Universität Tübingen
Contributions to computational geotechnics : non-isothermal flow in low-permeable porous media
Abstract
To better understand the coupling of thermal (T), hydraulic (H) and mechanical (M) processes (T-H-M processes) and their influence on the system behaviour, models allowing T-H-M coupling are developed. These models allow simulations in the near-field of the system. The modeling of non-isothermal thermo-hydraulic (TH) processes is important for applications such as geothermal energy generation, heat supported environmental remediation, and nuclear waste disposal. The work presented herein focuses on deep geological disposal of nuclear waste, and more specifically on the thermal and hydraulic processes in this application. Thermal processes result directly from the heat radiation of the waste and include heat transport from the core to the bentonite buffer. Other processes of importance are vaporization and condensation associated with phase changes between the liquid and gaseous phases. Hydraulic processes of importance include water intrusion from the host rock to the buffer and eventually to the core, as well as swelling and shrinking processes in the bentonite. Bentonite swells as a result of water intrusion from the host rock and dries as a result of the heat transport from the core. The objective of the work is to formulate the processes mathematically and to integrate them into the object-oriented simulator GeoSys/RockFlow. Buffer, host rock, and fluids in the gas and liquid phase form a multiphase-multicomponental system (porous medium). The TH model consists of a set of three balance equations. One balance equation for the water component, one balance equation for the air component and one energy balance equation. The three primary, or independent variables are gas pressure, water saturation, and temperature. To solve these balance equations, equations describing the material modelled are necessary. Material properties include for example capillary pressure-saturation relationships, density equations, or viscosity calculations. For those processes, material parameters and state variables are highly non-linear and mostly functions of temperature, saturation, and pressure. Other than describing the material, the thermodynamic state of the system has to be described. This is achieved with equations of state, as for example functions for the calculation of liquid pressure or mass fractions. When the material properties and the state functions are inserted into the balance equations, governing equations in the di.erential form are obtained. After numerical transformations, these equations are then solved by GeoSys/RockFlow. The implementation allows phase changes between the fluid phases (gas and liquid) to occur explicitly. The model allows the simulation of processes in very low permeability clays with high capillary pressures. Examples for code validation are shown, where low permeability clay is desaturated.
Author and committee
dc:creator, dc:contributor.*- Author
-
- De Jonge, Joëlle
Identifiers
dc:identifier.*- Identifier
- hdl:10900/48846