Page 357 - Reservoir Formation Damage

Inorganic Scaling and Geochemical Formation Damage 337

Aqueous Speciation Models
Aqueous Speciation models describe the thermodynamic properties of
aqueous solutions and they are an integral part of the geochemical models.
Plummer (1992)* summarizes the constituents of these models as:

1. Mass balance equations for each element,
2. Mass action equations and their equilibrium constants, for complex-
ion formation, and
3. Equations that define individual ion-activity coefficients.
Two types of aqueous specification models are popular: (a) ion-
association models and (b) specific interaction models. The ion association
and the specific interaction models facilitate, respectively, the extensions
and a complex expansion of the Debye-Hiickel theory to estimate the
individual ion activity coefficients of aqueous species (Plummer, 1992).
The specific interaction models are preferred for highly concentrated
solutions of mixed-electrolytes (Plummer, 1992). As pointed out by
Plummer (1992), aqueous geochemical models can be used for forward
and inverse geochemical modeling.

Geochemical Modeling—Inverse and Forward
Plummer (1992)* summarizes that "Two approaches to geochemical
modeling have evolved—"inverse modeling," which uses water and rock
compositions to identify and quantify geochemical reactions, and "forward
modeling," which uses hypothesized geochemical reactions to predict
water and rock compositions." However, the application of these models
is rather difficult because the basic data necessary for these models are
often incomplete and/or uncertain (Plummer, 1992).
Plummer (1992)* describes the most essential information necessary
for geochemical modeling and its applications as following:
1. The mineralogy, and its spatial variation in the system,
2. The surface area of reactants in contact with aqueous fluids in
ground-water systems,
3. The chemical and isotopic composition of reactants and products
in the system,
4. The hydrology of the system,
5. The extent to which the system is open or closed,

* Reprinted from "Water-Rock Interaction," Proceedings of the 7th international symposium,
WRI-7, Park City, Utah, 13-18 July 1992 Kharaka, Y. K. & A. S. Maest (eds.), 90 5410
075 3, 1992, 25 cm, 1730 pp., 2 vols., EUR 209.00/US$246.00 GBP147. Please order
from: A. A. Balkema, Old Post Road, Brookfield, Vermont 05036 (telephone: 802-276-
3162; telefax: 802-276-3837; e-mail: info@ashgate.com).