Research of key permeability enhancement technology by high-pressure jet helical slotting

To address the problem of low gas drainage efficiency in high-in-situ-stress and low-permeability coal seams, a high-pressure jet helical slotting technology was proposed based on the engineering background of the Zhaotong Tunnel on the newly built Chongqing-Kunming high-speed railway. The slotting mechanism and parameters of this technology were expounded through theoretical analysis. Numerical simulation was used to reveal the coal mass damage and seepage evolution laws of the high-pressure jet helical slotting technology, and the influences of helical slotting parameters on coal seam pressure relief were investigated. Field tests were carried out in combination with engineering practice. The results show that the multi-stage helical structure formed in coal by high-pressure jet helical slotting can effectively disperse and transfer stress. The gas drainage radius reaches 2.6 m, which is 6.5 times that of conventional boreholes and 1.37 times that of hydraulic slotting. Among the slotting parameters, the slotting pitch exerts a significant influence on in-situ stress relief and permeability improvement of coal seams. A smaller pitch leads to a smaller fluctuation amplitude of the coal seam stress curve and a denser stress-weakened zone, resulting in a significant stress reduction in the near-field coal mass and more sufficient pressure relief in the far-field coal mass. In field application, the gas volume fraction drained by high-pressure jet helical slotting ranges from 68.83% to 87.35%, which is 2.24 times that of conventional boreholes and 1.26 times that of hydraulic slotting, respectively. The net gas drainage rate is 0.058-0.123 m³/min, equivalent to 3.32 times that of conventional boreholes and 1.32 times that of hydraulic slotting, respectively.