Estimation of the minimum miscibility pressure for CO2–crude-oil systems by molecular dynamics simulation

Abstract

CO2 injection is an effective enhanced oil recovery technique for energy security with the benefits of carbon neutrality. To reach the maximum oil recovery, the miscible condition between CO2 and oil needs to be maintained in the reservoir, which requires the operation pressure to be higher than the minimum miscibility pressure (MMP). There are two types of MMPs: the first-contact MMP (FC-MMP) and the multi-contact MMP (MC-MMP). In this study, molecular dynamics simulations were performed for the CO2–oil interface system using two simplified digital oil models: a Bakken dead oil with four lumping components and a live-crude-oil model with 50 types of oil molecules but with no asphaltenes and heavy oil fractions. The vanishing interfacial tension method was used to predict the MMP. Different CO2–oil volume ratios were considered to mimic the different degrees of vaporization. To estimate the MMP accurately and rapidly, the interfacial tension in the low-pressure regime was used for the prediction. Consequently, different MMPs were obtained, where the MMP value increased with increasing CO2–oil volume ratio. FC-MMP can be predicted when the CO2–oil volume ratio is sufficiently high. When the CO2–oil volume ratio was approximately 9–10, MMP was closest to the actual MC-MMP value. The condensing and vaporizing mechanism was also studied at the molecular scale. Because pure CO2 was used, only the vaporizing effect on MMP occurred. It was found that the intermediate C2–C6 components have the main effect on the MMP calculation. This study can help to establish a computational protocol to estimate FC-MMP and MC-MMP, which are widely used in reservoir engineering.