Abstract
AbstractShales often show strong elastic anisotropy that originate from the alignment and platy nature of its constituent minerals. Despite its impact on amplitude variation with offset response and seismic time‐shifts, elastic anisotropy of shales is, however, often ignored since it is difficult to measure enough parameters in the field. Being able to correctly estimate anisotropy parameters of shales can therefore significantly improve seismic reservoir characterization. A predictive model is developed by combining existing theories. Properties of locally aligned clay platelets, called domains, are calculated using a rock physics model based on the anisotropic Hashin‐Shtrikman estimates. The effect of domain orientation is then accounted for by the orientation distribution function of domains. The applicability of the model was investigated using existing core measurements. Interesting findings include: (a) most of modeled anisotropy parameters are consistent with the measured values even though only limited information was used for the model parameters optimization, and (b) most of optimized interplatelet medium properties are consistent with the saturated fluid and small interplatelet medium shear modulus suggested by existing studies. These findings imply that the model can be used to predict anisotropy parameters from limited information.
Highlights
Most rocks are anisotropic to some extent
Interesting findings include: (a) most of modeled anisotropy parameters are consistent with the measured values even though only limited information was used for the model parameters optimization, and (b) most of optimized interplatelet medium properties are consistent with the saturated fluid and small interplatelet medium shear modulus suggested by existing studies
The effect of domain orientation is taken into account by the compaction orientation distribution function (ODF). This approach is an extension of Sayers and den Boer (2020) in which the impact of domain orientation is accounted for by two orientation distribution parameters W200 and W400; here, those are related by the compaction ODF
Summary
Most rocks are anisotropic to some extent. The origin of the anisotropy is always heterogeneities on a smaller scale than the volume under investigation, ranging from layered sequences of different rock types down to molecular configurations (Fjær et al, 2008). This approach is an extension of Sayers and den Boer (2020) in which the impact of domain orientation is accounted for by two orientation distribution parameters W200 and W400; here, those are related by the compaction ODF This compaction ODF-based model is similar to the existing methods (e.g., Hornby et al, 1994; Ruud et al, 2003; Yurikov et al, 2021), but differs from them in that the domain properties are calculated by the anisotropic Hashin-Shtrikman estimates. Vernik and Anantharamu (2020) demonstrated that the ODF given by Equation 1 fits measured pole density orientation data on low porosity shale samples reasonably well by using the compaction factor as the single fitting parameter (note that their formula for the ODF is in slightly different form; their formula uses the inverse of the compaction factor as Z and the normalization factor of 1/(8π2) is not included; with the normalization factor, their formula gives the same result as Equation 1). Initial porosity (if the compaction factor needs to be adjusted)
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