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 beddingnormal 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.
One of the major problems in the prediction of seismic velocities in shales is the unavailability of elastic properties of the individual clay minerals present in rocks. It is impossible to obtain individual crystals of clay large enough to measure acoustic properties. As a result, various studies tried to obtain the elastic properties of clay minerals using either theoretical calculations, a combination of theoretical and experiments, or empirical extrapolations. Reported values for clay moduli differ to a great extent. The values reported by Katahara (1996) and Wang et. al. (2001) (~50 GPa for bulk modulus, K and 20 GPa for shear modulus, ?) are much higher than those extrapolated (20 GPa for K and 7 GPa for ?) from both laboratory (Han, 1986; Tosaya, 1982) and well log measurements in shales and laminated shaley-sands (Eastwood and Castagna, 1987). On the other hand, atomic force acoustic microscopy measurements by Prasad et al. (2002) indicate very low Young''s modulus of clay particles (6.2 GPa). Berge and Berryman (1995), using theoretical models of shaley sandstones, have shown that the bulk modulus of clay should be around 10-12 GPa. Vanorio et. al. (2003) reported values of the solid clay phase ranging between 6 and 12 GPa for bulk modulus and between 4 and 6 GPa for shear modulus, respectively.
