Deep saline aquifers are considered as perfect storage sites to sequestrate CO 2 . Interfacial tensions (IFTs) and contact angles (CAs) are key parameters in the heat and mass transfer processes for CO 2 /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 CO 2 /brine/mineral systems at 20 MPa and 318.15 K. Four common brines were selected as NaCl, KCl, CaCl 2 , and MgCl 2 . 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 MgCl 2 and NaCl solutions, the contact angle increases with salinity. For CaCl 2 and KCl solutions, contact angle first increases and then remains constant with salinity. The product of IFT(CO 2 −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(CO 2 −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.

In recent years, the reduction of CO 2 emissions has become a joint effort throughout the world, and carbon capture and sequestration (CCS) is an effective approach to solving the problem of CO 2 emissions. In the present study, the effects of adding CH 4 , Ar, and H 2 S to CO 2 on the interfacial tension (IFT) and wettability (contact angle, CA) of the CO 2 /water/silica system have been investigated using molecular dynamics simulation methods at 20 MPa and 318 K when the molar concentration of impurity gas was fixed at 20%. For the conditions studied, (1) CH 4 has no significant effect; (2) Ar leads to a higher IFT, a larger CA on silica surfaces with a high hydroxyl density, and a smaller CA on silica surfaces with a low hydroxyl density; and (3) H 2 S causes a decrease of the IFT and an increase of the CA. Capillary pressure and gas storage capacity were predicted using IFT and CA data, and the variation of IFT and CA were explained based on density profiles normal to the gas/water and gas/silica interfaces. These findings might be helpful for better understanding the effects of impurities on CCS.