Study on mechanism of pressure relief and permeability enhancement in soft-hard composite coal seam by directional hydraulic flushing technology

The soft-hard composite coal seam composed of tectonic and primary structural coal has the problems, such as high gas content, high pressure, poor permeability and difficult extraction, which seriously affect the production safety in the mining. To study the evolution characteristics of pressure relief in soft-hard composite coal seam based on directional hydraulic flushing technology, taking Hudi coal mine in China as the engineering background, and the pressure relief effect of directional hydraulic flushing on 'two soft and one hard' composite coal seam was studied by theoretical analysis and numerical simulation. The stress evolution of the whole coal seam under a single hole and porous interaction of different flushing radii were analyzed. The field test of directional hydraulic flushing in tectonic soft coal seam similar to protective layer mining was conducted. Results show that when the flushing diameter is 3-4 m, the influence range of stress disturbance covers the whole coal seam, and the vertical stress relief of the primary coal seam is the largest, which provides a theoretical basis for coal seam pressure relief and permeability enhancement technology. The average daily gas extraction (ADGE) concentration has doubled, and the ADGE purity has increased from 0.03 m3 /d to 1.07m3 /d. The target period of gas extraction is shortened from the expected 6 months to 1 month, and the average daily driving speed of coal roadway increases from 2.32 m/d to 5.37 m/d. The gas extraction efficiency is significantly improved, and the risk of gas outburst is effectively reduced, which has important theoretical and practical significance for improving the pressure relief and permeability enhancement efficiency of the soft and low permeability coal seam.