Abstract:
A removable ball seat assembly is disclosed. It features a solid ball seat backed up by segmented dogs pinned to each other and mounted under the ball seat. Upon actuating a downhole tool with fluid pressure applied to a ball on the seat, the pressure is increased and the ball and seat move at a regulated rate. The dogs reach a recess and the ball moves through the seat. Subsequent, larger balls can pass through the seat, with the dogs in the recess, at much smaller pressure drops than the original ball.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of this invention relates to pump through ball seats used to build downhole pressure to actuate tools and more particularly to ball seats for use with liner hangers which must accommodate subsequent passage of wiper plugs during liner cementing or a larger ball for further downhole operations..  
         BACKGROUND OF THE INVENTION  
         [0002]    Downhole operations frequently involve the need to build up pressure to set a tool and/or to release from a tool. After the setting and release occurs, there is a need for access downhole. In the past ball seats have been used in combination with a ball or balls dropped from the surface to provide a way to close a tubular temporarily to allow for the requisite pressure buildup. The ball seats have to serve conflicting functions. They must be sturdy enough to withstand large differential pressures for a sufficient time to set the tool. They must cleanly release the ball to allow for subsequent objects such as wiper plugs or another, bigger, ball to pass through the spent ball seat with minimal pressure drop. They must be relatively easy to mill out of the way to accommodate subsequent downhole operations.  
           [0003]    Yet another problem is the potential to over pressure the formation below as the requisite pressure on the ball has been built up and needs to be released. In the past, this problem has been addressed by using a reduced shock mechanism as part of the ball seat design. As shown in U.S. Pat. No. 6,079,496, the ball seat is movably mounted with the landing collar and pressure buildup on the ball moves the ball seat to reduce the volume of a variable volume cavity whose outlet is restricted. The restrictor, in turn, regulates the flow out of the cavity, which forces the ball seat to move at a predetermined rate, to reduce shock on the formation below. This Patent also teaches the use of non-metallic materials to facilitate milling out of the landing collar. Millout must occur because the ball seat assembly is designed to remain downhole with the liner being set and cemented.  
           [0004]    Other prior designs have focused on construction of the ball seat. Some designs used segmented collets which shifted longitudinally under pressure with a ball on the seat formed by the segmented collets until a recess was reached allowing the segmented collets to spread and the ball to pass. Some examples of the segmented collet design are U.S. Pat. Nos. 5,244,044; 4,893,678; 4,823,882; 4,292,988; 3,220,481. Of these Patents, 4,292,988 is most notable because it also has a provision to regulate the movement of the ball seat after its securing shear pin is broken to reduce shock. Another design involved a solid ball seat which expanded when moved to an unsupported position to let the ball pass. Some examples of this design are U.S. Pat. Nos. 4,520,870; 4,510,994; 4,114,694; 3,090,442; 4,862,966 and 6,155,350 (which also incorporates a controlled release pressure feature). Still other designs contemplated plastic deformation of the seat or controlled breakage along scoring of the seat to allow the ball or balls to be pumped through. Examples of this variation are U.S. Pat. Nos. 5,146,992 and 5,960,881.  
           [0005]    Some of the drawbacks of the prior designs are addressed as the objectives of the present invention. The ball seat assembly is removable with the setting tool and running string so that it does not need to be milled out subsequently. The ball seat is firmly supported by segmented dogs held together with roll pins and disposed on the back side of the solid frusto-conically shaped ball seat. The problem of erosion of the ball due to rapidly moving fluid that could leak past segmented collets forming the ball seat is eliminated with the new ball seat design.  
           [0006]    Another drawback of prior designs which used solid ball seats, such as U.S. Pat. Nos. 5,146,992 and 5,960,881 is eliminated by the present invention. In the past after an initial ball was pushed through the seat, subsequent balls would require high pressures to clear through the ball seat because of the point of contact made with the ball seat by the bigger ball. This was undesirable as it was advantageous to get the next and larger ball through the seat at low pressure differentials to expedite the next downhole operation and to avoid setting off relief devices built into such subsequent balls. These and other advantages of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment, described below.  
         SUMMARY OF THE INVENTION  
         [0007]    A removable ball seat assembly is disclosed. It features a solid ball seat backed up by segmented dogs pinned to each other and mounted under the ball seat. Upon actuating a downhole tool with fluid pressure applied to a ball on the seat, the pressure is increased and the ball and seat move at a regulated rate. The dogs reach a recess and the ball moves through the seat. Subsequent, larger balls can pass through the seat, with the dogs in the recess, at much smaller pressure drops than the original ball. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a sectional view, in elevation of the invention, in the run in position:  
         [0009]    [0009]FIG. 2 is the view of FIG. 1 in the position just before the ball is blown through the seat;  
         [0010]    [0010]FIG. 3 is the view along lines  3 - 3  of FIG. 1;  
         [0011]    [0011]FIG. 4 is the view along lines  4 - 4  of FIG. 2;  
         [0012]    [0012]FIG. 5 is a section view, in elevation, of the ball seat; and  
         [0013]    [0013]FIG. 6 is a section view, in elevation, of one of the dog segments. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    Referring to FIG. 1, the apparatus A has a body  10  and a thread  12  adjacent the upper end. A thread  14  is disposed at the lower end of body  10 . In one application, a liner hanger setting and release tool (not shown) can be secured to thread  12  and another ball seat assembly can be secured to thread  14  to allow setting an external casing packer, for example. It is understood that body  10  is ultimately supported by “tubulars” from the well surface (not shown) and that at some point, body  10  is retrieved from the wellbore with such tubulars. “Tubulars” is defined as comprising coiled tubing or rigid pipe.  
         [0015]    Body  10  has a passage  16  that runs though it. Passage  16  has a recessed segment  18  in which sits sleeve  20 . Sleeve  20  defines an annular passage  22  in which restriction orifice  24  is disposed. Seal  26  is mounted on sleeve  20  to seal off the top of annular passage  22  as the sleeve  22  moves down. The restriction orifice  24  is secured to body  10 , such that downward movement of the sleeve  20  reduces the volume of annular passage  22  by squeezing fluid through restriction orifice  24  at a regulated rate. Appropriate seals between the sleeve  20  and the restriction orifice  24  allows for pressure buildup against restriction orifice  24  by reason of downward movement of sleeve  20 . Fluid displace through restriction orifice  24  exits body  10  through opening  28 .  
         [0016]    Retainer  30  is secured at thread  32  to sleeve  20 . Segmented support dogs  34  are doweled to retainer  30  using dowels or roll pins  36 . A ball seat  38  is supported by sleeve  20  using retainer  30 . The preferred material for ball seat  38  is 6061-T6 aluminum. Dogs  34 , in the run in position of FIG. 1, are also supported by the inner wall  40  of recessed segment  18 . A groove  42  is disposed at the lower end of wall  40  to allow the dogs  34  to become unsupported, when moved to the position shown in FIG. 2. FIG. 3 shows the dogs  34  fully supported by wall  40  during run in. FIG. 4 shows the dogs  34  separated after becoming aligned with groove  42 . FIG. 5 illustrates the ball seat  38  which is disposed at the lower end of sleeve  20 . FIG. 6 illustrates a dog  34  and the opening  44  for the dowel or roll pin  36 . Landing a ball  46  on the ball seat  38  initiates the process, which will be described below.  
         [0017]    The apparatus A is lowered downhole on tubing or a tubular string. Located above body  10  is a liner hanger. Located below body  10  may be receptacles for catching plugs for subsequent completion operations such as displacement of fluids or cement or setting an external casing packer (not shown). A ball  46  is dropped from the surface and lands on ball seat  38 . The pressure is built up to set, for example, the liner hanger (not shown), to a level in the order of 2000 pounds per square inch (PSI) surface pressure, which is equivalent to about 5,000 PSI in annular passage  22 , depending on dimensions. After the hanger is set, the surface pressure is increased further to about 2,500 PSI until rupture disc  48  located below restriction orifice breaks at a pressure closer to about 6300 PSI, in annular chamber  22 . The movement of sleeve  20  varies with the size of restriction orifice  24  and can be set to take several minutes, before dogs  34  reach groove  42 . Fluid is displaced out of opening  28 . If the restriction orifice  24  fails to function, a backup rupture disc  50  will break at about 4200 PSI applied from the surface or roughly 10,600 PSI in annular chamber  22 . If rupture disc  50  operates then restriction orifice  24  is bypassed and there is not shock reduction effect on the formation. This is because there is no longer a restriction limiting the exit rate of fluid from annular passage  22 , as the fluid now escapes abruptly through opening  52 .  
         [0018]    In normal operation, the breakage of rupture disc  48  allows sleeve  20  to move at a regulated rate until the dogs  34  come into alignment with groove  42 . The dogs then pivot about dowels  36  removing support for the tapered segment of the ball seat  38 . The ball seat  38  can then be expanded or extruded by ball  46  as ball  46  is blown through the ball seat  38  after landing on it, as shown in FIG. 2. The subsequent well operations may require wipers or plugs that exceed the diameter of ball  46  to pass through ball seat  38 . Because ball seat  38  has been deformed by the passage of ball  46  and is no longer supported by dogs  34 , very low differential pressure in the order of less than 500 PSI is required to force such subsequent plugs or past the former tapered segment  54 , see FIG. 5. These subsequent wipers, balls or plugs have built into them rupture discs, in the event they fail to travel all the way to their intended receptacle. Accordingly, because ball seat  38  is no longer supported by dogs  34  and further because it has been expanded by ball  46 , there is little danger of blowing rupture discs on subsequent plugs or balls as they try to pass through ball seat  38 . Ball seat  38  is preferably made of a solid piece without gaps as in the prior designs which used a collection of collets to form a ball seat. Rather, ball seat  38  is more akin to the ball seat in U.S. Pat. No. 5,146,992 insofar as it is a solid piece. However the function of ball seat  38  is different than the ball seat of U.S. Pat. No. 5,146,992 as described herein.  
         [0019]    If, for any reason the ball  46  will not go through the ball seat  38 , rupture disc  56  will blow at about 5000 PSI surface pressure and will provide a flowpath for subsequent operations through opening  58  in body  10 . It should be noted that rupture disc  56  is not in annular passage  22  and is therefor exposed directly to surface pressure at all times. In this manner the obstructed sleeve  20  can be bypassed for subsequent operations such a cementing the liner.  
         [0020]    The advantages of the apparatus A over the prior designs will now be readily apparent. The components such as the ball seat  38  can be made of metallic components since subsequent milling is not an issue in view of the fact that body  10  is removed when the requisite completion operations are accomplished. Using high strength components for the ball seat  38  and backing it with dogs  34  for additional support, allows high setting pressures for a sustained period to be applied to ball  46  for setting the liner hanger (not shown), for example. The ball seat can have a relatively thin tapered segment  58  which is about 0.020 inches plus or minus 0.002 with an initial outlet opening of about 1.28 inches and a slope of 30 degrees as measured from the longitudinal axis. With backing from dogs  34  it will readily hold the 2,500 PSI pressure from the surface necessary to break rupture disc  48  so sleeve  20  can move down. On the other hand, once the support from dogs  34  is removed, the ball  46  easily pushes through the tapered segment  54 . Furthermore, subsequent larger balls or plugs engage the now expanded and unsupported tapered segment  54  higher up than ball  46  or at the same height on the now expanded opening and therefore pass easily without large pressure differentials. Surface pressures of 500 PSI or less will allow such subsequent balls or plugs to pass uneventfully. On top of all these advantages, there is the reduced shock feature on the formation from the action of restrictor  24  after rupture disc  48  is broken.  
         [0021]    In the prior designs, downhole environments affected performance of the ball seats. Phenomena such as water hammer and fluid decompression at the time of ball landing due to well losses was loading these ball seats and causing a low shear, without surface pressure being applied. Because of this phenomenon, hydraulic hangers would not set and hydraulic running tools might not release. Another consequence was that subsequent cement jobs were performed without wiper plugs due to concerns over whether downhole equipment would function properly. The present invention addresses these concerns and overcomes these and other shortcomings of the prior art as described above.  
         [0022]    While the invention has been described and illustrated in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the scope of the claims below are the full scope of the invention being protected.