Ion-Exchange Conditioning of Sandstones for Chemical Flooding
Authors: Smith F.W. 
Published: December 2007

The ionic hardness of reservoir brines and clays can be destructive in some of the newer chemical flooding processes. Such aggressive conditions were treated in laboratory studies by preflushing with soft brines to displace the offending ions. Results for some cases show that successful preflushing requires large volumes of low- or moderate-salinity brine, whereas smaller volumes of more saline brines may be adequate.

Introduction
One current obstacle to the widespread use of certain tertiary oil recovery processes is the reservoir ionic environment that injected fluids must withstand. Those processes based on micellar fluids containing substantial processes based on micellar fluids containing substantial quantities of anionic surfactants (such as petroleum sulfonates) may be particularly vulnerable to the effects of high salinity or moderately high multivalent-cation concentration. Even when acceptable chemical formulations exist for specific ionic conditions, oil displacement efficiency may suffer if the reservoir brine composition is not reasonably constant throughout the reservoir. Reservoir conditioning is attempted primarily for these reasons. By injecting a brine of the desired composition ahead of a slug of micellar solution, the most favorable circumstances for effective performance of the chemicals may be established. This study examines the efficiency of preflushing in situations where ion-exchange phenomena are important. In particular, the replacement of divalent cations (such as calcium and magnesium) by less offensive monovalent cations (such as sodium) was studied. The effects of soluble minerals like gypsum or anhydrite were not considered; their behavior can be described adequately by relatively straightforward solubility relationships. Also, the use of chemicals in the preflush brine to react with divalent cations by precipitation or complex ion formation was not examined. It is hoped that this study can be used to determine whether such chemical treatments are needed.

Basic Considerations
Ion exchange is a process in which an ion bound to a surface is replaced by another ion of like electrical charge from a surrounding solution. In the simplest exchange cases, the total ionic charge on the surface does not change, nor does the bulk concentration of ionic charges in solution. Thus, even though the solution chemical composition may change, salinity will not change when defined as total equivalents of cations per unit volume (as done in this paper). Among the usual components of sandstone petroleum reservoirs, clay minerals are well known for their ion exchange properties. The cation exchange capacities (CEC) vary from about 10 meq (milliequivalents)/ 100 gm of kaolinite to about 100 meq/100 gm of montmorillonite. Other sandstone materials reported to possess ion-exchange properties are quartz, feldspars, and organic matter. The exchange capacities of quartz and feldspars are usually quite low, but the capacity of solid organic matter may be as high as 150 to 500 meq/100 gm. The major factors that appear to influence cation exchange and, thus, preflush efficiency are cation exchange capacity, brine composition, shape of exchange isotherm, and brine salinity. Fig. 1 shows two hypothetical exchange isotherms of an ion-exchanging substance in contact with a solution containing two cationic species. These curves represent the quantity of one species adsorbed on the ion exchanger as a function of its relative concentration in solution.
JPT
p. 959