Salinity gradients surfactant concentration

This section reviews information in the literature about optimum phase types, the relationship between optimum salinity and surfactant concentration, and optimum salinity gradients. 

This section further discusses the effects of k, curves, optimum phase type, and phase viscosity. The effect of negative salinity gradient is further discussed under conditions where different relationships between optimum salinity and surfactant concentrations occur. 

To the best of our knowledge, the work by Gupta and Trushenski (1979) and experimental data from Nelson (1982) are the only data published so far to support the concept of a negative salinity gradient. Gupta and Trushenski, and Nelson used the same kind of surfactant with a special phase behavior. My explanation to their observation on salinity effect is that for the surfactant they used, the IFTs for both microemulsion/oil and microemulsion/water in the type III system were high. Therefore, when a lower salinity was in the drive water, low IFT was obtained because the lower salinity matched the lower optimum salinity of surfactant as the surfactant concentration was diluted. 

For most surfactant systems used in chemical flooding, optimal salinity for oil displacement in the presence of multivalent cations decreases as surfactant concentration decreases. One reason for conducting a chemical flood in a salinity gradient is to keep the surfactants at optimal salinity as their concentration is reduced by adsorption and dispersion during the flood. 

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