Sensitivity of shale anisotropic parameters to core cutting rotation error

Abstract

In laboratories, core cannot always be cut exactly along the axis of symmetry (normal to the bedding plane), which leads to a minor core cutting rotation (CCR) error. The presence of a small CCR error can give rise to an error in computation of anisotropic parameters, which will result in erroneous P-wave and S-wave velocities (VP, VSV and VSH). In this study, we test the sensitivity of Thomson’s anisotropic parameters, epsilon (ε), gamma (γ) and delta (δ), to the CCR error. In the vertical transverse isotropy (VTI) system where no rotation exists, values of ε, γ and δ will not vary with azimuth. Similarly, in VTI the values of VP, VSV and VSH, measured at orientations with respect to the axis of symmetry, will also not vary with azimuth. We modelled and analysed the error generated, in anisotropic parameters and phase velocities, due to rotation error of 5° on VTI ANNIE model data, and the Niobrara Shale core sample measurements. For these two cases, we obtained values of P-wave and S-wave velocities along with anisotropy parameters ε, γ and δ, at 0°, 45° and 90° orientations with respect to the axis of symmetry, before and after a rotation of 5°, for different azimuth directions. This study shows that, when the CCR error exists, ε, γ and δ vary with azimuth. For Niobrara Shale samples, we observed that, among all three Thomsen’s parameters δ is the most sensitive parameter to the CCR error; when we varied azimuth, we observed a sign change of δ from positive to negative. For ε and γ, variation in azimuth lead to only slight changes in values without any change of sign. The CCR error affects VP and Vs measurements the most at 45° orientation, and the least at 90° orientation. The maximum error occurred at 0° azimuth, whereas the minimum error occurs at 90° azimuth. This analysis suggests that, if core cutting rotation exists, phase velocities should be measured at 90° azimuth for accurate results.