Horizontal transverse isotropy anisotropic parameter inversion via azimuthal seismic velocity anisotropy and its application to anisotropic 3D in-situ stress estimation

Abstract

The anisotropic parameters inversion in horizontal transverse isotropy (HTI) medium plays an important role in predicting the fracture density as well as the anisotropic in-situ stress for unconventional reservoirs. The current industry practice is to use the azimuthal PP-wave reflection coefficient to estimate the HTI anisotropic parameters. Based on the linear slip theory, this study adapts azimuthal P-wave phase velocity to calculate the HTI anisotropic parameters and demonstrates superiority against the conventional azimuthal PP-wave reflection coefficient. Specifically, we first verify that the azimuthal P-wave phase velocity is more feasible for the HTI elliptical fitting rather than the azimuthal PP-wave reflection coefficient due to the analytical formulations. Second, we sort the prestack wide-azimuth (WAZ) data into offset vector tile (OVT) sectors and perform the AVO (Amplitude Versus Offset) inversion at each azimuth. Third, the elliptical fitting is applied to the obtained azimuthal P-wave phase velocities to estimate the HTI anisotropic parameters, fracture density, and fracture direction. Fourth, based on the HTI mechanical earth model (MEM), we formulate the 3D in-situ stress as a function of the obtained elastic parameters and fracture compliance, which exhibits a potential for computational efficiency. Finally, field examples from the Zhaotong area, China demonstrate that the estimated fracture density and anisotropic in-situ stress present high accuracy and resolution compared with conventional methods. The dominant stress regime in the study area is a strike-slip faulting regime with a governing orientation of NE-SW and presents good alignments with well logs, which demonstrates the reliability and accuracy of our proposed method for predicting fracture density and anisotropic in-situ stress.