Attention to detail — thermo-osmosis and heterogeneity as explanation of the Mont Terri deep borehole observations

Understanding all relevant physical and chemical processes under repository-specific conditions is crucial for demonstrating long-term safety of underground nuclear waste storage at regulatory time scales (StandAG 2017). Among these processes, thermo-osmosis (TO) has been shown to generate fluxes comparable to or exceeding Darcy contributions under specific conditions and to contribute to the interpretation of anomalous pressures in clay formations (Gonçalvès et al. 2012, 2023). However, limited in-situ data obtained under repository-relevant conditions (geological conditions in formation intended to be used/explored for the purpose of nuclear waste storage, here: low-permeability clay) hinders quantitative assessment of TO's importance for nuclear waste repository safety. Most existing studies rely on retrospective analyses of data from experiments designed for unrelated purposes (Samper et al. 2008), highlighting the need for dedicated in-situ experiments. Accurately assessing TO's contribution without overfitting requires comprehensive understanding of the entire physical system. This can be achieved by verifying and calibrating models against existing field data and integrating into them knowledge from literature. This paper demonstrates this approach using data from the Mont Terri Deep Borehole experiment (DB) as preparatory analysis for the so-called ThORN project on experimental investigations on TO flow in clay-containing materials, which aims to develop an in-situ experiment specifically investigating TO under repository conditions.