Molecular insights into CO2-EOR/CCUS: Nanoconfinement effects on multi-component diffusion and miscibility in shale reservoirs

Abstract

CO₂ injection is a promising method for increase oil recovery, while also enabling secure CO₂ storage in shale reservoir, thereby reducing greenhouse gas emissions. Shale is distinguished by the abundance of small nanopores, where the molecular movement of fluids is influenced by the nanoconfinement effect, causing deviations from linear diffusion. Therefore, investigating the microscopic diffusion and interaction mechanisms of CO₂-reservoir fluid systems in nanopore is of significant guiding importance for enhanced oil recovery and carbon storage of shale reservoirs by using CO₂ injection. Shale nanopores with varying SiO₂ and kerogen contents are developed through molecular simulation to reveal the multicomponent diffusion and molecular-level interaction mechanisms interactions of CO₂ with light, medium, and heavy hydrocarbons in the presence of water, asphaltenes and methylbenzene across immiscible and miscible states. Compared to the immiscible condition, the diffusion coefficients of CO₂, methane, pentane, octane, and tetradecane in the miscible condition decrease by 48.46%, 77.68%, 39.16%, 51.26%, and 58.89%, respectively, while the diffusion coefficients of methylbenzene and asphaltenes increase by 16.96% and 33.15%, respectively. The volume expansion of crude oil and the reduction in viscosity caused by Oil–Gas miscibility facilitate the multicomponent diffusion of heavy components. As organic content in nanopores increases, the interaction between the wall and light hydrocarbons weakens, while heavy hydrocarbons tend to adsorb onto the wall. The diffusion coefficients of crude oil in the nano pores are decreased by 20% due to the presence of water. The study concludes that miscible CO₂-EOR significantly enhances shale oil recovery efficiency through the complex interactions of CO₂ and reservoir fluids in nanopore.