Effect of temperature on the water diffusion in claystone: Tournemire case

We measured the water pore diffusion coefficient of a sample from the Toarcian formation at the Tournemire Underground Research Laboratory at 4 temperatures (10, 30, 50 and 70 °C), corresponding to the range of temperatures expected during the thermal phase of a high level nuclear waste storage. Using a fast NMR based technique, diffusion coefficients could be obtained in about 20 h for each temperature with high accuracy. Effective diffusion coefficients at 20 °C (2.7 × 10 −11 m 2 /s) agree with existing data on similar claystones and tracer measurements on similar samples. Using an Arrhenius model an activation energy of 22.9 kJ/mol is found in the temperature range 30–70 °C, a value larger than those found by other authors in other claystones (17 to 21 kJ/mol). From NMR T 2 relaxation time data, the activation energy of surface diffusion is evaluated at 9.6 kJ/mol. When considering all data points in the range 10–70 °C, we propose a power law model already used for bulk water to represent the observed non-Arrhenius behavior. The model allows estimating a tortuosity value of 4.1 for the sample considered around 100 °C. We attribute the low diffusion coefficient observed at low temperatures to a balance between surface and volume diffusion, where surface diffusion becomes more dominant at lower temperatures.