Application of Molecular Simulations to CO2-EOR: Phase-Equilibria and Interfacial Phenomena

Abstract Molecular simulation is a powerful technique by which we can obtain thermodynamic properties of the system of given composition at desired temperature and pressure, and it enables us to observe microscopic phenomena by direct visualization. In this work, two important phenomena related to CO2-EOR were investigated by molecular simulations: interfacial phenomena and phase-equilibria. By molecular dynamics simulation, we have studied the effects of CO2 on oil-water interface properties. It was found that CO2 enriched in the interfacial region at all thermal conditions. While the oil-water interfacial tension (IFT) increases with pressure, the IFT could be reduced by CO2 about 1/3 at low pressure and half at higher pressure to a value ~ 25mN/m. Further analysis based on our MD trajectories show that the O=C=O is bonded to the water with "T-shaped structure", which provides the mechanism for the enrichment of CO2 at oil-water interface. The residual inneglectable IFT at high pressures implies that the connate or injected water existing in the reservoirs will strongly influence the transport of the CO2-oil solutes in a reservoir. By using Gibbs Ensemble Monte Carlo simulation method, we computed phase equilibria of CO2 and petroleum fluids to obtain ternary phase diagrams. As an example, we have obtained vapor-liquid equilibria of a system consisting of CO2, nbutane and N2 and a system consisting of CO2, n-butane and n-decane. The calculation of hydrocarbon fluids with CO2 and N2 mixture will allow us to evaluate the effects of N2 impurity for CO2-EOR. And the calculation of CO2 and hydrocarbon mixtures will allow us to study the minimum miscible pressure (MMP). Such two systems were chosen in our study as the experimental data were available. The application of GEMC techniques as well as MD simulations to a more realistic system including heavier hydrocarbons is straightforward.