The TOUGH Simulator: Can It Take the Heat?

Abstract

The TOUGH suite of nonisothermal multiphase flow and transport simulators has originally been developed in the early 1980s for applications in geothermal reservoir engineering, which required special attention to coupled fluid and heat flow, accurate description of thermophysical fluid properties, and an approach to represent fractured systems in a reasonable and computationally tractable manner. While the TOUGH simulators have evolved and are currently applied to address a wide variety of deep subsurface and near-surface problems, the highly nonlinear and coupled nature of geothermal system behavior still poses formidable challenges and provides the motivation to advance the codes. Attempts to significantly increase the worldwide production of geothermal energy led to the study and exploration of new systems for potential heat mining. Specifically, heat extraction from deeper systems with higher temperatures, the concept of enhanced geothermal system (EGS), the use of alternative working fluids (such as carbon dioxide), and the deployment of advanced drilling technologies require corresponding enhancements of simulation capabilities, including equation-of-state and reactive geochemistry modules with expanded temperature and pressure ranges, coupled thermal-hydrological-mechanical process simulations to address induced seismicity issues, and wellbore simulators linked to the reservoir model. Moreover, characterization methods need to be refined and the supporting analysis tools updated. This paper summarizes recent advances of the TOUGH suite of simulation and analysis tools, and discusses their limitations and applicability in geothermal reservoir engineering.