Abstract:
An apparatus that is usable with a well includes a string and a plurality of tools that are mounted in the string. The string includes a passageway. The tools are mounted in the string and are adapted to be placed in a state to catch objects (free-falling objects and/or pumped-down objects, as just a few examples) of substantially the same size, which are communicated downhole through the passageway.

Description:
[0001]     This application is a divisional of U.S. patent application Ser. No. 11/081,005 entitled, “TECHNIQUE AND APPARATUS FOR COMPLETING MULTIPLE ZONES,” filed on Mar. 15, 2005 which is a continuation-in-part of U.S. patent application Ser. No. 10/905,073 entitled, “SYSTEM FOR COMPLETING MUTLIPLE WELL INTERVALS,” filed on Dec. 14, 2004, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND  
       [0002]     The present invention generally relates to a technique and apparatus to complete multiple zones.  
         [0003]     For purposes of enhancing production from a subterranean well, the layers of the well may be fractured using a pressurized proppant-containing fracturing fluid or other treating fluids such as acid. The layers typically are fractured one at time by directing fracturing fluid to the layer being fractured and isolating the other layers.  
         [0004]     A conventional fracturing system includes surface pumps that pressurize fracturing fluid, which may be communicated downhole via the central passageway of a tubular string. The string extends downhole through a wellbore that traverses the various layers to be fractured; and the string may include valves (sleeve valves, for example) that are generally aligned with the layers so that the valves may be used to control fluid communication between the central passageway of the string and the layers. Thus, when a fracturing operation is performed on one of the layers, one of the valves is opened so that fracturing fluid may be communicated through the opened valve to the associated layer.  
         [0005]     To remotely operate the valves from the surface of the well, the valves may contain many different size ball seats. More specifically, to target and actuate the valves, differently sized balls may be dropped into the central passageway of the string from the surface of the well. Each ball size may be uniquely associated with a different valve, so that a particular ball size is used to actuate a specific valve. The smallest ball opens the deepest valve. More particularly, a free-falling ball lodges, or is “caught” by, a ball seat of the targeted valve. To discriminate between the different valves, each ball seat of the string has a different diameter.  
         [0006]     After a ball lodges in a ball seat, fluid flow through the central passageway of the string becomes restricted, a condition that allows fluid pressure to be applied from the surface of the well for purposes of exerting a downward force on the ball. The ball seat typically is attached to a sleeve of the valve to transfer the force to the sleeve to cause the valve to open.  
         [0007]     The annular area that is consumed by each ball seat restricts the cross-sectional flow area through the string (even in the absence of a ball), and the addition of each valve (and ball seat) to the string further restricts the cross-sectional flow area through the central passageway of the string, as the flow through each ball seat becomes progressively more narrow as the number of ball seats increase. Thus, a large number of valves may significantly restrict the cross-sectional flow area through the string.  
         [0008]     As an alternative to the ball seat being located in the string as part of the valves, a single activation tool may be selectively positioned in side the central passageway of the string to operate the valves. More specifically, a valve actuation tool may be lowered downhole by a conveyance mechanism (a slickline, for example) to the valve to be opened and to close previously-opened valves.  
         [0009]     A challenge with this alternative is that the fracturing pumps at the surface of the well may need to be idled after each layer is fractured. Furthermore, each valve typically is closed after its associated fracturing operation. The reclosure of the valves demands that the seals and sealing surfaces withstand the fracturing operations without damage.  
         [0010]     Thus, there is a continuing need for a technique and/or arrangement to address one or more of the problems that are set forth above as well as possibly address one or more problems that are not set forth above.  
       SUMMARY  
       [0011]     In an embodiment of the invention, an apparatus that is usable with a well includes a string and a plurality of tools that are mounted in the string. The string includes a passageway. The tools are mounted in the string and are adapted to be placed in a state to catch objects (free-falling objects and/or pumped-down objects, as just a few examples) of substantially the same size, which are communicated downhole through the passageway.  
         [0012]     In another embodiment of the invention, an apparatus that is usable with a well includes a tubular member, a first tool and a second tool. The tubular member includes a passageway. The first tool is attached to the tubular member, and the first tool is adapted to be placed in a state to catch a first object that is communicated through the passageway and perform an operation after catching the first object. The second tool is attached to the tubular member and is adapted to transition to a state to catch a second object communicated through the passageway in response to the operation.  
         [0013]     In yet another embodiment of the invention, a technique that is usable with a well includes providing a string that has a plurality of tools and a passageway that extends through the tools. The technique includes without running an activation tool into the passageway; and selectively activating the tools of the string to cause each activated tool to transition from a first state in which the activated tool is configured to allow a free-falling object to pass through the passageway to a second state in which the activated tool is configured to catch the free-falling object.  
         [0014]     Advantages and other features of the invention will become apparent from the following description, drawing and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]      FIG. 1  depicts a fracturing system according to an embodiment of the invention.  
         [0016]      FIGS. 2 and 3  depict a valve in a closed state and before being placed in a ball catching state according to an embodiment of the invention.  
         [0017]      FIG. 4  depicts the valve in a closed state and after being placed in a ball catching state according to an embodiment of the invention.  
         [0018]      FIGS. 5 and 6  depict the valve in its open state according to an embodiment of the invention.  
         [0019]      FIG. 7  is a flow diagram depicting a technique to fracture layers in a multiple layer well according to an embodiment of the invention.  
         [0020]      FIG. 8  is a perspective view illustrating surface features on a bottom end of a collet sleeve of the valve according to an embodiment of the invention.  
         [0021]      FIGS. 9 and 10  depict different states of a valve that uses a C-ring as a ball catcher in accordance with an embodiment of the invention.  
         [0022]      FIG. 11  is a perspective view of a valve housing according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Referring to  FIG. 1 , an embodiment  10  of a fracturing system includes a string  12  that extends into a wellbore  11  that traverses N layers  15  (layers  15   1 ,  15   2 ,  15   3  . . .  15   N−1  and  15   N , depicted as examples) of the well. As depicted in  FIG. 1 , the string  12  includes valves  14  (valves  14   1 ,  14   2 ,  14   3  . . .  14   N−1  and  14   N , depicted as examples), each of which is associated with a particular layer  15 . For example, the valve  14   3  is associated with the layer  15   3 . Thus, to fracture a particular layer  15 , the associated valve  14  (initially run downhole in a closed state) is opened by dropping a ball and pumping up, which shifts the sleeve valve open (as described below) to allow communication between the central passageway of the string  12  and the associated layer  15 . This communication, in turn, permits fracturing fluid and pressure to be routed to the associated layer  15 .  
         [0024]     More specifically, in some embodiments of the invention, each valve  14  controls communication between a central passageway of the string  12  and an annular region that surrounds the valve  14 . When the string  12  is run downhole, all of the valves  14  are initially closed. However, the valves  14  are successively opened one at a time in a predetermined sequence (described below) for purposes of fracturing the layers  15 .  
         [0025]     As a more specific example, in some embodiments of the invention, the valves are opened in a sequence that begins at the bottom of the string  12  with the lowest valve  14   N , proceeds uphole to the next immediately adjacent valve  14 , then to the next immediately adjacent valve  14 , etc. Thus, the valve  14   N  is opened before the valve  14   N−1 , the valve  14   3 , is opened before the valve  14   2 , etc.  
         [0026]     For purposes of opening a particular valve  14 , a free-falling or pumped-down object is deployed from the surface of the well into the central passageway of the string  12 . It is assumed below for purposes of clarifying the following discussion that the object is a spherical ball. However, it is understood that in other embodiments of the invention, other object types and/or differently-shaped objects may be used.  
         [0027]     In some embodiments of the invention, a ball of the same dimension may be used (although different size balls may be used in other embodiments of the invention) to open all of the valves  14 , as only one of the previously-unopened valves (called the “targeted valve” herein) is in a “ball catching state” at any one time. More specifically, in accordance with some embodiments of the invention, all of the balls that are pumped or dropped downhole for purposes of opening one of the valves  14  may have diameters that vary less than approximately 0.125 inches from each other.  
         [0028]     As described below, initially, all of the valves  14  are closed, and none of the valves  14  are in ball catching states. When a particular valve  14  opens, the valve  14  places the next valve  14  in the sequence in the ball catching state. When in the ball catching state, the valve  14  forms a seat that presents a restricted cross-sectional flow passageway to catch a ball that is dropped into the central passageway of the string  12 . For the sequence that is described above, the unopened valves  14  that are located above the unopened valve  14  that is in the ball catching state allow the ball to pass through.  
         [0029]     After the ball lodges in the ball catcher of the targeted valve  14 , the ball significantly restricts, if not seals off, the central passageway of the string  12  below the ball so that fluid pressure may be applied above the ball to generate a force to cause the valve to open, as further described below.  
         [0030]     As a more specific example, a ball may be dropped from the well&#39;s surface into the central passageway of the string  12  for purposes of opening a previously-unopened valve  14   N  that has previously been placed in a ball catching state. In response to the fluid pressure that is applied to the resultant restricted central passageway, the valve  14   N  opens to allow a fracturing operation to be performed on the associated layer  15   N . The opening of the valve  14   N , in turn, places the next valve  14   N−1  in the sequence in the ball catching state. Once the fracturing operation on the layer  15   N  is complete, another ball is dropped into the central passageway of the string  12  for purposes of opening the valve  14   N−1  so that the layer  15   N−1  can be fractured. Thus, this sequence continues until the last valve  14   1  is opened, and the associated layer  15   1  is fractured.  
         [0031]     As a more specific example, in accordance with some embodiments of the invention,  FIGS. 2 and 3  depict upper  14 A and lower  14 B sections of an exemplary valve  14  that is closed and has not been placed in ball catching state (i.e., the valve  14  is in its initial states when run into the well). Thus, as depicted in  FIGS. 2 and 3 , the valve  14  does not restrict its central passageway  24 . As further described below, the valve  14  may be subsequently placed in the ball catching state, a state in which the valve  14  compresses a collet sleeve  30  to form an annular seat to catch the ball.  
         [0032]     Turning now to the specific details of the embodiment that is depicted in  FIGS. 2 and 3 , in some embodiments of the invention, the valve  14  includes a generally cylindrical upper housing section  20  ( FIG. 2 ) that is coaxial with a longitudinal axis  26  of the valve  14 . The upper housing section  20  includes an opening  19  to communicate fluids (well fluid, fracturing fluid, etc.) with the portion of the string  12  that is located above and is attached to the upper housing section  20 . At its lower end, the upper housing section  20  is coaxial with and is connected to a generally cylindrical lower housing section  22  ( FIGS. 2 and 3 ). As depicted in  FIG. 2 , in some embodiments of the invention, a seal such as an  0 -ring  23  may be present between the upper  20  and lower  22  housing sections.  
         [0033]     The valve  14  includes a valve sleeve  60  ( FIG. 2 ) that is coaxial with the longitudinal axis  26  and is constructed to move longitudinally within an annular pocket  80  (see  FIG. 3 ) that is formed in the upper  20  and lower  22  housing sections of the valve  14 . The central passageway of the valve sleeve  60  forms part of the central passageway  24  of the valve  14 . Upper  62  and lower  64   0 -rings circumscribe the outer surface of the sleeve  60  and form corresponding annular seals between the outer surface of the sleeve  60  and the inner surface of the housing section  20  for purposes of sealing off radial openings (not shown in  FIG. 2 ) in the upper housing section  20  during the closed state (depicted in  FIGS. 2 and 3 ) of the valve  14 . As further described below, when the sleeve  60  moves in a downward direction to open the valve  14 , openings in the upper housing section  20  are exposed to place the valve  14  in an open state, a state in which fluid communication occurs between the central passageway  24  of the valve  14  and the region that surrounds the valve  14 .  
         [0034]     At its lower end, the valve sleeve  60  is connected to the upper end of the collet sleeve  30 , a sleeve whose state of radial expansion/contraction controls when the valve  14  is in the ball catching state. The collet sleeve  30  is generally coaxial with the longitudinal axis  26 . In some embodiments of the invention, the collet sleeve  30  includes a lower end  32  in which longitudinal slots  34  are formed, and these slots  34  may be regularly spaced about the longitudinal axis  26  of the collet sleeve  30 .  
         [0035]     In its expanded state (depicted in  FIG. 2 ), the lower end  32  of the collet sleeve  30  is flared radially outwardly for purposes of creating the maximum diameter through the interior of the collet sleeve  30 . Thus, as depicted in  FIG. 2 , in this state of the collet sleeve  30 , an opening  38  in the lower end  32  of the sleeve  30  has its maximum inner diameter, thereby leaving the central passageway  24  unobstructed.  
         [0036]     For purposes of radially compressing the lower end  32  of the collet sleeve  30  to place the valve  14  in its ball catching state, the valve  14  includes a mandrel  40 . The mandrel  40  is designed to slide in a downward longitudinal direction (from the position depicted in  FIG. 2 ) for purposes of sliding a sleeve  48  over the lower end  32  to radially compress the lower end  32 . The mandrel  40  is depicted in  FIG. 2  in a position to allow full radial expansion of the lower end  32  of the collet sleeve  30 , and thus, in this position, the mandrel  40  does not configure the collet sleeve  30  to catch a ball.  
         [0037]     For purposes of actuating the mandrel  40  to move the mandrel  40  in a downward direction, the mandrel  40  includes a piston head  43  that has an upper surface  44 . The upper surface  44 , in turn, is in communication with a fluid passageway  42  that may be formed in, for example, the upper housing section  20 . The upper surface  44  of the piston head  43  is exposed to an upper chamber  90  (having its minimum volume in  FIG. 2 ) of the valve  14  for the purpose of creating a downward force on the mandrel  40  to compress the lower end  32  of the collet sleeve  30 .  
         [0038]     As depicted in  FIG. 2 , an  0 -ring  47  forms a seal between the inner surface of the piston head  43  and the outer surface of the collet sleeve  30 ; and a lower  0 -ring  72  is located on the outside of the mandrel  40  for purposes of forming a seal between the exterior surface of the mandrel  40  and the interior surface of the upper housing section  20 . Due to these seals, the upper chamber  90  is sealed off from a lower chamber  75 , a chamber that is below a lower surface  73  of the piston head  43 . As an example, in some embodiments of the invention, the lower chamber  75  has gas such as air at atmospheric pressure or other low pressure or at a vacuum.  
         [0039]     The lower end of the mandrel  40  is connected to the sleeve  48  that has an inner diameter that is sized to approximately match the outer diameter of the section of the collet sleeve  30  located above the flared lower end  32 . Thus, when the pressure that is exerted on the upper surface  47  of the piston head  43  creates a force that exceeds the combined upward force exerted from the chamber  75  to the lower surface  73  and the reaction force that is exerted due to the compression of the lower end  32 , the sleeve  48  restricts the inner diameter of the lower end  32  of the collet sleeve  30  to place the valve  14  in its ball catching state.  
         [0040]      FIG. 4  depicts the upper section  14 A of the valve  14  when the valve  14  is in the ball catching state, a state in which the mandrel  40  is at its lowest point of travel. In this state, the valve sleeve  60  remains in its uppermost point of travel to keep the valve  14  closed. As shown, in this position, the outer diameter of the lower end  32  of the collet sleeve  40  is confined by the inner diameter of the sleeve  48 , and an interior annular seat  94  is formed inside the collet sleeve  30 . The seat  94 , in turn, presents a restricted inner diameter for catching a ball.  
         [0041]     The capture of the ball on the seat  94  substantially restricts, if not seals off, the central passageway of the valve  14  above the ball from the central passageway of the valve  14  below the ball. Due to this restriction of flow, pressure may be applied from the surface of the well for purposes of exerting a downward force on the collet sleeve  30 . Because the upper end of the collet sleeve  30  is connected to the lower end of the valve sleeve  60 , when pressure is applied to the lodged ball and collet sleeve  30 , a corresponding downward force is generated on the valve sleeve  60 . The sleeve  60  may be initially retained in the upward position that is depicted in  FIGS. 2 and 4  by such mechanism(s) (not depicted in the figures) as one or more detent(s), one or more shear pins, trapped low pressure, or vacuum chamber(s). However, when a sufficient downward force is applied to the valve sleeve  60 , this retention mechanism gives way to permit downward movement of the valve sleeve  60 .  
         [0042]     Thus, to open the valve  14 , a ball is dropped from the surface of the well, and then a sufficient pressure is applied (aided by the restriction presented by the lodged ball) to cause the valve sleeve  60  to shift from its uppermost position to its lowest position, a position that is depicted in  FIGS. 5 and 6 . More particularly,  FIGS. 5 and 6  depict the valve  14  in its open state. As shown in  FIG. 5 , in the open state, one or more radial ports  100  formed in the upper housing section  20  are exposed to the central passageway  24  of the valve  14 . Thus, in the open state, fluid, such as fracturing fluid (for example), may be communicated from the central passageway  24  of the string (see  FIG. 1 ) to the annular region that surrounds the valve  14 . It is noted that when the valve  14  is closed, the radial openings  100  are scaled off between the upper  62  and lower  64   0 -rings.  
         [0043]     Referring to  FIG. 6 , due to the pressure that is exerted on the valve sleeve  60 , the assembly that is formed from the valve sleeve  60 , collet sleeve  30 , mandrel  40  and sleeve  48  travels downwardly until the bottom surface of the collet sleeve  30  and the bottom surface of the sleeve  48  reside on an annular shoulder that is formed at the bottom of the annular pocket  80 .  FIG. 6  also depicts a sphere, or ball  150 , that rests on the seat  94  and has caused the valve  14  to transition to its open state.  
         [0044]     Referring back to  FIG. 5 , in the open state of the valve  14 , the passageway  70  is in fluid communication with the central passageway  24 . This is in contrast to the closed state of the valve in which the  0 -ring  68  forms a seal between the central passageway  24  and the passageway  70 , as depicted in  FIGS. 2 and 4 . Therefore, in the valve&#39;s open state, fluid pressure may be communicated to the passageway  70  (see  FIG. 5 ) for purposes of transitioning another valve  14  of the string  12  (see  FIG. 1 ) to its ball catching state.  
         [0045]     As a more specific example, in some embodiments of the invention, the passageway  70  may be in fluid communication with the passageway  42  of another valve  14  (the immediately adjacent valve  14  above, for example). Therefore, in response to the valve sleeve  60  moving to its lower position, a downward force is applied (through the communication of pressure through the passageways  70  and  42 ) to the mandrel  40  of another valve  14  of the string  12 . As a more specific example, in some embodiments of the invention, the passageway  70  of each valve  14  may be in fluid communication with the passageway  42  of the immediate upper adjacent valve in the string  12 . Thus, referring to  FIG. 1 , for example, the passageway  70  of the valve  14   3  is connected to the passageway  42  of the valve  14   2 , and the passageway  70  of the valve  14   2  is connected to the passageway  42  of the valve  14   1 . It is noted that the valve  14   1  in the exemplary embodiment that is depicted in  FIG. 1 , is the uppermost valve  14  in the string  12 . Thus, in some embodiments of the invention, the passageway  70  of the valve  14   1  may be sealed off or non-existent.  
         [0046]     For the lowermost valve  14   N , the passageway  42  is not connected to the passageway of a lower valve. Thus, in some embodiments of the invention, the lowermost valve  14   N  is placed in its ball catching state using a mechanism that is different from that described above. For example, in some embodiments of the invention, the valve  14   N  may be placed in its ball catching state in response to a fluid stimulus that is communicated downhole through the central passageway of the string  12 . Thus, the lowermost valve  14   N  may include a mechanism such as a rupture disc that responds to a remotely-communicated stimulus to permit a downward force to be applied to the mandrel  40 .  
         [0047]     As another example, in some embodiments of the invention, the above-described actuator may move the mandrel  40  in a downward direction in response to a downhole stimulus that is communicated via a slickline or a wireline that are run downhole through the central passageway of the string  12 . As yet another example, the stimulus may be encoded in an acoustic wave that is communicated through the string  12 .  
         [0048]     As another example of a technique to place the valve  14   N  in its ball catching state, in some embodiments of the invention, the mandrel  40  may have a profile on its inner surface for purposes of engaging a shifting tool that is lowered downhole through the central passageway of the string  12  for purposes of moving the mandrel  40  in a downward direction to place the valve  14   N  in its ball catching state. As yet another example of yet another variation, in some embodiments of the invention, the valve  14   N  may be run downhole with a collet sleeve (replacing the collet sleeve  30 ) that is already configured to present a ball catching seat. Thus, many variations are possible and are within the scope of the claimed invention.  
         [0049]     Because the valve  14   N  is the last the valve in the string  12 , other challenges may arise in operating the valve  14   N . For example, below the lowest layer  15   N , there is likely to be a closed chamber in the well. If a ball were dropped on the seat  94  (see  FIG. 14 , for example), the valve sleeve  60  of the valve  14   N  may not shift downwardly because any movement downward may increase the pressure below the ball. Thus, in some embodiments of the invention, the string  12  includes an atmospheric chamber  17  (see  FIG. 1 ) that is located below the valve  14   N . As an example, the chamber  17  may be formed in a side pocket in a wall of the string  12 . To initiate the valve  14   N  for operation, a perforating gun may be lowered downhole through the central passageway of the string  12  to the position where the atmospheric chamber  17  is located. At least one perforation formed from the firing of the perforating gun may then penetrate the atmospheric chamber  17  to create the lower pressure needed to shift the valve sleeve  60  in a downward direction to open the valve  14   N .  
         [0050]     In some embodiments of the invention, when the atmospheric chamber  17  is penetrated, a pressure signal is communicated uphole, and this pressure signal may be used to signal the valve  14   N  to shift the operator mandrel  40  in a downward direction to place the valve  14   N  in the ball catching state. More specifically, in some embodiments of the invention, the valve  14   N  may include a pressure sensor that detects the pressure signal so that an actuator of the valve  14   N  may respond to the pressure signal to move the mandrel  40  in the downward direction to compress the lower end  32  of the collet sleeve  30 .  
         [0051]     Alternatively, in some embodiments of the invention, the collet sleeve  30  of the valve  14   N  may be pre-configured so that the seat  94  is already in its restricted position when the string  12  is run into the well. A perforating gun may then be lowered through the central passageway of the string  12  for purposes of piercing the atmospheric chamber  17  to allow downward future movement of the sleeve valve  60 , as described above.  
         [0052]     Referring to  FIG. 7 , in some embodiments of the invention, a technique  200  may be used for purposes of fracturing multiple layers of a subterranean well. The technique  200  is used in conjunction with a string that includes valves similar to the ones that are described above, such as the string  12  that contains the valves  14  (see  FIG. 1 ).  
         [0053]     Pursuant to the technique  200 , the lowest valve of the string is placed in its ball catching state, as depicted in block  202 . Next, the technique  200  begins an iteration in which the valves are opened pursuant to a sequence (a bottom-to-top sequence, for example). In each iteration, the technique  200  includes dropping the next ball into the string  12 , as depicted in block  204 . Next, pressure is applied (block  206 ) to the ball to cause the valve to open and place another valve (if another valve is to opened) in the ball catching state. Subsequently, the technique  200  includes performing (block  208 ) fracturing in the layer that is associated with the opened valve. If another layer is to be fractured (diamond  210 ), then the technique  200  includes returning to block  204  to perform another iteration.  
         [0054]     As a more specific example, in some embodiments of the invention, the lowest valve  15   N  (see  FIG. 1 ) may be open via a rupture disc and an atmospheric chamber. More specifically, the string  12  is pressured up, the rupture disc breaks and then fluid pushes on side of a piston. The other side of this piston is in contact with an atmospheric chamber or a vacuum chamber.  
         [0055]     Contrary to conventional strings that use ball catching valves, the valves  14  are not closed once opened, in some embodiments of the invention. Furthermore, in some embodiments of the invention, each valve  14  remains in its ball catching state once placed in this state. Because the valves  14  are designed to trap a ball of the same size, the cross-sectional flow area through the central passageway of the string is not significantly impeded for subsequent fracturing or production operations.  
         [0056]     It is noted that for an arbitrary valve  14  in the string  12 , once the valve  14  is placed in its ball catching state, the restricted diameter formed from the lower end of the collet sleeve  30  prevents a ball from below the collet sleeve  30  below from flowing upstream. Therefore, during flowback, each ball may be prevented from flowing past the lower end  32  of the collet sleeve  30  of the valve  14  above.  
         [0057]     However, in accordance with some embodiments of the invention, each ball may be formed from a material, such as a dissolvable or frangible material, that allows the ball to disintegrate. Thus, although a particular ball may flow upstream during flowback and contact the bottom end of the collet sleeve  30  above, the ball is eventually eroded or at least sufficiently dissolved to flow upstream through the valve to open up communication through the string  12 .  
         [0058]     In some embodiments of the invention, captured ball used to actuate a lower valve  14  may push up on the collet sleeve  30  of a higher valve in the string  12  until the collet sleeve  30  moves into an area (a recessed region formed in the lower housing  22 , for example) which has a pocket in the inner diameter to allow the collet sleeve  30  to reopen. Thus, when the collet sleeve  30  reopens, the inner diameter is no longer small enough to restrict the ball so that the ball can flow uphole. Other variations are possible and are within the scope of the appended claims.  
         [0059]     Referring to  FIG. 8 , in accordance with some embodiments of the invention, a bottom surface  252  of the lower end  32  of the collet sleeve  30  is designed to be irregular to prevent a ball that is located below the collet sleeve  30  (and has not dissolved or eroded enough to pass through) from forming a seal that blocks off fluid communication. Thus, as depicted in  FIG. 8 , in some embodiments of the invention, the surface  252  may have one or more irregularities, such as a depression  252  that permits the surface  32  from becoming an effective valve seat. Other types of irregularities may be introduced to the surface  252 , such as raised portions, generally rough surfaces, etc., depending the particular embodiment of the invention.  
         [0060]     Other embodiments are within the scope of the appended claims. For example, referring to  FIG. 9 , in some embodiments of the invention, in a valve  290  (that replaces the valve  14 ) the collet sleeve  30  may be replaced by a C-ring  300 . The valve  290  has the same generally design of the valve  14 , except for the C-ring  300  and the following differences. The C-ring  300 , in some embodiments of the invention, includes a single open slot  309  when the valve is not in the ball catching state. Thus, as depicted in  FIG. 9 , in this state, a mandrel  302  is located above the C-ring  300  so that the open ends  307  of the C-ring  300  do not compress to close the slot  309 . As depicted in  FIG. 9 , an end  304  of the mandrel  302  may be inclined, or beveled, in some embodiments of the invention so that when the mandrel  302  slides downhole, as depicted in  FIG. 10 , the ends  307  meet to close the slot  309  ( FIG. 9 ) and thus restrict the inner diameter through the C-ring  300 . In the state that is depicted in  FIG. 10 , the valve is in a ball catching state, as the inner diameter has been restricted for purposes of catching a free-falling or pumped down object.  
         [0061]     The C-ring design may be advantageous, in some embodiments of the invention, in that the C-ring  300  includes a single slot  309 , as compared to the multiple slots  34  (see  FIG. 2 , for example) that are present in the collet sleeve  30 . Therefore, the C-ring design may be advantageous in that sealing is easier because less leakage occurs when the C-ring ring  300  contracts.  
         [0062]     Referring to back to  FIG. 1 , in some embodiments of the invention, the string  12  may be deployed in a wellbore (e.g., an open or uncased hole) as a temporary completion. In such embodiments, sealing mechanisms may be employed between each valve and within the annulus defined by the tubular string and the wellbore to isolate the formation zones being treated with a treatment fluid. However, in other embodiments of the invention, the string  12  may be cemented in place as a permanent completion. In such embodiments, the cement serves to isolate each formation zone.  
         [0063]     The cementing of the string  12  may potentially block valve openings, if not for certain features of the valve  14 . For example, referring back to  FIG. 5 , in some embodiments of the invention, the valve  14  may include lobes  101  that are spaced around the longitudinal axis  26 . Each lobe  101  extends radially outwardly from a main cylindrical wall  103  of the upper housing  20 , and each radial port  100  extends through one of the lobes  101 . The lobes  101  restrict the space otherwise present between the valve  14  and the wellbore to limit the amount of cement that may potentially block fluid communication between the central passageway  24  and the region outside of the valve  14 , as described in co-pending U.S. patent application Ser. No. 10/905,073 entitled, “SYSTEM FOR COMPLETING MUTLIPLE WELL INTERVALS,” filed on Dec. 14, 2004.  
         [0064]     In accordance with some embodiments of the invention, each radial port  100  is formed from an elongated slot whose length is approximately equal to at least five times its width. It has been discovered that such a slot geometry when used in a fracturing operating allows radial deflection when pressuring up, which increases stress in the rock and thus, reduces the fracturing initiation pressure.  
         [0065]     Depending on the particular embodiment of the invention, the valve may contain, as examples, three (spaced apart by 120° around the longitudinal axis  26 , for example) or six (spaced apart by 60° around the longitudinal axis  26 , for example) lobes  101 . In some embodiments of the invention, the valve  14  does not contain the lobes  101 . Instead, the upper housing section  20  approximates a circular cylinder, with the outer diameter of the cylinder being sized to closely match the inner diameter of the wellbore.  
         [0066]     Other variations are possible in accordance with the various embodiments of the invention. For example, depending on the particular embodiment of the invention, each radial port  100  may have a length that is at least approximately equal to ten or (in other embodiments) is approximately equal to twenty times its length.  
         [0067]     The radial slots  100  are depicted in  FIG. 5  as being located at generally the same longitudinal position. However, in other embodiments of the invention, a valve ( FIG. 11 ) may include a valve housing  400  (replacing the upper valve housing  20 ) that includes radial slots  420  that extending along a helical, or spiral path  422 , about the longitudinal axis  26 . As shown in  FIG. 11 , the valve housing  400  does not contain the radially-extending lobes. Thus, many variations are possible and are within the scope of the appended claims.  
         [0068]     Although directional and orientational terms (such as “upward,” “lower,” etc.) are used herein to describe the string, the valve, their components and their operations, it is understood that the specific orientations and directions that are described herein are not needed to practice the invention. For example, in some embodiments of the invention, the valve sleeve may move in an upward direction to open. As another example, in some embodiments of the invention, the string may be located in a lateral wellbore. Thus, many variations are possible and are within the scope of the appended claims.  
         [0069]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.