It has been well known for some time that obtaining reliable shear information from logging while drilling (LWD) dipole tools is a formidable challenge due to the strong coupling between the formation and collar flexural modes (Tang et al., 2003). However, the benefits that LWD dipole acquisition can bring, both in terms of the applications of the measurement and the time savings associated with the conveyance method, mean that it has remained an area of close focus. Obtaining dipole fast and slow shear anisotropy information (azimuth and shear slowness) would enable LWD sonic data to be useful for a wide range of applications where understanding of acoustic anisotropy is critical. These applications range from accurate time to depth migration of surface seismic to wellbore stability, completions and production optimization.
This paper presents dipole field data, acquired while drilling in fast formations, with a newly developed LWD multipole sonic tool, which acquires four-component dipole waveforms at the same data density as the standard acquisition of monopole and quadrupole waveforms. It discusses the main challenges of acquiring reliable dipole waveforms with a rotating LWD tool and how these challenges were addressed. Shear slowness evaluated with the low- and high-frequency processing algorithms are shown along with corresponding dipole dispersion curves (example shown in Fig.1). These curves are also validated with modeling results assuming a detailed acoustic tool structure for the new tool. Finally, the obtained LWD dipole shear slowness logs are compared with those from a wireline dipole tool. As a result, we demonstrate that the acquisition of reliable dipole data (in very fast formations, where there is a good slowness separation between the formation and the collar flexural modes) is feasible in a while-drilling environment. We also present adequately quality controlled data with consistent shear slowness logs comparable to a benchmark reference. The current limitations of the measurement are also analyzed and communicated. The results of the field data pave the way for an expansion of the measurement capability across a range of hole sizes and open the door to a variety of new applications for LWD measurements.
