Effects of heterogeneous pore closure on the permeability of coal involving adsorption-induced swelling: A micro pore-scale simulation

Adsorption-induced swelling of coal matrix tends to close some of the pores, and thus can significantly lower the permeability of coals. The influence of such a heterogeneous pore closure on coal permeability remains poorly understood. This study elucidated the effects of pore size distribution, pore closure behavior and extent of pore closure on the permeability of the coal with widely-developed pore spaces using three-dimensional pore network modeling. By the comparisons of the capillary pressure and relative and absolute permeabilities associated with varying pore size distribution models, the coefficient of variation could be utilized as the principal parameter for microstructural characterization of pores, in contrast to existing studies which rely on porosity. According to pore closure simulations, the irreducible water saturation and residual gas saturation are sensitive to the pore closure behavior and extent. A model of relative permeability involving the effects of pore closure was accordingly proposed, which is available for utilization in reservoir simulators. A parameter describing the permeability decline rate, along with a method for the subsequent determination of the decline index, was advanced. The proposed method can provide superior permeability data for coals associated with pore closure originating from adsorption-induced swelling.