A device and method prevent damage to an anti-extrusion device on a tool, such as a plug or a packer, prior to and during setting of the tool. Typically, the upper or outer edges of the anti-extrusion device are relatively delicate. A reinforcing band on the device's sheath strengthens or armors the upper or outer edge of the anti-extrusion device so the anti-extrusion device may be protected while running the tool into the well or casing. Longitudinal slots on the sheath allow the sheath to expand at least partially with the expansion of the sealing element, while the reinforcing band resists expansion of the distal edge of the sheath.

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Appl. No. 61,777,523, filed 12 Mar. 2013, which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    In connection with the completion of oil and gas wells, it is frequently necessary to utilize plugs, packers, or other sealing tools in both open and cased boreholes. The walls of the well or casing are plugged or packed from time to time for a number of reasons. For example, a section of the well may be packed off so pressure can be applied to a particular section of the well, such as when fracturing a hydrocarbon bearing formation, while protecting the remainder of the well from the applied pressure. 
         [0003]    A sealing element on a tool, such as a packer or a plug, typically has an initial diameter to allow the tool to be run into the well. The sealing element is then expanded to a radially larger size to seal in the wellbore. Such a tool typically consists of a mandrel about which other portions of the tool are assembled. For example, a fixed gage ring is attached to the lower end of the mandrel, and a push ring slidably surrounds the upper end of the mandrel. If desired, a slip assembly can be used on the mandrel to lock the tool longitudinally in place in the well. In any event, a sealing element is disposed on the mandrel between the fixed gage ring and the push ring. When compressed between the rings, the sealing element creates a seal between the mandrel and the surrounding wall, thereby preventing fluid flow past the tool. 
         [0004]    Typically, when the tool is set, the mandrel is held in place and force is applied to the push ring. The push ring moves towards one end of the mandrel, causing the various parts of the tool's sealing element to be longitudinally compressed but radially expanded. As the push ring slides down the mandrel, the sealing element is compressed longitudinally. Most sealing elements are an elastomeric material, such as rubber. When compressed longitudinally, the sealing element tends to then expand radially to form a seal with the well or casing wall. 
         [0005]    Unfortunately, the sealing element's expansion may not be limited to only being radially outward. Instead, due to the forces applied during expansion or the force of the pressurized fluid upon the sealing element, the sealing element may extrude longitudinally along the tool through the spaces between the fixed gage ring and the well wall and/or between the push ring and the well wall. Due to the unwanted possibility of extrusion, anti-extrusion rings can be used to prevent the sealing element from extruding beyond the fixed gage ring or push ring, which would cause the tool to fail. Such anti-extrusion rings are employed along the mandrel between the ends of the sealing element and any push or gage rings or other components on the tool. 
         [0006]    The anti-extrusion rings may be an elastomeric material, such as nylon, that may not seal as well as the sealing element. However, the anti-extrusion rings may deform enough to prevent the sealing element form extruding to the point of failure. In some instances, metal materials, such as lead, copper, or steel, have been used as well for anti-extrusion rings. 
         [0007]    One common structure used for an anti-extrusion device is a cup. The cup fits against the end of the sealing element so that the element's end fits partially in the interior of the cup. The outer bottom of the cup fits against a gage ring or push ring. As the sealing element expands, the cup opens by splaying into a petal like arrangement. The expanded cup or petals tend to limit the longitudinal expansion of the sealing element. To increase the efficiency of the anti-extrusion device, multiple layers of cups may overlay one another so that any gaps, such as between the petals of a split cup, will be overlapped by the adjacent cup. 
         [0008]    For example, a downhole tool  10  having a cup-style anti-extrusion ring  20  according to the prior art is shown in  FIG. 1A . The downhole tool  10  is an open-hole packer having a mandrel  12  on which are disposed a hydraulic piston  14  and an end ring  16 . A sealing element  18  is disposed between a push ring  15  of the piston  14  and the end ring  16 . When moved by the piston  14 , the push ring  15  compresses the sealing element  18  longitudinally against the end ring  16 , which causes the sealing element  18  to expand out radially. 
         [0009]    Cup-style rings  20  are provided on the ends of the sealing element  18  at the push and end rings  15 ,  16 . These Cup-style rings  20  help prevent over-extrusion of the sealing element  18 . For example,  FIG. 1B  depicts a side cut away view of a prior art anti-extrusion ring  20  after the sealing element  18  has been expanded against the casing C and the mandrel  12  to seal the annular area A, thereby preventing fluid flow past the tool  10 . As the sealing element  18  expands radially outward, the leading edge  26  of the sheath  22  of the prior art anti-extrusion ring  20  is also pushed radially outward to contact the casing C. 
         [0010]    Further details of the cup-style ring  20  are provided in cross-section in  FIG. 10 . This ring  20  is a petal-style foldback ring having a number of petals  22  connected at their proximal ends by a neck  24  and separate by gaps or slots  26  toward their distal ends. During use, the petal-style ring  20  opens by splaying into a petal-like arrangement as discussed above. 
         [0011]    Another cup-style ring  30  shown in  FIG. 1D  lacks petals and does not splay open into a petal-like arrangement. Instead, this ring  30  has a widened sidewall  32  that fits partially along the outside surface of the sealing element ( 18 ) and the element's end. The sidewall  32  extends over the end of the sealing element ( 18 ) from a wider neck  34  that fits at the mandrel ( 12 ) and push or end ring ( 15 ,  16 ) of the packer ( 10 ). The distal end of the sidewall  32  has an integrally formed lip  36 , which is rounded in shape. As can be particularly seen, the thickness of the sidewall  32  lessens from the wider neck  34  to the lip  36 . 
         [0012]    Unfortunately, cups may be easily damaged as they are run into a well. Additionally, they may be damaged during setting when they are radially expanded into sealing contact with the well or after the element and cups are set because the tool may move longitudinally due to varying forces acting on the tool in the wellbore. Therefore, a need exists for an anti-extrusion device that tends to limit or prevent any damage to the anti-extrusion device during run-in and use downhole. 
       SUMMARY 
       [0013]    An anti-extrusion device according to the present disclosure has a slotted foldback ring with an anti-hopping band. The device installs adjacent a sealing element on a sealing tool, such as a plug or a packer. Features of the device prevent damage to the end of the device while run into the well and when expanded. Typically, the distal edges of an anti-extrusion device are relatively delicate. To protect the disclosed anti-extrusion device, the distal edge is strengthened or armored so the anti-extrusion device may be protected while running the tool into the well or casing. The anti-extrusion device can also be protected as the tool moves in the wellbore due to a variety of forces such as pressure and temperature that act upon the tool once set. 
         [0014]    In one embodiment, the anti-extrusion device for use on a downhole tool, such as a plug or a packer, in a wellbore has a proximal edge or inner ring disposed on the tool adjacent to an end of the sealing element. A sheath extends from the inner ring and has a distal edge disposed at least partially over the end of the sealing element. A reinforcing band is disposed on the distal edge of the sheath. The sheath has longitudinal slots (i.e., slits or burst lines). The reinforcing ring may be a solid round ring or a solid flat ring, and the sheath may be metallic, plastic, or some other material. 
         [0015]    In a method of restraining a sealing element on a downhole tool, an anti-extrusion ring overlaps a portion of the sealing element. The anti-extrusion ring has a reinforcing band on its leading edge to protect the anti-extrusion ring. The sealing element, the anti-extrusion ring, and the reinforcing ring are run together into a well. Once the anti-extrusion ring, the sealing element and the reinforcing ring are properly located, the sealing element may be expanded as the anti-extrusion ring restrains the sealing element. 
         [0016]    The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  depicts an elevational view of an open-hole packer having anti-extrusion rings according to the prior art. 
           [0018]      FIG. 1B  depicts an anti-extrusion ring according to the prior art in an expanded condition relative to a compressed sealing element on a mandrel. 
           [0019]      FIG. 1C  depicts a side cross-section of a prior art anti-extrusion ring. 
           [0020]      FIG. 1D  depicts a side cross-section of a prior art cup-style ring. 
           [0021]      FIG. 2  depicts a downhole tool, such as a packer or a plug, having anti-extrusion devices according to the present disclosure. 
           [0022]      FIG. 3A  depicts a cross-sectional view of an anti-extrusion device according to the present disclosure. 
           [0023]      FIG. 3B  depicts an end-sectional view of the anti-extrusion device of  FIG. 3A . 
           [0024]      FIG. 3C  depicts an orthogonal view of the anti-extrusion device of  FIG. 3A . 
           [0025]      FIG. 4  depicts the anti-extrusion device according to the present disclosure in an expanded condition relative to a compressed sealing element on a mandrel. 
           [0026]      FIG. 5A  depicts a cross-sectional view of another anti-extrusion device according to the present disclosure. 
           [0027]      FIG. 5B  depicts an end-sectional view of the anti-extrusion device of  FIG. 5A . 
           [0028]      FIG. 6A  depicts a cross-sectional view of another anti-extrusion device according to the present disclosure, 
           [0029]      FIG. 6B  depicts a perspective view of the inner member of the anti-extrusion device of  FIG. 6A . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0030]    The description that follows includes exemplary apparatus, methods, techniques, and instruction sequences that embody techniques of the inventive subject matter. However, it is understood that the described embodiments may be practiced without these specific details. 
         [0031]      FIG. 2  depicts a downhole tool  50 , such as a plug, a packer, or the like, in an unset or run-in condition in casing C (although the tool  50  can be used in an open hole). The tool  50  has a mandrel  52 , an end gage ring  54 , a sealing element  56 , and a push ring  58 . The end gage ring  54  is fixed to the lower end of the mandrel  52  and may be secured to the mandrel  52  using known techniques. The push ring  58  as well as the sealing element  56  are movable along the outside of the mandrel  52 . In this way, a setting tool (not shown) can be used to hold the mandrel  52  and push the push ring  58  toward the fixed ring  54 , causing the sealing element  56  to be compressed and expand radially. 
         [0032]    In general, the sealing element  56  may be an elastomer or any other material that may be relatively easily deformed. Moreover, although the sealing element  16  has been described above as a compressible element, other types of sealing elements, such as a swellable sealing element, can be used and benefit from the teachings of the present disclosure. 
         [0033]    To prevent extrusion of the sealing element  56  through the annular spaces between the rings  54  and  58  and the casing C and into the annulus spaces between the mandrel  52  and the casing C, the tool  50  uses anti-extrusion devices  60  according to the present disclosure. One device  60  fits at one (downhole) end of the tool  50  between the end of the sealing element  56  and the fixed gage ring  54 , while another device  60  fits at the other (uphole) end between the opposite end of the sealing element  56  and the push ring  58 . 
         [0034]    Each anti-extrusion device  60  has a number of slots  64  formed into it to allow the middle section  66  to expand radially outward. The proximal section  62  may be relatively solid to prevent the proximal section  62  from expanding radially, thereby maintaining an anti-extrusion seal against the mandrel  52 . The distal section  68  may be relatively solid to prevent the distal section  68  from expanding radially outward. By having a relatively solid distal section  68 , the anti-extrusion device  60  is able to resist tearing or snagging as the tool  50  is run into the wellbore. In some instances, it may be desired to allow the distal section  68  to radially expand a certain amount. In these instances, the distal section  68  may have a separate set of expansion slots, or it may be reinforced by a reinforcing ring, where the reinforcing ring could be stretchable, split, or split with overlapping rings. 
         [0035]    The slots  64  are typically longitudinally elongated slits or splits cut through the material of the device  60 , but they could also be perforations, indentations, thinned areas, score lines, etc. (e.g., “burst lines”) formed partially through or on the anti-extrusion device  60  to allow the middle section  66  to split along the slots  64 , which would allow the anti-extrusion device  60  to expand against the wellbore or casing C and prevent the sealing element  56  from extruding past the anti-extrusion device  60 . In some instances, it may be desirable to overlap multiple anti-extrusion devices  60  on top of one another at each end of the sealing element  56  so that any gaps formed by the slots  64  in one layered device  60  are overlapped by the petals of the device  60  in an adjacent layer. 
         [0036]    When the tool  50  is a plug and is set in position downhole, a setting tool (not shown) is secured to the mandrel  52  and applies force in the direction of arrow P to the push ring  58 . Where the tool  50  is a packer and is set in position downhole, the components for setting the element would be part of the packer's assembly so that a separate setting tool may not be used. Either way, as the push ring  58  is forced downwards along the mandrel  52 , each of the slidably mounted components is also moved longitudinally downwards against the fixed gage ring  54 . A locking mechanism (not shown) may typically be used to hold the push ring  58  in place on the mandrel  52  once forced downward. 
         [0037]    At the same time, the sealing element  56  is longitudinally compressed and expands radially outwards to seal against both the mandrel  52  and the casing C, sealing the exterior of the mandrel  52  to fluid flow in either direction. As the sealing element  56  expands radially outward, portions of the sealing element  56  may tend to extrude longitudinally. The anti-extrusion devices  60  tend to limit the extrusion of the sealing element  56 . 
         [0038]      FIGS. 3A-3C  depict an embodiment of an anti-extrusion device  100  according to the present disclosure.  FIG. 3A  depicts a cross-sectional view of the anti-extrusion device  100 ,  FIG. 3B  depicts an end-sectional view of the anti-extrusion device  100 , and  FIG. 3C  depicts an orthographic view of the anti-extrusion device  100 . 
         [0039]    The anti-extrusion device  100  has an inner ring  110  at a proximal end or edge, a sheath  120  in a middle section, and a reinforcing ring or band  130  at a distal end or edge. The band  130  reinforces the distal edge  126  of the sheath  120  and, as noted herein, acts as anti-hooping band. The inner ring  110  is mounted on a tool's mandrel, such as the mandrel  52  from  FIG. 2 , and may have fastener holes  112  or the like. If used adjacent a fixed gage ring or other component, the inner ring  110  may be fixedly held on the mandrel  52 . If used adjacent a push ring or other movable component, the inner ring  110  may be slidably mounted on the mandrel  52 . 
         [0040]    The sheath  120  extends from the inner ring  110 , and has the distal edge  126  where the reinforcing band  130  is attached. When placed on a tool prior to the tool being set, the reinforcing band  130  and the sheath  120  fit over the end of the sealing element, such as sealing element  56  from  FIG. 2 . 
         [0041]    A distal portion of the sheath  120 , nearest to the reinforcing band  130  tends to have a relatively uniform diameter for a set longitudinal distance, such as distance  128 . This distance  128  is typically the distance that the anti-extrusion device  100  overlaps the sealing element  56 . The proximal portion of the sheath  120  nearest to the inner ring  110  has a rapidly diminishing diameter where it attaches to the inner ring  110 . 
         [0042]    Slots  124  are defined around the circumference of the sheath  120 . The slots  124  can be cut, formed, molded, or otherwise produced in the material of the sheath  120 . Typically, the slots  124  are disposed longitudinally along the sheath  120  and may extend from the inner ring  110  to the reinforcing band  130 . The slots  124  can be full slits or perforations defined through the material of the sheath  120 . In other instances, the slots  124  may not perforate through the material of the sheath  120 . Instead, the slots  124  may be creased, cut, or molded areas of reduced thickness, such as burst lines, in the sheath material so that the sheath material may break to form split slits when expanded. Either way, the sheath  120  may form a number of petals  122  upon expansion of the sealing element  56 . 
         [0043]    The anti-extrusion device  100  can be composed of plastic, metal, other material, or a combination thereof. The inner ring  110  and the sheath  120  may be integrally formed as one piece, while the reinforcing band  130  can be a separate component affixed, fused, embedded, molded, or otherwise attached to the distal end of the sheath  120 . The reinforcing band  130  may in fact be formed as a metal ring with a round, flat, or other cross-section that is molded, embedded, or affixed to the distal edge  126  of the sheath  120 , which may be formed of the same or different material. In another alternative, the inner ring  110  can be a flat metal ring affixed or disposed on the proximal end of the sheath  120 . In yet another alternative, the reinforcing band  130  can be integrally formed with the sheath  120  as one piece. 
         [0044]    In  FIG. 4 , an embodiment of the anti-extrusion device  100  according to the present disclosure is depicted in a side cut away view. The sealing element  56  has been expanded against the casing C and the mandrel  52  to seal the annular area A, thereby preventing fluid flow past the tool  50 . Prior to its radial expansion, the sealing element  56  and the anti-extrusion device  100  were arranged so that a portion of the sheath  120  as well as the reinforcing band  130  on the leading edge  126  of the sheath  120  overlaid a portion of the exterior of an end of the sealing element  56 . 
         [0045]    As the sealing element  56  radially expands, the sealing element  56  causes the portion of the sheath  120  to move radially outward to contact the casing C, thereby preventing the sealing element  56  from extruding past the point where the anti-extrusion device  100  contacts the casing C. 
         [0046]    As discussed previously, the leading edge  126  of the sheath  120  of the anti-extrusion device  100  is attached to the reinforcing band  130 . During run-in and after the sealing element  56  has been expanded, the reinforcing band  130  protects the leading edge  126  from snags that the leading edge  126  may encounter as it moves in the wellbore. The reinforcing band  130  also tends to limit the leading edge  126  from expanding with the sealing element  56  radially outwards to an extent towards the casing C that in certain instances may cause the anti-extrusion device  100  to have the appearance of a cresting wave in cross-section. In certain embodiments, the reinforcing band  130  may be of an expandable type of material or may be split to allow the leading edge  126  to expand at least to some extent with the sheath  120  and the sealing element  56 . It may also be desirable to have the reinforcing band  130  comprise overlapping reinforcing rings. 
         [0047]      FIGS. 5A and 5B  show another embodiment of an anti-extrusion device  100  according to the present disclosure. Rather than having a separate or round reinforcing band  130 , the device  100  of  FIGS. 5A-5B  has a reinforcing area  132  at the distal edge  126  of the sheath  120 . This reinforcing area  132  is not slotted and may not have an area of reduced diameter. In some instances, this reinforcing area  132  may be radially thicker than the adjacent leading edge  126 . 
         [0048]    Again, the anti-extrusion device  100  can be composed of plastic, metal, other material, or a combination thereof. The inner ring  110  and the sheath  120  may be integrally formed as one piece, while the reinforcing area  132  can be a separate component affixed, fused, embedded, molded, or otherwise attached to the distal end of the sheath  120 . The reinforcing band  130  may in fact be formed as a metal ring with a flat cross-section. Also, the reinforcing band  130  may also be integrally formed with the inner ring  110  and the sheath  120 . 
         [0049]    In some instances, it may be desirable to mount multiple anti-extrusion devices  100  adjacent to one another, but have the slots  124  of each anti-extrusion device  100  offset from an adjacent anti-extrusion device  100  on the tool's mandrel  52 . By mounting multiple anti-extrusion devices  100  adjacent to one another in this way, any gaps  124  between the petals  122  of one anti-extrusion device  100  can be covered by the petals  122  of the adjacent anti-extrusion device  100 . 
         [0050]    As one example,  FIG. 6A  depicts a cross-sectional view of another anti-extrusion device according to the present disclosure for use on one end of a sealing element (not shown). This device includes an inner device  200  disposed between an outer device  100  and the sealing element (not shown). The outer device  100  can be similar to those disclosed above having the reinforcing ring or band  130 . The inner device  200  can also be the same and can have such a reinforcing band (not shown). 
         [0051]    As depicted in  FIG. 6A , however, the inner device  200  may lack a reinforcing band. Instead, as best shown in the isolated perspective of  FIG. 6B , the inner anti-extrusion device  200  includes an inner ring  210  at a proximal end and a sheath  220  at an opposing end. The inner ring  210  is mounted on a tool's mandrel, such as the mandrel  52  from  FIG. 2 , and may have fastener holes  212  or the like. If used adjacent a fixed gage ring or other component, the inner ring  210  may be fixedly held on the mandrel  52 . If used adjacent a push ring or other movable component, the inner ring may be slidable mounted on the mandrel  52 . 
         [0052]    The sheath  220  extends from the inner ring  210  and has a distal edge  226 . When placed on a tool prior to the tool being set, the distal edge  226  and the sheath  220  fit over the end of the sealing element, such as sealing element  56  from  FIG. 2 . As shown, the distal edge  226  of the sheath  220  lacks a reinforcing ring in this embodiment. Instead, the slots  224  (e.g., slits or burst lines) are defined on the sheath  220  from the inner ring  210  to the device's distal edge  226  so that the inner device  200  has a number of free petals  222 . 
         [0053]    With the inner device  200  disposed inside of the outer device  100  as shown in  FIG. 6A , the inner device's distal edge  226  is preferably shorter than the extent of the outer device  100 . In this way, the reinforcing band  130  on the outer device  100  can overlap further on the sealing element (not shown) when disposed adjacent thereto. As further noted above and as shown in  FIG. 6A , the slots  224  (slits or burst lines) in the inner sheath  220  are preferably radially misaligned with the slots  124  (slits or burst lines) in the outer sheath  120 , although other arrangements are possible. For instance, the inner and outer devices  100  and  200  may have different numbers of slots  124  and  224  and may be offset from one another in different configurations. 
         [0054]    The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter. 
         [0055]    In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.