The impact of brine salinity and its ionic composition on oil recovery on chalk formations and to less extent in carbonate reservoirs have been investigated extensively in recent years due to the potential of extra oil recovery. Surface wettability was suggested to be the main reason behind such extra recovery. This study investigates the wettability alteration of calcite crystal and carbonates outcrop rock surface aged in model oils of total acid number of 2 and then treated with different brines. Model oils were prepared by mixing toluene with short chain (Heptanoic acid) and long chain (Stearic acid) carboxylic acids and the investigated brines included range of salinity and the effect of individual ions such as SO42−, Ca2+ and Mg2+. The results of this study showed that the long chain fatty acid (stearic acid) strongly adsorbs onto the calcite surface from the oil phase compared to the short chain (heptanoic acid) as confirmed by the measured contact angles. Twice dilution of Arabian Gulf seawater has been found to be a less effecient EOR fluid for wettability alteration as compared to undiluted Arabian Gulf seawater. This was confirmed by the changes in the measured contact angles toward more water-wet for aged calcite in heptanoic acid model oil, aged calcite in stearic acid model oil and aged carbonate in stearic acid model oil systems. Also, it was observed that significant wettability alteration was observed for the twice diluted Arabian Gulf seawater with higher concentrations of SO42− and Mg2+.

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