The effect of surfactant concentration on the Amott wettability index and residual oil saturation

The objective of this study was to determine the influence of surfactant concentration, surfactant adsorption and interfacial tension between oil and aqueous phase on the Amott wettability index and the residual oil saturation during waterflooding. The flooding experiment and Amott wettability tests were performed on 60 core plugs at a temperature of 50°C, with n-heptan and 1.5 wt.% NaCl brine with 12 different concentrations of surfactant. The anionic surfactant used in the experiments was n-dodecyl-o-xylene-sulfonate with a well-known interfacial tension vs. concentration relationship. The core material was Berea Sandstone with an average porosity of 19% and an average permeability of 104 md. Wettability indexes were determined by the standard Amott test involving spontaneous uptake of fluids and forced displacement steps. The Amott wettability index and residual oil saturation vs. surfactant concentration and an adsorption isotherm of the surfactant on Berea sandstone were the main quantities determined. The results show that increased surfactant concentration results in a change in wettability from strongly water wet to weakly oil wet. The residual oil saturation decreases with increasing surfactant concentration. A dramatic change in wettability from water wet to neutral and over to oil wet occurs in a narrow range of low concentrations which coincide with the critical micelle concentration. In this range the measured adsorption was only approximately 10% of the maximum value of the adsorption isotherm. Higher surfactant adsorption at higher concentrations did not have significant effect on the observed wettability.