Supercritical CO₂ Soaking Effect on the Permeability of Coal Fracture Under Shear Slip

CO₂ sequestered in deep coal seams (> 800 m) exists as supercritical state (ScCO₂) due to high temperature and pressure, and the injection induced non-uniform in-situ stress changes would cause shear slip, affecting fracture permeability and increasing CO₂-leakage risk. Thus, understanding the shear-induced permeability evolution of coal fracture subjected to ScCO₂ soaking is crucial. A series of shear-flow tests under different confining pressure were conducted to study the permeability evolution of un-soaked and ScCO₂-soaked fractured coal samples. The surface characteristics before and after the test was measured with 3D scanning methods to examine the shear-induced alteration of fracture geometries. The results show that, as the shear displacement increases, the permeability of coal fracture decreases at the early shearing stage, then increases due to shear dilation, but was offset by the gouge formation at the residual shear stages, leading to permeability reduction ranging from 9 to 70% under different confining pressure. After ScCO₂ soaking, the mechanical strength of surface asperities is greatly reduced, resulting in an increase of 26% in asperity damage volume and a decrease of 56–90% in permeability after shear. The shear behavior after peak stress of fractured coal samples shows shear hardening, and based on which a shear constitutive model of coal fracture was built to calculate the gouge production related to shear plastic work. A theoretical permeability model was built, showing a good application to describe the ScCO₂ effect on permeability of coal fracture subjected to shear stress.

Triaxial shear test was conducted to investigate the supercritical CO₂ soaking effect on the permeability of coal fracture under shear slip.