Key technology and application of ultra-high pressure water jet “cutting-pressure joint” pressure relief and permeability enhancement

To address the issues of low gas extraction efficiency and the difficulty of outburst prevention in deep, low-permeability coal seams, hydraulic measures such as hydraulic fracturing, hydraulic slotting, and hydraulic perforation have become important methods to improve coal seam permeability and reduce outburst risks. However, single hydraulic techniques have certain limitations. To further enhance pressure relief and permeability improvement in deep low-permeability coal seams, an ultra-high-pressure water jet “cutting-fracturing combination” pressure relief and permeability enhancement technology was proposed. By synergistically combining the precise pressure relief of hydraulic slotting with the large-scale permeability enhancement of hydraulic fracturing, and based on theoretical analysis, numerical simulation, and field tests, the synergistic mechanism of the “cutting-fracturing combination” was revealed: slots formed by hydraulic slotting induce stress redistribution in the coal mass, creating a plastic weakening zone; fracturing fluid preferentially extends along the plastic weak planes, constructing a connected three-dimensional fracture network, effectively avoiding the pressure relief blind zones and localized stress concentration issues present in single hydraulic fracturing. The key factors influencing the effectiveness of the cutting-fracturing combination were analyzed, and based on the specific conditions of the No. 1 coal seam in Xinji No. 2 Mine, numerical simulations optimized key process parameters: a slot spacing of 10 m ensures effective overlap of plastic zones, avoiding fracturing blank zones; a borehole spacing of 70 m facilitates efficient connectivity of the fracture network, forming a uniform permeability enhancement area. Two types of cutting-fracturing combination pressure relief technologies were proposed: the “controlled” type and the “enhanced” type. The core of the controlled cutting-fracturing technology lies in pre-intervention—control boreholes are pre-drilled at the boundaries of the controlled area to form guiding boundaries, directing the fractures generated by the cutting-fracturing combination to expand uniformly within the controlled area. The enhanced cutting-fracturing technology focuses on local post-treatment—after applying the cutting-fracturing combination, localized high-stress or weak permeability zones may exist, and supplementary hydraulic slotting measures are applied to further enhance pressure relief and permeability. Field applications at the 220106 working face of Xinji No. 2 Mine demonstrated that gas extraction efficiency improved by 2.3 to 2.9 times, borehole engineering workload was reduced by 33% to 38%, and the time to reach extraction standards decreased by 41% to 53%. The cutting-fracturing combination technology successfully achieved large-scale, uniform, and efficient pressure relief and permeability enhancement in low-permeability coal seams, providing theoretical and technical support for safe and efficient coal mining.