Increased permeability and coal and gas outburst prevention using hydraulic flushing technology with cross-seam borehole

Gas and coal outbursts seriously threaten the safe and effective production of coal mines with low-permeability and high-gas coal seams. To solve this problem, a hydraulic flushing technology with cross-seam boreholes was carried out. Based on the strain-softening constitution and Mohr-Coulomb failure criterion related to the gas pressure and second principal stress, a multi-physical coupling model was built. The failure range and permeability evolution after hydraulic flushing were studied, and the mechanism was presented. Furthermore, the optimal spacing of hydraulic flushing boreholes (HFB) was determined to provide the basis for field testing. Field tests were then carried out to verify the feasibility of the numerical model. The results indicate that hydraulic flushing can greatly improve the permeability of a coal seam; the failure range around the HFB increases with the gas drainage days; and the optimal radius and spacing of the HFB are 0.6 and 5.5 m, respectively.