New Correlation for Optimizing the Surfactant Structure for EOR
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 154262, ’New Correlation To Predict the Optimum Surfactant Structure for EOR,’ by Sriram Solairaj, SPE, Christopher Britton, SPE, Jun Lu, SPE, Do Hoon Kim, SPE, Upali Weerasooriya, SPE, and Gary A. Pope, SPE, The University of Texas at Austin, prepared for the 2012 SPE Improved Oil Recovery Symposium held in Tulsa, 14-18 April. The paper has not been peer reviewed.
The oil-recovery efficiency of chemical enhanced oil recovery (EOR) depends on the phase behavior of the microemulsion and on interfacial tension (IFT). The surfactants needed to obtain good phase behavior and ultralow IFT vary greatly, depending on oil characteristics and reservoir conditions. Many new surfactants have been introduced with widely different structures and formulations for application to a wider range of oils and temperature. A new correlation has been developed that uses an extensive data set, taking into account the effects of the propylene oxide number (PON), ethylene oxide number (EON), temperature, brine salinity, and the equivalent alkane carbon number (EACN) of the oil. The new correlation can help identify the most important variables and improve our understanding of the relationship between the variables that affect the optimum surfactant structure.
Introduction
There was a need for an explicit correlation between the optimum surfactant structure and the most important formulation variables, including parameters for new-generation surfactants, to assist with understanding the complex relationship among important formulation variables and as a guide for efficient selection of the best surfactant structures to be evaluated rigorously in the laboratory for specific oils and conditions. Because the objective was to determine the relationship among the variables for an optimum surfactant structure, only experimental data for optimized formulations were used in the regression analysis. This point is fundamental and must be understood clearly. Many measurements are made for surfactant formulations that have unacceptable behavior (e.g., the IFT is too high). These data were not used to develop the new correlation. Only data for good formulations were used, clearly affecting the outcome of the results.
Microemulsion-Phase-Behavior Procedure and Screening Criteria
The experiments consisted of micro-emulsion-phase-behavior tests and aqueous-stability tests. Microemulsion-phase-behavior observations are a fast and effective method to screen hundreds of surfactant mixtures and to determine the effects of formulation variables such as salinity, temperature, EACN, surfactant concentration, cosolvent concentration, and water/oil ratio. Qualitative observations regarding microemulsion type, microemulsion viscosity, IFT, equilibration time, and other factors are made visually on all samples over a period of days or weeks, where-as quantitative measurements are made on only a few samples and only after equilibration of the most-promising formulations. Formulations that formed a low-viscosity microemulsion with ultralow IFT and that passed the aqueous-stability test were selected as optimum formulations. Sealed ampoules, at reservoir temperature, were observed for several days, weeks, or in some cases months to determine the stability of the chemicals. The surfactants used in this study included mixtures of several anionic surfactants, including internal olefin sulfonates, alcohol propoxy sulfates, large-hydrophobe Guerbet alkoxy sulfates, and large-hydrophobe Guerbet alkoxy carboxylates.