Case Study Demonstrating the Estimation of Depth-Continuous Formation Anisotropy with Application to Geomechanics and Seismic Velocity Model Calibration

Abstract

This paper presents a case study in which new methods that use full-waveform sonic data are applied in an unconventional well setting to determine depth-dependent elastic anisotropy of formations penetrated by the well and estimate parameters of interest. The study objectives include the following: Estimate Thomsen's shear anisotropy parameter γ in an unconventional well that penetrates fast formationsUse rock physics and other approximations to further estimate a complete vertical transverse isotropic (VTI) elastic tensor at each depthCompare results with ground truth in terms of dynamic and static core measurementsUse these results to derive anisotropic geomechanical parameters for well completion and fracture treatment design and to compute upscaled seismic-equivalent elastic anisotropy for the calibration of anisotropic seismic velocity models Formation speeds in this well were extremely fast, typical for unconventional shale reservoirs, which created a challenging environment for estimating VTI Thomsen's parameter γ because of the extreme sensitivity of the inversion to the accuracy of the borehole fluid slowness estimate. The key to the study's success was development and application of methods to invert for a depth-dependent mud slowness curve. This allowed for much more accurate inversion of the VTI parameter γ than the conventional method that uses a constant mud slowness value. In addition to enabling a more accurate inversion, it is observed that the mud slowness curve not only varied with depth [likely because of pressure/temperature (P/T) changes and possible settling] but also reflected quite different properties across a drilling fluid pill that was placed around the reservoir formations. This analysis provides an additional benefit for drilling engineers because the mud slowness curve tracks mud property changes in the well and can determine the actual location of the drilling fluid pill after placement and stabilization. Additional work estimated the depth-continuous elastic tensor and geomechanics (anisotropic Poisson's ratios and Young's moduli necessary for computing horizontal stresses) for well completion and fracture treatment design. Seismic-scale properties were estimated using anisotropic Backus averaging for the calibration of the anisotropic seismic velocity model for prestack depth migration.