Complex conductivity of the Toarcian claystone at Tournemire (Southern France)

Summary The Tournemire Underground Research Laboratory (URL) operated by ASNR (Autorité de Sûreté Nucléaire et de Radioprotection) is a French test site specifically developed to study and comprehend the confinement properties of low-porosity claystone formations. More specifically, this site allows developing an improved knowledge of the transport properties of the Toarcian (Early Jurassic) shale formation. This claystone shares indeed similar characteristics to the Callovo-Oxfordian claystone formation encountered at the Meuse/Haute-Marne underground research laboratory operated by ANDRA (Agence Nationale pour la gestion des Déchets RAdioactifs) to study the feasibility of the long-tern storage of nuclear wastes. In the present study, 18 rock samples of the Toarcian formation are used to study (i) the induced polarization properties of this formation at full water saturation and (ii) the desiccation of this claystone by looking at the evolution of the complex conductivity spectra with the decrease of the water saturation in the frequency range 1 mHz-45 kHz. In this second set of experiments, the complex conductivity spectra are fitted with a double Cole Cole complex conductivity model. We then obtain the values of the first and second Archie’s exponents and estimates of both surface conductivity and normalized chargeability. We also test relationships between the surface conductivity, the normalized chargeability, the quadrature conductivity, and both the cation exchange capacity and water content of these clay-rocks. Furthermore, the laboratory data are favorably explained by the so-called Dynamic Stern Layer (DSL) model developed in the last decade. In order to test this new knowledge in field conditions, an in situ experiment is performed in a gallery of the Tournemire URL to test the ability of the method to image the damage zone around a 80-cm hole drilled normal to the gallery axis. For this purpose, a set of 5 wells equipped with a total of 240 non-polarizing electrodes is successfully used to get both electrical conductivity and normalized chargeability tomograms, which are in turn used to image the water content around the main hole. The results are analysed in terms of stress distribution including the effect of plasticity.