Microemulsion Phase Behavior: A Thermodynamic Modeling of the Phase Partitioning of Amphiphilic Species

A thermodynamic model is presented for modeling the partitioning of amphiphilic species between the different partitioning of amphiphilic species between the different phases of systems typically used for chemical flooding. phases of systems typically used for chemical flooding. The model, an extension of the pseudophase model by Biais et al. that can analyze only a four-component system, can work with five-component systems, including two partitioning amphiphilic species (e.g., two alcohols or one alcohol and a partitioning cosurfactant species). The self-association of alcohol in the organic phases, which results in a variable alcohol partition coefficient, is considered. Experiments to determine thermodynamic constants (which are entered into the model) are described for four-component systems, including one alcohol. The salinity dependence of these parameters is also studied. Brine/decane/isobutanol/TRS 10–410 as well as brine/nonane/ isopropanol/TRS 10–80 systems are considered. Some computations of pseudophase compositions for the five-component model and for various overall compositions are included. This partitioning model has been included in the chemical-flooding simulator developed at the U. of Texas; the results of this model have been presented in another paper. The model used for the presented in another paper. The model used for the binodal surface that is required to calculate phase compositions from pseudophase compositions is presented in this paper, as well as comparisons with experimental data for both four- and five-component systems. Reservoir simulation results are presented in Ref. 3. Introduction The possibility of reaching very low interfacial tensions (IFT) during the displacement of oil by surfactant solutions has been the subject of intense interest for some time. Because the decrease in IFT can be as much as several orders of magnitude, almost all the contacted oil can be mobilized by this process. However, the recovery rate has proved to be very sensitive to many parameters, and the process has to be designed carefully to achieve a good oil recovery. It is commonly recognized that the phase behavior is one of the most critical features for the phase behavior is one of the most critical features for the design of chemical oil-recovery processes. Many investigators have studied phase behavior of systems with various combinations of brine, oil, surfactants, and cosurfactants. Winsor introduced a very convenient classification of phase behavior for such systems. Type I is a lower-phase microemulsion (surfactant-rich phase) in equilibrium with an oleic phase; Type II is an phase) in equilibrium with an oleic phase; Type II is an upper-phase microemulsion in equilibrium with an aqueous phase, and Type III corresponds to a middle-phase microemulsion in equilibrium with both aqueous lower phase and oleic upper phase. The number of phases and their composition determined IFT's, viscosity, relative permeabilities and other hydrodynamic parameters on permeabilities and other hydrodynamic parameters on which the efficiency of the process is directly dependent. Components present in the reservoir during chemical flooding include water, electrolytes, oil, polymer, and the amphiphilic species surfactant and cosurfactant. From the viewpoint of chemical thermodynamics, the number of chemical species is very large if we consider every species of which oil, surfactant, and cosurfactant are made. Fortunately, some of these species behave collectively, so they can be considered a single pseudocomponent in the phase behavior description, thereby pseudocomponent in the phase behavior description, thereby making the study more tractable. For example, Vinatieri and Fleming considered brine a good pseudocomponent, which means that the ratio of salt to water is about the same in each phase. McQuigg et al.'s experiments yield similar conclusions. Even crude oil has been shown to be a good pseudocomponent with a fairly acceptable accuracy. Dealing with amphiphilic species is far more difficult. In some laboratory studies, surfactant can be a chemically pure component, but for field applications it is usually a complex blend, such as petroleum sulfonates. In the case of petroleum sulfonates, different monosulfonated or polysulfonated species are present with varied carbon polysulfonated species are present with varied carbon tails. Commercial nonionic surfactants, which generally are ethoxylated alcohols, show a broad distribution of ethylene oxide number (EON). In both cases, investigators have shown that these commercially available surfactants do not behave collectively but in some situations partition selectively between the phases. The cosurfactant generally is an alcohol or an ethoxylated alcohol. Although many research programs currently are devoted to the design of alcohol-free systems to avoid some of the drawbacks induced by its presence (lower solubilization parameters, higher IFT's), most of the commonly used systems include alcohol or even a blend of alcohols with different carbon chain lengths and/or branching. SPEJ P. 693