Modeling anisotropic static elastic properties of soft mudrocks with different clay fractions

Abstract

We have quantified the effects of clay fraction and fabric on the static elastic properties of soft mudrocks with emphasis on microlevel mechanisms. Soft mudrocks are treated as a mixture of nonclay minerals and clay-water composites. We have devised a simplified approach to estimate the fabric orientation distribution of soft mudrocks based on measured parameters such as clay fraction and porosity. A single parameter (fabric angle) that characterizes the fabric orientation distribution of soft mudrocks is related to the void ratio of clay-water composites. The static transversely isotropic (TI) elastic properties of soft mudrocks are modeled using an anisotropic differential effective medium approach. The effect of variation in fabric orientation distribution on the TI elastic parameters of clay-water composites is studied by applying the Voigt approximation. With an increase of clay fraction, soft mudrocks have decreasing trends in the deformation moduli because some nonclay minerals are replaced by clay-water composites. However, the deformation moduli of clay-water composites could increase when there is more anisotropy in the fabric due to an increase in the clay fraction. Thus, the correlations between anisotropic elastic moduli and volume fraction of clay-water composites will display some fluctuations. Such nonlinear relationships are validated against published experimental data on Colorado shale samples from the Western Canadian Sedimentary Basin.