The TOUGH Codes—A Family of Simulation Tools for Multiphase Flow and Transport Processes in Permeable Media

the same mathematical form, regardless of the nature and number of fluid components and phases. MULNumerical simulation has become a widely practiced and accepted KOM was a research code that served as a test bed for technique for studying flow and transport processes in the vadose developing much of the approaches and methodology zone and other subsurface flow systems. This article discusses a suite of codes, developed primarily at Lawrence Berkeley National Laborathat were subsequently implemented in TOUGH and tory (LBNL), with the capability to model multiphase flows with TOUGH2. A stripped-down version of MULKOM for phase change. We summarize history and goals in the development two-phase flow of water–air mixtures was released to of the TOUGH codes, and present the governing equations for multithe public in 1987 under the name TOUGH (Pruess, phase, multicomponent flow. Special emphasis is given to space dis1987). The acronym “TOUGH” stands for “transport cretization by means of integral finite differences (IFD). Issues of of unsaturated groundwater and heat,” and is also an code implementation and architecture are addressed, as well as code allusion to the tuff formations at Yucca Mountain, which applications, maintenance, and future developments. represented one of the chief application areas of the code at the time. A more comprehensive subset of MULKOM modules was later released under the name T is a numerical simulation program for TOUGH2 (Pruess, 1991a) through the Energy Science nonisothermal flows of multiphase, multicomponent and Technology Software Center (ESTSC; http://www. fluids in permeable (porous or fractured) media (Pruess, osti.gov/estsc/) of the USDOE and was more recently 1991a; Pruess et al., 1999). Developed primarily for apupdated to TOUGH2 version 2.0 (Pruess et al., 1999). plications to geothermal reservoir engineering, nuclear T2VOC (Falta et al., 1995) and TMVOC (Pruess and waste disposal, environmental remediation problems, and Battistelli, 2002) represent offshoots of TOUGH2 that vadose zone hydrology, TOUGH2 and related codes (Taare focused on environmental contamination problems ble 1) are currently in use in approximately 300 installawith non-aqueous phase liquids (NAPLs). Other related tions in more than 30 countries. This article gives a brief codes include iTOUGH2, which provides capabilities for summary of the scope, design, and methods used in inverse modeling, optimization, and sensitivity and uncerTOUGH2. It serves as an introduction to this special tainty analysis (Finsterle, 1999); TOUGHREACT, which section of Vadose Zone Journal, which features excouples TOUGH2 with a general chemical speciation panded versions of selected papers focusing on the vadose and reaction progress package (Xu and Pruess, 2001b); zone that were originally presented at the TOUGH Symand TOUGH-FLAC, which couples TOUGH2 with the posium 2003 (Finsterle et al., 2003; also available online commercial rock mechanics code FLAC3D (Rutqvist at http://esd.lbl.gov/TOUGHsymposium/). and Tsang, 2002). We begin with some brief remarks about the history Since its beginnings in the early 1980s, the developof TOUGH2 and then present the physical and mathement of TOUGH2 was driven by a desire to model matical model implemented in the code. Special emphaspecific types of flow systems. The early development sis is given to the IFD method used for space discretizawas focused on geothermal reservoir dynamics. Among tion and to general issues of code architecture and the important issues for geothermal reservoir modeling maintenance. The paper concludes with some comments are the nonisothermal nature of flow, the importance on current and future code developments. of phase change (boiling and condensation), and the highly nonlinear nature of two-phase (water-steam) HISTORICAL REMARKS flow. The first functional version of MULKOM was a single-porosity simulator that solved a mass balance for The ancestor of the current TOUGH codes is a simuwater and an energy balance—noncondensible gases lation program known as MULKOM, which was devel(NCGs) or dissolved solids were not included. In geooped at LBNL in the early 1980s (Table 1). The architecthermal reservoir problems the coupling between the ture and methodology of MULKOM was based on the mass and energy balance equations can be very strong, recognition that the governing equations for nonisotherseverely limiting the time step for which a sequential mal flows of multicomponent, multiphase fluids have iteration procedure will converge. For example, for a problem of cold water injection into a vapor-dominated Earth Sciences Division, Lawrence Berkeley National Laboratory, reservoir that would entail rapid vaporization with University of California, Berkeley, CA 94720. Received 8 Aug. 2003. strong latent heat effects, it was estimated that a sequenSpecial Section: Research Advances in Vadose Zone Hydrology through Simulations with the TOUGH Codes. *Corresponding author Abbreviations: EOS, equation of state; ESTSC, Energy Science and ([email protected]). Technology Software Center; IFD, integral finite differences; LBNL, Lawrence Berkeley National Laboratory; NAPL, nonaqueous phase Published in Vadose Zone Journal 3:738–746 (2004).  Soil Science Society of America liquid; NCG, noncondensible gas; PDE, partial differential equation; VOC, volatile organic compound. 677 S. Segoe Rd., Madison, WI 53711 USA