Relative Permeability Characteristics and Wetting Behavior of Supercritical CO₂ Displacing Water and Remaining Oil for Carbonate Rocks at Reservoir Conditions

Relative permeability characteristic and wetting behavior of reservoir rocks are crucial for oil recovery. Supercritical CO₂ (sc-CO₂) miscible flooding as an enhanced oil recovery (EOR) method has been successfully used in both sandstone and carbonate reservoirs. The sc-CO₂ is miscible with the remaining oil left after water flooding at injection pressures above MMP, and then higher recovery can be achieved. To describe the flow characteristics and performance of sc-CO₂ displacing remaining oil and water, characteristic parameters such as the water (Kᵣw) and miscible phase (Kᵣₘ) relative permeability curves and wetting behavior are required, which applies to reservoir numerical simulation for predicting production performance of sc-CO₂ miscible injection. Surprisingly, publications of experimental data including water and miscible phases are relatively rare due to the lack of proper experimental methods in laboratory. In this paper, we proposed a modified method based on the Corey model to calculate water and miscible phase relative permeability using endpoint values of oil/water system and water/miscible phase (sc-CO₂ dissolving into oil) system. In addition, relative permeability reduction and the change of wetting behavior of core plug after sc-CO₂ miscible injection were evaluated. Four core flooding experiments were carried out on carbonate composite cores using live oil at reservoir conditions. The experiments included seawater injection and sc-CO₂ injection for each core plug to obtain the endpoint values from both injections. The Corey model was used directly to calculate oil/water relative permeability of the carbonate composite cores. A modified Corey model proposed in this paper was used to calculate water and miscible phase relative permeability and obtain the relationship of relative permeability vs miscible phase saturation. The co-flow characteristics in water and miscible phase system were described using these endpoints and relative permeability curves. As a result, relative permeability to water and miscible phases can be calculated using the modified Corey model based on endpoint values during co-flow of water and miscible phases in core plug. The evaluation of relative permeability reduction of core plugs was made by comparing endpoint relative permeability of sc-CO₂ at residual state of water/phase phase system with that of water at residual oil saturation of oil/water system. The values of endpoint relative permeability to sc-CO₂ are extremely low, which are in the range of 1.57–5% after sc-CO₂ injection. The wetting behavior had slightly changed by observing photographs of water droplets and oil droplets on the surface of core plugs before and after sc-CO₂ injection. The relationship between relative permeability to water and miscible phases vs miscible saturation has been developed when the water saturation is decreasing during sc-CO₂ miscible injection process. There is an obvious influence of water and miscible phase relative permeability when the Corey exponents, Nw and Nₘ, are changed.