Original manuscript received in Society of Petroleum Engineers office Sept. 20, 1977. Paper accepted for publication June 2, 1978. Revised manuscript received Aug. 2, 1978. Paper (SPE 6846) was presented at SPE-AIME 52nd Annual Fall Technical Conference and Exhibition, held in Denver, Oct. 9-12, 1977. Abstract Results of experiments on the coalescence of crude oil drops at an oil-water interface and interdroplet coalescence in crude oil-water emulsions containing petroleum sulfonates and cosurfactant as surfactant systems with other chemical additives were analyzed in terms of interracial viscosity, interfacial tension, interfacial charge, and thickness of the films surrounding the microdroplets. A qualitative correlation was found between coalescence rates and interfacial viscosities; however, there appears to be no direct correlation with interfacial tension. New insight has been gained into the influence of emulsion stability in tertiary oil recovery by surfactant/polymer flooding in laboratory core tests. We concluded that those systems that result in relatively stable emulsions yield poor coalescence rates and, hence, poor oil recovery, Introduction The ability of the surfactant/polymer system to initiate and to propagate an oil bank is the single most important feature of a successful tertiary oil-recovery process. The mechanisms of oil-bank formation and development are yet unknown. It has been suggested that without the initiation of the oil bank, the process behaves more like the unstable injection of a surfactant solution alone, where the oil is produced by entrainment or emulsification in the flowing surfactant stream. In a laboratory study of the initial displacement of residual hydrocarbons by aqueous surfactant solutions, Childress and Schechter and Wade observed that those systems that spontaneously emulsified and coalesced rapidly yielded better oil recovery than those systems that spontaneously formed stable emulsions. Recently, Strange and Talash, Whitley and Ware, and Widmeyer et al. reported results of Salem (IL) low-tension, water-flood tests that used Witco TRS 10-80 TM petroleum sulfonate surfactant solution. They found stable oil-in-water emulsions at the observer well in addition to emulsion problems at the production well and reported that problems at the production well and reported that actual oil recovery was about one-quarter the target value. These studies clearly suggested that poor efficiency of oil recovery results from emulsion stability problems in the low-tension surfactant or micellar processes. Vinatieri presented results of experiments on the stability of crude-oil-in-water emulsions that coo be produced during a surfactant or micellar flood. More recently, we have assessed the rigidity of interfacial films and its relationship to coalescence rate through measurements of interfacial viscosities of crude oils contacted against aqueous solutions containing various concentrations of surfactants and other pertinent chemical additives. Our data clearly indicate that in the absence of a commercial surfactant, interfacial viscosity builds up rapidly, coalescence is inhibited, and the resulting emulsion is quite stable. These phenomena also have been observed by Gladden and Neustadter. Several studies were conducted on the structure of film-forming material at the crude oil/water interface, its effect on emulsion stability, and the role of such films in oil recovery by water or caustic solution displacements. Rigid films were found to reduce the amount of oil recovered. Our studies also have shown that the addition of a commercial surfactant lowered both the interfacial viscosity (ISV) and interfacial tension (IFT) of the crude oil-aqueous solution system. However, the concentration at which both the IFT and ISV are minimized cannot be identified by measuring IFT alone. We have conducted a cinephotomicrographic examination of spontaneous emulsification and a microvisual study of the displacement of residual crude oil by aqueous surfactant solutions in micromodel porous media. SPEJ P. 409