Patent Publication Number: US-10316620-B2

Title: Dart and sleeve mechanism for multiple zone actuation

Description:
FIELD 
     The subject disclosure generally relates to the field of devices in wellbores within subterranean rock formations. More particularly, the subject disclosure relates to techniques for actuation of downhole devices such as valves in wellbores. 
     BACKGROUND 
     There are many situations when it would be desirable to selectively activate multiple downhole devices. It is also preferable that one can do so not necessarily following a particular pre-determined sequence. 
     Typically there are multiple casing valves, each with a sliding sleeve and cemented in a well. Due to the heterogeneous nature of formation, one might not want to open all the valves simultaneously so that the fracturing operations can be performed separately for different layers of formations. A known method of selective actuation is to use different diameter balls to open the valves from bottom up. For example, each valve can have a restriction that will be blocked, or engaged, by a ball having a diameter at least as large as the restriction. The restrictions are then ordered so that the narrowest constriction is at the bottom, and the restriction size gradually is increased from the bottom up. A ball with smallest size is first dropped into the well. The size of the ball is designed so that it will go through all the valves except the bottom valve. The ball will be stopped by the bottom valve so that the sliding sleeve of the bottom valve will be pushed to the “open” position and expose the wellbore to cemented formation. Then the fracturing operation through the valve N can be executed. After that, the next size of ball will be dropped to activate the N−1 valve. 
     While the graduated ball technique works, it has an inherent limitation based on how many stages can be implemented in a string of valves downhole. Since the order of opening of the valves depends on the sizes of balls dropped, the radius of restriction in each stage needs to be different. Additionally, for a given size of the completion, the number of different sizes (radii) of restrictions is finite and is often less than desirable. Typically, a maximum of eight balls can be dropped, and this can severely impact the effectiveness of the multistage fracturing operation. For example, it is not uncommon to make wells economical that have 20 or more production zones. 
     Another known technique utilizes control lines between adjacent zones to activate restrictions. Once a restriction in a particular valve is activated, it is ready to catch a dart dropped from the surface in order to open this particular valve. However, there is a possibility for damage of the control lines during run-in-hole, especially in horizontal wells. A damaged control line might result in only those zones below the damaged zone being capable of production. Another drawback of this technique is that as the thickness of the valve increases, the internal diameter of the valve decreases in order to accommodate the complex hydraulic mechanisms in the valve. 
     Yet another known technique is described in commonly-owned U.S. Pat. Publ. No. 2015/0000935. This technique uses darts having moveable pieces that allow the dart to radially compress when engaging restrictions. However, since the darts have moveable pieces, during operation various portions of each dart piece are put under stresses and strains. As a result, a careful selection of material or materials should be made for the dart pieces. 
     Yet another known technique is described in commonly-owned U.S. Pat. Publ. No. 2014/0299319. This technique uses casing segments having a plurality of casing ribs. A sleeve member includes rocker members that are engaged by darts pumped through the well. Each dart that engages the rocking members causes the sleeve to move one rib position. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     According to some embodiments, a system is described for selectively actuating a plurality of devices positioned in a well. The system includes: a conduit, a plurality of devices with assemblies, and a plurality of untethered objects. The conduit is configured to be positioned within the well. The devices are disposed along the conduit such that a fluid passageway extends through each of the devices. Each device includes at least one assembly. Each of the assemblies includes: an assembly outer sleeve having on its inner surface a fulcrum and an indentation; and a plurality of rocking sleeve members circumferentially arranged within the outer sleeve such that the passageway extends through the sleeve members and axial movement of the sleeve members is constrained. Each sleeve member has an outer surface dimensioned and shaped to pivot on the fulcrum and to partially fit into the indentation when rocked toward the indentation. The untethered objects are configured to be communicated through the passageway. Each object is shaped and dimensioned so that it can either pass or be trapped by an assembly. The object is trapped by an assembly when its characteristic length and characteristic diameter prevent sufficient rocking action by the rocking sleeve members that would allow the object to pass the assembly. The devices are configured for actuation when an object is trapped by an included assembly. The device actuation is caused by a fluid pressure differential between the upper and lower sides of the trapped object. 
     According to some embodiments, each rocking sleeve member has first and second inwardly protruding portions, and the characteristic length for each object is a length of a portion of the object that has a maximum diameter. The object can be trapped by an assembly when its characteristic length is at least as long as a distance between the first and second inwardly protruding portions of the rocking sleeves of the assembly. According to some embodiments, each untethered object is a solid dart having a front portion shaped as a surface of revolution (for example spherical or conical), a cylindrical middle portion and tapered rear section. The second inwardly protruding portions of the rocking sleeve members are shaped to match the shape of the front portions, and the characteristic length for each dart is equal to a length of the cylindrical middle portion of each dart. According to some embodiments, when a dart is trapped by an assembly, engagement of the front portion of the dart with the second inwardly protruding portions of the rocking sleeve members forms a seal that facilitates the fluid pressure differential. 
     According to some embodiments, the conduit is a casing and the devices are sliding casing valves each including an outer sleeve having at least one opening and an inner sleeve. The inner and outer sleeves form a valve between the fluid pathway and an annular region outside of the casing. The valve is opened when openings on the outer sleeves are not obstructed by the inner sleeve due to sliding of the inner sleeve relative to the outer sleeve. According to some embodiments, the well penetrates a subterranean rock formation, such as a hydrocarbon-bearing formation, having a plurality of zones to be stimulated. The casing valves are normally closed, and the casing valves are positionable in the well with at least one casing valve in each zone such that each zone can be selectively stimulated by communicating an object that is configured to be trapped by an assembly of the casing valve in the zone and the fluid pressure differential causes the inner sleeve to slide relative to the outer sleeve so as to open the valve. The object (e.g. dart) can be made of a degradable material which is configured to dissolve in the wellbore environment after having been used to selectively actuate a device. 
     According to some embodiments, a method is described for selectively actuating devices in a well. The method includes installing a conduit in the well and pumping an untethered object through the well that becomes trapped and actuates a target device. The conduit installed in the well has a plurality of devices disposed thereon such that a fluid passageway extends through the conduit and through each device. Each device includes one or more assemblies. Each assembly includes an outer sleeve having on its inner surface a fulcrum and an indentation, and a plurality of rocking sleeve members circumferentially arranged within the outer sleeve such that the fluid passage extends therethrough. Each sleeve member has an outer surface dimensioned and shaped to pivot on the fulcrum and to partially fit into the indentation when rocked toward the indentation. The untethered object is selected and pumped through the passageway. The object is shaped and dimensioned to pass through certain assemblies and to be trapped by a target assembly that has sleeve members that are prevented from rocking by the object due to a characteristic length and a characteristic diameter of the object. The object is trapped by the target assembly, which is included in a target device. The target device is actuated using force from the fluid pressure differential created by the object being trapped in the assembly. 
     Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a diagram illustrating a wellbore penetrating a rock formation in which multiple zones are to be independently stimulated, according to some embodiments; 
         FIGS. 2A and 2B  are sectional views of a segmented rocking sleeve as it is placed inside the sliding valve sleeve, according to some embodiments; 
         FIG. 3  is a perspective view of a set of rocking sections used as part of a rocking sleeve assembly, according to some embodiments; 
         FIGS. 4A-4E  are a sequence of snapshot sectional views illustrating the rocking actions of a sleeve assembly as a dart is being allowed to pass, according to some embodiments; 
         FIGS. 5A-5C  are a sequence of snapshot sectional views illustrating the case when a dart is caught by a rocking sleeve assembly, according to some embodiments; 
         FIGS. 6A and 6B  are axial sectional views of a dart and rocking sleeve assembly forming part of a sliding valve positioned inside a casing, according to some embodiments; and 
         FIG. 7  is a cross sectional diagram showing a plurality of zones of a formation and a plurality of casing valves having rocking sleeve functionality, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the examples of the subject disclosure only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements. As used herein, the terms and phrases “deviated section or portion of the well”, “deviated section or portion”, “horizontal section or portion of the well”, and “horizontal section or portion” are used interchangeably to indicate the section of the well that departs from the vertical wellbore. 
     According to some embodiments, a new sleeve and dart design is combined with a segmented sleeve that exhibits a rocking (or pivoting) function. The rocking function of the sleeve segments allows the bottom end of the sleeve to open wider when the front of a dart is pressed against it, provided the length of the dart is shorter than a “target length” determined by the sleeve geometry. When the dart is at least as long as the “target length” of the sleeve, the back of the dart prevents the sleeve from rocking as the front of the dart is pressed against the bottom seat. The dart remains pressed against the sleeve, effectively isolating the wellbore section above the dart from the one below it. Multiple zones can thus be independently isolated by using a set of sleeves with similar diameter, but different “target lengths”. Additionally, sleeves and darts of different diameters (smaller towards the bottom of the well) can also be used. By using both dart length and dart diameter, rather than having a one-dimensional sequence of increasing radii (as in the known ball-restriction technique), we have a two-dimensional set of possible configurations. This greatly enhances the number of stages that could be isolated in a multi-stage fracturing operation; particularly because in contrast with the radial dimension, which is constrained by the well diameter, there is much more freedom along the axial direction. 
       FIG. 1  is a diagram illustrating a wellbore penetrating a rock formation in which multiple zones are to be independently stimulated, according to some embodiments. The subterranean formation  114  is penetrated by wellbore  112  from a surface wellsite  110 . The formation  114  has a plurality of zones that are to be independently stimulated, such as zones  120 ,  122 ,  124 ,  126  and  128 . Within well bore  112  is a casing (or production tubing)  130  that has multiple sliding casing valves  140 ,  142 ,  144 ,  146  and  148  that are positioned so as to be aligned with zones  120 ,  122 ,  124 ,  126  and  128 , respectively. The casing  130 , according to some embodiments, is cemented in place within wellbore  112 . Each of the sliding casing valves  140 ,  142 ,  144 ,  146  and  148  are equipped with a rocking sleeve mechanism, which will be described more fully, infra. According to some embodiments, a specifically dimensioned dart is dropped and/or pumped down the inner conduit of casing  130  so as to selectively target actuation of a specific sliding casing valve. The dart actuates the targeted sliding casing valve by interacting with the rocking sleeve mechanism such that it is “caught” by the rocking sleeve and fluid pressure forces the sliding valve to open. Although five zones and five sliding casing valves are shown in  FIG. 1  for purposes of clarity, in many applications there are other numbers of zones and valves. For example, it is not uncommon for 20 or more zones that are to be stimulated using 20 or more sliding valves. 
       FIGS. 2A and 2B  are sectional views of a segmented rocking sleeve as it is placed inside the sliding valve sleeve, according to some embodiments. Rocking sleeve assembly  200  is shown that forms part of a sliding casing valve, such as valves  140 ,  142 ,  144 ,  146  and  148  shown in  FIG. 1 .  FIG. 2A  shows the rocking sleeve assembly  200  with the segments closed at the bottom end (i.e. further to the right side). This is the “normal” configuration, meaning that the sleeve is in this position when not engaged by a dart. This is also the position of the rocking sections when a dart is not being allowed to pass. The sleeve assembly  200  is made up of an outer sleeve  250  that has a flared indentation  254  beginning at a location  252 , which acts as a fulcrum on which the rocking segments pivot. A plurality of rocking segments are disposed within the outer sleeve  250  of which two,  210  and  220 , are visible in  FIGS. 2A and 2B . In  FIG. 2A  the bottom surfaces  218  and  228  of rocking sections  210  and  220  respectively are in contact with each other as shown. The upper surfaces  216  and  226  of rocking sections  210  and  220  respectively are not in contact with each other. The upper end of each rocking section  210  and  220  includes a protruding portion,  214  and  224 , respectively. The bottom end of each rocking section  210  and  220  also includes a protruding portion  212  and  222 , respectively. A ramp member  260  ensures a smooth transition to the upper protruding portions  214  of section  210  and  224  of section  220 .  FIG. 2B  shows the rocking sleeve assembly  200  with the segments  210  and  220  closed at the top end. Is can be seen that the upper surfaces  216  and  226  are in contact while the lower surfaces  218  and  228  are not. The outer surfaces of the rocking portions  210  and  220  fit into the indentation  254  as shown. In the configuration shown in  FIG. 2B  the sleeve will allow the passage of darts having a characteristic length that is shorter than the target length of the sleeve. 
     Although two rocking sections  210  and  220  are visible in  FIGS. 2A and 2B , in general there can be two or greater numbers of rocking sections, such as two, three, four, five or six rocking sections. Each of the rocking sections exhibits a rocking (or pivoting) function between its front (uphole) end and its back (downhole) end. The space between the upper and lower protrusions give the rocking sections a characteristic length “L” that is also referred to as the “target length” of the sleeve. The rocking function allows darts having a shorter characteristic length than the “target length” of the sleeve to pass through the sleeve. 
       FIG. 3  is a perspective view of a set of rocking sections used as part of a rocking sleeve assembly, according to some embodiments. In the example shown, the rocking portion of the sleeve is segmented into four circumferential rocking sections, each spanning a 90 degree angle. Other numbers of rocking sections can be used including 2, 3, 5, 6 or more sections. The rocking sections  210 ,  310 ,  220  and  320  are dimensioned so as to fit into an appropriately shaped portion of the casing (or production tubing), such as into indentation  254  of sleeve  250  shown in  FIG. 2 . The casing (or tubing) is shaped to constrain the axial position of the rocking portions and to allow the segments to rock (or pivot) around an axis near the mid-length location of the sleeve (for example location  252  on sleeve  250  shown in  FIG. 2 ). Also visible in  FIG. 3  are the upper protrusions  214  and  314  of rocking sections  210  and  310 , upper surface  216  of rocking section  210 , and lower protrusions  212  and  312  of sections  210  and  310 . Note that the rocking sections are positioned in  FIG. 3  in the “normal” position, where the sleeve is either not being engaged by a dart, or not allowing a dart to pass. As can be seen, the bottom end surfaces of sections are in contact with each other. As can be seen, when the rocking sections are in this position, a relatively good fluid seal can be formed by the bottom end surfaces of sections being in contact with each other, and the lower protrusions of each rocking section mating with an appropriately shaped dart that is being “trapped” by the rocking sleeve. The fluid sealing characteristic is advantageous in providing zonal isolation and also for providing sufficient fluid pressure to actuate the sliding valve mechanism. As discussed, supra, although four rocking sections are shown in  FIG. 3 , other numbers of rocking sections can be used provided there are at least two rocking sections. When selecting the number of rocking sections to used for a particular application, consideration should be given to the requisite strength and bending stiffness of the sleeve segments and the material used. For example, in some cases two rocking sections are used which provides greater strength and bending stiffness. In such cases the ends of the protruding portions  212  and  222  (e.g. shown in  FIGS. 2A and 2B ) should be appropriately “flared” so that when the rocking sections pivot to open up at the bottom end (such as shown in  FIG. 2B ), the dart can pass through. This flare shaping becomes more important when fewer numbers of rocking sections are used. 
       FIGS. 4A-4E  are a sequence of snapshot sectional views illustrating the rocking actions of a sleeve assembly as a dart is being allowed to pass, according to some embodiments. The dart  410  is able to pass through the rocking sleeve assembly  200  since the characteristic length “l” is shorter than the target length “L” of the sleeve assembly. The shape of solid dart  410  can be thought of as three sections, although it may be manufactured from a single solid piece of material. The front section  412  in the example shown is semispherical and this shape should be matched to the “seat” formed by the bottom end protrusions of the sleeve sections (e.g.  212  and  222  in  FIGS. 2A-2B and 212 and 312  in  FIG. 3 ). The middle section  414  is preferentially cylindrical (or with cylindrical sub-sections at the ends, with some appropriately smooth shape in between). According to some embodiments, the length of the middle section determines the characteristic length l of the dart. The back section  416  is preferentially conical, or exhibiting an appropriate tapering towards the back end so as to allow the rocking segments to rock (without being constrained by the back of the dart) in cases where the dart is being allowed to pass through the sleeve. In particular, the back section  416  should be shaped such that it will clear the upper protrusions (e.g.  214  and  224  of sections  210  and  220  shown in  FIG. 2 ) of the rocking sections in cases where characteristic length l is equal to or shorter than the target length L of the sleeve. Note that while the front section of dart  412  is shown to be semispherical, in general other shapes can be used. In general any revolved surface (i.e. surface of revolution) can be used for the front section  412  and this shape should be matched to the seat formed by the bottom end protrusions of the rocking sections. For example, according to some embodiments, the front of the dart is conical and the seat formed by the bottom end protrusions of the rocking sections is a matching conical shape. 
     In  FIGS. 4B-4C , it can be seen that as the front of the dart  410  reaches the bottom seat of the rocking sections (e.g. bottom protrusion  212  of section  210 ) of the sleeve assembly  200 , the dart  410  pushes the rocking sections radially outwards. This rocking action of the rocking sections causes the top protrusions of the rocking sections (e.g. protrusion  214  of section  210 ) to move radially inwards in cases where the top protrusions are not blocked by the dart (which in this case is due to the dart length  1  being equal to or shorter than the sleeve target length L). As can be seen from  FIG. 4D , once the bottom seat opens, the dart can pass through. In  FIG. 4E  it can be seen that the rocking sections return to the “normal” position in which the bottom end seat (e.g. protrusion  212  of section  210 ) are pushed back radially inwards. According to some embodiments a spring can optionally be provided (not shown) which can be used to keep the sleeve segments in a normally closed position such as shown in  FIG. 4E , if desired. 
     As mentioned, the shape of the bottom protrusions (e.g.  212  and  222  in  FIGS. 2A, 2B, 4A -E, and  212  and  312  in  FIG. 3 ) can be conical, spherical or the shape of some other surface of revolution that forms a segmented “seat” at the bottom end of the sleeve assembly. This seat mates with the shape of the front section  412  of the dart. By matching the seat geometry with the geometry of the front of the dart, sufficient fluid isolation can be provided between the sections of the well above and below the sleeve. 
       FIGS. 5A-5C  are a sequence of snapshot sectional views illustrating the case when a dart is caught by a rocking sleeve assembly, according to some embodiments. The dart  510  has a characteristic length l, that is determined by the length of the cylindrical middle part of the dart. The sleeve assembly  200  has rocking sections (of which  210  and  220  are visible) with a target length of L, as measured between the bottom and top protrusions (e.g.  212  and  214  of section  210 ). In this case, l&gt;L. In  FIG. 5C , it can be seen that as the front of the dart  510  reaches the bottom seat formed by the bottom protrusions ( 212  and  224 ), the front of the dart tries to push the rocking sections radially outwards. However, the rocking sections are unable to rock because the upper protrusions ( 214  and  224 ) are pressed against the back of the dart  510 . Consequently, the dart  510  is not allowed to pass through. The dart  510  remains anchored in place as shown in  FIG. 5C . A seal is formed by the contact of the rocking sections with each other, and with the front of the dart  510  seated in the bottom protrusions of the rocking section. This seal prevents any fluid (or can allow an acceptable minimal amount of fluid) being pumped into the well to flow pass the dart  510 , and therefore isolate the lower section of the well below the dart. 
       FIGS. 6A and 6B  are axial sectional views of a dart and rocking sleeve assembly forming part of a sliding valve positioned inside a casing, according to some embodiments. Sliding casing valve  140  is shown positioned in zone  120  of the subterranean rock formation. In this case the valve  140  is cemented with cement  620  in place within the wellbore  112 . Sliding casing valve  140  includes sleeve assembly  200  mounted within recessed location a valve housing  610  such that it can slide axially.  FIG. 6A  shows sleeve assembly  200  in its initial configuration, which according to some embodiments is as it was conveyed downhole. The sleeve assembly  200  covers a plurality of fracturing ports in the casing, of which  612  and  614  are visible in  FIGS. 6A and 6B . A plurality of shear pins, of which pins  616  and  618  are visible in  FIG. 6A , keep the shifting sleeve assembly  200  in place. Dart  510  is shown and is not allowed to pass through sleeve assembly  200 , since dart  510  has a characteristic length l that is equal to or greater than the target length L of the sleeve assembly  200 , such as shown in  FIGS. 5A-5C .  FIG. 6B  shows the sleeve assembly  200  shifted towards the well bottom typically by the pressure differential between the top and bottom of the dart. The shear pins are designed to fail by shearing action under an appropriately chosen axial load on the sleeve, and allow the sleeve assembly  200  to shift downwards. The fracturing ports are now exposed to the inner fluid in the casing. Accordingly, when the valve  140  is in the open position, as shown in  FIG. 6B , fracturing fluid can flow into the rock as shown by the dashed arrows. 
       FIG. 7  is a cross sectional diagram showing a plurality of zones of a formation and a plurality of casing valves having rocking sleeve functionality, according to some embodiments. Zones  120 ,  122 ,  124 ,  126 ,  128  and  720  can be independently isolated and stimulated using sliding casing valves  140 ,  142 ,  144 ,  146 ,  148  and  740 , which are equipped with sleeve assemblies  200 ,  762 ,  764 ,  766 ,  768  and  770 , respectively. Each of the sleeve assemblies has a target length L as well as a characteristic diameter “D” which is a minimum diameter between protrusions of the sleeve assembly, which is also equal to the minimum diameter that a dart has to be trapped by the sleeve, assuming its characteristic length l is greater than or equal to the target length L of the sleeve assembly. In the example of  FIG. 7 , sleeve assemblies  200 ,  762 ,  764 ,  766 ,  768  and  770  have characteristic diameters D 1 , D 2 , D 3 , D 4 , D 5  and D 6 , and target lengths L 1 , L 2 , L 3 , L 4 , L 5  and L 6 , respectively. According to some embodiments, by using both, dart length and dart diameter, rather than having a one-dimensional sequence of increasing radii (as in the known ball-restriction technique), a two-dimensional set of possible configurations is provided. This greatly enhances the number of stages that could be isolated in a multi-stage fracturing operation; particularly because in contrast with the radial dimension, which is constrained by the well diameter, there is much more freedom along the axial direction. In the example of  FIG. 7 , the first three sleeve assemblies  200 ,  762 ,  764  all have equal diameters, which are all greater than the diameters of the last three sleeve assemblies  766 ,  768  and  770 . In other words: D 1 =D 2 =D 3 &gt;D 4 =D 5 =D 6 . Note that only D 2  and D 4  are labeled in  FIG. 7  for clarity. Further the target length of each of the first three sleeve assemblies is shorter than the previous such that L 1 &gt;L 2 &gt;L 3 . Similarly, the target length of each of the last three sleeve assemblies is shorter than the previous such that L 4 &gt;L 5 &gt;L 6 . In an example, a dart  712  having diameter d 2 , characteristic length l 2 , passes through all the sleeve assemblies  200 ,  762 ,  764 ,  766  and  768 , and is only trapped by sleeve assembly  770 . Because d 2 &lt;D 1 , D 2  and D 3 , dart  712  passes through sleeve assemblies  200 ,  762  and  764 . Because l 2 &lt;L 4  and L 5 , dart  712  passes through sleeve assemblies  766  and  768 . But dart  712  is trapped by sleeve assembly  770  because l 2 ≥L 6 , and d 2 ≥D 6 . Similarly, despite d 2 ≥D 1 , D 2  and D 3 , dart  710  passed through assemblies  200  and  762  since l 1 &lt;L 1  and L 2 , and is trapped by assembly  764  since l 1 ≥L 3 . 
     According to some embodiments the darts, either entirely or partially (e.g. appropriate sections of it), are made of dissolvable materials. This will facilitate the operation of opening the well for production after the fracturing operation has taken place. 
     According to some embodiments, the dart and sleeve mechanism described herein can be used for many other downhole applications whenever there is a need to selectively activate a series of actuations of devices (e.g. valves, packer, and etc.). 
     The techniques described, according to some embodiments, have advantages over techniques such as described in US 2015/0000935 that uses movable dart pieces. A single solid dart that has no moving parts is more structurally robust. As a result, designers have greater freedom in selecting dart material. This is important when darts are made from a dissolvable material. The techniques described herein also do not rely on control lines on the outside of the completion. 
     The techniques described, according to some embodiments, also have advantages over techniques such as described in US 2014/0299319 that use rocking members on a sleeve combined with a plurality of casing ribs. In this known technique, the sleeve assembly becomes longer with each additional zone by the rib-to-rib spacing amount. In contrast, in the techniques described herein, the sleeve assembly becomes longer for each additional zone by a much smaller amount (e.g. the width of the upper protrusions  214  and  224  in  FIGS. 2A and 2B ). Furthermore, the rocking sections can be configured to form a better seal with the dart since the rocking section bottom surfaces can meet each other such as shown in  FIG. 3 . 
     Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the examples without materially departing from this subject disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.