Field tests of the APX acoustic logging-while-drilling (LWD) system demonstrate significant improvement in the signal-to-noise ratio.

Acoustic compressional slowness measurements are critical for geomechanical, petrophysical and seismic applications. Conventional wireline acoustic data is often unavailable due to cost constraints or well problems, thus increasing the need for acoustic LWD. Tool strength requirements, drilling noise and slow data transfer rate have limited the LWD acoustic tool's ability to provide wireline-equivalent data quality. A new acoustic LWD tool, Acoustic Properties eXplorer (APX), overcomes several of these limitations. APX field tests have shown a significant improvement in the quality of the data acquired in real time.
Computer modeling
Extensive computer modeling was performed to optimize configuration of the acoustic tool and the operating parameters prior to building the first prototype. Modeling provided preliminary answers to most of the tool design questions. Subsequent lab testing allowed validation of the modeling results and fine-tuning of the tool parameters based on physical measurements. A finite element analysis (FEA) model showed improved measurement accuracy in the omnidirectional source-receiver arrangement over the unidirectional case. Centered and eccentered cases were considered. FEA also helped in evaluating the effects of different window patterns in the cover sleeves for the source and receivers.
The APX tool incorporates several design concepts found in advanced acoustic wireline logging tools (Figure 1).
Omnidirectional transmitter
Unlike their wireline counterparts, LWD tools typically are not omnidirectional. The APX transmitter uses a cylindrical piezo-electrical crystal set that provides 360° coverage in the wellbore and surrounding formations. The source is capable of monopole and dipole excitation and is tunable over a 10- to 18-kHz frequency range. As such, the source provides efficient compressional wave propagation through the formations to the receiver array and reduces concerns over the position of the collar in the wellbore.
Omnidirectional receiver array
The receiver section consists of 24 sensors in a series of four orthogonal, six-receiver arrays. This omnidirectional configuration, similar to wireline systems, aligns the receiver arrays with the appropriate source element to allow for directional multipole source excitation. The matched receiver response with respect to frequency, signal amplitude and phase matching is critical in a noisy drilling environment to provide quality slowness measurements.
Extended isolator section
The acoustic signal that propagates in the tool body from the transmitter to the receiver causes degradation of the data quality. In wireline tools, the transmitter-receiver (TR) spacing typically is long to minimize the problem. In LWD tools, the section must be rigid, so the spacing typically is 4 ft (1.2 m). In the APX system, the spacing has been extended to 10 ft (3 m) as a result of greater source power and multifrequency adjustable signature quality. Numerical modeling also was used to design an isolator sub that disrupts, alters and attenuates the acoustic path through the tool body. As a result, the tool body signal has been attenuated by -40 dB (Figure 2).
Source-to-collar coupling
Lab tests determined the acoustic coupling between the source and the tool's outer housing was greater for higher frequencies (10 kHz to 20 kHz), and the worst coupling took place at 16 kHz. Notches were found at various frequencies within the range of interest. The source frequency selection to use the notches allowed for improvement in the signal-to-noise ratio.
Receiver-to-collar coupling
Tests with the acoustic tool in the listening mode (no active source firing) showed that a majority of the drilling noise from a PDC bit was concentrated in the lower frequency range (below 5 kHz). Similar to the source, the receiver has a few select bands that have a much lower tool body signal. Optimal selection of the source firing frequency helped reduce the receiver-to-collar coupling.
Accelerometers for noise detection
The system uses three accelerometers mounted to the receiver collar to detect formation signals with respect to mud flow and collar noise as well as vibrations from the drilling process. By measuring these signals separately from the acoustic signals, it was possible to reduce these effects through advanced processing and optimized filtering.
Greater dynamic range
Enhancements in the electronics include mounting the source electronics section directly adjacent to the source elements to reduce electrical noise spikes within the tool's power system. The receiver electronics also are mounted directly beneath the receiver sensors.
The receiver stage consists of four independent acquisition sections. Each data channel is digitized with added gain normalization to increase the acquired signal dynamic range. Greater dynamic range improves the ability to enhance the formation arrival and extract it from the background or drilling noise.
Field test results
The APX system was evaluated in several environments, including vertical and deviated boreholes with fast and slow formations. Figure 3 shows data collected in a vertical Midcontinent well with a PDC bit through a consolidated sand-shale sequence. The figure illustrates the high-quality waveform data with no apparent tool mode, which would appear as straight bands in the waveform.
The agreement between wireline and APX data is excellent, and the LWD quality curve generally is 0.8 and higher (highest quality value being 1.0). One notable difference is that the wireline log has higher vertical resolution than the LWD due to the smaller array aperture and higher depth domain sampling of two samples per foot. Some of the finer detail in the wireline slowness correlates with the caliper that shows severe washouts in several of the shale zones, i.e., X900. As such, there is an advantage in acquiring LWD data right behind the bit.
Real-time data quality is critical to any LWD operation. Figure 4 shows acoustic data obtained while drilling in a Gulf of Mexico well, along with gamma ray, phase and amplitude resistivity. The acoustic slowness track shows two curves with the real-time data in blue. The red post-processed data and the real-time data display good agreement over the slowness range from 110+ to 80 msec/ft. In this sand-shale section, several sand intervals display increasing resistivity and compressional slowness (DTc) at the top zone (XX700 and XX750). This indicates the possible presence of hydrocarbons.
Dipole acquisition
The APX system's flexibility allows for dipole data acquisition, which is under evaluation. Tests in fast formations have shown that the low-frequency flexural mode agrees with projected shear arrival times obtained from monopole refracted-shear measurements. A stronger Stoneley-type component than is seen with wireline dipole data also is revealed.

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