ABSTRACT
Surfactant is used to extract remaining relatively immobile oil after completing the water or polymer-water flooding. Surfactant creates a mobile micro-emulsion phase and oil bank ahead of that. Depending on the salinity of the brine, three different types of phase environments such as Type II(−), Type II(+) and Type III can form. We considered a situation where water flooding was performed in water-wet reservoir before surfactant flooding. The viscous fingers due to immiscible viscous instability between oil and water during water flooding resulted in water channels and immobile patches of oil in the reservoir. Thus a heterogeneous distribution of residual oil saturation field was created. We took that heterogeneous oil saturation field as the initial condition for our numerical simulations. Since viscosity of micro-emulsion phase is lower than oil but higher than water, viscous instabilities are likely to happen. Past studies did not consider heterogeneous oil saturation field and did not model viscous instability during surfactant flooding. We numerically simulated all three types of surfactant flooding in one-quarter of five-spot setup using high-resolution computational grid. We discussed different phase behaviors and viscous fingering patterns for surfactant flooding with different salinity of the brine. For Type II(−), Type II(+) and Type III surfactant flooding, we compared the oil recovery and explained the difference with respect to phase behaviors and viscous fingering patterns. We found that immiscible viscous instability behind oil bank was most detrimental for Type II(−) surfactant flooding. However the oil content in the oil bank was the lowest for Type III flooding. Our studies showed the highest oil recovery by Type II(+) flooding and lowest oil recovery by Type III flooding. We demonstrated that injection of polymer could suppress the growth of fingers behind the oil bank. The increase of oil content in oil bank with polymer injection concentration was an additional advantage. Therefore oil production rate can be increased by injection of sufficient polymer during surfactant and polymer (SP) flooding. We performed quantitative comparisons of oil recovery for Type II(−) and Type II(+) flooding for different injection of concentration of polymer along with surfactant.
