Page 130 - Reservoir Formation Damage

112 Reservoir Formation Damage

lattice substitutions, and surface edge reactions are dominant for kaolinite
and chlorite (Kleven and Alstad, 1996). Expansion of swelling clays, such
as montmorillonite, increases their surface area of exposure and, therefore,
their cation-exchange capacity (Kleven and Alstad, 1996). Theoretical
description of the ion-exchange reactions between the aqueous phase and
the sedimentary formation minerals is very complicated because of various
effects, including ion composition, pH, and temperature (Kleven and
Alstad, 1996).
The methods used for measurement of the ion-exchange capacity vary
by the reported studies. For example, Kleven and Alstad (1996) measured
the CEC of clays using Ca 2+ brines without the presence of NaCl and
measured the AEC using SO%~ brines. Rhodes and Brown (1994) point
out the CEC measurement of clays by commonly used methods, such as
the ammonium ion and methylene blue dye adsorption methods, have
inherent shortcomings, leading to inaccurate results. Therefore, Rhodes
and Brown (1994) have used the adsorption of the colored Co(H 2O)
ion, which yields a very stable hydrated Co(If) complex. Rhodes and
+
Brown (1994) have determined that the CECs of four different Na -
montmorillonites measured by three different adsorption methods differ
appreciably. The methylene blue adsorption method generates significantly
different results from the cobalt and ammonium ion adsorption methods,
which agree with each other within acceptable tolerance.
Because the ion-exchange reactions in petroleum-bearing rock are usually
treated as equilibrium reactions for practical purposes, ion-exchange isotherms
relating the absorbed and the aqueous phase ion contents in equilibrium
conditions are desirable. For example, Kleven and Alstad (1996) deter-
mined the cation-exchange isotherms shown in Figures 6—4, 6-5, and 6-6,
respectively, for single cation-exchange reactions involving

Ca 2+ -> Na + (6-2)

and

l+
Ba -> Na + (6-3)
and binary cation exchange reactions involving

2+ 2+ + (6-4)
Ca + Ba -> Na
Similarly, Figure 6-7 by Kleven and Alstad (1996) shows the typical
anion-exchange isotherms for a single anion-exchange reaction involving
SOl ~^ d • When more than one ions are present in the system, some
are preferentially more strongly adsorbed than the others depending on