Wettability Alteration in Carbonate Rocks by the Low Salinity Water using a High-Speed Centrifuge

Abstract

Carbonate reservoirs are characterized by neutral or oil-wet surfaces and the injection of water as a secondary recovery method may result in low oil recovery factors. The injection of low salinity brines has shown to be a method that can lead to significant additional oil recoveries, since they promote alteration of the surface wettability towards a more water-wet condition. Several studies indicated that some ions present in the injected brine (Ca2+, Mg2+ and SO42-) are able to act in the ionic exchanges that promote the wettability alteration. Also, the decrease of the concentration of Na+ and Cl- resulted in an additional oil recovery factor in some studies. The studies of oil recovery at laboratory scale are usually performed in Amott's cells or by coreflooding. In this paper, the study of oil recovery using a high-speed centrifuge (HSC) is proposed in an innovative way with the aim to validate it as an apparatus for the study of wettability alteration. For this, six Silurian dolomite outcrop cores were used. Primary drainage process was performed on the HSC to establish the initial water saturation condition. Formation water and oil from a Brazilian pre-salt field with 2% weight of naphthenic acid were used as the saturation fluids. The cores were aged at 110 °C for 20 days to alter the initial water-wet condition. Two sets of tests were performed in the HSC to study the effects of the reduction of Na+ and Cl- ions in seawater composition. Additional oil recovery was observed using brine depleted of Na+ and Cl- ions. Besides, it was observed the influence of the sample permeability on oil recovery. The high concentration of Na+ and Cl- ions hinder the access of the potential ions (Ca2+, Mg2+ and SO42-) to the rock surface and, as consequence, hamper the wettability alteration mechanism. Removing these ions, the potential ions can reach easier to the surface and alters its wettability towards a more water-wet condition, promoting the additional oil recovery. Thus, the results showed that HSC could be used as preliminary apparatus for the wettability alteration studies in carbonate samples.