Salinity gradients optimum

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

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. 

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. 

As oil saturation is decreased (water saturation is increased), the acid content in the oil is decreased. Consequently, the soap molar fraction X oap is decreased, as Table 12.6 shows. As X oap is decreased, type 111 salinity limits are closer to those of surfactant. Thus, the limits are increased, and the optimum salinity is increased as well. The system is changed from type III to type I. This transition from type III to type I is exactly the salinity gradient we need. In practical alkaline-surfactant flooding, water saturation will be increased from the flood front to the upstream, and the microemulsion system will change from type III... 

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