Two-phase flow characterization and modelling in bentonite at various temperatures

Bentonite is considered a key buffer material for deep geological repositories. During repository evolution, water and vapour redistribution around the canisters governs several processes affecting the sealing properties of bentonite, such as swelling and density homogenisation. A thorough characterization of the material's thermal and hydraulic properties is essential for numerical model calibration and predicting the response of the Engineered Barrier System. In this work, we present an experimental campaign investigating the thermo-hydraulic response of bentonite in partially saturated conditions at two temperature levels, complemented by numerical THM modeling to analyse the contributions of vapour and liquid to the overall transport.

Water retention curves were obtained at 20°C and 50°C on free-swelling samples. At high suctions, water content is independent of void ratio since most water is expected to be in an adsorbed form, whereas at low suctions, results vary between constant volume and free swelling samples due to the influence of capillarity and the diffuse double layer. Results were compared with previous studies at room temperature and interpreted using the Van Genuchten equation. The retention curve was also compared with the model proposed by Bosch et al. (2023), which explicitly considers adsorbed and free water fractions.

Hydraulic conductivity was determined over a wide saturation range at 20°C and 50°C using the instantaneous profile method on two 25 cm-long cylindrical bentonite samples, initially at a homogeneous degree of saturation and compacted to a target dry density of 1.55 g/cm³. The samples were hydrated from one side with artificial pore water, to replicate in-situ brine composition. Upon contact with the brine, relative humidity (total suction) was periodically measured at five positions along the sample height to assess saturation evolution. These measurements, combined with the water retention curve, enabled the calculation of pressure gradients and water flux, allowing for the determination of hydraulic conductivity.

The adopted methodology enabled the evaluation of hydraulic conductivity evolution as a function of suction and degree of saturation, providing relative permeability curves and allowing for the calibration of appropriate models. The results also assess the impact of temperature on hydraulic conductivity. At laboratory temperature, hydraulic conductivity increased with saturation, spanning two orders of magnitude from 1×10⁻¹³ m/s (high saturation) to 1×10⁻¹⁵ m/s (low saturation). Higher water fluxes were recorded at 50°C, highlighting the role of temperature in enhancing water transport.

The collected data provide insight into how temperature influences overall fluid transport through its effects on vapor transport and water viscosity. These findings allowed the calibration of a THM numerical model to analyse the relative contributions of liquid and vapour transport mechanisms in bentonite, improving the predictive capability of engineered barrier system performance.

References

• A. Bosch, A. Ferrari, O. Leupin, and L. Laloui, “Modelling the density homogenisation of a block and granular bentonite buffer upon non-isothermal saturation,” Int. J. Numer. Anal. Methods Geomech., vol. 47, no. 11, 2023, doi: 10.1002/nag.3547.