Surfactant flooding salinity gradients

Nelson and Pope concluded that chemical flood design should be such as to maintain as much surfactant as possible in the type III phase environment. This condition can be accomplished by designing the micellar fluid such that the initial phase environment of the immiscible displacement is type II(+). A negative salinity gradient is imposed, and it moves the phase environment to type III and, eventually, to II(-). 

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... 

Dependence of Adsorption on Brine Salinity and Divalent Ion Content. Brine salinity and composition probably constitute the primary criteria for selecting surfactants for foam applications. Many Canadian pools that are being flooded with hydrocarbon solvents contain near-saturated formation brines. Some of these pools have been waterflooded with fresh water, and therefore, salinity gradients exist. In addition, the majority of hydrocarbon-miscible floods in Canada are conducted in carbonate formations that contain formation waters with high levels of hardness. 

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. 

Three-Phase Displacement. Provided that the alcohol free surfactant system shows a classic phase behavior, II( —) -> III II( -f) by increasing the salinity without forming gels or stable macroemulsions during the phase transitions, the chemical flood can be performed as a three-phase flood using a negative salt gradient. 

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