ABSTRACT
We developed a massively parallel simulator for modelling fully coupled multiphase fluid and heat flow and geomechanics in porous and fractured media. The simulator's starting point is the TOUGH2-MP fluid and heat transport formulation. We added to that formulation a geomechanical one, which combines the stress-strain, stress equilibrium, and strain-displacement relations to yield an equation for mean stress, pressure, and temperature. Individual stress tensor components are also calculated, from mean stress, pressure, and temperature, using relations derived from the thermo-multi-poroelastic Navier equation. In addition, permeability and porosity are functions of stress and other variables using correlations from the literature and poroelastic theory. Fractured media are represented using the MINC (Multiple Interacting Continua) model as well as an embedded discrete fracture model, and sorption tracer kinetics have been added. The simulator's conservation equations are solved using a finite difference approximation on a three-dimensional, unstructured grid. Example simulations are presented to illustrate the simulator's capabilities: a comparison of simulation with an analytical solution for a purely elastic problem, a demonstration of the dynamic adsorption formulation using a case from the literature, and a simulation of an EGS (Enhanced Geothermal System) using the discrete fracture model that is also from the literature.
A thermal-hydrological-mechanical (THM) simulator is a useful tool for modelling processes such as oil and gas production from low permeability and unconsolidated reservoirs, energy production from geothermal reservoirs, nuclear waste storage, and carbon dioxide storage in deep saline aquifers, since reservoir geomechanics and fluid and heat flow affect one another in these processes.
THM simulators have been described in the literature and some examples of them follow. TOUGH2-FLAC3D (Rutqvist et al., 2002) is a linkage of TOUGH2, a well-established code for geohydrological analysis with multiphase, multicomponent fluid flow and heat transport, and FLAC3D, a widely used commercial code that is designed for rock and soil mechanics with thermomechanical and hydromechanical interactions. CODE-BRIGHT (Olivella et al., 1996) is a finite element code that simulates non-isothermal multiphase flow of brine and gas in deformable media. FEHM (Bower and Zyvoloski, 1997) is a coupled thermal-hydrologic-mechanical dual porosity finite element code that allows permeability of up to two perpendicular fracture planes to vary with normal stress. OpenGeoSys (Wang and Kolditz, 2007; Goerke et al., 2011) is an object oriented open source thermal-hydrological-mechanical finite element code.
