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PRIORITY CLAIM/CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present document claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 61/655,126, filed on Jun. 4, 2012 and entitled, “Deployable Multiple Ball Seat System for Continuous Multi-Stage Stimulation”, the disclosure of which is incorporated herein by reference in its entirety. The present document also claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 61/709,642, filed on Oct. 4, 2012 and also entitled, “Deployable Multiple Ball Seat System for Continuous Multi-Stage Stimulation”, the disclosure of which is again incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses, added emphasis has been placed on efficiencies associated with well completions and maintenance over the life of the well. Over the years, ever increasing well depths and sophisticated architecture have made reductions in time and effort spent in completions and maintenance operations of even greater focus. 
         [0003]    Well stimulating applications which include perforating and fracturing of a cased well during completions constitute one such area were significant amounts of time and effort are spent. This is particularly true where increases in well depths and sophisticated architecture are encountered. Once the casing hardware is cemented in place, stimulating applications generally take place in a zone by zone fashion. For example, a terminal end of the well may be perforated and fractured followed by setting of a plug immediately uphole thereof. Thus, with the lowermost zone initially stimulated, the zone above the plug may now also be stimulated by way of repeating the perforating and fracturing applications. This time consuming sequence of plug setting, perforating and then fracturing is repeated for each zone. That is, likely 15-20 zones or more of a given well may be stimulated in this manner. Further, for any given zone, each step of plug setting, perforating and fracturing requires its own dedicated application trip into the well via wireline from surface or other appropriate conveyance. 
         [0004]    All in all, where stimulating operations are involved, the operator is likely faced with days&#39; worth of time dedicated to the task. In today&#39;s dollars this may translate into several hundred thousand dollars of lost time. Once more, footspace at the surface of the oilfield adjacent the well is taken up by simultaneously competing types of equipment. For example, since each zone requires separate dedicated applications of plugging, perforating and fracturing, all such equipment must remain at the oilfield surface throughout stimulation operations. Thus, so as to be available for later use, frac trucks are left running in place after use in one zone so as to be available for use in the next zone. In fact, this particular inefficiency is often exacerbated where a continuously running but intermittently utilized frac truck breaks down due to repetitive cycles of pumping and powering down to allow for plugging and perforating. 
         [0005]    Ultimately, once each zone has been stimulated, the well is left with twenty or so isolated zones. Thus, a milling application may ensue where a milling tool is dropped through the well which mills out all of the plugs. As such, flow through the central bore of the well may be restored. Unlike the previous steps, at least the milling may take place through each zone with only one trip into the well with the milling tool. 
         [0006]    Efforts have been undertaken to reduce the overall time and number of trips into the well that result from the zone by zone and stepped nature of stimulation operations. For example, the casing at each zone may be outfitted with a shifting sleeve that also includes a ball seat such that the sleeve may be opened and the wellbore exposed to the surrounding formation. That is, rather than separately introducing perforating and fracturing equipment into the well during separate dedicated trips to each zone, ball actuation may be used to open the sleeves one by one for targeted stimulation. That is to say, a ball of appropriate size may be dropped into the well, eventually finding the seat and sleeve of corresponding size and pressurizably opening that sleeve. The ball and seat may then serve the isolation function and the opened sleeve may obviate the need for perforating. Therefore, stimulation of the zone may take place with only the introduction of fracturing equipment. 
         [0007]    In theory the above ball drop technique may save a significant amount of time and trips into the well for sake of stimulation. Unfortunately, such a system renders a host of challenges to the rest of well operations. That is to say, as noted below, applications before and after stimulation are likely to be adversely affected by the use of conventional ball-drop and sleeve shifting hardware. 
         [0008]    Conventional ball-drop and sleeve shifting hardware requires fairly complex architecture that is incorporated into the casing and present from the outset of completions. This sophisticated architecture includes the noted sleeve which is likely to present a significant restriction into the main bore of the well. Further, complex mechanical parts such as springs, pressure support mechanisms, ratchets and other features of the ball seat are also likely to protrude into the main bore. Thus, as a practical matter, in spite of the potential time saving benefits, operators are likely to forego ball-drop sleeve shifting stimulation techniques. 
       SUMMARY 
       [0009]    A system is disclosed that is configured to accommodate multi-stage stimulation in a well. The system includes a casing with a frac sleeve that is of a diameter substantially that of the casing so as to support cementing therethrough. Additionally, a ball seat assembly is included for securing at the frac sleeve after the cementing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a partially sectional view of a casing stimulation region incorporating an embodiment of a passable sleeve and ball seat assembly for fracturing applications. 
           [0011]      FIG. 2A  is a perspective cross-sectional view of the casing stimulation region of  FIG. 1  pre-fitted with the passable sleeve. 
           [0012]      FIG. 2B  is a perspective cross-sectional view of the sleeve of the casing stimulation region of  FIG. 1  outfitted with the ball seat assembly. 
           [0013]      FIG. 3  is an overview of an oilfield with a cased well accommodating the stimulation region of  FIG. 1 . 
           [0014]      FIG. 4A  is as side partially sectional view of a stepped actuator delivery tool for placement of ball seat assemblies at sleeves of casing stimulation regions. 
           [0015]      FIG. 4B  is a side partially sectional view of the tool of  FIG. 4A  delivering the ball seat assembly to the sleeve of  FIG. 1 . 
           [0016]      FIG. 4C  is a side partially sectional view of the ball seat assembly of  FIG. 4B  actuated into set engagement with the sleeve. 
           [0017]      FIG. 5  is a flow chart summarizing an embodiment of carrying out near continuous multi-stage well stimulation operations in a manner taking advantage of passable sleeve and ball seat assembly hardware. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Embodiments are described with reference to certain types of downhole architecture and applications. For example, embodiments herein focus on a deviated well that is completed and subsequently outfitted with ball seat assemblies via wireline conveyance. However, a variety of different applications and well architecture types may take advantage of passable sleeve and ball seat assemblies as detailed herein. For example, vertical wells may include different regions outfitted with passable sleeve and ball seat assemblies that further cementing and/or allow for near continuous stimulation. Further, alternatives to wireline conveyance may be used, such as coiled tubing. Regardless, embodiments described herein include hardware that supports multi-stage stimulation in a manner that utilizes a frac sleeve and ball seat assembly without substantially compromising effective cementing operations. Thus, the sleeve and/or seat assembly may be referred to herein as passable. 
         [0019]    Referring now to  FIG. 1 , a partially sectional view of a casing stimulation region  101  is shown. This region  101  is part of a larger, more extensive casing  130  and other hardware that define a well  380  at an oilfield  300  such as that depicted in  FIG. 3 . In the depiction of  FIG. 1 , fracturing fluid  140  is shown emerging from slots or side ports  150  in the easing  130 . That is, as part of stimulation operations, ultimately directed at promoting the uptake of well fluids, fracturing may take place through the ports  150  as shown. However, such ports  150  are not configured to always be open throughout well operations. Rather, at the outset of operations, such ports  150  are to be closed. 
         [0020]    In order to keep the ports  150  closed at the outset of well operations, a frac sleeve  100  is provided that may be slid or shifted to an open position. Indeed, in the depiction of  FIG. 1 , the sleeve  100  within the main bore  180  of the casing  130  has been shifted downward such that the ports  150  of the casing  130  are now uncovered (see arrow  105 ). This is achieved by dropping of a ball  125  into the main bore  180  and pumping it through until it reaches a ball seat assembly  110 . With added reference to  FIG. 2 , this assembly  110  includes a seat portion  250  that is of a diameter corresponding to that of the ball  125 . Thus, the ball  125  may pass larger diameter seat portions at other stimulation regions  301 ,  305  of the well  380  without effecting any sleeve shifting thereat (see  FIG. 3 ). In other words, the ball  125  is sized to target a specific seat portion  250  and open a specific sleeve  100  at a specific region  101  for sake of fracturing thereat. 
         [0021]    The sleeve  100  described above may be referred to as a passable sleeve  100  that is nearly flush with the casing  130 . Indeed, with specific reference now to  FIG. 2A , a perspective cross-sectional view of the casing stimulation region  101  of  FIG. 1  is shown as it may appear during initial installation of the casing  130 . Specifically, at this point in time, the casing  130  is pre-fitted with the passable sleeve  100  covering over the adjacent ports  150 . The sleeve  100  may be held in place by a shear element or other conventional mechanism for at least temporary retention. Regardless, the sleeve  100  is passable in the sense that it does not present any significant restriction relative the bore  180 . Thus, during completions, as cement is driven through and out the bore  180 , no impediment is presented that might otherwise complicate or prevent effective installation of the casing  130 . 
         [0022]    In the embodiment of  FIG. 2A , a tapered portion  200  of the sleeve  100  is provided so as to help further ensure that the sleeve  100  does not present a significant hindrance to cementing as described above. Additionally, the profile of the sleeve  100  is not substantially different from that of the inner diameter of the casing  130 . This may be viewed in different ways. For example, in one embodiment the inner diameter of the sleeve  130  may be within about 5%-10% of that of the casing  130 . In another embodiment, the inner diameter of the sleeve  100  may be measured as within ½ of an inch of that of the casing  130 . Further, with reference to overall dimensions, in one embodiment, the sleeve  100  may be about 4.5 inches at its inner diameter whereas the inner diameter of the adjacent casing  130  is about 4.9 inches. 
         [0023]    Referring now to  FIG. 2B , a perspective cross-sectional view of the sleeve  100  at the casing stimulation region  101  is shown in a manner like that of  FIG. 1 . Specifically, the sleeve  100  is now outfitted with the ball seat assembly  110 . Thus, a ball  125 , such as that of  FIG. 1 , may be advanced to the assembly  110 , received by a the seat portion  250 , and the sleeve  100  moved toward a stop  201  at the inner diameter of the casing  130 . Upon reaching the stop  201 , the depicted ports  150  would no longer be covered by the sleeve  100 . Therefore, fluid running through the main bore  180  would be sealed off by the ball  125  and directed out the ports  150  (see the fracturing fluid  140  of  FIG. 1 .). 
         [0024]    Continuing with reference to  FIG. 2B , with added reference to  FIG. 1 , the ball seat assembly  110  is made up of two parts, an anchoring portion  275  and the above noted seat portion  250 . As referenced above, the seat portion  250  serves as a setting device and is also constructed with a seat for directly interfacing a ball  125  so as to seal off the bore  180  and responsively slide the sleeve  100  downhole. As detailed further below, these parts are delivered together by way of a stepped setting tool  400  (see  FIGS. 4A and 4B ). In order to attain this delivery, the anchoring, portion  275  may include a landing profile that is tailored for engagement with a particular sleeve  100 . More specifically, in the embodiment shown, the anchoring portion  275  is of a collet variety with matching size and profile for engaging with the specific sleeve  100  depicted. However, in another embodiment, a landing profile of the anchoring portion  275  may be constructed for reception by a locating catch  435  of the sleeve  100  for sake of locating the appropriate assembly  110  at the appropriate sleeve  100  (see  FIG. 4B ). 
         [0025]    Once placed, the anchoring portion  275  may be firmly set by shearing away of the seat portion  250  relative the anchoring portion  275  and moving in a downhole direction according to techniques detailed further below. Accordingly, the anchoring portion  275  may become anchored to the casing  130  and serve as a secure support for the seat portion  250 . Thus, the seat portion  250  may be reinforced as an effective seal when the seat thereof receives a ball  125  as shown in  FIG. 1 . In one embodiment, the seat portion  250  internally tapers down to a diameter of between about 0.7 and 6.5 inches to serve as the ball seat when receiving a ball  125  of slightly larger diameter. As a practical matter, this means that for the seat portion  250  of other ball seat assemblies installed further uphole in the well, a larger diameter seat and ball  125  will be utilized. That is, to ensure passage to the most downhole seat, a comparatively small ball  125  dropped from at oilfield surface  300  will need to attain passage through all other seats before reaching the most downhole seat/setting portion  250 . Otherwise, a premature engagement and sealing with another seat further uphole may take place, thereby preventing sleeve actuation at a location further downhole. 
         [0026]    Referring now to  FIG. 3 , an overview of an oilfield  300  is shown. A conventional rig  320  and pressure control equipment  330  are provided. Additionally, a deviated cased well  380  is depicted which accommodates the stimulation region  101  of  FIG. 1  along with other such regions ( 301 ,  305 ). Indeed, the well  380  traverses different formation layers  390 ,  395  and may include 15-20 or more different stimulation regions such as those depicted. However, as indicated above, the process of fracturing regions  101 ,  301 ,  305  such as these no longer requires that each region include a series of separate dedicated plugging and perforating interventions. Rather, a ball is dropped, a sleeve opened to expose ports  150  and the formation  395  adjacent a region  101  is stimulated by fracturing fluid at up to about 10,000 PSI. The result is shown in  FIG. 3  as formation cracks  375  adjacent the first region  101 . Subsequently, a slightly larger ball is dropped, and the same process repeated at another region  301  and then at yet another region  305  (again, with an incrementally larger ball). 
         [0027]    The above described manner of sequentially fracturing or “fracing” the formation  395  adjacent the various regions  101 ,  301 ,  305  is achieved in an efficient manner. For example, not only is the need for a multitude of dedicated interventional trips into the well  380  avoided, but this is done in a manner that allows frac pumps  310  to flirt nearly continuously. That is, fracturing requires the use of pumps  310 . They may be provided by way of frac trucks or on a skid or other less mobile form. In  FIG. 3 , they are depicted schematically in block form at the oilfield surface  300 . Regardless, operational efficiency of such high pressure inducing pumps is best attained when the pumps  310  are running and pumping at a significant rate. To the contrary, where repeated extended downtime is encountered for plug setting and/or perforating applications, the pumps  310  are more prone to inefficient operation or even breakdown. However, in the embodiment of  FIG. 3 , such significant downtime is not required. Rather, brief pumping pauses for sake of dropping one ball or another into the well  380  from the oilfield surface  300  is all that is necessary. The remainder of the time, the pumps  310  may function at the desired capacity and efficiency as determined by the operator. 
         [0028]    In addition to the efficiency of nearly continuous multi-stage stimulation that is provided by the overall system, the casing  130  and other hardware has also been installed in a practical and efficient manner. That is, with added reference to  FIG. 2A , the overall morphology of the internal sleeves  100  is such that the casing  130  may be cemented in place without undue obstruction to the main bore  180 . Rather, the cement  350  may pass through the entirety of the bore  180  and emerge outside the casing  130  to complete the installation process (see cement  350 ). 
         [0029]    Additional post-fracturing efficiencies are also provided via the system of  FIG. 3 . For example, the balls may be of a degradable or dissolvable form such that intervention for sake of restoring flow through the bore  180  may be avoided. In another embodiment, techniques may be employed to flow the balls back to surface. 
         [0030]    Referring now to  FIGS. 4A-4C , the manner of installation of the ball seat assembly  110  at the sleeve  100  is described in greater detail. More specifically,  FIG. 4A  is a side partially sectional view of a stepped actuator delivery tool  400  for delivery of the ball seat assembly  110  along with many others ( 410 - 415 ).  FIG. 4B  depicts the specific delivery of the assembly  110  to the sleeve  100  of  FIG. 1  and  FIG. 4C  reveals the anchored setting of the assembly  100  at the sleeve  100 . 
         [0031]    With specific reference to  FIG. 4A , the embodiment of the delivery tool  400  shown accommodates seven different ball seat assemblies  110 ,  410 - 415  in a stacked fashion. Thus, with added reference to  FIGS. 1 and 3 , following cementing of casing  130 , a single run of the tool  400  into the well may be used to place assemblies  110 ,  410 - 415  at up to seven different fracturing regions  101 ,  301 ,  305 . So, for example, in a well with 20 different regions, three different trips into the well  380  would be sufficient for fully outfitting each sleeve  100  at each region  101  with a ball seat assembly  110 . 
         [0032]    With specific reference to  FIG. 4B , a side partially sectional view of the tool  400  of  FIG. 4A  is shown in which the ball seat assembly  110  is delivered to the sleeve  100  of  FIG. 1 . The anchoring portion  275  of the assembly  110  is of a matching profile to that of the sleeve  100 . For example, with added reference to  FIGS. 1 and 3 , in one embodiment, the tool  400  bypasses all regions  101 ,  301 ,  305  of the well  380  and is then retracted back uphole. Upon reaching the first region  101  during the retraction, the matching profile of the assembly  110  will interlock with the sleeve  100  as shown in  FIG. 4B . 
         [0033]    With the assembly  110  in place, the tool  400  may be shifted downhole such that a first step  460  engages with the seat of the seat portion  250  of the assembly  110 . Thus, the seat portion  250  may sheared from its initial position and begin to shift downhole over an incline  430  of the anchoring portion  275 . Ultimately, as discussed further below, this may result in “wickets” or teeth  475  of the anchoring portion  275  biting into the sleeve  100  and securely retaining of the entire assembly  100  in place. 
         [0034]    It is of note that the movement of the tool  400  in order to set the first assembly  110  does not affect setting of the next assembly  410 . That is, the second step  465  of the tool  400  is distanced far enough from the seat of the second assembly  410  that it does not unintentionally begin to set the second assembly  410 . Rather, following setting of the first assembly  110 , the tool  400  is removed further uphole, taking the second assembly  410  and leaving the first assembly  110  in place. 
         [0035]    Referring now to  FIG. 4C , a side partially sectional view of the ball seat assembly  110  of  FIG. 4B  is shown now that it is fully actuated into set engagement with the sleeve  100 . With the tool  400  of  FIG. 4B  removed, the fully anchored assembly  110  is shown in place. As indicated above, the anchoring portion  275  is of a collet-type. Thus, as the seat portion  250  was shifted downhole, separate fingers  490 ,  495  of the anchoring portion  275  spread apart relative one another allowing the teeth  475  to come into full securing engagement with the sleeve  100 . Similarly, a rubber seal  450  has been energized into sealing engagement with the sleeve  100  such that the anchoring is both secure and sealed. The seat portion  250  is now poised for responsive reception of a ball having a diameter that is slightly above that of the seat (see diameter (d)). Once more, all of this installation is complete before any fracturing is begun. Thus, no interventional interruption of stimulation is necessary in order to achieve a sealing off of the bore  180  or for exposing of the adjacent formation. 
         [0036]    Referring now to  FIG. 5 , a now chart is shown summarizing an embodiment of carrying out near continuous multi-stage well stimulation operations. Specifically note that a ‘projectile’ or ball may be dropped to open a sleeve as indicated at  550 , a fracturing application undertaken as indicated at  565  and the process repeated (see  500 ) or terminated (see  580 ). That is, while the chart summarizes one particular ball drop and fracturing, the overall system is such that multi-stage stimulation may be undertaken merely by dropping another ball ( 550 ) and fracturing ( 565 ) at another location for as many times as necessary, as detailed hereinabove. Thus, the overall system may be referred to as supporting near continuous multi-stage stimulation with the only interruptions being brief pauses for the sake of dropping in another sized ball/projectile. 
         [0037]    Continuing with reference to  FIG. 5 , the practicality of the system is furthered by the use of a passable frac sleeve. That is, as indicated at  505 , a casing may be pre-fitted with one or more frac sleeves within the main bore that nevertheless allow for cementing through the main bore (see  520 ). As indicated at  595 , this may or may not be followed by a clean out run, for example, with a conventional wiper. Regardless, once the installation and cementing are complete, ball seat assemblies may be delivered and set as indicated at  535 . Thus, a repeatable ball drop stimulation technique may be undertaken as described above (see  550 ,  565 ,  500 ). 
         [0038]    Embodiments described hereinabove provide hardware and techniques that effectively reduce the number of trips into the well in order to perform multi-stage stimulation. Specifically, this is achieved via ball drop technique and hardware that allows for avoiding plug setting and perforating application trips separately directed at each zone. As a result, near continuous stimulation may be achieved without significant intervening disruption. Once more, this is achieved in a manner that avoids presenting any substantial obstructions to the main bore. Thus, effective cementing of the casing hardware is not sacrificed and follow-on intervention after stimulation is not materially impeded. 
         [0039]    The preceding description has been presented with reference to presently preferred embodiments. Persons skilled in the art and technology to which these embodiments pertain will appreciate that alterations and changes in the described structures and methods of operation may be practiced without meaningfully departing from the principle, and scope of these embodiments. Furthermore, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope.

Summary:
A system is provided that is conducive to multi-stage stimulation in a near-continuous fashion. That is, unlike conventional stimulation systems, embodiments herein may operate without the requirement of traditional plug-setting, perforating and fracturing interventions on a zone by zone basis for a cemented completion. Rather, the system is outfitted with frac sleeves that may be shifted open to expose the bore to the formation while simultaneously achieving a seal through a ball drop technique. Once more, this manner of operation is rendered practical by the sleeve being of a passable configuration such that cementing of the casing is not impeded.