CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patent application No. 63/185,519 filed May 7, 2021 , and entitled “Cluster Stimulation System with an Intelligent Dart”, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND

[0003] In some applications, following the drilling of a wellbore through an earthen subterranean formation, the formation may be stimulated to enhance fluid connectivity between the wellbore and the formation. In this manner, the amount of hydrocarbons produced from the formation via the wellbore may be maximized. In some applications, the formation may be stimulated by pumping a fracturing fluid through the wellbore and against a wall of the wellbore to thereby form one or more fractures within the formation which may later act as conduits for the communication of hydrocarbons from the formation to the wellbore. As an example, the fracturing fluid pumped through the wellbore may be directed through ports formed in sliding sleeve valves connected to a casing string which lines the wall of the wellbore and installed during the drilling of the wellbore. The sliding sleeve valves may control fluid communication between the wellbore and the formation during the process of stimulating or hydraulically fracturing the formation. In some instances, a flow-transportable ball or dart may be pumped through the wellbore to thereby actuate one or more sliding sleeve valves positioned in the wellbore between a run-in or closed configuration and an open configuration.

SUMMARY

[0004] An embodiment of a flow-transportable dart for actuating a sliding sleeve valve of a system for stimulating an earthen formation comprises a mandrel comprising a first end, a second end opposite the first end, and a central passage extending between the first end and the second end, a piston slidably disposed in the central passage of the mandrel, a landing ring positioned about the piston and in engagement with the second end of the mandrel, wherein the dart comprises a disarmed configuration in which the landing ring is positioned in a radially retracted position and an armed configuration in which the landing ring is locked in a radially expanded position, wherein the mandrel is configured to axially displace the landing ring between the retracted position and the expanded position in response to engagement between the landing ring and the second end of the mandrel. In some embodiments, the dart further comprises a housing comprising a central passage in which the piston is disposed and a locking arm located at an end of the housing, wherein the locking arm of the housing is received in a slot formed in the first end of the mandrel when the dart is in the armed configuration. In some embodiments, the dart further comprises a ring gear rotatably coupled to the piston and comprising a radial slot formed in an outer surface of the ring gear, wherein the locking arm of the housing is received in the radial slot formed in ring gear when the dart is in the armed configuration. In certain embodiments, the dart further comprises a housing comprising a central passage in which the piston is disposed and a locking arm located at an end of the housing, a pin coupled to the locking of the arm of the housing and received in an indexing groove of an indexer of the piston, wherein the guide pin rotatably couples the piston to the housing. In certain embodiments, the guide pin extends through a radial slot formed in an outer surface of the mandrel preventing relative rotation between the mandrel and the housing while permitting relative axial movement between the housing mandrel and the housing. In some embodiments, the dart further comprises a housing comprising a central passage in which the piston is disposed and an outer surface comprising a first engagement surface having a first outer diameter and a second engagement surface having a second outer diameter greater than the first outer diameter, wherein the landing ring is supported on the second engagement surface when the dart is in the armed configuration. In some embodiments, the dart further comprises a gear train rotatably coupled between the piston and the mandrel, wherein the dart is configured to actuate from the disarmed configuration to the armed configuration in response to the piston rotating about a central axis of the dart by a predefined angular degree of rotation, and wherein the predefined angular degree of rotation is based on a gearratio of the gear train. In certain embodiments, the gear train comprises a sun gear formed on an outer surface of the piston, a planet gear rotatably coupled to the mandrel and enmeshed with the sun gear, and a ring gear positioned about the sun gear and enmeshed with the planet gear. [0005] An embodiment of a flow-transportable dart for actuating a sliding sleeve valve of a system for stimulating an earthen formation comprises a mandrel comprising a first end, a second end opposite the first end, and a central passage extending between the first end and the second end, a piston rotatably disposed in the central passage of the mandrel, a landing profile positioned about the piston, a gear train rotatably coupled between the piston and the mandrel, wherein the dart comprises a disarmed configuration in which the landing profile is positioned in a radially retracted position and an armed configuration in which the landing profile is locked in a radially expanded position, wherein the dart is configured to actuate from the disarmed configuration to the armed configuration in response to the piston rotating about a central axis of the dart by a predefined angular degree of rotation, and wherein the predefined angular degree of rotation is based on a gear ratio of the gear train. In some embodiments, the gear train comprises a sun gear formed on an outer surface of the piston, a planet gear rotatably coupled to the mandrel and enmeshed with the sun gear, and a ring gear positioned about the sun gear and enmeshed with the planet gear. In some embodiments, the ring gear is permitted to rotate relative to the mandrel. In certain embodiments, the dart further comprises a housing comprising a central passage in which the piston is disposed, a pin coupled to the housing and received in an indexing groove of an indexer positioned on an outer surface of the piston, wherein the guide pin rotatably couples the piston to the housing. In certain embodiments, the indexing groove comprises an opening in which the guide pin is received when the dart is in the armed configuration. In certain embodiments, the indexer is configured to rotate the piston and the gear train in response to relative axial movement between the housing and the piston. In some embodiments, the landing profile comprises a landing ring and the mandrel is configured to axially displace the landing ring between the retracted position and the expanded position in response to engagement between the landing ring and the second end of the mandrel.

[0006] An embodiment of a system for stimulating an earthen formation comprises a casing string comprising a plurality of sliding sleeve valves and a plurality of landing profiles, wherein each of the plurality of sliding sleeve valves comprises a closed configuration and an open configuration, a flow-transportable dart comprising a disarmed configuration configured to allow the dart to be pumped through the plurality of sliding sleeve valves without actuating the plurality of sliding sleeve valves from the closed configuration to the open configuration, and an armed configuration configured to actuate each of the plurality of sliding sleeve valves through which the dart passes, wherein the dart is configured to register each landing profile through which the dart passes, wherein the dart is programmed with a predefined count in which the dart is configured to actuate from the disarmed configuration to the armed configuration in response to the dart passing through a number of the plurality of landing profiles equal to the predefined count, and wherein the predefined count is based on a gear ratio of a gear train of the dart. In some embodiments, the dart comprises a mandrel comprising a first end, a second end opposite the first end, and a central passage extending between the first end and the second end, a piston rotatably disposed in the central passage of the mandrel, a landing profile positioned about the piston, wherein the gear train is rotatably coupled between the piston and the mandrel, wherein the landing profile is positioned in a radially retracted position when the dart is in the disarmed configuration, and wherein the landing profile is locked in a radially expanded position when the dart is in the armed configuration. In some embodiments, the landing profile comprises a landing ring and the mandrel is configured to axially displace the landing ring between the retracted position and the expanded position in response to engagement between the landing ring and the second end of the mandrel. In certain embodiments, the dart comprises a housing comprising a central passage in which the piston is disposed, a pin coupled to the housing and received in an indexing groove of an indexer positioned on an outer surface of the piston, wherein the guide pin rotatably couples the piston to the housing. In certain embodiments, the indexer is configured to rotate the piston and the gear train in response to relative axial movement between the housing and the piston. In some embodiments, the indexing groove comprises an opening in which the guide pin is received when the dart is in the armed configuration. In some embodiments, the system further comprises a shifting tool deployable from an uphole end of the casing string and configured to engage a shifting profile of at least one of the sliding sleeve valves to actuate the sliding sleeve valve between the open configuration and the closed configuration an unlimited number of times.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] For a detailed description of disclosed exemplary embodiments, reference will now be made to the accompanying drawings in which:

[0008] Figures 1-3 are schematic views of a system for stimulating an earthen subterranean formation according to some embodiments; [0009] Figure 4 is a side view of a sliding sleeve valve of the stimulation system of Figures 1-3 according to some embodiments;

[0010] Figure 5 is a side cross-sectional view of the sliding sleeve valve of Figure 4 in a closed configuration;

[0011] Figure 5 is a zoomed-in, side cross-sectional views of the sliding sleeve valve of Figure 4 in the closed configuration;

[0012] Figure 7 is a side view of a sliding sleeve assembly of the sliding sleeve valve of Figure 4;

[0013] Figure 8 is a side cross-sectional view of the sliding sleeve valve of Figure 4 in an intermediate configuration according to some embodiments;

[0014] Figure 9 is a zoomed-in, side cross-sectional views of the sliding sleeve valve of Figure 4 in the intermediate configuration;

[0015] Figure 10 is a side cross-sectional view of the sliding sleeve valve of Figure 4 in an open configuration according to some embodiments;

[0016] Figure 11 is a zoomed-in, side cross-sectional views of the sliding sleeve valve of Figure 4 in the open configuration;

[0017] Figure 12 is a counting sub of the stimulation system of Figures 1-3 according to some embodiments;

[0018] Figure 13 is a perspective view of a dart for actuating the sliding sleeve valve of Figure 4 according to some embodiments;

[0019] Figure 14 is a side view of the dart of Figure 13;

[0020] Figure 15 is an exploded view of the dart of Figure 13;

[0021] Figure 16 is a side cross-sectional view of the dart of Figure 13;

[0022] Figure 17 is a cross-sectional view along lines 17-17 of Figure 14;

[0023] Figure 18 is a cross-sectional view along lines 18-18 of Figure 14;

[0024] Figure 19 is a perspective view of a housing of the dart of Figure 13 according to some embodiments;

[0025] Figure 20 is a front view of the housing of Figure 19;

[0026] Figure 21 is a gear view of a mandrel of the dart of Figure 13 according to some embodiments;

[0027] Figure 22 is a front view of the mandrel of Figure 21 ;

[0028] Figure 23 is a side cross-sectional view of the mandrel of Figure 21 ;

[0029] Figure 24 is a side view of a piston of the dart of Figure 13 according to some embodiments; and [0030] Figures 25-28 are side cross-sectional views of an exemplary process of actuating the sliding sleeve valve of Figure 4 using the dart of Figure 13 according to some embodiments.

DETAILED DESCRIPTION

[0031] The following discussion is directed to various embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

[0032] In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to..." Also, the term "couple" or "couples" is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection as accomplished via other devices, components, and connections. In addition, as used herein, the terms "axial" and "axially" generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms "radial" and "radially" generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. Any reference to up or down in the description and the claims is made for purposes of clarity, with “up”, “upper”, “upwardly”, “uphole”, or “upstream” meaning toward the surface of the borehole and with “down”, “lower”, “downwardly”, “downhole”, or “downstream” meaning toward the terminal end of the borehole, regardless of the borehole orientation.

[0033] As described above, a subterranean earthen formation may be stimulated to improved fluid conductivity between the formation and a wellbore extending therethrough prior to the production of hydrocarbons from the formation via the wellbore so as to maximize hydrocarbon production. In some applications, the formation may be stimulated by directing a hydraulic fracturing fluid through one or more sliding sleeve valves and into the formation to form one or more fractures therein. The fractures formed by the fracturing fluid may later act as conduits for hydrocarbons produced from the formation via the wellbore.

[0034] In some applications, the formation may be stimulated or fractured in a discrete number of stages, where each stage is associated with a particular region or zone of the formation. Each stage may also be associated with a plurality of separate sliding sleeve valves positioned in the wellbore. For example, a first stage of a hydraulic fracturing operation may be associated with a first plurality of sliding sleeve valves and a first zone of the formation while a second stage may be associated with a second plurality of sliding sleeve valves which are separate and spaced within the wellbore from the first plurality of sliding sleeve valves and a second zone which is distinct from the first zone.

[0035] During the first stage of the fracturing operation each sliding sleeve valve of the first plurality may be actuated from a run-in or closed position to an open position in which one or more ports of each sliding sleeve of the first plurality is exposed to the wellbore. The first plurality of sliding sleeve valves may be actuated from the closed position to the open position by a flow-transportable first ball or dart which is pumped through the wellbore and sequentially landed against a sliding sleeve of each of the first plurality of sliding sleeve valves. The sliding sleeves of each except for the final sliding sleeve valve of the first plurality may have a flexible seat to allow the first ball to pass through the flexible seat after actuating the sliding sleeve valve. The final sliding sleeve valve may include a rigid seat such that the first ball cannot pass through the final sliding sleeve valve of the first plurality and instead remains trapped against the rigid seat thereof.

[0036] Following the actuation of each sliding sleeve valve of the first plurality into the open position, fracturing fluid may then be pumped through the wellbore and into the formation via the ports of the first plurality of sliding sleeve valves (the second plurality of sliding sleeve valves remaining in the closed position) to fracture the first zone of the formation. The simultaneous fracturing of the formation at a plurality of discrete stimulation or fracturing points (the first plurality of sliding sleeve valves in this example) may be referred to as “cluster” fracturing and may provide enhanced fluid conductivity between the zone of the formation being fractured and the wellbore relative to fracturing at a single fracturing point via a single open sliding sleeve valve. [0037] Following the fracturing of the first zone a second ball may be pumped through the wellbore to actuate the second plurality of sliding sleeve valves into the open position. The second ball may remain trapped by a rigid seat of a final sliding sleeve valve of the second plurality so as to seal off the portion of the wellbore which includes the first plurality of sliding sleeve valves (positioned downhole from the second plurality of sliding sleeve valves). The second zone of the formation may then be fractured by pumping fracturing fluid through the ports of the second plurality of sliding sleeve valves as into the second zone of the formation.

[0038] In conventional fracturing operations, in order to allow the first ball to pass through the second plurality of sliding sleeve valves (positioned uphole from the first plurality) without actuating the second sliding sleeve valves, the first ball may have a smaller diameter than the second ball. The seats of the first plurality of sliding sleeve valves may correspondingly have a smaller diameter than the seats of the second plurality of sliding sleeve valves. Given that each stage of a conventional fracturing operation requires the use of a different diameter ball/seat, the maximum number of stages which may be used in a conventional fracturing operation may be undesirably limited. For example, a large number of stages will require the use of a first ball (for the first plurality of sliding sleeves) having an impractically small diameter. Additionally, the requirement of using sliding sleeve valves having relatively small seats (to allow the ball to pass through sliding sleeve valves positioned uphole from the valves to be actuated) may form a flow restriction in the wellbore during production which may reduce the amount of hydrocarbons which may be produced from the formation via the wellbore.

[0039] In some application a dart may be employed for actuating one or more sliding sleeve valves as part of a hydraulic fracturing operation. The dart may comprise a flexible collet beam and a counting mechanism configured to arm the dart and thereby lock the collet beam of the dart after the dart has passed through a predetermined number of sliding sleeve valves (or other downhole features registered by the counting mechanism). After the collet beam of the dart has been locked the dart may actuate the subsequent sliding sleeve valve entered by the dart from a closed configuration to an open configuration. However, conventional counting darts may not be employable in cluster frack operations as the counting dart, in at least applications, may only actuate the next single subsequent sliding sleeve valve following the arming of the conventional counting dart. Additionally, the use of a lockable collet beam may extend the axial length of the counting dart relative to balls and other obturating members, making the counting dart unsuitable for some applications in which the ball or dart must be passed through conduits having a relatively tight curvature. For example, hydraulic fracturing operations performed offshore may require the ball or dart to be conveyed through a curving fluid conduit extending between a surface vessel or platform and a seabed. A ball or dart having a relatively large axial length, such as a conventional counting dart comprising a collet beam, may be unsuitable for transport through the curving fluid conduit due to, for example, excessive friction between the counting dart and the curving fluid conduit.

[0040] Embodiments disclosed herein include flow-transportable darts for actuating sliding sleeve valves of stimulation or hydraulic fracturing systems. Embodiments od darts described herein are programmable to selectably actuate a desired sliding sleeve valve based on a count programmed into the dart prior to the deployment of the dart into a wellbore. The count may be assigned or programmed into the dart by tailoring a gear ratio of a gear train of the dart upon which the count is based. The count of the dart may correspond to the number of times the dart must be cycled before actuating from a run-in or disarmed configuration into an armed configuration in which the dart is configured to actuate one or more sliding sleeves from a closed configuration to an open configuration. The cycling of the dart may in-turn correspond to the displacement of the dart through a counting sub of a casing string comprising a plurality of sliding sleeve valves. A wide range of gear ratios may be obtained from the gear train, allowing the dart to be cycled tens or hundreds of times before actuating into the armed configuration. This may allow tens or hundreds of distinct production zones to be selectably stimulated or fractured using said darts described herein. Additionally, darts described herein may actuate one or more sliding sleeve valves using a landing profile or ring rather than a relatively lengthy collet beam, thereby minimizing the overall axial length of the dart.

[0041] Referring to Figures 1-3, a system 10 for stimulating an earthen subterranean formation 4 though which a wellbore 20 extends is shown. In this exemplary embodiment, wellbore 20 extends through a plurality of production zones 5A, 5B of subterranean formation 4. In some embodiments, system 10 comprises a system for hydraulic fracturing subterranean formation 4 and thus may also be referred to herein as hydraulic fracturing system 10. System 10 includes a tubular or casing string 12 comprising a plurality of casing joints and which extends along a wall 22 of wellbore 20. Additionally, casing string comprises a plurality of sliding sleeve valves 100. For clarity, a first plurality of sliding sleeve valves 100 of casing string 12 are associated with first or lower production zone 5A while a second plurality of sliding sleeve valves 100 of casing string 12 are associated with second or upper production zone 5B. Additionally, in this embodiment, casing string 12 comprises a plurality of counting subs 190. Particularly, a single counting sub 200 is associated with each production zone 5A, 5B and is positioned downhole of each sliding sleeve valve associated with the same production zone 5A, 5B. For example, the counting sub 200 associated with upper production zone 5B is positioned downhole of each sliding sleeve valve 100 associated with upper production zone 5B but uphole of each sliding sleeve valve associated with lower production zone 5B. Although only production zones 5A, 5B are shown in Figures 1-3, formation 4 may include dozens or even hundreds of production zones with one or more sliding sleeve valves 100 and a counting sub 200 associated with each production zone.

[0042] In this embodiment, each sliding sleeve valve 100 includes a sliding sleeve assembly 140 having a first or closed position associated with a first or run-in/closed configuration of the sliding sleeve valve 100, and a second or open position axially spaced from the closed position and associated with a second or open configuration of the sliding sleeve valve 100. Additionally, each sliding sleeve valve 100 includes one or more radial ports 112 configured to provide sleeve fluid communication between a central bore or passage 13 of casing string 12 and the formation 4. Particularly, when the sliding sleeve valve 100 is in the closed configuration, fluid communication between passage 13 and formation 4 via ports 112 of the sliding sleeve valve 100 is restricted. Conversely, when the sliding sleeve valve 100 is in the open configuration, fluid communication between passage 13 and formation 4 via ports 112 of the sliding sleeve valve 100 is permitted.

[0043] In this embodiment, wellbore 20 comprises a nonvertical or deviated wellbore 20 including a heel 22 and a toe 24 at a lower or downhole end of wellbore 20 that is laterally spaced from heel 22 relative to a surface 26 of wellbore 20. In this embodiment, the surface 26 of wellbore 20 is located at a seabed 5 positioned beneath a waterline 6. Thus, in this embodiment, stimulation system 10 comprises an offshore stimulation system; however, in other embodiments, stimulation system 10 may comprise a land-based stimulation system 10. In some embodiments, casing string 12 is then lowered through a fluid conduit 28 and into the wellbore 20 from a surface platform or vessel 30 disposed at the waterline 6 until the desired measured depth is reached so that sliding sleeve valves 100 are disposed adjacent production zones 5A, 5B, respectively. In this embodiment, well control equipment 29 (e.g., a wellhead, blowout preventers, etc.) is positioned at the surface 26 of wellbore 20 and is connected to a terminal end of the fluid conduit 28. In some embodiments, zonal isolation is accomplished by pumping cement towards the toe 24 of wellbore 20 and back towards the surface 26 via an annulus formed between an outer surface of casing string 12 and an inner surface of wellbore 20; however, in other embodiments, zonal isolation may be accomplished by positioning a plurality of annular packers in wellbore 20 to seal against the wall 22 of wellbore 20.

[0044] Stimulation system 10 includes one or more untethered, flow transportable darts or actuation tools 200 configured to selectively actuate one of the sliding sleeve valves 100 of casing string 12 from the closed configuration to the open configuration. As used herein, the term “flow transportable dart” encompasses any object that may be pumped through a wellbore or transmitted via fluid flow and used to cause movement of a tool or device disposed in the wellbore.

[0045] In this embodiment, following the installation of casing string 12 in wellbore 20 with sliding sleeve valves 100 in the closed configuration, a first dart 300 associated with lower production zone 5A is pumped through the central passage 13 of casing string 12 as shown particularly in Figure 1. As will be discussed further herein, the first dart 300 is programmed prior to being deployed from surface platform 30 with a predefined count associated, in this embodiment, with lower production zone 5A. Particularly, first dart 300 is configured to count each counting sub 200 through which it passes, and is configured to actuate from a first or run-in/disarmed configuration to a second or armed configuration after passing through a quantity of counting subs 190 equal in number to the predefined count programmed into the first dart 300. In this example, first dart 300 is programmed with a count equal to one and thus actuates from the disarmed configuration into the armed configuration after passing through the counting sub 200 associated with the upper production zone 5B. In other embodiments, the count of first dart 300 may be equal to ten or even a hundred or more allowing for the cluster fracking of tends to hundreds of distinct production zones of a given formation using the same number of darts 300. As will be discussed further herein, the count may be programmed into a given dart 300 mechanically via a gear train of the dart 300. Thus, in this embodiment, dart 300 does not include any fragile and expensive electrical and/or electromagnetic components which are prone to fail in the extreme operating environments such as that presented by wellbore 20. [0046] After passing through the counting sub 200 associated with upper production zone 5B, the first dart 300 continues towards sliding sleeve valves 100A/101A associated with lower production zone 5A. First dart 300 then actuates each sliding sleeve valves 100 into associated with the lower production zone 5A into the open configuration and subsequently lands and is trapped against a landing shoulder 206 of the counting sub 200 associated with lower production zone 5A. First dart 300 sealingly engages the counting sub 200 associated with lower production zone 5A and thereby seals off the toe 24 of wellbore 20 from the surface 26 of wellbore 20. As shown particularly in Figure 2, following the landing of first dart 300 in counting sub 200, hydraulic fracturing fluid may be pumped through the central passage 13 of casing string 12 and into the lower production zone 5A of formation 4 via the radial ports 112 of sliding sleeve valves 100 associated with lower production zone 5A, thereby forming fractures 6 in the lower production zone 5A. Thus, in a single run of a single first dart 300, a plurality of sliding sleeve valves 100 may be opened to allow for the cluster fracking of lower production zone 5A.

[0047] As shown particularly in Figure 3, following the cluster fracking of lower production zone 5A as described above, the first dart 300 landed within the counting sub 200 associated with lower production zone 5A may dissolve to permit fluid communication through the portion of casing string 12 extending across lower production zone 5A. As will be discussed further herein, each dart 300 may be comprised or formed from dissolvable materials configured to dissolve in the wellbore 20 after being exposed to a given temperature, pressure, or fluid for a predetermined period of time.

[0048] Following the dissolving of the first dart 300, a second dart 300 associated with upper production zone 5B may be deployed from surface platform 30 and pumped through the fluid conduit 28 and the central passage 13 of casing string 12. Second dart 300 actuates the sliding sleeve valves 100 associated with upper production zone 5B into the open configuration and subsequently lands within the counting sub 200 associated with upper production zone 5B. Second dart 20B is retained or captured within the counting sub 200 associated with upper production zone 5B whereby second dart 300 seals off the portion of wellbore 20 associated with lower production zone 5A from the surface 26 of wellbore 20. Following the landing of second dart 300 in the counting sub 200 associated with upper production zone 5B, hydraulic fracturing fluid may be pumped through the central passage 13 of casing string 12 and into the upper production zone 5B of formation 4 via the radial ports 112 of sliding sleeve valves 100, thereby forming fractures 6 in the upper production zone 5B. Thus, in a single run of a single second dart 300, a plurality of sliding sleeve valves 100 may be opened to allow for the cluster fracking of upper production zone 5B.

[0049] In other embodiments the process described above and shown in Figures 1-3 may be repeated for the cluster fracking of dozens of zones for a given formation by employing the same number of darts each programmed with a unique count associated with their respective production zone. Additionally, in this embodiment, sliding sleeve valves 100 associated with lower production zone 5A each have a similar minimum inner diameter as a minimum inner diameter of sliding sleeve valves 100 associated with upper production zone 5B. In other words, the sliding sleeve valves 100 associated with each succeeding downhole zone do not need to decrease in their respective minimum inner diameter in order to allow for the sequential fracking of discrete stages. Given that the minimum inner diameters of sliding sleeve valves 100 do not need to become successively smaller moving towards toe 24 of wellbore 20, restrictions to fluid flow through the central passage 13 of casing string 12 may be minimized in-turn maximizing the production of hydrocarbons from formation 4 via wellbore 20. Additionally, as will be discussed further herein, darts 300 of stimulation system 10 do not comprise a collet beam or other member having a relatively great axial length, and instead the axial length of each dart 300 is minimized such that darts 300 may be conveniently and easily transported through curving fluid conduits such as the fluid conduit 28 extending between surface platform 30 and well control equipment 29.

[0050] Referring to Figures 4-7, an embodiment of sliding sleeve valve 100 of the stimulation system 10 of Figures 1-3 is shown in Figures 4-7. Particularly, Figures 4- 7 show sliding sleeve valve 100 in a first or run-in/closed configuration. Sliding sleeve valve 100 has a longitudinal or central axis 105 and generally includes an outer housing 102, a first or upper sub 120, a second or lower sub 130, and a sliding sleeve assembly 140 slidably disposed in housing 102. Housing 102 of sliding sleeve valve 100 comprises a first or upper end 104, a second or lower end 106, a central bore or passage 108 defined by a generally cylindrical inner surface 110 extending between ends 104, 106. In this embodiment, the inner surface 110 of housing 102 includes an annular first or upper groove 114 which forms an annular shoulder 116 at a lower end thereof, and an annular second or lower groove 118. [0051] Upper sub 120 of sliding sleeve valve 100 comprises a first or upper end 121 , a second or lower end 123, a central bore or passage 124 extending between ends 121 , 123, and a generally cylindrical outer surface 125 extending between ends 121 , 123. The upper end 121 of upper sub 120 is configured to connect (e.g., threadably connect) to the casing string 12 shown in Figure 1. In this embodiment, an annular seal assembly 126 is positioned on the outer surface 125 of upper sub 120 for sealing a releasable connection 127 (e.g., a threaded connection) formed between the lower end 123 of upper sub 120 and the upper end 104 of housing 102.

[0052] Lower sub 130 of sliding sleeve valve 100 comprises a first or upper end 131 , a second or lower end 133, a central bore or passage 134 extending between ends 131 , 133, and a generally cylindrical outer surface 135 extending between ends 131 , 133. The lower end 133 of lower sub 130 is configured to connect (e.g., threadably connect) to the casing string 12 shown in Figure 1. In this embodiment, an annular seal assembly 136 is positioned on the outer surface 135 of lower sub 130 for sealing a releasable connection 137 (e.g., a threaded connection) formed between the upper end 131 of lower sub 130 and the lower end 106 of housing 102.

[0053] The sliding sleeve assembly 140 of sliding sleeve valve 100A generally comprises a closure sleeve 142, an actuation sleeve 160, a plurality of circumferentially spaced locking members or keys 180, and an annular collet sleeve 190. Closure sleeve 142 comprises a first or upper end 141 , a second or lower end 143, a central bore or passage 144 defined by a generally cylindrical inner surface 145 extending between ends 141 , 143, and a generally cylindrical outer surface 146 extending between ends 141 , 143.

[0054] In this embodiment, the inner surface 145 of closure sleeve 142 includes an annular first or upper groove 147 and an annular second or lower groove 148. As shown particularly in Figure 6, in this embodiment, the outer surface 146 of closure sleeve 142 includes an annular outer groove 150 extending from a first or upper end 152 to a second or lower end 153. Additionally, as shown particularly in Figures 6, 7, closure sleeve 142 comprises a plurality of circumferentially spaced radial openings or receptacles 154 positioned axially proximal the lower end 153 of outer groove 150. Each receptacle 154 extends entirely through both the inner surface 145 and the outer surface 146 of closure sleeve 142 and receives one of the keys 180.

[0055] A plurality of annular seal assemblies 155, 156, and 157 are each positioned in corresponding annular grooves formed on the outer surface 146 of closure sleeve 142 and are each configured to sealingly engage the inner surface 110 of housing 102. Seal assemblies 155, 156 are each positioned axially between the upper end 141 of closure sleeve 142 and the radial ports 112 of housing 102 when sliding sleeve assembly 140 is in the closed position. Additionally, seal assembly 157 is positioned axially between the lower end 143 of closure sleeve 142 and the radial ports 112 of housing 102 when sliding sleeve assembly 140 is in the closed position. In this configuration, seal assemblies 155, 156, and 157 are configured to restrict fluid communication between the central passage 13 of casing string 12 and the radial ports 112 of housing 102 when sliding sleeve assembly 140 is in the closed position. However, when sliding sleeve assembly 140 is in the open position, each of seal assemblies 155, 156, and 157 are positioned between axially between the lower end 143 of closure sleeve 142 and the radial ports 112 of housing 102, thereby permitting fluid communication between the central passage 13 of casing string 12 and radial ports 112 of housing 102.

[0056] As shown particularly in Figures 6, 7, each key 180 of sliding sleeve assembly 140 comprises a radially outer shoulder 182 configured to slidingly engage the inner surface 110 of housing 102. Each key 180 also includes an outer engagement surface 184 (shown in Figure 6) extending between a first or upper end of the key 180 to the shoulder 182 of the key 180. Keys 180 are received in the receptacles 154 of closure sleeve 142 such that relative axial movement is restricted between keys 180 and closure sleeve 142 is restricted while relative radial movement between keys 180 and closure sleeve 142 is permitted depending on the configuration of sliding sleeve valve 100. For example, in the closed configuration of sliding sleeve valve 100 shown in Figures 4-7, the shoulder 182 of each key 180 contacts the inner surface 110 of housing 102, restricting relative radial movement between keys 180 and the closure sleeve 142 of sliding sleeve assembly 140.

[0057] Collet sleeve 190 of sliding sleeve assembly 140 comprises a plurality of circumferentially spaced collet fingers 192 extending from a first or upper end of the collet sleeve 190 and which collectively define a second or lower end of the collet sleeve 190. Each collet finger 192 of collet sleeve 190 comprises a radially outer shoulder 194 located at a terminal end of the collet finger 192. Shoulders 194 of collet sleeve 190 may flex radially relative to closure sleeve 142 depending on the configuration of sliding sleeve valve 100. For example, in the closed configuration of sliding sleeve valve 100, the terminal end of each collet finger 192 is supported on or contacts the engagement surfaces 184 of keys 180, preventing shoulders 194 from flexing radially inwardly.

[0058] Actuation sleeve 160 of sliding sleeve assembly 140 is configured to interface and contact the dart 300 associated with the sliding sleeve valve such that the dart 300 may actuate the sliding sleeve assembly 140 from the closed position to the open position. Actuation sleeve 160 includes a first or upper end 161 , a second or lower end 163 opposite upper end 161 , and a central passage 164 defined by a generally cylindrical inner surface 165. In this embodiment, the inner surface 165 of actuation sleeve 160 comprises an annular radially inner shoulder 166 located at the lower end 163 thereof. Inner shoulder 166 may also be referred to herein as landing profile 166 as a dart 300 may land against landing profile 166 when in the armed configuration to thereby actuate sliding sleeve assembly 140 from the closed position to the open position. Additionally, a generally cylindrical outer surface of actuation sleeve 160 comprises a radially outer shoulder 168 located at the upper end 161 thereof. Outer shoulder 168 is received within the upper groove 147 of closure sleeve 142 and is configured to retain actuation sleeve 160 to closure sleeve 142 such that actuation sleeve 160 may not be ejected from the central passage 144 of closure sleeve 142. Actuation sleeve 160 may be coupled to closure sleeve 142 via mechanisms other than outer shoulder 168 in other embodiments.

[0059] In this embodiment, actuation sleeve 160 comprises a plurality of circumferentially spaced slots 170 extending axially into sleeve 160 from the lower end 163 thereof. Slots 170 permit the lower end 163, including landing profile 166, to flex radially outwardly depending on the configuration of the sliding sleeve valve 100. For example, in the closed configuration of sliding sleeve valve 100, each key 180 is locked into a first or radially inner position relative to closure sleeve 142 such that an inner surface of each key 180 is disposed directly adjacent or contacts the outer surface actuation sleeve 160 at the lower end 163 of sleeve 160, thereby preventing landing profile 166 from flexing radially outwardly.

[0060] In this embodiment, collet sleeve 190 isfrangibly coupled to closure sleeve 142 by a plurality of circumferentially spaced shear members or pins 196 which extend radially from collet sleeve 190 and into receptacles formed in the outer surface 146 of closure sleeve 142. While in this embodiment the plurality of circumferentially spaced shear pins 196 frangibly couple collet sleeve 190 with closure sleeve 142, in other embodiments other types of frangible connections may be provided between sleeves 190, 142 such as, for example, a shear ring.

[0061] Referring to Figures 5, 8-11 , as described above, sliding sleeve valve 100 may be actuated from the closed configuration (shown in Figure 5) to the open configuration (shown in Figures 10, 11) by the first dart 300 which will be discussed further herein. In the closed configuration of sliding sleeve valve 100, the collet sleeve 190 of sliding sleeve assembly 140 is received in the upper groove 114 of housing 102 with outer shoulders 194 of collet sleeve 190 located directly adjacent or contacting the shoulder 116 of housing 102. Contact between the engagement surfaces 184 of keys 180 and collet fingers 192 of collet sleeve 190 prevent outer shoulders 194 from flexing inwardly thereby retaining collet sleeve 190 within upper groove 114 and preventing sliding sleeve assembly 140 from travelling axially towards the lower sub 130. Additionally, in this configuration, keys 180 are forced into their lower radial positions thereby restricting the landing profile 166 of actuation sleeve 160 from expanding radially outward.

[0062] In addition to the open and closed configurations, sliding sleeve valve 100 includes an intermediate configuration shown in Figures 8, 9 through sliding sleeve valve 100 passes as valve 100 is actuated from the closed configuration to the open configuration. Particularly, in this embodiment, shear pins 196 have been sheared in the intermediate configuration such that closure sleeve 142 may travel axially towards lower sub 130 relative to collet sleeve 190 until the upper end of collet sleeve 190 contacts the upper end 152 of the outer groove 150 of closure sleeve 150, as shown particularly in Figure 9. With the upper end of collet sleeve 190 contacting the upper end 152 of outer groove 150, collet fingers 192 of collet sleeve 190 are no longer supported on the engagement surfaces 184 of keys 180, permitting outer shoulders 194 of sleeve 190 to flex inwardly such that sliding sleeve assembly 140 may travel towards lower sub 130 and thereby actuate the sliding sleeve valve 100 into the open configuration.

[0063] In the open configuration of sliding sleeve valve 100, radial ports 112 are exposed from sliding sleeve assembly 140, thereby permitting fluid flow from the central passage 13 of casing string 12 into radial ports 112. Additionally, in the open configuration, the lower end 143 of closure sleeve 142 is disposed directly adjacent or contacts the upper end 131 of lower sub 130 and the outer shoulders 182 of keys 180 are positioned within the lower groove 118 of housing 102. In the lower groove 118, keys 180 are permitted to displace radially outwards from their radially inner positions into radially outer positions, thereby also permitting landing profile 166 of actuation sleeve 160 to flex radially outwards in response to an axial force directed against the landing profile 166 by a dart 300, as will be discussed further herein. The flexibility of landing profile 166 when sliding sleeve valve 100 is in the open configuration allows profile 166 to function as a flex seat whereby a dart may exit the sliding sleeve valve 100 and proceed further downhole through casing string 12 after actuating the sliding sleeve valve 100 into the open configuration.

[0064] In this embodiment, the closure sleeve 142 of sliding sleeve assembly 140 additionally includes a pair of annular shifting profiles 158, 159 positioned at the opposing ends 141 , 143 thereof on the inner surface 145 of closure sleeve 142. Shifting profiles 158, 159 provide shoulders to which a surface deployable shifting tool may engage or latch against to thereby actuate the sliding sleeve valve 100 from either the closed configuration to the open configuration, or from the open configuration to the closed configuration an unlimited number of times. For example, sliding sleeve valve 100 may be shifted by a shifting tool from the open configuration to the closed configuration following the installation of sliding sleeve valve 100 in the wellbore 20. Sliding sleeve valve 100 may also be shifted by a shifting tool using shifting profiles 158, 159 from the open configuration to the closed configuration during a later point in the lifespan of wellbore 20 following an initial stimulation of formation 4 using darts 300. Sliding sleeve valves 100 may be reset to their closed configuration by the shifting tool to allow for a subsequent re-stimulation or fracturing of formation 4 using darts 300. For instance, the productivity of wellbore 20 during the production phase may decline overtime, making it advantageous to re-stimulate formation 4 using darts 300 to thereby boost the subsequent production of wellbore 20.

[0065] Referring briefly to Figure 12, an embodiment of a counting sub 200 of the stimulation system 10 of Figures 1-3 is shown. Counting sub 200 comprises a first or upper end 201 , a second or lower end 203, and a central bore or passage 204 defined by a generally cylindrical inner surface 205 extending between ends 201 , 203. The upper end 201 of counting sub may connect (e.g., threadably connect) to a casing joint or other component of casing string 12 including the lower sub 130 of one of the sliding sleeve valves 100. Similarly, the lower end 203 of counting sub 200 may connect (e.g., threadably connect) to a casing joint or other component of casing string 12, including the upper sub 120 of one of the sliding sleeve valves 100. In other embodiments, landing sub 200 may be incorporated directly into one of the sliding sleeve valves 100. As described above, the inner surface 205 of counting sub comprises landing shoulder 206 against which darts 200 are configured to against or engage with as they are pumped through casing string 12, as will be described further herein. Landing shoulder 206 may define a minimum inner diameter of the entire casing string 12.

[0066] Referring to Figures 13-17, an embodiment of a dart 300 of the stimulation system 10 of Figures 1-3 is shown. Dart 300 has a longitudinal or central axis 305 and generally includes a housing 302, a landing profile or ring 340, a mandrel 350, and a core or piston 390 receivable in housings 302, 350.

[0067] Referring to Figures 13-17, 19, and 20, the housing 302 of dart 300 generally includes a first or upper end 303, a second or lower end 307, a generally cylindrical outer surface 306 extending between ends 303, 307, and a central passage 308 extending partially into housing 302 from upper end 303 (passage 308 does not extend through lower end 307) and defined by a generally cylindrical inner surface 309. In this embodiment, the upper end 303 of housing 302 is defined by a plurality of circumferentially spaced locking arms 310, 311 , 312, and 313. The arcuate widths of locking arms 310-313 vary relative each other. In some embodiments, each locking arm 310-313 may have a unique arcuate width. While in this embodiment housing 302 comprises four separate locking arms 310-313, in other embodiments, the number of separate locking arms of housing 302 may vary. In this embodiment, a guide pin 314 extends radially through the locking arms 310, 312 of housing 302.

[0068] In this embodiment, the outer surface 306 of housing 302 comprises a first or upper engagement surface 316, a second or intermediate engagement surface 318, and a third or lower engagement surface 320. Intermediate engagement surface 318 is positioned axially between engagement surfaces 316, 320, where upper engagement surface 316 is positioned axially between intermediate engagement surface 318 and the upper end 303 of housing 302 while the lower engagement surface 320 is positioned axially between the intermediate engagement surface 318 and the lower end 307 of housing 302. In this embodiment, lower engagement surface 320 has a greater outer diameter than intermediate engagement surface 318. Additionally, intermediate engagement surface 318 has a greater outer diameter than upper engagement surface 316. In this embodiment, lower engagement surface 320 is separate from intermediate engagement surface 318 by a radially outer shoulder 322 having an outer diameter greater than the outer diameter of lower engagement surface 320.

[0069] The landing ring 340 is positionable on the engagement surface 316, 318, and 320 of housing 302. Landing ring 340 may radially expand and contract as it is displaced between engagement surfaces 316, 318, and 320 as described further herein. Landing ring 340 is positioned about the intermediate engagement surface 318 when dart 300 is in the unnamed configuration shown in Figures 13-17. In this embodiment, landing ring 340 comprises an angled orfrustoconical landing profile 342 at a lower end thereof.

[0070] An annular sealing element 344 is also positionable about the outer surface 306 of housing 302. Unlike landing ring 340, however, relative axial movement between sealing element 344 and housing 302 is restricted. Additionally, sealing element 344 forms an annular opening 346 at an upper end thereof and which extends axially over outer shoulder 322 of housing 302 such that shoulder 322 is received within annular opening 346. As will be described further herein, the angled profile 342 of landing ring 340 may engage the annular opening 346 of sealing element 344 to thereby radially expand the upper end of sealing element 344. In other embodiments, the upper end of sealing element 344 may not include annular opening 346 and may not extend axially over the outer shoulder 322 of housing 302.

[0071] Referring to Figures 13-17, 21-23, the mandrel 350 of dart 300 generally includes a first or upper end 353, a second or lower end 355, and a central passage 356 extending through mandrel 350 between ends 353, 355 and defined by a generally cylindrical inner surface 358. In this embodiment, a plurality of circumferentially spaced and axially extending radial slots 360 are formed in an outer surface of mandrel 350 and positioned proximal the upper end 353 of mandrel 350. Each radial slot 360 receives one of the guide pins 314 such that a delimited amount of relative axial movement is permitted between guide pins 314 and mandrel 350 while relative radial and angular (e.g., movement about central axis 305 of dart 300) movement between guide pins 314 and mandrel 350 is restricted.

[0072] In this embodiment, mandrel 350 comprises an annular wall or hub 362 located at the upper end 353 thereof and which reduces the diameter of the inner surface 358 of mandrel 350 at the upper end 353. A plurality of circumferentially spaced planet gears 370 extend axially from the hub 362 towards the lower end 355 of mandrel 350. Each planet gear 370 comprises a plurality of gear teeth 372 formed on an outer surface thereof. Each planet gear 370 disposed in the central passage 356 of mandrel 350 is rotatably coupled to the hub 362 whereby relative rotation between planet gears 370 and mandrel 350 is permitted. While in this embodiment dart 300 comprises three planet gears 370, in other embodiments, the number of planet gears 370 may vary. [0073] In this embodiment, a plurality of circumferentially spaced arcuate slots 363, 364, 365, and 366 extend through the hub 362 of mandrel 350. Slots 363-366 of mandrel 350 are associated with locking arms 310-313 of housing 302. Particularly, slot 363 is configured to receive locking arm 310; slot 364 is configured to receive locking arm 311 ; slot 365 is configured to receive locking arm 312; and slot 366 is configured to receive locking arm 313. Additionally, slots 363, 365 may also at least partially receive guide pins 314 which are coupled to locking arms 310, 312. Slots 363-366 permit axial movement of locking arms 310-313 relative to mandrel 350. Locking arms 310-313 remain angularly aligned with slots 363-366 via guide pins 314.. [0074] The central passage 356 of mandrel 350 also receives an annular ring gear 380 comprising a plurality of gear teeth 382 formed on an inner surface thereof. Gear teeth 382 mesh with the gear teeth 372 of planet gears 370 to form a geared connection between planet gears 370 and ring gear 380. In this embodiment, relative axial movement between ring gear 380 and mandrel 350 is restricted; however, ring gear 380 is permitted to rotate about the central axis 305 relative to mandrel 350.

[0075] Ring gear 380 also includes a generally cylindrical outer surface 383 in which a plurality of circumferentially spaced arcuate slots 384, 385, 386, and 387 are formed. Slots 384-387 of ring gear 380 are associated with locking arms 310-313 of housing 302. Particularly, slot 384 is configured to receive locking arm 310; slot 385 is configured to receive locking arm 311 ; slot 386 is configured to receive locking arm 312; and slot 387 is configured to receive locking arm 313. Additionally, slots 384, 386 may also at least partially receive guide pins 314 which are coupled to locking arms 310, 312. Given that the arcuate widths of locking arms 310-313 and corresponding slots 384-387 vary, locking arms 310-313 of housing 302 may only be inserted into slots 384-387 of ring gear 380 when housing 302 and ring gear 380 are in a predefined relative angular orientation therebetween associated with the armed configuration of dart 300. This is inaccurate 363-366 will always be aligned with 310- SI 3. Following is an example of how the dart 300 could be set up: 10 J-slots on 360°, 2 guide pins, 50 teeth in sun gear, 76 teeth in ring gear, grooves for guide pins in ring grar and slots/arms. 1.) If it was only the 10 J-Slots and 2 guide pins only 5 counts would be possible as guide pins would align every 180°. 2.) If the gears with grooves for guide pin are added: One j-slot count returns an angle change of 36°. At the same time the gear rotates 23,68421 ° for a total of 59,68421° for each count. Number of counts between each time guide pins aligns both with j-slot and grooves in ring gear is a whole number angle possible to divide by both 59,68421 and 180, +/- 5 just to accommodate tolerances. This gives 175 cycles 3.) If we add the slots/arms there is only alignment every 360° and number of counts becomes 380. From this one can say that the gear and j-slot is needed to get a high number of counts. Slots/arms are more of an optimization. Only keeping the j-slot or combination of j-slot and slots/arms will limit the amount of counts

[0076] Referring to Figures 13-17, 24, the piston 390 of dart 300 generally includes a first or upper end 393, a second or lower end 395, and a generally cylindrical outer surface 396 extending between ends 393, 395. As shown particularly in Figure 24, in this embodiment, an indexer 398 is formed on the outer surface 396 of piston 390 in which an indexing or J-slot groove 400 is formed. Indexing groove 400 is defined by a pair of opposed walls 401 , 403 which define a plurality of circumferentially spaced upper shoulders 402 and a plurality of circumferentially spaced lower shoulders 404. Additionally, pair of circumferentially spaced openings 406 are formed in an upper wall 401 of the pair of walls 401 , 403. Although in this embodiment indexing slot 400 comprises a pair of openings 406 formed approximately 180 degrees apart, in other embodiments, indexing slot 400 comprise a single opening 406 or more than two openings 406. The guide pins 314 coupled to housing 302 are received in the indexing slot 400. As will be discussed further herein, relative axial movement between housing 302 and piston 390 is translated into relative rotational movement as guide pins 314 are forced along indexing slot 400 and between shoulders 402, 404, and openings 406 by the pair of curving walls 401 , 403.

[0077] The outer surface 396 of piston 390 includes a sun gear 410 formed therein which comprises a plurality of gear teeth 412. In this embodiment, sun gear 410 is positioned axially between the upper end 393 of piston 390 and the indexer 398. The gear teeth 412 of sun gear 410 mesh with the gear teeth 372 of planet gears 370 to form a geared connection between sun gear 410 and planet gears 370. The geared connection formed between gears 370, 380, and 410 of dart 300 form a planetary gear system or train which comprises a counting mechanism of dart 300 as will be discussed further herein. [0078] In this embodiment, an outer hub 414 is connected to the upper end 393 of piston 390. Hub 414 is positioned external mandrel 350. Additionally, hub 414 has a greater outer diameter than an inner diameter of the hub 362 of mandrel 350 and thus retains piston 350 within the central passage 356 of mandrel 350 such that relative axial movement between piston 390 and housing 350 is restricted. Thus, while piston 390 may travel axially relative to mandrel 302, it is restricted from travelling axially relative to mandrel 350. In other embodiments, hub 414 and piston 390 may comprise a single, monolithically formed member.

[0079] An annular seal assembly 416 is positioned in a corresponding groove formed on the outer surface 396 of piston 390. Seal assembly 416 is configured to sealingly engage the inner surface 309 of housing 302, thereby forming a sealed chamber 315 therein. In this embodiment, sealed chamber 315 comprises a vacuum or atmospheric chamber that is filled with compressible fluid (e.g., air) at atmospheric or surface pressure (e.g., air pressure above the waterline 6). The pressure within atmospheric chamber 315 is less than the fluid pressure within the central passage 13 of casing string 12, and thus a net pressure force is applied against piston 390 in a first or downhole axial direction 317 (shown in Figure 16) when dart 300 is positioned in casing string 12.

[0080] Referring to Figures 1-3, 25-28, having described the structure of embodiments of sliding sleeve valves 100 and darts 300 of the stimulation system 10 above, an exemplary method of performing a stimulation or hydraulic fracturing operation using stimulation system 10 is provided below. In this embodiment, a first dart 300 associated is deployed in the disarmed configuration form surface platform 30 and programmed with a predefined count associated with lower production zone 5A. Following deployment from surface platform 30, dart 300 “counts” or registers each production zone (each zone including a single counting sub 200) through which dart 300 travels until dart 300 travels through.

[0081] For example, as shown particularly in Figure 25, dart 300 is shown flowing through a sliding sleeve valve 100 associated with a production zone uphole of lower production zone 5A in the disarmed configuration. In this embodiment, guide pins 314 are positioned against upper shoulders 402 of the indexing slot 400 of piston 390, restricting further travel of piston 390 in the downhole axial direction 317 relative to housing 302 when dart 300 is in the disarmed configuration. Additionally, in the disarmed configuration, landing ring 340 is positioned on the intermediate engagement surface 318 of housing 302 thereby providing landing ring 340 with a maximum outer diameter which is equal to or less than a minimum inner diameter of the landing profile of the actuation sleeve 160 of the sliding sleeve valve 100. Thus, dart 300 is permitted to pass through sliding sleeve valve 100 without landing the landing ring 340 of dart 300 against the landing profile 166 of the sliding sleeve valve 100.

[0082] However, as shown particularly in Figure 26, landing shoulder 206 of counting sub 200 has a smaller minimum inner diameter than the landing profile 166 of the sliding sleeve valve 100. Particularly, landing shoulder 206 of counting sub 200 has a smaller minimum inner diameter than the maximum outer diameter of the landing ring 340 of dart 300 when in the disarmed configuration, causing the landing ring 340 to land against landing shoulder 206 as dart 300 flows through the counting sub 200. Landing ring 340 is forcibly displaced by the landing shoulder 206 in an uphole axial direction 319 (opposite downhole axial direction 317) such that landing ring 340 may be positioned upon the upper engagement surface 316 of mandrel 302. The upper engagement surface 316, having a smaller outer diameter than intermediate engagement surface 318, permits landing ring 340 to radially contract such that dart 300 may travel through a reduced diameter section 208 of the inner surface 205 of counting sub 200.

[0083] Additionally, travel of the landing ring 340 in the uphole axial direction 319 also forces mandrel 350 upwards in the uphole axial direction 319 relative to piston 390 via contact between landing ring 340 and the lower end 355 of mandrel 350. The axial travel of mandrel 350 relative to piston 390 forces guide pins 314 to travel along indexing groove 400 of piston 390 until pins 314 contact lower shoulders 404 of groove 400. Travel of guide pins 314 through indexing groove 400 of piston 390 forces piston 390 to rotate relative to housings 302, 350. For example, piston 390 and ring gear 380 may each rotate in a clockwise direction (at different rotational rates) while housings 302, 350 each rotate in a counterclockwise direction at the same rotational rate.

[0084] The rotation of piston 390 induced by the axial displacement of landing ring 340 from the intermediate engagement surface 318 to the upper engagement surface 316 may be referred to herein as a cycling of the dart 300, where the predefined count programmed into dart 300 corresponds to a required number of times the dart 300 must be cycled before the dart 300 will actuate into the armed configuration. The total number of times the dart 300 must be cycled in-turn corresponds to a total degree of rotation of piston 390 that must occur before the dart 300 will actuate into the armed configuration. The total degree of rotation of piston 390 that occurs before dart 300 actuates into the armed configuration is based on a gear ratio of a gear train 430 (shown in Figure 16) of dart 300 comprising planet gears 370, ring gear 380, and sun gear 410. In other words, the number of times dart 300 may be cycled before actuating into the armed configuration may be tailored by adjusting the gear ratio provided by gear train 430. In this manner the gear train 430 of dart 300 may be referred to as a counting mechanism thereof given that the gear ratio of gear train 430 may be adjusted by an operator of dart 300 to provide a preferred number of cycles prior to actuation into the armed configuration. Moreover, a wide range of gear ratios (e.g., tens or hundreds of distinct gear ratios) may be provided by gear train 430, allowing darts 300 to be cycled tens or hundreds of times before actuating into the armed configuration. This may, in-turn, allowing for the fracturing of tens or hundreds of distinct production zones of formation 4.

[0085] As an example, only five cycles could be completed before dart 300 would be actuated into the armed configuration in an embodiment where dart 300 does not include gear train 430 and instead includes only two guide pins 314 and an indexing groove 400 having ten upper shoulders 402. Conversely, 175 cycles must be completed before dart 300 is actuated into the armed configuration in an embodiment in which ring gear 380 comprises 76 gear teeth 382, sun gear 410 comprises 50 gear teeth 412, and indexing groove 400 comprises ten upper shoulders 402 and receives two guide pins 314. Thus, gear train 430 allows for dramatically more stages to be fractured utilizing dart 300.

[0086] As described above, after having passed through enough counting subs 200 to achieve the predefined count programmed into the dart 300, the dart 300 actuates from the disarmed configuration into the armed configuration. Particularly, in the armed configuration of dart 300, guide pins 314 are circumferentially aligned with openings 406 of indexing slot 400. Additionally, in the armed configuration of dart 300, locking arms 310-313 are circumferentially aligned with both the corresponding slots 363-366 of mandrel 350 (e.g., locking arm 310 is circumferentially aligned with slot 363, locking arm 311 is circumferentially aligned with slot 364, etc.) and with the slots 384-387 of sun gear 380 (e.g., locking arm 310 is circumferentially aligned with slot 384, locking arm 311 is circumferentially aligned with slot 385, etc.). [0087] The circumferential alignment of locking arms 310-313 with their corresponding slots 363-366 of mandrel 350 and slots 384-387 of sun gear 380 permit locking arms 310-313 to be received within the slots 363-366 of mandrel 350 thereby permitting guide pins 314 to travel axially through openings 406 in indexing slot 400, in-turn permitting piston 390 to travel further in the downhole axial direction 317 than permitted in the disarmed configuration of dart 300 due to the pressure force applied against piston 390 in the axial direction 317. The further axial travel of piston 390 in downhole axial direction 317 in-turn forces mandrel 350 (via engagement between indexer 398 of piston 390 and the hub 362 of mandrel 350 through ring gear 380) to travel axially downwards in direction 317. The displacement of mandrel 350 in the downwards axial direction 317 forces landing ring 340 across outer shoulder 322 of housing 302 and onto the lower engagement surface 320 thereof in a radially expanded position. Landing ring 340 remains locked in a radially outer position supported on lower engagement surface 320 of housing 302 when dart 300 is in the armed configuration. Landing ring 340 has a greater outer diameter when in the radially expanded position relative to the outer diameters of landing ring 340 when in the radially retracted positions supported on engagement surfaces 316, 318 when dart 300 is in the disarmed configuration.

[0088] As shown particularly in Figure 27, with landing ring 340 disposed in a radially outer position supported on the lower engagement surface 320 of housing 320, the maximum outer diameter of landing ring 340 is greater than the minimum inner diameter of the landing profile 166 of the sliding sleeve valve 100 through which dart 30 travels (the sliding sleeve valve 100 being associated with lower production zone 5A in this example). Thus, landing ring 340 engages landing profile 166 and thereby forces the sliding sleeve assembly 140 of the sliding sleeve valve 100 from the closed position into the open position and thereby actuates the valve 100 from the closed configuration to the open configuration such that fluid communication may be provided between the central passage 13 of casing string 12 and the radial ports 112 of the sliding sleeve valve 100.

[0089] As described above, in the open configuration the keys 180 of sliding sleeve assembly in lower groove 118 of housing 102. Contact between the landing ring 340 of dart 300 and landing profile 166 of the sliding sleeve valve 100 when valve 100 is in the open configuration forces landing profile 166 to expand radially outwards (permitted by keys 180 displacing into their radially outer positions within lower groove 118). The radial expansion of landing profile 166 permits dart 300 to exit the sliding sleeve valve 100 with the sliding sleeve valve 100 remaining in the open configuration. Dart 300 may continue through casing string 12 actuating, in this example, each sliding sleeve valve 100 associated with lower production zone 5A into the open configuration.

[0090] After each sliding sleeve valve 100 associated with lower production zone 5A is actuated into the open configuration, dart 300, remaining in the armed configuration, lands against the landing shoulder 206 of the counting sub 200 associated with lower production zone 5A. In the armed configuration of dart 300, with guide pins 314 positioned in the openings 406 of indexing groove 400 and locking arms 310-313 of housing 302 received in slots 363-366 of mandrel 350, landing ring 340 remains locked into the radially expanded position supported on the lower engagement surface 320 of housing 302, thereby trapping dart 300 against the landing profile 206 of the counting sub 200. Additionally, in this position of dart 300 the sealing element 344 thereof sealingly engages the reduced diameter section 208 of counting sub 200 to thereby effect a seal across the central passage of counting sub 200 and fluidically isolate the portion of casing string 12 extending downhole from the counting sub 200 from the portion of casing string 12 extending uphole from counting sub 200.

[0091] In this example, with fluid flow through the landing sub 200 associated with lower production zone 5A restricted by dart 300, a stimulation or hydraulic fracturing fluid may be pumped from surface platform 30 through the central passage 13 of casing string 12 and into the radial ports 112 of each sliding sleeve valve 100 associated with lower production zone 5A. The fracturing fluid ejected from radial ports 112 may form fractures 6 in the lower production zone 5A as shown particularly in Figure 2. Following the fracturing of lower production zone 5A, the dart 300 landed within the counting sub 200 associated with lower production zone 5A may dissolve in-situ. Particularly, one or more components of dart 300 comprises a dissolvable material configured to dissolve in the wellbore 20 after being exposed to a given temperature, pressure, or fluid for a predetermined period of time. For example, housing 302, mandrel 350, and piston 390 may each comprise a dissolvable material. In some embodiments, each component of plug 300 comprises a dissolvable material. In some embodiments, housing 302, mandrel 350, and/or piston 390 of dart 300 may comprise a metal alloy such as a magnesium or aluminum alloy configured to dissolve the presence of certain brines with the rate of dissolution being proportional to downhole temperature and pressure. In other embodiments, components of dart 300 may comprise polymeric materials configured to dissolve in response to the elevated temperature and pressure encountered downhole.

[0092] The process described above with respect to the fracturing of lower production zone 5A may be repeated for upper production zone 5B using a second dart 300 as shown particularly in Figure 3, as well as for additional production zones not shown in Figures 103 using additional darts 300. For instance, the process described above may be used to cluster frack dozens or hundreds of distinct production zones from a single wellbore. Additionally, given that there is no difference in the minimum inner diameter of each sliding sleeve valve 100 or each dart 300, the addition of production zones does not require a concomitant decrease in the minimum diameter of the casing string (e.g., due to the inclusion of relatively smaller seats). A consistent and maximized inner diameter may thus be maintained irrespective of the number of production zones.

[0093] By varying the gear ratio provided by the gear train 430 of a given dart 300, the dart 300 may be tailored to actuate into the armed configuration after the dart 300 has been cycled tens or hundreds of times via tens or hundreds of corresponding counting subs 200. Thus, darts 300 may be used to selectably cluster rack tens or hundreds of distinct production zones utilizing landing ring 340 in lieu of a relatively lengthy collet beam, thereby minimizing the axial length of the dart 300. The minimal axial length of dart 300 may allow dart 300 to conveniently navigate flowpaths having relatively tight curves such as fluid conduit 28 shown in Figures 1-3.

[0094] While disclosed embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.