Fluids including both liquid and gas phases can be found within the crystal structure or along grain boundaries in all types of sedimentary rock and were formed by accumulating and reacting of different mineral and/or organic particles under pressure and temperature conditions during petrogenesis. These small inclusions range in size but are usually several micrometres and are usually invisible in detail without microscopic studies. However, these fluids usually dispersed in a very low amount can form local accumulations in a rock volume up to some cubic metres.
The initial fluid inclusions in undisturbed rock are situated under petrostatic pressure, which is much higher than hydrostatic pressure in deep sub-surface. Because of the very low rock permeability, migration of such fluids is almost impossible even under high pressure-gradient conditions. Fluid release from the crystal structure will take place if the stress state changes. In case of drilling or excavation, stress will be redistributed with the result of a deviatoric stress state. If fluid pressure is higher than the minimal principal stress, dilatancy-controlled fluid migration is expected to occur. This can result in the generation of micro-fissures between crystal structures with an increased permeability.
With regard to the long-term performance of a potential radioactive waste disposal repository, depending on the disposal concept, the presence and evolution of these fluids may be of significance to the understanding of the system. It is therefore potentially important to characterise the distribution, amount and interconnectivity of the fluid inclusions. It is also potentially important to determine the permeability of micro-fissures and to characterise the hydraulic properties after the fluid release.
Process understanding and model concept development are essential in this task. Modelling of the measured data needs a comprehensive effort, because a quantitative measurement at a microscale is not possible. An upscaling process from the microscale consideration to the macroscale observation is necessary.
In the construction phase, coupled HM process is important and in the post-closure phase, coupled THM processes should be considered. However, chemical reaction and mineralogical alteration processes, especially under high temperature condition, are indispensable to the full understanding of the system.
The emphasis of this work is to gain understanding of possible physical processes that can be used to explain the observations. It is expected that the teams will adopt different approaches but that in all cases the different approaches should all provide insight into the development of robust, predictive THMC analysis.
Extensive microscopic studies on the core structure using e.g. laser microscope, Computer Tomography, EBSD (electron backscatter diffraction) are available which may help in description of the morphology of the pore space and construction of numerical meshes for the pore scale simulation.
In addition, there are data about pressure, flow and geochemical components from more than 20 boreholes from underground laboratories, which allow us to understand mechanical, hydraulic and geochemical processes.
The current plan of the task includes three work packages with the focus on the understanding of the coupled thermal-hydro-mechanical and chemical processes involved in the system:
For further information, please contact the task leader, Dr Hua Shao.