The status of water in swelling claystones and shales

Pore water in smectite bearing clay rocks has a special status given the importance of clay-water interactions. An investigation of the changes in microstructure of the Callovo-Oxfordian (COx) claystone with respect to changes in suction was carried out by using Mercury Intrusion Porosimetry. The results were interpreted based on a simplified (brick) model of the microstructure (a 50% clay matrix containing detritic grains of quartz and calcite) and on the mechanisms of hydration of smectites (e.g. Mooney et al. 1952, Norris 1954, Saiyouri et al. 2004, Ferrage 2004) that are characterised by the ordered adsorption of a first, a second and then a third layer of water molecules along the smectite surface (as demonstrated by investigating the changes of the d001 inter basal space during hydration by X-ray diffraction techniques). The average number of smectite sheets by platelet is estimated around 20, and the ordered and step adsorption mechanisms of successive layers of water molecules provides a sound interpretation of the changes in microstructure, showing the utmost importance and power of the hydration mechanisms of smectites, even in clay rocks and shales. A relative proportion of adsorbed intra-platelet water (30-35%) and free inter-platelets water (65-70%) is deduced from these considerations.