The role of salinity and aging time on carbonate reservoir in low salinity seawater and smart seawater flooding

Abstract

Although the terms Low Salinity Sea Water and Smart Sea Water have been used interchangeably as one of Enhanced Oil Recovery methods in recent years, the present study shows that there are some differences between these two. To prove the claim, the role of salinity and aging time on interfacial tension reduction and contact angle alteration of a crude oil/brine/rock system is investigated using both smart sea water and low salinity sea water. Moreover, the governing mechanisms of the IFT variation and wettability alteration are discussed. For this purpose, the seawater solution is diluted with distilled water and at the optimal salinity NaCl, CaCl2, and Na2SO4 salts are added to the solution. Next, the IFT of brine and oil, and the CA of the oil drop are measured on the carbonate surface in the presence of brine. The results showed that IFT between low salinity sea water and oil was decreased with increasing salinity and then increased. In this regard, similar results were observed by adding NaCl, CaCl2, and Na2SO4 to a low salinity seawater (LSSW) solution. Additionally, reducing seawater salinity improved the wettability from oil-wet to neutral wet such that the optimum wettability range was achieved by adding Na2SO4 to the LSSW solution. The effect of aging time in the low salinity sea water solution and smart sea water resulted in a reduction in the interfacial tension and the contact angle, as well as improving the crude oil/brine/rock interface. The mechanisms governing the reduction of the interfacial tension of low salinity sea water and smart sea water were surface excess concentration, salting in effect, and boiling point reduction, while the dominant mechanism for changing the wettability of low salinity water and smart sea water were the pH rise, pseudo-alkaline behavior, double layer increase, carbonate surface dissolution and salting in effect.