Research of pressure relief and permeability improvement of hydraulic punching in soft and low permeability outburst coal seam

In response to the difficult problem of gas control in soft and low permeability outburst coal seam, gas control technology is proposed to use physical modification hydraulic punching to improve permeability. Theoretical calculations and FLAC^3D numerical simulation methods were used to analyze the plastic failure characteristics of coal bodies around holes under different drilling spacing conditions, and the corresponding stress evolution characteristics were quantitatively characterized. After hydraulic punching, the radial stress of coal bodies between holes significantly decreased, and the closer they were to the holes, the greater the reduction; the influence range of tangential stress is equivalent to the range of plastic failure zone; the peak vertical stress showed a significant increase, and as the spacing between boreholes decreased, the vertical stress curve evolved from a bimodal curve to a unimodal curve; it was found that the pore space provides a free surface for coal unloading, which can remove the confining pressure and transfer concentrated stress to the coal between the pores. Subsequently, based on the relationship between gas coal permeability and three-dimensional stress and gas pressure, the evolution characteristics of permeability under different drilling spacing conditions were quantitatively characterized. Compared with the original coal, the permeability of the coal around the holes was greatly increased, which promoted the gas desorption and flow, and determined the reasonable spacing of the holes, and a reasonable drilling spacing was determined. Field experiments have shown that hydraulic punching greatly increases the exposed area of soft coal bodies, provides pressure relief and transparency space for coal bodies, gas flow and concentration is significantly increased in high negative pressure extraction branch pipes, and the pure gas extraction volume fluctuates within the range of 6.20-9.8 m³/min. The gas concentration is above 34%.