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This application is based on Provisional Patent Application Ser. No. 61/744,435, filed Sep. 26, 2012, priority of which is claimed and which is incorporated herein by reference. 
     This invention relates to a technique for breaking a frangible isolation tool of a type run in a well to isolate a section of the well above the isolation tool from a section of the well below the tool. 
     BACKGROUND OF THE INVENTION 
     Isolation tools are used in hydrocarbon wells for a variety of purposes. They are commonly run in a well near the end of a tubing string and below a hydraulically set packer to isolate the packer from formation pressure and allow hydraulic operations above the isolation tool. They are run on the end of tubing strings or in order to pressure test the made up string. They are occasionally run on the bottom of casing strings before cementing the string in a well bore. Other uses will be apparent to those skilled in the art. 
     One type isolation tool comprises a pair of oppositely facing curved ceramic discs shown in U.S. Pat. No. 5,924,696. These discs have a strong side and a weak side, i.e. the convex side can resist considerably higher pressures than the concave side. These discs are arranged with the convex side facing toward the pressure to be resisted, i.e. the upper disc has its convex side facing upwardly and the lower disc has its convex side facing downwardly. 
     The upper ceramic disc disclosed in this patent is broken by dropping a weight or go-devil into the tubing string so this device is mainly usable in vertical wells. 
     It is desirable to provide an isolation tool comprising one or more ceramic domes which are usable in the horizontal or vertical leg of a hydrocarbon well. Such devices are shown in U.S. Pat. No. 7,806,189 and U.S. Printed Patent Application 20110017471 and application Ser. No. 12/800,622 which are incorporated herein by reference. 
     Other disclosures of interest are found in U.S. Pat. Nos. 3,831,680; 4,510,994; 4,658,902; 5,511,617; 6,155,350; 6,672,389; 7,044,230; 7,210,533 and 7,350,582 and U.S. Printed Patent Applications 20070074873; 20080271898; 20090056955; 20090020290 and 20120125631. 
     SUMMARY OF THE INVENTION 
     As used herein, upper refers to that end of the tool that is nearest the earth&#39;s surface, which is a vertical well would be the upper end but which in a horizontal well might be no more elevated than the other end. Similar, lower refers to that end of the tool that is furthest from earth&#39;s surface. 
     Three embodiments are disclosed. In two embodiments, the upper dome or disc is restrained by a shear device to withstand pressure to some value. When pressure from above exceeds the shear value, the upper dome or disc moves toward the lower disc and, in the process, disintegrates and causes the lower disc to shatter, either from shrapnel from the upper disc or from hydrostatic or dynamic pressure acting on the weak or concave side of the lower ceramic disc. 
     In the third embodiment, a sleeve mounted around the upper disc includes at least one spur on its upper end. When a shear device is broken by pressure from above, the sleeve moves downwardly around the upper disc so the spurs strike the convex side of the upper disc thereby fracturing it. This destroys the integrity of the upper disc which thereby fails. The lower disc shatters either from shrapnel from the upper disc or from the application of hydrostatic or dynamic pressure to the concave or weak side of the lower disc. 
     It is an object to provide an improved technique for removing frangible discs providing an isolating feature in a down hole well tool. 
     A further object is to provide an improved hydraulic technique for removing frangible discs in an isolation tool. 
     These and other objects and advantages will be apparent to those skilled in the art as this description proceeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view of one embodiment showing the tool as it is run into a well; 
         FIG. 2  is a view similar to  FIG. 1  showing an intermediate arrangement of elements during removal of the isolating discs; 
         FIG. 3  is a view of the device of  FIGS. 1-2  after the discs are broken and removed; 
         FIG. 4  is a vertical cross-sectional view of another embodiment showing the tool as it is run into a well; 
         FIG. 5  is a view similar to  FIG. 4  showing an intermediate arrangement of elements during removal of the isolating discs; 
         FIG. 6  is an enlarged partial view of a sleeve used in the embodiment of  FIGS. 4-5 ; 
         FIG. 7  is a vertical cross-sectional view of a third embodiment showing the tool as it is run into a well; 
         FIG. 8  is a view similar to  FIG. 7  showing an intermediate arrangement of elements during removal of the isolating discs; 
         FIG. 9  is a top view of the upper disc and its surrounding sleeve; and 
         FIG. 10  is an enlarged cross-sectional view of the sleeve and upper disc in the embodiment of  FIGS. 7-9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-3 , an isolation tool  10  may comprise a central body  12 , an upper coupling  14  and a lower coupling  16 . The body  12  and couplings  14 ,  16  captivate an upper disc or dome  18 , a shearable connection such as a plate or ring  20  and a lower disc or dome  22  against a shoulder or ledge  24 . A series of seals  26  may prevent leaks around the discs  18 ,  22  and a series of seals  28 ,  30  may prevent leaks between the body  12  and couplings  14 ,  16 . The tool  10  may typically be attached to a stinger (not shown) on the bottom of a packer (not shown) so that the tubing string (not shown) to which the packer is attached is isolated from formation pressure. The packer may be hydraulically set or other operations conducted above the tool  10  without interference or difficulty caused by pressure on the outside of the tubing string. In the alternative, the tool  10  may be run on the bottom of a tubing string or liner in order to pressure test the made up string. 
     The discs  18 ,  22  may be ceramic discs of the type shown in U.S. Pat. No. 5,924,696 or in application Ser. No. 12/800,622 or of any other suitable type frangible members that have the property of being stronger in one direction than in another. Preferably, the upper disc  18  may include an elongate skirt  32  allowing multiple seals  26  on the exterior and preferably is of an O.D. that is receivable in the I.D. of the lower disc  22 . 
     The shear plate  20  may include a lip  34  receiving the outside diameter of the upper disc  18  and a shoulder  36  abutting the bottom of the upper disc  18 . A circumferential notch or other weakened portion  38  shears off when pressure from above, as suggested by the arrow  40 , is sufficient. This allows the upper disc  18  to move toward the lower disc  22  as suggested in  FIG. 2 . The O.D. of the disc  18  may preferably be equal to or slightly less than the I.D. of the disc  22  so the collision between the upper and lower discs  18 ,  22  shatters at least the lower disc  22  and preferably both discs  18 ,  22 . This produces a full opening passage  42  through the tool  10 , i.e. the opening is as large as the minimum dimension through the central body  12  and couplings  14 ,  16  as shown in  FIG. 3 . Sometimes, the lower disc  22  is sheared off to leave the elongate skirt shown in  FIG. 3  and sometimes the elongate skirt is destroyed so that all of the ceramic components of the discs  18 ,  22  are removed from the tool body  12  and either fall by gravity out of the tool  10  or are pumped out by the fluid being pumped through the tool  10 . 
     In operation, the packer (not shown) may be set by pumping into the tubing string (not shown) until the pressure reaches a value sufficient to expand and set the packer against the inside of the casing string. Later, or immediately, pumping into the tubing string at an increased pressure reaches the shear value of the plate  20  whereupon the shear plate  20  fails releasing the upper disc  18  so the shear plate  20  and upper disc  18  move downardly into the lower disc  22  causing it to fail thereby providing communication across the tool  10  in preparation for additional operations. It is not completely clear whether the lower disc  22  is pulverized by the shear plate  20 , shrapnel from the upper disc  18 , the hydrostatic weight of liquid above the tool or the dynamic pressure resulting from pumping into the tool  10 . In any event, the lower disc  22  fails more-or-less immediately upon failure of the shear plate  20  providing an unobstructed passage through the tool  10 . 
     Referring to  FIGS. 4-5 , an isolation tool  44  may comprise a central body  46 , an upper coupling  48  and a lower coupling  50 . An upper disc  52  is provided. A plate or ring  54  may provide a passage therethrough and may connect to the central body  46  by a shearable connection such as a shear pin  56  and, together with the upper coupling  48 , captivates the upper disc  52 . A lower disc  58  may be captivated between the lower coupling  50  and a ledge  60 . A series of seals  62  prevents leaks around the discs  52 ,  58  and a series of seals  64 ,  66  may prevent leaks between the body  46  and couplings  48 ,  50 . The tool  44  may typically be attached to a stinger (not shown) on the bottom of a packer (not shown) so that the tubing string (not shown) to which the packer is attached is isolated from formation pressure. The packer may be hydraulically set or other operations conducted without interference or difficulty caused by formation or hydrostatic pressure on the outside of the tubing string. 
     The discs  52 ,  58  may be ceramic discs of the type shown in U.S. Pat. No. 5,924,696 or in application Ser. No. 12/800,622 or of any other suitable type frangible members that have the property of being stronger in one direction than in another. Preferably, the upper and lower discs  52 ,  58  may include an elongate skirt  68 ,  70  allowing multiple seals  62  on the exterior thereof. 
     The upper disc  52  is mounted for movement inside a sleeve  72  having a passage  74  therein. The sleeve  72  may comprise part of the central body  46  or may be captivated thereto. The passage  74  may be configured to disintegrate the upper disc  52  upon movement of the upper disc  52 . This may be accomplished in a variety of ways, such as tapering the passage slightly from an oversized upper end  76  to an internal diameter  78  that is substantially the same as or slightly smaller than the O.D. of the skirt  68 . In the alternative, the sleeve  74  may include a protrusion or point or otherwise be of smaller dimension than the skirt  68  to stress the skirt  68  during movement of the upper disc  52 . The lower edge of the disc  52  may rest on the upper edge. of the ring  54  so the shear pin  56  may initially constrain the upper disc  52  against movement downwardly. The ring  54  and shear pin  56  may preferably be of metal so the shear pin  56  operates in a conventional manner, i.e. it fails upon the application of a more-or-less predetermined or design force to free the ring  54  for downward movement. The concept is that when pressure applied as suggested by the arrow  80  is sufficient to shear the pin  56 , the plate  54  moves allowing the upper disc  52  to move downwardly into the sleeve  72  and fail. Failure of the upper disc  52  causes the lower disc  58  to fail, either due to shrapnel from the upper disc  52  or from hydrostatic or dynamic pressure inside the tubing string thereby providing communication through the tool  44 . 
     Exactly how the upper disc  52  fails may be subject to some argument because it is not completely clear whether the upper disc  52  shatters because it stops suddenly or whether it is squeezed by Constriction of the passage  74 . Initially, the intact upper disc  52  moves downwardly into the sleeve  72  but as its lower end approaches the I.D.  78 , the upper disc  52  fails. It may fail because of the sudden stop, either inside the sleeve  72  or against the plate  54 . It may fail because of the hoop stress applied to the skirt  68  by the constriction of the passage  74 . In any event, and without being bound by any theory, the upper disc  52  fails when it moves downwardly. This causes the lower disc  58  to fail. Tests run on a prototype show that the upper disc  52  shatters into relatively large pieces while the lower disc  58  is reduced to fine powder. 
     The embodiment of  FIGS. 4-6  has an advantage over the embodiment of  FIGS. 1-3  because, for a predetermined tool O.D., the tool  44  can have a larger I.D. than the tool  10 . The reason is that the embodiment of  FIGS. 4-6  does not have to size the lower disc to be of larger I.D. than the O.D. of the upper disc. This is of considerable importance because the radial dimension in a well tool is at a premium, particularly when compared to axial dimensions. It will be seen that the upper discs  18 ,  52  of the tools  10 ,  44  fail as a consequence of their motion. In the embodiment of  FIGS. 1-3 , failure occurs because the upper disc  18  collides with the lower disc  22 . In the embodiment of  FIGS. 4-6  the upper disc  52  fails either because it moves into the constricted passage  74  or because it stops. 
     Referring to  FIGS. 7-10 , an isolation tool  100  may comprise a central body  102 , an upper coupling  104  and a lower coupling  106 . An upper disc  108  abuts a ring  110  which in turn abuts a ledge  112  provided by the central body  102 . A sleeve  114  surrounds the disc  108  or at least a major portion of its skirt  116  and may initially be connected by one or more shear pins  118  to the ring  110 . It will accordingly be seen in  FIG. 7  that the sleeve  114  is spaced from the ledge  112 . For purposes more fully apparent hereinafter, the sleeve  114  includes one or more spurs  120  having sharp points  122  which may be adjacent the convex side of the disc  108 . One function of the spurs  120  is to constrain upward movement of the disc  108 . The sleeve  114  includes suitable seals  124  sealing against the disc  108  and seals  126  sealing against the central body  102  thereby preventing leakage around the disc  108 . 
     The tool  100  may also include a lower disc  128  having a skirt  130  sealed by multiple seals  132  against the central body. The lower disc  128  may be captivated against the ledge  112  by the coupling  106 . 
     The tool  100  may typically be attached to a stinger (not shown) on the bottom of a packer (not shown) so that the tubing string (not shown) to which the packer is attached is isolated from formation pressure. The packer may be hydraulically set or other operations conducted without interference or difficulty caused by formation or hydrostatic pressure on the outside of the tubing string. 
     The discs  108 ,  128  may be ceramic discs of the type shown in U.S. Pat. No. 5,924,696 or in application Ser. No. 12/800,622 or of any other suitable type frangible members that have the property of being stronger in one direction than in another. 
     When it is desired to provide communication through the tool  100 , pressure is applied from above as suggested by the arrow  134 . When the pressure produces a force sufficient to shear the pin or pins  118 , the sleeve  114  moves downwardly as suggested in  FIG. 8  so the spurs  120  contact the convex side of the upper disc  108  thereby fracturing the disc  108 . Failure of the disc  108  causes more-or-less immediate failure of the disc  128  thereby providing an open passage through the tool having a working I.D. which may be the same as the I.D. of the ledge  112  or the I.D. of the spurs  120 . 
     It will be seen that an important advantage of the tool  100  is that the spurs  120  contact the upper disc  108  at a location near the junction of the curved top of the disc  108  and the skirt  116  more-or-less aligned with or outboard of the interior surface of the shoulder  112 . This may be of advantage because the breaking mechanism does not utilize any radial space inside the passage through the shoulder  112 . Tools used in hydrocarbon wells have a great deal of leeway in an axial direction, i.e. along the well axis, but very little leeway perpendicular to the well axis. In other words, taking up radial space in a well tool is very costly. 
     Although this invention has been disclosed and described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred forms is only by way of example and that numerous changes in the details of operation and in the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

Summary:
A down hole pressure isolation tool is placed in a pipe string and includes a pair of pressure discs having one side that is highly resistant to applied pressure and one side that ruptures when much lower pressures are applied to it. The weak sides of the pressure discs face each other. In some embodiments, an upper disc is mounted for movement and restrained by a shearable connection. Upon the application of pressure to the tool, the connection shears allowing the upper disc to move into and collide with the lower disc thereby fracturing both discs and allowing pressure communication across the tool. In another embodiment, spurs carried by a sleeve fracture an upper disc in response to pressure delivered to an upper end of the tool.