Shear hardening characteristics and constitutive model of coal structural plane after supercritical CO₂ soaking

In the process of CO₂ geological storage in deep coal seam (> 800 m), CO₂ often exists in supercritical state under the influence of high temperature and pressure. Due to the non-uniform in-situ stress and the injected fluid pressure, a large number of joints, cracks and other fractures developed in coal seam will cause shear slip, which will further affect the mechanical properties of coal and increase the risk of CO₂ leakage. Therefore, revealing the shear mechanical properties of coal structural plane under the long-term effect of supercritical CO₂ is the key to ensure the safety of CO₂ storage. Through the direct shear mechanical tests of coal structural plane after soaking in supercritical CO₂ for different times, the results show that: ① With the increase of soaking time in supercritical CO₂, the shear strength and shear stiffness of coal structural plane decrease gradually. After soaking for 14 days, the shear strength and shear stiffness decreased by 40% and 28%, respectively. ② The increase of supercritical CO₂ soaking time will reduce the dilatancy deformation of coal structural plane, causing obvious shear contraction, at the same time, increasing the crushing volume of fracture asperity. After soaking for 14 days, the crushing volume of fracture asperity increased 1.5 times. ③ There is a strong correlation between shear stress and acoustic emission parameters during the shearing process of coal structural plane. The ringing down count increases to the maximum at the peak stress. With the increase of supercritical CO₂ soaking time, the cumulative ringing down count, hit number and energy of acoustic emission gradually decrease, and the brittle fracture characteristics of coal structural plane are weakened while the plasticity is enhanced. ④ The untreated coal structural plane samples show some obvious shear softening characteristics, but the shear characteristics change to hardening after soaking in supercritical CO₂. By establishing a new shear constitutive model of structural plane, the above transformation characteristics can be well characterized, and the theoretical value of the model has a high degree of fitting with the experimental value.