Page 363 - Reservoir Formation Damage

Inorganic Scaling and Geochemical Formation Damage 343

TS
log = log (13-32)
10 SPkaollm, \oK eq
A plot of K t versus Ks according to Eq. 13-32 yields the
SPcMoritt Pkaotii,
kaolinite-chlorite stability chart (Schneider, 1997) using the SOLMINEQ.88
program (Kharaka et al., 1988). Schneider (1997) points out that the
determination of the aqueous species activities is particularly complicated
in highly concentrated oilfield brines because of the complexing of cations
with inorganic and organic anions, and can be better accomplished by
means of a simulator such as the SOLMINEQ.88 program by Kharaka
et al. (1988).

pe- pH Charts
The pe - pH charts are constructed to describe the redox state of
reservoirs (Stumm and Morgan, 1996; Schneider, 1997). Considering the
+
electrons, e~, and protons, H , involved, chemical equilibrium reactions,
such as oxidation-reduction (redox) and acid-base reactions, are repre-
sented by

aA + bB +.. .ne~ + mH + <-> cC + dZ) +... (13-33)

The electron activity (pe) and potentiometric acidity (pH) can be con-
veniently expressed by the following equations, respectively:
= -log ]0[ (13-34)

and

(13-35)

The electrode potential (Eh) and electron activity (pe) are related by
(Schneider, 1997)

„, 2.30259RT
hh = pe (13-36)

in which T denotes the absolute temperature in K, /? = 8.31441
J - K~ ] - mol~ l is the universal gas constant and F = 9.64846 x 10 4 Coloumbl
mol is the Faraday constant. The electrode potential can be measured directly.
Eqs. 13-34 through 36 form the convenient mathematical bases for
constructing the pe- pH or Eh-pH charts. However, the pe - pH charts
are preferred over the Eh-pH charts because, while the sign of pH does
not change and the slopes of the stability boundaries are independent of
temperature, the sign of the Eh potential depends on the direction of