In the framework of the studies dealing on ability to store radioactive wastes in argillaceous formations, signification of interstitial pressures is an important point to understand water and solutes transport. In very low permeability argillaceous formations, like those studied in the Callovo-Oxfordian of the Paris basin by ANDRA, pore pressure is frequently higher than the theoretical hydrostatic pressure or than the pressure in the surrounding aquifers. Such an overpressure is also measured in the Toarcian/Domerian argillaceous formation (k = 10-21 m2), studied by the IRSN in the underground research laboratory of Tournemire (Aveyron, France). The hydraulic head profile has been specified in this manuscript and found to present a 30 +/- 10 m excess-head. This excess-head can be due to compaction disequilibrium of the argillaceous formation, diagenetic evolution of the rock, tectonic compression, changes in hydrodynamic boundary conditions or osmotic processes. Amongst these potential causes, chemical osmosis and thermo-osmosis, a fluid flow under a chemical concentration and a temperature gradient, respectively, are expected to develop owing to the small pore size and the electrostatic interactions related to the charged surface of clay minerals.
The goal of the work presented here was to study and quantify the contribution of each cause to the measured excess-head. Chemo-osmotic and thermo-osmotic permeabilities were obtained by experiments and using theoretical models. Theoretical models are based on the reproduction of the interactions occurring between the charged surface of clay minerals and pore solution and their upscaling at the representative elementary volume macroscopic scale. Chemical osmosis phenomenon is related to anionic exclusion and the determination of the chemo osmotic efficiency requires the resolution of an electrical interactions model. A triple-layer-model which considers diffuse layers overlapping was improved during this thesis to be able to take into account the effect of multi-ionic solutions, i.e. nearest than the natural waters composition, and, thus, to constrain better the chemo-osmotic efficiency. Thermo-osmosis process is poorly characterized so that no satisfactory macroscopic expression to calculate the thermo-osmotic permeability kT was available nor thermo-osmotic experiments performed on natural shales, so far. This process is interpreted as being related to changes properties of water sorbed at clay minerals surface compared with bulk water. A thermo-osmotic permeability predictive model is proposed here, based on the modifications of the hydrogen bounds associated with water molecules located at the vicinity of the solid surface. Input parameters of this model only consist in petrophysical parameters and medium conditions (porewater concentration and temperature).
Chemical osmosis and thermo-osmosis experiments were performed on Tournemire argillite samples and in a test interval equipped borehole at the Tournemire URL. These experiments have consisted in inducing a concentration or temperature gradient across a sample for the laboratory experiments and between the borehole test interval and the formation for the in situ experiments. Osmotic flows were identified by the interpretation of the pressure evolution in the test interval using a hydro-thermo-chemo-mechanical model based on the mass balance equations and the coupled-flow equations. Inversion of the measured pressure signals allowed identifying a chemo-osmotic efficiency ranging between 0.014 and 0.31 and a thermo-osmotic permeability kT ranging between 6.10-12 and 2.10-10 m2 K-1 s-1 for the Tournemire clay-rock.
In parallel to the characterization of the osmotic processes in the argillaceous formation of Tournemire, porewater composition and temperature profiles were established. Temperature pro le was obtained by direct measurement in different boreholes. Porewater composition profile was calculated by a geochemical model developed to reproduce the thermodynamic equilibrium reactions with mineral phases and cation exchange between the clayrock and the pore solution. Added to the requirement of the temperature and concentration profiles across the Tournemire argillaceous formation as force gradients to reproduce the osmotic flows through the formation, the porewater composition is also needed as it is an essential input parameter to predict the chemo-osmotic efficiency coefficient.
At last, the characterization of the osmotic processes and the different force gradient profiles allowed estimating the contribution of the osmotic and hydraulic processes to the measured excess-hydraulic head profile measured in the argillaceous formation of Tournemire. Considerations on the hydro-mechanical behaviour of the argillaceous formation allowed rule out the other possible causes of excess-head and lead to the conclusion that only the hydraulic processes, related to the intrinsic permeability variation across the formation, and osmotic processes can explain the pressure field in the Toarcian/Domerian formation. The results particularly highlight the importance of the spatial variations of the hydraulic and osmotic permeability coefficients in the generation of an excess-hydraulic head.