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FIELD OF THE INVENTION 
     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 that includes an expandable seat to allow balls to pass through its interior by expanding and then restricts expansion and locks when the designated ball is dropped. 
     BACKGROUND 
     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. Many gas wells are drilled with zone fracturing planned at the well&#39;s inception. Zone fracturing helps stimulate the well by creating conduits from the formation for the hydrocarbons to reach the well. A well drilled with planned fracturing zones 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. 
     Typically, 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. When the well stimulation in a particular zone is complete, the ball is removed from the fracture plug seat. 
     In order to maximize the number of zones and therefore the efficiency of the well, the difference in the axial opening diameter 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 ball during a hydraulic fracturing process, the balls often become stuck and difficult to remove from the fracture plug seats despite being designed to return to the surface due to pressure from within the formation. In such instances, the balls must be removed from the string by costly and time-consuming milling or drilling processes. 
       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  20 . Packing element  22  is disposed along tubing string  12  to restrict fluid flow in the annulus  20  formed between the earth  24  and the tubing string  12 . 
       FIG. 2  illustrates the prior art fracture plug seat assembly  10  of  FIG. 1 , but with a ball  26  landed on the metallic, high strength composite or other rigid material seat  14  and with sliding sleeve  16  in the second position. With ball  26  landed on the metallic, high strength composite or other rigid material seat  14 , fluid pressure  28  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 earth  24 . 
     As shown in  FIGS. 1 and 2 , the metallic, high strength composite or other rigid material seat  14  has a tapered surface  30  that forms an inverted cone for the ball or fracture plug  26  to land upon. This helps translate the load on the ball  26  from shear into compression, thereby deforming the ball  26  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  26 . 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  26 . For various reasons, including specific gravity and ease of milling, balls or fracture plugs  26  are often made of a composite plastic. 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  26  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  26  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 . 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art fracture plug seat assembly positioned in a well bore. 
         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. 
         FIG. 3  illustrates a cross-section of a fracture plug seat assembly incorporating an embodiment of the fracture plug seat of the present invention. 
         FIG. 4  illustrates the fracture plug seat assembly of  FIG. 3  with the fracture plug seat allowing a ball to pass to a deeper zone. 
         FIG. 5  illustrates a cross-section taken along line  5 - 5  of  FIG. 4 . 
         FIG. 6  illustrates the fracture plug seat assembly of  FIG. 3  with a ball landed on the seat of the fracture plug seat assembly and applying pressure to the fracture plug seat assembly which is in an unlocked position. 
         FIG. 7  illustrates the fracture plug seat assembly of  FIG. 3  with a ball landed on the seat of the fracture plug seat assembly and in which the fracture plug seat is in a position between the unlocked position shown in  FIG. 6  and a locked position shown in  FIG. 8 . 
         FIG. 8  illustrates the fracture plug seat assembly of  FIG. 6  with the fracture plug seat in the locked position. 
         FIG. 9  illustrates the fracture plug seat assembly of  FIG. 8  after the landed ball has been purged by reverse pressure and a downstream ball makes contact with the fracture plug seat which remains in the locked position. 
         FIG. 10  illustrates a magnified view of a portion of the fracture plug seat assembly as shown in  FIG. 9 . 
         FIG. 11  illustrates the fracture plug seat assembly of  FIG. 9  with a downstream ball passing through the fracture plug seat after it has been returned to an unlocked position by the downstream ball. 
         FIG. 12  illustrates a cross-section of an embodiment of a fracture plug seat assembly of the present invention in which the fracture plug seat incorporates a collet style expandable ring. In this illustration a ball is passing through the collet. 
         FIG. 13  illustrates the fracture plug seat assembly of  FIG. 12  with a ball landed on the seat of the fracture plug seat assembly and applying pressure to the fracture plug seat assembly so as to be in a locked position. 
         FIG. 14  illustrates a cross-section of an embodiment of a fracture plug seat assembly of the present invention with a ball landed on the seat of the fracture plug seat assembly. 
     
    
    
     DETAILED DESCRIPTION 
     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 a setting ring, an expandable ring and a lower ring that are capable of locking when a ball that is too large to pass through the setting ring is landed on the fracture plug seat assembly. The setting ring and lower ring collectively form what may be termed an expansion control portion of the overall fracture plug seat assembly. When a ball or fracture plug that is small enough to pass through the setting ring contacts the expandable ring, the expandable ring expands to allow the ball to pass. When the ball designed to plug the seat is launched, it engages the setting ring and actuates the expandable ring into a retracted and locked position in which further expansion is prevented, hence supporting the ball. 
       FIG. 3  illustrates a cross-section of an embodiment of a fracture plug seat assembly  40  according to the present invention. As shown in  FIG. 3 , the fracture plug seat assembly  40  includes an expandable ring  42  having an axial opening, a setting ring  44  having an axial opening and a lower ring  46  having an axial opening. According to the embodiment shown in  FIG. 3 , the lower ring  46  is also capable of expanding when sufficient force is applied by the expandable ring  42  thereby allowing the expandable ring  42  to move to a locked position. In certain embodiments, the setting ring  44  is integrated with the sleeve  48 . In certain other embodiments, the setting ring  44  may be held axially in the initial position shown in  FIG. 3  by means such as shear pins to prevent expandable ring  42  from moving prematurely to a locked position until the ball designed to plug the fracture plug seat assembly  40  is landed on the setting ring  44 . 
     The fracture plug seat assembly  40  shown in  FIG. 3  also contains a snap ring  50  which retains the assembly components, namely the expandable ring  42 , the setting ring  44  and the lower ring  46 , within the sleeve  48 . A Belleville washer or coned-disc spring  52  keeps pressure on the stack of rings, via an annular spacer  53  bearing on the top side of the setting ring  44 , so that contact between the rings is maintained and so that sand and cement cannot penetrate between the rings. Setting ring  44  has an O-ring seal  54  which prevents fluid from passing between the setting ring  44  and the sleeve  48 . Expandable ring  42  has a split  58  and a spring  56  which biases the split  58  of the expandable ring  42  to a closed position as shown in  FIG. 3 . The expandable ring  42  and the lower ring  46  have respective mating tapered surfaces  60  and  61  which maintain the expandable ring  42  and the lower ring  46  in an axial relationship and initiates expansion of the lower ring  46  when pressure is applied by the expandable ring  42 . The lower ring  46  includes an O-ring  47  for centering purposes. 
       FIG. 4  illustrates the fracture plug seat assembly  40  with a ball  62  passing through the expandable ring  42 . The diameter of the ball  62  is smaller than the diameter of the axial opening of the setting ring  44  and therefore is not large enough to engage and land on the setting ring  44 . The diameter of the ball  62  is larger than the diameter of the axial opening of the expandable ring  42  and exerts sufficient force on the expandable ring to overcome the spring force of spring  56  causing the split  58  to open and allow the ball  62  to pass through the axial opening of the expandable ring  42 . 
       FIG. 5  is an axial view of the fracture plug seat assembly taken along line  5 - 5  of  FIG. 4  showing the expandable ring  42  with the spring  56  in tension and the split  58  in the open position. The ball  62  is pressed within the inner diameter of the expandable ring  42 . 
       FIG. 6  illustrates the fracture plug seat assembly  40  with a ball  64  which has been dropped in the direction  66  and is engaged with and landed on the setting ring  44 . Significant pressure from the upstream side of the ball  64  forces the setting ring  44  downwardly against the expandable ring  42 . As the setting ring  44  is forced further downward toward the lower ring  46 , force builds on the tapered surface  60  of the expandable ring  42  and the tapered surface  61  of the lower ring  46  causing the lower ring  46  to expand. 
       FIG. 7  illustrates the fracture plug seat assembly  40  with a ball  64  which has been dropped in the direction  66  and is engaged with and landed on the setting ring  44 . Pressure from the upstream side of the ball  64  has caused the lower ring  46  to expand to the point at which tapered surface  61  of the lower ring  46  is disengaged from the tapered surface  60  of the expandable ring  42  and the expandable ring  42  is in a concentric relationship with the lower ring  46 . Continued pressure from the upstream side of the ball forces the expandable ring  42  downward with respect to the lower ring  46 . 
       FIG. 8  illustrates the fracture plug seat assembly  40  in the condition in which the expandable ring  42  has been forced downward with respect to the lower ring  46  until the tapered surface  60  of the expandable ring  42  engages shoulder  49  of the sleeve  48 . As shown in  FIG. 8 , the expandable ring  42  is in a retracted, locked position characterized by a concentric relationship with the lower ring  46 . The ball  64  is now supported by the setting ring  44  and the expandable ring  42 . Many prior art fracture plug seat designs only support a ball such as ball  64  with the engagement diameter A. This is because it is the smallest diameter of such designs that is capable of letting the preceding smaller ball  62  pass through. The engagement diameter B which corresponds to the diameter of the axial opening of the expandable ring  42  when it is in the locked position greatly adds to the support of ball  64  helping prevent the cracking or extrusion of the ball  64 . 
     When fracturing is complete, the balls are often purged to the surface.  FIGS. 9, 10 and 11  show the fracture plug seat assembly  40  with the larger ball  64  now purged up the well. In  FIGS. 9 and 10 , the smaller ball  62  has engaged the expandable ring  42  and pressure in the direction  72  is applying an upward force upon the fracture plug seat assembly  40 . As shown in  FIGS. 9 and 10 , the sleeve  48  includes a step  74  which prevents the lower ring  46  from moving upwards. Thus, as pressure in the direction  72  continues, the expandable ring  42  moves upward with respect to the lower ring  46  and pushes the setting ring  44  ahead of the expandable ring  42 . When the expandable ring  42  and setting ring  44  are moved to their original position as shown in  FIG. 3 , the expandable ring  42  is allowed to expand and the ball  62  passes through, as shown in  FIG. 11 . Tapered surface  76  on the annular spacer  53  prevents the setting ring  44  from moving upward any further and deflects any sand that might have accumulated during fracturing. 
     Another embodiment of the present invention is illustrated in  FIGS. 12 and 13 .  FIG. 12  shows a fracture plug seat assembly  80  which includes an expandable ring  82 , a setting ring  84  and a lower ring  86 . According to this embodiment, the expandable ring  82  is a collet with only one end expanding, and with one or more axial slits extending up the length of the expandable ring  82 . A shear tab  88  prevents the expandable ring  82  from sliding down the assembly  80 . In  FIG. 12 , a ball  90  is shown passing through expandable ring  82 . As shown in  FIG. 13 , when a ball  92  designed to be landed by the fracture plug seat assembly  80  is dropped onto the seat assembly  80 , it engages the setting ring  84  and moves the expandable ring  82  into a nested relationship with the lower ring  86 . In some embodiments, the lower ring  86  is integrated with the sleeve  94 . 
     Yet another embodiment of the present invention is illustrated in  FIG. 14  in which the lower ring is integrated into the sleeve and in which a shear member is included, both as mentioned above. Specifically,  FIG. 14  shows a fracture plug seat assembly  100  which includes an expandable ring  102  and a setting ring  104 . According to this embodiment, the expandable ring  102  rests upon a tapered shoulder  107  which is integrated into sleeve  108 . A shear tab  106  is provided on the expandable ring  102  and provides diametrical interference between the expandable ring  102  and the sleeve  108 . A ball  112  has been dropped in the direction  110  and is engaged with and landed on the setting ring  104 . Significant pressure from the upstream side of the ball  112  forces the setting ring  104  downward and into the expandable ring  102 . As the setting ring  104  is forced further downward toward the expandable ring  102 , force builds on the expandable ring  102  causing the shear tab  106  to shear and allow the expandable ring  102  to clear the tapered shoulder  107  and move downward with respect to the sleeve  108  until the expandable ring  102  is engaged with the shoulder  114  which is integrated into sleeve  108 . When this occurs, the expandable ring  102  is in a locked position characterized by a concentric relationship with the lower ring sleeve  108 . 
     In a manner similar to that described above with respect to  FIGS. 9, 10 and 11 , when fracturing is complete, the balls are often purged to the surface. When a ball smaller than ball  112  engages the expandable ring  102 , pressure in a direction opposite direction  110  applies an upward force upon the fracture plug seat assembly  100 . As pressure in the direction opposite direction  110  continues, the expandable ring  102  moves upward with respect to the sleeve  108  and pushes the setting ring  104  ahead of the expandable ring  102 . When the expandable ring  102  and setting ring  104  are moved to their original position as shown in  FIG. 14 , the expandable ring  102  is allowed to expand and the ball smaller than ball  62  passes through, similar to what is shown in  FIG. 11 . 
     It is understood that variations may be made in the foregoing without departing from the scope of the disclosure. 
     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. 
     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. 
     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. 
     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.

Summary:
An expandable fracture ball seat assembly for use in wellbore zone fracturing operations functions to permit passage therethrough and exit therefrom of fracture ball plugs of only diameters less than a predetermined magnitude. In a representative form, the seat assembly includes a ring stack disposed within a tubular member and formed from a first expandable ring coaxially sandwiched between a setting ring and a second expandable ring. When an oversized fracture ball plug is forced into the seat assembly it axially compresses the ring stack and reduces the diameter of the first expandable ring and telescopes it into the second expandable ring, with the first expandable ring and the setting ring blocking passage through and exit from the seat. A reverse passage of a suitably large diameter fracture ball plug through the seat assembly axially returns the setting ring and first expandable ring to their original positions.