Shales are anisotropic, but their anisotropy is often neglected in geomechanical field applications. A major reason for this omission is the need for input data, which includes at least five different elastic parameters that are not commonly available. Recent developments have pointed to internal correlations between elastic anisotropy parameters that can be linked to textural and compositional similarities between different shales. Oriented clay minerals constitute the main source of lithological anisotropy. Utilizing the internal correlations in a pragmatic and simple manner, we demonstrate that a reasonably good anisotropic characterization can be given, even if only one or two input parameters are available. Such a default anisotropic formulation is likely to yield more accurate results for several engineering applications rather than simply assuming isotropy. Several improvements are available, and some already exist, but at the cost of requiring more input from logs or core measurements. Further refinement of our current approach is a low-hanging fruit.
Shales serve as caprocks above hydrocarbon and CO2 storage as well as unconventional reservoir rocks. Shales are anisotropic, which has impact on reservoir compaction or extension, surface displacements, borehole stability during drilling, as well as depletion or injection induced stress and pore pressure changes. Still, most rock mechanical analyses assume isotropy, simplifying the computational process. Anisotropic analysis requires at least five elastic moduli, depending on symmetry, and if poroelasticity is incorporated, this number increases further.
Subsurface formations are normally not sufficiently characterized to enable determination of all the required elastic parameters. To determine all five static moduli for a transversely isotropic (TI) rock, at least three differently oriented cores would be needed. With a cylindrical core drilled with its axis aligned with the symmetry axis, an undrained triaxial test gives the undrained Young's modulus and Poisson's ratio for that specific orientation. In the field, gamma ray (to identify shale formations), density and sonic log data would be available, which may provide P-wave modulus and in most cases also shear modulus. However, three additional elastic moduli are required to fully characterize the anisotropy. Further, the discrepancy between static and dynamic moduli needs to be corrected for.
