Optimum formation depths for CO₂ structural trapping: Impact of salinity

Geological CO₂ storage is considered a promising solution to achieve net-zero goals. Structural trapping is one of the primary mechanisms that holds the injected CO₂ within the storage medium and prevents leakage via an impermeable seal/caprock. The capillary sealing efficiency of the caprock is also crucial in ensuring the safety of structural trapping. Capillary sealing and the associated CO₂ column height are determined by the balance of capillary threshold pressure and buoyancy pressure, which is strongly influenced by the CO₂/fluid/formation properties (i.e., density, interfacial tension (IFT), and wettability). However, subsurface formations typically exhibit a wide range of salt concentrations and depict heterogeneity in terms of wettability and pore radius, further influencing these critical properties. Thus, the impact of salinity, wettability, IFT, and pore radius on structural trapping efficiency is assessed in this study. Our analysis suggests that the optimal storage depth for structural trapping decreases as salinity increases; for instance, the highest CO₂ column height was observed at a depth of ∼1400 m in 5 wt% salinity formations. New correlations were also developed to quantify the CO₂ column height and mass under various formation depths and salinity conditions. Despite different dissolved salts (e.g., NaCl, CaCl₂, and MgCl₂) influencing the brine density and IFT, their impact on structural trapping is negligible. Additionally, the heterogeneity in formation properties (i.e., wettability, IFT, and pore radius) strongly influences the CO₂ column height, resulting in uncertainties in the CO₂ distribution prediction.