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 CO 2 (sc-CO 2 ) miscible flooding as an enhanced oil recovery (EOR) method has been successfully used in both sandstone and carbonate reservoirs. The sc-CO 2 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-CO 2 displacing remaining oil and water, characteristic parameters such as the water ( K rw ) and miscible phase ( K rm ) relative permeability curves and wetting behavior are required, which applies to reservoir numerical simulation for predicting production performance of sc-CO 2 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-CO 2 dissolving into oil) system. In addition, relative permeability reduction and the change of wetting behavior of core plug after sc-CO 2 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-CO 2 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-CO 2 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-CO 2 are extremely low, which are in the range of 1.57–5% after sc-CO 2 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-CO 2 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-CO 2 miscible injection process. There is an obvious influence of water and miscible phase relative permeability when the Corey exponents, N w and N m, are changed.