Active porosity in swelling claystone and shales: insight from the Callovo-Oxfordian claystone

International audience; In swelling claystones, significant clay-water interactions take place along the faces of the smectites minerals contained in the clay fraction, giving rise to the distinction between free water and adsorbed water. Further insight was recently gained by means of microstructure investigations, showing that the hydration mechanisms of pure or compacted smectites also hold to explain the hydration and swelling behaviour of the Callovo-Oxfordian (COx) claystone, a possible host rock for deep radioactive waste disposal in France. In this rock, the proportion of water molecules adsorbed in intra-platelets pores was recently estimated to around 25% of the total porosity, with 75% of the porosity containing free water. Based on these findings, the data of high precision poroelastic measurements conducted in an isotropic compression cell showed that the porosity to account for a proper calculation of the Skempton parameter is not the total porosity, but the 75% proportion corresponding to free water. This conclusion is believed to be of some interest with respect to the prediction of the rock response around the disposals at great depth, given that hydromechanical numerical simulations are often carried out within the framework of poroelasticity.