Macroscopic and Microscopic Flow Characteristics in CO2-Oil-Water Three-Phase System: Effect of Initial Oil Saturation and Injection Velocity.

The dual combination of CO2-enhanced oil recovery and geological storage is an important way to achieve carbon neutrality. The process involves a complex CO2-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 CO2 storage volume under different conditions were analyzed. The results indicate that the CO2 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 CO2 sweep efficiency. The number of water clusters is the highest, followed by oil clusters, and CO2 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 CO2 storage volume peak at an intermediate initial oil saturation. While increasing the injection velocity reduces the CO2 storage volume at breakthrough, it enhances the ultimate CO2 storage volume. This work elucidates three-phase displacement behavior from the pore scale to the macroscopic scale, providing a theoretical foundation for CO2 geological storage engineering.