Phase Relationships in Chemical Flooding
Authors: Nelson R.C. , Pope G.A. 
Published: December 2007

Abstract
Results of laboratory chemical floods are presented to show that equilibrium phases observed presented to show that equilibrium phases observed in test tubes are representative of phases produced in core flow experiments. Consequently, many performance characteristics of chemical floods can performance characteristics of chemical floods can be explained and predicted from equilibrium surfactant-brine-oil phase diagrams.
An oil reservoir under chemical flooding can be visualized as a series of connected cells with phase equilibrium attained in each. Fluid flow from phase equilibrium attained in each. Fluid flow from one cell to the next is governed, not so much by initial properties of the oil, brine, or chemical slug and drive, as by properties of equilibrium phases formed from those fluids.
Three types of equilibrium phase environment are defined. Results of interfacial tension measurements and laboratory flow experiments indicate that chemical floods should be designed to keep as much surfactant as possible for as long as possible in the "Type III" phase environment while the surfactant is traversing the reservoir.

Introduction
Recent research shows that when certain surfactants of interest in chemical flooding are equilibrated with brine and oil, the phases formed can be represented by relatively simple triangular phase diagrams. Furthermore, Healy and Reed phase diagrams. Furthermore, Healy and Reed revealed that surfactant-rich equilibrium phases, while immiscible with brine and oil, can displace waterflood residual oil effectively. Other papers involving phases of chemical flooding systems have appeared subsequently.
This paper extends the use of phase diagrams in chemical flooding research by presenting laboratory evidence that the same phases, observed when surfactant, brine, and oil are equilibrated in sample tubes, form and transport in a core under a chemical flood. This interrelationship between surfactant-brine-oil phase behavior and the characteristics of chemical flooding is reminiscent of the interrelationship between alcohol-brine-oil phase behavior and the characteristics of alcohol flooding as described by Tabor et al. We discuss here some consequences of local phase equilibrium in an oil reservoir under a chemical flood.
First, we review briefly the phase diagram representation. Next, visualizing the core as a series of connected mixing cells in each cell of which phase equilibrium is attained, we prescribe conditions for effluent liquids, based on phase diagrams for surfactant-brine-oil systems. Then, we compare results of flow experiments with those prescribed conditions. prescribed conditions. PHASE DIAGRAM REPRESENTATION PHASE DIAGRAM REPRESENTATION Following Healy et al., Fig. 1 illustrates three types of generalized phase diagram for three quasi-single components - surfactant, brine, and oil. These phase diagrams represent what we define as "phase environments." A surfactant-brine-oil system in any of the three phase-environment types can equilibrate as a single phase or as multiple phases, depending on the over-all composition of phases, depending on the over-all composition of the system.
At high-surfactant concentrations, all phase environments ideally are single phase. At lower-surfactant concentrations in a Type II(-) phase environment, two equilibrium phases are phase environment, two equilibrium phases are present. As indicated by the tielines in the present. As indicated by the tielines in the two-phase region, one phase is essentially pure oil and the other is a homogeneous phase containing surfactant, brine, and oil. Here, we shall call such a phase a "microemulsion." We use this term only to describe a phase containing surfactant, brine, and oil apparently in thermodynamic equilibrium with one or more other phases. The term does not suggest a particular concept regarding the structure of that phase.
Thus, in a Type II(-) phase environment, the maximum number of equilibrium phases is two. When surfactant, brine, and oil are plotted as in Fig. 1, the tielines in the two-phase region have a negative slope; hence the "II(-)" designation.
SPEJ
P. 325