Page 360 - Reservoir Formation Damage

340 Reservoir Formation Damage

Mineral stability charts can be more meaningfully developed by con-
sidering the incongruent equilibrium reactions of various solid phases
including the igneous and metamorphic reactions (Schneider, 1997).
Incongruent reactions represent the direct relationships of the various
solid minerals involved in aqueous solution systems. The expressions of
the incongruent reactions are derived from a combination of the relevant
mineral dissolution/precipitation reactions in a manner to conserve certain
key elements of the solid minerals so that the aqueous ionic species of
these elements do not explicitly appear in the final equation. For example,
the incongruent reactions of the alumino silicate minerals, including clay
minerals, feldspars, and chlorites, are usually expressed to conserve the
aluminum element (Fletcher, 1993; Schneider, 1997). Aluminum is a
natural choice as the conserved element because this element is mostly
immobile and the activities of the aqueous aluminum species are relatively
low (Hayes and Boles, 1992; Schneider, 1997). Consequently, the incon-
gruent mineral reaction equations do not involve the potential dissolved
+
3
aluminum species such as Af , Al(OH) 2 , Al(OH) 4~, Al(OH) +2 , and Al(OH) 3°
(Schneider, 1997). Thus, the aluminum element conserving incongruent
reaction to form the chlorite mineral from the kaolinite mineral reads as
(Schneider, 1997, p. 119):

+ 2A5Mg +2 + 2.25Fe +2
L4Al 2Si 2O 5(OH) 4
Kaolinite
+5.8//0 <-»
(13-25)
Chlorite

+0.1/f 4S*0 4°+8.8// +

The reactions for electrolyte dissolution in water can be represented by
(Schneider, 1997):

AB <->m^ +ne (13-26)
w w
Substituting unity for the activity of the solid phase, the expression of
the reaction quotient leads to the actual ion activity product, given by:

(13-27)
(actual) (actual)
At saturation, Eq. 13-27 yields the saturation ion activity product constant
given by: