Anisotropy correction for deviated‐well sonic logs: Application to seismic well tie

Abstract

Borehole sonic logs acquired in deviated wells penetrating the HRZ and Colville shales in the Niakuk field in Alaska's North Slope are seen to be significantly faster than vertical well logs. These differences are attributed to shale anisotropy. An iterative inversion scheme was created to invert for shale anisotropy parameters using multiple wells penetrating shale sections at different angles. The inversion involves fitting the sonic log data at a range of borehole angles to the compressional wave group velocity surface. The result is an estimate of the anisotropy parameters (ε and δ) and the vertical P‐wave velocity. The results show that the shales are strongly anisotropic, with compressional‐wave anisotropy (Thomsen's parameter ε) on the order of 40% and the anisotropy parameter δ (relates vertical velocity to short‐offset NMO velocity) around 10%. This large anisotropy can affect seismic imaging, AVO, and time–depth calculations.A procedure was created to estimate the anisotropy‐corrected vertical sonic logs from sonic data recorded in a deviated well. The inputs are well deviation, P‐wave sonic log, volume of shale or gamma ray data, and anisotropy parameters for rock with 100% shale volume. With these inputs the compressional‐wave group velocity surface is computed and the equivalent vertical P‐wave sonic log is output. The equivalent vertical sonic log can then be used for standard seismic applications using isotropic velocity assumptions. The application was applied to a well deviated at approximately 67°. Shale anisotropy parameters were taken from the sonic log inversion, and an anisotropy‐corrected sonic log was produced. Seismic well ties were attempted using both the recorded logs and the anisotropy‐corrected logs, with the result that the well tie using the measured logs was poor while a tie using the anisotropy‐corrected logs was good.