Stress-induced versus lithological anisotropy in compacted claystones and soft shales

Abstract

Shales are anisotropic. Most definitions of shale in-corporate this attribute, either by referring to fissility and existence of cleavage planes, or to anisotropic texture resulting in anisotropy of physical properties on many length scales. Definitions of shale scatter though; some focus on a high content of clay minerals as characteristic of a shale, while others consider a large amount of fine grains (< 2μm) as sufficient. In a rock mechanical context, it is natural to define shale as a rock in which clay minerals constitute the load-bearing framework. This means that “gas shales” in oil-field terminology are, strictly speaking, not shales according to a geological or a geomechanical perspective. Still, these materials have a lot in common with classically defined shales (e.g., low permeability) and anisotropy.

Modeling compaction effects on the elastic properties of clay-water composites

“…Its microstructure resembles a honeycomb structure without a dominant alignment of pores or clay particles. These observations agree well with measurements on similar samples performed by Holt et al (2011) who find that initial P-wave anisotropy was larger for a kaolinite (ε < 0.1) than for a smectite sample (ε ∼ 0.02). They also observe that the anisotropy of the kaolinite sample did not vary significantly with stress, whereas the smectite sample's anisotropy slightly increased with net stress.…”

Noncontacting benchtop measurements of the elastic properties of shales

“…Accurate estimation of elastic moduli has implications in understanding response and distribution of stress in shales (Dewhurst and Siggins, 2006;Holt et al, 2011), as well as in hydraulic fracturing (Suarez-Rivera et al, 2006). Shales can be represented as thin isotropic layers with a symmetry axis, also called transversely isotropic (TI), or hexagonal.…”

“…Shale anisotropy in the laboratory has been widely studied with transducer ultrasonic systems at variable saturation and pressure conditions (Jones and Wang, 1981;Vernik and Nur, 1992;Johnston and Christensen, 1995;Hornby, 1998;Wang, 2002;Dewhurst and Siggins, 2006;Bayuk et al, 2009;Holt et al, 2011;Sondergeld and Rai, 2011). Three directions of wave propagation on core samples are the minimum requirement to estimate the five elastic constants of the stiffness tensor.…”

Seismic sensitivity to variations of rock properties in the productive zone of the Marcellus Shale, WV

“…' Holt (2011) listed three attributes of Shale: (1) clay minerals should constitute the load-bearing framework; (2) shale's have nanometer pore sizes and nanodarcy permeability; (3) surface area is large, and water is adsorbed on surfaces or bound inside clay platelets. The above definition also applies for all fine grained sedimentary rocks.…”