Pressure-Dependent Pore-Scale Remobilization of Remaining Oil by CO2 Flooding in Waterflooded Tight Sandstone

Waterflooding in tight sandstone reservoirs preferentially mobilizes oil in large, well-connected pores, leaving substantial remaining oil in medium–small and poorly connected pore spaces. In this study, specific Chang 6 tight sandstone cores from the Ordos Basin were investigated using high-temperature and high-pressure online NMR displacement experiments, combined with CO2–oil phase behavior tests and CO2–brine–rock interaction experiments. The minimum miscibility pressure of the crude oil–CO2 system was estimated to be 19.4 MPa based on the available slim-tube data. CO2 dissolution enhanced oil mobility through swelling and viscosity reduction, while CO2–brine–rock reactions increased core permeability by 18.24–25.55%. Compared with waterflooding, CO2 flooding increased average oil mobilization in large, medium, and small pores by 33.66%, 39.56%, and 22.20%, respectively. Medium pores contributed most to incremental recovery, whereas small-pore oil was activated mainly under higher pressure. These results indicate that CO2 flooding can enhance recovery in the tested waterflooded Chang 6 tight sandstone cores by improving crude-oil mobility, modifying pore-throat connectivity, and expanding displacement from large pores to medium–small pores.