Microscopic mechanism of wettability alteration at the coal–water–CO2 interface under extreme conditions

Injecting hot flue gas from power plants into coal seams is a key strategy for promoting green and low-carbon coal mining. This study investigates the mechanisms underlying changes in coal–water interfacial wettability under high-temperature and high-pressure flue gas injection, with a specific focus on the coal–water–CO2 interface. Experimental results indicate that, under CO2 conditions ranging from 323 to 443 K and 2 to 8 MPa, the surface tension of de-ionized water decreases notably with increasing temperature and pressure. The coal–water contact angle decreases linearly with temperature but increases exponentially with CO2 pressure. Molecular dynamics simulations reveal that elevated temperatures enhance the spreading of water molecules on the coal surface, whereas increased CO2 density inhibits this effect. The concentration of water molecules in the strong adsorption layer, the interaction energy between coal and water, and the mean square displacement of water molecules all increase with temperature but decrease as CO2 density rises. Therefore, increasing temperature improves the wettability of the coal–water interface, while the introduction of CO2 impairs interfacial wettability. This change results from the competitive adsorption between water and CO2 molecules on the coal surface. These findings offer mechanistic insight into coal–water interfacial behavior under extreme conditions and support the application of hot flue gas to enhance permeability in coal seams.