Propagation of an Anionic Surfactant In Radial Sandstone Cores
Authors: Kwok Wellington , Nasr-El-Din Hisham A. , Hayes R.E. 
Published: March 2010

Abstract
An experimental study was conducted to examine the propagation with adsorption of an anionic surfactant in unfired Berea sandstone cores. The effects of sodium chloride concentration, pH, flow rate, surfactant concentration, and the presence of a liquid crystal phase on the rate of surfactant propagation were studied. To simulate the flow field around an injection well, a radial geometry was employed.
Experimental results showed dramatic surfactant loss and slower rate of surfactant propagation as sodium chloride concentration was increased. On the other hand, increasing the pH of the surfactant slug from 6 to 12 reduced surfactant loss by nearly 30 percent. At a given sodium chloride concentration and surfactant concentration, surfactant loss was found to decrease as the injection flow rate was increased.
At high surfactant concentrations and over a narrow range of sodium chloride concentrations, a liquid crystal phase formed. The presence of this phase resulted in a higher apparent viscosity, a dramatic increase in the pressure drop across the core, and significant surfactant retention in the core. The latter was due to the formation of viscous fingers at the tail of the surfactant slug.

Introduction
Surfactants are used in many enhanced oil recovery processes (EOR), including micellar, micellar/polymer alkl/surfactant, lkali/surfactant/polymer and foam flooding. Surfactant propagation in the reservoir is a significant factor which determines the effectiveness of these enhanced oil recovery processes. Surfactant loss in a typical reservoir occurs as a result of adsorption on the rock surface, precipitation by divalent cations partitioning into the oil phase and surfactant/polymer interactions leading to precipitation(1). The present study addresses propagation and adsorption of an anionic surfactant with high salinity tolerance in unfired Berea sandstone cores. Such surfactants are used in alkali/surfactant(2) and alkali/surfactant/polymer(3) flooding to increase the optimal salinity of petroleum soaps encountered in these processes.
Berea sandstone contains an average of 5 to 9 wt% clay minerals (mainly kaolinite and illite). Anionic sufactants are absorbed on the charged clay particles. Surface charges on the clay particles riginate from isomorphous substitution lattice defects, and broken bonds. Consequently, adsorption of anionic surfactants oil the surfaces of clay minerals is governed by electrostatic and van der Waals forces(4–9).
Imporlant factors influencing the adsorption of anionic surfactants on sandstone, such as salinity, pH, and type and composition of surfactants, have been extensively discussed in previous studies(10–20). Summaries of previous work were given by Meyers nd Salter(21) and recently by Rupprecht and Gu(22). Based on the discussion of Somasundaran and Hanna(15), higher adsorption of anionic surfactants on sandstone with increasing salinity is caused by an increase in the surface acidity and the compression of the electric double layer which reduces the electrostatic repulsion between the surfactant monomers and the surface of the clay minerals. Solution salinity also affects the formation of ionic surfactant micelles. Some recent studies on this effect will be referred to in the results and discussion section.
Another important parameter that influences the surface charge of the clay particles is the solution pH.