Geomechanics and Physics Related to Shale Strength, Stiffness and Anisotropy

For many years, the presence of large amounts of shales in many sedimentary basins worldwide has been regarded as a double edged sword. On the positive side, they form seals to hydrocarbon traps in the subsurface and in addition are regarded as potentially useful for the storage of CO2 and nuclear wastes. All three positives are enhanced by their often areally extensive nature. However, these large volumes of shaly material also have a downside in that exploration wells often have to drill through highly unstable shale formations; such instability can be caused by high pore pressures, drilling fluid-rock interactions coupled with the generally weak nature of many overburden shales. In recent times, understanding shale behaviour became more critical with the advent of shales as reservoirs, although it should be noted that these shales are not at all similar to typical overburden shales encountered for example in the North Sea or Gulf of Mexico. However, both types of shales are of interest in terms of geomechanical properties, overburden shales for seal integrity and gas shales for hydraulic fracturing. In addition, the coupled geomechanics and rock physics response is also of interest as overburden shales have shown anomalous responses in 4D seismic operations, while gas shales require an understanding of the mechanics and physics of partial saturation and presence/type of organic matter. This contribution will investigate the impacts of some critical factors governing shale geomechanical properties, outline the impact of changing stress on rock physics response through evaluation of velocity and its anisotropy and finally will look at the relationship between composition, dielectric properties and shale strength.