f Effects of Free and Bound Water on Stress Dependent Wave Velocities in Clays and Shales

Water has a profound control on wave velocities in clays and clay rich rocks like shale. Water exists as free pore water, but is also adsorbed on mineral surfaces and intercalated inside clay mineral sheets. The adsorbed or “bound” water is shown by various molecular dynamics simulations combined with Monte Carlo simulations (e.g. Leote de Carvalho and Skipper, 2001) to have an ordered crystal like structure. One may thus anticipate that bound water has properties different from bulk water, and in particular that it has shear stiffness. Even if bound water may be present only as one or a few layers of water molecules, it may fill a significant portion of the very small (nanometer size) pores in shale. Also, since it sits on mineral surfaces, it may control grain contact mechanics, which is known to be a major contributor to stress sensitivity of elastic properties in granular media.