Effect of ScCO2-H2O treatment duration on the microscopic structure of coal reservoirs: Implications for CO2 geological sequestration in coal

The microstructure of coal reservoirs, especially the pore structure, alters as a result of physicochemical reactions between the CO2 acidic fluid and coals. However, it is still unclear what controls this process and how the pore structure properties of coal alter by the action of CO2 acid fluids change over time. The objective of this study is to identify the underlying mechanism by analyzing the changes in the mineral composition, surface roughness, as well as micropore and mesopore structure of anthracite and bituminous coal when exposed to supercritical CO2 (ScCO2)-H2O. The results indicate that coal reservoirs primarily undergo processes of adsorption expansion and mineral dissolution/precipitation. Among these, adsorption expansion concluded or weakened after 7 days, with mineral precipitation gradually becoming the dominant factor, which results in more secure geological CO2 storage (GCS). Furthermore, comparative analysis of anthracite and bituminous coal with untreated and long-term treatment demonstrates that anthracite exhibits higher CO2 storage capacity and greater wettability. Consequently, anthracite is more suitable as a target reservoir for GCS compared to bituminous coal. These results, therefore, offer a theoretical foundation and a guide for choosing GCS sites in deep un-minable coal seams.