Rate Effects Attending The Flow Of Surfactant Solutions Through Porous Media
Authors: Fernandez M. , El Emary M. , Wade W.H. , Trogus F.J. , Schechter R.S. 
Published: December 2007

This paper presents data on the adsorption of pure sulfonates onto mineral oxide surfaces. The adsorption isotherms are seen to be simple and adsorption reaches a plateau at the CMC of the surfactant. Adsorption maxima and minima are not observed. This is interpreted to mean that complex isotherm behavior can occur with surfactant mixtures, but not with pure surfactants.

Rate studies have also been conducted by measuring the surfactant effluent from a Berea core The observed concentration has been modeled using a simple rate expression and the results of the modeling used to predict the minimum core length required for a laboratory experiment to model field scale results.

Finally, the model is extended to include multicomponent surfactant systems. It is shown that micellar/monomer equilibria must be taken into account to correctly account for the movement of surfactant mixtures through porous media.

Introduction

The adsorption of surfactant from the chemical slug determines, in part, the amount of chemical required for an enhanced recovery project. Other mechanisms also contribute to surfactant retention. These include surfactant precipitation by divalent ions or, in high salinity brines, trapping in dead-end pores and partitioning of surfactant into an immobile oil phase. This paper is concerned with the mechanism of adsorption and the development of a model which represents both equilibrium and nonequilibrium surfactant transport through a porous medium.

Trogus et al have presented a theory which contends that complex adsorption phenomena such as isotherm maxima and minima concentration overshoot in breakthrough curve: and fraction reversal at high concentrations are all attributable to the fact that the surfactant systems studied were mixtures. When two or more different types of surfactant are present in solution at concentrations in excess of the CMC, mixed micelles form which will contain a different proportion of surfactant types than does the monomer phase. Furthermore, it has been hypothesized that the adsorption of sulfonates on mineral oxide surfaces depends only on the monomer phase concentration and composition. Using this premise, isotherms for mixed surfactant systems which represent the total amount adsorbed as a function of total surfactant concentration can be shown to exhibit the complex behavior alluded to above, but these isotherms will also depend on the initial surfactant composition, the solid/liquid ratio as well as the properties of the individual surfactants (CMC, solubility, etc.).

The results presented in this paper support the hypothesized adsorption mechanism. Pure surfactant adsorption is shown to plateau at the system CMC. Adding salt and alcohol to the surfactant solution is shown to alter the plateau values but not the fact that adsorption ceases at the CMC.

Dynamic studies of pure surfactant adsorption have been conducted and the rate constants determined for selected alkyl aryl sulfonates. These rate constants can then be used to calculate the length of test section required to correctly model field scale processes.

Finally, a mathematical model for dynamic and equilibrium multicomponent adsorption is presented. The crucial issue is to correctly account for micellar/monomer equilibria.

EXPERIMENTAL

The substrate used in all experiments is a specially prepared kaolinite. The natural ions were exchanged for sodium by repeated washing in sodium chloride followed by distilled water. The free water was then removed by drying at 120 degrees C for 24 hours. The surface area of the kaolinite is 15.1 meters squared/gram as determined by nitrogen BET.