Pl. Alveskog et al., THE EFFECT OF SURFACTANT CONCENTRATION ON THE AMOTT WETTABILITY INDEXAND RESIDUAL OIL SATURATION, Journal of petroleum science & engineering, 20(3-4), 1998, pp. 247-252
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.