Study of the Mechanical Properties and Damage Characteristics of Coal after Interaction with Supercritical Carbon Dioxide

The injection of CO2 into deep unrecoverable coal seams can serve the dual purpose of facilitating the efficient extraction of coal-bed methane (CBM) and geological storage of CO2. However, due to the nonhomogeneous and anisotropic nature of coal seams, which are influenced by depositional environment, geological structure, and hydrogeology, the anisotropy of the deep unrecoverable coal seams can affect the CO2–water–coal reaction. The mechanical properties and damage of coal after the physical and chemical reactions of supercritical CO2 (SCO2)–water–coal at different seam inclinations have not been extensively studied. In this study, we analyze the mechanical properties and damage of coal before and after the reaction by utilizing noncontact full-field strain, acoustic emission, ultrasonic nondestructive testing, and nuclear magnetic techniques. The results demonstrate that the mechanical properties of the coal underwent significant changes after SCO2 treatment, with the largest decreases in compressive strength and elastic modulus observed in the 90° coal samples at 62.7 and 67.4%, respectively. Furthermore, the strain distribution of the treated coal samples exhibited distinct characteristics of laminar dip angle. Acoustic emission analysis revealed that the damage variable D1 of the 45° coal sample had the largest value, while the acoustic emission damage variable D1 of the 90° coal sample had the largest increase of 99.5%. The longitudinal wave velocity of a coal sample is highest at 0°, followed by 45°, and lowest at 90°. At 90°, the maximum value of the wave velocity damage factor is 0.577. Additionally, as the total porosity of the coal increases, the maximum increase observed after testing a coal sample at 90° is 1%. Moreover, the pore structure undergoes changes during this process. The number of micropores decreases, while the number of mesopores and macropores increases. The findings of this study provide theoretical guidance for CO2 injection into deep unrecoverable coal-bed methane.