Spatio temporal evolution of geostress and gas field around hydraulic punching borehole in coal seam

Hydraulic punchinging, as a strengthening measure of coal seam pressure relief and permeability enhancement, has been widely used in soft, low permeability and high-outburst coal seam. In view of the existing problems such as unclear distribution of coal field around hydraulic punching and parameters of hydraulic punching pressure relief radius and other parameters that are difficult to determine, the self-developed stress monitoring system and gas flow monitoring instrument were used to carried out synchronous monitoring of stress field and seepage field in hydraulic punching area in outburst coal seam in Liangbei coal mine, Henan province. The spatial and temporal distribution and evolution law of geostress field and gas field around hydraulic flushing hole were obtained. The results show that:①After hydraulic punching, there is a dynamic evolution process of the stress field of coal body around the hole, and the pressure relief area and stress concentration area gradually migrate outward and then basically stabilizes until 3 days later;②After the stress stabilizes,the stress relief zone is 4 m away from the center hole of hydraulic punching, the stress transition zone is within 4～5 m, and the stress concentration zone is over 5 m.The radius of pressure relief zone is more than 10 times of the equivalentradius of hydraulic punching hole;③The fracture field develops within 4 m from the center of punching hole, but the drainage gas flow and permeability become lower in surrounding stress concentration area.The research results provide theoretical and practical basis for accurately determining the gas drainage parameters of hydraulic punching, effectively reducing stress and improving permeability, and enhancing gas drainage effect.