Synergistic Optimization of CO2 Water-Alternating-Gas Enhanced Oil Recovery and Storage Efficiency

The high-pressure and low-temperature conditions of deep-water environments provide favorable stability for the long-term sequestration of CO2, rendering Water-Alternating-CO2 (WAG) flooding a highly promising technology within offshore CCUS-EOR synergistic development schemes. This approach is considered a key pathway for achieving low-carbon offshore oil and gas production. However, the development of low-permeability, heterogeneous, and inclined reservoirs are strongly influenced by complex mechanisms such as gravity segregation, which makes CO2-WAG performance highly sensitive to operation parameters. In this study, a three-dimensional reservoir model was established to systematically verify the applicability of CO2 WAG under such geological conditions, and to investigate how key operational parameters affect both oil recovery and CO2 geological storage efficiency. Results indicate that appropriate parameters can enhance sweep efficiency by suppressing gravity segregation and delaying gas breakthrough in high-permeability layers. In addition, a longer residence time of CO2 within the reservoir contributes to improved geological storage efficiency, and a cycle duration of 6 months, a water-gas ratio of 3:1 and an injection rate of 30 m3/d achieve optimal performance in terms of enhanced oil recovery and CO2 sequestration. Moreover, this study further proposes dynamic optimization strategies such as adjusting the water gas ratio to improve CO2 utilization efficiency. The findings provide theoretical guidance and practical reference for injection scheme design and storage efficiency improvement in CCUS-EOR projects targeting low-permeability, heterogeneous and inclined offshore reservoirs.