A multi-field coupling model for coal seam gas drainage based on the influence of double-pore medium–coal creep and its application

To address the issue of an unknown transportation mechanism and the difficulty of determining parameters in coal bed gas extraction processes, a coupled flow-solid model was created using the dual medium assumption for pore-fracture. This model considers the influence of the multi-mechanism coal bed gas flow as well as the creep effect. Coal seam gas extraction was simulated using the COMSOL numerical simulation program, which was based on the theoretical model mentioned above. The software calculated the appropriate hole spacing and pre-drainage time by analyzing the dynamic distribution law of pressure of gas over time. According to the study's findings, a single borehole's effective extraction radius follows a power function of time (r1=0.318⋅t0.621 R2=0.987). At 180 d, the effective extraction radius is 2.26 m. In multi-hole extraction, the borehole spacing needs to satisfy 3r≤L≤r+0.17R, and the optimal spacing is 6.8–9.2 m. The best spacing is 6.8–9.2 m; after 220 d of pre-drainage, the maximum gas pressure around the borehole decreased to 0.70 MPa, which is below the safety threshold of 0.74 MPa. The results reveal the dynamic coupling mechanism between seepage and stress fields and optimize the method of determining the spacing of boreholes and pre-drainage period. Through the field engineering application verification, the optimized hole spacing and pre drainage period can reduce the number of boreholes in the working face by 25%, shorten the gas pre-drainage time by 20 days, stabilize the residual gas pressure below the safety threshold, and significantly reduce the mine gas disaster risk.