Macroscopic and Microscopic Flow Characteristics in CO ₂ –Oil–Water Three-Phase System: Effect of Initial Oil Saturation and Injection Velocity

The dual combination of CO 2 -enhanced oil recovery and geological storage is an important way to achieve carbon neutrality. The process involves a complex CO 2 –oil–water three-phase displacement. The microscopic flow characteristics of three-phase displacement in the reservoir under the interaction of initial oil saturation and injection velocity were studied using digital rock simulations based on a CT-scanned pore geometry, and the displacement efficiency and CO 2 storage volume under different conditions were analyzed. The results indicate that the CO 2 displacement pattern exhibits strong viscous fingering characteristics. A larger dimensionless fingering width and a smaller fingering aspect ratio were found to be correlated with a higher CO 2 sweep efficiency. The number of water clusters is the highest, followed by oil clusters, and CO 2 clusters are the least. Moreover, the proportion of small oil and water clusters exceeds 80%, indicating that most of the displaced fluid remains in an isolated state, which restricts the overall displacement efficiency and storage potential. Furthermore, both oil displacement efficiency and CO 2 storage volume peak at an intermediate initial oil saturation. While increasing the injection velocity reduces the CO 2 storage volume at breakthrough, it enhances the ultimate CO 2 storage volume. This work elucidates three-phase displacement behavior from the pore scale to the macroscopic scale, providing a theoretical foundation for CO 2 geological storage engineering.