Changes of Surface Physicochemical Properties via Microspectroscopic Techniques in CO ₂ –H ₂ O–Coal System: Multifactor Synergistic Regulation Mechanism of Wettability

Wettability determines the efficiency of CO2–H2O-enhanced coalbed methane extraction, yet the synergistic regulation mechanism of CO2–H2O on coal wettability still needs further study. This study employs contact angle analysis, AFM, and microspectroscopic techniques (FTIR and Raman) to investigate the evolution of coal’s surface physicochemical properties and its intrinsic relationship with wettability. The results indicate that the hydrophilicity is significantly enhanced after the CO2–H2O treatment, with the contact angle decreasing from 78.56° (CG) to 55.68° (SWG-7). AFM analysis reveals that the CO2–H2O interaction induces mineral dissolution, thereby modifying the pore structure and apparent contour of coal. Specifically, the PV and SSA in the SWG-7 group increased by factors of 2.87 and 3.04, accompanied by a reduction in surface roughness. FTIR analysis shows a marked decrease in aromatic and aliphatic hydrocarbon structures post-treatment, while the hydroxyl group content increases remarkably. Furthermore, the aromaticity, condensation degree, and aliphatic side-chain length decrease, indicating that CO2–H2O significantly influences the macromolecular skeleton and functional group composition of coal. Raman parameters further reveal that the CO2–H2O treatment results in a substantial increase in small- and medium-sized aromatic rings. ID/IG increases from 1.802 to 2.256, while La decreases from 2.750 to 2.197. Results revealed that the CO2–H2O treatment significantly enhances the hydrophilicity of coal, concurrently modifying its surface physicochemical properties. This study offers experimental support and establishes a theoretical foundation for understanding how the evolution of CO2–H2O-induced surface physicochemical properties governs wettability behavior in coal seams.