Analyzing the permeability enhancement effect of hydraulic slotting for low-permeability coal seams based on a stress-diffusion-seepage coupling model

Objective and Methods This study aims to improve the permeability enhancement effect of coal seams for gas drainage. Targeting the soft, low-permeability coal seams with complex gas occurrence conditions in the Baiping mining area and considering the impacts of effective stress, gas migration, actual gas effects, and tortuosity, this study constructed a stress-diffusion-seepage coupling model based on the pore-fracture dual medium assumption. Using the coupling model, this study simulated the variation patterns of gas content in coal seams during gas drainage under varying gas contents and borehole spacings. Through field tests under different permeability enhancement measures and unloaded coal amounts across borehole intervals, this study compared the permeability enhancement effects of hydraulic flushing, hydraulic slotting, and mechanical reaming to determine the optimal permeability enhancement technique.

Results and Conclusions The stress-diffusion-seepage coupling model enables accurate characterization of the diffusion behavior and flow paths of gas in coal seams. The coal seam thickness and gas content produce significant impacts on the gas drainage effects, with a thicker coal seam and higher gas content associated with a smaller borehole spacing required to achieve effective gas drainage. Under gas contents of ≤7 m³/t, >7‒8 m³/t, and >8‒9.35 m³/t, borehole spacings sshould not exceed 11 m, 9 m, and 8 m, respectively in the case of coal seam thicknesses ≤1 m, should not exceed 10 m, 9 m, and 7 m, respectively if coal seam thicknesses range from > 1 m to 5 m, and should not exceed 10 m, 8 m, and 7 m, respectively if coal seam thicknesses vary from > 5 m to 9 m. Field tests indicate that hydraulic flushing, hydraulic slotting, and mechanical reaming all reduce the volume fraction of methane in coal seams and increase the effective gas drainage radius. Notably, these effects prove especially significant in the early drainage stage, while gradually tending to be gentle in the later stage. Under the same technical conditions, hydraulic slotting can deliver optimal performance, reducing the volume fraction of methane by 64.74 % and increasing the effective gas drainage radius by 0.35 m after 90 days of gas drainage.