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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 60/280,057 filed on Mar. 30, 2001, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to cup packers for use in wellbore tools and, more specifically, formation interval straddle tools that are employed for earth formation zone fracturing or other formation treating operations in wellbores. More particularly, the invention relates to improved cup packers which resist the intrusion of sand and debris into their open end. 
     2. Description of Related Art 
     Cup packers used on formation interval straddle tools and other wellbore tools for oilfield applications are generally formed of hardened rubber materials and are of an open-ended design. During formation fracturing or treating operations when pressurized fracturing or treating fluids are pumped through the straddle tool to the formation zone to be fractured or treated, the open ends of such cup packers fill with the treating fluid which often has sand and debris entrained therein. If a “screenout”, during which sand is left within the straddled interval of the wellbore following treatment, occurs, the fluid within the straddled interval can become dehydrated forming a dense sand pack between the cup packers and within the open ends of the cup packers. The mechanical wedging of sand between the cup packers can result in high pulling forces during retrieval of the straddle tool following treatment of the formation. Additionally, sand wedged within the open ends of the cup packers may impair their ability to properly seal the interval straddled between the upper and lower packers of the straddle tool to sustain the necessary differential pressure during subsequent treatments of the formation. 
     Therefore, there is a need for an improved cup packer that resists the intrusion of sand and debris into its open end. 
     BRIEF SUMMARY OF THE INVENTION 
     It is a principal feature of the present invention to provide a cup packer for use in wellbore tools and, more specifically, formation interval straddle tools, that resists the intrusion of sand and debris into its open end and thereby improves the operational characteristics and pressure-sealing performance of the wellbore tool. 
     Briefly, the invention is a cup packer wherein the open end is sealed or screened to prevent sand and debris intrusion therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained may be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the preferred embodiments thereof illustrated in the appended drawings. 
     FIG. 1 is a schematic representation of a formation interval straddle tool employed for earth formation zone fracturing or other formation treating operations deployed in a wellbore; 
     FIG. 2 is a longitudinal sectional view of a cup packer of a typical prior art design mounted on a formation interval straddle tool deployed in a wellbore; 
     FIG. 3 is a longitudinal sectional view of a cup packer in accordance with a first embodiment of the present invention mounted on a formation interval straddle tool deployed in a wellbore; and 
     FIG. 4 is a longitudinal sectional view of a cup packer in accordance with a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a formation interval straddle tool  10  of the type typically employed for earth formation zone fracturing or other formation treating operations in wellbores is shown positioned within a cased wellbore  12  which has been drilled in an earth formation  14 . The straddle tool  10  may be lowered into the wellbore  12  on a string of coiled or jointed tubing  16  to a position adjacent a selected zone  18  of the earth formation  14 . If the wellbore  12  has been cased with a casing  20 , the casing  20  will have been perforated at the selected zone  18  by the firing of the perforating shaped charges of a perforating gun or other perforating device, as illustrated by the perforations  22 , prior to the deployment of the straddle tool  10 . 
     Once the straddle tool  10  is in position adjacent the selected formation zone  18 , the straddle tool  10  is operated from the earth&#39;s surface to deploy anchor slips  24  to lock itself firmly into the casing  20  in preparation for fracturing or treating the selected formation zone  18 . The straddle tool  10  comprises one or more cup packers  26  which, when pressurized fracturing or treating fluid is pumped from the earth&#39;s surface through the string of coiled or jointed tubing  16  to the straddle tool  10 , are forced to engage the casing  20  by the pressure of fluid exiting the straddle tool  10  at one or more treating ports  28 . The open ends  29  of the cup packers  26  are arranged to face each other and the straddled interval  30  of the wellbore  12  between the cup packers. 
     When the cup packers  26  have fully engaged the casing  20 , the formation zone  18  and the straddled interval  30  between the cup packers  26  will be pressurized by the incoming fracturing or treating fluid. Upon completion of fracturing or treating of the formation zone  18 , the pumping of fracturing or treating fluid from the earth&#39;s surface is discontinued, and the straddle tool  10  is operated to dump any excess fluid, thereby relieving the pressure in the straddled interval  30 . 
     Referring to FIG. 2, a straddle cup packer  26  of a typical prior art design is illustrated in cross-section mounted on a straddle tool  10 . The cup packer  26 , having a body generally formed of a hardened rubber material, is shown in engagement with a casing  20  in a wellbore  12  such as would occur with the straddled interval  30  of the wellbore  12  under pressure from fracturing or treating fluid. As previously noted, the open end  29  of the cup packer  26  is arranged to face the straddled interval  30  between the cup packers mounted on the straddle tool  10 . On its open end  29  the bore of the cup packer  26  gradually enlarges thereby forming a gap  27  between the cup packer  26  and the wall of the straddle tool  10 . As the treating fluid often has sand and debris entrained therein, the open end  29  tends to collect such sand and debris when the straddled interval  30  is depressurized. 
     If a “screenout”, during which sand is left within the straddled interval  30  following treatment, occurs, the fluid within the straddled interval  30  can become dehydrated forming a dense sand pack between the cup packers  26  and within the open ends  29  of the cup packers. The mechanical wedging of sand between the cup packers  26  can result in high pulling forces during retrieval of the straddle tool  10  following treatment of the formation. Additionally, sand wedged within the open ends  29  of the cup packers  26  may impair the ability of the packers to properly seal the straddled interval  30  to sustain the necessary differential pressure during the subsequent treatment of another formation zone. Cup packers in accordance with the present invention alleviate the aforementioned problems as they prevent the accumulation of sand and debris in their open ends. 
     FIG. 3 is a longitudinal sectional view of a cup packer  26  in accordance with a first embodiment of the present invention mounted on a formation interval straddle tool  10  deployed in a wellbore  12 . The open end  29  of the cup packer  26  is filled with an elastomer filler  32 . The elastomer filler  32  may be added to a standard commercial cup packer after manufacture, or it may be integrally and seamlessly molded into the cup packer  26  during manufacture. In either case, the elastomer filler  32  fills the open end  29  of the packer bore such that the cross-sectional dimension of the filled bore is substantially the same as that of the straddle tool  10 , thereby effectively eliminating the possibility that the cup packer  26  may retain any sand or debris entrained in the fracturing or treating fluid. The term “substantially the same” as used herein means having a dimension allowing for normal fitting tolerances between components. 
     The longitudinal dimension of the packer, defined as the top of protruding section  34  to the bottom of packer  26 , is at its maximum at the bore. 
     The elastomer filler  32  is preferably formed of the same material as the cup packer  26 , for example 80A or 90A durometer nitrile butyl rubber (“NBR”) or hydrogenated nitrile butyl rubber (“HNBR”). Other materials, such as low durometer elastomers, for example 60A durometer NBR, are equally suitable for elastomer fillers  32  added to standard commercial cup packers after manufacture. 
     The configuration of the elastomer filler  32  in the embodiment of the invention illustrated in FIG. 3 provides for enhanced effectiveness of sealing between the cup packer  26  and the tool  10  on which it is mounted when treating fluid pressure is applied in the straddled interval  30 . The elastomer filler  32  has a section  34  which angularly protudes from the end surface  36  of the cup packer  26 . The pressure of treating fluid in the straddled interval  30  acting on the surface  38  of section  34  of the elastomer filler  32  forces the elastomer filler  32  into sealing contact with the surface of the tool  10  thereby minimizing the intrusion of sand and debris between the surface of the tool  10  and the cup packer  26 . Alternatively, where the elastomer filler  32  is integrally and seamlessly molded into the cup packer  26  at the time of manufacture, it may be advantageous to entirely eliminate the end surface  36  such that the angularly protruding section  34  extends to the outermost surface of the cup packer  26 . 
     FIG. 4 is a longitudinal sectional view of a cup packer  26  in accordance with a second embodiment of the present invention. A screen  39  is mounted in the open end  29  of the cup packer  26  and acts to prevent the intrusion of sand and debris therein. The inner end  41  of the screen  39  is sealingly attached to a screen sleeve  42  by welding or other suitable means. The screen assembly comprising the screen  39  and screen sleeve  42  is then mounted in the bore  44  of the cup packer  26  and sealingly secured to the end surface  36  of the cup packer  26  by screws  46  driven into threaded inserts  48  which may be molded into the cup packer  26  at the time of manufacture or inserted after manufacture of the cup packer  26 . Alternatively, the screen  39  may be integrally molded into the cup packer  26 , with or without a screen sleeve  42 . The screen sleeve  42  may also be integrally molded into the cup packer  26 . If a screen sleeve is not used, the screen  39  may be attached to the cup packer  26  only at open end  29  and need not be attached within the bore  44 . 
     The screen  39  may be made of any suitable material and the mesh size may be selected according to the expected size of the sand and debris particles to be excluded from the open end  29  of the cup packer  26 . A 40 mesh screen formed of 0.010 inch diameter  304  stainless steel wire has been found to be satisfactory for many applications. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design shown herein, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention.

Summary:
An improved cup packer for use on wellbore tools is disclosed. The open end of the cup packer bore is filled with an elastomer to prevent the intrusion of sand and debris into the space between the cup packer and the wellbore tool on which it is mounted. In an alternative embodiment of the invention, the packer body includes an angularly protruding section at at least one of its ends, wherein the longitudinal dimension of the packer body is at its maximum at its bore. In an additional alternative embodiment of the invention, the open end of the packer is sealed by a screen to prevent the intrusion of sand and debris into the space between the cup packer and the wellbore tool on which it is mounted.