Experiments on Three-Phase Relative Permeability in CO2 Flooding for Low Permeability Reservoirs

Abstract The three phase relative permeability is one of the most crucial parameters in reservoir engineering. Generally, experimental oil samples used in three phase flow are pure hydrocarbon samples instead of crude oil. Experiments are mostly carried out at low pressure with no chance of miscibility between the gas and oil phases. Because three phase lab data obtained by the steady-state method is expensive and time consuming, it is more common to use unsteady-state method to acquire two phase data. Many methods have been developed to calculate three phase relative permeability from two phase curves. However, there are no well recognized methods to perform the calculation, especially for carbon dioxide flooding due to the strong interaction among CO2, brine and oil phases. Diffusion coefficient and solubility of CO2 in crude oil and brine were measured. Furthermore, all parameters including viscosities, densities, volume factors and interfacial tensions of three phases in CO2 flooding were obtained. On the basis of these parameters, JBN method was revised to include the effects of CO2 on brine and oil. Relative permeabilities of oil and water phases were measured at conditions of oil and water two phases. Oil and gas relative permeabilities at different reservoir pressures were obtained at oil and gas phases, which show the effects of interfacial tension on relative permeabilities. Furthermore, Corey method and Stone Ⅱ method were improved and applied to calculate three phase relative permeability. Mathematic models of three phase relative permeability were proposed. The results show that oil phase isoperms is concave to 100% oil saturation. Water phase isoperms and gas phase isoperms are straight lines. Additionally, the scope of oil phase isoperms indicates that three phase flow zone is very narrow in CO2 flooding for low permeability reservoirs. The displacement effect is related to the saturation path, which contributes to oil recovery. All the production parameters can be illustrated through the ternary diagram combining three phase isoperms and saturation path change.