The Effect of Salinity on Surfactant Flooding: A Literature Review and an Experimental Study
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Surfactant flooding is a proven EOR technique, which mobilizes residual oil by reducing interfacial tension (IFT). Several parameters affect the phase behaviour of a microemulsion system and a successful surfactant flooding needs to be designed according to these parameters. Salinity is probably the main design parameter and has a substantial affect on phase behaviour. Injection of low salinity water is reported to mobilize additional oil both as a secondary waterflood and as a tertiary enhanced oil recovery technique. However, if capillary forces are high, the additional oil mobilized by the low salinity effect can easily be re-trapped. By combining low salinity water and reduction in capillary forces from surfactant flooding, re-trapping of oil mobilized can be avoided. Therefore, it is believed that a combination of these two EOR-methods can exceed recoveries of either of the techniques applied individually. For this study three commercial internal olefins sulfonates (IOS) were selected for an initial screening process using n-octane and water. The surfactant formulation used in the core flooding experiments was selected based on results from the initial screening, which showed good surfactant properties for surfactant O342. Its effect on oil recovery was investigated through core flooding where different injection scenarios were investigated using n-octane and Berea sandstone. Five core floods were conducted with different injection scenarios. The results showed a lower oil recovery for low salinity surfactant flooding, than for the surfactant flooding at optimal salinity (ultra low IFT). No additional oil was mobilized in either secondary flooding or tertiary surfactant flooding due to the low salinity effect. A tertiary dynamic surfactant flood supported the optimal salinity that was located during phase behaviour studies. Surfactants were injected at three different salinities; at optimal salinity, below optimal and above optimal salinity. A significant increase was experienced when shifting from below optimal to optimal salinity.