Study on the Damage and Seepage Evolution Laws of Gas-Bearing Coal During Instability and Failure

To investigate the influence of the confining pressure and axial loading rate on the damage-permeability characteristics of gas-containing coal, a solid-gas coupling experimental platform for coal was utilized to conduct damage-permeability experiments under varying confining pressures and axial loading rates. The study aimed to explore the coupling mechanism between coal damage and permeability under different influencing factors, providing a theoretical basis for determining the optimal extraction window during coalbed gas extraction. The results indicate that higher loading rates lead to a slight increase in the peak axial stress of coal samples in the stress-strain curve, with the permeability peak point lagging behind the axial stress peak point. As the confining pressure increases, the compressive strength of coal gradually improves but the permeability continuously decreases. Under uniaxial compression, coal samples exhibit an X-shaped conjugate inclined shear or single inclined shear failure. Higher loading rates result in greater damage to the coal samples, more complex failure patterns, and larger crack lengths and apertures, whereas higher confining pressures inhibit the further development of failure patterns. Based on the fractal dimension of coal surface damage, the surface fractal dimension of coal decreases linearly with increasing confining pressure after loading, further confirming that higher confining pressure reduces the complexity of crack development in coal samples. Therefore, in engineering practice, selecting the optimal window for gas extraction requires comprehensive consideration of the effects of mining speed and confining pressure as well as rational allocation of extraction negative pressure in different zones, to achieve efficient coalbed gas extraction.