The Influence of Borehole Breakout in Shear Wave Velocity Prediction: A Case Study of "Pleades" Well, South Sumatera, Indonesia

Septian Prahastudhi, Intan Andriani Putri, Ayi Syaeful Bahri, and Yuda Faisal Yushendri
Physics, Sepuluh Nopember Institute of Technology, Surabaya, Indonesia.

In seismic exploration, S-wave is an important attribute to characterize a reservoir. But, in the real condition, S-wave data often doesn't exsist, or the quality is very poor. Castagna and Krief equation can create a S-wave based on P-wave, but the this method only valid when the water saturation is 100%. In other hand, Biot-Gassmann fluid substitution need S-wave as an input and Gassmann approximation has a significant error for consolidate rocks. Dontown and Gunderson introduce a method to do fluid substitution with unknown Vs based on Biot-Gassmann equation, so P-wave velocity in the condition with water saturation 100% can be found. Then the S-wave velocity can be estimated using Castagna and Krief empirical relation before return it to the real condition using Gassmann Fluid Substitution. Compared with the FMI (formation micro imager) to analyze the borehole condition, the result shows that borehole breakout doesn't affect the quality of predicted S-wave directly, but the stress anisotropy caused by the borehole breakout influence the quality of the predicted S-wave. It indicated that there's still a consideration in using Gassmann equation with the appearance of borehole breakout.