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Investigation of the high-frequency wavefield of an off-center monopole acoustic logging-while-drilling tool

Wang Hua, Tao Guo and Michael C. Fehler
Investigation of the high-frequency wavefield of an off-center monopole acoustic logging-while-drilling tool
Geophysics (July 2015) 80 (4): D329-D341

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.


ISSN: 0016-8033
EISSN: 1942-2156
Coden: GPYSA7
Serial Title: Geophysics
Serial Volume: 80
Serial Issue: 4
Title: Investigation of the high-frequency wavefield of an off-center monopole acoustic logging-while-drilling tool
Affiliation: China University of Petroleum, Laboratory of Petroleum Resources and Prospecting, Beijing, China
Pages: D329-D341
Published: 201507
Other Date: 20150602
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 21
Accession Number: 2015-098578
Categories: Applied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Massachusetts Institute of Technology, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 201542
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