Coupled HM effects induced by gas injection and transport processes - modelling and validation against experiments
Report of the project:
WP GAS: What are we doing?
When heat generating waste package canisters are disposed of in a deep geological repository, various coupled thermo-hydraulic-mechanical effects and processes can be triggered (Fig. 1). Next to a change of the stress field due to excavation, thermal expansion and hydro-mechanic effects, the temperature rises due to the heat production of the radioactive waste. Furthermore, depending on the specific disposal concept, large amounts of metallic material are emplaced in the repository as well (canisters, drift structural supports etc.). The corrosion of these metals leads to the release of (mostly hydrogen) gas and subsequently, a gas pressure build up can facilitate the migration of gas in the geotechnical and geological barriers of the repository.
Activities in WP GAS focus on improving the understanding of the gas transport mechanisms as well as their impact on the barrier integrity anywhere between a microscopic scale and a full repository scale. Experimental teams and numerical modelling teams work in close collaboration to design experiments (from triaxial or oedometer gas injection experiments to hollow cylinder experiments on pre-fractured rock samples) and subsequently numerically model the results. One major challenge will also be the spatial and temporal upscaling of the process understanding from laboratory scale to repository scales.
Thermo-hydro-mechanical coupled processes in the repository and in the host rock can be induced or triggered by the emplacement of heat generating, radioactive waste canisters
Source: Grunwald et al. 2021
WP GAS: Why is it needed?
The migration of gas following a gas release and subsequent pressure build-up has two main consequences:
- Gas migration can lead to advective or diffusive transport of radionuclides in the geotechnical and geological barrier system and thus contribute directly to radionuclide release. In order to ensure that the magnitude of radionuclides that will be released from the repository remains below an admissible threshold, it is furthermore necessary to maintain the integrity of the geotechnical and geological barriers in the sense of their containment-providing properties.
- Depending on the hydraulic conductivity of the geotechnical and geological barrier, the gas release can lead to a significant build-up of gas pressures in the repository. Effective stresses decrease with increasing pore pressures, which leads to deformation potentially inducing an increase of the permeability. If the stresses exceed the dilatancy criterion or tensile effective stresses occur, micro fissures and fractures can form in the host rock, which could irreversibly change the mechanical and hydraulic properties of the host rock with potentially unfavourable
effects (Fig. 2).
Fig. 2: Gas transport mechanisms are dependent on the gas pressure build-up following the release of hydrogen gas due to metallic corrosion. Depending on the gas pressure, the mechanical and hydraulic properties may be changed reversibly or irreversibly
Source: Marschall et al., 2005
To improve our understanding of the aforementioned two processes, the experimental and modelling work of WP GAS takes place in three tasks. In the first task, the focus lies on the gas transport mechanisms, in the second tasks the focus lies on the impact of gas on the barrier integrity and in the third task, an analysis of the impact of gas generation on the scale of a generic full repository is conducted. In all the tasks, there are both experimental and modelling teams clustered together to facilitate an optimized workflow and ensure good communication between teams so that the scientific questions can be dealt with efficiently.
My particular work: continuum modelling of gas injection experiments with permeability modification approaches, benchmarking of a monolithic coupled TH²M model.
To simulate the impact of a high gas pressure on the hydraulic properties of the rock and, in turn, the impact of an elevated permeability of the rock on the migration of gas, different permeability modification approaches were proposed and implemented in the FEM code OpenGeoSys-5 (OGS-5) by Xu et al., 2013. The idea behind the permeability modification is that due to a poro-mechanical deformation and the creation of micro-fissures or fractures, the permeability of the porous medium increases. In the continuum modelling, the fractures are not modelled discretely and explicitly but they rather change the properties of the medium. This method does not model the creation of each discrete fracture, but it effectively reflects the hydro-mechanic effects of micro-fissures in a given volume and is efficient when it comes to upscaling the processes.
With the coupled, monolithic TH²M-implementation of OGS-6, which is currently being developed (Bilke et al., 2019; Grunwald et al., 2021), the aforementioned permeability modifications are currently in the process of integration and testing (Fig. 3).
Fig. 3: Experimental set up and modelling results of a gas injection experiment in a cylindrical sample of Opalinus Clay. Some permeability modification approaches exist already in the FEM code OpenGeoSys-5 (OGS-5) and are currently being implemented in the new implementation of OGS-6 as a TH²M-model
Source: Popp et al., 2007; Xu et al., 2013; Pitz et al., 2021
Fig. 4: Benchmarking example from the BenVaSim project. Comparison of results obtained with the FEM codes OGS-5 and 6
Source: Pitz et al., in preparation
Firstly, the new TH²M-implementation has to be tested and benchmarked against existing solutions (Fig.4), validated against experiments and compared to other, quality assured codes.
Next, experiments from the EURAD WP GAS will be focused on and the numerical results will be compared against the experimental results. Some of the experiments feature gas injection pressures close to or higher than the minimal principal stress, so that deformation and increased permeabilities are expected to occur. In Fig. 5, the triaxial gas injection experiment by BGS with the title “displacement vs. dilatant gas flow” (BGS, EURAD MS 20 experimental design report) is shown along with the first modelling results. The calculated temporal evolution of the water saturation suggest that the pore water is displaced by the injected gas. This comes along with an expansive deformation of the sample. The increasing saturation at the outlet shows that there is a strong influence of the type of outlet boundary condition. In this case, a Neumann-zero-flux boundary condition is applied.
Fig. 5: Experimental triaxial setup, FEM-mesh and some first modelling results. The gas flow boundary condition at the left side of the model is increased stepwise. Pore water is displaced by the gas injection and the sample expands due to increased pore gas pressure
Source: BGS, EURAD mile stone 20 experimental design report
Fig. 6: Schematic overview of the code implementation and validation cooperation between BGR, BGE, UFZ and TUBAF
Source: Pitz et al., 2021
Next steps
For the development of the numerical models in OGS-6, the BGR works in close cooperation with the BGE (Federal Company for Radioactive Waste Disposal), the UFZ (Helmholtz Centre for Environmental Research) and the TUBAF (University of Mining and Technology Freiberg). BGR and BGE act as the code validators whereas UFZ and TUBAF act as the code developers (Fig. 6).
The next steps are planned in close collaboration with the experimental partners within WP GAS. As experiments are conducted, exact descriptions of material properties and boundary conditions become available and the modelling of the experiments will continue. A detailed analysis of experimental and numerical results will allow the testing of the numerical models such as the permeability modification models introduced and discussed above.
To conclude: In Task 2, numerical models are developed and tested against WP GAS experiment, focusing on the gas transport mechanisms. In Task 3, the impact of these gas transport mechanisms on potential host rocks/EBS will be studied numerically, again with the help of experiments from the respective task. This we help to validate the process understanding and code implementation. With the validated permeability models, the next step will be to scale up to the full repository scale as defined in the final Task 4 of work package GAS. The objective of our work is the development of a numerical model, which takes into account the hydraulic-mechanical coupled effects induced by high gas pressures. The continuum approach allows for an analysis of this effect across the laboratory and repository scales. A final result will be a prediction of gas pressures to be expected with different disposal concepts.
EURAD Roadmap
Domain 4 of the roadmap has the title “JP Priorities and Activities of Common Interest that relate to Geoscience to understand rock properties, radionuclide transport and long‐term geological evolution”. Therein, the topic “perturbations (gas, temperature and chemistry)” represents the main motivations of the work presented here. Especially the points J.1.4.1, “to increase understanding of gas migration in different host rocks” and J.1.4.3 “Develop and implement two-phase flow numerical codes to Increase gas transient representation at the disposal scale” represent two main motivations of this work. Additionally, J.1.4.4 “Improved understanding of gas reactivity in the EBS and different host rocks” from Domain 4 and some topics from Domain 3 “Priorities and Activities of Common Interest that relate to Engineered barrier system (EBS) properties, function and long‐term performance” are of interest.
Acknowledgements
I would like to thank the entire OGS development team (www.opengeosys.org) and the members of the German Cluster (BGE TECHNOLOGY: Eric Simo, Christian Müller, Alireza Hassanzadegan; UFZ: Olaf Kolditz, Wenqing Wang, Norbert Grunwald) for the cooperation and collaboration within EURAD WP GAS. I would also like to thank the WP Gas leaders, Séverine Levasseur and Xavier Sillen, for their guidance and support. The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no 847593.
Powerpoint presentation:
Coupled HM effects induced by gas injection and transport processes – modelling and validation against experiments (WP GAS)
(EURAD Annual Event, March 17th 2021, Pitz, M.; Ziefle, G.; Grunwald, N.; Maßmann, J.; Nagel, T.)
Literature:
Bilke, L.; Flemisch, B.; Kalbacher, T.; Kolditz, O.; Helmig, R.; Nagel, T. (2019): Development of open-source porous media simulators: principles and experiences Transp. Porous Media 130 (1), 337 - 361
Grunwald, N. et al. (2021): Non-isothermal two-phase flow in deformable porous media: Systematic open-source implementation and verification procedure, Geomechanics and Geophysics for Geo-energy and Geo-Resources (submitted)
Marschall, P.; Horseman, S.; Gimmi, T. (2005). Characterisation of gas transport properties of the Opalinus Clay, a potential host rock formation for radioactive waste disposal. Oil & Gas Science and Technology 60 (1), 121–139
Pitz, M., Grunwald, N., Hassanzadegan, A., Kolditz, O., Maßmann, J., Müller, C., Nagel, T., Simo, E., Wang, W., Wengler, M., Ziefle, G. (2021). Modelling gas migration in tight barriers. Activities of the ‘German Cluster’ within the European Joint Programme on Radioactive Waste Management – Work package GAS. Tage der Standortauswahl 11./12. February 2021, Freiberg.
Popp, T.; Wiedemann, M.; Böhnel, H.; Minkley, W. (2007). Untersuchungen zur Barriereintegrität im Hinblick auf das Ein-Endlager-Konzept. Institut für Gebirgsmechanik GmbH, Leipzig, Germany.
Xu, W.J.; Shao, H.; Hesser, J.; Kolditz, O. (2014). Numerical modelling of moisture controlled laboratory swelling/shrinkage experiments on argillaceous rocks. Geological Society, London, Special Publications 400, 359 – 366.
