The elastic anisotrophy of shales (original) (raw)
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Geophysical Prospecting, 2005
Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology and pore pressure from seismic data, and to understand time-to-depth conversion errors and non-hyperbolic moveout. A single anisotropy parameter, Thomsen's δ parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to interpret the small-offset AVO response.
Effect of grain scale alignment on seismic anisotropy and reflectivity of shales
Geophysical Prospecting, 2004
The elastic properties and anisotropy of shales are strongly influenced by the degree of alignment of the grain scale texture. In general, an orientation distribution function (ODF) can be used to describe this alignment, which, in practice, can be characterized by two Legendre coefficients. We discuss various statistical ODFs that define the alignment by spreading from a mean value; in particular, the Gaussian, Fisher and Bingham distributions. We compare the statistical models with an ODF resulting from pure vertical compaction (no shear strain) of a sediment. The compaction ODF may be used to estimate how the elastic properties and anisotropy evolve due to burial of clayey sediments.
Mathematical modelling of anisotropy of illite-rich shale
Geophysical Journal International, 2009
The estimation of illite-rich shale anisotropy to account for the alignment of clays and gas-or brine-filled cracks is presented via mathematical modelling. Such estimation requires analysis to interpret the dominance of one effect over another. This knowledge can help to evaluate the permeability in the unconventional reservoir, stress orientation, and the seal capacity for the conventional reservoir.
Preferred orientation and elastic anisotropy in shales
GEOPHYSICS, 2007
Shales display significant seismic anisotropy that is attributed in part to preferred orientation of constituent minerals. This orientation pattern has been difficult to quantify because of the poor crystallinity and small grain size of clay minerals. A new method is introduced that uses high-energy synchrotron X-rays to obtain diffraction images in transmission geometry and applies it to an illite-rich shale. The images are analyzed with the crystallographic Rietveld method to obtain quantitative information about phase proportions, crystal structure, grain size, and preferred orientation ͑texture͒ that is the focus of the study. Textures for illite are extremely strong, with a maximum of 10 multiples of a random distribution for ͑001͒ pole figures. From the three-dimensional orientation distribution of crystallites, and single-crystal elastic properties, the intrinsic anisotropic elastic constants of the illite aggregate ͑excluding contribution from aligned micropores͒ can be calculated by appropriate medium averaging. The illitic shale displays roughly transverse isotropy with C 11 close to C 22 and more than twice as strong as C 33 . This method will lend itself to investigate complex polymineralic shales and quantify the contribution of preferred orientation to macroscopic anisotropy.
Estimation of the anisotropy parameters of transversely isotropic shales with a tilted symmetry axis
Geophysical Journal International, 2012
This paper reports a new approach for the estimation of Thomsen anisotropy parameters and symmetry axis orientation from ultrasonic P-wave traveltime measurements on transversely isotropic shale samples of arbitrary geometry. This approach can be used for core samples cut in any direction with respect to the bedding plane, because no a priori assumption regarding the symmetry axis orientation is made. This orientation is rather part of the solution of the inverse problem together with the anisotropy parameters themselves. Very fast simulated reannealing is used to search for the best possible estimate of the model parameters. The methodology is applied to spherical and cylindrical anisotropic shale samples.
The Elastic Properties of Clay in Shales
Journal Of Geophysical Research: Solid Earth, 2018
To accurately characterize shales, rock physics models must account for anisotropic clay minerals. Due to compliant regions between clay platelets, the elastic stiffness of clay in shales is much less than that of clay minerals. The clay in shales can be modeled as anisotropic clay platelets embedded in a softer interparticle region containing clay-bound water.