The improvement and influence mechanism of non-ionic surfactants on the extraction and expansion effects of CO2 on alkanes: Experimental and simulation approach

Abstract

The extraction and expansion effects are crucial mechanism for enhancing oil recovery (EOR) during CO₂ injection. It is usually hard to distinguish the extraction and expansion effects of CO₂ on oil because the both impacts are occurring simultaneously. In this work, the extraction and expansion effects of CO₂ on alkane were quantitatively distinguished and investigated. Moreover, the influence of CO₂-philic surfactants on the extraction and expansion effects, and the microscopic mechanism were examined through experiments and molecular dynamics simulations. The results show that with the increase of pressure, the extraction and expansion effects of CO₂ on hexadecane and white oil both gradually strengthen. Compared with hexadecane, the alkane molecules with long carbon chains exhibit stronger intermolecular interaction forces, making it difficult for CO₂ to enter the intermolecular space of the alkanes, which reduces the solubility of CO₂ in alkanes, as well as the extraction and expansion effects. Among the CO₂-philic surfactants, C₄EO₃PO₆ and C₄PO₆ can effectively promote the extraction and expansion effects of CO₂. Meanwhile, the surfactants added to the CO₂ phase can rapidly diffuse to the interface between CO₂ and oil, to improve the mutual dissolution of CO₂ and oil molecules. Surfactants can effectively promote both the dissolution of CO₂ into the alkane phase and the diffusion of alkanes into the CO₂ phase, as well as increase the dispersion force between CO₂ and hexadecane molecules and the diffusion coefficient of CO₂ in alkanes. This is the main mechanism of surfactants to enhance the extraction and expansion effects of CO₂.