Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture.

Molecular dynamics simulations are carried out to get insights into the interfacial behavior of the decane + brine + surfactant + CH4 + CO2 system at reservoir conditions. Our results show that the addition of CH4, CO2, and sodium dodecyl sulfate (SDS) surfactant at the interface reduces the IFTs of the decane + water and decane + brine (NaCl) systems. Here the influence of methane was found to be less pronounced than that of carbon dioxide. As expected, the addition of salt increases the IFTs of the decane + water + surfactant and decane + water + surfactant + CH4/CO2 systems. The IFTs of these surfactant-containing systems decrease with temperature and the influence of pressure is found to be less pronounced. The atomic density profiles show that the sulfate head groups of the SDS molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The sodium counterions of the surfactant molecules are located very close to their head groups. Furthermore, the density profiles of water and salt ions are hardly affected by the presence of the SDS molecules. However, the interfacial thickness between water and decane/CH4/CO2 molecules increases with increasing surfactant concentration. An important result is that the enrichment of CH4 and/or CO2 in the interfacial region decreases with increasing surfactant concentration. These results may be useful in the context of the water-alternating-gas approach that has been utilized during CO2-enhanced oil recovery operations.