EVALUATION OF STRESS AND GEOMECHANICAL CHARACTERISTICS OF A POTENTIAL SITE FOR CO 2 GEOLOGICAL STORAGE IN CENTRAL ALBERTA, CANADA

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1 EVALUATION OF STRESS AND GEOMECHANICAL CHARACTERISTICS OF A POTENTIAL SITE FOR CO GEOLOGICAL STORAGE IN CENTRAL ALBERTA, CANADA Kristine Haug Alberta Energy and Utilities Board, Alberta Geological Survey, Edmonton, Alberta, Canada Runar Nygaard and David Keith University of Calgary, Calgary, Alberta, Canada ABSTRACT Storage of CO in deep geological formations has been identified as the most likely near-future option to significantly reduce industrial greenhouse gas emissions in Alberta. In order to assess the risks associated with potential leakage of the injected CO the factors influencing the potential loss of containment need to be evaluated. Currently, there is a lack of publicly available, high quality, geomechanical data needed for this evaluation. This issue has been addressed in this paper by the utilization of log correlations to establish the geomechanical properties of the geological formations at a potential large scale CO sequestration site. RÉSUMÉ Quand des sols contaminés sont chauffés, la pression de vapeur des composés chimiques organiques augmente. L augmentation de la volatilité aide à l élimination des contaminants par des méthodes conventionnelles telles l extraction par vapeur et l extraction à phases multiples. On peut chauffer le sol par des méthodes électriques in-situ en utilisant le chauffage par conduction conventionnel, le chauffage électrique par résistance ou le chauffage par électromagnétisme. 1 INTRODUCTION The capture and geological storage of CO is currently being considered as an option for significantly reducing industrial greenhouse gas emissions to the atmosphere. With annual greenhouse gas emissions of more than Mt., Alberta is Canada s largest emitter of CO, but it also possesses significant potential for storing or disposing of these gases in subsurface rock formations, such as depleted oil and gas reservoirs, deep saline aquifers, and coal beds. As a result of many decades of exploration and production, oil and gas reservoirs in Alberta are generally well characterized and understood. While significant CO storage capacity in oil and gas reservoirs exists in Alberta, their use for CO storage is hindered by the fact that many of them are still producing and significant infrastructure investment would be required to bring CO from its industrial point source to a storage reservoir that has become available. Deep saline aquifers of regional extent are present in Alberta s subsurface beneath the locations of many industrial CO emitters and potentially provide an enormous capacity for CO storage; however, there is significantly less data and information available to assess their capacity and containment characteristics. The purpose of this study is to demonstrate an approach to assess the potential for loss of containment of injected CO at a large scale CO sequestration site through the evaluation of geomechanical properties of the potential injection horizon and cap rock. This study also addresses the sensitivity of geological containment to the range of potential values for geomechanical rock. STUDY AREA AND GEOLOGY The Wabamun Lake area southwest of Edmonton was identified as a potential site for future large-scale CO injection (Michael et al., 6, Michael et al., in press) (Figure 1). Several large industrial CO point sources, among them four coal-fired power plants with CO emissions between 3 and 16 Mt/year, are located in the vicinity, resulting in short transportation distances for the captured gas. The sedimentary succession in the Wabamun Lake area was previously characterized for the purpose of providing the necessary information regarding the capacity, injectivity and confinement of a prospective CO storage operation (Michael et al., 6, Michael et al., in press). It was determined that several deep aquifers in the study area have sufficient capacity to accept and store large volumes of CO in supercritical phase at the appropriate depth and are overlain by thick confining shale units. Supercritical phase CO is critical to maximize the storage capacity of the injection zone. This phase is reached at specific temperature and pressure conditions; therefore,

2 the deeper formations are a more suitable option for injection. The Upper Devonian Nisku Formation was identified as a potential injection zone with sufficient storage capacity and possessing the required injection characteristics. In addition the Nisku Formation in this area does not contain oil or gas resources therefore contamination of a reserve is not a concern and therefore the Nisku Formation is the preferred injection horizon in this study. The Nisku Formation is on average 7 meters thick, typically ranging from approximately 6 to 98 meters but thinning to less than 4 meters in the northwest corner of the study area. It is capped by the Calmar Formation shale ranging in thickness from to 1 meters. The cap rock is overlain by the upper Devonian-Lower Cretaceous aquifers (> metres). Ultimately, the thickness of the Colorado and Lea Park aquitards (> metres) above them would act as a final barrier to any vertically migrating CO. Figure 1. Location of the Wabamun Lake study area in Alberta, Canada, showing major CO emission sources (Michael et al., 6) Figure shows the stratigraphy in the Wabamun Lake area above the Nisku Formation. The sedimentary succession is divided by the thick Colorado Group shale aquitard and into two main hydrostratigraphic groups: a) the Upper Devonian-Lower Cretaceous and b) the post- Colorado. Each major hydrostratigraphic group contains several aquifers and intervening aquitards. The Nisku Formation is part of the Upper Devonian-Lower Cretaceous hydrostratigraphic group and is capped by the overlying Calmar Formation aquitard. Figure. Downhole model showing the stratigraphic succession at the Wabamun Lake study area from the Upper Devonian to surface 3 LOSS OF CONTAINMENT ASSESSMENT The critical issue surrounding the sequestration of CO is ensuring the long-term containment of the injected CO. An assessment of the potential for loss of containment is a crucial step in determining the feasibility of this option. Containment of the injected CO is controlled by the following factors: a) geometry of the injection zone and overlying cap rock, b) hydrodynamics of the system, c) geochemistry, d) interaction with the man-made structures (e.g. wells) and e) geomechanics (Figure 3). The geometrical characterization involves creating a 3- dimensional geological model of the study area and zones of interest. This includes the assessment of the thickness of the injection zone and the overlying barriers, both aquitard cap rocks and secondary aquifers (Figure 3a). Figure 3. Factors that effect containment of injected CO (a) geometry, (b) hydrodynamics, (c) geochemistry, (d) interaction with man-made structures and (e) geomechanics The hydrodynamical characterization of the site assesses the impact that the natural hydrogeological system has on the containment of the CO. This is impacted by the

3 geometry but also includes the porosity and permeability of the strata, as well as the direction and velocity of the flow of formation waters (Figure 3b). The Nisku aquifer has an average porosity of 9% and average permeabilities of 17 md and134 md from core and drill stem tests, respectively. The flow of formation water is generally up dip towards the east-northeast, and is driven most likely by past tectonic compression (Bachu, 199, Michael et al., in press). The geochemical barriers to leakage of the injected CO involve the following mechanisms (Gunter et al., ): dissolution of the CO within the formation waters (Solubility trapping) (Figure 3c), formation of ionic complexes from the reaction between formation waters and minerals present (ionic trapping) and precipitation of minerals (mineral trapping). The geochemical barriers play an important role in the long-term (>1, years) containment of the injected CO. For long-term CO storage the potential for leakage along poorly completed wells is an important issue to ensure containment of the injected CO (Figure 3d). Approximately 13 oil and gas wells have been drilled in the Wabamun Lake area (Figure 4). The oldest wells were drilling in 1948 and the number of wells drilled each year has cycled with oil prices and changes in technology. The majority of these wells were not drilled deep enough to penetrate the Nisku Formation. Nevertheless, the interaction with the wells needs to be assessed but this work is outside the scope of this paper. controlled by geomechanical parameters, such as the insitu stress regime, rock stiffness and rock strength. The potential for CO leakage through fractures and faults can be well managed through proper geological characterization and selection of the storage site and through proper operating procedures. Unfortunately, there is an absence of good geomechanical data with regards to saline aquifers therefore there is a need to collect this data to characterize the potential storage site. The determination of the geomechanical parameters will be discussed in greater detail in the next section. 4 GEOMECHANICAL CHARACTERIZATION The purpose for studying the geomechanical properties of the injection site is to ensure seal integrity during and after CO injection. To conduct a geomechanical characterization we need to obtain stress and pressure information, geomechanical information like strength and deformation properties of the injection horizon and cap rock. 4.1 Determination of Stress and Pressure Conditions Regional-scale studies of the stress regime indicate that in southern and central Alberta the vertical stress (S v ) is the largest principal stress, being greater than the maximum horizontal stress (S Hmax ) (Bell and Bachu, 3). The estimation of the minimum horizontal stress (S Hmin ) and S v in a well provides loose lower and upper bounds for the fracturing pressure in that well (Figure ). The S v magnitude is calculated by integrating the bulk density log from ground surface to the depth of interest. The trend of the S v magnitude follows the trend of formation depth, which increases from northeast to southwest. The S v gradient is approximately 3 kpa/m. The minimum horizontal stress can be evaluated using a variety of tests. The most accurate method for estimating the magnitude of the S Hmin is through micro-fracture testing, but mini-fracturing, leak-off tests and fracture breakdown pressures are also used (Bell, 3; Bell & Bachu, 3). The average gradient S Hmin in the Wabamun Lake study area is approximately kpa/m (Michael et al., in press). These two stress gradients (S v and S Hmin) provide loose lower and upper bounds for the fracturing pressure gradient that was found generally to be ~19 kpa/m for the entire Alberta Basin (Bachu et al., ). Figure 4. Distribution of wells in the Wabamun Lake study area The S Hmin orientations can be determined from borehole breakouts, which are spalled cavities that occur on opposite walls of a borehole (Bell, 3). The direction of S Hmin was approximately 14, in a general southeastnorthwest direction. This means that fractures will form and propagate in a vertical plane in a southwest-northeast direction (~6 ), basically perpendicular to the Rocky Mountain deformation front. In order to minimize the potential for CO leakage a geomechanical assessment must determine the leakage potential through natural, or induced, faults and fractures (Figure 3e). These potential CO leakage paths are

4 where E is Young s modulus (GPa), ρ is rock density, (g/cm 3 ), V s is shear velocity and ν d is dynamic Poisson s ratio calculated based on equation 1. The results for E d and ν d for 4 wells from the study area are given in Table 1. A 1 1 Figure. Vertical and least horizontal stress and pore pressure gradients (Michael et al., 6) 4. Geomechanical Rock Properties The most comprehensive and accurate methodology to establish geomechanical information on strength and deformation is by conducting rock mechanical laboratory tests of preserved core material. However, preserved core material is hard to find for most overburden formations and water filled reservoirs. This is especially true in the early phase of a storage projects when localization has not been finalized and detailed engineering work is not initiated. Nevertheless, if laboratory tests are performed they only represent a single point in a formation without taking into account the spatial variation in the geomechanical parameters. Therefore indirect measurements of these properties have to be utilized to obtain rock strength and strength and deformation properties Establish Deformation Behavior Dynamic Poisson's ratio B Dynamic Poisson's ratio With the lack of laboratory measurements of the deformation properties, dynamic deformation properties can be obtained by well logs. Dynamic elastic properties such as Poisson s ratio and Young s modulus can be calculated from logs. The Poisson s ratio and Young s modulus are determined from P- and S- wave velocity logs (sonic) and density logs. The dynamic Poisson s ratio (ν d ) is calculated from the relationship between the P wave velocity (V P ) and S wave velocity (V s ) as: v d VP V = ( V V P S S ) The Elastic Young s modulus can be calculated from velocity and rock density logs based on: [1] C Dynamic Poisson's ratio Figure 6. Histograms of dynamic Poisson s ratio for a) Upper Colorado, b) Calmar and c) Nisku formations E = ρv (1 + v d S d ) [] The logging intervals varied between the wells and therefore the different formations were covered in one or more wells. The data in Table 1 are based on a single well

5 or the combined wells if data was available from more than one well. The results in Table 1 are reported as the average value with the corresponding standard deviation. The distribution of Poisson s ratios and Young s moduli for the Upper Colorado, Calmar and Nisku formations are given in Figures 6 and 7, respectively A Dynamic Young's moduli (GPa) B Dynamic Young's moduli (GPa) C Table 1. Geomechanical properties (Young s Molulus, Poisson s ratio, and UCS) for each Geomechanical Unit (GMU) reported as average values and with standard deviation GMU E-dynamic ν- dynamic UCS Avg. Sdev. Avg. Sdev. Avg. Sdev. (GPa) % % (MPa) % Upper Colorado 18 1%.33 8% 6 1% Lower Colorado %.3 13% 3 1% Viking 33 9%. 1% 39 1% Joli Fou 3 41%.3 1% 33 3% Manville 9 9%.3 13% 39 3% Glauconitic 41 %.9 9% 6 48% Ostracod zone 44 3%.9 6% 69 3% Ellerslie 36 18%. 18% 41 % Nordegg 6 16%.3 17% 16 34% Banff 8 17%. 18% 87 48% Exhaw 4 6%.1 9% 6 8% Wabamun 74 1%.8 9% 13 6% Blueridge 79 9%.9 6% 17 % Calmar 67 8%.7 7% 18 31% Nisku 68 %.9 7% 19 3% 4.. Establish Rock Strength The use of sonic velocity logs to determine elastic properties of rock is well established. There exist several correlations between rock strength and sonic travel time or a combination of different logs. (e.g. Kasi et al., 1983, Onyia 1988, Hareland & Nygaard 7, Andrews et al., 7). Onyia (1988) conducted laboratory triaxial compressive tests from different lithologies to develop a continuous log based rock strength based on wireline compressional sonic travel. The experimental derived relationship for calculating unconfined compressive strength Onyia derived is given in Equation 3: 1 1 UCS 1. = k k ( ) 3 1 Δtc k + k 4 [3] Dynamic Young's moduli (GPa) Figure 7. Histograms of dynamic Young s moduli for a) Upper Colorado, b) Calmar and c) Nisku formations Where Δt c is travel time in μs/ft, UCS is sonic based unconfined compressive and k 1, k, k 3 and k 4 are lithology dependant constants (Table ). In this study we used correlations on limestone and dolomites from Onyia s work. For shales and sandstones the unconfined compressive strength laboratory results and sonic log data from Hareland & Nygaard (7) was used to derive

6 the lithology dependant constants used in equation 3. In addition to the shale and sandstone correlations a combined (combo) shale and sandstone correlation was derived from the same dataset to be used for shaly sandstones and sand rich shales. Gamma ray log readings were used to distinguish between the clastic lithologies where the sandstone correlation was used for all gamma ray readings below 4 API units, shale correlation were used for all gamma ray readings above 11 API units and the combined correlation was used for all readings in between. Figure 8 shows the correlations between UCS and sonic travel time from wireline log data for the different lithologies. The correlations are derived based on rock mechanical tests with reported UCS values in the range of 8 MPa to 1 MPa. The UCS results for the different formations are given as the average value with the correspondingly standard deviation in Table 1. The unconfined compressive strength for the upper Colorado, Calmar and Nisku formations are given in Figure 9. Table. Lithological dependant regression constants for UCS determination based on the sonic log correlation in Equation 3 Lithology Limestonstone Dolomite Sand- Shale Combo k k k k UCS (MPa) B A UCS (MPa) 18 Limestone Dolomite C UCS (MPa) Sandstone Shale Travel time (μs/ft) Figure 8. Correlations between unconfined compressive rock strength (UCS) and p-wave sonic travel time from wire line logs UCS (MPa) Figure 9. Histograms of UCS for a) Upper Colorado, b) Calmar c) Nisku formations DISCUSSION When characterizing the geomechanical properties of geological formations the formations are combined into similar geomechanical units (GMU). This is done to reduce the amount of data that needs to be entered into a geomechanical simulation at a later stage. It also helps to reduce the complexity of the geomechanical model and

7 reduces computational time. The geological formations were combined when the lithology and geomechanical properties were of similar values. But the vertical resolution of the GMUs was kept high enough so that the geomechanical properties were fairly constant with respect to depth within each GMU. Due to the limited number of wells studied we assumed that there was no lateral variation within each formation. This may be an oversimplification and should be further studied by including more wells and estimating inter-well geomechanical parameters by couple log data and seismic data (Jarvis 6). Within each GMU the geomechanical properties will vary dependant on the geological variation of the material. Most geological depositional systems create natural variation in properties. For instance, variations in grain sizes or mineralogical content will change the mechanical and chemical compaction of shales and hence their geomechanical properties (Rieke and Chilingarian 1974, Nygaard et al., 4). The vertical natural variation in the geomechanical properties can be seen in Figures 6, 7 and 9 for the Upper Colorado and Calmar and Nisku formations. For the dynamic Poisson s ratios in Figure 6 all three materials show a clear peak value and the shape of the curve represents a normal distribution (Figure 6a) or a skewed normal distribution (Figure 6b, c). The average value reported in Table 1 is representative for the dynamic Poisson s ratio properties to be used in further analysis. For the dynamic Young s moduli results given in Figure 7 the behaviour of the shale formations (Colorado and Calmar formations) mimic the dynamic Poisson s results. For the Nisku Formation, the dynamic Young s moduli data has two local maximum values with a global maximum around 8 GPa. The UCS value for the upper Colorado Formation is normally distributed around 7 MPa (Figure 9a). For the Calmar Formation the average reported value is 18 MPa but the most frequent value is 1 MPa (Figure 9b). The Nisku Formation has an average UCS of 19 MPa but Figure 9c shows that the material has strength around 8-1 MPa or -4 MPa. Which is partly outside the strength range the correlation are calibrated for which makes the higher values in the Nisku and Calmar formations unreliable. By taking the traditional approach and using only some average material properties for dynamic Young s modulus and UCS may lead to wrong conclusions in further simulations. If the distributions within each formation are known the empirical based probabilistic distribution model can be determined for each formation and material property. When doing the failure analysis probabilistically distributions or confidence intervals can be used as input. It is unclear how representative the dynamic properties are for studying static geomechanical problems such as CO sequestration. Empirical correlations have been established for determining UCS based on sonic travel time for sandstones (Nygaard et al., 7): E s = 111* UCS Where E s is static Young s modulus (MPa) and UCS is the unconfined compressive strength (MPa). To evaluate the applicability for the methodology of using dynamic deformation properties to evaluate static problems, like CO storage, Young s modulus dynamic (E d ) and Young s modulus static (E s ) has been compared. Figure 1 shows the dynamic and static Young s moduli compared for the Viking sandstone formation. Figure 1 shows that the static Young s moduli are significant lower than the dynamic Young s moduli. The reason for this discrepancy is due to the very different total strain and strain rate between triaxial compressional tests and log velocities. In the triaxial tests the material will experience permanent deformations. Sonic compressional waves will, on the other hand, not create permanent deformations in the material. Similar behavior may be expected for the dynamically measured Poisson s ratio. The effect will probably be less pronounced for Poisson s ratio, since both vertical and horizontal strain measurements will experience permanent deformation, and the effect will partly be negated when calculating the Poisson s ratio. However the results above show that relying on dynamic properties alone will under-predict the deformation which will occur in the formations when they are subjected to stress changes. Dynamic Young's moduli (GPa) Static Young's moduli (GPa) Figure 1 Comparison of dynamic Young s moduli based on wireline compressional and shear wave velocities and static Young s moduli based on UCS correlations 6 CONCLUSIONS A study of the Wabamun Lake area was conducted and a geomechanical model of the area was established which includes total and effective stresses, deformation [4]

8 properties and unconfined rock strength. The Nisku Formation is below the oil producing formations in the Wabamun Lake study area. Therefore there are few wells drilled into the Nisku Formation which makes the risk of well leakage less than in the permeable formations higher up in the stratigraphical column. The area is geologically undeformed so fracture and faults are also less of a concern. The Calmar Formation is the cap rock sealing the potential injection zone, the Nisku Formation, and is only a few meters thick. Any unknown variation of the thickness of the Calmar Formation may change the integrity of the injection site. The minimum horizontal stress gradient and the intact geomechanical properties for the Nisku and Calmar formations are critical data to complete the geomechanical study. Dynamic deformation properties (Young s modulus and Poisson s ratio) were established based on properties which overestimate the rock stiffness. The unconfined compressive strength (UCS) was established for each lithology based on correlations with log properties. A few laboratory triaxial tests should be conducted to confirm the accuracy of the UCS correlations for this specific site and to adjust dynamically derived properties. The statistical approach followed in this paper can be used as an input for geomechanical simulations of leakage risks and is a more appropriate approach for obtaining leakage probabilities compared with a pure deterministic approach. REFERENCES Andrews, R., Hareland, Nygård, R., G., Engler, T. & Virginillo, B. Method of using logs to quantify drillability, Presented at; SPE Rocky Mountain Oil and Gas Technology Symposium, April , Denver, Colorado USA. Bachu, S., 199, Synthesis and model of formation water flow in the Alberta Basin, Canada: AAPG Bulletin, v. 79, p Bell, J. S., 3, Practical methods for estimating in-situ stresses for borehole stability applications in sedimentary basins: Journal of Petroleum Science and Engineering, v. 38, no. 3-4, p Bell, S.J. and S. Bachu 3, In-situ stress magnitude and orientation estimates for Cretaceous coal-bearing strata beneath the plains area of central and southern Alberta: Bulletin of Canadian Petroleum Geology, v. 1, p Gunter, W.D., Pratt, A., Buschkuehle, B., and Perkins, E.. Acid Gas Injection in the Brazeau Nisku Q Carbonate Reservoir: Geochemical Reactions as a Result of the Injection of an HS-CO Waste Stream. In: Greenhouse Gas Control Technologies. E.S. Rubin, D.W. Keith and C.F. Gilboy (eds). Elsevier Ltd. Kiddlington, Oxford, UK. P Hareland, G., Nygård, R. Calculating unconfined rock strength from drilling data, Accepted in; 1st Canada- U.S. Rock Mechanics Symposium, May 7-31, 7, Vancouver, British Columbia, Canada. Jarvis. 6. Integrating well and seismic data for reservoir characterization: Risks and rewards. In: AESC 6, Melbourne, Australia. Kasi, A. Zekai, S. & Bahsa-Eldin, H., Relationship between Sonic Pulse Velocity and Uniaxial Compressive Strengths of Rocks. Proc. Of the 4th U.S. Symp. On Rock. Mech. Texas A&M University, -3 June 1983, TX, US: Michael, K., Bachu, S., Buschkuehle, M., Haug, K., Grobe, M., and A.T. Lytviak. 6. Comprehensive Characterization of a Potential Site for CO Geological Storage in Central Alberta, Canada, COSC Symposium, Lawrence Berkeley Laboratory, Berkeley, California, March -, 6, Michael, K., Bachu, S., Buschkuehle, M., Haug, K., and S. Talman. in press. Comprehensive Characterization of a Potential Site for CO Geological Storage in Central Alberta, Canada, AAGP Special Publication on Carbon Dioxide Sequestration. Nygard, R., Gutierrez, M., Gautam, R., Hoeg, K., 4. Compaction behavior of argillaceous sediments as function of diagenesis. Marine and Petroleum Geology 1 (3), Nygård, R., Bjorlykke, K., Høeg., K. and Hareland, G. The effect of diagenesis on stress-strain behaviour and acoustic velocities in sandstones, Accepted in; 1st Canada-U.S. Rock Mechanics Symposium, May 7-31, 7, Vancouver, British Columbia, Canada. Onyia, E.C., Relationships Between Formation Strength, Drilling Strength, and Electric Log Properties. 63rd Ann. Tech. Conf. Houston October , TX, USA. SPE Rieke, H.H., Chilingarian, G.V., Compaction of Argillaceous Sediments, Developments in Sedimentology, Vol. 16. Elsevier, Amsterdam, 44pp.