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.