Abstract
Molecular dynamics simulations were performed to understand the two-phase behavior of the decane + brine (NaCl) system in the presence of CH4, CO2, and their mixture at geological conditions. The simulated density profiles agree reasonably well with those estimated from the density gradient theory based on the cubic-plus-association equation of state with the Debye-Huckel electrostatic term. For these systems, the solubility of CH4 and/or CO2 in the water-rich phase decreased with increasing salt concentration due to the salting-out effect. We find that the interfacial tensions (IFTs) of the decane + brine system in the presence of CH4, CO2, and their mixture were relatively similar to those of the corresponding decane + brine system alone. For example, the IFTs of decane + brine and decane + brine + CO2/CH4 systems generally increase with pressure, whereas an opposite trend was observed for the brine + CO2/CH4 system. For all systems, the increase in IFT with pressure was attributed to the negative surface excess of decane. This surface excess is observed to decrease with temperature at a fixed pressure. This explains the significant dependence of IFT on the pressure at high temperatures. The positive surface excess of, for example, CO2 contributes to a less pronounced dependence of IFT on the pressure at high mole fractions of CO2 in the decane/CO2-rich phase. The presence of CO2 has a more significant effect on the IFT of the decane + brine system than the presence of CH4 because of the higher enrichment of CO2 at the interface. In contrast to the solubility behavior, the surface excesses of CH4 and CO2 increase with increasing salt concentration. This increase is possibly due to the stronger binding of water with CH4 and CO2 in the presence of salt. The salt ions exhibit a negative surface excess that explained the increase in the IFT with increasing salt concentration for the studied systems.