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,

6. The temporal variation of these properties,

7. The fundamental knowledge on the kinetics and mechanisms of important water-rock reactions,

8. The kinetics of sorption processes, and

9. The degradation pathways of organic matter.

Inverse Geochemical Modeling

Plummer (1992)* explains that "Inverse geochemical modeling combines information on mineral saturation indices with mass-balance modeling to identify and quantify mineral reactions in the system." The mass-balance modeling requires (Plummer, 1992):

1. Element mass balance equations,

2. Electron conservation equations,

3. Isotope mass balance equations, when applicable,

4. Aqueous compositional and isotopic data, and

5. Mineral stochiometry data for all reactants and products.

Plummer (1992)* warns that "The inverse-modeling approach is best suited for steady-state regional aquifers, where effects of hydrodynamic dispersion can often be ignored."

Forward Geochemical Modeling

The objective of the forward geochemical modeling is to predict mineral solubilities, mass transfers, reaction paths, pH and pe by using available solid-aqueous data in aqueous specification models (Plummer, 1992). Some of the important features of the advanced forward geochemical models are cited by Plummer (1992) as:

1. Access to a large thermodynamic data base,

2. Generalized reaction-path capability,

3. Provision for incorporation of reaction kinetics in both dissolution and precipitation,

4. A variety of activity coefficient models,

5. Treatment of solid solutions,

6. Calculation of pH and pe,

7. Calculation of mineral solubilities with and without accompanying irreversible reaction,

8. Calculation of boiling, cooling, wall-rock alteration, ground-water mixing with hot waters and evaporation, and

9. Equilibrium or partial equilibrium states in gas-solid-aqueous systems.

Plummer (1992)* states that forward geochemical modeling can be used "in developing reaction models that can account for the observed compositional-mineralogical relations in the deposit, if there are no aqueous or solid data for the system."

Reaction-Transport Geochemical Modeling

The reaction-transport models describe the geochemical reactions under the influence of fluid flow and convective and dispersive transport of various species in geological porous media. These models couple the geochemical reaction and the fluids and species transport submodels to accomplish temporal and spatial prediction of the evolution of geochemical reactions in compositionally-complex geological systems (Plummer, 1992). These models are more applicable in most petroleum reservoir exploitation and scale formation studies.

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