Anisotropy of experimentally compressed kaolinite-illite-quartz mixtures

Abstract

The anisotropy of physical properties is a well-known characteristic of many clay-bearing rocks. This anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling. Preferred orientation of clay minerals is an important factor causing anisotropy in clay-bearing rocks such as shales and mudstones that are the main cap rocks of oil reservoirs. The preferred orientation of clays depends mostly on the amount of clays and the degree of compaction. To study the effect of these parameters, we prepared several samples compressing ͑at two effective vertical stresses͒ a mixture of clays ͑illite and kaolinite͒ and quartz ͑silt͒ with different clay/quartz ratios. The preferred orientation of the phases was quantified with Rietveld analysis on synchrotron hard X-ray images. Pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction, increasing in strength with clay content and compaction pressure. Quartz particles have a random orientation distribution. Aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis. Calculated P-wave velocity anisotropy ranges from 0% ͑pure quartz sample͒ to 44% ͑pure clay sample, highly compacted͒, but calculated velocities are much higher than measured velocities. This is attributed to uncertainties about single-crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in the model. In this work, we present an effective method to obtain quantitative data, helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties of clay-bearing rocks.

Phased array compaction cell for measurement of the transversely isotropic elastic properties of compacting sediments

“…The change in texture resulting from one dimensional consolidation of the clay-rich sample is evident in the pre-and post-compaction SEM photomicrographs of Fig (Aplin et al, 2003;Wenk et al, 2007;Lonardelli et al, 2007;Voltolini et al, 2009). The increase in the maximum pole density with compaction is consistent with an improved alignment of the clay platelets that results from mechanical compaction (Ruud et al, 2003;Johansen et al, 2004).…”

The 3D quantitative lattice and shape preferred orientation of a mylonitised metagranite from Monte Rosa (Western Alps): Combining neutron diffraction texture analysis and synchrotron X-ray microtomography

“…The preferred orientation in terms of both lattice and shape of minerals (and pores) controls the anisotropy of their physical properties (e.g., Voltolini et al, 2008) such as the magnitude of seismic wave velocities as a function of direction in a clay þ silt material at different degrees of compaction. Accordingly, a comprehensive quantification of the LPO and SPO in rocks and in materials, more generally becomes of great importance for improving our understand and models concerning the anisotropy of mechanical properties in polyphase materials.…”

Permeability anisotropy and resistivity anisotropy of mechanically compressed mudrocks

“…Mudrocks with higher clay content reach higher degrees of preferred orientation (Voltolini et al, 2009). Martin and Ladd (1975) measured the particle orientation of Kaolinite slurries using peak ratio X-ray Diffraction (XRD) pole figure analysis (Figure 2-3 and Figure 2-4).…”