All rocks are anisotropic to some degree. Sedimentary basins worldwide comprise 80% of shales. Shales are anisotropic usually with vertically transverse isotropy (VTI). The rocks in many of the basins of the Asia Pacific region have been measured to have strong anisotropy (> 20%). Anisotropy can be measured through detailed core analysis, walk away VSPs, crosshole data and specialised sonic logging. However, these measurements are not collected very often. Anisotropy can be estimated from seismic velocity analysis and comparison of sonic velocities in wells with different deviations. Using the seismic velocities provides a low resolution estimate of anisotropy and this method of estimation can only resolve for p-waves. Estimating anisotropy from wells with a range of deviations presents its own practical problems. This study looks at the estimation of anisotropy from vertical and deviated wells to highlight a workflow and it’s limitations, and the correction of sonic logs for anisotropy and the quality control.
1. Ikon Science
2. Murphy Oil Sarawak