Mechanistic understanding of gas transport in clay materials (GAS)

Major highlights (June 2022 - May 2023)

- The plenary progress meeting in Delft in March 2023 (1st fully in person meeting  in 3 years) marked a turning point for the project by presenting the first conclusions of the integration phase of the results (objective of the second half of the project).
- The collaboration between experimenters and modellers is at its zenith and leads to fruitful achievements.

Forward look

- Now that experimental results and numerical models are available, broad trends are emerging and integrated interpretations are possible. Several experiments on different materials and under a wide range of conditions show comparable behaviour, communication on the reasons for this behaviour in terms of common concepts is progressing well. On the contrary, some experiments show differences in behaviour. The reasons for these differences are under investigation.
- Work progresses well on SOTA2 targeted to an audience of managers in agencies and structured along the needs of end-users.

'What'

This WP will provide data and develop process-level models to improve mechanistic understanding of transport processes in natural and engineered clay materials, including couplings with mechanical behaviour and impact on the clay properties. Experimental work will determine, for each identified gas transport regime, the conditions under which that regime is possible, in clay materials representative of host rock and clay EBS components. Data will be obtained that are pertinent for low (diffusion) to high (advection) gas transport rates.
Work will also show how knowledge gained from lab and in situ experiments is integrated in the conceptualisation of gas transport through different components of a repository system and how gas could affect (or not) the performance of the system. This will involve (i) development of phenomenological descriptions of gas transport and of its likely consequences at the relevant scale and (ii) testing different approaches to represent the effects of gas at repository scale and bounding its consequences in terms of repository performance.
 

'Why'

Theme 4 of the EURAD Roadmap (Geoscience to understand rock properties, radionuclide transport and long-term geological evolution), increasing the understanding of gas migration is a high priority topic. Gas generation and transport is a key issue as it is possible that gas could be generated at a faster rate than it can be removed through clay host rocks (and clay EBS components – Theme 3) without creating discrete, gas-specific pathways through these low-permeability components. In several disposal concepts, the potential for migration of free gas containing radionuclides to the biosphere is an important issue. Consequently, the raison d’être of this this WP is to answer two key end-users questions:

• How can gas migrate within the repository and which water soluble and volatile radionuclides could be associated with it?
• How and to what extend could the hydro-mechanical perturbations induced by gas affect barrier integrity and performance?

This WP will build on the outcomes of FORGE. Experiments in FORGE revealed complex mechanisms and emphasized the importance of the mechanical control exerted by the porous material on gas transport. It was suggested that this complexity can be addressed as long as one can bound the effects of these mechanisms using simple and robust descriptions for evaluation purposes (e.g. two-phase flow models for gas transient representation at repository scale, identified as a medium priority under Theme 4). A necessary condition for this is that the scientific bases are integrated properly, in a traceable way throughout the system conceptualisation process. Hence, the structure of this WP follows this process, imposing interactions at each step to ensure close cooperation between experimentalists, process modellers and those involved in evaluation of system performance. This should allow the development of robust evaluation approaches that support the expert judgement formulated at the end of FORGE that gas is not a feasibility challenging issue for geological disposal but more a challenge of managing uncertainties.