Thermal Recovery of Oil and Bitumen

llAR20 lgs! THERI$AL RECOVERY OF OIL AND BITUNIEN ROGERM. BUTLER Departmentof Chemical and PetroleumEngineering Univer...

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llAR20 lgs!

THERI$AL RECOVERY OF OIL AND BITUNIEN

ROGERM. BUTLER Departmentof Chemical and PetroleumEngineering Universityof Calgary Calgary,Alberta, Canada

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PrenticeHall,EnglewoodCliffs,New Jersey07632

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Confenfs PREFACE xii Chapter 1. INTRODUCTIONTO THERMAL RECOVERY Enhanced Oil RecoveryMethods z SteamStimulation z Steamflooding 3 Hot Waterflooding 4 In Situ Combustion 4 World Fuel Resources 5 The Oil Sand Resource 7 VenezuelanHeavv Oil 8 g Canadian Heavy Oil and Bitumen Correlation of Canadian Tar Sand Deposits Il Size of Alberta Oil Sand Deposits 11 Comparisonof Heavy Oil and ConventionalCXIResources 12 Deposits of Heavy Oil and Bitumen in the United States 12 The Nature of Heavy Oil and Bitumen Deposits 14 Solid Mineral Matter 16 Kaolinite 16 Montmorillonite 17 Illite 17 Chlorites 18 Water 18 Oil and Bitumen 19 Gas 19 Units of Measurement Z0 Use of ProgrammableCalculatorsand Microcomputers 22 Radial Flow to a Vertical Well 22 The Problem of Economic Exploitation 25 Bitumen Transportation 25 Bibliography 27 General References 29 Chapter 2. CONDUCTIONOF HEAT WtTHtN SOLTDS 30 Introduction 30 Thermal Conductivity 30 Fourier's Equation 3L Flow of Heat into a Semi-Infinite Solid 32 Significanceof Solution 36 Heat Transfer from a Spreading Hot Zone 37 Constant Heat Injection Rate into a Fracture 3g conduction from a Spreadingchamber That Advances to a Limit and Then Stops 39 Numerical problem 40 Conduction Ahead of an Advancing Front 43

ql AlLr ot a[ rtovlnc|lu rtull Tnmll lil TII[8 47 llort Ab.d of Rilt In Ttanrlont Foriod 48 Cctlnurllon of tho Prsvlous Numcrical Example 50 Elfcst of Ctarylng Ffont Vclocity Tho Situation Whcrc the Front Advance Velocity Is Inversely 51 Proportional to the SquareRoot of Time 52 Radial Heat Flow from a Well 55 Cumulative Heat Flow from Well Bore 56 FactorsAffecting Well Bore Heat Loss 56 Insulation of Wells to Reduce Heat Loss The Equivalent Well Radius with Multiple Resistances 58 59 Direct Injection of SteamDown the Well Casing Injection of Steamin the Tirbing with the Annulus Full of Gas ConvectiveHeat Transfer Between Two Concentric Vertical 60 Cylinders 63 Background Material on Well Bore Heat Loss 63 Numerical Example of Well Bore Heat Loss Calculation 68 Radial Conductive Heat Loss from a Buried Heated Cylinder 71. Bibliography

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rr 3. CONVECTIVE HEATINGWITHIN RESERVOIRS 72 72 Introduction 73 Simple Convective Heat Transfer Without Conductive Heat Loss 74 Overall Heat Balance Approach 75 Steam Injection 75 Lauwerier's Equation 78 Numerical Example Thermal Efficiency for Constant-Displacement Rate Steam-Drive Fraction of Heat in Steam-SaturatedChamberAfter the Critical 85 Time 86 Asymptote for As/A if tp : o Thermal Efficiency for Constant Steam-Injection Rate: Marx and 86 Langenheim'sTheory 90 Numerical Problem Using Marx-Langenheim's Equation 93 Simple Formulasfor Estimation of the Oil-Steam Ratio 95 Convective Transfer of Heat Beyond the Condensation Front Size of Steam Zone for Time Greater than the Mandl and Volek's 96 Critical Time 98 Effect of Non-Vertical Front 99 Steam Injection into a Thin Channel or Fracture Comparison of Fracture Filled with Steam for Constant Injection Rate 100 and for.ConstantArea Areal Growth Rate Calculation of Mandl-Volek Critical Time for a Numerical 100 Example Extension of Numerical Example to Injection into a Very Thin 101 Horizontal Layer or Fracture 103 Bibliography 104 rr 4. STEAMFLOODING 104 Introduction A Qualitative Discussionof Steam-injectionProcesses lv

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Stcrmlloodlng 105 Suitability of Spccific Rcscrvoirufor Stcamflooding 107 The Propcrtiesof Stcam 110 TemperatureDistributioninSteamflooding 122 Fingering 124 Gravity Override 124 SteamfloodingMechanisms t26 Reduction of Oil Viscosity 126 Changesin Relative Permeability 127 Myhill and Stegemeier'sApproach to Steamflooding L29 Summary of Myhill and Stegemeier's Assumptions 130 Outline of Method 130 Limitations 131 Comparisonsof Theoretical Predictionswith Data L33 Ten-Pattern Steamflood at Kern River 135 San Ardo Steamflood and Infill Drilling 137 Comparisonof Steamflood and_SteamSoak 139 SteamfloodingMulti-LayerReservoirs 140 Jones'SteamDrive Model L4L Jones Empirical Adjustment Factors i+Z Injectivity t44 Steady-stateDisplacementBetweenan Isolated Pair of Vertical Wells L45 Time for Breakthrough 147 Isolated Injection Well Surrounded by a Circle of Equally Spaced Producefs 148 Confined Patterns 149 Confined Horizontal Well Pair 150 RepeatedFive-Spot 151 Repeated Seven-Spot 152 SteamZone Shape:van Lookeren'sEquations t52 Numerical Example of the Use of van Lookeren's Theory t57 FiarouqAli's Unified Approach I57 Gomaa's Correlations for Predicting Oil Recovery 158 Vogelb Simplified Heat Calculation for Steamfloods 162 Comparison of Vogel's Predictions with Myhill-Stegemeier 165 Numerical Example 166 The Fast Process 167 Other Mechanismsin Steamflooding 168 Conversion of Mature Steamfloods to Hot Waterflooding 173 t74 Qualitative Review of Steamflooding Bibliography 175 chapter 5. THE DTSPLACEMENT OF HEAVY OtL ' Introduction 179 FiactorsAffectingDisplacement 179 Displacement Concepts 180 Piston Displacgqt.ent-,..180 '- Breakthroush 180-

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t04 Contents

Contents

179

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lgl fttont.l sl.blllty l8l Thc Thoorctlcal Approrcho to Displaccmcnt 182 Flood Intcrfacc Stability-Muskat's Model 182 Darcy's Law and Interfacial Stability 184 Effect of Interfacial Tension 185 A Simple Theory for Stabilizationby Interfacial Tension 188 Stability upon Interfacial Effect of Condensation L9l Miller'sTemperatureGradientStabilization 192 Darcy's Law for Two-PhaseFlow 192 Relative PermeabilityCurves t93 The Fractional Flow Equation 196 Effect of the Gravity Term on Fractional Flow Effect of SegregatedFlow on Apparent Relative Permeability 197 and Fractional Flow 200 The Buckley-Leverett Displacement Theory 200 The Velocity ofthe Shock Front 201 The SaturationBehind the Front 203 The Upper Shock Front 205 Conditions at Breakthrough 205 Recovery at and After Breakthrough 207 Effect of Viscosity Ratio 208 PressureGradients During Displacement 210 Numerical Problem on Buckley-Leverett Theory Comparisonof Displacementwith Diffuse and Segregated 2I3 Flows 213 Conditions at Breakthrough 214 Conditions When Oil-Water Ratio Falls to 0.025 2L4 Comparison of Oil Recoveries 214 Water Saturation Profiles 216 C.W. Nutt's Capillary Bundle Model 220 Analysis of Steamflood Using the Buckley-Leverett Theory 222 Buckley-Leverett Theory Applied to the Steam Chamber 222 Calculation of Volume of Steam Within the Reservoir 223 Heat Balance. 224 Numerical Example 225 Heat Balance, Saturations, and Recovery 227 Displacement of Oil Ahead of the Condensation Front 228 Effect of Shapeof Relative Permeability Curves 229 PressureDrop'forSteamflooding 232 SteamOverride 234 Effect of SteamQuality 238 Effect of Vertical Heat Loss 238 Effect of Increasing Steam Viscosity 238 General Conclusions on Displacement 239 Bibliography tr

24r 0. CYCLIC STEAM STIMULATION 241 Introduction The Stimulation of Wells with AppreciableCold Flow 243 Well Bore Skin 243 Near-Well Bore Region vl

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244 Bobcrl and Llntzb Modcl 246 Effect of ProcessVariables 248 Scaling of Thermal Models 250 Niko and Troost'sCyclic Steam Stimulation Model Experiments 250 Effect of ProcessVariables Simplified Analysis of Production Rate Decline During Reservoir 259 Cooling The Problem of the First Cycle in the Cyclic SteamStimulation of Tar 266 Sands TIl Cyclic Steamingof Vacca Tar, Oxnard, California 272 Compaction Drive in Conventional Heavy Oil Reservoirs 274 Fracturing and Reservoir Expansion During Steam Injection 275 StressDue to Gravity in a Semi-Infinite Strain-Free Solid 276 In Situ Reservoir Stresses 277 Fracturing Pressure 277 Ground Heave 279 Effect of Fracture Orientation on Productivity from Stimulation Possible Production of Orthogonal Vertical Fractures 280 from the Fracturing of a Line of Wells 281 Bibliography 285 Chapter 7. STEAM-ASSISTEDGRAVITY DRAINAGE 285 Introduction 285 Concept 286 Relationship to Convention Steamflooding 287 Gravity Drainage Theory 289 Darcy's Law 291 Integrated Flow 291 Material Balance 292 Velocity of the Interface 293 Position of the Interface 294 The Exponent m-An Extended Definition 295 Change of Variable of Integration 296 Original Scaled Visual Model 297 Dimensional Similarity 300 Original Scaled,PressurizedModels 300 Calculated Drain4ge Rates for Field Conditions 302 Extension to the Original SAGD Theory TANDRAIN-An 303 Effect of No Flow Boundary 305 Further Experimental Data 307 Extrapolation of the Model Experiment to the Field 307 The Rising Steam Chamber 309 Value of Proportionality Constant in Height Equation 310 The Oil-Production Rate 311 Shapeof SteamChamber 312 Available Head 312 Finger Rise Theory Effect of Steam Temperature, Reservoir Temperature, and Oil 313 Properties on Drainage Rates 313 Steam and Reservoir Temperatures

242

Contents

Contents

vlpol tnsm Thc ZlmPm'AEC Stcam Ccncntor BibliograPhY 411

316 Nuncrlcrl Problomo Stoem-Arldcd Orrvlty Dnln4o 321 Stoam-InJoctionWclls 321 Horizontal Injection Wells 325 Vcrtical lnjcctors 328 Avoiding the Steady-StateHeat-Distribution Assumption 330 Valuesof the ParameterBg 331 Heat Penetrationas a Function of Distance Along Interface 333 Predicted Oil-SteamRatios 335 Effect of Steam Pressure SAGD Results from Scaled Laboratory Reservoir Models Operating at High and Low Pressures 336 343 Oil Production After Stopping Steam Injection 344 Recovery of Heavy Oil Above Water 3,+8 Effects of Reservoir Heterogeneities 353 Fbrmation of WO EmulsionsWithin the Reservoir Well Bore Flow Resistance 356 357 Conclusions Bibliography 358 rpter 8. STEAM RECOVERYEOUIPMENTAND FACILITIES 3ffi Introduction 360 Steam Generation 364 Effect of Water Impurities 366 Deaeration and Oxygen Control 368 Oil Field Steam Generators 371 SteamQuality 371 Convection Section Radiant Section 373 373 Vertical Steam Generators 373 SteamDistribution System 375 Cluster6d Deviated Wells Thermal Well-Completions 375 378 Temperature Logging 380 Control of Heat Loss in Steam-InjectionWells 381 SelectiveSteamInjection Artificial Lift 381 387 Improving Well Performance 388 Treating ProducedFluids 393 Production Treatmentwith High Sand Production 393 Makeup Water Supply 394 Recycling ProducedWater 395 Produced Water Analyses 396 Treating Recycled Water 402 WastewaterManagement 443 Esso'sThermal Softening Process 4.03 ReducingTotal DissolvedSolids 404 Alternate Steam Generators 404 Coal-fired Steam Generators 405 Downhole SteamGeneration 447 Fluidized Bed CombustionBoilers vlll

410

41s Chaptcr 9. lN SITU COMBUSTION 415 Introduction 418 DrY Combustion bescriPtion of Phenomena 418 4t9 Combustion Tirbes 423 Alexander's Fireflood Pot 424 Fuel for Ratio otHlC Calculation Example of Stoichiometric Calculation for Combustion 425 Process 426 Fuel DePosition428 Oxidation LowjTemPerature 430 In Situ CombustionExperiments Using Oil Sands 432 Ignition 435 Temperature at the Combustion Front Combustion the upon Cooling Conductive of Effect 436 TemPerature 440 Examples o1 the Use of Ramey's Solutions 442 Oil Produced ProPerties of 442 Wet Combustion 445 LaboratorY Results 448 Water'to-Air Ratio 450 Sands Tar in In Situ Combustion 452 Use of OxYgenor Enriched Air 411 Potential Advantages for the Use of Oxygen . 454 Oxygen of Use the of Possible Disadvantages 455 The Cost of OxYgen The Effect of Pressureon Combustion Performance 457 with OxYgen 458 Design of In Situ Combustion Projects 459 Load Fuel Total 459 Air Requirement 4ffi Air Rate and Pressure 461 Oil DisPlaced per Volume Effect of Water-Air Ratio on Oil Recovery 464 Burned 466 Field Project Results 466 Lake Golden LloYdminster, 47L Ceityt Bellevue Field in Louisiana 473 Field ExPansionsat Bellevue 474 In Situ CombustionProjectsin Rumania 477 i BibliograPhY

360

481 Appendix 1. SYMBOLS 481 Lower Case 482 UPPer Case 484 EnthalPies 484 Greek Contents

Contents

Unltr and Convcnlon Fhcttm

4116

dlx 2. D€NSITIESOF OtL RESERVOTR MATERTALS Watcr at Boiling Point 487 SaturatedSteam 487 Brine Solutions 48i ReservoirOil 488 Rocks 490 ConversionFactors 490 Bibliography 490

Apprndlx !. THERMAT IN8ULATION Bibliography 520

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OF STEAM Appendlx 9. THERMAL PROPERTIES 521 521 Saturation Pressureand Temperature Enthalpies of SaturatedLiquid and Vapor Bibliography 523

TNDEX

dlx3. THERMAL coNDucflvrry oF orL REsERvorR MATERTALS 49l UnconsolidatedOil Sands 49L Comparisonof MeasuredThermal Conductivity of Tar Sand with Prediction from Somerton'sFormula 4g3 ConsolidatedPorousRocks 494 Comparisonof Thermal Conductivities of Consolidated and UnconsolidatedSandstones 495 Thermal Conductivity of Hydrocarbon Liquids 4g5 Thermal Conductivity of Liquid Water 495 Thermal Conductivity of Over- and UnderburdenMaterials Thermal Conductivitiesof MiscellaneousMaterials 4g7 ConversionFactors 497 Bibliography 497

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llx 4. HEAT CAPACTTTES AND ENTHALPTES 499 Sandstones 499 Carbonate 499 Clays 499 Oils 500 Water 501 Heat Capacitiesof Common Gases 502 Average Heat Capacities Betweeen T1 and T2 S0Z Changein Enthalpy Between T1 and T2 SW Volumetric Heat Capacitiesof Reservoir Materials 503 ConversionFactors 503 Bibliography 503 ix 5. VISCOSITIES 504 Viscosity of Crude Oil 504 Viscosity of Water and Steam ConversionFactors 513 Bibliography 514 x 6. HEATS OF COMBUSTTON Hydrocarbon Liquids 515 Fuel Gases 516 Solid Fuels 5I7 ConversionFactors 5L7 Bibliography 517 x

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511

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Contents

Contents

524

Prefsce

This book describesthe recovery of heavy oils and bitumen by in situ thermal methods.It is basedon the lecture notes,which have been developedby the author for an annual thirteen-weekgraduatecourseat the University of Calgary,to classes drawn from full-time graduate students and to a greater extent from engineers whose work is directly related to the oil industry. The author has presentedthe courseeachyear since 1982and the book has been written during this period. The first chapter is an introduction to the subject.The heavy oil and, more importantly, the bitumen depositsin Canadaare an enormousresourcewhich will become of great economic importance. Production from these sourcesis already equivalentto a very significant fraction of the Canadianrequirementfor crude oil. Other countries as well as Canadahave vast depositsof these crude oils. The depositsin the Canadianprovince of Alberta and thosein Venezuelaare eachapproximately equalin quantity to thoseof conventionalcrudesin placein the Middle East reservoirs.The purposeof this book is to discussthe technicalfactorsand problems which are involved in their production by those in situ methodswhich involve the heating of the reservoir. Although the book discusses,in a logical development,the theory and much of the practice in this area,it is not intendedto be an encyclopediaon the subject. It describesthe main ideasof the subjectwith the purpose of providing the reader with tools which can be usedto make further advances.In places,the book summarizes well establishedthinking whereasin others,it describesoriginal ideasand approaches;some of these have been publishedpreviously in paperswritten by the author and his collehgueswhile others appearhere for the first time. Chapter2 dealswith the transfer of heatwithin the reservoirbulk and within the adjacentregionsby thermal conduction.Equationsare presented,and many are derived,which allow the analysisand prediction of quantities such as the heat loss from the boundariesof a heatedreservoir.Numerical examplesin this chapter,like those in other chapters,provide the readerwith the meansfor the practical understandingand applicationof the theoreticalmaterial. The interspersingof numerical exampleswithin the book and, in some cases,the use of the results from the examplesfor the further developmentof concepts,are intended to make this book interestingand useful to the practical engineer.The approachemployedis practical and fundamentalwith a minimum of academicsophistication.The author'saddress is now in an invory tower but he camewith tar on his boots. xii

One of the conclusionsto be drawn from Chapter 2 is that simple thermal conductionis, in most instances,an inadequatemeansfor heating substantialreservoir volumesfrom small diameterwells. It is too slow. The third chapterdiscusses convective heating achieved by the injection of hot fluids such as steam or hot water. This allows heat to be introduced much faster and over substantialvolumes. Again a practical approachinvolving the use of illustrative numerical examplesis employed.One of the conclusionswhich the readerwill draw from this chapter is that a very substantialquantity of heat is required simply to raise a volume of reservoir to the steam temperature and that this quantity has to be augmented,frequently several-fold,in order to also supply the lossesof heat from the reservoir boundaries.The material in the third chapterprovidesthe readerwith tools which allow the estimation of these quantities and with a grasp of how the heat is distributed in steamrecoveryprocesses. Steamflooding and results from steamflooding field projects are discussed further in the fourth chapter. The chapter also extendsthe theoretical ideas developedpreviously.For examplethe tendencyfor steamto override the oil during lateral steamflooding and the contribution of steam distillation to recovery are discussed. Chapter 5 is concernedwith the mechanismby which oil is displacedby injectedfluids. A factor of major importancehere is that the displacingfluid is usually much lessviscousthan the oil. This causesinterfacial instabilitiesand the fingering of the displacingfluid-particularly if it is water. The situation can be different with steamsince it condenseswhen it intrudes into colder oil and it is the resulting aqueouscondensaterather than the steamwhich fingers.Also steamtends to float abovethe adjacentoil and override becauseof its low density. One of the subjects which is discussedwith practically-orientednumerical examplesis the displacement of oil by steam within a steam-saturatedregion using the Buckley-Leverett approach. This mechanismis surprisingly effective despite the sharp contrast between the viscosity of the steamand the oil. It is shown that the reasonfor this is that the flow of steamin suchsituationsis, on a volumetric basis,much higher than that of the oil. Steamcontainsmuch lessheat per unit volume than doeshot water and much larger volumes are required to heat a volume of reservoir. These much larger volumesare much more effective in displacingoil from the heatedzone even though the dynamicviscosity of steamis lessthan that of water. The cyclic steamstimulation processis describedin Chapter 6. This process was discoveredby accidentin 1959and it provided the main thrust for the early developmentof thermal recoveryin California, although most of those projectshave now been convertedto steamflooding.Steamstimulation is still the major process for the in situ recoveryof Alberta bitumen although it is likely that it too will be surpassedeventuallyby steamdisplacementprocessesbecauseof their potential to achievehigher oil recoveries. The Steam-AssistedGravity Drainageprocessis describedin Chapter7. This involves steamfloodingto horizontal production wells which are located near the baseof the reservoir. Steamis injected from wells which are higher in the formazonesform and grow abovethe productionwells. The growth tion. Steam-saturated of these steam chambers can be both vertical and sideways.The oil near the Preface

xIl

boundary of eachchamberis heatedand it flows by gravity downwardsto the production well. An important feature of the processis that the displacedoil remains heatedasit flows to the productionwells.The processhasbeentestedin field pilots, particularly in AOSTRAs Underground Test Facility in the Athabasca tar sands near Mclvturray.The performanceof this pilot is promising and recent reviewsby AOSTRA concludethit the processshouldbe economicallycompetitivewith steam stimulation even for projectJrecoveringthe extremelyviscouscrude of Athabasca. The facilities which are usedfor thermal recoveryincluding steamgenerators, wells, lifting practices,treating, and recyclingwater are discussedand analyzedin Chapter 8. Heavy oil and bitumen recoveryusing in situ combustionis reviewedin Chapter 9. The main attraction to this process,as comparedto steaming,lies in the much lower cost of the heat for the reservoir.This advantagecontinuesto generate interest in the processalthough developmentactivity appearsto be declining. The chapter includesa discussionof the principles involved and describesseveralsuccessfuland economicfield applications. The final chaptersin the book are Appendiceswhich contain data and correlations useful in the analysisof thermal recoveryprojects' The author is grateful to many peoplefor the help and advice'theyhave given to him in developingthis work to its presentstate:Chi Tak Yee, Viera Oballa, and philip Bakesaswelias many other students,made important contributions in identifying inconsistencies,and errors, both substantialand typographical, in earlier veisionsof the text. Riza Konak of EssoResourcesCanadaand Ken Porter of Gulf ResourcesCanadareviewed the material of Chapter 8 and suggestedvaluableimprovementsand additions. Gordon Moore and Matthew Ursenbachof the Univeriity of Ca[ary reviewed the material on in situ combustionand made important The author is also indebtedto his former colleaguesof the and useful-suggestions. Heavy Oil ReiearchDivision of EssoResourcesCanadawho contributed ideas,advice, and enthusiasmwhich became embeddedin his experience.He has clear, vivid memoriesof many stimulating and productive disussionswith G. S. McNab, H.Y. Lo, D. J. Stephens,M. weiss, F. Greebe,D.A. Best, S. Bharatha,P. N. Troffimenkoff, p. J. Griffin, R. Leaute, and many others. For him they were exciting yearsand exciting people.The encouragementand supportwhich the author hasreceived from the- Alberta Oil Sands Technology and ResearchAuthority (AOSTRA), and particularly from its first Chairman Dr. C.W. Bowman and its first Vice-ChairrnanDr. M. A. Carrigy, is also acknowledged.The Authority employed the author as Director of Technical Programsduring 1983and it was in this period that its plans for the UndergroundTest Facility were finalized. In March 1984the author's proposal for the Sleam Assisted Gravity Drainage process as the first processio be demonstratedin the UTF was presentedat a review organizedby AOSTRA for potential industry participants. The successof the subsequenttest and the enthusiasmwhich this has generatedin industry has done much to bolster his confidencein presentingthe material of Chapter 7. The author is also indebted to the Calgary Seition of itre PetroleumSociety of CIM and to the industries of calgary lor ttreir endowmentof the chair of PetroleumEngineeringwhich he has o""ipi"a since 1983.This support has made the writing of this book possible' xrv

Preface

I wish to thank the following for permission to use and copy material for which they hold the copyright; in eachcase,credit is also give to the author where the material appears: (1982)' Alberta Energy provided the data for Fig. 1.3 from their publication EnergyHeritage A'2'1' print Fig' permission to The American PetroleumInstitute granted The Alberta Oil SandsResearchand Technology Authority (AOSTRA) for works published in theAostra Journalof Researcft,the proceedingsof the UNITAR/UNDP International conferences of Heavy Ciude and Tar Sands, and proceedings of the AOSTRA and CANPET thru 8.19,8'29,8'30' 8.1,7 seminars:rigs. 1.4,7.30thru 7.32,7.45thru 7.54,7.67,7.70,8.14, 8 . 3 3 ,9 . 5 ,9 . 2 2 , 9 . 2 3 a, n d9 . 4 4 . Babcock& wilcox, Barberton,ohio, for Figures8.1 thru 8.3 reproducedfrom their publication Steam. Business Information Services (BIS), copyright holders for PetroleumEngineerInternational magazine,for permissionto use Fig. 9.64. The Canadian Institute of Mining and Metallurgy, publishers of the Journal of Canadian .34,7.39,7.44,7.54thru for the following:Figs.4.7,7.I2, 7.15,7.16,7 Technolagy Petroleum ' 7.56,7.68,7.69,and8.2'1 The canadian Journal of chemical Engineering(cichE) for the following: Figs. 1.8, 7'1 thru 7 . 3 , 7 . 5 ,a n d7 . 7 . The CanadianSocietyof PetroleumGeologistsfor Figs. 1.2 and 1'5' Corod ManufacturingLtd. provided the original drawing for Fig' 8'21' Editions Technip, Paris,Francegrantedpermissionto reproduceFigs. 9'2 and 9.34. EssoResourcesCanadaLimited for permissionto useFigures8.23,8.24,8.25,8.35'and 8.38. Mr. W. H. Fairfield and Mr. P. D. White for permissionto publish Figures9.60 to 9.63 and Tables9.7 to 9.10. Foster Wheeler Fired HeatersLtd., Calgary,Alberta provided us with the illustrationsfor Figs. 8.10(a)and 8.10(b)' Dr. G.W. Govier for permissionto use his data in Table 1.2. McGraw-Hill, N.y., publishers of the 1st UNITAR/UNDP International Conferenceon Heavy crude and Tai sands.for the following: Fig. 1.1,Table 1.9, and Figs. 8.31and 8.32. Natco canada, calgary, Alberta provided the drawingsfor Figs. 8.7, 8.28' and 8.34. The National Research Council, publishers of. the Canadian Journal of Earth Science,for Fig.6.29. Professorc.w. Nutt, granted permission to publish Figs. 5.32 thru 5.35 and Figs. 5'40 and 5.41. Oxford University Press,Oxford, U.K., for Fig.2.l2. Used by permissionof the Oxford University Press. The petroleum Societyof the CanadianInstitute of Mining and Metallurgy (CIM)' Calgary Section,for Figs. ?.58 thru 7.66 published in the preprints of the 40th Annual Technical Meeting of the Petroleum Society of CIM. The society of Petfoleum Engineers holds the copyright for all material published in their SpE papers, theJournalof PetioleurnEngineering,the Societyof PetroleumEngineeringlournal, and transcripts of ttre Spb,of AIME. Permissionhas been received and acknowledgedfor the following:Figs. 1.9,4.g,4.11thru 4.1,4,4.17thru 4.2L,4.29thru 4.40,4.43thru 4.5t'5.1, 5.7,5.g,6.2thru6.13,6.I5,6.21,6.23,6.24,8'39,8.43thru8'45,9'6thru9'21,9'25thru9'32' 9.35thru g.3g,g.45thru 9.47,9.50thru 9.55,and 9.65;Tables3'3, 4.1thru 4.5,4'8,6.2'8'4' and 9.6. Dr. P. G. Saffmangrantedpermissionto reproduceFigs' 5'3 and 5'4'

TOTRAN ServicesLimited, calgary, Alberta provided the photograph for Fig. 8.8. Eugene F. Traverse supplied Figs. 4.15 and 4.16'

The first draft of the book was typed by Mrs. Margaret McAuslan in 1'984and the author is grateful to her for her hard work and interest. Since then the annual revisionsand ixtensions of the lecture notesand the manuscriptfor this book have been typed in a world-classmanner by Mn. Patricia Stuart-Bakes.The author wishes-tothank her for her perseverance,moral support, and enthusiasm. Finally, the authorwishesto recognizethe encouragementand patienceof his wife Joyce*ho hur understoodand supportedhim. Writing books is an interesting and worthwhile endeavorbut it is time consumingand hard on one'sfamily. Thank you, Joyce. Roger M. Butler Calgary,Alberta

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Preface

1 Introduction to Thermsl

Recovery

The efficient and economicrecovery of heavy oil and bitumen from reservoirsin Canada,Venezuela,and elsewhereis a majortechnicalchallengeand taskl As will be seenlater in this chapter,the quantitiesof heavyoil and, p-articularly, bitumen in place are as large as and probablyfar larger than thoseof conventionaicrude oil. The challengeis twofold: recovering the oils from the reservoir and converting them to useful petroleumproducts. Heavy oils and bitumen contain much largei proportions of nondistillableresidualmaterial than do conventionaloils. The residuescontain larger proportionsof asphaltenes, and this makesthem particularly viscous.It is their high naturalviscositythat makesthe recoveryof heivy oils ani bitumen difficult. The samefactors that determine the viscosity of theseoils also greatly affect their conversioninto conventional petroleum products. The high contents of asphaltic residue make them particularly suitable for asphalt maiufacture but also greatly reducetheir suitability for most other purposes.Their conversionto distillate boiling-rangematerial involvesresidualcrackingprocessessuchas coking and/ or hydrocracking.The high contentsof sulphur and niirogen in the distillatesireate the need for extensivehydrotreating.The aromaticcontent of the middle distillates obtained reducestheir value as dieseland aviation jet fuels. Improvementof these propertiesrequiresfurther extensivehydrotreating. The productionand utilization of heavyoils and bitumensasbasicraw materials for the manufactureof the conventionalproductsof petroleumthus involvesextensivetechnology;there are great incentivesfor the extensionand improvementof this technology.This book concentrateson the first of the two areasdescribed.the

recoveryphase.Although it may appearthat this phaseis the more straightforward of the two, neverthelessit aboundsin interestingfacetsand opportunitiesfor development and invention. ENHANCEDOIL RECOVERYMETHODS As the availability of conventionalcrude oil has declined, there has developedan increasedincentive for the improvementof the recovery from known reservoirs, and methodsfor "enhancedoil recovery" have been developed.The most important of these are as follows: . r . .

RECOVERY THERMAL Steamstimulation Steamflooding Hot waterflooding In situ combustion CHEMICALPROCESSES

o Surfactantfloods o Polymerfloods r Alkaline floods MISCIBLEDISPLACEMENT

o Light hydrocarbonfloods r Carbondioxidefloods This book is concernedwith the first of these, thermal recovery, a subject areathat includesthe techniqueswhich havefound the most extensiveuse.Most of the applicationsof thermal methodsare for the recoveryof heavyoils that are too viscois at the original reservoirconditions to flow with economicrates and recoveries.The effectivenessof thesetechniquesdependslargely upon the reduction in oil viscosity that accompaniesheating. Although heating the oil requires energy, this is, in lconomic applications,considerablyless than the energy that the producedoil is capableof providing. A flctor which promotesthe useof thermal recovery processesis that miny of the depositsof heavycrudesare large,rich, and often *"ti kno*n. Thermal recovery projects are usually profitable and are frequently quite large. ' Th; fo[owing introduces the sgliqnt characteristicsof the common thermal recovery approaches. Steam Stimulation Shell discoveredthe processof steamstimulation by accidentin Venezuelawhen it was producingheavycrude by steamfloodingthe Mene Grande field near the eastern shoreof Lake Maracaibo' During the flood, a breakthroughof steamto the surface of the ground occurred and, in order to reducethe steampressurein the reservoir,the injectionwell lntroductionto Thermal Recovery

Chap. 1

wasallowedto flow back. Copiousquantitiesof oil were produced;from this accidental discoveryin 1959(reported by de Haan and van Lookeren 1969)came the steamstimulationprocess,which also goesby the nameof steamsoak andhuff and puff' There was a very rapid growth in the use of steam stimulation in the next decade,particularlyin California.By 1967there were 408 steamgeneratorsin use in Californiaproducingabout 120kB/d of oit (Burns 1969). In the steamstimulationprocess,steamis injectedinto the reservoirat rates of the order of 1000B/dl for a period of weeks;the well is then allowed to flow back and is later pumped.In suitableapplications,the productionof oil is rapid and the processis efficient, at leastin the early cycles.The processis usedexiensivelyin California and Venezuela;if the steampressureis high enoughto fracture the reservoir and thus allow injection,it can alsobe usedto producethe very viscousoil of the oil sands.For this operation,a steampressureofibout 1 psi per ioot of depthis requiredto overcomethe in situ rock stresses to causefracturing. Imperial oil-and later its productionwing, Esso Resourcescanada-has been the leadingdeveloperof the cyclic steamstimulation processfor the production of bitumen from the oil sandsat Cold Lake. This developmentstartedwith small-scalepilot experimentsin the early 1960s.With -ote-or-lesscontinuousdevelopmenton an ever-increasingscale,Esso'sCold Lake field is now producingover 80'000B/d'of bitumen and this, togetherwith its proportionateshareof the production from the Syncrude operation, has now converted tmperial oil, which is Canada'slargestoil producer,to one that is dependentfor about half of its production on Canadianbitumen.It is reasonable to expectthat thesetrendswill continue. The main drawbackof the cyclicsteamstimulationprocessis that it often allows only about l5Vo of the oil to be recoveredbefore ihe oil-to-steamratio becomesprohibitivelylow. Steamflooding In this processsteamis forced'continuously into specificinjectionwells and oil is driven to separateproduction wells. The zonesaround the injection wells become heatedto the saturationtemperatureof the steam,and thesezonesexpand toward the production wells. oil and water from the condensationof steamare removedfrom the producers. With viscousoil there is a considerabletendencyfor the steamto override the reservoir,and this tends to limit the downward penetrationof the heat and hence the recovery. Steamflooding can allow higher steam injection rates than steam stiinulation; this advantageoften offsetsthe rather lower thermal efficiency.Steam stimulationusuallyrequiresless(and in favorablecasesfar less)steamthan llooding initially but is lessefficient as depletionproceeds.Often it is economicto switch to steamfloodingafter initial operation of a field by steamstimulation. The recovery from steamfloodingcan approach50Voor even more. lln the oil fields steamquantitiesare normally measuredas the volume of water at standard conditionscontainedin the steam;a barrel of steamis thus 350 lb and a cubic meter is 1 tonne. Burning bitumenasfuel in a conventionaloil field steamgeneratorwould produceabout 14 to " 15 m3of 70% quality steam per cubic meter of fuel burned (or 14 to 15 B/B).

EnhancedOil RecoveryMethods

It is usualand desirableto produceoil first by steamstimulationfrom both the injectorsand producersin a steamflood project. This providesrapid initial production and better economicsand also allows effective steamfloodingto be achieved more rapidly. Hot Waterflooding Hot waterfloodingis usuallylesseffectivethan steamfloodingbecauseof the lower heatcontentof hot water comparedwith steam.Also, it is found that the residual oil level that can be achievedwith a hot waterflood is markedlyhigher than that found with steam-even at the sametemperature. It is thoughtthat steamis more effectivethan hot water in displacingoil becauseof the following: l. The extra pressuredifferentialresultingfrom the higher kinematicviscosity of steam.A comparablemassflow of steamresultsin much hieher fluid veloc_ ities and pressuredifferentials 2. A relatively low tendencyfor steamto finger comparedwith water. 3. Steam distillation effects, which allow volatile fractions of the crude oil to evaporateinto the steam and be carried by it. There are, thus, some of the characteristicsof a miscible flood in displacementby steam. These factors are discussedin subsequentchaprers. There is some application of hot waterflooding as a follow-up rreatment to steamflooding;this is practicedin severalareas. In a later chapterit will be shownthat, during a steamflood,oil is largely,and effectively,displacedfrom the steam-saturated zone (the steamchamber)und irunrferred through the condensationfront. As the oil proceedsthrough the condensation front, it cools rapidly and its viscosityincreases.In tar sani reservoirswith high initial oil viscosity,this displacedoil can rapidly sealoff any communication passagesthat may exist (see,for example,sufi 19gg).In this reference,sufi shows that the injection of steaminto a permeablewater-saturatedzoneat the baseof a model reservoir containing tar sandsresulted in rapid blockage;bitumen carried into the fracture plugged it as it cooled. on the other hand, ihe injection of hot waterresultedin the gradualheatingof the tar sandmasswithout blockage;aswill be seenlater, the reasonfor this is that hot water effectsrelatively little transport of bitumenascomparedto steam.This differencemay be usefulif it is desiredio heat tar sandsby the injectionof heat-carryingfluids into a relativelythin permeable zone or fracture. Under thesecircumstances,hot water is superiorto steambecause the permeablezone doesnot becomeblocked. In Situ Combustion In situ combustioninvolvesthe generationof heat by combustionwithin the reservoir. Air or (in somerecent tests)oxygenis suppliedto the combustionzone by injection into wells drilled from the surface.fhe main attraction of theseprocessesis Introductionto Thermal Recovery

Chap. 1

.

that heat is producedmore cheaplythan by surfacesteamgenerators.Although the fuel for heating comes from the reservoir itself, there is a substantialenergy requirementfor driving the compressors and-if oxygenis used-for operatingthe oxygenseparationplant. I As the combustion2oneadvancesthrough the reservoir,the oil aheadof the front becomesheated.Volatilefractionsare distilledfrom the oil and then. as the temperaturerises,thermal crackingreactionsoccur. The residualoil eventuallv forms a coke residue In the successfulapplicationsof this process,it is, for the mostpart, this coke that burnsand suppliesthe fuel; becauseof the distillationand crackingthat occur, the producedcrudetendsto be lighter and somewhatmorevaluablethan the original crude oil. Emulsionsproducedby in situ combustionare often very difficult to separate. In situ combustiontends to be lessstable2than steamprocesses,and premature arrival of the combustionfront at the productionwells is common. Thii often causeswell failure. Problemsare alsocreatedsometimesby the bypassingof oxygen containinggasaround the front into coolerparts ofthe reservoir.This resultsin low temperatureoxidation (LTO) reactionsin which the oxygenis addedchemicallyto the oil. The oxygenated productshavehigherviscosities,and this makesthe oil jess easily recovered.Also, valuableoxygenis consumedwastefullyby IIIO reactions. The inherent thermal advantageof in situ combustionas comparedwith steamshouldbe greatestwhereheat lossesfor steamprocessesare greatest-in thin reservoirsand in deeplyburied reservoirs.In in situ combustion,only the reservoir at and beyondthe fire front needsto be at high temperature,particularlyif wateris injectedaswell as air (wet combustion).Waterinjectiongenerates steambehindthe combustionfront. This steampassesthrough the front and condenses aheadof it. In this way, heat that would otherwisebe left behind is utilized in steamflooding the oil aheadof the front. The verticalsegregation, due to gravity,of the water and airloxygenbehind the front can be a problem.

WORLD FUELRESOURCES Table1.1comparesestimatesof the world'sreservesof oil, gas,shaleoil, and heavy oil and tar sandsexpressed in exajoules(1 EJ : 1018 J = 169 x 106B of oil or 0.95 x 1012 SCFof gas).The columnsin the tableare not comparablebecausethe first two are for recoverable reserves, whereasthe secondtwo representthe resourcein place. Howeverit is very clear that the oil sandand shaleoil resourcesare enormous. TableI.2 comparesCanadianenergyresourcesof differenttypesusingboth a proven, recoverablereservebasis and also an "ultimate" (recoverable)resource basis.The productionin the year 1982is also shownfor comparison. 2The

reasons for the greater stability of steam fronts are discussed in later chapters. Here it is sufficient to note that if a finger of steam tends to advance before a broadly moving front, the steam will tend to condense, leaving only water to advance, and this will become rapidly cooled. Thus a stable advancing steam front can have in froirt of it fingers of cold condensate running toward the well. It is the water that fingers, not the steam.

World FuelResources

in Exajoules TABLE1.1 WorldFuelResources Resource in Place

Established Reserves

3970 3189

oil(t) Gas(t) Shaleoil(2) Heavy oil and oil sands(3)

100,000 22,000to 36,000 (16000 of abovein Canada)

(1)R. Enright (1982) J. (2)F. Hart'iey,J. M. Hopkins and H' C' Huffman (1980) L. (3)J. Janisch(1979)

comparison of the upper and lower parts of the table showsthat opportunities relatively limited' for findinj conventionatoii in Canadamay be consideredto be gas' for discovering potential more be On the oiher hand, there appearsto for conventional than higher much are oil The presentreservesior synthetic potential includesoil oil, and the potential is very -oth high".. This is becausethe mining' open-pit from in situ recoveryas well as that from

TABLE 1.2 Canadian

Resourcesin Exaioulesand Exajoules

Year FRONTIERS

RESOURCE

1982 PRODUCTION

NONFRONTIER

ARCTIC

OFFSHORE

TOTAL

Proved Resources Conventional oil Syntheticoil from tar sands Natural gas Coal Uranium (CANDU eff.) Hydro (30 yr)(l)

2.4 0.3 3.0 1.0 4.8 2.7

29 150 82 430 131 89

1 0

Total L4.2

911

9

7 0

0 8

)t

150 82 430 139 89 927

Ultimate Resources Conventional oil Synthetic oil from tar sands Natural gas Coal Uranium Hydro

2.4 0.3 3.0 1.0 4.8 2.7 Total 14.2

60 1,170 147 16,000

252 2'70

2r7 1,170 475 16,270

201

50

257

11,5'78

661

144

18,383

(r)Hvdroelectricpower is a renewable,"*.rra", and the reservesare, in principle, infinite' To achieve a y trt" q"antitv of energy thai woul-dbe produced from 30 of comparison,the quantitiesshown here t#;;;;1 opefation. (from Govier 1983)

lntroductionto Thermal Recovery

Chap' 1

The potential coal resourceis now seento be enormousand much higher than that for oil sands.The data indicate that there is sufficient coal to supplyCanada's presentproduction of energy resourcesfor over 1000years at the present rate of consumption-assumingthat the coal can be convertedinto the requiredforms. THE OIL SAND RESOURCE Table 1.3 lists estimatesof the volume of oil in place within the major known depositsof oil sand.There is considerable uncertaintyin thesefigures-particularly thosefor Venezuelaand for Alberta's CarbonateTriangle. Nevertheless,it is apparent that the heavy oil resourceis, for the major part, divided betweenCanadaand Venezuela. Canadais not endowedwith much "conventional"crude oil (at leastwith easily accessibleconventionalcrude oil that can be found) but it does have tremendous TABLE 1.3 Major Heavy Oil and Oil Sands Deposits Volume in Place (Billion Barrels) Venezuela Orinoco heavyoil belt Canada Athabasca Cold Lake Wabasca PeaceRiver Lloydminster CarbonateTriangle

700-3000

869 270 119 92 32 1350 Subtotal

U.S.S,R Melekess Siligir Olenek

U.S.A Tar Triangle Circle Cliffs Sunnyside P.R. Springs Hill Creek Asphalt Ridge Variousheavyoils

The Oil Sand Resource

Lower CretaceousSands Lower CretaceousSands Lower CretaceousSands Lower CretaceousSands Lower CretaceousSands PaleozoicCarbonates

PermianSands CambrianCarbonates PermianSands

r44 16 1 4

4 I I 110

Subtotal 137 Four-countrytotal 37134013 (from Janisch1979)

Tertiary and Lower CretaceousSands

2732

r23 13 8 Subtotal

GeologicalAge

PermianSands PermianSands EoceneSands EoceneSands EoceneSands EoceneSands Tertiary, Mesozoic

T

Caribbean Sea

Venezuela

Legend

i il,y;j*"

cotombia

C Eastl-ake D Barinas E Apure F SouthGuarico G SouthAnzoategui& Monogas H Delta I Guanoco J Gutfol Paria K N.W.Trinidad

Figure 1.1 Heavy Oil and Bitumen in Venezuelaand Trinidad (after Gutierrez 1979)

quantitiesof oil sandsand very substantialamountsof conventionalheavy oil. Canadacontainsabout one-sixthof the world'sdiscoveredoil in place,but about 95%of it is bitumen.The recoveryand utilizationof this bitumenis a challensefor engineersand scientists. VENEZUELANHEAVYOIL The Venezuelan heavyoil fieldsand the extensionsto them lie in a band acrossthe northern end of South America, as may be seenfrom Figure 1.1 (Gutierrez 1979). The easternend of this band lies in Trinidad (K), where asphalthas been a productfor manyyears.To the west lies the Gulf of Paria (J) and Guanoco(I). To the southand west lies the orinoco tar belt (E, F, G, and H), which contains the bulk of the materialshownin the previoustable.Up until now it has not beendeveloped,althoughthereare significantplansto do so.Area D is the Barinas subbasin. The reservoirsaroundLake Maracaibo(A, B, and C) are the mosthighly developed.It is here that Shellfirst experimentedwith steamfloodingand discovered steamstimulation.Productionfrom the Bolivar coastis discussedin Chapter6. CANADIAN HEAVY OIL AND BITUMEN Although the origin of the Alberta oil sand depositsis speculative,the following seemsto be a likely description. Figure 1.2(Jardine1974)showsAlberta asit is thoughtto havebeenin Cretaceoustime (120million yearsago).The climatewastropicaland giant rivers,fed by water from the Canadian Shield in the east and from mountains to the west. Introductionto Thermal Recovery

Chap. 1

%w'\"-8ASK.

LT S.

too Mil,E5

-

EI % ffi E +

+

11,::rr..r.;l

\:i1i;'.1^'

MATN.Y sANosToNE \."'t'il)..ir. MATN.Y sHArE

i

lonox

\-5t',' \$ '''i

\,liil".i.x"'\r'..i,

H€AVY OII SANOS = 6I

1€

OE

d ^v

cr) e!

13

Billions of CubicMetres 100 50 lr,r,l,,"l

s r

Lloydminster Gold Lake Athabasca(Mining) Athabasca(in Situ) SaudiArabia 0 Sourca EnorgyMlnat lnd Rrtouril,

Oll In Place RecoverableOll

400 600 200 Billions of Barrels

800

Cmldt

Figure 1.6 Comparisonof CanadianBitumen & Heavy Oil with Conventional Reservesin SaudiArabia (after Allen 1979)

HeavyOil and Oil Sands

i Vonszuela Canada

u.s.s.R.

!! !l tl

i

Othe|si ! I

ConventionalOil in Place

I

iMiddleEasti NorlhAmerica

iu.s.s.R. I

i Others

12

Oil in PlaceTrilllonsof Barrels Figure 1.7 Comparisonof Quantitiesof Heavy Oil and Tar Sandsto those for ConventionalCrude Oil in Place(order of magnitudeestimates)(after Janisch r979)

There is a total volume of heavyoil in placeof about 107billion barrels.About half of this occursin California,and most of the remainderis in Texas. Most of the tar sandsin the United Statesare in Utah; the total volume in place is about 24-30 billion barrels. THE NATUREOF HEAVY OIL AND BITUMEN DEPOSITS Although this sectionis written with the Cretaceousdepositsof heavyoil and bitumen in Alberta in mind, most of the conceptsare applicableto other deposits'Qll sand as it occurs in a reservoir is a multiphasemixture with a very definite struclntroductionto Thermal Recovery

Chap. 1

TABLE 1.5 Heavy Crudesand Tar Sandsin the U.S.A. HEAVY CRUDE OIL

Billions of Barrelsin Place

Alabama and Mississippi Arkansas California Louisiana Oklahoma Texas Utah Wyoming

J

5 54 6

z JI I

5 Total

r07

TAR SANDS Utah Other

23-29 1 Total 24-30

(basedon Whiting 1979)

salq or sometimesweakly cqnsoUqglgd ture. It consistslargelyof unconsolidated ',-----..fr within the pore spacebetweenthe containingfluids-oil, waterandsomdtimE3Sas gitini. In addition to the sandgrains, other finer solids are present:silt and clays. '-**A characteristicfeature of many of the tar sanddepositsis that most or all of layer of water is spreadover the solid surface, the solid-fitfAiiai;is1ygtgkd-A pore spacefrom contractingthe sag$,913i;r_s, bitumiffi the th6 Whre!319-y9nts . Figure 1.8 (Takamura 1982)shows the structure of typical Athabasca tar sand.Waterwithin the sandis shownas occurringin three forms: 1. At the grain-to-graincontacts,water is presentas pendular rings, which derive their shapefrom interfacial tension. 2. Along the surfaceof the solid materials,there is a thin (about 10 nm, or 0.01lcm) film of water. Although this is only a few tens of moleculesthick4,it is sufficient to protect the solid surfacefrom contact with the bitumen. 3. Water is associatedwith clay and other fine material. These solids occur as clustersof fine particleswithin the pore spaceand are often distributed as a layer on the main sand particles. 3Sometar sandssuchasthosein Utah are oil-wet rather than water-wet;the hot-waterprocess is not effectivein separatingthese.The solid matrix in someheavyoil and bitumen reservoirsis car'Alberta carbonatetriangle" bonate rather than sand.The Grosmont and other formations in the contain important Canadiandeposits.Theseformationsare frequentlykarstic and contain fractures and reservoirvolume.The RospoMare reservoir and voidsthat can provide important flow passages in the Adriatic Seais a very largeEuropeandepositthat-even thoughit containsa very heavyoilcan be producedby nonthermalmethods.The productivity is greatlyinfluencedby the fracturesand void volumes. uThe averagedistancebetweenmoleculesin liquid water is of the order of 0.3 nm.

The Nature of Heavy Oil and Bitumen Deposits

15

7

Figure 1.8 Diagram showing the Structure of AthabascaOil Sand (Courtesv Takamura1982)

Solid Mineral Matter Solid mineral matter is often a very complexmixture in itself. Usuallyit is unconsolidated.lhg large grains are called sand. In some cases,particularly in the Mc_\41rrBy format-ionof Athabasca,the sandgrains are almostentirely quartz.ln ,J other cases,such as the Clearwaterand Grand Rapids sandsof{tbeita (see Figure 1.5) the grains are a complexmixture of various mineral types: quartz, av7lchert, feldspars,and volcanicfragments. In additionthere are fine particles(lessthan 300mesh),which vary from less than 5 wtVoin high-gradesamplesto over 20 wtVoin low-grademateriaiThe fines containsubstantialproportionsof clays(e.g.,30-60 wtVo).The claysincludekaolinite, smectiteor montmorillonite,illite, and chlorite.The particlesizeof the claysis generallylessthan 2 pm. Clay mineralsare hydratedaluminum silicatesthat frequentlycontain other cations.They have a sheetstructuresimilar to that of mica. On an atomic scale, there are two kinds of layersthat occur in eachof the precedingclay minerals. l. Silica tetrahedra.Theseare tetrahedraof oxygenions with central silicon ions. Theseunits are linked togetherto form a hexagonalsheetof indefinite size. 2. Alumina or aluminum hydroxide layers. In these, oxygen ions or hydroxyl ions form two parallel sheetswith aluminum ions arrangedbetweenthe oxygen octahedra that constitute the structure; only two-thirds of the possibie aluminum sites are occupied, and the averagearea occupiedper aluminum ion is the sameas the area per silicon ion in the silica tetrahedrallayer. The main differencebetweenthe structure of the different clay mineralsarises from the relative proportions of the two types of layers. raoiinite Kaoliniteis basedon a 1:1 combinationof the two typesof layer.Its compositionis (oH)8Al4si4o1o 16

Introduction to ThermalRecovery Chap.1

Stoichiometrically,this may be looked on as follows: Silica layer 4 SiOz

Hydrated aluminalayer 2 AlrO(OH)4

It shouldbe realizedthat eachsilicon ion is, in fact, at the centerof an oxygentetrahedron, and the aluminum ions are each betweensix oxygenor hydroxyl ions (an octahedronhas six corners). When water is addedto kaolinite, the lattice doesnot expand(i.e., the distancebetweenadjacentlayersdoesnot increase).Another characteristicof kaolinite is that substitutionof iron or magnesiumfor aluminum is not observed;kaolinite is usuallywhite in color. It was named in 1867after a hill near JauchauFu in China (reportedin Grim 1968). Montmorillonite Montmorillonite (named after Montmorillon in France) is frequently used interchangeablywith the mineral name smectite.Someauthorsimply the broad class of expanding lattice clays by the term smectite and reservethe term montmorillonite for clay minerals of this type having only a small replacementof Al by Mg (Grim 1968). The mineral is basedupon a layer structure having one layer of alumina/ aluminum hydroxide sandwichedbetween two silica tetrahedrallayers. In the idealizedcasewith no substitutionof Al it has the composition (OH)4Al4Si8O2s. nH2O The Si/Al atomic ratio is now 2, as comparedto 1 for kaolinite. The structure may be visualizedas follows: Silica layer 4 SiO2

Hydrated Alumina 2 AI2O2(OH)2

Silica layer 4 SiO2

Water layer nH2O

The water is presentas a layer of water that penetratesthe lattice, between the silica layer of one threeJayer,silica-alumina-silica unit and the silica layer of the adjacentone.This quantityof water is variable.The additionof water to montmorillonitecausesthe lattice to expandand the clay to swell.This is an important characteristic of smectiteclays. The swellingof montmorilloniteclay is greatlyreducedif potassiumor magnesiumions are presentin the water layer,sincetheseare ableto bind the triplelayer sandwichestogetherand preventthe intrusion of water. This sensitivityof smectite clays to fresh water is of practical importance in petroleum engineering, since swollenclayscan plug reservoirs. lllite Illite (the "clay mica," namedafter lllinois (Grim 1968))is of a similar structureto montmorillonite except that some of the silicon ions are replaced by aluminum ions, and the resultingdeficiencyin charge(Al3* is trivalent, whereasSi4* is teThe Natureof HeavyOil and BitumenDeposits

17

F

-

travalent)is made upby the presenceof potassiumatoms.Theseappear at the out_ side flat surfaceof the three larger units and bind them together.This prevents swelling when water is added. Other substitutionsof metal]on, ur" found commonly within illites. Chlorites Chlorites have a threeJayerstructure similar to montmorillonite but are magnesiarich. The three-layerunits are held togetherby a magnesiumhydroxide layerl Chlorite claysare nonswelling. For more information on clays,the readeris referred to F. J. pettijohn (1957) and R. E. Grim (1963). Effect of clays on permeability clays, and particularlyswellingclaysof the smectitetype, can influence the per_ meabilityof a porous-solid.Swellingclayscan expandto utoct pores and particularly pore throats,and claysin generalcan alsobecomedetachedfrom the surfaces to which they adhere.They can then be carriedby the movingfluid and deposited so that they block the throats of pores.This is often not a ierious problem with high-permeability sands. In a recentstudy(M. Kwan 19gg)it wasshownthat repacked,extracted cores from Cold Lake had a muchlower permeabilityif they*"r"'L"fo*d to freshwater. Presumably -clays migrated and swelled and pluggei the coie. However, it was found that if extractedpreservedcoreswereemptoyed(i.e.,cores that had not been broken up and repackedbut were used in their originar mechanical form), then fresh water had little effect on the permeability.The iiff"r"n.. between the behavior of.repackedand preservedcoreswasvery large, and this shouldbe a concernto experimenters wishingto test permeabilityana ines migrationphenomenain core material. Water The sandgrainsin oil sandsusuallyhavefine clay materialadheringto them; this clay is wetted with the connatewater.sThis water is salineand often also contains calciumand magnesiumsalts,which make it hard. As has been mentionedprevi_ ously,the salinity of the watervariesconsiderably from areato area. The wetting of the mineral matter by water rather than by bitumen is a very importantcharacteristicof someoil sands,particularlythe deplsits in Athabasca; this makespossiblethe separationof mined tar sandby the blark hot-waterpro_ cess,which is usedby Suncorand by Syncrude.In this processthe tar sandis mixed with hot water and a little causticsoda.Most of the sandseparates cleanly,leaving the liberatedoil to rise to the surfaceas dropletsadheringio bubblesof gas. rhJ cleanseparationis possiblebecausethe bitumen doesnot wet the solid ini-tially. It is becomingapparentthat the natureof the wetting of the matrix alsoplaysan important role in the recoveryof heavy oils by steaminjection. In pariicular, it is 5Connate water is relatedto the residualwater left in the reservoirmatrix after the bulk of the original water was displacedby oil during the filling of the reservoir.

18

Introductionto Thermal Recovery

Chap. 1

found that a matrix that is wetted initially with connatewater greatly reducesthe water in oil emulsificationthat occurs on steamins. Oil and Bitumen The most important physicalproperty of crude oil in recoveryprocessesis its viscosity.Figure 1.9showsthe viscositiesof severaltypical heavycrudesas a function of temperature(Buckles1979). The viscosity of heavy crude oils correlates,at least approximately,with the density of the oil. Figure 1.10shows a correlation preparedby Farouq Ali (1983), which can be used to estimate the viscosity as a function of temperatureand the densityof the oil. However,becausesignificant anomaliesare found when the gravities and viscositiesof heavyoils are compared,Figure 1.10shouldbe usedonly for rough estimates. Gas Heavy oil reservoirsoften contain pocketswith gas saturationand most heavy oils and bitumenscontain dissolvednatural gas.Also, during heating,a gaqpt!As-g_qel-d"q* {gb" fqrmed.The mechanismsinvolvedin ihfulnclutle the-evolutionof dissolved naturalgas,the decompositionof inorganiccarbonatesto form carbondioxide,and the decarboxylationof organic acids. RCO2H -+ RH + CO2 Decarboxylationof acids

The gas produced from the steam recovery of bitumen frequently contains about 50Vocarbondioxide, with the remainderbeing mostly methane.Most of the carbon dioxide probably comes from the chemical transformation of carbonaterocks. It has been suggestedthat carbon dioxide comesfrom the thermal decompositionof siderite(ferrouscarbonate),which is lessstablethan other carbonates. FeCOg-+ FeO + CO2 Carbonatedecomposition

i 11 i 1,m0,q!0 l^\iril 100,q)0 i -- -\x- -- - -- - - - -----i----10,(n0 --'i- - -\\lAtnaoacca--i-i - - r - - - \

10(x)

i

- - - - i - - - - - - - .r

-r-\q.

- - - - i - - - - - - f- - -

L

i

I

-

&1m 6

8ro

p

3

.,

,

PiLon\j

i

in"""roi, ' Kernj River'A' \

iconditions, jU=.*-

i r!!!

0

i

i^cld Lake -i ':Y: ---

tReawdter i

i

'100 150 200 250 50 Temperatureo C

The Nature of Heavy Oil and Bitumen Deposits

Figure 1.9 Viscosity of Heavy Crudes as a Function of Temperature(Courtesy Buckles1979)

19

Temperature ln degreesCelslus io7 106

25 50 ttttlttl

75

100

125 150 175 200 225 250 ll

10-

o to4 o .9, o 3000 CL 1000 tr (, o 300

.E 100

.e o 0 o

en

lt,

10 3 2 100

150

200

250

300

350

400 450 5oO

TemperatureIn degrees Fahrenhelt

Figure 1.10 ApproximateRelationship between Oil Viscosity, Gravity, and Temperature(after FarouqAli 1983)

However,a more important sourceof carbon dioxide is probablythe reactionof inorganiccarbonateswith quartz (SiO) to yield silicatesand carbondioxide.Gunter and Bird (1989),in a review, describeseveralhydrothermalreactionsin which quartz reactswith carbonatesto liberatecarbondioxide.For example, calcite + quartz + kaolinite = Ca-smectite+ CO2 + H2O dolomite + quartz + kaolinite + H2o = ca-Mg-smectite+ calcite + co2 One way of interpreting these reactionsis to look on the SiOz as an acid which is displacingCO2 from the carbonate. UNITS OF MEASUREMENT Measurements in the field of petroleumproduction-as in other areas-are in some confusionbecausemany countrieshaveswitchedfrom a hodgepodge of old, traditional units to the new,more consistentSI (SystdmeInternational)units. However, the United Statescontinues,for the mostpart, to use customaryunits; as a result, mostof the literaturecontinuesto be written in theseunits. Evenin Canada.where the SI systemhas been adopted,the old units still prevail in many cases.For example,the Canadiangovernmentcontinuesto discussthe price of oil in dollars (US$)per barrel rather than per cubic meter.Even beforethe presenttrend to SI, there was confusionin the ranks of the reservoirengineers.Someauthorspresent equationsthat are dimensionallyconsistentand into which one may substitute numericalvaluesdrawn from any dimensionallyconsistentset of units of measurement, whereasothers write equationsthat involve dimensionedconstants.This latter classof equationrequiresthe use of specifiedunits in order to provide the correctresult. For example,Darcy'slaw for the flow of a fluid in a porousmediummay be written as the dimensionallyconsistentequation1.1.6 6,{ll symbolsare listed

20

in Appendix 1.


Chap. 1

q=

T(E)

where ft is permeabilityI7 A is area # P is pressure MLlT-2 l.L is viscosityMrlT-1 x is distanceL q is flow fT-1

+

(1.1)

AX--.->

This equationwill give the correct answerproviding that any consistentset of units is employed.For example,it will work with SI units, with cgs.units, with fps units, and with any other setof units havinga consistentbasisfor mass,length,and time. In reservoir engineeringliterature, it has been (and still is in the United States) usualto measuretime in days,length in feet,viscositiesin centipoise,permeabilities in millidarcys, and volumes in barrels or sometimesin acre-feet.It has also beencustomaryto rewrite equationssuchas 1.1into forms in which the so-called field units can be substituteddirectly. Equation 1.2 is a frequently used dimensionalform of Darcy's equation. It is correct provided that the variablesare measuredin the particular units shown.

q = -0'0011'z|a(+\ p q,Bld

A, ft2;

k, mD

\AxI p, cp;

P, psi;

(1.2) x, ft

The numerical coefficient in equation 1.2 has the dimensions of (B cp ft)/ (daymD psi). Although lacking eleganceand sophistication,the field-unit systemhasprobably reducednumerical error by allowing the use of familiar and easily visualized quantities.However,the traditional field-unit systemhas the disadvantageof introducing awkward factors such as the 0.Nll27 of equation 1.2. It also requires that physicalpropertiesbe convertedto a rather rigid set of specific units. Conversion factors for various frequently employedquantities are shown in Tablesr.6 and 1.7. In this book, dimensionallyconsistentequationsare normally employed.In some cases,where dimensionalequationsare given, specificunits must be used.These are specifiedat the point wherethe equationis introduced.In descriptivematerial the authorhas employedthe units that are most familiar (to the author!). TABLE 1.6 ConsistentMeasurementUnits

Mass Length Time

SI

cgs

fps

Engineers

kg m s

gm cm s

lb ft

slug ft s

Units TABLE1.7 Oil Reservoir VOLUME 1 ac-ft : 7757.8B = 5.615ft3 : 0.159m3 1B PRESSURE I MPa = 145 psi = 106 Nm-2 where N : Newton I psi = 6.895kPa PERMEABILITY I D

: 1(cm3/s) (cp)(cm)(cm2)-'1atm;-' = 0.9869x 10-6cm2: 0.9869x 10-12m2 = 0.9869 r.r.m2

DYNAMIC VISCOSITY 1p lcP

: 1 g c m - l s - 1= 0 ' 1 k g m - r s - r o r 0 . 1P a ' s :0.01P:lmPa's

O : poise

KINEMATIC VISCOSITY 1st

I cm2 s-l : 0.0001 m2 s-t

I cst

I mm2 s-l

USE OF PROGRAMMABLECALCULATORSAND MICROCOMPUTERS Programmablecalculatorsand particularly microcomputersmake calculationsin this field much simpler.To usecomputerseffectively,it is important to have availablesimpleequationsthat allow the calculationof physicaland mathematicalquantities occurringin the problemat hand. For example,a microcomputercannotreadilyusea steamtable,but it can easily calculatethe desiredvaluefrom correlationequations.Justas engineersusedto use slide rules, so the modern engineerusescalculatorsand microcomputers.Often simplecorrelationsare sufficientlyaccurateto estimatephysicalquantitiesin view of the other uncertaintiesinvolvedin the problem.Slide-ruleaccuracyis sufficient for most engineeringcalculations. The practicing engineershould searchfor and collect equationsthat are of a suitableform to be includedin computercalculations.A selectionof usefulcorrelation equationsis given in the appendices. RADIAL FLOW TO A VERTICALWELL Figure 1.11showsthe plan view of a fully-perforatedwell that is producing oil in flow in a reservoirof height/2.It is assumedthat the boundary, radial,steady-state at radiusR", is at a constantpressureP, and that the well, of radiusR,, is at a constant lower pressureP,. The effective area for flow diminishesas the fluid approachesthe well; becauseof this, the absolutepressuregradientincreases. 22

Introductionto Thermal Recovery

Chap. 1

Figure 1.11

At someintermediateradiusR the pressuregradientto maintain the flow q is givenby substitutingthe area2rRh into Darcy'sequation(1.1).In this example,4 is consideredpositivefor flow to the well (i.e., in the oppositedirectionto R), so the minus sign in (1.1)is omitted. The resultis qp dPdR k(ZtrRh)

(1.3)

This equationmay be integratedto calculatethe flow arising from the pressuredifferenceA,P : P" - P-. D - D '-*[ * '-q P d R 'e J^.2trkh R

q:

Zrkh A'P

(1.4)

t,ln(R.lR.)

Equation1.4 is written for dimensionallyconsistentunits. If the dimensionalform of Darcy'sequation(equation1.2)is employed,the resultis equation1.5.This is the form found in many texts on reservoirengineering.

4=o.oo7o8;#h q,B/d; k, mD;

P, cP R", ft

h, ft;

R,, ft

(1.s)

AP, psi It is instructiveto substitutenumericalvaluesinto theseequationsto obtain an idea of the effect of viscosity on oil production rate. Table 1.8 showsvaluesof the production rate that havebeencalculatedfor a high-quality, thick reservoirthat is saturated with oils having viscositiesvarying from 1 cp (a low-viscosityconventional crude oil) up to 1,000,000cp, which correspondsto a material such as Athabasca bitumen. For a typical well bore radius of 0.3 ft, the production falls from 44,000B/d for the light crude to only 0.4B./dfor the bitumen.The first casecorresponds to a well of remarkableproductivity and the latter, to a well of little value. RadialFlow to a VerticalWell

TABLE 1.8 CalculatedWell Flow Rates Assumek = 1000mD (excellentsand);lr = 100ft; AP : 599 psi; R, : 1000ft. CALCULATED WELL FLOW RATES

10,000 100 R,,: 0.3fr 4.4 440 R, = 100ft 15.0 1,500

I

Oil viscosity(cP) Flow (B/d)

4.4 x 104

Flow (B/d)

1.5 x 105

100,000 0.4 l.)

using a largerwell bore will increasethe productivity.The lower line in the Altable showsthe iffect of using an imaginarywell having a radius of 100ft. by principle, in least at thoughsucha deviceis impractical,it maybe approximated' be might effect A similar the reservoirurorrnda well of normal dimensions. heati."ng in length. feet obtain;d by using a horizontalwell severalhundred Sucha strategymight, in the exampleshown,producea useful effect for the cp, but the productionwith the bitumenwould still oil having a viscosityotLO,gOO be too .-"ug", to be effective.A flow of 15 B/d is closeto the lower limit at which economicpioductioncould be anticipatedfor a practicalwell' Comparingthe resultsof calculationssuchasthis with the very sharpchanges of viscositywith temperature,which are shownby Figure 1.9,illustratesthe importancewhich reservoirtemperatureplaysin the recoveryof heavyoils. Figure 1.12showsthe oil recoveryachievablefor a number of Venezuelan (1979)' heavyoi fields as a function of the in situ viscosity,as given by Borregales for the viscosities the oil Also shown on the figure are points correspondingto Athabascaand Cold Lake fields' A majorreasonfor the higherviscosityof Canadianbitumensas comparedto Athathosein Venezuelais the loweireservoirtemperature'(Seedatafor Joboand

25 from'Physical

Fro

Principles

ol Oil Production"

by Muskal

.a'

'so- ^) \. .ra

o () o c15

_o

= o *10 J

t

Figure 1.12 APProximate Effect of

7 Viscosity on Oil Recoveryby Solution 6 5 "o 4 3 2 1 Gas Drive (after Borregales1979) Conditions) L;glO(Oil Viscosity in cp at Reservoir

24


Chap' 1

bascacrudesin Figure 1.12.)The climateof Venezuelamakesthe ground surface by the temperaturemuchhigherthan in Canada,and this differenceis exaggerated deeperburial of the Venezuelanreservoirs.It is this differencein reservoirtemperature rather than intrinsic differencesbetweenthe crude oils that causesmuch of the differencebetweenthe productivityof the Venezuelanheavyoil wells and the Canadianones. Although the Canadianbitumensmust be heatedsomewhatmore than the Venezuelan onesfor satisfactoryproduction,the largestdifficulty that the high initial viscositypresentsis that of gaininginitial accessto the reservoirin order to be ableto contactthe materialwith heatingmedia.In many respects,the problemof the productionof bitumenin Canadais that of trying to heata remote,very thick, impermeable,immobile,asphalticconcrete! THE PROBLEMOF ECONOMICEXPLOITATION There are other practicalproblemswhich are encounteredin the exploitationof the problem heavyoil resourcessuchas thosein Canada.So far we havediscussed problem of movingit to the surfaceof the ground. of recoveringthe crude-i.e., the The concernof this book is moving it to the surfaceby the use of in situ heating. Another approachto the sameproblemis to removethe tar sandby mining methodsand then to separateit using processessuch as the hot-waterprocess. Large operationsof this type are carried out in Athabascaby Suncor (formerly Great CanadianOil Sands)and by Syncrudenear Fort McMurray,Alberta. These plants are successful.However,the approachis very demanding;it dependson brute force and is suitableonly for thosedepositsin Alberta that are relativelyshallow. Ninety percentof the bitumen in Alberta and most elsewhereis too deeply buried for this to be a practicalapproach. The publishedeconomicsof the large Cold Lake commercialplant that was onceproposedby Essoshowthat recoveryusingthe cyclicsteamprocessis competitive with mining (McMillan 1979).An EssoCold Lake commercialplant wasorigiproductionof bitumen nally proposedin the late 1970sthat involvedthe large-scale by cyclic steamingfollowedby the upgradingof the bitumen to syntheticcrudeby fluid bed coking and hydrotreating.The projectwas shelvedbecauseof the questionableeconomicsand the enormouscapital outlay which would have been required. However,since then, Esso has realizedthat the productionof bitumen without upgradingcan be economic.This approachinvolvesthe productionof bituin quantimen and pipelinetransportationof the bitumendiluted with condensate ties that will soonbe far above100kB/d. BITUMEN TRANSPORTATION Transportationof the product is a major problemfor the bitumen producersince it cannotbe pumpedthrough a conventionalpipeline.Possiblesolutionsare shown next. All havebeenput into practice. o Move the bitumen in trucks or trains e Convert the bitumen to a more fluid material bv chemical transformation

o Dilute the bitumenwith a solventsuchas condensate and transportit by pipeline o Pump the bitumenwith water through a pipelineunder conditionsthat allow the water to flow as an annulussurroundinga bitumen core o Emulsify the bitumenin water and transportthe mixture by pipeline For a number of yearsbitumen was moved from Cold Lake in road trucks as hot cargoes. Suncorand Syncrudeboth convertthe bitumento an overheadproductusing coking. Hydrogenationprocessesprovide an alternative method of conversionberecentexpansioninvolvesthe addition ing developedby severalgroups.Syncrude's process plant; to their Husky plans an H-oil Unit for their upgrader of an LC-fining processes These in Lloydminster. also find use for the conversionof the residual material from the distillation of conventionalcrude oils. Table1.9showsa comparisonbetweenthe propertiesof bitumenand thoseof a typical conventionallight crudeand the upgradedcrudeproductthat wasto have beenproducedby the Cold Lake commercialproject.The upgradingcracksthe bitumen, and the crackedproductsare treatedwith hydrogento removesulphurand nitrogenand to saturatesomeof the aromatics. The dilution of bitumen with a solvent such as condensateto make it pumpablehas been practicedfor yearsin the Lloydminster area and more recently, and on a much larger scale,at Cold Lake. The main problem is the availability of a suitable diluent; about 30 LY% (basedon bitumen volume) of a material such as condensateis required. In somecasesdoublepipelineshave been constructed,with the diluent being returned to the field from the remote refinery by a secondline. by Sloan,Ingham, The shipmentof heavyoils by pipelinehasbeendiscussed and Mann (1981).They concludethat the crude oil viscosityshouldbe lessthan 150cst and that the temperatureshould be maintained lessthan 200'F in order to TABLE 1.9 Cold Lake Project-YieldComparisons(LV%) Cold Lake Bitumen

Butane(Cn) Naphtha(C5-180"C) (c5-350"F) Distillate (180-345"C) (350-6so'F) Gas oil (345-565"C) (650-1050"F+) Residuum(565"C+) (1050"F+) Total sulphur-wt7o Gravity-kg/m3 -.API

Typical Alberta Light Crude

Upgraded Crude Objective

3-4 30

1,5-20

t7

30

45-50

40

30

28-30

7 0.5 834 38

0 0

ConvectiveHeatingwithin Reservoirs

Chap.3

For lypl > 1:

4#)=(#) -(#)=' (#).(#)

Forlyrl = 1'

(3.r2)

(3.13)

andTf = 7;

rr =rt =roll;;:3

For rp = 6;

(3.14)

These equationswere solved by Lauwerier to give equation 3.15,which expresses the temperaturewithin the oil sand layer as a function of time and location'

zi' = *"(ffi^ rrxo< to then

.)

(3.1s)

and if xo > to then ?| = g the temperaturewithin the water layer is found by substitutinglo: equation3.16.

I to live

If xo < to then ?i =

"rf.(--+) \2Y 0(to- xr)l and if xp 2 tp then ?i = g

(3.16)

Figure 3.4 showsthe temperatureas a function of distancewithin the water layer for variousvaluesof time plotted using the dimensionlessvariablesjust shown with 0=1. 1 g 0.8 E G o

CL

E 0.6 o o

8E 0.4 o

e o.2

.E oo 00.5

1 1.5 Posltion x D Dlmenslonless

Equation Lauwerier's

Figure 3.4 Reservoir Temperatures Calculatedfrom Lauwerier'sEquation 77

Numerical ExamPle Hot water at 200'C is injected into a water layer 4 m thick containedwithin an oil sandreservoirat a rate;f 10 m3/h. The flow from the well is radial.The following propertiesmaYbe used: Porosity0.30; Tr : 10'C Water laYer; S, : 1.0 Oil sand:S, : 0.3; S, : 0'71 K :1'2 Btu/h ft'F Rock: heat capacity0.2 Btu/lb "F: SG = 2.2 Oil: heat caPacitY0'5; SG : 0.95 Lauwerier'sequation, as derived before, can be employedfor a radial systemif the a dimensionaldistanceis redefined.Plot the temperaturewithin the water layer as and 100, 10, of for times well injection the from R distance function of the radial if 1000days.Also plot the temperaturedistribution that would have been obtained temperathe Plot underburden. and overburden the to there hid been no heat loss well located ture distribution that would be expectedin a temperature-observation and below above oil sand in the temperatures Include 10 m from the injection well. the water laYer. Solution 1. Lauwerier'sequationin radial coordinates

oR=o(nR2)=2nRdR

- o.o,r,(#) oo:, o,r,(#) dA- 2K,(Ty),=,,0n (3.r7) Heat stored

Heatfrom water

Heat loss

hv"' This is similar to equation 3.8. ,4 has been substitutedfor x and Qn teplaced i'e'' substitutions; these making The solution can thus be written immediatelyby

*o =

4Kzx

6;Ci,

becomes

=

4KztrR2

"o ip,c,e,

(3.18)

andyo, tp, ?nd 0 are unchanged. Calculation: The results are given in the following tables' a. Temperature in the water laYer ConvectiveHeatingwithin Reservoirs

Chap' 3

Column3

'o :

4K2rrRz

(h p *c,e,)

Kz = 1.2Btu/h ft oF = 28.8Btu/d ft .F h=4m=13.123ft p.C, = 62.4Bfifft3 'F Q* = ro m3f h = 847.4ft3ld Column 4 t^

=

-

4Kzt

h'prC,

p,C, = 62.4x 2.2 x 0.2:27.46 Btuft3 "F prCr = 6p,C* + (1 - 6)p,C, = 37.94Btu/ft3'F S, = 0.3; So = 0.7 poCo= 62.4x 0.95x 0.5 :29.64 Btufr3.F pzCz= 65,p,C, I gS',p,Co+ (1 - 6)p,C, = 31.10Btuft3'F

o=P'l'= 1.221 Pzvz

Xp, x ='j l 7 (ro _ *d l -',,

Temp (in col.2) = 10 + 190erfc(X)

Radius(ft)

Temperature('C)

XD

Time:10d to=0.176 0 10 20 30 40 50 60

200.0 198.8 194.9 r87.4 t73.6 t42.4 10.0

0.000 0.00s 0.02r 0.047 0.083 0.130 0.188

0.000 0.006 0.024 0.059 0.r24 0.275

Time= 100d tD -- 1.763 0 10 20 30

200.00 199.6 198.5 196.5

0.000 0.005 0.021 0.047

0.000 0.002 0.007 0.016

Lauwerier'sEquation

79

continued

Radius(ft)

40 50 60 80 100 L20 140 160 180 200 Time = 1000d tD -- L7.631 0 10 20 30 40 50 60 80 100 120 160 200 250 300 350 400 500 600

rature ('C) 193.8 190.1 185.5

l'73.r 155.3 130.3 95.0 46.4 10.0 10.0

200.0 199.9 199.5 198.9 198.1 r97.0 195.6 r92.2 187.8 t82.3 t68.2 r49.6 r20.6 86.7 ) 2.5

25.2 10.0 10.0

0.083 0.130 0.188 0.334 0.522 0.751 1,.022 1.335 1.690 2.086

0.029 0.046 0.068 0.126 0.2r2 0.338 0.537 0.923 2.822

0.000 0.005 0.021 0.04'l 0.083 0.130 0.188 0.334 0.522 0.751 1.335 2.086 3.259 4.694 6.389 8.344 13.038 18.774

0.000 0.001 0.002 0.005 0.009 0.014 0.020 0.036 0.057 0.083 0.150 0.239 0.389 0.590 0.862 1.239 2.7s2

The predicted temperaturesin the reservoir are plotted againstthe distancefrom the injection well in Figure 3.5 for 10, 100,and 1000days' b. Temperaturein observationwell at R : 32.8ft (10 m) Golumn 4 Height abovewater sand in feet

Golumn3 Vn-

80

height -"| = h/2

ConvectiveHeatingwithin Reservoirs

Chap' 3

oo tso

; J

6 100 o CI

-

o t50

600

Figure 3.5 Predicted Temperaturesin Reservoir as a Function of Distance from Injection Well

Column2 1=

xD+ yD-l

- *df''' 210(tD

The other columns are as before. TEMP'C

to-1

184.3 106.0 51.6 23.9 13.5 10.7

Tirne= 10d; to = 0.176; xa = 0.056 0.073 0.000 0.47r 0.305 0.869 0.610 r.267 0.914 1,.664 1.219 2.062 1.524

195.8 t51.4 111.1 77.6 52.2 34.4 23.1 16.5

Time = 100d; to = 1.763; ro : 0.056 0.019 0.000 0.231 0.610 0.442 t.2t9 0.653 t.829 2.438 0.864 1.075 3.048 t.286 3.658 t.497 4.267

r98.7 184.6 170.'7 157.0 r43.6 t30.7 118.5 106.8

Time = 1000d; to -- 17.631;xa = 0.056 0.006 0.000 0.072 0.610 0.138 t.219 0.203 1.829, 0.269 2.438 3.048 0.335 0.401 3.658 4.267 0.467

Height(ft)

0 2 4 6 8 10

0 4 8 t2 16 20 .A

28

0 4 8 t2 16 20 24 28

The calculatedtemperaturesin the observationwell are plotted in Figure 3.6. Equation Lauwerier's

81

oo 150

; E roo o c o F50

01020 Helght abov€ water sand In feet

Figure 3.6 Predicted Temperaturesin Observation Well

c. Temperaturedistribution for no loss Heat Balance Q,p*C,(Ts - Ta)t = nR2hprC t(Ts - Ta) p = (Q*P'c*t|n \ rhPrct I

This equationyieldsvaluesofR = 58.1,183.7,and 581.4ft for 10,100and 1000days. Thesevalueshave been plotted as dotted lines in Figure 3.5. Note that the noloss radiusoccurswhenxo : 1o. THERMAL EFFICIENCYFORCONSTANT.DISPLACEMENT RATE STEAM.DRIVE Considerthe steam-drivesituation shown in Figure 3.7. Steamis injectedfrom the left side at a rate sufficientto causethe heatfront to advanceat a constantrateA. The shapeof the heatedareaA is not specified. The specification of the problem requires that the steam-injectionrate be raisedcontinuouslyto compensatefor the increasingheat losses.It is assumedthat the temperatureZs in the steamchamberis constantup to the front, where it falls abruptly to Zn.This assumptionis reasonableuntil the time when all the latent heat in the steamis consumedby the heat lossesand only sensibleheat is hvailableto advancethe heated region. The time at *hich this situation occurs is calculated later on. Rateof heatloss

Steam -4_----->

Figure 3.7

82

Conductive Heating within Reservoirs

Chap. 3

reservoirsurfaceof area2A (A at fhe cqgplafiye-heailossjrom-the-heated the top_gflhe reservoirand,4below)maybe calculated from equation2.27.

etc=r(! *,nt,-rr/h)

(3.1e)

where the subscript2 refers to the over- and underburden. The cumulative heat required to raise the reservoir and its residual contents from the initial temperatureZn to Zs is

(3.20)

Qrc= prCthA(Ts-Tn) where pl Cr is the volumetric heat capacityof the steamedreservoir.l The total cumulative heat injected is thus

f'\ lq Q,c: Qrc* Qn = z\!' x,eC' - ril\,1,*)

* ,,cftA(rs - rR)

(3.2r)

The instantaneousrate of heat injection may be found by differentiating equation 3.2I with respectto t.

.l

L

Ho=A(Ts-TR)14K, \

,*

+ ercrh)

(3.22)

The heat-injectionrate is equal to the sum of the losses(which increasewith the squareroot of time) plus the constantheat rate to expand the steamchamber. At the critical time t", when latent heat is no longeravailableat the heal,ftlr{rJ, the lossesare all suppliedby the lltent heat. When this occurs, the ratio of heatlossrate to stored-heatrate will be equal to the ratio of the latent heat injection rate to the sensibleheat injectionrate; i.e., I l-,1

lo*'r,l**) -- H^

Ho- H^

[prcrh]

,,=no,(ffi^#)'

(3.23)

For times lessthan t", latent heat is availableat the heat front, and a sharp temperaturegradient is maintained. The critical time is proportional to the square of the reservoirthickness,and it is independentof the rate. The fraction of the injected heat that remainsin the reservoirmay be looked upon as a thermal efficiency. It is given by -

Qt'

"o= en \

PtCth

x

ptCrh+ ;' 5

f,

K, r/|

7Td2

1, = __________;_

(3.24)

4 1.I ----'=' X 3Yzr

lThe value of prCrused in this equationis for the steamedreservoir-i.e., with fluid saturations correspondingto the depletedreservoir.The displacedfluids are cooledto the reservoirtemperatureas they pa$ through the heat front. Thermal Efficiency for Constant-displacement Rate Steam-drive

83

where nv -- -

2K,

t

,rcrh

Qz

Thedimensionlessterr.rXinequation3,2!isthesamevariableusedbyMarxan section' i" the descriptionof their theory in the next Langenheim;it also "rir". Otherwrite,,of,-*-"-ploythedimensionlesstime/o,whichisthesquare of X. 4K2P2Czt 4KZt (3.25) = x-" =

to

( i- cr n) hr = hr ( pr ci

IfthevolumetricheatcapacityoftheoverburdenandunderburdenarethesameaS tor tDcan be simplified to equation3'26' that of the steamzone,then the expression accurate' This approximationis often sufficiently then fP =

lf.prCr = PzCz,

4q.rt

(3.26)

i

also be expressedas The simplified dimensionlesstime, tp, rfrz! 4ozA

ro= -Ftr

(3.264)

Thethermalefficiency,E;,isplottedagainstthedimensionlesstimeinFigure3.8 dearea,A,increases)the thermal efficiency As time increases(o, u, tt" flooded inthe supply to of tire steamis required creases;i.e., a larger and larger fraction creasingheat losses. Theheightofthereservoirisaparticularlyimportantvariableinthedimen. inherently is squared.rire ttrirmal efficiency is sionlesstime becaus" iirl"r"" to corresponding '4 of ones.Smallervalues hieher for thick reservoirsthan for thin by

;r5.;;ffi;"r"g,

for a givenrateasmeasured prt"iJ" nign"refficiencies

"r* alsogivebetterthermalefficiencies' l. Uigh", heatinjectio;;".1i.e.,iigtrer,,i) ue Jmployedas an approlimationif If ,,4is not consrant,Figure3.g can stitt ro' The critical riln" i. ruu?itutedinto (r:o in order-tocalculate the total steaming

0.8

E o.t .9

E o.o o.2 0

84

0 Log19(t9)

2

Fisure 3.8 Valuesof Thermal EfficiencYFactorEr, for Constant DisPlacementRate

Reservoirs Conductive Heating within

ChaP' 3

time, t", from equation 3.23 can also be expressedas an equivalent dimensionless time toc given by -

arl

H,

toc=Xi=il:l

\2

+ 1Hs- H;l

(3.27)

Fraction of Heat in Steam-Saturated Chamber After the Critical Time -

2K2(Ts- To) fo' Yrraz

2K2(Ts- 7h) [" ----)dto Jg Yt-to Yra2

dAs

Jo \/t-tn

where /6 is the birth time of dA andl

is constant. Hence,

- \/t - r,)

a^ '' =!!{{14(\,, lrqz

(3.28)

where/5 is the birth time of the limit of As ar time t.

+ p,c,h\ Ho= )(rs- 7^1(+x, rlt Ytrdz \ I H^ Ho

(3.2e)

(3.30)

T, \

"*-

. Prcth 4K"

which reducesto 1 -

Ht

-t

T1 A- - -"- : A

V'

Ho

1, 1 +2

t:

(3.31)

t;

where

,o=5!44 h'(P' C t)' If As : A (3.31)becomes(3'27). In general,if tD > toc then (3.31)can be manipulated to become

2='n-n' where

n=

'.+c 1

(3.32)

(3.33)

lr

r*z!,* Rate Steam-drive Thermal Efficiencyfor Constant-displacement

85

Asymptote lor AslA if fp = o' The ratio,4sfA approachesa constantvalue for large times. This can be calculated as follows. Ht ft.-

:

1+ and

I

(3.34)

Ho

t

c).=- (*o)' nH^

"H,

(3.3s)

RATE: FoRcONsTANTSTEAM-lNJEciioN EFFTCTENCY THERMAL THEORY MARXANDLANGENHEIM'S imporMarx and Langenheim(1959)developedtheoretical relations describingthe of tant caseof a growing steamzone tirat is limited in its growth rate by the loss introheat to the ovJrburden and underburdenand by the rate at which steam is for duced.Their theory is similar to that describedin the previoussectionexcept rate a the assumptionthai the steamis introducedat a constantrate rather than at has equation Langenheim's and Marx rate. advance frontal that providesa constant important it is and field, this in studies subsequent the formed a basisfor many of to gain an understandingof it. by Marx and Langenheimis shown in Figure 3.9' The situationcons-idered rate into a steamzone that is spreadinglaterally. constant Steamis introduced at a this zone is specifiedas input to the problem' into 110, The rate of heat injection, in the growing steamzone and the losses stored heat the The heat goesto intrease that no heat is transferredahead assumed It is to the overburdenand underburden. is realistic only when the laassumption this previously, of the front. As discussed supplyall the losses-i.e.' to is sufficient steam tent heat suppliedin the injected condition will usually This front. heat the at when there is still latent heit arriving obtained' is being ratio steam to oil be satisfiedif a reasonablyhigh The areal shapeof th" it"urn zone is not specified.In the original theory it can was assumedthat the condensationfront remainsvertical, but this assumption Rateof healloss

TH

6A formedat tO to t0 + 6tO

86

-. condensate

Figure 3.9 Conductive Heating within Reservoirs

Chap' 3

be relaxed;this is discussedlater in this chapter.within the steamzone,the temperatureis assumedconstantat rs. outside it is zn, as shownin Fieure 3.10.

A

Figure 3.10

At any intermediate time / the vertical heat loss rate per unit area will be larger near the front (seeequation2.24). Equation 3.36 gives the rate of heat loss from the area26A (61 aboveand 64 below),which was initially heatedat time /0. The time /s can b€ looked upon as the "birth time" of any particular heatedarea.

6- Q r = Z A I E W

(3.36)

Vrd2G - to)

The total rate of heat loss,Q1, is found by integrating(3.36)over the whole area,as in (3.37).The time at which the elementof areawas formed,/6, is a function of ,4; alternatively,A may be considereda function of /e, and the variable of integration may be changed,as shown: dA=

Or:tf

(#)",

K2(Ts - TR)

ftdr(t

-T,;r

Equation 3.38 definesQ,, the rate at which heat is being stored as sensibleheat in the reservoir.

(3.38)

Qs=PrC&(Ts-ri4 dt

The overall heat balancefor the processis given by equation3.39.It may be solved by Laplace transformation,solution of the resulting algebraicequation for -4, and inversionof the transform to give,4(r). The inversionof the transform may be carried out by comparisonwith a table of standardforms.2

Ho= 21,' {ffi4(#)", Injection=

loss

- ril# r p1c1h(rs +

(3.3e)

storage

The solutionof (3.39)leadsto'theresultshownin equation3.40.The theoryfor this problemis similar to that developedby Carter for the growth of a fracture with side zSeeCRC Standard MathemnticalTables(22nd Ed.), Transform 45, page 510. Thermal Efficiency for Constant Steam-injection Rate

g7

leakageand a constantinjection rate (Carter 1957),and the mathematicalform of the problem is identical. ' A(t) = -

H o p r Ct h (".' z p z C z ( T s- 7]R)

erfc(X)+ 4

Yr

- ')

(3.40)

where

x=

2K' p r C t h Yq z

tn

Equation 3.40 gives the heatedarea as a function of time and differentiation gives the rate of area growth: dA dt

Hsex'erfc(X) p(th(Ts - Tn)

(3.41)

The rate at which oil is displacedfrom the steamzone may be calculatedby multiplying the rate of increaseof the volumeof the steamchamberby its porosityand by the changein oil saturation:

q,=hQ(S"-t)#

(3.42)

The residualoil saturationSoain the steamzone is usually quite small; typical valestimateif no other data are availuesare 0.05-0.2.A valueof 0.1_5isa reasonable variableX is employed.This is dimensionless the and 3.41 able.In equations3.40 It is the samedimensionless Langenheim. and Marx by the variable that was used to as the dimensionless X2 is referred Frequently earlier. group that was described time /a.

^

4K1t

to=X-=Trp*=

(3.43)

If the volumetric heat capacityof the overburdenand underburdenare the sameas that of the steam zone, then the expressionfor to ca;ltbe simplified to equation 3.44. 4azt

. tD = --;'-

n-

(3.44)

The fraction of the injectedheat that remainsin the reservoircan be determinedas a function of the dimensionlesstime by meansof equation3.45 or from Figure 3.11.

. ,l-b - tf u,= Ilr'""rrc{t/-t"l

(3.45)

The curve in Figure 3.11showsthat the fraction of the heat lost from the reservoir varies over a large rangewith the variable /o; this is proportional to time. As time continues,the fraction of the total heat injectedthat is lost grows. It should also be noted that there is only a slight differencebetweenthe curve drawn for a constant 88

ConductiveHeatingwithin Reservoirs

Chap' 3

1

-

o.B

o o E I!

-'-

o

u.a

tr 0) !

a a

t-- u.z

Figure 3.11 Comparisonof Thermal Efficiency.Eafor Constant Heat-InjectionRate and for Constant Displacement Rate

-202 Lo91s (tp )

injectionrate (i.e., from equation3.45) and that for a constant displacementrate from the much simplerequation3.24. Numericalvaluesof the term e'o erfc(f tp) or e*'erfc(X) in (3.45)can be obtained from Table 3.1. In computerprogramsthe rational approximationsdeveloped by Hastingsgiven on page35 may be employed. A major factor is that as time goeson and more and more steamis introduced (recallthat it is assumedthat the injectionrate is constant),the steamzonecontin-

TABLE 3.1 Valuesof the Functionex'?erfc(X) X

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65

q19 0.75 0.80 0.85 0.90 0.95

ex'efic(X)

1.000000 0.945990 0.896457 0.850936 0.809020 0.770347 0.734599 0.701496 0.670788 0.642252 0.615690 0.590927 0.s67805 0.546181 0,5:259i0, 0.506938 0.489101 0.472327 0.456532 0.441641

X 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 L.45 1.50 1.55 1.60 1.65 L.7g 1,.75 1.80 1.85 1.90 1.95

ex'erfc(X)

0.427584 0.4L4299 0.40r73L 0.389826 0.378537 0.367822 0.357642 0.347960 0.338743 0.329960 0.321,s84 0.313590 0.305952 0.298650 0.291663 0.284973 0.278561 0.272413 0.266513 0.260847

Thermal Efficiencyfor ConstantSteam-injectionRate

ex'erfc(X)

2.00 2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 ?.70 2.75 2.80 2.85 2.90 2.95

0.255403 0.250L67 0.245130 0.240281 0.235610 0.231108 0.226766 0.222576 0.218532 0.?r462s 0.210850 0.207199 0.203668 0.2d02sr 0.196943 0.193738 0.190632 0.187622 0.184702 0.181869

89

uesto grow, and a larger and larger areaof overburdenis heated.Eventually,nearly all the injected heat iJ Ueinglost. The thicknessof the reservoir,/r, is the most significant iactor involved in the expressionfor /o, since its value is squared' As an exampleof the useof Figure 3.11,considerthe caseof a reservoir100ft thick. Assumethit the thermal diffusivity of the reservoirand the overburdenare both 0.9 ftzfi. fhe horizontal scalein the figure correspondsto valuesof rp varying The correspondingvaluesof real time in days,for this rangeare from 0.001io 10001 given by hztpf4a,or 2778to: 2.8 to 2J78,000d. For a reservoir10 ft rather than IOOft tiri.t, the correspondingtimes are smaller by a factor of 100.

r days for h : I00 I days for /l = 10

0.001

0.1

2.8 0.028

2',78 2.8

1000 27,780 278

2,'778,000 27,780

NumericalProblemUsing Marx'Langenheim'sEquations : PzCz:33 Btuft3 "F), For the two differentcases(h = 10 and 100ft and ptCt assume: 6 = 0'35 Tn: 75"F S, : 0'7 So,: 0.15 measuredat 75"F Seventy percent quality steam is injected at a tate of 800 Bld at a pressureof 500 psia. Calculatethe following for eachreservoir thickness: 1. The area of the steamzone is acresas a function of time 2. The radius of the steamzone, assumingthe steamzone is cylindrical 3. The volume of disPlacedoil 4. The ratio of displacedoil rate to steaminjection rate 5. The ratio of cumulative displacedoil to cumulative injected steam Plot thesevariablesagainsttime for eachof the reservoir thicknesses. ForX > 3, use 2t e''- erfqX) : ---F'

\/r X+\E+2

(3.46)

Solution The solution to this problem is given in Table 3'2' 90


Chap' 3

TABLE3.2

YEARS

'D

Height= 100ft 0.00 0 0.50 0.07 1.00 0.r3 1.50 0.20 2.00 0.26 2.50 0.33 3.00 0.39 4.00 0.53 5.00 0.66 6.00 0.79 7.00 0.92 8.00 1.05 9.00 1.18 10.00 1.31 Height= 10ft 0.00 0 0.50 6.57 1.00 13.r4 1.50 t9.71 2.00 26.28 2.50 32.85 3.00 39.42 4.00 52.56 5.00 65.70 6.00 78.84 7.00 91.98 r05.t2 8.00 9.00 1,18.26 10.00 L31,.40

ACRES

RADIUS (ft)

CUMULAIIVE VOLUME (B)

0 99 136 162 183 201 217 245 268 288 305 322

0 0.71 1.33 1.89 ) a') 2.93 3.41 4.32 5.t7 5.97 6.73 7.46 8.16 8.83

JJt)

350

0 2.7r 4.t9 5.34 6.32 7.19 7.98 9.38 10.62 tt.74 12.77 t3.73 14.63 15.48

0 L94 241 272 296 3t6 333 361 384 403 ta1

436 450 463

BPD

osR

CUMULATIVE OSR

0 105,887 t97,965 282,630 362,0t1 437,267 509,139 644,675 771,442 891,128 1,004,989 1,1r3,876 1,218,450 t,319,229

694.20, 531.54 48152 448.18 422.96 402.66 385.68 358.39 336.99 319.48 304.74 292.07 28r.00 27r.20

0.868 0.664 0.602 0.560 0.529 0.503 0.482 0.448 0.421 0.399 0.381 0.365 0.351 0.339

0.868 0.725 0.678 0.645 0.620 0.599 0.581 0.552 0.528 0.509 0.492 0.477 0.464 0.452

0 40,495 62,547 79,786 94,433 107,393 rt9,t43 140,059 158,528 t75,249 190,640 204,97',| 2r8,449 23r,198

694.20 144.21 106.43 88.86 78.17 70.77 65.25 57.40 51.97 47.92 44.74 42.t5 39.99 38.15

0.868 0.180 0.133 0.111 0.098 0.088 0.082 0.072 0.065 0.060 0.056 0.053 0.050 0.048

0.868 0.277 0.214 0.182 0.162 0.t47 0.136 0.r20 0.109 0.100 0.093 0.088 0.083 0.079

Data: Thermal diffusivity: a : 0.9 ft2ld pC : 33 Btuft3 "F Heat capacity: Porosity: d : 0.35 Tn: 75'F Ts = 467'F from steamtable S" : 0.7 So.: 0.15 Steamrate : 800 BPD : 800 x 350 : 280,000lb/d Height h = 100ft

and 10 ft

ThermalEfficiency for ConstantSteam-injection Rate

two cases 91

1205Btu/lb 450 Btu/lb 43 Btu/lb

Enthalpy of vaPor Enthalpy of liquid at T5 Enthalpy of liquid at Tn Quality = 70Vo

from steamtables

Heat-injectionrate : 280,000(0'7x 1205+ 0'3 x 450 43) = 2.6194x 108Btu/d Column 1 in table:Time in Years Column 2: Dimensionlesstiine (equation3'44) to = 4 x0.9 x Yearsx 3651h2 Column 3: Area in acres(equation3'40) Areal(2.6194x 108h)/(4 x 2g.7 x 392 x 43,560)lf(tD) f(o) = (e*'erfc(X) .'l \Vzrl

- t\

(43,560ft2 = 1 acre)

and X = \/G

Colurnn 4: Radius in feet Radius = [(Col 3 x 43,560)lrrlos Column 5: Qumulativebarrelsdisplaced = A x 43,560x /t x 0.35 x 0.55/5.615 (5'615ft3 1 B) equation3'41) Column 6: Barrelsdisplacedper day (equation3'42 using 800' by 6 divided is column Column 7: Oil-steamratio 800 x years x 365' Column 8: Cumulative OSR is column 5 divided by

3.l2and the instantaThe heatedareasare shown as functionsof time in Figure 3.13.The heatedarea neousand cumulativeoil-steamratios are shown in Figure the thicknessis g.o*. onrv aboutT}Tofasterfor the thinner reservoir,even though lossesare a heat vertical ieduced by a factor of 10. For the thinner reservoir the muchlargerfractionoftheheatinput.ThisisShownclearlybythecurvesi 20 o

heightin ft' is reservoir Parameter

3ts 'S ,o

fo; s Tlme In Years

92

Figure 3.12 Heated Area as a Function of Time


Chap' 3

1

t -- - - - - . loorl 10ft

0'8 .9 .E E 0.6 E (!

fino o

|

|

Cumulative

.r.

, o.2 \'...-

cumutative Gumulative I Instantaneous \-.:::::::::--=--=----a--y'_-;::::::::::::::

od*

46 Tlme In Years

Figure 3.13 DisplacedOilSteam Ratios

10

Figure 3.13.The point on the vertical axis marked "No Loss" correspondsto the maximum possibleoil-to-steamratio requiredby a heatbalancewith no heat losses. It can be calculatedby equation 3.47. While both setsof curves start at the same no-losspoint, the curves for the thinner reservoir drop very much more rapidly. OSR."" =

/{sd(s"- s,,) p1C{Ts - Tp)

(3.47)

In the equation,F/s is the net heat per unit volume of steammeasuredas water.

SIMPLEFORMULAS FOR ESTIMATIONOF THE OIL.STEAM RATIO Often the physicalpropertiesof reservoirsare known only approximately,and simpler formulas may suffice to provide initial estirnates.One approachis to basethe estimate upon equation 3.21,.The cumulative injected heat given by (3.21),correspondsto a cumulativequantity of displacedoil of @AS,hA.^fhe oil-steamratio is thus

(3.48)

If it is assumedthat the thermal propertiesof the reservoir and of the overburden are equal,then (3.48)becomes

oSR= or**"(

';l -\

l*;

8 1

Simple Formulasfor Estimationof the Oil-steamRatio

(3.4e)

-l

rrh2|

93

Ifnumericalvaluesforlls,pC,andaequaltothetypicalonesusedinthepreviou exampleare emPloYed,namelY, pC = 33 Btuft3'F Hs = 58,375Btuft3 a = 09 ft2/d then equation3.49maYbe written

oSR=ffi

17696LS"

(3.50)

where { and AS' are fractions Is and Tnare in degreesFarenheit t is in daYs h is in feet

equation3.50with h : 100ft for The following valueshave been calculatedusing valuestaken from Table3'2' the previousnumerical example;they are compalredto OSR from (3.42)

Years

OSR from Table3.2

0.868 0.581 0.492 0.452

0.868 0.590 0.504 0.466

0 J

7 10

A s w o u l d b e e x p e c t e d f r o m t h e c o m p a r i s o n o f t h e e f f i c i e ntoc ythose c u r vfrom e s s hthe own are quite close Figure 3.11,the resuttstiom the simple formulas Mirx-Langenheim equation' that heat vogel.is discussedin which he suggests In chapter + u ffi'uy immedispreads that the steamchamber lossesshouldbe calculatedby assumrng atelvacrossthetopofthereservoir.Thisresultsinanequationsimilarto(3.4 wtricir ttre factor B/3 is replacedby 4'

oSR= osn,""/--l--r

\ + * rl#, I \r

f"t immediatesteamspreading

(3'51)

[fthesamenumericalvaluesareusedforthephysicalproperties,thisbecom

I

osR=l-l

ttog'das,

I

(3.s2

L,n-r^)lt+2:41F)l sameas in equation3'50' where the units for the variablesmust be the 94


Chap' 3

CONVECTIVETRANSFEROF HEAT BEYOND THE CONDENSATIONFRONT In the Marx-Langenheimtheory and in the previous numerical example,it is assumedthat no heat is transferredbeyond the heat front-i.e., that at the front the steamgives up all its heat, both latent and sensible.This idea is consistentwith the ideasdevelopedearlier,whereit wasshownthat the velocity of a condensationfront is greater than that of a thermal front carried forward by the sensibleheat of the condensatealone. Thus any heat carried before the condensationfront tends to be 'bverrun," and the two fronts remain combined.The assumptionof a single front madeby Marx and Langenheimis reasonableif there is still vapor left to condense at the front. However, the situation changesas the front becomesmore remote, since eventually all the steamis condensedby heat lossesbefore the heat front. Sincethe steamzon'eis at the steam-saturationtemperaturethroughout,3the only source of heat to supply the vertical lossesabove and below the zone is the latent heat of the steam.At the point where the latent heat has been completely consumedto supplylosses,the only remainingheat to be carried forward is the sensibleheat in the liquid water, and the processbecomesrather like that of Lauwerier, which was discussedearlier.It is not identical,however,becauseas the heatlosses are transferredto the overburdenand to the underburden,the rate atwhich steam mustbe condensedto supplytheselossesdecreases and excesssteambecomesavailableto advancethe condensationfront further. Thus eventhough essentiallyall the latent heat of the steamis being usedto supplythe vertical losses,the condensation front still advances;at the same time, substantialquantities of heat are carried beyond the condensationfront by the sensibleheat of the condensateand the heat front passesbeyond the condensationfront. Figure 3.14is a qualitative representation of the situation. Mandl and Volek (1969)and Hearn (1969)were the first to recognize this phenomenon;they each developedan equationthat predicts the time, /", at which it occurs. Let Hl be the rate at which latent heat is injectedinto the reservoir.The total heat-injectionrate is still taken as.F/0.AJ the critical time the latent heat-injection rate is just equal to the vertical heatJossrate. If this is so, then the rate at which heat is being stored must be equal to the rate at which sensibleheat is being injected-i.e., to FIo - I1r. This is shownby equation3.53,which reducesto equation 3.54.

= ptCth(Ts- Tn)Hyex'zerfc(X) -

Ho- H^ = Qs = p(ft(Ts - rf#

| -

prCth(Ts Ta)

*=

ex? efic(X")

(3.53)

(3.54)

'The steamtemperatureis assumedconstantin this chapter.In actual practicethere tendsto be a small reductionin steamsaturationtemperature,which is causedby the pressuredrop as the . steamflows awayfrom the injectionwell.

ConvectiveTransferof Heat beyondthe CondensationFront

95

_---_>

i Cond.i HeatFront i rtont i (fort > t.)

Latentheatis availableat AND STEAMOUALITYBEFOREtC TEMPERATURE

AND STEAMOUALITYAFTERI TEMPERATURE Steamcondensesbeforeit Figure 3.14

The critical time may be found by solvingequation3.54for the value of X" that correspondsto the particular value of.Htf Ho and then obtaining the time by the use of (3.43)or (3.44). equation ' Equation 3.54 was derived by Mandl and Volek (1969)and almost simultaneouslyby Hearn (1969).For given valuesof I{r and 110,the critical dimensionless time can be found by interpolation of Table 3'1' SIZE OF STEAM ZONE FOR TIME GREATERTHAN THE MANDL AND VOLEK'SCRITICALTIME Beyond the critical time, the vertical loss from the steamzone is given by equation 3.55,where-,4srefers to the area of the steamzone'

H", = fJsn " W a l

\/ rrazt - te)

(3.5s)

The time L at which the areadA of.the steamzonewas formed is found by calculating time as a functon of area from equation3.56. A(tn1=

96

HoptCth

4KzpzCz(Ts

^f(x)


(3.s6) Chap' 3

where

x =

2K' p1C1h\/ a2

\/,

*4 r r l*'e rfc( X) f(X .)\=\ / (e

- t\

This time may then be substitutedinto (3.55)and I{ calculatedas a functon of ,45 by evaluatingthe integral. From this the value of,,45may be determinedfor specific valuesof I4. This procedurewas followed by Hearn, who expressedhis resultsby equation3.57. As=

As=

HoprCrh

4K\gs

'(.,ft)

(3.s7)

_ ?]R)

HsP1Cft 4K2p2C2(Ts- f^)

'(.,*)

Hearn'sfunction F is given by Table3.3. For times lessthan the critical, F is identical to f. TABLE 3.3 Values of flXl' andrk,L\ 'Hol \

Values ofF for various 111/116

X 0.2 0.4 0.6 0.8 1.0 r.2 1,.4 1.6 1.8 2.0 2.5 3.0 3.5 4.0 4.5 5.0 6.0 7.0 8.0 9.0 10.0

f(X) 0.035 0.L22 0.245 0.392 0.556 0.733 0.918 1.111 1.310 1.512 2.032 2.566 3.105 3.650 4.200 4.753 5.863 6.978 8.097 9.218 10.340

0.6 0.035 0.103 0.r7r 0.240 0.309 0.378 0.448 0.517 0.586 0.656 0.829 1.003 1.t77 1.351 1.525 1.699 2.047 2.396 11AA

3.092 3.441

0.035 0.12L 0.22L 0.321 0.422 0.523 0.624 0.726 0.827 0.929 1.184 1.439 1.694 1.949 2.204 2.460 2.971 3.483 3.995 4.506 5.018

0.035 0.t22 0.243 0.372 0.502 0.632 0.762 0.894 1.025 1.156 1.485 1.814 2.1,44 2.475 2.805 3.136 3.798 4.460 5.L22 5.785 6.447

0.035 0.122 0.245 0.392 0.546 0.702 0.858 1.016 t.173 1.330 1.726 2.122 2.518 2.915 J.JIJ

3.710 4.506 5.303 6.099 6.896 7.693

0.035 0.122 0.245 0.392 0.556 0.732 0.910 1.089 t.268 1.448 1.899 2.352 2.806 3.260 3.714 4.169 5.079 5.990 6.902 7.8t4 8.726

0.7 0.035 0.122 0.245 0.392 0.556 0.733 0.918 1.111 1.308 1.506 2.002 2s01 2.999 3.499 3.999 4.500 s.503 6.506 7.510 8.514 9.518

0.8 0.035 0.122 0.245 0.392 0.556 0.733 0.918 1.111 1.310 r.5t2 2.032 2.563 3.094 3.628 4.1,62 4.696 s.766 6.837 7.908 8.980 10.052

(from Hearn 1969) Effect of a Nonvertical Front

97

The fraction of the total heat injectedthat remainswithin the steamzone may be calculatedby equation3.58. AshprC{Ts - TR) ^ un:

----------jj-l-

-

ITot

H')'(.,*)

(3.s8)

The resultsare shownin Table3'4 and plotted in Figure 3.L5. A similar figure has been derived by Myhill and Stegemeier(1978)basedon the theory of Mandl and Volek (1969)and modified by unpublishedwork of Prats and Vogiatzis.It is reproducedin Chapter 4 as Figure 4.10. EFFECTOF A NONVERTICALFRONT Myhill and Stegemeier(1978)point out that for the heat-lossequationusedby Marx and Langenheim(and the extensionsof it) to be applicable,it is not necessaryfor only that the total volumeof the steam the heatfront to be vertical.It is necessary the sum of by the expression(Ahlz), wherer4represents zonecan be represented the upper and lower surface areas. Exampleswherethis is true include 1. A sloped,but straight, front that is advancinglinearly, 2. An inclined front that is straight but advancingonly at the top, and 3. Cylindrical fronts. TABLE 3.4 ReservoirHeatingEfficiencyCalculatedfrom Hearn (1969) EFFTCIENCY FOR VARIOUS VALUES OF HtlHo X

X2

0.2 0.4 0.6 0.8 1.0 1".2 t.4 1.6 1.8 2.0 2.5 3.0 3.5 4.0 4.5 5.0 6.0 7.0 8.0 9.0 10.0

0.04 0.16 0.36 0.64 1.00 r.44 r.96 2.56 3.24 4.00 6.25 9.00 12.25 16.00 20.25 25.00 36.00 49.00 64.00 81.00 100.00

98

^) 0.875 0.644 0.475 0.375 0.309 0.262 0.229 0.202 0.181 0.164 0.133 0.111 0.096 0.084 0.075 0.068 0.057 0.049 0.043 0.038 0.034

0.3 0.875 0.756 0.614 0.502 0.422 0.363 0.318 0.284 0.255 0.232 0.189 0.160 0.138 0.122 0.109 0.098 0.083 0.071 0.062 0.056 0.050

0.6

0.4 0.875 0.763 0.675 0.581 0.502 0.439 0.389 0.349 0.316 0.289 0.238 0.202 0.175 0.155 0.139 0.125 0.105 0.091 0.080 0.071 0.064

0.875 0.763 0.681 0.613 0.s46 0.487 0.438 0.397 0362 0.333 0.276 0.236 0.206 0.182 0.t64 0.148 0.125 0.108 0.095 0.085 0.0'17

0.875 0.763 0.681 0.613 0.556 0.508 0.464 0.425 0.391 0.362 0.304 0.26r 0.229 0.204 0.183 0.167 0.141 0.122 0.108 0.096 0.087

0.7 0.875 0.763 0.681 0.613 0.556 0.509 0.468 0.434 0.404 0.37',7 0.320 0.278 0.245 0.219 0.t97 0.180 0.153 0.133 0.tr'7 0.105 0.095

1.0

0.8 0.875 0;763 0.681 0.613 0.556 0.509 0.468 0.434 0.404 0.378 0.325 0.285 0.253 0.227 0.206 0.188 0.160 0.140 0.124 0.111 0.101


0.875 0.763 0.681 0.613 0.556 0.s09 0.468 0.434 0.404 0.378 0.325 0.285 0.253 0.228 0.207 0.190 0.163 0.142 0.127 0.11 0.103

Chap.3

Parameteris H^/H

-g 0.8 UJ ,

\\

A0.6

\ o.e\

.9 ,9 o.a

o

e\

UJ 0.2

\

r-0.5

\

S

-ia

s.' S

0r0.03 0.03

0.1

0.3

1

10

3

30

100

Dimensionless Time tD Figure 3.1.5 ReservoirHeating Efficiency (basedon Hearn L969)

For conical fronts the volumevarieswith the degreeof truncation from an extreme otAhl3 toAhl2 asthe shapeapproachesa cylinder. Even this variation changesthe dimensionlesstime by a factor of only 4/9. As may be seenfrom the horizontal scale of Figure 3.15,changingtoby a factor of this magnitudedoesnot have a large effect on the predicted thermal efficiency. STEAM INJECTIONINTO A THIN CHANNELOR FRACTURE the limiting casewhereft is assumedto apIn his paper,Hearn (1969)discusses proach zero. His result may be obtained by allowing ft, in the right-hand side of equation 3.40, to approachzero. If the multiplier & is combined with each of the terms inside the bracketsof equation3.40, only the central term remains as ft approacheszero and the equationbecomes A(t) =

Ho\/ a2t

(3.se)

Kz(Ts- T){rr

Substitutingtt for t in equation3.59, solving the resulting equationfar L, substituting the result into equation3.55, and rearrangingleadsto H^=2Ho ["' n J6

ll

,

oo,= Ho

(3.60)

Y or.t

V\",rn-al);-"

.)

This may be integratedto give Ht=

-zH'l. lsln 1rl

Kz(Ts- 7h)\l"

H, )1,

(3.61)

which leadsto . ,t'=

Ho\/drt

.lr

H^\

*--o, _ nrrsntit\i'ar:/

(3'62)

This remarkablysimpleexpressionindicatesthat the steamzone remainsa constant fraction of the total with the fraction being a sine function involving the ratio SteamInjectioninto a Thin Channel or Fracture

99

2.29.lts use is disH^lHo.Equation 3.59was also derived in chapter 2 as equation cussedin a later numerical example' Comparison of Fracture Filleil with Steam for Constant Injection Rate and for Gonstant Areal Growth Rate be seen from equaFor a constant steam injection rate into a fraction it can to the total heated area saturated steam the tions 3.59 and 3.62 that ihe ratio of area is given bY, _,-l n_.H^\ = s''\T' HoJ 7

As

(3.63)

injection times the same For a constant injection rate it was shown that, for long ratio is given bY,

(3.64

- )ryt/4\ = f4\' ' uo \H'l \e l. These two valuesare quite similar'

Valuesof 15ft Ht

Constant Injection

Constant DisPlacementf = o

Ho

1 0.8 0.6 0.4 0.2 0

1 0.96 0.84 0.64 0.36 0

I

0.95 0.81 0.59 0.31 0

Example calculation of the Mandl-volek critical Time for a Numerical theory ignoredt The solution to the numerical exampleof the Marx-Langenheim by the vertical heat losses. possibility of the steambeing completelyco_ndensed critical time. Mandl-Volek the after this occurs tu, U"r" "*pfained, later' calculated &tQ tes, time, X, andthe dimensionless /{i : 800 x 350 x 755 x 0J = 148 x 106Btu/d Ho: Ht + 800 x 350 x 407 :262 x L06Btu/d Find X. from Table 3.i.

' 100

*=

o'435= exP(x3)erfc(X") ConductiveHeatingwithin Reservoirs

Chap'

The root is X" : 0.973;this is obtainedby interpolationof Table3.1' toc:X?=0.947 The correspondingactual times are calculatedas follows:

,"=*=26d

forh=roft

t" = 2630d

for&=100ft

For the thinner reservoirthe critical time correspondsto only 26 days(0.07years); for the thicker reservoir the time is 2630 days, or 7.2 years.At times later than thesethe condensationfront lagsbehind the heat front. This will tend to reducethe quantity of oil displacedbelow that calculatedpreviouslybecausethe residualoil in the waterfloodedregionwill be larger than in the steamfloodedzone.The displacement of oil by steamis comparedto that by water in Chapter 5. The sizeof the steamzone (asdistinct from the larger heatedzone)can be obtained by calculatingthe volume (and hencethe area)of the steamzone that would be obtained if there were no heat lossesat all and then multiplying this by the efficiency read from Figure 3.15.This is done for the previous numerical examplein Table3.5. Extension of Numerical Example to Injection Into a Very Thin Horizontal Layer or Fracture In the previouscalculation, it was seenthat the heatedarea increasedmore rapidly for the thinner reservoir.The reasonfor this is, of course,that lessheat is neededto heat a given area of reservoirwhen it is not as thick. The limiting casecorresponds to that of a very thin reservoiror fracture.This casemay be viewed in two ways: l. The limiting situation for the injection of steam into progressivelythinner reservoirs,or 2. The injection of steaminto a narrow fracture within a thick reservoir. In the first casethe injected heat is essentiallylost, whereasin the secondit heats the adjacenttar sands.The total heatedareaand the steamzone area can be calculated ior the data of the previous exampleusing Hearn'sequations3.59 and 3'62, respectively. Ho:262 x 106Btu/d t : 365tr"^, d2 : 0.9 ft2ld Kz:

29.7Btuft d'F

Ts : 467"F;Tn : 75"F or Fracture SteamInjectioninto a Thin Channel

101

TABLE 3.5 Calculationof Steam Zone Area Allowing for Mandl-VolekEffect Time Years

SteamZone

Dimension

Efficiency(2)

Reservoirthickness: 10 ft: 113 226 J

n

5 6 7

8 9 10

0.20 0.L4 0.t2 0.10 0.09 0.08

39 53 66 79 92 105 118 r3l

HeatedArea in Acres No Loss(l)

1a

34 51 68 85 102 119 t36 153

o'rt

r70

Reservoir thickness= 100ft: 1 0.13 2 0.26 3 0.39 4 0.53 5 0.66 6 0.79 7 0.92

With Loss

u./o

t.t

0.71

3.4

0.67

).1

0.63 0.61 0.59 0.56

6.8 8.5 10.2 11.9

SteamZone

ta

6.3 8.0 9.4 10.6 11.7 12.8 13.7 14.6 15.5

J.+

4.8 6.1 6.8 7.6 8.2 9.3

1.3 2.4 3.4 4.3 5.2 6.0 6.7

3.4 4.3 5.2 6.0 6.7

7.5 8.2 8.8

7.9 8.4

1.3 z.+

Critical time -- 7.2 y 8 9 10

1.05 1.18 1.31

0.535 0.515 0.495

t3.6 l).J

r'7.0

t.5

',t.)!1tt/lorC',h{ft- Tp))convertedto acres. *''FromFisure 3.1.5.

Area in acres= Ht:

Ho{drt

43,5601[rKz(Zs- r^)

= 5.283!G,

148 x 10"Btu/d

= 0.775of the total heatedarea As=A"{(;)e)] The resultsare given in Table3.6. Also shownin Table3.6 are calculatedaverage valuesfor the total thicknessof the heatedzoneaboveand belowthe heatedzone. Thesewereobtainedby dividing the total volumeof the heatedzonecorresponding to the quantityof steaminjectedby the total heatedarea.[t will be noted that the heat penetratesabout 50 tt (10212)on either side of the heatedzone in 10 years. With heat-penetration distancesof this order, it is easyto seewhy steamingthin reservoirsfor long periods is inefficient. 102


Chap.3

TABLE 3.6 CalculatedHeatedArea and Steam Zone Area for Injectioninto a Very Thin Layer Years

10

Hot zone in acres ).J

t -J

Steamzone in acres 4.1 5.8

9.t

10.6

11.8

7.r

8.2

9.2

12.9

10.8

r4.9

15.8

16.7

11.6

t2.3

12.9

Averageheatedzone thicknessin feet(l) 32 45 56 64 72 79 85 91 96 t02 (t)Assuming that all of the injectedheat remainsin a zoneof uniform thicknesshavingthe samearea as that calculatedfor the hot zone.

BIBLIOGRAPHY CentEn, R. D., Appendixl of.Optimum Fluid Characteristics for FractureExtensionby G. C. Howard and G. R. Fast,Drill. and Prod. Prac.,API (1957),267-268. HernN, C.L., "Effect of Latent Heat Content of Injected Steam in a Steam Drive," JPT, 374-375 (April 1969).o 1969SPE. I-euweruen, H. A., "The Transport of Heat in an Oil Layer Causedby the Injection of Hot Fluid," Appl. Scl. Res.A, 5: 145-150(1955). MeNoq G. and VoLnr, C.W., "Heat and MassTransportin SteamDrive Processes,"SPEI, 59-79 (March 1969). Menx, J.W., and LeNceuuuu, R. N., "Reservoir Heating by Hot Fluid Injection," Pet. Trans.AIME, 216:3I2-3I5 (1959). MvHrLL, N.A., and SrecuraerrR,G.L., "Steam-DriveCorrelationand Prediction," "IPI l7 3 -182 (February 1978). RAMEv,H. J., "How to CalculateHeat Transmissionin Hot Fluid Injection," in Fundamentals of Thermal Oil Recovery,Dallas, Tex.: Petroleum Engineer Publishing Company.

(1e6s).

VoceL, J. H., "Simplified Heat Calculationsfor Steamfloods," SPE lt2l9, (1982);IPT, tl271136(July 1984).

Bibliography

103

Steqmflooding

INTRODUCTION In this chapterthe ideasintroducedin Chapter3 are expanded,and it is shown how they may be used as the basisfor the analysisof field projects.The chapter also discussesimportant factors that were not included in the developmentof the ideasin Chapter3. Theseinclude the effect of gravity in causingoverrideof the steam,the effect of steamingupon the permeabilityof the matrix, depletion,and steamdistillation. A OUALITATIVEDISCUSSIONOF STEAM.INJECTIONPROCESSES i

Steam-injectionprocessesfor the recovery of heavy oils are divided into two categories: L. Cyclic stimulation 2. Steamflooding Cyclic Stimulation In stimulation,steamis injectedinto the reservoiratafate of up to about1000B/d (160tld or m3/d)for a period rangingfrom one to severalweeks,and then the well is producedby allowingfluids to flow back.When the pressureat the bottom of the well drops, the well is pumped. During the pumping period, the well temperature continuesto fall. Over a peiiod that can rangefrom severalmonths to a year or more, the oilproduction rate falls to the point where it is no longer advantageousto continue, 104

and the well is restimulatedby injectingmore steam.This cyclic processis continued until the quantity of oil recovered is no longer sufficient to justify further steaming.At this time the recoveryis typically of the order of I5%; the recovery dependson the natureof the reservoir,the economicvalueof the producedoil, the well spacing,and other variables. Steamflooding In the flooding process,steamis injectedcontinuouslyinto one or more wells and oil is driven to separateproductionwells. Usually the wells are placedin regular patterns.The steamfloodingprocessis alsoreferredto assteamdrive. In the examplesdescribedin this chapter,the objectiveis to drive the oil sidewaystoward productionwells.If the reservoirdips, it is advantageous to drive the oil downwardin order to utilize gravity to keep the steamfrom bypassingthe oil.l Frequentlythe two methodsof steaminjectionarecombinedandwellsareproducedby stimulationbeforeflooding is started.When it is desiredto producevery viscousoils suchasfrom oil sands,stimulationbeforeflooding is almostessentialin order to achieveflow communicationbetweenthe injectionand productionwells. Communicationcan be establishedbetweenpairs of wells even in cold tar sandby creatinga fracturebetweenthem. This can be doneby injectingsteamat a sufficientlyhigh pressure.In tar sandsdeeperthan about1000ft, suchfracturesare usuallyvertical, and they tend to have a definite azimuthal(compass) orientation. In much of Alberta this is approximatelySWNE. At shallowerdepths,horizontal fracturestend to be formed. lf steamis injectedinto a verticalfracturein cold tar sand,heatingwill occur and condensate will flow to the connectingproductionwell. There is a tendencyfor the steamto override,and the fracture can becomeheatedalongthe top without much heat penetratingto the lower parts. The pressuregradientalong a steamed communicatingfracturetendsto be smallbecauseof the needto preventexcessive steambypassing.As a result,while heat is transferredto the adjacentreservoir,oil productionis slowbecausethere is little driving force availableto movethe heated oil. Becauseof thesedifficulties and becauseof the attractiveness of early oil production, the preheatingof the reservoirbefore steamflooding appearsto be the preferableroute to achievingconventionalsteamfloodingin bitumen reservoirs. Although little practicalfield experienceis available,vertical steamflooding gravitydrainageapproach,which is describedin Chapter7, usingthe steam-assisted may be preferablein many circumstances-particularly for projectsinvolving thick reservoirs.In many cases,steamflood projectsthat were startedwith the idea of driving oil horizontally have endedup with more and more attention being paid to the importanceof gravity in providingdrive. It is recognizedthat downwardsteam drive in dippingreservoirsis a practicalmeansof achievinghigh injectionand productionrates.In commercialoperations,steamstimulationis often economicallyattlt is being realizedmore and more that downwardsteamfloodingoffers considerableadvantages.One way of accomplishingthis is to use horizontal productionwells locatednear the baseof the reservoir,with the steamintroducedabove.This approach,which has becomeknown ass/eamassistedgravity drainage,is discussedin Chapter 7.

A OualitativeDiscussionof Steam-injectionProcesses

105

tractive becauseit enablesrapid production of oil with acceptableand sometimes very high oil-to-steamratios. While the short-term economicsof stimulation are frequently satisfactory, only about 15 to 20% of the oil can be producedeconomically.After this, the oilsteamratio becomesrelativelypoor. At this stageit is common,at leastin fields containingmobileoil (particularlythosein California),to convertthe steamstimulation operationto a steamflood. Steamfloodscan produce recoveriesof the order of 50Voof the original oil in placewith oil-steamratios of the order of.0.2. Volumesof steamare traditionally measuredin terms of the volume of the equivalentwater; a barrel of steamis thus with the steam)and 1 m3is 1 t. 350lb of steam(includinganyliquid waterassociated nature upon the of the reservoir.Very deep ratio is dependent The oil-steam most) are uneconomic for conventional (deeper ft the very than 5000 at reservoirs pressures and correthe very high steam flooding because of and steamstimulation quantity steam reHeat losses and the of required. high temperatures spondingly Another factor become excessive. temperature quired to raisethe reservoir-to-steam the overburden. from the well bore to is the excessiveheat loss that can occur There is an increasein the well bore heat lossesas the depth of the reservoiris increased.As was discussedin Chapter2, this increaseis causedby the extra length of the well and also by the higher steamtemperatureassociatedwith the higher pressures. Thermal insulationcan be usedto extendthe practicaldepth for steam injection,but this tendsto be expensive. The next most important criterion for a successfulsteamrecovery project is that the reservoirshouldbe thick-certainly at least10 ft thick and preferablymuch thicker. The reasonfor this is that the heat lossesto the overburdenand underburden representan excessiveproportion of the total heat requirementfor thin reservoirs.This ideawas discussedin the last chapter. Typical successfulsteam-driveprojectsare in relativelyshallow,fairly thick reservoirs-e.g.,1000to 2000ft in depth and 100ft thick. Usuallythesereservoirs or looselyconsolidatedsandhavingreasonablyhigh perconsistof unconsolidated meability and porosity (e.g., 1 D and 30Voporosity) and high oil saturation. [t is usual to produceoil by stimulationfrom both the injection and the productionwells Stimulationis often continued,evenduring the drive, beforethe drive commences. if the temperatureof the producedfluids tendsto fall. It is alsobecomingcommon, as steamfloodedfields become depleted,to recover some of the remaining oil by waterflooding.In this situationit is still desirableto stimulatethe producersperiodically if the production tends to fall in temperature. Very shallowreservoirsare usuallynot suitablefor steamflooding(or for stimulation, either). The reasonfor this is that the steampressurethat can be utilized would have to be kept low to avoid fracturing to the surface of the ground above the reservoir. With the lower temperaturesthat correspondto the lower-pressure steam, the oil (particularly if it is bitumen) may not become sufficiently fluid to make recoverypracticable.Fracture pressureis, to a first approximation,equal to 1 psift of depth from the surface. The use of horizontal wells in place of conventionalones makes the use of steamfloodingprocessesin shallow reservoirsmore practical. Their greatercontact 106

Steamflooding

Chap.4

kB/d

,_

M//.4

l-l lffi t-l II

--- ---1 Sreamsoak1 SteamfloodI

rot"t

I

1968 1970 't972 1974 1976 1978 1980 1982 1984 1986 1988

Year Figure 4.1 Heavy Oil Recoverywith Steam in the United States(Sourceof Data Oil and Gaslournal\

with the reservoirallows more viscousoils to be producedat a useful rate. This ls discussedin Chapter7. During recentyearsthe trend in the heavyoil fieldsin Californiahasbeento switch from steam stimulation to flooding, and most heavy oil from there is now producedby steamflooding.Ihe main reasonfor this is the economicincentiveto improve the recovery.Figure 4.1.comparesthe historical recovery of oil by steam stimulationwith that by steamfloodingin California. In additionto providing a higher recovery,steamflooding-with its continuous injectionof heat-can produceoil significantlyfasterthan can the cyclicstimulation process.This, too, can have a significant economic impact. The main disadvantages of steamfloodingcomparedto stimulationare the following: o There is a lower oil-steamratio. In steamfloodingit is necessaryto heat a larger part of the reservoir,whereasin stimulation, at least in the early cycles,the heating is confined to a smaller region around the well. o There is a longerperiod of time before significant production starts. o Often flooding is not possibleinitially becauseof the lack of flow communication. FORSTEAMFLOODING SUITABILITY OF SPECIFICRESERVOIRS The choiceof steamfloodingas a meansfor the recoveryof petroleumhasbeendiscussedby a number of authors(including Farouq Ali 1974,FarouqAli and Meldau 1979,Geffen 1973,Matthews 1983,and Chu 1985).Whereas the suitability of a reservoir for production by steam stimulation can be determined relatively simply Suitabilityof SpecificReservoirsfor Steamflooding

107

and the nell ued until th steaming-Al dependsm rl well spriq; SteanrflooC

Sfeqmflooding

INTRODUCTION In this chapterthe ideasintroducedin Chapter3 are expanded,and it is shown how they may be used as the basisfor the analysisof field projects.The chapter also discusses important factorsthat were not includedin the developmentof the ideasin Chapter3. Theseincludethe effect of gravity in causingoverrideof the steam,the effect of steamingupon the permeabilityof the matrix, depletion,and steamdistillation. A OUALITATIVEDISCUSSIONOF STEAM.INJECTIONPROCESSES Steam-injectionprocessesfor the recoveryof heavy oils are divided into two categories: 1. Cyclic stimulation 2. Steamflooding Gyclic Stimulation

In the floodil oil is driren t patterns.Tb ples describa duction q-ellr order to utilil Fregrs duced bl srir viscousoib I order to rhi Comm sandbv creti sufficientl;*hi usuallvverth In much of A fracturested If sear and condens the steamto much heat pt communicdil steamblpasc productiut b oil. Becauscr duction.thc 1 preferableru Althqt usingthe srce may be prcfa reservoirs-lin driving cil h the impotal drive in di6i duction ratcs

In stimulation,steamis injectedinto the reservoiratarate of up to about1000B/d (160t/d or mt/d) for a period rangingfrom one to severalweeks,and then the well is producedby allowingfluids to flow back.When the pressureat the bottom of the well drops,the well is pumped.During the pumpingperiod, the well temperature continuesto fall. Over a period that can range from severalmonths to a year or more, the oilproduction rate falls to the point where it is no longer advantageousto continue,

'It is tri tages.One rryt the reservoir,ri assistedgranty I

104

A Oualitatiw I

and the well is restimulatedby injectingmore steam.This cyclic processis continued until the quantity of oil recoveredis no longer sufficient to justify further steaming.At this time the recoveryis typically of the order of I5Va;the recovery dependson the natureof the reservoir,the economicvalueof the producedoil, the well spacing,and other variables. Steamflooding

ded, and it is shown projects.The chapter l developmentof the usingoverrideof the ratrix.depletion,and

ES .i

re divided into two

up to about 1000B/d ks, and then the well 3 at the bottomof the the well temperature iear or more,the oilltageousto continue,

In the flooding process,steamis injectedcontinuouslyinto one or more wells and productionwells. Usually the wells are placedin regular oil is driven to s-eparate patterns.The steamfloodingprocessis alsoreferredto assteamdrive. In the examplesdescribedin this chapter,the objectiveis to drive the oil sidewaystoward productionwells.If the reservoirdips, it is advantageous to drive the oil downwardin order to utilize gravity to keep the steamfrom bypassingthe oil.1 Frequentlythe two methodsof steaminjectionare combinedandwellsareproducedby stimulationbeforeflooding is started.When it is desiredto producevery viscousoils suchasfrom oil sands,stimulationbeforeflooding is almostessentialin order to achieveflow communicationbetweenthe injectionand productionwells. Communicationcan be establishedbetweenpairs of wells even in cold tar sandby creatinga fracturebetweenthem. This can be doneby injectingsteamat a sufficientlyhigh pressure.In tar sandsdeeperthan about1000ft, suchfracturesare usuallyvertical, and they tend to have a definite azimuthal(compass) orientation. In much of Alberta this is approximatelySWNE. At shallowerdepths,horizontal fracturestend to be formed. lf steamis injectedinto a verticalfracturein cold tar sand,heatingwill occur and condensate will flow to the connectingproductionwell. There is a tendencyfor the steamto override,and the fracture can becomeheatedalongthe top without much heat penetratingto the lower parts. The pressuregradientalonga steamed communicatingfracturetendsto be smallbecauseof the needto preventexcessive steambypassing.As a result,while heat is transferredto the adjacentreservoir,oil productionis slowbecausethereis little driving force availableto movethe heated of early oil prooil. Becauseof thesedifficulties and becauseof the attractiveness duction, the preheatingof the reservoir before steam flooding appearsto be the preferableroute to achievingconventionalsteamfloodingin bitumen reservoirs. Although little practicalfield experienceis available,vertical steamflooding gravitydrainageapproach,which is describedin Chapter7, usingthe steam-assisted may be preferablein many circumstances-particularly for projectsinvolving thick reservoirs.[n many cases,steamflood projectsthat were startedwith the idea of driving oil horizontally have endedup with more and more attention being paid to the importanceof gravity in providingdrive. It is recognizedthat downwardsteam drive in dippingreservoirsis a practicalmeansof achievinghigh injectionand production rates.In commercialoperations,steamstimulation is often economicallyattlt is being realizedmore and more that downwardsteamfloodingoffers considerableadvantages.One way of accomplishingthis is to use horizontal productionwells locatednear the baseof the reservoir,with the steamintroducedabove.This approach,which has becomeknown as srearnassistedgravity drainage,is discussedin Chapter 7.

A OualitativeDiscussionof Steam-injectionProcesses

105

tractive becauseit enablesrapid production of oil with acceptableand sometimes very high oil-to-steamratios. While the short-term economicsof stimulation are frequently satisfactory, only about 15 to 20Voof the oil can be producedeconomically.After this, the oilsteamratio becomesrelativelypoor. At this stageit is common,at leastin fields containingmobileoil (particularlythosein California),to convertthe steamstimulation operationto a steamflood. Steamfloodscan produce recoveriesof the order of 50Voof the original oil in placewith oil-steamratios of the order of 0.2. Volumesof steamare traditionally measuredin terms of the volume of the equivalentwater; a barrel of steamis thus 350lb of steam(includinganyliquid waterassociated with the steam)and 1 m3is 1 t. The oil-steamratio is dependentupon the nature of the reservoir.very deep reservoirs(deeperthan 5CI0 ft at the very most) are uneconomicfor conventional steamstimulationand flooding becauseof the very high steampressures and correspondinglyhigh temperaturesrequired. Heat lossesand the quantity of steam required to raisethe reservoir-to-steam temperaturebecomeexcessive.Another factor is the excessiveheat loss that can occur from the well bore to the overburden. There is an increasein the well bore heat lossesas the depth of the reservoiris increased.As was discussedin Chapter2, this increaseis causedby the extra length of the well and also by the higher steamtemperatureassociatedwith the higher pressures. Thermal insulationcan be usedto extendthe practicaldepth for steam injection,but this tendsto be expensive. The next most important criterion for a successfulsteamrecovery project is that the reservoirshouldbe thick-certainly at least10ft thick and preferablymuch thicker.The reasonfor this is that the heatlossesto the overburdenand underburden representan excessiveproportion of the total heat requirementfor thin reservoirs.This idea was discussedin the last chapter. Typical successfulsteam-driveprojectsare in relativelyshallow,fairly thick reservoirs-e.g.,1000to 2000ft in depth and 100ft thick. Usuallythesereservoirs consistof unconsolidated or looselyconsolidatedsandhavingreasonablyhigh permeability and porosity (e.g., 1 D and 30vo porosity) and high oil saturation. It is usual to produceoil by stimulationfrom both the injection and the productionwells beforethe drive commences. Stimulationis often continued,evenduring the drive, if the temperatureof the producedfluids tendsto fall. It is alsobecomingcommon, as steamfloodedfields become depleted,to recover some of the remaining oil by waterflooding.In this situationit is still desirableto stimulatethe producersperiodicallyif the productiontendsto fall in temperature. Very shallowreservoirsare usuallynot suitablefor steamflooding(or for stimulation, either). The reasonfor this is that the steampressurethat can be utilized would have to be kept low to avoid fracturing to the surfaceof the ground above the reservoir. With the lower temperaturesthat correspondto the lower-pressure steam,the oil (particularlyif it is bitumen)may not becomesufficientlyfluid to make recovery practicable.Fracture pressureis, to a first approximation,equal to 1 psift of depth from the surface. The use of horizontal wells in place of conventionalones makes the use of steamfloodingprocessesin shallow reservoirsmore practical. Their greatercontact 106

Steamflooding

Chap.4

E' 50

3XD

FitDate O

with the rescr discussedin O During n switch from sf producedby c improve the rr stimulationwl In additi ous injectim d lation process disadvantager

e There b i largerpa cycles.th o Thereis, o Often fh commun

SUITABILITY OF SPC

The choiceof r cussedby a nu 1979, Geffen I reservoirfor p

Suitabilityof 59

rbleand sometimes uently satisfactory, After this, the oiln. at leastin fields rt the steamstimuf the original oil in m are traditionally rel of steamis thus am) and 1 m3is 1 t. :servoir.Very deep ic for conventional rressures and correantity of steamreive.Another factor lo the overburden. the reservoiris inb;-the extra length rd with the higher ;al depth for steam recoveryprojectis nd preferablymuch 'denand underburnentfor thin resertallow, fairly thick lly'thesereservoirs rasonably high peroil saturation.It is re productionwells n during the drive, rccomingcommon, e remainingoil by .he producerspericding (or for stimurat can be utilized the ground above the lower-pressure ufficiently fluid to ,ximation,equalto

kB/d

,_

--- --- -1

lV////,

Steamsoak I

IKffi

Steamflood

II

rotut

t_ l*

I

1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 t988

Year Figure 4.1 Heavy Oil Recovery with Steam in the United States (Source of Data Oil and Gas lournal)

with the reservoirallowsmoreviscousoils to be producedat a usefulrate. This is discussedin Chapter7. During recentyearsthe trend in the heavyoil fieldsin Californiahasbeento switch from steamstimulationto flooding, and most heavyoil from there is now producedby steamflooding.The main reasonfor this is the economicincentiveto improvethe recovery.Figure 4.1.comparesthe historicalrecoveryof oil by steam stimulationwith that by steamfloodingin California. In addition to providing a higher recovery,steamflooding-with its continuousinjectionof heat-can produceoil significantlyfasterthan can the cyclicstimulation process.This, too, can have a significant economic impact. The main disadvantages of steamfloodingcomparedto stimulation are the following: o There is a lower oil-steamratio. In steamfloodingit is necessaryto heat a largerpart of the reservoir,whereasin stimulation,at leastin the early cycles,the heatingis confinedto a smallerregionaroundthe well. r There is a longerperiod of time beforesignificantproductionstarts. o Often flooding is not possibleinitially becauseof the lack of flow communication. SUITABILITY OF SPECIFICRESERVOIRS FORSTEAMFLOODING

makesthe use of reir greatercontact

The choiceof steamfloodingas a meansfor the recoveryof petroleumhasbeen discussedby a number of authors(including Farouq Ali 1974,FarouqAli and Meldau 1979,Geffen 1973,Matthews L983,and Chu 1985).Whereasthe suitability of a reservoir for production by steamstimulation can be determined relatively simply

fboding

Suitabilityof SpecificReservoirs for Steamflooding

Chap.4

107

by meansof singlewell tests,field experimentationto determineits suitabilityfor steamfloodingis much more costly. Even the simplesttest must involve multiple wells and long periodsof operation. Severalquantitativeguidelineshave been developedto indicate whether a reservoirpropertymight respondfavorablyto steamfloods.Table4.1 is a summary of suchscreeningguides;it is taken from Chu (1985). Matthewslists the followingfactorsthat are unfavorablefor steamflooding. 1. 2. 3. 4. 5. 6.

Oil saturationlessthan 40% Porosity less than 20Vo Oil-zonethicknesslessthan 30 ft Permeabilitylessthan 100mD Ratio of net to grosspay lessthan 50% Layersof very low oil saturationand high permeabilityin the oil zonethat act as thief zones 7. Extremelyhigh viscosity 8. Fractures2 9. Large permeabilityvariationsin the oil zone 10. Poor reservoircontinuity betweeninjectorsand producers 11. Deep high-pressurereservoirsand shallowreservoirswith insufficientoverburdento permit steaminjectionwithout fracturing He points out that steamfloodsmay be successfuleven if one or two of the above conditionsare not met, providedthat the remainingfactorsare highly favorable. Chu (1985)describesan empiricalcorrelationthat predictsthe oil-steamratio (osR) or its reciprocal,the steam-oilratio (soR). His correlationequationsare givennext; note that the units employedare,in somecases,not the customaryones. tf soR < 5.0(osR> 0.201: Englishunits (asdefinedshortly): - 14J95" SOR = 18.744+ 0.001453D- 0.05088h- 0.0008864k- 0.0005915p. - 0.0002%8L! l.L

Metric units (asdefinedshortly): -14.795. SOR = 18.744+0.004767D-0.I6693h - 0.8981k- 0.5915pr

- 0.000s767LL l.L 2Fractures may be undesirablebecausethey promote bypassingof the steam.In the steamassistedgravity drainageprocess,however,which is operatedbelow the critical steam-coningrate, fracturesenhancethe processif they are vertical and have little effect if they are not.

108

Steamflooding

Chap.4

nine its suitability for nust involve multiple

<S

OOa

Nqi6

o indicate whether a Dle 4.1 is a summary I for steamflooding.

=

x-

orQ o -o

Vo N

'= 3u.? trv o

ln

t the oil zone that act

FoP A

8883= 338s+ \/\/\/on

c.l

ls ith insufficient over-

o

*

.s.i

o o Clo 9C'.1 o ci

F

or two of the above re highly favorable. ts the oil-steamratio lation equations are I the customarvones.

rJ) @ gt

^

i

AA-n o

'i ar E

6S

ho

i^^Nr^ Oni

Od)

d/\/\oY

C) o

nv?n

o) '6'

oON

G

- 14.795. Itr59151^c

E o

ot a

N

c.l a,

o) p

o

q

s

-vi oo €O€O6 hi-ii-i oAAAA

9l5p - 14.795,

.E o o

()

(t) t tI|

thc steam. In the steamitical steam-coningrate, lcy are not. rflooding

Chap. 4

@

r vx€=ir\FF 5.0(osR< 0.201: Englishunits (asdefinedshortly):

where I

- 0.00t3570+ 0.000007232p. oSR = -0.011253+ 0.0N02779D+ 0.0001579h

+ o.oo001o4z4 + o.5t2oos" l.L

Metric units (asdefinedshortly): - 0.077i50+ 0.007232p" OSR = -0.011253+ 0.00009117D + 0.0005180h

+ 0.0000346tU + 0.st20ds, p

English D = ft : I = S, : Soi= So,= I = d= p : 4 :

depth thickness permeability oil saturationat start initial oil saturation residualoil saturation temperature dip angle viscosity porosity

Metric

ftm ftm mD

mD

fraction of pore volume fraction of pore volume fraction of pore volume r

The term FY done per uni systemopctl equivalentI tem operatiq

"c

degree rad cp Pas fraction of bulk volume

These equationsmay be used quite simply becauseall they require are fairly basicmeasurements or estimatesof the reservoirproperties.Two formsof the equations are given,one for Englishunits and one for metric.Chu'spapercontainssummary data for 28 different steamflood field projects, including references.The equationsjust given were found to correlatewell with the data. Chu recommends that the equationfor SOR < 5 be tried first and that the secondequationbe used only if the answerfrom the first indicatesthe SOR to be greaterthan 5. Tables4.2, 4.3, 4.4, and 4.5 reproducethe field projectdata summariescollected by Chu. Referencesto the sourcesof the data are listed in Chu'spaper. It is interesting to note that of the 28 projects studied by Chu, only 7 gave oil-steam ratioshigher than 0.2.

where the lct ing the qrlcr For ert Wr(Hp - IJrl ing formulai tial energy,t the PV terme caseswhercd tional terms i At any 1 increaseste{ further heat i amount of hc is added,thc water vapor i mation. If fir The s plantssucha oil fields,wa that definest the weight fn thus liquid. I numericalfn The erl Table 4.6 as

THE PROPERTIES OF STEAM The most important properties of steamfor thermal recovery processesare those involvingenthalpy.Enthalpy is defined as

or, since

H:U*PV 110

Steamflooding

Chap.4

The Propertie

whereH

is the enthalpyin units of energyper unit mass,e.g.,kykg (or Btu/lb) U is the internal energy,k/kg (or Btu/lb) P is the pressure,kPa (or Btuft3) V is the specificvolume, m'lkg (lbft3)

i78 + 0.000007232p.

t7* + 0.007232pt

The term PV hasthe dimensionsof energyper unit mass;it is the work that mustbe done per unit massof material to introduce it at pressureP into a continuousflow systemoperatedin a steadystate.Similarly, material leaving the systemcan do an equivalentamount of work. The total heat effect in a continuous-flow,isolatedsystem operatingin a steadystate is thus Heat added= ) H" Wp- ZHrWr

Metric m m mD : r'olume : r'olume : volume

'c

rad Pas ; r'olume

:heyrequireare fairly '\r'o forms of the equa's papercontainssumrding references.The ata. Chu recommends ond equationbe used ater than 5. t data summariescold in Chu'spaper.It is only 7 gave oil-steam

where the terms Hp and Wprefer to the enthalpiesand massesof the productsleaving the systemand Hr and W are the correspondingterms for the feed. For example,the heat addedin a boiler to convert the feedwaterto steamis Wr(Hp - Hr) if the massof steamproductis equalto the massof feed.The preceding formulation of the law of conservationof energyneglectsterms such as potential energy,kinetic energy,electrical energy,and work other than that included in the PV terms;this is justifiablein the calculationsdescribedin this book. In other caseswhere theseother energyterms are significant, they must be included as additional terms in the energybalance. At any particular pressure,the temperatureand the enthalpyof liquid water increasesteadilyas heat is addeduntil the boiling point of the water is reached.If further heatis added,the waterboils at a constanttemperatureuntil an additional amountof heatequalto the latentheatof evaporationhasbeenadded.As this heat is added,the liquid is continuouslytransformedinto vapor until eventuallyonly water vapor is present;a very large increasein volume accompanies this transformation. [f further heat is added,the steambecomessuperheated. The steamemployedfor processheating and power generationin process plantssuchas refineriesand power stationsis usuallydry and superheated. In the oil fields,wet steam(i.e., a mixture of waterandvapor)is employed.The parameter that definesthe conditionof sucha mixture is the steamquality,/5;it is definedas the weightfractionof the steammixture that is vapor.A weightfraction (1 - /5) is thus liquid. The steamquality is often expressedas a percentagerather than as a numerical fraction. The enthalpy of steam of quality /s can be calculatedfrom the data of Table4.6 as IIs=(1 -/r)H"*fsH,

(4.1)

T DTOCeSSeS are those

or, since IIv=Hrl

amflooding

Chap.4

The Propertiesof Steam

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TABLE 4.6 Enthalpyof Water and Steam at SaturationConditions

T

Enthalpy kVkg

P (MPa)

('c)

Water

Evap.

0.006 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.r7 0 .l 8 0.19 0.20 0.21 0.22 0.23 o.24 0.25 0.26 0.27 0.28 0.29 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70 0.80 0.90 r.00 1.10 r.20 1.30 1.40

0.0 99.6 102.3 104.8 t07.1 109.3 rtt.4 113.3 rr5.2 116.9 118.6 120.2 rzr.8 123.3 t24.7 126.1 127.4 128.7 130.0 13r.2 132.4 133.5 138.9 143.6 t47.9 151.8 155.5 158.8 165.0 170.4 t75.4 179.9 184.1 188.0 191.6 195.0

0.0 417.5 428.8 439.4 449.2 458.4 467.1 475.4 483.2 490.7 497.9 504.7 511.3 5t7.6 523.7 529.6 535.4 540.9 546.2 551.5 556.5 561.4 584.3 604.7 623.2 640.1 655.8 670.4 697.1 720.9 742.6 762.6 781.1 ',198.4

2501,.6 2257.9 2250.8 2244.L 2237.8 2231,.9 2226.3 2220.8 2215.8 22t0.8 2206.t 2201.6 21,97.2 2193.0 2188.9 2t84.9 2181.0 21,77.3 2173.7 2170.0 2166.6 2163.2 2147.3 2132.9 21t9.7 2107.4 2095.9 2085.0 2064.9 2046.5 2029.5 2013.6 1998.6 L984.3 1970.7 1957.7

814.7 830.1

250t.6 2675.4 2679.6 2683.4 2687.0 2690.3 2693.4 2696.2 2699.0 270r.5 2704.0 2706.3 2708.5 2710.6 2712.6 27t4.5 2716.4 2718.2 2719.9 2721.5 2'723.r 2724.7 2731.6 2',737.6 2742.9 2'747.5 2'75r.7 2755,5 2762.0 2767.5 27"t2.1 27'/6.2 2779.'7 2782.'7 2785.4 278'7.8

T

P (MPa)

("c)

1.50 1.60 1.70 1.80 1.90 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 22.00 22.12

198.3 20r.4 204.3 207.1 209.8 212.4 223.9 233.8 242.5 250.3 257.4 263.9 269.9 275.6 280.8 285.8 290.5 295.0 299.2 303.3 30'7.2 311.0 318.0 324.6 330.8 336.6 342.1 347.3 352.3 357.0 361.4 365.7 369.8 373.7 374.2

Enthalpyk/kg

844.6 858.5 871.8 884.5 896.8 908.6 961.9 1008.3 t049.7 1087.4 Lt22.r 1154.5 1184.9 r2r3.7 t241.2 L267.5 1292.7 1317.2 1340.8 1363.8 1386.2 1408.1 1450.6 149L.7 1531.9 r57L.5 1610.9 1650.4 1691.6 1734.8 1778.7 1826.6 1886.3 2010.3 2107.4

t945.3 1933.2 192r.6 1910.3 1899.3 1888.7 1839.0 t794.0 1752.2 t7t2.9 1675.6 1639.7 1605.0 1571.3 1538.3 1506.0 1474.]. 1442.7 14rr.6 1380.8 1350.2 1319.7 1258.8 rr97.5 1135.1 1070.9 1004.2 934.5 860.0 '779.0

2789.9 2791.7 2793.4 2794.8 2796.r 2797.2 2800.9 2802.3 2801.9 2800.3 2797.7 2794.2 2789.9 2785.0 2779.5 2773.5 2766.8 2759.9 2752.4 2744.6 2736.4 2727.8 2709.4 2689.2 2667.0 2642.4 2615.1 2584.9 255t.6 2513.8 691.8 2470.5 591.6 2418.2 461.2 2347.5 186.3 2196.6 0.0 2101.4

Table {-( of evaporatkr temperature. The ralu unitsbv the ft

Simplerelatic tions of tempc Specific1 The latenthcr the criticalpo erating pressu 706'F), none i 2802kl/kg at In the fr is usual to ggr Typicallya gl than dry or sr water for the I Oil field the combustiq of about 70 to containshigh residualliquid

Abstractedfrom "U.K. SteamTablesin SI Units 1970",United Kingdom Committeeon Properties of Steam,Edward Arnold, London 1970.

Hs can also be expressed as

(4.2)

Hs=Hr*/sr\

where i is the latent heat of evaporationand the subscripts,S, L, and V refer to the steammixture, boiling liquid, and saturatedvapor,respectively. 120

Steamflooding

Chap.4

The Propertiesr

Table 4.6 and Figure 4.2 give the enthalpy of boiling water, the latent heat of evaporation,and the enthalpy of saturatedsteam as a function of pressureand temperature. The valuesin Table4.6 are in S.I. units. They may be convertedto Enelish units by the followingconversions:

Enthalpyk/kg bter '44.6 5E.5 i71.8 i{i4.5 iqt-8 oE.6 61.9 oE.3 49.'t E7.4 22.r 54.5 u.9 r3.7 1t.2 '67.5 92.7 t7.2 if0.E 63.8 ,t6.2 08.l 50.6 9r.7 31.9 71.5 I0.9 50.4 91.6 34.E 7E.7 26.6 E5.3 r0.3 0i.4

Evap.

Steam

1945.3 1933.2 1921.6 1910.3 1899.3 1888.7 1839.0 1794.0 t752.2 1712.9 1675.6 1639.7 1605.0 1571.3 1538.3 1506.0 1474.1 1442.7 14ll.6 1380.8 t350.2 t3t9.7 1258.8 rr97.5 1135.1 1070.9 1N4.2 934.5 860.0 779.0 691.8 59L.6 46t.2 186.3

2789.9 279t.7 2793.4 2794.8 2796.1 2797.2 2800.9 2802.3 280I.9 2800.3 2797.7 2794.2 2789.9 2785.0 2779.5 2773.5 2766.8 2759.9 2752.4 2744.6 2736.4 2727.8 2709.4 2689.2 2667.0 2642.4 2615.1 2s84.9 255t.6 25t3.8 2470.5 24t8.2 2347.5 2t96.6

0.0

2107.4

dom Committeeon Prop-

(Pressurein psia) : 145(pressure in MPa) (Temperaturein "F) = l.8(temperaturein 'C) + 32 (Enthalpy in Btu/lb) : (enthalpyin kJlkg)/2.326 Simplerelationsfor calculatingthe enthalpiesof saturatedliquid and vapor as functions of temperatureand pressureare given in Appendix 9. Specific points that should be noted with respectto Table 4.6 are as follows: The latent heat of evaporationdecreasesas the pressureis raised and disappearsat the critical point. Lesslatent heatis availableper unit massof steamwhen the operatingpressureis higher.Above the critical point (22.7MPa,374"C or 3208psia, 706"F), none is available.The enthalpy of saturatedsteamreachesa maximum of 2802kJlkg at 236'C and 3 MPa (1205Btu/lb at 457'F and 435 psia). ln the field, steamis generatedat pressuresup to about 15 MPa (2200psi). It is usualto generatewet steam-i.e., a mixture of saturatedsteamvapor and water. Typically a quality of 70 to 80Vois employed.The main reasonfor using wet rather than dry or superheatedsteamis to reduce the purity requirementsfor the feedwater for the steamgenerators. Oil field steamgeneratorsusually contain a single boiler tube coiled around the combustionzone.Water is pumped at high pressureinto one end and a mixture of about 70 to 80% vapor and 20 to 30Voliquid leavesthe other. The water usually containshigh concentrationsof dissolvedsolids.These remain dissolvedin the residual liquid water and are removedcontinuouslywith the steamproduct. More

IE

a15 -, o

06+ Quality

| o lzs I so I zs \ roo

J

810

o

o-

(4.2) l, andZ referto the ly. mflooding

Chap.4

2000 1000 EnthalpykJ/kg The Propertiesof Steam

3000

Figure4.2 Pressure-Enthalpy Diagram for Steam-Water

121

informationon water treating,steamgeneration,and steamdistributionis given in Chapter8. TEMPERATURE DISTRIBUTIONIN STEAMFLOODING Figure 4'3 shows,in an idealizedway, one conceptof the conditionsaround a steam-injection well. The temperaturein the vicinity of the well is nearlyconstant and is equalto the saturationtemperatureof the steam.This temperatuieprevails to the point wherethe last of the steamcondenses. Beyondthe condensation front thereis a hot-waterzonein which the temperature falls. The temperaturegradientjust beyond the front may be relatively ubrup, or moregentle,dependingupon the conditions.This wasdiscussed in the list chapter in connectionwith the works of Mandl and volek and of Hearn. Much of the heat introducedwith the steamis lost to the overburdenand to the underburdenby thermal conductionin the mannerdiscussedearlier. In the situation shown in the figure it is assumedthat the hot zone has reachedthe overburdenand underburden.In practice,it is possiblethat conditions may existin which the steamzonehasnot yet extendedto the upperand lower limits of the reservoir. A particularlycommonand importantsituationis that wherethe steamzone has risen, becauseof gravity effects,to the top of the reservoirbut has extended only part of the way to the bottom.Under theseconditionsthe oil belowthe steam zoneis beingheatedbut is producedslowly,and the potentialthermaladvantageof havinga thick reservoirto heatmay not be realized.lt is a challengeof thermil recoveryengineeringto devisesystemsby which the maximumavailablethicknessof reservoirmaterialis producedin orderto minimize the areaof over-and underburden to which heat is beinglost. Steamstimulationtendsto do this initially. In any case,as time goeson the steamzoneexpands,and the areatfiat is being heatedaboveand belowincreases. As a resultthe heatlossesalsoincrease,and lnjector

producerl

I i-->

.--->

Steam zone

warer flowtng lnrough

slow-moving oil bank___1,-

i->

Water

.9

E

Steam

E o

oil

n ------\

$ |

i

a smallerpod heating.The ited by inrerf The spr mining the u and underbu it takeslonga is greater. The dcs mal efficiency wells involrrcd the difficultv i in maintainiq when cold rig fingeringin C Tl.pical r of 2to 6 rres patternin Fgr seven-spotpea Figure4.5).At three prodrrc equal-the'ior from the -si& t ha : 10.m A featurt the project mr producersas e the bypassedo in this chaper Whenrh downdipin cr ment front. Th

---__..\

|

122

Figure 4.3 Diagram showingthe Distribution of Temperature,Pressure, and Saturationsin a Hypothetical One-DimensionalSteamflood

Steamflooding

Chap.4

l'!

TemperatureCIc

listributionis eivenin

conditionsaround a rell is nearlyconstant t temperatureprevails in w'hichthe temperarv be relatively abrupt ussedin the lastchapHearn. he overburdenand to ssedearlier. hat the hot zone has xsible that conditions : upp€rand lower limwherethe steamzone yoir but has extended te oil belowthe steam thermaladvantageof rallengeof thermalreavailablethicknessof rf over-and underburo this initially. nd the areathat is be;sesalsoincrease,and

a smaller portion of the heat in the injected steamis employedin useful reservoir heating.The heat lossesincreaseup to the point wherearealgrowth becomeslimited by interferencewith the neighboringpatterns(seeFigure 2.4). The spacingbetweeninjectorsand producersis an important factor in determining the utilization of heat. Large spacingsresult in large areasof overburden and underburdenhaving to be kept hot for longerperiods of time. For a given flow it takes longer to drain the oil betweenthe injector and the producer if the spacing is greater. The designof a steamfloodinvolves an economicbalancebetweenthe thermal efficiency of closespacingand the lower well investmentrequired for the fewer wells involvedwith larger spacings.Another factor, particularlywith tar sands,is the difficulty in establishing communication.Sometimesthere is alsothe difficulty in maintainingcommunication,sinceinterconnectingflow pathsmay tend to block when cold viscousoil drains into them by gravity drainage.(Seethe discussionon fingeringin Chapter5). Typical commercialsteamfloodprojectshave productionwells with spacings of 2 to 6 acreswith either one injection well per productionwell (inverted five-spot pattern in Figure 4.4) or one injection well for every two production wells (inverted seven-spotpattern in Figure 4.5). Line-drive configurationsare also common(see Figure4.5).Another popularconfigurationis the invertednine-spot;this resultsin three producersper injector.In this arrangement,the producingwells are not all equal-the "corner"wells (2, 4,6, and 8 in Figure4.4) havedifferent surroundings from the "side"wells (3,5,7, and 9). (Note:I acre= 43,560ft2 = 0.405ha and t ha : 10,000m2.) A featurethat is commonin manysteamfloodsis the additionof infill wellsas the projectmatures.Theseare frequentlyaddedwhen steambreaksthrough to the producersas a resultof gravity override.Infill wells allow the recoveryof someof the bypassedoil which lies belowthe steamzone.This is discussedfurther later on in this chapter. When there is a dip in the reservoirit is usuallyadvantageous to drive the oil downdip in order to make use of the gravitationalforce to stabilizethe displacement front. This is discussedin the next chapter. Injectionwellsareshownwithdiagonallinesthroughthem

d

(/

d

,g

a-,,--------a

aaooooao aaQaaaS0

o

O----------t

d

d

d

o

o a

e?e9oao ;^ ]"?'a'z

o

o

o o

o.-

ii

//,od INVERTEDFIVE SPOT PATTERN

)iagramshowingthe of Temperature,Pressure, rnsin a Hypothetical onal Steamflood

amflooding

Chap.4

producer 1 injector and1 (4quarters) perpattern

o

i9.-ez.-,a6oo

o.

INVERTED NINESPOTPATTERN 1 injectorand3 producers (4quarters+4halves) per panern

Figure 4.4 Inverted Five- and Nine-SpotWell Patterns Temperature Distribution in Steamflooding

123

Inloctlonwells ar6 shownwlth dlagonalllnesthroughthem.

i

fr q )J

V) i

@ i

3

a a

o

'12

/

i

iz

,at

164 ''r ac

6% oil satur.:: a steamf lft.i

i

o

STAGGEREDLINE DRME

II,IUER TEDSEVENSPOTPATTER N 1 Injeclor and two producers (slx one-thlrds)per pattern Figure4,5 InvertedSeven-Spot and Staggered-Line Drive patterns

1 Inlectorperproducer

FINGERING In the displacement processshown in Figure 4.3, the condensedwater runs more rapidly than the oil to the productionwell becauseit is much lessviscousthan the oil that it is displacing.Frequentlythe water runs as separaterivulets, or fingers, through the oil; the flow pattern can be visualizedas oil and water running togetheralongseparateflow paths,with the water velocitybeing much higherthan that of the oil. Thus, rather than dry oil, a mixture containingvery substantial quantitiesof water is produced.The fingeringof water through the oil may alsobe promotedby heterogeneities within the reservoir,including those createdby the fracturingthat resultsfrom steaminjectionat pressures abovethe minimum in situ stress.Passage of the water mustoccur if steamis to continueto supplyheatto the reservoir.If the removalof condensateis not possiblewith the availablepressure drops,then the processwill be slowedgreatly. Even if therewere no fingeringdue to the formationof unstablewater/oildisplacementfronts, the water would still run through the oil layer, with an early breakthroughbecauseof the adverseviscosityratio. It is shownin the next chapter that when an attemptis madeto displacea viscousoil with water,breakthroughof the water occursrapidly,becauseof the relativepermeabilityand viscositycharacteristics-even if the flow is diffuse rather than segregated (i.e., even if the water doesnot run as fingers). GRAVITYOVERRIDE A major difference between the practical situation and the flow depicted in Figure4.3 is that the differencein densitybetweenthe steamand the liquidsin the reservoircausesthe steamto override-i.e., to flow abovethe oil; the situationis as depictedin Figure 4.6. Eventuallysteambreaksthrough at the productionwell. The upper steam-swept regionhas a much lower residualoil saturationthan the lowerwater-floodedregion.For example,Blevinsand Billingsley(1975)report a 124

Steamflooding

Chap.4

:l

this project rcp ZOnerepre\':'l:( sure gradien: :r tion rate in o:J A sien::: w i l l r e s t r i c t: : e sure drop :rJ flooded zr.nu s t e a m ,u h r ; : ; . : 1 9 8 2 ,A l - K ; : : ; to be effectire ture and tha: t required. Foam meabilitrstr.:. Promisint the Midu ar-Su the injected rtci that the pro!-c: Friedman flooding of Bcr tant. Thel frrun oil saturationrt formed at high terial at lo* r el s a m el o \ . \ ' e l $ Mohamm test involr ine ir in California. t resulted in thc pounds of AOS R e s u l t sl r ' field in Califtrrr The test inrtrlr s t e a m .P o s i t i re causeof the un was reported c* The addit lated approach

GravityOverrrd

1a

lgh them.

Injection

CTPATTENN 0 Pfoducers Per Pattern 'rvePatterns

enseowater runs more 'h lessviscousthan the ate rivulets,or fingers, and water running toeing muchhigherthan ainingvery substantial rgh the oil may alsobe I thosecreatedby the 'e the minimum in situ re to supplyheatto the the availablepressure i unstable water/oildisil layer, with an early wn in the next chapter water,breakthroughof r and viscositycharac(r.e.,evenif the water

the flow depicted in t and the liquids in the he oil; the situation is at the production well. ual oil saturation than lingsley (1975)report a aamflooding

Chap.4

Production

Figure 4.6 Gravity Override of Steam

6Vooil saturationin the steam-swept zoneversus23Vofor the water-sweptzonefor a steamfloodin the Kern River field in California.The upper Steam-swept zonein this project representedabout one-third of the sweptvolume and the waterfloodetl zone representedtwo-thirds.Once steamhas broken through, there is little pressuregradientto removethe oil, particularlysinceit is necessary to reducethe injection rate in order to control steambypassing(i.e. steam"coning"). A significanteffort is being made currently to developsteamadditivesthat will restrict the flow of steamwithin the steamzone,therebyincreasingthe pressure drop and causingmore rapid encroachmentof the steam into the waterflooded zpne.A popular approachis the addition of surfactantmaterialsto the steam,which causethe formationof foamwithin the steamzone(e.g.,Dilgren et al. 1982,Al-Kahaafji et al. !982, and Eson and O'Nesky1982).For foamingmaterials to be effective,it is necessarythat they be chemicallystableat the steamtemperature and that their cost be low enoughfor them to be economicin the quantities required)'Foamadditivescan alsoreducethe bypassingof steamthroughhigh permeabilitystratain heterogeneous reservoirs. " Promisingresultswere obtainedby Ploegand Duerksen(1985)in field testsin the Midway-Sunset field in Californiain which sulphonatesolutionswere addedto the injectedsteam.Theseauthorsconcludedthat incrementaloil wasproducedand that the processwas economical. Friedmann and Jensen(1986)have reported an experimentalstudy of the flooding of Bereacoreswith foamspreparedusingChevronChaserSD1000surfactant. They found that the surfactantreducedthe relativepermeabilityto gas.High oil saturationsreducedthe degreeof foam formationand propagation.Foams,preformed at high velocitiesin sandpacks, could be propagatedthrough reservoirmaterial at low velocities.However,it wasnot possibleto generatefoamsin situ at the samelow velocities. Mohammadi,van Slyke,and Ganong(1989)reportedthat in a steamflooding test involving four five-spot patterns in the Potter sand in the Midway-Sunsetfield in California, the addition of NaCl, alpha olefin sodium sulphonate,and nitrogen resultedin the incrementalproductionof 207 kB of oil in 2 years.Four million pounds of AOS were injected. Resultsfrom a surfactant/steam-injection field test in the Guadalupeheavyoil field in Californiahavebeenreportedby Mohammadiand McCallumin California. The test involved the addition of alkyl toluene sulphonateand nitrogen to the steam.Positiveresultswere obtained,althoughthe test was stoppedabruptlybecauseof the unavailability of steam.An incrementalproduction of 29,400B of oil was reportedas the resultof the injectionof 257,0001bof activeAIS. The addition of thin film spreadingagents(TFSA) to the steamis another related approachin which there is interest.Thesematerialsare madeby treatingpheGravityOverride

125

nol with formaldehydeand then reactingthe resultingpolyolswith ethyleneoxide or propyleneoxide. Productsof this type are frequentlyusedasdemulsifiersto treat heavycrudes. In this applicationthey are thought to work by being adsorbedat the water-oilinmaterialsthat stabilizethe water terfaceand displacingthe bulky asphaltene-type in the oil emulsion.With the thinner demulsifiermoleculesat the interface,water dropletsare thought to approacheachother more closelyand then to coalesce.It is thoughtthat the effect of the TFSA in steamrecoveryis to promotethe waterwetting of the rock-i.e., to detachoil from oil-wet portionsof the surface. Blair, Scribner,and Stout (1982)describetestsin California in which indications of significantly improved performancewere obtained for such a chemical in cyclicsteamstimulationoperations.Further results(Stout,Blair, and Scribner1983) have shown that the effects of the TFSA appear to persist into subsequentcycles eventhough additionis stopped. STEAMFLOODINGMECHANISMS Reductionof Oil Viscosity The main physicaleffectof steamthat promotesthe recoveryof heavyoil is the reversiblereductionin viscositythat resultsfrom increasingthe temperature.This reduction in viscosityis very dramatic;with oil sand bitumen, it is almost of the nature of the meltingof a solid to form a fluid liquid. relationshipsfor a varietyof biFigure 1.9showstypicalviscosity-temperature tumensand heavyoils and alsofor lighter oils. Figure 4.7 showsthe effect of temperatureon the ratio of the viscosityof variousoils to that of water.The reduction in the viscosityof the oil makesit easierto push the oil at appreciablerateswith the pressuregradientsavailable.There are also other effects that promote the mobility of the oil. The first of theseeffectsis due to the improvementin the ratio of the viscosity of the oil to that of the water.This makeswaterpercolationableto dragoil at a 1,000,000

fasterrate tct thc waterfloodednq Even after mobility ratio. ll tendencvfor gee the steamcm& exceptwhenit is manner.the co fingers. This mech allowsfurther ct movedis largeri volume of rescrr (measuredas rz The calcu! from a reservtir I The quantitl of r ume of resentir the table repnesc steamtemp€ratu losses.In relatir doublingthe gce Changesin R*

Another phenoo floodsis that th thereis not a coo temperatureche tive permeabilit reduced.Anotha to be lower und

in cp at 100o C Parameter is oilviscosity

o

TABLE 4.7 Ouatl

1O0,000

G

TE '6 o o o

10,000

1,000

Stcr 100

10

0

100

300

200

in DegreesCelsius TemPerature Figure 4.7 The Effect of Temperatureon the Ratio of Oil Viscosity to Water Viscosity

126

Steamflooding

(t)or

Chap.4

in B/B.

Steamfloodirg lle

rls with ethyleneoxide ) to treat heavycrudes. led at the water-oilinhat stabilizethe water at the interface,water I then to coalesce. It is )romotethe waterwetthe surface. ornia in which indicafor sucha chemicalin air. and Scribner1983) into subsequent cycles

i of heavyoil is the rel temperature.This rern, it is almost of the hipsfor a varietyof biows the effect of temf water.The reduction appreciablerateswith that promotethe mothe ratio of the viscoson able to drag oil at a

fasterrate to the productionwell, which resultsin more effectivedepletionin the waterfloodedregionfor a givenvolumeof water (condensate). Even after heating,water still fingersthrough the oil becauseof the adverse mobility ratio. However,aswill be discussed in the next chapter,there is muchless tendencyfor steamto do so. It seemslikely that in most steamfloodcircumstances, the steamcondensationfront advancesin a stablemanner (i.e., without fingers) exceptwhenit is movingupward.While the condensation front advances in a stable manner, the condensatedrains through the oil to the productionwell, often in fingers. This mechanismremovesthe relativelylargevolumesof condensate and thus allows further condensationof the steam.Often the condensatethat must be removedis largerin volumethan the volumeof the oil produced.In order to heat a volume of reservoir to steam temperature,more than one pore volume of steam (measuredas water) is required. The calculatedquantity of steamrequired to raise a high-qualityreservoir from a reservoirtemperatureof 10'C to the steamtemperatureis given in Table4.7. The quantityof steamis expressed as the volumeof steamrequiredto heatthat volume of reservoirwhich containsa unit volumeof oil. The calculatedquantitiesin the table representthe heat required solelyto raise the reservoirand its contentsto steamtemperature.It is necessary, in addition,to provide steamto supplythe heat losses.In relativelyefficient situations,this will have the effect of approximately doublingthe steamrequirementsshown. Changesin Relative Permeability Another phenomenonthat plays a role in increasingthe effectivenessof steamfloods is that the relativepermeabilityeurveschangewith temper4tqre.Although thereis not a consensus on this, experimenters havegenerallyfound that raisingthe temperaturechangesrelativepermeabilitycurves.The main effect is that the relative permeabilityfor oil flow tendsto be increased,and the residualoil saturationis reduced.Another factoris that the relativepermeabilityfor liquid waterflow seems to be lower under steamfloodingconditionsthan it is with ordinary oils having TABLE a.7 Otlantityof Steam Requiredto Raisea High-QualityReservoirto Steam Temperature Basis: Porosity Oil Saturation Reservoir Temperature SteamQuality SteamTemperature

us iscosityto Water

32% 80% 10'c 70%

('c)

Ratio of Steam to oil (m3/m3f1)

100 150 200 250

0.52 0.86 t.2'1 1.81

(t)orin B7B. amflooding

Chap.4

Steamflooding Mechanisms

127

viscositiesat room temperaturesimilar to that of the heavy oil at steamflood conditions. A possiblepartial explanationfor theseeffectsis that waterhasa tendencyto form water-oilemulsions,within the reservoir,with bituminousoils under steaming conditions.This can explain the lower residualoil, since the residualoil droplets are "diluted" with micron-sizedropletsof water.In a way, a steamfloodcan be visualizedas beingpartially miscible.Another reasonfor a lower residualoil saturation which is applicablewhen there is steamsaturationis the steamdistillation effect;this is consideredlater.Emulsificationalsohasthe effectof reducingthe apparentwater-relativepermeabilitybecausesomeof the water is tied up with the slow movingoil phase. If in situ emulsificationdoesplay a role in the displacementof heavy oils, then it seemslikely that the conditionsof the experiment-such as thoseinvolved in the preconditioningof the core or sandpack-as well as the measuredsaturations,will play an important role. For example,changesthat affect the wettability of the core, the prefloodingconditions,and whether steamhas contactedthe oil may be expectedto have important influences.Experiments(Chungand Butler 1988,Jamaluddinand Butler 1988)have shown that water in oil emulsificationis promotedby the direct condensation of steamon colderbitumenand alsoby an oilwetted reservoirmatrix. There is less emulsionproduction,if any, when oil and phases.The effectof emulsificationupon the relawaterflow togetherascondensed tive permeabilityof the oil and wateris thus intertwinedwith the conditionsin the steam-saturated regionsof the reservoir,particularlyat the condensation interface. Although oil and water flowing together probably do not emulsify, water in oil emulsionformed at the condensationinterfacecan be pushedaheadof the steam chamberand then flow in the absenceof steam. Resultsfrom somepublishedstudiesof the effect of temperatureon relative permeabilityare given in Figures4.8 and 4.9.

100 -80

; = lt (E60

o

E o o40 o 620 E 020

u!

.{ paper I relativepcrE vent and tbeo I them. It appc very'deperdc follor*ed furrl Th€ sfr complicatedr effect an irry MYHILL AND STB(-

1.O Cetus oil, 22 o API Midway Sunset Unconsolidated sand

tr

.9 0.8 a) (u

*'o

r.

l!

I o.o

i\

E b o.c o. (, 2

74 o F \ r r

lt o o

..\ 2osoF

6 0.2 IE Knv

06

128

---.'

20 40 60 80 Water Saturation, 7o Pore volume

Figure 4.8 The Effect of Temperature on Relative Permeability(Data of Montgomeryreportedin Wu 1977)

Steamflooding

Chap.4

The paper by field. It usestl to providera, The be{ sizeof th€ $a (1959)apprd the possitility Thesemetbod The otf,r heatinjectedit to the saturali The rul ity of usinga t isee

aln d

Myhill and Stag

rvv oil at steamflood ater hasa tendencyto usoils under steaming e residual oil droplets steamfloodcan be vi'er residualoil saturathe steamdistillation :ct of reducingthe ap:r is tied up with the cementof heavy oils, ;uch as those involved the measuredsatura, affect the wettability has contactedthe oil ts (Chung and Butler n oil emulsification is ren and alsoby an oilif any, when oil and fication upon the relar the conditionsin the ondensation interface. emulsify,water in oil d aheadof the steam

tH

s.o

A--A

g

77 oF 340 oF

kro

t

(E60 o

l\

E o e+o

\

o .E

r,

?620

.|

G

20

40

60

.^ kr* 80

l(X)

Water Saturation,"/oPV

Figure 4.9 RelativePermeability Curves for BereaSandstoneCore (from Lo and Mungan 1973)

A paper by Bennion, Moore, and Thomas(1983)indicatesthat vastly different relative permeabilitycurves are obtainedif heavyoil coresare extractedwith a solvent and then restoredthan if they are preservedwith the originalreservoirfluid in them. It appearsthat the relativepermeabilitiesof corescontainingheavyoils are very dependentupon the state of wetting of the porous solid. This lead should be followedfurther.3 The effect of steam treatment and temperatureon relative permeabilitiesis complicatedand not understood.Overall, however,it appearsthat steamingdoes effect an improvement.

mperatureon relative MYHILL AND STEGEMEIER'S APPROACHTO STEAMFLOODING

'he Effect of Temperature 'ermeability(Data of reportedin Wu 1977)

rnflooding

Chap.4

The paper by Myhill and Stegemeier(1978)should be read by all workers in this field. It usesthe heat conductionand heat convectionideasof the previouschapter to provide an estimateof the efficiency of a steamflood. The basic idea used by Myhill and Stegemeierinvolvesthe calculationof the size of the steamzone from a simple energybalance using the Marx-Langenheim (1959)approachmodified by the ideasof Mandl and Volek (1969)in order to include the possibility of all the steambeing condensedbefore it reachesthe heat front. These methodswere discussedin the previouschapter. The objectiveis to calculatethe volume of the steamzonefrom the amount of heatinjectedinto the reservoir,the heat neededto raisea unit volume of steamzone to the saturationtemperature,and the heat lost to the overburdenand underburden. The method is simpleto use, is rapid, and gives a useful idea of the practicality of usinga steamflood in a particular situation. sSeealso the discussionof the work by M. Kwan (1988)in Chapter 1, page 18.

Myhill and Stegemeier's Approach to Steamflooding

129

Summary of Myhill and Stegemeier'sAssumptions

This p

The basicassumptions for the calculationare as follows: 1. The reservoircontainsa uniform amountof oil per unit bulk volume as defined by the productof porosity,net to grossthickness,and oil saturationin the net pay. Grossthicknessand areaper injectorare alsoconstantthroughout the reservoir2. Thermal properties,includinginitial formationtemperature,heat capacityof reservoirrock, and heat capacityand conductivityof cap and baserock are assumedconstantthroughoutthe zone. 3. Steamis injectedat a constantpressure,quality, and rate per injector. 4. Verticaltemperaturegradientsin the reservoirare zero. 5. Heat lossesfrom the steamzoneare by conductiononly and occur normal to the reservoirinto the cap and baserock. Heat is transferredin the reservoir by convectiononly, and heatpassesthroughthe condensation front only after Mandl and Volek'scritical time. 6. The quantity of residualoil remainingin the steamedchambercan be representedby an average,assumedresidualoil saturation.

u'here

f, f,.

Oncethe th culatedfs t capacitl'of r

or

q'here I I tpc

Outline of Method The heart of the methodis Figure4.10.It allowsthe thermalefficiencyof the heating to be obtainedfrom a knowledgeof the variablesin the dimensionless time numberand the steam-condition parameter,which is calledfi".

Myhill and S displacedfro saturationri

G

U Li

Z^6 N

u'hercq (

=

,s

U

'o L

0.t

L

t

o o z !! o

The rate of < rewritten fc

0.4

L

r

lrJ J

0.2 = G U

-

F

The valueof Limitatkrc D T M E N S T O N L ETSI M S E ,t D Figure 4.10 Fractionof Heat Injectedin Steamfloodthat Remainsin SteamZone (from Prats 1982)

130

Steamflooding

Chap.4

This approa aqueouscood

MyhillandSE

This parameteris the ratio of injected latent heat to injected total heat: H^ f'i In' = ,1, - 1a,= 110 bulk volume as dernd oil saturationin ;o constantthroughure,heatcapacityof p and baserock are , per injector.

wherefi I H, H*,

is the injected steamquality measuredat the bottom of the injection well is the latent heat of evaporationof water is the enthalpy of the injected steam is the enthalpy of liquid water at reservoir temperature

Oncethe thermalefficiencyis known, the volumeof the steamchambercan be calculated for the injection of a given amount of steam and a knowledgeof the heat capacityof a unit volumeof the chamber. Heat in steamchamber= HotEn,= VcbC)c(Ts- Tn)

rnd occur normal to ned in the reservoir rtion front only after amber can be repre-

lficiencyof the heatdimensionless time

Vc=

HotEn, (pC)c(Ts - Tn)

(4.3)

where Vc is the volume of the steamchamber Ho is the averageheat injection rate (pC), is the volumetric heat capacityof the steamchamberafter the oil has been displaced Myhill and Stegemeierrelate the volume of the steamchamberto the volume of oil displacedfrom the steamzone.To do this, they assumea value for the residualoil saturationwithin the steamzone: Q"=

Vc0(5" - 5",) Ho6(5. - So,)E6,t (pC)c(Ts- Tn)

(4.4)

where 4, is the cumulativevolume of oil displaced - 6 is the porosity otS, is the initial oil saturation {S,, is the residualoil saturation The rate of oil displacementat time / is obtainedfrom equation3.42, which may be rewritten for times before /" as

n = ffie'o

erfc(\/tp)

(4.s)

The value of the function of tp mal be obtained from Table 3.1. Limitations This approachneglectsthe oil removed ahead of the steam zone by the flowing aqueouscondensate.This amount is often quite small, but it can become signifimflooding

Chap.4

Approachto Steamflooding MyhillandStegemeier's

131

cant, particularlywheresignificantheat is carriedpast the condensation front, for injectiontimes greaterthan the Mandl-Volekcritical time. Unlessan allowanceis madefor it in choosingthe value of so.,the approach alsoneglectsthe smalloil bank (seeFigure4.3 and chapter 5) that buildsup behind the condensate front. The oil saturationin the steamzonetendsto be reducedfurther by the actionof the flowing steambehindthe front. The effect is due both to the sweepingaction of the steamin moving the oil and also to steamdistillation. The latter mechanismremovesthe lighter fractionof the oil selectively,leavingbehind a reducedsaturationof oil which is heavierthan the original crude. Figure 4.11(FarouqAli 1982)showsexperimentalvaluesfor the residualoil saturationtakenfrom a numberof experimentsand literaturedata.The meanvalue appearsto lie in the range70to ISVo.a There is a trend for lower residualoil saturation to be obtainedwith lower initial oil viscositiesand with highersreamtemperatures (pressures). The data are scattered,probablybecauseof the variationof other factorssuchas the propertiesof the reservoirmatrix. Myhill and Stegemeierassumethat the volume of the oil displacedis also equalto that produced.This is a weak part of their method,particularlyif an attempt is madeto predictthe oil productionduringthe earlypart of the flood. Also, 9.tlmay be displacedelsewherethan to the production*"it, particularlyin unconfined or only partially confinedpilots.It may alsobe left behlnd in the chamberas bypassedoil. The strict applicationof the Myhilr-Stegemeier approachwould predict the highestrate of production(for a constantsteaminjectionrate) at the start of the Figure 4.11 SteamfloodResidualOit as a Function of Temperatureand Oil Viscosity (from FarouqAli 1982).Some of the data (the solid circles)in this figure are from literature references and some(the open circles)from work reportedfor the first time in Farouq Ali (1982).The numbersin brackets are the steamtemperaturesin degrees Fahrenheit.The numberswithout bracketsare literature referencesas follows:

25 >20 o. * j15

o

Ero so

31 Blevinset al. (1969) 32 Bursellc. c. (1979) 33 Bursell,G. G. and Pitmann,G. M.

o E5

(Le7s) 100

ro1 fi2 103 104 Oil Viscosityai Tp in cp

105

34 Ozen,A. S. and FarouqAli, S. M. (1969) 35 Valleroy,V.V. et al. (1967)

"The tendencyof the steam to override introducesa difficulty in applying the Myhill and Stegemeiertheory, particularly in thick reservoirs.At the point of steambreakthrough,the average steamzonethicknessis lessthan the height of the reservoir.After breakthrough,there is a tendency for heatedoil to be bypassedbecauseof insufficient pressuregradientto move it to the production well. In this circumstance,the averageresidualoil saturationwithin the heatedregionis higher than that found in one-dimensionalsteamdisplacement.

132

Steamflooding

Chap.4

flood sirrc predicred suppll rhc Artq not predic speciflin r pracrice.rl impossibl the ecorn Th€ rates are ! overburdc high rares end of gea simpleapg useful.ft b rate of inF

Comparbo

Figure{-l? numberof r Each of ttr ment is g€D Figun fields.trr'or ditionalpm sent the tot obtaineds-i

Ff

F" 2l-

F" 0

Myhilland Sre

:nsationfront, for 5* the approach tbuildsup behind o be reducedfurect is due both to *eam distillation. tively, leavingbeil crude. lr the residualoil r. The meanvalue esidualoil saturaer steamtemperavariationof other displacedis also :ticularlyif an atrf the flood. Also, icularly in unconin the chamberas rould predict the I the start of the nflood ResidualOil emp€ratureand Oil rrouqAli 1982).Some ,lid circles)in this eraturereferences n circles)from work rst time in Farouq mbersin brackets peraturesin degrees umberswithout ture referencesas r%9) 1979) and Pitmann,G. M. d FarouqAli, S. M.

flood sincethe predictedthermal efficiency is highestthen. The rate would then be predicted to fall with time due to the increasingproportion of the heat neededto supplythe lossesabove and below the growing chamber. Another weaknessin the Myhill-Stegemeierapproachis that the theory does not predict what the experimental conditions will be. For example, one has to specifyin the calculationboth the steampressureand the injection rate,whereas,in practice,the injection pressureis dependentupon the rate. [n many cases,it may be impossibleto inject steamat the desiredrate without fracturing the reservoir.Often the economicswill dependupon the rate at which the processcan be conducted. The Myhill-Stegemeiermethod leads to the conclusion that high injection rates are most efficient becausethey allow production with less heat loss to the overburdenand underburden.However there are practical limitations to the use of high rates.Nevertheless,the method doesrationalize the resultsfound toward the end of steamfloodswhen most of the displacedoil has been recovered;for such a simple approach,the agreementbetween the predictions and the results make it useful. [t is also useful for prediction if someexperimentaldata are availablefor the rate of injection that may be achieved. Comparisonsof Theoretical Predictionswith Data Figure 4.12showsthe oil-to-steamratios predicted by Myhill and Stegemeierfor a number of scaledlaboratory steamfloodscomparedwith the experimentalvalues. Each of these points representsconditions well on into the flood, and the agreement is generallygood. Figure 4.13showsa similar comparisonfor field steamdrives. For many of the fields, two experimentalpoints are shown. The lower circlescorrespondto the additional production ascribedto the useof steam,whereasthe upper trianglesrepresent the total production;i.e., they include the productionthat would have been obtainedwithout steam. tt

afterMyhlllandStegemeler 1978

tr o

E o o. x ul 6 tt o

Mt.Poso (lowpressure)D,;

; 0.s o tr Midway-Sunset

E

o

tr SchoonEbeck Mt. Poso (highpressure)

o

et al. (1967)

c

rlying the Myhill and Ithrough, the average 3.h.there is a tendency rc it to the production d regionis higher than

looding

Chap. 4

.E (!

.Tatums Coalinga

.z t (t IIJ

0

00.5

1 CalculatedequivalentOSR

Myhill and Stegemeier'sApproach to Steamflooding

Figure 4.12 Comparisonof ExperimentalModel Resultswith CalculatedValues

133

Overril Another*av ( thicknessTh loss,eventhd is later hearcd becauseit hr availableto r Anolhcr age residualo spondingto tl one can cqri lower valued

1978 alterMyhlllandSt€gemeler o AdditionalOiusteamRatio(OSR) A TotalOSR

tr o o g

70% ol

(!

.u

6

f

cr

3G o.s tr

,9

A

d{

E9br

68,8

{EF/y #;EA"ti

It ll

I .9

€.-:Ad u-^Y

5

:^'cY'z'E-E 6

tt

Dl .60

l/ o (! c

-

J

6

F

{EP

Ten-Pattrn t

1 0.5 Calculated Addltlonal Equlvalent OSR

Figure 4.13 Comparisonof Field Steam-DriveResultswith Calculations

In general,the experimentalfield projectdatain Figure4.13tend to fall below the solid theoreticalline and lie mostlyin the rangeof 70 to I00% of the theoreti70Voof the theoreticalprediction.Myhill and Stegecal. The brokenline represents meierpoint out that there are severalreasonswhy field data might be expectedto be below the theoretical,includingthe fact that much of the field data comefrom patternsthat are unconfined.In suchpatterns,someof the mobilizedoil may be driven outsideof the pattern.Another reasonis that steamoverridemay resultin the averagethicknessof the steamzonebeinglessthan the reservoirthickness. As was shownin Chapter3, the followingequationpredictsalmostthe same OSR as doesthe more complicatedMarx-Langenheimexpression. OSR =

osR-", l---f-----

(3.4e)

r'7696 LS" ( T s- Z ^ ) ( 1+ L $ \ / W )

(3.s0)

1' -, 8 -\tll " t |.' J 'n'

ReservoirChra

ry

od1 :ir ' tt

T, 5, o t (Oglesbr et al ll

The resen'cirr 3.02x lff Bd FigureJ.l{. The cil il

?s and Za in "F, / in d, and h in ft. lt predictshigheroil-steamratiosfor the followingconditions: o o r o

As an exampl of the Kern I Billingslel-tl9'l in a patternc( spot averaged producenand given in the fr

Higher valuesof ASo-i.e., higher,.S,or lower S,, Higher porosity More rapid recovery,lower / Thicker reservoirs,high h

The lower oil-steamratiosfound in practiceas comparedto thosethat may be expectedfrom equation3.49 result from the mechanismbeing different from that postulated.

The recoven'r oil saturatiqr r m a i n i n gu i t h i n steam-s\r'epta

Chap.4

Myhill and Steg

134

Steamflooding

Override of the steamresultsin undisplacedoil remainingin the reservoir. Another way of looking at this is to saythat ft (in practice)is lessthan the reservoir thickness.The heatthat haspenetratedbelowthe steamchamberis equivalentto a loss,even thoughit resultsin heatingthe oil below.Even if much of this lower oil is later heatedto the steamtemperature,it tendsto staywithin the steamchamber becauseit has beenbypassedby the advancingfront and little pressuregradientis availableto move it. Another way of looking at the problem of bypassedoil is to saythat the average residualoil saturationin the steam-heated region is greaterthan that correspondingto the value for a one-dimensional steamflood.From this point of view, one can considerthe reservoirheightto be the appropriatevalue for ft, but a much lower value of AS" is requiredto allow for the bypassedoil. Ten-Pattern Steamflood As an exampleof this idea,we will considerthe Chevron"Ten-PatternSteamflood" of the Kern River Field in california, which has been discussedby Blevin and Billingsley(1975)and by Oglesbyet al. (1982).The projectconsistedof a steamflood in a pattern consistingof ten contiguousinvertedseven-spots. The areaper sevenspot averaged6.1 acresto give an averagespacingof 320 ft between injectorsand producersand alsobetweenadjacentproducers.Characteristics of the reservoirare given in the following table.

mparisonof Field sultswith Calculations

t3 tend to fall below Wc of the theoretir. Myhill and Stegeight be expectedto eld data come from obilized oil may be rrride may resultin ;€rvoirthickness. cts almostthe same on.

(3.4e)

I ReservoirCharacteristics: Ten-PatternFlood, Kern River

I

Depth Oil gravity Net sandthickness Tn r.s

s, q

(3.s0)

700-797 ft 14'API 97 ft 90"F Approx. 310'F 0.52 (after primary production) 0.34 4000mD

(Oglesbyet al. 1982)

The reservoirwassteamedfor 7 y; 18.58x 106B of steamwere injectedto produce 3.02 x 10"B of oil (i.e., OSR = 0.16;SOR : 6.15).Performancedata are shownin Figure4.14. The oil in placein the reservoirinitially is given by 6S,Ah=0.34x0.52x61 x 43560x97 = 45.6x 106ft3 or 8.1 x 106B rosethat may be exdifferent from that

The recoverywasthus 37Voof the oil at the end of the steamflood,and the average oil saturationremainingwas 0.52 x 0.63 = 0.328.This oil was madeup of oil remainingwithin the steam-swept zoneand of bypassedoil, suchas that beneaththe steam-sweDt zone.

nflooding


Chap,4

135

9.n

FE

6

o

F =

It is clea passedin this project,and m: that has beeno of the producti tion has beenr 4 by the endd 1000B/d (abqr will have beco

10

6 2

20,000 10,000

'$ S u,ooo =q

2'ooo

10,000 o 5,000 * cE _ 2,000

.EE t,o* I I .L

Ten-PatternStrr

5oo

100

| 6 S | 6 6 | 6 Z t 6 8 | 6 9 | Z OI t 1 t Z 2 t Z g t Z q t Z ' t Z 6 t Z t | 7 8 t 7 9 t g g I

Prrmer_ Stearofh \Aarcrfb

Years Figure 4.14 Performanceof Ten-PatternSteamflood(from Oglesbyet al. 1982)

Calculatingthe expectedOSR using equation3.50 and a residualoil saturation of 0.328leadsto OSR =

1769x 0.34x (0.52- 0.328)

San Ardo Str

(310- eo)(1 + r.43fi x 365m = 0.30

This value is much higher than the value of 0.16found in the field and, of course,very much higher than would be found if a lower residualoil fraction had been substitutedin the equation.Part of the reasonfor the high predictionis that someof the injectedheatbypasseddirectlyto the productionwells.[t wasestimated by_Blevinsand Billingsleythat this would reachl8Vo of the injectedheat.sIf allowance is made for this bypassedheat, then the expected OSR would be 0.82 x 0.30 = 0.246.This is still significantlyhigher than the value of 0.16 observedin the field. It is possiblethat the injectedsteammay have had a lower quality than was assumedin derivingequation3.50;steamquality data are not availablein the published information.Another similar factor is that no allowanceis made for heat lossesin the well bore in the precedingcalculation.However,it is unlikely that thesefactorswill accountfor the whole discrepancy. Another possibilityis that the spreadingof the heatedzone acrossthe patterns may have been much more rapid and that the heat lossesare underestimated. Equation3.52 is similar to equation3.50but is basedon the assumptionthat the steamzone spreadsimmediatelyacrossthe flooded area.Using equation3.52 insteadof 3.50for the precedingexampleleadsto a calculatedoil-steamratio of 0.225 or 0.184if allowanceis madefor the bypassedheat.This is muchcloserto the observedrates. tlt will be notedfrom Figure 4.14that the steam-injectionrate wasloweredfrom about 10,000 to 6000B/d during the period 1970to 1975in order to conservesteamafter breakthrough.

136

Steamflooding

Chap.4

Another largr. achievedis thc 1983).Most of t characteristict can be injected Properties of Arri

( T r a v e r s ee t a l l 9

The field has b pattern areaof with this sprin ductionrate.a0 pattern,as sbor Theseinfi zone,as shown MyhillandSteg

It is clear from the precedingcalculations that considerablehot oil was bypassedin this steamflood. This has been recognizedin the Chevron Kern River project, and muchof the remainingheatedoil hasbeenrecoveredby the waterflood that has been operatedsince 1975.During this waterflood, cyclic steamstimulation of the productionwells has beenused.As will be seenfrom Figure 4.14,this operation has beenvery successful,and the cumulative SOR has fallen from 6 to almost 4 by the end of 1980.During this period, the oil production rate remainedat about 1000B/d (about 50 B/d per production well). It is estimatedthat 78Voof.the OOIP will have been recoveredbv the end of the flood. Ten-PatternSteamflood-Oil Recovery 7o Recovery OOIP Primary Production Steamflood Waterflood

iby et al. 1982)

10 J+

34 (20 by end of 1980) 78

t residualoil satura-

San Ardo Steamflood and lnfill Drilling

in the field and, of lual oil fraction had gh prediction is that alls.It wasestimated injectedheat.sIf al:ed OSR would be re value of 0.16 ob-

Another large, successfulCalifornia steamflood in which a high recovery is being achievedis the Texacoproject in the SanArdo field (Traverse,Deibert, and Sustek 1983).Most of the steamfloodrecoveryhas been from the Aurignac zone.This has characteristicssimilar to the Kern River field. Although it is much deeper,steam can be injectedwith a bottom hole pressureof only 125 psig at 1300B/d per well. Propertiesof AurignacZone-San Ardo Area h Depth

ver quality than was availablein the pubae is made for heat r, it is unlikely that zoneacrossthe patare underestimated. assumptionthat the ng equation3.52 in-steamratio of 0.225 uchcloserto the ob-

a K

Tn Oil gravity

1755acres 97ft 2300ft 0.349 1000-3000 mD 100'F 13'API

(Traverseet al. 1982)

,rered from about 10,000 r breakthrough.

The field has been developedusing repeated,inverted nine-spot patterns with a pattern area of 20 acres.It has been concludedthat a 50Vorccovery is achievable with this spacing.In order to achievea higher recovery and to maintain the production rate, an infill drilling has been initiated. Four infill wells are addedto each pattern,as shownin Figure 4.15. Theseinfill wells have the objectiveof removing the oil from below the steam zone, as shown in the cross-sectionaldrawing at the right of Figure 4.15.

mflooding


Chap. 4

137

S A N A R D OF I E L D INTERVAL WITHINFILLS 9.SPOTSTEAM

C R O S S- F L O (

CURRENTFLOODPATTERN

o

d

ao

o

o

o

o

INJECTION

O-lNFlLL

o o + X - SECTION

o

t'igrrr

With furrl the use of foas patesthat a rcc INFILLS

COMPARISONOF SN

An interesting p switchingfrom Figure.l.l' 2.S-acrespacir

* BOP

;'P;'1

|

I

I

I

PRoDUCTION .IPRIMARY I WITHOUT STEAMFLOOD 70 7t 72 73 74 75 76 77 78 79 80 8t 82 Figure 4.15 Addition of Infill Wellsto SanArdo 9-SpotPattern(from Traverse et al. 1983)

In a nine-spotpattern there are three producersper injectionwell. [n the infilled pattern shown in Figure 4.15,there are sevenproducersper injectionwell. Texacoplans to reduce this ratio and to promote recoveryof additional oil by the conversionof the cornerwells of the original nine-spotpatternfrom producers to injectors.This idea is shownin Figure 4.16;it hasbeencalledcross-floodingby Texaco.Also shownin this figure is the conceptof how this conversionwill recover additionaloil from the bank which has accumulatedaround theseproducers.The conversionof the cornerwellswill resultin two injectorsper original 20-acre,ninespotpatternand six producers,or a ratio of three producersper injector.This convertsthe patternto a repeated10-acreinvertednine-spotpattern.Texacoestimates that the recoveryfrom their projectwill increasefrom 50Vofor the original pattern to 60% for the patternwith infill drilling. An important economicconsiderationis that the productionrate is maintained. 138

Steamflooding

Chap.4

;0.8 o o o

s-o.s E o o

6 o.q o

-g = 0.2 E

=

o

0

1

Comparison of St

SAN ARDO FIELD g-SPOTSTEAM INTERVAL IYITHINFILLSANDCROSS.FLOODING

C R O S S- F L O O D I N G

o

d ORIGINAL INJECTOR

INFILL WELL

F CORNER VT,ELL

CONVERTED TO INJECTOR

x-sEcTroN

O

.

.

ADDITIONALAREA TO BE SWEPTBY STEAM THROUGH CROSS-FLOODING Figure 4.16 Cross-Floodingat SanArdo Pattern(from Traverseet al. 1983) O

O

With further operationalchangessuch as waterflooding after steamflooding, the use of foam additives, and the selectiverecompletionof wells, Texaco anticipatesthat a recoveryof 79Vowill be achievableat San Ardo. COMPARISONOF STEAMFLOODAND STEAM SOAK An interestingpart of Myhill and Stegemeier's paper is concernedwith the effect of switching from steamstimulation to a steamflood. Figure 4.17showsthe injection rates,from a scaledmodel,for a steamsoakon 2.S-acrespacingthat was converted to a steam drive after 4.5 years comparedto 82

afier Myhllland Sl€gem€lar1978

| (from Traverse

rtion well. In the in:rs per injection well. of additional oil by ftern from producers lbd cross-flooding by nversion will recover these producers.The lriginal 20-acre,ninercr injector. This conErn.Texacoestimates n the original pattern omic considerationis

rnflooding

Chap.4

t o.s

.9 (t

o

s-o.e E IE

ffi

o

6 o.+ o .= -g 0.2 ? E I

o

o

0

05101520

"*"o"*t

Timein Years

Comparisonof Steamfloodand Steam Soak

Figure 4.17 Cumulative Steam Injection-Midway-Sunset Model Experiments

139

0.5 afierMyhilland Stegemeior1978

o.

5 0.4 o

G q,

g 0.3 o.

.z

5

o

o

g 2 0.4 E f

0.2

o

'F

s=

E 0.1 =

o

0.6

;

0,2

s,*ry4 51015 Time in Years

20

Figure 4.18 Cumulative Oil Productionin Midway-SunsetModel Experiments

Tin!r

JONES'STEAM Df,fY thosefor steamsoakingand to thosefor a soakprojectwith closerspacing.It was possibleto inject more steamwith the flood than with the steamsoak evenwith infilling. Figure4.18showsa comparisonof the cumulativeoil productionfor the same experiments;the parallelwith the injectioncurvesis very striking. Higher injection ratesgive higherproductionrates.The convergence of the cumulativeoil-to-steam ratio curves shown in Figure 4.19is also very interesting.One can seefrom this studywhy therehasbeena generaltrend to switchfrom steamstimulationto flooding in Californiaas a field matures.One may presumethat the sametendencywill developin the Canadianbitumenfieldsasthe projectsmature,asvirgin high-quality tar sandreservoirsbecomemore scarce,and as practicalexperienceis obtainedin the recoveryof tar sandoil by flooding. It is probablydesirableto extendthe cyclic steamstimulationphasein Alberta becauseof the generallyhigher initial oil viscosity.Also, the use of vertical steamfloodingwith horizontalwells (steam-assisted gravity drainage;seeChapter7) will probablyprove to be a superioralternativeto conventionalhorizontalsteamfloodingin many projects.

Therehasbec

clude factorsr Jonestl9 Figure 4.10 fro field with tha modelis alsosl The \trt earlyin the fb rateswhich are periodsin a Ee from one stag Stage 1 During this fin and, in somee 10 0 0

STEAMFLOODING MULTILAYERRESERVOIRS In somecases,multiple reservoirsseparated by impermeable barriersmaybe steamflooded sequentially.In thesecases,someof the heat lost during the flooding of one layer may be presentin the layer above(or below)when it is flooded.A study by Restine(1983)for two such operationsin Getty's Kern River Field shows,as might be expected,considerableimprovementsin the oil-steamratio and higher production ratesfor the preheatedoil sand.This effect affords greatereconomyfor the production of oil from stackedreservoirsthan from single ones. One wonders,for example,whether the extensivesteamingof the Clearwater sandsin the Cold Lake field will result in more economicproductionfrom the higher Grand Rapidsformation,perhapsusing the samewells that were drilled to exploit the Clearwaterformation. 140

Steamflooding

Chap.4

o

(L

E o

,' -n-n

E. c

.9 O I -c,

1n lv

(L

Jones' Steam Dri

1.0

afterMyhilland Stegem6ier1978

tr o

o.u ? .=

SteamSoak

G

o.+ E E f

o

SteamDrive

0.2

Figure 4.19 Cumulative Oil-Steam Ratio as a Function of Time from Start of Steam-Drivefor Midway-Sunset Experiments

05101s TlmeInYears,startlngat 4.5y

Cumulative Oil r Midway-SunsetModel

JONESSTEAMDRIVEMODEL closer spacing.It was seam soak even with oduction for the same king. Higher injection rmulativeoil-to-steam )ne can see from this t stimulation to floodhe sametendencywill , asvirgin high-quality erience is obtained in le to extend the cyclic higher initial oil visI wells(steam-assisted ;uperior alternative to

There has been progressin modifying the Myhill-Stegemeierapproachso as to include factorswhich were ignored in the original treatment. Jones (1981)describesan empirical approach that is simple and realistic. Figure 4.20 from his paper comparesthe reported recovery from the Kern River field with that predicted using the Myhill-Stegemeiertheory. A curve for Joned model is also shown. - The Myhill-Stegemeiermethod gives unrealistically high production rates early in the flood, reasonable onesin the middle,and as the flood reachesits end, rates which are severaltimes too high. Jonesconsidersthat there are three major periodsin a steamfloodand that the dominant factorschangeas the processmoves from one stageto the next. Stage 1 During this first stagethe dominant factor is the very high viscosity of the cold oil and, in somecases,the need to build an oil bank-i.e., to fill gas saturationwith 1000

rarriersmay be steamluring the flooding of it is flooded.A study River Field shows,as reamratio and higher ls greatereconomyfor le Ones. ring of the Clearwater r production from the tlsthat were drilled to

o (L rn

;

Myhilt-Stegemeier :-j ----- -- ---- --f ___.

100

o

E.

o

3 -o o

10

/

L

(L

1'

1968

1969

't970

1971

Yeor

amflooding Chap.4

Jones' Steam Drive Model

1972

1973

Figure 4.20 Comparison for Kern River, California, Steamflood Field Data with TheoreticalPredictions (after Jones198L)

141

oil. During this period, water channelsthrough the oil, and there is little production until warm oil can approachthe productionwell. It is during this period that steamstimulationof the producer(s)can be particularlyvaluableand with heavybitumens,almostessential.(Seelater discussionof tar sandflooding.) Stage 2 where r{

In the secondstagehot oil is movedto the productionwell relativelyeasily,and the productionrate is about equal to the rate of growth of the steamchamber.The Myhill-Stegemeier assumptionsare reasonablyvalid. The peak productionoccurs early in this stage. Stage 3 The Myhill-Stegemeiertheory would allow the secondstageto continue indefinitely, with the production rate dropping asymptoticallyto zero as the area for vertical heatlossesgraduallyincreased.In practicethe drainageareais finite, and the productionrate becomeslimited becauseof the depletionof the reservoir.No allowanceis madefor depletionin the Myhill-Stegemeier theory. Jones' EmpiricalAdjustment Factors

F'

Equatir doesnot l'ani viscosities. O flood. A visc sands;this is A concc volumetricba steamchamh must imagirr bypassed-ut Voo is tl tion 4.9.

Jonesallows for the effectsjust describedby multiplying the production rates predicted from the Myhill-Stegemeiertheory by three empirical factors: Vpo,Aco, and V6p. q-

VpoAcortrm

- S::),r,o erfc({G) - Z^)

(4.6)

Vpoallowsfor the effect of the initial gassaturation.It is given by equation4.7.

Jt

\43,560Ah"65s1

where-4 h^ 6 Ss 4,ini

orelse Wo-l

is effective pattern area in acres is net zone thicknessin feet is porosity is initial gassaturation is injectedsteamin barrels

(4.7)

Zp6,is equal to the squareof the injected steamvolume, measuredas liquid water,dividedby the initial volumeof free gasin the reservoir.When this ratio becomesunity, then Zrp is forced to be 1. ,4co allows for the effect of the initial oil viscosity.It is calculatedfrom equation4.8. It will be noted that the higher the value of poi, the lower is -,462.As the steamzone increasesin area,the value of A6p increases up to the forcedlimit of 1. For an initial oil viscosityof L06cp, the squareroot term is just equalto unity. 142

t

s

,* = ('^"-:tl,ut,=\' 0 R", it is an accurateapproximation. sAnalytical equationsfor the flows betweenwells have been discussedby a number of authors.The generalschemeof superimposingthe pressuredistributionof a numberof wellswasdeveloped by Muskat (1937)and subsequent work is basedto a large extent on this pioneeringeffort.

146

Steamflooding

Chap.4

Most of the bores,and tf, this meanstl the invasion creaseconsi< tive radiusol L/R" willchr 6.91to-1.61 of the prodr this too r.ill r

Time for Err

Considerthc sure gradied equation-l.l{

The average usingDarcyi Injectivity

)r the caseswherethe o solutions(4.11)and

(4.r4) E becomesP,. At the

(4.1s)

ity to refer to a dimensionalvalue in which q is measuredin barrels per day, pcin centipoise,ft in millidarcies,ft in feet, and AP in poundsper squareinch (these units are often termedfield units). Morel-Seytouxnormalizedinjectivity is thus given by (q inB/d)(p in cp)

(k in mD) (h in ft) (AP in psi)

100 200 500 1000 2000 5000

(4.16) ion wells is given by

e usedin (4.17). (in consistentunits). _

(4.18)

I ric arrangementsare escribesthe normalt normalizedinjectivre sourceand sink. It is, rcethe constantpressure ever, for practical situa-

(4'19)

Equation4.I9 may be usedto calculatethe injectivity in barrelsper day from the normalizedvaluesgiven here. It can be seenfrom equation4.18that the injectivitydecreases asl, increases; however,the effect is not very great becauseof the nature of the logarithmic function. Calculatedvaluesare shownin the followingtable.

L R"

(4.r7)

: l'127x 1o-3 (#)."",,,,"", "

NormalizedInjectivity in Consistent Units 0.682 0.593 0.506 0.455 0.413 0.369

Most of the resistanceto flow occursin the immediatevicinities of the two well bores,and the resistanceaddedby increasingl, is not very great.In steamflooding, this meansthat oncethe resistance aroundthe injectionwell decreases as a resultof the invasionof the low-viscositysteam,then the injectivitymay be expectedto increaseconsiderably.For example,supposethat steamflow has increasedthe effective radiusof the injectionwell from 0.1 to L0m. For a casewhereL is L00m, then L/R,willchange from 1000to 100/V10 x 0l = 100and ln(LlR*) will changefrom 6.91to 4.61;Ihe injectivitywill increaseby a factor of 6.9114.6I= 1.5.Stimulation of the productionwell can also result in an effectivelargerwell bore radius,and this too will increasethe injectivity. Time for Breakthrough, Considerthe straightstreamlinethat joins the two wellsjust discussed.The pressure gradient along this streamlinecan be obtained by setting Rp : L - R; in equation4.I4 and differentiatingP with respectto R;.

=- #("-' ., - *--L (**).=, )

(4.20)

ised by a number of aurmberof wellswasdevelhis pioneeringeffort.

The averagefluid velocity alongthe central streamlineis given by q/A$AS. and, usingDarcy'sequation,is

nflooding

Injectivity

Chap. 4

147

fr

4!=v==---g--dt

/aP\ -

q

._L

pr6LS,\aR,/"=o 2trhgAS, S(f -

A6LS"

S)-

g'21)

In this equation,S is the distancethat a particle of the fluid movesfrom the injection well in time /. The time for breakthroughcan be obtainedby integratingequation 4.21.with the result ThL26 LS,

LBT _

(4.22)

3q

Substitutingfor the value of q from equation4.18leadsto the expression .

6 AS"rtL2ln\lR*)

@'23)

"r:tftF-

The volumeof oil that is displacedduring the breakthroughperiod is givenby rearranging equation4.22: Qtar =

ThL26 AS"

(4.24)

3

This volumeof oil is independentof the rate of injection.It is equalto one-thirdof the volume of mobile oil containedwithin a cylinder of reservoirof heiehth and radiusL.

The dim circle of profi breakthroueh

It is a functiq Confined hl

lsolated Injection Well Surroundedby a Circle of Equally Spaced Producers The generalprocedurejust describedcan be extendedto the caseof an injectorsurroundedby a circle containingequallyspacedproducers,as shownin the left part of Figve 4.22. Resultsof this analysisare tabulatednext (Morel-Seytoux1966): qLL

-

kh LP

.

.RT

2nN (N + 1) ''(*)

(4.2s) - h(N)

: o

/r\ I | * ') r"(;il - rn(N)l 'LS.pL'?L(N

g E o

(4.26)

_

2(N + 2)kAP

N

(!

Qtnr= {TtnhL26LS"

E o

z

(4.27)

In these equations,N representsthe number of wells surroundingthe injector. Thus, for example,for an isolatedfive-spot there is one injector with four producers arrangedaroundit and N is equalto 4. The radiusof the surroundingcircle,.L, in this caseis half of the diagonallengthof the squarepattern.The injectivitiesfor a number of valuesof N are shown in Fisure 4.23. 148

In a repeated boundaries.Tl tern indefinitc infinite series example,the

Steamflooding

Chap.4

Figrr: Spacc

Injectivity

s(r - s)

(4.2r)

/

v

N equallyspacedwells of radiusRw

9r

/ --'*------

lr. io ! AI

t,'?

AO

novesfrom the injecl by integratingequa-

I

I

g)9*-t'

I i J.

_ _ . _ _ . { r ) _ _x_ _ I I I I

(4.22)

A v I

: expression

i I I

A v

(4.23)

I

"N+1 Spot"Pattern

Partof infiniterow of wells

Figure 4,22 Typical IsolatedWell Patterns

rriodis givenby rear-

(4.24) equalto one-thirdof 'voir of height h and

The dimensionless breakthroughtime for N = 1,. . . ,6 and for a continuous circle of productionwells is shownin Figure 4.24.In this figure the dimensionless breakthroughtime is definedas

Dimensionless breakthrough time = !-ô!'u' q c.sop,L2

(4.28)

It is a function of the systemgeometry-in this case,of N andLfR,. Gonfined Patterns

se of an injector surrown in the left part

In a repeatedregularpattern, planesof symmetrybetweenwells becomeno-flow boundaries.The flow in suchpatternscan be computedby extendingthe well pattern indefinitely and summing up the pressureterms for eachwell as one or more infinite series.[n somecasesthe resultinganswerscan be surprisinglysimple.For example,the pressureproducedby the seriesof equal injectionwells uniformly

rx 1966): Parameteris the numberof producersin the

(4.2s)

circle around the isolatediniector

:> 69

I

9C

..1

:;

)l I

8 8'E .N

(4.26)

(!0 Fo

5.E z

(4,27) unding the injector. 'with four producers oundingcircle,L, in 'he iniectivitiesfor a

mflooding

Chap.4

otot Radlueof clrcleAfVellbore radlus Figure 4.23 Injectivity of Isolated Well Surroundedby a Circle of Equally. SpacedProducers

Injectivity

149

Continuous

\,,,

..."2

No flow bo..rn

T-

---h I Parameter isthenumber of producing wells whichsurround theinjec,tor 100

1000

Distance to ProducerAltellbore Radius

10000 Figure 4.24 BreakthrouehTime for Isolatedpatterns

spa:edalonga straightline, which is shown in the right hand part of Figure4.22 and also in Figure 4.25,is given by (Muskat 1937)

P=Po-ffiLnfcorr,+-*'Tl

(4.2e)

Extendingth verticalu'elb

Repeated Fh

In equations4.29 and 4.30 the flow, q, is per unit length of well. Equation 4.29 canbe usedto predictthe flows betweena horizontalinjictor and a horizontal producer,asshownin the smalldiagramin Figure4.26.To do this it is necessary to write equationsfor the contributionof four seriesof regularlyspacedwellsand t-hen to combinethesewith the result shownin equation4.30. Gonfined Horizontal Well Pair qp_ kLP

(4.30)

The dimensionless injectivity from equation4.30hasbeenplotted againstthe logarithm of C/L in Figure4.26for a constantratio of R*fL:0.002.hhe injectivlty risesfrom a low value of 0.3 to the asymptotecorrespondingto a pair of isolatei wells (i.e., the injectivity given by equation4.18). c is equalto R, for the casewherethe two wells are immediatelybelow and above the reservoirboundaries.In this case,if it is also assumedtiat L ) R,, equation4.30 can be reducedto qp 0.5r -k^P: GF,r* 150

(4.3r)

Figrn Horra

Steamflooding

Chap.4

Injectivity

| )tmoge wetts No flow boundories .l'-/

9\

I )lmoge wells

{

rcatthrough Time for li

I part of Figure 4.22

(4.2e)

i

Figure 4.25 Infinite Vertical Column of Horizontal Wells to RepresentWell within HorizontalBoundaries

Extending this approachto repeatedinverted five-spot and seven-spotpatterns of vertical wells resultsin the injectivities given by the following two equations. Repeated Five-spot

3h of well. Equation stor and a horizontal this it is necessaryto pacedwells and then

qp kh AP

- 0'6174 r"(^fr)

(4.32)

L = distancefrom injector to producer

=)]l

(4.30) Asymplote- no effectol boundades Equation4.18

)ll

E

E o.o E

ao

ted againstthe logat.ffi2. The injectivity to a pair of isolated

.g E 6 -o O.2 tr o

.E o

mediatelybelow and nrmed that L > R.,

0

-3-2-101234 tog.,o(C/L)

(4.31)

Figure 4.26 Effect of Proximity to Reservoir Boundaries on Injectivity for Horizontal Wells

rnflooding

Chap. 4

Inje.ctivity

151

RepeatedSeven-spot

o E

qP khAP

tr3

4r

ct

1 ^1. lt\ - os6elj ,L',(fr)

tc z

Theseare plotted in Figure 4.27.eAlso shown in this figure is a curve for an injection well locatedwithin a continuousrow of producersa1the sameradius.comiar_ ing Figure 4.27 with Figure 4.23 showsthat the injector in the repeatedpattern has a significantly lower injectivity than that in an isolated pattern. A considerable fraction of the oil in the iso.latedpattern flows outsideof the pattern and then back toward the production wells. The quantity of oil that is producedat breakthroughis also considerablylarger for the isolatedpattern; much oi the producedoil has come from outside the pattern. This is shown in the following tabie. Volumeof oil Producedrtjlglth*g!

Initial mobile oil Fraction produced at breakthroueh

glyid"g

Isolated Repeated

!y Vorumeof oir Initiailywithin partern 2+ AS,L2 1.0472

2.5986 LS.L2 0.9069

0.7178

0.7437

o a a c gt

-

7

c E

-

0

(flow norm tions for ttr lar; onll-th Aba be characle tion 4.3-ial within the s

wherethe p The dimensionlessbreakthroughtimes (as defined by equation4.28) areplotted in Figure 4.28 for isolated and repeated5- and 7_spotpatterns. STEAM ZONE SHAPE: VAN LOOKERETVS EOUATTONS

and where y

W

.l/ I I k

Van Lookeren (197.7)developedequationsthat describethe degree of override that may be expectedin a steamflood. These equationsare based upon fundamental principles such as Darcy's law and make use of the assumption oi segregatedflow

v: (osR 3

.: Eg .gg EE o6 N . =

E E Fo E.E z Figwe 4,27 Injectivity in Confined Patterns

The ps ize the stati faces,u'here developmen In man it has been1 comparedto cousoil *'ith fingerinewil

equationsare given by Deppe (1961),who alsogives equationsfor the inverted ninespot pattern and for patternsat the boundary of a field development.

'oThis ir a n i n i t i a l h i g hr becauseof b1-p

152

Injectivity

esimilar

Steamflooding

Chap.4

o i:g C')

o 6a

a curve for an injecame radius.Comparrepeatedpattern has tern. A considerable lattern and then back ed at breakthroughis roducedoil has come

Within Pattern Seven-Spot 25986L5"L2 0.9069

o o o o o

El

o o

i5

0

10

100

1000

10ooo

Dlstanceto Producer/Wellbore Radius

Figure 4.28 BreakthroughTimes for Confined and IsolatedPatterns

(flow normal to bedding plane of the reservoir is neglected).He developedequations for the caseof linear flow and alsothe caseof radial flow. The two are similar; only the radial flow equationsare describedhere. A basic finding in van Lookeren'spaper is that the degreeof override may be characterized by a dimensionless number,which he termsz44;it is givenby equation 4.33 and is proportional to the squareroot of the ratio of the viscousforces within the steamzoneto the gravity forces.

o^=ffi'$-14*y

0.7437

(4.33)

where the pseudomobility

pt!,p,. (osR), tr4*_

n 4.28)are plotted in

l L ,k o P o

and where z" is kinematicviscosityof steam,m2/s IV,i is steaminjection rate, kg/s gree of override that d upon fundamental n of segregatedflow

njectivityin Confined

ros for the inverted nine-

rnflooding

Chap. 4

ap i fuksl^' g is graVity, m/s2

h k, p: (OSR)r

is thickness,m is permeability of steamzone to steam,pm2 is effectiveviscosityof oil, Pa . s is instantaneousoil-steamratio The pseudomobilityratio is analogousto the mobility ratio usedto characterize the stability of water floods. Valuesof M* lessthan unity lead to stableinterfaces,whereasthose greaterthan unity tend to lead to unstableinterfacesand the developmentof steamtongues. ln many cases,where the oil in the reservoiris not extremelyviscousor where it has been preheatedby stimulation before flooding, M* may be relatively small comparedto unity. On the other hand, whereattemptsare madeto drive cold viscousoil with steamdirectly,M* will be high and it will control the situation;steam fingeringwill then occur.'o loThisis relatedto the situationwhere,in Jones'method,the value of lco is lower becauseof an initial high oil viscosity.In both cases,oil that is heateddoesnot flow readily to the producer becauseof bypassing.

Injectivity

153

If M* is relativelysmall,then the value of -4n is controlledby the square-root term in equation4.33.Ot the variablesin this term, the only one that is in the direct control of the operator is the injection rate W,i.Higher rates give higher1ai.e., the viscousforcesincreasewhile the gravity forcesremain the same. Lower valuesof the permeabilityto steamwill alsogive highervaluesof Ap; aswill be seen,this leadsto steeperfronts and thicker steamzones.Accompanying this will be an increasein the oil recoveryand in the oil-steamratio. There is considerableresearchand developmentactivity that has the objectiveof reducingthe permeabilityof the steamzone to steamin order to increasethe pressuregradient belowthe steamzone.11 A promisingmethodinvolvesthe additionof surfactantsto causethe formationof foams. The effect of-4a on the predictedshapeof the steaminterfacecan be seen from Figure4.29.With low valuesof ,4p,the steamtendsto be confinedto the top of the reservoir,and the front is inclined at a low angle.As,4a increases, the front approaches the vertical. With a situationsuch as that shown in the top drawing in Figure 4.29, it is apparentthat steamwill break through early and that, for the amount of oil that will be produced,the heat lossesto the overburdenand to the unsweptreservoir belowthe steamzonemay be excessive. In this casethe advantagethat would have beenexpectedfor a thick reservoirfrom Myhill and Stegemeier's modelwill not be obtained.The productionrate and oil-to-steamratio will be almostindependentof Valueof A p

hst/h

^v - RadiuS +

t

-T'

Practical Range in Field

_l: -+,,

Rangeof Experiments Modelsin Laboratory

.-1,0

aftervanLookeren

Figure 4.29 Interface Profile during Injection (after vanlookeren) ttNot only doesthis increasethe recoveryof oil from below the steamzone but it also improvesthe recoverywithin the steamzone.The apparentviscosityof the steamis increasedand it is better able to displaceoil from the steamzone; this aspectis discussedfurther towardsthe end of Chapter5.

154

Steamflooding

Chap.4

reservoirthic the squarem height of re* evenif the n Doscbc considertbd thick resenti voir is renxru thicknessof t

Anothcr agramsof Fir to have to p|! helpingthe u the oil is allor assistedgrarit With rh ing force thl tween the ini At the starrtb voir. Once btt comesrelatiw steamdecreas below. For los'r not extendto I well or, if the , the formation .a particularlr light, tendsto I allowsone to a The parr the well bore n decreasing R. r move lower do In order t be able to cah in Figure{.31 cases,1 and J. fall in bet*'eeo that the avera Low valu reachor onll't Injectivity

I by.thesquare-root ne that is in the dirs give higher-46the same. righervaluesof .4p; nes.Accompanying ratio. There is con:ive of reducingthe re pressuregradient on of surfactantsto terfacecan be seen confinedto the top increases, the front in Figure4.29,it is amountof oil that 3 unsweptreservoir €e that would have 's modelwill not be nostindependentof

r'aciical Range 'Feid

:i &periments ^ ilboratory

okeren) am zone but it also ima m i s i n c r e a s e da n d i t i s ther towards the end of

rf looding

Chap.4

reservoirthickness.It is of interestto note (equation4.33)thatAp is proportionalto the squareroot of the ratio (Wtlh)lh-i.e., to the rate of steaminjectionper unit height of reservoirdivided by h.In thicker reservoirsthe overrideis thus greater evenif the rate of steaminjectionper unit heightis maintained. Doscherand Ghassemi(1981)and Doscher,omoregie,and Ghassemi(19g2), considerthat, in many practicalcases,the high oil-to-steamratios expectedfor thick reservoirsare never obtained becauseonly the oil from the top of the reservoir is removed.[n thesecases,accordingto van Lookeren'stheory, the average thicknessof the steamzonewill be hn=Q.Jl11*=Q.5

""'fir Lpgk,''

M*)

(4.34)

Another problem,which is apparentfrom the overrideshownin the upperdiagramsof Figure 4.29,is that after breakthroughthere is a tendencyfor the steam to have to push remainingoil up the slope.Gravity is playinga role, but it is not helpingthe movementof the oil. What would be moredesirableis a systemwhereby the oil is allowedto drain downwards.This is oneof the thoughtsbehindthe steamassistedgravity drainageprocessto horizontalwells discussedin chapter 7. with the situationshownin the upperdrawingsin Figure4.29,the only driving force that is moving oil to the productionwell comesfrom the differencebetween the injectionpressureof the steamand the pressureat the productionwell. At the start this is very large,but so is the resistingforce of the oil-saturatedreservoir. Once breakthroughoccurs,the resistanceto flow throughthe steamzonebecomesrelativelylow, and the driving force requiredto maintain a given flow of steamdecreases. Under theseconditionsmuchlessdrive is availableto movethe oil below. For low valuesof ,4a, Figure 4.29 showsthat the steam-liquidinterfacedoes not extendto the baseof the injectionwell. The steamescapes from the top of the welltr, if the well is perforatedonlyat the bottom, risesvery rapidly to thi top of the formation.The reasonfor this is not, as might be assumedat first, that theie is a particularly favorableopen streak at the top but simply that the steam,being light, tendsto float to the surface.Figure4.30,which is from van Lookeren'spaper, allowsone to estimatethe liquid level within the well. The parameterLNTM dependsupon the valuesof the drainageradius,R,, the well bore radius,R,, and the skin factorfor the well, s. For a givenvalueof ,4a, decreasing R, or increasingS hasthe effectof causingthe steam-water interfaceto move lower down the well. In order to estimatethe vertical conformanceof a steamflood,it is useful to be able to calculatethe average,area-weighted steamzone thickness;this is given in Figure 4.31.The curves in this figure are drawn for two extremetheoretical cases,1 and2, which are developedbyvan Lookeren.Actual casesare expectedto fall in betweenthesetwo theoreticalcurves,and it is suggested by van Lookeren that the averagecurve shouldbe used. Low valuesofr4p correspondto caseswhere the steamzone either doesnot reachor only barelyreachesthe baserock; seeFigure 4.29.when,4p is lessthan Injectivity

155

-til-

1.0 !

steam a F

Parameteris LNTNterm

.c 0.8 o o = 0.6 ,= ' E 0.4 = o

Sincc t for low valu thicknessal Altbq by Figure4given by Fg of the injecti fect the thirl

<jt>

LNTM=3 LNTM=gfor plugged --.->

:

nearwelboia

Numericel t

H

AS an ex2rilt

River steam culate,,{pan

.E 0.2 (, (!

il

llo

0

0.8 0.2 0.4 0.6 SteamZoneShapeFactorA p

Figure 4.30

SteaminF

1.0 Rate pcf

Predicted Water Level in Injection Well utl

L N r M = r n ( R , / R .-) t l 2 - R l / z R :+ S S is skinfactor;seeChapter6 (aftervanLookeren) 1.0, the mean steam zone thickness, as a fraction of the total thickness, is simply equal to half the -4n. This is also equal to the vertical conformance.

1.0

Aa

\.c r' O

Case1

z 0 . 5A n

l-c

g 0.8

The average

Averaoe Curve'

o

N

E o I 0.6 o

Case2

Using this tx an expected

5

o 6 th

o 0.4 E v .9

This is lesst that the pro< included.

F

o 0.2 C'I G L

afiervanLookeren

o

0 -

0

0.5

2.O 1.5 1.0 ShaPeParameterA R

FAROUOALI'S I..|II

2.5

FarouqAli ( many of the simplifiedag

Figure 4.31 RelativeAverageSteam-ZoneThicknessas a Functionof,4n (after van Lookeren)

156

Steamflooding

Chap.4

Farouq Ali'sU

ll tl

since the reservoir height occurs in the denominatorof ,4p, this meansthat, for low values of Ap, the mean steam zone height is independentof the reservoir thicknessand alsoof time. Although the steamheight within the well dependsupon R, and s, as shown by Figure 4.30, theseparametersdo not affect the averagesteamzone thicknessas given by Figure 4.31.The effect of R, and S is confined to the immediatevicinity of the injection well. Injecting steamonly at the bottom of the reservoirtendsto affect the thicknessof the steamzone only in the region closeto the injection well.

t-l J . .l ll -

*r>

v

Fii

tl tl tl

-ii

Numerical Example of the Use of van Lookeren'sTheory As an exampleof the use of van Lookeren'stheory, considerthe ten-patternKern River steamflooddiscussedpreviously.The valuesof the variablesrequired to calculateAp are as follows:

tl tl tl tl tl t_t

Steaminjection rate: 18.58x 106B of steamwere injected over a period of7 y, or 727 B/d per injection well average Rate per injector W,i = 727 x 350 x 0.4536/86,400= 1.3kg/s z, at 310oF= 5.0 x 10-6m2/s

I

-s

A^p= 960 kg/^, thickness,is simply ance.

I = 9.81m/s2 h = 97 x 0.3048= 29.6m k, = 0.4 x L0-12m2(assumingkn = 0.4) A^=(

Case1

5 x 10*6x 1,.3 \ r/2 = o''n' r r x 9 6 0 x 9S1,nR X 0.4 x 10-12 )

The averagesteamchamberheight would thus be

Averaqe Curve-

En = 0.5Anh = l'i..1m, or 38.5 ft

Case2

Using this height and an expectedresidualoil saturationof about 0.15would give an expectedrecoveryof 0.52- 0.15 38..s Ug "dx100Vo=28.27o This is less than the reported recovery of 37Vo.One reasonfor this difference is that the production of oil by waterflooding beneath the steamzone has not been included.

t 5

FAROUOALI'S UNIFIEDAPPROACH Farouq Ali (1982)has presenteda description of an approach that encompasses many of the conceptsdescribedpreviously in this chapter and unites thpm into a simplified approximatemodel.

on of,4p (after

nflooding

Chap. 4

FarouqAli's Unified Approach

157

His procedureinvolvesthe calculationof the steamzone thicknessft,, from van Lookeren'stheory and then, using the Mandl-Volekmethod,the calculationof the steamchambervolumefor successive time steps.At eachtime step,the flow of oil and waterfrom the regionbelowthe steamzoneis estimatedassumingthat the temperatureis uniform at a value determinedby the heat contentfor the heat loss calculated.Relativepermeabilities from Gomaa'scorrelationare used(Figure4.31). The procedureis repeateduntil the steamchambervolume grows to the breakthroughvolumecalculatedat the start.At this point the steaminjectionrate can be adjustedto control the amountof steambypass.FarouqAli providesan encouraging comparisonin his paperof the resultsof his calculationfor the Kern River data with the samecurvesdrawn by Jones.

tr,,-..

function ,.: reserroir Ft thick*a:,:rt were fou::i1! portion \': :l \ \ . a -. ,.1

C O I I C C I c ' Ci , x

then the c:-, w a s a l s o: ' : ; s u l t si f t n r . stant.There neither inr e'

GOMAAS CORRELATIONS FORPREDICTING OIL RECOVERY

'\ n

Gomaa(1980)developeda setof correlationchartsfor the predictionof steamflood oil recoveryand oil-to-steamratio as a function of reservoircharacteristics and operatingconditions.The correlationsare basedupon a seriesof numericalsimulation studies. Although Gomaa'sstudyis limited to a particularsetof fluid and rock properties and is dependenton the assumptionsinherent in the numerical simulation methodemployed,it developsinterestingconclusionsand ideas.The studyconsiders a reservoirwith the relativepermeabilitycurvesshown in Figure 4.32.These curveswere found to give a satisfactoryhistory match for an actual Kern River steamflood.Comparedto the valueswhich are commonlyfound for conventional oils, the relativepermeabilityof water is very low. It has been found necessaryto employrelativepermeabilitycurvesof this type to simulateheavyoil steamfloods in numericalsimulators.If conventionalcurvesare employedit is found that water is producedmuchtoo quickly.The distortedrelativepermeabilitiesemployedcompensatefor other problemswhich are involvedin the simulationsuch as the extremelylargetemperatureand viscositygradientswhich occur in the vicinity of the condensation front. The gravity of the oil in the studywas 14oAPI and the reservoirtemperature was 90"F. 1.0

'-.,

( 4 0 % 1u a . : o a given pr,rJ Dec:ca floodine rfi" through 'Fig this increa.< tion of thc n

a. o o |e \ F S*

g 0.8

o p ql

6

t

o

\J

3 o.o

?c-

.g 0.4 6

0q n

E e 0.2 Water note soecial scale 0.2

158

0.4 0.6 0.8 ( S s - S * 1 ) / ( 1 - S 1-aS; 6 1 )

Figure 4.32 NormalizedOil-Water RelativePermeabilities(from Gomaa 1980) Steamflooding

Chap. 4

Figun G.':.

Gomaa'sCo''e

e thickness8,, from d. the calculationof Lmestep,the flow of rd assumingthat the ent for the heat loss e used(Figure4.31). grows to the breakin|:ction ratecan be ovidesan encouragthe Kern River data

iction of steamflood aracteristics and oprumericalsimulation uid and rock properumerical simulation s. The studyconsidFigure4.32.These r actual Kern River rnd for conventional r found necessaryto :avv oil steamfloods t is found that water ities employedcomtion such as the exin the vicinityof the

Figure 4.33 showsthe oil recoveryfor reservoirsof variousthicknessesas a function of time with a constantsteam-injection rate of 1.7 B/d per acre foot of reservoir.For example,the steam-injectionrate for the caseof the reservoir 300 ft thick was30 timesgreaterthan that for the reservoir10ft thick. Higher recoveries werefound for thick reservoirs.This might be expectedbecauseof the smallerproportion of the total injectedheat that would be expectedto be lost vertically. A significantfinding from this studywas that if the heat injectionratesare correctedfor the vertical heat lossesto give the net heat injectedto the reservoir, then the diversecurvesof Figure4.33 all fall on the singlecurve of Figure 4.34.It was also found that neither the pattern shapenor the pattern size affectedthe results if the steam-injection rate per unit volume of reservoirwas maintainedconstant. There was a small effect of the rate per unit volume parameterthat was neither investigatednor includedin the correlation. interestingfeatureof thesestudiesis that an intermediatesteamquality (40Vo)wasfound to give the highestthermal efficiency.More heat was requirid for a given productionwith steamof lower or higher quality. Decreasingthe steamquality from l00Vaincreasesthe amount of hot waterflooding that occursbeneaththe steamzone and delaysthe time of steambreakthrough (Figure4.35);with the assumptions made in the simulationcalculations, this increases the recovery.Figure4.36showsthe calculatedoil recoveryas a function of the net heat injectedfor varioussteamqualities.

fttJECTION RATE : t.? B/O/Act. Fl

srEAileuaLrry:06 uoElLE olL saruRAror{ = o .la

q.

:servoirtemperature

I --J------=i

I RESERVoIR THICKNESS'Ft

60

a o le -40 \ e

so $

!zo { o

o ormalizedOil-Water abilities(from Gomaa

nflooding

Chap. 4

TtME , YEARS Figure4.33 Effectof Reservoir (from Thickness on Steamflood Oil Recoverv Gomaa1980) Gomaa'sCorrelationsfor PredictingOil Recovery

159

roo I

ft{JECTIOtR { ATE: l.? 8/Ollc?.F1. s T E A I Q U A L I T Y: 0 . 6 I O S I L E O I L s A T U R A T I O N: O . 4 2

x o

xo

60

a. o o ta .40 \ t \ s

q

INJECTil

o

t

300 too 40 20 to

o o

d o â oqcx

200

400

NEf

In using( reservoiruP ut heat loss read the effectof c Usingttr ageof the or[ the estimatedt the reservoiri initial oil satu stimulationbc Figure4. of reservoirth

RESERVOIR T H T C K N E SFSr,,

ax

b t{ 20 t

;:: .a *

x

600

HEAT INJEC\E?

800 MMBtu./Auc

1200

rooo Ft.

Figure 4.34 Oil Recoveryas a Functionof Net Heat Injectedfor VariousReservoir Thickness(from Gomaa 1980)

too OUALITY. % O 20 +r ++ ++ ++ 40 60 oooo 80

-+

Sol'5O7. Qlnj' 395 t{H8tu/AcrcFt. s .F

{ o

\ t\ +

l{J

_ooo oo-

{ deo o I { q

J(.

\

roo

T x

PROOUCER

INJECTOR

(A): 50% OIL SATURATION PROFILES (DISPLACEDOIL BANK)

++++ o ooo _H INJECTOR

\ o x40 \

+ + +

Sol. 507o Oini . 395 MMBtu/AcrrFt.

*

s

s

o sl{J 2 0

t_..' I

e

r

-J

PROOUCER

(B}: I5O"F ISOTHERMS (UNIFORMITY OF WELLBOREHEATING)

Steamflooding

s o

(fromGomaa Figure4.35 Effectof SteamQualityon Displacement Parameters 1980) 160

Qro

E

QUALITY. 7C

0 20 40 60 80 IOO

t

F€'

Chap.4

Vogel's Simdffi

,----

+ +-t.t r:I Sol. 5096 Svi.O Qlnl ' 390 MMBtU/Act Fl.

o o oo

60 go PROO{JCER

It{J€CTOR

(C): IO% VAPORSATURATIONPROFILES (STEAU ZONEGROWTHA BREAKTHROUGH) (continued)

Figure4.35

ln using Gomaa'scorrelation,one first calculatesthe net heat injectedinto the reservoirup until the end of the current time step.This is correctedfor the vertical heat loss read from Figure 4.37, and this net heat injection is adjustedto allow for thq effect of steamquality using the factor read from Figure 4.38. Using the effectiveheat injectionjust calculated,the oil recoveryas a percentageof the originalmobileoil (i.e., the oil saturationat the start of the flood minus the estimatedresidualoil saturationafter steaming)is obtainedusingFigure4.39.If the reservoir has been producedby steamstimulation prior to the flood, then the initial oil saturation should be adjustedfor the oil production during the steam stimulation before using Gomaa'scorrelation. Figure 4.40 showssometypical results from the correlation; the importance of reservoir thickness,oil saturationand the net-grosspay ratio are quite evident.

ERVOIR xltEss, Ft. 300 too 40 20 to

/ariousReser-

roo {

s

IOSIL

lrJ

O I L S A T I RATlON

,o oflf

Geo

.42

-l

PRODUCER

I

G60

b

s l{

o tQ40 \

s S

o sr{.12 0 t {

PROOUCER

tt

.t

s o

,r'

4

.\

3x :x

o

,fl r 200

400

STEAM OUALITY

t.o o.8 o.6 o.4 o.2 I 600

800

looo

1200

NET HEAT NJECTEO , llll&tu. /Acrc Ft.

,(fromGomaa

nflooding

t

{

(fromGomaa1980) Figure4.36 Effectof SteamQualityon Oil Recovery Chap.4

VogelisSimplifiedHeat Calculationsfor Steamfloods

161

\

\ao

d

.N

:i 80

\

=

b

6

N\

\

t60

\

I o

\

$ \

loo

T rtttl

$

\

--' HEAT IilJECTPil RATE un&u. /o/Acr. Fl. -).or lllrl

*,o

tb

\'l

_.2 \ \.4

s20

t40 \'

\

$

.b

lr

$ o

Bto a o

40

80

t20

t6o

200

240

ZAO

320

RESERVOIR THEKIIESS, FEET Figure 4.37 Heat Loss to Overlying and UnderlyingStrata(from Gomaa 1980)

{ o

o t

VOGETSSIMPLIFIEDHEATCALCULATIONFORSTEAMFLOODS

Figrn { N{obrlc I

Vogel(1984)haspresentedan approachto the calculationof the steamrequirements for a steamfloodthat is simple,practical,and conservative.The casethat Vogel considersis the one in which overrideof the steamchamberoccursrapidlyand the productionof oil is by gravity drainage,assistedby "steamdrag." As production proceeds,the steamchamberthickens.The generalconceptis shownin Figure 4.41.

I

s

Roo

H,O

No.

Fot

d

I

\

t-

!r)

\ oz

I N \

R \

s

I

t-

I

So'

L 0.6

$

I

t

I

o

o.2 0.4 0.6 0.8 INJECTEO STEATIOUALITY

l.o

o

Figure 4.38 Heat-Utilization Factor as a Function of Steam Quality (from Gomaa 1980)

162

o.5

Steamflooding

Figurc{ on Cum

Chap.4

Vogel's Simplif-n

roo

d

4

IIOBt ILE 5eo - tMtl.IAL ,IRAT, otL SATr i

K*

* V--hr-

I

(s.7)

t T

Itt _ ltz kt kz

\

As will be shown later, the Buckley-Leverett effects tend to improve the stability; it is shown on page 209 thal the condition for stability for a horizontal system then becomes k,of

Figurt 5 w a sE i t t

k,.f

-

This can be visualizedby sayingthat the M, is lessthan L for stabledisplacement. flooding fluid is not reallyjust waterbut a mixture of water and oil, which behaves asif it hasa muchhigherviscosity.This advantageis reallylargelyillusionary,however, because,as will be seen,the Buckley-Leverettmechanismcan leave vast quantitiesof oil behind the front when the oil is very viscous.

The effec modelsby Chu (1958).Models spacedglasspl dimensionsof t permeabilityol tween the plat Figure5instability.

Effect of Interfacial Tension

A Simple Tha

There are other stabilizing effects that tend to reduce viscous fingering. One of theseis the effect of interfacialtension;this tends to stabilizebecauseit acts to shortenthe interface. This stabilizingeffect is the largestfor smallfingersbecausethe pressuregradient acrossan interface is inverselyproportional to the radius of curvature. As a result,very smallperturbationstend to shrink; with the right conditions,largerones can grow. Fingeringcan be initiated by fluctuationscausedby nonuniformitiesin the reservoirmatrix; then those fingers that are larger than the critical dimension can grow.

Figure5.5 rep tion,whichis a is a similar ne than other*-is tendsto oppc If the increrna

M,=t*ethe pressuregraof curvature.As a rditions,Iargerones uniformitiesin the critical dimension

teavyOil

Chap.5

Figure5.5 represents an interfacewithin a poroussolid at a point wherea penetration, which is assumedto be hemisphericaland of radiusR, hasformed.Also shown is a similar negativepenetration.The pressurewithin the protuberanceis higher than otherwisebecauseof the tensionwithin the interface.This excesspressure tendsto opposethe growth of both positiveand negativeinterfacialprotuberances. If the incrementalpressuredue to interfacial tension is of the order2of 2o/R, then 2It can be shown that the excesspressurewithin a bubbleor droplet of radiusR is equal to 2olR by balancingthe pressureforce on a midplaneof the bubbleA,P(trR'?) to the tensilestressholding it togetheralongthe perimeterof the midplane o(2r.R).

Flood InterfaceStability-Muskat's Model

185

and Figure 5.4 Photographof ProgressiveFingering in Hele-ShawModel. Air is Displacing Glycerine Downwardsat a Velocity Greater Than the Critical One (from Saffman and Taylor 1958)

the incrementalpressuregradientnecessaryto make it grow is approximately2olR2 (equation5.8). dP dLt

-dP )dLt

2o R'

(5.8)

Obtainingthe pressuregradientterms from 5.1 and substitutingin 5.8 gives,with somerearrangement, the minimum radiusR of the protuberance which will be able to grow (equation5.9). The critical wavelength,\" (looking on the two adjacent hemispheresas a wave) is about 4R. The condition that the protuberancesshould grow is thus r\. =

4R>41

2o

,E-t).

11t2

-l

I

='l E-t),- )l

( P , - p 1 ) gs i n d l

2o

-l

J

II U2

(s.e)

I

V,

A more accurateand sophisticatedanalysisof this problem was publishedby Chuoke,van Meurs,and van der Poel(1959)with the resultgivenby equation5.10. This is the sameas5.9 exceptfor the constant2z', which replaces4V2. Essentially the sameequationwas publishedby Saffmanand Taylor (1958).

In addition u wavelengthol pitch of repee lent waveleng An irryt the dimensiq a critical pertr they might u flooding resd Chuokc porous soli&surface tensit propertiesau lows the rep terfacialarea This ide ger tends to I rounding rese water by imtf the absolute1 into it. If o'isl then equatic

U2

I,=2*l -n:l ,J E-fi)rv L 186

The Displacementof Heavy Oil

(s.10)

Chuokeassus tension and u Chap.5

Floodlnterfae

HemisphericolPerturbotion ExcessPressure: 20 /R Excesspressuregrodient required= 20 /R2

Figure 5.5

and Lrdel.Air is Displacing Ine rfrom Saffmanand

is approximately 2olR2

(s.8) uting in 5.8 gives,with ancewhich will be able rg on the two adjacent e protuberances should

-t 1','.', inal

(s.e) rlem was publishedby eivenby equation5.10. places4f2. Essentially 958).

(s.10)

)r- = trr{i In addition to the critical wavelengthfor finger growth, Chuoke calculatedthe wavelengthof "maximum instability,"A-. This is the perturbationwavelength(the pitch of repeatedfingers),which will grow at the fastestrate;it shouldbe the prevalent wavelength.It is shownwith equation5.10. An importantconceptin this theoryis that for fingeringof this type to occur, the dimensionsof the reservoirmustbe substantiallylargerthan the wavelengthof a critical perturbation.For example,while largefingerscan grow in field reservoirs, equipment.As a result, laboratory they might not be possiblein laboratory-scale predictions. flooding resultsmay lead to optimistic Chuokeet al. extendedthe ideajust describedto representinterfaceswithin poroussolids.To do this, they substituteda*, which they defined as the effective surfacetension,for o. The effective surfacetensiono* dependson the capillary propertiesand wetting of the matrix aswell ason the interfacialtension.Its useallows the representationof the idea that when a protuberanceforms, much new interfacialareais created,particularlyif the matrix is wettedby the displacingfluid. This ideais relatedto the conceptof imbibition.When a protrudingwaterfinger tends to form, the water within it is drawn away by imbibition into the surroundingreservoirif this is waterwet and at the irreduciblesaturation.Removalof water by imbibition tends to reduce the rate of growth of the finger by increasing the absolutepressuredifferential required to transfer the increasedflow of water into it. If o* is assumedproportionalto o and kr and kz are assumedto be equalto k, then equation5.10becomes

^.=rl

o*k

0r, - pr)(V -

,r]

\

(s.11)

Chuokeassumedthat the effectivesurfacetensionis proportionalto the interfacial tension and used equation5.11to predict the most probableperturbationwaveof HeavyOil

Chap.5

Flood lnterfaceStability-Muskat's Model

length. In this equation C is a constant (Chuoke'sconstant)for a particular reservoir material that includessomeof the precedingnumerical constantsas well as a proportionality constantfor the relationship between the effective interfacial tension and the actual surface tension. This ideawas studiedfurther by Petersand Flock at the University of Alberta (1981).Valuesof C taken from the literature are given in Table 5.1. They show the large effect that the wettability of the matrix has upon stability. The displacement of oil by water is stabilizedconsiderablyby the imbibition effect if the reservoir is water-wet. TABLE 5.1 Valuesof C, Chuoke'sConstant

Petersand Flock (1981) Chuoke

Oil-wet

Water-wet

25.4 30

190.5 200

Effect of Condensationupon Interfacial Stability

\dLl,

S,t i.e., to the upper right-handpart of the saturation curve. At the shock front, the saturation drops rapidly from S,y to S,i. For stratified flow, the whole range of effective water saturationsoccurs from S,r to 1 - So,becauseof the shapeof the fractionalflow curve. The Upper Shock Front In Figure5.19,the curve for the slopedoesnot fall completelyto zero at the righthand limit-i.e., at S, : (1 - So,): 0.9. This is becausethe assumedfractional flow curve has a small slopeat its upper end. This is a commonoccurrence.The resultis that the saturationcurvesin Figure5.22havea smallhorizontalpart where the water entersthe reservoir. The residualoil saturationextendsa finite distanceinto the reservoir.The performancepredictedby the Buckley-Leverett theoryis extremelysensitiveto ihe form of the fractionalwater flow curve at high water saturations. It is of interestto considerthe situationwherethe flood stream,which is introducedat the start of the reservoir.containsoil. While this circumstanceis not of importancein normal waterflooding,it is significantin consideringthe conditions downstreamof the condensation front in a steamflood.In this case,steamcondensateand oil are forced continuouslythrough the condensationfront. If the flood streamcontainsoil, then the fractionalwaterflow at the entrance is fixed, and this flowing-streamcondition persistsup to a shock front, which movesalong at a velocity correspondingto the velocity for the fractional water flow that is introduced.In the caseshown in Figure 5.19,this fractionalflow composition is 1.0. In Figures5.20 and 5.21,the diagramof Figure 5.19is redrawnfor flooding streamcompositionsof 0.95 and 0.5. In Figure 5.20 the flooding streamcomposition,/, : 0.95,corresponds to a water saturationof S, : 0.78.This saturationpersistsdownstreamup to a shockfront, which is moving with a velocity corresponding to the slope of f* at S.: 0.78. Beyond this front is the intermingled,BuckleyLeverett zone in which the water saturationfalls to the main shockfront saturation of 0.74. The Buckley-LeverettDisplacementTheory

203

4

,t

{

II

3 o

a constantsh the slopeof t (compare,5.{

4o

I

II

3

o

Conditions.l

ao "=

G

3 o.s G

tr .9 (,

l1gaklhlerrgh when-r; : 1. I

2o o

tity of watertl a rearrangedI

CL

-9

G L

1@

lr

00

00.5

1 WaterSaturation

Figure 5.20 Diagram for Flooding withf : 0.95

In the situationshownin Figure 5.21,the saturationof the upper shockfront falls below that of the lower front. In this circumstance,there is only one shock front, and the flowing-streamcompositionswitchesabruptly from l, : 0.5 to f" : 0 at the front. Note that in this case,the front velocity is higher than that which would correspondto the normal front velocifvdeterminedby the tangency condition. Figure 5.22 showsthe saturationdistributionfor the exampleof Figure 5.19. The uppercurvedparts of the lines aredxfdt (from equation5.24)multipliedby the appropriatetime from the start of the flood. At the point where the water saturation falls to the shockfront saturation5,6 the saturationcurve switchesto the verticalline with the water saturationfalling abruptlyto the connatewater saturation S,;. It shouldbe noted that identicalcurvescould have been obtainedby using equation5.42 over the whole saturationrangeand using the combinedfractional flow curve of Figure5.19,which is drawn by usingthe experimentalfractionalflow curve to the saturationfront conditionsand then the tansentline. This tansenthas

Recovery at I

At some tinre and the a!'em! equation 5..1{-

Subsrir gives

Substituting I results in equ tion 5.47.Thb

I -9 l!

o t!

3 o.s (!

.9

@

o

G tI.

= 0.5 WaterSaturation 204

Figure 5.21 Diagram for Flooding with, = 0.5

of HeavyOil The Displacement

Chap.5

The Buckley-La

a constantslopeover the rangeof saturationsthat exist acrossthe shockfront, and the slopeof this tangentgivesthe shockfront velocitywhen it is insertedin 5.42 (compare5.42 and 5.37).

4o

Conditions at Breakthrough

^c) "; 2o

Breakthroughoccurswhen the shockfront reachesthe limit of the reservoir-i.e., when.ry: L.up until this time only oil is displacedfrom the reservoir.The quantity of waterthat hasbeeninjectedcan be found asqrt from equation5.43,which is a rearrangedform of 5.42.

o

16

c''t , t = -

6AL (df,/ds*)f

(s.43) ,l

the uppershockfront :re is only one shock tlv from ,f, : 0.5 to 1 is higher than that ined by the tangency

No. of PV to breakthroush ._D.-=

Ii

a

+ @f./dS*)r

I

I

Recoveryat and after Breakthrough At sometime after breakthrough,the saturationprofile will be as in Figure 5.23, and the averagewatersaturationover the lengthof the reservoirL will be givenby equation5.44.This can be integratedby parts as shown.

t,=!rs.dx:i{rr;; fi, ,ds.}

ampleof Figure 5.19. i.2,1)multipliedby the rhere the water satu:urve switchesto the connatewater satura-

(s.44)

Substitutingx dS, : (q,tldA)df. from equation5.42 in 5.44 and integrating glves

s-,:s,r+ ffiO f;

en obtainedby using : combinedfractional nental fractionalflow line.This tangenthas

(5.4s)

SubstitutingL for x in equation5.42 and combiningthe resultwith equation5.45 results in equation 5.46, which, when rearranged,gives the remarkableequation 5.47. This was first publishedby Welge(1952).

@

o 6

= Diagram for Flooding Distancetrom Inlector

rf HeavyOil

Chap.5

The Buckley-LeverettDisplacementTheory

Figuri 5.22 Distribution of Water Saturation

205

*

S. : S,t -.

t!

--

l-

Figure 5.23 AverageWater Saturation after Breakthroush

r+- r; f, ft

S, _ S,I,

(s.46)

6.n)

In the more generalcasewhere the flooding fluid alreadycontainsoil, the lower to the limit of the integralin equation5.44 shouldbe the saturationcorresponding floodstreamwater fraction,f. Equation5.41then becomes

r: = Jr-lL -

JL

S"

S,r

The geometricsignificanceof this equationmay be seenfrom the construction shownin Figure 5.24. For any point on the fractionalflow curve, which lies at or abovethe point correspondingto the shock front (5"r, f"t), a tangent drawn upward intersectsthe line /, : f; at a water saturationcorrespondingto the averagewater saturationin the reservoir.This is indeeda remarkablysimpleanswer-an almostmagicalresult. method is employedas follows: The Buckley-Leverett-Welge 1. Draw the fractionalflow curve. 2. Draw the tangentfrom the foot of the curve and determine the conditions at the shockfront (S,r,/,r) and the averagewater saturationat breakthrough.S,1, from the intersectionwith the linefi, = f,(ft is usually1). 3. Calculatethe oil recoveryat breakthroughfrom Porevolumesof oil recoveredat breakthrough = S,/ - S,i

(s.48)

Effect of Visco

Figure 5.25sho the tangentsc( examplethe *z decreases. The breat in Figure5.25. As the vb that can be inir throughboth ia saturationat th slight increar through the lat tained at break

!

lr

a = a

4. Calculate the time of breakthrough from the total injection volume qi from 5.43and the injectionrate.6 5. For various arbitrary valuesof S,7.,draw the tangentsand calculatethe correspondingrecoveriesand injection volumes.

t

E lt

uAlthoughit hasbeenassumedthroughoutthis discussionof the Buckley-Leveretttheory that the injectionrate is constant,this is not necessaryfor caseswhere/, is assumedto be independentof rate, i.e., of q,/A. For a given injectionvolume, the sameconditionswill prevail even though the injection rate varies.

n

Chap.5

The Buckley-La

206


eragewater saturation ush

(s.46)

Figure 5.24 RelationbetweenOutlet Conditionsand AverageWater Saturation

(s.47)

t or abovethe point pu'ard intersectsthe 3 water saturationin lmostmagicalresult. r\\'s:

ine the conditionsat at breakthrough,S,y, '.

(

-s

(s.48)

I

Figure 5.25 showsthe seriesof fractionalflow curvesthat was drawn earlierwith the tangentscorrespondingto the conditionsat breakthroughdrawn in. In tbis examplethe water saturationat breakthroughincreasesas the viscosityof the oil decreases. The breakthroughconditionslistedin Table5.3wereobtainedfr"omthe curves in Figure5.25. As the viscosityof the oil is decreased, the fractionof a pore volumeof water that can be injectedbeforebreakthroughand the fractionalflow ofwater at breakthroughboth increase.It shouldalsobe notedthat with very viscousoils, the water salurationat the front is only slightlyhigherthan the irreduciblesaturation.Only a slight increasein water saturationis required to allow much water to percolate through the largely oil-saturatedreservoir.Only very low oil recoveriesare obtained at breakthroughwhen viscousoils are displacedwith water.

rm the construction

I

I li

tion volumeqi from Parameter is ltt ltw o

I calculatethe correk le1'-Leverettheory that med to be independent of evail even though the in-

1

00.5 WaterSaturation

Figure5.25 Effectof ViscosityRatioon Breakthrough Conditions HeavyOil

Chap.5

I

t

Effect of Viscosity Ratio

ntainsoil, the lower ;orrespondingto the

tI

The Buckley-LeverettDisplacementTheory

207

TABLE 5.3 Conditionsat Breakthroughfor VariousRatiosp,*fp,"

t",lt". 0 0.0001 0.001 0.01 0.1 I

10

f"r 0.60 0.73 0.75 0.81 0.90 0.98 1.0

Recovery(t)

Swf

0.2 0.24 0.29 0.39 0.54 0.73 0.88 0.90

0 0.06 0.13 0.28 0.41 0.58 0.69 0.7

0.2 0.26 0.33 0.48 0.61 0.78 0.89 0.90

0 0.09 0.19 0.40 0.59 0.83 0.99 1.0

shown as e$r by Hagoort(l'

Pressure Gradient Ratio(2)

FOR STAI

2.64 2.28 1.70 0.97 0.33

where

(r)Fraction of movableoil recovered. (')Pressure gradientin oil bank/pressuregradientjust behind front.

Table5.4 showsthe viscosityof Cold Lake crude and water as a function of temperature.It is instructiveto comparethe viscosityratios for this systemwith thosein Table5.3 above.Obviouslydisplacement with waterwould be much more effective at higher temperatures.Similar viscosityratio data are shown in Chapter 4, Figure 4.7. PressureGradients during Displacement

Graphsc with waterfor I dimensionles positionof the

The ratio of pressuregradients before and after the shock front are shown in Table 5.3 for the seriesof oils discussedpreviously.The pressuregradientsupstream and downstreamof the shock front were calculated by Darcy's equation (equations5.49). Upstream: Downstream:

(#) (#) -

-

-Qtf*flL*

_

Akk,,1

(s.4e)

QtlLo

AkkL

105

If the absolutepressuregradientupstreamof the front is smallerthan that downstream,then the systemwill be unstable,since any small penetrationof the front will be ableto grow.The conditionfor this to be so is givenby equation5.50,which may be developedas shown to give the condition for stability of the front (which is TABLE 5.4 Viscosity Ratio for Cold Lake Crude as a Functionof Temperature TEMPERATURE 'C "F

100 200 300 400

208

38 93 r49 204

Pr

ltw

(cp)

(cp)

15,300 181 20.9 6.3

0.70 0.30 0.19 0.14

PnlPo

0.000046 0.0017 0.009 0.022

tt"/tt"

22,000 603 110 45

tr o E io4 (t L

='to3 an o o t io2 o o -9 10l .E o c

E roo E

Dlmenslor The Displacementof Heavy Oil

Chap.5

TheBuckley-L

shownaq equation5.51).This may be rewritten as equation5.52,which was given by Hagoort (1974).

Pressure Gradient Ratio(2)

FORSTABILITY

(#)'-(#)

2.64 2.28 1.70 0.97 0.33

Fok,,f

f*rP.k|"

^,1 fwf =

(s.s1)

-----------:p*krof

.

, r -1--;-

rater as a function of ' for this system with uould be much more r are shown in Chap-

lLo Krwf

k,of , k,*f

(s.s2)

# Oil Sot

T e m p e r o t u r e=

MovingFluids:

Steom*Oil

Oil+Woter

i Oil only

Figure 5.42 Diagram of Adiabatic, One-DimensionalSteamflood Analysis of Steamflood Using the Buckley-Leverett Theory

221

ration conditions correspondto the fractional flow of oil and water that is leaving the front. If the fractionalwater-flowcurve is concaveto the left, as in Figure5.21, then theseconditionsremain constantin the reservoirup to the waterfloodfront, where the saturationchangesabruptlyto correspondto those in the initial reservoir. The waterflood front advancesmuch more rapidly than doesthe condensation front, and water breaksthrough long before the arrival of the condensationfront.

areamartl the areasI -t;; l"

The rolum

Buckley-LeverettTheory Applied to the Steam Chamber The generaldistribution of saturationsthat occur within the steamchamberare shown in Figure 5.43.The abscissain this diagramis the dimensionless distance, which is equalto df"/d9".The regionwith constantoil saturation,So.occursonly if the curve of f, is not tangentto the saturationaxis at its upper terminal point. The water saturationis constantat S,; throughoutbecausethere is no water flow within the steamfloodregion.As has been mentionedpreviously,if the injectedsteamis wet, then the water saturationwould be somewhathigherthan Su. The curve of steamsaturationversusdistancein Figure 5.43 is obtaineddirectly from relativepermeabilitydata for the flow of steamand oil in the reservoir matrix with an irreduciblesaturationof water.In Figure 5.43,the steamsaturation is plotted downward.

The quant in the losr

The correq ume bv thc

The fractkt vided b1'th

Calculationof Volume of Steam Within the Reservoir Considerthe positionwithin the steamfloodregionthat corresponds to the vertical dottedline shownin Figure 5.43;this represents somepoint behind the condensation front. The numberof pore volumesof steam(measuredasvapor and basedon the total pore volumeof the reservoirbetweenthe point of injectionand the point corresponding to the vertical dottedline) that hasbeeninjectedis equalto 1/fl (see equation5.42). Another way of looking at the horizontalscalein Figure5.43is to regardit as the distancealongthe reservoirexpressed in porevolumesof reservoirper volume of injected steam.At any particular time, some of the injected steam remains within the steamfloodregionto the left as steam,and the remainderhaspassedbeyond.The volumeof remainingsteamper volumeof injectedsteamis given by the WATER

-l

TI

E,"* o

E o

I

STEAM

222

(S

This I equation -i.5

S ol

f s , f ' s , a n dS

s

otL

H,:

!o,

I ' i f t

At anv part ance.At th tional cold r within the r denses:the lowing eqru

P*l

\.)

ot

Heat Balrr

's Dimensionless Distance

Figure 5.43 Saturationsin the Steam Chamber

of HeavyOil The Displacement

Chap.5

tl -

The h In. H. is the Analysisof !

ater that is leaving l. as in Figure 5.2L, e r.raterf lood front, in the initial reser:s the condensation 'ondensationfront.

area marked STEAM in the figure; it is convenientto expressthis as the sum of the areasmarked L and2. This sum is given by V

fi,=ur"u

I +arear=

f1_so,_swi

The volumeof water is given by the areamarkedWATER,which is equalto

v,_ V"i steamchamberare distance, :nsionless n. 5o,occursonly if terminalpoint. € there is no water :er.iously, if the inrat higherthan S,;. 5.-13is obtaineddioil in the reservoir he steamsaturation

(5.53)

fldS,+,fiS,=1-i+flS,

Jr,

S.if i

(5.54)

The quantityof oil remainingis calculatedby subtractingarea1 from the rectangle in the lowerleft corner.i.e..

#,="s'-1+i

(5.ss)

The corresponding volumeof rock matrix is givenby multiplyingthe total porevolume by the ratio (1 - il16. -w =-I

Vni

r-6", 6

(s.s6)

The fractionalrecoveryof oil is equal to the pore volume occupiedby steamdividedby the porevolumeinitially filled with oil, i.e., by

_ Recovery-ruds to the vertical :hindthe condensavapor and basedon :ction and the point is equalto 1//,1(see .{3 is to regardit as eservoirper volume :ted steam remains inderhaspassed beeam is given by the

r - [ + /;s, /;(l - s*,)

(s.s7)

Heat Balance At any particulartime, the limit of the steamchamberis determinedby a heatbalance.At the condensation front, the injectedsteamis condensingand heatingadditional cold reservoirmaterial.The balanceis determinedby equatingthe total heat within the contentsof the chamberto the heat suppliedby the steamwhen it condenses;the condensate coolsto the reservoirtemperature.This is shownby the following equation: (Steamdolumesupplied)' H, : (steamvolumein chamber). H, + (watervolumein chamber). H. * (oil volumein chamber). H, * (rock volumein chamber). H, This balance,based on a unit volume of injected steam,is expressedin equation5.58. **ftt, H , = ( 1 - i + l ; s " ) H " * s , , / J H , + ( / J S , - 1 + . [ ) H , 'o

rturations in the Steam

HeavyOil

Chap.5

(5.58)

The heat contentterms H; are all measuredabovethe rdservoirtemperature &. H, is the heatin a volumeof steamvapormeasuredaboveliquid water at 7n.In Analysis of SteamfloodUsingthe Buckley-LeverettTheory

223

t ,1

inr

il

I

any particular numerical example,all the terms on the right-hand side of this equation will be either constantsor known functionsoffl. It is thus possibleto plotlhe right-hand side against/J and thus determine the value of /j which balanies the equation. NumericalExample The following numerical example illustrates the use of the Buckley-Leverett methodto analyzea steamflood. Problem A one-dimensional, adiabaticsteamfloodis carried out using dry saturatedsteamat 3.5 MPa (243"C). The steamis injectedinto the core at a rateof 10kg m-2 h-1.The corehasthe following properties:

b. Plot a di along thr

c. Calculag sured as

Heat Balanca

The problemr gram. The fd flow of steam the computed Linear Stearnflot

Initial temperature Length Porosity Initial oil saturation Irreduciblewater saturation

25"C 50 cm 0.35 0.83 0.17

oil Saturation

both initially and for steamfloodedcore 0.06 after exhaustivesteamflood basedon oil at Tn 0.9 p,m2 1.0 p,,m2

Residualoil saturation Permeabilityto steamat S,i and .!o, Permeabilityto oil at S"i and S" : 0

The materialsinvolvedhave the followingproperties: PhysicalProperties

oil Density at 25'C kg/cu m Viscosityat25'Ccp Viscosityat Zs cp Mean heat capacitykykg "C Enthalpy at G above7n k/kg k/cu m H,, H,, H", H"

966 2,000 4 2.1, 458 442,235

Rock

Water

2,600

1,000 0.9

Steam 17.54 0.018

0.84 183 476,112

4.2 945 944,700

2,697 47,293

Assumethat the flow in the steamchamberis segregated-i.e.,that the relative permeabilitiesare linear functionsof So. a. calculate the time required for steamto breakthrough, the number of pore volumesof steam (measuredas liquid water) that have been injected at this time, and the correspondingpercent recoveryof the original oil in place. 224


Chap.5

0.060 0.060 0.095 0.130 0 . 16 5 0.200 0.235 0.270 0.305 0.340 0.375 0.410 0.445 0.480 0.515 0.518 0.550 0.585 (r)In kiloloulespcr

Figure5.. of the reservci ( i . e . , / ' ) .I t * i l l the steam for a steamchambe Alternati required to hea be convertedto of the densitie Analysisof Sts

I sideof this equapssible to plot the rhich balancesthe

Buckley-Leverett ried out using dry '. The corehasthe

b. Plot a diagramof saturation(water, steam,and oil) versusfractional distance along the core at steambreakthrough. Calculate the recovery as a function of pore volumesof steaminjected measured as liquid and plot a curve.

Heat Balance,Saturations, and Recovery The problemwas solvedby meansof a tabular calculationusing a spreadsheet program. The following table showsthe calculated relative permeabilities,fractional flow of steam, andf ' as a function of the oil saturation.The sixth column shows the computedheat content of the chamber. rl r,

LinearSteamfloodNumericalExample

;- and for rd core ;tive steamflood al Tn

Water

1,000 0.9

Steam

r7.54 0.018

A1

945 r-r.700

2,697 4'7,293

-i.e., that the relahe number of pore rn injected at this ral oil in place. eavy Oil

Chap.5

Permeabilitypm2

oil Saturation

Steam

0.060 0.060 0.095 0.130 0.165 0.200 0.235 0.270 0.305 0.340 0.375 0.410 0.445 0.480 0.515 0.518 0.550 0.585

0.900 0.900 0.860 0.820 0.780 0.740 0.700 0.650 0.610 0.570 0.530 0.490 0.450 0.410 0.370 0.360 0.330 0.290

Oil

0.000 0.000 0.050 0.090 0.140 0.180 0.230 0.270 0.320 0.360 0.410 0.450 0.500 0.550 0.590 0.590 0.640 0.680

f

f'

Steam

Steam

1.0000 1.0000 0.9998 0.9995 0.9992 0.9989 0.9985 0.9981 0.9977 0.9972 0.9966 0.9959 0.9950 0.9940 0.9928 0.9927 0.9913 0.9894

0.0000 0.0065 0.0071 0.0078 0.0087 0.0097 0.0108 0.0122 0.0139 0.0159 0.0185 0.0216 0.0257 0.0311 0.0382 0.0389 0.0483 0.0628

Chamber Heat(r)

Recovery Vo OOIP

0 7,193

92.77 92.77 92.58 92.01 91.05 89.72 88.00 85.91 83.43 80.57 77.32 73.70 69.69 65.31 60.54 60-20_ 55.39 49.86

? Rq5 I 715

9,678 10,818 12,181, 1,3,824 1,5,827 18,300 21,394 25,330 30,433 ?7 to?

46,408 47,271, 5q 171

78,385

(t)In kilo.loulesper cubic meter of injectedsteam.

Figure 5.44 showsthe heat required to heat the steamchamberas a function of the reservoir pore volumesthat have been heatedper volume of injected steam (i.e.,f ').It will be seenthat the heatrequirementsare equalto the availableheatin the steam for a reservoir pore volume of 0.0389.This is the pore volume in the steamchamberper volume of injected steam,measuredas vapor. Alternatively, we can saythat I/0.0389 : 25.7volumesof steam,asvapor, are required to heat and sweepone pore volume of reservoir.The steamvolumescan be convertedto the more conventionalwater equivalentby multiplying by the ratio of the densities,i.e., by I7.54/1000. Analysis of SteamfloodUsing the Buckley-LeverettTheory

225

, i

I

Required o

t

6oooo

b c.

40000

? x

Available

o-

o

b60

(0.0389, 47293)

t

o o40 o

6

fi zoooo ,; o o I0

*eo 0

0.02

0.o4

0,06

Figure 5.44 Heat Balancefor Steam Chamber

ReservolrPoreVolumeeper VolumeInlectedf'

Porevolumeof core : 50 x 0.35cm3/cm2 : 0.175^'l^' Displacemrrl

Steamrequiredfor breakthrough= 0.175x 25.7 x I7.54

The fluid mir water. Its cc chamber:this

: 78.9ksl^' Time requiredfor steambreakthrough: Z# = 7.89h 1o The saturationsare plotted as a function of /'in Figure 5.45,and the position of the condensate front is alsoshown.The water and oil saturationsbeyondthe front are discussedlater. The oil recoveryhasbeencalculatedin the seventhcolumn of the preceding table using equation5.57.The percentoil recoveryis plotted as a function of the steaminjectedin Figure 5.46.The pointsfor beyondthe steambreakthroughcome from the previoustable; up to breakthrough,the percentrecoveryvarieslinearly with the steaminjection.tIt shouldbe noted,however,that in general,the recovery is not volumetricallyequalto the steaminjection.

MaterialBalarrce

Steam Ch. Connatc Remaro Steam Total po

Injected sr Effluent r: Displaced

0.8

l" in effl*

z I 0.6

E P o.+

Condensationlronl

in Figure 5.46 should really be two straight lines, as in Figure 5.49. The changein slope occurs when water condensatebreaksthrough. However,with the linear relative permeabilityrelationsassumedhere,the oil displacedaheadof the condensationfront is very small and can be neslected.

The materialt for the left-ha (measured asli About 2% of r placedoil. Thc paniedby l.lO the steamchar sationfront aQ lar to that sbo curve is muchI

226

Analysisof Sre

o

0

0.01 0.02 0.03 0.04 0.0s 0.06 o.o7 o.o8 (l/PvInlected Distance measured asvapor)

Figure 5.45 Saturationsduring Steamflood

TThestraight line

The Displacement of HeavyOil

Chap.5

SteamIniected(PVas Vapor) 50 100

6 o E60 o40

SteambreaKhrough

o c)

seo Heat Balancefor Steam 0.5

1

1.5

2

Steam Iniected (PVas Liquid)

m.',/cm2

2.s

Figure 5.46 Oil Recovery as a Function of the Quantity of Steam Injected

:

I

The fluid mixture flowing from the steamchamberconsistsof viscouscold oil and water. Its compositionmay be obtained from a material balancefor the steam chamber;this is shownin the followine table.

rh i-5.and the position of ,tionsbeyondthe front 'lumn of the preceding 'd as a function of the rm breakthroughcome ecoveryvarieslinearly n general,the recovery

MaterialBalancefor Steam at Breakthrough Basis:One volumeof iniectedsteam Steam measured as vapor Steam Chamber Contents: Connate water Remaining oil Steam Total pore volume of chamber Injected steam Effluent water Displaced oil /" in effluent

S a t u r a t i o n sd u r i n g

n.\. as in Figure 5.49. The .:r.uith the linear relative : c : r s a t i o nf r o n t i s v e r y s m a l l

of HeavyOil

'{

Displacementof Oil ahead of the CondensationFront

- x 17.54

Chap.5

0.0066 0.0129 0.0194 0.0389 1 (vapor) 0.0172 0.0194 0.41

Steammeasured as liquid

0.377 0,73s 0.019 2.218 1 (asliquid) 0.981 1.108 0.47

The materialbalanceis basedon one volumeof injectedsteammeasuredasvapor for the left-hand column and liquid for the right. One volume of injectedsteam (measuredas liquid)producesa steamchamberhavinga total porevolumeof 2.218. About 2% of the injectedsteamremainsbehind to replacethe volume of the displacedoil. The steamcondensate or effluent watermeasures 0.981volumes,accompaniedby 1.108volumesof displacedoil. The fractionof waterin the effluent from the steamchamberis 0.47.The reservoirsaturationdownstreamfrom the condensationfront adjustsitself to correspondto this fractionalflow; the situationis similar to that shown in Figure 5.21,althoughin this casethe water fractional flow curve is muchsteeper.With the relativepermeabilitycurvesassumedhere and the Analysisof SteamfloodUsingthe Buckley-Leverett Theory

227

I

muchlowerviscosityof watercomparedto that of oil, only a slightincreasein water saturationis requiredto accommodatethe water flow. It is calculatedthat the water saturationwill rise from the irreduciblevalue of 0.17to about0.171-a very small change.A waterflood shock front racesaheadof the condensationfront, and it is characterizedby this very slight increasein water saturation. In this example,the waterflood effect downstreamof the condensationfront is very small. The main effect is that of the displacedoil moving through the reservoiras it leavesthe conwater condensate is ableto flow with very little densationfront; the accompanying changein water saturation.The calculatedsaturationsare shownby the solid lines as functionsof the distancealonsthe core of Fieure 5.47.

,r= o G

o

V

0L

Distancr I

EFFECTOF SHAPEOF RELATIVEPERMEABILITY CURVES In the proceedingnumericalcalculation,it wasassumedthat the relativepermeabilities were linear functionsof saturation.The calculationhas been repeatedin this sectionassumingthat the relativepermeabilitiesvary with the cube of the mobile saturation.The resultsare tabulatedin the followingtable.The calculatedsaturations are shown as dotted lines in Fisure 5.47. Assumedto Vary with the Cubeof the Mobile Flood NumericalExamplewith RelativePermeabilities Saturation Permeabilitiespm2

oil Saturation

Steam

0.060 0.095 0.130 0.165 0.200 0.235 0.2'70 0.305 0.340 0.375 0.40'r 0.410 0.445 0.480 0.515 0.550 0.585

0.90 0.78 0.68 0.58 0.49 0.42 0.35 0.29 0.23 0.19 0.15 0.15 0.11 0.08 0.06 0.04 0.03

f'

f

oil

Steam

Steam

0.00 0.00 0.00 0.00 0.01 0.01 0.02 0.03 0.05 0.0'7 0.09 0.09 0.13 0.16 0.21, 0.26 0.32

1.0000 1.0000 1.0000 1.0000 0.9999 0.9999 0.9997 0.9995 0.9991 0.9983 0.99'72 0.99'71 0.9950 0.9914 0.9852 0.9739 0.9531

0.0000 0.0000 0.0002 0.0006 0.0013 0.0025 0.0047 0.0082 0.0141 0.0240 0.0389 0.0407 0.0694 O,IZOL 0.2t21 0.3851 0.7224

Chamber Heat(r)

Recovery Vo OOIP

0 49 239 666 1,482 2,938 5,443 9,682 t6,823 28,901, 47,3tl 49,556 85,508 149,593 26'7,253 490,864 931,479

92.77 90.05 87.18 84.t7 81.02 77.72 74.29 70.71 66.99 63.t4 59.49 59.15 55.02 50.76 46.38 41.90 37.34

point and lessn ferencein the r a considerabl casethe \.\'ateri tial recoverr.is I ditions at the E The calcu Figure5.49. The follor

1. In the inir relativepe becauseol tion front2. At steam two exan{ 3. After stea lationship low oil sat

PRESSUREDROPFOR

The pressuredn particularlywitl reservoiris freg

(t)In kilojoulesper cubic meter of injectedsteam.

It is interestingto note that the quantityof oil remainingin the steamchamber is almostthe samefor the cubic relativepermeabilitycurvesas for the linear ones.However,it is distributeddifferently. More oil remainsnear the injection 228


Chap.5

Pressure Dropfor

ight increasein water ;ulatedthat the water t 0.171-a very small ntion front, and it is In this example,the ery small. The main r as it leavesthe conr flow with very little rwn by the solid lines

STEAM

.E 0.6

orL

(!

fr 0.4

""'f'v-J'o'"'' "''

Q

Linear Rel.Perm. Curves 0

0.02

0.04

0.05

0.08

Distance(1/PVinjected,measuredas vapor)

rerelativepermeabilibeenrepeatedin this re cube of the mobile 'he calculatedsatura-

rh rhe cube of the Mobile

lhamber Heat(t) 0 49 239 666 t,482 2,938 5,443 9,682 I 6,823 28,901 4 7 , 3 I1 19,556 85,508 149,593 ?67,253 490.864 93t,479

Recovery Vo OOIP

92.77 90.05 87.18 84.r7 81.02 77.72 74.29 70.71, 66.99 63.1,4 59.49 59.15 55.02 50.76 46.38 41.90 37.34

point and lessnear the front for the cubic curves.There is also a considerabledifferencein the waterflooding zone aheadof the steamfront. With the cubic curves, a considerablyhigher water saturation is required to provide oil mobility. In this casethe water is ableto sweepadditionaloil aheadof the steamfront, and the initial recoveryis better.The Buckley-Leverett diagramin Figure5.48showsthe conditions at the water front. The calculatedoil recoveryis plotted againstthe volume of steaminjected in Figure5.49. The following featuresshould be noted. 1. In the initial stages,additionaloil is recoveredin the examplewith the cubic relativepermeabilitycurvesas comparedto that with the linear ones.This is becauseof the additional oil displacedby water downstreamof the condensation front. 2. At steambreakthrough,the recoveriesare almost exactlythe samefor the two examples. 3. After steambreakthrough,the systemwith the linear relativepermeabilityrelationshipsshowsincreasinglybetter recoveries.The reasonfor this is that at low oil saturations,the oil is more mobilein this system. PRESSUREDROPFOR STEAMFLOODING The pressuredrop required to force steaminto the reservoiris of great importance, particularlywith viscousoils. The rate at which steamcan be introducedinto the reservoir is frequently controlled by the pressuredrop.

' o

0.8

tr b o.o 6

WaterFront Sw= 0 223 f*= o'+23

= G

v.e

.9

rg in the steamchamlrves as for the linear ins near the injection f HeavyOil

Chap.5

Figure 5.47 Saturationsduring Steamflood. Effect of RelativePermeability Curves

E 0., L

0

0,2

0.4 0.6 Water Saturallon

PressureDrop for Steamflooding

Figure 5.48 CalculatedConditionsat Water Front for Steamfloodwith AssumedCubic RelativePermeability Curves

229

f!

,t

H

100

q

Linearrelative

80

CubicrelativoDermeabilitv

E60

*

SteamBreakthrough

i40 o

e

&. zo 0

WaterBreakthrough

a

0123 SteamInlectedIn PoreVolumesas Water

Figure 5.49 CalculatedOil Recovery as a Function of SteamIniection

steamfloodusedin the numerical Consider,for example,the laboratory-scale exampledescribedpreviously.Resultsof the calculationpertainingto the pressure drop are given in the following table. PressureGradientsin SteamfloodNumericalExample Steam Zone At front

Inlet

Compositionof flowing stream Oil Va Water Steam

0 0 100

0.'73 0 99.27

Zone Ahead of Condensation Front

53 47 0

VolumetricFlow Rates oil m3/m'zh Water Steam

0 0 0.5701

0.0042 0 0.5660

0.0111 0.0098 0

Permeability Oil pm2 Water Steam

0 0 0.900

0.590 0 0.360

0.9996 0.0004 0

PressureGradient kPa/m psift

3.17 0.14

7.86 0.35

6160 272

This table showsthe conditionsat the inlet to the core,just beforethe condensationfront and just beyondthe condensationfront. The flowing streamconsists almost entirely of steam even immediatelybefore the condensationfront (99.27volVosteam).The very high ratio of steamto oil is the reasonfor the high of the steamin driving the oil forward.Immediatelybeyondthe front, effectiveness where the steamhas shrunk in volume becauseof its condensationto water, the ratio of oil volumeto watervolumein the flowing streamis much smaller;also,in this region,the temperaturehas fallen to the reservoirtemperatureand the oil is 230


Chap.5

much more viso zonesare shorl that in the ccld flow of the stcr and the temper cal examplesh 6 MPa/m. For ti 3 MPa, or over Steamhe but as it forces creasesenornxl Steamcan flog r dense,it mug c posedonly if th to becomeexpc out the other. Imaginea Steamis beingir by the continrn temperatureand The steam process.If the ra densationfront, voir beyond.thc pressuregoesup A problem very high value. fracture occurs. voirs when ecom steamfloodingm injectedfluids fl pressuredrop im Under thes rection of the fna duced. The pres in parting the re maybe verticalo within the reserv changeas the rer tions; it has bee originally were \ (Denbina, Bobeq It is interes the condensate n region-either w doesnot increase

Pressure Dropfor

alculated Oil Recovery rf Steam Injection

tsed in the numerical rining to the pressure

Zone Ahead of Condensation Front

53 47 0

0.0111 0.0098 0

0.9996 0.0004 0

6160 2',72

:, just beforethe con: flowing streamcone condensationfront re reasonfor the high rtelybeyondthe front, :nsationto water, the muchsmaller;also,in reratureand the oil is il HeavyOil

Chap.5

much more viscous.At the bottom of the table,the pressuregradientsin the two zonesare shown.The pressuregradientwithin the steamzoneis only about0.1%of that in the cold zone beyondthe condensationfront. There is little restrictionto flow of the steamand oil within the steamzone, but as soon as steamcondenses and the temperaturefalls, there is a very greatresistance. In the particularnumerical example shown, the pressuregradient within the cold flow zone is over 6 MPa/m. For the 500-cmcore of the numericalexample,the pressuredrop is about 3 MPa, or over 400 psi. Steamhas little problem in sweepingthe oil from the steam-saturatedzone, but as it forcesthe oil through the condensationfront, the resistanceto flow increasesenormously.The flows within the steamzone and beyond are coupled. Steamcan flow only if it can condenseat the perimeterof the steamzone.To condense,it must contact fresh cold reservoir. Fresh cold reservoir can become exposedonly if there is sufficient pressureto force the oil through it and thus allow it to becomeexposedto the hot steam.one phaseof the processcannotoccur without the other. Imagine a steamfloodingprocessin which the mechanismsare balanced. Steamis being introducedcontinuouslyand is condensedby the coolingprovided by the continuously uncoveredreservoir. The swept reservoir is heated to steam temperatureand the oil within it is forceddownstream. The steamchamberpressurereachesan equilibriumvalue that balancesthe process.If the rate of injectionof steamis increased, moreoil is suppliedto the condensationfront, more pressureis required to force this oil through the cold reservoir beyond, the required pressuredifferential increases,and the steam chamber pressuregoesup accordingly. A problemarisesif the oil is very viscousand the injectionpressurerisesto a very high value. In this situation the pressurecan reach a level at which reservoir fracture occurs.This is the situationin many, indeedmost,virgin bitumen reservoirs when economicsteam-injectionratesare achieved.Under theseconditionsthe steamfloodingmechanism,which hasbeendescribedpreviously,fails. Instead,the injectedfluids flow into the openingfracture beyond,and becausethere is little pressuredrop involved in their transport, the fracture extends. Under theseconditions,the heatingoccursas a finger extendingalongthe directionof the fracture.The amountof oil displacedby the processis drasticallyreduced. The pressureis dissipatednot in advancinga broad condensationfront but in parting the reservoirmatrix. The situationis shownin Figure5.50.The fracture may be verticalor horizontaldependingupon the initial in situ compressive stresses within the reservoir.This is discussed further in Chapter6. The in situ stresses may changeas the result of stressesintroduced by neighboringthermal recoveryoperations; it has been found at least in one project that reservoirsin which fractures originally were vertical change so that subsequentfractures become horizontal (Denbina,Boberg,and Rotter 1987). It is interesting to note that although the fracture may advancerapidly and the condensatemay be carried away alongits length, the areal extent of the heated region-either vertical or horizontal, dependingupon the fracture orientation)doesnot increasemuchfasterthan it would if a broad condensationfront were beins PressureDrop for Steamflooding

231

,l

;N

C O L DR E G I O N I M M O B I LOEI L

Figure 5.50 SteamFlow and Condensationwithin Fracture

carried forward by the samesteaminjectionrate in a reservoircontainingoil of a lower viscosity.The readershould refer to the resultsof the comparisonof the heatingof a reservoirfrom a fracturewith thoseof the Marx-Langenheimfrontal advancethat were given in Figure 3.I2. In that example,for instance,the heated area for the fracture was about double that for the frontal advancein a reservoir 100ft thick. When steam advancesinto the fracture, heat is still transferredto the oil sand,and the oil becomesfluid. The volume of oil heatedby a given amount of steamis approximatelythe same.However,the pressureavailableis largelyspentin disruptingthe reservoirmatrixorather than in moving the oil. In Figure 5.50,the oil is movedforward somewhatby the pressuregradientalongthe fracture,but this is generallyinadequateto achievemuchmovement.The hot oil remainsbesidethe reaches fracture,and the steamand waterrun throughit. Eventuallythe condensate the pressuresink, and steambreaksthrough without having displacednearly as much oil as is possiblein a nonfracturingsystem. STEAM OVERRIDE In a lateralsteamfloodin which steamis injectedat a pressurebelowthat required for fracturing,with the purposeof pushingoil horizontallytowardone or moreproduction wells, there is a tendencyfor the condensationfront to become tilted so that steamruns over the top of the colderoil below.This is shownin Figure 5.51. As the steamfront advances,the volumeof the steamchamberincreasesand oil is displaced.This oil tends to flow downward and sidewaystowards the production well, and it is accompaniedby condensate from the steam. The effectivenessof this displacementis generallysimilar to that described previously.However,becausethe steamis advancingrapidly under the colder over'The energyis expended in carrying out work againstthe stressesin the reservoirmatrix. This work, evenfor a vertical fracture,resultsin a lift of the surfaceof the ground abovethe reservoir. In cyclic steamprojects,someof this energybecomesavailable,during the productioncycle,to provide compactiondrive to move the reservoirfluids (Denbina,Boberg,and Rotter 1987).

232


Chap.5

burden.the b areathat mus ward and sidr horizontaldis for an easiert Another is that the cc increasingll's step-liketenp ual. In Chapt advancinefm vance(seeeq to estimateth Eventua the drive is m ity. Meansfr describedb1 ! ationsVogelp the productiu steambypass which is ecorx As the p well, the mecb oil is driven ar in a direction; parallelto the with the drag tivelysmallun well is hindere well with the r As is me horizontal pro( and by makir4 proach is to dr injectionand p SteamOverrb

_ Eo t L I II

I

i,,A

I

}

WATER

FlowPaths Oil + Condensate Figure 5.51 SteamOverride during Steamfloodingof Mobile Oil

rir containingoil of a re comparisonof the x-Langenheimfrontal ' instance,the heated dvancein a reservoir transferredto the oil br a given amount of ableis largelyspentin il. In Figure5.50,the r the fracture,but this oil remainsbesidethe reaches he condensate rg displacednearly as

re belowthat required ward one or more proIt to becometilted so shownin Figure5.51. rcr increases and oil is ruards the production rilar to that described underthe colderover:s in the reservoirmatrix. re ground abovethe reserng the productioncycle,to . and Rotter 1987).

rf HeavyOil

Chap.5

burden,the heat lossesare greaterthan for a linear flood becauseof the greater area that must be heated.The advancingsteamchambertends to produce a downward and sidewaysdrive over a larger area than would be achievedwith a simple horizontaldisplacement flow. The enlargedcondensation front areatendsto allow for an easierdisplacement of the oil. Another factor which becomesimportant with highly overridingsteamfronts is that the conductivetransferof heat beyondthe condensation interfacebecomes increasinglysignificantas the surfaceof the condensation front grows.The sharp step-liketemperaturegradientwhich occursinitially at the interfacebecomesgradual. In Chapter2 it wasshownthat the quantityof heatwhich is movedaheadof an advancingfront is, in the steadystate,inverselyproportional to the velocity of advance(seeequation2.45 et seq.).The methodsdescribedin Chapter2 can be used to estimatethe heatwhich is aheadof the advancingfront. Eventuallythe steamchamberreachesthe productionwell, and at this time the drive is mostlydownward,so the movementof oil is assistedsomewhatby gravity. Meansfor calculatingthe thermalefficiencyof processes suchasthis havebeen describedby Vogel(1984)and were describedin Chapters3 and 4. In thesesituationsVogelpointsout that the injectionof excesssteamresultslargelyin bypassto the productionwell. The rate of injectionshouldbe controlledso as to minimize steambypass;however,in suchsituationsthe rate of productionmay be belowthat which is economic. As the point is approachedwhere steamcan break through to the production well, the mechanismby which oil is moved gradually changesfrom one where the oil is driven awayfrom the advancinginterface through the colder oil beyond (i.e., in a direction awayfrom the interface) to one where the movementis more or less parallel to the interfacewith the driving force being provided by gravity combined with the drag of the steamflowing within the steamzone.This last effect is relasteambypassis permitted.The flow to the production tively smallunlessexcessive nature of the radial flow to it and the limited contactof the is hindered by the well well with the reservoir. As is mentionedin Chapter7, the processcan be improvedby using extended horizontal productionwells, which increasethe collection capacityfor draining oil, gravity drainageprinciple. A related apand by making use of the steam-assisted proach is to drill in-fill wells to recover someof the remaining oil left betweenthe injection and production wells; this is discussedin Chapter 4. Steam Override

233

i

H I

I

The useof hot water or low steamquality is also describedin Chapter-4as a meansfor recoveringfurther oil from mature steamfloods where steamhas broken throughto the producers.The densityof the waterallowsit to fall to the bottom of the steamchamberand thus invadethe regionoccupiedby the remainingoil. Another approachis to employlow quality steamthroughoutthe drive; this approach is discussed in Chapter4 in conjunctionwith the correlationsdevelopedby Gomaa.

In thE- | have been cm assumedthal t ing form:

EFFECTOF STEAM OUALITY In the analysisof the adiabatic,one-dimensional steamfloodshownin Figure5.42, it was assumedthat the steamwas dry and saturated.In practicalprocesses, wet steamis employed.Qualitatively,the effect of water in the steamis to increasethe watersaturationin the steamfloodregion,sinceliquid wateraswell as steamhasto flow. The heat in the liquid water that reachesthe condensation front is alsotransferred and this contributesto the advanceof the front. On a weightbasis,the total heat of wet steamis lessthan that of dry steam;however,on a volumebasis,with the volumemeasuredas that of the wet steammixture, the heat contentis larger. Water at its boiling point has a higher heat content per unit volume than does steam.For a given quantityof injectedheat,the condensation front movesforward a slightlysmallerdistancethan it doesfor dry steaminjection.The reasonfor this is that the flooded steamchambercontainsslightlymoreheatbecausethe watersaturation and oil saturationare both slightly higher.However,the differenceis relatively small,as will be seen. The conditionswithin the steamfloodedregioncan be approximatedby assuming that the flowing ratio of steamvapor to liquid water remains constant within the steamfloodregion. This assumptionis reasonableexcept in extreme casesbecausethe amountof water left behind to provide the increasedwater saturation is only a small fraction of the total steamand water flow. The fractionalsteamquality,f,, is definedby the following equation. t

QtPt

-

J'-

q*pJ

q'p'

(s.60)

Q" P"L I' J The flows of the individual componentsare given by the Darcy equations: Qo

=

-

Qn

=

dX

k*A AP 0x Pw

234

where

Thesum

Eliminat

This ma1 tion as a furrti

The rr.ate

(s.61)

Equation for any given o before;the com

k,A aP Qt=

This ma1'bere termsof the s&

K"A AP &o

--

Equating leadsto

(s.se)

This may be rearrangedas follows to give the ratio of water flow to steam flow (on a volumetricbasis):

q!=e:ft;nl =o

wheren.C-C From thc the ratio R can

lt,

6x


Chap.5

Effectof Stean t

d in Chapter 4 as a :e steam has broken all to the bottom of remaining oil. Anirive; this approach r elopedby Gomaa.

In thesethree equations,the relativepermeabilitiesand absolutepermeability have been combinedas singleterms. For the purposeof this analysis,it will be by equationsof the followassumedthat the three permeabilitiescan be expressed ing form: ko = Co(So -

k, = C,(S* -

t,,)'l

r"'rl

(s.62)

ft, = C,(S,)' oun in Figure5.42, tical processes, wet m is to increasethe *ell assteamhasto r front is alsotrans:iqhtbasis,the total , rolumebasis,with at contentis larger. volume than does ront movesforward 'he reasonfor this is ausethe watersatue differenceis relarpproximatedby as'r remainsconstant except in extreme rcreasedwater sature equation.

wheren, Co,C*, and C, are constants. From the expressions fot q, and 4" in equation5.61,a secondexpressionfor the ratio R can be obtained.

Q *= ! ' P ' = R Qt

(s.63)

KtF*

rt

Equating the right-hand sides of equations5.60 and 5.63 and rearranging leadsto

?=ff^=?(+)'

(s.64)

I

This may be rearrangedto give the followingexpressionfor the water saturationin termsof the steamsaturation: S,=S,r*FS, where

B_

(?r^)"

(s.6s)

The sum of the saturationsof the three individual phasesmustbe unity.

(s.se) later flow to steam

(s.60)

S,+S,*So=l

(s.66)

Eliminating S, from 5.65and 5.66leadsto

s,,+(L+p)s,*s,=l

(s.67)

This may be rearrangedto give the followingexpressionfor the steamsaturation as a function of the oil saturation:

Darcy equations:

-S,-S,;)

s,=( 1 ( 1+ p )

(s.68)

The water saturationis obtainedbv difference.

(s.61)

S,=L-S,-So

(5.69)

Equations5.68and 5.69allow the steamandwatersaturationsto be calculated for any given oil saturation.The Buckley-Leverettmethod may now be applied as before;the combinedwater plus steamstreamis treatedas a singlecomponent. teavy Oil

Chap,5

Effect of Steam Ouality

;! I

235

We define the fractional flow of water plus steamas " rwr

Q,*Qs qo + q, + q,

(1 +R)4, q, + (I + R)q,

'* n*J;=)

(s.70) .9 0.6

The ratio of oil flow to steam(vapor)is given by

L f

Qo

KoF,

p,C"ls"-so,\n

Q,

K,Fo

lroC,\

S,

t

a

I

0.4

(s.71)

Combiningequations5.70and 5.71leadsto the following expressionfor f^.

0.02

I

Pore Volun

Jws

The calc the distance6 per Gigai:ule r heat, the frff qualities.The , of the steamfi steam saturat 'l alsobe seen. through the cl remainingbet injected stearn interface. The cakr Figure5.53.R of the oil at ba would be abh

(s.72) - 'So'\' / S' ,*r[T/

where

n-tL:C'l

t

\

tlo C" \1 + R/

This may be differentiatedto give an expressionfor f!",. c-cl df,, df*,_---"r/s,-.\'-trI _ , , _= -K= = * /; r"f,",\= +sJ d(s, s-l ilt ffil

rs.z:l

The equationsdescribedhere can be usedto predict the saturationsand recovery from a steamfloodby meansof a tabular calculationsimilar to that describedpreviously.The positionof the condensate front is determinedby the same type of heat balance;it is necessaryto treat the wet steamas a singlecomponent and to use the volume of the combinedvapor and liquid in the calculation.The heat available in a cubic meter of the wet steam (measuredas mixture) will be higherthan that in a cubic meterof vapor. The resultsof a seriesof calculationsof this type are shown in Figures5.52 and 5.53.Thesenumericalexampleswere calculatedusingthe data from the previous numerical example.The only difference in the input data is the steamquality. Figure 5.52 showsthe oil, steam,and, by difference,water saturationin the steamchamberfor caseswherethe steamqualityis l00Vo,50Vo, and25Vo.As before, the horizontal axis representsthe distancefrom the injector measuredin pore volumesper pore volume of injectedsteam,the steambeing measuredas the volume of the mixture of vapor and water. Decreasingthe quality of the steamhas the effect of increasingthe heat supply per pore volume and the distanceto which the condensationfront advances. The Displacementof Heavy Oil

Chap.5

0.8 E

o

^^

L f

o (n

nr v.a

v

u.a Cubic Mr

Effect of Stean

1\ +Rl

(s.70) .9 0.6 L

J -^

nA

(n

Heot in Steom

(s.71)

100% 50% 25%

I expressionforfi". o

rs.z:t

: saturationsand rer similar to that de:rmined by the same ; a single component the calculation.The as mixture) will be rown in Figures5.52 data from the previis the steamquality. rter saturationin the and25Vo. As before, neasuredin pore volrred asthe volumeof steamhasthe effect ce to which the con-

l-leavyOil

Chap.5

0.o4 0.06 0.08 0.02 PoreVolumesper lnjectedPoreVolume

Figure 5.52 Effect of SteamQuality on Saturations.Linear RelativePermeability Functions

The calculatedsaturationsare also shownin Figure 5.53.Here the scalefor the distancefrom the injector has been expressedas cubic meters of pore volume per Gigajoule(GJ) of injected heat. These curves show that for a given quantity of heat, the frontal advanceis nearly the samefor each of the three injected steam qualities.The effect of lowering the steamquality is to reduce,slightly, the advance of the steamfront becauseof the higher heat capacityof the chamber.The smaller steam saturations and the larger oil and water saturationsof the chamber can also be seen.The water saturationis higher becauseof the need for water to flow through the chamber(i.e., saturationsaboveS,; are required).The oil saturations remainingbehind the front are also'higherbecauseof the lower volume of the injected steam. Less steam flow is available to drag the oil to the condensation interface. The calculated percent recoveriesof the OOIP are tabulated in the box in Figure 5.53.For a given injection of heat, the dry steamis predictedto remove60Vo of the oil at breakthrough,whereas,for the sameheat injection, 25Voquality steam would be able to remove about5l% from a slightly smaller steamedvolume.

(s.72)

!o-Jo'l ,(t + B )l

47293 62758 92142

t

, 1 O O %Q u o l i t v ,/ 50% ---./

/'

25%

:" :' 1'1':t':' ; :.:.S-:-- :'

0.8 't

,/

u.o

;'m T a a

t

l

-S

I

I

6a

0.2

,"t

t/ i

tl / O

o:n

-,i

lsoz lccq-

0.6 0.8 0.4 0.2 Cubic Metres of Pore Volume per GJ

Effect of Steam Ouality

1

Figure 5.53 Effect of Heat Input on Linear RelativePermeSaturations. abilityFunctions

237

l;

H

{

EFFECTOF VERTICAL HEAT LOSSES In the analysisjust described,it is assumedthat there are no vertical heat losses from the expandingsteamchamber.Considerthe situationshown in Figure 5.42, in which heatis lost by verticalconduction,both upwardsand downwardsfrom the steamfloodregion.Suchlosseshave the effect of reducingthe heat availableto advancethe front and also of reducingthe quality of the steamthat is flowing. The effectwill vary with time. Initially in the flood, therewill be little areafor heat to be conductedaway,and the effect on the processwill be small.As the flood progresses, the rate of heatlossincreases, and a smallerand smallerfractionof the injected heat is availableto advancethe front. Heat balancesfor this effect were discussedin Chapter 3 for severalsituations.These may also be applied to the presentsituationusing the averagesteamchamberheat capacity,as predictedbefore. It is suggested that for practicalpurposes,the magnitudeof the effect upon the recoveryof oil can be estimatedby includingthe lossesin the calculationof the heat availableto extendthe front and by using an averagequality of the steamin order to estimateits effect upon recovery. More elaborateproceduresfor carrying out suchcalculations,which invorve the approximationof the steamchamberas a number of discretevolumes,have beenproposed(Shutlerand Boberg 1972;Boberg1987).Thesemethodsare rather involved and complicated;in many cases,the accuracyof the input data would probablynot justify the complexity.The methodsdescribedhere are relativelysimple and can be readily carried out using tabular calculationswith a spreadsheettype microcomputerprogram. EFFECTOF INCREASINGSTEAM VISCOSITY If the steamhad a higherviscosityit would be more effectivein displacingthe oil from the steamchamber.As an exampleof this effect, the numerical example which was describedstartingon page224 wasrepeatedassumingthat the viscosity of the steamwas 0.05cp insteadof the true value of 0.018cp. This changehad the resultof decreasing, substantially,the oil saturationswhich remainedin the steam chamberand improving the recoveries.The oil recoveriesare plotted againstthe volume of steaminjectedin Figure 5.54.There was also a slight reductionin the quantityof steamrequiredto reachbreakthrough.This differenceresultedfrom the lower heat capacityof the steamchamber. This calculationindicatesthat additivessuch as foam producingmaterials should,becauseof their effectin increasingthe apparentviscosityof the steam,result in a lower residualoil saturationin the steam-swept regionsaswell as increase the steam-drageffect.

o E60 o o40

*eo

2. In the 1l6rth greater prodr.rc 3. Becaus steamb floods t strongtl 4. InaEa fingen I a succe sateas i fer. tbc (steamf shownb tempere vancein Until al the sen With ho more fin 5. With ve practica ture as flo*'s al to the fr the prod

GENERALCONCLUSIONSON DISPLACEMENT The following qualitativeconclusionson the nature of displacementprocesses for heavyoils can be drawn: 1. when heavyoils are displacedby water,the watertendsto breakthroughvery quickly.This would be so evenif therewere no frontal instability. 238


Chap.5

Ann,teNro.\t. I Media."SPE Bibliography

ro vertical heat losses hown in Figure5.42, downwardsfrom the : heatavailableto adl that is flowing. The little areafor heat to all. As the flood proller fractionof the in; for this effect were so be applied to the rcitv,as predictedbede of the effect upon the calculationof the rality of the steamin ations,which involve scretevolumes,have e methodsare rather :he input data would 3reare relativelysims with a spreadsheet-

: in displacingthe oil e numericalexample ringthat the viscosity This changehad the lmainedin the steam 'e plotted againstthe ight reductionin the nce resultedfrom the producingmaterials rsity of the steam,rens aswell as increase

80

6 o E60 ([t

B qo o

szo

0.5

Steam Iniected (PVa8 Liquid)

HeavyOil

Chap.5

Figure 5.54 Effect of Higher Steam Viscosity upon Calculated Oil Recoveries. Steam Injected (PZ as Vapor)

2. In the production of heavy oil by waterflooding,the water tends to run of this processis throughthe oil and dragsomeoil with it. The effectiveness greater the lower the oil viscosity. Higher temperaturesgive better oil-water productionratiosbecauseof the betterviscosityratio. 3. Becauseof the stabilization resulting from the shrinkage on condensation, steamhas a much lower tendencyto finger than doeswater. In many steamfloods the steamcondensationfront is more or lessstable,although there is a strongtendencyfor the steamto overridethe liquids. tendsto drain throughthe oil either as 4. In a steamflood,the watercondensate fingers or in diffuse flow. This is often not undesirable,since in order to have to removethe condena successful heatingprocessusingsteamit is necessary in the section on convectiveheat transit is formed. As was discussed sateas fer, the heat in the condensatecan be effectively transferred to the front (steamfront) as long as uncondensedsteamremains at the front. As has been shown by Miller, there is also a tendencyfor the front to be stabilizedby the temperaturegradient. Steamfloodingfronts can be relatively stable and advancein a regularmanner through the reservoirwith the steamoverriding. Until all the latent heat of the steamis lost supplyingvertical losses,most of the sensibleheat of the condensateis given up at the steaminterface as well. With hot waterflooding, the front is much more unstableand there is much more fingering of heat into the reservoir. 5. With very viscousoils, reservoir!_4cturingcan occur if steamis injectedat a practical rate. This resultsin the heating of the reservoiradjacentto the fracture as the steam flows into the fracture and condenses.The condensate flows alongthe fracture. Although largevolumesof oil can be heatedadjacent to the fracture, there is little driving force availableto move the heatedoil to the production well, and oil production is small.

rcementprocesses for to breakthroughvery instability.

11.522.5

BIBLIOGRAPHY Anl,rENro,M.E., and MrLLrR, C.A., "stability of Moving CombustionFronts in Porous Media," SPEJ,423-430,December1977.@ 1977SPE, Bibliography

239

f f!

,t

I

BAKEn,P. E., "Effect of Pressureand Rate on SteamZone Developmentin Steamflooding," SPEJ,274-284, October 1973. BoneRG,R. C., "Thermal Methods of Oil Recovery,"J. Wiley, New york, 1987. BucrLey, S.E., and Lnvnnrrr, M. c., "Mechanismof Fluid Displacement in Sands,"Trans. AIME, 146,t07-n6, (L942). cuuore, R. L., vaNMEuns,P., and vANDERPonr-,c., "The Instabilityof Slow,Immiscible, viscousLiquid-Liquid Displacements in PermeableMedia," pet. Trans.AIME, 216,1gg194,(t959). @ 1959SPE. Dere, L. P., "Fundamentalsof ReservoirEngineeringChapter L0," Elsevier Scientific PublishingCo., New York, 1978. DeunrNA,E.S., Bonrr.c, T.c., and Rorrrn, M.B., "Evaluation of Key ReservoirDrive Mechanismsin the Early Cyclesof SteamStimulationat Cold Lake," SPE 16737,Dallas, 1987. Haooonr, J., "DisplacementStability of Water Drives in Water-WetConnate-Water-Bearing Reservoirs,"SPEJ,63-74, February1974. LrvenErr, M.c., "Flow of oil-water Mixtures Through UnconsolidatedSands,"Trans. AIME, L32,I49-t7t, (1939). O 1939SpE. MILLln, C.A., "Stabilityof Moving Surfacesin Fluid Systemswith Heat and MassTransport, III. Stabilityof Displacement Frontsin PorousMedia," ArchE Journal,21,474-479, (May 1975). MusKer,M., "Flow of Homogeneous Fluids," McGraw Hill, New york, 1937. Nurr, C.W., "The PhysicalBasis of the Displacementof Oil from PorousMedia by other Fluids: a CapillaryBundle Model," Proc. Roy. Soc.Lond., A3BZ,I55-I78, (1982). PErnns,8.J., and FLocK, D.L., "The onset of InstabilityDuring Two-phaseImmiscible Displacementin PorousMedia," SPEJ,249-258,April 1981. SannveN,P.G., and reyLoR, G.I., "The Penetrationof a Fluid into a porousMedium or Hele-Shawcell containing a More ViscousLiquid," Proc. Roy. Soc., 4245, 3rz-329,

(1es8).

sHurr-en,N. D., and BoneRG, T. c., 'A one-Dimensional,Analytic Techniquefor predicting Oil Recoveryby Hot Water or Steamflooding,"SpEJ, 489-498,Dec. 1972. Suunro, R., and BanooN,C., rn EuropeanSymposiumon Enhancedoil Recovery(ed. J. Brown), Edinburgh: Institute of Offshore Engineering,Heriot-Watt University, 303-334, (1e78). vaNMnuns, P., and vANDERPou, C., 'A TheoreticalDescriptionof Water-DriveProcesses InvolvingViscousFingering,"Pet. Trans.,AIME, 2I3,103-112,(1958). voceL, J. H., "SimplifiedHeat calculationsfor steamfloods,"Jpr, rrzT-t136,July 19g4. wELGe,H. J., "Simplified Method For computing oil Recovery By Gas or water Drive," Trans. AIME, 195,91,-98,(1952).

240


Chap,5

Cycli

INTRODUCTION

The useof crr proven to be ar ditions,and th A signiF tratednearto I ents are highe do the mostgs tional steamfb heated as it flo mustpassthru At one eo ing oil thatis x hereis to "meh requirementsft quiredto raiser The othe preciablemotilr circumstancetl tance;this canI ment is relatedt be much lower In the fin make it mobile by reducingthe injection decrea the injectionc.u generalreservc

ent in Steamflooding," turk, 1987. ment in Sands,"Trans. t1-of Slow, Immiscible, rans.AIME, 216,I88:lsevierScientificPub'Key ReservoirDrive ;e." SPE 16737,Dallas,

CyclicSfedm Stimulqtion

lon nate-Water-Bearing

f

lidated Sands,"Trans. Heat and Mass Transi Journal,21,474-478, rk, 1937. lorous Media by other 155-178,(1982). Two-PhaseImmiscible o a PorousMedium or Soc., 4245, 312-329, :chniquefor Predicting :c. 1972. i Oil Recovery(ed. J. rt University,303-334, Water-DriveProcesses b8). ,r?'t-1136,July 1984. Gas Or Water Drive,"

HeavyOil

Chap.5

f INTRODUCTION The use of cyclic injection of steamto increasethe flow of oil from reservoirshas proven to be an effective technique.It is useful over a wide rangeof reservoirconditions, and the mechanismby which it worksvaries. A significant feature of steamstimulation is that the injected heat is concentrated near to the well bore wherethe streamlinesconvergeand the pressuregradients are highest.Steamstimulationtends,inherently,to put the heat whereit will do the most good. A major differencebetweencyclic steamstimulation and conventional steamfloodingis that in stimulation,the displacedoil becomesand remains heated as it flows to the production well whereasin conventionalflooding the oil must passthrough cooler reservoir until the flood becomesmature. At one end of the scaleis the cyclic injection of steaminto reservoirscontaining oil that is soviscousthat it may be consideredas almostsolid.The role of steam here is to "melt the solid" and thus allow it to flow through the reservoir.The steam requirementsfor this mode of operation are related to the quantity of steam required to raise the reservoirto steamtemperatureafter an allowanceof heat losses. The other extremecaseis where the oil within the reservoiralreadyhas appreciablemobility and conventionalproduction is possiblebut at a low rate. In this circumstancethe role of steam injection is to decreasethe near-well bore resistalaei thii can be looked on as a true stimulationof production.The steamrequirement is relatedto the heat requiredfor the near-well bore region; normally this will be much lower than that required for generalreservoir heating. In the first case,the role of steamis to heat oil throughout the reservoir to make it mobile. In the second,the role of steamis to increasethe production rate by reducingthe near-well bore flow resistance.In both cases,the effect of steam injection decreasesas the heated region cools, and it becomesnecessaryto repeat the injection cycle.Also, in both cases,subsequentcyclesbecomelesseffective. In generalreservoir heating it is necessaryfor successivecyclesto heat the reservoir

{

which is more and more remote from the production well. For the near-well bore cyclesdeteriorates stimulationmechanism,the effectof subsequent as the reservoir pressure(or other driving mechanism)becomesdissipated. At any point in the spectrumof applicationsof the cyclic steamstimulation process,theremustbe an effectivemeansto force the oil to the productionwell. If the oil alreadyhassubstantialmobility and can be producedby conventionalmeans without steamstimulationat appreciablerates,then the samedriving force, the reservoirpressure,can transportthe oil to the well. The flow is fasterthan in conventionalproductionbecauseof the reductionin the near-wellbore resistance; this is discussedlater. Reservoirpressureis inadequateto move the oil at a practical rate to the productionwell when the cold oil is initially immobileor nearlyso. In this case,other driving forcesare required. In some reservoirs,compactiondrive resultsfrom the consolidationof the reservoirsand,with an accompanying in averageporosityasthe pore presdecrease provide pressure transport the oil. The oil is squeezed surefalls; this can drive to from the porousrock as it compactswhen the pore pressureis lowered.This mechanismhasbeenimportant in the productionof oil from the Bolivar Coastof Lake Maracaiboin Venezuela. Another form of compactionrecognizedasbeingimportantto the production of oil in the earlycyclesof steamstimulationin the bitumenreservoirof Cold Lake is the compactionthat followsreservoirexpansionas the resultof steaminjectionat fracturing pressure.In this reservoir,injection at fracturing pressureis the only meansby which steamcan be injectedat practicalrates.Steaminjectioncausesan increasein the pore volumeof the reservoir,which is reflectedby an increasein the elevationof the ground surfaceabove.Someof the energyusedto injectsteaminto the reservoiris storedaspotentialenergyby lifting the ground.When the well pressureis lowered,fluids can be squeezed towardsthe well by the settlingof the lifted ground.The effect is not reversible,sincemovementof the sandgrainsin the vicinity of the fracturewill preventthem from shifting backto their initial position: there is hysteresis. A very important sourceof drive to moveoil to the well in steamstimulation projects,particularlythoseproducingbitumen,is gravity.This can only have a significant effect if there is a low-densityphaseto replacethe oil as it drains downwards. Steamcan fill this role. As oil is drained from the reservoir,an existing steamchambercan expandto replaceit. The cyclic steamstimulation processis also known as huff and puff, as steam soaking,and as steamstimulation;theseare all acceptabledescriptions. COLD FLOW THE STIMULATIONOF WELLS WITH APPRECIABLE

CyclicSteamStimulation

Well Bore Sti

Theremar.bc not character may occur as I forations.chea migratineresc to flow is repr

Even if the sL tance,-\P./9.< reducesthe vir in the skin ef! the factorS. T] other deposits sistance bl'tcz lies in the welt neededto hea Near-Well 8q

The steadl.sg around the s'd

;tt :3

Rei

Steaminjected into reservoirs,which are saturatedwith relatively mobile oil, flows into the formation by displacingreservoir fluids away from the well. At the same of steamoccurs. time, heatis transferredto the reservoirmatrix and condensation The condensatefrom the steam is cooled as it flows into the reservoir and more heat is transferred.Heat is also lost to the overburdenand underburden. 242

The effc by consideri throughthrec

Chap.6

t The Stimuhtbn

the near-well bore atesas the reservoir ic steamstimulation l productionwell. If conventionalmeans e driving force, the s fasterthan in conthis bore resistance; tical rateto the pror. In this case,other :onsolidationof the ;ity asthe pore presThe oil is squeezed lowered.This mechrlivar Coastof Lake nt to the production rrvoir of Cold Lake of steaminjection at pressureis the only n injection causesan by an increasein the I to inject steaminto When the well pressettlingof the lifted and grainsin the vitheir initial position:

The effect of steamstimulation can be visualizedin an approximatemanner by consideringthe flow to the well as being controlled by the steady-stateflow through three concentriccylindricalregions,as shownin Figure 6.1. Well Bore Skin There may be specialrestrictionsto flow in the immediatewell bore regionthat are not characteristicof flow through the reservoiras a whole. Resistancein the region may occur as the result of damagedue to mud invasion,inadequateor blocked perforations,chemicaldamagesuchas that causedby clay swelling,and damagedue to migratingreservoirfines blockingthe pore structureof the matrix. The resistance to flow is representedby the effect of a skin factor, g in the formula AP" _ pS q Zrrkh

(6.1)

Even if the skin factor remains unchangedduring stimulation, the well bore resistance, A,P,fq,decreaseswith steam stimulation becausethe increasedtemperature reducesthe viscosity,p.rn additionto this, steammay effect further improvement in the skin effect by cleaningthe poresin the well bore region;i.e., it may reduce the factor S. This is particularly important where the skin effect is causedby wax or other depositswhich can be removedby steam.The reductionof well bore skin resistanceby heating can have a very dramatic effect if much of the flow resistance lies in the well bore skin. This effect is largelyindependentof the quantityof heat neededto heat the bulk of the reservoir. Near-Well Bore Region The steady-stateresistanceto flow in the cylinder of radius Rr, that is heated aroundthe well bore is given by

in steamstimulation can only have a sigil as it drains downeservoir, an existing

DrainageRadius Re

ff and puff, as steam scriptions. Y

Resistances in series:

rely mobileoil, flows he well. At the same tion of steamoccurs. : reservoir and more derburden. itimulation

Chap.6

Skin

HotZone

trhs znkh

IrhLn(Rh/R$)

Cold Zone IrcLn(Re/Rh)

Figure 6.1 Steady-State Radial Flow to a Steam-Stimulated Well The Stimulation of Wells with Appreciable Cold Flow

243

LPn_ p" ln(Rn/R*) q 2rrkh

(6.2)

This resistanceto flow is reduced if the region is heatedbecauseof the effect of temperaturein changingthe viscosity,pr,. Far-Well Bore Region Beyondthe heat front of radiusRa, the resistanceto flow is given by LP, p.ln(R"lRn) = q Zrrkh

(6.3)

If the sum of the resistances to flow for the cold situationis divided by the similar sum for the hot, the resultis

M

p.S. * p.,ln(R"/R,) pnSn* p.nln(RnfR*) * p"ln(R"/R)

(6.4)

lroon],-

Alltrr methodssr The re equationd6

If the total pressuredrop, )AP, is the samefor both cases,this becomes Qn_

e,

p"S. * p,, ln(R"/R,) * FnSn p.lln(R6lR*) * t",ln(R"/R)

(6.s)

Two extremecasesof this are of interest. l. If p.ais negligiblecomparedto pr..,then q n_ 5 , * l n ( R " f R , ) ln(R,/R1) Q,

(6'6)

2. If Rh = R"-i.e., the wholereservoiris heated-and S, = S",then

q!=y: Q,

(6.7)

ltn

This assumesthat the skin factor remainsconstant.If Sr,is lessthan S., then this too will tend to improve the flow and the ratio could, in favorablecircumstances, be largerthan that given by equation6.7. BOBERGAND LANTZ'S MODEL A quantitativeanalysisof the processoutlinedin the previoussectionis described in a classicalpaperby Boberg andLantz (1966);their methodpredictsthe performancefor isolatedsteam-stimulatedwells in reservoirscontaining relatively low viscosity oil. The basicidea involvedis shownin Figure 6.2. It is assumedthat steamflow is radial and that the heatedzone is a cylinder centeredon the well.''The reservoirmay, if it is appropriate,be representedas shownby a numberof thin sandsdivided by horizontal, impermeableshalebarriers. 2M

Cyclic Steam Stimulation

Chap.6

After injecri< tically and r estimatedas dition, allorl ducedfluids. The cal ing equationt

In this equar oil producrim verticalhearI wise integrat

rln rherr p as shown in Figr memberand lr is lationof ro. lf rhr reservolras a sil Boberg and La

(6.2) ruse of the effect of olt SAND SHA!E

ven by

OIT SAND

(6.3) 5l{AtE

vided by

ott 'AND

thesimilar

-?'t-

Allowanceis madefor the heat lossfrom the well bore during injectionusing methodssuchas that describedin Chapter2. The radiusof the steamheatedzoneis calculatedusingthe Maix-Langenheim equationdescribedpreviously.l

(6.4)

-?-

ris becomes

I

(6.s)

= S., then (6.7) :ssthan S", then this rrablecircumstances,

{

Stimulation Chap.6

(6.8) -1

After injectionstops,the temperatureof the heatedzonefalls as it losesheat vertically and also horizontally to the colder surroundings.These heat lossesare estimatedas a function of time from solutionsto the conductivityequation.In addition, allowanceis madefor the heat removedfrom the heatediegionby the producedfluids. The calculationis carried out in a stepwisemannerin time usingthe following equationto estimatethe averagetemperatureof the heatedcylinder at eachstep: = Tn + (fs - TR)lrRrzg - 6) - 6l Tuue (6.e)

(6.6)

ed zone is a cylinder e. be representedas neableshalebarriers.

+ 2

In this equation,6 is a term that accountsfor the energyremovedby the water and oil production, and la and v7 are dimensionless factorsthat allow for the radial and verticalheatlossesfrom the heatedcylindricalvolume.6 is obtainedfrom the stepwise integrationof the heat balanceequation, ^

I {

I

A-

H'dt [' | J,,mih(pC),(fs - f*)

(6.10)

'In their paper, Boberg and Lanz considera numberof equal-sized,separatesandmembers, as shown in Figure 6.2,with thick shalemembersbetween.In this case,11,is the heat injection per memberand y'lis the thicknessof eachindividual member,both in equation6.4 and alsoin the calculation of rn. If the shalemembersarethin, then their effect shouldbi allowedfor by consideringthe reservoiras a singleentity.

Bobergand Lantz'sModel

IF st

p

if

{

4K2p2C2t 4qzt =h'(p,C,)'- h2

.

rH

:

hH"f (tD) 4IGr(Ts - TR)

f(td = e'o erfc({t)

s sectionis described I predictsthe perforing relativelylow vis-

Figure 6,2 Boberg and Lantz,s Steam_ stimulation Model (from Boberg and Lantz 1966)

245

where1{ is the rate at which heat is withdrawn with the products.At eachstep,it is changedusingthe quantitiesof the products,their heat capacities,and the production temperature.There is a decline in the production rate and temperature with time. The factors 7aand i2 are obtainedfrom Figure 6.3. Thesewere obtainedfrom approximatetheoreticalsolutionsto the conductionequation(lp can be obtained from equation2.85). The productionrate at eachtime is calculatedusing the idea that the flow is through two concentricreservoircylindersand the skin, with allowancebeing madefor the changingresistanceof the skin and heatedcylinderas the hot-oil viscosityfalls. At the end of the production cycle, Boberg and Lantz add the heat remaining in the reservoirto the heat injectedwith the steamin the next injectionperiod in order to calculatethe total heat injected.This is usedto estimatethe new heated radius.This approximationis conservative,sinceit neglectsthe heat storedin the overburdenand underburdenat the end of the cycle.The methodhasbeenshown to producegoodpredictionfor reservoirshavingoil with an in situ viscosityof a few hundredcentipoises. For the assumedmechanismto be effective,it is necessarythat there be an effectivereservoirpressurethat is ableto move the oil to the well bore in the cold condition. The increasedproductionof the stimulatedwell comesfrom the increasedflow that this natural drive can producewhen the effectiveradiusof the well bore is increasedby heatingthe reservoiraroundit. Effect of ProcessVariables Bobergand Lantz describea studyof the theoreticaleffect of variousprocessvariablesand draw a numberof interestinsconclusions: 1. The methodis intendedfor reservoirswith substantialdrive and cold mobility; their method is not suitablefor tar sands. 2. Wells having a high skin factor respondmost favorablyto stimulation even if no cleanup(i.e. reductionof S) is achieved.Figure 6.4 showsthe calculated effect of steamstimulation on the oroduction for a well assumingseveralskin

fl

Figure 6.3 Chart for Estimating7p andzyin Equation 6.6. (from Boberg and Lantz 1966) Cyclic Steam Stimulation

Chap. 6

ol@ c

o

?ro I.,

!co

z o

U.o o

92q

c

6l FE:r Bobq

factorsneck- c

n

tive. re ties srr (negati 1967l. effect d Chapre the skil 3. The fa o Higb r thar i o Lon, proxi greatl fecr d ofal shour ratesI from I

o Lorl p rated r beas. o High c

increa Bobergard La

lcts. At eachstep,it aiities, and the proIte and temperature

o G

o ao I tll

were obtainedfrom (vncan be obtained

e

z I

: idea that the flow ith allowancebeing ler as the hot-oil vis-

L'

D

o

o

e a J

J the heatremaining ;t injectionperiod in natethe new heated re heat storedin the hod hasbeen shown ;itu viscosityof a few

o

ve and cold mobility; o stimulationevenif showsthe calculated ssumingseveralskin

I

Chap.6

IF F

B

factors.Steamstimulationallowsoil to flow more easilythrough the ,,bottleneck" createdby a high skin factor.

varlousprocessvan-

Stimulation

t{

Figure 6.4 Effect of Skin Damage on steam stimulation Response(from Boberg and Lantz 7966)

ary that there be an well bore in the cold comesfrom the infective radiusof the

hart for Estimating 7p ation 6.6. (from Boberg 6l

40 60 80 t00 120 140 160 rso IIMT SINCESIARIOF SIEAMINJECIION . DAYS

The skin factor is a variablethat representsthe additional(or, if negative, reduced)pressuredrop around the well bore causedby local irregularities such as plugging (positive) or cleaning treatments such as acidizing (negative,it is hoped).It is definedby equation6.1(seeMatthewsand Russell 1967).The effect of the skin in radial fluid flow to a well is analogousto the effect of insulationon the flow of heatthat wasdiscussed in the latter part of Chapter2. In somecasesan additional effect of steamstimulationis to reduce the skin factor S by cleaningout depositsaroundthe well bore. 3. The factorswhich tend to give higher incrementaloil to steamratios are: o High oil saturationand high oil sandto shaleratios.Thesereducethe heat that is requiredper unit volumeof reservoiroil. o Low producedwater-oilratio (woR). water has a heat capacitythat is approximatelytwice that of oil and thus water productiontendsto accelerate greatlythe drainageof heatfrom the stimulatedreservoir.The predictedeffect of producedwoR ratio can be seenin Figure 6.5 which showsresults of a processvariable study made by Boberg and Lantz. Figure 6.5 also shows the beneficial effect which is predictedfor using higher injection ratesand also larger steaminjectionquantities.Theseimprovementsstem from the lower fraction of the heat which is lost. o Low producedgasto oil ratio. This is beneficialsinceproducedgasis saturatedwith watervapor and the heatin this, particularlythe latentheat,can be a seriousdrain on the heat pool. o High oil viscosity.The viscosityof very viscousoils dropsmorerapidlywith increasingtemperaturethan doesthat of lessviscousonesand, as a result. Bobergand Lantz's Model

247

{

J 2.4 (o o 4 2.0 E

E t.2 H t/t :.8

zut

E.4

ttl 4

\,

o ro

I

o ol

o rO

I

I

= STEAM RATE(LB/HR)/NETFT = 0.5(5T8/Ol/FT cotD Ol[ RATE

mr/h,

lot

I

m./hn = $g

/

rI 7 5 7

r6

I

mr/hn = 151

I WOR=O 2,/\

rt

Pt

a,

mrlhn = l5QO;

(n

t.a

E t.2 trl

ffit

J

t.o

o

o.t z

lt

E 0.6

lt

e (,,

-o loo

150

300 250 200 m,t;/h - M LB STEAM/FIOF GROSSINTERVAL

50

Figure 6.5 Theoretical Prediction of Incremental Oil-Steam Ratio versus SteamInjected(from Bobergand Lantz 1966)

the effect of steam stimulation can be larger with very viscous oils. Figure 6.6 showsthe predictedeffect for a particular set of conditions. o Large sandthickness.This improvesthe OSR becauseof the reducedfraction of the injectedheatwhich is lost. 4. Back-pressuring the well during the earlypart of the productioncyclecan be beneficialby reducingheatedzonecoolingcausedby the flashingof water. Bobergand Lantz'smethodhasbeenextendedand coupledwith calculations for gasJifting wells using the Orkiszewskicorrelationsfor calculatingthe twophasepressuredrop in a vertical pipe (Boberg,Penberthy,and Hagedorn 1973).

SCALINGOF THERMALMODELS Physicallaboratorymodelsare usuallyscaledto the field situationby employingdimensionalanalysis.The most commonschemeemployedfor doing this is to make the physicalmodelgeometricallysimilar to the field situationand to usethe same fluids in the model,i.e., oil, waterand steam.Scalingis usuallycarriedout by makine the Fourier number

z 0.4 I Figrrt andLr

equal for the I gree of heat pc the time scah will be smaller

or if a.oa.i = o

whereR is the Thus. fm t h in the mod The othe the sameratio I i.e.,

,o=# 248


Chap.6

Scalingof Tfrern

o

t.8

!o !o

ro

t.6

I

o

1 .4

4,

E 1.2 ul

(n

t.o

6 0.8

5 f,l

z rlj

HI

il

€ w oc

0.6

z

0.4 L 40

fif

Fl

5

t",

250 300 TERVAI.

t00

I

t000

I

- cP ortvtscostTY

Ratio versus

h very viscous oils. set of conditions. r of the reducedfracduction cycle can be e flashingof water. rledwith calculations calculatingthe twod Hagedorn1973).

Figure 6.6 Effect of Viscosity on Incrementaloil-Steam Ratio (from Bobers a n dL a n t z I 9 6 6 )

equalfor the model and the field at corresponding times.This meansthat the degreeof heatpenetrationby conductionwill be the samefor each.It alsomeansthat the time scalewill be shortenedbecausethe correspondinglengthsin the model will be smaller. lmodet a.oo.r _ /Lroa.r\2 =

t*"

"r"r

\t*"

/

of if a-o6"1 = @fierd,

+*=(L,"*,)'_n, Iri.ro \ lri"ro / tion by employingdidoing this is to make and to usethe same y'carriedout by mak-

whereR is the geometricscalingfactor. Thus, for example,if 1 cm in the modelequals1 m in the field, R : 0.01and t h in the modelwill be equalto 104h : 1.14y in the field. The other criterionusedis to makethe pressuregradientsdue to oil flow bear the sameratio to the potentialgradientdue to gravity in the modeland in the field; i.e.,

=(#fr),,,,, (m)^.,,,

itimulation

Chap.6

Scalingof ThermalModels

249

The velocityV in eachcaseis proportionalto Llt.If the samefluids are usedin the model as in the field, then Ap and lr.owill be the samein each,so

=(f),,",, (f)..,",

and k m o c e_t kr,",o

t Rl,,",o lmodel R

I

Becauseof the need to shortenthe time scaleby the factor R2 in the model,it is necessaryto increasethe permeabilityof the modelby a factor of llR in order to maintain the viscousdrag forcesproportionalto the gravity forces.This scaling procedurewas describedby Pujol and Boberg(1972). More elaboratescalingprocedureswhich allow modelsto be operatedat pressuresmuch lower than thosein the reservoirare discussedby Stegemeier, Laumbach, and Volek (1980).The approachof operatingscaledphysicalmodelsunder low pressureconditionsusing oils different from those in the reservoirand with steamof a differentquality hasbeenusedby Shelland othersfor the physicalmodel simulationof oil recoveryprocesses usingsteam. NIKO AND TROOST'SCYCLICSTEAM STIMULATIONMODELEXPERIMENTS Niko and Troost (197I) carried out an interestingseriesof low-pressure, scaledmodel studiesof the steamstimulationprocess.Their physicalmodel represented the near-wellregionof a reservoirin which therewas adequatecold-oilmobility to providedrive. In sucha model,it is necessary to representthe ability of the reservoir beyondthe model to supplyand to receivecold oil during the productionand stimulationcycles. Niko and Troostovercamethis problemby usinga seriesof resistanceand capacitortubesconnectedto the end of the sandpack.This arrangementis shownin Figure 6.7. The seriesof tubesand capillariesprovideda volumeinto which liquid from the sandpackcould be squeezed. The conditionswere arrangedso that the heat remainedwithin the sandpack. A numberof processvariablestudieswere carriedout. The followingconclusionsare expressed in termsof the full-scalefield that wasmodeled.The field data which were represented were for a typical VenezuelanBolivar coastfield.

Figrrt r Troog I

Soak time

Soaktiru that hot-oil sro future lifting c

r977). Differential p

The prod the resen'oirp Oil viscosity

The prod stimulationto I larger proporti Reservoirthil

Thickerk did not penetr reservoir,the s and per unit ol

Effect of ProcessVariables Injection rate

TABLE 6.1 Effed

A steam-injectionrate in the rangeof 19 to 60 t/d into a 9-m layer of reservoir had little effect on subsequent performancefor a given total quantity of injected steam.

250


Chap,6

Viscositl r

Productrrr

Niko and Troost'

fluids are usedin the ;h. so

ELEVATION

R: in the model,it is tor of L/R in order to i' forces.This scaling o be operatedat presb1'Stegemeier, Laumrh1'sical modelsunder he reservoirand with for the physicalmodel

lXPERIMENTS low-pressure, scaled:al model represented te cold-oilmobility to re ability of the reserrg the productionand

-l

F

Figure 6,7 Niko and Troost's Steam Stimulation Apparatus (after Niko and Troost 1971)

$(

H bl

Soak time

F

Soaktime wasnot a significantvariablein the rangeof 1 to 160d. This means that hot-oil storagewithin the reservoircan be looked upon as a cushionto meet future lifting demands.This has also been found in field experience(Borregales

r977). Differential pressure The production rate was found to be proportional to the difference between the reservoirpressureand the well pressure. Oil viscosity

s of resistance and ca'angementis shownin ume into which liquid arrangedso that the

The productivityindex improvement(the ratio of the productivityindex after stimulationto that before)was greaterfor more viscousoils becauseheatinghad a largerproportionaleffect on their viscosity(Table6.1).

The followingconcluodeled.The field data rr coastfield.

Thicker layersdid not respondaswell asmight be expectedbecausethe steam did not penetrateto the bottom.For a fixed injectionof steamper unit thicknessof reservoir,the steamwas found to penetratefarther horizontallyfor thicker layers and per unit of original oil in place.For a 980-cpoil, a 1133-tsteamslug injected

Reservoirthickness

TABLE 6.1 Effect of InitialOil Viscosity on ProductivityRatio

r 9-m layerof reservoir al quantity of injected

Stimulation

Chap.6

Viscosityin centipoise Productivity ratio

980 6

4000 IJ

Niko and Troost's CyclicSteam StimulationModel Experiments

8000 20

25',l

I {

into a 9-m layergavean increasedproductivityratio of 4.5; three times this quantity of steaminjectedinto a27-m layer gavea ratio of 3.8.

4

Steam slug size

-9 @

:3

The effect of steamslug size was found to be rather complex.A given quantity of steaminjected as a number of small treatmentsrather than as fewer larger treatments(see Figures 6.8 and 6.9) gave a higher initial oil-to-steamratio (Figure6.10)and higher cumulativeproductioninitially (Figure6.11).

E

a o o

z2 o

30

6

=1

E f

Steam-soak experiment

!,

e)

o

Steam-slug size: 3400tons Cyclelength:1230days

E

20 at

0

o E tr o

F€r

Ero :

g a),u

tt

o o-

E

c o o

0

1000 Figure 6.8

2000 Time in days

f !,

3000

o CL

- 10,(n o

Large Steam-Slug Size (after Niko and Troost 1971)

o

30

G f

E

Steam-soak experiment

!t G)

f

o

Steam-slug size:1133tons Cyclelength:625days

E

20 ot tg G

o r!

tr o

E 10 I

After a 1-e passedthat frou steamratios apg

!t

o o. 0

Cycle length

1000

2000 Time in days

3000

The effect of c1c eachcycle.It *'as earlierin eachcr ducedthe cumu

Figure 6.9 SmallerSteam-Slugs (after Niko and Troost 1971)

252


Chap.6

SteamStimulatio

three times this quan-

4 "9 (6 -o

rmplex.A given quanrr than as fewer larger :ial oil-to-steamratio ure 6.11).

E

(E

o o

E2

o .: g =1

---------,I

E 5

)eriment I I

l*3',t,=l I

6800 tons/cycle

o 0

1000

2000 Time in days

3000

Figure 6.10 Cumulative Oil-SteamRatio (after Niko and Troost 1971)

oa 20.000 E i o

l

--l

1133 tons/cycle 3400 tons/cycle

(t :t tt

ir.r*ronrt';%;19,'n

o CL

- 10,000 o o .:

rost1971)

_,

)€riment

|

1

tl 1133tons | @5 days |

\d

(E

E E 5

|

o

| |

1000

Il

2(X)o Time in days

3000

Figure 6.11 CumulativeProduction(after Niko and Troost 1971)

After a year or so the cumulative production from the large treatment surpassedthat from the smallertreatments(Figure 6.11)and the cumulativeoil-tosteamratios approachedthe samelevel (Figure 6.10). Cycle length The effect of cyclelength was also studiedfor a fixed injection quantity of steamto eachcycle.It was found that decreasingthe cyclelength (i.e., cutting off production earlier in each cycle)increasedthe cumulative oil production (Figure 6.12)but reduced the cumulative oil to steamratio (Figure 6.13).

ost 1971) Stimulation

0

Chap,6 ,

Steam StimulationProductionMechanism

STEAM STIMULATIONPRODUCTIONMECHANISM

6

In the Boberg-Lantzanalysisof the steamstimulationprocessand also in the experimentsof Niko and Troost discussedin the previoussection,the production mechanismfor the steam-stimulated well was assumedto be basicallythe sameas that for cold production.A reservoirpressurepushedthe oil to the well; the effect of the steamwas to make the oil flow more easilyby heatingthe reservoiradjacent to the well bore. The pressurethat moved the oil was the reservoirpressureexistingbefore the operationstarted.In the exampleof the Quirequirefield usedby Bobergand Lantz as a field example,this view of the mechanismis a reasonable one. In this casethe cold-flow rate was already135B/d, and this was increasedto 350 B/d by steaming. Factorsignored in the Boberg-Lantzmethod include the following: 1. The movementof the oil from around the well by displacement with steam during the injectioncycleand the refilling of the steam-saturated regionduring the productionare ignored.The Boberg-Lantztheory assumesthat the steam heats the near-well bore region but does not move oil away from the well bore.This is inconsistent with the ideasdescribedin the previouschapter. During the productioncycle,the oil mustfirst build an oil bank as it flows to the productionwell. Also as the pressurearoundthe well bore is decreased during the onsetof production,there will be vaporizationof water and the generationof steam.Eventually,however,the steamwill be displacedfrom the systemand liquids will flow

20,000 c,

E E

o o J

t,

o 010,000 o o

"z (E E

c =

Sizeof steamslug: 1133tons/cycle

o

0

1000

2000 Time in days


6

E

t4 6

o o

Ez : E 5

o 0

o

Figrn I and Tm

2. The rheq falling.ra cool fluid in the cfli conducti< partialh.I coolingwr considere den. This tially'ar rl heatedsq losses.cal the adjre heat loss h 3. As Boberg of oil rl ithi ration.Th in this ma men)is so evenif tha even ttK}q becauseit is produca

This las pt fornia fields.thc thanthatwhich well bore radius

3000

Figure 6.12 Effect of Cycle Length on Cumulative Oil Production(after Niko and Troost 1971)

254

-9

Chap.6

SteamStimulatb

6 , and also in the ex:ion, the production rasicallythe sameas r the well; the effect he reservoiradjacent ;sureexistingbefore usedby Bobergand sonableone. In this reasedto 350 B/d by

lacementwith steam :aturatedregiondur)rv assumesthat the ;e oil away from the the previouschapter. ril bank as it flows to e is decreased during nd the generationof r the systemand liq-

..-4

..-Go-) 'l, t t

I

I rnmary | rduction I

II

tons/cycle

I I

I

//

E

E4

,r'\

-.

| l, l. Ir-/l-

-1

..t -rt

I t l.

Cyclelength InOayS 1l7o,------?'-

o o o

E2 J

E J

o

Sizeof steam-slug: 1133tonspercycle

0

1000

2000 Time in days

3000

i{

ti ll|

lt

Figure 6,13 Effectof CycleLength onCumulative Oil-Steam Ratio(afterNiko andTroost1971)

|i fl

2. The theory assumes that the near-wellbore regionremainsat a uniform, but falling, temperature.In practice,muchof the coolingcomesfrom the flow of cool fluids into the perimeter,and a temperaturegradientwill be established in the cylindricalregion.The heatedcylinderis assumedto cool by thermal conduction.In the radial direction, this conductionwill be offset, at least partially, by the flowing fluids carrying heat back by convection.The radial coolingwill be lessthan estimated.The other mechanismof cooling that is consideredis the verticalconductionof heatto the overburdenand underburden. This estimationassumesthat the overburdenand underburdenare initially at the reservoirtemperature.In practicethey will have alreadybeen heatedsomewhatduring the spreadingof the heat chamber,and the heat losses,calculatedfrom the Marx-Langenheimformula,are alreadypresentin the adjacentreservoirboundaries.This, too, will tend to make the estimated heat losshigh. 3. As BobergandLantz point out, the methoddoesnot allow for any depletion of oil within the heatedzone-i.e., replacement of oil saturationby steamsaturation. They recognizethat in many casesthe major part of the oil is produced in this manner.Thesecasesare thosein which the cold oil (or usuallybitumen)is soviscousthat it cannotflow at a practicalrate to the heatedboundry evenif that boundaryis very largein radius.Thesecasesare alsothosewhere, eventhough the reservoiris thick, there is a relativelylow oil-to-steamratio becauseit is necessary to heat the entire reservoirvolumefrom which the oil is produced. This lastpoint hasbeendiscussed by Burns(1969)who pointsout that in California fields, the increasein oil rate found in steamstimulation is often much larger than that which would be expectedfrom the conceptof an increasein the effective well bore radius.

3000 ;tion (afterNiko

Stimulation

/

0

following:

@ -tazc

o o

Chap. 6

Steam StimulationProductionMechanism

255

i(

rl

I

I

Assumingsteady-state conditionsand neglectingany contributionof skin factor, the ratio of the productionrate for a well surroundedby a heatedregionof radiusRl, to that for an unheatedwell shouldbe given by equation6.11(compare with equation6.5): R"

F,, Qn Q"

*'" RR n , F,, R. rn + R. *t" R,

(6.11)

If the ratio of viscositiesis very large,then this equationmay be reducedto equation 6.12;i.e., the effectivewell bore radiusis increasedby heatingfrom R, to Rr (comparewith equation6.6): qo_ i

!

l

Q'

tn&

R*

(6.72)

t., -- &

Rr,

As Burns points out, for reasonable valuesof R7,,the productionratio from equation 6.12 is limited to relatively low values.For example,if R, : 0.25 ft ind R" : 1000ft, the followingvaluesfor the productionratio may be calculated: Heatedzone radius,feet: 50 Productionratio: 2.8

100 ' 3.6

200 5.2

However,asmaybe seenfrom Table6.2 (takenfrom Burns'paper),the ratiosfound in the field are usuallymuchlarger;the averagevalueof qnfq.in this tableis r2.g or 9.0 if the extremelyhigh value for the third row of data is left out. Although large skin factorsand/orreductionsin the value of the skin factor could causetheoreticalincreasesin the productionratio that are as largeas those shown in Table 6.2, this is not consistentwith the valuesfound for the oil-steam ratio. When the improvementin productionrate ariseslargelyfrom its effect upon the skin factor,then it would be expectedthat a relativelysmall amountof steam would be sufficient,i.e., that there would be a relativelyhigh oil-steamratio. In Table4.7 it was shownthat for generalreservoirheatingto be achieved,steam-tooil ratios of at least0.5 to 2 would be required.Thesevaluesare for production without heatloss.If allowanceis madefor lossesand for incompletedisplacement, then considerablymore-probably severaltimes more-steam would be required. Assumingthat at leasttwice asmuchsteamwould be requiredthen the SORwould be expectedto be at least1 to 4 (i.e. oSR would be no more than 0.25 to 1) for generalreservoirheating.[n Table6.2, only the first three or possiblyfour of the projects shown display oil-to-steamratios sufficiently high for skin and nearwellborestimulationto be the main causeof the improvedproductionrate. In the other cases,the quantityof steamthat wasrequiredwould be expectedto be sufficient for there to havebeenextensiveheatineof the reservoirwell bevondthe well bore zone.

256


Chap.6

:ributionof skin fac, a heatedregionof ration6.11(compare

i-N:UF

O.OOi66F O@hNSfl--> FI\

a o

x u.l

'1.-:-

Value of ho;

E

E

y

H

.;.

|-t,

s t-s

st\

tl

ooSN-*\Fis{'

:I E

:_r .r I

H I :

c,

Equation7.{l first brackets secondbracke The init perimentsimi heightof the c ples implante measuredtenq eachthermoc steamtemper

TheRisingSta

The factor 2 outside the squareroot sign in equation7.36 recognizesthat oil is draining to the well from both sides,whereas7.16 gavethe quantity draining from only one side. (7.36)

Q:2

Sinceit is assumedthat the steamchamberremainsgeometricallysimilar asit grows, the cumulative oil productionwill be proportional to the mobile oil per unit areamultipliedby the squareof the chamberheight.This is shownby equation7.37, wherethe constant7 is determinedby the shapeof the chamber;the area of the chamberis 7h2.

e c u= f o n o , = y d a s " h 2

(7.37)

DifferentiatingT.3Twith respectto time givesanotherexpressionfor the production rate, equation7.38.

q = zvSA^S,hff

(7.38)

setting the right-handside of 7.36 equal to the right-handside of 7.38 resultsin equation7.39,which may be integratedas in 7.40 to give an expressionfor the heightof the chamber,h, as a function of time. This is equation7.41.

=z 2tSAS,hff

kga$ L,S"h

(7.3e)

*rt

[orr,r*r=irlffifr*

(7.40)

^=(+i)"(m)"

=(i#)"0#n1'''u'

(7.41)

r

Value of Proportionality Constant in Height Equation ci F:

tl > il vil ao

9 ll .€S

ii

st\

€lE =l !

:

EI

Y

Equation 7.41 showsthat the chamberheight ft should be equal to a constant(the first brackets),multiplied by a factor involving the reservoir and oil properties(the secondbracketedterm), and multipliedby the time raisedto the J power. The initial constantterm in 7.4Ihas beenevaluatedfrom a scaled-model experimentsimilar to that shownin the earlierphotographs.In this experimentthe height of the chamberwas determinedfrom the behaviorof a seriesof thermocou: ples implantedinto the model in a vertical line above the productionwell. The measuredtemperatures are shownin Figure 7.17.As the steamchamberapproached eachthermocouple,the temperatureincreasedabruptly from the initial level to the steamtemperature. The RisingSteam Chamber

309

100

o

o_ o J

i: 50 e, CL

E

o

F

Timein Minutes Figure 7.17 MeasuredTemperaturesaboveproduction Well

From these measurements it was possibleto plot the height of the steam chamberagainsttime, as in Figure 7.18.Also shownin Figure 7.1gis a theoretical line basedon equation7.41with the constantset equal to 2; i.e., this theoretical curve is basedon equation7.42.The slopeof the experimentalcurve is closeto the theoreticalvalue of ?.

,=,(ffi)',f''

(7.42)

Equation7is verr':atis

Shape of S

If it is assu circle.then of gamma! ment*ith t

The Oil-productionRate When the valueof ft from equation7.42 is substitutedinto equation7.37, the result is equation7.43.This haswithin it theshapefactor, y. By cuive fitting the oil-rateproductiondata from the experiment,7 hasbeenfound to be about *9. Qcum

=

^r(ffi)" roLSo)1/3 f/z

(7.43)

From curve fitting, = 2.25 or y

= zzs(tv)"($ Q"u^

q 10

AS.)rt3t4t3

310

dt

= t(Bg\'''(,fAS,;r/3rrl3 \mvsl

Steam-AssistedGravity Drainage

; o o o o 810

o (7.44)

Using this value leadsto equation1.44 for the cumulativeoil production,and differentiatingthis with respectto time resultsin equation7.45 for the instantaneous productionrate.

q- =+

s

0

(7.4s)

Chap.7

The RisirgS

-t

tr

I

200

I eisht of

150 q)

c

I

Theory Experiment

100

rermocouple I lvewell,mml

q)

a 0)

II

50

I

rrl

...*..

?n q)

trn

71020 fime in Minutes

-

60

Figure 7.18 SteamChamberRise

on Well

: height of the steam re 7.18is a theoretical ll i.e., this theoretical al curve is closeto the

(7.42)

luation7.37,the result rve fitting the oil-ratee about*9. (7.43)

Equation7.44is comparedto the experimentaldata in Figure 7.19.The agreement is very satisfactory. Shape of Steam Chamber. If it is assumedthat the shapeof the steamchamberis approximatelya sectorof a circle,then the anglesubtendedby the sidesof the sectorthat gives*aas the value of gammais 64"; this is shownin Figure 7.20.This shapeis in approximateagreement with the observedshapesof rising steamchambers.

20

Equation7.u14

I f o o C) o 810

o (7.44)

O Experiment I production,and diffor the instantaneous

(7.4s)

ty Drainage

Chap.7

20 40 Time in Minutes

60

Figure7.19 Oil Recovery DuringChamber-Rise Period

The RisingSteamChamber

311

o G

E o ct) G^ o 't6r E ^= qt

-@ gr?

H

Eg,

E .9 th

Figure 7.20 ApproximateShapeof Rising ChamberSectorwith y : 9116

'(

o

E i5

AvailableHead It is also possibleto calculatethe value of the head availabilityfactor B. This is donein equation7.46; the result-!, or 1.125-indicatesthat nearly half the availableheadis requiredto move the oil laterally.

Figrr

4v2

B=8r-i-

9 8

(7.46)

This value is lessthan the value of 1.5 found for the TANDRAIN equation.The photographs in Figure 7.15indicatethat duringthe chamber-rise period,the top of the steamchamberis ill-defined becauseof the instabilityof the rising front (but the sidewaysspreadingfront hasa stableinterface).Becauseof the raggednatureof the front, it is not surprisingthat lessof the headis availableto effect drainageto the productionwell. The photographsshowclearlythe effect of gravity in stabilizing and destabilizingsteamfronts. Equation 7.39 (with F = 1.125)predictsa lower drainagerate at the point where the chamberreachesthe top of the reservoirthan does the TANDRAIN equationfor the samevalue of h. It is practicalto continueto use equations7.44 and 7.45 after the chamberreachesthe top until the rate reachesthat predictedby the TANDRAIN equation.After this point the TANDRAIN equationshouldbe employedto follow the further depletionof the reservoir.This approachis shown in the numericalexamplewhich is developedstartingon page316. The ratepredictedby equation7.45 for the rising chamberperiod and that by equation7.35for the depletionperiod can be plottedon a singlechart, as shownin Figure 7.21.

ture. It alsopr eachother an gerswhich int Tl pical 1 of the fineen

Visctrsi:J;'

Velocits oi rt.c I Steam temp.er Steam tenr;'crr Steam temprcr

Finger dimcns:tx Steam tr-mP'e Steam tenp=::: S t e a m t em ; r : a tThe

top oi trc f

EFFECT OFSTEAM ON I PROPERTIES FINGERRISETHEORY

Steam and R Another approachto the predictionof the rate of rise of a steamchamberhasbeen describedby Butler (1987).This paperdescribesa theoreticalapproachin which the frictional drag for the falling oil around a rising finger is balancedwith the frictional dragwithin the finger and the driving force providedby gravity. The theory predictsa rate of rise for the fingerswhich is proportionalto the permeabilityand inverselyproportionalto the viscosityof the oil at steamtempera-

The equation effect of rere Figurc tion 7.1-ias a of 129.1mm:

312

Effect of Steat


Chap.7

3

o G

E o ct) Q^

'EF0 ,p =o _o 9C

vao

E o o tr o

pproximateShapeof :r Sectorwith y = 9/16

.E o lity factor B. This is nearlyhalf the avail-

1

$ a

*

,'--_..-,/ Parameteris w/h w is (wellspacing)/2

'.. \

Drainage from

0.5

tl

TANDRAIN- startingwith l- - - verticalhot plane

\

risingunconfinedchamber

\

\

\

o'25 o'zs d = 1.a5(w/h) 1R""ouery1

a 0

\

\ I

00.s1

t r

Fractionof UltimateRecovery Figure 7.21 CalculatedDrainageRatesfor Rising and DepletingChambers

(7.46) IAIN equation.The iseperiod,the top of the rising front (but the raggednatureof to effect drainageto of gravity in stabilizLgerate at the point es the TANDRAIN o use equations7.44 res that predictedby i equationshouldbe rs approachis shown 316. :r period and that by le chart, as shownin

tl

r{

ture. It alsopredictsthe curvatureof the top of the risingfinger.Fingersrisebeside eachother and the oil flowing downwardsbetweenthem falls as meanderingfingerswhich interferewith eachother and with the rising steam. Typical predictedvaluesof the rate of rise and of a characteristicdimension of the fingersare given in the followingtable:

i

fI i{

q

ilrl

Viscosity of reservoiroil at 100'C Velocity of rise mfd Steamtemperature f00'C Steamtemperature200"C Steamtemperature 300"C Finger dimension/o m (seeequationbelowl) Steamtemperature 100"C Steamtemperature 200"C Steamtemperature300"C tThe

0.0163 0.0822 0.190

0.0082 0.0571 0.149

0.0043 0.0415 0.r21

8.97 4.47 2.56

4.57 3.29 2.22

2.65 2.53 L.94

top of the finger is parabolic according to the equation

lYlfot = $lft')

AND OIL TEMPERATURE, RESERVOIR EFFECTOF STEAM TEMPERATURE, ON DRAINAGERATES PROPERTIES Steam and ReservoirTemperatures m chamber has been pproach in which the anced with the fricv gravity. s proportional to the :lil at steam tempera-

The equationusedfor the generaldefinitionof m (7.25)allowsthe predictionof the effect of reservoirtemperatureon production rate (Butler 1981). Figure 7.22 showsa plot of the integrandin the right-hand side of equation 7.25 as a function of temperaturefor a typical heavycrude havinga viscosity --t/s (or cs) at 100"Cand 6.8 mm'/s at200"C. of 129.1,

' Drainage

Effect of Steam Temperature,ReservoirTemperature,and Oil Properties

Chap,7

{

313

!

{

1.6

. ^

PARAMETERIS RESERVOIRTEMPERATUREOC

E

1.4

'i t.,

OF ?r r.o

3

o

6 ?oa l!\

(J

!; ou i 0.4 = o.z o.o

j

!

a I

o 2 50

150 200 250 STEAM TEMPERATUREOC Figure 7.22 Effect of remperatureon value of Integrandin Equation 7.25

3

o

curves are showncorresponding to reservoirtemperatures of 0, 20, and 40.c. Increasingthe reservoirtemperaturehas the effect of increasingthe value of the integrand and, thereby, the correspondingvalue of the integral and the oilproductionrate. The curvesin Figure 7.22 wereintegratednumericallyand the corresponding valuesof mwere calculated, with the resultsshownin Figure7.23.rtis found thai z changessomewhatwith steamtemperatureand decreases with increasingreservoir temperature.A lower value of m for a given set of conditionshasthe effect of increasingthe drainagerate. In general,it is found that the rateof drainagevariesinverselywith the square root of mv*This factor is plotted againststeamtemperaturefor the samecrudeoil in Figure7.24.This figure showsthe significanteffectof the reservoirtemperature and how it may be allowedfor by the extendedtheory. Oil Properties

very vlscous ( more fluid m termed a con Lor'r'ert more slo'* lr drainagerate temperatureI The cal ure 7.26. Tlx but the overa

The three curvesin Figure 7.25 givecalculatedvalues of m as a function of steam temperaturefor three differenthypotheticaloils. The uppercurve corresponds to a a/,J

a a

R E S E R V O ITRE M P E R A T U R E

o uJ

Ij

2 o

J

z o ah

c 2

fie

I

I

o E

= I

E

t

o

lr

O3 u.l

I

f

80

120 200 150 240 'C STEAMTEMPERATURE Figure 7.23 Effect of Reservoirand SteamTemperatureson rn

314

Steam-Assisted Gravity Drainage

Chap.7

Effect of Stea

n'""_l

R E S E R V O ITRE M P E R A T U R E

)

.rs/1

I\ IF

/,Y1 'l '///

0.2 ct o

|

F

o rr 0,1 u,l

I

o.oo fr o

I 0

= 0.04

250

E,

o

u.oo=129.1SQ MM/SEC

r Equation7.25

rresof 0, 20, and 40'C. asingthe value of the integral and the oiland the corresponding e 7.23.It is found that *'ith increasingreseritionshasthe effect of rerselywith the square for the samecrude oil reservoirtemperature

80

160 200 120 'C STEAMTEMPEBATURE

240

'l

rl

Figure 7.24 Effect of Tn and Ts on Rate Factor

veryviscousoil similar to Athabascabitumen.The middle curve is for a somewhat morefluid crudesimilar to Cold Lake crude.The lowestcurve is for what might be termeda conventionalheavyoil similar to the oils found in the Lloydminsterarea. Lower valuesof m are calculatedfor the lessviscousoils becausez changes more slowly with temperature;this has the effect of tending to give predicted drainagerateshigherthan would be expectedfrom the viscosityof the oil at steam temperaturealone. The calculateddrainage-ratefactorsfor these three oils are plotted in Figure7.26. The calculatedeffect of the oil propertieson drainagerate is significant, but the overall effect is not overwhelmins.

as a function of steam lurve corresponds to a

R E S E R V O ITRE M P E R A T U R=E1 3 ' C t! J

z 9

4.5

o

z 4.0 UT

=

3.5 lr

o UJ f J

3.0 2.5

ty Drainage

PARAMETER IS CRUDE vtscoslTY AT 100'c/200'c SQ MM/SEC

,.0--;;, OC STEAMTEMPERATURE Figure 7.25 Valuesof rn for Different Crude Oils

tes on /n

Chap.7

5 t.

i

1

t

ilrl

I q

{ I

1 {

I o o

:

1 q

Effect of Steam Temperature, Reservoir Temperature, and Oil Properties

315

bitumenand r the sand used representI )(a) Obtain wt obtainingan :

PARAMETERIS CRUDE vtscostTYAT 100"C IN SQ MM/SEC

t\

s t

o o lt lrl F E |rJ

(, =

R E S E R V O ITRE M P E R A T U R=E1 3 " C

t

o

80

120

160 200 240 OC STEAMTEMPERATURE Figure 7.26 Drainage-RateFactorsfor Three Heavy Crudes

Estimating lrt

There are,of course,othervariablesin equationssuch as7.16that are equally effectivein modifying the drainagerate;these(e.g.,thicknessand permeability),ai well as the oil properties,can vary significantlybetweenreservoirs.

(b) Tabulatcp

NumericalProblemon Steam-Assisted Gravity Drainage A tar sandreservoirhas the following properties: Reservoirtemperature Oil viscosityat f Bitumendensity Bitumenviscosityat 100"C Reservoirthickness Thermal diffusivity Porosity Initial oil saturation Residualoil saturation Effective permeability for oil flow

15"C 100,000cs 7.00glcc 80 cs 20 m 0.0j m2lD 0.33 0.75 0.13 0.4 d8

(c) Calcularc

The field is to be drainedby a seriesof parallelhorizontalwells with a spacingof 75 m betweenwells.The wellswill be located2.5 m abovethe baseof the ieservoir. Steamwill be injectedfrom separatehorizontalinjectionwells placed above the producers.Assumethat initial thermalcommunicationis achievedand that the system will be operatedwith a steampressureof 1.2MPa. Estimatethe percentrecovery of the original oil in placeas a function of time for a period of 7 y. It is plannedto carry out a model experimentrepresentingthe field in the laboratory.The modelwill have a heightof 35 cm and will operateusingthe same

(d) Equation

sAssumed to correspondto an absolutepermeabilityof 1.0 D.

316


Chap.7

Effect of Stean

bitumen and at the samepressureas the field. What shouldbe the permeabilityof the sand used in the model? How many minutes in the model will be required to represent1 y in the field? (a) Obtain value of parameterm: Determine m using either equation 7.25 or by obtaining an approximatevalue by interpolation from Figwe 7.25.

-

I

Zs = 188'C

I

Zn = 15"C vs = 7.8 cs (by interpolation on Figure A.5.2)

=13ccI

=ll'l.rt*,,"'"^''"'

-_l x)

Estimating m from Figure 7.25 leadsto

mdcs

m=3.4

7.16that are equally nd permeability),as xin.

(b) Tabulateparametersfor problem:

6 = 033 AS,=0.75-0.13=0.62 k = 0.4 x 10-12mz I = 9.81ry/s2 q. = 0.07/(24x 3600)m'fs = 8.10x l0-7 m2fs h=20-2.5=17.5m m=3.4 vs = 7.8 x 10-6m2/s w = 75/2 = 37.5m (c) Calculatefactor to convertq to q*:

lls with a spacingof nse of the reservoir. ls placed above the vedand that the syste the percent recovodofTy. ting the field in the ef,ateusing the same

F a c t o=r - l - *

'-10-

for the transic Chapter l. F! smaller valu6 Figure 3. of an advancin for larger valu Thus for a gir. steadystate\a t h e s m a l l e rt h This erp ure 7.35.The t steady state ua ahead of the fi permeabilitf it steadystate hc oil drains. bur gravity. high t thin la1'enof r thick la1ers.O lished becaus The buil becausethe fn (Figure1.8t./ state is includ vance of the ir will be noted i the reserroir a

Avoidingthe S

)N Porometer is volue of 83

n assumedthat the rrresponding to the en by equation7.4. rnt advancingfrom heat aheadof the to the steadystate, hasbeendescribed rd the heat storage ing an approximate the elementis calm materialbalance ed using this techrr 83 set equalto 8. rt on 83; this is dei similar to thoseof sed. This tendency rves shown in Fig, a constantdimenequation7.21;this advancinginterface from the improved 'ith the calculations

|ell

Drainage

Chap.7

c 0.8 q,

3 o.o

t* = 0.5

o o

i5 E 0.4 o

E

I o.z 0

Dottcd line is from originol equotion 7,2'l

0.s

1.0

1.5

HorizontolDistoncex/h Figure 7.35

r

'r

Effect of Parameter 83 on Shape of Interface

rl

for the transienttemperatureprofile aheadof an advancingfront that were madein Chapter 2. Figure 2.8 showsthat the approachto the steadystate increasesfor smallervaluesof a. Smallvaluesof a alsogive largervaluesof B: in Figure 7.35. Figure2.8 showsthat the approachto the steady-state heatequilibriumahead of an advancingfront is a function of U2tla.For a giventime, /, this approachis less for largervaluesof a; theselargervaluesof a correspondto smallervaluesof .B3. Thus for a given value of /* the degreeof heat penetration(i.e. the fraction of the steadystatevaluewhich is achieved)will tend to be lessthe largerthe valueof a or the smallerthe value of B:. This explainsthe effect of B: on the position of the interfacecurvesin Figure 7.35.The smallerthe valueof B: the farther awaythe heatpenetrationfrom the steadystatevalue.As a result there is a lower drainagerate while the heat bank aheadof the front is building. Largevaluesof ,B3correspondto the casewherethe permeabilityis large comparedto the thermal diffusivity. In this circumstancea steadystateheat distribution is achievedrapidly and only a very thin layer of heated oil drains,but it drains quickly becauseof the favorablepermeability(and/orhigh gravity,high head,low viscosity,and so on). High valuesof ,B3result in relatively thin layersof rapidly-flowingmobileoil whereaslow valuesof ,B3resultin the flow of thick layers.Considerable time is requiredfor a thick flowing layerto becomeestablishedbecauseof the need to build up the heat bank beyondthe advancingfront. The buildup of the heat bank takes longerin the lower part of the reservoir becausethe frontal advancerate is slower there and for a given time U2tfais lower (Figure 2.8). As a result, for the calculationwhere the deviationfrom the steady stateis included(Figure7.34)comparedto that whereit is not (Figure7.4),the advanceof the interfaceis slowernear the baseof the reservoirthan it is at the top. It will be notedin Figure 7.34 that initially the steamchamberdoesnot advanceinto the reservoirat the bottom; it overridesat the top without steampenetratingto the

Avoidingthe Steady-stateHeat-distributionAssumption

329

ll \ {

F rl

r

il tl

xl

c{

s

rl

u

I

,I

rI

AthabascaCrude; 300cs at 1000C; 10 cs at 2OOoC q = 0.069mzlday; ReservoirTemperature 12 oC

Coid ---.

C

a=38

Parameteris kh in darcy metres

Paramelef 6 Er an darcy neinla

20

(')

(f)

dl

co

= >10

-(u 1n

c.)

=

1oo

oc steam r#fierature

3oo

Figure 7.36 Effect of SteamTemperatureand kh on 83

productionwell.l1This is reminiscentof the behaviorpredictedby van Lookeren's theorydescribedin chapter 4 (seeFigures4.29 an44.30).The phenomenonshown for earlytimesin Figure7.34, andwhich would be evenmoreevidentif the valueof 83 had beensmaller,occurseventhoughit is assumedthat initially the entirevertical plane abovethe productionwell is at steamtemperature.The flow of draining oil preventsthe advanceof the steamdownwards. Values of the Parameter83 Figures7.36,7.37,and 7.38 give valuesof -83as a function of steamtemperature and the productkh for three different crude oils using typical valuesof a and rn. Valuesof 83 for a whole rangeof situationscan be estimatedfrom thesediagrams by interpolation. ttThe theory describeddoesnot allow for steambeing introduced at a higher pressurethan the draining oil. In practicethe steamchambercould be forced to the productionwell more rapidly by either increasingthe steaminjection pressureor by lowering the pressurewithin the production well. Strategiessuchas this are desirablebecausethey make the wholedrainageheadavailablemore rapidly. Also there is a needto allow for the resistanceto radial f low in the immediatevicinity of the productionwell; this too requiresan increasein the pressuredifferencebetweenthe injectorand the producer.Another factor which requiresan increasein the pressuredifferenceis the resistanceto the radial flow of steamaround the injector.Neverthelessthere remainsthe conceptof the continuous steam-assisted gravity drainagewith the productionof oil controlledso that oil is withdrawn at a rate equalto that of the drainagearound the perimeterof the chamberwithout allowinglive steam to bypassin excessivequantities.This can also be looked upon as the productionof oil from below an expandingsteam (gas) cap without allowing the coning or, perhapsmore accurately,cresting of steam.

330


Chap.7

LloydminslerTy;r o = 0Gtr

o

(9

9ro

Heat Penetr

Figure7.39: a casein\.ol ure 7.12.it $ what lower.1 The rez is lower thar '-

I hls lt

the initial renr

Avoidingthe

2oooc 2tc

oC C o l dL a k eC r u d e ;l o o c s a t l o o o C ; 6 c s a t 2 o o oC q = o.oo9m2/day; ReservoirTemperature 12 Parameteris kh in darcymetres

20 (f)

(D (!)

=

*=tF^"'* 100

200

300

oC SteamTemperature

Figure 7.37 Effect of SteamTemperature and kh on Bs

ed by van Lookeren's e phenomenon shown evidentif the valueof tially the entireVertiThe flow of draining

il q i

tl ltl

tI

20

[,

ill

tf

(9 dl

of steamtemperature ll valuesof a and Z1q. from thesediagrams

at a higher pressurethan rductionwell more rapidly ure within the production rnageheadavailablemore immediatevicinity of the t*een the injectorand the :renceis the resistance to heconceptof the continuso that oil is withdrawnat ithout allowinglive steam rductionof oil from below more accurately,cresting

y Drainage

Chap.7

lt

r{

Lloydminster TypeCrude; 30cs at 1OOo C; 3.5 at 2OO oC "s o = 0.069m27day;ReservoirTemperature= 12oC

nB,

Ir

9ro

ltl

ru r( rr --10

il! rd

Parameteris kh in darcymetres

I i{

ril

200 oC SteamTemperature

300

Figure 7.38 Effect of SteamTemperature and kh on Bz

Heat Penetration as a Function of Distance Along Interface Figure 7.39 showsthe interfacepositionscalculatedusing the sameapproachfor a caseinvolvinga confinedwell. Although the curvesare similar to thoseof Figure 7.12,it will be found, on carefulexamination,that the productionrate is somewhat lower, particularlyinitially. The reasonfor the lower rate is that the heatpenetrationbeyondthe interface is lower than that correspondingto the steadystate.l2The fraction of the steadyr2Thisis particularly true near the start where it is assumedthat the reservoiris cold beyond the initial verticalhot olane.


331

o .G

a a CE o

0-8

o

tG o t, G ()

(}r -g

0.6

a

o a a

0.4

! C o

E o

a g a

E o 0

0,2

0.4 0.8 1.0 0.6 Horizontaldistance fuh

1.2

1.4

Figure 7.39 Positionof Interfacefor a Confined Well

stateheatpenetrationthat is achievedis plottedagainstthe verticalpositionon the interfacefor a dimensionless time of 0.3 and for l, = 8 in Figure 7.40. The fraction of the steady-state heat penetrationachievedis above0.8 over most of the interfaceand is muchlower near the well. It is this deviationfrom the steady-state heat penetrationthat makesthe drainagerate somewhatlessthan that predictedby the earlier equations. Figure 7.41 comparesthe drainageratespredicted from the earlier equations with thosecalculatedfrom the curvesin Figure 7.39.The three dotted curves in this diagramare the sameas thosein Figure 7.13.The solid curve showsthe rate

E 100 o tr

1. The hea 2. The hea p€ratun This as actuall3. The hea calcula 4. The hc method

t80 .U

o r60 o IE

@+o t t6 o

620

332

Predicted Ol

The steamcc matedusingt In gena

tr o

o bs0

from the nen 7.34.After th exceedsthe n ized that duri drainagerate curve for the r larities in thc calculation.

0 0.2 0.4 0.6 0.8 1.0 Vertical Height Along Interface y/h

Figure 7.40 Heat Penetration Along the Interface as Percent of Steady State:,* : 0.3; 83 : 8


Chap.7

ttlt is rc|l (T^ Tn)/lTs' with the oil bcy

Avoiding the S

* Originaltheory Q :,/Z

a

TandrainQ* : y't.s _----*_ \'----

o (U E o

ED{ ^

\r..

E G

Tandrainwith depletion

o o v, o c

Nonsteady-state theory withB, : A

-9 v, c o

-E o 0 12

1.4

0

0.5 DimensionlessTime

1.0

+ rl

{

Figure 7.41 Comparisonof PredictedProductionRates

lr tical positionon the ure 7.40. rd is above0.8 over ; deviationfrom the en'hatlessthan that he earlierequations reedotted curvesin urve showsthe rate

from the newer theory. It startsmuch lower and then reachesthe level predictedby 7.34. After this, it fallsbecauseof depletion.Although the solidcurvein Fig,neT.41 exceedsthe rate predictedby equation7.35aoverpart of the range,it shouldbe realized that during this period,the reservoiris lessdepletedbecauseof the earlierlow drainage rate. If the curves had been plotted againstthe fractional recovery, the curvefor the new theorywould havefallen belowthe other throughout.The irregularities in the curve from the new theory are due to instabilitiesin the methodof calculation. PredictedOil-Steam Ratios The steamconsumptionfor the processesdescribedin this chaptermay be estimatedusingthe equationsdevelopedin Chapter2. In general,steamis required to provide the following: 1. The heat to raise the steamchamberfrom Ta to Ts. 2. The heat required to raise the produced oil from Zn to the production temperature. It may be assumedthat this is the sameas the steamtemperature. This assumptionis somewhatpessimisticbecausethe oil leavingthe systemis actuallybelow the steamtemperature.13 3. The heat lossesto the overburdenabove the steamchamber.These may be calculatedusingequation2.28. 4. The heat in the reservoir beyond the advancing front. An approximate method of evaluatingthis is to assumethat it is equal to the heat loss to the

eat Penetration Along s Percent of Steady B.=8

Drainage

Chap.7

13Itis relatively simple to show that the mixing temperatureof the draining oil is given by (T^ Til/Qs - D : ml@ + L). It seemslikely that the draining condensatewill also intermingle with the oil beyondthe interfaceand reachthis sametemperature.


333

I

I q r l

(

,l

x

d

q r{

I!

ri i

overburden.Alternatively,equation2.48 can be employedusing an average valuefor U. A more accuratemethodinvolvesthe useof the heat-penetration function describedby Butler (1984).It will be found that whicheverof the The heat to the reservoirincreasesrapidly at first as heat penetratesthe side approximatelythe same.

o ! a

The heatlossesfor the unconfinedwell of Figure7.34 areplottedagainsttime in Figure 7.42. In this diagramthe cumulativeheatrequirementsare convertedto dimensionlessvaluesby meansof the following equation:

qi=

u.

a a !g

9 a

c a

E cl

(7.s0)

h2pC(75 - Tp)

In usingthis equation,the value of pC shouldbe for the reservoiror for the overburden, as appropriate.In Figure 7.39 the curve for the cumulativeheat to the chambergivesthe total heatto the chamberplusthe producedoil if the valueof pC is for the fully saturatedreservoir. The curvefor the chambermay alsobe lookedon asa curvefor the cumulative producedoil in appropriateunits. It is the integralof the production-ratecurve. The heat to the reservoirincreasesrapidly at first as heat penetratesthe side of the initial hot plane.The rate of heat supplyto the reservoirdecreases after this initial period and then growsas the extentof the heatedinterfaceincreases. The heat requirementsfor the confinedwell of Figure 7.39 areshownin Figure 7.43 as thinner lines; the thicker lines are for the unconfinedwell and are taken from Figure 7.42. Lines for the confined well deviate starting at the point where the steam chambermeetsthe one growingfrom the neighboringwell. At this point the rates of increaseof the heat loss to the overburdenand to the reservoirdecreasevery

tl3rrr Thrc\c Srstel

markedll' and chamber.Thc increases. A comp ure 7.44. In this fi that is usedto for the confin Effect of Ste

Table7.-lshor performanceI o

=

F t'o o o .9 tr o o tr

E o.s

o

OJ NA F Lrl

F

tr n4 Q)

o ' )p

na

v,z

= :l

DimensionlessTime Figure7.42 Production HeatDistribution RateandCumulative

334


D Chap.7

Avoiding the Stl

d using an average he heat-penetration *'hicheverof the penetratesthe side

o : F 1.0

plottedagainsttime

o o g c

.9

ertedto dimension-

ao c

o.t -E o

(7.s0) 'oir or for the overulative heat to the ril if the valueof pC : for the cumulative rction-ratecurve. penetratesthe side decreases after this ace increases. 9 are shownin Figrfined well and are rt where the steam this point the rates rvoir decreasevery

0

0.5 1.0 Dimensionless Time Figure 7.43 Effect of Well Confinement on Cumulative Heat Requirements Thicker Lines are for Unconfined Systemand Thinner Lines are for Confined System.

markedly and proportionatelymore so than the decreasein the growth of the chamber.The net result is that the thermal efficiencyfor the confined well case increases. A comparisonof the thermal efficiencyfor the two casesis shown in Figure 7.44. In this figure the thermal efficiency representsthe fraction of the steamheat that is usedto heat the steamchamberand product.The efficiencyis much larger for the confinedcase;the effect is very significant. Effect of Steam Pressure Table7.4 showsthe effect of varying the operatingsteampressureon the predicted performance.In eachof the three calculationsshown,the reservoirwasassumedto

I

I

I

o c

)

.Q)

.9

I

Heatin I erburdenl

a-

Confined w:2h

LrJ

-41I

E E 0.4

Unconfined

L Q)

II

,l

B3:8

u.o

F

c)

II

.F

o

o .2

E 1.0

0

bution Drainage

00.5 Dimensionless Time %a t /h2

Chap.7

1

Figure 7.44 Effect of Well Confinement on Thermal Efficiency


33s

TABLE 7,4 Effectof Steam Pressureon the Performanceand ThermalEfficiencyof an Unconfined HorizontalWell Steampressure,MPa Steamtemperature,"C

0.45 148

8.7 0.029 0.37 0.37 4

Yearsto produce 92 m3fm Averagerate, m3/m D Thermal efficiency Oil-steamratio Bz

2.0 213

3.8 0.068 0.49 0.36 8

5.7 272 1A

0.104 0.56 0.34 1,2

(from Butler 1985)

havethe propertiesshown at the bottom of the table;thesepropertiescorrespond to a reservoirsimilar to that at Cold Lake. The effect of operatingat higher steampressuresis to raisethe temperatureof the steamchamber;this allowsthe oil to drain morerapidly.Becauseof the shorter time involvedin the operationat the highestpressure,the thermal efficiencyis also highest,i.e., a smallerfraction of the injectedheat is lost to the overburdenand reservoirbeyondthe steamchamber. There is, however,a counteractingeffect that offsets the improved efficiency at high pressure.This resultsfrom the increasedheatneededto raisethe systemto steamtemperature;this is higher simplybecausethe steamtemperatureis higher. The net result is that the overall oil-steamratios in Table7.4 are almostindependent of the steampressureemployed.There may, however,be significant economic advantagesin operating at higher pressuresbecauseof the faster production that is obtained. Calculationswere basedon following parameters:

3.55MPa (t steadystatc I Cold Lr senso that tl quirementn field. In or& meability'pr field pressun In ttrc I Figure 7.4fl lowed.Durin Figure 7.{6 s ment for c within the rq in Figure 7.{ photograpts ure 7.46 is so the fingering The wavy nI which drew t and is not si It was fr preheatingrl adjustmentit rate and thc 1 mentaldata.

P,""= 2040kgl^'

SteamQuality = 9.7

Tn = 6"C

Crock= 963J/kg"C

H =22m d = 0.35

Cor: 2093Jlkg'C Kot : 1.73J/s m"C

AS, = 9.61

pos : 2400kg/-'

k : 0.5 x 10-12m2 K,", = 1.3 /s m "C

Coa: 837J/kg"C z = 100cs at 99"C

SAGD RESULTSFROM SCALED LABORATORYRESERVOIRMODELS OPERATING AT BOTH HIGH AND LOW PRESSURES Chung and Butler (1989b)describeexperimentalstudiesof the Steam-Assisted verticallaboGravity DrainageProcesswhich were carriedout in two-dimensional ratory scaledmodels.Someof the experimentswere carried out with steampressuresnear to atmosphericand others,using a strongerapparatus,with a pressureof 336


Chap.7

SAGD Res.*ts

Eiencvof an Unconfined 5.7 272

2.4 0.104 0.56 0.34 l2

ropertiescorrespond e the temperatureof rcauseof the shorter malefficiencyis also the overburdenand improved efficiency o raisethe systemto mperatureis higher. are almostindepensignificanteconomic ster production that

3.55 MPa (507 psia). The resultswere comparedwith predictionsfrom the nonsteadystate theory which was describedpreviously (Butler 1985). Cold Lake Bitumen was employedand the experimentalconditionswere chosen so that the value of ,83would be the samein the model as in the field. This requirement meansthat a coarserpacking is required in the model than that in the field. In order to model the samefield conditions it is necessaryto use higher permeability packing for low pressureexperimentsthan for ones carried out at the field pressureand temperature. In the first experimentsa preheatedvertical injection well was employed(see Figure 7.45) and steamwas circulated within the well before production was allowed. During the production period steamwas injected at the top of the reservoir. Figure 7.46 showsphotographsof the steamchamberat various stagesof development for one of the experimentsand Figure 7.47 showsthe position of isotherms within the reservoirfor the sametimes. It is interestingto comparethe photographs in Figure 7.46 for a steamchamberspreadingfrom a central hot well to the initial photographsin Figure 7.15 which show a rising chamber. The interface in Figure 7.46 is stableand advancesin a steadyand systematicmanner as contrastedto the fingering displacementwhich occurs above the rising chamberin Figure 7.15. The wavy nature of the isothermsin Figure 7.47is causedby the computerprogram which drew the contours from the limited temperaturemeasurementinformation and is not significant. It was found necessaryto modify the theory slightly to allow for the reservoir preheatingwhich occurred during the initial steamcirculationperiod. With this adjustmentit was found that the theoretical predictions of both the oil production rate and the position of the interface were in excellentagreementwith the experimentaldata.

/kg'C

;'{

r{

,

i lff

"i

fkg'c I/sm'C kgl^' lkg"C s at 99'C )ELS OPERATING '

y Drainage Chap.7

4

i l-l

kg/m'

the Steam-Assisted nsionalvertical laboout with steampresus,with a pressureof

/, {

HORIZONTAL PRODUCTION WELL

Figure 7.45 PreheatedVertical Steam Injector usedin Model Experiments (from Chung and Butler 1989b)

SAGD Resultsfrom ScaledLaboratoryReservoirModels

*S min.

60 mln*

9S nin.

1P0min.

Figurt 7f Figurc 7.4 Chungend

Figure 7.46 Photographsof SteamChamberDevelopingAbout a PreheatedVertical IniectionWell in a Low PressureExperimentwith a TransparentCell Wall (from Chungand Butler 1989b)

The oil production rate is compared to the theoretical predictions in Figure7.48.Two theoreticalcurvesare shown.The dottedcurve is the theoretical curve which is obtainedif it is assumedthat the reservoiris all at the initial temperatureat the start of production.The solid curve makes an allowancefor the preheatingbeforeproduction.Figure 7.49 shows,for the sameexperiment,a comparisonof the position of the interfaceas determinedfrom photographswith the predictedvalues.The agreementwith the measureddata is satisfactory. In experimentsof this type the oil-steamratiosare far lower than thosewhich would be found in the field becauseof the excessiveheatlossesfrom the largevertical surfacesof the two-dimensionalmodel.It is howeverquite practicableto predict the oil-steamratiosfor the field situationand examplesof this are given in the paper(Chungand Butler 1989b). A similar agreementbetweentheoreticalpredictionsand experimentaldata was alsofound for the high pressureexperiments.Figure 7.50showsa comparison for a high pressurerun. Experimentswere also conductedusing multi-well modelsat both high and pressures and againthe resultswere similar.Figure 7.51showsisothermsfrom low pressure model experimentin which five parallelhorizontalwell pairs were a high representsa two-dimensionalsectionthroughthe reservoir.In The model modeled. the steamwas injectedfrom five injectionwells eachof which theseexperiments was locatedimmediatelyabovea producer. 338


Chap.7

This ga pilot at AG facility consis In the steam wards from tl well in eachp o1:

o, qt

e,200 c o o !t o tt

100

Co5 tr.-r

o

SAGD Reglte

lO min.

20 min.

60 min.

t

T {1

120min.

90 min.

h

tr

Scaled time, years

.r

o

5

300

P.sdicled and expetimental resulls tot a preheated v€rtical inleclor

tt\.oor'

ot t

.15

C E E

a

gt

c

.9 o T' 'loo o

o

..?

200

.i\ i ;

j

.9

o'..

,' \

'

\

cold f.acture

\'..... o

... \ oN'

CI

a

a

o

!

t - \ - - - . _ .

\

o o.o5 E o t -9 (! o(h

2

Time,hours

ity Drainage ChaP.7

ll

This geometricarrangementis similar to that employedin the steamchamber pilot at AOSTRA's UndergroundTest Facility (Edmundset al. 1987).This test facility consistsof tunnelsimmediatelybelow a sectionof the Athabascareservoir. In the steamchamberpilot three pairs of parallel deviatedwells are drilled, upwards from the tunnels and then horizontallyinto the reservoir.The production well in eachpair lies near to the baseof the reservoirand its steaminjectionwell is

etical predictions in urve is the theoretical all at the initial teman allowancefor the re experiment,a comwith the photographs ,atisfactory. ru'er than thosewhich sesfrom the largeverrite practicableto prerf this are given in the

rdelsat both high and showsisothermsfrom izontalwell pairs were roughthe reservoir.In rn wells eachof which

r

Figure 7,47 Temperature Distribution within the Reservoir for the Experiment of Figure 7.42. Isotherms are Labelled in "C. Cell Dimensionsare Marked in cm (from Chung and Butler 1989b)

reheatedVerticalInlall (from Chungand

rnd experimentaldata i0 showsa comparison

H q

l2O min

I

Figure 7.48 Predicted and Experimental Oil Production Rates for a Low Pressure Experiment (from Chung and Butler 1989b)

SAGD Resultsfrom ScaledLaboratoryReservoirModels

339

c

:u 1 t

ft ll

E o +i E

.9 ro o I

Figure 7.49 A Comparisonof the Predicted and ExperimentalPositionsof the Interface (from Chung and Butler 1989b)

51015

Half well spacing,cm

parallel and slightly above.The project was adoptedby an industry steeringcommittee following a proposalmadeby the author to AOSTRA in 1984(Butler 1984). The project has been constructed and operated and it has been very successful; there are plans to developan expandedproject. One of the factors studied in the laboratorywork was the effect of the steam chambersinterfering with each other before they grew to the top of the reservoir. This can happenif the wellsare locatedwith a closehorizontalspacing.The ability to drill closelyspacedwells economicallymight possiblybe the majorjustification for a commercialtunnel-basedbitumen recoveryproject. Projectswith large spacingswould be more economicalusingwells drilled from the surface.la

o 19 zoo G

a

.9 o

a

a aaaa-

.\. t

=

t.\.' a

loo

€

o-.o

o

o 123

Time,hours

Figure 7.50 A Comparison of the Predicted and ExperimentalOil Production Ratesfor a High Pressure Experiment (from Chung and Butler 1989b)

Figurr m e n t .! Separe Steam Chamb Exccp (from (

toThereis doubt as to whether the cost of developingundergroundmine-workingsas a base for the constructionof horizontal wells is economicallyjustifiable. A significant part of the mine cost is the provisionof safeworking conditions,escaperoutesand the like, for the undergroundfacility in the event of a well failure or of a steamor hot oil leak. At the AOSTRA projectthere are

two alternate es refugeroom to br the mine approsurfacewhich ha

Chap.7

Oil Production t

340


10min.

, Comparisonof the Prerrimental Positionsof from Chung and Butler

; {

dustry steeringcomn 1984(Butler 1984). ,€en very successful; e effect of the steam top of the reservoir. I spacing.The ability re major justification pcts with large spacurface.to

I{ {

r

ill

h s n

60 min.

i I i q

I

\ Comparisonof the PreperimentalOil Produca High Pressure rom Chung and Butler

. 90 min. Figure 7.51 TemperatureDistribution for a High Pressure5 Well-pair Experiment. Steamis InjectedJustAbove Each of the Five ProductionWellsThrough SeparateInjectors.The Upper Part of the Figure ShowsFive SeparateRising Steam Chambers.At 60 Minutes These Have CoalescedTo Form a Separate Chamber.By 90 Minutes EssentiallyAll of the Model is Saturatedwith Steam Except for Colder SpotsBetweenthe ProductionWellsand the Ends of the Cell (from Chung and Butler 1989b).

mine-workingsas a base nificant part of the mine :. for the undergroundfaOSTRA project there are

two alternate escaperoutes throughout; these are based upon dual mine shafts. There is also a refugeroom to be occupiedby personneltrappedwithin the mine. A major factor that detractsfrom the mine approachis the substantialimprovementin the cost of drilling horizontalwells from the surfacewhich has occurred during recentyears.

ry Drainage Chap.7

Oil Productionafter StoppingSteam lnjection

341

Scaled time, years

OIL PRODUCTION A

12

200

E o

3

)'a'

) rso

..A^ 6_ A

at S roo

ot

A ; .â

ix_r

. ,,

^ \i^

rL

e---r

" . t ] ] . _ort -a-, - -

.9 o

" a. r .

Aa

a a

[-

a t -''!

3uo o

^

70

Well spacing 11.7 cm

.9 o o.os!

=

,a."o,, ..

"

a.o.!

o

|E

u !

'a'a-rr_e. sO

Time,hours

E

-g o

Figure 7.52 Oil ProductionRatesfor High PressureExperimentswith Varied Horizontal SpacingsBetween AdjacentWells.The Curve for 11.'7cm Spacingis for the Experimentwhich is Depictedin Figure7.47(from Chung and Butler 1989b)

Formatlon lleight 2O m

In the steamof the prodtjt1 this operation the rocks u ith steam is produ boundaries.11 The pru that in a confi mately the tim the adjacent r lowing the ces fore, altho,'gh production ral of 60, 90. and Curvesshor tl bers interming

o .} ,oo

E

Wsll Spacing 67 m

.9 150

(,

!

9 roo

IL

o

.z -g

50

tr

o

4

Time, years

1

i

\

Figure 7.53 PredictedCumulative Oil Productionfor the Field at AOSTRA's UTF Site for VariousHorizontal Well Spacings;FormationHeight 20 m (from Chung and Butler 1989b)

-._Q?tq!9rnce*a|llgjlgamrchimbers occurred in the experiment depicted in Figure7.51.This coalescence did not resultin a reducedrateof productionper well ashad beenfeared.In fact the experimentindicateda higherrate of productionper well after the interferencethan from wells which were more widely separatedfor the sametime in the experiment. Figure7.52showsthe rate of productionas a function of time for variouswell spacings. In eachexperimentit took aboutoneyearof time (scaledto the Athabasca reservoirconditions)for the steamchamberto reachthe top of the reservoir.Howby the solid circlesthe rate was ever for the most closelyspacedwells represented somewhathigher and the peak rate which correspondsto the chambersreaching the top of the reservoirwas achievedearlierthan in the other experiments. The cumulativeproductionfor the Athabascafield conditionswhich would be predictedfrom theseexperimentsis shownin Figure 7.53.This diagramcorrespondsto a steamtemperatureof 200"C (i.e. steamat 1.6 MPa or 232 psia). 342


Chap.7

Recovervof Hg

OIL PRODUCTION AFTERSTOPPINGSTEAM INJECTION In the steam-assisted gravity drainageprocess,it is possible,toward the latter part of the productivelife, to continueproductionof oil without steaminjection.During this operation,the pressurein the steamchamberfalls as the systemcools.Heat in the rockswithin the steamchamberis transferredto waterin the pores,and further steamis produced.This heat is transferredfrom within the steamchamberto the boundaries,whereit heatsthe oil and promotesgravity drainage. The processhas been studiedby Fergusonand Butler (198s).They showed that in a confineddevelopment,the steaminjectionshouldbe stoppedat approximatelythe time that the adjacentsteamchambersmeeteachother halfwaybetween the adjacentwells.Under thesecircumstances, the productionof oil obtainedfollowing the cessationof steaminjectionis approximately507oof that producedbefore, although the rate drops off fairly rapidly. Figure 7.54 showsthe predicted productionrate as a function of time for a Cold Lake reservoirwith half-spacings of 60, 90, and 120m. The rate risesrapidly and rhenfalls off with time. The three curvesshowthe rate predictedif the steamis shut off at the time the steamchambers intermingle (the confinement time).

I ProductionRatesfor .xperiments with al Spacings Between The Curve for 11.7cm LeExperimentwhich is tre 7.47 (from Chung DI

;

tl 0

200

5

150

;u ll

rt

IJ F

:dictedCumulativeOil he Field at AOSTRA's rious Horizontal Well rtion Height 20 m (from :r 1989b)

F r{

(r 100 z I

I

\w= 120m 90\

F

:)

50

E

o12 16 YEARS

)erimentdepictedin f productionper well ateof productionper widely separatedfor

20

y Drainage Chap.7

28

o

o.z+ Q E

W=60m

= o.20 lrj

time for variouswell rledto the Athabasca f the reservoir.HowI circlesthe rate was e chambersreaching r experiments. rditionswhich would This diagramcorrea or 232psia).

?4

Figure 7.54 Predictedproduction Rate for ParallelHorizontal Wells in Cold Lake Reservoir Well Length 1000m; ReservoirHeight 30 m; Injectionto Time of Confinement; ParameterW : Well HalfSpacingin Meters (from Fergusonand Butler 1988)

120

fi o.re J

o o.lz trJ

P o.oe E j o.oe l (J

oo'

8121620 TIME (yeors)

Recoveryof Heavy Oil above Water

?4

28

Figure 7.55 Predicted Oil-Steam Ratio for Parallel Horizontal Wells in Cold Lake Reservoir Reservoir Height 30 m; Injection to Time of Confinement; Parameter W : Well Half-Spacing in Meters (from Ferguson and Butler 1988)

343

$t

ilrt il

fr ll

I

0.3

J

6 9F

IInjectionfl

ifl

E =o . e =Ft )a

STEAMINJECTION OMPo P R E S S U RI E

()

0.1

o

23

75 50 ( % R E C O V E R Y o f m o b i l eo i l )

100

Figure 7.56 Cumulative Oil-Steam Ratio versusRecoveryfor Parallel Horizontal Wells in Cold Lake Reservoir ReservoirHeight 30 m; Parameters: Injection Time as Percentof /., Half-well Spacingin Meters (from Fergusonand Butler 1988)

The cumulative oil-steam ratio is shown for the same three cases in Figure7.55.In eachcase,the COSR increases rapidlyafter the steamis shutoff; oil is producedduring this period without the further consumptionof steam.Higher oil-steamratiosare obtainedwith the closerwell spacingsbecauseof the morerapid production and reducedtime for heat loss. The generallylow level of oil-to-steam ratiosthat are shownin Figure 7.55and 7.56reflect the choiceof conditionsused pressure(10 MPa) ' for the case-in particular, an extremelyhigh steam-injection with its associated extremetemperaturei:fftl. It is thoughtthit muchmore economic OSRswould have beenfound if a lower injectiontemperaturehad been assumed.Nevertheless, similar trendswould be expected. In Figure7.56 the cumulativeoil-steamratio is plottedagainstthe percentrecoveryof mobile oil (the mobile oil lying abovethe productionwells).Curves are shown for the samethree well spacingsand also for the time at which the steam injectionwas stopped.This time is expressedas a percentageof the time of confinement.For eachwell spacing,the overallrecoveryincreases asthe time of steaming increases, and the cumulativeoil-steamratio risesto rather flat maxima in the vicinity of t" = 1; at this point the recoveryof the mobile oil is about75Vo.

o

1(( \ Produlr

ry

>

u

Y,/,/, -
! o60

ha *E

aE -9o != 9Q

>= 3- - 3E boo

e2 o'.

HF oo :c oo = e

.Y;

2e h'F

9H rm

O_

=|€

.10 F EN

The formation of water-in-oil emulsionsis very commonin thermal recoveryoperations.The mechanismof formationof emulsionswithin the reservoirhasbeendiscussedby Jamaluddinand Butler (1988).They considerthat the main causeof emulsionformationwithin the reservoirduring recoveryprocesses which involve steamis the condensationof steamon cooler bitumen surfaces.The tendencyof bitumen to spreadon water surfacescausessmall dropletsof water which are created by the condensation of steamto becomeburied within the bulk of the bitumen. For small water droplets to form it is necessary,becauseof the effect of the small radius of curvature of the droplet on vapour pressure,for the steam to be (i.e. the partial pressureof the water vapor needsto be somewhat supersaturated abovethe vapor pressureof liquid water at the temperatureof the condensate).It is this supersaturation which providesthe driving force for the emulsification.The degreeof supersaturationwhich can be achieveddependsupon how easyit is for water to condenseelsewhere.In particular if the reservoir rock is water-wet then there is considerablewater availablewith a flat surface on which steamcan condensewithout droplet formation. From this reasoningit would be expectedthat emulsionformation would be greatestin circumstances where steamcan contact coolerbitumensurfaceswithout contactingrelativelyflat water surfaces.Jamaluddin and Butler show from a thermodynamic argument that the work required to dispersewaterwithin circular capillariesis lessif the capillariesare oil wet than if they are water-wet. Experimentaldata which are in support of the above ideas have been reported by Chungand Butler (1988and 1989a)as well as by Jamaluddinand Butler (1988).Jamaluddinand Butler showedthat when oil is displacedby steamfrom a stronglyoil-wet packedbed (Teflon beadsor toluene-washed, dried sand)higher ratios of emulsifiedwater to oil were found in the productthan in similar experimentswhich employedwater-wetsand (sandwashedprior to run with detergent). The measuredratiosfor theseexperimentsare shownin Figure 7.67. Measurementsof the emulsified-wateroil ratio were reported by Chung and Butler (1988and 1989a)for the productsfrom scaledSAGD experimentscarried out at both low and high pressures.It was found that more emulsificationwas found when there was a rising steamchamberthan when the steamchamberwas spreading. This is consistentwith the theoreticalideaswhich were describedpreviously sincethere is more opportunity for steamto contactbitumen as the steamfingers rise into the cold reservoir. Figure 7.68showsa comparisonof the emulsified-water oil ratiosfor the products from two companionexperiments;one was carried out with a steampressure of 153kPa (22 psia)and the other with a steampressureof 790 kPa (115psia).In both experimentsthe modelwas saturatedinitially with bitumenand therewas no connatewater.It wasfilled by upwardsflooding of the dry packingand it is likely that the packingwas oil-wet initially. The resultsfrom the high pressureand low pressureruns were very similar. In both, the emulsifiedwater-oil ratio was relatively high during the period when the steamchamberwas rising and then it fell as the steamchamberspreadsidewards.The low initial valuesof the ratio are the reFormationof $O

Emulsionswithin the Reservoir

o o b

o a

3

g

a I

:

E o o -9 a

tr

Fi3rrt p€ruE tained

(c) Figure 7.66 1989)

sult of rhe po experiments The resu run in which I

(d)

Photographs of Experiment Shown in Figure 7.65 (from Yang and Butler

'6 1.0

C

o b 0.8 3

E a E o g

1o

1A

1' 0.6 o '6

WATER-WET .-tI AA

II.

b

ta

+

I

E o.z

o A

o

I

Teflonpacking Toluene-washed sand Detergent-washed

IE

4

Water-saturated sand

E0

t

OIL-WET

G

= o.4 E o

o !

50 100 Time in minutes

t G

150

Figrrc i perirncr menttL Saruret

Figure 7.67 The Effect of SandPretreatmenton the Ratio of EmulsifiedWater to Oil in the Productfrom SAGD Experimentswith Cold Lake Crude Bitumen (from Jamaluddinand Butler 1988)

354

' o .9

Gravity Drainage Steam-Assisted

Chap.7

Well Bore ResB

Steam Grain pressuresize kPa mm o 153 2.0 r 790 0.85

o 0.8

L o (E

=

rr 0.6

o 6

= o.4

E

o

o o.2

o (u

0

2 4 Time in hours Figure 7.68 Ratio of Emulsified Water to Oil in the product from SAGD Experimentswith SteamInjectedJust abovethe production well. Reservoircontained No water at the start of the Experiments(from chung and Butler 19g9a)

sult of the productionof the bitumenwhich waswithin the well at the start of the experiments. The resultsfrom run 1 of Figure7.68 arecomparedwith thosefrom a similar run in which the packinghad a saturationof 12.5%of connatewater at the start

Yang and Butler

l-Ft

I

5 1.0 I

f

E 0.8 o

s =(J 0.6 o b 0.4

I

o =

I

-----=l!r----1r..r

rrrll

. \I

\

i.. .t1

'

r irr-rl!-r-.r.i

.

E 0.2 o

Eo 150

Figure 7.69 Ratio of Emulsified Water to Oil in the Product from SAGD Experimentswith SteamInjectedJustAbove the ProductionWell. In One Experiment the ReservoirwasDry Initially and in the Other It Contained72.5VoWater Saturation(from Chung and Butler 1989a)

Emulsified Water e Crude Bitumen

ity Drainage

24 Time in hours

Chap.7

Well Bore Resistance

1.0 o

+o

0.8

o o =

0.6

Hatschek's Equation u o l l re : 1 - " 1 / 3 wherex is the volume fractionof water

r.

applications/ bore has been, as occurringil

a I

i) Gravitr r has been ii) Flo* of hereis tt iii) The pres to achie the *ell-

(E !,

o

0.4

f

aD (U

q)

=

o.2 0

Figure 7.70 Comparisonof Viscosities of Cold Lake EmulsionsPredictedus-

0

0.2 0.4 0.6 predicted vatue of

0.9

1.0

It ollr e

ing Hatschek's Equationwith Measand Jamaluddin ;'r",Ln*:l(rrom

(seeFigure 7.69).Although the resultsof this secondexperimentshow the same trends the connatewater had alarge effect and much lower levelsof emulsification wereobserved.This is in agreement with the theoreticalideaswhich werediscussed previously.l5 The viscositiesof water-in-oilemulsionsare higherthan thoseof the baseoil. A convenient,approximateequationto predict the viscosityis that of Hatschek (1911);this is given below, tt": tt,/(l-

xll3)

(7.s1)

Ong and Butk slopein the h pressurediffer smallunlessth cold then then An inter sider the well havingrelativ sure gradient( laboratorvrno( simplegeome

coNcLUsroNs

15Inthe experiment with a high permeabilityreservoirlayer below a lower permeabilitylayer that wasdescribedon page348 and illustratedin Figure 7.60,morewater emulsificationand a lower drainagerate were found than in the experimentwith the high permeabilitylayer at the top. It is thoughtthat this differencewascausedby the greatercontactof steamwith bitumenwhich occurred becauseof the underminins effect.

In this chapte This processir near to the bor jectionwells.lt productionratc The proc of the improva tal wells, much operateat satr manceand he[ The proo oils. Although promising indk promisingfield area,havejoin In Athatx is believedtha ning stages. Th steamfloodinr

356

Conclusions

where ;.r,, is the viscosityof the emulsion is the viscosityof the pure oil at the sametemperature Po and x is the volumefraction of water in the emulsion. Emulsionviscositiespredictedfrom this equationare comparedto measuredvalues for Cold Lake crude emulsionsin Figure 7.70.In Chapter8, Figure 8.25,which is taken from Chung and Butler 1989,showsmeasuredvaluesof viscositiesof Cold Lake crude emulsionsas a function of temoerature. WELL BORERESISTANCE Although in the analysisin this chapterit is assumedthat the pressurewithin the horizontalproductionwell is constantthere is a need to considerthis in practical


Chap.7

applications.An analysisof the effect of pressuredrop along the horizontal well bore hasbeendescribedby Ong and Butler (1989).They consideredthree processes as occurringin series:

Comparisonof Viscosities EmulsionsPredictedus's Equationwith Measfrom Jamaluddinand

:iment show the same evelsof emulsification ; whichwerediscussed

i) Gravity drainagearound the steamchamber.The rate at which this occurs hasbeendiscussedpreviously. ii) Flow of oil from below the chamberto the productionwell. The resistance here is that due to the radial convergingflow. iii) The pressuredrop along the length of the well bore. The pressuregradient to achievethis increasesfrom zero at one end to a maximum at the outlet of the well. . ong and Butler show that the effect of the well bore pressuredrop is to causea slopein the bottom of the steamchamberalongthe well. This slopereflectsthe pressuredifferencealongthe well. In practicalfield situationsthe effect is relatively small unlessthe oil viscositywithin the well is high becauseit is cold. If the well is cold then there is an advantagein heatingit by circulatingsteamor otherwise. An interestingfinding in their paperis that it is particularlyimportantto consider the well bore pressuredrop in three-dimensionalscaledlaboratorymodels having relatively long horizontal wells.A well scaledto have the samerelative pressure gradient(measuredas the slopeof the bottom of the steamchamber)in the laboratorymodel should have a diameter larger than that which would come from simplegeometricscaling.

emperature ,ion. 'ed to measuredvalues , Figure 8.25,which is of viscositiesof Cold

[e pressurewithin the rsiderthis in practical a lower permeabilitylayer emulsification and a lower bility layerat the top. It is th bitumenwhichoccurred ity Drainage

Chap. 7

l

rl

rl

1 'i

1 {l

r thoseof the baseoil. y is that of Hatschek

(7.s1)

r

4

il f

CONCLUSIONS

,l

In this chapterthe SteamAssistedGravity DrainageProcesshas been described. This processinvolvesthe use of one or more horizontalproductionwells located near to the bottom of the reservoirwith steamintroducedabovefrom separateinjection wells. It hasbeen shown that suchan arrangementcan lead to satisfactory productionrateswith good recoveryand oil-to-steamratios. The processis a logicalextensionof conventionalsteamfloodingbut, because of the improvedcontactwith the reservoirwhich is achievedby the useof horizontal wells, much higher ratesper productionwell can be obtained.It is possibleto operateat satisfactoryrateswithout steam-coning.Becauseof this, better conformanceand hencerecoverycan be obtainedthuswith conventionalsteamflooding. The processcan be usedfor the productionof bitumenor conventionalheavy oils. Although extensivefield demonstrationdata are not yet available there are promisingindicationsof success which are in line with expectations.Recentlytwo promisingfield demonstrations, one in Athabascaand one in the Lloydminster area,havejoined the long-standingEssopilots which are at Cold Lake. In AthabascaAOSTRA hasbeentestingthe processat their UTF site and it is believedthat the resultsare successful. An expansionis said to be in the planning stages.The AOSTRA demonstrationis believedto be the most promisingfield steamfloodingoperationthat hasbeenconductedyet in the Athabascafield. Conclusions

357

ff

:

1

The SceptreResourcesprojectin the Tangleflagsfield near Lloydminsteris alsovery promising.Very high productionrateshavebeenobtainedwith reasonable waterto oil ratiosand steamrequirements. The resultsare notablenot only because of the very high production rates (up to 1000B/d or more of oil from a producer which is 420m long)but becausethey are obtainedin a field which, with conventional production,is uneconomicbecauseof excessivewater productionfrom the underlyingaquifer.

BIBLIOGRAPHY Bezernn,G. E. and MaRrrw, I. A., "EssoResources HorizontalHole Projectat Cold Lake," CIM 79-30-10, 30th Annual TechnicalMeetingof the PetroleumSocietyof CIM (1979). ButLEn, R. M.: "New Interpretationof the Meaningof the Exponent"m" in the Gravity DrainageTheory for ContinuouslySteamedWells,"AOSTRA J. of Research,2, 67-71 (1985). ButLen, R.M.,'A SteamChamberPilot for AOSTRA's UndergroundTest Facility," presentedat AOSTRA's UTF-IndustryMeetingin the GlenbowMuseumAuditorium, Calgary (May 8, 1984). BurLER,R. M., 'A New Approachto the Modellingof Steam-Assisted Gravity Drainage," JCPT, 42-51.(May-June 1985). Burr-en,R.M., "Rise of InterferingSteamChambers,"JCPT, Yol.26, No. 3, 70-75 (MayJune 1987). ButLen, R. M. and Perela, G., "TheoreticalEstimationof BreakthroughTime and InstantaneousShapeof SteamFront During VerticalSteamflooding,"AOSTRA J. of Research, Vol. 5, No. 4 (1989),pp.359-382. Burr-en,R. M., McNen, G. S., and Lo, H.Y., "TheoreticalStudieson the Gravity Drainage of Heavy Oil During SteamHeatinE,"Can.l. Chem.Eng., 59: 455-460(August1981). ButLrR, R. M. and SrerHeNs,D. J., "The Gravity Drainageof Steam-Heated Heavy Oil to ParallelHorizontalWells,"JCPT, 90-96 (April-June 1981). Burlen, R.M., SrepHeNs, D.J., and Werss,M., "The VerticalGrowth of SteamChambers in the In-Situ Thermal Recoveryof Heavy Oils," Proc. 30th. Can. Chem. Eng. Conf., 4: 1152-1160,(October 19-22, 1980). Burlrn, R. M. and Yee, C.T., 'A TheoreticalStudyof SteamCondensation in the Presence of Non-Condensable Gasesin PorousSolids,".4OSTR A J. of Research,3, no.1: 1-14 (September1986). ButLen, R. M. and Yee, C.T., 'An ExperimentalStudyof SteamCondensation in the Pressureof Non-Condensable A J. of Research, 3, no. 1,:15-24 Gasesin PorousSolids,",4OSZR (September 1986). CaRlwEt-t-,W.T. and PensoNs,R. L., "Gravity Drainage Theory," Trans.AIME 179, t99-2r1 (1949).

Cnurc. K i{ : A s s i s t e ;( i : , : FourthL \ lT V o l .- 1 :I ; : - : : DrerRrcu. -l ii 935-9li \-; Dvxsrn r. ll T 7978). Eptvruro:.\ R . O i l a n dO : v gar!'(19\Fencusor.F R i

^r e-t -i n_o_-F_- ,-.-' . :- ..

Sept.-O.: . -e G n r n p r r .P . , T - ' : ; Drainagc P: ..: FlarscHrx. E . {

JeueLuoo;r. \ W a t e r - i n - O :I :

Josur, S. D -::.1 D r a i n a g eL ' r : : i

ONc, Ter. S ::.. Drainagc. -r( I Pnars,M.. '.{ (':

SucIANTo.S. i:J with Bott,.,:: \ (March--A.pr:.-" TEnwrlrrcrri. P I perimenta: r:i AIME IJ$. ),-

YeNc, Gurn'. i. .r coverv br S:r.: A n n . T e c h .\ l : i

CnuNc,K. H. and Burr-en,R. M., "GeometricalEffect of SteamInjectionon the Formation of Emulsionsin the Steam-Assisted Gravity DrainageProcess,"JCPT, Yol. 27, No. 1 (January-February 1988). CHuNc,K. H. and BurLEn, R. M., "In-Situ Emulsificationby the Condensation of Steamin (1989). Contactwith Bitumen,"ICPT, Vol. 28, No. 1 (January-February 358

GravityDrainage Steam-Assisted

Chap.7

Bibliography

near Lloydminster is ained with reasonable able not only because I oil from a producer x'hich, with convenproduction from the

r Projectat Cold Lake," iociety of CIM (1979). ent "m" in the Gravity of Research, 2,67-71 ,undTest Facility," prerseumAuditorium, CalstedGravity Drainage," 16, No. 3, 70-75 (MayroughTime and InstanIOSTRA J. of Research, rn the Gravity Drainage ;5-450(August1981). rm-HeatedHeavy Oil to wth of SteamChambers )an. Chem. Eng. Conf,

CHuNc,K.H. and Burlrn, R.M.,'A Theoreticaland ExperimentalStudy of SteamAssistedGravity DrainageProcess,"in R. F. Meyersand E. J. wiggins (Editors),The Fourth UNITAR/UNDP International Conferenceon Heavy Crude and Tar Sands, Vol. 4: In-Situ Recovery,AOSTRA, Edmonton,(1989b),pp. 191-210. Drrrnrcu, J. K., "The Kern River Horizontalwell SteamPilot," spE ReservoirEngineering, 935-944 (August 1988). DyrsrRA, H., "The Predictionof Oil Recoveryby Gravity Drainage,""fp?l 818-830(May 1978). Eorr.ruNos, N. R., WoNc,A., McConrr.recr, M. E. and Succrrr, J.C., "Fourth Annual Heavy Oil and Oil SandsTechnicalSymposium,"Universityof Calgary,February18, 1987,Calgary (1987). FeRcusoN,F. R. S. and Burr-nn, R. M., "Steam-Assisted Gravity DrainageModel Incorporating Energy Recoveryfrom a Cooling SteamChamber,"JCPT, Yol.27, No.5,75-83, Sept.-Oct.,1988. GRInrtN,P. J. and Tnontvnwrorr, P. N., "LaboratoryStudiesof the Steam-Assisted Gravity Drainage Process,"AOSTRA J. of Research,2, no. 4: 197-203(1986). HanscuEr,E., Kolloid-Z.,8, 34 (1911). JeuaLUDDrN,A.K.M. and BurLen, R.M., "Factors Affecting the Formation of Water-in-OilEmulsionsDuring Thermal Recovery,",4OSTRA J. of Research(May, 1988). Josru, S.D. and Tnnnr-relo, C. B., "Laboratory Studies of Thermally Aided Gravity DrainageUsingHorizontalWells,",4OSTRAJ. of Research,2, no. 1: 11-19(1985). ONc, Tne, S. and Burr-pn, R. M., "Wellbore Flow Resistancein Steam-Assisted Gravity Drainage,"JCPT,YoL29, No. 2 (March-April 1990). Pnars,M., 'A CurrentAppraisalof Thermal Recovery,"JPT, \129-1136(August1978). Suct,lNto, S. and BurleR, R. M., "The Productionof ConventionalHeavy Oil Reservoirs with Bottom Water Using Steam-Assisted Gravity Drainage,"JCPT, YoL. 29, No. 2 (March-April 1990). TrnwrLLrceR,P.L., Wrlsny, L. E., Halr-, H. N., Bnroces,P. M., and MorsE,R. A., 'An Experimental and Theoretical Investigationof Gravity Drainage Performance,"Trans. AIME 146,28-53(1951). YaNG,GurHua,and Burlen, R.M., "Effectsof ReservoirHeterogeneities on HeavyOil Recoveryby Steam-Assisted Gravity Drainage,"PaperNo. 89-40-72,presentedat the 40th Ann. Tech.Mtg. of the PetroleumSocietyof CIM (May 28-31 1989).

ensationin the Presence v c h , 3 , n o . 1 : 1 - 1 4( S e p in the Presondensation ?.esearch, 3, no.l: 15-24 ry." Trans. AIME 179, iectionon the Formation " JCPT, Vol. 27, No. 1 .ondensation of Steamin (1989). ty Drainage

Chap.7

Bibliography

359

N

il fll

:if

1 '1

rl 4 d{

u

n

1

fit

t! ll

:

rt

It s.-,..n extended hca I n g e n e r a l ": r

Sfeqm Recovery Equipment

snd Focilifies

INTRODUCTION In this chapterthe equipmentand surfacefacilitiesneededfor thermalrecoveryare discussed-the equipmentfor steamgenerationand steamdistribution,the wells, the facilitiesfor treatingthe crude,and, finally, the processes involvedin treating producedwater to make it suitablefor recyclingto steamgenerators. In a typical steamrecoveryoperation,the volumeof producedwater may be aboutthree to five times largerthan the volumeof producedoil. As a result,tlere are very significantcapital and operatingcostsfor the water-treatingand waterhandlingfacilities.Sincethe performanceand servicefactor of the whole project dependsupon the satisfactoryhandlingof water and generationof steam,lt i, uital to developgood designsand operatingprocedures. For thesereasons,a goodunderstandingof the water reuseprocessis essentialif oil recoveryis to be successful technicallyand economically.

1. Fire tub that are fuel arc flou thr In scrme throu€:h 2. \t\ater tr

f lou ins the outs

Fire tub i n s t a l l a t i o n :e are limited rr fire tube:. F i g u r e: i n 1 8 7 7 .\ a t u inclined tulre from the liqu boiler in orJe N{odern shown in Fre water tube tr

STEAM GENERATION The main roots of the industrialrevolutionare to be found in the discoveryand developmentof practicalsteampower in England at the end of the seventeenth century. Early steamboilerssuchas the one shownin Figure8.1wereof simpledesign and were limited in capacityand pressureby the small sizeof the steelplatesavailable.There were many explosionsand accidentsas a result of improperoperating practicesand design.

360

Steam Gerpra

It soon becameapparentto boiler designersthat it was necessaryto provide extendedheat-transfersurface area in order to build boilers of increasedcapacity. In general,two approacheswere followed:

ment

r thermalrecoveryare listribution, the wells, esinvolvedin treating nerators. roducedwater may be Loil. As a result,there er-treatingand waterr of the whole project ion of steam,it is vital reasons,a goodunder:ry is to be successful

1. Fire tubes.in which combustionoccurswithin the insideof one or moretubes belowthe surfaceof the waterin the boilervessel.Air and that are submerged fuel are introduced into one end of thesetubes,and the combustionproducts flow throughthe remainderof the tube; this providesadditionalheattransfer. \n sornecases,ttre convectirretreat tranrsteris extended,b; passirrgthe t\ue gas through the boiler within a set of parallelsmallerdiametertubes. 2. Water tube boilers in which the water is heatedand boiled with the water flowing insidetubesthat are exposedto the fire and combustionproductson the outside. Fire tube boilersare usedfor smallerinstallations,particularlylower-pressure installationsand for portableusessuchas railway locomotives.Their applications are limited by the needto build a high-pressure vessellargeenoughto contain the fire tubes. Figure8.2 showsa crosssectionof an earlywatertube boiler,which wasbuilt in 1877.Natural convectioncausedwater to circulateto the bottom of the bank of inclined tubesand the boiling mixture of water and steamto rise. Steamseparated from the liquid water in the upper drum. Blowdownwaterwas removedfrom the boiler in order to limit the concentrationof dissolvedsolids. Modern, large,high-pressure boilers are of the water tube type. The boiler shown in Figure 8.3 showsthe type of constructionemployedin a large,modern water tube boiler. This boiler seneratessteamwithin vertical tubesthat form the

Lin the discoveryand nd of the seventeenth rn'ereof simpledesign f the steelplatesavailof improperoperating

Figure 8.1 HaycockBoiler (1720) (from Babcockand Wilcox 1972)

Steam Generation

361

a t a

o c E o

Fi3rn $'ikq Figure 8.2 coal-Fired, Babcock and wircox water Tube Boiler (1g77)(from Babcockand Wilcox 1972)

Tempering

362

Figure 8.3 Stirling Boiler for 925 psi and 900"FSteamTemperature.This Boiler Is Fired By PulverizedCoal. The Walls of the CombustionChamber Are Lined With Water Tubes Which Absorb Radiation from rhe Flames.SensibleHeat from the Combustion Gas Product Is AbsorbedIn ConvectionTube Banks (from Babcock and Wilcox 1972)

Steam RecoveryEquipmentand Facilities

Chap.g

wall of the c steamis supe Heat is ation from tl may be caus atedby purq Water tr and they'can The dir externalll'ry scale repres steam.The q The three up different lera Near th ing point. th is being heat and a largert larger heat flt ference betlr transfer coef As bdU mechanism d form at the b by a layer of 'The

rcq the liquid is gsr are formed colb b e p o s s i b l et o b r of the liquid hrl

Steam Gersl

.

i+ll

o I a!

o

CL

E o

Practical tubeoutletconditions Tubewalltemperature

-ll9!I"-1t-rlYl--i-----i*j-- onQ i \. i -__i/ -lte_d-I'_e_aJjl:u]--j-__-______ Low heatflu5- - -i- - - - - - - - - - - - - - - - - - - - - - -., -\ |

\-

FluidTemperature

100 SteamQuality7o (afterBabcock Figure8,4 BoilerTubeFluidandTubeWallTemperatures and Wilcox1972) 0

1877)(from

ng Boilerfor 925 psi Temperature. This ; PulverizedCoal. CombustionChamith Water Tubes :diationfrom the Heatfrom the Comuct Is AbsorbedIn Banks(from Babcock

Facilities

Chap.8

wall of the combustionchamberand also in an independentbank of tubes.The in a separatesectionof tubes. steamis superheated Heat is transferredthroughthe surfaceof the boiler tubesboth by direct radiation from the fire and by convectionfrom the hot gases.Flow through the tubes may be causedby naturalcirculation,as in the designshownhere,or it may be createdby pumping. Watertube boilersare fired usingany of the commonfuels-gas, oil, or coal; and they can alsobe adaptedfor specialfuels suchas refinerycoke. The diagramin Figure 8.4 showsthe temperaturealonga tube that is heated externallyby a furnace;wateris flowing insidethe tube and boiling.The horizontal scale representsthe cumulativeheat transfer representedby the quality of the steam.The ordinatedepictsthe temperatureof the tube wall and alsoof the fluid. The three upper broken curvesshow the metal temperatureof the tube for three different levelsof heat flux. Near the entranceto the tube, assumingthat the water entersbelow its boiling point, the temperatureof the tube risesalongwith that of the liquid water that is being heated.The temperatureof the tube wall is higher than that of the fluid, and a larger temperaturedifferencebetweenthe wall and the fluid is requiredfor a largerheat flux. At the point wherethe water beginsto boil,r the temperaturedifbecauseof the increasedheatferencebetweenthe wall and the fluid decreases due to the boiling. transfer coefficient that resultsfrom the agitation the tube where the boiling point along As boiling continues,there comesa where separatebubblesof vapor mechanismchangesfrom that of nucleateboiling, the surfacebecomescovered form at the hot surface,to that of film boiling, where by a layer of vapor through which the heat must be transferred.At this point a lThe temperaturegradientat the heatingsurfacecausesthe liquid to boil at the surfacebefore the liquid is generallyheatedto the boiling point. At moderateheat fluxes, the bubblesof vapor that are formed collapseas they rnix with the bulk of the liquid. At extremelyhigh heat fluxes, it would be possibleto havea completefilm of vapor coveringthe inner wall of the tube eventhough the bulk of the liquid has not reachedthe boiling point; this is not a desirablecondition.

Steam Generation

363

I

tr

I I

E

( {

!t

E

H P d

f

r{

o

qt t (!

Practical tubeoutletconditions Tubewalltemoerature

+f

o

CL

E o

F

100 Steam Quality 7o Figure 8,4 Boiler Tube Fluid and Tube Wall Temperatures(after Babcockand Wilcox 1972)

r1877)(from

ng Boilerfor 925psi Temperature. This ; PulverizedCoal. CombustionChamrth Water Tubes rdiationfrom the Heat from the Comuct Is AbsorbedIn Banks(from Babcock

Facilities

Chap.8

wall of the combustionchamberand also in an independentbank of tubes.The in a separatesectionof tubes. steamis superheated Heat is transferredthroughthe surfaceof the boiler tubesboth by direct radiation from the fire and by convectionfrom the hot gases.Flow through the tubes may be causedby naturalcirculation,as in the designshownhere,or it may be createdby pumping. Watertube boilersare fired usingany of the commonfuels-gas, oil, or coal; and they can also be adaptedfor specialfuels such as refinery coke. The diagramin Figure 8.4 showsthe temperaturealonga tube that is heated externallyby a furnace;wateris flowing insidethe tube and boiling.The horizontal scale representsthe cumulativeheat transfer representedby the quality of the steam.The ordinatedepictsthe temperatureof the tube wall and alsoof the fluid. The three upper broken curves show the metal temperatureof the tube for three different levelsof heat flux. Near the entranceto the tube, assumingthat the water entersbelow its boiling point, the temperatureof the tube risesalongwith that of the liquid water that is being heated.The temperatureof the tube wall is higher than that of the fluid, and a larger temperaturedifferencebetweenthe wall and the fluid is requiredfor a largerheat flux. At the point wherethe waterbeginsto boil,l the temperaturedifbecauseof the increasedheatferencebetweenthe wall and the fluid decreases due to the boiling. agitation transfer coefficient that resultsfrom the point the tube where the boiling along As boiling continues,there comesa where separatebubblesof vapor mechanismchangesfrom that of nucleateboiling, the surfacebecomescovered form at the hot surface,to that of film boiling, where by a layer of vapor through which the heat must be transferred.At this point a lThe temperaturegradientat the heatingsurfacecausesthe liquid to boil at the surfacebefore the liquid is generallyheatedto the boiling point. At moderateheat fluxes, the bubblesofvapor that are formed collapseas they rnix with the bulk of the liquid. At extremelyhigh heat f luxes,it would be possibleto havea completefilm of vapor coveringthe inner wall of the tube eventhough the bulk of the liquid has not reachedthe boiling point; this is not a desirablecondition.

Steam Generation

363

much larger temperaturedifference is required to maintain the heat flux: as a result, the temperatureof the boiler tube tends to rise rapidly. The film of vapor forms an insulating blanket through which the heat must be tiansferred. The point wherethe boiling mechanismchangesis known as the departure from nucleatiboiling (DNB). Boilers are normally designedto maintain nucleateboiling within the tubesin which evaporationis occurring. For large heat fluxes, this limits the evaporation per passto low values.In once-throughoil field steamgenerators,the evaporation (in one pass)is higher than in typical drum-type boilerJ,but the heat flux is much lower. The expenseof the additional heat-transfersurfacethat is required is offset by the mechanical and operating simplicity. Typical heat fluxes are given in Table8.1. TABLE8.1 Typical

Heat Flux in kBT

High capacitypower boilers Oil field generators

I

t2 h{r)in RadiantSection g0-190(2)

15- 1g(3) (t)1 kBtu6t'h = 3.1546kWm2. of tube wall. Low_ervalue is for pulverized coal firing and upper value is iT* fuel oil :,1 (Blokh H?]::]""*j"t"a for 1988);seealso Delibert (1987). (3)Based on tube area3l-in. oD on 6-in. spacing(Kerby, Kense,and peacheyr9g4).

The lower flux in oil field steamgeneratorsmakes them much more tolerant to the overheatingcausedby the depositionof scaleon the heatingtubes.Scaledeposits due to such causesas water hardness provide a heat-trinsfer resistance betweenthe wall and the water.The heatflux flowing throughthis resistanceproducesan increasedtemperaturedrop that is proportional to the resistanceuni to the flux' Although this effect is lessin oil field generators,it is still very important to soften the feedwaterto minimize scaleformation. There is, however,considerable toleranceto other dissolvedsolids suchas sodium chloride. The feedwaterflow rate to a steamgeneratormust be kept within a certain range.Low velocities,for a givenfiring rate,resultin excessive vaporization,DNB, and tube failure. On the other side, high feed rates result in low-quality steamand tube erosion. EFFECTOF WATERIMPURITIES The quality of feedwateremployedfor conventionalsteamboilers is frequently .the critical to their operation if corrosion and scale formation are to be avoided. Table8.2 gives specificationsfor feedwaterand for the water within the boiler that have been recommendedby the ASME Committee on Water in Thermal power Systems.These recommendationsare for typical water tube boilers; oil field steam generatorsare much more tolerant becauseof the lower heat flux.

364


Chap.8

TABLEE2

r f

Drum pressure MPat

I | 0.1-2.2 | 2.2-3.2 | 3.242 4.2-5.3 | | 5.3-6.3 | 6.3-7.0 7.0-10.4 | 10.4-13.9 |

tTo converl llt 2Minimumterd regard to silbr 3Atkalinityu aZero in tbccc r amount of tofd treatment uscd

The co When oxyga ferric oxi&; senceof oryl

It is recomn to 3 MPa pr Appr€o in boiler fecd versesolutil poundsdecrt and Mg(OHl heatingsurfi Silica ir depositsco

2A cc are shut do?t a in contact rhl recommendcd 1

Effect of \ir!

e heat flux; as a reThe film of vapor rnsferred.The point zfrom nucleateboillwithin the tubesin rits the evaporation Jrs, the evaporation te heat flux is much is required is offset luxes are given in

iring and uppervalueis r 1984).

TABLE 8,2 RecommendedFeedwaterGuidelinesfor Modern IndustrialWater Tube Boilers for ReliableContinuousOperations(from Kirk-Othmer1978-84) Boiler feedwater

Drum pressure MPal

0.1-2.2 2.21.2 3.24.2 4.2-5.3 5.3-6.3 6.3-'7.0 7.0-10.4 10.4-13.9

Boiler water

lron, ppm Fe

Copper, ppm Cu

Total hardness, ppm CaCO3

Silica ppm SiO2

Total alkalinity2, ppm CaCO3

Specific conductance, pS/cm

0.100 0.050 0.030 0.025 0.020 0.020 0.010 0.010

0.050 0.025 0.020 0.020 0.015 0.015 0.010 0.010

0.300 0.300 0.200 0.200 0.100 0.050 0.000 0.000

150 90 40 30 20 8 2 1

7003 60d 5003 4003 3003 2003 04 04

7000 6000 5000 4000 3000 2000 150 100

1To convert MPa to psi, multiply by 145. 'Minimum level of OH alkalinity in boilers 3ou>

q)

Sa b=

iN6+h\Ot'-0Ocl\ ay G

ojr

i- ii =c z.v S=

.< vo'

uJ to

;0 .q -

dNo$h\Ot'-ooo\

^J

Lli

379

can be made if the temperaturemeasuringtool is at the bottom end of the logging string' The designof the roggingtool is discussedin their paper. when they emproyeda conventional,"stackable,'temperature-rogging tool in which the sensingelementwas containedwithin un op"n sectionin the centerof the tool, it was found that the logs obtained were uuriubt. and dependedon the direction of logging (up or down)lnd on the rate of ,n*"-"rrt of the toor. This was causedby the thermal capacityof the fluid that was urong'uy i;:1f# "ur.i"i

I

CONTROLOF HEAT LOSS IN STEAM.INJECTION WELLS The effect of using the tubing for injection of steam with insulation betweenit and the casingwas.discussedin chaptei z. rnl, is desirable from the point of view of reducingheat losses,providing higher-quality steam at the sandfaie and reducing the mechanicalstresseswittrin thi casing.ih, lutt", is a particularly important factor if it is planned to employexistingwills that are not designedr- irr"iiJ-"peration in a thermal project. The current practicesin reducingwell bore heat losses havebeenreviewedby Meldau (1gSg). In manycases'becauseof the complicationof using a thermalpackerand becauseof the need.tovent gasesup the annulusbetweerithe tubing and the casing during cyclic production,operationwith no insulation has been used; the casing must be designedto operateat the steamtemperature. This approachis common in the Cold Lake area. Someoperatorsinject steamdown the tubing and inject a small amount of gas, either natural gasor nitrogen,down the annulus-in order to preventheat transfer by refluxing in the annrrlus-i.e., by liquid waterboiling on ,t hot tubing and con_ densingon the casing.This has blen done uoth witriuare " tubing and, more recently,with insulatedtubing (Meldau 19gg;Cormier l9g7). Insulationby isolatingthe annuluswith a packerand venting the annulusis often usedas a simpleform of insulation in steamfloodr; tt i, is com-"mon pi"fii." in california. If the annurusis vented,any_steam leaking at the packe.oi.oupiing. passesthrough the annuluswithout condensation. The use of insulatedtrrbingis quite practicalas a meansof savingheat and reducingcasingtemperature.Howevei,it is expensive; costsare of the order of $25 to $35 U.S. per foot (Meldau 19gg). The general arrangement for the use of insulated tubing is shown in Figure8.17. As is shownin Figure 8.18,insulatedtubing is constructed with a hollowwall with the annulusfilled with layersof foil (to ,"iu." radiation losses)and ceramic fiber' The insulation spaceis evacuated.A major problem with earlier versionsof in-sulated tubing was the heat lossthat occurrei uittr" couplings betweensections of the insulatedtube. The probrem is aggravate d by reftixing; this involves the boiling of water in contact with the hot ioupling roitowlo by its condensationon the casing.The heatloss.hasbeengreatlyreoucejuy uning thl couprings*ith ;; lation. Typical constructionis shownin Fisure g.lti. 380


Chap.8

STEATI SELECTIVE

SeveralmeaE specific horia that have bec pack with a o ure 8.19).Thit so the pluggiq The usc of steam duril ARTIFICIALLIFT

Artificial lift i steam prodtd pumping is ru Conveo for lifting cil metal-to-rnet ventional nig practice.The t The c'tt significantpt wear can be t around the pc

tAn inscrt string. It is s..I

Artificial Lift

-^4 ==l

m end of the logging €r. ature-loggingtool in rion in the centerof nd dependedon the :nt of the tool. This wascarried alongby

Thermalpacker joint and expansion

ationbetweenit and the point of view of rdface and reducing rticularly important ;nedfor thermalopwell bore heatlosses rmal packerand berbingand the casing en used;the casing rroachis commonin ;mall amountof gas, revent heat transfer hot tubing and conrbing and, more rexting the annulusis commonpracticein packeror couplings of savingheat and : of the order of 925 ubing is shown in J with a hollowwall losses)and ceramic r earlierversionsof qs betweensections f: this involvesthe its condensationon ;ouplingswith insu-

Facilities

Chap.8

--

Figure 8.17 Use of InsulatedTubing for SteamInjection (after Meldau 1988)

STEAMINJECTION SELECTIVE Severalmeanshave been developedto allow the selectiveinjection of steaminto specifichorizontal layers.Borregales(1977) and Burkill (1977)describemethods that have been usedin Venezuela.These allow the partial pluggingof the gravel pack with a cement material that is forced through a specialport collar (seeFigure 8.19).This equipmentallowsports in a blank sectionof the liner to be opened so the plugging agentcan be squeezedinto the gravel' The use of speciallysized and placedperforationsto allow selectiveinjection of steamdurins a steamfloodis describedby Gates and Brewer (1975). ARTIFICIALLIFT in both steamfloodingand cyclicsteamprojects.In cyclic Artificial lift is necessary steam production, reservoir pressuredrives the fluids up the well initially, but pumping is required when the reservoir pressurefalls' conventional pump jacks with tubing insert pumpst are generally employed for lifting oil from ihermal wells (Peacheyand Nodwell 1981).The pumps have a metal-to-metalpiston sealthat will withstand high temperaturesin placeof the conventional nipple seals.Longer and slower strokes are employed than in normal practice.Thi constructionof a typical pump is shownin the diagramin Figure8.20. The wear of the couplingsusedwith conventionalsuckerrods has presenteda significant problem, partiiularly when pumping deviated wells. The effect of this wear can be reducedby rotating the rods during operation.This spreadsthe wear around the perimeterof the couplingsrather than allowing it to concentrateat one 5An insert pump can be run into the hole as a completeassemblyon the end of the suckerrod string. It is seatedwithin a sealthat is previouslyinstalledin the tubing'

Artificial Lift

381

Buttress c o up li n g

ilti

SrEu SELECTTVE txro rtC Lt8

ragn lltl

Tubing I n s ul a t io n

Insulation

? N

Foil layers Csramicfiber Vacuum

#

I nsert

T H E RM A L P A CK E R

Coupling l i n er

$

I

I

Tubing 27l8"N.80 4 1 / 2 "K - 5 5

Buttress coupling

I

I

I Figure 8.18 Typical InsulatedTubing and Coupling (after Meldau 1988)

tional oil fields the 1500operar

location.The rotationis achievedby a mechanicaldeviceon the pumpjack that imparts a slight rotation during eachstroke. . Another approachto reducingcouplingwear is to usea type of couplingcontaining smallwheelsthat roll on the surfaceof the well tubing.Theseare available with plasticwheelsfor operationbelow250"Fand with steelwheelsand journalsfor high-temperature operation(DDS Calgary). Another meansfor alleviating the problemof suckerrod wear involvesthe use of a continuoussuckerrod without couplings(Corod).6This continuoussuckerrod is suppliedin long lengthsin large-diameter (18-ft)coils, and theseare weldedinto still longerlengthsat the site as the rod is installedinto the well. Figure 8.21illustrates the procedurefor feeding the rod from the transportationreei into the well. Another featureof Corod that reduceswear is that it can be madewith a flattenedratherthan a circular crosssection.If the oval sectionis used,it is chosenso that the flattenedfaceshaveapproximatelythe sameradiusof curvatureasthe tubing in which it is installed;this increasesthe areaof contactand reducesthe wear. Pump maintenanceis a major sourceof expensein many heavyoil projects. Elgert, Chambers,and Suzuki (1989)report that at the Essocyclicsteamprojectin cold Lake, averagepump life was only 200d, as comparedto 1 to 2 y in conven-

Trt

I

oCorod6 is a registeredtrademarkof corod ManufacturingLtd., Nisku, Alberta.

382


Chap.8

ArtificialLifl

SEL€CTIVE S1EAT INJECTION INTO T}€ t'PPEi ZONE

SELECTIV€ STEAM IIIJECTDI

ftro T|€ LOtrenzoilE

Tubing

THERUAL PACX€R

Figure 8,19 SelectiveInjectionof Steam(after Borregales1977)

tional oil fields.Pump repairsand relatedservicework cost $2 million per year for the 1500operatingwells in the project.

rl

rt

rl tf

Sucker rod

'pe of couplingconTheseare available :els and journals for

lt

Tubing

In

tl Plunger

ear involvesthe use ntinuoussuckerrod ese are welded into ll. Figure8.21illusn reel into the well. rc madewith a flatused,it is chosenso urvatureasthe tubd reducesthe wear. r heavy oil projects. ;lic steamprojectin I to 2 y in conven-

ti

Travellingvalve

j

Barrel

Seal Standingvalve

Rising Plunger Standing valve oPen

(u. Alberta.

Fallmg Plunger Travellmg valve oqen

Figure 8.20 Diagrarnof Tubing Insert Pump

Chap,8

ll !l

) pumpjack that im-

Facilities

lr

ArtificialLift

383

In manl th nificantpm T . R -' Californiar in this field the special1 This p

l. Therc carria 2. The sl contiE enced tects tl 3. The p the flu ing in I diamd 4. The di Huskyl prodrr

SURFACEDROD IN REEL

THECORODSYSTEM Figure 8.21 The Instailation of continuous Sucker Rod (courtesy of corod ManufacturingLtd.)

Their analysisof the problemindicatedthat the primary causesof pump failure were related to the followins: Sand flowing into the wells, particularly during flowback and particularly during the first cycle. 2. ScalescontainingCaCO: and SiOzadheringto pump barrels. Theseproblemshave beenalleviatedby thesemeasures: 1. Throttling the productionduring initial blowbackto reducesandproduction. A choke-operatingguideline has been developedfor well operation in which the well chokeis progressively openedas the AP acrossthe chokedecreases. 2. Using chromium-platedpump barrels.These are resistantto corrosion,and scaledoesnot adhereto the smooth surface.It is not practical to use chromium-platedplungersinside chromium-platedbarrelsbeiausethe two hard materials gall. However, Esso has found that plungers with a sprayednickel coating work effectively with chromium-platedbarrels. Steam RecoveryEquipmentand Facilities

Chap.g

The pl conventiond the smalhr 1 the new pul The sa scaleis the c small and is Small amouo slowlyin thc The ch heavyoil wd the producti< valve. After through ancl by forcinga g plungerpiscr shuttingoff tl rising liquid. In the c ductionand tl cally. The lil chamber.Th Anothcr by downholcI ArtificialLift

In many thermal recoveryprojectsthe production of sandwith the oil is a very significant problem.Somereservoirsare particularlyprone to this problem T. R. vonde (1979)describesthe production or o" apt cai canyon crude in California where the averageproduction contains27 wt% of sand.Someproduction in this field has containedas much as70vo.To handlethis, Husky has developed the specialpump shown inFigure 8.22. This pump has the following features: 1. There are two tubing strings.one carriesdiluent to the pump and the other carriesthe diluted productionto the surface. 2. The suckerrod is containedwithin the diluent tubing and doesnot comeinto contactwith the productionstream.This avoidsthe slow fall that is experiencedwhen the rod must move through the viscousproduct, and it also protects the rod and plunger from the abrasiveaction oi the sand. 3. The pump deliversfluid to the surface on the downstroke(most pumps pull the fluid upward;this one pushesit). In order to minimize compressive loading in the suckerrod, a counterweightconsistingof 2900to 3g5bkg of 2-in.diametersteelbars is fastenedto the end of the suckerrod. 4. The diluent flow is controlled by adjustingthe addition rate atthe surface.In Husky's application at cat canyon, the rate is controlled to give a 12. Apr product.

rrtesyof corod

causesof pump fail,ack and particularly rrrels.

uce sandproduction. ll operationin which the chokedecreases. rnt to corrosion,and rcticalto usechromiusethe two hard maith a sprayednickel

d Facilities Chap.8

The pump is consideredsuccessfur and, althoughits cost is more than for a conventionalpump,the extra is saidto be paid for by the reducedmaintenanceand the smallerpower requirement.It is reportedthat, in one lease,installingfive of the new pumps increasedproduction from 200 to 700 B/d of oil. The sandproblemjust describedis an extremecase.At the other end of the scaleis the experienceof Essoat Cold Lake, wheresandproductionis usuallyvery small and is handledby the small sectionof a slottedliner shown in figure 4.19. Smallamountsof fine solids,which are carriedwith the crude,tend to accumulate slowlyin the separatorsand tanks, and theserequireoccasionalcleaning. The chamber-liftprincipleis anothermeansthat has been usedfor pumping heavyoil wells (Elfarr 1976).rnthis technique,a downholechamberconnectedto the production tubing is allowed to fill with produced fluid through a nonreturn valve. After the chamber has filled, the fluid contents are displacedupward through anothernonreturn valve, through the productiontubing, and to the surface by forcing a gasinto the top of the chamber.The gasthus repla-esthe conventional plungerpiston.After the dischargecycle,the pressureis reducedin the chamberby shutting off the gas supply and allowing excessgas to blow to the surfacewith thl rising liquid. In the chamberlift systemtwo tubing strings are employed:one for the production and the other to transfer the lifting gasdown to the pump chamberperiôaically. The lift is assistedby the buoyant effect of the eihauited gas from the chamber.The pump has beenusedsuccessfully in the slocum field in Texas. Another lifting techniquewhich has been tried is the use of pumps operated by downholehydraulicmotors.The CanterralTennecoin situ pilot in Athabascahas ArtificialLift

385

t ll il

ril

'1 tt Il

ri

Itr & ri

CONVENTIONAL DILUENTPUMP

T Y P EY

TYPE X orlr.rfrt lxf,cl|or tI'O PUIP o[Utrr liJ€cllor nto lozlLt

t

a

; =

H0(L0f

sucrtl Portr stRn6

a

HEP unit.r the Hrdrabc An irq the dor.r'nal ductionnbcr the needto t casesthereh very cold*c

J

toLLol s\JcxtR f,00 srnlrc

IMPROVINGWELL I

In cyclicsea Keelirq peratureand cyclicsteam1 As prod surefall as in reservoirpre$ ing this perb nulusof the r afterthis rhe flowing b1 irs gasis separat ratelyup thc the pumping maintained-

sPRil€ CHECTY LVE

I rA't0,!01 tuLL lult

rn'_ TUBIIG

PI,IP SEAI 6 SE L

sPirs Loltf,o vllvE

clfct

2' PUIP EARi€L

HOLLOI PULL R00

XI OILU€

cH^r6€R 0rLrftl PORIS I !/{. PUIP urffL

2tE PUIP tl

ryt'

C ?t rt. ttttfL

E intL c|{cr VALYT

2 Va' PLUIGEi

I

orLUErl PORT

txtt YALY€

gr

sTAr0n6 vrlv€ ttTirxct YALYE

'll' t/

\l t\

ll

0nouL sPtY tozzlt

tr rLrtlrt0t F'Si---\]

Figure 8.22 Husky Diluent Pump (from Vonde 1977)

used Kobe downhole hydraulic pumps; these have also been used at the Suncor World Wide Energy pilot at Fort Kent. Another developmentof interest is the use of drivers, driven by hydraulic cylinders,in place of conventionalcrank-drivenpump jacks. One of theseis the 386


Chap.8

Figrrr I LakcR

lmproving Wel I

IVEIITIONAL IENT PUMP

HEP unit, which is manufacturedin Calgaryby ForemostEngineering;another is the Hydrabeamunit; a third is the curtis Hoover Hydraulic Fump raik. An importantpotentialadvantageof this type of driver is its ability to adjust the down- and upstroketiming independently. This is imporrantwith heavyoil ductionwherethe speedof the downstroke,due to the viscousnatureof the oil iroand the needto avoidcompressive bucklingof the rod, can be a limiting factor.In some casesthere havebeenproblemswith hydraulicallyoperatedpumpingequipment in very cold weather. IMPROVINGWELL PERFORMANCE In cyclic steamprojects,eachwell is subjectedto a seriesof changinqconditions. Keeling (1985)has discussedmeansfor improving well perfor-ma-nce. The temperatureand pressureof a typical well during the productioncycle in the Esso cyclic steamprojectat Cold Lake are shownin Figure g.23. As production continues,the wellheadtemperatureand the bottom hole pressurefall as indicated.Initially, the entire productionfrom the well flows, driven bv reservoirpressure,throughthe tubing and a choketo the productionflowline. Duiing this period, there is considerable steamin the produit. At somepoint the annulusof the well is connectedto the flowline, and the gasflows separitely.Shortly after this the productionpump is seatedand pumpingsiarts.At thii point th" gu, i, flowing by itself to the productionline, and the oit ir ueinglifted by ir,. purnpltn" gasis separatedfrom the producedliquidsat the bottom of the weli and ilowi separately up the annulus.This separationof the gas at the bottom of the well makes the pumping much more efficient and allows a lower bottom hole pressureto be maintained. MPa 9auge

k-

II

t:

ll

ll

WellheadTemperature \i

1

i\

100 Bottom hole - lressure

:acilities Chap.8

:1 tl'

Vent gas compressionstarted

\!i

it \l

riven by hydraulic )ne of theseis the

ru

i{

\i i

,sedat the Suncor

til

!tl

oc

Oil pumpingstarted

j.iT-

\ 2

Vent gas divertedto flow line from annulus

l#-

i il

Flow I

Oitpumped

Time (up to 250 days total) Figure8.23 well remperature andpressure Duringproductioncyclefor cold LakeReservoir (fromKeeling1985) lmprovingWell Performance

387

When the wellheadpressurefalls to the point where gasflow into the producsystemis startedand the tion line is no longerpossible,the vent gas-compression gas is compressedseParatelY. A flow plan showingthe compressionsystememployedby Esso at Cold Lake is shown in Figure 8.24; it is the result of considerabledevelopment. The casinghead product from the wells flows to the vent gas separator.The liquid that separatesis pumped to the product line. The gas from the separatoris cooled,and someof the liquid is condensedin an exchanger.The cooledstream passesto anotherseparator.The liquid from this separatoris pumpedto join the plant product stream. The gas from the secondseparatorpassesto a liquid ring gas,after passingthrough a knockoutdrum to remove the compressed compressor; joins the remainderof the productstream' liquid, the compressor facility such as this Oni of the problemsin designinga well-gas-compression from the wells. feed stream well gas in the is the variability of the amount of of time. as a function plant a in such Figure8.25showsthe flow of gasto be treated compresfor two calls design In order to handle the variability of flow, Esso's sors.Both compressorsare used in the first two cycles;after that, the secondcompressorbecomesan on-line sPare.

the oil. A rar ply oil field r what worksr Whrerof demulsifi

FLUIDS TREATINGPRODUCED The productionfrom steamedheavyoil wells is usually a mixture of hot oil and water with somegas.In many casesit containssubstantialportionsof solidssuchas clay and sand. The production is nearly always emulsified; some of the water is emulsifiedin the oil and someof the oil is emulsifiedin the water. Figure 8.26 showsa typical schemeof the treatmentof the productionfrom thermal recoveryoPerations. A demulsifieichemical is addedbefore the production streamfrom the wells reachesthe treatment plant; this promotesthe separationof dropletsof water from

_

]J J-_--i-

J-

FEE)

+

Pi

c{

fi3rrc and \o

J

lEmulsrm

Figure 8.24 Esso'sCold Lake Casing Scheme(from KeelGas-Compression ing 1985)

388


Chap.8

in-oil emulsionr. oil, and oil-in-re water. Attemp{r l flow by the addrt d i s c u s s e di n C h q allow pipelinc tn

Treating ftodr

# into the producis startedand the Essoat Cold Lake lent.

Figure 8.25 Expected Flow of Vent Vapor in Esso'sCold Lake Commercial Project (from Keeling 1985). The Diagram Showsthe Vapor Flow from a Gas CompressionFacility Serving a Pad of 20 Wells.The Wells are Steamed10 at a Time. The Numbers abovethe PeaksRefer to the Production Cycle.There Are Two Peaksfor Each Cycle, Correspondingto the Two Batchesof l0 Wells Goins on Production

3asseparator.The m the separatoris fhe cooledstream umpedto join the :s to a liquid ring rt drum to remove t.

acility suchas this m from the wells. a functionof time. s for two comprest, the secondcom-

YEARS

the oil. A variety of demulsifierchemicalsis availablefrom the companiesthat supply oil field additives.There is an art in selectingthe bestone, and it is found that what works well in one location is unsuitablein another. Water-in-oil emulsionsare more viscousthan the oil itself,Tand the addition of demulsifier at the wells can reduce the pressuredrop in the gathering lines as

: of hot oil and wars of solids such as me of the water is

t;

it

rll

rt€f .

rfl

,r

re production from

'1 gamfrom the wells plets of water from

r!l I I I

i

ELECTROSTATIC TRFATER

Figure 8.26 ProductionTreatmentUsed by Esso at Cold Lake (after Peachey and Nodwell 1981)

so'sCold Lake Casing n Scheme(from Keel-

Facilities

Chap. 8

TEmulsions are dispersionsof one liquid in another.There is a largedifferencebetweenwaterin-oil emulsions,where the oil is the continuousphaseand which have higher viscositiesthan the oil, and oil-in-wateremulsions,which are lessviscousthan the oil althoughmgre viscousthan the water. Attempts have been made to createoil-in-water emulsionswithin the rbservoirto promote flow by the addition of chemicalssuch as causticsoda to the steam(Doscheret al. 1963).As was discussedin Chapter1, there is currently interestin making concentratedoil-in-wateremulsionsto allow pipeline transportationof very heavyoils.

TreatingProducedFluids

389

well as allowingpremixingof the demulsifier.Premixingalsogivesthe demulsifier more time to act. Figure 8.27 showsthe measuredviscositiesof Cold Lake crude containing various fractions of emulsifiedwater as functions of temperature. At the plant, the streamis cooled and introducedinto a baffled horizontal separatorvessel.The oil, which still containsemulsifiedwater(about3Va),is cooled further and treatedin an electrostatictreaterto producethe final bitumenproduct (0.5%BS & W) and morewater,which can be sentto the recycleplant or disposed of otherwise.Figure 8.28 is a diagramof a modern electrostatictreater.Electrostatictreatersare often combinedwith a fired heater(it is then a "heatertreater") to raise the temperatureof the oil. This is required for conventionalproduction which is cold. In thermal projects,it is more commonfor the oil to be cooled. Electrostatictreatersutilize an electricalfield betweenimmersedelectrode water droplets.The role of the coagrids to promotecoalescence of the suspended lescershouldbe confinedto removingsmallresidualamountsof waterfrom the oil ratherthan largequantities.The separationdependsupon the effect of the electrical field in causinga motionof the waterdroplets.Althoughboth AC and DC fields have been used,AC is more popular becauseit is simpler,althoughperhapsnot quite so effective.More recenttechnologyusesboth AC and DC fieldsin the same unit-the so-calleddual polarity treater such as that shown in Figure 8.28. The electrodesin this equipmentare connectedto the electricalpower in the manner shownin Figure8.28. Electrostaticfields promotedropletcoalescingbecause in the elec1. The waterdropletsbecomepolarizedand tend to alignthemselves trical field, with one sideof the dropletpositivelychargedand the other,negatively charged.There is thus a tendencyfor dropletsto attracteachother; this promotescoalescence. woter/Oilvolumerotio Emulsjfied 0.015.(cold.Lokebitumen) 0.19(R'un1) 2 0.30'(Run2) 0.30'(Run

0.70Gun frun 2)

i

l= o

8 .q

toooo

\ l.-

FEr

Eu

\

:-:\l

Fior natc Sra r rth

2. The dr electr droplc

Temperoture,'C Figure 8.27 Effect of Emulsifiedwater on the viscosity of cold Lake Bitumen (from Chung and Butler 1988)

390


Chap.8

Treatirpftoc

ivesthe demulsifier e crude containing 'e. r baffled horizontal tbout3Vo),is cooled ral bitumenproduct le plant or disposed tic treater.Electrot a "heatertreater") entionalproduction il to be cooled. immersedelectrode fhe role of the coarf water from the oil effectof the electrirh AC and DC fields though perhapsnot C fieldsin the same in Figure8.28.The o*,er in the manner DUAL POLARITYDESALTER

in the elecemselves and the other,negatracteachother: this

WATEROUT

E M U L S I O NI N

Figure 8.28 Diagram of a Dual-Polarity ElectrostaticTreater. Water in Oil Emulsion Feed Is Introduced Beneaththe Inverted Distributor Trough and It Flows Upwards Between the Vertical Electrodes.These are Charged Alternately + and - by the Electrical SystemShown in the Lower Right. At the SameTime an Alternating Voltageis Applied to the Whole Electrode System with Respectto Ground (CourtesyNatco)

2. The dropletstend to have electricalchargesinitially and to migrate in the electricalfield. However,there is a differencein the velocitywith which large dropletsand smaller onesmigrate, and there is an increasedtendencyfor col-

J Lake Bitumen

d Facilities

ChaP.8

TreatingProducedFluids

391

whereI

->ioperotingl

Pr P.

i Ronge I

o

Leming Produced Woter

Leming

L

I

lL'

(J

\20 t./

q)

o (n

o APt Oil

i

\

i

\

0.9

Other r SI units shor For the dropletsintcr by a factr fi

I I I I I I I

\

I I I I

I

200

100 Temperotureo C

0

Figure 8.29 Densitiesof Cold Lake Oil and Water (after Peacheyand Nodwell 1981)

lision. Also, particularly in a DC field, dropletshaving oppositeelectrical chargesmove in oppositedirectionsand tend to collidewith eachother. 3. The electrical field may weaken the film of emulsifier on the surface of the upon collision. droplets.This promotescoalescence If there is too much water suspendedin the oil, then the dropletsmay form chains betweenthe electrodesand producea short circuit. Adjustmentsthat can be made to the operationof an electrostatictreater include the spacingof the grids and the appliedvoltage. The temperatureof operationof the separatingfacilities is important as it controlsthe densitydifferencebetweenthe oil and the water(Figure8.29)and also the oil viscosity(Figure8.30).Both of theseaffect the rate of settling' The rate at which water droplets settle from oil is determinedby Stokeslaw. This may be written for a singlesphericalwater droplet as: 2 R2(p,- p)g (8.1) Y == 9

l"o

where F = 6 As rhc t to increasett maximum,:lt settlingrareBoth fr This is parriq If the addirir be dilutedro r add diluenrlo formance {cq pipelinecorry than 0.5%in r the pipelines

PRODUCTIONTREA

Figure8.31sh at Cat Canlu Somewhatorr which is firred out periodiceX emulsiontre-

3% BS& \r. 6 o

MAKEUPWATERSII

o

Evenif rhefir 100 Temperatureo g

392

200

Figure 830 Viscosity of Cold Lake Oil (after Peacheyand Nodwell 1981)


Chap.8

907a),there ri that has beeo I ume. There ri and cooling p

Maker.p WaE !

where V pw po g po

is falling viscosity of water droplets (m/s) is density of water (kg/m') is density of oil (kg/m3) is accelerationdue to gravity (9.81m/s'z) is oil viscosity(Pa . s)

Other sets of dimensionallyconsistentunits can be used in place of the SI units shown. For the caseof an actual emulsion, the velocity is lower becausethe falling dropletsinterfere with eachother. The effect may be calculatedby multiplying 8.1 by a factor f'5 (Steinour 1944).

sitiesof Cold Lake ter Peacheyand Nod-

oppositeelectrical ith eachother. the surfaceof the ts may form chains s that can be made rf the grids and the ; is important as it igure8.29)and also €ttling. ined by Stokeslaw.

v : +R2(P' P)EF, 9

(8.2)

l"o

rztt-a;and e : volume whereF5 : 62119-i fractionof oil in the emulsion. As the temperatureis raised,the viscosityof the oil decreases, and this tends to increasethe settling velocity. However, the density difference passesthrough a maximum, and there is thus an optimum temperaturethat provides the maximum settlingrate. Both factorsjust mentionedcan be improvedby addingdiluent to the system. This is particularlyvaluableif the densitiesof the bitumenand waterarevery close. If the addition of diluent is practicable,as it is, for example,when the bitumen is to be diluted to transport it eventually through a pipeline, then it is very desirableto add diluent to the mixture before separatingthe water. This will increasethe performance (capacity and/or product quality) of the separation equipment. Most pipeline companiesrequire BS & W to be less than lVo by volume and usually less than 0.5Voin order to prevent corrosion and to reducethe frequencyof pigging in the pipelines. PRODUCTION TREATMENTWITH HIGHSAND PRODUCTION

(8.1)

Figure 8.31showsthe facilities used by Husky in the treatment of their production at Cat Canyon. The averageproduction from this project contains 27 wtTo sand. Somewhatover half of the sand is separatedin the direct-fired desandervessel, which is fitted with a conicalbottom and containshydraulicjets to flush the sand out periodically. The remainder of the sand is removed from the bottom of the emulsiontreater.The producedoil containslessthan 0.2 wtVosand and lessthan 3VoBS &W. MAKEUP WATERSUPPLY

icosity of Cold Lake et and Nodwell1981) I Facilities

ChaP. 8

Even if the fraction of the producedwater that is recycledis very high (greaterthan 907o),there will still be a need for makeup water supply.In the reservoir, the oil that has been removedis normally replacedby injectedwater of an equivalentvolume. There will alsobe a need for additional water in a recoveryprojectfor service and coolingpurposes. MakeupWater Supply

393

Produced U TAX€-UP PROCESS WA'ER I

L._

_.- _ _.

E:

l1 Al 8 ! Gl dl

sl;

tl

iJ

I 1..'-

I

lr r.., !

3 r tl

L[€

4'rEsr Lrir€

PRESCJR€ }tGE

PROqffIOTT

-

-

TDS SS

I UAPHRAil I ArRACTUATED ar-r Sxlll

RJHP

MANIFOLO

so.

HCO, Sulphrd

sio:

wAtER r{JECTrOr,r IO REOUCE LINE

BROOKSZONE A TREATING FAcrury C'ATHERING

\{g CI

Tempcil

PNESSLRE

pH

'tF El..trut ( 2 ) K"ro n a k and Gr (3)Suspende d ol as total organt

*'."B.rI63Ii*

Figure E.31 Processfor TreatingHigh-Sand-ContentHeavy Crude (from Vonde 1979)

Conventional sourcesof water supply such as from undergroundreservoirs, rivers,and lakesare normallyused.However,other water suppliesmay be consideredif thereis a shortageof waterin the area.For exampleSuncor,in their thermal recoveryproject at Bonneville,Alberta, useswastewatereffluent from a nearby municipality. Freshwateris normallytreatedby chlorineoxidation,lime/sodasoftening,filtration, ion exchange,and deaeration(Kloepfer,Card, and Kus i983). RECYCLINGPRODUCEDWATER In the previous section it was shown how the production from steam recovery projects can be separatedinto a marketable oil product and a produced water stream.In manycasesit is desirableto treat this water so that it may be recycledto the steamgenerators. Recyclingwater reducesthe impact of the thermal recoveryprojecton the environment; it not only provides an acceptablemeansfor disposingof the tainted produced-waterstream,but it also greatly reducesthe needfor fresh water. In areas wherewateris scarce,as in California,this reductionin the needfor freshwater is a very significant factor. Another advantageof water recycling is that the heat in the recycledwater reducesthe heat requirementfor steamgenerationsomewhat. 394

Oil and (

Na. K

SANO 8 WATER ' t otJp sYstEM|

{FJ

rcr comt

TABLE 8.5 q

Ca

tfc

't ti tZ

Analy'sesof a correspol once-throrr


Chap.8

The pn dissolvedsol the water at from the lea material. In the t to removeth ica, althoug iron content Thesecondit vent the foru TableE basca.The a impuritiesan The lor peratures of t Essopilot at It seemslikel concentrali becomemore Recycling ftc

ProducedWater Analyses UAKE-UP PROC€SSWAIER ! _-._l-._

-.-

-

-1

Analysesof a typical producedwater samplefrom the Essopilot at cold Lake and a correspondingtreated produced water sample that is suitable for feeding to a once-through,oil field steamgeneratorare given in Table8.5. TABLE 8.5 Compositionof Producedand TreatedRecycleWater{l)at The LemingCold Lake Pilgt(2)

;;l

-E--- - 1 6 II.ER

^-\. :0 clflr.l

I F

i:

srmra I I rx ---te ll

---r

:tl Fl ;

o:i

IU :F

_15

m Vonde 1979)

dergroundreservoirs, rpliesmay be considncor,in their thermal :luent from a nearby

UNTREATED

ppm

TREATED

Oil and Grease 5,000-10,000 0-100(3) TDS 4,000-10,000 4,000-10,000 SS 10-120 0-5 Ca 4M0 0-1 Mg 4-8 0-1 Na, K 1,000-4,000 1,00H,000 CI 2,00M,000 2,000-6,000 SO+ 4U200 4V200 HCO: 10H00 0-r0 Sulphide 10+0 5-10 sio2 150-300 15-30 Temperature("C) 80=90 80-90 pH 7-8 8-9 (l)From ElectrostaticTreatersand Water RecvclePilot (2)Konak and Grisard(1979). (3)Suspended oil (usuallynear 0). There are alsoabout250 ppm dissolvedorganicmaterialsmeasured as total organic carbon (TOC\.

ery projecton the enposingof the tainted r freshwater.In areas reedfor freshwater is ng is that the heat in nerationsomewhat.

The producedwater containsrelativelylarge quantitiesof dispersedoil. The dissolvedsolidsare largelysodium chloride,arisingfrom the reservoirwater, but the water also containsappreciableamountsof hardnesssalts and silica, arising from the leachingof the reservoirrocks.There is alsosubstantialdissolvedorganic material. In the treatmentof this materialfor feed for steamgeneration,it is necessary to removethe suspended oil and the hardness.It is alsodesirableto reducethe silica, althoughconsiderablesilica can be toleratedif the calcium,magnesium,and iron contentsare kept low and if the wateris alkalinewhenit is fed to the generator. Theseconditionswill tend to keep the silica in solutionas sodiumsilicateand prevent the formationof insolubleiron, magnesium,and calciumsilicatescales. Table8.6 showsan analysisof producedwater from the Texacopilot in Athabasca.The analysisis similar to that from Cold Lake but the concentrations of the impuritiesare lower. The lower silica contentmay reflect the lower solubilityof silica at the temperaturesof the steamedreservoir;the Texacopilot is at a shallowerdepththan the Essopilot at Cold Lake, and one would expectthe injectionpressuresto be lower. It seemslikely that the lower levelof dissolvedsolids(salt)arisesbecauseof a lower concentrationin the reservoirconnatewater-possibly the original seawaterhas becomemore diluted by surfacewater.

d Facilities Chap.8

RecyclingProducedWater

ne,/soda softening,filus 1983).

trom steamrecovery rd a produced water it maybe recycledto

395

l

:l

ii

TABLE 8.6 ProducedWater Analysisfrom TexacoSteamfloodingPilot Near McMurrayAthabasca Qu"ntitutiu" AnulysesofDissolved

u Ptodrr.tion Wut"t"'

Sodium Potassium Calcium Magnesium Chloride Sulfate Aluminium Iron Silicon

529 11 56 20 820 80 E

l5'C is equivalent

6 OYTAIA SATO - FFP ) a 8ER€A sAtDsroilE-LorGTUaE a orTAw sANo-Lorc tuBE

iqi

o//

ot-

-/" ooto

ts 3: E F

J3 €o

i9

tt

,/

l- a i J

/o

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or/"

z f6

=G q-j
Combustion&

/ z- Evooorotion

NORMAL

Laboratory F Figure 9.33 Effect of Water Addition

Normal wet combustionis shown by the two diagramsin the upper right part of the figure. Steampassesthrough the front and later condenses.The steammoves oil aheadof the combustionfront as well as movinq heat. There are thus two beneficial effects:

Figure9.35sh was injected ir were measutca

o The rocks are preheatedbefore the combustionfront reachesthem, and this tends to increasethe temperature of the combustion front ([ is higher in equation9.3). o The steamreducesthe concentrationof residualoil remainingin the path of the advancingcombustionfront. This resultsin a substantialreduction in the fuel concentrationand reducesthe air required to burn through a given volume of reservoir.This effect tends to lower the temperatureof the combustion front (Ifzis lower in equation9.3). Also, becauseof the displacementcausedby the steamaheadof the front, it may not be necessaryto burn all the way through the reservoirin order to achieveeffective recovery.In the extreme,wet combustioncan be looked upon as a meansof generatingsteam within the reservoir rather than in a surface steam generator. When sufficient fuel has beenburned to generatethe steamrequired for the recovery, then the processcan be terminated,leavingthe remainingresidualoil uncoked and unburned. The lower left diagramsin Figure 9.33 show the condition obtained if the amount of water is increasedto the point where the evaporationfront just trails the combustionfront. [n this condition the maximum amount of steam is generated without liquid water reachingthe combustionzone. The combustionzone is not itself being cooled by the direct evaporationof water. The steamcondensationzone 4M

ln Situ Combustion

Chap.9

I

u

Itt.

0 I'[ut'l Latil lll Wet Comhstb

rStron nsotion l-

T] gJ, OUENCHEDI

extendsfar beyondthe combustionfront, and the fuel laydownis decreasedbecause of the effect of the steamin reducingthe residual.Essentiallyall the oxygenis consumedin the high-temperaturecombustionprocessat the front. lf the water-to-airratio is increasedstill further, the situationshown in the diagramin the lower right-handcorneris produced.Now liquid watercan enterthe combustionzone, and this zone is cooled below the levels found with lesswater. Someunburnedfuel is left behind. In this condition there can be excessivelowtemperaturecombustionand production of viscoustars. When someliquid water entersthe combustionzone and vaporizescompletely, the processis describedaspartially quenchedcombustion.At higher water-to-air ratios, liquid water passesright through the oxidation zone, and the operationis describedasquenchedcombustion,or superwetcombustion. Figure 9.34 showstypical temperatureand saturationprofiles for a normal wet combustionprocess.It may be comparedwith the similar diagramgiven earlier for dry combustion(Figure9.1). Laboratory Results

-91-

r the upper right part ses.The steammoves re are thus two bene-

Figure9.35showstemperatures measuredin a combustiontube run in which water was injectedinto the air feed partway through the experiment.The temperatures were measuredby a seriesof thermocouplesplaced along the path of the combus-

nchesthem, and this front (4 is higher in rainingin the path of ntial reductionin the through a given volature of the combusd of the front, it may rrderto achieveeffecI upon as a meansof iace steam generator. equiredfor the recovg residualoil uncoked

t E

t

'-6

E ig

t

E 3 o

lition obtainedif the on front just trails the rf steam is generated bustionzone is not itrm condensationzone bmbustion

Chap.9

s

t

Figure 9.34 Temperature and Saturation Profiles for Wet Combustion (from Latil 1980) Wet Combustion

445

Rua !2 l,. t.ot raL. |.Fd}l

I I

llrmni .rala?

d cailnrara lalach-

r) .a

I ta a

t

lraa.tn|.|

Figure 935 Tcq (from Burger ll t

Figure 9.35 Temperaturesas a Function of Time for Various Locations along CombustionTube (from Burger and Sahuquet1973)

tion. As the combustionfront approachedeachmeasurementpoint, the temperature increased,reacheda maximum,and then fell as the front passedalong. Severalfeaturesshouldbe noticed: o The width of the high-temperaturezone decreasedmarkedly when the water was added. o Water addition did not greatly affect the peak temperature.Presumably,the two effectsof water on the combustionfront temperaturethat were discussed previouslyalmost compensatedeachother. o A distinct steamplateauformed when the water was added. The sametemperaturemeasurements are plotted as instantaneoustemperatureprofiles in Figure 9.36, and the sameobservationscan be made from an examination of this diagram. Figure 9.37showsanalytical data for the producedgasfrom the sameexperiment. The gas composition showed no change as a result of the water addition. This indicatesthat the fuel was of the samecompositionwith water addition as it waswithout it. Although the composition of the fuel did not changewith the addition of water,there was a large changein the quantity of fuel. This is shown in Figure 9.38, wherethe positionsof the combustionfront, the condensationfront, and the vaporization front are plotted againsttime. At the point where the water was added,the 46

In Situ Combustion

Chap,9

I

i

I

E

IT Figure 937 Cq Sahuquet1973) Wet Comh.stin

lnr.rrrer

rns along

l. gll

Figure 9.36 Temperatures as a Function of the Distance along the CombustionTube (from Burger and Sahuquet1973)

fnt, the temperature ed along. edly when the water ure. Presumably,the ) that were discussed led. oustemperatureprotom an examination om the sameexperii the water addition. t water addition as it with the addition of rhownin Figure9.38, front, and the vaporiwaterwas added,the ornbustion Chap.9

Tha.lnll Figure 937 Composition of Produced Gas from CombustionTube (from Burger and Sahuquet1973) Wet Combustion

47

Brsrnninsof I rotrinircfion I

I

|

tlt

t50 E ct a c o

li

roo

o

o o-

R u n8 2

// coodrnrolion fmnf

ll

o E o c .9

i

/ / / /

comburlion fnonl voporirotion front

and materia the rocks ld addedis abo of air). The ct Chiu (l988f. the poresbd tion zoneby mum $aterestimatedfn pies 8ff2 of t

wherc/ I I

I

I

//'/

Timr,houn 60

70

Figure 9.38 Effect of WaterAddition upon the Velocitiesof the Condensation, Combustion,and VaporizationFronts (from Burger and Sahuquet1972)

condensationfront acceleratedas more heat was transported ahead of the front. The additional steamhad the effect of reducingthe residualoil left in the path of the combustionfront, and, as this decreasedthe availablefuel, the burn could movefastereventhough the air rate was kept constant. Similar resultshave been reported by Josephand Pusch(1980)for a field pilot studyin the Bellevuefield in Louisiana.The test, carriedout by Cities Serviceinfive-spotpatterns.One of thesepatternswas operateddry volvedtwo side-by-side and the other, wet. Someresultsfrom this paperare shownin Figure 9.39,where the heated reservoirvolumes are comparedas a function of the volume of air injected.[t wasconcludedthat with wet combustion,higherrecoveriesof oil could be expectedbecauseof better volumetric sweep,that the air volume required to processa given reservoirvolume was reducedby 63%, and that lesstime would be required becauseof the lower air requirement.

Chiu also ce steamzoneplus the hea jectedwater percentageo so doesthis percentage b

I

:

6 3

o I o o

o

Water-to-Air Ratio

!

The ratio of water to air that shouldbe usedin wet combustiondependsupon such factorsas the fuel concentration,the water content alreadypresentin the reservoir, and the possibility of water intruding into the combustionregion from outside the pattern. In principle, the amount of water to be addedcan be calculatedfrom heat

M8

In Situ Combustion

Chap.9

e

Wet Cornhrsl

Brglnningof woftn injtction

I

I ! !

R u n8 2

,l

t50 E a, , C

condanrction fr!|rf

o c

conburlion front voporirotion fFonf

o

o c .9

and materi the rocksI addedi; ah of airr. The c Chiu r l9EE the poresh tion zone ! mum \r'ate estimatedfi pies 8{r? o{

roo

6 o

ii/

o

xherc

/ Timl,hounr 40

50

60

70

{)

Figure 9.38 Effect of WaterAddition upon the Velocitiesof the Condensation, Combustion,and VaporizationFronts (from Burger and Sahuquet1972)

condensationfront acceleratedas more heat was transported ahead of the front. The additionalsteamhad the effect of reducingthe residualoil left in the path of the combustionfront, and, as this decreasedthe availablefuel, the burn could movefastereventhough the air rate was kept constant. Similar resultshavebeenreportedby Josephand Pusch(1980)for a field pilot studyin the Bellevuefield in Louisiana.The test,carriedout by Cities Serviceinvolvedtwo side-by-side five-spotpatterns.One of thesepatternswas operateddry and the other, wet. Someresultsfrom this paper are shownin Figure 9.39,where the heatedreservoirvolumesare comparedas a function of the volume of air injected.It wasconcludedthat with wet combustion,higherrecoveriesof oil could be expectedbecauseof better volumetricsweep,that the air volume requiredto processa given reservoirvolumewasreducedby 63Vo,and that lesstime would be requiredbecauseof the lower air requirement.

Chiu also c steam zon€. plus the trca jected$atcf percenlage ( so does thb percentaeet

t I

Water-to-Air Ratio

t

The ratio of water to air that shouldbe usedin wet combustiondependsupon such factorsas the fuel concentration,the water content alreadypresentin the reservoir, and the possibilityof water intruding into the combustionregionfrom outsidethe pattern. In principle, the amount of water to be addedcan be calculatedfrom heat

448

In Situ Combustion

Chap.9

-

Wet Comhrst

and material balancesusing the objectiveof removingmost of the excessheat from the rocks left behind the combustionfront. In many cases,the amount of water addedis about200Io 250B per million SCF of air (0.92to 1.15lb waterper pound of air). The choice of the water-to-air ratio to be employed has been discussedby Chiu (1988).If low water-additionratesare employed,the addedwaterpartially fills the poresbehind the combustionfront and doesnot removeheat from the combustion zone by forming steamto condensebeyondthe combustionfront. The minimum water-to-air ratio required for water to be available for evaporationcan be estimatedfrom the followingequation;this assumesthat, at the limit, water occupies 80Voof the pore volume behind the front.

n-"= uffi6 whereR,o Ro, 6 Bs pw

Condensation, | 1972)

aheadof the front. ril left in the path of uel, the burn could 1980)for a field pilot by Cities Serviceinrnswas operateddry n Figure 9.39,where he volumeof air inveriesof oil could be rme required to pro:sstime would be re-

(chiu 1e88)

(e.s)

is the massof water per unit volumeof air is the volumeof air requiredto burn a unit volumeof reservoir is the porosity is the formation volume factor of the air behind the front is the densityof water.

Chiu also calculatesthe heat carried forward from the combustionfront into the steamzone.This is equalto the heatin the dry gasat the combustiontemperature plus the heat in the watervapor-both the vapor from the vaporizationof the injectedwater and that from the water formed by combustion.He expressesthis as a percentageof the heat of combustion.As the quantity of injected water increases, so doesthis percentage. At the limiting conditionfor normalwet combustion,this percentagebecomeslffiVo.This is an absoluteupper limit. Chiu recommendsthat

1o' 30

I

o

'o

F

= o U F u

t0

o

: F

n dependsupon such :sentin the reservoir. lion from outsidethe calculatedfrom heat

cmbustion

Chap.9

U

UMSC'

lo 20 CUMULATIVE AIR IT{JECTEO

Wet Combustion

Figure 9.39 Comparisonof Heated Volumes for Side-by-SideWet and Dry Five-SpotPatterns(from Josephand Pusch 1980)

449

the percentageof the heat of combustioncarried forward to the steamzone should not exceed85% and that combustiontube experimentsshould be conductedto showthat this is workablefor a particular reservoirsituation. Figures9.40 and 9.41show the resultsof Chiu'scalculationsof the water-toair ratio requiredto carry variouspercentages of the heat of combustionforward for typical reservoirconditions.Figure 9.40showsthe water-oilratio as a function of the air requirementfor air ISC. Figure 9.41showsthe effect of using enriched air for a particularreservoir. IN SITU COMBUSTIONIN TAR SANDS A processfor utilizing in situ combustionwithin tar sanddepositsmust overcome two fundamentalobstacles: o There is little or no initial injectivity in tar sanddeposits. o The low volatility of bitumen, togetherwith its asphalticnature, makesthe fuel depositionload very high. The depositioncan amountto Z to 3 lbft3 of reservoir,whereast lb/ft3 would be sufficientto raisethe reservoirto 500'F. There havebeensomeattemptsto producebitumenfrom tar sandsby in situ combustionwithout prior heating.AMOCO operatedseveralpilots at GregoireLake in Athabasca.Their processis describedin a paperby Jenkinsand Kirkpatrick (1979). lt involvedthe injectionof air into wellslocatedwithin invertedfive-spots.The formation was ignited by first injectingsteamand then air. The processwas carried out in three phases. In the first phaseof the process,the objectivewas to heat the reservoirby combustion.Injection was continueduntil the combustionfront approachedthe productionwells.Relativelylittle bitumenwasproducedduring this phase. In the secondphase,air injectionwas stoppedand the reservoirpressurewas loweredby allowing productionat the productionwells. It was thought that this productionwasassistedby the flashingof connatewaterto steamwithin the heated reservoir.

'i\*: \NIi. l\

-€ g16 v

kt

SupaNot Combudion

3oo

E

200

ai

t

: 6

s

"."-"t :

,

i Dry Comburrion

;

loo

i , :

: I

r6{,

200

250

300

m aeouneuerr h.(sn/m'l

450

360

400

Figure 9.40 Effect of Air Requirementupon the Water-Air Ratio Requiredto Achieve Various Percentages of Carry Forward of Heat of Combustion.The ParameterIs the Percentof Heat of CombustionThat Is Carried Forward to the SteamZone (from Chiu 1988). In Situ Combustion

Chap. 9

In the f producebitu Apart I information , AOSTRA. h When r reservoir.tlt may alrea$tion, combud front advanc front adranc may fall. Lr from the nm in the cased tial availablc This pn usingin situ meansof pro cyclic steamr nomic produ ing the reser This co Companvin 11.5'APIcrr Westernpill waspredicte than 30cZ.Tl ing designprt cessful.and i possibleb1 o as high as tlu was conclud

In Situ Corrb.

eam zone should be conducted to ; of the water-tonbustion forward rtio as a function rf using enriched

1200

ro

5 E. t !

|m c

l(n !o0

G E

aoo

2

m

is must overcome

o

m

,ro

60

tO

oxtcCl{ Mou PEaCEiIT

ature,makesthe to 2 to 3 lbft3 of :servoirto 500'F. ls by in situ comGregoireLake in ,irkpatrick(1979). !'e-spots. The forlcesswas carried : the reservoirby t approachedthe his phase. 'l'oir pressurewas thought that this *'ithin the heated

:t of Air I the Water-Air Ratio :r e Various rr.r Forwardof Heat ne ParameterIs the iCombustionThat Is o the SteamZone

x.rstion

Chap. 9

=I

E

+ o ()o

g

Figure 9.41 Effect of OxygenUpon the Water-Air Ratio Requiredto Carry ForwardVarious Percentages of the Heat of Combustion.The ParameterIs the Percentof Heat of Combustion That Is Carried Forward to the Steam Zone (from Chiu 1988)

In the final stageof the process,air and waterwereinjectedsimultaneously to producebitumenby wet combustion. Apart from the paper just referred to, there appearsto be little published information on the AMOCO project. An expandedpilot was built jointly with AOSTRA, but it was apparentlyunsuccessful. When in situ combustionoccurs in an initially cold, bitumen-containing reservoir,the gas presumablyflows along relativelythin permeablepaths.These may alreadyexist in the reservoir,or they may be formedby fracturing.With ignition, combustioncan occur alongthe surfacesof thesepaths,with a combustion front advancingat right anglesto the direction of flow. However,as the combustion front advances, the supplyof air to it will tend to decline,and the front temperature may fall. Under these circumstances,low-temperatureoxidation may take over from the normalcombustionprocesswith little mobilizationof the oil. Also, just as in the caseof steamfloodingalonga fracture,there may be little pressuredifferential availableto drive the oil toward the producer. This problemmay be overcomeby preheatingthe reservoirwith steambefore using in situ combustion.It appearsto be practicalto use in situ combustionas a meansof producingadditionaloil from a reservoirthat has been producedusing cyclic steamstimulation.Cyclic steamingcan thus serveas a meansfor the economicproductionof perhaps15%of the oil in placeaswell as a meansfor preheating the reservoirto allow the productionof further oil by combustion. This combination of processeswas pioneeredby the Chanslor-WesternOil Companyin the Midway Sunsetfield in California. This reservoircontains an 11.5"API crude oil, which can be producedby cyclic steaming.In the ChanslorWesternpilot, about I5Voof the oil in placewas producedby cyclic steaming,and it waspredictedthat the ultimate recoverywithout combustionwould havebeenless than 30Vo.The projectwas convertedto dry in situ combustionusing the engineering designproceduresdescribedby Nelsonand McNeil (1961).The project was successful,and it was concludedthat oil was producedmore economicallythan was possibleby continuing the cyclic steamrecovery.The production rate was at least as high as that which could be obtainedby steamdrive in the samepattern,and it was concluded that the in situ combustion processwas more economic in this In Situ Combustionin Tar Sands

451

instance.An expandedproject involving 10 injection and 40 productionwells is operatedby SanteFe Energy on the samelease.The project is describedas promising and is producing 1000B/d of oil, of which 800 is enhancedrecovery production (Aalund 1988). BP ResourcesCanada (Donnelly, Hallam, and Duckett 1985; Nzekwu, Hallam, and Williams 1988)is following a similar approachat its combustionpilot at MargueriteLake in the Cold Lake oil sandsdepositin Alberta. [n its process, BP first producedoil from the tar sandsby steamstimulation; this required the initial steaminjection to be at abovefracturing pressure.After a number of cyclesof steamstimulation,the operationwas convertedto in situ combustion.Initially, BP used air in situ combustion,as in its original plans.Subsequently they converted from the use of air to oxygenand have now had considerableexperiencewith pure oxygeninjection. BP found that the combustionzone overridesthe reservoirand movesrapidly along the fracture paths opened during cyclic steaming.To avoid production well damage,they developedcyclic techniquesin which the injection of oxygenis interrupted. Water injection, either as slugsor intermittently, is important in controlling the temperatures and in distributingthe heat(Nzekwu,Hallam, and Williams1988). The cyclic steamingphaseof the processrequires 5 to 7 y to produce 15 to 20Voof the original bitumen in place.It is anticipatedthat with combustion,the recoverywill be doubledto about 30 to 50Vo.BP startedpilot operationsat Marguerite Lake in 1977 and carried out cyclic steamexperiments,wet-air combustion,and, since March 1983,enriched air and high-purity oxygencombustion. A larger semicommercialoperation was built in 1983at nearby Wolf Lake; this started operationsin early 1985.This project has L92start-up wells that were directionally drilled from 10 satellites;it is designedto produce 1100m3/d of bitumen. After the cyclic steamphase,it is planned to incorporatecomtustion (probably using oxygen)in the early 1990s. Wet combustionhas considerablepotential and many advantageswhen used with tar sands.It provides a meansfor reducingthe quantity of injected air that would be neededto consumethe very high fuel load that arisesbecauseof the composition of the bitumen. Wet combustionproducesa steamfloodaheadof the combustion front that drives oil ahead,and this reducesthe fuel load. [n addition, it is probablypractical to look upon the wet in situ combustionmechanismas an in situ steamgenerator.This leadsto the concept of not burning completelythrough the reservoir,but stoppingafter adequatesteamfor recoveryhas been generated.Thus, in practical bitumen, in situ combustionprojects,it is probably economicto leave behind considerablevolumesof unburned reservoir. USE OF OXYGEN OR ENRICHEDAIR

Potentid Ar

f. Elinir givea o Fctin tinr.I speiq inirt. for sla couE3| 2. The cq highcr, the a! the vir analyr tively (l

a

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{

a a

r5

a

a (:'

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One of the most interestingcurrent areasof developmentin in situ combustionis the use of enriched air or oxygenin place of air. This possibility was suggestedby H. Ramey (1954). For the useof oxygento becomean economic,commercialreality, it is necessary for its advantagesto outweigh its disadvantages. As it is seenat present,the following are the major advantagesand disadvantages. In Situ Combustion

Chap.9

and C

3one SCt reservoir.Slcinjectionof l([

0.7x ld SCE may reducctha

Useof Oxtga

luctionwellsis opribed aspromising covery production :t 1985; Nzekwu, s combustionpilot rta. In its process, s requiredthe iniumberof cyclesof stion.Initially, BP lly they converted rcriencewith pure and movesrapidly id productionwell of oxygenis intertant in controlling nd Williams1988). r to produce 15 to ombustion,the reions at Marguerite combustion,and,

Potential Advantages for the Use of Oxygen 1. Elimination of nitrogen reducesthe gasvelocity at the producingwells for a given oxygen-injectionrate. This in turn can allow a higher rate of oxygeninjection and can give much more rapid heat production and shortenproduction time. As has been mentioned previously,it is also possibleto use larger well spacing.With air, the rate of production of heat within the reservoirfrom an injection well is frequently lessthan can be achievedby using the samewell for steam injection. with oxygen injection, the rate of heat production becomesapproximatelyequal to that which is achievablewith steam.3 2. The concentrationof carbon dioxide in the gaswithin the reservoir is much higher, and it hasbeen suggestedthat this will improve recoveryby increasing the amountof carbondioxidedissolvingin the crude;this will tend to lower the viscosityand causeswelling.Figures9.42 and 9.43 comparethe flue gas analysesfound in combustiontube experimentsusing air and oxygen,respectively (Mossand Cady 1982).

o I

ton.

rearby Wolf Lake; up wells that were 1100m3/dof bituombustion(probalntageswhen used rf injectedair that ecauseof the comaheadof the comJ. In addition,it is anismas an in situ rletelythrough the n generated.Thus, economicto leave

{

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-f

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llne, situ combustionis y was suggestedby reality,it is necesgen at present,the rbustion

Chap.9

hours

Figure 9.42 CombustionTube Run with Air 12' Lindbergh Crude (from Moss and Cady 1982) 3OneSCF air generates of about 100Btu when the oxygenwithin it reactswith the fuel in the reservoir.Steaminjection gives about 1000Btu per pound of steam.Thus, to be equivalentto the injection_of1000B/d of steam(350 x 106Btu/d), one would have to inject 3.5 x 106scF)/dof air, or 0.7 x 10"SCF/dof 02. In practice,the higher efficiencyof utilization of the heat from combustion may reducetheserequirements.

Use of Oxygenor EnrichedAir

100

I coz -..r

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r-

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+, 6

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3. The heatingvalue of the producedgaswill be much higher becauseit is not diluted with nitrogen. Also, the high concentrationof carbon dioxide may makethe gasusefulasa sourceof carbondioxide.It is possible,then, that the producedgas may have a positivOrather than a negativeeconomicvalue. It could, for example,be possibleto separatethe COz for sale and utilize the resultanthigh-heating-value tail gas as a fuel. The combustionof the tail gas from conventional in' situ combustion is difficult and requires special equipment. 4. Theremaybe lessoverridethan with air for the sameoxygen-injection ratebecauseof the reducedvolumeof gasand perhapsbecauseof its higherdensity. PossibleDisadvantagesof the Use of Oxygen 1. There is a hazard in using high-pressure, high-concentrationoxygenin cirwhereit can be mixedwith hydrocarbons. cumstances A particularconcernis to eliminatemixing of oxygenwith oil in the injectionwell. This can occur if the pressurein the injectionwell is allowedto fall during operationand thus allow oil to back up into the well. A likely causefor suchan eventwould be the failure of the oxygensupply.Precautionsthat can be usedto preventthis includethe provisionof backupoxygenand meansfor injectingwaterinto the In Situ Combustion

Precr tro& nece lociri l9&l: minil tion i 3. The ir air ct the n 4. As is with r

The Cost t

Figure 9.43 CombustionTube Run with 02 12" Lindbergh Crude (from Moss and Cady 1982)

454

well r wirb For c able I nelly 2. Spec riqls tisr r

Chap.9

There is al The mosl c practicalf(t and fractio Coolingis r batic expa comes fro0 90 psig is s pressure(se for air liqu O*r-g in insulate by BritishP by Arco ar Roberts1S The li provide a s oxygen and startup.arx Systemsfu Duckerr(19 The o fifth of rhc

Useof Oxyg

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+, a &

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rt (, I c

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B T 0t o l{ a o o

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well in the event of failure of the oxygen supply. The mechanicalproblems with oxygen in situ combustionhave been largely solved in pilot operations. For example,BP in their Cold Lake pilot has injected oxygenfor a considerable period and are contemplatingits use on a large commercialscale(Donnelly, Hallam, and Duckett 1985). 2. Specialprecautionsare also necessaryto handle oxygen.For example,precautions must be taken to avoid sourcesof ignition that could causethe combustion of ordinary steel pipe carrying high-pressureoxygen. Although these precautionsare well known in the conventionalhandling of oxygen,they introduce a new degreeof complexityinto oil field operations.It is, for example, necessaryto minimize dust particlesand to stay within maximum line velocities to avoid ignition from static electricity (Henningson and Duckett 1984;Hvizdos,Howard, and Roberts1983).The choiceof materialsthat will minimize corrosion problemsand be compatiblewith oxygenin situ combustion is discussedby Zawieruchaet al. (1988). 3. The investmentcostfor oxygen-separation plantsis higherthan that for simple air compressors. This is partially-and in somecasescompletely-offset by the reducedpower requirementto compressthe smallervolume of oxygen. 4. As is discussedlater, low-temperature oxidationseemsto occur more readily with oxygenthan with air-particularly at higherpressures. The Cost of Orygen

: tfrom Moss

er becauseit is not arbon dioxide may sible,then, that the economicvalue. It saleand utilize the rbustionof the tail nd requiresspecial en-injectionrateberf its higherdensity.

ltion oxygenin cirrarticularconcernis ll. This can occurif r operationand thus r an eventwould be usedto preventthis :ctingwaterinto the mbustion

Chap.9

There is alreadya vast experiencein the manufactureof oxygen on a large scale. The most economicmethod for manufacture,and the only developedone that is practical for considerationfor large-scaleoperations,involvesthe liquefactionof air and fractional distillation to separatethe oxygen.The processis very efficient. Cooling is achievedby countercurrentheat exchangewith the products and adiabatic expansionof the compressedfeed. The free energy to operate the process comes from the compressionof the feed. An input air pressureof about 75 to 90 psig is sufficient to drive the processand produce pure oxygen at atmospheric pressure(seeTable9.2).Newton(1979)givesa concisedescriptionof the technology for air liquefaction and fractionation. Oxygenfor small combustionpilots is usuallytransportedto the site as liquid in insulatedtrailers.Productionpilots of this type have been operatedin Canada by British Petroleum,by Husky Oil, and by Dome Petroleum.There havebeentests by Arco and by the GreenwichOil Companyin Texas(Hvizdos, Howard, and Roberts1983). The liquid oxygenis pumpedunderpressurethrough avaporizer.Itis usualto provide a similar facility for the vaporizationof liquid nitrogen so that a blend of oxygen and nitrogen can be injected. A blend correspondingto air is used for startup, and the oxygenconcentrationis increasedgraduallyas the burn progresses. Systemsfor the supply of oxygen to fire floods are discussedby Henningsonand Duckett (1984).Figure 9.44 showsa systemsuitablefor a field pilot. The compressionof the oxygenfrom a separationplant requiresonly about a fifth of the energythat would be required for the compressionof the sameamount Use of Oxygenor EnrichedAir

455

YsrsrEr uqro orvB€xarPzL -5 ,

:: l\

z! s
r

{ :c

l/lgodt

FbrComola

tqhlrtg

Figure 9.44 Liquid Oxygen Vaporization Systemfor In Situ Pilot Operations (from Henningsonand Duckett 1984)

of oxygenas air. This savingin compression work can offset the work neededto separatethe oxygenfrom air. Whetherthe overallenergyis lower or higherdependsupon the final delivery pressure.Figure 9.45 (Hvizdos,Howard, and Roberts1983)showsa comparison of the power requirementsto produce4 million SCF/dof oxygenas air and as pure oxygen.At pressuresabove about 175 psia, the production of oxygen requires lesspower. The costsfrom the samereferencearecomparedin Figure9.46.Which source of oxygenis cheaperdependsnot only upon the pressurerequiredbut also on the volume.This is becauseof the very substantialeconomyin scalein the manufacture of oxygen;largeplantsare more economicalthan small ones.This is a significant problemwhen it is desiredto experimenton a small pilot scalewith oxygento developthe method. The discontinuitiesin the curves of Figure 9.46 reflect the by switchingfrom recipeconomiesthat can be madein the costof air compression rocatingto turbocompressors if the requirementis large enough.

Rathr-: l e a s t ,i n u s : ; l ima in the.c other factr.n r tration.It:ce pureox\qen. An inte is given br k c o m b u s t i o nr s c o m p a r et h c I o r e n r i c h e d:

The Effect o{ with Oxygen

The precedrn fected bv dc'g by Moore an'J case (Nloorc tube, ther ra: I t}_

i I

\ RELAT IVE OXYGEN COST

nvvGEN

40 60 100 200 O E L I V E RPYR E S S UP RS EI A

456

Figure 9.45 Electrical Energy to Produce4MSCF/dof Oz or Air Containing4MSCF/dof 02 (from Hvizdos, Howard and Roberts1983)

ln Situ Combustion

Chap.9

' lrri I I

: I-+tc

Use of Oxvgre.

r s[

F;EI

;fi hi

EgHI t ;l Figure 9.46 Differential Cost for Oxygen Comparedto Air (from Hvizdos, Howard, and Roberts1983)

M M S C F DO F C O N T A I N E DO X Y G E N

)perations (from

re work neededto n the final delivery ows a comparison as air and aspure rf oxygen requires l.tl6. Which source ed but also on the in the manufacture 'his is a significant ile with oxygen to re 9.46 reflect the from recipltchine

Rather than produce pure oxygen there is some economy,in oxygen cost at least,in usingenrichedair. This is shownin Figure 9.47.Although there are minima in these curves,the differencesare not very great, and it seemslikely that other factorssuchas reservoirperformancewill determinethe best oxygenconcentration. It seemsreasonable to expectthat this will likely turn out to be essentially pure oxygen. An interestingdiscussionof the potential for in situ combustionusing oxygen is given by Fairfield and White (1982).A state-of-the-artreview of oxygen in situ combustionis presentedby Garon, Kumar, and Cala (1986).[n their review,they comparethe physicalcharacteristicsof nine different field projectsthat use oxygen or enrichedair. The Effect of Pressureon Combustion Performance with Oxygen The precedingdiscussionof oxygencostspresumesthat burn performanceis unaffected by degreeof oxygen enrichment. Combustiontube experimentsperformed by Moore and Bennion at the University of Calgarysuggestthat this may not be the case (Moore et al. 1987).Using a 4-in.-diameter,6-ft-long adiabaticcombustion tube, they ran a seriesof dry combustiontube testsusing95%oxygen-enriched air LI

s

RELATIVE OXYGEN

cosT

:clricalEnergyto ,d of 02 or Air CFld of Oz (from d and Roberts1983) nbustion

Chap.

60

70

80

90

% O X Y G E NI N P R O D U C T

Use of Oxygenor EnrichedAir

t00

Figure 9.47 Effect of OxygenPurity on Cost (from Hvizdos, Howard, and Roberts1983)

457

Effect of Pressureon Oxygenand FuelRequirements

Nelsm follo*'ing: 120

o Total ei o Rate aa r Total I

(D

E B ro .Y

110 #

c o

100E '=

c o

=(o

Eoo o

Total RrC Lr

soe s

.: = ct o tr' 20 o

The fuel load ent porositf i

L

6s

so Itr X-:

o 70E

5 IL i:

. Rate d ' OPeratir

E

where ll F

o

10

19

a

a

60o

o

The acre-fod tional petrolcr

50 024681012

TotalPressure(MPa) Figure 9.48 Effect of Pressureon Oxygen and Fuel Requirements-Combustion and Tube Experimentswith AthabascaSandCore and95VaOz (from Moore et al. 1987)

Air Retpircrn

and Athabascaoil sandcore.Figure9.48showsthat the overalloxygenand fuel requirementsappearedto increaselinearly with operating pressure,nearly doubling over the rangeof 2700to 10,300kPa (400-1500psi). They attributedthis increase to the preoxidizingeffect causedby the high oxygenpartial pressures. This is consistent with the observationsof Alexander, Martin, and Dew (1962)already presented (see Figure 9.17). Both Moore and Alexander noted that oxygen partial pressurehad a much smallereffect on normal air (21%oxygen)combustionparameters. Further observationson the relative performanceof oxygenand air in situ combinationin a large number of combustiontube testsare summarizedby Moore, Bennion,and Ursenbach(1988). DESIGNOF IN SITU COMBUSTIONPROJECTS The practical design and sizing of facilities for in situ combustion projects have beendiscussedby Nelsonand McNeil (1961)and by Gatesand Ramey(1980).These are two excellentpapersthat will be of considerableassistanceto an engineerfaced with the planningand designof a new project.Chiu (1988)discusses a relatedanalytical model that extendsthe theory. In Situ Combustion

Chap.9

The air consu and this is pn volume of res

where z{

u w

The total air r per unit r-oluo volumeis egu sweepefficiea of 62.6%strm

AN Air

Designof h 9t

Nelson and McNeil describe means for making simple following:

110 #

c o

E

100E '=

r . r o r

of the

Total air requirements. Rate and pressureat which the air must be supplied. Total amountof oil that will be produced. Rate at which the oil will be produced. Operatingexpense.

=(o

Total Fuel Load

60

The fuel load measuredin a combustiontube test is adjustedto allow for the different porosityin the reservoiras comparedto the laboratorysandpack.

eoes oo Itc

lb fuel/acre-ft burned = 43,560WF

X-

o 70E o

ooo 50

whereW

F

(e.6)

is lb fuelft3lab test

is (1 - 6il1$ - 6,)

0^ is reservoirporosity 6o is sand-packporosity The acre-footmeasureof reservoirvolume is often used when employing conventional petroleum-measuring units. 1 acre-ft = 43,560ft3

rents-Combus02 (from Moore

rll oxygenand fuel re;sure,nearly doubling tributed this increase rressures. This is conw (1962)already preI that oxygenpartial ) combustionparamexygenand air in situ rmmarizedby Moore,

rbustionprojects have I Ramey(1980).These e to an engineerfaced iscusses a relatedana)ombustion Chap.9

Air Requirement The air consumptionin the laboratorytube is expressedas SCF per pound of fuel, and this is proratedas shown in equation9.6 to give the air requirementper unit volume of reservoir. V^ t=fiwF

(e.7)

whereA is SCF airlCF reservoir V, is SCF air in lab test We is total lb of fuel burned in lab test The total air requiredfor the project is estimatedby multiplying the requirement per unit volumeby the estimatedvolumeof the reservoirthat will be burned.This volumeis equalto the volumeof the patternmultipliedby an estimatedvolumetric sweepefficiency.For a five-spot,Nelsonand McNeil suggestthat a sweepefficiency of 62.6Voshouldbe employed;this leadsto equation9.8. MScF/acre-ft burned Air per acre-ft burned : 43,56041106 Air per acre-ft pattern = 0.626 x 43,560A/106

(e.8)

= 27,269A/106MscF/acre-ft pattern Designof In Situ Combustion Projects

459

Air Rate and Pressure The rate at which air is introduced controls the burning rate.

(e.e)

U=Au where U is air flux SCFft'z d A is SCF air/CF reservoir z is burning front advance,ftfd

As hasbeen discussedpreviously,avery low burning rate is insufficient to maintain combustion,and a very high rate causeserosionproblemsat the production wells. The practical maximum gas rate at a production well seemsto be about 500,000to 600,000SCF/day. It is commonto choosean injectionrate that will maintain, initially, a constant burning front velocity. A velocity of 0.5 ft/d is recommendedby Nelson and McNeil. As the front advances,the air-injectionflow is increasedup to the capacity of the compressor. Followingthis, the injectionis maintainedat the maximumrate. During this main period, the rate of advancedecreasesbecauseof the increasing area of the front. Figure 9'49 shows how the air-injection rate and cumulative injection vary with time. The gradualdecreasein the rate at the end of the production is required to minimize oxygenbypassing.[n a large multiple-patterndevelopment,the air capacity that is madefree during this period can be utilized to start up a new pattern. Nelson and McNeil discussthe schedulingof air for production from a number of staggeredpattern operations. Nelsonand McNeil considerthe lowestburning velocity at which satisfactory combustioncan be obtained is about 0.125tt/d. The methodsof Ramev described earliercan alsobe usedto studythis.

Gat.'. e to gire a n::' this flur iilr reserrc-ri::i The erl tain a rni::nr s i n c ei t r r : 1 .a b e s to b t a : : e c Asarr e q u a t i o nv : i l to the f lou .-l permeat'iiitr abilitl of 5-. Equa:i. the marinut mum value.

*here P D

rl

T j

: t1

c

o .P

(E G tr

.9 .H

o o tr

.9 {

InjectionRate

Oil Displaced

C'

o

A s s u m i n et h a displaced ::ruThis is giren I

E o .z +. (g

E E J

L

o

* here

t' c (g 'T.\s:'::::

Time

w e l la su p c : : : c temnerer,rr.

Figure 9.49 Air Injection Programm(after Nelsonand McNeil 1961)

460

In Situ Combustion

i

-.

( 1 9 7 7r)e 3 . : : sr r :

Chap.9

Design of In ft

(e.e)

ufficient to maintain he production wells. ' be about 500,000to rain, initially, a con:nded by Nelson and ed up to the capacity rt the maximum rate. rse of the increasing Llativeinjection vary roduction is required elopment,the air carrt up a new pattern. rn from a number of at which satisfactory of Ramey described

Gatesand Ramey(1980)considerthat the air capacityshouldbe great enough to give a minimum burning rate of 0.15ft/d or an air flux of 2.15 scF/h ft2, but this flux shouldbe calculatedas if the air were passingthrough only ] to ] of the reservoirthickness.a The existenceof a limiting gas-production rateper well and the needto maintain a minimum burning advancerate tend to make the use of oxygenattractive, sinceit will allow largerratesand wider patterns.The injectionpressurerequiredis best obtained from actual field injection test data. As a means of making a prior estimate, Nelson and McNeil recommended equation9.10.This requiresan estimateof the permeabilityof the cold formation to the flow of gas,i.e., the permeabilityof the formationmultipliedby the relative permeability.If there is no specificinformation, they suggestthat a relative permeability of 5Vocan be used. Equation 9.10gives the pressureat the time the air-injectionrate first reaches the maximum value. This is the point where the pressurereachesits maximum value.

P ? . = P*' zl.i'" p " z t \ f ' | 4 _ \ - 1 . 2 3 8 . l \oro3/.1/ L'n\',,,r,r I where Pi, Pio Ita Tf a ty ks h rw v1

(e.10)

is injection well bottom hole pressure,psia is production well bottom hole pressure,psia is maximum air rate, SCF/d is air viscosityat Ty,cp is absoluteformation temperature,R is well spacingfor the five spot pattern, ft is time to reachmaximumrate, d is effectivepermeabilityto air, mD is formationthickness,ft is production well radius, ft is initial rate of advanceof burning front, ft/d

Oil Displaced Assumingthat there is no oil left in the burned-outzone,then the amountof oil displacedmust be equal to the original oil in place minus that consumedas fuel. This is given by equation9.11. Oil displaced= oil at start - fuel

- y:\ B/acre-rt = n.soo1:9i '"'"""\ -' s.ot 35o l

where So Qo 43,560 5.6r 350

is fractional oil saturation is reservoirporosity, fraction ftz facre ft3/B (assumed) lb fue/B

*The temperature at the firefront for a given rate of advance dependsupon the fuel load as well as upon the air rate. Thus a very heavy oil that gives a high fuel load will give a higher front temperaturefor the samerate of advance.Thus, for example,in the Midway Sunsetfield, Counihan (1977)reports excellentcombustionresultswith a designrate of only 1 in./d.

Neil 1961) omtustion

(e.11)

Chap.9

Design of In Situ Combustion Projects

461

Gatesand Rameypointedout that, at an intermediatetime, the cumulativeamount of oil producedcan be greateror lessthan the amount of oil displacedfrom the burned zonebecauseof two opposingfactors: 1. Oil aheadof the front may be displacedto the productionwell by other mechanisms"includinghot water drive, steamdrive, hot gasdrive, vaporization, misciblephasedisplacement,expansionand gravity drainage."These mechanismsare all made more effective by the elevatedtemperatures. 2. Oil may have to form a bank aheadof the combustionzone in order to fill someof the gassaturationthat is presentinitially in the reservoir. Basedon this conceptand usingexperimentaldatafrom the long-standing and successfulSouthBelridgepilot, Gatesand Rameydevelopedthe chart in Figure 9.50. This correlationpredictsthe cumulativeoil productionas a function of the fraction of the reservoirthat has been burned and the initial gas saturation.It is basedupon considerabledata from cored wells at the pilot and also upon laboratory experimentsthat demonstrated the effect of initial gassaturation. Figure 9.50 predictsthat there is a delayperiod before oil is producedthat corresponds to the formation of the oil bank and that the initial production,when it eventuallyoccurs,is at a muchhigherrate than the average. A delayin the initial productionof oil is shownby the data shownpreviouslyin Figure 9.10. In using the correlationit is necessaryto know the fuel concentration.This may be obtained from combustiontube experimentsor, if these are not available, from the correlationin Figure9.51.Figure 9.52 maybe usedto estimatethe air requirementto burn the fuel. Using the correlationin Figure 9.50,we can measurethe slopeof the appropriate curve and calculate the instantaneousoil rate as a function of the fraction burned.Using this informationand the fuel requirement,we can calculatethe instantaneousair-oil ratio as a function of recovery.The resultsof such a calculation for the SouthBelridgefield are shownin Figure 9.53. The initial current air-oil ratio is muchlower than the averagerate shownby the dottedhorizontalline. The cumulativeratio is also lower than the averageexcept at the end, where it converges.

roo: roo-I

f

?oo-

e 'oo0!

o

In prrti that the prodr uneconomic h the dottedlic The calc the combusir gas,and allorr zone.The eto Gatesand Rel air and that it Figure 9. pared to predi that the sprhc when the prod The rcad verv interesliq centrationfru ll-

t aq-

lrl tilfl4.

J

U 5 L o q J

\

F

3

'/fiI

F J

//V

o o It

/. '/,

G 5 o I ! G J

o

462

t

aas safuRAfloil I at I

a

3

I

I t

/lt

+ I

f

3 : + 3

st

7

3

,/

/,

'/t

/xl

t

o o

Figure 9.50 Chart for Estimating Oil Recoveryas a Function of the Percent of the ReservoirBurned (from Gates and Ramey 1980) ln Situ Combustion

Chap. 9

I a

I

>-

i I I olo

Designof In Sitt

cumulativeamount displacedfrom the

F

U

tion well by other ;as drive, vaporizai drainage."These emperatures. ure in order to fill eservoir. g-standingand sucrart in Figure 9.50. s a function of the gassaturation.It is lso upon laboratory )n. il is producedthat rl production,when ' delayin the initial rre 9.10. ;oncentration.This € are not available, estimatethe air reslope of the approtion of the fraction an calculatethe in,f sucha calculation :ragerate shown by han the averageex-

)

@

z o U E

OIL GRAVTTY. 'API

Figure 9.51 Chart for Predicting Fuel Concentration(from Gates and Ramev 1980)

ln practiceit is likely that the processwould not be taken to the very end but that the production would be terminated when the current air-oil ratio reachedan uneconomiclevel.In this casethe final cumulativeratio would alsobe lower than the dottedline. The calculationso far has assumedthat the oxygenis completelyconsumedin the combustionprocess.In practicethere is someunreactedoxygenin the produced gas, and allowanceshouldbe made for this in estimatingthe size of the burned zone. The excessair requirementsmay be estimatedfrom Figure 9.54; however, Gatesand Rameyconsiderthat this figure may overestimatethe amountof excess air and that it is conservative. Figure 9.55 shows the experimental air-oil ratios from South Belridge compared to predictionsmadeusing the precedingmethod. Gatesand Rameypoint out that the spikesin the experimentaldata in this figure correspondlargely to periods when the production wells at the pilot were partly shut in for mechanicalreasons. The reader is referred to the original paper for further details, including a very interestingdescriptionof severalparallel methodsfor estimatingthe fuel concentrationfrom the field data. F E d

I J 2

o

) J

I

\

J

u 2 E

e 0 o

c. t art for EstimatingOil unction of the Percent 'Burned (from Gates rl mbustion

Chap. 9

o U E

c cRuoE otl GRAYITY,.APt

Designof In Situ CombustionProjects

Figure 9.52 Air Requirementsper Acre-Foot of Reservoir (from Gates and Ramey 1980)

463

.o

a

I

I

t

]o.

;

o F

t

20

G J o

- _DlslLlc_E[ENr_\_ _-z_

;

to

W

_ _

lg.

a <e Figure 9.53 ComputedAir-Oil Ratio for South BelridgeField (from Gates

o l L R E C O V E R Y - ? o OOFI L A T S T A R TL E S S F U E L

and Ramey 1980)

Effect of Water-Air Ratio on Oil Recoveryper Volume Burned The correlationof Gatesand Ramey(Figure9.50)for predictingthe oil recoveryas a function of the burned reservoirfraction and the initial gassaturationis for dry combustionin a particularreservoir.Different correlationsmaybe expectedfor differentsituations.For example,in wet combustion,oil is movedaheadby the steam, and oil production occurs more rapidly. This phenomenonhas been studied by Moore et al. (1988)in a seriesof combustiontube testswith varyingratiosof water to air. Someof their data are shownin Figures9.56-9.59. Figure 9.56 showsthe effect of adding2 kglm3(ST) of water to the combustion air. Oil production starts when a smaller percentageof the core has been burned, and for a given fraction burned, the oil recoveryis larger.Also, because the fuel loadis lessfor wet combustion,lessair is requiredto achievethe samefraction burned.As a result, the curvesof percentoil recoveryversusquantity of air injected(Figure9.57)showan evenlargerdifference. Resultsfrom similar experimentsusingcoresof Athabascareservoirsandand bitumenthat were reconstitutedby flooding are shownin Figures9.58and 9.59.In this seriesof experiments,runs with much higher water-injectionratios are compared to a dry run.

-.-p The periment.::r slightlr hrg:er peak temptera was coolinerl reasonfor rte It can hr causedoil t.. i tage of more ! unburned fuel t e d a g a i n s :l x Furthcrr quantitr of ar problem de:.-n bustion tut'c tr a mathemati!the quantitr o volvesthe calc fication of the

$ g o o g

o x u

roo o 20 40 60 80 OIL RECOVERY.'6OF LESS FIfL OIL AT START ExcEss^ri .mffii**--aer

464

x roo* rcr.ritxyt

Figure 9.54 ExcessAir from South BelridgePilot (from Gates and Ramey 1980) In Situ Combustion

Chap. 9

Designof In Srt

XOITHLY IN.'ECTEOAh . PR'UJC€O OIL RATIO

o a t

9 F c J

o c

Air-OilRatio mputed geField(fromGates

oi.0ro60oo OIL IECOVERY.Tf

€d g the oil recoveryas iaturation is for dry be expectedfor difaheadby the steam, as been studied by l ing ratios of water ater to the combusthe core has been [ger. Also, because hievethe samefrac:ISusquantity of air a reservoirsand and res9.58and 9.59.In tion ratios are com-

roo OIL AT START LESS

FI,EL

Figure 9.55 South BelridgeCurrent Air/Oil Ratio (from Gatesand Ramey 1980)

The "optimal" water-airratio for this seriesof runswas about4.6.In this experiment, the averagepeak combustion front temperaturewas 608"c. This was slightly higher than that for the dry run (569'c). on the other hand, the average peak temperaturefor the run with a ratio of 6.9 was only 225'C.In it, liquid water was cooling the combustionzone,and unburnedfuel was left behind. This is the reasonfor the relatively low final recovery. It can be seenfrom Figure 9.58that, as in Figure 9.56,the additionof water causedoil to be recoveredsooner.However, in the superwetrun, the early advantage of more productionwas not maintained becauseof the material left behind as unburned fuel. The difference is even more pronouncedwhen the recoveryis plotted againstthe cumulativeair injectionin Figure 9.59. Further insight into the factorsthat affect the oil recoveryas a function of the quantity of air injected can be obtained from the mathematical analysis of the problemdescribedby Chiu (1988).In his paper,Chiu showshow the resultsof combustion tube testssuchas those shown in Figures9.56-9.59can be developedfrom a mathematicalmodel involving steamfloodingaheadof the combustionzone,with the quantity of steambeing calculatedfrom a heat balance.The heat balance involvesthe calculationof the vertical heat lossesfrom the steamzone using a modification of the Marx-Langenheimmethoddescribedin Chapter4. 100

t

o o (J o

e o .{ rcessAir from Pilot(fromGates t0) cmbustion

Chap.9

200 PVof Air Iniected

Designof In Situ CombustionProjects

400

Figure 9.56 Effect of Water-Air Ratio on Oil Recovery;CombustionTube Test with Packof AthabascaBitumen and Silica Sand(after Moore et al.)

465

Parameter is Water/Air Ratio xgim3 1s1

o o o o

8so = o o

s 100 S0 c/oof VolumeBurned

Figure 9.57 Effect of WaterAddition upon Oil Recoveryas a Function of CumulativeAir; InjectionTestswith AthabascaBitumen-Silica and Sand Pack(after Moore et al.) TABLE9.7 }:

FIELDPROJECT RESULTS Lloydminster,Golden Lake As Fairfieldand White (1982)havediscussed, the Lloydminsterareacontainsmany reservoirsof heavyoil containedin thin, Lower Cretaceous sands.The oils arevery viscous,and the oil saturationis usuallyhigh; the sandsare very permeable.The reservoirsare estimatedto contain a total of 50 to 70 billion barrelsof oil. The Lloydminsteroils are lessviscousthan thosein the Cold Lake field to the north, and primary productionis possible;well productivitiesof the order of 20 Bld are obtained,but the primary recoveryis only 3 to 8Vo.Becauseof the high viscosityand the fingeringthat occurs,waterfloodingis not very effective;Fairfield and White indicatethat the incrementalrecoveryfrom waterfloodingis of the order of only 2Vo. Figure 9.60 showsthat about 90Voof this oil occurs in sandsthinner than 20 ft, and 50Vooccxrsin sandslessthan 10ft. Becausethe sandsare so thin, steam drive is generallynot applicable.For thermal efficiency reasons,a thicknessof about25 ft or more is necessaryfor steamdrive to be practical. In situ combustionis suitablefor sandsdown to about 10 ft in thicknessand has promisefor allowing efficient recoveryof much of the Lloydminstercrude.It hasthe advantages of usingcheaperenergythan steamrecovery,of not requiringall of the reservoirto be maintainedat the thermal recoverytemperaturethroughout the project,and of removingsomeof the most refractorymaterialas fuel.

( F a i r f i e l da n : \ A

100

wet 4.6 /

lJ ^^

66u tt

E o o o

tso o o

*

.: Suoer Wetb,9 ..

60u

=

1."

o,^

Dryo Parameteris

a

Water/AirRatio

dzo

rg/m31sr1 50 o/oof VolumeBurned

466

100

Figure 9.58 Effect of Water-Air Ratio on Oil RecoveryTestswith ReconstitutedAthabascaTar Sand (after Moore et al.)

In Situ Combustion

Chap.9

: v0

FieldProjectRe

e o o o t

o ;x

iffect of WaterAddition rveryas a Function of ir: InjectionTestswith umen-Silicaand Sand rore et al.)

0

400 200 PVof Alr Iniected

Figure 9.59 Effect of Water Addition upon Oil Recoveryas a Function of CumulativeAir Injection Testswith ReconstitutedAthabascaTar Sand (after Moore et al.)

TABLE 9.7 Propertiesof GoldenLake Reservoir Sand Depth, ft Net sand,ft Original pilot Expansion#1 Porosity, Vo Permeability,md Core data Calculatedfrom productiondata Saturation,7o oil Watcr Originalreservoirpressure. psig Reservoirtemperature,"F Reservoirf luid properties Oil gravity, "API Oil formation volume factor Solutiongas oil ratio, SCF)/B Dead oil viscosityat 70"F.,cp Live oil viscosityat 500psig,cp

'-r areacontarnsmany nds. The oils are very very permeable.The barrelsof oil. te Cold Lake field to vities of the order of ,. Becauseof the high :ry effective; Fairfield Ioodingis of the order n sandsthinner than ndsare so thin, steam asons,a thickness of )al. l0 ft in thicknessand .loydminstercrude. It ry, of not requiring all mperaturethroughout terial as fuel.

Sparky 1600 LJ

20.8 35 1200 8000 82 l8 510 70 t2-13 1.01 45 6300 3500

(Fairfield and White 1982) 100 o

E ao rL o

.z

!:6 0 9+o o o

Ezo Effect of Water-Air Ratio ery Testswith I AthabascaTar Sand et al.) Sombustion

Chap. 9

: v0

0

Field Project Results

48 Sand Thicknessm

12

Figure 9.60 Oil in Placein the Lloydminsterfields as a Function of SandThickness(after Fairfield and white 1982)

467

TABLE 9.8 CombustionCharacteristicsof GoldenLake ReservoirMaterial Molecularweight (unit) Atomic hydrogen- carbon ratio Fuel, lbft3 Fuel, B/acre-ft Unit air requirement at (100% efficiency) SCF/lb carbon SCF/lb fuel scF/ft3 Water formed by combustion, Bfacre-f.t Oil displacedby combustionfront, B/acre-ft TheoreticalAOR, dry combustion,kSCF/B Water-airratio, B/MSCF Residualoil saturation,steamzone: Vo pore space(avg.) B/acre-ft (avg.) Displacement in steam zone, Bfacre-ft

I J.+

1.4 1.93 250 200 t79 345 ))4

t957 7.68 205 22.6 614 1593

(Fairfield and White 1982)

Propertiesof the Golden Lake Reservoir and the combustioncharacteristics are shownin Tables9.7 and 9.8.The oil hasa viscosityof 3500cp at reservoirconditions. There is a large difference between the permeabilitiesmeasuredon core samplesand those calculated from production data. This is thought to be due to "worm holes" in the reservoir,possiblyformed as a result of sand production. The well layout for the Golden Lake projectis shown in Figure 9.61.The original five-spotpattern aroundwell 815-11was ignited in July 1969and expansion1, consistingof two seven-spotpatterns,was ignited in 1974.Water injection was startedat a designrate of 205 B per million SCF in 1972in the original pattern and in July 1976in the expansionpattern. Productionresultsare shown in Figure 9.62 and are summarizedin Table 9.9. An analysisof the producedgasis given in Table9.10.

tfuur Fairfid

TABLE 9.9 GoldenLake Injectionand Production(81-09-30)

Pattern

Cumulative Air Million SCF

Cumulative Water KB

Burned Volume Percent(1)

oil Recovered KB

TABLE9.10 G(

Recovery Percent OOIP

t7.1 39.8 1240 165 567 Original Pattern Expansion1 438 18.3 4.4 D7 Pattern r07 507 J.t 378 t9.7 116 B9 Pattern 562 19.0 5.0 816 Total Expansion1 1069 223 (t)It was anticipated that it would be economic to continue the burn until the burned volume was about 20Vo.There are thus quite a few yearsof production aheadof even the original pattern (Fairfieldand white 1982).

Gffffi !

468

In Situ Combustion

Chap.9

I

I

Field Project Rc

1 0 1. 2 5a c 2329 ac-n

13.4

Primarydrainagearea

l.+

c-15

1.93 :,50 100 179

20 ac .7 B-15 t 460 ac-fl

A-147

_u5 17d

A-15

98.75ac 2049 ac-ft

1 9 57 7.68

D-9

r05 c-10 30 ac

)2.6 611 1,s93

B-10

\

A . 1O T

o-

,

< a_v

623 ac-ft u-d

rustioncharacteristics 0 cp at reservoirconies measuredon core thought to be due to sandproduction. Figure9.61.The orig,969and expansion1, \later injection was e originalpatternand rmarizedin Table9.9.

oil Recovered KB


567

39.8

{38 378 816

18.3 t9.7 19.0

,

.115 acres 2386 ac-tt O--'--

Figure 9.61 Well Arrangementfor Husky'sPilot Projectsat Golden Lake (from Fairfield and White 1982)

TABLE 9.10 GoldenLake ProducedGas Analvsis Component Carbon dioxide Carbon monoxide Methane Nitrogen Oxygen Argon

until the burned volume eren rhe originalpattern

Volume Percent

16.0 0.4 t.2 81.4 0.0 1.0 100.0


lornbr.rstion

Chap.9


469

TABLE 9.8 combustion characteristicsof GoldenLake ReservoirMateriar Molecular weight (unit) Atomic hydrogen-carbon ratio Fuel, lbft3 Fuel, B/acre-ft Unit air requirement at (100Voefficiencv) SCVIb carbon SCF/lb fuel

13.4 1.4 1.93 250 200 179 345

scFrt3 Water formed by combustion, B/acre-ft Oil displacedby combustionfront, B/acre-ft TheoreticalAOR, dry combustion,kSCF/B Water-airratio, B/MSCF Residualoil saturation,steamzone: Vo pore space(avg.) B/acre-ft (avg.) Displacement in steam zone, B/acre-ft

laA

1957 7.68 205 22.6 614 1593


Propertiesof the Golden Lake Reservoir and the combustioncharacteristics are shownin Tables9.7 and9.g.The oil hasa viscosityof 3500cp at reservoir con_ ditions. There is a large difference between the permeabilitiesmeasured on core samplesand those calculated from production data. This is thought to be due to "worm holes" in the reservoir,possiblyformed as a result of sand production. The well layout for the Golden Lake project is shown in Figuri 9.61.The orig_ , inal five-spotpattern aroundweil 815-11wasignited in July p(9 andexpansion i, consisting of two seven-spotpatterns, was ignited in r9i4. water injection was started at a designrate of 205B per million SCF in 7972|n the original pattern and in July 1976in the expansionpattern. Productionresultsare shown in Figure 9.62 and are summarizedin Table 9.9. An analysisof the producedgasis given in Table9.10.

Ftrc F.irfi

TABLE 9.9 GoldenLake Injectionand production(81-09-301

Pattern

Cumulative Air Million SCF

Cumulative Water KB

Burned Volume Percent(l)

oil Recovered KB


Original Pattern 1240 165 L7.l 561 39.8 Expansion1 D7 Pattern 507 107 4.4 438 18.3 89 Pattern 562 116 5.7 318 19.7 Total Expansion1 1069 223 5.0 816 19.0 (t)It was anticipated that it would be economic to continue the burn until the burned volume was about 20Vo.There are thus quite a few yearsof production aheadof even the original pattern (Fairfield and White 1982).

468

In Situ Combustion

Chap.9

TABTT 9.IO C

(Fairficld

eod tl

Fa*t Prqsr ft

Sparkyformationoriginalpattern

E

E i .o

E

E

1000=

C'

=

p

c o

e o

E

:tr

o

66 67 68 69 7A 71 72 79

. Lo*er g r Enharro o Aserof! and its s A series Mehrotn bitumeo o Rapid co o Valuable o Lessorg

75 76 T7 78 7g 80 81

Sparkyformationexpansion#1 10000 5000 E

I

E

i o

,rffi

E

t I'

e CL

1000 !E

h'v'1

500

i o

E

.:E

o 100 50 wet combustlon Drv ' l combustlonI

10 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 Figure 9.62 Productionfrom Golden Lake In Situ CombustionPilots. The Air Injection Is In kSCVd. (from Fairfield and White 1982)

470

In Situ Combustion

TrouHc permeabilit;-t drillingof a u waterlaversh be verl' prmi Sandco the oil to be p heavy rods an Ven'hig ployed.and tb The pc Fairfieldand I

Chap.9

It appea minstercrudei ideal type of n the top of a di1 of 30 to 5ff'r o More reo Miller and Jr scribedb1'Fair which were igr culty in ignitiq was shut dovo encounteredh Getty's B€tr

This project is t original pilot b may be seenft about 350 well The rese is broken up b There is vertic

FieldProjectRa

10000

so00 T'

E

1000 s00

c o

E

F

o Lower gasvelocities o Enhancedoil mobility (seeFigure 9.63) o A set of generalizedcorrelationsfor predicting the solubility of carbon dioxide and its effect on swellingand viscosityis given by simon and Grave (1965). A series of papers by Mehrotra and Svrcek (Svrcek and Mehrotn 1982; Mehrotra and Svrcek 1984, 1985a,1985b,1985c)contain measurementsfor bitumen-carbondioxide systems. o Rapid contactwith carbondioxide and swelling o Valuableproducedgas o Lessoverride

100 50 10 7A 80 81

-t ----1oooo ___15000 At |rl|A

I

E E

I

i .9

---*-Jtooo ,r

I

;T=*

E

.F

fl

___-l100

--ls0 rbn ,

'

It appearsthat the in situ combustionapproachto the production of Lloydminstercrudeis successfuland that the useof oxygenmay be very promising.The ideal type of reservoirfor this processis one in which oxygencould be injectedat the top of a dipping reservoir,and the authorsconsiderthat recoveriesof the order of 30 to 50Voof the oil in placeshouldbe practicable. More recent experienceat the Golden Lake fireflood project is describedby Miller and Jacques(1987).The pilot was expanded from the three patterns describedby Fairfield and White by adding two further inverted seven-spotpatterns, which were ignited in 7982,and three adjacentpatterns in 1985.There was difficulty in igniting the lastthreepatterns,and this wasnot solvedby the time the pilot was shut down in 1986becauseof the low price of oil. There were also problems encounteredbecauseof the encroachmentof water into the pilot area. Getty's BellevueField in Louisiana

I

lto

79 80 81 ;. The Air Injection

rnbustion

Trouble was found with air short-circuiting from the injector through a high permeabilitythief zone in the original pattern, and this eventuallyrequiredthe drilling of a new injection well. Severalwell workoversand squeezejobs to shut off water layershave also been required.Nevertheless,the operationhasbeen found to be very promising,and a secondexpansionhasbeencompleted. Sandcontrol is a continuing problem, and Husky has found it better to allow the oil to be producedrather than to try to restrict it from the well bore. They use heavyrods and large(3.5-in.)tubing. Very high permeability channelsform when high injection pressuresare employed,and they limit the pressurein order to preventthese. The possibleadvantagesof using oxygen rather than air are emphasizedby Fairfield and White:

Chap.9

This project is the largestin situ combustionproject in the United States.Sincethe original pilot beganoperationsin 1963,it has been expandednumeroustimes, as may be seenfrom Figure 9.64.In 1978about 2900 B/d of oil was produced from about 350 wells (most of it from 223 fireflooded wells). The reservoiris shallow (depth is 300-420 ft) and relatively thin (20-90 ft). It is broken up by numerousfaults and by a 4-ft layer of noncontinuouslimestone. There is vertical communicationbetweenthe upper and lower zones. Field ProjectResults

IJts f-]1967expansionA rrpans,m B @ 1970 1972 expanso tr f-l Ii973 exoans'u'tr

g

Remaining areassci€dra for futuredeveloprr|?n

I

O Location

3

a Producing weli

t

1ma O:e e

126 O,e

0.9

3. : \'"i \l

F

-"

1.",

20

rF

o nnr.

0

!F

{F

I

sn

pl

Figure 9.63 Effect of COz on Viscosity of Lloydminster Crude (from Fairfield and White 1982)

Figrrc lJ ana. U-S Cop;-ril

Field Exp.l Although the project is very successful,it requires a very high air-to-oil ratio-about 19,000SCF per barrel. The high air-to-oil ratio is believed to be due to the crude depositingan abnormalamountof fuel, to the relatively low oil saturation (about 52%\, and to the difficulty in moving all the heatedoil to the production wells becauseof the reservoir heterogeneities.

472

In Situ Combustion

ChaP.9

1963: 1967: 1970: I9T2:

FieldPrciqr

Legsnd !

1962expansion A$74

Q 19zoexpansion Q$ts f| t9z2exoansion Qfi76 flt973 expansion n

expansion exgansion expansion

tg77expansion

QlgTB rrp ntion Remaining areas scheduled forfuturedevelopment O Location a Producing well

well @ Injection well t Abandoned

Cities Service Co.

Co, Service Cities

Figure 9.64 Getty'sIn Situ CombustionProjectin BellevueField, BossierParish,Louisiana, U.S.A. (from Bleakely 1978).Reprinted by permissionof Edgell Communications. Copyright @ November,1978,PETROLEUM ENGINEER INTERNAIIONAL.

rfrom Fairfield

FieldExpansions at Bellevue a very high air-to-oil r is believedto be due lativelylow oil saturaIted oil to the produc-

Combustion

Chap.9

1963: 1967: 1970: 1972:

Pilot 1.-4 5-8 9-24

Field ProjectResults

1973: 25-28 1974: 29-31, 1975: 36-47

1976: 1977: 1978: 1979:

32 60-73 & 12I-123,except64 85-88 52-55

473

A major factor in the economicsuccessof the projectis the shallowdepth of the reservoirand the low reservoirpressure.This makeswells cheapto driil and reducescompression costs;the dischargepressureof the compressors is only about 100psi. Although the air consumptionis large,the compressed air is cheap. The wells are drilled on inverted nine-spotpatterns.originally, the pattern sizeswerechosento give reservoirvolumesof 185acre-ftper pattern;i.e., the wells were spacedfarther apart wherethe reservoirwas thinner. Somepatternswere as small as 2.2 acresand others,as largeas 8.5 acres.slow responsewas found in the large patterns,and thesewere infilled with additionalproducers.This gavemuch better results. Originally dry injection was used; then, after about50Voof the theoretical burn was achieved,air injectionwas stoppedand waterfloodingwas usedto scavengeand utilize the residualheat.This procedurehasnow beenmodifiedto include a period of simultaneous water and air injectionafter the dry operationand before the waterflood. Getty expectsto recover60Voof the originaloil in placein the Bellevuefield usingin situ combustion.To do this will requirethe combustionto 6 of 15Voof the oil in placeas fuel. Cities ServiceCompanyhas a wet combustionprojectsimilar to that developedby Getty on their leasein the Bellevuefield; this leaselies directly to the east of the Getty lease(Figure9.64).Josephand Pusch(1982)report that Cities Service expectsto recovernearly 40Voof the oil in place.Well productivity is about20 to 30 B/d. [n their paperJosephand Puschalsoprovide an interestingbreakdownof the operatingcostsfor the field, which, exclusiveof taxes,were$17.2j/B in 19g0-1.

The lin of the count a 2000-cpri are shownT A prh 1967.This h injectedand bustionin tl Oil pn 1974 to Effi Figure 9.66. The in capacitvard 1979this*a The rl During this m3/d1te.+n operationis I

In Situ Combustion Projects in Rumania There are somevery interestingand largein situ combustionprojectsbeingcarried out in Rumania (Gadelleet al. 1981;Turta and pantazi 1982;Carcoanar9g2:Aldea. Turta, and Zamfir 1988).These four papersalso contain many related references. TABLE 9.11 Suplacude BarcauReservoirProperties Sandcharacter Depth Effective thickness Dip Initial reservoirpressure Reservoirtemperature Porosity Absolute permeability Initial oil saturation Oil specificgravity Oil viscosity at 18'C (heavyasphalt-base crude oil)

Unconsolidated 35-220 m 4-24 m 5"-8. 0.4-2.2MPa 18"C(at a depth of 80 m) 0.32 1.7 p.m2 0.85 0.96(16'APr) 1.8-2Pa . s

16

(Turta and Zamfir 1988).

474

In Situ Combustion

Chap,9

Field ProjectF

shallowdepth of heapto drill and sorsis only about ir is cheap. rally, the pattern rrn; i.e.,the wells patternswere as wasfound in the This gavemuch rf the theoretical *'asusedto scavrdified to include ration and before :he Bellevuefield o 6 of.l5Vo of the lar to that develirectly to the east rat Cities Service ity is about 20 to ing breakdownof 7.27/Bin 1980-1.

The largestprojectis in the Suplacude Barcaufield, which is in the northwest of the country.The reservoirconsistsof a high-quality,shallowsandsaturatedwith a 2000-cpoil; the reservoirdips at 5 to 8'toward the south. Reservoirproperties are shownTable9.11.Figure 9.65showsa map of the field. A pilot testwasinitiated in 1964,and semi-industrialoperationwasstartedin 1967.This has sincebeen expandedto the point where "given the amountsof air injectedand oil produced,this projectmay be consideredthe mostimportantcombustionin the world" (Gadelleet al. 1981). Oil production due to combustionhas increasedfrom 340 tld, (2200 B/d) in 1974to 800 t/d in 1977and to 1000t/d in 1979.Productionrates are plotted in Figure 9.66. The increasein productionwas achievedby the additionof air-compression capacityand by the extensionof the length of the combustionfront; at the end of 1979this was more than 4 km long. The numberof injectionwells was increasedfrom 11 in 1974to 38 in 1979. During this period the air-compression capacityrose from 0.55 million standard m3lagS.+ million SCF/d)to 1.8million. The air-to-oil ratio achievedin long-term operationis between1500and 2000std m3/t (8.4 andlL2 kSCF/B)of oil.

'r,:i/

o

tr!

o

o

ato

a

204

?a

aa(,

o

at2

o

o

O

o

l0a

o

!al 0

.ri

tl

att

||t

o

atl

6

t0

o

o

9!

o

aaa

Itl

o

lcts beingcarried nna 1982;Aldea, llatedreferences.

o

!tt

a

l?a

o

ta

!rI o\

20t org

l?lt

||l

o

,

I!

@ o

'.

.)\

o

o

!T

It

ed

o

_9_: l.

@

a;-:--j

E0m)

PtlotArco

?\ _

..a 0t

o

att

ll

t,

7/1,, ti..

6+'",d

o I

€8

o

to o

0| I

IA

llt

lioboth

rl)

@

Arr or wotsr npction u.ll in Oct.1977

lOOn

o

ftodtttidr

nll

an Oct.B77

Figure 9.65 Suplacude Barcau Field (from Gadelleet al. 1981)

rustion

Chap.9


475

START x gTU COIE Srbx

Ttr€sc l. 2. 3. 4.

€c t

r
3300

Engine Vo Efficiency(t)

Fuel Requirement SCVHP h

28 32 36 38

10.0 8.8 7.8

(l)Based on LHV of methanefuel (908Btu/SCF). (')Based on Perry, R. H. and Chilton, C.H.., ChemicalEngineerbHandbook,5th ed., New york: McGraw-Hill (1973),24-14.

Basis:Inlet conditions60"F and 1 atmosphere. Discharge Pressure Psia

100 200 400 800 1200

HP h per 1000SCF Compressed Isothermal Compression(r)

2.0s 2.79 3.53 4.28 4.71

(t)Hp = t.o7 tn(p2lp). (')Assuming practicalHP is 40Vogreaterthan isothermal,i.e., HP = 1.5 ln(pz/pt). 518

Practical(2)

2.88 3.92 4.96 6.00 6.60

TABLE A.8.1 TherrnalCor

Material Polyurethane Polystyrene Cellular elastomeric Cork pipe insulation Cellulosefiber board Mineral fiber blanket block board pipe insulation Cellularglass Calciumsilicate block board r('

Diatomaceousearth Diatomaceousearth Expandedperlite Neisel and Verschoor , l95lr

Appendix8 Thermql lnsulstion

Thermal insulationpracticeswere reviewedin an article by Neiseland Verschoor (1981),which contains59 references. SeeTableA.8.1. Fuel Requirement scF/HP h 10.0 8.8 7.8 7.4 5th ed., New York:

I Compressed Practical(2)

2.88 3.92 4.96 6.00 6.60 t(PzlP).

TABLE A.8.1 ThermalConductivitiesof Block,Board,and pipe Insulation Temp range or maximum,'C

Material Polyurethane Polystyrene Cellular elastomeric Cork pipe insulation Cellulosefiber board Mineral fiber blanket block board pipe insulation Cellular glass Calcium silicate block board Diatomaceousearth Diatomaceousearth Expandedperlite

-73 to 110

Thermal Conductivity,m$(m "C)

0'c 1A JI

4l

24'C

100'C

400'c

540'C

25 35 +J

48 55

204 204 982 650 -268 to 427 649 650 (577kg/m3or 36 lbft3) 870 1040 816

37 4l

5t 52

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