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Ultralow oil−water interfacial tension (IFT) has provided an important basis for screening optimum surfactant formulation for improving the oil washing efficiency. Thus, it is of great significance to further investigate the selection method for surfactant systems with ultralow IFT. In this study, a selection of surfactant systems with ultralow IFT was simplified by a method of comparing the equivalent alkane carbon number (EACN) of crude oil with the minimum alkane carbon number (n min ) of surfactant mixtures. The results show that the ultralow IFT can be achieved when the n min of optimum surfactant formulation is equal to the EACN of crude oil. Meanwhile, the oil washing efficiency experiments show that the oil washing efficiency increases with the decrease of IFT, and the optimum surfactant formulation with ultralow IFT has the highest oil washing efficiency. This study provides a more efficient way for selecting optimum surfactant formulation systems with ultralow IFT for improving the oil washing efficiency.

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