Complete coal permeability models from initial to ultimate equilibrium

Abstract

Coal permeability is a crucial property for coal seam gas production. In previous studies, it was assumed that both matrix blocks and coal bridges share an average equalized gas pressure. This assumption results in an equalized swelling strain within the REV (representative elementary volume). As a result of this assumption, no change in fracture aperture is induced by gas sorption. However, this assumption may contradict the conceptual model under stress-controlled conditions. We hypothesize that this contradiction is due to the inability of current permeability models to consider the impact of non-uniform swelling on the evolution of permeability from initial to ultimate equilibrium. In this study, the aperture change is represented by an averaging method after obtaining the distribution of matrix swelling within the REV. The approach shows the matrix swelling initially reduces the fracture aperture, and the aperture recovers from initial to ultimate equilibrium. This approach has led to a new set of non-equilibrium permeability models under different distributions of matrix deformation. The model results clearly demonstrate that coal permeability evolves even under both constant confining pressures and constant effective stresses. Equilibrium permeability models represent the bounds of permeability change while new non-equilibrium models can fill the gap from the initial equilibrium state (pre-gas injection/extraction) to the ultimate equilibrium state (post-gas injection/extraction) to generate a map of permeability change. Therefore, our non-equilibrium permeability models represent complete evolutions of coal permeability.