Effect of CO2 Pre-Extraction on Waterflooding in Nanopores: Insights from Molecular Dynamics Simulations

Summary Shale oil primarily resides within low-porosity, low-permeability nanopores, where conventional waterflooding yields low recovery rates, impeding economic development. Although carbon dioxide (CO2) enhanced oil recovery (EOR) (CO2-EOR) offers potential benefits—including viscosity reduction, swelling, and promoted desorption—existing studies often rely on bulk fluid parameters. These studies generally lack a quantitative consideration of nano-confinement effects and mineralogical heterogeneity, failing to accurately quantify the relationship between CO2 concentration, desorption efficiency, and fluid mobility, thereby constraining the precise optimization of gas injection schemes. For this study, we used equilibrium and nonequilibrium molecular dynamics (MD) simulations to characterize quantitatively the density profiles, competitive adsorption behaviors, and rheological properties of shale oil/CO2 mixtures within quartz and kaolinite nanopores. Microscopic simulation results indicate that CO2 preferentially accumulates at pore walls, displacing adsorbed oil and significantly weakening oil/rock interactions. Nano-confinement increases fluid viscosity by a factor of two to three compared with the bulk phase, with distinct negative slip observed in kaolinite pores. As CO2 concentration increases to 20–40%, system viscosity decreases significantly, and boundary slip behavior improves, transforming the displacement front from a fingering pattern to a more efficient piston-like displacement. Building upon these findings, a multiscale coupling model integrating molecular simulation parameters with reservoir numerical simulation was developed to perform 3D field-scale predictions for the Gulong Sag. Macroscopic simulations reveal that predicted recovery factors incorporating nanoconfinement parameters (8–20%) align more closely with field data, whereas the use of bulk phase parameters significantly overestimates development performance. Sensitivity analysis further identifies distinct economic CO2 concentration windows for different minerals (>20% for quartz pores; 30–40% for kaolinite pores), verifying the necessity of differentiated injection strategies. The integrated multiscale approach established in this study bridges the gap in traditional models regarding microscopic interfacial effects. It provides a theoretical basis and parameter support for enhancing CO2 utilization efficiency and achieving synergistic optimization of CO2-EOR and carbon sequestration in unconventional reservoirs.