In order to facilitate the mobilization of trapped oil in porous reservoirs by injecting a surfactant solution for enhanced oil recovery (EOR), it is critical to have low (< 1 mN·m
-1) or ultralow (< 10
-2 mN·m
-1) interfacial tension (IFT) values between a crude oil and the surfactant aqueous solution even after significant interphase mass transfer has occurred. We report on the IFT and phase behavior of pre-equilibrated mixtures of aqueous surfactant solutions and a crude oil. The surfactant used here is a commercial anionic surfactant, PETROSTEP S-13D HA, which is a single-extended-isopropoxylated-chain sodium sulfate salt. The synthetic brine used is similar to the one present in an actual oil reservoir. It contains 9,700 ppm of NaCl and NaHCO
3, primarily, with smaller concentrations of CaCl
2, MgCl
2, BaCl
2, KCl, Na
2SO
4, and MnCl
2 and an ionic strength of 150 mM; see Chung, J.; Boudouris, W.B.; and Franses, E.I
Colloids and Surfaces A: Physicochemical and Engineering Aspects 2018,
537, 163-172. The low-viscosity crude oil samples used were prepared from oil produced from an oil reservoir. The apparent phase behavior at 22 ± 2 °C of surfactant/water/crude oil mixtures with water and with brine, and the dynamic interfacial tensions (DIFTs) and the equilibrium interfacial tensions (EIFTs) were found to depend strongly on the methods used for stirring and equilibrating the biphasic dispersions and on the water-to-oil volume ratio (WOR) used. The initial surfactant concentration in the aqueous solution was 8,000 ppm in both cases. For all systems, two phases were observed, and there was no evidence of a third middle-phase microemulsion phase that is often reported for such systems. The stirring methods used here were (a) mild mixing by turning the sample vials upside down and bringing them back to its upright position; (b) magnetic stirring; and (c) shaking vigorously by hand. With water, the EIFTs at 24 °C for the pre-equilibrated aqueous phase against the pre-equilibrated oil phase were about 1 mN·m
-1 for all three stirring methods, and were similar to the un-pre-equilibrated EIFT. However, with brine solutions, there was more surfactant transfer from the aqueous phase to the oil phase with the method c than with methods a and b. The pre-equilibrated EIFTs were higher, or much higher, for WOR=1 than the un-pre-equilibrated EIFT of 0.014 mN·m
-1. They were 0.4 mN·m
-1, 0.04 mN·m
-1, and 0.020 mN·m
-1 for methods c, b, and a, respectively. These results indicate that the mixtures were closer to the phase equilibrium with method c. Moreover, as the WOR varied from 2.33 to 0.43, the pre-equilibrated EIFTs with method c increased from 0.016 mN·m
-1 to ~2 mN·m
-1. These changes can be attributed to the substantial partitioning of the surfactant from the aqueous phase to the oil phase. The surfactant concentration at the aqueous layer, which was 8,000 ppm before stirring, decreased from 8,000 ppm to the range of 6,000 to 2,100 for the range of WORs of 2.33 to 0.43. The results indicate that the pre-equilibrated EIFT values can be quite different from the un-pre-equilibrated EIFTs. The reason is not simply the decrease in the total surfactant concentration in the equilibrated aqueous phase, because the EIFTs of the un-pre-equilibrated systems at lower surfactant concentrations are also quite low (Chung et al. 2018). The actual reason is rather that changes in the relative concentrations of the various components in the aqueous and the oil phases lead to significant changes in the two coexisting phases, and hence to major EIFT changes.
The results have significant implications for screening surfactants for use in EOR processes. The results on the mixing methods effects suggest that caution is needed in conducting and interpreting phase behavior experiments in a laboratory setting, and then predicting the phase behavior in flow tests A single EIFT cannot describe adequately the interfacial tension behavior of a mixture of fixed initial concentration of a surfactant and a fixed salinity, and in fact may lead to erroneous conclusions or choices. A spectrum of EIFTs actually characterizes a certain formulation. As the aqueous solution that is injected into a reservoir comes into contact, and undergoes potential phase equilibration, with increasing amounts of oil as it flows through a porous medium, the effective WOR decreases. Then, the IFT and the partitioning of the various components in the various phases, those present initially or those formed thereafter, can change continuously. These phenomena must be taken into account in designing EOR processes, or in assessing core-flood and field tests.
