Combined permeability enhancement technology of ultra-high pressure hydraulic slot and hydraulic fracturing in low permeability coal seam

In order to solve the problems existing in the mine gas pre-drainage, improve the mine gas drainage utilization efficiency, and prevent the occurrence of gas disaster accidents,aimed at the technical problems of tight replacement of mining and excavation, poor permeability of coal seam and low gas drainage rate, taking gas pre drainage technology of Xinji No.2 Coal Mine as the research background, the joint permeability enhancement technology of ultra-high pressure hydraulic cutting and hydraulic fracturing is proposed.Based on the theory of rock mechanics and fluid mechanics, the combined permeability enhancement mechanism of ultra-high pressure hydraulic slotting and hydraulic fracturing was analyzed.A numerical simulation method was used to determine the fracture direction, fracture zone, plastic zone, elastic zone and original rock stress zone along the slot extension direction from the slot to the deep part of the coal body.Under the influence of high pressure water jet shear and cutting stress, the original rock stress zone transfers to the deep part of coal body, and the permeability of coal body increases.The results show that the arrangement of hydraulic fracturing boreholes in the plastic zone formed by ultra-high pressure hydraulic slits can achieve better permeability enhancement and the combined technology of ultra-high pressure hydraulic cutting and hydraulic fracturing was designed: The hydraulic pressure of cutting is 95～100 MPa, the rotating tail speed is 40 r/min, the gap spacing between the slits is 1.0～1.2 m, and the single punching time is 12 min, the diameter of hydraulic fracturing borehole is 95 mm, and the hole expanding device with the diameter of 100 mm is adopted.The application results of No.2 drilling field of No.220108 floor roadway in No.2201 mining area of Xinji No.2 coal mine show thatthe ultra-high pressure hydraulic cutting and hydraulic fracturing synergistic permeability enhancement technology can significantly improve the permeability of coal seam.After 30 days of gas extraction, the average gas extraction volume of ultra-high pressure hydraulic slotted boreholes is 10.3 times that of ordinary boreholes, and the average gas extraction volume of collaborative hydraulic fracturing boreholes is 6.4 times of that of ordinary boreholes, and it can continuously guarantee higher flow and concentration of gas drainage effect.