(465c) A New Surfactant Evaluation Protocol Based on Interfacial Tension, Phase Behavior, and Lab-Scale Coreflood Tests for Chemical Enhanced Oil Recovery (cEOR) Applications

The robust screening and evaluation of surfactant formulations are critical for accelerating the design of chemical formulations, because laboratory-scale evaluation processes usually require much less time and resources compared to field tests. Conventional laboratory-scale surfactant evaluation processes include phase behavior tests of surfactant brine solution and oil mixtures, followed by laboratory-scale core-flood tests with either model core materials or actual reservoir rock samples. It is generally assumed that for a formulation to be most effective the interfacial tensions (IFTs) between the aqueous solution and the oil phase, and other possible phases present, such as middle-phase microemulsions, should be ultralow (< 0.01 mN·m^-1). In many studies, however, the IFT values are not measured or reported, and are often estimated based on phase behavior results only. Here, we describe a robust protocol for screening chemical formulations including experimentally determined IFT data. The first step involves the measurement of the surfactant solubility in the specific brine solution in order to use solutions below the surfactant solubility. Here “surfactant” is meant one or two products, and possibly include a “co-surfactant” or an alcohol. The second step involves determining reliable equilibrium IFT (EIFT) values between the aqueous phases and the specific oil or hydrocarbon of interest. Some additional steps using not only surfactant but also a polymer to control the solution viscosity may be added later to make the protocol more comprehensive. In our two previous publications (Chung, J.; Boudouris, B.W.; Franses, E.I. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2018, 537, 163-172 and Chung, J.; Boudouris, B.W.; Franses, E.I. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2019, 571, 55-63), we defined two types of EIFTs: the “un-pre-equilibrated IFT” (EIFT_up) and the “pre-equilibrated IFT (EIFT_p)”. One can find the exact definitions and the protocols for their experimental determination in these articles. The EIFT_up is the EIFT of the aqueous surfactant/brine solution against an oil drop, and the equilibrium interfacial tension is established prior to the equilibration of the oil and aqueous phase components among the two phases. After such equilibration is accomplished, the EIFT_p is established. To accelerate such overall phase equilibrium at laboratory conditions, intensive mixing was applied. It was recently found that the two EIFTs can be significantly different for several examples, and that the EIFT_p can depend on the mixing protocol and the oil/water volume ratio. In all our data for these systems, the surfactant concentrations were quite low, about 1 wt%, and no third phase microemulsion was observed. Hence, for a given surfactant formulation, brine composition, and oil, there is no single EIFT that can fully represent a formulation. The results of the above two publications, for a specific extended anionic surfactant, and a specific reservoir oil and brine, and some more recent results with certain other formulations will be reviewed. For several examples of laboratory-scale core-flood tests with a Berea Sandstone^TM core, the oil recovery efficiency was poor even though the EIFT_up values were ultralow, and probably because the EIFT_p values were high. Therefore, it is critical to find surfactants or surfactant formulations that have low or ultralow EIFT_up and EIFT_p. Then, such chemical formulations can be used to predict and design efficient EOR recovery processes for field tests.