Geological Input to Selection andEvaluation of CO2 Geosequestration Sites

John G. Kaldi, Catherine M. Gibson-Poole, and Tobias H. D. Payenberg

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Australian School of Petroleum,University of Adelaide, Australia

ABSTRACT

Coal, oil, and natural gas currently supply about 85% of the world’s energy

needs. Unfortunately, the burning of these fossil fuels is the major source of

anthropogenic carbon dioxide, which is also the main greenhouse gas released

to the atmosphere. One promisingmeans by which to reduce CO2 emissions, and so the

atmospheric buildup ofCO2, is geosequestration.Geosequestration, also known as carbon

capture and storage (CCS), involves the long-term storage of CO2 in deep subsurface

geological reservoirs. Geosequestration comprises several steps that include the capture

ofCO2, the transport ofCO2, the injectionofCO2 into suitable reservoirs, and finally, the

storage and monitoring of the CO2 that has been introduced into the reservoir.

Geological input into the evaluation of storage sites, including injection, storage, and

monitoring and verification of volumes and movement of CO2 plumes, is critical for ac-

ceptance of CCS technologies. Detailed characterization and realistic modeling of reser-

voir and seal properties, as well as of rock and fault integrity, will permit a more viable

analysis of risks associated with the subsurface containment of injectedCO2. Geoseques-

tration can be a significant factor in the portfolio of CO2 emissions reduction strategies

because by reducing CO2 emissions while still allowing for the continued use of fossil

fuels, geosequestration buys time for the transition to renewable energy sources.

INTRODUCTION

Coal, oil, and natural gas currently supply about85% of the world’s energy needs. Moreover, given therelatively low cost and abundance of fossil fuels togetherwith the huge sunken investment in fossil-fuel-based in-frastructure, fossil fuels will likely continue to be used forat least the next 25 to 50 yr. The burning of fossil fuels is,however, themajor sourceof anthropogenic (man-made)carbon dioxide (CO2). Carbon dioxide is themain green-

house gas released to the atmosphere (Intergovernmen-tal Panel on Climate Change [IPCC], 2005).

Geosequestration, also known as carbon capture andstorage (CCS), is a means to reduce anthropogenic CO2

emissions to the atmosphere. Geosequestration involvesthe long-term storage of captured CO2 emissions in deepsubsurfacegeological reservoirs.Carbon sequestrationcanbe pursued as part of a portfolio of greenhouse gas abate-ment options, when this portfolio also includes improv-ing the conservation and efficiency of energy use and

1Kaldi, J. G., C. M. Gibson-Poole, and T. H. D. Payenberg, 2009, Geological input

to selection and evaluation of CO2 geosequestration sites, in M. Grobe, J. C.Pashin, and R. L. Dodge, eds., Carbon dioxide sequestration in geologicalmedia—State of the science: AAPG Studies in Geology 59, p. 5–16.

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Copyright n2009 by The American Association of Petroleum Geologists.

DOI:10.1306/13171230St59227

utilizing nonfossil energy forms such as renewable (solar,wind, and tidal) and nuclear energy (Kaldi, 2005). Geo-sequestration may contribute significant reductions toanthropogenic CO2 emissions. Estimates by the IPCCindicate that a technical potential of at least about 2000billionmetric tonnes of CO2 storage capacity in geolog-ical formations likely exists (Table 1) (IPCC, 2005).

Geosequestration comprises several steps: first, theCO2 is captured at the source,which can be a power plantor other industrial facility; the capturedCO2 is then trans-ported, typically via pipeline, from the source to the geo-logical storage site; next, the CO2 is injected deep under-groundviawells into the geological reservoir; and finally,the CO2 is stored in the geological reservoir, where itsmovement is carefullymonitored and thequantity storedis regularly verified (Figure 1). The capture, transport, andinjection processes do require additional energy to beexpended (and hence more CO2 is emitted); however,the net CO2 emission reduction is still a significantlylarge volume tomake deep reductions in anthropogenicgreenhouse gas emissions. For example, a power plant

with CCS could reduce net CO2 emissions to the at-mosphere by approximately 80–90% compared to aplant without CCS (Figure 2) (IPCC, 2005).

CARBON DIOXIDE CAPTURE

Carbon dioxide capture can be conducted at a point(stationary) source of CO2 such as a power plant. Carbondioxide capture involves capturing the produced CO2

instead of allowing it to be released to the atmosphere.This captured CO2 is then compressed to make it moredense and much easier, and less costly, to transport tothe geological storage site.

AnthropogenicCO2 that canbe captured is producedby threemain types of activity: industrial processes, elec-tricity generation, and hydrogen (H2) production. Indus-trial processes that lend themselves to CO2 capture in-clude natural gas processing, ammonia production, andcementmanufacture.Note, however, that the total quan-tity of CO2 produced by these processes is limited. A

FIGURE 1. A simplified viewof the steps involved in thegeosequestration process (im-age courtesy of CooperativeResearch Centre for Green-house Gas Technologies[CO2CRC]).

TABLE 1. Storage capacity for several geological storage options.*

Reservoir Type Lower Estimate ofStorage Capacity (Gt CO2)

Upper Estimate ofStorage Capacity (Gt CO2)

Oil and gas fields 675** 900**

Unmineable coal seams in ECBMy recovery 3–15 200

Deep saline formations 1000 Uncertain but possibly 10,000

*The storage capacity includes storage options that are not economical (from IPCC, 2005).**These numbers would increase by 25% if undiscovered oil and gas fields were included in this assessment.yECBM = enhanced coalbed methane.

6 Kaldi et al.

far larger source, accounting for one-third of total CO2

emissions, is fossil-fueled power production. The typesof power plants that are best suited to CO2 capture arepulverized coal (PC), natural gas combinedcycle (NGCC),and integrated gasification combined cycle (IGCC) plants(Davison et al., 2006). Finally, a potentially large futuresource of CO2 for capturewill beH2 production, wherebythe producedH2 is used to fuel a hydrogen economy, i.e.,it is used in turbines toproduce electricity and in fuel cellsto power cars. Technologies for capturing CO2 from elec-tricity generation fall into two general categories: post-combustion and precombustion (Kentish et al., 2006).

Postcombustion

Currently, the most widely used postcombustiontechnology for CO2 capture is chemical absorption(Figure 3). This capture process involves the flue gasbeing blown through a solvent such as monoethanol-amine (MEA) in an absorption column and the CO2 inthe flue gas being absorbed in the MEA solvent by theformation of a chemically bonded compound (Jassimand Rochelle, 2006). A very similar process using MEAhas been used for decades to remove acid gases, suchas CO2 and hydrogen sulfide (H2S), from natural gas

streams (Franco et al., 2006). Chemical absorption ismost likely to be used for PC and NGCC power plants(Roberts et al., 2005).

Precombustion

In the case of IGCC plants, using the precombus-tion CO2 capturemethod of physical absorption wouldbe possible (Figure 3). This capture method involves gas-ifying the coal to produce a synthetic gas (syngas) com-posed of carbon monoxide (CO) and hydrogen (H2)(Veawab et al., 2002). The CO is reacted with water toproduce CO2 and H2, and the H2 is sent to a turbine toproduce electricity. The CO2 is captured bymeans of dis-solving it in a physical solvent such as methanol. Sev-eral IGCCand coal gasification facilities existworldwideto produce syngas and various other by-products. Onesuch example of a gasification facility is an ammoniamanufacturing plant (Kentish et al., 2006).

CARBON DIOXIDE TRANSPORT

Carbon dioxide transport involves moving, or trans-porting, the captured CO2 from the CO2 point source tothegeological storage site. TheCO2 is typically transportedin a compressed form via pipeline, although the CO2

could also be transported by truck, rail, or in the case of ageological storage site located offshore, ocean tanker.

Transport via Pipeline

The CO2 is transported via pipeline as a supercriticalor dense phase fluid. Above the critical point, whichoccurs at a temperature of 31.18C and a pressure of7.38MPa, CO2 exists in the supercritical or dense phase.The CO2 in this phase has a significantly higher densitythan either gaseous or liquidCO2. Transporting theCO2

in this phase, and also at higher density, has significanteconomic benefits (Shaw and Bachu, 2002).

The transport of CO2 by pipeline already occurs quiteextensively in the United States as well as, to a smallerextent, in other countries where CO2 is used for en-hanced oil recovery (EOR) operations. In the UnitedStates, some 2400 km (1491 mi) of CO2 pipelines tosupply 72 EOR projects using CO2 floods exist. Many ofthese pipelines have been in operation since the early1980s (Shaw and Bachu, 2002). Most of the transportedCO2 is obtained fromhigh-pressure, high-purity naturalunderground deposits, with a small percentage of theCO2 from anthropogenic sources. The longest and oneof the most significant CO2 pipelines currently in oper-ation is theWeyburn pipeline, which is 325 km (202mi)in length and transports 2.7 millionm3 (95.3million ft3)of CO2 per day from the Great Plains Synfuels plant inNorth Dakota, to the Weyburn CO2-EOR project in

FIGURE 2. Comparison of CO2 emissions from powerplants with andwithout CO2 capture and storage. A powerplant with CCS (lower bar) has increased CO2 productionresulting from loss in overall efficiency because of the ad-ditional energy required for capture, transport, and stor-age. However, available technology can capture 85–95%of the CO2 processed in a capture plant, resulting in anet CO2 emission reduction (CO2 avoided) of 80–90%compared to the reference power plant (upper bar)withoutcapture. CCS = carbon capture and storage.

Geological Input to Selection and Evaluation of CO2 Geosequestration Sites 7

Saskatchewan, Canada (International Energy Agency[IEA], 2005).

CARBON DIOXIDE INJECTION

Carbon dioxide injection involves taking the CO2

from the surface and injecting it deep underground intoa reservoir rock. The CO2 is injected into the reservoirvia a single well or array of wells. Both EOR using CO2

floods and acid gas injection (AGI) are mature technol-ogies that involve significant quantities of CO2 beinginjected underground and are therefore very good ana-logs for CO2 injection as part of geosequestration ac-tivities. The first project using CO2 for EOR began in1972, and by 1999, 84 operational projects worldwideexisted (72 in the United States) injecting an estimatedtotal of more than 15million t of CO2 per year (ElectricPower Research Institute [EPRI], 1999).

CARBON DIOXIDE STORAGE

Carbon dioxide storage involves keeping the CO2

secured deep underground in a geological reservoir. Car-bon dioxide can be stored geologically in a variety ofdifferent options (Figure 4). These include depletedoil and gas fields, EOR, deep saline formations, deepunmineable coal seams, enhanced coalbed methanerecovery (ECBMR), and other opportunities such as saltcaverns (Cook, 1998; Bachu and Gunter, 1999; Cooket al., 2000; IPCC, 2005).

The CO2 can be geologically stored in oil and gasfields once they have been depleted and are no longerproducing or can be used to enhance oil recovery infields that are still producing. The main advantages ofstorage in depleted oil and gas fields are that the con-tainment potential of the site has been proven by theretention of hydrocarbons for millions of years and typi-cally large amounts of geological and engineering data

FIGURE 3. Overview of carbon dioxide capture processes (image courtesy of Cooperative Research Centre for GreenhouseGas Technologies [CO2CRC]). IGCC = integrated gasification combined cycle; LNG = liquefied natural gas.

8 Kaldi et al.

are available for detailed site characterization (Hollowayand Savage, 1993; IPCC, 2005). Possible drawbacks maybe the physical size of the structural or stratigraphic trap(i.e., potential storage capacity may be limited), the pos-sibility that pore-pressure depletion has led to pore col-lapse (which will reduce the potential storage capacity),and the timing of availability of depleted fields with re-spect to the source of CO2 (Bradshaw and Rigg, 2001;Bradshaw et al., 2002; Streit and Siggins, 2005). In EOR,the CO2 is used to incrementally increase the amount ofoil extracted by either immiscible (notmixed) ormiscible(mixed together) flooding, thus providing an economicbenefit while additionally storing CO2. As with depletedoil and gas fields, the potential storage capacity may belimited because of the physical size of the field (Islamand Chakma, 1993; Cook et al., 2000; IPCC, 2005).

TheCO2 storage in coalbeds is very different from thestorage in oil and gas fields or saline formations becausethe trapping mechanism is by adsorption as opposed tostorage in rock pore space. The CO2 is preferentially ad-sorped onto the coal micropore surface, displacing theexisting methane (CH4) (Gunter et al., 1997; BradshawandRigg, 2001; IPCC,2005). TheCO2 canbe geologicallystored in coalbeds that are considered economically un-mineable or can be used to enhance coalbed methanerecovery. Technical challenges for CO2 storage in coalseams include the ability to inject theCO2, causedby thetypically lowpermeability characteristics of the coal cleatsystem (especially with increasing depth and coal ma-turity), and the economic viability, caused by the largenumber of wells that may need to be drilled (Gunteret al., 1997; Bradshaw and Rigg, 2001; IPCC, 2005).

Saline formations are deep sedimentary rocks satu-rated with formation waters that are unsuitable for hu-manconsumptionor agriculture. Theyhavebeen identi-fied bymany studies as one of the best potential optionsfor CO2 geological storage (e.g., Bachu, 2000; Bradshawet al., 2002). Possible drawbacks of saline formations arethat the containment potential of the seal is commonlyuntested and limited amounts of data are commonlyavailable for site characterization. However, theirmainadvantages are that they are distributed widely over theworld and their potential storage capacity is large (Koideet al., 1992; Hendriks and Blok, 1993; Rigg et al., 2001;IPCC, 2005).

The main geological constraints for finding theright place to store CO2 include a porous and permeablereservoir rock overlain by an impermeable cap rock. Be-cause the stored CO2 is less dense than the formationwater, it will naturally rise to the top of the reservoir,and a trap is needed to ensure that it does not reach thesurface. The CO2 can be trapped by several differentmechanisms (such as structural or stratigraphic, hydro-dynamic, residual gas, solubility, andmineral trapping),with the exact mechanism depending on the specificgeological conditions. Structural or stratigraphic trap-ping relates to the free-phase (immiscible) CO2 that isnot dissolved into formation water. When supercriticalCO2 rises upward by buoyancy, it can be physicallytrapped in a structural or stratigraphic trap in exactlythe same manner as a hydrocarbon accumulation. Thenature of a structural or stratigraphic trap depends onthe geometric arrangement of the reservoir and sealunits. The CO2 can be hydrodynamically trapped in

FIGURE 4. Options for thegeological storage of CO2

(image courtesy of Coopera-tive Research Centre forGreenhouse Gas Technolo-gies [CO2CRC]).

Geological Input to Selection and Evaluation of CO2 Geosequestration Sites 9

horizontal or dipping reservoirs with no defined struc-tural closures when the dissolved and immiscible CO2

travels with the formationwater for very long residence(migration) times of the order of thousands to millionsof years (Bachu et al., 1994). Residual gas trapping oc-curs when the saturation of CO2 falls below a certainlevel and it becomes trapped in the pore spaces by cap-illary pressure forces and ceases to flow (Ennis-King andPaterson, 2001; Holtz, 2002; Flett et al., 2005). Solubilitytrapping relates to the CO2 dissolved into the forma-tion water (Koide et al., 1992). The time scale for com-plete dissolution is critically dependent on the verticalpermeability and the geometry of the top seal but ispredicted to occur on a scale of hundreds to thousandsof years (Ennis-King and Paterson, 2002). Mineral trap-ping results from the precipitation of new carbonateminerals following the interaction of the CO2 with thein-situ formationwater and theminerals of the host rock(Gunter et al., 1993). This storagemechanism is themostpermanent of the trapping types discussed because itrenders the CO2 immobile (Bachu et al., 1994).

In any geological storage site, the injected CO2 willultimately be trapped by several of the mechanisms de-scribed above. The type of trapping that occurs, andwhen, is dependent on the dynamic flow behavior ofthe CO2 and the time scale involved. With increasingtime, the dominant storage mechanism will changeand typically the storage security also increases. Figure 5shows how the initial storage mechanism will domi-nantly be physical structural and stratigraphic trappingof the immiscible-phase CO2.With increasing time andmigration, more CO2 is trapped residually in the porespace or is dissolved in the formation water, increasingthe storage security. Finally, mineral trapping may oc-cur after the geochemical reaction of the dissolved CO2

with the host rock mineralogy, permanently trappingthe CO2.

SITE CHARACTERIZATION

The subsurface behavior of CO2 is influenced bymany variables, including reservoir and seal structuralgeometry, stratigraphic architecture, reservoir hetero-geneity, relative permeability, faults and fractures, pres-sure and temperature conditions, mineralogical compo-sition of the rock framework, and hydrodynamics andchemistry of the in-situ formation fluids (Root, 2007).The nature of geological variability means that each po-tential storage site needs to be assessed individually; how-ever, a similar workflow can be applied to all site evalu-ations. The geological complexity of any potential CO2

storagesite isbest addressedbyamultidisciplinary researcheffort, which can provide an integrated and comprehen-sive site evaluation for the geological storage of CO2

(Gibson-Poole et al., 2005; Gibson-Poole, 2009).

Different levels of site characterization can be under-takendependingon thematurity of theproject (Figure 6).Initially, a regional characterization process is neededto establish the potential of an area for CO2 geologicalstorage before an actually site location is selected. Sedi-mentary basins across a state or country can be screenedand ranked as to their overall suitability for CO2 stor-age, using criteria suggested by Bachu (2003), such astectonic setting, basin size and depth, intensity of fault-ing, hydrodynamic and geothermal regimes, existingresources, and industrymaturity. Once a basin has beenidentified as suitable, a regional assessment can be un-dertaken to locate possible storage sites (Bradshawand Rigg, 2001; Rigg et al., 2001; Bradshaw et al., 2002)(Figure 6). The stratigraphy is reviewed to identify suit-able rock combinations that may provide reservoir andseal pairs, and data gathered to assess five key factors:storage capacity (will it meet the volume requirementsof currently identified CO2 sources, e.g., pore volume,area, and temperature or pressure?); injectivity potential(are the reservoir conditions viable for injection, e.g.,permeability, porosity, and thickness?); site details (isthe site economically and technically viable, e.g., on-shore or offshore, distance from source, and depth to topreservoir?); containment (will the seal and trap workfor CO2, e.g., seal capacity and thickness, trap type, andfaults?); and existing natural resources (are there viable

FIGURE 5. Schematic representation of the change ofdominant trapping mechanisms and increasing CO2

storage security with time.

10 Kaldi et al.

natural resources at the site that may be compromised,e.g., proven petroleum system, groundwater, coal, orother natural resource?) (Bradshaw and Rigg, 2001; Rigget al., 2001; Bradshaw et al., 2002). These five factorsprovide a useful ranking scheme for describing the keyelements of any potential CO2 geological storage siteand can be used to compare and contrast the relativemerits of one potential site over another site.

Once apreferred site has been selected, it canproceedto a detailed site evaluation (Gibson-Poole et al., 2005;Gibson-Poole, 2009), the first step of which is the estab-lishment of a structural and stratigraphic framework(Figure 6). A sequence-stratigraphic approach is adoptedbecause it focuses on key surfaces that allow litho-facies distributions to be predicted. This is vital in un-derstanding the likely distribution and connectivity ofreservoirs and seals. Of the five key factors discussedabove, the ones that require detailed geological assess-

ment are injectivity, containment, and capacity. Injec-tivity issues include the geometry and connectivity ofindividual flow units, the nature of the heterogeneitywithin those units (i.e., the likely distribution and im-pact of baffles), and the physical quality of the reservoirin terms of porosity and permeability characteristics.Containment issues include the distribution and con-tinuity of the seal, the seal capacity (maximum CO2

column height retention), CO2-water-rock interactions(potential for mineral trapping), potential migrationpathways (structural trends, distribution and extent ofintraformational seals, and formation water flow direc-tion and rate), and the integrity of the reservoir and seal(fault and fracture stability and maximum sustainablepore fluid pressures). Potential CO2 storage capacity canbeassessedgeologically in termsof availablepore volume;however, the efficiency of that storage capacity will bedependent on the rate of CO2 migration, the dip of the

FIGURE 6. Site characteriza-tion methodology for thegeological storage of CO2

(from Gibson-Poole, 2009).

Geological Input to Selection and Evaluation of CO2 Geosequestration Sites 11

reservoir, the heterogeneity of the reservoir and the po-tential for fill-to-spill structural closures encounteredalong the migration path, and the long-term prospectsof residual gas trapping, dissolution into the formationwater, or precipitation into new minerals. The geologi-cally calculated pore volumeprovides the basis for numer-ical flow simulations of CO2 injection and storage, whichwill give a more accurate assessment of howmuch of theavailable pore volume is actually used (sweep efficiency).

The engineering characterization phase continueson from the geoscience characterization (Figure 6). Short-termnumerical simulationmodels of the injection phaseare needed to provide data on the injection strategy re-quired to achieve the desired injection rates (e.g., numberof wells, well design, injection pattern). Postinjection-phase numerical simulations evaluate the long-termstorage behavior, modeling the likely migration, distri-bution, and form of the CO2 in the subsurface. Coupledsimulation models, such as geochemical reactive trans-port, can also be undertaken to further evaluate the CO2

storage potential of a site.The final stage in a detailed site evaluation is the

socioeconomic characterization (Figure 6). This includeseconomicmodeling to establish such aspects as the like-ly capital and operating costs, as well as the cost permetric ton of CO2 avoided. Risk and uncertainty anal-ysis is crucial to establish whether a selected site can beclassed as a safe and effective storage site for thousandsof years. The design of a monitoring and verificationprogram is dependent on the geological characteristicsof the selected site and needs to be carefully evaluatedto produce an optimum program both in terms of effi-ciency and cost.

GEOLOGICAL INPUT TO SITECHARACTERIZATION

The ideal characterization of geological storage sitesfor CO2 requires a thorough integration of all geo-scientific data. Data types change depending on thestage of characterization. Regional assessment requireslow-resolution, long-range data sets, such as two-dimensional (2-D) seismic and stratigraphic drill holes.However, site-specific assessment requires more de-tailed data such as high-density 2-D or 3-D seismic,core, and many wells and logs. These different types ofdata are commonly available from petroleum explora-tion and production.

The regional data sets should be reviewed to eval-uate the structural configuration and regional distribu-tion of lithofacies. This is best done using a large 2-Dseismic data set complemented with available well-logcontrol and petrophysical data. The aim during thisphase is to identify reservoir units with appropriate

storage properties (such as adequate porosity for stor-age of substantial volumes of CO2 and permeability forinjectivity and subsequent dissemination into the poresystem) overlain by suitably extensive and thick seals.

The high-density data sets are needed to evaluatethe reservoir and seal geometries and architectures ofthe identified storage site inmore detail. The aimof a fulldata set integration and interpretation is to assess themigrationpathwayof injectedCO2 aswell as to assess thepotential storage volume. This is best done by building adetailed static 3-D reservoir model, which can be up-scaled for dynamic fluid flow simulations. Various itera-tions of the dynamic simulations should be incorporatedbyan integrated teamtobetter understand thegeologicaleffects on CO2 injection, migration, and storage.

Data challenges are frequently encountered whentrying to assess geological storage sites for CO2 becauseeither the basinunder assessment hasnot been exploredby the petroleum industry or, more typically, the siteunder investigation lies just off-structure and thus com-monly outside the major structurally controlled hydro-carbon fieldswith their densedata sets. In addition, datachallenges are also common because of incomplete datasets, data loss, or simple data deterioration with time.Two types of solutions can be considered to overcomethe data challenges. The best butmost costly solution isdata acquisition. Paying several millions of dollars fordrilling a well is common, whereas the acquisition andprocessing of seismic data are equally expensive. A farmore cost-effective but also less accuratemethod of over-coming data challenges is to use outcrop and subsurfaceanalog data sets to model the subsurface geology at thestorage site. Analog data sets are useful in that they pro-vide generic quantitative data of a range of parametersparamount to a specific geological setting. For example,analogs can be used to predict sand body and shale geo-metries, connectivities, and heterogeneities. They canalso be used for providing ranges and distributions ofporosities and permeabilities and for providing esti-mates on likely seal capacities. Analog data sets to char-acterize geological storage sites forCO2 are currently themost affordable and accessible data sets for reservoircharacterization.

MONITORING

In addition to the careful selectionof the subsurfaceformation, a comprehensive monitoring system needsto be put in place to verify that the CO2 remains in thesubsurface. Monitoring of the activities of stored CO2

includes an extensive range of established direct andremote sensing technologies, including petrophysical,geophysical, and geochemicalmethodologies deployedon the surface and in the borehole. These are used forrepeated assessments from a reservoir, containment,

12 Kaldi et al.

wellbore integrity, near-surface, and atmospheric per-spective (Dodds et al., 2006). Wellbore properties suchas pressure, temperature, resistivity, and sonic responsescan be recorded in injection and observationwells. Geo-physical monitoring involves quantification of 3-D andseismic time-lapse imaging of the plume and its migra-tion. This is done using an array of methodologies, in-cluding vertical seismic profile (VSP),microseismic data,electromagnetic imaging (EM), and gravity to track themovement ofCO2 in the subsurface (Dodds et al., 2006).This process involves calibration with laboratory deter-mination of in-situ geophysical properties associatedwith CO2 and developing predictive forward modelingof the behavior of CO2. Detailing results of such mod-eling andpossible acquisition effects on seismic imagingare provided by Arts et al. (2009), who describe the injec-tion of CO2 in the Utsira Sand at Sleipner (ongoing since1996 with almost 10 million t of CO2 injected to date).

Nonseismic techniques, such as electrical proper-ties, the monitoring of injection processes with changesin stress state, and detecting potential fracture processesthroughpassive seismicmeasurements,mayalsobeaddedto themonitoring array. Including geochemical and hy-drodynamic sampling to ensure that the injected CO2

has not leaked from its container and hence verify theintegrity of seals is also important. Adding tracers to theinjected CO2, combined with sampling at surface local-ities, allows rapid detection of any seepage or leakage inthe unlikely circumstance that this should occur. Near-surface and surface (soil, water well, and atmospheric)monitoring devices, including tracer and isotope anal-ysis, can be deployed to determine the flux and com-position of CO2 and to distinguish anthropogenic andnatural sources of CO2 from injected CO2.

RISKS

Risks surrounding CO2 geosequestration are sum-marized by Bowden and Rigg (2004). Essentially, themain concerns relate to the potential for unanticipatedCO2 leakage either caused by unanticipated CO2move-ment up the wellbore or along an unexpected geolog-ical migration path or caused by induced or natural seis-micity. The risks associated with CO2 storage, althoughconsidered very low, are characterized by a greater de-gree of uncertainty than those connectedwithCO2 trans-port and injection. This is because of the fact that oncethe CO2 enters the geological reservoir, its fate is trans-ferred from mostly human control to a natural system.Although most of the existing knowledge of CO2 be-havior in the subsurface exists from the long history ofCO2 floods associatedwith EOR, the risks associatedwithlarge-scale storage are at a different scale. The quantitiesof CO2 stored for EOR floods are smaller, and the CO2

residence times are shorter than required for large-scale

carbon geosequestration. For geosequestration of CO2,the risk of leakage depends onnot only the likelihoodofexistence of potential leakage pathways (such as wells,faults, permeable zones in the seal, etc.), but also the like-lihood that these potential pathways will intersect CO2

while it is in a mobile phase and finally the likelihoodthat the potential leakage pathway will leak (Rigg et al.,2006). Asmany containment risk assessments are bench-marked against an impact of 1% leakage over 1000 yr(IPCC, 2005), the frequency, duration, and volume of po-tential leakage events need to be assessed for this timeframe (Rigg et al., 2006).

Any geological CO2 storage project resulting in acatastrophic release of CO2 is highly unlikely. This is be-cause the pressure of CO2 injected into a geological res-ervoir reduces as it moves away from the injection welland is diffused over large areas of the formation, thusavoiding large pressure buildups (Streit andHillis, 2002).No record of a catastrophic CO2 release from a naturalCO2 deposit to date is observed, and any potential re-lease from a CO2 storage project should be preventablethrough careful site selection, operation, and monitor-ing. The critical geological input tominimize the risk ofsuch occurrence is seal analysis and geomechanics. TheCO2 columnheight calculations canbe used to assess thesafe storage volumes of CO2 at any storage site (Danieland Kaldi, 2009), whereas geomechanical analysis ofthe faults can be used to assess the maximum sustain-able pore fluid pressures at potential storage sites (Streitand Hillis, 2004). Understanding the rock fluid inter-action potential betweenCO2 and contactedminerals isalso important. The CO2 creates carbonic acid when itmixes with H2O, and the risk of whether this could po-tentially result in the chemical erosion of some seal li-thologies leading to leakage needs to be addressed. Inmost instances, because of their low permeability andcapillary properties, CO2 is unlikely to enter seals. There-fore, any potential reactions are likely to be limited tothe base of the seal. In addition, because the pH buf-fering capabilities of the seal lithology are generallygreater than the dissolution capabilities of carbonic acid,reactions are likely to be mineral precipitation insteadof dissolution, thus leading to seal capacity enhance-ment instead of degradation (Watson et al., 2005).

Induced seismicity isnot expected tobe a significantproblem at geological CO2 storage sites. Induced seismic-ity has been documented during hydrocarbon produc-tion, EOR, AGI, natural gas storage, and waste injectionoperations (Wallroth et al., 1996; Maxwell et al., 1998;Jupe et al., 2000). These induced seismic events havebeen caused by poor engineering practices such as theinjection of the CO2 at too high a pressure, which inturn can result in microfracturing of the reservoir rockand/or small movement along existing fracture lines.Note, however, that most of the recorded events havebeenof a very smallmagnitude andhave causednoharm.

Geological Input to Selection and Evaluation of CO2 Geosequestration Sites 13

Moreover, the risk of induced seismicity can be reducedthrough careful siting and placement of injectionwells,adherence to proper pressure guidelines, and a soundunderstanding of the geomechanical properties of thestorage reservoir. A range of technologies can be identi-fied by a rigorous process of risk assessment and confor-mance to clearly identify performance criteria, whichcanbe subsequently verified. These criteria are agreed inconjunction with the regulatory authorities to managethe project through all phases, addressing responsibil-ities and liabilities and providing assurance of safe stor-age to the satisfaction of the public at large.

CONCLUSIONS

The selection of potential CO2 geosequestrationsites uses techniques developed dominantly in the pe-troleum industry for the exploration and developmentof hydrocarbons. The geological input into the site se-lection, characterization, risking, andmonitoring is para-mount. Detailed site evaluation requires an accurateassessment of structural and stratigraphic configurationof trap, faults, reservoir, and seal lithologies. Sedimen-tological and sequence-stratigraphic principles are usedto determine facies distributions on a basin scale and tocharacterize the reservoir and seal at the reservoir level.A detailed high-resolution sequence-stratigraphic andsedimentological analysis integrating all available datais optimal. However, data coverage at many potentialsites is sparse, and a detailed characterization of the sub-surface needs to rely heavily on modern and outcropanalogs. Numerical simulation, using realistic geologi-cal input parameters, is used tomodel the flow behaviorof CO2 in the subsurface. Monitoring of the stored CO2

includes petrophysical, seismic, and geochemicalmeth-odologies.Wellbore properties such as pressure, temper-ature, resistivity, and sonic responses need to be recordedin injection and observation wells. Seismic monitoringusing an array of methodologies allows the tracking ofmovement of CO2 in the subsurface. Geochemical sam-pling at both surface and subsurface will allow rapiddetection of any seepage or leakage, should this occur.

Geosequestration could be a significant factor inany portfolio of options for CO2 emissions reduction.By reducing CO2 emissions while still allowing for thecontinued use of fossil fuels, carbon geosequestrationallows time for the transition to renewable energy sourcesfrom fossil fuels. Effective geosequestration of CO2 in-volves capture of CO2 at stationary source locations,transportation of CO2 from the source to the geologicalstorage site, injection of CO2 into subsurface reservoirs,storage of CO2 in the subsurface, and effective moni-toring and verification of CO2 storage. Given the largenumber of known geological formations suitable forgeosequestration, the opportunity exists for significant

volumes of CO2 storage around the world. Much of thetechnologyneeded for carbon geosequestrationprojectsis ready today. The key elements in its deployment aregoing to be economics, politics, and public perception.

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