Abstract:
Coal and gas outburst is one of the primary disasters restricting the safe and efficient mining of deep coal resources. Traditional gas control technologies for coal roadway strips are generally plagued by long treatment cycles and low efficiency due to the influence of high in-situ stress and low permeability of coal seams. To improve the control efficiency of coal roadway strips and eliminate the outburst danger at the heading face, this paper proposes a "blasting fracturing + fracturing driving" technology for uniform pressure relief and permeability enhancement. This technology first involves constructing control boreholes along the outer contour of the coal roadway strip. Through hydraulic flushing of these control boreholes, the local stress field is reconstructed to relieve stress concentration. Subsequently, a blasting-fracturing borehole is constructed at the geometric center of the control boreholes to perform pre-splitting blasting on the coal seam, forming uniform blast-induced cracks within the borehole. Furthermore, the stress concentration effect at the tips of the blast-induced cracks is utilized to guide the initiation of hydraulic fractures. Combined with the pressure relief space created by the control boreholes, this promotes the directional propagation of hydraulic fractures until they connect with the control boreholes. Ultimately, through the cascade guiding effect of the blast-induced cracks and control boreholes, a uniform hydraulic fracturing fracture network is established, achieving uniform pressure relief and permeability enhancement within the controlled area. Based on this, the paper reveals the mechanism by which control boreholes relieve coal seam stress concentration and the mechanism of constructing a uniform fracture network via stepwise blasting and fracturing. The results indicate that when the connecting line between the control borehole and the blasting-fracturing borehole forms a 45° angle with the direction of the maximum principal stress, and the spacing is 5 times the control borehole radius, the control borehole can regulate the secondary stress field at the tips of the blast-induced cracks to guide fracture propagation. Simultaneously, a complete set of technical equipment was developed to achieve safe and efficient operations for water-sealed coupled blasting and controlled fracturing. Field tests were conducted in the Ji-15 coal seam of the No. 8 Mine of Pingmei Co., Ltd. The results show that compared with traditional techniques, the drilling workload was reduced by 75%, and the control cycle was significantly shortened. After 90 d of extraction, the gas drainage concentration increased by 2.4 times, the drainage flow rate increased by 2.5 times, and the cumulative gas extraction volume over 90 d was 1.5 times that of traditional control technologies. This technology achieves the organic integration of “targeted fracturing, synergistic pressure relief, and efficient extraction”, providing an innovative solution for disaster control in deep high-gas coal seams.