Download A Fully-Coupled Finite Element Code for Modelling Thermo

FEM Modelling of Coupled Thermo-Hydro-Mechanical Processes in Porous
Geological Media
Lanru Jing
Royal Institute of Technology, Stockholm, Sweden
The presentation describes a new FEM method and code for simulating fully-coupled
thermo-hydro-mechanical processes in porous geological media, which may be
applicable for design, operation, performance and safety assessments of rock/soil
engineering in general and geological radioactive waste repositories in particular.
The governing equations are based on the theory of mixtures applied to the multiphysics
of porous media. Among the phenomena accounted for are solid-phase deformation,
liquid-phase flow, gas flow, heat transport, thermally-induced water flow, phase change
of water, and swelling deformation. For three-dimensional problems, eight governing
equations are presented to describe the coupled THM processes. Three displacement
components, temperature, water pressure, gas pressure, vapor pressure and porosity of teh
media are chosen as the eight primary variables. Parameters such as the density,
viscosity, thermal expansion coefficient and bulk modulus of the water and gas are
expressed in terms of the basic variables. The elastic moduli, permeability and thermal
expansion coefficient of the solid are taken to be constant, or to depend on stress,
pressure and temperature without requiring fundamental modifications to the code. The
relative permeability of both the liquid and vapor phase are functions of the liquid
saturation.
The eight governing continuum equations are discretized using the Galerkin finite
element formulation. This leads to a non-symmetric matrix equation that has many small
entries along its diagonal, and is therefore ill-conditioned. An interlaced solution
approach was developed to solve the global matrix equation.
The code was validated against several classical analytical solutions to problems in
poroelasticity and thermoelasticity, including the Mandel-Cryer problem for a porous
cylinder. It was applied for simulating a laboratory experiment on compacted MX-80
bentonite (as a buffer material between radioactive waste canisters and the host rock in a
geological repository), which was performed by the French Commission of Atomic
Energy from 2003 to 2004. Good agreement was found between the numerical
predictions and experimental data, and continued development is in progress for more
comprehensive constitutive models of rock and fluids, and equation solution efficiencies.