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BACKGROUND 
       [0001]    A variety of packers are used in wellbores to isolate specific wellbore regions. A packer is delivered downhole on a conveyance and expanded against the surrounding wellbore wall to isolate a region of the wellbore. Once set against the surrounding wellbore wall, the packer can be subjected to substantial heat, pressures and forces. Consequently, flexible rubber packer layers can undergo undesirable extrusion which has a detrimental effect on the function of the packer. 
         [0002]    Some packers employ anti-extrusion layers to limit the undesirable extrusion. However, expansion/inflation of the packer under high temperature and high-pressure conditions can cause unwanted movement and/or disruption of the anti-extrusion layer. Often, existing packer designs are not suitable for controlling anti-extrusion layer movement during packer inflation and particularly during multiple inflation cycles. 
         [0003]    Undesirable movement of the anti-extrusion layer can be induced by a variety of factors related to operation of the packer in a downhole environment. By way of example, rubber materials may creep under the influence of inflation pressure which can result in unwanted additional expansion of the anti-extrusion layer and an undesirable shortening ratio. Expansion of the packer also can cause disorganization of anti-extrusion layer fibers which again increases the shortening ratio. The crossing or disruption of fibers in the anti-extrusion layer also may create high friction points that again influence the shortening ratio. The combination of excessive friction and unwanted local shortening ratio creates a substantial pulling force on the anti-extrusion layer at the packer extremities. If the pulling force is not controlled or countered, the anti-extrusion layer can be pulled free of the packer extremities or otherwise damaged in a manner that detrimentally affects the functionality of the packer. 
       SUMMARY 
       [0004]    In general, the present invention provides a system and method for controlling anti-extrusion layer movement during packer inflation. A packer is designed with a packer nipple having retention features which uniquely engage layers of the packer. The retention features are positioned at a plurality of distinct retention regions. Each retention feature is designed to engage a specific layer of the packer. An individual retention feature may be designed to secure the anti-extrusion layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0006]      FIG. 1  is a schematic front elevation view of a well system having a packer and completion deployed in a wellbore, according to an embodiment of the present invention; 
           [0007]      FIG. 2  is a partial cross-sectional view of one example of the packer illustrated in  FIG. 1 , according to an embodiment of the present invention; 
           [0008]      FIG. 3  is a schematic representation of a fiber based anti-extrusion layer, according to an embodiment of the present invention; 
           [0009]      FIG. 4  is an enlarged view of a section of the packer illustrated in  FIG. 2 , according to an embodiment of the present invention; 
           [0010]      FIG. 5  is a cross-sectional view of one example of a wall of a skirt member that may be employed in a mechanical extremity of the packer, according to an embodiment of the present invention; 
           [0011]      FIG. 6  is a cross-sectional view of one example of a wall of an inner packer nipple, according to an embodiment of the present invention; 
           [0012]      FIG. 7  is an enlarged portion of the inner packer nipple wall illustrated in  FIG. 6 , according to an embodiment of the present invention; 
           [0013]      FIG. 8  is an enlarged view similar to that of  FIG. 4  but showing an alternate embodiment for securing the anti-extrusion layer, according to an embodiment of the present invention; and 
           [0014]      FIG. 9  is an enlarged view similar to that of  FIG. 4  but showing another embodiment for securing the anti-extrusion layer, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0016]    The present invention generally relates to a system and method for constructing a packer designed to better withstand the rigors of actuation in a downhole environment. According to one embodiment, the packer is designed to maintain the placement of an anti-extrusion layer during multiple inflation cycles of the packer while in the high temperature well environment. Maintaining desired placement of the anti-extrusion layer helps ensure packer efficiency over the life of the packer. 
         [0017]    Generally, the packer comprises a plurality of expandable layers that are held at their opposed, longitudinal ends by mechanical extremities. In one example, the plurality of expandable layers comprises inner and outer bladders, an anti-extrusion layer, and a mechanical layer that may be formed of cables or other mechanical components to strengthen the packer. At least one of the mechanical extremities comprises retention features designed to individually retain selected packer layers. For example, retention features may be provided to independently secure the anti-extrusion layer. Additionally, the retention features also may be designed to independently secure other packer layers, such as the inner bladder layer and/or the mechanical layer. 
         [0018]    One or more of the improved packers can be used in a given well system to isolate regions of the wellbore. For example, packers can be used in cooperation with, or formed as part of, a variety of well completions. The well completions are deployed downhole for use in various production operations, servicing operations, or other well related operations. 
         [0019]    Referring generally to  FIG. 1 , one embodiment of a well system  20  is illustrated as deployed in a wellbore  22 , however many other types of well systems can be designed with individual or multiple packers. The illustrated well system  20  comprises a conveyance  24  employed to deliver at least one packer  26  downhole to a desired wellbore location. In many applications, packer  26  is deployed by conveyance  24  in the form of a tubing string, but conveyance  24  may have other forms, including wirelines or slick lines, for other applications. In the embodiment illustrated, conveyance  24  extends downhole from a wellhead  28  positioned at a surface location  30 . The packer  26  cooperates with or is part of a completion  32 . Furthermore, packer  26  is designed with one or more features that help preserve the packer and its functionality in a harsh downhole environment. In many downhole environments, packer  26  will be subjected to substantial differential pressures and high temperatures that can have detrimental impacts on the shortening ratio of various packer layers, such as the anti-extrusion layer. 
         [0020]    Referring generally to  FIG. 2 , one embodiment of packer  26  is illustrated via a partial cross-sectional view of an end of the packer. In this embodiment, packer  26  comprises an expandable portion  34  connected to a mechanical extremity  36  at each longitudinal end of the packer  26 . (Only one end of the packer  26  and one mechanical extremity  36  are illustrated for purposes of explanation.) By way of example, each mechanical extremity  36  comprises an inner packer nipple  38 , having an interior passage  39 , and an outer skirt  40 . In at least some embodiments, outer skirt  40  comprises a crimped skirt system  42 . The crimped skirt system  42  may comprise a single, unitary skirt portion or a plurality of independent skirt portions. In one alternate embodiment, crimped skirt system  42  comprises separate skirt portions for crimping an elastomeric layer, an anti-extrusion layer, and a mechanical layer, respectively. Basically, the inner packer nipple  38  and outer skirt  40  cooperate to hold and retain longitudinal ends of packer layers that form expandable portion  34 . Each mechanical extremity  36  may comprise other components, such as end connectors  44  by which packer  26  can be connected into a tubing string, completion, or other well equipment. 
         [0021]    The expandable portion  34  is constructed with a plurality of packer layers  46  arranged adjacent one another in a radial direction. In the embodiment illustrated, packer layers  46  comprise an inner bladder layer  48  that may be formed of a rubber material or other elastomeric material. Radially outward of inner bladder layer  48  is an anti-extrusion layer  50  positioned to prevent unwanted extrusion of the elastomeric material. The anti-extrusion layer  50  may be formed from a variety of materials that enable expansion and contraction of expandable portion  34 . For example, anti-extrusion layer  50  may be formed from fibers or a combination of fibers and mechanical cables. Additionally, the anti-extrusion layer  50  may extend from one mechanical extremity to the other or may extend axially inward a desired, but limited, distance from each mechanical extremity  36 . 
         [0022]    Packer layers  46  also may comprise a mechanical layer  52  which may be formed with metal, e.g. steel, cables or other structural components. By way of example, mechanical layer  52  comprises a plurality of steel cable layers  54 , e.g. two steel cable layers, and is located radially outward of anti-extrusion layer  50 . Depending on the application, mechanical layer  52  also may be designed with four cable layers, six cable layers, or other numbers of cable layers  54 . Packer layers  46  may further include an outer bladder or seal layer  56  which is the radially outermost layer of expandable portion  34 . Seal layer  56  is designed to seal against the surrounding wellbore wall, e.g. casing, to isolate the desired region of wellbore  22 . At least some of the individual packer layers  46  are independently retained at one or both of the mechanical extremities  36  via retention features  58  located at unique retention regions, as described in greater detail below. In one example, the retention features  58  are located along a radially outward exterior of the packer nipple  38 . 
         [0023]    In various applications, it is important to independently retain the anti-extrusion layer  50  against pulling forces that can occur during actuation, e.g. expansion, of packer  26 . The design of anti-extrusion layer  50  can affect the forces experienced by the anti-extrusion layer  50  and by the specific retention feature  58  used to independently retain the anti-extrusion layer. In the schematic example illustrated in  FIG. 3 , a portion of one embodiment of anti-extrusion layer  50  is illustrated as having layers of fibers  60 . Each layer of fibers  60  may be oriented at an opposite angle with respect to an axis of the packer  26  to provide controlled shortening of the anti-extrusion layer  50  during expansion of packer  26 . In some embodiments, the fibers  60  of anti-extrusion layer  50  are set along the packer length with an angle comprised between, for example, 0° and 20°. The fibers are set to form an anti-extrusion layer  50  that provides 100% surface coverage with no holes or other gaps between fibers before and after expansion, e.g. inflation, of packer  26 . The uniform coverage ensures the surrounding rubber material is protected against extrusion when inflation pressure is applied. 
         [0024]    Referring generally to  FIG. 4 , an enlarged view of a portion of packer  26  is provided to better illustrate various retention features  58 . In this example, inner bladder layer  48 , anti-extrusion layer  50 , and mechanical layer  52  are independently secured via unique retention features  58  for each layer. By way of example, retention features  58  may comprise an inner bladder layer gripping region  62 , an anti-extrusion layer gripping region  64 , and a mechanical layer retention region  66 . Each of these retention features is located at a unique position in the mechanical extremity  36  to enable independent engagement, e.g. gripping, of the specific layer. By way of example, each retention feature  58  may be axially offset from the other retention features  58 , as illustrated. The anti-extrusion layer gripping region  64  is designed to securely hold the anti-extrusion layer  50  against pulling forces, represented by arrow  67 , which can result during packer expansion due to a variety of factors, including an uncontrolled shortening ratio. 
         [0025]    The actual engagement of individual layers  46  by retention features  58  may be accomplished by a variety of mechanisms. According to one embodiment, mechanical layer  52  comprises a crimped end portion  68  that is retained between inner packer nipple  38  and outer skirt  40 . For example, crimped skirt system  42  may comprise a crimped engagement region  70 , as illustrated in  FIG. 5 , designed to engage crimped end portion  68  on one side of mechanical layer  52 . The crimped skirt system  42  also may comprise an anti-extrusion crimping feature  72 , such as a tooth, dedicated to helping secure the anti-extrusion layer  50 . As described above, crimped skirt system  42  may comprise a single, unitary skirt portion or a plurality of independent skirt portions  73  that may be separated at, for example, one or both of the dashed vertical lines in  FIG. 5 . As a result, various combinations of independent skirt portions  73  can be designed for specific applications. For example, the skirt system  42  may be separated at both vertical dashed lines to form independent skirt portions dedicated to the mechanical layer, the anti-extrusion layer, and the elastomeric layer, respectively. In other applications, the skirt system  42  can be separated at one of the vertical dashed lines or at other desired locations. By separating the skirt system  42  at one of the vertical dashed lines illustrated in  FIG. 5 , the skirt system forms two independent skirt portions in which one skirt portion comprises an independent elastomeric layer skirt portion or, alternatively, an independent mechanical layer skirt portion. Use of independent skirt portions  73  provides an ability to crimp specific packer layers without any interaction between those layers during operation of the packer. 
         [0026]    The mechanical layer retention region  66  of inner nipple  38  also may comprise a crimped surface region  74 , as illustrated in  FIG. 6 . The surface region  74  is designed to engage crimped end portion  68  on an opposite side of mechanical layer  52  relative to crimped engagement region  70 . Consequently, the mechanical layer  52  is securely and independently gripped and retained between inner packer nipple  38  and outer skirt  40 . 
         [0027]    Retention features  58  also may comprise a variety of other mechanisms and surfaces. For example, inner bladder gripping region  62  may comprise a series of surface undulations  76 , as illustrated in  FIG. 6 . The surface undulations  76  are designed to sink into and grip the elastomeric material of inner bladder layer  48  when the inner bladder layer is crimped or otherwise secured in mechanical extremity  36 . The anti-extrusion layer gripping region  64  also may comprise a variety of configurations. In the embodiment illustrated in  FIGS. 6 and 7 , the anti-extrusion layer gripping region  64  comprises a multilevel surface or profile  78  designed to engage the anti-extrusion layer  50 . The multilevel profile  78  also may be disposed at a desired angle to facilitate insertion and retention of the anti-extrusion layer  50 . 
         [0028]    In the example illustrated, the multilevel profile  78  is positioned and designed to cooperate with crimping feature  72  of skirt portion  42  to securely grip the anti-extrusion layer  50  when crimped between inner packer nipple  38  and outer skirt  40 . In this particular design, the anti-extrusion layer  50  is longer than inner bladder  48  to enable independent retention of the individual layers via distinct retention features  58 . This avoids use of the inner bladder layer  48 , which has a relatively poor shear resistance, to secure the anti-extrusion layer  50 . Instead, the anti-extrusion layer  50  is held by the multilevel profile  78  which may be formed in a metal material used to construct inner nipple  38 . The skirt and nipple designs may be optimized for efficient retention of the anti-extrusion layer  50  after crimping of the plurality of layers between the inner packer nipple  38  and outer skirt  40 . 
         [0029]    Referring generally to  FIG. 8 , another embodiment of a system for holding the anti-extrusion layer  50  is illustrated. In this embodiment, a binding  80 , such as a fiber binding, is disposed around a longitudinal end  82  of the anti-extrusion layer  50 . The binding  80  may be disposed along a surface of anti-extrusion layer  50  on an opposite side of the anti-extrusion layer relative to the anti-extrusion layer gripping region  64 . Also, the binding  80  may be squeezed between anti-extrusion layer  50  and mechanical layer  52 . Binding  80  is used to improve retention of anti-extrusion layer  50  in the mechanical extremity  36 . If binding  80  is formed as a fiber binding, it may be constructed with fibers that are wound around longitudinal end  82  of anti-extrusion layer  50  with sufficient tensile force to ensure efficient gripping of the layer via the anti-extrusion layer retention feature  64 . The fibers used to construct fiber binding  80  may be formed of a variety of materials, and may comprise aramid fibers, carbon fibers, metallic fibers, thermoplastic fibers, or other suitable fibers or mixtures of fibers. In some applications, the strength of the binding  80  is further improved by impregnating binding  80 , e.g. impregnating the wound fibers, with an additional material. By way of example, the impregnating material may comprise glue, thermo-hardened resin, epoxy resin, or other materials that create a strong composite. The binding  80  also can be continued above the inner bladder gripping region, as illustrated by dashed line  84 , to help ensure perfect inner bladder bonding to the nipple before crimping. 
         [0030]    In  FIG. 9 , another embodiment of a system for holding anti-extrusion layer  50  is illustrated. In this embodiment, the longitudinal end  82  of anti-extrusion layer  50  is impregnated with a stiffener material  84  to convert longitudinal end  82  into a stiff composite that can be better gripped via gripping region  64 , in at least some applications. The stiffener material may comprise a glue, a thermo-hardened resin, or another suitable material that helps ensure efficient holding of anti-extrusion layer  50 . 
         [0031]    In any of the embodiments described above where a component is described as being formed of rubber or comprising rubber, the rubber may include an oil resistant rubber, such as NBR (Nitrile Butadiene Rubber), HNBR (Hydrogenated Nitrile Butadiene Rubber) and/or FKM (Fluoroelastomers). In a specific example, the rubber may be a high percentage acrylonytrile HNBR rubber, such as an HNBR rubber having a percentage of acrylonytrile in the range of approximately 21 to approximately 49%. Components suitable for the rubbers described in this paragraph include, but are not limited to, inner bladder layer  48 , outer seal layer  56 , and elastomeric material between expandable layers. 
         [0032]    As described above, well system  20  and packer  26  may be constructed in a variety of configurations for use in many environments and applications. The packer  26  may be constructed from many types of materials and with components/layers positioned in various arrangements. Additionally, individual packer retention features can be arranged to hold a variety of selected, expandable packer layers. The specific surfaces and features used to independently grip specific packer layers are selected according to the packer design and layer material. Additionally, a variety of components and/or materials can be used in cooperation with select packer layers, e.g. the anti-extrusion layer  50 , to aid in retention during repeated expansion and contraction of the packer. The design and components of the packer extremities also can be adjusted according to the design of the overall packer. In some applications, the retention features are used at both packer extremities while in other applications the retention features are used at only one of the packer extremities. 
         [0033]    Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Such modifications are intended to be included within the scope of this invention as defined in the claims.

Summary:
A technique enables control over anti-extrusion layer movement during packer inflation. A packer employs a packer nipple having an exterior which uniquely engages distinct layers of the packer. The exterior comprises a plurality of distinct retention features, and each retention feature is designed to engage a specific layer of the packer. An individual retention feature can be uniquely designed to secure an anti-extrusion layer.