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.
The shear waves from a four-component cross-dipole acoustic tool were analyzed to provide an important method for the single-well acoustic imaging application. We analyzed the effects of wave radiation, reflection, and borehole acoustic response on the shear-wave reflection measurements. In particular, we studied shear-wave radiation from a dipole source and the wave's reflection from a formation reflector, showing that the shear waves generated by a dipole source in borehole have a wide radiation pattern. This allows for imaging reflectors at various dip angles crossing the borehole. More importantly, the azimuthal variation of the shear waves, in connection with the multi-component nature of a cross-dipole tool, can be used to determine the strike azimuth of the reflector. The analysis results establish a theoretical foundation for the borehole shear-wave imaging application. The theoretical results are utilized to formulate an inversion procedure for field data processing. The field data application not only validates the theoretical results but also demonstrates advantages of shear-wave imaging.
The effect of formation anisotropy on single-well acoustic imaging was analyzed to provide a simple and effective correction method for the imaging application. We used analytical and synthetic modeling to analyze acoustic reflection moveout in a transversely isotropic (TI) formation for borehole configuration. Far-borehole reflections from a bed generally have small reflection angles to the bed. Thus the reflection moveout away from the borehole is controlled largely by the velocity along the TI-symmetry axis. Consequently, reflection imaging using measured velocity along the borehole can cause errors in the imaged reflector position. By correcting the anisotropy effect in the velocity, reflectors can be imaged correctly. A simple correction is to use the velocity along the TI-symmetry axis, which effectively maps the bed reflector to its correct position. Application of the correction method to synthetic and field-data examples demonstrate the anisotropy effect on reflector imaging and the effectiveness of the method.
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