High-efficiency regional gas drainage model after hydraulic fracturing of comb-shaped long boreholes in the roof of broken soft and low permeability coal seam

Advanced management of gas enrichment area is an important guarantee to realize safe, efficient and intelligent mining in coal mine. Aiming at the problem of high-efficiency gas drainage in broken and soft coal seam region, taking No.3 coal seam in Hancheng mining area as the research object, a gas drainage mode of comb-shaped long borehole hydraulic fracturing in the roof of coal seam is proposed. The applicability of the model is verified by means of theoretical analysis, numerical simulation and field test, and the fracture propagation law, gas drainage mechanism, and fracturing curve features of comb-shaped borehole in adjacent roof are clarified. Then, a key technology system of open-hole staged hydraulic fracturing for long comb boreholes in the roof is established, which is suitable for 500 m borehole depth. The system integrates dynamic analysis of geological conditions, staged fracturing, packer blockage release and continuous exploration of fracturing range, realizing the main hole track of comb borehole in coal seam roof about 5 m away from the coal seam, multi-stage uniform fracturing, whole hole monitoring and efficient accident handling. On this basis, the engineering practice of two holes has been carried out in Sangshuping No.2 well. The length of the main fracturing hole is 588 m, about 2.0 m away from No.3 coal seam, and there are 6 fractured sections of single hole. The exploration depth of the fracturing range is 381 m, and the overall fracturing influence radius is more than 20 m. After fracturing, the average gas drainage concentration of borehole is more than 40%, and the gas drainage volume is more than 1 m³/min. The drainage effect is 4 times that of the conventional boreholes, and the effective radius of gas drainage reaches up to 9 m in 120 days, realizing efficient gas drainage in broken soft coal seam area. Furthermore, this study puts forward the technical ideas of large-scale application, which is suitable for large-scale gas drainage in broken soft coal seam and remote regional drainage and pressure relief in broken soft and strong outburst coal seam with high gas pressure.