Seawater-based surfactant formulation for supercritical CO2 injection into coastal saline aquifers: Implications for fresh-water conversion and carbon management

Abstract

Coastal saline aquifers exhibit salinity levels comparable to seawater due to the intrusion of seawater. To effectively utilize the storage potential of aquifers, researchers have investigated the enhancement of carbon dioxide injection performance through the use of chemical additives such as polymers, surfactants, and nanofluids. In field applications, a significant amount of freshwater is typically required to prepare aqueous solutions containing these chemical additives. However, the use of salt-free freshwater necessitates additional transportation infrastructure and procurement costs, which in turn increases the overall cost of subsurface storage. In light of this, the present study evaluates the feasibility of utilizing seawater—which is more readily available than freshwater—for the offshore preparation of chemical additive solutions. For this evaluation, a surfactant known to be effective under deionized water conditions was introduced into a NaCl solution with a concentration similar to that of seawater. The interfacial characteristics between supercritical carbon dioxide and the aqueous solution, along with the injection efficiency in a porous medium, were subsequently assessed. The results indicate that the presence of NaCl induces salt screening and salting-out effects, leading to a further reduction in interfacial tension compared to that in pure water. While the contact angle exhibits only minor variations compared to interfacial tension, the capillary factor—defined as the product of interfacial tension and the cosine value of the contact angle—is predominantly influenced by the interfacial tension. Nonetheless, within the range of conditions and surfactant formulations tested in this study, the reduction in the capillary factor driven by interfacial tension did not lead to any measurable additional increase in injection efficiency. These results indicate that, for the present system, seawater-based surfactant solutions with added NaCl can achieve injection efficiencies comparable to those prepared with freshwater, suggesting that seawater is a promising substitute in practical applications. However, the extent to which this behavior holds in other reservoir conditions and for different surfactant systems should be examined in future studies.