Almost 90% of Iranian oil reservoirs are carbonate (limestone, dolomite or chalk). They are normally highly fractured, mixed to oil wet and have low matrix permeability. Since 1980’s, it has been discovered that water flooding with different ion composition compared to formation water may cause incremental oil recovery in carbonate reservoirs; especially after experience of seawater flooding in Ekofisk chalk field in North Sea.Despite all the attempts during last years, the detailed mechanism and the chemical/physical backgound of the process of ionically modified water injection is not yet fully understood. Austad and his coworkers [1,2,3] attributed the wettability alteration to the chemical reaction of potentail determining ions (PDI) with calcite mineral surface, which is positively charged in the pH range of formation brine. However, the influence of water salinity (multivalent ions of brine) on surface charge density and electrical double layer thickness were mentiond by some researchers [4,5,6] in this field as the main mechanism during advanced water flooding.

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