Molecular Dynamics Simulation Study of Carbon Dioxide, Methane, and Their Mixture in the Presence of Brine

We perform molecular dynamics simulation study of CO₂, methane, and their mixture in the presence of brine over a broad range of temperature (311-473 K), pressure (up to about 100 MPa), and NaCl concentration (up to about 14 wt %). The general decrease in the interfacial tension (IFT) values of the CH₄-brine system with pressure and temperature is similar to that obtained for the corresponding CH₄-water system. The IFT of methane and brine is a linearly increasing function of salt concentration, and the resulting slopes are dependent on the pressure. A similar behavior as methane is observed for such systems containing CO₂ and CO₂-CH₄ mixture. The IFT of CO₂ and brine increases linearly with increasing salt content; however, the resulting slopes are independent of pressure. The simulations show that the presence of CO₂ decreases the IFT values of the CH₄-water and CH₄-brine systems, but the degree of reduction depends on the amount of CO₂ in each sample, which is consistent with experimental evidence. These IFT values show a linear correlation with the amount of CO₂, and the resulting slopes are dependent on the temperature and pressure. Furthermore, our results for the mole fractions of the different species in the CO₂-CH₄-water system at 323 K and 9 MPa are in agreement with those of experiments. The mole fractions of methane and CO₂ in the water-rich phase decrease with increasing salt concentration, whereas that of H₂O in the methane- or CO₂-rich phases remains almost unaffected in all of the studied cases. Our results could be useful because of the importance of carbon dioxide sequestration and shale gas production.