Examples from the Niger Delta indicate that a new-generation monopole and crossed-dipole acoustic tool can overcome acquisition challenges.

Acquisition of shear and compressional sonic data can be troublesome in the Niger Delta. In general, a refracted compressional arrival representing shear is not present due to the slow velocities encountered. Hole sizes can be large, and frequently a need exists to acquire shear in large hole sizes of more than 20 in. Field experience in the Niger Delta has pointed out that oil-base or synthetic drilling fluids, employed in a majority of Nigeria wells, attenuate sonic energies in the enlarged borehole sections. A high number of boreholes are deviated, and perfect centralization of a sonic tool may be impossible to achieve. In many cases drillpipe conveyance is required - due to deviation, overpressures or hole conditions - and combinability of a robust shear sonic with other sensors is highly desirable.

A new monopole and crossed dipole acoustic logging tool (the WaveSonic tool) has been designed to overcome the acquisition challenges present in Nigeria. Big-hole performance in holes with bit size up to 171/2-in. is excellent. The robust bender bar shear source and the ability to select, while logging, the optimal frequency for flexural wave propagation helps overcome the attenuation effects of oil-base or synthetic mud. Numerous commercial logging jobs have confirmed good-quality data acquired by non-perfectly centralized tools that are a characteristic of drillpipe-conveyed logging. The isolator section of the tool (a section that is needed to eliminate low-frequency tool mode) is designed to withstand compressional forces encountered during drillpipe-conveyed logging. The tool is completely combinable with magnetic resonance and pump-through formation testing tools that may be located above or below the sonic device.

The tool acquires 96 waveforms per depth sample, and all in-line and cross-line data is recorded for anisotropy analysis. A logging speed of approximately 1,800 ft/hr (30 ft/min) can be achieved with full monopole and crossed dipole acquisition of two samples per foot. The tool can be combined with a quad-combo logging suite.

Tool design

Transmitter section. This tool utilizes three transmitters: one omni-directional monopole transmitter and two dipole transmitters. The monopole transmitter consists of a piezoelectric crystal of cylindrical geometry mounted in an arrangement that allows the transmitted acoustic energy to be almost completely uniform around the circumference of the tool. The monopole transmitter is energized in the typical "pulsed" mode, where an essentially pure monopole wave is emitted with a center frequency around 5-6 kHz and a bandwidth of 1 kHz to 12 kHz. This center frequency is approximately 2 to 3 times lower than the monopole transmitter frequency of traditional monopole, full-waveform tools. The generation of pulses with these frequency characteristics results in a much greater depth of investigation for the P wave and refracted shear wave.

The dipole source is an on-depth quad arrangement of bender bars. The orthogonal positioning of the bender bars allows for controlled X-X and Y-Y flexural wave generation. The sources are mounted in a way such that very little energy is coupled into the tool housing assembly, thus minimizing the excitation of unwanted acoustic waves in the tool itself. In addition, the source mounting ensures that there is no cross-excitation from one pair of the bender bars to the other, thus ensuring a proper acoustic dipole signature.

Tool programmability. The tool allows for full control of just about every aspect of the dipole source signature. In addition, the electronics in the tool allows for almost limitless control of the source "firing" sequence and the timing between consecutive firings.

Isolator section. This section has two very distinct and very diverse (one could consider opposite) requirements:

• It has to be extremely flexible so as to isolate acoustic waves traveling in or at the surface of the tool; and
• It has to be rather strong in order to allow tubing and/or pipe-conveyed wireline logging operations.
In addition, it has to be strong enough to support the weight of other tools attached below it in the wireline tool string. The tool can withstand 100,000 pounds of push or pull, and it provides for more than 90 dB of acoustic isolation over an extended frequency range, extending as low as about 500 Hz to 600 Hz.
Receiver array. The receiver array consists of 32 receiver crystals arranged in eight co-planar rings. Each ring has four receivers mounted perpendicular to the tool axis and evenly distributed at 90 degrees from each other. The radial positioning of the receivers is oriented with the dipole sources so that there are two in-line arrays and two cross-line arrays for both the X-X and Y-Y dipole sources.

Log examples

The first example is located in an offshore environment at a measured depth of approximately 4,800 ft (1,464 m) in a 121/4-in. hole section. The water depth is approximately 1,500 ft (525 m), and the log interval presented is around 3,300 ft (1,000 m) below the mud line. The formations shown are Pleistocene in age, and the lithology is predominately shale and siltstones.

The monopole P-wave slowness, X-X dipole slowness and Y-Y dipole slowness values are presented in track 1 (Figure 2). The monopole P wave and dipole semblance coherency data is presented in an image format in tracks 2, 3, 4, 5 and 6. In the selected depth interval, the P wave 6 slowness values range from 170 to 180 µsec/ft, and the shear wave travel times range from 600 to 700 µsec/ft. The color scheme, blue through red, indicates the quality of semblance, with red indicating the highest values.

The crossed dipole data was acquired from two logging passes using 1.2 kHz and 1.5 kHz source center frequencies. The semblance and slowness values for the X-X and Y-Y are presented in tracks 3, 4, 5 and 6. The semblance coherency values are of much higher quality for the 1.2 kHz transmitter center frequency logging pass (tracks 3 and 5) than the data from the 1.5 kHz pass (tracks 4 and 6). This data indicates that a low frequency (1.2 kHz) is required to excite and propagate flexural waves in poorly consolidated formations, and the lower frequency flexural waves are less attenuated in the enlarged borehole.

The quality control (QC) plots (Figure 3) are from the depth indicated by the arrow on Figure 2. Two passes of the X-X dipole data were acquired at 1.2 and 1.5 kHz. The slowness values from the two logging passes give exactly the same slowness value of 630 µsec/ft direction. The QC plots confirm that the 1.5 kHz center frequency data has poorer semblance coherence and its energy is highly attenuated when compared to the 1.2 kHz data. This example illustrates that there is an optimal frequency to excite and propagate flexural waves, and it varies with formation properties.

In the second example, the high quality of the data is illustrated in extremely poor borehole conditions. A 171/2-in. bit was used to drill this interval. The monopole source has a center frequency of 5.5 kHz, which is approximately four times lower in frequency than the monopole sources used in competitive wireline dipole sonic tools. By acoustic wave theory, a lower frequency (monopole signal) is less attenuated than higher frequency (monopole signal). Therefore, low frequency monopole data will provide higher-quality slowness measurements in poor borehole conditions. For a given P-wave slowness, the lower-frequency monopole signal from the tool will have a much greater depth of investigation than a higher frequency (20 kHz) monopole measurement. The P-wave slowness in this log section (Figure 4) varies between 170 and 180 µsec/ft, and the semblance coherency in track 2 confirms the high quality of the measurement.
The crossed dipole data was acquired using a 1.2 kHz center frequency. The caliper data in the depth interval of X600 to X630 indicates that the borehole is washed out to approximately 23 in.

Conclusions

Log examples have clearly shown that there is an optimal frequency for exciting flexural waves. This frequency varies with formation properties. The programmable features of this new crossed-dipole source tool allow for optimizing the flexural wave excitation (and propagation) by its capability to select the optimal dipole transmitter frequency based on formation characteristics.
The tool is fully combinable with all logging suites, thus minimizing the number of logging trips required for formation evaluation. The low-frequency monopole transmitter (in contrast to other full waveform and dipole sonic tools) provides Vp/Vs measurements within similar depths of investigation well beyond any near wellbore-altered region.

Lastly, the on-depth crossed dipole sources and transmitter firing sequence allows for all 64 dipole waveforms from the eight-level receiver array to be reliably used for anisotropy analysis without the need of depth shifting or normalization of waveform data.

Editor's note: This paper was prepared for and presented at the 6th Offshore West Africa Conference & Exhibition in Abuja, Nigeria, March 20-22, 2002, and is reprinted with permission from the authors.

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