Understanding the Irreversible Evolution of Coal Permeability Under Cyclic Axial Deviatoric Stress

Coalbeds are subject to diverse load conditions stemming from hydraulic fracturing, mining activities, and geological tectonic forces. Understanding how coalbed permeability evolves under various stress conditions—such as effective stress, peak stress, axial deviatoric stress, and stress cycling—is crucial for optimizing coalbed methane flow dynamics. In this study, coal sample permeability evolution was assessed using the steady-state method under various loading paths. The study revealed insights into the impact of irreversible deformation induced by different axial deviatoric stresses on coal permeability. Our results indicate that confining pressure has a greater impact on axial permeability than axial stress does. Initial stress cycles involving confining pressure notably reduce coal permeability, an effect that is less pronounced in subsequent cycles. Different levels of axial deviatoric stress have varied consequences for coal fractures. Specifically, high axial deviatoric stress conditions promote fracture propagation, thereby enhancing coal seam permeability. Conversely, under low axial deviatoric stress, the cyclical application of axial and confining pressures results in coal compaction and fracture closure, leading to a decrease in permeability after unloading. To visualize microcrack development and propagation in coal under differing axial deviatoric stress conditions, we integrated the discrete element method with the Mohr–Coulomb model in a particle flow program. The findings from our triaxial seepage experiments corroborate well with this computational model, providing a robust validation and deeper insight into the observed permeability changes.