Anisotropic permeability model for coal containing methane based on anisotropic structure ratio

The coal permeability is the key parameter for the gas seepage characteristics and the migration law, and the structural anisotropy of the coal seam makes the permeability have obvious orientation dependence. By assumeing that the coal structure is anisotropic, an anisotropic permeability model is deduced using anisotropic structure ratio for the key parameters, and the Comsol computing platform is implanted. The effects of coal anisotropy on gas diffusion and penetration are investigated. The theoretical and numerical results show that based on the comparative analysis of anisotropic permeability experiments, the proposed model is verified to be reasonable and used for different conditions. There is an exponential relationship between the anisotropic structure ratio and the permeability ratio of the parallel direction of bedding plane to the vertical direction, in other words, the larger the anisotropic structure ratio, the greater the permeability ratio of the parallel direction of bedding plane to the vertical direction. Under the constant confining pressure, with the increasing pressure, the anisotropic structure ratio and the permeability ratio of the parallel direction of bedding plane to the vertical direction increase. Under the constant confining volume, with the increasing pressure, the anisotropic structure ratio and the permeability ratio of the parallel direction of bedding plane to the vertical direction decrease.