Regional gas control technology using long-borehole perforation–fracturing composite permeability enhancement in medium-hard coal seams

To address technical problems such as high fracture initiation pressure and uncontrollable fracture propagation when conventional hydraulic fracturing is used for gas control in medium-hard coal seams, a regional gas control technology based on directional long-borehole perforation–fracturing composite permeability enhancement was systematically investigated against the engineering background of regional gas control at Shaanxi Huangling No. 2 Coal Mine Co., Ltd. through theoretical analysis, numerical simulation, and field tests, and a complete technical system covering parameter optimization, staged fracturing operations, and performance verification was established. The core of this technology was to use high-pressure water-jet perforation to form directional guide slots, thereby reducing the initiation pressure of subsequent hydraulic fracturing by approximately 30% and inducing stable fracture propagation along the perforation direction. Numerical simulations and field tests determined the matching perforation and fracturing parameters as a nozzle diameter of 3.0 mm and a jet pressure of 5 MPa. Field hydraulic fracturing operations were completed in four boreholes using a fracturing-stage spacing of 30–50 m and a fluid injection volume of at least 80 m³ per stage. The application results showed that after the perforation–fracturing composite permeability enhancement technology was applied, the coal-seam fracture initiation pressure was effectively controlled at 11.5–13.7 MPa, lower overall than the conventional fracturing initiation pressure. The radial fracture propagation distance reached 50 m; the average pure gas drainage rate per 100 m of borehole reached 0.226 m³/(min·hm), 2.9 and 3.8 times those of slotted boreholes and ordinary boreholes, respectively. The gas drainage volume fraction remained above 67% over the long term.