CO2 flooding is considered as one of the most effective enhanced oil recovery (EOR) methods in low-permeability reservoirs. In our work, we studied CO2 miscible/immiscible flooding in low-permeability sandstones, using nuclear magnetic resonance (NMR) and volume of fluid (VOF) method. The experimental results indicated that the oil recovery after CO2 miscible flooding is 68.13%, which is twice as much as the one after CO2 immiscible flooding; oil in large pores is mainly displaced in the process of CO2 immiscible flooding, whereas in the case of CO2 miscible flooding, the oil comes from all kinds of pores. On the basis of VOF simulation results, it was fond that oil recovery after CO2 miscible flooding is also two times the one after CO2 immiscible flooding, which are dependent on the characteristic of CO2-oil contact. Moreover, oil recovery of CO2 miscible/immiscible flooding significantly decreased with the increase of oil viscosity. The interesting observation is that piston displacement happened at the injection part and finger displacement did at the production part during CO2 miscible flooding. In the end, we found that CO2 storage rate of miscible flooding is higher than that of immiscible flooding, and CO2 storage rate also significantly decreased with the increase of oil viscosity.
Keywords miscible/immiscible, NMR, VOF, oil recovery, CO2 storage