Effects of permeability anisotropy on coal mine methane drainage performance

Abstract Coal mine methane (CMM) drainage using in-seam boreholes is an effective method to reduce the methane content in coal seams, ensure coal mine safety production and exploit fossil fuel resources. The dominant direction of gas seepage changes due to the anisotropy of coal seam permeability, which causes differences in the amount of methane extracted from boreholes drilled in different directions. The main purpose of this study is to investigate the control effect of coal permeability anisotropy on CMM drainage performance. First, a mathematical model coupling the orthotropic permeability and gas flow in coal seams was established. The methane drainage parameters of the coal mass surrounding in-seam boreholes were simulated. The simulation results showed that the amount of methane extracted from a borehole perpendicular to the permeability advantage direction was much greater than that from a borehole parallel to the permeability advantage direction, and the amount of methane increased with the increase of the angle between the boreholes and the permeability advantage direction. Then, permeability differences of coal samples in three directions, with Gaohe Coal Mine as the test site, were obtained through laboratory tests, and it was determined that the direction of methane flow along the inclined direction of the coal seam was the permeability advantage direction. Finally, five groups of boreholes with different angles from the coal seam inclination, were drilled to investigate their drainage performance. The field test results showed that the group of the largest-angle boreholes had the most positive effect of methane drainage, and the amount of extracted methane was positively proportional to the angle between the borehole and the coal seam inclination. This behavior is consistent with the simulation results. These study outcomes can provide scientific guidance to maximize the methane drainage performance by designing a reasonable layout of boreholes considering the coal permeability anisotropy.