Study on coal pressure relief and permeability increase through ultra-high pressure hydraulic slotting in high outburst and low permeability coal seam

Aiming at solving the problem of gas control in high outburst and low permeability coal seam of No.8 coal seam in Wulunshan Coal Mine No. 1809 coal mining face, this paper proposes a gas extraction technology using ultra-high hydraulic slotting in coal seams. The plastic failure characteristics and stress evolution of coal around the hydraulic slots under different slot spacings are analyzed theoretically and numerically. The numerical simulation is conducted using FLAC3D. Then, bas ed on the relationship between gas permeability and triaxial stress and gas pressure, the permeability distributions of coal around the hydraulic slots are analyzed. It is found that the free surface of hydraulic slots can reduce the confining pressure around the slots, which results from the concentrated stress transferring to the area outs ide of the slots and the coal between the slots. Coal permeability is significantly increased, followed by the increase of gas desorption and gas flow. The field practice s hows that the ultra-high pressure hydraulic cutting increases coal exposed area and coal disturbance volume.The gas flow rate and gas concentration of high negative pressure extraction pipes are then significantly increased. The field measurement data shows that the pure quantity of drainage is between 0.7 and 1.1 m3/min and the gas concentration is above 50%. This paper provides reference for gas control in other working faces of the mine.