Effect of triaxial stress on permeability and enhancement rate of coal seams: Implications for gas extraction efficiency

The permeability enhancement of coal seams plays a critical role in gas extraction efficiency. The true triaxial stress changes generated during the coal mining process can cause alterations in the permeability of the coal. Experiments were conducted to investigate the deformation and permeability evolution of coal under true triaxial stress conditions, and an analytical model of stress–strain was developed to quantify the seepage behavior under true triaxial stress. To clarify the spatial distribution and evolution of permeability enhancement within coal seams under mining-induced stress, a permeability enhancement model was established. The results indicate: (1) Both anisotropic stress significantly influence coal permeability; (2) Stress unloading leads to matrix shrinkage and fracture expansion, resulting in an exponential increase in permeability with unloading; (3) Under true triaxial conditions, horizontal stress constraint has a greater impact on permeability evolution than vertical stress. (4) The coal seam can be divided into four stages: Stage I: Maximized Permeability Enhancement after Coal Failure; Stage II: Increased Permeability Enhancement with Pressure Depletion; Stage III: Decreased Permeability Enhancement with Rising Pressure; Stage IV: Stable Permeability Enhancement under Original In Situ Stress. Permeability enhancement primarily occurs during Stages II and I. Propose field construction recommendations tailored to the varying stages of coal seams to enhance gas extraction efficiency.