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[0001]    This invention relates to an improved tool or plug for use in hydrocarbon wells having a modified segmented back up ring to restrain deformation of a seal. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are many well tools that incorporate a sealing member that is deformed into sealing engagement with a casing string. Typically such tools are called plugs, one species of plugs being packers. Many plugs are designed to be soluble, meltable or drillable, i.e. they incorporate a modest amount of materials that not easily drillable and are typically mostly made of composites, polymers, aluminum, brass and the like which are easily removed from a well in any of a variety of ways. 
         [0003]    These type tools usually incorporate slips that grip the interior of a casing string, an expansion device or devices to expand the slips into gripping engagement with the casing string and a deformable or resilient seal member that is compressed during actuation of the plug so it expands more-or-less radially into sealing engagement with the casing string. An element often used in such devices is known as a back up ring, a support ring, a back up shoe, a gage ring or the like, the purpose of which is to restrain axial expansion of the deformable seal so it is directed radially against the casing string. In other words, these devices are anti-extrusion devices which minimize or prevent extrusion of the malleable seal axially along the tool and thereby minimize or prevent leakage past the seal. 
         [0004]    Disclosures of some interest relative to this invention are found in U.S. Pat. Nos. 3,554,280; 4,397,351; 4,730,835; 5,024,270; 5,540,279; 6,739,491; 7,578,353; 8,066,065 and 8,336,616. 
       SUMMARY OF THE INVENTION 
       [0005]    As disclosed herein, a plug has a collapsed or running in position so it an be run in a well, such as a hydrocarbon well, and an expanded or operative position where a deformable seal is pressed against the inside of a casing string or well bore in the case of an open hole packer. Such plugs include the deformable seal, slips that anchor the plug in a desired position, some way allowing manipulation of the tool so it can be expanded from the running in position to the operative position and a back up ring to restrain deformation of the seal so it efficiently expands against the casing string. 
         [0006]    Many current generation plugs are used during completion of wells and are designed to be readily drilled up in order to minimize completion costs. Current generation back up rings are made of composite material and are segmented so that when the plug is set or expanded, the segments flare out against the casing string in much the same manner as flower petals opening and thereby prevent extrusion of the deformable seal axially. This directs the deformable seal radially toward the casing string. It has been learned that current model segmented back up rings sometimes fail in laboratory tests of extended reach plugs such as shown in U.S. application Ser. No. 13/737,223, filed Nov. 8, 2011, the disclosure of which is incorporated herein by reference. Although such back up rings often fail during laboratory tests, no field failures have yet been seen which is not surprising because down hole failures are unusual and because the cause is almost never known. 
         [0007]    Extensive tests are run by Magnum Oil Tools, Ltd. on many different types of plugs. On extended reach plugs where the tool, in its running in condition, is relatively small compared to its expanded condition and necessarily undergoes considerable expansion, it is common for the petals of back up rings to fracture and detach from the main part of the ring during testing. 
         [0008]    The failure rate of back up rings has, by use of the construction disclosed herein, has so far fallen to zero. This is accomplished, as disclosed hereinafter, by moving the connection between the segment or petal and the ring body toward the exterior of the back up ring. 
         [0009]    It is an object of this invention to provide an improved segmented back up ring and a plug incorporating the same. 
         [0010]    Another object of this invention is to provide an improved segmented back up ring that allows flaring of the segments without fracturing the segment from the body to which it is attached. 
         [0011]    These and other objects and advantage of this invention will become more fully apparent as this description proceeds, reference being made to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a partial vertical cross-sectional view of a plug equipped with a segmented back up ring; 
           [0013]      FIG. 2  is a cross-sectional view of a conventional segmented back up ring; 
           [0014]      FIG. 3  is a cross-sectional view of an improved segmented back up ring; 
           [0015]      FIG. 4  is an end view of the segmented back up ring of  FIG. 3 ; 
           [0016]      FIG. 5  is an exploded view of an improved segmented back up ring, as in  FIG. 3 , in conjunction with a separate additional annular support; 
           [0017]      FIG. 6  is a schematic view of the relationship between a segment of a conventional back up ring and a casing string which it abuts in an expanded condition of the plug; and 
           [0018]      FIG. 7  is a view similar to  FIG. 6  illustrating the relationship between a segment of an improved back up ring and the casing string. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    As used herein, upper refers to that end of the tool that is nearest the earth&#39;s surface, which in a vertical well would be the upper end but which in a horizontal well might be no more elevated than the opposite end. Similarly, lower refers to that end of the tool that is furthest from earth&#39;s surface. Although these terms may be thought to be somewhat misleading, they are more normal than the more correct terms proximal and distal ends. 
         [0020]    Referring to  FIGS. 1 , a plug  10  may comprise, as major components, a body or mandrel  12  having a passage  13  therethrough, one or more sets of slips  14 ,  16 , one or more wedge sections  18 ,  20 , a rubber or packing element  22  and an anti-rotation device or mule shoe  24 . The body  12  may include an upper section  26  and a lower section  28  connected together in a suitable manner, such as by threads  30 . The tool  10  is illustrated as of a type that can be converted between a bridge plug, a flow back plug, a check valve plug or otherwise by installing or removing a component in an insert  32  such as shown in U.S. Pat. No. 8,307,892, the disclosure of which is incorporated herein by reference. The component may be a plug, a valve ball, a soluble ball or the like as shown in U.S. patent application Ser. No. 12/317,497, filed Dec. 23, 2008, the disclosure of which is incorporated herein by reference. 
         [0021]    The insert  32  may be attached to the upper body  26  by suitable threads  34  and may include internal threads  36  for connection to a conventional setting tool (not shown) connected to a wire line extending to the surface. The setting tool (not shown) may act in a conventional manner by pushing down on the top of a collar  38  and pulling up on the threads  36 . This shears a pin (not shown) and allows the collar  38  to move downward relative to the slips  14 ,  16  thereby expanding the slips  14 ,  16  into gripping engagement with the casing  40 . 
         [0022]    The slips  14 ,  16 , the wedges  18 ,  20  and the packing element  22  may be of a conventional type as shown in U.S. patent application Ser. No. 12/317,497, filed Dec. 23, 2008 so the tool is set in a conventional manner. During setting of the tool  10 , the slips  14 ,  16  ride along the wedges  18 ,  20  to expand the slips  14 ,  16  and fracture them into a number of segments in gripping engagement with the interior of a casing string  40  which may be cemented in a well bore (not shown). At the end of the setting of the tool  10 , the insert  32  fails or breaks at a neck  42  thereby detaching the threads  36  and the setting tool (not shown) so the setting tool and wire line may be removed from the well. 
         [0023]    The anti-rotation device  24  acts to minimize or prevent rotation of the tool when it is being drilled up by interacting with a subjacent tool. This may be accomplished in a number of ways, one of which is to provide angled faces  44 ,  46  on the bottom of a body  48  of the anti-rotation device  24 . 
         [0024]    The plug  10  may also include one or more back up rings  50 ,  52  which may be part of the wedges  18 ,  20  or may be separate members. In addition, the back up rings  50 ,  52  may abut the packing element  22  or may abut an intermediate annular support as discussed hereinafter which may be a drillable material, soluble material or meltable material such as a drillable metal, polymer or composite. As shown in  FIG. 2 , a conventional wedge or expander  18  may be of conventional shape and can comprise a body  54  having a central passage  56 , a tapered exterior or conical section  58  and one or more set screw passages  60  for securing the lowermost wedge  20  to the body  12 . Pulling up on the insert  32  causes the lowermost wedge  20  to rise relative to the uppermost wedge  18  thereby setting the slips  14 ,  16  and expanding the seal  22 . 
         [0025]    The back up rings  50 ,  52  may be part of the bottom of the wedges  18 ,  20  and may include a series of tapered segments  62  extending circumferentially around the passage  84 . The segments  62  can act like flower petals and flare out against the casing  40  during setting of the plug  10  and thereby constrain movement of the seal  22  into generally radial movement into sealing engagement with the casing  40 . In drillable plugs, the back up rings  50 ,  52  may preferably be of a conventional composite material or polymer. Current composite or polymer materials are rigid at room temperature but become somewhat pliable or flexible at typical temperatures found in hydrocarbon wells. To promote the flexibility of the segments  62 , an exterior notch  64  has been provided. Those skilled in the art will recognize the plug  10  as being of a type commercially available from Magnum Oil Tools International of Corpus Christi, Tex. 
         [0026]    Some fraction of laboratory tests with the conventional back up rings  50 ,  52  in plugs similar to the plug  10  have experienced failure of the segments  62 , i.e. a fracture or complete break sometimes develops in the joint  66  between the end of the notch  64  and the central passage  56  as represented by the jagged line  68 . When a segment  62  detaches from the body  54 , this allows the seal  22  to expand axially into the gap left by the detached segment  62  thereby reducing the ability of the seal  22  to move radially into sealing engagement with the casing  40  thereby reducing the ability of the seal  22  to seal against the casing  40 . No field failures have yet been reported even though several thousand plugs with the design of  FIG. 2  have been run and set in hydrocarbon wells, have sustained fracing pressures when the wells were fraced and have then been drilled up. The absence of reported field failures may be simple good luck, it may be that a small seal leak is not consequential in light of the high volume pumped during frac jobs or it may be that frac sand bridges off the plug, even if it is leaking. In any event, it is desirable to provide a back up ring that does not fail by fracturing at the joint between the segment  62  and the body  54 . 
         [0027]    To this end, the segmented back up ring  80  is provided. The back up ring  80  may be integral with the wedges  18 ,  20  or may be separate, as illustrated in  FIG. 3 , from an expander dome (not shown) which may be affixed to the back up ring  80  by suitable threads or other means. Integral and separate segmented back up rings are illustrated in U.S. application Ser. No. 13/373,223, filed Nov. 8, 2011, which is incorporated herein by reference. The back up ring  80  may comprise a body  82  having a cone (not shown) on the upper end or an integral cone which acts to fracture or expand the slips  14 ,  16  in a conventional manner. A passage  84  through the back up ring  80  allows the back up ring  80  to be received on the body  12 . Instead of the groove  64  on the outside of the back up ring, a groove  86  on the inside of the back up ring  80  opens into the passage  84  and imparts some flexibility to the petals or segments  88  at reservoir temperature. As in the prior art, the segments  88  are separated by a gap or kerf  90  which may be formed in any suitable manner, as by cutting with a saw. 
         [0028]    The back up ring  80  accordingly provides a connection or joint  92  between the segments  88  and the body  82 . The outside of the junction  92  may be on the outside diameter of the body  82  or adjacent the outside diameter of the body  82  or, in any event, is closer to the outside diameter than to the inside diameter. The back up ring  80  may be made of a soluble, meltable or drillable material such as aluminum, brass, a composite material or polymer either by machining, injection molding or otherwise. The kerfs  90  separating the segments  88  may preferably extend through the junction  92  and separate it into segments. Thus, kerfs in the junction  92  may be coplanar with kerfs through the segments  88 . 
         [0029]    As suggested in  FIG. 1 , the back up ring  80  may abut the packing element  22  or may abut an intermediate annular support or second back up ring  94  as shown in  FIG. 5  which is in load transferring relation between the back up ring  80  and the packing element or seal  22 . The annular support  94  may be a soluble, meltable or drillable metal, plastic or composite material. The annular support  94  may comprise a rim or body  96  having a passage  98  therethrough from which depend segments  100  resembling the segments  88  of the back up ring  80 . The segments  100  may be separated by kerfs or slots  102  and may flare outwardly to nest in a cavity  104  in one end of the back up ring  80 . It will be seen that the back up ring  80  is in force transmitting relation with the seal  22 , either in direct contact as in the embodiment of  FIG. 3  or in indirect contact through the annular support  94  as in the embodiment of  FIG. 5 . 
         [0030]    Lab tests of plugs incorporating the improved back up ring  80  show that the connection or joint  92  does not fracture or fail under conditions where the segments  62  of the prior art back up ring  50  are prone to fail. There appear to be several reasons. One reason may be the junction  92  between the segments  88  and the body  82 , being on or adjacent the outside diameter of the body  82 , is necessarily longer and therefore has more material than a comparably thick junction on the inside diameter, as in  FIG. 2 . 
         [0031]    Second, it may be that the geometry of the segments  88  is more favorable than the geometry of the segments  62 , i.e. the junctions  66 ,  92  act analogously to a pivot about which the petals  62 ,  88  rotate. Because the junction  66  is further from the inside wall of the casing  40 , the base of the petals  62  have to undergo more movement than the base of the petals  88  in order for the tips of the petal to reach the I.D. of the casing  40 . This is shown by a comparison of  FIGS. 6 and 7 . 
         [0032]    Third, the thickness of the junction  92  may be thicker than in the prior art for reasons which are not immediately apparent. It may be that the segments  88  have to move so much less, as discussed above, that a thicker junction  92  can still allow sufficient flexibility. One would think that the junction  66  of the prior art might be thickened but the depth of the notch  64  is needed to provide the necessary flexibility of the segments  62 . 
         [0033]    In  FIG. 6 , a conventional segment is connected to the body of the back up ring at a junction  66  and basically pivots about a point  106  from a solid line position  108  to a dashed line position  110  to engage the inside of the casing string  40  upon expansion of the plug  10 . The solid line position  108  represents the centerline of the unstressed segment  62  and the dashed line position  110  represents the centerline of the stressed segment  62  when it abuts the casing  40 . The angle  112  is accordingly defined by the length of the solid line  108  and the distance between the pivot point  106  and the inside of the casing string  40 . 
         [0034]    In  FIG. 7 , a segment of the improved back up ring  80  is connected to the body at a junction  92  and basically pivots about a point  114  from a solid line position  116  to a dashed line position  118  to engage the inside of the casing sting  40  upon expansion of the plug  10 . The solid line  116  represents the centerline of the unstressed segment  88  and the dashed line represents the centerline of the stressed segment  88  when it abuts the casing  40 . The angle  120  is accordingly defined by the length of the solid line  116  and the distance between the pivot point  114  and the inside of the casing string  40 . The angle  120  will be found to be smaller than the angle  112  and is necessarily smaller than the angle  112 . The same idea can be visualized by extending one&#39;s arm slightly away from one&#39;s body and asking: is the angle between the arm and the side of the body smaller than the angle between the arm and the centerline of the thigh. 
         [0035]    It will be seen that a preferred location of the pivot point  114  may be as close as possible to the outer diameter of the tool  10  represented by the line  120  in  FIG. 7  but some advantages accrue as the pivot point is moved from the inner diameter of the tool toward the outer diameter. In other words, a preferred location of the outside of the junction  92  may be the outer diameter of the tool  10 . 
         [0036]    Although this invention has been disclosed and described in one of its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred form 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. 
         [0037]    represents the centerline of the stressed segment  88  when it abuts the casing  40 . The angle  120  is accordingly defined by the length of the solid line  116  and the distance between the pivot point  114  and the inside of the casing string  40 . The angle  120  will be found to be smaller than the angle  112  and is necessarily smaller than the angle  112 . The same idea can be visualized by extending one&#39;s arm slightly away from one&#39;s body and asking: is the angle between the arm and the side of the body smaller than the angle between the arm and the centerline of the thigh. 
         [0038]    It will be seen that a preferred location of the pivot point  114  may be as close as possible to the outer diameter of the tool  10  represented by the line  120  in  FIG. 7  but advantages accrue as the pivot point is moved from the inner diameter of the tool toward the outer diameter. In other words, a preferred location of the outside of the junction  92  may be the outer diameter of the tool  10 . 
         [0039]    Although this invention has been disclosed and described in one of its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred form 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.

Summary:
A down hole tool or plug includes a segmented back up ring acting to minimize extrusion of a seal in an axial direction thereby promoting radial expansion of the seal into engagement with the internal diameter of a casing string. The segments of the ring are joined to a ring body having a passage thereby by a junction having, as its outer dimension, the outside diameter of the tool and an inside dimension provided by a groove opening into the passage.