A statistical review of mudrock elastic anisotropy
ABSTRACT
Mudrocks, defined to be fine-grained siliclastic sedimentary rocks such as siltstones,claystones, mudstones and shales, are often anisotropic due to lamination and mi-croscopic alignments of clay platelets. The resulting elastic anisotropy is often non-negligible for many applications in the earth sciences such as wellbore stability, wellstimulation and seismic imaging. Anisotropic elastic properties reported in the openliterature have been compiled and statistically analysed. Correlations between elasticparameters are observed, which will be useful in the typical case that limited infor-mation on a rock’s elastic properties is known. For example, it is observed that thehighest degree of correlation is between the horizontal elastic stiffnesses C11and C66.The results of statistical analysis are generally consistent with prior observations.In particular, it is observed that Thomsen’s ε and γ parameters are almost alwayspositive, Thomsen’s ε and γ parameters are well correlated, Thomsen’s δ is mostfrequently small and Thomsen’s ε is generally larger than Thomsen’s δ. These obser-vations suggest that the typical range for the elastic properties of mudrocks span asub-space less than the five elastic constants required to fully define a Vertical Trans-versel Isotropic medium. Principal component analysis confirms this and that fourprincipal components can be used to span the space of observed elastic parameters.
Key words: Anisotropy, Petrophysics, Rock physics.
