Enhanced oil recovery (EOR) by altering the ionic composition of the injected water is an attractive, cost effective and environmentally friendly technique, especially for highly fractured (with low permeable matrix) and oil wet carbonate reservoirs. However, understanding and relating the undergoing mechanism(s) in this EOR method is usually a challenge. In this work, different mechanisms - including surface charge alteration, double layer expansion and potential determining ions (PDI) - were evaluated for the core plugs of an Iranian limestone oil reservoir under the influence of Persian Gulf sea water (SW). Spontaneous imbibition (SI) and zeta potential experiments were conducted in presence of formation brine (FW) and SW, which contained higher sulfate ions and lower salinity, compared to FW. The estimated electrical double layer thickness was increased by changing water composition from FW to SW. Also, the measured zeta potential in rock-brine interface was altered to more negative values with SW, compared to FW. Almost 2% incremental recovery in SI test, by switching FW to SW, may be explained then by either carbonate surface charge modification, which resulted in expanded water film, or higher concentration of sulfate ion in SW which activated the PDI mechanism.

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