Alteration of Potential Storage Space of Shale Gas Reservoirs Induced by Mineral Dissolution and Precipitation during scCO2–H2O Shale Reactions

Long-term storage of CO2 in geological reservoirs can be achieved by mineral trapping through the formation of carbonate minerals. Hence, investigating the precipitation and dissolution processes of carbonate minerals and their effects on the storage space of the target reservoir are of great significance. In this study, a suite of shale samples collected from the southern Sichuan Basin were selected for supercritical carbon dioxide (scCO2)–H2O–shale experiments. Mineralogical composition, surface morphology, and gas adsorption feature variations before and after scCO2–H2O–shale reaction were studied. The results reveal that the carbonate, clay, and feldspar mineral contents in the samples decrease, while the concentrations of Ca2+, Mg2+, and K+ cations in the reacted filtrate increase, indicating the dissolution process. The elevated concentrations of K+ and Na+ cations in the flushed filtrate are ascribed to the redissolution of the precipitated carbonates. The precipitation process has a tendency to decrease the mesopore volume. However, this influence on micropores merely occurs in samples with lower calcite content. After flushing, mesopore volume exhibits an increasing trend, because of the redissolution of soluble precipitates. Precipitation and redissolution also affect pore geometrical characteristics, characterized by increased hysteresis coefficient, initiating blocked pore structure and greater pore heterogeneity. This mainly develops in ink bottle pores, uneven capillary pores, and parallel plate pores. In summary, mineral dissolution, precipitation, and redissolution during scCO2–H2O-shale experiments can significantly alter the storage space of reservoirs. Understanding these dynamic variations and their underlying mineralization mechanism during CO2 storage is a matter of considerable concern for "Carbon Neutrality".