A Thermodynamic Model for Low Interfacial Tensions in Alkaline Flooding

Abstract

Abstract Many experimental studies have been undertaken to measure interfacial tensions (IFT's) as a function of pH, salinity, temperature, and divalent ion concentrations. This behavior varies from one crude oil to another and is critical in determining its suitability as a candidate for caustic flooding. A model based on a thermodynamic analysis of crude oil/caustic interfaces has been developed that predicts the IFT behavior of such systems. The model accounts for IFT variations with changes in pH, salinity, and temperature by using parameters that must be estimated from a preliminary experimental study. The model predictions compare well with earlier reported experimental results. It is shown that the application of such a model to empirical correlations for residual oil saturations and fractional flow curves enables us to predict the changes in the fractional flow and relative permeability curves for an alkaline flood with self-sharpening fronts if the injection pH and salinity are specified. The basic equations needed to extend this model to account for divalent ions and more complex flow systems are provided in the Appendices. Introduction In the currently available literature on the subject of caustic flooding, divergent views have been expressed on the basic operating mechanism. Johnson' has summarized them into these categories: emulsification with entrainment, emulsification with entrapment, oil phase swelling, disruption of rigid films, wettability alteration, and wettability reversal. All these mechanisms have been studied and written about in some detail. Which one of them predominates in a particular flooding process depends on a variety of parameters like pore structure, injection concentrations, rock chemistry, etc. One fact, however, has emerged undisputed: The alkali reacts with the crude oil in place and generates a surface-active species, in situ, that substantially lowers the IFT and initiates the operating mechanisms. To understand these mechanisms better, we must have a more fundamental understanding of low IFT'S. It is well known that slightly water-soluble salts adsorb on interfaces and cause changes in IFT. Species like surfactants adsorb very strongly at an interface, whereas species like NaCl may actually be excluded from the interface. The presence of adsorbed surfactant molecules, besides lowering interfacial tension, may alter the mechanical properties of the interface--e.g., surface viscosity, surface film thickness, and surface viscoelasticity. As a brief overview. there have been two fundamental approaches to this problem of IFT behavior of liquid/liquid interfaces. The molecular approach involves a statistical mechanical calculation of the intermolecular forces operating at the interfaces between two phases. SPEJ P. 125^