Analytical solutions of borehole acoustic fields for an eccentric drill collar in the monopole acoustic LWD

摘要

Analytical solutions of acoustic fields are derived for the case of an eccentric drill collar inside a fluid-filled borehole, which often troubles us a lot during logging while drilling due to complex string movements. With this method, the borehole wavefields in such condition are investigated excited by a monopole source with the center frequency of 8 kHz. The effects of varying tool eccentricity on the waveforms at different azimuths, are analyzed. The results reveal that the wavefields properties in some direction are closely related to the thickness of the fluid annulus between the collar and the formation toward that azimuth, especially for collar waves and Stoneley waves. In addition, based on the two-dimensional spectrum technique, the coupling of multipole modes is revealed, when the drill collar is off center. Particularly, in the case of severe eccentricity, the flexural mode can be strong enough that it may interfere the recognition of Stoneley waves. These studies will be helpful for tool design and data processing.