Properties Of Amphiphile/Oil/Water Systems At An Optimum Formulation For Phase Behavior
Authors: Bourrel Maurice , Lipow Andrew M. , Wade W.H. , Schechter R.S. , Salager Jean-Louis 
Published: December 2007

Abstract
The oil recovery effectiveness of a chemical flood has been shown to be related to the phase behavior of the brine-oil-surfactant system. In particular, it is advantageous to formulate the system particular, it is advantageous to formulate the system so that middle phases are formed. There are, however, an infinite number of surfactant cosurfactant (alcohol) combinations which will give the desired phase behavior. The selection of the preferred system out of this infinity of possible systems is an optimization problem and is the subject of this paper. problem and is the subject of this paper. An extensive study of the interfacial tension of dodecyl ortho xylene sulfonate sodium salt, as a function of salinity, alcohol and hydrocarbon molecular weight has been conducted. The results reveal that certain formulations may be preferred since the interfacial tension of some systems at optimum conditions is smaller than others. Indeed, conditions giving a global interfacial tension minimum were found.
It is also known that for enhanced oil recovery, it is desirable to maintain miscibility between the chemical slug and the reservoir fluids as long as possible. This means that the height of the possible. This means that the height of the multiphase region should be minimized. This study show that the height can be minimized by a proper formulation.

Introduction
A correlation relating the variables deft optimum systems for improved oil recovery has be reported. This equation gives the optimum salinity, S*, as a function of the alkane carbon number of the oil, ACN, the alcohol and a parameter, sigma, which is characteristic of the surfactant as follows:
(1)
where K is a constant equal to 0.16 for all alkylaryl sulfonates and f(A) is a function of alcohol type and concentration known for alcohols heavier than C5. The dots indicate that other variables, such as the water-oil ratio (WOR) and the temperature, have been omitted since they are not considered in this work. Equation 1 applies if WOR = 4 and T = 25 degrees C. The ratio sigma/K has been called the EPACNUS and is the extrapolated preferred alkane carbon number at unit salinity and preferred alkane carbon number at unit salinity and without alcohol, since f(A) is defined so that it vanishes as the alcohol concentration goes to zero.
Equation 1 represents a plane in the space in S,ACN and f(A) as shown in Figure 1, and any point on this plane is an optimum system in the following sense. For a given oil and surfactant system, a middle phase will be observed over a range of salinities and the midpoint of this salinity range will be called the optimum salinity. Furthermore, the surfactant system is said to be an optimum one. This implies that a series of experiments in which a property important to improved oil recovery, such property important to improved oil recovery, such as interfacial tension or amount of surfactant needed to solubilize certain volumes of water and oil, will yield a minimum value of the property when a point on the plane is reached. Thus, a series of interfacial tension measurements will reveal a minimum value at the point where line 4 shown in Figure 1 pierces the plane. (Perhaps, it would be more pierces the plane. (Perhaps, it would be more precise to assert that the minimum will occur in the precise to assert that the minimum will occur in the vicinity of the plane rather than on it. This point is further clarified in a subsequent section.) If the experiment is repeated at a different salinity or alcohol concentration or both, then the minima of the properties in question will be found at the new point at which a vector parallel to line 4 intersects the plane. If the values of the minima are different at the two points, then one point on the plane may be preferred to the other with regard to plane may be preferred to the other with regard to efficiency of oil recovery. Thus, it is appropriate to ask if all points on the optimum plane are equivalent with regard to oil recovery. This is the question addressed in this paper.