Shale Anisotropy Estimation From Logs in Vertical Wells

Anisotropic elastic parameters for shales are widely needed in seismic imaging, reservoir characterization, and carbon sequestration monitoring. Unlike other elastic parameters such as vertical P and S wave velocities, anisotropy parameters are not measured directly from the acoustic well logs due to the single‐directional nature of a well. We assume that shale anisotropy is induced by thin cracks that are filled with liquid in a background isotropic medium, whose bulk and shear moduli are obtained from the vertically measured P and S wave velocities, density, and porosity from corresponding well logs through a formalized inversion scheme. We show that the estimated anisotropy using the proposed method is consistent with the mineralogy and agrees with the published laboratory measurements. This framework allows us to quantify the uncertainties in the anisotropy parameters estimated from the inversion, which can be used as a measure to evaluate the validity of the chosen rock physics model. Plain Language Summary The differences in the wave speeds as they propagate in different angles, defined as anisotropy, are widely needed for imaging the subsurface and understanding the tectonic processes. However, measurements made in a vertical borehole cannot provide this directional information. In this study, we assume a rock physics model for shales that adds anisotropy‐inducing thin inclusions in an isotropic background, the elastic and fluid properties of which are inverted formally from the measured well logs. The proposed method generates anisotropy estimates that agree with published laboratory measurements and that are consistent with the mineralogy log. Moreover, the proposed method quantifies the uncertainties in the estimated anisotropy, which can be used to evaluate the applicability of the chosen model on a particular rock formation. Key Points A formalized inversion framework to estimate anisotropy from single‐directional well log measurements is proposed Parameters inverted from the framework are physically meaningful and bounded Uncertainties are quantified based on the framework and indicate the applicability of the chosen rock physics model