Numerical Simulation and Field Testing of Coal Seam Drilling Hole Gas Discharge Characteristics Based on Fluid–Solid Interaction

The effectiveness of gas discharge depends on the geological conditions and drilling parameters. Investigating gas seepage behavior near boreholes under fluid–solid coupling conditions can provide theoretical support for scientifically determining the effective discharge radius (EDR) and ensuring mining safety. In this study, taking Xinyuan Coal Mine as the engineering background, a fluid–solid coupled model describing gas migration was developed. The effects of the discharge duration, borehole diameter, permeability, and borehole layout on the spatiotemporal evolution of gas around boreholes and EDR were investigated. The results indicate that gas pressure around the borehole continuously decreases with time, and the affected zone expands elliptically. The EDR exhibits a power-law relationship with time. Increasing the borehole diameter enlarges the EDR, with the effect being particularly significant in the initial stage of gas discharge. After 5 h of gas discharge, the EDR in high-permeability coal seams is approximately twice that in low-permeability coal seams. Compared to the triple-flower patterns, the square pattern produces a larger EDR at the same time. The EDR calculated based on the measured values of the drill cuttings volume S value and drill cuttings desorption gas volume K1 value shows a high degree of consistency with the simulation results. After 5 h of gas discharge using the square pattern, the gas volume fraction at the upper corner of the working face dropped to the safe level of 6%, enabling mining to resume.