Cation Exchange and Chemical Flooding
Authors: Hill H.J. , Helfferich F.G. , Lake L.W. , Reisberg J. , Pope G.A. 
Published: December 2007

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The ionic composition of surfactant systems is an important determinant of system performance. This is reflected by the use of prefloods to displace formation waters considered too saline and/or too "hard" and by limitations placed on the ionic composition of the water used in the surfactant fluid. Recent literature reflects a growing concern about the role of the reservoir clay in determining the in-situ composition of the surfactant formulation.
In this article we show a laboratory example of a low-salinity preflood that, because of cation exchange with clays, results in a threefold increase in divalent cation (Ca++ and Mg++) concentration in the front of the surfactant flood, We outline the basic concepts required to model the reversible changes in cationic composition that result from cation exchange and indicate the general direction of our research efforts on this important aspect of surfactant flooding.
Disaggregated Tar Springs sandstone with a cation exchange capacity of 0.44 meq/100 gm was used to prepare a 1 -in.-diameter, 2-ft-long sand pack. We prepare a 1 -in.-diameter, 2-ft-long sand pack. We saturated the pack with a saline Tar Springs formation water (Fig. 1) and flowed 3.8 PV of this water through the pack at a frontal advance rate of 1 ft/D and a temperature of 85 deg. F. We then displaced the formation water with a "preflood" containing 7.5 percent formation water mixed with a fresh lake water. After 1.5 PV of preflood injection, steady-state composition was achieved. The flood was continued to a total preflood injection of 4.4 PV. After this pack equilibration period, 3.7 PV of a PV. After this pack equilibration period, 3.7 PV of a chemical slug prepared in lake water containing 5 percent formation water were injected. Steady-state composition was achieved after 1.5 PV.
As shown in Fig. 1, the preflood "loaded" the clays with calcium and magnesium. The "denuded" chemical slug water and the front portion of the chemical slug "unloaded" these cations from the clays. The concentration of divalent cations in the front part of the chemical slug increased from 0.0153 to 0,051 meq/ml. Calculated as Ca++ only, this is equivalent to an increase from 306 to 1,020 ppm, an amount sufficient to radically alter interfacial and viscometric properties of the system.
Various equations describing the relation between cation distribution on a clay surface and composition of the equilibrium solution have been given in the literature. For the experiment reported, simple mass-action equations adequately describe the equilibria between clays and brine solutions but, as used, do not adequately do scribe the equilibria with the surfactant system. These equations are
(1) and
(2)
where C and C are the concentrations of the indicated cation associated with the clay and equilibrium solution, respectively, both expressed as milliequivalents per milliliter of pore volume. Applicable values of K and K may vary with the type of clay, an ionic strength parameter. and temperature. Thus, each reservoir may present a parameter. and temperature. Thus, each reservoir may present a new experimental evaluation problem.
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