Effect of CO₂-H₂O Interaction with High-Rank Coal on Mechanical Properties and Acoustic Emission Characteristics...

The adsorption of CO₂ by coal leads to changes in its mechanical properties, particularly when considering supercritical CO₂ and water with supercritical CO₂ adsorption. This is strongly linked to the efficiency of CO₂-enhanced coalbed methane (CO₂-ECBM) extraction and the safety of CO₂ geological storage. This study focuses on 3^# coal from the Datong Mine in Gaoping City, Shanxi Province. The high-rank coal's mechanical properties, including the triaxial compressive strength and elastic modulus, were examined under the combined effects of CO₂ injection pressure, CO₂ injection time, and moisture content. The triaxial compressive strength and elastic modulus of the coal showed a decrease following CO₂ injection. Increasing the CO₂ injection pressure, prolonging the CO₂ injection time, and increasing the moisture content were favorable for coal softening. In particular, the triaxial compressive strength and elastic modulus of the coal sample after 144 h of water and supercritical CO₂ softening decreased by 67.67 and 64.15%, respectively. Injecting CO₂ into coal changed its failure mode. The dry raw coal sample exhibited a brittle shear failure mode, while the coal samples showed transitional shear failure after injecting 6 MPa CO₂ and 8 MPa CO₂ and ductile nondilatant barreling failure after injecting water and 8 MPa CO₂ (with a moisture content of 3.02%). Moreover, the cumulative acoustic emission energy of the coal samples followed a similar trend to the decrease in mechanical properties under different conditions. The physical and chemical interactions among coal, CO₂, and water caused the softening of coal; these included the generation of the swelling stress, the dissolution of minerals by carbonate solutions, the reduction in surface energy of coal owing to CO₂ adsorption, and the extraction and plasticization reactions of organic matter in coal. The research results are of great significance for further understanding CO₂-ECBM and CO₂ geological sequestration.