Numerical simulation of solute transport in argillaceous rock under thermal gradient with a coupled THM-solute transport model

Argillaceous rocks have many favourable characteristics for deep geological disposal of high-level radioactive waste (HLW) such as low permeability resulting in slow solute transport dominated by diffusion processes. However, waste-generated heat can increase pore pressure through Thermal-Hydraulic-Mechanical (THM) coupled processes, potentially enhancing advective transport. In this study, the authors developed a mathematical model to simulate a laboratory and a large-scale in situ experiment at an underground research facility (URF), to investigate (1) T-solute transport coupling (via the Soret effect and temperature-dependent diffusion coefficient) and (2) THM-solute transport coupling in argillaceous rock. The findings suggest that the Soret effect is significant in the laboratory experiments with relatively high thermal gradient, but negligible in the URF experiment where the thermal gradient is much smaller. Instead, the effect of temperature on the diffusion coefficient appears to play a more crucial role for the URF experiment. In addition, the advection enhancement due to thermal pressurization as a result of THM processes shows an insignificant effect on solute transport. The modelling of the URF experiment, as confirmed by observational evidence, shows the importance of anisotropy of the THM-transport properties as well as the effects of the excavation damage zone (EDZ). Finally, the model captures the key features of both experiments, highlighting its capability in enhancing comprehension of transport processes from a deep geological repository (DGR) built in argillaceous rocks. This improved understanding is valuable for safety assessments of DGRs in such rock types. • Elevated temperatures in clayrocks can enhance solute transport by thermal pressurization, by the Soret effect and by increasing the molecular diffusion. • A mathematical model of coupled Therma-Hydraulic-Mechanical-transport processes was developed to evaluate the above three effects. • The model was used to simulate experiments of solute transport in clayrock in the laboratory and at the Tournemire Underground Research Facility in France. • It was found that the increase in molecular diffusion is important in both cases, while the Soret effect is only significant in the laboratory experiment. • Thermal pressurization has negligible influence on transport, however anisotropy and rock damage need be considered in the underground experiment.