A review on the interfacial properties of caprock/CO2/brine system-implications for structural integrity of deep saline aquifers during geological carbon storage

Geological carbon storage (GCS) in deep saline aquifers can potentially lessen the detrimental environmental effects of carbon dioxide (CO2). However, the long-term viability of a GCS project depends on preventing CO2 leakage through caprock, which is typically shale. The most crucial petro-physical factors in assessing the containment security of shale-caprock are the wettability and interfacial tension (IFT) of the shale-CO2_brine system. Therefore, this study extensively examines existing research related to the wettability and interfacial tension of the Shale-CO2-brine system, as well as caprock representative minerals such as mica, kaolinite, and illite, in order to offer perspective on precise CO2-leak risk assessment. Review suggested that the long-term interaction of CO2 with clay-rich shale, as well as prevailing temperature and pressure during the interaction, could have an adverse impact on the wettability and IFT of the Shale-CO2-brine system. Such changes in petro-physical parameters could potentially increase the risk of CO2 leakage in the longer term. Other parameters such as rock mineralogy, organic matter concentration and maturation, brine chemistry, and the existence of organic acids can all affect wettability and IFT, limiting the caprock seal's efficiency. Nonetheless, the utilization of nanoparticles (NPs) and surfactants has been proven to produce favorable results in changing the wettability and IFT of shales from CO2-wet towards water-wet. The review additionally disclosed some knowledge deficits in wettability and IFT investigations of the Shale-CO2-brine system. Particularly, studies on the implications of induced acidic conditions caused by CO2 dissolving in brine are limited. Similarly, the vast majority of CO2-shale interaction experiments have been conducted in the absence of free water. Nevertheless, in real subsurface sit-uations, CO2 will be interacting with shale in an environment of water that may have varying salinities. Therefore, further researches into the effects of CO2-Shale interactions in the presence of carbonic acid and free water on wettability and IFT are strongly recommended. Review concludes that the wettability and IFT of the shale-CO2-brine system is extremely important for assessing the structural entrapment of CO2. The challenges pertaining to caprock structural integrity can be addressed by identifying all possible wettability and IFT alteration mechanisms and applying suitable and cost-effective remedies to control them.