Computation of effective elastic properties of clay from X-ray texture goniometry data

Abstract

We have developed a new method for estimating the contribution of a pure clay fraction (i.e., devoid of organic matter) to the total effective rock stiffness. The method is based on published clay mineral stiffness data and on an original preferred clay mineral orientation data set obtained by X-ray texture goniometry on 56 samples of Kimmeridgian and Devonian age from two North American shale plays. We find that (1) large variability in preferred orientation of clay results in moderate variability in effective clay elastic anisotropy and (2) the effect of variations in the preferred orientation on effective rock properties is small compared with the effects of variations in clay abundance. As a result, a single clay elastic tensor is computed to be used in effective medium models. In addition, to account for various degrees of hydration, water is incorporated into the dry clay tensor through inclusion models. In situations in which isotropic approximations are necessary, we also provide apparent bulk and shear moduli for a hydrated clay fraction as a function of porosity and propagation angle.