Patent Description:
Communication systems, and in particular telemetry systems may be deployed in oil or gas wells to measure and/or transmit or relay data for purposes of, for example, production monitoring, well integrity monitoring or communicating signals during a particular well operation (e.g. drill stem testing). Data transmitted by such telemetry systems may be observed at surface with appropriate receiver systems.

In some wells, the well structure itself or the environmental conditions may not be conducive to communication using conventional wired telemetry systems. For example, certain wells may comprise various tools and valves that are ill-suited to the presence of wired telemetry systems. Further, wired solutions typically require some form of planning for deployment at the time of completing the well and so retrofitting such solutions may be more complex than for example, a wireless hybrid solution.

As such, downhole telemetry systems may employ wireless data communication techniques as a means to transmit and receive data without incurring the costs, overheads and inconveniences associated with conventional wired systems.

Wireless downhole telemetry systems may employ techniques such as EM/RF communication, pressure/flow rate modulation of mud or the like, and/or acoustic transmission. In some examples, wireless communication may use multiple wireless techniques, and further sections of the signal path may be wired.

In some examples, it may be possible to employ acoustic telemetry in oil and gas wells using either the structure of the well itself or available production tubing or other continuous tubing in the well to transmit/receive signals. Limitations of acoustic telemetry include the need for regular repeaters due to attenuation, and limitations to data transfer rates and the reliability of received data.

<CIT> describes an electromagnetic (EM) telemetry system with telluric referencing for use with downhole equipment. The system includes a downhole transceiver comprising an encoded signal transmitter; a downhole sensor disposed to monitor the downhole equipment, the downhole sensor coupled to the transceiver; an encoded signal receiver; a reference receiver spaced apart from the encoded signal receiver and communicatively coupled to the encoded signal receiver; and a telluric voltage module coupled to one of the encoded signal receiver and the reference receiver.

<CIT> describes downhole telemetry systems and methods which employ time-reversal pre-equalization. The system includes an acoustic transducer that transmits an acoustic signal to a distant receiver via a string of drillpipes connected by tool joints, and a digital signal processor that drives the acoustic transducer with an electrical signal that represents modulated digital data convolved with a time-reversed channel response.

<CIT> describes an apparatus for transmitting EM telemetry data from a downhole location to surface comprising: an EM signal transmitter configured to generate positive and negative polarity EM pulses corresponding to a telemetry signal; and an electronics subassembly communicative with the EM signal transmitter and comprising a processor and a memory having encoded thereon program code executable by the processor to perform a method comprising encoding measurement data into an EM telemetry signal using a modulation scheme comprising mapping a symbol set to at least one positive polarity EM pulse and one negative polarity EM pulse; and sending a control signal to the EM signal transmitter to generate EM pulses corresponding to the EM telemetry signal.

<CIT> describes a method comprising identifying each carrier frequency signal or channel, not only by its base or reference frequency, but also by a rate of change of the carrier frequencies, such that for each carrier frequency assignment, an additional number of transmission channels, each having a different linear rate of change of frequency starting simultaneously from the reference carrier frequency, is provided where the instantaneous carrier frequency of these channels changes continuously over the frequency band, but each with a different incremental rate of change.

<CIT> describes a fiber optic sensor system that includes a coherent-detection optical time domain reflectometry system to extract phase information from optical signals returned from a fiber optic sensor arrangement in response to a plurality of interrogating pulses, and a frequency-shifting circuit to repeatedly translate the frequency of an optical pulse generated by a narrowband source to generate a train of interrogating pulses of multiple frequencies.

This background serves only to set a scene to allow a skilled reader to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the invention may or may not address one or more of the background issues.

Aspects of the present disclosure are defined in the independent claims provided herewith. Some optional features are described in the dependent claims.

According to a first aspect of the present disclosure is a transmitter according to claim <NUM>.

The transmitter may comprise an acoustic transducer configurable to provide an acoustic signal based on, or derived from, the output signal. The transducer may be adapted to be coupled to a well structure and/or the transducer is operable to propagate a signal via a well structure.

The transmitter may comprise an electromagnetic (e.g. RF) transducer configurable to provide an electromagnetic signal based on, or derived from, the output signal. The transmitter may comprise a plurality of mixers configured to generate the output signal based on one or more frequency modulated signals.

The signal generation apparatus may be configured to provide a sweep signal, such as a glide sweep or stepped sweep, to the mixer. The sweep signal may comprise a linear or exponential up-chirp and/or down-chirp signal. The signal generation apparatus may comprise a tuneable oscillator, which may be configured to provide a signal to the mixer. The signal generation apparatus may comprise a signal generator, which may be configured to provide a signal to the tuneable oscillator. The tuneable oscillator may be a Voltage Controlled Oscillator.

The mixer may be a double balanced mixer.

The modulator may be configurable to provide on-off keying. The modulator may be configurable to provide phase shift keying and/or binary phase shift keying and/or frequency shift keying and/or amplitude shift keying, which forms of modulation may be applied to the fixed-frequency oscillator.

The transmitter may comprise a driver circuit. The driver circuit may be configured to provide one or more signals based on the output signal to the transducer. An output of the driver may comprise a differential signal. An output of the driver may comprise a pulse-width modulation (PWM) signal.

The modulator may be operable to modulate at least one of: a signal provided by the signal generator; a supply or signal provided to the driver; a supply or signal provided to the fixed-frequency oscillator; a supply or enabling signal provided to the mixer; and a supply or signal provided to the transducer.

The transmitter may be configured to transmit data signals having a frequency of in the region of from <NUM> to <NUM>.

According to a second aspect of the present disclosure is a repeater for use in downhole telemetry and/or control. The repeater comprises the transmitter of the first aspect. The repeater further comprises a receiver. The repeater may be configured to transmit a received signal. The repeater may be adapted to transmit an electromagnetic signal and/or an acoustic signal. The repeater may be adapted to receive an acoustic signal and/or an electromagnetic signal.

According to a third aspect of the present disclosure is a telemetry system for use in wirelessly transmitting data downhole. The telemetry system comprises a transmitter of the first aspect. The transmitter may be configured to wirelessly transmit data signals, e.g. for propagation via a well structure.

The system may comprise a receiver configurable to receive the data signals. The system may comprise at least one repeater disposed on or within the well structure. The at least one repeater may be communicably coupled to the transmitter and/or a further repeater and/or the receiver. The repeater and/or the further repeater may be a repeater according to the second aspect. The telemetry system may comprise at least a portion of the well structure, wherein the well structure may be a metallic structure.

According to a fourth aspect of the present disclosure is a production well or an abandoned well comprising the telemetry system of the third aspect.

According to a fifth aspect of the present disclosure is a method according to claim <NUM>. The method may further comprise the step of providing the output signal to an acoustic transducer, the acoustic transducer being coupled to a well structure.

According to a sixth aspect of the present disclosure is a method for monitoring a well. The method comprises collecting data associated with the well, the collected data being derived from data signals having been transmitted via a metallic well structure of the well using a transmitter of the first aspect.

In some examples, there is described a computer program product that when programmed into a suitable controller configures the controller to perform any methods disclosed herein. There may be provided a carrier medium, such as a physical or tangible and/or non-transient carrier medium, comprising the computer program product. The carrier medium may be a computer readable carrier medium.

It will be appreciated that one or more embodiments/aspects may be effective in providing downhole communication, and in particular acoustic communications (e.g. for the purpose of telemetry methods and systems control).

The above summary is intended to be merely exemplary and non-limiting.

A description is now given, by way of example only, with reference to the accompanying drawings, in which:-.

For ease of explanation, the following examples have been described in relation to an oil and gas well, and in particular a well structure extending below the surface, or the like. However, systems and methods described herein may be equally used and applicable in respect of flow lines associated with oil and gas production, or indeed injection wells, etc. As such, while the following examples may be described in relation to oil and gas wells, and in particular production and appraisal wells, the same systems and methods, etc., may be used beyond oil and gas applications. A skilled artificer will be able to implement those various alternative embodiments accordingly.

Generally, disclosed herein are apparatuses, systems and methods for communicating data signals from downhole to at least one receiver at a ground region near the well, or vice versa. In particular, apparatuses, methods and systems disclosed are arranged to communicate data signals from a well having an essentially continuous well structure or well structure signal path (e.g. either the casing of the well, or tubing components positioned within the well), wherein the well structure may be used as a medium to propagate the data signals from downhole to the receivers at the surface, or vice versa. It is noted that the well structure need only be suitable for propagating signals, such as acoustic and/or electromagnetic signals.

Referring now to <FIG>, there is shown a block diagram of a transmitter, generally denoted <NUM>, according to an example. In this example, the transmitter <NUM> comprises a mixer <NUM>, a modulator <NUM> for providing on-off keying (OOK) and a signal generation apparatus <NUM>.

An oscillator, such as a fixed frequency oscillator <NUM> may be coupled to the mixer <NUM>. That is, the fixed frequency oscillator <NUM> provides a signal <NUM> to the mixer <NUM>, the signal comprising a frequency component at a fixed frequency. An example of the signal <NUM> provided by the fixed frequency oscillator <NUM> is shown in <FIG>.

The mixer <NUM> is balanced with respect to both its inputs (i.e. double-balanced). The mixer <NUM> is configured to generate an output signal <NUM> derived from a frequency modulated signal <NUM>. A frequency spectrum of the output signal <NUM> of the mixer <NUM> comprises upper and lower sidebands disposed about a suppressed or reduced frequency modulated signal <NUM>, as will be described in more detail with reference to <FIG>. In particular, the output from the mixer <NUM> is a dual-tone chirp signal <NUM>, an example of which is shown in <FIG>.

The mixer <NUM> in this example is a double balanced mixer. As such, the mixer <NUM> suppresses or reduces frequency components on the output signal <NUM> of the mixer <NUM> of both input signals <NUM>, <NUM> to the mixer <NUM>.

Advantageously, by suppressing the frequency components of both input signals <NUM>, <NUM> to the mixer <NUM> on the output signal <NUM> from the mixer <NUM>, the power of the output signal <NUM> may be completely, or predominantly, confined to sidebands i.e. frequencies of interest, and not in a carrier signal, i.e. not at a frequency of the fixed frequency oscillator <NUM> or the signal generation apparatus <NUM>.

The signal generation apparatus <NUM> comprises a signal generator <NUM>. In the example embodiment shown, the signal generator <NUM> is configured to generate a saw-tooth waveform signal <NUM>. It will be appreciated that the signal generator <NUM> may be capable of generating other waveforms. The signal generator <NUM> may be a general purpose signal generator or an otherwise configurable signal or function generator, or may be a dedicated circuit for specifically generating the saw-tooth waveform signal <NUM> of interest.

As can be seen from <FIG>, the saw-tooth waveform signal <NUM> generated by the signal generator <NUM> comprises an upward saw-tooth waveform i.e. the signal periodically ramps upwards from a low-voltage to a high-voltage before sharply dropping to the low-voltage. In other embodiments falling within the scope of the invention, the saw-tooth waveform signal <NUM> may be a downward saw-tooth waveform signal (e.g. a signal comprising an inverse saw-tooth waveform).

In this example, the signal generation apparatus <NUM> comprises a tuneable oscillator <NUM>. Here, the signal generator <NUM> and the tuneable oscillator <NUM> are operable to generate the input signal <NUM> to the mixer <NUM>, as described below.

The tuneable oscillator <NUM> is a voltage controlled oscillator. The saw-tooth waveform signal <NUM> is input to the tuneable oscillator <NUM>. An output <NUM> from the tuneable oscillator <NUM>, i.e. the frequency modulated signal <NUM>, is an input to the mixer <NUM>. The frequency modulated signal <NUM> has a frequency component that is proportional to a voltage of the saw-tooth waveform signal <NUM>. As such, the tuneable oscillator <NUM> is configured to generate a signal <NUM> with a frequency component that periodically linearly increases in frequency from a low frequency to a high frequency. That is, the tuneable oscillator <NUM> is configured to generate an upwards chirp signal <NUM>.

In the example embodiment described above, an upward chirp signal <NUM>, and in particular a linear upward chirp signal, is generated. It will be appreciated that a downward saw-tooth waveform signal generated by the signal generator <NUM> would generate a downward chirp signal. An example of a downward chirp signal <NUM> that could be provided by the tuneable oscillator <NUM> is shown in <FIG>. Furthermore, other chirp signals, such as exponential chirp signals may be generated by means of configuring the signal generator <NUM> to generate alternative waveforms such as, for example, exponential saw-tooth waveform signals. In yet further examples, a function generator may directly provide a chirp signal to the mixer <NUM>, i.e. without need for a discrete VCO.

The output signal <NUM> from the mixer <NUM> is an input to a driver <NUM>. The driver <NUM> provides an output signal <NUM> to a transducer <NUM>. The driver <NUM> may be capable of providing a higher current output than the mixer <NUM>. The driver <NUM> may boost the voltage of the signal from the mixer <NUM>. In the example embodiment shown, the driver <NUM> is a differential driver, e.g. the driver <NUM> provides a differential signal <NUM>. However, it will be appreciated that the type and rating of the driver <NUM> may be adapted or selected to suit the transducer <NUM> to which the driver is providing a signal <NUM>, and/or to suit the input signal <NUM> (voltage and/or drive strength) provided to it from the mixer <NUM>. For example, in other examples, the driver <NUM> may provide a single output (i.e. non-differential output). Similarly, the driver <NUM> may be or may comprise a solid-state driver and/or a pulse-width modulation (PWM) driver, and/or discrete components and circuitry. In examples, the driver <NUM> may comprise a plurality of drivers, arranged as a multi-stage driver circuit. In an example embodiment, the portion of circuitry of the transmitter comprising the mixer <NUM> may be rated at, for example, +/- 5V and < 1A, whereas signals <NUM> provided to the transducer <NUM> may be up to 1kV, or currents exceeding 10A. Furthermore, drive circuitry, such as some or all of driver <NUM> may be a component of, or generally formed as part of, the transducer <NUM>.

Here, the transducer <NUM> is an acoustic transducer, which in <FIG> is shown coupled to the well structure and labelled as DL acoustic channel, i.e. Down Link. It will however be appreciated that the transducer may also serve in an UL acoustic channel, i.e. Up Link. In a preferred embodiment, the transducer <NUM> is a piezoelectric transducer, such as a PZT transducer or the like. In use, the transducer <NUM> may be coupled to a component of a downhole well, such as a drill string, a tool, casing, or other tubular or liner, as will be described below.

The transmitter <NUM> comprises a modulator <NUM>. In the example embodiment shown the modulator <NUM> provides a modulation signal <NUM> to the driver <NUM>. The modulation signal <NUM> modulates data to be transmitted by the transmitter <NUM> onto a signal <NUM> output by the driver <NUM>. In one embodiment, the modulator <NUM> is configurable to provide on-off keying and/or amplitude shift keying (ASK) based on the data. However, in other examples, the modulator may be configurable to provide phase shift keying and/or binary phase shift keying and/or frequency shift keying to the "fixed-frequency" oscillator <NUM>.

The modulator <NUM> may be implemented at various stages of the circuitry of the transmitter <NUM>. For example, turning now to <FIG>, there is shown a transmitter <NUM> which corresponds to an alternative example. The transmitter <NUM> comprises generally the same features as the transmitter <NUM> of <FIG>, and is annotated by references incremented by <NUM> relative to the references of <FIG>, i.e. mixer <NUM>, tuneable oscillator <NUM>, signal generator <NUM>, fixed frequency oscillator <NUM>, modulator <NUM> and transducer <NUM>.

As shown in <FIG>, modulation is applied by the modulator <NUM> directly to the mixer <NUM>. That is, the modulator <NUM> may, for example, modulate a power supply (not shown) to the mixer <NUM>, or to a portion of circuitry comprising the mixer <NUM>. Alternatively the modulator <NUM> may modulate signals <NUM>, <NUM>, <NUM> provided to or from the mixer <NUM>. Alternatively the modulator <NUM> may modulate power supplies to, or signals to or from, one or more components of the transmitter <NUM>, for example the signal generator <NUM>, the driver <NUM> or the transducer <NUM>.

Referring now to <FIG>, there is shown a frequency spectrum of an example use case, and in particular a use case downhole. Specific frequencies shown are for example purposes only, and it will be understood that in practical implementation of this example, signals comprising other frequencies may be used. In particular, although frequencies shown in <FIG> are in the range of <NUM> to <NUM>, lower frequencies such as <NUM>, and higher frequencies, such as <NUM> or ultrasonic frequencies, may be used, depending on specific application - as will be appreciated.

In the example shown in <FIG>, the frequency of the signal, i.e. signal <NUM>, <NUM>, provided by the signal generation apparatus <NUM>, <NUM> ranges from <NUM> to <NUM>. The signal when at <NUM> is denoted fin1MIN and the signal when at <NUM> is denoted fin1MAX.

Furthermore, in the example shown in <FIG>, the fixed frequency signal, e.g. the signal <NUM>, <NUM> provided by the fixed frequency oscillator <NUM>, <NUM> as shown in <FIG>, is <NUM>, and is denoted fin2.

In one embodiment, the mixer <NUM>, <NUM> is a double balanced mixer. As such, in the output signal <NUM>, <NUM> from the mixer <NUM>, <NUM>, frequency components corresponding to the fixed frequency signal <NUM>, <NUM> and the signal <NUM>, <NUM> provided by the signal generation apparatus are suppressed or reduced.

The mixer provides an output signal <NUM>, <NUM> comprising a lower frequency component f<NUM> with a frequency corresponding to the difference between the frequency components of the frequencies of the input signals <NUM>, <NUM>, <NUM>, <NUM>. That is, the output signal from the mixer comprises a lower frequency component with a frequency that ranges from fin1MIN-fin2 to fin1MAX-fin2. As shown in <FIG>, this lower frequency component f<NUM> ranges from F1LO to F1HI and is denoted "SIDEBAND_LOW".

The output signal <NUM>, <NUM> of the mixer also comprises an upper frequency component f<NUM> with a frequency corresponding to the sum of the frequency components of the frequencies of the input signals <NUM>, <NUM>, <NUM>, <NUM>. That is, the output signal from the mixer comprises frequency components with a frequency that ranges from fin1MIN+fin2 to fin1MAX+fin2. As shown in <FIG>, this upper frequency component f<NUM> ranges from f2LO to f2HI and is denoted "SIDEBAND_HIGH".

In an example embodiment, the fixed frequency oscillator <NUM>, <NUM> may provide a <NUM> signal <NUM>, <NUM> to the mixer <NUM>, <NUM>. The signal generation apparatus <NUM>, <NUM> may provide a linear upwards chirp signal <NUM>, <NUM> to the mixer <NUM>, <NUM>, wherein the frequency of the signal <NUM>, <NUM> periodically linearly increases from <NUM> to <NUM>. As such, an output signal <NUM>, <NUM> from the mixer <NUM>, <NUM> comprises a lower frequency component that ranges from <NUM> to <NUM>, termed the SIDEBAND_LOW, and a higher frequency component that ranges from <NUM> to <NUM>, termed SIDEBAND_HIGH. As previously described, in other embodiments alternative frequencies and waveforms may be provided to the mixer <NUM>, <NUM>.

Examples of the waveforms described herein are shown in <FIG>, which shows the fixed frequency waveform <NUM>, <NUM> output by the fixed frequency oscillator <NUM>, <NUM>, the sawtooth driven chirp waveform <NUM> (in this case a descending chirp waveform) output from the tuneable oscillator <NUM>, <NUM> (the shown portion being at the transition between the end of one chirp and the start of the next chirp), and the dual-tone spread spectrum chirp <NUM>, <NUM> output by the mixer <NUM>, <NUM>.

Referring now to <FIG>, there is shown an example of data modulated onto an output signal <NUM> output by the driver <NUM>. The signal <NUM>, <NUM> in this example is an upwards chirp signal generated as described with reference to <FIG>. The signal <NUM>, <NUM> comprises upper and lower main frequency components, generated as described with reference to <FIG>. In the example shown in <FIG>, On-Off Keying (OOK) is used to modulate a binary string of data, shown as "<NUM>", onto the signal <NUM>, <NUM>.

The upwards chirp signal of <FIG> comprises two main frequency components, f<NUM> and f<NUM>, which corresponds to SIDEBAND_LOW and SIDEBAND_HIGH respectively. SIDEBAND_LOW increases in frequency linearly from f1LO to f1HI. Similarly, SIDEBAND_HIGH increases in frequency linearly from f2LO to f2HI. As such, each chirp comprises two frequency components, i.e. 'a dual-tone' chirp.

A binary "<NUM>" is indicated by the presence of the signals, i.e. at least part of the upwards chirp signals as described above, and as shown <FIG>. A binary "<NUM>" is provided by the absence of signals. This exemplifies On-Off keying. In this manner, a binary string or sequence may be transmitted by the transmitter <NUM>, <NUM>.

Advantageously, the provision of dual-tone chirp signal provides a robust and reliable data transmission means for downhole communication. In particular, detrimental characteristics that may otherwise be associated with Chirp Spread Spectrum (CSS) communication, such as channel fades, multipath propagation, periodic transmission passbands, jamming signals and the like may be overcome, or sufficiently mitigated by using dual-tone chirp signals in accordance with the disclosed invention.

<FIG> show examples of use cases of the devices and methods. <FIG> shows a telemetry system deployed in a well, and generally denoted <NUM>. The system <NUM> comprises a transmitter 410a according to any of the abovementioned examples. The transmitter 410a is coupled to a tubular <NUM> or otherwise form of well structure.

The transmitter is also coupled to a measurement or downhole tool <NUM>, for measuring or logging data for transmission by the transmitter 410a. It will be appreciated that the examples provides are for illustration, and that the transmitter may be coupled to or form part of a communication system provided with a tools string, or the like, and equally with specific well structure like casing, or indeed be provided to communicate along a signal path comprising tubing, well structure, gauges, etc..

In the embodiment shown in <FIG>, the tubular <NUM> is the casing string, which is cemented into the well <NUM> using cement <NUM>. The reader will appreciate that the transmitter 410a for use in downhole telemetry and/or control may be affixed to other tubulars or tools, such as production tubing, or other string in the well, e.g. drill string, or the like.

The transmitter 410a comprises a transducer (not shown), as exemplified in <FIG>. The transducer of the transmitter 410a is coupled to the casing <NUM>, and is thus operable to transmit acoustic signals via the casing <NUM>.

Due to attenuation of the acoustic signal as it propagates along the casing <NUM> towards surface <NUM>, it may in some situations be necessary to employ one or more repeaters <NUM>. In one embodiment, such a repeater <NUM> comprises a receiver <NUM> adapted to receive the acoustic signal transmitted by the transmitter 410a. Furthermore, such a repeater <NUM> also comprises a transmitter 410b, wherein the transmitter is a transmitter according to any of the abovementioned embodiments of the invention.

In the example shown in <FIG>, a single repeater <NUM> is employed. It will be appreciated that, dependent upon one or more factors which may include the distance between transmitter 410a and receiver <NUM> at surface <NUM>, and the nature of any medium within the casing and/or the nature of a surrounding formation, more than one repeater <NUM> may be required. Furthermore, the number of repeaters <NUM> required may depend upon a frequency and amplitude of a transmitted signal, and a general attenuation of the acoustic signal within the well.

Advantageously, by transmitting data using dual-tone chirp spread spectrum communication, according to the disclosed examples, the signal may be more reliably received, and may be less prone to the effects of channel fades, multipath propagation, periodic transmission passbands, jamming signals and the like. As such, dual-tone chirp spread spectrum communications may permit the use of fewer repeaters than may otherwise be required when using conventional downhole acoustic telemetry systems.

<FIG> is a simple schematic diagram of a further example of a telemetry system deployed in a well, and is generally denoted <NUM>. The system <NUM> comprises a transmitter 510a according to any of the abovementioned examples. The transmitter 510a is coupled to a tubular <NUM>. However, in contrast to <FIG>, the transmitter 510a is deployed in the well using a tool, and may therefore be moved to different positions within the well. That is, the transmitter 510a is not installed on a wall of the casing <NUM> or other tubular. The transmitter 510a (or at least the transducer of the transmitter 510a) may be brought into contact with the casing <NUM> using, for example, arms of a centralizer <NUM>. As such, the transmitter 510a is operable to transmit acoustic signals, such as dual-tone chirp signals according to the present examples, via the structure of the casing <NUM>. Different types of tool or centralizer <NUM>, such as a bow spring, rigid, semi-rigid or mould-on centralizer, could be used, and each may have differing signal transfer properties. However, a skilled worker in the field could, based on the present disclosure, select a suitable tool, e.g. centralizer, for the specific situation in which the system is to be used to achieve acceptable signal transfer.

Again, for purposes of example only, a single repeater <NUM> is employed to relay the transmitted signal to a receiver <NUM> at surface <NUM>.

The repeater <NUM> comprises a receiver <NUM> adapted to receive the acoustic signal transmitted by the transmitter 510a. Furthermore, the repeater <NUM> also provides a means for the transmitter 510b to couple to the metallic structure of the casing <NUM>, such as via arms of a centraliser 580b, thus enabling the receiver <NUM> to receive signals and the transmitter 510b to transmit signals via the casing <NUM>.

Turning now to <FIG>, there is shown an example of a control system deployed in a multilateral producing well, generally denoted <NUM>. The well comprises two branches 630a, 630b, extending from the primary well 630c. Each of the branches 630a, 630b and the main well 630c comprises a corresponding receiver 670a, 670b, 670c. Each receiver 670a, 670b, 670c is coupled to a corresponding tool 690a, 690b, 690c. As such, data received by each receiver 670a, 670b, 670c may be used to actuate the corresponding tool 690a, 690b, 690c.

A transmitter <NUM> according to any of the abovementioned examples is located at surface <NUM>. The transmitter <NUM> is coupled to the structure of the casing <NUM> of the well. As such, the transmitter <NUM> is operable to transmit data to the receiver 670a, 670b, 670c via the well structure, using dual-tone chirp signals as described above. As described with reference to <FIG>, one or more repeaters (not shown) may also be employed to compensate for attenuation of the transmitted signal as it propagates down the well.

Thus, the transmitter <NUM> at surface <NUM> may be used to control one or more tools 690a, 690b, 690c downhole by means of data transmitted to receivers 670a, 670b, 670c associated with the downhole tools 690a, 690b, 690c via the structure of the casing <NUM> of the well. For example, such tools 690a, 690b, 690c may be valves, which may be operable to restrict a flow of production fluids to surface <NUM>.

Claim 1:
A transmitter (<NUM>, <NUM>) for use in downhole telemetry and/or control, the transmitter (<NUM>, <NUM>) comprising:
a mixer (<NUM>, <NUM>);
modulator (<NUM>, <NUM>);
signal generation apparatus (<NUM>, <NUM>) configured to generate a frequency modulated signal (<NUM>, <NUM>); and
a fixed-frequency oscillator (<NUM>, <NUM>), the fixed frequency oscillator (<NUM>, <NUM>) being coupled to the mixer (<NUM>, <NUM>);
wherein the mixer (<NUM>, <NUM>) is configured to generate an output signal (<NUM>, <NUM>) based on the frequency modulated signal (<NUM>, <NUM>), a frequency spectrum of the output signal (<NUM>, <NUM>) comprising upper and lower sidebands disposed about a suppressed or reduced frequency of the frequency modulated signal (<NUM>, <NUM>),
wherein the modulator (<NUM>, <NUM>) is operable to modulate an input signal onto the output signal (<NUM>, <NUM>), and
wherein the mixer (<NUM>, <NUM>) is configured to suppress or reduce a frequency component corresponding to the fixed-frequency oscillator (<NUM>, <NUM>) from the output signal (<NUM>, <NUM>).