Effects of the water-oil volume ratio and premixing or pre-equilibration on the interfacial tension and phase behavior of biphasic mixtures

The interfacial tension (IFT) between an aqueous surfactant solution and displaced crude oil in a tertiary oil recovery process is the most important parameter affecting the oil mobilization. The key question, which has not been addressed previously in a systematic way, is which IFT is the most relevant to the oil recovery: the un-pre-equilibrated IFT, or the IFT obtained after the aqueous solution has equilibrated, totally or partially, with the oil. The un-pre-equilibrated IFT values may be close to the IFT values of oil/water during the initial stages of the oil recovery process before phase equilibration. Conversely, the pre-equilibrated IFT values may be close to the IFTs during the later oil recovery stages, because then the phases are closer to equilibrium. Here, we compare the IFT behavior in the laboratory at 24 degrees C and at 1 atm of un-pre-equilibrated and pre-equilibrated systems of a crude oil, a brine containing eight salts, and a commercial surfactant. The synthetic brine used is similar to the one present in an actual oil reservoir. The surfactant used here is a commercial anionic surfactant, PETROSTEP (R) S-13D HA, which is the sodium salt of a single-extended-isopropoxylated-chain sulfate. The pre-equilibrated equilibrium IFT (EIFT) values are quite different from the un-pre-equilibrated EIFT values. In addition, the effects of three mixing methods and the water-to-oil volume ratio (WOR) on the pre-equilibrated IFT were evaluated. Of the three methods examined here, (A) mild mixing, (B) magnetic stirring, and (C) shaking vigorously by hand, method C combined with centrifugation is the best method to evaluate the phase behavior and EIFT of premixed systems and produces mixtures that are the closest to the equilibrium state. For method C, the surfactant concentration in the aqueous layer after equilibration was the lowest due to surfactant partitioning into the oil phase. Moreover, the WOR affects the pre-equilibrated EIFT in brine systems significantly because of the different proportions of surfactant components that partition into the oil phase. For the surfactant at an initial concentration of 0.8 g.mL(-1) in the aqueous phase, as the WOR decreases from 2.33 to 0.43, or as the oil volume fraction in the mixture, phi, increases from 0.3 to 0.7, the surfactant concentration in the aqueous layer drops to the range from 5.9 g.mL(-1) to 2.2 g.mL(-1), and the EIFT increases by a factor of (similar to)70. In addition, the pre-equilibrated EIFTs are different from the un-pre-equilibrated EIFTs at the same surfactant concentration in the aqueous layer evidently because of preferential partitioning of the various surfactant components. This phenomenon can be accounted for with a simple two-phase extraction model, if the more surface-active component partitions preferentially in the oil. Therefore, the effects of the mixing method and centrifugation, the WOR, and the preferential partitioning of surfactant components into the oil phase should be evaluated for general surfactant screenings for uses in EOR applications.