Interfacial Tension and Phase Behavior in Surfactant-Brine-Oil Systems
Authors: Anderson D.R. , Bidner M.S. , Davis H.T. , Manning C.D. , Scriven L.E. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., March 22–24, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
In surfactant and micellar flooding processes the surface-active agents are injected in a slug at concentrations generally in excess of two weight percent. At front and rear of the slug, dilution lowers the average concentration continuously to zero. At various compositions within this range, systems of surfactants, brine and oil exist as one, two, or sometimes three or more phases at equilibrium. Low interfacial tensions phases at equilibrium. Low interfacial tensions are observed in two distinct regimes:in the brine-oil interface at low surfactant concentrations; andin the brine-microemulsion and microemulsion-oil interfaces over ranges of higher concentrations.
The relationship of two phases at low concentrations to three phases at higher concentrations is demonstrated by finding the 'bottom' tie line of the three-phase region in sample systems. These contain either the nonionic, dodecyl phenoxy polyethylenoxy ethanol, or one of the anionics polyethylenoxy ethanol, or one of the anionics monoethanolamine salt of dodecyl orthoxylene sulfonate and sodium salt of a sulfonated petroleum fraction.
With the nonionic surfactant it is demonstrated that both regimes of low tension can occur in the same system at different surfactant concentrations. This raises questions about interpretation of results of recovery processes designed to operate on low tension in one regime or the other.
The situation is summarized with ternary diagrams, within which a few features constitute indices to phase behavior and interfacial tension. Possibilities for simulating these diagrams are Possibilities for simulating these diagrams are discussed.

Introduction
The key to mobilizing residual oil in surfactant and micellar flooding processes is attainment of interfacial tensions less than 0.01 dyne/cm. Two distinct regimes of low tension have been found. At surfactant concentrations well under 1 wt. % the brine-oil interface may display a low minimum of tension. At moderate surfactant/cosurfactant concentrations above 1 wt. % there may be a surfactant-rich microemulsion phase and low tension at a microemulsion-brine interface or at a microemulsion-oil interface, or at both if both are present. An interface is between two phases and so at the crux are the conditions under which a brine-oil-surfactant system separates into two or more coexisting phases. Equilibrium separations must be understood before nonequilibrium situations can be fully identified and analyzed. At low surfactant concentrations two phases, brine and oil, coexist at equilibrium.