Experimental study on gas seepage characteristics of coal around boreholes in hard and low-permeability coal seams

To investigate the seepage characteristics of coal around boreholes in hard, low-permeability coal seams and enhance gas extraction efficiency in mines, this study analyzed the radial stress distribution and fracture development degree around boreholes. Using a triaxial seepage experimental system with zonal loading, the complete process from elastic deformation to failure of hard, low-permeability coal samples was simulated by progressively increasing axial load under different confining pressures. Combining steady-state and transient methods, experimental research was conducted on the gas seepage characteristics of coal around boreholes in hard, low-permeability coal seams. The permeability throughout the entire deformation and failure process of coal samples was obtained, revealing the gas seepage patterns in different stress zones around the boreholes.The research results indicate: ① Under stress influence, the coal around boreholes gradually undergoes elastic and plastic deformation. Once the strength limit is exceeded, fracturing begins. Radially, from near to far, the areas are categorized as the fractured zone, plastic zone, and elastic zone. The permeability first rapidly decreases from its maximum value to a minimum and then slowly recovers to the original permeability of the coal seam, overall presenting a "V" shape. ② Experiments revealed that confining pressure strongly inhibits permeability. When the confining pressure increased to 4 MPa, the permeability of the coal sample was only 4.57% of that at 3 MPa. Therefore, under deep mining conditions, pressure relief techniques should be prioritized for gas extraction. ③ The steady-state method can measure the permeability of loose or highly permeable coal samples, while the transient method yields higher permeability results and is suitable for measuring samples under high stress or those with inherently dense structures.