TOUGHREACT Version 2.0: A simulator for subsurface reactive transport under non-isothermal multiphase flow conditions
Introduction
TOUGHREACT is a numerical simulation program for chemically reactive nonisothermal flows of multiphase fluids in porous and fractured media (Xu and Pruess, 2001, Spycher et al., 2003, Sonnenthal et al., 2005, Xu et al., 2006, Xu, 2008, Zhang et al., 2008, Zheng et al., 2009). The program was written in Fortran 77 and developed by introducing reactive chemistry into the multiphase fluid and heat flow simulator TOUGH2 (Pruess, 2004). The program can be applied to one-, two- or three-dimensional porous and fractured media with physical and chemical heterogeneity. The code can accommodate any number of chemical species present in liquid, gas, and solid phases. A variety of subsurface thermal, physical, chemical, and biological processes are considered under a wide range of conditions of pressure, temperature, water saturation, ionic strength, and pH and Eh.
Processes for fluid flow and heat transport are the same as in the original TOUGH2. Transport of aqueous and gaseous species by advection and molecular diffusion is considered in both liquid (aqueous) and gas phases. Depending on computer memory and CPU performance, any number of chemical species in the liquid, gas, and solid phases can be accommodated. In the 2004 version (Version 1.0) of TOUGHREACT, aqueous complexation, acid-base, redox, gas dissolution/exsolution, and single-site cation exchange were considered under the local equilibrium assumption. Mineral dissolution and precipitation could proceed either subject to local equilibrium or kinetic conditions.
Over the past several years, many new capabilities have been developed within different research projects at Lawrence Berkeley National Laboratory. We have incorporated these new capabilities into Version 2.0 of TOUGHREACT. Major additions and improvements in Version 2.0 include:
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intra-aqueous reaction kinetics and biodegradation;
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surface complexation models including double layer;
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multi-site cation exchange;
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improvements on reactive surface area algorithm for mineral–water reactions, and fugacity coefficient corrections for gas–water reactions;
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improvements on mineral solid solution model that is an ideal model and only available for minerals reacting under kinetic constraints;
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improvements in coupling and mass balances between the chemistry and physics parts, including changes in rock and fluid properties due to reactions, and accounting for CO2 fixed as carbonates in the flow simulation (for using ECO2N module; Pruess and Spycher, 2007);
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improvement on functionalities such as printouts of mineral reaction rates, and aqueous component and species concentrations in different user-selectable units; and
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improvements in computational efficiency.
TOUGHREACT V2.0 is written in FORTRAN 77 with some Fortran-90 extensions. It has been tested on various computer platforms, including Microsoft Windows- and Linux-based PCs, Apple Macintosh G4 and G5, and Intel-based computers. An effort was made for the TOUGHREACT source code to comply with the ANSIX3.9-1978 (FORTRAN 77) standard, and on most machines the code should compile using Fortran 95, Fortran-90, and some Fortran 77 compilers, and run without modification. TOUGHREACT (like TOUGH2 V2) requires 64-bit arithmetic (eight byte word length for floating point numbers) for successful execution; the code is intrinsically double-precision, to achieve 64-bit arithmetic on the commonly used 32-bit processors. The computer memory required by TOUGHREACT depends on the problem size such as numbers of grid blocks, aqueous and gaseous species, and minerals. Array dimensions are set in parameter statements in the INCLUDE files.
The correct implementation, setup, problem formulation, and interpretation of the results of TOUGHREACT requires knowledge of the basic equations of multiphase non-isothermal fluid flow and transport in geologic media, and a basic understanding of the numerical solution of the equations that are used to describe these processes. In addition, the formulation of the geochemical problem requires familiarity with geochemical modeling and an in-depth understanding of the system that is being modeled and of the data used for input to the model. The model boundary conditions, time step length, convergence criteria, and grid properties are crucial elements for a realistic and accurate solution to a problem.
We first present the general formulation for solving flow, transport, and reaction equations. To illustrate new features and applicability of TOUGHREACT Version 2, we then present two applications examples.
Section snippets
Mathematical formulation
TOUGHREACT uses a sequential iteration approach similar to Yeh and Tripathi (1991).
After solution of the flow equations, the fluid velocities and phase saturations are used for chemical transport simulation. The chemical transport is solved on a component-by-component basis. The resulting concentrations obtained from solving transport equations are substituted into the chemical reaction model. The system of mixed equilibrium-kinetic chemical reaction equations is solved on a grid block by grid
Multiple interacting continua
For chemical transport in variably saturated fractured rocks, global fluid flow and transport of aqueous and gaseous species occurs primarily through a network of interconnected fractures, while chemical species may penetrate into tight matrix blocks primarily through relatively slow diffusive transport in gas and liquid phases. Methods developed for fluid flow in fractured rock can be applied to the geochemical transport.
The method of “multiple interacting continua” (MINC) is used to resolve
Application examples
TOUGHREACT V2.0 has been applied to a wide variety of geological and environmental problems. An application to denitrification and sulfate reduction was presented in Xu (2008). The possible mobilization of inorganic constituents such as Pb and As in groundwater in response to CO2 leakage from deep geological storage has been modeled in Zheng et al. (2009), who considered aqueous complexation, mineral dissolution/precipitation, and desorption/adsorption via surface complexation, using the double
Conclusions
A non-isothermal reactive transport program, TOUGHREACT, has been developed, which allows comprehensive modeling of chemical interactions between liquid, gaseous, and solid phases that are coupled to solute transport and subsurface multiphase fluid and heat flow. The program 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 of pressure, temperature, water saturation, ionic
Acknowledgment
The development of TOUGHREACT was supported by various DOE program offices, and documentation of Version 2.0 was supported by the Zero Emission Research and Technology project (ZERT) of the U.S. Department of Energy under Contract no. DE-AC02-05CH11231 with Lawrence Berkeley National Laboratory.
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Current address: Shell International E&P Inc., Houston, TX 77079, USA.