The Effect of Low Aspect Ratio Pores On the Seismic Anisotropy of Shales

Summary

As a result of the increasing stress that develops during burial, plate-shaped clay minerals in shales tend to align with planes oriented approximately perpendicular to the maximum stress direction. This partial alignment results in shale anisotropy, and this needs to be quantified to reliably extract reservoir fluid, lithology and pore pressure from seismic data and to understand time-to-depth conversion errors and non-hyperbolic moveout. The low aspect ratio pores between clay particles play an important role in determining the character of the anisotropy of shales and can be represented by a normal compliance BN and shear compliance BT that describe the deformation of the interparticle regions under an applied stress. The relations among the various anisotropy parameters for shales depend on the ratio BN/BT of these low aspect ratio pores. For perfectly aligned clay particles, Thomsen''s anisotropy parameter ? is a function only of the shear compliance BT, but ? and ? increase with increasing BN/BT. The presence of a fluid with non-zero bulk modulus in the regions between clay particles acts to decrease BN and may lead to significant reductions in e and d for sufficiently high fluid bulk modulus.

Introduction

Shales are a major component of sedimentary basins, and play an important role in fluid flow and seismic imaging because of their low permeability and anisotropic properties. The seismic anisotropy of shales results from the partial alignment of anisotropic plate-like clay minerals (Kaarsberg, 1959; Tosaya, 1982; Sayers, 1994, 1999, 2005). Figure 1, for example, shows a scanning electron micrograph of a shale sample from the Kimmeridge Clay Formation. This shows a matrix of clay sheets, containing more-or-less isolated silt particles. The clay particles are seen to vary in orientation, but are aligned locally.

Because the low aspect ratio pores between the clay particles are expected to be more compliant than the clay particles themselves, it is important to account for the additional compliance of interparticle regions in any model of elastic wave propagation through shales. In this paper, the effect of the interparticle regions between clay particles on the anisotropy of shales is investigated. An understanding of shale anisotropy is important 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. Figure 1: Scanning electron micrograph of a shale from the Kimmeridge Clay Formation in Dorset, UK. Photograph by John Cook, Schlumberger Cambridge Research. Many shales encountered in the subsurface can be described, to a good approximation, as being transversely isotropic with a vertical axis of rotational symmetry. A transversely isotropic medium has five independent elastic stiffnesses.

The effect of interparticle regions on seismic anisotropy

The displacement discontinuity may be related to the applied traction vector twith components ti by [ui] = Bij tj . In the simplest case, Bij may be represented in terms of a normal compliance BN and shear compliance BT , where BN gives the displacement discontinuity normal to the particle surfaces for unit normal traction, while BT gives the displacement discontinuity parallel to the particle surfaces for unit shear traction.