Injection of a Smart Water, with a modified and optimized ionic composition, is an environmentally friendly and cheap EOR method. To be able to optimize the ionic composition to cause wettability alteration in the reservoir, one must understand the initial wetting of the reservoir. Experimental studies have confirmed that acidic material in the crude oil, especially negatively charged carboxylates, R-COO-, are the most important wetting parameters towards positively charged carbonate surfaces that dictate the rock wettability. The carboxylate molecules bond strongly to the carbonate surface and these crude oil anchor molecules can only be removed from the calcite surface by chemical reactions. Generating representative core wettability during core restoration in the laboratory is important for doing realistic oil recovery studies, capillary pressure and relative permeability measurements.

Very water-wet outcrop chalk cores showing good reproducibility were used to study adsorption of carboxylic material onto chalk. Crude oil with a known acid number (AN) was flooded through water-wet chalk cores with 10 % water saturation. The AN of the eluted oil was measured and the amount of adsorbed acidic organic material was determined. It is a general assumption that aging of a core is a requirement to generate a mixed-wet core. Therefore the wettabilities of aged and non-aged cores were determined and compared by spontaneous imbibition and chromatographic wettability tests. The results of this study first and foremost showed that both the aged and non-aged core behaved mixed-wet, thus aging is not a requirement to generate a mixed-wet core. The two parallel cores adsorbed similar amounts of acidic material, and the chromatographic wettability test results showed similar water-wet surface area in both the aged and non-aged cores. However, since spontaneous imbibition is very sensitive to the location of the oil-wet surface, a difference in capillary forces between the aged and non-aged cores was observed. The non-aged core behaved mixed-wet in a spontaneous imbibition test, while the aged core behaved slightly less water-wet than the non-aged core. It seems that during the aging process the oil components were distributed in such a way to influence the capillary forces to some degree. To conclude, aging is not necessary to change the wettability of an initially water-wet core that has been flooded with crude oil. The acidic polar oil components attach to the carbonate surface immediately upon contact, resulting in a mixed-wet system.


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