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BACKGROUND 
     The present invention relates to packer and bridge plug type tools used in wellbores and more particularly to limiter assemblies, which resist extrusion of packer elements when exposed to borehole conditions, especially high pressure and high temperature. 
     In the drilling or reworking of oil wells, a great variety of downhole tools are used. For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of the well, such as when it is desired to pump cement or other slurry down the tubing and force the cement or slurry around the annulus of the tubing or out into a formation. It then becomes necessary to seal the tubing with respect to the well casing and to prevent the fluid pressure of the slurry from lifting the tubing out of the well or for otherwise isolating specific zones in a well. Downhole tools referred to as packers and bridge plugs are designed for these general purposes and are well known in the art of producing oil and gas. 
     Packers generally rely on a packer sealing assemblies to seal the wellbore. Traditionally such assemblies are comprised of at least one elastomeric sealing element and at least one mechanically set slip. Typically, a setting tool is run in with the packer to set it. The setting can be accomplished hydraulically due to relative movement created by the setting tool when subjected to applied pressure. This relative movement causes the slips to ride up on cones or wedges and extended into biting engagement with the surrounding casing or wellbore. At the same time, the sealing element is compressed into sealing contact with the surrounding casing or wellbore. 
     Packer element back-up shoes and rings have been employed to support the ends of the packer sealing elements as the elements are expanded into contact with a borehole wall. These back-up shoes or rings also may limit the axial extrusion of the packer sealing elements; thus they are sometimes called limiters or extrusion limiters. The shoes are typically segmented and, when the tool is set in a well, spaces between the expanded segments have been found to allow undesirable extrusion of the backer elements, at least in high pressure and high temperature wells. This tendency to extrude effectively sets the pressure and temperature limits for any given tool. Various improvements have been developed in ongoing efforts to prevent the extrusion of the packer elements between the segmented gaps and, while some have been effective to some extent, they have been complicated and expensive. 
     SUMMARY 
     The present invention provides a less complicated and expensive system of restraining the extrusion of the packer element by utilizing a simplified design to serve as a fixed extrusion limiter for a drillable tool. Additionally, the present invention does not suffer from the pressure and temperature limitations caused by the gaps in segmented limiters. 
     In one embodiment of the invention there is provided a downhole tool for use in a wellbore. The tool has a packer mandrel having a longitudinal axis. Disposed about the mandrel is an expandable sealing element, wherein the expandable sealing element is radially expandable outwardly from an unsealed position when the tool is in an unset position to a sealed position when the tool in a set position. In the sealed position, the expandable sealing element sealingly engages the wellbore. Additionally, the tool has a slip ring disposed about the mandrel and radially expandable outwardly from a disengaged position when the tool is in the unset position to an engaged position when the tool is in the set position, wherein the slip ring grippingly engages the wellbore in the engaged position. A slip wedge is disposed about the mandrel, having a radially outer surface containing a channel therein and an abutment end that abuts the expandable sealing element when the tool is in the set position. When the tool is moved from the unset position to the set position, the slip wedge interacts with the slip ring so as to expand the slip ring to its engaged position. A limiter ring is positioned in the channel of the slip wedge. When the tool is in the set position the limiter ring and the abutment end of the slip wedge act to retain the expandable sealing element and resist extrusion of the expandable sealing element. 
     In another embodiment of the invention there is provided a downhole tool for use in a wellbore. The tool has a packer mandrel having a longitudinal axis and an expandable sealing element disposed about the packer mandrel. The expandable sealing element is radially expandable outwardly from an unsealed position when the tool is in an unset position to a sealed position when the tool in a set position, and wherein the packer element assembly sealingly engages said wellbore in the sealed position. The tool also has a slip ring disposed about the mandrel. The slip ring is radially expandable outwardly from a disengaged position when the tool is in the unset position to an engaged position when the tool is in the set position. The slip ring grippingly engages the wellbore in the engaged position. A slip wedge is disposed about the mandrel. The slip wedge has a wedge portion having a generally conical shape with a first end having first outer radius and a second end having a second outer radius greater than said first outer radius. Additionally, the slip wedge has a back-up portion adjacent to the second end of the wedge portion. The back-up portion has a generally cylindrical shape with a third outer radius greater than the second outer radius and an abutment end that abuts the expandable sealing element when the tool is in the set position. The abutment end of the back-up portion acts to retain the expandable sealing element and resist extrusion of the expandable sealing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of a packer apparatus in the unset position having a wedge and limiter ring in accordance with one embodiment of the present invention. 
         FIG. 2  is a cross-sectional side view of the packer apparatus of the embodiment of  FIG. 1  in the set position. 
         FIG. 3  is a cross-sectional side view of the upper right section of the packer apparatus illustrated in  FIG. 1 . 
         FIG. 4  is a cross-sectional side view of the lower right section of the packer apparatus illustrated in  FIG. 1 . 
         FIG. 5  is a perspective view of limiter ring in accordance with an embodiment of the present invention. 
         FIG. 6  is a cross-sectional side view of a packer apparatus having a wedge in accordance with second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1-4 , downhole tool, or downhole apparatus  10  is shown in an unset position  11  in a well  15  having a wellbore  20 . The wellbore  20  can be either a cased completion with a casing  22  cemented therein as shown in  FIG. 1  or an openhole completion. Downhole apparatus  10  is shown in set position  13  in  FIG. 2 . Casing  22  has an inner surface  24 . An annulus  26  is defined by casing  22  and downhole tool  10 . Downhole tool  10  has a packer mandrel  28 , and may be referred to as a bridge plug due to the downhole tool  10  having a plug  30  being pinned within packer mandrel  28  by radially oriented pins  32 . Plug  30  has a seal means  34  located between plug  30  and the internal diameter of packer mandrel  28  to prevent fluid flow therebetween. The overall downhole tool  10  structure, however, is adaptable to tools referred to as packers, which typically have at least one means for allowing fluid communication through the tool. Packers may therefore allow for the controlling of fluid passage through the tool by way of one or more valve mechanisms which may be integral to the packer body or which may be externally attached to the packer body. Such valve mechanisms are not shown in the drawings of the present document. Packer tools may be deployed in wellbores having casings or other such annular structure or geometry in which the tool may be set. 
     Packer mandrel  28  has an outer surface  36 , an inner surface  38 , and a longitudinal central axis, or longitudinal axial centerline  40 . Also, as referred to herein the term “radially” will refer to a radial direction perpendicular to the longitudinal axial centerline. An inner tube  42  is disposed in, and is pinned to, packer mandrel  28  to help support plug  30 . 
     Downhole tool  10 , which may also be referred to as packer apparatus  10 , includes the usage of a spacer ring  44  which is preferably secured to packer mandrel  28  by pins  46 . Spacer ring  44  provides an abutment, which serves to axially retain slip ring  48  which is positioned circumferentially about packer mandrel  28 . Slip ring  48  may be composed of slip segments positioned circumferentially around packer mandrel  28  in order to form the slip ring  48 . Slip retaining bands  50  serve to radially retain slip ring  48  in an initial circumferential position about packer mandrel  28  as well as slip wedge  52 . Bands  50  are made of a steel wire, a plastic material, or a composite material having the requisite characteristics of having sufficient strength to hold the slip ring  48  in place prior to actually setting the downhole tool  10  and to be easily drillable when the downhole tool  10  is to be removed from the wellbore  20 . Preferably, bands  50  are inexpensive and easily installed about slip ring  48 . Slip wedge  52  is initially positioned in a slidable relationship to, and partially underneath, slip ring  48  as shown in  FIGS. 1 and 3 . Designs of slip ring  48  are described in U.S. Pat. No. 5,540,279, which is incorporated herein by reference. 
     Slip wedge  52  has a radially outer surface  54  containing a channel  56  therein. Additionally, slip wedge  52  has an abutment end  58  that abuts expandable sealing element  72 , located below slip wedge  52 . A limiter ring  60  is positioned in channel  56 . Limiter ring  60  has abutment end  62  that abuts expandable sealing element  72 . Limiter ring  60  is pressed into wedge  52  and can be held in place by frictional forces and/or adhesives. The limiter ring  60  can also serve to hold slip wedge  52  in place prior to setting the downhole tool. 
     As can be seen from  FIG. 3 , slip wedge  52  is designed as a partial cone with a first outer radius R 1  at first end  64  and a second outer radius R 2 , wherein R 2  is greater than R 1 . In one embodiment, slip wedge  52  has the wedge portion  66 , preferably having a generally conical shape, and a tongue portion  67 , preferably having a generally cylindrical shape. Tongue portion  68  has an outer radius R 3 , which is less than R 2 . In this embodiment, limiter ring  60  has an inner radius that is substantially equal to R 3  and an outer radius R 4  that is greater than R 2 . In a second embodiment, illustrated in  FIG. 6 , slip wedge  100  has a wedge portion  102  and a limiter portion or back-up portion  104  so that the limiter ring is an integral part of the slip wedge. In this second embodiment back-up portion  104  will preferably be generally cylindrical in shape and have an outer radius of R 4 . 
     Limiter ring  60  is design so that its outer surface  68  is close to inner surface  24  of casing  22  in order to minimize the gap between the two. Accordingly, the outer diameter of limiter ring  60  should be no more than 0.25 inch less than the inner diameter of the inner surface  24  to assure minimum extrusion of the expandable sealing element. In other words, outer radius R 4  should be no more than 0.125 inches less than the radius of inner surface  24  when the tool is in the set position. Additionally, the outer diameter of ring  60  should be no less than 0.125 inch less than the inner diameter of inner surface  24  to assure adequate clearance during insertion of the tool in the wellbore. In other words, radius R 4  should be no less than 0.0625 inch than the inner radius of inner surface  24  when the tool is in the unset position. 
     Limiter ring  60  can be a solid ring and applied to the downhole tool during assembly. In another embodiment, illustrated in  FIG. 5 , limiter ring  60  has an expansion joint  70 , which allows the limiter ring to be installed after assembly of the downhole tool; that is after the spacer ring, slip rings, slip wedges and expandable sealing elements have been assembled on the packer mandrel. Expansion joint  70  can be a z-cut type joint. 
     Slip wedge  52  can be composed of composition material as is known in the art. Generally, limiter ring  60  can be made form any suitable material that will withstand the downhole use and yet can be readily cut or ground up by drilling with a drill bit. While limiter ring  60  may be composed of a similar material to slip wedge  52 , generally limiter ring  60  will be formed from a material having a higher wear resistance such as brass or zirconia ceramic. Additionally, non-metallic engineering grade plastics can be used for the limiter ring, such as composite materials or structural phenolic materials. A suitable phenolic materials are available from General Plastics &amp; Rubber Company, Inc., 5727 Ledbetter, Houston, Tex. 77087-4095. Alternatively, structural phenolics available from commercial suppliers may be used. 
     Located below slip wedge  52  is a expandable sealing element  72 . The packer assembly of downhole tool  10  includes at least one such expandable sealing element, as shown in the figures, but may include two, three or more such expandable sealing elements. Expandable sealing element  72  has upper end  74  and lower end  76 . Expandable sealing element  72  has unset and set positions  78  ( FIG. 1) and 80  ( FIG. 2 ) corresponding to the unset and set positions  11  and  13 , respectively, of downhole tool  10 . The expandable sealing element  72  is radially expandable from the unset position  78  to a set position  80  in response to the application of axial force on the expandable sealing element  72 . Preferably, in unset position  78 , expandable sealing element  72  has an unset radius that is less than the outer radius R 4  of limiter ring  60 . Also preferably, in set position  80 , expandable sealing element  72  has a set radius that is greater than outer radius R 4  of limiter ring  60 . In the set position  80  the expandable sealing element  72  engages the wellbore  20  to create a seal to prevent flow through annulus  26 . 
     Slip wedge  52  and limiter ring  60  are disposed at the upper end  74  of expandable sealing element  72 . There is a second slip wedge  82  and limiter ring  84  disposed at the lower end  76  of expandable sealing element  72 . Slip wedge  82  and limiter ring  84  are similar to slip wedge  52  and limiter ring  60 ; accordingly, like parts have been given the same reference numerals. As shown in  FIGS. 1 ,  2  and  3  upper end  74  of expandable sealing element  72  resides directly against the abutting ends of upper slip wedge  52  and upper limiter ring  60 . Additionally, lower end  76  of expandable sealing element  72  reside directly against lower slip wedge  82  and lower limiter ring  84 . Thus, as illustrated in  FIG. 2 , by minimizing the gap between the outer surface  68  and the casing  22 , the upper and lower limiter rings retain the expandable sealing element in the set position and limit extrusion of the expandable sealing element; generally, this will be axial extrusion. Thus, the current limiter rings and slip wedges provide for a fixed extrusion limiter as opposed to the complex expanding extrusion limiter systems of prior art. 
     Located below slip wedge  82  is slip ring  86 . Slip wedge  82  and slip ring  86  are like slip wedge  52  and slip ring  48 . At the lowermost portion of downhole tool  10  is an angled portion, referred to as mule shoe  88 , secured to packer mandrel  28  by pin  90 . The lowermost portion of downhole tool  10  need not be mule shoe  88  but can be any type of section which will serve to terminate the structure of the downhole tool  10  or serve to connect the downhole tool  10  with other tools, a valve or tubing, etc. It will be appreciated by those in the art that pins  32 ,  46  and  79 , if used at all, are preselected to have shear strengths that allow for the downhole tool  10  to be set and deployed and to withstand the forces expected to be encountered in the wellbore  20  during the operation of the downhole tool  10 . 
     Although the disclosed invention has been shown and described in detail with respect to a preferred embodiment, it will be understood by those skilled in the art that various changes in the form and detailed area may be made without departing from the spirit and scope of this invention as claimed. Thus, the present invention is well adapted to carry out the object and advantages mentioned as well as those which are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.

Summary:
The invention relates to an improved downhole tool apparatus for limiting the extrusion of a sealing elements in downhole tools. The apparatus provides for using a limiter ring or shoe located in a channel on the slip wedge so as to abut the sealing element. The limiter ring extends outward to the casing to minimize the gap through which the sealing element can extrude when the tool is in a set position.