Field Test Results Demonstrating Improved Real-Time Data Quality in an Advanced LWD Acoustic System

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAcoustic compressional slowness measurements are critical for geomechanics, petrophysical and seismic applications. Conventional acoustic wireline data is often unavailable due to cost constraints or well problems, thus increasing the need for acoustic Logging While Drilling (LWD). The LWD environment creates additional demands on borehole acoustic measurements as compared to wireline, due to tool strength requirements and drilling noise. These issues have consistently limited the LWD acoustic tool's ability to provide wireline equivalent data quality. The conventional LWD methods have incorporated single direction or single axis acoustic systems with short transmitter-receiver (TR) spacing (typically 4 ft). Technical features of a new acoustic LWD tool incorporating long TR spacing (+10 ft) with an array containing six fourelement receiver stations to obtain omni-directional measurements will be discussed. Field test data examples will show the improved quality of this real-time measurement.The tool configuration was optimized as a result of comprehensive modeling and lab testing of key system components. Greater source power combined with multifrequency adjustable signature capability allowed the TR spacing increase. The system quality was enhanced due to improved software and hardware functionality. An improved receiver array configuration allows for better noise isolation and response to formation arrivals. In addition, real-time downhole monitoring of the drilling noise coupled directly into the collar allows for further data quality enhancement. Redundancy and a modular design approach were key for providing a robust and reliable system with fast tool setup and data downloading.The field test examples will present data acquired in different drilling modes to demonstrate the improved signalto-noise ratio for raw and stacked data in both fast and slow formations. As compared to industry standards, faster Rate of Penetration (ROP) is achievable due to enhanced downhole processing capabilities. The real time slowness results compared to conventional wireline shows the systems' ability to make comparable measurements. These field test results demonstrate an improved data quality with this new LWD acoustic system.

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

“…However, if the velocity difference is not large, the P-wave may be influenced by the collar wave and special means such as tool grooves (e.g., Leggett et al, 2001;Kinoshita et al, 2010) should be adopted to eliminate the interference. 2) More collar modes, including monopole, dipole, and even quadrupole modes, appear on receivers in the direction of the tool offset.…”

“…A great deal of research in ALWD leads to the conclusion that the velocities of the P-and S-waves can be reliably measured in fast formations by a tool with a monopole source. Successful measurements require that the effect of the drill collar on the waveforms are eliminated by some means (e.g., Leggett et al, 2001;Wang et al, 2009b;Kinoshita et al, 2010;Zhan et al, 2010). The effects of the drill collar on the processing and interpretation of ALWD data are analyzed by Moore et al (2008).…”

Investigation of collar properties on data-acquisition scheme for acoustic logging-while-drilling

“…Figure 2b shows a velocity-time semblance plot for the waveforms. We can see that the P-wave is submerged in the collar wave due to the short spacing in the ALWD tool, which makes it difficult to determine the P-velocity (Aron et al, 1994(Aron et al, , 1997Minear et al, 1995;Joyce et al, 2001;Leggett et al, 2001;Tang et al, 2002). This collar-wave interference needs to be eliminated to analyze the P-wave.…”

“…It is difficult to identify the P-wave in a fast formation because of the interference between it and the collar wave. The main method to eliminate the interference is by using a sound isolator on the tool, in which the isolator is set either between the sensors (receivers and source) and drill pipe to decouple the collar and formation waves (Varsamis et al, 1999) or between the source and receiver to attenuate the amplitude of the collar wave or change the travel path of the collar wave (Aron et al, 1994;Leggett et al, 2001). Usually, grooves are cut periodically in the inner (Kinoshita et al, 2010) or outer wall (Varsamis et al, 1999) of the collar, and they act together as an acoustic isolator between the source and receiver (Aron et al, 1997;Joyce et al, 2001).…”

Characteristics of group velocities of backward waves in a hollow cylinder

“…Acoustic isolation is essential for the accurate measurement of compressional wave velocities of formations using a LWD monopole tool. 15 When the transmitter, which is mounted to the collar, starts to work, it excites motion in both the formation and the collar. Thus, signals received by the receivers are composed of the formation waves and the tool waves.…”