BACKGROUND

After a borehole is drilled, it is often useful to gain information about the quality and condition of certain areas of the wellbore. One way to obtain this information is through use of the borehole imaging system. The borehole imaging system provides an output signal, which is indicative of the nature of the borehole. The surface is illuminated with acoustic pulses and the acoustic pulse return signal is used in some fashion to obtain an indication of the surface of the surrounding borehole. This procedure is normally carried out in an open-hole condition where the well is filled with drilling fluid. The wall is intended to be at a controlled and specific distance from the transducer, which transmits and then receives the acoustic pulse. For optimum resolution, the acoustic energy is focused at some specific distance from the logging tool.

It is expected that focusing the acoustic energy will provide two advantages.

First, the return signal from a focused acoustic pulse generally has a higher amplitude, which improves the signal-to-noise ratio of the measurement. Second, the focused pulse provides the measurements with increased distance sensitivity, which translates into an improved depth of field. Such sensitivity improves the system's response to surface roughness and other rugosity. Both of these anticipated advantages would contribute to improved detection of formation characteristics, boundaries between formation beds, and faults or other voids intersected by the borehole.

One way to focus the acoustic energy is to employ an annular ring transducer such as that described in U.S. Pat. 5,044,462 titled "Focused Planar Transducer" and filed July 31 , 1990 by inventor V. Maki. However, this and other existing annular ring transducer designs require deeply cut grooves for their operation. Previous fabrication methods cut grooves with a minimum depth of 80% of the piezoelectric material thickness to form annular rings at the surface. Such grooves can be difficult and expensive to cut, and may be expected to reduce yield and reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the various disclosed system and method embodiments can be obtained when the following detailed description is considered in conjunction with the drawings, in which:

Fig. 1 shows an illustrative borehole imaging system;

Fig. 2 shows one embodiment of an existing annular ring transducer;

Fig. 3 shows a cross-section of the transducer in Fig. 2;

Fig. 4 shows an illustrative focused acoustic transducer;

Fig. 5 shows a cross-section of the illustrative transducer in Fig. 4;

Fig. 6 shows an illustrative focused acoustic transducer package;

Fig. 7 shows illustrative transmitter and receiver electronics;

Fig. 8 is a flow diagram of an illustrative fabrication method; and

Fig. 9 is a graph demonstrating operation of a planar, ungrooved transducer.

DETAILED DESCRIPTION

The issues identified in the background above can be addressed, at least in part, by devices and methods employing an improved focused acoustic transducer. In at least one embodiment, a focused acoustic transducer for use in a downhole environment includes a disk of piezoelectric material with low planar coupling and low Poisson's ratio mounted on a backing material and sealed inside an enclosure. The piezoelectric material disk has a pattern of electrodes deposited on an otherwise smooth, ungrooved surface. Despite the lack of grooves, the material's low planar coupling and low Poisson's ratio enables the electrodes to operate independently and provide focused acoustic pulses similar to those created by cut or deeply grooved transducers from the prior art. Moreover, the elimination of deep cuts offers a much lower cost of construction.

In at least some embodiments, the focused acoustic transducer is created by depositing a layer of silver or other conductive material on opposite surfaces of planar pieces of piezoelectric material. The conductive layer on one side provides a ground or reference electrode and the conductive layer on the other side can be patterned into annular rings or other desired shapes. This patterning can occur during the deposition process (e.g., using a silk-screen or other printing technique) or afterwards (e.g., with an etch technique that uses a pre- or post-deposition photoresist layer). The patterns may also be cut into the electrode material using mechanical processes. Wires or conductive lines are then provided to couple each electrode to phased transmit and receive electronics that provide for the creation of a focused acoustic wave.

In at least one system embodiment, the focused acoustic transducer is part of a borehole imaging system that further includes a logging tool with a processor coupled to a telemetry system. The processor is coupled to the planar focused transducer to generate an acoustic signal by driving the pattern of electrodes in a phased manner. In addition, the processor is further configured to receive an acoustic signal by combining signals from the pattern of electrodes in a phased way. Characteristics of the received acoustic signal are measured and communicated to the surface where they can be displayed as a log or image of the borehole wall.

Turning now to the figures, Fig. 1 shows an illustrative borehole imaging system. The numeral 10 identifies an acoustic measuring device supported in a sonde 12. The sonde 12 encloses a telemetry system 14, which provides an output signal on a logging cable 16 that extends to the surface. The sonde 12 includes a rotator 18 for rotating a transducer 20 in accordance with the present disclosure. The transducer is mounted on a rotatable mechanism 22 so that the emitted acoustic pulse travels radially outwardly along a propagation line 24 and impinges on the sidewall 26 of the borehole. The sonde 12 is constructed with a housing 28, which is elongate and cylindrical. The transducer 20 is preferably submerged in the borehole fluid 30 to provide better acoustic coupling, though operation in air is possible and contemplated.

Although the well borehole 26 has been represented as a relatively smooth surface, it can be irregular depending on the nature of the drilling process and the nature of the formations penetrated by the borehole 26.

The conductor 16 extends to the surface where it passes over a sheave 38. The sheave 38 directs the logging cable 16 to a drum 40 where it is spooled for storage. The conductors in the cable 16 are connected with surface located electronics 42. In at least some embodiments, the surface electronics 42 take the form of a digital controller or a general purpose digital processing system such as a computer, and they operate on the received signals to map the measured characteristics of the acoustic signals to the corresponding position and orientation of the transducer 20 in the borehole to form a log or image of the borehole wall. The output data is displayed on a display 44. The data is recorded electronically 48, simultaneously with depth and time. The time is obtained from a real time clock 52 with millisecond resolution. The depth may be provided by an electrical or mechanical depth measuring apparatus 46 which is connected with the sheave 38 and which also connects to the recorder 48. Alternatively, position and orientation sensors can be provided in the downhole tool. Such sensors can include accelerometers, gyroscopes, magnetometers, and inertial tracking systems.

The present apparatus further includes acoustic electronics 50 which are supported in the sonde 12 and coupled to transducer 20. Though the transducer in Fig. 1 is shown rotating relative to the body of the sonde 12, other embodiments have the transducer affixed to a rotating sonde body. While Fig. 1 shows a wireline embodiment, the focused acoustic transducer can alternatively be employed in a logging-while-drilling (LWD) tool that communicates with the surface via a LWD telemetry system. As the drill string rotates and extends the borehole, the acoustic transducer scans the borehole wall in a helical pattern. Depending on the relative rates of rotation and axial motion, the acoustic imaging tool may be able to collect multiple measurements, which can be combined to make more accurate measurements for each pixel in the resulting borehole wall image or each point in the log of acoustic properties of the formation.

Fig. 2 is a diagram of an existing annular ring transducer. In this transducer embodiment, a disk of piezoelectric material 202 is cut with annular grooves 204. The piezoelectric disk member has a circular shape and the grooves have a depth of at least 80% of the transducer's thickness. As Fig. 3 shows in a cross-section of disk 202, the grooves 305 need not fully penetrate the ceramic disk. Rather, they are made deep enough to substantially isolate the acoustic and electrical excitations of one ring from the next, while leaving enough of a mechanical connection to maintain the spatial arrangement of the rings during the manufacturing process. The illustrated transducer has a circular center region surrounded by a sequence of five annular rings. The center and ring regions are each coated with an electrically conductive electrode material. Electrical attachments are made to the electrodes using solder or conductive epoxy. A ground wire is attached to the back surface before the ceramic is bonded to the backing material.

An improved focused acoustic transducer 402 is illustrated in Figs. 4 and 5. The annular spaces 404 that define the annular electrodes are created by patterning or etching the electrode material only and not by cutting deep grooves into the piezoelectric material. (In some manufacturing methods, there may be incidental (shallow) grooves produced by over-exposure to the etching solution, but such incidental grooves are not expected to exceed 10% of the thickness of the material.) The transducer relies on the low planar coupling and low Poisson's ratio of the piezoelectric material to isolate the acoustic excitations of the rings rather than deep grooves or kerfs. One suitable piezoelectric material is lead metabionate (e.g., material K-81 or K-91 sold by Piezo Technologies of Indianapolis, Indiana). Other transducer materials may be selected in accordance with good engineering practice in the design of high temperature transducer modules. The normal operating frequency can be anywhere from 50kHz to 500kHz. The thickness of the ceramic can be adjusted in some embodiments to achieve a center frequency of 350 kHz +/- 5% (e.g., roughly 0.17 inches or 0.4 cm for K-81 ). Concentric electrode surfaces can be produced by cuts through a whole-surface electrode deposited on the ceramic and possibly a small depth into the ceramic, no more than 10% into the substrate. Alternatively, the electrode surfaces can be printed or patterned as the electrode material is deposited on the surface of the ceramic disk. Each isolated electrode surface is connected to a wire leading out of the back of the transducer package. The electrode on the opposite side is the common electrode, which is also connected to a wire leading out the back of the transducer package. Contemplated electrode materials include silver, silver alloys, gold, and aluminum, though in principle any conductive material can be used to form the electrodes.

The illustrative transducer is expected to withstanding harsh, downhole environment conditions. For example, the presented transducer may experience a normal operating pressure range of up to 20,000 to 30,000 psi gauge pressure, and may be expected to survive without permanent degradation following exposures to 30,000 psi gauge pressure. Further, the expected operating temperature range of the transducer may be a range of 150° to 200° C, and no permanent degradation is expected to result from storage or operation at temperatures between -40 to 185° C. Moreover, the transducer assembly is expected to withstand vibration levels of 15 - 25G rms from 5 Hz to 500 Hz. In regards to shock, the transducer assembly may be expected to remain operable after shock levels up to 1000G's. For some tool embodiments, the ceramic has a thickness of about 0.17 inches and a diameter of about 1 .25 inches. The ceramic thickness to diameter ratio is about 0.12, though any value above 0.0625 may be regarded as acceptable.

Fig. 6 shows an illustrative embodiment of a fully packaged transducer. The illustrated transducer has a solid backing 606, which acts as a highly attenuative medium absorbing the acoustic energy which is radiated into it. The ceramic 602 and backing 606 are enclosed in a housing 612 having a small thickness separating the ceramic 606 from the borehole fluid. This material has a proper acoustic impedance, and is a well known technique for improving the transfer of acoustic energy from the ceramic which has a high impedance to the borehole fluid (e.g., water) which can have a lower impedance. In at least some embodiments, the housing 612 is made from a glass-filled PolyEther Ether Ketone (PEEK) and encapsulates the transducer. In the illustrated embodiment the backing material 606 is a tungsten-polymer mix. The tungsten polymer mix may be formed from a mixture of Viton, tungsten crystalline powder, and tungsten powder. The coupled wires 604 are routed between the electrodes. To improve pressure performance all compressible gasses may evacuated and replaced by a fluid such as oil, and a passage 608 can be provided for this purpose.

Fig. 7 shows an illustrative set of electronics for driving the focused acoustic transducer. The electronics employ the annular electrodes in a phased relationship to transmit and receive focused acoustic energy. Each of the rings (ranging from the smallest on the inside to the largest on the exterior) is used as a separate transmitting transducer. They are each connected to their own dedicated transmitter and receiver units. For example, if there are five rings in the acoustic transducer assembly (including a center electrode), then five duplicate circuits are provided. The phase delays used by the electronics determine the focal distance of the transducer, both for the transmit mode and the receive mode. The transmit focus may be controlled independently from the receive focus. The transmit pulse is delayed by the difference in travel time required for the acoustic energy to propagate from each ring to the desired focal point as the ring diameter decreases. The outer ring typically has no delay, and the inner disk has the most delay. The signal out of the transmitter circuit may be either a single pulse or a burst (typically a square wave) signal at the resonant frequency of the transducer. Again, the signal from the center disk will typically be delayed the most since it will be the closest to the focus, and the outer ring signal will be delayed the least since it is the farthest from the focus. As the focal distance increases, the total range of delays decreases. The acoustic electronics 50 include the range select logic 90 which determines the focal distance. The transmit focal distance is sent to the timing driver logic 82 which controls the signals going to each of the transmitter circuits 84. The transmit/receive switches 94 are used to protect the preamp circuits 86 from the high voltage transmit pulse.

When the transmit/receive switch is in the receive position, the receive signals pass through a delay line 88 having taps at different signal delays. (Alternatively, the signals can be digitized and the multi-tap delay line implemented digitally.) The range select logic 90 controls the tap selection and thereby controls the delays which determine the receiver focal distance. The appropriately-delayed signals from each of the electrodes are summed in the summing amplifier 98 to produce the focused signal output 102. A second output 104 is also made available which is the signal from only the center element, amplified by amplifier 100. The peak of the envelope of the signal 102 forms the amplitude signal. The time location of the onset of this signal is used to derive the travel time, indicating the range to the borehole wall. This forms the typical output signal provided to the surface through the telemetry so that the borehole imaging system presents an image of what is seen by the equipment in the borehole.

Fig. 8 shows an illustrative fabrication process for the focused acoustic transducer. In block 802, a piezoelectric material with reduced or low planar coupling is provided. One suitable piezoelectric material is lead metabionate, which has a planar coupling coefficient (k ₃ i ) of less than 0.05 and Poisson's ratio of less than 0.2. Other materials with higher planar coupling coefficient values (e.g., up to about 0.1 ) and Poisson ratios (e.g., up to about 0.25) can be used, though such materials would place greater demands on the performance of the driving electronics. The material is given a circular shape with no grooves, cuts, or kerfs. In block 804 an electrode material is deposited (e.g., silver). In block 806, the electrode material is etched into an annular ring pattern. Next, the wires are coupled to the electrodes in block 808 before the transducer is mounted on a backing material (e.g., a tungsten-polymer mix) in block 810. Finally, in block 812, the transducer and backing material are encapsulated in a sealed housing (e.g., PEEK). The encapsulation process may include the provision of pathways for pressure compensation oil to displace any compressible gasses from the housing. Teflon tape may be used to create these pathways. An epoxy having low shrinkage such as Duralco 4700 or equivalent is appropriate for encapsulation. Pressure compensation oil may be allowed to permeate the ceramic and backing before encapsulation. Preferably at least 65% of the cylindrical surface of the backing is bonded to the PEEK housing to ensure the structural integrity of the device. An alternative material for the backing could be used. For example, Viton could be replaced with an epoxy such as Duralco 4538.

As another alternative, other polymers used in the construction of the transducer could be compatible with specific environmental conditions. Duralco 4460, Duralco 4700, Duralco 4538, Duralco 120, 124 or equivalent, high temp epoxy, rated to at least 185°C can be used where appropriate. Procedures can be used to minimize the formation of voids in the epoxy and backing material. Epoxies should be fully degassed where appropriate (by stirring under vacuum) prior to their use.

As an alternative to lead metaniobate, an equivalent material with a low planar coupling and low Poisson's ratio and that can withstand very high temperatures while maintaining extremely stable piezoelectric activity can be used. For example, bismuth titanate is also suitable and may be preferred if the temperature requirements are much higher. Bizmuth titanate has a slightly higher planar coupling coefficient and Poisson's ratio, but can withstand very high temperatures while maintaining extremely stable piezoelectric activity. Other materials with high stability of dielectric constant and piezoelectric constant at various temperatures and pressures will be suitable for an equivalent.

Fig. 9 shows calculated and measured responses for a transducer designed to focus the acoustic signal at a distance 0.48 times the diameter of the transducer. The vertical axis is the signal amplitude in dB. The horizontal axis is the distance from the centerline of the transducer. The solid line represents the measured amplitude while the broken line represents the computed response. The close correspondence between the actual response and the computed response indicate that the desired performance can be achieved without cutting deep grooves into the piezoelectric material.

These and other variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.