Abstract:
Aspects of the present invention provide apparatus and methods of releasing a plug. In one aspect, a plug and ball seat assembly for releasing a plug is disclosed. The assembly is selectively maintained in a housing using a mounting system. The plug and ball seat assembly includes a ball seat selectively connected to the plug. The mounting system is designed such that disengaging the ball seat from the plug also disengages the plug from the housing, thereby releasing the plug. In another aspect, a method of releasing a plug includes selectively connecting a ball seat to the plug and disposing the plug and ball seat assembly within a tubular. When the plug is ready for release, the ball seat is disconnected from the plug and allowed to move axially relative to the plug. In turn, the plug and ball seat assembly disconnects from the tubular and releases downhole.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to fluid circulation operations for a wellbore. More particularly, the invention relates to a plug and ball seat assembly for releasing a wiper plug during cementing operations. 
   2. Description of the Related Art 
   In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons. 
   It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second “liner” string is then fixed or “hung” off of the surface casing. Afterwards, the liner is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter. 
   In the process of forming a wellbore, it is sometimes desirable to utilize various plugs. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. 
   Plugs are commonly used, for example, during the cementing operations for a liner. During a cementing operation for a liner, the liner is typically run into the wellbore at the end of a run-in string. The run-in string usually comprises a string of drill pipe. At the same time, a liner wiper plug is run into the wellbore at the lower end of the drill pipe. The run-in string places the liner and the wiper plug at the desired depth in the wellbore. After a wiper plug is released, it is pumped downhole in order to clean or “wipe” the inside of the liner of mud and debris. In this respect, the liner wiper plug employs radial wipers to contact and wipe mud from the liner as the plug travels downhole. 
   The process of releasing a liner wiper plug downhole is typically accomplished by pumping a drill-pipe dart down the drill string. The dart is pumped downward by injecting cement or other desired circulating fluid into the wellbore under pressure. The fluid forces the drill pipe dart downward into the wellbore until it contacts a seat in the wiper plug at or near the base of the drill pipe. The dart sealingly lands into the wiper plug. Hydraulic pressure from the injected fluid ultimately causes a releasable connection between the wiper plug and the drill pipe to release, thereby allowing the dart and the wiper plug to be pumped downhole as a single plug. This consolidated wiper plug separates the fluid above the plug, such as cement, from the drilling mud or other circulating fluid below the plug. 
   As noted, wiper plugs are retained through a releasable connection. A variety of mechanisms have been employed to retain and subsequently release liner plugs. These include mechanisms such as shear screws, dogs, collets, and retaining rings. Many of these utilize a sliding sleeve that is held in place by a shearable device. When the drill-pipe dart lands in the sliding sleeve, the shearable device is sheared and the sleeve moves down, allowing the plug to release. 
   Certain disadvantages exist with the use of the above release mechanism arrangements. For example, during well completion operations, the release mechanism, such as the shear screws or the sliding sleeve, is subjected to various stresses which may cause premature release of the wiper plug. In some situations the sliding sleeve is subjected to an impact load by a ball or other device as it passes through the inside of the plug. In other situations, a pressure wave may impact the releasable connection. In either of these situations, it is possible for the sliding sleeve or other release mechanism to shear and to thereby inadvertently or prematurely release the wiper plug. 
   There is a need, therefore, for a more effective release mechanism for a wiper plug. There is a further need for a releasing mechanism for a liner wiper plug which will not prematurely release the plug in the event of a pressure surge. Still further, there is a need for a liner wiper plug releasing mechanism which does not rely upon a sliding sleeve. 
   SUMMARY OF THE INVENTION 
   Aspects of the present invention provide apparatus and methods of releasing a plug. In one aspect, a plug and ball seat assembly for releasing a plug is disclosed. The plug and ball seat assembly includes a ball seat selectively connected to the plug. The assembly is selectively maintained in a housing using a mounting system. The mounting system is designed such that disengaging the ball seat from the plug also disengages the plug from the housing, thereby releasing the plug. 
   In another aspect, a method of releasing a plug includes selectively connecting a ball seat to the plug and disposing the plug and ball seat assembly within a tubular. When the plug is ready for release, the ball seat is disconnected from the plug and allowed to move axially relative to the plug. In turn, the plug and ball seat assembly disconnects from the tubular and releases downhole. 
   In another aspect still, a method of circulating a fluid within a wellbore includes positioning a plug and ball seat assembly in a path of the fluid. Thereafter, the fluid path is selectively blocked to prevent movement of the fluid through the path. The plug and ball seat assembly is then released into the path of the fluid, thereby allowing the movement of the fluid to continue. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1A  is a cross-sectional view of a plug and ball seat assembly according to aspects of the present invention. The plug and ball seat assembly is connected to a housing. 
       FIG. 1B  shows the plug and ball seat assembly released from the housing. 
       FIG. 2  is a view of the body of the plug in the assembly. 
       FIG. 2A  is a cross-sectional view of the body of the plug of  FIG. 2  along line  2 A— 2 A. 
       FIG. 3  is a cross-sectional view of the lower backup member of the assembly shown in FIG.  1 A. 
       FIG. 4  is a top view of the mounting plate of the assembly shown in FIG.  1 A. 
       FIG. 4A  is a cross-sectional view of the mounting plate of  FIG. 4  along line  4 A— 4 A. 
       FIG. 5A  is a schematic cross-sectional view of the plug and ball seat assembly disposed in a wellbore just after a ball has landed in the ball seat. 
       FIG. 5B  is a schematic cross-sectional view of the plug and ball seat assembly landing in the float assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A  is a cross-sectional view of a plug and ball seat assembly  100  according to aspects of the present invention. The plug and ball seat assembly  100  is designed to be independent of any running tools. The assembly  100  includes a mechanism for releasing the plug  10  downhole. Such an assembly  100  may have many applications, including the release of a liner wiper plug from an upper portion of a liner  205  during a liner cementing operation as shown in FIG.  5 A. 
   As shown in  FIG. 1A , the plug and ball seat assembly  100  is disposed in a housing  20 . The housing  20  defines a tubular having connection means  22  for connecting to another tubular such as a liner. As shown, threads  22  are formed at the ends of the housing  20  for connecting to another tubular. The connection means  22  facilitate the attachment of the plug and ball seat assembly  100  to the liner or attachment of additional plug and ball seat assemblies. A groove  25  is formed on an inner surface of the housing  20  for maintaining the assembly  100  in the housing  20  before it is ready for release, as will be discussed in more detailed below. Although the housing  20  is shown as a separate tubular, it must be noted that the aspects of the present invention also contemplates disposing the assembly  100  directly in any tubular in which a plug  10  is to be released. For example, the assembly  100  may be disposed directly in the liner by forming the groove  25  in the liner itself. In which case, the liner also functions as the housing  20  for the assembly  100 . 
   The plug and ball seat assembly  100  includes a plug  10  and a ball seat  30  disposed therein. In one embodiment, the plug  10  includes a tubular body  110  having a slotted lower portion  115  as illustrated in  FIGS. 2 and 2A .  FIG. 2A  shows a cross-sectional view of the body taken along line  2 A— 2 A. Preferably, the slotted lower portion  115  is formed by providing a plurality of longitudinal slots  116  on the body  110 . The body  110  further includes a stop member  120  to separate the slotted portion  115  from the remaining portions of the body  110 . Referring back to  FIG. 1A , a nose adapter  40  connects a nose plug  50  to the body  110 . Preferably, the nose adapter  40  is threaded onto the end of the slotted portion  115 , thereby maintaining the spatial relationship of the slots  116  of the slotted portion  115 . A plug landing bushing  60  is installed on an upper end of the body  110  for mating with the nose plug of a successive plug (not shown) dropped into the wellbore during circulation operations. 
   The plug  10  may be equipped with a plurality of fins  70 . The fins  70  allow the plug  10  to wipe or clean an inner surface of the housing  20  and/or the liner  205  as the plug  10  travels along the liner  205 . Preferably, the fins  70  are made of an elastomeric material. However, other types of fins  70  are equally applicable as is known to a person of ordinary skill in the art. 
   The fins  70  may optionally be supported by one or more backup members  72 ,  74 ,  76 . Preferably, the backup members  72 ,  74 ,  76  are in the form of annular rings made of metal. A backup member  72 ,  74  may be disposed on each end of the fins  70 . In one embodiment, the lower backup member  72  is disposed between the stop member  120  and the fins  70 . As illustrated in  FIG. 3 , the lower backup member  72  has a partially threaded inner surface  230  for mating with a threaded surface  130  of the body  110 . The lower backup member  72  may also have a cone-shaped inner surface  235  to complement the shape of the fins  70 . The upper backup member  74  works with the lower backup member  72  to prevent axial movement of the fins  70  relative to the plug  10 . The upper backup member  74  has a cone shaped interior to accommodate the plug landing bushing  60 . Intermediate backup members  76  may also be disposed between each fin  70 . The intermediate backup members  76  are designed to provide support between each fin  70 . The backup members  72 ,  74 ,  76  allow the elastomeric fins  70  to maintain maximum contact with the inner surface of the liner  205 . 
   In another aspect, the ball seat  30  is selectively connected to the inner surface of the plug  10  to form the plug and ball seat assembly  100  shown in FIG.  1 A. An example of the ball seat  30  includes a swab cup ball seat or any applicable ball seat known to a person of ordinary skill in the art. In one embodiment, the ball seat  30  is selectively connected to the plug  10  using a shearable connection. Preferably, one or more holes  80  are formed on an outer surface of the ball seat  30  for mating with a shearable member  85 . The holes  80  are formed in alignment with an annular groove  125  on the inner surface of the plug  10 . The shearable member  85 , which is a shearable pin in this embodiment, is at least partially disposed in both the recess groove  125  of the plug  10  and a respective hole  80  in the ball seat  10 . The shearable pins  85  are designed to shear at a predetermined pressure. The number of pins  85  used to maintain the ball seat  30  in the unreleased position depends on the desired ball seat release pressure. In this manner, the ball seat  30  may be maintained in the plug  10  until the proper time for release. It must be noted that aspects of the present application also contemplate other types of shearable connections for selectively maintaining the ball seat  30  in the plug  10  as is known to a person of ordinary skill in the art 
   The plug and ball seat assembly  100  is selectively maintained in the housing  20  using a mounting system that is actuated by the release of the ball seat  30  from the plug  10 . The mounting system includes a mounting plate  90  and one or more mounting members  95 .  FIG. 4  illustrates a top view of a preferred embodiment of the mounting plate  90 . The mounting plate  90  is in the form of an annular ring having four arcuate slots  92  formed concentric to the center of the mounting plate  90  for slidable connection with the slotted portion  115  of the body  110 . Additionally, the mounting plate  90  has a threaded inner surface  97  that mates with a threaded lower portion  35  of the ball seat  30  as shown in FIG.  1 A. In this manner, the mounting plate  90  is movable with the ball seat  30 . 
   A plurality of mounting supports  94  are formed on an outer surface of the mounting plate  90  for receiving a mounting member  95  as illustrated in  FIGS. 1A ,  4 , and  4 A.  FIG. 4A  shows a cross-sectional view of the mounting plate  90  along line  4 A— 4 A. As shown, eight radially spaced mounting supports  94  are formed on the mounting plate  90  and designed to at least partially receive the mounting members  95 . Referring to  FIG. 1A , each mounting member  95  at least partially resides in the mounting support  94  and is trapped between the mounting plate  90  and the lower backup ring  72 . In one embodiment, each mounting member  95  comprises a ball made of metal or some other material capable of supporting the weight of the assembly  100 . Preferably, the total outer diameter of the mounting plate  90  with the mounting members  95  disposed therein is greater than an inner diameter of the housing  20 . In this respect, the mounting member  95  may at least partially reside in the groove  25  of the housing  20 , thereby maintaining the plug and ball seat assembly  100  in the housing  20  until it is ready to be released. 
   In operation, the plug and ball seat assembly  100  disposed in the housing  20  may be attached to an upper portion of a liner  205  as shown in FIG.  5 A. The liner  205  may be operatively connected to a liner hanger assembly (not shown) and lowered into the wellbore (not shown) using a run-in string (not shown). The liner hanger assembly may be a hydraulically operated liner hanger assembly or any other liner hanger assembly known to a person of ordinary skill in the art. The liner  205  is lowered into the wellbore until it reaches the desired depth. For example, the liner  205  may be placed in an overlapping position with an existing casing (not shown) in the wellbore. Before its release, the plug and ball seat assembly  100  is supported in the housing  20  through the mounting plate  90  and the mounting members  95 . 
   When the liner  205  is ready to be hung off of the casing, a ball  3  is dropped into the wellbore. The ball  3  may travel through the drill pipe and the running tool, and land in the ball seat  30  of the assembly  100  as shown in  FIGS. 1B and 5A , thereby closing off fluid communication between the wellbore below the ball seat  30  and the wellbore above the ball seat  30 . Thereafter, pressure is supplied to the liner hanger assembly to set the hanger to hang the liner  205  in the casing. 
   After the liner  205  is hung, pressure is increased to shear the pins  85  connecting the ball seat  30  to the plug  10 . When the pressure reaches the predetermined pressure necessary to shear the pins  85 , the ball seat  30  detaches and travels along the interior of the plug  10  toward the nose plug  50  as shown in FIG.  1 B. Because the mounting plate  90  is attached to the ball seat  30 , the mounting plate  90  moves downward along with the ball seat  30 . As the mounting plate  90  moves away from the lower backup ring  72 , the mounting members  95  are treed from the mounting plate  90  and allowed to fall out of the groove  25  of the housing  20  and travel downward. In this manner, the plug  10  is disengaged from the housing  20  and released downhole toward the float assembly  215 . As the plug  10  descends in the wellbore, the ball seat  30  and the mounting plate  90  will move toward the nose plug  50  until it contacts the nose plug  50 . 
   Fluid behind the plug and ball seat assembly  100  pumps the assembly  100  along the liner  205  until it lands in the float assembly  215  disposed at the bottom of the liner  205  as shown in FIG.  5 B. The float assembly  205  prevents the assembly  100  from further axial movement after landing. 
   As can be seen, a plug and ball seat assembly  100  has been provided. In this respect, the ball seat  30  may be located near the top of the liner  205  without being attached to any running tool. In this manner, the ball seat  30  may be positioned high enough in the liner  205  to not impact the formation with surge pressure and prevent the premature launch of the plug  10 . 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. In this respect, it is within the scope of the present invention to use the plug and ball assembly  100  disclosed herein to release plugs  10  for various cleaning and fluid circulation procedures in addition to cementing operations for liners. The plug and ball seat assembly  100  of the present invention has utility in the context of subsea operations and for cementing operations of any string of casing.