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FIELD OF THE INVENTION 
   The invention relates to cementing wiper plugs used in cementing casing downhole and particularly to top cementing plugs used for cementing production casing. 
   BACKGROUND OF THE INVENTION 
   It is conventional practice, in the drilling and completion of wells, to case an open hole by cementing tubular casing in place in a wellbore. Thus, the open hole is prevented from caving in, fragile formations are protected, inter-zonal communication is restricted and contamination of groundwater is prevented. In the course of cementing the casing, components are placed in the well which can later migrate and possibly interfere with well operations. To understand the phenomena, cementing operations are reviewed herein. 
   A string of casing is made up and lowered into the open wellbore. Prior to the placement of cement, the casing and hole are filled with drilling mud, which must be displaced for placing cement. 
   In the case of surface and intermediate casing, in order to reduce contamination of the interface between the displaced mud and the cement, a bottom cementing plug is placed in the casing and pumped ahead of the cement slurry. The bottom plug is typically constructed with a one piece hollow metallic or a one piece non-metallic core having an elastomer covering molded to the core. The elastomer cover typically incorporates a plurality of wipers. The function of the wipers is to wipe the internal surface of the casing, maintain the separation of fluids during the displacement of the cement slurry down the casing and provide a means of sealing upon displacement of the plug. The bottom plug incorporates a rupture diaphragm or valve that will rupture or open upon the bottom plug reaching or resting on a float shoe, float collar or landing collar located near or at the bottom of the casing. An increase in fluid pressure above the supported bottom plug results in the diaphragm rupturing, allowing the cement slurry to pass though the bottom plug and continue out the bottom of the casing, beginning to fill the annular space between the casing and the well bore. 
   When the necessary volume of cement has been placed into the casing, a top plug is positioned on top of the cement for separating the cement from a driving slug of mud. The top plug is typically constructed having a solid elastomer, one piece metallic or one piece non-metallic core having an elastomer covering molded to the core, the elastomer cover incorporating a plurality of wipers. Optionally, the top plug may also have a rupture element, as described in U.S. Pat. No. 5,191,932 and incorporated herein by reference in its entirety, so that if the top and bottom plugs are inadvertently reversed, in operation, cementing can continue without removal of the plug or removal of cement placed into the wellbore before the error was discovered. Pressures required to rupture the diaphragm are such that the diaphragm will not rupture during normal operations. The function of the wipers is to wipe the internal surface of the casing, maintain the separation of fluid during the displacement of cement slurry down the casing using drilling fluid and to provide a method of providing a sealing mechanism across the casing upon landing the top plug on top of the bottom plug. When displacement of the cement slurry is complete the top plug will land on top of the bottom plug and is expected to remain in this position once the cement hardens. 
   After the cement slurry has become hard, the top and bottom plugs are drilled out. Additional drilling can then proceed through the cemented casing. Additional lengths of casing are hung in the cemented casing and the cementing operation is repeated. 
   The last segment of casing to be positioned in the wellbore is the production casing. It is typically smaller in diameter than either the surface or intermediate casing and extends to the bottom of the wellbore. As no further drilling will occur after the production tubing has been run in and cemented, the plugs are not drilled out, but instead are left cemented into the bottom of the hole. As with the previous cementing operations, a bottom plug is run ahead of the cement and a top plug is run behind. Once the top plug rests on the bottom plug, pressure sufficient to keep the plugs at the bottom of the hole, but not to rupture the diaphragm in the top plug, if present, is maintained on the plugs for approximately 8 hours to permit the cement to properly set. 
   Once the wellbore has been cased, the casing is perforated above the plugs at a zone of interest and the wellbore is ready for production. A tubing string and pump are lowered into the casing and fluids are produced up the tubing string to surface. 
   Applicant is aware that in many cases, often a year or more after the cementing of the casing, the top cementing plug can migrate up the production casing to the pump intake and cause fouling of the pump. Typically, most wellbores have a minimum overhole, that is to say that the bottom of the casing is not far below the zone to be perforated. The Applicants believe that during perforation of the casing, the cement surrounding the plugs and outside the casing may be fractured. If sufficient fracturing occurs, the plugs are no longer held securely inside the casing and can migrate upwards. It is also possible that gas from the formation can travel downward through the fractured cement outside the casing and rise at the bottom of the casing to apply pressure on the plugs. If the one-way valves in the float equipment are also damaged as a result of pressure pulses during perforation, the plugs may be forced upwards due to the increased pressure from below. 
   Traditionally, whenever the pump intakes are fouled, production is lost and the tubing is tripped out of the well to repair the pump, at great expense. A solution that has been employed to prevent plugs from migrating upwards into the pump intake is to run a bridge plug into the casing and set it down on the top cementing plug to anchor the plug in position. Whether repairing the pump or setting a bridge plug, significant expense is involved in both equipment and rig time. 
   Regardless of the reason or hypothesis for plug migration, clearly there is a need for means to prevent the cementing plug from migrating up the casing. Ideally, such means would be incorporated directly into the plug, thus realizing significant cost and time savings. 
   SUMMARY OF THE INVENTION 
   A self-retaining cementing wiper plug comprises two or more holddown fingers biased radially therefrom and extending outward for engaging an inner surface of the casing once the plug is positioned at the bottom of the casing. Substantially, regardless of the formation, the novel plug is prevented from migration. The holddown fingers are angled uphole, as are the wipers, to enable insertion into the casing bore and are flexible relative to the plug only in so much as the elastomeric body in which they are embedded flexes or the attachment to the core of the plug permits limited flex, to permit insertion. The fingers themselves are substantially inflexible so as to resist flexing once engaged with the casing to prevent movement of the plug uphole. 
   In a broad aspect of the invention, an improvement to conventional cementing wiper plugs is provided having a plurality of radially extending, elastomeric wipers extending therefrom for insertion into a production casing for wiping an inner casing surface is provided, the improvement comprising one or more upwardly angled, substantially inflexible projections biased radially outward from the cementing plug wherein the projections are moveable inwardly sufficiently so as to permit insertion of the plug downwardly into the casing and are sufficiently inflexible to restrict uphole movement of the plug in the casing. 
   In a preferred embodiment of the invention, the substantially inflexible holddown fingers are steel or carbide-tipped fingers embedded at a first end in the elastomeric covering of the plug and extending outward to engage the inner surface of the casing at a second end. The fingers extend at least the extent of the flexed wipers and can be positioned between the wipers or embedded within the wipers. Even one hold down finger, but preferably two or more holddown fingers positioned 180 degrees from one another, are sufficient to secure the plug in the casing. 
   The holddown fingers can be individually embedded into the elastomeric covering or can be attached, such as adjacent their base, to a ring which is positioned about the core and embedded in the elastomeric covering. Further, the first end of the fingers or an inner edge of the ring can be formed into an anchor for more securely embedding the fingers in the elastomer. 
   Optionally the fingers can be attached to and extend outward from the core of the plug or be slips biased outwardly by the elastomer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of a wellbore casing string having a top cementing plug of the present invention positioned on or adjacent a bottom end of the production casing; 
       FIG. 2  is a cross-sectional view of a first embodiment of the invention according to FIG.  1  and showing holddown fingers embedded in the cementing plug and protruding between the wipers for engaging the production casing; 
       FIG. 3  is a cross-sectional view of a second embodiment of the invention showing spring steel holddown fingers and carbide-tipped holddown fingers embedded in the wipers of the cementing plug for engaging the production casing; 
       FIG. 4  is a partial perspective view of a ring to be secured around a core of the cementing plug and having a plurality of angled holddown fingers radially extending therefrom for engaging the casing; 
       FIG. 5  is a cross-sectional view of a third embodiment of the invention showing a plurality of slips having carbide tips at a bottom end of the plug for engaging the production casing, the slips in a non-engaged position for insertion into the casing; and 
       FIG. 6  is a cross-sectional view of the third embodiment according to  FIG. 5  wherein a shear surface has been sheared and the slips are caused to be positioned in an engaged position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1 , a cementing plug  10  of the present invention is shown positioned adjacent a bottom  11  of a casing string  12  comprising, in order beginning from surface (not shown): surface casing  13 , intermediate casing  14  and production casing  15 . The cementing plug  10  is located below a plurality of perforations  16  in the production casing  15  and below a pump  17  lowered into the casing  12  at the end of a production string  18 . 
   As shown in  FIG. 2 , and in a preferred embodiment of the invention, the cementing plug  10  comprises a core  20 . The core  20  is covered with an elastomeric covering  24  having a plurality of wipers  25  formed thereon. Preferably, the core  20  defines a bore  21  therethrough. A top end  22  of the core  20  is fitted with a rupture element  23  to permit the passage of cement slurry during cementing should the plug  10  be used inadvertently as a bottom wiper plug. Typically, the rupture element  23  is designed to rupture only at a predetermined pressure. The wipers  25  extend radially outward from the core  20  and covering  24  and are angled uphole slightly to permit flexing for insertion through the production casing  15 . The wipers  25  act to wipe an inner surface  19  of the casing  15  and maintain separation between fluids above and below the cementing plug  10  during its insertion. 
   Two or more substantially inflexible projections, preferably radially extending holddown fingers  26 , are formed in a space  27  defined by two of the plurality of wipers  25  extending from the cementing plug  10 . A first end  28  of the holddown fingers  26  is embedded in the elastomeric covering  24  and a second end  29  extends at least equal to the extent of the flexed wipers  25  so that when the cementing plug  10  is positioned in the production casing  15 , the second end  29  engages the inner wall  19  of the casing  15 . The holddown fingers  26  are positioned to angle slightly uphole and are permitted limited flexing to aid in insertion of the cementing plug  10  into the casing  15  as a result of flexing of the elastomeric covering  24 , however, once positioned at the bottom  11  of the casing  15 , any uphole movement of the cementing plug  10  is prohibited as a result of limited rotation and compression of the holddown fingers  26  through engagement of the second end  29  of the holddown fingers  26  with the casing&#39;s inner wall  19 . Typically, the holddown fingers  26  are manufactured from spring steel and may be tipped with carbide. The fingers  26  are substantially inflexible so as to be incapable of flexing or displacing overly so as to prevent the second ends  29  from losing their grip and disengaging from the casing&#39;s inner wall  19  in response to pressure from below the plug  10 . 
   In a preferred embodiment of the invention, as shown in  FIG. 2 , two spring steel holddown fingers or carbide holddown fingers  26  are positioned 180 degrees circumferentially from one another about the plug  10 . The holddown fingers  26  are blade-like, being approximately 1.5 inches in length and  1  inch in width and are angled to approximately the same degree as the wipers  25 . Preferably, the first end  28  of each finger  26  is profiled or curved to form an anchor  30  so as to be more securely embedded in the elastomeric covering  24 . Applicant has found that two holddown fingers  26  are sufficient to secure the cementing plug  10  in the casing  15 , under test conditions. One holddown finger  26  may be sufficient as the wipers  25  already act to center the plug  10 . 
   A plurality of holddown fingers  26  may be spaced circumferentially about the plug individually, or joined as shown in FIG.  4 . Each finger  26  may be separately embedded in the elastomeric covering  24  or, as shown in  FIG. 4 , for ease of production, the plurality of angled holddown fingers  26  may extending radially and cantilevered from a ring  40 . The ring  40  can be embedded in the elastomeric covering  24  about the core  20 . Further, an inner edge  41  of the ring  40  can be profiled as an anchor  42  for better securing the ring  40  in the elastomeric covering  24 . 
   Having reference again to  FIG. 3 , and in a second embodiment of the invention, the holddown fingers  26  can be embedded within the elastomeric wipers  25 . As is the case with the previously described embodiment, the holddown fingers  26  can be discrete and embedded individually within the wipers  25  or can extend periodically from a ring  40  which can be embedded about the core  20 . Individually, the holddown fingers  26  may extend from the cementing plug&#39;s core  20  or may have an anchor  30  formed at the first end  28  permitting the finger  26  to extend from within the elastomeric covering  24 . 
   For imparting further compressive strength, the holddown fingers  26  extending from the core  20  rest upon a shoulder  43  formed about the core  20  and provide additional resistance to inward flexing of the fingers  26 . 
   Having reference to  FIG. 5 , a third embodiment of the invention is shown. The plugs core  20  is formed in two portions, an upper core  100  and a lower core  101 . A unitary elastomeric covering  103  is formed over both the upper and lower core  100 ,  101  from which a plurality of upwardly angled wipers  104  extend. A plurality of slips  105  are shearably connected between the upper and lower core  100 ,  101 , preferably by shear tabs  106 . The slips  105  reside in openings or ports  107  in the elastomeric covering  103 , proximate to the bottom of the core&#39;s upper portion  100 . In a casing non-engaging position, the slips  105  are retracted sufficient to permit insertion of the plug  10  into the casing  12 . A stop  108  is formed in the elastomeric covering  24  adjacent a base  109  of the slip  105  and is deformed outwardly by the slip  105 , biasing the slip  105  into the port  107 . 
   In operation, as shown in  FIG. 6 , once the top plug  10  is set upon the bottom plug (not shown) and pressure is applied as a result of drilling fluid, the upper core  100  is forced downward into a recess  110  formed in the lower core  101  causing the shear tabs  106  to shear. The elastomeric covering  103  deforms inward forcing the slips  105  outward into engagement with the production casing  15 . The movement of the slip  105  releases the stop  108  from compression and the stop  108  is permitted to return inwardly to an upstanding position and aid in maintaining the position of the slips  105 , tipped in the casing-engaging position.

Summary:
A self-retaining cementing wiper plug has two or more steel or carbide tipped holddown fingers or slips extending radially outward from the plug for engaging the inner surface of the casing and preventing the plug from moving uphole over time and potentially interfering with other downhole apparatus such as a pump.