Development of an anisotropic coal permeability model incorporating damage and its application in gas drainage

Coal is characterized as a complex geological material with significant anisotropic features. Permeability in coal is critically influenced by anisotropic characteristics, resulting in impacts on efficiency during gas drainage processes. To systematically examine dynamic changes in directional flow anisotropy behavior during gas drainage, the anisotropic coal permeability model incorporating damage was formulated. The model was established by extending the existing model governed by adsorption–desorption and effective stress effects, with the influence of damage induced fracturing processes on mechanical degradation in coal systematically integrated. Validation analysis was conducted using experimental data from coal seepage tests under triaxial loading in orthogonal directions. The anisotropic permeability variations during triaxial compression were effectively characterized by the developed anisotropic coal permeability model incorporating damage. Finally, an anisotropic permeability ratio was introduced to analyze the directional heterogeneity of permeability variations in coal under orthogonal directions. Furthermore, simulations of the gas desorption–diffusion process were conducted using the developed anisotropic coal permeability model incorporating damage. Moreover, coupled simulations capturing gas desorption processes and stress-dependent permeability evolution in coal seams were carried out by applying the anisotropic permeability model with integrated damage mechanisms. The temporal evolution of gas drainage under varying drainage durations and borehole diameters was analyzed in depth, and the impacts of critical factors, including elastic modulus, porosity, and initial permeability, on anisotropic permeability were systematically investigated.