Formation density and acoustic velocities are required over the entire overburden interval for proper seismic calibration. However, these data are often not acquired owing to poor borehole conditions encountered in the shallow sections of the borehole. This paper describes log acquisition, processing, and interpretation required to provide the necessary simulated log information using neutron capture spectroscopy and the latest-generation borehole acoustic logs acquired through casing. The method presented is the result of a combined effort of a team of geophysicists and petrophysicists.
In an offshore Abu Dhabi well, openhole logging tools could not be lowered in the 17½-in. borehole owing to severe washouts. Only one thousand feet out of five thousand feet could be logged and the data were affected by poor pad-formation contact of the density tool. After setting 13 3/8-in. casing, the petrophysical operation team decided to acquire the neutron capture spectroscopy data in addition to the latest-generation sonic data over the entire five thousand foot interval. Cement bond data indicated poor cement over much of the interval.
Excellent shear and lithology information was acquired over most of the interval and reliable compressional velocity was extracted over 40% of the interval. The paper describes, in detail, the method utilized to combine the lithology and shear data to reconstruct the compressional velocity and formation density over the entire interval. Validation of the reconstructed data against measured data, where available, is presented. Finally, we present the comparison of elastic models generated using this method vs. those models using existing techniques of estimating the density and velocity information and show that our approach provides a much-improved match to the vertical seismic profile data acquired on the well.
The improved logging capability of the modern tools in large-cased boreholes opens up an opportunity for acquiring better geophysical data needed for proper tie-in to seismic information.
A well was drilled in an offshore Abu Dhabi field in late 2005 with approximately 20 degree constant inclination. Intensive data gathering including multi-azimuth walk-away and walk-around VSP's were acquired in order better tie existing 3D seismic data and, most importantly, aid in an ongoing 3D seismic reprocessing project for AVO and seismic anisotropy analysis. Since all seismic waves propagate through overburden from surface to reservoir, acquisition of reliable formation elastic properties information (Vp, Vs, density) in the shallow section is critical to understanding these overburden effects on the target reservoir seismic units and to make proper seismic-VSP tie. Reliable acoustic information also allows one to model parameters such as multiple generators (for eventual removal) and gain anisotropy (VTI & HTI) information for the seismic processing in combination with the acquired VSP data.