Injecting CO2 into coal seams can be an important strategy for reducing carbon emissions. Although CO2 has a strong affinity for coal through the sorption mechanism, resulting potentially in securing long-term carbon storage, it also causes severe coal matrix swelling (along with displacement of water and methane) that can drastically reduce the coal permeability and CO2 injectivity. Coal is essentially an anisotropic organic-matter-rich ‘rock’ with complex cleat systems and bedding planes; these fabrics can significantly affect the reservoir and geomechanical properties of coal and hence impact the stored CO2 volume. Understanding the dynamics of coal behavior during CO2 injection is critical for evaluating the practical storage capacity of coal under economic and operational constraints.
A parametric study is performed herein to investigate the effect of anisotropic permeability changes on CO2 sequestration in deep Mannville coals of western Canada. The investigated ranges of parameters are mainly derived from an analog field case (i.e., the Fenn-Big Valley micropilot) in Alberta, Canada. The isotropic Palmer and Mansoori (P-M) and the anisotropic Palmer and Higgs (P-H) permeability models are compared to model the cleat anisotropy and gas content changes during CO2 injection. In addition, the extended Langmuir isotherm is employed to model sorption-induced swelling as well as competitive adsorption. The P-M and P-H models are used as input to a 3D numerical reservoir simulator to examine the relative importance of sorption strain over compaction under simulated field conditions. By introducing an anisotropic factor g (<1) into the P-H model, permeability reduction related to the swelling effect is amplified, which in turn suppresses the compaction effect on permeability increase, lowering the injectivity by up to 20% and limiting the practical storage capacity of CO2. Cleat porosity directly controls the fracture compressibility, and coals with lower porosity tend to have better injectivity and larger CO2 storage capacity. The impact of sorption swelling parameters is found to be minor for a reasonable range of values due to the small differential swelling strain associated with competitive sorption.
In this study, for the first time, the combined effect of various parameters impacting the evolution of the anisotropic coal permeability during CO2 injection is evaluated systematically. Using an example from the Mannville coal (Alberta, Canada), the findings are beneficial to operators attempting to design an optimum CO2 injection pilot program in support of CO2 sequestration activities in deep, low permeability coals.