| Abstract | During logging-while-drilling (LWD) operations, complex
drill string movements and the weight of the drill pipe often lead
to a measurement tool that is not centralized. Therefore, studies
of the response of an off-center acoustic LWD tool are essential
to facilitate better interpretation of measurements made in an
actual drilling environment. Such studies will be helpful for tool
design and data processing. We used a finite-difference method
to simulate the response of a noncentralized monopole acoustic
LWD tool at high frequency (10 kHz). We analyzed the effects
on the waveforms for receivers at different azimuths caused by
an off-center tool with differing amounts of offset. We used
velocity-time semblance and dispersion analysis methods to
help us to understand the modes in the waveforms at different
azimuth receivers for different tool offsets. We have found that
the waveforms in the direction of the tool offset, that is, where
the fluid column is smallest, were affected the most.Waveforms
in the orthogonal direction were less affected by tool offset. Collar
flexural and collar quadrupole modes appear when the tool is
off center. In addition, the formation flexural and quadrupole
modes contaminate the Stoneley wave. Waveforms in a fast formation
are more strongly affected by the offset of the tool than
those in a slow formation. In a fast formation, the new collar
modes make it difficult to determine the P-wave velocity in
the direction of tool offset whereas it is easier in the orthogonal
direction. However, P-waves are less contaminated by new
modes in a slow formation. Due to the significant changes in
waveforms with azimuth when the tool is off center, the simple
addition of all waveforms from an azimuthal distribution of
receivers will not result in a clean waveform that is sensitive
to only the surrounding formation. |
