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BACKGROUND OF THE INVENTION 
   The present invention relates to safety features in downhole tools and in particular to a safety trip button to prevent premature setting in weight set downhole tools. 
   A number of downhole tools as used in the oil and gas industry are operated within a well bore by contacting or landing part of the tool onto a formation located within the well bore. Typically a sleeve of the tool is landed on a liner top PBR (polished bore receptacle), causing the weight of the tool to force the tool into the liner while the sleeve remains stationary on the liner top. The relative movement of the sleeve on the tool body operates the tool, for example by opening radial ports or by compressing a packer. 
   In order that the sleeve does not move when the tool is inserted or run into the well bore, shear pins are typically inserted between the tool body and the sleeve to hold the sleeve in place. When the tool is landed on the liner top, the relative movement of the sleeve to the tool body causes the pins to shear thereby allowing operation of the tool. 
   A major disadvantage of these weight set tools is that they can be operated by the sleeve contacting any formation in the well bore. For example if the sleeve comes into contact with debris adhering to the walls of the casing or at a casing joint where the internal casing diameters are mismatched, the sleeve may be jarred or stick at that point in the well bore. Once stationary the tool body may be free to fall with a sufficient relative force to shear the pins. Thus the tool will be activated and operate at the incorrect position in the well bore. 
   It is an object of at least one embodiment of the present invention to provide a safety mechanism to prevent premature setting of a weight set tool in a well bore. 
   It is a further object of at least one embodiment of the present invention to provide a safety trip button which shears only when a weight set tool contacts a selected formation in a well bore. 
   It is a yet further object of at least one embodiment of the present invention to provide a compression set packer tool which includes a safety mechanism to prevent premature setting of the packer. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a safety mechanism for use in a weight set downhole tool to prevent the tool from setting before an operating element of the tool has landed on a selected formation in a well bore, the mechanism comprising a button mounted in a first position to lock the operating element 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 operating element thereby setting the tool. 
   As the tool cannot be set until the selected formation provides the dual role of holding the button in the second position and contacting the operating element, it is unlikely that any unintended formation in the well bore could achieve this and thus the tool will not operate until it reaches the selected formation. 
   Preferably the button comprises a cylindrical body which is mounted through a portion of the operating element and a portion of the tool body to lock each together. Locking prevents the operating element from moving in relation to the tool body so that the tool can be run into the well bore. 
   Preferably also the safety mechanism includes retaining means to hold the button to the operating portion and the tool body once the tool is set. The retaining means may be one or more bissell pins. The one or more bissell pins may be shearable. The retaining means may be a magnet, the magnet being mounted on a surface to attract another surface and hold the two surfaces together. 
   Preferably the face of the button is held proud of the tool in the first position. Preferably the surface is located facing the selected formation. More preferably the face is a plane surface located at an acute angle to the tool in the first position. Thus as the formation rides past the face it forces the button towards the tool into the second position. 
   More preferably the button includes a shearable section. The shearable section may be a narrower portion of the button, a portion of the button made of a differing material or a combination thereof. When the button is in the second position the lock is disengaged by the movement of the shearable section to a position where it may be sheared. 
   In a preferred embodiment the operating element is a sleeve and the selected formation is a polished bore receptacle. 
   According to a second aspect of the present invention there is provided a weight set downhole tool, the tool including a tool body mountable on a work string, an operating element slidably mounted on the tool body which operates the tool by contacting a formation in a well bore, shearable retaining means to hold the operating element to the tool body until such time as adequate force is applied to shear the retaining means, and a safety mechanism to prevent shearing of the retaining means until the operating element has contacted a selected formation. 
   Preferably the operating element is a sleeve. 
   Preferably the shearable retaining means is one or more shear pins. 
   Preferably the selected formation is a polished bore receptacle. 
   Preferably the safety mechanism is according to the first aspect. 
   Preferably the downhole tool is a circulation tool. Preferably also the downhole tool may be a packer tool. 
   Preferably the downhole tool further comprises an integral bypass means to allow fluid to pass through the tool as it is run into the well bore. More preferably the bypass means are ports or channels. The bypass means may by opened or closed by virtue of the movement of the operating element when the tool is set. 
   Preferably also the downhole tool includes cleaning means. The cleaning means may be brushes, scrapers or milling elements. Preferably the cleaning means are mounted below the operating element so that they reach the formation prior to the operating means. Alternatively the cleaning means may be located on the operating element so that unwanted formations such as debris can be removed before contacting the safety mechanism. 
   According to a third aspect of the present invention there is provided a method of preventing a weight set downhole tool setting prematurely before an operating element of the tool has landed on a selected formation, the method comprising the steps: 
   (a) running a weight set downhole tool including a safety mechanism into a well bore on a work string; 
   (b) engaging a face of the safety mechanism on to the selected formation to move the safety mechanism to a released position; and 
   (c) engaging the operating element on to the selected formation to set the tool while the selected formation maintains the safety mechanism in the released position. 
   Preferably the method includes the step of shearing the safety mechanism when the tool is set. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described by way of example only with reference to the following drawings of which: 
       FIG. 1  is a schematic cross sectional view through a downhole tool including a safety mechanism in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a schematic cross sectional view through section A–A′ of  FIG. 1 ; 
       FIG. 3  is a schematic cross sectional view of the tool of  FIG. 1  in the set position; and 
       FIG. 4  is a schematic cross sectional view of a packer tool including a safety mechanism in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference is initially made to  FIG. 1  of the drawings which illustrates a safety mechanism, generally indicated by reference numeral  10 , in accordance with a preferred embodiment of the present invention. Safety mechanism  10  comprises a cylindrical body or button  12 . On one end  14  of the button  12  there is a contact face  16 . Face  16  is planar and located at an acute angle to the button  12 . At the opposing end  18  there is located a magnet  20 . Located between opposing ends  14 , 18  is a narrowed section  22  of the button  12 . The narrowed section provides a weak point on the button  12  making it susceptible to shearing across the narrow section  22 . 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 be equally susceptible to shearing. 
   Further features of the button  12  can be seen with the aid of  FIG. 2 . Through the button  12  is located a channel  24  at each end of which are inserted bissell pins  26 , 28 . At one end of the channel  24  there is an opening  30  wide enough to clear the bissell pin  28 . No such opening is located at the other end of the channel  24 . Located at end  14  there is an aperture  32 . Aperture  32  includes a screw thread such that the button may be removed by insertion of a mating screw into the aperture  32 . 
   In use, the button  12  is inserted through a portion of a sleeve  34  and a tool body  36 , to which the sleeve  34  is located on. The bissel pins  26 ,  28  locate into the sleeve  34  such that the narrow section  22  is kept away from the shear plane  38  located between the sleeve  34  and the tool body  36 . 
   When the tool is inserted in a well bore (not shown) the sleeve  34  may come into contact with any irregularities or protrusions from the walls of the well bore. For instance debris or cuttings may adhere to walls of a casing while joints in the casing or liner may be mismatched leaving ledges. On contacting these formations the sleeve  34  may stick or become jarred. Once stationary the weight bearing down on the tool body  36  will cause a force to exist between the sleeve  34  and the tool body  36  along the shear plane  38 . Ordinarily this force may be sufficient to cause the standard shear pins  42 , retaining the sleeve  34  to the tool body  36 , to shear and as a result the tool would set at that point in the well bore. However, with the safety mechanism  10  in the position shown in  FIG. 1 , the first position, the sleeve  34  and tool body  36  are prevented from shearing apart by virtue of the section  40  of the button  12  lying across the shear plane  38 . Thus premature setting of the tool is avoided. 
   When the tool reaches a selected formation  44 , in this case a polished bore receptacle (PBR) on a liner top, upper surface  46  of the PBR  44  will engage with the contact face  16  of the button  12 . As the tool moves into the liner the face  16  and surface  44  will ride over each other with the result that the button  12  will be pushed in towards a recess  48  in the tool body  36 . The force exerted by the PBR  44  on the button  12  is sufficient to break a first bissell pin  26 . The opening  30  around the second bissell pin  28  allows the button to shift from a first position, sitting proud of the sleeve  34 , to a second position within recess  48 , while still retaining the button  12  to the sleeve  34 . 
   In the second position, the magnet  20  is located in the base of the recess  48  and the narrow section  22  lies on the shear plane  38 . The button  12  is held in this position by the surface  50  of the PBR  44 . This is illustrated in  FIG. 3  where the tool has moved further into the well bore such that the surface  46  of the PBR  44  has contacted a surface  52  of the sleeve  34 . Further this contact has caused the sleeve  34  to remain stationary relative to the tool body  36 . Weight applied to the tool body  36  now causes the shear pins  42  to shear along with the narrow section  22  of the safety mechanism  10 . Once sheared the sleeve  34  moves relative to the tool body  36  to set the tool as shown in the Figure. While the tool is being set the button  12  is always held in the second position, where it may be sheared, by the continued contact of the surface  50  of the PBR  44  with the face  16  of the button  12 . Thus the dual function of the PBR  44  in both holding the button  12  in the second position while contacting the sleeve  34  to set the tool allows the tool only to be set by the PBR  44 . 
   Once the button  12  has been sheared when the tool is set, the second bissell pin  28  holds the upper section  54  of the button  12  to the sleeve  34  to prevent it from becoming free and lodging somewhere in the tool where it may cause damage. Similarly, magnet  20  holds the lower section  40  of the sheared button  12  in the recess  48  against the tool body  36  and prevents it from interfering with the operation of the tool. 
   When the tool is retrieved, the button  12  may be removed from the sleeve  34  by inserting a screw into the aperture  32  and withdrawing the button  12 . The lower section  40  may be removed via a magnet or by simply pulling on the remains of the narrow section  22 . 
   Reference is now made to  FIG. 4  of the drawings which illustrates a packer tool, generally indicated by reference numeral  100 , in accordance with an embodiment of the present invention. In  FIG. 4  like parts to those of the other Figures have been given the same reference numeral with the addition of 100. 
   Packer tool  100  comprises a one piece full strength drill pipe mandrel  60 , making up the tool body  136 , and having a longitudinal bore  62  therethrough. A box section  64  connection is located at a top end of the mandrel  60  and a threaded pin section  66  is located at a bottom end of the mandrel  60 . Sections  64 , 66  provide for connection of the packer tool  100  to upper and lower sections of a drill pipe (not shown). 
   Mounted on the mandrel  60  is a compression set packer  68  with integral by pass means  70  which will be described hereinafter with regard to operation of the tool  100 . 
   Below the packer  68  is a stabiliser sleeve  72 . Sleeve  72  is rotatable with respect to the mandrel  60 . Raised portions or blades  74  provide a ‘stand off’ for the tool  100  from the walls of the well bore and lower torque on the tool  100  during insertion into the well bore. 
   Located below the stabiliser sleeve  72  is a Razor Back Lantern (Trade Mark)  76 . The Lantern  76  provides a set of scrapers for cleaning the well bore prior to setting the packer  68 . Though scrapers are shown it will be appreciated that the scrapers could be replaced by brushes or other suitable cleaning means. 
   The safety mechanism  110  and the contact surface  152  of the sleeve  134  are located on a top dress mill  78  at a lower end of the tool  100 . The top dress mill  78  can be used to dress off the PBR (not shown) top if required, while a section  80  of the top dress mill  78  can be used to clean the inside walls of the PBR. 
   The tool  100  operates as described hereinbefore with reference to  FIGS. 1 to 3 . When run in the bypass means  70  is open allowing fluid to flow around, behind the packer and thus reduce the amount of debris contacting the packer outer surface  82 . Setting down a weight of approximately 12,000 lbs will cause the sleeve  134  to engage with the PBR and the safety mechanism  110  and the shear pins  142  to shear. A ‘shear shudder’ will be felt on the drill string at the surface. Additionally the sleeve  134  will move across the bypass channel around the packer and it will be closed off. At the same time the sleeve contacts the base of the packer  68 . The upper end of the packer is fixed to the tool body  136 . Setting down further weight on the tool  100 , typically 20,000 lbs of applied weight, forces the base of the packer against the sleeve  134  which causes the packer to be compressed and as a result the rubber material of the packer  68  is compressed axially while expanding radially. Thus the packer  68  expands until the outer surface  82  meets and seals against the wall of the well bore or casing, if used. The packer is therefore set. Additionally the packer  68  can be unset by merely lifting the tool off the PBR whereupon the sleeve will fall back to its original position thereby releasing the packer  68  and opening the bypass means  70 . 
   The principle advantage of the present invention is that it prevents premature setting of a weight set downhole tool before the tool has landed on the selected formation. 
   A further advantage of the present invention is that it provides a failsafe compression set packer tool which allows an inflow or negative test to be carried out on a liner over-lap and the liner shoe-track on the same trip as the well bore clean-up. The tool further eliminates the need for a controlled displacement of the whole well to lighter density fluid through use of the retrievable packer to perform the test. 
   Modifications may be made to the embodiments described herein without departing from the scope thereof.

Summary:
A safety trip button ( 12 ) is described for a weight-set downhole tool. The button operates between the tool body ( 36 ) and a sleeve ( 34 ) of the tool, locking them initially together. When the tool reaches a selected formation ( 44 ) in a well bore, the button engages the formation ( 44 ) which unlocks the body and sleeve. The button is kept in the unlocked position by virtue of the formation while the tool is set. The button prevents premature setting of the tool and finds application on weight set tools such as packers and circulation tools.