A Group Contribution Activity Coefficient Model for Enhanced Oil Recovery Process Involving Electrolyte Solutions: Part II—Solutions in the Presence of Partial Molecular Association

Abstract

ABSTRACT In Part I of this series, a group contribution activity coefficient model for electrolyte solutions was developed based on Debye-Hückel theory, solvation theory and local composition theory. It was demonstrated that the model can satisfactorily correlate the complex thermodynamic properties of electrolyte solutions in the absence of partial molecular association. In this work, the above described model is extended to electrolyte solutions which exhibit partial molecular association. In addition to the thermodynamic theories introduced in the preceding paper, the present model further includes the chemical equilibrium theory to represent chemical association between molecular species in the solution. The equilibrium thermodynamic properties are then solved by invoking a recently developed simultaneous phase and chemical equilibrium calculation scheme. The reliability of the proposed model is tested for two important thermodynamic behaviors occurring in related solutions, namely, micellization and the phase transition upon salt addition. First, experimental mean ionic activity coefficients of the homologous sodium carboxylates in aqueous solutions are compared with the correlation. Chemical equilibria are introduced to account for the micellization of sodium carboxylates. The chemical equilibrium constant and aggregation number are found to increase with the length of hydrocarbon tail in the ampliphile. The sudden drop in the mean ionic activity coefficient caused by micellization is correctly predicted. The average deviations between the predicted mean ionic activity coefficients and experimental-data are generally less than 5%. Second, published experimental phase behavior data of water-NaCl-t-butanol-n-paraffin mixtures are compared with the correlations. It is found that the aggregation between water and tbutanol is responsible for the abnormal amounts of water in the upper phase, while the aggregation between water and oil is responsible for the large amounts of oil in the middle phase. The aggregations are modelled by chemical equilibria. The type II-/III/II+ phase transition for the above system upon successive addition of salts are correctly predicted. The critical salinities and phase compositions are also well correlated. Studies also show that the vertices of the three phase region are very sensitive to the alcohol concentration and suggest empirical correlations, such as Hand plots, may have to consider the surfactant effect as well as the salinity effect on the vertices of the three phase region. Our studies show that the model is capable of modelling micellization and the complex phase transion of oil-water mixture containing an amphilphile, and that it can be incorporated in a simulator to study the mechanisms of recovery processes involving electrolyte solutions or to design such processes.