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TOUGHREACT Software

Summary

TOUGHREACT is a numerical simulation program for chemically reactive non-isothermal flows of multiphase fluids in porous and fractured media, developed by introducing reactive chemistry into the multiphase flow code TOUGH2. Interactions between mineral assemblages and fluids can occur under local equilibrium or kinetic rates. The gas phase can be chemically active. Precipitation and dissolution reactions can change formation porosity and permeability, and can also modify the unsaturated flow properties of the rock.

Features & Capabilities

TOUGHREACT is applicable to one-, two-, or three-dimensional geologic domains with physical and chemical heterogeneity and can be applied to a wide range of subsurface conditions. The temperature (T) and pressure (P) range is controlled by the applicable range of the chemical thermodynamic database, and the range of the EOS module employed. Because the equilibrium constants are generally not as sensitive to pressure as to temperature, the temperature dependence of equilibrium constants is taken into account by TOUGHREACT but the pressure dependence is not. The temperature and pressure range of thermodynamic data can be extended by changing the thermodynamic database without code modifications. It is the user’s responsability to ensure that the thermodynamic data used with this software is appropriate for the temperature and pressure range of the simulated systems. Water saturation can vary from completely dry to fully water-saturated. The model can deal with ionic strengths from dilute to moderately saline water (up to ionic strengths in the 2–4 molal range, for an NaCl-dominant solution, depending on the system being modeled).

The major processes for fluid and heat flow are: (1) fluid flow in both liquid and gas phases occurs under pressure, viscous, and gravity forces; (2) interactions between flowing phases are represented by characteristic curves (relative permeability and capillary pressure); (3) heat flow by conduction and convection, and (4) diffusion of water vapor and air. Thermophysical and geochemical properties are calculated as a function of temperature, such as fluid (gas and liquid) density and viscosity, and thermodynamic and kinetic data for mineral-water-gas reactions. Transport of aqueous and gaseous species by advection and molecular diffusion are considered in both liquid and gas phases. In principle, any number of chemical species in the liquid, gas and solid phases can be accommodated. Aqueous complexation, acid-base, redox, gas dissolution/exsolution, and cation exchange are considered under the local equilibrium assumption. Mineral dissolution and precipitation can proceed either subject to local equilibrium or kinetic conditions. Linear adsorption and decay can be included.

The primary governing equations for multiphase fluid and heat flow, and chemical transport have the same structure, derived from the principle of mass (or energy) conservation (see TOUGH2).  The transport equations are written in terms of total dissolved concentrations of chemical components, which are concentrations of the basis species plus their associated aqueous secondary species. If kinetically-controlled reactions occur between aqueous species, then additional ordinary differential equations need to be solved to link the total concentrations of the primary species with the evolving concentrations of the secondary species. Advection and diffusion processes are considered for both the aqueous and gaseous species. Aqueous species diffusion coefficients are assumed to be the same. Gaseous species, having a neutral valence, can have differing diffusion coefficients calculated as a function of T, P, molecular weight, and molecular diameter. The local chemical interactions in the transport equations are represented by reaction source/sink terms.

The primary governing equations must be complemented with constitutive local relationships that express all parameters as functions of fundamental thermophysical and chemical variables. Mass conservation in the closed chemical system is written in terms of basis (component) species. The species distribution must be governed by the total concentrations of the components. Oxygen is used for formulating redox reactions by attributing the oxidizing potential to the dissolved oxygen. In contrast to the free electron in the hypothetical electron approach, oxygen can be present and can be transported in natural subsurface flow systems. For kinetically-controlled mineral dissolution and precipitation, a general form of the rate law is used. Thermodynamic and kinetic data are functions of temperature.

Temporal changes in porosity, permeability, and unsaturated hydrologic properties owing to mineral dissolution and precipitation can modify fluid flow. Changes in porosity during the simulation are calculated from changes in mineral volume fractions. Several porosity-permeability and fracture aperture-permeability relationships are included in the model. The code can also be set to monitor changes in porosity and permeability during the simulation without considering their effects on fluid flow. In unsaturated systems, capillary pressure can be modified via permeability and porosity changes using Leverett scaling.

TOUGHREACT-Pitzer provides reactive geochemical transport modeling of concentrated aqueous solutions based on the Pitzer ion-interaction model.

Applications

TOUGHREACT is applicable to a variety of reactive fluid and geochemical transport problems, including (a) contaminant transport with linear Kd adsorption and radioactive decay, (b) natural groundwater chemistry evolution under ambient conditions, (c) assessment of nuclear waste disposal sites, (d) sedimentary diagenesis, and CO2 disposal in deep formations, (e) mineral deposition such as supergene copper enrichment, and (f) mineral alteration and silica scaling in hydrothermal systems under natural and production conditions.

Licensing & Download

TOUGHREACT V2.0 and TOUGREACT-Pitzer V1.21 are distributed by the Berkeley Lab Software Center.

Documentation