Geological Storage of Carbon Dioxide (Geological Society of London Special Publication No. 233)

Geological Storage of Carbon Dioxide

Geological Society Special Publications Society Book Editors R. J. PANKHURST (CHIEF EDITOR) P. DOYLE F. J. GREGORY J. S. GRIFFITHS A. J. HARTLEY R. E. HOLDSWORTH

J. A. HOWE P. T. LEAT A. C. MORTON N. S. ROBINS J. P. TURNER Special Publication reviewing procedures The Society makes every effort to ensure that the scientific and production quality of its books matches that of its journals. Since 1997, all book proposals have been refereed by specialist reviewers as well as by the Society's Books Editorial Committee. If the referees identify weaknesses in the proposal, these must be addressed before the proposal is accepted. Once the book is accepted, the Society has a team of Book Editors (listed above) who ensure that the volume editors follow strict guidelines on refereeing and quality control. We insist that individual papers can only be accepted after satisfactory review by two independent referees. The questions on the review forms are similar to those for Journal of the Geological Society. The referees' forms and comments must be available to the Society's Book Editors on request. Although many of the books result from meetings, the editors are expected to commission papers that were not presented at the meeting to ensure that the book provides a balanced coverage of the subject. Being accepted for presentation at the meeting does not guarantee inclusion in the book. Geological Society Special Publications are included in the ISI Index of Scientific Book Contents, but they do not have an impact factor, the latter being applicable only to journals. More information about submitting a proposal and producing a Special Publication can be found on the Society's web site: www.geolsoc.org.uk.

It is recommended that reference to all or part of this book should be made in one of the following ways: BAINES, S. J. & WORDEN, R. H. (eds) 2004. Geological Storage of Carbon Dioxide. Geological Society, London, Special Publications, 233. GUNTER, W. D., BACHU, S. & BENSON, S. 2004. The role of hydrogeological and geochemical trapping in sedimentary basins for secure storage of carbon dioxide. In: BAINES, S. J. & WORDEN, R. H. (eds) 2004. Geological Storage of Carbon Dioxide. Geological Society, London, Special Publications, 233,129-145.

GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 233

Geological Storage of Carbon Dioxide

EDITED BY SHELAGHJ.BAINES BP Exploration and Production Company, Sunbury, UK and

RICHARD H. WORDEN Department of Earth & Ocean Sciences, University of Liverpool, Liverpool, UK

2004

Published by The Geological Society London

THE GEOLOGICAL SOCIETY

The Geological Society of London (GSL) was founded in 1807. It is the oldest national geological society in the world and the largest in Europe. It was incorporated under Royal Charter in 1825 and is Registered Charity 210161. The Society is the UK national learned and professional society for geology with a worldwide Fellowship (FGS) of 9000. The Society has the power to confer Chartered status on suitably qualified Fellows, and about 2000 of the Fellowship carry the title (CGeol). Chartered Geologists may also obtain the equivalent European title, European Geologist (EurGeol). One fifth of the Society's fellowship resides outside the UK. To find out more about the Society, log on to www.geolsoc.org.uk. The Geological Society Publishing House (Bath, UK) produces the Society's international journals and books, and acts as European distributor for selected publications of the American Association of Petroleum Geologists (AAPG), the American Geological Institute (AGI), the Indonesian Petroleum Association (IPA), the Geological Society of America (GSA), the Society for Sedimentary Geology (SEPM) and the Geologists' Association (GA). Joint marketing agreements ensure that GSL Fellows may purchase these societies' publications at a discount. The Society's online bookshop (accessible from www.geolsoc.org.uk) offers secure book purchasing with your credit or debit card. To find out about joining the Society and benefiting from substantial discounts on publications of GSL and other societies worldwide, consult www.geolsoc.org.uk, or contact the Fellowship Department at: The Geological Society, Burlington House, Piccadilly, London W1J OBG: Tel. +44 (0)20 7434 9944; Fax +44 (0)20 7439 8975; E-mail: [email protected]. For information about the Society's meetings, consult Events on www.geolsoc.org.uk. To find out more about the Society's Corporate Affiliates Scheme, write to [email protected]. Published by The Geological Society from: The Geological Society Publishing House Unit 7, Brassmill Enterprise Centre Brassmill Lane BathBA13JN,UK (Orders: Tel. +44 (0)1225 445046 Fax+ 44 (0)1225 442836) Online bookshop: http://bookshop.geolsoc.org.uk The publishers make no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility for any errors or omissions that may be made. © The Geological Society of London 2004. All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with the provisions of the Copyright Licensing Agency, 90 Tottenham Court Road, London W1P 9HE. Users registered with the Copyright Clearance Center, 27 Congress Street, Salem, MA 01970, USA: the item-fee code for this publication is 0305-8719/047$ 15.00. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 1-86239-163-7 Typeset by Servis Filmsetting Limited, Manchester, UK Printed by Cromwell Press, Trowbridge, UK

Distributors USA AAPG Bookstore PO Box 979 Tulsa OK 74101-0979 USA Orders: Tel. + 1 918 584-2555 Fax+ 1918 560-2652 E-mail [email protected] India Affiliated East-West Press PVT Ltd G-l/16 Ansari Road, Darya Ganj, New Delhi 110002 India Orders: Tel. +91 11 2327-9113, 2326-4180 Fax+ 91 112326-0538 E-mail [email protected] Japan Kanda Book Trading Company Cityhouse Tama 204 Tsurumaki 1-3-10 Tama-shi, Tokyo 206-0034 Japan Orders: Tel. + 81 (0)423 57-7650 Fax+ 81 (0)423 57-7651 E-mail [email protected]

Contents BAINES, S.J. & WORDEN, R.H. Geological storage of carbon dioxide GALE, J. Why do we need to consider geological storage of CO2? CHADWICK, R.A., HOLLOWAY, S., BROOK, M.S. & KIRBY, G.A. The case for underground CO2 sequestration in northern Europe PEARCE, J., CZERNICHOWSKI-LAURIOL, I., LOMBARDI, S., BRUNE, S., NADOR, A., BAKER, J., PAUWELS, H., HATZIYANNIS, G., BEAUBIEN, S. & FABER, E. A review of natural CO2 accumulations in Europe as analogues for geological sequestration SHIPTON, Z.K., EVANS, J.P., KIRSCHNER, D., KOLESAR, P.T., WILLIAMS, A.P. & HEATH, J. Analysis of CO2 leakage through 'low-permeability' faults from natural reservoirs in the Colorado Plateau, east-central Utah BAINES, S.J. & WORDEN, R.H. The long-term fate of CO2 in the subsurface: natural analogues for CO2 storage ROCHELLE, C, CZERNICHOWSKI-LAURIOL, I. & MILODOWSKI, A.E. The impact of chemical reactions on CO2 storage in geological formations: a brief review JOHNSON, J.W, NITAO, J.J. & KNAUSS, K.G. Reactive transport modelling of CO2 storage in saline aquifers to elucidate fundamental processes, trapping mechanisms and sequestration partitioning GUNTER W.D., BACHU, S. & BENSON, S. The role of hydrogeological and geochemical trapping in sedimentary basins for secure geological storage of carbon dioxide HOVORKA, S.D., DOUGHTY, C., BENSON, S.M., PRUESS, K. & KNOX, PR. The impact of geological heterogeneity on CO2 storage in brine formations: a case study from the Texas Gulf Coast ZWEIGEL, P., ARTS, R., LOTHE, A.E. & LINDEBERG, E.B.G. Reservoir geology of the Utsira Formation at the first industrial-scale underground CO2 storage site (Sleipner area, North Sea) ARTS, R., EIKEN, O., CHADWICK, A., ZWEIGEL, P., VAN DER MEER, B. & KIRBY, G. Seismic monitoring at the Sleipner underground CO2 storage site (North Sea) LAENEN, B., VAN TONGEREN, P., DREESEN, R. & DUSAR, M. Carbon dioxide sequestration in the Campine Basin and the adjacent Roer Valley Graben (North Belgium): an inventory WORDEN, R.H. & SMITH, L.K. Geological sequestration of CO2 in the subsurface: lessons from CO CO2 injection enhanced oil recovery projects in oil fields BACHU, S. & GUNTER, W.D. Acid-gas injection in the Alberta Basin, Canada: a CO2-storage experience STENHOUSE, M.J. & SAVAGE, D. Monitoring experience associated with nuclear waste disposal and its application to CO2 sequestration projects Index

1 7 17

29

43 59 87 107 129 147 165 181 193

211 225 235 249

This page intentionally left blank

Geological storage of carbon dioxide SHELAGH J. BAINES1 & RICHARD H. WORDEN2 1

BP Exploration and Production Company, Chertsey Road, Sunbury-on-Thames, TW16 7LN, UK (e-mail: bainess @bp. com) 2 Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK (e-mail: [email protected])

Abstract: Carbon dioxide is the main compound identified as affecting the stability of the Earth's climate. A significant reduction in the volume of greenhouse gas emissions to the atmosphere is a key mechanism for mitigating against climate change. Geological storage of CO2, or the injection and stabilization of large volumes of CO2 in the subsurface in saline aquifers, existing hydrocarbon reservoirs or unmineable coal-seams, is one of the more technologically advanced options available. A number of studies have been carried out aimed at understanding the behaviour and long term fate of CO2 when stored in geological formations.

Global warming, and through it, climate change has been generally accepted as a problem; the consensus proffered by the International Panel on Climate Change (IPCC) in 2001 was that climate change is real and that its causes and effects should be mitigated. Recent studies suggest that the earth's surface has warmed by 0.6° over the past 100 years (Hulme & Jenkins 1998). A significant reduction in the volume of greenhouse gas emissions to the atmosphere was identified as a key mechanism for achieving stability. Carbon dioxide (CO2) is the main compound identified as affecting the stability of the Earth's climate, representing 62.5% of all greenhouse gases generated globally (IPCC 1996, 2001; review by Gale). Geological storage of CO2, or the injection and stabilization of large volumes of CO2 in the subsurface, is one option being employed against rising CO2 emissions. Most of the additional CO2 added to the atmosphere has been generated by human activity; mainly through fossil fuel combustion (e.g. power plants, refineries, motor vehicles). Roughly one third of CO2 emissions in the USA come from power plants and other large point sources (US DOE 2004). In the UK alone, the volume of CO2 emitted has increased by one third since the industrial revolution (IPCC 1996). Predictions of CO2 emissions suggest the increase is, and will remain, exponential under the current energy-use pattern.

Geological storage options There are a number of ways by which CO2 emissions can be reduced. The technology exists to capture the CO2 at a power plant, separating it from the flue

gases and producing a stream of concentrated gas. In addition, a number of oil and gas reservoirs contain significant volumes of CO2 (e.g. Baines & Worden) which must be stripped from the hydrocarbon before processing or sale. Historically, this CO2 would have been vented to the atmosphere. Once the stream of CO2 is produced, it needs to be removed from the earth's atmosphere for a period of time sufficient to allow climate stabilization. The actual time period required is not well understood, however, the general consensus is that CO2 would need to be prevented from reaching the atmosphere for hundreds, if not thousands of years (Gunter et al.). The three main options for subsurface storage of CO2 are saline aquifers, existing oil and gas fields, and unmineable coal seams. These can be split into two economic end-members. First are purely storage options, whereby the CO2 is injected without any positive benefit other than to the atmosphere (e.g. saline aquifers), although carbon tax credits may improve the financial viability of this option. The second are utilization options where the CO2 injection process has additional, economic benefits through enhanced oil recovery or coal-bed methane production (e.g. existing oil and gas fields and coal seams). Alternative options for captured CO2 exist (Chadwick et al; Gale) but these are at an early stage of technology development. They include deep ocean sequestration (e.g. Omerod et al. 1999), carbonate production, and injection into deep carbonate sediments. Research into geological storage of CO2 is still at a relatively early stage, although the technology has matured over the past decade from feasibility and theoretical studies (e.g. Cox etal 1996; CzernichowskiLauriol et al. 1996; Holloway 1997; 2001; Gupta et

From: BAINES, S. J. & WORDEN, R. H. (eds) 2004. Geological Storage of Carbon Dioxide. Geological Society, London, Special Publications, 233,1-6.0305-8719/047$ 15.00 © The Geological Society of London 2004.

2

S. J. RAINES & R. H. WORDEN

al. 2002) to application in the hydrocarbon industry in storage (Sleipner field, North Sea) and utilization (Weyburn field, Canada) projects.

Saline aquifers Saline aquifers can be defined as units which do not have any potential to act as sources of potable water. The volumetric potential for saline aquifer storage is huge (Holloway 1997; Gunter et al. 1998). Studies suggest that, on a global scale, saline aquifers could account for between 20% and 500% of the projected total CO2 emissions to 2050 (Davidson et al. 2001). It has been estimated that deep saline formations in the United States alone could potentially store up to 500 billion tonnes of CO2 (US DOE 2004). Chadwick et al. suggest that within the UK, the Bunter sandstone can provide a storage volume equivalent to around 350 years of UK derived emissions. A similar study of the Campine Basin in Northen Belgium (Laenen et al.} has identified a CO2 storage capacity of several hundred million tonnes in Triassic and Carboniferous formations. Economically, saline aquifer storage is less attractive than other utilization options. However, carbon tax credits, or carbon trading systems may improve the economics. Gas from the Sleipner field in the Norwegian sector of the North Sea contains 4.0-9.5% CO2; the sales gas agreement allows a maximum CO2 content of 2.5% (Baklid et al. 1996). Venting to atmosphere of the approximate 1 Mt/a of stripped CO2 would increase the total Norwegian emissions by 3% but also cost the operator in carbon taxes under Norway's tax scheme. The solution was to strip the CO2 from the produced gas at the offshore top-side facility and then re-inject it, as a supercritical phase, into a saline aquifer 800m below the sea-bed. Although not 100% cost-effective (DOE EIA 2002), the project was the world's first geological storage programme, providing a demonstration of the technology required to store CO2. An accompanying research project studied the first five years of CO2 injection (Torp & Gale 2003; Zweigel et al. 2002; Arts et al.\ Zweigel et al.}. Further storage projects are already being planned, for example, the Gorgon field on the North West Shelf, Australia where the produced gas contains up to 14% CO2. CO2 stripped from the gas will be reinjected 2000m below the producing reservoir into a saline formation (IEA 2003).

Existing oil and gas fields The alternative to storage of CO2 is to use the gas to improve the economics of the capture process. The oil industry uses CO2 to improve the recovery effi-

ciency of heavy oils, a process known as enhanced oil recovery (EOR). Around 32 million tons of CO2 per year are injected into oil reservoirs for EOR in the US (NETL web-site). The two main EOR processes are discussed by Holm & Josendal (1974), Monger et al. (1991), and briefly by Worden & Smith. Historically, the long term storage of CO2 was not considered by EOR operators; CO2 which reached the surface during production was either vented or captured for recycling. However, technology to increase the volume of CO2 that remains in the subsurface does exist. An additional economic benefit of CO2 injection into oil reservoirs is the extension of field life by improving rate of recovery. CO2 injection into gas fields for improved pressure support is also being considered as an option. Oil and gas fields also have potential as storage reservoirs after they cease to be economic. The geological understanding of old petroleum fields is typically much greater than that for saline aquifers and potential exists for recycling of production and enhanced oil recovery (injection) infrastructure already in place. A major research project, combining EOR with CO2 storage was started in 2000 at the Weyburn field, Canada. CO2 purchased from a synthetic fuel plant in North Dakota, USA was piped 320km to the field in Saskatchewan province. The EOR project is expected to produce 130 million barrels of incremental oil through CO2 displacement, extending field life by 25 years (Moberg 2001). Studies on the phase behaviour of the CO2-reservoir fluids, the chemical reactions between the CO2 and reservoir rocks, the impact of the CO2-rock interaction on fluid flow, formation stability, seal integrity and storage capacity were undertaken (Moberg 2001). Detailed time-lapse monitoring programmes, using various remote and direct techniques, were planned to study the physical distribution of the CO2 during the injection period, providing data for history matching reservoir models. The overall aim was to provide a comprehensive risk and economic assessment of CO2 storage in the area. Acid gas, a variable mixture of hydrogen sulphide (H2S) and CO2 derived from the 'sweetening' of sour gas, is also a candidate for geological storage. In Alberta, Canada, instead of flaring the acid gas or stripping the sulphur (desulphurization) and storing it in solid form, the gas mixture has been injected into several geological formations, including both saline aquifers, old oil and gas reservoirs and the water leg of producing reservoirs (Bachu & Gunter).

Unmineable coal-seams Coal seams typically contain economically large amounts of methane-rich gas that is adsorbed onto

GEOLOGICAL STORAGE OF CARBON DIOXIDE

the surface of the coal. Extraction of the gas requires depressurization, usually by pumping water out of the reservoir. Roughly twice as much CO2 can be adsorbed on coal as methane such that injected CO2 has the potential to displace methane and remain sequestered in the coal seam (Reeves 2003). A pilot project in San Juan, New Mexico suggests that methane production from the extensive coal beds there could be increased 75% by injecting CO2 (Reeves etal. 2001). For any CO2 storage or utilization process to be successful, it must be economically competitive compared to alternative options, and effective in significantly reducing emissions. It must be a long-term option that can prevent emissions from reaching the atmosphere for sufficient periods of time and be safe both to the population and to the environment. In addition, the technology must be flexible to allow storage in a range of geological regimes, ideally, close to the point source of CO2. A number of studies have been carried out aimed at understanding the behaviour of CO2 when stored in geological formations and to help create guidelines for the selection and development of CO2 storage facilities.

Natural analogues for CO2 storage CO2 occurs naturally in the subsurface, often in large volumes (Pearce et al. 1996; Baines & Worden; Pearce et al; Ship ton et al). There are a number of possible sources (Baines & Worden, and references therein); CO2 is often associated with igneous processes, with high temperature metamorphism of carbonate-bearing rocks and de-volatilization of CO2-bearing fluids. Alteration of organic matter can also produce abundant CO2. The retention time of these CO2 accumulations can vary from extremely short term, active vents (Pearce et al.; Shipton et al), to fields that have undergone CO2-EOR with a CO2 residence time of months to several years (Wolcott et al 1989; Worden & Smith), to CO2 in subsurface traps that have been in place for many thousands or even millions of years (Baines & Worden). The study of these accumulations provides ideal natural analogues for studying the longterm consequences, both physical and chemical, of introducing CO2 into the subsurface in a range of geological environments.

Geochemical fate of CO2 CO2 is a reactive gas and, unlike methane, interacts with both the rock and formation fluid in the injected reservoirs. A number of studies have attempted to predict the behaviour of CO2 under a range of physi-

3

cal and chemical conditions (Czernichowski-Lauriol et al 1996; Gunter et al 1998; Gunter et al, Rochelle et al; Worden & Smith). CO2 solubility is dependent on the temperature, pressure and composition of the formation fluids. The proportion of an injected volume which dissolves is controlled by the surface area of CO2-water interaction and the degree of mixing. Injection rate also plays a role. Dissolution of CO2 creates chemical disequilibria in the storage formation, resulting in reactions involving mineral dissolution and precipitation (Bachu et al. 1994; Baines & Worden; Rochelle et al., Worden & Smith). Precipitation of carbon-bearing minerals results in much more stable, longer term trapping, or sequestration, of the CO2; however, studies from natural analogues suggest that mineral precipitation is relatively minor, slow, and controlled by the concentration of divalent cations available for reaction (Baines & Worden). The extent of CO2-water-rock interaction during migration of the injected CO2 is the main control on the ultimate fate of the CO2. Reactive transport modelling of a Sleipner-like storage reservoir suggested that only 1% of the CO2 precipitated as carbonate minerals whereas 15-20% was still dissolved in the formation fluids after 20 years. The remainder stayed as an immiscible phase (Johnson etal.). Migration and trapping of CO2 The pressure, hydrodynamic and geothermal regimes in a basin have a significant impact on its potential for CO2 storage (Gunter et al). At the basin scale, the tectonic regime affects the rate at which fluids migrate up-dip, or along fault zones. The physical and chemical controls on CO2 state post-injection affect the migration of the injected volume. CO2 that remains in an immiscible phase is driven by buoyancy forces, displacing the pore fluids, up-dip to the surface unless constrained by stratigraphical or structural traps in the same manner as oil and gas accumulations. This upward migration may be extremely slow taking more than a million years (Gunter et al), thus forming transient 'hydrodynamic traps' (Bachu et al. 1994). CO2 dissolved in formation fluid migrates at an even slower pace, at the rate of the in situ formation fluids of the basin, which may have subsurface residence times of many millions of years. A key point is that fluids and gases do not remain stationary in the subsurface but migrate at rates related to their composition, state and the 'plumbing' of the basin. Oil and gas accumulations provide a good analogy for the long-term security of injected CO2; hydrocarbons form continually migrating fronts; however, their progress is slowed by subsurface traps allowing significant accumulations to form.

4

S. J. BAINES & R. H. WORDEN

The geological fill of a basin and the internal heterogeneities at all scales, including faults and fractures, depositional patterns, and porositypermeability trends, affect the capacity and effectiveness of CO2 storage (Hovorka et al.; Johnson et al.). The buoyancy-driven migration of immiscible CO2 is controlled by barriers and baffles to flow, increasing the CO2-water-rock interaction. This internal heterogeneity can also have a negative impact on storage effectiveness. Barriers to flow, particularly close to the injection point can create pressure gradients resulting in barrier or seal failure and continued migration of the CO2 along the path of least resistance. It is necessary to understand the storage volume at a greater scale than merely the primary trapping mechanism; secondary or even tertiary traps may occur over time. Studies carried out at the Sleipner site in Norway provide some understanding of how CO2 migrates immediately post-injection (Arts et al. 2000; Chadwick et al 2000; Lindeberg et al 2000; Zweigel et al 2000; Arts et al., Zweigel et al.}. Reservoir models built for the injected reservoir, the Utsira Formation, were used to model CO2 migration. These were history-matched using data from time-lapse 3D seismic over the injected volume. The results demonstrated the impact of the internal heterogeneity, present as shale bands of variable thickness and shallow structural doming, of the Utsira Formation on CO2 migration.

Monitoring CO2 storage sites Monitoring provides confidence that the distribution of the injected CO2 can be proven both immediately post-injection and in the medium term after the injection period is completed. If mitigation is required at any point, i.e. through unexpected migration direction, an accurate picture of CO2 distribution would be necessary. Verification of the injected volume may also become a requirement. The monitoring process is an integral part of the storage development plan. A number of monitoring options have potential, including both surface and downhole (cross-well and vertical seismic profiling) methods. Any monitoring option should be cheap, repeatable, easy to acquire, and have a short interpretation turnaround time The use of 3D seismic has been demonstrated at two CO2 injection sites. At Sleipner, a pre-injection 3D seismic survey was shot over the area (Arts et al 2000) and, although not optimized for the Utsira Formation, formed the baseline survey. Subsequent surveys indicated that time-lapse seismic allows the CO2 distribution to be mapped effectively (Arts et al.) in this offshore field. For the onshore Weyburn CO2-EOR project, a comprehensive multicompo-

nent 3D seismic survey was shot over the injection area prior to any CO2 injection. This was followed by a second survey after 14 months of CO2 injection. Strong seismic anomalies around the CO2 injector wells in the second survey suggest good imaging of the CO2 migration fronts (Jazwari 2002).

Regulation of CO2 storage Regulatory frameworks exist for the disposal of hazardous wastes; radioactive and chemical. Monitoring is a key component of the development and operation of nuclear waste repositories (Stenhouse & Savage), requiring continuing geotechnical, groundwater and environmental checks to ensure the continued security of the stored material. Injection or long-term storage in geological formations has been taking place for a number of years. Although there are distinct physical and chemical differences between CO2 and liquid or radioactive waste, the experience gained from these industries may be valuable when designing guidelines for CO2 storage. The technology for CO2 storage is developing rapidly. If successful, a large number of wells, both onshore and offshore could be required for injecting the volumes of CO2 available for storage. At present, no dedicated regulatory framework is in place in any country. The two areas should be developed together, to ensure technology meets the needs of the regulatory controls, i.e. monitoring guidelines and operational constraints.

The future of geological storage of CO2 Geological storage of CO2 remains a controversial concept amongst the public and environmental groups. Even if all technical obstacles to geological storage are met and if sufficient capacity at an economic rate is available to mitigate much of the present CO2 emissions, the safety and acceptability of storage has to be demonstrated. However, national governments are beginning to realize that today's policies regarding carbon emissions and the environment cannot be sustained and will not meet the targets set for future emissions. Between 2002 and 2004 both the US and UK governments set targets for reducing greenhouse gas emissions. In 2002, the UK government issued a White Paper (UK DTI 2002) stating their energy goals which included plans to reduce CO2 emissions by 60% by 2050, with the aim of making significant progress by 2020. Similarly, in 2004, the USA government Global Climate Change Initiative (US DOE 2004) stated an ambition to reduce greenhouse gas intensity by 18% by 2012. Both countries highlighted geological storage as the technologically most advanced option for achieving

GEOLOGICAL STORAGE OF CARBON DIOXIDE these aims, with the US indicating it would have commercially-ready storage or sequestration technologies available for assessment by 2012. The focus of future carbon management is on energy efficiency and alternative energy supplies such as lower carbon fuels (e.g. switching from coal to natural gas-fired power stations) and renewable energy mechanisms. However, the current global energy demand is such that these options cannot supply sufficient energy at present and the timeframe for development means that fossil fuels will remain the main fuel source for the short to medium term. However, by capturing the carbon at source and removing it from the atmosphere and surface environment of the planet, fossil fuels can become a low-carbon option, thus reducing greenhouse gas emissions. CO2 storage therefore becomes a stopgap mechanism, buying time whilst development of efficient, low- or no-carbon energy sources continues. This means that the technology required to carry out effective, safe, CO2 storage is needed now. Many of the papers in this volume arose from a technical session held at BUG 2001, entitled 'Greenhouse Gas Disposal' chaired by Shelagh Baines and John Gale. The aim of the session was to provide an introduction to the science of geological storage of carbon dioxide to a wider technical audience. The editors would like to thank the following for undertaking the technical reviews of the papers in this volume: Etienne Brosse, John Bunney, Charles Byrer, Bob Chaplow, Max Coleman, Daniel Garcia, Hal Gluskotter, Neeraj Gupta, Adrian Hartley, Stuart Haszeldine, Howard Herzog, Susan Hovorka, Ian Hutcheon, Yousef Kharaka, Rob Lander, Steve Laubach, Craig Lewis, Eric Lindeburgh, Ross McCartney, Tony Milodowski, Larry Meyers, Paul Nadeau, Tim Needham, Euan Nisbet, Sue Raikes, Beverly Saylor, Bruce Sass, Craig Smalley, Scott Stevens. Angharad Hills and Sally Oberst of the Geological Society dealt with the organization and editing of the volume. John Gale is thanked for helping with the early stages of the volume.

References ARTS, R., BREVIK, L, EIKEN, O., SOLLIE, R., CAUSSE, E. & VAN DER MEER, B. 2000. Geophysical methods for monitoring marine aquifer CO2 storage - Sleipner experiences. In: WILLIAMS, D. J., DURIE, B., MCMULLAN, P., PAULSON, C. & SMITH, A. (eds) Greenhouse Gas Control Technologies, CSIRO Publishing, Collingwood, Australia, 366-371. BACHU, S., GUNTER, W.D. & PERKINS, E. H. 1994. Aquifer disposal of CO2: hydrodynamic and mineral trapping. Energy Conversion and Management, 35, 269-279. BAKLID, A., KORB0L, R. & OWREN, G. 1996. Sleipner Vest CO2 disposal, CO2 injection into a shallow underground aquifer. Paper presented on the 1996 SPE Annual technical Conference and Exhibition, Denver, Colorado, USA, SPE paper 36600,1-9. CHADWICK, R. A., HOLLOWAY, S., KIRBY, G. A., GREGERSEN, U. & JOHANNESSEN, P. N. 2000. The Utsira Sand,

5

Central North Sea - an assessment of its potential for regional CO2disposal. In: WILLIAMS, D. J., DURIE, B., MCMULLAN, P., PAULSON, C. & SMITH, A. (eds) Greenhouse Gas Control Technologies, CSIRO Publishing, Collingwood, Australia, 349-354. Cox, H., HEEDRICK, J. P., VAN DER MEER, B., VAN DER STRAATEN, R., HOLLOWAY, S., METCALFE, R., FABRIOL, H. & SUMMERFIELD, I. 1996. Safety and stability of underground storage. In: The Underground Disposal of Carbon Dioxide. Joule II project no: CT92-0031, Final report. CZERNICHOWSKI-LAURIOL, I., SANJUAN, B., ROCHELLE, C., BATEMAN, K., PEARCE, J. & BLACKWELL, P. 1996. Analysis of the geochemical aspects of the underground disposal of CO2. In: APPS J.A. & TSANG C.F. (eds) Deep Injection Disposal of Hazardous and Industrial Waste. Academic Press Inc, pp. 565-583. DAVIDSON, J., FREUND, P. & SMITH, A. (2001). Putting Carbon Back in the Ground. IEA Greenhouse Gas R&D Programme, February. DOE EIA. 2002. Norway: Environmental Issues, http ://w w w. eia.doe. gov/emeu/cabs/norenv.html GUNTER, W. D., WONG, S., CHEEL, D. B. & SJOSTROM, G. 1998. Large CO2 sinks: Their role in the mitigation of greenhouse gases from an international, national (Canadian) and provincial (Alberta) perspective. Applied Energy, 61, 209-227. GUPTA, N., SASS, B. M., SMINCHAK, J. R. & HICKS, J. E. 2002. Feasability of long term carbon dioxide storage in deep saline formations, http://www.netl.doe.gov/ publications/proceedings/98/98ps/ps4-7.pdf HOLM, L. W. & JOSENDAL, V. A. 1974. Mechanisms of oil displacement by carbon dioxide. Journal of Petroleum Technology, December 1974. HOLLOWAY, S. 1997. An overview of the Joule II project: The underground disposal of carbon dioxide. Energy Conversion and Management, 38S, S193-S198. HOLLOWAY, S. 2001. Storage of fossil fuel-derived carbon dioxide beneath the surface of the earth. Annual reviews of Energy and the Environment, 26, 145-166. HULME, M. & JENKINS, G. J. 1998. Climate Change Scenarios for the U.K.: Scientific Report. TJKCIP Technical Report No:l, Climate Research Unit, Norwich. IEA. 2003. Australia's Gorgon gas development will reinject reservoir CO2. Greenhouse Issues, 66. http://www.ieagreen.org.uk/ IPCC. 1996. Climate Change 1995: The Science of Climate Change, Summary for Policymakers and Technical Summary of the Working Group I Report. Cambridge University Press, Cambridge, UK, 56pp. IPCC. 2001. Climate Change 2001: The Scientific Basis, Summary for Policymakers and Technical Summary of the Working Group I Report. JAZWARI, W. 2002. Monitoring CO2 injection at Weyburn. Greenhouse Issues, 61, 5-6. IEA Greenhouse Gas Programme, U.K. LlNDEBERG, E., ZWEIGEL, P., BERGMO, P., GHADERI, A. &

LOTHE, A. 2000. Prediction of CO2 dispersal pattern improved by geology and reservoir simulation and verified by time lapse seismic. In: WILLIAMS, D. J., DURIE, B., MCMULLAN, P., PAULSON, C. & SMITH, A. (eds) Greenhouse Gas Control Technologies, CSIRO Publishing, Collingwood, Australia, 372-377.

6

S. J. BAINES & R. H. WORDEN

MOBERG, R. 2001. The Weyburn CO2 monitoring and storage project. Greenhouse Issues, 57, 2-3. MONGER, T. G., RAMOS, J. C. & THOMAS, J. 1991. Light oil recovery from cyclic CO2 injection: influence of low pressures, impure CO2 and reservoir gas. SPE Reservoir Engineering, 6, 25-32. OMEROD, W. G., FREUND, P. & SMITH, A. 1999. Ocean Storage of CO2. IEA Greenhouse Gas R&D Programme Report. NETL. http://www.netl.doe.gov PEARCE, J. M., HOLLOWAY, S., WACKER, H., NELIS, M. K., ROCHELLE, C. & BATEMAN, K. 1996. Natural occurrences as analogues for the geological disposal of carbon dioxide. Energy Conversion and Management, 37,1123-1128. REEVES, S. 2003. Enhanced CBM recovery: Coal-bed CO2 sequestration assessed. Oil and Gas Journal, July 14th. REEVES, S., PEKOT, L. & CLARKSON, C. 2001. Geologic sequestration of CO2 in deep, unmineable coal-beds: An integrated research and commercial scale field demonstration project. SPE 71749. Proceedings of the SOE Annual Technical Conference and Exhibition, New Orleans, September 30-October 3 2001. TORP, T. & GALE, J. (2003), Demonstrating Storage of CO2

in Geological Reservoirs: The Sleipner and SACS Projects. In: GALE, J. & KAYA, Y. (eds) Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies, Kyoto, Japan, Elsevier Science. UK DTI. 2002. Our Energy Future - Creating a Low Carbon Economy, http://www.dti.gov.uk/energy/ whitepaper/index. shtml US DOE. 2004. http://www.fe.doe.gov/programs/ sequestration WOLCOTT J. M., MONGER T. G., SASSEN, R. & CHINN, E. W. 1989. The effects of CO2 flooding on reservoir mineral properties. 1989 SPE International Symposium, Houston, Texas. SPE Paper 18467, 101-109. ZWEIGEL, P., HAMBORG, M., ARTS, R., LOTHE A. & T0MMERAs, A. 2000. Prediction of migration of CO2 injected into an underground depository: Reservoir geology and migration modelling in the Sleipner case (North Sea). In: WILLIAMS, D. J., DURIE, B., MCMULLAN, P., PAULSON, C. & SMITH, A. (eds) Greenhouse Gas Control Technologies, CSIRO Publishing, Collingwood, Australia, pp. 360-365.

Why do we need to consider geological storage of CO2? JOHN GALE IE A Greenhouse Gas R&D Programme, Stoke Orchard, Cheltenham, Glos., GL52 4RZ, UK (e-mail: [email protected]) Abstract: To meet targets for greenhouse gas emission reduction set by the Kyoto Protocol, many countries are considering a range of near-term options such as, fuel switching, energy efficiency improvements and use of renewable sources of energy, to reduce their greenhouse gas emissions. However, to meet the goal of the UN Framework Convention on Climate Change, namely stabilization of greenhouse gas concentrations in the atmosphere, it is likely that deeper reductions in emissions will be needed. This will require additional measures such as the geological storage of CO2. Geological storage of CO7 would be used to sequester CO2 captured from large anthropogenic sources, such as power and large industrial plants. There are a number of reservoirs suitable for geological storage of CO2 including depleted oil and gas fields and deep saline aquifers. Many of these reservoirs have stored hydrocarbons and fluids for million of years, which gives confidence that CO2 can be stored for similar durations, but research is needed to confirm this.

The topic of geological storage of CO2 forms the main focus of this volume. This opening paper sets the scene by debating why this particular mitigation option should be considered in the context of reducing greenhouse gas emissions globally. The paper reviews the international arena and reviews what actions are underway to reduce greenhouse gas emissions globally. Currently CO2 capture and storage is not included in the list of internationally agreed mitigation measures. However, the paper aims to discuss the issue of why this mitigation option should be included in the future as a complementary action to those already in place. The paper is not a technical article that is consistent with the normal scope of this journal and, therefore, contains references to different sorts of evidence in presenting its conclusions. Three sorts of references are included; scientific peer-reviewed articles, conference proceedings which have not been peerreviewed and reports by a number of organizations that may not be considered to have the same rigour of quality of evidence of the other references. The paper is based on work undertaken by the IEA Greenhouse Gas R&D Programme which is an international collaborative programme focusing on the assessment of technologies for reducing greenhouse gas emissions. The Programme is supported by sixteen countries and the European Commission and aims to provide expert reports for its members on measures for the mitigation of the full range of greenhouse gases. The Programme's work is used by its members to support national and international policy development.

International actions to reduce greenhouse

gas emissions In the late 1980s, increased public awareness of international environmental issues moved the climate change debate from the scientific to the political arena. Concerns about the possibility of global warming due to anthropogenic emissions of greenhouse gases prompted governments to form the International Panel on Climate Change (IPCC) in 1988 (Grant 1999). The key aims of the IPCC were to assess the available scientific information on climate change; to examine the potential environmental and social impacts of climate change and to formulate national and international response options. The IPCCs third assessment report was published in 2001, in the report the scientific committee stated that there was 'newer and stronger evidence' (from modelling of the global climate, reconstructions of past records and studies of the temperature records) that 'most of the observed warming over the last 50 years is likely to have been due to the increase of greenhouse gas concentrations' (IPCC 2001). The findings of the IPCC provide evidence that global climate change is a real effect and the potential consequences of its impact are now well acknowledged by governments and the public alike. International actions to reduce global emissions of greenhouse gases are undertaken through the United Nations Framework Convention on Climate Change (UNFCC). The UNFCC was signed by 155 countries at the so called 'Earth Summit' held in Rio de Janeiro in 1992 and came into force in 1994 after ratification by 50 countries (Grant 1999). A national government becomes a party to the convention by ratifying it. The ultimate objective of the Framework

From: BAINES, S. J. & WORDEN, R. H. (eds) 2004. Geological Storage of Carbon Dioxide. Geological Society, London, Special Publications, 233, 7-15. 0305-8719/047$ 15.00 © The Geological Society of London 2004.

8

J. GALE

Convention was 'to achieve stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system'. The implementation of the convention is shaped by the Conference of the Parties (COP) which convenes at regular intervals (Grant 1999). The third Conference of the Parties (COP-3) was held in Kyoto, Japan in December 1997 and was where the parties debated and adopted the Kyoto Protocol. The main features of the Kyoto Protocol were that it called on the developed countries to reduce their greenhouse gas emissions by an average of 5.2% below 1990 levels by the end of the first five-year commitment period, 2008 to 2012. In recognition of their different circumstances, countries agreed different reduction targets. For example the European Union agreed an 8% reduction, whereas Norway and Australia were actually allowed to increase their emissions by 1 and 8% respectively, relative to their 1990 levels (Weyant & Hill 1999). The ratification of the Kyoto Protocol has been more protracted than was initially conceived. Several of the parties, led by the European Commission have now ratified the treaty but many such as the USA and Australia remain opposed to ratification.

Greenhouse gas abatement under the Kyoto Protocol Under the Kyoto Protocol, the greenhouse gas reduction commitments apply to six gases or groups of gases namely; carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6). Those substances that contribute to ozone depletion, namely chlorofluorocarbons (CFCs) and halons are covered by the Montreal Protocol, a separate international agreement (Weyant & Hill 1999). The contributions of the different gases are weighted according to their Global Warming Potentials (GWP) (Weyant & Hill 1999). GWP is defined by IPCC as the time-integrated commitment to climate forcing from the instantaneous release of 1kg of a trace gas expressed relative to that of a reference gas (CO2). The time horizon used for the GWP index is typically 100 years (IPCC 2001). The contribution of CO2 to climate change is the most significant of all the basket of gases covered by the Kyoto Protocol. The contributions to global warming from anthropogenic sources from pre-industrial times to date are shown in Figure 1. Therefore, to make significant long-term reductions in global wanning, significant reductions in global anthropogenic CO2 emissions will be needed, as well as cuts in the other gases (Weyant & Hill 1999). The abatement measures proposed under the Kyoto Protocol to reduce emissions were:

Fig. 1. Contribution to global warming by greenhouse gas emissions from anthropogenic sources: pre-industrial times to date.

improved energy efficiency both in end-use and in the supply and conversion sectors; fuel switching to reduce the carbon intensity of fossil fuel use, such as substituting natural gas for coal; use of renewable energy; and use of nuclear power. The nuclear power option was promoted strongly by a number of parties at the outset of the process but technical doubts remain, primarily relating to safety which, along with attendant political issues, mean that nuclear power is not universally accepted as a mitigation measure. Many countries are focusing their greenhouse gas reduction targets on the first commitment period for the Kyoto Protocol (2008-2012) and will concentrate on the low cost, easily achieved options. These options can include: fuel switching (coal to natural gas), abatement of N2O emissions at adipic acid plants and methane emission reduction from natural gas pipelines and from coal mining. However, the low cost easy to achieve options will soon be used up and other more expensive abatement options will then be required, for later commitment periods. There is a growing recognition that if the UNFCC target of stabilization of atmospheric greenhouse gas emissions is going to be achieved then deep reductions in greenhouse gas emissions and in particular CO2 emissions will be required. To achieve deep reductions, wide-scale changes in the world's energy system would be needed, for example wide-scale use of renewable energy, substantial improvements in energy efficiency and fuel switching. Such changes are potentially attainable but the complexity of the change needed to the world's energy system is substantial and is unlikely to be practically achievable for at least another decade (Edmonds et al 2000; Lewis & Shinn 2001). The alternative would be to introduce CO2 capture at existing power and industrial plants combined with CO2 sequestration, which would have three advantages:

THE NEED FOR CO2 STORAGE

It would significantly reduce the complexity of any change in the world's energy system. It would allow the development of alternative sustainable technologies to take place at a reasonable technical pace (Edmonds et al. 2000). It could act as a stepping stone to a renewable hydrogen-based economy by helping to establish the necessary infrastructure to support such a low-carbon economy (Simbeck 2002). One criticism often levelled at CO2 capture and sequestration is that it is a high-cost abatement option (Edmonds et al. 2000). At its current technical status the costs are significant (of the order US $50 per tonne of CO2 abated); however, extensive research work is underway to reduce these costs and it has been estimated that by 2020 the costs of CO2 capture and sequestration will be cost-competitive with other alternative power generation technologies (Edmonds etal 2000).

CO2 sequestration options The options for sequestering CO2 fall into two broad categories. The first category includes the enhancement of natural sinks, such as forests and soils, and the second is capture and storage of CO2.

Enhancement of natural sinks The use of forests and soils to capture and store CO2 is considered by many to be an environmentally attractive method of reducing global CO2 emissions. However, in practical terms there are a number of disadvantages to both forestry and soils as CO2 sequestration options. Large land masses are required to store relatively modest amounts of CO2 (Chadwick et al 2000). IPCC has estimated that some 700 million hectares of forestry might be available globally, principally through reforestation schemes in the tropics, which could sequester between 220-320 Gt CO2 (IPCC 2001). This is about 11-16% of the atmospheric CO2 that would need to be absorbed to stabilize global emissions (IPCC 2001). Increasing use of land for forestry could also compete with agricultural use which would not be acceptable, particularly in areas of the world where population growth is increasing the demand for food. Agricultural practices can be modified to promote the uptake of carbon in soils but the process cannot be considered as either permanent or irreversible (IPCC 2001). The IPCC has estimated that another 84-161 Gt CO2 could be sequestered in agricultural soils, that is 4-8% of the total CO2 that needs to be absorbed (IPCC 2001). In both cases there is the issue of permanence

9

(Trexler 1999; Lewis & Shinn 2001). The carbon (CO2) stored in trees could be held for 50-100 years or more provided that the area remains forested. Problems with permanence will arise if deforestation occurs due to logging or clearance activities or if the increased temperature due to climate change makes the areas unsuitable for forestry. Ideally, the CO2 should be stored well past the end of the fossil fuel economy which could conceivably last for 200 years or more with the development of new exploration technology and development of non-conventional fossil fuel sources like natural gas hydrates. Another issue, particularly relevant to international carbon trading, is that of verification of sequestered CO2 It is extremely difficult to monitor the amount of CO2 that is stored physically within forests with a reasonable degree of accuracy; hence it is extremely difficult to verify how much CO2 will be stored in a forestry sequestration scheme (Trexler 1999; Lewis & Shinn 2001). Recently there have been concerns that some tropical forests, in particular the Amazon rainforest may not be a net absorber of CO2 despite previous assertions (Di Paola 2002). Also, afforestation may take place for reasons other than greenhouse gas reduction and it will be difficult to distinguish between those measures that would have occurred anyway and those that are deliberate sequestration schemes. Sequestration of CO2 in soils is another option that has been advocated. Most of the issues relating to forestry are also common to those for soils. In particular, there are concerns over the permanence of CO2 in soils. In certain regions of the world agricultural practices, such as no-till, have been introduced to reduce soil erosion and such measures also indirectly improve CO2 sequestration. However, the very nature of farming means that agricultural practices can change quickly, even year by year, due to factors other than the need for greenhouse gas reduction and such changes mean that the transition from CO2 sequestration to a net release of CO2 could occur within several years. Concerns have also been raised that areas of the world that are currently net absorbers of CO2 could release CO2 if the global temperature increases much further (White 1998; Di Paola 2001). Overall, neither of these methods for enhancing natural sinks of CO2 are, on their own or in combination, going to make deep reductions in CO2 emissions achievable.

CO2 capture and storage CO2 can be captured from a variety of anthropogenic sources such as power and large industrial plants and either stored in the oceans or in geological reservoirs. The oceans already act as a large natural sink for CO2; therefore, there is some merit in attempting

10

J. GALE

to increase their storage potential (Ormerod et al 1999). There are two potential routes for ocean storage of CO2: direct injection of captured CO2 into the deep ocean; and fertilization of the oceans with nutrients to increase the draw down of CO2 from the atmosphere. Although very different in concept, both these methods have the potential to increase the amount of CO2 storage in the oceans significantly (Ormerod et al. 1999). Of the two options, there exist greater uncertainties regarding the concept of ocean fertilization. The principal concerns include: the impact on the marine community and ecological balance (Ormerod et al 1999). Modelling work has indicated that the increased amount of CO2 sequestered could be offset by biological production (Ormerod et al 1999). The science associated with the injection of CO2 into the deep ocean, although imperfect, is somewhat better understood. However, there are considerable uncertainties over the environmental impact of deep ocean disposal (Omerod et al 1999). There is also a political dimension to ocean storage in that the disposal of CO2 in the ocean is currently not allowed under international law. In addition, there is considerable opposition from the NGO community to ocean storage that makes it a difficult option for governments to consider. Because of the uncertainties over the science and the attendant political issues, there is considerable reluctance internationally to consider ocean storage as a mitigation option at present. The alternative option is the use of geological reservoirs for the injection and storage of CO2 This option is discussed in detail in the following sections of the paper.

The geological options for storing CO2 Several potential geological reservoirs can be used to store captured CO2 from power and large industrial plants. These include: depleted and disused oil and gas fields, deep saline aquifers and deep unminable coal seams (Lewis & Shinn 2001). The global storage capacity for these geological reservoirs has been estimated and is compared with the projected total emissions between 2000 and 2050 according to IPCCs 'business as usual' scenario in Table 1 (Davison etal 2001). The capacity estimates for these reservoirs show that geological storage of CO2 can make a substantial impact on CO2 emissions reduction. The estimates for storage in aquifers were made in the early 1990s; research work underway should assist in firming up these numbers. A study completed for NW Europe suggested some 800 Gt CO2 could be stored in that region alone, mostly in deep saline aquifers (Gale etal 2001).

Table 1. Global storage capacity of potential geological reservoirs for carbon dioxide.

Global capacity Storage option

GtCO 2

% of emissions to 2050

Depleted oil and gas fields 920 45 Deep saline aquifers 400-10 000 20-500 Unminable coal seams 20 Scotland, UK (e-mail: [email protected]) 2 Department of Geology, Utah State University, Logan, UT 84322, USA 3 Department of Earth and Atmospheric Sciences, Saint Louis University, St Louis, MO 63103, USA Abstract: The numerous CO2 reservoirs in the Colorado Plateau region of the United States are natural analogues for potential geological CO2 sequestration repositories. To understand better the risk of leakage from reservoirs used for long-term underground CO2 storage, we examine evidence for CO2 migration along two normal faults that cut a reservoir in east-central Utah. CO2-charged springs, geysers, and a hydrocarbon seep are localized along these faults. These include natural springs that have been active for long periods of time, and springs that were induced by recent drilling. The CO2-charged spring waters have deposited travertine mounds and carbonate veins. The faults cut siltstones, shales, and sandstones and the fault rocks are fine-grained, clay-rich gouge, generally thought to be barriers to fluid flow. The geological and geochemical data are consistent with these faults being conduits for CO2 moving to the surface. Consequently, the injection of CO2 into faulted geological reservoirs, including faults with clay gouge, must be carefully designed and monitored to avoid slow seepage or fast rupture to the biosphere.

Effective design and implementation of geological CO2 sequestration projects require that we understand the storage capacity of a candidate site, the trapping mechanisms for gas, and the hydrodynamics of the system. The CO2must be segregated effectively from the atmosphere for periods of thousands of years (Rochelle et al 1999). Natural sources of CO2 include mantle degassing, metamorphism or dissolution of carbonates, oxidation or bacterial degradation of organic matter, and thermal maturation of hydrocarbons (Selley 1998). Numerous, large naturally occurring CO2 fields provide analogues for the integrity of stored gas systems (e.g. Allis et al. 2001). In some CO2 fields, however, gas leaks into the atmosphere, primarily along faults. We can study these active leaks to understand the factors that might control the feasability and safety of future CO2 injection projects and guide the design and implementation of such projects. In this contribution we examine the hydrology, stratigraphy, structural geology, and geochemistry of a naturally degassing CO2 reservoir in the Colorado Plateau of east-central Utah. The CO2 discharges from the hydrocarbon-rich Paradox Basin along the Little Grand Wash and Salt Wash faults. These faults cut sandstones, shales, and siltstones producing zones of clay-rich gouge that should theoretically be a barrier to flow (e.g. Freeman et al 1998). CO2charged springs and geysers, travertines (both active and ancient), and carbonate-filled veins are localized

along the fault traces. The faults are presently conducting CO2-rich fluids and the fault has conducted fluids for a substantial amount of time. Abandoned hydrocarbon boreholes are also active conduits for CO2 to the surface (Doelling 1994). In order to quantify the volume of CO2 that has leaked through the Little Grand Wash and Salt Wash faults, the CO2 sources, pathways, volumes, and rates of flow must be known. We have examined the distribution of the fault-related rocks and associated outcrop-scale structures of travertines and carbonate veins to characterize the flow paths. We have used the geochemistry of the spring waters, carbonates, and travertines to identify the potential sources of the fluids in the system. We have combined these data to develop a conceptual model for the groundwater/CO2 flow system (source, pathways, reservoir and caprocks).

Geological setting The Colorado Plateau and Four Corners region of the western United States contains at least nine producing or abandoned CO2 fields with up to 28 trillion cubic feet of CO 2 gas (Allis et al 2001). Most of these fields are fault-bounded anticlines with fourway anticlinal closure or fault seal along one margin of the field. We focus on the Little Grand Wash and Salt Wash faults, which are at the northern end of the

From: BAINES, S. J. & WORDEN, R. H. (eds) 2004. Geological Storage of Carbon Dioxide. Geological Society, London, Special Publications, 233,43-58.0305-8719/047$ 15.00 © The Geological Society of London 2004.

44

Z.K.SHIPTON£rAL.

Paradox Basin. The Paradox Basin contains a number of actively producing oil and natural gas fields, as well as CO2 fields including the Lisbon field and McElmo Dome field. Immediately south of the study area is the abandoned Salt Wash oil field (Peterson 1973). To the north of the Paradox Basin are the active methane and CO2 fields of the San Rafael Swell (e.g. Drunkards Wash, Perron Dome). Many of the methane fields in the area produce significant amounts of CO2 (Cappa & Rice 1995), much of which is vented to the atmosphere. The Paradox Basin is defined by the extent of organic-rich Pennsylvanian and Permian marine limestones, shales and evaporites (Fig. 1). Hydrocarbon source rocks occur in the Ismay-Desert Creek and Cane Creek cycles of the Paradox Formation (Nuccio & Condon 1996), a mixed sequence of dolostone, black shales, anhydrite, and halite. These are overlain by Triassic and Jurassic fluvial and aeolian redbeds. The oldest lithologies that crop out in the study area are red-brown finegrained sandstones of the Middle Jurassic aeolian Entrada and Curtis Formations. The Middle Jurassic Summerville Formation forms characteristic low cliffs with thin bedding and seams of gypsum. The Upper Jurassic Morrison Formation consists of stacked fluvial channels of the Salt Wash Sandstone member, overlain by the bentonite-rich lacustrine shales of the Brushy Basin member. The Lower Cretaceous Cedar Mountain Formation and the Upper Cretaceous Dakota Sandstone are conglomeratic channel sandstones. The youngest formation exposed in the field area is the Upper Cretaceous Mancos Shale, a dark organic-rich marine shale. Approximately 2500m of Cretaceous and Tertiary rocks have been eroded from the area (Nuccio & Condon 1996). The Little Grand and Salt Wash faults (Fig. 1) affect the present-day flow of gas and water. Carbonate springs, an active CO2-charged geyser, and actively forming travertine deposits are localized along the Little Grand Wash fault zone (Baer & Rigby 1978; Campbell & Baer 1978; Doelling 1994) and numerous CO2-charged springs occur in the region of the Salt Wash faults (Doelling 1994). The faults are part of a WNW trending set of 70-80° dipping normal faults in the region. Timing of continued movement along these faults is poorly known, though we present arguments below for Early Tertiary and Quaternary slip. The faults cut the Mancos Shale, consistent with substantial fault activity having occurred at least up to the Middle Cretaceous. The faults cut a north-plunging anticline (Figs 1 & 2), which could be related to salt movement in the Paradox Formation at depth. A basinwide system of salt anticlines initiated when the salt was loaded by the Pennsylvanian/Permian elastics shed off the Uncompahgre uplift to the NE.

Reactivation of the salt-related anticlines and faults occurred during Laramide (Eocene) contraction (Chan etal 2000).

The Little Grand Wash fault The Little Grand Wash fault is a south-dipping arcuate normal fault with a surface trace length of 61 km (Fig. 1). The Little Grand Wash fault consists of two parallel strands from 3.2km east to O.lkm west of the Green River; elsewhere it has only one strand (Fig. 3). Total vertical separation on the fault near the Green River is 180-210m, most of which is accommodated by the southern fault strand. The two strands of the fault were encountered at depth in an abandoned well (Amerada Hess, Green River no. 2 drilled in 1949, total depth 1798m) at 805m and 970m. Drilling records state that the deeper of the two faults has Cutler Group sediments in the hanging wall and Hermosa Group sediments in the footwall. It is therefore unclear what the offset of the fault is at this depth, or whether the fault cuts the Paradox Formation (Fig. 2). The fault is cut by several stream channels that provide excellent cross-sectional exposures of the fault zone and associated host-rock alteration. Between the two main strands of the fault, smaller faults define structural terraces with varying dips (Fig. 4a). Slickensides on subsidiary fault surfaces indicate mostly dip-slip with some oblique leftand right-lateral movement. The fault zone contains 70cm to 3 m of foliated clay gouge with occasionally well-defined, sub-planar slip-surfaces. Smaller faults with offsets less than 1 m are decorated with a thin (millimetre-thick) foliated purpleblack, clay-rich fault gouge and occasional thin calcite veins (1-2 mm thick) with subhorizontal fibres. The normally dark purple-red Summerville Formation is bleached to a pale yellow for up to several metres into the footwall. This alteration is localized along the subsidiary faults and within certain beds.

CO2 emissions and springs Several active CO2-charged springs are localized along the two strands of the Little Grand Wash fault zone (Fig. 3). The Crystal Geyser erupts to heights of up to 25m at 4-12-hour intervals (Fig. 5a). This is not a geyser in the strict sense of the term; the water in the geyser is cool and the eruptions are powered by CO2-charged waters rather than a heat source. The geyser began erupting when the Glen Ruby #1X well was drilled in 1935. This abandoned exploration well was completed to the base of the Triassic section (TD 801m). Occasionally the geyser water

CO2 LEAKAGE THROUGH LOW-PERMEABILITY FAULTS

45

Fig. 1. (a) Regional geology of the Little Grand Wash and Salt Wash faults. Stars mark the location of known CO2charged springs. Dot marks the town of Green River, Utah. Compiled from Doelling 2001; Williams 1964; Williams & Hackman 1971. The line of section shown in Figure 2 is indicated. Inset shows location of study area and approximate boundaries of the Paradox Basin, (b) Stratigraphic column, (after Doelling 2001, thicknesses converted to metres from Hintze 1993). Stippled areas indicate likely reservoir or aquifer rocks, cross hatched areas represent likely cap rocks or seals.

46

Z.K.SHIPTONZTAL.

Fig. 2. Schematic cross-section across the Little Grand Wash and Salt Wash faults, approximately along the northplunging anticline axis. Depths are above or below sea level. Abandoned oil wells (projected onto the line of section) give control on the stratigraphy. The structure of the Salt Wash faults is not constrained at depth by any well data. The line of section is indicated on Figure 1.

Fig. 3. Local map of the Little Grand Wash fault showing the location of the travertine deposits, the gas seep, oil seep, and abandoned oil wells mentioned in the text.

has a strong sulphur smell, and/or a thin film of hydrocarbons coating the water pooled around the drill pipe. The driller's records document that the well was spudded into a travertine mound and that the travertine thickness was 21.5m before hitting bedrock (Baer & Rigby 1978). The spring system must have been active prior to the well being drilled. Anecdotal reports and scant completion records indicate that the geyser erupted more regularly (at 11 hour intervals) in the past. The hole is currently open to c. 130m depth. Much damage is reported to have been done to the borehole, including dynamiting of the hole, attempts at cementing, and dumping of railroad ties. CO2-charged emissions continue despite this damage. A 1.5 m high steel pipe was added to the top of the well casing in 2001. This pipe has had no discernable effect on the geyser's eruption pattern. Three other springs are located on the system of travertine mounds around the geyser. To the NE, a water-filled pool and a chocolate-brown, mud-filled pool erupt penecontemporaneously with the geyser. These are located on the eastern edge of the travertine mound where it has been badly damaged by vehicles.

To the north, another small water-filled pool is located on the active travertine slope. The close correlation of the timing of the geyser and activity of these springs suggests that the latter either reflects some escape of CO2-charged waters from the well bore at shallow levels, or that these pools could be the original, pre-well flow paths for the CO2-charged waters to the surface. It is common for hydrothermal travertine spring sources to switch locations when the flow paths become cemented (Chafetz & Folk 1984). Although in this cool water system, cementation may not be as rapid as in hydrothermal systems, it is still likely that the flow paths switch with time when they become sealed by calcite precipitation. Within the Green River, small gas seeps produce small streams of bubbles. These can be observed on both banks of the river and are parallel to the trace of the fault. A kilometre to the east of the Crystal Geyser, in a low-lying area in the footwall of the fault, gas seeps audibly from the ground (Fig. 3). A small amount of water flows sporadically from upstream of this location leaving salty deposits and wet sand in the base of the wash. Further to the east,

CO, LEAKAGE THROUGH LOW-PERMEABILITY FAULTS

47

Fig. 4. Photos of Little Grand Wash fault, (a) Cross-sectional view looking east along fault strike, about 30m east of Crystal Geyser. At this location the fault consists of several strands separating structural terraces. The southern fault strand accommodates most of the offset here. Note the thick calcite/aragonite veins cutting the travertine that sits in the hanging wall of the central fault strand. Js, Jurassic Summerville Formation; Kmu, Cretaceous Mancos Shale. Photo about 500m across, (b) Boxwork of veins beneath a travertine mound centred on the northern fault strand. Note that the fault does not clearly cut all the way up the travertine mound, and appears to be cut by some of the veins. The host rocks are Jurassic Morrison Formation; Jmb, Brushy Basin shale member; Jms, Salt Wash sandstone member. Three people in the centre for scale.

an oil seep is located on the southernmost fault strand (Fig. 3). A shallow pit contains fresh oil indicating that there is active flow of petroleum to the surface. The outcrop close to this seep (Salt Wash member of the Morrison Formation) contains patches of oil staining.

Mineral deposits Modern travertines at the Little Grand Wash fault consist of bedded travertine mounds that were deposited from the Crystal Geyser and surrounding springs (Fig. 5). The surface of the active geyser mound has a classic rimstone texture and lobes of travertine have built out to form sub-metre scale caves with stalactites. An ochre colour indicates a small component of iron oxide. These travertines successively bury the vegetation that surrounds the geyser. The travertine surface has regions of actively forming and inactive travertines, presumably con-

trolled by switching of the source spring location or by lateral migration of flow across the surface of the mound. To the SW of Crystal Geyser, older carbonate deposits are in the process of being covered by the present day mound. These deposits are in the form of two distinct mounds of carbonates and breccias, one at the level of the river cutting an older one to the east (Fig. 5a). From a distance, it appears as though variably dipping veins are visible within these mounds, though these are not veins in the usual structural geological sense. They consist of centimetre-thick to tens of centimetres-thick subhorizontal tabular masses of radiating acicular calcite and aragonite crystals 6-15 cm long with botryoidal or mammilated top surfaces (Fig. 5b). Fresh surfaces are bright white, with occasional pale yellow banding. These veins often have paired banding and/or mammilated surfaces, which face towards the centre of the vein. Deposits of this form (described as ray-crystal crusts by Folk et al. 1985) have been interpreted to form

48

Z.K.SHIPTONE7AL.

Fig. 5. (a) Eruption of the Crystal Geyser looking east along fault. Active and inactive travertine mounds can be seen around geyser, and ancient travertine mounds and carbonates are located along fault surface. Strands of fault are marked. Je, Entrada Formation; Jet, Curtis Formation; Js, Summerville Formation; Jms, Salt Wash Sandstone (Morrison Formation); Jmb, Brushy Basin member of Morrison Formation; Kmu, Mancos Shale, (b) Thick veins cutting the lower inactive travertine mound at the edge of the Green River. Veins dip into mound, but roll over to subvertical at front of photo. Top of this mound consists of a breccia with clasts of vein material. Modern travertine deposits are covering the ancient mound in bottom left of photo. This section is about 2m high.

underwater, with the apex of the radiating crystals pointing towards the source of fluids (i.e., in the centre of the veins). The centres of some of these veins contain stalactite-like structures suggesting that subhorizontal fissures were infilled above the water table. Above these deposits lie travertinecemented breccias that include clasts of ray-crystal calcite and sandstone clasts. The surfaces of the inactive mounds have some rimstone textures preserved, though they have been extensively damaged by vehicles. The 1867 Powell expedition documented 'satin spar' at this location (Powell 1895), which we interpret to be either the travertine terraces or the bright white ray-crystal calcite veins. Other ancient travertine deposits along the fault occur at higher elevations (up to 37m; Baer & Rigby 1978) than the one presently forming. These deposits tend to form resistant caps to small buttes. The hanging wall of the fault in Figure 4a contains a thicker ancient travertine deposit than the footwall, though it is unclear if this deposit filled in the space

left by faulting, erosion of the hanging wall, or if movement on the fault cut a pre-existing deposit. The ancient travertines consist of dense-bedded layers, 1-2 mm thick, interbedded with vuggy open carbonate 1-3 cm thick. Horizontal and vertical carbonate ray-crystal veins up to 30cm thick cross cut these deposits. In an outcrop east of the geyser, an impressive array of millimetre to centimetre-thick veins with a boxwork pattern has completely obliterated the original fabric in the fault gouge (Fig. 4b). The veins in this array are not parallel to the original fault-parallel gouge fabric. This boxwork is cross cut and offset by thicker subvertical veins with occasional stalactite textures. The variation in the locations of the inactive deposits shows that the loci of active effusion of CO2-rich waters have changed in the past. The carbonate mounds form predominantly between the two strands of the Little Grand Wash fault, or in the footwall of the fault. The spatial correlation of the ray-crystal calcite/aragonite veins and the

CO, LEAKAGE THROUGH LOW-PERMEABILITY FAULTS

49

Fig. 6. Map of Salt Wash fault outcrops mentioned in text. Locations of springs and travertine/carbonate vein deposits are marked including ones not specifically mentioned in text. Abandoned oil wells are shown with their total depth (TD) and depth to top of the Paradox salt.

travertines is consistent with the latter having been the 'plumbing system' to the travertines. The Salt Wash faults The Salt Wash faults are a set of N 70° W striking normal faults. The map-scale structure of the faults reveals two normal fault systems that form a shallow graben over 15km long (Fig. 1). The faults offset Jurassic Entrada Sandstone in their footwalls against Cretaceous and Jurassic Cedar Mountain Formation in the centre of the graben. The Salt Wash faults consist of two linked en echelon graben segments (Doelling 2001) and may be structurally linked to the Moab fault system to the SE (Fig. 1), though Quaternary deposits obscure the area where this linkage potentially occurs. The depth to which these faults extend is uncertain (Fig. 2), but they may sole into the Paradox salt sequence, and could be related to salt tectonics in the region. We have studied two areas along the northernmost Salt Wash fault in detail: the Tenmile Geyser and Torrey's spring areas (Fig. 6). Tenmile Geyser The Tenmile Geyser is centred on an abandoned well 200m into the hanging wall of the northern fault (Fig.7a). A drill pipe sits within a low mound

of flaky travertine with poorly developed rimstone textures. The Tenmile Geyser erupted infrequently in the past (Doelling 1994) and continues to erupt with infrequent 1-1.5m high eruptions. A second mineral-charged spring sits on a low mound 100m into the footwall of the fault. There is anecdotal evidence that a set of travertine terraces with rimstone textures used to exist at this locality. This mound has since been excavated into a pit about 2 by 3 m in size and 1.5m deep. The bottom of this pit does not reach the base of the travertine deposit. There is an almost constant stream of CO? from three vents in the base of the pool, but this spring has not been documented to have geyser-style eruptions. There are extensive inactive travertines up to 4m thick at elevations up to 30m above the level of the present-day spring, some with well-developed rimstone textures. These travertines are presently being quarried. A 2-1 Om thick zone of fractures, intense alteration, and veining obliterates the primary sedimentary structures beneath portions of the travertine deposits. Bedding parallel carbonate veins, 2-5 cm-thick, extend up to 50m away from the fault zone. In some places mammilated ray-crystal veins change from vertical to horizontal orientation within the outcrop. These locally contain open vuggy deposits with rhombohedral calcite crystals, interpreted as forming in subaerial or spelean pools. Fractures up to two metres deep that cut the mounds have been filled with bedded travertine. The

50

Z.K.SHIPTONETAL.

gies separated by clay-rich foliated gouge (Fig. 7b). In the footwall north of the Tenmile Geyser, the Entrada Sandstone (usually red) has been extensively bleached to a light tan to pale yellow. In other places along the fault zone, the Entrada Formation is bleached in zones close to the fault and along fractures (Fig. 7c). Close to the fault, poikilotopic aragonite cements occur preferentially in certain horizons of the host rock.

Torrey 's spring

Fig. 7. Photos of Salt Wash fault, (a) Aerial view of northern Salt Wash fault looking north, showing distribution of springs and travertines in vicinity of Tenmile Geyser, (b) Cross-sectional view looking east along fault strike, about 800m west of Tenmile Geyser. At least three fault strands, consisting of thin gouge zones, can be identified based on differing lithologies. (c) Alteration focused along fractures and bedding planes adjacent to fault zone. At top of outcrop the base of an ancient travertine mound can be seen. Je, Entrada Formation; Js, Summerville Formation; Jms, Salt Wash Sandstone (Morrison Formation); Jmb, Brushy Basin member of the Morrison Formation; Kcm, Cedar Mountain Formation.

travertines tend to form resistant caps to a line of small buttes along the fault trace. All the travertines along the Salt Wash faults are localized either on the northernmost fault trace or in the footwall of this fault. The only activity seen within the graben is the Tenmile Geyser. The fault gouge is locally well-exposed, and consists of a zone up to 5 m thick of slices of host litholo-

This spring occurs in the Entrada Sandstone in the footwall to the northern fault (Fig. 6). The spring is located in the centre of a fresh-looking travertine mound about 15 m in diameter, which has slope of c. 8°. The saline spring bubbles almost constantly and occasionally smells of sulphur. To the authors' knowledge, it has never erupted in geyser-style eruptions. This spring appears to be close to the location of the Delaney Petroleum Corporation #1 drill hole drilled in 1949, which had a total depth of 299m. There was no oil show in this well. The spring was first visited by the authors in June 2000, and again in June 2001. During this interval, the travertine had advanced significantly over a large area and had created a small travertine 'frozen waterfall' into a dry river bed to the west. This is evidence of the rapid growth rate of these travertine mounds and the high CaCO2 content of the effusing waters. Growth rates of 1 mm to 10cm per year have been documented in other travertine deposits, though these were areas of warm or hot water deposition (Folk et al 1985). The main Salt Wash fault outcrops to the south of Torrey's spring. A line of small buttes (10-20 m high) capped with travertines parallels the location of the fault. Commonly, the steeply dipping veins are larger than, and cross cut, the subhorizontal veins. Fissures have been infilled with travertines, and the infill breccias contain fragments of the vein material. These outcrops also contain gypsum, though it is unclear if they are associated with the fault or if they are from the original host rock (Entrada Sandstone). Alteration of the host rock to pale yellow 'bleached' sandstone is focused along fractures close to the fault (Fig.7c). Bedding-parallel calcite veins 2-5 cm thick extend up to 350m from the fault in the footwall.

Water chemistry We collected and analysed the chemistry of waters from the Crystal Geyser on the Little Grand Wash fault, and from two CO2-charged springs along the Salt Wash fault (Torrey's spring and the excavated

51

CO, LEAKAGE THROUGH LOW-PERMEABILITY FAULTS

Table 1. Chemical data from field and laboratory testing and associated cation-anion balance errors. Samples from the Crystal Geyser are labelled CG, from Torrey 's spring T, and from the excavated spring near Tenmile Geyser SW. Cations (mmol/1) Sample number

Temp (°C)

pH

TDS (mg/1)

Ca

Mg

Na

K

Fe

Mn

Sr

CG92.1 CG92.2 CG92.3 T1/T2 SW1/SW2

16.0 16.0 16.0 19.4 23.0

6.96 6.7 6.46 6.37 6.26

13920 13685 13555 21188 20020

2.37 9.71 10.78 20.83 20.76

8.72 8.80 9.30 7.86 8.27

174.52 171.74 167.82 218.63 236.38

7.62 7.67 7.65 10.44 9.39

0.00 0.00 0.00 0.14 0.08

0.00 0.02 0.02 0.02 0.00

0.01 0.05 0.05 0.14 0.16

Anions(mmol/l) Sample number

HCO3

C03

Cl

S04

% Balance error

CG92.1 CG92.2 CG92.3 T1/T2 SW1/SW2

61.00 50.00 46.40 75.70 58.90

0.00 0.00 0.00 0.00 0.00

91.15 95.43 98.23 204.52 186.38

24.33 24.64 25.07 29.47 30.81

0.9 5.3 5.1 -8.3 -0.4

spring at Tenmile Geyser). Water samples were collected in polyethylene bottles and samples for cation analyses were acidified with reagent-grade nitric acid to pH 2 or less. Field analyses included pH and temperature. Alkalinity was not determined in the field, but relatively low balance errors for the analyses show that this did not cause significant error. Samples were kept refrigerated until analysed for major element composition. Table 1 presents the chemical data. Dissolution of calcite cannot account for all of the CO2 in the water because there is substantially less calcium in the water than bicarbonate. There must be an alternative or additional source for the CO2. Similarly, the excess sodium in the waters (Cl~/Na+ ratios range from 0.6-0.8 for Crystal Geyser system waters) could not be solely from the dissolution of halite and therefore must be derived from other minerals such as Na-bearing montmorillonite. The water we collected from the main Crystal Geyser is supersaturated with respect to calcite, aragonite and dolomite (Table 2). Modelling of our data and the water chemistry of Mayo (1991) with the programme Wateq (Truesdel & Jones 1974) indicates that the Crystal Geyser water is supersaturated with respect to aragonite, calcite, dolomite, fluorite and gypsum (Table 2). This suggests that the fluid source for the veins could have had a similar composition to present-day fluid. Results of modelling with Salt Norm (Bodine & Jones 1986) are consistent with the chemistry of the geyser water being a mixture of meteoric water and brine from redissolved marine evaporites. For a given water analysis this program calculates the salt

composition that would result if that water were evaporated to dryness. The water composition will reflect the materials with which the water came into contact; therefore the calculated salt composition can provide clues about the composition of those materials. Bodine & Jones (1986, p.37, Fig. 4) list criteria that define water types based on the calculated salt composition. Applying those criteria to our water samples suggests that the water in the Crystal Geyser system primarily is a mixture of redissolved marine evaporites and meteoric water (Table 3). Evaporites occur in the Paradox salt, and also in shallower units such as the Carmel Limestone and Summerville Formation. Water chemistry was monitored during two separate Crystal Geyser eruptions to see what chemical changes occur during an eruption cycle. There were larger changes in water chemistry for the longer eruption (30 minutes as opposed to less than 10 minutes) consistent with an influx of relatively low salinity groundwater recharging the well bore during an eruption (Fig. 8).

Isotope data Samples of carbonate veins and travertine were analysed in the stable isotope laboratory at Saint Louis University using both continuous-flow and conventional techniques. For continuous flow analysis, submilligram aliquots of sample powder were digested in orthophosphoric acid at 90 °C for several hours in an automated extraction device. Liberated gas was entrained in a helium stream, passed through

Z.K.SHIPTON£TAL.

52

Table 2. Saturation indices of water samples from the Crystal Geyser, Torrey's Spring and the Salt Wash excavated spring compared with samples from Mayo (1991). Samples labelled as in Table 1.

Aragonite Calcite Dolomite Fluorite Gypsum

Mayo 1991

CG92.1

CG92.2

CG92.3

T1/T2

SW1/SW2

1.140 1.289 2.134 3.283 0.153

0.167 0.317 1.259 NA -0.977

0.442 0.592 1.201 NA -0.372

0.217 0.367 0.730 NA -0.320

0.595 0.743 1.169 NA -0.108

0.434 0.579 0.912 NA -0.098

Table 3. Comparison of Little Grand Wash fault and Salt Wash fault spring waters with Salt Norm criteria for identifying water type. Samples labelled as in Table 1.

Re-solution of marine evaporites Meteoric source Meteoric source Meteoric source

Normative salts

CG92.1

CG92.2

CG92.3

T1/T2

SW1/SW2

>78% halite presence of thenardite presence of magnesite presence of dolomite

76.9%

80.2% 4.2%

77.3% 10.3%

80.3% 8.3%

79.3% 10.4%

6.7%

6.6%

3.7%

3.5%

a GC column for isolation of the CO2 and then transferred on-line to a gas-source, isotope-ratio mass spectrometer. Duplicates and some triplicate analyses were made for most samples. Most of the samples were also analysed by conventional (manual) techniques following the laboratory procedure described in Kirschner et al (2000). The average values of these analyses are depicted in Figure 8a. Carbonate standards (two in-house standards) were analysed with the samples. Analytical precision of 813C was below 0.10%0 (N=23- la); o18O was below 0.26%0 (N= 20 excluding one set of 3 standards; Icr). Several trends can be observed in the data (Fig. 9a). Travertine data from individual localities lie on sub vertical trends. In contrast, most ray crystal calcite/aragonite veins have lower 818O and similar or lower 813C values relative to the travertines from the same localities (dashed trends in Fig. 9a). The 818O values are consistent with precipitation of the carbonate at low temperature (c. 15-35 °C) from water with a 818O value less than -?%«?. This is similar to partly evolved (isotopically altered) meteoric water that is an important component of the water in the faults and geysers (cf. IAEA 2001). The variation in 818O values among sites could be due to variable mixing of meteoric water with other fluids or partial exchange with carbonate rocks or CO2 at depth. The vertical 813C trend of the travertines is consistent with CO2 degassing during discharge and surface flow of the spring water. Light isotopes of carbon and oxygen preferentially fractionate into the gas during degassing, resulting in an appreciable increase in carbon isotope values of the dissolved inorganic carbon. Although the same phenomenon

5.3% 2.0%

occurs for the oxygen isotopes, the resulting shift in isotopic values is not large due to the overwhelming abundance of oxygen in the water. Similar trends have been documented in other studies of travertine deposits and associated spring waters (Fig. 9b and references therein). The positive increase in 818O and 813C values between veins and travertines from individual sites is most likely associated with increased isotopic fractionation due to cooling of water during upward migration and surface discharge. Assuming the water is cooling as it is ascending and there is an abundance of fluid in the system, then isotopic values of carbonates precipitating from cooler water will be higher (heavier) for both carbon and oxygen, resulting in a positive 818O-813C trend. Evaporation of the water could also have produced the positive trend. If a significant amount of water evaporates, then the water that remains becomes isotopically heavier (because water molecules with the lighter isotopes evaporate preferentially). Carbonates precipitated from water that has undergone significant evaporation will consequently have higher (heavier) oxygen isotopic values (and carbon values, assuming that CO2 is degassing at the same time). Either mechanism can produce positive trends.

Discussion In order to evaluate the effectiveness of the Little Grand Wash and Salt Wash faults as conduits for leakage of CO2 we need to know the source of the CO2, the volume of the gas reservoir, and the rate at

CO2 LEAKAGE THROUGH LOW-PERMEABILITY FAULTS

53

Fig. 8. Variation in sodium (dotted lines) and potassium (solid lines) concentrations from the Crystal Geyser during two separate eruptions (time from start of eruption in minutes along ;t-axis). The squares show data from an eruption that lasted 30 minutes, the diamonds show data from an eruption that lasted less than 15 minutes.

which this reservoir is being depleted. Although our work is still in its initial stages, the preliminary results can be used to build a working model for the source, pathways, and timing of flow in the Little Grand Wash and Salt Wash fault system.

Source of CO2 A number of processes can produce CO2 within basins including mantle degassing, metamorphism or decarbonation of carbonates, oxidation and/or bacterial degradation of organic matter, and maturation of hydrocarbons (Selley 1998). Helium isotope data of gas from the Crystal Geyser and a spring on the Salt Wash fault suggests only a minor component of mantle-derived helium (Heath et al 2002; Heath 2004), thus excluding a mantle source for the CO2. The gas in the Paradox basin is therefore likely to be sourced from one or more of the remaining processes, though this source cannot be unequivocally identified solely from the carbon and oxygen isotope data of the carbonates or water chemistry. The 813C values of +4 to +5%o for the veins are probably more closely associated with the subsurface fluid than the higher 813C values of the travertine samples. Barring the presence and dissolution of carbonates with unusually high 813C values in the basin, two probable sources of isotopically heavy CO2 in the field area are biologically mediated hydrocarbon generation and thermallyinduced decarbonation of carbonates. Many previous studies have focused on existing oil and gas deposits in the Paradox Basin (Hansley 1995; Nuccio & Condon 1996) and on the evidence for a palaeohydrocarbon play in and around the Moab fault (Chan et al 2000; Garden et al. 2001). The presence of the oil seep at the Little Grand Wash

Fig. 9. Stable isotope data for travertines and ray-crystal calcite/aragonite veins analysed in this study, and for travertines and associated spring waters from published data, (a) 813C (PDB) v. 818O (SMOW) for samples of veins (solid symbols) and travertines (unfilled symbols) at the Little Grand Wash and Salt Wash faults. Differences between veins and travertine at individual localities could be due either to decrease in temperature when fluids evulse onto land surface or different sources of fluid. Vertical arrays formed by travertine data are consistent with progressive CO2 degassing during surface flow of water, (b) Vertical data arrays of travertine and travertine-forming spring waters in 813C 818O space are common and result from downstream CO2 degassing of spring water, seasonal variations in isotopic values of source waters, and variable microbial influence in facilitating the precipitation of travertine carbonates. All six studies are from active travertine deposits: (1) Mammoth Hot Springs, Yellowstone (Friedman 1970), spring temperature in °C; (2) Durango, Colorado (Chafetz et al 1991£); (3) Oklahoma (Chafetz et al 19910); (4) Coast Range, California (Amundson & Kelley 1987); (5) near Florence, Italy (Guo et al. 1996); (6) Central Italy (Pentecost 1995).

54

Z.K.SHIPTONCTAL.

fault suggests that the fault is acting as a conduit for oil as well as CO2. The geochemistry of the oil may be consistent with it coming from Lower Permian Formations (P. Lillis pers. comm. 2001), which are the source for much of the oil in the Upper Palaeozoic rocks of the northern and central Rocky Mountains (Claypool et al 1978). Sanford (1995) concluded, however, that the Permian was an unlikely source rock for hydrocarbons in the Paradox Basin based on a study of palaeo-groundwater flow in the White Rim Sandstone, which has been bleached by hydrocarbon-bearing reducing fluids (Hansley 1995). Alternative hydrocarbon source rocks in the area include the late Proterozoic Chuar group, the Mississippian Chainman shale, the Lower Triassic Sinbad Limestone, and shales within the Paradox Formation. Although isotopically light 813C values of hydrocarbons and associated CO2 are the norm for hydrocarbon deposits (e.g. Schoell 1983), it is possible for hydrocarbon-associated CO2 to have isotopically heavy 813C values due either to biologically mediated reactions (e.g. Coleman et al 1988; Jenden et al. 1988) or to low-temperature carbon isotope exchange between hydrocarbons and CO2. Neither can be excluded with the present data set. A more probable source of isotopically heavy and abundant CO2 is the thermal decarbonation of carbonates (cf. Baumgartner & Valley 2001). Cappa & Rice (1995) presented evidence that some of the CO2 in the gas fields of southern Utah and Colorado was produced by high-temperature thermal decomposition of the Mississippian Leadville Limestone or decomposition of kerogen within the Leadville Limestone or the Paradox Formation. We suggest that this may have occurred within the contact aureoles of Tertiary intrusions (the La Sal and Henry mountains). Both the Leadville Limestone and the potential hydrocarbon source rocks lie within or below the Paradox Formation. Therefore, regardless of the source of CO2, it must have migrated through the salt.

at least as thick as those at the Crystal Geyser occur in areas where no drilling has occurred. The various ancient travertines are evidence that the migration pathways within and next to the fault zones have switched over time. Some of the ancient travertines are located up to 37m above the level of the present day springs. Assuming that they were not initially deposited at the tops of the buttes, the amount of incision could be used to infer the age of the travertines. Baer & Rigby (1978) suggested a date of 200000 years for the highest-level inactive travertines based on Colorado Plateau uplift rates. The uplift rate on the Colorado Plateau is currently a focus of debate; however, these spring systems have been active for a substantial amount of time.

Timing of fault activity Although we do not yet have geochronological constraints on the age of activity on the Little Grand and Salt Wash faults, structural relations between faults and travertine deposits are consistent with recent fault movement. Travertine filled fractures within the ancient travertine deposits could be related to seismic events (cf. Hancock et al. 1999). Some of the travertines appear to be nestled within a hanging wall half-graben, suggesting that a scarp existed at the time of travertine deposition. This could be the result of differential erosion of the relatively soft Mancos shale in the hanging wall, or due to syndepositional movement along the faults. Quaternary activity has been reported along the Salt Wash fault (Hecker 1993). These faults may be related to movement of the Paradox salt, which has resulted in numerous episodes of fault motion SE of our study area (Chan et al. 2000; Garden et al 2001). Pevear et al (1997) used K-Ar techniques on fine-grained illite of the Moab fault to show that this fault was active between 60-50 Ma consistent with one of the regional episodes of salt movement and dissolution (Chan et al 2000; Garden et al 2001).

Timing of fluid flow The ancient travertine terraces and veins preserved at Little Grand Wash and Salt Wash demonstrate that the faults have been the focus of CO2-charged waters for a substantial amount of time. Although many of the springs in the area are due to recent drilling activities, there continues to be flow from springs and gas seeps that are not associated with wells. The flow at the Crystal Geyser was active prior to 1935, when the Ruby 1-X well was drilled, and is likely to have produced the deposit that was observed by the Powell expedition in 1867. The rest of the drillingassociated springs have only poorly developed (incipient) travertine mounds. Deposits of travertine

Flow paths for CO2 and water All except one of the potential sources of CO2 are below the Paradox salt. The Paradox salt is an interbedded unit with halite beds up to several metres thick. Salt can deform by crystal-plastic deformation at high strain rates, which can result in rapid healing of faults. Therefore faults that cut salt are not usually considered to be conduits for fluid flow unless faulting forms a juxtaposition of non-salt rocks between the hanging wall and foot wall. From driller's records of wells in the study area, the thickness of the salt below the Little Grand Wash fault is

CO, LEAKAGE THROUGH LOW-PERMEABILITY FAULTS

of the order of 650-1300 m but the fault offset is only c. 200m. Thus flow along the faults is the likely pathway through the salt. Given the clay-rich nature of the fault zones within the Jurassic and Cretaceous sequences we would generally expect a low-permeability fault rock. However, the localization of springs and travertines, the presence of hydrocarbon seeps along the Little Grand Wash fault, and the close association of faulting and bleached sandstones (this study; Chan et al 2000; Garden et al 2001) suggests that there is a component of up-dip flow within or adjacent to these fault zones. Outcrops of the fault show that the foliated fault gouge has a strong fabric anisotropy, and that in places slip is localized onto discrete slip surfaces. This fabric anisotropy will result in anisotropy in fault gouge permeability. Specifically, the permeability is likely to be substantially higher along the fault surface than in the crossfault direction. Regional aquifers that could act as reservoirs for the CO2 include the Entrada Sandstone, the Navajo/Wingate sandstones and the White Rim sandstone. The water temperature (18°C) indicates that shallow aquifer waters are producing the springs, unless slow upwards flow rates allow thermal equilibration with the surrounding rocks. Heat flow values in the Green River area are consistent with geothermal gradients of 34 °C km"1 (Nuccio & Condon 1996) suggesting that the Navajo-Wingate aquifer (200-450m below the footwall to the Little Grand Wash fault) is the reservoir for shallow groundwater that is being charged with deep CO2. Several other CO2-charged springs occur in this region. The once spectacular Woodside Geyser, situated at the old town of Woodside approximately 40km north of the study area, now only erupts sporadically to a few metres from an abandoned drill hole. The Tumbleweed and Chaffin Ranch geysers erupt occasionally from drill holes to the south of the faults in this study (Fig. 2). These geysers, and the springs along the faults generally fall along the line of the regional north-plunging anticline axis (Fig. 2). This suggests that the flow of CO2 or CO2-charged fluids is focused along the anticline axis. The active springs along the Little Grand Wash and Salt Wash faults all occur in the footwall, apart from the Tenmile Geyser, where the host drill hole may penetrate the fault. The fault may be acting as a barrier to regional south-directed groundwater flow in the Navajo/Wingate aquifer (Hood & Patterson 1984), concentrating the CO2-charged fluid in a footwall reservoir. It would be contributing to the flow path through the impermeable layers above the aquifer (the Carmel limestone, Morrison Formation etc.). In this case, the fault would have higher up-dip permeability than cross-fault permeability. The travertines may be located at structural complexities along the

55

fault zone, such as at the ends of segments or at stepovers. This relationship is commonly observed in hydrothermal systems (Curewitz & Karson 1997; Hancock^ al 1999). The bleaching and euhedral aragonite cements that are found in the footwall host rock close to the faults are evidence that fluid infiltrated the host rock as well as simply being channelled up the fault. Chan et al (2000) and Garden et al (2001) suggest that such bleaching is the result of movement of a reducing fluid through the system. They argue that this was meteoric water driven by topographic flow to salt depth where the fluid became saline and reducing and reacted with hydrocarbons. They argue for fault-controlled upward flow and movement of the fluid out into the Jurassic aquifer sequence.

Implications for CO2 sequestration projects The preliminary results of this study have implications for the design of CO2 sequestration projects. These projects require that CO2 be isolated from the atmosphere on a timescale of hundreds to thousands of years (Rochelle et al 1999). The Little Grand Wash and Salt Wash faults have been conducting fluids for a substantial period of time; however, more work remains to be done on the Little Grand Wash and Salt Wash faults to evaluate the fraction of the total CO2 reservoir that is leaking from the faults. A relatively low volume leakage of a large reservoir may well be deemed acceptable in a large sequestration project. The volume of carbon that is trapped in carbonate deposition at the surface must be estimated from future mapping of the total volume of travertines and carbonates around the wells and springs. The fact that many of the springs in the region are centred on abandoned oil wells suggests that care should be taken with well-bore integrity during the design and monitoring of CO2 sequestration projects. As suggested by the correlation of CO2 degassing and earthquake faults, the stress state in and around faults may significantly influence whether a fault is conductive or not (Barton et al. 1995). Zoback and Townend (2001) suggested that many faults in the crust are critically stressed and are very near the point of failure as predicted from a Mohr-Coulomb failure analysis. Present-day seismicity in the Colorado Plateau is dominated by extension along NW to NNW striking normal faults (Wong & Humphrey 1989) and the present state of stress in the region is consistent with NNE extension (Mueller et al 2000). The Little Grand Wash and Salt Wash faults are therefore optimally oriented for failure in the present stress field and are likely conduits for the upward migration of fluids. In addition to mapping the occurrence of faults in the vicinity of potential

56

Z.K.SHIPTONCTAL.

CO2 disposal sites, the orientation of faults with respect to the current stress state should be taken into account. Given the possibility of Quaternary to Recent fault activity, it is possible that seismic activity has played a part in the migration of the CO2 to the surface; earthquake activity has often been correlated with outgassing of CO2 reservoirs (Irwin & Barnes 1980; Sugisaki et al 1983; Chiodini et al 1995; Guerra & Lombardi 2001) and there is often a link between earthquakes and the activity of geothermal geysers. The Little Grand Wash and Salt Wash faults cut rocks that are good analogues for low net-to-gross reservoir rocks such as those found in the North Sea. The sealing capacity of faults depends on the type of structures that occur in the fault zone, how they are arranged, the contribution of each structure type to flow, and geochemical processes in the faults, which may add or remove sealing capacity. Due to the poor preservation potential of faults in outcrops of shalerich rocks, few field analogues have been studied in detail. Consequently, there is little data with which to make predictions regarding the behaviour of these faults. Such faults are often accounted for in hydrocarbon reservoirs using simple shale smear or gouge algorithms (e.g. Freeman et al 1998). These make substantial assumptions about the fault, specifically that the seal is formed by physical smearing or mixing of the low permeability clays in the host rock with no diagenesis of the fault rocks. Fault-rock alteration and diagenesis is clearly taking place in this locality and would almost certainly take place within any CO2 sequestration schemes that included faults. Much further work needs to be done on such systems before the implications for long-term storage of CO2 can be fully understood.

Conclusions The Little Grand Wash and Salt Wash faults, eastcentral Utah, affect the present-day CO2-charged groundwater flow regime over reservoir-scale distances. The CO2 flows upward from one or more deep sources that include the thermal decarbonation of carbonates deep in the basin and/or microbially mediated hydrocarbon generation. Most of the potential CO2 source rocks are below or within the Pennsylvanian Paradox Formation, approximately 1.5km below the surface. Observations of fault-controlled cementation, bleaching, and hydrocarbon accumulation elsewhere in the basin suggest that faults have been a conduit for reducing fluids and hydrocarbons. It is therefore likely that faults have been a conduit for deep-sourced CO2 through the Paradox salt and other low-permeability rocks in the basin. The low temperature of the water that effuses at the springs and geysers suggests that it is sourced

from the shallow Navajo/Wingate aquifer. The units that provide top seals to this aquifer are the Jurassic Summerville and Morrison Formations. The Cretaceous Mancos Shale and Cedar Mountain Formation would provide a juxtaposition seal in the hanging wall to the faults. The location of well-bore-related springs in the area show that gas-charged groundwater is ponded at the apex of a regional north-plunging anticline. Both natural and drill hole-related spring and travertine activity is concentrated along the fault zones, specifically in the northern footwalls. This suggests that the faults have a relatively low crossfault permeability and form a seal to migration of gas up the plunge of the anticline. The faults must therefore be the conduit for the gas to move through the sealing units, and have relatively high hydraulic conductivity in the vertical (up-dip) direction. The gas either seeps upward continually or is transported during fault slip events. The springs and travertines are concentrated at structural complexities and linkage zones along the faults. The long-lived nature of the fluid flow is demonstrated by ancient travertines and vein systems that are localized along the fault zone. The faults cut rocks that are analogous to the North Sea reservoirs in which it is proposed to dispose of significant quantities of CO2. Even if a relatively small proportion of the gas reservoir is able to escape along fault zones, then over periods of hundreds to thousands of years the efficiency of CO2 sequestration projects could be called into question. We have determined the basic regional structure of the Little Grand Wash and Salt Wash fault system, making these faults an ideal candidate for a more detailed study of the interaction between fluids and low-permeability faults in clay-rich rocks at the oil-field scale. Thanks to Bill Shea for introducing us to the field site, and to Torrey Copfer for re-discovering some of the studied springs. This work was funded in part by the Carbon Capture Project, a joint industry-government consortium. Steve Laubach, Tim Needham and Richard Worden provided thoughtful reviews of the manuscript. We thank Scott Imbus for insights into the geochemistry of petroleum systems, and to John Gale and Shelagh Baines for organizing the 2001 session on CO2 at BUG. We thank the people of Green River, Utah who provided information on historical eruptions of the Crystal Geyser.

References ALLIS, R., CHIDSEY, T, GWYNN, W., MORGAN, C, WHITE, S., ADAMS, M. & MOORE, J. 2001. Natural CO2 reservoirs on the Colorado Plateau and southern Rocky Mountains: candidates for CO2 sequestration. In: Proceedings of the National Energy Technology Laboratory, First National Conference on Carbon Sequestration. World Wide Web address:http://www.netl.doe.gov/ publications/proceedings/01/carbon_seq/6a2.pdf.

CO? LEAKAGE THROUGH LOW-PERMEABILITY FAULTS AMUNDSON, R. G. & KELLY, E. 1987. The chemistry and mineralogy of a CO2-rich travertine depositing spring in the California Coast Range. Geochimica et CosmochimicaActa, 51,2883-2890. BAER, J. L. & RIGBY, J. K. 1978. Geology of the Crystal Geyser and the environmental implications of its effluent, Grand County, Utah. Utah Geology, 5, 125-130. BARTON, C. A., ZOBACK, M. A. & Moos, D. 1995. Fluid flow along potentially active faults in crystalline rock. Geology, 23,683-686. BAUMGARTNER L. R & VALLEY J. W. 2001. Stable isotope transport and contact metamorphic fluid flow: In: VALLEY, J. W. & COLE, D. R. (eds) Stable Isotope Geochemistry, Reviews in Mineralogy and Geochemistry, 43,415^68. BODINE, M. W. & JONES, B. F 1986. The Salt Norm: A quantitative chemical-mineralogical characterization of natural waters. US Geological Survey Water Resources Investigations, 86-4086,130 p. CAMPBELL, J. A. & BAER, J. L. 1978. Little Grand Wash fault-Crystal Geyser area. In: FASSET, J. E. & THOMAIDIS, N. D. (eds) Oil and gas fields of the Four Corners area. Four Corners Geological Society, 666-669. CAPPA, J. A. & RICE, D. D. 1995. Carbon dioxide in Mississippian rocks of the Paradox Basin and adjacent areas, Colorado, Utah, New Mexico and Arizona. US Geological Survey Bulletin 2000-H, 21pp. CHAFETZ, H. S. & FOLK, R. L. 1984. Travertines: Depositional morphology and the bacterially constructed constituents. Journal of Sedimentary Petrology, 54, 289-316. CHAFETZ, H.S., UTECH N. M. & FITZMAURICE S. P. 19910. Differences in the 818O and 813C signatures of seasonal laminae comprising travertine stromatolites. Journal of Sedimentary Petrology, 61,1015-1028. CHAFETZ, H. S., RUSH, P. & UTECH, N. \99lb. Microenvironmental controls on mineralogy and habit of CaCO3 precipitates: an example from an active travertine system. Sedimentology, 38,107-126. CHAN, M.A., PARRY, W. T. & BOWMAN, J. R. 2000. Diagenetic hematite and manganese oxides and faultrelated fluid flow in Jurassic sandstones, southeastern Utah. American Association of Petroleum Geologists Bulletin, 84, 1281-1310. CHIODINI, G., FRONDINI, F. & PONZIANI, F. 1995. Deep structures and carbon dioxide degassing in Central Italy. Geothermics, 24, 81-94. CLAYPOOL, G. E., LOVE, A. H. & MAUGHAN, E. K. 1978. Organic geochemistry, incipient metamorphism, and oil generation in black shale members of Phosphoria Formation, Western Interior United States. American Association of Petroleum Geologists Bulletin, 62, 98-120. COLEMAN, D. D., Liu, C.-L. & RILEY, K. M. 1988. Microbial methane in the shallow Paleozoic sediments and glacial deposits of Illinois, USA. Chemical Geology, 71,23^0. CUREWITZ D. & KARSON J. A. 1997. Structural settings of hydrothermal outflow; fracture permeability maintained by fault propagation and interaction. Journal of Volcanology and Geothermal Research, 79, 149-168. DOELLING, H. 1994. Tufa deposits in western Grand

57

County. Survey Notes - Utah Geological Survey, 26, 8-10, 13. DOELLING, H. 2001. The geologic map of the Moab and eastern San Rafael Desert 30X60' quadrangles. Utah Geological Survey Map M-180. FREEMAN, B., YIELDING, G., NEEDHAM, D. T & BADLEY, M. E. 1998. Fault seal prediction: the gouge ratio method. In: COWARD, M. P. DALTABAN, T. S., & JOHNSON, H. (eds) Structural Geology in Reservoir Characterization. Geological Society, London, Special Publications, 127,19-26. FRIEDMAN, I. 1970. Some investigations of the deposition of travertine from Hot Springs -1. The isotopic chemistry of a travertine-depositing spring. Geochimica et CosmochimicaActa,34,1303-1315. FOLK, R. L., CHAFETZ, H. S. & TIEZZI, P. A. 1985. Bizarre forms of depositional and diagentic calcite in hotspring travertines, Central Italy. In: SCHNEIDERMANN, N. & HARRIS, P. M. (eds) Carbonate Cements. Society for Sedimentary Geology Special Publication, 36, 349-379. GARDEN, I. R., GUSCOTT, S. C., BURLEY, S. D., FOXFORD, K. A., WALSH, J. J. & MARSHALL, J. 2001. An exhumed palaeo-hydrocarbon migration fairway in a faulted carrier system, Entrada Sandstone of SE Utah, USA. Geofluids, 1,195-214. GUERRA, M. & LOMBARDI, S. 2001. Soil-gas method for tracing neotectonic faults in clay basins: the Pisticci field (Southern Italy). Tectonophysics, 339, 511-522. Guo, L., ANDREWS, J., RIDING, R., DENNIS, P. & DRESSER, Q. 1996. Possible microbial effects on stable carbon isotopes in hot-spring travertines. Journal of Sedimentary Research, 66,468^73. HANCOCK, P. L., CHALMERS, R. M. L., ALTUNEL, E. & CAKIR, Z. 1999. Travitonics; using travertines in active fault studies. Journal of Structural Geology, 21, 903-916. HANSLEY, P. L. 1995. Diagenetic and burial history of the White Rim Sandstone in the Tar Sand Triangle, Paradox Basin, southeastern Utah. US Geological Survey Bulletin, 2000-1,41pp. HEATH, J. E. 2004. Hydrogeochemical characterization of CO2 charged fault zones in east-central Utah. M.S. Thesis, Utah State University, Logan. 166pp. HEATH J. E., LACHMAR T. E., SHIPTON Z. K., NELSON, S. & EVANS J. P. 2002. Hydrogeochemical analysis of leaking CO2-charged fault zones: the Little Grand Wash and Salt Wash fault zones, Emery and Grand counties, Utah. Geological Society of America Abstracts with Programs, 34 (6), p. 392. HECKER, S. 1993. Quaternary tectonics of Utah with emphasis on earthquake-hazard characterization. Utah Geological Survey Bulletin, 127, p. 157. HINTZE, J. F. 1993. Geologica History of Utah. 2nd edn. Brigham Young University Geology Studies, Special Publication 7.202p. HOOD, J. W & PATTERSON, D. J. 1984. Bedrock aquifers in the northern San Rafael Swell area, Utah, with special emphasis on the Navajo Sandstone. State of Utah Department of Natural Resources Technical Publication no. 78. IAEA (2001). GNIP Maps and Animations, International Atomic Energy Agency, Vienna. World Wide Web address: http://isohis.iaea.org.

58

Z.K.SHIPTON£TAL.

IRWIN, W. P. & BARNES, I. 1980. Tectonic relations of carbon dioxide discharges and earthquakes. Journal of Geophysical Research, 85, 3115-3121. JENDEN, P. D., KAPLAN, I. R., POREDA, R. J. & CRAIG, H. 1988. Origin of nitrogen-rich natural gases in the California Great Valley - evidence from helium, carbon, and nitrogen isotope ratios. Geochimica et Cosmochimica Acta, 52, 851-861. KIRSCHNER, D., ENCARNACION, J., & AGOSTA, F. 2000. Incorporating stable isotope geochemistry in undergraduate laboratory courses. Journal of Geoscience Education, 48, 209-215. MAYO, A. L., SHRUM, D. B. & CHIDSEY, T. C, JR. 1991. Factors contributing to exsolving carbon dioxide in ground-water systems in the Colorado Plateau, Utah. In: CHIDSEY, T. C, JR (ed.) Geology of east-central Utah. Utah Geological Association Publication, 19, 335-342. MCKNIGHT, E.T. 1940. Geology of area between Green and Colorado rivers, Grand and San Juan Counties, Utah. US Geological Survey Bulletin, 908,147 pp. MUELLER, B., REINECKER, J., HEIDBACH, O. & FUCHS, K. 2000. The 2000 release of the World Stress Map (available online at www.world-stress-map.org). Nuccio, V. F. & CONDON, S. M. 1996. Burial and thermal history of the Paradox Basin, Utah and Colorado, and the petroleum potential of the Middle Pennsylvanian Paradox Formation. US Geological Survey Bulletin, 2000-O,41pp. PETERSON, P. R. 1973. Salt Wash field. Utah Geological Survey oil and gas field studies, no 4. 3pp. PENTECOST, A. 1995. Geochemistry of carbon dioxide in six travertine-depositing waters of Italy. Journal of Hydrology, 167, 263-278. PEVEAR, D. R., VROLIJK, P. J. & LONGSTAFFE, F. J. 1997. Timing of Moab fault displacement and fluid movement integrated with burial history using radiogenic and stable isotopes. In: HENDRY, J., CAREY, P., PARNELL, J., RUFFEL, A. & WORDEN, R. (eds) Geofluids II1997 Extended abstract volume, 42-45. POWELL, J. W. 1895. The Canyons of the Colorado (now

published as The exploration of the Colorado River and its canyons. 1997. Penguin Books 416 pages.) ROCHELLE, C. A., PEARCE, J. M. & HOLLOWAY, S. 1999. The underground sequestration of carbon dioxide: containment by chemical reactions in the deep geosphere. In: METCALFE, R. & ROCHELLE, C. A. (eds) Chemical containment of waste in the lithosphere, Geological Society, London, Special Publications, 157,117-129. SANFORD, R. F. 1995. Groundwater flow and migration of hydrocarbons to the Lower Permian White Rim Sandstone, Tar Sand Triangle, Southeastern Utah. US Geological Survey Bulletin, 2000-J, 24pp. SCHOELL, M. 1983. Genetic characterization of natural gases. American Association of Petroleum Geologists Bulletin, 67,2225-2238. SELLEY, R. C. 1998. Elements of Petroleum Geology, 2nd edn, Academic Press. 470pp. SUGISAKI, R. IDO, M., TAKEDA, H. et al. 1983. Origin of hydrogen and carbon dioxide in fault gases and its relation to fault activity. Journal of Geology, 91, 239-258. TRUESDELL, A. H. AND JONES, B. F. 1974. WATEQ, a computer program for calculating chemical equilibria of natural waters. Journal of Research of the US Geological Survey, 2(2), 233-248. WILLIAMS, P. L. 1964. Geology, structure, and uranium deposits of the Moab Quadrangle. Colorado and Utah US Geological Survey Map I- 360. WILLIAMS, P. L. & HACKMAN, R. J. 1971. Geology, structure, and uranium deposits of the Salina Quadrangle, Utah. US Geological Survey Map 1-591. WONG, I. G. & HUMPHREY, J. R. 1989. Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau. Geological Society of America Bulletin, 101, 1127-1146 ZOBACK, M. D. & TOWNEND, J. 2001. Implications of hydrostatic pore pressure and high crustal strength for the deformation of intraplate lithosphere. Tectonophysics, 336,19-30.

The long-term fate of CO2 in the subsurface: natural analogues for CO2 storage SHELAGH J.BAINES1 & RICHARD H. WORDEN2 1

BP Exploration and Production Company, Chertsey Road, Sunbury-on-Thames, TW16 7LN UK (e-mail: [email protected]) 2 Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK Abstract: CO2 is a common gas in geological systems so that planned storage of CO2 in the subsurface may do no more than mimic nature. Natural CO2 has a wide number of sources that can be at least partly identified by carbon stable isotope geochemistry. Three pairs of case studies with different reservoir characteristics and CO2 contents have been examined to assess the natural impact of adding CO2 to geological systems. Carbonate minerals partially dissolve when CO2 is added simply because the CO2 dissolves in water and forms an acidic solution. Therefore, carbonate minerals in the subsurface are not capable of sequestering secondary CO2. The addition of CO2 to a pure quartz sandstone (or a sandstone in which the supply of reactive aluminosilicate minerals has been exhausted by excess natural CO2 addition) will have no consequences: the CO2 will simply saturate the water and then build up as a separate gas phase. The addition of CO2 to carbonate cemented sandstone without reactive aluminosilicate minerals will induce a degree of carbonate mineral dissolution but no solid phase sequestration of the added CO2. When CO2 is naturally added to sandstones it will induce combined aluminosilicate dissolution and carbonate cementation if the aluminosilicate minerals contain calcium or magnesium (or possibly iron). Examination of a CO2filled porous sandstone with abundant reactive aluminosilicate minerals that received a huge CO2 charge about 8000 to 100000 years ago reveals minimal evidence of solid phase sequestration of the added CO2. This indicates that either dissolution of reactive aluminosilicates or precipitation of carbonate minerals is relatively slow. It is very likely that the slow dissolution of reactive aluminosilicates is the rate-limiting step. Solid phase sequestration of CO2 occurs only when reactive aluminosilicates are present in a rock and when the system has had many tens to hundreds of thousands of years to equilibrate. The two critical aspects of the behaviour of CO2 when injected into the subsurface are (1) that the rock must contain reactive Ca and Mg aluminosilicates and (2) that reaction to produce carbonate minerals is extremely slow on a human timescale. The reactive minerals include anorthite, zeolite, smectite and other Fe- and Mg-clay minerals. Such minerals are absent from clean sandstones and limestones but are present in 'dirty' sandstones (lithic arenites which are mineralogically immature) and some mudstones. The analysis of geological analogues shows that injection of CO2 into carbonate-bearing rocks that do not contain reactive minerals will induce dissolution of the carbonate, whether it is a matrix cement, rock fragment, fault seal or part of a top-sealing mudstone.

Subsurface storage of CO2 is considered by both industry and national governments (e.g. UK DTI 2002; US DOE 2004) to be a key mechanism for reducing the emission of greenhouse gases to the atmosphere. CO2 captured either directly from a petroleum stream or from the exhaust products of electricity generating stations and industrial plants can be introduced into suitable subsurface reservoirs for long term, safe, storage. Simple logic suggests that since non-aqueous fluids and gases, such as petroleum accumulations, are naturally trapped in the subsurface for geological periods of time, so captured CO2 can also be injected and stored for the 1000 to 10000 year time period considered necessary for stabilization and remediation of the earth's atmosphere (Pacala pers. comm.). There are several options under investigation including injection into

deep, saline aquifers (e.g. Sleipner field, Baklid^^/. 1996), injection into former or current oil and gas accumulations and injection into coal beds. CO2 is already injected into oil fields during enhanced oil recovery thus reducing the issue to development of existing technology rather than invention of radically new technology. Two broad scenarios are envisaged here upon injection of the CO2 into the subsurface. The first is storage, or simple retention, similar to present day petroleum accumulations in which the CO2 is present as either a gas, aqueous or dense (supercritical) phase. This scenario retains the ability for the CO2 to migrate if the integrity of the site is breached over time, or if the geological framework is radically altered as has occurred in volcanically active areas, e.g. LakeNyos (Cameroon) and Mammoth Mountain

From: BAINES, S. J. & WORDEN, R. H. (eds) 2004. Geological Storage of Carbon Dioxide. Geological Society, London, Special Publications, 233,59-85.0305-8719/047$ 15.00 © The Geological Society of London 2004.

Fig. 1. Global distribution of high (>20%) CO2-content basins. The sites discussed in this study are indicated.

NATURAL ANALOGUES

(USA). The second is CO2 sequestration, in which the CO2 is locked into newly formed minerals that are stable under reservoir conditions. These approaches have one problem; CO2 is a reactive gas that has significantly more influence upon the host rocks and their formation waters than petroleum fluids. The long-term consequence of increasing the gross volume of CO2 in the subsurface is unknown, but is a key screening criterion in the evaluation of potential subsurface storage sites. Resorting to a modelling approach suffers from the perennial problem that the rates of the various component geochemical and physiochemical processes under geological conditions are not well known. Previous studies aimed at understanding the impact of CO2 on the subsurface environment have commonly been designed around forward-modelling, using geochemical software combined with short term experimental studies to replicate the expected conditions during CO2 injection and retention (e.g. Czernichowski-Lauriol et al. 1996; Gunter et al. 1997, 2004; Gupta & Sass 1999; Johnson et al. 2004; Rochelle et al. 2004). These use either measurements of present day physical and chemical conditions, or a range of values as primary inputs for forward modelling the final state of the CO2. The short-term experiments, necessarily performed at elevated temperatures to produce results on a measurable timescale, are expensive and thus limited, deal with only single variables instead of the huge complexity of nature and do not mimic nature. Other studies have used data from enhanced oil recovery (EOR) CO2 floods to study the interaction of CO2 with both the reservoir rock and the formation fluids (e.g. Wolcott et al. 1989; Smith 1998; Worden & Smith 2004). Both of these approaches, however, rely on generally incomplete datasets over relatively short timescales (months to one or two decades); at least two or more orders of magnitude less than those required for long-term storage. Many geological systems have abundant evidence that CO2 has been added in large volumes. Indeed there are numerous geological sites that today have naturally elevated CO2 concentrations (Fig. 1). The geological timescale is typically variable but can stretch from a few tens of thousands of years to many millions of years (and beyond). Thus the rock record, in association with current fluid compositional data, affords the opportunity for assessment of the long-term fate of CO2 in the subsurface. Rather than anthropogenic sources, the CO2 in question has natural sources.

CO2 in geology CO2 is an important gas in many geological formations and is responsible for, or involved in, many

61

natural geochemical processes. CO2 affects porosity since it can influence both mineral dissolution and growth in a given formation. The occurrence of high-pressure CO2 may lead to weakened fault rocks and may trigger faulting and thus earthquakes. Overpressured fluids, including CO2, in the subsurface can lead to seal failure and the escape of fluids to the surface. CO2 seepage via faults at the seabed from submarine fault zones can lead to deep-water reefs and bioherms. CO2 is one of the most common volatiles found in volcanic gases and is important in determining eruption cycles (e.g. Krauskopf 1979). There are a large number of possible sources of CO2 in the subsurface (e.g. Clayton 1991; Emery & Robinson 1993; Irwin et al 1977 Wycherley et al 1997; Table 1 & Fig. 2). The isotopic ratio of 13C to 12 C is commonly used to help type CO2. Both isotopes are non-radioactive and are not influenced directly by time. The ratio of 13C to 12C is conventionally compared to a standard (Peedee Formation belemnite) and the result is quoted as a per mil quantity (813C %0 PDB). It is commonly difficult to identify an exact source of any sample of CO2 based on either or both of geological location and carbon stable isotopes since there are ambiguities in both. Broadly, though, there are four main groups of CO2 sources (Fig. 2). (1)

(2)

Deep sources of CO2 are common in tectonically active regions of the earth's crust. Thermal metamorphism and juvenile sources of CO2 are associated with igneous processes (both intrusive and extrusive) prevalent at plate collision zones (e.g. the Andes) and plate rifting zones (e.g. the East African rift). High temperature metamorphism leads to devolatilization and loss of CO2 from minerals (e.g. Yardley 1989). Any carbonate-mineral bearing, prograde metamorphic, rock will tend to lose CO2 to the fluid phase that then will be free to ascend under normal buoyancy to the surface or a transient trapping structure. Deepseated faults have routinely been implicated in the release of very deep sources of CO2. Regional metamorphic terrains have their fluid phase dominated by CO2 and N2 so that release of fluid via deep faults (during seismicity) will tend to release CO2. Atmospherically derived CO2 in the subsurface has been suggested but only in regions where there are active groundwater systems with a constant flux of meteoric water. Such conditions are found in artesian basins. CO2 solubility in water is a function of many controls including temperature, pressure and salinity. The rate of passage of water through aquifers is sufficiently slow to permit equilibration of CO2 with the surrounding rock

62

S. J. BAINES & R. H. WORDEN

Table 1. Geological sources ofCO2 and their stable isotopic compositions. Data from Clayton (1991); Emery & Robinson (1993); Irwin et al. (1977); Wycherley et al (1997). CO9 source

Stable carbon isotope range (S 13 C%cPDB)

Main occurrence

Deep crust

-6%o

Near very deep faults

Juvenile (volcanic)

-6%c

Associated with plate spreading and subduction

Atmospheric CO2 exsolved from meteoric waters

Variable over geological time, approx. -6%c

In artesian sandstones and limestones, typically at the outlet of the aquifer

Fermentation of organics

-20%o

Very shallow buried organic rich sediments

Bacterial oxidation of organics

-23%o

Very shallow buried organic rich sediments Petroleum source rocks and coal beds

Kerogen and coal decarboxylation

-25%o

Burial diagenesis of impure limestones

Depends on limestone age, Most sedimentary basins, many clastic sediments approx -2 to +3%o

Burial diagenesis of carbonate cemented sandstones

Depends on 813C of carbonate cement

Most sedimentary basins, many clastic sediments

Thermal metamorphism of limestones

Depends on limestone age, approx -2 to +3%o

Contact metamorphism due to intrusion

Sulphate reduction of petroleum (petroleum oxidation)

-20to-40% 0

Bacterial = near surface, Thermochemical = > 120°C

Fig. 2. Schematic illustration of the natural origins of CO2 in geological systems (Clayton 1991; Emery & Robinson 1993; Irwin et al 1977; Wycherley et al. 1997)

(3)

matrix. Consequently this source of subsurface CO2 is only likely to be volumetrically significant in situations where water in relatively deep aquifers moves upwards rapidly before equilibrium can be achieved. The consequence is mineralized springs and geysers that are well known for their dissolved CO2 contents (e.g. Shiptonetal.2004). Organic sources of CO2 take many forms but all have a root cause: some sedimentary rocks, usually mudstones, contain organic detritus

when they are buried. Much of the organic matter that is initially buried with sediments undergoes decomposition within a few hundred metres burial (e.g. Irwin et al. 1977). The decomposition processes include fermentation, bacterial oxidation, and bacterial sulphate reduction (plus minor nitrate reduction, ferric iron reduction, and manganese reduction). CO2 from these sources can dominate gases in the local subsurface and typically has low 813C values, reflecting the isotopic composition of the organic matter. Methanogenesis is a bacterial process that synthesizes hydrocarbon gas from CO2 resulting in isotopically enriched CO2 although inevitably reducing the quantity of CO2. Organic matter that survives initial burial is known as kerogen. Kerogen undergoes thermal breakdown processes, known as maturation, as a function of time and temperature. Kerogen decarboxylation (loss of C-O functional groups) occurs relatively early in the maturation process and produces CO2 with fairly low 813C values (e.g. Tissot & Welte 1984). Petroleum, also produced by kerogen thermal breakdown, can undergo oxidation by reaction with sulphate or ferric iron. Sulphate and ferric iron reduction, whether bacterial or thermochemical, produces abundant CO2 although much is locally reprecipitated as carbonate minerals. Within inverted (uplifted) oil fields, ingress of meteoric water may introduce sulphate-reducing bacteria and possibly dis-

63

NATURAL ANALOGUES Table 2. End member case study locations and accumulation type. End member type

Carbonate

Quartz arenite

Arkose

High CO2 case (% C02)

Blue Whale, Da Nang Basin (>70%)

Miller field (-28%)

Bravo Dome (100%)

Accumulation type (high CO2 case)

Offshore gas

Offshore oil and gas

Onshore CO2

Low CO2 case (% CO2)

Dolphin, Da Nang Basin (