Yibal oil reservoir, a fractured carbonate formation located in the north of Oman and characterized by high salinity brine and high temperatures, is a potential candidate for enhanced oil recovery (EOR) projects. This experimental study focused on the possibility to use surfactant injection as an EOR process in the Yiabl field. On the basis of the results from our previous investigations, ethoxylated sulfonates and amphoteric surfactants were used in this study. Selected surfactants showed a great tolerance to high salinity and temperatures. All surfactant systems displayed dynamic interfacial tension (IFT) behavior, in which transient ultra-low IFTs were detected. Dilute surfactant solutions (0.1 wt %) were considered for core-flooding tests on limestone plugs. In one set of experiments, surfactant solutions were injected into a fully water-flooded core (surfactant tertiary recovery). In another set, surfactant solutions were injected without pre-water-flooding (surfactant-modified water-flooding or surfactant secondary recovery). Results of tertiary recovery were found to be between 1 and 7% of original oil in place (OOIP), which correspond to 6 and 24% of residual oil in place (percentage based on the remaining oil in place after water-flooding). Tertiary surfactant injection therefore appears to be an attractive option for pre-water-flooded zones for additional oil recovery. In the other hand, results of secondary recovery for some surfactants showed significant oil recovery compared to water-flooding, whereas for other surfactants, water-flooding was more effective. Secondary surfactant recovery resulted in faster rate recovery as compared to water-flooding. Thus, minimum pore volume (PV) injected at ultimate oil recovery for 7-58 surfactant was found to be significantly lower (2.3 PV) than the corresponding PV in water-flooding (9.1 PV).
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