Microscopic production characteristics and fluid behavior of shale oil in CO2–cosolvent SAG process: Insights into EOR and carbon storage

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Non-equilibrium molecular dynamics method was employed to simulate dimethyl ether (DME) and ethyl acetate (EA) assisted CO 2 flooding, huff-n-puff, and SAG in organic–inorganic composite nanopores. The mechanisms and performances of enhanced oil recovery (EOR) and CO 2 storage (CS) under cosolvent influence were compared across various injection methods. Finally, the applicability of CO 2 -cosolvent SAG was assessed based on oil compositions, nanopore diameters, and reservoir depths (under miscible conditions). The results indicate that increasing polar component content, smaller nanopore diameters, and greater reservoir depth reduce free-phase oil proportion while increasing adsorbed and dissolved oil proportion. During SAG, early soaking ensures full CO 2 –oil contact, and later flooding provides a sustained driving force. Besides, adsorbed oil shows optimal replacement, and relatively more CO 2 can be stored in adsorbed and dissolved states. DME primarily enhances CO 2 to carry oil molecules, whereas EA mainly promotes the competitive adsorption between CO 2 and oil near walls. Cosolvent polarity closer to oil enhances CO 2 –oil miscibility, and improves EOR and CS performances. The oil recovery and CO 2 storage rate (CSR) increase first and then stabilize with increasing nanopore diameter, but the continuous increase of the free CO 2 proportion is not conducive to long-term storage. CO 2 and oil transition from immiscible to near-miscible and then to miscible state with the increasing reservoir depth. EOR and CS performances first improve significantly and then slightly. Factors such as oil compositions, nanopore diameters, and reservoir depths cannot change the respective advantages of the two cosolvents in promoting CO 2 -EOR and CS.

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