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FIELD OF THE INVENTION 
     The present invention relates to remote actuation of a downhole tool. In particular, the invention utilises RFID technology to communicate data and operating instructions to/from static readers coupled to a downhole tool such as a valve or sliding sleeve. 
     BACKGROUND OF THE INVENTION 
     During downhole drilling operations, mud and drilling fluids are circulated within the wellbore by being pumped down through the drill string and returning to the surface via the borehole annulus. Drill cuttings produced during drilling are carried up to the surface through the annulus by the drilling mud. However, in extended reach wells and/or highly deviated or slim diameter wells, the pressure of the drilling mud along the circulation path can drop from that at the surface, which results in a lower cutting lifting performance which in turn can lead to restrictions/obstructions arising in the annulus caused by accumulating cuttings. 
     In order to alleviate this problem, it is conventional to include one or more downhole circulating subs in the drill string which allow fluid circulation rates to be varied by selectively opening a path from the interior of the drill string to the annulus. Ports in the circulating subs can be opened and closed to enable the flow path of drilling fluids to take a different course, thereby altering the circulation time. 
     Conventional circulating subs typically comprise a ball seat and, in the event of a restriction in the circulation path at a location in the annulus above that of the circulating sub, a ball, of greater diameter than the seat at its narrowest point, is dropped or pumped through the drill string such that it lands on the ball seat. Once in position, the area above the ball and ball seat becomes sufficiently pressurised to move the ball seat downwards thereby uncovering the ports which enables the drilling fluids to flow through ports in the sidewall of the circulating sub and string into the annulus. 
     Typically, a series of circulating subs is provided within the drill string at vertically spaced apart points. In view of the method of operation of the ball seats, vertically higher ball seats necessarily have a greater inner diameter than vertically lower ball seats allowing smaller balls destined for the lower seats to bypass higher circulating subs when dropped downhole. Due to the progressively narrower inner diameter required towards the bottom of the casing, a drill string can usually only accommodate a maximum of six such circulating subs. 
     The aim of the present invention is to provide an improved circulation sub and an improved method of actuating downhole tools which alleviates problems associated with the prior art described hereinbefore and also provides a means of sending instructions and/or data from/to downhole tools. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided apparatus for operating a downhole tool located in a conduit for the passage of fluid therethrough, the apparatus comprising:—
         at least one reader associated with the conduit, wherein the at least one reader is arranged to read data and wherein the at least one reader is also arranged for the passage of fluid therethrough;   a downhole tool coupled to the at least one reader; and at least one tag moveable through at least a portion of the conduit and the reader wherein the or each tag is capable of containing data;   such that the reader is capable of reading data from the tag when the tag passes through the reader, thereby enabling remote actuation of the tool.       

     The inner diameter of the reader can be similar to the inner diameter of the conduit such that the reader does not cause a restriction in the conduit. 
     The conduit can comprise any downhole tubing string such as a drill string. One example of the downhole tool may be any valve such as a sliding sleeve. “Sliding sleeve” as used herein is intended to refer to any device that can be operated to selectively provide and prevent a flow path between the drill string and the annulus. Sliding sleeves incorporate one or more ports that can be opened or closed by a sliding component and can be used as a circulation sub. 
     Preferably, the reader can also transmit data and information to the tag regarding operating conditions of the tool or the external environment. 
     The at least one tag is preferably added to fluid circulating through the conduit. The tag may be recoverable after use in the conduit. 
     Two or more readers and respective coupled tools can be provided, the readers being individually identifiable or selectable, wherein the tags may be selectively coded with data, such that data from each tag is capable of being received by an individual reader. Therefore, the apparatus may preferably comprise several readers coupled to respective downhole tools and a plurality of tags, with certain tags encoded with data which may be read only by a particular reader with a unique identity for operation of a specific tool. 
     According to a second aspect of the present invention there is provided a method for operating a downhole tool comprising the steps of: 
     a) providing a conduit for the passage of fluid therethrough, the conduit comprising at least one reader also arranged for the passage of fluid therethrough, wherein the at least one reader can read data; 
     b) coupling a downhole tool to the or each reader; 
     c) providing at least one tag wherein the or each tag is capable of containing data; and 
     d) moving the or each tag within the conduit and at least partially through the reader such that the reader is capable of reading data from the tag, when the tag passes through the reader, enabling remote operation of the tool. 
     The method typically comprises the step of running the downhole conduit into a borehole in between steps b) and c) or c) and d) 
     The method may further comprise the step of matching the inner diameter of the reader and conduit such that the inner diameter of the conduit is not restricted by the reader. 
     The tool coupled to a reader may be any valve such as a sliding sleeve. The conduit can be a drill string. The reader may also be arranged to transmit data. 
     Fluid may be circulated through the conduit and the at least one reader. Tags can be added to the circulating fluid. The method may comprise the additional step of recovering the tag after use. 
     Several readers may be arranged in series. The readers may have portions of conduit therebetween. The method may further comprise the step of providing each reader with a unique identity and selectively coding each tag such that a particular tag is arranged to communicate with a reader having a particular identity. In this way it is possible to target specific tools and send different operating instructions to each tool. 
     According to a third aspect of the present invention there is provided an antenna for use in a downhole tubular, the antenna comprising:
         a generally cylindrical housing;   and a coiled conductor located within a portion of the housing and being separated from the portion of the housing by insulating material,   wherein the portion of the housing has a greater internal diameter than the external diameter of the coiled conductor.       

     At least one antenna can be provided for arrangement in a tubular. 
     The insulating material can be any suitable non-conducting material, such as air, glass fibre, rubber or ceramic. The antenna may further comprise a liner, wherein the coiled conductor is located or wrapped around the liner, preferably in a helical coaxial manner. Preferably, the housing and liner form a seal around the coiled conductor and insulating material. The housing can be made of steel. Preferably the liner should be non-magnetic and non-conductive to prevent eddy currents. Since the antenna is provided for use downhole, all components comprising the antenna are preferably capable of withstanding the high temperatures and pressures experienced downhole. 
     The antenna may operate in the frequency range 50 to 200 Khz. The optimum frequency band for the downhole work is 100 to 200 Khz. The most preferable frequency operating band is 125 to 134 Khz. The antenna should be of sufficient length to charge and read the RFID tag while passing through the antenna, allowing all data to be transferred. Preferably the length of the antenna is less than 10 m. 
     The antenna according to the third aspect of the invention can be used as the reader for the apparatus and method according to the first and second aspects of the invention. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments of the invention will be described with reference to and as shown in the accompanying drawings in which:— 
         FIG. 1  is a sectional view of a borehole with drill string inserted therein, the drill string having attached apparatus according to the present invention; 
         FIG. 2  shows a sectional view of circulation sub apparatus in accordance with the present invention; 
         FIG. 3  is a top sectional view of the circulation sub of  FIG. 2 ; 
         FIG. 4  is a perspective view of liner and coiled conductor required for construction of an antenna according to the present invention; and 
         FIG. 5  is a sectional view through the antenna of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a borehole  10  lined in the upper region with a casing  12 . A drill string  14  made up of lengths of drill pipe  26  is provided within the borehole  10 . A drill bit  16  attached to the lower end of the drill string  14  is acting to drill the borehole  10  to thereby extend the borehole  10 . The drill string  14  shown in  FIG. 1  has four circulation subs  18   a ,  18   b ,  18   c  and  18   d  provided therein with drill pipe  26  therebetween. It should be noted that  FIG. 1  is not to scale and that there may be many lengths of drill pipe  26  provided in between each of the circulating subs  18 . The drill pipe  26  and circulation subs  18  are joined by conventional threaded torque pin and box connections. Each circulation sub  18  shown in  FIG. 1  comprises a sliding sleeve valve  20 , a port  22  and an antenna  24 . 
       FIG. 2  shows a more detailed sectional view of the circulation sub  18 . The circulation sub  18  has three main sections; a top sub  36 , hydraulic housing  58  and bottom sub  66 . 
     Towards the upper (in use) end of the circulation sub  18  there is provided the top sub  36  in which the antenna  24  is located where the antenna is typically in the region of 10 meters or less in length. As shown in the perspective view of  FIG. 4  and sectional view of  FIG. 5 , the antenna  24  comprises an inner liner  38  located in an enlarged bore portion of the top sub  36 , where the liner  38  is formed from a non-magnetic and non-conductive material such as fibreglass, moulded rubber or the like, having a bore  96  extending longitudinally therethrough. The inner bore  96  is preferably no narrower than the inner bore of the drill string  14 . A coiled conductor (not shown) typically formed of, for example, a length of copper wire is concentrically wound around the liner  38  within grooves  94  in a helical coaxial manner. Referring again to  FIG. 2 , insulating material  40  formed from fibreglass, rubber or the like separates the coiled conductor  94  from the recessed bore of the top sub  36  in the radial direction. The antenna  24  is formed such that the insulating material  40  and coiled conductor are sealed from the outer environment and the inner throughbore by the inner liner  38  and the inner bore of the recess of the top sub  36 . 
     The top sub  36  is joined to the hydraulic housing  58  via a pin and box threaded torque connection  42 . O-ring seals  44  are also provided to create a fluid tight seal for the connection  42 . 
     Within the hydraulic housing  58 , a bulkhead  32  is positioned between outlet ports  70 ,  71 . The outlet ports  70 ,  71  are ports for a hydraulic pump  46  which lies adjacent a gearbox  48 . A motor  50  is connected to an electronics pack  52 , both of which are powered by a battery pack  54 . 
     The lower end of the hydraulic housing  58  is connected to a bottom sub  66  which has ports  22  extending through its side wall such that the throughbore of the bottom sub  66  can be in fluid communication with the annulus  28  (shown in  FIG. 1 ) when the ports  22  are uncovered by the sliding sleeve  20 . The bottom sub  66  is attached to the hydraulic housing  58  in the usual manner, by threaded connection  42  which are sealed with an O-ring  44 . The sliding sleeve  20  is shown in a first position in  FIG. 2  covering ports  22 . 
     The inner diameter of the bottom sub  66  is stepped inwardly to create a shoulder  68  against which a piston  60  abuts in the first position when the fluid channel provided by the ports  22  between the throughbore of the bottom sub  66  and the annulus  28  is closed. The piston  60  can also occupy a second position in which the piston  60  abuts a shoulder  56  provided towards the lower end of hydraulic housing  58 .  FIG. 2  shows the piston  60  occupying the first position with the piston  60  in abutment with the shoulder  68  thereby creating a piston chamber  62 . The piston chamber  62  is bordered by the sliding sleeve  20 , piston  60 , a portion of the hydraulic housing  58  and the shoulder  56 . Piston seals  64 U and  64 M are used to create a fluid tight seal for the chamber  62 . 
       FIG. 3  is a top view of a portion of the hydraulic housing  58  of the circulation sub  18 . Connecting lines  78  connect the first pump outlet port  70  with a first hydraulic line  72  and the second pump outlet port  71  with a second hydraulic line  73 . At one end, the hydraulic lines  72 ,  73 ,  78  are sealed by plugs  88 . The other ends of the first and second hydraulic lines  72 ,  73  are provided with a first chamber opening  76  and a second chamber opening  74  respectively. The openings  74 ,  76  are arranged such that they are always located within the piston seals  64 U,  64 L. 
     The hydraulic line  72  is in fluid communication with a floating piston  80  having a screw plug  82  at one end thereof. 
     RFID tags (not shown) for use in conjunction with the apparatus described above can be those produced by Texas Instruments such as a 32 mm glass transponder with the model number RI-TRP-WRZB-20 and suitably modified for application downhole. The tags should be hermetically sealed and capable of withstanding high temperatures and pressures. Glass or ceramic tags are preferable and should be able to withstand 20 000 psi (138 MPa). Oil filled tags are also well suited to use downhole, as they have a good collapse rating. 
     In operation, a drill string  14  as shown in  FIG. 1  is positioned downhole. The drill bit  16  suspended on the end of drill string  14  is rotated to extend the borehole  10 . Nozzles (not shown) provided on the drill bit  16  expel fluid/mud at high velocity. The drilling fluid/mud is used for bit lubrication and cooling and is also circulated up the annulus created between the outside of the drill string  14  and the inner surface of the borehole to retrieve cuttings from the bottom of the borehole  10 . If higher circulation rates are desired, ports  22  can be opened to create a path between the throughbore of the drill string  14  and the annulus  28  at the location of the respective ports  22 . This can be achieved using the method and apparatus of the present invention, as described below. 
     Initially, the ports  22  are closed as they are covered by the sliding sleeve  20 , shown in  FIG. 1  and in greater detail in  FIG. 2 . 
     An RFID tag (not shown) is programmed at the surface by an operator to generate a unique signal in a frequency range which is preferably 125-134 Hz. Similarly, each of the electronics packs  52  coupled to the respective antenna  24 , prior to being included in the drill string  14  at the surface, is separately programmed to respond to a specific signal within the preferred frequency range 125-134 Hertz. The RFID tag comprises a miniature electronic circuit having a transceiver chip arranged to receive and store information and a small antenna within the hermetically sealed casing surrounding the tag. 
     The pre-programmed RFID tag is then weighted, if required, and dropped or flushed into the well with the drilling fluid. After travelling through the inner bore of the drill string  14 , the selectively coded RFID tag reaches the specific circulation sub  18  the operator wishes to actuate and passes through the inner liner  38  thereof. During passage of the RFID tag (not shown) through the top sub  36  in the upper end of the circulation sub  18 , the antenna  24  housed therein is of sufficient length to charge and read data from the tag. The tag then transmits certain radio frequency signals, enabling it to communicate with the antenna  24 . The data transmitted by the tag is received by the adjacent receiver antenna  24 . This data is processed by electronics pack  52 . 
     As an example the RFID tag in the present embodiment has been programmed at the surface by the operator to transmit information instructing that a particular sliding sleeve  20  (such as that of the second from bottom circulating sub  18   c ) is moved into the open position. The electronics pack  52  processes the data received by the antenna  24  as described above and recognises a flag in the data which corresponds to an actuation instruction data code stored in the electronics pack  52 . The electronics pack  52  then instructs motor  50 , powered by battery pack  54 , to drive the hydraulic pump  46  of that circulating sub  18   c . Hydraulic fluid is then pumped out of pump outlet  70 , through connecting line  78  and hydraulic line  72  and out of chamber opening  76  to cause the space between piston seals  64 M and  64 L to fill with fluid thereby creating a new hydraulic fluid containing chamber (not shown). The volume of hydraulic fluid in first chamber  62  decreases as the piston  60  is moved towards the shoulder  56 . Fluid exits the chamber  62  via chamber opening  74 , along hydraulic line  73  and is returned to a hydraulic fluid reservoir (not shown). When this process is complete the piston  60  abuts the shoulder  56 . This action therefore results in the sliding sleeve  20  moving towards the hydraulic housing  58  of the circulation sub  18  to uncover port  22  and opens a path from the interior of the drill string  14  to the annulus  26 . 
     Therefore, in order to actuate a specific tool, for example sliding sleeve  20   b , a tag programmed with a specific frequency is sent downhole. Sliding sleeve  20   b  is part of circulating sub  18   b  and is coupled to an antenna  24  responsive to the specific frequency of the tag. In this way tags can be used to selectively target certain tools by pre-programming readers to respond to certain frequencies and programming the tags with these frequencies. As a result several different tags may be provided to target different tools. 
     Several tags programmed with the same operating instructions can be added to the well, so that at least one of the tags will reach the desired antenna  24  enabling operating instructions to be transmitted. Once the data is transferred the other RFID tags encoded with similar data can be ignored by the antenna  24 . 
     The tags may also be designed to carry data transmitted from antennas  24 , enabling them to be re-coded during passage through the borehole  10 . In particular, useful data such as temperature, pressure, flow rate and any other operating conditions of the tool etc can be transferred to the tag. The antenna  24  can emit a radio frequency signal in response to the RF signal it receives. This can re-code the tag with information sent from the antenna  24 . The tag is typically recoverable from the cuttings lifted up the annulus from the borehole  10 . 
     Modifications and improvements may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example the sliding sleeve can be replaced by other types of movable tools that require remote actuation. In this case the tools may be operable directly by electrical power from the battery  54 , rather than by hydraulic actuation.

Summary:
A method and apparatus for operating a downhole tool is disclosed. The method involves providing a conduit for the passage of fluid. The conduit includes at least one reader which can read data and is arranged for the passage of fluid. A downhole tool is coupled to the reader and has at least one tag capable of containing data. The tag is moved within the conduit and at least partially through the reader such that the reader can read data from the tag when the tag passes. This enables remote operation of the downhole tool. An antenna is provided for use in a downhole tubular. The antenna comprises a generally cylindrical housing and a coiled conductor located within a portion of the housing and separated therefrom by insulating material. The portion of housing has a greater internal diameter than an external diameter of the coiled conductor.