CO2/N2 Displacement in Illite Nanopores: Molecular Mechanisms and Storage Implications

Abstract

Shale has a small pore size and complex mineral composition, and the process of gas injection for enhanced oil recovery is controlled by multiple factors, among which the microscopic displacement mechanism still lacks systematic understanding. This study is based on molecular dynamics (MD) to investigate the displacement behavior of CO2/N2 in nanopores, exploring the effects of pore size differences, displacement rate, temperature, pressure, and crude oil components on oil displacement efficiency, as well as the CO2/N2 synergistic displacement mechanism. The research results indicate that when CO2 and N2 enter the Illite nanopores at different ratios, the oil displacement efficiency significantly improves with the increase of the CO2 ratio in the injected gas. During the displacement process, CO2 effectively promotes the flow of crude oil due to its high solubility in crude oil and stronger interfacial adsorption ability with mineral surfaces, while N2 mainly achieves the overall propulsion of the fluid by providing a pressure gradient. The increase in temperature will reduce displacement efficiency and will increase CO2 storage capacity. The increase in pressure enhances the interaction energy between CO2 and Illite, which is beneficial for improving the recovery rate. Microscopic mechanism analysis shows that the increase in the mean square displacement (MSD) of oil, changes in the radial distribution function (RDF), and changes in interfacial energy during the displacement process jointly reveal the controlling effect of gas–oil–mineral interface interaction. This study deepens the understanding of the oil displacement mechanism of mixed gas (CO2/N2) at the microlevel and provides technical support for promoting carbon neutrality goals.