AU2012356431A1

AU2012356431A1 - Acoustic telemetry

Info

Publication number: AU2012356431A1
Application number: AU2012356431A
Authority: AU
Inventors: Philip March; Mark TOLLEY; Matthew Waters
Original assignee: Acoustic Data Ltd Australia
Current assignee: Acoustic Data Ltd Australia
Priority date: 2011-12-19
Filing date: 2012-12-18
Publication date: 2014-06-26
Also published as: GB2497744A; WO2013093445A3; GB201121772D0; WO2013093445A2

Abstract

An acoustic telemetry transmitter includes transducer means having internal capacitance coupled with charging means, and switch means arranged to be switched by signals that are dependent upon control signals derived from sensor means and to produce a step response drive signal to the transducer means. The internal capacitance of the transducer means is used as an energy store in the production of a transmission output signal from the transducer means.

Description

WO 2013/093445 PCT/GB2012/053170 ACOUSTIC TELEMETRY This invention relates to acoustic telemetry and, in particular, to an acoustic telemetry transmitter for transmitting data through tubular structures, particularly, 5 although not exclusively, through tubing in wellbores, to a remote receiver for receiving the data, and to a system incorporating the transmitter and receiver. After a well has been drilled it is highly desirable to monitor, for example, downhole temperature and formation pressure, amongst other downhole parameters. To allow better reservoir management and improved hydrocarbon recovery, a 10 premium exists in obtaining real time reservoir temperature and pressure data from downhole sensors. For many years, such data has been derived from a memory gauge which has been lowered into a well, and that remains downhole for a period of days or weeks, logging pressure and temperature data into an associated memory storage device. At 15 the end of a well test, the memory storage device is retrieved and the data that has been stored is downloaded for analysis. Although the use of a memory gauge is relatively inexpensive, it does not allow engineers to continually monitor the well inflow and general reservoir conditions whilst production continues, and the oil and gas reserves are depleted. This limitation has led to the development of techniques to 20 transmit data from sensors that are permanently located downhole to the surface in real time. One system that is used is to run a cable downhole, but this is extremely expensive in both time and materials and the operating environment is not suited to the use of communication cables that often become damaged during deployment, and 25 in the course of production activities. Another technique that is used is to use acoustic or electromagnetic telemetry and such acoustic telemetry devices are described in, for example, US-A-3252225 and US-A-3900827. Thus, US-A-3900827 discloses transmitting acoustic waves in a drill string, the waves being modulated to transmit data derived from one or more sensors exposed to conditions to be monitored. The 30 modulated waves that, in the fore-noted patent, are, preferably, torsional acoustic waves of zero order, are demodulated at a remote receiver to recover the transmitted information. EP-A-1996794 discusses the problem of Brillouin scattering of acoustic 1 WO 2013/093445 PCT/GB2012/053170 signals within tubing in a wellbore due to different impedances of the tube sections and especially the connection collars that interlink the tube sections; these collars causing undesirable reflections which act in anti-phase to the required transmitted signal. In EP-A-1996794, the problem of reflections is overcome by transmitting 5 signals in the form of tone bursts having a wavelength and temporal length of predetermined characteristics. The systems and devices hitherto have primarily focussed upon transmitting high volumes of data and are generally only suitable for deployment during relatively short time scales, e.g. days rather than months. 10 In general, the known, prior art, acoustic telemetry systems include an acoustic transducer coupled to a power source, for example a battery, or some other energy storage device, such as a capacitor, that is capable of delivering the energy required for transmission. A typical prior art acoustic transmitting station, known from US-A 15 20060077760, is shown in block schematic form in Figure 1. In Figure 1, a power voltage source 15, such as a battery, which may be a lithium thionyl chloride battery, or some other lithium composition, is connected to a microcontroller and pulse width modulation (PWM) generating circuitry 20. The PWM signal is amplified by a power amplifier 30, the output of the power amplifier 20 30 being connected to a primary coil 41 of an impedance matching step-up transformer 40. A secondary coil 42 of the transformer 40 is connected across a series connection of a tuning coil 50 and an electromechanical transducer 60, that is typically a piezoelectric ceramic transducer. The matching step-up transformer 40, is used to match the low output impedance of the power amplifier 30 to the high input 25 impedance of the transducer 60. When designing small size and low cost acoustic telemetry systems, it is extremely desirable to limit the amount of energy consumed and, hence, the size of the battery. The commercially available batteries that are capable of high temperature operation are currently limited to lithium compositions, but such battery technology is 30 not well suited to provide the high current required for the power amplifier 30. The active element of the piezoelectric transducer 60 is characterised by having a high input impedance, and to achieve the desired output power, the 2 WO 2013/093445 PCT/GB2012/053170 impedance matching transformer 40 is required to step-up the voltage from the power amplifier 30. The requirement for a matching transformer presents a further limitation in that the transformer must be capable of the same power throughput as the acoustic transducer. This produces, in essence, an energy density problem because if 5 the acoustic transducer 60 is to achieve its maximum output, as determined by the power density capability of its ceramic material, the impedance matching transformer 40 must have a similar energy density capability as that of the transducer 60. However, in practice, the energy density of a typical impedance matching transformer core is somewhat lower than the capability of the piezoelectric ceramic material of the 10 transducer 60. Thus, for maximum power output, the matching transformer 40 needs to be increased in size, which imposes a severe limitation when attempting to produce a transmitting system that is small in size and cost effective. Further, in the prior art, the power source is required to supply power to the power amplifier during transmission, thereby providing a drain on the source. 15 The present invention seeks to at least partially mitigate the foregoing disadvantages. According to a first aspect of this invention there is provided an acoustic telemetry transmitter including transducer means having internal capacitance coupled with charging means, and switch means arranged to be switched by signals that are 20 dependent upon control signals derived from sensor means and to produce a step response drive signal to the transducer means, whereby the internal capacitance of the transducer means is used as an energy store in the production of a transmission output signal from said transducer means. Advantageously the transmission output signal is a pulse position modulated 25 (PPM) signal responsive to the step response drive signal modulated by signals from said sensor means and having a data rate in the order of a few seconds per bit. Preferably, the transducer means is a piezoelectric ceramic transducer having said internal capacitance, and the step response drive output signal is produced by the switching means short circuiting the transducer. 30 Advantageously, the piezoelectric ceramic transducer is connected in series with a tuning coil so as to control the electrical resonance of said transducer. 3 WO 2013/093445 PCT/GB2012/053170 Conveniently, the switch means is a switchable electronic device, such as a semiconductor device, and, preferably, said switch means is one of a transistor, thyristor or solid-state relay. Advantageously, the output signal has a frequency in the range 750Hz 5 1500Hz, and, preferably, in the range 900Hz - 1300Hz. Preferably, the step response drive signal has a duration that is less than ten times the resonant frequency of said transducer means. Advantageously, the charging means includes a signal generator coupled with a charging circuit and connected to a power source. 10 Preferably, the charging means is connected to be controlled by output signals from one or more sensor means and said charging means is arranged to control opening and closing of said switch means. Conveniently, the charging circuit is one of a diode capacitor charge pump, a switch mode converter and a piezoelectric transformer. 15 Advantageously, the sensor means is arranged to measure one of temperature and pressure. According to a second aspect of this invention there is provided a receiver including a decoder for evaluating the output signal transmitted by the acoustic telemetry transmitter of the first aspect. 20 According to a third aspect of this invention there is provided a telemetry system including a transmitter in accordance with the first aspect of this invention, said transmitter being connected to at least one sensor means arranged to operate said switch means, whereby, in operation, the transducer means provides a pulse position modulated (PPM) signal to a receiver in accordance with the second aspect of this 25 invention, said transmitter and receiver being arranged remotely from one another along a tubular structure. Preferably, the tubular structure comprises a plurality of sections of tubing, each second being interconnected by coupling means. The invention will now be described, by way of example, with reference to the 30 accompanying drawings, in which Figure 1 shows in block schematic form a prior art acoustic telemetry transmitting station, 4 WO 2013/093445 PCT/GB2012/053170 Figure 2 shows in block schematic form an acoustic telemetry transmitter in accordance with this invention, and Figure 3 shows in schematic an acoustic telemetry system in a downhole structure in which the transmitter in accordance with this invention may be used, 5 In the Figures like reference numerals denote like parts. Referring to Figure 2, the power, voltage, source 15, shown as a battery 16 is connected to a microcontroller and signal generator circuit 21 being arranged to generate a square wave drive signal 23 and a switch signal 24. The signal 23 is connected to drive a charging circuit 25 and the switch signal 24 controls a 10 selectably closable transmission switch 80 that is arranged to be opened and closed by data signals from one or more sensors measuring, for example, temperature and/or pressure and which sensors control circuit 21. The switch 80 may be any suitable switching device such as a transistor, thyristor or solid state relay. The switch 80 is connected across the tuning coil 50 and transducer 60. 15 In this invention, the internal capacitance of the piezoelectric ceramic transducer is charged to a required level by the charging circuit 25 which may be formed from a diode capacitor charge pump, a switch mode converter, or a piezoelectric transformer, all of which are known per se. When the terminals of the transducer 60 are short circuited by the switch 80 20 closing, the transducer vibrates at its primary resonant frequency with a certain energy that dissipates in other parasitic resonances. Thus, the drive signal to the transducer is a step response and once the internal capacitance of the transducer 60 is charged and then short circuited, a step voltage change occurs. In the present invention, the internal capacitance of the transducer is used as the sole energy store in the production 25 of a transmission signal. Also, because the transducer internal capacitance is used as the energy store, during transmission no drain on the power source 15 occurs. The transducer is arranged to supply a signal having any desired wave mode, preferably the LO,1 mode, in a frequency range 750Hz - 1500Hz and, preferably, 900Hz 1300Hz. The step response signal typically is arranged to provide a duration less than 30 ten times the resonant frequency of the transducer. 5 WO 2013/093445 PCT/GB2012/053170 The signal transmitted along the tubular structure, formed by tube sections 3, and collars 4 to which the transducer is connected, is remotely decoded by the receiver 14 for evaluation. Without the tuning coil 50, the precise control of the output frequency of the 5 transducer 60 is difficult to achieve. Thus, the accuracy of the output frequency is improved by the provision of the tuning coil 50 that is arranged to control the electrical resonance of the transducer. Thus, the output signal of the transducer is then a function of both the electrical and mechanical resonance of the transducer that is within the fore-noted desired frequency range. 10 Hitherto, most communication systems have relied upon transmitting signals having significant control over the transmission frequency, length and amplitude and/or phase. The control of such systems is performed by the power amplifier and associated output circuitry. However, the effects of Brillouin scattering make many of these traditional forms of signal transmission and data encoding techniques 15 difficult, if not impossible, to achieve in practice. The present invention is principally designed to provide only a few data signal measurements per day, for example representative of temperature and pressure, but is intended to operate for a period of months rather than the period of days or weeks of the prior art. Further, the data rate of this invention is in the order of a few seconds. 20 Because the present invention is arranged to send modest amounts of data with high energy efficiency, it is unnecessary to maintain the level of control over the output of the transducer 60 that was previously necessary. In this respect, when the transmission switch 80 is closed, the transducer 60 naturally responds at its primary resonant frequency. This output eventually 25 decreases at a rate that depends on the natural Q factor of the transducer. The Q factor is defined by the mechanical parameters (motion or mass and stiffness) of the transducer 60 and its associated losses (losses due to heat and radiated acoustic energy). The output is analogous to the step response of a simple tuned circuit. In this invention there is no power amplifier as such to control the transducer to respond 30 at the amplifier's output frequency and/or power level. A series of signals are produced to form a string of signal transmissions from the transducer. In this form, 6 WO 2013/093445 PCT/GB2012/053170 the timing between the signal transmissions forms a Pulse Position Modulation (PPM) signal. It is believed any one of many different wave propagation modes may be used but the L(O,1), or first longitudinal, mode is currently preferred. 5 This invention may be used in a system described in relation to Figure 3. Figure 3 shows an acoustic telemetry system in a downhole structure. The structure is formed by a wellbore casing 1 located within a formation 2. Located axially within the casing 1 is a string of wellbore production tube sections 3, typically having a typical length of about 9 metres, each connected by a connection collar 4. 10 Mounted between one of the tube sections is a mounting device 10 for an acoustic telemetry transmitter 8 in accordance with this invention, that is arranged to receive signals from a pressure sensor (not shown) and a temperature sensor (not shown) that may be located within an enclosure of the acoustic telemetry transmitter 8. In many applications, alternatively, mounted inside one of the tube sections is a mounting 15 device 5 for an acoustic telemetry transmitter 6 in accordance with this invention, that is arranged to receive signals from a pressure sensor (not shown) and a temperature sensor (not shown) that may be located within an enclosure of the acoustic telemetry transmitter 6. Located at the surface is a well head 12, connected to which is a receiving station having a sensor formed by an accelerometer 13 and a receiver 14 20 that includes a decoder. In use, signals from the acoustic telemetry transmitter 8, or alternatively transmitter 6, are acoustically transmitted through the tube sections 3 to be decoded by the receiver 14. The present invention thus provides a simplified, low cost acoustic telemetry 25 transmission system. 7

Claims

1. An acoustic telemetry transmitter including transducer means having internal capacitance coupled with charging means, and switch means arranged to be switched 5 by signals that are dependent upon control signals derived from sensor means and to produce a step response drive signal to the transducer means, whereby the internal capacitance of the transducer means is used as an energy store in the production of a transmission output signal from said transducer means. 10

2. A transmitter as claimed in claim 1, wherein the transmission output signal is a pulse position modulated (PPM) signal responsive to the step response drive signal modulated by signals from said sensor means and having a data rate in the order of a few seconds per bit. 15

3. A transmitter as claimed in claim 1 or 2, wherein the transducer means is a piezoelectric ceramic transducer having said internal capacitance, and the step response drive signal is produced by the switching means short circuiting the transducer. 20

4. A transmitter as claimed in claim 3, wherein the piezoelectric ceramic transducer is connected in series with a tuning coil so as to control the electrical resonance of said transducer.

5. A transmitter as claimed in any preceding claim, wherein the switch means is 25 a switchable electronic device.

6. A transmitter as claimed in claim 5, wherein said switch means is one of a transistor, a thyristor or a solid-state relay. 30

7. A transmitter as claimed in any preceding claim, wherein the output signal has a frequency in the range 750Hz - 1500Hz. 8 WO 2013/093445 PCT/GB2012/053170

8. A transmitter as claimed in any preceding claim, wherein the output signal has a frequency in the range 900Hz - 1300Hz.

9. A transmitter as claimed in any preceding claim, wherein the step response 5 drive signal has a duration that is less than ten times the resonant frequency of said transducer means.

10. A transmitter as claimed in any preceding claim, wherein the charging means includes a signal generator coupled with a charging circuit and connected to a power 10 source.

11. A transmitter as claimed in any preceding claim, wherein the charging means is connected to be controlled by output signals from one or more sensor means and said charging means is arranged to control opening and closing of said switch means. 15

12. A transmitter as claimed in claim 11, wherein the charging circuit is one of a diode capacitor charge pump, a switch mode converter and a piezoelectric transformer. 20

13. A transmitter as claimed in any preceding claim, wherein the sensor means is arranged to measure one of temperature and pressure.

14. A receiver including a decoder for evaluating the output signal transmitted by the acoustic telemetry transmitter as claimed in any preceding claim. 25

15. A telemetry system including a transmitter as claimed in any of claims 1 to 13, wherein said transmitter is connected to at least one sensor means arranged to operate said switch means, whereby, in operation, the transducer means provides a pulse position modulated (PPM) signal to a receiver, as claimed in claim 14, said 30 transmitter and receiver being arranged remotely from one another along a tubular structure. 9 WO 2013/093445 PCT/GB2012/053170

16. A system as claimed in claim 15, wherein the tubular structure comprises a plurality of sections of tubing, each section being interconnected by coupling means.

17. An acoustic telemetry transmitter substantially as herein described with 5 reference to and as shown in Figure 2 of the accompanying drawings.

18. A telemetry system substantially as herein described with reference to and as shown in Figures 2 and 3 of the accompanying drawings. 10 15 20 25 30 35 40 10