Schlumberger

Technical Paper: Slow Formation Shear from an LWD Tool: Quadrupole inversion with a Gulf of Mexico Example

Society: SPWLA
Paper Number:
Presentation Date: 2010
 

Abstract

The general consensus in the industry is that the best way to measure slow formation shear in the logging-while-drilling (LWD) environment is by utilizing quadrupole waveforms. The use of quadrupole acoustic logging to determine shear velocities in slow formations requires an inversion method that takes into account the dispersion of the quadrupole mode, including the effects of the tool presence. This implies that the quadrupole shear inversion requires careful and well-validated modeling of the tool acoustics.

The impact of the tool presence on the quadrupole dispersion is largest in the smaller boreholes where the tool takes up the largest percentage of hole volume. A log example was obtained using a 4.75-in LWD multipole tool from a 5.75-in hole in the Gulf of Mexico where the slowness ranged from 150 to above 250 us/ft. Monopole shear was available only in short intervals, providing a comparison for the quadrupole log in these intervals, as well as illustrating the need for the quadrupole measurement to fill in the missing depths.

The resulting logs must be accompanied by convincing quality control (QC) displays to provide the user with the means to QC the final shear slowness product. Log displays compare the waveform dispersion with the inverted model dispersion and the resulting shear log, and single-depth displays provide details where needed.

This quadrupole inversion methodology was used with high-quality, very wideband quadrupole waveforms and dispersion curves from a Gulf of Mexico well. Together they supplied the information needed to produce a set of QC displays that enable the resulting shear slowness log to be judged with confidence.

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