Surfactant-assisted carbon dioxide huff-and-puff for enhanced recovery of Jiyang shale oil: interfacial regulation, miscibility enhancement, and pore-scale mobilization

Shale oil recovery is constrained by strong capillary forces, restricted matrix transport, and insufficient carbon dioxide (CO2)–oil contact in confined pores. This study evaluates sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-assisted CO2 huff-and-puff for wax-rich Jiyang shale oil under representative reservoir conditions. High-temperature and high-pressure experiments were conducted to characterize phase behavior, fluid properties, CO2 solubility, interfacial tension, minimum miscibility pressure (MMP), hydrocarbon extraction, core-scale recovery, and nuclear magnetic resonance (NMR) transverse relaxation time (T2). Compared with pure CO2, the AOT-assisted CO2 system increased measured CO2 solubility in oil–water systems by 32%–38%, reduced CO2–oil and oil–water interfacial tensions, enhanced light-hydrocarbon extraction, and lowered the MMP from 30.77 to 28.74 MPa. The oil recovery factor increased from 6.24% to 10.71%, whereas CO2 containing ethanol alone achieved 6.86%. NMR T2 analysis indicated greater mobilization of oil in small and strongly confined pores. The novelty of this work lies in integrating evidence from fluid-phase behavior, interfacial properties, compositional analysis, core-scale recovery, and NMR to link system-level changes to pore-scale oil mobilization.