Study on influence laws of coal flushing rate on coal seam stress relief and gas extraction

Reasonable determination of the hydraulic punching rate is the key to ensure the hydraulic punching measures can achieve good pressure relief and permeability improvement effects, and it is of great significance to meet the standard of coal seam gas drainage in the specified pre-drainage period for different coal seam gas conditions. According to the geological conditions of coal seam gas in Mengjin Coal Mine, the influence laws of coal punching rate on coal seam pressure relief, permeability improvement and gas extraction were studied by means of numerical simulation and field test.The results show that the hydraulic punching technology can effectively improve the pressure relief range of the coal mass around the boreholes, and with the increase of the coal punching rate, the stress state of the coal body between the boreholes gradually converts from stress relief zone and the stress concentration zone to stress relief zone. The higher the coal punching rate, the greater the degree of pressure relief of the coal body between the boreholes, resulting in an order of magnitudes increase for the coal permeability. The coal permeability adjacent to the boreholes increases by thousands of times, thus resulting in the greater reduction of the coal seam gas content after extraction, and the better the pre-extraction effect of coal seam gas. According to the gas occurrence conditions of the coal seam, a coal seam gas pre-extraction plan with reasonable coal punching rate should be selected. For thick coal areas and gas-enriched areas, the hydraulic punching rate should be increased. The process of gas pre-drainage, hydraulic punching-second gas extraction can reduce the coal seam gas content and gas pressure around the boreholes before hydraulic punching,so that the numbers and intensity of spraying holes during hydraulic piercing can be significantly reduced, which is conducive to prevent and control the occurrence of spraying holes and abnormal gas emission during hydraulic punching. The longer the gas pre-extraction time, the lower the residual gas content of coal seam. However, the reduction rate of residual coal seam gas content tends to slow down with the increase of the extraction time. The research results successfully guided the safe and efficient gas pre-drainage of coal roadway strip for working face 12050 with different coal seam gas conditions in Mengjin Coal Mine.