Numerical simulation and test of gas drainage with water jet layered pressure relief and permeability enhancement in soft coal seam

Hydraulic punching is the main technical measure to enhance permeability and drainage of coal seam gas. The pressure relief effect is mainly evaluated by the coal output. However, there are some problems such as large coal output, uneven pressure relief and easy stress concentration. Therefore, this paper puts forward the soft coal interlayer water jet layered pressure relief and permeability increasing gas drainage technology. Considering the influence of gas pressure compression and coal matrix adsorption gas expansion on the coal mass deformation, a multiphysics theoretical model under the coupling condition of stress field, fracture field and seepage field was established. With COMSOL numerical simulation software, the branch number of water jet、the variation law of gas pressure and gas content of coal within the influence range of pressure relief in soft coal interlayer were analyzed. The results show that when the long half axis of water jet branch is 2 m and the short half axis is 0.22 m, the number of jet branch number is 6 in numerical model which is more reasonable. Under the same coal output rate, the gas pressure and content decrease as the distance from the borehole increases. After 180 days of drainage, the effective drainage radius of layered pressure relief by water jet is about 2.14 times that of the conventional hydraulic punching. When the effective influence range is 2 m, the gas content of layered pressure relief by water jet is reduced by 7 m³/t, while that of conventional hydraulic punching is reduced by 4.1 m³/t. In the field application of Xinyi Coal Mine, it is found that when the layered coal discharge rate of water jet is 0.29-0.71 times of the conventional punching, and the time of extracting high concentration gas is still 2 times of the conventional hydraulic punching. The soft coal interlayer water jet layered pressure relief and permeability increasing gas drainage technology is of great significance to the development of soft coal interlayer hydraulic punching technology in the future, providing a new method and direction for coal mine gas control.