Selection Of Pseudo-Components In Surfactant-Oil-Brine-Alcohol Systems
Authors: Salter Stephen J. 
Published: December 2007

In this work, phase diagrams at three salinities are presented in quaternary representation which demonstrate the nature of three different forms of phase behavior. At all three salinities, regions of phase behavior. At all three salinities, regions of the phase diagram exist in which partitioning of each of the components among the phases present is significant. The consequences of attempting to reduce the representation of such a system from pseudoquaternary to pseudoternary are investigated for each of the three salinities. A mechanism is proposed by which the three-phase region is formed and increases in size. Also a region of high alcohol content microemulsion is observed, the nature of which is almost independent of brine salinity.

Mixtures of pure hydrocarbons were investigated which indicate that even though the constituents individually result in different phase behavior, the mixture can be considered as a single component, oil. The results for these mixtures also serve to bolster the concept of "equivalent alkane carbon number." Finally, it is shown that when fractional flow or dispersion are significant, mass transfer between the liquid phases can cause separation of components which were injected together.

Introduction

After primary and secondary recovery, a great deal of residual oil remains in most reservoirs. Presently a number of tertiary recovery methods aimed Presently a number of tertiary recovery methods aimed at recovering this residual oil are being investigated. These processes involve the sequential injection of a number of fluids which are designed to either increase the portion of the reservoir being contacted creased sweep efficiency) or to do a better job of mobilizing oil than water does in those portions of the reservoir contacted during waterflooding (increased microscopic displacement efficiency). In many cases the residual oil is trapped in the rock pores as blobs or ganglia. In order to mobilize a ganglion, the capillary forces holding it in the pore must be reduced. This effect can be accomplished by lowering the oil-water interfacial tension. one method of lowering the interfacial tension involves injecting surfactant into the reservoir. The surfactant molecules aggregate at the oil-water interface and cause phase changes to occur. These phase changes, the phase changes to occur. These phase changes, the formation of microemulsion, reduce the interfacial tension between the oil- and water-rich phases. If this interfacial tension is lowered sufficiently (typically 3 to 4 orders of magnitude), the available pressure gradient becomes sufficient to mobilize the pressure gradient becomes sufficient to mobilize the oil ganglia.

The phase behavior of these oil-water-surfactant systems is, therefore, very important and has been the object of intensive study for some time. Previous research has shown that the phase behavior of these systems, frequently characterized on pseudoternary phase diagrams, can be directly related to the phase diagrams, can be directly related to the interfacial tensions which arise between the various phases. phases. A typical surfactant flooding process involves the injection of a single-phase surfactant-containing fluid in a slug. This slug is generally followed by a slug of polymer solution used to "push" the surfactant slug through the reservoir. A comprehensive review of the processes which utilize such materials has been presented recently by Reed and Healy. The components of the surfactant slug are characteristically a mixture of petroleum sulfonates, alcohols, cosurfactants, brine, and perhaps oil. Upon injection, this slug mixes with both the reservoir oil-brine system at the slug front and the polymer solution at tie slug rear. Thus, the physics-chemical system which requires modeling actually consists of a large number of chemical components: multi-component surfactants, diverse hydrocarbon mixtures, alcohols (possibly blends), polymer, water and various inorganic salts. The method used to divide these many components into pseudo-components has an effect on not only the phase diagram which results but also the physical properties among and between the phases which physical properties among and between the phases which result at any total composition.