Study on gas seepage from coal seams in the distance between boreholes for gas extraction

In order to research the influence of coal gas seepage law between boreholes for gas extraction, the gas seepage model was established based on the mass conservation equation, Darcy's law, Langmuir adsorption analytic equation, Klinkenberg equation of coal gas, effective stress equation, as well as porosity and permeability dynamic equation. Three-dimensional numerical simulation of coal gas seepage in the surrounding borehole of gas extraction was conducted by the COMSOL Multiphysics software. The law of gas pressure distributions, gas seepage velocity distributions and permeability change were given between two drilled boreholes and around the two boreholes. In order to determine the reasonable spacing of the drilling holes, by simulating the result of the gas pressure drop in different spacing, the arrangement position of the observation hole of the gas pressure is 1/2 along the distance of the drilling test along the center line of the two extraction drilling line. If the gas pressure can be reduced by more than 50% in the pre pumping period, it is indicated that the interval between two holes is a reasonable distance, which will not cause the blank area of extraction, and will not form the superposition area of invalid extraction, which can guarantee the effectiveness of coal mine gas extraction.