Application Of The Ion Exchange Process To Reservoir Preflushes

Abstract

Introduction The ionic environment of a reservoir is important in micellar flooding and in any other process where ion sensitive agents, such as caustic process where ion sensitive agents, such as caustic or wettability alteration fluids, are flooded through a reservoir. Ton exchange can have a major, if not the controlling influence on the composition of brines in petroleum reservoirs. Ion exchange operations are chemical reactions between an electrolyte (the ions) in solution and an insoluble electrolyte (the clays) contacted by the solution. Clays in reservoirs behave as ion exchangers. Normally, they are in equilibrium with the reservoir brine. When fluids of a different composition, not in equilibrium with the clays, contact the clays, an exchange of ions between the clays and the fluid takes place until the two are in equilibrium. The exchange is primarily with cations (positively charged ions) primarily with cations (positively charged ions) with little, if any, anion (negatively charged ions, such as chloride) exchange occurring. To preserve electrical neutrality, the exchange is preserve electrical neutrality, the exchange is stoichiometric with respect to the charges of the ions. Material balances show that two monovalent cations (sodium) are exchanged for one divalent cation (calcium). A relatively hard though fresh Madison water has been injected during the waterflood of the Second Wall Creek Reservoir of the Salt Creek Field. The reservoir is working as a large water softener, exchanging the calcium and magnesium from the injected water for sodium on the clays. The result is that the clays near the injectors have been hardened by the Madison water, while the clays near the producer can still remove the divalent ions and soften the injected water. This hardening process can be reversed by injection of a soft brine containing low levels of calcium and magnesium. This process is analogus to the regeneration of a water softener. Injection of a micellar fluid more saline than the Madison water would reverse the process causing high concentrations of hardness which could be detrimental to an ion sensitive micellar fluid. The purpose of this paper is to present experimental data and model studies showing how the ion exchange process has affected and will affect the Second Wall Creek reservoir. A method for preflushing the reservoir to remove the divalent ions is also presented. Additional insights into the ion exchange process and the nature of the ion equilibrium are discussed. Background Theory The ion exchange reaction is described by Equation 1 (1) as a reversible chemical reaction between free ions (Na and Ca) in solution and ions adsorbed on the clays where X is an ion exchange site. To preserve electrical neutrality, two monovalent preserve electrical neutrality, two monovalent (sodium) ions exchange for one divalent (calcium) ion. The clay ion exchange sites can be thought of as a separate chemical species involved in the reaction.