When CO2 comes into contact with crude oil a process of dissolution occurs thereby causing swelling. The degree of swelling depends on pressure, temperature and oil composition. Swelling is important for two reasons: Firstly, the residual oil saturation is inversely proportional to swelling factor. The residual oil saturation is an important point in relative permeability curves and determines ultimate recovery. Secondly, swollen oil droplets will force fluids out of the pores, creating a drainage process. This process causes the trapped droplets that cannot move under present pressure gradient, to move toward production well. Also effect of oil swelling on oil saturation is undeniable. The oil swelling increases oil saturation, therefore increases oil relative permeability, too. Overall effects of CO2 and N2 on IFT, oil viscosity and swelling factor are summarized.
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.
Last date updated on July, 2014