Experimental Study on Permeability Variations in Fractured Coal Under Stress Changes in the Mining Area of the Ordos Basin, North China

Deep coalbed methane (CBM) reservoirs are characterized by high in situ stress, and the effective stress during CBM production is significant, leading to substantial damage to reservoir permeability. Studying the variation patterns of coal permeability during stress unloading is crucial for revealing the mechanisms by which CBM stimulation through slotting and cavity creation modifies in situ stress. To understand the permeability variations in fractured coal under stress changes, gas seepage experiments were conducted using seven deep coal samples obtained from the Linxing–Shenfu mining area in the Ordos Basin of North China. Through these experiments, permeability variations in coal under different confining, axial, and gas pressures were investigated, and their implications for permeability enhancement through hydraulic slotting in deep coal seams were analyzed. The results show that during loading, permeability decreases with increasing effective stress, and the rate of permeability damage increases. During unloading, the changes in coal permeability transition from slow to rapid, with the stress sensitivity coefficient increasing and the stress sensitivity becoming more pronounced. Regardless of the loading or unloading process, lower axial pressure leads to higher permeability, greater permeability recovery and damage rate, a larger stress sensitivity coefficient, and stronger stress sensitivity of the coal. For every 4 MPa decrease in the axial pressure, the permeability increases by approximately 0–10%, and the permeability recovery rate increases by about 6%. This is because the lower axial pressure reduces the effective stress acting on the coal matrix and fractures, thereby widening the flow channels and enhancing both the permeability and its recovery capacity. In addition, for every 0.3 MPa increase in the gas pressure, the permeability increases by approximately 10–50%, and the permeability recovery rate increases by about 20%. This indicates that elevating pore pressure effectively counteracts effective stress, expands fracture apertures, and promotes fracture connectivity. This work demonstrates that fractured coal is highly sensitive to stress and that stress relief plays a crucial role in enhancing the permeability of deep coal seams.