Use of the Rate-Transient Analysis Method to Evaluate Permeability to Multiple Gases in a Fractured Coal Sample

Deep coal seams can be suitable candidates for CO2 storage due to their high adsorption capacity. However, CO2 injection into coal is known to be affected by CO2 adsorption-induced swelling, offset to a certain degree by the reduction in effective stress near the injection well during injection. To evaluate the net result of these competing mechanisms, as well as the impact of coal properties and fabric on CO2 transport and storage, a systematic laboratory study was conducted on a coal sample from the Mannville Formation of Alberta, Canada. For the first time, the rate-transient analysis (RTA) permeability and porosity (‘RTAPK’) method, a new core analysis approach developed for low-permeability reservoir samples, was applied to examine the evolution of pressure- and stress-dependent permeability of a coal sample containing fractures. Using a coal core plug obtained from the Cretaceous Mannville Formation in western Canada, helium (He), argon (Ar), methane (CH4) and carbon dioxide (CO2) permeabilities were measured using the RTAPK method. The permeability measured with CO2 was consistently lower than CH4 due to its stronger adsorption onto coal and greater coal swelling in the presence of CO2. The baseline permeability, measured with He and Ar (which do not adsorb to a significant degree on the coal), was reduced substantially (>50%) after CH4 injection, indicating significant matrix swelling and associated fracture aperture reduction after CH4 injection. However, He permeability (i.e., baseline permeability) after CO2 injection was comparable to that after CH4 injection. This is likely due to (1) the coal sample reaching its maximum deformation (swelling and fracture closure) during CH4 injection and/or (2) the interaction with CO2 causing the coal sample to become structurally weaker, generating more fractures during the gas injection/soaking period. The findings of this study will be of interest to operators working on carbon capture and sequestration in deep coal seams, offering valuable insights for field-scale pilot testing of the technique.