Anisotropic Flow-Solid Coupling Model for Gas Extraction from Cis-Layer Boreholes and Its Application

To investigate the effect of anisotropy of coal body on the gas extraction effect of cis-borehole, the anisotropy permeability model of coal based on structural anisotropy ratio and flow-solid coupling model were established at a working face of Zhongmacun mine Henan Province, China, as the research object, and COMSOL numerical simulation software was used. The results show that considering coal anisotropy, the gas pressure decreases more faster than that without coal anisotropy, and the farther away from the borehole, the smaller the difference between them. The extraction time was a logarithmic function of the effective extraction radius, the negative extraction pressure was an exponential function of the effective extraction radius, and the borehole diameter satisfies a power function relationship with the effective extraction radius. The variation of gas pressure with extraction time in different stratigraphic directions was analyzed, and gas pressure decreases faster in parallel stratigraphic directions and slower in vertical stratigraphic directions. Considering the complexity and safety of gas extraction at the working face, a 30% redundancy factor is added to determine the maximum magnitude and range of gas pressure drop when the spacing of cascade drill holes in a working face of Zhongmacun mine Henan Province, China, is 6 m, which can avoid the superposition of "blank zone" and ineffective extraction.