Quadrupole shear is now a ubiquitous measurement from Logging While Drilling (LWD) acoustic tools and offers a robust and reliable method of delivering formation shear information in slow formations. The general increase in the use of LWD measurements worldwide has meant that LWD acoustics measurements are being made in a greater variety of hole sizes, borehole conditions and formation types than ever before. These include a variety of anisotropic formations such as those encountered in unconventional operations. Where anisotropy is present then its correct characterization can be critically important for a variety of applications including seismic processing, geomechanics and wellbore stability, completions design and production management.
The non-uniquely directional nature of the quadrupole mode means that it is not yet possible to reliably extract the fast and slow shear anisotropy information from quadrupole data. Dipole tools are commonly used as solutions when this information is needed. Dipole measurements are currently only routinely available on wireline devices due to the large coupling of the dipole mode with the collar mode. Unipole devices are also used in certain situations. However in many cases quadrupole and/or monopole may be the only shear measurement methods used on a particular well. This brings up the question - what does the quadrupole actually measure in anisotropic formations and how is the data sensitive to such an environment?
This paper explores the effect of anisotropy on the quadrupole mode, its azimuthal sensitivity, what it measures and its limitations in anisotropic formations. The effects of anisotropy on the quadrupole dispersions are discussed along with ways to determine the presence of anisotropy based upon the results of the quadrupole inversion processing. Numerical modeling in anisotropic formations coupled with data examples are presented, including dispersion plots, that demonstrate these effects and explain the techniques used.