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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     This invention relates in general to cementing a casing string within a wellbore, and in particular to a pump down cement retaining device that prevents backflow of cement. 
     BACKGROUND OF THE INVENTION 
     Most oil and gas wells are drilled with a drill string comprised of drill pipe. After reaching a certain depth, the drill string is removed and casing is lowered into the wellbore. A cement valve, is normally attached to the lower end of the casing. The cement valve allows cement to be pumped down through the casing and up the annulus surrounding the casing, and prevents backflow of cement from the annulus back into the casing. Another type of casing string, referred to as a liner, may be installed in a similar manner. A casing string extends all the way back to the upper end of the well, while a liner string is hung off at the lower end of a preceding string of casing. 
     In another drilling technique, the casing is used as part or all of the drill string. The bit may be attached to the lower end of the casing string permanently, in which case it is cemented in place. Alternatively, it may be retrieved after reaching desired depth, such as by using a wireline, drill pipe, or pumping the bit assembly back up the casing. While drilling, the casing string may be rotated by a gripping mechanism and a top drive of the drilling rig. With liner drilling, the liner string serves as the lower end of the drill string, and a string of drill pipe is attached to upper end of the liner string. 
     In casing and liner drilling, if the bottom hole assembly, which includes a drill bit and optionally measuring instruments and steering devices, is to be retrieved before cementing, the operator will install a cement valve at the lower end of the liner after retrieval of the bottom hole assembly. The cement valve may be lowered into place on a wire line or a string of drill pipe and locked to a profile at the lower end depth of the liner string. Also, it is has been proposed to pump the cement valve down the casing, rather than convey it on a wire line. The cement valve may have a flapper valve to prevent back flow of cement. It may also have a frangible barrier to allow the cement valve to be pumped down the casing string. Once in place, increased fluid pressure causes the barrier to break and the fluid to flow out the lower end of the cement valve. 
     It has also been proposed to pump a receptacle down the casing string and latch it into a profile at the lower end prior to cementing. The receptacle has a passage that allows the downward flow of cement, but does not have a valve to prevent backflow. At the conclusion of cementing, a wiper plug or prong is pumped down into engagement with the receptacle. The prong stabs into the upper end of the receptacle to form a seal and retain the plug to prevent backflow of cement. 
     After the cement is cured, if the operator intends to drill the well deeper, the drill string must drill through the receptacle and wiper plug. It is thus desirable to make the receptacle and wiper plug of easily drillable materials. These materials must meet the requested specifications of the tools. 
     SUMMARY OF INVENTION 
     The method of this invention utilizes a receptacle that is positioned at the lower end of the casing string. A wiper plug is pumped down the string of casing following the pumping of cement. The wiper plug has a prong on its end with a seal that seals within a lower portion of the receptacle. The positioning of the seal places the receptacle under a compressive force when a pressure differential exists due to uncured cement in the annulus. Since the force is compressive, many of the components of the receptacle can be made of more easily drillable materials, such as plastic and resin composites, than in the prior art design. The prior art design had to accommodate at least some tensile forces. 
     In the preferred embodiment, the lower end of the prong is substantially flush with a lower end of the axial passage through the receptacle once locked in place. Preferably, the seal is also located at the lower end of the axial passage. The latching members of the prong and receptacle may comprise a ratchet sleeve and a grooved profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a receptacle installed in a profile at the lower end of a string of casing in accordance with this invention. 
         FIG. 2  is a sectional view of the receptacle of  FIG. 1 , with the burst disk broken to allow fluid to be pumped through the axial passage. 
         FIG. 3  is a sectional view of the receptacle of  FIG. 1 , showing a wiper plug and prong being pumped down the string of casing. 
         FIG. 4  is a sectional view of the wiper plug and receptacle of  FIG. 3 , but showing the prong fully engaged with the receptacle. 
         FIG. 5  is a sectional view of the wiper plug, prong and receptacle of  FIG. 4 , but showing fluid pressure acting upward on the lower end of the receptacle. 
         FIG. 6  is an enlarged sectional view of the wiper plug and prong of  FIG. 3 . 
         FIG. 7  is a further enlarged sectional view of a lower portion of the wiper plug prong landed within the receptacle as shown in  FIGS. 4 and 5 . 
         FIG. 8  is a sectional view of an alternate embodiment of a wiper plug and prong. 
         FIG. 9  is a sectional view of an alternate embodiment of a receptacle, and showing the wiper plug and prong of  FIG. 8  installed. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to  FIG. 1 , a string of casing  11  comprises tubular members secured together by threads for installation in a wellbore. The term “casing” is used broadly herein to include also a liner string, which is normally constructed the same as casing but does not extend fully to the surface, rather its upper end is hung off near the lower end of the preceding string of casing. 
     A lower or profile sub  13  is attached to the lower end and forms part of the string of casing  11 . Profile sub  13  has number of internal grooves that in this embodiment were used previously to secure a bottom hole assembly (not shown) for drilling. Profile sub  13  also has an annular recess  15  located therein that has a larger inner diameter than the inner diameter of the remaining portion of the string of casing  11 . Recess  15  is defined by an upper shoulder  17  and a lower shoulder  19 . 
     A cement plug receptacle  21  is shown latched into profile sub  13 . Cement plug receptacle  21  has a body  23  with an axial passage  25  extending through it. Body  23  has at least one and optionally a plurality of circumferential grooves  27  on its exterior. In this embodiment, grooves  27  are configured in a triangular fashion, resulting in a downward-facing conical flank  29  intersecting an upward-facing conical flank  31 . When viewed in cross-section, flanks  29  of grooves  27  are parallel to each other and flanks  31  are parallel to each other. 
     An outward-biased collar  33  surrounds body  23  at grooves  27 . Collar  33  is of a resilient material and is split so as to radially expand and contract. Collar  33  has at least one and optionally a plurality of internal grooves  35  for mating with grooves  27  of body  23 . The resiliency of collar  35  causes it to spring outward from grooves  27  when it reaches profile sub recess  15 . As receptacle  21  moves down casing  11 , prior to reaching recess  15 , the outer diameter of collar  33  will slidingly engage the inner diameter of casing  11 . Anti-rotation keys  37 , one at the upper end and one at the lower end of body  23 , engage collar  33  to prevent collar  33  from rotating relatively to body  23 . Grooves  35  have same configuration as grooves  27 , but body  23  is capable of axial movement from a lower position relative to collar  33 , shown in  FIG. 4 , to an upper position, shown in  FIG. 5 . In the lower position, downward-facing flanks  29  of body grooves  27  are engagement with collar grooves  35  but upward-facing flanks  31  are not in engagement with collar grooves  35 . In the upper position of  FIG. 5 , upward-facing flanks  31  are engagement with grooves  35 , but downward-facing flanks  29  are not in engagement with grooves  35 . 
     Referring still to  FIG. 1 , body  23  has a lower body extension  39  that has a threaded neck  41  that secures it to the lower end of body  23 . Lower body extension  39  could optionally be integrally formed with body  23 . Axial passage  25  extends through lower body extension  39 . A latch member sleeve  43  with internal grooves is mounted within lower body extension  39 . 
     A lower seal  45  is attached to the lower end of lower body extension  39  by a threaded neck  47 . Lower seal  45  is illustrated as a cup seal, having a downward-facing concave interior; but it could be other types. Pressure acting on the lower side of lower seal  45  pushes seal  45  outward and upward into sealing engagement with profile sub  13 . A cylindrical seal member  48  is preferably located in the portion of axial passage  25  that extends through lower seal  45 . 
     An upper seal  49  is mounted to the upper end of body  23  by a threaded neck  51  in this example. Upper seal  49  may have the same general shape as lower seal  45 . Axial passage  25  extends through upper seal  49  but it is initially closed by a frangible barrier, which comprises a burst disk  53  in this example. Burst disk  53  closes axial passage  25  until the differential pressure acting on it exceeds a selected level, at which time it breaks or ruptures to allow flow through axial passage  25 . Burst disk  53  is secured to upper seal  49  by a shear cylinder retainer  55 .  FIG. 1  shows burst disk  53  as initially installed and  FIG. 2  shows burst disk  53  after being ruptured. Rather than the barrier device being a rigid frangible member, burst disk  53  could be a flexible elastomeric member or diaphragm that ruptures, or other types of devices. 
       FIG. 3  shows a wiper plug  57  being pumped down following the dispensing of cement. Wiper plug  57  has flexible ribs  59  on its outer side that seal against the inner diameter of casing  11  as it moves downward. A prong  61  is mounted to the lower end of wiper plug  57  and protrudes downward. Prong  61  comprises a rod located on the axis of wiper plug  57 . A plurality of transverse ports  67  optionally may be formed along its length. A nose  69  is attached to the lower end of prong  61 . Referring to  FIG. 7 , nose  69  has one or more seal  71  that extends around it. Seals  71  seal against seal sleeve  48  located within lower seal  45 . A latch member comprising a ratchet sleeve  73  is mounted just above nose  69 . Ratchet sleeve  73  is a split cylindrical sleeve that is biased outward due to its internal resiliency. Ratchet sleeve  73  has grooves  75  on its exterior that will mate with the grooves in latch sleeve  43 . Grooves  75  and the mating grooves in latch sleeve  43  are configured to allow downward movement of prong  61  but not upward movement. During downward movement, the saw-tooth shape of grooves  75  in ratchet sleeve  73  cause ratchet sleeve  73  to retract and expand. 
     An annular retainer  77  located below ratchet sleeve  73  on the upper end of nose  69  has a tapered surface  79  on its upper end that faces upward and outward for urging ratchet sleeve  73  outward into tighter engagement due to internal pressure acting against nose seals  71 . 
     Preferably, most, if not all the components of cement plug receptacle  21  and wiper plug  57  are constructed of easily drillable materials to allow the operator to readily drill out the assembly after the cementing operation is over and the cement is secured. These materials may include composite materials, such as resin reinforced fiber as well as plastic materials. They may also include metallic materials such as aluminium. 
     In operation, after drilling to a desired depth and retrieving the bottom hole assembly (not shown), the operator places cement plug receptacle  21  into the upper end of the string of casing  11  and applies fluid pressure to casing  11  to pump it downward, typically with water. When cement plug receptacle  21  reaches recess  15 , the outward-biased collar  33  springs outward and secures cement plug receptacle  21  to profile sub  13 , as shown in  FIG. 1 . Once in engagement, downward movement is prevented by upward-facing shoulder  19  and upward movement is prevented by downward-facing shoulder  17 . 
     Continued fluid pressure after cement plug receptacle  21  has landed shears burst disk  53 , as shown in  FIG. 2 . Once burst disk  53  ruptures, the operator may pump cement through casing  11 , which flows through axial passage  25  and up the annulus surrounding casing  11 . When the desired quantity of cement has been dispensed, the operator places wiper plug  57  in casing string  11 , as shown in  FIG. 3 , and pumps wiper plug  57  downward, normally with water. Wiper plug  57  pushes the cement in casing string  11  downward through axial passage  25 . Eventually, prong  61  will stab into axial passage  25 , as shown in  FIG. 4 , and wiper plug  57  will land on retainer  55 . At this point, the tip of wiper plug nose  69  will be located substantially flush with the lower end of axial passage  25 . Seals  71  on nose  69  will be sealing engagement with seal sleeve  48  ( FIG. 7 ). Ratchet sleeve  73  will be in locking engagement with latch sleeve  43 . Downward-facing flanks  29  on body  23  will be in engagement with grooves  35  in collar  33 . Most, if not all, of ribs  59  of wiper plug  57  will be located above receptacle  21  and do not perform any latching function or any sealing function against upward acting pressure. 
     The operator may then release the fluid pressure from above wiper plug  57 . The weight of the cement in the casing annulus tends to cause it to flow back upward into casing string  11 . Wiper plug  57  and body  23  will initially move upward slightly in unison due to the differential pressure force as shown in  FIG. 5 . This upward movement will stop once upward-facing flanks  31  on body  23  engage grooves  35  in collar  33 , as shown in  FIG. 5 . The load path due to the pressure of the cement in the annulus passes through lower seal  45 , lower body extension  39  and body  23  into collar  33 , which transfers the load to profile sub  13  through upper shoulder  17 . The load path also passes from nose  69  through latch sleeve  43  into lower body extension  39 . Lower body extension  39 , body  23 , nose  69  and collar  33  will be in compression. No components of receptacle  21  or wiper plug  57  will be in tension as a result of the upward acting pressure. 
     After the cement has cured, the operator may run a new drill string, which could comprise drill pipe or a smaller diameter string of casing. A drill bit on the lower end will drill out cement plug receptacle  21 , leaving only profile sub  13 . 
     An alternate embodiment is shown in  FIGS. 8 and 9 . Referring to  FIG. 8 , prong  81  differs from the first embodiment in that is does not have holes such as ports  67  ( FIG. 2 ) extending through it perpendicular to its axis. Also, its internal cavity  82  is deeper than the internal cavity of prong  61  ( FIG. 6 ). Nose  83  is longer than nose  69  of the first embodiment; however, seals  85  are positioned about the same distance from the lower end as seals  71  on nose  69  of the first embodiment. Nose  83  may have an axially extending internal cavity  84 , as shown. A split ratchet ring  87  is attached near the lower end of prong  81  as in the first embodiment. Wiper plug  89  on the upper end of prong  81  has seal ribs  91  that protrude radially less distance from the body of wiper plug  89  than seal ribs  59  of the first embodiment. 
     Referring to  FIG. 9 , receptacle  93  is shown anchored in a profile sub  95  that may the same as lower sub  13  of the first embodiment. Receptacle has a lower cup seal  97  that differs from lower seal  45  ( FIG. 1 ) in that it is carried on a tubular cup mandrel  99  of a more rigid material than the material of seal  97 . An annular load ring  101  encircles cup mandrel  99  for transmitting upward compressive force from lower seal  97  to a tubular extension member  103 . The first embodiment does not have a load ring. The upper end of cup mandrel  99  is secured to extension member  103 , and the lower end of cup mandrel  99  extends below load ring  101  into lower seal  97 . Ratchet or internally grooved sleeve  105  is mounted within extension member  103  for engagement with ratchet ring  87  on prong  81  as in the first embodiment. 
     Body  107  is attached to the upper end of extension member  103  and may be constructed the same as body  23  of the first embodiment. A collar  109  encircles body  107  and springs outward into a recess  111  of profile sub  95  as in the first embodiment. An upper cup seal  113  similar to upper seal  49  ( FIG. 1 ) is mounted on top of body  107 . A seat  115  containing a burst disc  117  is mounted within upper seal  113 . The operation of the embodiment of  FIGS. 8 and 9  is the same as the operation of the first embodiment. 
     While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Summary:
A wall casing cement plug assembly includes a receptacle with an axial passage. The receptacle is pumped to a lower end of the casing string and locked in place. The receptacle has a casing seal that engages the string of casing and a retainer mechanism on its exterior that engages a profile in the string of casing. Cement is pumped through the receptacle by rupturing a blocking device in the axial passage of the receptacle. A wiper plug is pumped down the string casing. The wiper plug has a prong on its lower end that stabs into the axial passage of the receptacle. A latch located in the lower portion of the receptacle locks the wiper plug to the body.