The safety of a radioactive waste repository in a deep geological formation requires the identification of all processes affecting the undisturbed and disturbed geological barrier. The occurrence of overpressures in semipermeables seems to be the rule in most of the well-compacted and deep clayey formations. These overpressures generally result from a combination of processes among which compaction, diagenesis, hydrocarbon migration, tectonic stress, changes in hydrodynamic conditions, long-term transient osmotic effects and others... This brings us to ask the following question: are we capable of identifying all processes responsible for such overpressures and of estimating their respective contributions? It is especially to try answering this question that a new borehole (PH4) has been drilled in the Tournemire Experimental Station from the tunnel ground between October, 2006 and January, 2007.
An overpressure of about 50m was measured at Tournemire in 1994 in a deep borehole (ID180) equipped with a double packer device. This device isolated an 80m height test section crossing a water-bearing fractures in the Toarcian argillite (Matray et al., 2007). Such overpressure would be of the same order of magnitude to that measured in the Callovo-Oxfordian argillite in Bure which was attributed to chemical osmosis "that seems to be capable of explaining the amplitude of the measured overpressure" (ANDRA, 2005). In the case of osmotic transfers, overpressures would result in a water movement from a low concentrated solution (aquifers) towards the semipermeable (the Toarcian/Domerain clayey formation), the osmotic transfer being one of the components of the electrochemical flow couplings. The assessment of such transport phenomena therefore requires the acquisition of (i) transport parameters and water activity profiles between the semipermeable and the surrounding aquifers and (ii) pore-water pressure profile. (i) Transport parameters and porewater compositions Water activity can be deduced from the chemical composition of interstitial waters. However, direct water collection was impossible due to the very low water content (3-4wt%) and permeability (10-14 - 10-15 m/s) of the semipermeable. The methodology finally adopted has consisted in developing a geochemical model at thermodynamic equilibrium that uses rock core analysis. It was decided to fully air core the semipermeable (i.e. 200m of argillite and marls) to obtain a complete profile of the water isotopic composition in order to keep the samples as little disturbed as possible (see Savoye et al., in a companion abstract). Every 10m, specific core samples were preserved in cells with epoxy resin for the acquisition of the mobile anion concentration and of the geochemical porosity by radial diffusion. Other core samples were dedicated to on-site petrophysical measurements at 150°C and some others were preserved in aluminium bags under vacuum for their mineralogy, populations of exchangeable cations, organic matter and other geomechanical properties. At last, specific core samples were preserved in epoxy resin and in triaxial cells for lab experiments dedicated to the acquisition of osmotic parameters like reflexion coefficients. (ii) Pressure profile The verification of the occurrence of an overpressure has needed the installation of a multipacker device right after the drilling and the subsequent diagraphies (diameter, video, BIPS, gamma-ray, neutron porosity, seismic acoustic core borehole...). This has been done in February 2007 by installing a Westbay® device isolating 6 test sections: five in the semipermeable and a sixth one in the lower aquifer. Four of the test zones in the clayey formation were selected in unfractured and short (60cm) areas when the fifth was selected to record pressure in a 60m height zone affected by breakouts. This installation has been followed by the realization of pulse tests after several months acquisition of an historic of pressure. The goals of these pulse tests are the acquisition of hydraulic parameters in the test sections and the acceleration of their static pressure recovery.
The structural parameters i.e. the petrophysical and electro-chemical parameters acquired in the experimental process are used in a modeling approach. The osmotic permeability can be determined using various theoretical expressions (Bresler, 1973; Leroy and Revil, 2004). Usually, these expressions require the resolution of an electrical model that reproduces the interactions between the pore solution and the negatively charged surface of clay minerals. The electrical model that is implemented here is a Triple Layer Model (TLM) that is thought to capture with a better accuracy than the double diffuse layer models these electrical interactions. We developed a 2D-model of coupled flow using the classical formulation for these transfers (see e.g. Gonçalvès et al. 2004 ; Revil and Leroy, 2004; Gueutin et al., 2006). This coupled flow model includes a TLM module that provides values for the so-called osmotic efficiency. This code is used at the sample scale as well as at the formation scales for the interpretation of the experiments. Other transient processes than osmosis can be evaluated as already attempted by Gonçalvès et al. (2004). Some variations in the hydraulic boundary conditions can be tested through a classical hydrogeological model. The geological time scale variation in tectonic stress can also be tested introducing a source term in the equation of diffusivity as shown by e.g. Gonçalvès et al. (2004). Provided that a temporal evolution of this tectonic stress can be traced, this simulation can be processed using a classical fluid flow model.