Patent Publication Number: US-10774619-B2

Title: Downhole completion system

Description:
This application claims priority to EP Patent Application No. 17162048.7 filed Mar. 21, 2017, the entire contents of which are hereby incorporated by reference. 
     The present invention relates to a downhole completion system for completing a well having a borehole. Furthermore, the present invention relates to a sensor unit for use with a downhole completion system according to the present invention. 
     Various methods and systems for monitoring a well and the production have been proposed over the years. However, so far these methods are associated with a number of drawbacks. For example, it has been suggested to monitor downhole conditions using a submerged tool which is retrieved to download the data. The tool may be arranged in order to measure downhole parameters such as pressure, temperature, position etc. Such parameters may also be of great importance during completion and during production. As is evident, these solutions may only monitor downhole conditions during the time span in which the monitoring tool is positioned at the specific location downhole. When the tool is to measure parameters, e.g. 10 km from the top, the tool needs to emerge to surface every time data needs to be unloaded. Prior art tools are only capable of sending control signals to the tool via a wireline powering the tool when the tool is several kilometres down the well. Prior art tools cannot perform real time monitoring of a well over many years, both due to the lack of sufficient uploading possibilities and since the tool needs power, and the wireline cannot stay in the well as this hinders production. 
     In order to solve the problem associated with running monitoring equipment downhole, and to allow for a more permanent monitoring, sensor systems have been developed. These sensors are positioned downhole and may provide monitoring independently of the presence of any downhole tool. These sensors may either be powered by an external power supply, such as a wireline, or by an embedded battery. While the wired alternative requires the undesired need for long cables, the stand-alone battery-powered alternative suffers from a limited operating time. 
     Hence, it would be advantageous to provide an improved system and method enabling monitoring of downhole conditions over a longer period of time. 
     It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to provide an improved method and system for monitoring of downhole conditions for a longer period of time. 
     The above objects, together with numerous other objects, advantages and features, which will become evident from the below description, are accomplished by a solution in accordance with the present invention by a downhole completion system for completing a well having a borehole, said downhole completion system comprising:
         a well tubular metal structure arranged in the borehole forming an annulus and comprising:
           a wall, and   a plurality of sensor units forming a mesh network,
 
wherein at least a number of said sensor units is provided with a self-powering device configured to harvest energy downhole.
   
               

     By having a mesh network of sensor units having a self-powering device configured to harvest energy downhole, any kind of tool without a wireline or any kind of sensor module can be more permanently arranged in the well as measured data is sent to surface using the mesh network when there is data to be sent. In the meantime, the self-powering device harvests energy downhole and accumulates enough energy to be able to receive and transmit when the next set of data is to be communicated to surface. 
     Thus, the plurality of sensor units forming a mesh network provides for a reliable data path even though at least some of the sensor units are out of range from the data collection provided at surface or at seabed level. 
     The self-powering device may be configured to harvest energy downhole from fluid flowing in the well. 
     Said self-powering device may be configured to harvest energy downhole from fluid flowing in the annulus and/or in the well tubular metal structure. 
     Moreover, the sensor units may be arranged at least partly in the wall of the well tubular metal structure. 
     Further, the sensor units having a transmitting and receiving distance and the sensor units may be arranged with a mutual distance of half the transmitting and receiving distance. 
     The self-powering device may be configured to convert kinetic energy to electrical energy. 
     Moreover, the self-powering device may comprise a vibrating member. 
     Also, the self-powering device may comprise a piezoelectric member. 
     Further, the self-powering device may comprise a magnetostrictive member. 
     In addition, the self-powering device may comprise a thermoelectric generator. 
     Furthermore, the self-powering device may further comprise at least one capacitor. 
     Each sensor unit may be configured to receive wirelessly transmitted data from an adjacent sensor unit, and to forward the received data to adjacent sensor units. 
     The downhole completion system according to the present invention may further comprise a surface system configured to receive downhole data from said sensor units. 
     The surface system may be at least partly arranged at the seabed level. 
     Moreover, said surface system may further be configured to determine the position of at least one sensor unit. 
     Further, the surface system may be configured to determine the position of at least one sensor unit by Monte Carlo simulation and/or Shortest Path simulation and/or acoustic pinging time of flight. 
     Also, the mesh network may be a self-healing mesh network. 
     Furthermore, the sensor units may use the inside of the well tubular metal structure as a waveguide for communication between the sensor units. 
     At least one of said sensor units may comprise a sensor for measuring one or more conditions of the well fluid surrounding the well tubular metal structure. 
     Further, each one of said sensor units may comprise at least one detector. 
     Additionally, the detector may comprise an accelerometer and/or a magnetometer, and position data may comprise inclination and/or azimuth. 
     Moreover, at least one of said sensor units may be positioned in the annulus formed between the well tubular metal structure and a borehole wall. 
     Cement characteristics may comprise acoustic impedance, and the detector may comprise a transducer for measuring a reflected signal for determining the acoustic impedance. 
     In addition, the detector of at least one of said sensor units may be configured to detect borehole characteristics such as flow conditions and/or water content. 
     The downhole completion system according to the present invention may further comprise a sensor module comprising additional sensors. 
     Said sensor module may comprise a temperature sensor and/or a pressure sensor and/or a flow condition sensor and/or a water content sensor. 
     Also, the well tubular metal structure may further comprise annular barriers, each annular barrier comprising:
         a tubular metal part having an expansion opening and being mounted as part of the well tubular metal structure, and   an expandable metal sleeve surrounding and connected with the tubular metal part, and the expandable metal sleeve being expandable by means of fluid entering through the expansion opening.       

     Furthermore, the well tubular metal structure may further comprise flow devices. 
     The well tubular metal structure may comprise several lateral well tubular metal structures. 
     The downhole completion system according to the present invention may further comprise a downhole autonomous tool configured to move within the well tubular metal structure, the downhole autonomous tool comprising a communication unit configured to communicate with the sensor units for sending information to surface via the network of sensor units. 
     The present invention also relates to a sensor unit for use with a downhole completion system as described above, wherein said sensor unit may be provided with a self-powering device configured to harvest energy downhole. 
     It should be noted, that within this specification, the term “mesh network” should be interpreted as a network of which each associated sensor forms a network node being configured to relay data for the network. All network sensors thus cooperate in the distribution of data in the network. In a mesh network within the context of this specification, data transfer is accomplished by routing data between the sensors until the data reaches its destination. The data path is not constant, but it is re-routed if any existing sensors are unavailable. 
    
    
     
       The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which: 
         FIG. 1  shows a downhole completion system, 
         FIG. 1A  shows an enlarged view of one of the sensor units in  FIG. 1 , 
         FIG. 2  shows a downhole completion system with a downhole autonomous tool, 
         FIG. 2A  shows an enlarged view of one of the sensor units of  FIG. 2 , 
         FIG. 3  shows a downhole completion system having laterals, 
         FIG. 4  is a schematic view of a downhole completion system, 
         FIG. 5  is a schematic view of a sensor unit for use with a downhole completion system, 
         FIG. 6  is a schematic view of a self-powering device of a sensor unit, and 
         FIG. 7  is a diagram showing data communication between different sensor units of a downhole completion system. 
     
    
    
     All the figures are highly schematic and not necessarily to scale, and they show only those parts which are necessary in order to elucidate the invention, other parts being omitted or merely suggested. 
     In the following description a downhole completion system  100  will be described, and in particular sensor units  10  forming a mesh network  130  for use with such downhole completion system  100  will be described. 
       FIG. 1  shows a downhole completion system  100  for completing a well  2  having a borehole  3 . The downhole completion system comprises a well tubular metal structure  1  arranged in the borehole forming an annulus  4  between a wall  6  of the borehole and the well tubular metal structure  1 . The well tubular metal structure has a wall  5  and comprises a plurality of sensor units  10  forming the mesh network  130 . At least a number of said sensor units  10  is provided with a self-powering device  11  configured to harvest energy downhole in order that the mesh network in the downhole completion system is self-powering over time. The self-powering device  11  is configured to harvest energy downhole from fluid flowing in the well, e.g. during production, but also during fracking, wash-out and/or cementing operations. Thus, the self-powering device  11  is configured to harvest energy downhole from fluid flowing in the annulus and/or in the well tubular metal structure. As shown in  FIG. 1 , the sensor units  10  are arranged at least partly in the wall of the well tubular metal structure and are thus able to harvest energy from the fluid flowing in the annulus, as indicated by the arrows, before the fluid enters through openings  17  in the well tubular metal structure. An enlarged view of one of the sensor units  10  is shown in  FIG. 1A . 
     The sensor units  10  have a transmitting and receiving distance D which is the distance over which the sensor units are able to reach out to transmit and receive signals/data from an adjacent sensor unit. Thus, the transmitting and receiving distance D is the distance between two sensor units which are able to communicate with each other, i.e. transmit data/signals to each other and receive data/signals from each other. The sensor units  10  are arranged with a mutual distance of half the transmitting and receiving distance. In this way, each sensor unit is capable of sending data/signals to an adjacent sensor unit and to the neighbour of the adjacent sensor unit, so that if the adjacent sensor unit is not functioning, the sensor unit can send data/signals past the adjacent sensor unit to the neighbour on the other side of that adjacent sensor unit, and the mesh network is established without the dysfunctional sensor unit. In this way, information can still be sent upwards towards the top  77  of the well and/or downwards towards the bottom of the well. 
     In  FIG. 2 , the downhole completion system  100  further comprises annular barriers  40  for isolating a first zone  101  from a second zone  102 . Each annular barrier comprises a tubular metal part  41  having an expansion opening  42 . The tubular metal part  41  is mounted as part of the well tubular metal structure  1 . Each annular barrier further comprises an expandable metal sleeve  43  surrounding and connected with the tubular metal part. The expandable metal sleeve is configured to be expanded by means of fluid entering through the expansion opening  42 , e.g by pressurising the well tubular metal structure from within and thus expanding several expandable metal sleeves substantially simultaneously, or by isolating a zone opposite the expansion opening by means of an expansion tool or a drill pipe with cups. The well tubular metal structure further comprises a flow device  44  arranged in the second zone so that fluid from that zone may enter through the opening  17 , when the flow device is in its open position as shown in  FIG. 2 . The sensor units are arranged partly in the wall of the well tubular metal structure, as shown in the enlarged view  FIG. 2A , but the self-powering device  11  does not have fluid contact with the fluid in the annulus. The self-powering device  11  of each sensor unit  10  harvests energy downhole from fluid flowing in the well tubular metal structure. 
     The downhole completion system  100  of  FIG. 2  further comprises a downhole autonomous tool  50  configured to move within the well tubular metal structure  1 . The downhole autonomous tool comprises a communication unit  51  configured to communicate with the sensor units for sending information to surface via the network of sensor units  10 . In  FIG. 2 , the downhole completion system  100  comprises a downhole power supply unit  52  which is arranged on the outer face of the well tubular metal structure and is powered through a cable  53  from surface through the main barrier  54 . The downhole autonomous tool  50  is thus able to be powered up before entering the well in order to complete an operation. The downhole autonomous tool  50  may, as it submerges or later emerges, download or transmit information/data and/or power to or from the sensor units. The well tubular metal structure  1  has, at its top, a receptacle into which a second well tubular metal structure is inserted. The main barrier is arranged above the receptacle and provides a barrier against the second well tubular metal structure  1 A, so that the well tubular metal structures can move in relation to each other. 
     To save power in each sensor unit, the sensor units may enter into “beacon mode” in which the network, at regular predetermined time intervals, wakes up and controls if any signals need to be communicated to another neighbouring sensor unit. Thus, the sensor units are programmed with a delay between each beacon ping. 
     In  FIG. 3 , the well tubular metal structure  1  of the downhole completion system  100  comprises several lateral well tubular metal structures  1 B,  1 C. The downhole autonomous tool  50  is situated in one of the lateral well tubular metal structures  1 C, and while the downhole autonomous tool  50  performs an operation or after the operation, the downhole autonomous tool  50  sends up information through the mesh network  130  of sensor units  10 . In this way, the downhole autonomous tool  50  is able to remain in the lateral well tubular metal structure and it will not have to emerge to the top of the well between two operations to unload data. Furthermore, the downhole autonomous tool  50  can be arranged in the lateral well tubular metal structure for a very long period of time and may activate itself every 6 months, measure some characteristics of its surroundings, e.g. temperature, pressure and flow density, and send the measured data to surface if some characteristics have changed, and then enter into “sleep mode” for a new period of e.g. 6 months. When the downhole autonomous tool  50  lacks power it emerges and re-loads in the downhole power supply unit  52 . The emergence of the downhole power supply unit  52  is assisted by the production fluid entering through the openings  17  or through the flow devices  44  in the well tubular metal structure. The mesh network of sensor units forms a network when required, and in the meantime, the sensor units harvest energy. Thus, the harvesting process does not need to be very efficient since the downhole completion system only uses the mesh network for a short period of time. Furthermore, the mesh network is formed when required so that non-functioning sensor units are skipped. 
     As will be explained in the following, this is realised by configuring the sensor units  10  to establish a physically distributed independent and localised sensing network, preferably with peer-to-peer communication architecture. As will be understood from the following description, the mesh network being established by the sensor units  10  as a self-healing mesh network, will automatically provide for a reliable and self-healing data path, even though at least some of the sensor units  10  are out of range from the final destination, i.e. the data collection provided at the surface level. 
     In  FIG. 3 , a yet further example of the use of a downhole completion system  100  is shown. Here, the sensor units  10  are arranged at the well tubular metal structure wall, either on the inner side, the outer side, or embedded within the downhole completion wall. The sensor units  10  are arranged at the downhole completion in order to form a “smart casing/liner”, i.e. to provide information to the surface relating to well characteristics along the borehole over time. As will be explained in the following, this is realised by configuring the sensor units  10  to establish a physically distributed independent and localised sensing network, preferably with peer-to-peer communication architecture. 
     All sensor units  10  are preferably identical, although provided with a unique ID. An example of a downhole completion system  100  is schematically shown in  FIG. 4 . The downhole completion system  100  comprises a surface system  110  and a sub-surface system  120 . The sub-surface system  120  comprises a plurality of sensor units  10 , although only one sensor unit  10  is shown in  FIG. 4 . Each sensor unit  10  is provided with a number of components configured to provide various functionality to the sensor unit  10 . As shown in  FIG. 4 , each sensor unit  10  includes a power supply in the form of a self-powering device  11 , a digital processing unit  12 , a transceiver  13 , and optionally a detector  14  and a sensor module  15  comprising additional sensors. For at least one sensor unit  10 , the power supply is formed by means of a self-powering device  11  (POWER in  FIG. 4 ) as will be explained in more detail below. Preferably, all sensor units  10  are provided with a self-powering device. 
     As shown in  FIG. 5 , the sensor module  15  may e.g. comprise a temperature sensor  15   a  and/or a pressure sensor  15   b  and/or a flow condition sensor  15   c  and/or a water content sensor  15   d . The detector  14  can for example be used together with the digital processing unit  12  to form a detecting unit for determining position data of the sensor unit  10 . The detector  14  may, in such embodiments, comprise an accelerometer and/or a magnetometer and/or a transducer. By providing a transducer as the detector  14 , it is possible to determine specific characteristics of the surroundings, such as cement integrity etc. 
     The power supply in the form of the self-powering device  11  is configured to supply power to the other components  12 - 15  of the sensor unit  10  by converting energy of the surrounding environment to electrical energy. 
     The digital processing unit  12  of  FIG. 4  preferably comprises a signal conditioning module  21 , a data processing module  22 , a data storage module  23  (STORAGE in  FIG. 4 ) and a micro controller  24 . The digital processing unit  12  is configured to control operation of the entire sensor unit  10 , as well as temporarily storing sensed data in the memory of the data storage module  23 . 
     The transceiver  13  is configured to provide wireless communication with transceivers of adjacent sensor units  10 . For this, the transceiver  13  comprises a radio communication module and an antenna. The radio communication module may be configured to communicate according to well-established radio protocols, e.g. IEEE 801.1aq (Shortest Path Bridging), IEEE 802.15.4 (ZigBee) etc. The radio communication module may also be configured to position the sensor units in relation to each other, i.e. configured to perform a distance measurement. 
     The surface system  110  also comprises a number of components for providing the desired functionality of the entire downhole completion system  100 . As is shown in  FIG. 4 , the surface system  110  has a power supply  31  for providing power to the various components. As the surface system  110  may be permanently installed, the power supply  31  may be connected to mains power, or it may be formed by one or more batteries. The surface system  110  also comprises a transceiver  32  for receiving data communicated from the sensor units  10 , and also for transmitting data and control signals to the sensor units  10 . Hence, the transceiver  32  is provided with a radio communication module and an antenna for allowing for communication between the surface system  110  and the sensor units  10  of the sub-surface system  120 . The surface system  110  also comprises a clock  33 , a human-machine interface  34 , and a digital processing unit  35 . The digital processing unit  35  comprises the same functionality as the digital processing unit  12  of the sensor unit  10 , i.e. a signal conditioning module, a data processing module, a data storage module, and a micro controlling module. 
     Before describing the operation of the downhole completion system  100 , a sensor unit  10  is schematically shown in  FIGS. 5 and 6 . The sensor unit  10  has a housing  19  which is configured to enclose the components previously described, as well as to form a protection which is capable of withstanding any impact, e.g. with potential collisions with the borehole wall  6 . Although shown as rectangular, the shape of the housing  19  may of course be chosen differently. For example, it may be advantageous to provide the housing  19  with only rounded corners. The housing  19  may for such embodiment have a spherical shape. Inside the housing  19 , the following is fixedly mounted: the self-powering device  11 , the digital processing unit  12 , the transceiver  13 , the detector  14  and optionally the sensor module  15 . 
     In  FIG. 6 , the self-powering device  11  is shown in further detail. The self-powering device  11  is configured to provide electrical power to the various electrical components of the sensor unit  10  by harvesting energy from the downhole environment. The self-powering device  11  therefore comprises an energy harvesting module  1100 . The harvesting module  1100  may be selected from the group comprising a vibrating member  1101 , a piezoelectric member  1102 , a magnetostrictive member  1103 , and a thermoelectric generator  1104 . As is shown in  FIG. 6 , any of these members is possible. In case of using a vibrating member  1101 , a piezoelectric member  1102  or a magnetostrictive member  1103 , the energy harvesting module  1100  is configured to convert mechanical vibrations of the surrounding environment, such as in the well tubular metal structure or in downhole fluid, to electrical energy. In case of using a thermoelectric generator  1104 , such as a Peltier element, the harvesting module  1100  is configured to convert thermal energy of the surrounding energy to electrical energy. 
     The harvested energy is preferably supplied to a rectifier  1105 . The rectifier  1105  is configured to provide a direct voltage and comprises a switching unit  1106  and a rectifier  1107 . It should be noted that the position of the switching unit  1106  and the rectifier  1107  could be changed, in order that the rectifier  1107  is directly connected to the harvesting module  1100 . As is shown in  FIG. 6 , the rectifier  1107  is preferably connected to a capacitor  1108  for storing the harvested energy. The electrical components  12 - 15  of the sensor unit  10  are therefore connected to the capacitor  1108  forming the required power source or storage buffer. Optionally, the self-powering device  11  is further provided with an amplifier (not shown) and/or with control electronics (not shown) for the switching unit  1106 . Additional capacitors may also be provided. 
     Now turning to  FIG. 7 , the configuration of the downhole completion system will be described further, and in particular the downhole or sub-surface system  120  will be described. The sensor units  10 A-F, representing parts of a sub-surface system  120 , are arranged at the well tubular metal structure wall. The communication between the sensor units  10 A-F is preferably based on a relay model, which means that the surface system communicates with the sensor units  10 A-F via a sensor unit network. Preferably, each signal that is transmitted from a sensor unit  10 A-F comprises information relating to a unique ID of the sensor unit  10 A-F. Further, data echoing and cross-talk are reduced by limiting the number of possible re-transmissions between the sensor units  10 A-F. By reducing data echoing, the possibility of one sensor unit sending the same data more than once to the same neighbouring sensor unit is eliminated. The network knows its neighbours by their unique IDs, and hereby the transmitter can target the transmission of data, and thus the situation in which data is sent back and forth can be avoided in that the neighbouring sensor unit “knows” from which sensor unit the data is received and will consequently not send that data back again. 
     Each sensor unit  10 A-F is preferably configured to operate in two different modes. The first mode, relating to activation for the purpose of receiving data relating e.g. to the position or trajectory of the borehole or cement or borehole characteristics, preferably comprises a step of gathering data (optionally including data from the additional sensors  15   a ,  15   b  (shown in  FIG. 5 ), and transmitting the data upon request. In the second mode, the sensor units  10 A-F are configured to re-transmit received signals. 
     The location of each sensor unit  10 A-F may also be determined by a round-trip elapsed time measured by the surface system  110 . The surface system  110  may thus be configured to ping a specific sensor unit  10 A-F using the unique ID, whereby the specific sensor unit  10 A-F replies by transmitting a response signal with a unique tag. The surface system  110  receives the transmitted signal with elapsed times, and either Monte Carlo simulation and/or Shortest Path simulation may be used to determine the specific position of the sensor unit  10 A-F. 
     Using Monte Carlo simulation, a simulated sensor unit location model may be created having a uniform probability distribution. For such method, it may be possible to assume that the sensor units  10 A-F are distributed along a specific borehole or well tubular metal structure length, and that these locations, for a given time, are known in the simulated model. The simulated model also includes a relay model with specific individual sensor processing delays. 
     For each distribution, the shortest round-trip travel time is calculated for each sensor units  10 A-F. This results in a map of travel time versus location of sensor units  10 A-F. It is then possible to compare the measured elapsed time with the map to determine the location of the sensor unit  10 A-F. 
     For Shortest Path simulation, once the surface system  110  pings a sensor unit  10 A-F, the round-trip times of multiple received signals, each one from a specific relay path, is recorded. The shortest time for the particular sensor unit  10 A-F is then determined by calculating the distance from the surface system  110  using the speed of light. 
     It would also be possible to use the detectors  14  of the sensor units  10 A-F for determining the distance between adjacent sensor units  10 A-F, especially if the detectors  14  are realised as transducers. As the sonic pulse transmitted by the detector  14  will travel with the speed of sound, more time for computing will be available. Hence the detector  14  is used not only for cement bond evaluation, but also for distance measurements. The radio communication module may also be used for the distance measurements, e.g. in smart mud. All information will, however, be communicated wirelessly using radio frequency. For example, the sensor units  10 A-F may be programmed to transmit a signal, via the transceiver, to its neighbouring sensor units  10 A-F, whereby the signal contains information that a sonic pulse will be transmitted at a predetermined time, e.g. 10 ms from transmittance of the signal. When one of the neighbouring sensor units  10 A-F detects the transmitted sonic pulse, it is possible, for each receiving sensor unit  10 A-F, to determine the time elapsed from transmission of the sonic pulse to receipt of the sonic pulse. The time of flight for the acoustic pulse is then converted to a distance between the transmitting sensor unit  10 A-F and each receiving sensor unit  10 A-F. Absorption based energy consideration and reverberation measurements are other examples of possible implementations for the range estimation between two neighbouring sensor units. 
     In the example shown in  FIG. 7 , each sensor unit  10 A-F forms a node in the mesh network  130 . Each node is configured to receive and transmit data signals, as well as add ID and timestamp with each data package. Each node will send a signal corresponding to its current state (i.e. the detected signals representing cement characteristics) asynchronously with respect to other nodes. In the table below, data communication in the mesh network  130  is explained further. In the table, nX represents the node ID, TnX represents the timestamp for the particular node, and sX represents the sensed data from the particular node. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Node 
                 Forwarded signal 
                 Received signal 
               
               
                   
               
             
            
               
                 10A 
                 nA:TnA:sA 
                   
               
               
                 10B 
                 nB:TnB:nA:TnA:sA 
                 nA:TnA:sA 
               
               
                 10C 
                 nC:TnC:nA:TnA:sA 
                 nA:TnA:sA 
               
               
                 10D 
                   
                 nB:TnB:nA:TnA:sA 
               
               
                   
                   
                 nC:TnC:nA:TnA:sA 
               
               
                   
                 nD:TnD:nB:TnB:nA:TnA:sA 
                   
               
               
                   
                 nD:TnD:nC:TnC:nA:TnA:sA 
                   
               
               
                 10E 
                   
                 nB:TnB:nA:TnA:sA 
               
               
                   
                   
                 nC:TnC:nA:TnA:sA 
               
               
                   
                 nE:TnE:nB:TnB:nA:TnA:sA 
                   
               
               
                   
                 nE:TnE:nC:TnC:nA:TnA:sA 
                   
               
               
                   
                   
                 nD:TnD:nB:TnB:nA:TnA:sA 
               
               
                   
                   
                 nD:TnD:nC:TnC:nA:TnA:sA 
               
               
                   
                   
                 nE:TnE:nB:TnB:nA:TnA:sA 
               
               
                   
                   
                 nE:TnE:nC:TnC:nA:TnA:sA 
               
               
                   
               
            
           
         
       
     
     Accordingly, data is communicated through the mesh network  130  until the signals are received by the surface system  110 . 
     Due to the provision of the self-powering device  11  of the sensor units  10 , data may be measured and transmitted to the surface without the need for expensive wires, and the sensor units  10  may operate for a much longer period of time compared to if batteries or other embedded power supplies are used. 
     By fluid or well fluid is meant any kind of fluid that may be present in oil or gas wells downhole, such as natural gas, oil, oil mud, crude oil, water etc. By gas is meant any kind of gas composition present in a well, completion, or open hole, and by oil is meant any kind of oil composition, such as crude oil, an oil-containing fluid etc. Gas, oil, and water fluids may thus all comprise other elements or substances than gas, oil, and/or water, respectively. 
     By an annular barrier is meant an annular barrier comprising a tubular metal part mounted as part of the well tubular metal structure and an expandable metal sleeve surrounding and connected to the tubular part defining an annular barrier space. 
     By a well tubular metal structure, casing or production casing is meant any kind of pipe, tubing, tubular, liner, string etc. used downhole in relation to oil or natural gas production. 
     In the event that the tool is not submergible all the way into the well tubular metal structure, a downhole tractor can be used to push the tool all the way into position in the well. The downhole tractor may have projectable arms having wheels, wherein the wheels contact the inner surface of the well tubular metal structure for propelling the tractor and the tool forward in the well tubular metal structure. A downhole tractor is any kind of driving tool capable of pushing or pulling tools in a well downhole, such as a Well Tractor®. 
     Although the invention has been described in the above in connection with preferred embodiments of the invention, it will be evident for a person skilled in the art that several modifications are conceivable without departing from the invention as defined by the following claims.