Experimental study on damage evolution and post-peak seepage characteristics of gas-bearing coal under horizontal stress compression

Abstract Following sedimentation, underground coal seams experience tectonic activity that compresses them horizontally, generating fractures that serve as pathways for coalbed methane migration. Understanding coal seam permeability under horizontal stress compression—especially post-peak permeability—is therefore critical. This study used true triaxial experiments to simulate the deformation and permeability evolution of coal seams under horizontal stress. The loading protocol increased horizontal stress before the peak and gradually decreased it afterward, revealing fracture development during loading and unloading. During loading, pores and fractures compress, increasing particle contact and friction, which promotes fracture initiation and propagation; horizontal compression also reduces primary fracture tortuosity, causing permeability ( k ) to increase with volumetric strain. During unloading, as stress is relieved, volumetric deformation decreases, slightly restoring particle and fracture positions, increasing the tortuosity of primary and secondary fractures, and slightly reducing permeability. Based on these characteristics, a damaged coal seam permeability model was proposed. Established using gas pressure and coal seam strain and incorporating a damage factor D for both pre- and post-peak segments, the model effectively reveals permeability evolution with changes in strain and gas pressure and fits the experimental results well..