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This patent application claims an international filing date of 16 Jan. 2003 and a priority date of 18 Jan. 2002. 
   CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to downhole tools for use in the oil and gas industry and in particular although not exclusively to a disengagable downhole tool which allows a tool to be disengaged from a work string in a well bore and later retrieved on the work string when the work string is removed from the well bore. 
   In drilling or completing a well, each stage requires a work string to be made-up which includes any tools required within the well bore. Typically once made-up the work string is inserted in to the well until the tool reaches the desired location, the job is undertaken and then the work string including the tool is returned to the surface. As a number of jobs are required in a well in order to drill and complete the well, this can require a great number of trips into the well by a work string. Each time the string is retrieved, made up and reinserted time is lost which increases the cost involved in drilling and completing the well. 
   It has long been known to combine tools on a single work string. However, some tools such as the drill bit can only be located at a single position on the work string e.g. at the base. 
   An example where a single tool is required to be placed on a work string is in the milling of a polished bore receptacle. A polished bore receptacle is typically positioned at the top of a production liner in conjunction with the liner hanger disposed in a well bore. The polished bore receptacle typically has a long polished bore, which slideably and sealingly receives a sealing assembly on the end of a tubing string. Due to its function of requiring a surface against which a seal can be made, the polished bore receptacle which is inserted into the well is generally milled and dressed to provide an ideal surface finish. Such milling and dressing of the receptacle and in particular the top portion of the liner requires a single trip into the well with a suitable milling assembly. 
   Where multiple tools can be mounted on a single work string, difficulties can arise in the need to co-ordinate the activities of each of the tools independently from each other, particularly, if one tool is required to work before or after the operation of another tool. 
   Further difficulty arises when a tool, in order to operate, must come into contact with a portion of the well bore lining or casing. In these circumstances, the tool must be capable of being retracted or moved away from the well bore lining or casing so that the work string can be repositioned without the tool making unwanted contact to other parts of the well bore. 
   An example of a retractable tool is that disclosed in U.S. Pat. No. 2,346,629. This tool is mounted on a work string and operates by the application of fluid through the work string. Fluid pressure causes cleaning members in the form of brushes or scrapers to be expanded radially outwards to contact the walls of the well casing or liner. The work string can be rotated so that the brushes or scrapers clean the walls of the casing or liner. When cleaning is complete a change in fluid pressure causes the cleaning members to be retracted back into the work string so that the work string may be lowered further into the well or be retrieved from the well without the cleaning members making any further contact with the casing or liner. A disadvantage of such systems is that they require changes in fluid pressure from the surface and due to the mechanical components used to assist in the expansion and retraction of the cleaning members they can be prone to failure in hostile environments. 
   It is an object of at least one embodiment of the present invention to provide a tool located on a work string, which when it has completed its function in a well bore can be disengaged from the work string such that the work string be run further into the well bore and when retrieved ‘pick-up’ the tool and remove it from the well bore. 
   It is a further object of at least one embodiment of the present invention to provide a tool for insertion in a work string including at least one further tool, which when the string is inserted into the well bore can mill a polished bore receptacle, remain at the polished bore receptacle while the one or more further tools perform their function(s) below the polished bore receptacle and is retrieved when the work string is retrieved from the well. 
   It is a yet further object of at least one embodiment of the present invention to provide a tool for insertion in a work string which includes a safety feature such that a portion of the tool will disengage only when the tool has reached a desired location in the well bore. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a downhole tool for location on a work string, the tool including an assembly operable in a well bore via the work string, wherein the assembly is disengagable from the work string at a selected location in the well bore, and wherein the tool further includes retrieval means to pick up the assembly on retrieval of the work string from the well bore. 
   When disengaged the work string can move freely through the tool so that functions can be performed by other tools on the work string. 
   Preferably the tool comprises a substantially tubular body upon which is located the assembly. 
   Preferably the assembly is a sleeve positioned on an outer surface of the tool. More preferably the assembly is a milling sleeve. Advantageously, the assembly is a milling sleeve suitable for milling a polished bore receptacle in a well bore. Preferably, also the sleeve includes one or more longitudinally arranged milling ribs to dress an internal diameter of the top of the polished bore receptacle. Further the sleeve may include an additional milling portion, scrapers or brushes on an outer surface. Advantageously, the sleeve has a length equal to the length of the polished bore receptacle. 
   Preferably, the assembly is operated from the work string by a hex-drive system. 
   Preferably the body has a portion of an outer surface having a plurality of longitudinally extending planar sections arranged around a circumference of the body. 
   Preferably the assembly includes an inner surface, a portion of which has a plurality of longitudinally extending sections matching those of the body, such that when the body is rotated by virtue of the work string being rotated, the assembly is rotated also. 
   Preferably the assembly includes a shoulder on the inner surface thereof, the shoulder providing a ledge upon which a portion of the body engages when the tool is retrieved from the well bore. Advantageously, a portion of the body is that portion provided as a ledge by the plurality of longitudinally extending planar sections. 
   Preferably the assembly is detachably coupled to the body. More preferably, the detachable coupling is by one or more shear pins. 
   Preferably the assembly has an outer shoulder, the outer shoulder contacting a formation in the well bore to cause the shear pins to shear and decouple the assembly from the body thereby disengaging the assembly from the work string. 
   More preferably, the shear pins are arranged so that they take no stress on operation of the assembly from the work string. Advantageously the pins include a constricted portion positioned at a plane between the assembly and the body so that no stress is exacted on the pins when the body is rotated, rotating the assembly with it. 
   Preferably also the body and the assembly include means for retaining sheared parts of the sheared pins to prevent them from dispersing into the well bore. 
   Preferably the means for retaining sheared parts of the shear pins is by pockets located in the body and the assembly. 
   Advantageously the tool includes a safety mechanism to prevent premature decoupling of the assembly prior to the assembly reaching a selected location in the well bore. 
   Preferably a safety mechanism operates when the assembly reaches a selected formation at the selected location for the assembly to operate. 
   Preferably the safety mechanism comprises a button mounted in a first position to lock the assembly to the tool body, the button having a face engageable with the selected formation, whereupon engagement with the selected formation moves the button from the first position to a second position, disengaging the lock and wherein the selected formation maintains the button in the second position while the selected formation contacts the assembly thereby disengaging the assembly from the work string. 
   According to a second aspect of the present invention there is provided a method of running a work string in a well bore to operate more than one tool on a single trip, the method comprising the steps:
     a) locating a first tool, including an assembly, operable in a well bore on the work string, the work string including one or more further tools located below the assembly,   b) running the work string into the well bore until the assembly reaches a selected location and at this location operating the first tool via the work string;   c) disengaging the assembly from the work string at the selected location;   d) passing the work string beyond the assembly until the one or more further tools have reached desired locations and performed their functions;   e) removing the work string from the well bore; and   f) picking up the assembly on the work string as the work string is retrieved.   

   Preferably, the assembly is a milling assembly for milling and dressing a polished bore receptacle in the well bore. 
   Preferably also the assembly is disengaged from the work string by contacting the assembly with a formation in the well bore and setting down weight on the work string. 
   Preferably also the assembly is picked up by the work string by contacting a ledge on the work string with a shoulder on the assembly. 
   According to a third aspect of the present invention, there is provided a method of milling a polished bore receptacle in a well bore on the same trip as other functions are performed in the well bore, the method comprising the steps:
     a) mounting a milling assembly in the form of a sleeve including one or more milling elements onto a body in a work string;   b) connecting a drive between the assembly and the body and coupling the assembly to the body;   c) running the work string in the well bore until the milling assembly reaches the polished bore receptacle;   d) rotating the work string and thereby through the drive rotating the milling assembly to mill and dress the polished bore receptacle;   e) resting a portion of the assembly on the top of the polished bore receptacle and setting down weight on the work string to disengage the coupling between the assembly and the body;   f) running the work string further into the well bore and operating one or more further tools from the work string;   g) retrieving the work string from the well bore and engaging a portion of the body to a shoulder on the assembly so that the assembly is picked up by the work string and retrieved from the well bore.   

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example only with reference to the following figures in which: 
       FIG. 1  shows a part cross-sectional view of a downhole tool in accordance with a first embodiment of the present invention; 
       FIG. 2  shows a cross-sectional along section line A-A of the tool of  FIG. 1 ; 
       FIG. 3  shows an enlarged portion of the downhole tool of  FIG. 1  illustrating the detachable coupling arrangement; and 
       FIG. 4  shows a portion of a downhole tool including a safety mechanism according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference is initially made to  FIG. 1  of the drawings which illustrates a downhole tool, indicated by reference numeral  10 , in accordance with a first embodiment of the present invention. Tool  10  comprises an annular body  12  having an axial bore  14  there through. At an upper end  16  of the body  12  there is a box section  18  allowing connection of the body  12  onto a work string (not shown). At a lower end  20  of the body  12  is a pin section  22  to allow the body  12  to be threadably connected to the work string (not shown). Therefore the body  12  of tool  10  may be mounted within a work string. This work string may of course have one or more tools located upon it and preferably a tool will be mounted on the work string below the lower end  20  of the tool  10 . 
   Mounted on the body  12  is a sleeve  24  comprising two sections. The first is a longitudinal section  26  which mates to the body  12  and typically can be extended to ensure that the sleeves are the required length for the jobs. For example, if the job is to mill a polished bore receptacle (PBR) as shown in  FIG. 1 , section  26  will be of a length so that the lower milling section  28  can reach to the base of the polished bore receptacle  30 . The second is outer section  32  which provides the function of the tool. Section  32  in the first embodiment includes on its surface a plurality of milling ribs  34  which when turned will mill and dress the top  36  and inside wall  38  of the PBR  30 . 
   Body  12  and sleeve  24  are mated together via a plurality of shear pins  40  shown schematically on  FIG. 1  together with a drive system shown in detail on  FIG. 2 . The drive section is generally referenced by numeral  42 . In the embodiment shown here, there are six shear pins holding the body  12  to the sleeve  24 . At mating portions  44   a  and  44   b  of the body  12  and sleeve  24 , respectively, the inner surface  46  of sleeve  24  is provided on a polygon cross-sectional area. In the embodiment shown there are six planar surfaces making up the inner surface  46 . A corresponding outer surface  48  is machined on the circumference of the body  12  at mating portion  44 . The drive section  42  is thus a hex drive and ensures that when the body  12  is rotated on the work string sleeve  24  will also rotate when surfaces  46  and  48  are aligned. The body  12  and sleeve  24  are held together by the shear screws or shear pins  40 . 
   With the aid of  FIG. 3  the arrangement of each shear pin  40  can be seen clearly. Shear pin  40  comprises a screw thread section  50 , a constrictive section  52  and a head  54 . An aperture  56  in the sleeve  24  ensures that each pin  40  can be inserted and screwed to a matching screw thread in body  12 . Pin  40  is inserted to a point such that the head  54  engages a lip  60  on the sleeve  24 . At this point, the constricted section  52 , is against the plane  62  formed between the surfaces  46 ,  48  of the sleeve  24  and body  12  respectively. Constricted section  52  ensures that when the body  12  is rotated the torque is applied to the surface  46  and not to the shear pins  40 . When inserted a plug  43  is screwed into the sleeve  24  behind the pin  40  to prevent the ingress of debris to the aperture  56  and to ensure that when pin  40  is sheared the head  54  and sheared portion of the constricted section  52  are retained within aperture  56  and cannot exit into the well bore. Conversely the threaded portion  50 , which is sheared, remains attached to the body  12  and is likewise prevented from dislodging and entering into the well bore. 
   In use sleeve  24  is mounted onto body  12  aligning surfaces  46 ,  48 . Shear pins  40  are inserted through apertures  56  so that sleeve  24  is coupled to the body  12 . Tool  10  is then connected into a work string by virtue of sections  18 ,  22 . The work string can then be inserted into the well in the standard manner. 
   When the outer portion  32  of sleeve  24  reaches the top  36  of the PBR  30 , the PBR  30  is then milled and dressed using the milling ribs  34  together with the bore milling section  28  on the sleeve  24 . Sleeve  24  is operated purely through rotation of the work string which in turn rotates the body  12  via the hex drive  42 . Sleeve  24  is turned and thus milling ribs  34  and milling section  28  can mill and dress the inside walls  38  and the top  36  of the PBR  30 . 
   Once this function is complete the sleeve  24  is located against the PBR  30  at the top  36 . The work string is then set down on a weight of approximately 10,000 lbs, top  36  acting as a formation in the well bore and the tool then becomes a weight set tool. Setting weight on the tool causes the pins  40  to shear due to the planar arrangement of surfaces  46 ,  48 . The body  12  together with the work string will disengage from the sleeve  24 , sleeve  24  will remain at the liner top  36  while the body  12  and work string can descend further into the well bore. A tool connected to the work string (not shown) can then perform any required function at a greater depth in the well bore. 
   When all functions are complete in the well bore the work string is removed in the standard manner. When the string brings the mating portion  44   a  of the body  12  up to the mating portion  44   b  of the sleeve  24  if the surfaces  46 ,  48  do not align the sleeve  24  will be picked up by virtue of the top ledge formed from the surface  46  meeting the bottom ledge formed from surface  48 . 
   Alternatively, if the surfaces  46 ,  48  match then the body mating portion  44   a  slides into the matching mating portion  44   b  of the sleeve. The sleeve  24  is then picked up by the upper ledge formed from the planar sections of the surface  48  now mating with a shoulder  64 , see  FIG. 1 , in the sleeve  24 . The shoulder  64  is further illustrated in  FIG. 3 . On picking up the sleeve  24 , the sleeve  24  moves with the body  12  and is removed from the well bore. 
   Reference is now made to  FIG. 4  of the drawings which illustrates a safety mechanism mounted on the sleeve  24  of the tool  10  according to a second embodiment of the present invention. The safety mechanism  66  is as disclosed in UK Patent Application No 0126550.3 to the Applicants and is incorporated herein by reference. 
   Safety mechanism  66  comprises a cylindrical body or button  68 . In one end  70  of the button  68  there is a contact face  72 . Face  72  is planar and located at an acute angle to the button  68 . At an opposing end  74  there is located a magnet  76 . Located between opposing ends  70 , 74  is a narrowed section  78  of the button  68 . The narrowed section provides a weak point on the button  68  making it susceptible to shearing across the narrow section  78 . It will be appreciated that instead of a narrow section the button could include a section of differing material which is weaker than the remaining material and can be equally susceptible to shearing. 
   In use, button  68  is inserted through a portion or recess  80  of the sleeve  24  and the body  12 . The button  68  is positioned such that the narrow section  78  is kept away from plane  62  between the body  12  and sleeve  24 . This is achieved by the insertion of bissel pins (not shown). When the tool  10  is inserted into the well bore, the safety mechanism  66  remains in this position until the polished bore receptacle  30  contacts the face  72  of the button  68 . On contacting the face  72  the PBR  30  causes the button  68  to be pushed into the recess  80 . Once pushed fully in the PBR  30  comes to rest on an upper face  82  of the sleeve  24 . In this position the PBR  30  maintains the button  68  in a position where the narrow section  78  lies on the shear plane  62 . The button  68  is held in this position by the inner surface  38  of the PBR  30 . This contact has caused the sleeve to remain stationary relative to the body  12 . Weight applied to the body  12  now causes the shear pins  40  to shear along a narrow section  78  of the safety mechanism  66 . Once sheared the sleeve  24  moves relative to the body  12  to allow the work string to become free of the sleeve  24  disengaging the milling assembly. While the tool  10  is being set the button  68  is always held inwards where it may be sheared by the continued contact of the surface  38  of the PBR  30  with the face  72  of the button  68 . This dual function of the PBR  30  in both holding the button  68  inwards while contacting the sleeve  24  to set the tool allows the tool only to be set by the PBR  30 . 
   As setting of the tool occurs only when the PBR provides the dual role of holding the button inwards while contacting the sleeve, it is unlikely that any unintended formation in the well bore could achieve both these functions simultaneously and thus the tool will not operate until it reaches the PBR. 
   The principle advantage of the present invention is that it provides a downhole tool which can perform more than one function on a single trip into the well bore. In particular, the tool provides for the milling of a polished bore receptacle while allowing the work string to travel further into the well and perform other functions. 
   A further advantage of the present invention is that it provides a tool, which retrieves all parts thereof on removal from the well. 
   A yet further advantage of the present invention is that it includes a safety mechanism to ensure that the assembly only disengages at a selected location in the well bore. 
   It will be appreciated by those skilled in the art that various modifications may be made to the invention disclosed herein without departing from the scope thereof. For instance, the description relates to a milling assembly being disengaged on the work string, however other tools can be disengaged such as sensors or cleaning equipment. Presently we have not disclosed other tools suitable for operation in the work string and it will be appreciated by those skilled in the art that any tool may be placed below the assembly to operate on the work string. In fact, it would be possible to include one or more of these assemblies on a single work string, each disengaging at a selected location, as the casing or liner diameter decreases with the depth of the well bore. Further, it will be appreciated that although a hex drive mechanism is shown to drive the assembly from the work string, any drive mechanism which can disengage and decouple allowing free passage of a work string through the assembly would be suitable. Additionally, in this respect the number of shear pins required to hold the assembly to the work string on its descent into the well may be varied.

Summary:
A disengageable downhole tool ( 10 ) for use in the oil and gas industry. The tool can be decoupled from a work string to allow further tools to be operated from the work string on being run deeper into a well. The tool is then picked-up by the work string on retrieval from the well. In one embodiment a milling tool ( 28, 32 ) is described having a hex-drive system for operation of the tool from the work string with coupling and decoupling being achieved via shear pins ( 40 ) and a mating ledge and shoulder.