Gas Drainage Effect and Parameter Optimization of Long Borehole along Roof Strike based on CFD Numerical Simulation

In order to improve the gas extraction effect of long borehole along roof strike in 4301 fully mechanized working face of Longquan Coal Mine, Fluent numerical simulation software was used to study the distribution characteristics of gas in goaf before and after long borehole extraction. Through comparison and analysis of the gas extraction effect of boreholes under different layout of the final hole, the basic principle and the optimal layout layer of long borehole along roof strike were determined, and the spacing and number of boreholes were optimized. The field verification results show that:with the increase of borehole spacing, the net gas flow of gas drainage increases first and then decreases. The gas concentration in the upper corner shows a change trend of "high-low-high", and when the borehole spacing is 10 m, the net flow of gas drainage reaches 15.72 m³/min, while the gas concentration in the upper corner is about 0.54%; with the increase of borehole number, the gas concentration in the upper corner decreases and the rate of decrease decreases gradually, the net flow of gas extraction increases gradually but the rate of increase decreases. Therefore, considering the effect of gas drainage and the drilling quantity, it is determined that the vertical height of the final borehole is 25 m, the number of boreholes is 5, and the borehole spacing is 10 m, the gas extraction effect is better under this condition.