Dynamic Characteristics and Storage Potential of Water-Alternating-CO₂ in Low Permeability Reservoirs

Oil reservoirs in China, largely characterized by low permeability and heterogeneity, face challenges with conventional water flooding, such as rising water cuts and declining recovery rates. Over 60% of reserves are in low and ultralow permeability reservoirs. To improve oil recovery and CO2 storage after water flooding, water-alternating-gas (WAG) injection is gaining attention. However, uncertainties remain regarding the effects of water cuts on minimum miscibility pressure (MMP) and the influence of slug ratios and sizes on oil recovery and storage efficiency during WACO2 injection. This study addresses these issues in the L block of the Jiangsu oilfield through slim tube tests, long-core flooding, and online NMR experiments. Key findings include: water cuts had minimal impact on MMP but affected oil recovery under nonmiscible conditions. The optimal CO2 displacement and storage synergy occurred with a slug size of 0.1 PV and a slug ratio of 1:1, though a 1.5:1 ratio was also viable. Online NMR results showed that most recoverable oil can be extracted under miscible conditions, while some oil migrated under immiscible conditions, recoverable with increased pressure. CO2 override and gravity separation were absent during WACO2 but observed in continuous CO2 injection. This research provides valuable insights into the feasibility of CO2 flooding, the impact of water cuts on miscibility, and the recovery mechanisms during WACO2 injection, offering guidance for enhanced oil recovery in low-permeability reservoirs.