VTI Anisotropy Estimation of the Regional Shale from Onshore Abu Dhabi Walkaway VSP

Abstract Seismic anisotropy is the directional dependence of the velocity of seismic waves propagating in a medium. It can affect how the subsurface structures are seismically imaged. Vertical Transverse Isotropy (VTI) is caused by fine geological layering such as in shales. To correct for VTI effects, we need to measure its parameters, which describe how much the seismic wave speed varies with the angle of propagation. Walkaway VSP offers the best dataset for determination of anisotropy parameters, since the direct P-wave arrivals are usually well determined, and the depths are known accurately (Thomsen 2014). In addition, mode-converted S-waves are also available for analysis. Walkaway VSP was recorded in an ongoing time-lapse monitoring project using a 36 level 7.6 m spaced 3C downhole receiver array with a 5 km extended line with 25 m source spacing on each side of a vertical well in an onshore field in Abu Dhabi. The VSP data quality was excellent, the extended Walkaway VSP source line provided wide coverage of the incident angles required for anisotropy estimation. Anisotropy estimation was performed using two methods; the slowness polarization method which requires 3C data and the phase slowness method which requires direct P-wave arrival travel times only i.e. 1C data. Walkaway VSP provides direct measurements of the upgoing and downgoing primary P- and secondary S-waves. 3C data was oriented along the direct P-wave ray path followed by 3C wavefield separation to obtain slowness and polarization of the upgoing and downgoing compressional and shear waves. Thomson’s anisotropy parameters (ε and δ) at receiver depths were estimated by modelled curve fitting on the slowness polarization cross-plot. Local anisotropy estimation using local slowness and polarization of compressional and shear waves is not affected by the overburden complexity. Strong primary to shear wave mode conversion at the top of the shale formation helped to obtain reliable results. The geological structure in the overburden is relatively flat and simple, hence local anisotropy was also estimated by phase slowness inversion which uses the direct P-wave arrival travel times to compute the vertical and horizontal components of the slowness with respect to depth and offset, respectively. Modelled curve fitting on the vertical and horizontal slowness cross-plot provided a local anisotropy estimate in the shale formation. The estimated anisotropy parameters from both the methods were quite robust. The improved anisotropic model resulted in improved Walkaway VSP imaging and hence improved time-lapse results. This paper provides anisotropy estimate of the highly anisotropic regional shale cap rock. Thomsen parameters from this onshore Abu Dhabi case study are presented which can also benefit other seismic imaging projects in the region.