Surfactant-Polymer (SP) involves adding low concentrations (0.1% to 2%) of a surfactant and/or cosurfactant and/or cosolvent and/or salt to the injection water in order to reduce the oil-water interfacial tension. Polymer is added to increase the solution viscosity in order to overcome viscous instability of low interfacial tension displacement. The SP solution is injected for 20% to 40% pore volume of the target oil-bearing zone, followed by similar volumes of polymer flush.
Conceptually SP flooding addresses both sweep efficiency and displacement efficiency, limited only to those conditions limiting polymer flooding. Practically, the process is limited to the amount of surfactant and polymer that can be blended together because at some concentration of one in the presence of the other, the solution begins to separate. The concentration of surfactant used must be greater than the critical micelle concentration (CMC) at which point the interfacial tensions become very low for surfactant at "optimum" conditions. The CMC is often quite low. However, both polymer, and to a much greater degree surfactant are adsorbed by the reservoir rock surface, which decreases their concentration as they progress through the reservoir. Thus the surfactant concentration must be high enough to overcome this adsorption loss.
Designing an SP flood is more complex than polymer flooding. The conditions at which a surfactant formulation is "optimum" is related to the water salinity, temperature, and pH, as well as crude oil properties. This "optimum" condition is expressed by very low interfacial tension between the crude oil and surfactant solution. Certain types of surfactant can handle the different reservoir temperature, water salinity, etc. conditions, but no surfactant is the silver bullet that can handle them all. Although it is not difficult to develop an SP formulation that is optimum for an oil reservoir's environment, the process has not been demonstrated successfully in the field
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