Elastic anisotropy of clay

Abstract

Elastic anisotropy of shale is mainly controlled by the intrinsic anisotropy of individual clay minerals as well as by the textural alignment of grains, pores, and fractures. One of the major challenges in predicting the elastic anisotropy of shales, while using rock physics models, is that the elastic properties of rock‐forming clay minerals are poorly known. Since it is impossible to find single and large enough clay crystals for acoustic measurements and ab initio calculations are still incomplete, few data exist on the elastic moduli of clay minerals. In an attempt to derive the intrinsic anisotropy of pure clay minerals, we present laboratory measurements of compressional and shear wave anisotropy in compacted clay powders at different porosities. In the present work, we focus on the anisotropy of montmorillonitic clays. We used a cold‐press method by applying uniaxial compaction in order to obtain compacted mineral aggregates. Different degrees of compaction enable us to obtain samples with variable porosities and crystallite alignments. We measure ultrasonic P‐ and S‐ wave velocities along the bedding‐normal and the parallel directions. The textural orientation of compacted clay aggregates is found to be controlled by compaction. We obtain the orientation distribution of the clay minerals using synchrotron X‐ray diffraction. Increasing anisotropy of the clay assemblages corresponds to an increase in the preferred orientation of the clay minerals. The combined usage of P‐ and S‐anisotropy measurements with orientation distributions allows us to better constrain the inversion of clay mineral moduli. Our work provides laboratory data on elastic anisotropy of pure clay minerals while linking them to the variation of clay orientation distribution with porosity.