Nanoconfinement effect on the miscible behaviors of CO2/shale oil/surfactant systems in nanopores: Implications for CO2 sequestration and enhanced oil recovery

Currently, CO₂ flooding is the most promising carbon capture, utilization, and storage (CCUS) technology in the energy industry. Understanding the nanoconfinement effect on the CO₂-oil miscible process is crucial for accurately determining the minimum miscibility pressure (MMP) of CO₂/oil in shale reservoirs. In this study, we conducted molecular dynamics (MD) simulations to investigate the effects of pore size, surfactants, and pore type on the MMP of nanoconfined CO₂/shale oil/surfactant systems. Validations against experimental data show a deviation of 2.98 % in the CO₂ MMP. The MMPs under nanoconfinement are found to be significantly lower than those in bulk phase conditions (up to 22.94 %). The simulation results reveal that decreasing pore size can enhance the miscibility of CO₂ and oil by increasing the CO₂ adsorption ratio, improving CO₂-surfactant interactions, and inhibiting the tendency of CO₂ molecules to self-aggregate. The enhancement of CO₂-oil miscibility caused by surfactants is ranked by CFP > SF > SDS according to the mixing degrees (D_mix) and the spatial distribution of CO₂ around surfactant molecules. In addition, pore type exhibits various abilities in influencing the MMP, owing to their different mineral surface properties and ability to influence CO₂-surfactant interactions. Nanopores with stronger hydrophobicity and a denser CO₂ distribution around surfactant molecules have lower MMPs. The results show that the order of MMP in terms of pore types is Quartz < Kaolinite < I/M clay. This study elaborates the micro-mechanisms of surfactant-assisted CO₂-oil miscibility under nanoconfinement, offering valuable insights for effectively designing CO₂ miscible flooding in shale oil reservoir development.