Clays and clay‐bearing rocks like shale are extremely water sensitive. This is partly due to the interaction between water and mineral surfaces, strengthened by the presence of nanometer‐size pores and related large specific surface areas. Molecular‐scale numerical simulations, using a discrete‐element model, show that shear rigidity can be associated with structurally ordered (bound or adsorbed) water near charged surfaces. Building on these and other molecular dynamics simulations plus nanoscale experiments from the literature, the water monolayer adjacent to hydrophilic solid surfaces appears to be characterised by shear stiffness and/or enhanced viscosity. In both cases, elastic wave propagation will be affected by the bound or adsorbed water. Using a simple rock physics model, bound water properties were adjusted to match laboratory measured P‐ and S‐wave velocities on pure water‐saturated kaolinite and smectite. To fit the measured stress sensitivity, particularly for kaolinite, the contribution from solid‐grain contact stiffness needs to be added. The model predicts, particularly for S‐waves, that viscoelastic bound water could be a source of dispersion in clay and clay‐rich rocks. The bound‐water‐based rock physics model is found to represent a lower bound to laboratory‐measured velocities obtained with shales of different mineralogy and porosity levels.

Shales are anisotropic. Most definitions of shale in-corporate this attribute, either by referring to fissility and existence of cleavage planes, or to anisotropic texture resulting in anisotropy of physical properties on many length scales. Definitions of shale scatter though; some focus on a high content of clay minerals as characteristic of a shale, while others consider a large amount of fine grains (< 2μm) as sufficient. In a rock mechanical context, it is natural to define shale as a rock in which clay minerals constitute the load-bearing framework. This means that “gas shales” in oil-field terminology are, strictly speaking, not shales according to a geological or a geomechanical perspective. Still, these materials have a lot in common with classically defined shales (e.g., low permeability) and anisotropy.

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