Wettability of Supercritical CO₂–Brine–Mineral: The Effects of Ion Type and Salinity

Deep saline aquifers are considered as perfect storage sites to sequestrate CO₂. Interfacial tensions (IFTs) and contact angles (CAs) are key parameters in the heat and mass transfer processes for CO₂/brine/mineral systems in porous media. In the present study, a molecular dynamics simulation method was used to investigate the effects of brine salinity and ion type on wettability of CO₂/brine/mineral systems at 20 MPa and 318.15 K. Four common brines were selected as NaCl, KCl, CaCl₂, and MgCl₂. Interfacial tensions, water contact angles, and hydrogen bond structure and dynamics have been analyzed. The effects of brine salinity and ion type on water contact angles were found to be very complicated. For MgCl₂ and NaCl solutions, the contact angle increases with salinity. For CaCl₂ and KCl solutions, contact angle first increases and then remains constant with salinity. The product of IFT­(CO₂–brine) and the cosine of CA was found to be constant for all brine solutions studied. In the context of large uncertainty of experimentally measured contact angles, this finding is very useful to predict contact angles using interfacial tension data. Due to the fact that IFT­(CO₂–brine) × cos­(CA) is usually related with capillary pressure and residual trapping capacity, this finding is also very helpful to predict these parameters at different brine conditions. More work is required to study the effects of pressure, temperature, and solid surface structure on this relationship