The oil-recovery effectiveness of a chemical flood has been proved related to the phase behavior of the brine/oil/surfactant system. In particular, it is advantageous to formulate the system so that optimal threephase behavior is obtained. However, it also has been demonstrated that all the optimized systems are not equivalent in terms of solubilization. interfacial tensions (IFT's), and oil-recovery efficiency. This paper addresses the conditions that promote high solubilization in microemulsions, a property correlated to the values of the IFT and therefore correlated to the ability of such systems to displace the oil in porous media. When one formulation parameter is changed, another parameter must be varied at the same time for compensation to reoptimize the system. The mechanism of solubilization is investigated experimentally by considering the usual formulation parameters: salinity, oil type, alcohol type and concentration, and surfactant structure and type (anionics and nonionics). The results are interpreted in terms of interaction energies between surfactant, oil, and water. In particular, the role of the alcohol and its impact on the solubilization by amphiphilic systems are discussed in detail and interpreted. Moreover, the concepts developed in this paper explain the effect of the surfactant structure and therefore aid in the design of amphiphilic molecules exhibiting a high solubilizing power for given conditions of brine, temperature, etc. Introduction Mobilization and transport of residual oil by chemical-flooding processes involve various mechanisms that must be considered when formulating a surfactant slug, but, among them, it is well known that IFT's between phases play a major role. Reed and Healy have shown phases play a major role. Reed and Healy have shown that ultralow IFT's can be attained when a microemulsion phase (surfactant-rich phase, the so-called "middle phase (surfactant-rich phase, the so-called "middle phase") is in equilibrium simultaneously with an oil phase") is in equilibrium simultaneously with an oil phase and a water phase. They first have defined the phase and a water phase. They first have defined the concept of optimal salinity as being the point where the IFT's at the oil-middle phase and middle phase/water interfaces are equal. At that point, the volumes of oil and water solubilized in the middle phase generally are identical, although there is no theoretical basis for that. A correlation between the values of the quantities of oil and water solubilized in the middle phase and the values of the IFT's between the phases also has been found: the lower the tension, the higher the solubilization. Therefore, it appears judicious to start the screening procedure of surfactant systems for enhanced oil procedure of surfactant systems for enhanced oil recovery (EOR) by looking for the point where equal volumes of oil and water are solubilized in the surfactant phase of a three-phase system. During recent years, phase of a three-phase system. During recent years, much time has been devoted to discovering that point, and the rules for compensating changes in the formulation variables have been established for anionic and non-ionic surfactants. We must emphasize that, if we start from an optimized system and we change a formulation variable defining the system, the optimal state is lost, and another formulation variable must be changed to reach a new optimal state. All optimized systems are not equivalent, as shown in previous results, and consideration of the amount of previous results, and consideration of the amount of oil and water solubilized in such systems provides a criterion to compare them. In a previous paper, we carried out a systematic study of the effect of the formulation variables on the solubilization at optimum by anionic surfactants. Some results concerning nonionics have been presented recently presented recently. SPEJ p. 327

Surfactant/oil/water phase diagrams have become the most important screening tool used to select microemulsion systems for enhanced oil recovery. The number of phases coexisting at a given salinity, the extent of the single-phase region, and the position of the phase boundaries all have relevance with respect to oil displacement efficiency. It is shown that the phase diagrams can be made to take on different configurations depending on the alcohol cosurfactant, the salinity, the impurities present in the surfactant, and the dispersity of the surfactant mixture. Besides the importance of the phase boundary shape, this study provides further insight into factors determining the height of the binodal surface on the pseudoternary phase diagram. Results show the effect of salinity as well as the surfactant, alcohol, and hydrocarbon types on the height of the binodal surface. It is shown that salinity is the main factor; other parameters have little or no influence once a surfactant has been selected. Finally the microemulsion viscosity is shown to be related to the proximity of the formulation to phase boundaries. Extensive data for one system are presented. Introduction It is now recognized that formulating surfactant/oil/brine systems that exhibit desirable phase behavior is an important step in optimizing performance of microemulsion systems for enhanced oil recovery. Oil is displaced by a combination of mechanisms-miscible displacement, swelling of the oil phase, and low tension displacement all of which are related to the topology of the phase boundaries in composition space. To predict the outcome of a particular project, a representation of the phase boundaries and their evolution when diluted with oil or brines having various proportions of divalent ions is required. For example, successful application of the salinity gradient concept demands phase relationships specially structured to accommodate the variations in salinity experienced by the surfactant slug during the course of the flood. Recent publications have dealt with the optimal salinity as a function of total amphiphile concentration (surfactant plus cosurfactant), and reported trends that are quite different from those found if the cosurfactant (alcohol) concentration is held constant. One purpose of this paper is to demonstrate that contorted phase boundaries found by Glover et al are caused by the variation of alcohol concentration when the concentration of total amphiphile is varied and because the direction that the phase boundaries twist or rotate is controlled by the nature of the alcohol. Another important factor is the extent of the single-phase region. More precisely, the height of the demixing curve in the pseudoternary representation should be minimized. This would permit, in principle, the amount of surfactant and cosurfactant in the micellar slug to be minimized. A correlation permitting the determination of the oil, salinity, alcohol, and surfactant at which the height of the demixing curve is minimized has been reported, but few data giving the value of the minimum height have been presented. This height is an important feature of the phase boundary topology and extensive measurements are reported here. The microemulsion viscosity must be high enough to help maintain mobility control. It is sometimes difficult to achieve the required levels of viscosity. Studies of microemulsion viscosity have been reported. We provide further data here and have related the microemulsion viscosities to phase behavior. Materials and Experimental Techniques The phase diagrams have been established by two techniques: a titration procedure and a grid-point technique. SPEJ P. 28^

Re~u le 24 mars 1977, accepte le lerjuin 1977) Résumé. 2014 On a mesuré par la méthode des battements lumineux le coefficient de diffusion de micelles formées au sein de mélanges quaternaires eau-alcool-tensioactif-toluène, pour différentes valeurs de la fraction volumique de la phase dispersée. Les résultats obtenus conduisent d'une part à admettre une monodispersion du diamètre des micelles, d'autre part à un bon accord 2014 expliqué théoriquementavec les mesures de turbidité. Abstract. 2014 The translational diffusion coefficient of microemulsions formed from water, toluol, sodium dodecyl-sulfate and alcohol, has been measured using light beating spectroscopy. The experimental results show that the microemulsions are monodisperse, the micelles being electrically charged.

Summary Systems optimized for micellar flooding are not all equivalent in terms of solubilization, interfacial tensions (IFT's), and oil recovery efficiency. The basic conditions that promote high solubilization into microemulsions, a property correlated to the values of IFT'S, were established in a previous paper. Changes in oil type, for example, have to be compensated for by adjustment of the lipophile of the surfactant; however, this is not always sufficient to guarantee a high solubilization. The purpose of this paper is to investigate the influence on solubilization of the relationship between the oil type to be solubilized and the surfactant lipophile. We found that this relationship is a determining factor for solubilization at a given temperature and salinity. For example, we observed that for the alkane series, solubilization is not affected by the alkane carbon number (ACN), provided that the compensation be achieved by adjustment of the surfactant lipophile length. For other oils or mixture of oils, like crudes, however, this is not necessarily the case; the solubilization may be found to be more or less different from that of alkanes, depending on the structure of the surfactant lipophile. A particularly interesting case of oil mixtures is mixtures of hydrocarbons and methane (under pressure), because this gas is often dissolved in substantial amounts in live crudes. The incidence of the presence of methane is shown to be a function of the amount of gas presence of methane is shown to be a function of the amount of gas dissolved and of the nature of the dissolving hydrocarbon. An interpretation of the results is proposed, based on the oil/lipophile, oil/oil, and lipophile/lipophile interaction energies, which are involved in Winsor's R-theory. Introduction The design of the micellar or microemulsion slug for surfactant flooding requires careful attention to attain the low IFT's essential for mobilizing the residual oil trapped in the reservoir. Ultralow IFT's have been shown to occur when a surfactant-rich phase-called middle phase-is in equilibrium with both excess oil and water phases. 2 The IFT values were found to correlate to the amount of oil and water solubilized in the middle phase: the higher the solubilization, the lower the tensions. It is then convenient to compare various systems through their solubilizing power. For meaning comparison, however, a reference state has to be chosen, generally the three-phase system in the middle phase of which equal volumes of oil and water have been solubilized. Such systems are commonly said to be "at optimum" and can be obtained by scanning any formulation variable. Comparison of optimized systems through their solubilizing power will reveal that they are not all equivalent. This raises the problem of finding the best surfactants from the solubilization and IFT standpoints. A corollary question is whether a meaningful comparison of surfactants, though taken at optimum, can be achieved, whatever the composition of the system. This question has been addressed in a previous paper, which showed that a key factor was the level of the interaction energies of the surfactant with oil and water. At optimum, the compensation for the change in a formulation variable that has affected one side of the water/oil interface (where the surfactant is adsorbed) has to be carried out on the same side so as not to decrease the solubilization. For example, a shortening of the surfactant tail compensated for by an increase in salinity entails a decrease in solubilization. In the same way, an augmentation in the hydrophobicity of the oil-e.g., through its ACN-decreases the solubilization if the compensation is achieved through an increase in salinity. On the other hand, a variation of normal ACN can be compensated for by adjustment of the surfactant tail length at constant salinity without serious damage to solubilization. Regarding the effect of oil on solubilization, however, some difficulties have appeared in some cases when hydrocarbons different from the normal alkane series were used, even though the compensation had been carried out through the surfactant tail length. The purpose of this paper is to investigate in detail the relationship between paper is to investigate in detail the relationship between the surfactant lipophile and the oil to be solubilized. The results obtained when the oil change is compensated for by adjustment of the surfactant tail length will be presented first. Compensation by changing the surfactant hydrophile will be discussed next. Oils of various natures and molecular weights will be used, as well as mixtures. The effect of the presence of dissolved methane in the oil will be seen as an interesting particular case of mixture. SPERE P. 41

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