Stress Dependent Anisotropy in Shales — Measurements and Modelling

Shales comprise a large proportion of the sedimentary pile in many hydrocarbon basins and impact significantly on exploration, development and production costs through the effect of seismic anisotropy on imaging and depth conversion. Shales play an important role in time-lapse seismic response and pore pressure prediction. To understand stress dependent anisotropy on shales, stress dependencies of the five elastic constants are measured for four sets of shales. These new measurements are used to test the method that calculates the shale elastic constants using the silt fraction, clay packing density and set of elastic constants derived on the basis of nano-indentation technique. The results of our analysis show considerable variability of clay properties for a given clay packing density. We also relate the variation of clay elastic constants with the amount of very small and compliant pores (soft porosity). For the first time, soft porosity of shales is estimated from the experimental data. The soft porosity correlates with stress dependent variations of elastic moduli. This suggests that stress dependency of shale properties results from the closing of the soft porosity. Variations of the compressibility depend on exponential functions of the effective confining stress and the soft porosity can be estimated from the fitting coefficients. The stress dependencies of the measured elastic compliances of wet shales and of the dry compressibilities estimated using Gassmann equation are approximated by exponential functions using a nonlinear fitting based on the Levenberg–Marquardt algorithm. The soft porosities estimated from the fitting coefficients for dry compressibilities are shown to be in a reasonable agreement with the measured values.