Shale swelling/shrinkage and water content change due to imposed suction and due to direct brine contact

Analyses were performed on nine different preserved shales, representing in situ states of 5-15% water content and 0.13-0.42 void ratio. Under varying total suction (controlled humidity), each shale shows well-defined relationships among suction, volume change, water content and saturation, with the lower-porosity shales undergoing less volume and water content change than the higher-porosity shales. A decrease in in situ porosity is also associated with a much higher native state suction as well as full saturation extending to suction values beyond 40 MPa. Only part of the high suction is due to capillary tension. Under direct brine exposure, the shales almost always swell, even when the brine has an equivalent suction greater than the shale. This is likely due to the reduction in some component of the matric suction. The shale pore water is found to equilibrate with the solute content of the surrounding brine, due to ion diffusion. Much or all of the swelling, and water increase, appears to take place in the clay-bound water and not in the main (free water) pore space. The swelling magnitude is consistent with the amount of water content increase. Swelling usually corresponds to less than one additional water layer being added between the clays. Swelling, and water increase, is very small for the low-porosity shales. Some osmotic effects are observable in all the shales, and cation exchange on the clays also takes place. Swelling is best inhibited with potassium, followed by sodium, followed by calcium, for brines of equal water activity ranging from 0.8 to 0.9.