Adsorption and Interfacial Tension Dynamics of Surfactants in Porous Media

Abstract

ABSTRACT The objective of this work is an understanding of the dynamic mechanisms associated with the adsorption of surfactants on porous media and the dynamics associated with the lowering of the interfacial tension between oil and a surfactant solution. Most studies on surfactant adsorption have assumed that the adsorption mechanism occurs rapidly as compared with convection and dispersion. Trogus et al. (1977) have proposed a kinetic model which at equilibrium yields the Langmuir isotherm. Results of using this model have shown that the kinetic rate constant increases with an increase in phase velocity. An explanation for this variation is to postulate separate mass transfer and kinetic adsorption mechanisms. A new model is proposed and tested which consists of the solute moving to the rock-fluid interface by a mass transfer mechanism and then adsorbing to the rock via a kinetic mechanism. The mass transfer rate constant has been evaluated and correlated as a function of phase velocity. The correlation obtained for Berea sandstone and Triton X-100 (non-ionic) surfactant is in good agreement with existing mass transfer correlations for packed-bed reactors, indicating the validity of our mass transfer model for describing the transport of surfactant through porous media. Using a spinning drop tensiometer, the dynamic response of the interfacial tension between n-decane, and a brine solution of Texas No. 1 surfactant or TRS 10–80 surfactant was obtained. A kinetic adsorption/ desorption rate model for the dynamic behavior was developed. At high surfactant concentrations the normal interfacial tension decrease stops abruptly at a specific plateau value. This plateau breakpoint seems to be due to an abrupt phase change at the interface which prevents the continued adsorption of surfactant monomer molecules. The breakpoint disappears at low surfactant concentrations allowing a normal equilibrium condition to be reached. An increase in surfactant concentration increases the interfacial tension dynamic response rate. Close to the optimal salinity the dynamic behavior is slow with Texas No. 1 and fast with TRS 10–80. We have linked these observations to the Van der Waals energy of desorption and the Gouy surface potential of the double layer.