THE EFFECT OF SURFACTANT CONCENTRATION ON THE AMOTT WETTABILITY INDEXAND RESIDUAL OIL SATURATION

The objective of this study was to determine the influence of surfacta nt concentration, surfactant adsorption and interfacial tension betwee n oil and aqueous phase on the Amott wettability index and the residua l oil saturation during waterflooding. The flooding experiment and Amo tt wettability tests were performed on 60 core plugs at a temperature of 50 degrees C, with n-heptan and 1.5 wt.% NaCl brine with 12 differe nt concentrations of surfactant. The anionic surfactant used in the ex periments was n-dodecyl-o-xylene-sulfonate with a well-known interfaci al 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 st eps. The Amott wettability index and residual oil saturation vs. surfa ctant concentration and an adsorption isotherm of the surfactant on Be rea sandstone were the main quantities determined. The results show th at increased surfactant concentration results in a change in wettabili ty from strongly water wet to weakly oil wet. The residual oil saturat ion decreases with increasing surfactant concentration. A dramatic cha nge in wettability from water wet to neutral and over to oil wet occur s in a narrow range of low concentrations which coincide with the crit ical 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 s ignificant effect on the observed wettability. (C) 1998 Elsevier Scien ce B.V. All rights reserved.