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 (see, for example, Green et al., 2016 and references therein). Conventionally, in shale dominated sequences the pressures and stresses are estimated by either using deterministic, empirical equations (Zhang, 2011) or by building holistic, geologically-driven pressure and stress models (Edwards et al., 2017).
If we focus on the most common methods for predicting pore pressures using both deterministic and empirical equations, 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.
It is widely known that anisotropic properties can be measured from multiple sources and at varying scales. These include; seismic processing velocity move out analysis, walk-away VSP, well log data and laboratory core measurements. Mitigation for Vertical Transverse Isotropy, VTI (and other forms of anisotropy) is now common during seismic processing and correction of elastic measurements in deviated wells. Despite steady advances in the understanding of shale elasticity the full effects of anisotropic rocks through the entire subsurface workflow is often neglected at varying stages and, as such, fudging (or ignoring) of particular parameters in pore pressure and stress equations in order to fit observed behaviour, such as well bore breakouts, has now become quite common practice, and often overlooked in the most simplistic of cases (e.g. mature basin, clastic sand/shale sequences).