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 SummaryThe 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.
Digital Rock Physics (DRP) is a rapidly developing technology for rock property characterization, which plays an important role in a wide range of fields, such as hydrogeology, petroleum exploration and production, and CO 2 capture and sequestration (Berg et al., 2017;Blunt et al., 2013;Nur et al., 2011;Singh et al., 2017). The basic workflow of DRP is "image-and-compute," which is to image and digitize the 3D structure of rock samples and then perform numerical simulations of various physical processes in the digital objects to obtain rock properties like permeability, elastic moduli, and electrical resistivity (Andrä et al., 2013a(Andrä et al., , 2013bBlunt et al., 2013). Digital rock models are typically a few millimeters in diameter, which is about an order of magnitude smaller than that of laboratory samples. Hence, larger digital rock images are preferred to fully characterize rock properties. Compared to laboratory measurements, DRP is able to derive more rock properties of the same rock sample in a shorter time, and thereby emerges as a promising and valuable source to reveal the causal relations between rock structure and properties and the cross-property relations (
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