To address the disadvangates associated with the use of alkali during surfactant flooding, such as reduction of polymer viscosity, formation scaling, corrosion hazards, and emulsion formed by the produced fluids, this study focuses on the formulation of surfactants without alkali for either very low salinity (<10,000 ppm TDS) or very high salinity (>100,000 ppm TDS) surfactant flooding. A series of laboratory tests were conducted to investigate the synergism among anionic, cationic, nonionic and zwitterionic surfactants for EOR purposes based on IFT and phase behavior measurements. All the surfactants used in this study are commercially available and no alkali is added for all cases. The results demonstrate that strong interactions between anionic/cationic and anionic/zwitterionic surfactants make them form highly active aggregates which behave more like nonionic surfactants and exhibit great salt tolerance capability. Ultra-low interfacial tensions (IFTs) with the order of magnitude of 10−3 mN/m are achieved by these mixtures. By adjusting the mixing ratio, they can be tailored easily to reservoirs with different salinities and solid surface charges. The anionic/zwitterionic surfactant mixtures at certain pH show exceptional performance, which are able to form middle microemulsion phase from as low as 0 till up to 180,000 ppm TDS salinity. The study also shows the addition of cosolvents (short chain alcohols) should be thoroughly justified during surfactant formulation because they can improve the surfactant solubility and speed phase coalescence. However, they may adversely afftect the water and oil solubilization capacity.

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