Optimum Formulation of Surfactant/Water/Oil Systems for Minimum Interfacial Tension or Phase Behavior

A screening test used to help select surfactant systems potentially effective for oil recovery is to identify those formulations that yield middle-phase microemulsions when mixed with sufficient quantities of oil and brine. A correlation is presented to link these variables regarding their contributions to middle-phase formation: structure of the sulfonated surfactant, alkane carbon number (ACN), and alcohol type and concentration. WOR and temperature effects are introduced as correction terms added to the empirical correlation. Sets of variables that give middle-phase microemulsions are shown as identical to those defining the low tension state without observable middle phases. This generally occurs for low surfactant concentrations. INTRODUCTION Healy and Reed and Healy et al. have shown that the phase behavior of surfactant/brine/oil systems is a key factor in interpreting the performance of oil recovery by microemulsion processes. By systematically varying salinity, they found low interfacial tensions and high solubilization of both oil and water in the microemulsion phase to occur in or near the salinity ranges giving three phases. Since both low interfacial tensions and a high degree of solubilization are considered desirable for oil recovery, the conditions for three-phase formation assume added importance. Similar conclusions have been reported in other recent papers. Several investigators have considered the effect of different variables on the range of salinities for which Original manuscript received in Society of Petroleum Engineers office Feb. 13, 1978. Paper accepted for publication Aug. 1, 1978. Revised manuscript received Jan. 31, 1979. Paper (SPE 7054) first presented at the SPE-AIME Fifth Symposium on Improved Methods for Oil Recovery, held in Tulsa, April 16-19, 1978. *Now at U. de Los Andes, Venezuela. *Now with British Petroleum Ltd. †Now with Lagoven S.A. 0037-9999/79/0004-7054$00.25 © 1979 Society of Petroleum Engineers of AIME three phases form. This optimum salinity (a more precise definition is given in a subsequent section) has been found to decrease with increasing surfactant molecular weight, and to increase with increasing chain length of the alcohol cosurfactant. Studies on the effect of alcohols by Jones and Dreher and Salter provided results similar to those reported by Hsieh and Shah. The interfacial tension at surfactant concentrations low enough so that a discernible third phase does not form has been the subject of considerable investigation regarding surfactant molecular weight and structure, oil ACN, salinity and surfactant concentration, and alcohol addition. A recent paper was a first attempt to tie together the low tension state observed at low surfactant concentrations and the three-phase region observed at higher surfactant concentrations. All indications point to an inextricable intertwining of phase behavior, surfactant partitioning, solubilization, and low tensions. This paper corroborates the equivalence of three-phase behavior and minimum tension as criteria for optimum formulation and presents a correlation that quantifies the trends observed previously.