Synergistic Conformance Control in Fractured Reservoirs Using CO ₂ -Responsive Gel and Foam during CO ₂ Flooding

In fractured reservoirs, strong heterogeneity often promotes early CO2 breakthrough along dominant flow paths, significantly reducing sweep efficiency and oil recovery. A staged injection strategy combining a CO2-responsive gel (CRG) and a CO2-responsive foam (CRF) can synergistically seal high-permeability channels and enhance displacement efficiency. In this work, two CRGs (CR-DDN and CR-DAN) were synthesized via free-radical polymerization of N-(3-(dimethylamino)propyl)methacrylamide, acrylamide, N,N-dimethylacrylamide, and N-vinylpyrrolidone. At 80 °C, their swelling ratios reached 39 and 30 °C after CO2 exposure, respectively. Rheological analysis and SEM imaging indicated that the layered-stacking morphology of CR-DDN provided greater structural integrity than the irregular sponge-like network of CR-DAN. Correspondingly, at 80 °C, the elastic modulus of CR-DDN (3518 Pa) was approximately 3.5 times that of CR-DAN (993 Pa). Core-scale EOR tests in artificial fractured cores treated with CR-DDN showed that the gel not only restricted gas mobility by sealing fractures but also contributed to residual oil mobilization through its viscoelastic properties, yielding incremental recoveries of 14.5%–25.5% during gel injection depending on matrix permeability. Incorporating CRF further enhanced the CO2-breakthrough control. Compared with coinjection, two surfactant-alternating-gas (SAG) cycles established a more sustained resistance plateau and increased the ultimate recovery to 76.91%. These findings provide guidance for designing robust CRGs and demonstrate the potential of staged foam-gel treatments to enhance the CO2 flooding performance in fractured reservoirs.