Prediction of microemulsion phase behavior from surfactant and co-solvent structures




Chang, Leonard Yujya

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Structure-property models were developed to predict the optimum salinity, optimum solubilization ratio, and the aqueous stability limit from the molecular structures of surfactants and co-solvents used for enhanced oil recovery. The models are sufficiently accurate to provide a useful guide to experimental testing programs for the development of chemical formulations for enhanced oil recovery and other similar applications requiring low interfacial tension. This is the first time a structure-property model has been developed to predict the optimum solubilization ratio. The solubilization ratio can be used in the Huh equation to predict the interfacial tension, which is the most important property in enhanced oil recovery applications. The UTCEOR Database was constructed and used to develop the models. The database is a collection of highest-quality experimental chemical EOR data conducted at The University of Texas at Austin from 2005 to 2018. It contains several thousand phase behavior experiments using 34 unique crude oils, 294 unique surfactants, and 70 unique co-solvents. The structures of the surfactants and co-solvents were characterized and include variations in the type of hydrophobe (carbon number, degree of branching, polydispersity, and aromaticity), number of alkoxylate groups (propylene oxide and ethylene oxide), and the type of head group. The model focuses on blends of anionic surfactants and nonionic co-solvents. Both the optimum salinity and the optimum solubilization ratio were modeled as a function of monovalent and divalent cations in the brines. The oils were characterized using their equivalent alkane carbon number. The models include the effect of soaps generated from the neutralization of acidic crude oils. Previous models for optimum salinity have not included the effects of divalent cations, soap, and co-solvents among other limitations. Most importantly, the new model can be used to predict interfacial tension as well as optimum salinity whereas previous models were used to predict only optimum salinity. In this research, the structure-concentration and structure-property effect of co-solvents were modeled separately, whereas previous models convoluted both effects and were not predictive. New measurements were made and combined with literature data to develop improved correlations for the oil-water partition coefficient and the interface-water partition coefficient of co-solvents. These correlations were used with pseudophase theory to more accurately model the structure-concentration effect. A structure-property model was developed for the aqueous stability that predicts the coacervation of chemical formulations. The interactions between surfactant hydrophobes and the PO groups were modeled because they influence the stability of micelles. The effects of co-solvent, polymer, and divalent cations were included for the first time. The structure-property models can be used to predict formulations for a given oil, brine and temperature that are likely to achieve ultra-low IFT with aqueous stability at optimum salinity and thus greatly accelerate the process of finding the best formulations to test for chemical EOR.


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