Foaming systems for foam flooding with both high foaming performance and ultralow oil/water interfacial tension

Abstract

In foam flooding ultralow oil/water IFT and superior foaming performance are usually not compatible, because both are achieved by surfactants at different conditions. In this paper we constitute a mixed surfactant system which can achieve both ultralow oil/water IFT and high foaming performance concurrently by means of synergistic effects between different surfactants.Aiming at the reservoir conditions of Daqing oilfield, China (45 °C, total salinity 6,778 mg/L, pH = 8–9), a ternary surfactant mixture SDS/DTAB/APG at a mole fraction ratio of 0.36/0.24/0.4 was found to be an optimal surfactant system by which ultralow IFT and high foaming performance could be achieved concurrently at wide total concentrations (2–6.5 mM). Specifically, at a total concentration of 5 mM the IFT can be reduced to as low as 2.9 × 10-5 mN/m whereas the composite foaming index in the presence of crude oil can reach as high as 936 Ls. The SDS/DTAB binary mixture dominates the mixed monolayer at both air/water and oil/water interfaces as indicated by a high pseudo mole fraction of 0.94. The strong attraction between anionic/cationic surfactants endows densely packed monolayers which are responsible for the achievement of low surface tension and ultralow IFT, whereas the nonionic surfactant APG behaves as a solubilizer of the SDS/DTAB binary mixture in bulk as well as a stabilizer of the bubbles and emulsified oil droplets, which synergistically endows a high foaming performance to the ternary surfactant system. Based on the relevant principle more mixed surfactant systems may be found which are significant for improving efficiency of foam flooding.