Numerical analysis of water-alternating-CO2 flooding for CO2-EOR and storage projects in residual oil zones

Residual oil zones (ROZs) have high residual oil saturation, which can be produced using CO₂ miscible flooding. At the same time, these zones are good candidates for CO₂ sequestration. To evaluate the coupled CO₂-EOR and storage performance in ROZs for Water-Alternating-CO₂ (WAG) flooding, a multi-compositional CO₂ miscible model with molecular diffusion was developed. The effects of formation parameters (porosity, permeability, temperature), operation parameters (bottom hole pressure, WAG ratio, pore volume of injected water), and diffusion coefficient on the coupled CO₂-EOR and storage were investigated. Five points from the CO₂ sequestration curve and the oil recovery factor curve were selected to help better analyze coupled CO₂-EOR and storage. The results demonstrate that enhanced performance is observed when formation permeability is higher and a larger volume of water is injected. On the other hand, the performance diminishes with increasing porosity, molecular diffusion of gas, and the WAG ratio. When the temperature is around 100 °C, coupled CO₂-EOR and storage performance is the worst. To achieve optimal miscible flooding, it is recommended to maintain the bottom hole pressure (BHP) of the injection well above 1.2 minimum miscibility pressure (MMP), while ensuring that the BHP of the production well remains sufficiently high. Furthermore, the tapered WAG flooding strategy proves to be profitable for enhanced oil recovery, as compared to a WAG ratio of 0.5:1, although it may not be as effective for CO₂ sequestration.