Abstract:
A method of high frequency data transmission for transmitting data over a three phase power system between a surface and a subsurface location, method using a first data transmit frequency and a second data transmit frequency, the data transmit frequencies being numerically distinct to each other and the data is transmitted on at least the second transmit frequency with a time delay between transmissions at each of the transmit frequencies to provide an uncorrupted data signal without interference. The invention finds specific application monitoring down hole equipment such as an electrical submersible pump in a wellbore.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to data transmission to and from down hole equipment and in particular, though not exclusively, to an improved method of data transmission through a three phase power system between the sub surface and a surface location. 
         [0002]    “Down hole equipment” is understood to refer to any tool, equipment or instrument that is used in a wellbore. 
         [0003]    Data needs to be transmitted between down hole equipment and the surface for various purposes, for example: monitoring performance of motors/pumps; transmission of control signals for control of valves; measuring device orientation and position; and making physical measurements. 
         [0004]    For motorised down hole equipment, data needs to be sent from below the equipment in a circuit that includes motor windings and the equipment&#39;s power cable which can be considered as a three phase power system. The rationale is that since there are already power cables present the cost of the solution using these should be proportionately less than one where you must supply the appropriate length of communications cable. 
         [0005]    Due to the motor and power cable properties of a three phase power system, DC current based devices which are coupled to the power system using inductive couplings have been developed and are extensively used. Examples of digital and processor based devices are disclosed in U.S. Pat. No. 5,515,038, GB 2283889 and U.S. Pat. No. 6,396,415. These systems utilise DC current injected onto the power signal and extracted through inductive Y-point couplings. These are all susceptible to failure when insulation on the power cable is lost or damaged. They are also typically either analogue in nature introducing noise into the measurements or where digital data is transmitted, it is at a very slow data rate. 
         [0006]    AC based systems which make use of AC power and/or signal transmission have been developed to overcome these problems. However, these AC based systems introduce disadvantages of their own as frequency at which the signal is transmitted becomes critical. Significant issues arise with attenuation of the signal in the motor cable system and also interference with the signal from both the instrument system power source and the motor power system which often is a variable speed drive generating switching noise with harmonics at very high frequencies. The combination of the attenuation of the signal and interference from the other power sources in the system mean that AC based systems are not in widespread use today because of the practical problems of signal recovery, and power delivery in the presence of cable faults. 
         [0007]    In order to recover data in this environment frequency selective techniques such as highly tuned filters are needed . In any case, any noise or interference which has a component at the same frequency as the data or carrier will interfere with or if it is much larger in amplitude erase any trace of the data which needs to be recovered. To overcome this, GB 2416097 suggests altering the frequency of the surface power frequency to reduce noise. Unfortunately, this only reduces noise from the instrument AC power and has no effect on noise from the main motor supply which is not within the control of the instrument system. GB 2352150 suggests synchronising the data transmit with the power frequency and/or the motor power frequency. While this appears to be an effective technique, in practice it is extremely difficult to fully implement because of the nature of the motor supply waveforms which are difficult to measure, and also because the motor supply can change relatively rapidly and quite often. 
         [0008]    US 2012/0026003 to Layton describes systems and methods for reliably communicating data between surface and down hole equipment over a power cable, regardless of the length of the power cable, wherein a transmitter modulates a common data stream onto multiple high frequency carrier signals, each of which has a different frequency. Each of the different frequencies is best suited to communication over a different length of cable. The resulting modulated high frequency data signals are impressed on the power cable and are recovered from the cable by a receiver. The receiver is configured to recover signals at each of the different carrier frequencies, at least one of which should be transmitted with little enough attenuation and interference that the data stream can be accurately recovered from the corresponding modulated high frequency data signal. This technique describes using a plurality of high frequency signals which are transmitted both as square waves so that they contain harmonics, and also superimposed simultaneously on the power cable. This provides disadvantages in that: the square wave transmission is phase shifted and distorted when it is received at surface making it harder to detect; the harmonics from the transmissions will mean that some portion of each carrier will be detected in each of the other carrier based data streams, causing interference and degradation of the signal; and even if they were not square waves any impurity in the signal transmission will mean you get some portion of each carrier in each other carrier, causing corruption of the data, because they are transmitted at the same time. 
       SUMMARY OF THE INVENTION 
       [0009]    It is therefore an object of the present invention to provide a method of data transmission for transmitting data over a three phase power system wherein the data will not be lost or become corrupted in the presence of noise at a selected data transmit frequency. In this specification, the term data transmit frequency will also include the data carrier frequency in FM transmission. 
         [0010]    According to a first aspect of the present invention there is provided a method of high frequency data transmission for transmitting data over a three phase power system between a surface and a subsurface location, the method using a first data transmit frequency and a second data transmit frequency, the data transmit frequencies being numerically distinct to each other and the data is transmitted on at least the second transmit frequency with a time delay between transmissions at each of the transmit frequencies. 
         [0011]    In this way, the second transmit frequency can be selected to not be numerically related to or be transmitted at the same time as the first data transmit frequency so that any noise present at the first data transmit frequency will not interfere with the data transmitted at the second frequency. In this way, an uncorrupted data signal without interference is transmitted. By introducing a time delay, any harmonics from the data transmit frequencies will not interfere with the other data channels and the data recovery is simplified. 
         [0012]    In an embodiment, each data transmit frequency is not a harmonic multiple of any other data transmit frequency. The data transmit frequencies may be in different frequency bands. In an embodiment, there are a plurality of data transmit frequencies each numerically distinct from each other and the first data transmit frequency. In this way, the data can be transmitted at multiple transmit frequencies to increase the probability of successful data transmission. 
         [0013]    The second data transmit frequency may be pre-programmed at the subsurface. In an embodiment, the second data transmit frequency may be pre-programmed at the surface. In this way, all data transmitted between the surface and subsurface is transmitted at a first and at least a second transmit frequency. 
         [0014]    Alternatively, the second data transmit frequency is selected at the surface and communicated to the subsurface for use in transmitting data. In this way, if test data is transmitted between the surface and subsurface and the test data is corrupted by interference, then a signal can be sent to the subsurface to transmit the data at the second frequency to avoid interference. This may be considered as channel hopping. 
         [0015]    In an embodiment of the present invention there are first and second data transmit frequencies at 70 and 106 KHz. In an alternative embodiment of the present invention there are first and second data transmit frequencies at 90 and 123 KHz. 
         [0016]    In this way, even when there is noise which interferes with the first data transmit frequency the other frequency(ies) are unaffected. 
         [0017]    In an embodiment, data at each transmit frequency is transmitted sequentially. This reduces the time to obtain a recoverable signal. 
         [0018]    In an embodiment, the three phase power system includes down hole equipment and the data is transmitted between the down hole equipment and a surface. 
         [0019]    In an embodiment, the down hole equipment comprises a component of an artificial lift system. 
         [0020]    In an embodiment, the down hole equipment includes an electrical submersible pump (ESP). 
         [0021]    The data may be analogue or digital. The transmitted data may be frequency modulated. In such an arrangement the data transmit frequency will be the carrier frequency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0023]      FIG. 1  shows the typical set up of a down hole equipment in a well, showing the positions of the equipment, the motor and the control interfaces at the surface; 
           [0024]      FIG. 2  is a functional block diagram of a data transmission system according to an embodiment of the present invention; 
           [0025]      FIGS. 3A ,  3 B, and  3 C are spectral plots illustrating ( 3 A) a data signal at a single transmission frequency, ( 3 B) an illustrative noise signal with noise at the transmission frequency, and ( 3 C) the recovered signal; and 
           [0026]      FIGS. 4A and 4B  are spectral plots illustrating ( 4 A) a data signal and ( 4 B) a recovered signal according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    One category of down hole equipment is artificial lift systems, for use in wells where there is insufficient pressure in the reservoir to lift the well&#39;s fluid (e.g. oil, water or gas) to the surface. Types of artificial lift systems include hydraulic pumps, Rod pumps, Electric Submersible Pumps (ESPs), Jet Pumps, Progressing-Cavity pumps (PCPs) and gas lift. 
         [0028]    Reference is initially made to  FIG. 1  of the drawings which illustrates a typical ESP completion in a wellbore. An ESP motor  10  is coupled through a seal  12  to a centrifugal pump  14  and used to lift the fluids through a tubing  16  to a surface  18  of the well  20  in a manner known to those skilled in the art. In order to monitor the operation, sensors or gauges  22  are located below the ESP  10 . Typically, the motor  10  is a three phase Y configuration. The motor is driven by a variable speed drive system  24  and is connected via a three phase power cable  26 . The system can be considered to comprise two distinct parts, a surface system, generally indicated by reference numeral  28 , and a down hole system, generally indicated by reference numeral  30 . These two parts  28 ,  30  communicate using the ESP power cable  26 . 
         [0029]    Surface equipment relating to the gauge system is shown in  FIG. 1  where there is a HV unit  13  connected directly to the 3 phase power supply to the down hole motor and there is a further LV or low voltage unit  8  which is safely isolated from the high voltage system. The LV system is primarily for data recovery and processing and data display etc. The HV unit is used to inject AC power and also make recovery of raw data from the 3-phase power system, in separate couplings as will be described. 
         [0030]    Referring now to  FIG. 2  of the drawings there is illustrated a functional block diagram of a data transmission system, generally indicated by reference numeral  40 , according to an embodiment of the present invention. In this arrangement data can be transmitted onto the three phase power cable  26  in either direction between the surface equipment  28  and subsurface or down hole equipment  30 . 
         [0031]    At surface  28  the equipment is divided into a high voltage side  32  and a low voltage side  34 . The high voltage side  32  provides the power to the down hole system  30 . Tuned high-voltage AC coupling  36  is used to connect to each of the phases in the power cable  26 . Thus a tripling of circuitry is used in the high-voltage equipment  32 . A microprocessor  38  controls the power distribution on to the three-phase cable  26  and is linked to a corresponding microprocessor  40  on the low voltage side  34 . Additionally the high-voltage side  32  uses tuned high-voltage AC coupling  36 , in parallel to pick off the data signals on the three-phase cable  26 . These signals are then filtered  42  and de-modulated  44  by known methods. Data signals then pass via the microprocessor  40  for display  46  or transport to a data logger or SCADA system. Additionally, the process can work in reverse where microprocessor  40  provides data on to the power lines  26  via the tuned high-voltage AC coupling  36  on the high-voltage side  32  as is known in the art. 
         [0032]    Down hole an ESP system  48  is provided as described herein with reference to  FIG. 1 . Like parts have the same reference numerals to aid clarity. Below the motor  10  is a standard Y-point connector  50 . At the Y-point connector  50  is arranged a down hole system  52 . The down hole system  52  provides monitoring in the form of measurement devices sensors or gauges  54 , hooked up via a microprocessor  56 . Power to drive the gauges  54  is provided via tuned HV AC coupling circuits  36  to a power regulator  58 . Similarly, data from the measurement devices  54  is processed in the microprocessor  56 . Using a signal driver  60  and tuned HV AC coupling circuits  36 , the data is transmitted on to the power line  62  for transmission to the Y-point  50  and onward transmission up the three-phase power cable  26  to the surface units  28 . 
         [0033]    In an embodiment of the present invention, unlike the prior art, instead of a single transmission frequency being selected to carry the data to the surface, two distinct transmission frequencies are selected. These frequencies are numerically distinct and are specifically not harmonics of each other or any other known frequencies in the system  40 . Each frequency is pre-programmed into the microprocessor  56  on the down hole side  30 , and the data is frequency modulated at these carrier frequencies on to the power line  62  and cable  26  in a known manner. The data is sent at each of the transmission frequencies with a time delay there between. The data can be sent sequentially at each transmission frequency. This improves the signal processing and simplifies the demodulation  44  stage on the surface equipment  28 . The microprocessor  40  at the surface  28 , analyses the received signals and if one is corrupted then this is discarded and the data recovered from the other signal. It will be apparent that multiple data transmit frequencies can be used to send the same data at different carrier frequencies or at different data rates to ensure the data is received without interference. 
         [0034]    In an alternative embodiment of the present invention, a data transmit frequency is pre-programmed into the microprocessor  56  of the down hole equipment  30 . A data test signal is sent from the surface  28  to the microprocessor  56  down hole. This signal is sent back across the cable  26 . Microprocessor  40  at surface, analyses the received signal for any signs of interference of corruption of the data. If the received test signal does not match the sent signal then interference is determined and the microprocessor  40  sends a command signal to the microprocessor  56  down hole, to select a second data transmit frequency. This second frequency is numerically distinct from the first, being neither a multiple or harmonic of the first data transmit frequency. Data is then transmitted at the second data transmit frequency to surface. Periodically, further test signals may be sent to ensure that interference has not been introduced at the second data transmit frequency. If the signal is corrupted by interference, then a command signal can be sent to switch to a third data transmit frequency, selected to be distinct from the first and second data transmit frequencies. This process of testing and channel hopping can be repeated to ensure clean uninterrupted data transmission. 
         [0035]    Interference and the consequential corruption of data transmitted over three phase power systems is known, particularly in the application of ESP. Each part of the system can introduce spurious components to the signal at multiple frequencies. If we consider a typical data signal  64  as shown in  FIG. 3A . This plot gives voltage  66  versus frequency  68  and shows the amplitude of a data signal at a frequency F2  70 . The signal  64  has smaller side band harmonic components. At  FIG. 3B  there is illustrated a plot of frequency versus voltage for noise generated by the motor power supply. This noise signal  74  is at a much lower frequency  76  than our data signal  64 , but it has a harmonic side band at a lower amplitude which is at a frequency F2  78  that matches the frequency F2  70  of our data signal  64  ( FIG. 3A ). If we now consider the recovered signal  80 , illustrated in  FIG. 3C , again as a plot of voltage versus frequency, it is apparent that in the combination  80  of the data signal  64  and the noise signal  78 , only the noise harmonic is recovered at F2  82 . Accordingly if the data signal  64  and the noise signal  74  both exist in the same system, the data signal  64  while still present cannot be recovered regardless of how much filtering is applied because of the larger amplitude harmonic data from the motor supply. 
         [0036]    Referring now to  FIGS. 4A and 4B , we consider the same data but now being transmitted at multiple signal transmit frequencies according to an embodiment of the present invention.  FIG. 4A  illustrates a voltage  66  versus frequency  68  plot of the data  64  with several different signal transmit frequencies F1, F2 and so on potentially to Fn. Now, if we assume that the noise signal is that illustrated in  FIG. 3B , then our combination and recovered signal is shown in  FIG. 4B . In this plot of voltage versus frequency, it is seen at that the noise at F2  84  is larger than the signal so the data transmitted at F2 cannot be decoded. 
         [0037]    However, the signal at F1  86 , is not affected and so the data transmitted at the frequency F1 can be recovered. 
         [0038]    The data transmit frequencies are best selected from different frequency bands. Suggested frequencies could be 70 and 106 KHz or 90 and 123 KHz and so on. 
         [0039]    The principle advantage of the present invention is that it provides a method of data transmission over a three phase power system where the data transmit frequency or the data carrier frequency in the case of FM transmission, is not lost or corrupted by the presence of noise at the exact data or carrier frequency. 
         [0040]    A further advantage of the present invention is that it provides a method of data transmission over a three phase power system which can be implemented in signal processing and does not require additional equipment in the well. 
         [0041]    Various modifications may be made to the invention herein described without departing from the scope thereof. For example, the principle can be applied to analogue or digital signals, signalling in either direction between the surface and the subsurface, and the transmission of control signals. Additionally, while an ESP application has been described this data transmission system can be applied across any system where there is a remote ac powered motor present and there is a need to transfer data across a system in proximity to the motor with a single star-point connection without the addition of dedicated wires. Other applications where this transmission system could be applied include: Sub-sea control valves; Chokes—Sub-sea power transformer monitoring; ESPCP systems; Remote surface motor/transformer systems; and general telemetry. The motor can also be in any phase configuration. It is also to be appreciated that for more powerful or more complex systems, the transmission system can be used across a number of similar motors arranged in a stacked manner. 
         [0042]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.