A dual poroelastic model for CO2-enhanced coalbed methane recovery

Although CO₂-enhanced coalbed methane (ECBM) recovery has been comprehensively investigated, the impact of coal matrix-fracture interactions on the evolution of coal permeability under in-situ conditions is still unclear. In prior studies on this issue, the influences of coal matrix-fracture interactions have not rigorously coupled with the binary gas transport system. In this work, general porosity and permeability models are developed to explicitly quantify the interactions between binary mixtures (CO₂ and CH₄) and dual solid media (coal matrix and fracture) under the full spectrum of mechanical conditions spanning prescribed in-situ stresses through constrained displacement. These models are implemented into a fully coupled finite element (FE) model of coal deformation, binary gas flow and transport in the matrix system, and binary gas flow and transport in the fracture system. The FE model represents important non-linear responses due to the effective stress effects that cannot be recovered where mechanical influences are not rigorously coupled with the binary gas transport system. The FE model is applied to simulate the results of a single well injection micro-pilot test performed in the anthracitic coals of the South Qinshui basin, Shanxi Province, China. The modeled CH₄ production rates are in good agreement with the observed production history. In addition to this agreement, model results also demonstrate (1) CO₂ injection increases the total pressure gradients; (2) as the CO₂ injection progresses the partial CO₂ pressure increases while the partial CH₄ pressure decreases; (3) without CO₂ injection the CH₄ content at a specific point decreases almost linearly while with the CO₂ injection the CH₄ content at a specific point decreases exponentially; (4) without CO₂ injection the CH₄ production rate decreases linearly while with CO₂ injection the CH4 production rate increases dramatically; (5) without CO₂ injection coal permeability increases almost linearly while with CO₂ injection coal permeability decreases near exponentially; (6) CO₂ injection enhances cumulative CH₄ production and the enhancement is proportional to the injection pressure; and (7) cumulative CO₂ injection volume is also proportional to the injection pressure.