An apparatus, system, and method useful for discouraging the extrusion of at least one deformable member of a device into a space adjacent to the deformable member when the device is disposed within a borehole and the deformable member extends from the device to the borehole wall involves at least one expandable member disposed upon the device. The expandable member is expandable into the annulus to reduce the width thereof and retractable therefrom to allow movement of the device within and out of the borehole and redeployment of the expandable member. The apparatus, system, and method are particularly suited for limiting the extrusion of packer elements.

CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/395,037, filed Jul. 11, 2002, which is incorporated herein by reference. 


   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to methods, apparatus and systems for reducing the size of one or more spaces adjacent to a deformable component of a device. In one embodiment, for example, the invention relates to methods, apparatus and systems for discouraging the extrusion of one or more borehole sealing elements, or members, of a downhole tool into a gap formed between the tool and the borehole. The invention is particularly applicable to packers, including cup and tension set packers. 
   2. Description of Related Art 
   Various operations involve the use of devices having one or more deformable members that may undesirably move, or extrude, into a space adjacent to the deformable member. As used throughout this patent, the term “deformable member” means a component, part, or member that may deform under pressure. This often occurs, for example, with devices that are inserted into a borehole, wherein the deformable member is used to form a seal between the device and the borehole wall or other item(s). As used throughout this patent, the term “borehole” means a hole, passageway or area, such as, for example, a wellbore having a casing, within which a device having a deformable member may be deployed. To fit the device into the borehole, the outer width or diameter of the device is often smaller than the inner width or diameter of the borehole. After the device is positioned as desired in the borehole, the deformable member is extended from the device across the annulus formed between the device and the borehole wall, or other item(s), against which it will seal. In such instances, the deformable member may be undesirably forced or extruded into the open annulus adjacent to the deformable member, such as when subject to a differential pressure. Extrusion of the deformable member may not be desirable, such as when it causes the deformable member to become damaged or lose its seal. 
   This occurs, for example, in the petroleum exploration and recovery industries in operations involving the formation of seals around various types of tools and other equipment in subsurface wells. An example device having one or more deformable members that may be subject to undesirable extrusion is a packer. Packers are often used to secure the position of tubing or other equipment in a borehole, and to isolate the borehole above and below the packer to allow one or more treatment, or operation, to be conducted. A typical packer, which may, for example, be of the inflatable, cup, or tension set type, includes, among other components, one or more elastomeric members (the deformable member) that are extended across an annulus formed between the packer and the borehole wall to form a seal and isolate the borehole above and below itself. 
   Various solutions have been proposed to address the problem of undesirable extrusion of deformable members. Examples of proposed solutions for preventing or discouraging the extrusion of deformable members in packers and other devices in the petroleum exploration and recovery industries are disclosed in U.S. Pat. Nos. 6,167,963 B1; 6,102,117; 5,988,276; 5,904,354; 5,701,959; 5,603,511; and 5,924,696. These proposed solutions include the use of metal or phenolic pieces and/or garter springs embedded into the elastomeric member, a ceramic seat in which the elastomeric member is located, shoe-type supports held together by c-rings, and split rings or multiple slips. A complete review of each example patent will provide a thorough description of the disclosed design and indicate various limitations thereof. 
   Thus, there remains a need for methods, apparatus and/or systems for discouraging the undesirable extrusion of a deformable member having one or more of the following capabilities or features: reduces the width, or size, of the gap into which the deformable member may extrude; is removable from the borehole, resettable and reusable; is not deformable; does not hinder removal of the device from the borehole; does not leave substantial debris, or residual material, in the borehole that could obstruct removal of the device from the borehole; has a continuous, or unbroken, outer surface proximate to the deformable member; involves an anti-extrusion device that does not have openings into which the deformable member may extrude; does not significantly increase the length of the device with which it is used; is capable of providing forces upon the deformable member to assist in maintaining its position or seal; assists the deformable member to maintain an isolation seal in a borehole at significant differential pressures, such as, for example, greater than approximately 6000 psi, and high temperatures, such as greater than approximately 300° F.; assists in providing an isolation seal across a large gap in the borehole; assists in reducing the size of the gap sufficient to increase the pressure rating of the device; does not require an expensive rubber embedding process for its manufacture or assembly; and does not rely upon the bonding of rubber for its manufacture or assembly. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with the present invention, certain embodiments involve an apparatus that is useful for discouraging the extrusion of a deformable member of a device into a space located on at least one side of the deformable member when the device is disposed within a borehole, the deformable member extending between the device and borehole wall. The apparatus includes an annular expandable member disposed upon the device and movable between at least one radially non-expanded position and at least one radially expanded position. In a radially expanded position, the annular expandable member reduces the width of the space on at least one side of the deformable member around the circumference of the device, has a continuous outer surface proximate to the deformable member and assists in asserting forces upon the deformable member to maintain the deformable member in engagement with the borehole wall. When the annular expandable member is in a radially non-expanded position, the device is deliverable into and removable from the borehole without obstruction caused by the annular expandable member. In these embodiments, the annular expandable member is redeployable. 
   The deformable member may be an elastomeric seal and the device may be a retrievable packer. The annular expandable member may be located adjacent to and up-hole of the deformable member. The annular expandable member may be integral to, or connected with, the deformable member, or may be a separate component. A plurality of annular expandable members positioned on at least one among the opposing sides of the deformable member may be included. 
   The annular expandable member may include at least one spring-acting member, such as, for example, a wave spring, compression spring or bellville washer. A ramped ring upon which the spring-acting member is movable between radially non-expanded and radially expanded positions may be included. A mandrel upon which the deformable member and the annular expandable member are carried, and a centralizer associated with the annular expandable member which is capable of generally maintaining the annular expandable member centered upon the mandrel may be included. The apparatus may include a load ring associated with the annular expandable member and capable of asserting force upon the deformable member to maintain the deformable member in engagement with the borehole wall. 
   The annular expandable member may experience insubstantial permanent structural deformation after being moved between radially non-expanded and radially expanded positions. The annular expandable member may be disposed in a flexible cover. 
   Various embodiments of the invention involve an anti-extrusion apparatus useful for reducing the width of an extrusion gap adjacent to a seal member of a downhole device deployed in a borehole, the seal member extending between the downhole device and the borehole wall. The anti-extrusion apparatus includes a redeployable spring-acting member disposed upon the downhole device, the spring-acting member being movable between a radially non-expanded position and a radially expanded position. In the radially expanded position, the spring-acting member reduces the width of the extrusion gap on at least one side of the seal member around the circumference of the downhole device and assists in imparting forces upon the seal member to maintain the seal member in engagement with the borehole wall. In the radially non-expanded position, the spring-acting member is movable within and from the borehole without obstructing movement of the downhole device. 
   A load ring associated with the spring-acting member and capable of asserting force upon the seal member to maintain the seal member in engagement with the borehole wall may be included. A ramped ring upon which the spring-acting member is movable between the radially non-expanded position and the radially expanded position may be included. Two spring-acting members may be included. The spring-acting member may include at least one wave spring. The spring-acting member may have a continuous outer surface proximate to the seal member. The seal member may be an elastomeric seal and the downhole device may be a packer. 
   Certain embodiments of the invention involve a downhole tool capable of sealing an annulus formed between the downhole tool and a borehole wall in the presence of a pressure differential in the annulus. The downhole tool includes an inner mandrel deployable into the borehole, at least one elastomeric member, and at least one annular expandable member carried upon the inner mandrel. The elastomeric member is extendable from the downhole tool and sealingly engageable with the borehole wall. The annular expandable member is extendable into the annulus around the circumference of the downhole tool, reducing the width of the annulus proximate to the elastomeric member to discourage extrusion thereof into the annulus. The annular expandable member is also capable of assisting in asserting forces upon the at least one elastomeric member to maintain the at least one elastomeric member in sealing engagement with the borehole wall. The annular expandable member is further capable of being retracted from the annulus to allow movement of the downhole tool within and from the borehole, and the annular expandable member is redeployable. 
   The differential pressure in the annulus may be at least approximately 6000 psi. The annular expandable member may have a continuous outer surface sufficient to discourage extrusion of the elastomeric member into the annular expandable member during normal operations. The annular expandable member may include a spring-acting member. 
   Some embodiments of the invention involve a method for discouraging the extrusion of a seal member of a device into an annulus formed between the device and the wall of a borehole, the seal member extendable from the device across the annulus and sealingly engageable with the borehole wall, the device also carrying an annular expandable member. The method includes applying compressive force to the annular expandable member, causing the annular expandable member to extend into and reduce the width of the annulus on at least one side of the seal member around the circumference of the device. The annular expandable member asserts force upon the seal member to assist in maintaining the seal member in engagement with the borehole wall. Compressive force is removed from the annular expandable member, causing the annular expandable member to retract from the annulus and allow movement of the device within and out of the borehole, and enabling redeployment of the annular expandable member and seal member. If desired, the device may be repositioned in the same or another borehole. The seal member and annular expandable member may be redeployed. 
   Accordingly, the present invention includes features and advantages which are believed to enable it to advance the technology associated with discouraging the extrusion of a deformable member. Characteristics and advantages of the present invention described above and additional features and benefits will be readily apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments and referring to the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of preferred embodiments of the invention, reference will now be made to the accompanying drawings wherein: 
       FIG. 1  is a partial cross-sectional view of a conventional, or prior art, tension or compression-set mechanical packer disposed in a borehole; 
       FIG. 2  is a partial cross-sectional view of an embodiment of an anti-extrusion device having a wave spring form of expandable member in accordance with the present invention shown used with a packer in a borehole; 
       FIG. 3  is a partial cross-sectional view of the embodiment of  FIG. 2  showing the expandable member in an expanded position; 
       FIG. 4  is a perspective view of an example wave spring that may be used in one or more embodiment of an anti-extrusion device in accordance with the present invention; 
       FIG. 5  is a partial cross-sectional view of an embodiment of an anti-extrusion device that includes a centralizer in accordance with the present invention; 
       FIG. 6  is a partial cross-sectional view of another embodiment of an anti-extrusion device having a compression spring form of expandable member in accordance with the present invention shown used with a packer in a borehole; 
       FIG. 7  is a partial cross-sectional view of the embodiment of  FIG. 6  showing the expandable member in an expanded position; 
       FIG. 8  is a partial cross-sectional view of another embodiment of an anti-extrusion device having a wave spring form of expandible member in accordance with the present invention shown in use with cup packers in a straddle arrangement while running into a cased borehole; 
       FIG. 9  is a partial cross-sectional view of the embodiment of  FIG. 8  shown with the cup packers energized by pressure in the straddled interval; 
       FIG. 10  is a partial cross-sectional view of the lower cup packer and anti-extrusion device of the embodiment of  FIGS. 8 and 9  as it encounters a restricted diameter section in the cased borehole; and 
       FIG. 11  is a partial cross-sectional view of the lower cup packer and anti-extrusion device of  FIG. 10  as the anti-extrusion device passes through the restricted diameter section in the cased borehole. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   Presently preferred embodiments of the invention are shown in the above-identified figures and described in detail below. It should be understood that the appended drawings and description herein are of preferred embodiments and are not intended to limit the invention or the appended claims. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. In showing and describing the preferred embodiments, like or identical reference numerals are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
   As used herein and throughout all the various portions (and headings) of this patent, the terms “invention”, “present invention”, and variations thereof are not intended to mean the claimed invention of any particular appended claim or claims, or all of the appended claims. These terms are used to merely provide a reference point for subject matter disclosed herein. The subject or topic of each such reference is thus not necessarily part of, or required by, any particular claim(s) merely because of such reference. Accordingly, the use herein of the terms “invention”, “present invention”, and variations thereof is not intended and should not be used in arriving at the construction or scope of the appended claims. 
   For background purposes, reference will now be made to  FIG. 1 , wherein a downhole tool  10  is shown. The illustrated downhole tool  10  is a mechanical packer  12 . The packer  12  is shown including an inner mandrel  18  carrying an upper gage ring  22 , lower gage ring  26 , and a deformable member  28 , as are or become known. The deformable member  28  of this example is an elastomeric member  30 . In some embodiments, there may be numerous such components. Thus, the references herein to a component in the singular tense includes embodiments having one or more such component(s) (e.g. the term “elastomeric member” means one or more elastomeric member). 
   In the example shown, the axial position of the elastomeric member  30  and lower gage ring  26  are fixed upon the inner mandrel  18 . However, the inner mandrel  18 , the elastomeric member  30 , and lower gage ring  26  are axially movable relative to the upper gage ring  22 . This enables setting of the illustrated packer  12 , as is or becomes known. It should be understood that these components of the packer  12 , if included, may take any other suitable form and configuration. Moreover, the packer  12  may include different or additional components. 
   The inner mandrel  18  of the illustrated example includes a conduit  20  in fluid communication with the bore of a tubing (not shown), which is used to convey the tool  10  into a borehole  40 . The tubing (not shown) may be coiled tubing, or any other suitable tubing or component(s). Thus, as used herein and throughout the various portions of this patent, the term “tubing” and variations thereof means coiled tubing, jointed drill string elements, or any other desirable component(s) capable of deploying a tool, or other device, into a borehole. 
   Still referring to  FIG. 1 , the illustrated packer  12  is shown in a run-in-hole position in the borehole  40 . While the borehole  40  of  FIG. 1  appears vertically-oriented, the present invention is not limited to any particular orientation of the borehole  40 . For example, the tool  10  may be used in a borehole  40  that is non-vertical, such as a “horizontal” or “deviated” well. 
   In typical use, the exemplary packer  12  is inserted into the borehole  40  via the tubing (not shown). To enable such insertion, the outer diameter of the packer  12  is smaller than the inner diameter of the borehole  40 . When the packer  12  is moved into the borehole  40 , an area, or annulus, is thus generally formed between the packer  12  and the borehole wall  42 . This area, or annulus, is referred to herein as the “extrusion gap”  44 . After the desired depth of the packer  12  in the borehole  40  is achieved, the elastomeric member  30  is expanded to form a seal between the packer  12  and the borehole wall  42 , creating isolated zones in the borehole  40  above and below the elastomeric member  30 . These borehole zones are referred to herein as the upper and lower zones  46 ,  48 , respectively (see e.g. FIG.  3 ). 
   Specifically with reference to the packer  12  of  FIG. 1 , the elastomeric member  30  is expanded by imparting axial forces upon it, as is known in the art. For example, the upper gage ring  22  may be secured to the borehole wall  42 . The inner mandrel  18 , elastomeric member  30 , and lower gage ring  26  are then moved upwardly relative to the fixed upper gage ring  22 . This action effectively squeezes the elastomeric member  30  between the upper and lower gage rings  22 ,  26 , causing it to expand. The elastomeric member  30  is thus compressed under axial forces, causing it to expand outwardly into the borehole  40  to ultimately engage and form a seal with the borehole wall  42 , as is or becomes known. 
   If the pressure in the upper and lower zones  46 ,  48  differs, the elastomeric member  30  is subject to a pressure differential, which may cause the elastomeric member  30  to extrude or deform into the extrusion gap  44  adjacent to it. In such instances, the elastomeric member  30  may become damaged and/or lose its seal with the borehole wall  42 , potentially compromising or disrupting isolation of the upper and lower zones  46 ,  48 . The ability to maintain isolation of the zones  46 ,  48  may thus be dependent upon the size or width of the extrusion gap  44  adjacent to the elastomeric member  30 . 
   Further details of the components, arrangement, and operation of the packer  12 , as well as alternate components and arrangements therefore, are, or will be, known to persons skilled in the art, and can be found in various patents and printed publications, such as, for example, U.S. Pat. Nos. 6,257,339; 4,862,961; and 4,665,977, each of which is incorporated herein by reference. 
   The above description of the illustrated packer  12  and its operation is provided for illustrative purposes only and is not limiting upon the present invention. Moreover, the present invention, embodiments of which will be described below, is not limited to use with packers, but can be incorporated in, or associated with, any tool, or device, having a deformable member disposable across an area, or which is used to separate two or more zones. Thus, the type, operation, components and arrangement of the packer  12 , or other tool  10 , and the environment within which it is used are in no way limiting upon the present invention. 
   Referring now to  FIG. 2 , an example anti-extrusion device  50  in accordance with the present invention is shown associated with the tool  10 . The illustrated anti-extrusion device  50  includes an expandable member  54  that is expandable into the extrusion gap  44  to reduce the width, or size, of the gap  44  on at least one side of the deformable member  28 . 
   The illustrated expandable member  54  is located adjacent to and up-hole of the elastomeric member  30  and, as shown in  FIG. 3 , is expandable entirely across the extrusion gap  44 , thus abutting, or engaging, the borehole wall  42 . This embodiment is thus capable of closing off or blocking the extrusion gap  44  up-hole of the elastomeric member  30  and may be desirable, for example, when the upper zone  46  of the borehole  40  has, or is expected to have, a lower pressure than the lower zone  48 . It should be understood, however, that the expandable member  54  may instead extend only partially into, or across, the extrusion gap  44 , reducing its width. Further, if desired, the anti-extrusion device  50  may include multiple expandable members (not shown) on either side, or both sides, of the deformable member  28 . For example, it may be desirable to locate the expandable member  54  on the downhole side of the deformable member  28  when the pressure in the upper zone  46  is expected to be higher than the pressure in the lower zone  48 . 
   Referring specifically to  FIG. 2 , the expandable member  54  may take any suitable form and configuration. For example, the expandable member  54  may be an annular spring-acting member  56  that is radially expandable into the extrusion gap  44  around the circumference of the tool  10  after the tool  10  is positioned as desired in the borehole  40 . In the embodiment shown, the expandable member  54  is a spring-acting member  56  disposed around the inner mandrel  18  between the elastomeric member  30  and the upper gage ring  22 . The illustrated spring-acting member  56  is a wave spring  58 , as is or becomes known. An example embodiment of a wave spring  58  is shown in FIG.  4 . However, the present invention is not limited to the inclusion of a wave spring  58 . Any other suitable expandable member  54  may be used. For example, the expandable member  54  may include one or more bellville washers (not shown) or compression spring  76  (FIG.  6 ). 
   The example expandable member  54  of  FIGS. 2 and 3  is a separate component of the anti-extrusion device  50 . However, the expandable member  54  may instead be molded into, integral with, or attached to the deformable member  28 , or another component. Further, the illustrated expandable member  54  has a continuous, or unbroken, outer surface proximate to the deformable member  28  to prevent extrusion of the deformable member  28  into the expandable member  54  itself. If desired, the expandable member  54  may be contained in, or include, a cover or carrier (not shown), such as a flexible elastomeric, or high-elasticity rubber, sheath. Such configuration may be desirable, for example, to assist in preserving and/or excluding debris from the expandable member  54  during use, providing a continuous outer surface of the expandable member  54 , or any combination thereof. However, the continuous outer surface and use of a cover may not be included in some embodiments of the invention. 
   Referring again to  FIG. 2 , the expandable member  54  is shown associated with a carrier  62 . The illustrated carrier  62  is a piston-shaped load ring  64  engaged around the inner mandrel  18  in a fixed axial position. The load ring  64  includes a body  66  and a shoulder  68 . The body  66  carries the wave spring  58 , while the shoulder  68  engages, and applies axial forces to the elastomeric member  30 . However, the carrier  62  need not be a load ring  64  having such features, but can be any suitable device for carrying the wave spring  58  and/or engaging the elastomeric member  30 . Yet further, if desired, a different component may be used for engaging the deformable member  28 . Moreover, a carrier  62  or other such component(s) may not be included in some embodiments. 
   Now referring to  FIG. 5 , the anti-extrusion device  50  may include a centralizer  72  useful for assisting in centering the expandable member  54  on the tool  10 . In the particular embodiment shown, the centralizer  72  includes a body  73  and shoulder  75  and is used in place of the carrier  62 . The body  73  carries the wave spring  58 , while the shoulder  75  engages and applies axial forces to the elastomeric member  30 . The example centralizer  72  also includes a relatively steep-angled cone surface  74 , which assists in maintaining the wave spring  58  centered on the inner mandrel  18  throughout operations. However, the centralizer  72  can take any other suitable form and configuration. For example, the centralizer  72  may be used in addition to a carrier  62 , or may be part of another component. Moreover, a centralizer  72  may not be included in some embodiments. 
   Referring to  FIG. 3 , the expandable member  54  may be expanded into the extrusion gap  44  with any suitable technique. For example, when the expandable member  54  is a spring-acting member  56 , axial forces may be used to expand and allow the retraction of the spring-acting member  56 . In the particular embodiment shown, axial forces are placed upon the wave spring  58  when the inner mandrel  18  is moved relative to the anchored upper gage ring  22  to expand the elastomeric member  30  and set the packer  12 . As the inner mandrel  18  is drawn upwardly, the carrier  62  and wave spring  58  are sandwiched between the upper gage ring  22  and elastomeric member  30 . The wave spring  58  is compressed, causing its outer diameter to expand around its circumference, the wave spring  58  thus extending into the extrusion gap  44 . Since the exemplary wave spring  58  has a continuous outer surface (as shown in FIG.  4 ), the elastomeric member  30  should not extrude into the wave spring  58 . 
   In another aspect of the invention, the anti-extrusion device  50  may, if desired, be designed to assist the elastomeric member  30  in maintaining its position, or seal, in the borehole  40 . In the embodiment of  FIG. 3 , for example, when the wave spring  58  is expanded and the elastomeric member  30  is set, the wave spring  58  maintains spring force upon the carrier shoulder  68 , which, in turn, applies generally continuous axial force upon the elastomeric member  30 . Such forces maintained on the elastomeric member  30  by the anti-extrusion device  50  may be sufficient to retain the elastomeric member  30  in a set, or sealingly engaged, state during typical use conditions. In an exemplary embodiment, the anti-extrusion device  50  may be designed to assist in maintaining an isolation seal at the elastomeric member  30  at high differential pressures, such as over 6000 psi, and high temperatures, such as over 300° F., or within large extrusion gaps  44  at lower temperatures and pressures. It should be understood, however, that this aspect may not be included in some embodiments. 
   Referring again to  FIGS. 2 and 3 , in yet another aspect of the invention, the expandable member  54  may, if desired, be retractable from an expanded to a non-expanded position, so as to allow movement of the tool  10  within or out of the borehole  40  and/or redeployment of the tool  10  and expandable member  54  within the same or other boreholes  40 . In the embodiment shown, for example, when the packer  12  is disengaged from the borehole wall  42  by lowering the inner mandrel  18 , axial forces on the expandable member  54  are released. The expandable member  54  moves to its original, or a substantially non-expanded, state, thus retracting out of the extrusion gap  44  and removing axial forces placed upon the elastomeric member  30  thereby. In a non-expanded state, the expandable member  54  should not catch on the borehole wall  42  or otherwise hinder the mobility of the tool  10 . Further, the exemplary anti-extrusion device  50  should leave no debris or residual material in the borehole  40  that could disrupt movement of the tool  10  therein. The tool  10  is thereafter movable within and from the borehole  40 , and redeployable without obstruction caused by the anti-extrusion device  50 . 
   If desired, the anti-extrusion device  50  may be designed so that the expandable member  54  in a non-expanded state is set back from the outermost diameter of the tool  10 , such as, for example, the outer diameter of the upper and/or lower gage rings  22 ,  26 . In such instance, other components of the tool  10  will generally protect the expandable member  54  in a non-expanded state from damage during movement into, within, and out of the borehole  40 . The expandable member  54  may thereafter be redeployed by repeating the expansion process as described above or another suitable technique. It should be understood, however, that retraction and redeployment of the expandable member  54  may not be included in some embodiments. 
     FIGS. 6 and 7  illustrate another embodiment of an anti-extrusion device  50  in accordance with the present invention. In this example, the expandable member  54  is a compression spring  76  used with a cone-shaped ring  80  that is disposed around the inner mandrel  18 . The ring  80  includes a ramp-shaped, or angled, surface  82  upon which the compression spring  76  is disposed and a shoulder  84  engageable with the elastomeric member  30 . 
   When deploying the packer  12  of this embodiment, generally axial forces placed upon the compression spring  76  push the spring  76  along the ramped surface  82  of the ring  80 . The first coil  86  of the spring  76  partially unwinds and the outer diameter of the spring  76  is forced to expand into the extrusion gap  44 . Also, the expanded spring  76  places forces upon the ring shoulder  84 , which, in turn, maintains generally axial force upon the elastomeric member  30  to assist in retaining it in a set, or sealingly engaged, position during use. Upon removal of axial forces on the exemplary compression spring  76 , the spring  76  returns to its nominal, or non-expanded, state enabling movement of the tool  10  and redeployment of the expandable member  54 . 
   The compression spring  76  and ring  80  may, if desired, be sized to achieve the desired expansion of the spring  76 . For example, the compression spring  76  may be formed with a relatively small length and its spring element  77  tightly wound. For another example, the angled surface  82  of the ring  80  may have a specific desired angle, such as approximately thirty degrees (30°). For still a further example, the combined size of the largest diameter of the ring  80  along its surface  82  and twice the diameter of the compression spring element  77  may be specifically selected to cause the compression spring  76  to extend into the extrusion gap  44  to a desired, or pre-determined, extent. Other than as described above, the characteristics, capabilities, and operation of this embodiment of the anti-extrusion device  50  may, if desired, be generally similar to those described above for the embodiment of  FIGS. 2 and 3 . 
   Referring now to  FIG. 8 , the anti-extrusion device  50  of the present invention is illustrated in use with cup packers on a straddle tool  11  such as is used for isolating a borehole interval for treatment. The straddle tool  11  comprises an upper cup packer  14  and a lower cup packer  16  mounted on the inner mandrel  18  by means of mounting rings  15  and  17 , respectively. The wave springs  58  of the anti-extrusion device  50  associated with each cup packer are mounted between the respective mounting ring  15 ,  17  and a backup ring  19 ,  21 . When the straddle tool  11  is run into the borehole  40 , in the example illustrated in  FIG. 8  a borehole  40  having a casing  41  therein, the wave springs  58  are uncompressed so as to pass easily through the casing  41 . The mounting ring  15  of the upper cup packer  14  is secured to the mandrel  18  by a shear member  24 , which may, for example, be in the form of a shear pin or pins, or a shear ring. The backup ring  21  associated with lower cup packer  16  is likewise secured against movement along mandrel  18  by a shear member  25 , which may also, for example, be in the form of a shear pin or pins, or a shear ring. 
   Referring now to  FIG. 9 , once the straddle tool  11  has been positioned at the proper depth in the borehole  40 , the borehole interval  43  straddled by the packers  14 ,  16  may be treated by pressurized fluid injected into the borehole interval  43  from the conduit  20  inside mandrel  18  through a port or ports  23 . The pressurized fluid entering the borehole interval  43  forces the packers  14 ,  16  apart and seats them against the casing  41  forming a seal therewith. The force created by the pressurized fluid in the borehole interval  43  causes the shear pin or pins  24  to shear, allowing movement of the packer  14  along the mandrel  18  to compress the wave springs  58  of the anti-extrusion device  50 . Likewise, pressurization of the borehole interval  43  causes packer  16  to move along the mandrel  18  until its mounting ring  17  seats against shoulder  27  on mandrel  18  thereby compressing the wave springs  58  of the anti-extrusion device  50  associated therewith. 
   Once treatment of the borehole interval  43  is complete, the interval  43  is depressurized in preparation for removal of the straddle tool  11  from the borehole  40 . As illustrated in  FIG. 10 , the wave springs  58  will generally remain in the compressed state, thereby presenting an obstruction to removal of the straddle tool  11  when the lower cup packer  16  encounters a restricted diameter section  45 , such as a nipple, in the casing  41 . When this occurs, the force created by the restricted diameter section  45  against the compressed wave springs  58  causes the shear pin or pins  25  to shear, allowing the wave springs  58  to relax until the backup ring  21  seats against the shoulder  29  in the mandrel  18 . 
   As illustrated in  FIG. 11 , the straddle tool  11  can then pass through the restricted diameter section  45  of the casing  41 . 
   The present invention includes additional features, capabilities, functions, methods, uses and applications that have not been specifically addressed herein but are, or will become, apparent from the description herein, the appended drawings and claims. Preferred embodiments of the present invention thus offer advantages over the prior art and are well adapted to carry out one or more of the objects of the invention. 
   It should be understood that the present invention does not require each of the techniques or acts described above. Moreover, the present invention is in no way limited to the above methods. Further, the methods described above and any other methods which may fall within the scope of any of the appended claims can be performed in any desired suitable order and are not necessarily limited to the order described herein or listed in the appended claims. Yet further, the methods of the present invention do not require use of the particular embodiments shown and described in the present specification, such as, for example, the expandable member  54  of  FIGS. 2-7 , but are equally applicable with any other suitable structure, form and configuration of components. 
   Also, it should be understood that the present invention does not require all of the above features and aspects. Any one or more of the above features or aspects may be employed in any suitable configuration without inclusion of other such features or aspects. Further, while preferred embodiments of this invention have been shown and described, many variations, modifications and/or changes of the apparatus and methods of the present invention, such as in the components, details of construction and operation, arrangement of parts and/or methods of use, are possible, contemplated by the patentee, within the scope of the appended claims, and may be made and used by one of ordinary skill in the art without departing from the spirit or teachings of the invention and scope of the appended claims. All matter herein set forth or shown in the accompanying drawings should thus be interpreted as illustrative and not limiting. Accordingly, the scope of the invention and the appended claims is not limited to the embodiments described and shown herein.