Shale anisotropy and the elastic anisotropy of clay minerals

Abstract

As shales are usually anisotropic, models that neglect this anisotropy may lead to incorrect estimates of rock and fluid properties derived from inversion of seismic amplitude versus offset (AVO) data. Such models may also fail to describe geomechanical behavior correctly. An important contribution to shale anisotropy results from the partial alignment of anisotropic plate-like clay minerals. Clay minerals are examples of sheet silicates, and are among the minerals with the highest elastic anisotropy. Because of technical difficulties associated with small grain size, single crystal elastic moduli of clay minerals have not been measured experimentally. However, calculations of the complete elastic tensors of several sheet silicates using first-principle calculations based on density functional theory have been reported. The elastic stiffness tensor resulting from such calculations can be approximated to good accuracy as a transversely isotropic (TI) medium. The large interlayer space in the crystal structure of clay minerals plays an important role in determining the character of their anisotropy, and can be represented by a normal compliance BN and shear compliance BT that describe the deformation of the interlayer space under an applied stress. Relations between the various clay mineral anisotropy parameters depend on the ratio BN/BT of these interlayer regions. A simple model of clay minerals, as isotropic layers interacting via BN and BT, is shown to reasonably describe their elastic anisotropy. This model can be used to derive the elastic anisotropy of shales, if the orientation of clay particles, the compliant regions between them, and silt particles are also taken into account.