Effects of CO₂-H₂O on pore-fracture structure and mechanical properties of bituminous and anthracite coals

To investigate the effects of CO₂ -water on the pore-fracture structure and mechanical properties of coal,this study employed a self-developed high-temperature and high-pressure acidification experimental system. SJG bituminous coal and DF anthracite coal were selected as test samples,and their pore-fracture structural changes and mechanical responses before and after treatment at 40 ℃ under 2 MPa CO₂ pressure were examined using CT scanning,SEM-EDS,and ultrasonic testing. The results showed that the acidification treatment significantly altered the pore-throat structure of the coal samples. For SJG bituminous coal, the pore-fracture proportion increased by 4. 33%, while the mineral content decreased by 0. 38%. After dissolution,the pore-fracture structure extended along the bedding planes,and the proportion of pore throats with equivalent diameters exceeding 1 000 μm increased,exhibiting the characteristics of an interconnected fracture network. In contrast,DF anthracite exhibited a weak acidification response,owing to its high degree of metamorphism,dense structure,and enrichment of silicate minerals. Its pore-fracture proportion increased by only 0. 08% and mineral content decreased by 0. 05%,while the proportion of pore throats exceeding 200 μm showed only a slight increase,indicating restricted fracture development. Mechanical testing revealed that under CO₂ pressurization,both coal samples exhibited decreased wave velocity,deteriorated elastic modulus, and increased Poisson's ratio,with more significant damage observed in SJG bituminous coal. DF anthracite,however,showed a sudden drop in wave velocity,attributable to dissolution inhibition and stress concentration. The study revealed that metamorphic grade and mineral composition regulate the sensitivity of coal to CO₂ acidification. Medium- to low-rank bituminous coal (SJG) contains more pre-existing pore-fracture structures,which makes it easier for stress to break through fracture tips and form an interconnected fracture network. In contrast,the acidification effect in high-rank anthracite (DF) is inhibited by inert minerals, resulting in only localized fracture development. These findings provide important insights for CO₂ -enhanced coalbed methane recovery in both bituminous and anthracite seams.