Numerical Investigation of an Anisotropic Permeability Model for Bedded Coal Based on the Equivalent Fracture Aperture Coefficient

Abstract

Permeability evaluation of bedded coal is always a difficult problem, thereby an improved permeability model is established based on the equivalent fracture aperture coefficient. The equivalent fracture aperture coefficient can reflect the anisotropy of coal in mechanical and seepage properties. This model is executed into COMSOL Multiphysics to perform numerical simulations. The evolutions of permeability, gas pressure, and cumulative gas production on three typical boundary conditions are investigated. Our simulation results show that this model is competent to represent evolution of anisotropic permeability in bedded coal during gas production, which is governed by the competitive effects of desorption and effective stress. Under the same boundary conditions, different equivalent fracture aperture coefficients lead to significantly different trends in anisotropic permeability evolution. Anisotropic permeability changes with gas pressure and time during production, and the magnitude of change depends on both the equivalent fracture aperture coefficient and applied boundary conditions. Our model shows that the bedding characteristics of permeability have a significant effect on gas production forecasts.