Characterization of coal seam permeability stage evolution and gas dynamic transport before and after reaming

Mechanical reaming technology can significantly improve the permeability of coal seams, thus promoting gas extraction from low-permeability coal seams, and is an effective measure to reduce the risk of gas dynamic disasters. To elucidate the mechanism by which mechanical reaming affects gas extraction, this study employed a gas-solid coupling model and numerical simulation to investigate the evolution of coal seam permeability before and after reaming, thereby deriving analytical solutions for gas extraction. Subsequently, field tests were conducted at the Yangquan mining area to validate the findings. The results of the study show that coal seam gas pressure under reaming stress disturbance decreases continuously with the increase of extraction time. The pattern of change in the rate of decline can be divided into two types of laws: gradually decreasing with pumping time (Type Ⅰ) and unchanged (Type Ⅱ). The gas pressure of the reamed coal seam is significantly lower than that of the normal borehole coal seam. As the distance from the center of the borehole increases, the permeability of the coal seam gradually decreases. The permeability of the coal seam increases with time. The permeability of the reamed coal seam is significantly higher than that of the normal borehole coal seam. Compared with normal boreholes, the gas seepage rate of reamed boreholes increases by about 8 times, and the effective extraction radius is enlarged by 1.70~1.78 times. The gas extraction density and purity of the coal seam in the mechanically reamed face were improved by 6.2 times and 12.6 times, and the content of residing gas was reduced by 19 %. Reaming significantly improves the coal seam permeability and gas extraction effect. The reliability of the coupled model solution and the effectiveness of the mechanical reaming technique were verified.