h i l e f a c e c l e a t s a r e c o n t i n u o u s t h r o u g h o u t t h e r e s e r v o i r , b u t t c l e a t s a r e d i s c o n t i n u o u s and end a t t h e f a c e c l e a t s . T h i s r e l a t i o n s h i p c r e a t e s t h e a n i s o t r o p i c a l p e r m e a b i l i t y nat u r e o f t h e c o a l seam macropore s t r u c t u r e , w i t h p e r m e a b i l i t y i n f a c e c l e a t d i r e c t i o n b e i n g c o n s i d e r a b l y l a r g e r t h a n t h a t i n b u t t c l e a t d i r e c t i o n . Some asp e c t s o f c l e a t system o r i e n t a t i o n and m u l t i p l e - w i n g b o r e h o l e s a r e o u t l i n e d as follows.
4.8.6.2.1.3.1. Cleat system orientat ion W i t h r e s p e c t t o a v e r t i c a l h y d r a u l i c f r a c t u r e t h a t has more o r l e s s random o r i e n t a t i o n t o t h e c l e a t system which i s governed by t h e s t r e s s p a t t e r n i n t h e g e o l o g i c a l sequence, h o r i z o n t a l b o r e h o l e s i n c o a l seams have t h e d e c i s i v e advantage t h a t t h e i r o r i e n t a t i o n t o f a c e and b u t t c l e a t s i s c o n t r o l l a b l e , and a l s o p e n e t r a t i o n l e n g t h o f h o r i z o n t a l b o r e h o l e s can be much e a s i e r v a r i e d t h a n propag a t i o n d i s t a n c e o f h y d r a u l i c f r a c t u r e s (ERTEKIN, SUNG & SCHWERER 1986; LOGAN 1988). V e r t i c a l h y d r a u l i c f r a c t u r e s u s u a l l y e x t e n d p a r a l l e l t o t h e maximum
586 s t r e s s o r f a c e c l e a t d i r e c t i o n and t h e r e f o r e may n o t a d e q u a t e l y a c c e s s an a n i s o t r o p i c r e s e r v o i r , and a h o r i z o n t a l b o r e h o l e p l a c e d p e r p e n d i c u l a r t o t h e f a c e c l e a t s r e p r e s e n t i n g t h e maximum p e r m e a b i l i t y d i r e c t i o n may b e a more e f f e c t i v e s t i m u l a t i o n t e c h n i q u e due t o p r o v i d i n g maximum access t o p r i m a r y f l o w c h a n n e l s . Shortand medium-radius directional d r i l l i n g techniques ( c f . section 4.8.6.1.2.) a r e f r e q u e n t l y used t o s u c c e s s f u l l y p l a c e h o r i z o n t a l d r a i n h o l e s i n t o c o a l seams. H o r i z o n t a l b o r e h o l e s d r i l l e d p e r p e n d i c u l a r l y t o t h e f a c e c l e a t d i r e c t i o n p r o v i d e t h e h i g h e s t gas p r o d u c t i o n p o t e n t i a l (ERTEKIN, SUNG & SCHWERER 1 9 8 6 ) .
4.8.6.2.1.3.2.Mult iple-wing boreholes F u r t h e r improvement o f h o r i z o n t a l w e l l d r i l l i n g i n t h i n c o a l seams c a n be achieved by m u l t i p l e boreholes c o n s i s t i n g o f several wings, w i t h a two-wing w e l l b o r e o r i g i n a l l y b e i n g a b l e t o i n f l u e n c e a drainage area t h a t i s two times l a r g e r than t h a t a f f e c t e d by a four-wing borehole d u r i n g e a r l y e x p l o i t a t i o n , whereas in l a t e p r o d u c t i o n , t h e f o u r - w i n g b o r e h o l e s y s t e m i s c h a r a c t e r i z e d b y a more u n i f o r m movement o f p r e s s u r e t r a n s i e n t due t o i t s p o s i t i o n in t h e c e n t e r o f a s q u a r e r e s e r v o i r (ERTEKIN, SUNG & SCHWERER 1986; LOGAN 1 9 8 8 ) . Coal seam t h i c k n e s s i s a more c r i t i c a l f a c t o r i n f l u e n c i n g v e r t i c a l c r a c k p e r f o r m a n c e t h a n t h a t o f a h o r i z o n t a l b o r e h o l e , because f r a c t u r e s u r f a c e a r e a i n c r e a s e s as c o a l seam t h i c k n e s s i n c r e a s e s , whereas t h e exposed s u r f a c e a r e a f o r a g i v e n - d i a m e t e r b o r e h o l e i n c r e a s e s o n l y when i t s l e n g t h i s i n c r e a s e d . C o m p a r a t i v e m o d e l l i n g based on i d e n t i c a l exposed s u r f a c e a r e a s r e v e a l s t h a t h y d r a u l i c f r a c t u r e s i n i t i a l l y p r o d u c e gas a t h i g h e r r a t e s t h a n h o r i z o n t a l b o r e h o l e s , because t h e d i s t a n c e f o r t h e gas t o t r a v e l i s s h o r t e r i n a f u l l y c o n t a i ned c r a c k t h a n i n t h e h o r i z o n t a l w e l l . I n l a t e r e x p l o i t a t i o n , d e p l e t i o n o f t h e c l e a t s y s t e m and movement o f p r e s s u r e t r a n s i e n t s t o d i s t a n c e s f a r t h e r away f r o m v e r t i c a l f r a c t u r e and h o r i z o n t a l b o r e h o l e r e s u l t s i n w e l l b o r e l e n g t h becoming t h e g o v e r n i n g p a r a m e t e r and t h e h o r i z o n t a l b o r e h o l e t h e n p e r f o r m i n g b e t t e r t h a n t h e v e r t i c a l f r a c t u r e , w i t h t h e d i f f e r e n c e i n gas p r o d u c t i o n r a t e s b e f o r e and a f t e r c r o s s o v e r a l s o r i s i n g w i t h i n c r e a s i n g c o a l seam t h i c k n e s s .
4.8.6.2.1.4.Single- vs. mult iple-layer reservoirs W h i l e h o r i z o n t a l w e l l s a r e v e r y e f f e c t i v e and s u p e r i o r i n p e r f o r m a n c e t o b o t h u n s t i m u l a t e d and v e r t i c a l l y f r a c t u r e d v e r t i c a l b o r e h o l e s i n t h i n s i n g l e l a y e r r e s e r v o i r s , p a y zones w i t h h i g h v e r t i c a l p e r m e a b i l i t y and p r o s p e c t i v e i n t e r v a l s w i t h g a s - and w a t e r - c o n i n g p r o b l e m s , v e r t i c a l w e l l s o f e i t h e r f r a c t u r e d o r unstimulated nature are better solutions f o r t h i n multiple-layer reservoirs and p a y zones w i t h l o w v e r t i c a l p e r m e a b i l i t y where h o r i z o n t a l b o r e h o l e s a r e uns u i t a b l e (JOSH1 1986 a ) . V e r t i c a l w e l l s w h i c h a r e s t i m u l a t e d b y v e r t i c a l f r a c t u r e s a r e g e n e r a l l y more e f f e c t i v e t h a n u n f r a c t u r e d b o r e h o l e s , and m u l t i p l e v e r t i c a l f r a c t u r i n g o f h o r i z o n t a l w e l l s c a n be a c o m p e t i t i v e p r o c e d u r e in i n t e r m e d i a t e r e s e r v o i r s . I f the length o f a horizontal borehole is s i g n i f i c a n t l y l a r g e r than pay thickness, then horizontal w e l l production i s i n the long time the same as t h a t o b t a i n e d f r o m a f u l l y p e n e t r a t i n g i n f i n i t e - c o n d u c t i v i t y v e r t i c a l fracture. V e r t i c a l p e r m e a b i l i t y d i s t r i b u t i o n has c o n s i d e r a b l e i m p a c t on p r o d u c t i v i t y a u g m e n t a t i o n t h r o u g h h o r i z o n t a l w e l l s , w i t h l o w and h i g h v e r t i c a l p e r m e a b i l i t y r e d u c i n g and e n h a n c i n g h o r i z o n t a l b o r e h o l e p r o d u c t i v i t y , r e s p e c t i v e l y . A h o r i z o n t a l w e l l r e p l a c e s more v e r t i c a l b o r e h o l e i n a t h i n - l a y e r r e s e r v o i r t h a n i n a t h i c k - b e d p a y zone, whereas in c o n t r a s t t o h o r i z o n t a l w e l l s , s l a n t b o r e h o l e s ( c f . s e c t i o n 4.8.6.2.2.2.) a r e more p r o d u c t i v e i n t h i c k r e s e r v o i r s t h a n i n t h i n p a y zones. Thus m a x i m i z a t i o n o f h y d r o c a r b o n p r o d u c t i o n c a n b e p e r f o r m e d b y sel e c t i o n between v e r t i c a l , s l a n t and h o r i z o n t a l w e l l s d e p e n d i n g on r e s e r v o i r t h i c k n e s s and a n i s o t r o p y .
587
4.8.6.2.1.5. Orientat ion to natural fractures and permeab i 1 it y an i sot r op i es I n terms o f t h e g e o m e t r i c a l r e l a t i o n s h i p between n a t u r a l f r a c t u r e s and p e r m e a b i l i t y a n i s o t r o p i e s i n t h e r e s e r v o i r f o r m a t i o n , h o r i z o n t a l w e l l s can be v e r y e a s i l y d r i l l e d i n any d i r e c t i o n t o a d j u s t t o t h e optimum c o n f i g u r a t i o n o f n o r mal p e n e t r a t i o n o f t h e w e l l b o r e t r a j e c t o r y through t h e n a t u r a l j o i n t s . The p r o p a g a t i o n of h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s i s i n most cases d i r e c t i o n a l l y det e r m i n e d by t h e e x i s t i n g g e o s t r e s s f i e l d , and w i t h o u t thermal i n f l u e n c e , o n l y a l t e r e d - s t r e s s f r a c t u r i n g (WARPINSKI & BRANAGAN 1988; c f . s e c t i o n 4.9.4.1.3.1.) can g i v e r i s e t o d i f f e r e n t o r i e n t a t i o n s o f crack e x t e n s i o n and p e r m i t t h e opt i o n t o model t h e a n g l e o f i n t e r s e c t i o n o f n a t u r a l c r a c k s and h y d r a u l i c f r a c t u r e s . A l t e r e d - s t r e s s f r a c t u r i n g may u l t i m a t e l y even l e a d t o i n t e r c o n n e c t e d n e t works c o n s i s t i n g o f c r a c k s p r o p a g a t i n g i n a l t e r n a t i n g s t a n d a r d d i r e c t i o n and i n t h a t p e r p e n d i c u l a r t o i t . The o r i e n t a t i o n o f b o t h h y d r a u l i c f r a c t u r e s a n d / o r hor i z o n t a l w e l l s i n hydrocarbon r e s e r v o i r s i s a l s o i m p o r t a n t w i t h r e s p e c t t o t e c t o n i c a l d i p d i r e c t i o n as w e l l as sand body geometry and e x t e n s i o n .
4.8.6.2.2.Horizontal hole dri 1 1 ing in combination with fracturing SUNG & ERTEKIN (1987) p r e s e n t a performance comparison o f v e r t i c a l and h o r i z o n t a l h y d r a u l i c f r a c t u r e s and h o r i z o n t a l b o r e h o l e s i n l o w - p e r m e a b i l i t y r e s e r v o i r s . The r e s u l t s o f a s u i t e o f numerical s i m u l a t i o n e x e r c i s e s show t h a t f o r equal s u r f a c e area, t h e p r o d u c t i o n from h o r i z o n t a l b o r e h o l e s i s expected t o be g r e a t e r than t h a t from h y d r a u l i c f r a c t u r e s i n v e r t i c a l w e l l s . P r o d u c t i o n r a t e s f r o m b o t h h o r i z o n t a l w e l l s and h y d r a u l i c f r a c t u r e s r i s e w i t h i n c r e a s i n g r e s e r v o i r p e n e t r a t i o n l e n g t h , b u t achievement o f g r e a t e r i n t e r s e c t i o n l e n g t h s i s t e c h n i c a l l y more f e a s i b l e i n case o f h o r i z o n t a l b o r e h o l e s . Some comments a r e g i v e n as f o l l o w s on r e s e r v o i r t h i c k n e s s and p e r m e a b i l i t y , and w e l l b o r e angle b u i l d u p t e c h n o l o g y and s l a n t h o l e s .
4.8.6.2.2.1. Reservoir thickness and permeabi 1 i t y Concerning r e s e r v o i r t h i c k n e s s and p e r m e a b i l i t y , t h e s u p e r i o r i t y o f h o r i z o n t a l w e l l s becomes more apparent i n t h i n n e r and t i g h t e r f o r m a t i o n s , whereas t h e e f f e c t i v i t y o f h y d r a u l i c f r a c t u r e s improves i n t h i c k e r pay zones. I n case o f ani s o t r o p i c r e s e r v o i r s , h o r i z o n t a l b o r e h o l e s s h o u l d be d r i l l e d and h y d r a u l i c f r a c t u r e s s h o u l d be propagated p e r p e n d i c u l a r l y t o t h e l a r g e s t p e r m e a b i l i t y d i r e c t i o n . Under comparable c o n d i t i o n s , t h e p r o d u c t i o n r a t e f r o m a h o r i z o n t a l crack system i s g r e a t e r than t h a t f r o m a v e r t i c a l f r a c t u r e system, because a more u n i f o r m p r e s s u r e d e c l i n e o v e r t h e e n t i r e r e s e r v o i r caused by t h e h o r i z o n t a l c r a c k helps achieving b e t t e r drainage. The best, b u t c e r t a i n l y a l s o t h e most expensive s o l u t i o n , however, i s a comb i n a t i o n o f h o r i z o n t a l d r i l l i n g and h y d r a u l i c f r a c t u r i n g t h a t p r o v i d e s t h e maximum d r a i n a g e area i n t h e r e s e r v o i r . P a r t i c u l a r l y s u i t a b l e t a r g e t s f o r t h i s dual approach a r e o i l - b e a r i n g c h a l k s ( c f . s e c t i o n 4 . 5 . 4 . ) and g a s - b e a r i n g s h a l e s ( c f . s e c t i o n 4 . 4 . 4 . ) . MERCER, PRATT & YOST (1988) document t h a t two h y d r a u l i c a l l y f r a c t u r e d o r f o u r u n s t i m u l a t e d h o r i z o n t a l w e l l s can produce 1 . 5 t i m e s as much gas as 16 u n t r e a t e d v e r t i c a l w e l l s i n case o f b o t h i n f i l l d r i l l i n g and v i r g i n r e s e r v o i r development ( c f . s e c t i o n 4 . 4 . 3 . 6 . ) . Comparative performance eval u a t i o n o f h o r i z o n t a l and v e r t i c a l w e l l s i s c a r r i e d o u t by BORNER, HALDORSEN, HARRISON, HOPWOOD & MACDONALD (1988); K I N G & ERTEKIN (1988) and WOOTEN, MUKHERJEE & MACH ( 1 9 8 8 ) . S p e c i a l f e a t u r e s o f h y d r a u l i c f r a c t u r i n g o f h i g h l y - d e v i a t e d o r h o r i z o n t a l b o r e h o l e s a r e d i s c u s s e d i n c h a p t e r 4.11. F i e l d aspects o f hydraul i c f r a c t u r i n g i n h o r i z o n t a l w e l l s e c t i o n s a r e a l s o discussed by G I G E R , R E I S S & JOURDAN (1984) ; YOST, OVERBEY, SALAMY, OKOYE & SARADJI (1987) ; PEMPER, FLECKER, McWHIRTER & OLIVER (1988); SOLIMAN, HUNT & EL RABAA (1988) and SPREUX, GEORGES
588 & LESS1 ( 1 9 8 8 ) .
DOSCHER, Y I N G & KOSTURA (1988) d i s c u s s t h e u t i l i z a t i o n o f s p e c i a l l y f r a c t u r e d h o r i z o n t a l w e l l s t o improve s t e a m - d r i v e enhanced r e c o v e r y o f h i g h - v i s c o s i t y hydrocarbons. P r o p e r l y p o s i t i o n e d and f r a c t u r e d h o r i z o n t a l p r o d u c t i o n w e l l s p e r m i t commercial e x p l o i t a t i o n o f many heavy o i l and bitumen r e s e r v o i r s which o t h e r w i s e would be c o m p l e t e l y e c o n o m i c a l l y u n f e a s i b l e .
4.8.6.2.2.2.Wellbore angle buildup technology and slant holes Apart from v e r t i c a l d r i l l i n g a t the beginning o f h o r i z o n t a l w e l l generation, b u i l d i n g o f a n g l e i n s h a l l o w s l a n t h o l e s can be a c c e l e r a t e d by i n c l i n e d r i g pos i t i o n (VRIELINK & HIPPMAN 1988). Using s l a n t w e l l technology, the required w e l l spacing u n i t i s achieved w i t h minimum s u r f a c e l a n d usage, optimum h o l e qual i t y and minimum e n v i r o n m e n t a l impact, w i t h s l a n t w e l l d r i l l i n g t h e r e f o r e p r o v i d i n g an a t t r a c t i v e a l t e r n a t i v e t o c o n v e n t i o n a l d i r e c t i o n a l m u l t i w e l l pad p r o j e c t s . S l a n t borehole d r i l l i n g i s p a r t i c u l a r l y s u i t a b l e f o r shallow r e s e r v o i r s w i t h reduced i n t e r w e l l spacing, h i g h f a c i l i t i e s and t i e - i n c o s t p e r b o r e h o l e head, h i g h o p e r a t i n g and maintenance c o s t p e r l o c a t i o n , and r e q u i r e m e n t t o d r i l l d e v i a t e d s t r a i g h t h o l e s i n o r d e r t o m i n i m i z e t u b u l a r wear (RICHARDSON 1985). M o d e l l i n g o f spud i n c l i n a t i o n , spud azimuth and s u r f a c e l o c a t i o n can be done f o r o p t i m i z a t i o n o f s u r f a c e d i s t a n c e , w e l l b o r e c u r v a t u r e and downhole separ a t i o n . V e r t i c a l l y spudded w e l l s a r e n o t t o be c r o s s - d r i l l e d i n o r d e r t o e l i m i n a t e t h e c o l l i s i o n and i n t e r p r e t a t i o n danger. R i g i n c l i n a t i o n i n s l a n t h o l e d r i l l i n g i s u s u a l l y a b t . 10 - 30 degrees, b u t can i n extreme cases a l s o r e a c h 45 - 60 degrees.
4.8.6.2.3. Preperforated 1 iner
corn let ion and proppant requirement
Long h o r i z o n t a l w e l l s a r e t y p i c a l l y completed w i t h p r e p e r f o r a t e d l i n e r s t o p r e s e r v e h o l e i n t e g r i t y (YOST, OVERBEY, WILKINS & LOCKE 1988). The disadvantage o f t h i s type o f completion i s the associated r i s k o f p u l l i n g the l i n e r a t a l a t e r stage o f p r o d u c t i o n h i s t o r y and r e - r u n n i n g and cementing a c a s i n g s t r i n g such t h a t s e l e c t i v e placement o f f r a c t u r i n g f l u i d s can be accomplished. An a l t e r n a t i v e approach i s zone i s o l a t i o n performed by i n s t a l l a t i o n o f e x t e r n a l cas i n g packers and p o r t c o l l a r s as an i n t e g r a l p a r t o f a c a s i n g s t r i n g r u n a l o n g t h e h o r i z o n t a l s e c t i o n . Such a c o m p l e t i o n arrangement p r o v i d e d s t i m u l a t i o n i n t e r v a l s w i t h ready-made p e r f o r a t i o n s i n j e c t i n g f r a c t u r i n g f l u i d s i n t o an openh o l e s t i m u l a t i o n c o n d i t i o n b e h i n d p i p e . The l a t t e r method a v o i d s t h e problems o f f o r m a t i o n damage a s s o c i a t e d w i t h cementing and e l i m i n a t e s t h e need f o r t u bing-conveyed p e r f o r a t i n g o f numerous t r e a t m e n t i n t e r v a l s . Comparison o f d i f f e r e n t f r a c t u r e o p e r a t i o n designs i n h o r i z o n t a l w e l l s t r a v e r s i n g s h a l e gas r e s e r v o i r s i n d i c a t e s t h a t carbon d i o x i d e s t i m u l a t i o n can be more than f o u r t i m e s b e t t e r than n i t r o g e n f r a c t u r i n g i n i t i a l l y and can l a s t t w i c e as l o n g p r i o r t o c r a c k c l o s u r e ( c f . s e c t i o n 4 . 4 . 4 . 2 . ) . The d e n s i t y cont r a s t between n i t r o g e n and carbon d i o x i d e may have caused t h e p r o p a g a t i o n o f more and w i d e r f r a c t u r e s w i t h carbon d i o x i d e r e s u l t i n g i n h i g h improvement r a t i o s . Proppants a r e d e f i n i t e l y r e q u i r e d f o r f r a c t u r i n g o f l o w - s t r e s s s h a l e gas r e s e r v o i r s t o s u s t a i n gas p r o d u c t i o n beyond b a s e l i n e c o n d i t i o n s .
4.8.6.2.4.Mu1 t ip le vertical f rac tur ins o f highly-inclined boreholes A s u i t a b l e t e c h n i q u e t o maximize d r a i n a g e e f f e c t i v i t y o f h i g h l y - i n c l i n e d t o h o r i z o n t a l b o r e h o l e s i s m u l t i p l e v e r t i c a l f r a c t u r i n g (STRUBHAR, FITCH & GLENN 1974; STRUBHAR & GLENN 1974; c f . s e c t i o n 4.11.) which i n t e g r a t e s e x t e n s i o n o f
589 geometry and f l o w c a p a c i t y o f e x i s t i n g c r a c k s as w e l l as c r e a t i o n o f new f r a c t u r e s (OVERBEY, YOST & WILKINS 1988). Using m u l t i p l e f r a c t u r e s i n a h o r i z o n t a l w e l l , t h e o r e t i c a l l y t h e same amount o f t o t a l p r o d u c t i o n w i l l be r e c o v e r e d a t an a c c e l e r a t e d r a t e and t h u s economical payout o f t h e b o r e h o l e w i l l be o b t a i n e d i n a s h o r t e r p e r i o d o f t i m e (AUSTIN, ROSE & SCHUH 1988). Aspects o f s e t s o f p a r a l l e l f r a c t u r e planes, n a t u r a l f r a c t u r e i n t e r s e c t i o n , m u l t i p l e - vs. l i m i t e d - e n t r y f r a c t u r i n g , f r a c t u r e morphology and o r i e n t a t i o n , and i n f l u e n c e o f i n j e c t i o n r a t e s a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.8.6.2.4.1.Sets of parallel fracture planes The o b j e c t o f t h e m u l t i f r a c t u r e c o m p l e t i o n concept i s t o o b t a i n g r e a t e r f r a c t u r e area t h r o u g h c l o s e r c r a c k spacing than c o u l d be achieved f r o m t h e same numb e r o f w e l l s w i t h s i n g l e v e r t i c a l f r a c t u r e s (PASINI & OVERBEY 1969). O p t i m i z a t i o n o f m u l t i p l e s t i m u l a t i o n i n h i g h l y - d e v i a t e d b o r e h o l e s r e q u i r e s t h e azimuth o f t h e w e l l b o r e t o be a t h i g h a n g l e ( p r e f e r e n t i a l l y p e r p e n d i c u l a r ) t o t h e f r a c t u r e azimuth. Some aspects o f a n g u l a r r e l a t i o n s h i p between w e l l b o r e t r a j e c t o r y and induced c r a c k s as w e l l as f r a c t u r e spacing vs. d r a i n a g e e f f e c t i v i t y a r e o u t l i n e d as f o l l o w s .
4.8.5.2.4.1.1. Angular relationship between we1 lbore trajectory and induced cracks There a r e a l l stages o f t r a n s i t i o n o f d i s t a n c e s o f t h e i n d i v i d u a l f r a c t u r e s and a n g u l a r r e l a t i o n s h i p t o t h e w e l l b o r e t r a j e c t o r y between t h e end p o i n t s o f a s i n g u l a r v e r t i c a l crack p r o p a g a t i n g p a r a l l e l t o t h e h o r i z o n t a l b o r e h o l e t r a j e c t o r y i n case o f o r i e n t a t i o n o f t h e l a t t e r p a r a l l e l t o f r a c t u r e azimuth, and mult i p l e p a r a l l e l v e r t i c a l cracks extending perpendicularly t o the h o r i z o n t a l boreh o l e t r a j e c t o r y i n case o f o r i e n t a t i o n o f t h e l a t t e r o r t h o g o n a l t o f r a c t u r e a z i muth. P r e d i c t i o n o f t h e induced f r a c t u r e azimuth can be made f r o m s t r a i n r e l a x a t i o n o f o r i e n t e d c o r e s ( c f . s e c t i o n 6.2.2.3.3.4.), borehole breakout elongation a n a l y s i s ( c f . s e c t i o n 6 . 2 . 1 . 8 . ) , o r d i r e c t i o n a l d e t e r m i n a t i o n o f c r a c k s generat e d d u r i n g m i c r o - o r m i n i f r a c t u r e c a l i b r a t i o n t r e a t m e n t s i n a d j o i n i n g o r preced i n g v e r t i c a l w e l l s (AUSTIN, SCHUH & ROSE 1988; c f . s e c t i o n 4 . 8 . 2 . ) .
4.8.6.2.4.1.2. Fracture spacing vs. drainage effectivity The d i f f e r e n t f r a c t u r e s have t o be k e p t p a r a l l e l and s e p a r a t e i n o r d e r n o t t o d e t e r i o r a t e t h e d r a i n a g e e f f e c t by c r a c k i n t e r f e r e n c e and i n t e r s e c t i o n , w i t h t h e l a t t e r drawback b e i n g a b l e t o be avoided by s u f f i c i e n t l a t e r a l spacing o f t h e i n d i v i d u a l f r a c t u r e s and c a r e f u l c o n t r o l o f t h e i r azimuth, i n c l i n a t i o n and l e n g t h f o r making sure t h a t c o n t a c t o f two independent c r a c k s does n o t o c c u r (ROEGIERS & DETOURNAY 1988). Several v e r t i c a l i n d e p e n d e n t l y p r o d u c i n g c r a c k s c r o s s i n g a s i n g l e d e v i a t e d w e l l b o r e a r e much more economical than i n d i v i d u a l v e r t i c a l f r a c t u r e s i n separate v e r t i c a l b o r e h o l e s and r e p r e s e n t t h e most f e a s i b l e c o m b i n a t i o n o f h o r i z o n t a l w e l l d r i l l i n g and h y d r a u l i c f r a c t u r i n g , and c e r t a i n l y a l t h o u g h b e i n g t h e most expensive r e s e r v o i r t r e a t m e n t , d e f i n i t e l y r e p r e s e n t t h e most e f f e c t i v e means o f d r a i n a g e m a x i m i z a t i o n . F i e l d examples of m u l t i p l e v e r t i c a l f r a c t u r e s i n h o r i z o n t a l b o r e h o l e s and enhanced p r o d u c t i v i t y m o d e l l i n g i n comparison t o s i n g l e - f r a c t u r e d v e r t i c a l w e l l s a r e p r e s e n t e d b y STRUBHAR, FITCH & GLENN (1974); GIGER, COMBE & R E I S S (1983); GIGER (1985); KARCHER, GIGER & COMBE (1986); ANDERSEN, HANSEN & FJELDGAARD (1988) and OVERBEY, YOST & WILKINS ( 1 9 8 8 ) .
590
4.8.6.2.4.2. Natural fracture intersect ion Similar aspects apply f o r optimization of drainage of n a t u r a l l y fra c ture d res e r v o i r s where e f f e c t i v e depletion i s reached when a h o r i z o n t a l well i s d r i l l e d perpendicularly t o the o r i e n t a t i o n of the n a t u r a l cracks thereby i n t e r s e c t i n g a whole bundle of p a r a l l e l p r e- ex i s t i n g f r a c t u r e s (JOSH1 1987; c f . s e c t i o n 4.8.6.1.5.1.). Influences of natural cracks on horizontal borehole performance are a l s o discussed by CARVALHO & ROSA (1988). Insights t o hydraulic f r a c t u r i n g of horizontal wells i n n a t u r a l l y j o i n t ed formations a r e a l s o given by LAYNE & SIRINARDANE (1988).Mu1 t i p l e v e r t i c a l f r a c t u r e s p r o p a g a t i n g from h o r i z o n t a l boreholes a r e a l s o the optimum s o l u t i o n f o r gas drainage from coal seam r e s e r v o i r s because of s u i t a b l e combination of b o t h elements, as the former a n d the l a t t e r give r i s e t o higher production r a t e s i n e a r l y a n d l a t e e x p l o i t a t i o n s t a ges, r e s p e c t i v e l y (ERTEKIN, SUNG & SCHWERER 1986; c f . se c tion 4.4.3.6.). O p t i m i z a t i o n of number a n d s i z e of multiple hydraulic f r a c t u r e s i n t e r s e c t i n g horizontal wells can be performed w i t h r es er voir simulators based on the ne t present value concept ( M E N G & BROWN 1987; BALEN, MENG & ECONOMIDES 1988; MUKHERJEE & ECONOMIDES 1988; c f . s ect i o n s 1.4.12.2. and 4.8.11.4.). I n horizontal w e l l s, the sum of the maximum n et present values of the individual m ultiple f r a c t u r e s must be s u b s t a n t i a l l y l a r g e r than the net present value of a v e r t i c a l crack i n t e r s e c t i n g a v e r t i c a l borehole i n order to be comparatively more p r o f i table.
4.8.6.2.4.3. Mu1 t iple- vs. limited-entry f ractur ins AUSTIN, R O S E & SCHUH (1988) present simultaneous m ultiple -e ntry hydraulic f r a c t u r i n q i n h o r i z o n t a l wel l s . Limited-entry pe rfora tion a t several crack i n i t i a t i o n points allows equal placement of proppants a n d f l u i d s i n one stimulation treatment, and i n carbonates, proppant f r a c t u r i n g can a l s o be replaced by acid f r a c t u r i n g i f economically f e a s i b l e . The number of crack i n i t i a t i o n points can be maximized i f the horizontal borehole i s orie nte d perpendicularly t o the azimuth of the induced f r a c t u r e s . Unless the s p e c i f i c re se rvoir re quire s something e l s e , the individual crack i n i t i a t i o n points should ge ne ra lly be spaced 250 - 300 f t a p a r t . Before the actual treatment i s c a r r i e d out, each s e t of perf o r a t i o n s a t each f r a c t u r e i n i t i a t i o n point should be broken down a n d a s t a b l e in j e c t i o n r a t e e s t a b l i s h e d .
4.8.6.2.4.4.Fracture morphology and orientation Si m i l a r l y as multiple v e r t i c a l hydraulic f r a c t u r e s emanating from highly-inc li n e d t o horizontal boreholes represent s u b sta ntia l r e s e r v o i r drainage improvement with r e sp e c t t o s i n g u l ar cracks f r a c t u r e s i n t e r s e c t i n g deviated wells o r even v e r t i c a l boreholes in some cas es , success proba bility of e xplora tion a n d appraisal of n a t u r a l l y f r act u r ed pay zones can be considerably increased i f d i r e c t i o n a l iqells ar e d r i l l e d perpendicularly t o the orie rtta tion of the natural cracks instead of normal boreholes ( A G U I L E R A 1983; c f . s e c t i o n s 4 . 8 . 6 . 1 . 5 . 1 . a n d
4.8.6.2.4.2.).
BEN NACEUR & R O E G I E R S (1988) i n v es t i g at e simultaneous propagation of m u 1 t i ple v e r t i c a l hydraulic f r a c t u r e s . Di s t i n ct i o n can be made between c o l l i n e a r s e t s emanating from v e r t i c a l boreholes a n d p a r a l l e l s e t s i n t e r s e c t i n g horizont a l wellbores. The e l a s t i c i n t e r a c t i o n between c o l l i n e a r f r a c t u r e s i n v e r t i c a l wells r e s u l t s i n l a r g e r f r a c t u r e widths o r lower apparent s t i f f n e s s e s , whereas the consequences f o r p a r a l l e l f r a c t u r e s in horizontal wells a r e opposite. Theref o r e individual f r a c t u r e s a r e i n v ar i ab l y shorte r in the f i r s t case ( c f . a l s o s e c t i o n 4.2.2.1.3.4.).
Se l e c t i v e multiple f r a c t u r i n g w i t h introduction of several e q u i d i s t a n t p l a n p a r a l l e l f r a c t u r e s from horizontal drainholes can considerably improve producti-
591 v i t y index (ANDERSEN, HANSEN & FJELDGAARD 1988). W h i l e i n t h e e a r l y stage o f exp l o i t a t i o n t h e r e i s no i n t e r f e r e n c e between t h e c r a c k s , a f t e r some p e r i o d of p r o d u c t i o n t h e f r a c t u r e planes s t a r t i n t e r f e r i n g , w i t h t h e degree o f i n t e r f e rence s t e a d i l y i n c r e a s i n g .
4.8.6.2.4.5.Influence o f injection rates OVERBEY, YOST & WILKINS (1988) and YOST, OVERBEY, W I L K I N S & LOCKE (1988) s t u dy m u l t i p l e v e r t i c a l h y d r a u l i c f r a c t u r e o r i g i n i n h o r i z o n t a l b o r e h o l e s t r a v e r s i n g s h a l e gas r e s e r v o i r s t h a t a r e e x t e n s i v e l y n a t u r a l l y j o i n t e d . N u l t i p l e hyd r a u l i c f r a c t u r e s can be propagated i n a w e l l b o r e which has been completed t o p r o v i d e adequate access t o m u l t i p l e n a t u r a l cracks. Low i n j e c t i o n r a t e s d u r i n g a hydraulic f r a c t u r i n g operation allow the propagation o f n a t u r a l cracks w i t h a low-angle r e l a t i o n s h i p t o t h e p r i n c i p a l s t r e s s o r i e n t a t i o n , whereas h i g h e r i n j e c t i o n r a t e s g e n e r a l l y r e s u l t i n i n d u c i n g f r a c t u r e s c o n t r o l l e d by t h e s t r e s s f i e l d . Some comments a r e o f f e r e d as f o l l o w s on d e n d r i t i c f r a c t u r i n g and s t r e s s f i e l d r e l a t i o n s h i p s as w e l l as i n f l a t i o n and b a l l o o n i n g o f n a t u r a l c r a c k s .
4.8.6.2.4.5.1. Dendr i t ic fracturing and s t r e s s field relationships Low i n j e c t i o n r a t e s thus l e a d t o development o f a d e n d r i t i c f r a c t u r e p a t t e r n by p r e f e r e n t i a l l y e n t e r i n g and b a l l o o n i n g p r e - e x i s t i n g n a t u r a l c r a c k s ( c f . sect i o n s 4.5.4.4.3. and 4 . 8 . 8 . 4 . ) , whereas h i g h pumping r a t e s g i v e r i s e t o abandonment o f t h e o r i g i n a l j o i n t s c r e a t e d by t h e p a l a e o s t r e s s f i e l d and g e n e r a t i o n o f new f r e s h f r a c t u r e s t h a t a r e i n accordance t o t h e a c t u a l s t r e s s f i e l d i n case o f changes o f palaeomagnetical and p a l a e o t e c t o n i c a l s t r e s s s i t u a t i o n between c r e a t i o n o f n a t u r a l c r a c k s and i n d u c t i o n o f h y d r a u l i c f r a c t u r e s . I n t h e s p e c i a l case o f n e i t h e r r o t a t i o n n o r t i l t i n 9 o f t h e c r u s t a l b l o c k s between f o r m a t i o n o f n a t u r a l j o i n t s and t r i g g e r i n g o f h y d r a u l i c f r a c t u r e s , a l s o h i g h e r i n j e c t i o n r a t e s can o n l y r e s u l t i n l a t e r a l and l o n g i t u d i n a l expansion o f p r e - e x i s t i n g n a t u r a l c r a c k s i n c l u d i n g some breakthroughs between i n d i v i d u a l j o i n t s which a l s o leads t o d e n d r i t i c f r a c t u r i n g . D i s t r i b u t i o n o f t h e propagated f r a c t u r e s i s more random a t low than a t h i g h i n j e c t i o n r a t e s , w i t h t h e l a t t e r seeming t o t r i g g e r a u t o - s e l e c t i o n by t h e format i o n c o n c e r n i n g spacing and d i s t r i b u t i o n o f c r a c k s . M u l t i p l e h y d r a u l i c v e r t i c a l f r a c t u r e s can be induced f r o m a h o r i z o n t a l w e l l b o r e d u r i n g a s i n g l e pumping e v e n t . Even though s t r e s s r a t i o s i n d i c a t e t h a t t h e i n v e s t i g a t e d area i s n e a r l y t e c t o n i c a l l y r e l a x e d , proppants a r e s t i l l needed t o m a i n t a i n permeable f l o w paths f o r more than s e v e r a l weeks. D i f f e r e n t c l o s u r e p r e s s u r e s h a v i n g been r e corded i n d i f f e r e n t s t i m u l a t i o n t r e a t m e n t s can be t h e consequence o f i n d u c t i o n o f d i f f e r e n t f r a c t u r e s h a v i n g n e a r - a d j a c e n t angles, and subsequent p r e s s u r e drops i n d i c a t e t h a t some o f t h e c r e a t e d c r a c k s were c l o s i n g o f f .
4.8.6.2.4.5.2.Inflation and ballooning o f natural cracks Improvement o f gas r e c o v e r y e f f i c i e n c y o f h i g h l y - i n c l i n e d o r h o r i z o n t a l boreh o l e s i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s by e x t e n s i o n o f geometry and f l o w capac i t y o f e x i s t i n g c r a c k s and c r e a t i o n o f new h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s can be b e s t accomplished by s t i m u l a t i n g t h e n a t u r a l j o i n t s which e x i s t i n t h e r e s e r v o i r by i n f l a t i n g them w i t h a non-damaging f l u i d and p r o p p i n g t h e b a l l o o n e d f r a c t u r e s t o m a i n t a i n t h e enhanced f l o w c a p a c i t y and t o induce a d d i t i o n a l f r a c t u r e s b y i n c r e a s i n g i n j e c t i o n r a t e (OVERBEY, YOST & WILKINS 1988; c f . a l s o sect i o n 4.8.8.6.). M u l t i p l e n a t u r a l c r a c k s e t s a r e generated by b o t h normal o r b l o c k f a u l t i n g and t h r u s t i n g . F i e l d e x p e r i e n c e r e v e a l s t h a t d u r i n g s t i m u l a t i o n operations, two s e t s o f n a t u r a l f r a c t u r e s were opened and propagated, and i n d u ced c r a c k s were c r e a t e d a l o n g a t h i r d d i r e c t i o n c o n t r o l l e d by t h e r e g i o n a l s t r e s s f i e l d . M u l t i p l e - o r i e n t e d h y d r a u l i c f r a c t u r e s can be generated f r o m a ho-
592 r i z o n t a l w e l l b o r e under openhole c o n d i t i o n s where t h e and induced c r a c k s f o r m an a n g l e o f a b t . 15 degrees.
orientations
of
natural
4 . 8 . 7 . Uptake capacity enhancement o f injection wells M i n i f r a c t u r i n g and o t h e r s m a l l e r - s c a l e s t i m u l a t i o n a r e a l s o regarded t o be s u i t a b l e means t o i n c r e a s e t h e s u r f a c e o f t h e f o r m a t i o n s u r r o u n d i n g t h e b o r e h o l e f o r enhanced uptake o f water, t e n s i d , polymer, a c i d o r steam i n i n j e c t i o n w e l l s , and t o a m e l i o r a t e t h e c o n d u c t i v i t y o f n a t u r a l l y j o i n t e d r e s e r v o i r s by s m a l l e r - o r l a r g e r - s c a l e a r t i f i c i a l f r a c t u r i n g . The most s i g n i f i c a n t i t e m s o f a p p l i c a t i o n o f i n j e c t i o n - w e l l uptake c a p a c i t y improvement by h y d r a u l i c f r a c t u r i n g a r e o i l - f i e l d w a t e r - i n j e c t i o n w e l l s and w a s t e - l i q u i d d i s p o s a l w e l l s .
4 . 8 . 7 . 1 . Oil-field water-injection wells The most abundant u t i l i z a t i o n o f uptake c a p a c i t y a m e l i o r a t i o n o f i n j e c t i o n w e l l s by h y d r a u l i c p r o p p a n t s m a l l - o r l a r g e - s c a l e f r a c t u r i n g i s t h e s t i m u l a t i o n o f o i l - f i e l d w a t e r - i n j e c t i o n w e l l s . D i s t i n c t i o n can be made between c o n t r o l l e d f r a c t u r i n g by a p r o p e r l y designed h y d r a u l i c s t i m u l a t i o n t r e a t m e n t and accident a l f r a c t u r i n g by i n j e c t i o n o p e r a t i o n above f o r m a t i o n f r a c t u r e g r a d i e n t .
4 . 8 . 7 . 1 . 1 . Control led f ractur ins I n many cases, t h e p e r m e a b i l i t y o f sandstone r e s e r v o i r s i s p o o r e r i n t h e a q u i f e r zone s u r r o u n d i n g t h e o i l f i e l d than i n t h e o i l - b e a r i n g column due t o e a r l y o i l i m m i g r a t i o n i n t o embryonic s t r u c t u r e s which r e s u l t e d i n r e t a r d e d d i a genesis and t h u s b e t t e r p e r m e a b i l i t y p r e s e r v a t i o n i n t h e o i l - s a t u r a t e d b e l t w i t h r e s p e c t t o t h e w a t e r - c o n t a i n i n g m a r g i n a l seam t h a t was o f t e n more t i g h t l y cemented. T h e r e f o r e i n many f i e l d s t h e o i l - p r o d u c t i o n w e l l s do n o t o r o n l y t o s u b o r d i n a t e amounts r e q u i r e s t i m u l a t i o n , whereas t h e w a t e r - i n j e c t i o n w e l l s need h y d r a u l i c f r a c t u r i n g i n o r d e r t o a m e l i o r a t e t h e i r uptake c a p a c i t y and t h u s enhance r e s e r v o i r p r e s s u r e improvement and maintenance by i n c r e a s i n g i n j e c t i o n r a t e s . A p a r t f r o m enhanced r e s e r v o i r d e p l e t i o n by secondary r e c o v e r y , w a t e r f l o o d i n g a m e l i o r a t i o n i s an i m p o r t a n t measure f o r c o u n t e r a c t i n g pay recompaction and r e s u l t i n g o i l - f i e l d subsidence ( c f . s e c t i o n 4 . 5 . 4 . 1 . 6 . ) . A f t e r discussion o f some g e n e r a l aspects, a few p o i n t s o f p r o p p a n t s i z e and r e s e r v o i r p r e s s u r e as w e l l as d e c r e a s i n g i n - s i t u s t r e s s by c o l d w a t e r i n j e c t i o n , and sweep e f f i c i e n c y vs. i n j e c t i v i t y a r e o u t l i n e d .
4 . 8 . 7 . 1 . 1 . 1 . General aspects An a r t i f i c i a l f r a c t u r i n g o p e r a t i o n o f s m a l l e r o r l a r g e r s i z e performed i n a l ready n a t u r a l l y j o i n t e d r e s e r v o i r s can a l s o c o n s i d e r a b l y enhance t h e c o n d u c t i v i t y o f t h e l a t t e r and can be t e c h n i c a l l y handled much e a s i e r by j u s t e n l a r g i n g and p r o p p i n g t h e a l r e a d y e x i s t i n g zones o f weakness ( a s p e c t s o f n a t u r a l l y f r a c t u r e d r e s e r v o i r s a r e d i s c u s s e d by P I N E & BATCHELOR 1982, RUMFlEL 1985, BLANTON 1986; GATENS, OLAREWAJU & LEE 1986; GUEVARA & TYLER 1986, HARRISON & TILOEN 1986, NELSON 1986; TANSOE, K I N G & HOLMAN 1986; RUMMEL, HEUSER, MULLER & OTTO 1987; c f . s e c t i o n 4 . 8 . 8 . ) . Comments on h y d r a u l i c f r a c t u r i n g o f i n j e c t i o n w e l l s a r e g i v e n by RILEY (1959), JONES (1960), HOWELL & THOMAS (1978), ILSENG & CORTEZ (1985), K O N I N G ( 1 9 8 5 ) , KONING & N I K O (1985), PERKINS & GONZALEZ (1985); B I O T , MASSE & MEDLIN (1986); CHEN ( 1 9 8 6 ) , R E V I E R E & WU (1986), HOLZHAUSEN & EGAN (1987) and WILLIAMS, SHERRARD & LIN ( 1 9 8 7 ) .
As a consequence o f i n j e c t i o n w e l l f r a c t u r i n g , i n j e c t i o n r a t e may be i n c r e a sed a t a n e a r - c o n s t a n t i n j e c t i o n pressure, o r i f necessary i n j e c t i o n p r e s s u r e may be reduced w h i l e a c o n s t a n t i n j e c t i o n r a t e i s m a i n t a i n e d (GUERREDO 1958, POWELL & JOHNSTON 1960, HARTSOCK & SLOBOD 1961). W i t h i n c r e a s i n g w a t e r i n p u t , t h e
593 f l o o d o u t t i m e f o r a p a r t i c u l a r r e s e r v o i r would be reduced (HOWARD & FAST 1970 a ) . Large-volume f r a c t u r e s t i m u l a t i o n s i n a u a t e r f l o o d o p e r a t i o n a r e r e p o r t e d by ILSENG & CORTEZ ( 1 9 8 5 ) . F r a c t u r i n g f o r a s s i s t a n c e o f s t e a m - f l o o d i n g i s eval u a t e d by BXITTON, MARTIN, LEIBRECHT & HARMON (1983) and STANG & SON1 (1987; c f . a l s o s e c t i o n 4 . 9 . 4 . ) . F l u i d f l o w and l e a k o f f p r o p e r t i e s f r o m f r a c t u r e s i n duced i n i n j e c t i o n o p e r a t i o n s a r e d i s c u s s e d by SETTARI ( 1 9 8 8 ) . H y d r a u l i c s t i m u l a t i o n o f o i l - f i e l d w a t e r - i n j e c t i o n w e l l s has i n t h e p a s t a l r e a d y f r e q u e n t l y been performed i n t h e USSR.
4.8.7.1.1.2. Proppant size and reservoir pressure HOOVER & ADAMS (1988) o u t l i n e t h a t proppant s e l e c t i o n f o r w a t e r f l o o d p r o j e c t s has t o be based on s i z e and u l t i m a t e performance of i n j e c t i o n o p e r a t i o n . S w i t c h i n g f r o m 20/40 t o 12/20 mesh proppants i s o f t e n f a v o u r a b l e i n o r d e r t o a v o i d o r t o d i m i n u i s h f i l t e r cake b u i l d u p on t h e f r a c t u r e f a c e which a f f e c t s proppant performance and i s i n essence a r e d u c t i o n i n u l t i m a t e propped c r a c k w i d t h . Pumping o f l a r g e q u a n t i t i e s o f l a r g e - s i z e d proppants p a r t i a l l y o f f s e t s t h e d i m i n u t i o n o f propped f r a c t u r e w i d t h a t t r i b u t e d t o s t i m u l a t i o n f l u i d - c a u s e d f i l t e r cake remnants. The damage aspects a r e a c o m b i n a t i o n o f f l u i d r e t e n t i o n , f i n e s p r e s e n t i n t h e p r o p p a n t package, f i n e s h a v i n g m i g r a t e d f r o m t h e format i o n , and embedment due t o c l o s u r e s t r e s s .
The pay h o r i z o n s t o be t r e a t e d by more o r l e s s m i n i f r a c t u r i n g and o t h e r s m a l l - s c a l e h y d r a u l i c s t i m u l a t i o n i n c l u d e b o t h hydropressured and geopressured f o r m a t i o n s ( q u e s t i o n s o f geopressured r e s e r v o i r s a r e r e p o r t e d by SAMUELS 1979, TUCKER 1979, WEEKS 1979; McCOY, HARTSOCK & DOBSON 1980; AMIRIJAFARI, SHARER & R I G B Y 1982; HAMMERSHAIMB & KUUSKRAA 1982, ROSE & DOHERTY 1982; c f . a l s o s e c t i o n 4.4.2.), w i t h normal h y d r o s t a t i c p r e s s u r e s b e i n g a b t . 0.45 p s i / f t and hyperhyd r o s t a t i c p r e s s u r e s r e a c h i n g up t o 0.9 p s i / f t o r even more (BRIM 1986). KUO, HANSON & BRISAY (1984) e v a l u a t e p r e d i c t i o n o f f r a c t u r e e x t e n s i o n d u r i n g w a t e r f l o o d o p e r a t i o n , and GARON, Y I N & DUNAYEVSKY (1988) s i m u l a t e t h e r m a l l y - i n d u c e d w a t e r f l ood f r a c t u r i n g .
4.8.7.1.1.3. Decreasing in-situ stress by cold water injection S p e c i a l f e a t u r e s i n water-, thermal- o r c h e m i c a l - i n j e c t i o n p r o j e c t s a r e dec r e a s i n g i n - s i t u s t r e s s o r f r a c t u r e c l o s u r e p r e s s u r e w i t h d e c l i n i n g pore pressur e when f i n a l l y c l o s u r e p r e s s u r e i n sandstone r e s e r v o i r s i s much lower t h a n i n t h e s u r r o u n d i n g s h a l e s (SMITH 1981), w i t h such an i n - s i t u s t r e s s c o n t r a s t p r o v i d i n g a s t r o n g b a r r i e r t o v e r t i c a l growth, and p e n e t r a t i o n o f t h e f r a c t u r e t i p beyond t h e r e l a t i v e l y p r e s s u r i z e d area o f t h e i n j e c t i o n p r o j e c t i n t o r e g i o n s o f low r e s e r v o i r p r e s s u r e and/or c l o s u r e s t r e s s . Some aspects o f r e s e r v o i r p r e s s u r e d i s t r i b u t i o n and f r a c t u r e p e n e t r a t i o n as w e l l as coupled f o r m a t i o n c o o l i n g and f r a c t u r e e l o n g a t i o n a r e o u t l i n e d as f o l l o w s .
4.8.7.1.1.3.1. Reservoir pressure distribution and fracture penetration I n c r e a s e and r e d i s t r i b u t i o n o f r e s e r v o i r p r e s s u r e by i n j e c t i o n o f l i q u i d s f o r p r e s s u r e maintenance o r enhanced r e c o v e r y has t h e r e s u l t t h a t as t h e f r a c t u r e p e n e t r a t e s beyond t h e i n c r e a s e d p r e s s u r e r e g i o n , i t would be e n t e r i n g a l o wer c l o s u r e s t r e s s area t h u s p o s s i b l y p r o d u c i n g e x c e s s i v e a f t e r f l o w o f t h e f r a c t u r i n g f l u i d . The l a t t e r e f f e c t t a k e s p l a c e when a f t e r s h u t - i n o f t h e w e l l , t h e s l u r r y continues t o flow u n t i l the f r a c t u r e closes t o lock the proppant i n p l a c e ( c f . s e c t i o n 4.12.2.2.1.), w i t h t h e l e n g t h o f t h i s a f t e r f l o w p e r i o d depending on f l u i d - l o s s r a t e and p r o p p a n t c o n c e n t r a t i o n . T h i s s p e c i a l problem o f p r e s s u r e v a r i a t i o n i n r e s e r v o i r s where i n j e c t i o n p r o j e c t areas a r e a c t i v e needs s e r i o u s a t t e n t i o n f o r i n t e r p r e t i n g and r e c o g n i z i n g these s y n t h e t i c pay zone p r e s s u r e d i s t r i b u t i o n c o r r e c t l y and t o a d j u s t t h e s t i m u l a t i o n t r e a t m e n t t o i t .
534 I n - s i t u s t r e s s i s a l s o s i g n i f i c a n t l y a f f e c t e d by f l u i d ring.
leakoff
during
fractu-
I n j e c t i o n o f c o o l w a t e r can r e d u c e e a r t h s t r e s s e s a r o u n d u p t a k e w e l l s c o n s i d e r a b l y , c a u s i n g them t o f r a c t u r e a t p r e s s u r e s s u b s t a n t i a l l y l o w e r t h a n w o u l d be e x p e c t e d i n t h e absence o f t h e t h e r m o e l a s t i c e f f e c t (PERKINS & GONZALEZ 1 9 8 5 ) . Depending on t h e shape o f t h e c o o l r e g i o n and t h e d i f f e r e n c e between m i nimum and maximum i n - s i t u h o r i z o n t a l e a r t h s t r e s s e s , m i n o r f r a c t u r e s p e r p e n d i c u l a r t o t h e m a i n f r a c t u r e e v e n t u a l l y open, t h u s c r e a t i n g a j o i n t e d s y s t e m o f p r i mary and s e c o n d a r y f r a c t u r e s ( c f . a l s o s e c t i o n 4 . 9 . 5 . ) .
4 . 8 . 7 . 1 . 1 . 3 . 2 . Coupled formation cooling and fracture elongation R e s e r v o i r behaviour a s s o c i a t e d w i t h c o l d water i n j e c t i o n i n t o a h o t format i o n a r e v e r y complex because o f t h e c o u p l e d n a t u r e o f t h e m e c h a n i s m i n v o l v e d (GARON, L I N & DUNAYEVSKY 1 9 8 8 ) . Water i n j e c t i o n c o o l s t h e r e s e r v o i r b y conduct i o n and c o n v e c t i o n . P r i o r t o f r a c t u r i n g , f l o w d i s t r i b u t i o n i s c o n t r o l l e d b y r e s e r v o i r c h a r a c t e r i s t i c s , and t e m p e r a t u r e f i e l d i s g o v e r n e d b y f l o w p a t t e r n . Ins i t u s t r e s s d i s t r i b u t i o n i s s u b s e q u e n t l y d e t e r m i n e d b y t e m p e r a t u r e and p r e s s u r e f i e l d s , and f r a c t u r e g r a d i e n t may be d e c r e a s e d u n t i l a t some i n j e c t i o n p r e s s u r e s , f o r m a t i o n p a r t i n g o c c u r s . F l o w p a t t e r n i s s t r o n g l y i n f l u e n c e d by any f r a c t u r e s d e v e l o p i n g w h i c h c o n t r o l s u b s e q u e n t t e m p e r a t u r e and s t r e s s d i s t r i b u t i o n s . X e d u c t i o n s o f s t r e s s n e a r t h e f r a c t u r e t i p s r e s u l t i n f r a c t u r e e l o n g a t i o n and f u r t h e r stress diminution, w i t h t h i s coupling e f f e c t being able t o g i v e r i s e t o r a p i d f r a c t u r e extension ( c f . s e c t i o n 4.9.5,). Other aspects o f thermally-induc e d w a t e r f l o o d f r a c t u r i n g a r e d i s c u s s e d b y PERKINS & GONZALEZ ( 1 9 8 4 ) ; MORALES, ABOU-SAYED, JONES & AL-SHAFFAR ( 1 9 8 6 ) and WILLIAMS, SHERRARD & L I N ( 1 9 8 7 ) .
4 . 8 . 7 . 1 . 1 . 4 . Sweep efficiency vs. injectivity HOlJELL & THOMAS ( 1 9 7 8 ) ernphasize t h e r o l e o f i n j e c t i o n w e l l f r a c t u r i n g f o r m a x i m i z a t i o n o f sweep e f f i c i e n c y . F i e l d e x p e r i e n c e has shown t h a t i n j e c t i v i t y o f s h o t h o l e s i s i n f e r i o r t o a l l o t h e r t y p e s o f s t i m u l a t i o n , and i n p a r t i c u l a r f r a c t u r i n g o f s h o t h o l e s i s l e s s e f f e c t i v e t h a n f r a c t u r i n g i n gauge h o l e s . Some w e l l s t r e a t e d w i t h l o w - v i s c o s i t y f l u i d have g r e a t e r d e c l i n e b e f o r e s t a b i l i z i n g t h a n t h o s e s t i m u l a t e d w i t h h i g h - v i s c o s i t y f l u i d s , whereas h i g h - v i s c o s i t y f l u i d j o b s e x h i b i t fewer mechanical screenout problems than l o w - v i s c o s i t y ones. Shot h o l e s may p r o v i d e o n l y s m a l l open f r a c t u r e s .
In a d d i t i o n t o o r as an a l t e r n a t i v e o f h y d r a u l i c p r o p p a n t f r a c t u r i n g , s e l e c t i v e a c i d i z i n g o f sandstone r e s e r v o i r s i n i n j e c t i o n w e l l s can d i s c r e t e l y stimul a t e i n e f f e c t i v e i n t e r v a l s and s i g n i f i c a n t l y i m p r o v e i n j e c t i v i t y p r o f i l e s (ELLENBERGER & ASELTINE 1976; c f . s e c t i o n 4 . 5 . 3 . ) . S t i m u l a t i o n o f w a t e r i n j e c t i o n w e l l s i s a l s o d i s c u s s e d b y CLEMENTZ ( 1 9 8 2 ) .
4 . 8 . 7 . 1 . 2 . Acc iden ta 1 fracturing I n j e c t i o n w e l l s c a n a l r e a d y be a c c i d e n t a l l y s t i m u l a t e d by e x c e e d i n g t h e f o r m a t i o n f r a c t u r e g r a d i e n t by t o o h i g h i n j e c t i o n r a t e s and p r e s s u r e s . Such a c c i d e n t a l u n p l a n n e d f r a c t u r i n g i n w a t e r - f l o o d o r - d i s p o s a l p r o j e c t s have t o be a v o i d e d i n many c a s e s f o r r e s e r v o i r e n g i n e e r i n g and e n v i r o n m e n t a l s a f e t y r e a sons. Some o f t h e m a i n drawbacks o f a c c i d e n t a l i n j e c t i o n - w e l l f r a c t u r i n g a r e i l l u s t r a t e d as f o l l o w s a l o n g t h e l i n e s o f f l u i d b r e a k t h r o u g h v s . sweep e f f i c i e n c y , and open c r a c k s above f r a c t u r e p r e s s u r e and n o n - p r o p p a n t h y d r a u l i c s t i m u l a tion.
595
4.8.7.1.2.1.
F l u i d breakthrough vs. sweep e f f i c i e n c y
P r e m a t u r e f r a c t u r i n g o f i n j e c t i o n w e l l s may r e s u l t i n bypass o f r e s e r v o i r r e g i o n s w i t h s i g n i f i c a n t o i l and gas r e s e r v e s (AMAEFULE & MASUO 1 9 8 4 ) , t h e r e b y det e r i o r a t i n g sweep p a t t e r n and e f f i c i e n c y as w e l l as r e c o v e r y f a c t o r . E x c e s s i v e i n j e c t i o n p r e s s u r e s c a n a l s o g i v e r i s e t o w a t e r c h a n n e l l i n g i n t h e cement s h e a t h between c a s i n g and b o r e h o l e w a l l o r w a t e r passage t h r o u g h t h e n a t u r a l c r a c k s y s t e m i n t h e p a y s a n d s t o n e i n t o v e r t i c a l f r a c t u r e s and h o r i z o n t a l bedd i n g planes o f o v e r l y i n g boundary shales, w i t h t h e water causing s w e l l i n g o f c l a y m i n e r a l s w h i c h l e a d s t o s e r i o u s c a s i n g damage (CHEN 1 9 8 6 ) . A c c i d e n t a l i n j e c t i o n - w e l l f r a c t u r i n g may l e a d t o p r e m a t u r e b r e a k t h r o u g h o f i n j e c t e d f l u i d s , p o o r sweep e f f i c i e n c y , r e d u c e d r e c o v e r y and l o s s o f c o s t l y i n j e c t i o n f l u i d s (SINGH & AGARWAL 1 9 8 8 ) . I n some r e s e r v o i r s , f r a c t u r i n g i s n o t r e commended a t a l l , because r a d i a l f l o w has a more f a v o u r a b l e i m p a c t o n w a t e r f l o o d performance than t h e l i n e a r f l o w paths which u s u a l l y r e s u l t f r o m f r a c t u r i n g (SLOAT 1988; c f . s e c t i o n 4 . 1 1 . 3 . ) . Aspects o f a c c i d e n t a l i n j e c t i o n - w e l l f r a c t u r i n g by i n j e c t i o n pressure exceeding formation o r f r a c t u r e p a r t i n g press u r e ( f r a c t u r e e x t e n s i o n / p r o p a g a t i o n p r e s s u r e ) a r e a l s o d i s c u s s e d b y DICKEY & ANDRESEN ( 1 9 4 5 ) , JONES ( 1 9 6 0 ) , IIILLIAMS ( 1 9 7 7 ) ; HAGOORT, WEATHERHILL & SETTARI ( 1 9 8 0 ) ; HAGOORT ( 1 9 8 1 ) , KONING & N I K O ( 1 9 8 5 ) ; TANG; L I U & J I A N G ( 1 9 8 6 ) and PRADO, KRUYSDIJK, NIKO, SANCEVIC & RODRIGUEZ ( 1 9 8 8 ) .
4.8.7.1.2.2. Open cracks above f r a c t u r e pressure and non-proppant h y d r a u l i c s t i m u l a t i o n Injection well fracturing, however, has t o t a k e a l w a y s i n t o c o n s i d e r a t i o n t h a t i n case o f c o n t i n u o u s i n j e c t i o n o p e r a t i o n above f r a c t u r e p r e s s u r e , open c r a c k s e x i s t w h i c h a r e k e p t open b y t h e permanent o v e r p r e s s u r e and c o n d u c t i v i t y m i g h t be s e v e r e l y d e t e r i o r a t e d i f t h e f r a c t u r e s a r e i n f i l l e d w i t h p r o p p a n t s (BARBY & BARBEE 1 9 8 7 ) . I f an i n j e c t i o n w e l l has i n s u f f i c i e n t u p t a k e c a p a c i t y a t o r above f r a c t u r e p r e s s u r e , i t may be p o s s i b l e t o i n c r e a s e i n j e c t i v i t y b y h y d r a u l i c s t i m u l a t i o n o f t h e r e s e r v o i r u s i n g a high-rate, low f l u i d - l o s s , s l i c k water treatment w i t h o u t proppant. The h i g h r a t e and l a r g e volume may e x t e n d v e r t i c a l and h o r i z o n t a l communicat i o n t o more p e r m e a b l e l a y e r s o f t h e r e s e r v o i r . Once t h e b a r r i e r s a r e b r o k e n t h r o u g h , t h e r e g u l a r i n j e c t i o n above t h e f r a c t u r e p r e s s u r e s h o u l d keep t h e f r a c t u r e s open, t h e r e b y n o t n e c e s s i t a t i n g a p r o p p a n t t o s u p p o r t t h e c r a c k and t h u s n o t r e q u i r i n g t o r e d u c e t h e maximum f l o w c a p a c i t y o f an open f r a c t u r e b y i n f i l l i n g w i t h proppants. Non-proppant h y d r a u l i c r e s e r v o i r s t i m u l a t i o n i s a l s o s i g n i f i c a n t i n some m i n i f r a c t u r i n g t r e a t m e n t s o f s a n d s t o n e s and c a r b o n a t e s ( c f . sect i o n 4.8.2.5.), water f r a c t u r i n g o f chalks ( c f . s e c t i o n 4.5.4.5.2.) and g e o t h e r mal r e s e r v o i r s ( c f . s e c t i o n 4 . 7 . 3 . 5 . ) , and n i t r o g e n f r a c t u r i n g o f gas s h a l e s ( c f . s e c t i o n 4.4.4.2.).
4.8.7.1.2.3.
Other aspects
A c c i d e n t a l f r a c t u r i n g o f w a t e r - i n j e c t i o n w e l l s has u n f o r e s e e n consequences o n w a t e r f l o o d sweep e f f i c i e n c y (DIKKEN & NIKO 1 9 8 7 ) . F r a c t u r i n g o f t e n causes c o o l i n g o f h o t formations by c o l d i n j e c t i o n water. Another undesired e f f e c t i s c r a c k w a l l i m p a i r m e n t as a r e s u l t o f f i l t e r c a k e b u i l d - u p due t o p o o r - q u a l i t y i n j e c t i o n w a t e r . On t h e o t h e r hand, i n j e c t i o n o p e r a t i o n f a r b e l o w t h e f o r m a t i o n o r f r a c t u r e p a r t i n g p r e s s u r e l e a d s t o i n j e c t i o n volumes much l o w e r t h a n t h e a l l o w a b l e maximum and t h e r e b y r e s u l t i n a c o n s i d e r a b l y r e d u c e d r a t e o f o i l r e c o v e ry. A r e l i a b l e e s t i m a t e o f t h e f r a c t u r e o r f o r m a t i o n p a r t i n g pressure i s thus o f c r i t i c a l i m p o r t a n c e i n c o n d u c t i n g s e c o n d a r y and t e r t i a r y r e c o v e r y p r o j e c t s (SINGH & AGARWAL 1988; c f . s e c t i o n 4 . 8 . 2 . 3 . 1 . ) . WILLIAMS ( 1 9 7 7 ) e v a l u a t e s t h e i n f l u e n c e o f i n d u c e d v e r t i c a l c r a c k s on o v e r a l l r e s e r v o i r p e r f o r m a n c e i n a c c i d e n t a l l y f r a c t u r e d w a t e r - i n j e c t i o n w e l l s as a consequence o f o p e r a t i o n above
596 f r a c t u r e pressure. P r i o r t o i n t r o d u c t i o n o f h y d r a u l i c f r a c t u r i n g t o t h e hydrocarbon i n d u s t r y some 40 y e a r s ago, a c c i d e n t a l f r a c t u r i n g as a consequence o f i n j e c t i o n p r e s s u r e s exceeding f o r m a t i o n f r a c t u r e g r a d i e n t o c c u r r e d d u r i n g a c i d i z i n g (HOWARD & FAST 1970 a ) , w a t e r i n j e c t i o n (GREBE 1943, DICKEY & ANDERSON 1945, YUSTER & CALHOUN 1945) and squeeze cementing (TORREY 1940, REISTLE 1945, TEPLITZ & HASSEBROEK 1946, HOWARD & FAST 1950) as r e f l e c t e d by acceptance o f c o n s i d e r a b l y l a r g e r volumes by t h e w e l l than p r e d e t e r m i n e d by r e s e r v o i r e v a l u a t i o n and c a l c u l a t i o n . F o r m a t i o n l i f t i n g and p r e s s u r e p a r t i n g r e s u l t e d i n much h i g h e r u p t a k e c a p a c i t y o f t h e pay than u s u a l .
4.8.7.2.Waste-liquid disposal wells Cheap m i n i f r a c t u r e s may a l s o be t h e o n l y f e a s i b l e way f o r some s p e c i a l a p p l i c a t i o n s o f s t i m u l a t i o n such as t o open zones f o r take-up o f disposed waste liq u i d s d e r i v i n g f r o m d r i l l i n g , gas p r o d u c t i o n and p u r i f i c a t i o n ( m a i n l y sour-gas t r e a t i n g ) , o i l and w a t e r s e p a r a t i o n i n f i e l d s p r o d u c i n g w i t h h i g h e r w a t e r c u t s , steam-drive enhanced o i l r e c o v e r y (excess w a t e r ) , and o t h e r i n d u s t r i a l processi n g , i n o r d e r t o remove them s a f e l y i n t o p r o p e r l y s e a l e d underground f o r m a t i o n s ( u n l e s s t h e y a r e s t o r e d w i t h i n s a l t caverns) w i t h i n p r e f e r e n t i a l l y i s o l a t e d dep l e t e d o i l and gas r e s e r v o i r s . A f t e r commenting on s a f e removal o f waste liq u i d s by underground d i s p o s a l , some aspects o f p r o p p a n t v s . non-proppant d i s p o sal f r a c t u r i n g are o u t l i n e d .
4.8.7.2.1. Safe removal o f waste liquids by underground disposal The e x p e r i e n c e o f l o o k i n g f o r s u i t a b l e d i s p o s a l h o r i z o n s t h a t c o u l d t a k e up reasonable q u a n t i t i e s o f waste l i q u i d s o f d i f f e r e n t v i s c o s i t y and w i t h v a r y i n g amounts o f d i s p e r s e d s o l i d s has shown t h a t t h e r e a r e n o t many g e o l o g i c a l l y s a f e and u n o b j e c t i o n a b l e harmless f o r m a t i o n s f o r t h i s a p p l i c a t i o n , and t h e few poss i b l e t a r g e t l a y e r s p r e d o m i n a n t l y r e q u i r e p e r m e a b i l i t y enhancement by f r a c t u r i n g i n o r d e r t o a v o i d r a p i d p l u g g i n g o f t h e i n j e c t i o n w e l l s by p a r t i c l e s f i l t r a t e d o u t o f t h e disposed f l u i d s . I t i s t h e r e f o r e assumed t h a t h y d r a u l i c p r o p p a n t f r a c t u r i n g o f underground w a s t e - l i q u i d d i s p o s a l w e l l s w i l l change f r o m b e i n g u n c o n v e n t i o n a l t o more o r l e s s p o p u l a r i n l i g h t o f t h e i n c r e a s i n g need of s a f e c l e a r a n c e o f v a r i o u s f l u i d s i n t h e near f u t u r e when c o n v e n t i o n a l i n j e c t i o n c a p a c i t i e s a r e approachi n g e x h a u s t i o n ( t h i s a p p l i e s p a r t i c u l a r l y f o r waste l i q u i d s t h a t cannot be d i s posed i n t o a q u i f e r s s u r r o u n d i n g p r o d u c i n g o i l and gas f i e l d s w i t h o u t r i s k i n g t o d e t e r i o r a t e t h e hydrocarbon q u a l i t y o r t o s e r i o u s l y d i s t u r b o i l and gas d r a i nage). F r a c t u r e containment i n v e r t i c a l d i r e c t i o n i s i n t h i s s p e c i a l a p p l i c a t i o n o f p a r t i c u l a r s i g n i f i c a n c e f o r t h e reason o f e n v i r o n m e n t a l and r e s e r v o i r e n g i n e e r i n g s a f e t y ( c f . s e c t i o n 4 . 2 . 2 . ) . Aspects o f so f a r common p r a c t i c e s o f d i s p o s a l o f excess w a t e r a r e d i s c u s s e d by SPRINGER & MOLLER ( 1 9 8 5 ) , and econornic a l q u e s t i o n s o f d i s p o s a l methods f o r N o r t h Sea o i l and gas f i e l d s a r e s t u d i e d by FISHMAN ( 1 9 8 6 ) . WILSON & K I S E R (1987) a l s o comment on aspects o f hazardous w a s t e - l i q u i d d i s p o s a l by i n j e c t i o n i n t o subsurface f o r m a t i o n s , and SUN & MONGAN (1974), DOSCHER & EL-ARAB1 (1983) and McBANE & JEU (1988) r e p o r t examples o f waste w a t e r d i s p o s a l a t p r e s s u r e s above f r a c t u r e g r a d i e n t o r breakdown p r e s sure.
4.8.7.2.2. Proppant vs. non-proppant disposal fracturing I t has been r e c o g n i z e d v e r y e a r l y i n h y d r a u l i c f r a c t u r i n g h i s t o r y t h a t s t i m u l a t i o n o f d i s p o s a l b o r e h o l e s can e s t a b l i s h a low-pressure, h i g h - f l u i d - i n j e c t i o n c a p a c i t y w e l l almost everywhere i n t h e hydrocarbon f i e l d complex (HOWARD & FAST 1970 a ) , w i t h t h u s d i s p o s a l f r a c t u r i n g b e i n g an easy s o l u t i o n o f l a c k i n g waste
597 d i s p o s a l c a p a c i t i e s i n r e a s o n a b l y c l o s e s i t u a t i o n t o o i l and gas p r o d u c t i o n and p r o c e s s i n g . I n terms o f f r a c t u r e propping, t h e h y d r a u l i c a l l y - i n d u c e d c r a c k may be l e f t open i f s u f f i c i e n t l y supported by permanent i n j e c t i o n above f r a c t u r e p r e s s u r e ( c f . s e c t i o n 4.8.7.1.2.2.), because p r o p p a n t packages p l u g g i n g t h e c r a c k may be s u b j e c t e d t o r a p i d p e r m e a b i l i t y d e t e r i o r a t i o n by i n v a s i o n o f f i n e s o l i d s d i s p e r s e d w i t h i n t h e disposed waste f l u i d s i n t o t h e proppant i n t e r s t i t i a l s and t h u s d e s t r o y i n g uptake c a p a c i t y . I f w a s t e - l i q u i d disposal i s n o t continuously c a r r i e d o u t a t s u f f i c i e n t overp r e s s u r e t o keep t h e f r a c t u r e open o r t o enable u n p r o b l e m a t i c r e p e a t e d reopeni n g o f t h e c r a c k upon r e t u r n t o i n j e c t i o n o p e r a t i o n , t h e f r a c t u r e s h o u l d be plugged w i t h coarse t o v e r y coarse proppants i n bank o r p i l l a r d i s t r i b u t i o n ( c f . s e c t i o n 4.3.3.2.) i n s t e a d o f c o n t i n u o u s i n f i l l i n g , and wide c r a c k s h o u l d be supported by p r o p p a n t m u l t i l a y e r s i n o r d e r t o p r o v i d e as much poros t y and p e r m e a b i l i t y as p o s s i b l e and t o e n a b l e maximum f l u i d acceptance c a p a c i t y
4.8.8. Combination of natural and artificial fractures Smaller- o r larqer-scale hydraulic proppant f r a c t u r i n q w i t h i n natural y j o i n t e d r e s e r v o i r s i s p a r t i c u l a r l y s i g n i f i c a n t ; as t h e p r e - e x i s t i n g c r a c k network can be s u c c e s s f u l l y employed f o r enhancement o f t h e e f f e c t i v i t y o f t h e a r t i f i c i a l s t i m u l a t i o n operation. N a t u r a l l y j o i n t e d ( a l s o c a l l e d double-porosity) pays a r e sedimentary f o r m a t i o n s which were a f t e r d e p o s i t i o n f r a c t u r e d by t e c t o n i c a l o r chemical processes, and t h e r e f o r e have a p r i m a r y i n t e r g r a n u l a r m a t r i x p o r o s i t y which i s c o n t r o l l e d by sedimentary environment and d i a g e n e t i c a l e v o l u t i o n , and a secondary f r a c t u r e p o r o s i t y t h a t i s governed by f r a c t u r i n g o r j o i n t i n g as a r e s u l t o f f a i l u r e d u r i n g mechanical d e f o r m a t i o n (BARUA, KUCUK & GOMEZANGULO 1985; HENSEL 1988). N a t u r a l l y j o i n t e d r e s e r v o i r s can be t h u s regarded as d o u b l e - p o r o s i t y media c o n s i s t i n g o f i s o t r o p i c heterogeneous p r i m a r y r o c k m a t r i x and a n i s o t r o p i c heterogeneous secondary f r a c t u r e system (HOUZE, HORNE & RAMEY 1984; NAKORNTHAP & EVANS 1986). ABDASSAH & ERSHAGHI (1986) i n t r o d u c e a t r i p l e p o r o s i t y model f o r r e p r e s e n t i n g n a t u r a l l y c r a c k e d pays. Some i m p o r t a n t n a t u r a l l y f r a c t u r e d o i l and gas r e s e r v o i r s i n c l u d e Monterey F o r m a t i o n and o v e r t h r u s t sandstone f o r m a t i o n s i n t h e western USA, c h a l k f o r m a t i o n s i n t h e N o r t h Sea i n Europe ( c f . s e c t i o n 4.5.4.), and carbonate f o r m a t i o n s i n t h e M i d d l e E a s t (BERGOSH, LORD & OWEN 1987). U n f o r t u n a t e l y some c o n f u s i o n occurs due t o simultaneous a p p l i c a t i o n o f t h e t e r m secondary p o r o s i t y i n sedimentary p e t r o g r a p h y t o d e s c r i b e renewed o r i g i n o f v o i d spaces by m i n e r a l t r a n s f o r m a t i o n and d i s s o l u t i o n w i t h i n t h e r o c k m a t r i x d u r i n g course o f b u r i a l d i a g e n e s i s independent f r o m any i n f l u e n c e o f f r a c t u r e s (SCHMIDT & McDONALD 1979 a, 1979 b; AL-SHAIEB & NALKER 1986), and i n p e t r o p h y s i c s f o r c h a r a c t e r i z a t i o n o f r e s e r v o i r s i n c l u d i n g p r i m a r y m a t r i x and secondary crack p o r o s i t y . T h e r e f o r e f r a c t u r e p o r o s i t y i n p e t r o p h y s i c a l sense s h o u l d b e t t e r named t e r t i a r y p o r o s i t y t o a l l o w f o r p r o p e r d i s t i n c t i o n o f a l l t h e p o s s i b l e p o r o s i t y t y p e s ( t h e same a p p l i e s f o r p e r m e a b i l i t y ) . The v a r i o u s phenomena a r i s i n g f r o m c o e x i s t e n c e o f n a t u r a l and h y d r a u l i c f r a c t u r e s a r e summarized i n sect i o n s commenting on f r a c t u r e i n t e r f e r e n c e and s u p e r i m p o s i t i o n , f r a c t u r e d i v e r gence, temporary n a t u r a l f r a c t u r e damage, d e n d r i t i c f r a c t u r i n g technique, t a i l o r e d p u l s e l o a d i n g , proppant p l u g g i n g o f n a t u r a l f r a c t u r e s , and o t h e r aspects.
4.8.8.1. Fracture interference and superimposition I n t e r f e r e n c e and s u p e r i m p o s i t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r e s a r e t h e r e s u l t s o f c a r r y i n g o u t a r t i f i c i a l s t i m u l a t i o n treatments i n o r i g i n a l l y j o i n t e d f o r m a t i o n s . Aspects o f d i s t r i b u t i o n and g e n e r a t i o n o f n a t u r a l f r a c t u r e s , intera c t i o n between n a t u r a l and h y d r a u l i c f r a c t u r e s , and i n t e r f r a c t u r e communication a r e o u t l i n e d as f o l l o w s .
5 98
4.8.8.1.1.Distribution of natural fractures Two t y p e s of n a t u r a l crack systems can be d i s t i n g u i s h e d comprising m u l t i p l e f r a c t u r e s e t s a s s o c i a t e d with s t r u c t u r a l l y deformed s t r a t a and s i n g l e r e g i o n a l c r a c k s e t s ( L O R E N Z & FINLEY 1987, NORTHROP & FROHNE 1 9 8 7 ) . F r a c t u r e s of t h e f i r s t type a r e caused by l o c a l f a u l t i n s o r foldincj and commonly c u t i n d i s c r i m i n a n t l y a c r o s s l i t h o l o g i c a l b o u n d a r i e s . F r a c t u r e s of t h e second type a r e caused by r e s i o n a l s t r e s s e s of much l e s s magnitude i n c o n j u n c t i o n w i t h hiqh pore p r e s s u r e s and o c c u r in s t r u c t u r a l l y undeformed f o r m a t i o n s . Some a s p e c t s of s t r e s s d i f f e r e n c e s and d e p o s i t i o n a l environment a s well a s r e s e r v o i r p e r m e a b i l i t y and communication are o u t l i n e d a s f o l l o w s ( v a r i o u s t y p e s and d i s t r i b u t i o n s of n a t u r a l f r a c t u r e s a r e i l l u s t r a t e d on p l a t e s I - X I i n s e c t i o n 3 . 1 3 . ) .
4.8.8.1.1.1.Stress differences and depositional environment The d i s t r i b u t i o n of r e g i o n a l f r a c t u r e s i s s e n e r a l l y c o n t r o l l e d by s u b t l e s t r e s s d i f f e r e n c e s of the v a r i o u s l i t h o l o g i e s , w i t h thus d e p o s i t i o n a l e n v i r o n ment t h a t governs t h e s e d i m e n t a r y h e t e r o g e n e i t y of t h e r e s e r v o i r a l s o s u p e r v i s i n g t h e d i s t r i b u t i o n of r e g i o n a l f r a c t u r e s w i t h i n t h e pay zone ( L O R E N Z & FINL E Y 1987, NORTHROP & FROHNE 1 9 8 7 ) . T h e r e f o r e chancjes i n c r a c k - r e l a t e d product i o n from r e s e r v o i r s w i t h r e g i o n a l f r a c t u r e s can be c o r r e l a t e d w i t h changes i n d e p o s i t i o n a l m i l i e u , and some s e d i m e n t a r y environments have a b e t t e r p r o s p e c t i v i t y f o r c r a c k - c o n t r o l l e d hydrocarbon p r o d u c t i o n than o t h e r s . F r a c t u r i n g s t y l e o f t e n changes with depth from s h e a r f r a c t u r i n g n e a r t h e s u r f a c e t o e x t e n s i o n f r a c t u r i n g a t d e p t h i n t h e hydrocarbon r e s e r v o i r ( C L A R K 1983; c f . a l s o s e c t i o n 6 . 2 . 2 . 3 . ) . Shear c r a c k s a r e a s s o c i a t e d w i t h movement p a r a l l e l t o t h e p l a n e w i t h no p e r p e n d i c u l a r motion, whereas e x t e n s i o n f r a c t u r e s a r e c r e a t e d a s the w a l l s move a p a r t ( H E N S E L 1 9 8 8 ) . AGUILERA ( 1 9 8 7 ) d i s t i n g u i s h e s t e c t o n i c a l , r e g i o n a l and c o n t r a c t i o n a l n a t u r a l f r a c t u r e s .
4.8.8-1.1.2.Reservoir permeability and comnunication R e s e r v o i r p e r m e a b i l i t y i s s i g n i f i c a n t l y enhanced by t h e n a t u r a l f r a c t u r e s y s -
tem, b u t t h e i r r e g u l a r l y d i s t r i b u t e d o r i g i n a l c r a c k s allow o n l y i m p e r f e c t commu-
n i c a t i o n a c r o s s t h e pay and r e p r e s e n t an u n i d i r e c t i o n a l accomodating p e r m e a b i l i t y enhancement o n l y i n a p r e f e r r e d d i r e c t i o n ( L O R E N Z & FINLEY 1987, NORTHROP & FROHNE 1 9 8 7 ) . T h e r e f o r e h y d r a u l i c proppant f r a c t u r i n q s t i m u l a t i o n i s r e q u i r e d t o f u l l y c o n n e c t a l l r e s e r v o i r segments t o t h e w e l l b o r e , b u t t h e f r a c t u r e perm e a b i l i t y system i s e a s i l y damaged by t r e a t m e n t f l u i d s and s t r e s s e s . Crack-domin a t e d p e r m e a b i l i t y systems a r e thus an i m p o r t a n t c o n s i d e r a t i o n i n t i g h t g a s res e r v o i r modelling ( c f . s e c t i o n 3 . 4 . 1 . ) , w i t h in c e r t a i n c a s e s f a c t o r s such a s r e s e r v o i r pressure, s i z e s and h e t e r o g e n e i t i e s p r o v i d i n g secondary c o n t r o l s on fracture-dominated production. Typical n a t u r a l l y f r a c t u r e d r e s e r v o i r s a r e composed of b r i t t l e rock w i t h low i n t e r g r a n u l a r p o r o s i t y (HENSEL 1 9 8 8 ) . Occurrence and extent of n a t u r a l j o i n t s i s governed by i n i t i a l s h e a r s t r e n g t h , i n t e r n a l f r i c t i o n , rock d u c t i l i t y , e f f e c t i v e c o n f i n i n g stress, t e m p e r a t u r e , d e p t h o f b u r i a l , rock t y p e , bed t h i c k n e s s , and a d j a c e n c y t o o t h e r l a y e r s . Crack p o r o s i t y i s mainly a f u n c t i o n of block s i z r and f r a c t u r e w i d t h . F r a c t u r e s have such a d r a m a t i c impact on r e s e r v o i r p e r formance t h a t t h e i r hydrocarbon s t o r a g e c a p a c i t i e s tend t o be i n f l a t e d .
4.8.8.1.2.Generation of natural fractures Natural f r a c t u r e s a r e g e n e r a t e d by d i a s t r o p h i s m in c a s e of fGlding ( c f . p l a t e I / 1 - 7 ) and f a u l t i n g ( c f . p l a t e X I / 3 - 4 ) where c r a c k s a r e c r e a t e d along the l i n e of t h e f a u l t which in turn produce a zone of d i l a t a n c e , deep e r o s i o n of t h e overburden t h a t p e r m i t s t h e upper p a r t s t o expand, u p l i f t and f r a c t u r e through p l a n e s of weakness ( f r e q u e n t l y i n c o n n e c t i o n w i t h o r i g i n of unconformi-
559 t i e s ; c f . p l a t e I / 1 - 7), and volume s h r i n k a g e as i n c a s e o f c o o l i n g o f i g n e o u s r o c k s ( c f . p l a t e XI/5 - 6) and d e s i c c a t i o n o f s e d i m e n t s ( c f . p l a t e s V I / 1 - 7 and X I / 7 ; AGUILEXA 1983). A s t h e p r o b a b i l i t i e s o f i n t e r c e p t i n g v e r t i c a l f r a c t u r e s w i t h a v e r t i c a l h o l e a r e v e r y s l i m , e x p l o r a t i o n and a p p r a i s a l s u c c e s s i n nat u r a l l y f r a c t u r e d r e s e r v o i r s w o u l d be g r e a t e r i f d i r e c t i o n a l w e l l s a r e d r i l l e d perpendicular t o the orientation o f the natural cracks (cf. sections 4.8.6.1.5.2. and 4.8.6.2.4.2.). The common o f f s h o r e e x p l o r a t i o n s t r a t e g y o f d r i l l i n g a v e r t i c a l w e l l i n t h e c r e s t a l a r e a o f a n t i c l i n a l s t r u c t u r e s may be a d o u b l e p i t f a l l , because f i r s t the v e r t i c a l hole might not intercept vertical o r high-inclination natural fract u r e s and second, i n c o n t r a s t t o p o p u l a r b e l i e f , t h e c r e s t a l a r e a o f asymrnetric a l a n t i c l i n e s m i g h t c o n t a i n o n l y l i t t l e f r a c t u r e p o r o s i t y . The d i s c u s s i o n as f o l l o w s c o n c e n t r a t e s on i m p a c t o f r e s e r v o i r h e t e r o g e n e i t i e s , f r a c t u r e s p a c i n g v s . bed t h i c k n e s s , and f r a c t u r e m i n e r a l i z a t i o n and s t r e s s s e n s i t i v i t y .
4.8.8.1.2.1. Impact of reservoir heterogeneities LORENZ, BRANAGAN, WARPINSKI & SATTLER (1586); LORENZ & FINLEY (1587), NORTHROP (1988) and NORTHROP & FROHNE (1988) d e s c r i b e r e s e r v o i r s c o n t a i n i n g a s y s t e m
o f u n i d i r e c t i o n a l s u b p a r a l l e l n a t u r a l f r a c t u r e s which occur p r i n c i p a l l y i n s a n d s t o n e s and s i l t s t o n e s and t e r m i n a t e a t mudstone c o n t a c t s b o t h i n v e r t i c a l and h o r i z o n t a l d i r e c t i o n as w e l l as b o t h a t i n t e r r e s e r v o i r and i n t r a r e s e r v o i r l i t h o l o g i c a l d i s c o n t i n u i t y p l a n e s . T h i n mudstone i n t e r b e d d i n g s i n s a n d s t o n e comp l e x e s such as o v e r b a n k f l o o d p l a i n members o f f l u v i a l b r a i d e d - r i v e r c y c l o t h e m s ( c f . p l a t e s I I I / l - 2, IV/3 - 5, VII/3, V I I I / 3 and IX/1 - 3), i n t e r d u n e pond d e p o s i t s i n a e o l i a n dune and s h e e t sand as w e l l as p l a y a sequences ( c f . p l a t e V I I I / 1 - 2), and l o w - w a t e r d r a p e s i n f l u v i a l c h a n n e l sand u n i t s ( c f . p l a t e s IV/6 - 8, V I I / 5 - 6, VIII/8 and IX/4 - 8) a r e a l r e a d y s u f f i c i e n t m e c h a n i c a l p r o p e r t y and r o c k s t r e s s c o n t r a s t s t o s t o p n a t u r a l f r a c t u r e p r o p a g a t i o n . D i s c o n t i n u i t i e s such as u n c o n f o r m i t i e s between f o l d e d basement and f l a t - l y i n g c o v e r s t r a t a ( c f . p l a t e I / 1 - 7 ) o r m a s s i v e c a r b o n a t e r e e f domes i n t e r s p e r sed i n bedded c a r b o n a t e s e d i m e n t s ( c f . p l a t e I/8) a r e a l s o a r e a s o f f r a c t u r e e x t e n s i o n t e r m i n a t i o n and change o f g e o m e t r i c a l c o n s t e l l a t i o n o f t h e c r a c k s a t t h e t r a n s i t i o n between two d i f f e r e n t j o i n t systems o f d i f f e r e n t g e n e s i s . N a t u r a l f r a c t u r e s o c c u r i n a w i d e s p e c t r u m o f l e n g t h s , w i d t h s , s p a c i n g s and d e n s i t i e s , and t h e c r a c k s and t h e i r i n t e r c o n n e c t i o n s a r e o f t e n n a r r o w a n d / o r m i n e r a l i z e d , r e s u l t i n g i n a s t r e s s - s e n s i t i v e , e a s i l y damaged system.
4.8.8.1.2.2.Fracture spacing vs. bed thickness The a v e r a g e f r a c t u r e s p a c i n g i n h e t e r o g e n e o u s r e s e r v o i r s i s a l w a y s s i g n i f i c a n t l y l e s s t h a n w o u l d b e s u g g e s t e d b y a l i n e a r r e l a t i o n s h i p between j o i n t spac i n g and bed t h i c k n e s s . The n a t u r a l c r a c k s f o r m e d u n d e r c o n d i t i o n s o f h i g h p o r e p r e s s u r e s and r e l a t i v e l y l o w h o r i z o n t a l s t r e s s e s (LORENZ, BRANAGAN, WARPINSKI & SATTLER 1986) w h i c h were p r o v i d e d by t e c t o n i c a l c o m p r e s s i o n o f t h e b a s i n . F r a c t u r i n g o c c u r r e d a t h i g h i n - s i t u p o r e p r e s s u r e s t h a t were caused b y gases r e l e a sed d u r i n g m a t u r a t i o n o f o r g a n i c m a t e r i a l i n t h e f o r m a t i o n w h i c h d e c r e a s e d t h e average e f f e c t i v e s t r e s s s u f f i c i e n t l y t o a l l o w f a i l u r e t o take p l a c e a t r e l a t i v e l y l o w h o r i z o n t a l s t r e s s e s (NORTHROP 1 9 8 8 ) .
4.8.8.1.2.3. Fracture mineralization and stress sensitivity I n many t i g h t sands, gas p r o d u c t i o n i s p r i n c i p a l l y c o n t r o l l e d b y a n e t w o r k o f open and p a r t l y m i n e r a l i z e d n a t u r a l f r a c t u r e s (PITMAN & STRUNT 1986) w h i c h formed i n response t o h i g h p o r e - f l u i d pressures t h a t developed d u r i n g hydrocarbon g e n e r a t i o n as w e l l as t o w i d e s p r e a d t e c t o n i c a l s t r e s s w i t h l a t e - s t a g e b a s i n s u b s i d e n c e and s e d i m e n t l o a d i n g a s s o c i a t e d w i t h p e r i o d s o f u p l i f t and e r o s i o n .
600 S p a c i n g , o r i e n t a t i o n and mode o f c r a c k s a r e r e l a t e d t o r e s e r v o i r l i t h o l o g y ( v a r i o u s examples o f g e o m e t r y and d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s a r e i l l u s t r a t e d on p l a t e s I - X I i n s e c t i o n 3.13.). F r a c t u r e s a r e e i t h e r open and n o n - m i n e r a l i z e d o r a r e p a r t l y t o c o m p l e t e l y occ l u d e d b y f i n e - t o c o a r s e - c r y s t a l l i n e c a l c i t e (LORENZ, BRANAGAN, WARPINSKI & SATTLER 1986; LORENZ & FINLEY 1 9 8 7 ) . I n c o m p l e t e c a l c i t e c r y s t a l cjrowths a l o n g j o i n t w a l l s a r e v e r y common and may keep t h e c r a c k s p r o p p e d open w h i c h h e l p s t o m a i n t a i n t h e i n c r e a s e d p e r m e a b i l i t y p r o d u c e d b y f r a c t u r i n g . On t h e o t h e r hand, j o i n t s c o m p l e t e l y cemented b y c a l c i t e may b a r f l u i d f l o w a t d e p t h . V e r t i c a l f r a c t u r e s i n s a n d s t o n e s p r e d o m i n a n t l y r e s u l t f r o m e x t e n s i o n , because t h e y a r e i n m o s t c a s e s open w i t h r e a s o n a b l e w a l l s e p a r a t i o n and show l i t t l e i f any e v i dence o f l a t e r a l d i s p l a c e m e n t . C r a c k s i n s h a l e s a r e numerous and t e n d t o be n o n - m i n e r a l i z e d and c l o s e d . T h e i r h i g h l y i r r e g u l a r s u r f a c e s w h i c h a r e i n c l i n e d a t l o w a n g l e s t o b e d d i n g and w h i c h a r e commonly p o l i s h e d and s l i c k e n s i d e d i n d i c a t e t h a t t h e c r a c k s o r i g i n a t e d b y s h e a r , and p e r m e a b i l i t y a l o n g t h e s e f r a c t u r e s under overburden s t r e s s i s n e g l i g i b l e . A c c o r d i n g t o t h e s t r e s s s e n s i t i v i t y o f t h e r e s e r v o i r , p r o d u c t i o n c a n be e s s e n t i a l l y s h u t o f f b y r e d u c i n g f o r m a t i o n p r e s s u r e w h i c h e f f e c t i v e l y squeezes t h e c r a c k s c l o s e d , whereas c o n v e r s e l y h i g h - p r e s s u r e gas i n j e c t i o n g i v e s r i s e t o d r a m a t i c a l l y i n c r e a s e d p a y p e r m e a b i l i t y due t o j o i n t o p e n i n g (RANDOLPH, SOEDER & CHOWDIAH 1984; NORTHROP & FROHNE 1 9 8 8 ) . O t h e r n a t u r a l f r a c t u r e s w h i c h a p p e a r t o be d i s c o n t i n u o u s o n c o r e s c a l e a r e b e l i e v e d t o enhance m a t r i x p e r m e a b i l i t y and i n c r e a s e p e r m e a b i l i t y a n i s o t r o p y r a t h e r t h a n a c t i n t h e manner o f c r a c k s ass o c i a t e d w i t h t h e d u a l - p o r o s i t y s y s t e m (BELFIELD 1 9 8 8 ) .
4.8.8.1.3. Interaction between natural and hydraulic fractures N a t u r a l c r a c k s have an i m p o r t a n t i n f l u e n c e o n m o d i f i c a t i o n o r a l t e r a t i o n o f propagation o f h y d r a u l i c a l l y induced f r a c t u r e s ( c f . a l s o s e c t i o n 4.5.1.). Pree x i s t i n g n a t u r a l j o i n t s a r e s e c o n d a r i l y e n l a r g e d f i r s t b y d r i l l i n g mud and l a t e r b y s t i m u l a t i o n f l u i d s (KULANDER & DEAN 1988; NURMI, WATERHOUSE, KHOBAR & AKBAR 1 9 8 8 ) . A n a t u r a l c r a c k w h i c h i s c r o s s e d b y a h y d r a u l i c a l l y - g e n e r a t e d f r a c t u r e behaves l i k e a h i g h - p e r m e a b i l i t y s t r e a k o r a r e a s o n a b l e l i t h o l o g i c a l d i s c o n t i n u i t y ( c f . a l s o s e c t i o n 4 . 2 . 4 . 2 . ) and i s a b l e t o d e f l e c t o r c o m p l e t e a r r e s t i t s growth ( c f . s e c t i o n 4.2.4.3.). A s i g n i f i c a n t increase i n bottom h o l e t r e a t i n g p r e s s u r e i s o f t e n r e q u i r e d t o r e i n i t i a t e f r a c t u r e p r o p a g a t i o n and r e s u l t s i n f o r m a t i o n o f a m u l t i p l e c r a c k n e t w o r k w i t h v e r y s h o r t e f f e c t i v e penet r a t i o n i n t o t h e r e s e r v o i r (BLANTON 1980; CONWAY, McGOWEN, GUNDERSON & KING 1 9 8 5 ) . C o n n e c t i v i t y o f t h e s y s t e m o f n a t u r a l f r a c t u r e s i s a s i g n i f i c a n t paramet e r (NIJESINGHE 1985, WIJESINGHE & KECECIOGLU 1986 a ) . Many n a t u r a l l y j o i n t e d f o r m a t i o n s a r e f a u l t - r e l a t e d , and s i n g l e and i n t e r c e p t i n g s e a l i n g f a u l t s have an i m p o r t a n t i m p a c t o n r e s e r v o i r c o m m u n i c a t i o n (AGUILERA 1 9 8 5 ) . A s p e c t s o f c o m b i n a t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r i n g a r e a l s o d i s c u s s e d b y BEN NACEUR ( 1 9 8 8 ) , and e x p e r i m e n t a l m o d e l l i n g o f t h e i n t e r a c t i o n between h y d r a u l i c a l l y - i n d u c e d and p r e - e x i s t i n g f r a c t u r e s i s p e r f o r m e d b y BLANTON ( 1 9 8 2 ) . N a t u r a l c r a c k s c o n t r o l t h e p e r m e a b i l i t y o f many t i g h t gas r e s e r v o i r s and c a n c r e a t e t e n s - o f - m i l l i d a r c y p a y zones o u t o f s u b m i c r o d a r c y r o c k s (NOIITHROP 1988; c f . s e c t i o n 3 . 4 . 1 . ) . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l a s p e c t s , o r i e n t a t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r e s as w e l l as r e l a t i o n s h i p between a n c i e n t and a c t u a l s t r e s s s y s t e m a r e o u t l i n e d .
4.8.8.1.3.1.General aspects ?lost t i g h t r e s e r v o i r s are n a t u r a l l y fissured, w i t h the cracks e i t h e r being v e r y s m a l l and i r r e l e v a n t t o f l o w t h u s r e d u c i n g t h e pay t o a homogeneous and lik e l y i s o t r o p i c medium, o r w i t h p r o d u c t i o n b e i n g s i g n i f i c a n t l y a f f e c t e d b y i n t e r s e c t i o n o f w e l l b o r e and c r a c k s i n m a s s i v e n a t u r a l l y j o i n t e d f o r m a t i o n s (BEN NA-
601
CEUR & ECONOMIDES 1988). D u r i n g e x e c u t i o n o f h y d r a u l i c f r a c t u r i n g treatments, t h e presence o f f i s s u r e s and c r a c k s a f f e c t s induced f r a c t u r e geometry, because major n a t u r a l c r a c k s l e a d t o a change o f h y d r a u l i c f r a c t u r e p a t h (JEFFREY E VANDAMME 1987; c f . s e c t i o n 4.2.4.3.) t h e r e b y c a u s i n g problems f o r p r o p p a n t t r a n s p o r t , and t h e h i g h p e r m e a b i l i t y o f f i s s u r e s r e s u l t s i n t h e i r a c t i o n as t h i e f zones where f r a c t u r i n g f l u i d - l o s s i s enhanced. Depending on c r a c k magnitude, i t i s p o s s i b l e t h a t l e a k o f f c o n t r o l w i t h o u t s u i t a b l e d i v e r t i n g agents does n o t work because f i l t e r cakes f r o m polymer r e s i d u e s may be i m p o s s i b l e t o o r i g i n a t e . I n l i m e s t o n e f o r m a t i o n s and d u r i n g a c i d f r a c t u r i n g , f i s s u r i n g may enhance wormInfluences h o l e growth (BEN NACEUR & ECONOMIDES 1988; c f . s e c t i o n 4.5.4.2.1.). o f n a t u r a l c r a c k s on p r o p a g a t i o n o f h y d r a u l i c f r a c t u r e s a r e a l s o d i s c u s s e d by TEUFEL E WARPINSKI ( 1 9 8 8 ) .
4.8.8.1.3.2.Orientation of natural and hydraulic fractures JEFFREY, VANDAMME & ROEGIERS (1987) comment on mechanical i n t e r a c t i o n s i n branched o r s u b p a r a l l e l h y d r a u l i c f r a c t u r e s . H y d r a u l i c s t i m u l a t i o n o f n a t u r a l l y j o i n t e d r e s e r v o i r s can r e s u l t i n g e n e r a t i o n o f complex geometries such as o f f s e t s a t c r o s s c u t t i n g n a t u r a l f r a c t u r e s . Pumping f l u i d i n t o a c r a c k e d r o c k f o r m a t i o n can i n f l a t e t h e n a t u r a l j o i n t s , and t h e r e s u l t i n g h y d r a u l i c f r a c t u r e may propagate as a system o f p a r a l l e l t o s u b p a r a l l e l f r a c t u r e s w i t h b o t h opening and s l i d i n g movement across t h e c r a c k s u r f a c e s (PINE & BATCHELOR 1984). Some aspects o f c r o s s - c u t t i n g and mu1 t i p l e b r a n c h i n g o f induced f r a c t u r e s as w e l l as h y d r a u l i c f r a c t u r e p r o p a g a t i o n p a r a l l e l t o n a t u r a l c r a c k s a r e o u t l i n e d as f o l l o w s .
4.8.8.1.3.2.1.Cross-cutting and multiple branching of induced fractures I f t h e n a t u r a l j o i n t s a r e more widely-spaced, t h e h y d r a u l i c f r a c t u r e may p r o pagate as a s i n g l e f r a c t u r e b u t i n t e r a c t w i t h n a t u r a l c r a c k s one by one as i t approaches them, w i t h o f f s e t t i n g and b r a n c h i n g o f t h e h y d r a u l i c f r a c t u r e r e s u l t i n g f r o m such i n t e r a c t i o n s (JEFFREY, VANDAMME & ROEGIERS 1987; c f . s e c t i o n 4 . 2 . 4 . 3 . ) . S l i p o c c u r r i n g on weakness p l a n e s a f t e r t h e h y d r a u l i c f r a c t u r e c r o s ses them l e a d s t o a r e d u c t i o n i n c r a c k w i d t h a t t h e s i t e o f o f f s e t which may cause p r o p p a n t b r i d g i n g .
The s t u d y o f t h e p r o p a g a t i o n o f m u l t i p l e branched f r a c t u r e s and i n t e r a c t i o n o f a h y d r a u l i c f r a c t u r e w i t h a c r o s s - c u t t i n g n a t u r a l crack r e v e a l s t h a t m u l t i p l e branched f r a c t u r e s a r e a q u i t e common r e s u l t o f such i n t e r f e r e n c e s . The i n t e r a c t i o n between t h e two ( o r more) branches o f t h e f r a c t u r e g i v e s r i s e t o i n c r e a s i n g t r e a t i n g p r e s s u r e as compared w i t h t h e s i n g l e c r a c k . H y d r a u l i c s t i m u l a t i o n o f sandstone r e s e r v o i r s c h a r a c t e r i z e d by s u b p a r a l l e l u n i d i r e c t i o n a l n a t u r a l f r a c t u r e s s u f f e r i n e f f e c t i v i t y from the f a c t t h a t the hydraulic cracks propagate p a r a l l e l t o t h e n a t u r a l f r a c t u r e t r e n d (LORENZ & FINLEY 1987).
4.8.8.1.3.2.2. Hydraulic fracture
propagation parallel to natural cracks As h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s a r e g e n e r a l l y p a r a l l e l t o n a t u r a l and t o t h e p r i n c i p a l h o r i z o n t a l s t r e s s o f t h e system, t h e a r t i f i c i a l f r a c t u r e s conseq u e n t l y o f t e n f a i l t o i n t e r c e p t t h e main n a t u r a l f r a c t u r e s (AGUILERA 1983). Conj u g a t e c r a c k s m i g h t be c r o s s e d and connected by h y d r a u l i c f r a c t u r e s , b u t have lower t r a n s m i s s i b i l i t i e s than t h e main n a t u r a l j o i n t s which g u i d e t h e propagat i o n d i r e c t i o n o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s t o e x t e n d more o r l e s s p a r a l l e l t o them. I n a d d i t i o n , s i n c e c o n j u g a t e c r a c k s a r e under compressive s t r e s s , t h e y p r o b a b l y t e n d t o c l o s e as t h e p r e s s u r e w i t h i n t h e h y d r a u l i c f r a c t u r e s i s reduced, t h e r e b y p r o v o k i n g sharp d e c l i n e s i n p r o d u c t i o n r a t e s . Another s t i m u l a -
602 tio n problem a r i s e s from the usually l ar g e thickness of n a t u r a l l y fra c ture d re s e r v o i r s w i t h o u t p l a s t i c l ay er s i n bet\Jeen. Even i f only a s h o r t inte rva l i s perforated a n d t r e a t e d according t o the 1 irni ted-entry technique, the hydraulic f r a c t u r e grows upwards a n d downwards u n t i l i t reaches the p l a s t i c b a r r i e r s bounding the r e s er v o i r complex, and consequently the e xte nt of the f r a c t u r e away from the wellbore i n l a t e r a l d i r e c t i o n i s lim ite d ( c f . se c tion 4 . 2 . 3 . 2 . ) .
4.8.8.1.3.3.Relationship between
ancient and actual stress system The combination of impacts of depositional environment, natural c ra c ks, a n d i n - s i t u s t r e s s e s places c o n s t r a i n t s on the ef fe c tive ne ss of hydraulic f r a c t u ring as a means of s t i mu l at i n g n a t u r a l l y j o i n te d re se rvoirs (NORTHROP 1988, NORTHROP & FROHNE 1988). I f p al aeo s t r es s s t a t e having c re a te d the n a t u r a l crack system a n d actual s t r e s s s t a t e g u i d i n g the o r i g i n of hydraulic f r a c t u r e s have the same o r i e n t a t i o n , the hydraulic f r a c t u r e w i l l p a r a l l e l the natural j o i n t s and thus i n t e r s e c t r e l a t i v e l y few of the more conductive pa ths, w i t h thus short hydraulic f r a c t u r e s perpendicular t o the n a t u r a l crack d i r e c t i o n being j u s t as e f f e c t i v e as a much l ar g er hydraulic f r a c t u r e which p a r a l l e l s the n a t u r a l j o i n t system (BRANAGAN, C I P O L L A , L E E & CHEN 1987). Ac c e ssibility of gas re se rve s i n such n a t u r a l l y cracked r e s e r v o i r s t h er ef o r e sometimes even renders a r t i f i c i a l a l t e r a t i o n of the s t r e s s s t a t e f e a s i b l e i n order t o t r i g g e r hydraulic f r a c t u r e propagation perpendicular t o the n a t u r a l j o i n t system (WARPINSKI & BRANAGAN 1988; c f . s e c t i o n 4 . 9 . 4 . 1 . 3 . 1 . ) . I f n a t u r a l f r a c t u r e s ar e o r i en t ed in an acute angle t o the princ ipa l s t r e s s d i r e c t i o n , they can be i n f l a t e d during s t i mu la tion and can propagate f u r t h e r bef o r e the t r e a t i n g pressure i s inducing f a i l u r e i n the stress-determined d i r e c t i o n (OVERBEY, YOST & YOST 1988). The sequential development of t h i s p a t t e r n i s describsd by YOST, OVERBEY, NILKINS & LOCKE (1988) as a succession of f i r s t opening a n d widening of natural f r a c t u r e s a t lower i n j e c t i o n pressures i n a low-angle p o s i t i on t o the p r i n ci p al s t r e s s a n d second c re a tion of newly induced cracks i n the predetermined d i r e c t i o n a t higher pre ssure s. EASON (1985) a l s o p o i n t s o u t t h a t a successful hydraulic stimulation comprises l a t e r a l penetratio n of the induced f r a c t u r e away from the wellbore and i n t e r s e c t i o n of a la rge number of n a t u r a l cr ack s .
4.8.8.1.4. Interfracture comnunication LORENZ, BRANAGAN, NARPINSKI & SATTLER (1986) describe a system of unidire c tio n a l s u b p a r a l l el or anastomosed n a t u r a l f r a c t u r e s t h a t origina te d under conditio n s of h i g h pore pressures and r e l a t i v e l y low d i f f e r e n t i a l horizontal s t r e s ses r a t h e r than by s t r u c t u r a l deformation which control the evolution of secondary permeability i n many t i g h t l e n t i c u l a r non-marine f l a t - l y i n g c l a s t i c r e s e r v o i r rocks. The cracks a r e s u b p ar al l el w i t h infrequent low-angle i n t e r s e c t i o n s allowing only l i mi t ed i n t e r f r a c t u r e communication, a n d orthogonal connecting j o i n t s a r e r a r e . Some aspects of permeability c ontinuity a n d s t r e s s s e n z i t i v i t y a r e o u t l i n e d as follows.
4.8.8.1.4.1. Permeability continuity Permeability co n t i n u i t y across the f r a c t u r e pa tte rn within r e s e r v o i r s does e x i s t , b u t r e q u i r es t h a t f l u i d s t r av el along the j o i n t s a n d z i g - z a g back a n d f o r t h t h r o u g h low-angle i n t e r s e c t i o n s of adjacent subpa ra lle l c ra c ks, w i t h these paths being a n order of magnitude or more longer t h a n the d i r e c t route (LORENZ, BRANAGAN, WARPINSKI & SATTLER 1986). Co-linear f r a c t u r e s a re connected by en-echelon o f f s e t s t h a t may coalesce t o become a n apparent s i n g u l a r crack d u r i n g erosional s t r e s s r e l i e f , a n d j o i n t s occur i n a spectrum uf lengths a n d spac in g s. The en-echelon connections commonly c r e a t e e f f e c t i v e f r a c t u r e lengths
603 t h a t a r e bounded o n l y by t h e l i m i t s o f t h e r e s e r v o i r o r by l a r g e - s c a l e heterogeneities.
internal
4.8.8.1.4.2. Stress sensitivity Most f r a c t u r e s t e r m i n a t e v e r t i c a l l y w i t h i n r e s e r v o i r sandstones a t m i n o r lit h o l o g i c a l d i s c o n t i n u i t i e s such as mudstone i n t e r b e d s and l e n s e s i n f l u v i a l channel sands ( c f . p l a t e s I I / 7 - 8, III/l- 2, I V / 7 - 8, V I I / 5 - 6 and I X / 4 - 8 ) as w e l l as mudstone c o n t a c t s bounding t h e sandstone pays ( c f . p l a t e s I V / 4 - 5, V I I / 3 , V I I I / 6 - 8 and I X / 1 - 3 ) , w i t h t h e r e f o r e f r a c t u r e s w i t h i n d i f f e r e n t r e s e r v o i r s t o r e y s b e i n g p o o r l y connected t o c r a c k s w i t h i n t h e o v e r - and u n d e r l y i n g beds which r e s u l t s i n r e s t r i c t e d v e r t i c a l f r a c t u r e c o n t i n u i t y (LORENZ, BRANAGAN, WARPINSKI & SATTLER 1986). As a consequence o f t h e i r m a i n l y n a r row w i d t h , most o f t h e j o i n t s a r e c o n s i d e r e d t o be h i g h l y s t r e s s - s e n s i t i v e due t o c l o s u r e o f gaps between a s p e r i t i e s i n t h e same way t h a t l o w - p e r m e a b i l i t y mat r i x r o c k s a r e s t r e s s s e n s i t i v e t o m i c r o c r a c k s , and many o f t h e f l o w p a t h s may be s u f f i c i e n t l y narrow t o c r e a t e l a r g e c a p i l l a r y f o r c e s t h e r e b y i n h i b i t i n g liq u i d f l o w which emphasizes t h e n o n - l i n e a r r e l a t i o n s h i p o f f r a c t u r e w i d t h and transmi s s i b i 1 it y . H i g h t r e a t m e n t p r e s s u r e s d u r i n g h y d r a u l i c s t i m u l a t i o n o f such n a t u r a l l y j o i n t e d r e s e r v o i r s i n d i c a t e complex f r a c t u r i n g due t o i n t e r a c t i o n w i t h t h e p r e e x i s t i n g c r a c k s . Decreasing f l o w r a t e and average f o r m a t i o n c a p a c i t y o f t h e nat u r a l l y j o i n t e d pay d e s p i t e t h e presence o f a new a r t i f i c i a l h i g h - c o n d u c t i v i t y f r a c t u r e r e f l e c t s damage o f t h e p r e - e x i s t i n g c r a c k s h a v i n g been i n t e r s e c t e d by the hydraulic f r a c t u r e ( c f . section 4.8.8.3.).
4.8.8.2. Fracture divergence The i n i t i a l l y p a r a l l e l elements o f t h e f r a c t u r e bundle d i v e r g e f r o m one ano t h e r a t an a n g l e t h a t d i m i n i s h e s w i t h i n c r e a s i n g i n j e c t i o n r a t e , f l u i d v i s c o s i ty, f i e l d s t r e s s , and i n i t i a l spacing (LORENZ, BRANAGAN, WARPINSKI & SATTLER 1986). Such u n i f o r m d i v e r g e n c e and simultaneous p r o p a g a t i o n a r e n o t expected t o be observed i n a c t u a l h y d r a u l i c f r a c t u r e s , because s l i g h t d i f f e r e n c e between t h e two branches would l i k e l y l e a d t o one branch d o m i n a t i n g and c o n t i n u i n g t o grow a t t h e expense o f t h e o t h e r . Branching i s n e v e r t h e l e s s expected t o a f f e c t t h e w i d t h o f t h e dominant f r a c t u r e n e a r t h e d i v i s i o n by p o s s i b l y c a u s i n g l o c a l r e s t r i c t i o n s t h a t m i g h t g i v e r i s e t o p r o p p a n t b r i d g i n g and screenout (BLANTON 1982, 1986; JEFFREY, VANDAMME & ROEGIERS 1987). Abnormal t r e a t i n g p r e s s u r e s i n h y d r a u l i c f r a c t u r i n g o p e r a t i o n s a r e a l s o r e p o r t e d by MEDLIN & FITCH ( 1 9 8 3 ) . Thus
a c o m b i n a t i o n o f n a t u r a l and a r t i f i c i a l f r a c t u r i n g s h o u l d p r e f e r e n t i a l than creat i o n o f an independent system o f s y n t h e t i c f i s s u r e s p e r p e n d i c u l a r o r o b l i q u e t o them, thus i m p l y i n g t h a t p r e - f r a c t u r i n g d e t e r m i n a t i o n o f t h e g e o s t r e s s f i e l d by w e l l b o r e b r e a k o u t e l o n g a t i o n i n t e r p r e t a t i o n ( c f . s e c t i o n 6.2.1.8.1.) and p r o b a b l y a l s o n a t u r a l c r a c k m o n i t o r i n g by b o r e h o l e t e l e v i s i o n (FLECKENSTEIN 1984, CLERKE & AKKEREN 1986, DARILEK 1986; c f . s e c t i o n s 6.2.1.9. and 6.2.1.10.1.) to be necessary f o r optimum d e s i g n and performance o f a r t i f i c i a l h y d r a u l i c f r a c t u r i n g i n a n a t u r a l l y c r a c k e d o r j o i n t e d r e s e r v o i r . Aspects o f i n t e r a c t i o n b e t ween p r e - e x i s t i n g ( p r i m a r y ) and h y d r a u l i c a l l y - i n d u c e d (secondary) f r a c t u r e s a r e a l s o discussed by BLANTON ( 1 9 8 2 ) . HOUZE, HORNE & RAMEY (1984); LANCASTER & GATENS (1986), MURPHY & FEHLER (1986); BRANAGAN, CIPOLLA, LEE & CHEN (1987) and BEN NACEUR & ECONOMIDES (1988) a l s o comment on h y d r a u l i c f r a c t u r i n g o f n a t u r a l l y j o i n t e d f o r m a t i o n s . I n terms o f o r i e n t a t i o n o f n a t u r a l f r a c t u r e s w i t h i n t h e g e o s t r e s s f i e l d , DULA (1981) comments on c o r r e l a t i o n between d e f o r m a t i o n l a m e l l a e m i c r o f r a c t u r e s , m a c r o f r a c t u r e s and i n - s i t u s t r e s s measurements. l y aim on enlargement o f t h e n a t u r a l c r a c k s p a r a l l e l t o t h e n r a t h e r
604
4.8.5.3. Temporary natural fracture damage Due t o the l a r g e hydrocarbon reserves held by n a t u r a l l y fra c ture d pays, t h e i r e x p l o i t a t i o n has become a challenging task f o r the petroleum industry ( P R A D O & PRAT 1987) and much p o t en t i al i s seen f o r the near f u t u r e . A r t i f i c i a l enhancement of natural f r a c t u r e systems i s expected t o play a major r o l e f o r i n c r e a s i n g p r o d u c ti v i t y of these r e s e r v o i r s . There i s , however, the danger t h a t hydraulic f r a c t u r i n g may r e s u l t i n a t l e a s t temporary subsequent decrease of the flow capacity of the natural j o i n t system near the wellbore and the a r t i f i c i a l l y induced crack, thus l i mi t i n g production capacity of the well (BRANAGAN, CIPOLLA, L E E & YAN 1987). Natural f r a c t u r e damage by water a n d stim ula tion f l u i d s can be u p t o such an ex t en t t h a t post-treatment o f f t a k e i s l e s s than pre-job e x p l o r a t i o n r a t e s (SATTLER, R A I B L E & GALL 1985). Aspects of i n t e r a c t i o n s between natural f r a c t u r e s a n d stimulation f l u i d s , f l u i d l o s s , lim ite d f r a c t u r e interconnectedness, el ev at ed treatment pressure, a n d s t r e s s s e n s i t i v i t y behaviour a n d a s p e r i t y shearing a r e o u t l i n e d as follows.
4.8.8.3.1. Interactions between
natural fractures and stimulation fluids Natural f r a c t u r e damage can be the r e s u l t of residual treatment l i q u i d s and/ o r st i m u l a t i o n f l u i d blockage ( s i m i l a r as water blocking due t o c a p i l l a r y press u r e ; HOLDITCH 1979; c f . s ect i o n s 3 . 1 1 . 2 . 4 . 2 . ) impairing the permeability o f the natural j o i n t s i n t er s ect ed by the a r t i f i c i a l crack, b u t i s ofte n of t r a n s i to r y nature a n d can be overcome by a n extended shut-in period t o enable s i g n i f i c a n t clean-up. As l i q u i d impairment o f natural f r a c t u r e s could se ve re ly reduce t h e i r r e l a t i v e gas permeability, the high c a p i l l a r y pressures of the matrix rock cause during a n extended s h u t - i n period the l i q u i d i n the natural cracks t o imbibe i n t o the m a t r i x a n d thus the almost t o t a l l i q u i d s a t u r a t i o n of the natu r a l j o i n t s decreases with time. Fracturing f l u i d residue blocking a l s o degrades due t o increasing molecular weight of the polymers with time and temperature, with thereby b o t h processes leading t o increasing gas permeability. The worst case of n a t u r a l crack damage occurs i n a n i so t r o p i c systems where a d i r e c t i o n of the subsequently induced hyd r a u l i c f r a c t u r e p a r a l l e l t o the maximum-permeability natural j o i n t s can magnif y the d e t e r i o r a t i o n e f f e c t s a n d minimize production enhancement (BRANAGAN, C I POLLA, L E E & YAN 1987). GALL, MALONEY, SATTLER & R A I B L E (1988) r eport permeability d e t e r i o r a t i o n of n a t u r a l f r a c t u r e s caused by s t i mu l at i o n f l u i d polymers, a n d SATTLER, RAIBLE, GALL & GILL (1988) describe s u i t a b l e f r a c t u r i n g f l u i d s f o r n a t u r a l l y cracked t i g h t gas sandstones in order t o minimize natural j o i n t damage. I n many t i g h t gas sandstones, natural f r a c t u r e s dominate pre-stimulation production, and a very important question of post-stimulation offta ke i s how well the n a t u r a l cracks remain open. I n t e r a c t i o n s between n a t u r a l f r a c t u r e s a n d treatment f l u i d s a r e r e f l e c t e d by high operating p r es s u r es , acc e le ra te d or dual leakoff ( c f . sect i o n 4 . 8 . 8 . 3 . 2 . 3 . ) , and highly increased values of a c c e ssible r e s e r v o i r surfa c e upon reversal of gas flow. The summary as follows includes comments on natural crack width, d u a l f l u i d l eak o f f , c a p i l l a r y f o r c e s , impact of breaker systems a n d water blocking.
4.8.8.3.1.1. Natural crack width The narrowness of many natural cracks makes them susc e ptible b o t h t o f l u i d damage during s t i mu l at i o n a n d t o changes in the local s t r e s s f i e l d as a r e s u l t of hydraulic f r a c t u r i n g o r drawdown from production. Very h i g h pore c a p i l l a r y pressures may tend t o keep the narrow natural j o i n t s r e l a t i v e l y f r e e o f water. Abnormally h i g h treatment pressures above the c losure s t r e s s e s or even above the containment b a r r i e r s as determined from i n - s i t u s t r e s s t e s t i n g a n d a l s o oc-
605 c u r r i n g d u r i n g screenouts ( c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) suggest t h a t t h e n a t u r a l f r a c t u r e s open up under such h i g h t r e a t i n g , p r e s s u r e s o r t h a t s t i m u l a t i o n g e l i s f o r c e d i n t o them o r b o t h . F r a c t u r i n g f l u i d once f o r c e d i n t o t h e narrow n a t u r a l c r a c k s would be d i f f i c u l t t o r e c o v e r a t e x i s t i n g r e s e r v o i r p r e s s u r e .
4.8.8.3.1.2. Dual fluid leakoff Opening up o f n a t u r a l f r a c t u r e s w i t h t r e a t m e n t f l u i d i n j e c t i o n a t h i g h e r s t i m u l a t i o n p r e s s u r e s i s a l s o i n d i c a t e d by dual f l u i d l e a k o f f c o n s i s t i n g o f normal l e a k o f f a t l o w e r p r e s s u r e s and a c c e l e r a t e d l e a k o f f a t a p r e s s u r e h i g h enough t o t r i g g e r t h e n a t u r a l c r a c k s t o open up (WARPINSKI 1988 a ) . I f g e l s t a b i l i z e d by methanol and c o n t a i n i n g l i t t l e b r e a k e r i s pumped i n t o t h e n a t u r a l f r a c t u r e system, p o s t - s t i m u l a t i o n workover o p e r a t i o n s i n v o l v i n g l a r g e q u a n t i t i e s o f w a t e r may suppress f o r m a t i o n temperatures and r e t a r d d e g r a d a t i o n o f t h e g e l t r a p p e d i n t h e n a t u r a l c r a c k network. 100 mesh sand does n o t i n c r e a s e p e r m e a b i l i t y damage i n t h e presence o f a b r e a k e r , b u t seems t o i n c r e a s e p e r m e a b i l i t y d e t e r i o r a t i o n w i t h o u t a breaker, and b r e a k e r added t o b i o g e l may m i t i g a t e p e r m e a b i l i t y damage (SATTLER, RAIBLE, GALL & GILL 1988). The d i s c u s s i o n as f o l l o w s focusses on 100 mesh sand as f l u i d - l e a k o f f a d d i t i v e and microproppant, c o n v e n t i o n a l f l u i d - l o s s a d d i t i v e s vs. 100 mesh sand, and f l i p - f l o p i n s e r t i o n o f 100 mesh sand.
4.8.8.3.1.2.1. 100 mesh sand as f hid-loss additive and microproppant 100 mesh sand i s v e r y e f f e c t i v e as a c o m b i n a t i o n o f f l u i d - l o s s a d d i t i v e and m i c r o p r o p p a n t by r e d u c i n g f l u i d l e a k o f f due t o h a i r l i n e f i s s u r e p l u g g i n g , b u t i n s t a l l i n g a s u f f i c i e n t l y permeable s u p p o r t i n g wedge w i t h i n t h e m i c r o c r a c k s t o a l l o w them l a t e r t o e f f e c t i v e l y c o n t r i b u t e t o r e s e r v o i r d r a i n a g e (WARPINSKI 1988 a; c f . a l s o s e c t i o n 4 . 8 . 8 . 3 . 2 . 3 . ) . 100 mesh sand d i m i n i s h e s secondary f l u i d loss t h r o u g h i n t e r s e c t i n g h a i r l i n e f i s s u r e s , b u t does n o t r e s t r i c t f l u i d t r a v e l down t h e p r i n c i p a l f r a c t u r e and t h u s p e r m i t s t h e subsequent s t i m u l a t i o n f l u i d t o e x t e n d t h e main c r a c k t o t h e d e s i r e d d i s t a n c e i n t o t h e r e s e r v o i r (THOMPSON 1977; c f . s e c t i o n 1.4.11.2.1.). 100 mesh sand i s p r i m a r i l y used as an added c o n d i t i o n i n g s t e p i n t h e pad f l u i d s . I n p a r t i a l l y d e p l e t e d zones which have been h e a v i l y a c i d i z e d , i t i s necessary t o use l a r g e volumes o f 100 mesh sand i n o r d e r t o f i l l a l l t h e p r e v i o u s l y c r e a t e d v o i d s so t h a t t h e new t r e a t ment can e n t e r e a r l i e r u n s t i m u l a t e d p o r t i o n s o f t h e i n t e r v a l . H a i r l i n e f i s s u r e s t h i e v i n g o f f some o f t h e f r a c t u r i n g f l u i d and t h u s p r e v e n t i n g achievement o f the predicted penetration are e i t h e r n a t u r a l l y occurring o r are stress f r a c t u r e s caused d u r i n g t h e process o f h y d r a u l i c f r a c t u r i n g and a s s o c i a t e d d i l a t a n c y ( c f . section 4.8.1.3.1.).
4.8.8.3.1.2.2. Conventional fluid-loss agents vs. 100 mesh sand The p a r t i c l e s i z e o f c o n v e n t i o n a l f l u i d - l o s s a d d i t i v e s i s t o o small t o e f f e c t i v e l y b r i d g e o f f and l i m i t f l u i d l e a k o f f i n t h e m i n i a t u r e c r a c k s (NARPIMSKI 1988 a ) . The h a i r l i n e f i s s u r e s a r e a c t u a l l y propagated w i t h f l u i d o n l y and serve as escape r o u t e s f o r t h e f l u i d t o l e a v e t h e p r i m a r y c r a c k and t o l i m i t i t s e x t e n s i o n . F l u i d l o s s t h r o u g h t h e h a i r l i n e f i s s u r e s causes a b u i l d u p o f proppant c o n c e n t r a t i o n i n t h e p r i n c i p a l f r a c t u r e which aggravates t h e s i t u a t i o n , and when t h e p r e s s u r e i s r e l e a s e d a f t e r t h e t r e a t m e n t , t h e m i c r o f r a c t u r e s c o n t a i n i n g o n l y f l u i d - l o s s a d d i t i v e s can o n l y v e r y l i t t l e c o n t r i b u t e t o hydrocarbon p r o d u c t i o n . 100 mesh sand p e r f e c t l y b r i d g e s o f f t h e f i s s u r e s b e f o r e t r a v e l l i n g v e r y f a r down t h e h a i r l i n e and b u i l d s up a secondary g r a n u l a r m a t r i x w i t h i n t h e c r a c k s which serves as a base f o r o t h e r f l u i d - l e a k o f f a d d i t i v e s t o work a g a i n s t , t h e r e by f u r t h e r l i m i t i n g f l u i d loss i n t o t h e h a i r l i n e s and keeping t h e r e s t o f t h e
605 f l u i d w i t h i n t h e p r i n c i p a l f r a c t u r e . I n c o m b i n a t i o n w i t h 100 mesh sand, o n l y sol u b l e f l u i d - l e a k o f f a g e n t s s h o u l d be used, because i n v a s i o n i n t o t h e 100 mesh sand may cause such a d e g r e e o f p l u g g i n g t h a t a l o w - p r e s s u r e f o r m a t i o n c a n n o t k i c k o u t . 100 mesh sand p e r m i t s t h e m a i n f r a c t u r e t o p e n e t r a t e d e e p e r i n t o t h e r e s e r v o i r w h i c h i n t u r n r e s u l t s i n c o n n e c t i o n o f more and more h a i r l i n e s t o t h e p r i n c i p a l c r a c k t h a t u l t i m a t e l y l e a d s t o more p r o d u c t i o n avenues c a u s i n s b o t h higher i n i t i a l offtake, l o n g e r l a s t i n g w e l l p r o d u c t i v i t y , and g r e a t e r c u m u l a t i v e e x p l o i t a b i l i t y . 100 mesh sand p l u g g i n g o f h a i r l i n e c r a c k s c a n a l s o h e l p t o c o n t a i n v e r t i c a l f r a c t u r e p r o p a g a t i o n ( c f . s e c t i o n 4.2.2.4.).
4.8.8.3.1.2.3. Flip-flop insertion of 100 mesh sand I n t e r m s o f o p e r a t i o n s , t h e f l i p - f l o p method has t u r n e d o u t t o be t h e b e s t t e c h n i q u e , c o n s i s t i n g o f a l t e r n a t i n g s l u g s o f p a d f l u i d and 100 mesh sand s l u r r y i n t h e same f l u i d b e i n g pumped ahead o f t h e a c i d o r p r o p p a n t s l u r r y (WARPINSKI 1988 a ) . These s p a c e r s s e r v e as make-up f l u i d f o r t h a t f l u i d l o s t t o t h e h a i r l i n e f r a c t u r e s i n t h e b r i d g i n g p r o c e s s and m a i n t a i n an e v e n f l o w o f cons t a n t 100 mesh sand c o n c e n t r a t i o n . The o v e r a l l r e s u l t o f t h e f r a c t u r i n g t r e a t ment seems t o be b e t t e r i f t h e t o t a l volume o f 100 mesh sand i s a b t . e q u a l t o t h e t o t a l p r o p p a n t volume i n s m a l l e r j o b s o r i f a t l e a s t 50,000 l b s o f 100 mesh sand a r e used i n l a r g e r o p e r a t i o n s . 100 mesh sand c a n be a p p l i e d i n v a r i o u s c o n c e n t r a t i o n s i n t h e range o f 1 - 6 l b s / g a l . I n many c a s e s , u s i n g 100 mesh sand pads a l l o w s t o p l a c e l a r g e r s a t u r a t i o n s and h i g h e r q u a n t i t i e s o f p r o p p a n t i n t o t h e m a i n f r a c t u r e a t l o w e r pumping r a t e s and w i t h l e s s e f f i c i e n t s t i m u l a t i o n f l u i d s t h a n i n c a s e o f t r e a t m e n t w i t h o u t 100 mesh sand pads. E f f e c t i v e t r a p p i n g o f 100 mesh sand i n t h e h a i r l i n e c r a c k s when p r e s s u r e i s r e l e a s e d a t t h e e n d o f t h e j o b l e a v e s a p r o p p e d m i c r o f r a c t u r e t h a t i s f u l l y capable o f feeding accelerated hydrocarbon p r o d u c t i o n i n t o t h e main f r a c t u r e .
4.8.8.3.1.3.Capi 1 lary forces N a t u r a l f r a c t u r e c o n d u c t i v i t y damage b y f r a c t u r i n g f l u i d s i s m a i n l y c o n t r o l l e d b y i n i t i a l p a y p e r m e a b i l i t y and p o l y m e r g e l t y p e (GALL, SATTLER, MALONEY & RAIBLE 1 9 8 8 ) . C a p i l l a r y f o r c e s c o u l d cause r e d i s t r i b u t i o n o f l i q u i d s w i t h i n t h e r e s e r v o i r and r e s u l t i n r e m o v a l o f w a t e r f r o m t h e n a t u r a l c r a c k s , and p o l y m e r r e s i d u e s i n t h e f r a c t u r e c o u l d decompose a t p a y t e m p e r a t u r e s o f 200 OF. As g e l l e d p o l y m e r f l u i d s c a n e n t e r n a r r o w n a t u r a l j o i n t f l o w c h a n n e l s and l e a v e beh i n d r e s i d u e s w h i c h c o u l d n o t be removed b y gas f l o w , a t l e a s t p a r t s o f t h e nat u r a l f r a c t u r e c o n d u c t i v i t y damage caused by p o l y m e r r e m n a n t s has t o be assumed t o be p e r m a n e n t . I n c r e a s e d l e a k o f f r a t e s w i t h p o l y m e r s o l u t i o n s c o n t a i n i n g b r e a kers suggest t h a t b u i l d u p o f polymer residues i n the crack i s slower than w i t h non-breaker-bearing g e l s . Polymer remnants deposited i n n a t u r a l c r a c k s i n t e r s e c t e d by h y d r a u l i c f r a c t u r e s r e s u l t i n r e s t r i c t e d gas f l o w , w i t h t h e o v e r a l l i n crease a f t e r t h e s t i m u l a t i o n treatment being considerably l e s s than expected. The a d d i t i o n o f b r e a k i n g a g e n t s a t l o w c o n c e n t r a t i o n s may be o f some h e l p i n r e d u c i n g p o l y m e r r e s i d u e s and damage t o gas f l o w i n n a r r o w f r a c t u r e s . As c r a c k p e r m e a b i l i t y i s a l s o s e n s i t i v e t o s t r e s s changes, p r e s s u r e drawdown i n a n a t u r a l l y j o i n t e d r e s e r v o i r c o u l d cause r e d u c t i o n s i n gas p e r m e a b i l i t y . As i n s t a l l a t i o n o f a c o n d u c t i v e h y d r a u l i c f r a c t u r e i s o f t e n accompanied b y diminution o f flow capacity o f the natural j o i n t s that are i n contact w i t h the induced crack, e f f e c t i v e n e s s o f h y d r a u l i c f r a c t u r e s t i m u l a t i o n i n t i g h t l e n t i c u l a r n a t u r a l l y j o i n t e d gas sands i s dependent on p r e s e r v i n g t h e f l o w c a p a c i t y o f t h e e x i s t i n g n a t u r a l f r a c t u r e systems (BRANAGAN, CIPOLLA, LEE & bJILNER 1 9 8 5 ) .
607
4.8.8.3.1.4. Impact of breaker systems A b r e a k e r system g e n e r a l l y s h o u l d a l l o w t h e v i s c o s i t y o f a s t i m u ? a t i o n f l u i d t o remain r e a s o n a b l y h i g h and s t a b l e o v e r t h e s h o r t e r term t o c a r r y proppants and f i n e - g r a i n e d f l u i d - l o s s a d d i t i v e sand o u t i n t o t h e h y d r a u l i c a l l y - c r e a t e d f r a c t u r e , whereas i n t h e l o n g e r term, t h e v i s c o s i t y o f t h e s t i m u l a t i o n f l u i d system s h o u l d degrade t o f a c i l i t a t e cleanup and r e c o v e r y o f t h e polymer (GALL, SATTLER, MALONEY & RAIBLE 1988). Oamage f r o m t r e a t m e n t f l u i d polymers i s respons i b l e f o r most o f t h e e a r l y p o s t - f r a c t u r i n g p r o d u c t i o n problems, and o t h e r forms o f mechanical d e t e r i o r a t i o n as w e l l as c l o s u r e due t o drawdown can make t h e n a t u r a l c r a c k s even more narrow and can compound polymer damage which m i g h t be m i t i g a t e d by use o f a prepad c o n s i s t i n g o f a b r e a k e r s o l u t i o n . T h i s approach would p u t a d d i t i o n a l b r e a k e r i n t o t h e f o r m a t i o n and would ent a i l l i t t l e r i s k o f premature v i s c o s i t y degradation, because t h e g e l has no cont a c t w i t h t h e a d d i t i o n a l b r e a k e r u n t i l b e i n g i n t h e r e s e r v o i r i t s e l f . F i e l d and l a b o r a t o r y evidence i n d i c a t e t h a t f l u i d systems i n c l u d i n g a b r e a k e r prepad a r e a p p a r e n t l y s a t i s f a c t o r y f o r s t i m u l a t i o n s o f n a t u r a l l y f r a c t u r e d t i g h t gas sandstone r e s e r v o i r s , whereas f a i l u r e t o use a p r o p e r l y v i s c o s i f i e d base f l u i d w i t h adequate l e a k o f f and foam s t a b i l i t y c h a r a c t e r i s t i c s would r i s k an uphole o r downhole screenout. BRANAGAN, LEE, CIPOLLA & WILMER (1988) a l s o emphasize t h e extreme s e n s i t i v i o f n a t u r a l c r a c k systems i n some t i g h t gas sandstones t o i n v a d i n g c o m p l e t i o n and h y d r a u l i c s t i m u l a t i o n l i q u i d s . A l t h o u g h r e s e r v o i r m a t r i x damage i s consider e d t o be d e t r i m e n t a l , t h e more s e r i o u s problem i s c l e a r l y m a i n t a i n i n g t h e h i g h l y p r o d u c t i v e f l o w p a t h s o f t h e n a t u r a l c r a c k s i n t h e i r o r i g i n a l s t a t e . The det e r i o r a t i o n mechanisms t o t h e j o i n t s a r e so severe t h a t f r a c t u r e f l o w c a p a c i t y c o u l d be d i m i n i s h e d t o t h e p o i n t o f r e n d e r i n g them u s e l e s s as c u r r e n t c o n d u i t s (BRANAGAN & WILHER 1988). ty
4.8.8.3.1.5. \dater blocking Water b l o c k i n g o f f r a c t u r e and/or f o r m a t i o n o c c u r s i f t h e drawdown p r e s s u r e g r a d i e n t i n t h e r e s e r v o i r near t h e c r a c k f a c e does n o t exceed t h e r o c k c a p i l l a r y p r e s s u r e s u f f i c i e n t l y f o r e n a b l i n g gas t o f l o w (HOLDITCH 1979; ABRAMS & V I N E GAR 1985; c f . s e c t i o n 3.11.2.4.2.). Water b l o c k i n g i s g e n e r a l l y n o t a s e r i o u s problem i n most t i g h t f o r m a t i o n s because drawdown p r e s s u r e and gas m o b i l i t y a r e u s u a l l y h i g h enough f o r e f f i c i e n t displacement o f s t i m u l a t i o n f l u i d f r o m t h e r e s e r v o i r , b u t may develop i f pay p r e s s u r e g r a d i e n t s i n t h e n e a r f r a c t u r e - f a c e r e g i o n a r e low o r i f f l u i d m o b i l i t i e s a r e c o n s i d e r a b l y reduced by f o r m a t i o n o r c r a c k damage. C a p i l l a r y p r e s s u r e i n c r e a s e s when c a p i l l a r i e s a r e s m a l l , s u r f a c e and i n t e r f a c i a l t e n s i o n s a r e high, and t h e c o n t a c t a n g l e between l i q u i d and r o c k i s l e s s than 90 degrees (BROADDUS 1988). When c a p i l l a r y p r e s s u r e exceeds bottom-hole pressure, t h e i n t e r v a l i s w a t e r - b l o c k e d . The b e s t s o l u t i o n t o a v o i d w a t e r b l o c k i n g i s p e r f o r m i n g t h e s t i m u l a t i o n oper a t i o n w i t h o n l y p a r t i a l l y aqueous o r even non-aqueous t r e a t m e n t media such as foam, e n e r g i z e d f l u i d s , n i t r o g e n o r carbon d i o x i d e ( c f . s e c t i o n s 3.11. and 4.8.8.3.2.1.). P r e v e n t i o n can a l s o be made by adding a s u r f a c t a n t t o t r e a t i n g f l u i d s which i m p a r t s low s u r f a c e t e n s i o n and changes t h e a n g l e a t which f l u i d contacts the rock face.
4.8.8.3.2. Fluid loss Damage o f f l o w and uptake c a p a c i t i e s o f n a t u r a l l y f r a c t u r e d i n t e r v a l s o f r e s e r v o i r complexes may a l s o be caused d u r i n g d r i l l i n g when s e r i o u s l o s t c i r c u l a t i o n problems occur, e s p e c i a l l y when complete mud l o s s e s t a k e suddenly p l a c e and l e a k o f f r a t e s exceed r i g pumping c a p a b i l i t i e s as a consequence o f p r e s s u r e s w i t h i n t h e d i s t u r b e d zone b e i n g commonly w e l l below t h a t o f a f r e s h w a t e r g r a -
608 d i e n t (CANSON 1985; c f . s e c t i o n 4 . 8 . 8 . 6 . ) . P a r t i a l p l u g g i n g o f t h e n a t u r a l f r a c t u r e system i s achieved by b l i n d d r i l l i n g o r i n s e r t i o n o f b r i d g i n g s o l i d s i n t o t h e f l u i d stream i n o r d e r t o c o n t r o l t h e b o r e h o l e and t o a l l o w f u r t h e r d r i l l i n g p r o g r e s s , o r even by r e p e a t e d cementing o f t h e i n t e r v a l s t a k i n g up e x c e s s i v e amounts o f mud. P e n e t r a t i o n o f a low-pressure n a t u r a l l y f r a c t u r e d r o c k i n t e r v a l i s u s u a l l y s i g n a l e d by a sudden l o s s o f r e t u r n s a t a h i g h r a t e accompanied by a n o t a b l e i n c r e a s e i n r o t a r y t o r q u e and r e l a t i v e d r i l l i n g roughness w h i c h a r e a consequence o f c r o s s i n g openings t h a t a r e l a r g e enough f o r b i t t e e t h t o f a l l i n t o o r hang up i n . Some aspects o f breakdown f r a c t u r i n g procedures, c a p i l l a r y p r e s s u r e and s u r f a c e t e n s i o n , and dual l e a k o f f b e h a v i o u r a r e o u t l i n e d as f o l lows.
4.8.8.3.2.1. Breakdown fracturing procedures BRANAGAN & WILMER (1988) d i s c u s s breakdown procedures designed t o m i n i m i z e n a t u r a l l y f r a c t u r e d r e s e r v o i r damage ( c f . s e c t i o n 4 . 8 . 2 . 2 . ) . I n t h e course o f d r i l l i n g and c o m p l e t i n g a w e l l , a number o f mechanisms degrades t h e o r i g i n a l h i g h c o n d u c t i v i t y o f n a t u r a l cracks, i n c l u d i n g i n v a s i o n o f d r i l l i n g mud t h a t may u l t i m a t e l y a c t as a p l u g , cements squeezed i n t o t h e f r a c t u r e , p e r f o r a t i o n s and n e a r - w e l l b o r e s t r e s s changes t h a t c o u l d tend t o c l o s e t h e c r a c k , and t h e i n t e r f a c i a l t e n s i o n a l f o r c e s g e n e r a l l y d e s c r i b e d by c a p i l l a r y p r e s s u r e o f invaded c o m p l e t i o n l i q u i d s which l i m i t f r a c t u r e f l o w c a p a c i t y . Some aspects o f f r a c t u r e a p e r t u r e r e s t r i c t i o n by w a t e r and non-aqueous t e c h n i q u e s f o r a v o i d i n g o f w a t e r b l o c k i n g a r e d i s c u s s e d as f o l l o w s .
4.8.8.3.2.1.1. Fracture aperture restriction by water L i q u i d s p r e s e n t i n t h e n a t u r a l j o i n t system r e s t r i c t t h e a v a i l a b l e a p e r t u r e t h r o u g h which gas can f l o w . I f w a t e r i s used i n t h e breakdown i t w i l l u n d o u b t e d l y f i l l most o f t h e nearby c r a c k s . When t h e w e l l i s on p r o d u c t i o n , t h e i n j e c t e d w a t e r must e i t h e r be removed f r o m any t h r o u g h t h e b o r e h o l e o r i t must be a l l o w e d t o imbibe i n t o t h e m a t r i x o t h e r w i s e gaseous f l o w w i l l be almost t o t a l l y impeded due t o t h e i n a b gas t o f l o w h o r i z o n t a l l y t h r o u g h t h e w a t e r ( t h i s phenomenon i s known h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s as w a t e r b l o c k i n g ; HOLDITCH 1979; c f . 3.11.2.4.2. and 4 . 8 . 8 . 3 . 1 . 5 . ) .
fracture process, ater put fracture because lity of l s o from sections
4.8.8.3.2.1.2.Non-aqueous techniques
for avoiding of water blocking
Non-aqueous m i c r o f r a c t u r i n g breakdown t r e a t m e n t s a v o i d i n g w a t e r b l o c k i n g comp r i s e h y d r a u l i c n i t r o g e n f r a c t u r i n g u s i n g s u r f a c e pump t r u c k s , p e r f o r a t i n g w i t h an underbalanced n i t r o g e n column, h i g h - p r e s s u r e n i t r o g e n impulse p e r f o r a t i n g / m i c r o f r a c t u r i n g , n i t r o g e n impulse i n j e c t i o n u s i n g a t u b i n g pump-out p l u g , and t a i l o r e d pulse explosive f r a c t u r i n g . N i t r o g e n impulse perforating/microfracturing s i m u l t a n e o u s l y p e r f o r a t e s t h e w e l l and c r e a t e s c r a c k s t h a t e x t e n d f r o m t h e b o r e h o l e and i n t e r s e c t t h e n a t u r a l j o i n t system i n t h e w e l l b o r e v i c i n i t y . Dynamically, n i t r o g e n impulse f r a c t u r i n g i s more e x p l o s i v e r a t h e r than i n j e c t i v e , as t h e compressional energy s t o r e d i n t h e h i g h - p r e s s u r e n i t r o g e n column i s almost i n s t a n t a n e o u s l y r e l e a s e d i n t o t h e p e r f o r a t i o n s i n t h e moment when t h e y a r e c r e a t e d . T a i l o r e d p u l s e e x p l o s i v e breakdown procedure ( c f . s e c t i o n 4 . 2 . 2 . 1 . 3 . 3 . ) e n t a i l s t h e use o f a r e l a t i v e l y slow b u r n i n g p r o p e l l a n t designed t o c r e a t e a m u l t i p l e s e t o f f r a c t u r e s r a d i a t i n g l i k e spokes f r o m t h e p e r f o r a t e d w e l l b o r e . I n o r d e r t o c r e a t e t h e c o r r e c t environment f o r t h e t a i l o r e d p u l s e e x p l o s i v e t o p r o p e r l y i n i t i a t e m u l t i p l e f r a c t u r e s , a f l u i d w i t h a d e n s i t y about t h a t of w a t e r b u t c o n s i d e r a b l y i n excess o f most a v a i l a b l e gases i s r e q u i r e d as tamping medium i n t h e c a s i n g , w i t h l i q u i d carbon d i o x i d e b e i n g an i d e a l w e l l b o r e tamping f l u i d . P u l s e d e x p l o s i v e s can be
609 successfully applied f o r c r e a t i o n o f m u l t i p l e sets o f unobstructed f r a c t u r e s t h a t connect w e l l b o r e and n a t u r a l c r a c k system. The same e f f e c t can be achieved by p e r f o r a t i n g i n a d r y underbalanced w e l l b o r e column o f n i t r o g e n .
4.8.8.3.2.2. Capillary pressure and surface tension Enhanced s t i m u l a t i o n f l u i d r e c o v e r y can a l s o be achieved by i m p r o v i n g p r o p p a n t pack d r a i n a g e and m i n i m i z i n g c a p i l l a r y p r e s s u r e i n t h e f o r m a t i o n t h r o u g h j u d i c i o u s a p p l i c a t i o n o f s u r f a c e a c t i v e agents which i s a p p l i c a b l e p a r t i c u l a r l y i n t i g h t gas r e s e r v o i r s and low-pressure o i l w e l l s and may be used i n a l l aqueous f r a c t u r i n g t r e a t m e n t s i n c l u d i n g s l i c k and g e l l e d w a t e r as w e l l as foamed and c r o s s l i n k e d systems (PHILLIPS & WILSON 1984 a ) . The t e c h n i q u e i n v o l v e s u t i l i z a t i o n o f a s o l v e n t i n t h e pad f l u i d s and a s u r f a c t a n t system (CLARK, PIKE & RENGEL 1979, 1980) t h r o u g h o u t t h e e n t i r e f r a c t u r i n g t r e a t m e n t , w i t h t h i s combin a t i o n i m p r o v i n g p r o p p a n t pack d r a i n a g e and m i n i m i z i n g c a p i l l a r y w a t e r b l o c k s i n t h e invaded f o r m a t i o n w h i l e m a i n t a i n i n g t h e water-wet c o n d i t i o n o f b o t h propp a n t pack and f o r m a t i o n (PHILLIPS & WILSON 1984 a ) . T h i s dual c o m b i n a t i o n accounts f o r t h e complex r e l a t i o n s h i p o f e f f e c t i v e o i l p e r m e a b i l i t y and t h u s o i l r e c o v e r y b e i n g h i g h e s t when t h e s u b s t r a t e i s s t r o n g l y water-wet and w a t e r s a t u r a t i o n i s low, and c o n v e r s e l y a s t r o n g l y o i l - w e t s u b s t r a t e and low o i l s a t u r a t i o n r e s u l t i n g i n h i g h r e l a t i v e p e r m e a b i l i t y t o w a t e r and enhanced l o a d w a t e r r e c o v e r y (OWENS & ARCHER 1971). Another i m p o r t a n t e f f e c t m i n i m i z i n g w a t e r damage i s t h a t u l t r a - l o w s u r f a c e t e n s i o n p r o v i d e d by t h e s u r f a c t a n t s h e l p s r e d u c i n g c a p i l l a r y i m b i b i t i o n o f i n j e c t e d w a t e r i n t o t h e f o r m a t i o n a t t h e f r a c t u r e f a c e (PHILLIPS & WILSON 1984 a ) , and m i n i m i z a t i o n o f c a p i l l a r y i m b i b i t i o n o f t h e i n i t i a l l e a k o f f f l u i d i s achieved by adding methanol t o t h e pre-pad and pad stages ( c f . s e c t i o n 3 . 1 1 . 4 . ) . R e s e r v o i r s w i t h low c l o s u r e s t r e s s and g r e a t e r development o f n a t u r a l f r a c t u r e s r e q u i r e an e n e r g i z e d system f o r improved r e t u r n o f s t i m u l a t i o n f l u i d s , w i t h up t o 40 % n i t r o g e n w i t h i n delayed c r o s s l i n k e d f l u i d s f o r m i n g a s t a b l e foam s t r u c t u r e i n s i d e t h e f l u i d and s u p p l y i n g adequate energy f o r t h e r e t u r n o f l o a d w a t e r (EASON 1985; c f . s e c t i o n 3.11.2.). D i m i n u t i o n o f f l u i d l o s s d u r i n g t h e p r o p p a n t - l a d e n main f r a c t u r i n g t r e a t m e n t stages can be s i m i l a r l y as f o r r e s e r v o i r c o o l i n g be achieved by f o r m a t i o n s a t u r a t i o n w i t h l a r g e amounts o f f l u i d due t o pre-pad and pad enlargement ( c f . sect i o n 4.3.4.6.1.4.), w i t h t h e cheap and s i m p l e pre-pad and pad f l u i d s b e i n g sac r i f i c e d t o save t h e expensive and complex main s t a g e f l u i d s . The purpose o f r u n n i n g e x t r e m e l y l a r g e pads i s t o a l l o w ample l e a k o f f i n t o n a t u r a l f r a c t u r e s and m a t r i x i n o r d e r t o b l o c k t h e p r e - e x i s t i n g c r a c k s and t o a v o i d f u r t h e r f l u i d l o s s i n t o t h e t e c t o n i c a l and sedimentary d i s c o n t i n u i t y p l a n e s and f i s s u r e s dur i n g t h e p r o p p a n t - l a d e n main t r e a t m e n t stages. I n e x c e p t i o n a l cases, t h e pad stage can comprise up t o 60 - 80 % o f t h e whole t r e a t m e n t q u a n t i t y (TAN, McGOWEN, LEE & SOLIMAN 1988).
4.8.8.3.2.3. Dual leakoff behaviour N a t u r a l l y f r a c t u r e d t i g h t gas r e s e r v o i r s w i t h more o r l e s s w e l l i n t e r c o n n e c t e d j o i n t systems a r e c h a r a c t e r i z e d by dual l e a k o f f b e h a v i o u r o f f r a c t u r i n g f l u i d s d u r i n g course o f h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s (WARPINSKI 1988 a ) . A t r e l a t i v e l y low i n j e c t i o n pressures, t h e base f l u i d - l o s s c o e f f i c i e n t i s v e r y low and has a magnitude t h a t i s c o n s i s t e n t w i t h e f f e c t i v e r e s e r v o i r p e r m e a b i l i t i e s o f t e n s o f m i c r o d a r c i e s . Above some c r i t i c a l pressure, however, l e a k o f f a c c e l e r a t e s and becomes as much as 50 t i m e s g r e a t e r t h a n t h e base l e v e l . When p r o p p a n t - l a d e n stages e n t e r t h e p e r f o r a t i o n s under these e l e v a t e d secondary l e a k o f f c o n d i t i o n s , t h e s l u r r y r a p i d l y s u f f e r s s i g n i f i c a n t i n c r e a s e i n v i s c o s i t y and p r o p p a n t c o n c e n t r a t i o n due t o f l u i d d i s s i p a t i o n , and d e h y d r a t i o n s c r e e n o u t f a i l u r e o c c u r s . The a s s o c i a t e d c h a r a c t e r i s t i c p r e s s u r e b e h a v i o u r i s r a p i d f l a t t e n i n g o f t h e p r e s s u r e c u r v e above some c r i t i c a l p r e s s u r e v a l u e .
610
A s e x c e s s i v e f r a c t u r e h e i g h t g r o w t h c a n b e e x c l u d e d due t o s i m i l a r c o m p l i a n ces o f a l l t h e r o c k s i n t h e i n v e s t i g a t e d i n t e r v a l o f t h e case s t u d i e s , high l e a k o f f i s t h e l i k e l y candidate f o r t h i s behaviour, although the pressure d e c l i ne p a t t e r n has i n v a r i a b l y y i e l d e d a l o w f l u i d - l o s s c o e f f i c i e n t even a t e a r l y t i mes. Thus an a c c e l e r a t e d l e a k o f f above a t h r e s h o l d p r e s s u r e t a k e s p l a c e t h r o u g h t h e n a t u r a l f r a c t u r e system. Some a s p e c t s o f mechanisms o f p r i m a r y and secondar y f l u i d l o s s , i n s e r t i o n o f 100 mesh sand, i m p a c t on r e s e r v o i r s t i m u l a t i o n , 100 mesh sand v s . o t h e r f l u i d - l o s s a d d i t i v e s , h i g h - q u a l i t y 100 mesh m i c r o p r o p p a n t s , and d e g r a d a b l e f l u i d - l e a k o f f a d d i t i v e s a r e o u t l i n e d as f o l l o w s .
4.8.8.3.2.3.1. Mechanisms o f primary and secondary fluid loss Dual f l u i d l e a k o f f c a n be t r i g g e r e d b y two mechanisms. The f i r s t p o s s i b i l i t y i s o p e n i n g o f n a t u r a l f r a c t u r e s w i t h i n t h e s a n d s t o n e pays, w i t h t h r e s h o l d p r e s s u r e t h e n b e i n g d e p e n d e n t upon d i f f e r e n c e s i n h o r i z o n t a l i n - s i t u s t r e s s e s and o t h e r r e s e r v o i r and t r e a t m e n t f a c t o r s (WARPINSKI 1988 a ) . The second way i s due t o f r a c t u r e h e i g h t g r o w t h i n t o a d j a c e n t s i l t s t o n e s and s a n d s t o n e s c o n t a i n i n g wide n a t u r a l cracks, w i t h t h r e s h o l d pressure being r e l a t e d t o the pressure a t w h i c h enough h e i g h t p r o p a g a t i o n o c c u r s f o r t h e s e open j o i n t s t o be i n t e r s e c t e d . I n some t i g h t gas f o r m a t i o n s , s t r e s s e s and o t h e r p a r a m e t e r s have such m a g n i t u des t h a t i t i s d i f f i c u l t t o d e t e r m i n e w h i c h o f t h e two modes i s t h e r e a l l y d e c i s i v e one. I n c a s e o f r e c o g n i t i o n o f t h e p r o b l e m b e i n g f l u i d l e a k o f f and n o t f r a c t u r e h e i g h t g r o w t h , complex s t i m u l a t i o n o r some o t h e r p r o c e s s , f l u i d - l o s s c o n t r o l m u s t be e f f e c t e d t h r o u g h some a d d i t i v e o r change i n t h e t r e a t m e n t s c h e d u l e t o keep i n j e c t i o n p r e s s u r e b e l o w t h r e s h o l d v a l u e . A s i t has t u r n e d o u t t o be imposs i b l e t o l o w e r t h e pumping p r e s s u r e s i g n i f i c a n t l y and s t i l l t r a n s p o r t r e a s o n a b l e amounts and c o n c e n t r a t i o n s o f p r o p p a n t s , l e a k o f f a d d i t i v e s have t o be sel e c t e d which are n o t e n t i r e l y plugging the n a t u r a l f r a c t u r e s t h a t represent t h e p r i m a r y p r o d u c t i o n p a t h s . 100 mesh sand i s an optimum a d d i t i v e , because i t b r i d ges o f f e x c e s s i v e f l u i d l e a k o f f , b u t a l s o l e a v e s p r o p p e d c o n d u c t i v e f l o w p a t h s f o r s u b s e q u e n t p r o d u c t i o n ( c f . a l s o s e c t i o n s 1 . 4 . 1 1 . 2 . 1 . and 4 . 8 . 8 . 3 . 1 . 2 . ) .
4.8.8.3.2.3.2.Insertion o f 100 mesh sand E x p e r i m e n t a l s t i m u l a t i o n work has r e v e a l e d t h a t 100 mesh sand can r e d u c e t h e l e a k o f f d u r i n g pumping by a f a c t o r o f 3 . 5 f r o m 70 t i m e s t h e base l e v e l t o o n l y 20 t i m e s t h e i n i t i a l r a t e i f added o n l y i n s m a l l c o n c e n t r a t i o n s . 100 mesh sand a l s o helps t o b r i d g e - o f f f r a c t u r e h e i g h t growth a t l e a s t w h i l e i t i s a c t i v e l y i n j e c t e d , and a l s o g i v e s r i s e t o i n c r e a s i n g f l u i d - l o s s c o e f f i c i e n t d u r i n g p r e s s u r e d e c l i n e (WARPINSKI 1988 a; c f . a l s o s e c t i o n 4 . 8 . 8 . 3 . 1 . 2 . ) . This behaviour i s t r i g g e r e d b y p r o p p i n g o f t h e n a t u r a l j o i n t s b y t h e 100 mesh sand, t h e r e b y p r o v i d i n g an enhanced f l o w p a t h f o r h y d r o c a r b o n p r o d u c t i o n i n t o t h e h y d r a u l i c fracture. Optimum p e r f o r m a n c e o f 100 mesh sand i s g i v e n b y c o n t i n u o u s a d d i t i o n o f s m a l l s a t u r a t i o n s t h r o u g h a l m o s t t h e e n t i r e t r e a t m e n t w i t h t h e e x c e p t i o n o f an i n i t i a l pad, w i t h t h e q u a n t i t i e s , however, h a v i n g t o be l i m i t e d t o such an amount t h a t c o n d u c t i v i t y d e t e r i o r a t i o n as a consequence o f i n t e r m i x i n g o f 2 0 / 4 0 p r o p p a n t s and 100 mesh sand ( c f . s e c t i o n 1 . 4 . 1 1 . 2 . 1 . ) i s s t i l l s u b o r d i n a t e and i n s i g n i f i c a n t . A f r a c t u r e c o n d u c t i v i t y r e d u c t i o n b y a b t . a f a c t o r o f two w h i c h can be a c c e p t e d i n m o s t o f t h e l o w - p e r m e a b i l i t y r e s e r v o i r s because s t i l l a v e r y h i g h c o n d u c t i v i t y c o n t r a s t i s l e f t p e r m i t s t o add a l r e a d y such c o n c e n t r a t i o n s o f 100 mesh sand t h a t t h e d u a l t a s k o f m i c r o p r o p p i n g t h e h a i r l i n e c r a c k s and c o n t r o l l i n g f l u i d l e a k o f f r a t e c a n be s u c c e s s f u l l y c a r r i e d o u t .
611
4.8.8.3.2.3.3. Impact on reservoir stimulation Dual f l u i d - l o s s m o d e l l i n g and c o n t r o l i s a s u i t a b l e mechanism t o improve s t i mulation r e s u l t s i n low-permeability highly-anisotropic n a t u r a l l y f r a c t u r e d t i g h t gas r e s e r v o i r s which are o t h e r w i s e unable t o be t r e a t e d w i t h o u t e n c o u n t e r i n g an e a r l y screenout t h a t g i v e s r i s e t o o n l y s h o r t propped f r a c t u r e l e n g t h s and h i g h p r e s s u r e s above c l o s u r e (WARPINSKI 1988 a ) . D e h y d r a t i o n s c r e e n o u t i s a r e s u l t o f h i g h l e a k o f f c o n d i t i o n s a t e l e v a t e d i n j e c t i o n p r e s s u r e s and can be t r i g g e r e d by e i t h e r opening o f n a t u r a l f r a c t u r e s w i t h i n t h e pay o r h e i g h t propag a t i o n i n t o nearby l a y e r s c o n t a i n i n g many wide open c r a c k s ( c f . s e c t i o n 6.2.4.2.1.). I f the f r a c t u r e s i n the r e s e r v o i r d i l a t e a t pressure conditions above some t h r e s h o l d , t h e damage i s p r o b a b l y due t o i n j e c t i o n o f l a r g e amounts o f s t i m u l a t i o n f l u i d i n t o t h e n a t u r a l f r a c t u r e s , and cleanup o f these c r a c k s would be d i f f i c u l t because o f o c c u r r i n g under low-pressure c l o s e d c o n d i t i o n s . R e c o g n i t i o n o f t h e dual l e a k o f f phenomenon r e q u i r e s a n a l y s i s o f b o t h p r e s s u r e d e c l i n e a f t e r s h u t - i n and i n j e c t i o n p r e s s u r e e v o l u t i o n i n c o n j u n c t i o n w i t h a h i s t o r y match o f t h e t r e a t m e n t p r e s s u r e .
4.8.8.3.2.3.4. 100 mesh sand vs. other fluid-loss additives W h i l e i n some t i g h t gas r e s e r v o i r s a c c e l e r a t e d f l u i d l e a k o f f due t o opening o f n a t u r a l f r a c t u r e s can be avoided by s i m p l y keeping t r e a t m e n t p r e s s u r e s below t h e c r i t i c a l l e v e l , i n o t h e r f o r m a t i o n s economical s t i m u l a t i o n w i t h correspondi n g low i n j e c t i o n r a t e s i s n o t p o s s i b l e and f l u i d - l o s s a d d i t i v e s a r e r e q u i r e d i n o r d e r t o a l l o w t o r a i s e o p e r a t i n g p r e s s u r e s t o t h e l e v e l needed f o r p r o f i t a b l e t r e a t m e n t (WARPINSKI 1988 a ) . As t h e n a t u r a l f r a c t u r e s a r e a l s o t h e primar y gas p r o d u c t i o n pathways, f l u i d l e a k o f f a d d i t i v e s t o t a l l y p l u g g i n g t h e h a i r l i n e c r a c k s such as s i l i c a f l o u r cannot be employed because t h e r e i s no guarant e e t h a t complete cleanup o f t h e f i s s u r e s i n c l u d i n g e j e c t i o n o f t h e b l o c k i n g mat e r i a l s c o u l d be e f f e c t e d . 100 mesh sand i s a b l e t o c o n t r o l b o t h f l u i d l o s s i n t o n a t u r a l f r a c t u r e s and p r o v i d e enhanced p e r m e a b i l i t y p a t h s by a c t i n g as a p l u g g i n g m i c r o p r o p p a n t i n t h e narrow j o i n t s ( c f . a l s o 4 . 8 . 8 . 3 . 1 . 2 . ) . Some enhanced b r i d g i n g o f n a t u r a l c r a c k s may even r e s u l t f r o m i n t e r a c t i o n w i t h t h e 20/40 mesh main p r o p p a n t s . As a d d i t i o n o f 100 mesh sand does n o t c o m p l e t e l y s t o p a c c e l e r a t e d f l u i d l e a k o f f a t c o n c e n t r a t i o n s which do n o t y e t s e r i o u s l y d e s t r o y p r o p p a n t c o n d u c t i v i t y ( c f . s e c t i o n 1 . 4 . 1 1 . 2 . 1 . ) , r e t a r d e d f l u i d loss s t i l l l i m i t s t h e amount o f proppants t h a t can be p l a c e d i n t o such r e s e r v o i r s w i t h o u t s u f f e r i n g f r o m an e a r l y screenout. U n d e r e s t i m a t i o n o f f l u i d - l e a k o f f c o e f f i c i e n t g i v i n g r i s e t o premat u r e s c r e e n o u t t e r m i n a t i o n o f t h e s t i m u l a t i o n j o b i s a l s o r e p o r t e d by NORTHCUTT, ROBERTSON & HANNAH (1988). The use o f 100 mesh sand i n n a t u r a l l y f r a c t u r e d r e s e r v o i r s as a t r a n s i t i o n a l stage between pad and p r o p p a n t - l a d e n f l u i d ( c f . a l s o HOLDITCH, ROBINSON, WHITEHEAD & ELY 1987) serves as f l u i d - l o s s a d d i t i v e and as i n i t i a l p r o p p a n t and a l l o w s s i g n i f i c a n t i n c r e a s e o f proppant concent r a t i o n due t o e f f e c t i v e l e a k o f f c o n t r o l .
4.8.8.3.2.3.5.High-qua1 i ty 100 mesh microproppants and degradable fluid-leakoff additives I n deeper r e s e r v o i r s w i t h h i g h e r c l o s u r e s t r e s s , i t would be d e s i r a b l e t o use 100 mesh i n t e r m e d i a t e - t o h i g h - s t r e n g t h s y n t h e t i c proppants as an e f f e c t i v e m i c r o p r o p p a n t f o r h a i r l i n e cracks, b u t u n f o r t u n a t e l y t h i s g r a i n s i z e o f a r t i f i c i a l h i g h - q u a l i t y proppants i s n o t y e t c o m m e r c i a l l y a v a i l a b l e ( c f . s e c t i o n s 1 . 3 . 4 . and 1 . 5 . 1 . 1 . ) . A s u i t a b l e a l t e r n a t i v e would be r e s i n - c o a t e d 100 mesh sand which s i m i l a r l y as c o n v e n t i o n a l 100 mesh sand has a b t . 5 - 7 d a r c y permeab i l i t y a t almost any c l o s u r e s t r e s s (FRACFAX 1988 f ) , w i t h r e s i n bonding c r e a t i n g a s t a b l e m i c r o p r o p p a n t package i f no flowback can be t o l e r a t e d ( c f . sect i o n 1.2.6.).
612
Another approach t o dual f l u i d leakoff control i s a degradable a d d i t i v e which l a t e r a f t e r f u l f i l l i n g of i t s function of re ta rding o r even preventing f l u i d escape i s removed by d i s s o l u t i o n or d i s i n t e g r a t i o n a n d ultim a te ly leaves the natural f r a c t u r e s open in order t o enable them t o c ontribute t o production without any r e s t r i c t i o n (CANTU & BOYD 1988; NORTHCUTT, ROBERTSON & HANNAH 1988; c f . s e c t i o n 4 . 3 . 4 . 6 . 4 . ) . On the o t h e r hand, however, primarily plugged a n d secondarily reopened h a i r l i n e cracks as a consequence of l a t e r degradation of the o r i g i n a l l y i n se r t ed f l u i d - l o s s ad d i t i v e may r e s u l t in c losure of the open f i s s u r e s in high-pressure r e s e r v o i r s as a consequence of absence of support a ga inst the compressive s t r e s s , w i t h the natural f r a c t u r e s thus no longer being a ble t o c o n t r i b u t e t o gas production. Micropropping of h a i r l i n e cracks with m a t e r i a l s t h a t can adequately r e s i s t t o the cl o s u r e s t r e s s i s the re fore the p r e f e r e n t i a l s o l u t i o n of d u a l f l u i d leakoff c o n t r o l . The signific a nc e of secondary f l u i d lo ss i n t o n a t u r a l and s y n t h et i c h a i r l i n e f r a c t u r e s and the successful control of dual f l u i d leakoff by 100 mesh sand i s a l s o discussed by CROW (1977).
4.8.8.3.3.
Limited fracture interconnectedness
LORENZ, BRANAGAN, WARPINSKI & SATTLER (1986) a l s o i l l u s t r a t e temporal damage of a complex n a t u r a l crack system due t o influence by l i q u i d s d u r i n g hydraulic f r a c t u r e s t i m u l a t i o n , N a t u r a l j o i n t s ar e common i n numerous geological s e t t i n g s where major s t r u c t u r a l movements a r e important, b u t they a l s o form in provinces of l i t t l e o r no s t r u c t u r a l deformation i f s u f f i c i e n t i n - s i t u pore pressures a n d some horizontal s t r e s s anisotropy e x i s t . These conditions give r i s e t o a f r a c tu r e network c o n s i s t i n g of u n i d i r ect i o n al subpa ra lle l cracks t h a t a r e aligned w i t h the maximum horizontal s t r e s s and which a re limited v e r t i c a l l y by irre gul a r l i t h o l o g i c a l d i s c o n t i n u i t i e s in the r e s e r v o i r . Fra c ture s r a r e l y extend across intervening s h al es between major sandstones and thus each r e s e r v o i r sandstone st o r e y r ep r es en t s a n i s o l a t e d s t r a t i g r a p h i c a l t r a p i f surrounded by mudstone a l s o in terms of crack d i s t r i b u t i o n . With s u f f i c i e n t time of shut-in a f t e r the hydraulic f r a c t u r i n g operation, l i q u i d s in the natural j o i n t s could then be imbibed i n t o the even smaller pore spaces of the matrix rock, thus a l l o w i n g gas flow i n the natural cracks t o r et urn t o the origina l undamaged condition. Naturally f r a ct u r ed r e s e r v o i r s w i t h l i mite d crack interconnectedness a r e s t r e s s - s e n s i t i v e (NORTHROP 1988). Production can be e s s e n t i a l l y shut-off by re ducing the bottomhole p r es s u r e, w i t h the increasing e f f e c t i v e s t r e s s being suff i c i e n t t o squeeze the n a t u r a l f r a c t u r e s shut. Conversely i n j e c t i o n of gas a t high pressures r e s u l t s i n a d r amat i cal l y increased volume of a c c e s s i b l e r e s e r v o i r s u r f a c e due t o opening of the f r a c t u r e s . Depending on s t a b i l i t y o r m o b i l i t y of s t r e s s d i r e c t i o n s during course of the geological h i s t o r y of the petroleum-prospective basin, h y d r au l i cal l y induced f r a c t u r e s w ill propagate in a d i r e c t i o n p a r a l l e l o r a t an angle t o the o r i e n t a t i o n o f the na tura l cracks ( c f . s e c t i o n 4 . 8 . 8 . 1 . 3 . 2 . ) . The most favourable s i t u a t i o n f o r interconnection of nat u r a l j o i n t s by the hydraulic f r a c t u r e i s a r o t a t i o n of the princ ipa l s t r e s s d i r e c t i o n a f t e r formation of the n a t u r a l cracks t o enable an acute angle between n a t u r a l f r a c t u r e s a n d i n t e r s e c t i n g hydraulic f r a c t u r e t o occur.
I n case of s t i l l p a r a l l e l s t r e s s d i r e c t i o n , the hydraulic f r a c t u r i n g process e i t h e r only includes i n f l a t i o n a n d enlargement of e x i s t i n g n a t u r a l c ra c ks, or the s t r e s s s t a t e has t o be a r t i f i c i a l l y a l t e r e d by mechanical o r thermal i n fluences ( c f . sect i o n 4 . 9 . 4 . ) . G R I (1988) de sc ribe s t i g h t gas sandstones i n an area of t e c t o n i c a l quiescence where natural f r a c t u r e s , d r i l l i n g - a n d coring-induced cracks a n d hydraulic f r a c t u r e s have a l l the same trend in the f i e l d .
4.8.8.3.4.
Elevated treatment pressure
Temporary n a t u r a l f r a c t u r e damage i s b es t expressed by abnormally high t r e a t ment pressures being u p t o four times g r e a t e r t h a n pre dic te d. As the hydraulic
613 f r a c t u r e i n t e r s e c t s s u b p a r a l l e l c r a c k s t h a t a r e d e v i a t e d a t some angle, i t i s l i k e l y t h a t some o r many o f these n a t u r a l j o i n t s open up and i n i t i a t e secondary h y d r a u l i c f r a c t u r e s t r a n d s , g i v i n g r i s e t o a wide zone o f f r a c t u r i n g . The e x t r a t o r t u o s i t y and a d d i t i o n a l w a l l s u r f a c e s o f t h e many c r a c k s t r a n d s i n c r e a s e f l u i d f r i c t i o n and t h u s g i v e r i s e t o c o n s i d e r a b l y h i g h e r i n j e c t i o n p r e s s u r e s . The damage o f t h e n a t u r a l f r a c t u r e system i s a s e l f - d r i v e n mechanism which temp o r a r i l y i m p a i r s p r o d u c t i v i t y by e l i m i n a t i n g t h e most f a v o u r a b l e d e p l e t i o n mechanism of t h e t i g h t gas sands. The h i g h e r t r e a t m e n t p r e s s u r e s t r i g g e r e d by i n c r e a s i n g f r i c t i o n i n t h e n a t u r a l j o i n t system r e s u l t i n g r e a t e r f l u i d l e a k o f f i n t o t h e n a t u r a l f r a c t u r e s , l a r g e r shear s t r e s s e s and h i g h e r p o r e p r e s s u r e s t o induce shear s l i p p a g e , and g r e a t e r e f f e c t i v e propped w i d t h s t o e l e v a t e proppant-induced s t r e s s e s . I n c r e a s i n g o p e r a t i n g p r e s s u r e i n response t o t h e presence o f t h e n a t u r a l f r a c t u r e system g i v e s a l s o r i s e t o i n c r e a s i n g d e t e r i o r a t i o n . The damage s c e n a r i o i s most eas i l y seen f o r small h y d r a u l i c f r a c t u r e s . L a r g e r t r e a t m e n t s c o n t a c t more e x t e n s i v e m a t r i x areas and t h u s l o s t p r o d u c t i o n through t h e n a t u r a l f r a c t u r e s i s compensated somewhat by i n c r e a s e d m a t r i x f l o w . W i t h s m a l l jobs, productivity loss i s a c l e a r signal o f deleterious operation. Concerning damage o f narrow n a t u r a l c r a c k systems by s t i m u l a t i o n f l u i d s , h i g h f r a c t u r i n g p r e s s u r e s e i t h e r open t h e i n t e r s e c t i n g n a t u r a l j o i n t s o r f o r c e f l u i d and i t s a s s o c i a t e d o r g a n i c m a t e r i a l s i n t o them under h i g h d i f f e r e n t i a l p r e s s u r e (NORTHROP 1988, NORTHROP & FROHNE 1988). Opening o f t h e c r a c k system i s i n d i c a t e d by s i g n i f i c a n t l y i n c r e a s e d l e a k o f f c o e f f i c i e n t s above a t h r e s h o l d p r e s s u r e . O t h e r reasons o f v e r y h i g h t r e a t m e n t p r e s s u r e s a r e h i g h s t r e s s e s i n t h e c o n f i n i n g l i t h o l o g i e s , backstresses, m u l t i s t r a n d f r a c t u r i n g , o r presence o f t h i n h i g h - s t r e s s s t r i n g e r s i n t h e r e s e r v o i r . A p a r t f r o m damage o f t h e n a t u r a l c r a c k system, a n o t h e r d e l e t e r i o u s e f f e c t o f h i g h s t i m u l a t i o n p r e s s u r e s i s t h e c r e a t i o n o f s h o r t e r and w i d e r f r a c t u r e s w i t h l e s s e f f e c t i v e d r a i n a g e . E f f e c t i v e ness o f h y d r a u l i c f r a c t u r i n g i s l i m i t e d b y an a n i s o t r o p i c n a t u r a l j o i n t system, h i g h f r a c t u r i n g pressures, a c c e l e r a t e d o r dual f l u i d l e a k o f f ( c f . s e c t i o n 4.8.8.3.2.3.), and damage t o t h e n a t u r a l c r a c k network. Breakdowns and extended cleanup and t e s t t i m e s a r e r e q u i r e d f o r a c c u r a t e r e s e r v o i r assessment, and i n s i t u s t r e s s b e h a v i o u r v a r i e s w i t h d e p t h and l i t h o l o g y , i s complex, and s i g n i f i c a n t l y a f f e c t s s t i m u l a t i o n and gas p r o d u c t i o n .
4.8.8.3.5. Stress-sensit ivity behaviour and asperity shearing Damage mechanisms o f n a t u r a l f r a c t u r e systems by h y d r a u l i c s t i m u l a t i o n t r e a t ments i n c l u d e a p a r t f r o m l i q u i d s problems a l s o s t r e s s - s e n s i t i v i t y b e h a v i o u r and a s p e r i t y s h e a r i n g w i t h subsequent f r a c t u r e c l o s u r e . L i q u i d s problems comprise g e l r e s i d u e phenomena, b u t may a l s o c o v e r c a p i l l a r y p r e s s u r e e f f e c t s ( c f . sect i o n 4.8.8.3.2.2.) o f l i q u i d s i n t h e narrow passageways o f t h e n a t u r a l c r a c k s . I f many o f t h e passages through t h e f r a c t u r e s a r e e x t r e m e l y s m a l l , w a t e r movement becomes d i f f i c u l t and cleanup v e r y slow. C a p i l l a r y e f f e c t s a r e exacerbated by l a r g e m o l e c u l a r g e l r e s i d u e s i n t h e f r a c t u r e . S t r e s s - s e n s i t i v i t y problems o r i g i n a t e i f t h e propped h y d r a u l i c f r a c t u r e i m p a r t s a l a r g e s t r e s s on a d j a c e n t n a t u r a l c r a c k s thus r e d u c i n g t h e i r e f f e c t i v e w i d t h s . As t h i s s t r e s s cannot be g r e a t e r t h a n t h e t r e a t m e n t i n j e c t i o n p r e s s u r e above c l o s u r e s t r e s s , t h i s mechanism may n o t e n t i r e l y s h u t down f l o w i n t h e nat u r a l f r a c t u r e s , b u t may a l s o a c t i n c o n c e r t w i t h g e l o r c a p i l l a r y p r e s s u r e p r o blems ( c f . s e c t i o n 4.8.8.3.1.5.). A s p e r i t y - s h e a r i n g a l o n g n a t u r a l f r a c t u r e s and t h e r e s u l t a n t c l o s u r e and t r a p p i n g o f f i n e s , g e l s o r w a t e r i s caused by f l u i d l e a k o f f and/or induced t r e a t m e n t s t r e s s e s , w i t h unknown parameters b e i n g e x t e n t o f t h e shear s l i p p a g e r e g i o n and amount o f c l o s u r e r e p r e s e n t i n g damage s e v e r i t y . Aspects o f temporary n a t u r a l f r a c t u r e d e t e r i o r a t i o n a r e a l s o d i s c u s s e d by BRANAGAN, CIPOLLA, LEE & YAN (1987).
614
4.8.8.4.
Dendrit ic fracturing technique
A s p e c i a l h y d r a u l i c t r e a t m e n t method f o r v a r i o u s n a t u r a l l y j o i n t e d r e s e r v o i r s i s t h e d e n d r i t i c f r a c t u r i n g t e c h n i q u e ( K I E L 1977; LOPUS, SEIFERT & S C H E I N 1 9 8 7 ) . T h i s p r o c e s s i s t o overcome t h e p r o b l e m s a s s o c i a t e d w i t h g a i n i n g v e r t i c a l s e l e c t i v i t y and c o n n e c t i n g o r o p e n i n g t h e n a t u r a l c r a c k n e t w o r k , whereas c o n v e n t i o n a l s t i m u l a t i o n i n t h e p a s t c o u l d o n l y a f f e c t a modest p o r t i o n o f t h e p a y i n t e r v a l . A s p e c t s o f c o n n e c t i o n o f n a t u r a l and h y d r a u l i c f r a c t u r e s as w e l l as s h e a r s l i p p a g e v s . t e n s i l e f a i l u r e ae summarized as f o l l o w s .
4.8.8.4.1. Connection o f natural and hydraulic fractures The d e n d r i t i c f r a c t u r i n g p r o c e d u r e c o n f i n e s t h e o p e r a t i o n t o a l r e a d y e x i s t i n g n a t u r a l j o i n t s b y t h e use o f l a r g e volumes o f l i g h t l y g e l l e d f r e s h w a t e r pumped a t h i g h r a t e s and v e l o c i t i e s r a t h e r t h a n c r e a t i n g new c r a c k s as more v i s cous f l u i d s w o u l d ( K I E L 1977; LOPUS, SEIFERT & S C H E I N 1 9 8 7 ) . The h i g h r a t e s e f f e c t i v e l y d i s s o l v e f r a c t u r e - b r i d g i n g c a l c i t e , s a l t and o t h e r s e c o n d a r y m i n e r a l d e p o s i t s t o improve t h e i r p e r m e a b i l i t y . Various stages o f t h e t r e a t m e n t incorpor a t e s m a l l amounts o f p r o p p a n t s i n o r d e r t o a i d i n a b r a s i o n o f e x i s t i n g f r a c t u r e s as w e l l as f l u i d - l o s s c o n t r o l , o r b r i d g i n g t o cause s l i g h t changes i n d i f f e r e n t i a l p r e s s u r e w i t h i n a c r a c k t o a s s i s t i n d i v e r t i n g i n t o o t h e r j o i n t s . The f r a c t u r e g r a d i e n t s measured d u r i n g b o t h c o n v e n t i o n a l and d e n d r i t i c s t i m u l a t i o n treatments a r e considerably lower than the u n f r a c t u r e d m a t r i x gradient, which i n d i c a t e s t h a t c r a c k c l o s u r e i n t h e s t a n d a r d sense does n o t o c c u r , b u t t h e r e a sons f o r t h e w e l l r e s p o n s e t o t h e d e n d r i t i c s t i m u l a t i o n p r o c e s s i n c l u d e d i s s o l u t i o n o f s e c o n d a r y m i n e r a l d e p o s i t s f r o m f r a c t u r e o p e n i n g s and f r a c t u r e w a l l s , thereby p r o v i d i n g a consequential increase i n f r a c t u r e c o n d u c t i v i t y . I n s o l u b l e m a t e r i a l s p l u g g i n g t h e c r a c k s a r e e r o d e d b y t h e p r o p p a n t - l a d e n f l u i d s t r e a m and t r a n s p o r t e d away f r o m t h e w e l l b o r e d u r i n g t r e a t m e n t . C o n v e n t i o n a l f r a c t u r i n g o p e r a t i o n s i n v a r i o u s f o r m a t i o n s s u f f e r f r o m t h e beh a v i o u r o f t h e p r o p p a n t wedge i n t h e c r a c k as a f i l l e r c a u s i n g t h e s o l u b i l i z e d m i n e r a l s i n t h e r e t u r n t r e a t m e n t f l u i d s t o r e p r e c i p i t a t e s a l t and c a l c i t e w i t h i n t h e p r o p p a n t package, w i t h c e m e n t a t i o n b e i n g p o s s i b l e t o t h e p o i n t t h a t f l u i d f l o w i s s e v e r e l y r e s t r i c t e d and a r e s e r v o i r o f l i m i t e d e x t e n s i o n and r a p i d d e p l e t i o n i s m i m i c k e d b y t h e l o w b o t t o m h o l e f r a c t u r i n g p r e s s u r e . In s u c h situations, h y d r o c h l o r i c a c i d u t i l i z e d i n the d e n d r i t i c s t i m u l a t i o n process w i t h h i g h f l u i d v e l o c i t y has s u c c e s s f u l l y h e l p e d i n removal o f t h i s damage. Some p o i n t s o f d i v e r t i n g s t a g e s and r e v e r s e - f l o w p u l s e s t i m u l a t i o n a r e s k e t c h e d as f o l l o w s .
4.8.8.4.1.1.Diverting stages P a s t a t t e m p t s a t l i m i t e d e n t r y and s e l e c t i v e p e r f o r a t i n g ( c f . s e c t i o n 4.2.2.2.) i n c l u d i n g t h e use o f b a l l s e a l e r s ( c f . s e c t i o n 4 . 2 . 2 . 1 . 1 . ) have p r o ven t o be i n e f f e c t i v e i n a m e l i o r a t i n g z o n a l c o v e r a g e , because r e g a r d l e s s o f f l u i d c o n t r o l i n s i d e o r d i r e c t l y behind the pipe, t h e i n t e r c o n n e c t i n g f r a c t u r e n e t w o r k r e s u l t e d i n o n l y t h e w e l l - d e v e l o p e d p o r t i o n s o f t h e p a y zone a c c e p t i n g m o s t o f t h e t r e a t i n g f l u i d ( K I E L 1977; LOPUS, SEIFERT & S C H E I N 1 9 8 7 ) . The dend r i t i c f r a c t u r i n g t e c h n i q u e p r o v i d e s d i v e r t i n g s t a g e s u s i n g a g e l l e d f l u i d cont a i n i n g r o c k s a l t and p r o p p a n t s pumped a t s u f f i c i e n t r a t e s t o a c h i e v e d i v e r s i o n away f r o m t h e b o r e h o l e . S m a l l v a r i a t i o n s i n s u r f a c e and b o t t o m - h o l e t r e a t i n g p r e s s u r e s d u r i n g t h e d i v e r t i n g s t a g e s a r e t h e consequence o f l o w g r a d i e n t s i n many n a t u r a l l y f r a c t u r e d f o r m a t i o n s , w i t h t h e r e f o r e t h e m a g n i t u d e o f p r e s s u r e changes r e q u i r e d f o r good d i v e r s i o n b e i n g l o w and o f t e n a p p e a r i n g i n s i g n i f i cant. I n d e p e n d e n t o f number of d i v e r t e r s t a g e s and amount o f s o l i d d i v e r t e r used, i t i s imperative during treatment t o maintain the highest possible velocity w i t h i n t h e f r a c t u r e s , w i t h t h e o n l y r e a s o n f o r r e d u c t i o n o f r a t e d u r i n g t h e ope-
615 r a t i o n b e i n g t o mix and pump t h e d i v e r t e r stages a t p r o p e r c o n c e n t r a t i o n s (HARR I S O N 1971 g i v e s a h i s t o r i c a l r e v i e w o f t h e use o f v a r i o u s d i v e r t i n g agents i n o i l - and g a s - f i e l d o p e r a t i o n ) . The h i g h f l u i d v e l o c i t y c r e a t e s a mechanism f o r c a r r y i n g t h e sand used f o r i t s b r i d g i n g and a b r a s i v e c h a r a c t e r i s t i c s . D u r i n g r e t r e a t m e n t s o f c o n v e n t i o n a l l y f r a c t u r e d w e l l s , h i g h f l u i d v e l o c i t y i s necessary t o c a r r y t h e i n i t i a l proppant q u a n t i t y away f r o m t h e w e l l b o r e ( c f . s e c t i o n 4.8.9.5.), w i t h h y d r o c h l o r i c a c i d having t o be i n c l u d e d i n these i n s t a n c e s t o break t h e p r o p p a n t package f r e e o f c a l c i t e and s a l t cementing i t t o g e t h e r . I f c o n t i n u e d pumping p e r m i t s t h e r a t e t o be i n c r e a s e d w i t h no s i g n i f i c a n t r i s e i n s u r f a c e o p e r a t i o n pressure, t h i s c o u l d i n d i c a t e a change in i n j e c t i v i t y because o f reduced d i v e r s i o n e f f e c t i v e n e s s o r t h e r e s u l t o f pumping i n t o p r e v i o u s l y untreated f r a c t u r e s c r e a t i n g less flow r e s t r i c t i o n . I f the slope o f the p r e s s u r e versus t i m e graph f o r t h e v a r i o u s s t i m u l a t i o n stages u s u a l l y shows a p o s i t i v e t r e n d , t h i s r e f l e c t s r e s t r i c t i o n o r r e s i s t a n c e t o f l o w caused by t h e new f r a c t u r e e n t r y , o r t h a t t h e c a r r i e r f l u i d has m i n i m a l l y e f f i c i e n t p r o p p a n t t r a n s p o r t c a p a b i l i t i e s l e a d i n g t o f o r m a t i o n o f a sand bank t h a t , as i t b u i l d s , w i l l a c t t o c r e a t e p r e s s u r e d i f f e r e n t i a l across t h e c r a c k c a u s i n g f l u i d t o ent e r o t h e r secondary f r a c t u r e s . Aspects o f d e n d r i t i c s t i m u l a t i o n o f n a t u r a l l y j o i n t e d r e s e r v o i r s a r e a l s o d i s c u s s e d by HANNAH (1976) and HUBBARD & PIERSON ( 1 9 8 6 ) . A p p l i c a t i o n o f r o c k s a l t as d i v e r t i n g agent f o r s t i m u l a t i o n b l o c k s t a ges i s a l s o r e p o r t e d by HUCKABEE (1988).
4.8.8.4.1.2.Reverse-f low pulse st imulat ion D e n d r i t i c f r a c t u r i n g i s a reverse-flow s t i m u l a t i o n technique w i t h respect t o c o n v e n t i o n a l o r normal p l a n a r h y d r a u l i c f r a c t u r i n g and i s v e r y s u i t a b l e f o r geothermal w e l l s t i m u l a t i o n , because i t o f f e r s t h e b e s t chance o f i n t e r s e c t i n g maj o r n a t u r a l cracks, whereas a s i n g l e p l a n a r f r a c t u r e m i g h t o n l y p a r a l l e l and n o t c r o s s t h e p r i n c i p a l n a t u r a l j o i n t s (CAMPBELL, HANOLD, SINCLAIR & VETTER 1981; c f . s e c t i o n 4 . 8 . 8 . 1 . 3 . 2 . ) . D e n d r i t i c f r a c t u r e s a r e provoked by r e v e r s e f l o w p u l s i n g o f t h e r e s e r v o i r , t h e r e b y i n d u c i n g f o r m a t i o n s p a l l i n g and d i v e r s i o n o f t h e f r a c t u r e wings by downhole s t r e s s m o d i f i c a t i o n . M u l t i p l e i n j e c t i o n p e r i o d s a r e used w i t h each stage u t i l i z i n g l o w - v i s c o s i t y f l u i d s , sand s l u g s , and s e v e r a l flow-back times, w i t h h i g h f l o w r a t e s and f r i c t i o n r e d u c t i o n b e i n g a p p l i e d i n these t r e a t m e n t s . The b e s t r e s u l t s o f d e n d r i t i c f r a c t u r i n g a r e a c h i e ved i n n a t u r a l l y j o i n t e d f o r m a t i o n s where m a j o r and m i n o r c r a c k systems a l r e a d y e x i s t b u t may n o t have f l o w c a p a b i l i t y .
4.8.8.4.2. Shear slippage vs. tensile failure The o r i g i n o f a m u l t i p l e branched j o i n t system o r d e n d r i t i c f r a c t u r i n g i s n o t c o n t r o l l e d by t e n s i l e r o c k f a i l u r e as assumed by KIEL (1977), b u t i s r a t h e r governed by shear s l i p p a g e which r e s u l t s i n l o c a l r e d i s t r i b u t i o n o f s t r e s s e s (MURPHY & FEHLER 1986; c f . s e c t i o n 4.7.7.). I f f r i c t i o n a l r e s i s t a n c e t o shear s l i p p a g e i s low and maximum d i l a t a n c y due t o shear i s l a r g e , o n l y a s i n g l e j o i n t i s s t i m u l a t e d . I n case o f h i g h shear r e s i s t a n c e o r small d i l a t a n c y ( c f . section 4.8.1.3.1.), however, m u l t i p l e c r a c k s t i m u l a t i o n o c c u r s . Shear s l i p p a g e a l o n g t h e j o i n t s i s accompanied by s h e a r - s t r e s s drops, and t h e i n t e r a c t i o n o f these s t r e s s drops w i t h t h e a c t i n g e a r t h s t r e s s e s r e s u l t s i n opening o f c r a c k s more p e r p e n d i c u l a r t o t h e maximum s t r e s s , t h e r e b y g i v i n g r i s e t o f o r m a t i o n o f a branched o r d e n d r i t i c p a t t e r n . D e n d r i t i c f r a c t u r i n g i n hydrocarbon r e s e r v o i r s t i m u l a t i o n (KIEL 1977) i s t e c h n i c a l l y performed by r e p e t i t i v e f r a c t u r i n g w i t h p r o p p a n t - b e a r i n g f l u i d s , s h u t t i n g - i n and v e n t i n g . The f i r s t c y c l e o f p r e s s u r i z a t i o n r e s u l t s i n s p a l l i n g and s e l f - p r o p p i n g o f t h e main f r a c t u r e ( c f . s e c t i o n 4.7.7.4.), and i n t h e subsequent c y c l e s , t h e proppant p u r p o s e l y i n t r o d u c e d i n t h e t r e a t i n g f l u i d b r i d g e s the spa]]-proppants, t h e r e b y l e a d i n g t o p r e s s u r e r i s e and p r o p a g a t i o n o f l a t e r a l f r a c t u r e s p e r p e n d i c u l a r l y t o t h e main c r a c k ( K I E L 1977). T h i s model, how-
616 e v e r , does n o t work when c o n s i d e r a b l e s t r e s s d i f f e r e n c e s occur, because t h e p r e s s u r e r i s e i n t h e b l o c k e d main f r a c t u r e then s i m p l y r e s u l t s i n f u r t h e r l i f t o f f o f t h e l a r g e c r a c k , overcoming t h e temporary blockage and a l l o w i n g t h e main f r a c t u r e t o c o n t i n u e i t s p r o p a g a t i o n (MURPHY & FEHLER 1986). D e n d r i t i c f r a c t u r i n g i s r a t h e r c o n t r o l l e d by shear which p e r m i t s t h e necessa-
r y r e d u c t i o n o f t h e e a r t h s t r e s s p a r a l l e l t o t h e main f r a c t u r e t o i n i t i a t e open-
i n g o f l a t e r a l j o i n t s . Aspects o f shear s l i p p a g e g i v i n g r i s e t o a s p e r i t y sheari n g a l o n g n a t u r a l f r a c t u r e s and temporary damage o f t h e p r e - e x i s t i n g c r a c k system due t o h y d r a u l i c r e s e r v o i r s t i m u l a t i o n a r e a l s o d i s c u s s e d by LORENZ, BRANAGAN, W A R P I N S K I & SATTLER ( 1 9 8 6 ) . F i e l d e x p e r i e n c e o f d e n d r i t i c f r a c t u r i n g i s a l s o r e p o r t e d by McKETTA, K O Z I A R & COOK ( 1 9 8 0 ) .
4.8.8.5. Tailored pulse loading T a i l o r e d p u l s e l o a d i n g comprises i n t e r m e d i a t e s t r a i n r a t e r o c k f r a c t u r i n g processes t h a t a r e capable o f g e n e r a t i n g m u l t i p l e c r a c k networks which have a much h i g h e r p r o b a b i l i t y o f i n t e r s e c t i n g n a t u r a l j o i n t systems t h a n s t a n d a r d exp l o s i v e and c o n v e n t i o n a l h y d r a u l i c f r a c t u r i n g (STOLLER 1985; c f . s e c t i o n s 4.2.2.1.3.3. and 4.7.3.). C o n d u c t i v i t y maintenance o f t h e non-propped m u l t i p l e f r a c t u r e s can be achieved by c h a n n e l l i n g and gouging which o c c u r on t h e c r a c k s u r f a c e s , and by t r a n s l a t i o n o f one f r a c t u r e s u r f a c e r e l a t i v e t o t h e o t h e r i f t h e c r a c k s a r e n o t o r i e n t e d i n t h e p r i n c i p a l s t r e s s d i r e c t i o n s , w i t h t h e sheari n g s t r e s s i n f l u e n c e on t h e o f f - a x i s f r a c t u r e s b e i n g v e r y i m p o r t a n t . I n terms o f c r e a t i o n o f m u l t i p l e f r a c t u r e s , d i s t i n c t i o n has t o be made b e t ween e a r l y stage when f r a c t u r e - f r a c t u r e i n t e r a c t i o n s must be c o n s i d e r e d t o a l low a l l t h e i n d i v i d u a l c r a c k s t o grow, and l a t e stage when o p t i m i z a t i o n o f t h e p r e s s u r e p u l s e i s r e q u i r e d t o a c h i e v e maximum f r a c t u r e l e n g t h s . The c r a c k geom e t r y c r e a t e d by t a i l o r e d p u l s e l o a d i n g tends towards r e c t i l i n e a r as opposed t o r a d i a l , which r e s u l t s f r o m t h e more dynamic n a t u r e o f t h e f r a c t u r i n g process t h a t masks o u t t h e i n f l u e n c e o f s t a t i c e f f e c t s such as i n - s i t u s t r e s s e s .
4.8.8.6.Proppant plugging of natural fractures Hydraulic s t i m u l a t i o n treatments o f n a t u r a l l y f r a c t u r e d formations o f t e n suff e r f r o m t h e u n c e r t a i n t y whether t h e r e s e r v o i r has been r e a l l y broken down w i t h c r e a t i o n of new c r a c k p l a n e s o r whether o n l y p r e - e x i s t i n g n a t u r a l j o i n t s have been e n l a r g e d and i n f i l l e d w i t h p r o p p a n t s . I n many n a t u r a l l y f r a c t u r e d f o r mations, f l u i d and proppants cannot be p u t down f a s t enough t o a v o i d l e a k o f f i n t o t h e p r e - e x i s t i n g c r a c k system o r even i n t o t h e r o c k m a t r i x . T h i s i s a l r e a dy h i g h l i g h t e d by t h e f a c t t h a t n a t u r a l l y j o i n t e d r e s e r v o i r s f r e q u e n t l y cause s e r i o u s d r i l l i n g problems due t o l o s t c i r c u l a t i o n d u r i n g p e n e t r a t i o n , w i t h mud l o s s r a t e s o f t e n exceeding r i g pumping c a p a b i l i t i e s and r e q u i r i n g temporary p r o g r e s s by b l i n d d r i l l i n g u n t i l t h e zone i s c r o s s e d (CANSON 1985; c f . s e c t i o n 4.8.8.3.2.). I n terms o f h y d r a u l i c s t i m u l a t i o n o f e x t e n s i v e l y naturally f r a c t u r e d r e s e r v o i r s , two p o s s i b i l i t i e s e x i s t c o m p r i s i n g p l u g g i n g o f n a t u r a l c r a c k s w i t h p r o p p a n t s and o r i g i n of i n c l i n e d f r a c t u r e s by Coulomb f a i l u r e . The d i s c u s s i o n as f o l l o w s i n c l u d e s aspects of p r o d u c t i o n a c c e l e r a t i o n by p r o p p i n g o f n a t u r a l c r a c k s , f r e s h r e s e r v o i r volume a c q u i s i t i o n and r e c o v e r y enhancement by a r t i f i c i a l f r a c t u r i n g , and n a t u r a l p r o p p i n g o f sedimentary s h r i n k a g e c r a c k s .
4.8.8.6.1. Production acceleration by propping of natural cracks N a t u r a l f r a c t u r e propping, however, does n o t r e s u l t i n a d d i t i o n a l h y d r o c a r bon e x p l o i t a t i o n , because t h e i n i t i a l peak p r o d u c t i o n i s l a t e r compensated b y f a s t e r d e c l i n e o f o u t p u t r a t e s and c u m u l a t i v e o f f t a k e volume. The reason f o r t h i s e f f e c t i s t h e l a c k of newly c r e a t e d f r a c t u r e p l a n e s w h i c h c o u l d access i n cremental hydrocarbon q u a n t i t i e s t h a t a r e n o t reached by t h e e x i s t i n g d r a i n a g e
617 system. O n l y h y d r a u l i c f r a c t u r i n g can g i v e r i s e t o s u b s t a n t i a l p r o d u c t i v i t y i m provement by exposing f r e s h surfaces and g e n e r a t i n g new i n t e r s e c t i o n s through t h e t i g h t m a t r i x a l o n g w h i c h h i t h e r t o i n a c c e s s i b l e r e s e r v e s can be a c q u i r e d , w i t h t h e n n a t u r a l c r a c k s t h a t a r e c r o s s e d b y t h e induced f r a c t u r e and t h u s t i e d i n t o t h e a c t i v e system connected d i r e c t l y t o t h e b o r e h o l e a l s o a c h i e v i n g c o n s i d e r a b l e s i g n i f i c a n c e f o r p r o d u c t i o n c o n t r i b u t i o n , and i n t h i s case p r o p p i n g o f n a t u r a l f r a c t u r e s a s c e r t a i n i n g t h a t t h e y s t a y open a l s o w i t h r e t u r n i n g c l o s u r e s t r e s s and keep b e i n g permeable and c o n d u c t i v e hydrocarbon d e l i v e r y p a t h s t o t h e main h y d r a u l i c f r a c t u r e stem and t h e r e b y u l t i m a t e l y t o t h e w e l l b o r e . N a t u r a l f r a c t u r e p r o p p i n g r e s u l t s i n s i m p l e a c c e l e r a t i o n by f a s t e r f l o w o f t h e hydrocarbons t h a t a r e a l r e a d y w i t h i n t h e reach o f t h e e x i s t i n g d r a i n a g e p a t t e r n t o t h e w e l l b o r e as a consequence o f i n c r e a s i n g c o n d u c t i v i t y o f t h e f l o w paths. Concerning comparative assessment o f e f f e c t i v i t y , h y d r a u l i c f r a c t u r e s have t h e advantage o f b e i n g f r e s h s u r f a c e s , whereas many n a t u r a l c r a c k s can be c l a y - c o v e r e d o r m i n e r a l i z e d which may c o n s i d e r a b l y reduce t h e i r p e r m e a b i l i t y and/or c o n d u c t i v i t y . I n a d d i t i o n , t h e d i s t r i b u t i o n o f n a t u r a l f r a c t u r e s i n t h e g e o l o g i c a l column i s heterogeneous, w i t h t i g h t gas sands c o n t a i n i n g v e r y few nat u r a l cracks, whereas h i g h - p e r m e a b i l i t y sandstones and carbonates have a l o t o f natural fractures.
4.8.8.6.2. Fresh reservoir volume acquisition and recovery enhancement by artificial fracturing Enhancement o f t h e d r a i n a g e e f f e c t i v i t y o f n a t u r a l j o i n t s t o g e t h e r w i t h i m provement o f c u m u l a t i v e p r o d u c t i v i t y can o n l y be achieved by i n t e r c o n n e c t i n g nat u r a l and h y d r a u l i c o r e x p l o s i v e f r a c t u r e s . A r t i f i c i a l l y c r e a t e d c r a c k s propagat i n g a t h i g h r a t e s r u n through n a t u r a l f r a c t u r e s and c o n t i n u e t o e x t e n d i n t o t h e m a t r i x , whereas those t r a v e l l i n g a t low r a t e s t u r n i n t o t h e d i r e c t i o n o f nat u r a l j o i n t s once a c r i t i c a l v e l o c i t y o f p r o p a g a t i o n i s no l o n g e r exceeded. I n terms o f e x p l o s i v e f r a c t u r i n g , however, h i g h p r o p a g a t i o n r a t e s r e q u i r e h i g h det o n a t i o n r a t e s w h i c h r e s u l t i n p u l v e r i z a t i o n o f t h e r o c k and c r e a t i o n o f a l o t o f f i n e s , u n l e s s t h e e x p l o s i v e charge i s s e l e c t i v e l y p u t i n t o n a t u r a l f r a c t u r e s and i g n i t e d t h e r e i n ( c f . s e c t i o n 4.7.3.). A s u i t a b l e c o m b i n a t i o n o f h y d r a u l i c and e x p l o s i v e f r a c t u r i n g ( c f . s e c t i o n 4.8.9.6.) would be t h e performance o f a h y d r a u l i c m i n i f r a c t u r e i n t o which t h e e x p l o s i v e charge i s i n s e r t e d . I f l o n g - t e r m p r o d u c t i o n m a x i m i z a t i o n i s d e s i r e d t o be achieved, h y d r a u l i c o r e x p l o s i v e f r a c t u r e s s h o u l d be i n t e r c o n n e c t e d w i t h t h e s m a l l e s t n a t u r a l c r a c k system f o r r e c o v e r y f a c t o r o p t i m i z a t i o n . On t h e o t h e r hand, i f opening-up o f t h e f o r m a t i o n and h i g h i n i t i a l p r o d u c t i o n r a t e s a r e wanted, h y d r a u l i c o r e x p l o s i v e f r a c t u r e s s h o u l d be i n t e r c o n n e c t e d w i t h t h e l a r g e s t n a t u r a l c r a c k system. T h i s guarantees good f l o w r a t e s , b u t as a consequence o f o n l y l a r g e m a t r i x b l o c k s b e i n g i n c l u d e d i n t h e coarse network, r e c o v e r y f a c t o r s a r e low, whereas e n c l o s u r e o f s m a l l m a t r i x b l o c k s by t h e f o r m e r t e c h n i q u e r e s u l t s i n good a r e a l d r a i n a g e and h i g h r e c o v e r y f a c t o r s .
ZUBER, LEE & GATENS (1987) r e v i e w v a r i o u s s t i m u l a t i o n t e c h n i q u e s f o r Devon i a n s h a l e gas r e s e r v o i r s . The h i s t o r i c a l l y abundantly a p p l i e d s t a n d a r d e x p l o s i ve f r a c t u r i n g method by d e t o n a t i n g g e l a t i n a t e d n i t r o g l y c e r i n e i n t h e b o r e h o l e ( w e l l shooting; c f . s e c t i o n 4.4.4.6.) has i n t h e l a s t y e a r s been more and more r e p l a c e d by s i n g l e h y d r a u l i c f r a c t u r i n g w i t h o r w i t h o u t p r o p p a n t i n s e r t i o n as w e l l as m u l t i p l e r a d i a l t a i l o r e d - p u l s e f r a c t u r i n g r e p r e s e n t i n g s e l e c t i v e and c o n t r o l l e d e x p l o s i v e s t i m u l a t i o n ( c f . s e c t i o n 4.7.3.). The n a t u r a l f r a c t u r e system i n t e r s e c t i n g t h e s h a l e c r e a t e s an a n i s o t r o p i c a l p e r m e a b i l i t y d i s t r i b u t i o n . T h i s r e s e r v o i r h e t e r o g e n e i t y and t h e access o f a d d i t i o n a l gas volumes o n l y by opening o f new c r a c k s r a t h e r than e n l a r g i n g p r e - e x i s t i n g j o i n t s a r e t h e reason f o r a much b e t t e r performance o f w e l l s w i t h induced f r a c t u r e s r u n n i n g p e r p e n d i c u l a r t o t h e n a t u r a l f r a c t u r e t r e n d t h a n such w i t h a r t i f i c i a l l y c r e a t e d c r a c k s b e i n g p a r a l l e l t o t h e e x i s t i n g n a t u r a l j o i n t system.
618
4.8.8.6.3.
Natural propping o f sedimentary shrinkage cracks
Sedimentary shrinkage cracks of s u b aer i al de sic c a tion o r subaqueous synaereo r i g i n i n f l u v i a l and l a c u s t r i n e mudstones a r e e x c e l l e n t examples t o i l l u s t r a t e a n a t u r a l p r o p p i n g mechanism of f r a c t u r e s ( c f . p l a t e s VI/1 - 7 and X I / 7 ) . Contraction of the mud s u r f ace i n q u i e t environment e i t h e r by atmospher i c d r y i n g or subaqueous shrinkage gives r i s e t o opening of extension c ra c ks. Subsequent high-energy f l u v i a l or aeolian depositional events lead t o re turn of sand accumulation which does n o t only cover the sedimentary surfa c e , b u t a l s o plugs the open syndepositional cr ack s . The i n f i l l i n g of the shrinkage f r a c t u r e s by sand of d i f f e r e n t grain s i z e according t o the granulometrical spectrum supp li e d by the provenance area a n d transported by wind or water re pre se nts n a t u ral p r o p p i n g of natural f r a c t u r e s which r e s u l t s in keeping the cracks open a n d a s c e r t a i n i n g t h e i r function of high-permeability avenues a l s o in post-deposit io n a l h i s t o r y . sis
4.8.8.7.
Other aspects
KOHLHAAS (1982) describes a r e s e r v o i r with l a t e r a l change of patches w i t h and w i t h o u t natural f r a c t u r e s . The zones w i t h natural cracks have b e t t e r prea n d post-treatment r e s u l t s than the s p o t s without natural j o i n t s . I n j e c t i o n of f l u i d a n d proppants leads t o enlargement of natural f r a c t u r e s and p a r t i a l p l u g ging with proppants i n addition t o cr eat i on of new hydra ulic a lly induced c r a c k s . T h i s example underlines the e f f e c t i v i t y of a combination o f na tura l and a r t i f i c i a l f r a c t u r i n g f o r t h e successful stimulation of hydrocarbon r e s e r v o i r s o f limited thickness a n d extension. Reservoir management aspects i n n a t u r a l l y fra c ture d o r f i s s u r e d pay format i o n s , e s p e c i a l l y in terms of g r av i t y drainage, a r e discussed by JACQUIN, L E GAIT, MARTIN, NECTOUX, ANTERION & R I O C H E (1987) and NECTOUX (1987). NELSON (1986) presents a summarizing geological an alysis of n a t u r a l l y fra c ture d formatio n s with emphasis on s t r a t e g i e s of r es er v o ir management, de te c tion a n d predict i o n of f r a c t u r e o r i e n t a t i o n a n d spacing, r ese rvoir anisotropy a n d s t a t i s t i c a l modelling, a n d p o s s i b i l i t i e s of s t i mu l at i o n of n a t u r a l l y jointe d pays. H E N N I N G TON (1980) reviews unconventional exploration f o r natural f r a c t u r e s , and RUOTSALA & WILLIAMS (1982) s t r e s s the s i g n i f i can ce of r e f l e c t i o n seismology as a n exp lo r a t i o n tool f o r f r act u r ed zones. ABDASSAH & ERSHAGHI (1986) de fine n a t u r a l l y cracked r e s e r v o i r s as t r i p l e - p o r o s i t y systems, whereas WARREN & ROOT (1963) id e a l i z e d the n a t u r a l l y j o i n t ed pay as co n s i sting of two d i s t i n c t systems being matrix a n d f r a c t u r e s . NAJURIETA (1979) a n d D E R U Y C K , BOURDET, PRAT & RAMEY (1982) comment on the s i g n i f i can ce of i n t er f ere nc e a n d pulse t e s t i n g f o r the evaluation of communication within n a t u r a l l y fra c ture d r e s e r v o i r s . B O U R D E T & G R I N G A R T E N (1980) as s es s determination of f i s s u r e volume a n d block s i z e i n n a t u r a l l y f r act u r ed formations. ULDRICH & ERSHAGHI (1979) pre se nt e s t i mations o f the i n t e r p o r o s i t y flow parameter. Well log e va lua tion of n a t u r a l l y jo i n t e d r e s e r v o i r s i s discussed by AGUILERA (1976) a n d AGUILERA & ACEVEDO ( 1 9 8 2 ) , a n d REYNOLDS, C H A N G , Y E H & RAGHAVAN (1985) comment on wellbore pressure response. Various points of n a t u r a l l y f r act u r e d pays a re summarized by A G U I L E R A ( 1 9 8 0 ) . Aspects of n a t u r a l l y cracked r es e rvoirs a re a l s o t r e a t e d by A G U I L E R A (1975, 1976, 1985), KOSTURA & RAVENSCROFT (1977); BRANAGAN, COTNER & L E E ( 1 9 8 4 ) ; BLASINGAME & L E E ( 1 9 8 6 ) ; BRANAGAN, C I P O L L A & L E E (1987) and EVANS & UBANI ( 1 9 8 7 ) .
619
4.8.9. Late-stage refracturing o f old wells Another mHF o r r e l a t e d s m a l l - s c a l e s t i m u l a t i o n a p p l i c a t i o n , b u t o c c a s i o n a l l y a l s o l a r g e r - s c a l e t r e a t m e n t t o even MHF u t i l i z a t i o n , i s r e f r a c t u r i n g o f p a r t i c u l a r l y s h a l l o w o i l w e l l s c o m p r i s i n g renewed f r a c t u r i n g i n h o r i z o n s which had a l ready been f r a c t u r e d once l o n g t i m e ago ( o r e q u i v a l e n t l y o r i g i n a l f r a c t u r i n g i n advanced d e p l e t i o n stages o f r e s e r v o i r s i n t e r s e c t e d by b o r e h o l e s t h a t had never been s t i m u l a t e d b e f o r e ) . A f t e r an o u t l i n e o f g e n e r a l r e f r a c t u r i n g s i g n i f i c a n c e , t h e d i s c u s s i o n as f o l l o w s comments on r e f r a c t u r i n g c a n d i d a t e w e l l choice, a r e a l d i s t r i b u t i o n o f p o t e n t i a l r e s e r v o i r s , m o b i l i z a t i o n o f a d d i t i o n a l hydrocarbon r e serves, p r o p p a n t s e l e c t i o n f o r r e f r a c t u r i n g , f r a c t u r e r e o p e n i n g and proppant placement d u r i n g r e f r a c t u r i n g , and c o m b i n a t i o n o f h y d r a u l i c and e x p l o s i v e f r a c turing.
4.8.9.0. General refracturing significance R e f r a c t u r i n g can be v e r y s u c c e s s f u l by reopening t h e o l d f r a c t u r e ( t h a t has i n t h e meantime become c o m p l e t e l y c l o s e d due t o c r u s h i n g o f n e a r l y a l l t h e f o r m e r l y i n s e r t e d p r o p p i n g m a t e r i a l ) which a c t s as a p r e - e x i s t i n g zone o f weakness and i s i n f i l l e d again w i t h proppants, w i t h r e f r a c t u r i n g becoming i n c r e a s i n g l y i m p o r t a n t f o r workover o f o l d o i l f i e l d s . Since t h e advent o f h y d r a u l i c p r o p p a n t f r a c t u r i n g , p e r i o d i c a l r e s t i m u l a t i o n has been employed as a v i a b l e means o f e c o n o m i c a l l y i n c r e a s i n g p r o d u c t i o n r a t e s and c a p t u r i n g r e c o v e r a b l e r e s e r v e s , and i n many i n s t a n c e s , r e t r e a t m e n t o f e x i s t i n g w e l l s has become a f e a s i b l e a l t e r n a t i v e t o new e x p l o r a t i o n o r d r i l l i n g a d d i t i o n a l development w e l l s . I n some g e o g r a p h i c a l areas, p r o d u c t i o n i n c r e a s e s f r o m r e s t i m u l a t i o n have demonstrated t h a t r e c o m p l e t i o n o f t a r g e t e d w e l l s can be one o f t h e b e s t i n v e s t m e n t o p p o r t u n i t i e s a v a i l a b l e t o t h e producer (HALLIBURTON 1987). R e f r a c t u r i n g o f w e l l s h a v i n g a l r e a d y passed c o n s i d e r a b l e p r o d u c t i o n h i s t o r y and h a v i n g been e a r l i e r s t i m u l a t e d s h o r t e r o r l o n g e r t i m e ago has become p a r t i c u l a r l y i n t e r e s t i n g a f t e r t h e 1986 o i l p r i c e c r a s h as a consequence o f b e i n g a c o m p a r a t i v e l y cheap investment l e a d i n g t o r a p i d payout i n comparison t o much more expensive f r a c t u r i n g o f new w e l l s w i t h a much more u n f a v o u r a b l e d i s c o u n t e d cash f l o w r a t e ( c f . s e c t i o n 2.2.1.5.3.). R e f r a c t u r i n g i s a remedy o p e r a t i o n i n o r d e r t o improve p r o d u c t i v i t y o f such w e l l s t h a t have a l r e a d y been f r a c t u r e d some t i m e ago w i t h poor t e c h n i c a l p e r f o r mance, i n s u f f i c i e n t r e s e r v o i r response, o r inadequate s t i m u l a t i o n designs (MART I N S 1987) o r i n a more u n f a v o u r a b l e economical environment. C o n s i s t e n t success can be achieved by r e f r a c t u r i n g i f t h e o r i g i n a l t r e a t m e n t has been performed some t i m e ago and b o t h t e c h n i c a l and economical framework have changed, whereas i f r e c e n t l y a good l a r g e - s c a l e j o b has been c a r r i e d o u t w i t h s a t i s f a c t o r y r e s u l t s , an enhancement by r e f r a c t u r i n g may be d i f f i c u l t t o achieve. I f t h e f o r mer c r a c k i s t o o densely propped, t h e new s t i m u l a t i o n t r e a t m e n t p r o b a b l y does n o t a m e l i o r a t e t h e r a t e and c o u l d even by p o o r e r t h a n t h e o l d o p e r a t i o n . There i s a complete range between j u s t b r e a k i n g down t h e p e r f o r a t i o n s o r pumping t h e whole o l d t r e a t m e n t away when c a r r y i n g o u t r e f r a c t u r i n g . The environment f o r r e f r a c t u r i n g o f a l r e a d y e a r l i e r s t i m u l a t e d b o r e h o l e s as w e l l as i n i t i a l f r a c t u r i n g o f l a t e - s t a g e i n f i l l p r o d u c t i o n p o i n t s i s d e p l e t e d r e s e r v o i r pore p r e s s u r e which i n c r e a s e s t h e c o n f i n i n g s t r e s s c o n t r a s t between pay zone and s h a l e b a r r i e r s , t h e r e b y a l l o w i n g b e t t e r f r a c t u r e h e i g h t growth cont r o l ( c f . s e c t i o n 4.2.2.) which l e a d s t o l o w e r p r o p p a n t q u a n t i t y b e i n g r e q u i r e d t o p l u g t h e s m a l l e r c r a c k (HUEBINGER, WEBSTER, CHISHOLM, VENDITTO & HUNT 1988).
4.8.9.1. Refracturing candidate well choice I n t h e c u r r e n t u n f a v o u r a b l e economical c l i m a t e caused by u n c e r t a i n o i l p r i c e ( c f . s e c t i o n 2.2.1.1.3.) and a c u r t a i l e d gas m a r k e t ( c f . s e c t i o n 2.2.1.6.3.; in Europe and o t h e r p a r t s o f i n t e r n a t i o n a l markets a l s o u n f a v o u r a b l y superimposed
620 by t h e s i m u l t a n e o u s US $ weakness; c f . s e c t i o n 2 . 2 . 1 . 3 . ) , r e s t i m u l a t i o n o f e x i s t i n g w e l l s t o increase production r a t e s and/or economically recoverable r e s e r v e s has become an a l t e r n a t i v e t o e x p l o r a t i o n o r d r i l l i n g d e v e l o p m e n t w e l l s (CONWAY, McMECHAN, McGOWEN, BROWN, CHISHOLM & VENDITTO 1985; c f . s e c t i o n 2.2.1.5.3.). Production increases r e a l i z e d from r e f r a c t u r i n g i n several areas have d e m o n s t r a t e d t h a t r e c o m p l e t i o n o f c e r t a i n w e l l s c a n be one o f t h e b e s t i n v e s t m e n t o p p o r t u n i t i e s p r e s e n t l y a v a i l a b l e . Improvement o f t h e e f f e c t o f t h e o r i g i n a l s t i m u l a t i o n b y t h e renewed o p e r a t i o n c a n b e a c h i e v e d because o f r e c e n t advances i n f r a c t u r i n g t e c h n o l o g y e m p l o y i n g more e f f i c i e n t f l u i d s t h a t a r e c a pable o f transporting higher proppant concentrations, with i n combination with m o d i f i e d s u r f a c e e q u i p m e n t now t r e a t m e n t s b e i n g a b l e t o b e p e r f o r m e d t h a t i n t e r m s of f l u i d and p r o p p a n t volume as w e l l as p r o p p a n t s a t u r a t i o n w e r e p r e v i o u s l y n o t possible. P e r i o d i c a l r e f r a c t u r i n g has been employed as a method f o r i n c r e a s i n g p r o d u c t i o n r a t e s and/or economically r e c o v e r a b l e reserves s i n c e t h e advent o f hydraul i c f r a c t u r i n g , and f o c u s s e s p a r t i c u l a r l y on w e l l s w h i c h f o r m e r l y have been und e r e s t i m a t e d b y u t i l i z a t i o n o f t o o l o w p r o p p a n t c o n c e n t r a t i o n s and t o o s m a l l j o b s . S I N H A & ELBEL ( 1 9 8 3 ) comment o n p a r t i a l r e h e a l i n g o f f r a c t u r e s as p r e s s u r e d e c l i n e s w i t h p r o g r e s s i v e d e p l e t i o n o f t h e r e s e r v o i r , w i t h a n advanced s t a g e o f c l o s u r e o f t h e o r i g i n a l c r a c k b e i n g an i n d i c a t o r f o r r e f r a c t u r i n g p r o v i d e d s a t i s f a c t o r y p r o d u c t i o n r a t e s have been o b t a i n e d a f t e r t h e p r i m a r y o p e r a t i o n . W e l l p e r f o r m a n c e a n a l y s i s a f t e r o r i g i n a l s t i m u l a t i o n as a base f o r f u r t h e r t r e a t m e n t p l a n n i n g i n c l u d e s a c o m p a r a b l e a p p r o a c h f o r r e f r a c t u r i n g c a n d i d a t e sel e c t i o n and m i n i f r a c t u r e c a l i b r a t i o n t e s t e v a l u a t i o n ( c f . s e c t i o n 4 . 8 . 2 . ) . As a consequence o f p r o g r e s s i v e l y i n c r e a s i n g e x p e r i e n c e f r o m e a r l i e r t r e a t ments and r e s u l t i n g d e s i g n o p t i m i z a t i o n s f o r s u c c e s s i v e o p e r a t i o n s b o t h i n connection w i t h r e f r a c t u r i n g o f o l d w e l l s long time a f t e r the o r i g i n a l s t i m u l a t i o n j o b s and i n r e l a t i o n s h i p w i t h f r a c t u r i n g campaigns in new f i e l d s w i t h s h o r t p e r i o d s o f t i m e between t h e i n d i v i d u a l t r e a t m e n t s , an e v o l u t i o n o f f r a c t u r e o p e r a t i o n d e s i g n and p e r f o r m a n c e (TINDELL, NEAL & HUNTER 1981) t a k e s o f t e n p l a c e f r o m e a r l y more o r l e s s e x p e r i m e n t a l s t a g e s t h r o u g h advanced r o u t i n e phases u p t o a m a t u r e h i g h - t e c h n o l o g y l e v e l . HOWARD & FAST ( 1 9 7 0 a ) summarize t h e p u r p o s e o f r e f r a c t u r i n g as b e i n g e x t e n s i o n o f t h e e x i s t i n g f r a c t u r e system, r e o p e n i n g o f p r e v i o u s l y g e n e r a t e d c r a c k s , w a s h i n g o f f r a c t u r e f a c e s , r e p l e n i s h i n g o f embedded p r o p p a n t s , and o p e n i n g o f new c r a c k s i n p r e v i o u s l y u n s t i m u l a t e d i n t e r v a l s . The o u t l i n e as f o l l o w s i l l u s t r a t e s w e l l p e r f o r m a n c e a f t e r o r i g i n a l s t i m u l a t i o n t r e a t m e n t and c h a r a c t e r i z e s a l s o t h e c o m p a r a b l e a p p r o a c h o f r e f r a c t u r i n g c a n d i d a t e s e l e c t i o n and m i n i f r a c t u r e c a l i b r a t i o n t e s t e v a l u a t i o n .
4.8.9.1.1.We1 1 performance after original st imulat ion treatment Concerning r e f r a c t u r i n g candidate s e l e c t i o n , p r e v i o u s l y f r a c ured w e l l s w i t h s u f f i c i e n t r e m a i n i n g r e s e r v e s f o r c o n s i d e r a t i o n c a n be d i v i d e d n t o f o u r c a t e g o r i e s . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l a s p e c t s , s u c c e s s v s . f a i l u r e o f o r i g i n a l f r a c t u r e j o b i s analyzed.
4.8.9.1.1.1.General aspects The f i r s t c l a s s c o m p r i s e s w e l l s w i t h i n s u f f i c i e n t p r o d u c t o n i n c r e a s e f r o m t h e f i r s t h v d r a u l i c s t i m u l a t i o n .job due t o p r e m a t u r e s c r e e n o u t f a i l u r e . A n a l y s i s o f t r e a t i n g p r e s s u r e f r o m 'the f i r s t o p e r a t i o n i s n e c e s s a r y t o i d e n t i f y t h e p r o b l e m and t o m o d i f y t h e second t r e a t m e n t such as t o overcome t h e f o r m a t i o n c h a r a c t e r i s t i c s t h a t cause t h e f i r s t j o b t o f a i l ( c f . CONWAY, McGDWEN, GUNDERSON & KING 1 9 8 5 ) . The second g r o u p i n c l u d e s b o r e h o l e s w i t h i n s u f f i c i e n t p r o d u c t i o n i n c r e a s e f o l l o w i n g s u c c e s s f u l pumping o f t h e f i r s t f r a c t u r e o p e r a t i o n . A m o d i f i e d r e f r a c t u r i n g p r o c e d u r e has t o b e s e t u p w h i c h i s p r o m i s i n g i n t e r m s of b o t h t e c h n i c a l and e c o n o m i c a l a s D e c t s .
62 1 The t h i r d c l a s s c o n t a i n s w e l l s t h a t were f o r m e r l y a l r e a d y s u c c e s s f u l l y f r a c t u r e d , b u t p r o d u c t i o n has f a l l e n below p r o j e c t e d d e c l i n e l e v e l s i n t h e meant i m e . S e v e r i t y and t i m e frame o f t h e abnormally reduced p r o d u c t i o n h e l p t o det e r m i n e t h e degree o f m o d i f i c a t i o n o f t h e o r i g i n a l d e s i g n which i s necessary f o r s u c c e s s f u l r e s t i m u l a t i o n . The f o u r t h assemblage bears b o r e h o l e s w i t h successful o r i g i n a l treatment t h a t are s t i l l producing a t projected decline r a t e s . I n t h i s case, t h e main q u e s t i o n i s whether improvements i n f r a c t u r i n g t e c h n o l o gy have made i t p o s s i b l e t o achieve a l a r g e enough p r o d u c t i o n i n c r e a s e o v e r t h a t from the f i r s t s t i m u l a t i o n j o b t o j u s t i f y the c o s t o f the r e f r a c t u r i n g t r e a t m e n t (CONWAY, McMECHAN, McGOWEN, BROWN, CHISHOLM & VENDITTO 1 9 8 5 ) .
4.8.9.1.1.2. Success vs. failure of original fracture job I f a w e l l responded f a v o u r a b l y t o o r i g i n a l s t i m u l a t i o n , i t i s probably a good c a n d i d a t e f o r r e f r a c t u r i n g , because b o r e h o l e s d r i l l e d i n areas o f h i g h pot e n t i a l b u t t r e a t e d w i t h c o n v e n t i o n a l d e s i g n show i n many cases s a t i s f a c t o r y e a r l y p r o d u c t i o n t h a t l a t e r d e c l i n e s r a p i d l y (VENDITTO, CHISHOLM, W I G G I N S & CONWAY 1986). Where t h e i n i t i a l f r a c t u r i n g j o b screened o u t , good p o t e n t i a l may e x i s t f o r r e f r a c t u r i n g a f t e r i d e n t i f i c a t i o n o f t h e s c r e e n o u t mode and s o l u t i o n Soo f t h e problem which caused t h e premature f a i l u r e ( c f . s e c t i o n 6 . 2 . 4 . 2 . 1 . ) . metimes a s t i m u l a t i o n o p e r a t i o n i s a t e c h n i c a l success, b u t an economical f a i l u r e due t o d i s a p p o i n t i n g r e s u l t i n g p r o d u c t i o n . I f i t can be determined what t h e o r i g i n a l t r e a t m e n t accomplished and i f i t was d e f e c t i v e i n some way, r e f r a c t u r i n g may be e c o n o m i c a l l y j u s t i f i e d . R e s t i m u l a t i o n i s a l s o t h e a p p r o p r i a t e sol u t i o n where a w e l l was s u c c e s s f u l l y f r a c t u r e d , b u t e x p l o i t a t i o n d e c l i n e i n d i c a t e s t h a t t h e c r a c k i s no l o n g e r e f f e c t i v e . W e l l s h a v i n g been s t i m u l a t e d and p r o d u c i n g a t p r o j e c t e d d e c l i n e r a t e s may even be c a n d i d a t e s i f t h e y were o r i g i n a l l y f r a c t u r e d u s i n g c o n v e n t i o n a l l o a d s pumped a t low p r o p p a n t c o n c e n t r a t i o n s . R e f r a c t u r i n g o f s t i l l p r o d u c i n g w e l l s w i t h massive t r e a t m e n t s and h i g h p r o p p a n t s a t u r a t i o n s has h i g h economical f e a s i b i l i t y p o t e n t i a l .
O l d e r w e l l s t h a t were never s t i m u l a t e d b u t have good p r o d u c t i o n h i s t o r i e s can o f t e n be r e j u v e n a t e d by l a t e - l i f e f r a c t u r i n g . R e s t i m u l a t i o n i s g e n e r a l l y a v i a b l e way t o r e a c h untapped hydrocarbon r e s e r v e s . W h i l e r e f r a c t u r i n g by MHF technology i s h i g h l y s u c c e s s f u l , a l s o s m a l l - s c a l e f r a c t u r i n g and a c i d i z i n g have given e x c e l l e n t production increase returns. Other r e f r a c t u r i n g candidates are w e l l s where f o r m e r l y bad and damaging f l u i d s , i n s u f f i c i e n t p r o p p a n t c o n c e n t r a t i o n s o r inadequate p r o p p a n t t y p e s have been used. CROWELL & JENNINGS (1979) analyze r e f r a c t u r i n g c a n d i d a t e s e l e c t i o n i n l o w - p e r m e a b i l i t y gas r e s e r v o i r s and u n d e r l i n e t h e d e f i n i t e need o f w e l l performance d i a g n o s i s p r i o r t o r e s t i m u l a t i o n . An i m p o r t a n t source o f i n f o r m a t i o n f o r r e f r a c t u r i n g d e c i s i o n i s p r e s s u r e development o b t a i n e d a f t e r o r i g i n a l s t i m u l a t i o n (TUCKER 1979).
4.8.9.1.2. Comparable approach of refracturing candidate selection and minifracture calibration test evaluation I n some t i g h t gas f i e l d s , w e l l s which had i n i t i a l l y been s u c c e s s f u l l y complet e d w i t h small-volume f r a c t u r e t r e a t m e n t s can be s u c c e s s f u l l y r e s t i m u l a t e d by MHF o p e r a t i o n s , whereas w e l l s t h a t responded p o o r l y t o o r i g i n a l small-volume f r a c t u r e j o b s w i l l n o t s u f f i c i e n t l y r e a c t t o r e f r a c t u r i n g on l a r g e r s c a l e t o pay o u t t h e expenses o f t h e r e s t i m u l a t i o n t r e a t m e n t (PARROT & LONG 1979). I n o r d e r t o f i n d o u t these r e l a t i o n s h i p s as a t o o l o f c o n t a i n i n g f r a c t u r i n g expenses and t o t e s t t h e f o r m a t i o n i n r e a l i t y r a t h e r than r e l y i n g c o m p l e t e l y on more o r l e s s t h e o r e t i c a l p l a n n i n g , m i n i f r a c t u r i n g w i t h i n s e r t i o n o f small q u a n t i t i e s of proppants ( c f . s e c t i o n 4.8.2.3.) s h o u l d be done b e f o r e MHF t r e a t m e n t s espec i a l l y i n areas w i t h l i t t l e e x p e r i e n c e from o t h e r w e l l s i n t h e f i e l d . The s i g n i f i c a n c e o f p r o p p a n t m i n i f r a c t u r i n g o f t i g h t gas sands r e q u i r i n g MHF s t i m u l a t i o n f o r u l t i m a t e economical p r o d u c t i o n i s n o t o n l y f r a c t u r e p r e s s u r e g r a d i e n t , f l u i d - l e a k o f f c o e f f i c i e n t , and c l o s u r e s t r e s s d e t e r m i n a t i o n by c a l i b r a t i o n t e s t i n g ( c f . s e c t i o n 4.8.2.1.), b u t a l s o f e a s i b i l i t y c h e c k i n g by p r o b i n g f o r m a t i o n
622 r e s p o n s e , a n d thus comprehensive MHF t r e a t m e n t p l a n n i n g i n c l u d i n g p r e c e d i n g min i f r a c t u r e can be compared t o l a r g e - s c a l e r e f r a c t u r i n g of w e l l s having been f o r merly s u b j e c t e d t o s m a l l - s c a l e s t i m u l a t i o n j o b s .
4.8.9.2. Areal distribution of potential reservoirs High-rank t a r g e t s f o r r e f r a c t u r i n g i n Europe a r e seen e s p e c i a l l y i n Lower C r e t a c e o u s o i l - b e a r i n g s a n d s t o n e s i n the l a r g e o i l f i e l d s i n the boundary b e l t between Germany FRG and the N e t h e r l a n d s a s well a s i n v a r i o u s o t h e r o l d e r p r o d u c t i o n p a t c h e s i n the seam of J u r a s s i c and Lower C r e t a c e o u s o i l f i e l d s i n t h e Lower Saxony Basin i n the n o r t h e r n p a r t of Germany FRG ( f o r l o c a t i o n maps c f . B O I G K 1980, SCHRODER 1986), and i n T e r t i a r y o i l - and g a s - b e a r i n g s a n d s t o n e s i n t h e b i g f i e l d s i n t h e Vienna Basin i n A u s t r i a ( D O L A K & PERNTHANER 1976) and Czechoslovakia. A high p o t e n t i a l f o r r e f r a c t u r i n g i s a l s o c o n s i d e r e d t o be p r e s e n t i n l a r g e o i l f i e l d s in t h e USSR such a s f o r example around t h e Caspian Sea and in S i b e r i a . Another r e s t i m u l a t i o n goal a r e g a s - b e a r i n g R o t l i e g e n d and C a r b o n i f e r o u s sands t o n e s i n the Mid-European Basin ( S o u t h e r n Permian Basin; c f . s e c t i o n 2 . 4 . 1 . and 3 . 3 . ) which had been a l r e a d y f r a c t u r e d b e f o r e the a d v e n t of i n t e r m e d i a t e and h i g h - s t r e n g t h ceramic p r o p p a n t s and where s t i l l s u f f i c i e n t reserves a r e l e f t t o j u s t i f y an improvement of the u n s a t i s f y i n g e a r l i e r t r e a t m e n t by performing a modern high-technology o p e r a t i o n .
4.8.9.3. Mobi 1 izat ion o f additional hydrocarbon reserves The m o b i l i z a t i o n of a d d i t i o n a l hydrocarbon r e s e r v e s by r e f r a c t u r i n g of e a r l i e r s t i m u l a t e d b o r e h o l e s o r even f i r s t f r a c t u r i n g i n t h e l a t e s t a g e of well and f i e l d h i s t o r y i s s u p p o r t e d by an a m e l i o r a t e d f e a s i b i l i t y due t o a l r e a d y a m o r t i z a t i o n of t h e o r i g i n a l d r i l l i n g and completion investment through p r o l o n ged p r o d u c t i o n . The t e c h n i c a l e x e c u t i o n of t h e r e s t i m u l a t i o n t r e a t m e n t i s e a s i e r than t h a t of an o r i g i n a l j o b because of t h e a l r e a d y e x i s t e n c e of a weakn e s s p l a n e l e a d i n g t o f o r m a t i o n opening a t lower i n j e c t i o n p r e s s u r e s than usual ( c f . section 4.8.9.5.), and the c r e a t i o n of new d r a i n a g e p a t h s improves flow c o n d i t i o n s and a c q u i s i t i o n a r e a and thus l e a d s t o r e c o v e r y enhancement independ e n t from r e s e r v o i r d e p l e t i o n s t a g e . Some comments a r e a l s o o f f e r e d on product i o n d e c l i n e o f f s e t t i n g and r e s e r v o i r t y p e s s u i t a b l e f o r r e f r a c t u r i n g .
4.8.9.3.1.Ameliorated feasibility due to well amortization The main s i g n i f i c a n c e of r e f r a c t u r i n g o r p a r t i a l l y a l s o even f i r s t s t i m u l a t i o n i n t h e l a t e s t a g e of hydrocarbon e x p l o i t a t i o n and r e s e r v o i r pressure d e p l e t i o n i s t o m o b i l i z e p a r t s of t h e remaining hydrocarbon q u a n t i t i e s from t i g h t e r s t r e a k s and l a y e r s in o r d e r t o o p t i m i z e the r e c o v e r y f a c t o r and thus more e f f e c t i v e l y d r a i n p a r t i c u l a r l y huge s h a l l o w o i l f i e l d s c o n t a i n i n g enormous amounts of r e s e r v e s ( a t l e a s t in terms of r e l a t i v e view when comparing t h e s i z e of a l l the domestic p r o d u c t i o n p a t c h e s w i t h i n t h e i n d i v i d u a l c o u n t r i e s ) p r i o r t o plugg i n g and abandoning of the w e l l s . L a t e - s t a g e r e f r a c t u r i n g of h o r i z o n s t h a t have a l r e a d y been e x p l o i t e d f o r p a r t i a l l y more than h a l f a c e n t u r y can be i n p l a c e s combined w i t h s t i m u l a t i o n of marginal r e s e r v o i r s over- o r u n d e r l y i n g the main producing s t o r e y i n l i t t l e d i s t a n c e which had been p r e v i o u s l y n o t p e r f o r a t e d due t o the much l a r g e r reserves i n the main t a r g e t f o r m a t i o n and the much b e t t e r o u t p u t r a t e s from t h e p e r f o r a t e d i n t e r v a l s t h e r e i n , w i t h the marginal l a y e r s t h e r e f o r e having been p u t on a b a c k s e a t p o s i t i o n i n t h e ranking l i s t a s long a s o f f t a k e from t h e main h i g h - c a p a c i t y r e s e r v o i r was s a t i s f a c t o r y . Once the main producer i s more o r l e s s d e p l e t e d , however, the f e a s i b i l i t y s i t u a t i o n f o r r e f r a c t u r i n g of s e n i l e h o r i z o n s along w i t h f i r s t t r e a t m e n t of juven i l e marginal pays i s very f a v o u r a b l e , because t h e o n l y expenses f o r then a c c e s -
623 s i n g t h e o i l i n t h e m a r g i n a l r e s e r v o i r a r e t h e c o s t f o r r e c o m p l e t i o n and s t i m u l a t i o n , as t h e w e l l i s a l r e a d y t h e r e and would a f t e r w a r d s anyway have t o be plugged w i t h cement, and a l s o a f t e r up t o s e v e r a l decades o f p r o d u c t i o n f r o m t h e main t a r g e t h o r i z o n t h e r e i s n o t h i n g l e f t f o r s t i l l w r i t i n g o f f , and l o n g term e x p l o i t a t i o n has r a t h e r n o t o n l y p a i d o f f a l l t h e p r e v i o u s investment, b u t a l s o g i v e n r i s e t o c o n s i d e r a b l e p r o f i t . Aspects o f l a t e d e p l e t i o n stage and end o f f i e l d l i f e economics i n N o r t h Sea hydrocarbon patches a r e o u t l i n e d by KEMP, KELLAS & ROSE ( 1 9 8 7 ) .
4.8.9.3.2. Product ion decline offsett ins GRIFFITH & MADISON (1988) o u t l i n e f i e l d examples where p r o d u c t i o n d e c l i n e s have been p a r t i a l l y o f f s e t by r e f r a c t u r i n g . I t i s n o t uncommon f o r a w e l l t o have been r e f r a c t u r e d s e v e r a l times t h r o u g h o u t i t s p r o d u c i n g l i f e . W h i l e a c i d s t i m u l a t i o n s have t r a d i t i o n a l l y r e s u l t e d i n s h o r t - l i v e d p r o d u c t i o n i n c r e a s e s w i t h l i m i t e d e f f e c t s on t h e u l t i m a t e r e c o v e r y o f t h e borehole, r e f r a c t u r i n g has l e d t o a greater production increase sustained over a longer period o f time. Ext e n s i o n o f propped f r a c t u r e l i f e and p r e v e n t i o n o f e a r l y c o n d u c t i v i t y d e s t r u c t i o n by m o b i l e f i n e s and s c a l e d e p o s i t i o n can be achieved w i t h l a r g e r f r a c t u r e w i d t h and i n c r e a s i n g proppant s a t u r a t i o n , and t h e r e f o r e d u r i n g course o f s t i m u lation evolution, small multi-staged low-proppant-concentration f r a c t u r i n g t r e a t m e n t s were r e p l a c e d by l a r g e r s i n g l e - s t a g e l i m i t e d - e n t r y h i g h - p r o p p a n t - s a t u r a t i o n f r a c t u r i n g j o b s . The b e n e f i t s o f t h e s i n g l e - s t a g e l i m i t e d - e n t r y f r a c t u r e o p e r a t i o n i n c l u d e l a r g e r propped f r a c t u r e w i d t h , b e t t e r a r e a l coverage o f n e t pay zone, and s t i m u l a t i o n o f p r e v i o u s l y u n f r a c t u r e d pay zones ( c f . s e c t i o n 4.2.2.2.).
4.8.9.3.3. Easier technical treatment
performance
by
presence of weakness plane
Restimulation options include r e f r a c t u r i n g , reacidizing, reperforating, heat e d a c i d t r e a t m e n t s ( c f . s e c t i o n 4.5.1.2.4.), closed f r a c t u r e acidizing, water f r a c t u r i n g ( c f . s e c t i o n 4.5.4.4.2.), subzone redevelopment, and h o r i z o n t a l w e l l o r d r a i n h o l e d r i l l i n g ( c f . s e c t i o n 4 . 8 . 6 . ) , o f which r e f r a c t u r i n g i s by f a r t h e most e f f e c t i v e method (JOURNAL PETROLEUM TECHNOLOGY 1987). W h i l e f r a c t u r i n g o f a d e p l e t e d w e l l f o r t h e f i r s t t i m e i s much more d i f f i c u l t t h a n s t i m u l a t i n g a v i r g i n w e l l because i t r e q u i r e s a much h i g h e r f r a c t u r e i n i t i a t i o n pressure, r e f r a c t u r i n g i s f a c i l i t a t e d by a c r a c k a l r e a d y e x i s t i n g r e p r e s e n t i n g a weakness p l a n e i n t h e r o c k f a b r i c which does no l o n g e r r e q u i r e t o overcome t h e t e n s i l e s t r e s s o f t h e f o r m a t i o n ( c f . s e c t i o n 4 . 8 . 9 . 5 . ) . T h e r e f o r e l e s s p r e s s u r e i s needed t o reopen t h e o l d f r a c t u r e . The proppant package p l u g g i n g t h e o r i g i n a l c r a c k i s a l r e a d y suspended, and i n many cases, t h e r e i s an e f f i c i e n t b a r r i e r a t t h e b o t t o m o f t h e f r a c t u r e as a consequence o f p r o p p a n t b a n k i n g which a c t s as a b l o c k l i m i t i n g downwards f r a c t u r e p r o p a g a t i o n ( c f . s e c t i o n 4 . 2 . 2 . 4 . 3 . ) and t h u s p r e v e n t s t h e c r a c k f r o m m i g r a t i n g i n t o t h e u n d e r l y i n g w a t e r zone. WYMAN, HOLDITCH & RANDOLPH (1979) r e p o r t s p e c i a l displacement techniques f o r proppants w i t h i n o l d f r a c t u r e s a t t h e b e g i n n i n g o f r e s t i m u l a t i o n o p e r a t i o n s . I n o r d e r t o ensure t h a t a s c r e e n o u t would n o t r e s u l t f r o m p i c k u p o f t h e p r o p p a n t i n s e r t e d d u r i n g t h e e a r l i e r t r e a t m e n t by t h e l e a d i n g edge o f t h e pad d u r i n g t h e r e f r a c t u r i n g j o b , combined and a l t e r n a t e d n i t r o g e n , methanol and w a t e r m i x t u r e i n j e c t i o n i s c a r r i e d o u t . I n t h e m i d d l e o f t h i s displacement procedure, a small foam s l u g i s pumped i n t o t h e annulus f o r t h e purpose o f f u r t h e r s c o u r i n g any o l d proppant f r o m t h e f r a c t u r e w a l l s , w i t h t h e f o l l o w i n g n i t r o g e n b r e a k i n g i t up and a l l o w i n g any e n t r a i n e d p r o p p a n t t o s e t t l e t o t h e bottom.
624
Recovery enhancement independent from reservoir depletion stage
4.8.9.3.4.
L a t e - s t a g e h y d r a u l i c p r o p p a n t s t i m u l a t i o n a l s o o f d e p l e t e d w e l l s i s support e d by t h e f a c t t h a t f r a c t u r i n g i s t h e most e f f e c t i v e t r e a t m e n t t e c h n i q u e by superimposing a n e w l y - c r e a t e d h i g h - p e r m e a b i l i t y s t r u c t u r e w i t h i n t h e r e s e r v o i r which i s i n communication w i t h t h e w e l l b o r e , a l t h o u g h t h e o v e r a l l pay permeabil i t y cannot be changed (ECONOMIDES 1987 b ) , and t h i s i n c r e a s e o f f o r m a t i o n s u r f a c e which i s a c c e s s i b l e f r o m t h e b o r e h o l e through t h e f r a c t u r e system and t h a t can c o n t r i b u t e t o d r a i n a g e o f hydrocarbons f r o m t h e r o c k m a t r i x t o t h e b o r e h o l e through t h e c r a c k network i s t h e reason f o r r e c o v e r y enhancement i n pay sect i o n s r e g a r d l e s s o f t h e i r d e p l e t i o n stage. I n t i g h t gas r e s e r v o i r s , f l u i d l o a d r e c o v e r y i n o r i g i n a l t r e a t m e n t s can r e q u i r e a few weeks t o s e v e r a l y e a r s , and f o r r e f r a c t u r i n g , i t may need even l o n g e r t i m e because o f d e c l i n e d r e s e r v o i r p r e s s u r e due t o p r o d u c t i o n (PARROT & LONG 1979). R e f r a c t u r i n g o f w e l l s which were a l r e a d y once t r e a t e d b e f o r e i s a l s o r e p o r t e d by KOHLHAAS (1982) who a l s o d e s c r i b e s s u c c e s s f u l f i r s t f r a c t u r i n g i n a matur e o i l f i e l d i n l a t e stages o f d e p l e t i o n , r e s u l t i n g i n access t o a d d i t i o n a l r e m a i n i n g r e s e r v e s t h a t would never have come o u t o f t h e r e s e r v o i r w i t h o u t s t i m u l a t i o n . Renewed f r a c t u r i n g o f a gas and condensate w e l l a f t e r a f i r s t s m a l l e r t r e a t m e n t by a second l a r g e r o p e r a t i o n i s o u t l i n e d by ECONOMIDES, C I K E S , PFORTER, U D I C K & URODA ( 1 9 8 6 ) . WATERS (1980) a l s o emphasizes t h e n e c e s s i t y o f many s h a l l o w s t r i p p e r w e l l s t o be f r a c t u r e d i n o r d e r t o remain p r o d u c t i v e . Aspects of r e f r a c t u r i n g a r e a l s o d i s c u s s e d by COULTER & M E N Z I E (1973), CROWELL & JENN I N G S (1978), PARROT & LONG (1979), HUNTER (1985) and SAUCIER (1988), and S I N H A & ELBEL (1983) i n v e s t i g a t e s t i m u l a t i o n o f o l d p r o d u c i n g w e l l s . Recompletion proppant f r a c t u r i n g f o r improving p r o d u c t i v i t y o f multipay i n t e r v a l s i s also e v a l u a t e d by DARR & CARLTON ( 1 9 8 8 ) . GRIFFITH & MADISON (1988) o u t l i n e o p t i m i z a t i o n o f p r o p p a n t c o n c e n t r a t i o n and p r o f i t a b i l i t y f o r r e f r a c t u r i n g . DOLAK & PERNTHANER (1976) document t h a t a l s o i n o l d h i g h l y - w a t e r e d r e s e r v o i r s w i t h v e r y low f o r m a t i o n pressure, c o m p a r a t i v e l y cheap and r e l a t i v e l y s m a l l - s c a l e f r a c t u r i n g t r e a t m e n t s can l e a d t o p r o d u c t i o n o f s i g n i f i c a n t amounts o f a d d i t i o n a l o i l .
4.8.9.3.5.
Reservoir types suitable for refracturing
W h i l e most o f t h e r e f r a c t u r i n g work t a k e s p l a c e i n sandstone r e s e r v o i r s , a l s o g a s - b e a r i n g s h a l e s can be an e c o n o m i c a l l y a t t r a c t i v e and t e c h n i c a l l y c h a l l e n g i n g t a r g e t f o r p r o p p a n t r e s t i m u l a t i o n (LANCASTER, GUIDRY, GRAHAM, CURTIS, SHAW & BLAKE 1987; c f . s e c t i o n 4 . 4 . 4 . ) . Coal r e f r a c t u r i n g i s o u t l i n e d by JEU, LOGAN & McBANE (1988; c f . s e c t i o n 4 . 4 . 3 . ) . R e s t i m u l a t i o n can a l s o comprise a c i d non-proppant f r a c t u r i n g e s p e c i a l l y i f g e l l e d o r c r o s s l i n k e d a c i d i s a p p l i e d , and i n some cases, such a t r e a t m e n t can f o l l o w an u n s u c c e s s f u l h y d r a u l i c p r o p p a n t t r e a t m e n t t h a t was t e r m i n a t e d by s c r e e n o u t f a i l u r e i n carbonate r e s e r v o i r s (JOHNSON, FOX, BURNS & O'MARA 1988). BRINKMANN (1982) d e s c r i b e s an example o f unwanted a c c i d e n t a l r e f r a c t u r i n g of an e a r l i e r s t i m u l a t e d i n t e r v a l d u r i n g course o f a MHF campaign c a r r i e d s u c c e s s i v e l y o u t i n s e v e r a l h o r i z o n s w i t h i n one we1 1 .
4.8.9.4.
Proppant select ion for ref ractur ins
I m p o r t a n t g u i d e l i n e s f o r p r o p p a n t s e l e c t i o n f o r advanced-stage f i r s t o r r e p e a t e d f r a c t u r i n g o f o i l - o r gas-bearing r e s e r v o i r s a r e t h e r e l a t i o n s h i p s b e t ween d i s c o u n t e d o p e r a t i n g cash income, t i m e and p r o p p a n t t y p e (HOLDITCH 1984). Some aspects o f s y n t h e t i c proppants vs. n a t u r a l sand as w e l l as r e s e r v o i r p r e s s u r e vs. p r o p p a n t c o n c e n t r a t i o n a r e o u t l i n e d as f o l l o w s .
625
4.8.9.4.1. Synthetic proppants vs. natural sand Examination o f f i e l d examples r e v e a l s t h a t d i s c o u n t e d income d u r i n g t h e f i r s t few y e a r s i s g r e a t e r when u s i n g s y n t h e t i c b a u x i t e proppants i n s t e a d o f nat u r a l sand due t o h i g h e r i n i t i a l gas f l o w r a t e s as a consequence o f g r e a t e r f r a c t u r e c o n d u c t i v i t y which r e s u l t s i n p r o d u c t i o n o f a g r e a t e r percentage o f gas d u r i n g e a r l y f i e l d h i s t o r y and b e t t e r a c c e l e r a t i o n o f c u m u l a t i v e gas recover y . On t h e o t h e r hand, however, t h e average p r o d u c t i o n r a t e o f a f r a c t u r e p r o p ped w i t h sand i s u s u a l l y s l i g h t l y h i g h e r i n comparison t o p l u g g i n g o f t h e c r a c k w i t h b a u x i t e proppants d u r i n g t h e l a t e r p r o d u c t i o n p e r i o d s as a consequence o f l e s s pronounced a c c e l e r a t i o n , w i t h t h e s l i g h t l y h i g h e r r a t e produced through t h e s a n d - i n f i l l e d f r a c t u r e i n comparison t o t h a t coming s t i l l o u t o f t h e baux i t e - p l u g g e d c r a c k ( w h i c h has a l r e a d y f a r more s t r o n g l y d e p l e t e d t h e r e s e r v o i r w i t h r e s p e c t t o t h a t t r e a t e d w i t h sand) b e i n g t h e e x p l a n a t i o n f o r t h e h i g h e r d i s c o u n t e d p r o f i t generated d u r i n g l a t e f i e l d l i f e f o r t h e case o f a sand-propped f r a c t u r e . Some c o n t r i b u t i n g f a c t o r s which g i v e r i s e t o low w e l l p r o d u c t i v i t y and char a c t e r i z e r e f r a c t u r i n g c a n d i d a t e s a r e low proppant c o n c e n t r a t i o n , polymer degrad a t i o n o r f i l t e r cake p l u g g i n g , crushed proppants o r f o r m a t i o n p a r t i c l e s , and embedment and flowback o f proppants (FRACFAX 1988 h ) . Curable r e s i n - c o a t e d p r o p pants are f r e q u e n t l y a s u i t a b l e material f o r r e f r a c t u r i n g ( c f . section 1.2.6.). When t h e y a r e i n j e c t e d i n t o an o l d f r a c t u r e f i l l e d w i t h c o n v e n t i o n a l proppants, t h e v i s c o u s pad pushes back t h e o l d proppant, and c u r a b l e r e s i n - c o a t e d p a r t i c l e s a r e t h e n pumped i n s u f f i c i e n t q u a n t i t y t o cover t h e p e r f o r a t e d i n t e r v a l up t o a d e s i r e d f r a c t u r e l e n g t h , w i t h r e s i n bonding l o c k i n g t h e c o a t e d p a r t i c l e s i n p l a c e a t t h e f r a c t u r e f a c e t o g e t h e r t o f o r m a s t a b l e mass which p r e v e n t s flowback o f u n s t a b i l i z e d o l d proppants as w e l l as guarantees i n v a l i d i z a t i o n o f new proppants ( c f . s e c t i o n 4 . 1 2 . 3 . 3 . ) .
4.8.9.4.2.Reservoir pressure vs. proppant concentration I n terms o f t r e a t m e n t s i z e and proppant s a t u r a t i o n , r e f r a c t u r i n g u s u a l l y t a kes p l a c e i n an environment o f d e p l e t e d r e s e r v o i r p o r e p r e s s u r e which c r e a t e s more e f f e c t i v e boundaries between pay zone and a d j a c e n t s h a l e s because o f t h e increased f r a c t u r e g r a d i e n t and i n - s i t u s t r e s s d i f f e r e n c e ( c f . s e c t i o n s 4.2.2.3.1. and 4.2.3.2.2.) between t h e two media (HUEBINGER, WEBSTER, CHISHOLM, VENDITTO & HUNT 1988). The i n c r e a s e d f r a c t u r e g r a d i e n t f o l l o w i n g somewhat deplet i o n o f r e s e r v o i r pore pressure allows t o design f r a c t u r i n g operations t h a t w i l l be more c o n t a i n e d w i t h i n t h e p r o s p e c t i v e i n t e r v a l . W i t h r e s p e c t t o t h e o r i g i n a l s t i m u l a t i o n jobs, b o t h l e s s t o t a l p r o p p a n t q u a n t i t y and l e s s p r o p p a n t sat u r a t i o n a r e r e q u i r e d t o f i l l i n a s h o r t e r f r a c t u r e d u r i n g r e t r e a t m e n t which i s c o n c e n t r a t e d t o t h e c o r e o f t h e pay zone, and thus r e s t i m u l a t i o n j o b s t u r n o u t t o be c o n s i d e r a b l y cheaper t h a n i n i t i a l t r e a t m e n t s when r e s e r v o i r p o r e p r e s s u r e was a t i t s v i r g i n l e v e l (case s t u d i e s show t h a t p r o p p a n t volume can be reduced up t o a b t . 40 % t o a c h i e v e a comparable e f f e c t ) . T h i s i s t h e reason why r e f r a c t u r i n g can s t i l l be economical even w i t h d e c l i n i n g p r o d u c t i o n and f a l l i n g o i l p r i c e . The o t h e r p o s s i b i l i t y opened by h i g h e r f r a c t u r e g r a d i e n t i s p r o p a g a t i o n o f a l a r g e r c r a c k i n f i l l e d by l a r g e r amounts o f proppants t h a n would have been s a f e l y enabled by i n i t i a l r e s e r v o i r p r e s s u r e , t h e r e b y g i v i n g r i s e t o deeper pay p e n e t r a t i o n by a more c o n d u c t i v e f r a c t u r e which c o n s i d e r a b l y i n c r e a s e s hydrocarbon d r a i n a g e and c u m u l a t i v e o f f t a k e , b u t also requires higher i n i t i a l c a p i t a l expenditure r e s u l t i n g i n longer amortizat i o n time. Proppant pumping i n r e f r a c t u r i n g s h o u l d f o l l o w an a g g r e s s i v e schedule (VENDITTO, CHISHOLM, WIGGINS & CONWAY 1986; c f . a l s o s e c t i o n 4 . 1 3 . ) . The b e s t way t o achieve l o n g f r a c t u r e s and h i g h volumes o f p r o p p a n t d e p o s i t i o n i s t o p r o gress q u i c k l y f r o m t r e a t m e n t pad t o heavy p r o p p a n t s a t u r a t i o n w h i c h propagates t h e c r a c k r a p i d l y and h e l p s t o ensure p r o p p a n t d e l i v e r y s u f f i c i e n t l y f a s t t o
626 the fracture l i m i t s . I n terms o f c h o i c e o f sand v s . h i g h - s t r e n g t h p r o p p a n t s , f i e l d e x p e r i e n c e has shown t h a t h i g h sand c o n c e n t r a t i o n s c a n be more e f f e c t i v e t h a n m o d e r a t e s a t u r a t i o n s o f i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p p a n t s t h a t have been k e p t l i m i t e d due t o e c o n o m i c a l r e a s o n s .
4.8.9.5. Fracture reopening and proppant placement during r e f r a c t u r i n g P r o p p a n t s e l e c t i o n f o r r e f r a c t u r i n g has t o be combined w i t h t r e a t m e n t des i g n . An i m p o r t a n t p o i n t o f c o n s i d e r a t i o n i s t h e r e s i d u a l p r o p p a n t package i n t h e e a r l i e r c r a c k w h i c h has t o be e f f e c t i v e l y removed when r e o p e n i n g t h e f r a c t u r e i n o r d e r t o p r e v e n t premature screenout f a i l u r e o f t h e r e f r a c t u r i n g operat i o n by g e t t i n g s t u c k a t t h e o l d e r p l u g w i t h i n t h e c r a c k ( c f . s e c t i o n 4.8.9.3.3.). The f o l l o w i n g summary c o n t a i n s e v a l u a t i o n o f i n c r e a s i n g f r a c t u r e w i d t h and p r o p p a n t c o n c e n t r a t i o n as w e l l as p o s s i b i l i t i e s o f r e i n j e c t i o n c o m p l i cations.
4.8.9.5.1.
Increasing f r a c t u r e width and proppant concentrat i o n
F r a c t u r e mouth c l e a r a n c e b y d i s p l a c e m e n t o f t h e e a r l i e r i n s e r t e d s u p p o r t i n g m a t e r i a l can be a c h i e v e d by c o n s i d e r a b l y i n c r e a s i n g f r a c t u r e w i d t h and p r o p p a n t c o n c e n t r a t i o n , w i t h t h e optimum s h i f t i n g e f f e c t o f t h e h i g h - d e n s i t y c a r r i e r s l u r r y being obtained i f the m i x t u r e t r a v e l s by p l u g f l o w . Transport c a p a c i t y o f t h e f l u i d i s c e r t a i n l y enhanced by c h o o s i n g a l i g h t p r o p p a n t t y p e ( c f . sect i o n 4 . 3 . 4 . ) , p l u g f l o w b e h a v i o u r c a n be i m p r o v e d b y a d d i n g r e s i n as g r a i n p e l l i c l e s o r d i s p e r s e d p a r t i c l e s ( c f . s e c t i o n 1 . 2 . 6 . and 5 . 8 . 5 . ) , and p l a c e m e n t a m e l i o r a t i o n and embedment m i n i m i z a t i o n o f t h e p r o p p a n t g r a i n s c a n be s e c u r e d b y r e s i n f i l m s s u r r o u n d i n g t h e p a r t i c l e s w h i c h g i v e r i s e t o a r t i f i c i a l cementat i o n o f t h e p r o p p a n t package t o a s t a b l e r i g i d wedge s u p p o r t i n g t h e c r a c k . The f o l l o w i n g o u t l i n e f o c u s s e s on a s p e c t s o f a r r a n g e m e n t o f p r i m a r y and s e c o n d a r y p r o p p a n t l o a d , r e f r a c t u r i n g t r e a t m e n t s i z e , p r e s s u r e and s t r e s s d i s t r i b u t i o n , and p e r f o r a t e d i n t e r v a l c o n f i g u r a t i o n .
4.8.9.5.1.1. Arrangement o f primary and secondary proppant load P r a c t i c a l e x p e r i e n c e has shown t h a t t h e o r i g i n a l f r a c t u r e s g e n e r a l l y do n o t d i s t u r b t h e r e s t i m u l a t i o n o p e r a t i o n . As t h e o l d c r a c k i s s t i l l a weakness p l a n e and opens e a r l i e r t h a n an u n f r a c t u r e d f o r m a t i o n , t h e f r a c t u r e g r a d i e n t i s l o w e r t h a n u s u a l , b u t on t h e o t h e r hand f l u i d l o s s i s h i g h e r due t o t h e t h i e f e f f e c t o f the e x i s t i n g f r a c t u r e . Displacement o f the o l d proppant p l u g c o n s i s t i n g o f e i t h e r s t i l l i n t a c t o r a l r e a d y c r u s h e d g r a i n s i s p r i n c i p a l l y no p r o b l e m p r o v i ded s u f f i c i e n t w i d t h o f t h e n e w l y e n l a r g e d f r a c t u r e . Arrangement o f p r i m a r y and s e c o n d a r y p r o p p a n t l o a d i s t h e r e f o r e g e n e r a l l y v e r y good, and o n l y o c c a s i o n a l l y some s u b o r d i n a t e r e i n j e c t i o n c o m p l i c a t i o n s o c c u r ( c f . s e c t i o n 4 . 8 . 9 . 5 . 2 . ) . F i e l d examples o f l a r g e - s c a l e r e f r a c t u r i n g w i t h h i g h p r o p p a n t c o n c e n t r a t i o n s i n r e s e r v o i r s t h a t have f o r m e r l y been s t i m u l a t e d w i t h s m a l l - s c a l e t r e a t m e n t s and l o w p r o p p a n t s a t u r a t i o n s r e v e a l a s i g n i f i c a n t i n c r e a s e i n p r o d u c t i o n r a t e and c o m p a r a t i v e l y r a p i d p a y - o u t e v e n i f t h e f r a c t u r i n g j o b expenses a p p r o a c h 50 % o f t h e t o t a l w e l l c o s t s (BRINKMANN 1982; GREGORCZYK, PAULS, HOLTMYER, VENDITTO & CHISHOLM 1984). R e f r a c t u r i n g does n o t o n l y a p p l y f o r renewed t r e a t m e n t s months o r y e a r s a f t e r t h e o r i g i n a l o p e r a t i o n s , b u t a l s o f o r c o n t i n u a t i o n o f j o b s a f t e r s e v e r a l days i n c a s e o f n e c e s s a r y i n t e r r u p t i o n o f t h e pumping sched u l e due t o bad w e a t h e r c o n d i t i o n s o r s e v e r e m e c h a n i c a l p r o b l e m s w i t h t h e e q u i p ment (PEARSON, LYNCH, SCHMIDT & McCASLIN 1 9 8 8 ) .
627
4.8.9.5.1.2. Ref ractur ing treatment size R e f r a c t u r i n g can be c a r r i e d o u t as s m a l l - s c a l e j o b s w i t h a b t . 10,000 l b s o f proppants t h r o u g h a l l stages o f c o n v e n t i o n a l and massive o p e r a t i o n s up t o jumbo t r e a t m e n t s c o m p r i s i n g 3 Mio. l b s (1,500 t ) o f proppants (VENDITTO, CHISHOLM, W I G G I N S & CONWAY 1986). I n many cases, massive h y d r a u l i c r e f r a c t u r i n g t r e a t m e n t s w i t h h i g h proppant c o n c e n t r a t i o n s a r e c a r r i e d o u t i n w e l l s h a v i n g e x p e r i e n c e d s m a l l - s c a l e s t i m u l a t i o n w i t h low p r o p p a n t s a t u r a t i o n i n a p e r i o d o f 1 - 40 y e a r s ago. The b e n e f i t s o f massive h i g h - c o n c e n t r a t i o n r e f r a c t u r i n g i n c l u d e r e d u c t i o n o f e f f e c t s o f f i n e s on f r a c t u r e f l o w c a p a c i t y as a consequence o f t h e p r o p p a n t bed b e i n g s u b j e c t e d t o l e s s p l u g g i n g by f i n e s m i g r a t i o n , more e f f e c t i v e d r a i n a g e o f s t r i n g e r sands, l e a v i n g o f t h e c r e a t e d f r a c t u r e propped f o r a much h i g h e r percentage o f i t s e n t i r e h e i g h t and l e n g t h , and improvement o f b o t h c o n d u c t i v i t y and l e n g t h o f f r a c t u r e t h u s a m e l i o r a t i n g p r o d u c t i o n r a t e and t o t a l r e c o v e r a b l e hydrocarbons i n v e r y t i g h t r e s e r v o i r s .
(5 t )
4.8.9.5.1.3. Pressure and stress distribution When l o w - p e r m e a b i l i t y gas w e l l s a r e f r a c t u r e d upon i n i t i a l c o m p l e t i o n , r e s e r v o i r p r e s s u r e and h o r i z o n t a l s t r e s s a r e u n i f o r m t h r o u g h o u t t h e pay f o r m a t i o n (HOLDITCH, ROBINSON, ELY & R A H I M 1988). I f t h e b o r e h o l e i s produced p r i o r t o t h e s t i m u l a t i o n t r e a t m e n t o r i f l a t e r - s t a g e r e f r a c t u r i n g i s c a r r i e d o u t , however, t o a g r e a t e r o r l e s s e r amount b o t h a p r e s s u r e d i s t r i b u t i o n and s t r e s s g r a d i e n t a r e developed i n t h e r e s e r v o i r , w i t h s t r e s s i n t h e r o c k b e i n g l a r g e r i n t h e areas away f r o m t h e w e l l b o r e where p r e s s u r e has n o t been a f f e c t e d by product i o n . I n such a s i t u a t i o n , t h e s t r e s s a t t h e f r a c t u r e t i p and t h e r e s u l t i n g b o t tomhole t r e a t i n g p r e s s u r e i n c r e a s e as t h e c r a c k propagates i n t o r e s e r v o i r areas n o t h a v i n g been d r a i n e d so f a r by t h e e x i s t i n g w e l l b o r e . S t r e s s changes a t t h e f r a c t u r e t i p t h e r e f o r e deserve s e r i o u s a t t e n t i o n d u r i n g r e f r a c t u r i n g d e s i g n .
4.8.9.5.1.4. Perf orated interval configuration I n many cases, h y d r a u l i c p r o p p a n t o r a c i d f r a c t u r i n g has t o be performed by l i m i t e d - e n t r y t e c h n i q u e i n o r d e r t o a v o i d unwanted e x c e s s i v e f r a c t u r e e x t e n s i o n i n e i t h e r upward o r downward d i r e c t i o n i n t o n o n - p r o d u c t i v e h o r i z o n s (HUCKABEE 1988; c f . s e c t i o n 4 . 2 . 2 . 2 . ) . W h i l e i n new completions t h i s goal can be achieved by s e l e c t i v e p e r f o r a t i o n placement a c c o r d i n g t o r e s e r v o i r requirements, r e s t i m u l a t i o n i s o f t e n confronted w i t h the f a c t t h a t a long perforated section e x i s t s i n t h e b o r e h o l e and communication occurs w i t h a much t h i c k e r segment o f t h e pay complex t h a n can be a l l o w e d f o r f r a c t u r e i n i t i a t i o n . R e f r a c t u r i n g o p e r a t i o n s t h e r e f o r e f r e q u e n t l y have t o a p p l y mechanical d i v e r s i o n methods such as sand plugbacks, s t r a d d l e packer assemblies, and r e t r i e v a b l e b r i d g e p l u g s and packers. On t h e o t h e r hand, t h e h i g h e r i n - s i t u s t r e s s c o n t r a s t between pay and b a r r i e r as a consequence o f d e p l e t e d r e s e r v o i r p r e s s u r e d i m i n i s h e s t h e danger o f u n c o n t r o l l e d e x c e s s i v e f r a c t u r e h e i g h t growth and t h u s l i m i t e d - e n t r y t e c h n i que which i s necessary t o i n e v i t a b l y i n o r i g i n a l s t i m u l a t i o n a t v i r g i n r e s e r v o i r pressure i s f r e q u e n t l y n o t required i n late-stage r e f r a c t u r i n g .
4.8.9.5.2. Poss i bi 1 it ies o f reinject ion cornp 1 icat ion W h i l e i n most cases no d i f f i c u l t i e s have been encountered d u r i n g reopening o f o l d f r a c t u r e s , replacement o f t h e o r i g i n a l proppant i n f i l l and i n s e r t i o n o f t h e new p r o p p a n t l o a d d u r i n g course o f r e s t i m u l a t i o n o p e r a t i o n s , BEGNAUD & CLAIBORNE (1985) r e p o r t t h e o c c a s i o n a l occurrence o f p r e s s u r e - o u t c o m p l i c a t i o n s due t o i n t e r f e r e n c e o f e a r l i e r and c u r r e n t p r o p p a n t l o t s , e s p e c i a l l y i n case o f new t r e a t m e n t s b r e a k i n g t h r o u g h b a r r i e r s t o g e t i n communication w i t h o l d f r a c t u r e s i n a d j o i n i n g h o r i z o n s . Such i n t e r f e r e n c e s can r e s u l t i n p r o p p a n t bed r e d i s t r i b u t i o n which may have t h e e f f e c t t h a t a g a i n o f p r o d u c t i v i t y i n one s t o r e y i s j e o p a r d i z e d by a r e d u c t i o n o f e x p l o i t a b i l i t y i n t h e o t h e r l a y e r .
628 Occasional h i g h e r f r a c t u r e reopening p r e s s u r e s i n c o a l seam gas r e s e r v o i r r e f r a c t u r i n g can be c r e a t e d by accumulation o f c o a l f i n e s near t h e w e l l b o r e which b r i d g e t h e c r a c k and c o n s t r i c t t h e b o r e h o l e v i c i n i t y (ZUBER, REEVES, JONES & SCHRAUFNAGEL 1988; c f . s e c t i o n s 4.2.2.7. and 4 . 4 . 3 . 3 . ) . Coal seams can become p a r t i c u l a r l y f r i a b l e i f l o o s e n i n g o f t h e f a b r i c i s exacerbated by shear s t r e s ses imposed by f a u l t s i n t h e s u r r o u n d i n g s o f t h e b o r e h o l e . I n r a r e events, i t can be imagined t h a t pushing o f t h e o l d p r o p p a n t package i n f i l l i n g t h e o r i g i n a l c r a c k i n f r o n t o f t h e pad i n t o t h e now w i d e r opening and deeper p e n e t r a t i n g f r a c t u r e can t a k e p l a c e f a s t e r than c r a c k e x t e n s i o n and thus r e a c h i n g o f t h e o l d proppant l o t a t t h e f r o n t o f t h e r e s t i m u l a t i o n t r e a t m e n t t o t h e f r a c t u r e t i p c o u l d provoke s c r e e n o u t t e r m i n a t i o n ( c f . s e c t i o n 6.2.4.2.1.). As most o f t h e r e f r a c t u r i n g j o b s , however, p e r f o r m smoothly w i t h o u t s c r e e n o u t problems, propagation o f the crack during r e f r a c t u r i n g probably takes place a t a r a t e fast e r than t h a t o f p r o p p a n t - l a d e n f l u i d advance. O t h e r c o m p l i c a t i o n s may o c c u r i n r e s e r v o i r r o c k s w i t h m u l t i p l e n a t u r a l j o i n t systems t h a t a r e p a r t i a l l y h e a l e d (DOLAK & PERNTHANER 1976). I n such cases, one o f t h e main problems i s opening up o f a d d i t i o n a l f i s s u r e systems f o r uptake o f a d d i t i o n a l amounts o f p r o p p a n t s .
4.8.9.6. Combination of hydraulic and explosive fracturing L a t e - s t a g e r e f r a c t u r i n g o f w e l l s where an e a r l i e r h y d r a u l i c s t i m u l a t i o n t r e a t m e n t has been performed some t i m e ago can c o n s i d e r a b l y p r o f i t f r o m e x p l o s i v e f r a c t u r i n g superimposed on t h e o l d e r h y d r a u l i c t r e a t m e n t (YOUNG, BARKER & CLARK 1984). T h i s i s p a r t i c u l a r l y due t o t h e independency o f o r i e n t a t i o n o f c r a c k s generated by stem-induced h y d r a u l i c f r a c t u r i n g which a r e v e r t i c a l even i n r e s e r v o i r columns where t h e i n - s i t u s t r e s s s t a t e would d i c t a t e t h e o r i g i n o f h o r i z o n t a l c r a c k s . The i n t e r s e c t i o n o f e a r l i e r h o r i z o n t a l h y d r a u l i c f r a c t u r e s by l a t e r v e r t i c a l e x p l o s i v e f r a c t u r e s t h e r e f o r e i s a v e r y e f f e c t i v e means o f i n c r e a s i n g t h e d r a i n a g e area by combining s e v e r a l d i s t i n c t c r a c k systems ( c f . also section 4.9.4.2.1.4.). I n a d d i t i o n , f r a c t u r e s generated by e x p l o s i v e d e t o n a t i o n may o f t e n a l s o i n t e r s e c t l a r g e r p o r t i o n s o f t h e pay zones t h a t were n o t a d e q u a t e l y tapped by t h e e a r l i e r h y d r a u l i c s t i m u l a t i o n t r e a t m e n t s . The l a r g e f l u s h i n p r o d u c t i o n r e s u l t i n g f r o m e x p l o s i v e r e f r a c t u r i n g o f w e l l s t h a t had been p r e v i o u s l y h y d r a u l i c a l l y s t i m u l a t e d u n d e r l i n e s t h e s i g n i f i c a n t b e n e f i t o f stem-induced e x p l o s i v e r e t r e a t ments i n o r d e r t o a c q u i r e new p a r t s o f t h e r e s e r v o i r t h a t have so f a r n o t been d r a i n e d by t h e h y d r a u l i c a l l y - i n d u c e d c r a c k . Combination o f h y d r a u l i c and e x p l o s i v e f r a c t u r i n g a r e p a r t i a l l y r e v i v i n g o i l - w e l l s h o o t i n g by f i r i n g e x p l o s i v e charges which has been abundantly c a r r i e d o u t p r i o r t o i n t r o d u c t i o n o f hydraul i c f r a c t u r i n g t o t h e hydrocarbon market, w i t h w e l l s h o o t i n g a i m i n g on f r a c t u r i n g o r r u b b l i z i n g o f t h e r e s e r v o i r i n o r d e r t o i n c r e a s e b o t h i n i t i a l f l o w and u l t i m a t e r e c o v e r y o f o i l , w a t e r and gas w e l l s (GRANT, DUVALL, OBERT, ROUGH & ATCHISON 1950).
4.8.10. Combination of acid and proppant fracturing M i n i a t u r e h y d r a u l i c p r o p p a n t f r a c t u r i n g c o u l d a l s o be a p r o m i s i n g improvement o f a c i d f r a c t u r i n g by h a v i n g a c o m b i n a t i o n o f t h e e f f e c t o f d i s s o l u t i o n by t h e a c i d and p r o p p i n g o f t h e f r a c t u r e t h a t was opened by t h e pad, and i n case of good r e s u l t s c o u l d be t h e i n i t i a l s t e p f o r changing towards l a r g e r - s c a l e hyd r a u l i c p r o p p a n t f r a c t u r i n g o f carbonate f o r m a t i o n s i n Europe. Some combined p r o p p a n t - a c i d - f r a c t u r i n g j o b s have a l r e a d y been c a r r i e d o u t i n Cretaceous c h a l k i n t h e Norwegian and Danish N o r t h Sea (TANSOE, K I N G & HOLMAN 1986; c f . s e c t i o n 4 . 5 . 4 . 2 . ) and i n M i d d l e T r i a s s i c carbonates i n A u s t r i a . A p a r t i c u l a r l y import a n t marketing p o t e n t i a l o f m i n i f r a c t u r e s c a r r i e d out w i t h intermediate- t o h i g h - s t r e n g t h proppants i s seen a t l e a s t i n p a r t s of E a s t e r n Europe where admin i s t r a t i v e i n h i b i t i o n s o f f a s t approval o f m a j o r i n v e s t m e n t s would f a v o u r t o p e r f o r m some i n i t i a l t r e a t m e n t s as m i n i f r a c t u r e s w i t h own equipment and s e r v i c e
629 ( c f . s e c t i o n 2.4.4.4.) and t h u s o n l y c o m p a r a t i v e l y l i t t l e amounts o f proppants would have t o be purchased i n t h e west. Aspects o f p r o p p a n t d i s s o l u t i o n , t r e a t ment schedule and a c i d improvement, and s p e c i a l a p p l i c a t i o n s a r e b r i e f l y o u t l i ned as f o l l o w s .
4.8.10.1.
Proppant d i s s o l u t i o n
Concerning c o m b i n a t i o n o f a c i d and proppant f r a c t u r i n g (cf. section 4.5.4.2.), a t t e n t i o n has t o be p a i d t o t h e f a c t t h a t under a c i d c o n d i t i o n s , nat u r a l q u a r t z sand has t h e l o w e s t s o l u b i l i t y i n HCl/HF a c i d (CHEUNG 1985, COBB & FARRELL 1986). S i n t e r e d b a u x i t e and alumina o x i d e and s i l i c a t e proppants d i s s o l v e more o r l e s s q u i c k l y when exposed t o HCl/HF-acid by l e a c h i n g and format i o n o f p i t s i n t h e p a r t i c l e s . The r a t e o f s o l u b i l i t y i n c r e a s e s w i t h h i g h e r HF c o n c e n t r a t i o n s , l o n g e r exposure t i m e t o a c i d , and a g i t a t i o n o f t h e f l u i d ( c f . s e c t i o n 1 . 3 . 3 . 2 . ) . HCl/HF-mixtures i n t h e r a t i o 12 : 3 a t t a c k a l l t h e proppants much more r a p i d l y than e i t h e r h y d r o c h l o r i c a c i d o r h y d r o f l u o r i c a c i d i n d i v i d u a l l y . The presence o f c o r r o s i o n i n h i b i t o r s ( c f . s e c t i o n 4.5.1.1.3.2.) which a r e t o p r o t e c t t h e t u b i n g s o n l y slow down, b u t do n o t s t o p t h e d i s s o l u t i o n r a t e o f alumina o x i d e - c o n t a i n i n g p r o p p a n t s . I n a d d i t i o n t o chemical d i s s o l u t i o n , t h e alumina oxide-based proppants l o s e c o n s i d e r a b l e percentages o f t h e i r mechanical s t r e n g t h , and s i n t e r e d b a u x i t e even becomes weaker than n a t u r a l q u a r t z sand t h a t has been s i m i l a r l y t r e a t e d . T h e r e f o r e c o m b i n a t i o n o f proppant and a c i d f r a c t u r i n g has t o f o c u s on a p p l i c a t i o n o f n a t u r a l q u a r t z sand as p r o p p i n g agent when HCl/HF-mixtures a r e used as a c i d phase, and f o r deep w e l l s propped w i t h alumina oxide-based proppants, e x treme c a u t i o n s h o u l d be taken i n t h e use o f HCl/HF-acid and p r e f e r a b l y o n l y hyd r o c h l o r i c a c i d s h o u l d be a p p l i e d which does n o t s i g n i f i c a n t l y d i s s o l v e o r change t h e p h y s i c a l p r o p e r t i e s o f g r a v e l s and p r o p p a n t s .
4.8.10.2.
Treatment schedule and a c i d imrovement
I n terms o f t r e a t m e n t schedule, d i s t i n c t i o n has t o be made between j o i n t o r separated o p e r a t i o n o f a c i d and proppant f r a c t u r i n g . As a c i d a l o n e i s o n l y a poor f r a c t u r i n g f l u i d w i t h i n s u f f i c i e n t proppant t r a n s p o r t p r o p e r t i e s , i t s e f f e c t i v i t y f o r p r o p p a n t c a r r i a g e has t o be f r e q u e n t l y improved by g e l l i n g and/or c r o s s l i n k i n g o f t h e a c i d ( c f . s e c t i o n s 4.3.4.3.4.1. and 4.5.1.2.). Some comments on s u i t a b l e successions o f stages a r e a l s o o f f e r e d .
4.8.10.2.1.
Joint or separated operation
Combination o f a c i d and proppant f r a c t u r i n g has t o d i s t i n g u i s h between tempor a l l y j o i n t o r separated o p e r a t i o n . CHEUNG (1985) i s f a v o u r i n g a c i d i z i n g a f t e r g r a v e l p a c k i n g o r h y d r a u l i c f r a c t u r i n g , b u t t h i s can i n c l u d e o n l y a c i d washing o r m a t r i x a c i d i z i n g . A c i d and p r o p p a n t f r a c t u r i n g always s h o u l d be executed mor e o r l e s s s i m u l t a n e o u s l y , because i n case o f f i r s t p r o p p a n t f r a c t u r i n g and second a c i d s t i m u l a t i o n o f carbonates o r c a r b o n a t e - b e a r i n g sandstones, t h e c r a c k would be e n l a r g e d by a c i d l e a c h i n g and t h e proppant d i s t r i b u t i o n would be no l o n g e r s u i t a b l e t o s u p p o r t t h e f r a c t u r e and p r e v e n t i t s c l o s u r e . I n v e r s e l y , f i r s t a c i d i z i n g o r a c i d f r a c t u r i n g and second h y d r a u l i c p r o p p a n t f r a c t u r i n g could r e s u l t i n propagation o f the large p r o p p a n t - i n f i l l e d crack i n another d i r e c t i o n o r a l o n g o t h e r predetermined cleavage p l a n e s than t h e a c i d - i n duced v o i d spaces, which would mean t h a t no c o m b i n a t i o n e f f e c t i s achieved, b u t t h e two s e p a r a t e s t e p s o f o p e r a t i o n r a t h e r a c t i n d i v i d u a l l y and s p a t i a l l y d i s t i n c t . Simultaneous p r o p p a n t and a c i d f r a c t u r i n g guarantees t h a t a f t e r t h e main r e a c t i o n p e r i o d o f t h e a c i d , t h e h y d r a u l i c a l l y induced and c h e m i c a l l y e n l a r g e d f r a c t u r e i s i n f i l l e d by t h e proppants which s u p p o r t t h e whole f i n a l geometry and shape o f t h e c r a c k .
630
4.8.10.2.2.Acid gelling and crosslinking On t h e o t h e r hand, a c i d i s a v e r y u n s u i t a b l e f r a c t u r i n g o r g r a v e l p a c k i n g f l u i d w h i c h p e r f o r m s much w o r s e t h a n w a t e r - o r o i l - b a s e d g e l s due t o p o o r l e a k o f f c o n t r o l and l o w v i s c o s i t y u n l e s s u p g r a d e d ( c f . a l s o s e c t i o n 4 . 1 1 . ) . A c i d i m provement, however, c a n be done b y g e l l i n g o r c r o s s l i n k i n g , o r by u s i n g a l t e r n a t i n g s t a g e s o f a c i d and g e l o r f l u i d - l o s s a d d i t i v e (ELPHICK 1985; c f . a l s o sect i o n s 4.3.4.3.4.1. and 4 . 5 . 1 . 2 . 2 . ) . H i g h - s t r e n g t h c r o s s l i n k e d a c i d systems w i t h e l e v a t e d t e m p e r a t u r e s t a b i l i t y (PABLEY & HOLCOMB 1982; SMITH, DAWSON & SCOGGINS 1983) have e x c e l l e n t p r o p p a n t t r a n s p o r t c a p a c i t y and a r e t h e r e f o r e t h e optimum medium f o r a c o m b i n a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g . C r o s s l i n k i n g o f t h e a c i d c a n be done b o t h i n c o n v e n t i o n a l and d e l a y e d manner ( c f . s e c t i o n 4 . 3 . 4 . 3 . ) which i s considerably i n c r e a s i n g t h e p o s s i b i l i t y o f reducing t u b u l a r f r i c t i o n pressure. The l o w pH o f a c i d g e l systems m i n i m i z e s c l a y damage and t h u s a l l o w s c r o s s l i n k e d a c i d systems t o be a r e a s o n a b l e a l t e r n a t i v e t o foam and e n e r g i z e d f r a c t u r i n g f l u i d s i n w a t e r - s e n s i t i v e sandstone r e s e r v o i r s . F l u i d - l o s s c o n t r o l i d e n t i c a l o r l a r g e r t h a n t h a t o b t a i n e d b y c r o s s l i n k e d w a t e r - b a s e d f l u i d s i n s u r e s deep p e n e t r a t i o n o f l i v e a c i d , and an e x t r e m e l y r e t a r d e d r e a c t i o n r a t e a l l o w s i n many c a s e s t h e c r o s s l i n k e d a c i d t o be p l a c e d i n t o t h e f o r m a t i o n w i t h l i t t l e o r no d e c r e a s e i n a c i d s t r e n g t h . A s p e c t s o f e f f e c t i v e a c i d p e n e t r a t i o n i n f r a c t u r e systems a r e d i s c u s s e d b y REN & XIONG ( 1 9 8 6 ) , and h i g h - t e m p e r a t u r e s t a b i l i t y o f a c i d g e l l i n g p o l y m e r s i s i n v e s t i g a t e d by NORMAN, CONWAY & WILLIAMS ( 1 9 8 1 ) and NORMAN & CONWAY ( 1 9 8 3 ) .
4.8.10.2.3. Succession of stages C o n c e r n i n g t e c h n i c a l r e a l i z a t i o n o f combined a c i d and p r o p p a n t f r a c t u r i n g , i t may be i n many c a s e s t h e optimum s o l u t i o n t o pump a c i d f i r s t w i t h o u t p r o p p a n t s , open t h e f r a c t u r e h y d r a u l i c a l l y w i t h t h e a c i d pad, and f o l l o w t h e n w i t h i n j e c t i o n o f a g e l l e d c a r r i e r f l u i d t h a t i s s a t u r a t e d w i t h p r o p p a n t s once t h e r e a c t i o n o f the a c i d i n the f r a c t u r e d formation i s almost terminated. Caution has t o be e x e r c i s e d c o n c e r n i n g p r o p p a n t c h o i c e and a c i d s e l e c t i o n , because w h i l e h y d r o c h l o r i c a c i d i s g e n e r a l l y n o t h a z a r d o u s t o any p r o p p a n t t y p e , HC1H F - m i x t u r e s s e v e r e l y c o r r o d e p r o p p a n t s c o n s i s t i n g o f a l u m i n a o x i d e and s i l i c a t e , whereas n a t u r a l q u a r t z sand i s t h e c h e m i c a l l y m o s t s t a b l e p r o p p i n g mater i a l i n a c i d e n v i r o n m e n t (CHEUNG 1985, COBB & FARRELL 1986) ( a s p e c t s of sandstone a c i d i z i n g are discussed i n s e c t i o n 4.5.5.).
4.8.10.3. Special appl icat ions A s p e c i a l t y p e o f c o m b i n a t i o n o f a c i d and p r o p p a n t f r a c t u r i n g has t o b e app l i e d i n some c h a l k f o r m a t i o n s where s h o r t w i d e c r a c k s a r e i n d u c e d h y d r a u l i c a l l y and s u b s e q u e n t l y e n l a r g e d b y c r e a t i o n o f w i d e l y - s p a c e d e t c h e d r i d g e s b y a c i d i z i n g i n o r d e r t o make t h e f r a c t u r e s s u f f i c i e n t l y w i d e f o r i n f i l l i n g w i t h p r o p p a n t m u l t i - l a y e r s t h a t m i n i m i z e embedment o f g r a i n s i n t o t h e s o f t f o r m a t i o n (SHELL 1979, HARTLEY & BOSMA 1985; TANSOE, K I N G & HOLMAN 1986; c f . s e c t i o n A s u i t a b l e t e c h n i q u e f o r such weak u n s t a b l e r e s e r v o i r s i s t i p 4.5.4.2.2.). and s c r e e n o u t f r a c t u r i n g (SMITH, MILLER & HAGA 1987; c f . s e c t i o n s 4 . 5 . 4 . 4 . 1 . 4.8.5.3.). From t h e d i s c u s s e d p o i n t s , c o n c l u s i o n c a n t h u s b e made t h a t m i n i f r a c t u r i n g and r e l a t e d s m a l l - s c a l e s t i m u l a t i o n i s a h i g h - r a n k means t o expand t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g m a r k e t i n b o t h Western and E a s t e r n Europe b y n o t o n l y a l l o w i n g t o c u t s t i m u l a t i o n c o s t , b u t a l s o p e r m i t t i n g t o b e t t e r j u s t i f y t h e app r o a c h o f u n c o n v e n t i o n a l o i l and gas r e s e r v o i r s a t l e a s t a t an e x p e r i m e n t a l s t a g e where so f a r no sound t e c h n i c a l and e c o n o m i c a l s t i m u l a t i o n c o n c e p t has
631 been devel oped,
4.8.11. Fracture length and height optimization A d i s c u s s i o n of mHF vs. MHF and o t h e r s m a l l e r - o r l a r g e r - s c a l e p r o p p a n t s t i m u l a t i o n s a u t o m a t i c a l l y has t o f o c u s on t h e s i g n i f i c a n c e o f f r a c t u r e l e n g t h ( e x pressed as h a l f l e n g t h f r o m t h e w e l l b o r e t o t h e t i p o f one wing) and f r a c t u r e h e i g h t and t h e i r c o n t r o l l i n g f a c t o r s and i n f l u e n c e s . H y d r a u l i c f r a c t u r i n g t r e a t ments do n o t o n l y r e p r e s e n t a l a r g e f r a c t i o n o f i n i t i a l w e l l expenses ( c f . sect i o n s 2.3.4. and 3 . 7 . ) , b u t a l s o determine t h e economical v i a b i l i t y of a p a r t i c u l a r b o r e h o l e o r f i e l d (NEWBERRY, NELSON & AHMED 1985). A s t i m u l a t i o n j o b b e i n g t o o l a r g e can be an unnecessary waste o f c o m p l e t i o n funds, whereas a f r a c t u r i n g o p e r a t i o n b e i n g t o o small may r e s u l t i n such i n e f f i c i e n t r e s e r v o i r d r a i nage as t o r e n d e r a w e l l u n p r o f i t a b l e . T h i s economical double-edged sword n e c e s s i t a t e s t h a t h y d r a u l i c f r a c t u r i n g t r e a t m e n t s have t o be designed i n such a way t h a t optimum d e p l e t i o n o f t h e pay zone i s reached, w i t h p r o p p a n t t y p e and g r a i n s i z e s e l e c t i o n b e i n g one o f t h e key elements o f t h i s procedure ( c f . s e c t i o n 1 . 4 . 1 2 . ) , b u t an almost e q u i v a l e n t c o n t r o l e x e r t e d by c h o i c e o f adequate f r a c t u r e l e n g t h which i s t h e i t e m governi n g t o t a l j o b s i z e g i v e n more o r l e s s c o n s t a n t f r a c t u r e h e i g h t w i t h i n t h e lim i t s o f r e s e r v o i r t h i c k n e s s between t h e upper and lower bounding s e a l s . The o u t l i n e as f o l l o w s focusses on r e s e r v o i r p e r m e a b i l i t y and w e l l spacing vs. f r a c t u r e l e n g t h , f r a c t u r e c o n d u c t i v i t y vs. f r a c t u r e l e n g t h , f r a c t u r e f l o w c a p a c i t y vs. proppant d i s t r i b u t i o n , n e t p r e s e n t v a l u e o p t i m i z a t i o n and f r a c t u r e h e i g h t . Some f i e l d examples a r e a l s o mentioned.
4.8.11.1. Reservoir Permeability and
well spacing vs. fracture length
R e s e r v o i r p e r m e a b i l i t y and w e l l spacing a r e i m p o r t a n t f a c t o r s i n f l u e n c i n g f r a c t u r e l e n g t h s e l e c t i o n . Long and s h o r t f r a c t u r e s have t o be chosen i n lowand h i g h - p e r m e a b i l i t y pay zones, r e s p e c t i v e l y , and t h e t i g h t e r t h e r e s e r v o i r , t h e c l o s e r t h e b o r e h o l e s have g e n e r a l l y t o be spaced. The f o l l o w i n g d i s c u s s i o n a l s o i n c l u d e s comments on r e s e r v o i r and f r a c t u r e parameters v s . economical asp e c t s , p r e s s u r e d e c l i n e and f l u i d f l o w p a t t e r n , and f r a c t u r e e x t e n s i o n vs. r e s e r v o i r shape.
4.8.11.1.1. Long and short fractures in low- and high-permeability reservoirs G e n e r a l l y , f r a c t u r e l e n g t h i s s i m i l a r l y as w e l l spacing determined by t h e d r a i n a g e r a d i u s i n t h e hydrocarbon-bearing f o r m a t i o n which i s i n t u r n a f u n c t i o n o f r e s e r v o i r p e r m e a b i l i t y (MONTGOMERY & STEANSON 1985, POULSEN & SOLIMAN 1987). T h i s r e l a t i o n s h i p a l r e a d y l e a d s t o c o n c l u s i o n t h a t l o n g f r a c t u r e s i n conn e c t i o n w i t h MHF t r e a t m e n t s a r e r e q u i r e d i n l o w - p e r m e a b i l i t y pay h o r i z o n s and p a r t i c u l a r l y t i g h t gas sands, whereas moderate- and h i g h - p e r m e a b i l i t y format i o n s can be s u c c e s s f u l l y approached by s h o r t e r c r a c k s o f h i g h c o n d u c t i v i t y ( c f . s e c t i o n s 4.5.4.2.2. and 4 . 6 . ) and s m a l l e r j o b s i z e s and thus a r e s u i t a b l e t a r g e t s f o r m i n i f r a c t u r i n g and i t s a p p l i c a t i o n i n t h e v a r i o u s examples o u t l i n e d i n the preceding sections. As
fracture length i s also linked w i t h fracture flow capacity o r conductivit h i s means t h a t s u p p o r t o f l o n g f r a c t u r e s i n t i g h t r e s e r v o i r s has t o be p r o v i d e d by i n t e r m e d i a t e - and h i g h - s t r e n g t h s y n t h e t i c proppants, whereas i n f i l l i n g o f s h o r t e r c r a c k s i n moderate- t o h i g h - p e r m e a b i l i t y pay h o r i z o n s may a l s o i n c l u d e n a t u r a l sand i n many cases, a l t h o u g h a b e t t e r cond u c t i v i t y c o n t r a s t i s always achieved by t h e a p p l i c a t i o n o f a high-performance t y ( c f . s e c t i o n 1.4.10.),
632 man-made p r o p p a n t . S h o r t e r f r a c t u r e s i n m o d e r a t e - t o h i g h - p e r m e a b i l i t y r e s e r v o i r s r e q u i r e a l s o e f f i c i e n t m u l t i l a y e r p r o p p i n g t o a v o i d embedment ( c f . sect i o n 4.3.3.1.). The f o l l o w i n g d i s c u s s i o n c o n c e n t r a t e s on r e s e r v o i r p e r m e a b i l i t y and f r a c t u r e l e n g t h , n e a r - w e l l b o r e damage zone bypass, and f r a c t u r e f l o w c a p a c i t y and l e n g t h o p t i m i z a t i o n .
4.8.11.1.1.1. Reservoir permeability and fracture length As l o w - p e r m e a b i l i t y r e s e r v o i r s w i t h b r o a d d r a i n a g e r a d i i r e s p o n d f a v o u r a b l y t o c r e a t i o n o f l o n g p r o p p e d f r a c t u r e s w h i c h i s a c h i e v e d b y l a r g e f l u i d and p r o p p a n t volumes, f r a c t u r i n g r e s e a r c h has f o r y e a r s c o n c e n t r a t e d o n f r a c t u r e l e n g t h improvement (CRAMER & SONGER 1 9 8 8 ) . T h i s f o c u s has u n f o r t u n a t e l y l e d f o r a l o n g t i m e t o b e l i e f t h a t w e l l p r o d u c t i v i t y i s d i r e c t l y p r o p o r t i o n a l t o t r e a t m e n t volume and f r a c t u r e l e n g t h r e g a r d l e s s o f r e s e r v o i r c h a r a c t e r i s t i c s . A d i f f e r e n t approach, however, i s r e q u i r e d f o r m o d e r a t e - t o h i g h - p e r m e a b i l i t y p a y zones where o p t i m a l r e s u l t s a r e a c h i e v e d b y c r e a t i o n o f s h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s ( c f . s e c t i o n s 4.5.4.2.2. and 4 . 6 . ) . I n h y d r o c a r b o n - b e a r i n g i n t e r v a l s i n t e r s e c t e d by closely-spaced w e l l s , t h i s i s most e f f i c i e n t l y accomplished b y u s i n g a r e l a t i v e l y s m a l l s l u r r y volume c o n t a i n i n g h i g h p r o p p a n t c o n c e n t r a t i o n . W e l l b o r e t r a n s m i s s i b i l i t y i s i n c r e a s e d by b o r e h o l e v i c i n i t y damage zone b y p a s s ( c f . s e c t i o n 4 . 8 . 3 . ) and e x t e n s i o n o f e f f e c t i v e w e l l b o r e r a d i u s .
4.8.11.1.1.2. Near-wellbore damage zone bypass Damage zone b y p a s s i s a c h i e v e d b y c r e a t i o n o f a f l o w p a t h w i t h s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t f r o m w e l l b o r e t o v i r g i n a l r e s e r v o i r f o r m a t i o n and i s n o r m a l l y r e s p o n s i b l e f o r m o s t o f t h e p r o d u c t i v i t y improvement a t t a i n e d f r o m f r a c t u r e s t i m u l a t i o n o f m o d e r a t e - t o h i g h - p e r m e a b i l i t y p a y zones (CRAMER & SONGER 1988; c f . s e c t i o n 4 . 8 . 3 . ) . E x t e n s i o n o f e f f e c t i v e w e l l b o r e r a d i u s a l s o r e q u i r e s e x i s t e n c e o f a l a r g e c o n d u c t i v i t y c o n t r a s t between p r o p p e d f r a c t u r e and r e s e r v o i r p o r e n e t w o r k ( c f . s e c t i o n s 4 . 5 . 1 . 1 . 2 . and 4 . 6 . 1 . ) . W i t h a l l o t h e r t h i n g s b e i n g c o n s t a n t , t h e b e n e f i t s o f c o n d u c t i v i t y c o n t r a s t and r e s u l t i n g t r a n s m i s s i b i l i t y c o n t i n u a l l y diminuish w i t h i n c r e a s i n g f r a c t u r e length. W i t h i n t h e range o f achievable f r a c t u r e c o n d u c t i v i t i e s , optimal f r a c t u r e h a l f - l e n g t h v a r i e s from 10 - 30 % o f t h e w e l l d r a i n a g e r a d i u s . The p r i m a r y t r e a t m e n t o b j e c t i v e s i n moder a t e - t o h i g h - p e r m e a b i l i t y r e s e r v o i r s a r e t o bypass w e l l b o r e damage, m a x i m i z e f r a c t u r e c o n d u c t i v i t y , a v o i d e x c e s s i v e f r a c t u r e h e i g h t and l e n g t h , and m i n i m i z e operation cost. L e n t i c u l a r t i g h t gas sands ( c f . s e c t i o n 3 . 4 . 1 . ) r e q u i r e more c o m p l i c a t e d sol u t i o n s , because t h e r e l a t i o n s h i p between sand body l o n g a x i s o r i e n t a t i o n and f r a c t u r e a z i m u t h as a consequence o f h o r i z o n t a l s t r e s s d i r e c t i o n has i m p o r t a n t i m p a c t on t h e p o s s i b i l i t y o f f r a c t u r e i n t e r f e r e n c e and t h u s s t i m u l a t i o n e f f e c t d e t e r i o r a t i o n ( c f . s e c t i o n 4.10.5.1.). Given f a v o u r a b l e c o n d i t i o n s , square d r a i nage p a t t e r n s w i t h r e l a t i v e l y s h o r t f r a c t u r e s a r e more e f f i c i e n t i n r e s e r v o i r s w i t h h i g h e r p e r m e a b i l i t y ( o v e r 0 . 1 md), whereas r e c t a n g u l a r d r a i n a g e c o n f i g u r a t i o n s w i t h l o n g f r a c t u r e s a r e t h e optimum p o s s i b i l i t y f o r p a y zones w i t h l o w e r p e r m e a b i l i t y ( b e l o w 0 . 1 md; KUUSKRAA, BRASHEAR, ELKINS & MORRA 1 9 7 9 ) .
4.8.11.1.1.3. Fracture flow capacity and length optimization Based on known r e s e r v o i r p e r m e a b i l i t y , i t i s i m p o r t a n t t o o p t i m i z e f r a c t u r e l e n g t h and c o n d u c t i v i t y by c o m p a r i n g t r e a t m e n t c o s t and e x p e c t e d p r o d u c t i o n r e venue (MONTGOMERY & STEANSON 1 9 8 5 ) . P r e s s u r e d r o p a l o n g a p r o p p e d f r a c t u r e ( o f f i n i t e c o n d u c t i v i t y ; i n f i n i t e - c o n d u c t i v i t y cases a r e h a r d l y , i f a t a l l , developed i n n a t u r e ; c f . R E I N I C K E , BRINKMANN, SCHWARZ & H U E N I 1985) t h a t has an i n s u f f i c i e n t f l o w c a p a c i t y w i l l s e r i o u s l y l i m i t w e l l p r o d u c t i v i t y , whereas a f r a c t u r e w i t h e x c e s s i v e f l o w c a p a c i t y i s n o t e f f e c t i v e and i s r a t h e r a w a s t e o f money, because t h e i n c r e m e n t a l i n v e s t m e n t i s n o t compensated and r e t u r n e d b y f u r -
633 t h e r a d d i t i o n a l p r o d u c t i o n ( t h i s a p p l i e s f o r excess o f f r a c t u r e l e n g t h , whereas a d d i t i o n a l c r a c k h e i g h t may have a p o s i t i v e impact on w e l l p r o d u c t i v i t y ; CIPOLLA & LEE 1987). When t h e e f f e c t i v e i n - s i t u r e s e r v o i r p e r m e a b i l i t y i s g r e a t e r than a b t . 0 . 1 md, t h e d e s i r e d f r a c t u r e l e n g t h i s g e n e r a l l y a b t . 1,000 f t ( a b t . 300 m) o r l e s s , whereas i n t i g h t pay h o r i z o n s w i t h l e s s than 0 . 1 md, p r o d u c t i o n can be a l most d i r e c t l y p r o p o r t i o n a l t o c r a c k l e n g t h b e f o r e boundary c o n d i t i o n s a r e r e a ched and w i t h adequate f r a c t u r e f l o w c a p a c i t y , t h e g e n e r a l r u l e i s t h e l o n g e r t h e crack, t h e h i g h e r t h e p r o d u c i n g r a t e . I n t i g h t r e s e r v o i r s , thus f r a c t u r e l e n g t h i s u s u a l l y a b t . 2,500 - 4,500 f t ( a b t . 750 - 1,500 m; BAKER 1981, VEATCH 1983), w i t h l o n g e r f r a c t u r e s p a r t i c u l a r l y b e i n g necessary i n l e n t i c u l a r i n s t e a d MEEHAN, o f c o n t i n u o u s b l a n k e t sandstone pay segments ( c f . s e c t i o n 3 . 4 . 1 . ) . HORNE & A Z I Z (1988) o u t l i n e e f f e c t s o f r e s e r v o i r h e t e r o g e n e i t y and f r a c t u r e a z i muth on o p t i m i z a t i o n o f f r a c t u r e l e n g t h and w e l l spacing.
4.8.11.1.2. Reservoir and f r a c t u r e parameters vs. economical aspects F r a c t u r i n g s t i m u l a t i o n t r e a t m e n t o p t i m i z a t i o n has t h r e e b a s i c s t e p s compris i n g e v a l u a t i o n o f i n c r e a s e d income which m i g h t be expected f r o m o i l - o r gasp r o d u c i n g performance r e s u l t i n g f r o m v a r i o u s f r a c t u r e l e n g t h s and c o n d u c t i v i t i e s , d e t e r m i n a t i o n o f c o s t s r e q u i r e d t o achieve t h e d i f f e r e n t l e n g t h s and cond u c t i v i t i e s , and assessment o f n e t revenue vs. f r a c t u r e l e n g t h t o p i c k up t h e t r e a t m e n t d e s i g n which y i e l d s t h e maximum payback (VEATCH & M O S C H O V I D I S 1986). A l t h o u g h f r a c t u r e l e n g t h r e q u i r e m e n t s depend g r e a t l y on r e s e r v o i r p e r m e a b i l i t y , fracture conductivity, f r a c t u r e p e n e t r a t i o n and f o r m a t i o n p e r m e a b i l i t y , o t h e r f a c t o r s such as f r a c t u r e h e i g h t and n e t pay t h i c k n e s s can become i m p o r t a n t cons i d e r a t i o n s i n f r a c t u r e economics by h a v i n g s i g n i f i c a n t i n c r e m e n t a l e f f e c t s . Some comments on f r a c t u r e p e n e t r a t i o n vs. n e t pay t h i c k n e s s and o t h e r aspects a r e o f f e r e d as f o l l o w s .
4.8.11.1.2.1.
Fracture p e n e t r a t i o n vs. net pay thickness
Optimum f r a c t u r e p e n e t r a t i o n g e t s l o n g e r as n e t pay t h i c k n e s s i n c r e a s e s f o r moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s , b u t remains r a t h e r c o n s t a n t f o r lowpermeabi 1 it y hydrocarbon-bearing i n t e r v a l s (VEATCH & MOSCHOVIDIS 1986). Optimum c r a c k l e n g t h can t h e r e f o r e v a r y w i d e l y f o r a g i v e n pay p e r m e a b i l i t y and f r a c t u r e c o n d u c t i v i t y depending on n e t pay magnitude. F r a c t u r e h e i g h t can have s i g n i f i c a n t impact on optimum economical c r a c k p e n e t r a t i o n which i n t u r n a f f e c t s w e l l spacing, because t h e d i s t a n c e between n e i g h b o u r i n g b o r e h o l e s can be i n c r e a sed i f f r a c t u r e l e n g t h and/or h e i g h t i n c r e a s e , t h e r e b y r e p l a c i n g r e l a t i v e l y h i g h d r i l l i n g expenses by l o w e r s t i m u l a t i o n c o s t s . O p t i m i z a t i o n o f f r a c t u r e l e n g t h and f r a c t u r e h e i g h t t h e r e f o r e has i m p o r t a n t consequences on p l a n n i n g o f w e l l spacing and t h u s f i e l d development expenses.
4.8.11.1.2.2.
Other aspects
Additional factors requiring consideration f o r fracturing stimulation treatment o p t i m i z a t i o n a r e d u r a t i o n o f p r o d u c t i o n f o r e c a s t f r o m which n e t p r e s e n t w o r t h ( c f . s e c t i o n s 1.4.12.2. and 4.8.11.4.) i s c a l c u l a t e d , n e t d i s c o u n t e d p r o d u c t i o n revenue, and amount o f investment r e q u i r e d t o achieve t h e d e s i g n o p t i o n (WAREMBOURG, KLINGENSMITH, HODGES & ERDLE 1985), as w e l l as hydrocarbon p r i c e ( c f . s e c t i o n 2 . 2 . 1 . ) , i n t e r e s t ( d i s c o u n t ) f a c t o r s , t e c h n o l o g y l e v e l and r i s k (ROSENBERG, O'SHEA, MERCER, MORRA & BRASHEAR 1983; BRASHEAR, ROSENBERG & MERCER 1984). BRITT (1985) documents economical b e n e f i t s o f s h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s i n m o d e r a t e - p e r m e a b i l i t y o i l r e s e r v o i r s ( c f . s e c t i o n 4.6.) and documents t h e impact o f d i f f e r e n t f r a c t u r e l e n g t h s and f l o w c a p a c i t i e s on increment a l present worth.
634 I n r e s e r v o i r s where f r a c t u r i n g i s an i n t e g r a l p a r t o f t h e d e v e l o p m e n t p r o gram, w e l l s p a c i n g and a p p r o p r i a t e f r a c t u r e l e n g t h r e q u i r e m e n t s a r e paramount (VEATCH 1986; c f . s e c t i o n 2 . 3 . 4 . ) . The i m p a c t o f l a t e t i m e w e l l i n t e r f e r e n c e f r o m o v e r l a p p i n g d r a i n a g e a r e a s o f i n e f f i c i e n t w e l l p a t t e r n s on economics i s c o n s i d e r a b l y i n f l u e n c e d b y f r a c t u r e a z i m u t h w h i c h has a p r o m i n e n t c o n t r o l on w e l l s p a c i n g i n o r d e r t o p r e v e n t unwanted c r a c k c o n n e c t i o n and c u t t i n g o f f d r a i nage p a t h s . BARBA ( 1 9 8 8 ) comments on r e s e r v o i r j u s t i f i c a t i o n f o r m a x i m i z i n g hydraulic fracture length.
4.8.11.1.3.
Pressure d e c l i n e and f l u i d flow p a t t e r n
L o w - p e r m e a b i l i t y r e s e r v o i r s i n t e r s e c t e d by l o n g h y d r a u l i c f r a c t u r e s a r e char a c t e r i z e d b y r a p i d d e c l i n e r a t e s d u r i n g t h e f i r s t months o f p r o d u c t i o n a f t e r t h e t r e a t m e n t (HOLDITCH & LEE 1 9 7 9 ) . U n s u c c e s s f u l s t i m u l a t i o n r e s u l t s as a consequence o f e x t r e m e l y q u i c k r a t e d i m i n u t i o n beyond n o r m a l r a n g e s a r e t h e r e s u l t o f an e x t r a o r d i n a r i l y l o w f o r m a t i o n p e r m e a b i l i t y ( l e s s t h a n 0 . 0 0 1 md), insuffic i e n t f r a c t u r e l e n g t h w h i c h i s u s u a l l y caused b y f r a c t u r i n g o u t o f zone o r b y p o o r p r o p p a n t t r a n s p o r t , i n s u f f i c i e n t f r a c t u r e c o n d u c t i v i t y due t o p r o p p a n t c r u s h i n g and embedment, o r i n s u f f i c i e n t g a s - i n - p l a c e . I n v e r y t i g h t r e s e r v o i r s , l o n g e r f r a c t u r e s c a n be c r e a t e d b y pumping l a r g e r t r e a t m e n t volumes, and conduct i v i t y improvement c a n be made b y e m p l o y i n g h i g h e r g e l c o n c e n t r a t i o n s , s m a l l e r p r o p p a n t g r a i n s i z e s , and d e n s i t y - c o n t r o l l e d f l u i d s ( c f . s e c t i o n 4 . 2 . 2 . 6 . ) . I n many c a s e s , however, t h e a v e r a g e t r e a t m e n t a c h i e v e s o n l y a b t . 70 % o f t h e designed f r a c t u r e length, i m p l y i n g t h a t t h e a c t u a l c r a c k i s w i d e r and s h o r t e r and f l u i d l o s s i s l a r g e r t h a n p r e d i c t e d b y c o n v e n t i o n a l d e s i g n t e c h n i q u e s , and t h e b a r r i e r s c o n t r o l l i n g f r a c t u r e h e i g h t m i g h t be u n d e r e s t i m a t e d . W h i l e i n r e s e r v o i r s w i t h p e r m e a b i l i t i e s o f 0 . 0 0 1 - 0 . 0 5 md and w i t h f r a c t u r e l e n g t h s o f 500 - 1,500 f t ( 1 5 0 - 500 m), e v e n c r u s h e d sand o r p r o p p a n t s u s u a l l y p r o v i d e s u f f i c i e n t c o n d u c t i v i t y t o a d e q u a t e l y d r a i n t h e p a y zone ( c f . s e c t i o n 4 . 1 2 . 4 . ) , c r u s h i n g a n d / o r embedment o f p r o p p a n t s c a n c r e a t e s e r i o u s p r o b l e m s i n o t h e r h y drocarbon-bearing i n t e r v a l s . Low f r a c t u r e c o n d u c t i v i t y g i v e s r i s e t o an e s s e n t i a l l y r a d i a l f l o w p a t t e r n i n t h e r e s e r v o i r ( c f . s e c t i o n 4 . 1 1 . 3 . ) , whereas h i g h f r a c t u r e c o n d u c t i v i t y t r i g g e r s an u n i q u e f l o w p a t t e r n , and an u n i q u e h i s t o r y m a t c h i s o n l y o b t a i n e d when the dimensionless f r a c t u r e c o n d u c t i v i t y i s 10 o r g r e a t e r ( c f . s e c t i o n 4 . 8 . 1 1 . 3 . ) . A s t h e m a j o r i t y o f p r o p p e d f r a c t u r e l e n g t h s a p p e a r s t o have r e a c h e d o n l y a b t . 70 % o f t h e d e s i g n e d l e n g t h s (HOLDITCH & LEE 1979, SCOTT 1 9 7 9 ) , i t may be a d v i s a b l e t o o v e r d e s i g n a s t i m u l a t i o n t r e a t m e n t i n t e r m s o f l e n g t h , espec i a l l y when s e l e c t i o n o f f r a c t u r e b a r r i e r s i s d i f f i c u l t . C o n c e r n i n g e x c e s s i v e c r a c k l e n g t h , l a r g e f r a c t u r e s u r f a c e a r e a s do n o t n e c e s s a r i l y p r o d u c e p r o p o r t i o n a l l y more, w i t h o f f t a k e f r o m any g i v e n f o r m a t i o n a l s o b e i n g c o n t r o l l e d b y nonhomogeneous p e r m e a b i l i t y d i s t r i b u t i o n , c a p i l l a r y p o r e s i z e , c r u s h e d p r o p p a n t beds and f l u i d i n t e r f a c i a l t e n s i o n (DERBY & SMITH 1 9 7 9 ) .
4.8.11.1.4.
Fracture extension vs. r e s e r v o i r shape
F r a c t u r e l e n g t h m o d e l l i n g has c o n s i d e r a b l e s i g n i f i c a n c e i n d r a i n a g e o f l e n t i c u l a r t i g h t gas sands where c r a c k l e n g t h a l s o depends on t h e a n g u l a r r e l a t i o n s h i p between h o r i z o n t a l s t r e s s d i r e c t i o n and sand l e n s a x i s o r i e n t a t i o n (KUUSKRAA, BRASHEAR, ELKINS & MORRA 1 9 7 9 ) . D i s t i n c t i o n has t o be made between f r a c t u r e p r o p a g a t i o n p a r a l l e l t o l o n g and s h o r t axes o f t h e sand l e n s as w e l l as i n t e r m e d i a t e o b l i q u e e x t e n s i o n . The optimum c a s e i s f r a c t u r e o r i e n t a t i o n p a r a l l e l t o t h e l o n g a x i s o f t h e l e n t i c u l a r sand body where maximum f r a c t u r e l e n g t h c a n be a c h i e v e d i n o r d e r t o a l l o w f o r e f f i c i e n t and f e a s i b l e gas r e c o v e r y . When r e gional tectonical forces are essentially perpendicular t o lens direction, fract u r e s i n t e r s e c t i n g i s o l a t e d l e n s e s have t o be k e p t s h o r t i n o r d e r t o a v o i d waste o f f r a c t u r e e x t e n s i o n i n surrounding b a r r i e r r o c k s . F r a c t u r e l e n g t h can
635 o n l y be maximized i f l e n s spacing i s so narrow t h a t s e v e r a l s e p a r a t e l e n s e s can be connected t o g e t h e r and t o t h e b o r e h o l e by a l o n g f r a c t u r e . I f t h e r e l a t i o n s h i p between c r a c k azimuth and sand l e n s l o n g a x i s i s random, optimum f r a c t u r e e x t e n s i o n i s a l s o l i m i t e d by t h e l e n s boundary u n l e s s o t h e r a d j o i n i n g l e n s e s can be reached by t h e same c r a c k . Thus f r a c t u r e l e n g t h i n l e n t i c u l a r t i g h t gas sandstones i s m a i n l y a f u n c t i o n of l e n s diameter, c r a c k o r i e n t a t i o n w i t h r e s p e c t t o l e n s a x i s , and frequency and spacing o f lenses i n comparison t o e c o n o m i c a l l y f e a s i b l e f r a c t u r e l e n g t h . I n c o n t r a s t t o l e n t i c u l a r t i g h t gas sands, crack azimuth has no s i g n i f i c a n c e i n b l a n k e t sandstones e x c e p t o f w e l l p a t t e r n p l a n n i n g f o r a v o i d i n g f r a c t u r e i n t e r f e r e n c e ( c f . s e c t i o n 4.10.5.1.), and f r a c t u r e l e n g t h can be designed e x c l u s i v e l y a c c o r d i n g t o p e r m e a b i l i t y d i s t r i b u t i o n and r e c o v e r a b l e gas r e s e r v e s based on n e t p r e s e n t v a l u e o p t i m i z a t i o n ( c f . s e c t i o n 1.4.1.2.2.and 4.8.11.4.). Connection o f i s o l a t e d sand l e n s e s does n o t o n l y i n c l u d e a c q u i s i t i o n o f s t r i c t l y l a t e r a l l y a d j o i n i n g lenses, b u t i n c o r p o r a t e s a l s o o b l i q u e l y d i s p l a c e d lower and h i g h e r l e n s e s i n u n d e r l y i n g and o v e r l y i n g r e s e r v o i r l e v e l s . T h e r e f o r e d r a i n a g e o p t i m i z a t i o n i n l e n t i c u l a r t i g h t gas sands a l s o i n c l u d e s f r a c t u r e h e i g h t m o d e l l i n g which a u t o m a t i c a l l y a l s o has i t s i n f l u e n c e on f r a c t u r e l e n g t h . The s i g n i f i c a n c e o f m a x i m i z a t i o n o f h y d r a u l i c crack e x t e n s i o n t o t h e r e s e r v o i r l i m i t s o f sand lenses i s a l s o emphasized by PETERSON & KOHOUT (1983).
4.8.11.2. Fracture conductivity vs. fracture length The r e l a t i o n s h i p o f f r a c t u r e c o n d u c t i v i t y and c r a c k l e n g t h can be summarized i n t h e r e q u i r e m e n t o f s h o r t e r h i g h e r - c o n d u c t i v i t y f r a c t u r e s f o r moderate- t o h i g h - p e r m e a b i l i t y r e s e r v o i r s and l o n g e r f r a c t u r e s f o r t i g h t pay i n t e r v a l s which do n o t n e c e s s a r i l y have t o be o f h i g h - c o n d u c t i v i t y n a t u r e , b u t can a l s o be o f a b s o l u t e l o w - c o n d u c t i v i t y type, as even i n the l a t t e r case s t i l l a s i g n i f i c a n t r e l a t i v e c o n d u c t i v i t y c o n t r a s t between t h e l o w - p e r m e a b i l i t y r o c k m a t r i x and t h e p r o p p a n t package i n t h e f r a c t u r e i s c r e a t e d ( c f . s e c t i o n s 4.5,l.l.Z.and 4.6.1.). The f o l l o w i n g o u t l i n e a l s o focusses on impact o f p r o p p a n t type, f r a c t u r e f l o w c a p a c i t y o p t i m i z a t i o n , f r a c t u r e p r o d u c t i v i t y and f l u i d f l o w p a t t e r n , a c c e l e r a t i o n vs. u l t i m a t e r e c o v e r y enhancement, and d i f f e r e n c e s between c r e a ted, propped and e f f e c t i v e f r a c t u r e l e n g t h .
4.8.11.2.1.
Impact of proppant type
Fracture length i s l i n k e d w i t h crack conductivity, w i t h longer f r a c t u r e s i n t i g h t e r formations j u s t i f y i n g the a p p l i c a t i o n o f a better-performing proppant i n o r d e r t o maximize d r a i n - o f f r a t e s o f hydrocarbons t o t h e w e l l b o r e . T h i s i s p a r t i c u l a r l y due t o t h e f a c t t h a t most o f t h e t i g h t f o r m a t i o n s which have t o be s t i m u l a t e d b y MHF t r e a t m e n t s i n v o l v i n g l o n g f r a c t u r e s a r e s i t u a t e d i n g r e a t e r depth where h i g h e r c l o s u r e s t r e s s e s i n d i s p e n s i b l y r e q u i r e t h e a p p l i c a t i o n o f i n t e r m e d i a t e - o r h i g h - s t r e n g t h s y n t h e t i c proppants, and even i n case o f doubt o f t h e i r n e c e s s i t y because o f t h e c l o s u r e s t r e s s s t i l l b e i n g i n t h e range o f t h e boundary s t a b i l i t y o f sand, i t i s f o r t h e reason o f s e c u r i n g l o n g - t e r m product i o n h i s t o r y up t o 20 - 30 y e a r s recommended t o go on t h e a b s o l u t e l y s a f e s i d e and choose a proppant t y p e t h a t would be a b l e t o r e s i s t t o any p o s s i b l e p r e s s u r e drawdown d u r i n g course o f p r o g r e s s i v e d e p l e t i o n o f t h e r e s e r v o i r ( c f . sect i o n s 1.3.2. and 2.4.1.). F r a c t u r e l e n g t h has t o be o p t i m i z e d by v a r i a t i o n o f c r a c k c o n d u c t i v i t y , b u t a l s o c a u t i o n has t o be e x e r c i s e d t o c o n t a i n t h e f r a c t u r e w i t h i n t h e planned v e r t i c a l i n t e r v a l f o r r e s e r v o i r e n g i n e e r i n g and e n v i r o n m e n t a l s a f e t y reasons ( c f . s e c t i o n 4.2.2.). As a r u l e o f thumb, h i g h - p e r m e a b i l i t y r e s e r v o i r s r e q u i r e h i g h f r a c t u r e c o n d u c t i v i t i e s b u t do n o t need deeply p e n e t r a t i n g cracks, whereas lowp e r m e a b i l i t y f o r m a t i o n s demand d e e p l y p e n e t r a t i n g f r a c t u r e s b u t can t o l e r a t e l o wer c r a c k c o n d u c t i v i t i e s (VEATCH 1983).
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4.8.11.2.2. Fracture flow capacity optimization D e e p l y p e n e t r a t i n g f r a c t u r e s a r e u s u a l l y recommended f o r l o w - p e r m e a b i l i t y r e s e r v o i r s , b u t t h e i n c r e m e n t a l improvement on w e l l p r o d u c t i o n d i m i n i s h e s w i t h c r a c k l e n g t h due t o t h e f a c t t h a t t h e d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t y o r f l o w c a p a c i t y decreases w i t h i n c r e a s i n g crack length, w i t h thus a balance b e t ween f r a c t u r e c h a r a c t e r i s t i c s and r e s e r v o i r p r o p e r t i e s h a v i n g t o be a c h i e v e d in o r d e r t o o p t i m i z e p a y d e l i v e r a b i l i t y (MENG & BROWN 1 9 8 7 ) . I n r e a l i t y , however, t h e s e l e c t i o n o f an optimum p r o p p e d f r a c t u r e l e n g t h based o n r e s e r v o i r c o n d i t i o n s i s i n f l u e n c e d t o g r e a t e x t e n t b y n o n - t e c h n i c a l a s p e c t s such as o p e r a t o r b u d g e t and common a r e a p r a c t i c e s (PHILLIPS & ANDERSON 1985; ANDERSON & PHILLIPS 1986, 1 9 8 7 ) , and based on t h e s e c o n s i d e r a t i o n s as w e l l as on p a y p r o p e r t i e s , pen e t r a t i o n d i s t a n c e and p r o p p a n t c o n c e n t r a t i o n p r o f i l e a r e o b t a i n e d u s i n g f r a c t u r e d e s i g n p r o g r a m s ( c f . s e c t i o n 1 . 4 . 1 2 . ) . T h e r e f o r e t h e optimum d i m e n s i o n l e s s c o n d u c t i v i t y o r f l o w c a p a c i t y o f a t l e a s t 10 i n t i g h t r e s e r v o i r s can n o t always be e c o n o m i c a l l y o r o p e r a t i o n a l l y o b t a i n e d . I n t e r m s o f f r a c t u r e c o n d u c t i v i t y v s . l e n g t h , a t t e n t i o n has a l s o t o b e p a i d t o c o n d u c t i v i t y v a r i a t i o n s a l o n g t h e e x t e n s i o n o f t h e f r a c t u r e w i n g s away f r o m t h e b o r e h o l e ( c f . s e c t i o n 4 . 3 . 6 . ) . F r a c t u r e c o n d u c t i v i t y i s i n p r a c t i c e a dec r e a s i n g f u n c t i o n o f d i s t a n c e f r o m t h e w e l l b o r e (BENNETT, ROSATO, REYNOLDS & RAGHAVAN 1 9 8 1 ) . I f f r a c t u r e c o n d u c t i v i t y d e c r e a s e s m o n o t o n i c a l l y w i t h d i s t a n c e from the borehole, then a t l a t e production time t h e v a r i a b l e f r a c t u r e conductiv i t y s o l u t i o n s behave l i k e a c o n s t a n t c o n d u c t i v i t y c r a c k w i t h c o n d u c t i v i t y equal t o t h e a r i t h m e t i c c o n d u c t i v i t y average. Decreasing f r a c t u r e c o n d u c t i v i t y a l o n g t h e c r a c k e x t e n s i o n c a n be due t o c h a n g i n g p r o p p a n t t y p e s a n d / o r g r a i n s i z e as a consequence o f t a i l - i n o p e r a t i o n s , o r due t o n a r r o w i n g f r a c t u r e w i d t h away f r o m t h e w e l l b o r e i n d i r e c t i o n o f t h e c r a c k t i p . I n c a s e o f p r e s e n c e o f a l o w e r c o n d u c t i v i t y zone i n t h e f r a c t u r e r e a c h , t h e c r a c k l i m i t s t h e c a p a c i t y o f h i g h e r c o n d u c t i v i t y i n t e r v a l s beyond i t t o s u p p l y f l u i d o r gas t o t h e w e l l . F r a c t u r e c o n d u c t i v i t y n e a r t h e b o r e h o l e c a n be d e t e r m i n e d o n l y i f d a t a a r e r e corded d u r i n g t h e b i l i n e a r f l o w p e r i o d .
4.8.11.2.3.Fracture productivity and fluid flow pattern A w i d e and v a r i e d a s s o r t m e n t o f methods o f b o t h g r a p h i c and c o m p u t e r i z e d n a t u r e a r e a v a i l a b l e t o e s t i m a t e e f f e c t s of f r a c t u r e l e n g t h and c o n d u c t i v i t y on r e s e r v o i r p r o d u c t i v i t y , and v a r i o u s c r a c k g e o m e t r y m o d e l s e x i s t w h i c h may p r e d i c t d i f f e r e n t f r a c t u r e l e n g t h s f o r a g i v e n amount o f i n j e c t e d f l u i d a t a g i v e n r a t e w i t h a l l t h e o t h e r p a r a m e t e r s b e i n g t h e same, w i t h t h e d i f f e r e n t r e s u l t s b e i n g a consequence o f t h e b a s i c p r e m i s e s u s e d t o d e v e l o p t h e m o d e l s (VEATCH 1 9 8 3 ) . I n h i g h - p e r m e a b i l i t y r e s e r v o i r s where s t e a d y - s t a t e o r s e m i - s t e a d y s t a t e f l o w i s dominant, l e s s than a t h r e e - f o l d p r o d u c t i o n increase i s a v a i l a b l e r e g a r d l e s s o f f r a c t u r e l e n g t h and c o n d u c t i v i t y , whereas i n l o w - p e r m e a b i l i t y p a y s e c t i o n s w i t h c h i e f l y u n s t e a d y ( t r a n s i e n t ) p e r f o r m a n c e b e h a v i o u r , t h e more deepl y p e n e t r a t i n g f r a c t u r e s enhance t h e f o l d s o f p r o d u c t i o n r a t e i n c r e a s e , b u t cond u c t i v i t y - f u n c t i o n v a l u e s o f 5 o r more a r e r e q u i r e d t o r e a l i z e s i g n i f i c a n t o f f t a k e enhancement, and i n o r d e r t o m a x i m i z e e x p l o i t a t i o n r a t e , a d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t y i n t h e r a n g e o f 100 - 500 s h o u l d be a c h i e v e d . CALLANAN, CIPOLLA & LEE ( 1 9 8 3 ) o u t l i n e t h a t d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t i e s g r e a t e r t h a n 10 i m p l y enough p e r m e a b i l i t y t o h a n d l e a l l t h e gas w h i c h t h e f o r m a t i o n c a n s u p p l y , whereas v a l u e s i n e x c e s s o f 100 a r e r e p r e s e n t i n g i n f i n i t e l y c o n d u c t i v e f r a c t u r e s . DAVIES & K U I P E R ( 1 9 8 8 ) a r g u e t h a t a d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t y of 15 i s an a p p r o p r i a t e d e s i g n v a l u e f o r p s e u d o - s t e a dy-state flow conditions, but t i g h t reservoirs w i t h high i n i t i a l transient prod u c t i o n r a t e s r e q u i r e h i g h e r v a l u e s because o f t h e p r o l o n g e d d u r a t i o n o f t h e t r a n s i e n t r a t e s ( c f . section 4.8.11.3.).
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4.8.11.2.4. Acceleration vs. ultimate recovery enhancement I n h i g h - p e r m e a b i l i t y r e s e r v o i r s w i t h s t e a d y - s t a t e o r semi-steady s t a t e f l o w , hydraulic fracture s t i m u l a t i o n increases e a r l y - l i f e production r a t e s which increases cash f l o w , b u t has l i t t l e o r no impact on u l t i m a t e r e c o v e r y and t h u s bas i c a l l y represents acceleration o f r e s e r v o i r depletion ( c f . section 4 . 6 . ) . I n l o w - p e r m e a b i l i t y f o r m a t i o n s w i t h u n s t e a d y - s t a t e ( t r a n s i e n t ) performance behav i o u r , however, d e e p l y p e n e t r a t i n g f r a c t u r e s can s i g n i f i c a n t l y improve u l t i m a t e r e c o v e r y , w i t h advanced t e c h n o l o g y such as l o n g and c o n d u c t i v e f r a c t u r e s b e i n g a b l e t o i n c r e a s e r e c o v e r a b l e r e s e r v e s f r o m t i g h t gas sands by 40 - 75 % (BAKER 1981, VEATCH 1981). LANCASTER, GUIDRY, GRAHAM, CURTIS, SHAW & BLAKE (1987) c a r r y o u t f r a c t u r e l e n g t h o p t i m i z a t i o n i n s h a l e gas r e s e r v o i r s and conclude t h a t w h i l e l o n g e r f r a c t u r e s l e a d t o i n c r e a s e d gas p r o d u c t i o n a t some p o i n t of time, t h e i n c r e a s e d e x p l o i t a t i o n may become i n s u f f i c i e n t t o o f f s e t t h e i n c r e a s e d c o s t s a s s o c i a t e d w i t h pumping of l a r g e r t r e a t m e n t s .
4.8.11.2.5. Created, propped and effective fracture length The c r e a t e d c r a c k l e n g t h always has t o be a b t . 10 % l a r g e r than t h e subs e q u e n t l y propped f r a c t u r e l e n g t h i n o r d e r t o m i n i m i z e t h e p o s s i b i l i t y o f a s c r e e n o u t a t t h e c r a c k t i p (POULSEN & SOLIMAN 1987). F o r any f r a c t u r e l e n g t h , c r a c k w i d t h has t o be s u f f i c i e n t l y l a r g e f o r p r e v e n t i o n o f b r i d g i n g o f proppants between t h e f r a c t u r e w a l l s . Crack w i d t h a t t h e d e s i r e d propped f r a c t u r e l e n g t h a t t h e t i m e o f j o b c o m p l e t i o n has t o exceed t w i c e t h e d i a m e t e r o f t h e p r o p p a n t p a r t i c l e s i n o r d e r t o a l l o w unhindered passage o f t h e proppants throughout the crack. F o r m a t i o n p e r m e a b i l i t y , f r a c t u r e c o n d u c t i v i t y and c r a c k p e n e t r a t i o n i n terms o f length o f the drainage path are a f f e c t i n g production r a t e f o l d s o f increase and r e c o v e r a b l e r e s e r v e i n c r e a s e s (VEATCH 1986). Cash f l o w f r o m r a t e a c c e l e r a t i o n has a s i g n i f i c a n t impact on economics even w i t h o u t any r e s e r v e i n c r e a s e . D i s t i n c t i o n has t o be made n o t o n l y between f l u i d - c r e a t e d and a c t u a l l y - p r o p p e d f r a c t u r e l e n g t h ( c f . s e c t i o n 6.2.1.5.2.), b u t a l s o between propped and e f f e c t i v e f r a c t u r e l e n g t h (ZAHNER & CRAFTON 1985). E f f e c t i v e f r a c t u r e l e n g t h i s t h e p o r t i o n o f propped f r a c t u r e l e n g t h which c o n t r i b u t e s t o p r o d u c t i o n and i s small e r than t o t a l propped f r a c t u r e l e n g t h due t o f o r m a t i o n damage a d j o i n i n g t o t h e c r a c k w a l l by i n c r e a s i n g r e l a t i v e w a t e r p e r m e a b i l i t y and w a t e r i m b i b i t i o n which a r e r i s i n g w i t h b o t h t i m e and s u r f a c e area o f exposure t o i n v a d i n g f r a c t u r i n g f l u i d s . R e d u c t i o n o f c r a c k area by i n c r e a s i n g r e l a t i v e w a t e r p e r m e a b i l i t y and water i m b i b i t i o n thus represents reduction o f f r a c t u r e length a t the bottom o f t h e l i n e i f c r a c k h e i g h t i s c o n s i d e r e d t o be c o n s t a n t .
4.8.11.3. Fracture f l o w capacity vs. proppant distribution I n terms o f d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t y o r f l o w c a p a c i t y , t h e o p t i mum l e n g t h - t o - w i d t h r a t i o i n h i g h - p e r m e a b i l i t y r e s e r v o i r s exceeding 1 md permeab i l i t y o r i n case o f c u m u l a t i v e p r o d u c t i o n d u r i n g l o n g - t e r m f i e l d o p e r a t i o n i s 1.26 (PRATS 1961). F o r t i g h t f o r m a t i o n s , however, t h e e a r l y - t i m e t r a n s i e n t p r o d u c t i o n r a t e s p r i o r t o r e a c h i n g pseudo s t e a d y - s t a t e c o n d i t i o n s a r e i m p o r t a n t when c o n s i d e r i n g t h e economics o r p a y - o u t o f a f r a c t u r i n g t r e a t m e n t (MORSE & GONTEN 1972), and t h e r e f o r e h i g h e r values o f f r a c t u r e f l o w c a p a c i t y a r e more f e a s i b l e . ELBEL (1985) documents t h a t i n t h e p e r m e a b i l i t y range o f 0 . 1 - 0.001 md, a d i m e n s i o n l e s s f r a c t u r e f l o w c a p a c i t y o f 5 s h o u l d be c o n s i d e r e d when t h e proppant volume has been predetermined, and HOLDITCH & LEE (1979) and PHILLIPS & ANDERSON (1985) p o i n t o u t t h a t an u n i q u e h i s t o r y match can o n l y be o b t a i n e d when t h e d i m e n s i o n l e s s f r a c t u r e f l o w c a p a c i t y i s 10 o r g r e a t e r . V a r y i n g p r o p p a n t d i s t r i b u t i o n f o r a g i v e n proppant volume and crack l e n g t h can a f f e c t p r o d u c t i o n r a t e s depending on f r a c t u r e f l o w c a p a c i t y ( c f . s e c t i o n
638
SAMANIEGO & DOMINGUEZ ( 1 9 7 8 ) and AGARWAL, CARTER & POLLOCK 4.3.6.). CINCO-LEY, ( 1 9 7 9 ) recommend as a r u l e - o f - t h u m b t o d e s i g n f r a c t u r e f l o w c a p a c i t y t o be a t l e a s t 10 w h i c h a c c o r d i n g t o t h e i n v e s t i g a t i o n s o f ELBEL ( 1 9 8 5 ) has o b v i o u s benef i t s , b u t on t h e o t h e r hand r e q u i r e s e i g h t t i m e s t h e amount o f p r o p p a n t t h a n f o r a v a l u e o f 1 . 2 6 . O p t i m i z a t i o n m o d e l l i n g can be p e r f o r m e d g i v i n g p r o d u c t i o n f o r e c a s t s f o r v a r i o u s e q u a l p r o p p a n t volume f r a c t u r e g e o m e t r i e s w i t h v a r i o u s formation permeabilities, constant f r a c t u r e length w i t h d i f f e r e n t proppant volumes, and e q u a l f r a c t u r e l e n g t h and p r o p p a n t volume w i t h v a r y i n g p r o p p a n t d i s t r i b u t i o n s . Square d r a i n a g e s i m u l a t i o n c a n be r e p l a c e d b y more a p p r o p r i a t e r e c t a n g u l a r d r a i n a g e m o d e l l i n g i n c a s e o f l o n g f r a c t u r e s i n t i g h t r e s e r v o i r s (HOLDITCH, JENNINGS, NEUSE & WYMAN 1978), and i f f r a c t u r e a z i m u t h (SMITH, HOLMAN, FAST & COVLIN 1978) i s known and t h e w e l l s c a n be spaced f o r r e c t a n g u l a r d r a i nage, t h e n i t may be p o s s i b l e t o economize t h e t r e a t m e n t b y d e s i g n i n g f o r l o w e r f r a c t u r e f l o w c a p a c i t i e s . Crack c o n d u c t i v i t y i n c r e a s e can be f u r t h e r a c h i e v e d b y improvement o f gas p e r m e a b i l i t y w i t h i n t h e f r a c t u r e p o s s i b l y by r e m o v a l o f s t i m u l a t i o n f l u i d o r b y f l u i d s e g r e g a t i o n , as w e l l as b y s u s t a i n e d p r o d u c t i o n (SCOTT 1 9 7 9 ) . Proppant t r a n s p o r t e f f e c t i v i t y i s d e c i s i v e f o r t h e success o f a h y d r a u l i c f r a c t u r i n g j o b , because f r a c t u r e c o n d u c t i v i t y i s c o n t r o l l e d b y p r o p p a n t d i s t r i b u t i o n (NOVOTNY 1 9 7 7 ) . As t h e s l u r r y t r a v e l s away f r o m t h e w e l l b o r e a l o n g t h e c r a c k , p r o p p a n t s and f l u i d a r e h e a t e d and t h e r e s e r v o i r r o c k i s c o o l e d ( c f . s e c t i o n 4.3.4.6.1.4.). Because c a r r i e r f l u i d i s c o n t i n u o u s l y l o s t t o t h e f o r m a t i o n b y l e a k o f f , p r o p p a n t c o n c e n t r a t i o n and s l u r r y v i s c o s i t y i n c r e a s e and f l u i d v e l o c i t y d i m i n i s h e s . F r a c t u r e w i d t h a l s o d e c r e a s e s away f r o m t h e w e l l b o r e w h i c h a l t e r s f l u i d v e l o c i t y . As i s c a n n o t b e assumed t h a t e v e n a c o m p l e t e l y s u s p e n d i n g f l u i d c a r r i e s t h e proppants t o t h e t i p s a t t h e end o f t h e f r a c t u r e , l e n g t h o f the p r o p p a n t - f l u i d s l u r r y i s u s u a l l y a b t . 2/3 o f t h e t o t a l f r a c t u r e l e n g t h .
4.8.11.4. Net present value optimization N e t p r e s e n t v a l u e ( b e i n g d e f i n e d a s h y d r o c a r b o n r e v e n u e by s t i m u l a t i o n m i n u s c f . s e c t i o n 1.4.12.2.) d r a s t i c a l l y increases with r i s i n g f r a c treatment cost; t u r e h a l f l e n g t h , i n c r e a s i n g p r o p p a n t c o n c e n t r a t i o n and i n c r e a s i n g p r o p p a n t g r a i n s i z e . A f t e r some d i s t a n c e o f s t e e p r i s e , n e t p r e s e n t v a l u e a c h i e v e s a p l a t e a u and does n o t i n c r e a s e f u r t h e r when f r a c t u r e l e n g t h i s c o n t i n u e d t o be p r o g r e s s i v e l y i n c r e a s e d . The u n f e a s i b i l i t y o f e x c e s s i v e c r a c k l e n g t h i s b e s t r e f l e c t e d b y t h e n e t p r e s e n t v a l u e even a g a i n d e c l i n i n g a f t e r t h e p l a t e a u section, because t h e c r e a t i o n o f s u c c e s s i v e l y l o n g e r f r a c t u r e s i s l i n k e d w i t h a g a i n i n c r e a s i n g t r e a t m e n t expenses t h a t a r e n o t compensated b y r i s i n g a d d i t i o n a l h y d r o c a r b o n r e c o v e r y and t h u s t h e i n c r e a s i n g o p e r a t i o n a l c o s t s g i v e r i s e t o d i m i n u t i o n o f t h e d i f f e r e n c e d e f i n i n g n e t p r e s e n t v a l u e . Many s t i m u l a t i o n j o b s , however, r e m a i n i n t h e p r e - p l a t e a u s t a g e o f t h e f r a c t u r e l e n g t h v s . n e t p r e s e n t v a l u e p l o t w h i c h t e s t i f i e s t o l a c k o f a c h i e v e m e n t o f optimum f r a c t u r e l e n g t h (ECGNGMIDES & NGLTE 1987). I t i s in p r a c t i c e c e r t a i n l y n o t e a s y t o r e a c h e x a c t l y t h e optimum p o i n t o f t h e n e t revenue curve a t which c o s t t o achieve l o n g e r f r a c t u r e s s t a r t s t o exceed t h e r e v e n u e g e n e r a t e d b y p r o d u c t i o n f r o m t h e a d d i t i o n a l f r a c t u r e l e n g t h (VEATCH 1983, 1 9 8 6 ) , b u t t h i s optimum s o l u t i o n c a n be c l o s e l y approached espec i a l l y i n l a r g e f i e l d - w i d e campaigns a f f e c t i n g up t o s e v e r a l dozens o f w e l l s i n one b i g p a t c h where s u f f i c i e n t c o m p a r i s o n o f t r e a t m e n t s u c c e s s and e c o n o m i c a l payback c a n be done between t h e i n d i v i d u a l w e l l s . ZAHNER & CRAFTON ( 1 9 8 5 ) a l s o emphasize t h a t t h e r e i s no c o r r e l a t i o n between p e r f o r m a n c e and p r o p p e d f r a c t u r e l e n g t h beyond a maximum and c o n s e q u e n t l y optimum v a l u e o f t h e l a t t e r .
I n terms o f o p e r a t i o n s , i t i s e a s i e r t o c r e a t e l o n g f r a c t u r e s i n t h i n r e s e r v o i r s t h a n i n t h i c k p a y f o r m a t i o n s (JOSH1 1 9 8 7 ) . W h i l e g e n e r a t i o n o f s h o r t f r a c t u r e s o f a b t . 100 - 200 f t ( 3 0 - 70 m) l e n g t h i s q u i t e e a s y and cheap, c r e a t i o n o f l o n g f r a c t u r e s o f a b t . 3,000 f t (1,000 m) l e n g t h i s b o t h d i f f i c u l t and expens i v e (FAST, HOLMAN & COVLIN 1 9 7 7 ) . A s p e c t s o f f r a c t u r e t r e a t m e n t o p t i m i z a t i o n
639 by n e t p r e s e n t v a l u e m o d e l l i n g a r e a l s o SCHULER ( 1 9 8 0 ) .
discussed
by
COCIANCIG,
GRIFFITHS
&
4.8.11.5. Field examples MONTGOMERY & STEANSON (1985) p r e s e n t an example showing t h a t i n t h e g i v e n d r a i n a g e r a d i u s and t h u s a l s o f r a c t u r e l e n g t h , an u n s t i m u l a t e d w e l l would p r o duce 8 % o f t h e i n i t i a l gas i n p l a c e i n 20 years, whereas i n case o f c r e a t i n g a c r a c k o f a b t . 1,300 f t ( a b t . 400 m) l e n g t h , t h e same w e l l would d e p l e t e 34 % o f t h e i n i t i a l gas i n p l a c e i n t h e same amount o f t i m e . The i n i t i a l r a t e s f o r t h e f r a c t u r e d w e l l would be h i g h e r t h a n those f o r t h e u n s t i m u l a t e d one and u l t i m a t e r e c o v e r y would a l s o be g r e a t e r . These two f a c t o r s a r e key p o i n t s i n determinat i o n o f t h e economics o f f r a c t u r e l e n g t h and w e l l spacing. Aspects o f f r a c t u r e l e n g t h a r e a l s o d i s c u s s e d by POOLLEN, TINSLEY & SAUNDERS (1958); McGUIRE & S I KORA (1960), PRATS & LEVINE (1963); BENNETT, ROSATO & REYNOLDS (1981); HOLDITCH, LEE & G I S T (1981); CLARK (1983) and CAMACHO, RAGHAVAN & REYNOLDS ( 1 9 8 4 ) . BARBA (1988) e v a l u a t e s r e s e r v o i r j u s t i f i c a t i o n f o r maximizing h y d r a u l i c f r a c t u r e l e n g t h , and BARBA (1986) comments on i m p r o v i n g r e t u r n on h y d r a u l i c t r e a t m e n t i n v e s t m e n t w i t h w i r e l i n e i n p u t s . The importance o f v e r t i c a l f r a c t u r e h e i q h t on s t e a d y - s t a t e p r o d u c t i o n i n c r e a s e i s commented by TINSLEY, WILLIAMS, TINNER & MALONE ( 1 9 6 9 ) . The MHF o p e r a t i o n s i n R o t l i e g e n d and C a r b o n i f e r o u s g a s - b e a r i n g f o r m a t i o n s i n Germany FRG which a r e p a r t i a l l y t h e l a r g e s t s t i m u l a t i o n j o b s c a r r i e d o u t i n Europe w i t h i n t h e most i m p o r t a n t gas r e s e r v o i r s i n t h i s area ( c f . s e c t i o n 2.4.1. and 3 . 3 . ) have been performed i n pay h o r i z o n s w i t h p e r m e a b i l i t i e s o f l e s s than 0 . 1 md and i n extreme cases as low as 0.5 m i c r o d a r c y (REINICKE, BRINKMANN, SCHWARZ & HUENI 1985). Proppant q u a n t i t i e s up t o 550 t (1,100,000 l b s ) have been i n j e c t e d i n t o f r a c t u r e s r e a c h i n g i n l e n g t h between 200 and 550 m (600 and 1,600 f t ) and i n h e i g h t between 10 and 115 m ( 3 0 and 345 ft; KLOSE & KRUMER 1983). FAST, HOLMAN & COVLIN (1977) o u t l i n e MHF t r e a t m e n t s i n t i g h t gas sands i n Colorado/USA where proppant volumes up t o 500 t have been pumped i n t o f r a c t u r e s up t o 3,000 f t (1,000 m) i n l e n g t h .
4.8.11.6. Fracture height I n c o n t r a s t t o f r a c t u r e l e n g t h which beyond an optimum w i l l be i n e f f e c t i v e w i t h r e s p e c t t o f u r t h e r i n c r e a s e o f t h e p r o d u c t i v i t y index and t h e r e f o r e i s econ o m i c a l l y n o t j u s t i f i e d ( c f . s e c t i o n 4.8.11.4.), excess crack h e i g h t may have a p o s i t i v e impact on t h e r e s u l t o f t h e s t i m u l a t i o n j o b (CIPOLLA & LEE 1987). W h i l e c o n v e n t i o n a l f r a c t u r e d e s i g n i s based on c o n s t a n t c r a c k h e i g h t b e i n g equal o r l e s s t h a n r e s e r v o i r t h i c k n e s s (PERKINS & KERN 1961, GEERTSMA & DEKLERK 1969; TINSLEY, WILLIAMS, TINNER & MALONE 1969; NORDGREN 1972), more r e c e n t f r a c t u r e modelling i n d i c a t e s t h a t hydraulic f r a c t u r e s are o f t e n n o t l i m i t e d t o res e r v o i r t h i c k n e s s (CLEARY 1980, PALMER & CARROLL 1982, SETTARI & CLEARY 1982; ABOU-SAYED, SINHA & CLIFTON 1984; MEYER 1985). F r a c t u r e h e i g h t s e l e c t i o n i s t h e most i m p o r t a n t v a r i a b l e i n h y d r a u l i c t r e a t ment design, because i t d i c t a t e s volumes and r a t e s r e q u i r e d f o r e x e c u t i o n o f t h e o p e r a t i o n (GARBIS, BROWN & MAURITZ 1985). F r a c t u r e h e i g h t can be d i v i d e d i n t o n e t h e i g h t r e p r e s e n t i n g t h e amount o f zone where f l u i d l e a k o f f occurs, and gross h e i g h t b e i n g t h e d i s t a n c e between zone b a r r i e r s . Three-dimensional r e s e r v o i r s i m u l a t i o n s t u d i e s i n d i c a t e t h a t propped f r a c t u r e h e i g h t i n excess o f t h e p r o d u c t i v e t h i c k n e s s o f t h e r e s e r v o i r can add t o w e l l p r o d u c t i v i t y , p a r t i c u l a r l y when f r a c t u r e c o n d u c t i v i t y i s r e l a t i v e l y low compared t o f o r m a t i o n p e r m e a b i l i t y (CIPOLLA & LEE 1987). The magnitude o f t h e p r o d u c t i v i t y increase i s a f u n c t i o n o f the dimensionless crack flow capacity and excess h e i g h t . F r a c t u r e h e i g h t e x t e n d i n g above and below t h e r e s e r v o i r i n t o n o n - p r o d u c t i v e s t r a t a can s i g n i f i c a n t l y a f f e c t t r a n s i e n t p r e s s u r e b e h a v i o u r o f
640 v e r t i c a l l y f r a c t u r e d w e l l s . I n t e r m s o f p r o p p a n t s e l e c t i o n and c r a c k d e s i g n , e x cess f r a c t u r e h e i g h t can r e s u l t i n requirement o f l e s s proppant q u a n t i t y f o r a given crack length ( c f . section 4.3.), necessity o f a less conductive (and ther e f o r e e x p e n s i v e ) p r o p p a n t t y p e ( c f . s e c t i o n 1 . 3 . 2 . ) , i n c r e a s e o f t h e optimum economic f r a c t u r e l e n g t h ( c f . s e c t i o n 4 . 8 . 1 1 . ) a n d / o r i n c r e a s e d e s t i m a t e d p o s t s t i m u l a t i o n p r o d u c t i o n . TINSLEY, WILLIAMS, TINNER & MALONE (1969) p r e s e n t c u r v e s s h o w i n g p r o d u c t i o n i n c r e a s e as a f u n c t i o n o f f r a c t u r e h e i g h t , f r a c t u r e length, r e s e r v o i r height, drainage radius, f r a c t u r e f l o w capacity, formation p e r m e a b i l i t y and w e l l b o r e r a d i u s .
4.9. Oil-reservoir fracturing In c o n t r a s t t o t h e USA where b o t h o i l - and g a s - r e s e r v o i r f r a c t u r i n g i s common sense and h y d r a u l i c p r o p p a n t s t i m u l a t i o n has a l r e a d y e v o l v e d t o t h e s t a g e o f b e i n g one o f t h e m o s t i m p o r t a n t a s p e c t s o f w e l l c o m p l e t i o n (WATERS 1980; ROB I N S O N , HOLDITCH & WHITEHEAD 1986; c f . s e c t i o n 2 . 3 . 4 . ) , i n E u r o p e and o t h e r p a r t s o f t h e w o r l d p r e f e r e n t i a l l y gas w e l l s a r e t r e a t e d . A f t e r d i s c u s s i o n o f some g e n e r a l p o i n t s , s e v e r a l a s p e c t s a r e o u t l i n e d w h i c h i l l u m i n a t e t h e b e n e f i t o f o i l - r e s e r v o i r f r a c t u r i n g w i t h high-performance s y n t h e t i c proppants i n view o f c o n d u c t i v i t y c o n t r a s t and e c o n o m i c a l f e a s i b i l i t y . Comments a r e a l s o o f f e r e d o n s t e a m - d r i v e e f f e c t s on f r a c t u r e p r o p a g a t i o n and c o l d w a t e r i n j e c t i o n e f f e c t s on h y d r a u l i c f r a c t u r i n g .
4 . 9 . 1 . General aspects The o u t l i n e o f some g e n e r a l i n t r o d u c t o r y a s p e c t s c o m p r i s e s s k e t c h i n g o f v s . o i l r e s e r v o i r s t i m u l a t i o n as w e l l as p r o p p a n t a p p l i c a t i o n p r i n c i p l e s .
gas
4 . 9 . 1 . 1 . Gas vs. oil reservoir stimulation I n comparison t o t h e s t i m u l a t i o n o f gas-bearing formations, o i l - r e s e r v o i r f r a c t u r i n g i s i n Europe and o t h e r p a r t s o f t h e w o r l d o n l y p e r f o r m e d i n a c o n s i d e r a b l y l o w e r s h a r e and i n s m a l l e r s c a l e and i s p r e d o m i n a n t l y r e s t r i c t e d t o spec i a l c a s e s where a p a r t f r o m a f e w e x c e p t i o n s , a l m o s t e x c l u s i v e l y n a t u r a l sand i s u s e d as p r o p p i n g a g e n t due t o s h a l l o w d e p t h r e a s o n a b l y b e l o w t h e c r i t i c a l p r e s s u r e s t a b i l i t y o f s a n d and as a consequence o f s e r i o u s c o s t c o n t a i n m e n t f o r economical reasons o f investment l i m i t a t i o n . Although t h e general f e a s i b i l i t y s i t u a t i o n i n t h e y e a r s a f t e r t h e 1986 o i l p r i c e c r a s h ( c f . s e c t i o n 2 . 2 . 1 . 1 . 2 . ) does n o t a l l o w i n t r o d u c t i o n o f new t e c h n i q u e s , c o n s i d e r a b l e improvement o f o i l p r o d u c t i o n f r o m m a r g i n a l r e s e r v o i r s c a n be a c h i e v e d i n t h e n e a r f u t u r e b y i n c r e a s i n g t h e s t i m u l a t i o n a c t i v i t y i n o i l - b e a r i n g p a y h o r i z o n s and p e r f o r m i n g t h e o p e r a t i o n s i n r i s i n g amounts w i t h s y n t h e t i c p r o p p a n t s i n s t e a d o f u s i n g sand, w i t h t h e e f f e c t o f o b t a i n i n g a more p r o n o u n c e d c o n d u c t i v i t y c o n t r a s t b e t ween f r a c t u r e and e s p e c i a l l y m o d e r a t e - t o h i g h - p e r m e a b i l i t y f o r m a t i o n s and t h u s a b e t t e r d r a i n a g e o f t h e o i l ( c f . s e c t i o n s 4 . 5 . 1 . 1 . 2 . and 4 . 6 . 1 . ) . W h i l e s e v e r a l a s p e c t s o f o i l - r e s e r v o i r f r a c t u r i n g have a l r e a d y been d i s c u s sed i n t h e d i v i s i o n s on c h a l k s t i m u l a t i o n ( s e c t i o n 4 . 5 . 4 . ) , high-permeability reservoir fracturing (section 4.6.) and l a t e - s t a g e r e f r a c t u r i n g o f o l d w e l l s ( s e c t i o n 4 . 8 . 9 . ) , some p o i n t s a r e c o m p i l e d h e r e t o o u t l i n e i n a s u m m a r i z i n g mann e r t h e advantages o f o i l - p a y f r a c t u r i n g w i t h placement o f s y n t h e t i c high-cond u c t i v i t y p r o p p a n t s i n t h e c r a c k as a p e r s p e c t i v e o f n e a r - f u t u r e enhanced o i l recovery.
4.9.1.2.Proppant application principles The e f f e c t i v e n e s s o f h y d r a u l i c f r a c t u r i n g f o r i m p r o v i n g w e l l p r o d u c t i v i t y i s a f u n c t i o n o f f r a c t u r e l e n g t h and f r a c t u r e c o n d u c t i v i t y (TINSLEY, WILLIAMS, TIN-
641
NER & MALONE 1969; c f . s e c t i o n s 1.4.10. and 4.8.11.) and i n o r d e r t o achieve t h e most c o s t - e f f e c t i v e s t i m u l a t i o n , p a r t i c u l a r l y o p t i m i z a t i o n o f f r a c t u r e f l o w c a p a c i t y i s e s s e n t i a l which i s p r i m a r i l y governed by t y p e and c o n c e n t r a t i o n o f I n moderatet h e proppants (NORMAN, CIPOLLA & WEBB 1983; c f . s e c t i o n 1.3.2.). t o h i g h - p e r m e a b i l i t y o i l r e s e r v o i r s , h i g h p r o p p a n t c o n c e n t r a t i o n s p l u g g i n g wide c r a c k s i n m u l t i l a y e r s a r e one way t o improve s t i m u l a t i o n r e s u l t s (COULTER & WELLS 1972; SHAH, SMITH & DONALDSON 1983; c f . a l s o s e c t i o n 4 . 3 . ) , b u t a more s i g n i f i c a n t a m e l i o r a t i o n can be o b t a i n e d by v a r y i n g p r o p p a n t t y p e and p a r t i a l l y a l s o g r a i n s i z e . W h i l e i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants a r e w i d e l y app l i e d i n deep gas r e s e r v o i r s where t h e i r usage i s i n e v i t a b l e due t o correspondi n g c l o s u r e s t r e s s e s , so f a r c o m p a r a t i v e l y l i t t l e f i e l d e x p e r i e n c e e x i s t s on t h e p o t e n t i a l o f these h i g h - c o n d u c t i v i t y proppants i n low- t o m o d e r a t e l y - s t r e s sed r e l a t i v e l y s h a l l o w o i l - b e a r i n g pay h o r i z o n s (KOHLHAAS 1982; NORMAN, CIPOLLA & WEBB 1983). The f o l l o w i n g o u t l i n e p a r t i c u l a r l y emphasizes c o n d u c t i v i t y cont r a s t and economical f e a s i b i l i t y . Aspects o f s h a l l o w t o moderate-depth o i l - r e s e r v o i r f r a c t u r i n g a r e a l s o d i s cussed by BRITT (1985), B R I T T & BENNETT (1985), BRITT & LARSEN (1986), L I & ZHU (1986) and MARPLE, RULEY, WORLEY & FREEMAN ( 1 9 8 7 ) . GRAHAM, MONAGHAN & OSOBA (1959) o u t l i n e t h e i n f l u e n c e o f p r o p p i n g sand w e t t a b i l i t y on p r o d u c t i v i t y of hyd r a u l i c a l l y f r a c t u r e d o i l w e l l s . R I C E & SHURR (1978); BLACK, RIPLEY, BEECROFT & PAMPLIN (1979) and NYDEGGER, R I C E & BROWN (1979) comment on s t i m u l a t i o n o f lowpermeabil i t y , low-pressure s h a l l o w gas r e s e r v o i r s . SINHA & ELBEL (1983) p r e s e n t p r o d u c t i v i t y i n c r e a s e o f many o l d w e l l s by p r o p e r l y designed s t i m u l a t i o n t r e a t ments.
4.9.2.
Conductivity contrast
I n t e r m e d i a t e - and h i g h - s t r e n g t h proppants have n o t o n l y t h e advantage o f r e s i s t i n g t o h i g h e r c l o s u r e s t r e s s e s than sand, b u t t h e y a l s o o f f e r up t o t e n t i mes t h e c o n d u c t i v i t y o f sand even a t t h e low c l o s u r e s t r e s s l e v e l s encountered i n most o i l w e l l s as a consequence o f t h e c o m b i n a t i o n o f b e t t e r roundness and s p h e r i c i t y , l e s s c r u s h i n g and narrower s i z i n g i n comparison t o sand (NORMAN, C I POLLA & WEBB 1983). The h i g h e r b u l k d e n s i t y o f t h e ceramic proppants t o g e t h e r w i t h t h e i r b e t t e r c o n d u c t i v i t y w i t h r e s p e c t t o sand e n a b l e t o reduce t h e p r o p ped f r a c t u r e w i d t h s and t h u s t o s t r e a m l i n e t h e s t i m u l a t i o n o p e r a t i o n . Comments a r e o f f e r e d as f o l l o w s on p r o p p a n t t y p e and g r a i n s i z e .
4.9.2.1.
Proppant type
I n many m o d e r a t e - p e r m e a b i l i t y o i l r e s e r v o i r s , p r o d u c t i v i t y i n c r e a s e s and optimum f r a c t u r e l e n g t h f r o m t h e i n d i v i d u a l w e l l s a r e l i m i t e d by f r a c t u r e conduct i v i t y . G e n e r a l l y , l o n g e r f r a c t u r e s r e q u i r e more c o n d u c t i v e , high-performance proppants i n o r d e r t o achieve i n c r e a s i n g p r o f i t a b i l i t y (NORMAN, CIPOLLA & WEBB 1983). I n c r e a s i n g f r a c t u r e l e n g t h can s i g n i f i c a n t l y a m e l i o r a t e hydrocarbon p r o d u c t i o n when p r o p p a n t e f f e c t i v e n e s s i s moderate t o h i g h ( c f . a l s o s e c t i o n As t h e u l t i m a t e c o n t r o l l i n g f a c t o r o f t h e success o f t h e s t i m u l a t i o n 4.8.11.). j o b i s t h e c o n d u c t i v i t y c o n t r a s t between f r a c t u r e and f o r m a t i o n ( c f . s e c t i o n s and 4.6.1.; which a c c o r d i n g t o WATERS 1980 has t o be a t l e a s t 100 t i 4.5.1.1.2. mes c o n d u c t i v i t y d i f f e r e n c e , and P A I , GARBIS & HALL 1983 recommend f l o w c a p a c i t y c o n t r a s t r a t i o s between f r a c t u r e and r e s e r v o i r m a t r i x o f 1,000 - 1,000,000, w i t h f r a c t u r e c o n d u c t i v i t y b e i n g a f u n c t i o n o f type, s i z e , s t r e n g t h , q u a l i t y , and d e n s i t y o f t h e p r o p p a n t as w e l l as f o r m a t i o n overburden p r e s s u r e ) , a h i g h e r - q u a l i t y i n t e r m e d i a t e - o r h i g h - s t r e n g t h proppant has t o be s e l e c t e d i n s t e a d o f sand p a r t i c u l a r l y i n case o f h i g h e r pay i n t e r v a l p e r m e a b i l i t y i n o r d e r t o accomplish t h e necessary c o n d u c t i v i t y c o n t r a s t f o r p r o d u c t i o n i n c r e a s e s maxim ization. I n terms o f comparative e f f e c t i v e n e s s o f n a t u r a l sand and s y n t h e t i c ceramic proppants, t h e performance d i f f e r e n c e s a r e i n c r e a s i n g and d i m i n i s h i n g a t h i g h e r
642 and l o w e r c l o s u r e s t r e s s e s , r e s p e c t i v e l y . The p r o p p a n t s w i t h t h e h i g h e s t conduct i v i t y a l s o show t h e l e a s t r e v e n u e l o s s due t o f r a c t u r e damage b y p r o p p a n t c r u s h i n g , embedment, f l o w b a c k o r p l u g g i n g (NORMAN, CIPOLLA & WEBB 1983; c f . also section 4.12.).
4.9.2.2. Proppant g r a i n s i z e C r e a t i o n o f s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t between t h e m o d e r a t e - t o h i g h p e r m e a b i l i t y s h a l l o w o i l sand and t h e f r a c t u r e i n f i l l i n g c a n a l s o be p e r f o r m e d b y s e l e c t i n g c o a r s e r p r o p p a n t g r a i n s i z e s such as 1 0 / 2 0 o r e v e n 8 / 1 2 (MARPLE, RULEY, WORLEY & FREEMAN 1987; c f . s e c t i o n 4 . 3 . 5 . 1 . ) . A d d i t i o n a l b e n e f i t i n cond u c t i v i t y c a n be a c h i e v e d b y i n c r e a s i n g f r a c t u r e w i d t h and p l a c e m e n t o f h i g h e r p r o p p a n t c o n c e n t r a t i o n s p e r u n i t a r e a o f t h e f r a c t u r e w a l l s b y m u l t i l a y e r depos i t i o n ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) . Case s t u d i e s show t h a t m o d i f i e d t r e a t m e n t s u s i n g o n e - t h i r d l e s s t o t a l volume and t w i c e as much p r o p p a n t s w i t h r e s p e c t t o t h e o r i g i n a l o p e r a t i o n d e s i g n g i v e r i s e t o s i g n i f i c a n t improvements i n w e l l p r o d u c t i v i t y . The p e r f o r m a n c e seen on n e w l y c o m p l e t e d b o r e h o l e s even s u g g e s t s t h a t r e f r a c t u r i n g o f o l d e r w e l l s w i t h t h e r e v i s e d j o b s c h e d u l e may have e c o n o m i c a l v i a b i l i ty ( c f . section 4.8.9.). NIEMEYER & R E I N A R T ( 1 9 8 6 ) r e p o r t e f f e c t i v e bypass o f n e a r - w e l l b o r e damage and r e s e r v o i r s t i m u l a t i o n b y s h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s i n m o d e r a t e - p e r m e a b i l i t y o i l zones. Complex f o r m a t i o n damage i n t h e s e p r o s p e c t i v e i n t e r v a l s r e s u l t e d f r o m i n t e r a c t i o n s between mud f i l t r a t e , cement f i l t r a t e and c o n n a t e wat e r l e a d i n g t o p r e c i p i t a t i o n o f c a l c i u m c a r b o n a t e and b a r i u m s u l p h a t e , w a t e r b l o c k f r o m c o m p l e t i o n f l u i d s ( c f . s e c t i o n s 3 . 1 1 . 2 . 4 . 2 . and 4 . 8 . 8 . 3 . 1 . 5 . ) , clay d i s p e r s i o n b y a d r i l l i n g mud a d d i t i v e and m o b i l e f o r m a t i o n f i n e s ( c f . s e c t i o n 3 . 5 . 2 . ) . I n o r d e r t o compensate f o r e x c e s s i v e l e a k o f f i n t o t h e moderate-permeab i l i t y m a t r i x , a l a r g e p a d e x c e e d i n g h a l f o f t h e t o t a l j o b volume had t o be pump e d ( c f . s e c t i o n 4.3.4.6.1.4.), and s i l i c a f l o u r and 100 mesh sand were a l s o i n c l u d e d t o r e s t r i c t f l u i d l o s s ( c f . s e c t i o n s 1 . 4 . 1 1 . 2 . 1 . and 4 . 8 . 8 . 3 . 2 . 3 . ) . W h i l e f r a c t u r i n g w i t h c o a r s e p r o p p a n t s o f 12/20 mesh s i z e has t u r n e d o u t t o be a p r o b l e m due t o b o t h p e r f o r a t i o n and f o r m a t i o n s c r e e n o u t f a i l u r e , satisfact o r y c o n d u c t i v i t y c o n t r a s t s have been a c h i e v e d w i t h 2 0 / 4 0 p r o p p a n t s i n v a r i o u s s t i m u l a t i o n f l u i d s . S m a l l - s c a l e t r e a t m e n t s w i t h r e d u c e d r a t e s and c u m u l a t i v e q u a n t i t i e s have been p e r f o r m e d w i t h s u c c e s s f u l f r a c t u r e h e i g h t l i m i t a t i o n in w e l l s where t h e r e s e r v o i r i s o n l y capped b y a t h i n s e a l o f i n s u f f i c i e n t t h i c k n e s s t o p r e v e n t b r e a k t h r o u g h t o a n o t h e r p a y s t o r e y w i t h d i f f e r e n t f l u i d and p r e s s u r e r e g i m e . Some a s p e c t s o f c o n d u c t i v i t y improvement b y c o a r s e p r o p p a n t g r a i n s i z e s i n o i l r e s e r v o i r s a r e a l s o d i s c u s s e d b y LAMBERT, DOLAN & GALLUS (1983).
4.9.3. Economical feas i b i 1i t y E x a m i n i n g t h e c o m p a r a t i v e e f f e c t i v e n e s s o f n a t u r a l sand and s y n t h e t i c p r o p p a n t s i n a v a r i e t y o f o i l r e s e r v o i r s in d i f f e r e n t d e p t h s and p r e s s u r e r e g i m e s , e c o n o m i c a l a n a l y s i s i n d i c a t e s f r e q u e n t l y t h a t t h e i n c r e m e n t a l p r e s e n t v a l u e obt a i n e d b y f r a c t u r i n g c a n be a m e l i o r a t e d b y u s i n g a h i g h - p e r f o r m a n c e p r o p p a n t e v e n t h o u g h p r o p p a n t c o s t i s h i g h e r , because c e r a m i c p r o p p a n t s o f f e r s i g n i f i c a n t l y more p e r m e a b i l i t y t h a n t h e same s i z e sand even a t l o w c l o s u r e s t r e s s e s as e n c o u n t e r e d in m o s t o i l w e l l s (NORMAN, CIPOLLA & WEBB 1 9 8 3 ) . Some a s p e c t s o f s y n t h e t i c p r o p p a n t s v s . n a t u r a l sand as w e l l as f r a c t u r e d e s i g n and p l u g g i n g a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.9.3.1. Synthetic proppants vs. n a t u r a l sand S i m u l a t i o n s i n d i c a t e t h a t i f a h i g h - p e r f o r m a n c e p r o p p a n t i s used i n s t e a d o f sand, g r e a t e r r e c o v e r y e f f i c i e n c i e s are obtained i n a shorter p e r i o d o f time
643 and up t o 25 % g r e a t e r i n t e r m e d i a t e - d e p t h oil come o f 5.6 : 1 and 23 pants f o r p l u g g i n g t h e
r e t u r n s can be achieved. KOHLHAAS (1982) r e c o r d s i n an f i e l d return-on-investment r a t i o s from the a d d i t i o n a l i n : 1 when u s i n g n a t u r a l sand and man-made b a u x i t e propfracture, respectively.
High-performance s y n t h e t i c proppants a r e e c o n o m i c a l l y d e s i r a b l e f o r format i o n p e r m e a b i l i t i e s o f l - 10 md o r more even a t c l o s u r e s t r e s s l e v e l s n o t exceeding 2,000 p s i , and below 1 md r e s e r v o i r p e r m e a b i l i t y , t h e use o f manufactur e d proppants can be e c o n o m i c a l l y j u s t i f i e d a t c l o s u r e s t r e s s e s o f a b t . 4,000 p s i o r more. As t h e a p p l i c a t i o n o f i n t e r m e d i a t e - o r h i g h - s t r e n g t h proppants enables t o p l u g l o n g e r , more f e a s i b l e f r a c t u r e s and because o f t h e much h i g h e r c o n d u c t i v i t y t h a n sand even a t low c l o s u r e s t r e s s e s ( c f . s e c t i o n 1 . 3 . 2 . ) , NORMAN, CIPOLLA & WEBB (1983) recommend t o c o n s i d e r man-made proppants f o r use i n oil r e s e r v o i r s even below t h e c r u s h i n g p o i n t o f sand. KOHLHAAS (1982) emphasizes t h e p r o f i t a b i l i t y o f such an investment n o t o n l y i n e a r l y - s t a g e f i e l d p r o d u c t i o n , b u t a l s o i n l a t e - s t a g e o f f t a k e w i t h advanced r e s e r v o i r d e p l e t i o n when s t i l l c o n s i d e r a b l e q u a n t i t i e s o f o i l can be d r a i n e d t h r o u g h a h i g h - c o n d u c t i v i t y f r a c t u r e t h a t would o t h e r w i s e never come o u t o f t h e pay h o r i z o n ( c f . s e c t i o n
4.8.9.).
4.9.3.2.
Fracture design and plugging
BRITT (1985) documents t h e economical b e n e f i t s o f s h o r t h i g h - c o n d u c t i v i t y f r a c t u r e s even i n m o d e r a t e - p e r m e a b i l i t y o i l r e s e r v o i r s undergoing b o t h p r i m a r y and secondary d e p l e t i o n . LI & ZHU (1986) p r e s e n t comparative performance e v a l u a t i o n o f sand and b a u x i t e proppants i n moderate- t o h i g h - d e p t h o i l r e s e r v o i r s . F i e l d e x p e r i e n c e has shown t h a t i n v a r i o u s cases, f r a c t u r e s propped w i t h lower c o n c e n t r a t i o n s o f b a u x i t e have b e t t e r c o n d u c t i v i t i e s t h a n those propped w i t h h i g h e r s a t u r a t i o n s o f sand. The use o f h i g h - s t r e n g t h proppants as a t a i l - i n a f t e r t h e i n j e c t i o n o f sand as main proppant l o a d g i v e s l i t t l e s u p e r i o r i t y i n obt a i n i n g b e t t e r f r a c t u r e c o n d u c t i v i t y , w i t h t h e optimum b e i n g p r o v i d e d by an u n i f o r m p r o p p a n t package. Moderate- t o h i g h - p e r m e a b i l i t y o i l r e s e r v o i r s can f r e q u e n t l y a l r e a d y be s u c c e s s f u l l y s t i m u l a t e d by m i c r o f r a c t u r i n g o r m i n i f r a c t u r i n g (CRAMER & SONGER 1988; c f . c h a p t e r 4 . 8 . ) .
4.9.4. Steam-drive e f f e c t s on f r a c t u r e propagation Steam-drive o p e r a t i o n i s a common means o f enhanced o i l r e c o v e r y i n heavyo i l r e s e r v o i r s and has a l r e a d y been abundantly a p p l i e d i n many p a r t s o f t h e w o r l d . S t e a m - f l o o d i n g r e s u l t s i n h e a t i n g o f t h e pay zone t h a t f o r m e r l y belonged t o t h e r e g i o n a l g e o t h e r m i c a l temperature regime up t o an a r t i f i c i a l l e v e l acc o r d i n g t o t h e temperature o f steam o r condensed h o t w a t e r e n t e r i n g t h e i n j e c t i o n w e l l and m i g r a t i n g towards t h e p r o d u c t i o n b o r e h o l e . I n many h e a v y - o i l sand r e s e r v o i r s , i n j e c t i o n o f s i g n i f i c a n t amounts o f steam o f t e n r e q u i r e s c r e a t i o n o f f r a c t u r e s , and m o n i t o r i n g of i n i t i a t i o n , p r o p a g a t i o n and c o l l a p s e o f f r a c t u r e s may p r o v i d e i m p o r t a n t i n f o r m a t i o n about t h e d i s t r i b u t i o n o f i n j e c t e d f l u i d s w i t h i n t h e r e s e r v o i r (SINGHAI & CARD 1988). A p a r t f r o m s h a l l o w - d e p t h h e a v y - o i l r e s e r v o i r s , s t e a m - f l o o d i n g can a l s o be s u c c e s s f u l l y a p p l i e d i n i n t e r m e d i a t e - and h i g h - d e p t h l i g h t - o i l pay zones where steam d i s t i l l a t i o n i s more i m p o r t a n t and g r a v i t y o v e r r i d e i s l e s s s i g n i f i c a n t (SARATHI, ROARK & STRYCKER 1988). Steam-drive i s c a r r i e d o u t i n v e r t i c a l , i n c l i ned and h o r i z o n t a l w e l l s (JAIN & KHOSLA 1985, PETZET 1987). T e r t i a r y thermal o i l r e c o v e r y f r e q u e n t l y r e q u i r e s f r a c t u r e - a s s i s t e d s t e a m - f l o o d t e c h n o l o g y (SON1 & HARMON 1986), because c o n v e n t i o n a l s t e a m - f l o o d i n g i n t h i n r e s e r v o i r s g e n e r a l l y s u f f e r s f r o m e x c e s s i v e h e a t l o s s e s t o t h e o v e r - and underburden, and h i g h r a t e steam i n j e c t i o n i s o f t e n c o n s t r a i n e d by l i m i t e d r e s e r v o i r i n j e c t i v i t y . F r a c t u r e - a s s i s t e d steam-drive a l l o w s h i g h r a t e s o f steam i n j e c t i o n i n t o s h a l l o w pay sands t h r o u g h h y d r a u l i c a l l y - c r e a t e d h o r i z o n t a l f r a c t u r e s which l i m i t s h e a t l o s s e s by s h o r t e n i n g s t e a m f l o o d p r o j e c t l i f e .
644 Heating o f the r e s e r v o i r formation associated w i t h steam-drive operations c a n p r o v o k e a f u n d a m e n t a l change o f t h e i n - s i t u s t r e s s s t a t e o f t h e g e o l o g i c a l column, l e a d i n g from a geostress regime f a v o u r i n g p r o p a g a t i o n o f v e r t i c a l f r a c t u r e s a t t h e c o l d g e o t h e r m i c a l t e m p e r a t u r e l e v e l b e f o r e steam i n j e c t i o n o n s e t t o a g e o s t r e s s framework s u p p o r t i n g c r e a t i o n o f h o r i z o n t a l c r a c k s a t t h e h o t t e m p e r a t u r e n i v e a u t h a t was a r t i f i c i a l l y c r e a t e d b y s t e a m - f l o o d i n g (HOLZHAUSEN, CARD, RAISBECK & DOBECKI 1 9 8 5 ) . F o l l o w i n g an o u t l i n e o f t h e c h a n g i n g s t r e s s r e gime b y t h e r m a l r e s e r v o i r e x p a n s i o n , t h e c o n c e p t o f c o m b i n a t i o n o f h o r i z o n t a l and v e r t i c a l f r a c t u r e s f o r s t e a m - d r i v e r e s e r v o i r s t i m u l a t i o n i s i l l u s t r a t e d . Comments a r e a l s o o f f e r e d o n changes o f f r a c t u r e a z i m u t h o r i e n t a t i o n .
4.9.4.1.Changing stress regime by thermal reservoir expansion B a s i c a l l y t w o t y p e s o f h o t w a t e r and steam i n j e c t i o n p r o c e s s e s i n c l u d i n g f l u i d d i s p l a c e m e n t a r e i n use i n enhanced o i l r e c o v e r y (BRITTON, MARTIN, L E I BRECHT & HARMON 1 9 8 3 ) . The f i r s t t y p e i n c l u d e s d r i v e o r m a t r i x f l o w p r o c e s s i n w h i c h h o t w a t e r o r steam o r some i n t e r m e d i a t e m i x t u r e i s c o n t i n u o u s l y i n j e c t e d i n t o a r e s e r v o i r a t r e l a t i v e l y l o w r a t e s and p r e s s u r e s t o h e a t and d i s p l a c e o i l i n a m o d i f i e d w a t e r f l o o d i n g manner. T h i s t e c h n i q u e w o r k s s a t i s f a c t o r i l y i f o i l a t n a t u r a l p a y c o n d i t i o n s i s s u f f i c i e n t l y m o b i l e t o be moved a t p r a c t i c a l r a t e s by h o t f l u i d i n j e c t i o n w i t h o u t pressure p a s t i n g t h e r e s e r v o i r o r u n c o n t r o l l e d f i n g e r i n g . The second t y p e c o m p r i s e s c o n d u c t i o n h e a t i n g o f t h e p a y b y h o t f l u i d p a s s i n g t h r o u g h a h i g h l y p e r m e a b l e zone such as a h o r i z o n t a l f r a c t u r e , a gas cap a t t h e t o p o f t h e p r o s p e c t i v e s e c t i o n , o r a r e l a t i v e l y t h i n i n t e r v a l w i t h i n o r a d j a c e n t t o t h e m a i n r e s e r v o i r complex such as a w a t e r h o r i z o n a t t h e b o t t o m o f t h e d e p o s i t . H y d r a u l i c f r a c t u r i n g f o r enhanced u p t a k e o f i n j e c t e d steam i s p a r t i c u l a r l y s i g n i f i c a n t f o r t h e l a t t e r t y p e o f enhanced o i l r e c o v e r y o p e r a tion. The r e s e r v o i r s e c t i o n a d j a c e n t t o t h e h i g h l y p e r m e a b l e zone i s h e a t e d b y v e r t i c a l c o n d u c t i o n o f h e a t f r o m steam o r h o t w a t e r i n t h e c h a n n e l and a l s o b y c o n v e c t i o n o f steam o r h o t w a t e r w h i c h may have l e a k e d f r o m t h e c h a n n e l . When steam i s i n j e c t e d i n t o a p a y c o n t a i n i n g v i s c o u s i m m o b i l e f l u i d s , steam t e n d s t o e i t h e r c h a n n e l and push i t s way t h r o u g h t h e i m m o b i l e f l u i d s t o w a r d s t h e l o w p r e s s u r e s i n k , o r steam f l o w s t o p s m o m e n t a r i l y and t h e p r e s s u r e b u i l d s u p u n t i l t h e f o r m a t i o n p a r t s s o t h a t f l u i d i n j e c t i o n c a n c o n t i n u e (GESHELIN, GRABOWSKI & PEASE 1981; S E T T A R I & RAISBECK 1 9 8 1 ) . As such f r a c t u r e s seldom d e v e l o p a t t h e b e s t l o c a t i o n and w i t h t h e m o s t s u i t a b l e o r i e n t a t i o n , i t i s h e l p f u l t o h y d r a u l i c a l l y s t i m u l a t e t h e w e l l s p r i o r t o steam i n j e c t i o n , w i t h m a i n l y h o r i z o n t a l f r a c t u r e s o r i g i n a t i n g i n t h e s h a l l o w r e s e r v o i r d e p t h o f m o s t h e a v y - o i l sands (REYNOLDS, SCOTT, POPHAM & COFFER 1961; MARTIN, ALEXANDER, DEW & TYNAN 1972; DOSCHER, LABELLE, SAWATSKY & ZWICKY 1964; CLOSMANN 1971, SETTARI & RAISBECK 1981, CHHINA 1986; c f . s e c t i o n 1 . 2 . 8 . ) . O t h e r a s p e c t s o f f r a c t u r e p r e h e a t i n g i n t h e r mal o i l r e c o v e r y a r e d i s c u s s e d b y BRITTON, MARTIN, McDANIEL & WAHL ( 1 9 8 1 ) and RESTINE (1983). Low steam i n j e c t i v i t y in many h e a v y - o i l r e s e r v o i r s n e c e s s i t a t e s f r a c t u r i n g o f t h e f o r m a t i o n f o r enhanced u p t a k e i n o r d e r t o a c h i e v e c o m m e r c i a l r a t e s o f steam i n j e c t i o n , w i t h e n e r g y d i s t r i b u t i o n and o i l d e p l e t i o n f o l l o w i n g t h e f r a c t u r e t r e n d s (VITTORATOS 1 9 8 6 ) . The change i n g e o s t r e s s s t a t e i s caused b y t h e r mal e x p a n s i o n o f t h e o i l p a y d u r i n g steam i n j e c t i o n . The f o l l o w i n g o u t l i n e c o n c e n t r a t e s on s t r e s s i n v e r s i o n due t o a n i s o t r o p i c r e s e r v o i r e x p a n s i o n as w e l l as o n c y c l i c f r a c t u r e c o l l a p s e and o p e n i n g . Comments a r e a l s o o f f e r e d on o t h e r t y pes o f c h a n g i n g s t r e s s d i r e c t i o n .
645
4.9.4.1.1. Stress inversion due
to anisotropic reservoir expansion W i t h p r o g r e s s i v e h e a t i n g , some v e r t i c a l expansion o f t h e f o r m a t i o n m a t e r i a l s h o u l d be p o s s i b l e i n s p i t e o f r e s t r i c t i o n by overburden and l i t h o s t a t i c p r e s sure, whereas h o r i z o n t a l enlargement may be suppressed by t h e c o n f i n i n g e f f e c t o f t h e s u r r o u n d i n g r o c k s which a r e compacted and l i t h i f i e d by overburden s t r e s s and d i a g e n e t i c a l cementation, r e s p e c t i v e l y (HOLZHAUSEN, CARD, R A I S B E C K & OOBECK1 1985). Suppression o f l a t e r a l expansion would g i v e r i s e t o an i n c r e a s e i n t h e magnitude o f h o r i z o n t a l i n - s i t u s t r e s s components, l e a v i n g t h e v e r t i c a l stress l a r g e l y unaffected. T h e r e f o r e h o r i z o n t a l f r a c t u r e s s t a r t t o o r i g i n a t e when h o r i z o n t a l s t r e s s e s exceed t h e v e r t i c a l i n - s i t u compression, w i t h more than two weeks o f steam i n j e c t i o n being required f o r t r i g g e r i n g t h i s stress d i s t r i b u t i o n m o d i f i c a t i o n . Inj e c t e d steam and h o t w a t e r may i n i t i a l l y e n t e r t h e f o r m a t i o n t h r o u g h v e r t i c a l f r a c t u r e s t h a t have been h y d r a u l i c a l l y c r e a t e d p r i o r t o t h e o n s e t o f steam f l o o d i n g , whereas w i t h i n c r e a s i n g r e s e r v o i r temperature, t h e v e r t i c a l f r a c t u r e s may c l o s e due t o thermal expansion o f t h e o i l sands u n l e s s propped w i t h s u i t a b l e m a t e r i a l s , and h o r i z o n t a l c r a c k s open i f enhanced o i l r e c o v e r y o p e r a t i o n i s performed above f o r m a t i o n f r a c t u r i n g p r e s s u r e w h i c h g u i d e t h e i n j e c t e d steam and h o t w a t e r away f r o m t h e b o r e h o l e i n t o t h e f o r m a t i o n . H o r i z o n t a l f r a c t u r e g e n e r a t i o n d u r i n g s t e a m - d r i v e o p e r a t i o n s o f t e n takes p l a c e i n c y c l e s c o m p r i s i n g p r o p a g a t i o n and c o l l a p s e (HOLZHAUSEN, CARD, RAISBECK & OOBECKI 1985). As f l u i d e n t r y i n t o t h e h e a v y - o i l r e s e r v o i r i s r e s t r i c t e d by t h e h i g h v i s c o s i t y o f t h e p o r e f l u i d , p r e s s u r e r a p i d l y b u i l d s up w i t h c o n t i n u ous steam i n j e c t i o n a f t e r t h e b e g i n n i n g o f t h e o p e r a t i o n . Once a c r i t i c a l p r e s s u r e i s reached, f o r m a t i o n breakdown r e s u l t s i n p r o p a g a t i o n o f a h o r i z o n t a l f r a c t u r e away f r o m t h e w e l l b o r e .
4 . 9 . 4 . 1 . 2 . Cyclic fracture collapse and opening The volume i n c r e a s e o f t h e system due t o t h e aforementioned processes causes f l u i d p r e s s u r e t o drop which i n t u r n g i v e s r i s e t o f l a s h i n g o f some w a t e r i n t h e w e l l b o r e i n t o steam (VITTORATOS 1986). Aspects o f w a t e r f l a s h i n g , thermal r e s e r v o i r expansion and p l a s t i c i t y changes, steam i n j e c t i v i t y improvement by r e s e r v o i r f r a c t u r i n g , i n t e r w e l l communication and f r a c t u r e i n t e r f e r e n c e , and g r a v i t y d r a i n a g e t h r o u g h t h e f r a c t u r e s a r e o u t l i n e d as f o l l o w s .
4 . 9 . 4 . 1 . 2 . 1 . Water flashing The p r e s s u r e drop i n t h e newly c r e a t e d f r a c t u r e f i n a l l y d e c l i n e s t o a v a l u e t h a t i s t o o low t o c o n t i n u e t o l i f t t h e overburden, w i t h t h e c r a c k then c o l l a p s i n g and c l o s i n g (HOLZHAUSEN, CARD, R A I S B E C K & DOBECKI 1985; VITTORATOS 1986). Some p r e s s u r e l o s s b e f o r e c o l l a p s e may be t h e consequence o f steam condensation a f t e r t h e f r a c t u r e propagated i n t o c o o l e r p o r t i o n s o f t h e o i l sand. R e h e a l i n g o f t h e c r a c k a f t e r c o l l a p s e provokes a g a i n r e s t r i c t e d f l u i d e n t r y , p r e s s u r e b u i l d u p and f r a c t u r e g e n e r a t i o n a t t h e b e g i n n i n g o f t h e n e x t c y c l e . T i l t m e t e r m o n i t o r i n g r e v e a l s t h a t i n t e r r u p t i o n s o f steam i n j e c t i v i t y correspond t o openi n g and c l o s i n g episodes o f f r a c t u r e s (SINGHAI & CARD 1988; c f . s e c t i o n 6.2.2.3.4. and 6.2.2.4.1.). C y c l i c f r a c t u r e opening and r e h e a l i n g depending on steam i n j e c t i o n p r e s s u r e i s a l s o r e p o r t e d by ZIEGLER ( 1 9 8 8 ) . Water f l a s h i n g t a k e s p l a c e i f t h e p r e s s u r e i n t h e f o r m a t i o n heated by steam i n j e c t i o n f a l l s below t h e steam s a t u r a t i o n p r e s s u r e (VITTORATOS 1986). T h i s occ u r s i n t h e v i c i n i t y o f t h e s t i m u l a t e d w e l l b o r e as t h e p r e s s u r e d e c l i n e s a t t h e end o f steam i n j e c t i o n , and a l s o f u r t h e r o u t i n t h e r e s e r v o i r a t l o w e r temperat u r e s i f t h e p r e s s u r e drops s u f f i c i e n t l y due t o h i g h f l u i d p r o d u c t i o n r a t e s .
646 F l a s h i n g o f t e n occurs i n c o n j u n c t i o n w i t h convective f l o w which b o t h g i v e r i s e t o r e g i o n s o f u n i f o r m r e s e r v o i r t e m p e r a t u r e . GUDMUNDSSON, MENZIES & HORNE ( 1 9 8 3 ) a n a l y z e s t e a m t u b e r e l a t i v e p e r m e a b i l i t y f u n c t i o n s f o r f l a s h i n g steam-wat e r flow i n fractures.
4.9.4.1.2.2.Thermal reservoir expansion and plasticity changes The change o f t h e g e o s t r e s s s t a t e by t h e r m a l e x p a n s i o n d u r i n g s t e a m - d r i v e o p e r a t i o n c a n a l s o be e x p l a i n e d b y c h a n g i n g p l a s t i c i t y o f t h e r e s e r v o i r r o c k and d e c r e a s i n g m a t r i x s t r e s s c o n t r a s t between v a r i o u s l i t h o t y p e s (CONSTANT & BOURGOYNE 1 9 8 6 ) . I n g e o t h e r m i c a l t e m p e r a t u r e s i t u a t i o n s , the cohesive r e s i s t a n c e o r i n t e r n a l f r i c t i o n o f a s a n d s t o n e i s g r e a t e r thari t h a t o f many m u d s t o nes, t h e r e b y p e r m i t t i n g a g r e a t e r d i f f e r e n c e between v e r t i c a l and h o r i z o n t a l mat r i x s t r e s s t o develop ( c f . s e c t i o n s 4.2.2.3.1. and 4 . 2 . 3 . 2 . 2 . ) . With r i s i n g t e m p e r a t u r e due t o a r t i f i c i a l h e a t i n g b y steam i n j e c t i o n , t h e f o r m a t i o n s become more p l a s t i c a l ( c f . s e c t i o n s 4 . 2 . 3 . 2 . 3 . and 4 . 9 . 4 . 2 . 1 . 1 . 1 and t h u s t h e r e a r e l e s s d i f f e r e n c e s between h o r i z o n t a l and v e r t i c a l m a t r i x s t r e s s . Comments on i n s i t u s t r e s s changes d u r i n g s t e a m - d r i v e s t i m u l a t i o n o f o i l r e s e r v o i r s a r e a l s o g i v e n b y CLOSMANN & PHOCAS ( 1 9 7 6 ) , DUSSEAULT & SIMMONS ( 1 9 8 2 ) and WONG ( 1 9 8 6 ) .
4.9.4.1.2.3. Steam injectivity improvement by reservoir fracturing C y c l i c steam s t i m u l a t i o n e f f e c t s i n m u l t i - w e l l systems ( i n s t e a d o f s i n g l e w e l l systems as u s u a l l y assumed and m o d e l l e d ) a r e d i s c u s s e d b y VITTORATOS, SCOTT & BEATTIE ( 1 9 8 8 ) i n v o l v i n g c o n s i d e r a b l e i n t e r w e l l c o m m u n i c a t i o n o r i n t e r f e r e n c e upon f o r m a t i o n f a i l i n g ( c f . s e c t i o n 4 . 1 0 . 5 . 1 . 4 . ) . Steam i n j e c t i v i t y i s g e n e r a l l y a c h i e v e d b y r e s e r v o i r f r a c t u r i n g (BUCKLES 1979, VITTORATOS 1 9 8 6 ) . T h e r e i s e s s e n t i a l l y no w a t e r o r steam i n j e c t i v i t y u n l e s s f o r m a t i o n f a i l u r e o c c u r s (VITTORATOS, SCOTT & BEATTIE 1 9 8 8 ) . B o t h v e r t i c a l and h o r i z o n t a l f r a c t u r i n g t a k e p l a c e a t c r a c k i n i t i a t i o n , w i t h h o r i z o n t a l f r a c t u r e s b e i n g more common n e a r a r e a s t h a t have undergone i n t e n s i v e steam i n j e c t i o n . E x t e n s i v e i n t e r w e l l i n t e r a c t i o n s a r e t h e r e s u l t o f p a y f a i l u r e t h a t i s r e q u i r e d in o r d e r t o a c h i e v e c o m m e r c i a l l e v e l s o f steam i n j e c t i v i t y . R e s e r v o i r f r a c t u r i n g c a n e x t e n d t o d i s t a n c e s e q u a l t o t h a t o f w e l l s p a c i n g o r more and t h u s o v e r l a p p i n g o f t h e f a i l e d p o r t i o n s o f t h e p a y zone may o c c u r . The d i s t u r b e d i n t e r v a l s a s s o c i a t e d w i t h t h e w e l l s c r e a t e c o n d u i t s o f enhanced p r e s s u r e and f l u i d p r o p a g a t i o n b e t ween t h e b o r e h o l e s , w i t h t h e c o n d u i t s embedded i n a m a t r i x w i t h s i g n i f i c a n t l y s l o w e r p r e s s u r e and f l u i d p r o p a g a t i o n .
4.9.4.1.2.4. Interwell communication and fracture interference I n f e r e n c e s o n t h e shape o f t h e i n t e r w e l l c o m m u n i c a t i o n r e g i o n c a n b e made f r o m t h e h e a t t o w a t e r b r e a k t h r o u g h r a t i o , w i t h c y l i n d r i c a l c h a n n e l s h a v i n g val u e s o f 2 - 3, whereas f r a c t u r e s have v a l u e s o f 6 and more, because t h e i r l a r g e r s u r f a c e f o r a g i v e n r e s e r v o i r volume p e r m i t s more h e a t t o be t r a n s f e r r e d i n t o t h e p a y zone (VITTORATOS, SCOTT & BEATTIE 1 9 8 8 ) . S i n c e f r a c t u r i n g i s n e c e s s a r y f o r steam i n j e c t i v i t y , i n t e r w e l l c o m m u n i c a t i o n and c r a c k i n t e r f e r e n c e c a n o n l y be a v o i d e d b y i n c r e a s i n g w e l l s p a c i n g , o r b y p l a c i n g t h e p r o d u c t i o n and i n j e c t i o n w e l l s i n f a v o u r a b l e geometrical p a t t e r n s according t o t h e geostress system ( c f . s e c t i o n 4 . 1 0 . 5 . 1 . ) . A n o t h e r method i s t o l a y o u t t h e s y s t e m i n such a way t h a t t h e f i e l d i s d i s s e c t e d i n t o c o m m u n i c a t i o n c o u p l e t s between i n j e c t i o n and p r o d u c t i o n w e l l s . The h i g h e s t r e c o v e r y , however, i s a c h i e v e d w i t h z e r o o v e r l a p o f f r a c t u r e s e x t e n d i n g f r o m n e i g h b o u r i n g b o r e h o l e s , b u t i n p r a c t i c e , t h e h i g h i n t e r w e l l commun i c a t i o n a s s o c i a t e d w i t h n o o v e r l a p l e a d s t o h i g h w e l l downtime because o f s a n d - r e l a t e d pump f a i l u r e s . T h e r e f o r e a 50 % o v e r l a p appears t o be t h e optimum f o r s p r e a d i n g t h e h e a t between t h e w e l l s , i n c r e a s i n g o f f - t r e n d movement o f
647 steam, r e t a r d i n g t h e tendency o f t h e r e s e r v o i r t o f o r m c o u p l e t s , and o p e r a t i o n a l l y d i s r u p t i v e borehole i n t e r a c t i o n s .
minimizing
4.9.4.1.2.5. Gravity drainage through fractures D u r i n g f r a c t u r e - a s s i s t e d s t e a m - f l o o d i n g , t h e o i l d r a i n s by g r a v i t y i n t o t h e open h o r i z o n t a l f r a c t u r e where i t i s r a p i d l y t r a n s p o r t e d as d i s p e r s i o n o r ent r a i n m e n t t o t h e p r o d u c t i o n w e l l v i a t h e c r a c k , t h u s bypassing t h e r e l a t i v e l y c o l d m a t r i x between i n j e c t o r and o f f t a k e p o i n t (SON1 & HARMON 1986). Growth o f t h e steam-heated zone above t h e h o r i z o n t a l f r a c t u r e i s s u b s t a n t i a l l y more t h a n t h a t below (CLOSMANN & SMITH 1983), r e f l e c t i n g a h o r i z o n t a l c r a c k which r i s e s s l o w l y by slumping o f t h e u n c o n s o l i d a t e d sand above t h e f r a c t u r e as a g r a v i t y dominated h e a t t r a n s f e r i n which t h e movement o f t h e upper boundary o f t h e v e r t i c a l l v crrowinq steam zone i s l i m i t e d bv c o u n t e r - c u r r e n t f l u i d r e d i s r i bu t i on. As a consequence o f t h i s upwards growth, o r i g i n a l i n s t a l l a t i o n o f t h e h o r i z o n t a l c r a c k by h y d r a u l i c f r a c t u r i n g should be performed low i n t h e pay.
4.9.4.1.2.6. Other aspects Comparable thermal expansion o f t h e r e s e r v o i r r o c k as i n steam-dr ve operat i o n s i s known f r o m i n - s i t u combustion enhanced o i l r e c o v e r y where p a r t i a l ' c l o s i n g o f t h e n a t u r a l f r a c t u r e s i n t e r s e c t i n g t h e pay zone m a t r i x may o c c u r due t o f o r m a t i o n h e a t i n g and may a m e l i o r a t e r e s e r v o i r d r a i n a g e and sweep e f f i c i e n c y (SCHULTE & V R I E S 1985). C y c l i c steaming o f t a r sands t h r o u g h h y d r a u l i c a l l y - i n d u ced f r a c t u r e s i s d i s c u s s e d by D I E T R I C H (1983), and WATKINS, KALFAYAN & BLASER (1987) o u t l i n e c y c l i c steam s t i m u l a t i o n o f t i g h t c l a y - r i c h r e s e r v o i r s . MILHEM & AHMED (1987) and SYLVESTER & CHEN (1988) r e p o r t f i e l d e x p e r i e n c e o f c y c l i c steam s t i m u l a t i o n p r o j e c t s . Improvement o f steam s t i m u l a t i o n performance by chem i c a l emulsion v i s c o s i t y r e d u c t i o n i s assessed by CURRENT ( 1 9 8 8 ) . Aspects o f c y c l i c steam s t i m u l a t i o n above f r a c t u r e p r e s s u r e a r e a l s o d i s c u s s e d by I T 0 (1984), KENNEDY (1984) ; PETHRICK, SENNHAUSER & HARDING (1986), LlN (1988) and TORTIKE & ALI ( 1 9 8 9 ) . As a consequence o f t h e s h a l l o w r e s e r v o i r depth o f most h e a v y - o i l sand depos i t s , p r e d o m i n a n t l y h o r i z o n t a l f r a c t u r e s o r i g i n a t e (REYNOLDS, SCOTT, POPHAM & COFFER 1961; DOSCHER, LABELLE, SAWATSKY & Z W I C K Y 1064; WAHL 1965, CLOSMANN 1971; MARTIN, ALEXANDER, DEW & TYNAN 1972; SETTARI & RAISBECK 1981, CHHINA 1986; c f . s e c t i o n 1.2.8.). W e l l s a r e notched and h o r i z o n t a l l y f r a c t u r e d and then s t i m u l a t e d w i t h h i g h - p r e s s u r e steam i n o r d e r t o p r o v i d e a heated t a r g e t (BRITTON, MARTINS, LEIBRECHT & HARMON 1982). H o r i z o n t a l f r a c t u r e s a r e then c r e a t e d f r o m t h e i n j e c t i o n w e l l t o t h e p r o d u c t i o n boreholes, and steam i s i n j e c t e d a t v e r y h i g h r a t e s and p r e s s u r e s t o h o l d t h e f r a c t u r e open, p r e h e a t t h e format i o n and m o b i l i z e t h e heavy o i l .
4.9.4.1.3. Other possibilities o f changing stress direction A p a r t f r o m s t e a m - d r i v e r e s e r v o i r h e a t i n g , expansion and f r a c t u r i n g , o t h e r p o s s i b i l i t i e s e x i s t t o change t h e s t r e s s d i r e c t i o n i n t h e r o c k complex and t o e x p l o i t t h i s advantage o f m a n i p u l a t i o n o f main f o r m a t i o n o r i e n t a t i o n f o r d i s t i n c t i v e h y d r a u l i c f r a c t u r i n g . Some techniques a r e b r i e f l y i l l u s t r a t e d as f o l lows by commenting on a l t e r e d - s t r e s s f r a c t u r i n g as w e l l as s t r u c t u r a l c o n t r o l vs. s t r e s s c o n t r o l .
4.9.4.1.3.1. Altered-stress fracturing W A R P I N S K I & BRANAGAN (1988) p r e s e n t a l t e r e d s t r e s s f r a c t u r i n g which i s a l s o a t l e a s t p a r t i a l l y t a k i n g advantage o f a r t i f i c i a l changes o f t h e g e o s t r e s s p a t t e r n . The purpose o f a l t e r e d - s t r e s s f r a c t u r i n g i s t o c r e a t e h y d r a u l i c f r a c t u r e s
648 propagating perpendicularly o r obliquely t o the natural crack system and across the p r e - e x i st i n g j o i n t s in t i g h t gas sandstones instead of p a r a l l e l t o them ( c f . s e c t i o n 4 . 8 . 8 . 1 . 3 . ) , thereby opening u p a much l a r g e r surfa c e of the r e s e r v o i r a n d connecting numerous natural f r a c t u r e s t o the hydraulically-induced drainage p a t h . Al t er ed - s t r es s f r a c t u r i n g i s a concept whereby in most cases a hydraulic f r a c t u r e in one well i s r eo r i ente d by another hydraulic crack i n a nearby l o c a t i o n . While t r a d i t i o n a l l y most e f f o r t s t o control f r a c t u r e height growth a n d azimuth propagation have been modifications of stim ula tion design a n d d r i l l i n g p a t t e r n with emphasis on well spacing a n d c onfigura tion, a l t e r i n g the s t r e s s f i e l d t o a more favourable o r i e n t a t i o n o r magnitude i s a s u i t a b l e method p a r t i c u l a r l y in o l d e r f i e l d s where borehole d i s t r i b u t i o n i s given and can be n e i t h e r changed nor supplemented by renewed d r i l l i n g . As s i g n i f i c a n t s t r e s s p e r t u r b a t i o n s a r e induced by any process t h a t changes r e s e r v o i r pressure o r f r a c t u r e s the rock, a hydraulic crack i s an e f f e c t i v e mechanism f o r a l t e r i n g the s t r e s s f i e l d because of the high pressures needed f o r f r a c t u r e d i l a t i o n and the l a r g e su r f a c e area cr eat ed .
I n order t o overcome coalignment of n a t u r a l a n d hydraulic f r a c t u r e s , e a r l i e r approaches included p r o p el l an t f r a c t u r i n g and d i r e c t i o n a l d r i l l i n g . Prope lla nt f r a c t u r i n g or t a i l o r e d pulse loading (CUDERMAN & NORTHROP 1984; c f . se c tions 4 . 2 . 2 . 1 . 3 . 3 . a n d 4 . 8 . 8 . 5 . ) induces multiple r a d i a l l y orie nte d c ra c ks, b u t f r a c tu r e length t h a t can be achieved i s very lim ite d. Directional high-angle o r even h o r i z o n t a l d r i l l i n g i n l e n t i c u l a r sands i n t e r s e c t s more natural j o i n t s i f the hole i s o r i en t ed perpendicular t o natural f r a c t u r e azimuth ( c f . s e c t i o n s 4 . 8 . 6 . 1 . 5 . 1 . and 4 . 8 . 6 . 2 . 4 . 2 . ) , b u t i f d r i l l i n g damages the natural cracks o r i f too few natural f r a c t u r e s a r e encountered i n a r e s t r i c t e d sand body, conventio n a l s t i m u l a t i on s i n a d i r e c t i o n a l l y d r i l l e d well a r e not more e f f e L t i v e than in v e r t i c a l boreholes. Al t er ed - s t r es s f r a c t u r i n g i s discussed a s follows f o r two-well a n d one-well ap p l i cat i o n s . Some comments a r e a l s o offe re d on s t r e s s c o n t r a s t a n d leakoff c o n t r o l .
4 . 9 . 4 . 1 . 3 . 1 . 1 . Two-well a p p l i c a t i o n A l t e r e d - st r e ss f r a c t u r i n g i s a n attempt t o r e o r i e n t the s t r e s s f i e l d in a region by conducting a hydraulic f r a c t u r e nearby (SHUCK 1977). While the o r i g i n a l concept of s t r e s s a l t e r a t i o n using mu l t i p le w e lls i s somewhat lim ite d because of borehole spacing c r i t e r i a , r ecen t innovation allowed the technique t o be a l s o a p p l i c a b l e in a s i n g l e deviated hole ( U H R I 1987), and some s t r e s s modificatio n s can a l s o be c a r r i e d o u t i n v e r t i c a l wells (WARPINSKI & BRANAGAN 1988). An o f f s e t well i s used t o c r e a t e a hydraulic f r a c t u r e t h a t a l t e r s the s t r e s s f i e l d around the borehole under d i s cu s s i o n , w i t h the o f f s e t well being fra c ture d f i r s t and then being continued t o be t r e a t e d with h i g h pressure while the production borehole i s f r a c t u r e d . I f the s t r e s s d i f f er en ce i s not too l ar g e, the w e lls a r e r e l a t i v e l y c l o s e , a n d treatment pressure and f r a c t u r e s i z e in the o f f s e t well a r e s u f f i c i e n t l y l a r g e , enough s t r e s s can be added t o the v i r g i n minimum horizontal i n - s i t u s t r e s s t o convert i t t o the maximum horizontal s t r e s s . Pumping has t o be maintained in the o f f s e t well f r a c t u r e , because s t r e s s pe rturba tion w i l l decay a s pressure d e c l i n es . I f a treatment in the production borehole i s s t a r t e d under these c o n d i t i o n s, the hydraulic f r a c t u r e i n the production well w ill propagate perpendicular t o the u s u a l d i r e c t i o n of propagation and extends a c e r t a i n d i s tance before i t i s turning i n t o the o r i g i n al o r i e n t a t i o n because of weakening conversion pressure f a r t h e r away from the pressurized f r a c t u r e in the o f f s e t w e l l . Thus a l t e r e d - s t r e s s f r a c t u r i n g enables to c r e a t e an inte rloc king network of orthogonally-oriented induced crack systems f o r optimum i n t e r s e c t i o n and drainage of the r e s e r v o i r . As s t r e s s p e r t u r b at i o n decays r ap i d l y w i t h dista nc e in both horizontal and v e r t i c a l d i r e c t i o n representing f r a c t u r e length and he ight, the two w e lls should be as c l o s e together as p o s s i b l e, and a l s o the diffe re nc e between the
649 two h o r i z o n t a l s t r e s s e s must n o t be l a r g e , because s t r e s s p e r t u r b a t i o n i s p r o p o r t i o n a l t o p r e s s u r e i n t h e o f f s e t w e l l f r a c t u r e which has o p e r a t i o n a l l i m i t s . High t r e a t m e n t p r e s s u r e s i n t h e o f f s e t w e l l can be o b t a i n e d by r e s e r v o i r f e a t u r e s such as good s t r e s s c o n t r a s t s o r by induced f e a t u r e s such as screenouts. S t r e s s c o n t r a s t s a r e a l s o i m p o r t a n t because f r a c t u r e w i d t h i n h i g h - s t r e s s r e g i o n s w i l l be s m a l l so t h a t l e s s s t r a i n w i l l be induced and t h e e f f e c t o f e x t r a h e i g h t growth w i l l be l e s s t h a n i n a homogeneous f o r m a t i o n . High s t r e s s e s i n s u r r o u n d i n g mudstones reduce t h e magnitude o f t h e s t r e s s change, w i t h t h e e f f e c t i n c r e a s i n g w i t h c r a c k s i z e , w h i l e l o w - s t r e s s l a y e r s o f course enhance t h e induced s t r e s s change. Lower and h i g h e r a n i s o t r o p y r a t i o s reduce and increase, r e s p e c t i v e l y , t h e advantage o f t h e a1 t e r e d - s t r e s s f r a c t u r e .
4.9.4.1.3.1.2. One-we1 1 application As a p p l i c a b i l i t y o f a l t e r e d - s t r e s s f r a c t u r i n g i n s e p a r a t e w e l l s i s l i m i t e d , emphasis has t o be p u t on c a r r y i n g o u t t h e o p e r a t i o n i n a s i n g l e d i r e c t i o n a l l y d r i l l e d b o r e h o l e ( U H R I 1987). The f i r s t t r e a t m e n t i s conducted down t h e annulus w h i l e t h e second j o b i s performed down t h e t u b i n g , r e s u l t i n g i n much c l o s e r d i s tances t h a n c o u l d be o b t a i n e d i n m u l t i p l e w e l l s and t h e r e f o r e p r o f i t i n g f r o m l e s s pressure decline. I n addition, i f the f i r s t operation t o a l t e r the stress i s i n i t i a t e d i n a h i g h - s t r e s s muddy b a r r i e r ( c f . s e c t i o n s 4.2.2.3.1. and 4.2.3.2.2.), h i g h t r e a t i n g p r e s s u r e s and l a r g e h e i g h t a r e f a c i l i t a t e d so t h a t much g r e a t e r s t r e s s changes o c c u r . A l t e r e d - s t r e s s f r a c t u r i n g cannot be used t o r e v e r s e t h e h o r i z o n t a l s t r e s s i n a v e r t i c a l w e l l when overburden s t r e s s i s a l s o vertical, b u t i f d i p p i n g s t r a t a , b u r i e d topography ( c f . p l a t e I / 1 - 7 ) o r o t h e r e f f e c t s a r e m i s a l i g n e d w i t h t h e normal d i r e c t i o n , a l t e r e d - s t r e s s f r a c t u r i n g g i ves f e a s i b l e r e s u l t s (WARPINSKI & BRANAGAN 1988). I f c a r r i e d o u t i n one w e l l , however, i t may n o t be p o s s i b l e t o guarantee s u f f i c i e n t pressure i n the o f f s e t f r a c t u r e t o reverse the stress f i e l d , and a l s o t h e c o m p l e x i t y o f t h e o p e r a t i o n s can be a problem. Unusual f r a c t u r e designs f o r such cases a r e t i p s c r e e n o u t ( c f . s e c t i o n 4.5.4.4.1.) f o r the f i r s t f r a c t u r e which ensures h i g h p r e s s u r e s as w e l l as making maintenance o f those easy by a l l o w i n g f o r r e l a t i v e l y slow pumping of o n l y l i q u i d s a f t e r t h e t i p s c r e e n o u t has formed. I t may a l s o be p o s s i b l e t o induce a t i p screenout and pack enough p r o p p a n t s i n t o t h e f r a c t u r e so t h a t no a d d i t i o n a l pumping i s needed, w i t h t h e p r o p ped f r a c t u r e w i d t h and t h e s t r a i n i t produces b e i n g e q u i v a l e n t t o t h e p r e s s u r e r e q u i r e d t o c r e a t e t h a t w i d t h . As s t r e s s p e r t u r b a t i o n a t some d i s t a n c e f r o m a f r a c t u r e i s independent o f modulus, t h e compliance o f n a t u r a l f r a c t u r e s i s n o t a t t e n u a t ing t h e decay d i stance .
4.9.4.1.3.1.3. Stress contrast Another p o s s i b l e a p p l i c a t i o n o f t h e s t r e s s - m o d i f i c a t i o n concept i s f o r a l t e r a t i o n o f t h e v e r t i c a l d i s t r i b u t i o n o f t h e minimum h o r i z o n t a l i n - s i t u s t r e s s , because a h y d r a u l i c f r a c t u r e n o t o n l y c r e a t e s a l a r g e compressive s t r e s s i n t h e l a y e r i t r e s i d e s , b u t a l s o induces l a r g e t e n s i l e s t r e s s e s above and below which c o u l d be used t o advantage i f h y d r a u l i c c r a c k s a r e p r o p a g a t i n g i n t o u n d e s i r a b l e zones such as a q u i f e r s (WARPINSKI & BRANAGAN 1988). Changing o f i n s u f f i c i e n t l y Tow t o s a t i s f a c t o r i l y h i g h s t r e s s c o n t r a s t can be performed i n a v e r t i c a l w e l l , a d e v i a t e d w e l l o r two a d j a c e n t w e l l s . A l t e r e d - s t r e s s f r a c t u r i n g does n o t change f r a c t u r e o r i e n t a t i o n from v e r t i c a l t o h o r i z o n t a l as provoked by steam f l o o d i n g o f t h e r e s e r v o i r and r e s u l t i n g pay expansion due t o h e a t i n g , b u t r a t h e r g i v e s r i s e t o changes o f c r a c k azimuth i n a d i r e c t i o n r u n n i n g perpendicul a r l y t o t h a t determined by t h e u n d i s t r i b u t e d g e o s t r e s s f i e l d . The e f f e c t o f a l t e r e d - s t r e s s f r a c t u r i n g can be even accentuated by staggered steam i n j e c t i o n which combines g e o s t r e s s p a t t e r n d i s t u r b a n c e by p r e s s u r e impact f r o m a hydraul i c f r a c t u r e and f r o m r e s e r v o i r expansion as a consequence o f i n c r e a s i n g pay temperature i n s t e a m - d r i v e o p e r a t i o n s ( c f . s e c t i o n 4.9.4.3.2.).
650 I f a n a l t e r e d - s t r e s s concept i s used t o r e o r i e n t the f r a c t u r e , the i n i t i a l breakdown technique in the production well should be c a r e f u l l y considered. Normal breakdown o r b al l - o u t ( c f . s ect i o n 4 . 4 . 2 . 3 . ) i n i t i a t e s hydraulic f r a c t u r e s in the usual o r i e n t a t i o n , and afterwards attempts t o s t a r t a n a l t e r e d - s t r e s s crack may be d i f f i c u l t a n d s t r e s s p er t u r b at ions may be m itiga te d. I t i s the re f o r e b e t t e r t o break down under reversed s t r e s s conditions so t h a t the f r a c t u r e i s o r i e n t e d c o r r e c t l y from the s t a r t . An a l t e r n a t e breakdown technique i s using p r o p e l l a n t f r a c t u r i n g (CUDERMAN & NORTHROP 1984) through pe rfora tions with a t l e a s t 90 degrees phasing, w i t h cracks i n i t i a t i n g o u t of each s e t of pe rfora tio n s giving reasonable i n i t i a t i o n s i t e s f o r the follow-on treatment (WARPINSKI & BRANAGAN 1988).
4.9.4.3.1.4. Leakof f control Another important consideration f o r a l t e r e d - s t r e s s operations i s leakoff cont r o l . I f the hydraulic f r a c t u r e i s a l t e r e d t o c ross the most permeable natural cracks r a t h e r than running p a r a l l e l t o them, then f l u i d leakoff may be much g r e a t e r t h a n normally found in the r e s e r v o i r . Height containment c onditions may a l s o s u f f e r p a r t i c u l a r l y i f maximum a n d minimum s t r e s s have d i f f e r e n t v e r t i c a l d i s t r i b u t i o n , which occurs i f v i s c o e l a s t i c s h a le s a r e present (WARPINSKI & TEUFEL 1987) a n d i f t ect o n i c or o t h e r s t r a i n s a r e induced d i f f e r e n t l y i n the two principal d i r e c t i o n s . Accurate f r a c t u r e height e stim a tions can be made i f s t r e s s measurements ar e c a r r i e d out in an o f f s e t well which enables t o r e l a t i v e ly e a s i l y determine the beginning of s i g n i f i c a n t f r a c t u r e height growth. Another p o s s i b i l i t y t o c r e a t e v e r t i c a l f r a c t u r e s in shallow r e s e r v o i r s t o reys where u s u al l y horizontal cracks o r i g i n a t e i s limited-entry stim ula tion ( c f . s e c t i o n 4 . 2 . 2 . 2 . ) of mu l t i p l e l ay er s u n t i l proppant abrasion of perforat i o n tunnels r e s u l t s in t h e i r enlargement a n d consequently pressure d r o p f o l l o wed by screenout of various p er f o r at i o n s , a s well as leading t o i n i t i a t i o n o f a v e r t i c a l f r a c t u r e a f t e r termination of propagation of the e a r l i e r horizontal crack (SCHRAUFNAGEL & LAMBERT 1988). The change of o r i e n t a t i o n from an o r i g i n a l h o r i z o n t a l f r a c t u r e t o a subsequently v e r t i c a l crack due t o pressure r i s e a t screenout s t a g e i s p a r t i c u l a r l y pronounced when p r o p p a n t bridging and s t i c k i n g leads t o considerably increasing treatment pressure e a r l y d u r i n g proppant inje c tio n (HANSON 1988).
4.9.4.1.3.2. Structural control vs. stress control McLENNAN, R O E G I E R S & MARX (1983) even conclude t h a t the re i s l i t t l e argument t h a t hydraulic f r a c t u r i n g i s a s t r e s s - c o n t r o l l e d phenomenon wherein the crack tends t o open perpendicularly t o the s mal l es t princ ipa l s t r e s s . I n a ddition t o s t r e s s government, s t r u c t u r a l supervision in the te c tonic a l framework of the o i l a n d / o r gas d e p o s i t e x e r t s a modifying influence on f r a c t u r e o r i e n t a t i o n . Therefore s i g n i f i c a n t anisotropy due t o layering a n d ofte n consequently higher horizontal permeability can enhance the likelihood of horizontal f r a c t u r e gener a t i o n a t s l i g h t l y g r e a t e r depths (KOMAR & FROHNE 1973, SUN & MONGAN 1974). T h e s t r u c t u r a l control i s most important when v e r t i c a l s t r e s s and minimum horizont a l s t r e s s a r e approximately equal (McLENNAN, R O E G I E R S & MARX 1983). This c o n s t e l l a t i o n f r eq u en t l y r e s u l t s i n development of a zone w i t h horizont a l f r a c t u r e s followed by a t r a n s i t i o n a l region where horizontal and v e r t i c a l f r a c t u r e s may co ex i s t ( o r may be generated vice ve rsa ) t h a t i s g r a d i n g i n t o an in t e r v a l w i t h ex cl u s i v el y v e r t i c a l cracks in g r e a t e r depth ( c f . se c tion 1 . 2 . 8 . ) . I n considerably t e c t o n i c a l l y deformed a r e a s , f r a c t u r e propagation trend may p a r a l l e l the f a u l t i n g t r en d , w i t h e i t h e r the f a u l t s themselves cont r o l l i n g crack extension d i r e c t i o n o r the regional s t r e s s f i e l d b e i n g dominant. Under some circumstances, i t may be f e a s i b l e t o a l t e r i n - s i t u s t r e s s c o n t r a s t s by pore-pressure manipulation, f l u i d v i s c o s i t y a n d r a t e c o n t r o l , pe rfora tion placement, a n d prenotching t o provoke generation of v e r t i c a l f r a c t u r e s instead
651 o f h o r i z o n t a l ones G I E R S & MARX 1983).
in
s h a l l o w e r depth s t o r e y s (CLEARY 1980 b; McLENNAN, ROE-
4.9.4.2. Combination of horizontal and vertical fracturing Steam-drive enhanced o i l r e c o v e r y can be c o n s i d e r a b l y s u p p o r t e d by u t i l i z i n g t h e changing g e o s t r e s s regime ( c f . s e c t i o n 4.9.4.1.) f o r an e x t r a o r d i n a r y design o f special hydraulic s t i m u l a t i o n treatments comprising a combination o f v e r t i c a l and h o r i z o n t a l f r a c t u r e s which would be i m p o s s i b l e i n o t h e r r e s e r v o i r s n o t s u b j e c t e d t o a r t i f i c i a l h e a t i n g and t h u s c o n t r o l l e d thermal expansion. Asp e c t s o f c r e a t i o n , management and d e t e r m i n a t i o n o f h o r i z o n t a l and v e r t i c a l f r a c t u r e s a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.9.4.2.1. Fracture creation C r e a t i o n o f a v e r t i c a l f r a c t u r e by h y d r a u l i c s t i m u l a t i o n p r i o r t o t h e o n s e t o f steam i n j e c t i o n p r o v i d e s a base f o r improved e n t r y o f steam i n t o t h e heavyo i l - b e a r i n g pay zone. Aspects o f thermal r e s e r v o i r expansion and i n c r e a s i n g c l o s u r e s t r e s s , h o r i z o n t a l f r a c t u r e s and h o r i z o n t a l vs. v e r t i c a l boreholes, mult i p l e h o r i z o n t a l and v e r t i c a l f r a c t u r e propagation, and c o m b i n a t i o n o f hydraul i c and e x p l o s i v e f r a c t u r i n g a r e o u t l i n e d as f o l l o w s .
4.9.4.2.1.1. Thermal reservoir expansion
and increasing closure stress
I f a v e r t i c a l c r a c k i s generated h y d r a u l i c a l l y b e f o r e t h e s t a r t o f steam f l o o d i n g , c a u t i o n has t o be e x e r c i s e d f o r adequate p r o p p i n g o f t h e v e r t i c a l f r a c t u r e , because thermal expansion o f t h e r e s e r v o i r due t o h e a t i n g by t h e steam g i v e s r i s e t o i n c r e a s i n g c l o s u r e s t r e s s e s as a consequence o f an a d d i t i o n a l p r e s s u r e component coming f r o m volume enlargement o f t h e r o c k m a t r i x . Propp a n t c h o i c e t h e r e f o r e has t o r e s p e c t t h e n e c e s s i t y o f h i g h e r r e s i s t i v i t y t h a n a c c o r d i n g t o d e p t h and f o r m a t i o n p r e s s u r e t o a v o i d c o l l a p s e o f t h e c r a c k by p r o p p a n t f a i l u r e due t o thermal expansion s t r e s s , and a l s o has t o be made acc o r d i n g t o t h e subsequent h i g h - t e m p e r a t u r e environment i n terms o f thermal s t a b i l i t y o f the m a t e r i a l supporting the f r a c t u r e walls, and r e s i s t i v i t y a g a i n s t a g g r e s s i v e a l c a l i n e s o l u t i o n s ( s i m i l a r l y as a p p l y i n g f o r s t e a m - i n j e c t i o n w e l l g r a v e l packing; c f . s e c t i o n 5 . 7 . ) .
Another aspect which has t o be taken i n t o c o n s i d e r a t i o n i s i n c r e a s i n g p l a s t i c i t y o f t h e r e s e r v o i r r o c k w i t h r i s i n g temperature ( c f . s e c t i o n 4.2.3.2.3. and 4.9.4.1.2.2.) which provokes i n c r e a s i n g embedment o f t h e proppants, and t h u s m u l t i l a y e r p r o p p i n g o f t h e c r a c k has t o be planned ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) . Conc e r n i n g p r o p p a n t type, a l l t h e mentioned adverse c o n d i t i o n s i n t h e high-temperat u r e h i g h - a l c a l i n i t y s t e a m - i n j e c t i o n w e l l s can o n l y be s a t i s f a c t o r i l y met by s i n t e r e d b a u x i t e and n i c k e l - c o a t e d sand ( c f . s e c t i o n 5 . 7 . ) .
4.9.4.2.1.2. Horizontal fractures and horizontal v s . vertical wells Horizontal f r a c t u r e s i n t e r s e c t i n g v e r t i c a l w e l l s s u f f e r considerably from t h e v e r y l i m i t e d communication i n t e r v a l between w e l l b o r e and f r a c t u r e p l a n e i n a s i m i l a r way as a p p l y i n g f o r v e r t i c a l c r a c k s c r o s s i n g h i g h l y - d e v i a t e d o r h o r i z o n t a l b o r e h o l e s ( c f . s e c t i o n 4 . 1 1 . 1 . 2 . ) . The o n l y d i f f e r e n c e i n thermal enhanced o i l r e c o v e r y p r o j e c t s i s t h e steam-induced c o m b i n a t i o n w i t h v e r t i c a l f r a c t u r e s t h a t g i v e s r i s e t o a c r o s s i n g p a t t e r n o f two f r a c t u r e s p e r p e n d i c u l a r t o each o t h e r and r e p r e s e n t i n g a s p e c i a l case which cannot be achieved b y e i t h e r h o r i z o n t a l cracks i n t e r s e c t i n g h o r i z o n t a l boreholes o r v e r t i c a l f r a c t u r e s c u t -
652
t i n g v e r t i c a l wellbores. The e f f e c t i v i t y of horizontal f r a c t u r e s generated by s t r e s s regime inversion due t o steam flooding ( c f . s ect i o n 4 . 9 . 4 . 1 . ) , however, can be considerably i m proved w i t h horizontal steam-injection wells where the horizontal f r a c t u r e s have a l o n g communication i n t er v al with the wellbore comparable to t h a t of vert i c a l f r a c t u r e s i n t e r s e c t i n g v e r t i c a l boreholes. Thus horizontal w e lls i n steam-flood operations provide the unique opportunity t o increase the d i r e c t c o n t a c t between wellbore a n d pay zone, between wellbore and f r a c t u r e , and between f r a c t u r e and pay zone, with t h i s combination being the optimum solution and leading t o maximum hydrocarbon recovery from the r e s e r v o i r . Other impacts of h o r i z o n t a l wells on steam-drive processes a re outline d by BUTLER & STEPHENS (1980), R I A L ( 1 98 4 ) , GOODE & THAMBYNAYAGAM (1985) a n d HUANG & HIGHT (1986; c f . also section 4 . 8 . 6 . ) . F i e l d examples of hydraulic cr eat i o n of a v e r t i c a l f r a c t u r e p r i o r t o steam in j e c t i o n a n d subsequent formation of a horizontal crack as a r e s u l t of steam s t i m u l a t i o n a r e reported by SINGHAI & CARD (1988). Aspects of f r a c t u r i n g of heavy-oil sand r e s e r v o i r s d u r i n g course of steam i n j e c t i o n a r e a l s o discussed by BABCOCK ( 1 9 7 5 ) ; GESHELIN, GRABOWSKI & PEASE (1981); G E S H E L I N & GRABOWSKI ( 1 9 8 2 ) , ABAD & HENSLEY (1984) and PEAKE ( 1 9 8 4). I t has t o be emphasized t h a t only the combined e f f e c t w i t h the v e r t i c a l cracks renders the horizontal f r a c t u r e s s u f f i c i e n t l y e f f e c t i v e , because they alone would be hampered in performance by l i m i t e d c r o s s - s ect i o n with the borehole in a s i m i l a r way a s applying f o r vert i c a l cracks crossing highly-deviated or horizontal boreholes ( c f . se c tion 4 . 1 1 . 1 . 2 . ) . The very small communication i nte rva l between pe rfora te d zone a n d f r a c t u r e plane h a s a s u i t e of negative impacts on treatment performance a n d e f f e c t which, however, i n the s p eci al case of combined design a r e compensated by the c o e x i st i n g v e r t i c a l f r a c t u r e .
4 . 9 . 4 . 2 . 1 . 3 . Multiple horizontal
and vertical fracture propagation Developing interwell communication between i n j e c t o r s a n d producers i n e a r l y steam-flood operation i s very important f o r e f f e c t i v e enhanced o i l recovery ( K U IAR, C H H I N A , BEST & MACKENZIE 1988). For more e f f e c t i v e r e s e r v o i r heating, the p r e f e r r e d interwell communication p a t h i s e i t h e r low o r in the middle o f the pay zone, w i t h f r a c t u r e geometry playing an important r o l e . Princ iple s a n d s i g n if i c a n c e o f m u l t i p l e horizontal and v e r t i c a l f r a c t u r e propagation a r e outline d a s follows.
4 . 9 . 4 . 2 . 1 . 3 . 1 . Principles Achieving horizontal f r a c t u r e s a t d e p t h where i n - s i t u s t r e s s i s in ne a r-isotr o p i c s t a t e i s uncertain because of pore-pressure increase due t o f l u i d leako f f , thermal expansion a n d f r a c t u r e loading which may a l t e r the s t r e s s f i e l d , thereby inducing a change i n f r a c t u r e o r i e n t a t i o n o r r e s u l t i n g i n m u l t i d i r e c t i o nal cracks (KULAR, CHHINA, BEST & M A C K E N Z I E 1988). Heat i n j e c t i o n a n d various f r a c t u r e loadings induce changes of i n - s i t u s t r e s s regime. Crack loading which i s defined as d i f f er en ce between f r a c t u r e extension pressure a n d formation c l o sure s t r e s s has s i g n i f i c a n t influences on changes o f i n - s i t u s t r e s s ( C H H I N A & AGAR 1985, WONG 1986). I n case of smaller f r a c t u r e loadings, horizontal cracks may s t a y horizontal and may only be moderately inc line d, whereas f o r g r e a t e r f r a c t u r e loadings, an i n i t i a l l y horizontal crack may progress i n t o m ultiple f r a c t u r e s with varying o r i e n t a t i o n s which i s caused by s t r e s s r e v e r s a l s occurring i n a n e a r - i s o t r o p i c s t r e s s environment. Q u a l i t a t i v e l y h i g h crack loadings can induce changes i n the horizontal t o v e r t i c a l s t r e s s r a t i o s t o favour m u l t i p l e fracturing.
653 While i m m e d i a t e l y a d j a c e n t t o t h e f r a c t u r e t h e i n i t i a l t o t a l s t r e s s r a t i o does not change s i g n i f i c a n t l y because b o t h h o r i z o n t a l and v e r t i c a l t o t a l s t r e s s increase, some d i s t a n c e away f r o m t h e c r a c k f a c e t h e s t r e s s r a t i o has changed, w i t h a r e v e r s a l o f t h e minimum s t r e s s d i r e c t i o n h a v i n g o c c u r r e d f r o m o r i g i n a l l y v e r t i c a l t o now h o r i z o n t a l . I f a p r o p a g a t i n g h o r i z o n t a l f r a c t u r e o r h i g h f l u i d p r e s s u r e reaches t h e v e r t i c a l f r a c t u r e zone o r s t r e s s r e v e r s a l i n t e r v a l , a conj u g a t e v e r t i c a l c r a c k s t a r t s p r o p a g a t i n g . I n i t i a t i o n and e x t e n s i o n o f t h i s secondary v e r t i c a l f r a c t u r e depends on sand f a i l u r e mechanism and pore p r e s s u r e d i s t r i b u t i o n o f f t h e c r a c k f a c e ( a s p e c t s o f composed T-shaped o r mushroom f r a c ture complexes are also discussed i n s e c t i o n s 1.2.8.4., 4.4.3.4.3., 4.9.4.2.3.2. and 6 . 2 . 2 . 4 . 4 . ) . Pore p r e s s u r e can i n c r e a s e i n t h e f o r m a t i o n due t o l e a k o f f o r due t o f r a c t u r e l o a d i n g under u n d r a i n e d c o n d i t i o n s . As o i l sands a r e c h a r a c t e r i z e d by h i g h f l u i d l o s s and low t e n s i l e s t r e n g t h , h y d r a u l i c f r a c t u r e s propagate by b o t h shear and t e n s i l e f a i l u r e (DUSSEAULT 1985), w i t h shear f a i l u r e generally occurring p r i o r t o t e n s i l e f a i l u r e .
4.9.4.2.1.3.2. Significance Shear f a i l u r e r e g i o n , h i g h f l u i d l e a k o f f , o r pore p r e s s u r e g e n e r a t i o n due t o u n d r a i n e d f r a c t u r e l o a d i n g a r e some o f t h e mechanisms by w h i c h t h e h o r i z o n t a l c r a c k c o u l d p e n e t r a t e i n t o t h e v e r t i c a l f r a c t u r e zone p a s t t h e b u f f e r zone where t h e i n i t i a l s t r e s s r a t i o d i d n o t change. Pressure drop i s p r i m a r i l y a f f e c t e d by s t i m u l a t i o n f l u i d v i c o s i t y and o n l y s e c o n d a r i l y by f l u i d i n j e c t i o n r a t e (KULAR, CHHINA, BEST & MACKENZIE 1988). F o r u n c o n s o l i d a t e d f o r m a t i o n s , t r e a t ment f l u i d v i s c o s i t y i s a c r i t i c a l f r a c t u r e d e s i g n parameter due t o i t s i n f l u e n c e on c r a c k l o a d i n g which i n t u r n d i c t a t e s f r a c t u r e o r i e n t a t i o n . Apparent s l u r r y v i s c o s i t y tends t o i n c r e a s e crack l o a d i n g as p r o p p a n t i n j e c t i o n i s i n i tiated. Mu1 t i p l e h o r i z o n t a l f r a c t u r e s emanating i n d i f f e r e n t r e s e r v o i r l e v e l s and from d i f f e r e n t w e l l s which are i n t e r l i n k e d w i t h a conjugate v e r t i c a l crack are e f f i c i e n t means o f pay zone and b o r e h o l e c o n n e c t i o n t h r o u g h a network o f f r a c t u r e s o f d i f f e r e n t o r i e n t a t i o n . T h i s g e o m e t r i c a l combinatory c o n s t e l l a t i o n p r o v i des an i m p o r t a n t p o s s i b i l i t y o f b u i l d i n g b r i d g e s between v a r i o u s r e s e r v o i r segments i n h o r i z o n t a l and v e r t i c a l d i r e c t i o n and thus much more e f f i c i e n t pay comp l e x i n t e r s e c t i o n t h a n c o u l d be achieved by h o r i z o n t a l o r v e r t i c a l f r a c t u r e s a1 one.
4.9.4.2.1.4,
Combi nat ion of hydrau1 ic and explos ive fracturing
The secondary h i g h - t e m p e r a t u r e environment o f s t e a m - d r i v e p r o j e c t s i n primar y low-temperature r e s e r v o i r s and t h e a s s o c i a t e d change i n g e o s t r e s s s t a t e which leads t o i n v e r s i o n o f f r a c t u r e p r o p a g a t i o n o r i e n t a t i o n f r o m o r i g i n a l l y v e r t i c a l t o subsequently h o r i z o n t a l a l s o enables p r o f i t a b l e c o m b i n a t i o n o f hyd r a u l i c and e x p l o s i v e f r a c t u r i n g . W h i l e t h e c r a c k s generated by e x p l o s i v e d e t o n a t i o n i n the borehole are u s u a l l y propagating i n v e r t i c a l d i r e c t i o n , the primar y g e o s t r e s s s t a t e i n many s h a l l o w o i l r e s e r v o i r s and t h e secondary g e o s t r e s s s t a t e i n s t e a m - f l o o d o p e r a t i o n s t r i g g e r h y d r a u l i c a l l y - g e n e r a t e d c r a c k s t o penet r a t e t h e pay i n t e r v a l i n h o r i z o n t a l o r i e n t a t i o n . T h i s r e l a t i o n s h i p p e r m i t s i n b o t h mentioned cases a s u i t a b l e a d d i t i o n a l p e r m e a b i l i t y improvement o f t h e r e s e r v o i r by h o r i z o n t a l h y d r a u l i c f r a c t u r i n g superimposed by v e r t i c a l e x p l o s i v e s t i m u l a t i o n , e n a b l i n g enhanced o i l o f f t a k e i n p r o d u c t i o n w e l l s ( c f . s e c t i o n 4.8.9.6.) and a m e l i o r a t e d steam uptake c a p a c i t y i n thermal i n j e c t i o n b o r e h o l e s .
4.9.4.2.2.
Fracture management
Once a s u i t a b l y equipped v e r t i c a l h y d r a u l i c f r a c t u r e has been i n s t a l l e d w i t h i n t h e r e s e r v o i r , steam i n j e c t i o n a t h i g h r a t e s w i l l f i r s t l e a d t o f l u i d e n t r y p r e f e r e n t i a l l y v i a t h e v e r t i c a l c r a c k . I n c r e a s i n g i n j e c t i o n r a t e s exceeding up-
654 t a k e c a p a c i t y o f t h e d r a i n a g e p a t h w i l l t h e n g i v e r i s e t o p r e s s u r e b u i l d u p , and achievement o f a c r i t i c a l pressure l e v e l r e s u l t s i n g e n e r a t i o n o f h o r i z o n t a l f r a c t u r e s i n t e r s e c t i n g t h e e a r l i e r v e r t i c a l c r a c k s . C o n t i n u o u s steam i n j e c t i o n w i t h s u f f i c i e n t l y h i g h r a t e s keeps t h e h o r i z o n t a l f r a c t u r e open and t h u s a l l o w s f l u i d e n t r y t h r o u g h two p e r p e n d i c u l a r c r a c k systems. As p r o p p a n t i n s e r t i o n i n t o the horizontal f r a c t u r e s i s n o t possible i n the h o t high-pressure steam-drive e n v i r o n m e n t w h i c h a l s o m u s t n o t be i n t e r r u p t e d o r d i s t u r b e d i n o r d e r t o a c h i e v e an optimum e f f e c t o f o i l m o b i l i z a t i o n , t h e h o r i z o n t a l c r a c k s c a n o n l y b e k e p t open b y m a i n t a i n i n g i n j e c t i o n p r e s s u r e a t a s u f f i c i e n t l y e l e v a t e d n i v e a u , o r c y c l i c a l g e n e r a t i o n and c o l l a p s e o f h o r i z o n t a l f r a c t u r e s a c c o r d i n g t o t h e model o f HOLZHAUSEN, CARD, RAISBECK & DOBECKI ( 1 9 8 5 ) has t o be a c c e p t e d .
I n v e r y s h a l l o w r e s e r v o i r d e p t h above 3,000 f t (1,000 m), an i n v e r s e p r o c e dure i s possible w i t h f i r s t hydraulic generation o f a h o r i z o n t a l f r a c t u r e which i s s t a b l e i n t h i s l o w - o v e r b u r d e n e n v i r o n m e n t , and second e i t h e r e n l a r g i n g t h e e x i s t i n g f r a c t u r e b y steam p r e s s u r e o r c r e a t i o n o f new c r a c k s r u n n i n g p r e f e r e n t i a l l y p a r a l l e l t o t h e o l d e r f r a c t u r e . V e r t i c a l c r a c k s w o u l d i n such s h a l l o w d e p t h o n l y o r i g i n a t e when t h e f o r m a t i o n i s s p l i t i n t o s e a l e d f a u l t b l o c k s o r t h e r e s e r v o i r sand i s p r e s e n t as d i s c o n t i n u o u s l e n s e s and c h a n n e l s p i n c h i n g o u t w i t h i n t h i c k e r mudstone complexes, w i t h t h u s l a t e r a l e x p a n s i o n o f t h e p a y zone upon h e a t i n g b e i n g c o n s i d e r a b l y r e s t r i c t e d and i n some c a s e s a c c o r d i n g t o t h e s h a l l o w d e p t h b e i n g e v e n more l i m i t e d t h a n v e r t i c a l e x p a n s i o n . I n a d d i t i o n , p e r f o r a t i o n f r i c t i o n o c c u r s due t o l i m i t e d c o m m u n i c a t i o n between w e l l b o r e and a hor i z o n t a l f r a c t u r e i n s h a l l o w d e p t h , and t h i s e f f e c t c a n cause a s m a l l v e r t i c a l f r a c t u r e t o open w h i c h b r e a k s down t h e w e l l b o r e , b u t t h e n soon t u r n s i n t o t h e proper d i r e c t i o n . Aspects o f f r a c t u r e - a s s i s t e d steam-flood technology a r e a l s o d i s c u s s e d b y BRITTON, MARTIN, LEIBRECHT & HARMON ( 1 9 8 3 ) and SON1 & HARMON (1986).
4.9.4.2.3.
Fracture determination
The method m o s t commonly u s e d t o d e t e r m i n e w h e t h e r a h y d r a u l i c f r a c t u r e i s v e r t i c a l o r h o r i z o n t a l i s comparison o f t h e pressure r e q u i r e d t o propagate t h e f r a c t u r e (which i s represented by t h e instantaneous s h u t - i n pressure) t o t h e v e r t i c a l i n - s i t u s t r e s s a t t h e i n j e c t i o n d e p t h (HOLZHAUSEN, CARD, RAISBECK & DOBECK1 1 9 8 5 ) . I f t h e p r e s s u r e i s l e s s t h a n t h e v e r t i c a l s t r e s s , t h e f r a c t u r e c a n n o t be h o r i z o n t a l because t h e p r e s s u r e i s i n s u f f i c i e n t t o l i f t t h e o v e r b u r d e n . The a s s u m p t i o n t h a t t h e v e r t i c a l s t r e s s i s a p r i n c i p a l s t r e s s l e a d s t o c o n c l u s i o n t h a t t h e f r a c t u r e i s v e r t i c a l , because i t s h o u l d f o r m i n a p l a n e p e r p e n d i c u l a r t o t h e l e a s t c o m p r e s s i v e p r i n c i p a l s t r e s s w h i c h m u s t be h o r i z o n t a l .
On t h e o t h e r hand, i f t h e p r e s s u r e i n t h e f r a c t u r e exceeds t h e v e r t i c a l stress, t h e c r a c k p l a n e i s taken. t o be h o r i z o n t a l u n d e r t h e a s s u m p t i o n t h a t t h e v e r t i c a l s t r e s s i s t h e l e a s t c o m p r e s s i v e p r i n c i p a l s t r e s s . Thus i n g e n e r a l o n l y e i t h e r v e r t i c a l o r h o r i z o n t a l f r a c t u r e s c a n o r i g i n a t e i n a g i v e n r e s e r v o i r , and t h e g e n e r a t i o n o f b o t h t y p e s o f c r a c k s one a f t e r e a c h o t h e r t o f i n a l l y c o e x i s t i n t h e f o r m a t i o n and make up t w o p e r p e n d i c u l a r i n t e r s e c t i n g d r a i n a g e p a t h s i s n o t p o s s i b l e u n l e s s s p e c i a l c o n d i t i o n s a r e present, w i t h t h e most i m p o r t a n t means o f c h a n g i n g g e o s t r e s s r e g i m e b e i n g t h e r m a l e x p a n s i o n o f t h e r o c k c o l u m n as d i s c u s s e d above. O t h e r methods o f d i s c r i m i n a t i o n between h o r i z o n t a l and v e r t i c a l c r a c k s a r e t i l t m e t e r m o n i t o r i n g ( c f . s e c t i o n s 6 . 2 . 2 . 3 . 4 . and 6 . 2 . 2 . 4 . 1 . ) and t e m p e r a t u r e p r o f i l i n g (VITTORATOS 1 9 8 6 ) . Some a s p e c t s o f f r a c t u r e g r a d i e n t v s . o v e r b u r d e n s t r e s s g r a d i e n t as w e l l as r e s e r v o i r e x p a n s i o n and c h a n g i n g i n s i t u s t r e s s a r e i l l u s t r a t e d as f o l l o w s .
4.9.4.2.3.1.
Fracture gradient vs. overburden stress gradient
I n t e r m s o f c r a c k d e t e r m i n a t i o n in s h a l l o w - d e p t h r e s e r v o i r s , f r a c t u r e g r a d i e n t s e x c e e d i n g t h e o v e r b u r d e n s t r e s s g r a d i e n t may r e f l e c t v e r t i c a l c r a c k s , whereas f r a c t u r e g r a d i e n t s a p p r o x i m a t e l y e q u i v a l e n t t o t h e o v e r b u r d e n s t r e s s
655 g r a d i e n t a r e i n t e r p r e t e d as i n d i c a t i n g c r e a t i o n o f h o r i z o n t a l c r a c k s (STOLLER 1985). Temperature p r o f i l e m o n i t o r i n g i s a s u i t a b l e i n s t r u m e n t t o determine f r a c t u r e shape and s i z e , thermal d i f f u s i v i t y o f t h e f o r m a t i o n , and f l u i d f l o w v e l o c i t i e s i n c o n v e c t i v e r e g i o n s (VITTORATOS 1986). T i l t m e t e r survey can d i s t i n g u i s h v e r t i c a l and h o r i z o n t a l f r a c t u r e s which produce a r a d i a l symmetrical upl i f t o f t h e ground and a t r o u g h a t t h e s u r f a c e p a r a l l e l t o f r a c t u r e s t r i k e , r e s p e c t i v e l y (HOLZHAUSEN, CARD, RAISBECK & DOBECKI 1985; c f . s e c t i o n s 6.2.2.3.4. and 6 . 2 . 2 . 4 . 1 . ) .
4.9.4.2.3.2.
Reservoir expansion and changing i n - s i t u stress
Steam-drive o p e r a t i o n s i n h e a v y - o i l f i e l d s thus p r o v i d e t h e u n i q u e p o s s i b i l i o f r e s e r v o i r s t i m u l a t i o n w i t h a c o m b i n a t i o n o f h o r i z o n t a l and v e r t i c a l f r a c t u r e s as a consequence o f t h e changing i n - s i t u s t r e s s regime by thermal pay expansion which would be i m p o s s i b l e i n o t h e r f o r m a t i o n s t h a t a r e n o t s u b j e c t e d t o t e m p e r a t u r e - r e l a t e d enhanced o i l r e c o v e r y . Aspects o f h o r i z o n t a l f r a c t u r e s i n s h a l l o w h e a v y - o i l r e s e r v o i r s s t i m u l a t e d by steam f l o o d i n g a r e a l s o d i s c u s s e d by CLOSMANN & PHOCAS (1976); REDDY, PROWSE & REDFORD (1980); CLOSMANN & SMITH (1983) and VITTORATOS (1986), and DUSSEAULT & SIMMONS (1982) analyze i n j e c t i o n induced s t r e s s and f r a c t u r e o r i e n t a t i o n changes. ty
CLOSMANN & PHOCAS (1976) conclude t h a t f a v o u r a b l e c o n d i t i o n s f o r h o r i z o n t a l f r a c t u r e s can be c r e a t e d by h e a t i n g a l o n g two p a r a l l e l v e r t i c a l c r a c k s . DUSSEAULT & SIMMONS (1982) document t h a t p o r e p r e s s u r e has more s i g n i f i c a n t e f f e c t s than temperature on i n d u c i n g s t r e s s changes, and t h a t t h e s t r e s s f i e l d i s a l t e r e d enough f r o m f l u i d i n j e c t i o n a l o n e t o a l l o w h o r i z o n t a l f r a c t u r e s t o be c r e a t e d . CLEARY, CROCKETT, MARTINEZ, NARENDRAN & SLUTSKY (1983) p u t f o r w a r d t h a t f l u i d l e a k o f f i n c r e a s e s t h e p o r e p r e s s u r e around t h e c r a c k and t h a t t h e induced b a c k s t r e s s e s a c t t o c l o s e t h e f r a c t u r e subsequently. CROCKETT, VO & CLEARY (1984) show t h a t t h e r m a l l y - i n d u c e d s t r e s s e s a r e s m a l l compared w i t h those gener a t e d by f l u i d l o s s o v e r t h e t i m e s c a l e o f t y p i c a l h y d r a u l i c f r a c t u r i n g i n t i g h t reservoirs. Combinations o f v e r t i c a l and h o r i z o n t a l f r a c t u r e s o r i g i n a t e a l s o i n r e s e r v o i r s w i t h normal geotemperature w i t h o u t i n f l u e n c e o f a r t i f i c i a l f o r m a t i o n h e a t i n g by opening o f a p r i m a r y v e r t i c a l c r a c k which s e c o n d a r i l y t u r n s i n t o a h o r i z o n t a l f r a c t u r e i n s h a l l o w depth t h a t breaks o u t a t t h e t o p o f t h e v e r t i c a l c r a c k (WOOD, PARKIN, YOTAM, HANSON, SMITH, ABBOTT, COX & O'SHEA 1983; c f . sect i o n 6.2.3.). T-shaped f r a c t u r e s c o n s i s t i n g o f v e r t i c a l and h o r i z o n t a l components a l s o o r i g i n a t e i n c o a l seams t h a t a r e bounded by s h a l e s due t o a b n o r m a l l y h i g h t r e a t i n g p r e s s u r e s (JEU, LOGAN & McBANE 1988; c f . s e c t i o n 4 . 4 . 3 . 8 . ) . O t h e r aspects o f composed mushroom-shaped f r a c t u r e assemblages a r e d i s c u s s e d i n sect i o n s 1.2.8.4., 4.4.3.4.3., 4.9.4.2.1.3. and 6.2.2.4.4.
4.9.4.3.
Changes o f f r a c t u r e azimuth o r i e n t a t i o n
I n - s i t u s t r e s s changes d u r i n g steam s t i m u l a t i o n o f h e a v y - o i l sand r e s e r v o i r s do n o t o n l y g i v e r i s e t o g e n e r a t i o n o f h o r i z o n t a l f r a c t u r e s i n o r i g i n a l geos t r e s s regimes f a v o u r i n g f o r m a t i o n o f v e r t i c a l c r a c k s w i t h o u t thermal i n f l u e n c e , b u t a l l o w a l s o changes o f v e r t i c a l f r a c t u r e o r i e n t a t i o n w i t h r e s p e c t t o t h e azimuth o f t h e c r a c k p r e f e r e n t i a l l y i n deeper r e s e r v o i r s t o r e y s where hor i z o n t a l f r a c t u r e s a r e d i f f i c u l t t o c r e a t e a l s o under a r t i f i c i a l l y e l e v a t e d pay zone temperatures and p r e s s u r e s (WONG 1986). T h i s has i m p o r t a n t consequences f o r p l a n n i n g o f b o r e h o l e p a t t e r n and spacing as w e l l as u t i l i z a t i o n o f f r a c t u r e i n t e r f e r e n c e and c r e a t i o n o f c r a c k s p e r p e n d i c u l a r l y t o each o t h e r f o r enhanced hydrocarbon e x p l o i t a t i o n . The o u t l i n e as f o l l o w s focusses on o n - t r e n d and o f f t r e n d w e l l p a t t e r n s , staggered steam i n j e c t i o n i n t o o f f - t r e n d w e l l p a i r s , and mechanisms o f f r a c t u r e azimuth o r i e n t a t i o n changes.
656
4.9.4.3.1. On-trend and o f f - t r e n d w e l l p a t t e r n s A n a l y z i n g two w e l l s , d i s t i n c t i o n can be made between o n - t r e n d p a t t e r n when both boreholes are aligned along the f r a c t u r e d i r e c t i o n according t o the undist u r b e d g e o s t r e s s f i e l d , and o f f - t r e n d arrangements when b o t h w e l l s a r e p l a c e d i n such a way t h a t t h e c o n n e c t i o n l i n e r u n s p e r p e n d i c u l a r l y t o t h e c r a c k d i r e c t i o n determined by t h e i n h e r e n t g e o s t r e s s d i s t r i b u t i o n (WONG 1986). I n terms o f steam s t i m u l a t i o n , d i s c r i m i n a t i o n can be made between simultaneous and stagger e d steam i n j e c t i o n o f two n e i g h b o u r i n g b o r e h o l e s . I f two o n - t r e n d w e l l s a r e steamed s i m u l t a n e o u s l y o r i n a staggered manner, t h e f r a c t u r e s which a r e c r e a t e d a r e o r i e n t e d i n i t i a l l y i n t h e n a t u r a l c r a c k d i r e c t i o n where t h e y do n o t i n t e r a c t w i t h each o t h e r b e f o r e t h e y s t a r t t o grow c o n s i d e r a b l y i n l e n g t h , w i t h t h e amount and r a p i d i t y o f i n t e r a c t i o n by i n t e r f e r e n c e and c o n n e c t i o n depending on w e l l s p a c i n g and i n j e c t i o n p r e s s u r e . Continued growth o f t h e f r a c t u r e s i n o n - t r e n d d i r e c t i o n w i t h p r o g r e s s i v e steam i n j e c t i o n occurs because t h e p r e f e r r e d c r a c k o r i e n t a t i o n ahead o f t h e f r a c t u r e does n o t change. I n c o n t r a s t t o t h e o n - t r e n d case, t h e i n j e c t i o n sequence has s i g n i f i c a n t i m p a c t on t h e o r i e n t a t i o n s o f f r a c t u r e s i n two o f f - t r e n d w e l l s . Simultaneous steam i n j e c t i o n i n t o two o f f - t r e n d b o r e h o l e s r e s u l t s again i n c r a c k o r i e n t a t i o n and growth i n t h e v i r g i n a l f r a c t u r e d i r e c t i o n as l o n g as t h e y do n o t i n t e r a c t w i t h each o t h e r , and once a f r a c t u r e l e n g t h has been reached t o a l l o w f o r conn e c t i o n and i n t e r f e r e n c e , t h e c r a c k s a l s o c o n t i n u e t o propagate i n t h e o r i g i n a l fracture orientation.
4.9.4.3.2.
Staggered steam i n j e c t i o n i n t o o f f - t r e n d w e l l p a i r s
Remarkable changes i n f r a c t u r e o r i e n t a t i o n , however, a r e provoked by stagger e d steam i n j e c t i o n i n t o two o f f - t r e n d w e l l s (WONG 1986). I f steam s t i m u l a t i o n i s s t a g g e r e d i n such a way t h a t i n j e c t i o n a t t h e f i r s t b o r e h o l e has g e n e r a t e d a c r a c k o f l e n g t h i n t h e o r d e r o f w e l l spacing, t h e n i n j e c t i o n a t t h e second w e l l causes a f r a c t u r e growing p e r p e n d i c u l a r l y t o t h e f i r s t one and p r o p a g a t i n g i n o f f - t r e n d d i r e c t i o n as a consequence o f s i g n i f i c a n t changes i n t h e p r i n c i p a l o r i e n t a t i o n s along the o f f - t r e n d d i r e c t i o n from a crack. Staggered steam i n j e c t i o n o f b o r e h o l e p a i r s p l a c e d i n an o f f - t r e n d p a t t e r n i s t h e r e f o r e a p o s s i b i l i t y t o make use o f i n - s i t u s t r e s s d i s t r i b u t i o n changes by r e s e r v o i r h e a t i n g i n o r d e r t o c r e a t e f a b r i c s o f two f r a c t u r e systems r u n n i n g p e r p e n d i c u l a r l y t o each o t h e r which a l l o w t o d r a i n t h e pay zone i n much w i d e r e x t e n t t h a n c o u l d be done by a number o f p a r a l l e l f r a c t u r e s , because a d d i t i o n a l q u a n t i t i e s o f hydrocarbons can be accessed w i t h fewer w e l l s by normal connect i o n s o f p a r a l l e l f r a c t u r e s and g e n e r a t i o n o f o r t h o g o n a l l y l i n k e d o r approachi n g c r a c k s i n s t e a d o f a s u i t e o f p a r a l l e l f r a c t u r e s . Staggered steam i n j e c t i o n o f o f f - t r e n d w e l l p a i r s t h u s r e p r e s e n t s a s p e c i a l method o f a l t e r e d - s t r e s s f r a c t u r i n g by b e i n g a c o m b i n a t i o n between g e o s t r e s s d i s t r i b u t i o n d i s t u r b a n c e and t r a n s f o r m a t i o n by t h e f i r s t h y d r a u l i c f r a c t u r e and t h e p r e s s u r e necessary t o gen e r a t e i t and t o keep i t open ( c f . s e c t i o n 4 . 9 . 4 . 1 . 3 . 1 . ) , and t h e a d d i t i o n a l i m p a c t o f r e s e r v o i r h e a t i n g by steam f l o o d i n g which provokes changes o f g e o s t r e s s p a t t e r n due t o pay expansion.
4.9.4.3.3.
Mechanisms o f f r a c t u r e azimuth o r i e n t a t i o n changes
Steam s t i m u l a t i o n t h e r e f o r e e f f e c t s s i g n i f i c a n t changes i n magnitudes and o r i e n t a t i o n s o f s t r e s s e s (WONG 1986). Steam i n j e c t i o n b a s i c a l l y i n c r e a s e s t h e i n - s i t u stresses, w h i l e the stress a l t e r a t i o n s are recovered mainly e a r l y i n t h e p r o d u c t i o n c y c l e . A t t h e end o f an i n j e c t i o n c y c l e , most o f t h e s t r e s s changes a r e around t h e m i d d l e o f t h e f r a c t u r e i n the o f f - t r e n d d i r e c t i o n , whereas ahead o f t h e crack, o n l y small a l t e r a t i o n s i n t h e p r i n c i p a l o r i e n t a t i o n s o c c u r . F r a c t u r e l o a d i n g i s t h e dominant mechanism r e s u l t i n g i n changes o f b o t h magnitude and d i r e c t i o n o f s t r e s s , because t h e magnitude o f f r a c t u r e l o a d i n g i s r e l a -
657 t i v e l y h i g h compared w i t h t h e average p r e s s u r e / t e m p e r a t u r e a l t e r a t i o n . Pressure and temperature changes s i m p l y a l t e r t h e s t r e s s magnitudes, b u t l e a v e t h e p r i n c i p a l s t r e s s o r i e n t a t i o n s almost u n a f f e c t e d . The maximum a l t e r a t i o n s t a k e p l a c e around t h e t i p o f t h e d i s t u r b e d zones o f p r e s s u r e / t e m p e r a t u r e changes where t h e l a r g e s t g r a d i e n t s o c c u r . A t t h e end o f production cycles, the i n - s i t u stresses r e t u r n e s s e n t i a l l y t o t h e i r i n i t i a l stat e s as a consequence o f l i n e a r - e l a s t i c behaviour and small average pressure/temp e r a t u r e changes, b u t r e s i d u a l s t r e s s e s and d e f o r m a t i o n s a r e expected once nonl i n e a r b e h a v i o u r i s t a k e n i n t o account. From t h e a n a l y s i s o f s t r e s s a l t e r a t i o n s d u r i n g c y c l i c steam s t i m u l a t i o n , f r a c t u r e o r i e n t a t i o n can be m a n i p u l a t e d t o a c e r t a i n e x t e n t by choosing an a p p r o p r i a t e c o m b i n a t i o n o f w e l l l a y o u t and operat i n g schedule, t h e r e b y o p t i m i z i n g hydrocarbon p r o d u c t i o n . S t r e s s changes i n d u ced by p o r e f l u i d p r e s s u r e and/or temperature changes a r e a l s o i n v e s t i g a t e d by CLOSMANN & PHOCAS (1976), DUSSEAULT & SIMMONS (1982); CLEARY, CROCKETT, M A R T I NEZ, NARENDRAN & SLUTSKY (1983); CROCKETT, VO & CLEARY (1984) and P E R K I N S & GONZALEZ (1984, 1985). An a l t e r n a t i v e t e c h n o l o g y t o s t e a m - d r i v e m o b i l i z a t i o n o f heavy o i l i s m i n i n g o f t h e h i g h - v i s c o s i t y d e p o s i t s (HARDING & ALI 1984; LYMAN, PIPER & RIDDELL 1984) which, however, i s f r e q u e n t l y o f v e r y l i m i t e d economical f e a s i b i l i t y i n comparison t o thermal enhanced o i l r e c o v e r y . The e f f e c t o f o i l m i n i n g can be enhanced by h o r i z o n t a l w e l l b o r e s c o n n e c t i n g v e r t i c a l s h a f t s and p a r t i a l l y r e p l a c i n g h o r i z o n t a l g a l l e r i e s o r t o c r e a t e o r t h o g o n a l communication tubes between p a r a l l e l g a l l e r i e s (BEST, CORDELL & HASTON 1987, ERDUL-ERDGAS-KOHLE 1988 b, MOORE 1988 c ) .
4.9.5. Cold water injection effects on hydraulic fracturing A p a r t f r o m steam d r i v e c o m p r i s i n g t e r t i a r y - r e c o v e r y h o t steam and w a t e r i n j e c t i o n , a l s o c o l d w a t e r i n j e c t i o n by c o n v e n t i o n a l secondary-recovery w a t e r f l o o d i n g i n t o o i l r e s e r v o i r s can have s i g n i f i c a n t e f f e c t s on h y d r a u l i c f r a c t u r i n g by m o d i f y i n g o r changing t h e i n i t i a l i n - s i t u s t r e s s d i s t r i b u t i o n i n t h e pay i n t e r v a l (PERKINS & GONZALEZ 1985). F o r t y p i c a l w a t e r f l o o d i n g o f a moderat e l y deep o i l r e s e r v o i r , h o r i z o n t a l e a r t h s t r e s s e s may be reduced by s e v e r a l hundred p s i . I f t h e i n j e c t i o n p r e s s u r e i s t o o h i g h o r i f suspended s o l i d s i n t h e w a t e r p l u g t h e f o r m a t i o n f a c e a t t h e p e r f o r a t i o n s , t h e pay w i l l be f r a c t u r e d h y d r a u l i c a l l y . As t h e c r a c k grows, t h e f l o w system e v o l v e s f r o m an essent i a l l y c i r c u l a r geometry i n t h e p l a n view t o one c h a r a c t e r i z e d more n e a r l y as e l l i p t i c a l . Aspects o f i n - s i t u s t r e s s and f r a c t u r e p r e s s u r e r e d u c t i o n , r e s e r v o i r p r e s s u r e and f r a c t u r e g r a d i e n t r e d u c t i o n , f o r m a t i o n p a r t i n g and s t r e s s r e d u c t i o n a t f r a c t u r e t i p , p r i m a r y and secondary f r a c t u r e s ; and r e s e r v o i r p r e s sure, i n j e c t i v i t y and f r a c t u r e l e n g t h a r e o u t l i n e d as f o l l o w s .
4.9.5.1. In-situ stress and fracture pressure reduction Temperatures o f i n j e c t e d f l u i d s a r e t y p i c a l l y l o w e r than i n - s i t u r e s e r v o i r temperatures, and t h e r e f o r e a r e g i o n o f c o o l e d r o c k forms around each i n j e c t i o n w e l l and grows as a d d i t i o n a l f l u i d i s i n j e c t e d (PERKINS & GONZALEZ 1985). Format i o n r o c k w i t h i n t h e c o o l e d r e g i o n c o n t r a c t s , and t h i s l e a d s t o d e c r e a s i n g h o r i z o n t a l e a r t h s t r e s s near t h e i n j e c t i o n b o r e h o l e . I n a d d i t i o n , w a t e r s n o r m a l l y a v a i l a b l e f o r i n j e c t i o n c o n t a i n suspended s o l i d s , and c o n s e q u e n t l y p l u g g i n g o f f o r m a t i o n r o c k occurs t h e r e b y p o s s i b l y s e v e r e l y l i m i t i n g i n j e c t i v i t y . I n f i e l d operations, i n j e c t i o n o f t e n s i m p l y c o n t i n u e s a t a bottomhole p r e s s u r e t h a t i s h i g h enough t o i n i t i a t e and e x t e n d h y d r a u l i c f r a c t u r e s which then enable t h e i n j e c t e d f l u i d t o l e a k o f f r e a d i l y through t h e l a r g e c r a c k f a c e area ( c f . s e c t i o n 4.8.7.1.2.). Because o f t h e l o w e r i n g o f h o r i z o n t a l e a r t h s t r e s s e s t h a t r e s u l t f r o m c o l d f l u i d i n j e c t i o n , h y d r a u l i c f r a c t u r i n g p r e s s u r e s can be much lower than would be expected f o r an o r d i n a r y l o w - l e a k o f f h y d r a u l i c s t i m u l a t i o n t r e a t ment.
658
I n terms of i n i t i a l f r a c t u r e o r i e n t a t i o n , moderately deep formations a r e t y p i c a l l y i n a s t a t e of t r i a x i a l compression w i t h the l a r g e s t princ ipa l s t r e s s o r i e n t e d v e r t i c a l l y , w i t h t h er ef o r e v e r t i c a l hydraulic cracks tending t o i n i t i a t e and propagate perpendicularly t o the minimum horizontal s t r e s s ( c f . sect i o n 1 . 2 . 8 . ) . When cold f l u i d i s i n j ect ed i n t o an unfractured w e ll, the region of cooled rock might be considered r a d i a l l y symmetrical w i t h re spe c t t o the borehole. Superimposition of the thermoelastic s t r e s s thus w ill decrease the maximum a n d minimum horizontal s t r e s s e s by an equal amount. Although the well can be f r a c t u r e d h yd r au l i cal l y a t a reduced pressure, the re w i l l be no e f f e c t of a r a d i a l l y symmetrical cooled region on the i n i t i a l o r i e n t a t i o n of an induced crack.
4.9.5.2. Reservoir temperature and fracture gradient reduction Considerable r e s e r v o i r temperature reductions in waterflood ope ra tions lead t o s i g n i f i c a n t d ecl i n e of f r a c t u r e g r ad i en t a n d re quire s diminution of i n j e c t i o n pressures t o avoid accidental formation p a r t i n g (GARON, LIN & D U N A Y E V S K Y 1988). Thermally induced s t r e s s reduction a n d f r a c t u r i n g f a c i l i t a t i o n i s of cons i d e r a b l e importance f o r proper waterflood design and modelling, because changes i n r e s e r v o i r temperature and pore pressure f i e l d s r e s u l t i n g from cold water in j e c t i o n can cause changes in i n - s i t u s t r e s s l e v e l s . Decreasing r e s e r v o i r temperature r e s u l t s in t o reduction of compressive s t r e s s , while r i s i n g pore pressure i n c r e a se s i n - s i t u s t r e s s . Uncontrolled v e r t i c a l a n d l a t e r a l f r a c t u r e growth could s e r i o u s l y impact v e r t i c a l a n d areal sweep e f f i c i e n c i e s , whereas re duction of i n j e c t i o n pressures could provoke severe underinjection irl many wat e r f l o o d p a t t e r n s . Reservoir behaviour associated with cold water i n j e c t i o n i n t o a hot formation a r e very complex because of the coupled nature of the mechanisms involved. Water i n j e c t i o n cools the pay by conduction a n d convection. P ri o r t o f r a c t u r i n g , flow d i s t r i b u t i o n i s c ontrolle d by r e s e r v o i r c h a r a c t e r i s t i c s , and temperature f i e l d i s governed by flow p a t t e r n . The i n - s i t u s t r e s s d i s t r i b u t i o n i s subsequently determined by temperature a n d pressure f i e l d s , and f r a c t u r e g r a d i e n t may be decreased.
4.9.5.3.
Formation parting and stress reduction at fracture tip
A t some i n j e c t i o n p r es s u r e, formation p ar ting occurs. Flow pa tte rn i s obviously s t r o n g l y influenced by any f r a c t u r e s which develop a n d t h i s c o n t r o l s the subsequent temperature a n d s t r e s s d i s t r i b u t i o n s . S t r e s s d i m i n u t i o n near the crack t i p s r e s u l t s in f r a c t u r e elongation a n d f u r t h e r s t r e s s reduction, with t h i s coupling e f f e c t being able t o give r i s e t o rapid f r a c t u r e extension ( P E R KINS & GONZALEZ 1985). The maximum level of s t r e s s reduction inc re a se s a n d the region of s t r e s s d i s t r i b u t i o n expands as the cooling region grows with c o n t i n u i n g water i n j e c t i o n . The maximum s t r e s s decline occurs a t some dista nc e from the wellbore because of the combination of thermal a n d pore-pressure e f f e c t s . Simulation of f r a c t u r e opening a n d continuous crack propagation can be performed by u t i l i z a t i o n of a pressure-dependent permeability m u l t i p l i e r which below f r a c t u r e pressure has a co n s t an t value of one a n d above crack pressure equals the assigned f r a c t u r e permeability divided by the product of o r i g i n a l re s e r v o i r permeability and r e l a t i v e permeability to water and re sidua l o i l s a t u r a t i o n . Vertical permeability b a r r i e r s o f t en have a higher undisturbed minimum hor i z o n t a l s t r e s s t h a n the adjacent pay sandstones a n d a re l e s s a ffe c te d by cooling, thereby providing a r e l a t i v e b a r r i e r t o i n h i b i t v e r t i c a l f r a c t u r e g r o w t h . Other aspects of thermally induced waterflood f r a c t u r i n g a re a l s o discussed by PERKINS & GONZALEZ ( 1 9 8 4 ) ; MORALES, ABOU-SAYED, JONES & AL-SAFFAR (1986) a n d WILLIAMS, SHERRARO & L I N ( 1 9 8 7 ) .
659
4.9.5.4. Primary and secondary fractures D i f f e r e n t i n j e c t i o n c o n d i t i o n s can be d i s t i n g u i s h e d . I f t h e i n j e c t i o n r a t e exceeds s l i g h t l y t h e a b i l i t y o f t h e u n f r a c t u r e d f o r m a t i o n t o accept t h e f l u i d , a s h o r t c r a c k extends f r o m t h e w e l l b o r e , and t h e s u r r o u n d i n g r e g i o n o f c o o l e d r o c k i s n e a r l y c i r c u l a r i n shape and o n l y s l i g h t l y e l l i p t i c a l , w i t h t h e r e f o r e the thermoelastic reduction o f h o r i z o n t a l stresses being n e a r l y uniform i n a l l d i r e c t i o n s (PERKINS & GONZALEZ 1985). I f t h e i n j e c t i o n r a t e i s much l a r g e r o r some f r a c t u r e - f a c e damage has occurred, t h e c r a c k extends a g r e a t e r d i s t a n c e from the well, and t h e c o o l e d r e g i o n becomes more e l o n g a t e d i n shape. As t h e c o o l e d area e l o n g a t e s , t h e t h e r m o e l a s t i c r e d u c t i o n i n s t r e s s p a r a l l e l t o t h e f r a c t u r e exceeds t h e t h e r m o e l a s t i c s t r e s s d i m i n u t i o n p e r p e n d i c u l a r l y t o t h e c r a c k which tends t o decrease t h e d i f f e r e n c e between s t r e s s e s w i t h i n t h e c o o l e d r e g i o n t h a t a r e p a r a l l e l and p e r p e n d i c u l a r t o t h e f r a c t u r e , r e s p e c t i v e l y . I f c o n t i n u e d i n j e c t i o n causes t h e f r a c t u r e t o e x t e n d f u r t h e r , a f l a t t e r shape o f t h e c o o l e d area causes t h e s t r e s s e s p a r a l l e l t o t h e c r a c k t o become l e s s than those across t h e f r a c t u r e , and t h u s m i n o r c r a c k s o r j o i n t s perpendicul a r t o t h e main f r a c t u r e would open p r e f e r e n t i a l l y . The shape o f t h e j o i n t e d r e g i o n then presumably a d j u s t s i t s e l f c o n t i n u a l l y t o m a i n t a i n open f r a c t u r e s i n b o t h d i r e c t i o n s as t h e s i z e o f t h e c o o l e d area expanded. The opening o f secondar y c r a c k s p e r p e n d i c u l a r t o t h e main f r a c t u r e system i s a l s o known f r o m g e o t h e r mal w e l l s t i m u l a t i o n where a l s o c o l d w a t e r i s i n j e c t e d i n t o a h o t r e s e r v o i r (BAZANT 1979, BARR 1980; c f . s e c t i o n 4 . 7 . ) .
4.9.5.5. Reservoir pressure, injectivity and fracture length F r a c t u r e e x t e n s i o n p r e s s u r e s a r e a f u n c t i o n o f r e s e r v o i r pressure, permeabil i t y and s i z e o f t h e r e g i o n c o o l e d by i n j e c t i o n w a t e r . Low pay p r e s s u r e enables t h e f l u i d t o l e a k o f f t h r o u g h a r e l a t i v e l y s h o r t crack, whereas h i g h e r r e s e r v o i r p r e s s u r e r e q u i r e s a l o n g e r f r a c t u r e t o accommodate f l u i d a t t h e i n j e c t e d r a t e . I n j e c t i o n o f c o o l w a t e r can reduce e a r t h s t r e s s around i n j e c t i o n w e l l s s u b s t a n t i a l l y , c a u s i n g them t o f r a c t u r e a t p r e s s u r e s c o n s i d e r a b l y l o w e r than would be expected i n t h e absence o f t h e t h e r m o e l a s t i c e f f e c t . Depending on t h e shape o f t h e c o o l r e g i o n and t h e d i f f e r e n c e between minimum and maximum i n - s i t u h o r i z o n t a l e a r t h s t r e s s e s , m i n o r f r a c t u r e s p e r p e n d i c u l a r t o t h e main c r a c k open t h u s c r e a t i n g a j o i n t e d f r a c t u r e system c o n s i s t i n g o f p r i m a r y and secondary f r a c t u r e s . S h o r t c r a c k s r e s u l t i n g f r o m low i n j e c t i o n r a t e , h i g h p e r m e a b i l i t y o r low r e s e r v o i r p r e s s u r e do n o t s i g n i f i c a n t l y change t h e shape o f t h e w a t e r f l o o d f r o n t f r o m t h e c i r c u l a r shape t h a t would be expected f o r an u n f r a c t u r e d w e l l , whereas l o n g e r c r a c k s d e r i v i n g f r o m h i g h i n j e c t i o n r a t e s , low p e r m e a b i l i t y , h i g h pay p r e s s u r e o r poor w a t e r q u a l i t y cause t h e w a t e r f l o o d e d r e g i o n t o become n o t i c e a b l y e l l i p t i c a l i n shape.
4.10. Comnunication fracturing Conventional h y d r a u l i c proppant f r a c t u r i n g i s c o n f i n e d t o one s i n g u l a r hydrocarbon-bearing s t o r e y i n t h e p r o s p e c t i v e column. When s e v e r a l s t i m u l a t i o n j o b s have been c a r r i e d o u t so f a r i n one w e l l i n Europe, t h e r e was g e n e r a l l y a c l e a r temporal succession o f f i r s t p e r f o r m i n g t h e t r e a t m e n t i n t h e lowermost sand, a f t e r w a r d s t e s t i n g and/or p r o d u c i n g i t u n t i l d e p l e t i o n i n case o f success o f t h e s t i m u l a t i o n j o b o r up t o d e c l a r a t i o n as a f a i l u r e , then p a r t i a l l y p l u g g i n g back t h e b o r e h o l e by cementing, sanding-up and/or s e t t i n g b r i d g e s s e a l i n g o f f t h e l o wer h o r i z o n which i s t o be abandoned, and subsequently f r a c t u r i n g s e p a r a t e l y t h e n e x t p o t e n t i a l sandstone h i g h e r up i n t h e s e c t i o n . F o l l o w i n g an o u t l i n e o f c o n v e n t i o n a l s t o r e y - w i s e f r a c t u r i n g i n d i s c r e t e t i m e steps, some aspects o f app l i c a t i o n o f simultaneous communication f r a c t u r i n g i n p r o d u c t i o n and a p p r a i s a l d r i l l i n g as w e l l as f r a c t u r e i n t e r f e r e n c e a r e discussed. Comments a r e a l s o o f f e r e d on i n f l u e n c e o f p e r f o r a t i o n arrangement.
660
4.10.1. Conventional storey-wise f ractur ins Conventional h y d r a u l i c f r a c t u r i n g by s t o r e y - w i s e t r e a t m e n t o f v e r t i c a l l y succ e s s i v e s e p a r a t e r e s e r v o i r s u b u n i t s and i n t e r v a l s i n d i s c r e t e t i m e s t e p s has been a p p l i e d v e r y widespread i n Europe and o t h e r p a r t s o f t h e w o r l d d u r i n g t h e l a s t decades. S t a r t i n g a t t h e b o t t o m o f t h e b o r e h o l e and c l i m b i n g up t h e s t a i r case o f pay zones r e q u i r e s s e a l i n g o f f o f t h e i n f e r i o r u n i t and p e r f o r a t i n g o f t h e s u p e r i o r u n i t a f t e r p l u g g i n g o f t h e b o r e h o l e f l o o r i n each s t i m u l a t i o n s t e p . Some aspects o f r e p e a t e d s t e p w i s e f r a c t u r i n g i n R o t l i e g e n d and C a r b o n i f e r o u s o f Germany FRG, commingling o f separate l a y e r s a f t e r i n d i v i d u a l f r a c t u r i n g , m u l t i l a y e r r e s e r v o i r s w i t h and w i t h o u t c r o s s f l o w , and c o n n e c t i o n o f separ a t e l a y e r s by j o i n t f r a c t u r i n g a r e i l l u s t r a t e d as f o l l o w s .
4.10.1.1. Repeated stepwise fracturing i n Rot 1 iegend and Carboniferous of Germany FRG Examples o f such r e p e a t e d s t e p w i s e f r a c t u r i n g w i t h f o l l o w i n g e x p l o i t a t i o n i n v e r t i c a l l y s u c c e s s i v e r e s e r v o i r s a r e p r e d o m i n a n t l y t h e g a s - b e a r i n g sandstone columns i n t h e Upper C a r b o n i f e r o u s and s u b o r d i n a t e l y a l s o i n p a r t s o f t h e R o t l i e gend ( c f . s e c t i o n s 2.4.1.1. and 3.3.2.) o f N o r t h w e s t Germany ( m a i n l y t h e Wechs e l f o l g e in t h e Hannover-Formation o r Ten Boer-Formation a t t h e t o p where s i m i l a r l y as i n t h e Upper C a r b o n i f e r o u s t h e more o r l e s s t h i n i n d i v i d u a l sandstone h o r i z o n s a r e separated by t h i c k e r o r t h i n n e r mudstone i n t e r v a l s ; BRINKMANN 1982; R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985; c f . s e c t i o n 3 . 2 . 1 . 3 . ) . S e p a r a t i o n o f t h e i n d i v i d u a l t r e a t m e n t s i n R o t l i e g e n d and C a r b o n i f e r o u s w e l l s was achieved by sand p l u g g i n g . I n some cases, t o o small i n t e r v a l s between p e r f o r a t i o n s and i n s u f f i c i e n t cement bond caused communication b e h i n d c a s i n g and reduced t h e s t i m u l a t i o n r a t i o . M u l t i s t a g e t r e a t m e n t s i n c l u d i n g p e r f o r a t i n g , f r a c t u r i n g , s e t t i n g a sand p l u g t o i s o l a t e t h e i n t e r v a l , and moving u p h o l e f o r t h e n e x t stage have a l s o been performed i n t h e C o t t o n V a l l e y F o r m a t i o n (Upper J u r a s s i c ) i n E a s t Texas and Louisiana/USA (JENNINGS & SPRAWLS 1975, HUFFT 1977) and i n m u l t i p l e c o a l seam sequences (SCHRAUFNAGEL & LAMBERT 1988; c f . s e c t i o n 4 . 4 . 3 . ) . Successive m u l t i s t a g e f r a c t u r i n g can be conducted f r o m one h o r i z o n t o t h e n e x t i n upwards d i r e c t i o n by mechanical d i v e r s i o n w i t h sand plugback, s t r a d d l e packer assemblies, and r e t r i e v a b l e b r i d g e p l u g s and packers (HUCKABEE 1987; c f . s e c t i o n 4 . 2 . 2 . 1 . 3 . ) . Conventional s t o r e y - w i s e f r a c t u r i n g w i t h i s o l a t i o n o f t h e l o w e r zone a f t e r t e r m i n a t i o n o f t h e t r e a t m e n t and upwards movement f o r t h e n e x t j o b i s u n a v o i d a b l e i f v e r t i c a l l y s u c c e s s i v e h o r i z o n s have t o be s t i m u l a t e d w i t h d i f f e r e n t f r a c t u r i n g f l u i d types (HOEL 1988) o r c r a c k s w i t h i n them have t o be i n f i l l e d w i t h d i f f e r e n t p r o p p a n t t y p e s due t o s p e c i f i c r e s e r v o i r c o n d i t i o n s t h a t a r e cons i d e r a b l y changing w i t h i n t h e p r o s p e c t i v e complex. I f t h e same f r a c t u r i n g f l u i d and t h e same p r o p p a n t t y p e can be u t i l i z e d f o r s e v e r a l s u c c e s s i v e l a y e r s , then m u l t i p l e f r a c t u r i n g i n one r u n w i t h o u t d i s c r e t e h a n d l i n g o f i n d i v i d u a l h o r i z o n s i s possible ( c f . section 4.2.2.1.3.) provided the i n - s i t u stresses i n a l l the l e v e l s i s comparable t o a l l o w more o r l e s s simultaneous opening o f t h e s e p a r a t e f r a c t u r e s i n a l l t h e zones ( c f . s e c t i o n 6 . 2 . 5 . ) . I n case o f s i g n i f i c a n t i n - s i t u s t r e s s d i f f e r e n c e s between t h e i n d i v i d u a l h o r i z o n s , c o n v e n t i o n a l s t o r e y - w i s e s t i m u l a t i o n i n s t e a d o f m u l t i s t a g e f r a c t u r i n g has t o be performed.
4.10.1.2. Comingling of separate layers after individual fracturing Many o f t h e R o t l i e g e n d and Carboniferous w e l l s i n Germany FRG were s t i m u l a t e d i n d i v i d u a l l y i n s e v e r a l h o r i z o n s and then commingled f o r p r e s s u r e b u i l d - u p a n a l y s i s and p a r t i a l l y a l s o p r o d u c t i o n ( t h u s r e p r e s e n t i n g m u l t i l a y e r l m u l t i f r a c -
661 t u r e systems; BENNETT, REYNOLDS & RAGHAVAN 1982 a, 1982 b; c f . s e c t i o n s 2.4.1.1. and 3 . 3 . 2 . ) . Commingled systems comprise m u l t i s t o r e y r e s e r v o i r s where t h e i n d i v i d u a l l a y e r s do n o t communicate i n terms o f f l u i d f l o w through t h e f o r mation, b u t a l t h o u g h c r o s s f l o w i s absent, t h e y may be completed t o g e t h e r and produced by t h e same w e l l b o r e (KUCHUK & WILKINSON 1988). Many d i f f e r e n t t y p e s of commingled systems e x i s t , w i t h t h e b o r e h o l e b e i n g v e r t i c a l , h o r i z o n t a l , i n c l i n e d , f r a c t u r e d o r p a r t i a l l y p e n e t r a t e d , and t h e i n d i v i d u a l r e s e r v o i r s b e i n g homogeneous, heterogeneous o r f r a c t u r e d , w i t h d i f f e r e n t o u t e r boundary c o n d i t i o n s such as i n f i n i t e e x t e n t , c o n s t a n t p r e s s u r e o r n o - f l o w b e i n g developed. I n t h e R o t l i e g e n d and C a r b o n i f e r o u s f o r m a t i o n s i n Germany FRG, t h e mudstone beds between t h e d i f f e r e n t f r a c t u r e d h o r i z o n s a r e t h i c k e r t h a n necessary f o r containment o f t h e cracks, w i t h communication between t h e h o r i z o n s t h u s o n l y b e i n g expected t o o c c u r t h r o u g h t h e w e l l b o r e (REINICKE, BRINKMANN, SCHWARZ & HUENI 1985), w i t h t h e r e f o r e no t r u e commingling by c o n t a c t and exchange o n l y v i a t h e f r a c t u r e i n c o n t r a s t t o v e r t i c a l communication everywhere i n t h e r e s e r v o i r (BENNETT, REYNOLDS & RAGHAVAN 1982 b ) b e i n g g i v e n (commingfing comprises communication o f s e v e r a l l a y e r s o f t h e r e s e r v o i r s t o r e y complex o n l y through t h e w e l l b o r e s ; ONUR, REYNOLDS & RAGHAVAN 1986).
4.10.1.3.
Multilayer reservoirs with and without crossflow
Aspects o f m u l t i p l e - z o n e f r a c t u r i n g a r e a l s o d i s c u s s e d by AHMED, NEWBERRY & CANNON (1985); EASON (1985), CHOO & WU (1987) and SULLIVAN, LEE & HOLDITCH (1987), and some p o i n t s o f commingled mu1 t i l a y e r e d r e s e r v o i r s a r e r e p o r t e d by COBB, RAMEY & MILLER (1972); LARSEN (1981), GREENFIELD & AHMED (1983); ONUR, REYNOLDS & RAGHAVAN (1986); CHEN (1987); HATZIGNATIOU, OGBE, DEHGHANI & ECONOMIDES (1987) and SMITH, MILLER & HAGA ( 1 9 8 7 ) . SULLIVAN, LEE & HOLDITCH (1987) p r e s e n t a s p e c i a l case o f m u l t i l a y e r r e s e r v o i r s where small t h i n h i g h - p e r m e a b i l i t y channel sands o f v e r y l i m i t e d a r e a l e x t e n t a r e a l r e a d y n a t u r a l l y commingled w i t h much l a r g e r t h i c k e r l o w e r - p e r m e a b i l i t y b l a n k e t sands o f r e l a t i v e l y l a r g e areal extent. DEANS
&
GAO (1983), GAO (1983), GAO & DEANS (1983), PRATS (1983); PRIJAMBO-
DO, RAGHAVAN & REYNOLDS (1985); WIJESINGHE & KECECIOGLU (1986 b ) and LEE & OLA-
REWAJU (1988) o u t l i n e m u l t i l a y e r r e s e r v o i r s w i t h c r o s s f l o w , and EARLOUGHER, KERSCH & KUNZMAN (1974); RAGHAVAN, TOPALOGLU, COBB & RAMEY (1974); OTUOMAGIE & MENZIE (1976); KUCUK, KARAKAS & AYESTARAN (1984); KUCUK, SHAW, AYESTARAN & N I CHOLSON (1986); FETKOVICH, BRADLEY, WORKS & TRASHER (1988) and KUCHUK & WILKINSON (1988) comment on m u l t i l a y e r pays w i t h o u t c r o s s f l o w . OLAREWAJU & LEE (1988) d i s c u s s combinations o f c r o s s f l o w and commingling. LEUNG (1986 b) p r e s e n t s d u a l - p o r o s i t y and - p e r m e a b i l i t y a q u i f e r s as w e l l as two i n t e r c o n n e c t e d s i n g l e porosity aquifers.
4.10.1.4. Connection of separate layers by joint fracturing J o i n t f r a c t u r i n g o f s e v e r a l v e r t i c a l l y superimposed sandstones by i n j e c t i o n o f proppants and f l u i d s through s e v e r a l p e r f o r a t e d i n t e r v a l s ( c f . s e c t i o n 4.2.2.) w i t h i n one j o b (BENNETT, REYNOLDS & RAGHAVAN 1982 b ) , w i t h t h e i n d i v i dual f r a c t u r e s r e m a i n i n g c o n t a i n e d w i t h i n t h e i n d i v i d u a l h o r i z o n s , have h i t h e r t o n o t been c a r r i e d o u t i n Europe beyond t h e p o i n t o f e x p e r i m e n t a l stage. I n a l l t h e cases, so f a r c r e a t i o n of a v e r t i c a l o r h o r i z o n t a l communication f r a c t u r e between s e v e r a l s i m i l a r o r d i f f e r e n t r e s e r v o i r f a c i e s zones t h r o u g h a s e p a r a t i n g b a r r i e r has g e n e r a l l y n o t been attempted i n European hydrocarbon p r o d u c t i o n w e l l s and has o n l y i n a few cases been made i n w a t e r - i n j e c t i o n w e l l s near t h e margin o f t h e f i e l d i n o r d e r t o i n c r e a s e t h e w a t e r uptake, w i t h t h e des i g n o f f r a c t u r e p r o p a g a t i o n h a v i n g g e n e r a l l y aimed on confinement w i t h i n one h o r i z o n i n o r d e r n o t t o break t h r o u g h t h e bounding s e a l s o f t h e r e s e r v o i r . L i m i t e d communication f r a c t u r i n g between two v e r t i c a l l y superimposed pay i n t e r v a l s
662 has been p e r f o r m e d i n t h e N o r t h Sea c h a l k (SMITH, MILLER & HAGA 1987; c f . sect i o n 4.5.4.) where o n l y one zone has been p e r f o r a t e d and t h e f r a c t u r e has been extended i n t o t h e o t h e r horizon by i n t e r f a c e f r a c t u r i n g ( c f . s e c t i o n 4.5.4.4.2; some a s p e c t s o f t h i s t e c h n i q u e a r e a l s o d i s c u s s e d b y HOLM 1975; AHMED, WILSON & STRAWN 1983 and BRITT & LARSEN 1 9 8 6 ) . M u l t i p l e - s t a g e f r a c t u r i n g and communicat i o n t r e a t m e n t s , however, have a l r e a d y been f r e q u e n t l y c a r r i e d o u t i n t h e USA and o c c a s i o n a l l y a l s o i n o t h e r p a r t s o f t h e w o r l d . Some comments a r e o f f e r e d h e r e c o n c e r n i n g a p p l i c a t i o n o f c o m m u n i c a t i o n f r a c t u r i n g in p r o d u c t i o n a n d app r a i s a l d r i l l i n g as w e l l as f r a c t u r e i n t e r f e r e n c e as f o l l o w s .
4.10.2. Application in production drilling P r o d u c t i o n p r o b l e m s i n h y d r o c a r b o n - b e a r i n g f o r m a t i o n s w i t h c o m p l i c a t e d geom e t r y o f sand b o d i e s i n h o r i z o n t a l a n d / o r v e r t i c a l d i r e c t i o n , however, s u p p o r t a t l e a s t f o r t h e f u t u r e a m e l i o r a t i o n o f e x p l o i t a t i o n ( g i v e n an e c o n o m i c a l l y more f a v o u r a b l e o i l p r i c e s c e n a r i o w h i c h i s e x p e c t e d t o be r e a c h e d s o o n e r o r l a t e r ; OEHME 1986; and i n Europe a l s o n e c e s s a r i l y enhanced b y an a g a i n s t r o n g e r U S $; c f . s e c t i o n 2 . 2 . 1 . 3 . ) t h e p o s s i b i l i t y o f p a r t i a l l y b e i n g s o l v e d w i t h unc o n v e n t i o n a l c o m m u n i c a t i o n f r a c t u r i n g where h o r i z o n t a l o r v e r t i c a l c o n n e c t i o n i s made between s e v e r a l f a c i e s zones, t h e r e b y a d a p t i n g t o p r o c e d u r e s w h i c h a r e a l r e a d y common i n t h e USA s i n c e q u i t e some t i m e . Some a s p e c t s o f c o n n e c t i o n o f i s o l a t e d sand b o d i e s and s e p a r a t e d r e s e r v o i r s t o r e y s a r e b r i e f l y o u t l i n e d as follows.
4.10.2.1. Connection o f isolated sand bodies Examples o f a p p l i c a t i o n o f c o m m u n i c a t i o n s t i m u l a t i o n c o m p r i s e f r a c t u r i n g f r o m one i s o l a t e d c h a n n e l s a n d s t o n e i n t o a n o t h e r i n h o r i z o n t a l d i r e c t i o n i n o r d e r t o i m p r o v e w a t e r - and s t e a m - f l o o d p r o j e c t s o r t o i n c r e a s e t h e p r o d u c t i o n r a dius, f r a c t u r i n g f r o m one h i g h - p e r m e a b i l i t y zone t o a n o t h e r i n h o r i z o n t a l o r v e r t i c a l d i r e c t i o n t h r o u g h a s e p a r a t i n g l o w - p e r m e a b i l i t y sandy o r muddy b a r r i e r ( w h e r e some o p e r a t i o n a l d i f f i c u l t i e s o f s e l f - c o n t a i n m e n t o f t h e f r a c t u r e w i t h i n one s a n d s t o n e body s t i l l have t o be overcome by f i n d i n g t h e r i g h t p r e s s u r e d i s t r i b u t i o n f o r p e r f o r m i n g t h e c o m m u n i c a t i o n f r a c t u r e a c r o s s l i t h o f a c i e s boundar i e s , b u t n e v e r t h e l e s s k e e p i n g h o r i z o n t a l and v e r t i c a l c r a c k p r o p a g a t i o n r e a s o n a b l y u n d e r c o n t r o l ) i n o r d e r t o enhance t h e d r a i n a g e r a d i u s o r t o a l l o w j o i n t i n s t e a d o f s e l e c t i v e c o m p l e t i o n i n one w e l l , and f r a c t u r i n g f r o m one b o r e h o l e t o a n o t h e r f o r i m p l e m e n t i n g a g u i d e c h a n n e l f o r t h e p r o p a g a t i o n o f t e n s i d s , pol y m e r s o r steam d u r i n g t e r t i a r y o i l r e c o v e r y i n m a r g i n a l r e s e r v o i r s o r f o r a l l o w i n g e x p l o i t a t i o n o f g e o t h e r m a l e n e r g y b y pumping c o l d w a t e r t h r o u g h a h o t d r y f o r m a t i o n where i t h e a t s up ( c f . c h a p t e r 4 . 7 . ) . Some a s p e c t s o f v e r t i c a l i n t e r f e r e n c e t e s t i n g a c r o s s a l o w - p e r m e a b i l i t y zone w h i c h i s p a r t i a l l y based on c o m m u n i c a t i n g f r a c t u r e s a r e d i s c u s s e d b y EHLIG-ECONOMIDES & AYOUB ( 1 9 8 6 ) . The o u t l i n e as f o l l o w s f o c u s s e s on m a g n i t u d e o f i n - s i t u s t r e s s c o n t r a s t , f r a c t u r e w i d t h v s . l e n s a x i s o r i e n t a t i o n , and sand body s i z e and s p a c i n g .
4.10.2.1.1. Magnitude o f in-situ stress contrast Communication f r a c t u r i n g f o r t h e p u r p o s e o f c o n n e c t i o n o f many i s o l a t e d sands t o n e u n i t s o f l e n t i c u l a r o r r i b b o n - t y p e c h a r a c t e r t o t h e w e l l b o r e , however, i s o n l y f e a s i b l e i n c a s e o f p r e s e n c e o f s m a l l t o a l m o s t n e g l i g i b l e (in t e r m s o f r o c k m e c h a n i c s ) i n - s i t u s t r e s s c o n t r a s t s between d i f f e r e n t l i t h o l o g i e s (NORTHROP 1988, NORTHROP & FROHNE 1 9 8 8 ) . I n c a s e o f l a r g e i n - s i t u s t r e s s d i f f e r e n ces, a h y d r a u l i c f r a c t u r e does n o t e a s i l y b r e a k o u t o f a l o w - s t r e s s s a n d s t o n e , traverse the high-stress confining rocks i n horizontal o r v e r t i c a l direction, and c r o s s a n o t h e r s a n d s t o n e o f c o n t i n u o u s o r i s o l a t e d n a t u r e . Thus i n t e r s e c t i o n , p r o p p i n g and d r a i n a g e o f a l a r g e number o f r e m o t e r e s e r v o i r u n i t s i s h a r d l y f e a s i b l e i n t h e presence o f l a r g e s t r e s s c o n t r a s t s . Models o f f r a c t u r e g r o w t h t h r o u g h l i t h o l o g i c a l o r t e c t o n i c a l b a r r i e r s a r e p r e s e n t e d b y LUISKUTTY,
663
TOMUTSA & PALMER (1986). R e s e r v o i r communication can be checked by m u l t i p l e i n t e r f e r e n c e t e s t i n g which enables d e t e c t i o n o f pay l i m i t s and e x t e n s i o n o f t h e c o r r e s p o n d i n g b l o c k s (EARLOUGHER & RAMEY 1973; LEAVER, SAGEEV & RAMEY 1986; c f . section 4.2.4.1.2.2.).
4.10.2.1.2. Fracture azimuth vs. lens axis orientation Connection o f i s o l a t e d sand bodies by communication f r a c t u r i n g i s v e r y import a n t i n l e n t i c u l a r t i g h t gas sandstones where r e c o v e r y e f f i c i e n c y and t h u s economical f e a s i b i l i t y i n c r e a s e s w i t h c r a c k p e n e t r a t i o n t h r o u g h s e p a r a t e i e n s e s t h a t a r e n o t i n d i r e c t c o n t a c t w i t h t h e b o r e h o l e (KUUSKRAA, BRASHEAR, ELKINS & MORRA 1979; c f . s e c t i o n 3 . 4 . 1 . 1 . 2 . ) . A t a r a t e o f one a d d i t i o n a l l e n s accessed by t h e h y d r a u l i c f r a c t u r e f o r each l e n s encountered a t t h e w e l l b o r e , t h e i n i t i a l ten-year gas r e c o v e r y c o u l d be up t o 70 % h i g h e r than f o r t h e s i n g l e - l e n s case. F r a c t u r e azimuth as a consequence o f h o r i z o n t a l s t r e s s d i r e c t i o n i n r e l a t i o n s h i p t o l o n g a x i s o r i e n t a t i o n o f sand l e n s e s i s a d e c i s i v e f a c t o r i n c o n t r o l l i n g f r a c t u r e l e n g t h , and spacing and diameter o f sand l e n s e s i n h o r i z o n t a l and v e r t i c a l d i r e c t i o n a l s o i n f l u e n c e s c r a c k h e i g h t . Maximum f r a c t u r e l e n g t h can be achieved i f t h e c r a c k propagates p a r a l l e l t o t h e l o n g a x i s o f t h e i s o l a t e d sand body. I f t h e f r a c t u r e i s o r i e n t e d p e r p e n d i c u l a r o r o b l i q u e t o sand l e n s l o n g a x i s , f r a c t u r e l e n g t h has t o be k e p t s h o r t e r u n l e s s n e i g h b o u r i n g sand l e n s e s can be accessed by t h e same c r a c k and can a l s o be connected t o t h e w e l l b o r e . The importance o f f r a c t u r e e x t e n s i o n t o t h e r e s e r v o i r l i m i t s o f l e n t i c u l a r sand b o d i e s i s a l s o s t r e s s e d by PETERSON & KOHOUT (1983), and MEEHAN, HORNE & A Z I Z (1988) o u t l i n e e f f e c t s o f r e s e r v o i r h e t e r o g e n e i t y and c r a c k azimuth on o p t i m i z a t i o n o f f r a c t u r e l e n g t h and w e l l spacing.
4.10.2.1.3. Sand body size and spacing Communication f r a c t u r i n g between separated sand l e n s e s depends i n d e s i g n on s i z e and s p a c i n g o f t h e i s o l a t e d b o d i e s which i s a f u n c t i o n o f p r i m a r y deposit i o n a l environment and secondary e r o s i o n a l condensation o f t h e sedimentary sequence ( c f . MADER 1985 a ) . L i m i t e d n a t u r a l i n t e r c o n n e c t i o n o f o r i g i n a l sand bod i e s may even s t i l l p e r s i s t i n m u l t i s t o r e y and m u l t i l a t e r a l complexes o f v e r t i c a l l y and h o r i z o n t a l l y amalgamated p o l y l e n t i c u l a r aggregates due t o s e p a r a t i n g mud drapes ( c f . p l a t e s III/l- 2, I V / 4 - 8, V I I / 5 - 6 and I X / 1 - 8 ) and g r a i n s i z e f l u c t u a t i o n s i n sand-dominated cyclothems. T h e r e f o r e even f r a c t u r e i n t e r s e c t i o n o f such heterogeneous sand l e n s assemblages cannot c o m p l e t e l y remove t h e phenomenon t h a t some r e s e r v o i r i n t e r v a l s a r e q u i c k l y d r a i n e d w h i l e o t h e r nearby segments a r e o n l y s l o w l y d e p l e t e d (WARPINSKI, BRANAGAN, SATTLER, CIPOLLA, LORENZ & THORNE 1988). I n s u f f i c i e n t n a t u r a l o r i g i n a l sand body i n t e r c o n n e c t i o n by p r i m a r y - d e p o s i t i o n a l o r secondary-erosional j u x t a p o s i t i o n has t o be a r t i f i c i a l l y compensated by communication f r a c t u r i n g i n o r d e r t o a l l o w f o r p r o f i t a b l e r e s e r v o i r a c q u i s i t i o n and d r a i n a g e .
4.10.2.2. Connection o f separated reservoir storeys Communication f r a c t u r i n g f r o m one r e s e r v o i r s t o r e y t o a n o t h e r a i m i n g on l a t e r j o i n t c o m p l e t i o n and p r o d u c t i o n i s o n l y reasonable i f b o t h h o r i z o n s a r e n o t t o o f a r away i n v e r t i c a l d i r e c t i o n and t h e i r t h i c k n e s s i s i d e a l l y each c l o s e t o m a r g i n a l , w i t h t h e r e f o r e s i n g l e c o m p l e t i o n and p r o d u c t i o n b e i n g h a r d l y f e a s i b l e . While two superimposed hydrocarbon-bearing s t r a t a can a l s o be e x p l o i t e d by two pumps i n dual b o r e h o l e c o m p l e t i o n , m u l t i p l e l a y e r - c a k e - t y p e o i l - and gas-bearing s e c t i o n s w i t h c r i t i c a l t h i c k n e s s e s , p e t r o p h y s i c a l p r o p e r t i e s and t h u s a l s o p r o d u c t i v i t i e s o f t h e i n d i v i d u a l sheets can o n l y be e c o n o m i c a l l y exp l o i t e d by communication f r a c t u r i n g and t h e n f o l l o w i n g j o i n t d e p l e t i o n .
664 Considerable s i g n i f i can ce of communication f r a c t u r i n g i s seen f o r c re a ting a connection path between l a t e r a l l y or diagonally c oe xisting s i m i l a r f a c i e s bod ie s t h a t a r e separated by t i g h t i n t er v en t i o ns, in order t o allow proper performance of secondary and t e r t i a r y o i l recovery by guaranteing the a r r i v a l of the in j e c t e d f l u i d s o r gases a t the margin of the drainage cone of the production well and not being stuck a t f a c i e s an i s otropie s i n heterogeneous r e s e r v o i r s (the experience of o i l e x p l o i t a t i o n in Middle Europe in the l a s t decades has shown t h a t f r e q u en t l y f a i l u r e s of water flooding were caused by i n t e r c a l a t e d impermeable f a c i e s u n i t s which prevented communication between i n j e c t i o n and production w e l l s ) . A special type of communication f r a c t u r i n g i s generation of connections from steam i n j e c t i o n wells t o neighbouring production boreholes via a horizontal f r a c t u r e according t o the shallow r e s e r v o i r depth of most heavy o i l de posits (BRITTON, MARTIN, LEIBRECHT & HARMON 1982; c f . se c tion 4 . 9 . 4 . 2 . ) . An a l t e r n a t i v e method t o communication f r a c t u r i n g i s connection of v e r t i c a l wellbores by l a t e r a l boreholes or drainholes which resemble hydraulic f r a c t u r e s t h a t a re reduced from a plane t o a l i n e and condensed in the wellbore tube ( F I N C H E R 1985; c f . s e c t i o n 4 . 8 . 6 . 1 . ) . This method, however, i s only s u i t a b l e f o r t h i n r e s e r v o i r sands, whereas connection of boreholes through thick pay zone bodies can only be performed by l ar g e- s cal e hydraulic p r o p p a n t f r a c t u r i n g .
4.10.3. I n f l u e n c e o f p e r f o r a t i o n arrangement The success of communication f r a c t u r i n g between several boreholes can be optimized by an o r i e n t a t i o n of the l i n e connecting the boreholes p a r a l l e l t o the g e o s t r e ss f i e l d by ap p r o p r i at e well l o cat i o n p r i o r t o d r i l l i n g of the t a r g e t boreholes, a n d by improving f r a c t u r e propagation through both well axes by o r i e n t e d p r e c i s i on p er f o r at i o n s (JARED, SCOTT & EVANS 1984). Aspects of perforatio n placement optimization a r e a l s o discussed by McLEOD (1978); AHMED, THOMPSON, KELKAR, VEGHTE & HATHAWAY (1984); TARIQ, ICHARA & AYESTARAN (1985); HUSHBECK (1986), SCOTT (1986) a n d I C H A R A & CANNON (1987). CHENEVERT (1985) comments on p e r f o r a t i o n s t a b i l i t y in low-permeability gas r e s e r v o i r s , a n d TODD & BRADLEY (1986) i l l u s t r a t e the influence of p er f o ra tion geometry on well produc tivity and f r a c t u r e performance. AHMED & K E L L E R (1984) document the s i g n i f i c a n c e of p e r f o r a t i o n placement techniques f o r modification of hydraulic f r a c t u r i n g t r e a t ments. The following o u t l i n e d i s cu s s es some aspects of pe rfora tion o r i e n t a t i o n and s t r e s s d i r e c t i o n , angular d i s t r i b u t i o n of pe rfora tions, a n d breakdown of perforations.
4.10.3.1. P e r f o r a t i o n o r i e n t a t i o n and s t r e s s d i r e c t i o n Concerning r e l a t i o n s h i p s between p er f o r at i ons and hydraulic f r a c t u r e s , the influence of p e r f o r at i o n s on the created hydraulic crack depends on s t r e s s d i s t r i b u t i o n a r o u n d the wellbore, because cracks o r i g i n a t e whenever the maximum t e n s i l e s t r e s s induced a t the borehole w a l l exceeds t h a t of the formation ( D A NESHY 1973 b ) . I n terms of arrangement, a h elic a l d i s t r i b u t i o n of the perforatio n s enables b e t t e r matching of the r e s e r v o i r s t r e s s d i r e c t i o n s than l i n e a r rows where coincidence with the o r i e n t a t i o n of formation s t r e s s may be d i f f i c u l t t o achieve, because p er f o r at i o n l i n e s may not be in a d i r e c t i o n c lose t o the f r a c t u r e plane. I n comparison t o open holes, the e xiste nc e of a pe rfora te d casing g r e a t l y i ncr eas es breakdown pressure t o almost twice i t s value. W i t h progressive deviation o f the p er fora tions from the expected f r a c t u r e plane, the chances t h a t cracks i n i t i a t e from pe rfora tions decreases, with t h u s hydraulic f r a c t u r e s not n eces s ar i l y i n s e r t i n g a t the pe rfora tions. BRANAGAN & WILMER (1988) i l l u s t r a t e the two extreme cases of la rge - or small-scale f r a c t u r e s r u n n i n g p a r a l l e l o r orthogonal t o p er f ora tion tunne ls. Cracks propagating p a r a l l e l t o p e r f o r at i o n holes a r e the continuous f u r t h e r extension of them away from the wellbore in case of p er f o r at i o n tunnel d i r e c t i o n matching with geo-
665 s t r e s s o r i e n t a t i o n ( w i t h then g r a v e l - f i l l e d p e r f o r a t i o n h o l e s p r o v i d i n g a smooth t r a n s i t i o n f r o m mechanical sand c o n t r o l t o r e s e r v o i r f r a c t u r i n g s t i m u l a t i o n ; c f . s e c t i o n 5.3.5.). Propagation o f the large- o r small-scale cracks perp e n d i c u l a r l y t o t h e p e r f o r a t i o n t u n n e l s and i n s e r t i n g a t t h e end o f it, however, p r o v i d e s o n l y v e r y l i m i t e d c o n t a c t t o t h e p e r f o r a t i o n h o l e i n c a s i n g and cement sheath and t h u s i s c o n s i d e r a b l y i m p a i r i n g t h e e f f e c t o f t h e s t i m u l a t i o n o r breakdown t r e a t m e n t i n case o f p e r f o r a t i o n t u n n e l d i r e c t i o n b e i n g normal t o geostress o r i e n t a t i o n .
4.10.3.2. Angular distribution of perforat ions Experiments w i t h two d i a m e t r i c a l l y o p p o s i t e l i n e s o f p e r f o r a t i o n s i n t h e cas i n g showed t h a t f o r an a n g l e between p e r f o r a t i o n s and expected f r a c t u r e p l a n e up t o 30 degrees, most o f t h e c r a c k s s t a r t t o propagate f r o m p e r f o r a t i o n s on b o t h s i d e s o f t h e hole, whereas f o r an a n g l e up t o 60 degrees, t h e f r a c t u r e s u s u a l l y e x t e n d o n l y f r o m one s i d e and i g n o r e t h e p e r f o r a t i o n s on t h e o t h e r s i d e (DANESHY 1973 b, BRANAGAN & WILMER 1988). F o r an a n g l e up t o 90 degrees, t h e f r a c t u r e m o s t l y i g n o r e s a l l t h e p e r f o r a t i o n s and i n i t i a t e s on t h e b o r e h o l e w a l l perpendicularly t o the l e a s t l a t e r a l p r i n c i p a l stress, w i t h s t i m u l a t i o n f l u i d e n t e r i n g t h e p e r f o r a t i o n s and t r a v e l l i n g between f o r m a t i o n and o u t e r c a s i n g w a l l b e f o r e r e a c h i n g t h e f r a c t u r e . T h i s f l u i d m i g r a t i o n can e a s i l y become t h e source o f s c r e e n o u t f a i l u r e due t o p r o p p a n t b r i d g i n g i n t h e narrow p a t h which causes p l u g g i n g o f t h e c r a c k by s l u r r y s t i c k i n g and p r o p p a n t b l o c k i n g . H e l i c a l c o n f i g u r a t i o n o f p e r f o r a t i o n arrangement assures t h a t a t l e a s t some p e r f o r a t i o n s a r e always v e r y c l o s e t o t h e f r a c t u r e p l a n e i n terms o f a n g u l a r r e l a t i o n ship.
4.10.3.3. Perforat ion breakdown Breakdown p r e s s u r e d i f f e r e n c e s show t h a t i f t h e f r a c t u r e o r i e n t a t i o n i s known w i t h s u f f i c i e n t c e r t a i n t y , i t i s b e t t e r t o p e r f o r a t e t h e w e l l a l o n g a l i n e i n t h e c r a c k p l a n e than t o p e r f o r a t e h e l i c a l l y . S h o r t e r p e r f o r a t i o n s have a l o w e r breakdown p r e s s u r e than l o n g e r ones, and l a r g e r p e r f o r a t i o n s l e a d t o a l o w e r t e n s i l e s t r e n g t h o f t h e f o r m a t i o n t h a n s m a l l e r ones. P r e c i s i o n p e r f o r a t i o n breakdown h e l p s t o i n c r e a s e t h e e f f e c t i v i t y o f f r a c t u r i n g s t i m u l a t i o n s (HUSHBECK 1986). The b e n e f i t s o f t h i s t e c h n i q u e i n c l u d e c l e a n i n g o u t o f p e r f o r a t i o n s and s l i g h t f o r m a t i o n breakdown i n o r d e r t o g i v e t h e t r e a t m e n t an opening t o b e g i n an a c i d j o b o r h y d r a u l i c f r a c t u r i n g o p e r a t i o n . I n a d d i t i o n , t h e s t a t u s o f p e r f o r a t i o n s i s e s t a b l i s h e d , which i s i m p o r t a n t due t o t h e s e n s i t i v i t y o f pumping r a t e s and p r e s s u r e s t o number and diameter o f p e r f o r a t i o n s . Limited-entry s t i m u l a t i o n ( c f . section 4.2.2.2.) i s considerably aided by d e t a i l e d knowledge o f p e r f o r a t i o n s t a t u s .
4.10.4. Application in appraisal drilling Communication f r a c t u r i n g f r o m a poor f a c i e s b e l t i n t o t h e a d j o i n i n g b e t t e r zone o f r e s e r v o i r development c o u l d s u p p o r t and improve t h e a p p r a i s a l d r i l l i n g s t r a t e g y concepts o f f i e l d s i n c o m p l i c a t e d f o r m a t i o n s w i t h f a c i e s changes t h a t a r e d i f f i c u l t t o p r e d i c t and i n s i t u a t i o n s where hydrocarbon-bearing sandstones l a p on and o f f a basement w i t h pronounced t o p o g r a p h i c a l h i g h s and lows ( s u c h as a s s o c i a t e d w i t h u n c o n f o r m i t i e s and p a l a e o r e l i e f s ; c f . p l a t e I / 1 - 7 ) . Convention a l f i e l d a p p r a i s a l so f a r i n c l u d e d p l u g g i n g back and p o s s i b l e s i d e t r a c k i n g o f a w e l l t h a t encountered a n o n - r e s e r v o i r f a c i e s b e l t o r a basement h i g h , w i t h t h e d e v i a t i o n o f t h e b o r e h o l e a i m i n g on r e a c h i n g a b e t t e r f a c i e s zone o r meeti n g t h e r e s e r v o i r r o c k s i n depressions between t h e basement e l e v a t i o n s ( a s p e c t s o f palaeotopography on e x p l o r a t i o n and a p p r a i s a l w e l l s i t i n g and c o n s e q u e n t l y a l s o h y d r a u l i c p r o p p a n t f r a c t u r i n g p l a n n i n g a r e discussed by THOMAS, SONNENBERG, LANGENBAHN & MURPHY 1988).
666 The p u r p o s e o f r e s e r v o i r f a c i e s b e l t b r i d g i n g and a p p r a i s a l d r i l l i n g i m p r o v e ment c o u l d a t l e a s t i n some c a s e s be r e a c h e d c o n s i d e r a b l y c h e a p e r by s m a l l e r o r l a r g e r - s c a l e h y d r a u l i c p r o p p a n t f r a c t u r i n g i n s t e a d o f c o m p a r a t i v e l y expensive directional sidetrack d r i l l i n g , with the p a r t i a l l y big difference i n inv e s t m e n t a t l e a s t j u s t i f y i n g t o t r y t o t u r n t h e p r e s e n t f a i l u r e i n t o a commerc i a l producer by c a r r y i n g o u t a communication f r a c t u r i n g t r e a t m e n t b e f o r e i n case o f n e g a t i v e r e s u l t o f t h e s t i m u l a t i o n , p a r t i a l p l u g g i n g back and d e v i a t i o n d r i l l i n g i s s t i l l a r e m a i n i n g p o s s i b i l i t y b e f o r e f i n a l l y c e m e n t i n g and abandoni n g t h e w h o l e w e l l . Thus a l s o c o m m u n i c a t i o n f r a c t u r i n g i s e x p e c t e d t o be i n var i o u s c a s e s a s u i t a b l e method t o r e d u c e t h e o v e r a l l d e v e l o p m e n t expenses p a r t i c u l a r l y in l i g h t o f n e c e s s a r y c o s t c o n t a i n m e n t when t h e g e n e r a l e c o n o m i c a l s i t u a t i o n r e s t r i c t s t h e d r i l l i n g budget. D r i l l i n g o f a p p r a i s a l w e l l s in f r a c t u r e d r e s e r v o i r s and s u b s e q u e n t a i m i n g o n connection o f t h e borehole s e c t i o n t o a nearby l a r g e - s c a l e n a t u r a l crack o r f a u l t b y a h y d r a u l i c f r a c t u r i n g t r e a t m e n t c a n be c o n s i d e r a b l y o p t i m i z e d b y immed i a t e l y d r i l l i n g t h e w e l l on i n t e r s e c t i o n s o f m a j o r n a t u r a l f r a c t u r e s as r e v e a l e d f r o m p h o t o g r a m m e t r y and i m a g e r y (REYNOLDS 1987; c f . s e c t i o n 6 . 2 . 7 . ) .
4.10.5.
Fracture interference
W h i l e c o m m u n i c a t i o n f r a c t u r i n g f r o m one b o r e h o l e t o a n o t h e r i n c l u d e s o n l y one h y d r a u l i c s t i m u l a t i o n o p e r a t i o n in o n l y one s o u r c e w e l l , w i t h t h e c r a c k then p r o p a g a t i n g t o t h e t a r g e t borehole, i n t e r f e r e n c e o f several f r a c t u r e s can o c c u r when a c o u p l e o f n e i g h b o u r i n g w e l l s in gas o r o i l f i e l d s a r e s e p a r a t e l y t r e a t e d and i f t h e i r c o n n e c t i o n l i n e i s i n a c c o r d a n c e w i t h t h e g e o s t r e s s s y s tem. I n c o n t r a s t t o t h e d e s i r e d e f f e c t o f c o m m u n i c a t i o n f r a c t u r i n g o f s e v e r a l b o r e h o l e s b y one c r a c k , f r a c t u r e i n t e r f e r e n c e b y o v e r l a p p i n g o r c o n n e c t i o n o f s e v e r a l c r a c k s p r o p a g a t i n g f r o m d i f f e r e n t w e l l s may be unwanted in o i l and gas exploitation, as t h i s e f f e c t s e r i o u s l y r e d u c e s t h e a t t a i n a b l e p r o d u c t i v i t y imp r o v e m e n t f o r each b o r e h o l e b y d i m i n i s h i n g d r a i n a g e r a d i u s and o f f t a k e a r e a o f t h e i n d i v i d u a l w e l l (VISSER & DOMSELAAR 1974, SCHOLS 1983, LEICHT 1 9 8 5 ) . C a r e f u l p l a n n i n g o f b o r e h o l e s p a c i n g , f r a c t u r e l e n g t h ( w h i c h depends o n d r a i nage r a d i u s and i s a l s o l i n k e d t o f r a c t u r e c o n d u c t i v i t y ; McDANIEL & WILLINGHAM 1978, MONTGOMERY & STEANSON 1985; c f . s e c t i o n 4 . 8 . 1 1 . ) and w e l l o r i e n t a t i o n has t h u s t o be made p r i o r t o j o b e x e c u t i o n i n o r d e r t o o p t i m i z e h y d r o c a r b o n p r o d u c t i o n w i t h i n f i e l d s of l i m i t e d e x t e n s i o n and r e s o u r c e s . The s i g n i f i c a n c e o f f r a c t u r e a z i m u t h p r o g n o s i s f o r t h e improvement o f h y d r o c a r b o n r e c o v e r y i s a n a l y z e d b y HASSAN ( 1 9 8 2 ) . F r a c t u r e i n t e r f e r e n c e assessment i s d i f f e r e n t i n h y d r o c a r b o n and g e o t h e r m a l r e s e r v o i r s .
4.10.5.1. O i l
and gas reservoirs
I n o i l and gas f i e l d s w i t h more o r l e s s n a r r o w s p a c i n g o f w e l l s w i t h i n p r e s s u r e - and flow-homogeneous r e s e r v o i r b l o c k s , c a u t i o n has n o t o n l y t o be e x e r c i sed f o r a v o i d i n g d i r e c t f r a c t u r e i n t e r f e r e n c e b y merging o f s e v e r a l intit ;dual cracks t o a d e n d r i t i c system ( c f . s e c t i o n 4.8.8.4.), b u t a l s o f o r p r e v e n t i n g i n t e r f e r e n c e o f cracks emanating from t h e i n d i v i d u a l boreholes w i t h t h e drainage cones o f o t h e r w e l l s . T h e r e f o r e t h e t i p - t o - t i p l e n g t h o f t h e f r a c t u r e s does n o t o n l y have t o be r e a s o n a b l y i n t o l e r a n c e w i t h t h e w e l l - t o - w e l l distances, but s h o u l d be d e s i g n e d i n such a way t h a t i n t e r s e c t i o n o f n e i g h b o u r i n g d r a i n a g e c o nes does n o t o c c u r . The f o l l o w i n g d i s c u s s i o n i l l u s t r a t e s a s p e c t s o f f r a c t u r e l e n g t h and w e l l d r a i n a g e r a d i u s , f r a c t u r e o r i e n t a t i o n p r e d i c t i o n and i n t e r f e r e n c e a v o i d a n c e , l e n t i c u l a r t i g h t gas sands, and s t e a m - d r i v e o p e r a t i o n .
667
4.10.5.1.1. Fracture length and well drainage radius I n t i g h t gas r e s e r v o i r s o f m i c r o d a r c y p e r m e a b i l i t y range, f r a c t u r e l e n g t h s g r e a t e r than normal w e l l d r a i n a g e r a d i u s a r e o f t e n necessary t o achieve economic a l l y f e a s i b l e p r o d u c t i o n (SMITH, REN, SORRELS & TEUFEL 1985). I n these cases, i t i s necessary t o determine f r a c t u r e azimuth p r i o r t o i n f i l l d r i l l i n g i n o r d e r n o t t o d e s t r o y t h e e f f e c t i v i t y o f t h e l o n g l i n e a r d r a i n a g e p a t h s by premature t e r m i n a t i o n o f t h e f l o w channels a t n e i g h b o u r i n g b o r e h o l e s . F r a c t u r e azimuth mon i t o r i n g techniques ( c f . s e c t i o n 6 . 2 . 2 . 3 . ) represent invaluable support f o r w e l l p a t t e r n p l a n n i n g and s t i m u l a t i o n design i n o r d e r t o a v o i d i n t e r f e r e n c e w i t h p r e - e x i s t i n g w e l l s and t o p r e v e n t i n t e r f e r e n c e as f i e l d development p r o ceeds (OVERBEY, YOST & YOST 1988). Knowledge o f a z i m u t h a l o r i e n t a t i o n o f i n d u ced f r a c t u r e s i n h y d r a u l i c a l l y s t i m u l a t e d b o r e h o l e s i n t i g h t gas r e s e r v o i r s can i n c r e a s e r e c o v e r a b l e gas r e s e r v e s by as much as one t h i r d t h r o u g h o p t i m a l spang o f p r o d u c t i o n w e l l s w i t h i n t h e gas f i e l d .
4.10.5.1.2. Fracture orientation prediction
and interference avoidance
Even w i t h massive h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n , much o f t h e gas r e s e r v e s t i g h t sandstone r e s e r v o i r s may be l e f t unrecovered because t h e d r a i n a g e p a t t e r n around a f r a c t u r e - t r e a t e d w e l l i s e l l i p t i c a l r a t h e r than r a d i a l i n low-perm e a b i l i t y pay zones (TEUFEL 1981), w i t h t h e e l l i p t i c a l d r a i n a g e network b e i n g even accentuated and enhanced as t h e f r a c t u r e i n c r e a s e s i n l e n g t h . T h e r e f o r e b o r e h o l e l o c a t i o n and spacing based on r a d i a l d r a i n a g e may n o t be t h e optimum economical gas r e c o v e r y procedure i n a p a r t i c u l a r f i e l d due t o o v e r l a p p i n g d r a i nage a t c r a c k t i p s as a consequence o f f r a c t u r e i n t e r f e r e n c e and due t o dead zones between c r a c k f l a n k s . Thus r e l i a b l e methods o f f r a c t u r e o r i e n t a t i o n p r e d i c t i o n a r e o f g r e a t importance t o gas i n d u s t r y , as knowledge o f c r a c k d i r e c t i o n enables t o a v o i d f r a c t u r e i n t e r f e r e n c e and a l l o w s t o achieve a h i g h e r d e p l e t i o n stage i n r e s e r v o i r e x p l o i t a t i o n ( t e c h n i q u e s f o r f r a c t u r e azimuth p r o g n o s i s and d e t e r m i n a t i o n a r e d i s c u s s e d i n s e c t i o n s 6.2.1.8. and 6.2.2.3., r e s p e c t i v e l y ) . Another reason f o r f r a c t u r e o r i e n t a t i o n f o r e c a s t and d e t e r m i n a t i o n i s because massive h y d r a u l i c c r a c k s propagate p a r a l l e l t o t h e maximum h o r i z o n t a l s t r e s s d i r e c t i o n , knowledge o f t h i s o r i e n t a t i o n p r i o r t o s t i m u l a t i o n p e r m i t s opt i m a l placement o f s e i s m i c sensors which can t r a c k p o s i t i o n and advance o f t h e f r a c t u r e (CLARK 1983). E f f e c t s o f s t r e s s s t a t e on f r a c t u r e o r i e n t a t i o n a r e a l s o discussed by ZOBACK, ZOBACK & SEEBURGER ( 1 9 8 1 ) . FORGOTSON, FORGOTSON & BROWN (1980) i l l u s t r a t e p r e d i c t i o n o f h y d r a u l i c f r a c t u r e o r i e n t a t i o n .
4.10.5.1.3. Lenticular tight gas sands Achievement o f optimum r e s e r v o i r d r a i n a g e and avoidance o f f r a c t u r e i n t e r f e rence i s p a r t i c u l a r l y i m p o r t a n t i n l e n t i c u l a r t i g h t gas sands where w e l l spac i n g and crack l e n g t h a r e a f u n c t i o n of h o r i z o n t a l s t r e s s o r i e n t a t i o n c o n t r o l l i n g f r a c t u r e p r o p a g a t i o n d i r e c t i o n and azimuth o f sand body l o n g a x i s h a v i n g s i g n i f i c a n t impact on c r a c k l e n g t h and h e i g h t m o d e l l i n g . The g e o m e t r i c a l r e l a t i o n s h i p between sand l e n s o r i e n t a t i o n , f r a c t u r e azimuth and w e l l p a t t e r n governs d r a i n a g e c o n f i g u r a t i o n where depending on p r e v e n t i o n o f crack i n t e r f e rence and c o n t r o l l e d by p e r m e a b i l i t y l e v e l , e i t h e r square d r a i n a g e networks w i t h r e l a t i v e l y s h o r t f r a c t u r e s o r rectangular patterns w i t h long f r a c t u r e s are t h e more e f f i c i e n t and f e a s i b l e s o l u t i o n (KUUSKRAA, BRASHEAR, ELKINS & MORRA 1979).
4.10.5.1.4. Steam-drive operations VITTORATOS, SCOTT & B E A T T I E (1988) r e p o r t f r a c t u r e i n t e r f e r e n c e i n steamd r i v e o p e r a t i o n s i n heavy o i l sand r e s e r v o i r s . I n t e r w e l l communication w i t h
668 f r a c t u r e b r e a k t h r o u g h f r o m i n j e c t o r t o p r o d u c e r and f o r m a t i o n o f numerous o f such b o r e h o l e c o u p l e t s c o n s i d e r a b l y r e d u c e s sweep e f f i c i e n c y and p a y zone e x p l o i t a t i o n ( c f . section 4.9.4.1.2.). The e x t e n s i v e i n t e r w e l l i n t e r a c t i o n s a r e a r e s u l t o f r e s e r v o i r f a i l u r e t h a t i s r e q u i r e d i n o r d e r t o achieve commercial l e v e i s o f steam i n j e c t i v i t y . As f a i l u r e c a n e x t e n d u p t o d i s t a n c e s e q u a l t o b o r e h o l e s p a c i n g , o v e r l a p p i n g o f t h e f a i l e d p a y p o r t i o n s o c c u r s , and t h e d i s t u r b e d zones a s s o c i a t e d w i t h t h e w e l l s c r e a t e c o n d u i t s o f enhanced p r e s s u r e and f l u i d p r o p a g a t i o n between t h e b o r e h o l e s , w i t h t h e p a t h s embedded i n a m a t r i x w i t h s i g n i f i c a n t l y s l o w e r p r e s s u r e and f l u i d p r o p a g a t i o n . A p a r t f r o m w e l l s p a c i n g and f r a c t u r e o r i e n t a t i o n o p t i m i z a t i o n b y p r o p e r g e o m e t r i c a l p l a n n i n g o f t h e enhanced o i l recovery operations according t o t h e geostress p a t t e r n i n o r d e r t o a v o i d f r a c t u r e i n t e r f e r e n c e and steam e x p a n s i o n s h o r t c u t t i n g a l o n g b o r e h o l e conn e c t i o n c o u p l e t s c r e a t e d b y f r a c t u r e b r e a k t h r o u g h , s t e a m i n g o f r o w s o f w e l l s seq u e n t i a l l y w i t h 50 % o v e r l a p i n i n j e c t i o n t i m e between a d j a c e n t rows s i g n i f i c a n t l y r e d u c e s w a t e r t r a n s f e r between b o r e h o l e s and i n c r e a s e s t h e c o n f o r m a n c e o f i n j e c t e d heat.
4.10.5.2.
Geothermal reservoirs
On t h e o t h e r hand, i n t e r f e r e n c e f r a c t u r i n g may be a n o t h e r means t o i n c r e a s e t h e p o s s i b i l i t i e s o f geothermal h e a t e x p l o i t a t i o n ( c f . c h a p t e r 4.7.) by having f l o w f r o m and t o t h e b o r e h o l e f r o m s e v e r a l d i r e c t i o n s and t h u s o p t i m i z i n g pumpi n g o f t h e i n j e c t e d c o l d w a t e r through a h o t d r y formation, w i t h i n case o f a s u i t a b l e o r i e n t a t i o n o f t h e w e l l s t o t h e g e o s t r e s s f i e l d even a l l o w i n g t o save some i n j e c t i o n w e l l s and p u t more i n v e s t m e n t i n p r o d u c t i o n f a c i l i t i e s and s t i m u l a t i o n design. F r a c t u r e i n t e r f e r e n c e m o d e l l i n g in e i t h e r n e g a t i v e manner t o a v o i d i t t o happen o r i n a p o s i t i v e way t o a c h i e v e t h i s e f f e c t , however, has t o t a k e i n t o a c count t h a t i r r e s p e c t i v e o f the general r u l e t h a t f r a c t u r e o r i e n t a t i o n takes p l a c e a c c o r d i n g t o t h e r e g i o n a l s t r e s s f i e l d , l o c a l d e v i a t i o n s up t o 15 d e g r e e s a r e p o s s i b l e as a consequence o f p a t c h y c o m p r e s s i o n a l i n f l u e n c e s p a r t i c u l a r l y i n anticlinal structures.
4.11. Fracturing o f deviated wells W i t h i n c r e a s i n g o f f s h o r e d e v e l o p m e n t o f t i g h t gas f i e l d s where l a r g e s t i m u l a t i o n campaigns have t o be c a r r i e d o u t and i n some p a t c h e s n e a r l y a l l t h e w e l l s have t o b e t r e a t e d , h y d r a u l i c p r o p p a n t f r a c t u r i n g o f m o d e r a t e l y - t o h i g h l y - d e v i a t e d h o l e s i s becoming i n c r e a s i n g l y s i g n i f i c a n t . T h i s i s a consequence o f t h e standard o f f s h o r e development s t r a t e g y t o d r i l l d i r e c t i o n a l l y i n a l l p o s s i b l e ways f r o m a c e n t r a l p l a t f o r m f r o m w h i c h a l o t o f p r o d u c t i o n w e l l s emanate i n a s t a r - o r s p i d e r - l i k e p a t t e r n . One o f t h e m a j o r examples o f o f f s h o r e h y d r a u l i c p r o p p a n t f r a c t u r i n g campaigns i n an a r e a w i t h numerous h i g h l y - d e v i a t e d w e l l s i s t h e R o t l i e g e n d r e s e r v o i r b e l t i n t h e B r i t i s h S o u t h e r n N o r t h Sea ( c f . s e c t i o n s 2.4.1.2. and 3 . 7 . t o 3 . 9 . ) . The d i s c u s s i o n as f o l l o w s f o c u s s e s on f r a c t u r e p r o p a g a t i o n and o r i e n t a t i o n , p r o p p a n t s t r a t i f i c a t i o n and f l u i d l a y e r i n g , h y d r o c a r bon p r o d u c t i o n and f l o w r e g i m e s , and improvements o f f r a c t u r e e f f e c t i v i t y i n deviated wells.
4.11.1. Fracture propagation and orientation The e f f e c t s o f f r a c t u r e p r o p a g a t i o n and o r i e n t a t i o n in h i g h l y - d e v i a t e d t o hor i z o n t a l b o r e h o l e s a r e i l l u s t r a t e d b y commenting on w e l l b o r e a x i s b e i n g n o t par a l l e l t o i n - s i t u p r i n c i p a l s t r e s s d i r e c t i o n , and l i m i t e d c o m m u n i c a t i o n i n t e r v a l between f r a c t u r e p l a n e and w e l l b o r e s e c t i o n .
669
4.11.1.1. Wellbore axis not parallel to
in-situ principal stress direction
The key f e a t u r e o f d e v i a t e d w e l l s i s t h a t t h e i r a x i s i s g e n e r a l l y n o t a l i g ned w i t h t h e d i r e c t i o n o f t h e i n - s i t u s t r e s s components i n t h e r o c k medium, b u t i s o r i e n t e d a t an a n g l e o t h e r than normal and/or p a r a l l e l t o i t (YEW & L I 1987) u n l e s s o r i e n t e d h i g h l y - d e v i a t e d t o h o r i z o n t a l d r i l l i n g i s made a l o n g o r o r t h o g o nal t o the geostress pattern. Therefore the t r a j e c t o r i e s o f deviated boreholes a r e n o t p a r a l l e l t o t h e l e a s t p r i n c i p a l s t r e s s (BUCHANAN 1987), and t h e medium s u r r o u n d i n g t h e w e l l i s under t h e combined a c t i o n o f normal and shear s t r e s s e s and i s t h u s i n a t h r e e - d i m e n s i o n a l s t r e s s s t a t e (BRADLEY 1979, YEW & L I 1987). H y d r a u l i c f r a c t u r e s i n i t i a t e when t h e maximum t e n s i l e s t r e s s induced on t h e w e l l b o r e w a l l exceeds t h e t e n s i l e s t r e n g t h o f t h e f o r m a t i o n r o c k ( K I M , CHAMPION & COOPER 1985), and propagate i n a p l a n e p e r p e n d i c u l a r t o t h e l e a s t i n - s i t u p r i n c i p a l s t r e s s . I n v e r t i c a l w e l l s , one o f t h e i n - s i t u p r i n c i p a l s t r e s s e s i s u s u a l l y p a r a l l e l t o t h e b o r e h o l e a x i s , and t h e r e s u l t i n g f r a c t u r e p l a n e i s h o r i z o n t a l i n s h a l l o w e r and v e r t i c a l i n deeper f o r m a t i o n s ( c f . s e c t i o n 1 . 2 . 8 . ) . I n d e v i a t e d w e l l s , none o f t h e i n - s i t u p r i n c i p a l s t r e s s e s i s p a r a l l e l o r n o r mal t o t h e b o r e h o l e a x i s , and t h e r e f o r e i n i t i a t i o n o f a f r a c t u r e t a k e s p l a c e f r o m two o p p o s i t e p o i n t s a l o n g t h e c i r c u m f e r e n c e o f t h e w e l i b o r e . The c r a c k sweeps t h e e n t i r e p e r f o r a t e d o r open zone i n t h e d e v i a t e d w e l l o n l y i n case o f t h e b o r e h o l e azimuth b e i n g v e r y n e a r t o t h a t o f t h e f r a c t u r e p l a n e . As a consequence o f t h e g e o m e t r i c a l c o n s t e l l a t i o n , t h e b e s t case o f c r a c k o r i e n t a t i o n i s r e p r e s e n t e d by c o i n c i d e n c e o f c r a c k p l a n e and w e l l t r a j e c t o r y azimuth ( e s p e c i a l l y i f t h e b o r e h o l e i s d e v i a t e d t o h o r i z o n t a l ) , whereas t h e w o r s t case comprises a w e l l b o r e b e i n g normal t o t h e f r a c t u r e p l a n e (BUCHANAN 1987). I t i s a l s o poss i b l e t o c r e a t e s e v e r a l p a r a l l e l f r a c t u r e s i n t h e same w e l l b o r e ( c f . s e c t i o n 4.8.6.2.4.). On t h e o t h e r hand, AADNOY (1987, 1988) p o s t u l a t e s t h a t i n d e v i a t e d w e l l s , v e r t i c a l h y d r a u l i c f r a c t u r e s a r e always i n i t i a t e d a l o n g t h e w e l l b o r e a x i s r e g a r d l e s s o f b o r e h o l e i n c l i n a t i o n , which means t h a t f r a c t u r e s c u t t i n g t h e h o l e a t an a n g l e a r e n o t expected, w i t h t h e r e f o r e t h e l e n g t h o f t h e cracked w e l l sect i o n d e t e r m i n i n g s t i m u l a t i o n success. As t h e y propagate away f r o m t h e borehole, t h e f r a c t u r e s change d i r e c t i o n and o r i e n t themselves normal t o t h e l e a s t i n - s i t u s t r e s s . SNOW & HOUGH (1988) r e p o r t t h a t i n h i g h l y - d e v i a t e d w e l l s , a t l e a s t i n t h e n e a r - w e l l b o r e r e g i o n c r a c k growth i s n o t i d e a l , b u t w i l l e i t h e r r e s u l t i n m u l t i p l e f r a c t u r e s o r i n a c o n t i n u o u s o r en-echelon bending o f t h e c r a c k t o v e r t i c a l p o s i t i o n ( c f . a l s o STRUBHAR, FITCH & GLENN 1988).
4 . 1 1 . 1 . 2 , Limited comnunicat ion interval between fracture plane and we1 lbore sect ion Unless t h e a z i m u t h a l o r i e n t a t i o n s o f f r a c t u r e p l a n e and w e l l t r a j e c t o r y c o i n c i d e , t h e r e i s o n l y a v e r y small communication i n t e r v a l between p e r f o r a t e d boreh o l e s e c t i o n and crack p l a n e i n m o d e r a t e l y - t o h i g h l y - d e v i a t e d w e l l s (BUCHANAN 1987) i n comparison t o a v e r t i c a l f r a c t u r e i n a v e r t i c a l b o r e h o l e where a f t e r a successful s t i m u l a t i o n treatment, the e n t i r e v e r t i c a l e x t e n t o f the formation s h o u l d be i n communication w i t h t h e p e r f o r a t i o n s t h r o u g h t h e f r a c t u r e (HOLDITCH & LEE 1979). H o r i z o n t a l w e l l s and d r a i n h o l e s even a r e c h a r a c t e r i z e d b y v e r y l i m i t e d f r a c t u r e h e i g h t i n t e r s e c t i o n which i s equal t o w e l l b o r e d i a m e t e r (JOSH1 1987). A f t e r d i s c u s s i o n o f some drawbacks i n o p e r a t i o n and performance o f v e r t i c a l f r a c t u r e s i n highly-deviated t o horizontal boreholes t r a v e r s i n g s l i g h t l y d i p p i n g r e s e r v o i r s , t h e comparable s i t u a t i o n o f v e r t i c a l w e l l s c r o s s i n g s t e e p l y dipping prospective formations i s i l l u s t r a t e d .
670
4 . 1 1 . 1 . 2 . 1 . Drawbacks in operation and performance The u n f a v o u r a b l e g e o m e t r i c a l r e l a t i o n s h i p o f v e r t i c a l f r a c t u r e s i n t e r s e c t i n g highly-deviated t o horizontal wells w i t h only a very r e s t r i c t e d contact plane r e s u l t s i n h i g h pressure losses d u r i n g treatment, low s t i m u l a t i o n r a t e s , small f r a c t u r e s o f l i m i t e d e x t e n s i o n away f r o m t h e b o r e h o l e i n t e r s e c t i o n , and w e l l b o r e p l u g g i n g and p e r f o r a t i o n l e a k o f f . M u l t i p l e f r a c t u r i n g t h r o u g h d i f f e r e n t p e r f o r a t i o n s may o c c u r as a consequence o f r e p e a t e d o n s e t o f t h e c r a c k a t sever a l s p a t i a l l y s e p a r a t e d p l a c e s a l o n g t h e o b l i q u e b o r e h o l e s e c t i o n , because a t any o f t h e s e p o i n t s , t h e b o u n d a r y c o n d i t i o n s f o r f r a c t u r e i n i t i a t i o n a r e s i m u l t a n e o u s l y f u l f i l l e d and t h e i n d i v i d u a l c r a c k s p r o p a g a t e e a c h i n t h e d i r e c t i o n normal t o t h e l e a s t p r i n c i p a l s t r e s s , g i v i n g r i s e t o v a r i o u s p a r a l l e l t r a v e l l i n g s m a l l e r f r a c t u r e p l a n e s i n s t e a d o f one l a r g e c o h e r e n t c r a c k f a c e i n v e r t i c a l w e l l s . Some comments a r e o f f e r e d as f o l l o w s on w e l l b o r e i n s t a b i l i t y and f r a c t u r e h e i g h t determination by w e l l logging.
4 . 1 1 . 1 . 2 . 1 . 1 . Wellbore instability T w i s t i n g e f f e c t s o c c u r as t h e f r a c t u r e r e o r i e n t a t e s i t s e l f f r o m t h e d e v i a t e d w e l l b o r e i n case i t s t a r t e d t o propagate i n d i f f e r e n t d i r e c t i o n s . I n h i g h l y - d e v i a t e d and h o r i z o n t a l w e l l s , s t i m u l a t i o n b o a t c a p a c i t y i s t h e l i m i t i n g f a c t o r f o r o f f s h o r e f r a c t u r i n g , w i t h t h e l o n g e r b o r e h o l e s r e s t r i c t i n g pumping r a t e and p r e s s u r e t h a t c a n be a c h i e v e d and m a i n t a i n e d b y t h e v e s s e l s (LOUVIERE 1987; c f . s e c t i o n 3 . 8 . 1 . 2 . ) . F r a c t u r i n g i n h i g h l y - d e v i a t e d b o r e h o l e s even b e a r s c o n s i d e r a b l e r i s k o f w e l l b o r e s c r e e n o u t due t o h i g h f l u i d l e a k o f f t h r o u g h t h e p e r f o r a t i o n s . With i n c r e a s i n g borehole i n c l i n a t i o n , w e l l b o r e i n s t a b i l i t y problems i n c r e a s e and g i v e more f r e q u e n t l y r i s e t o b o r e h o l e c o l l a p s e and f o r m a t i o n f r a c t u ring (WILSON & WILLIS 1986; FUH, WHITFILL & SCHUH 1988; c f . s e c t i o n 4.8.6.1.5.8.). I t i s t h e r e f o r e e s s e n t i a l t o p r e d i c t any s i g n i f i c a n t s t r e s s changes a r o u n d t h e w e l l b o r e when t h e h o l e a n g l e i n c r e a s e s u n d e r c e r t a i n g e o l o g i c a l and i n - s i t u s t r e s s c o n d i t i o n s . B o r e h o l e c o l l a p s e p r e s s u r e g r a d i e n t i n c r e a s e s w i t h w e l l a n g l e , whereas f r a c t u r e g r a d i e n t r e m a i n s t h e same, and as a r e s u l t , t h e w o r k i n g r a n g e between t h e two g r a d i e n t s i s n a r r o w e r a t h i g h e r h o l e a n g l e s . The same r e l a t i o n s h i p as a p p l y i n g f o r h i g h l y - i n c l i n e d t o h o r i z o n t a l w e l l s traversing gently-dipping t o f l a t - l y i n g formations i s also true f o r v e r t i c a l boreholes i n t e r s e c t i n g steeply-dipping rocks ( c f . section 4.11.1.2.2.). Optimum mud w e i g h t f o r w e l l b o r e s t a b i l i t y i n h i g h l y - i n c l i n e d h o l e s r a n g e s between b o r e h o l e c o l l a p s e p r e s s u r e g r a d i e n t r e p r e s e n t i n g t h e l o w e r l i m i t and f o r m a t i o n f r a c t u r e g r a d i e n t b e i n g t h e upper boundary ( c f . s e c t i o n 6.2.3.2.6.). Aspects o f ins t a b i l i t y o f h i g h l y - d e v i a t e d b o r e h o l e s a r e a l s o d i s c u s s e d b y BRADLEY ( 1 9 7 9 ) ; HOTTMAN, S M I T H & PURCELL ( 1 9 7 9 ) ; AADNOV & CHENEVERT ( 1 9 8 7 ) , H S I A O ( 1 9 8 7 ) , MAURY & SAUZAY ( 1 9 8 7 ) and YEW & L I ( 1 9 8 7 ) .
4 . 1 1 . 1 . 2 . 1 . 2 . Fracture height determination by well logging The l i m i t e d c o m m u n i c a t i o n i n t e r v a l between v e r t i c a l f r a c t u r e and h i g h l y - d e v i a t e d w e l l b o r e t r a j e c t o r y has a l s o n e g a t i v e i m p a c t o n t h e e f f e c t i v i t y o f f r a c t u r e h e i g h t d e t e r m i n a t i o n b y w e l l l o g g i n g t e c h n i q u e s (PEETERS & HARTLEY 1984; c f . section 6.2.1.10.8.5.). As a l l methods o f f r a c t u r e h e i g h t d e t e c t i o n b y w i r e l i n e l o g g i n g have o n l y a s m a l l r a d i u s o f i n v e s t i g a t i o n and c a n n o t r e g i s t e r c r a c k h e i g h t f a r t h e r away f r o m t h e w e l l b o r e , m o n i t o r i n g o f f r a c t u r e h e i g h t i s p r o g r e s s i v e l y d e t e r i o r a t e d w i t h i n c r e a s i n g b o r e h o l e i n c l i n a t i o n and s u c c e s s i v e l y s h o r t e r c o n t a c t i n t e r v a l s between w e l l b o r e t r a j e c t o r y and c r a c k p l a n e . Hydraulic fracture direction determination i n highly-deviated wells i s also disc u s s e d by EL RABAA (1988).
67 1
4.11.1.2.2. Comparable situation in vertical wells t raver s i ng steep 1 y dipping r eservoi r s A geometrical situation comparable to horizontal drilling traversing flat-lying to gently-inclined beds can be generated by vertical wells penetrating steeply-dipping formations (ELBEL & SCHOTTLER 1977). This constellation also gives a long borehole intersection with the reservoir and represents the most favourable stimulation situation in vertical wells by enabling a very long contact interval between vertical fracture, highly-inclined pay formation and wellbore trajectory, in contrast to horizontal drilling of flat-lying or low-angle dipping strata where the vertical fracture in greater depth allows only a limited communication path between borehole and crack and a favourable geometrical arrangement comparable to vertical fractures intersecting steeply-dipping reservoir rocks in vertical wells can only be reached by horizontal fractures crossing slightly-inclined pay zones in highly-deviated to horizontal boreholes in shallower depth storeys. The same geometrical constellation of very small communication interval between perforated section and fracture plane as applying for vertical cracks crossing highly-deviated or horizontal boreholes is valid for horizontal fractures traversing vertical wells in shallow reservoir depth.
In order to improve the geometrical constellation of wellbore trajectory and vertical hydraulic fracture, extended-reach drilling should whenever technically possible and economically feasible not be carried out in a J-shaped manner, but in case of necessity of subsequent vertical hydraulic fracturing or gravel packing, preference should be given to a S-shape which is generated by again reducing borehole inclination to an angle below 45 degrees prior to penetration of the pay complex (BRUIST, JEFFERIS & BOTTS 1983; cf. section 4.8.6.1.). Such adjustments, however, are not possible in case of horizontal drainholes that are installed parallel to the reservoir boundaries for the purpose of hydrocarbon exploitation improvement by prolongation of the contact path between wellbore trajectory and pay stratification.
4.11.2. Proppant stratification and fluid layering Proppant transport in highly-deviated wells follows tortuous paths. Dehydration of the proppant-fluid-slurry is very high due to extensive fluid loss through the perforations. Proppant stratification during pumping also occurs, because segregation into high-density slurry on the lower side and low-density slurry at the upper side of tubing or casing takes place which gives higher friction pressure and looses hydrodynamic head. The heavier the proppant grains, the more pronounced is the settling and bedding effect (a similar banking and bedding of proppants occurs during gravel pack operations in highly-deviated wells; cf. section 5.8.3.1.). Extreme cases of stratification might even lead to origin of a settled bank of proppants. Gravity layering of fluids in deviated wells during displacement flushing also occurs.
4.11.3. Hydrocarbon product ion and flow regimes The influences of the special geometrical relationship between highly-deviated boreholes and intersecting hydraulic fractures are highlighted by discussing aspects of reduction of long-term well productivity by changing flow pattern, and non-Darcy flow and skin factor as follows.
4.11.3.1. Reduct ion of long-term we1 1 productivity
by changing
flow pattern
Concerning production in formations where the wellbore is not in communication with the created fracture over its total height, but is restricted to only
672
a small i n t e r v a l of i t as a consequence of the deviated borehole ( c f . se c tion 4 . 1 1 . 1 . 2 . ) , major drawbacks ar e high-pre'ssure losse s d u r i n g e x p l o i t a t i o n , high v e l o c i t i e s in the f r a c t u r e near the wellbore i n t e r s e c t i o n , diminished f r a c t u r e conductivity because of non-Darcy flow, and reduced ove ra ll well produc tivity ( L E E & HOLDITCH 1979, SCHULTE 1984). I f the f r a c t u r e i s in communication with the wellbore over i t s t o t a l height in v e r t i c a l boreholes, a b i l i n e a r flow regime i s developed co n s i s t i n g of t r a n s i e n t l i n e a r horizontal flow within the f o r m a t i o n towards the f r a c t u r e i n combination w i t h t r a n s i e n t l i n e a r horizontal flow within the f r a c t u r e towards the wellbore. The d i f f e r e n t flow i n t e r v a l types i n v e r t i c a l a n d horizontal wel l s ar e o u t l ine d as follows.
4.11.3.1.1. Flow interval types in vertical wells D i st i n c t i o n can be made between f o u r d i f f e r e n t flow i n t e r v a l s f o r drawdowns v e r t i c a l l y f r act u r ed wells comprising f r a c t u r e - l i n e a r , b i l i n e a r , formationl i n e a r a n d pseudoradial flow ( C I N C O - L E Y & SAMANIEGO 1981). If the communication between wellbore and f r a c t u r e i s l i mi t ed in deviated boreholes, a l i n e a r / r a d i a l flow p a t t e r n i s developed which comprises t r a n s i e n t l i n e a r horizontal flow w i t h in the formation towards the f r a c t u r e in combination w i t h t r a n s i e n t ra dia l curr e n t within the crack towards the i n t er v al of the f r a c t u r e i n t e r s e c t i n g the wellbore ( L E E & HOLDITCH 1979, SCHULTE 1979). I n a semiradial f r a c t u r e , the flow regime following l i n e a r / r a d i a l cu r r en t i s t r a n s i e n t l i n e a r flow in the f o r mation a n d s t e a dy - s t at e c u r r e n t in the crack, whereas in a very elongated f r a c t u r e , the l i n e a r / r a d i a l flow regime i s followed by the b i l i n e a r c u r r e n t regime. I n both c a s e s , pressure l o s s r e s u l t i n g from the lim ite d inflow inte rva l a t the wellbore i s seen as a skin f a c t o r . A n intermediate behaviour i s pseudo-radial c u r r e n t (RUSSELL & TRUITT 1 9 6 4 ) . I
CINCO-LEY & MENG (1988) introduce two additional flow periods being t r i l i near and formation b i l i n e a r c u r r e n t , thereby increasing the number of flow int e r v a l s t o be d i s t i n g u i s h ed t o s i x . T r i l i n e a r c urre nt i s present inste a d of the b i l i n e a r flow regime during the s t a b i l i z a t i o n period f o r small volumes of dimens i o n l e s s f r a c t u r e s t o r a t i v i t y , and formation b i l i n e a r c urre nt occurs during the pseudolinear flow period when most of the f l u i d expansion i s provided by the mat r i x i n l i n e a r c u r r en t regime under the same c onditions.
4.11.3.1.2. Flow intervals in horizontal wells I n horizontal wel l s , the typical i n f i n i t e a c ting ra dia l flow pa tte rn of vert i c a l wells i s complicated by f l u i d cu r r en t i n the wellbore v i c i n i t y perpendic u l a r as well as p a r a l l e l t o the sedimentary bedding ( K U C H U K , GOODE, WILKINSDN & THAMBYNAYAGAM 1988). As a consequence of r e s e r v o i r anisotropy, horizontal wells s t a r t w i t h an e l l i p t i c c y l i n d r i c a l flow pa tte rn which l a t e r gives way t o r a d i a l c u r r e n t t h a t i n turn i s followed by a hemi-cylindrical flow pe riod. An intermediate time l i n e a r cu r r en t may develop once the pressure t r a n s i e n t reaches the upper a n d lower boundaries t h a t pass i n t o intermediate time ra dia l flow a f t e r the e f f e c t s of t o p and bottom boundaries reach s t e a d y - s t a t e .
4.11.3.2. Non-Darcy flow and skin factor The l i m i t a t i o n of the communication between wellbore a n d f r a c t u r e t o only a p a r t of the f r a c t u r e height in highly-deviated boreholes s e r i o u s l y a f f e c t s the long-term p r o d uct i v i t y of the wel l . The flow i n the formation follows Darcy's law, whereas i n si d e the f r a c t u r e , non-Darcy flow e f f e c t s have t o be taken i n t o account which may s i g n i f i c a n t l y increase the skin f a c t o r as a r e s u l t of p a r t i a l communication. The d e t e r i o r a t i n g influence can be so la rge t h a t the semi-steady s t a t e flow i s only one t h i r d of t h a t where wellbore a n d crack a r e in f u l l commun ic a t i o n . I n a d d i t i o n , the apparent f r a c t u r e conductivity may be an orde r lower t h a n the real value as a consequence o f non-Darcy flow e f f e c t s . Marked layering
673 o f t h e pay zone i s a l s o c o n t r i b u t i n g t o t h e o v e r a l l r e s u l t s , because t h e var i o u s beds s t a r t t o d e l i v e r hydrocarbons t o t h e c r a c k a t d i f f e r e n t t i m e s and p o s s i b l y a l s o w i t h d i f f e r e n t r a t e s . The d e t e r i o r a t i o n o f l o n g - t e r m p r o d u c t i v i t y due t o a w e l l i n f l o w i n t e r v a l b e i n g much s m a l l e r than t o t a l c r e a t e d f r a c t u r e h e i g h t must be t a k e n i n t o s e r i o u s c o n s i d e r a t i o n p a r t i c u l a r l y f o r t h e d e s i g n o f c r a c k s f r o m h i g h l y - d e v i a t e d o r even h o r i z o n t a l w e l l s and d r a i n h o l e s i n t o t i g h t gas r e s e r v o i r s ( o t h e r aspects o f non-Darcy f l o w a r e d i s c u s s e d i n s e c t i o n 4.3.3.3.).
4.11.4. IrrQrovement of fracture effectivity in deviated wells H y d r a u l i c f r a c t u r i n g i n h i g h l y - d e v i a t e d w e l l s can be c o n s i d e r a b l y improved i f t h e pay zone c o u l d be p e n e t r a t e d by an almost v e r t i c a l b o r e h o l e s e c t i o n which, however, causes h i g h a d d i t i o n a l d i r e c t i o n a l d r i l l i n g expenses. The b o r e h o l e t r a j e c t o r y s h o u l d be planned i n such a way t h a t a t l e a s t across t h e r e s e r v o i r i n t e r v a l , azimuth o f w e l l b o r e a x i s and f r a c t u r e p l a n e a r e more o r l e s s coi n c i d i n g , because t h i s g i v e s maximum i n t e r s e c t i o n o f t h e b o r e h o l e by t h e f r a c t u r e . An example o f such f r a c t u r e - s u i t a b l e d i r e c t i o n a l d r i l l i n g m o d e l l i n g i s r e p o r t e d by OIL GAS JOURNAL (1988 b) where a l t h o u g h maximum i n c l i n a t i o n a n g l e o f t h e b o r e h o l e was 76 degrees, a n g u l a r w e l l b o r e d e v i a t i o n was reduced t o 27 degrees f o r p e n e t r a t i o n t h r o u g h t h e pay zone which i s r e a s o n a b l y s u b v e r t i c a l t o c o n s i d e r a b l y d i m i n u i s h o r even c a n c e l t h e problems t h a t would have been occurr e d i n the high-angle hole. On t h e o t h e r hand, an azimuth o f t h e w e l l b o r e a x i s p e r p e n d i c u l a r t o t h a t o f the crack plane gives r i s e t o m u l t i p l e p a r a l l e l f r a c t u r e s crossing the borehole l i k e a s e t o f s l i c e s w h i c h m i g h t be d e s i r a b l e i n some s p e c i a l a p p l i c a t i o n s ( c f . s e c t i o n 4.8.6.2.4.). O t h e r i m p o r t a n t measures f o r i m p r o v i n g s t i m u l a t i o n a r e i n c r e a s i n g e f f e c t i v e w e l l b o r e diameter, u s i n g m u l t i p l e d e e p - p e n e t r a t i n g p e r f o r a t i o n charges, e n l a r g i n g p e r f o r a t i o n s and p l a n n i n g a staged f r a c t u r i n g t r e a t m e n t ( c f . section 4.2.2.1.3.). P e r f e c t p r o p p a n t t r a n s p o r t can be achieved w i t h shear-independent f l u i d s ( c f . s e c t i o n 4 . 3 . 4 . 2 . 1 . ) .
4.12. Fracture damage F o l l o w i n g t h e o u t l i n e o f v a r i o u s p o s s i b i l i t i e s o f t e c h n o l o g i c a l improvement o f t h e p r e d o m i n a n t l y European f r a c t u r i n g p o t e n t i a l by more o r l e s s u n c o n v e n t i o n a l s t i m u l a t i o n techniques i n m a r g i n a l r e s e r v o i r s , t h e d i s c u s s i o n c o m p r i s i n g a s u i t e o f g e o l o g i c a l , r e s e r v o i r e n g i n e e r i n g and t e c h n i c a l aspects w i l l be c o n c l u ded i n t h i s s e c t i o n by r e v i e w i n g s e v e r a l i m p o r t a n t p o i n t s which a r e c o n t r o l l i n g a m e l i o r a t i o n o r d e t e r i o r a t i o n of f r a c t u r e e f f e c t i v i t y . The summary i s p r e s e n t e d i n t h e f i n a l d i v i s i o n o f t h e r e v i e w and s t a t u s r e p o r t on h y d r a u l i c p r o p p a n t f r a c t u r i n g because o f i t s s i g n i f i c a n c e a l s o f o r c r o s s l i n k i n g t h e c h a p t e r on p r o p p a n t s e l e c t i o n w i t h t h e account o f t h e t e c h n o l o g i c a l p e r s p e c t i v e s . F o r e f f e c t i v e p r o p p a n t c h o i c e and a p p l i c a t i o n as w e l l as f o r s u c c e s s f u l performance o f t h e s t i m u l a t i o n t r e a t m e n t , i t i s e s s e n t i a l t o c o n s i d e r aspects o f p o s s i b l e f r a c t u r e damage a f t e r t h e end o f t h e o p e r a t i o n and ways o f i t s m i n i m i z a t i o n o r even complete p r e v e n t i o n which a r e i l l u s t r a t e d as f o l l o w s . A f t e r d i s c u s s i o n o f some g e n e r a l aspects, t h e s i g n i f i c a n c e o f proppant s e t t l i n g , p r o p p a n t flowback and p r o p p a n t c r u s h i n g a r e o u t l i n e d . The impact o f f l u i d s i s a l s o b r i e f l y s k e t ched.
4.12.1. General aspects Important p o i n t s f o r i l l u s t r a t i o n o f s t i m u l a t i o n f r a c t u r e c o n d u c t i v i t y vs. f o r m a t i o n p e r m e a b i l i t y c r u s h i n g and embedment vs. c l o s u r e s t r e s s .
r e s u l t d e t e r i o r a t i o n are damage as w e l l as p r o p p a n t
674
Fracture conductivity vs. formation permeability damage 4.12.1.1.
F r a c t u r e c o n d u c t i v i t y damage c a n o c c u r b y p r o p p a n t f l o w b a c k i n t o t h e w e l l b o r e , embedment o r c r u s h i n g o f p r o p p a n t g r a i n s ( c f . s e c t i o n 4 . 3 . 3 . 1 . 2 . ) , and plugging o f t h e proppant p o r o s i t y by formation f i n e s , i n s o l u b l e f l u i d - l o s s addit i v e s and g e l r e s i d u e s (ROBINSON, HOLDITCH & WHITEHEAD 1986) and has c o n s i d e r a b l y d e t r i m e n t a l e f f e c t s on w e l l p e r f o r m a n c e (COOKE 1973, HOLDITCH 1 9 7 9 ) . I n comparison, damage o f t h e f o r m a t i o n a r o u n d t h e f r a c t u r e c o u l d be caused b y s w e l l i n g c l a y s ( c f . s e c t i o n 3 . 5 . 2 . ) , m e c h a n i c a l movement o f s o l i d f i n e s , chemical p r e c i p i t a t i o n s i n t h e p o r e space, p l u g g i n g o f i n t e r s t i t i a l s b y u n b r o k e n g e l , and v a r i o u s o t h e r p h y s i c a l o r c h e m i c a l r e a c t i o n s between t h e i n j e c t e d s y n t h e t i c m a t e r i a l s and t h e f o r m a t i o n and i t s n a t u r a l c o n s t i t u e n t s . Some a s p e c t s o f cont r a s t m a g n i t u d e , r e l a t i v e s i g n i f i c a n c e , and b r e a k e r t y p e and c o n c e n t r a t i o n a r e i l l u s t r a t e d as f o l l o w s .
4.12.1.1.1.
Contrast magnitude
F r a c t u r e damage i s much more i m p o r t a n t t h a n f o r m a t i o n damage a r o u n d t h e c r a c k , because d e t e r i o r a t i o n o f p r o p p a n t p o r o s i t y i s much s t r o n g e r d i m i n i s h i n g t h e p e r m e a b i l i t y c o n t r a s t between f r a c t u r e and f o r m a t i o n t h a n d e s t r u c t i o n o f p a r t s o f r e s e r v o i r r o c k m a t r i x p o r o s i t y (ROBINSON, HOLDITCH & WHITEHEAD 1986 e s t i m a t e t h a t i t r e q u i r e s a 1000 f o l d d e c r e a s e i n f o r m a t i o n p e r m e a b i l i t y b e f o r e t h e e f f e c t s a r e c o m p a r a b l e t o o n l y a 40 f o l d d e c r e a s e i n f r a c t u r e c o n d u c t i v i t y ) . The o n l y m a j o r e x c e p t i o n a r e p r o b a b l y l o w - p r e s s u r e w a t e r - s e n s i t i v e r e s e r v o i r s where f o r m a t i o n damage c a n s e v e r e l y c u r t a i l gas o r o i l p r o d u c t i o n . Other types o f f r a c t u r e d e t e r i o r a t i o n a r e r e l a t e d t o non-achievement o f t h e i d e a l r e s u l t o f a highly-conductive crack i n t e r s e c t i n g the r e s e r v o i r m a t r i x i n i t s o r i g i n a l c o n f i g u r a t i o n and i n f u l l v e r t i c a l t h i c k n e s s o f t h e p a y i n t e r v a l (ECONOMIDES 1 9 8 6 ) . Damaged f r a c t u r e s a r e s u r r o u n d e d b y an e l l i p s o i d a l o r c y l i n d r i c a l zone o f f o r m a t i o n m a t r i x d e t e r i o r a t i o n as a consequence o f s t i m u l a t i o n f l u i d l e a k o f f i n t o t h e r e s e r v o i r m a t r i x o r w a t e r b l o c k i n g i n t h e p a y zone a d j a c e n t t o t h e f r a c t u r e ( c f . s e c t i o n 3.11.2.4.2.). Choked c r a c k s s u f f e r f r o m r e d u c e d c o n d u c t i v i t y and p e r m e a b i l i t y i n t h e i m m e d i a t e w e l l b o r e v i c i n i t y as a r e s u l t o f p r o p p a n t f l o w b a c k o r c r u s h i n g due t o e i t h e r u n s u i t a b l e c l o s u r e t i m e o r i n a d e q u a t e c l e a n u p and p r o d u c t i o n s t a r t - u p p r o c e d u r e s (CINCO-LEY & SAMANIEGO 1 9 8 1 b ) . P a r t i a l l y p e n e t r a t i n g f r a c t u r e s f a i l t o span t h e e n t i r e r e s e r v o i r t h i c k n e s s i n h e i g h t due t o i n s u f f i c i e n t volume and p r e s s u r e d u r i n g t r e a t m e n t execution.
4.12.1.1.2.
Relative significance
P r o p p a n t p a c k p e r m e a b i l i t y damage m i n i m i z a t i o n i s c o n s i d e r a b l y more i m p o r t a n t t h a n f o r m a t i o n d e t e r i o r a t i o n m i n i m i z a t i o n , because r e s e r v o i r m a t r i x damage has a r e l a t i v e l y s h o r t - t e r m e f f e c t o n p r o d u c t i v i t y , whereas p r o p p a n t p a c k d e t e r i o r a t i o n has a v e r y p r o l o n g e d d e l e t e r i o u s e f f e c t (ECONOMIDES 1 9 8 6 ) . F r a c t u r i n g f l u i d s become more c o n c e n t r a t e d as t h e t r e a t m e n t p r o g r e s s e s , because p o l y m e r s and s o l i d f l u i d - l o s s a d d i t i v e s a r e f i l t e r e d o u t on t h e r o c k s u r f a c e . W h i l e e a r l i e r s t u d i e s assumed t h a t t h e f i l t e r c a k e i s r e d i s p e r s e d and d i s t r i b u t e d e v e n l y t h r o u g h o u t t h e p r o p p a n t package (STEPHEN-PYE & S M I T H 1 9 7 3 ) , l a t e r i n v e s t i g a t i o n s have shown t h a t t h i s model i s i n c o r r e c t and t h e f i l t e r c a k e p r o g r e s s i v e l y b u i l d s on t h e f r a c t u r e w a l l s ( c f . s e c t i o n s 1 . 4 . 1 0 . 3 . 5 . and 4 . 3 . 4 . 1 . 4 . ) and g i ves r i s e t o p r o p p a n t embedment (McDANIEL 1987, 1988; c f . s e c t i o n 4 . 3 . 3 . 1 . 2 . ) . R e d u c t i o n o f f r a c t u r e c o n d u c t i v i t y i s c a u s e d b y a c o m b i n a t i o n of p r o p p a n t embedment i n t o f i l t e r c a k e a n d / o r f o r m a t i o n , p r o p p a n t c r u s h i n g , proppant compaction and pack c o n f i g u r a t i o n changes, and p r o p p a n t i n t e r s t i t i a l p l u g g i n g b y f o r m a t i o n f i n e s and r e s i d u e f r o m f r a c t u r i n g f l u i d s (ECONOMIDES 1 9 8 6 ) .
675
4.12.1.1.3. Breaker type and concentration F r a c t u r e c o n d u c t i v i t y can be c o r r e l a t e d t o m o l e c u l a r s i z e o f t h e f l u i d r e s i due, and complete breakage o f t h e f l u i d i s e s s e n t i a l t o achieve good c o n d u c t i v i t y (ALMOND 1982). E x c e s s i v e l y h i g h i n j e c t i o n p r e s s u r e s a r e r e q u i r e d t o pump p a r t i a l l y broken f r a c t u r i n g f l u i d s through t h e c r a c k . H i g h e r b r e a k e r c o n c e n t r a t i o n s i n c r e a s e f l o w c a p a c i t y , whereas f l o w r e d u c t i o n i n c r e a s e s as c r o s s l i n k e r and/or g e l l i n g agent s a t u r a t i o n i n c r e a s e s . F o r t h e purpose o f s t u d y i n g f r a c t u r e c o n d u c t i v i t y damage, each s t i m u l a t i o n f l u i d must be e v a l u a t e d as a system composed o f polymer, g e l l i n g agent, c r o s s l i n k e r , b r e a k e r and a d d i t i v e s . Enzyme o x i d i z e r and thermal b r e a k i n g mechanisms have d i f f e r e n t d e g r a d a t i o n e f f i c i e n c y f o r d i f f e r e n t polymer f l u i d systems (ALMOND & BLAND 1984, GALL & RAIBLE 1985, K I M & LOSACANO 1985). The l o w e r t h e proppant c o n c e n t r a t i o n , t h e h i g h e r t h e f r a c t u r e c o n d u c t i v i t y damage (COOKE 1975). Residue s a t u r a t i o n i n t h e f r a c t u r i n g f l u i d i s a f u n c t i o n o f g e l l i n g agent, b r e a k e r t y p e and b r e a k e r c o n c e n t r a t i o n , and f a c t o r s i n f l u e n c i n g f r a c t u r e c o n d u c t i v i t y damage a r e proppant p o r o s i t y and permeab i l i t y , r e s i d u e s a t u r a t i o n i n t h e f l u i d , and r e t a i n e d r e s i d u e i n t h e f r a c t u r e .
4.12.1.2. Proppant crushing and embedment vs. closure stress F r a c t u r e damage o f t e n may o c c u r by p r o p p a n t c r u s h i n g o r embedment. I n h a r d f o r m a t i o n s , proppants t e n d t o crush, w h i l e i n s o f t f o r m a t i o n s , t h e y a r e l i k e l y t o embed i n t o t h e r e s e r v o i r r o c k ( c f . s e c t i o n 4 . 3 . 3 . 1 . 2 . ) . Simultaneous severe proppant c r u s h i n g and embedment, however, can t a k e p l a c e i n deep s o f t r e s e r v o i r s (such as t h e V i c k s b u r g F o r m a t i o n i n South Texas; ROBINSON, HOLDITCH & WHITEHEAD 1986). I n t h i s r e s p e c t , an i m p o r t a n t c o n s i d e r a t i o n i s p r o p p a n t c o n c e n t r a t i o n and i t s succession d u r i n g course o f t h e t r e a t m e n t . High p r o p p a n t s a t u r a t i o n s r e s u l t i n wide f r a c t u r e s i n f i l l e d w i t h proppant m u l t i l a y e r s and t h u s f r a c t u r e c o n d u c t i v i t y i s p r o p o r t i o n a l l y i n c r e a s e d and t h e e f f e c t s o f c r u s h i n g and embedment a r e n o t so c r i t i c a l . S i m i l a r l y as a p p l y i n g f o r h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s , t h e normal g r a d u a l c l o s u r e b e h a v i o u r o f p r e - e x i s t i n g n a t u r a l c r a c k s around a b o r e h o l e d u r i n g v a r i o u s stages o f d e p l e t i o n when t h e e f f e c t i v e normal s t r e s s o r c l o s u r e s t r e s s ( b e i n g e q u i v a l e n t t o t o t a l p r e s s u r e minus p o r e p r e s s u r e ) i s p r o g r e s s i v e l y r i s i n g has a m a j o r impact on changes o f f r a c t u r e p e r m e a b i l i t y and subsequent w e l l p r o d u c t i o n (BAKHTAR, BARTON, RAKOP & JONES 1985). While d e t e r i o r a t i o n o f n a t u r a l c r a c k s ( c f . s e c t i o n 4.8.8.3.) cannot o r o n l y t o m i n o r e x t e n t be i n f l u e n c e d , t h e damage o f h y d r a u l i c a l l y - i n d u c e d f r a c t u r e s can be c o n t r o l l e d by b o t h l a y o u t o f t h e proppant package and d e s i g n o f clean-up and e a r l y p r o d u c t i o n schedules.
4.12.2. Proppant settling H y d r a u l i c f r a c t u r i n g t r e a t m e n t s a r e g e n e r a l l y performed t o i n c r e a s e w e l l p r o d u c t i v i t y by e n l a r g i n g t h e s u r f a c e area a v a i l a b l e f o r f l o w f r o m t h e f o r m a t i o n and by p r o v i d i n g a more permeable c o n d u i t f o r c u r r e n t from t h e e x t r e m i t i e s o f t h e d r a i n a g e area t o t h e w e l l b o r e (POULSEN & LEE 1984). Proppants a r e p l a c e d i n t h e f r a c t u r e by m i x i n g them w i t h t h e s t i m u l a t i o n f l u i d , t h e r e b y g e n e r a t i n g a s l u r r y which i s pumped i n t o t h e b o r e h o l e t o c r e a t e t h e f r a c t u r e , w i t h i d e a l l y on c o m p l e t i o n o f t h e t r e a t m e n t t h e p r o p p a n t b e i n g d i s t r i b u t e d such t h a t i t cov e r s t h e e n t i r e exposed p r o d u c i n g i n t e r v a l . As p r o p p i n g agents a r e g e n e r a l l y denser than t h e f l u i d t r a n s p o r t i n g them, t h e y may s e t t l e toward t h e f r a c t u r e bottom t h r o u g h o u t t h e j o b and c o n t i n u e s e t t l i n g u n t i l t h e c r a c k c l o s e s , t h e r e b y p o s s i b l y l e a v i n g t h e upper p o r t i o n o f t h e f r a c t u r e unsupported as a consequence o f banking i n t h e l o w e r p a r t o f t h e f r a c t u r e . Proppant s e t t l i n g v e l o c i t y i s m a i n l y c o n t r o l l e d by f l o w b e h a v i o u r of t h e t r a n s p o r t i n g f l u i d which i s a l t e r e d t h r o u g h s e l e c t i o n o f g e l l i n g agent, c r o s s l i n k e r , s t a b i l i z i n g a d d i t i v e s and t h e i r r e s p e c t i v e c o n c e n t r a t i o n s . F o r a g i v e n f l u i d , f l o w b e h a v i o u r as w e l l as proppant t r a n s p o r t and suspension a b i l i t y change w i t h t i m e and temperature.
676 Since f r a c t u r i n g t r e a t m e n t s o f t e n t a k e hours t o complete and f l u i d temperature can change r a d i c a l l y f r o m w e l l b o r e t o c r a c k t i p , b o t h t i m e and temperature a r e i m p o r t a n t f a c t o r s i n s t i m u l a t i o n o p e r a t i o n design. Proppant s e t t l i n g d u r i n g t h e t i m e r e q u i r e d f o r f r a c t u r e c l o s u r e a f t e r a hyd r a u l i c s t i m u l a t i o n j o b can have an i m p o r t a n t b e a r i n g on p r o p p a n t d i s t r i b u t i o n i n t h e c r a c k and may determine success o r f a i l u r e o f t h e o p e r a t i o n (NOVOTNY 1977). I n a h y d r a u l i c s t i m u l a t i o n t r e a t m e n t , a f r a c t u r e i s c r e a t e d by r u p t u r i n g t h e f o r m a t i o n a t h i g h p r e s s u r e by means o f a s p e c i a l f l u i d t r a n s m i t t i n g t h e hyd r a u l i c energy, and proppants t r a n s p o r t e d by t h e c a r r i e r f l u i d a r e p l a c e d i n t h e induced f r a c t u r e channel t o p r e v e n t t h e c r a c k f r o m c l o s i n g and r e h e a l i n g a f t e r p r e s s u r e r e l e a s e (ROODHART 1985). P r o d u c t i v i t y improvement i s m a i n l y determined by p r o p p a n t dimensions and d i s t r i b u t i o n which i n t u r n a r e l a r g e l y c o n t r o l l e d by p r o p p a n t s e t t l i n g v e l o c i t y i n the f l u i d , w i t h high s e t t l i n g v e l o c i t i e s r e s u l t i n g i n t r i g g e r i n g o f proppant banking a t t h e f r a c t u r e bottom, whereas a v e r y low s e t t l i n g v e l o c i t y p e r m i t s t h e p r o p p a n t t o remain i n suspension d i s t r i b u t e d over t h e t o t a l f r a c t u r e h e i g h t . Some aspects o f p o s t - f r a c t u r i n g p r e s s u r e d e c l i n e , p r o p p a n t t r a n s p o r t and banking, c a r r i e r f l u i d v i s c o s i t y , pad and c a r r i e r f l u i d volumes, and s l u r r y c o n s i s t e n c y and proppant c l u s t e r i n g a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.12.2.1.Post-f ractur ing pressure decline A n a l y s i s o f s t i m u l a t i o n i n j e c t i o n p r e s s u r e s d u r i n g t h e j o b and p r e s s u r e dec l i n e a f t e r t h e t r e a t m e n t (NOLTE 1979, NOLTE & S M I T H 1981; ROBINSON, HOLDITCH & WHITEHEAD 1986) i s i m p o r t a n t f o r d e t e r m i n a t i o n o f t h e f r a c t u r e c l o s u r e p r e s s u r e which i s a p p r o x i m a t e l y equal t o t h e l e a s t p r i n c i p a l s t r e s s . The c o n t r o l by p r e s s u r e e v a l u a t i o n i s e s s e n t i a l , because t h e c r a c k must have a l r e a d y c l o s e d b e f o r e t h e w e l l is opened f o r c l e a n - u p and t h e t i m i n g of f r a c t u r e c l o s u r e must be p r i o r t o b e g i n n i n g o f g e l breakage i n o r d e r t o t r a p t h e p r o p p a n t s b e f o r e t h e y can s e t t l e i n t h e decomposing f r a c t u r i n g f l u i d . The r a t e o f p o s t - f r a c t u r i n g p r e s s u r e d e c l i n e depends on f l u i d l e a k o f f which i s a f u n c t i o n o f f o r m a t i o n p e r m e a b i l i t y ( b o t h p r i m a r y m a t r i x and secondary j o i n t p e r m e a b i l i t y ; c f . s e c t i o n 4.8.8.1.), w i t h i n high-permeability r e s e r v o i r rocks the pressure f a l l i n g o f f r a p i d l y , thereby allowing the f r a c t u r e t o close i n a r e l a t i v e l y short p e r i o d o f time, whereas i n t i g h t pay h o r i z o n s , f l u i d l e a k o f f may be r e t a r d e d and t h u s t h e f r a c t u r e w i l l s t a y open f o r some l o n g e r t i m e .
4.12.2.2.
Proppant transport and banking
Proppant t r a n s p o r t p r e d i c t i o n models a r e g e n e r a l l y based on S t o k e s ' law f o r l a m i n a r o r c r e e p i n g f l o w o f Newtonian f l u i d s a t low Reynolds numbers as w e l l as on Newton's law f o r t u r b u l e n t f l o w a t h i g h t o v e r y h i g h Reynolds numbers (CLARK & Q U A D I R 1981, SHAH 1982, VEATCH 1 9 8 3 ) . While some f r a c t u r i n g f l u i d s e x h i b i t a Newtonian f l o w b e h a v i o u r , o t h e r s a r e non-Newtonian f l u i d s b u t e s s e n t i a l l y behave as power-law f l u i d s , and t h e c r o s s l i n k e d polymers commonly used f o r MHF t r e a t m e n t s a r e v e r y complex non-power-law m i x t u r e s ( c f . s e c t i o n 4 . 3 . 4 . 3 . ) . Propp a n t s e t t l i n g v e l o c i t i e s i n Newtonian f l u i d s a r e a f u n c t i o n o f g r a v i t a t i o n a l acc e l e r a t i o n , f l u i d d e n s i t y , p a r t i c l e d e n s i t y , p a r t i c l e diameter, f l u i d v i s c o s i t y and s u r f a c e roughness. I n s l u r r i e s , s e t t l i n g takes p l a c e in a somewhat d i f f e r e n t manner because o f p a r t i c l e i n t e r f e r e n c e and/or clumping (CLARK & GULER 1983) and c l u s t e r e d p r o p p a n t t r a n s p o r t which i s h i n d e r i n g g r a v i t y s e g r e g a t i o n (SIEVERT, WAHL, CLARK & HARKIN 1981). Aspects o f p r o p p a n t t r a n s p o r t and s e t t l i n g a r e a l s o d i s c u s s e d by LOCKETT & AL-HABBOOBY (1973), BARNEA & MEDNICK (1975), NOVOTNY (1977), DANESHY (1978 b ) ; HARRINGTON, HANNAH & WILLIAMS (1979); M I R Z A & RICHARDSON ( 1 9 7 9 ) , ZIGRANGE & SYLVESTER (1981), SHAH (1982, 1986), PEDEN & LUO (1987) and CLIFTON & WANG (1988, 1989). Proppant s e t t l i n g , p r o p p a n t banking and f o r m a t i o n g r a i n s i z e d i s t r i b u -
677 t i o n , f r a c t u r e c l o s u r e and f l u i d v i s c o s i t y , and o t h e r aspects a r e b r i e f l y d i s cussed as f o l l o w s .
4.12.2.2.1,
Proppant s e t t l i n g
While proppant i s i n j e c t e d i n t o the fracture, i t t r a v e l s along w i t h the f l u i d away from t h e w e l l b o r e and s e t t l e s downward a t a r a t e t h a t depends on f l u i d p r o p e r t i e s and s u r r o u n d i n g c o n d i t i o n s (NOVOTNY 1977). Movement p a t h s o f proppants a r e o f t e n t o r t u o u s due t o secondary f l o w caused by i r r e g u l a r morphol o g y of t h e c r a c k w a l l s (VEATCH & MOSCHOVIDIS 1986). As c a r r i e r f l u i d i s l o s t t h r o u g h t h e permeable w a l l s o f t h e f r a c t u r e , t h e c r a c k s t a r t s t o c l o s e a t t h e end o f t h e t r e a t m e n t , w i t h proppant c o n t i n u i n g t o s e t t l e u n t i l a p r o p p a n t bank i s formed a t t h e bottom o f t h e crack, t h e p r o p p a n t c o n c e n t r a t i o n i n t h e s l u r r y becomes so h i g h t h a t i t can no l o n g e r s e t t l e , o r t h e f r a c t u r e c l o s e s on t h e s l u r r y thereby trapping the proppant. Proppant banking beneath t h e p r o d u c t i v e r e s e r v o i r sand i n t e r v a l b e f o r e c l o s u r e o f t h e f r a c t u r e s t o p s s e t t l i n g by t r a p p i n g o f t h e p r o p p a n t s between t h e w a l l s o f t h e crack, and t h i s process can r e s u l t i n such f r a c t u r e d e t e r i o r a t i o n t h a t almost a l l t h e s t i m u l a t i o n e f f e c t i s gone. As i t i s n o t u n l i k e l y t h a t t h e c r a c k c l o s e s i n s t a n t a n e o u s l y a t t h e end o f p r o p p a n t i n j e c t i o n , p r o p p a n t s e t t l i n g d u r i n g t h e t i m e t h e f r a c t u r e s t a y s open has t o be reduced as much as poss i b l e by m o d i f y i n g f l u i d v i s c o s i t y and proppant c o n c e n t r a t i o n i n o r d e r t o i m prove p r o p p a n t suspension i n t h e c a r r i e r medium. Some aspects o f c l u s t e r e d p r o p p a n t t r a n s p o r t and h i n d e r e d s e t t l i n g , f o r c e d f r a c t u r e c l o s u r e , and w e l l b o r e v i c i n i t y and t a i l - i n a r e o u t l i n e d as f o l l o w s .
4.12.2.2.1.1.
Clustered proppant transport and hindered s e t t l i n g
I n non-Newtonian f l u i d s , p r o p p a n t g r a i n s a r e t r a n s p o r t e d i n c l u s t e r s r a t h e r t h a n as s i n g l e p a r t i c l e s (SIEVERT, WAHL, CLARK & HARKIN 1981). When t h e f l o w i s stopped, t h e c l u s t e r s f a l l a t a much f a s t e r r a t e t h a n s i n g l e p a r t i c l e s t o t h e b o t t o m of t h e crack, w i t h t h e suspended proppants t h u s s e t t l i n g i n a v e r y s h o r t time. I f grains f a l l through a l i q u i d containing other p a r t i c l e s , the terminal s e t t l i n g v e l o c i t y i s d i f f e r e n t than t h a t o f a s i n g l e g r a i n f a l l i n g t h r o u g h t h e same l i q u i d as a consequence o f h i n d e r e d s e t t l i n g (CLARK & QUADIR 1981; c f . a l s o s e c t i o n 4.12.2.5.1.). Terminal s e t t l i n g v e l o c i t i e s o f proppants i n hydraul i c f r a c t u r i n g t r e a t m e n t s a r e governed by f l u i d v i s c o s i t y , f l u i d v e l o c i t y , f r a c t u r e w i d t h , p a r t i c l e s i z e and p a r t i c l e d e n s i t y , and a r e a l s o a f u n c t i o n o f g r a v i t a t i o n a l and d r a g f o r c e s . An absence o f e x t e r n a l shear on t h e f l u i d produces l o w e r s e t t l i n g v e l o c i t i e s and hence l o n g e r p r o p p a n t packages. Apparent p r o p p a n t s e t t l i n g v e l o c i t i e s decrease w i t h i n c r e a s i n g p r o p p a n t c o n c e n t r a t i o n (SIEVERT, WAHL, CLARK & HARKIN 1981). Hindered s e t t l i n g e f f e c t s which account f o r t h e i n f l u e n c e o f i n t e r a c t i o n s between p a r t i c l e s i n a s l u r r y on t h e f r e e - f a l l v e l o c i t y o f t h e g r a i n s a r e n o t s u f f i c i e n t l y pronounced t o e x p l a i n t h e whole magnitude o f d e v i a t i o n between app a r e n t and a c t u a l s e t t l i n g v e l o c i t i e s . I f t h e p r o p p a n t p a r t i c l e s move a t a h o r i z o n t a l r a t e l e s s than t h a t o f t h e c a r r y i n g f l u i d , p r o p p a n t c o n c e n t r a t i o n i n t h e s l u r r y i n c r e a s e s which i n non-Newtonian f l u i d s i n t u r n i n c r e a s e s pack growth r a t e , w h i l e Newtonian f l u i d s do n o t e x h i b i t a c l e a r - c u t i n f l u e n c e o f p r o p p a n t s a t u r a t i o n . On t h e o t h e r hand, d o u b l i n g o f t h e v i s c o s i t y o f Newtonian f l u i d s has o n l y a s m a l l e f f e c t on t r a n s p o r t b e h a v i o u r i n these systems, whereas i n non-Newtonian systems, a s e v e r a l f o l d i n c r e a s e i n t r a n s p o r t and suspension capab i l i t y takes p l a c e . Impingement o f p r o p p a n t g r a i n s a g a i n s t f r a c t u r e w a l l s dur i n g t r a n s p o r t and s e t t l i n g i s n o t c o n s i d e r e d t o be a m a j o r i n f l u e n c i n g f a c t o r , because most o f t h e p a r t i c l e s s t a y i n t h e c e n t r a l h i g h - v e l o c i t y zone and a r e n o t t e n d i n g t o c o n c e n t r a t e i n t h e m a r g i n a l l o w - v i s c o s i t y zone.
678
4.12.2.2.1.2.Forced fracture closure ROSE (1988) presents a technique allowing a f t e r placement of the proppants i n the f r a c t u r e t o induce cl o s u r e of the crack so t h a t the proppants a r e t r a p ped in the producing i n t er v al instead of s e t t l i n g t o the bottom of the f r a c t u r e . Simple methods o f as cer t ai n i n g proppant deposition within the prospective in t e r v a l and avoiding s i g n i f i c a n t a n d unacceptable s e t t l i n g a re se pa ra te t r e a t ment of each i n t er v al of the r es er v o i r column ( c f . se c tion 4.10.1.) and limited-entry p e r f o r a t i o n a n d f r a c t u r i n g ( c f . s ec tion 4 . 2 . 2 . 2 . ) . A s crack height may s t i l l be excessive or n o t completely contained within the perforated i n t e r v a l , a forced f r a c t u r e cl o s u r e technique can be applied t o forc e the crack t o s h u t before proppant s e t t l i n g can occur. Forced f r a c t u r e cl o s u r e can be accomplished by c o n t r o l l i n g flowback r a t e of t r e a t i n g f l u i d s thus reducing the pressure d i f f e r e n t i a l near the wellbore a n d allowing f r a c t u r e f aces t o cl o s e and t o t r a p the high p r o p p a n t c onc e ntra tion, thus stopping flowback of e a r l i e r p r o p p a n t s t age s a n d r e s u l t i n g i n b e t t e r well p r o d u c t i v i t y . Rapid r et u r n of carbon dioxide foam f l u i d s supports forced crack c lo su r e by quick pressure drop within wellbore and f r a c t u r e during f l u i d recovery, thereby allowing the crack walls t o shut upon proppant suspended within the perforated i n t e r v a l .
4.12.2.2.1.3. Wellbore vicinity and tail-in The most s i g n i f i c a n t proppant s e t t l i n g occurs a t the place where f r a c t u r e width i s l a r g e s t which i s i n the immediate surroundings of the wellbore ( E L Y 1988). As p a r t i a l f r a c t u r e rehealing as a consequence of proppant s e t t l i n g i s p a r t i c u l a r l y detrimental in the most proximal se c tion of the crack, a s s i s t a n c e has t o be given t o f r a c t u r e cl o s u r e by immediate flowback of the well a f t e r t e r mination ( c f . sect i o n 4 . 1 2 . 3 . 1 . ) of the treatment or t o enhancement of cleanup by energizing components or foams ( c f . s ection 3 . 1 1 . 2 . ) . Excessive proppant s e t t l i n g e i t h e r i n pipe or f r a c t u r e during p u m p i n g may r e s u l t i n premature screenout termination of the job (SHAH, SMITH & DONALDSON 1983; c f . se c tion 6 . 2 . 4 . 2 . 1 . ) . F r act u r e cl o s u r e can be observed by monitoring surfa c e pressure following a treatment. By insuring t h a t the crack i s completely closed before returning the well t o hydrocarbon o f f t a k e , proppant production can be mitigate d .
P r o p p a n t s e t t l i n g does not stop when i n j e c t i o n i s terminated, b u t continues u n t i l the proppant e i t h e r s e t t l e s on t o p of the e x i s t i n g proppant bed i n the lower p a r t of the crack or becomes trapped between the walls as the f r a c t u r e c los e s (POULSEN & L E E 1984). While near the wellbore the proppants a re nearly perf e c t l y suspended a n d only very l i t t l e s e t t l i n g occurs, the suspended p r o p p a n t height drops more r ap i d l y c l o s e r t o the leading edge of the proppant-laden f l u i d f a r t h e r outwards within the crack, thereby indic a ting f l u i d degradation which provokes higher s e t t l i n g v e l o c i t i e s . If la rge portions of the p r o p p a n t load a r e deposited a t the f r a c t u r e bottom, much of the upper inte rva l of the crack i s l e f t unpropped. Near the wellbore where conditions of time a n d temperatu r e a r e l e a s t severe, the l a r g e s t amount of p r o p p a n t i s suspended when the f r a c t u r e c l o se s a t t h a t l o cat i o n . This e f f e c t even occurs although crack w i d t h i s i n i t i a l l y g r e a t e s t next t o the wellbore a n d the f r a c t u r e takes the longest time t o c l o s e t h e r e . A s f l u i d s t a b i l i t y in the immediate borehole surroundings i s very good, howe v e r , the pronounced f r a c t u r e width and extended time of crack c losure a r e n o t e f f e c t i v e f o r giving r i s e t o enhanced proppant s e t t l i n g . Two means e x i s t f o r increasing propped f r a c t u r e height a t cl o s u r e comprising decreasing p r o p p a n t s e t t l i n g r a t e by s e l e c t i n g a more s t a b l e or viscous f l u i d , or increasing f l u i d loss to provoke sooner crack cl o s u r e. Since f l u i d leakoff a f f e c t s the f l u i d portion a v a i l a b l e f o r f r a c t u r e c r e a t i o n , the l a t t e r solution may re quire some compromise as t o w h a t f l u i d l o s s c h a r a c t e r i s t i c s a r e d e s i r a b l e . T a i l - i n p r o p p a n t sche-
679 d u l i n g ( c f . s e c t i o n s 2.4.1.2.3. and 4.12.3.3.) increases proppant s e t t l i n g i n t h e w e l l b o r e v i c i n i t y due t o h i g h e r d e n s i t y o r l a r g e r g r a i n s i z e o f t h e t a i l - i n material, w i t h the t a i l e d - i n proppant l o t therefore also r e q u i r i n g greater gel s t r e n g t h and s t a b i l i t y t o keep i t suspended.
4.12.2.2.1.4.
Other aspects
F r a c t u r e temperature c a l c u l a t i o n s which a i m on p r o p p a n t s e t t l i n g e s t i m a t i o n have t o employ more a c c u r a t e l y r e p r e s e n t e d f l o w r a t e s and f l u i d - l o s s r a t e s . F r a c t u r e c l o s u r e c a l c u l a t i o n s a r e d e t e r m i n i n g proppant d i s t r i b u t i o n i n t h e c l o sed c r a c k . Treatment c o s t s a r e reduced by s t a g i n g g e l , s t a b i l i z e r and b r e a k e r c o n c e n t r a t i o n s based on f l u i d b e h a v i o u r i n t h e f r a c t u r e . F l u i d l o s s t o t h e f o r m a t i o n by l e a k o f f decreases f l u i d v e l o c i t y and i n c r e a ses proppant s a t u r a t i o n and s l u r r y v i s c o s i t y , t h e r e b y d i m i n i s h i n g p r o p p a n t s e t t l i n g p o s s i b i l i t i e s . Drag o f f l u i d and p r o p p a n t towards t h e f r a c t u r e w a l l s i s neg l i g i b l e f o r most s t i m u l a t i o n c o n d i t i o n s . S e t t l i n g v e l o c i t y o f t h e p r o p p a n t f l u i d s l u r r y i s i n f l u e n c e d by t h e c o m b i n a t i o n o f f r a c t u r e geometry, f l u i d l o s s , f l u i d h e a t i n g and r e s e r v o i r f o r m a t i o n c o o l i n g (BABCOCK, PROKOP & KEHLE 1967; NOVDTNY 1977).
4.12.2.2.2. Proppant banking and formation g r a i n size d i s t r i b u t i o n Proppant banking i n t h e f r a c t u r e decreases t h e c r o s s - s e c t i o n a l area f o r t h e f l o w which i n c r e a s e s f l u i d v e l o c i t y . Since proppant bank p e r m e a b i l i t y i s much g r e a t e r than f o r m a t i o n p e r m e a b i l i t y , f l u i d l o s s can be assumed t o o c c u r o v e r t h e e n t i r e permeable r e s e r v o i r sand even i f i t i s covered by a p r o p p a n t bank. W i t h c o n t i n u i n g b u i l d i n g o f t h e p r o p p a n t bank, f l u i d v e l o c i t y across i t s t o p may become so l a r g e t h a t no more p r o p p a n t can be deposited, w i t h t h e p r o p p a n t bank then r e a c h i n g i t s e q u i l i b r i u m h e i g h t . Some aspects o f f i n i n g - and coarsening-upwards g r a i n - s i z e sequences as w e l l as hydrocarbon d i s t r i b u t i o n i n g r a n u l o m e t r i c a l cyclothems a r e d i s c u s s e d as f o l l o w s .
4.12.2.2.2.1.
Fining- and coarsening-upward grain-size sequences
Proppant banking may be d e s i r e d t o c r e a t e a p a r t i c u l a r l y dense p r o p p a n t package i n t h e l o w e r p o r t i o n o f t h e f r a c t u r e i n such r e s e r v o i r s where most of t h e hydrocarbon p r o d u c t i o n i s coming f r o m t h e b o t t o m o f t h e p r o s p e c t i v e i n t e r v a l (LEE & DANESHY 1985), such as f o r example i n f i n i n g - u p w a r d s g r a i n s i z e sequences where t h e h i g h e s t p e r m e a b i l i t i e s o c c u r a t t h e base o f t h e pay zone. I n t h e r e v e r s e case o f coarsening-upwards g r a n u l o m e t r i c a l successions, however, propp a n t banking w i t h c o n c e n t r a t i o n o f t h e m a t e r i a l s u p p o r t i n g t h e f r a c t u r e a t t h e b o t t o m o f t h e r e s e r v o i r would cause c o n s i d e r a b l e d e t e r i o r a t i o n o f e x p l o i t a t i o n p o s s i b i l i t i e s , because t h e b e s t s e c t i o n a t t h e t o p o f t h e pay zone would h a r d l y r e c e i v e any proppant p l u g . Proppant banking i s a l s o i n v e s t i g a t e d by SCHOLS & VISSER (1974). S i m i l a r l y as a p p l y i n g f o r coarsening-upwards c l a s t i c r e s e r v o i r s w i t h f u l l o i l s a t u r a t i o n , p r o p p a n t s e t t l i n g and banking i s d e t r i m e n t a l t o sandstone pay i n t e r v a l s o f e i t h e r g r a i n s i z e d i s t r i b u t i o n and o r g a n i z a t i o n t h a t a r e d i v i d e d i n t o an upper o i l - b e a r i n g s e c t i o n and a l o w e r w a t e r - b e a r i n g p o r t i o n by t h e o i l w a t e r - c o n t a c t . Proppant s e t t l i n g p r i o r t o f r a c t u r e c l o s u r e would l e a d i n such r e s e r v o i r s t o reduced proppant c o n c e n t r a t i o n s i n t h e upper i n t e r v a l w i t h h i g h o i l s a t u r a t i o n s and t o i n c r e a s e d p r o p p a n t package d e n s i t y a d j a c e n t t o t h e l o w e r w a t e r - p r o d u c t i v e s e c t i o n (CRAMER & SONGER 1988).
680
4.12.2.2.2.2. Hydrocarbon distribution
in granulometrical cyclothems
I n i d e a l cases r e p r e s e n t i n g f u l l y h y d r o c a r b o n - b e a r i n g f i n i n g - u p w a r d s o r c o a r sening-upwards sandstone sequences, p r o p p a n t b a n k i n g can be t h e optimum s o l u t i o n i f f r a c t u r e p r o p a g a t i o n i n downwards d i r e c t i o n s t o p s a t t h e r e s e r v o i r base t o a l l o w upwards bank growth f r o m t h e a c t u a l b e g i n n i n g o f t h e n e t pay i n t e r v a l and n o t w a s t i n g p r o p p a n t m a t e r i a l by s e t t l i n g i n t o t h e b a r r e n s e c t i o n below t h e t a r g e t zone. I n such cases, banking r e s u l t s i n p r o p p a n t s e t t l i n g i n t o t h e f u l l c r e a t e d f r a c t u r e w i d t h i n s t e a d o f t h e c r a c k c l o s i n g on a l e s s c o n c e n t r a t e d and n a r r o w e r pack such as s e t i n p l a c e by p e r f e c t - t r a n s p o r t f l u i d s (HOEL 1988). I f h i g h - q u a l i t y proppants a r e used, a s u f f i c i e n t c o n d u c t i v i t y c o n t r a s t i s always g i v e n everywhere i n t h e f r a c t u r e r e g a r d l e s s o f whether t h e p r o p p a n t banks a r e o p p o s i t e f i n e r - g r a i n e d l o w - p e r m e a b i l i t y o r c o a r s e r - g r a i n e d h i g h - p e r m e a b i l i t y bas a l members o f c o a r s e n i n g - o r f i n i n g - u p w a r d s cyclothems, r e s p e c t i v e l y , o r whet h e r t h e open f l o w channel i s s i t u a t e d n e x t t o t h e c o r r e s p o n d i n g upper coarse and f i n e u n i t s .
4.12.2.2.3. Fracture closure and fluid viscosity C r o s s l i n k e d f r a c t u r i n g f l u i d s have t h e u n i q u e advantage o f y i e l d i n g h i g h v i s c o s i t y f o r modest polymer c o n c e n t r a t i o n which makes them i d e a l f o r p l a c i n g h i g h e r p r o p p a n t s a t u r a t i o n s due t o i n h e r e n t h i g h e f f i c i e n c y , the a b i l i t y t o c r e a t e wide f r a c t u r e s , and e x c e l l e n t p r o p p a n t t r a n s p o r t and suspension c h a r a c t e r i s t i c s (HANNAH, HARRINGTON & POTTER 1980; c f . s e c t i o n 4 . 3 . 4 . 3 . ) . I n case o f l a r g e t r e a t m e n t s , f r a c t u r e c l o s u r e does n o t o c c u r f o r s e v e r a l hours w h i c h would a l l o w ample t i m e f o r complete p r o p p a n t s e t t l i n g i n water-based g e l s , whereas c r o s s l i n k e d f r a c t u r i n g g e l s can r e t a i n t h e a b i l i t y t o s u p p o r t p r o p p a n t s f o r sev e r a l hours a t e l e v a t e d temperatures u n l e s s t h e i r v i s c o s i t y i s degraded t o some c r i t i c a l p o i n t by b r e a k e r a c t i o n o r a b s o l u t e thermal d e g r a d a t i o n . I f b r e a k e r a c t i o n s i g n i f i c a n t l y reduces t h e t i m e d u r i n g w h i c h p e r f e c t p r o p p a n t s u p p o r t i s p r e s e n t and f r a c t u r e c l o s u r e i s r e t a r d e d , t h e n i m p e r f e c t p r o p p a n t d i s t r i b u t i o n w i l l r e s u l t and t h e o v e r a l l performance may be no b e t t e r than i f a s i m p l e g e l were used. On t h e o t h e r hand, i f breakers s i g n i f i c a n t l y i n crease f l u i d l o s s d u r i n g c l o s u r e , then f r a c t u r e c l o s u r e t i m e may be reduced and t h u s a f a v o u r a b l e p r o p p a n t d i s t r i b u t i o n may be s a f e l y t r a p p e d between t h e s h u t t i n g c r a c k w a l l s . I n l o w - v i s c o s i t y f r a c t u r i n g f l u i d s , p r o p p a n t s may s e t t l e r a p i d l y , t h e r e b y l e a v i n g a s h o r t c r a c k w i t h low bed h e i g h t which c o u l d r e s u l t i n o n l y t h e b o t t o m few f e e t o f t h e zone o f i n t e r e s t a c t u a l l y b e i n g propped w i t h a c o n d u c t i v e f r a c t u r e (KUNDERT & SMINK 1979). F r a c t u r e c l o s u r e u s u a l l y occurs a t t h e end o f l i n e a r f l o w (KULAR, CHHINA, BEST & MACKENZIE 1988; c f . s e c t i o n 4.11.3.).
4.12.2.2.4. Other aspects P r i n c i p a l l y p r o p p a n t s e t t l i n g i s a f u n c t i o n o f f l u i d shear r a t e , b u t when p a r t i c l e s a r e f l o w i n g t h r o u g h a f r a c t u r e , d r a g g i n g near t h e c r a c k w a l l s h i n d e r s g r a i n s e t t l i n g , w i t h w a l l roughness a l s o b e i n g an i m p o r t a n t f a c t o r (ACHENBACH 1974). Proppant s a t u r a t i o n p r o b a b l y has t h e g r e a t e s t e f f e c t on s e t t l i n g v e l o c i t y o f t h e s o l i d p a r t i c l e s , w i t h p r o g r e s s i v e f l u i d l o s s d u r i n g course o f t h e hyd r a u l i c s t i m u l a t i o n treatment r e s u l t i n g i n successively higher f l u i d v i s c o s i t y and p r o p p a n t c o n c e n t r a t i o n and t h e r e b y p r o p p a n t s e t t l i n g o c c a s i o n a l l y b e i n g severe d u r i n g i n i t i a l stages, b u t b e i n g i n h i b i t e d more and more towards t h e end o f t h e o p e r a t i o n by b e t t e r suspending and s u p p o r t i n g c o n d i t i o n s i n t h e more sat u r a t e d and more v i s c o u s c a r r i e r medium (NOVOTNY 1977). S t a t i c and dynamic proppant s e t t l i n g v e l o c i t i e s are d i f f e r e n t i n shear-sensitive f l u i d s . Experimental work by B I O T &
MEDLIN
(1985)
and
MEDLIN,
SEXTON
&
ZUMWALT
681 (1985) f o r t h i n u n c r o s s l i n k e d f l u i d s has r e v e a l e d f o u r r e g i o n s o f d i f f e r e n t p r o p p a n t t r a n s p o r t phenomena. The l o w e s t zone i s t h e s e t t l e d bank where concent r a t i o n i s a f u n c t i o n o f p r o p p a n t c h a r a c t e r i s t i c s . I n upwards d i r e c t i o n , t h e f o l l o w i n g segments o f t h e f l o w p r o f i l e comprise bed l o a d b e i n g a f l u i d i z e d l a y e r o f r e l a t i v e l y small h e i g h t , zone o f v i s c o u s d r a g t r a n s p o r t where p r o p p a n t s a t u r a t i o n i s more o r l e s s c o n s t a n t , and i n t e r v a l o f t u r b u l e n t t r a n s p o r t through which c o n c e n t r a t i o n d e c l i n e s t o zero. Aspects o f p r o p p a n t t r a n s p o r t a r e a l s o d i s c u s s e d by KERN, PERKINS & WYANT (1959), LOWE & H U I T T (1966); CLARK & QUADIR (1981); SIEVERT, WAHL, CLARK & HARK I N (1981); CLARK & GULER (1983); GOTTSCHLING, ROYCE & SHUCK (1983); AHMED (1984); CLARK, HALVACI, GHAELI & PARKS (1985); ROODHART (1985) and ACHARYA (1986, 1987). S i m i l a r e f f e c t s o f t r a n s p o r t and s e t t l i n g as e x h i b i t e d by propp a n t s i n f r a c t u r i n g s t i m u l a t i o n f l u i d s a p p l y f o r r o c k c u t t i n g s i n d r i l l i n g mud (PEDEN & LUO 1987).
4.12.2.3. Carrier fluid viscosity O p t i m i z a t i o n o f p r o p p a n t d i s t r i b u t i o n w i t h i n t h e f r a c t u r e and m i n i m i z a t i o n o f p r o p p a n t s e t t l i n g can be b e s t achieved by u s i n g h i g h p r o p p a n t s a t u r a t i o n s ( c f . s e c t i o n 4.3.) and s h o r t e n i n g c l o s u r e t i m e o f t h e f r a c t u r e (BARBY & BARBEE 1987). The f o l l o w i n g overview p r e s e n t s some aspects o f c r o s s l i n k i n g types, propp a n t t r a n s p o r t c a p a c i t y , and f l u i d r h e o l o g y s t a b i l i t y .
4.12.2.3.1. Cross1 inking types Proppant s e t t l i n g r a t e s a r e p a r t i c u l a r l y c r i t i c a l i n l o w - v i s c o s i t y c a r r i e r f l u i d s and w i t h l o n g f r a c t u r e c l o s u r e times. Concerning v i s c o s i t y m o d e l l i n g , d i f f e r e n t t y p e s o f c r o s s l i n k e r s can be used, and shear f o r c e s caused b y pumping o f t h e v i s c o u s c r o s s l i n k e d g e l down t h e t u b u l a r s c o u l d provoke r e d u c t i o n o f t h e e f f e c t i v e v i s c o s i t y ( c f . s e c t i o n 4.3.4.3.). Conventional c r o s s l i n k i n g agents l e a d t o immediate c r o s s l i n k i n g , whereas a delayed c r o s s l i n k e r c o u l d be t i m e d t o c r o s s l i n k as moving downhole, t h e r e b y b u i l d i n g up v i s c o s i t y d u r i n g t h e e n t i r e pumping t i m e . D u a l - v i s c o s i t y c r o s s l i n k e r s f o r m a t once a s l i g h t c r o s s l i n k i n g and t h e n i n c r e a s e v i s c o s i t y by f u r t h e r c r o s s l i n k i n g w h i l e b e i n g d i s p l a c e d . Del a y e d and d u a l - v i s c o s i t y c r o s s l i n k e r s s h o u l d n o t be as e x t e n s i v e l y a f f e c t e d by t h e s h e a r i n g f o r c e i n t h e t u b u l a r s as t h e c o n v e n t i o n a l c r o s s l i n k e r (GARDNER & EIKERTS 1982).
4.12.2.3.2. Proppant transport capacity Concerning f l u i d v i s c o s i t y vs. p r o p p a n t s e t t l i n g , l o w - v i s c o s i t y f l u i d s (10 20 c p ) l e a d t o g r a v i t a t i o n a l d i f f e r e n t i a t i o n and c o n c e n t r a t i o n o f much p r o p p a n t m a t e r i a l t o t h e b o t t o m o f t h e f r a c t u r e , and bed h e i g h t can be i r r e g u l a r i f d i f f e r e n t p a r t i c l e s i z e s a r e used d u r i n g t h e t r e a t m e n t ( c f . s e c t i o n 2.4.1.2.3.). M e d i u m - v i s c o s i t y f l u i d s (50 - 200 cp) p r o v i d e good, b u t n o t p e r f e c t p r o p p a n t t r a n s p o r t , and p r o p p a n t beds can s t i l l be i r r e g u l a r . M e d i u m - v i s c o s i t y f l u i d s c a r r y o n l y 20/40 o r s m a l l e r proppants f o r l o n g d i s t a n c e s , whereas l a r g e r g r a i n s w i l l s e t t l e more r a p i d l y ( t h e dependency o f s e t t l i n g r a t e f r o m p a r t i c l e s i z e i s a l s o i l l u s t r a t e d by LOCKETT & AL-HABBOOBY 1973 and M I R Z A & RICHARDSON 1979). H i g h - v i s c o s i t y f l u i d s can t r a n s p o r t a l l p r o p p a n t g r a i n s i z e s w i t h o n l y minimal s e t t l i n g , and t h e r e s u l t a n t f r a c t u r e u s u a l l y i s v e r y u n i f o r m (HOLDITCH 1984). Aspects o f p r o p p a n t d i s t r i b u t i o n i n v e r t i c a l f r a c t u r e s a r e a l s o r e p o r t e d by CLARK & QUADIR (1981); SIEVERT, WAHL, CLARK & HARKIN (1981) and BABCOCK, PROKOP & KEHLE ( 1 9 8 7 ) . Proppant s e t t l i n g i s d i s c u s s e d by KIRKBY & ROCKEFELLER (1985), ROODHART (1985) and SHAH (1986). CLARK, HALVACI, GHAELI & PARKS (1985); DUNAND & SOUCEMARIANADIN (1985) and ACHARYA (1986, 1987) comment on p a r t i c l e t r a n s p o r t i n v i s c o u s and v i s c o e l a s t i c f r a c t u r i n g f l u i d s . SHAH & LEE (1986) a n a l y z e f r i c -
682 t i o n pressures o f proppant-laden hydraulic f r a c t u r i n g f l u i d s .
4.12.2.3.3. Fluid rheology stability The r h e o l o g i c a l p r o p e r t i e s o f t h e c a r r i e r f l u i d can v a r y s i g n i f i c a n t l y due t o temperature changes in t h e f r a c t u r e and t h e t i m e d u r a t i o n of exposure t o each temperature (LEE & DANESHY 1985). V i s c o s i t y l o s s o f t h e c a r r i e r due t o t i m e and temperature can l e a d t o i n a b i l i t y o f f u r t h e r p r o p p a n t suspension, b u t p r o p p a n t s e t t l i n g can be p r e v e n t e d by replacement by two-stage g e l l e d f l u i d s where t h e second g e l l i n g agent i s a c t i v a t e d a t h i g h e r temperature. Many f l u i d s , however, can m a i n t a i n t h e i r v i s c o s i t y even up t o temperatures approaching 300 OF and t h u s p r o p p a n t suspension a t l e a s t f o r s e v e r a l hours i s guaranteed. T a i l i n pumping schedules w i t h i n s e r t i o n o f f i r s t s t r o n g e r and second weaker g e l s o r v i c e v e r s a a r e a l s o p o s s i b l e . D r i l l i n g and f r a c t u r i n g f l u i d s a r e g e n e r a l l y r e p r e s e n t i n g power-law-type f l u i d s , and t h e problem o f r o c k c u t t i n g s e t t l i n g o u t f r o m d r i l l i n g mud and o f proppants f r o m f r a c t u r i n g f l u i d s i s c o m p l i c a t e d by t h e shear-dependent c h a r a c t e r i s t i c s o f t h e f l u i d s (PEDEN & LUO 1987).
4.12.2.4. Pad and carrier fluid volumes The danger o f e x c e s s i v e p r o p p a n t s e t t l i n g can a l s o be m i n i m i z e d by f a v o u r a b l e v a r i a t i o n s o f pad and c a r r i e r f l u i d volumes (LEE & OANESHY 1 9 8 5 ) . When f l u i d e f f i c i e n c y i s low, e i t h e r a l a r g e amount o f p r i m a r y pad i s r e q u i r e d t o complete t h e t r e a t m e n t , o r i f t h e l e a d i n g p r o p p a n t - l a d e n elements l o s e t h e i r suspended proppants, t h e y can serve as secondary pad and t h u s t h e q u a n t i t y o f p r i m a r y pad can be reduced. D e t e r m i n a t i o n o f an optimum amount o f p r i m a r y pad has t o be based on t h e j o i n t c o n s i d e r a t i o n o f f l u i d l o s s and p r o p p a n t s e t t l i n g . I f t h e l e a d i n g p r o p p a n t does n o t s e t t l e o u t , propped f r a c t u r e l e n g t h can be cont r o l l e d b y v a r y i n g t h e p r i m a r y pad volume. The l e s s pad r e m a i n i n g a t t h e end o f t h i s stage, t h e l o n g e r t h e propped f r a c t u r e w i l l be. As f r a c t u r e w i d t h a t t h e t i p i s narrow and a l i t t l e e x t r a volume o f pad w i l l occupy a l o n g r e g i o n , p r o p ped f r a c t u r e l e n g t h may t h e r e b y be reduced. Too l i t t l e pad, however, w i l l i n crease t h e chance o f p r o p p a n t r e a c h i n g t h e f r a c t u r e t i p , t h e r e b y c r e a t i n g h i g h p r e s s u r e . A l t e r n a t i v e t r e a t m e n t s i n c l u d e two-stage g e l l e d f l u i d pumping where t h e second g e l l i n g agent i s a c t i v a t e d a t h i g h e r temperatures.
I n t i g h t f o r m a t i o n s w i t h low f l u i d l e a k o f f r a t e s , f r a c t u r e c l o s u r e t i m e i s i n e v i t a b l y v e r y h i g h , and t h u s i t i s p r a c t i c a l l y i m p o s s i b l e t o keep p r o p p a n t f r o m s e t t l i n g t o t h e f r a c t u r e b o t t o m a t c l o s u r e . In o r d e r t o p r o v i d e an adequate amount o f proppant i n t h e pay zone, proppant c o n c e n t r a t i o n and volume s h o u l d be as h i g h as p o s s i b l e w i t h o u t r i s k i n g premature s c r e e n o u t f a i l u r e ( c f . section 6.2.4.2.1.). F r a c t u r e geometry i s most i n f l u e n c e d by t h e f l u i d v i s c o s i t y a t t h e t i p o f t h e c r a c k , w i t h t h e h i g h e r t h e v i s c o s i t y , t h e s h o r t e r the f r a c t u r e l e n g t h f o r a g i v e n i n j e c t e d volume. T h e r e f o r e i n o r d e r t o c r e a t e narrow l o n g f r a c t u r e s , f l u i d v i s c o s i t y a t t h e t i p o f t h e c r a c k has t o be m a i n t a i n e d low, whereas f o r w i d e r f r a c t u r e s , f l u i d sequences r e s u l t i n g in h i g h e r v i s c o s i t y a t t h e c r a c k t i p have t o be s e l e c t e d .
4.12.2.5. Slurry consistency and proppant clustering K I R K B Y & ROCKEFELLER (1985) analyze p r o p p a n t s e t t l i n g v e l o c i t i e s i n n o n - f l o w ing slurries, because s t a t i c s e d i m e n t a t i o n b e h a v i o u r i s i m p o r t a n t i n p r o p p a n t t r a n s p o r t n o t o n l y p r i o r t o c l o s u r e when t h e p l a c e d s l u r r y i s s t a n d i n g i n t h e c r a c k , b u t a l s o d u r i n g pumping and f l u i d i n s e r t i o n i n t o t h e f r a c t u r e , as a h i g h percentage o f t h e p r o p p a n t s l u r r y r e s i d e s i n r e g i o n s o f low-shear r a t e . Sediment a t i o n b e h a v i o u r o f f r a c t u r i n g f l u i d s i s dominated by p a r t i c l e c l u s t e r i n g which g i v e s r i s e t o average s t a t i c s l u r r y s e t t l i n g v e l o c i t i e s many t i m e s f a s t e r t h a n
683 t h a t o f s i n g l e p a r t i c l e s (CLARK, HARKIN, WAHL & SIEVERT 1977; SIEVERT, WAHL, CLARK & HARKIN 1981; c f . s e c t i o n 4.12.2.2.1.), w i t h t h e consequence o f c l u s t e r i n g b e i n g an inhomogeneous s l u r r y and a time-dependent d i s t r i b u t i o n o f s e t t l i n g v e l o c i t i e s (TORY & PICKARD 1977). Some comments a r e o f f e r e d as f o l l o w s on p r o p p a n t c l u s t e r i n g and f l u i d v i s c o s i t y v a r i a t i o n as w e l l as s l u r r y s e d i m e n t a t i o n and proppant c l u s t e r f o r m a t i o n .
4.12.2.5.1. Proppant clustering and fluid viscosity variation Proppant c l u s t e r i n g occurs i n a l l f r a c t u r i n g f l u i d s a t t y p i c a l p r o p p a n t conc e n t r a t i o n s , w i t h s e t t l i n g v e l o c i t y depending on proppant s a t u r a t i o n i n a much d i f f e r e n t manner t h a n assumed by c o n v e n t i o n a l proppant t r a n s p o r t models (NOVOTNY 1977). Proppant c l u s t e r i n g can t h e r e f o r e s i g n i f i c a n t l y i n c r e a s e p r o p p a n t s e t t l i n g v e l o c i t y independent o f f l u i d t y p e (KIRKBY & ROCKEFELLER 1985). F r a c t u r i n g f l u i d v i s c o s i t y v a r i e s g r e a t l y w i t h i n t h e c r a c k due t o wide v a r i a t i o n s i n l o c a l shear r a t e (KIRKBY & ROCKEFELLER 1985), w i t h t h e v i s c o s i t y o f some o f t h e f r a c t u r i n g f l u i d s b e i n g e x t r e m e l y s e n s i t i v e t o shear r a t e (CLARK 1979). The p r o p p a n t p l u g f l o w i n g i n t h e c e n t e r o f t h e crack r e s i d e s i n a r e g i o n o f v e r y low shear r a t e (NOLTE 1982). As a consequence o f v i s c o s i t y d i f f e r e n c e s a t low shear r a t e s and s t a t i c s e t t l i n g v e l o c i t y , r h e o l o g i c a l c h a r a c t e r i z a t i o n o f t h e f l u i d s a t low shear r a t e s i s e s s e n t i a l f o r d i f f e r e n t i a t i n g suspension performance ( c f . a l s o CLARK, HALVACI, GHAELI & PARKS 1985).
4.12.2.5.2. Slurry sedimentation and proppant cluster formation The s l u r r y s e d i m e n t a t i o n b e h a v i o u r i n d i c a t e s s i g n i f i c a n t d i f f e r e n c e s between Newtonian and f r a c t u r i n g f l u i d s (KIRKBY & ROCKEFELLER 1985). Newtonian f l u i d s e x h i b i t r e l a t i v e l y u n i f o r m c o n c e n t r a t i o n i n t h e s l u r r y phase as w e l l as d e f i n e d f l u i d / s l u r r y and s l u r r y / p r o p p a n t bank i n t e r f a c e s , whereas f r a c t u r i n g f l u i d s d i s p l a y m u l t i p l e c o n c e n t r a t i o n phases w i t h o u t d i s t i n c t i n t e r f a c e s . As t h e p r i m a r y s l u r r y body s e t t l e s , a s l o w - s e t t l i n g d i l u t e - s l u r r y phase remains above. Dense p r o p p a n t banks f o r m v e r y s l o w l y , w i t h a s l o w - s e t t l i n g , l e s s dense s l u r r y phase a p p e a r i n g above t h e bank. Some f r a c t u r i n g f l u i d s d e p o s i t dense p r o p p a n t banks t h a t a r e h i g h l y f l u i d and can r e a d i l y be r e d i s p e r s e d , whereas Newtonian f l u i d s l a y down immobile p r o p p a n t banks. An i m p o r t a n t a s p e c t o f c l u s t e r e d s e d i m e n t a t i o n i s t h e dependence o f s l u r r y s e t t l i n g v e l o c i t y on d i s t a n c e , t i m e and l o c a l c o n c e n t r a t i o n (TORY & P I C K A R D 1977, K I R K B Y & ROCKEFELLER 1985). R a p i d s e t t l i n g does n o t o c c u r u n t i l t h e p a r t i c l e s have t r a v e r s e d s u f f i c i e n t d i s t a n c e t o a l l o w t h e f o r m a t i o n o f c l u s t e r s . The n a t u r a l i n s t a b i l i t y o f t h e d e p o s i t i o n process l e a d s t o inhomogeneous propp a n t c o n c e n t r a t i o n (THACKER & LAVELLE 1978). I f f l o w w i t h i n a f r a c t u r e i s h i g h l y non-uniform, average s e t t l i n g v e l o c i t y i s determined by t h e suspension behav i o u r o f t h e slow-moving zones which t e n d t o have h i g h e r v i s c o s i t i e s . Slow-mov i n g h o r i z o n t a l f l u i d bands w i t h h i g h apparent y i e l d s t r e s s can g r e a t l y i n h i b i t p r o p p a n t s e t t l i n g , t h e r e b y a c t i n g as a s e d i m e n t a t i o n b a r r i e r .
4.12.3. Proppant flowback Proppant flowback a f t e r c o m p l e t i o n o f t h e h y d r a u l i c s t i m u l a t i o n can s e r i o u s l y damage f r a c t u r e c o n d u c t i v i t y by removing t h e s u p p o r t o f t h e c r a c k t o s t a y open and t h u s a l l o w i n g t h e f r a c t u r e t o p a r t i a l l y c l o s e and t o d i s c o n n e c t t h e d r a i n a g e path. Proppant flowback i n t o t h e w e l l b o r e i s t h u s g e n e r a l l y u n d e s i r a b l e s i n c e t h e p r i m a r y goal o f h y d r a u l i c f r a c t u r i n g i s t o p l a c e p r o p p a n t i n t h e c r e a t e d f r a c t u r e t o i n c r e a s e c r a c k c o n d u c t i v i t y and t h e r e b y a m e l i o r a t e r e s e r v o i r p r o d u c t i v i t y v i a a g r e a t e r e f f e c t i v e w e l l b o r e r a d i u s (HOLCOMB 1983). A p a r t f r o m f r a c t u r e c o n d u c t i v i t y damage e s p e c i a l l y i n t h e p r o x i m a l seam around t h e borehole, p r o p p a n t flowback causes w e l l b o r e f i l l which must be c i r c u l a t e d
684 o r b a i l e d o u t b e f o r e t h e b o r e h o l e can be produced e f f i c i e n t l y , and e r o s i o n a l and p l u g g i n g damage t o t u b u l a r s , w e l l h e a d assemblies, valves, chokes and f l o w l i nes, w i t h t h i s equipment d e s t r u c t i o n b e i n g a s e r i o u s d e t e r i o r a t i o n o f b o t h t e c h n i c a l performance and economical f e a s i b i l i t y . Proppant d i s t r i b u t i o n and t h u s a l s o o v e r a l l f r a c t u r e q u a l i t y a r e s e v e r e l y a f f e c t e d by p r o p p a n t i n f l o w i n t o t h e w e l l b o r e (CLARK 1983). I n a d d i t i o n , flowback o f h i g h l y a b r a s i v e s i n t e r e d b a u x i t e p r o p p a n t s can be e x t r e m e l y hazardous, w i t h n o t o n l y c u t t i n g o u t o f v a l v e s and Christmas t r e e s occ u r r i n g where l a r g e q u a n t i t i e s o f s i n t e r e d b a u x i t e proppants a r e r e t u r n e d w i t h w e l l f l u i d s , b u t a l s o i n extreme cases blowouts and f i r e s t a k i n g p l a c e (OSBORNE, McLEOD & SCHROEDER 1981; c f . s e c t i o n 1 . 4 . 6 . ) . The most i m p o r t a n t procedures o f c o n t r o l l i n g and a v o i d i n g p r o p p a n t flowback a r e c a r e f u l cleanup o f t h e w e l l a f t e r t h e f r a c t u r i n g j o b , pumping of a t a i l - i n of c u r a b l e r e s i n - c o a t e d sand i n t h e t e r m i n a l s t a g e o f t h e h y d r a u l i c s t i m u l a t i o n j o b , u t i l i z a t i o n o f foam and e n e r g i z e d f r a c t u r i n g f l u i d s , and s h u t - i n t i m e adjustment.
4.12.3.1. We1 1 cleanup Well cleanup has a v e r y i m p o r t a n t impact on f r a c t u r e performance due t o i t s c o n t r o l on p r o p p a n t p a r t i c l e a d j u s t m e n t d u r i n g c r a c k c l o s u r e w i t h d e c l i n i n g p r e s s u r e l e v e l . The i n f l u e n c e s o f b o r e h o l e cleanup a r e summarized by b r i e f l y commenting on f r a c t u r e c l o s u r e and proppant t r a p p i n g , p r o p p a n t flowback c o n t r o l and d i m i n u t i o n , and e f f e c t s o f f r a c t u r i n g f l u i d and c a p i l l a r y p r e s s u r e on w e l l cleanup.
4.12.3.1.1. Fracture closure and proppant trapping The e f f e c t i v i t y o f a h y d r a u l i c f r a c t u r i n g t r e a t m e n t i s m a i n l y c o n t r o l l e d by f i n a l pumped p r o p p a n t c o n c e n t r a t i o n and c r a c k w i d t h a t t h e w e l l b o r e , w i t h these two f a c t o r s p r e d o m i n a n t l y d e t e r m i n i n g optimum w e l l p r o d u c t i v i t y (OSBORNE, McLEOD & SCHROEDER 1981). O f f t a k e c a p a b i l i t y o f t h e f r a c t u r e d w e l l i s t h u s o f t e n governed by t h e c o n n e c t i o n o f t h e l a s t volume o f i n j e c t e d proppants t o t h e w e l l b o r e . T h e r e f o r e n o t o n l y crack e v a c u a t i o n has a c o n s i d e r a b l y d e t r i m e n t a l e f f e c t on f r a c t u r e performance, b u t a l s o proppant o v e r d i s p l a c i n g can be d i s a s t r o u s when p r o p p a n t i s d i s c o n n e c t e d f r o m t h e b o r e h o l e (RAYMOND & BINDER 1967, SMITH 1973). I n t h e l a t t e r case, f l u i d must f l o w o u t o f t h e f r a c t u r e and through t h e low f o r m a t i o n p e r m e a b i l i t y i n t o t h e w e l l b o r e o v e r t h e d i s c o n n e c t e d d i s t a n c e . Some aspects o f p r e s s u r e d e p l e t i o n and f l u i d l e a k o f f as w e l l as p r e s sure d e c l i n e a s s i s t a n c e by c o n t r o l l e d f l u i d p r o d u c t i o n a r e i l l u s t r a t e d as f o l lows.
4.12.3.1.1.1. Pressure depletion and fluid leakoff I n l o w - p e r m e a b i l i t y r e s e r v o i r s , 12 - 24 h r s may be r e q u i r e d b e f o r e t h e p r e s sure d e p l e t e s s u f f i c i e n t l y t o p e r m i t f r a c t u r e c l o s u r e and t r a p p i n g o f t h e proppants between t h e c r a c k w a l l s , whereas i n h i g h - p e r m e a b i l i t y pay h o r i z o n s , f r a c t u r e c l o s u r e may a l r e a d y be achieved i n a t i m e between a q u a r t e r t o a c o u p l e o f h o u r s . As 12 - 24 hours i s a l s o t h e t i m e range when most o f t h e added g e l b r e a k e r s s t a r t t o become a c t i v e and b e g i n t o decompose t h e g e l i n c l u d i n g v i s c o s i t y l o w e r i n g , i t may be necessary t o a s s i s t f r a c t u r e c l o s u r e by s l o w l y and car e f u l l y f l o w i n g t h e w e l l back on low r a t e s and t h r o u g h a narrow choke i n o r d e r t o b l e e d o f f p r e s s u r e f r o m t h e c r a c k . P r o d u c t i o n r a t e s , however, have t o be adj u s t e d t o keep p r o p p a n t flowback l i m i t e d t o a minimum i n o r d e r n o t t o d e t e r i o r a t e p r o p p a n t p l u g g i n g o f t h e f r a c t u r e p a r t i c u l a r l y in t h e w e l l b o r e surroundi n g s . A f t e r f r a c t u r e c l o s u r e i s d e t e c t e d o r when p r e s s u r e has f a l l e n down below t h e known v a l u e o f c l o s u r e s t r e s s , t h e w e l l has t o be s h u t i n t o a l l o w t h e g e l t o break. F o r f r a c t u r i n g and c l o s u r e p r e s s u r e p r e d i c t i o n and r e c o g n i t i o n as w e l l as f o r d e s i g n o f mud, c a s i n g and cementing programs, f r a c t u r i n g t e c h n i q u e
685 and p r o p p a n t type, a c c u r a t e v a l u e s o f i n - s i t u s t r e s s a r e needed. C l o s u r e t i m e which i s v e r y c r i t i c a l f o r p r o p e r w e l l cleanup m o d e l l i n g can be understood as t h e p e r i o d n e c e s s i t a t e d f o r e i t h e r complete f l u i d l e a k o f f o r t o t a l p r o p p a n t s e t t l i n g and t r a p p i n g (LEE & DANESHY 1985). The t i m e r e q u i r e d f o r a l l suspended proppants e i t h e r t o s e t t l e down o r t o be trapped between t h e f r a c t u r e w a l l s (POULSEN & LEE 1984) g i v e s a b e t t e r approach, because t h e p e r i o d needed f o r t h e c r e a t e d f r a c t u r e volume t o l o s e a l l o f i t s f l u i d when pumping s t o p s (NOLTE 1979) can be e x c e s s i v e l y h i g h i n case o f slow f l u i d l e a k o f f i n t i g h t r e servoir s .
4.12.3.1.1.2. Pressure decline assistance by controlled fluid production I t i s c e r t a i n l y a c r i t i c a l d e c i s i o n t o produce f l u i d back i n o r d e r t o a s s i s t p r e s s u r e d e c l i n e which a u t o m a t i c a l l y i n v o l v e s flowback o f a t l e a s t l i t t l e quant i t i e s o f proppants from the f r a c t u r e i n t o the wellbore, b u t i t i s o f t e n the l e s s e r o f two e v i l s , because p r o p p a n t s e t t l i n g down t o t h e b o t t o m o f a p e r s i s t e n t l y open wide c r a c k and even p o t e n t i a l l y o u t o f zone i s t h e w o r s t p o s s i b l e case of f r a c t u r e damage and would d e t e r i o r a t e t h e e f f e c t o f t h e whole t r e a t m e n t (ROBINSON, HOLDITCH & WHITEHEAD 1986). T h e r e f o r e e v e r y t h i n g has t o be u n d e r t a ken t o ensure t h a t t h e c r a c k h e a l s b e f o r e t h e g e l breaks and a l l o w s p r o p p a n t s e t t l i n g , and cleanup and p r o d u c t i o n s h o u l d n o t be s t a r t e d b e f o r e t h e p r o p p a n t i s trapped i n the closed f r a c t u r e .
S p e c i a l c a r e has t o be taken t o p r e v e n t proppant d i s c o n n e c t i o n i n case i t i s necessary t o s h u t down pumping o p e r a t i o n s t o r e p a i r l e a k s o r t o work on e q u i p ment m a l f u n c t i o n (OSBORNE, McLEOD & SCHROEDER 1981). Shut-down has t o be made w i t h c r o s s l i n k e d g e l and proppants s t i l l b e i n g i n t u b i n g and b l e n d e r u n t i l r e p a i r s a r e made and then enough volume o f base g e l w i t h o u t any f l u i d - l o s s a d d i t i v e , c r o s s l i n k e r o r proppants i s pumped t o r e - e s t a b l i s h r a t e and p r e s s u r e which i s f o l l o w e d by s t a r t i n g of s i m u l t a n e o u s l y pumping c r o s s l i n k e r , f l u i d - l e a k o f f a d d i t i v e and p r o p p a n t a t t h e same c o n c e n t r a t i o n as b e f o r e shut-down. It i s n o t advantageous t o s t a r t up a g a i n w i t h a l o w e r p r o p p a n t c o n c e n t r a t i o n t h a n p r e v i o u s l y pumped o r r u n n i n g any c r o s s l i n k e d g e l w i t h o u t p r o p p a n t . The t h i n nonc r o s s l i n k e d g e l i n t h e i n t e r m e d i a t e stage f i n g e r s i n t o t h e more v i s c o u s c r o s s l i n k e d g e l and by t h i s as w e l l as by f l u i d l e a k o f f w i l l be absorbed t o t h e p o i n t where no d i s c o n n e c t i o n w i l l o c c u r between t h e p r o p p a n t - l a d e n elements.
4.12.3.1.2. Proppant flowback control and diminution Well flowback and cleanup a f t e r c l o s u r e o f t h e c r a c k has t o be performed v e r y c a r e f u l l y , because i n t h e f i r s t hours t o days a f t e r t h e f r a c t u r e operat i o n , t h e p r o p p a n t p a r t i c l e s a r e a d j u s t i n g and p a c k i n g due t o t h e c l o s u r e s t r e s s which i s e x e r t e d on t h e proppant. I f t h e f r a c t u r i n g f l u i d s a r e produced back t o o f a s t a t t o o h i g h r a t e s , p r o p p a n t f a i l u r e o r flowback i n t o t h e b o r e h o l e can o c c u r . Some p o i n t s o f f l u i d f l o w v e l o c i t y and p r e s s u r e drop, and f l u i d v i s c o s i t y and w e l l cleanup a r e i l l u s t r a t e d as f o l l o w s .
4.12.3.1.2.1. Fluid flow velocity and pressure drop Broken g e l s t i l l r e t a i n s some v i s c o s i t y and t h u s imposes a much g r e a t e r d r a g on t h e proppants t h a n does d r y gas, and t h e r e f o r e a d j u s t i n g and p a c k i n g propp a n t s can be c a r r i e d away f r o m t h e f r a c t u r e by t h e produced f l u i d s . As p r e s s u r e drop and f l u i d v e l o c i t y reach t h e i r maximum i n t h e immediate v i c i n i t y o f t h e w e l l b o r e , p r o p p a n t flowback and c r u s h i n g i s g r e a t e s t i n t h e v e r y s u r r o u n d i n g s o f t h e b o r e h o l e , and t h i s e f f e c t can o n l y be avoided by c a r e f u l l y a d j u s t i n g cleanup t o v e r y low r a t e s and m a x i m i z i n g back-pressure a g a i n s t t h e f o r m a t i o n . Proppant flowback can a l s o be r e t a r d e d by i n t e r m i x i n g a n g u l a r g r a i n s i n t o t h e
686 p o p u l a t i o n o f round p a r t i c l e s (LARSEN & SMITH 1985; c f . s e c t i o n s 1.3.5. and 1 . 4 . 3 . ) o r t h e a p p l i c a t i o n o f r e s n-coated m a t e r i a l (POPE, WILES & P I E R C E 1987; c f . section 4.12.3.3.). F l u i d f l o w v e l o c i t y reaches t s maximum i n t h e n e a r e s t zone o f t h e w e l l b o r e because o f f u n n e l l i n g o f t h e f l o w l i n e s f r o m t h e whole f r a c t u r e h e i g h t t o t h e p e r f o r a t e d i n t e r v a l which ( a c c o r d i n g t o t h e l i m i t e d - e n t r y technique; HOWARD & FAST 1970 a, SMALL 1985, CRAMER 1987) comprises u s u a l l y o n l y p a r t s o f t h e f o r mer and p r e s s u r e drawdown becomes p a r t i c u i a r l y c r i t i c a l due t o c o n c e n t r a t i o n t o the narrow f r a c t u r e where l i n e a r f l o w i n s t e a d o f r a d i a l f l o w i n u n s t i m u l a t e d r e s e r v o i r s occurs ( c f . s e c t i o n 4 . 1 1 . 3 . ) and focusses t h e e f f e c t o f i n c r e a s i n g c l o s u r e s t r e s s t o some small s e c t i o n s o f t h e pay h o r i z o n (such p r e s s u r e d i s t r i b u t i o n f u n n e l l i n g i s i l l u s t r a t e d f o r R o t l i e g e n d and C a r b o n i f e r o u s gas w e l l s i n Germany FRG by BRINKMANN, KRUMER & R E I N I C K E 1980 and R E I N I C K E , BRINKMANN, SCHWARZ & HUENI 1985; c f . s e c t i o n s 2 . 4 . 1 . 1 . and 3 . 3 . 2 . ) .
4.12.3.1.2.2.Fluid viscosity and well cleanup improved w e l l c l e a n u p w i t h s i g n i f i c a n t l y decreased r i s k o f p r o p p a n t flowback can be p r o v i d e d by delayed foamed c r o s s l i n k e d f r a c t u r i n g g e l s which i n comparison t o c o n v e n t i o n a l l i n e a r g e l s have much b e t t e r proppant t r a n s p o r t c a p a b i l i t y and s u f f e r much l e s s f r o m f l u i d l o s s due t o t h e i r h i g h e r v i s c o s i t y and b e t t e r developed two-phase i n t e r l o c k i n g s t r u c t u r e (FREEMAN, BILDEN & H O S S A I N I 1986; c f . s e c t i o n 4 . 3 . 4 . 3 . ) . C o n v e n t i o n a l l i n e a r foamed g e l s have t h e d i s a d v a n t a g e t h a t i n o r d e r t o m i n i m i z e n i t r o g e n o r carbon d i o x i d e l e a k o f f f o r t h e purpose o f m a x i m i z i n g l i q u i d r e c o v e r y and l i m i t i n g p r o p p a n t s e t t l i n g , w e l l cleanup has t o s t a r t a l r e a d y a c o u p l e o f hours a f t e r t e r m i n a t i o n o f pumping, w i t h energy and v e l o c i t y o f foam r e t u r n s combined w i t h t h e r e l a t i v e l y s h o r t s h u t - i n t i m e o f t e n g i v i n g r i s e t o e x c e s s i v e p r o p p a n t flowback and r e c o v e r y . The h i g h e r v i s c o s i t y and s t a b i l i t y o f delayed foamed c r o s s l i n k e d g e l s enables b e t t e r s u p p o r t and p r o f i l e o f t h e p r o p p a n t package w i t h i n t h e f r a c t u r e , and because phase s e p a r a t i o n o r gas l e a k o f f i s s i g n i f i c a n t l y reduced w i t h a c r o s s l i n k e d foam, the s t a r t o f l o a d r e c o v e r y by w e l l cleanup can be delayed which a l l o w s f o r s u f f i c i e n t c r a c k c l o s u r e r e s t r a i n i n g t h e p r o p p a n t package. The s i g n i f i c a n c e o f i n i t i a l product i o n v i a a s m a l l choke a t r e s t r i c t e d r a t e s f o r maintenance o f o p t i m a l f r a c t u r e performance i s a l s o u n d e r l i n e d by KUNDERT & SMINK ( 1 9 7 9 ) . OSBORNE, McLEOD & SCHROEDER (1981) recommend adding o f e x t r a b r e a k e r t o t h e c r o s s l i n k e d g e l i n t h e l a s t f l u i d pumping stage which h e l p s t o o b t a i n l o w e r f l u i d v i s c o s i t y a f t e r f r a c t u r e closure i n the section a d j o i n i n g t o the w e l l bore, t h e r e b y i m p r o v i n g f l u i d r e c o v e r y and w e l l cleanup. A s s i s t a n c e can be g i ven by t a k i n g samples f r o m t h e l a s t f l u i d i n j e c t i o n stage i n t o t h e l a b o r a t o r y and s u p e r v i s i n g b r e a k e r r e a c t i o n and v i s c o s i t y e v o l u t i o n i n o r d e r t o b e t t e r det e r m i n e t h e most s u i t a b l e t i m e f o r t e r m i n a t i o n o f w e l l s h u t - i n and b e g i n n i n g o f f l u i d cleanup.
4.12.3.1.3. Effects of fracturing fluid and capillary pressure on well cleanup SOLIMAN & HUNT (1985) i l l u s t r a t e some e f f e c t s o f f r a c t u r i n g f l u i d and c a p i l l a r y p r e s s u r e on w e l l cleanup which a l s o have i m p l i c a t i o n s on p r o p p a n t p e r f o r mance i n t h e f r a c t u r e . A f t e r t h e t o t a l s t i m u l a t i o n t r e a t m e n t volume has been pumped, f l u i d l o s s w i l l c o n t i n u e w h i l e t h e p r e s s u r e i n t h e f r a c t u r e d i s s i p a t e s . I m b i b i t i o n may cause t r e a t m e n t f l u i d t o p e n e t r a t e even deeper i n t o t h e format i o n a f t e r t h e f l u i d p r e s s u r e i n t h e c r a c k has e q u a l i z e d w i t h t h e pore p r e s s u r e o f t h e r e s e r v o i r . As a r e s u l t o f t h e f r a c t u r i n g o p e r a t i o n , s t i m u l a t i o n f l u i d e x i s t s i n s i d e t h e c r a c k and f o r a c e r t a i n d e p t h w i t h i n t h e f o r m a t i o n around t h e f r a c t u r e . The invaded zone has a reduced r e l a t i v e p e r m e a b i l i t y t o gas due t o t h e presence o f b o t h gas and l i q u i d . W i t h b e g i n n i n g o f f t a k e , i n i t i a l l y m o s t l y t r e a t m e n t f l u i d i s produced t h u s c l e a n i n g up t h e pay zone and l e a d i n g t o i n c r e a -
687 s i n g gas s a t u r a t i o n i n t h e invaded i n t e r v a l , w i t h t h e cleanup p a t t e r n depending on f o r m a t i o n and c r a c k p r o p e r t i e s . Aspects o f dimensionless f r a c t u r e c o n d u c t i v i t y and c a p i l l a r y p r e s s u r e a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.12.3.1.3.1. Dimensionless fracture conductivity The dimensionless f r a c t u r e c o n d u c t i v i t y o r f l o w c a p a c i t y has p a r t i c u l a r l y s i g n i f i c a n t impact on w e l l cleanup (SOLIMAN & HUNT 1985), w i t h s p e c i a l emphasis b e i n g on t i m e and l o c a t i o n o f gas b r e a k t h r o u g h i n t h e invaded zone. A t low d i mensionless f r a c t u r e c o n d u c t i v i t i e s , t h e gas b r e a k t h r o u g h i n t o t h e c r a c k occurs near t h e w e l l b o r e , i m p l y i n g t h a t e x c e p t f o r t h e b o r e h o l e v i c i n i t y , gas f l o w i n t o t h e f r a c t u r e i s o b s t r u c t e d by t h e presence o f s t i m u l a t i o n f l u i d t h u s g r e a t l y r e d u c i n g c r a c k e f f e c t i v e n e s s . I f p r e s s u r e t e s t s a r e conducted under such cont h e f r a c t u r e w i l l behave as i f b e i n g o f much s h o r t e r l e n g t h c o m p r i s i n g ditions, o n l y a b t . 20 - 30 % o f t h e a c t u a l l e n g t h . As t h e d i m e n s i o n l e s s f r a c t u r e c o n d u c t i v i t y increases, t h e change i n t r e a t ment f l u i d s a t u r a t i o n i n s i d e t h e c r a c k becomes more g r a d u a l , and a t a v a l u e o f a b t . 50, s t i m u l a t i o n f l u i d c o n c e n t r a t i o n i n t h e f r a c t u r e g e t s almost u n i f o r m . A t h i g h e r d i m e n s i o n l e s s f r a c t u r e f l o w c a p a c i t i e s between 200 and 1,000, a m i l d gas b r e a k t h r o u g h occurs a t t h e c r a c k t i p . I n o r d e r t o h e l p a v o i d i n g e a r l y and severe gas b r e a k t h r o u g h a t t h e w e l l b o r e , t h e dimensionless f r a c t u r e c o n d u c t i v i t y s h o u l d be a t l e a s t 50. The optimum dimensionless f r a c t u r e f l o w c a p a c i t y i s i n t h e neighbourhood of 100 o r a b t . t h r e e times t h e v a l u e r e q u i r e d t o produce t h e f o r m a t i o n f l u i d e f f i c i e n t l y and q u i c k l y ( c f . a l s o s e c t i o n 4.8.11.1.1.).
4.12.3.1.3.2. Capillary pressure I n t i g h t gas r e s e r v o i r s , c a p i l l a r y p r e s s u r e can be v e r y h i g h t h u s r e s t r i c t i n g f r a c t u r i n g f l u i d f l o w and r e q u i r i n g a v e r y h i g h p r e s s u r e drawdown f o r c l e a n up t o t a k e p l a c e (SOLIMAN & HUNT 1985). H i g h c a p i l l a r y p r e s s u r e reduces t h e e f f e c t i v e n e s s o f a f r a c t u r e t o produce s t i m u l a t i o n f l u i d s by p r e v e n t i n g w a t e r f r o m f l o w i n g e a s i l y , w i t h a minimum d i f f e r e n t i a l p r e s s u r e b e i n g r e q u i r e d b e f o r e any w a t e r can b e g i n t o f l o w . High c a p i l l a r y p r e s s u r e a l s o i n h i b i t s e a r l y gas b r e a k t h r o u g h a t t h e w e l l b o r e , w i t h gas breakthrough o c c u r r i n g i n areas w i t h l e a s t w a t e r s a t u r a t i o n which a r e u s u a l l y s i t u a t e d a t t h e c r a c k t i p . W i t h i n c r e a s i n g c a p i l l a r y pressure, c u m u l a t i v e f r a c t u r i n g f l u i d p r o d u c t i o n a t a c e r t a i n t i m e d e c l i n e s . High c a p i l l a r y p r e s s u r e means l e s s w a t e r p r o d u c t i o n w h i c h i n t u r n r e s u l t s i n l e s s r e s e r v o i r cleanup. Concerning w a t e r b l o c k e f f e c t s on gas o f f t a k e and w e l l cleanup, t h e phenomenon has no s e r i o u s consequence as l o n g as no permanent damage t o t h e f o r m a t i o n had o c c u r r e d (HOLDITCH 1979; c f . s e c t i o n s 3.11.2.4.2. and 4.8.8.3.1.5.). Waterb l o c k i s a l s o o f no s i g n i f i c a n c e when drawdown p r e s s u r e g r a d i e n t i n t h e near f r a c t u r e f a c e r e g i o n o f t h e m a t r i x i s s e v e r a l hundred p s i / i n . higher than the c a p i l l a r y e n t r y p r e s s u r e (ABRAMS & VINEGAR 1985). As a consequence o f a l l these e f f e c t s , i t i s recommended t o d e s i g n c r a c k c o n d u c t i v i t y s u f f i c i e n t l y h i g h t o adequately cleanup t h e s t i m u l a t i o n f l u i d s (SOLIMAN & HUNT 1985). I n some i n s t a n ces, t h e optimum f r a c t u r e f l o w c a p a c i t y necessary t o c l e a n up t h e f o r m a t i o n p r o p e r l y and r e a s o n a b l y q u i c k l y may be much h i g h e r than t h a t r e q u i r e d f o r gas p r o d u c t i o n f r o m t h e r e s e r v o i r . I t i s a l s o d e s i r a b l e t o a t t e m p t t o reduce c a p i l l a r y p r e s s u r e e f f e c t by any means p o s s i b l e .
4.12.3.2. Proppant grain size G e n e r a l l y t h e l a r g e r t h e p r o p p a n t g r a i n s i z e , t h e b e t t e r t h e p r o p p a n t would s t a y i n t h e f r a c t u r e mouth and flowback can be a l s o m i n i m i z e d by s e l e c t i n g l a r g e r proppants (BARBY & BARBEE 1987). A d d i t i o n a l b e n e f i t c o u l d be achieved by car e f u l s e l e c t i v e o v e r f l u s h i n g o f t h e m i d d l e and/or upper p a r t o f t h e p e r f o r a t e d
688 i n t e r v a l , w i t h s l i g h t o v e r f l u s h i n g sometimes b e i n g a b l e t o save tremendous expenses f o r pump r e p a i r and w e l l b o r e c l e a n o u t . Even w i t h l a r g e p r o p p a n t g r a i n s i z e , however, some embedment must o c c u r t o h o l d t h e p r o p p a n t i n p l a c e . I f t h e f r a c t u r e f a c e a t t h e w e l l b o r e i s s e v e r e l y eroded by h i g h l y - a b r a s i v e l a r g e p r o p p a n t s o r a n n i h i l a t e d by l a r g e volumes o f a c i d , e x c e s s i v e p r o p p a n t flowback may o c c u r . These e f f e c t s can a l s o be avoided by t a i l i n g - i n r e s i n - c o a t e d p r o p p a n t i n o r d e r t o f o r m a s t a b l e wedge near t h e f r a c t u r e mouth ( c f . s e c t i o n 4 . 1 2 . 3 . 3 . ) . Proppant g r a i n s i z e s t a b i l i z a t i o n can a l r e a d y be achieved t o some e x t e n t by conventional t a i l - i n comprising i n s e r t i o n o f coarser proppant grains i n the f i n a l stage t h a n i n t h e main p a r t o f t h e t r e a t m e n t ( c f . s e c t i o n 2 . 4 . 1 . 2 . 3 . 4 . ) . The l a r g e r p r o p p a n t g r a i n s near t h e f r a c t u r e mouth a r e themselves s t a y i n g b e t t e r w i t h i n t h e c r a c k due t o h i g h e r d r a g r e s i s t a n c e and f r i c t i o n , b u t i n a d d i t i o n t o t h a t , t h e coarse proppant t a i l - i n a c t s l i k e a g r a v e l pack on t h e f i n e r g r a i n s o f t h e medial t o d i s t a l p o r t i o n s o f propped f r a c t u r e l e n g t h ( c f . s e c t i o n 5 . 8 . 8 . ) . I n case o f p r o p e r g r a i n s i z e s e l e c t i o n , t h e c o a r s e r t a i l - i n q u a n t i t y grains bridge the f i n e r main-lot p a r t i c l e s a t the i n t e r f a c e o f both proppant stages and p r e v e n t t h e i r f u r t h e r movement towards t h e w e l l b o r e and o u t o f t h e f r a c t u r e mouth. The e f f e c t o f t h e c o a r s e r p r o p p a n t t a i l on t h e f i n e r p r o p p a n t main stage i s comparable t o t h e impact o f p e r f o r a t i o n t u n n e l f i l l i n g s o r b o r e h o l e w a l l l i n i n g s w i t h g r a v e l on r e s e r v o i r f o r m a t i o n sand i n mechanical sand c o n t r o l ( c f . s e c t i o n 5 . 6 . 1 . ) . Aspects o f proppant r e t u r n m i n i m i z a t i o n by t a i l i n f u n c t i o n l i k e a screen a r e a l s o d i s c u s s e d by CROW ( 1 9 7 7 ) .
4.12.3.3. Resin-coated proppant tail-in Proppant package s t a b i l i t y improvement by a p p l i c a t i o n o f r e s i n - c o a t e d g r a i n s i s a common and e f f i c i e n t means o f p r e v e n t i o n o f f r a c t u r i n g o p e r a t i o n f a i l u r e by p r o p p a n t flowback. The most s i g n i f i c a n t aspects o f r e s i n - c o a t e d p r o p p a n t t a i l - i n a r e o u t l i n e d as f o l l o w s .
4.12.3.3.1. Fracture operation failure by proppant flowback D i f f e r e n t p o s s i b i l i t i e s o f f r a c t u r e o p e r a t i o n f a i l u r e by p r o p p a n t flowback can be d i s t i n g u i s h e d c o m p r i s i n g damage s h o r t l y a f t e r t h e h y d r a u l i c s t i m u l a t i o n , s h o r t l y a f t e r w a t e r - f l o o d response, s h o r t l y a f t e r a c i d t r e a t m e n t and a t random (POPE, WILES & P I E R C E 1983). Most o f t h e f a i l u r e s o c c u r w i t h i n 100 days o f t h e f r a c t u r i n g o p e r a t i o n and can be a t t r i b u t e d t o low c l o s u r e p r e s s u r e and i n c r e a sed f l u i d v e l o c i t y t h r o u g h t h e p e r f o r a t i o n s i n t h e s h a l l o w o i l w e l l s . The h i g h p r o d u c i n g f l u i d l e v e l i n b o r e h o l e s t h a t respond w e l l t o t h e f r a c t u r i n g t r e a t ment corresponds t o a h i g h bottomhole f l o w i n g p r e s s u r e and c o n s e q u e n t l y a low c l o s u r e p r e s s u r e which can be i n s u f f i c i e n t t o keep t h e p r o p p a n t i n p l a c e , and i n c r e a s e d f l u i d v e l o c i t y t h r o u g h t h e p e r f o r a t i o n s can d i s p l a c e t h e p r o p p a n t i n t o t h e w e l l b o r e . H i g h e r f l u i d v e l o c i t i e s and r a t e s t h r o u g h t h e p e r f o r a t i o n s a r e a l s o reasons f o r p r o p p a n t flowback a f t e r response t o w a t e r f l o o d i n g and a c i d i z i n g . A c i d i z i n g can even d e s t r o y a c a l c i t e s c a l e which a c t s as a cement h o l d i n g the proppant i n place i n the surroundings o f the wellbore. Proppant flowback does n o t o n l y r e s u l t i n f r a c t u r e damage by removal o f t h e s u p p o r t o f t h e c r a c k and t h e r e b y t r i g g e r i n g i t s p a r t i a l c l o s u r e , b u t s i m i l a r l y as does sand p r o d u c t i o n a l s o l e a d s t o b o r e h o l e o p e r a t i o n a l problems such as s t u c k pump p l u n g e r s , eroded pump v a l v e s , sucker r o d f a i l u r e s , s t u c k t u b i n g and p r o p p a n t fill (POPE, WILES & P I E R C E 1987; c f . s e c t i o n 5 . 2 . 1 . 3 . ) . Proppant f l o w back can a l s o f i l l s u r f a c e equipment w i t h sand which g i v e s r i s e t o i n a c c u r a t e f l u i d measurements and e v e n t u a l l y r e q u i r e s e x t e n s i v e c l e a n i n g . I n many cases, t h e c o s t t o r e t u r n a w e l l t o o i l p r o d u c t i o n a f t e r f a i l u r e by p r o p p a n t flowback has been so e x c e s s i v e t h a t t h e w e l l had t o be p r e m a t u r e l y abandoned. I n terms o f f r a c t u r e e v a c u a t i o n by p r o p p a n t flowback i n t h e n e a r - w e l l b o r e area, RAYMOND & BINDER (1967) document t h a t n e a r l y a l l o f t h e p o t e n t i a l s t i m u l a -
689 t i o n i s l o s t i f the u n f i l l e d p a r t o f the crack i s o n l y 0.5 % o f the t o t a l f r a c t u r e l e n g t h and i f t h i s p o r t i o n i s adjacent t o the borehole ( c f . a l s o SINCLAIR, GRAHAM & SINCLAIR 1983). I n a d d i t i o n , crack evacuation by proppant flowback causes f r a c t u r e diameter narrowing t o t h a t w i d t h where crack h e a l i n g occurs, and even i f complete c l o s u r e does n o t take place, a narrow f r a c t u r e can be e a s i l y plugged by s p a l l e d pieces and f i n e f o r m a t i o n p a r t i c l e s .
4.12.3.3.2. Package stability inprovement
by resin-coated proppants
Several a l t e r n a t i v e methods have been used t o c o n t r o l proppant flowback f o l lowing f r a c t u r e s t i m u l a t i o n comprising r a i s i n g the pumping equipment above the p e r f o r a t i o n s , u s i n g s p e c i a l sand pumps, i n s t a l l i n g a gravel pack, and r u n n i n g a tubing-conveyed sand f i l t e r , b u t s u f f i c i e n t success was n o t achieved w i t h any o f these procedures. S a t i s f a c t o r y c o n t r o l o f proppant flowback, however, c o u l d be obtained by performing the f r a c t u r i n g treatment w i t h a t a i l - i n o f c u r a b l e r e sin-coated sand ( c f . s e c t i o n 1.2.6.) o f a b t . 10 - 20 % o f the t o t a l proppant q u a n t i t y . The f o l l o w i n g d i s c u s s i o n summarizes some p o i n t s o f proppant flowback c o n t r o l as w e l l as t a i l - i n l e n g t h and a d d i t i o n a l gravel packing s t a b i l i z a t i o n .
4.12 3.3.2.1. Proppant f lowback cont r ol Under c o n d i t i o n s o f r e s e r v o i r temperature and pressure, the r e s i n c h e m i c a l l y bonds together t o form a c o n s o l i d a t e d permeable b a r r i e r a g a i n s t proppant f l o w back once c a r r i e d o u t p r o p e r l y ( c f . s e c t i o n 1.2.6.). Resin-coated proppant t a i l - i n e f f e c t i v e l y prevents f r a c t u r e evacuation i n the near-wellbore area, because the c o n s o l i d a t e d proppant package f i r m l y couples borehole and crack thus l e a d i n g t o optimum drainage o f the p r o d u c t i v e f o r m a t i o n (SINCLAIR, GRAHAM & S I N CLAIR 1983). I n a d d i t i o n t o i n h i b i t i o n o f f r a c t u r e evacuation, proppant f l o w back p r e v e n t i o n g r e a t l y reduces w e l l maintenance, wear, e r o s i o n and abrasion problems. F a i l u r e r a t e s i n f r a c t u r e s w i t h a wedge o f r e s i n - c o a t e d proppants i n the t e r minal i n t e r v a l near the w e l l b o r e are much lower than i n c o n v e n t i o n a l l y equipped boreholes. F a i l u r e s o f t h e r e s i n - c o a t e d sand t o c o n t r o l proppant flowback can be a t t r i b u t e d t o f a i l u r e o f r e s i n bonding and f a i l u r e o f the r e s i n - c o a t e d sand t o cover the whole p e r f o r a t e d i n t e r v a l . I f the r e s i n cures b e f o r e the gel breaks and thus b e f o r e the proppant g r a i n s come i n t o contact, the a b i l i t y o f c o n s o l i d a t i o n o f the proppant package i s l o s t . I f the proppant i s being banked near the w e l l b o r e d u r i n g s t i m u l a t i o n , the c u r a b l e resin-coated sand i n the t a i l - i n w i l l o v e r r i d e the top o f t h e banked proppant and cover o n l y the p e r f o r a t i o n s t h a t are t a k i n g f l u i d . Subsequent a c i d i z i n g does n o t a f f e c t the s t a b i l i t y o f the c o n s o l i d a t e d package due t o r e s i n i n e r t n e s s . P r o d u c t i v i t y losses due t o f r a c t u r e evacuation near the w e l l b o r e by flowback o f uncoated proppants have turned o u t t o be much h i g h e r than r e d u c t i o n s o f e x p l o i t a t i o n due t o s l i g h t l y lower f l o w e f f i c i e n c i e s o f r e s i n - c o a t e d proppant c o n s o l i d a t e d packages w i t h r e s p e c t t o those o f cracks propped w i t h conventional sand, thus u n d e r l i n i n g t h a t a t a i l - i n o f resin-coated sand i s an e f f e c t i v e method o f c o n t r o l l i n g proppant flowback f o l l o w i n g f r a c t u r e s t i m u l a t i o n . Proppant flowback d i m i n u t i o n o r e l i m i n a t i o n by r e s i n - c o a t e d proppant t a i l - i n i n - combination w i t h small-choke cleanup i s - a l s o o u t l i n e d by' GARBIS, BROWN & MAURITZ (1985).
4.12.3.3.2.2. Tail-in length and
additional gravel packing stabi 1 izat ion I n terms o f l e n g t h o f the r e s i n - c o a t e d proppant t a i l - i n ,
the optimum i s
abt.
690
100 f t i n m o s t t r e a t m e n t s o f s t a n d a r d o r a v e r a g e s i z e , because i n c a s e o f s h o r t e r t a i l s some p r o p p a n t f l o w b a c k c a n s t i l l o c c u r due t o n o n u n i f o r m p r o p p a n t d i s t r i b u t i o n and l o n g e r t a i l s do n o t g i v e a s u p e r i o r e f f e c t , b u t a r e o n l y a p r e mium i n v e s t m e n t ( c f . s e c t i o n 2 . 4 . 1 . 2 . 3 . ) . Proppant flowback m i n i m i z a t i o n a f t e r f r a c t u r i n g treatments by a p p l i c a t i o n o f resin-coated t a i l - i n is a l s o discussed b y JEU, PEREZ & WAY ( 1 9 8 8 ) . S i m i l a r l y as a p p l y i n g f o r t o t a l p r o p p a n t q u a n t i t y , t a i l - i n volume and l e n g t h have t o be a d j u s t e d t o t o t a l c r a c k l e n g t h i n o r d e r t o be e f f e c t i v e . W i d e r f r a c t u r e s n e a r t h e b o r e h o l e r e q u i r e l a r g e r t a i l - i n q u a n t i t i e s t o g e n e r a t e t h e m u l t i l a y e r p a c k i n g t o t h e d e s i g n e d l e n g t h , and may a l s o demand l o n g e r t a i l - i n s i n o r d e r t o p r o v i d e a d d i t i o n a l package s t a b i l i t y . R e s i n b o n d i n g o f t a i l - i n l o t s f o r p r o p p a n t s t a b i l i z a t i o n and c r e a t i o n o f a c o n s o l i d a t e d wedge i n t h e b o r e h o l e v i c i n i t y i s p a r t i c u l a r l y s i g n i f i c a n t i n w i d e r c r a c k s where a t l e a s t i n t h e c e n t r a l p a r t some more i n s t a b i l i t y t h a n u s u a l c a n o c c u r . S h o r t e r t a i l - i n s t h a n a c c o r d i n g t o s t a n d a r d d e s i g n c a n be s u f f i c i e n t i f t h e i r p r i m a r y purpose i s t o c a r r y r a d i o a c t i v e markers e i t h e r w i t h i n t h e propp a n t c r y s t a l l a t t i c e o r as s u r f i c i a l p e l l i c l e s f o r f r a c t u r e h e i g h t m o n i t o r i n g b y gamma-ray l o g g i n g ( c f . s e c t i o n 6 . 2 . 1 . 3 . ) . I n e x t r e m e cases, t a i l - i n length may r e a c h f r o m a b t . 1 m ( 3 f t ) i n v e r y s m a l l - s c a l e t r e a t m e n t s o f m i n i - o r m i c r o - s i z e t o a b t . 1,000 f t ( 3 3 0 m) i n j u m b o - t y p e m a s s i v e h y d r a u l i c s t i m u l a t i o n j o b s where t o t a l f r a c t u r e l e n g t h c a n be up t o 4,500 f t (1,500 m). In t e r m s o f c o m p a r i s o n o f j o b s i z e , p r o p p a n t q u a n t i t y and f r a c t u r e l e n g t h p o r t i o n r e q u i r e d f o r a t a i l - i n o f a MHF o p e r a t i o n c a n b e l a r g e r t h a n t h o s e f o r a w h o l e s m a l l e r scale hydraulic s t i m u l a t i o n treatment. P a r t i c u l a r l y i n p o o r l y - c o n s o l i d a t e d f o r m a t i o n s , b o t h p r o p p a n t and f o r m a t i o n sand f l o w b a c k i n t o t h e w e l l c a n be e f f e c t i v e l y c o n t r o l l e d b y s e t t i n g a g r a v e l pack a f t e r t h e t e r m i n a t i o n o f t h e h y d r a u l i c f r a c t u r i n g t r e a t m e n t ( t h i s a p p l i e s e s p e c i a l l y f o r s o f t sands and c h a l k s ) . G r a v e l pack s t a b i l i z a t i o n o f b o t h r e s e r v o i r sand and p r o p p a n t package i n t h e f r a c t u r e i s t h e s a f e s t way o f maximum c o n d u c t i v i t y and e f f e c t i v i t y p r e s e r v a t i o n i n u n c o n s o l i d a t e d p a y zones, b e c a u s e p r o p e r l y s e t g r a v e l pack m a n t l e and p e r f o r a t i o n t u n n e l p l u g s h o l d back e v e r y t h i n g w h i c h t r i e s t o move ( a s p e c t s o f c o m b i n a t i o n o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g a r e a l s o d i s c u s s e d i n s e c t i o n s 4 . 5 . 4 . 3 . and 5 . 3 . 5 . ) .
4.12.3.4. Foam and energized fracturing fluids P r o p p a n t f l o w b a c k p r e v e n t i o n c a n a l s o be a c h i e v e d w i t h u t i l i z a t i o n o f foam and e n e r g i z e d f r a c t u r i n g f l u i d s (HOLCOMB 1983; c f . s e c t i o n 3 . 1 1 . 2 . ) . I f p r o p e r l y c a r r i e d o u t , w e l l c l e a n u p a f t e r foam o r e n e r g i z e d s t i m u l a t i o n t r e a t m e n t s p r o v i d e s maximum f l u i d l o a d r e c o v e r y and minimum p r o p p a n t f l o w b a c k i n t o t h e b o r e h o l e o r t o t h e s u r f a c e . C a u t i o n has t o be e x e r c i s e d , however, on v e l o c i t y o f w e l l cleanup, because t o o r a p i d f l u i d p r o d u c t i o n c o u l d cause foam r e g e n e r a t i o n w h i c h w o u l d t h e n a g a i n be a b l e t o c a r r y p r o p p a n t f r o m t h e f r a c t u r e mouth i n t o t h e b o r e h o l e . W h i l e some p r o p p a n t f l o w b a c k i s a s s o c i a t e d w i t h a l m o s t any t y p e o f f r a c t u r i n g t r e a t m e n t and causes l i t t l e harm i f k e p t t o a minimum, t h e added i n f l u e n c e o f g a s a s s i s t f r o m a foam f r a c t u r i n g o p e r a t i o n c a n i n c r e a s e t h e rnagnitude o f t h i s problem s i g n i f i c a n t l y i f cleanup i s n o t p r o p e r l y c a r r i e d o u t . Propp a n t f l o w b a c k c a n be m i n i m i z e d b y a d e q u a t e l y f l u s h i n g t h e p r o p p a n t s a t l e a s t t o t h e p e r f o r a t i o n s , s l i g h t foam o v e r f l u s h i n g i n o r d e r t o a n t i c i p a t e some r e g e n e r a t i o n , and u t i l i z a t i o n of an a d j u s t a b l e w e l l h e a d choke assembly w i t h a h i g h s t e e l a l l o y o r tungsten carbide m a t e r i a l i n t h e valve seating. Proper procedure f o r foam f l o w b a c k a l l o w s t h e foam t o b r e a k back u n i f o r m l y as i t l e a v e s t h e f r a c t u r e t h r o u g h t h e p e r f o r a t i o n s and t r a v e l s u p t h e w e l l b o r e t u b u l a r s t o t h e s u r face, thus l e a v i n g t h e proppant i n t h e f r a c t u r e . A l t h o u g h i t i s g e n e r a l l y assumed t h a t once c r a c k c l o s u r e i s a c h i e v e d , i t i s i r r e v e r s i b l e , i t c o u l d be p o s s i b l e f o r gas e x p a n s i o n w i t h i n t h e f r a c t u r e t o r e open t h e c r a c k and a l l o w some p r o p p a n t f l o w b a c k , w i t h c o n t i n u o u s c l e a n u p m o n i t o r i n g helping t o prevent the occurrence o f t h i s e f f e c t . Therefore i t i s desir a b l e t o m a i n t a i n t h e f l o w b a c k p r e s s u r e a t a l e v e l a p p r o x i m a t e l y 200 p s i b e l o w
691 t h e p r e s s u r e necessary t o keep f r a c t u r e c l o s u r e s t r e s s and proppant t r a p p e d w i t h i n t h e a p p a r e n t l y h e a l e d c r a c k system.
thereby
leave
the
4.12.3.5. Shut-in time adjustment S i g n i f i c a n t i n f l u e n c e on w e l l cleanup and proppant flowback can a l s o be achieved by s h u t - i n t i m e adjustment (PA1 & GARBIS 1983 b ) . P r o d u c t i v i t y as r e l a t e d t o f l u i d r e t e n t i o n i s a f u n c t i o n o f s h u t - i n p e r i o d a f t e r t r e a t m e n t , and subs t a n t i a l p r o d u c t i v i t y impairment occurs i f t h e w e l l i s l e f t s h u t - i n a f t e r s t i m u l a t i o n f o r l o n g p e r i o d s o f time. I f f o r some reason t h e w e l l has t o be l e f t s h u t - i n , i t s h o u l d be s t i m u l a t e d j u s t p r i o r t o opening. I n o r d e r t o e x p e d i t e cleanup, t h e w e l l s h o u l d be opened w i t h i n a b t . 30 min a f t e r an a c i d t r e a t m e n t and w i t h i n a b t . 2 - 4 h r s a f t e r a f r a c t u r i n g o p e r a t i o n , w i t h f r a c t u r i n g s t i m u l a t i o n s h a v i n g t o be l e f t s h u t - i n l o n g e r i n o r d e r t o a l l o w f o r t h e g e l t o break and t h e c r a c k t o h e a l . E x c e s s i v e l y l o n g s h u t - i n times l e a d t o d i s s i p a t i o n o f s t i m u l a t e d gas i n t o t h e f o r m a t i o n , g i v i n g r i s e t o s u b s t a n t i a l energy and p r e s s u r e l o s s which i s then l a c k i n g i n f l u i d r e c o v e r y .
4.12.4. Proppant crushing A p a r t f r o m o r i n a d d i t i o n t o p r o p p a n t flowback, more o r l e s s severe p r o p p a n t c r u s h i n g can occur d u r i n g cleanup and b a c k - p r o d u c t i o n as a consequence o f l a r g e p r e s s u r e drawdown and r e s u l t i n g h i g h c l o s u r e s t r e s s (ROBINSON, HOLDITCH & WHITEHEAD 1986). F o l l o w i n g t e r m i n a t i o n o f t h e h y d r a u l i c s t i m u l a t i o n o p e r a t i o n , t h e r e i s always a p r e s s u r e drop i n t h e f r a c t u r e due t o i t s f i n i t e c o n d u c t i v i t y (CINCO-LEY, SAMANIEGO & OOMINGUEZ 1978; CINCO-LEY & SAMANIEGO 1981), as i n f i n i t e f r a c t u r e c o n d u c t i v i t i e s (GRINGARTEN, RAMEY & RAGHAVAN 1974, 1975) where no p r e s s u r e d r o p o c c u r s i n t h e c r a c k a r e h a r d l y , i f a t a l l , developed i n n a t u r e (REINICKE, BRINKMANN, SCHWARZ & HUENI 1985). Some aspects o f proppant shock l o a d i n g , p r e s s u r e drawdown and hydrocarbon p r o d u c t i o n r a t e s , and p r o d u c t i o n c y c l i n g by r e p e a t e d w e l l s h u t - i n a r e b r i e f l y o u t l i n e d as f o l l o w s .
4.12.4.1. Proppant shock loading F i e l d e x p e r i e n c e has shown t h a t because t h e maximum c l o s u r e s t r e s s d i d n o t r e a c h t h e boundary l e v e l o f g e n e r a l p r o p p a n t s t a b i l i t y i n a number o f cases, t h e e x p l a n a t i o n o f abundant c r u s h i n g i s t h a t l a r g e r a p i d i n c r e a s e s i n c l o s u r e s t r e s s l e a d t o shock l o a d i n g o f t h e proppants t h u s c a u s i n g g r e a t e r f a i l u r e than u s u a l l y observed w i t h gradual l o a d i n g (ROBINSON, HOLDITCH & WHITEHEAD 1986). App l i c a t i o n o f h i g h - s t r e n g t h proppants m i n i m i z e s proppant c r u s h i n g by shock l o a d ing, b u t i n w e l l s where t h e f r a c t u r e i s i n f i l l e d w i t h low- t o i n t e r m e d i a t e s t r e n g t h proppants, shock l o a d i n g w i t h e x c e s s i v e c l o s u r e s t r e s s e s can be severel y d e t r i m e n t a l t o b o r e h o l e performance, and s i g n i f i c a n t c r u s h i n g o f p r o p p a n t s i n t h e v i c i n i t y o f t h e w e l l b o r e where p r e s s u r e drawdown and f l u i d f l o w r a t e s are greatest i s s e r i o u s l y d e t e r i o r a t i n g the effectiveness o f the e n t i r e created f r a c t u r e (McDANIEL & WILLINGHAM 1978, CARROLL & BAKER 1979). Aspects o f sand s t r e s s e s around t h e b o r e h o l e a r e a l s o d i s c u s s e d by RISNES, BRATLI & HORSRUD ( 1 9 8 2 ) . Some comments a r e o f f e r e d as f o l l o w s on bottom-hole f l o w i n g p r e s s u r e and choke s i z e as w e l l as s t r e s s c o r r o s i o n c r a c k i n g .
4.12.4.1.1. Bottom-hole flowing pressure and choke size F r a c t u r e damage can a l s o o c c u r l o n g t i m e a f t e r t h e f l u i d s have been c l e a n e d up. I n s t a l l a t i o n o f l a r g e chokes on w e l l s i n o r d e r t o i n c r e a s e o i l o r gas p r o d u c t i o n can a l s o cause r a p i d l y r i s i n g c l o s u r e s t r e s s and shock l o a d i n g o f t h e p r o p p a n t s . T h e r e f o r e i t i s v e r y c r i t i c a l t o a v o i d e x e r t i n g q u i c k and e x c e s s i v e c l o s u r e s t r e s s e s on t h e proppants by m i n i m i z i n g drawdown d u r i n g b o t h f r a c t u r i n g f l u i d cleanup and l a t e r hydrocarbon p r o d u c t i o n . F l o w i n g t h e b o r e h o l e s t o o h a r d
692 i n o r d e r t o a t t e m p t t o produce more o i l o r gas o f t e n o n l y l e a d s t o f r a c t u r e det e r i o r a t i o n , because i n most r e s e r v o i r s , t h e a d d i t i o n a l p r e s s u r e drawdown does n o t r e s u l t i n a s u b s t a n t i a l i n c r e a s e i n e x p l o i t a t i o n , as i l l u s t r a t e d by gas del i v e r a b i l i t y r e l a t i o n s h i p s . I f the bottom-hole flowing pressure o f the w e l l i s equal t o 50 % o f t h e average r e s e r v o i r pressure, t h e b o r e h o l e w i l l produce a b t . 75 % o f t h e a b s o l u t e open f l o w . Reducing t h e bottom-hole f l o w i n g p r e s s u r e below t h i s p o i n t w i l l n o t r e s u l t i n reasonable a m e l i o r a t i o n o f p r o d u c t i o n , b u t may cause f r a c t u r e damage (ROBINSON, HOLDITCH & WHITEHEAD 1986). O n l y g r a d u a l l o a d i n g o f t h e proppants occurs i f t h e w e l l i s produced on a small choke, and t h e proppant w i l l never be exposed t o t h e h i g h c l o s u r e s t r e s ses which m i g h t o c c u r i f a l a r g e choke i s used, Concerning a p p l i c a t i o n o f lowt o i n t e r m e d i a t e - s t r e n g t h proppants, o n l y a minimal amount o f p r o p p a n t c r u s h i n g may o c c u r when p r o d u c i n g w i t h a small choke, whereas i r r e p a r a b l e damage c o u l d t a k e p l a c e when u s i n g a l a r g e choke. Severe p r o p p a n t c r u s h i n g due t o h i g h c l o s u r e s t r e s s e s i s a m a j o r problem when e x c e s s i v e p r e s s u r e drawdown o c c u r s e i t h e r i n e a r l y o r l a t e stage o f p r o d u c t i o n , because p r o p p a n t c r u s h i n g reduces f r a c t u r e c o n d u c t i v i t y t o t h e p o i n t where w e l l performance i s s e r i o u s l y a f f e c t e d and u l t i m a t e r e c o v e r y i s d r a m a t i c a l l y reduced (ROBINSON, HOLDITCH & LEE 1 9 8 3 ) .
4.12.4.1.2. Stress-corrosion cracking Another aspect i s s t r e s s c o r r o s i o n c r a c k i n g o f p r o p p a n t g r a i n s depending on the fluid medium (CUTLER, ENNISS, JONES & CARROLL 1983; c f . s e c t i o n 1 . 4 . 1 0 . 3 . 2 . ) . The e a r l i e r decrease i n c o n d u c t i v i t y i n w a t e r as compared t o t h a t i n n i t r o g e n i n proppant t e s t i n g i s ascribed t o stress corrosion cracking, i n d i c a t i n g t h a t f r a c t u r e p r o p a g a t i o n r a t e s depend on t h e environment a t t h e c r a c k t i p and h i g h e r w a t e r vapor c o n c e n t r a t i o n s i n c r e a s e f r a c t u r e growth r a t e . Theref o r e proppant c o n d u c t i v i t y t e s t i n g should be c a r r i e d o u t i n aqueous e n v i r o n ments s i n c e t h i s r e p r e s e n t s t h e w o r s t p o s s i b l e case s i t u a t i o n . Proppant p l a c e ment i n t h e f r a c t u r e i n t h e r e s e r v o i r s h o u l d t h u s be performed w i t h foamed f l u i d s o f h i g h q u a l i t y t o guarantee t h a t f o l l o w i n g q u i c k cleanup o f t h e foam w i t h h i g h c o n t e n t o f gaseous phase, t h e p r o p p a n t package i s i m m e d i a t e l y invaded by f o r m a t i o n gas and t h e r e f o r e has o n l y l i m i t e d p o s s i b i l i t i e s o f c o n t a c t i n g aqueous phases.
4:12.4.2.
Pressure drawdown and hydrocarbon production r a t e s
Aspects o f p r e s s u r e drawdown and hydrocarbon p r o d u c t i o n r a t e s a r e along the l i n e s o f careful slow s t a r t i n g o f w e l l production a f t e r f r a c t u r i n g and g r a v e l packing, e f f e c t o f c r u s h i n g on c o n d u c t i v i t y o f bauxite, f r a c t u r e c o n d u c t i v i t y degradation vs. residual preservation, p a n t c r u s h i n g and c l o s u r e s t r e s s r e s i s t i v i t y i n t e r v a l s .
discussed hydraulic sand and and prop-
4.12.4.2.1. Careful slow s t a r t i n g o f w e l l production a f t e r h y d r a u l i c f r a c t u r i n g o r gravel packing A s h u t - i n o r p r o d u c i n g w e l l s h o u l d t h e r e f o r e never be opened on a l a r g e choke i n s t a n t a n e o u s l y . S t a r t i n g w i t h small chokes, i n c r e a s i n g choke s i z e i n g r a dual increments i n reasonably wide t i m e s t e p s should be performed up t o a c a r e f u l l y s e l e c t e d optimum and e q u i l i b r i u m l e v e l o f hydrocarbon p r o d u c t i o n and f r a c t u r e damage p r e v e n t i o n . I n terms of c r a c k w i d t h v s . proppant t y p e w i t h r e s p e c t t o c r u s h i n g r a t i o , MONTGOMERY & STEANSON (1985) r e p o r t t h a t even though f r a c t u r e s propped w i t h n a t u r a l sand o f t e n a r e w i d e r owing t o t h e l o w e r s p e c i f i c g r a v i t y o f sand, t h e c o n d u c t i v i t y o f c r a c k s supported by b a u x i t e proppants i s up t o a b t . 30 t i m e s h i g h e r because o f t h e h i g h e r s t r e n g t h and v e r y l i m i t e d c r u s h i n g o f b a u x i t e proppants. Experience r e p o r t s o f proppant c r u s h i n g by shock l o a d i n g and c a r e f u l slow p r o d u c t i o n t o m i n i m i z e f r a c t u r e d e t e r i o r a t i o n a r e g i v e n by ROBINSON, HOLDITCH & LEE ( 1 9 8 3 ) . Well cleanup problems i n c l u d i n g r e p e a t e d blow-
693 downs can o f t e n have t h e consequence t h a t e f f e c t i v e s t r e s s a c t i n g on t h e p r o p p a n t package approaches t o t a l c l o s u r e s t r e s s (WARPINSKI, BRANAGAN, SATTLER, LORENZ, NORTHROP, MANN & FROHNE 1983). When combined w i t h c y c l i c f a t i g u e l o a d i n g , proppant c r u s h i n g can c o n s i d e r a b l y reduce f r a c t u r e c o n d u c t i v i t y . S i m i l a r l y as a p p l y i n g f o r h y d r a u l i c f r a c t u r e c o n d u c t i v i t y damage p r e v e n t i o n , g r a v e l pack p e r m e a b i l i t y d e t e r i o r a t i o n can be i n h i b i t e d by c a r e f u l l y and s l o w l y b e g i n n i n g w e l l p r o d u c t i o n a f t e r t e r m i n a t i o n o f t h e sand c o n t r o l t r e a t m e n t (HALLIBURTON 1988). T o t a l f l u i d r a t e and f o r m a t i o n drawdown have t o be k e p t t o a m i nimum d u r i n g t h e f i r s t weeks o f p r o d u c t i o n , w i t h o f f t a k e s t a r t i n g o u t w i t h small choke s i z e s and p o s s i b l y a l s o reduced volumes o f l i f t gas. Such a p r o c e dure s h o u l d m i n i m i z e i n i t i a l g r a v e l movement i n w e l l b o r e and p e r f o r a t i o n t u n n e l s and a l l o w t h e b o r e h o l e t o c l e a n up w i t h minimum drawdown across t h e newly p l a c e d g r a v e l pack. Any r a p i d r e a d j u s t m e n t o f t h e f r e s h l y i n s t a l l e d pack i s lik e l y t o cause f o r m a t i o n f i n e s t o invade p e r f o r a t i o n t u n n e l s and pack m a n t l e and i n t e r m i x w i t h g r a v e l which would l e a d t o p e r m e a b i l i t y l o s s . Care s h o u l d be t a ken d u r i n g swabbing n o t t o o v e r s t r e s s t h e g r a v e l pack. C a u t i o n has p a r t i c u l a r l y t o be e x e r c i s e d i n deep gas w e l l s where t h e most severe c l o s u r e s t r e s s c o n d i t i o n s happen i n e a r l y p r o d u c t i o n h i s t o r y , because t h e amount o f s t r e s s r e q u i r e d t o open t h e f r a c t u r e i s a t t h e h i g h e s t v a l u e . I f t h e b o r e h o l e i s a l l o w e d t o produce a t maximum r a t e o r i s swabbed h a r d d u r i n g i n i t i a l completion, an e x t r e m e l y low p r e s s u r e w i l l be t e m p o r a r i l y c r e a t e d w i t h i n t h e f r a c t u r e and w i l l cause an u n d u l y h i g h c l o s u r e s t r e s s w i t h t h e danger o f severe c o n d u c t i v i t y damage by p r o p p a n t c r u s h i n g (MONTGOMERY & STEANSON 1985).
4.12.4.2.2. Effect o f crushing on
conductivity of sand and bauxite
W h i l e i n some t i g h t gas r e s e r v o i r s even crushed proppants may p r o v i d e s u f f i c i e n t c o n d u c t i v i t y t o reasonably d r a i n t h e hydrocarbon r e s e r v e s (HOLOITCH & LEE 1979; HUEBINGER, WEBSTER, CHISHOLM, VENDITTO & HUNT 1988), proppant c r u s h i n g gen e r a l l y causes s t r a t i f i c a t i o n o f t h e p r o p p a n t bed which i n t u r n w i l l behave l i k e an inhomogeneous f o r m a t i o n and l i m i t f l u i d f l o w (DERBY & SMITH 1979). Propp a n t c r u s h i n g i s evidenced by c o r r e l a t i o n s between c l o s u r e s t r e s s and l o s s i n f r a c t u r e f l o w c a p a c i t y , p r e s s u r e t r a n s i e n t t e s t i n g r e l a t i o n s h i p s , and d i r e c t obs e r v a t i o n o f c o l l a p s e d g r a i n s which have been produced t h r o u g h t u b i n g t o t h e s u r f a c e equipment o r have been b a i l e d o u t o f t h e w e l l d u r i n g workover (TUCKER 1979). Proppant c r u s h i n g i s a l s o t h e reason f o r d i f f e r e n t p r o d u c t i o n h i s t o r i e s e x h i b i t e d by deep gas w e l l s t h a t have been f r a c t u r e d w i t h n a t u r a l sand and s i n t e r e d b a u x i t e (CLARK 1983). Comparison o f performance r e v e a l s t h a t b o r e h o l e s which u t i l i z e d n a t u r a l sand as p r o p p i n g medium possess a much more r a p i d d e c l i n e i n p r o d u c t i o n t h a n w e l l s o f equal c l o s u r e s t r e s s t h a t have been t r e a t e d w i t h s i n t e r e d b a u x i t e . As sandf r a c t u r e d b o r e h o l e s which e x p e r i e n c e l e s s e r values o f c l o s u r e s t r e s s do n o t show such an abnormal d e c l i n e o f f l o w r a t e , t h e comparative assessment c l e a r l y r e v e a l s t h a t t h e more severe d e c l i n e o f some w e l l s i s t h e consequence o f sand c r u s h i n g i n c o n t r a s t t o b a u x i t e s t a b i l i t y . S i m i l a r l y as a p p l y i n g f o r p r o p p a n t c r u s h i n g , choke s i z e c h o i c e i s a v e r y c r i t i c a l aspect a l s o i n view o f p r o p p a n t flowback. Under normal circumstances, i n c r e a s i n g choke s i z e l e a d s t o i n c r e a s i n g p r o d u c t i o n r a t e and d e c r e a s i n g f l o w i n g p r e s s u r e due t o l a r g e r drawdown, b u t t h i s occurs o n l y as l o n g as t h e a d d i t i o n a l c l o s u r e s t r e s s e x e r t e d on t h e p r o p pants does n o t reduce f r a c t u r e c o n d u c t i v i t y by proppant flowback and/or c r u s h ing.
694
4.12.4.2 - 3 . Fracture c o n d u c t i v i t y degradation vs. r e s i d u a l preservation F r a c t u r e c o n d u c t i v i t y d e g r a d a t i o n as a consequence o f p r o p p a n t c r u s h i n g and s u b s e q u e n t m i g r a t i o n o f f i n e s c a n o c c a s i o n a l l y o c c u r even t h o u g h f l o w i n g p r e s s u r e drawdowns were m i n i m i z e d and c l o s u r e s t r e s s e s m a i n t a i n e d w i t h i n t h e e s t a b l i s h e d l i m i t s f o r t h e o r i g i n a l l y chosen p r o p p a n t t y p e (HUEBINGER, WEBSTER, CHISHOLM, VENOITTO & HUNT 1 9 8 8 ) . Case s t u d i e s , however, r e v e a l o c c a s i o n a l l y t h a t some w e l l s h a v i n g s u s p e c t e d f r a c t u r e d e g r a d a t i o n have l i m i t e d p o s t - r e f r a c t u r i n g h i s t o r i e s and o n l y l i t t l e improvement w i t h r e s p e c t t o t h e o r i g i n a l f r a c t u r e ( c f . s e c t i o n 4 . 8 . 9 . ) , and o t h e r b o r e h o l e s have p r o d u c t i o n h i s t o r i e s t h a t show no s i g n s o f f r a c t u r e damage b y p r o p p a n t c r u s h i n g and a p p e a r t o b e c a p a b l e o f draining the reservoir e f f i c i e n t l y without requiring a refracturing treatment. N a t u r a l sand i n s t e a d o f i n t e r m e d i a t e - o r h i g h - s t r e n g t h p r o p p a n t s c a n be sel e c t e d because t h e economics o f h i g h e r - q u a l i t y p r o p p a n t s i n v i r g i n - p r e s s u r e d w e l l s c a n n o t be s u p p o r t e d because o f t h e l a r g e volumes r e q u i r e d f o r an e f f e c t i v e f r a c t u r e , and c l o s u r e s t r e s s e s in p a r t i a l l y d e p l e t e d i n f i l l b o r e h o l e s a r e l o w enough t h a t t h e l i k e l i h o o d o f c r u s h i n g i s n o t s i g n i f i c a n t . These r e l a t i o n s h i p s o u t l i n e t h a t c r u s h i n g o f n a t u r a l sand o r c r y s t a l l i n e a l u m i n a o x i d e and s i l i c a t e p r o p p a n t s w h i c h l e a d s t o f o r m a t i o n o f h a l v e s and q u a r t e r s l e a v e s i n some cases s t i l l s u f f i c i e n t c o n d u c t i v i t y f o r p r o f i t a b l e d r a i n i n g o f t h e r e s e r v o i r , a l t h o u g h c e r t a i n l y t h e optimum i s a v o i d i n g o f any p r o p p a n t damage.
4.12.4.2.4. Proppant crushing and closure s t r e s s r e s i s t i v i t y i n t e r v a l s TUNN ( 1 9 7 1 ) r e p o r t s f r a c t u r e damage b y sand c r u s h i n g f r o m deep s t i m u l a t e d gas w e l l s p r i o r t o c o m m e r c i a l i n t r o d u c t i o n o f s y n t h e t i c h i g h - s t r e n g t h p r o p p a n t s . The m o s t i m p o r t a n t r e a s o n f o r c o m p a r a t i v e l y l o w p r o d u c t i v i t y i n c r e a s e i n dexes i s t h e i n s u f f i c i e n t p r e s s u r e r e s i s t i v i t y o f t h e sand, w i t h r a p i d c r u s h i n g o f t h e sand r e s u l t i n g i n q u i c k p a r t i a l o r a l m o s t c o m p l e t e c l o s u r e o f t h e c r a c k s . The r e d u c t i o n o f t h e h y d r o c a r b o n f l o w r a t e a f t e r i n i t i a l improvement back t o more o r l e s s t h e o r i g i n a l l e v e l t o o k p l a c e in a f e w days t o a c o u p l e o f weeks t i m e . The e x p l a n a t i o n p o s s i b i l i t i e s o f t h i s phenomenon c o m p r i s e l a c k i n g p r e s s u r e r e s i s t i v i t y o f t h e sand t o f o r m a t i o n c l o s u r e s t r e s s a f t e r d e c l i n e o f the treatment i n j e c t i o n pressure, i n s u f f i c i e n t pressure r e s i s t i v i t y o f t h e r e s e r v o i r rock which covers, s u r r o u n d s and i n c l u d e s t h e u n d e s t r o y e d p r o p p a n t g r a i n s , and l a c k i n g sand g r a i n s in t h e p r o x i m a l p a r t o f t h e f r a c t u r e n e a r t h e w e l l b o r e , w i t h t h e consequences o f a l l t h e s e mechanisms b e i n g f r a c t u r e c l o s u r e and c o n d u c t i v i t y d e s t r u c t i o n . As a c o n c l u s i o n o f t h e e v a l u a t i o n o f f r a c t u r e damage and p o s s i b i l i t i e s o f i t s p r e v e n t i o n , ROBINSON, HOLDITCH & WHITEHEAD ( 1 9 8 6 ) recommend maximum c l o s u r e s t r e s s e s o f 4,000 p s i f o r sand, 7,000 p s i f o r i n t e r m e d i a t e - s t r e n g t h and 10,000 p s i f o r h i g h - s t r e n g t h proppants. I n comparison t o most r e s u l t s o f proppant t e s t i n g , however, t h e s e v a l u e s a r e c o n s i d e r e d t o be t o o p e s s i m i s t i c , b u t i n v i e w o f c a r e f u l p l a n n i n g f o r l o n g p r o d u c t i o n h i s t o r y up t o 20 - 30 y e a r s , i t s h o u l d be p r e f e r r e d t o go a b s o l u t e l y o n t h e s a f e s i d e and t o b e t t e r t a k e a more e x p e n s i v e h i g h e r - p e r f o r m a n c e p r o p p a n t r a t h e r t h a n r i s k i n g f r a c t u r e damage b y e x c e s s i v e p r e s s u r e drawdown in l a t e s t a g e s o f r e s e r v o i r d e p l e t i o n .
4.12.4.3.
Production c y c l i n g by repeated
well s h u t - i n
C o n s i d e r a b l e f r a c t u r e c o n d u c t i v i t y damage b y b o t h p r o p p a n t c r u s h i n g and embedment ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) c a n a l s o be caused b y p r o d u c t i o n c y c l i n g due t o r e p e a t e d b o r e h o l e s h u t - i n and a g a i n s t a r t i n g o f f t a k e o f h y d r o c a r b o n s (KIM & WILLINGHAM 1 9 8 7 ) . S h u t - i n o f gas w e l l s h a v i n g been s t i m u l a t e d b y h y d r a u l i c p r o p -
695 p a n t f r a c t u r i n g i s f r e q u e n t l y performed f o r s h o r t e r o r l o n g e r d u r a t i o n due t o demand r e s t r i c t i o n s , c o m p l e t i o n workover o r o t h e r reasons. The s t a b i l i z e d gas p r o d u c t i o n r a t e f o l l o w i n g each s h u t - i n p e r i o d , however, i s l o w e r than t h a t o f t h e p r e v i o u s o f f t a k e c y c l e a t t h e same w e l l b o r e p r e s s u r e (REINICKE, BRINKMANN, SCHWARZ & HUENI 1985; VENTURA 1985), because f r a c t u r e c o n d u c t i v i t y c o n t i n u e s t o decrease f o l l o w i n g t h e subsequent s h u t - i n s as r e f l e c t e d by p r e s s u r e b u i l d u p analysis. Repeated p r o d u c t i o n and s h u t - i n o f t h e f r a c t u r e - s t i m u l a t e d b o r e h o l e causes c y c l i c l o a d i n g o f t h e proppant package i n t h e f r a c t u r e , r e s u l t i n g i n p r o g r e s s i v e c l o s u r e o f t h e c r a c k as a consequence o f b o t h proppant c r u s h i n g and embedment and t h u s g i v i n g r i s e t o s u c c e s s i v e l y decreasing w i d t h o f t h e propped f r a c t u r e w i t h c y c l e s (KIM & WILLINGHAM 1987). Some comments a r e o f f e r e d as f o l l o w s on i r r e c o v e r a b l e f r a c t u r e c o n d u c t i v i t y d i m i n u t i o n as w e l l as c o m b i n a t i o n o f proppant embedment and c r u s h i n g . Comments a r e a l s o o f f e r e d on impact o f s h u t - i n time.
4.12.4.3.1. Irrecoverable fracture conductivity diminution Experimental work has shown t h a t t h e c o n d u c t i v i t y o f t h e propped f r a c t u r e r e mains a t t h e v a l u e a t t a i n e d when maximum c l o s u r e s t r e s s i s e x p e r i e n c e d even i f t h e l o a d i s l a t e r reduced f r o m t h e maximum t o a l o w e r l e v e l . Thus t h e r e d u c t i o n o f c r a c k c o n d u c t i v i t y i s an i r r e c o v e r a b l e process, and c o n d u c t i v i t y d i m i n u t i o n due t o s u s t a i n e d l o a d i n g a t a c o n s t a n t c l o s u r e s t r e s s below t h e a n t i c i p a t e d maximum c l o s u r e s t r e s s i s n o t s i g n i f i c a n t f o r e s t i m a t i n g u l t i m a t e propped f r a c t u r e c o n d u c t i v i t y . C l o s u r e s t r e s s a c t i n g on t h e propped f r a c t u r e c o n t i n u e s t o i n c r e a s e a t a slow r a t e w i t h p r o g r e s s i v e r e s e r v o i r d e p l e t i o n and i s approaching maximum l o a d . Upon s h u t - i n , c l o s u r e s t r e s s i s s l o w l y r e l e a s e d w i t h o u t , however, h a v i n g a r e c o v e r y e f f e c t on f r a c t u r e c o n d u c t i v i t y , and r e o p e n i n g o f t h e w e l l r e p e a t s t h e process o f c y c l i c l o a d i n g , t h e r e b y r e s u l t i n g i n f u r t h e r f r a c t u r e cond u c t i v i t y reduction.
4.12.4.3.2.
Combination of proppant embedment and crushing
The o v e r a l l decrease i n f r a c t u r e c o n d u c t i v i t y upon p r o d u c t i o n c y c l i n g w i t h r e p e a t e d w e l l s h u t - i n i s t h e combined e f f e c t o f proppant embedment i n t o t h e f o r m a t i o n r o c k a t t h e f r a c t u r e w a l l s and o f proppant c r u s h i n g which a r e b o t h perman e n t damage e f f e c t s ( c f . s e c t i o n 4 . 3 . 3 . 1 . ) . The s i g n i f i c a n c e o f p r o p p a n t embedment i s r e f l e c t e d by t h e r e l a t i o n s h i p t h a t t h e h a r d e r t h e r e s e r v o i r r o c k , t h e l e s s e r i s t h e c y c l i c d e t e r i o r a t i o n e f f e c t on f r a c t u r e c o n d u c t i v i t y . L a r g e r proppants o f 12/20 mesh s i z e p e r f o r m b e t t e r i n s o f t f o r m a t i o n r o c k than i n h a r d r e s e r v o i r s under c y c l i c l o a d i n g , whereas a t h i g h c l o s u r e s t r e s s e s above 6,000 p s i , s m a l l e r p r o p p a n t o f 20/40 mesh s i z e show l e s s c y c l i c l o a d i n g damage e f f e c t i n h a r d pay i n t e r v a l s . H i g h proppant c o n c e n t r a t i o n s l e a d i n g t o m u l t i l a y e r propp i n g have an a d d i t i o n a l b e n e f i t by m i n i m i z i n g c y c l i c l o a d i n g d e t e r i o r a t i o n o f f r a c t u r e c o n d u c t i v i t y , because proppant embedment which f r e q u e n t l y causes more damage t o f r a c t u r e f l o w c a p a c i t y than p r o p p a n t c r u s h i n g i s s i g n i f i c a n t l y d i m i n i shed i n m u l t i l a y e r p r o p p a n t packages. As f r a c t u r e c o n d u c t i v i t y cannot be r e g a i n e d by r e l e a s i n g c l o s u r e s t r e s s , det e r i o r a t i o n o f f r a c t u r e f l o w c a p a c i t y s h o u l d be k e p t a t a minimum by a v o i d i n g u n d u l y h i g h c l o s u r e s t r e s s d u r i n g i n i t i a l c o m p l e t i o n and e a r l y p r o d u c t i o n , and l i m i t i n g w e l l s h u t - i n p e r i o d s i n advanced o f f t a k e stages t o a b s o l u t e l y necessar y events, because e v e r y c y c l i n g o f t h e p r o d u c t i o n process i s i r r e v o c a b l y l i n ked w i t h a t l e a s t some f r a c t u r e c o n d u c t i v i t y damage.
696
4.12.4.3.3. Impact o f shut-in time S u b s t a n t i a l w e l l p r o d u c t i v i t y impairment can o c c u r i n case o f l o n g - t i m e s h u t - i n a f t e r f r a c t u r i n g s t i m u l a t i o n (PA1 & G A R B I S 1983). A l t h o u g h a c e r t a i n t i m e o f w e l l s h u t - i n i s necessary t o a l l o w f o r g e l breakage and f r a c t u r e h e a l i n g , e x c e s s i v e l y l o n g s h u t - i n times g i v e r i s e t o d i s s i p a t i o n o f s t i m u l a t i o n gas i n energized treatments i n t o the formation leading t o substantial l o s s i n pressure and energy t h a t i s r e q u i r e d i n l o a d r e c o v e r y . Drainage o f foam d u r i n g t o o l o n g w e l l s h u t - i n b u i l d s up a h i g h h y d r o s t a t i c f l u i d column w h i c h a f t e r recover y o f a gas cap causes t h e flowback t o cease (HOLCOMB 1983). Gas i n t h e foam l o c a t e d i n t h e f r a c t u r e a l s o d i s s i p a t e s i n t o t h e r e s e r v o i r as foam l i f e i s exhausted, which negates t h e advantage o f gas a s s i s t t o r e c o v e r t h e l i q u i d f r o m t h e f a r t h e s t reaches o f t h e c r e a t e d f r a c t u r e system. On t h e o t h e r hand, when w e l l s a r e opened t o o much and t o o q u i c k l y a f t e r a foam f r a c t u r i n g t r e a t m e n t , c a s i n g c o l l a p s e m i g h t occur due t o n e g a t i v e p r e s s u r e s caused by t h e r a p i d v e n t i n g o f gas f r o m t h e w e l l h e a d o v e r r i d i n g t h e expansion p r e s s u r e f a c t o r due t o f r i c t i o n h e a t i n g and b o t t o m - h o l e temperature. V a r i o u s aspects o f f r a c t u r e damage a r e a l s o d i s c u s s e d by AHMED, ABOU-SAYED & JONES (1979); CARROLL & BAKER (1979); BRANAGAN, CIPOLLA, LEE & YAN (1987) and HAWKINS & K I N G ( 1 9 8 8 ) .
4.12.5. Fluid aspects SOLIMAN & HUNT (1986) summarize t h e e f f e c t o f s t i m u l a t i o n f l u i d and i t s cleanup on w e l l performance and c r a c k p r o p e r t i e s . H y d r a u l i c f r a c t u r i n g u s u a l l y i n v o l v e s i n j e c t i o n o f a l a r g e volume o f p r o p p a n t - l a d e n f l u i d i n t o t h e c r a c k i n duced w i t h i n t h e r e s e r v o i r . W i t h c o n t i n u i n g i n j e c t i o n , t h e f r a c t u r e e n l a r g e s and p a r t o f t h e f l u i d l e a k s o f f i n t o t h e m a t r i x . F l u i d l o s s w i l l c o n t i n u e a f t e r t h e t e r m i n a t i o n o f t h e j o b w h i l e f r a c t u r e p r e s s u r e d i s s i p a t e s . I m b i b i t i o n may cause t r e a t m e n t f l u i d t o p e n e t r a t e even deeper i n t o t h e f o r m a t i o n a f t e r t h e f l u i d p r e s s u r e i n t h e f r a c t u r e has e q u a l i z e d w i t h t h e p o r e p r e s s u r e o f t h e r e s e r v o i r . Due t o i n f i l t r a t i o n , t h e invaded pay zone w i l l have a reduced r e l a t i v e p e r m e a b i l i t y t o gas p r i o r t o p r o d u c t i o n o f b o t h gas and w a t e r d u r i n g cleanup. The h i g h w a t e r s a t u r a t i o n w i t h i n f r a c t u r e and f o r m a t i o n r e p r e s e n t s a temporary damage e f f e c t which i s u s u a l l y r e v e r s i b l e by c a r e f u l cleanup o f t h e w e l l ( c f . a l s o s e c t i o n 4 . 8 . 8 . 3 . ) . F r a c t u r e l e n g t h , f r a c t u r e c o n d u c t i v i t y and c a p i l l a r y p r e s s u r e a r e i m p o r t a n t i n f l u e n c e s on t h e c l e a n u p p a t t e r n . An i m p o r t a n t mechanism damaging m a i n l y f o r m a t i o n and s u b o r d i n a t e l y a l s o f r a c t u r e i s w a t e r b l o c k i n g (HOLDITCH 1979, BROWER & MORROW 1983; PENNY, SOLIMAN, CONWAY & B R I S C O E 1983; ABRAMS & VINEGAR 1985; c f . s e c t i o n s 3.11.2.4.2. and 4.8.8.3.1.5.). SOLIMAN & HUNT (1986) f i n d a d i r e c t r e l a t i o n s h i p between f r a c t u r e c o n d u c t i v i t y and s t i m u l a t i o n f l u i d cleanup, which means t h a t e f f e c t i v e f r a c t u r e f l o w c a p a c i t y i n c r e a s e s w i t h p r o g r e s s i v e cleanup o f t h e c r a c k , and t h u s p r o p e r cleanup removes temporary f r a c t u r e d e t e r i o r a t i o n . The optimum c r a c k c o n d u c t i v i t y f o r sound cleanup m i g h t be h i g h e r than t h a t necessary f o r economic a l l y f e a s i b l e gas p r o d u c t i o n . I n v e r y t i g h t r e s e r v o i r s , t h e cleanup o f f r a c t u r i n g f l u i d s m i g h t be l a r g e l y c o n t r o l l e d by t h e c a p i l l a r y p r e s s u r e e f f e c t . The e x i s t e n c e o f a m o b i l e w a t e r s a t u r a t i o n i n t h e c r a c k r e p r e s e n t i n g an apparent f r a c t u r e damage has a pronounced e f f e c t on t h e b u i l d u p p r e s s u r e response which must be r e c o g n i z e d i n o r d e r t o a v o i d m i s i n t e r p r e t a t i o n o f t h e b u i l d u p d a t a ( S O LIMAN & HUNT 1 9 8 6 ) .
4.13. Aggressive fracturing design The s u c c e s s f u l improvement o f t h e European hydrocarbon s t i r n u l a t i o n p o t e n t i a l by a p p l i c a t i o n o f many o f t h e concepts o u t l i n e d above and b e i n g a l r e a d y more o r l e s s common i n t h e USA and o t h e r p a r t s o f t h e w o r l d a l s o s t r o n g l y depends on t h e o p e r a t i n g m e n t a l i t y and t h e encouragement t o t a k e some r i s k i n t o c o n s i d e r a -
697 t i o n , t o o . Many m a r g i n a l r e s e r v o i r s w i l l never be a b l e t o become approached i n an adequate manner u n l e s s a c h a l l e n g i n g procedure i s c a r r i e d o u t which a u t o m a t i c a l l y i n v o l v e s some r i s k . I t may p a r t i a l l y be a d i f f i c u l t d e c i s i o n e s p e c i a l l y i n t h e poor economical s i t u a t i o n i n t h e y e a r s a f t e r t h e 1986 o i l p r i c e c r a s h ( c f . s e c t i o n 2.2.1.1.; i n Europe and o t h e r i n t e r n a t i o n a l markets even accentuat e d by t h e US $ weakness; c f . s e c t i o n 2 . 2 . 1 . 3 . ) , b u t s i m i l a r l y as many g r e a t successes i n t h e p a s t have been performed by r i s k y p i l o t t r e a t m e n t s , i t i s a l s o i n t h e f u t u r e e s s e n t i a l t o open new p o s s i b i l i t i e s by encouraged s o l u t i o n s and an a g g r e s s i v e d e s i g n o f j o b s . Some comments a r e o f f e r e d as f o l l o w s on p r o g r e s s i n s t e a d o f r o u t i n e and r i s k y approaches o f m a r g i n a l r e s e r v o i r s .
4.13.1. Progress instead o f routine An o p e r a t i n g m e n t a l i t y o f a g g r e s s i v e f r a c t u r i n g d e s i g n i n c l u d e s t h e p h i l o s o phy t h a t n o t h i n g must become r o u t i n e , b u t t h e r e has always t o be p r o g r e s s . I n t h i s connection, however, i t i s i n d i s p e n s i b l e t h a t successes a r e accompanied by f a i l u r e s , and i t depends on s k i l l and sometimes a l s o a b i t on l u c k whether t h e r a t i o i s a t t h e b o t t o m o f t h e l i n e e c o n o m i c a l l y f e a s i b l e i n t o t a l o r n o t . On t h e o t h e r hand, much p r o f i t can be missed by a c o n t i n u o u s l y t o o c o n s e r v a t i v e des i g n . I t i s n i c e t o see e v e r y t h i n g r u n n i n g smoothly, b u t one always has t o ask t h e q u e s t i o n i f t h e r e would n o t be a d e s i g n s t i l l i m p r o v i n g t h e performance. WAREMBOURG, KLINGENSMITH, HODGES & ERDLE (1985) emphasize t h a t a s u c c e s s f u l t r e a t ment i s n o t an o p e r a t i o n t h a t has been pumped w i t h o u t problems, b u t r a t h e r a j o b t h a t p r o v i d e s t h e p r o d u c t i o n performance enhancement p r e d i c t e d by t h e des i g n process o r even exceeding t h e f o r e c a s t . HOLDITCH (1984) p r e s e n t s a s u i t a b l e example t o i l l u s t r a t e t h e above suggest i o n s . While a b t . 30 - 50 % o f t h e t o t a l c o s t o f a f r a c t u r i n g t r e a t m e n t i n v o l ves equipment and manpower t h a t i s used d u r i n g t h e j o b and removed a f t e r t h e operation i s finished, t h e overwhelming p a r t o f t h e expenses o f most o f t h e j o b s i s f o r proppants and f l u i d s . As f l u i d l e a k s o f f and i s produced back, t h e o n l y t h i n g t h a t remains w i t h i n t h e f r a c t u r e and determines t h e f a t e o f t h e whole t r e a t m e n t b e i n g e i t h e r success o r f a i l u r e i s t h e p r o p p i n g agent w h i c h was pumped downhole and t h a t i s s u p p o r t i n g t h e c r a c k t o s t a y open and t o a c t as a d r a i n a g e pathway ( c f . a l s o PHILLIPS & ANDERSON 1985, HALL & LARKIN 1986). Theref o r e a w i s e approach t o s t i m u l a t i o n i s t o g e t as much g r a i n s o f t h e designed proppant q u a n t i t y i n t o t h e f r a c t u r e as p o s s i b l e . I f , however, e v e r y j o b goes p e r f e c t l y w i t h no problems o f p u t t i n g t h e proppants away, t h e n t h e f r a c t u r e des i g n i s t o o c o n s e r v a t i v e . E i t h e r money i s b e i n g wasted by u s i n g t o o l a r g e pads, g e l , f l u i d - l o s s a d d i t i v e s o r o t h e r s , o r t h e proppant c o n c e n t r a t i o n b e i n g pumped i s n o t s u f f i c i e n t t o pack t h e f r a c t u r e . A t t e m p t i n g t o determine t h e optimum p r o p p a n t s a t u r a t i o n f o r a zone, t h e p o s s i b i l i t y o f screenouts and p r e s s u r e - o u t s i n c r e a s e s w i t h approaching t h e p h y s i c a l boundary o f uptake and/or placement cap a b i l i t y (SHAH, SMITH & DONALDSON 1983). I t i s c e r t a i n l y n o t necessary t o p r o voke a screenout i n e v e r y t r e a t m e n t , b u t money s h o u l d n o t be wasted by overdesigning the operation. GREGORCZYK, PAULS, HOLTMYER, CHISHOLM & VENDITTO (1984) r e p o r t f i e l d exper i e n c e where b e s t s t i m u l a t i o n r e s u l t s have been o b t a i n e d when an a g g r e s s i v e pumping schedule was used. CONWAY & HARRIS (1982) a l s o emphasize t h a t optimum r e s u l t s o f designed t r e a t m e n t s and m a j o r advantages o f i n n o v a t i v e and c o n v e n t i o n a l components can o n l y be determined f r o m f i e l d t r i a l s and experiments which a u t o m a t i c a l l y have t o i n c l u d e some r i s k .
4.13.2. Risky approach o f marginal reservoirs Thus a t t h e end o f t h e d i s c u s s i o n o f t e c h n o l o g i c a l p o s s i b i l i t i e s o f enhanc i n g t h e h y d r a u l i c p r o p p a n t f r a c t u r i n g p o t e n t i a l i n Europe and o t h e r p a r t s o f t h e w o r l d , t h e g e n e r a l recommendation i s g i v e n t o p e r f o r m more a g g r e s s i v e f r a c t u r i n g designs and a l s o t o c a r r y o u t r i s k y approaches t o m a r g i n a l o i l - and gas-
698 bearing formations. Aggressive s t r a t e g i c a l concepts a r e the o n l y p o s s i b i l i t y o f a c h i e v i n g t h e n e c e s s a r y p r o g r e s s w i t h access t o h i t h e r t o u n c o n v e n t i o n a l r e s e r ves w h i c h a r e i n d i s p e n s i b l e f o r t h e s a t i s f a c t i o n o f t h e i n c r e a s i n g p r i m a r y e n e r gy demand and f o r t h e s e c u r i t y o f some d o m e s t i c independence b y more o r l e s s own h y d r o c a r b o n s u p p l y i n t h e coming decades. NEWENDORP (1975, 1 9 8 3 ) , MEGILL (1984, 1 9 8 5 ) , BLESSING ( 1 9 8 8 ) and GARB ( 1 9 8 8 ) o u t l i n e s t r a t e g i e s f o r i m p l e m e n t i n g r i s k a n a l y s i s i n v a r i o u s h y d r o c a r b o n e x p l o r a t i o n and d e v e l o p m e n t p r o j e c t s . Some s p e c i a l i z e d t e c h n i q u e s f o r a p p l i c a t i o n i n m a r g i n a l o r u n c o n v e n t i o n a l r e s e r v o i r s even a i m p r i n c i p a l l y on s c r e e n o u t e v e n t s i n some s t a g e s o f t h e t r e a t ment o r phases o f f r a c t u r e p r o p a g a t i o n ( s u c h as t i p s c r e e n o u t f r a c t u r i n g ; SMITH, MILLER & HAGA 1987; c f . s e c t i o n 4 . 5 . 4 . 4 . 1 . ) .
4.14. Conclusion The o u t l i n e o f v a r i o u s t e c h n o l o g i c a l p e r s p e c t i v e s o f enhancement o f t h e hyd r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g p o t e n t i a l i n E u r o p e i n t h e p r e s e n t r e v i e w and s t a t u s r e p o r t , as w e l l as t h e a r e a l m a r k e t i n g and e c o n o m i c a l a s sessment c o m p r i s i n g r e v i e w , s t a t u s and f o r e c a s t o f t e c h n i c a l , e n g i n e e r i n g and f i n a n c i a l a s p e c t s o f p r o p p a n t s t i m u l a t i o n i n W e s t e r n and E a s t e r n E u r o p e (MADER 1987; c f . c h a p t e r 2 ) and t h e e v a l u a t i o n o f p r o p p a n t s e l e c t i o n as a consequence o f r e s e r v o i r c o n d i t i o n s and p r o p p a n t p r o p e r t i e s ( c f . c h a p t e r 1) p e r m i t t h e c o n c l u s i o n t h a t numerous p o s s i b i l i t i e s o f h y d r a u l i c p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g e x i s t i n E u r o p e and o t h e r i n t e r n a t i o n a l m a r k e t s i n t h e n e x t y e a r s . I n c a s e o f t h e n e c e s s a r y back-up o f t h e a c t i v i t y b y an a g a i n more c o n s t a n t o i l p r i c e a t a r e a s o n a b l e n i v e a u , p a r t i a l l y a l s o s u p p o r t e d by a s t r o n g e r US 8 , and b y a d e q u a t e p r i c i n g o f p r o p p a n t s and s e r v i c e based o n h i g h t e c h n i c a l q u a l i t y and m a r k e t i n g r e q u i r e m e n t s , b o t h Western and E a s t e r n Europe a r e c o n s i d e r e d t o be e x p a n d i n g s t i m u l a t i o n m a r k e t s w i t h i n c r e a s i n g demand o f h y d r a u l i c p r o p p a n t f r a c t u r i n g i n c l u d i n g promotion o f treatments i n marginal formations i n t h e near future. I t i s , however, e s s e n t i a l t h a t t h e t e c h n i c a l recommendations b y an i n t e g r a t e d m a r k e t i n g s t r a t e g y i n c o r p o r a t i n g a l s o g e o l o g i c a l and r e s e r v o i r e n g i n e e r i n g concepts a r e supported by a s u i t a b l e approach o f t h e i n d i v i d u a l m e n t a l i t i e s i n t h e s u i t e o f d i f f e r e n t European c o u n t r i e s n o t o n l y s p e a k i n g d i f f e r e n t l a n g u a ges, b u t a l s o c o m p r i s i n g a v a r i e t y o f r e s e r v o i r r o c k s w i t h s p e c i f i c p r o b l e m s , and a r e n o t a g a i n i n t e r r u p t e d b y a n o t h e r m a j o r d i s t u r b a n c e o f t h e o i l - and gasp r o d u c i n g i n d u s t r y by a renewed d r a m a t i c a l change o f U S $ exchange r a t e and o i l price.
699
5 . G r a v e l 5.0.Summary
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Gravel p a c k i n g and h y d r a u l i c f r a c t u r i n g a r e t h e main a p p l i c a t i o n s o f n a t u r a l sand and s y n t h e t i c proppants i n o i l and gas r e s e r v o i r s t i m u l a t i o n and complet i o n . Gravel p a c k i n g i s t h e most common sand c o n t r o l method c o m p r i s i n g mechanic a l s t a b i l i z a t i o n o f p e r f o r a t i o n t u n n e l s and b o r e h o l e w a l l a g a i n s t i n v a s i o n o f l o o s e f o r m a t i o n sand. Sand c o n t r o l methods comprise mechanical b r i d g i n g i n s t a l l a t i o n s such as g r a v e l packs, s l o t t e d l i n e r s , wire-wrapped screens o r p r e packs; chemical c o n s o l i d a t i o n by i n j e c t i o n o f s o l i d i f y i n g f l u i d s ( m a i n l y p l a s t i c r e s i n s ) i n t o t h e f o r m a t i o n t o p r o v i d e an i n - s i t u g r a i n - t o - g r a i n cementat i o n , and maintenance of a h i g h l e v e l o f g r a i n - t o - g r a i n s t r e s s i n t h e r e s e r v o i r sand w i t h i n t h e b o r e h o l e w a l l and i t s near s u r r o u n d i n g s t h r o u g h sand a r c h i n g i n t h e pay zone g r a i n f a b r i c , w i t h t r i g g e r i n g o f t h e g e o m e t r i c a l arrangement b e i n g supported o r enhanced by u s i n g l o n g i n f l a t a b l e packers. Gravel p a c k i n g i s by f a r t h e most d i s t r i b u t e d and p o p u l a r sand c o n t r o l method due t o i t s comparativel y s i m p l e t e c h n i c a l procedure, good economical f e a s i b i l i t y , h i g h success r a t e and widespread a p p l i c a b i l i t y . A l t h o u g h many pay zones may produce up t o a c e r t a i n l i m i t r a t e w i t h o u t sand i n f l u x o r w i t h t e c h n i c a l l y t o l e r a b l e sand i n v e n t i o n l e v e l s , p r o f i t a b l e r e s e r v o i r e x p l o i t a t i o n can o n l y be performed i f t h e f o r m a t i o n sand i s s u f f i c i e n t l y s t a b i l i z e d by e i t h e r n a t u r a l o r a r t i f i c i a l means. Gravel p a c k i n g i n c l u d e s r u n n i n g o f a s l o t t e d l i n e r o r wire-wrapped screen i n t o t h e open o r cased b o r e h o l e and s u r r o u n d i n g i t w i t h a g r a v e l m a n t l e t h a t c o v e r s t h e w e l l b o r e w a l l and s u p p o r t s t h e p e r f o r a t i o n t u n n e l s which a r e a l s o f i l l e d w i t h g r a v e l i n cased h o l e s o r p r e v e n t s t h e r e s e r v o i r f a c e s e c t i o n f r o m c o l l a p s e i n open h o l e s . Gravel p a c k i n g d e s i g n p r i m a r i l y i n c l u d e s s e l e c t i o n o f g r a v e l t y p e a c c o r d i n g t o r e s e r v o i r depth, temperature and b r i n e composition; c h o i c e o f g r a v e l g r a i n s i z e i n o r d e r t o c o m p l e t e l y r e t a i n movement o f f o r m a t i o n sand, sel e c t i o n o f screen s i z e and/or l i n e r s l o t openings f o r t h e purpose o f p r e v e n t i n g b o t h g r a v e l and r e s e r v o i r f i n e s t o e n t e r t h e gravel-pack hardware, g r a v e l pack t h i c k n e s s aiming on achievement o f maximum f i l t r a t i o n c a p a c i t y and minimum hydrocarbon f l o w r a t e impairment, and p e r f o r a t i o n t u n n e l l e n g t h f o r g u a r a n t e i n g s t a b l e p e r f o r a t i o n h o l e s and c r e a t i n g a s u f f i c i e n t c a p t u r e r a d i u s f o r t h e e f f l u e n t hydrocarbons, as w e l l as b o r e h o l e s i z e and g r a v e l placement t e c h n i q u e . W h i l e most o f t h e i n s i d e g r a v e l packs w i t h i n t h e w e l l b o r e a r e n o t u n d e r g o i n g h i g h e r p h y s i c a l s t r e s s e s and t h e r e f o r e w i t h e x c e p t o f steam-drive enhanced o i l r e c o v e r y o p e r a t i o n s n a t u r a l sand i s s t i l l s u i t a b l e as g r a v e l m a t e r i a l , t h e g r a i n s p l u g g i n g t h e p e r f o r a t i o n t u n n e l s may be s u b j e c t e d t o c o n s i d e r a b l e d i f f e r e n t i a l s t r e s s e s and p r e v e n t i o n o f p e r f o r a t i o n h o l e c o l l a p s e r e q u i r e s i n some cases s e l e c t i o n o f h i g h - q u a l i t y s y n t h e t i c proppants w i t h s u f f i c i e n t c l o s u r e s t r e s s r e s i s t i v i t y . The main reasons f o r t h e predominant t o almost e x c l u s i v e app l i c a t i o n o f n a t u r a l sand as g r a v e l p a c k i n g m a t e r i a l a r e i t s ready n e a r l y u n l i m i t e d a v a i l a b i l i t y and i t s cheap p r i c e , w i t h payment o f a premium f o r a h i g h q u a l i t y s y n t h e t i c p r o p p a n t t y p e i n s t e a d o f n a t u r a l sand i n most o f t h e s h a l l o w r e s e r v o i r depths where g r a v e l p a c k i n g i s c a r r i e d o u t n o t b e i n g t e c h n i c a l l y and e c o n o m i c a l l y j u s t i f i e d . I n boundary cases when deeper pay zones r e q u i r e g r a v e l p a c k i n g o r s h a l l o w e r u n c o n s o l i d a t e d sands a r e overpressured, t h e s e c t i o n s o f t h e g r a v e l pack which a r e most s u s c e p t i b l e t o d e t e r i o r a t i o n by g r a i n c r u s h i n g due t o s u p e r c r i t i c a l c l o s u r e s t r e s s a r e t h e plugged p e r f o r a t i o n t u n n e l s and t h e g r a v e l nodes a t t h e d i s t a l ends o f t h e p e r f o r a t i o n tubes where t h e more o r l e s s f u l l l i t h o s t a t i c p r e s s u r e i s r e l e a s e d and t h e e f f e c t i v e c l o s u r e s t r e s s a l s o r i ses w i t h r e s e r v o i r p r e s s u r e d e c l i n e d u r i n g p r o g r e s s i v e d e p l e t i o n . A s t h e p e r f o r a t i o n t u n n e l s a r e s i t u a t e d i n t h e immediate v i c i n i t y o f t h e w e l l b o r e where c l o s u r e s t r e s s upon p r e s s u r e drawdown reaches i t s maximum, a t t e n t i o n has t o be p a i d t o d e s i g n t h e m a t e r i a l i n f i l l i n g h y d r a u l i c f r a c t u r e p a t h as w e l l as g r a vel-packed p e r f o r a t i o n t u n n e l s i n t h e n e a r e s t s u r r o u n d i n g s o f t h e w e l l b o r e i n such a manner t h a t t h e p l u g g i n g i s adequate t o t h e h i g h e s t p o s s i b l e c l o s u r e s t r e s s t h r o u g h o u t b o r e h o l e l i f e . Gravel g r a i n s i z e c h o i c e i s t h e key i t e m i n g r a v e l pack d e s i g n and has a much g r e a t e r importance than i n h y d r a u l i c f r a c t u r i n g m o d e l l i n g where proppant t y p e i s t h e most d e c i s i v e p o i n t t h a t i n t u r n i s
700 o f c o m p a r a t i v e l y s u b o r d i n a t e s i g n i f i c a n c e i n g r a v e l p a c k i n g . In o r d e r t o complet e l y b r i d g e f o r m a t i o n g r a i n s m i g r a t i n g t o t h e s a n d / g r a v e l i n t e r f a c e and f o r t h e p u r p o s e o f t o t a l l y s t o p p i n g i n v a s i o n o f sand i n t o t h e b o r e h o l e , t h e mean s i z e o f t h e g r a v e l s h o u l d be a b t . f i v e t o s i x t i m e s l a r g e r t h a n t h a t o f t h e sand. F o r m o s t o f t h e r e s e r v o i r s r e q u i r i n g sand c o n t r o l b y g r a v e l p a c k i n g , g r a v e l g r a i n s i z e s e l e c t i o n c a n be summarized t h a t a r a t i o o f g r a v e l median d i a m e t e r / f o r m a t i o n sand median d i a m e t e r between 5 and 6 i s t h e i d e a l r a n g e p r o v i d i n g abs o l u t e s t o p p a g e o f sand movement and u n r e s t r i c t e d i n i t i a l g r a v e l p e r m e a b i l i t y . R a t i o s b e l o w 5 g i v e r i s e t o l i m i t e d h y d r o c a r b o n p r o d u c t i o n , because p r o p e r b r i d g i n g a t t h e g r a v e l i n t e r f a c e i s n o t p o s s i b l e , b u t y e t no i n v a s i o n o f t h e g r a v e l pack b y f o r m a t i o n sand o c c u r s . R a t i o s between 6 and 11 p r o v o k e i n t r u s i o n o f r e s e r v o i r sand i n t o t h e g r a v e l pack w h i c h d r a s t i c a l l y r e d u c e s g r a v e l p e r m e a b i l i ty, b u t s t i l l p r o v i d e s e f f e c t i v e sand c o n t r o l once t h e g r a v e l pack i s c o m p l e t e l y p l u g g e d w i t h f i n e p a r t i c l e s and f u r t h e r m o t i o n o u t o f t h e p a y zone i s i n h i b i t e d . A t r a t i o s i n e x c e s s o f 11, t h e p o r e s i n t h e g r a v e l package become s o l a r g e t h a t f o r m a t i o n sand i s p a r t i a l l y m i g r a t i n g t h r o u g h t h e g r a v e l i n t o t h e w e l l bore, thereby i n c r e a s i n g g r a v e l p e r m e a b i l i t y again, but already progressively d e t e r i o r a t i n g h y d r o c a r b o n e x p l o i t a t i o n b y h a v i n g o n l y one e f f e c t o f s l o w i n g down o f sand p r o d u c t i o n . A t a r a t i o o f a b t . 15, o r i g i n a l g r a v e l pack p e r m e a b i l i t y i s r e a c h e d a g a i n w h i c h r e f l e c t s t h a t a t r a t i o s above 15, f o r m a t i o n sand c a n i n v a d e t h e b o r e h o l e b y passage t h r o u g h t h e g r a v e l pack w i t h o u t any r e s t r i c t i o n , w i t h i n t h i s c a s e t h e g r a v e l pack b e i n g c o m p l e t e l y u s e l e s s . A l t h o u g h t h e g r a v e l pack i t s e l f r e p r e s e n t s some t y p e o f f o r m a t i o n damage b y c a u s i n g p r e s s u r e l o s s e s a c r o s s t h e p e r f o r a t i o n s and f l u i d f l o w r e s t r i c t i o n t h r o u g h t h e c y l i n d r i c a l g r a v e l mantle, the p o s i t i v e e f f e c t i s s t i l l b e t t e r than i t s negative influence, and p r e s s u r e d r o p and f l o w a r e a c a n be i m p r o v e d b y i n c r e a s i n g p e r f o r a t i o n d e n s i t y and d i a m e t e r o r even s w i t c h i n g t o o p e n - h o l e c o m p l e t i o n i f t e c h n i c a l l y p o s s i b l e . Gravel-pack completions w i t h plugging o f long deeply p e n e t r a t i n g p e r f o r a t i o n t u n n e l s o f w i d e d i a m e t e r and n a r r o w s p a c i n g a l o n g t h e b o r e h o l e w a l l a r e app r o a c h i n g m o r p h o l o g y and e f f e c t i v i t y o f h y d r a u l i c p r o p p a n t m i n i f r a c t u r i n g and t h u s s i g n i f i c a n t l y i n c r e a s i n g t h e e f f e c t i v e b o r e h o l e c r o s s - s e c t i o n . In some r e s e r v o i r s , s l o t t e d l i n e r o r w i r e - w r a p p e d s c r e e n c a n even b e r u n w i t h o u t i n s e r t i n g g r a v e l a r o u n d i t , because i f f o r m a t i o n sand g r a i n s i z e i s s u f f i c i e n t l y c o a r se t o p r o v i d e p e r m e a b i l i t y , i n f l u x o f p a y zone sand w o u l d c r e a t e s a n a t u r a l g r a v e l pack a r o u n d t h e sand c o n t r o l h a r d w a r e i n s t a l l e d i n t h e open b o r e h o l e . I n terms o f c a r r i e r f l u i d s , l o w - v i s c o s i t y f l u i d s can o n l y t r a n s p o r t low g r a v e l conc e n t r a t i o n s and have t o be pumped a t h i g h v e l o c i t i e s w h i c h can p r o v o k e i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand, whereas h i g h - v i s c o s i t y f l u i d s c a n suspend h i g h g r a v e l s a t u r a t i o n s and c a n be i n j e c t e d a t l o w e r v e l o c i t i e s t h u s e l i m i n a t i n g g r a i n c o n t a m i n a t i o n . The m o s t i m p o r t a n t a s p e c t o f g r a v e l p a c k i n g c a r r i e r f l u i d s i s s u f f i c i e n t l e a k o f f i n t o t h e f o r m a t i o n i n o r d e r t o achieve t h e necessar y t i g h t p a c k i n g o f t h e g r a v e l , w h i c h i s t h e second m a j o r d i f f e r e n c e t o h y d r a u l i c f r a c t u r i n g where f l u i d l o s s has t o be m i n i m i z e d i n o r d e r t o a v o i d p r o p p a n t b r i d g i n g and p r e m a t u r e s c r e e n o u t f a i l u r e . G r a v e l p a c k i n g r e s u l t s a r e i m p r o v i n g w i t h p r o g r e s s i v e f l u i d l e a k o f f and t h e r h e o l o g i c a l p r o p e r t i e s o f sand c o n t r o l t r a n s p o r t f l u i d s t h e r e f o r e have t o b e a d j u s t e d such t h a t s a t i s f a c t o r i l y h i g h r a t e s o f f l u i d l e a k o f f a r e achieved. F l u i d l o s s i n t o l i n e r o r screen and/or format i o n d u r i n g g r a v e l p a c k i n g i s n e c e s s a r y t o t a k e p l a c e i n h i g h r a t e s in o r d e r to a l l o w d e p o s i t i o n o f t h e g r a v e l on t h e b o r e h o l e w a l l where i t f o r m s a m a n t l e c o a t i n g t h e r e s e r v o i r / w e l l b o r e i n t e r f a c e , spearheads p l u g g i n g t h e p e r f o r a t i o n t u n n e l s , a b e l t b r i d g i n g t h e a n n u l u s between b o r e h o l e b o u n d a r y f a c e and s c r e e n o r l i n e r , o r a p l u g i n f i l l i n g t h e whole diameter o f t h e w e l l b o r e . Steam-drive enhanced o i l r e c o v e r y i s a p a r t i c u l a r l y h o s t i l e e n v i r o n m e n t , because t h e h i g h pH s o l u t i o n s a t t a c k f o r m a t i o n sand and p a c k i n g g r a v e l and d i s s o l v e i t , t h e r e b y d e t e r i o r a t i n g t h e sand c o n t r o l i n s t a l l a t i o n . S i n t e r e d b a u x i t e and n i c k e l - c o a t e d sand have been p r o v e n t o r e p r e s e n t s u i t a b l e m a t e r i a l s w h i c h a r e m e c h a n i c a l l y and c h e m i c a l l y s t a b l e u n d e r t h e s e h a r s h c o n d i t i o n s , and w h i l e s i n t e r e d b a u x i t e has a l r e a d y been a b u n d a n t l y a p p l i e d i n t h e f i e l d s i n c e many y e a r s , n i c k e l - c o a t e d sand i s s t i l l i n e x p e r i m e n t a l s t a g e . S p e c i a l g r a v e l pack a p p l i c a t i o n s i n c l u de c o n s o l i d a t e d pack i n s t a l l a t i o n and r e s i n - c o a t e d g r a v e l p l a c e m e n t where t h e i n d i v i d u a l g r a v e l g r a i n s a g g r e g a t e t o a s t a b l e m a t r i x due t o t h e c e m e n t i n g a c t i o n o f t h e r e s i n p e l l i c l e s , r e s u l t i n g i n o r i g i n of a s y n t h e t i c s a n d s t o n e . The
701 main advantage of these systems i s t h a t o n l y an o u t s i d e g r a v e l pack i n f i l l i n g t h e p e r f o r a t i o n t u n n e l s has t o be performed, because b a c k f l o w o f g r a v e l f r o m t h e p e r f o r a t i o n h o l e s i s n o t p o s s i b l e and t h e r e f o r e s u p p o r t o f t h e c a s i n g w a l l by a g r a v e l m a n t l e i s n o t necessary, t h u s l e a v i n g t h e b o r e h o l e f r e e and unobs t r u c t e d by any sand c o n t r o l hardware. Gravel p a c k i n g i n d e v i a t e d w e l l s i s mainl y i n f l u e n c e d by t h e phenomenon o f g r a v e l duning which causes i n c o m p l e t e annul u s p l u g g i n g and can even provoke premature annulus b r i d g i n g . Adequate g r a v e l p a c k i n g i n h i g h l y - i n c l i n e d t o h o r i z o n t a l b o r e h o l e s can be achieved by f l e x i b l e b a f f l e s c r e a t i n g f l o w r e s i s t a n c e , l a r g e - d i a m e t e r washpipe, s t r i n g r o t a t i o n and p e r f o r a t i o n p r e p a c k i n g . L i n e r v i b r a t i o n d u r i n g g r a v e l p a c k i n g changes t h e pack c o n f i g u r a t i o n f r o m c u b i c t o hexagonal and a l l o w s t o u t i l i z e c o n s i d e r a b l y c o a r s e r g r a v e l g r a i n s i z e s w i t h t h e same r e s u l t a n t p a c k i n g d e n s i t y than a p p l y i n g f o r f i n e r g r a v e l w i t h o u t v i b r a t i o n , w i t h t h e advantage b e i n g much h i g h e r permeab i l i t y i n case o f shaking. I n terms o f e x e c u t i o n o f g r a v e l pack t r e a t m e n t s dur i n g course o f p r o d u c t i o n h i s t o r y o f p r e d o m i n a n t l y o i l and s u b o r d i n a t e l y a l s o gas w e l l s , t h r e e cases have t o be d i s t i n g u i s h e d : p r i m a r i l y i n s u f f i c i e n t l y cohes i v e pay zones, as w e l l as s e c o n d a r i l y u n s t a b l e hydrocarbon-bearing s e c t i o n s due t o r e s e r v o i r p r e s s u r e d e c l i n e , and w a t e r c u t i n c r e a s e and c o r r e s p o n d i n g f l u i d v i s c o s i t y decrease. The f i r s t group o f f o r m a t i o n s comprises c o m p l e t e l y unc o n s o l i d a t e d sands and c h a l k s which r e q u i r e s t a b i l i z a t i o n by g r a v e l p a c k i n g f r o m t h e v e r y b e g i n n i n g o f p r o d u c t i o n . The main pays i n t h i s group a r e s i l t y and a r g i l l a c e o u s , m a i n l y medium- t o f i n e - g r a i n e d sands as w e l l as s o f t f l o w i n g c h a l k s and diatomaceous e a r t h s which a c c o r d i n g t o t h e i r f i n e g r a i n s i z e , i n s u f f i c i e n t b i n d i n g n a t u r e and l a c k i n g cementation have n o t enough i n t e r n a l cohes i o n t o be s a t i s f a c t o r i l y s t a b l e w i t h o u t a r t i f i c i a l improvement. The second assemblage i n c o r p o r a t e s weakly-cemented sands o f v a r i o u s g r a i n s i z e i n c l u d i n g t h e whole spectrum f r o m almost s i l t t o n e a r l y g r a v e l where t h e problems o f sand i n f l u x i n t o t h e b o r e h o l e s t a r t w i t h p r o g r e s s i v e d e p l e t i o n due t o d e c r e a s i n g r e s e r v o i r p r e s s u r e t h a t i s so f a r m a i n t a i n i n g a more o r l e s s s e l f - s u p p o r t i n g framework o f g r a i n s , w i t h t h e f o r m a t i o n f a i l i n g a t once a c r i t i c a l boundary p r e s s u r e i s reached and underpassed. E a r l y p r e s s u r e maintenance methods such as w a t e r o r gas i n j e c t i o n which a r e v e r y common i n o i l - f i e l d e x p l o i t a t i o n can h e l p t o p r e v e n t t h e sand f r o m becoming s o f t and f l o w i n g , o r can a t l e a s t d e c e l e r a t e t h e coh e s i v i t y transformation, b u t t h e changing phase c o m p o s i t i o n and v i s c o s i t y o f t h e r e s e r v o i r f l u i d s a l s o has i t s impact. The t h i r d a s s o c i a t i o n i n c l u d e s s l i g h t l y - l i t h i f i e d sands o f d i f f e r e n t g r a i n s i z e c o m p r i s i n g a broad range t h r o u g h o u t t h e g r a n u l o m e t r i c a l i n t e r v a l o f sand where g r a v e l p a c k i n g becomes necessary due t o r e s e r v o i r breakdown and d i s i n t e g r a t i o n w i t h i n c r e a s i n g w a t e r c u t i n t h e o i l d u r i n g advanced p r o d u c t i o n w i t h r i s i n g o i l - w a t e r - c o n t a c t accompanying p r e s s u r e d e p l e t i o n , as a consequence o f e x c e s s i v e w a t e r i n j e c t i o n due t o p r e s s u r e m a i n t e nance o p e r a t i o n s , o r as a r e s u l t o f w a t e r c o n i n g and breakthrough due t o u n f a v o u r a b l e d i f f e r e n c e s i n r e l a t i v e p e r m e a b i l i t y and p r e f e r r e d w a t e r c o n d u c t i v i t y a l o n g some h i g h - p e r m e a b i l i t y s t r e a k s o r through t h e j o i n t system. Changing f l u i d v i s c o s i t y r e s u l t s i n d i f f e r e n t cohesion o f t h e g r a i n f a b r i c , and g e n e r a l l y induces l e s s s t a b i l i t y than b e f o r e and t h u s u l t i m a t e l y causes t h e sand t o f l o w . The main members o f t h e second and t h i r d groups o f sand-problem r e s e r v o i r s a r e weakly-cemented f r i a b l e sands which m i g h t be o r i g i n a l l y s t i l l s u f f i c i e n t l y s t a b l e , b u t w i t h f a l l i n g r e s e r v o i r p r e s s u r e and/or changing f l u i d v i s c o s i t y , i n t e r n a l s u p p o r t i s taken away and t h e g r a i n f a b r i c s t a r t s t o c o l l a p s e and t o r e l e a s e more and more l o o s e sand g r a i n s which move towards t h e l e a s t r e s t r i c t e d s p o t t h a t i s r e p r e s e n t e d by t h e b o r e h o l e . Other sand c o n t r o l methods m a i n l y comprise r e s i n c o n s o l i d a t i o n o f t h e r e s e r v o i r sand which i s performed by i n j e c t i o n o f r e s i n i n t o t h e pay zone where i t hardens and forms an a r t i f i c i a l cement o f t h e sand. R e s i n c o n s o l i d a t i o n and g r a v e l p a c k i n g r e p r e s e n t i n g chemic a l and mechanical sand c o n t r o l treatments, r e s p e c t i v e l y , have each t h e i r p r e f e r e n t i a l a p p l i c a t i o n s , b u t can i n some cases a l s o be combined, p a r t i c u l a r l y i n case of c o n s o l i d a t e d pack i n s t a l l a t i o n w i t h accompanying a d d i t i o n a l f o r m a t i o n stabilization. O t h e r chemical sand c o n t r o l o p e r a t i o n s a r e hydroxy-aluminum cons o l i d a t i o n , e l e c t r o l e s s n i c k e l p l a t i n g , s i l i c a cementation and warm a i r c o k i n g . The c o n s t r u c t i o n o f s t a b l e sand arches around t h e p e r f o r a t i o n h o l e s i s t h e exp l a n a t i o n why w i t h i n c r e a s i n g pay depth, sand p r o d u c t i o n t r o u b l e s a r e more and more d i m i n i s h i n g and f i n a l l y v a n i s h i n g . I n s h a l l o w r e s e r v o i r s , s t a b l e sand a r -
702 c h e s c a n be a r t i f i c i a l l y c r e a t e d b y a g i t a t i n g t h e g r a i n f a b r i c b y i n f l a t a b l e p a c k e r s o r v i b r a t i n g s t r i n g s . Open-hole g r a v e l p a c k i n g p r o v i d e s t h e g r e a t e s t i n f l o w a r e a f o r t h e h y d r o c a r b o n s and t h e b e s t f i l t r a t i o n e f f e c t o f t h e sand p l u g , w i t h t h e e f f e c t i v e b o r e h o l e d i a m e t e r b e i n g i n c r e a s e d b y u n d e r r e a m i n g . The m o s t c r i t i c a l p o i n t i n c a s e d - h o l e g r a v e l p a c k i n g i s adequate i n f i l l i n g o f a l l t h e p e r f o r a t i o n t u n n e l s w h i c h have t o b e a b s o l u t e l y c l e a n p r i o r t o g r a v e l i n s e r t i o n , w i t h p e r f o r a t i o n p r e p a r a t i o n b e i n g p o s s i b l e b y washing, s u r g i n g o r u n d e r balanced shooting. 5.1.
I n t r o d u c t i o n ........................... . . . . . . . . . . . . . . 710 p a c k i n g ..... 710 5.1.1. R e s e r v o i r composition spectrum r 5.1.2. Sand s t a b i l i z a t i o n and p r o d u c t i v i t y m a i n t e n a n c e . . 5.1.3. O r g a n i z a t i o n o f t h e r e v i e w and s t a t u s r e p o r t . . . . .
5.2.
Gravel types and grain sizes 5.2.1.
5.2.2.
5.2.3.
........................................ 711 Sand p r o d u c t i o n and i t s c o n t r o l . . . . . . . . . . . . . . . . . 712 5.2.1.1. G e n e r a l a s p e c t s ................................... 712 . . . . . . . . . . . . . . . . . 712 5.2.1.2. Reasons o f sand p r o d u c t i o n ... 5.2.1.3. W e l l d e t e r i o r a t i o n b y sand p r o d u c t i o n . . . 713 5.2.1.3.1. Damage s p e c t r u m caused b y sand i n f l u x . 713 5.2.1.3.1.1. E q u i p m e n t a b r a s i o n . . . . . . 713 5.2.1.3.1.2. S a c r i f i c a t i o n o f hydroc a r b o n e x p l o i t a t i o n t i m e 714 5.2.1.3.2. Economical and t e c h n i c a l l o s s o f w e l l s 714 714 5.2.1.3.3. O t h e r a s p e c t s ......................... 5.2.1.4. C r i t i c a l hydrocarbon o f f t a k e r a t e ... . . . . . . 715 5.2.1.4.1. Toleration level o f j o i n t h ........... 715 and sand p r o d u c t i o n . . . . 5.2.1.4.1.1. E x c e p t i o n a l l y h i g h sand c o n t e n t s i n oil p r o d u c 715 t i o n ...__.._............ 5.2.1.4.1.2. Sand p r o d u c t i o n v s . h y drocarbon e x p l o i t a t i o n 715 r a t e .................... 5.2.1.4.2. I n f l u e n c e o f p e r f o r a t i o n s . . . . . . . . . . . . . 716 5.2.1.4.2.1. G e n e r a l a s p e c t s . . . . . . . . . 716 5.2.1.4.2.2. Selective perforation o f h a r d l a y e r s i n i n t e r b e d s 716 5.2.1.5. N e c e s s i t y and aims o f sand c o n t r o l t r e a t m e n t s . . . . . 717 5.2.1.5.1. Sand c o n t r o l method s p e c t r u m . . . . . . . . . . 717 5.2.1.5.2. Formation c vs. gravel . . . . . . . . . . . . . . . 717 p a c k i n g ... 5.2.1.5.3. T i m i n g o f sand c o n t r o l i n s t a l l a t i o n ... 718 -. . 118 G r a v e l t y p e . . . . . . ........................................... a s p e c t s o f g r a v e l t y p e c h o i c e . . . . . . . . . . . . . 718 5.2.2.1. General 719 5.2.2.2. N a t u r a l sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 . 2 . 2 . 2 1. R e s e r v o i r d e p t h and c l o s u r e s t r e s s i n p e r f o r a t i o n t u n n e l s . . _ _ . . . . . . . . . . . . . . .719 5 . 2 . 2 . 2 2 . Provenance o f g r a v e l p a c k i n g sand . . . . . 720 720 5.2.2.3. Synthet c proppants ............................... 5 . 2 . 2 . 3 1. Sand c o n s o l i d a t i o n and c l o s u r e s t r e s s t r a n s f e r .............................. 720 5 . 2 . 2 . 3 2. P e r f o r a t i o n t u n n e l and b o r e h o l e w a l l 721 s t a b i l i t y ............................. 5 . 2 . 2 . 3 . 3. A n g l e o f r e p o s e and g r a v e l i n f i l l i n g . . 7 2 1 721 Gravel g r a i n s i z e ........................................... 5.2.3.1. G e n e r a l a s p e c t s o f g r a v e l g r a i n s i z e s e l e c t i o n . . . . 722 5 . 2 . 3 . 1 . 1. Common q r a v e l q u a n t i t i e s and q r a i n s i z e 722 5.2.3.1.2. P r e v e n t i o n o f f o r m a t i o n sand j n t r u s i o n 722 5.2.3.1.3. G r a v e l q u a l i t y demand . . . . . . . . . . . . . . . . . 723 5.2.3.2. Gravel g r a i n s i z e s e l e c t i o n vs. f o r m a t i o n g r a i n 723 s i z e ..............................................
703 Packing g r a v e l / f o r m a t i o n sand s i z e r a t i o s . . . . . . . . . 5.2.3.3.1. A b s o l u t e stoppage o f moving f o r m a t i o n sand .................................. 5.2.3.3.2. Gravel p e r m e a b i l i t y and p a c k i n g d e n s i t y 5.2.3.3.2.1. P o s s i b l e range o f pack gravel/paysand s i z e r a t i o 5.2.3.3.2.2. I d e a l l e v e l o f pack g r a v e l / p a y sand s i z e r a t i o . 5.2.3.3.3. F o r m a t i o n p e r m e a b i l i t y and g r a n u l o m e t r y 5.2.3.3.4. Impact o f f o r m a t i o n and g r a v e l f i n e s .. 5.2.3.4. Width o f g r a v e l m a n t l e ............................ 5.2.3.5. Impact o f g r a v e l pack hardware .................... 5.2.3.5.1. S l o t t e d l i n e r s vs. wire-wrapped screens 5.2.3.5.1.1. General aspects ......... 5.2.3.5.1.2. S l o t and w i r e spacing sel e c t i o n ................. 5.2.3.5.1.3. Mechanical l i m i t a t i o n o f s l o t and w i r e w i d t h . . . . . 5.2.3.5.2. Packing hardware i n f l u e n c e on g r a v e l q u a l i t y ............................... 5.2.3.5.2.1. P o t e n t i a l g r a v e l breakup p o i n t s and improvement .. 5.2.3.5.2.2. F l u i d v i s c o s i t y and v e l o c i t y .................... 5.2.3.6. Use o f screen o r l i n e r w i t h o u t g r a v e l pack . . . . . . . . 5.2.3.6.1. G r a v e l - l e s s hardware i n s t a l l a t i o n ..... 5.2.3.6.1.1. S l o t / w i r e w i d t h and n a t u r a l g r a v e l pack b u i l d i n g 5.2.3.6.1.2. T r i p l e - w r a p p e d screen ... 5.2.3.6.2. L i n e r l e s s g r a v e l placement ............ 5.2.3.7. Formation g r a i n s i z e evaluation . . . . . . . . . . . . . . . . . . . 5.2.3.7.1. G r a i n s i z e v a r i a t i o n s i n heterogeneous r e s e r v o i r s ............................ 5.2.3.7.2. Adjustment o f g r a v e l g r a i n s i z e t o f i n e s t pay beds ......................... 5.2.3.8. G r a i n s i z e i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand 5.2.3.8.1. H i g h g r a v e l c o n c e n t r a t i o n and h i g h f l u i d v i s c o s i t y ....................... 5.2.3.8.2. Tackifying a c t i o n o f r e s i n i n the s l u r r y .................................... 5.2.3.9. Hydrocarbon p r o d u c t i v i t y vs. sand c o n t r o l ......... 5.2.3.9.1. Sand r e t e n t i o n vs. hydrocarbon f l o w cap a c i t y ................................ 5.2.3.9.2. Other aspects ......................... Gravel g r a i n s u r f a c e and shape .............................. 5.2.4.1. Angular vs. round g r a v e l i n mechanical sand c o n t r o l 5.2.4.2. D i f f e r e n c e s between f r a c t u r i n g and g r a v e l p a c k i n g . 5.2.3.3.
5.2.4.
5.3.
Gravel packing completion fluids and gravel saturation 5.3.1.
..............
F l u i d v i s c o s i t y and g r a v e l c o n c e n t r a t i o n .................... 5.3.1.1. Achievement o f s u f f i c i e n t f l u i d l e a k o f f ........... 5.3.1.1.1. I n f l u e n c e o f c a r r i e r f l u i d v i s c o s i t y .. 5.3.1.1.1.1. Gravel placement and f l u i d b r e a k i n g .......... 5.3.1.1.1.2. Gel c o n c e n t r a t i o n vs. s l u r r y d e h y d r a t i o n ...... 5.3.1.1.1.3. F l u i d v i s c o s i t y and f o r m a t i o n damage ........... 5.3.1.1.1.4. F l u i d v i s c o s i t y and l e a k o f f r a t e s ............... 5.3.1.1.2. Improvement by s u r f a c t a n t - b e a r i n g wat e r - b a s e d f l u i d s ......................
724 724 724 725 725 725 726 726 727 727 727 727 728 728 728 729 729 729 729 730 730 731 731 731 732 732 732 732 733 733 734 734 734 734 735 735 736 736 737 737 737 738
704 Gel s t r u c t u r e ........... M i c r o g e l b r e a k i n g and r e .... moval 5.3.1.1.2.3. Gel f i ract u r i n g and g r a v e l p a c k i n g 5.3.1.1.3. Impact o f formation p e r m e a b i l i t y . . . . . . 5.3.1.1.3.1. Permeability classificat i o n f o r gravel packing . 5.3.1.1.3.2. Reservoir permeability and c o m p o s i t i o n . . . . . . . . . 5.3.1.2. Prevention o f excessive f l u i d l o s s ................ 5.3.1.2.1. Drawbacks o f p o s t - p a c k i n g a c i d i z i n g . . . 5.3.1.2.2. S o l i d f l u i d - l o s s a d d i t i v e s and f o r m a t i o n p r e - p a c k i n g ...................... T r a n s p o r t and p a c k i n g c a p a c i t y o f t h e s l u r r i e s . . . . . . . . . . . . . . 5.3.2.1. S u s p e n s i o n symmetry ............................... 5.3.2.2. Dual c a r r i e r f l u i d v i s c o s i t y ...................... 5.3.2.3. E v o l u t i o n o f g r a v e l p a c k i n g t e c h n i q u e s ............ 5.3.2.3.1. G e n e r a l a s p e c t s ....................... 5.3.2.3.2. Normal and r e v e r s e c i r c u l a t i o n p a c k i n g 5.3.2.3.3. Squeeze p a c k i n g . . . . . . . . . . . . . . . . . . . 5.3.2.3.4. Combined c i r c u l a t i o n and squeeze p a c k i n g ................................... 5.3.2.3.5. Four-position longstroke packing ...... 5.3.2.3.6. Wash down and p r e s s u r e p a c k i n g . 5.3.2.3.7. O t h e r t e c h n i q u e s ...................... 5.3.2.4. G r a v e l volume and p l a c e m e n t ....................... 5.3.2.4.1. C i r c u l a t i o n packing ................... 5.3.2.4.2. Squeeze p a c k i n g ....................... 5.3.2.4.3. R e s e r v e b l a n k s e c t i o n above s c r e e n and 5.3.1.1.2.1. 5.3.1.1.2.2.
5.3.2.
5.3.3.
5.3.4.
5.3.5.
5.4.
738 738 739 739 739 739 740 740 740 741 741 741 742 742 742 743 744 744 745 745 745
. . . . . . . . . . . . . . . 745 G r a v e l s e t t l i n g d i m i n u t i o n i n t h e c a r r i e r f l u i d . . . . . . . . . . . . . 746 5.3.3.1. Pressure drop m i n i m i z a t i o n . . . 5.3.3.2. Decreasing p a r t i c l e densi t y / c a r r i e r r a t i o .............................. 5.3.3.3. High-density f l u i d s vs. low-density 5.3.3.4. A p p l i c a t i o n s p e c t r u m o f l o w - d e n s i t y p a r t i c l e s . . . . . 747 Gel s u r f a c e f i l t r a t i o n f r o m p o l y m e r s o l u t i o n s . . 5.3.4.1. Screenout mimicking by face-layer induced pressure .......................................... 748 e c t s ......................... C o m b i n a t i o n o f g r a v e l p a c k i n g and p r o p p a n t f r a c t u o f combined g r a v e l p a c k i n g and f r a c t u ......................................... 749 Adequate s t a b i l i z a t i o n o f l o o s e sands 749 and c h a l k s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.5.1.2. Proper i n f i l l i n g o f p e r f o r a t i o n tunnels and cement s h e a t h d e f e c t s . . . . . . . . 5.3.5.1.3. O p e r a t i o n a l sequence o f h y d r a u l i c t u r i n g f o l l o w e d by gravel packing oppant f r a c t u r ................... 7 5 1
Reservoir s t a b i l i t y and f l u i d dynamics .............................. 5.4.1. F o r m a t i o n p r e s s u r e and f 5.4.1.1. Insufficient c sure ......... ......... 5.4.1.2. Changing f l u i d 5.4.1.3. Achievement o f t h e c r i t i c a l stage o f f o r m a t i o n c o l lapse ........ ......... 5.4.2. F l u i d dynamics .............................................. 5.4.2.1. Sand b r i d g e c o n s t r u c t i o n and breakdown . . . . . . . . . . . .
751
752 753 753
705 5.4.2.2. I n f l u e n c e o f p r o d u c t i o n r a t e s ..................... Rock mechanics .............................................. 5.4.3.1. Shear and t e n s i l e f a i l u r e ......................... 5.4.3.2. I n - s i t u s t r e s s s t a t e and r e s e r v o i r d e p l e t i o n ...... 5.4.3.3. C a p i l l a r y bonding ................................. 5.4.3.4. Sand b r i d g i n g ..................................... 5.4.3.5. P e r f o r a t i o n t u n n e l s t a b i l i t y ...................... Pressure drop and f l o w r e s i s t a n c e i n p e r f o r a t i o n 5.4.3.6. t u n n e l s ........................................... P e r f o r a t i o n t u n n e l morphology ............................... Water c u t i n t h e hydrocarbons ............................... D i f f e r e n c e s between p r o p p a n t f r a c t u r i n g and g r a v e l p a c k i n g . .
753 754 754 754 755 755 755
Areal d i s t r i b u t i o n o f potential reservoirs .......................... 5.5.1. General aspects ............................................. 5.5.2. Centres o f g r a v e l p a c k i n g ................................... 5.5.3. Gravel p a c k i n g p o t e n t i a l i n Europe .......................... 5.5.3.1. Sandstones ........................................ 5.5.3.2. Chalks ............................................ 5.5.4. Impact o f r e s i n - c o a t e d sand and proppants ................... 5.5.4.1. R e s i n bonding and a g g r e g a t i o n ..................... 5.5.4.2. A p p l i c a t i o n spectrum o f r e s i n - c o a t e d m a t e r i a l ..... 5.5.4.3. P o t e n t i a l f o r p r o m o t i o n o f m a r g i n a l sands and c h a l k s ............................................
757 757 758 758 758 760 760 760 761
P a r t i c l e transport through perforations ............................. 5.6.1. P e r f o r a t i o n p a c k i n g and p a r t i c l e b r i d g i n g ................... 5.6.1.1. I n s i d e and o u t s i d e g r a v e l p a c k i n g ................. 5.6.1.2. P e r f o r a t i o n diameter/particle diameter r a t i o 5.6.1.3. Number and s i z e o f p e r f o r a t i o n s ................... 5.6.1.3.1. P r e s s u r e loss i n t h e p e r f o r a t i o n t u n n e l 5.6.1.3.2. P e r f o r a t i o n and screen e r o s i o n by g r a v e l s u r g i n g ........................... 5.6.1.4. Tubing-conveyed underbalanced p e r f o r a t i n g ......... 5.6.1.4.1. Underbalanced vs . overbalanced p e r f o r a t i n g .................................. Advantages o f tubing-conveyed o p e r a t i o n 5.6.1.4.2. 5.6.2. Impact o f f l u i d v i s c o s i t y ................................... 5.6.2.1. V e r t i c a l vs . r a d i a l p a c k i n g o f t h e g r a v e l ......... 5.6.2.2. P l u g f l o w b e h a v i o u r o f c o n c e n t r a t e d s l u r r i e s ...... 5.6.3. Achievement o f p e r f o r a t i o n t u n n e l p l u g g i n g .................. 5.6.3.1. Two-stage s a n d - o i l squeeze i n l o n g zones .......... 5.6.3.2. Wikker D i k k e r t e c h n i q u e i n s h o r t zones ............ 5.6.3.3. Pressure p a c k i n g and c a v i t y f i l l i n g ............... 5.6.3.4. Sequence o f c i r c u l a t i o n and squeeze s t e p s ......... 5.6.3.5. Wash-down and t h r o u g h - t u b i n g p a c k i n g .............. 5.6.4. I n f l u e n c e o f p e r f o r a t i o n t e c h n i q u e .......................... 5.6.4.1. F o r m a t i o n damage by p e r f o r a t i o n d e b r i s ............ 5.6.4.2. P e r f o r a t i o n c l e a n i n g .............................. 5.6.4.2.1. General aspects ....................... 5.6.4.2.2. P e r f o r a t i o n washing ................... 5.6.4.2.2.1. Pressure p a r t i n g o f r e s e r v o i r sand ............ 5.6.4.2.2.2. Comparative e f f e c t i v i t y o f p e r f o r a t i o n washing .. 5.6.4.2.3. P e r f o r a t i o n s u r g i n g ................... 5.6.4.2.3.1. P e r f o r a t i o n opening and sand removal ............ 5.6.4.2.3.2. Long and s h o r t r e s e r v o i r i n t e r v a l s ............... 5.6.4.3. Proppant-induced p e r f o r a t i o n e r o s i o n .............. 5.6.5. I n f l u e n c e o f p e r f o r a t i o n geometry ...........................
761 762 762 763 763 763
5.4.3.
5.4.4. 5.4.5. 5.4.6.
5.5.
5.6.
......
755 756 756 757
761
764 764 764 765 765 766 766 767 767 767 767 767 768 768 768 769 769 769 770 770 771
771 771 772 772
706 5.6.6.
5.6.7.
5.6.8. 5.7.
I n f l u e n c e o f r e s e r v o i r s e c t i o n and t r e a t m e n t v a r i a b l e s . . . . . . 5.6.6.1. F o r m a t i o n p e r m e a b i l i t y v a r i a t i o n s and f l u i d v i s c o s i t y .............................................. ............ 5.6.6.2. Multiple intervals 5.6.6.3. F l u i d v e l o c i t y and Gravel prepacking o f p e r f o r a t i o n tunnels ....... ....... 5.6.7.1. High-permeability p e r f o r a t i o n tunnel f No sand p r o d u c t i o n p r i o r t o g r a v e l p r e p a c k i n g . . . . . 5.6.7.2. 5.6.7.3. Sand p r o d u c t i o n p r i o r t o g r a v e l p r e p a c k i n g . . . . . . . . Other aspects . . . . . .......................................
Steam-drive enhanced o i l recovery ................................... 5.7.1. S i n t e r e d b a u x i t e ........................... .............. 5.7.2. N i c k e l - c o a t e d sand ......................... .............. 5.7.3. A p p l i c a t i o n p o t e n t i a l i n Europe ............................. 5.7.4. G r a v e l d e n s i t y .............................................. 5.7.5. A l c a l i n i t y r e d u c t i o n o f t h e aqueous i n j e c t e d phase . . . . . . . . . . 5.7.5.1. F o r m a t i o n sand v s . g r a v e l pack sand d i s s o l u t i o n . . . 5.7.5.2. C y c l i c steam i n j e c t i o n and downhole pH v a l u e 5.7.5.3. M e c h a n i c a l g r a v e l pack s t a b i l i t y . . . . . . . . . . . . . . . . . . 5.7.5.4. O t h e r a s p e c t s .......... .........
5.8. Special gravel pack applications 5.8.1.
5.8.2.
....................................
C o n s o l i d a t e d pack i n s t a l l a t i o n .............................. 5.8.1.1. P e r f o r a t i o n t u n n e l p l u g g i n g ....................... 5.8.1.1.1. A p p l i c a t i o n s p e c t r u m ..... 5.8.1.1.2. Exclusive outside gravel p 5.8.1.2. D i f f e r e n c e s between c o n s o l i d a t e d and packs . ........ ....... 5.8.1.3. A r t i f i c i a l s t a b l e b o r e h o l e w a l l ................... 5.8.1.4. O t h e r a s p e c t s ..................................... Resin-coated g r a v e l placement . . . . . 5.8.2.1. Gravel mantle s t a b i l i z a t i 5.8.2.1.1. E x t e r n a l and i n t e r n a l c a t a l y z a t i o n . . . . 5.8.2.1.2. Thermal and c h e m i c a l c u r i n g . . . . . . . . . . . 5.8.2.1.3. A p p l i c a t i o n i n sand c o n t r o l and f r a c ........ ture stimulation ......... 5.8.2.1.3.1. Compressive s t r e n g t h o f resin-coated aggregates . 5.8.2.1.3.2. S i n g l e and m u l t i p l e g r a v e l pack c o m p l e t i o n s . . . . 5.8.2.1.4. Consolidati t i o n sand . 5.8.2.2. S p e c i a l a p p l i c a t i o n s .. 5.8.2.2.2. 5.8.2.2.3.
772 773 774 775 775 776 776 776 777 778 778 779 779 780 781 781 781
783 783 784 784 785 785 785
S t e a m - d r i v e p r o j e c t s ... T r a n s i t i o n between g r a v e fracturing .............. 5.8.2.2.3.2.
5.8.2.2.4.
5.8.3.
M i n i f r a c t u r e s vs. p e r f o r a t i o n t u n n e l r o w s . . . . . . 788 C a r r i e r f l u i d and f o r m a t i o n . . . . . . . . . . . 788 5.8.2.2.4.1. I n t e r f a c i a l f l u i d propert y a d j u s t m e n t . . . . . . . . . . . 788
G r a v e l p a c k i n g of d e v i ......... 5.8.3.0. General aspects 5.8.3.1. Gravel dunin 5.8.3.1.1. E x t e n d e d - r e a c h d r i l l i n g shape . . . . . . . . . 790 5.8.3.1.2. Dune g r o w t h and m i g r a t i o n ... 5.8.3.1.3. C a r r i e r f l u i d v i s c o s i t y and g r a v e l dune
707 dynamics .............................. Gravel placement and e q u i l i b r i u m bank p o s i t i o n .............................. 5.8.3.1.5. E q u i l i b r i u m v e l o c i t y and g r a v e l bank h e i g h t ................................ 5.8.3.1.6. F l u i d v e l o c i t y and dune movement . . . . . . 5.8.3.1.7. F l u i d v e l o c i t y and g r a v e l t r a n s p o r t ... 5.8.3.2. Gravel compaction ................................. 5.8.3.2.1. Flow r e s i s t a n c e c r e a t e d by f l e x i b l e b a f f l e s ............................... 5.8.3.2.1.1. B a f f l e spacing and s t i f f ness .................... 5.8.3.2.1.2. Rubber d i s c s ............ 5.8.3.2.2. Large-diameter washpipes and o t h e r asp e c t s ................................. 5.8.3.2.2.1. F l u i d v e l o c i t y and v i s c o s i t y .................... 5.8.3.2.2.2. P r e v e n t i o n o f f l u i d chann e l l i n g t h r o u g h screen .. 5.8.3.2.3. F l u i d v i s c o s i t y and c a s i n g - l i n e r r e l a t i o n s h i p .............................. 5.8.3.2.4. Gravel pack q u a l i t y improvement ....... 5.8.3.3. P e r f o r a t i o n p a c k i n g and s t r i n g r o t a t i o n ........... 5.8.3.3.1. P e r f o r a t i o n phasing ................... 5.8.3.3.1.1. S i n g l e - vs . d o u b l e - s i d e perforating ............. 5.8.3.3.1.2. Packing e f f i c i e n c y i n t o p - s i d e p e r f o r a t i o n s ... 5.8.3.3.2. P e r f o r a t i o n p r e p a c k i n g ................ 5.8.3.3.3. L i n e r v i b r a t i o n and d r i l l p i p e r o t a t i o n 5.8.3.4. Gravel s e t t l i n g ................................... 5.8.3.4.1. Low p a r t i c u l a t e densi t y l c a r r i e r f l u i d d e n s i t y r a t i o ......................... 5.8.3.4.2. Impact o f l o w - d e n s i t y g r a v e l p a c k i n g m a t e r i a l s ............................. 5.8.3.5. I n v e r t e d g r a v e l s l u r r y c i r c u l a t i o n ................ 5.8.3.5.1. Conventional v s . i n v e r t e d g r a v e l p a c k i n g 5.8.3.5.2. Void prevention w i t h i n v e r t e d s l u r r y c i r c u l a t i o n ........................... 5.8.3.5.3. Comparison t o normal and r e v e r s e c i r c u l a t i o n p a c k i n g ........................ 5.8.3.6. Viscous g r a v e l p a c k i n g s l u r r i e s ................... 5.8.3.7. G r a v e l - l e s s s l o t t e d l i n e r s and pre-packed screens . 5.8.3.8. O t h e r aspects ..................................... Gravel p a c k i n g w i t h l i n e r v i b r a t i o n ......................... 5.8.4.1. Compensation o f i n i t i a l undercompaction by b l a n k l i n e r s e c t i o n ..................................... 5.8.4.2. P r e v e n t i o n o f a f t e r - p a c k s e t t l i n g ................. 5.8.4.3. Achievement o f optimum pack d e n s i t y by l i n e r v i b r a t i o n .............................................. 5.8.4.3.1. Gravel p a c k i n g ........................ 5.8.4.3.2. Pump p r o t e c t i n g pre-packed l i n e r s ..... 5.8.4.3.3. P r i m a r y cementing ..................... 5.8.4.4. Gravel and p e r f o r a t i o n washing .................... Concentrated g r a v e l s l u r r y pumping .......................... 5.8.5.1. B e t t e r g r a v e l t r a n s p o r t and t i g h t e r g r a v e l p a c k i n g 5.8.5.2. T a c k i f y i n g a c t i o n o f i n t e r m i x e d r e s i n ............. D e n s i t y s l u r r y and foam g r a v e l p a c k i n g ...................... Viscous polymer g r a v e l p a c k i n g and d e p o s i t b u i l d u p .......... 5.8.7.1. General aspects ................................... 5.8.7.2. Nodular d e p o s i t b u i l d u p a t t h e p e r f o r a t i o n s . . . . . . . 5.8.7.3. Annular d e p o s i t b u i l d u p on t h e screen . . . . . . . . . . 5.8.3.1.4.
5.8.4.
5.8.5. 5.8.6. 5.8.7.
791 791 791 792 792 793 793 793 794 794 794 794 795 795 795 796 796 796 796 796 797 797 798 798 798 799 799 799 800 800 800 801 801 801 802 802 802 803 803 803 803 804 804 804 805
708 5.8.8. 5.8.9.
5.9.
5.8.7.4. Benefits o f high carrier f l u i d viscosity ... P r e v e n t i o n o f p r o p p a n t f l o w b a c k b y f r a c t u r e t a i l - i n .. P u d d l e p a c k i n g .......................................
Other sand control methods 5.9.1.
5.9.2. 5.9.3.
.....................
.........
R e s i n c o n s o l i d a t i o n .................................. 5.9.1.1. General aspects ... . . . . . . . . . . . . . 808 5.9.1.1.1. Mechani f o r m a t i o n c o n s o l i d a t i o n . . . . . . . . . . . . . . . 808 5.9.1.1.2. R e s i n m o b i l i z a t i o n and m i g r a t i o n . . . . . . 808 5.9.1.1.3. R e s i n c u r i n g and c a t a l y z a t i o n . . . . . . . . . 809 5.9.1.2. R e s i n v i s c o s i t y and a c t i v i t y ...................... 809 5.9.1.2.0. G e n e r a l a s p e c t s ....................... 809 5.9.1.2.1. I n t e r n a l l y a c t i v a t e d systems . . . . . . . . . . 810 5.9.1.2.2. E x t e r n a l l y a c t i v a t e d o r o v e r f l u s h systems .................................. 810 5.9.1.2.3. R e s i n y i e l d and f o r m a t i o n p e r m e a b i l i t y r e t e n t i o n ............................. 811 5.9.1.2.3.1. R e s e r v o i r c o m p o s i t i o n and r e s i n y i e l d . . . . . . . . . . . . . 811 5.9.1.2.3.2. Consolidation strength and d i r t y p a y s t a b i l i z a t i o n ............... 5.9.1.3. Sources o f c o n s o l i d a t i o n f a i l u r e . . . . . . . . . . . . . . . . . . 8 1 1 5.9.1.3.1. Requirements f o r successful r e s i n con812 s o l i d a t i o n ............................ 5.9.1.3.2. U n s u i t a b l e d r i l l i n g and c o m p l e t i o n p r a c t i c e s ............................. 812 5.9.1.3.2.1. C e m e n t a t i o n . . . . . . . . . . . . . 812 5.9.1.3.2.2. P e r f o r a t i o n . . . . . . . . . . . . . 813 5.9.1.3.3. I n t e r m i x i n g o f r a t h o l e f l u i d and sand 813 c o n t r o l f l u i d ......................... 5.9.1.3.3.1. R e s i n c o n s o l i d a t i o n ..... 813 5.9.1.3.3.2. G r a v e l p a c k i n g . . . . . . . . . . 813 . . . . . . . . . . . . . . . 814 5.9.1.4. Advantages and p r o b l e m s .... ..... 814 5.9.1.4.1. Multistage p uni erfo814 r a t i o n c o v e r a g e ....................... 5.9.1.4.2. S p e c i a l c o n d i t i o n i n g o f d i r t y sands . . . 815 5.9.1.5. P o s s i b i l i t y of i m p r o v e d r e s i n cons mance ............................ . . . . . . . . . . . . . 815 815 5.9.1.5.1. Formation prepacking . 5.9.1.5.1.1. Pressure pa v e l p l a c e m e n t i n f r a c t u r e 815 5.9.1.5.1.2. Influence o f perforation w a s h i n g and s u r g i n g . . . . . 816 5.9.1.5.2. F o r m a t i o n p r e h e a t i n g . . . . . . . . . . . . . . . . . . 816 5.9.1.5.3. F o r m a t i o n p r e f l u s h i n g . . . . . . . . . . . . . . . . . 816 5.9.1.5.4. Two- and t h r e e - s t a g e sand c o n s o l i d a t i o n 817 5.9.1.5.4.1. Two-stage sand s tion ........... 5.9.1.5.4.2. Formation f r a c t u r i n g r e s i n c o n s o l i d a t i o n 817 5.9.1.5.4.3. T h r e e - s t a g e sand s t a b i l i -
5.9.1.5.5. Other aspects 5.9.1.6. Formation c o n s o l i d a t i o n vs 5.9.1.7. P e r m e a b i l i t y d e s t r u c t i o n and p l u g g i n g o f t h i H y d r o x y - a l u m i n u m and e l e c t r o l e s s n i c k e l c o n s o l i d a t i o n 5.9.2.1. Hydroxy-aluminum s t a b i l i z a . . . . . . . 819 5.9.2.2. Electroless nickel plating S i l i c a c e m e n t a t i o n .............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820
709
5.9.4. 5.9.5.
5.9.3.1. S i l i c a l o c k concept ................................ Solder g l a s s method ............................... 5.9.3.2. Warm a i r coking c o n s o l i d a t i o n ............................... S t a b l e sand arch formation .................................. 5.9.5.1. General aspects ................................... 5.9.5.1.1. C a v i t y face p e r m e a b i l i t y vs . d i l a t i o n . 5.9.5.1.2. Sand package d i s i n t e g r a t i o n vs . g r a i n i n t e r l o c k i n g .......................... 5.9.5.1.3. Wellbore s t r e s s s t a t e c o n t r o l ......... 5.9.5.1.3.1. Pore pressure changes and arch l o a d a l t e r a t i o n s ... 5.9.5.1.3.2. Radial e f f e c t i v e stress and imposing techniques 5.9.5.2. Sand a r c h i n g combined w i t h r e s i n c o n s o l i d a t i o n .... 5.9.5.3. G r a i n surface roughness ........................... 5.9.5.4. F l u i d m i x i n g r e l a t i o n s h i p s ........................ Pre-pack l i n e r s and multiwire-wrapped screens ............... 5.9.6.1. Pre-pack l i n e r s ................................... 5.9.6.1.1. Gravel packing ........................ 5.9.6.1.2. Pump p r o t e c t i o n ....................... 5.9.6.2. Multiwire-wrapped screens .........................
.
5.9.6.
5.10. I n f l u e n c e o f completion type on g r a v e l packing ...................... 5.10.1. I n s i d e gravel packing ....................................... 5.10.1.1. S i g n i f i c a n c e o f p e r f o r a t i o n tunnel packing ........ 5.10.1.2. Two-stage and o n e - t r i p systems .................... 5.10.1.2.1. M i n i m i z a t i o n o f f o r m a t i o n damage and o p e r a t i n g time ........................ 5.10.1.2.2. I n t e r v a l l e n g t h vs . r e s e r v o i r permeabil i t y .................................. 5.10.1.3. Other aspects ..................................... 5.10.2. Open h o l e g r a v e l packing .................................... 5.10.2.1. Advantages and drawback o f open-hole gravel packing 5.10.2.2. Borehole underreaming ............................. 5.10.2.3. P o r t c o l l a r implementation and pack washing ....... 5.10.3. Factors improving g r a v e l pack e f f i c i e n c y .................... 5.10.4. L i n e r placement ............................................. 5.10.5. Wellbore damage i n gravel packed holes ...................... 5.10.6. Water c o n t r o l i n g r a v e l packed completions .................. 5.11.
Formation damage d u r i n g g r a v e l packing .............................. 5.11.1. P e r f o r a t i o n tunnel p l u g g i n g ................................. 5.11.1.1. R e s e r v o i r impairment by the gravel pack ........... 5.11.1.2. Covering o f damaged f o r m a t i o n by gravel packing ... 5.11.1.3. Necessity o f clean open p e r f o r a t i o n holes f o r t i g h t g r a v e l i n f i l l i n g .................................. 5.11.1.4. Impact o f p e r f o r a t i o n h o l e diameter ............... 5.11.2. D r i l l i n g mud l o s s ........................................... 5.11.3. Completion f l u i d l o s s ....................................... 5.11.3.1. General aspects ................................... 5.11.3.1.1. Mechanical f l u i d - l e a k o f f c o n t r o l ...... 5.11.3.1.2. Types o f f l u i d - l o s s a d d i t i v e s ......... 5.11.3.2. A p p l i c a t i o n o f gravel and d i v e r t i n g agent ......... 5.11.3.3. I n s e r t i o n o f s o l i d f l u i d - l o s s a d d i t i v e s ........... 5.11.3.4. Removal o f s o l i d f l u i d - l o s s a d d i t i v e s ............. 5.11.3.4.1. A c i d i z i n g ............................. 5.11.3.4.2. Problems o f incomplete f l u i d - l o s s addit i v e removal .......................... 5.11.4. Formation damage removal by a c i d i z i n g ....................... 5.11.4.1. A c i d i z i n g p r i o r t o gravel packing ................. 5.11.4.1.1. Gravel and o t h e r m a t e r i a l s as d i v e r t i n g agents ................................
820 820 820 821 821 821 822 822 822 823 823 824 824 824 825 825 825 826 826 827 827 827 828 828 829 829 829 830 830 831 831 831 832 832 832 833 833 833 834 834 834 835 835 835 835 836 836 836 837 837 837 837
710
5.11.5.
5.11.4 1 . 2 . D i v e r t i n g a g e n t i n j e c t i o n and d i s t r i b u t i o n .................................. 838 5 . 1 1 . 4 1 . 3 . F o r m a t i o n wetness and c l a y s t a b i l i z a t i o n ................................. 838 5 . 1 1 . 4 . 2 . A c i d i z ng a f t e r g r a v e p a c k i n g .................... 839 5.11.4.2.1. S e l e c t i v e a c i d p e n e t r a t i o n vs. p e r f o r a t i o n t u n n e l c o n t e n t . . . . . . . . . . . . . . . . . . . 839 5.11.4.2.2. D i f f i c u l t y o f f l u i d - l o s s agent removal t h r o u g h t h e g r a v e l pack . . . . . . . . . . . . . . . 8 4 0 5 . 1 1 . 4 . 3 . G r a v e l u n c o n s o l i d a t i o n and d i s s o l u t i o n b y a c i d i z i n g 8 4 0 5.11.4.3.1. G r a v e l and f o r m a t i o n p l u g g i n g b y m i g r a 840 t i n g f i n e s ............................ 841 5 . 1 1 . 4 . 3 . 2 . G r a v e l d i s s o l u t i o n .................... 5.11.4.3.2.1. G r a v e l g r a i n s i z e and a c i d c o n c e n t r a t i o n . . . . . . 841 5.11.4.3.2.2. M o b i l i t y o f loose packing g r a v e l and p a y sand . . . . . 8 4 1 5 . 1 1 . 4 . 3 . 3 . G r a v e l u n c o n s o l i d a t i o n . . . . . . . . . . . . . . . . 842 F l u i d and h a r d w a r e c l e a n l i n e s s .............................. 842
5.1. Introduction G r a v e l p a c k i n g f o r sand c o n t r o l i s t h e second a p p l i c a t i o n o f b o t h n a t u r a l sand and s y n t h e t i c p r o p p a n t s i n o i l and gas w e l l s t i m u l a t i o n and c o m p l e t i o n (ROGERS 1971; SUMAN 1974, 1975 a; ANAND & JONES 1977; GURLEY, COPELAND & HENDRICK 1977; MCLEOD & CRAWFORD 1983; SUMAN, ELLIS & SNYDER 1983; LEDLOW, SAUER & T I L L 1985; NEAL & CARROLL 1985, PENBERTHY 1985, SPARLIN & HAGEN 1985; COULTER, MARTINEZ & FISCHER 1987; GURLEY 1988) and i s c a r r i e d o u t i n o i l - , gas- and w a t e r - p r o d u c t i o n w e l l s ; w a t e r - , steam- and c h e m i c a l - i n j e c t i o n w e l l s ; u n d e r g r o u n d g a s - s t o r a g e w e l l s and o c c a s i o n a l l y a l s o m i n e r a l - l e a c h i n g and g e o t h e r m a l h e a t - e x p l o i t a t i o n w e l l s (SOLUM 1 9 8 4 ) . G r a v e l p a c k i n g was i n t r o d u c e d t o t h e h y d r o c a r b o n i n d u s t r y more t h a n 60 y e a r s ago and has been a p p l i e d i n w a t e r w e l l s s i n c e more t h a n 100 y e a r s (SOLUM 1 9 8 6 ) . S i n c e t h a t t i m e , i t has p r o g r e s s i v e l y e v o l v e d and i s a l r e a d y s i n c e many decades t h e m o s t common sand c o n t r o l t e c h n i q u e . The g r a v e l pack s y s t e m b a s i c a l l y cons i s t s o f a s c r e e n o r l i n e r t o p r e v e n t g r a v e l p r o d u c t i o n , and a g r a v e l m a n t l e t o s t o p f o r m a t i o n sand i n v a s i o n . G r a v e l p a c k i n g aims on r e d u c t i o n o r c o n t r o l l i n g o f sand p r o d u c t i o n , m i n i m i z a t i o n o f f o r m a t i o n damage, and i n c r e a s e o f h y d r o c a r bon p r o d u c t i o n . The i n t r o d u c t o r y r e m a r k s as f o l l o w s i n c l u d e comments on s i g n i f i cance and n e c e s s i t y o f g r a v e l p a c k i n g f o r sand s t a b i l i z a t i o n and p r o d u c t i v i t y m a i n t e n a n c e as w e l l as o r g a n i z a t i o n of t h e p r e s e n t r e v i e w and s t a t u s r e p o r t . An o u t l i n e i s a l s o g i v e n on t h e r e s e r v o i r composition spectrum r e q u i r i n g support by gravel packing.
5.1.1. Reservoir composition spectrum requiring gravel packing G r a v e l p a c k i n g i s done in o r d e r t o a r t i f i c i a l l y s t a b i l i z e m a i n l y o r i g i n a l l y i n s u f f i c i e n t l y consolidated, s u b o r d i n a t e l y a l s o s e c o n d a r i l y and t e r t i a r i l y des t r e n g t h e n e d sands and o t h e r r e l a t e d g r a i n y s e d i m e n t a r y r o c k s . I n s u f f i c i e n t l y c o n s o l i d a t e d ( d u e t o p o o r c a r b o n a t e , s i l i c a o r o t h e r c e m e n t a t i o n ) and l i t h i f i e d ( d u e t o o n l y s l i g h t p r e s s u r e s o l u t i o n ) o r n o t enough c o h e s i v e ( a s a consequence o f o n l y l i t t l e b i n d i n g b y o n l y s m a l l amounts o f d i s p e r s e d mud) t o even a l m o s t l o o s e f r i a b l e sands o f v a r i o u s g r a i n s i z e a r e t h e m o s t common t r o u b l e - m a k i n g dep o s i t s i n t e r m s o f sand p r o d u c t i o n . A p a r t f r o m u n c o n s o l i d a t e d c o a r s e , medium o r f i n e , c l e a n o r d i r t y sands, t o m i n o r amounts a l s o s i l t y and muddy sands, sand-mud s a n d w i c h - t y p e i n t e r b e d d i n g s , c a r b o n a t e s h e l l l a y e r s and s o f t c h a l k r e q u i r e s t a b i l i z a t i o n t r e a t m e n t s i n t h e s u r r o u n d i n g s o f t h e w e l l b o r e i n o r d e r t o p r e v e n t i n f l u x o f sand, mud o r c a r b o n a t e p a r t i c l e s i n t o t h e b o r e h o l e and t o s t o p p r o d u c t i o n o f s o l i d f o r m a t i o n f r a g -
711 ments ( m a i n l y sand) t o g e t h e r w i t h t h e hydrocarbons. While g r a v e l p a c k i n g i n sandy r e s e r v o i r s i s q u i t e abundantly a p p l i e d and has been performed t h r o u g h o u t t h e w o r l d s i n c e many decades, g r a v e l p a c k i n g i n c h a l k (ATTARD, MATHES & MOWER 1988; c f . s e c t i o n 4.5.4.3.) and c o a l (JEU, LOGAN & McBANE 1988) has so f a r o n l y occas i o n a l l y been u t i l i z e d .
5.1.2.
Sand stabilization and productivity maintenance
The g r a v e l pack a c t s as a f i l t e r f o r f i n e s m i g r a t i n g o u t o f t h e f o r m a t i o n and i n h i b i t s t h e f i n e s t o p l u g t h e s l o t s i n t h e l i n e r o r t h e i n t e r w i r e spaces i n t h e screen, hut a l l o w s r e s e r v o i r f l u i d s t o f l o w e a s i l y i n t o t h e w e l l b o r e . The permeable g r a v e l pack f i l t e r p e r m i t s maximum p r o d u c t i o n r a t e s w h i l e i t mec h a n i c a l l y props t h e f o r m a t i o n and p r o h i b i t s t h e passage o f sand i n t o t h e w e l l bore. B r i d g i n g o f t h e f o r m a t i o n sand a g a i n s t t h e g r a v e l pack i n o r d e r t o a l l o w o n l y n a t u r a l f l u i d f l o w i s t h e key t o sand movement c o n t r o l (SCHWARTZ 1969). L i m i t e d amounts o f sand i n f l u x i n t o t h e b o r e h o l e can a l s o be a t l e a s t tempor a r i l y handled by c u t t i n g back o i l o r gas p r o d u c t i o n r a t e s (STEIN, ODEH & JONES 1974) which r e s u l t s i n r e d u c t i o n o f t h e tendency o f t h e sand t o move ( e s p e c i a l l y i n weak sands c o n t a i n i n g h i g h - v i s c o s i t y o i l ; c f . s e c t i o n 5.2.1.4.), but larg e r q u a n t i t i e s o f sand i n t e r v e n t i o n can o n l y be stopped by g r a v e l p a c k i n g o r o t h e r sand c o n t r o l methods. PEOEN & YASSIN (1986) i n v e s t i g a t e t h e e f f e c t o f p e r f o r a t i o n c o n d i t i o n s upon maximum sand-free p r o d u c t i o n r a t e i n f r i a b l e sandstones. A l t h o u g h g r a v e l p a c k i n g as t h e most common sand c o n t r o l t r e a t m e n t i s q u i t e expensive, i t i s n o t n e a r l y as c o s t l y as l o s i n g a p r o d u c t i o n w e l l due t o sand problems ( c f . s e c t i o n 5 . 2 . 1 . 3 . ) . The economical i m p l i c a t i o n s o f t h e damage which f l o w i n g f o r m a t i o n sand can c r e a t e a r e c r i t i c a l enough t o r e q u i r e c o n t i nuous improvements i n sand c o n t r o l techniques and sand p r o d u c t i o n p r e d i c t i o n methods (GHALAMBOR, KOLIBA, HAYATDAVOUDI & ALCOCER 1988).
5.1.3. Organization o f the review and status report As t h i s r e v i e w and s t a t u s r e p o r t m a i n l y focusses on aspects o f p r o p p a n t app l i c a t i o n i n o i l - and g a s - w e l l s t i m u l a t i o n and completion, t h e d i s c u s s i o n o f sand c o n t r o l t e c h n i q u e s c o n c e n t r a t e s on g r a v e l p a c k i n g where q u e s t i o n s o f t y p e and g r a i n s i z e o f n a t u r a l sand o r s y n t h e t i c proppants a r e v e r y d e c i s i v e f o r adequate performance o f t h e o p e r a t i o n . Gravel p a c k i n g i s s t i l l t h e most e f f e c t i v e , p r a c t i c a l , s u c c e s s f u l , r e l i a b l e , s t r a i g h t f o r w a r d , and i n many cases a l s o cheap e s t sand c o n t r o l t r e a t m e n t , as u n d e r l i n e d by t h e f a c t t h a t 60 - 90 % o f a l l s a n d - s t a b i l i z a t i o n j o b s i n t h e USA a r e c a r r i e d o u t by g r a v e l p a c k i n g (ALLEN & ROBERTS 1982, SUMAN & SNYDER 1982; SUMAN, ELLIS & SNYDER 1983). F o r t h e sake o f completeness and comparison w i t h g r a v e l packing, t h e o t h e r sand c o n t r o l methods a r e a l s o b r i e f l y discussed, w i t h s p e c i a l emphasis on t h e r o l e o f n a t u r a l sand o r s y n t h e t i c proppants i n some o f these a p p l i c a t i o n s . A f t e r some general comments, aspects o f a p p l i c a t i o n o f p r o p p a n t o r g r a v e l t y pes and g r a i n s i z e s , g r a v e l p a c k i n g c o m p l e t i o n f l u i d s and g r a v e l s a t u r a t i o n , r e s e r v o i r s t a b i l i t y and f l u i d dynamics, a r e a l d i s t r i b u t i o n o f p o t e n t i a l r e s e r v o i r s , p a r t i c l e t r a n s p o r t t h r o u g h p e r f o r a t i o n s , steam-drive enhanced o i l recover y and o t h e r techniques a r e d i s c u s s e d i n c o n n e c t i o n w i t h g r a v e l pack q u e s t i o n s . An account o f s p e c i a l g r a v e l pack a p p l i c a t i o n s forms a t r a n s i t i o n t o an o u t l i n e o f o t h e r sand c o n t r o l methods, and f i n a l l y some comments a r e o f f e r e d on t h e i n f l u e n c e o f c o m p l e t i o n t y p e on g r a v e l p a c k i n g and f o r m a t i o n damage d u r i n g g r a v e l packing.
5.2. Gravel types and grain sizes S i m i l a r l y as p r o p p a n t t y p e and g r a i n s i z e s e l e c t i o n i s t h e key element o f hyd r a u l i c f r a c t u r i n g design, g r a v e l t y p e and g r a i n s i z e c h o i c e i s a v e r y c r i t i c a l
712 and c e n t r a l p a r t o f g r a v e l p a c k i n g d e s i g n . Gravel g r a i n s i z e has t o be c a r e f u l l y determined i n o r d e r t o s a t i s f y t h e dual r e q u i r e m e n t s o f t h e pack t o be p e r meable t o f o r m a t i o n f l u i d s ( p r i m a r i l y hydrocarbons) and w e l l c o m p l e t i o n f l u i d s , and t o a c t as a f i l t e r p r e v e n t i n g r e s e r v o i r sand f r o m moving i n t o t h e w e l l b o r e . The term g r a v e l used i n sand c o n t r o l i s d i f f e r e n t f r o m t h a t i n sedimentary pet r o g r a p h y i n n o t r e l a t i n g t o g r a n u l o m e t r i c a l assemblages c o a r s e r t h a n sand, b u t c o m p r i s i n g t h e whole g r a i n s i z e spectrum o f sand and p r o b a b l y a l s o c o a r s e r f r a c t i o n s t h a t a r e used as p a c k i n g g r a v e l . The s y n t h e t i c i n t e r m e d i a t e - and h i g h - s t r e n g t h p e l l e t s c o n s i s t i n g o f low- o r h i g h - d e n s i t y alumina o x i d e a n d / o r s i l i c a t e o r l o w - d e n s i t y z i r c o n i a - s i l i c a t e ( c f . s e c t i o n s 1.3. and 5.2.2.3.) are c a l l e d proppants i n hydraulic f r a c t u r i n g t o express t h e i r f u n c t i o n o f p r o p p i n g t h e open c r a c k and p r o v i d i n g s u p p o r t a g a i n s t i t s c l o s u r e , whereas t h e same p e l l e t s a r e named g r a v e l i n mechanical sand c o n t r o l even i f t h e i r t a s k i s p a r t i a l l y approaching t h a t o f p r o p p a n t s i n h y d r a u l i c f r a c t u r i n g when p l u g g i n g p e r f o r a t i o n t u n n e l s i n a d d i t i o n t o composing a c y l i n d r i c a l g r a v e l m a n t l e a l o n g ' t h e b o r e h o l e w a l l o r between c a s i n g and screen o r l i n e r . A f t e r a b r i e f g e n e r a l o u t l i n e o f sand p r o d u c t i o n and i t s cont r o l , aspects o f g r a v e l type, g r a v e l g r a i n s i z e , and g r a v e l g r a i n s u r f a c e and shape a r e d i s c u s s e d as f o l l o w s .
5.2.1. Sand production and i t s c o n t r o l Sand p r o d u c t i o n i s one o f t h e o l d e s t o i l - f i e l d problems (ALLEN & ROBERTS 1982) and i s a p a r t f r o m i n c r e a s i n g w a t e r c u t w i t h p r o g r e s s i v e r e s e r v o i r d e p l e t i o n t h e most s i g n i f i c a n t f a c t o r d e t e r i o r a t i n g n e t o i l e x p l o i t a t i o n r a t e s . I n c o n t r a s t t o w a t e r i n v e n t i o n s h i f t i n g t h e r e l a t i o n s h i p o f o i l t o w a t e r i n t o an uneconomical d i r e c t i o n , sand i n f l u x i n t o t h e b o r e h o l e i s n o t o n l y an o i l product i o n problem, b u t a l s o provokes c o n s i d e r a b l e damage o f t h e w e l l b o r e and s u r f a c e i n s t a l l a t i o n s which i n t h e w o r s t case may l e a d t o complete l o s s o f t h e w e l l . The b r i e f overview as f o l l o w s i n t r o d u c e s t h e s a n d - p r o d u c t i o n t r o u b l e as a base o f t h e succeeding d i s c u s s i o n o f g r a v e l p a c k i n g techniques and m a t e r i a l s . A f t e r some general aspects, t h e o u t l i n e focusses on reasons o f sand p r o d u c t i o n , w e l l d e t e r i o r a t i o n by sand p r o d u c t i o n , c r i t i c a l hydrocarbon o f f t a k e r a t e , and necess i t y and aims o f sand c o n t r o l t r e a t m e n t s .
5.2.1.1.
General aspects
Sand g r a i n s composing a r e n i t i c r e s e r v o i r r o c k s a r e s t a b i l i z e d by compressive f o r c e s due t o t h e w e i g h t o f t h e overburden, by c a p i l l a r y cohesion and by p a r t i c l e cementation (ALLEN & ROBERTS 1982, BJ-HUGHES 1986). Causes o f sand product i o n above a c r i t i c a l hydrocarbon o f f t a k e r a t e a r e r e l a t e d t o d i s l o d g i n g o f g r a i n s by drag o r f r i c t i o n a l f o r c e s of f l o w i n g f l u i d which i n c r e a s e w i t h h i g h e r c u r r e n t r a t e s and h i g h e r f l u i d v i s c o s i t y , d i m i n u t i o n i n f o r m a t i o n s t r e n g t h o f t e n a s s o c i a t e d w i t h w a t e r p r o d u c t i o n due t o d i s s o l u t i o n o r d i s p e r s i o n o f cementi n g m a t e r i a l s between sand g r a i n s , r e d u c t i o n i n c a p i l l a r y f o r c e s w i t h i n c r e a s i n g w a t e r s a t u r a t i o n , d e c l i n e d r e l a t i v e p e r m e a b i l i t y t o o i l due t o r i s i n g wat e r s a t u r a t i o n which i n c r e a s e s r e s e r v o i r p r e s s u r e drawdown f o r a g i v e n o i l w i t h drawal r a t e , d i m i n i s h i n g cohesion due t o f i n e s m o b i l i z a t i o n and w e t t a b i l i t y a l t e r a t i o n s as a consequence o f changing f l u i d t y p e f r o m o i l t o water, and d e c l i n i n g f o r m a t i o n p r e s s u r e w i t h advancing r e s e r v o i r d e p l e t i o n which i n c r e a s e s o v e r burden s t r e s s and thus compaction f o r c e s and may d i s t u r b c e m e n t a t i o n between g r a i n s . Sand p r o d u c t i o n can be b a s i c a l l y c o n t r o l l e d by t h r e e mechanisms : reduc i n g d r a g f o r c e s by i m p r o v i n g w e l l c o m p l e t i o n p r a c t i c e s , mechanical b r i d g i n g o f sand by g r a v e l packing, and i n c r e a s i n g f o r m a t i o n s t r e n g t h by chemical sand consolidation.
5.2.1.2.
Reasons o f sand product i o n
Sand p r o d u c t i o n i n o i l and gas r e s e r v o i r s can be caused by t o t a l l y u n c o n s o l i d a t e d f o r m a t i o n s , h i g h hydrocarbon o f f t a k e r a t e s , o n s e t o f w a t e r p r o d u c t i o n , subsidence due t o r e s e r v o i r d e p l e t i o n , and improper w e l l c o m p l e t i o n p r a c t i c e s
713 (HALLIBURTON 1988). T o t a l l y u n c o n s o l i d a t e d f o r m a t i o n s r e a c t t o any a t t e m p t t o w i t h d r a w o i l o r gas w i t h l a r g e amounts o f sand i n f l o w i n t o t h e b o r e h o l e . O t h e r r e s e r v o i r s produce s a n d - f r e e i f o f f t a k e r a t e s a r e r e s t r i c t e d , b u t i n c r e a s i n g gross p r o d u c t i o n r a t e s c r e a t e e x c e s s i v e s t r e s s e s on i n s u f f i c i e n t l y l i t h i f i e d pay zones and may exceed t h e a b i l i t y o f weak cements t o bond sand g r a i n s t o g e t h e r . C l a y m i n e r a l s and s i l t which a r e s l i g h t l y s t a b i l i z i n g o t h e r l o o s e sandstones may be s e r i o u s l y a f f e c t e d by w a t e r and when w a t e r p r o d u c t i o n s t a r t s , t h e bonding i s weakened o r d e s t r o y e d and f o r m a t i o n sand i s f l o w i n g i n t o t h e boreh o l e . R e d u c t i o n o f r e s e r v o i r p r e s s u r e may cause t h e overburden t o subside, c r u s h a p o o r l y c o n s o l i d a t e d r e s e r v o i r and l e a d t o sand p r o d u c t i o n and s e r i o u s c a s i n g damage. Improper w e l l c o m p l e t i o n p r a c t i c e s i n c l u d e misuse o f a c i d f o r s t i m u l a t i o n which may remove t h e small amount o f c a l c a r e o u s bonding m a t e r i a l i n c e r t a i n w e a k l y - c o n s o l i d a t e d pay zones g i v i n g r i s e t o sand p r o d u c t i o n . Swabbing a w e l l t o o h a r d o r b r i n g i n g t h e b o r e h o l e t o d e s i r e d o f f t a k e c a p a c i t y t o o q u i c k l y can cause e x c e s s i v e s t r e s s on s l i g h t l y l i t h i f i e d r e s e r v o i r s and provoke sand f l o w . I n s t a n t l y opening some w e l l s t o maximum p r o d u c t i o n r a t e can t r i g g e r w a t e r t o be drawn i n t o b o r e h o l e and a d j a c e n t f o r m a t i o n m a t r i x , w i t h such w e l l c o m p l e t i o n p r a c t i c e s b e i n g a b l e t o t r i g g e r premature sand i n f l u x i n b o r e h o l e s t h a t m i g h t have o p e r a t e d s a n d - f r e e f o r l o n g e r p e r i o d s o f time.
5.2.1.3. Well deterioration by sand production V a r i o u s p o s s i b i l i t i e s o f w e l l d e t e r i o r a t i o n by sand p r o d u c t i o n e x i s t which range from damage o f o n l y a few p a r t s o f downhole and s u r f a c e i n s t a l l a t i o n t o complete d e s t r u c t i o n o f t h e equipment w h i c h u l t i m a t e l y can have t h e consequence t h a t f o r t e c h n i c a l and/or economical reasons t h e w e l l has t o be abandoned. The o u t l i n e as f o l l o w s c o n c e n t r a t e s on damage spectrum caused b y sand i n f l u x , and economical and t e c h n i c a l l o s s o f w e l l s .
5.2.1.3.1. Damage spectrum caused by sand influx Sand f l o w f r o m u n c o n s o l i d a t e d f o r m a t i o n s i n t o t h e b o r e h o l e c r e a t e s v a r i o u s problems i n hydrocarbon e x p l o i t a t i o n such as premature f a i l u r e o f a r t i f i c i a l l i f t equipment; sand f i l l - u p and b r i d g i n g i n p e r f o r a t i o n s , c a s i n g , t u b i n g and/ o r f l o w l i n e s t h e r e b y s h u t t i n g - o f f p r o d u c t i o n ; f a i l u r e and c o l l a p s e o f c a s i n g o r l i n e r as a consequence o f removal and m i g r a t i o n o f s u r r o u n d i n g f o r m a t i o n s which l e a d s t o s u p e r c r i t i c a l d i f f e r e n t i a l p r e s s u r e and r e s e r v o i r compaction o r s u b s i dence, e r o s i o n and a b r a s i o n o f downhole and s u r f a c e t u b u l a r equipment; c u t t i n g o f p i p e s , chokes, v a l v e s , and a r t i f i c i a l l i f t t o o l s ; f i l l i n g o f s e p a r a t o r t a n k s and t r a n s p o r t f a c i l i t i e s , downhole c a s i n g and f o r m a t i o n damage t h a t sometimes causes premature abandonment o f completions, and h a n d l i n g and d i s p o s a l o f produced r e s e r v o i r m a t e r i a l s (SUMAN, ELLIS & SNYDER 1983). Some comments a r e o f f e r e d on equipment f a i l u r e and s a c r i f i c a t i o n o f hydrocarbon e x p l o i t a t i o n t i m e as f o l lows.
5.2.1.3.1.1. Equipment abrasion Sand p r o d u c t i o n f r o m u n c o n s o l i d a t e d r e s e r v o i r s as w e l l as p r o p p a n t evacuat i o n f r o m i n s u f f i c i e n t l y s t a b i l i z e d f r a c t u r e i n f i l l i n g s ( c f . s e c t i o n 4.12.3.) l e a d t o b o r e h o l e o p e r a t i o n a l problems such as s t u c k pump p l u n g e r s , eroded pump valves, sucker r o d f a i l u r e s , s t u c k t u b i n g and p r o p p a n t f i l l , and above a l l t o d r a s t i c a l d e t e r i o r a t i o n o f t h e e f f e c t i v i t y o f r e s e r v o i r s t i m u l a t i o n and w e l l c o m p l e t i o n f o r hydrocarbon p r o d u c t i o n . Damage o f s u r f a c e and downhole equipment by sand e r o s i o n and impingement can a l s o be t h e reason o f f i r e o r b l o w o u t (ANAND & JONES 1977, HALLIBURTON 1988). R e s e r v o i r compaction due t o sand i n f l u x i n t o t h e b o r e h o l e l e a d s t o l o a d d e f o r m a t i o n o f c a s i n g and g r a v e l - p a c k screens (WOOLEY & PRACHNER 1984). I n a d d i t i o n t o f o r m a t i o n sand i n f l u x , d e t e r i o r a t i n g
714
e f f e c t s on wellbore hardware a r e a l s o caused by proppant flowback from f r a c t u r e mouths in hydraulic r e s e r v o i r s t i mu l at i o n ( c f . se c tion 4.12.3.) and a l s o during pumping of very abrasive proppant types downhole during f r a c t u r i n g treatments ( c f . se c t i o n 1 . 4 . 6 . ) .
5.2.1.3.1.2. Sacr i f icat ion o f hydrocarbon exploitation time Hydrocarbon e x p l o i t a t i o n time must be s a c r i f i c e d t o empty s e p a r a t o r s o r t o replace damaged completions components which causes high a dditiona l unnecessary c o s t in terms of damaged equipment, expensive workovers a n d l o s t o i l a n d g a s production, and i n many cas es , the c o s t t o r eturn a well t o hydrocarbon e xploit a t i o n a f t e r extensive sand i n f l u x o r proppant flowback can be so excessive t h a t the well has t o be prematurely abandoned (POPE, WILES & PIERCE 1987) o r can only be saved by s i d et r ack i n g (MURER & STRAUCH 1980, MURER 1981). I n addit i o n t o the expenses f o r the s a n d problems, unexpected pipe leaks o r equipment f a i l u r e s r e p r e sen t s er i o u s s a f e t y and s p i l l h a z a r d s p a r t i c u l a r l y i n offshore and inland water l o cat i o n s (GHALAMBOR, KOLIBA, HAYATDAVOUDI & ALCOCER 1988). Much money has t o be invested every year t o combat sand production in petroleum industry (DURRETT, GOLBIN, MURRAY & T I G H E 1977).
5.2.1.3.2. Economical and technical loss o f wells A p a r t from economical l o s s of he well f o r the reason of unfe a sible remedy expenses, the ul t i mat e danger of s er i o u s sand flow i n t o the borehole can be technical l o s s of the well by cas ng f a i l u r e i n the producing inte rva l due t o non-uniform l a t e r a l loading as the sediment i s flowing and pushing and/or high a x i a l compressive load due t o slumping of the overlying formations in i n i t i a l l y f a i r l y competent r es er v o i r rocks, o r due t o s e rious pay zone compaction subje c ting the e n t i r e casing s t r i n g t o unusual load conditions in i n i t i a l l y unconsolidated pays (DEAN 1939; JARDINS 1959). Large-scale s a n d influx can even r e s u l t in tremendous r e s e r v o i r s e t t l i n g and consequently o i l - f i e l d subsidence ( A L L E N 1969, 1973; SULAK & DANIELSEN 1988, WAAL & SMITH 1988), w i t h pay zone shorteni n g a l s o being transmitted t o casing, tubing, l i n e r and screen which can be p l a s t i c a l l y deformed a t loads exceeding the e l a s t i c a l l i m i t of s t e e l , leading t o severe buckling when l a t e r a l r e s t r a i n t i s l o s t due t o sand production.
5.2.1.3.3. Other aspects Deliberate disturbance of t h e r e s e r v o i r framework by sand production can a l s o cause s i g n i f i c a n t decrease i n pay zone permeability although porosity may only s l i g h t l y change (HALLIBURTON 1988), with t h i s type of formation damage making successful sand control more d i f f i c u l t . In a ddition, sand which i s produced during e a r l y well l i f e may be t h e supporting matrix f o r sha le s t r e a k s which upon l o s s of support could f a l l across some of the pe rfora tions a n d prevent sat i s f a c t o r y placement of remedial sand control m a te ria ls. Slumping and intermixing of overlying s h al es with r e s e r v o i r s a n d can se ve re ly r e s t r i c t f l u i d flow because permeability l o s s es up t o 95 % may occur (RENSVOLD 1982). The n e c e s s i t y of gravel packing hydrocarbon production w e lls in order t o avoid t h e i r d e s t r u ct i o n by sand flow i s p a r t i c u l a r l y urgent in subsea i n s t a l l a t i o n s where many wells a r e t i e d t o the cen t r al template by subsea flow line s, because the r i s k of leaving wells unpacked i n a subsea completion i s considerably higher than normal due t o t h e ex t r ao r d i n ar y expenses involved (SCRIMGEOUR & SHEPHERD 1988).
715
5.2.1.4. Critical hydrocarbon offtake rate S l i g h t sand i n f l u x and f o r m a t i o n o f sand b r i d g e s i n c a s i n g o r t u b i n g can a t l e a s t t e m p o r a r i l y be handled by r e d u c i n g o i l o r gas p r o d u c t i o n r a t e s i n o r d e r t o decrease t h e tendency f o r sand t o e n t e r t h e b o r e h o l e ( e f f e c t s o f f l o w r a t e on s t a b i l i t y o f u n c o n s o l i d a t e d sands a r e a l s o d i s c u s s e d by TIPPIE & KOHLHAAS 1973), by i n c r e a s i n g p r o d u c e d - f l u i d r i s i n g v e l o c i t i e s i n t u b u l a r s t o m i n i m i z e sand s e t t l i n g by u t i l i z i n g small t u b i n g s t r i n g s o r by p r e m a t u r e l y a p p l y i n g gas l i f t , o r by i n c r e a s i n g r i s i n g v e l o c i t i e s by r e c y c l i n g p a r t o f t h e f l o w back i n t o t h e w e l l . Some aspects on t o l e r a t i o n l e v e l o f j o i n t hydrocarbon and sand p r o d u c t i o n , and i n f l u e n c e o f p e r f o r a t i o n s a r e o u t l i n e d as f o l l o w s .
5.2.1.4.1. Toleration level o f joint hydrocarbon and sand product ion A l t h o u g h t h e t e c h n i c a l l y and e c o n o m i c a l l y optimum s o l u t i o n i s always z e r o sand p r o d u c t i o n r a t e , r e l a t i v e l y weak f o r m a t i o n s c o n t a i n i n g h i g h - v i s c o s i t y o i l may a l l o w hydrocarbon o f f t a k e i n s p i t e o f some sand i n f l u x w h i c h does n o t cause problems i f t h e f l o w r a t e i s s u f f i c i e n t l y low (WEISSENBURGER, MORITA, MARTIN & WHITFILL 1987). On t h e o t h e r hand, c o m p a r a t i v e l y s t r o n g r e s e r v o i r s may produce sand once t h e e f f e c t i v e i n - s i t u s t r e s s i s i n c r e a s e d due t o d e c l i n i n g r e s e r v o i r p r e s s u r e by p r o g r e s s i v e d e p l e t i o n . F o l l o w i n g t h e o u t l i n e o f some examples o f exc e p t i o n a l l y h i g h sand c o n t e n t s i n o i l p r o d u c t i o n , comments a r e o f f e r e d on sand p r o d u c t i o n vs. hydrocarbon e x p l o i t a t i o n r a t e .
5.2.1.4.1.1. Exceptionally high sand contents in oil production CHEN, ZHOU & LIU (1986) r e p o r t some f l o w i n g w e l l s t h a t o c c a s i o n a l l y produce o i l even w i t h a sand c o n t e n t up t o 40 % a t l e a s t f o r some t i m e o f e x p l o i t a t i o n (MULLINS, BALDWIN & BERRY 1974; MORSE 1975 and FOSTER & LINVILLE 1979 p r e s e n t methods of d e t e c t i o n and m o n i t o r i n g o f sand p r o d u c t i o n i n f l o w i n g w e l l s , and CHAMBERLAIN & FORSYTH 1982 a l s o comment on sand removal f r o m gas streams), b u t g e n e r a l l y t h e p r a c t i c a l l i m i t on sand i n f l u x i s a b t . 0 . 1 % o r even l e s s (ALLEN & ROBERTS 1982). VONDE (1980) documents t h a t w i t h s p e c i a l l y designed pumping and t r e a t i n g equipment, crude o i l c o n t a i n i n g up t o 70 v o l % o f e n t r a i n e d sand can be withdrawn i n e x t r a o r d i n a r y cases, and t h a t even h i g h e r o f f t a k e r a t e s t h a n w i t h c o n v e n t i o n a l r o d pumps and sand c o n t r o l c o m p l e t i o n s can be achieved, b u t on t h e o t h e r hand, e x c e s s i v e sand p r o d u c t i o n l e a d s t o r e s e r v o i r damage. I n t h e e x c e p t i o n a l case s t u d y r e p o r t e d b y VONDE (1980), one l i m i t a t i o n t o t h e a p p l i c a t i o n o f t h e s p e c i a l pumps i s t h e assurance t h a t adequate v e l o c i t y w i l l be m a i n t a i n e d i n t u b i n g and w e l l b o r e t o e l i m i n a t e sand s e t t l i n g . W h i l e sand e x c l u s i o n c o m p l e t i o n s have r e s u l t e d i n severe w e l l b o r e p l u g g i n g and l i m i t e d o i l p r o d u c t i v i t y , s i g n i f i c a n t o f f t a k e i n c r e a s e s have been o b t a i n e d by a l l o w i n g u n r e s t r a i n e d sand e n t r y i n t o t h e b o r e h o l e . The o n l y l o n g - t e r m e f f e c t i v e and g e n e r a l l y a p p l i c a b l e t r e a t m e n t a g a i n s t sand f l o w , however, i s g r a v e l p a c k i n g o r another sand c o n t r o l o p e r a t i o n (SUMAN, ELLIS & SNYDER 1983).
5.2.1.4.1.2. Sand product ion vs . hydrocarbon exploitation rate Sand p r o d u c t i o n i s a r a t e - s e n s i t i v e process, w i t h a c r i t i c a l hydrocarbon o f f t a k e r a t e e x i s t i n g below which no o r o n l y n e g l i g i b l e amounts o f sand w i l l be produced (BJ-HUGHES 1986). T h i s s a n d - f r e e r a t e , however, i s o f t e n uneconomical and t h e r e f o r e t h e r e s e r v o i r sand has t o be c o n t r o l l e d i n o r d e r t o make o i l exp l o i t a t i o n f e a s i b l e . The c r i t i c a l hydrocarbon o f f t a k e r a t e above which sand p r o d u c t i o n becomes e x c e s s i v e and u n t o l e r a b l e i n o i l - f i e l d maintenance can be d e t e r mined b y beaning-up f l u i d f l o w r a t e s (ALLEN & ROBERTS 1982). As f l u i d f l o w r a t e i s i n c r e a s e d stepwise, sand c o n c e n t r a t i o n jumps a t each i n c r e m e n t and t h e n t a pers o f f t o the o r i g i n a l saturation.
716 The surge e f f e c t a p p a r e n t l y breaks u n s t a b l e sand b r i d g e s w h i c h r e b u i l d a t t h e h i g h e r r a t e i n s u b c r i t i c a l f l o w regimes. Under s u p e r c r i t i c a l c o n d i t i o n s when t h e t o l e r a n c e boundary has been exceeded, sand b r i d g e s do n o t r e f o r m due t o o v e r p a s s i n g o f t h e s t r e n g t h o f t h e s t r u c t u r e and sand i n f l o w c o n t i n u e s a t h i g h e r r a t e s , w i t h hydrocarbon o f f t a k e r a t e t h e n h a v i n g t o be reduced s i g n i f i c a n t l y below t h e c r i t i c a l range t o a l l o w r e i n s t a l l a t i o n o f sand b r i d g e s . Beani n g - u p o f o i l p r o d u c t i o n has t h e s i d e b e n e f i t t h a t t h e g r a d u a l l y i n c r e a s i n g r a t e s p r o v i d e i d e a l clean-up o f induced and i n h e r e n t f i n e s i n t h e p o r e channels around t h e w e l l b o r e , t h e r e b y o f t e n r e s u l t i n g i n s i g n i f i c a n t l y h i g h e r p r o d u c t i v i t y indexes. PEDEN & Y A S S I N (1986 b) d i s c u s s d e t e r m i n a t i o n o f optimum c o m p l e t i o n conditions f o r sand-free o i l production. As a consequence o f e x t r e m e l y h i g h f l o w v e l o c i t i e s , i n gas w e l l s even a small amount o f sand f o r a v e r y s h o r t d u r a t i o n can r e s u l t i n complete e r o s i o n o f s t e e l p i p e a t c r i t i c a l areas (ANAND & JONES 1977), whereas i n o i l w e l l s due t o l o w e r v e l o c i t y and h i g h e r v i s c o s i t y , h i g h e r c o n c e n t r a t i o n s o f sand can be t o l e r a t e d o v e r l o n g e r p e r i o d s o f t i m e a t l e a s t i n some cases. I n a few v e r y uns t a b l e o i l sands, t h e c r i t i c a l r a t e a t which sand s t a r t s f l o w i n g i s l e s s t h a n 1 m3 o f o i l p e r day (MURER 1981).
5.2.1.4.2. Influence of perforations F o r some r e s e r v o i r s s u f f e r i n g f r o m sand e m i g r a t i o n , maximum hydrocarbon o f f t a k e r a t e w i t h o u t e x c e s s i v e sand p r o d u c t i o n i s p r o p o r t i o n a l t o t h e l e n g t h o f t h e p e r f o r a t e d i n t e r v a l , w i t h t h e r e b y some sand i n f l u x problems a l s o b e i n g a b l e t o be s o l v e d by opening an i n c r e a s e d l e n g t h o f pay zone s e c t i o n , o r i m p r o v i n g p e r f o r a t i o n d e n s i t y i n case i n t e r v a l l e n g t h has reached i t s t e c h n i c a l boundary by b e i n g i d e n t i c a l t o n e t r e s e r v o i r t h i c k n e s s . F o l l o w i n g d i s c u s s i o n o f some gen e r a l aspects, t h e advantage o f s e l e c t i v e p e r f o r a t i o n o f h a r d l a y e r s i n i n t e r beddings o f s o f t and h a r d sands i s i l l u s t r a t e d .
5.2.1.4.2.1. General aspects O t h e r f a c t o r s a f f e c t i n g maximum o f f t a k e r a t e a r e p r o d u c t i v i t y i n d e x and f o r m a t i o n s t r e n g t h o r drawdown f a c t o r . The s i g n i f i c a n c e o f i n e r t i a l e f f e c t s ( c f . s e c t i o n 4.3.3.3.) on sand p r o d u c t i o n can be shown by f l u c t u a t i n g c u r r e n t r a t e experiments where e i t h e r a sharp i n c r e a s e o r a sudden decrease i n f l o w r a t e causes temporary i n c r e a s e o f sand i n f l u x (SAUCIER 1974). Given u n i f o r m c u r r e n t r a t e a f t e r t h e change, sand p r o d u c t i o n decreased, t h e r e b y a p p a r e n t l y i n d i c a t i n g r e e s t a b l i s h m e n t o f b r i d g i n g e f f e c t s . Rate changes may t h u s ( i n d e p e n d e n t o f t h e a b s o l u t e o f f t a k e l e v e l s ) be more d e t r i m e n t a l t o r e s e r v o i r s t a b i l i t y t h a n magnitude o f a c e r t a i n f l o w r a t e once i t i s k e p t more o r l e s s c o n s t a n t . PEDEN & YASS I N (1986 a ) a n a l y z e t h e e f f e c t o f p e r f o r a t i o n c o n d i t i o n s upon maximum sandf r e e p r o d u c t i o n r a t e i n f r i a b l e sandstones.
5.2.1.4.2.2. Selective perforation of hard layers in interbeds I n a d d i t i o n t o o r a l t e r n a t i v e l y t o reduced hydrocarbon o f f t a k e r a t e s , s e l e c t i v e p e r f o r a t i o n can be a l r e a d y an e f f e c t i v e means o f sand c o n t r o l i n t h i c k r e s e r v o i r s u n l e s s p o o r l y - c o n s o l i d a t e d m a t e r i a l i s dominant (WEISSENBURGER, M O R I TA, M A R T I N & WHITFILL 1987). T h i s a p p l i e s p a r t i c u l a r l y f o r i n t e r b e d d e d sequences o f s o f t and l i t h i f i e d h o r i z o n s . W h i l e t h e absence o f induced f o r m a t i o n damage and a p o s s i b l e s i g n i f i c a n t improvement o f p r o d u c t i v i t y i n d e x by a c c e p t a b l e m i l d sand f l o w a r e advantages, s e l e c t i v e p e r f o r a t i o n r e q u i r e s s t r i c t adherence t o o p e r a t i o n a l g u i d e l i n e s w i t h s o p h i s t i c a t e d p e r f o r a t i o n i n t e r v a l c h o i c e based on a mechanical p r o p e r t i e s l o g t h a t has been c a l i b r a t e d by c o r e s , because any e r r o r s c o u l d r e s u l t i n s e r i o u s sand problems. L e a v i n g o u t t h e weak u n s t a b l e hor i z o n s from b e i n g p e r f o r a t e d i n i n t i m a t e i n t e r b e d d i n g s o f s o f t and h a r d sands
717 p r o v i d e s hydrocarbon o f f t a k e t h r o u g h t h e c o n s o l i d a t e d l a y e r s , w i t h t h e format i o n sand loosened w i t h i n t h e u n s t a b l e i n t e r v a l s a l r e a d y b e i n g b r i d g e d a t t h e i n t e r f a c e t o t h e h a r d i n t e r v e n t i o n s and t h u s even n o t r e a c h t h e s u r r o u n d i n g s o f the p e r f o r a t i o n tunnels.
5.2.1.5. Necessity and aims o f sand control treatments R e s e r v o i r e n g i n e e r i n g p l a n n i n g o f f i e l d e x p l o i t a t i o n u s u a l l y does n o t have any n e c e s s i t y and u n d e r s t a n d i n g f o r h a n d l i n g sand f l o w problems by d e c r e a s i n g hydrocarbon p r o d u c t i o n r a t e s , b u t i n many f i e l d s t h e economical framework r e q u i r e s p r o g r e s s i v e l y a c c e l e r a t e d o i l o r gas o f f t a k e q u a n t i t i e s ( c f . a l s o STRACKE 1987), and moreover secondary and t e r t i a r y o i 1- r e c o v e r y methods become sensel e s s once t h e necessary l a r g e t o tremendous f l u i d volumes w i t h c o n s i d e r a b l e excess w a t e r cannot be managed i n c l u d i n g t h e i r r e i n j e c t i o n f o r t h e purpose o f p r e s s u r e maintenance, r e s e r v o i r f l u i d pushing and e n v i r o n m e n t a l l y s a f e and haz a r d l e s s underground d i s p o s a l ( c f . a l s o SCHWARTZ 1969). Therefore performance o f g r a v e l pack o r another sand c o n t r o l o p e r a t i o n i s a w i d e l y a p p l i e d means t o o p t i m i z e o f f t a k e q u a n t i t i e s by s t a b i l i z i n g t h e g r a i n framework o f t h e sandy pay f o r m a t i o n and t o p r e v e n t i t s c o l l a p s e which would d i s t u r b r e g u l a r and/or enhanced r e s e r v o i r d e p l e t i o n . Comments a r e o f f e r e d as f o l l o w s on sand c o n t r o l method spectrum; f o r m a t i o n consol d a t i o n vs. g r a v e l packing, and t i m i n g o f sand c o n t r o l i n s t a l l a t i o n .
5.2.1.5.1. Sand control method spectrum Sand c o n t r o l methods comprise mechanical b r i d g i n q i n s t a l l a t ons such as g r a v e l packs, s l o t t e d 1 i n e r s , . wire-wrapped screens-or-pre-packs; chemical c o n s o l i d a t i o n by i n j e c t i o n o f s o l i d i f y i n g f l u i d s ( m a i n l y p l a s t i c r e s i n s ) i n t o t h e f o r m a t i o n t o p r o v i d e an i n - s i t u g r a i n - t o - g r a i n cementation ( c f . s e c t i o n 5 . 9 . 1 . ) , and maintenance o f a h i g h l e v e l o f g r a i n - t o - g r a i n s t r e s s i n t h e r e s e r v o i r sand w i t h i n t h e b o r e h o l e w a l l and i t s near s u r r o u n d i n g s t h r o u g h sand a r c h i n g i n t h e pay zone g r a i n f a b r i c ( c f . s e c t i o n 5.9.5.), w i t h t r i g g e r i n g o f t h e g e o m e t r i c a l arrangement b e i n g supported o r enhanced by u s i n g l o n g i n f l a t a b l e packers. Grav e l p a c k i n g i s by f a r t h e most d i s t r i b u t e d and p o p u l a r sand c o n t r o l method due t o i t s c o m p a r a t i v e l y s i m p l e t e c h n i c a l procedure, good economical f e a s i b i l i t y , h i g h success r a t e and widespread a p p l i c a b i l i t y . Gravel p a c k i n g i s p a r t i c u l a r l y s u i t a b l e f o r w e l l s which have a l r e a d y produced q u a n t i t i e s o f sand, t h e r e b y h a v i n g caused e i t h e r v o i d s b e h i n d t h e c a s i n g and/or development o f a zone o f d i l a t e d pay zone around t h e b o r e h o l e (RENSVOLD 1982). Sand c o n t r o l t r e a t m e n t s aim on r e t e n t i o n o f l o a d - b e a r i n g s o l i d s which upon movement and e m i g r a t i o n would weaken t h e r o c k f a b r i c and u l t i m a t e l y g i v e r i s e t o pay zone c o l l a p s e independent f r o m t h e i r d e t r i m e n t a l e f f e c t w i t h i n t h e w e l l b o r e i t s e l f and t h e t e c h n i c a l i n s t a l l a t i o n s t h e r e i n (ALLEN & ROBERTS 1982). F i n e s o l i d s a s s o c i a t e d w i t h framework i n t e r s t i t i a l s o r f o r m a t i o n f l u i d s a r e n o t p a r t o f t h e mechanical s t r u c t u r e and a r e always produced i n small amounts, w i t h t h i s b e i n g i n f a c t b e n e f i c i a l as o t h e r w i s e t h e y would e v e n t u a l l y b l o c k t h e f l o w channels.
5.2.1.5.2. Formation consolidat ion vs. gravel packing W h i l e t h e chemicals f o r s y n t h e t i c sand framework cementation a r e r e l a t i v e l y expensive and t h u s c o n s o l i d a t i o n i s n o r m a l l y more c o s t l y t h a n g r a v e l p a c k i n g ( c f . s e c t i o n 5.9.1.), t h e performance o f g r a v e l p a c k i n g r e q u i r e s i n any case r i g o r h o i s t o p e r a t i o n which i s n o t i n e v i t a b l y necessary f o r c o n s o l i d a t i o n by chemical i n j e c t i o n t h a t can even be r u n t h r o u g h c o i l e d t u b i n g (LANMON & GILL 1966 even p r e s e n t a w i r e - l i n e compl.etion technique f o r sand c o n t r o l ) . As a r t i f i c i a l g l u e i n g and cementing of l o o s e f o r m a t i o n sands can c o m p l e t e l y r u i n t h e po-
718 roperm p r o p e r t i e s i f t h e j o b f a i l s , however, g r a v e l p a c k i n g has t u r n e d o u t dur i n g t h e l a s t decades t o be t h e most e f f e c t i v e , p r a c t i c a l and s u c c e s s f u l sand c o n t r o l method s i n c e i t s i n t r o d u c t i o n t o w a t e r - p r o d u c t i o n i n d u s t r y o v e r 100 y e a r s ago ( A P I 1961, CASHION & M A R K I E 1970, SOLUM 1986) and i t s f i r s t a p p l i c a t i o n s i n o i l and gas f i e l d s o v e r 60 y e a r s ago (SCOTT 1928, SAWDON 1937, COBERLY & WAGNER 1938, ECKEL & REISTLE 1940, HILL 1941, WEST 1951).
5.2.1.5.3.
Timing o f sand control installation
I n terms o f t i m i n g o f i n s t a l l a t i o n o f sand c o n t r o l measures, e a r l y w e l l l i f e i s i n many cases a more s u i t a b l e p e r i o d f o r i n v e s t m e n t than l a t e p r o d u c t i o n h i s t o r y (HALLIBURTON 1 9 8 8 ) . Delay o f sand c o n t r o l t r e a t m e n t s bears t h e r i s k t h a t once sand i n f l o w commences w i t h i t s a s s o c i a t e d disadvantages i f n o t i n h i b i t e d i n i t i a l l y , c o s t o f remedial a c t i o n s may by f a r exceed t h a t o f p r i m a r y sand cont r o l , and r e p a i r o p e r a t i o n s may f a i l due t o r e s e r v o i r damage i n c o n t r a s t t o p r i mary c o n t r o l which i s much s u r e r and does n o t o n l y save c o s t f o r h a n d l i n g o f sand i n v a s i o n problems, b u t a l s o a l l o w s f a s t e r hydrocarbon e x p l o i t a t i o n and t h u s d r i l l i n g and c o m p l e t i o n a m o r t i z a t i o n . While i n i t i a l c o s t may be f a i r l y acc u r a t e l y e s t i m a t e d , t h e expenses o f r e m e d i a l work a r e l e s s p r e d i c t a b l e .
5.2.2.
Gravel type
Gravel p a c k i n g d e s i g n p r i m a r i l y i n c l u d e s s e l e c t i o n o f g r a v e l t y p e a c c o r d i n g t o r e s e r v o i r depth, temperature and b r i n e composition; c h o i c e o f g r a v e l g r a i n s i z e i n o r d e r t o c o m p l e t e l y r e t a i n movement o f f o r m a t i o n sand, s e l e c t i o n o f screen s i z e and/or l i n e r s l o t openings f o r t h e purpose o f p r e v e n t i n g b o t h g r a v e l and f o r m a t i o n f i n e s t o e n t e r t h e g r a v e l - p a c k hardware, g r a v e l pack t h i c k ness aiming on achievement o f maximum f i l t r a t i o n c a p a c i t y and minimum h y d r o c a r bon f l o w r a t e impairment, and p e r f o r a t i o n t u n n e l l e n g t h f o r g u a r a n t e i n g s t a b l e p e r f o r a t i o n h o l e s and c r e a t i n g a s u f f i c i e n t c a p t u r e r a d i u s f o r t h e e f f l u e n t hydrocarbons, as w e l l as b o r e h o l e s i z e and g r a v e l placement t e c h n i q u e . S e l e c t i o n o f g r a v e l i n terms o f t y p e and g r a i n s i z e i s t h e key element o f sand c o n t r o l des i g n t r e a t m e n t . Some comments on n a t u r a l sand and s y n t h e t i c p r o p p a n t s a r e o f f e r e d as f o l l o w s .
5.2.2.1.
General aspects o f gravel type choice
Both n a t u r a l sand and s y n t h e t i c p r o p p a n t s can be a p p l i e d f o r h y d r a u l i c f r a c t u r i n g as w e l l as g r a v e l p a c k i n g . The most i m p o r t a n t c r i t e r i o n f o r p r o p p a n t o r g r a v e l t y p e s e l e c t i o n i s c l o s u r e s t r e s s which i n h y d r a u l i c f r a c t u r i n g a f f e c t s t h e whole p r o p p a n t wedge i n f i l l i n g t h e crack, whereas i n g r a v e l packing, o n l y t h e proppants p l u g g i n g t h e p e r f o r a t i o n t u n n e l s and composing g r a v e l nodes a t t h e d i s t a l ends o f t h e p e r f o r a t i o n tubes a r e s u b j e c t e d t o more o r l e s s f u l l c l o s u r e s t r e s s , b u t t h e g r a v e l m a n t l e e i t h e r along t h e b o r e h o l e w a l l o r between cas i n g and screen o r l i n e r does n o t have t o r e s i s t t o t h e f u l l p r e s s u r e l o a d due t o t h e absence o f t h e l i t h o s t a t i c component i n t h e open w e l l b o r e . The d i s t a l g r a v e l nodes r e p r e s e n t a more o r l e s s b a l l - s h a p e d replacement i n f i l l i n g o f t h e r u b b l i z e d zone a t t h e end o f t h e p e r f o r a t i o n t u n n e l where u s i n g up o f t h e charge r e s u l t e d i n f u r t h e r f o r m a t i o n p e n e t r a t i o n , and must n o t be conf u s e d w i t h t h e g r a v e l nodes a t t h e p r o x i m a l ends o f t h e p e r f o r a t i o n t u n n e l s which a r e d e p o s i t s o r i g i n a t i n g a f t e r f i l l i n g o f t h e s h o t h o l e s on t h e c a s i n g w a l l where t h e y t e n d t o b r i d g e t h e annulus by meeting w i t h a n n u l a r sediments cov e r i n g t h e o u t e r f a c e o f screen o r l i n e r ( c f . s e c t i o n 5 . 8 . 7 . ) . A d d i t i o n a l more i r r e g u l a r g r a v e l nodes o r i g i n a t e a t t h e c e m e n t / f o r m a t i o n i n t e r f a c e o r even a t t h e casing/cement boundary as a consequence o f i n f i l l i n g o f v o i d s t h a t d e r i v e f r o m i n c o m p l e t e cement d i s t r i b u t i o n w i t h h o l e s and channels i n t h e cement sheath. S i g n i f i c a n c e , s u i t a b i l i t y and n e c e s s i t y o f n a t u r a l sand and s y n t h e t i c proppants as g r a v e l i n mechanical sand c o n t r o l a r e o u t l i n e d as f o l l o w s .
719
5.2.2.2. Natural sand As deep t i g h t gas-bearing sandstones and moderately-deep o i l - b e a r i n g sandstones a r e u s u a l l y s u f f i c i e n t l y cemented and do n o t d i s i n t e g r a t e i n t o loose g r a i n s d u r i n g t e s t i n g and p r o d u c t i o n even t h r o u g h o u t l o n g e x p l o i t a t i o n h i s t o r y , sand c o n t r o l t r e a t m e n t s a r e i n most cases r e s t r i c t e d t o c o m p a r a t i v e l y s h a l l o w o r even v e r y s h a l l o w w e a k l y - l i t h i f i e d o r n e a r l y uncemented f o r m a t i o n s c o n t a i n i n g m a i n l y o i l and s u b o r d i n a t e l y a l s o gas r e s e r v o i r s . I n o t h e r areas, however, sand problems may be encountered up t o r e s e r v o i r depths o f 12,000 f t (4,000 m) o r more (ALLEN & ROBERTS 1982, BJ-HUGHES 1986). P r o d u c t i o n d r i l l i n g on ground w a t e r a l s o has abundantly t o i n c l u d e g r a v e l p a c k i n g o f t h e u n s t a b l e , o f t e n g e o l o g i c a l l y q u i t e young sandstones and conglomer a t e s i n f r e q u e n t l y v e r y s h a l l o w d e p t h p a r t i c u l a r l y i n case o f c o n v e n t i o n a l d r i n k i n g water; m i n e r a l i z e d o r thermal w a t e r s commonly d e r i v e f r o m deeper s t o r e y s o f t h e l o c a l g e o l o g i c a l column which a r e b e t t e r l i t h i f i e d . Aspects o f g r a v e l t y p e s e l e c t i o n a r e summarized as f o l l o w s along t h e l i n e s o f r e s e r v o i r d e p t h and c l o s u r e s t r e s s i n p e r f o r a t i o n t u n n e l s . An o v e r v i e w o f t h e provenance of g r a v e l p a c k i n g sand i s a l s o p r o v i d e d .
5.2.2.2.1. Reservoir depth and closure
stress in perforation tunnels
As a consequence o f t h e low c l o s u r e s t r e s s e s i n t h e g e n e r a l l y s h a l l o w d e p t h o f t h e o i l , w a t e r o r gas r e s e r v o i r s r e q u i r i n g sand c o n t r o l , n a t u r a l sand i s i n most cases t h e adequate m a t e r i a l f o r g r a v e l p a c k i n g i n view o f demanded mechanic a l r e s i s t i v i t y , w i t h t h e c l o s u r e s t r e s s regime i n most o f t h e pay f o r m a t i o n s w i t h sand i n f l u x problems b e i n g s u b c r i t i c a l f o r sand. Sand c r u s h i n g may g e n e r a l l y o n l y o c c u r i n l i m i t e d amounts d u r i n g s u r f a c e h a n d l i n g and downhole pumping, b u t n o t a t t h e bottom o f t h e w e l l as a consequence o f e x c e s s i v e c l o s u r e s t r e s s i n most o f t h e r e s e r v o i r depth ranges where g r a v e l p a c k i n g i s c a r r i e d o u t ( c f . s e c t i o n 1.2.2.). The main reasons f o r t h e predominant t o almost e x c l u s i v e a p p l i c a t i o n o f n a t u r a l sand as g r a v e l p a c k i n g m a t e r i a l i n mechanical sand c o n t r o l a r e i t s ready n e a r l y u n l i m i t e d a v a i l a b i l i t y and i t s cheap p r i c e , w i t h payment o f a premium f o r a h i g h - q u a l i t y s y n t h e t i c p r o p p a n t t y p e i n s t e a d o f n a t u r a l sand i n most o f t h e s h a l l o w r e s e r v o i r depths where g r a v e l p a c k i n g i s c a r r i e d o u t n o t b e i n g t e c h n i c a l l y and e c o n o m i c a l l y j u s t i f i e d . I n boundary cases when deeper pay zones r e q u i r e g r a v e l p a c k i n g o r s h a l l o w e r u n c o n s o l i d a t e d sands a r e overpressured, t h e s e c t i o n s o f t h e g r a v e l pack which a r e most s u s c e p t i b l e t o d e t e r i o r a t i o n by g r a i n c r u s h i n g due t o s u p e r c r i t i c a l c l o s u r e s t r e s s a r e t h e plugged p e r f o r a t i o n t u n n e l s and t h e g r a v e l nodes a t t h e d i s t a l ends o f t h e p e r f o r a t i o n tubes where t h e more o r l e s s f u l l l i t h o s t a t i c p r e s s u r e i s r e l e a s e d and t h e e f f e c t i v e c l o sure stress also r i s e s w i t h r e s e r v o i r pressure decline during progressive deplet i o n ( s i m i l a r l y as a p p l y i n g f o r propped f r a c t u r e s i n h y d r a u l i c s t i m u l a t i o n ; c f . s e c t i o n 4.12.4.). The d i f f e r e n t i a l p r e s s u r e l a s t i n g on t h e g r a v e l i n t h e p e r f o r a t i o n t u n n e l s i s i n any case c o n s i d e r a b l y h i g h e r t h a n t h a t a c t i n g on t h e g r a vel mantle i n the wellbore. As t h e p e r f o r a t i o n t u n n e l s a r e s i t u a t e d i n t h e immediate v i c i n i t y o f t h e w e l l b o r e where c l o s u r e s t r e s s upon p r e s s u r e drawdown reaches i t s maximum, a t t e n t i o n has t o be p a i d n o t o n l y i n h y d r a u l i c f r a c t u r i n g , b u t a l s o i n g r a v e l packi n g , t o go on t h e a b s o l u t e l y s a f e s i d e and d e s i g n t h e m a t e r i a l i n f i l l i n g hydraul i c f r a c t u r e p a t h as w e l l as g r a v e l - p a c k e d p e r f o r a t i o n t u n n e l s i n t h e n e a r e s t s u r r o u n d i n g s o f t h e w e l l b o r e i n such a manner t h a t t h e p l u g g i n g i s adequate t o the highest possible closure stress throughout the l i f e o f the borehole. Perfor a t i o n c o l l a p s e (ANTHEUNIS, VRIEZEN, SCHIPPER & VLIS 1976) i n f r i a b l e s h a l l o w sandstones and weakly-cemented r e s e r v o i r s i n g r e a t e r d e p t h has t o be a v o i d e d by
720 e a r l y propping g r a i n size.
5.2.2.2.2.
of
the
perforation
tunnels
w i t h g r a v e l o f s u i t a b l e t y p e and
Provenance of gravel packing sand
H i g h - q u a l i t y sand o f b o t h American and European provenance has been commonly used f o r g r a v e l packing, w i t h t h e u t i l i z a t i o n o f cheaper m a t e r i a l f r o m l o c a l sources even h e l p i n g t o reduce t h e c o s t o f t h e o p e r a t i o n s i n case q u a l i t y i s acc e p t a b l e (examples where expensive i m p o r t o f sand f r o m remote sources c o u l d be avoided due t o l o c a l presence o f sand i n s u i t a b l e q u a l i t y and s u f f i c i e n t q u a n t i t i e s were i n t h e l a s t y e a r s p a r t i c u l a r l y Libya, I n d o n e s i a , and USSR, and t h e same c e r t a i n l y a p p l i e s f o r o t h e r p a r t s o f A f r i c a and t h e F a r E a s t ) . I n terms o f q u a l i t y demand, t h e expensive t o p - c l a s s Ottawa Sand i s f r e q u e n t l y n o t i n e v i t a b l y necessary as g r a v e l p a c k i n g m a t e r i a l and can be r e p l a c e d by cheaper sands f r o m areas c l o s e r s u r r o u n d i n g t h e o i l - f i e l d o p e r a t i o n , t h e r e b y s a v i n g b o t h p r o d u c t and t r a n s p o r t c o s t ( t h e same a p p l i e s f o r s m a l l - t o m o d e r a t e - s c a l e sand f r a c t u r i n g i n s h a l l o w t o i n t e r m e d i a t e r e s e r v o i r depth; c f . s e c t i o n 1 . 3 . 1 . ) . Sand p r o d u c t i o n d i f f i c u l t i e s u s u a l l y i n c r e a s e w i t h s h a l l o w i n g f o r m a t i o n depth where t h e r e s e r v o i r r o c k s a r e i n l e s s mature stages o f b u r i a l d i a g e n e s i s and t h u s l i t h i f i c a t i o n and t h e g r a i n f a b r i c i s a l s o p o o r e r s u p p o r t e d by l o w e r l i t h o s t a t i c and h y d r o s t a t i c pressures, as w e l l as w i t h r i s i n g percentage o f f i nes (RIKE 1970). T h i s r e l a t i o n s h i p c l e a r l y u n d e r l i n e s t h a t i n most o f t h e cases o f sand c o n t r o l demand, r e s e r v o i r depth and t h u s bottomhole p r e s s u r e a r e so low t h a t n a t u r a l sand i s s t i l l enough s t a b l e w i t h s u f f i c i e n t s e c u r i t y t o be used as gravel packing m a t e r i a l .
5.2.2.3. Synthetic proppants I n t e r m e d i a t e - and h i g h - s t r e n g t h l o w - d e n s i t y o r h i g h - d e n s i t y alumina o x i d e and s i l i c a t e ( c e r a m i c ) o r l o w - d e n s i t y z i r c o n i a - s i l i c a t e p r o p p a n t s ( c f . s e c t i o n 1 . 3 . ) have so f a r o n l y been a p p l i e d i n a few e x c e p t i o n s f o r g r a v e l packing, w i t h t h e i r w i d e r d i s t r i b u t i o n h a v i n g been m a i n l y l i m i t e d by t h e c o n s i d e r a b l y h i g h e r p r i c e s o f t h e h i g h - q u a l i t y expensive s y n t h e t i c m a t e r i a l s ( c f . s e c t i o n 2 . 2 . 2 . 3 . ) . S i n t e r e d b a u x i t e has been p a r t i c u l a r l y u t i l i z e d i n s t e a m - f l o o d i n j e c t i o n - w e l l g r a v e l p a c k i n g ( c f . s e c t i o n 5.7.1,), and n i c k e l - c o a t e d sand has been r e c e n t l y i n t r o d u c e d f o r t h e same purpose because o f b e i n g even s u p e r i o r t o s i n t e r e d b a u x i t e i n t h e thermal h i g h - a l c a l i n i t y environment ( c f . s e c t i o n 5 . 7 . 2 . ) . R e s i n - c o a t e d sand and proppants have a l s o r e p e a t e d l y been s e l e c t e d f o r a p p l i c a t i o n i n t h e g r a v e l p a c k i n g m a r k e t segment ( c f . s e c t i o n 5 . 8 . 2 . ) . L o w - d e n s i t y i n t e r m e d i a t e - s t r e n g t h alumina s i l i c a t e p r o p p a n t s a r e t h e most s u i t a b l e m a t e r i a l f o r optimum pumping and placement, because t h e i r s p e c i f i c g r a v i t y i s almost i d e n t i c a l t o t h a t o f sand which a l l o w s e x c e l l e n t suspension and t r a n s p o r t w i t h o u t s e t t l i n g i n most o f t h e g r a v e l p a c k i n g f l u i d s a t t h e b e n e f i t o f t h e much h i g h e r c o n d u c t i v i t y o f t h e h i g h - q u a l i t y s y n t h e t i c m a t e r i a l . R e c e n t l y an u l t r a l i g h t a r t i f i c i a l g r a v e l p a c k i n g m a t e r i a l has been i n t r o d u c e d f o r s p e c i a l a p p l i c a t i o n i n h i g h l y - d e v i a t e d w e l l s ( c f . s e c t i o n s 5 . 3 . 3 . and 5 . 8 . 3 . ) . The f o l l o w i n g o u t l i n e focusses on sand c o n s o l i d a t i o n and c l o s u r e s t r e s s t r a n s f e r , p e r f o r a t i o n t u n n e l and b o r e h o l e w a l l s t a b i l i t y , and a n g l e o f repose and g r a v e l i n f i l l i n g .
5.2.2.3.1.
Sand consolidation and closure stress transfer
F o r t h e purpose o f s e c u r i n g u n l i m i t e d success o f g r a v e l p a c k i n g o p e r a t i o n s , recommendation i s made t o f a v o u r t h e more expensive h i g h - q u a l i t y s y n t h e t i c proppants i n a l l cases where a t any stage d u r i n g l o n g - t e r m f i e l d o p e r a t i o n and hydrocarbon p r o d u c t i o n h i s t o r y , e f f e c t i v e c l o s u r e s t r e s s i n t h e immediate s u r r o u n d i n g s o f t h e w e l l b o r e m i g h t upon t h e h i g h e s t p o s s i b l e r e s e r v o i r p r e s s u r e drawdown exceed t h e r e s i s t i v i t y boundary o f n a t u r a l sand, because c r u s h i n g o f t h e
721 m a t e r i a l i n f i l l i n g t h e p e r f o r a t i o n t u n n e l s and f o r m i n g t h e g r a v e l nodes a t t h e d i s t a l ends o f t h e p e r f o r a t i o n tubes would be t h e w o r s t d e t e r i o r a t i o n o f t h e e f f e c t i v i t y o f t h e g r a v e l pack j o b . C l o s u r e s t r e s s becomes c r i t i c a l on t h e proppants i n t h e p e r f o r a t i o n t u n n e l s p a r t i c u l a r l y i n moderately-cemented sandstones i n g r e a t e r r e s e r v o i r depth. I n s h a l l o w u n c o n s o l i d a t e d sands, c l o s u r e s t r e s s i s n o t f u l l y e x e r t e d on t h e propp a n t s i n t h e p e r f o r a t i o n t u n n e l s because o f compensation o f t h e s t r e s s by comp a c t i o n of t h e s o f t r o c k and embedment o f t h e p r o p p a n t p a r t i c l e s . The w e i g h t i s t r a n s f e r r e d f r o m t h e p o o r l y - l i t h i f i e d l a y e r s t o under- or o v e r l y i n g i n t e n s e l y cemented h o r i z o n s such as carbonates and c o n s o l i d a t e d sandstones. An i m p o r t a n t a p p l i c a t i o n of i n t e r m e d i a t e - and h i g h - s t r e n g t h proppants i s p r e v e n t i o n o f p e r f o r a t i o n t u n n e l c o l l a p s e i n l o w - p e r m e a b i l i t y overpressured gas sandstones where e l e v a t e d p r e s s u r e drawdown l e a d s t o c l o s u r e o f t h e h o r i z o n t a l p e r f o r a t i o n h o l e s and impedes gas p r o d u c t i o n (CHENEVERT & THOMPSON 1985).
5.2.2.3.2. Perforation tunnel and borehole wall stability P e r f o r a t i n g i n t o t h e s i d e o f a b o r e h o l e s e t s up a v e r y h i g h - s t r e s s e n v i r o n ment which i s v e r y c r i t i c a l f o r p e r f o r a t i o n t u n n e l s t a b i l i t y , and c o l l a p s e o f t h e h o r i z o n t a l h o l e s can b e s t be p r e v e n t e d by i n f i l l i n g them w i t h p r e s s u r e - r e s i s t a n t p r o p p a n t s which p l u g t h e v o i d s and guarantee t h e a p p l i c a b i l i t y o f h i g h p r e s s u r e drawdowns f o r r e s e r v o i r - a d e q u a t e o f f t a k e r a t e s i n s t e a d o f keeping drawdown v e r y l i m i t e d i n o r d e r t o a v o i d c o l l a p s e o f unplugged p e r f o r a t i o n t u n n e l s and t o renounce f r o m economical gas p r o d u c t i o n r a t e s ( a s p e c t s o f p e r f o r a t i o n s t a b i l i t y under h i g h s t r e s s a r e a l s o d i s c u s s e d by PATTILLO & SMITH 1982). STEIN (1976) uses c o n v e n t i o n a l w e l l l o g d a t a f o r s e l e c t i o n o f g r a v e l p a c k i n g mater i a l s f o r sand c o n t r o l .
5.2.2.3.3. Angle of repose and gravel infilling The a n g l e o f repose o f t h e g r a v e l - p a c k m a t e r i a l should be small i n o r d e r t o a l l o w r e a d y i n f i l l i n g o f a l l c r e v i c e s and p o c k e t s i n t h e f o r m a t i o n i n b a r e f o o t completed w e l l s (COBERLY & WAGNER 1938). A c c o r d i n g t o t h e i r v e r y l o w a n g l e o f repose, h i g h - s t r e n g t h z i r c o n i a - s i l i c a t e p r o p p a n t s s h o u l d a l s o i n terms o f t h e i r low s p e c i f i c g r a v i t y be t h e i d e a l gravel-pack m a t e r i a l , b u t u n f o r t u n a t e l y t h e y a r e f a r t o o expensive f o r most o f t h e a p p l i c a t i o n s ( c f . s e c t i o n s 1.4.3. and 2.2.2.3.). Low d e n s i t y and cheap p r i c e o f i n t e r m e d i a t e - s t r e n g t h alumina s i l i c a t e proppants c h a r a c t e r i z e t h i s t y p e o f g r a v e l t o be t h e most s u i t a b l e mater i a l t o r e p l a c e n a t u r a l sand i n a l l t h e u t i l i z a t i o n s where h i g h e r c l o s u r e s t r e s s r e s i s t i v i t y and h i g h e r pack c o n d u c t i v i t y o f t h e g r a v e l i s r e q u i r e d . The low s p e c i f i c g r a v i t y o f l o w - d e n s i t y alumina s i l i c a t e proppants r e s u l t s i n a pumping b e h a v i o u r o f t h i s m a t e r i a l s i m i l a r t o t h a t o f n a t u r a l sand w h i l e a l l t h e advantages o f t h e h i g h - q u a l i t y p r o p p a n t a r e o b t a i n e d . The h i g h s p e c i f i c g r a v i t y o f s i n t e r e d b a u x i t e i s a c o n s i d e r a b l e drawback f o r t h e a p p l i c a t i o n o f g r a v e l m a t e r i a l a p a r t f r o m i t s e l e v a t e d p r i c e , b u t as a consequence o f i t s p e r formance b e i n g s u p e r i o r t o a l l c o m p e t i t i v e m a t e r i a l s i n some s p e c i a l a p p l i c a t i o n s such as s t e a m - d r i v e enhanced o i l r e c o v e r y ( c f . s e c t i o n 5.7.1.), t h e u t i l i z a t i o n o f t h e expensive and heavy s i n t e r e d b a u x i t e i s i n some cases i n e v i t a b l e f o r a c h i e v i n g s u c c e s s f u l sand c o n t r o l .
5.2.3. Gravel grain size The most e x t e n s i v e l y used s a n d - c o n t r o l t e c h n i q u e c o n s i s t s of p l a c i n g a screen i n t h e w e l l b o r e and p a c k i n g g r a v e l around i t (GRUESBECK, SALATHIEL & ECHOLS 1979). The screen i s s i z e d t o h o l d back t h e g r a v e l which i n t u r n i s s i zed t o r e t a i n t h e f o r m a t i o n sand. G r a i n s i z e s e l e c t i o n o f t h e g r a v e l i s t h e r e f o r e a f u n c t i o n of mean p a r t i c l e s i z e of t h e r e s e r v o i r sand, and screen open-
722 i n g s have t o be a d j u s t e d t o chosen g r a v e l g r a i n s i z e . S e l e c t i o n o f g r a v e l g r a i n s i z e i n sand c o n t r o l i s t h e r e f o r e o f much g r e a t e r s i g n i f i c a n c e t h a n g r a v e l t y p e choice, which i s i n c o n t r a s t t o h y d r a u l i c f r a c t u r i n g where emphasis i s on s e l e c t i o n of p r o p p a n t t y p e and a l t h o u g h b e i n g a l s o c o n s i d e r a b l y i m p o r t a n t f o r conduct i v i t y o p t i m i z a t i o n , p r o p p a n t g r a i n s i z e c h o i c e i s i n most cases o n l y o f subord i n a t e p r i o r i t y ( c f . s e c t i o n s 1.3. and 1 . 4 . ) . F o r m a t i o n p a r t i c l e s i z e d i s t r i b u t i o n d a t a a r e t h e b a s i s f o r g r a v e l s i z e and screen opening s p e c i f i c a t i o n s and have a l s o pronounced e f f e c t s on c o m p l e t i o n c o n f i g u r a t i o n (ECONOMIDES 1986), because f i n e r r e s e r v o i r sands r e q u i r i n g small e r g r a v e l f o r p r o p e r c o n t r o l need more p e r f o r a t i o n s than normal o r even openh o l e c o m p l e t i o n i n o r d e r t o p r o v i d e an adequate f l o w area. A f t e r t h e o u t l i n e o f some g e n e r a l f e a t u r e s , t h e b r i e f account as f o l l o w s c o n c e n t r a t e s on aspects o f g r a i n s i z e s e l e c t i o n vs. f o r m a t i o n g r a i n s i z e , p a c k i n g g r a v e l / f o r m a t i o n sand s i z e r a t i o s , w i d t h o f g r a v e l mantle, impact o f g r a v e l pack hardware, f o r m a t i o n g r a i n s i z e e v a l u a t i o n , g r a i n s i z e i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand, and hydrocarbon p r o d u c t i v i t y vs. sand c o n t r o l . Some aspects o f u s i n g screen o r lin e r w i t h o u t g r a v e l pack a r e a l s o discussed.
5.2.3.1. General aspects of gravel grain size selection C a r e f u l s e l e c t i o n o f sand o r p r o p p a n t g r a i n s i z e f o r sand c o n t r o l i s essent i a l , as t o o f i n e m a t e r i a l l e a d s t o p l u g g i n g o f t h e r e s e r v o i r , whereas t o o coarse m a t e r i a l g i v e s r i s e t o g r i n d i n g o f t h e l i n e r by a l l o w i n g f o r m a t i o n p a r t i c l e s t o pass t h e i n e f f e c t i v e f i l t e r and t o be produced t o g e t h e r w i t h t h e hydrocarbons. Gravel s i z e r e d u c t i o n may d i m i n i s h t h e h e i g h t o f t h e g r a v e l - p a c k e d i n t e r v a l and l e a v e some zones w i t h o u t sand c o n t r o l , c r e a t e v o i d s w i t h i n p e r f o r a t i o n s and cause t h e i r c o l l a p s e , change g r a v e l s i z e d i s t r i b u t i o n and decrease sand c o n t r o l e f f i c i e n c y , and generate f i n e s and p l u g t h e g r a v e l pack, whereas f o r h y d r a u l i c f r a c t u r i n g , p r o p p a n t s i z e r e d u c t i o n may decrease f r a c t u r e conduct i v i t y and generate f i n e s (CHEUNG 1985). Some aspects o f common g r a v e l q u a n t i t i e s and g r a i n s i z e s , p r e v e n t i o n o f f o r m a t i o n sand i n t r u s i o n , and g r a v e l q u a l i t y demand a r e o u t l i n e d as f o l l o w s .
5.2.3.1.1. Comnon gravel quantities and grain sizes As
t h e q u a n t i t i e s o f sand t h a t a r e needed f o r g r a v e l p a c k i n g a r e a b t . 1 - 20
t p e r w e l l and i n t h e s m a l l e r - s c a l e a p p l i c a t i o n s o f t e n o n l y amounts between a few hundreds o f kg and some t o n s a r e necessary, t h e t o t a l demand o f m a t e r i a l
f o r g r a v e l p a c k i n g i s much l e s s t h a n t h a t r e q u i r e d f o r h y d r a u l i c s t i m u l a t i o n and i s i n most areas c u m u l a t i v e l y f o r f i e l d s o r complexes o f patches o n l y a b t . 5 - 20 % and f o r t h e i n d i v i d u a l j o b o n l y a b t . 1 - 10 % o f t h e q u a n t i t y used f o r fracturing. I n a d d i t i o n , common g r a i n s i z e s f o r g r a v e l p a c k i n g a r e o f t e n 30/50 and 40/60 mesh i n s t e a d o f p r e d o m i n a n t l y 20/40 and 16/20 i n h y d r a u l i c f r a c t u r i n g , whereas i n o t h e r r e s e r v o i r s , a l s o 12/20 and 20/40 a r e usual g r a i n s i z e s f o r g r a v e l packi n g i n s t e a d o f o r i n a d d i t i o n t o t h e f i n e r m a t e r i a l . I n v e r y coarse pay zones o r i n case o f reasonable g r a v e l compaction by l i n e r v i b r a t i o n ( c f . s e c t i o n 5.8.4.), a l s o 8/12 and 6/10 s i z e s have been o c c a s i o n a l l y a p p l i e d ( c f . FLANIGAN 1 9 7 9 ) , whereas e x t r e m e l y f i n e sands t o s i l t s , diatomaceous e a r t h s and c h a l k s may r e q u i r e even f i n e r g r a v e l o f 40/70 o r 70/140 s i z e u n l e s s s p e c i a l d e s i g n has t o be made such as i n case o f mass-flowing c h a l k s w i t h m o t i o n o f p e l l e t s composed o f numerous i n d i v i d u a l m i c r o p a r t i c l e s ( c f . s e c t i o n 4 . 5 . 4 . 3 . ) .
5.2.3.1.2. Prevention of formation sand intrusion Choice o f g r a v e l s i z e has t o f o l l o w t h e g u i d e l i n e s t h a t an e f f e c t i v e g r a v e l pack i s t o c o m p l e t e l y p r e v e n t t h e i n t r u s i o n of f o r m a t i o n sand b o t h i n t o t h e
723 w e l l b o r e and t h e g r a v e l pack i t s e l f , t o p r o v i d e maximum p r o d u c t i v i t y by m i n i m i z i n g t h e pseudo-skin a s s o c i a t e d w i t h t h e a d d i t i o n a l f l o w r e s i s t a n c e t o t h e hydrocarbons generated by t h e g r a v e l pack, and t o m i n i m i z e p e r m e a b i l i t y i m p a i r ment w i t h i n t h e f o r m a t i o n (PEDEN, RUSSELL & OYENEYIN 1985). The o b j e c t i v e o f a good g r a v e l pack i s t o connect t h e i n t e r s t i t i a l tubes o f t h e f o r m a t i o n t o those o f t h e g r a v e l i n such a way t h a t t h e r e i s no movement o f s o l i d s a t a l l when t h e produced hydrocarbons pass t h r o u g h t h e pack (SOLUM 1984). Gravel s i z e s e l e c t i o n r e l a t i v e t o t h e s i z e o f t h e f o r m a t i o n p a r t i c l e s i s one of t h e most i m p o r t a n t aspects o f d e s i g n i n g g r a v e l pack f o r maximum p r o d u c t i v i t y (GURLEY, COPELAND & HENDRICK 1977). Gravel should be p r i n c i p a l l y s i z e d t o p r e v e n t i n v a s i o n o f t h e f i n e s t f o r m a t i o n sand w i t h i n t h e p e r f o r a t e d hydrocarbonb e a r i n g i n t e r v a l o f t h e g e o l o g i c a l column i n case o f m u l t i p l e - h o r i z o n complet i o n s i n o r d e r t o assure e f f e c t i v e sand c o n t r o l and l o n g e r - l i v e d g r a v e l packs, w i t h t h e d e s i r e d r e s u l t b e i n g s t a b i l i z e d g r a i n b r i d g i n g and t h e r e f o r e a b s o l u t e stoppage of sand p r o d u c t i o n w i t h o u t undue l i m i t a t i o n s o f f l o w c a p a c i t y and t h u s e x p l o i t a t i o n r a t e s (SAUCIER 1974). Coarser pay zone sands a r e r e l a t i v e l y easy t o c o n t r o l , b u t c o n s i d e r a b l e d i f f i c u l t i e s o c c u r i n f i n e r r e s e r v o i r sands w i t h l a r g e r amounts o f s i l t and c l a y p a r t i c l e s . S m a l l e r g r a v e l may r e q u i r e a l a r g e r p e r f o r a t i o n a r e a o r even open h o l e c o m p l e t i o n f o r i t s i n s t a l l a t i o n , and i s more s u s c e p t i b l e t o p e r m e a b i l i t y r e d u c t i o n caused by m i x i n g w i t h f o r m a t i o n d e b r i s and c o n t a m i n a t i o n w i t h r e s e r v o i r sand d u r i n g placement.
5.2.3.1.3. Gravel quality demand I n terms o f q u a l i t y demand, a minimum o f 96 % o f t h e g r a v e l s h o u l d f a l l b e t ween t h e d e s i g n a t e d s i e v e mesh s i z e s , and n o t more t h a n 2 % should be s m a l l e r t h a n t h e l a s t mesh s i z e o f t h e g r a n u l o m e t r i c a l i n t e r v a l . The g r a v e l s h o u l d cons i s t o f i n d i v i d u a l g r a i n s and n o t of c l u s t e r s o f two o r more g r a i n s . Sand cont a i n i n g more t h a n 1 % o f a c i d - s o l u b l e contaminates i s n o t recommended f o r g r a v e l p a c k i n g because o f r e p r e s e n t i n g a source o f f i n e s and g r a i n f r a c t u r i n g which reduces g r a v e l pack l i f e . Gravel c o n t a i n i n g more t h a n 0.5 % o f c l a y - s i z e d f i n e p a r t i c l e s p a s s i n g 325 mesh screen s h o u l d a l s o be r e j e c t e d , as f i n e s q u i c k l y d e s t r o y pack p e r m e a b i l i t y .
5.2.3.2. Gravel grain size selection vs. formation grain size I n terms of granulometry, e x p e r i e n c e d u r i n g t h e l a s t decades has shown t h a t mean s i z e of t h e g r a v e l pack sand o r proppants s h o u l d be g e n e r a l l y f i v e t o s i x t i m e s t h e mean s i z e o f t h e f o r m a t i o n sand expressed by t h e 50 % d i a m e t e r o f t h e g r a i n s i z e c u r v e (SCHWARTZ 1969, SAUCIER 1974, SPARLIN 1974; GURLEY, COPELAND & HENDRICK 1977; PENBERTHY & COPE 1979) w h i c h i s c o n s i d e r e d t o be t h e optimum f o r complete p r e v e n t i o n o r stoppage o f movement o f f o r m a t i o n sand i n t o o r t h r o u g h t h e g r a v e l w i t h l i t t l e o r no e f f e c t on i n i t i a l g r a v e l p e r m e a b i l i t y . The e a r l i e r g u i d e l i n e s o f s e l e c t i n g a mean s i z e o f t h e g r a v e l pack m a t e r i a l o f e i g h t t o t e n t i m e s t h e 10 % coarse p e r c e n t i l e o f t h e c u m u l a t i v e s i e v e a n a l y s i s c u r v e o f t h e f o r m a t i o n sand (COBERLY & WAGNER 1938, HILL 1941) o r f o u r t o t h i r t e e n t i m e s t h a t g r a i n s i z e percentage o f t h e g r a n u l o m e t r i c a l d i s t r i b u t i o n (SCHWARTZ 1969; WILLIAMS, ELLIOTT & WEAVER 1972) o r o t h e r r u l e s o f thumb (WINTERBURN 1947, TAUSCH & CORLEY 1958) have t u r n e d o u t t o r e s u l t i n t o o many f a i l u r e s (SAUCIER 1974). Gravel pack m a t e r i a l / f o r m a t i o n sand r a t i o s o f f i v e t o s i x have p a r t i c u l a r l y been proven t o be s u c c e s s f u l i n w e l l s w i t h severe f l o w d i s t u r b a n c e where b r i d g i n g i s u n s a t i s f a c t o r y under t h e same c o n d i t i o n s where s t i l l good r e s u l t s a r e achieved f o r u n i f o r m u n d i s t u r b e d f l o w . O v e r s i z e d g r a v e l can be invaded and plugged by f o r m a t i o n f i n e s d u r i n g product i o n due t o i n s u f f i c i e n t b r i d g i n g p r o p e r t i e s (SPARLIN & COPELAND 1972, SAUCIER 1974). P r o p e r l y s i z e d g r a v e l s h o u l d be p l a c e d i n such a manner t h a t p e r f o r a t i o n t u n n e l s a r e g r a v e l - f i l l e d and even a g r a v e l node i s c r e a t e d a t t h e d i s t a l ends o f t h e p e r f o r a t i o n tubes (BELL 1982), because o t h e r w i s e pay zone sand may f i l l
724 t h e p e r f o r a t i o n s and cause severe p r o d u c t i v i t y r e s t r i c t i o n (SUMAN, ELLIS & SNYDER 1 9 8 3 ) . Aspects o f g r a v e l s i z e s e l e c t i o n and g r a v e l - s a n d s i z e r a t i o s a r e a l s o d i s c u s s e d by COBERLY & WAGNER (1938), TAUSCH & CORLEY (1959) and STEIN (1983), and COBERLY (1937) o u t l i n e s q u e s t i o n s o f screen w i r e and l i n e r s l o t openings. OYENEYIN (1987 b ) p r e s e n t s f o r m a t i o n sand s i z e p r e d i c t i o n f r o m w e l l 1og c o r r e l a t i o n s .
5.2.3.3. Packing gravel/formation sand size ratios As r e s e r v o i r f l o w c o n d i t i o n s may change d u r i n g hydrocarbon w i t h d r a w a l t i m e and a l t e r e d f l u i d f o r c e s cause pay i n s t a b i l i t i e s t o occur, g r a v e l b r i d g e s t e n d t o breakdown and more sand i s produced u n t i l new b r i d g e s f o r m under new c o n d i t i o n s o f s t a b i l i t y (SAUCIER 1974). T h i s i s t h e reason why i n c o n t r a s t t o e a r l i e r g r a v e l pack d e s i g n aiming on b r i d g i n g and r e a s o n a b l y s l o w i n g down o f format i o n sand m i g r a t i o n w i t h l a r g e r g r a v e l - s a n d mean diameter r a t i o s i n t h e range o f 6 - 14, tendency has been d u r i n g t h e l a s t decade more and more towards small e r g r a v e l - s a n d d i a m e t e r r a t i o s o f a b t . 5 - 6 i n o r d e r t o a b s o l u t e l y s t o p format i o n sand moving and f l o w i n g i n t o o r through t h e g r a v e l (SPARLIN 1969; FLANIGAN 1979; SUMAN, ELLIS & SNYDER 1983). O t h e r p o i n t s which have t o be c o n s i d e r e d a r e g r a v e l p e r m e a b i l i t y and p a c k i n g d e n s i t y , f o r m a t i o n p e r m e a b i l i t y and granulomet r y , and impact o f f o r m a t i o n and g r a v e l f i n e s .
5.2.3.3.1. Absolute stoppage o f moving formation sand A l t h o u g h a b s o l u t e stoppage o f sand i n t r u s i o n may be d e s i r a b l e f r o m t h e sand c o n t r o l p o i n t o f view (SAUCIER 1974), an overdesign o f sand p a r t i c l e r e s t r a i n t b e a r s t h e danger o f p o t e n t i a l r e d u c t i o n in w e l l p r o d u c t i v i t y (PEOEN, RUSSELL & OYENEYIN 1985), w i t h t h u s a sound compromise b e i n g necessary between maximum r e s t r i c t i o n o f sand movement and h i g h e s t p o s s i b l e hydrocarbon o f f t a k e r a t e . I n c o n t r a s t t o g r a v e l - p a c k sand s i z i n g , wrapped-screen w i r e s p a c i n g o r l i n e r s l o t w i d t h has o n l y a minimal, g e n e r a l l y almost n e g l i g i b l e d e t r i m e n t a l e f f e c t on w e l l p r o d u c t i v i t y and t h u s screen and l i n e r opening s i z i n g e x c l u s i v e l y aims on a b s o l u t e containment o f g r a v e l and f o r m a t i o n sand ( c f . s e c t i o n 5 . 2 . 3 . 5 . ) . A l t h o u g h t h e i n i t i a l p e r m e a b i l i t y o f t h e g r a v e l pack i s i n case o f a mater i a l / f o r m a t i o n sand s i z e r a t i o o f f i v e t o s i x c o n s i d e r a b l y l e s s t h a n f o r a r a t i o o f a b t . ten, t h i s e f f e c t i s s i g n i f i c a n t l y compensated by much l e s s subseq u e n t g r a v e l pack impairment by p o r e b r i d g i n g and p l u g g i n g i n case o f s m a l l e r r a t i o s w i t h r e s p e c t t o l a r g e r r e l a t i o n s h i p s (SAUCIER 1974). I f g r a v e l pack g r a i n s i z e i s t o o l a r g e , f o r m a t i o n sand p a r t i c l e b r i d g i n g f a i l s and e n t e r i n g r e s e r v o i r sand g r a i n s may even reduce e f f e c t i v e pack p e r m e a b i l i t y t o a v a l u e l e s s than t h e i n i t i a l one o f a f i n e r g r a v e l pack t h a t undergoes l e s s impairment. The l o w e s t g r a v e l pack d e t e r i o r a t i o n i s achieved when g r a v e l s i z e and f o r m a t i o n sand s i z e a r e e q u i v a l e n t , b u t f o r t h i s c o n s t e l l a t i o n , a l s o w e l l p r o d u c t i v i t y i s n e g l i g i b l e and approaches i t s minimum. F o r any g r a v e l s i z e , t h e p e r m e a b i l i t y o f t h e r e s e r v o i r sand/gravel pack i n t e r f a c e i s s u b s t a n t i a l l y t h e same as t h a t o f t h e f o r m a t i o n i t s e l f o r l a r g e r (COBERLY & WAGNER 1938).
5.2.3.3.2. Gravel permeability and packing density S m a l l e r g r a v e l - s a n d mean d i a m e t e r r a t i o s below t o t h e low a b s o l u t e q r a v e l D e r m e a b i l i t v and may on l y d i s t u r b e d r e s e r i o i r f i o w c o n d i t i o n s (SAUtIER c u r r e n t s i t u a t i o n s (ALLEN & ROBERTS 1982), because c o n t r o l o n l y a t t h e expense o f hydrocarbon produc p e c i a l l y i n cased-hole c o m p l e t i o n s where s m a l l e r , packages cause an i n c r e a s e d p r e s s u r e d r o p across r e d as f o l l o w s on p o s s i b l e range and i d e a l l e v e l sand s i z e r a t i o .
5 u s u a l l y c r e a t e problems due
y be d e s i r a b l e under s e r i o u s -
1974) and under h i g h - v e l o c i t y t h e y u s u a l l y p r o v i d e sand i v i t y (HIMES & R U I Z 1988) esl e s s permeable p e r f o r a t i o n them. Some comments a r e o f f e o f packing gravel/formation
725
5.2.3.3.2.1. Possible range o f pack gravel/pay sand size ratio F o r v e r y h i g h o i l o f f t a k e r a t e s o r i n case o f expected w a t e r p r o d u c t i o n , even a g r a v e l - s a n d r a t i o o f 2 may be h e l p f u l (STEIN 1983). S u b s t a n t i a l l y l a r g e r g r a v e l d i a m e t e r f o r c o n t r o l l i n g t h e same s i z e o f f o r m a t i o n sand can be chosen i f hexagonal i n s t e a d o f c u b i c g r a v e l g r a i n p a c k i n g can be achieved by l i n e r v i b r a t i o n (SOLUM 1984; c f . s e c t i o n 5.8.4.). W i t h 8/12 mesh g r a v e l i n t h e hexagon a l pack, v i b r a t i n g w i l l g i v e t h e same i n t e r s t i t i a l tube s i z e as 20/30 mesh g r a v e l i n t h e c o n v e n t i o n a l c u b i c arrangement t h r o u g h f l o w - t y p e placement, and a l s o t h e w i d t h o f l i n e r s l o t s and wrapped-screen w i r e s can be a c c o r d i n g l y i n c r e a s e d . On t h e o t h e r hand, GULATI & MALY (1974) p r e s e n t evidence a r g u i n g f o r s e l e c t i o n o f s m a l l e r g r a i n s i z e s i n grave7 p a c k i n g . S m a l l e r g r a v e l - s a n d s i z e r a t i o s a r e a l s o r e q u i r e d when u s i n g l o o s e i n s t e a d o f t i g h t p a c k i n g f a b r i c s (ALLEN & ROBERTS 1982). I n some f o r m a t i o n s o f p r e f e r a b l y c o a r s e r g r a i n s i z e , g r a v e l - s a n d s i z e r a t i o s o f 6 - 14 a l l o w sand i n v a s i o n i n t o t h e pack and r a t i o s o v e r 14 even p e r m i t unres t r a i n e d sand p r o d u c t i o n through t h e pack (FLANIGAN 1979, PENBERTHY & COPE 1979). I n such r e s e r v o i r s , complete stoppage o f sand movement can be reached w i t h g r a v e l - s a n d s i z e r a t i o s o f 3 - 6. The more u n i f o r m t h e g r a v e l m a t e r i a l , t h e h i g h e r t h e p e r m e a b i l i t y o f t h e g r a v e l pack, and a v o i d i n g excess f i n e s makes twice screening o f the gravel f e a s i b l e .
5.2.3.3.2.2. Ideal level o f pack gravel/pay sand size ratio F o r most o f t h e r e s e r v o i r s r e q u i r i n g sand c o n t r o l by g r a v e l packing, g r a v e l grain s i z e s e l e c t i o n can be summarized t h a t a r a t i o o f g r a v e l median d i a m e t e r / f o r m a t i o n sand median d i a m e t e r between 5 and 6 i s t h e i d e a l range p r o v i d i n g a b s o l u t e stoppage o f sand movement and u n r e s t r i c t e d i n i t i a l g r a v e l p e r m e a b i l i t y (ECONOMIDES 1986). R a t i o s below 5 g i v e r i s e t o l i m i t e d hydrocarbon p r o d u c t i o n , because p r o p e r b r i d g i n g a t t h e g r a v e l i n t e r f a c e i s n o t p o s s i b l e , b u t y e t no i n v a s i o n of t h e g r a v e l pack by r e s e r v o i r sand occurs. R a t i o s between 6 and 11 provoke i n t r u s i o n o f f o r m a t i o n sand i n t o t h e g r a v e l pack which d r a s t i c a l l y reduces g r a v e l p e r m e a b i l i t y , b u t s t i l l p r o v i d e e f f e c t i v e sand c o n t r o l once t h e g r a v e l pack i s c o m p l e t e l y plugged w i t h f i n e p a r t i c l e s and f u r t h e r m o t i o n o u t o f t h e pay zone i s i n h i b i t e d . A t r a t i o s i n excess o f 11, t h e pores i n t h e g r a v e l package become so l a r g e t h a t r e s e r v o i r sand i s p a r t i a l l y m i g r a t i n g through t h e g r a v e l i n t o t h e w e l l bore, t h e r e b y i n c r e a s i n g g r a v e l p e r m e a b i l i t y again, b u t a l r e a d y p r o g r e s s i v e l y d e t e r i o r a t i n g hydrocarbon e x p l o i t a t i o n by h a v i n g o n l y one e f f e c t o f s l o w i n g down o f sand p r o d u c t i o n . A t a r a t i o o f a b t . 15, o r i g i n a l g r a v e l pack p e r m e a b i l i t y i s reached a g a i n which r e f l e c t s t h a t a t r a t i o s above 15, f o r m a t i o n sand can invade t h e b o r e h o l e by passage t h r o u g h t h e g r a v e l pack w i t h o u t any r e s t r i c t i o n , w i t h i n t h i s case t h e g r a v e l pack b e i n g c o m p l e t e l y u s e l e s s .
5.2.3.3.3. Formation permeability and granulometry Gravel g r a i n s i z e s e l e c t i o n depends on b o t h f o r m a t i o n p e r m e a b i l i t y and granulometry. While coarser gravel g r a i n sizes are required f o r high-permeability r e s e r v o i r s b e i n g c l e a n medium- t o c o a r s e - g r a i n e d sands, s m a l l e r g r a v e l g r a i n s i zes a r e needed f o r t o w - p e r m e a b i l i t y pay zones b e i n g f i n e - g r a i n e d sands c o n t a i n i n g d i s p e r s e d c l a y p a r t i c l e s . As f i n e s m i g r a t i o n i s t h e w o r s t impairment i n l o w - p e r m e a b i l i t y sands, s m a l l e r g r a v e l g r a i n s i z e s have t o be s e l e c t e d f o r b r i d g i n g and h o l d i n g back o f c l a y m i n e r a l s , w i t h b e t t e r r e s u l t s even b e i n g achieved i f a c c e p t i n g l o w e r g r a v e l pack p e r m e a b i l i t y w i t h r e s p e c t t o t h a t o b t a i n e d f r o m l a r g e r g r a v e l g r a i n s i z e s . I n many cases, 40/60 o r 50/70 mesh g r a v e l pack mater i a l i s a p p l i e d f o r c o n t r o l l i n g f i n e - g r a i n e d l o w - p e r m e a b i l i t y sands. A l t h o u g h
726 such d i r t y r e s e r v o i r s can be e x p l o i t e d w i t h o u t s t a b i l i z a t i o n by r e d u c i n g product i o n r a t e s , enough i n l e t area i s necessary t o achieve r e a s o n a b l e o f f t a k e r a t e s . C r e a t i o n o f s u f f i c i e n t i n f l o w a r e a and d i m i n u t i o n o f f l o w v e l o c i t y i n h i g h - p r e s sure o i l w e l l s may r e q u i r e i n s t a l l a t i o n o f l a r g e r amounts o f p e r f o r a t i o n s o r open-hole c o m p l e t i o n , and impairment can be c o u n t e r a c t e d by i n c r e a s i n g i n l e t area and r e d u c i n g v e l o c i t y . As most o f t h e w e l l s w i t h g r a v e l p a c k i n g p o t e n t i a l a r e nowadays completed in low- t o m o d e r a t e - p e r m e a b i l i t y gas r e s e r v o i r s , a b t . 80 % o f t h e j o b s a r e c a r r i e d o u t u s i n g 40/60 mesh o r even s m a l l e r g r a v e l p a c k i n g m a t e r i a l i n c o n t r a s t t o t h e s i t u a t i o n a b t . 10 y e a r s ago when p r e f e r e n t i a l l y 20/40 mesh g r a v e l has been app l i e d f o r h i g h - p e r m e a b i l i t y o i l r e s e r v o i r sand c o n t r o l . 40/60 mesh g r a v e l has t u r n e d o u t t o p e r f o r m much b e t t e r i n p r e v e n t i n g f o r m a t i o n f i n e s i n v a s i o n t h a n 20/40 mesh g r a v e l . L o w - p e r m e a b i l i t y sands r e q u i r i n g g r a v e l p a c k i n g a r e p a r t i c u l a r l y widespread i n t h e G u l f o f Mexico.
5.2.3.3.4. Impact of formation and gravel fines Gravel pack performance i s even more s u s c e p t i b l e t o f o r m a t i o n and g r a v e l f i nes than h y d r a u l i c f r a c t u r i n g b e h a v i o u r . Contents o f c l a y m i n e r a l s and o t h e r f i n e m a t e r i a l s above 1 % in t h e sand t o be c o n t r o l l e d c r e a t e problems u n l e s s c l a y i n h i b i t o r s o r s t a b i l i z e r s a r e i n c l u d e d , and 2 - 4 % o f c l a y m i n e r a l s i n a sand can screw up a g r a v e l pack j o b i f f r e s h w a t e r systems a r e i n t r o d u c e d . T h i s s e n s i b i l i t y o f g r a v e l p a c k i n g a g a i n s t even s l i g h t i m p u r i f i c a t i o n s o f f i n e s a l s o i m p l i e s t h a t g r a v e l pack sand o r proppants have t o be much n a r r o w e r screened than t h e m a t e r i a l used i n h y d r a u l i c f r a c t u r i n g . A l t h o u g h t h e p r e p a r a t i o n o f spec i a l l y s i e v e d g r a v e l pack sand i s more c o s t l y than t h e p r o d u c t i o n o f f r a c t u r i n g sand, much more money can be wasted once t h e i n f e r i o r p r o d u c t i m p a i r s g r a v e l pack p e r m e a b i l i t y and hence o i l p r o d u c t i o n . T h e r e f o r e even m u l t i p l e s c r e e n i n g o f g r a v e l t o e x t r a c t f i n e s below a t o l e r a b l e minimum r e s i d u e pays o f f v e r y we1 1 . The aforementioned g u i d e l i n e s f o r g r a v e l g r a i n s i z e s e l e c t i o n as a f u n c t i o n o f r e s e r v o i r g r a n u l o m e t r y a l l r e f e r e x c l u s i v e l y t o sandy pay zones. I n c h a l k formations, these r u l e s a r e n o t a p p l i c a b l e , and a n o t h e r approach has t o be und e r t a k e n . D e v i a t i o n f r o m c o n v e n t i o n a l g r a v e l pack d e s i g n c r i t e r i a i n t h e c h a l k i s caused by t h e e x t r e m e l y f i n e g r a i n s i z e o f t h i s r e s e r v o i r r o c k t y p e and i t s unique f l o w b e h a v i o u r i n case o f l a c k i n g c o n s o l i d a t i o n and occurrence o f i n t e r n a l o v e r p r e s s u r e (ATTARD, MATHES & MOWER 1988; c f . s e c t i o n 4 . 5 . 4 . 3 . ) . Instead o f a p r o h i b i t i v e l y s m a l l g r a v e l g r a i n s i z e which would have been r e q u i r e d t o ensure i n d i v i d u a l f o r m a t i o n p a r t i c l e stoppage a c c o r d i n g t o t h e s t a n d a r d g u i d e l i n e s , 20/40 g r a i n s i z e has t u r n e d o u t t o r e t a i n s u b s t a n t i a l f l o w c a p a c i t y o f t h e o i l and t o h o l d back t h e c h a l k m a t r i x which i s c h a r a c t e r i z e d by mass f l o w b e h a v i o u r once becoming u n s t a b l e and m o b i l e .
5.2.3.4. Width o f gravel mantle A g r a v e l pack w i d t h o f 5 g r a i n d i a m e t e r s w i l l a l r e a d y c r e a t e an e f f e c t i v e b r i d g e (SAGE & LACEY 1942), b u t t h e accepted minimum g r a v e l pack t h i c k n e s s f o r o i l - f i e l d a p p l i c a t i o n s i s 3 i n . (SCHWARTZ 1969) a c c o r d i n g t o t h e problem o f p l a c i n g g r a v e l and f l u c t u a t i n g f l o w c o n d i t i o n s . T h e r e f o r e open h o l e s have g e n e r a l l y t o be underreamed t o p r o v i d e 3 i n . o r even more between screen and f o r m a t i o n a t t h e b o r e h o l e w a l l (ALLEN & ROBERTS 1 9 8 2 ) . W a t e r - w e l l c o m p l e t i o n has shown t h a t a pack t h i c k e r than 8 in. cannot be p r o p e r l y developed. The i d e a l g r a v e l pack t h i c k n e s s f o r p r o t e c t i n g a h i g h - r a t e secondary-recoveo i l - p r o d u c t i o n w e l l i s 6 i n . , b u t t o o l c a p a b i l i t i e s and c a s i n g o r screen schemes o f t e n l i m i t g r a v e l b e l t t h i c k n e s s t o 5 in. T h i c k e r g r a v e l packs commonl y p e r m i t h i g h e r p r o d u c t i o n r a t e s p e r u n i t l e n g t h o f r e s e r v o i r i n t e r v a l . I n cased holes, screen d i a m e t e r should be as l c i r g e as p o s s i b l e , b u t adequate room ry
727 f o r g r a v e l p a c k i n g s h o u l d be l e f t i n t h e annulus ( a s p e c t s o f g r a v e l m a n t l e i g i d t h i n open and cases h o l e s a r e a l s o d i s c u s s e d i n s e c t i o n s 5.10.1.2. and 5.10.2.1.). MURER (1981) recommends a g r a v e l pack w i d t h o f a t l e a s t 100 - 150 mm t o c r e a t e a s u f f i c i e n t l y s t a b l e sand annulus. LEWIS (1985) argues t h a t a t l e a s t i n some cases, g r a v e l pack t h i c k n e s s p l a y s key f u n c t i o n i n sand c o n t r o l .
5.2.3.5. Impact o f gravel pack hardware I n a d d i t i o n t o g r a v e l s i z e s e l e c t i o n , c o n s i d e r a b l e improvement o f t h e g r a v e l pack performance can be made by v a r i o u s o t h e r t e c h n i c a l f e a t u r e s (GURLEY, COPELAND & HENDRICK 1977). The g r e a t e s t p r e s s u r e drop i n a g r a v e l - p a c k e d w e l l occ u r s i n t h e p e r f o r a t i o n t u n n e l (WILLIAMS, ELLIOTT & WEAVER 1972, SAUCIER 1974), w i t h minimum p r e s s u r e drawdown b e i n g achieved by i n c r e a s i n g t h e i n l e t area by i n c r e a s i n g d e n s i t y o r d i a m e t e r o f t h e p e r f o r a t i o n s , o r even by e l i m i n a t i n g t h e p e r f o r a t i o n s w i t h open-hole c o m p l e t i o n (SUMAN 1972). Some aspects o f s l o t t e d lin e r s v s . wire-wrapped screens and p a c k i n g hardware i n f l u e n c e on g r a v e l q u a l i t y a r e summarized as f o l l o w s .
5.2.3.5.1. Slotted liners vs. wire-wrapped screens While c o a r s e r g r a v e l can be s u f f i c i e n t l y o p e r a t e d w i t h a s l o t t e d l i n e r , wire-wrapped screens a r e much more s u i t a b l e f o r f i n e r g r a v e l , because t h e w i r e spacing can be m e c h a n i c a l l y much b e t t e r a d j u s t e d t o be s m a l l e r t h a n t h e diamet e r o f t h e s m a l l e s t g r a v e l than c o u l d be made adequate s l o t spacing i n a s l o t t e d l i n e r , and i n a d d i t i o n t o t h a t wire-wrapped screens have h i g h e r f l o w c a p a c i t i e s than s l o t t e d l i n e r s w i t h t h e same s l o t w i d t h as t h e spacing o f t h e w i r e . O t h e r b e n e f i t s o f wire-wrapped screens w i t h r e s p e c t t o s l o t t e d l i n e r s a r e c l o g r e s i s t a n c e and d u r a b i l i t y a g a i n s t c o r r o s i o n , whereas t h e o n l y c o m p e t i t i v e advantage o f s l o t t e d l i n e r s o v e r wire-wrapped screens i s c o s t and t h e r e f o r e s l o t t e d l i n e r s may be f e a s i b l e i n b o r e h o l e s where v e r y l o n g i n t e r v a l s have t o be g r a v e l packed (HALLIBURTON 1988). F o l l o w i n g d i s c u s s i o n o f some g e n e r a l aspects, comments a r e o f f e r e d on s l o t and w i r e spacing s e l e c t i o n as w e l l as on mechanical l i m i t a t i o n o f s l o t and l i n e r w i d t h .
5.2.3.5.1.1. General aspects I n e a r l i e r years, many sand p r o d u c t i o n problems were s i m p l y handled b y i n d i c i o u s s e l e c t i o n o f opening w i d t h i n p e r f o r a t e d l i n e r s (COBERIY 1937, COBERLY & WAGNER 1938). I n many f i e l d s , however, t h i s method o f r e d u c t i o n o r e l i m i n a t i o n o f sand t r o u b l e does n o t l e a d t o s a t i s f a c t o r y r e s u l t s because o f t o o f i n e g r a i n s i z e o f t h e f o r m a t i o n sand, poor compaction o f t h e r e s e r v o i r , low bottom-hole p r e s s u r e and o t h e r adverse c o n d i t i o n s , and g r a v e l pack i n s t a l l a t i o n i n combinat i o n w i t h s l o t t e d l i n e r s o r wire-wrapped screens has t u r n e d o u t t o be t h e o p t i mum s o l u t i o n .
S l o t t e d l i n e r s a r e p a r t i c u l a r l y s u c c e s s f u l i n r e s e r v o i r s where g r a i n s i z e d i s t r i b u t i o n and framework cohesion i s such t h a t f o r m a t i o n sand f i l t e r s o u t and due t o b r i d g i n g tendency generates i t s own pack. Wire-wrapped screens i n c l u d e wire-wrapped f l u s h w i t h t h e p i p e f a c e w i t h no spacers between w i r e and p i p e , and wire-wrapped o v e r - r i b s h o l d i n g t h e w i r e wrapping away f r o m t h e p i p e . The l a t t e r s t a n c l i f f t y p e o f wire-wrapped screens has a g r e a t l y i n c r e a s e d area a v a i l a b l e f o r f l o w i n t o t h e w e l l b o r e which reduces f l o w v e l o c i t y i n t h e b o r e h o l e v i cinity.
5.2.3.5.1.2. Slot and wire spacing selection L i n e r s l o t s and w i r e spacings must have a w i d t h equal t o o r l e s s t h a n t h e d i a m e t e r o f t h e s m a l l e s t g r a i n s o f t h e g r a v e l pack (COBERLY & WAGNER 1938) o r
728 f o r m a t i o n sand depending on whether t h e screen i s r u n i n c o m b i n a t i o n w i t h g r a v e l o r n o t (ALLEN & ROBERTS 1982). While a s l o t w i t h a w i d t h t w i c e t h e g r a i n s i z e a t t h e t e n p e r c e n t i l e o f t h e g r a v e l pack would h o l d t h e g r a v e l i n t h e acc e p t e d sense, t h e small q u a n t i t y o f g r a v e l which i s produced d u r i n g t h e p e r i o d when t h e s l o t b r i d g e i s b e i n g b u i l t up m i g h t be enough t o cause t h e g r a v e l pack t o s e t t l e , t h e r e b y a l l o w i n g p r o d u c t i o n o f s u f f i c i e n t f o r m a t i o n sand t o b r a n d the i n s t a l l a t i o n a f a i l u r e . S l o t w i d t h s h o u l d be as narrow as p o s s i b l e t o r e t a i n sand g r a i n s , b u t on t h e o t h e r hand as l a r g e as p o s s i b l e f o r n o t r e s t r i c t i n g f l o w o f f l u i d s and i n t e r s t i t i a l f i n e s (ALLEN & ROBERTS 1982). I n o r d e r t o go on t h e a b s o l u t e l y s a f e s i d e , w i r e spacing o r s l o t w i d t h o f h a l f o r two t h i r d s t h e d i a m e t e r o f t h e s m a l l e s t g r a v e l pack g r a i n i s d e s i r a b l e (HALLIBURTON 1988). A b s o l u t e stoppage o f format i o n sand and optimum g r a v e l r e t e n t i o n r e q u i r e a maximum number o f l i n e r s l o t s w i t h a w i d t h l e s s t h a n t h e d i a m e t e r o f t h e s m a l l e s t g r a v e l used ( N I N I & OWEN 1983). U n d e r s i z i n g has proven t o reduce t h e tendency o f n e a r - s i z e g r a v e l t o p l u g s l o t s d u r i n g g r a v e l placement (SHRYOCK & MILLHONE 1979). LEDLOW & SPARLIN (1980) and SOLUM & RAMEZAMI (1987) r e p o r t downhole p r o t e c t i o n o f sand c o n t r o l screens. Good g r a v e l pack i n s e r t i o n t o o l s a r e t h e base f o r s a t i s f a c t o r y g r a v e l pack i n s t a l l a t i o n and a penny saved f o r b e t t e r hardware c o u l d r e s u l t i n l o s s o f thousands o f d o l l a r s once g r a v e l pack performance i s n o t a p p r o p r i a t e due t o poor placement.
5.2.3.5.1.3. Mechanical limitation of slot and wire width As c u t t i n g o f l i n e r s l o t and wrapping o f w i r e - s c r e e n w i d t h i s m e c h a n i c a l l y limited, i t may be i n some v e r y f i n e f o r m a t i o n sands d i f f i c u l t t o keep t h e r u l e t h a t t h e maximum opening s h o u l d n o t be l a r g e r than t h e s m a l l e s t g r a v e l used (FLANIGAN 1979). A l t h o u g h on average, screen opening s h o u l d be a b t . h a l f o r two t h i r d s t h e g r a v e l s i z e , s m a l l s l o t w i d t h s a r e p a r t i c u l a r problems i f screen o r l i n e r has t o be c o a t e d w i t h polymers i n o r d e r t o p r e v e n t c o r r o s i o n . Screen o r l i n e r l e n g t h s h o u l d be a b t . 10 f t . l o n g e r than t h e p e r f o r a t e d i n t e r v a l and s h o u l d e x t e n d above and below t h e p e r f o r a t i o n s where i t passes i n t o b l a n k l i n e r (HALLIBURTON 1988). Screen o r l i n e r d i a m e t e r s h o u l d be such t h a t a c l e a r a n c e o f a t l e a s t 3 / 4 i n . c o m p l e t e l y around t h e g r a v e l pack hardware i s p r o v i d e d when pos i t i o n e d i n t h e c a s i n g i n o r d e r t o a l l o w room f o r g r a v e l t o f l o w f r e e l y , and p r o v i d e e x t r a g r a v e l r e s e r v e f o r a f t e r - p a c k s e t t l i n g and adequate space f o r f i s h i n g t o o l s ( N I N I & OWEN 1983). I n c r e a s i n g screen o r l i n e r d i a m e t e r i s more i m p o r t a n t i n h i g h e r - r a t e w e l l s e s p e c i a l l y i n case o f gas because o f t u r b u l e n c e , and l e s s i m p o r t a n t i n l o w - r a t e w e l l s (LEOLOW, SAUER & T I L L 1985).
5.2.3.5.2. Packing hardware influence on gravel quality Due t o t h e h i g h c o s t o f a c c u r a t e l y screened g r a v e l and t h e importance o f n o t b r e a k i n g o r c r u s h i n g i t , t r e a t m e n t and placement o f g r a v e l w i t h s u r f a c e e q u i p ment i s a p r i m a r y f a c t o r i n p r o p e r sand c o n t r o l (SOLUM 1 9 8 6 ) . S a t i s f a c t o r y r e s u l t s o f g r a v e l p a c k i n g o p e r a t i o n s s t r o n g l y depend on h i g h g r a v e l q u a l i t y , accur a t e c o n t r o l o f t h e f l u i d - g r a v e l r a t i o , reduced g r a v e l break-up and w e l l - f i l t e r e d c a r r i e r f l u i d s . SOLUM (1986) p r e s e n t s a h i s t o r i c a l o v e r v i e w o f i n t r o d u c t i o n and m o d i f i c a t i o n o f v a r i o u s s u r f a c e and downhole equipment and methods d u r i n g course o f t h e t e c h n o l o g i c a l e v o l u t i o n o f g r a v e l p a c k i n g . Gravel breakup causes l i n e r p l u g g i n g and reduces pack p e r m e a b i l i t y . Some aspects o f p o t e n t i a l g r a v e l breakup p o i n t s and improvement as w e l l as f l u i d v i s c o s i t y and v e l o c i t y a r e o u t l i n e d as f o l l o w s .
5.2.3.5.2.1. Potential gravel breakup points and improvement The most p o t e n t i a l g r a v e l breakup p o i n t s a r e h i g h - a n g l e bends i n t h e m a n i f o l d i n g as w e l l as c e n t r i f u g a l pump, r e s t r i c t e d f l o w p o i n t s , c r o s s o v e r t o o l and ca-
729 s i n g w a l l . C o n s i d e r a b l e m i n i m i z a t i o n o f g r a v e l breakup has been achieved d u r i n g t h e l a s t y e a r s by i n t r o d u c t i o n o f a s p h e r i c a l b a l l v a l v e i n t h e f l u i d pump. A l though g r a v e l breakup can never be e l i m i n a t e d c o m p l e t e l y d u r i n g s u r f a c e and downhole h a n d l i n g , t h e e f f e c t s o f most o f t h e mentioned t r o u b l e p o i n t s can be s u f f i c i e n t l y m i n i m i z e d by q u a l i t y improvement o f equipment and t e c h n o l o g y . Gravel c r u s h i n g and e r o s i o n o f c a s i n g and f o r m a t i o n f a c e can o c c u r i n case o f a p p l i c a t i o n o f s l i m - s i z e c r o s s o v e r p a r t s (FLANIGAN 1979). E n l a r g e d c r o s s o v e r p a r t s f o r u t i l i z a t i o n of v i s c o u s s l u r r i e s d i m i n u i s h s i g n i f i c a n t l y t h e tendency o f g r a v e l c r u s h i n g . Pumping o f g r a v e l - l a d e n w a t e r m i x t u r e s t h r o u g h a narrow c r o s s o v e r p a r t causes f r a c t u r i n g o f some of t h e g r a i n s , w i t h t h e b l a s t i n g e f f e c t o f sand h i t t i n g t h e c a s i n g w a l l s b e i n g a b l e t o produce enough f i n e s t o comp l e t e l y p l u g l i n e r s l o t s (HALLIBURTON 1988). I n c o n t r a s t t o water-based c a r r i e r f l u i d s , g e l l e d polymers r e q u i r e l e s s v e l o c i t y t o move t h e g r a v e l e f f i c i e n t l y through t h e c r o s s o v e r t o o l , t h e c u s h i o n i n g e f f e c t o f t h e g e l reduces g r a i n - t o g r a i n a b r a s i o n which can a l s o generate f i n e s , and v i r t u a l l y no p l u g g i n g o f s l o t s occurs.
5.2.3.5.2.2. Fluid viscosity and velocity T h i n l o w - v i s c o s i t y p a c k i n g f l u i d s have t o be pumped a t h i g h r a t e and v e l o c i i n o r d e r t o be a b l e t o c a r r y g r a v e l (HALLIBURTON 1988). L o w - v i s c o s i t y g r a v e l s l u r r i e s , however, have a v e r y severe s c o u r i n g e f f e c t on work s t r i n g s and cause t h e pack t o c o l l e c t d e b r i s . H i g h f l u i d v e l o c i t y provokes g r a i n c r u s h i n g d u r i n g g r a v e l placement, and l o w - v i s c o s i t y f l u i d s c a r r y i n g g r a v e l a t h i g h pump r a t e s can cause i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand due t o j e t t i n g e f f e c t s . When u s i n g t h i n p a c k i n g f l u i d s , g r a i n c r u s h i n g i n pumps can be a v o i d e d b y downstream sand i n j e c t o r s . ty
5.2.3.6. Use o f screen or liner without gravel pack I n a d d i t i o n t o g r a v e l p a c k i n g where t h e screen h o l d s t h e g r a v e l i n p l a c e t h a t i n t u r n r e t a i n s t h e r e s e r v o i r sand, f o r m a t i o n sand c o n t r o l i s a l s o poss i b l e by s t a b i l i z i n g t h e pay zone w i t h t h e screen a l o n e w i t h o u t g r a v e l o r v i c e v e r s a (ALLEN & ROBERTS 1982). Comments a r e g i v e n as f o l l o w s on g r a v e l - l e s s hardware i n s t a l l a t i o n and l i n e r l e s s g r a v e l placement.
5.2.3.6.1. Gravel-less hardware installat ion The economical b e n e f i t o f g r a v e l - l e s s hardware i n s t a l l a t i o n i s t h a t a r t i f i c i a l l y - i n t r o d u c e d g r a v e l i s a m a j o r c o s t f a c t o r i n sand c o n t r o l o p e r a t i o n s which can be saved p a r t i c u l a r l y i n coarse c l e a n r e s e r v o i r sands where f l o w i n g f o r m a t i o n sands c o n s t r u c t a n a t u r a l g r a v e l pack around t h e wire-wrapped screen o r s l o t t e d l i n e r . Ribbed wire-wrapped screens c o s t two o r t h r e e t i m e s more t h a n s l o t t e d p i p e s , b u t f o r t h e same s l o t w i d t h , t h e f o r m e r have perhaps e i g h t t o t e n times t h e open area. A l l - w e l d screens have t w i c e t h e open area as r i b b e d screens, b u t c o s t n e a r l y t w i c e as much. Wire-wrapped screens a l s o have t h e advantage o f more e r o s i o n - and c o r r o s i o n - r e s i s t a n t m a t e r i a l s . V e r t i c a l l y s l o t t e d p i p e has more a x i a l s t r e n g t h and bending s t r e n g t h than h o r i z o n t a l l y s l o t t e d p i p e . The f o l l o w i n g o u t l i n e i l l u m i n a t e s some aspects o f s l o t / w i r e w i d t h and nat u r a l g r a v e l pack b u i l d i n g as w e l l as t r i p l e - w r a p p e d screens.
5.2.3.6.1.1. Slot/wire width and natural gravel pack building The use o f p r o p e r l y s i z e d wire-wrapped screens o r s l o t t e d l i n e r s w i t h o u t g r a v e l pack can be a l o w - c o s t means o f c o n t r o l l i n g sand f o r low p r o d u c t i o n r a t e s p e r u n i t l e n g t h o f r e s e r v o i r s e c t i o n . S l o t s i z e s h o u l d be more o r l e s s t w i c e t h e 10 p e r c e n t i l e sand s i z e f o r n o n - u n i f o r m sands, whereas s l o t w i d t h p r o b a b l y
730 s h o u l d n o t be g r e a t e r than t h e 10 p e r c e n t i l e s i z e f o r u n i f o r m sands (COBERLY 1937, ALLEN & ROBERTS 1 9 8 2 ) . S e l e c t i o n o f wire-wrapped screen o r s l o t t e d l i n e r once t h e s l o t s i z e has been determined depends on w e l l c o n d i t i o n s . Sawcut s l o t s a r e cheaper, whereas wire-wrapped screens p e r m i t t h e a p p l i c a t i o n o f h a r d e r , more e r o s i o n - r e s i s t a n t m e t a l . Screens s e t i n s i d e c a s i n g u s u a l l y reduce p r o d u c t i v i t y s i n c e f i n e - g r a i n e d f o r m a t i o n sand moving t h r o u g h t h e p e r f o r a t i o n s f i l l s t h e annulus between screen and casing, w i t h t h u s o p t i m i z a t i o n b e i n g reached by u s i n g t h e l a r g e s t p o s s i b l e screen d i a m e t e r . T h i s a p p l i e s p a r t i c u l a r l y f o r open h o l e s i n o r d e r t o p r e v e n t c a v i n g o f s h a l e l a m i n a t i o n s , w i t h under these c o n d i t i o n s underreaming o f t h e w e l l b o r e b e i n g n e i t h e r necessary n o r d e s i r a b l e . Such p r e p a c k i n g o f l i n e r s w i t h b o r e h o l e c o l l a p s e o n t o t h e l i n e r and g e n e r a t i o n o f n a t u r a l g r a v e l p r e p a c k i n g i s a l s o a p p l i c a b l e i n h i g h l y - d e v i a t e d t o h o r i z o n t a l w e l l s as a c o m p l e t i o n t e c h n i q u e i n u n c o n s o l i d a t e d sands which c r e a t e c o n s i d e r a b l e b o r e h o l e i n s t a b i l i t y (AUSTIN, ROSE & SCHUH 1988). I n s t a l l a t i o n o f screen o r l i n e r a l o n e w h i c h r e s u l t s i n b u i l d i n g o f a nat u r a l g r a v e l pack c o n s i s t i n g o f f o r m a t i o n sand around t h e hardware, however, may promote r e s e r v o i r d i s t u r b a n c e s and p e r m e a b i l i t y damage (HALLIBURTON 1988; c f . section 5.9.6.2.).
5.2.3.6.1.2. Triple-wrapped screens Improved hardware f o r g r a v e l - l e s s i n s t a l l a t i o n a r e t r i p l e - w r a p p e d screens (LIKWARTZ 1976). T r i p l e - w r a p p e d screens f o r s e l f - p a c k i n g o f f o r m a t i o n sand around them a r e designed such t h a t t h e m u l t i p l e wraps b r i d g e t h e l a r g e s t r e s e r v o i r sand g r a i n s a t t h e o u t e r screen and t h e s m a l l e s t p a r t i c l e s a t t h e i n n e r most screen. T r i p l e - w r a p screens a r e e s p e c i a l l y s u i t a b l e f o r w e l l s where bottomh o l e o r e s s u r e i s t o o low t o a l l o w s r a v e l Dackinq ( c f . a l s o SPURLOCK & DEMSKI 1972).
5.2.3.6.2. Linerless gravel placement The i n v e r s e s o l u t i o n t o screen o r l i n e r i n s t a l l a t i o n w i t h o u t g r a v e l i s 1 in e r l e s s q r a v e l p a c k i n q (HALLIBURTON 1988) where a b t . 20 - 25 % o f coarse q r a v e l has to-be u n i f o r m l y mixed i n t o t h e main g r a v e l i n o r d e r t o p r o v i d e a b r i d g i n g e f f e c t on t h e p e r f o r a t i o n s , w i t h round p a r t i c l e s b r i d g i n g a h o l e t h r e e t i m e s as g r e a t as t h e d i a m e t e r o f one o f t h e p a r t i c l e s . The main advantage o f l i n e r l e s s g r a v e l p a c k i n g i s complete absence o f any hardware o b s t r u c t i o n i n t h e h o l e . L i n e r l e s s g r a v e l packing, however, s h o u l d never be a t t e m p t e d i n cased h o l e s unl e s s e v e r y e f f o r t has been undertaken t o open up e v e r y p e r f o r a t i o n , because one u n t r e a t e d o r i n s u f f i c i e n t l y packed p e r f o r a t i o n may serve as e n t r a n c e f o r format i o n f i n e s which then p l u g t h e g r a v e l c y l i n d e r and u l t i m a t e l y r e s u l t i n sand c o n t r o l f a i l u r e . I n o r d e r t o keep t h i s drawback p o s s i b i l i t y r e l a t i v e l y low, t o t a l p e r f o r a t i o n number s h o u l d n o t exceed t h a t necessary t o p r o v i d e adequate p r o d u c t i o n . S p h e r i c a l g r a i n s f o r m s t a b l e b r i d g e s on openings l a r g e r t h a n g r a i n d i a meter, and g r a i n a n g u l a r i t y and shape do n o t m a t e r i a l l y a f f e c t t h e opening s i z e f o r a s t a b l e b r i d g e , b u t do i n c r e a s e b r i d g i n g range (COBERLY 1937). I n open holes, l i n e r l e s s g r a v e l placement can p r o f i t f r o m t h e absence o f hardware i n t h e w e l l b o r e by e n a b l i n g t o i n s e r t t h e g r a v e l w i t h h i g h e r o p e r a t i n g p r e s s u r e which p e r m i t s b e t t e r i n f i l l i n g and p l u g g i n g o f a l l t h e c a v i t i e s and i r r e g u l a r i t i e s c r e a t e d a t t h e b o r e h o l e w a l l o f t h e u n s t a b l e sand by underreaming. L i n e r l e s s g r a v e l packs r e p r e s e n t i n f a c t complete i n f i l l i n g o f t h e t o t a l i n n e r b o r e h o l e d i a m e t e r w i t h a dense c y l i n d r i c a l g r a v e l pack which a t t h e same t i m e i s an e x c e l l e n t f i l t e r f o r t h e o i l t h a t f l o w s o u t a t t h e g r a v e l t o p i n c o n t r a s t t o a tube-shaped g r a v e l m a n t l e between screen o r l i n e r and c a s i n g o r b o r e h o l e w a l l generated by c o n v e n t i o n a l g r a v e l p a c k i n g w i t h hardware u t i l i z a t i o n .
731
5.2.3.7. Formation grain size evaluation F o r m a t i o n g r a i n s i z e e v a l u a t i o n has t o be c a r r i e d o u t w i t h maximum accuracy, because i t s p r o p e r d e t e r m i n a t i o n i s i n e v i t a b l y necessary f o r s e l e c t i o n o f t h e s u i t a b l e g r a v e l g r a i n s i z e f o r t h e sand c o n t r o l t r e a t m e n t . F o r m a t i o n g r a i n s i z e assessment i s a l s o t h e g r e a t e s t p o s s i b i l i t y o f erroneous g r a v e l pack d e s i g n and t h u s a v e r y p o t e n t i a l source o f f a i l u r e o f t h e sand c o n t r o l o p e r a t i o n i f n o t l e a d i n g t o a r e s u l t t h a t i s more than c l o s e l y approaching t h e r e a l i t y downhole. Some comments a r e o f f e r e d as f o l l o w s on g r a i n s i z e v a r i a t i o n s i n heterogeneous r e s e r v o i r s and a d j u s t m e n t o f g r a v e l g r a i n s i z e t o f i n e s t pay beds.
5.2.3.7.1. Grain size variations in heterogeneous reservoirs While g r a i n s i z e d e t e r m i n a t i o n may be r a t h e r s i m p l e i n homogeneous r e s e r v o i r s , many pay zones e x h i b i t c o n s i d e r a b l y heterogeneous c o m p o s i t i o n ( c f . sect i o n 4.2.4.1.) w i t h g r a i n s i z e s changing h o r i z o n t a l l y and/or v e r t i c a l l y a t random o r i n an o r d e r e d manner on v a r i o u s s c a l e s , and a d d i t i o n a l c o m p l i c a t i o n s a r e g i v e n by t h e d i f f e r e n t e f f e c t i v i t y o f v a r i o u s b o r e h o l e sampling t e c h n i q u e s . The d e c i s i v e drawback i s t h a t s i g n i f i c a n t g r a i n s i z e changes can t a k e p l a c e on sample s c a l e i n v e r y a n i s o t r o p i c and heterogeneous pay zones such as c r o s s - s t r a t i f i e d small a e o l i a n dune sands ( c f . p l a t e I I / 2 and 6 i n s e c t i o n 3.13.), h o r i z o n t a l - l a m i n a t e d a e o l i a n i n t e r d u n e and p l a y a sediments ( c f . p l a t e VIII/1-5) and f l u v i a l b r a i d e d - r i v e r channel b a r d e p o s i t s ( c f . p l a t e s III/1-2, V/1-8, VII/5-6 and IX/l-8). T y p i c a l successions o f p r o g r e s s i v e l y changing g r a i n s i z e s i n a more o r l e s s o r d e r e d manner a r e c o a r s e n i n g - and f i n i n g - u p w a r d s sequences w h i c h a r e c h a r a c t e r i s t i c f e a t u r e s o f d e l t a i c and f l u v i a l sedimentary complexes, r e s p e c t i v e l y ( c f . a l s o s e c t i o n 4.2.2.6.). M i c r o c y c l i c r e p e t i t i o n s o f s e v e r a l members o f these g r a i n - s i z e t r e n d cyclothems r e p r e s e n t h e t e r o g e n e i t i e s on a l o w e r h i e r a r c h i c a l o r d e r , and i n t e r c a l a t i o n s o f mud drapes i n t o sand bodies ( c f . p l a t e s III/1-2, VII/5-6 and IX/l-8) a r e s m a l l e r - s c a l e a n i s o t r o p i e s o f g r a i n s i z e d i s t r i b u t i o n and depending on e x t e n s i o n and t h i c k n e s s o f hydrocarbon-bearing i n t e r v a l s can q u i t e s e r i o u s l y d i s t u r b c o r r e c t g r a i n s i z e d e t e r m i n a t i o n f o r mechanical sand c o n t r o l design.
5.2.3.7.2. Adjustment of gravel grain size to finest pay beds T h e r e f o r e i n many heterogeneous and a n i s o t r o p i c r e s e r v o i r s , f o r m a t i o n g r a i n s i z e may v a r y c o n s i d e r a b l y o v e r t h e i n t e r v a l t o be g r a v e l packed (SAUCIER 1974). I n o r d e r t o achieve maximum e f f e c t i v i t y , g r a v e l pack d e s i g n and g r a i n s i z e s e l e c t i o n s h o u l d be based on t h e f i n e s t sand-size segment o f t h e product i v e f o r m a t i o n t h a t i s p e r f o r a t e d . T h i s procedure a s c e r t a i n s t h a t even i n t h e w o r s t case, complete stoppage o f a l l g r a i n s i z e s o f moving f o r m a t i o n sand i s achieved. A l t h o u g h t h i s may r e s u l t i n c h o i c e o f a s m a l l e r g r a v e l g r a i n s i z e than would be s u i t a b l e f o r many i n t e r v a l s o f t h e p r o s p e c t i v e s e c t i o n and t h u s hydrocarbon o f f t a k e c a p a c i t y i s s a c r i f i c e d due t o a h i g h e r p r e s s u r e drop through a l e s s permeable g r a v e l pack t u n n e l p l u g and mantle, i t i s b e t t e r t o p r e v e n t d r a s t i c a l g r a v e l p e r m e a b i l i t y d e s t r u c t i o n by i n v a d i n g f o r m a t i o n sand. Pay zone g r a i n s i z e d e t e r m i n a t i o n has t o be c a r r i e d o u t w i t h c o n s i d e r a b l e acc u r a c y and c a u t i o n i n c l u d i n g compensation and c o r r e c t i o n o f d i s t u r b a n c e s and c o n t a m i n a t i o n in o r d e r n o t t o l e a d t o improper g r a v e l pack l a y o u t (MALY & KRUEGER 1971). C a u t i o n has t o be e x e r c i s e d t o r e s t r i c t sand p r o d u c t i o n i n o r d e r n o t t o t r i g g e r d e s t r u c t i o n o f t h e n a t u r a l s t r a t i f i c a t i o n o f t h e r e s e r v o i r (SPARLIN & COPELAND 1972). I f t h e o r i g i n a l bedding f a b r i c c o l l a p s e s due t o e x c e s s i v e sand i n f l u x i n t o t h e borehole, nearby s h a l e lenses and l a y e r s w i l l mix w i t h t h e u n c o n s o l i d a t e d sand when s h i f t i n g towards t h e w e l l b o r e . Such m i x t u r e s can become severe p e r m e a b i l i t y b a r r i e r s i f stopped and h e l d i n p l a c e by t h e g r a v e l pack. E s p e c i a l l y when f o r m a t i o n g r a i n s i z e v a r i e s c o n s i d e r a b l y i n t h e i n v e s t i g a -
732 t e d i n t e r v a l i n h o r i z o n t a l and v e r t i c a l d i r e c t i o n , a t t e n t i o n h a s t o be p a i d i n g r a v e l - s a n d s i z e r a t i o d e s i g n t o t h e s m a l l e r r e s e r v o i r sand g r a i n s and t h e f i n e r p a y zone i n t e r b e d s , p a r t i c u l a r l y i n c a s e o f h i g h e r f l o w v e l o c i t y , more nonu n i f o r m sand, f l u c t u a t i n g f l o w r a t e and h i g h g a s - o i l r a t i o (ALLEN & ROBERTS 1982).
5.2.3.8. Grain size intermixing o f gravel and formation sand C o n c e r n i n g g r a i n s i z e m i x i n g , s i m i l a r l y as a p p l y i n g f o r c o n d u c t i v i t y d e t e r i o r a t i o n i n h y d r a u l i c proppant f r a c t u r i n g ( c f . s e c t i o n 1.4.11.2.), intermixing o f r e l a t i v e l y s m a l l amounts o f f o r m a t i o n sand ( a b t . 10 - 25 % ) b e i n g c o n s i d e r a b l y f i n e r t h a n t h e s a n d - c o n t r o l medium l o w e r s g r a v e l m i x t u r e p e r m e a b i l i t y t o n e a r l y t h a t o f t h e c o n t a m i n a t i n g p a y zone sand a l o n e o r e v e n l e s s (MORROW, HUPPLER & SIMMONS 1969; S P A R L I N 1 9 7 4 ) . T h e r e f o r e i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand has t o be m i n i m i z e d d u r i n g p e r f o r m a n c e o f t h e t r e a t m e n t , w h i c h c a n be a c h i e v e d b y t a c k i f y i n g a g e n t s i n t h e f l u i d t h a t cause t h e g r a v e l t o f l o w as a p l u g o r a c o n s i s t e n t s l u r r y , o r b y h i g h g r a v e l c o n c e n t r a t i o n and h i g h f l u i d v i s c o s i t y .
5.2.3.8.1. High gravel concentration and high fluid viscosity S i g n i f i c a n t means o f r e d u c i n g i n t e r m i x i n g o f g r a v e l pack m a t e r i a l and r e s e r v o i r sand a r e s u s p e n s i o n o f g r a v e l a t h i g h c o n c e n t r a t i o n s ( a b t . 15 l b s o f g r a v e l added t o 1 g a l o f c a r r i e r f l u i d o r even more) i n h i g h - v i s c o s i t y f l u i d s and pumping t h e s l u r r y t h r o u g h p e r f o r a t i o n s a t v e r y s l o w r a t e s ( S P A R L I N & COPELAND 1972; SUMAN, E L L I S & SNYDER 1 9 8 3 ) , w h i c h a l s o r e q u i r e s l e s s c a r r i e r f l u i d and thus c u t s r i g time, minimizes f o r m a t i o n plugging, reduces screen e r o s i o n d u r i n g placement, and d e c r e a s e s g r a v e l c r u s h i n g a t t h e c r o s s o v e r p a r t and p l u g g i n g o f s l o t t e d l i n e r o r w i r e - w r a p p e d s c r e e n (UNDERDOWN, DAS & NGUYEN 1 9 8 4 ) . The d a n g e r o f m i x i n g o f r e s e r v o i r sand and g r a v e l pack m a t e r i a l i s p a r t i c u l a r l y h i g h i f t h e g r a v e l i s squeezed o u t s i d e t h e c a s i n g and i n j e c t e d w i t h h i g h v e l o c i t i e s ( S P A R L I N 1969, S P A R L I N & BOND 1969, S P A R L I N & COPELAND 1 9 7 2 ) .
5.2.3.8.2. Tackifying action o f resin i n the slurry The amount o f m i x i n g o f g r a v e l w i t h f o r m a t i o n sand can be s i g n i f i c a n t l y r e d u c e d b y s u s p e n d i n g h i g h g r a v e l c o n c e n t r a t i o n s i n a v i s c o u s o i l as c a r r i e r f l u i d and add r e s i n as a t a c k i f y i n g a g e n t t o t h e s l u r r y in o r d e r t o make t h e g r a v e l g r a i n s s l i g h t l y s t i c k y . The s l u r r y i s t h e n squeezed i n t o t h e p e r f o r a t i o n s and a s c r e e n i s washed i n p l a c e a f t e r t h e c o n c e n t r a t e d g r a v e l s l u r r y has been i n j e c t e d . M i n i m i z a t i o n o f g r a v e l and p a y zone sand m i x i n g i s m a i n l y a c h i e v e d due t o p u s h i n g o f t h e l o o s e r e s e r v o i r sand away f r o m t h e w e l l b o r e b y t h e h i g h - v i s c o s i t y f l u i d , t h e p r o m o t i o n o f p l u g f l o w o f t h e g r a v e l b y t h e h i g h s a t u r a t i o n and t h e c o h e s i v e f o r c e o f t h e t a c k i f y i n g a g e n t t h e r e b y b r i n g i n g l a r g e amounts o f g r a v e l t o t h e f o r m a t i o n a t t h e same t i m e , and t h e s l o w pumping r a t e p r e v e n t i n g any b l a s t i n g , f i n g e r i n g o r f r a c t u r i n g o f t h e r e s e r v o i r .
5.2.3.9. Hydrocarbon productivity vs. sand control The m a i n q u e s t i o n when t r e a t i n g sand p r o b l e m s i n o i l w e l l s i s how t o p e r f o r m optimum sand c o n t r o l w i t h o u t r e s t r i c t i n g h y d r o c a r b o n p r o d u c t i o n ( S P A R L I N & COPELAND 1 9 7 2 ) . A s m o s t o p e r a t o r s a r e now more c r i t i c a l l y t h a n e v e r b e f o r e r e v i e w i n g the w e l l s i n l i g h t o f p o s s i b i l i t i e s t o increase o f f t a k e r a t e s a t the lowest p o s s i b l e i n v e s t m e n t and m a i n t e n a n c e expenses and t o d r a s t i c a l l y l o w e r w i t h d r a w a l c o s t w i t h o u t l o o s i n g t o o much o u t p u t w h i c h h a s t o b e r e m a i n e d a t a n optimum feasibility level, improvement o f g r a v e l p a c k i n g t e c h n i q u e s as one p o t e n t i a l l y p r o f i t a b l e way t o a m e l i o r a t e w e l l p r o d u c t i v i t y i n c o m b i n a t i o n w i t h g i v i n g b e t t e r sand c o n t r o l i s o f p r o g r e s s i v e l y g r e a t e r s i g n i f i c a n c e . Sand r e t e n t i o n v s . h y d r o c a r b o n f l o w c a p a c i t y and o t h e r a s p e c t s a r e b r i e f l y o u t l i n e d as f o l l o w s .
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5.2.3.9.1.
Sand r e t e n t i o n vs. hydrocarbon
flow capacity
A l t h o u g h d e s i g n o f t h e s a n d - r e t e n t i o n g r a v e l i s based on t h e f i n e s t format i o n material present i n the productive i n t e r v a l , gravel selected according t o sand s t a b i l i z a t i o n c r i t e r i a c o u l d a d v e r s e l y a f f e c t w e l l f l o w c a p a c i t y . High success r a t i o s o f t h e g r a v e l pack o p e r a t i o n , however, a r e f r e q u e n t l y o b t a i n e d when c a r e f u l l y c o n s i d e r e d adjustments a r e made i n design c o n d i t i o n s . Gravel p l u g g i n g w i t h f o r m a t i o n sand may be i n t e n t i o n a l l y p e r m i t t e d o p p o s i t e some f i n e r low-perm e a b i l i t y p o r t i o n s of m u l t i - p a y i n t e r v a l s t o a l l o w h i g h e s t p r o d u c t i v i t y f r o m t h e c o a r s e r good r e s e r v o i r s e c t i o n s t o which g r a v e l s i z e i s o p t i m a l l y a d j u s t e d . A l t e r n a t i v e s o l u t i o n s i n case o f l o w a b s o l u t e g r a v e l p e r m e a b i l i t y w i t h h i g h p r e s s u r e d r o p across t h e p e r f o r a t i o n t u n n e l s a r e i n c r e a s e d p e r f o r a t i o n d e n s i t y and/or diameter, and open-hole g r a v e l pack i f t e c h n i c a l l y a l l o w e d by t h e d i s t r i b u t i o n o f r e s e r v o i r t y p e s and f l u i d compositions i n t h e g e o l o g i c a l column penet r a t e d by t h e b o r e h o l e s i n c e t h e l a s t c a s i n g was r u n . I n any case, however, g r a v e l - s a n d d i a m e t e r r a t i o s t h a t a l l o w c o n t i n u e d sand p r o d u c t i o n and l e a d t o poss i b l e l i n e r e r o s i o n have t o be a v o i d e d (SUMAN, ELLIS & SNYDER 1983). The most p r e v a l e n t t h e o r y a t t r i b u t e s sand p r o d u c t i o n t o f r i c t i o n and r e s u l t a n t p r e s s u r e drop as r e s e r v o i r f l u i d s pass through t h e p o r e s o f t h e f o r m a t i o n sand body (RIKE 1970). When t h e p r e s s u r e d r o p i s h i g h and cementing m a t e r i a l s between sand g r a i n s a r e weak o r absent, t h e i n d i v i d u a l p a r t i c l e s o f t h e sand body a r e d i s l o d g e d and c a r r i e d i n t o t h e w e l l b o r e . Another e x p l a n a t i o n suggests t h a t environmental w a t e r which was p r e s e n t d u r i n g o r i g i n a l d e p o s i t i o n o f t h e sand g r a i n s i s c h e m i c a l l y d i f f e r e n t f r o m t h e w a t e r c o n t a i n e d i n t h e a q u i f e r s u r r o u n d i n g t h e o i l f i e l d . Water p r o d u c t i o n can a c t u a l l y c h e m i c a l l y d i s s o l v e a p o r t i o n o f t h e cement between t h e sand g r a i n s , t h e r e b y l o o s e n i n g t h e g r a i n f a b r i c . Another concept assumes t h a t t h e r e s e r v o i r compacts as f o r m a t i o n p r e s s u r e d e c l i nes w i t h p r o g r e s s i v e d e p l e t i o n , and t h e changing l o a d tends t o s h i f t sand g r a i n s and m e c h a n i c a l l y shear e x i s t i n g cementing m a t e r i a l s .
5.2.3.9.2.
Other aspects
An i m p o r t a n t a p p l i c a t i o n o f g r a v e l p a c k i n g i n c o m b i n a t i o n w i t h h y d r a u l i c f r a c t u r i n g i s p r e v e n t i o n o f p r o p p a n t and f o r m a t i o n sand flowback i n t o t h e borehole a f t e r termination o f hydraulic s t i m u l a t i o n treatments p a r t i c u l a r l y i n s o f t sands and c h a l k s . Double i n s u r a n c e a g a i n s t g r a i n flowback can be made by t a i l i n g - i n r e s i n - c o a t e d proppants i n t h e t e r m i n a l stage o f t h e h y d r a u l i c f r a c t u r e j o b ( c f . s e c t i o n 4.12.3.3.2.) and p e r f o r m i n g a combined o u t s i d e and i n s i d e g r a v e l pack o p e r a t i o n w i t h r e s i n - c o a t e d g r a v e l ( c f . s e c t i o n 5 . 8 . 2 . ) . I n case o f p r o p e r g r a i n s i z e design, t h e c o a r s e r t a i l - i n o f t h e p r o p p a n t wedge f i l l i n g t h e h y d r a u l i c a l l y - g e n e r a t e d c r a c k a c t s i t s e l f as a g r a v e l pack by b r i d g i n g t h e f i n e r proppants o f t h e main f r a c t u r e - p l u g g i n g l o t a t t h e i n t e r f a c e o f b o t h g r a i n s i z e c l a s s e s ( c f . s e c t i o n 5 . 8 . 8 . ) . Drawbacks o f any t y p e o f g r a v e l p a c k i n g a r e m a i n l y l i n k e d w i t h s o f t h i g h - p e r m e a b i l i t y t h i c k r e s e r v o i r i n t e r v a l s which a r e s u b j e c t t o mechanical f o r m a t i o n damage when b e i n g c o a t e d w i t h t h e g r a v e l mantle, and h o r i z o n s where m o b i l e f i n e s o r h i g h - v i s c o s i t y waxy crude o i l may c r e a t e d e p o s i t s near t h e w e l l b o r e o r w i t h i n t h e g r a v e l pack t h a t c o u l d lower p r o d u c t i v i t y (WEISSENBURGER, MORITA, MARTIN & WHITFILL 1987). Experience o f p r o d u c t i v i t y d i f f e r e n c e s as a consequence o f g r a v e l g r a i n s i z e v a r i a t i o n s shows t h a t h i g h e r o f f t a k e r a t e s a r e o b t a i n e d w i t h c o a r s e r g r a v e l due t o b e t t e r p e r m e a b i l i t y o f t h e pack which a l l o w s e a s i e r hydrocarbon f l o w through t h e s a n d - c o n t r o l f i l t e r (STRACKE (1987). GURLEY, COPELAND & HENDRICK (1977) o u t l i n e d e s i g n i n g and p l a n n i n g o f g r a v e l - p a c k o p e r a t i o n s f o r maximum p r o d u c t i v i t y . R e l a t i o n s h i p s between g r a v e ? pack d e s i g n and w e l l p r o d u c t i v i t y a r e a l s o examined by SCHWARTZ (1969) and LYBARGER, SCHEUERMAN & WILLARD ( 1 9 7 4 ) .
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5.2.4. Gravel grain surface and shape I n terms of surface morphology, a p p l i c a t i o n of g l a s s beads o r z i r c o n i a - s i l i c a t e proppants which have b o t h p e r f e c t l y r o u n d grain shapes a n d smooth g r a i n s u r f a c e s ( c f . s e c t i o n s 1 . 3 . 2 . and 1 . 4 . 1 . ) r e q u i r e s reduction of the gravel-sand diameter r a t i o i n o r d e r t o promote bridging on the even s u r f a c e of the g l o b u l a r o r s p h e r i c a l g r a i n s (SUMAN, ELLIS & SNYDER 1983) o r intermixing of some subspher i c a l o r angular g r a i n s w i t h rough surface of s i m i l a r s i z e i n o r d e r t o provide the necessary f r i c t i o n . Following discussion of some aspects of r o u n d vs. angul a r sands f o r mechanical sand c o n t r o l , d i f f e r e n c e s i n g r a i n shape requirement between hydraulic p r o p p a n t f r a c t u r i n g a n d gravel packing a r e i l l u s t r a t e d .
5.2.4.1. Angular vs. round gravel in mechanical sand control Rough-surface natural sand a n d s y n t h e t i c alumina s i l i c a t e a n d oxide p r o p pants have b e t t e r bridging p r o p e r t i e s t h a n smooth-surface a r t i f i c i a l g l a s s beads a n d z i r c o n i a - s i l i c a t e proppants ( c f . s e c t i o n s 1 . 3 . 2 . and 1 . 4 . 1 . ) . Angular gravel-pack sand i s even l e s s l i k e l y t o be invaded because s a n d bridges more r e a d i l y on the i r r e g u l a r shapes and uneven s u r f a c e s , b u t more c o n s i s t e n t packi n g i s obtained w i t h rounded gravel w i t h i r r e g u l a r rough s u r f a c e ( G U L A T I & MALY 1974). Rounded gravel pack g r a i n s r e s u l t on average i n lower pressure drops a n d apparently show l e s s packing variance than more a n g u l a r m a t e r i a l s as a consequence of i n e r t i a l e f f e c t s of flow ( S A U C I E R 1974; c f . s e c t i o n 4 . 3 . 3 . 3 . ) . Angul a r sand has a higher tendency t o form premature bridges t h a n round sands ( H A L LIBURTON 1988). Angular sand, however, i s a l s o more s u b j e c t t o chipping and fragmenting during placement which reduces pack permeability a n d can clog s l o t s and screens.
5.2.4.2. Differences between fracturing and gravel packing The important d i f f e r e n c e of s u r f a c e morphology s i g n i f i c a n c e between hydraul i c proppant f r a c t u r i n g a n d gravel packing i s t h a t i n hydraulic s t i m u l a t i o n , bridging of proppants i n the crack i s undesirable because i t leads t o premature screenout f a i l u r e , a n d t h e r e f o r e round smooth p a r t i c l e s a r e p r i n c i p a l l y b e t t e r s u i t e d f o r hydraulic f r a c t u r i n g . I n gravel packing, however, bridging i s necessary f o r the success of the operation, w i t h thus i r r e g u l a r gravel s u r f a c e s t h a t provide f r i c t i o n being more favourable t h a n smooth s u r f a c e s where intermixing of o t h e r g r a i n s i s necessary t o t r i g g e r bridging. S t a b i l i t y improvement of a l l gravel types a n d grain s i z e s can be best achieved by l i n e r v i b r a t i o n (SOLUM 1984; c f . s e c t i o n 5 . 8 . 4 . ) which gives r i s e t o o r i g i n of a hexagonal p a r t i c l e framework instead of the standard cubic f a b r i c t h a t i s c r e a t e d by conventional flow-type gravel placement. Consistency of the hexagonal g r a i n arrangement i s a l s o increasing with more pronounced s u r f a c e roughness o r shape a n g u l a r i t y of the g r a i n s a n d subordinately a l s o w i t h l a r g e r s i z e of the g r a v e l .
5.3. Gravel packing completion fluids and gravel saturation Gravel packing t r a n s p o r t f l u i d s a r e in many cases more viscous than the hyd r a u l i c f r a c t u r i n g c a r r i e r f l u i d s , while o t h e r mechanical sand control a p p l i c a t i o n s a r e a l s o performed w i t h low-viscosity t r a n s p o r t f l u i d s . Viscosity a n d thus suspension capacity of f l u i d s f o r gravel packing, however, i s in comparison t o hydraulic f r a c t u r i n g a much smaller concern i n terms of proppant o r gravel d i s p e r s i o n , because a s a consequence of the mainly low c l o s u r e s t r e s s i n the p r e v a i l i n g amount of r e s e r v o i r s , predominantly natural q u a r t z sand and l i g h t , low-density s y n t h e t i c proppants ( e i t h e r a l u m i n a - s i l i c a t e or z i r c o n i a - s i l i c a t e proppants according t o the p r e s e n t l y commercially a v a i l a b l e spectrum; c f . chapter 1 . 3 . ) a r e s e l e c t e d f o r sand-control treatments. High-density proppants a r e only used i n special a p p l i c a t i o n s such as steam-drive p r o j e c t s ( c f . s e c t i o n 5 . 7 . 1 . ) , because the operation causes d i f f i c u l t i e s of proper placement
735 by p a r t i c u l a r l y s t r o n g s e t t l i n g e f f e c t s o f heavy alumina o x i d e and s i l i c a t e p r o p p a n t g r a i n s i n t h e f l u i d s ( a s p e c t s o f gravel-pack c a r r i e r f l u i d s a r e d i s c u s sed by GURLEY, COPELAND & HENDRICK 1977; CONSTIEN & MAYER 1978, COSTEIN & WALLACE 1980, TORREST 1982, SCHEUERMAN 1984, SKAGGS 1984; UNDERDOWN, OAS & NGUYEN 1984; LEDLOW & SAUER 1985; MURPHEY, ROLL & WONG 1985; PENBERTHY 1985). Gravel p a c k i n g s h o u l d aim on s e t t i n g t h e g r a v e l w i t h o u t c o n t a m i n a t i o n a t t h e p r o p e r l o c a t i o n as t i g h t l y as p o s s i b l e and then h o l d i n g i t i n p l a c e f o r t h e l i f e t i m e o f t h e w e l l (ALLEN & ROBERTS 1982). The most i m p o r t a n t aspect o f g r a vel packing c a r r i e r f l u i d s i s s u f f i c i e n t l e a k o f f i n t o the formation i n order t o achieve t h e necessary t i g h t p a c k i n g o f t h e g r a v e l , which i s t h e second m a j o r d i f f e r e n c e t o h y d r a u l i c f r a c t u r i n g where f l u i d l o s s has t o be m i n i m i z e d i n o r d e r t o a v o i d p r o p p a n t b r i d g i n g and premature screenout f a i l u r e . Gravel p a c k i n g r e s u l t s a r e i m p r o v i n g w i t h p r o g r e s s i v e f l u i d l e a k o f f and t h e r h e o l o g i c a l p r o p e r t i e s o f sand c o n t r o l t r a n s p o r t f l u i d s t h e r e f o r e have t o be a d j u s t e d such t h a t s a t i s f a c t o r i l y h i g h r a t e s o f f l u i d l e a k o f f a r e achieved. The most i m p o r t a n t asp e c t s a r e f l u i d v i s c o s i t y and g r a v e l c o n c e n t r a t i o n , t r a n s p o r t and p a c k i n g capac i t y o f the s l u r r i e s , gravel s e t t l i n g diminution i n the c a r r i e r f l u i d , gel surf a c e f i l t r a t i o n f r o m polymer s o l u t i o n s and s c r e e n o u t m i m i c k i n g , and c o m b i n a t i o n o f g r a v e l p a c k i n g and h y d r a u l i c p r o p p a n t f r a c t u r i n g .
5.3.1. Fluid viscosity and gravel concentration D i s t i n c t i o n can g e n e r a l l y be made between c o n v e n t i o n a l c i r c u l a t i o n g r a v e l p a c k i n g where l o w - v i s c o s i t y f l u i d s a r e used t o c a r r y low g r a v e l c o n c e n t r a t i o n s (up t o a b t . 1.5 l b s / g a l which i s e q u i v a l e n t t o a b t . 180 kg/m3) i n t o t h e w e l l a t h i g h c i r c u l a t i o n r a t e s and l a r g e t o t a l t r e a t m e n t q u a n t i t i e s ( h i g h v o l u m e / r a p i d displacement method), and h i g h - d e n s i t y g r a v e l p a c k i n g where h i g h - v i s c o s i t y , pol y m e r - g e l l e d f l u i d systems a r e a p p l i e d t h a t t r a n s p o r t h i g h g r a v e l s a t u r a t i o n s (up t o a b t . 15 l b s / g a l which i s e q u i v a l e n t t o a b t . 1,800 kg/m3) t o achieve packi n g w i t h low t o t a l volume handled (SPARLIN & COPELAND 1972; SUMAN, ELLIS & SNYDER 1983) by t h e s l u r r y pack method (PEDEN, RUSSELL & OYENEYIN 1985) o r h i g h d e n s i t y squeeze p a c k i n g t e c h n i q u e where t h e sequence o f prepad, g r a v e l s l u r r y and postpad i s s u c c e s s i v e l y pumped (ELSON, OARLINGTON & MANTOOTH 1984). Highv i s c o s i t y c a r r i e r f l u i d s and h i g h g r a v e l c o n c e n t r a t i o n s r e s u l t i n l e s s tendency o f i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand (GAJDOSIK & WILLINGHAM 1976). L o w - v i s c o s i t y f l u i d s w i t h low g r a v e l c o n c e n t r a t i o n a r e m a i n l y used i n c i r c u l a t i o n p a c k i n g i n open h o l e s o r d e v i a t e d cased h o l e s where t h e p e r f o r a t i o n s have been prepacked by v i s c o u s s l u r r i e s t r a n s p o r t i n g h i g h g r a v e l s a t u r a t i o n s , whereas h i g h - v i s c o s i t y f l u i d s w i t h h i g h g r a v e l c o n c e n t r a t i o n s a r e p r e f e r e n t i a l l y u t i l i z e d i n squeeze p a c k i n g i n cased v e r t i c a l h o l e s . H i g h g r a v e l c o n c e n t r a t i o n s i n h i g h - v i s c o s i t y f l u i d s do n o t p l u g t h e screen so s e v e r e l y as do low g r a v e l s a t u r a t i o n s i n l o w - v i s c o s i t y f l u i d s and a r e a l s o much b e t t e r p r o m o t i n g adequate p e r f o r a t i o n p a c k i n g . A r e c e n t o u t s t a n d i n g i n n o v a t i o n a r e s u r f a c t a n t - b e a r i n g water-based h i g h - v i s c o s i t y g r a v e l p a c k i n g f l u i d s which combine t h e f a v o u r a b l e aspects o f h i g h f l u i d l o s s r a t e s o f c o n v e n t i o n a l l o w - v i s c o s i t y water-based f l u i d s and h i g h g r a v e l t r a n s p o r t c a p a c i t y o f t r a d i t i o n a l h i g h - v i s c o s i t y polymer-based fluids (DOWELL SCHLUMBERGER 1988, NEHMER 1988; c f . s e c t i o n 5 . 3 . 1 . 1 . 2 . ) . Some comments on achievement o f s u f f i c i e n t f l u i d l e a k o f f and p r e v e n t i o n o f e x c e s s i v e f l u i d l o s s a r e g i v e n as f o l l o w s .
5.3.1.1. Achievement of sufficient fluid leakoff H i g h - v i s c o s i t y f l u i d s a r e t h u s a b l e t o c a r r y a b t . t e n t i m e s t h e g r a v e l quant i t y p e r f l u i d u n i t which can be t r a n s p o r t e d by l o w - v i s c o s i t y f l u i d s . The v i s c o s i t y o f t h e c a r r i e r f l u i d i s d i r e c t l y r e l a t e d t o t h e designed g r a v e l c o n c e n t r a t i o n s f o r t h e s l u r r y (PEDEN, RUSSELL & O Y E N E Y I N 1985). H i g h - v i s c o s i t y f l u i d s c h a r a c t e r i z e d by h i g h g r a v e l t r a n s p o r t c a p a c i t y and thus r e s u l t i n s h o r t e r g r a v e l placement times, and a r e t h e r e f o r e an e f f e c t i v e means f o r e l i m i n a t i n g b l a n k
736
s e c t i o n s i n the w e l l b o r e provided pack d e h y d r a t i o n can be a c h i e v e d by f l u i d l e a k o f f through l i n e r o r s c r e e n (ELSON, DARLINGTON & MANTOOTH 1 9 8 4 ) . Low-visc o s i t y f l u i d s may p r o v i d e t i g h t e r g r a v e l placement i n the annulus ( A L L E N & ROBERTS 1 9 8 2 ) , b u t do n o t promote a d e q u a t e p e r f o r a t i o n packing. While the most s i g n i f i c a n t advantages of high g r a v e l s a t u r a t i o n s i n h i g h - v i s c o s i t y f l u i d s a r e e x c e l l e n t p e r f o r a t i o n packing, l e s s impairment of s c r e e n flow c a p a c i t y , and quick performance t h e r e b y s a v i n g r i g t i m e , s e v e r a l drawbacks a l s o e x i s t . After-pack g r a v e l slumping may c r e a t e v o i d s i n t h e packs which i s a p a r t i c u l a r l y s e v e r e problem i n d e v i a t e d w e l l s ( c f . s e c t i o n 5 . 8 . 3 . ) . The high-dens i t y g r a v e l - l a d e n s l u r r y may f a l l through t h e l i g h t e r p r e v i o u s l y pumped pad f l u i d , w i t h the r a t e of f a l l t h r o u g h becoming g r e a t e r when l a r g e - d i a m e t e r work s t r i n g s a r e used. I f c i r c u l a t i o n i s taken above the packer and n o t through a s c r e e n , the roping e f f e c t ( c f . a l s o s e c t i o n 5 . 9 . 1 . 3 . 3 . ) has t o be c o n s i d e r e d t o p r e v e n t g e t t i n g g r a v e l on t o p of the p a c k e r . I n c o n t r a s t t o h y d r a u l i c f r a c t u r i n g where e x t r a o r d i n a r i l y high f l u i d l o s s t o th;. formation can r e s u l t i n premature s c r e e n o u t t e r m i n a t i o n of the t r e a t m e n t by f l u i d d e h y d r a t i o n , proppant b r i d g i n g and s l u r r y s t i c k i n g ( c f . a l s o s e c t i o n s 4 . 2 . 2 . 2 . 3 . and 6 . 2 . 4 . 2 . 1 ) , and e x c e s s i v e f l u i d l e a k o f f r a t e s a r e t h e r e f o r e undes i r e d , grave7 pack o p e r a t i o n s r e q u i r e h i g h r a t e s of f l u i d l o s s i n t o the format i o n i n o r d e r t o t r a n s p o r t and d e p o s i t l a r g e amounts of g r a v e l and thus t o e n a b l e high plugging e f f i c i e n c y of the p e r f o r a t i o n t u n n e l s (NEHMER 1 9 8 8 ) . I n f l u e n c e s of c a r r i e r f l u i d v i s c o s i t y a s well a s improvement by s u r f a c t a n t - b e a r ing w a t e r - b a s e d f l u i d s a r e b r i e f l y i l l u s t r a t e d a s f o l l o w s . Some comments a r e a l s o given on t h e impact of f o r m a t i o n p e r m e a b i l i t y .
5.3.1.1.1. Influence of carrier fluid viscosity Gravel packing can be performed with t h i n l o w - v i s c o s i t y o r t h i c k h i g h - v i s c o s i t y f l u i d s (HALLIBURTON 1 9 8 8 ) . Thin l o w - v i s c o s i t y f l u i d s a r e a b l e t o c a r r y sand in lower c o n c e n t r a t i o n s o n l y a t h i g h e r pump r a t e s which may Cause i n t e r m i xing of g r a v e l and f o r m a t i o n sand w i t h a s s o c i a t e d p e r m e a b i l i t y damage, and some g r a i n breakage and f i n e s g e n e r a t i o n may o c c u r due t o pumping of sand s l u r r i e s a t high r a t e s through t h e t o o l s a s a consequence of impact. In e x t r e m e l y long p r o d u c t i v e i n t e r v a l s , t h e a p p l i c a t i o n of l o w - v i s c o s i t y f l u i d s may be d i c t a t e d by economical c o n s i d e r a t i o n s . H i g h - v i s c o s i t y f l u i d s allow g r a v e l packing a t low i n j e c t i o n r a t e s which reduces i n t e r m i x i n g of g r a v e l and f o r m a t i o n sand when the s l u r r y i s pumped through t h e p e r f o r a t i o n s . Aspects of g r a v e l placement and f l u i d b r e a k i n g , gel c o n c e n t r a t i o n vs. s l u r r y d e h y d r a t i o n , f l u i d v i s c o s i t y and f o r m a t i o n damage, and f l u i d v i s c o s i t y and l e a k o f f r a t e s a r e b r i e f l y o u t l i n e d as follows.
5.3.1.1.1.1. Gravel placement and fluid breaking High f l u i d - l o s s r a t e s of l o w - v i s c o s i t y g r a v e l pack c a r r i e r media can be achieved by b u i l d i n g a p a r t i c l e b r i d g e on the formation f a c e a g a i n s t which hyd r o s t a t i c p r e s s u r e can be a p p l i e d (ALLEN & ROBERTS 1 9 8 2 ) . H i g h - v i s c o s i t y t r a n s p o r t media p l a c e t h e g r a v e l more q u i c k l y and w i t h l e s s f l u i d l e a k o f f t o the f o r mation which b e a r s a l s o l e s s p o s s i b i l i t i e s of r e s e r v o i r damage. I n c r e a s i n g c a r r i e r - f l u i d v i s c o s i t y minimizes g r a v i t y s e g r e g a t i o n of t h e g r a v e l d u r i n g p l a c e ment and t h e r e b y minimizes plugging of t h e l i n e r s l o t s w i t h g r a v e l f i n e s which develop i n t h e s e t t i n g p r o c e s s . Concerning a p p l i c a t i o n of h i g h - v i s c o s i t y w a t e r based f l u i d s , t y p e and c o n c e n t r a t i o n of v i s c o s i t y b r e a k e r must be determined depending on f o r m a t i o n t e m p e r a t u r e , and the sump volume below the s c r e e n has t o be minimized i n o r d e r t o reduce f u r t h e r s e t t l i n g of the g r a v e l a f t e r t h e v i s c o s i t y has broken, t h e r e b y l o o s e n i n g t h e pack f a b r i c . E f f e c t i v i t y and time of breakage of t h e c a r r i e r f l u i d gel have a d i r e c t b e a r ing upon g r a v e l t r a n s p o r t e f f i c i e n c y , compaction and s e t t l i n g of t h e g r a v e l
737 pack, and p o s s i b l e impairment o f g r a v e l pack p e r m e a b i l i t y by f l u i d r e s i d u e s (ALMOND & BLAND 1984, SCHEUERMAN 1984; UNDERDOWN, DAS & NGUYEN 1984). Minimum c a r r i e r f l u i d d e n s i t y r e q u i r e d depends on r e s e r v o i r p r e s s u r e which i s a f u n c t i o n o f s t r u c t u r a l d e p t h (PEDEN, RUSSELL & OYENEYIN 1985).
5.3-1.1.1.2. Gel concentrat ion vs. slurry dehydration I n t e r m e d i a t e g e l c o n c e n t r a t i o n s have h i g h e r l e a k o f f a t t h e same p r e s s u r e than h i g h g e l s a t u r a t i o n s which a i d s i n s l u r r y d e h y d r a t i o n b o t h i n t o t h e format i o n and t h r o u g h t h e screen (LEDLOW, SAUER & T I L L 1985) i f t h e r e i s no s k i n and no p r e s s u r e l o s s t h r o u g h t h e p e r f o r a t i o n s . The pad volume i n a s t r a i g h t squeeze pack should be decreased t o a minimal amount such t h a t less f l u i d has t o l e a k o f f i n o r d e r t o achieve a t i g h t pack, whereas i n a c i r c u l a t i n g pack, t h e pad volume should be i n c r e a s e d i n o r d e r t o a l l o w some pad l e a k o f f i n t o t h e f o r m a t i o n and f a v o u r c i r c u l a t i o n up t h e wash p i p e . I n c r e a s i n g pump r a t e i n c r e a s e s p a c k i n g e f f i c i e n c y and p r e v e n t s premature screenout o r r o p i n g .
5.3.1.1.1.3. Fluid viscosity and formation damage A special type o f h i g h - v i s c o s i t y c a r r i e r f l u i d s w i t h h i g h gravel saturations a r e c o n c e n t r a t e d g r a v e l s l u r r i e s (SPARLIN & BOND 1969, SPARLIN & COPELAND 1972) where a r e s i n a d d i t i v e i s i n t e r m i x e d as a t a c k i f y i n g agent i n o r d e r t o make t h e i n d i v i d u a l g r a v e l g r a i n s s t i c k y and t o promote p l u g f l o w o f t h e s l u r r y ( c f . sect i o n 5 . 8 . 5 . ) . I n t h e l a s t years, t h e t r e n d o f a p p l i c a t i o n i s more and more t o wards h i g h - v i s c o s i t y f l u i d s and h i g h g r a v e l c o n c e n t r a t i o n s f o r t h e purpose o f a c h i e v i n g s a f e g r a v e l placement i n a h i g h e r p a c k i n g d e n s i t y , because e x p e r i e n c e has shown t h a t l o w - v i s c o s i t y systems w i t h low g r a v e l s a t u r a t i o n s e f f e c t i v e l y c o n t r o l f o r m a t i o n sand, b u t a l s o r e s t r i c t hydrocarbon p r o d u c t i o n . L o w - v i s c o s i t y f l u i d s c a r r y i n g g r a v e l a t h i g h pumping r a t e s can cause i n t e r m i x i n g o f t h e c o a r s e r p a c k i n g g r a v e l and t h e f i n e r f o r m a t i o n sand due t o t h e j e t t i n g e f f e c t (HALLIBURTON 1988) w h i c h can provoke severe p e r m e a b i l i t y damage. The h i g h e s t amount o f damage occurs a t a r a t i o o f a b t . 70 v o l % p a c k i n g g r a v e l and 30 v o l % f o r m a t i o n sand, w i t h t h e g r e a t e r t h e r a t i o between g r a v e l and r e s e r v o i r p e r m e a b i l i t y , t h e g r e a t e r t h e damage p o t e n t i a l . High pumping r a t e s a l s o r e s u l t i n generated f i n e s due t o g r a v e l s h a t t e r i n g d u r i n g placement which may p l u g t h e s l o t s o f t h e screen ( N I N I & OWEN 1983).
5.3.1.1.1.4. Fluid viscosity and leakoff rates F l u i d l e a k o f f r a t e t o t h e r e s e r v o i r i s n o t t h e most d e c i s i v e aspect f o r est a b l i s h i n g g r a v e l packs through t h e p e r f o r a t i o n s (RENSVOLO & DECKER 1978). I t i s , however, e s s e n t i a l t h a t a v o i d a l r e a d y e x i s t s b e h i n d t h e c a s i n g o r t h a t t h e s l u r r y d i s p l a c e s f o r m a t i o n sand t o c r e a t e room f o r t h e pack. T h i c k h i g h - v i s c o s i t y c a r r i e r f l u i d s w h i c h have a c o n s i d e r a b l y lower f l u i d - l o s s r a t e t o t h e pay than t h i n l o w - v i s c o s i t y t r a n s p o r t media can more e f f e c t i v e l y d i s p l a c e r e s e r v o i r sand o u t s i d e t h e p e r f o r a t i o n s t o generate space f o r t h e g r a v e l pack than t h i n l o w - v i s c o s i t y f l u i d s which a r e c h a r a c t e r i z e d by h i g h l e a k o f f r a t e s , b u t which have i n s u f f i c i e n t a b i l i t y t o compress l o o s e r e s e r v o i r sand i n t h e immediate v i c i n i t y o f t h e p e r f o r a t i o n t u n n e l s and t h e r e f o r e can o n l y manage t o i n s t a l l a l e s s compact g r a v e l p l u g than h i g h - v i s c o s i t y c a r r i e r s . I n c o n t r a s t t o polymer f l u i d s which s u f f e r f r o m l i m i t e d l e a k o f f and cause f o r m a t i o n damage by m i c r o g e l s and f i s h e y e s , h i g h - v i s c o s i t y water-based g e l systems ( w h i c h a r e o u t l i n e d i n t h e f o l l o w i n g s e c t i o n ) do n o t c o n t a i n any polymer and t h e r e f o r e have e x c e l l e n t l e a k o f f and g r a v e l suspension c h a r a c t e r i s t i c s (DOWELL SCHLUMBERGER 1988 a ) .
738
5.3.1.1.2. Improvement by surfactant-bearing water-based fluids Improvement of f l u i d leakoff i n t o the r e s e r v o i r in order t o achieve s a t i s f a c tory p e r f o r a t i o n tunnel packing a n d enhancement of gravel suspension p r o p e r t i e s which a r e p a r t i c u l a r l y d e c i s i v e in highly-deviated boreholes can be reached with a v i s c o e l a s t i c s u r f a c t a n t - b a s e gravel-pack c a r r i e r f l u i d which has d i s t i n c t advantages over conventional polymer g e l l e d f l u i d systems ( N E H M E R 1988) a n d r e p r e s e n t s a major recent innovation. Comments on gel s t r u c t u r e , microgel breaking a n d removal, a n d gel f i l t e r cakes i n f r a c t u r i n g a n d gravel packing a r e o f f e r e d as follows.
5.3.1.1.2.1. Gel structure S u r f a c t a n t s develop viscous g e l s by formation of rod-shaped m i c e l l e s which i n t e r a c t with each o t h e r a t h i g h concentrations, thereby hindering movement of the micelles a n d t r i g g e r i n g an increase of f l u i d v i s c o s i t y , with thus t h e rodshaped m i c e l l e s a c t i n g s i m i l a r t o polymer molecules. A s a l t - i n d u c e d t r a n s i t i o n from spherical t o rod-shaped m i c e l l e s i s a major mechanism f o r increasing viscoe l a s t i c i t y of the f l u i d . The v i s c o e l a s t i c s u r f a c t a n t - b a s e c a r r i e r f l u i d does n o t cause formation damage in c o n t r a s t t o many polymer-gelled f l u i d s where f i s h eyes a n d / o r microgels which a r e even not removed by breakers p l u g pore t h r o a t s and thus lead t o impaired f l u i d l e a k o f f . Intense manipulation of polymer-gelled f l u i d s u s i n g shear a n d f i l t e r devices helps removing the f i s h e y e and/or microgel p a r t i c l e s , b u t makes the treatment more expensive i n terms of equipment a n d manpower, and excessive shear degrades the polymer, g i v i n g r i s e t o poor gravel suspension. The v i s c o e l a s t i c s u r f a c t a n t g e l l e d f l u i d system t h e r e f o r e r e p r e s e n t s a super i o r s o l u t i o n because of absence of formation damage a s well a s presence of f a vourable f l u i d l e a k o f f , gravel suspension, gravel t r a n s p o r t a n d gravel placement options (DOWELL SCHLUMBERGER 1988 a ) . The s u r f a c t a n t - b e a r i n g water-based system combines the favourable a t t r i b u t e s of high f l u i d - l o s s r a t e s of t r a d i t i o n a l low-viscosity water-based f l u i d s and high gravel t r a n s p o r t c a p a c i t y of conventional h i g h - v i s c o s i t y polymer-based f l u i d s . Another promising a l t e r n a t i v e i s the low p a r t i c l e d e n s i t y l c a r r i e r f l u i d d e n s i t y system introduced by H U D S O N & MARTIN (1988; c f . s e c t i o n 5 . 3 . 3 . 2 . ) .
5.3.1.1.2.2. Microgel breaking and removal The main s i g n i f i c a n c e of v i s c o e l a s t i c s u r f a c t a n t g e l l e d f l u i d systems i s f a s t leakoff of carrying f l u i d s which a s s i s t s the gravel t o p e n e t r a t e a l l the way t o the ends of the p e r f o r a t i o n tunnels, because i n order t o achieve minimum pressure drawdown during production, screen/casing annulus a n d p e r f o r a t i o n s must be completely packed with gravel t o keep o u t formation s’and (DOWELL SCHLUMBERGER 1988 a ) . Rapid f l u i d l o s s i s e s p e c i a l l y d e s i r a b l e i n squeeze packs ( c f . s e c t i o n 5 . 3 . 2 . 4 . 2 . 1 , because the e n t i r e prepad must be leaked i n t o the format i o n before the s l u r r y can even reach the p e r f o r a t i o n s . Aspects of f l u i d q u a l i t y enhancement by microgel breaking and removal in polymer-gelled gravel packing f l u i d operations predominantly conducted under cons t a n t flow r a t e a r e discussed by HASHEMI, CAOTHIEN & R O V I G ( 1 9 8 8 ) . Both mechan i c a l and hydraulic shearing break a n d remove the p a r t i a l l y hydrated microgels, w i t h the r e s u l t i n g g e l s having higher f l u i d leakoff f o r a t i g h t e r gravel pack. Polymer shear degradation occurs i n a d d i t i o n t o chemical, b i o l o g i c a l and t h e r mal decomposition and has t o be taken i n t o account. Gel s h e a r i n g as a s t e p t o break u p f i s h - e y e s or m i c e l l e s a n d augment t h e i r hydration can u l t i m a t e l y cause g e l s t o lose the v i t a l property of gravel suspension, a n d t h e r e f o r e must be mon i t o r e d c l o s e l y p r i o r t o mixing a gravel pack s l u r r y ( C O L E , SHAW, C A V E N Y & BELLENGER 1988).
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5.3.1.1.2.3. Gel filter cakes in fracturing and gravel packing S i m i l a r l y as g e l f i l t e r cakes o r i g i n a t i n g on t h e f o r m a t i o n f a c e a t f r a c t u r e w a l l s d e t e r i o r a t e crack c o n d u c t i v i t y due t o proppant embedment, g e l s u r f a c e f i l t r a t i o n f r o m polymer s o l u t i o n s d u r i n g g r a v e l p a c k i n g covers t h e r e s e r v o i r f a c e w i t h a l o w - p e r m e a b i l i t y drape and d e t e r i o r a t e s f u r t h e r l e a k o f f o f g r a v e l c a r r i e r f l u i d s i n t o t h e pay m a t r i x , t h e r e b y having d e l e t e r i o u s e f f e c t s on g r a v e l placement and t h u s q u a l i t y and performance o f mechanical sand c o n t r o l (TORREST & MARCANO 1986; c f . s e c t i o n s 1.4.10.3.5.3. and 4.3.4.1.4.). F i l t r a t i o n o f microg e l s d u r i n g polymer i n s e r t i o n q u i c k l y b u i l d s up a r e l a t i v e l y impermeable s u r f a c e l a y e r , and t h e p r e s s u r e d r o p f o r w a t e r f l o w through t h i s p e l l i c l e i s linear i n r a t e . Once formed, t h e g e l l a y e r s can undergo changes i n c h a r a c t e r i s t i c s w i t h w a t e r f l o w a t s u f f i c i e n t r a t e s . The l a y e r b u i l d u p tends t o g i v e f a l s e s i g n a l s o f j o b c o m p l e t i o n s as pumping p r e s s u r e s r i s e r a p i d l y by m i m i c k i n g screenout t e r m i n a t i o n o f t h e o p e r a t i o n , and reduced g r a v e l pack q u a l i t y and p e r formance c o u l d r e s u l t f r o m f a i l u r e t o account f o r t h e e f f e c t o f s u r f a c e g e l f i l t r a t i on.
5.3.1.1.3. Impact of formation permeability F l u i d l o s s d u r i n g g r a v e l p a c k i n g i s s t r o n g l y dependent f r o m r e s e r v o i r format i o n p e r m e a b i l i t y . H i g h - p e r m e a b i l i t y sands a r e c h a r a c t e r i z e d by good f l u i d l e a k o f f and t h u s a l l o w h i g h - q u a l i t y g r a v e l packing, whereas l o w - p e r m e a b i l i t y sands p e r m i t o n l y poor f l u i d l o s s and a r e t h e r e f o r e d i f f i c u l t t o g r a v e l pack w i t h r e a sonable e f f e c t i v i t y . F o l l o w i n g a b r i e f c h a r a c t e r i z a t i o n o f f o r m a t i o n p e r m e a b i l i t y c l a s s i f i c a t i o n f o r g r a v e l p a c k i n g i n comparison t o h y d r a u l i c p r o p p a n t f r a c t u r i n g , some aspects o f r e s e r v o i r p e r m e a b i l i t y and c o m p o s i t i o n a r e o u t l i n e d .
5.3.1.1.3.1. Permeabi 1i ty classification for gravel packing I n view o f g r a v e l p a c k i n g a p p l i c a t i o n s , l o w - p e r m e a b i l i t y sands a r e r e s e r v o i r s w i t h p e r m e a b i l i t i e s i n t h e range o f a b t . 20 - 75 md ( w h i c h would have t o be c h a r a c t e r i z e d as h i g h - p e r m e a b i l i t y pays i n l i g h t o f h y d r a u l i c p r o p p a n t f r a c t u r i n g ; c f . s e c t i o n 4 . 6 . ) , and h i g h - p e r m e a b i l i t y sands a r e f o r m a t i o n s w i t h p e r m e a b i l i t i e s i n t h e range o f a b t . 75 md - 4 d. W h i l e polymer f l u i d s have h i g h v i s c o s i t y and b r i d g e o f f i n l o w - p e r m e a b i l i t y sands t h e r e b y r e s u l t i n g i n o n l y l i m i t e d e f f e c t i v i t y o f g r a v e l packing, s u r f a c t a n t - b a s e d non-polymer f l u i d s have lower v i s c o s i t y and t h u s e x h i b i t good l e a k o f f r a t e s a l s o i n l o w - p e r m e a b i l i t y sands, t h e r e f o r e e n a b l i n g s a t i s f a c t o r y g r a v e l placement (DOWELL SCHLUMBERGER 1988 a) and a v o i d i n g problems caused by o t h e r c a r r i e r f l u i d s such as permeabil i t y loss f r o m f o r m a t i o n damage, low pack e f f i c i e n c y due t o slow f l u i d l e a k o f f , and r a p i d s e t t l i n g - o u t o f g r a v e l f r o m t h e s l u r r y .
5.3.1.1.3.2. Reservoir permeability and composition While high-permeability r e s e r v o i r s w i t h gravel packing p o t e n t i a l are clean medium- t o c o a r s e - g r a i n e d u n c o n s o l i d a t e d sands, l o w - p e r m e a b i l i t y pay zones r e q u i r i n g s t a b i l i z a t i o n a r e f i n e - g r a i n e d sands c o n t a i n i n g d i s p e r s e d c l a y m i n e r a l p a r t i c l e s . E s p e c i a l l y c o m p l i c a t e d s i t u a t i o n s f o r g r a v e l p a c k i n g i n view o f s u f f i c i e n t f l u i d l e a k o f f r a t e s a r e i n t e r b e d d e d sequences o f h i g h - and low-permeabil i t y sands, w i t h again p r o p e r f l u i d loss o n l y b e i n g a b l e t o be achieved by s u r f a c t a n t - b a s e d non-polymer f l u i d s w i t h low v i s c o s i t y ( a l t e r n a t i n g low- and h i g h p e r m e a b i l i t y r e s e r v o i r s t o r e y s a l s o have a n e g a t i v e impact on f l u i d l e a k o f f i n hydraulic f r a c t u r i n g which i s d i f f i c u l t t o l i m i t i n c o n t r a s t t o the necessity o f i t s p r o m o t i o n i n g r a v e l packing; c f . s e c t i o n 4 . 3 . 4 . 6 . ) .
740
5.3.1.2. Prevention o f excessive fluid loss H i g h f l u i d - l o s s r a t e s d u r i n g g r a v e l p l a c e m e n t a r e n e c e s s a r y t o a c h i e v e good pack q u a l i t i e s , b u t h i g h t o e x c e s s i v e f l u i d l e a k o f f p r i o r t o g r a v e l p a c k i n g c a n cause s e r i o u s p r o b l e m s i f s o l u b l e f l u i d - l o s s a g e n t s had t o be added, because i n v a s i o n o f f o r m a t i o n pores, b r i d g i n g a t t h e entrance o f t h e r e s e r v o i r t o b u i l d up a f i l t e r cake, and f i l l i n g o f p e r f o r a t i o n t u n n e l s may p r e v e n t c o m p l e t e g r a v e l p l a c e m e n t a n d r e d u c e w e l l p r o d u c t i v i t i e s (SPARLIN & HAGEN 1986). E x c e s s f l u i d - l o s s a d d i t i v e s i n h i b i t c o m p l e t e c o v e r a g e o f an i n t e r v a l w i t h g r a v e l due t o e i t h e r inadequate l e a k o f f t o t h e formation o r p a r t i a l plugging o f the screen. F i n e g r i n d s o f c a l c i u m c a r b o n a t e o r s i l i c a f l o u r a r e t h e m o s t common s o l i d l e a k o f f a d d i t i v e s t o g e l l e d f l u i d s and f o r m l o w - p e r m e a b i l i t y f i l t e r c a k e s and p a c k s as t h e y c o v e r r e s e r v o i r f a c e and p e r f o r a t i o n t u n n e l s , r e s p e c t i v e l y , b u t t h e s e f i n e p a r t i c l e s c a n cause s e r i o u s f o r m a t i o n damage, as t h e y a r e d i f f i c u l t t o remove ( c f . s e c t i o n 5 . 1 1 . 4 . ) . Due t o t h e f a c t t h a t any p e r f o r a t i o n s f i l l e d w i t h f l u i d - l o s s a d d i t i v e s w i l l n o t a c c e p t g r a v e l and where f l u i d f l o w i s r e stricted, g r a v e l w i l l n o t be packed s u f f i c i e n t l y t i g h t l y , a l l t h e f l u i d - l e a k o f f a g e n t s u s e d b e f o r e s e t t i n g t h e g r a v e l pack m u s t be removed f r o m w e l l b o r e , p e r f o r a t i o n s and n e a r - b o r e h o l e f o r m a t i o n t o e n s u r e a c o m p l e t e p a c k . Some drawbacks o f p o s t - p a c k i n g a c i d i z i n g as w e l l as a few p o i n t s o f s o l i d f l u i d - l o s s a d d i t i v e s and p a r t i a l f o r m a t i o n p r e - p a c k i n g a r e i l l u s t r a t e d as f o l l o w s .
5.3.1.2.1. Drawbacks o f post-packing acidizing A c i d i z i n g a f t e r a g r a v e l pack i s i n p l a c e s h o u l d be a v o i d e d i f p o s s i b l e , because f l u i d i n j e c t e d i n t o t h e g r a v e l m a n t l e may d i s t u r b t h e pack and p u s h t h e g r a v e l away f r o m t h e w e l l b o r e , and where m a t e r i a l i s d i s s o l v e d , t h e g r a v e l w i l l move i n t o t h e v o i d e d space, r e s u l t i n g i n h o l e s i n t h e pack a n d / o r g r a v e l and sand m i x i n g t o g e t h e r . As a c i d c a n b e i n j e c t e d t h r o u g h p e r f o r a t i o n s f i l l e d w i t h g r a v e l much more e a s i l y t h a n t h r o u g h t h o s e p l u g g e d w i t h c a l c i u m c a r b o n a t e f l u i d - l o s s a d d i t i v e as a consequence o f p e r m e a b i l i t y and p r e s s u r e d r o p cont r a s t , t h e a c i d w i l l bypass t h e c a l c i u m - c a r b o n a t e f i l l e d p e r f o r a t i o n s and t h e r e f o r e not. c o m p l e t e l y remove t h e f i n e p l u g g i n g m a t e r i a l . I t is t h u s f a r b e t t e r t o c i r c u m v e n t t h e need f o r an a c i d j o b a f t e r g r a v e l pack i n s t a l l a t i o n b y r e m o v i n g a l l p r e v i o u s l y i n s e r t e d f l u i d - l e a k o f f a d d i t i v e s by a pre-gravel-pack a c i d t r e a t ment w h i c h p r o v i d e s h i g h e r w e l l p r o d u c t i v i t y and more p o s i t i v e sand c o n t r o l ( o t h e r a s p e c t s o f a c i d i z i n g p r i o r t o and a f t e r g r a v e l pack i n s t a l l a t i o n a r e d i s c u s sed i n s e c t i o n 5 . 1 1 . 4 . ) .
5.3.1.2.2. Solid fluid-loss additives and formation pre-packing The m o s t p r a c t i c a l s o l u t i o n i s t o use a c o m b i n a t i o n o f g r a v e l and v i s c o u s f l u i d s as a d i v e r t i n g s y s t e m f o r t h e a c i d w h i c h h e l p s t o p r e v e n t e x c e s s i v e f l u i d loss ahead o f t h e g r a v e l s l u r r y and p a r t i a l l y p r e p a c k s t h e f o r m a t i o n w i t h g r a v e l . A c i d has t o be d i v e r t e d t o a l l p e r f o r a t i o n s i n o r d e r t o a l l o w f o r comp l e t e f l u i d - l o s s a d d i t i v e r e m o v a l and t o p r e v e n t a c i d bypass t h a t l e a v e s some damaged p a t c h e s i n t h e f o r m a t i o n . The g r a v e l d i v e r t e r s t a g e s s h o u l d be d e s i g n e d such t h a t n o t more t h a n 10 % o f t h e p e r f o r a t i o n s a r e f i l l e d w i t h g r a v e l i n e a c h s t a g e , as t h e e f f e c t o f t h e g r a v e l - f i l l e d p e r f o r a t i o n s on a c i d i n j e c t i o n p r e s s u r e depends on amount o f p e r f o r a t i o n s packed b y g r a v e l , number o f p e r f o r a t i o n s c l e a n e d b y a c i d , s e v e r i t y o f p l u g g i n g o f o t h e r p e r f o r a t i o n s , undamaged i n j e c t i v i t y o f t h e c o m p l e t e d i n t e r v a l , downhole dynamic v i s c o s i t y o f t h e g e l l e d b r i n e , g r a v e l p e r m e a b i l i t y and i n j e c t i o n r a t e . I n j e c t i o n o f caiciurn carbonate l e a k o f f a d d i t i v e s i n t o a g r a v e l pack i s o f t e n necessary t o c o n t r o l excessive f l u i d l o s s w h i l e preparing the w e l l f o r product i o n a f t e r a g r a v e l p a c k o r w h i l e i n s t a l l i n g a g r a v e l pack i n an u p p e r zone, b u t c a n l e a d t o s e r i o u s p r o b l e m s . As t h e s e f l u i d - l e a k o f f a d d i t i v e s a r e o f t e n de-
741 signed such t h a t t h e p a r t i c l e s f l o w through t h e g r a v e l and b u i l d a f i l t e r cake on t h e f o r m a t i o n f a c e a t t h e boundary between r e s e r v o i r sand and p a c k i n g g r a v e l , t h e p o r e s o f t h e g r a v e l i n some o f t h e p e r f o r a t i o n t u n n e l s become p l u g g e d and t h e m a t e r i a l i s v e r y d i f f i c u l t t o remove due t o s e l e c t i v e d i s s o l u t i o n by t h e a c i d f o l l o w e d by a c i d bypassing o f p e r f o r a t i o n s f i l l e d w i t h f i n e - g r a i n e d sol i d s due t o d i f f e r e n c e s i n p r e s s u r e drops. T h e r e f o r e t h e median p a r t i c l e s i z e o f t h e f l u i d - l o s s a d d i t i v e s h o u l d a p p r o x i m a t e l y equal t h e median p o r e s i z e o f t h e g r a v e l which m i n i m i z e s i n v a s i o n o f c a l c i u m carbonate i n t o p e r f o r a t i o n t u n n e l s and f o r m a t i o n and s t i l l a d e q u a t e l y c o n t r o l s f l u i d l o s s r a t e .
5.3.2. Transport and packing capacity o f the slurries T r a n s p o r t e f f i c i e n c y o f h i g h - d e n s i t y g r a v e l pack s l u r r i e s g e n e r a l l y improves w i t h i n c r e a s i n g f l u i d v e l o c i t y , g r a v e l c o n c e n t r a t i o n and a n n u l a r speed (SKAGGS 1984). W i t h i n c r e a s i n g g r a v e l s a t u r a t i o n , l e s s f l u i d v i s c o s i t y i s r e q u i r e d t o m a i n t a i n a g i v e n l e v e l o f t r a n s p o r t e f f i c i e n c y which improves placement by b e t t e r l e a k o f f r a t e s and reduced p r e s s u r e d r o p through t h e p e r f o r a t i o n s w h i l e bei n g packed. T r a n s p o r t e f f i c i e n c y i s p r e d o m i n a n t l y c o n t r o l l e d by s l u r r y c o n s i s tency, w i t h s i m i l a r c o n d i t i o n s b e i n g a b l e t o be achieved w i t h l o w - v i s c o s i t y f l u i d s and h i g h g r a v e l c o n c e n t r a t i o n s o r h i g h - v i s c o s i t y f l u i d s and low g r a v e l s a t u r a t i o n s w i t h i n t h e range o f g r a v e l suspension c a p a c i t y o f t h e d i f f e r e n t f l u i d types, t h e r e b y g i v i n g ample p o s s i b i l i t i e s o f design. The overview as f o l lows i n c l u d e s comments on suspension symmetry, dual c a r r i e r f l u i d v i s c o s i t y , e v o l u t i o n o f g r a v e l p a c k i n g techniques, and g r a v e l volume and placement.
5.3.2.1. Suspension symnetry D i s t i n c t i o n can g e n e r a l l y be made between symmetrical and asymmetrical suspensions f o r s l u r r y d e f i n i t i o n (WAHLMEIER & ANDREWS 1988). Symmetrical suspens i o n s i n which t h e s o l i d s a r e u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t t h e l i q u i d a p p l y f o r s l u r r i e s having extremely high s o l i d concentrations, h i g h v e l o c i t y , f i n e p a r t i c l e s i z e , e l e v a t e d f l u i d v i s c o s i t y , and/or small d e n s i t y d i f f e r e n c e s b e t ween s o l i d s and l i q u i d . Asymmetrical suspensions e x h i b i t a pronounced c o n c e n t r a t i o n g r a d i e n t normal t o f l o w d i r e c t i o n and a r e r e p r e s e n t e d by s l u r r i e s u s u a l l y h a v i n g l o w e r s a t u r a t i o n s and/or c o n t a i n i n g l a r g e r p a r t i c l e s i z e s than those o f symmetrical suspensions. Asymmetrical suspensions r e p r e s e n t i n g heterogeneous f l o w can be s u b d i v i d e d i n t o t h e stages o f f l o w w i t h i n a d i s t o r t e d c o n c e n t r a t i o n p r o f i l e and no p a r t i c u l a t e d e p o s i t , s e d i m e n t a t i o n w i t h i n a moving bed o f p a r t i c u l a t e s , d e p o s i t e d s t a t i o n a r y bed w i t h s a l t a t i o n , and complete s e t t l i n g o f t h e p a r t i c l e s f r o m t h e l i q u i d . The c r i t i c a l o r e q u i l i b r i u m v e l o c i t y o f t h e s l u r r y i s t h e v e l o c i t y f o r g i v e n l i q u i d and/or f l o w c o n d i t i o n s where a bed o f s o l i d s begins t o form.
5.3.2.2. Dual carrier fluid viscosity I n c r e a s i n g g r a v e l pack c a r r i e r f l u i d v i s c o s i t y f a c i l i t a t e s m i x i n g , pumping and t r a n s p o r t o f s l u r r i e s w i t h h i g h g r a v e l c o n c e n t r a t i o n s , improves g r a v e l movement through p e r f o r a t i o n s , s t a b i l i z e s p e r f o r a t i o n c a v i t i e s and t u n n e l s i n t h e f o r m a t i o n , decreases e r o s i o n and screen p l u g g i n g by f i n e s , and i n h i b i t s g r a v i t y s e g r e g a t i o n o f p a r t i c l e s i z e s and s e t t l i n g o f g r a v e l i n t h e s l u r r y (GRUESBECK & COLLINS 1978; SHRYOCK, DUNLOP & MILLHONE 1979; SHRYOCK & MILLHONE 1979). On t h e o t h e r hand, however, i n c r e a s i n g v i s c o s i t y can cause i n c o m p l e t e p a c k i n g due t o d e c r e a s i n g f l u i d l e a k o f f r a t e s and i n c r e a s i n g tendency f o r g r a v e l l a y e r i n g on t h e screen and node b u i l d i n g a t t h e p e r f o r a t i o n s (SHRYOCK 1980, SCHEUERMAN 1984; c f . s e c t i o n 5 . 8 . 7 . ) . As t h e f l u i d v i s c o s i t y needed f o r m i x i n g and pumping o f g r a v e l s l u r r i e s a t t h e s u r f a c e i s h i g h e r t h a n t h a t r e q u i r e d f o r optimum performance downhole, the necessary dual v i s c o s i t y i s a t t a i n e d by a d d i t i o n o f b r e a k e r s t o degrade t h e v i s -
742 c o s i f y i n g p o l y m e r (SCHEUERMAN 1 9 8 4 ) . T h i s d u a l v i s c o s i t y m o d e l l i n g o f g r a v e l p a c k i n g f l u i d s i n o r d e r t o a c h i e v e t h e r e q u i r e d downhole v i s c o s i t y n o t b e f o r e c o n t a c t i n g t h e r e s e r v o i r c a n be t o some e x t e n t compared w i t h d e l a y e d c r o s s l i n k i n g o f f r a c t u r i n g f l u i d s which a l s o provides a u s e f u l combination, although i n r e v e r s e o r i e n t a t i o n , b y h a v i n g l o w e r v i s c o s i t y upon e n t r a n c e t o t h e f l o w l i n e s t h e r e b y r e d u c i n g t u b u l a r f r i c t i o n , and r e a c h i n g h i g h e r v i s c o s i t y downhole t h u s b e i n g a b l e t o suspend and t r a n s p o r t h i g h p r o p p a n t c o n c e n t r a t i o n s i n t o t h e f r a c t u r e ( c f . s e c t i o n 4.3.4.3.).
5.3.2.3. Evolution o f gravel packing techniques I n t h e e a r l y y e a r s o f g r a v e l p a c k i n g s i n c e t h e l a t e 1 9 3 0 ' s when t h i s method became a v i a b l e means o f c o n t r o l l i n g sand p r o d u c t i o n i n o i l and gas w e l l s , g r a v e l was p l a c e d downhole i n t h e i n i t i a l submature phase b y s i m p l e d r y dumping, and t h e n w i t h b e g i n n i n g a m e l i o r a t i o n o f t e c h n i q u e s and a c c u m u l a t i o n o f knowl e d g e and e x p e r i e n c e m a i n l y b y t h e normal c i r c u l a t i o n method w h i c h c o m p r i s e s pumping o f t h e g r a v e l down t h e t u b i n g s t r i n g and t h r o u g h a f l o a t c o l l a r a t t h e b o t t o m o f t h e l i n e r and back u p t h e a n n u l a r space (SOLUM 1 9 8 6 ) . L a t e r r e v e r s e c i r c u l a t i o n , squeeze p a c k i n g and f o u r - p o s i t i o n l o n g s t r o k e o p e r a t i o n w e r e succ e s s f u l l y i m p r o v i n g g r a v e l p l a c e m e n t e f f e c t i v i t y . F o l l o w i n g d i s c u s s i o n o f some g e n e r a l a s p e c t s , some comments a r e g i v e n on normal and r e v e r s e c i r c u l a t i o n p a c k i n g , squeeze p a c k i n g , combined c i r c u l a t i o n and squeeze p a c k i n g , f o u r - p o s i t i o n l o n g s t r o k e p a c k i n g , wash down and p r e s s u r e p a c k i n g , and o t h e r t e c h n i q u e s .
5.3.2.3.1. General aspects A f t e r t h e e a r l y s t r a i g h t f o r w a r d b e g i n n i n g s , sand c o n t r o l t e c h n o l o g y has l a t e r p r o g r e s s i v e l y e v o l v e d w i t h b o t h l o c a l a d a p t a t i o n and i n c r e a s i n g s o p h i s t i c a t i o n (MURPHEY, BILA & TOTTY 1 9 7 4 ) . C l o g g i n g of t h e f l o a t c o l l a r and t h e i n a b i l i t y t o pack t h e l i n e r t o p were m a j o r drawbacks o f t h e normal c i r c u l a t i o n m e t h o d . D u r i n g t h e l a s t decades, t h e r e v e r s e c i r c u l a t i o n t e c h n i q u e was d e v e l o p e d w h i c h c o n s i s t s o f pumping o f t h e g r a v e l down t h e a n n u l a r space and a r o u n d t h e o u t s i d e o f t h e l i n e r . As t h e g r a v e l r e a c h e s i t s s e t t i n g p o i n t , t h e c a r r y i n g f l u i d s w o u l d c o n t i n u e downhole and r e t u r n t o t h e s u r f a c e t h r o u g h a p e r f o r a t e d n i p p l e a t t h e b o t t o m o f t h e l i n e r . E a r l i e r drawbacks such as b r i d g i n g o f f due t o p o o r downhole t o o l s and s u r f a c e p r o c e d u r e s have been overcome d u r i n g c o u r s e o f t h e t e c h n o l o g i c a l e v o l u t i o n , w i t h r e v e r s e c i r c u l a t i o n i n c o m b i n a t i o n w i t h squeeze p a c k i n g b e i n g t h e c u r r e n t l y m o s t f r e q u e n t l y a p p l i e d g r a v e l p l a c e m e n t method ( S O LUM 1 9 8 6 ) . The o r i g i n a l n o r m a l c i r c u l a t i o n t e c h n i q u e has r e c e n t l y been r e v i v e d and u t i l i z e d i n an i m p r o v e d v e r s i o n f o r g r a v e l p a c k i n g o f h i g h l y - d e v i a t e d w e l l s (STILES 1985; STILES, COLOMB & FARLEY 1986; c f . s e c t i o n 5 . 8 . 3 . 5 . ) .
5.3.2.3.2. Normal and reverse circulation packing The s i m p l e s t sand c o n t r o l measure i s i n s t a l l a t i o n o f s c r e e n o r l i n e r a l o n e w i t h o u t g r a v e l , w i t h i n f l u x o f f o r m a t i o n sand i n t o t h e b o r e h o l e t h e n r e s u l t i n g i n b u i l d i n g o f a n a t u r a l g r a v e l pack (HALLIBURTON 1988; c f . s e c t i o n 5 . 2 . 3 . 6 . ) . No i n f l u e n c e c a n b e e x e r t e d on g r a v e l g r a i n s i z e d i s t r i b u t i o n i n s u c h n a t u r a l p a c k s a r i s i n g f r o m p a y c o l l a p s e a l o n g t h e b o r e h o l e w a l l w h i c h i s g i v e n by r e s e r v o i r granulometry, and sand e n t e r i n g t h e w e l l b o r e may cause f o r m a t i o n d i s t u r bance and p e r m e a b i l i t y damage. G r a v i t y p a c k i n g i s t h e m o s t p r i m i t i v e means of g r a v e l i n s e r t i o n where a s c r e e n i s r u n i n t o t h e b o r e h o l e and g r a v e l i s s l o w l y p o u r e d down t h e a n n u l u s . C a r e m u s t be t a k e n t o a v o i d g r a v e l b r i d g i n g a r o u n d t h e t u b i n g c o l l a r s and i t i s v e r y u n l i k e l y t h a t any g r a v e l i s f o r c e d t h r o u g h t h e p e r f o r a t i o n s , w i t h consequently hydrocarbon f l o w i n t o t h e w e l l b o r e being r e s t r i c t e d by f o r m a t i o n f i n e s i n v a d i n g t h e p a c k . C i r c u l a t i n g p a c k s a r e g e n e r a l l y p l a c e d i n a w e l l b y pumping f l u i d s down t h e w o r k s t r i n g t h r o u g h a c r o s s o v e r t o o l and r e t u r n i n g t o t h e s u r f a c e (LEDLOW, SAUER
743
& TILL 1985). A c i r c u l a t i n g pack i s b e s t f o r g r a v e l placement i n t h e screen-cas i n g annulus b u t has poor performance i n p e r f o r a t i o n t u n n e l p l u g g i n g ( c f . sect i o n 5 . 6 . 3 . ) . I t i s t h e r e f o r e c h i e f l y a p p l i e d i n open-hole c o m p l e t i o n s i n v e r t i c a l w e l l s ( c f . s e c t i o n 5.10.2.) where g r a v e l squeezing i n t o p e r f o r a t i o n s i s n o t r e q u i r e d and i n w e l l s where p e r f o r a t i o n p r e p a c k i n g ( c f . s e c t i o n 5.6.7.) has been performed. F o r c i r c u l a t i o n packing, l o w e r t e l l - t a l e screen and s e a l sub a r e e l i m i n a t e d , and an upper t e l l - t a l e screen i s p l a c e d between shear s a f e t y sub and b l a n k p i p e ( c f . s e c t i o n 6 . 3 . 5 . ) . C i r c u l a t i o n passes t h r o u g h t h e p r i m a r y screen u n t i l i t i s covered w i t h g r a v e l and i s t h e n d i v e r t e d t h r o u g h t h e upper t e l l - t a l e screen, w i t h g r a v e l f a l l i n g by g r a v i t y t o f i l l t h e b l a n k l i n e r annul u s u n t i l t h e upper t e l l - t a l e screen i s covered and an a b r u p t p r e s s u r e i n c r e a s e occurs. C i r c u l a t i o n gravel packing i s u s u a l l y c a r r i e d o u t w i t h l o w - v i s c o s i t y f l u i d s w i t h low g r a v e l c o n c e n t r a t i o n s . R e v e r s e - c i r c u l a t i o n p a c k i n g enables t o p l a c e some g r a v e l through t h e p e r f o r a t i o n s by s h u t t i n g o f f t h e r e t u r n s a t t h e s u r f a c e and pumping c o n t i n u o u s l y u n t i l a p r e s s u r e i n c r e a s e i n d i c a t e s t h a t g r a v e l has covered t h e t e l l - t a l e screen. I f no p o s i t i v e p r e s s u r e i s e x e r t e d , o n l y g r a v e l p a c k i n g i n s i d e t h e w e l l b o r e i s l i k e l y t o be achieved. Two o r t h r e e j o i n t s o f uns l o t t e d t u b i n g a r e g e n e r a l l y r u n above t h e screen t o p r o v i d e space f o r g r a v e l t o f i l l t h e annulus above t h e screen which n o r m a l l y p r e v e n t s f l u i d and g r a v e l movement up t h e annulus and p r o v i d e s a r e s e r v e pack i f t h e g r a v e l s e t t l e s ( c f . s e c t i o n s 5.3.2.4.3. and 5 . 8 . 4 . 1 . ) .
5.3.2.3.3.
Squeeze packing
Squeeze p a c k i n g i s s u i t a b l e f o r v e r t i c a l w e l l s w i t h s h o r t p e r f o r a t e d i n t e r v a l s (HALLIBURTON 1988). No t e l l - t a l e screen i s implemented a t a l l , and t h e t o o l i s f i r s t placed i n reversing position, b u t instead o f reversing, the f l u i d s a r e c i r c u l a t e d t a k i n g r e t u r n s above t h e packer. Once t h e f i r s t f l u i d s t o be squeezed i n t o t h e f o r m a t i o n r e a c h a p o i n t j u s t above t h e packer, t h e t o o l i s c y c l e d t o t h e squeeze p o s i t i o n and g r a v e l i s squeezed i n t o t h e f o r m a t i o n u n t i l a p r e d e t e r m i n e d p r e s s u r e i s a t t a i n e d and t h e t o o l i s c y c l e d t o r e v e r s e p o s i t i o n t o remove excess g r a v e l . Squeeze g r a v e l p a c k i n g i s u s u a l l y c a r r i e d o u t w i t h h i g h - v i s c o s i t y f l u i d s w i t h h i g h g r a v e l c o n c e n t r a t i o n s . A squeeze pack i s p l a c e d i n t o a w e l l by u l t i m a t e l y pumping a l l f l u i d s i n t o t h e f o r m a t i o n w i t h no r e t u r n s t o t h e s u r f a c e (LEDLOW, SAUER & T I L L 1985). Squeeze p a c k i n g i s most e f f i c i e n t i n p l a c i n g g r a v e l i n t h e p e r f o r a t i o n t u n n e l s and l e a s t e f f e c t i v e i n p l a c i n g g r a v e l i n t h e screen-casing annulus. Unless c a r r i e r f l u i d v i s c o s i t y i s decreased o r t h e p r e s s u r e d i f f e r e n t i a l i s increased, a s t r a i g h t squeeze pack o f t e n r e s u l t s i n poor g r a v e l placement.
5.3.2.3.4.
Combined c i r c u l a t i o n and squeeze packing
As a consequence o f t h e i r i n v e r s e e f f e c t i v i t y , a c o m b i n a t i o n o f c i r c u l a t i o n and squeeze g r a v e l pack i s o f t e n t h e b e s t s o l u t i o n f o r reasonable p a c k i n g o f b o t h p e r f o r a t i o n t u n n e l s and screen-casing annulus (HALLIBURTON 1988). Gravel i s p l a c e d i n t h e w e l l by c i r c u l a t i n g s l u r r y down u n t i l a s c r e e n o u t i s o b t a i n e d on t h e lower t e l l - t a l e screen, and when t h e s l u r r y crosses t h e screen, r e t u r n s a r e s h u t o f f and f l u i d i s pumped i n t o t h e f o r m a t i o n . Gravel i s c i r c u l a t e d t o t h e lower t e l l - t a l e screen u n t i l an a b r u p t p r e s s u r e i n c r e a s e i n d i c a t e s t h a t t h e lower t e l l - t a l e screen i s covered t h e r e b y r e s t r i c t i n g c i r c u l a t i o n . A t t h i s p o i n t , t h e t o o l i s c y c l e d i n t o squeeze p o s i t i o n and pumping i s c o n t i n u e d u n t i l a p r e d e t e r m i n e d p r e s s u r e i s reached, w i t h pumping proceeding a t t h e h i g h e s t p r a c t i c a l r a t e which a s s i s t s i n p e r f o r a t i o n packing. The c o m b i n a t i o n o f c i r c u l a t i o n and squeeze g r a v e l p a c k i n g i s p a r t i c u l a r l y s u i t a b l e f o r w e l l s w i t h l e s s than 45 f t o f c o m p l e t i o n i n t e r v a l o r l e s s than 45 degrees o f h o l e d e v i a t i o n . The combined method a l s o a l l o w s f o r p r e s s u r e d e p l e t i o n by temporary c i r c u l a t i o n when g e l - o u t o r screenout d u r i n g squeeze p a c k i n g
.
744
i s c r e a t i n g pressure-up c y c l e s and m i m i c k i n g h i g h p r e s s u r e l o s s e s downhole which c o u l d cause premature t e r m i n a t i o n o f t h e j o b .
5.3.2.3.5. Four-posi t ion longstroke packing F o u r - p o s i t i o n l o n g s t r o k e p a c k i n g i s performed w i t h adding an upper c i r c u l a t i o n p o s i t i o n t o t h e combined c i r c u l a t i o n and squeeze t o o l . The f o u r - p o s i t i o n i n s t r u m e n t i s o p e r a t e d by v e r t i c a l w o r k s t r i n g movements i n t o squeeze (no r e turns), l o w e r c i r c u l a t i o n ( r e t u r n s o n l y through l o w e r t e l l - t a l e s c r e e n ) , upper c i r c u l a t i o n ( r e t u r n s t h r o u g h main screen) and r e v e r s e c i r c u l a t i o n (DOWELL SCHLUMBERGER 1988 b ) . S l u r r y i s f i r s t c i r c u l a t e d down t o s c r e e n o u t on t h e l o w e r t e l l - t a l e screen, and t h e n second t h e t o o l i s lowered i n t o squeeze p o s i t i o n which c l o s e s t h e c i r c u l a t i n g p a r t t h e r e b y p r e v e n t i n g r e t u r n s f r o m g o i n g t o t h e s u r f a c e and i n s t e a d o f t h a t i n j e c t i n g i n t o t h e r e s e r v o i r . A f t e r s c r e e n o u t on t h e f o r m a t i o n , t h e i n s t r u m e n t i s r a i s e d t o t h e upper c i r c u l a t i n g p o s i t i o n i n t h e t h i r d stage and t h e r e m a i n i n g s l u r r y i s dehydrated i n t h e s c r e e n - c a s i n g annulus, and a f t e r a l l t h e p o s i t i o n s have been t e s t e d t o v e r i f y a s c r e e n o u t i n each p o s i t i o n , t h e excess s l u r r y l e f t i n t h e t u b i n g i s r e v e r sed o u t . The f o u r - p o s i t i o n l o n g s t r o k e c r o s s o v e r i n s t r u m e n t w i t h r e v e r s i n g b a l l i n p l a c e o f f e r s t h e f l e x i b i l i t y t o s u c c e s s f u l l y g r a v e l pack most w e l l s i n c l u d i n g those w i t h l o n g e r zones and h i g h e r degrees o f h o l e d e v i a t i o n .
5.3.2.3.6. Wash down and pressure packing Wash down p a c k i n g c o n s i s t s o f d e p o s i t i n g g r a v e l t o a p r e d e t e r m i n e d h e i g h t above p e r f o r a t i o n s f o l l o w e d by r u n n i n g screen and l i n e r assembly w i t h a wash p i p e and a c i r c u l a t i n g - t y p e shoe (HALLIBURTON 1988). The screen i s washed down through the gravel, and when t h e shoe i s on bottom, g r a v e ? i s a l l o w e d to f a l l back around screen and l i n e r . T h i s approach has minimum p r o v i s i o n f o r compacti n g t h e g r a v e l i n t h e s c r e e n - c a s i n g annulus and no p r o v i s i o n f o r squeezing g r a v e l through t h e p e r f o r a t i o n s . The w e l l , however, may be pressure-packed p r i o r t o washing down t h e screen, and t h e r e may be some s e g r e g a t i o n o f g r a v e l s i z e s d u r i n g t h e wash down. Pressure g r a v e l p a c k i n g enables t o f i l l t h e p e r f o r a t i o n t u n n e l s w i t h g r a v e l i n o r d e r t o keep them unclogged by f o r m a t i o n sand and f i n e s , t o f i l l v o i d spaces b e h i n d t h e c a s i n g , and t o r e s t r e s s t h e r e s e r v o i r . C i r c u l a t i n g p r e s s u r e pack p e r m i t s s l u r r y i n j e c t i o n down t h e t u b i n g which may p r e v e n t s c o u r i n g o f mud, r u s t , p i p e dope and s c a l e f r o m t h e t u b i n g - c a s i n g annulus t h a t c o u l d damage pack permeability. I n order t o place gravel through the perforations, surface r e t u r n s may be stopped and s l u r r y squeezed a g a i n s t t h e pay zone b e f o r e t h e p e r f o r a t i o n s a r e covered w i t h s l u r r y .
5.3.2.3.7. Other techniques B u l l h e a d o r bradenhead p r e s s u r e p a c k i n g o p e r a t e s w i t h o u t t e l l - t a l e screen o r wash p i p e because no r e t u r n s a r e taken (HALLIBURTON 1988). Pumping p r e s s u r e s i n crease as more p e r f o r a t i o n s a r e covered w i t h g r a v e l , and a f t e r f i n a l sandout, t h e packer i s r e l e a s e d f r o m screen and b l a n k p i p e a t t h e c r o s s o v e r t o o l . Combin a t i o n o f s e v e r a l methods can enhance t h e p r o s p e c t o f a c h i e v i n g a t i g h t u n i f o r m pack w i t h g r a v e l occupying each p e r f o r a t i o n t u n n e l and r e s t r e s s i n g a l l areas o f t h e sand-producing f o r m a t i o n . B u l l h e a d p r e s s u r e p a c k i n g f o l l o w e d by c i r c u l a t i o n p r e s s u r e p a c k i n g can be b e n e f i c i a l l y combined t o f o r m an o u t s t a n d i n g composite design. Reasons f o r u n s a t i s f a c t o r y sand c o n t r o l performance a r e m a i n l y annulus b r i d g i n g , i n t e r m i x i n g o f g r a v e l and f o r m a t i o n sand, and m i x i n g o f b o r e h o l e f l u i d s w i t h t h e g r a v e l p a c k i n g s l u r r y (RENSVOLD & DECKER 1 9 7 8 ) .
745
5.3.2.4. Gravel volume and placement The volume r e q u i r e d f o r a n n u l a r and p e r f o r a t i o n p a c k i n g depends on placement techniques as w e l l as on t h e f a c t whether p r e v i o u s sand p r o d u c t i o n has o c c u r r e d o r n o t i n o l d o r new boreholes, r e s p e c t i v e l y (ECONOMIDES 1986). D i f f e r e n t g r a v e l volumes a r e r e q u i r e d i n c i r c u l a t i o n and squeeze packing, and a r e s e r v e b l a n k s e c t i o n above screen and p e r f o r a t i o n s has t o be i n c l u d e d i n o r d e r t o compensate g r a v e l s e t t l i n g .
5.3.2.4.1. Circulation packing I n case o f s i m p l e c i r c u l a t i o n pack w i t h no i n t e n t i o n o f squeezing g r a v e l i n t o t h e f o r m a t i o n , two times t h e a n n u l a r volume o f screen and b l a n k l i n e r above t h e zone a r e u s u a l l y an adequate g r a v e l volume (ECONOMIDES 1986). I n case o f squeezing, g r a v e l q u a n t i t y can be v e r y d i f f e r e n t depending on f o r m a t i o n char a c t e r i s t i c s , because s c r e e n o u t o r p r e s s u r e - o u t p r e v e n t i n g f u r t h e r i n j e c t i o n i s h i g h l y dependent on r e s e r v o i r p e r m e a b i l i t y . I n s e c t i o n s w i t h c l e a n h i g h l y p e r meable pay sand, g r a v e l s l u r r y d e h y d r a t i o n occurs e a r l y i n t h e squeeze, thereby r e d u c i n g t h e g r a v e l requirement, whereas i n d i r t y l o w - p e r m e a b i l i t y r e s e r v o i r i n t e r v a l s , f r a c t u r i n g may t a k e p l a c e as a consequence o f r e t a r d e d l e a k o f f and and a s c r e e n o u t may have p r e s s u r e b u i l d u p ( c f . s e c t i o n s 5.6.4. and 5.6.6.3.), t o be f o r c e d by r e d u c i n g i n j e c t i o n r a t e s t o a bare minimum. Long zones t a k e l e s s g r a v e l p e r f t t h a n s h o r t i n t e r v a l s , because t h e r e i s more area a v a i l a b l e f o r s l u r r y d e h y d r a t i o n . Long s e c t i o n s w i t h highly-permeable p o r t i o n s a t t h e t o p o f t h e i n t e r v a l may s c r e e n o u t e a r l y due t o d e h y d r a t i o n i n t o t h a t zone, t h e r e b y c a u s i n g b r i d g i n g which p r e v e n t s i n j e c t i o n i n t o t h e l o w e r p a r t .
5.3.2.4.2. Squeeze packing Squeeze placements i n g e n e r a l r e q u i r e more o r l e s s one t i m e t h e a n n u l a r volume o f screen and b l a n k l i n e r above t h e zone p l u s 0 . 5 - 1.5 f t 3 o f p e r f o r a t i o n s (ECONOMIDES 1986). The d i f f e r e n c e s between c i r c u l a t i o n and squeeze packi n g i n terms o f necessary amount o f g r a v e l o f a b t . one t i m e o f a n n u l a r volume o f screen and b l a n k l i n e r can be m a i n l y e x p l a i n e d w i t h more e f f i c i e n t compact i o n o f t h e g r a v e l by squeezing under h i g h e r p r e s s u r e s where p r e d o m i n a n t l y t h e p e r f o r a t i o n s a r e p r o p e r l y f i l l e d and then t h e annulus i s a l s o plugged, whereas c i r c u l a t i n g always i s p r o b l e m a t i c a l c o n c e r n i n g p e r f o r a t i o n f i l l i n g and p a c k i n g d e n s i t y i s l o w e r i n t h e annulus t h a n achieved by squeezing.
5.3.2.4.3. Reserve blank section above screen and perforations A p a r t f r o m p a c k i n g t h e whole l e n g t h o f p e r f o r a t e d i n t e r v a l and s l o t t e d l i n e r o r wire-wrapped screen, a r e s e r v e a n n u l a r s e c t i o n o f g r a v e l above t h e zone has t o be p l a c e d a g a i n s t a b l a n k l i n e r (ECONOMIDES 1986). The r e s e r v e a l l o w s excess g r a v e l t o compensate f o r s e t t l i n g a f t e r screenout o c c u r s ( c f . a l s o s e c t i o n 5.8.4.1.), and i t a l s o p r e v e n t s g r a v e l f l u i d i z a t i o n d u r i n g hydrocarbon product i o n . I n i n j e c t i o n w e l l s , t h e r e s e r v e i s a v a i l a b l e t o r e p l a c e g r a v e l which i s pumped away w h i l e i n j e c t i n g . When g r a v e l i s p l a c e d i n near v e r t i c a l w e l l s by c i r c u l a t i n g , g r a v e l i s c a r r i e d i n a l o w - v i s c o s i t y f l u i d and r e s e r v e g r a v e l i s s t o r e d i n t h e b l a n k p i p e annulus below t h e upper t e l l - t a l e screen and above t h e main screen. I n v e r t i c a l w e l l s , c i r c u l a t i o n p r o v i d e s complete f i l l i n g o f t h e b l a n k p i p e annulus. A f t e r t h e p r i m a r y screen i s covered, c i r c u l a t i o n c o n t i n u e s t h r o u g h t h e upper t e l l - t a l e screen and g r a v e l s e t t l e s o u t u n t i l i t i s covered as w e l l , w i t h r e s e r v e g r a v e l b e i n g s i m p l y a f u n c t i o n o f b l a n k l i n e r l e n g t h and i t s a n n u l a r volume. H i g h - v i s c o s i t y f l u i d s have r e s t r i c t e d f l o w t h r o u g h t h e upper t e l l - t a l e screen and b r i d g i n g occurs b e f o r e g r a v e l placement i s complete. U s i n g h i g h - v i s c o s i t y f l u i d s , r e s e r v e g r a v e l can o n l y be d e r i v e d by s e t t l i n g o f g r a v e l o u t o f
746 the s l u r r y column, w i t h the higher the gravel s a t u r a t i o n , the more gravel being a v a i l a b l e t o form the reserve volume. The length of b l a n k l i n e r required i s a f f e c t e d n o t only by gravel co n cen t r at i o n , b u t a l s o by screen le ngth. I n openhole completions, the open-hole s ect i o n may be considerably l a r g e r a s a consequence of underreaming t h a n the cased hole diameter where the reserve i s locate d . Due t o d i f f i c u l t i e s of placing reserve gravel below the upper t e l l - t a l e screen i n highly-deviated wel l s , continuous screen should extend above the perf o r a t i o n s t o allow i n s e r t i o n of reserve g r av e l.
5.3.3. Gravel settling diminution in the carrier fluid The success of a gravel-pack completion depends on placement of gravel in the p e r f o r a t i o n tunnel area a n d the screen-casing annulus (HUDSON & MARTIN 1988). As wellbore deviation i n cr eas es , g r a v i t a t i o n a l forc e s a n d c a r r i e r f l u i d l i m i t a t i o n s may r e s u l t in gravel s e t t l i n g or duning ( c f . se c tion 5 . 8 . 3 . ) and i n complete packing of much of the p er f o r at i on tunnel area as well as premature screenout of the l i n er - cas i n g annulus. Factors influencing gravel placement e f f e c t i v i t y include f l u i d v i s c o s i t y , f l u i d de nsity, pumping r a t e , gravel s i z e , gravel c o n c e n t r a t i o n , gravel d en s i t y , wellbore deviation a n d downhole assembly. The discussion as follows focusses on pressure drop minimization, decreasing p a r t i c u l a t e d e n s i t y l t r a n s p o r t f l u i d d en s i t y r a t i o , high-density f l u i d s vs. lowd e n si t y p a r t i c l e s , a n d ap p l i cat i o n spectrum of low-density p a r t i c l e s .
5.3.3.1. Pressure drop minimization I n order t o maximize p r o d u ct i v i t y , the pressure drop across the gravel pack f i l t e r must be minimized which r eq u i r es choice of the l a r g e s t gravel p a r t i c l e t h a t prevents movement of the formation material (HUDSON & MARTIN 1988; c f . sectio n 5 . 2 . 3 . 3 . ) . Poor gravel placement with voids i n pe rfora tion tunnels a n d screen-casing annulus decreases well e x p l o i t a b i l i t y . Low produc tivity a s a consequence of i n s u f f i c i e n t gravel i n s e r t i o n i s a common problem e s p e c i a l l y assoc i a t e d with highly-deviated w e l l s , because s e t t l i n g of gravel in the s l u r r y ( c f . s e c t i o n 5 . 8 . 3 . 1 . ) as i t flows down the annulus causes a gravel concentrat i o n g r a d i e n t i n the f l u i d from the high t o the low s i d e of the flow a re a . When the c a r r i e r f l u i d leaking off t h r o u g h the p erfora tions has only low gravel sa tur a t i o n or even does not contain gravel as a consequence of s e t t l i n g , i t i s d i f f i c u l t t o completely pack the p er f o r at i o n s , i f possible a t a l l . Therefore s e t t l i n g of gravel in the c a r r i e r f l u i d must be reduced as much a s possible in order t o improve gravel pack e f f i c i e n c y ( c f . a l s o se c tion 5 . 8 . 3 . 4 . ) . The pressure drop across a gravel pack i s r a t h e r i n s e n s i t i v e t o the annular clearance between casing and screen ( Y I L D I Z & LANGLINAIS 1988) a n d i s a l s o independent of the p er f o r at i o n phasing angle, i n dic a ting t h a t flow from nearby perf o r a t i o n s does not i n t e r f e r e . Shot d en s i ty and pe rfora tion diameter have the g r e a t e s t influence on pressure d r o p , while media anisotropy has only subordinate c o n t r i b u t i o n t o t o t a l pressure d r o p .
5.3.3.2. Decreasing particle density/carrier fluid density ratio Gravel placement i s g en er al l y improved by decreasing the r a t i o of pa rtic ul a t e d e n s i t y t o c a r r i e r f l u i d d en s i t y (HUDSON & MARTIN 1988). Low p a r t i c u l a t e d e n s i t y l c a r r i e r f l u i d d en s i t y r a t i o s a r e obtained using e i t h e r gravel of var io u s type i n high-density b r i n es o r low-density gravel-pack m a te ria ls in commonly used c a r r i e r f l u i d s . Regardless of c a r r i e r f l u i d de nsity, the s e t t l i n g r a t e of the gravel g r a i n s decreases as the p a r t i c l e d e n s i t y l c a r r i e r f l u i d densit y r a t i o decreases. Addition of a v i s c o s i f i e r a l s o reduces the s e t t l i n g r a t e s of gravel g r a i n s a t each p a r t i c l e d e n s i t y l c a r r i e r f l u i d de nsity r a t i o , and th e r e f o r e p a r t i c l e t r an s p o r t improvement can be achieved with e i t h e r lower part i c l e d e n s i t y l c a r r i e r f l u i d d en s i t y r a t i o s or higher f l u i d v i s c o s i t i e s . Par-
747 t i c l e s e t t l i n g i s f u r t h e r i n f l u e n c e d by g r a v e l c o n c e n t r a t i o n i n t h e c a r r i e r f l u i d , w i t h s e t t l i n g r a t e decreasing w i t h increasing p a r t i c l e saturation. Gravel s e t t l i n g i s t h u s reduced w i t h use o f h i g h - v i s c o s i t y c a r r i e r f l u i d s and systems w i t h low p a r t i c l e d e n s i t y l c a r r i e r f l u i d d e n s i t y r a t i o s . H i g h - v i s c o s i t y c a r r i e r f l u i d s , however, do n o t p r o v i d e t h e suspension p r o p e r t i e s r e q u i r e d i n s u r f a c e m i x i n g , pumping equipment o r under downhole c o n d i t i o n s . Polymer conc e n t r a t i o n s r e q u i r e d t o y i e l d h i g h v i s c o s i t y l e a d t o i m p a i r e d f l u i d l e a k o f f and f a l s e screenouts as w e l l as r e d u c t i o n i n f o r m a t i o n p e r m e a b i l i t y . The low p a r t i c l e d e n s i t y l c a r r i e r f l u i d d e n s i t y system u t i l i z e s buoyancy f o r c e s t o c o u n t e r t h e g r a v i t a t i o n a l f o r c e s on t h e g r a v e l p a r t i c u l a t e r a t h e r t h a n h i g h v i s c o s i t y t o reduce p a r t i c l e s e t t l i n g (HUDSON & MARTIN 1988). Because t h e polymer s a t u r a t i o n can be reduced, i t does n o t have t h e disadvantages of t h e h i g h polymer conc e n t r a t i o n / h i g h v i s c o s i t y system. Another a l t e r n a t i v e a r e s u r f a c t a n t - b e a r i n g wat e r - b a s e d h i g h - v i s c o s i t y f l u i d systems which a r e c h a r a c t e r i z e d by h i g h f l u i d l e a k o f f r a t e s and h i g h g r a v e l suspension and t r a n s p o r t c a p a c i t y (DOWELL SCHLUMBERGER 1988 a, NEHMER 1988; c f . s e c t i o n 5 . 3 . 1 . 1 . 2 . ) .
5.3.3.3. H igh-densi ty fluids vs . low-dens i ty particles Low p a r t i c l e d e n s i t y l c a r r i e r f l u i d d e n s i t y r a t i o s can be o b t a i n e d u s i n g e i t h e r standard gravel material i n high-density b r i n e f l u i d s o r low-density part i c l e s i n commonly a p p l i e d c a r r i e r f l u i d s . The use o f h i g h - d e n s i t y f l u i d s i n t r o duces a d d i t i o n a l drawbacks, because t h e overbalance c r e a t e d by these f l u i d s can l e a d t o severe f l u i d l o s s and w e l l c o n t r o l problems. A g r a v e l pack system design u t i l i z i n g conventional f l u i d s c a r r y i n g low-density p a r t i c u l a t e material t h e r e f o r e improves p a r t i c l e t r a n s p o r t w i t h o u t t h e problems a s s o c i a t e d w i t h h i g h polymer l o a d i n g o r h i g h - d e n s i t y f l u i d s , whereas w i t h c o n v e n t i o n a l g r a v e l , t h e r e q u i r e d f l u i d d e n s i t y r e s u l t s i n h i g h s l u r r y d e n s i t y and h i g h h y d r o s t a t i c p r e s sures. Concerning h i g h l y - d e v i a t e d w e l l b o r e s , HUDSON & MARTIN (1988) t h e r e f o r e recommend t h e use o f l i g h t p a r t i c l e s o f p r e f e r a b l y 1.5 - 1.8 g/cm3 s p e c i f i c d e n s i t y as a replacement f o r s i z e d g r a v e l t o y i e l d t h e r e q u i r e d p a r t i c l e d e n s i t y / c a r r i e r f l u i d d e n s i t y r a t i o when combined w i t h c o n v e n t i o n a l c a r r i e r f l u i d s . E x p e r i mental work has shown t h a t under these c o n d i t i o n s , complete p a c k i n g o f t h e p e r f o r a t i o n t u n n e l s a l s o a t t h e upper s i d e o f t h e h i g h l y - d e v i a t e d b o r e h o l e s can be achieved ( c f . a l s o s e c t i o n 5 . 8 . 3 . 4 . ) . The low s p e c i f i c g r a v i t y o f t h e u l t r a l i g h t g r a v e l p a c k i n g m a t e r i a l i s achieved by c o a t i n g an i n n e r c o r e w i t h p l a s t i c , w i t h t h i s c o m p o s i t i o n h a v i n g v e r y low a c i d s o l u b i l i t y and p r o v i d i n g good p e r m e a b i l i t y a t 4,000 p s i c l o s u r e s t r e s s .
5.3.3.4. App 1 icat ion spectrum o f low-dens i ty part ic les The h i g h c o s t o f t h e u l t r a l i g h t g r a v e l p a c k i n g m a t e r i a l (HUDSON & MARTIN 1988) r e s t r i c t s i t s a p p l i c a t i o n t o h o r i z o n t a l and h i g h l y - i n c l i n e d b o r e h o l e s where t h e achieved maintenance o f a n n u l a r f l o w by low f r i c t i o n p r e s s u r e d r o p and improved p a c k i n g d e n s i t y pays o f f , whereas t h e m a t e r i a l i s t o o expensive and does n o t have s u f f i c i e n t c l o s u r e s t r e s s r e s i s t i v i t y f o r u t i l i z a t i o n i n v e r t i c a l - h o l e g r a v e l p a c k i n g and h y d r a u l i c f r a c t u r i n g where ( i n c o n t r a s t t o o n l y l i t t l e amounts r e q u i r e d f o r g r a v e l p a c k i n g ) l a r g e r q u a n t i t i e s o f m a t e r i a l a r e needed. I n a d d i t i o n , p r o p e r d e p o s i t i o n o f t h e u l t r a l i g h t m a t e r i a l i n s a t i s f a c t o r y c o n c e n t r a t i o n s and p a c k i n g d e n s i t y can o n l y be achieved by v e r y h i g h f l u i d l o s s r a t e s which a r e d e s i r a b l e i n g r a v e l packing, b u t unacceptable i n h y d r a u l i c f r a c t u r i n g where e x c e s s i v e f l u i d l e a k o f f a u t o m a t i c a l l y l e a d s t o premature screenout f a i l u r e o f t h e t r e a t m e n t due t o proppant b r i d g i n g and s l u r r y s t i c k i n g as a consequence o f f l u i d d e h y d r a t i o n ( c f . s e c t i o n s 4.2.2.2.3. and 6 . 2 . 4 . 2 . 1 . ) . I n c r e a s i n g s l u r r y d e n s i t y i n c r e a s e s f r i c t i o n p r e s s u r e which l e a d s t o b r i d g i n g a t t h e end o f t h e screen. Buoyant l o w - d e n s i t y g r a v e l pack m a t e r i a l , however, does n o t a l l o w g r a v i t y s e t t l i n g o f t h e s o l i d p a r t i c l e s , w i t h t h e r e f o r e p a c k i n g
o n l y b e i n g a b l e t o be a c h i e v e d t h r o u g h f l u i d l e a k o f f o r s t a g e d t r e a t m e n t s w i t h s e c t ion-1.r ise p 1acemen t .
5.3.4. Gel surface filtration from polymer solutions G r a v e l p a c k i n g w i t h v i s c o u s aqueous p o l y m e r s o l u t i o n s a l l o w s r a p i d p l a c e m e n t w i t h s u s p e n s i o n s o f h i g h s o l i d s c o n c e n t r a t i o n s (TORREST & MARCANO 1 9 8 6 ) . H i g h v i s c o s i t y f l u i d s g e n e r a l l y f a c i l i t a t e i n f i l l i n g o f p e r f o r a t i o n s and p l a c e m e n t o f g r a v e l d e p o s i t s beyond t h e p e r f o r a t i o n s w h i l e m i n i m i z i n g m i x i n g w i t h f o r m a t i o n sand. V i s c o u s aqueous p o l y m e r s o l u t i o n s , however, c r e a t e a r e l a t i v e l y i m p e r m e a b l e s u r f a c e l a y e r w h i l e f l o w i n g t h r o u g h p o r o u s m e d i a s u c h as g r a v e l pack and f o r m a t i o n sand, g i v i n g r i s e t o anomalous p r e s s u r e d r o p s and i n l e t b u i l d - u p which i s n o t removable w i t h subsequent w a t e r f l u s h i n g . Surface l a y e r g e n e r a t i o n i s m a i n l y due t o m i c r o g e l p l u g g i n g o f l a r g e p o r e d i a m e t e r s a t l o w c u r r e n t r a t e s , w i t h a b t . 4 % o f t h e t o t a l p o l y m e r volume c o n t r i b u t i n g t o t h e b u i l d - u p o f t h e g e l l a y e r . The p r e s s u r e d r o p f o r w a t e r f l o w t h r o u g h t h i s l a y e r i s l i n e a r i n c u r r e n t r a t e a t l e a s t d u r i n g t h e i n i t i a l s t a g e s , and t h e g e l l a y e r s once f o r m e d c a n u n d e r g o changes i n c h a r a c t e r i s t i c s w i t h w a t e r f l o w a t s u f f i c i e n t r a t e s . S c r e e n o u t m i m i c k i n g b y s u r f a c e l a y e r - i n d u c e d p r e s s u r e r i s e and o t h e r a s p e c t s a r e b r i e f l y d i s c u s s e d as f o l l o w s .
5.3.4.1. Screenout mimicking by face layer-induced pressure rise The s i g n i f i c a n c e o f s u r f a c e i m p a i r m e n t and r e t e n t i o n i s t h a t u n e x p e c t e d b u t s u b s t a n t i a l f i l t r a t i o n a t t h e f o r m a t i o n f a c e w i t h v i s c o u s p o l y m e r s o l u t i o n s dur i n g g r a v e l p a c k i n g i s m a n i f e s t e d l a r g e l y as h i g h p r e s s u r e s w h i c h i s c l o u d i n g t h e i n t e r p r e t a t i o n o f p l a c e m e n t b e h a v i o u r and a f f e c t e d g r a v e l pack q u a l i t y (TORREST & MARCANO 1 9 8 6 ) . As p r e s s u r e i n c r e a s e s o f t e n a s s o c i a t e d w i t h p r e m a t u r e s c r e e n o u t d u r i n g p a c k i n g c o u l d t h u s a l s o be caused b y t h e r a p i d b u i l d - u p o f a g e l s u r f a c e l a y e r a t t h e f o r m a t i o n f a c e , m i s t a k i n g such f a l s e s i g n a l s as i n d i c a t o r f o r s c r e e n o u t t e r m i n a t i o n and based on t h i s d e c i s i o n b e i n g made t o s t o p pumping l e a v e s t h e f l o w l i n e s f u l l o f g r a v e l and t h e r e f o r e t h e pack i s b u i l d i n g i n a way v e r y d i f f e r e n t f r o m t h a t o r i g i n a l l y i n t e n d e d . R a p i d m i c r o g e l l a y e r b u i l d - u p w i t h p l a c e m e n t o f v e r y s m a l l s o l u t i o n volumes c o u l d t h e r e f o r e a c t as a p r o f i l e c o n t r o l d u r i n g g r a v e l p a c k i n g w i t h c o n c e n t r a t e d p o l y m e r s o l u t i o n s i n t h e w e l l b o r e f l o w i n g t h r o u g h t h e p e r f o r a t i o n s and i n t o t h e f o r m a t i o n . I f t h e r i s i n g i n j e c t i o n p r e s s u r e s a r e e r r o n e o u s l y i n t e r p r e t e d as i n d i c a t i n g s c r e e n o u t t e r m i n a t i o n l o n g b e f o r e t h e g r a v e l p a c k has a c t u a l l y f o r med, s t o p p i n g o f t h e j o b based o n r a p i d p r e s s u r e b u i l d - u p d u r i n g g r a v e l p l a c e ment l e a d s t o i n c o m p l e t e g r a v e l pack g e n e r a t i o n and r e d u c e d g r a v e l p a c k q u a l i t y as a consequence o f f a i l u r e t o a c c o u n t f o r t h e e f f e c t o f s u r f a c e g e l f i l t r a t i o n . I t i s t h e r e f o r e e s s e n t i a l t o be a b l e t o r e c o g n i z e such s c r e e n o u t m i m i c k i n g caused b y s u r f a c e g e l l a y e r d e p o s i t i o n and t o c o n t i n u e pumping o f g r a v e l - s a t u r a t e d s l u r r y u n t i l t h e r e a l end o f t h e j o b .
5.3.4.2. Other aspects On t h e o t h e r hand, m a i n t e n a n c e o f h i g h pumping p r e s s u r e s c o u l d g i v e r i s e t o f r a c t u r i n g o f t h e r e s e r v o i r and s t r o n g l y u n i f o r m g r a v e l p l a c e m e n t p r e d o m i n a n t l y w i t h i n t h e p r e s s u r e - p a r t e d c r a c k ( c f . s e c t i o n s 5 . 3 . 5 . and 5 . 6 . 4 . 2 . ) . A l t h o u g h v i s c o s i t y b r e a k e r s c a n i n p r i n c i p l e be chosen t o a l l o w pumping away o f m a t e r i a l t o the formation, t h e f i n e t u n i n g needed t o r e t a i n h i g h v i s c o s i t i e s f o r c o n c e n t r a t e d s u s p e n s i o n s and l o w v i s c o s i t i e s i n t o t h e r e s e r v o i r i s n o t a l w a y s p o s s i b l e . I n view o f t h e apparent successes o f g r a v e l packing w i t h viscous aqueous p o l y m e r s o l u t i o n s , t h e k i n d s o f b e h a v i o u r caused b y g e l s u r f a c e f i l t r a t i o n and l a y e r g e n e r a t i o n s u g g e s t t h a t o t h e r f a c t o r s i n c l u d i n g more c a r e f u l c o m p l e t i o n s and c h o i c e s f o r a p p l i c a t i o n as w e l l as g e n e r a l improvement i n f l u i d p r e p a r a t i o n and c l e a n l i n e s s may be i m p o r t a n t c o n t r i b u t i o n s .
749 I n c o n t r a s t t o g e n e r a t i o n o f t h e s u r f a c e g e l l a y e r , a d s o r p t i o n f r o m polymer molecules i n s o l u t i o n o r m o l e c u l a r entanglement i n c o n c e n t r a t e d s o l u t i o n s a r e n o t s i g n i f i c a n t f a c t o r s i n gravel packing applications i n c o n t r a s t t o t h e i r part i a l l y c o n s i d e r a b l e importance i n h y d r a u l i c proppant f r a c t u r i n g ( c f . s e c t i o n 4.3.4.). D i s c u s s i o n o f aspects o f polymer a d s o r p t i o n a r e a l s o c a r r i e d o u t by CHAUVETEAU (1981), GRAMAIN & MYARD (1981) and COHEN & METZNER (1982). CHAUVETEAU (1981) comments on hydrodynamic r e t e n t i o n , and BURCIK (1969), KOHLER & CHAUVETEAU (1981) and CHAUVETEAU & KOHLER (1984) i l l u s t r a t e m i c r o g e l p l u g g i n g .
5.3.5. Combination of gravel packing and proppant fracturing Viscous g r a v e l pack c a r r i e r f l u i d s have an i n c r e a s e d d r a g e f f e c t on b o t h g r a v e l and f o r m a t i o n sand, a l l o w i n g b e t t e r r e s t r e s s i n g o f t h e r e s e r v o i r , o r i g i n o f a more t i g h t l y packed g r a v e l mantle, and t o h e l p pushing f o r m a t i o n sand o u t o f t h e p e r f o r a t i o n t u n n e l s . Knowledge o f t h e q u a l i t y o f t h e f o r m a t i o n hydrocarbonb e a r i n g sand enables p r e d i c t i o n o f t h e pay zone a c c e p t a b i l i t y f o r g r a v e l (GURLEY, COPELAND & HENDRICK 1977). D i r t y r e s e r v o i r s c o n t a i n i n g l a r g e r amoul;ts o f s i l t and c l a y almost always t a k e up more g r a v e l b e f o r e s c r e e n o u t than p r e v i o u s l y expected and planned, as t h e p e r m e a b i l i t i e s o f these l o w - m a t u r i t y sands a r e poor and t h e f l u i d c a r r y i n g t h e g r a v e l w i l l n o t l e a k o f f a t a h i g h r a t e , w i t h t h e f o r m a t i o n t h u s becoming f r a c t u r e d and t h e g r a v e l b e i n g t r a n s p o r t e d away f r o m t h e w e l l b o r e i n t o t h e p r e s s u r e - p a r t e d c r a c k i n o r d e r t o p r o p i t i n an unwanted h y d r a u l i c f r a c t u r i n g o p e r a t i o n u n l e s s an a r t i f i c i a l s c r e e n o u t i s t r i g g e r e d by d e c r e a s i n g pumping r a t e s . I n v e r s e l y , c l e a n and h i g h l y - p e r m e a b l e r e s e r v o i r s a r e c h a r a c t e r i z e d by r a p i d f l u i d l e a k o f f and q u i c k s c r e e n o u t a f t e r i n j e c t i o n o f o n l y small amounts o f g r a v e l , and consequently a low p r o b a b i l i t y o f t r i g g e r i n g t h e opening o f a h y d r a u l i c f r a c t u r e d u r i n g t h e g r a v e l pack operation. Pressure p a r t i n g w i t h opening o f a h y d r a u l i c a l l y - i n d u c e d f r a c t u r e can a l s o o c c u r i n v a r i o u s s i t u a t i o n s o f g r a v e l p r e p a c k i n g o f p e r f o r a t i o n s independent f r o m whether t h e f o r m a t i o n has produced sand p r i o r t o g r a v e l p r e p a c k i n g o r n o t , and whether p e r f o r a t i o n washing o r s u r g i n g i s c a r r i e d o u t , w i t h t h e o n l y cont r o l l i n g parameter b e i n g o p e r a t i n g p r e s s u r e (PENBERTHY 1985; c f . s e c t i o n s 5.6.4.2. and 5 . 6 . 6 . ) . Advantages o f combined g r a v e l p a c k i n g and f r a c t u r i n g as w e l l as f o r m a t i o n s t r e n g t h s u p p o r t by h y d r a u l i c f r a c t u r i n g a l o n e a r e b r i e f l y discussed as f o l l o w s .
5.3.5.1. Advantages o f combined gravel packing and fracturing I n some cases, i t may be d e s i r a b l e f r o m r e s e r v o i r e n g i n e e r i n g p o i n t o f view t o c a r r y o u t a combined h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g t r e a t m e n t i n one t r i p by p r o v o k i n g excess o f t h e f r a c t u r e g r a d i e n t , opening o f t h e format i o n , p l u g g i n g o f t h e c r a c k by proppants o r g r a v e l , and t h e n t r i g g e r i n g a t i p screenout by l o w e r i n g pumping r a t e s i n o r d e r t o s t o p f u r t h e r p r o p a g a t i o n o f t h e f r a c t u r e , p r o p i t c o m p l e t e l y f r o m t h e w i n g t i p s up t o t h e w e l l b o r e , f i l l t h e p e r f o r a t i o n t u n n e l s w i t h g r a v e l i n cased holes, and subsequently l i n e t h e boreh o l e w a l l a l o n g t h e r e s e r v o i r i n open h o l e s o r t h e p e r f o r a t e d c a s i n g f a c e w i t h t h e g r a v e l pack m a n t l e . The d i s c u s s i o n as f o l l o w s focusses on adequate s t a b i l i z a t i o n o f l o o s e sands and c h a l k s , p r o p e r i n f i l l i n g o f p e r f o r a t i o n t u n n e l s and cement sheath d e f e c t s , and o p e r a t i o n a l sequence o f h y d r a u l i c p r o p p a n t f r a c t u r i n g f o l l o w e d by g r a v e l p a c k i n g .
5.3.5.1.1. Adequate stabilization o f loose sands and chalks Combined s m a l l - s c a l e h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g t r e a t m e n t s o f c o n t r o l l e d and n o t random o r a c c i d e n t a l t e c h n i c a l performance a r e v e r y import a n t i n v a r i o u s l o w - p e r m e a b i l i t y u n s t a b l e f o r m a t i o n s (such as s h e l l d e b r i s ,
750 c h a l k and s i l t y s a n d s ) and w i l l become i n c r e a s i n g l y i m p o r t a n t i n t h e n e a r f u t u r e w i t h p r o g r e s s i v e a p p r o a c h o f more and more m a r g i n a l r e s e r v o i r s ( c f . a l s o s e c t i o n 4 . 5 . 4 . 3 . ) . S t a b i l i z a t i o n o f l o o s e sands and c h a l k s c a n e v e n be enhanced by t a i l i n g - i n o f resin-coated gravel i n the terminal stage o f t h e h y d r a u l i c f r a c t u r i n g t r e a t m e n t ( c f . s e c t i o n 4.12.3.3.2.) and a l s o p e r f o r m i n g t h e g r a v e l pack w i t h r e s i n - c o a t e d g r a i n s ( c f . s e c t i o n s 4 . 5 . 4 . 3 . and 5 . 8 . 2 . ) . H y d r a u l i c f r a c t u r i n g o f t h e p a y zone t o be g r a v e l packed has r e p e a t e d l y o c c u r r e d d u r i n g sand c o n t r o l t r e a t m e n t s , as i n d i c a t e d by t h e c o n s u m p t i o n o f unu s u a l l y h i g h volumes o f g r a v e l i n some w e l l s and a l s o b y c o n s i d e r a b l y i n c r e a s e d p o s t - o p e r a t i o n a l p r o d u c t i o n r a t e s (SPARLIN & COPELAND 1 9 7 2 ) . The e x p e r i e n c e has shown t h a t i f f r a c t u r i n g h a s t a k e n p l a c e d u r i n g g r a v e l pack j o b s , i t has appar e n t l y n o t r e d u c e d t h e e f f e c t i v i t y o f t h e sand c o n t r o l measure, t h u s c o n t r a d i c t i n g t h e e a r l i e r f e a r t h a t i n case o f f r a c t u r i n g , t h e g r a v e l w o u l d a l l go i n t o one p l a c e b y p l u g g i n g t h e c r a c k and w o u l d n o t a l l o w a p r o p e r s o l u t i o n o f t h e sand p r o b l e m . A s p e c t s o f combined h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a r e a l s o d i s c u s s e d b y POOLLEN & MALONE ( 1 9 5 9 ) and SOLUM ( 1 9 8 6 ) .
5.3.5.1.2. Proper infilling of perforation tunnels and cement sheath defects LEDLOW, SAUER & T I L L ( 1 9 8 5 ) p r e s e n t a p o s i t i v e assessment o f h y d r a u l i c f r a c t u r i n g during gravel packing operations. I n p a r t i a l l y depleted reservoirs, o f t e n t h e h y d r o s t a t i c p r e s s u r e g e n e r a t e d b y a column o f g r a v e l p a c k s l u r r y i s a l r e a d y s u f f i c i e n t t o exceed f o r m a t i o n f r a c t u r e p r e s s u r e (CIRIGLIANO & LEIBACH 1967, PENBERTHY & COPE 1979, TRAHAN & SPIES 1 9 8 1 ) . A c h i e v e m e n t o f p a y zone f r a c t u r i n g , i n f i l l i n g o f t h e c r a c k w i t h g r a v e l and s u b s e q u e n t l y p a c k i n g p e r f o r a t i o n s and a n n u l u s w i t h g r a v e l i s i n many cases t h e o n l y p o s s i b i l i t y t o o b t a i n a good g r a v e l p a c k w h i c h p r o p e r l y i n f i l l s a l l t h e p e r f o r a t i o n s and v o i d s and s e t s a d e c e n t i n s i d e m a n t l e . The b e n e f i t o f h y d r a u l i c f r a c t u r i n g and - f o l l o w i n g f o r m a t i o n breakdown - c o m p l e t e f i l l i n g o f t h e w e l l b o r e w i t h g r a v e l i s t h a t i f pumpi n g i s s t o p p e d t o o e a r l y , t h e f r a c t u r e c l o s e s and t h e b o r e h o l e i s n o t f u l l o f sand because e q u i l i b r i u m b e d h e i g h t has n o t s u f f i c i e n t l y r i s e n , t h e i n c o m p l e t e l y f i l l e d w e l l b o r e e x h i b i t s l i t t l e o r no s t i m u l a t i o n and i s s u s c e p t i b l e t o s a n d production. Even f r a c t u r i n g i n t o an u n d e r l y i n g w a t e r - b e a r i n g zone i s more p o s i t i v e t h a n n e g a t i v e , because t h e cement s h e a t h i n many w e l l s i s d e f e c t i v e and o f i n s u f f i c i e n t q u a l i t y , and i f a p r e s s u r e d i f f e r e n t i a l b r e a k s down t h e cement and c r e a t e s a c h a n n e l , t h e o n l y p o s s i b i l i t y t o p l u g a l l t h e v o i d s i n b o t h f o r m a t i o n and cement m a n t l e i s t o i n f i l l them w i t h g r a v e l u n t i l t e r m i n a l s c r e e n o u t i n d i c a t e s t h a t maximum p a c k i n g has been a c h i e v e d . E x c e e d i n g f r a c t u r e p r e s s u r e does n o t prevent gravel screenout, b u t i n f a c t leads t o higher screenout pressures which t o g e t h e r w i t h mu1 t i p l e p r e s s u r e c y c l e s d e h y d r a t e g r a v e l pack s l u r r i e s b e t t e r .
5.3.5.1.3. Operational sequence of hydraulic followed by gravel packing
proppant fracturing
C o n c e r n i n g o p e r a t i o n a l sequence, e i t h e r t h e h y d r a u l i c f r a c t u r i n g t r e a t m e n t o r t h e g r a v e l p a c k i n g j o b c a n be c a r r i e d o u t f i r s t . S t a r t i n g w i t h h y d r a u l i c f r a c t u r i n g i n new w e l l s has t h e a d v a n t a g e t h a t a c o n t i n u o u s g r a v e l m a n t l e c a n be i n s t a l l e d l a t e r w h i c h t h e n i s no l o n g e r i n t e r s e c t e d b y p e r f o r a t i o n s h o o t i n g and f r a c t u r e e x t e n s i o n . I n o l d w e l l s w h i c h have a l r e a d y been e q u i p p e d w i t h a g r a v e l pack, h y d r a u l i c f r a c t u r i n g s t i m u l a t i o n i n a more advanced s t a g e o f d e p l e t i o n r e q u i r e s c r o s s i n g o f t h e g r a v e l pack b y b o t h p e r f o r a t i o n s and p r o p a g a t i n g hydraulic fracture. te,
As new p e r f o r a t i o n h o l e s have t o be s h o t f o r a l l o w i n g t h e f r a c t u r e t o i n i t i a optimum c o m p l e t i o n i n c l u d e s p e r f o r m a n c e o f renewed g r a v e l p a c k i n g f o r t h e
751 purpose o f p l u g g i n g o f t h e p e r f o r a t i o n h o l e s which were newly s h o t f o r p r e p a r a t i o n o f t h e f r a c t u r e j o b ( a s p e c t s o f r e p e r f o r a t i o n and r e - g r a v e l p a c k i n g a r e mentioned by BONOMO & YOUNG 1983). T h e r e f o r e t h e i d e a l sequence i s s t a r t i n g w i t h h y d r a u l i c proppant f r a c t u r i n g and, upon t e r m i n a t i o n o f i n f i l l i n g o f t h e crack w i t h a t i g h t p r o p p a n t package, c o n t i n u i n g w i t h p l u g g i n g o f p e r f o r a t i o n t u n n e l s and s e t t i n g o f t h e i n s i d e g r a v e l pack m a n t l e . F i e l d examples o f hydraul i c f r a c t u r i n g i n o l d e r w e l l s equipped w i t h an e a r l i e r g r a v e l pack a r e r e p o r t e d by DARR, BROWN & MURPHY (1985) and DARR & CARLTON (1988). Aspects o f combinat i o n s o f h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a r e a l s o o u t l i n e d by SOLUM (1986).
5.3.5.2. Formation strength support by proppant fracturing alone C r e a t i o n o f a c o n d u c t i v e p a t h some d i s t a n c e i n t o t h e r e s e r v o i r f o r m a t i o n by means of a propped h y d r a u l i c f r a c t u r e i s a l s o w i t h o u t c o m b i n a t i o n w i t h g r a v e l p a c k i n g a s u i t a b l e means o f s u p p o r t i n g f o r m a t i o n s t r e n g t h maintenance o r even a m e l i o r a t i o n o f r e s e r v o i r s t a b i l i t y as a consequence o f r e d u c t i o n o f d r a g o r f r i c t i o n a l f o r c e s on t h e pay zone by i n c r e a s i n g f l o w area (ALLEN & ROBERTS 1982). S m a l l - s c a l e h y d r a u l i c f r a c t u r i n g t o g e t h e r w i t h i m p r o v i n g p e r f o r a t i o n geom e t r y and d i s t r i b u t i o n can a l r e a d y e f f e c t i v e l y reduce an o t h e r w i s e s e r i o u s sand c o n t r o l problem and t h u s i n some cases h e l p t o a v o i d t h e n e c e s s i t y o f g r a v e l packing, t h e r e b y l e a v i n g a f u l l open w e l l b o r e and n o t g i v i n g r e s i s t a n c e t o f l u i d f l o w t h r o u g h p e r f o r a t i o n t u n n e l p l u g s and borehole-seaming g r a v e l m a n t l e . Such f r a c t u r e p a c k i n g can be e f f e c t i v e f o r stacked t h i n r e s e r v o i r i n t e r v a l s , i n c l i n e d h o l e s and u n s t a b l e w e l l s t h a t would be d i f f i c u l t t o g r a v e l pack (WEISSENBURGER, MORITA, MARTIN & WHITFILL 1987). F r a c t u r e packing, however, r e q u i r e s sol i d f o r m a t i o n a d j a c e n t t o t h e p o o r l y - c o n s o l i d a t e d zones o f i n t e r e s t , and t h i n s o f t l a y e r s a r e b e t t e r s u i t e d because o f t h e r e l a t i v e l y h i g h f l o w r e s t r i c t i o n i n the f r a c t u r e .
5.4. Reservoir stability and fluid dynamics R e s e r v o i r s t a b i l i t y and f l u i d dynamics a r e m u t u a l l y depending f a c t o r s . The f o l l o w i n g overview demonstrates how changing f l u i d dynamics and c o m p o s i t i o n can a f f e c t r e s e r v o i r c o n s i s t e n c y and how decreasing f o r m a t i o n p r e s s u r e and s h i f t i n g h y d r o c a r b o n l w a t e r r a t i o s can c r e a t e sand c o n t r o l problems i n p r e v i o u s l y s t a b l e pay zones. Comments a r e a l s o g i v e n on r o c k mechanics, p e r f o r a t i o n t u n n e l morphol o g y , and w a t e r c u t i n t h e hydrocarbons. Some d i f f e r e n c e s between h y d r a u l i c f r a c t u r i n g and g r a v e l p a c k i n g a r e a l s o o u t l i n e d .
5.4.1. Formation pressure and fluid c m o s i t i o n I n terms o f e x e c u t i o n o f g r a v e l pack t r e a t m e n t s d u r i n g course o f t h e product i o n h i s t o r y o f p r e d o m i n a n t l y o i l and s u b o r d i n a t e l y a l s o gas w e l l s , t h r e e cases have t o be d i s t i n g u i s h e d . The s u i t e o f p o s s i b i l i t i e s comprises p r i m a r i l y n o t enough c o h e s i v e pay zones as w e l l as s e c o n d a r i l y u n s t a b l e hydrocarbon-bearing s e c t i o n s due t o r e s e r v o i r p r e s s u r e d e c l i n e , and w a t e r c u t i n c r e a s e and c o r r e s ponding f l u i d v i s c o s i t y decrease. The spectrum o f cases i s summarized i n a d i s c u s s i o n o f i n s u f f i c i e n t c o n s o l i d a t i o n and d e c l i n i n g r e s e r v o i r pressure, and changing f l u i d c o m p o s i t i o n and v i s c o s i t y as f o l l o w s . Some comments a r e a l s o o f f e r e d on achievement o f t h e c r i t i c a l stage o f f o r m a t i o n c o l l a p s e .
5.4.1.1, Insufficient consolidation and declining pay pressure The f i r s t group o f r e s e r v o i r s comprises c o m p l e t e l y u n c o n s o l i d a t e d sands and c h a l k s which r e q u i r e s t a b i l i z a t i o n by g r a v e l p a c k i n g from t h e v e r y b e g i n n i n g o f p r o d u c t i o n . The main pay f o r m a t i o n s i n t h i s group a r e s i l t y and a r g i l l a c e o u s , m a i n l y medium- t o f i n e - g r a i n e d sands as w e l l as s o f t f l o w i n g c h a l k s and diatoma-
752 ceous e a r t h s w h i c h a c c o r d i n g t o t h e i r f i n e g r a i n s i z e , i n s u f f i c i e n t b i n d i n g nat u r e and l a c k i n g cementation have n o t enough i n t e r n a l cohesion t o be s a t i s f a c t o r i l y s t a b l e w i t h o u t a r t i f i c i a l improvement. Some l o o s e s c h i l l carbonates a l s o belong t o t h i s d i v i s i o n o f r e s e r v o i r t y p e s . The second assemblage i n c o r p o r a t e s weakly-cemented sands o f v a r i o u s g r a i n s i z e i n c l u d i n g t h e whole spectrum f r o m almost s i l t t o n e a r l y g r a v e l where t h e problems o f sand i n f l u x i n t o t h e b o r e h o l e s t a r t w i t h p r o g r e s s i v e d e p l e t i o n due t o d e c r e a s i n g r e s e r v o i r p r e s s u r e t h a t i s so f a r m a i n t a i n i n g a more o r l e s s s e l f - s u p p o r t i n g framework o f g r a i n s , w i t h t h e f o r m a t i o n f a i l i n g once a c r i t i c a l boundary p r e s s u r e i s reached and underpassed. E a r l y p r e s s u r e maintenance methods such as w a t e r o r gas i n j e c t i o n which a r e v e r y common i n o i l - f i e l d e x p l o i t a t i o n can h e l p t o p r e v e n t t h e sand f r o m becoming s o f t and f l o w i n g , o r can a t l e a s t d e c e l e r a t e t h e c o h e s i v i t y t r a n s f o r m a t i o n , b u t t h e changing phase composit i o n and v i s c o s i t y o f t h e r e s e r v o i r f l u i d s a l s o has i t s impact. V A Z I R (1986) r e c o g n i z e s changes o f p e r m e a b i l i t y and r o c k p r o p e r t i e s around t h e we l b o r e t o be t h e main reasons t r i g g e r i n g sand p r o d u c t i o n .
5.4.1.2. Changing fluid comosition and viscosity The t h i r d a s s o c i a t i o n i n c l u d e s s l i g h t l y - l i t h i f i e d sands o f d i f f e r e n t g r a i n s i z e c o m p r i s i n g a broad range t h r o u g h o u t t h e g r a n u l o m e t r i c a l i n t e r v a l o f sand where g r a v e l p a c k i n g becomes necessary due t o r e s e r v o i r breakdown and d i s i n t e g r a t i o n w i t h i n c r e a s i n g w a t e r c u t i n t h e o i l d u r i n g advanced p r o d u c t i o n w i t h a r i s i n g o i l - w a t e r - c o n t a c t accompanying p r e s s u r e d e p l e t i o n , as a consequence o f e x c e s s i v e w a t e r i n j e c t i o n due t o p r e s s u r e maintenance o p e r a t i o n s , o r as a r e s u l t o f w a t e r c o n i n g and b r e a k t h r o u g h due t o u n f a v o u r a b l e d i f f e r e n c e s i n r e l a t i v e p e r m e a b i l i t y and p r e f e r r e d w a t e r c o n d u c t i v i t y along some h i g h - p e r m e a b i l i t y s t r e a k s o r t h r o u g h t h e j o i n t system. The changing f l u i d v i s c o s i t y l e a d s t o d i f f e r e n t cohesion o f t h e g r a i n f a b r i c , g e n e r a l l y induces l e s s s t a b i l i t y than b e f o r e and thus u l t i m a t e l y causes t h e sand t o f l o w . The main members o f t h e second and t h i r d groups o f sand-problem r e s e r v o i r s a r e weakly-cemented f r i a b l e sands which m i g h t be o r i g i n a l l y s t i l l s u f f i c i e n t l y s t a b l e ( a l t h o u g h sand p r o d u c t i o n may o c c u r f r o m t h e v e r y beg i n n i n g o f b e i n g p u t on stream), b u t w i t h f a l l i n g r e s e r v o i r p r e s s u r e and/or changing f l u i d v i s c o s i t y , i n t e r n a l s u p p o r t i s taken away and t h e g r a i n f a b r i c s t a r t s t o c o l l a p s e and t o r e l e a s e more and more l o o s e sand g r a i n s which move t o wards t h e l e a s t r e s t r i c t e d s p o t t h a t i s r e p r e s e n t e d by t h e b o r e h o l e .
5.4.1.3. Achievement o f the critical stage of formation collapse I n a l l t h e t h r e e d i f f e r e n t cases as o u t l i n e d above, s o l i d c o n t r o l problems commonly s t a r t w i t h more o r l e s s unprevented m i g r a t i o n o f sand, mud and c h a l k t h a t i s n o t s e r i o u s l y d i s t u r b i n g hydrocarbon p r o d u c t i o n , b u t l a t e r t h e stage i s reached when p l u g g i n g o f r e s e r v o i r pores and l i n e r s l o t s as w e l l as i n f i l l i n g o f t h e t u b i n g and e x c e s s i v e p r o d u c t i o n o f s o l i d s s t o p s o i l o r gas f l o w and r u i n s t h e equipment. I n some cases, sand p r o d u c t i o n problems a r e o r i g i n a l l y abs e n t and a r e o n l y s e c o n d a r i l y c r e a t e d by i n s u f f i c i e n t o r non-adequate complet i o n p r a c t i c e s (ALLEN & ROBERTS 1982). I n some v e r y u n s t a b l e o i l sands, t h e c r i t i c a l o f f t a k e r a t e a t which sand s t a r t s t o f l o w i s l e s s than 1 m3 o f o i l p e r day (MURER 1981). D i s t i n c t i o n has t o be made between more o r l e s s c o n s t a n t sand c o n c e n t r a t i o n i n t h e produced o i l as a consequence o f slow and g r a d u a l sand i n f l u x , and p u l s a t o r y c o l l a p s e o f channels and caverns as w e l l as i n t e r m i t t e n t a r r i v a l o f l a r g e amounts o f l o o s e sand i n t h e b o r e h o l e a l t e r n a t i n g w i t h p e r i o d s o f apparent sand s t a b i l i t y .
753
5.4.2. Fluid dynamics Gas e v o l u t i o n f r o m t h e f l o w i n g f l u i d a l s o has s i g n i f i c a n t e f f e c t s on sand p r o d u c t i o n (SAUCIER 1974), and t h e same a p p l i e s t o f l o w d i s t u r b a n c e s caused by r a t e changes and s u r g i n g (such as t a k i n g p l a c e d u r i n g a r t i f i c i a l l i f t ) . Reduct i o n o f f l u i d r a t e l e a d s t o v e r y l a r g e g a s / l i q u i d r a t i o s and r e s u l t s i n d e c l i n i n g sand p r o d u c t i o n which i s n o t t h e consequence o f pack f l u i d i z a t i o n , as e v i denced by t h e occurrence o f t h e same f e a t u r e s i n c o n s o l i d a t e d packs i n f u l l s c a l e model v e r i f i c a t i o n t e s t s . Rate changes a f f e c t g r a v e l pack b e h a v i o u r more s i g n i f i c a n t l y t h a n does t h e magnitude o f t h e f l o w r a t e . Sand b r i d g e c o n s t r u c t i o n and breakdown as w e l l as i n f l u e n c e o f p r o d u c t i o n r a t e s a r e o u t l i n e d as f o l lows.
5.4.2.1. Sand bridge construction and breakdown F o r a g i v e n s e t o f r e s e r v o i r f l o w c o n d i t i o n s , p r i m a r y and secondary sand b r i d g e s f o r m which a r e s t a b l e f o r t h e e x i s t i n g geometries and hydrodynamic f o r ces. I f f l u i d f o r c e s a r e a l t e r e d , i n s t a b i l i t i e s occur, b r i d g e s break down, and more sand i s produced u n t i l new b r i d g e s o r i g i n a t e under new c o n d i t i o n s o f s t a b i l i t y ( c f . s e c t i o n 5 . 9 . 5 . ) . H i g h f l o w r a t e s a t t h e b e g i n n i n g o f hydrocarbon exp l o i t a t i o n cause h i g h e r i n i t i a l q u a n t i t i e s o f sand p r o d u c t i o n and f l o w i m p a i r ment, thus n e c e s s i t a t i n g t o s t a r t e x p l o i t a t i o n o f o i l r e s e r v o i r s w i t h sand cont r o l problems w i t h low t o moderate o f f t a k e r a t e s . Hydrodynamic f a c t o r s a f f e c t i n g g r a v e l pack d e s i g n a r e a l s o d i s c u s s e d by SAGE & LACEY ( 1 9 4 2 ) . Concerning s p e c i a l treatments, steam-soak o r h u f f and p u f f w e l l s o f t e n s u f f e r f r o m sand i n f l u x , because t h e c y c l i c a l r e p e t i t i o n o f steam i n j e c t i o n and hydrocarbon, wa,ter and steam-condensate p r o d u c t i o n r e s u l t s i n c o n t i n u o u s l y chang i n g s w e l l i n g and s h r i n k i n g o f t h e f o r m a t i o n due t o a l t e r n a t i n g h e a t i n g and c o o l i n g , which a d d i t i o n a l l y t o t h e a l r e a d y o r i g i n a l l y f r i a b l e n a t u r e f u r t h e r loosens t h e g r a i n f a b r i c (TOMA, LIVESEY & H E I D R I C K 1986). I n o r d e r t o maximize r e l a t i v e p e r m e a b i l i t y t o o i l , t h e g r a v e l has t o be water-wet b e f o r e i t i s added t o t h e placement f l u i d p a r t i c u l a r l y i n case o f o i l - b a s e d c a r r i e r media (ALLEN & ROBERTS 1982).
5.4.2.2. Influence of production rates S i m i l a r l y as a p p l y i n g f o r h y d r a u l i c f r a c t u r i n g ( c f . s e c t i o n 4.11.1, damage o f t h e g r a v e l pack a f t e r i t s i n s t a l l a t i o n has t o be avoided by c a r e f u l l y p u t t i n g t h e w e l l on stream. The f o r m a t i o n pores around t h e w e l l b o r e a r e loaded w i t h f i n e s c a r r i e d i n by f l u i d s f i l t e r i n g i n t o t h e r e s e r v o i r o r by i n h e r e n t f i nes made movable by f l u i d f i l t r a t e e f f e c t s . T h i s assembly o f f i n e s s h o u l d be s t a r t e d t o be t r a n s p o r t e d back towards t h e w e l l b o r e as soon as p o s s i b l e a f t e r c o m p l e t i o n o f t h e g r a v e l p a c k i n g j o b , b u t a t a low r a t e i n o r d e r t o m i n i m i z e p l u g g i n g due t o h i g h v e l o c i t y . P r o d u c t i o n r a t e s should be g r a d u a l l y i n c r e a s e d i n o r d e r t o promote b r i d g i n g o f f o r m a t i o n sand on t h e g r a v e l . M i g r a t i o n o f f i nes i n t o t h e g r a v e l pack can be m i n i m i z e d by t h e use o f c l a y - s t a b i l i z i n g organ i c polymers i n t h e placement f l u i d which a r e s t r o n g l y absorbed by c l a y s , f e l d spars and o t h e r n e g a t i v e l y charged s i l i c a t e s , t h e r e b y e x t e n d i n g g r a v e l pack life. Water-wet f o r m a t i o n s produce hydrocarbons w i t h o u t i n f l u x o f sand i f t h e r e i s no t u r b u l e n t f l o w i n t h e r e s e r v o i r and i f w a t e r s a t u r a t i o n i s l e s s than t h e c r i t i c a l c o n c e n t r a t i o n (COLLE 1988). S u r f a c e t e n s i o n o f t h e w a t e r h o l d s sand g r a i n s , s h a l e p a r t i c l e s and o t h e r f i n e s i n p l a c e d u r i n g n o n - t u r b u l e n t f l o w , whereas t u r b u l e n t f l o w overcomes s u r f a c e t e n s i o n and causes t h e w a t e r t o move w i t h c o r r e s p o n d i n g t r a n s p o r t o f f i n e s and sand. may
Sand i n f l u x may n o t be a s e r i o u s problem d u r i n g p r i m a r y gas p r o d u c t i o n , b u t o n l y show up a f t e r c o n v e r s i o n o f w e l l s and r e s e r v o i r f o r secondary gas s t o -
754 r a g e (ANAND & JONES 1977), e s p e c i a l l y i f p l a n n e d gas o f f t a k e r a t e s f r o m t h e d e r g r o u n d s t o r e a r e c o n s i d e r a b l y g r e a t e r t h a n p r i m a r y gas w i t h d r a w a l r a t e s .
un-
5.4.3. Rock mechanics I n t e r m s o f r o c k m e c h a n i c a l a p p r o a c h o f r e s e r v o i r and p a r t i c u l a r l y p e r f o r a t i o n t u n n e l s t a b i l i t y v s . c o l l a p s e , two t y p e s o f f a i l u r e have t o be d i s t i n g u i shed (MORITA, WHITFILL, FEDDE & L Q V I K 1 9 8 7 ) . Shear f a i l u r e o c c u r s i f w e l l p r e s s u r e i s t o o low, whereas t e n s i l e f a i l u r e t a k e s p l a c e i f t h e n o r m a l i z e d p r e s s u r e g r a d i e n t i s t o o h i g h . B o t h s h e a r and t e n s i l e f a i l u r e e n v e l o p e s a r e a f f e c t e d b y c y c l i c l o a d i n g due t o r e p e a t e d s h u t - i n and r e s t a r t i n g f l o w o f t h e w e l l . Sand p r o d u c t i o n i s c o n t r o l l e d by two independent f a c t o r s b e i n g w e l l pressure o r t o t a l drawdown, and l o c a l p r e s s u r e g r a d i e n t a r o u n d t h e p e r f o r a t i o n c a v i t y o r comp l e t i o n p r e s s u r e l o s s . The o u t l i n e as f o l l o w s f o c u s s e s on s h e a r and t e n s i l e f a i l u r e , i n - s i t u s t r e s s s t a t e and r e s e r v o i r d e p l e t i o n , c a p i l l a r y b o n d i n g , sand b r i d g i n g , p e r f o r a t i o n t u n n e l s t a b i l i t y , and p r e s s u r e d r o p and f l o w r e s i s t a n c e i n perforation tunnels.
5.4.3.1. Shear and tensile failure Shear f a i l u r e s u f f e r s f r o m t h e w e a k e n i n g e f f e c t due t o p l a s t i c f a t i g u e , wher e a s t h e s h i f t o f t h e t e n s i l e f a i l u r e e n v e l o p e depends o n t h e d e g r e e o f s h e a r ing. Once p l a s t i c i z e d , t h e p e r f o r a t i o n c a v i t y s u r f a c e c a n n o t r e c o v e r t h e d e f o r mation, and t h e s t r e s s i s r e l e a s e d a t t h e c a v i t y s u r f a c e w h i c h i s s u s c e p t i b l e t o t e n s i l e f a i l u r e , as t h e r e i s n o t enough s t r e s s t o b i n d t h e sand g r a i n s . T h e r e f o r e t h e d e g r e e o f s h e a r i n g and t h u s t h e p r e s s u r e drawdown s h o u l d be l i m i t e d d u r i n g t h e p r o d u c t i o n c y c l e p r e c e d i n g w e l l s h u t - i n . In a d d i t i o n , s h e a r f a i l u r e u s u a l l y o c c u r s more a b r u p t l y t h a n t e n s i l e f a i l u r e . The u n l o a d i n g e f f e c t o f t h e p l a s t i c zone a r o u n d t h e p e r f o r a t i o n c a v i t i e s i s s e r i o u s when p r o d u c t i o n r e s t a r t s a f t e r t e r m i n a t i o n o f t h e s h u t - i n p e r i o d o f t h e w e l l , and t e n s i l e f a i l u r e o c c u r s w i t h i n a r e l a t i v e l y s m a l l f l o w r a t e i f t h e drawdown d u r i n g t h e p r e v i o u s o f f t a k e cycle i s too large.
5.4.3.2. In-situ stress state and reservoir depletion The sand p r o b l e m s a r e s e n s i t i v e t o t h e i n - s i t u s t r e s s s t a t e , w i t h i n c a s e o f h i g h i n - s i t u s t r e s s s h e a r f a i l u r e t e n d i n g t o o c c u r , whereas t h e p o s s i b i l i t y o f t e n s i l e f a i l u r e i s diminished. Reservoir pressure depletion increases the e f f e c t i v e i n - s i t u s t r e s s , and t h e r e f o r e a t e n s i l e - d o m i n a n t sand p r o b l e m may be r e d u ced, b u t a s h e a r - d o m i n a n t one becomes s e r i o u s i n weak f o r m a t i o n s w i t h d e c l i n i n g r e s e r v o i r pressure. Since shear f a i l u r e i s u s u a l l y o f c a t a s t r o p h i c a l nature, r e s e r v o i r p r e s s u r e s h o u l d be m a i n t a i n e d o r drawdown s h o u l d be l i m i t e d i n c a s e o f p e r f o r a t i n g and p r o d u c i n g a weak p a y zone. P e r f o r a t i o n t u n n e l s t a b i l i t y i s a l s o s e n s i t i v e t o d e f o r m a t i o n and s t r e n g t h c h a r a c t e r i s t i c s o f t h e r o c k . Two t y p e s o f f a i l u r e e n v e l o p e s u s u a l l y a p p l y comp r i s i n g octahedral shear s t r e s s vs. e f f e c t i v e h y d r o s t a t i c s t r e s s f o r b r i t t l e r o c k s and p l a s t i c s t r a i n v s . e f f e c t i v e h y d r o s t a t i c s t r e s s f o r m a l l e a b l e r o c k s . The s h e a r s t a b i l i t y e n v e l o p e v a r i e s w i t h c r i t i c a l p l a s t i c s t r a i n , although the t e n s i l e f a i l u r e e n v e l o p e r e m a i n s unchanged. I n h i g h l y d e f o r m a b l e r e s e r v o i r s , p e r f o r a t i o n t u n n e l s a r e d i s t o r t e d due t o f l u i d f o r c e s and t h u s t h e c a v i t i e s a r e d e s t a b i l i z e d . T e n s i l e s t r e n g t h i m p r o v e s c a v i t y s t a b i l i t y when t h e drawdown p r e s s u r e i s s m a l l ; however, a r o c k n o r m a l l y l o o s e s i t s t e n s i l e s t r e n g t h once i t i s sheared. I f t h e cementation m a t e r i a l i s n o t b r i t t l e , a c e r t a i n degree o f tens i l e s t r e n g t h s t i l l r e m a i n s between t h e sand g r a i n s .
755
5.4.3.3. Capillary bonding When t h e w a t e r s a t u r a t i o n i n t h e r e s e r v o i r sand around t h e p e r f o r a t i o n cav i t i e s approaches t h e i r r e d u c i b l e connate c o n c e n t r a t i o n d u r i n g e f f l u x o f hydrocarbons i n t o t h e w e l l b o r e , t h e c a p i l l a r y p r e s s u r e r a p i d l y i n c r e a s e s (MORITA, WHITFILL, FEDDE & L0VIK 1987). C a p i l l a r y bonding i s n o t weakened d u r i n g t h e s h e a r i n g process and i s t h e r e f o r e n o t a f f e c t e d by c y c l i c a l f l o w . Thus even i f a p r e v i o u s drawdown i s c l o s e t o t h e c r i t i c a l shear f a i l u r e drawdown, no sand p r o blems occur w i t h a s i g n i f i c a n t l y h i g h f l o w r a t e a f t e r s h u t - i n o f t h e w e l l , because t h e f l o w r a t e t o break c a p i l l a r y bonding i s v e r y h i g h s i n c e t h e p l a s t i c zone i s v e r y t h i n w i t h r e s p e c t t o c a v i t y s i z e .
5.4.3.4. Sand bridging Sand b r i d g i n g i s a l s o i m p o r t a n t i f g r a i n s i z e i s l a r g e compared w i t h t h e p e r f o r a t i o n t u n n e l r a d i u s and has a s i g n i f i c a n t e f f e c t i f c a p i l l a r y bonding a l s o e x i s t s (MORITA, WHITFILL, FEDDE & L0VIK 1987). I f s t r e s s around a c a v i t y i s r e l e a s e d , m i c r o c r a c k s appear around i t . Unless c a p i l l a r y bonding e x i s t s , d i s p l a cement a t t h e c a v i t y s u r f a c e i s n o t u n i f o r m and c r a c k growth occurs a t one s i d e when s t r e s s i s r e l e a s e d . I f displacement exceeds a c r i t i c a l f r a c t i o n o f g r a i n s i z e , t h e sand g r a i n i s d i s l o d g e d which i s f o l l o w e d by sand f l o w u n t i l a n o t h e r s t a b l e sand a r c h i s formed ( c f . s e c t i o n 5 . 9 . 5 . ) . C a p i l l a r y bonding tends t o g i v e more u n i f o r m d e f o r m a t i o n and t o p r e v e n t d i s l o d g i n g o f r e s e r v o i r sand p a r t i c l e s . Cohesive s t r e n g t h o f t h e sand g r a i n package i n c r e a s e s w i t h w e t t i n g phase s a t u r a t i o n t o a maximum a t a b t . 80 % c o n c e n t r a t i o n , b u t beyond t h i s p o i n t , any i n c r e a s e i n w e t t i n g phase s a t u r a t i o n causes a r a p i d decrease i n cohes i v e s t r e n g t h (DURRETT, GUBIN, MURRAY & TIGHE 1977).
5.4.3.5. Perforat ion tunnel stability P e r f o r a t i o n t u n n e l s i n f r i a b l e sandstones t h a t a r e c o n s o l i d a t e d t o a c e r t a i n e x t e n t a r e s t a b l e when t h e f o r m a t i o n i s i n i t i a l l y p e r f o r a t e d (YASSIN 8, PEDEN 1988). F a i l u r e o f t h e t u n n e l s can occur i n an advanced stage i f t h e g r a i n l o a d i s i n c r e a s e d enough t o break t h e cementation bonds, which i s a s s o c i a t e d w i t h f o r c e s generated by f l u i d d e p l e t i o n through t h e permeable r o c k and i s a m p l i f i e d w i t h r e s t r i c t i o n t o f l o w caused by f o r m a t i o n damage due t o d r i l l i n g and p e r f o . r_ i a t n g o p e r a t i o n s . I f t h e p l a s t i c zones expand t o reach t h e m i d p o i n t d i s t a n c e between a d j a c e n t p e r f o r a t i o n t u n n e l s , i n s t a b i l i t y o r f a i l u r e occurs by l a r g e d e f o r m a t i o n o r s t r a i n r e s u l t i n g i n sand p r o d u c t i o n . Thus t h e r e l a t i o n s h i p o f t h e p l a s t i c zone t o a s p e c i f i e d d i s t a n c e f r o m t h e c e n t r e o f t h e p e r f o r a t i o n t u n n e l r e p r e s e n t s t h e s t a b i l i t y r a t i o o f t h e g r a i n f a b r i c . O t h e r aspects o f p e r f o r a t i o n t u n n e l s t a b i l i t y a r e d i s c u s s e d by VRIEZEN, SPIJKER & VLIS (1975); ANTHEUN I S , VRIEZEN, SCHIPPER & VLIS (1976); NORDGREN (1977) and CHENEVERT & THOMPSON (1985).
5.4.3.6. Pressure drop and flow resistance in perforation tunnel Gravel-packed p e r f o r a t i o n t u n n e l s u s u a l l y o f f e r l i t t l e f l o w r e s i s t a n c e when p e r f o r a t i n g d e n s i t y i s adequate (McLEOD 1984). When g r a v e l i s n o t packed t i g h t l y enough d u r i n g placement, g r a v e l s h i f t i n g i n p e r f o r a t i o n t u n n e l s o c c u r s (TORREST 1982, STEIN 1983). Damage t o f o r m a t i o n sand b e f o r e g r a v e l i n s t a l l a t i o n causes premature p r e s s u r e o u t s r e s u l t i n g f r o m v i s c o u s f l u i d s e n t e r i n g damaged o r reduced p e r m e a b i l i t y near t h e p e r f o r a t i o n s (McLEOD 1984). As a consequence o f h i g h pressures, pumping may be h a l t e d b e f o r e t h e g r a v e l has c o n c e n t r a t e d adequat e l y i n t h e p e r f o r a t i o n h o l e s and i f t h u s t h e t u n n e l s a r e o n l y p a r t i a l l y f i l l e d w i t h g r a v e l , r e s e r v o i r sand e n t e r s t h e h o l e s upon w e l l p r o d u c t i o n , t h e r e b y b r i d g i n g on t h e g r a v e l i n s i d e t h e t u n n e l and p a c k i n g t h e p a r t i a l l y v o i d h o l e w i t h f o r m a t i o n sand which i s much l o w e r i n p e r m e a b i l i t y than g r a v e l . I f r e s e r v o i r sand f i l l s t h e p e r f o r a t i o n t u n n e l s , p r e s s u r e drop through t h e c o m p l e t i o n i n c r e a -
756
ses and f l o w r a t e d e c l i n e s f g r a v e l i s p l a c e d c o r r e c t l y i n t h e p e r f o r a t i o n t u n n e l and l i t t l e o r no g r a ve 1 i s i n s e r t e d o u t s i d e t h e h o l e pay zone sand abuts t h e t u n n e l e n t r a n c e a t t h e c e m e n t - r e s e r v o i r and g r a v e l - o r m a t i o n i n t e r f a c e s . I f t h i s p r o s p e c t i v e i n t e r Val sand i s c l e a n and Dermeable. t h e p r e s s u r e drop caused by s p h e r i c a l f l o w t h r o u g h t h e pay sand t o t h e t u n n e l w i l l be s m a l l , b u t i f any damage e x i s t s f r o m polymer r e s i d u e , p i p e dope o r f o r m a t i o n f i n e s , t h e p r e s s u r e d r o p can be substant i a l and f l o w g r e a t l y reduced. Successful remedy by a c i d i z i n g depends on damage s e v e r i t y and s o l v e n t c h o i c e . I f damage i s moderate ( l e s s than 90 % p e r m e a b i l i t y loss), a c i d can u s u a l l y d i s s o l v e t h e damage, whereas i n case o f severe damage (more than 99 % l o s s i n p e r m e a b i l i t y ) , a c i d may n o t e n t e r t h e p e r f o r a t i o n s f a s t enough t o d i s s o l v e t h e damage.
5.4.4.
Perf orat ion tunnel morphology
P e r f o r a t i o n t u n n e l s a r e u s u a l l y l o n g and o f c y l i n d r i c a l n a t u r e i m m e d i a t e l y a f t e r c r e a t i o n ( M O R I T A , WHITFILL, FEDDE & LQVIK 1987). S p h e r i c a l c a v i t i e s a r e g e n e r a l l y more s t a b l e t h a n c y l i n d r i c a l t u n n e l s f o r shear f a i l u r e , whereas almost no d i f f e r e n c e s e x i s t f o r e x t e n s i o n f a i l u r e . The n o r m a l i z e d p r e s s u r e g r a d i e n t has more e f f e c t upon t h e f a i l u r e envelope shape o f a c y l i n d r i c a l h o l e than t h a t o f a c y l i n d r i c a l c a v i t y . T h e r e f o r e a h i g h s a n d - f r e e hydrocarbon f l o w r a t e can be achieved by e n l a r ging long c y l i n d r i c a l p e r f o r a t i o n tunnels i n t o spherical c a v i t i e s . Although a l o n g p e r f o r a t i o n h o l e i s l e s s s t a b l e than a s h o r t one, i t has l e s s c o m p l e t i o n p r e s s u r e drop, and t h u s t h e use o f e x c e s s i v e l y s h o r t o r l o n g p e r f o r a t i o n s s h o u l d be avoided i n weak f o r m a t i o n s . As a consequence o f c a v i t y enlargement dur i n g p r o d u c t i o n , t h e c o m p l e t i o n p r e s s u r e drop i s r e l a t i v e l y small i n a weak r e s e r v o i r . N e v e r t h e l e s s t h e use o f e x c e s s i v e l y l o n g p e r f o r a t i o n t u n n e l s f o r reduct i o n o f c o m p l e t i o n p r e s s u r e l o s s s h o u l d n o t be overemphasized, even though l o n g p e r f o r a t i o n holes are p r e f e r r e d f o r strong formations. Three t y p e s o f p e r f o r a t i o n t u n n e l i n s t a b i l i t y have t o be d i s t i n g u i s h e d . Shear f a i l u r e occurs i n weak f o r m a t i o n s i f e f f e c t i v e i n - s i t u s t r e s s i s h i g h o r w e l l p r e s s u r e i s low. T e n s i l e f a i l u r e d u r i n g l o a d i n g takes p l a c e f o r an abnorm a l l y h i g h p r e s s u r e g r a d i e n t around a c a v i t y due t o h i g h l y damaged p e r m e a b i l i t y o r a sudden i n c r e a s e i n f l o w r a t e such as f o l l o w i n g w e l l s h u t - i n . T e n s i l e f a i l u r e f o r an unloaded zone o c c u r s w i t h a r e l a t i v e l y low f l o w r a t e i f a p r e v i o u s drawdown b e f o r e a w e l l s h u t - i n exceeds t h e c r i t i c a l drawdown.
5.4.5. Water cut i n the hydrocarbons Sand problems by shear o r t e n s i l e f a i l u r e o f t h e g r a i n f a b r i c a r e enhanced by appearing and r i s i n g w a t e r c u t i n t h e hydrocarbons, because c a p i l l a r y p r e s sure h o l d i n g sand g r a i n s t o g e t h e r i s l o s t when w a t e r s a t u r a t i o n i n c r e a s e s reason a b l y above t h e i r r e d u c i b l e connate c o n c e n t r a t i o n , and f l o w f r i c t i o n s i g n i f i c a n t l y i n c r e a s e s due t o l o w e r i n g o f r e l a t i v e hydrocarbon p e r m e a b i l i t y by r i s i n g w a t e r s a t u r a t i o n and lowered r e s e r v o i r p r e s s u r e when w a t e r c u t appears i n t h e o i l and/or gas (MORITA, WHITFILL, FEDDE & LQVIK 1987; GHALAMBOR, KOLIBA, HAYATDAVOUOI & ALCOCER 1988). I n addition, i n o r d e r t o m a i n t a i n an e c o n o m i c a l l y a c c e p t a b l e hydrocarbon f l o w r a t e , t h e w e l l i s i n c l i n e d t o be produced w i t h a h i g h e r t o t a l f l o w r a t e which causes low p r e s s u r e i n t h e w e l l and h i g h p r e s s u r e g r a d i e n t a t t h e c a v i t y s u r f a c e . O t h e r p o i n t s t r i g g e r i n g i n c r e a s i n g sand problems w i t h a p p e a r i n g and r i s i n g w a t e r c u t a r e i n c r e a s i n g p l u g g i n g r a t e when w a t e r s p i l l s f i n e p a r t i c l e s away which p r e v i o u s l y adhered t o sand g r a i n s , enlargement o f c a v i t y shape r e s u l t i n g i n c a v i t y i n t e r a c t i o n , and cement d i s s o l u t i o n between sand g r a i n s b y t h e f l u s h i n g w a t e r . As shear f a i l u r e and c y c l i c a l t e n s i l e f a i l u r e sand problems a r e
757 s e r i o u s f o r weak f o r m a t i o n s , low w e l l p r e s s u r e should be avoided by u s i n g second a r y maintenance, and t o t a l f l o w r a t e o f t h e w e l l should be decreased a f t e r appearance o f w a t e r c u t i n t h e hydrocarbons.
5.4.6.
Difference between proppant fracturing and gravel packing
I n c o n t r a s t t o h y d r a u l i c p r o p p a n t f r a c t u r i n g which c o n c e r n i n g r e s e r v o i r r o c k can be s u c c e s s f u l l y c a r r i e d o u t i n a l l stages o f p r o d u c t i o n h i s t o r y , b u t i s o f t e n performed i n e a r l y w e l l e v o l u t i o n f o r economical reasons based on absol u t e hydrocarbon r e s e r v e s and r e l a t i v e o i l o r gas s a t u r a t i o n w i t h r e s p e c t t o t h a t o f w a t e r i n t h e gas zone, sand c o n t r o l o p e r a t i o n s have i n most cases t o be done b e f o r e p r o d u c t i o n s t a r t s o r s h o r t l y a f t e r b e g i n n i n g o f e x p l o i t a t i o n o f t h e hydrocarbon-bearing i n t e r v a l f o r t e c h n i c a l reasons. Gravel p a c k i n g i s o n l y o p t i m a l l y e f f e c t i v e when i n s t a l l e d b e f o r e t h e r e s e r v o i r r o c k i s s e r i o u s l y d i s t u r b e d by sand removal, w i t h f u r t h e r sand f l o w b e i n g v e r y d i f f i c u l t t o c o n t r o l as t h e volume o f produced sand i n c r e a s e s . Experience i n d i c a t e s t h a t i n i t i a l sand cont r o l i n s t a l l a t i o n s a r e f a r more s u c c e s s f u l than remedial t r e a t m e n t s which may even c o n s i d e r a b l y t o s e r i o u s l y i m p a i r p r o d u c t i v i t y (SUMAN, ELLIS & SNYDER
1983).
I n c o n t r a s t t o h y d r a u l i c f r a c t u r i n g where f l u i d l e a k o f f has t o be l i m i t e d t o a minimum i n o r d e r t o p r e v e n t e x c e s s i v e proppant c o n c e n t r a t i o n i n t h e f r a c t u r e t h a t u l t i m a t e l y l e a d s t o a screenout ( c f . s e c t i o n 4 . 3 . 4 . 6 . ) , f l u i d l o s s i n t o l i n e r o r screen and/or f o r m a t i o n d u r i n g g r a v e l p a c k i n g i s d e s i r e d and necessary t o t a k e p l a c e i n h i g h r a t e s i n o r d e r t o a l l o w d e p o s i t i o n o f t h e g r a v e l on t h e b o r e h o l e w a l l where i t forms a m a n t l e c o a t i n g t h e r e s e r v o i r / w e l l b o r e i n t e r f a c e , spearheads p l u g g i n g t h e p e r f o r a t i o n t u n n e l s , a b e l t b r i d g i n g t h e annulus b e t ween b o r e h o l e boundary f a c e and screen o r l i n e r , o r a p l u g i n f i l l i n g t h e whole diameter o f t h e w e l l b o r e (depending on t y p e o f w e l l c o m p l e t i o n and g r a v e l pack i n s t a l l a t i o n ; c f . s e c t i o n 5.10.).
5.5. Areal distribution o f potential reservoirs The most i m p o r t a n t areas o f p a s t performance and f u t u r e p o t e n t i a l o f g r a v e l p a c k i n g a r e b r i e f l y o u t l i n e d i n a d i s c u s s i o n o f general aspects, c e n t r e s o f g r a v e l packing, p o t e n t i a l i n Europe and impact o f r e s i n - c o a t e d sand and proppants as f o l l o w s .
5.5.1. General aspects U n s t a b l e r e s e r v o i r s r e q u i r i n g sand c o n t r o l t r e a t m e n t s a r e p a r t i c u l a r l y widespread i n younger g e o l o g i c a l f o r m a t i o n s which a r e s t i l l i n an e a r l y stage o f t h e d i a g e n e t i c a l e v o l u t i o n and a l s o p a r t i a l l y have n o t y e t achieved t h e stage o f f u l l mechanical compaction. Sandy sediments can, however, a l s o p r e s e r v e a s o f t n a t u r e t h r o u g h o u t c o n s i d e r a b l e s e c t i o n s o f t h e g e o l o g i c a l h i s t o r y i f reason a b l e cementation has been p r e v e n t e d by p r e f e r a b l y e a r l y o i l and s u b o r d i n a t e l y a l s o e a r l y gas i m m i g r a t i o n o r by p e r s i s t e n t s h a l l o w b u r i a l under low compress i v e s t r e s s , o r i f undercompaction i s conserved by i n t e r n a l f l u i d overpressur i n g . S e c o n d a r i l y , i n s t a b i l i t y can be generated i f e x i s t i n g i n - s i t u s t r e s s e s a r e a l t e r e d by d r i l l i n g and/or c o m p l e t i o n o p e r a t i o n s such t h a t t h e r o c k m a t r i x i s weakened by movement o f t h e b o r e h o l e w a l l . A f o r m e r l y h a r d sandstone m a t r i x can a l s o become f r i a b l e again i n case o f t e c t o n i c a l s h e a r i n g and f a u l t i n g o f t h e sedimentary column ( t h i s p a r t i c u l a r l y a p p l i e s f o r narrower o r b r o a d e r p e r t u r b a t i o n o r even m y l o n i t e zones along graben boundary f a u l t s o r main nappe o v e r t h r u s t s ) . T e r t i a r i l y , weakening o f t h e g r a i n f a b r i c by l o o s i n g s u p p o r t can o r i g i n a t e d u r i n g hydrocarbon e x p l o i t a t i o n , w i t h t h e most i m p o r t a n t r e s e r v o i r eng i n e e r i n g f a c t o r s r e s p o n s i b l e f o r o n s e t t i n g sand p r o d u c t i o n problems b e i n g dec l i n i n g f o r m a t i o n p r e s s u r e as a consequence o f p r o g r e s s i v e d e p l e t i o n and chang i n g f l u i d c o m p o s i t i o n and t h u s v i s c o s i t y due t o i n c r e a s i n g w a t e r c u t i n t h e
758 oil ( c f . section 5.4.1.).
5.5.2. Centres o f gravel packing Reviewing world-wide d i s t r i b u t i o n of g r a v e l packing c e n t r e s , sand p r o d u c t i o n problems o c c u r most commonly i n T e r t i a r y and C r e t a c e o u s d e p o s i t s such a s i n Gulf Coast (Texas and L o u i s i a n a ) and C a l i f o r n i a i n USA, Gulf of Mexico, Niger D e l t a and I v o r y C o a s t , T a r Sand B e l t i n Canada, and i n I n d o n e s i a , T r i n i d a d and Venezuela (SUMAN, ELLIS & SNYDER 1983; OYENEYIN 1 9 8 8 ) . The c u r r e n t h i g h l i g h t s of g r a v e l packing o p e r a t i o n s a r e Gulf of Mexico and Western A f r i c a . O f f s h o r e g r a v e l packing can be performed from both p l a t f o r m s w i t h mounted r i g s and f l o a t ing v e s s e l s such a s semi-submersibles and d r i l l s h i p s (ZALESKI & DONOVAN 1 9 8 6 ) . The main o f f s h o r e g r a v e l packing a r e a s i n the l a s t y e a r s comprise North Sea, Gulf of Mexico, o f f s h o r e C a l i f o r n i a , I n d o n e s i a and I v o r y C o a s t . Most of t h e r e s e r v o i r s i n Europe r e q u i r i n g g r a v e l packing a l s o a r e of T e r t i a r y and s u b o r d i n a t e l y of C r e t a c e o u s and J u r a s s i c age. The h o r i z o n s needing s a n d - c o n t r o l t r e a t ments a r e mainly o i l - b e a r i n g f o r m a t i o n s , b u t g a s - b e a r i n g pay zones a r e a l s o a f f e c t e d ( A N A N D & JONES 1 9 7 7 ) , w i t h t h e problem i n gas w e l l s o f t e n becoming even more pronounced a s a consequence of high p r e s s u r e and high flow v e l o c i t y which a r e both s e r i o u s f a c t o r s c o n t r i b u t i n g t o e a s y sand p r o d u c t i o n from i n s u f f i c i e n t ly consolidated reservoirs (OYENEYIN 1988).
5.5.3. Gravel packing p o t e n t i a l i n Europe The main t a r g e t r e s e r v o i r rock t y p e s f o r g r a v e l packing i n Europe a r e sands t o n e s and c h a l k s , with in many c a s e s combinations of h y d r a u l i c proppant f r a c t u r i n g and g r a v e l packing having t o be c a r r i e d o u t w i t h both c o n v e n t i o n a l and r e s i n - c o a t e d g r a i n s i n t h e c h a l k , whereas most of t h e s a n d s t o n e s r e q u i r e o n l y s t a n d a r d g r a v e l packing ( c f . t a b s . 5 and 11). Coal and s h a l e g r a v e l packing a s o c c a s i o n a l l y a l r e a d y having been c a r r i e d o u t a t l e a s t i n e x p e r i m e n t a l s t a g e i n t h e USA has i n Europe so f a r n o t been c o n s i d e r e d .
5.5.3.1. Sandstones Gravel packing i n Europe has so f a r been predominantly done ( a n d w i l l a l s o be n e c e s s a r y i n the f u t u r e ) i n T e r t i a r y o i l - and p a r t i a l l y a l s o g a s - b e a r i n g sands i n A u s t r i a ( f i e l d s Matzen and H o c h l e i t e n ; SAMHABER 1977, MURER 1 9 8 1 ) , Czec h o s l o v a k i a , I t a l y and Yugoslavia ( o f f s h o r e A d r i a t i c Sea; c f . a l s o B E U D E L L 1985 a ) , and Yugoslavia and Hungary o n s h o r e . I n the Norwegian Northern North S e a , some J u r a s s i c o i l - b e a r i n g s a n d s and C r e t a c e o u s o i l - b e a r i n g c h a l k s need g r a vel packing (MASSIE, NYGAARD & MORITA 1 9 8 7 ) , whereas sand c o n t r o l t r e a t m e n t s i n t h e B r i t i s h Northern North Sea a r e so f a r o n l y n e c e s s a r y i n a few e x c e p t i o n s . While the J u r a s s i c o i l - b e a r i n g s a n d s t o n e s i n the l a t t e r a r e a a r e i n most c a s e s s u f f i c i e n t l y s t a b l e , t h e T e r t i a r y Eocene o i l - b e a r i n g s a n d s t o n e s c o u l d r e q u i r e sand c o n t r o l once they a r e going t o be developed ( B E U D E L L 1987 b ) . Gravel packing ( p a r t i a l l y a s an a l t e r n a t i v e o r replacement of s m a l l - s c a l e s a n d - o i l - f r a c t u r i n g ) i s a l s o done i n some Lower C r e t a c e o u s o i l - b e a r i n g s a n d s t o nes i n Germany FRG ( f i e l d s Georgsdorf and Ruhlermoor; GESEMANN & BADURA 1976, LILLIE & WIETHOFF 1985) and N e t h e r l a n d s ( f i e l d Schoonebeek; HARMSEN 1979, CHUOY 1 9 8 1 ) , and o c c a s i o n a l l y a l s o i n Lower C r e t a c e o u s and Middle J u r a s s i c s a n d s t o n e s in v a r i o u s p a r t s of t h e Northwest German o i l - f i e l d b e l t ( f o r maps c f . BOIGK 1 9 8 0 ) . The s h a l l o w Lower C r e t a c e o u s o i l - b e a r i n g s a n d s t o n e s i n Germany FRG and N e t h e r l a n d s have been f r e q u e n t l y t h e t a r g e t of open-hole g r a v e l packing by sanding-up t h e b o r e h o l e f o l l o w i n g underreaming ( c f . a l s o s e c t i o n s 4 . 1 1 . 2 . and 5 . 1 0 . 2 . ) . G e n e r a l l y a l m o s t no sand c o n t r o l problems o c c u r i n t h e g a s - b e a r i n g R o t l i e g e n d s e c t i o n in t h e B r i t i s h and Dutch Southern North Sea a s well a s i n Net h e r l a n d s and Germany FRG onshore ( i n t h e l a t t e r a r e a s w i t h l o c a l e x c e p t i o n s such a s the Groningen f i e l d i n t h e N e t h e r l a n d s ; D A V I E S , ZWOLLE & MEIJS 1983;
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