New Scheme for Sonic Verticalization in a Highly Deviated Well, in a Complex Carbonate Field, Onshore Abu Dhabi, UAE

Abstract

Reliable sonic and density logs are key inputs for seismic driven reservoir characterization. Anisotropic surface vertical velocities are essential for seismic well ties, estimation of wavelets and low frequency modeling. These help us to understand the relationship between reservoir and acoustic properties. In this study, sonic logs were acquired in highly deviated wells penetrating highly anisotropic shale formations. These logs needed corrections before integrating in the reservoir characterization workflows. In allusion to problem existed in traditional methods, an iterative inversion method based on multi-well models composed of adjacent vertical and deviated wells was proposed. Well data was optimized after matching depth of vertical and deviated wells. Thomson approximation formula was re-derived based on multivariate linear fitting and least square coefficient matrix was constructed to obtain delta and epsilon respectively. Best Thomsen parameters were obtained to address shale anisotropy. The results show that the Nahr-Umr shale formation have strong anisotropy for highly deviated wells in the field of study. The correction was not required for deviation less than 20 degrees. The correction gradually increases and reaches as much as 20% to 30% at 60 degrees. It was observed that there is a huge difference between the measured velocity of highly deviated wells and the true vertical velocity in the shale formation. This anisotropy can affect seismic imaging, time-depth conversion, AVO, well-seismic calibration. Therefore, sonic log correction is necessary for highly deviated wells in the shale formations. Corrected sonic logs in deviated wells provide accurate acoustic velocities for further reservoir characterization & seismic inversion studies. The real acoustic data of deviated wells can be obtained from shale formations with strong anisotropy such as Nahr-Umr and Laffan formations.