A specially designed rotary indexing system and associated operational methods are incorporated in a downhole control device, representatively a sliding sleeve valve, having an outer tubular member in which an annular plug seat is coaxially disposed. The plug seat is resiliently expandable between a first diameter and a larger second diameter and is illustratively of a circumferentially segmented construction. The rotary indexing system is operative to detect the number of plug members that pass through and diametrically expand the plug seat, and responsively preclude passage of further plug members therethrough when such number reaches a predetermined magnitude. Such predetermined magnitude is correlated to the total rotation of an indexing system counter ring portion rotationally driven by axial camming forces transmitted to the rotary indexing system by successive plug member passage-generated diametrical expansions of the plug seat.

CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of the filing date of provisional U.S. patent application No. 61/644,887 filed May 9, 2012. The entire disclosure of the provisional application is hereby incorporated herein by this reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a fracture plug seat assembly used in well stimulation for engaging and creating a seal when a plug, such as a ball, is dropped into a wellbore and landed on the fracture plug seat assembly for isolating fracture zones in a well. More particularly, the present invention relates to a fracture plug seat assembly that includes a mechanical counter allowing plugs to pass through the seat then locking to a rigid seat position after a designated number of plugs from the surface have passed through the seat. The locking mechanism disengages when flow is reversed and plugs are purged. 
       BACKGROUND 
       [0003]    In well stimulation, the ability to perforate multiple zones in a single well and then fracture each zone independently, referred to as “zone fracturing”, has increased access to potential reserves. Zone fracturing helps stimulate the well by creating conduits from the formation for the hydrocarbons to reach the well. Many gas wells are drilled for zone fracturing with a system called a ball drop system planned at the well's inception. A well with a ball drop system will be equipped with a string of piping below the cemented casing portion of the well. The string is segmented with packing elements, fracture plugs and fracture plug seat assemblies to isolate zones. A fracture plug, such as a ball or other suitably shaped structure (hereinafter referred to collectively as a “ball”) is dropped or pumped down the well and seats on the fracture plug seat assembly, thereby isolating pressure from above. 
         [0004]    Typically, in ball drop systems a fracture plug seat assembly includes a fracture plug seat having an axial opening of a select diameter. To the extent multiple fracture plugs are disposed along a string, the diameter of the axial opening of the respective fracture plug seats becomes progressively smaller with the depth of the string. This permits a plurality of balls having a progressively increasing diameter, to be dropped (or pumped), smallest to largest diameter, down the well to isolate the various zones, starting from the toe of the well and moving up. 
         [0005]    A large orifice through an open seat is desired while fracing zones below that seat. An unwanted consequence of having seats incrementally smaller as they approach the toe is the existence of pressure loss across the smaller seats. The pressure loss reduces the efficiency of the system and creates flow restrictions while fracing and during well production. 
         [0006]    In order to maximize the number of zones and therefore the efficiency of the well, the difference in the diameter of the axial opening of adjacent fracture plug seats and the diameter of the balls designed to be caught by such fracture plug seats is very small, and the consequent surface area of contact between the ball and its seat is very small. Due to the high pressure that impacts the balls during a hydraulic fracturing process, the balls often become stuck and are difficult to purge when fracing is complete and the well pressure reverses the flow and produces to the surface. If a ball is stuck in the seat and cannot be purged, the ball(s) must be removed from the string by costly and time-consuming milling or drilling processes. 
         [0007]      FIG. 1  illustrates a prior art fracture plug seat assembly  10  disposed along a tubing string  12 . Fracture plug seat assembly  10  includes a metallic, high strength composite or other rigid material seat  14  mounted on a sliding sleeve  16  which is movable between a first position and a second position. In the first position shown in  FIG. 1 , sleeve  16  is disposed to inhibit fluid flow through radial ports  18  from annulus  20  into the interior of tubing string  12 . Packing element  24  is disposed along tubing string  12  to restrict fluid flow in the annulus  20  formed between the earth  26  and the tubing string  12 . 
         [0008]      FIG. 2  illustrates the prior art fracture plug seat assembly  10  of  FIG. 1 , but with a ball  28  landed on the metallic, high strength composite or other rigid material seat  14  and with sliding sleeve  16  in the second position. With ball  28  landed on the metallic, high strength composite or other rigid material seat  14 , fluid pressure  30  applied from uphole of fracture plug seat assembly  10  urges sliding sleeve  16  into the second position shown in  FIG. 2 , thereby exposing radial ports  18  to permit fluid flow therethrough, diverting the flow to the annulus  20  formed between the earth  26  and the tubing string  12 . 
         [0009]    As shown in  FIGS. 1 and 2 , the metallic, high strength composite or other rigid material seat  14  has a tapered surface  32  that forms an inverted cone for the ball or fracture plug  28  to land upon. This helps translate the load on the ball  28  from shear into compression, thereby deforming the ball  28  into the metallic, high strength composite or other rigid material seat  14  to form a seal. In some instances, the surface of such metallic, high strength composite or other rigid material seats  14  have been contoured to match the shape of the ball or fracture plug  28 . One drawback of such metallic, high strength composite or other rigid material seats  14  is that high stress concentrations in the seat  14  are transmitted to the ball or fracture plug  28 . For various reasons, including specific gravity and ease of milling, balls or fracture plugs  28  are often made of a composite plastic or aluminum. Also, efforts to maximize the number of zones in a well has reduced the safety margin of ball or fracture plug failure to a point where balls or fracture plugs can extrude, shear or crack under the high pressure applied to the ball or fracture plug during hydraulic fracturing operations. As noted above, when the balls  28  extrude into the metallic, high strength composite or other rigid material seat  14  they become stuck. In such instances, the back pressure from within the well below is typically insufficient to purge the ball  28  from the seat  14 , which means that an expensive and time-consuming milling process must be conducted to remove the ball  28  from the seat  14 . 
         [0010]    Other prior art fracture plug seat assembly designs include mechanisms that are actuated by sliding pistons and introduce an inward pivoting mechanical support beneath the ball. These designs also have a metallic, high strength composite or other rigid material seat, but are provided with additional support from the support mechanism. These fracture plug seat assembly designs can be described as having a normally open seat that closes when a ball or fracture plug is landed upon the seat. Such normally open fracture plug seat assembly designs suffer when contaminated with the heavy presence of sand and cement. They also rely upon incrementally sized balls so such systems suffer from flow restriction and require post frac milling. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a prior art fracture plug seat assembly positioned in a well bore. 
           [0012]      FIG. 2  illustrates the prior art fracture plug seat assembly of  FIG. 1  with a ball landed on the seat of the fracture plug seat assembly. 
           [0013]      FIG. 3  illustrates a cross-section of a fracture plug seat assembly incorporating an embodiment of the present invention with a cam driven rotating counter in the unlocked position. 
           [0014]      FIG. 4  illustrates a cross-section of the fracture plug seat assembly illustrated in  FIG. 3  with a ball passing through the assembly and actuating an expandable seat. 
           [0015]      FIG. 5  illustrates a side view of an embodiment of a counting mechanism of the present invention for use in a fracture plug seat assembly with a semi-translucent counting ring. 
           [0016]      FIG. 6  illustrates an isometric view of an embodiment of a counting ring of the present invention for use in a fracture plug seat assembly. 
           [0017]      FIG. 7  illustrates a side view of the embodiment of a counting mechanism of the present invention illustrated in  FIG. 5  with the components in position to actuate the counter. 
           [0018]      FIG. 8  illustrates a side view of the embodiment of a counting mechanism of the present invention illustrated in  FIG. 5  with a locking ring in a locked position. 
           [0019]      FIG. 9  illustrates a cross-section of the fracture plug seat assembly illustrated in  FIG. 3  with a locking ring in a locked position. 
           [0020]      FIG. 10  illustrates a cross-section of the fracture plug seat assembly illustrated in  FIG. 9  with a ball plugging the seat. 
           [0021]      FIG. 11  illustrates a cross-section of the fracture plug seat assembly illustrated in  FIG. 9  with a ball purging to the surface. 
           [0022]      FIG. 12  is a cross-section of a fracture plug seat assembly of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The method and apparatus of the present invention provides a fracture plug seat assembly used in well stimulation for engaging and creating a seal when a plug, such as a ball, is dropped into a wellbore and landed on the fracture plug seat assembly for isolating fracture zones in a well. The fracture plug seat assembly has a fracture plug seat that includes an expandable ring that enables the seat to expand when a ball passes through and actuates a counting mechanism so that balls are allowed to pass until the counting mechanism reaches a predetermined position which will enable the actuation of a locking mechanism. When actuated, the locking mechanism prevents expansion of the seat when the next ball lands on the seat and pressure is applied from the upstream direction. When flow is reversed, the seat is free to disengage from the locking mechanism and allow expansion and hence, balls that had previously passed through the seat pass through from downstream and return to the surface. 
         [0024]    According to the fracture plug seat assembly of the present invention, all balls have the same size and, therefore, flow restriction is greatly reduced at the lower zones, since the seat orifices do not become incrementally smaller. Also, according to the fracture plug seat assembly of the present invention, when dropping balls from the surface, it is not required to drop sequential ball sizes which eliminates a potential source of errors. Moreover, only one size of seat assembly and ball must be manufactured, instead of sometimes 40 different sizes, making manufacturing more cost effective. Finally, according to the fracture plug seat assembly of the present invention, the resulting production flow from the string can eliminate the need to mill out the seats. 
         [0025]      FIG. 3  illustrates a cross-section of a fracture plug seat assembly incorporating an embodiment of the present invention. Specifically, sliding sleeve assembly  40  is illustrated in a position to receive balls which will pass through and be counted. Sliding sleeve  41  is sealably retained within a tubing string. A segmented expandable seat assembly  42  is in a first closed position and positioned between a lower seat nut  43  and an upper piston  44 . The lower seat nut  43  is threadably connected to and does not move relative to the sliding sleeve  41 . The upper piston  44  is biased in the downstream direction  51  against the seat assembly  42  by a spring  46 . The spring  46  engages a shoulder  45  on the sliding sleeve  41 . 
         [0026]      FIG. 4  illustrates the fracture plug seat assembly of  FIG. 3  with a ball  50  passing through the sliding sleeve assembly  40  in the direction  51  with the direction of flow moving upstream to downstream. In  FIG. 4 , the ball  50  is engaged with the expandable seat assembly  42  and has driven the seat radially outward into a pocket  52  of a locking ring  53 . The upper piston  44  is wedged to move in the upstream direction  54  and further compresses the spring  46 . When the upper piston  44  moves in the upstream direction  54  it actuates a counting ring  55  via radial pins  56  which are rigidly connected to the upper piston  44  by engaging a cam surface  57  located on the end of the counting ring  55 .  FIG. 5  illustrates an embodiment for actuating the counting ring  55 . As the radial pins  56  move axially in the upstream direction  54  and into the counting ring  55 , the counting ring  55 , which is shouldered axially to the sliding sleeve  41  is forced to rotate as the radial pins  56  slide along the cam surface  57 . When the ball  50  has passed through the expandable seat assembly  42 , the spring  46  forces the upper piston  44  to return to the position shown in  FIG. 3 . According to the counting mechanism embodiment illustrated in  FIG. 5 , a second set of radial pins  58  engages a cam surface  59  on the upstream end of the counting ring  55  and force further rotation of the counting ring  55  by sliding across the cam surface  59 . As shown in  FIG. 7 , axial pin(s)  61  prevent the counting ring  55  from moving in the downstream direction since they are rigidly connected to the locking ring  53  which is biased in the upstream direction  54  by spring  63  ( FIG. 3 ). 
         [0027]      FIG. 6  illustrates an isometric view of the downstream side of counting ring  55 . As depicted, counting ring  55  has two synchronized sets of cam surfaces  57 , each set spanning nearly  180  degrees. Two holes  60  are located in the downstream face of the counting ring  55 . As shown in  FIG. 7 , a partially translucent counting ring  55  is shown in a side view with a radial pin  56  engaging a cam surface  57 . Also, as shown in  FIG. 7 , yet another radial pin  64  keeps the locking ring  53  from rotating relative to the upper piston  44 .  FIG. 7  is consistent with the position shown in  FIG. 4 . Further, as shown in  FIG. 7 , an axial pin  61  is fixed to the locking ring  53  and slides across the smooth surface  62  of counting ring  55  ( FIG. 6 ). An additional axial pin is diametrically opposite the axial pin  61  and is fixed to the locking ring  53  and slides across the smooth surface  62  of counting ring  55 . When a predetermined number of balls have passed through the seat assembly  42  and have thus rotated the counting ring  55  in relation to the locking ring  53 , the pin(s)  61  engage hole(s)  60  and a spring  63  ( FIG. 3 ) forces the locking ring  53  in the upstream direction  54 , as shown in  FIG. 8 .  FIG. 9  shows the sliding sleeve assembly  40  in the position where the locking ring  53  has shifted upstream and is in contact with the counting ring  55 . The pocket  52  is no longer in a position to allow expansion of the expandable seat assembly  42  from a ball passing in the direction  51 .  FIG. 10  illustrates the sliding sleeve assembly  40  with a ball  70  that has landed on the expandable seat assembly  42  when the locking ring  53  is in the locked position. The expandable seat assembly  42  is restricted from expanding due to the locking ring  53  and hence the ball  70  cannot pass in the downstream direction  51 . A seal  71  can assist in preventing fluid from passing by the ball  70  in the downstream direction  51  and a seal  73  prevents fluid from passing between the upper piston  44  and the sliding sleeve  41 . Pressure applied to the ball in the downstream direction  51  results in the force necessary to actuate the sliding sleeve assembly  40  to an opened position so its corresponding zone can be fractured. 
         [0028]    When pressure in the downstream direction is relieved, the ball  70  is purged to the surface in the direction  54  by accumulated pressure from downstream.  FIG. 11  illustrates a ball  72  that had previous passed through the sliding sleeve assembly  40  in the downstream direction  51  and actuated the counting ring  55 . Now pressure from the downstream side of the ball  72  forces the expandable seat assembly  42  to slide in the upstream direction  54  until it reaches the pocket  52 . Ball  72  can now pass through the expandable seat assembly  40  and freely purge to the surface. 
         [0029]      FIG. 12  is a cross-section of a fracture plug seat assembly of the present invention in a position ready to count a ball. As shown in  FIG. 12 , an upper wave spring  83  which helically spirals around axis  84 , biases an upper piston  81  in the downstream direction  51 . A wave spring  85  similar to the upper wave spring  83  biases a locking ring  82  in the upstream direction  54 . An expandable seat assembly  94  is clamped by the biased upper piston  81  and a lower seat nut  93  into a cinched position. The expandable seat assembly  94  is free to expand into a pocket  95  when a ball passes through. When a ball actuates the expandable seat assembly  94 , the upper piston  81  carries radial pins  96  into a cam profile of counting ring  97  to initiate rotation of the counting ring  97 . After the final ball to be counted passes through the expandable seat assembly  94 , an axial pin  98  falls into a mating hole in counting ring  97  and the locking ring  82  is free to be pushed in the upstream direction  54  by the wave spring  85 . 
         [0030]    Also illustrated in  FIG. 12  are an upper wiper seal  86 , a lower seal  87  and a nut seal  88 . According to the embodiment shown in  FIG. 12 , both upper wiper seal  86  and lower seal  87  engage the upper piston  81  at the same diameter so there is no change in volume in annulus  89  when the upper piston  81  is actuated. While not essential to the function of this embodiment of the fracture plug seat assembly, this embodiment resists the accumulation of dirty fluid in the annulus  89 . Also, the nut seal  88  guards against the incursion of debris into the space  91 . Expandable seat assembly  94  may be formed from any suitable material such as a segmented ring of drillable cast iron. Those of ordinary skill in the art will understand that the expandable seat assembly  94  may also be encapsulated in rubber so as to guard against the entry of contaminants into pocket  95  and to shield the cast iron from the abrasive fluid passing through the expandable seat assembly  94 . 
         [0031]    It is to be understood that the means to actuate the counter could be a lever or radial piston that is not integrated into the expandable seat. It is convenient to use the expandable seat as the mechanism to actuate the counter. It is also to be understood that the counter could actuate a collapsible seat. 
         [0032]    It is understood that variations may be made in the foregoing without departing from the scope of the disclosure. 
         [0033]    In several exemplary embodiments, the elements and teachings of the various illustrative exemplary embodiments may be combined in whole or in part in some or all of the illustrative exemplary embodiments. In addition, one or more of the elements and teachings of the various illustrative exemplary embodiments may be omitted, at least in part, and/or combined, at least in part, with one or more of the other elements and teachings of the various illustrative embodiments. 
         [0034]    Any spatial references such as, for example, “upper,” “lower,” “above,” “below,” “between,” “bottom,” “vertical,” “horizontal,” “angular,” “upwards,” “downwards,” “side-to-side,” “left-to-right,” “left,” “right,” “right-to-left,” “top-to-bottom,” “bottom-to-top,” “top,” “bottom,” “bottom-up,” “top-down,” etc., are for the purpose of illustration only and do not limit the specific orientation or location of the structure described above. 
         [0035]    In several exemplary embodiments, while different steps, processes, and procedures are described as appearing as distinct acts, one or more of the steps, one or more of the processes, and/or one or more of the procedures may also be performed in different orders, simultaneously and/or sequentially. In several exemplary embodiments, the steps, processes and/or procedures may be merged into one or more steps, processes and/or procedures. In several exemplary embodiments, one or more of the operational steps in each embodiment may be omitted. Moreover, in some instances, some features of the present disclosure may be employed without a corresponding use of the other features. Moreover, one or more of the above-described embodiments and/or variations may be combined in whole or in part with any one or more of the other above-described embodiments and/or variations. 
         [0036]    Although several exemplary embodiments have been described in detail above, the embodiments described are exemplary only and are not limiting, and those skilled in the art will readily appreciate that many other modifications, changes and/or substitutions are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications, changes and/or substitutions are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.