|Oil viscosity is reduced dramatically with dissolving CO2 in oil. The overall reduction of viscosity depends on the initial viscosity, where there is greater reduction for higher viscous crudes. Reducing oil viscosity increases relative permeability of oil and reduces residual oil saturation. Lefebvre du Prey was shown that decreasing oil viscosity increases end point relative permeability of oil, but has no effect on relative permeability ratio.
Oil permeability is higher in CO2 injection compared to N2 injection. Interfacial tension and viscosity reduction, and oil swelling are mechanism that accounts for oil relative permeability improvement. These causes the recovery factor of CO2 injection becomes higher than N2 injection.
Recovery factor in CO2 injection is higher due to swelling effect of CO2 and lower interfacial tension result in lower residual oil saturation. Also sooner breakthrough was seen in N2 injection due to higher gas relative permeability. Thus Gas production and gas-oil ratio is higher in N2 injection compared to CO2 injection. The comparison of gas permeabilities shows that at the start of injection, N2 and CO2 relative permeability are equal, but as the gas saturation increased, N2 relative permeability becomes higher than CO2 relative permeability, because single phase flow occurs sooner in N2 injection due to higher residual oil saturation. This causes the gas production to become higher in N2 injection.