Elastic anisotropy modeling of Kimmeridge shale

Abstract

[1] Anisotropy of elastic properties in clay-rich sedimentary rocks has been of long-standing interest. These rocks are cap rocks of oil and gas reservoirs, as well as seals for carbon sequestration. Elasticity of shales has been approached by direct velocity measurements and by models based on microstructures. Here we are revisiting the classical Kimmeridge shale studied by Hornby (1998) by first quantifying microstructural features such as phase volume fractions, grain shapes and grain orientations, and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties. It is shown that by application of a self-consistent algorithm based on Eshelby's (1957) model of inclusions in a homogeneous medium, it is possible to explain most experimental elastic constants, though some discrepancies remain which may be due to the interpretation of experimental data. Using a differential effective medium approach, an almost perfect agreement with experimental stiffness coefficients can be obtained, though the physical basis of this method may be questionable. The influence of single crystal elastic properties, grain shapes, preferred orientation, and volume and shapes of pores on elastic properties of shale is explored.

Crack deflection in brittle media with heterogeneous interfaces and its application in shale fracking

“…Such natural weak zones have a great influence on the mechanical properties of shales ( Zeng and Wei, 2016 ). Commonly, shales are regarded as brittle and transversely isotropic materials, with a symmetry axis perpendicular to the sedimentary planes ( Vasin et al, 2013 ). For simplicity, but without loss of physics, we consider the bedding interfaces and natural fractures as material discontinuities that have no thickness and different fracture toughness values than that of the shale matrix.…”

The influence of crack-orientation distribution on the mechanical properties of pre-cracked brittle media

“…To further simplify the problem, we assume that the plate is orthotropic after being permeated by the distributing cracks. This assumption is based on experimental observations from different groups that shale is approximately a vertical transversely isotropic solid ( Hornby et al, 1994;Sayers, 1994;Sone and Zoback, 2013a;Vasin et al, 2013 ). We define the direction parallel to the crack angle expectation as principle direction 1, the corresponding perpendicular direction as principle direction 2 and the direction inclined at a 45 °angle to direction 1 as direction x, as shown in Fig.…”

“…Ideally, we desire to construct a sound physical connection between the microstructure characteristics of shales and the macroscopic properties of the media, especially the pre-existing cracks in the media. Typically, shales display significant anisotropy ( Sarker and Batzle, 2010;Sondergeld and Rai, 2011;Sone and Zoback, 2013a ) and are brittle and transversely isotropic materials, with a symmetry axis vertical to their sedimentary plane ( Vasin et al, 2013 ). The elastic anisotropy is partly caused by the preferred orientation of mineral components ( Lonardelli et al, 2007 ).…”

“…The elastic anisotropy is partly caused by the preferred orientation of mineral components ( Lonardelli et al, 2007 ). To address this anisotropy, theoretical works have been developed to predict effective elastic properties ( Hornby et al, 1994;Lonardelli et al, 20 07;Sayers, 20 05;Vasin et al, 2013 ). Because natural * Corresponding author.…”

Antigorite crystallographic preferred orientations in serpentinites from Japan

“…Based on CPO and single crystal elastic constants for antigorite Table 2) elastic properties of the serpentinites B and C were calculated using a self-consistent method that takes particle shapes into account (Matthies, 2012;Vasin et al, 2013). Stiffness tensors based on X-ray data for serpentinites B and C are listed in Table 2 and compared with single crystal values.…”