Shale Capillarity, Osmotic Suction and Permeability, and Solutions to Practical Testing Issues

For typical shales (void ratio 0.15 to 0.42), modal pore throat sizes range from a few nm to a few tens of nm. Unstressed shales, even when fully saturated, have negative pore water pressure (capillary tension). However, the total suction is often greater than this, especially for highly-compacted shales with extremely small pore size. The additional suction is due to effects associated with clay surfaces. Osmotic pressures can be directly measured, and they can easily be several MPa, a combination of solute suction and clay-related effects. The small pore sizes in shales also result in extremely low values of permeability and of consolidation coefficient. All these characteristics directly impact testing protocols. The first step in any test should be to apply sufficient confining stress to raise the pore pressure up to a positive, measured value. Undrained consolidation, combined with undrained triaxial compression and with small sample sizes (and drainage screens when necessary), results in acceptable test durations. A range of effective consolidation stress values is attained by first equilibrating shale samples in varying amounts of suction, to vary the water content. Non-aqueous fluids are required when sampling, to avoid swelling, and are often necessary for pore lines if osmotic pressures are to be avoided.