Pressure and Stress Assessment in Anisotropic Rocks

Many studies have shown the important role that pore pressure and stresses play in the full life cycle of a field, not least in the determining of a safe drilling margin. Conventionally, in shale dominated sequences the pressures and stresses are estimated by either using deterministic, empirical equations or by building holistic, geologically-driven pressure and stress models.

If we focus on the most common methods both deterministic and empirical equations for predicting pore pressures, it is clear that all of these methods assume isotropic constitutive properties within shale sequences. This seems non-intuitive since shales are widely accepted to be anisotropic due to their fabric (e.g. phase composition, crystallographic preferred orientation, crystal structure, and microstructure). Given that shales are anisotropic, models that neglect the anisotropy may lead to incorrect estimates of rock and elastic properties and thus ultimately fail to describe the true geomechanical behaviour correctly. We believe that the effects of anisotropic rocks should be fully considered in building geomechanical models, which are often utilised in the design of safe drilling margins.