Injection of CO2 into subsurface coal seams is a viable technology for reducing the carbon footprint. The primary storage mechanism in coal, gas adsorption, is distinctively different from other subsurface reservoirs, providing secure and long-term storage for carbon; however, CO2 adsorption can reduce coal permeability and injectivity due to matrix swelling. In this work, a reservoir simulation study was performed to assist with the design of a field pilot for injecting CO2 into the deep Mannville coals of Alberta. The proposed field pilot consists of a vertical well for injection of CO2, and a closely spaced offset vertical well for observation (pressure measurement and fluid sampling). Extensive numerical modeling was carried out before the pilot implementation to aid with pilot design, assess injectivity, and optimize pilot operations.

Because of the scarcity of reservoir information in the study area, most reservoir attributes were obtained by history-matching the Fenn Big Valley (FBV) micro-pilot (single vertical well) injection data, the closest analog field case performed in the Mannville coal. Accordingly, the reservoir simulation study was conducted in two phases: (1) testing of the numerical model setup using the FBV micro-pilot data and (2) construction of a new pilot area-specific simulation model, corresponding to the new pilot area. During the testing phase, the FBV injection well bottomhole pressure and produced gas compositions were adequately matched. During the new pilot area-specific simulation phase, a full field model (multilayer, two-well) covering a drainage area of 40000 m2 was constructed to represent the target coal seams and the bounding zones. Because the studied coal reservoir is considered to be geomechanically anisotropic with complex cleat systems, the anisotropic Palmer-Higgs model was integrated into the flow simulation to accurately simulate the stress-dependent permeability changes during CO2 injection.

Utilizing geologic information and analog field studies, the new pilot area-specific simulation suggests that the target amount of 1500 tonnes CO2 can be securely stored in the Mannville coal seam at the planned pilot site. To optimize the injection scheme operations, and maximize injectivity, two hypothetical injection scenarios were considered: a constant-rate injection scheme at 5 tonnes per hour and a variable- rate injection scenario at a rate of up to 15 tonnes per hour. Both pre-field simulation scenarios suggest that 1500 tonnes of CO2 can be securely injected into the target coal seam (at 1500 m, with an initial permeability of 1.5 md). However, the time to inject the target amount of CO2 in the variable-rate scenario is significantly less than for the constant-rate scenario. Therefore, a variable injection rate schedule with a progressive increase of 5, 10, and 15 tonnes per hour was suggested for the actual field trial. Additionally, the effect of coal anisotropy on CO2 migration was accounted for in the well-spacing design. The simulation results demonstrate that geomechanical and permeability anisotropy do not substantially affect the CO2 distribution in the coal seam because most of the injected CO2 will be adsorbed onto the coal matrix, with a rate that is mainly controlled by diffusion (not permeability). Analysis of simulation results reveals that the simulated sweep zone at the end of 1500 tonnes CO2 injection ranges from about 42 to 50 m from the injection point. Consequently, an injection/observation well spacing of 44 m was suggested for the new pilot to ensure that the offset well (to injector) can serve as an effective subsurface monitoring well.

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