Abstract:
A cementing plug has a plurality of body segments which are secured together, preferably by adhesive, to form a plug body. Elastomer wipers are clamped between the body segments. This cementing plug may be readily made shorter or longer in length with an increased or decreased number of wipers. Elastomer covers only the wipers and not the plug body.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a cementing plug and, more particularly to an easily drillable plug for use in well cementing operations. 
     BACKGROUND OF THE INVENTION 
     It is conventional practice, in the drilling and completion of wells, that prior to the placement of cement, the casing and hole are filled with drilling mud. In order to reduce contamination of the interface between the 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 that will rupture upon the bottom plug reaching a float shoe, float collar or landing collar located near or at the bottom of the casing. When the bottom plug reaches the float shoe, float collar or landing collar, the increase in fluid pressure will result in the diaphragm rupturing allowing the cement slurry to pass though the bottom plug and continue out the bottom of the casing, filling the space between the casing and the well bore. 
     Upon placing all the cement into the casing a top plug is located on top of the cement. 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. 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 and provide a method of providing a sealing mechanism 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 remaining in this position. The required length for the top plug and the bottom plug varies with the intended application, and is based upon a number of factors including the pressure being contained. 
     After the cement slurry has become hard, the top and bottom plugs are drilled out using a drilling bit. Typical prior art cementing plug sets incorporate teeth, lugs or threads that act to prevent the plugs from rotating during the drill-out procedure, thereby reducing the time required to perform the drill-out operation. 
     SUMMARY OF THE INVENTION 
     What is required is an alternative configuration of cementing plug. 
     According to the present invention there is provided a cementing plug which includes a plurality of body segments. Means is provided for securing the plurality of body segments together to form a plug body. Elastomer wipers clamped between the body segments. 
     The use of a plurality of body segments enables plugs to be produced of varying lengths without having to produce additional molds for molding the plastic cores and elastomer covers. Prior art cement plugs used a one-piece molded core ith a one piece molded elastomer covering. To manufacture the prior art cementing plugs, molds for the plastic cores and molds for the elastomer covers were produced with each mold producing a plug of a set length. With the prior art should it be desired to produce a plug of shorter or longer length of a plug with an increased or decreased number of wipers a new set of molds would be produced to manufacture the new plug. The present invention allows the plug length and the number of wipers incorporated in the plug to be varied by the adding additional segmented plastic cores and wipers. It is intended that each of the body segments be substantially identical, but this is not essential. There are a variety of ways of securing the body segments together, several of which will hereinafter be further described. 
     Although beneficial results may be obtained through the use of the cementing plug, as described above, even more beneficial results may be obtained when the body segments are made from polymer plastic. Metal and rubber are difficult to drill. With the cementing plug, as described above, the metal and rubber can largely be replaced with more drillable materials, such as plastics. 
     Although beneficial results may be obtained through the use of the cementing plug, as described above, even more beneficial results may be obtained when a variety of elastomer wipers are used made from elastomers with differing properties. The prior art cementing plugs covered a core with an elastomer cover. This necessarily required that the elastomer wipers be made of the same elastomer materials as the elastomer cover. The selection of elastomer materials, therefore, was of importance. Some elastomers work more effectively for wiping, other elastomers work better at elevated temperatures, other elastomers have better wear properties and other elastomers have better compatibility with specific wellbore fluids. With the cementing plug, as described above, the elastomer wipers need not all have the same properties. A number of elastomer wipers with differing properties can, therefore, be configured to improve performance of the plug in a wide range of downhole operating conditions and fluids. In addition, the cementing plug described above has reduced the amount of elastomer material used, by using individual wipers that are locked between the molded plastic segments which replaces much of the elastomer material with plastic. The elastomer cover that normally covers the core between the wipers is eliminated, which improves the ability to drill out the plugs. 
     Although beneficial results may be obtained through the use of the cementing plug, as described above, it is important that the elastomer wipers be securely attached to the plug body. Even more beneficial results have, therefore, be obtained when the body segments have opposed mating faces with a receptacle on each of the faces, and each of the elastomer wipers has radial projections. The radial projections are received in the receptacles of mating body segments to lock the elastomer wipers to the plug body. 
     Although beneficial results may be obtained through the use of the cementing plug, as described above, it slow the drilling process if the body segments rotate when a force is exerted upon them by a drill bit during drilling out of the cementing plug. Even more beneficial results may, therefore, be obtained when the body segments have a projecting lug on one face and a lug receiving aperture on an opposed face. When the body segments are secured together, the projecting lug of one of the body segments engages the lug receiving aperture of another of the body segments to prevent relative rotation of the body segments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, wherein: 
     FIG. 1, labelled as PRIOR ART, is a cross-sectional view of a prior art non-metallic plug set. 
     FIG. 2, labelled as PRIOR ART, is a cross-sectional view of a prior art non-metallic, non-rotating plug set. 
     FIG. 3 is a cross-sectional view of one embodiment of the non-metallic segmented plug set encompassing threaded segments for the purpose of assembly. 
     FIG. 4 is a cross-sectional view of another embodiment of the non-metallic segmented plug set encompassing adhesive bonding for the purposes of assembly. 
     FIG. 5 is a cross-sectional view of another embodiment of the non-metallic segmented plug set encompassing adhesive bonding for the purpose of assembly and non-rotating lugs. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment, a cementing plug will now be described with reference to FIGS. 1 through 5. 
     Referring to FIG. 1, a prior art non-metallic plug set  10  is shown. The top plug  11  comprises a plastic body  12  having a central cavity  13  and a one piece elastomer cover  14  thereon having a plurality of wipers  15 . The bottom plug  21  comprises a plastic body  22  having a bore  23  throughout and an elastomer cover  24  thereon having a plurality of wipers  25  and a diaphragm  26 . 
     Referring to FIG. 2, a prior art non-metallic, non-rotating plug set  30  is shown. The top plug  31  comprises a plastic body  32  having a central cavity  33  therein and a bore  34  in the lower end, in which a non-rotating insert  35  having lugs  36  thereon is retained by adhesive bonding of the insert  35  to the body member  32 . Top plug  31  has an elastomer cover  37  with a plurality of wipers  38  thereon. The bottom plug  41  comprises a plastic body  42  having a bore  43  throughout and a bore  44  in the lower and upper ends, in which a non-rotating inserts  45  having lugs  46  thereon is retained by adhesive bonding of the inserts  45  to the body member  42 . Bottom plug  1  has an elastomer cover  47  having a plurality of wipers  48  hereon and a diaphragm  49 . 
     Referring to FIG. 3, plug set  50  is illustrated which represents a first embodiment constructed in accordance with the teachings of the present invention is shown. The plug set comprises a top plug  51  and a lower plug  61 . The top plug  51  is comprised of molded plastic body segments  52 . Each segment has a threaded male projection  53  on one face  150  and a threaded female aperture  152  on an opposed face  154 . Body segments  52  are secured together by mating threaded male projection  53  of one of body segments  52  with threaded female aperture  152  of another of the body segments  52 . Each molded segment  52  has a receptacle  54  in each of faces  150  and  154  for retaining a plurality of wipers  55 . Each of wipers  55  have radial projections  156  which are received in receptacles  54 . The engagement between radial projections  156  and receptacles  54  prevents wipers  55  from being withdrawn when clamped between adjacent ones of molded segment  52 . The bottom plug  61  comprises molded plastic segments  62 . Each segment  62  has a threaded male projection  64  on one face  150  and a threaded female aperture  152  on an opposed face  154 . Body segments  62  are secured together by mating threaded male projection  64  of one of body segments  62  with threaded female aperture  152  of another of the body segments  62 . Each molded segment  62  has a receptacle  65  in each of faces  150  and  154  for retaining a plurality of wipers  66 . Each of wipers  66  have radial projections  156  which are received in receptacles  65 . Bottom plug  61  differs from top plug  51  in that bottom plug  61  has a central cavity  63  with a diaphragm  67  located in central cavity  63 . 
     Referring to FIG. 4, a plug set  70  is illustrated which respresents a second embodiment of a form of cementing plug constructed in accordance with the teachings of the present invention. Plug set  70  includes a top plug  71 , a bottom plug  81  and plug seat  110 . The top plug  71  is comprised of molded plastic segments  72 , each segment having interlocking lugs  73  on one face  200  and a lug receiving aperture  202  on an opposed face  204 . When segments  72  are secured together projecting lug  73  of one of the segments  72  engages lug receiving aperture  202  of another of the segments  72 . This prevents relative rotation of the segments  72 . Each of segments  72  has a receptacle  74  for retaining a wiper  75 . Each of wipers  75  have radial projections  206  which are received in receptacles  74 . The bottom plug  81  includes molded plastic body segments  82 . Each segment has interlocking lugs  83  on one face  200  and a lug receiving aperture  202  on an opposed face  204 . When segments  82  are secured together projecting lug  83  of one of the segments  82  engages lug receiving aperture  202  of another of the segments  82 . This prevents relative rotation of the segments  82 . Each segment  82  has a receptacle  84  for retaining a plurality of wipers  85 . Each of wipers  85  have radial projections  206  which are received in receptacles  84 . Bottom plug  81  differs from top plug  71  in that a central cavity  208  is provided in which is positioned a diaphragm  86 . 
     The plug seat  110  has integral interlocking lugs  111  for use with plug set  70  during cementing operations. The lug receiving apertures  202  on plug  71  interlock with the lugs  83  on the top of plug  81 . Lug receiving aperture  202  on the bottom of plug  81  interlock with the lugs  111  on the plug seat  110  so that rotation of both top plug  71  and bottom plug  81  is prevented. The plug seat  110  may be installed in any suitable cementing equipment such as a float collar, float shoe, guide shoe or landing collar. 
     Referring to FIG. 5, a plug set  90  is illustrated which represents a third embodiment of the present invention. The plug set comprises of a top plug  91  and a bottom plug  101 . The top plug  91  is comprised of plastic segments  92  bonded together by adhesive with each segment  92  having a receptacle  93  to retain a plurality of wipers  94 . Each of wipers  94  has radial projections  300  which are received in receptacles  93 . The bottom plug  101  is comprised of molded plastic segments  102  bonded together by adhesive. Each segment has a receptacle  103  to retain a plurality of wipers  104 . Each of wipers  104  has radial projections  300  which are received in receptacles  93 . Bottom plug  101  differs from top plug  91  in that bottom plug  101  has a central bore  106  in which is positioned a diaphragm  105 . 
     It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the claims.