Cation Exchange in Chemical Flooding: Part 2--The Effect of Dispersion, Cation Exchange, and Polymer/Surfactant Adsorption on Chemical Flood Environment

Abstract

Abstract In this paper, we extend some of the results of Part 1 by discussing aspects of the effects of Part 1 by discussing aspects of the effects of dispersion, two-cation exchange, and adsorption on a chemical flood:we show that when chemical slug sizes are small and dispersion is moderate, the inclusion of dispersion in the calculation can modify results that are calculated by methods which assume no dispersion;we describe a new phenomenon in which dispersion itself can initiate cation exchange; andfinally, we describe an asymptotic concentration profile toward which dispersive, self-sharpening fronts tend. Introduction One of the most important factors affecting the interfacial activity, phase behavior, and mobility control of a chemical flood is the ionic environment in which the chemical slug passes through the reservoir. Factors affecting this environment are the in-place and injected-fluid compositions and dispersive mixing. Equally important, but perhaps not so obvious, is the role that cation exchange can play in determining the chemical slug's environment. The importance of surfactant and polymer adsorption to the practicality and economics of the chemical flood process long has been recognized. This is because adsorption directly affects the quantity and rate at which surfactant and polymer can be propagated through the reservoir. The interfacial activity and, to a lesser extent, the mobility of the surfactant and polymer are as strongly affected by their ionic environment, particularly the divalent ion concentration, as by particularly the divalent ion concentration, as by their own concentration levels for some ranges. Concern about this sensitivity has led to formulations that include cosurfactants to increase salinity tolerance in the chemical slug, and to preflooding schemes that lower reservoir salinity to acceptable levels. Since cation exchange profoundly affects the flowing phase ionic environment, it must play a central role in the design and implementation of a chemical flood. Toward this goal, this paper tries to illustrate some consequences of cation exchange and polymer/surfactant adsorption in porous media, polymer/surfactant adsorption in porous media, including the effect of fluid-dynamic dispersion. This paper is a companion to Part 1, dealing with cation exchange in nondispersing systems. We, therefore, use some conventions and definitions from that paper. ASSUMPTIONS The assumptions used here are similar to those in Part 1, except thatfluid-dynamic dispersion is not neglected,all material-balance relations are restricted to single-phase flow (there is no oil in the system), andthe system can have no more than five components: calcium (C1 - meq/ml) - a divalent cationic component; sodium (C2 - meq/ml) - a monovalent cationic component; chloride or total salinity (C3 - meq/ml) - a nonadsorbing anionic component; surfactant (C4 - meq/mi) - an adsorbing component; and polymer (C5 - ppm) - a nonionic adsorbing component. Note that all concentrations for C1 through C4 (including the stationary phase concentrations) are in milliequivalents per milliliters of pore volume. The foregoing assumptions restrict somewhat the usefulness of the following results when predicting the behavior of real systems. Nevertheless, these results effectively illustrate the combined influence of dispersion, cation exchange, and polymer/ surfactant adsorption on a chemical flood's environment. SPEJ P. 435