Study on the Evolution Mechanism of Permeability and Porosity of Coal under Repeated Mining Stress Path

Existing studies have established that the permeability evolution of coal under repeated loading and unloading stress paths and the distribution and changes of its internal microstructure are crucial for the efficient and safe mining of coal and coalbed methane. However, the specific impact of cyclic loading and unloading with gradually increasing axial pressure on coal permeability and porosity, and the underlying response mechanisms, remain inadequately understood. This study systematically examines the permeability evolution pattern of coal samples subjected to these stress paths. The permeability evolution law and internal microstructure response of coal samples were investigated through five cyclic loading and unloading experiments under confining pressure and varying axial pressures, combined with six nuclear magnetic resonance (NMR) tests. The results indicate that increasing axial pressure during stress loading and unloading leads to two distinct damage stages in the coal samples. Initial damage occurs at a bias stress of 30 MPa, marked by maximum permeability and fissure signal intensity. As loading and unloading progress, stress sensitivity gradually decreases, with significant differences in permeability unloading curves at various axial pressures. Secondary damage appears at a bias stress of 58 MPa. Microstructural changes, including increased NMR signal intensity and porosity, are more pronounced during unloading. This study provides new insights into the dynamic relationship between permeability and microstructural changes in coal under complex stress conditions, contributing to safer and more efficient coalbed methane extraction.