Immiscible Microemulsion Flooding
Authors: Healy Robert N. , Reed Ronald L. 
Published: December 2007

Abstract
Economical microemulsion flooding inevitably involves microemulsion phases that are immiscible with water, oil, or both. Oil recovery is largely affected by displacement efficiency in the immiscible regime. Therefore, it is pertinent to study this immiscible aspect in relation to variables that affect phase behavior and interfacial tension between phases. This is accomplished through core flooding experiments wherein microemulsions immiscible with oil and/or water are injected to achieve enhanced oil recovery. One advantage of such an immiscible microemulsion flood is that surfactant concentration can be small and slug size large, thereby reducing deleterious effects of reservoir heterogeneity; a disadvantage is that the temporary high oil recovery accompanying locally miscible displacement before slug breakdown is reduced.
Final oil saturation remaining after lower, middle, and upper-phase microemulsion floods is studied as a function of salinity, cosolvent, temperature, and surfactant structure; and results are related to interfacial tension, phase behavior and solubilization parameters. A conclusion is that immiscible microemulsion flooding is an attractive alternative to conventional microemulsion processes.
Oil recovery obtained from microemulsion slugs is correlated with capillary number based on what is called the controlling interracial tension. Physically, this means the least effective of the Physically, this means the least effective of the displacement processes at the slug front or rear determines the flood outcome.
Finally, a screening procedure is developed that is useful for either immiscible or conventional microemulsion floods and that can reduce the number of core floods required to estimate oil recovery potential for a candidate microemulsion system. potential for a candidate microemulsion system

Introduction
This is the fourth in a sequence of papers dealing with miscible and immiscible aspects of microemulsion flooding.
The first of these papers identified micellar structures above the binodal curve and showed how the region of miscibility could be maximized at the expense of the multiphase region, thereby prolonging locally miscible displacement. This was accomplished by varying salinity, and the notion of optimal salinity was introduced as that which minimized the extent of the multiphase region. Interfacial tensions within the multiphase region were measured and found to vary nearly three orders of magnitude, depending on WOR and surfactant concentration. Careful isothermal pre-equilibration of bulk phases was a requisite to all interfacial tension measurements.
The second paper emphasized core flooding behavior and distinguished locally miscible displacement before slug breakdown, from immiscible displacement occurring thereafter. Fractional oil flow was correlated with capillary number and it was found that an effective immiscible displacement cannot be distinguished from the locally miscible case. Further, during an effective flood, the greater part of the oil was recovered during the immiscible regime. Finally, it was shown that micellar structure within the miscible region is not of itself an important variable.
Having determined that the immiscible aspect of a microemulsion flood was important and dominant, the third paper dealt extensively with the multiphase region. Microemulsions were classified as lower-phase (1), upper-phase, (u), or middle-phase (m) in equilibrium with excess oil, excess water, or both excess oil and water, respectively. Transitions among these phases were studied and systematized as functions of a number of variables. Solubilization parameters for oil and water in microemulsions were introduced and shown to correlate interfacial tensions. The middle-phase was identified as particularly significant because microemulsion/excess-oil and microemulsion/ excess-water tensions could be made very low simultaneously.
In this paper, the sequence is continued by introducing the notion of an immiscible microemulsion flood as one having an injection composition in the neighborhood of the multiphase boundary.
SPEJ
P. 129