Interfacial Characterization and Minimum Miscible Pressure Study of CO2 Flooding Based on Molecular Dynamics

DOI: 10.14800/IOGR.1207

Abstract

CO₂ flooding can not only effectively improve the recovery of oil reservoirs, but also permanently store CO₂ underground to alleviate the greenhouse effect. In the process of CO₂ flooding, CO₂ is injected into the oil reservoirs to affect the properties of crude oil through adsorption on oil interface, which reduces the interfacial tension and the density of the crude oil, thereby enhancing the recovery factor. Conventional core flooding experiments can only observe the oil displacement effect from a macroscopic perspective, but it is difficult to characterize the interface of CO₂ flooding from a microscopic perspective. Furthermore, the experiment to obtain the minimum miscible pressure (MMP) usually requires a long time and high cost.

In this study, an all-atom molecular dynamics simulation method was used to establish a CO₂ model and a crude oil model. Using the CO₂ force field of ZHU and the alkane force field of NERD, the interface interaction, interface characteristics, and minimum miscible pressure between CO₂ and crude oil were analyzed. The research results show that when CO₂ mixes with the crude oil, CO₂ accumulates at the interface to form an adsorption layer; with the increase of system pressure, the phase interface and the CO₂ adsorption layer gradually become thicken, and the interfacial tension (IFT) between CO₂ and crude oil decreases linearly. The addition of the light hydrocarbon component allows CO₂ to be more readily miscible with crude oil. At a low pressure, IFT decreases with increasing temperature, while at a high pressure, IFT increases with increasing temperature and MMP increases with increasing temperature as well. The MMP calculated using linear extrapolation of the interfacial tension was in good agreement with the experiment measurements.

The main innovations of this study was the use of molecular dynamics analysis to simulate the interfacial properties of a system composed of CO₂ and n-alkanes, and it is applicable to apply the simulated MMP in real oilfields.This study can significantly reduce the human, material and financial resources of the experiment in estimating MMP and also provide constructive suggestions on the practical application of CO₂ flooding on oil reservoirs.