The present invention relates generally to monitoring and control of downhole instrumentation systems for use in activities related to oil and gas.

In particular the invention relates to an electric power supply network providing power to downhole instrumentation and control modules and a communications network for providing communication between such downhole instrumentation and control modules and between each module and a common network control unit.

In subterranean wells there is an increasing demand for monitoring and controlling downhole devices and systems, for example sensors and instrumentation or control devices. There is a trend towards attempting to bring such instrumentation closer and closer to the actual hydrocarbon reservoir regions and towards multi-well exploration techniques for enabling the extraction of a higher fraction of the hydrocarbon resources available in such reservoirs. At the same time it is desirable to distribute instrumentation and measurement devices at a larger number of locations in order to be able to measure and assess the performance of the well production at more measurement points. A main objective of such measurements is to be able to obtain as accurate measures as possible of the reservoir pressure and temperature, and a secondary objective is to be able to locate possible problems at an early stage.

Hence it is vital to be able to provide systems capable of operating under the relevant conditions and which provides an operator with improved data on the conditions and performance of completed wells. An example of such a system is discussed in

US8330615, showing a power supply and a monitoring system, for controlling downhole sensors and instrumentation being provided with an interface module.

Another known system is described in US3991611 describing a telemetry system for communication with subsurface instrumentation. In this application, a carrier signal is used transmitting information between the subsurface and surface equipment, the carrier signal thus being active when the signal is transmitted. Standard gain and noise suppression techniques are not well suited to high temperature downhole gauge networks. In downhole gauge networks as described in US8330615 there can be a multitude of nodes placed at varying distances from the controller electronics. Each node will receive a different amplitude of input signal from the controller card. The receiver electronics utilize AGC (Automatic Gain Control) to varying the gain of the receiving amplifier whilst the receiver searches for incoming data signals. The weakness of this design is that the receiver can amplify noise and occasionally this noise signal is interpreted as a valid data package and decoded. If a valid data response arrives during the decoding process the valid package could be missed and the receiver will not respond causing the transmitter to timeout.

Traditionally a number of techniques can be used to suppress noise in transceiver architectures such as a squelch function or a noise gate. Both of these functions derive a signal strength threshold below which the receiver will mute until the received signal strength is above the threshold. Often this threshold can be manually adjusted to optimize the transmission. In a downhole network manual adjustment of this threshold is obviously impossible. Further, traditional noise suppression techniques assume the background noise will be roughly constant. In a downhole network the imposed noise is difficult to determine and can come from an unlimited range of 3rd party tools which is difficult to determine or model. A preamble is a common technique in digital signal transmission to allow the receiving electronics to lock on to the incoming signal and allow the receiver time to optimize its configuration.

The receiver architecture as described in patent US8330615 utilizes a preamble to help focus the receiver architecture. However in systems that require a large dynamic range on the receiver due to greatly varying incoming signal strengths a standard 2 byte preamble may not give the receiver time enough to optimize its settings. The preamble could be extended to allow the receiver more time to optimize its setting but this reduces the available bandwidth and consequently data throughput. Further to add reliability for high temperature electronics the receiver design should be kept as simple as possible. The present invention overcomes these draw backs in traditional receiver architectures to allow noise suppression for high temperature electronics for use in the downhole oil and gas environment.

US2008/0310457 describes a system for transmitting signals on a power line, including a pilot signal. A receiver receives the pilot signal and based on this communicates information about that channel to the transmitter. Based on this a noiseless channel is chosen. Other methods for handling noise in the system may be squelch functions such as a tone coded squelch in which the receiver has to analyze the received signal to be able to lock out all signals except signals having the right tone. This will require processing of the received signals and possibly resulting in not receiving the right signal.

It is therefore an object of the present invention to provide a way for reducing the time used in decoding noise, and this way increasing the performance and enhancing the throughput of the signal transmission.

Also it is an object of the present invention to provide a system and method for improving the signal transmission between the transmitters and receiver nodes in a downhole network. These objects are is obtained as specified in the accompanying claims.

The invention will be described more in detail below, with reference to the

accompanying drawings.

Figure 1 illustrates the per se downhole network system using the invention. Figure 2 illustrates a communication sequence according to the invention.

The present solution relates in its simplest form to an integrated downhole network implementing a downhole network controller as described in US8330615, connected to a cable connected to individual or multiple sensors positioned in downhole modules 3. Referring to figure 1 the network controller 1 with a conductor 2 having a number of downhole modules 3 connected in a series along the conductor 2 as discussed in US8330615. As discussed in US8330615 each module 3 draws a chosen voltage, e.g. 10V, and the cable draws a voltage V resulting from a resistance in the cable, and as described a coded signal 4, e.g. using a so-called Manchester code, is sent through the cable to be pickup up by the relevant instruments. The present solution is based on the implementation of a pilot tone in the intermediate idle periods between the signal transmissions, as described below with reference to figure 2a and 2b, where figure 2a illustrates the known solution and figure 2b illustrates the present invention. The figures illustrate the amplitude A of the transmitted signal as a function of time. In the known solution, a TX frame 12 including a signal from the transmitter is followed by a short idle time 14 (fig. 2b)and then a RX frame 13 allowing for a response from receiver. After the RX frame 13 a long idle time 10 follows, which includes background noise. The noise amplitude may be sufficiently high or have characteristics that activate the receiver 3 which would start trying to interpret the signal. The instruments will use valuable resource interpreting the background noise signal which may result in the instrument not being able to detect the real transmitted signals 12. The problems are related to the following characteristics of the system as described in figure 2a:

A typical receiver design in a downhole module 3 has an automatic level control (ALC) input circuit as part of the receiving amplifier that search for incoming data signals. These ALC circuits can modify the attenuation and the expected baud rate of the received signal.

A typical ALC could latch onto noise, amplify it and then send it to the decoder. This could result in the decoder spending time decoding noise The ALC and decoder can then be unable to track and decode the real communication packages when they were transmitted, resulting in corrupt data or timeouts in the communication.

The problem usually only occurs when the receiver is searching for a data signal, i.e when the downhole network is in an "idle" state and the network controller 1 or a modules 3 are not communicating.

According to the invention the system provides an improvement over the known solutions by utilizing a pilot tone generator. As illustrated in figure 2b a known signal 11 is applied in the communication line 2 during the long idle periods 10 in the communication. This forces the downhole modules 3 to listen to an invalid

communication message 11 which is easily discarded by the module as having no relevant information for the module to interpret. When a valid communication signal 12 is sent to the modules 3 decodes the package and acts accordingly. Using the pilot tone 11 the chance of an invalid noise signal being decoded by the downhole module is virtually eliminated by minimizing the time the downhole module 3 containing the receiver is in a "searching" mode. According to the present invention this results in a system with a dropout rate magnitudes lower than if the pilot tone is not utilized under certain well conditions..

The communication system according to the preferred embodiment of the present invention is operated by modulating a clear carrier signal on the downhole cable 2 into which all the modules 3 are able to lock onto and stay tuned to.

During active communication the network controller 1 will disrupt the pilot signal 1 with a valid message 12 and the module addressed will easily detect the transition and decode the message before providing an answer 13 during the following time window.. The disruption is performed directly by changing seamlessly from pilot tone to active signal or by stopping the pilot signal a short time period before transmitting the active signal. This way the method avoids the problem with modules being busy decoding noise and in that way overlooking real signals.

More in detail, during the "idle" phases the network controller 1 is programmed to output a digital signal 4,11, thus driving the communication line with a known signal and baud rate. The ALC circuitry in the downhole modules lock onto the pilot tone 11 and as an additional feature the attenuation and baud rate settings of the receiving module 3 may be optimized for the actual data package to be received in the signal package 12. The pilot tone 11 is a digital signal 4 having predetermined characteristics that are easier for the decoder in the module to discard than the noise otherwise present in the system. This way the use of pilot tone reduces the "idle" time of the system to a minimum as well as minimizing the time that the modules can search for an active signal and inadvertently try to decode noise.

Preferably there is a short idle time 14 between the active signal 12 and the expected return signal 13, which is sent during a predetermined time window, and a second idle time 15 after the return signal and before the pilot signal 11 is continued. In addition, a third idle time 16 may be used before the transmission of the next TX signal 12 from the network controller. This '"idle" time between communication packages can be tuned in SW for the particular installation to maximize the improvements. In practice the improvements from > 20 % timeout to <3 % timeout have been achieved.

To summarize the present invention thus relates to a downhole communication system or method where the system includes a network controller 1 for transmitting signals 4 through a downhole network. The system comprises at least one sensor or module 3 connected to the network controller by a downhole cable 2. The network controller being adapted to transmit a first TX signal 12 addressed to said modules 3 into said cable 2 during chosen time frames or periods 12, the signal time periodes being separated by idle time periodes 10, 14, 15 and said at least one module 3 being adapted to receive signals from the network controller. The module(s) may be adapted to respond by sending a response signal RX 13 back to the network controller The network controller according to the invention comprises at least one pilot tone generator transmitting a pilot tone signal 11 in at least part of the idle time between said TX or RX signal time frames periods 12,13. The pilot tone signal 11 having predetermined characteristics and said at least one module 3 being adapted to maintain active while receiving said pilot tone signal. The pilot signal according to the invention is therefore not a carrier signal for the transmitted signal, but an "empty" signal keeping the receiver active without requiring processing power.

Each module is adapted to recognize the pilot tone characteristics, and to discard the signal as containing no information, while waiting for a signal addressed to said module, so as to minimize the processing time in the modules while waiting for the transmitted signal 12.. The pilot signal period is disrupted and changes directly into an addressed, active TX signal or stops for a chosen idle time before the active signal is transmitted, thus separating the pilot signal from the information signal.

After receiving the TX signal the system allows for a time frame for receiving return signals 13 from the modules, the length of the RX time frame may be fixed or may depend on the number of modules expected to respond. A chosen idle time is provided between the active signal in the RX time frame from the receiver or receivers and the pilot tone. The idle time being chosen to be longer than the expected response time for said modules.

The method of the invention may thus be summarized as being related to a method for communicating with a downhole network including at least one network controller for transmitting signals into said network and receiving signals from modules in the network. The method includes the steps of transmitting from said network controller a first TX signal 12 into said network in a TX time frame 12 and receiving a return RX signal from said modules in a RX time frame 13 following said TX time frame. A pilot tone signal 11 is transmitted from the network controller in the period following the RX time frame, where the pilot tone signal has predetermined characteristics being recognized by said downhole modules as a signal not requiring additional processing. The pilot tone signal is transmitted until or just before the transmission of the next TX signal from the network controller.