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 microscopic alignments of clay platelets. The resulting elastic anisotropy is often non‐negligible for many applications in the earth sciences such as wellbore stability, well stimulation and seismic imaging. Anisotropic elastic properties reported in the open literature have been compiled and statistically analysed. Correlations between elastic parameters are observed, which will be useful in the typical case that limited information on a rock's elastic properties is known. For example, it is observed that the highest degree of correlation is between the horizontal elastic stiffnesses C11 and C66. The results of statistical analysis are generally consistent with prior observations. In particular, it is observed that Thomsen's ɛ and γ parameters are almost always positive, Thomsen's ɛ and γ parameters are well correlated, Thomsen's δ is most frequently small and Thomsen's ɛ is generally larger than Thomsen's δ. These observations suggest that the typical range for the elastic properties of mudrocks span a sub‐space less than the five elastic constants required to fully define a Vertical Transversel Isotropic medium. Principal component analysis confirms this and that four principal components can be used to span the space of observed elastic parameters.

Microseismic joint location and anisotropic velocity inversion for hydraulic fracturing in a tight Bakken reservoir

“…We then assumed ϵ ¼ γ, roughly consistent with other observations (Horne, 2013). Finally, we assumed δ ¼ 0.5ϵ, which lies within the large scatter in observations (Vernik and Liu, 1997;Havens, 2012;Horne, 2013). In fact, Thomsen's δ is poorly constrained by well data.…”

The Role of Texture, Cracks, and Fractures in Highly Anisotropic Shales

“…Although, theoretically, the existence of shear wave triplications has been known for some time, we know of only a few examples of inferred or observed triplications in field data (Hake et al, ; Jolly, ; Slater et al, ). This is despite a recent statistical analysis of published anisotropy parameters which suggests that over 25% of shales should be expected to produce triplications (Horne, ). This may in part be due to the fact that the reflection angles of converted waves in typical surface seismic reflection experiments rarely extend to the inclinations where triplications would be expected.…”

“…The ϵ and γ parameters describe the fractional difference between the horizontal and vertical velocities of the P and S H waves, respectively. In shales ϵ and γ both tend to be positive (i.e., higher horizontal velocities than vertical) and correlate with each other (Horne, ; Pervukhina & Rasolofosaon, ). The δ parameter is more difficult to describe because its effect depends on its value compared to the ϵ parameter.…”

“…Note that the S H wavefront is elliptical for all VTI cases. In most shales, however, δ is less than ϵ (Horne, ). This distorts the P and S V velocity surfaces at oblique angles, producing velocities that are slower and faster, respectively, compared to the elliptical situation, which may result in the S V and S H wavefronts crossing each other (Figure b).…”

A wellbore stability analysis model with chemical-mechanical coupling for shale gas reservoirs

“…2012a, 2012b; Josh et al, 2012;Cho et al, 2012;Kim et al, 2012;Vasin et al, 2013;Horne, 2013;Das, 2013;Ma and Chen, 2014b;Tang, 2014;Heng et al, 2014). The data on the anisotropic strengths of different shales collected from tri-axial compression experiments and the literature are listed in Table 2.…”