Research on mechanism of fracture formation, pressure relief, and permeabilityenhancement with composite technology of cavitation and CO₂-frac

Coal mine disasters caused by coal and gas outbursts are severe in China. Currently, static and dynamic technologies are mainly used to address the difficult gas control problem. The gas control mechanism of the composite technology of cavity making and gas phase fracturing is studied by comprehensively using three technical methods: 185 MPa high pressure CO₂ impact coal sample, numerical simulation, and fiber optic strain monitoring. The research results indicate that ① under the action of CO₂ gas phase fracturing stress waves, typical microstructures such as damage pits (DM) and three wing cracks (TRW) are newly formed on the surface of the coal matrix in coal samples. ② Under the action of static and dynamic loads, the original fractures in the coal seam within the range near the borehole center are reopened along the direction of vertical fracture, forming a radial multi-scale, tensile-dominant fracture with the borehole as the center. The fracture can fully equalize stress concentration zones and cause irreversible damage to the coal seam, with a stress disturbance range of 24 m. ③ Within the influence range of effective gas phase fracturing hole making composite technology, hole making holes help to improve the energy utilization efficiency of gas phase fracturing stress waves, avoid energy dissipation in ineffective vibration areas, fully connect the multi-scale fractures of the fracturing hole and hole making hole, making the coal seam fracture area wide, continuous, and fully depressurized, which fundamentally solves the problem of low permeability, difficult extraction, and high ground stress in the coal seam.