Temporal and spatial evolution mechanism of large-diameter borehole pressurerelief and permeable gas seepage in high gas coal seam

Mine gas extraction has the dilemma of low extraction rate, short effective extraction cycle, and substandard gas pre-extraction concentration. In order to realize the precise pressure relief and permeability of high gas and low gas permeability coal seam, taking the working face of high gas and low gas permeability coal seam in Shunhe Coal Mine 2404 as the research object, based on the methods of rheological theory modeling, COMSOL numerical analysis and field experiment, a multi-field coupling model of gas transport of large-diameter borehole pressure relief coal seam was constructed, and the determination index of the effective extraction radius of large-diameter borehole gas was proposed - the residual gas pressure did not exceed 0.22 MPa. The mechanism of pressure relief and permeability enhancement, as well as the law of gas migration in large diameter coal seam drilling, are unveiled: the gas pressure within a single large diameter coal seam drilling exhibits an elliptical distribution with the center of the drilling circle serving as its symmetrical center. In other words, the gas pressure gradually increases from the center towards the outer edges of the drilling circle. Furthermore, with prolonged drainage time, there is a gradual expansion of the standard drainage area. The stress transfer around the borehole is characterized by the fact that with the increase of extraction time, the vertical stress of the peripheral points of a single large-diameter borehole shows a trend of first increasing and then decreasing, and the technology of high-gas and low-gas permeability coal seam large-diameter borehole pressure relief and enhanced penetration extraction is proposed. The simulation determined the effective extraction radius of the borehole, and the drilling hole with a hole spacing of 6.0 m and a diameter of 300 mm had the best pressure relief and penetration enhancement effect, and the average permeability within 3.0 m on both sides was 1.58×10⁻¹⁵ m², which was much greater than the initial value of 1.15×10⁻¹⁷ m². The field application demonstrates that gas extraction in a large diameter borehole is significantly superior to that in an ordinary borehole. In a 300 mm large diameter borehole, the maximum gas volume fraction reaches 47.2%, whereas it only amounts to 10.6% in a 75 mm original borehole. Furthermore, the extraction duration of high gas volume fraction increases by 40.2% in the large diameter borehole. This approach effectively addresses the challenge of controlling small diameter gas boreholes and provides an efficient method for precise pressure relief and permeability enhancement of high gas coal seams.