Investigation of the high-frequency wavefield of an off-center monopole acoustic logging-while-drilling tool
Name
Wang-2015-Investigation of the.pdf
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Author(s)
Wang, Hua
Tao, Guo
Fehler, Michael
Date Issued
May 2015
Journal
Geophysics
Publisher
Society of Exploration Geophysicists
Citation
Wang, Hua, Guo Tao, and Michael C. Fehler. “Investigation of the High-Frequency Wavefield of an off-Center Monopole Acoustic Logging-While-Drilling Tool.” Geophysics 80, no. 4 (July 2015): D329–41. © 2015 Society of Exploration Geophysicists
Version
Final published version
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.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology. Earth Resources Laboratory
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DOI of Published Version
https://doi.org/10.1190/GEO2014-0426.1
