A Laboratory Investigation into Wettability Alteration of Carbonate Rock by Surfactants: The Effect of Salinity, pH, and Surfactant Concentration

Wettability alteration is an important method for increasing oil recovery from oil-wet carbonatereservoirs. Chemical agents like surfactants are known as wettability modifiers in carbonate systems.Oil can be recovered from initially oil-wet carbonate reservoirs by wettability alteration from oil-wetto water-wet condition with adding dilute surfactant and electrolyte solutions. This paper investigatesthe effects of brine concentration, surfactant concentration, and the pH of injection water on thewettability alteration of carbonate reservoirs by different class of surfactants. Scanning electronmicroscopy images verified the formation of surfactant layer surfaces and the adsorption of surfactantmolecules on the rock. The results revealed that TX-100, as a nonionic surfactant, and CTAB, as acationic surfactant, were better wettability modifiers than SDS, as an anionic surfactant, for carbonaterocks. At the concentration of 1 wt.% and higher, the contact angle reduction was approximatelyunchanged. The results also proved that there was an optimum salinity for the maximum wettabilityalteration by surfactants. Increasing the pH of aging fluid resulted in better wettability alteration byCTAB, while, in the case of SDS, the wettability alteration was reduced. Acidic conditions had anegligible effect on the wetting behavior of TX-100.