The Influence of Injection Modes on CO2 Flooding and Storage in Low-Permeability Reservoirs

Low-permeability reservoirs have poor reservoir properties and are difficult to develop by conventional water flooding. CO2 flooding can significantly improve oil recovery while achieving carbon storage, and is widely recognized as an effective solution for the development of low-permeability oil reservoirs. In order to address the lack of a comparative quantitative analysis of the tradeoff between oil recovery factor, CO2 storage rate, and total CO2 storage volume for the main injection modes in low-permeability reservoirs, this study systematically evaluated the performance of CO2-enhanced oil recovery (EOR) and geological storage under different pressures and injection modes through core flooding experiments. The results indicate that displacement pressure and injection strategy significantly influence the CO2 flooding performance. Continuous miscible flooding (30 MPa) substantially reduced the displacement pressure differential (maximum 6.1 MPa) and achieved the highest oil recovery (78.96%) and the greatest CO2 storage capacity (5916 cm3). Miscible WAG flooding effectively delayed gas breakthrough (extended to 1.90 pore volumes), homogenized the displacement front, and yielded the best overall outcome: the highest ultimate oil recovery (83.8%) coupled with the optimal CO2 storage rate (89.1%). The study further reveals that the pre-breakthrough stage is critical for contributing to oil recovery and achieving efficient storage, regardless of the injection modes. These findings clarify the technical characteristics and applicable conditions of different injection modes, providing crucial theoretical insights and practical guidance for optimizing CO2 EOR and storage projects in low-permeability reservoirs.