Rock Wettability and Its Implication for Caprock Integrity in CO2-Brine Systems: A Comprehensive Review

Carbon geo-sequestration (CGS) is considered a mitigation technology for removing harmful CO2 accumulations from the environment contributing toward a low-carbon future. One of the key concerns with CGS in deep geological formations is to prevent CO2 leakage to the environment requiring long-term integrity of caprock (seal rock), which is affected by its wettability. This state-of-the-art review explores the key parameters that affect seal rock wettability in shale-CO2-brine systems, such as the pressure, temperature, brine salinity, and CO2-aging time. The caprock sealing efficiency may also be affected by the presence of common shale minerals (i.e., mica, illite, kaolinite, and montmorillonite), which adversely affect wettability. The review indicates that increased pressure generally enhances CO2-wetness (i.e., increases contact angle) across different minerals, with shale showing the most pronounced shifts. On the other hand, an increase in the temperature tends to reduce CO2-wetness (i.e., decreases contact angle, making it more water-wet), particularly for shale. However, the effect of higher salinity on the wettability of shale had no clear trend, either increasing, decreasing, or having no effect, depending upon the mineral composition. The long-term exposure (aging) of shale in a CO2-rich environment changed the wettability from water-wet to CO2-wet, which would reduce the caprock sealing capacity. Additionally, this review critically analyzes the published literature, highlighting the discrepancies and pointing out the areas where further research is needed. Moreover, the use of nanoparticles and surfactants alone or in a mixture was shown to increase the rock sealing capacity due to alteration in its wettability toward a more water-wet state. The effect of these ancillary materials is synergistic in enhancing caprock sealing integrity. This review also highlights the significance of caprock wettability for structural trapping integrity and, thus, CO2 containment security.