Compositional Effects On Surfactantflood Optimization
Authors: Hedges James H. , Glinsmann Gilbert R. 
Published: December 2007

Abstract
This paper presents results of phase behavior studies and oil displacement tests conducted in Berea sandstone cores which show that surfactant flood system based on optimal salinity design displace oil with different efficiencies. The optimal salinity for a given oil-surfactant system was varied by using alcohol cosurfactants with different water solubilities. The results suggest that there exists a preferred optimal salinity (herein referred to as the unique salinity) at which oil recovery is maximized. The value of this preferred optimal salinity was shown to increase with preferred optimal salinity was shown to increase with decreasing equivalent weight of the surfactant and with increasing effective alkane carbon number (EACN) of the oil displaced. For displacement of a given EACN oil, the cosurfactant type yielding the unique salinity design was substantially the same for various surfactant types. The existence of the unique salinity is attributed to the dominance of deleterious effects from cosurfactant dilution at salinities less than the unique value and surfactant retention at salinities greater than the unique value. The results of these studies provide a basis for optimal design based on maximized oil recovery. In addition, displacement efficiency was shown to be increased with "non-unique" optimal systems when the operating salinity was shifted toward the unique value.

Introduction
Early approaches to micellar techniques for enhanced oil recovery were developed where oil was displaced miscibly with water or oil external surfactant systems. However, more recent studies showed that, under economically practical conditions, most of the oil was displaced via immiscible mechanisms This led to studies of the use of immiscible microemulsion flooding. Other studies with micellar phase behavior indicated that the surfactant solution could be injected directly to contact with residual oil and allow for the in situ generation of microemulsion. When the system is properly designed, the interfacial tensions (IFT's) between the different phases generated are sufficiently low such that effective oil displacement can be achieved in porous media.
Oil displacement behavior in porous media can be associated with standard laboratory equilibration experiments. Phase and IFT behavior obtained from equilibration experiments are reported in the literature. At low salinities, a "lower" phase microemulsion system is formed with an upper oil phase and a lower microemulsion phase. At intermediate salinities, a "middle" phase phase. At intermediate salinities, a "middle" phase microemulsion system is formed with an upper oil phase, a middle microemulsion phase and a lower brine phase. At high salinities, an "upper" phase microemulsion system is formed with an upper microemulsion phase and a lower brine phase. IFT's are minimized within the "middle" microemulsion region near that salinity at which equal volumes of brine and oil are solubilized into the microemulsion phase. This salinity is frequently referred to as the "optimal" salinity and is generally associated with maximum oil recovery. However, recent results have suggested that maximum oil recovery can be deviated from the optimal salinity.
In the work reported here, results of oil displacement tests are reported using aqueous surfactant solutions which are designed to generate a multiphase microemulsion in situ. The oil displacement results were used to explore the relationship between optimal salinity and oil displacement efficiency. Using various oils, surfactants and cosurfactants, conditions for maximum oil recovery were investigated by a systematic study of phase and oil displacement behavior as a function of salinity.

EXPERIMENTAL
Phase behavior studies were conducted by equilibrating equal volumes of the surfactant solution and an oil of interest. The relative volume fractions of the resulting fluid phases were plotted versus the salinity of the surfactant solution (see Figure 1a). These plots are referred to as phase volume diagrams.