Gas drainage and optimal arrangement of boreholes based on permeability anisotropy

Based on the true triaxial dynamic anisotropic permeability model of coal and rock, a multi field coupling mathematical model of structurally anisotropic coal seams under true triaxial stress was established, and a numerical simulation experiment of porous dry gas drainage in anisotropic coal seams was conducted using the COMSOL Multiphysics numerical simulation software. It was found that with an increase in the permeability anisotropy ratio, the greater the ratio of the long and short axes of the elliptical pressure isoline was, the denser was the gas streamline in the dominant seepage direction. As the permeability anisotropy ratio increased, the gas extraction volume and rate decreased. When the permeability was isotropic, the gas drainage volume of the uniformly arranged rectangular boreholes was the largest. However, when the permeability was anisotropic, the gas extraction volume and rate were the largest along the y-direction layout (perpendicular to the dominant seepage direction), followed by the rhombus layout, and the smallest along the x-direction layout. Based on the test results, an optimal arrangement principle of multi holes pumping boreholes along the vertical dominant seepage direction was proposed.