Numerical assessment of the influences of coal permeability and gas pressure inhomogeneous distributions on gas drainage optimization

In this paper, significant effort was devoted to theoretical/numerical modeling to make an optimal gas drainage design for a pre-drainage coal seam considering the inhomogeneity of coal permeability and gas pressure. A fully coupled model of gas flow, gas diffusion and permeability evolution was developed to evaluate the borehole drainage performances of safety needs (primary consideration) and economic efficiency (secondary consideration). A novel method was applied to rebuilt the non-uniform initial condition in COMSOL to realize the inhomogeneous distribution of coal permeability and gas pressure in the numerical simulations. The significant influences of the inhomogeneity of coal permeability and gas pressure on gas drainage were discussed, and two main disadvantages were visually revealed, including unnecessary engineering cost and enormous mining risk. The optimal gas drainage design consists of three different borehole spacings available for 6 zones, being able to save about 21.5% engineering cost compared to that without considering the reservior inhomogeneity. Numerical simulation results are also helpful to the gas drainage technological innovations.