IEA GHG Weyburn CO 2 monitoring and storage project

Transcription

1 Fuel Processing Technology 86 (2005) IEA GHG Weyburn CO 2 monitoring and storage project C. Preston a, M. Monea b, W. Jazrawi b, K. BrownT,c, S. Whittaker d, D. White e, D. Law f, R. Chalaturnyk g, B. Rostron h a Natural Resources Canada, 1 Oil Patch Drive, Devon, Alberta, Canada T9G 1A8 b Petroleum Technology Research Centre, 6 Research Drive, Regina, Saskatchewan, Canada S4S 7J7 c Geological Storage Consulting, Inc., 17 Royal Oak Crescent, Calgary, Alberta, Canada T3G 4X8 d Saskatchewan Industry and Resources, 201 Dewdney Avenue, Regina, Saskatchewan, Canada S4N 4G3 e Natural Resources Canada, 615 Booth Street, Ottawa, Ontario, Canada K1A 0E9 f Alberta Research Council, 250 Karl Clark Road, Edmonton, Alberta, Canada T6N 1E4 g University of Alberta, Civil and Environmental Engineering, 220 Civil Building, Edmonton, Alberta, Canada T6G 2G7 h University of Alberta, Earth and Atmospheric Sciences, 3-19D Earth Science Building, Edmonton, Alberta, Canada T6G 2E3 Abstract This paper presents an integrated overview of the results from over 50 individual technical research projects conducted under the auspices of the International Energy Agency Greenhouse Gas R&D Programme [1] [International Energy Agency Greenhouse Gas R&D Programme, The overall project, called the IEA GHG Weyburn CO 2 Monitoring and Storage Project [2] [IEA GHG Weyburn CO 2 Monitoring and Storage Project, was created to predict and verify the ability of an oil reservoir to securely and economically store CO 2. Research activities in the project were divided into four bthemesq that applied leading-edge science and engineering in geophysics, geomechanics, geochemistry, geology, reservoir engineering, risk assessment, and economics. D 2005 Elsevier B.V. All rights reserved. Keywords: IEA GHG (International Energy Agency Greenhouse Gas R&D Programme); PTRC (Petroleum Technology Research Centre); Weyburn; CO 2 (carbon dioxide); Storage; Monitoring T Corresponding author. Tel.: address: ken.brown@browngsc.com (K. Brown) /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.fuproc

2 1548 C. Preston et al. / Fuel Processing Technology 86 (2005) Introduction Geologic storage of carbon dioxide (CO 2 ) has been proposed as a viable means for reducing anthropogenic CO 2 emissions [3]. There is significant and active worldwide interest in geological storage projects from a wide range of stakeholders industry, regulators, reservoir owners, environmental organizations, public interest groups, and the general public. Important issues concerning geological storage must be addressed before stakeholders, including financial markets, accepted as a solution for reducing CO 2 emissions. These issues include:! Demonstration of the safety and long-term security of geological CO 2 storage.! The general effect of economic factors, including incentives and taxes.! What factors should be considered for permitting, operation, and abandonment of storage sites.! Determining long-term monitoring capabilities and requirements to manage long-term liability for industry and the public sector. To develop confidence in the geological storage of CO 2 as a safe and environmentally acceptable mitigation option, it is necessary to provide sound scientific information that CO 2 injected into reservoirs can be stored for geological timescales. Study of actual CO 2 storage projects is an ideal source of the required technological information. In July 2000, the IEA GHG Weyburn CO 2 Monitoring and Storage Project (the project) used to study geological storage and sequestration of CO 2 was launched by the Petroleum Technology Research Centre (PTRC) [4] located in Regina, Saskatchewan, in close collaboration with EnCana of Calgary, Alberta [5], which is the operator of the CO 2 enhanced oil recovery (EOR) project in the Weyburn Field. EnCana began storage operations in late September 2000 following baseline data collection surveys by the project. The baseline data set makes this geological monitoring and storage project truly unique [6]. The Weyburn Oilfield is one of the most studied fields in the world due to its horizontal drilling technology projects and the major world-class EOR project [8]. The Weyburn field is an exceptional natural laboratory for the study of CO 2 storage, based on the extensive historical field and well data that are publicly available [9], the abundant core material, and year-round accessibility to the site. Located in the southeast corner of the province of Saskatchewan in Western Canada, the Weyburn Unit is a 180-km 2 (70 square miles) oil field that is part of the large Williston sedimentary basin which straddles Canada and the United States (see Fig. 1). Production is API medium gravity sour crude from the Midale beds of the Mississippian Charles formation. Water flooding was initiated in 1964 and significant field development, including the use of horizontal wells, was begun in In September 2000, EnCana initiated the first phase of a CO 2 -enhanced oil recovery scheme in 18 highly modified inverted nine-spot patterns. The flood is expected to be rolled out in phases until the year 2015 for a total of 75 patterns.

3 C. Preston et al. / Fuel Processing Technology 86 (2005) Weyburn Field HUDSON BAY ALBERTA MANITOBA EDMONTON SASKATCHEWAN PRINCE ALBERT SASKATOON CANADA CALGARY REGINA WEYBURN BRANDON WINNIPEG U.S.A. MONTANA NORTH DAKOTA HELENA BISMARCK Williston Sedimentary Basin WYOMING PIERRE SOUTH DAKOTA Weyburn Unit: Field Size: 70 sq. miles OOIP: 1.4 billion bbls Oil Recovered: 366 million bbls CO2 IR: 130 million bbls Fig. 1. Location of the Weyburn Field. The CO 2 is 95% pure and the initial injection rate is 5000 tonnes/day (equivalent to 95 mmscf/day) [8]. A total of approximately 20 million tonnes of CO 2 is expected to be stored in the reservoir over the EOR project life. The net storage will be approximately 14 million tonnes after deducting the atmospheric emissions created by compressing the CO 2 for shipment and the extended operational life of the Weyburn Oilfield [10]. The CO 2 is a purchased byproduct from the Dakota Gasification synthetic fuel plant in Beulah, North Dakota, and is transported through a 320-km pipeline to Weyburn (see Fig. 2). An operations update for the Weyburn Unit EOR project is given in Figs. 3 and 4. The IEA GHG Weyburn Project was funded by 15 sponsors from governments and industry, among them the Natural Resources Canada, United States Department of Energy, Alberta Energy Research Institute, Saskatchewan Industry and Resources, the European Community, and 10 industrial sponsors in Canada, United States, and Japan. 2. Research objectives The overall project objective was to predict and verify the ability of an oil reservoir to securely store and economically contain CO 2. This was done through a comprehensive analysis of the various process factors as well as monitoring/modeling methods intended to address the migration and fate of CO 2 in a specific EOR environment.

4 1550 C. Preston et al. / Fuel Processing Technology 86 (2005) Dakota Gasification Company 250 mmscfd CO 2 by-product of coal (lignite) gasification 95 mmscfd (5000 tonnes/day) contracted and injected at Weyburn CO2 purity 95% (H 2 S less than 2%) EnCana currently injects 120 mmscfd (i.e. 21% recycle) Fig. 2. The source of the CO 2. The scope of work focused on understanding mechanisms of CO 2 distribution and containment within the reservoir into which the CO 2 is injected and the degree to which CO 2 can be permanently sequestered. The technology, design, and operating know-how thus obtained can then be applied in screening and selecting other CO 2 storage sites and in designing and implementing successful CO 2 storage projects worldwide. Weyburn Unit Oil Production BOPD (Gross) 50,000 Original Verticals Infill Verticals Hz Infill CO2 45,000 40,000 35,000 Actual Forecast 30,000 25,000 20,000 15,000 10,000 5,000 0 Jan-55 Jan-58 Jan-61 Jan-64 Jan-67 Jan-70 Jan-73 Jan-76 Jan-79 Jan-82 Jan-85 Jan-88 Jan-91 Jan-94 Jan-97 Jan-00 Jan-03 Jan-06 Jan-09 Jan-12 Jan-15 Jan-18 Jan-21 Jan-24 Jan-27 Date 75 patterns to be added 32 patterns active, 10 additional in 2003 Peak rate 30,000 bopd Incremental recovery 130 MMbbls Fig. 3. Weyburn EOR project forecast.

5 C. Preston et al. / Fuel Processing Technology 86 (2005) Weyburn CO 2 Project Initial EOR Area Actual Production 10,000 9,000 8,000 Actual Base Waterflood Total Production (BOPD) 7,000 6,000 5,000 4,000 3,000 2,000 1,000 CO2 Injection Begins Sept, bopd Incremental 0 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Fig. 4. Weyburn initial EOR area Actual production. A secondary objective was the application of economic realities to such an undertaking by predicting the point at which a CO 2 storage project reaches its economic limit. The application of customized economic models to the various storage cases helped in assessing not only cases of CO 2 storage in conjunction with EOR operations but also of CO 2 storage in non-eor situations such as saline aquifers, which have a significantly larger CO 2 storage potential compared to depleting oil pools [7]. The ultimate deliverable from the IEA GHG Weyburn Project was a credible assessment of the permanent containment of injected CO 2 through formal risk analysis techniques including long-term predictive reservoir simulations not only in the Williston Basin but also at other sedimentary basins where CO 2 storage may be contemplated. 3. Results and discussion The IEA GHG Weyburn Project was completed in June Results show strong support for both the feasibility and safety of geological CO 2 storage [11]. Clearly, CO 2 storage can safely take place without impacting EOR operations [12]. In fact, economic studies demonstrated that implementation of incentives used to motivate additional CO 2 storage, beyond that associated with EOR, could also ultimately result in additional oil recovery [13]. A Phase 2 of the IEA GHG Weyburn Project began in mid The following has been organized into four main bthemes,q which were chosen to group over 50 research subtasks in a manner corresponding to the main objectives of the project.

6 1552 C. Preston et al. / Fuel Processing Technology 86 (2005) Geological characterization of the geosphere and biosphere Purpose The principal aim of geological characterization was to assess the integrity of the geological bcontainerq encompassing the Weyburn Unit for effective long-term storage of CO 2 [11]. Data obtained during this assessment were used to develop a three-dimensional system model that includes features and properties of an area extending 10 km beyond the CO 2 flood extent to provide the geological framework for the risk assessment of the longterm fate of CO 2 injected into the subsurface at Weyburn (see Fig. 5) Technical approach The Weyburn Oil Pool is a giant oilfield containing about 1.4 billion barrels of oil in place in limestones and dolostones (Midale Beds) of Mississippian age. Carbonates of the Midale reservoir occur at about 1.5 km depth in the northeastern portion of the Williston Basin, a sedimentary basin broadly similar to the Illinois and Michigan basins of North America and numerous intracratonic basins that occur elsewhere around the world. Characterization of the Weyburn geological system for CO 2 storage targeted the delineation of primary and secondary trapping mechanisms and the identification of any potential pathways of preferential CO 2 migration [11]. To place these components within a regional or basinal context, the geological framework was constructed for a region Saskatchewan Alberta System Model (10 km beyond EOR) Manitoba Montana North Dakota Wyoming Williston Basin Regional Study (200 x 200 km) South Dakota Fig. 5. Geoscience framework.

7 C. Preston et al. / Fuel Processing Technology 86 (2005) extending km around the Weyburn Field that includes portions of Saskatchewan, North Dakota, and Montana [17]. Large-scale studies such as this more effectively reveal basin hydrogeological flow characteristics and the underlying tectonic framework that can greatly influence depositional patterns of sedimentary packages and fracture development. Increased detail was focused within an area extending 10 km beyond the limits of the CO 2 flood that forms the basis for the system model used in risk assessment. The development of a comprehensive geological model for use in risk assessment required a focused and highly integrated multidisciplinary approach. Lithostratigraphic mapping identified over 140 individual surfaces from the Precambrian basement to ground surface. The lithostratigraphic units were used to define larger flow packages, or hydrostratigraphic units, that were mapped and characterized using extensive data analysis to provide fundamental information on fluid behavior within the basin as required by performance assessment [14]. Much of the 2000 km of 2D seismic data processed to refine the characterization of subsurface features and basement tectonics was integrated with high-resolution aeromagnetic data to augment fracture and regional fault delineation [15]. Detailed geological studies performed on primary seals (those in contact with the reservoir) and secondary seals (barriers to flow higher in the stratigraphic column) included core descriptions, petrography, isotope geochemistry, and fluid inclusion studies [18]. Shallow hydrogeological surveys defined the distribution and continuity of potable aquifers in near-surface sediments of the study region. Remotely sensed imagery analysis was used to determine whether structural elements observed in the deep subsurface are related to linear surface features identified through air photo and satellite imagery. Soil gas surveys, designed to transect some of the linear surface features, were performed regularly around the Weyburn Unit to monitor for changes in CO 2 fluxes in soils that may be due to potential anthropogenic CO 2 migration. Other specialized studies undertaken included obtaining cores from selected strata above the reservoir for petrophysical measurements, till sampling for soil gas characterization, shallow aquifer demarcation, and natural analog comparisons. Integration of these diverse data provided a coherent and representative geological model that can be tailored for use in risk assessment Results and conclusions A good geological description of the reservoir and a large surrounding region was developed from both existing and newly generated geological, geophysical, and hydrogeological information. A robust system model of the geosphere and the biosphere was constructed to serve as the platform for the long-term risk assessments of the Weyburn CO 2 storage site [16]. The main conclusion of the work was that the geological setting at the Weyburn field appears to be highly suitable for long-term geological storage of CO 2 [11]. One of the most important results from this work was the development of a tremendous geoscience dataset pertinent to understanding the geological storage of CO 2 in the Williston Basin and other sedimentary basins. A great deal of information was accumulated within a relatively short time span so there remains an additional opportunity for more advanced interpretation and integration of this world-class database.

8 1554 C. Preston et al. / Fuel Processing Technology 86 (2005) Prediction, monitoring, and verification of CO 2 movements Purpose An underlying goal of the IEA GHG Weyburn Project was to optimize effective management of the reservoir for enhanced oil recovery and storage of CO 2. To accomplish this, an improved understanding of the reservoir properties and the nature of how the injected CO 2 spreads and interacts with the rock matrix and reservoir fluids was required. The specific objectives of this work were to test and improve conventional geological-based simulator predictions of how the CO 2 flood will progress, and to assess the chemical reactions and mechanisms for long-term storage of CO 2 within the reservoir. Monitoring entailed observing the physical and chemical effects of CO 2 injection on the state of the reservoir system with a focus on tracking the spread of CO 2 within and potentially outside the reservoir. Verification was defined as the substantiation of the interpreted monitoring results to allow reliable estimation of the volume and distribution of CO 2 in the subsurface Technical approach Initial predictions of how the CO 2 flood would progress were based on flow simulations using an existing reservoir model that was constructed with the well-bore geology from the dense network of wells in the Weyburn field (see Fig. 6) [13]. A variety of seismic and geochemical sampling methods were subsequently used to monitor the CO 2 injection process and characterize the reservoir between boreholes. Seismic imaging of the CO 2 in the subsurface was accomplished primarily by time-lapse 3D multi-component surface seismic reflection imaging complemented by time-lapse and static borehole (VSP and crosswell) seismic surveys and passive seismic monitoring (see Table 1 for a detailed list). Rock/fluid property measurements, combined with reservoir simulation and production history matching including seismic constraints, were used to calibrate the seismic observations to known CO 2 injection volumes and to update the reservoir simulation model [15]. The geochemistry of produced oil, gas, and brine was regularly monitored and analyzed for a broad range of chemical and isotopic parameters to infer injection-related chemical processes within the reservoir and to track the path of injected CO 2. This analytical work was supported by model calculations and laboratory studies on geochemical reactions. Soil gas sampling was designed to detect injected CO 2 that may have escaped from the reservoir and migrated to the surface [14] Results and conclusions Seismic surveys were highly successful and were used in bground-truthingq reservoir modeling. The seismic surveys clearly demonstrated an ability to detect anomalies in the reservoir induced by CO 2 invasion (see Fig. 7) [15]. Geochemical fluid sampling gave good insights into the movement of CO 2 within the reservoir and gave strong indication of incipient CO 2 breakthrough at wells (see Fig. 8) [14]. Tracer surveys were not as successful due to a variety of technical and operational problems. Geochemical modeling to determine the long-term CO 2 material capture in various sequestration forms (trapping mechanisms) was reasonably concluded. However, further efforts in reactive transport

9 Fig. 6. Fence diagram of predicted CO 2 saturation distribution after 26 months of CO 2 injection using the geological-based reservoir simulation model. C. Preston et al. / Fuel Processing Technology 86 (2005)

10 1556 C. Preston et al. / Fuel Processing Technology 86 (2005) Table 1 Data acquisition schedule Schedule of geochemical and seismic monitoring activities within the Phase 1A CO2 injection area. 3D multicomponent surface seismic surveys (CSM=Colorado School of Mines; EnCana) were supplemented by borehole surveys (3D-VSP and X-well). BL=baseline (pre-injection) survey; M1=Monitor 1 Survey; M2=Monitor 2 Survey; X-Well=crosswell; VSP=vertical seismic profile; VX=vertical crosswell seismic survey; HX=horizontal crosswell seismic survey; CO 2=injected volume of CO2; fluid/gas=production fluid sampling. modeling to complete the geochemical picture will be made in Phase 2. There was no evidence from either the time-lapse seismic or the soil gas sampling to indicate migration of measurable amounts of CO 2 into the overburden or seepage to the surface [14,15] CO 2 storage capacity and distribution predictions and the application of economic limits Purpose There were several objectives within this theme: to estimate the maximum CO 2 storage capacity achievable both physically and economically at a geological storage site, to predict the CO 2 distribution and trapping mechanisms within the storage site, and to determine if the CO 2 storage performance can be improved through the application of conformance control treatments Technical approach A multi-phase, multi-component compositional reservoir simulation model was used to predict the CO 2 storage capacity in the Weyburn Unit reservoir [13]. The approach taken in modeling the size and complexity of 75 EOR patterns was to start with fine-grid single-pattern simulations and end with a coarse-grid 75-pattern simulation. The process involved three levels of upscaling: (1) from a detailed geological model of the Weyburn reservoir to a fine-grid reservoir simulation model; (2) from three fine-grid single-pattern models to coarse-grid models of the same

11 C. Preston et al. / Fuel Processing Technology 86 (2005) D-3C Time-Lapse Seismic Surveys vs. Baseline survey (Sept. 2000) Marly Zone Courtesy: EnCana Corporation Fig. 7. Time-lapse seismic surveys vs. baseline survey (September 2000). patterns; and (3) from three coarse-grid single-pattern models to a 75-pattern model using the same grid resolution. Laboratory measurements of oil properties and CO 2 oil phase equilibrium behaviour using oil samples collected periodically from different wells provided information to tune the equation-of-state parameters in the PVT model used in the reservoir simulation. The reservoir simulation model was validated by both laboratory-scale and field-scale simulations. In the laboratory-scale simulation, CO 2 coreflood experiments conducted with different oil samples were history-matched, while in the fieldscale simulation, field production histories in three different patterns with different CO 2 injection strategies (i.e., bsimultaneous but separate water and gas injectionq (SSWG), bvuggy water-alternating gasq (VWAG), and bmarly, Vuggy water-alternating gasq (MVWAG)) were history-matched. Then, the reservoir simulation model was used to predict the CO 2 storage performance during the EOR period, first in the three single patterns and then in the entire 75 EOR patterns. EnCana s operating strategies were followed as closely as possible. This was labeled the base case. Alternative CO 2 storage cases after EOR were also investigated with a focus on promoting additional CO 2 storage. Using the predicted CO 2 distribution in the reservoir at the end of EOR, a geochemical model was used to provide a preliminary assessment of the amount of CO 2 that will be stored in the reservoir through different trapping mechanisms (solubility, ionic, and mineralogical trappings) [14]. The geochemical modeling also used formation and

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13 C. Preston et al. / Fuel Processing Technology 86 (2005) injection fluid compositions, detailed mineralogical assessment of each of the major flow units in the reservoir, and evaluation of mineral kinetic data. The performance of both CO 2 storage and EOR depends on achieving maximum sweep efficiencies (conformance). These sweep efficiencies can be improved through conformance control techniques [19]. The Weyburn reservoir pay zone is a fractured carbonate with large permeability contrasts, which allows the injected CO 2 to finger and bypass a significant fraction of the recoverable oil. Laboratory evaluation of commercially available technologies for conformance control such as CO 2 foam, gel, and gel foam processes was conducted to select the most suitable options for the Weyburn reservoir. Well production histories provided by EnCana were analyzed to select candidate wells with high production gas-to-oil ratios for future conformance control field trials. The analysis included reservoir simulation modeling using existing fine-grid single-pattern simulations to design the field trial and predict the field trial performance. With the prediction of CO 2 storage capacities and EOR performance, an economic model was used to apply economic constraints to the CO 2 storage cases [13]. This storage economic model has the capability to calculate CO 2 capture, transportation, and storage costs, in addition to the conventional economic evaluation of an EOR process. The model can be run either for stand-alone CO 2 storage options (e.g., depleted oil or gas reservoirs, saline aquifers, etc.) or storage in conjunction with CO 2 EOR projects. The objective of the storage economics model was to guide geological storage decisions where not only estimates of the maximum amount of CO 2 that can be physically stored can be determined, but also how much of that CO 2 is actually economically stored under different gas credits assumptions Results and conclusions Modeling started with fine-grid, individual well patterns and gradually upscaled to a coarser, 75-pattern grid. Good history match was achieved with actual production data (see Fig. 9). Also, predictions of total CO 2 injected matched reasonably well EnCana s internal estimates [21]. Other CO 2 storage cases were also investigated including continuing with CO 2 injection past the termination of the commercial EOR project (approximately year 2033), while continuing to produce incremental oil from wells still operating under a certain gas-to-oil ratio limit and disposing of produced water elsewhere to make room for additional CO 2 injected [13]. Conformance control treatments developed in this project predicted a substantial improvement in volumetric sweep efficiency from the application of specially formulated gel treatments to the best candidate wells. If successfully applied, conformance control may contribute another 10% additional recovery of the total EOR oil from the wells treated. This in turn could accommodate another 1.8 million tons of additional CO 2 Fig. 8. Geochemical fluids in the reservoir. Contour maps of total alkalinity, [Ca], and d 13 C HCO3 across the initial injection area (outlined in blue). Pre-injection (baseline) contoured measurements are shown on the far left while post-co 2 injection values (after 10, 21, and 31 months of continuous injection) are contoured to the right. Dots represent well locations sampled during each trip. CO 2 dissolution is evident by 10 months, and carbonate dissolution is evident by 21 months, as can be seen in the d 13 C HCO3 values (becoming more negative and more positive, respectively).

14 1560 C. Preston et al. / Fuel Processing Technology 86 (2005) Fig. 9. Oil production rate history match (figure shows one of the initial patterns). stored, assuming a case where 20% of the EOR patterns received a gel treatment [19]. Detailed mineralogy of the Weyburn reservoir was determined from microscopic examination, X-ray diffraction (XRD) results, and LPNORM analysis of approximately 100 samples that established the presence and abundances of minerals for each of EnCana s reservoir flow units. Results show that even in a carbonate reservoir such as Weyburn, silicate minerals are present in sufficient quantity to react with CO 2 -charged fluid and enable mineral fixation of CO 2. Using estimates of the porosity and the volume of each of the flow units and the reactions determined through the geochemical modeling, the maximum potential amount of trapping in each flow unit was estimated (see Table 2). After 5000 years, it was determined that a free supercritical CO 2 gas phase will no longer exist, having been effectively trapped [14]. A storage economic model was successfully developed. Alternate economic scenarios were tested (e.g., the above case predicated on continued CO 2 injection in the Weyburn Unit past the economic limit of the EOR operation). The EOR phase allowed 23 MT of CO 2 to be physically and economically stored. The post-eor phase allows for up to an additional 31 MT of CO 2 to be physically stored. However, the portion of the 31 MT that can be economically stored would depend on the amount of the CO 2 credits received and the desired rate of return for the operation. Table 2 Geochemical modeling results Geochemical modeling results summary Maximum potential CO 2 trapping in the Midale reservoir 22.5 million tons of solubility trapping of CO million tons of trapping of CO million tons of mineral trapping of CO 2 Up to 49% of the injected CO 2 can be trapped in bnewq carbonate minerals Summary 45 million tons of CO 2 potential trapping 20 million tons of CO 2 planned injection

15 C. Preston et al. / Fuel Processing Technology 86 (2005) Long-term risk assessments of the storage site Purpose The risk assessment was done to identify and evaluate the risks associated with geological storage of CO 2 within the Weyburn reservoir and assess the reservoir s ability to securely store CO Technical approach Risk assessment embodies the overall process of risk analysis and risk evaluation. Risk analysis involves the systematic use of project information to identify sources of potential CO 2 leakage and to estimate their probability and magnitude. Risk evaluation examines the acceptability of these risks considering the needs, issues, and concerns of stakeholders. Geological storage of CO 2 is a developing technology and, as such, does not have a sufficient knowledge base from which to extract historical data on all leakage risks. Consequently, the risk analyses conducted in the IEA GHG Weyburn Project focused on assessing storage system performance or behavior to increase our understanding of crucial processes. These processes will form a critical component of the final risk assessment in Phase 2. This risk assessment process will ultimately mature into a framework that considers social, economic, and political factors associated with geological storage; evaluates the risks associated with a geological storage reservoir; and assesses the effectiveness of remedial actions that can be taken to minimize both near-term and long-term probabilities and consequences arising from CO 2 leakage. Equally important, this process will provide the basis for communication about the existence, nature, form, magnitude, and acceptability of risks associated with the geological storage of CO 2. As with many engineered or natural systems, the Weyburn bsystem,q which was comprised of the geology of the reservoir and overlying and underlying layers, varying well types, groundwater flow regimes, fluid characteristics, and so on, is very complex. This complexity was managed through application of a rigorous and formal systems analysis approach, firstly to identify and define the system and, secondly, to define base and alternative scenarios for the long-term fate of CO 2 within the system. Scenarios are the plausible and credible ways in which the Weyburn System might evolve over decades to thousands of years. Integration of the performance assessment with the major research themes of the project remains an essential element in its success. Geological characterization research led to a detailed three-dimensional System Model description. Embedded within the System Model is the entire 75 EOR pattern area planned for CO 2 flood rollout and used to predict the CO 2 storage capacity in the Weyburn reservoir. These large-scale simulation results provided the necessary fluid phase and pressure distributions at the end of EOR for the long-term risk assessment out to 5000 years. From an assessment perspective, the two main elements of the System Model are the geosphere and biosphere. The geosphere, which includes the reservoir, incorporates all geological, hydrogeological, and petrophysical information assimilated for the System Model. The biosphere extends to a depth of about 300 m below ground surface and includes soil, surface water, and the atmosphere, and flora and fauna

16 1562 C. Preston et al. / Fuel Processing Technology 86 (2005) found within these areas. To assist in identifying the processes that could be relevant to the evolution or performance of the system, a list of features, events, and processes (FEPs) was developed (see Fig. 10). Features are physical characteristics of the system (e.g., permeability), events are discrete occurrences influencing the system (e.g., earthquakes), and processes identify the physics of change within the system (e.g., diffusion). Fig. 11 provides a small sample of the FEPs list defined for this project. An evaluation of these FEPs, including their interactions, was used to describe how the system will evolve over the timeframe of the risk assessment and form the foundation for the development of a scenario that describes how the system is expected to evolve (called the base scenario) in the far future, and other scenarios that describe alternative but feasible futures. Based on reviews of the FEPs by project researchers and stakeholders, a base scenario was developed and is summarized in Table 3 and Fig. 12. As part of Category WEYBURN FEP TITLE Category WEYBURN FEP TITLE SYSTEM FEPs SYSTEM FEPs (continued) Rock properties Other gas Mechanical properties of rock (including stress field) Gas pressure (bulk gas) Mineralogy Release and transport of other gases Organic matter (solid) Presence and nature (properties) of faults / lineaments Geology Presence and nature (properties) of fractures Seismicity (local) Cap-rock integrity Temperature / thermal field Uplift and subsidence (local) Hydrogeological properties Cross-formation flow Abandoned Wells Fluid characteristics of rock Annular space (quality / integrity) Geometry and driving force of groundwater flow system Boreholes - unsealed (extreme case) Groundwater flow (including rate and direction) Corrosion of metal casing (abandoned wells) Hydraulic pressure Expansion of corrosion products (abandoned well metal casing) Hydrogeological properties of rock Incomplete borehole sealing / Early seal failure Pore blockage Incomplete records of abandonment / sealing Saline (or fresh) groundwater intrusion Transport pathways NON-SYSTEM FEPs EFEPs Chemical/Geochemical Artificial CO 2 mobility controls Carbonation Climate change Colloid generation Cross-formation flow (fast pathways) Degradation of borehole seal (cement / concrete) Depth of future wells drilled Dissolution of minerals/precipitates/organic matter Earthquakes Dissolution / exsolution of CO 2 EOR-induced seismicity Dissolved organic material Exreme erosion Groundwater chemistry (basic properties) Fault activation Methanogenesis Future drilling activities Microbial activity Glaciation Mineral surface processes (including sorption/desorption) Hazardous nature of other gases Precipitation/Coprecipitation/Mineralisation Hydraulic fracturing (EFEP?) Reactive gaseous contaminants Hydrothermal activity Redox environment / heterogeneities Igneous activity Salinity gradient Major rock movement Metamorphic processes CO2 Properties and Transport Mining and other underground activities Advective flow of CO 2 Monitoring (future) Colloid transport Regional uplift and subsidence (e.g. orogenic, isostatic) Diffusion of CO 2 Rock properties - undetected features Dispersion of CO 2 (e.g. faults, fracture networks, shear zone, etc.) Gas flow Sea-level change Source term (CO 2 distribution) Seismic pumping Thermodynamic state of CO 2 Seismicity (EXTERNAL) Transport of CO 2 (including multiphase flow) Fig. 10. Features, events, and processes relevant to the Weyburn CO 2 storage system.

17 C. Preston et al. / Fuel Processing Technology 86 (2005) Risk Assessment of CO 2 Sequestration A number of escape scenarios are being analyzed: 1. Rapid short-circuit release (via fracture, borehole, or unconformity) 2. Potential long-term release 3. Induced seismic event 4. Disruption of host rock Release to aquifer Fig. 11. Escape scenarios. the systems analysis approach, alternative scenarios (modifications to the base scenario) were also developed (see Table 4). Storage system performance simulations in the IEA GHG Weyburn Project were performed primarily on the base scenario Results and conclusions This was the most challenging theme in the project. A comprehensive deterministic risk assessment (DRA) numerical simulation approach was employed in simulating the potential of CO 2 migration away from the Weyburn Unit and into the geosphere and the biosphere over a period of up to 5000 years following the conclusion of the commercial EOR project. Augmenting the deterministic assessment was a smaller, stochastic (probabilistic risk assessment or PRA) simulation of the same systems model but using a compartment model and analytical methods. A benchmarking exercise was also undertaken to ensure that the two PRA/DRA approaches gave similar results on a simple, idealized test case. The benchmarking proved reasonably successful, illustrating both the challenges and value in comparing deterministic and stochastic simulation approaches and highlighting the limitations inherent in both approaches. Initial numerical simulation results on a single pattern indicated that an estimated 2.7% of the initial CO 2 in place may migrate out of the 75 patterns, 5000 years after the end of EOR, most of it igrating laterally into the unconfined eastern areas of the Midale reservoir. The migration is carried out by diffusion through the oil and water phases, pushed along by the action of slow-moving aquifers. However, no CO 2 appears to ever reach or penetrate the Watrous formation, a regionally extensive and thick aquitard above the main anhydrite cap rock, which forms the primary seal for the Midale reservoir [20].

18 1564 C. Preston et al. / Fuel Processing Technology 86 (2005) Table 3 Base scenario! System model domain: the Weyburn 75-well patterns and a 10-km zone surrounding it.! Time frame: inception of EOR using injected CO 2 and with an nominal end time taken as the earlier of 5000 years or the time at which there is 50% loss (to the biosphere) of CO 2 that was in place within the geosphere at the end of EOR.! The caprock may have natural fractures or discontinuities but all are isolated or sealed such that caprock integrity is not impaired.! There is a series of aquifer/aquitards above and below the reservoir horizon. These media may contain fractures and fissures.! Will consider physical trapping features, which have naturally contained the oil/gas within the reservoir.! Will consider geochemical effects (formation of carbonate minerals and CO 2 removal by solubility and ionic trapping) in the aqueous phase of all aquifers.! The biosphere starts from the deepest possible potable aquifer and technically includes all of the glacial till and surficial deposits (i.e., it extends to a depth of about 300 m below ground surface). It includes soil, surface water, atmosphere, flora, and fauna.! Includes the presence of all wells found within the system model domain.! All wells assumed to have been abandoned following current field abandonment procedures applicable at the time of abandonment. Note that this includes wells that may have been sealed in earlier years according to different abandonment procedures and regulations.! Well seals may degrade after abandonment. Well seals are primarily the cement used to fill the annulus between the casing and borehole, cement and metallic plugs used to fill the casing bore, and the cap welded onto the casing approximately 4 m below ground surface. Consideration should also be given to degradation of the casing itself within the reservoir and all aquifers and aquitards penetrated by the casing.! The base scenario includes consideration of FEPs that could affect the storage and movement of CO 2. These include, but are limited to, processes such as hydrodynamics, geochemistry, buoyancy and density-driven flow, dissolution of CO 2 in water and residual oil, and pressure temperature changes occurring within the geologic formations. Early simulations confirmed that wells and their integrity strongly influence leakage from the storage reservoir [18], that the Marly permeability controls CO 2 leakage rates through boreholes, and that storage within the geosphere is greatly Defined as the expected evolution of the Weyburn CO 2 storage system CO 2 migration pathways will be a combination of natural and man-made pathways Wellbore casing seals will be assumed not to leak at time zero CO 2 -rock -water interactions may occur (long-term geochemical modeling) Fig. 12. Weyburn base scenario.

19 C. Preston et al. / Fuel Processing Technology 86 (2005) Table 4 Description of the alternative scenarios Alternative scenario name Unique characteristics Engineering options for EOR: (a) maximize CO 2 storage; (b) water flush at the end of EOR Well abandonment options Leaking wells Fault movement of reactivation, including undetected faults Tectonic activity Deliberate and accidental human intrusion: (a) destruction of surface casing; (b) resource extraction Option (a) involves larger reservoir pressures; overpressurisation and caprock fractures are possible problems. Option (b) would result in changes to CO 2 distributions in the reservoir and could also decrease CO 2 storage Emphasis on improved long-term sealing capabilities Involves extreme failures only as the base scenario has dnormalt leakage Could represent a new and fast CO 2 transport pathway; could affect several formations Low probability but possible Likely scenario involves intrusion into the reservoir in search for CO 2 or petroleum. Option (a) could affect the uppermost seal in one or more wells. Option (b) likely involves extraction of some shallower resource, but could lead to CO 2 blowout from CO 2 trapped in formations above the reservoir enhanced where there is groundwater flow above about 1 m/year in any upper aquifer zones. Synthesis of all available well information within the initial EOR area of the project provided the performance assessment studies with ranges of well types and their associated transport properties. Cement degradation models incorporating sulphate attack, mechanical fatigue, carbonation, and leaching have provided wellbore cement hydraulic conductivities in the range of m 2 for most well types (see Fig. 13). For historical injection and production pressures within aging wellbores, modeling predicted minimal impact on the sealing capability of the wellbores over the life of the EOR project. Performance assessment studies to date show clear support for the conclusion reached within the geological characterization studies the geological setting at the Weyburn Field is highly suitable for long-term subsurface storage of CO 2. These studies highlighted the significant capacity of the geosphere region surrounding the reservoir to effectively sequester CO 2 and prevent its migration to the biosphere. The performance assessment studies also clearly identified wellbores as a potential primary CO 2 leakage pathway to the biosphere in the Weyburn Field [20]. 4. Conclusions 4.1. General conclusions Very encouraging results have been achieved to date. The information and datasets have added significantly to all the technical disciplines that are represented within the project.

20 1566 C. Preston et al. / Fuel Processing Technology 86 (2005) Fig. 13. Bounding seal. The IEA GHG Weyburn Project has developed a suite of leading-edge monitoring and verification technologies. The technologies are applicable to many sites around the world not just CO2-EOR projects. The established technical reputation of the research providers was important to the credibility of the project and the results. There was great cooperation and support among the sponsors and the research providers Major challenges in completing the IEA GHG Weyburn Project Integration of all the elements of the project within and between technical disciplines required a significant effort. Effective integration was critical to the understanding of the overall system and process that is required for CO 2 storage Projects.

21 C. Preston et al. / Fuel Processing Technology 86 (2005) The project required substantial effort by all concerned to insure that deliverables were completed in sufficient time for the integration process to occur. Final project documentation required extensive effort. The project results were prominently featured in papers and a special session at the IEA GHG R&D Programme s GHGT-7 Conference in September Refereed publications were featured prominently and provided a strong body of knowledge for the Intergovernmental Panel on Climate Change (IPCC) special report on CO 2 capture and storage, which is expected to be released in early Recommendation for continuing research In view of the commercial Weyburn CO 2 flood continuing for another 20+ years, initiating a Phase 2 of the IEA GHG CO 2 Monitoring and Storage project has been deemed synergistic and would help answer questions that could not be addressed in Phase I. Phase 2 began in mid Acknowledgments The authors would like to thank the Natural Resources Canada for their continuing support and funding through the Government of Canada s bclimate Change Action FundQ and the bclimate Change Action Plan.Q Also, thanks is extended to the United States Department of Energy for their support and funding through the barrangement on Energy Research and DevelopmentQ between the USDOE and Natural Resources Canada. Finally, we wish to thank the Petroleum Technology Research Centre in Regina, Canada, for support and permission to publish this paper. References [1] International Energy Agency Greenhouse Gas R&D Programme; [2] IEA GHG Weyburn CO 2 Monitoring and Storage Project; [3] Intergovernmental Panel on Climate Change; [4] Petroleum Technology Research Centre; [5] EnCana Corporation; [6] M. Monea, IEA Weyburn CO 2 monitoring and storage project, Proceedings of CO 2 Capture and Storage Technology Roadmap, Natural Resources Canada, September [7] J. Bradshaw, CO 2 storage potential of the world s sedimentary basins, GHGT7 Proceedings, Vancouver, BC, Canada, September [8] R. Hattenbach, M. Wilson, K. Brown, Capture of carbon dioxide from coal combustion and its utilization for enhanced oil recovery, GHGT4 Conference Proceedings, Interlaken, Switzerland, [9] Saskatchewan Industry and Resources; [10] S. Hancock, A. Lukes, Weyburn Unit Carbon Dioxide Miscible Flood Project Activities Implemented Jointly Submission, Department of Foreign Affairs and international Trade Office, Ottawa, Canada, 2000 (March). [11] S. Whittaker, Geological framework, GHGT7 Proceedings, Vancouver, BC, Canada, September [12] D. Mudie, D. Hassan, Industrial perspective on the monitoring project, GHGT7 Proceedings, Vancouver, BC, Canada, September 2004.

22 1568 C. Preston et al. / Fuel Processing Technology 86 (2005) [13] D. Law, Reservoir simulation and economic modelling, GHGT7 Proceedings, Vancouver, BC, Canada, September [14] B. Gunter, E. Perkins, Geochemical monitoring and modelling, GHGT7 Proceedings, Vancouver, BC, Canada, September [15] D. White, Geophysical monitoring, GHGT7 Proceedings, Vancouver, BC, Canada, September [16] W. Zhou, M. Stenhouse, T. Royer, D. Law, R. Chalaturnyk, S. Whittaker, W. Jazrawi, The Weyburn CO 2 monitoring and storage project modelling of the long-term migration of CO 2 from the Weyburn field, GHGT7 Proceedings, Vancouver, BC, Canada, September [17] F. Haidl, M. Yurkowski, S. Whittaker, K. Kreis, C. Gilboy, R. Burke, The importance of regional geological mapping in assessing site of CO 2 storage with intracratonic basins: examples from the Weyburn CO 2 monitoring and storage project, GHGT7 Proceedings, Vancouver, BC, Canada, September [18] F. Moreno, R. Chalaturnyk, J. Jimenez, Methodology for assessing integrity of bounding seals (Wells and Caprock) for geological storage of CO 2, GHGT7 Proceedings, Vancouver, BC, Canada, September [19] K. Asghari, A. Al-Dliwe, Optimization of carbon dioxide sequestration and improved oil recovery in oil reservoirs, GHGT7 Proceedings, Vancouver, BC, Canada, September [20] R. Chalaturnyk, Risk assessment, GHGT7 Proceedings, Vancouver, BC, Canada, September [21] K. Hirsche, R. Adair, G. Burrowes, D. White, Direct estimation of in situ CO 2 storage at the Weyburn Field using integrated reservoir simulation and time lapse seismic monitoring, GHGT7 Proceedings, Vancouver, BC, Canada, September 2004.