Molecular dynamics simulation to estimate minimum miscibility pressure for oil with pure and impure CO2

Abstract

CO2 enhanced oil recovery (CO2-EOR) has gained great attention worldwide for increasing oil production and reducing greenhouse emissions. In CO2-EOR project, the minimum miscibility pressure (MMP) is the key factor to determine the possibility of CO2 miscible-phase displacement. In this study, we used molecular dynamics (MD) simulation to estimate the MMP of pure and impure CO2 with oil by obtaining interfacial tensions (IFTs) based on the vanishing interfacial tension technique. To get accurate IFTs, modified Lorentz-Berthelot rules were developed for all simulations. Then, we performed MD simulations for CO2 + n-hexane binary system, CO2 + n-hexane + n-decane ternary system and complex CO2 + crude oil system to calculate the MMPs. Firstly, MMPs predicted by MD simulation show good agreement with previous experimental data. Secondly, the effects of injected gas components, oil components and reservoir temperature on the MMP are investigated. The results demonstrate that methane and N2 will increase the MMP of CO2-oil system significantly; the effects of a certain oil component on the MMP depend on the properties of crude oil, generally speaking, heavy oil components will increase the MMP while light oil components decrease it; temperature will increase the MMP of CO2-oil system, while it will decrease the MMP of N2-oil system, which is due to their different solubility in oil. Thirdly, sensitivity analysis is conducted to evaluate the effects of those influence parameters on the MMP of CO2-oil system, finding that N2 concentration in CO2 and reservoir temperature have more significant impacts on the MMP.