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[0001]     This application claims priority from United Kingdom Patent Application No. 0412927.6, filed Jun. 9, 2004.  
         [0002]     The present invention generally relates to apparatus and methods for transmitting information using a radio frequency transponder in a turbulent flow environment. More specifically, it relates to such apparatus and methods to carry information through a flow of well bore effluents.  
       BACKGROUND OF THE INVENTION  
       [0003]     Radio frequency identification (RFID) tags are found in an ever-increasing number of applications within a wide variety of different environments. Within the oilfield such tags can be used to identify equipment and/or transfer information along the well bore. The use of RFID tags in well bore flows poses a number of challenges not encountered in the field of inventory control and device tracking where such tags are commonly applied.  
         [0004]     The flow of effluents from the well bore is often turbulent and the effluents may consist of multiple phases such as hydrocarbons, water and gas. RFID tags are convected along in the flow of the produced fluids. At a surface location the tags are energized and read with an appropriate reader antenna. The lack of orientation of a device free to move within the flow, the temperature of the flow, and the metal pipe surrounding the flow are all factors that either alone or jointly prevent the use of most of the known RFID tags. For example, known tags often have simple antennas with a directional characteristics. In conventional inventory control systems it is possible to overcome this problem by controlling the orientation. For most of these known systems it can be assumed that the tag does not change its orientation during the read-out phase, which is typically in the order of 0.05 second at the low frequencies of interest. However as a tag in a turbulent flow will tumble, it may not couple to the field produced by the reader antenna. In a turbulent flow it is likely that the tag changes its orientation during the read-out phase thus losing contact with the reader. While it is possible to shorten the read times by using higher frequencies, such frequencies have a higher attenuation in conductive fluids and are therefore not a practical solution for many oilfield applications.  
         [0005]     A number of technologies have been proposed around the idea of sending some object or element up or down the borehole. A raw piece of semiconductor memory onto which data is written by a downhole device has been disclosed for example in GB Patent Application 1 549 307. A more sophisticated and robust vessel containing memory has been disclosed by GB Patent No. 2 352 041 and the co-pending U.S. patent application Ser. No. 10/049,749 assigned to Schlumberger Technology Corporation. Alternatively, even more complex vessels containing a variety of sensors and data storage have been disclosed. Examples of this solution can be found in GB Patent 2 352 042, co-pending U.S. patent application Ser. No. 10/030,587 assigned to Schlumberger Technology Corporation, and PCT Published Application WO 99/66172.  
         [0006]     U.S. Pat. No. 6,443,228 discloses the use of flowable devices in well bores to provide communication between surface and downhole instruments, among downhole devices, to establish a communication network in the well bore, to act as a sensor, and to act as power transfer devices. In some embodiments, the upward communication is proposed by writing information on the flowable devices downhole which are bound for the surface.  
         [0007]     Co-pending GB Patent Application No. 0229328.0 filed 14 Dec. 2002 (incorporated herein by reference) discloses a well control system enabling the control of various downhole well control functions by instructions from the surface. The well or downhole tool conveyance mechanism can be used without necessarily being equipped with electrical power and control cables extending from the surface and without the use of complex and inherently unreliable mechanical shifting or push/pull techniques requiring downhole movement controlled remotely from the surface. The invention of this co-pending application makes use of a downhole well control apparatus that is responsive to instructions from elements such as fluids or physical objects such as darts and balls that are embedded with tags for identification and for transmission of data or instructions. According to at least one disclosed embodiment, a downhole device may also write information to the element for return to the surface. In these disclosed embodiments, where information is being sent from a downhole location to the surface, information is written to the device (or acquired by the device itself) downhole.  
         [0008]     Co-pending published U.S. patent application Ser. No. 20040020643, filed 30 Jul. 2002, under obligation to assign to Schlumberger Technology Corporation (incorporated herein by reference) discloses a well control system enabling the control of various downhole well control functions by instructions from the surface without necessitating the well or downhole tool conveyance mechanism being equipped with electrical power and control cables extending from the surface and without the use of complex and inherently unreliable mechanical shifting or push/pull techniques requiring downhole movement controlled remotely from the surface. The invention of this co-pending application makes use of downhole well control apparatus that is responsive to instructions from elements such as fluids or physical objects such as darts and balls that are embedded with RF tags for identification and for transmission of data or instructions. According to at least one disclosed embodiment, a downhole device may also write information to the element for return to the surface.  
         [0009]     Co-pending GB Patent Application Serial No. 0229328.0, filed 14 Dec. 2002, discloses a downhole communication system with a plurality of releasable vessels positioned at downhole locations, the vessels containing signal information affixed to the vessels prior to placement of the vessels downhole, and the signal information indicating the presence of at least one of three or more predetermined downhole conditions. A detecting system is positioned on the surface such that the signal information can be detected on one or more of the vessels. A processing system is located on the surface and is programmed to establish the presence of the predetermined downhole condition based on the signal information. The vessels can carry RFID tags.  
         [0010]     In the light of the above prior art, it is seen as an object of the present invention to provide RF based communication devices with improved read-out capabilities for use in turbulent flows. It is seen as a further object of the invention to provide tags that are more stable during a read-out phase.  
       SUMMARY OF THE INVENTION  
       [0011]     According to a first aspect of the invention there is provided a system and vessels for communicating information from a downhole location in a hydrocarbon borehole to the surface with one or more releasable vessels positioned at the downhole location, the vessels carrying radio frequency (RF) activated circuitry and conveying signal information indicative of downhole conditions or parameters, and a RF reading system on the surface positioned and adapted to detect presence and/or the signal information of the one or more vessels, the vessels being adapted to maintain RF coupling with the reader in a fluid flow, particularly in a turbulent flow.  
         [0012]     In other words the invention provides means to counter or reduce the effect of a turbulent flow on the read-out of RFID tags.  
         [0013]     The fluid flow to which the vessels of the invention are best adapted to can be characterized as at least temporarily turbulent. Turbulent is understood to mean a flow beyond its laminar flow range. RF circuitry or tags are incorporated into the vessel, which in turn floats passively within the well bore liquid.  
         [0014]     The RF frequency range best suitable for use in conductive water and oil flow is within the lower range of radio frequencies. A preferred range is 40 to 500 kHz. A more preferred range is 80 to 300 kHz.  
         [0015]     The vessels, also referred to herein as passive autonomous devices, preferably encapsulate the tag in such a way as to allow its survival in the challenging downhole environment and facilitate the collection of data without interrupting the well bore flow.  
         [0016]     For an inductively coupled tag to be read, its antenna must be aligned with the magnetic field produced by the reader. As the device moves in the flow, it experiences torque due to turbulence. This force, acting in ever changing directions and magnitudes, will cause the object to spin randomly, therefore preventing the data being read from the enclosed tag. In order for reading to be possible, the spin must be controlled or, alternatively, the read-out must be less sensitive to the rotation of the tag.  
         [0017]     Therefore according to a further aspect of the invention, an RFID tag of the invention may include two, three or more antennas, each antenna generating a preferred spatial direction for coupling to a reader antenna different from the other antennas. Each of the antennas can be regarded as dipole with a respective emission and/or reception characteristic. By orienting the antennas and hence the dipoles in different spatial, preferably orthogonal directions, any rotation of the RFID tag or device is compensated for by a coupling of another antenna to the stationary receiver.  
         [0018]     In a preferred embodiment of this aspect of the invention, the antennas are connected by an electronic circuit designed to pass through to the RFID tag the signal from the antenna with the strongest coupling and, hence, best reception from the reader antenna.  
         [0019]     In a further preferred variant of this embodiment, the antennas are coupled by half bridge or full bridge rectifiers. The rectifiers can be assembled from passive elements such as diodes, however, an equivalent transistor circuit or more complex electronic circuitry may be used to couple the antennas at the expense of higher power consumption.  
         [0020]     According to another aspect of the invention, the carrier of the RFID tag is stabilized in the turbulent flow by establishing a preferred orientation of the device. This is preferably achieved using a carrier or device that assumes a preferred orientation in either the force field generated by the flow itself or by the gravity field.  
         [0021]     According to a preferred embodiment of this aspect of the invention the device has a geometry and weight distribution such that a force field like fluid drag or gravity generates a torque that rotates the device into its preferred orientation.  
         [0022]     A possible method of achieving this is to ensure the device has a non-uniform density creating a stabilizing torque. If the stabilizing torque is greater than the perturbations due to the turbulence, the orientation of the device can be stabilized in a preferred orientation. The orientation is chosen to optimize the coupling with a reader coil.  
         [0023]     A number of different configurations are feasible to achieve this effect, however, in a preferred variant the device is essentially spherical with the centre of mass offset from the geometrical centre of the sphere. The term essentially spherical includes elliptical, oval or similar shapes. However, it was found that the spherical shape is the least sensitive to turbulent effects.  
         [0024]     The device is preferably buoyant in the flow into which it is released. More preferable, it has a density of less than 1.1 g/ccm. Even more preferable, it has a density of 0.7 to 1.0 g/ccm. Hence, mean or average density of the device is preferably less than or equal to water. The device can be for example manufactured using fluid-filled capsules of heat and pressure resistant non-conductive material such as glass or epoxy loaded capsules with a small amount of high-density material at an off-centre position.  
         [0025]     To withstand the pressure in the well while maintaining sufficient buoyancy, the device preferably includes sections or parts of a castable hardening material, such as epoxy resin, loaded or filled with hollow particles, such as glass micro-bubbles.  
         [0026]     According to a further aspect of the invention, the reader antenna is located inside a metal conveyance system. In the case of hydrocarbon exploration and production, the metal conveyance system includes sections of steel pipe at the entry of the well. The antenna preferably includes a solenoid placed at the inner wall of the pipe but isolated from it by a layer of ferrite material.  
         [0027]     These and other aspects of the invention will be apparent from the following detailed description of non-limitative examples and drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  shows a subterranean well bore completion system with a reservoir of RFID tags released in intervals to carry information to the surface;  
         [0029]      FIG. 2A  is a diagram of a reader antenna circuit for energizing and read-out of RFID tags;  
         [0030]      FIG. 2B  is a horizontal cut through a schematic perspective view of a surface pipe section including a reader antenna in accordance with an aspect of the invention;  
         [0031]     FIGS.  3 A,B show circuit diagrams of the antenna of tags in accordance with an example of the invention;  
         [0032]      FIG. 4  compares the signal strength of a conventional tag with a tag using a design in accordance with the invention;  
         [0033]      FIG. 5  illustrates the improvements in data recovery probability afforded by tags using a design in accordance with the invention;  
         [0034]     FIGS.  6 A-C are variants of devices or vessels for RFID tags in accordance with examples of the invention; and  
         [0035]     FIGS.  7 A,B compare the orientations of tags in a turbulent flow.  
     
    
     DETAILED DESCRIPTION  
     Examples  
       [0036]     In  FIG. 1  there is shown a system for communicating information from downhole to the surface. The system generally comprises one or more downhole sensors and associated release mechanisms  11 , floating passive autonomous devices or vessels  12 , and surface detection system  13 . There are four sensor/release mechanisms  11  positioned in the lower end of well  10 . Well  10  is producing hydrocarbons from reservoir region  141  in the earth  14 . The vessels  12  are constructed to have a high probability of surviving downhole pressures and temperatures, and will be carried to the surface by the flowing liquids in the well  10 . Surface detection system  13  detects and/or recovers vessels and interprets the signals conveyed by vessels  12 .  
         [0037]     Reader circuit  20  of surface detection system  13  is shown in greater detail in  FIG. 2A . The reader circuit includes reader coil  21  of 170 mm length and 90 mm diameter. It is wound with approximately 440 turns of 0.3 mm insulated copper wire mounted onto a flow pipe  131  in a manner shown in  FIG. 2B  below. The coil has an inductance of 7.8 mH and a resistance of 27.8 Ohms. To allow for fine adjustment of the resonance frequency, parallel network  22  of fixed capacitor (150 pF) and adjustable capacitor (15-65 pF) is used.  
         [0038]     The coil and capacitor have a calculated Q of 218, which results in a bandwidth of 568 Hz. To dampen the resonance and thus increase the bandwidth, resistor  23  (510 Ohms) is added in series with coil  21  and capacitor network  22 . This reduces Q to 11.3 and increases the bandwidth to 10 kHz. Damping resistor  23  is also used to sense the current flow through coil  21 . Coil  21  is driven by signal generator  24  and amplifier  25 . The tuning of the coil to a specific frequency is best done in-situ on the flow from the well, as the resonance shifts with the fluid in the flow.  
         [0039]     A demodulation circuit includes diode detector  26  to half-wave rectify the signal and remove the carrier signal as generated by signal generator  24  and amplifier  25 . Diode detector  26  includes a RC network acting as a low pass filter and DC component blocking circuit. The output of diode detector  26  is fed into low path filter and amplifier  27 . The low path filter has a corner frequency of 20 kHz. The filtered and amplified signal is then fed into comparator  28  the level of which was set just above background noise. The comparator lifts the signal to TTL levels via a Schmitt gate.  
         [0040]     The TTL level data stream is fed into processor board  29  carrying processor and memory together with I/O ports to decode the signal and covert it into machine-readable data. Processor board  29  also controls signal generator  24 .  
         [0041]     An arrangement to house coil  21  inside a metal section of surface pipe  131  is shown in  FIG. 2B .  
         [0042]     It is beneficial to isolate the reader coil from the enclosing pipe to prevent the occurrence of large eddy currents. Thus, part of inner surface  133  of the pipe is sheathed with layer  134  of ferrite. The ferrite material may be chosen from commercially available material, such as manganese-zinc or nickel-zinc. Sheath  134  of ferrite material separates coil  21  from metal pipe section  131  thereby reducing the eddy current losses and radiation emission. In the example shown, concentrically mounted inner pipe  135  protects the coil from being damaged by abrasive material carried along with the fluid flow. Inner pipe  135  is made of reinforced polyetheretherketone (PEEK). Two connectors  136  provide an electrically conductive path to the exterior of pipe  131 . Pipe section  131  terminates in flanges  132 .  
         [0043]     Instead of using a ferrite sheath, pipe section  131  can be made of high-strength, non-conductive materials, in which case reader coil  21  is wound around the exterior of the pipe. Suitable materials for such non-conductive pipe include ceramics and composites like reinforced PEEK.  
         [0044]     In operation, coil  21  generates an axial magnetic field. Magnetic fields in diametrical direction, if required, may be generated using an extra layer of conductors (not shown) running in axial direction along the inside of ferrite sheath  134 . The magnetic field is to a large extent confined to the interior of pipe  131 . Ferrite layer  134  reduces losses of energy in the wall of steel pipe  131  through eddy currents or other residual losses.  
         [0045]     Whilst the above configuration improves the reader antenna, it is another aspect of the invention to enhance the read-out by improving the vessel to be less sensitive to the flow turbulence.  
         [0046]     In  FIG. 3  there are shown two different antenna configurations to improve the efficiency of the coupling of a tag to the reader coil.  
         [0047]     Two mutually orthogonally oriented antennas  311 ,  312  are shown in  FIG. 3A  connected by a rectifier in a half-bridge configuration  313 . In  FIG. 3B , full-bridge  323  replaces half-bridge  313  of  FIG. 3A . The diodes used in both configurations have a low forward voltage drop, e.g., Schottky diodes. The full-bridge configuration has a higher component count.  
         [0048]     Using the antenna configuration of  FIG. 3A , i.e., half-bridge rectifier  313  to couple the two orthogonal antennas  311 ,  312 , the peak output voltages varies by less than 30 percent as the tag is rotated in a (constant) magnetic field. In  FIG. 4  this effect is demonstrated with lower curve  41  indicating the peak output voltage from a single coil device. Upper curve  42  indicates the output from the two-coil device.  
         [0049]     A Monte Carlo simulation illustrated by  FIG. 5  can evaluate the effectiveness of the dual antenna configuration shown in  FIG. 3  and a treble antenna configuration, which is not shown in  FIG. 3 . However, the treble antenna configuration can be regarded as an extension of the dual antenna configuration with a third antenna mounted with an orientation orthogonal to those of the other two antennas.  
         [0050]     Assuming that the antenna is spinning around an axis that is randomly orientated with respect to both the exciting field and one of the antennas, the probability can be estimated for a tag being read as a function of the exciting field. This field is measured in units of the field necessary to read the tag when it&#39;s most favorably orientated. The single antenna (curve  51  of  FIG. 5 ) has a low probability, remaining below 20 percent even at high field strengths. The dual antenna (curve  52  of  FIG. 5 ) is not one hundred percent effective because there is a chance that it will spin around an axis that is normal to its plane and also along the field. The treble antenna (curve  53 ) is easily one hundred percent effective once the necessary field strength is reached.  
         [0051]     The read-out efficiency of the devices of the present invention in a turbulent flow environment can be further improved by a strong asymmetry that generates a torque on the device itself when placed in a force field.  
         [0052]     It is feasible to create such an asymmetry through the outer shape of the device by, for example, stabilizers such as fins and tails. However, for application in well bores and wellbore pipes, deviation from an approximately spherical shape may cause extended devices to get stuck. The invention therefore contemplates a way of rendering a spherical shape such that a preferred orientation in a turbulent flow is accomplished.  
         [0053]     In  FIG. 6A  there is shown the cross-section of hollow (sacrificial) polypropylene shell  61  acting as the carrier of RFID tag  62 . The tag is embedded in matrix  63  of low-density epoxy resin having a density of 0.62 g/ccm and being rated for temperatures up to 180 degrees Celsius. A second part  64  of sphere  61  is filled with a much denser material. This denser material is preferably electrically non-conductive to not interfere with the electromagnetic field of the reader. A suitable material is Barium sulphate powder with a bulk density of 4.5 g/ccm. As compacted powder, the density drops but remains above 2 g/ccm. The two parts of the sphere are glued together using the setting reaction of the epoxy filling.  
         [0054]     The thus weighted sphere has an average density of 0.8 g/ccm thus being slightly buoyant in a mixture of oil and water. It has a torque that can be calculated by integration of the density distribution times the distance from the center of the sphere. The stabilizing torque acts to turn the sphere such that the cap  64  of barite is located at the bottom.  
         [0055]     The above example of  FIG. 6A  can be further refined to increase the torque. One way of increasing the torque is to increase the difference in density of cap material  64  and of matrix or body material  63 . In the device of  FIG. 6B , the body material includes two layers  65 ,  66  of powder filling. The layer  65  is a low-density epoxy powder with the trade name Expancel supplied by Boud Minerals &amp; Polymers of Marsden, Kent, England. The powder has a very low density of 0.03 g/ccm. The epoxy material used in the example of  FIG. 6A  above serves as intermediate layer  66 . Tag  62  and barite cap  64  remain unchanged from the example of  FIG. 6A .  
         [0056]     The above examples of  FIGS. 6A and 6B  have a shell  61  and an epoxy filling that may collapse under higher pressure. In an improved variant of the invention, as illustrated by  FIG. 6C , the shell is replaced by a molded sphere.  
         [0057]     In the example of  FIG. 6C , the sphere comprises a first section  67  made of Bisphenol-A based epoxy using 50 micron diameter hollow glass beads  671  (commercially available from 3M™) as filler material. Tag  62  and barite cap  64  remain unchanged from the example of  FIG. 6A .  
         [0058]     The barite may be replaced by higher density materials such as tungsten oxide (density 7.1 g/ccm).  
         [0059]     The devices of  FIG. 6  have an improved read-out probability in a turbulent flow. In  FIG. 7 , a comparison is illustrated of the vertical projection of the tag inside a sphere in a turbulent vertical flow. The vertical projection can be regarded as a measure indicating a possible read-out. It is conservatively estimated that a read-out is possible when the tag is inclined less than 30 degrees from the vertical axis, equivalent to a value of approximately 0.87 in the plots of  FIG. 7 . This threshold is shown as horizontal line  71  of marks. Scattered marks  72  show the orientation of the device as measured.  
         [0060]     In the example of  FIG. 7A , the spherical device has a homogeneous weight distribution and hence a low torque. The orientation is found to be scattered between projection values of 0.3 to 0.65. The vertical projection never rises above the level required to read a tag reliably.  
         [0061]     In the example of  FIG. 7B , the sphere has an asymmetric weight distribution as described above. The torque caused by the gravity force field is sufficient to increase the vertical projection values  72  above threshold level  71  under most circumstances.  
         [0062]     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Summary:
A system for communicating information, particular for use in a hydrocarbon well, is described including one or more releasable vessels having radio frequency (RF) circuitry responsive, when in use, to an RF reader system, the vessel including counteracting means to reduce the effect of destabilizing flow forces thereby extending the coupling time with the reader system.