Application of Molecular Simulations to CO2-Enhanced Oil Recovery: Phase Equilibria and Interfacial Phenomena

Summary Molecular simulation is a powerful technique for obtaining thermodynamic properties of a system of given composition at a specific temperature and pressure, and it enables us to visualize microscopic phenomena. In this work, we used simulations to study interfacial phenomena and phase equilibria, which are important to CO2-enhanced oil recovery (EOR). We conducted molecular dynamics (MD) simulation of an oil/water interface in the presence of CO2. It was found that CO2 was enriched at the interfacial region under all thermal conditions. Whereas the oil/water interfacial tension (IFT) increases with pressure, CO2 reduces the IFT by approximately one-third at low pressure and one-half at higher pressure. Further analysis on the basis of our MD trajectories shows that the O=C=O bonds to the water with a “T-shaped” structure, which provides the mechanism for CO2 enrichment at the oil/water interface. The residual nonnegligible IFT at high pressures implies that the connate or injected water in a reservoir strongly influences the transport of CO2/oil solutes in that reservoir. We used Gibbs ensemble Monte Carlo (GEMC) simulation to compute phase equilibria and obtain ternary phase diagrams of such systems as CO2/n-butane/N2 and CO2/n-butane/n-decane. Simulating hydrocarbon fluids with a mixture of CO2 and N2 enables us to evaluate the effects of N2 impurity on CO2-EOR. It also enables us to study the phase behavior, which is routinely used to evaluate the minimum miscibility pressure (MMP). We chose these two systems because experimental data are available for them. Our calculated phase equilibria are in fair agreement with experiments. We also discuss possible ways to improve the predictive capability for CO2/hydrocarbon systems. GEMC and MD simulations of systems with heavier hydrocarbons are straightforward and enable us to combine molecular-level thinking with process considerations in CO2-EOR.