An experimental investigation into carbon dioxide flooding and rock dissolution in low-permeability reservoirs of the South China Sea

Low-permeability reservoirs in the western South China Sea hold substantial hydrocarbon reserves; yet, their development via water injection remains suboptimal due to challenges such as high CO 2 content (22–42%) in associated gas. To enhance recoverable reserves and unlock residual oil potential, this study investigates the interplay of gas–water–rock interactions during CO 2 flooding under extreme reservoir conditions: high temperature (140°C), high pressure (33 MPa), and significant dip angles (5–20°). Laboratory experiments and mechanistic analyses were conducted to evaluate phase-state transitions of crude oil, CO 2 -driven permeability enhancement, and recovery efficiency. Results demonstrate that CO 2 injection induces a phase shift from liquid-dominant to gas-dominant systems, elevating the gas–oil ratio and saturation pressure while expanding the gas–liquid phase boundary. Notably, steeper reservoir dip angles synergistically amplify recovery rates, achieving a peak efficiency of 88.8 at 80% CO 2 injection. Rock dissolution, driven by CO 2 –water interactions, selectively dissolves feldspar (45% plagioclase) and clay minerals (22% kaolinite), enlarging pore radii (0.8 → 1.2 μm) and increasing permeability by 10%. Dynamic displacement experiments further reveal a 10% rise in gaseous-phase permeability post-dissolution. These findings highlight the dual role of CO 2 flooding in enhancing oil recovery and enabling carbon storage, with annual sequestration potential projected at 2.5 × 10 4 tons. The study provides a technical framework for optimizing CO 2 -EOR strategies in offshore low-permeability reservoirs, offering critical insights for sustainable hydrocarbon extraction and carbon management in analogous high-temperature, high-pressure environments.