Patent Publication Number: US-9410399-B2

Title: Multi-zone cemented fracturing system

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure generally relates to a multi-zone cemented fracturing system. 
     2. Description of the Related Art 
     Hydraulic fracturing (aka fracing or fracking) is an operation for stimulating a subterranean formation to increase production of formation fluid, such as crude oil and/or natural gas. A fracturing fluid, such as a slurry of proppant (i.e., sand), water, and chemical additives, is pumped into the wellbore to initiate and propagate fractures in the formation, thereby providing flow channels to facilitate movement of the formation fluid into the wellbore. The fracturing fluid is injected into the wellbore under sufficient pressure to penetrate and open the channels in the formation. The fracturing fluid injection also deposits the proppant in the open channels to prevent closure of the channels once the injection pressure has been relieved. 
     In a staged fracturing operation, multiple zones of a formation are isolated sequentially for treatment. To achieve this isolation, a liner string equipped with multiple fracture valves is deployed into the wellbore and set into place. A first zone of the formation may be selectively treated by opening a first of the fracture valves and injecting the fracturing fluid into the first zone. Subsequent zones may then be treated by opening the respective fracture valves. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure generally relates to a multi-zone cemented fracturing system. In one embodiment, a method of cementing a liner string into a wellbore includes deploying a liner string into the wellbore to a portion of the wellbore traversing a productive formation using a workstring. The liner string includes a first fracture valve and the workstring includes a first wiper plug. The method further includes: pumping cement slurry into the workstring; and pumping a dart through the workstring, thereby driving the cement slurry into the liner string. The dart engages the first wiper plug and releases the first wiper plug from the workstring. The dart and engaged first wiper plug drive the cement slurry through the liner string and into an annulus formed between the liner string and the wellbore. The dart and engaged first wiper plug land onto the first fracture valve. The dart releases a first seat into the first wiper plug. The dart engages a second wiper plug connected to the first fracture valve and releases the second wiper plug from the first fracture valve. 
     In another embodiment, a fracture valve for use in a wellbore includes: a tubular housing having threaded couplings formed at each longitudinal end thereof and one or more ports formed through a wall thereof; and a sleeve disposed in the housing and releasably connected thereto in a closed position. The sleeve is longitudinally movable relative to the housing between an open position and the closed position. The sleeve covers the ports in the closed position. The sleeve exposes the ports in the open position. The valve further includes: a collar connected to the first sleeve and made from a millable material and a wiper plug releasably connected to the collar and having a first seat formed therein. 
     In another embodiment, a dart for use with a fracture valve system includes: a mandrel made from a millable material; one or more fins connected to the mandrel and made from an elastomer or elastomeric copolymer; and a seat stack. The seat stack includes: a lower seat fastened to the mandrel by one or more lower shearable fasteners and having an outer sealing surface and an inner sealing surface; and an upper seat fastened to the lower seat or mandrel by one or more upper shearable fasteners and having an outer sealing surface and an inner sealing surface. A shear strength of the lower shearable fasteners is greater than a shear strength of the upper shearable fasteners. An outer diameter of the upper seat is greater than an outer diameter of the lower seat. A diameter of the inner sealing surface of the upper seat is greater than a diameter of the inner sealing surface of the lower seat. 
     In another embodiment, a method of fracturing a productive formation includes deploying a liner string into a wellbore to a portion of the wellbore traversing the productive formation using a workstring. The liner string includes a first cluster valve and the workstring includes a first wiper plug. The method further includes: pumping cement slurry into the workstring; and pumping a dart through the workstring, thereby driving the cement slurry into the liner string. The dart engages the first wiper plug and releases the first wiper plug from the workstring. The dart and engaged first wiper plug drive the cement slurry through the liner string and into an annulus formed between the liner string and the wellbore. The dart and engaged first wiper plug land onto the first cluster valve. The first wiper plug releases the dart. The dart engages a second wiper plug connected to the first cluster valve and releases the second wiper plug from the first cluster valve. The method further includes deploying a ball through the liner string to the first cluster valve. The ball lands onto the first wiper plug and opens the cluster valve. The first wiper plug releases the ball. 
     A fracture valve for use in a wellbore includes: a tubular housing having threaded couplings formed at each longitudinal end thereof and one or more ports formed through a wall thereof; a sleeve disposed in the housing and releasably connected thereto in a closed position. The sleeve is longitudinally movable relative to the housing between an open position and the closed position. The sleeve covers the ports in the closed position. The sleeve exposes the ports in the open position. The valve further includes: a collar connected to the sleeve and made from a millable material; a wiper plug releasably connected to the collar; and a seat releasably connected to the wiper plug in an extended position, wherein the seat is movable relative to the wiper plug among the extended position, a first retracted position, and a second retracted position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1A  illustrates a drilling system in a cementing mode, according to one embodiment of the present disclosure.  FIG. 1B  illustrates a well being completed using the system. 
         FIG. 2A  illustrates a fracture valve of  FIG. 1B .  FIG. 2B  illustrates a dart of  FIG. 1A .  FIG. 2C  illustrates a seat stack of the dart.  FIGS. 2D-2F  illustrate wiper plugs of  FIG. 1B .  FIG. 2G  illustrates an additional wiper plug usable with a liner string of  FIG. 1B . 
         FIGS. 3A-3J  illustrate a cementing operation performed using the system. 
         FIG. 4  illustrates a fracturing system. 
         FIGS. 5A-5E  illustrate a fracturing operation performed using the system. 
         FIG. 6A  illustrates a portion of an alternative fracture valve usable with the liner string, according to another embodiment of the present disclosure.  FIG. 6B  illustrates an alternative dart usable with the liner string, according to another embodiment of the present disclosure. 
         FIGS. 7A-7E  illustrate a cluster fracture valve and dart (and operation thereof) usable with the liner string, according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  illustrates a drilling system  1  in a cementing mode, according to one embodiment of the present disclosure.  FIG. 1B  illustrates a well being completed using the system  1 . The drilling system  1  may include a drilling rig  1   r , a fluid system  1   f , and a pressure control assembly (PCA)  1   p . The drilling rig  1   r  may include a derrick  2  with a rig floor  3  at its lower end having an opening  4  through which a workstring  5  extends downwardly through the PCA  1   p . The PCA  1   p  may be connected to a wellhead  7   h . The wellhead  7   h  may be mounted on a casing string  7   c  which has been deployed into a wellbore  8   w  drilled from a surface  8   s  of the earth and cemented  9  into the wellbore. The wellbore  8   w  may include a vertical portion and a deviated, such as horizontal, portion. The workstring  5  may also be connected to a cementing head  6 . The cementing head  6  may also be connected to a Kelly valve  10 . 
     The Kelly valve  10  may be connected to a quill of a top drive  11 . A housing of the top drive  11  may be suspended from the derrick  2  by a traveling block  12   t . The traveling block  12   t  may be supported by wire rope  13  connected at its upper end to a crown block  12   c . The wire rope  13  may be woven through sheaves of the blocks  12   t,c  and extend to drawworks  14  for reeling thereof, thereby raising or lowering the traveling block  12   t  relative to the derrick  2 . Alternatively, a Kelly and rotary table (not shown) may be used instead of the top drive  11 . 
     The workstring  5  may include a liner deployment assembly (LDA)  5   d  and a deployment string, such as joints of drill pipe  5   p  connected together, such as by threaded couplings. An upper end of the LDA  5   d  may be connected a lower end of the drill pipe  9   p , such as by threaded couplings. The LDA  5   d  may releasably connect a liner string  15  to the workstring  5 . The LDA  5   d  may include a diverter valve, a junk bonnet, a setting tool, a running tool, a stinger, a packoff, a spacer, a release, a plug release system, and a cementing plug, such as wiper plug  19   a . The plug release system may releasably connect the wiper plug  19   a  to the LDA spacer. 
     The cementing head  6  may include an actuator swivel  6   a , a cementing swivel  6   c , and a launcher  6   p . Each swivel  6   a,c  may include a housing torsionally connected to the derrick  2 , such as by bars, wire rope, or a bracket (not shown). Each torsional connection may accommodate longitudinal movement of the respective swivel  6   a,c  relative to the derrick  2 . Each swivel  6   a,c  may further include a mandrel and bearings for supporting the housing from the mandrel while accommodating relative rotation therebetween. 
     The cementing swivel  6   c  may further include an inlet formed through a wall of the housing and in fluid communication with a port formed through the mandrel and a seal assembly for isolating the inlet-port communication. The cementing swivel inlet may be connected to a cementing pump  16   c  via shutoff valve  17   b . The shutoff valve  17   b  may be automated and have a hydraulic actuator (not shown) operable by a rig controller, such as a programmable logic controller (PLC)  18 , via fluid communication with a hydraulic power unit (HPU) (not shown). Alternatively, the shutoff valve actuator may be pneumatic or electric. The cementing mandrel port may provide fluid communication between a bore of the cementing head  6  and the housing inlet. 
     The actuator swivel  6   a  may be hydraulic and may include a housing inlet formed through a wall of the housing and in fluid communication with a passage formed through the mandrel, and a seal assembly for isolating the inlet-passage communication. Each seal assembly may include one or more stacks of V-shaped seal rings, such as opposing stacks, disposed between the mandrel and the housing and straddling the inlet-port interface. Alternatively, the seal assembly may include rotary seals, such as mechanical face seals. The passage may extend to an outlet of the mandrel for connection to a hydraulic conduit for operating a hydraulic actuator  6   h  of the cementing head  6 . The actuator swivel  6   a  may be in fluid communication with the HPU. Alternatively, the actuator swivel and cementing head actuator may be pneumatic or electric. The Kelly valve  10  may also be automated and include a hydraulic actuator (not shown) operable by the PLC  18  via fluid communication with the HPU. The cementing head  6  may further include an additional actuator swivel (not shown) for operation of the Kelly valve  10  or the top drive  11  may include the additional actuator swivel. Alternatively, the Kelly valve actuator may be electric or pneumatic. 
     The launcher  6   p  may include a housing, a diverter, a canister, a latch, and the actuator  6   h . The housing may be tubular and may have a bore therethrough and a coupling formed at each longitudinal end thereof, such as threaded couplings. Alternatively, the upper housing coupling may be a flange. To facilitate assembly, the housing may include two or more sections (three shown) connected together, such as by a threaded connection. The housing may also serve as the cementing swivel housing (shown) or the launcher and cementing swivel  6   c  may have separate housings (not shown). The housing may further have a landing shoulder formed in an inner surface thereof. The canister and diverter may each be disposed in the housing bore. The diverter may be connected to the housing, such as by a threaded connection. The canister may be longitudinally movable relative to the housing. The canister may be tubular and have ribs formed along and around an outer surface thereof. Bypass passages may be formed between the ribs. The canister may further have a landing shoulder formed in a lower end thereof corresponding to the housing landing shoulder. The diverter may be operable to deflect cement slurry  109  or displacement fluid  110  away from a bore of the canister and toward the bypass passages. A cementing plug, such as dart  20 , may be disposed in the canister bore for selective release and pumping downhole to activate the wiper plug  19   a . Alternatively, the wiper plug  19   a  may be omitted. 
     The latch may include a body, a plunger, and a shaft. The body may be connected to a lug formed in an outer surface of the launcher housing, such as by a threaded connection. The plunger may be longitudinally movable relative to the body and radially movable relative to the housing between a capture position and a release position. The plunger may be moved between the positions by interaction, such as a jackscrew, with the shaft. The shaft may be longitudinally connected to and rotatable relative to the body. The actuator  6   h  may be a hydraulic motor operable to rotate the shaft relative to the body. Alternatively, the actuator may be linear, such as a piston and cylinder. Alternatively, the actuator may be electric or pneumatic. Alternatively, the actuator may be manual, such as a handwheel. 
     In operation, the PLC  18  may release the dart  20  by operating the HPU to supply hydraulic fluid to the actuator  6   h  via the actuator swivel  6   a . The actuator  6   h  may then move the plunger to the release position (not shown). The canister and dart  20  may then move downward relative to the housing until the landing shoulders engage. Engagement of the landing shoulders may close the canister bypass passages, thereby forcing displacement fluid  110  to flow into the canister bore. The displacement fluid  110  may then propel the dart  20  from the canister bore into a lower bore of the housing and onward through the drill pipe  5   p  to the wiper  19   a.    
     The PCA  1   p  may include a blow out preventer (BOP)  21 , a flow cross  22 , and a shutoff valve  17   a . Each component of the PCA  1   p  may be connected together and the PCA may be connected to the wellhead  7   h , such as by flanges and studs or bolts and nuts. The casing string  7   c  may extend to a depth adjacent a bottom of an upper formation and the liner string  15  may extend into a portion of the wellbore  8   w  traversing a lower formation. The upper formation may be non-productive and the lower formation may be a hydrocarbon-bearing reservoir. 
     The liner string  15  may include a plurality of liner joints  15   j  connected to each other, such as by threaded connections, one or more centralizers  15   c  spaced along the liner string at regular intervals, one or more fracture valves  50   a - c , a toe sleeve  15   s , a float shoe  15   f , a liner hanger  15   h , a packer  15   p , and a polished bore receptacle (not shown). The liner hanger  15   h  may be operable to engage the casing  7   c  and longitudinally support the liner string  15  from the casing  7   c . The liner hanger  15   h  may include slips and a cone. The liner hanger  15   h  may accommodate relative rotation between the liner string  15  and the casing  7   c , such as by including a bearing (not shown). The packer  15   p  may be operable to radially expand into engagement with an inner surface of the casing  7   c , thereby isolating the liner-casing interface. The liner hanger  15   h  and packer  15   p  may be independently set using the LDA  5   d . Each liner joint  15   j  may be made from a metal or alloy, such as steel, stainless steel, or a nickel-based alloy. The centralizers  15   c  may be fixed or sprung. The centralizers  15   c  may engage an inner surface of the casing  7   c  and/or wellbore  8   w . The centralizers  15   c  may operate to center the liner string  15  in the wellbore  8   w . Alternatively, the centralizers  15   c  may be omitted. 
     The shoe  15   f  may be disposed at the lower end of the liner string  15  and have a bore formed therethrough. The shoe  15   f  may be convex for guiding the liner string  15  toward the center of the wellbore  8   w . The shoe  15   f  may minimize problems associated with hitting rock ledges or washouts in the wellbore  8   w  as the liner string  15  is lowered into the wellbore  8   w . An outer portion of the shoe  15  may be made from the liner joint material, discussed above. An inner portion of the shoe  15  may be made of a drillable or millable material, such as cement, cast iron, non-ferrous metal or alloy, engineering polymer, or fiber reinforced composite, so that the inner portion may be drilled through if the wellbore  8   w  is to be further drilled. The shoe  15   f  may include a check valve for selectively sealing the shoe bore. The check valve maybe operable to allow fluid flow from the liner bore into the wellbore  8   w  and prevent reverse flow from the wellbore into the liner bore. 
     The toe sleeve  15   s  may include a housing and a piston. The housing and piston may be made from any of the liner joint materials, discussed above. The housing may be tubular, have a bore formed therethrough, and have couplings, such as a threaded pin and a threaded box, formed at longitudinal ends thereof for connection to other components of the liner string  15 . The housing may also have one or more flow ports formed through a wall thereof for providing fluid communication between the housing bore and the annulus  8   a . To facilitate manufacture and assembly, the housing may include two or more sections connected together, such as by threaded connections and fasteners, such as set screws and sealed, such as by o-rings. The piston may be disposed in the housing bore and be longitudinally movable relative thereto subject to engagement with upper and lower shoulders of the housing. The piston may be releasably connected to the housing in a closed position (shown). The releasable connection may be a shearable fastener, such as one or more shear screws. The piston may cover the flow ports in the closed position and a piston-housing interface may be sealed, such as by seals carried by the piston and spaced longitudinally there-along to straddle the flow ports in the closed position. The piston may also carry a fastener, such as a C-ring, adjacent a lower end thereof for engaging a complementary profile, such as a groove, formed in an inner surface of the housing. 
     A hydraulic chamber may be formed between the piston and the housing. The hydraulic chamber may be in fluid communication with an annulus  8   a  (formed between an inner surface of the casing  7   c  and wellbore  8   w  and an outer surface of the workstring  5  and liner string  15 ) via the flow ports. The piston may have an enlarged inner shoulder exposed to the housing bore and an outer shoulder exposed to the hydraulic chamber. The piston may be operated by fluid pressure in the housing bore exceeding fluid pressure in the annulus  8   a  by a substantial differential sufficient to fracture the shear screws. Once released from the housing, the piston may move downward relative to the housing until a bottom of the piston engages the lower housing shoulder, thereby exposing the flow ports to the housing bore ( FIG. 5A ). As the piston is nearing the open position, the C-ring may engage the groove, thereby locking the piston in the open position. 
     The fluid system if may include one or pumps  16   c,m , one or more shutoff valves  17   b - d , a drilling fluid reservoir, such as a pit  23  or tank, a solids separator, such as a shale shaker  24 , one or more sensors, such as one or more pressure sensors  25   m,c,r  one or more stroke counters  26   m,c , and a cement mixer, such as a recirculating mixer  27 . The fluid system if may further include one or more flow lines, such as a mud line connecting a mud pump  16   m  to the top drive  11 , a cement line connecting a cement pump  16   c  to the cementing swivel  6   c , a return line connecting the flow cross  22  to the shale shaker  24 , a mud supply line connecting the pit  23  to the pumps  16   c,m , and a cement supply line connecting the mixer  27  to the cement pump. The cement slurry  109  ( FIG. 3B ) may be formulated to resist flash setting due to multiple releases of the wiper plugs and dart seats. 
     The valve  17   a  and pressure sensor  25   r  may be assembled as part of the return line. The valve  17   b  and pressure sensor  25   c  may be assembled as part of the cement line. The valve  17   c  may be assembled as part of the cement supply line. The valve  17   d  may be assembled as part of the mud supply line. The pressure sensor  25   m  may be assembled as part of the mud line. Each sensor  25   m,c,r ,  26   m,c  may be in data communication with the PLC  18 . The pressure sensor  25   r  may be operable to monitor wellhead pressure. The pressure sensor  25   m  may be operable to measure standpipe pressure. The stroke counter  26   m  may be operable to measure a flow rate of the mud pump  16   m . The pressure sensor  25   c  may be operable to measure discharge pressure of the cement pump  16   c . The stroke counter  26   c  may be operable to measure a flow rate of the cement pump  16   c.    
     To prepare for the cementing operation, a conditioner  108  may be circulated by the mud pump  16   m . The conditioner  108  may flow from the mud pump  16   m , through the standpipe and a Kelly hose to the top drive  11 . The conditioner  108  may continue from the top drive  11  into the workstring  5  via the Kelly valve  10  and cementing head  6 . The conditioner  108  may continue down the liner string bore and exit the shoe  15   f . The conditioner  108  may flush drilling fluid, such as mud  107 , up the annulus  8   a . The displaced mud  107  may exit from the annulus  8   a , through the wellhead  7   h , and to the shaker  24  via the flow cross  22  and the valve  17   a . The displaced mud  107  may then be processed by the shale shaker  24  and discharged into the pit  23  for storage. The conditioner  108  may also wash cuttings and/or mud cake from the wellbore  8   w  and/or adjust pH in the wellbore for pumping the cement slurry  109 . Alternatively, the conditioner  108  may be pumped by the cement pump  16   c  through the valve  17   b . The workstring  5  and liner  15  may also be rotated  30  from the surface  8   s  by the top drive  11  during circulation of the conditioner  108 . 
       FIG. 2A  illustrates the fracture valve  50   a . The fracture valve  50   a  may include a housing  51 , a sleeve  52 , a collar  53 , and a cementing plug, such as wiper plug  19   b . The housing  51  and sleeve  52  may be made from any of the liner joint materials, discussed above. The housing  51  may be tubular, have a bore formed therethrough, and have couplings, such as a threaded pin  51   e  and a threaded box  51   b , formed at longitudinal ends thereof for connection to other components of the liner string  15 . The housing  51  may also have one or more fracturing ports  51   p  formed through a wall thereof for providing fluid communication between the housing bore and the annulus  8   a . To facilitate manufacture and assembly, the housing  51  may include two or more sections  51   a - c  connected together, such as by threaded connections and fasteners, such as set screws  54   u,b , and sealed, such as by o-rings  55   u,b.    
     The sleeve  52  may be disposed in the housing bore and be longitudinally movable relative thereto subject to engagement with upper  58   u  and lower  58   b  shoulders of the housing  51 . The shoulders  58   u,b  may be formed by longitudinal ends of the respective housing sections  51   a,c . The sleeve  52  may be releasably connected to the housing  51  in a closed position (shown). The releasable connection may be a shearable fastener, such as shear ring  57   s . The shear ring  57   s  may have a stem portion disposed in a recess  59   u  formed in an inner surface of the housing  51  adjacent the upper shoulder  58   u  and a lip portion extending into a groove formed in the outer surface of the sleeve  52 . The sleeve  52  may cover the ports  51   p  in the closed position and a sleeve-housing interface may be sealed, such as by seals  56   u,b  carried by the sleeve and spaced longitudinally there-along to straddle the ports  51   p  in the closed position. The seals  56   u,b  may each be single element or seal stacks, as discussed above. 
     The sleeve  52  may also carry a fastener, such as a C-ring  61 , adjacent a lower end thereof for engaging a complementary profile, such as a groove  59   b , formed in an inner surface of the housing  51  adjacent the lower shoulder  58   b . Once released from the housing  51 , the sleeve  52  may move downward relative to the housing until a bottom of the sleeve engages the lower shoulder  58   b , thereby exposing the ports  51   p  to the housing bore ( FIG. 5E ). As the sleeve  52  is nearing the open position, the C-ring  61  may engage the groove  59   b , thereby locking the sleeve in the open position. 
     The collar  53  may be disposed in a bore of the sleeve  52  and connected, such as longitudinally and torsionally, thereto, such as by one or more fasteners (i.e., set screws  54   m ). The collar  53  may be made from any of the millable/drillable materials, discussed above. The collar  53  may be annular and have a bore formed therethrough. The collar  53  may have a landing shoulder  53   u  and a mounting shoulder  53   b , each shoulder formed in an inner surface thereof. The mounting shoulder  53   b  may be mated with a top of the wiper plug  19   b.    
     The wiper plug  19   b  may have a body  19   y  and a wiper seal  19   w . The body  19   y  may be annular and have a bore formed therethrough. The body  19   y  may have a seat formed in an inner surface thereof, a mounting shoulder formed in an outer surface thereof, and a stinger portion  19   s  forming a lower end thereof for landing in the collar (see collar  53 ) of the adjacent fracture valve  50   b . The wiper seal  19   f  may be molded, bonded, or fastened onto an outer surface of the body  19   y  and seated against the mounting shoulder. The wiper seal  19   f  may be made from an elastomer or elastomeric copolymer. The wiper plug  19   b  may be releasably connected to the collar  53  and seated against the mounting shoulder  53   b . The releasable connection may include a set  57   w  of one or more (one shown) shearable fasteners, such as shear screws. 
       FIGS. 2D-2F  illustrate wiper plugs  19   a,c,e  of the LDA plug release system/fracture valves  50   b - c .  FIG. 2G  illustrates an additional wiper plug  19   d  usable with the liner string  15 . The wiper plug  19   a  may be identical to the wiper plug  19   b  except for having a seat diameter  65   a  greater than a seat diameter  65   b  of the wiper plug  19   b  and having a slight modification for connection to the LDA plug release system. The wiper plug  19   c  may be identical to the wiper plug  19   b  except for having a seat diameter  65   c  less than the seat diameter  65   b . The wiper plug  19   d  may be identical to the wiper plug  19   b  except for having a seat diameter  65   d  less than the seat diameter  65   c . The wiper plug  19   e  may be identical to the wiper plug  19   b  except for having a seat diameter  65   e  less than the seat diameter  65   d  and having a landing shoulder for engagement with the shoe  15   f  instead of the stinger portion  19   s.    
     The other fracture valves  50   b,c  may each be identical to the fracture valve  50   a  except for the substitution of the wiper plug  19   c  for the wiper plug  19   b  in the valve  50   b  and the substitution of the wiper plug  19   e  for the wiper plug  19   b  in the valve  50   c . The liner string  15  may further include an additional fracture valve (not shown) disposed between the fracture valves  50   b,c  identical to the fracture valve  50   a  except for the substitution of the wiper plug  19   d  for the wiper plug  19   b.    
       FIG. 2B  illustrates the dart  20 .  FIG. 2C  illustrates a seat stack  60  of the dart. The dart  20  may include a mandrel  20   m , a fin stack  20   c,f , and the seat stack  60 . The fin stack  20   c,f  may include one or more (three shown) fins  20   f , each fin bonded, molded, or fastened to an outer surface of a respective fin collar  20   c . Each fin  20   f  may be made from an elastomer or elastomeric copolymer. An outer surface of the mandrel  20   m  may have an upper mounting shoulder for receiving the fin collars  20   c  and an upper thread for receiving a fastener, such as a threaded nut  20   n , thereby connecting the fin stack  20   c,f  to the mandrel. The mandrel  20   m , seat stack  60 , fin collar  20   c , and nut  20   n  may be made from any of the millable/drillable materials, discussed above. 
     The seat stack  60  may include one or more seats  60   a - d  and a retainer  60   r . A top seat  60   a  of the stack  60  may be releasably connected to a first intermediate seat  60   b  of the stack  60 . The releasable connection may include a set  62   a  of one or more (two shown) shearable fasteners, such as shear screws. The first intermediate seat  60   b  of the stack  60  may also be releasably connected to a second intermediate seat  60   c  of the stack  60 . The releasable connection may include a set  62   b  of one or more (three shown) shearable fasteners, such as shear screws. The second intermediate seat  60   c  of the stack  60  may also be releasably connected to a bottom seat  60   d  of the stack  60 . The releasable connection may include a set  62   c  of one or more (four shown) shearable fasteners, such as shear screws. A bottom seat  60   d  of the stack  60  may also be releasably connected to the retainer  60   r . The releasable connection may include a set  62   d  of one or more (five shown) shearable fasteners, such as shear screws. 
     A shear strength of each set  62   a - d  of shearable fasteners may be greater or substantially greater than a shear strength of each set  57   w  of shearable fasteners. A shear strength of the shear ring  57   s  may be greater or substantially greater than the shear strength of each set  62   a - d  of shearable fasteners and may be greater or substantially greater than the shear strength of each set  57   w  of shearable fasteners. The shear strength of the bottom set  62   d  of shearable fasteners may also be greater or substantially greater than the shear strength of the second intermediate set  62   c  of shearable fasteners. The shear strength of the second intermediate set  62   c  of shearable fasteners may also be greater or substantially greater than the shear strength of the first intermediate set  62   b  of shearable fasteners. The shear strength of the first intermediate set  62   b  of shearable fasteners may also be greater or substantially greater than the shear strength of the top set  62   a  of shearable fasteners. 
     Each seat  60   a - d  may have an outer seating surface for engagement with a seat of the respective wiper plug  19   a - c ,  19   d  and an inner seating surface for receiving a respective pump-down plug, such as balls  170   a - c  ( FIG. 4 ) (ball for seat  20   d  not shown). The top seat  60   a  may have an outer diameter greater than an outer diameter of each successive seat  60   b - d  (and the retainer  60   r ) and corresponding to the seat diameter  65   a  such that the top seat may engage the seat of the wiper plug  19   a . The successive seats  60   b - d  (and the retainer  60   r ) may each have an outer diameter less than the seat diameter  65   a  such that the rest of the seats  60   b - d  may pass through the wiper plug seat unobstructed. The first intermediate seat  60   b  may have an outer diameter greater than an outer diameter of each successive seat  60   c - d  (and the retainer  60   r ) and corresponding to the seat diameter  65   b  such that the first intermediate seat may engage the seat of the wiper plug  19   b . The successive seats  60   c - d  (and the retainer  60   r ) may each have an outer diameter less than the seat diameter  65   b  such that the rest of the seats  60   c - d  may pass through the wiper plug seat unobstructed. The second intermediate seat  60   c  may have an outer diameter greater than an outer diameter of the bottom seat  60   d  (and the retainer  60   r ) and corresponding to the seat diameter  65   c  such that the second intermediate seat may engage the seat of the wiper plug  19   c.    
     The bottom seat  60   d  (and the retainer  60   r ) may each have an outer diameter less than the seat diameter  65   c  such that the bottom seat  60   d  may pass through the wiper plug seat unobstructed. The bottom seat  60   d  may have an outer diameter greater than an outer diameter of the retainer  60   r  and corresponding to the seat diameter  65   d  such that the bottom seat may engage the seat of the wiper plug  19   d . The retainer  60   r  may have an outer diameter less than the seat diameter  65   d  such that the retainer  60   r  may pass through the wiper plug seat unobstructed. The retainer  60   r  may have an outer seating surface and a threaded inner surface and the outer surface of the mandrel  20   m  may have a lower shouldered thread for receiving the retainer  20   r , thereby connecting the seat stack  60  to the mandrel  20   m . A bottom of the retainer  60   r  may form a seat having an outer diameter corresponding to the seat diameter  65   e  such that the retainer seat may engage the seat of the wiper plug  19   e.    
       FIGS. 3A-3J  illustrate a cementing operation performed using the system  1 . Referring specifically to  FIG. 3A , rotation  30  may be halted and the LDA  5   d  may be operated to set the liner hanger  15   h  mechanically by articulation of the workstring  5  or hydraulically by pumping a setting plug, such as a ball (not shown), through the deployment string to a seat of the LDA  5   d . Alternatively, the liner hanger  15   h  may be set using a control line (not shown) extending along the workstring to the actuator swivel  6   a . Once the liner hanger  15   h  has been set, the LDA running tool may be operated to release the liner string  15  therefrom. Setting of the liner hanger  15   h  and release of the liner string  15  may be confirmed by raising and lowering of the LDA  5   d  using the deployment string. 
     Referring specifically to  FIG. 3B , rotation  30  may resume and the cement slurry  109  may be pumped from the mixer  27  into the cementing swivel  6   c  via the valve  17   b  by the cement pump  16   c . The cement slurry  109  may flow into the launcher  6   p  and be diverted past the dart  20  via the diverter and bypass passages. Once the desired quantity of cement slurry  109  has been pumped, the dart  20  may be released from the launcher  6   p  by the PLC  18  operating the actuator  6   h . Displacement fluid  110  may be pumped into the cementing swivel  6   c  via the valve  17   b  by the cement pump  16   c . The displacement fluid  110  may flow into the launcher  6   p  and be forced behind the dart  20  by closing of the bypass passages, thereby propelling the dart into the workstring bore. Pumping of the displacement fluid  110  by the cement pump  16   c  may continue until residual cement slurry in the cement discharge conduit has been purged. Pumping of the displacement fluid  110  may then be transferred to the mud pump  16   m  by closing the valve  17   b  and opening the Kelly valve  10 . Alternatively, the cement pump  16   c  may be used to continue pumping of the displacement fluid  110  instead of switching to the mud pump  16   m . The dart  20  may be driven through the workstring bore by pumping of the displacement fluid  110  until the dart (specifically seat  60   a ) lands onto the seat of wiper plug  19   a , thereby closing a bore of the wiper plug. Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   a ,  20  until the wiper plug  19   a  is released from the LDA plug release system. 
     Referring specifically to  FIG. 3C , once released, the combined dart and plug  19   a ,  20  may be driven through the liner bore by the displacement fluid  110 , thereby driving cement slurry  109  through the float shoe  15   f  and into the annulus  8   a . Pumping of the displacement fluid  110  may continue and the combined dart and plug  19   a ,  20  may land on the shoulder  53   u  in the first fracture valve  50   a , thereby closing a bore of the collar  53 . Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   a ,  20  until the seat  60   a  is released from the dart  20  by fracturing the set  62   a  of shear screws. 
     Referring specifically to  FIG. 3D , release of the seat  60   a  may free the rest of the dart  20  from the combined wiper plug and seat  19   a ,  60   a  and continued pumping of the displacement fluid  110  may force the fin stack  20   c,f  into the first wiper plug bore until the rest of the dart (specifically seat  60   b ) lands onto the seat of the wiper plug  19   b . Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   b ,  20  until the wiper plug  19   b  is released from the collar  53  by fracturing the set  57   w  of shear screws. 
     Referring specifically to  FIG. 3E , once released, the fin stack  20   c,f  may be driven through the collar bore and the combined dart and plug  19   b ,  20  may be driven through the first fracture valve bore by continued pumping of the displacement fluid  110 , thereby ensuring the first fracture valve bore is free from residual cement slurry that may otherwise cause malfunction of the first fracture valve  50   a . Travel of the combined dart and plug  19   b ,  20  may also continue to drive cement slurry  109  through the float shoe  15   f  and into the annulus  8   a . Pumping of the displacement fluid  110  may continue and the combined dart and plug  19   b ,  20  may land on the shoulder (see shoulder  53   u ) in the second fracture valve  50   b , thereby closing a bore of the collar (see collar  53 ). Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   b ,  20  until the seat  60   b  is released from the dart  20  by fracturing the set  62   b  of shear screws. 
     Referring specifically to  FIG. 3F , release of the seat  60   b  may free the rest of the dart  20  from the combined wiper plug and seat  19   b ,  60   b  and continued pumping of the displacement fluid  110  may force the fin stack  20   c,f  into the second wiper plug bore until the rest of the dart (specifically seat  60   c ) lands onto the seat of the wiper plug  19   c . Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   c ,  20  until the wiper plug  19   c  is released from the collar (see collar  53 ) by fracturing the set (see set  57   w ) of shear screws. 
     Referring specifically to  FIG. 3G , once released, the fin stack  20   c,f  may be driven through the collar bore and the combined dart and plug  19   c ,  20  may be driven through the second fracture valve bore by continued pumping of the displacement fluid  110 , thereby ensuring the second fracture valve bore is free from residual cement slurry that may otherwise cause malfunction of the second fracture valve  50   b . Travel of the combined dart and plug  19   c ,  20  may also continue to drive cement slurry  109  through the float shoe  15   f  and into the annulus  8   a . Pumping of the displacement fluid  110  may continue and the combined dart and plug  19   c ,  20  may land on the shoulder (see shoulder  53   u ) in the third fracture valve  50   c , thereby closing a bore of the collar (see collar  53 ). Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   c ,  20  until the seat  60   c  is released from the dart  20  by fracturing the set  62   c  of shear screws. 
     Referring specifically to  FIG. 3H , release of the seat  60   c  may free the rest of the dart  20  from the combined wiper plug and seat  19   c ,  60   c  and continued pumping of the displacement fluid  110  may force the fin stack  20   c,f  into the third wiper plug bore until the rest of the dart (specifically retainer  60   r ) lands onto the seat of the wiper plug  19   e . As discussed above, if a fourth fracture valve (not shown) is used, the dart  20  may instead land onto a shoulder of the wiper plug  19   d . Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  19   e ,  20  until the wiper plug  19   e  is released from the collar (see collar  53 ) by fracturing the set (see set  57   w ) of shear screws. 
     Referring specifically to  FIG. 3I , once released, the fin stack  20   c,f  may be driven through the collar bore and the combined dart and plug  19   e ,  20  may be driven through the third fracture valve bore by continued pumping of the displacement fluid  110 , thereby ensuring the third fracture valve bore is free from residual cement slurry that may otherwise cause malfunction of the third fracture valve  50   c . Travel of the combined dart and plug  19   e ,  20  may also continue to drive cement slurry  109  through the float shoe  15   f  and into the annulus  8   a . Pumping of the displacement fluid  110  may continue and the combined dart and plug  19   e ,  20  may land on a shoulder of the float shoe  15   f , thereby increasing pressure in the liner  15  and workstring bore which may be detected by the PLC  18  monitoring the standpipe pressure. 
     Referring specifically to  FIG. 3J , once landing has been detected, pumping of the displacement fluid  110  and rotation  30  of the liner  15  may be halted and the packer  15   p  set hydraulically or mechanically using the LDA setting tool. The LDA  5   d  may be raised from the liner hanger  15   h  and displacement fluid  110  circulated to wash away excess cement slurry (no excess shown). Pressure in the workstring  5  and liner bore may be bled. The float valve  15   f  may close, thereby preventing the cement slurry  109  from flowing back into the liner bore. The workstring  5  may then be retrieved to the rig  1   r  and the rig dispatched from the well site. Once the workstring  5  has been retrieved, the cement slurry  109  may be allowed to cure for a predetermined period of time. 
       FIG. 4  illustrates a fracturing system  101 . The fracturing system  101  may be deployed once the rig  1   r  has been dispatched from the wellsite. The fracturing system  101  may include a fluid system  101   f  and a production tree  101   t . The production tree  101   t  may be installed on the wellhead  7   h . The production tree  101   t  may include a master valve  121   m , the flow cross  22 , and a swab valve  121   s . Each component of the production tree  101   t  may be connected together, the production tree may be connected to the wellhead and an injector head  122 , and the cap may be connected to the injector head, such as by flanges and studs or bolts and nuts. The fluid system if may include the one or more shutoff valves  17   b - d , the PLC  18 , the pit  23  (or other fluid reservoir, such as a tank), one or more sensors, such as the pressure sensors  25   c,r  and the stroke counter  26   c , one or more launchers  106   a - c , a fracture pump  116 , the injector head  122 , and a fracture fluid mixer, such as a recirculating mixer  127 . Each sensor  25   c,r ,  26   c  may be in data communication with the PLC  18 . The pressure sensor  25   r  may be connected to the head cap and may be operable to monitor wellhead pressure. The pressure sensor  25   c  may be connected between the fracture pump  116  and the valve  17   b  and may be operable to measure discharge pressure of the fracture pump  116 . The stroke counter  26   c  may be operable to measure a flow rate of the fracture pump  116 . 
     Each launcher  106   a - c  may include a housing, a plunger, and an actuator. The balls  170   a - c  may be disposed in the respective plungers for selective release and pumping downhole to activate respective fracture valves  50   a - c . The plunger may be movable relative to the housing between a capture position and a release position. The plunger may be moved between the positions by the actuator. The actuator may be hydraulic, such as a piston and cylinder assembly. Alternatively, the actuator may be electric or pneumatic. Alternatively, the actuator may be manual, such as a handwheel. In operation, the PLC  18  may release one of the balls  170   a - c  by operating the HPU to supply hydraulic fluid to the respective actuator. The actuator may then move the plunger to the release position (not shown). The carrier and ball  170   a - c  may then move into a discharge pipe connecting the fracture pump  116  to the injector head  122 . The pumped stream of fracturing fluid  111  ( FIG. 5A ) may then carry each ball  170   a - c  from the respective launcher  106   a - c  and into the wellhead  7   h  via the injector head  122  and tree  101   t.    
     The first ball  170   a  may have a diameter greater than a diameter of each successive ball  170   b - c  and corresponding to a seat diameter of the top seat  60   a  such that the first ball may engage the top seat. The successive balls  170   b - c  may each have an outer diameter less than the seat diameter of the top seat  60   a  such that the rest of the balls  170   b - c  may pass through the top seat unobstructed. The second ball  170   b  may have a diameter greater than a diameter of the third ball  170   c  and corresponding to a seat diameter of the first intermediate seat  60   b  such that the second ball may engage the first intermediate seat. The third ball  170   c  may have a diameter less than the seat diameter of the first intermediate seat  60   b  such that the third ball  170   c  may pass through the first intermediate seat. The third ball  170   c  may have a diameter corresponding to a seat diameter of the second intermediate seat  60   c  such that the third ball may engage the second intermediate seat. 
       FIGS. 5A-5E  illustrate a fracturing operation performed using the system  101 . Referring specifically to  FIG. 5A , the third ball  170   c  may be released from the launcher  106   c  by the PLC  18  operating the respective actuator and fracturing fluid  111  may be pumped from the mixer  127  into the injector head  122  via the valve  17   b  by the fracture pump  116 . As discussed above, the fracturing fluid  111  may be a slurry including: proppant (i.e., sand), water, and chemical additives. Pumping of the fracturing fluid  111  may increase pressure in the liner bore until the differential is sufficient to open the toe sleeve  15   s . Once the toe sleeve  15   s  has opened, continued pumping of the fracturing fluid  111  may force the displacement fluid  110  in the liner bore through the cured cement  109  and into the lower formation by creating a first fracture  130 . 
     Referring specifically to  FIG. 5B , continued pumping of the fracturing fluid  111  may drive the third ball  170   c  toward the third fracture valve  50   c  until a desired quantity for a third zone of the lower formation has been pumped. Once the desired quantity has been pumped, the second ball  170   b  may be released from the launcher  106   b  by the PLC  18  operating the respective actuator. Continued pumping of the fracturing fluid  111  may drive the balls  170   b,c  until the third ball lands onto the second intermediate seat  60   c , thereby closing a bore of the third fracture valve  50   c.    
     Referring specifically to  FIG. 5C , continued pumping of the fracturing fluid  111  may exert pressure on the combined ball  170   c , seat  60   c , wiper plug  19   c , collar (see collar  53 ), and sleeve (see sleeve  52 ) of the third fracture valve  50   c  until the sleeve is released from the housing (see housing  51   a ) by fracturing the shear ring (see shear ring  57   s ). Continued pumping of the fracturing fluid  111  may move the ball/seat/wiper plug/collar/sleeve combination longitudinally relative to the housing of the third fracture valve  50   c  until the sleeve is stopped by the lower shoulder (see lower shoulder  58   b ) and locked into place by the C-ring (see C-ring  61 ), thereby opening the fracture ports (see fracture ports  51   p ). Continued pumping of the fracturing fluid  111  may force the fracturing fluid (below the second ball  170   b ) through the cured cement  109  and into the third zone of the lower formation by creating a second fracture  131 . As discussed above, proppant may be deposited into the second fracture  131  by the fracturing fluid  111 . Continued pumping of the fracturing fluid  111  may also drive the second ball  170   b  toward the second fracture valve  50   b  until a desired quantity for a second zone of the lower formation has been pumped. Once the desired quantity has been pumped, the first ball  170   a  may be released from the launcher  106   a  by the PLC  18  operating the respective actuator. The fracturing fluid  111  may continue to be pumped into the third zone until the second ball  170   b  lands onto the first intermediate seat  60   b , thereby closing a bore of the second fracture valve  50   b.    
     Referring specifically to  FIG. 5D , continued pumping of the fracturing fluid  111  may exert pressure on the combined ball  170   b , seat  60   b , wiper plug  19   b , collar (see collar  53 ), and sleeve (see sleeve  52 ) of the second fracture valve  50   b  until the sleeve is released from the housing (see housing  51   a ) by fracturing the shear ring (see shear ring  57   s ). Continued pumping of the fracturing fluid  111  may move the ball/seat/wiper plug/collar/sleeve combination longitudinally relative to the housing of the second fracture valve  50   b  until the sleeve is stopped by the lower shoulder (see lower shoulder  58   b ) and locked into place by the C-ring (see C-ring  61 ), thereby opening the fracture ports (see fracture ports  51   p ). Continued pumping of the fracturing fluid  111  may force the fracturing fluid (below the first ball  170   a ) through the cured cement  109  and into the second zone of the lower formation by creating a third fracture  132 . As discussed above, proppant may be deposited into the third fracture  132  by the fracturing fluid  111 . Continued pumping of the fracturing fluid  111  may also drive the first ball  170   a  toward the first fracture valve  50   a  until a desired quantity for a first zone of the lower formation has been pumped. The fracturing fluid  111  may continue to be pumped into the second zone until the first ball  170   a  lands onto the top seat  60   a , thereby closing a bore of the first fracture valve  50   a.    
     Referring specifically to  FIG. 5E , continued pumping of the fracturing fluid  111  may exert pressure on the combined ball  170   a , seat  60   a , wiper plug  19   a , collar  53 , and sleeve  52  of the first fracture valve  50   a  until the sleeve is released from the housing  51   a  by fracturing the shear ring  57   s . Continued pumping of the fracturing fluid  111  may move the ball/seat/wiper plug/collar/sleeve combination longitudinally relative to the housing of the first fracture valve  50   a  until the sleeve is stopped by the lower shoulder  58   b  and locked into place by the C-ring  61 , thereby opening the fracture ports  51   p . Continued pumping of the fracturing fluid  111  may force the fracturing fluid through the cured cement  109  and into the first zone of the lower formation by creating a fourth fracture  133 . As discussed above, proppant may be deposited into the fourth fracture  133  by the fracturing fluid  111 . Pumping of the fracturing fluid  111  may continue until the desired quantity for the first zone has been pumped. Once the desired quantity has been pumped, displacement fluid  112  may be pumped to force the remaining fracturing fluid  111  into the first zone via the fourth fracture  133 . The displacement fluid  112  may be water, drilling mud  107 , conditioner  108 , or the displacement fluid  110 . Alternatively, fracturing fluid  111  may be used instead of the displacement fluid  112 . 
     Alternatively, depending on parameters for a specific wellbore  8   w , the balls  170   a - c  and desired quantities of fracturing fluid  111  may be pumped before the third ball  170   c  lands onto the second intermediate seat  60   c . The displacement fluid  112  may then be pumped before and during opening of the fracture valves  50   a - c.    
     Once the fracturing operation has been completed, the injector head  122  may be removed from the tree  101   t . The flow cross  22  may be connected to the pit  23  and fluid allowed to flow from the wellbore to the pit. One or more of the balls  170   a - c  may or may not be recovered. A milling system (not shown) may then be deployed. The milling system may include a coiled tubing unit and a bottomhole assembly (BHA). The CTU may include an injector, a reel of coiled tubing, and a PCA. The BHA may include a drilling motor, such as a mud motor, and one or more mill bits. The BHA may be loaded into a tool housing of the PCA and connected to the coiled tubing. The PCA and injector may be connected to the tree  101   t . The injector may be operated to lower the coiled tubing and BHA into the wellbore and the BHA operated to mill the millable portions of the fracture valves. The BHA and coiled tubing may then be retrieved and the milling system dispatched from the wellsite. A production choke may be connected to the flow cross and to a separation, treatment, and storage facility (not shown). Production of the lower formation may commence. 
       FIG. 6A  illustrates a portion of an alternative second fracture valve  150   b  usable with the liner string  15 , according to another embodiment of the present disclosure. The alternative fracture valve  150   b  may include the housing  51 , the sleeve  52 , a collar  153 , an alternative wiper plug (not shown, similar to illustrated alternative wiper plug  119   b ), and one or more sets  154   a,t  of fasteners. The fracture valve  150   b  may be identical to the fracture valve  50   b  except for the substitution of the collar  153  for the collar  53  and substitution of the alternative wiper plug for the wiper plug  19   c.    
     The collar  153  may be disposed in a bore of the sleeve  52  and connected longitudinally and torsionally thereto by the set screws  54   m . The collar  153  may be made from any of the millable/drillable materials, discussed above. The collar  153  may be annular and have a bore formed therethrough. The collar  153  may have a landing shoulder  153   u  and the mounting shoulder  53   b , each shoulder formed in an inner surface thereof. The mounting shoulder  53   b  may be mated with a top of the alternative wiper plug. The wiper plug  119   b  may have a body  119   y  and the wiper seal  19   w . The body  119   y  may be annular and have a bore formed therethrough. The body  119   y  may have a seat formed in an inner surface thereof, a mounting shoulder formed in an outer surface thereof, and a stinger portion  119   s  forming a lower end thereof. The wiper plug  119   b  may be releasably connected to a collar (not shown) of an alternative first fracture valve (not shown, identical to the fracture valve  150   b  except for having the alternative wiper plug  119   b ) and seated against the respective mounting shoulder. The releasable connection may include the set  57   w  of shear screws. 
     A set  154   a  of one or more longitudinal fasteners, such as dogs, may be connected to the collar  153  and a set  154   t  of one or more torsional fasteners, such as dogs may be connected to the collar  153 . Each dog may be radially movable between an extended position and a retracted position and may be biased toward the extended position by a spring. Each dog may have a cammed upper surface for being pushed inward to the retracted position by a cammed bottom of the stinger portion  154   s . The stinger portion  119   s  may have a first complementary profile, such as a groove  155   a , for receiving the longitudinal set  154   a  of fasteners and a second complementary profile, such as a set  155   t  of one or more slots, for receiving the torsional set  154   t  of fasteners. Since the torsional fasteners  154   t  may facilitate milling of the wiper plug  119   b , the torsional fasteners need not be engaged with the set  155   t  of slots upon landing but may engage in response to contact of a mill bit (not shown) with the wiper plug  119   b . A set  156  of one or more longitudinal fasteners, such as dogs, may be connected to the plug body  119   y  for receiving an alternative dart (only seat  160   b  shown). The set  156  may be similar to the collar set  154   a . The seat  160   b  may be identical to the seat  60   b  except for the addition of a shoulder  161  for receiving the longitudinal set  156  of fasteners. 
     Alternatively, the collar  153  may have a set of threaded dogs (not shown) instead of the sets  154   a,t  of fasteners and the stinger portion  119   s  may have a threaded outer surface instead of the profiles  155   a,t . Each dog may have a portion of a thread complementing the stinger portion thread. Each thread/thread portion may be a ratchet thread allowing longitudinal movement of the wiper plug  119   b  toward the collar landing shoulder  153   u  and preventing longitudinal movement of the wiper plug away from the collar landing shoulder. The ratchet thread/thread portions may also torsionally connect the collar  153  and the wiper plug  119   b . Alternatively, a C-ring may be used instead of the set  154   a  and the set  156  of fasteners. 
     Alternatively, a C-ring may be used instead of the set  156  of threaded dogs to longitudinally connect the seat  160   b  to the plug body  119   y . Alternatively, the plug body  119   y  may include an additional set of torsional fasteners and the seat  160   b  may have a mating torsional profile or the plug body may have the threaded dogs and the seat may have a complementary thread. 
     Additionally, the float shoe  15   f  may include any of the sets of longitudinal and/or torsional fasteners and the alternative dart may have complementary profile(s). Connection of the dart to the float shoe may obviate need for the check valve so that the check valve may be omitted from the float shoe. 
       FIG. 6B  illustrates an alternative dart  120  usable with the liner string  15 , according to another embodiment of the present disclosure. The dart  120  may include the mandrel  20   m , the fin stack  20   c,f , and a seat stack  180 . The seat stack  180  may include one or more (three shown) seats  180   a - c  and a retainer  180   r . Instead of the seats  180   a - c  being releasably connected to each other as for the dart  20 , each seat  180   a - c  may be separately connected to the retainer  180   r  by a respective set  182   a - c  of one or more (two shown) shearable fasteners. A shear strength of each set  182   a - c  of shearable fasteners may be greater or substantially greater than a shear strength of each set  57   w  of shearable fasteners. A shear strength of the shear ring  57   s  may be greater or substantially greater than the shear strength of each set  182   a - c  of shearable fasteners and may be greater or substantially greater than the shear strength of each set  57   w  of shearable fasteners. A shear strength of each set  182   a - c  of shearable fasteners may be the same or different relative to one another. 
     Each seat  180   a - c  may have an outer seating surface for engagement with a seat of the respective wiper plug  19   a - c  and an inner seating surface for receiving the respective ball  170   a - c . The top seat  180   a  may have an outer diameter greater than an outer diameter of each successive seat  180   b - c  (and the retainer  180   r ) and corresponding to the seat diameter  65   a  such that the top seat may engage the seat of the wiper plug  19   a . The successive seats  180   b - c  (and the retainer  180   r ) may each have an outer diameter less than the seat diameter  65   a  such that the rest of the seats  180   b - c  may pass through the wiper plug seat unobstructed. The intermediate seat  180   b  may have an outer diameter greater than an outer diameter of a bottom seat  180   c  (and the retainer  180   r ) and corresponding to the seat diameter  65   b  such that the intermediate seat may engage the seat of the wiper plug  19   b . The bottom seat  180   c  (and the retainer  60   r ) may each have an outer diameter less than the seat diameter  65   b  such that the rest of the bottom seats  180   c  may pass through the wiper plug seat unobstructed. The bottom seat  180   c  may have an outer diameter greater than an outer diameter of the retainer  180   r  and corresponding to the seat diameter  65   c  such that the bottom seat may engage the seat of the wiper plug  19   c . The retainer  180   r  may have an outer diameter less than the seat diameter  65   c  such that the retainer  180   r  may pass through the wiper plug seat unobstructed. The retainer  180   r  may have an outer seating surface and a threaded inner surface and the outer surface of the mandrel  20   m  may have a lower shouldered thread for receiving the retainer  20   r.    
       FIGS. 7A-7E  illustrate a cluster fracture valve  250  and dart  220  (and operation thereof) usable with the liner string  15 , according to another embodiment of the present disclosure. The cluster valve  250  may include the housing  51 , the sleeve  52 , the collar  53 , and a wiper plug  219   c , and one or more (two shown) buttons  251 . A cluster of one or more (two at least partially shown) of the cluster valves  250  and the fracture valve  50   c  may be assembled with the liner string  15  instead of the valves  50   a - c . The fracture valve  50   c  may be located at the bottom of the cluster. Each valve  250  in the cluster may be identical except that the cluster valve (not shown) adjacent the fracture valve  50   c  may have a slightly modified cluster wiper plug (not shown). An additional cluster wiper plug (not shown) may be slightly modified for connection to the LDA plug release system, as discussed above for the wiper plug  19   a . Alternatively, each cluster valve  250  and/or the dart  220  may be modified to include any of the sets of longitudinal and/or torsional fasteners, discussed above for the fracture valve  150   b.    
     Each button  251  may be disposed in a respective port  51   p  and connected to the housing  51 , such as by a threaded connection. A series of small orifices may be formed through each button  251  and may allow leakage through the ports  51   p  when the sleeve  52  is in the open position. Each button  251  may be made from an erosion-prone material, such as aluminum, polymer, or brass. The orifices may be arranged in a peripheral cross-pattern around the button&#39;s center and joined slots may be formed in the inner surface of each button and may extend through the peripheral orifices and the center of each button  251 . A hex-shaped orifice may be formed at the center of each button  251  for screwing each button  251  into the respective housing port  51   p . Once the sleeve  52  has moved to the open position ( FIG. 7D ), the leakage through the button orifices may be small enough to not compromise differential pressure between the housing bore and the annulus  8   a  until the bottom valve of the cluster has been opened. As fracturing fluid  111  leaks through the orifices, rapid erosion may be encouraged by the pattern of the orifices and the slots. 
     The fracture valve  50   c  may or may not have the buttons  251 . Alternatively, the buttons  251  may be omitted in favor of relying on the cured cement  109  to limit flow of fracturing fluid through the open ports  51   p  until the bottom valve of the cluster has been opened. Alternatively rupture disks may be used instead of the buttons  251 . 
     Each of the wiper plugs  219   b,c  may include a body  219   y , the wiper seal  19   w , a seat  265   a,b , and one or more sleeves, such as an inner sleeve  218   i  and an outer sleeve  218   o . The body  219   y  may be annular and have a bore formed therethrough. The body  219   y  may have a mounting shoulder formed in an outer surface thereof and a stinger portion  219   s  forming a lower end thereof. The wiper plug  219   c  may be releasably connected to the collar  53  and the wiper plug  219   b  may be releasably connected to a collar (not shown) of another identical cluster valve (not shown) and seated against the respective mounting shoulder. Each releasable connection may include the set  57   w  of shear screws. The body  219   y , sleeves  218   i,o , and seat  265   a,b  may each be made of one of the millable/drillable materials, discussed above. The seat  265   a,b  may include a plurality of dogs, such as a first dog  265   a  and a second dog  265   b . Each dog  265   a,b  may have a stem portion and a tab portion and may be movable between an extended position ( FIG. 7A ), a first retracted position ( FIG. 7B ) and a second retracted position ( FIG. 7E ). Each dog  265   a,b  may be received by a respective opening formed through a wall of the inner sleeve  218   i . Each opening may include a through portion for receiving a respective dog stem portion and a recess portion for engaging the respective tab portion. 
     The outer sleeve  219   o  may have slots  217   i  formed through a wall thereof for receiving an outer portion of the respective dog  265   a,b . The body  219   y , such as at the stinger portion  219   s , may have slots  217   o  formed in an inner surface thereof also for receiving an outer portion of the respective dog  265   a,b . Each sleeve may  218   i,o  may be longitudinally movable relative to the body subject to interaction with the seat  265   a,b , an upper shoulder formed in an inner surface of the body, and a lower shoulder formed by a fastener, such as C-ring. The inner sleeve-outer sleeve interface and the outer sleeve-body interface may each be sealed, such as by respective seals carried by the sleeves. The seals may each be single element or seal stacks, as discussed above. The outer sleeve  219   o  may be releasably connected to the body  219   y  in an upper position by a set  257   o  of one or more shearable fasteners, such as shear screws. The inner sleeve  219   i  may be releasably connected to the outer sleeve  219   o  in an upper position by a set  257   i  of one or more shearable fasteners, such as shear screws. To maintain alignment of the dogs  265   a,b  and slots  217   i,o , the sleeves  218   i,o  may be torsionally connected and the outer sleeve and the body  219   y  may be torsionally connected, such as by pin-slot connections (not shown). 
     A shear strength of each outer set  257   o  of shearable fasteners may be greater or substantially greater than a shear strength of the shear ring  57   s , may be greater or substantially greater than the shear strength of each inner set  257   i  of shearable fasteners, and may be greater or substantially greater than the shear strength of each set  57   w  of shearable fasteners. A shear strength of the shear ring  57   s  may be greater or substantially greater than the shear strength of each inner set  257   i  of shearable fasteners and may be greater or substantially greater than the shear strength of each set  57   w  of shearable fasteners. A shear strength of each inner set  257   i  of shearable fasteners and may be greater or substantially greater than the shear strength of each set  57   w  of shearable fasteners. 
     The dart  220  may include the mandrel  20   m , the fin stack  20   c,f , and an actuator, such as a bung  260 . The bung  260  may have an outer seating surface and a threaded inner surface for connection to the mandrel  20   m.    
     In operation, the dart  220  may be driven through the workstring bore by pumping of the displacement fluid  110  until the dart (specifically seat bung  260 ) lands onto the seat of the LDA (first) cluster wiper plug, thereby closing a bore of the first cluster plug. Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  220  until the first wiper plug is released from the LDA plug release system. Once released, the combined dart and plug  220  may be driven through the liner bore by the displacement fluid  110 , thereby driving cement slurry  109  through the float shoe  15   f  and into the annulus  8   a . Pumping of the displacement fluid  110  may continue and the combined dart and plug  220  may land on the shoulder (see  53   u ) in the first cluster valve (see  250 ), thereby closing a bore of the collar  53 . 
     Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  220  until the dart  220  is released from the LDA wiper plug by operation of the seat (see  265   a,b ) to the first retracted position. Continued pumping of the displacement fluid  110  may force the fin stack  20   c,f  into the first wiper plug bore until the dart  220  (specifically bung  260 ) lands onto the seat  265   a,b  of the second cluster wiper plug  219   b . Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  219   b ,  220  until the wiper plug  219   b  is released from the collar (see collar  53 ) by fracturing the set  57   w  of shear screws. Once released, the fin stack  20   c,f  may be driven through the collar bore and the combined dart and plug  219   b ,  220  may be driven through the first fracture valve bore by continued pumping of the displacement fluid  110 , thereby ensuring the first fracture valve bore is free from residual cement slurry that may otherwise cause malfunction of the first fracture valve. 
     Referring specifically to  FIG. 7A , travel of the combined dart and plug  219   b ,  220  may also continue to drive cement slurry  109  through the float shoe  15   f  and into the annulus  8   a . Pumping of the displacement fluid  110  may continue and the combined dart and plug  219   b ,  220  may land on the shoulder  53   u  in the second fracture valve  250 , thereby closing a bore of the collar  53 . 
     Referring specifically to  FIG. 7B , continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  219   b ,  220  until the inner sleeve  218   i  is released from the outer sleeve  218   o  by fracturing the inner set  257   i  of shear screws. Continued pumping of displacement fluid  110  may drive the combined dart and inner sleeve  218   i ,  220  downward relative to the second plug body  219   y  until the seat  265   a,b  aligns with the inner slot  217   i . The bung  260  may then push the seat  265   a,b  into the inner slot  217   i , thereby moving the seat to the first retracted position and releasing the dart  220  from the second wiper plug  219   b . Continued pumping of the displacement fluid  110  may force the fin stack  20   c,f  into the second wiper plug bore until the dart  220  (specifically bung  260 ) lands onto the seat  265   a,b  of the third wiper plug  219   c.    
     Continued pumping of the displacement fluid  110  may exert pressure on the combined dart and wiper plug  219   c ,  220  until the wiper plug  219   c  is released from the collar  53  by fracturing the set  57   w  of shear screws. Once released, the fin stack  20   c,f  may be driven through the collar bore and the combined dart and plug  219   c ,  220  may be driven through the second cluster valve bore by continued pumping of the displacement fluid  110 , thereby ensuring the second cluster valve bore is free from residual cement slurry that may otherwise cause malfunction of the second cluster valve. The cementing operation may continue until the dart  220  has traveled through the rest of the cluster valves  250  and lands onto the fracture valve  50   c  and releases the wiper plug  19   e  therefrom and the combined dart and wiper plug  19   e ,  220  land in the float shoe  15   f.    
     Referring specifically to  FIG. 7C , once the cement slurry  109  has cured, the ball  270  may be released from one of the launchers  106   a - c  by the PLC  18  operating the respective actuator and fracturing fluid  111  may be pumped from the mixer  127  into the injector head  122  via the valve  17   b  by the fracture pump  116 . Pumping of the fracturing fluid  111  may increase pressure in the liner bore until the differential is sufficient to open the toe sleeve  15   s . Once the toe sleeve  15   s  has opened, continued pumping of the fracturing fluid  111  may force the displacement fluid  110  in the liner bore through the cured cement  109  and into the lower formation by creating the first fracture  130 . Continued pumping of the fracturing fluid  111  may drive the ball  270  until the ball lands onto the seat of the first wiper plug, thereby closing a bore of the first fracture valve. Continued pumping of the fracturing fluid  111  may exert pressure on the combined ball/seat/wiper plug/collar/sleeve until first fracture valve opens and the ball  270  is released by moving the seat to the second retracted position. Even though the sleeve has moved to the open position, the ports may still be choked by the buttons  251 . Continued pumping of the fracturing fluid  111  may drive the ball  270  until the ball lands onto the seat of the second wiper plug  219   b , thereby closing a bore of the second fracture valve  50   b.    
     Referring specifically to  FIG. 7D , continued pumping of the fracturing fluid  111  may exert pressure on the combined ball  270 , seat  265   a,b , wiper plug  219   b , collar  53 , and sleeve  52  of the second fracture valve  250  until the sleeve is released from the housing  51   a  by fracturing the shear ring  57   s . Continued pumping of the fracturing fluid  111  may move the ball/seat/wiper plug/collar/sleeve combination longitudinally relative to the housing of the second fracture valve  50   b  until the sleeve is stopped by the lower shoulder (see lower shoulder  58   b ) and locked into place by the C-ring  61 , thereby opening (choked by buttons  251 ) the fracture ports  51   p.    
     Referring specifically to  FIG. 7E , continued pumping of the fracturing fluid  111  may exert pressure on the combined dart and wiper plug  219   b ,  220  until the outer sleeve  218   o  is released from the plug body  219   y  by fracturing the outer set  257   o  of shear screws. Continued pumping of the fracturing fluid  111  may drive the combined dart and inner sleeves  218   i,o ,  220  downward relative to the second plug body  219   y  until the seat  265   a,b  aligns with the outer slot  217   o . The ball  270  may then push the seat  265   a,b  into the outer slot  217   o , thereby moving the seat to the second retracted position and releasing the ball  270  from the second wiper plug  219   b . The fracturing operation may continue until all the ball  270  has traveled through to the fracture valve  50   c  (having the modified cluster wiper plug seated therein) and lands onto the seat of the modified cluster wiper plug. The modified cluster wiper plug may be similar to the other wiper plugs  219   b,c  except for not having a second retracted position, thereby catching but not releasing the ball  270 . Once the ball  270  is caught, continued pumping of the fracturing fluid  111  may quickly erode the buttons  251  so that the fracturing fluid may flow freely through the fracturing ports and create the fractures  131 - 133 . 
     Additionally, a second (or more) cluster (not shown) having one or more cluster valves may be added to the liner string  15 . The second cluster may include one or more cluster valves and the fracture valve having the wiper plug  19   d  located at the bottom of the second cluster, each cluster valve identical to the cluster valve  250  except for having different cluster wiper plugs. The second cluster wiper plugs may each be similar to the wiper plugs  219   b,c  except for having a larger seat size. The dart  20  (having only the seat  60   d  and retainer  60   r ) may be used with the dual cluster system. The two (or more) clusters may be arranged in series with the second (larger seat size) cluster located above the first (smaller seat size) cluster. The dart  20  may be launched after the cement slurry is pumped and may be propelled by the displacement fluid  110  to the LDA cluster plug. The dart may travel through the workstring and launch the LDA cluster plug (second cluster seat size). The combined dart and LDA wiper plug  20  may land in the second cluster valve and launch the second cluster wiper plug as discussed above. The combined dart and second cluster wiper plug  20  may land in the fracture valve (having the wiper plug  19   d ) and launch the wiper plug  19   d . The combined dart and wiper plug  19   d  may land in a top of the first cluster valves  250 . The dart  20  may release the seat  60   d  in the wiper plug  19   d  and launch the top first cluster wiper plug  219   b  using the retainer  60   r . The dart  20  and top first cluster wiper plug  19   b  may then land in the next first cluster valve  250  and launch the next first cluster wiper plug  219   c . The cementing process may conclude as discussed above. For the fracturing operation, the ball  270  may be launched to operate the first cluster valves (minus the top first cluster valve) and then a second larger ball (not shown) may be launched to operate the second cluster valves (plus the top first cluster valve). 
     Alternatively, each seat  265   a,b  may have a C-ring instead of the dogs  265   a,b . Alternatively, the wiper plugs  219   b,c  may each have a resettable seat, such as a collet and spring, instead of the seat  265   a,b  and sleeves  218   i,o . Alternatively, the dart  220  may have a retractable actuator, such as a C-ring, and the ball  270  may be deformable instead of the wiper plugs  219   b,c  having the retractable seats  265   a,b.    
     Alternatively, any of the fracture valves, wiper plugs, and/or darts may be used in other types of stimulation operations besides fracturing. Alternatively, any of the fracture valves, wiper plugs, and/or darts may be used in a staged cementing operation of a casing or liner string instead of a cementing and fracturing operation. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the invention is determined by the claims that follow.