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

The injection of CO2 into deep unrecoverablecoal seamscan serve the dual purpose of facilitating the efficient extractionof coal-bed methane (CBM) and geological storage of CO2. However, due to the nonhomogeneous and anisotropic nature of coalseams, which are influenced by depositional environment, geologicalstructure, and hydrogeology, the anisotropy of the deep unrecoverablecoal seams can affect the CO2-water-coalreaction. The mechanical properties and damage of coal after the physicaland chemical reactions of supercritical CO2 (SCO2)-water-coal at different seam inclinations have notbeen extensively studied. In this study, we analyze the mechanicalproperties and damage of coal before and after the reaction by utilizingnoncontact full-field strain, acoustic emission, ultrasonic nondestructivetesting, and nuclear magnetic techniques. The results demonstratethat the mechanical properties of the coal underwent significant changesafter SCO2 treatment, with the largest decreases in compressivestrength and elastic modulus observed in the 90 & DEG; coal samplesat 62.7 and 67.4%, respectively. Furthermore, the strain distributionof the treated coal samples exhibited distinct characteristics oflaminar dip angle. Acoustic emission analysis revealed that the damagevariable D (1) of the 45 & DEG; coal samplehad the largest value, while the acoustic emission damage variable D (1) of the 90 & DEG; coal sample had the largestincrease of 99.5%. The longitudinal wave velocity of a coal sampleis highest at 0 & DEG;, followed by 45 & DEG;, and lowest at 90 & DEG;.At 90 & DEG;, the maximum value of the wave velocity damage factoris 0.577. Additionally, as the total porosity of the coal increases,the maximum increase observed after testing a coal sample at 90 & DEG;is 1%. Moreover, the pore structure undergoes changes during thisprocess. The number of micropores decreases, while the number of mesoporesand macropores increases. The findings of this study provide theoreticalguidance for CO2 injection into deep unrecoverable coal-bedmethane.