Numerical analysis and field tests of self-relief pressure zone ahead of mining workface: implications for methane extraction during underground mining

Coalbed methane extraction not only reduces natural emissions of a greenhouse gas, it also allows for safer coal mining and reduces the risk of underground disasters. The pressure environment of coal is an important factor in the gas extraction effect, so it is important to accurately assess the pressure distribution in front of the working face. In this paper, we analyzed the pressure distribution of the coal mass in front of the working face by means of numerical simulations. We then proposed the concept of self-relief pressure zone based on the numerical simulation results to accurately determine the efficient gas extraction area. Finally, a field test was conducted in the Qinshui Basin of Shanxi Province, China. Numerical simulations indicated that there is a self-pressure relief zone of about 4.86 m to 5.06 m in front of the working face. The total stress of coal seam in self-relief pressure zone is obviously small, showing obvious plastic deformation characteristics. Field tests showed poor methane extraction when the borehole was in a stress concentration zone. The amount of pure methane extracted significantly increases when the borehole enters the self-relief pressure zone in front of the working face. For the test working face, the width of the pressure relief zone in front of the working face is about 8.9 m to 14.7 m. Furthermore, we analyze the mechanism of the effect of self-relief zones on gas extraction and discuss how self-relief zones can be used to improve gas extraction in coal seams. Thus, these findings have important theoretical foundations for the field application of gas extraction with pressure relief in coal mines.