Patent Publication Number: US-11047202-B2

Title: Top plug with transitionable seal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Division of U.S. application Ser. No. 15/335,118, filed on Oct. 26, 2016. The aforementioned patent application is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field of the Invention 
     Embodiments of the present disclosure generally relate to plugs for casing floatation and/or pressure testing, and methods of use and assembly thereof. 
     In well completion operations, a wellbore is formed by drilling to access hydrocarbon-bearing formations. After drilling to a predetermined depth, the drill string and drill bit are removed, and a section of casing (or liner or pipe or tubular) is lowered into the wellbore. An annular area is formed between the string of casing and the formation, and a cementing operation may then be conducted to fill the annular area with cement. 
     In some operations, insertion of casing is problematic due to the characteristics of the wellbore. For example, in a highly deviated wellbore (e.g., high inclination, extended horizontal reach, or multiple directional changes), there may be high friction between the wellbore wall and the casing. In such operations, techniques include filling a section of the casing with a buoyancy fluid (a liquid or a gas) that has a lower density than the liquid contained inside the wellbore. As the casing is lowered into the wellbore, this difference in fluid density provides partial or complete buoyancy of the section of casing containing the buoyancy fluid. This buoyancy may reduce the friction, thus aiding in casing insertion. 
     Following insertion of the casing, the buoyancy fluid may be removed from the section of casing, either uphole or downhole, depending on factors such as equipment configuration, buoyancy fluid properties, formation properties, operational considerations, etc. Cement may then be pumped through the casing to fill the annular area. Typically a pressure test will follow to confirm the casing and plug connections. Once the casing is free of obstructions, production of formation fluids can begin. 
     However, equipment and techniques applicable to trapping and releasing buoyancy fluid in a section of casing can often impede cementing, pressure testing, and production. For example, plugs used in trapping buoyancy fluid may obstruct the bore of the casing, requiring drill-out before production. Accordingly, there is a need for an improved equipment and methodology that allows buoyant insertion of casing without additional delay or drilling prior to production. 
     SUMMARY 
     The present disclosure generally provides plugs for casing floatation and/or pressure testing, and methods of use and assembly thereof. 
     In an embodiment, a top latch-in plug includes a housing having: a head end; a tail end; and a bore from the head end to the tail end; and a transitionable seal, wherein: the transitionable seal seals the bore of the housing when in a first configuration, the transitionable seal unseals the bore when in a second configuration, and the transitionable seal is triggerable to transition from the first configuration to the second configuration. 
     In an embodiment, a method of well completion includes floating a casing in a wellbore; pumping cement downhole through the casing to supply cement between the casing and the wellbore; sequentially engaging a lower bottom latch-in plug and a top latch-in plug to a landing collar of the casing, wherein the top latch-in plug includes a transitionable seal sealing a bore of the top latch-in plug; pressure testing the casing; and triggering the transitionable seal to unseal the bore of the top latch-in plug. 
     In an embodiment, a method of well completion includes causing a casing to be floated in a wellbore; causing cement to be pumped downhole through the casing to supply cement between the casing and the wellbore; sequentially engaging a lower bottom latch-in plug and a top latch-in plug to a landing collar of the casing, wherein the top latch-in plug includes a transitionable seal sealing a bore of the top latch-in plug; causing the casing to be pressure tested; and causing a triggering of the transitionable seal to unseal the bore of the top latch-in plug. 
     In an embodiment, a casing floatation system includes a casing having a pre-load collar and a landing collar; and a lower bottom latch-in plug comprising: a catch mechanism compatible with the pre-load collar; and a landing mechanism compatible with the landing collar. 
     In an embodiment, a method of well completion includes floating a casing in a wellbore, wherein the casing includes a pre-load collar located uphole from a landing collar, the floating the casing comprising: disposing the casing in the wellbore; disposing buoyancy fluid in the casing between the pre-load collar and the landing collar; and sealing the buoyancy fluid in the casing by engaging a lower bottom latch-in plug with the pre-load collar; discharging the buoyancy fluid from the casing; releasing the lower bottom latch-in plug from the pre-load collar; and engaging the lower bottom latch-in plug with the landing collar. 
     In an embodiment, a method of assembling a latch-in plug includes obtaining a casing having a pre-load collar and a landing collar; disposing buoyancy fluid in the casing between the pre-load collar and the landing collar; catching a forward portion of a latch-in plug with the pre-load collar, thereby sealing the buoyancy fluid in the casing; and securing an aft portion of the latch-in plug to the forward portion. 
     In an embodiment, a method of well completion includes causing a casing to be floated in a wellbore, wherein: the casing includes a pre-load collar located uphole from a landing collar, and floating the casing comprises: disposing the casing in the wellbore; disposing buoyancy fluid in the casing between the pre-load collar and the landing collar; and sealing the buoyancy fluid in the casing by engaging a lower bottom latch-in plug with the pre-load collar; discharging the buoyancy fluid from the casing; causing a lower bottom latch-in plug to be released from the pre-load collar; and engaging the lower bottom latch-in plug with the landing collar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  illustrates a casing having a pre-load collar and a landing collar downhole from the pre-load collar according to embodiments of the invention. 
         FIG. 2  illustrates a lower bottom latch-in plug caught in a pre-load collar according to embodiments of the invention. 
         FIG. 3  illustrates an upper bottom latch-in plug uphole from a pre-load collar according to embodiments of the invention. 
         FIG. 4  illustrates an upper bottom latch-in plug latched-in with a lower bottom latch-in plug according to embodiments of the invention. 
         FIG. 5  illustrates a bottom latch-in plug released from a pre-load collar according to embodiments of the invention. 
         FIG. 6  illustrates a bottom latch-in plug proximate to a landing collar according to embodiments of the invention. 
         FIGS. 7A-C  illustrate a top latch-in plug according to embodiments of the invention. 
         FIG. 8  illustrates a top latch-in plug proximate to a bottom latch-in plug according to embodiments of the invention. 
         FIG. 9  illustrates an unsealed top latch-in plug proximate to a bottom latch-in plug that is proximate to a landing collar according to embodiments of the invention. 
         FIGS. 10A-D  illustrate an alternative top latch-in plug according to embodiments of the invention. 
         FIGS. 11A-E  illustrate another alternative top latch-in plug according to embodiments of the invention. 
         FIG. 12  illustrates a forward portion of a lower bottom latch-in plug according to embodiments of the invention. 
         FIG. 13  illustrates a forward portion of a lower bottom latch-in plug proximate to a pre-load collar according to embodiments of the invention. 
         FIG. 14  illustrates an aft portion of a lower bottom latch-in plug according to embodiments of the invention. 
         FIG. 15  illustrates an aft portion of a lower bottom latch-in plug proximate to a forward portion of a lower bottom latch-in plug according to embodiments of the invention. 
         FIG. 16  illustrates a catch mechanism of a lower bottom latch-in plug according to embodiments of the invention. 
         FIG. 17A-B  illustrate methods of well completion according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure generally relate to plugs for casing floatation and pressure testing, and methods of use and assembly thereof. 
       FIG. 1  illustrates a casing  100  having a pre-load collar  102  and a landing collar  104  downhole from the pre-load collar  102 . A float shoe with a check valve may be connected at the end of the casing string, downhole from the landing collar  104 . The check valve may be biased closed until the pressure inside the casing  100  equals or exceeds the pressure outside the casing  100 . For example, the check valve may allow fluid (a liquid or gas) to exit the casing  100  when the pressure inside the casing  100  exceeds the pressure outside the casing  100  by a selected amount. The check valve may close to prevent entry of fluid into the casing  100  when the pressure outside the casing  100  exceeds the pressure inside the casing  100  (or when the pressure inside the casing  100  does not exceed the pressure outside the casing  100  by the selected amount). Between the pre-load collar  102  and the landing collar  104  may be a stimulation tool  106 . During operation, the casing  100  will typically be located in a wellbore so that the landing collar  104  is near the bottom of the wellbore. Cement may then be circulated downhole through the casing  100 , through the landing collar  104 , out of the casing string through the check valve of the float shoe, and uphole through an annulus between the casing  100  and the wellbore. Once the cement sets, the formation surrounding the stimulation tool  106  may be stimulated, for example by perforating the casing  100  at the stimulation tool  106 . In some embodiments, one or more toe sleeves may be utilized with, or in lieu of, stimulation tool  106 , and may be located near stimulation tool  106 , near landing collar  104 , or between stimulation tool  106  and landing collar  104 . A toe sleeve is a ported collar that is run downhole as part of the casing string. A toe sleeve may be opened (for example, with a pressure signal) to communicate with the wellbore. Multiple toe sleeves may be run, and the toe sleeves may be distributed to cover large production zones or multiple production zones. Typically, to provide a clear (free of cement) communication path through the toe sleeves to the wellbore, a quantity of displacement fluid may be pumped downhole following the pumping of cement (known as “over-displacement” of the cement). 
     To assist in locating the casing  100  in the wellbore, especially if the wellbore is highly deviated (e.g., high inclination, extended horizontal reach, or multiple directional changes), the casing  100  may be “floated” into the wellbore. In some embodiments, a buoyancy fluid may be disposed in the casing  100  between the pre-load collar  102  and the landing collar  104  prior to moving the casing  100  downhole. For example, the buoyancy fluid may be sealed in the casing  100  between the pre-load collar  102  and the landing collar  104 . Suitable buoyancy fluids include a gas, a liquid, or a gas and liquid mixture having a density that is less than the density of the fluid in the wellbore. The lighter density fluid may cause the casing to “float” in the heavier density fluid in the wellbore. In this respect, the buoyancy fluid sealed inside the casing may reduce frictional forces between the casing  100  and the wellbore as the casing  100  is floated into place. In some instances, a heavier pumping fluid may fill the casing  100  uphole from the pre-load collar  102 , thereby adding weight to assist with running the casing  100 . Suitable pumping fluids include any of a variety of fluids typically pumped in a well completion operation, such as water, mud, drilling fluid, spacer fluid, chemical wash, cement, etc. The buoyancy fluid may be introduced into the casing  100  while the casing  100  is at or near the surface of the wellbore. For example, air at atmospheric pressure may be used as a buoyancy fluid. Other fluids may be introduced into the casing  100  to displace air at atmospheric pressure. 
     The casing  100  may move downhole while the buoyancy fluid is introduced, or the casing  100  may remain near the surface of the wellbore until the buoyancy fluid is sealed in the casing  100 . In some embodiments, the casing  100  with the pre-load collar  102  and landing collar  104  may be constructed prior to introduction into the wellbore. In other embodiments, casing  100  may be constructed in segments. For example, a first casing segment having a landing collar  104  and float shoe may be introduced into the wellbore at the surface. A second casing segment having a stimulation tool  106  may then be connected to the first casing segment, thereby moving the casing  100  downhole by the length of the second casing segment. A third casing segment having a pre-load collar  102  may then be connected to the second casing segment, thereby moving the casing  100  downhole by the length of the third casing segment. The buoyancy fluid may then be introduced into casing  100  and sealed at the downhole end by the check valve of the float shoe, and at the uphole end by coupling a lower bottom latch-in plug  200  in the pre-load collar  102 . For example, the check valve may seal the downhole end of the casing  100  by remaining closed in response to the external pressure exceeding the internal pressure (or when the pressure inside the casing  100  does not exceed the pressure outside the casing  100  by the selected amount). 
       FIG. 2  illustrates a first bottom plug  200  caught in and/or coupled to the pre-load collar  102  of casing  100 . As shown, the first bottom plug  200  is a lower bottom latch-in plug  200  having a housing  210 , a head end  220 , a tail end  230 , a bore  240  in the housing  210  extending from the head end  220  to the tail end  230 , one or more fins  250 , a pressure seal  260 , and a catch mechanism  270  that is compatible with, configured to releasably connect with, and/or configured to releasably engage the pre-load collar  102 . Head end  220  may have a landing mechanism that is compatible with, configured to connect with, and/or configured to engage landing collar  104 . Tail end  230  may have a retaining mechanism to receive other latch-in plugs. Fins  250  may be made of a flexible material, such as rubber or polyurethane, and may extend radially outward and/or at an angle towards the tail end  230 . Fins  250  may comprise short fins, long fins or a combination thereof as operationally desired. 
     Lower bottom latch-in plug  200  is introduced, head end  220  first, into casing  100  behind the buoyancy fluid. Lower bottom latch-in plug  200  forms an uphole seal for the buoyancy fluid. In particular, fins  250  of lower bottom latch-in plug  200  contact and seal against the interior wall of casing  100 , and pressure seal  260  of lower bottom latch-in plug  200  seals the bore  240  of lower bottom latch-in plug  200 . Once introduced into the casing  100 , lower bottom latch-in plug  200  travels downhole through the casing  100 , until reaching pre-load collar  102 . Lower bottom latch-in plug  200  may travel downhole by gravity, by pumping of a pumping fluid behind the lower bottom latch-in plug  200 , or by an assembly tool  800  (discussed below). The catch mechanism  270  causes lower bottom latch-in plug  200  to be caught by the pre-load collar  102 . In some embodiments, the catch mechanism  270  may include a collet and a shear ring. The catch mechanism  270  may beneficially provide few or no obstructions in the interior of the casing  100  at the pre-load collar  102  after the lower bottom latch-in plug  200  is released. Once the pre-load collar  102  catches the lower bottom latch-in plug  200 , the buoyancy fluid is sealed in the casing  100 . The casing  100  may then be moved further downhole in the wellbore until reaching the desired landing location. As used herein, “seal”, “sealed”, “block”, “blocked”, and similar wording refers to preventing fluid communication to within acceptable error tolerances. In other words, a bore is “sealed” if no fluid can pass through, but also if fluid can pass through at a rate that is sufficiently low to allow the sealing feature to perform its intended function. As used herein, “unseal”, “unsealed”, “unblock”, “unblocked”, and similar wording refers to allowing fluid communication at desired flow rates to within acceptable error tolerances. In other words, a bore is “unsealed” if fluid can pass through at a rate that is sufficiently high to allow the fluid communication feature to perform its intended function. 
     The pressure seal  260  may operate to seal and/or block the bore  240  at the tail end  230  of the housing  210  until the downhole pressure reaches a specific level, at which point the pressure seal  260  releases, and the bore  240  is no longer blocked. For example, the pressure seal  260  may be a rupture disk that is sensitive to a specific pressure signal. As will be appreciated with the discussion that follows, in some embodiments the pressure seal  260  is selected to release at a downhole pressure that is relatively low, while still being higher than the downhole pressure expected to be used to pump lower bottom latch-in plug  200  downhole to pre-load collar  102 . For example, in some embodiments the pressure seal  260  may be a rupture disk configured to rupture at a predetermined pressure such as 2,500 psi. 
     Once pre-load collar  102  catches lower bottom latch-in plug  200 , pumping of pumping fluid behind the lower bottom latch-in plug results in an increase in downhole pressure. Such downhole pressure increase may be detected at the surface as an indication that lower bottom latch-in plug  200  has sealed the buoyancy fluid in the casing  100 . Surface operations may shift from pumping of pumping fluid to moving the casing  100  further downhole in the wellbore. Once the casing  100  reaches the desired landing location, surface operations may resume pumping of pumping fluid. Continued pumping of pumping fluid behind the lower bottom latch-in plug results in an increase in downhole pressure until reaching a level that causes pressure seal  260  to release. In some operations, downhole pressures may be monitored, and a selected pressure signal may be used to cause pressure seal  260  to release. The buoyancy fluid, being less dense than the expected wellbore liquids at the intended location for the casing  100 , may then travel uphole through bore  240 . Likewise, the pumping fluid behind the lower bottom latch-in plug may replace the buoyancy fluid in the casing  100  between the pre-load collar  102  and the landing collar  104 . In some embodiments, some or all of the buoyancy fluid may exit the casing  100  through the landing collar  104  and through the check valve of the float shoe. The buoyancy fluid may thus be discharged from the casing  100 . 
       FIG. 3  illustrates a second bottom plug  300  uphole from pre-load collar  102  of casing  100 . As shown, the second bottom plug  300  is an upper bottom latch-in plug  300  having a housing  310 , a head end  320 , a tail end  330 , a bore  340  in the housing  310  extending from the head end  320  to the tail end  330 , one or more fins  350 , and a pressure seal  360 . Fins  350  may be made of a flexible material, such as rubber or polyurethane, and may extend radially outward and/or at an angle towards the tail end. Fins  350  may comprise short fins, long fins or a combination thereof as operationally desired. Upper bottom latch-in plug  300  is introduced, head end  320  first, into casing  100  and travels downhole through the casing  100 , until reaching lower bottom latch-in plug  200 . Upper bottom latch-in plug  300  may travel downhole by gravity and/or by pumping of a pumping fluid behind the upper bottom latch-in plug  300 . 
       FIG. 4  illustrates the upper bottom latch-in plug  300  latched-in with and/or engaged with lower bottom latch-in plug  200 . The head end  320  of upper bottom latch-in plug  300  is designed to mate with the tail end  230  of lower bottom latch-in plug  200 , thereby coupling the upper bottom latch-in plug  300  to the lower bottom latch-in plug  200 . For example, a retaining mechanism may be used to latch-in upper bottom latch-in plug  300  with lower bottom latch-in plug  200 . An example of a suitable retaining mechanism is available from Weatherford® as described in product brochure Doc No. 5-3-GL-GL-CES-00029, Revision 2, Date 17 Aug. 2015. The combined upper bottom latch-in plug  300  and lower bottom latch-in plug  200  will be referred to as “bottom latch-in plug  200 / 300 .” 
     Continued pumping of pumping fluid behind the bottom latch-in plug  200 / 300  raises the downhole pressure. The catch mechanism  270  is designed to release in response to a selected pressure signal. It should be appreciated that the level of downhole pressure selected for the pressure signal to cause the catch mechanism  270  to release may be greater than the level of downhole pressure selected to release for previously-discussed pressure seal  260 . For example, in some embodiments the catch mechanism  270  may utilize a 3000 psi shear ring. Once the downhole pressure rises to the selected level, catch mechanism  270  releases, and the bottom latch-in plug  200 / 300  moves downhole from pre-load collar  102 , as illustrated in  FIG. 5 . 
     In some embodiments, the pumping fluid behind bottom latch-in plug  200 / 300  includes cement. Bottom latch-in plug  200 / 300  may wipe the interior surface of casing  100  in advance of the cement. The pumping fluid may also include one or more chemical washes and/or spacer fluids to better prepare the interior of casing  100  for the cement. 
     As illustrated in  FIG. 6 , bottom latch-in plug  200 / 300  travels downhole until it reaches landing collar  104 . Bottom latch-in plug  200 / 300  then latches-in with landing collar  104 . The head end  220  of lower bottom latch-in plug  200  is designed to mate with and securely couple to landing collar  104 . For example, a landing mechanism may be used to latch-in bottom latch-in plug  200 / 300  with landing collar  104 . Commonly available landing mechanisms may be used to meet operational needs. 
     Continued pumping of pumping fluid (including cement) behind the bottom latch-in plug  200 / 300  raises the downhole pressure. Such downhole pressure increase may be detected at the surface as an indication that bottom latch-in plug  200 / 300  has reached the landing collar  104 . Continued pumping of pumping fluid (including cement) behind the bottom latch-in plug  200 / 300  results in an increase in downhole pressure until reaching a level that causes pressure seal  360  to release. In some operations, downhole pressures may be monitored, and a selected pressure signal may be used to cause pressure seal  360  to release. It should be appreciated that the level of downhole pressure selected for the pressure signal to cause the pressure seal  360  to release may be greater than the level of downhole pressure selected for previously-discussed catch mechanism  270 . For example, in some embodiments the pressure seal  360  may be a 4000 psi rupture disk. Release of pressure seal  360  opens the bore  240 / 340  of bottom latch-in plug  200 / 300 . Cement can thus be pumped through the casing  100 , the bottom latch-in plug  200 / 300 , the landing collar  104 , and the check valve of the float shoe to enter and/or fill the annulus between the casing  100  and the wellbore. In some embodiments, a quantity of displacement fluid may be pumped through the casing  100  behind the cement. For example, when one or more toe sleeves are utilized, a sufficient quantity of displacement fluid may be pumped to over-displace the cement, allowing for a clear (free of cement) communication path between the toe sleeves and the wellbore. 
     Following the desired amount of cement and/or displacement fluid, a top plug is introduced into casing  100 , as illustrated in  FIGS. 7A-C . As shown, the top plug is a top latch-in plug  700  having a housing  710 , a head end  720 , a tail end  730 , a bore  740  in the housing  710  extending from the head end  720  to the tail end  730 , and one or more fins  750 . Fins  750  may be made of a flexible material, such as rubber or polyurethane, and may extend radially outward and/or at an angle towards the tail end. Fins  750  may comprise short fins, long fins or a combination thereof as operationally desired. Top latch-in plug  700  also includes a transitionable seal. In some embodiments, the transitionable seal may be a cap (for example, expendable cap  780 , discussed below). In the initial configuration (when top latch-in plug  700  is introduced into and pumped down casing  100 ), the cap  780  seals the bore  740  at the tail end  730  of the housing  710 . Top latch-in plug  700  is introduced, head end  720  first, into casing  100  and travels downhole through the casing  100 , until reaching bottom latch-in plug  200 / 300 . Top latch-in plug  700  may travel downhole by gravity and/or by pumping of a pumping fluid behind the top latch-in plug  700 . In some embodiments, the pumping fluid behind the top latch-in plug may be a tail slurry and/or displacement fluid. It should be appreciated that the tail slurry may be free of cement or other materials that might obstruct casing  100 , stimulation tool  106 , any toe sleeves, the float shoe, the check valve, and/or bores  740 ,  340 ,  240 ,  140  (see  FIG. 9 ) after pressure testing. 
     As illustrated in  FIG. 8 , top latch-in plug  700  travels downhole until it reaches bottom latch-in plug  200 / 300 . Top latch-in plug  700  then latches-in with bottom latch-in plug  200 / 300 . The head end  720  of top latch-in plug  700  is designed to mate with and securely couple to the tail end  330  of upper bottom latch-in plug  300 . For example, a retaining mechanism may be used to latch-in top latch-in plug  700  with upper bottom latch-in plug  300 . An example of a suitable retaining mechanism is available from Weatherford® as described in product brochure Doc No. 5-3-GL-GL-CES-00029, Revision 2, Date 17 Aug. 2015, which is incorporated herein. Note that lower bottom latch-in plug  200  is latched-in with landing collar  104 , that upper bottom latch-in plug  300  is latched-in with lower bottom latch-in plug  200 , and that top latch-in plug is latched-in with upper bottom latch-in plug  300 . Any of the latch-in plugs may be thereby considered sequentially latched-in with the downhole latch-in plugs and/or landing collar  104 . 
     Continued pumping of pumping fluid behind the top latch-in plug  700  raises the downhole pressure. Such downhole pressure increase may be detected at the surface as an indication that top latch-in plug  700  has reached the landing collar  104 . This may be an indication that most or all of the cement has traveled downhole through the casing  100 , the bottom latch-in plug  200 / 300 , the landing collar  104 , and the check valve of the float shoe to enter and/or fill the annulus between the casing  100  and the wellbore. Surface operations may shift to allow the cement in the annulus to harden, forming a cement shell around casing  100 . After it is determined that the cement has hardened (for example, with the passage of a period of time), the casing and/or the plug connections may be pressure tested. In other words, downhole pressure may be increased and held over time to confirm that the casing  100  is capable of withstanding certain downhole pressures. Some types of pressure tests include one or more pressure levels, each held for a designated period of time. It should be appreciated that the level of downhole pressure selected for the lowest pressure level of the pressure test may be greater than the level of downhole pressure selected for previously-discussed pressure seal  360 . For example, in some embodiments the downhole pressure during the pressure test may be between about 10 k psi and 12 k psi. It is currently believed that downhole pressure greater than about 12 k psi may rupture the casing  100 . 
     In conjunction with and/or following the pressure test, the transitionable seal of top latch-in plug  700  may be triggered to transition from sealing the bore  740  to unseal the bore  740 . In some embodiments, the transitionable seal may be triggered to transition with a pressure signal. In some embodiment, the transitionable seal may be triggered to transition with multi-step triggering. For example, a first triggering event may initiate the transition, a second triggering event may advance the transition, and the transitionable seal may transition from sealing the bore  740  to unseal the bore  740 . In some embodiments, the transitionable seal may be triggered to transition with a multi-step pressure signal. In some embodiments, following the pressure test, an expendable cap  780  may transition from sealing the bore  740  to unseal the bore  740 . In one configuration of such embodiment, the expendable cap  780  seals the bore  740  at the tail end  730  of the housing  710  of top latch-in plug  700 . For example, in the configuration illustrated in  FIG. 7A , the expendable cap  780  seals the bore  740  at the tail end  730  of the housing  710 . In some embodiments, the expendable cap  780  may have a lid portion  781  and a stopper portion  785 . There may be a recess  784  between the lid portion  781  and the housing  710 . The stopper portion  785  may sealingly fit in the bore  740 . One or more O-rings  786  may be located around the stopper portion  785  to create a seal with the interior of the housing  710 . Other configurations may be envisioned so that the expendable cap  780  may seal the bore  740  at the tail end  730  of the housing  710 . The expendable cap  780  may be triggered to transition from a configuration wherein the expendable cap  780  seals the bore  740  at the tail end  730  of the housing  710  to a configuration wherein expendable cap  780  unseals the bore  740 . For example, the expendable cap  780  may unseal the bore  740  by blocking no more than half of a cross-sectional area  790  of the bore  740  at the tail end  730  of the housing  710 , as in the configuration illustrated in  FIG. 7C . In the illustrated embodiment, a spring element  788  is located in the bore  740  and, when compressed by expendable cap  780 , is biased to eject the expendable cap  780  from the housing  710 . Other post-triggered configurations may be envisioned so that the expendable cap  780  unseals the bore  740 . In some embodiments, the transitionable seal may seal the bore of the housing in a post-triggered configuration. For example, in the configuration illustrated in  FIG. 7B , the expendable cap  780  seals the bore  740  at the tail end  730  of the housing  710 . Other transitionable seals of top latch-in plug  700  may be envisioned so that, in conjunction with and/or following the pressure test, the transitionable seal may be triggered to transition from sealing the bore  740  to unseal the bore  740 , such as with a hydraulic port collar, a sliding sleeve, or a staging baffle plate (see for example the discussion in relation to  FIGS. 10 and 11  below). 
     The transitionable seal may be triggered to transition from sealing the bore  740  to unseal the bore  740 , but the transitionable seal may seal the bore  740  at least until completion of the pressure test. In some embodiments, the completion of the pressure test may be indicated by a pressure-drop signal proximate the tail end  730  of the housing  710 . The transitionable seal may thereby seal the bore of the housing in a post-triggered configuration. For example, in the illustrated embodiment, the lid portion  781  of expendable cap  780  may have one or more shear pin receptacles  783  for receiving shear pins  782 . The shear pins  782  hold the expendable cap  780  in the housing  710 . The shear pins  782  are designed to shear in response to a selected pressure signal. In some embodiments, the level of downhole pressure selected for the pressure signal to cause the shear pins  782  to shear may be greater than the level of downhole pressure selected for the previously-discussed pressure seal  360 . For example, in some embodiments the shear pins  782  may be 11 k psi shear pins. Moreover, the transitionable seal may seal the bore  740  at least until the completion of the previously-discussed pressure test, as indicated by a pressure-drop signal. Therefore, while the level of downhole pressure selected for the pressure signal to cause the shear pins  782  to shear may be near, at, or above the level of downhole pressure selected for the lowest pressure level of the pressure test, the transitionable seal may seal the bore  740  until downhole pressure drops to a level below the level of downhole pressure selected for the lowest pressure level of the pressure test. As illustrated, at the selected downhole pressure for triggering the expendable cap  780 , the shear pins  782  shear, allowing the lid portion  781  of expendable cap  780  to enter the recess  784 . This further compresses spring element  788  in bore  740 . The spring element  788  may be biased to apply pressure to the expendable cap  780  in a direction away from housing  710 . In some embodiments, the downhole pressure may be increased, possibly in conjunction with a pressure test, thereby holding the lid portion  781  in the recess  784 . In some embodiments, the force of compressed spring element  788  is sufficient to overcome the downhole pressure and eject expendable cap  780  (as illustrated in  FIG. 7C ). In some embodiments, pumping pressure may be reduced to provide a pressure-drop signal, for example at the end of the pressure test, so that the force of compressed spring element  788  is sufficient to overcome the downhole pressure and eject expendable cap  780 . In some embodiments, spring element  788  includes small charges, electromagnets, or other devices to provide impulsive force to assist in ejecting expendable cap  780 . In some embodiments, spring element  788  may be replaced by a reservoir of dissolving fluid. For example, movement of expendable cap  780  into recess  784  may puncture the reservoir of dissolving fluid, causing expendable cap  780  to at least partially dissolve over a period of time. As discussed below in relation to  FIGS. 10 and 11 , other configurations may be envisioned so that, in conjunction with and/or following the pressure test, the transitionable seal may be triggered to transition from sealing the bore  740  to unseal the bore  740 , such as with a hydraulic port collar, a sliding sleeve, or a staging baffle plate. 
     As illustrated in  FIG. 9 , once the transitionable seal has transitioned from sealing the bore  740  to unseal the bore  740 , the casing  100  has an open pathway through bores  740 ,  340 ,  240 ,  140  to reach the formation through the check valve of the float shoe. In some embodiments, the check valve may be opened or disabled to allow fluid flow from the wellbore into the casing  100  through the open pathway. For example, the check valve may be sheared-out of the float shoe with a pressure signal. In other embodiments, the check valve may be otherwise opened with a pressure signal, an electronic signal, a wireless signal, or another suitable signal. In some embodiments, one or more toe sleeves may be opened to allow fluid to flow from the wellbore into the casing  100 . For example, the toe sleeves may be opened with a pressure signal, an electronic signal, a wireless signal, or another suitable signal. Stimulation of the formation and/or production of formation fluids from downhole in the wellbore can then begin. For example, stimulation fluids (e.g., fracturing or acidizing fluids) may be pumped downhole through the casing  100  and the bores  740 ,  340 ,  240 ,  140 . As another example, formation fluids may be produced from downhole through the bores  140 ,  240 ,  340 ,  740 , and the casing  100 . In some embodiments, following the pressure test, casing  100  may be perforated to allow for stimulation of and/or fluid production from the formation around stimulation tool  106 . In some embodiments, expendable cap  780  travels uphole with the production fluids. Top latch-in plug  700  and bottom latch-in plug  200 / 300  may remain latched-in with landing collar  104  during production of fluids through casing  100 . In some embodiments, one or more of the latch-in plugs  200 ,  300 ,  700  may have an anti-rotation feature, such as an anti-rotation mill profile, locking teeth, and/or plug inserts, which would allow for more efficient drill-out. For example, were it desirable to further open casing  100 , latch-in plugs  200 ,  300 ,  700  may be drilled-out. Rather than rotating in response to the drill-out tool, the anti-rotation feature of the latch-in plugs  200 ,  300 ,  700  would at least partially resist the rotational forces of the drill. 
       FIG. 10  illustrates an alternative top plug as an example of other envisioned configurations that provide a transitionable seal that, in conjunction with and/or following a pressure test, may be triggered to transition from sealing the bore  740  to unseal the bore  740 . As shown, the top plug is a top latch-in plug  700 ′ having a housing  710 ′, a head end  720 ′, a tail end  730 ′, a bore  740 ′ in the housing  710 ′ extending from the head end  720 ′ to the tail end  730 ′, and one or more fins  750 ′. Top latch-in plug  700 ′ also includes a transitionable seal. In some embodiments, the transitionable seal may be a sleeve (for example, sleeve  880 , discussed below). In the initial configuration shown in  FIG. 10A  (when top latch-in plug  700 ′ is introduced into and pumped down casing  100 ), the sleeve  880  seals the bore  740 ′ of the housing  710 ′. 
     As with top latch-in plug  700 , top latch-in plug  700 ′ may latch-in with bottom latch-in plug  200 / 300 . The casing and/or the plug connections may be pressure tested. In conjunction with and/or following the pressure test, the transitionable seal of top latch-in plug  700 ′ may be triggered to transition from sealing the bore  740 ′ to unseal the bore  740 ′. In some embodiments, following the pressure test, a sleeve  880  may transition from sealing the bore  740 ′ to unseal the bore  740 ′. For example, in the configuration illustrated in  FIG. 10A , the sleeve  880  seals the bore  740 ′ of the housing  710 ′ by blocking ports  885 . In some embodiments, the sleeve  880  may have a lid portion  781 ′ and a stopper portion  785 ′. There may be a recess  784 ′ between the stopper portion  785 ′ and the housing  710 ′. In the illustrated embodiment, a spring element  788 ′ is located in recess  784 ′ of the housing  710 ′, biasing the sleeve  880  towards the tail end  730 ′ of the housing  710 ′. The stopper portion  785 ′ may sealingly fit in the bore  740 ′. One or more O-rings  786 ′ may be located around the stopper portion  785 ′ to create a seal with the interior of the housing  710 ′. Other configurations may be envisioned so that the sleeve  880  may seal the bore  740 ′ of the housing  710 ′. The sleeve  880  may be triggered to transition from a configuration wherein the sleeve  880  seals the bore  740 ′ of the housing  710 ′ to a configuration wherein sleeve  880  unseals the bore  740 ′. For example, the sleeve  880  may unseal the bore  740 ′ as in the configuration illustrated in  FIG. 10C , wherein ports  885  are shown fluidly connected to bore  740 ′ through sleeve passages  890 . As illustrated, housing  710 ′ has four ports  885 , and sleeve  880  has four sleeve passages  890 , but various numbers, sizes, and distributions of ports  885  and sleeve passages  890  may be envisioned to accommodate operational requirements and designs. Further, other post-triggered configurations may be envisioned so that the sleeve  880  unseals the bore  740 ′. 
     As with top latch-in plug  700 , the transitionable seal of top latch-in plug  700 ′ may be triggered to transition from sealing the bore  740 ′ to unseal the bore  740 ′, and the transitionable seal may seal the bore  740 ′ at least until completion of the pressure test. In some embodiments, the completion of the pressure test may be indicated by a pressure-drop signal proximate the tail end  730 ′ of the housing  710 ′. For example, in the illustrated embodiment, the lid portion  781 ′ of sleeve  880  may have one or more shear pin receptacles  783 ′ for receiving shear pins  782 ′. The shear pins  782 ′ hold the sleeve  880  in the housing  710 ′. The shear pins  782 ′ are designed to shear in response to a selected pressure signal. The transitionable seal may seal the bore  740 ′ at least until the completion of the previously-discussed pressure test, as indicated by a pressure-drop signal. While the level of downhole pressure selected for the pressure signal to cause the shear pins  782 ′ to shear may be near, at, or above the level of downhole pressure selected for the lowest pressure level of the pressure test, the transitionable seal may seal the bore  740 ′ until downhole pressure drops to a level below the level of downhole pressure selected for the lowest pressure level of the pressure test. As illustrated, at the selected downhole pressure for triggering the sleeve  880 , the shear pins  782 ′ shear, compressing the stopper portion  785 ′ against spring element  788 ′. This further compresses spring element  788 ′ in the recess  784 ′. 
     As illustrated in  FIG. 10D , there may be a J-slot  895  on the exterior of sleeve  880 . A pin on an interior surface of housing  710 ′ may engage the J-slot  895 . In the initial configuration shown in  FIG. 10A  (when top latch-in plug  700 ′ is introduced into and pumped down casing  100 ), the pin may engage J-slot  895  at point  895 -A. In addition to shearing of shear pins  782 ′, triggering the sleeve  880  may further include moving the pin relative to J-slot  895  from point  895 -A to point  895 -B. Sleeve  880  may thereby rotate relative to housing  710 ′. Sleeve  880  blocks ports  885  of housing  710 ′ both with the pin in J-slot  895  at point  895 -A and with the pin in J-slot  895  at point  895 -B. Sleeve  880  thereby seals the bore  740 ′ when the pin is in J-slot  895  at point  895 -A and at point  895 -B. In some embodiments, following triggering sleeve  880  with a selected downhole pressure, the downhole pressure may be increased, possibly in conjunction with a pressure test, thereby holding the pin in J-slot  895  point  895 -B (as illustrated in  FIG. 10B ). The transitionable seal may thereby seal the bore of the housing in a post-triggered configuration. In some embodiments, the force of compressed spring element  788 ′ is sufficient to overcome the downhole pressure and move the pin relative to J-slot  895  from point  895 -B to point  895 -C. Sleeve  880  aligns sleeve passages  890  with ports  885  of housing  710 ′ with the pin in J-slot  895  at point  895 -C. Sleeve  880  thereby unseals the bore  740 ′ when the pin is in J-slot  895  at point  895 -C. In some embodiments, pumping pressure may be reduced to provide a pressure-drop signal, for example at the end of the pressure test, so that the force of compressed spring element  788 ′ is sufficient to overcome the downhole pressure and move the pin to point  895 -C (as illustrated in  FIG. 10C ). In some embodiments, spring element  788 ′ includes small charges, electromagnets, or other devices to provide impulsive force to assist in moving pin to point  895 -C. In some embodiments, subsequent pressure signals (either pressure increases or pressure decreases) may further move the pin relative to the J-slot  895 , thereby rotating sleeve  880  to either seal or unseal the bore  740 ′ of the housing  710 ′. A variety of other configurations may be envisioned so that, in conjunction with and/or following the pressure test, the transitionable seal may be triggered to transition from sealing the bore  740  to unseal the bore  740 . 
       FIG. 11  illustrates another alternative top plug as an example of other envisioned configurations that provide a transitionable seal that, in conjunction with and/or following a pressure test, may be triggered to transition from sealing the bore  740  to unseal the bore  740 . As shown, the top plug is a top latch-in plug  700 ″ having a housing  710 ″, a head end  720 ″, a tail end  730 ″, a bore  740 ″ in the housing  710 ″ extending from the head end  720 ″ to the tail end  730 ″, and one or more fins  750 ″. Top latch-in plug  700 ″ also includes a transitionable seal. In some embodiments, the transitionable seal may be a sleeve (for example, sleeve  880 ′, discussed below). In the initial configuration shown in  FIG. 11A  (when top latch-in plug  700 ″ is introduced into and pumped down casing  100 ), the sleeve  880 ′ seals the bore  740 ″ of the housing  710 ″. 
     As with top latch-in plug  700 , top latch-in plug  700 ″ may latch-in with bottom latch-in plug  200 / 300 . The casing and/or the plug connections may be pressure tested. In conjunction with and/or following the pressure test, the transitionable seal of top latch-in plug  700 ″ may be triggered to transition from sealing the bore  740 ″ to unseal the bore  740 ″. In some embodiments, the triggering may be a multi-step triggering. For example, a first triggering event may initiate the transition, a second triggering event may advance the transition, and the transitionable seal may transition from sealing the bore  740 ″ to unseal the bore  740 ″. For example, in the configuration illustrated in  FIG. 11A , the sleeve  880 ′ seals the bore  740 ″ of the housing  710 ″ by blocking ports  885 ′. In some embodiments, the sleeve  880 ′ may have a lid portion  781 ″ and a stopper portion  785 ″. There may be a recess  784 ″ between the stopper portion  785 ″ and the housing  710 ″. In the illustrated embodiment, a spring element  788 ″ is located in recess  784 ″ of the housing  710 ″, biasing the sleeve  880 ′ towards the tail end  730 ″ of the housing  710 ″. The stopper portion  785 ″ may sealingly fit in the bore  740 ″. One or more O-rings  786 ″ may be located around the stopper portion  785 ″ to create a seal with the interior of the housing  710 ″. Other configurations may be envisioned so that the sleeve  880 ′ may seal the bore  740 ″ of the housing  710 ″. The sleeve  880 ′ may be triggered to transition from a configuration wherein the sleeve  880 ′ seals the bore  740 ″ of the housing  710 ″ to a configuration wherein sleeve  880 ′ unseals the bore  740 ″. For example, the sleeve  880 ′ may unseal the bore  740 ″ as in the configuration illustrated in  FIG. 11D , wherein ports  885 ′ are shown fluidly connected to bore  740 ″ through sleeve passages  890 ′. As illustrated, housing  710 ″ has four ports  885 ′, and sleeve  880 ′ has four sleeve passages  890 ′, but various numbers, sizes, and distributions of ports  885 ′ and sleeve passages  890 ′ may be envisioned to accommodate operational requirements and designs. Further, other post-triggered configurations may be envisioned so that the sleeve  880 ′ unseals the bore  740 ″. 
     As with top latch-in plug  700 , the transitionable seal of top latch-in plug  700 ″ may be triggered to transition from sealing the bore  740 ″ to unseal the bore  740 ″, and the transitionable seal may seal the bore  740 ″ at least until completion of the pressure test. In some embodiments, the completion of the pressure test may be indicated by a pressure-drop signal proximate the tail end  730 ″ of the housing  710 ″. For example, in the illustrated embodiment, the lid portion  781 ″ of sleeve  880 ′ may have one or more shear pin receptacles  783 ″ for receiving shear pins  782 ″. The shear pins  782 ″ hold the sleeve  880 ′ in the housing  710 ″. The shear pins  782 ″ are designed to shear in response to a selected pressure signal. The level of downhole pressure selected for the pressure signal to cause the shear pins  782 ″ to shear may be near, at, or above the level of downhole pressure selected for the lowest pressure level of the pressure test. As illustrated, a first triggering event that initiates the transition of the transitionable seal may be a pressure signal, such as a selected downhole pressure that causes shearing of the shear pins  782 ″. The pressure signal may compressing the stopper portion  785 ″ against spring element  788 ″. This may further compresses spring element  788 ″ in the recess  784 ″. 
     As illustrated in  FIG. 11E , there may be a multi-step J-slot  895 ′ on the exterior of sleeve  880 ′. A pin on an interior surface of housing  710 ″ may engage the J-slot  895 ′. In the initial configuration shown in  FIG. 11A  (when top latch-in plug  700 ″ is introduced into and pumped down casing  100 ), the pin may engage J-slot  895 ′ at point  895 ′-A. A first triggering event may initiate the transition of the transitionable seal by shearing shear pins  782 ″. The first triggering event may further include moving the pin relative to J-slot  895 ′ from point  895 ′-A to point  895 ′-B, thereby rotating sleeve  880 ′ relative to housing  710 ″. Sleeve  880 ′ blocks ports  885 ′ of housing  710 ″ both with the pin in J-slot  895 ′ at point  895 ′-A and with the pin in J-slot  895 ′ at point  895 ′-B. Sleeve  880 ′ thereby seals the bore  740 ″ when the pin is in J-slot  895 ′ at point  895 ′-A and at point  895 ′-B. In some embodiments, following the first triggering event, the downhole pressure may be increased, possibly in conjunction with a pressure test, thereby holding the pin in J-slot  895 ′ point  895 ′-B (as illustrated in  FIG. 11B ). In some embodiments, the transitionable seal may thereby seal the bore of the housing in a post-triggered configuration. In some embodiments, the force of compressed spring element  788 ″ is sufficient to overcome the downhole pressure and move the pin relative to J-slot  895 ′ from point  895 ′-B to point  895 ′-C. Sleeve  880 ′ may thereby further rotate relative to housing  710 ″. In some embodiments, pumping pressure may be reduced to provide a pressure-drop signal, for example at the end of the pressure test, so that the force of compressed spring element  788 ″ is sufficient to overcome the downhole pressure and move the pin to point  895 ′-C (as illustrated in  FIG. 11C ). In some embodiments, spring element  788 ″ includes small charges, electromagnets, or other devices to provide impulsive force to assist in moving pin to point  895 ′-C. Sleeve  880 ′ blocks ports  885 ′ of housing  710 ″ with the pin in J-slot  895 ′ at point  895 ′-C, thereby sealing the bore  740 ″. 
     A second triggering event may advance the transition of the transitionable seal by moving the pin relative to J-slot  895 ′ from point  895 ′-C to point  895 ′-D, thereby further rotating sleeve  880 ′ relative to housing  710 ″. For example, a pressure signal or series of pressure signals may selectively move stopper portion  785 ″ relative to housing  710 ″ by alternatively decompressing and compressing spring element  788 ″. As illustrated by J-slot  895 ′, the pin moves relative to J-slot  895 ′ from point  895 ′-C to point  895 ′-D with a single decompression followed by a single compression, but other J-slot configurations may be envisioned to respond to a variety of pressure signals to accommodate operational requirements and designs. The second triggering event may advance the transition by alternatively decompressing and compressing stopper portion  785 ″ against spring element  788 ″. As illustrated in  FIG. 11D , when the pin is in J-slot  895 ′ at point  895 ′-D, sleeve  880 ′ aligns sleeve passages  890 ′ with ports  885 ′ of housing  710 ″. Sleeve  880 ′ thereby unseals the bore  740 ″ subsequent to the second triggering event. In some embodiments, subsequent pressure signals (either pressure increases or pressure decreases) may further move the pin relative to the J-slot  895 ′, thereby rotating sleeve  880 ′ to either seal or unseal the bore  740 ″ of the housing  710 ″. A variety of other configurations may be envisioned so that, in conjunction with and/or following the pressure test, the transitionable seal may be triggered to transition from sealing the bore  740  to unseal the bore  740 . 
     As would be appreciated by one of ordinary skill in the art with the benefit of this disclosure, more complex well completions could be conducted using a multiplicity of bottom latch-in plugs. For example, separation between various additional pumping fluids could be achieved with additional bottom latch-in plugs. Additional bottom latch-in plugs may also provide for additional wiping of the interior of the casing prior to cementing. The bottom latch-in plugs may be designed to sequentially latch-in, ultimately with the landing collar. Each bottom latch-in plug may have a pressure seal, wherein the downhole pressures selected to release each of the pressure seals may be incrementally increased, starting from the lowest bottom latch-in plug and increasing with each bottom latch-in plug in uphole sequence. Surface operations may detect and react to downhole pressure increases prior to each pressure seal release, providing information regarding the location of boundaries between various pumping fluids. It is currently believed that as many as 10 bottom latch-in plugs may be used. Likewise, more complex well completions could be conducted using a multiplicity of top latch-in plugs. Additional top latch-in plugs may also provide for additional wiping of the interior of the casing prior to production. However, only the uphole-most top latch-in plug may have a transitionable seal. 
     In some embodiments, the lower bottom latch-in plug  200  may be assembled in the casing  100 . For example, as illustrated in  FIGS. 12-15 , lower bottom latch-in plug  200  may include a forward portion  200 - f  ( FIG. 12 ) and an aft portion  200 - a  ( FIG. 14 ). 
     Forward portion  200 - f  may include housing  210 , head end  220 , bore  240 , fins  250 , pressure seal  260 , and catch mechanism  270 . Head end  220  may have a landing mechanism that is compatible with and/or configured to connect with landing collar  104 . Forward portion  200 - f  is introduced, head end  220  first, into casing  100  behind the buoyancy fluid. Forward portion  200 - f  forms an uphole seal for the buoyancy fluid. In particular, fins  250  of forward portion  200 - f  contact and seal against the interior wall of casing  100 , and pressure seal  260  of forward portion  200 - f  seals the bore  240  of forward portion  200 - f . Once introduced into the casing  100 , forward portion  200 - f  travels downhole through the casing  100 , until reaching pre-load collar  102 . Forward portion  200 - f  may travel downhole by gravity, by pumping of a pumping fluid behind the forward portion  200 - f , or by an assembly tool  800  ( FIG. 13 ). The catch mechanism  270  causes forward portion  200 - f  to be caught by the pre-load collar  102 . In some embodiments, assembly tool  800  may actuate catch mechanism  270  to cause forward portion  200 - f  to be caught by the pre-load collar  102 . As previously discussed, the buoyancy fluid may be introduced into the casing  100  while the casing  100  is at or near the surface of the wellbore. Therefore, assembly of bottom latch-in plug  200 , including catching forward portion  200 - f  by the pre-load collar  102  to form an uphole seal for the buoyancy fluid, may also occur at or near the surface of the wellbore. Assembly tool  800  thus may be no longer than 5 meters. 
     Aft portion  200 - a  may include housing  210 , tail end  230 , bore  240 , and fins  250 . Tail end  230  may have a retaining mechanism to latch-in with other latch-in plugs. Aft portion  200 - a  is introduced, tail end  230  last, into casing  100  behind forward portion  200 - f . Once introduced into the casing  100 , aft portion  200 - a  travels downhole through the casing  100 , until reaching forward portion  200 - f  at pre-load collar  102 . Aft portion  200 - a  may travel downhole by gravity, by pumping of a pumping fluid behind the aft portion  200 - a , or by an assembly tool  800  ( FIG. 15 ). Aft portion  200 - a  is secured to forward portion  20 - f . In some embodiments, assembly tool  800  may actuate a locking mechanism to cause aft portion  200 - a  to be secured to forward portion  200 - f . In some embodiments, the locking mechanism may be similar to the previously-discussed retaining mechanism for latch-in plugs. Forward portion  200 - f  and aft portion  200 - a  may thereby form a unified lower bottom latch-in plug  200  that is caught in pre-load collar  102 , forming an uphole seal for the buoyancy fluid. 
     As illustrated in  FIG. 16 , catch mechanism  270  of lower bottom latch-in plug  200  may be a collet  275  with a shear ring  279 . In the illustrated embodiment, the housing  210  has a profile that includes a shoulder  211  and a waist  213 , wherein the shoulder  211  has a larger diameter than the waist  213 . In one configuration, the collet  275  is held open by the shoulder  211 . When the collet  275  is held open, the collet  275  may be caught by pre-load collar  102 . In another configuration, the collet  275  may be collapsed against the waist  213 . When the collet  275  is collapsed, the lower bottom latch-in plug  200  may be released by the pre-load collar  102 . Collet  275  may be prevented from collapsing against the waist  213  by shear ring  279 . Downhole pressure applied to lower bottom latch-in plug  200  may cause shear ring  279  to shear. As previously discussed, the catch mechanism  270  may be designed to release (e.g., shear ring  279  shears) in response to a selected pressure signal. When shear ring  279  shears, collet  275  may be free to slide relative to housing  210 , for example in groove  277 . Collet  275  may thus transition from a configuration in which lower bottom latch-in plug  200  may be caught by pre-load collar  102  to a configuration in which lower bottom latch-in plug  200  may be released by pre-load collar  102 . Other configurations may be envisioned so that catch mechanism  270  releases in response to a selected pressure signal. More specifically, other configurations may be envisioned that provide few or no obstructions in the interior of the casing  100  at the pre-load collar  102  after the lower bottom latch-in plug  200  is released. 
     Such methods and devices may provide a number of advantages, such as allowing a casing pressure test after cementing without additional trips or drilling before production. The latch-in plugs (sometimes referred to in the industry as “latch-down plugs”) discussed herein may beneficially serve multiple functions, such as: separation of fluids inside of pipe; wiping of materials from the inner surface of pipe; operation of a downhole tool; surface indication of a downhole event; and formation of a temporary pressure barrier. A full-bore toe sleeve could also be used with this system. Use of the plugs in this system may improve wiping performance during displacement of cement, reducing the likelihood of a coil tubing cleanout run before well completions. 
     Casing floatation systems disclosed herein may be useful in locating a casing in a wellbore, especially if the wellbore is highly deviated. A method  921  of floating a casing into a wellbore is illustrated in  FIG. 17B . In some embodiments, the method begins with disposing the casing in the wellbore at step  931 . The casing may be at or near the surface of the wellbore, and only a downhole portion of the casing may be within the sidewalls of the wellbore at step  931 . The casing may be constructed in segments, and only a subset of the segments may be disposed in the wellbore at step  931 . The method continues as buoyancy fluid is disposed in the casing at step  932 . The buoyancy fluid may be disposed between a pre-load collar and a landing collar. At step  933 , the buoyancy fluid is sealed in the casing. The buoyancy fluid may be sealed between the pre-load collar and the landing collar. The casing may move downhole at step  934 . In some embodiments, the casing may also move downhole while the buoyancy fluid is disposed in the casing at step  934 ′. In some embodiments, the method begins with disposing buoyancy fluid in the casing at step  932 . For example, the casing may be constructed with a pre-load collar and a landing collar prior to introduction into the wellbore. The buoyancy fluid may be disposed between the pre-load collar and the landing collar prior to introduction of the casing into the wellbore. At step  933 , the buoyancy fluid is sealed in the casing. The buoyancy fluid may be sealed between the pre-load collar and the landing collar. The casing may then be disposed in the wellbore at step  931 , and moved downhole at step  934 . The casing moves downhole until reaching a designated location. The method  921  of floating a casing into a wellbore completes and progresses to a next step of well completion at step  935  when the buoyancy fluid is discharged. 
     Method  921  of floating a casing into a wellbore may be useful in well completion operations, such as method  900  of well completion illustrated in  FIG. 17A . Method  900  begins at step  921 , floating a casing into a wellbore, as previously discussed. The casing may have a pre-load collar uphole from a landing collar. A bottom plug may be disposed at the pre-load collar. The method continues at step  922  when the bottom plug is released from the pre-load collar. The bottom plug may wipe the interior surface of the casing. In some embodiments, the bottom plug may travel downhole until it reaches the landing collar. The bottom plug may engage with the landing collar. At step  923 , cement is pumped downhole through the casing. The cement may be pumped through the casing, the bottom plug, the landing collar, and a float shoe to enter and/or fill an annulus between the casing and the wellbore. Following pumping a desired amount of cement and/or displacement fluid, a top plug may be introduced into the casing. The top plug may include a transitionable seal. The top plug may travel downhole through the casing until reaching the landing collar and/or any plugs previously engaged with the landing collar. At step  924 , the top plug may engage with the landing collar (or sequentially engage therewith via any plugs previously engaged with the landing collar). A pressure test of the casing may be conducted at step  925 . In some embodiments, the pressure test may trigger the transitionable seal of the top plug to transition from a configuration sealing the bore of the top plug to a configuration unsealing the bore. At step  926 , the bore of the top plug is unsealed, completing the well for production and/or further operations. 
     In an embodiment, a top latch-in plug includes a housing having: a head end; a tail end; and a bore from the head end to the tail end; and a transitionable seal, wherein: the transitionable seal seals the bore of the housing when in a first configuration, the transitionable seal unseals the bore when in a second configuration, and the transitionable seal is triggerable to transition from the first configuration to the second configuration. 
     In one or more embodiments disclosed herein, the transitionable seal seals the bore of the housing when in a post-triggered configuration. 
     In one or more embodiments disclosed herein, the transitionable seal is an expendable cap. 
     In one or more embodiments disclosed herein, the top latch-in plug also includes one or more shear pins holding the expendable cap in the housing when in the first configuration; and a spring element biased, when in the first configuration, to eject the expendable cap from the housing. 
     In one or more embodiments disclosed herein, the expendable cap transitions from the first configuration to the second configuration by forcibly ejecting from the housing. 
     In one or more embodiments disclosed herein, the expendable cap blocks no more than half of a cross-sectional area of the bore at the tail end of the housing when in the second configuration. 
     In one or more embodiments disclosed herein, the transitionable seal is a sleeve. 
     In one or more embodiments disclosed herein, the sleeve includes a plurality of sleeve passages that align with ports in the housing when in the second configuration; and a j-slot that engages with a pin of the housing. 
     In one or more embodiments disclosed herein, the transitionable seal is triggerable by a pressure signal. 
     In one or more embodiments disclosed herein, the transitionable seal is triggered to transition with multi-step triggering. 
     In one or more embodiments disclosed herein, the top latch-in plug also includes a recess between the transitionable seal and the housing when in the first configuration, wherein the transitionable seal enters the recess during transition between the first configuration and the second configuration. 
     In one or more embodiments disclosed herein, the transitionable seal comprises: a lid portion; one or more shear pin receptacles in the lid portion; a stopper portion; and one or more O-rings around the stopper portion. 
     In one or more embodiments disclosed herein, the transitionable seal transitions from the first configuration to the second configuration by at least partially dissolving. 
     In one or more embodiments disclosed herein, a pressure-drop signal causes the transitionable seal to unseal the bore. 
     In one or more embodiments disclosed herein, a multi-step pressure signal causes the transitionable seal to unseal the bore. 
     In an embodiment, a method of well completion includes floating a casing in a wellbore; pumping cement downhole through the casing to supply cement between the casing and the wellbore; sequentially engaging a lower bottom latch-in plug and a top latch-in plug to a landing collar of the casing, wherein the top latch-in plug includes a transitionable seal sealing a bore of the top latch-in plug; pressure testing the casing; and triggering the transitionable seal to unseal the bore of the top latch-in plug. 
     In one or more embodiments disclosed herein, the casing includes a pre-load collar located uphole from the landing collar; the method further comprising releasing the lower bottom latch-in plug from the pre-load collar. 
     In one or more embodiments disclosed herein, the transitionable seal is a cap. 
     In one or more embodiments disclosed herein, the transitionable seal is a sleeve. 
     In one or more embodiments disclosed herein, the transitionable seal seals the bore of the top latch-in plug at least until completion of the pressure testing. 
     In one or more embodiments disclosed herein, pressure testing the casing triggers the transitionable seal to unseal the bore of the top latch-in plug. 
     In one or more embodiments disclosed herein, a pressure-drop signal causes the transitionable seal to unseal the bore of the top latch-in plug. 
     In one or more embodiments disclosed herein, the pressure testing comprises increasing the downhole pressure; the increasing the downhole pressure triggers the transitionable seal; and the transitionable seal unseals the bore of the top latch-in plug after completion of the pressure testing. 
     In one or more embodiments disclosed herein, the triggering includes a first triggering event that initiates the transition, and a second triggering event that advance the transition. 
     In one or more embodiments disclosed herein, the triggering comprises a multi-step pressure signal. 
     In one or more embodiments disclosed herein, the method also includes, after pumping the cement and before sequentially engaging the lower bottom latch-in plug and the top latch-in plug to the landing collar, pumping an additional top latch-in plug downhole through the casing. 
     In one or more embodiments disclosed herein, the method also includes producing fluid from the wellbore through the casing. 
     In one or more embodiments disclosed herein, drilling does not occur between the triggering the transitionable seal and the producing fluid. 
     In one or more embodiments disclosed herein, the method also includes perforating the casing between the pre-load collar and the landing collar. 
     In one or more embodiments disclosed herein, the method also includes, after releasing the lower bottom latch-in plug and before pumping the cement, pumping an additional bottom latch-in plug downhole through the casing. 
     In an embodiment, a method of well completion includes causing a casing to be floated in a wellbore; causing cement to be pumped downhole through the casing to supply cement between the casing and the wellbore; sequentially engaging a lower bottom latch-in plug and a top latch-in plug to a landing collar of the casing, wherein the top latch-in plug includes a transitionable seal sealing a bore of the top latch-in plug; causing the casing to be pressure tested; and causing a triggering of the transitionable seal to unseal the bore of the top latch-in plug. 
     In an embodiment, a casing floatation system includes a casing having a pre-load collar and a landing collar; and a lower bottom latch-in plug comprising: a catch mechanism compatible with the pre-load collar; and a landing mechanism compatible with the landing collar. 
     In one or more embodiments disclosed herein, the catch mechanism comprises a collet with a shear ring. 
     In one or more embodiments disclosed herein, the lower bottom latch-in plug further comprises a pressure seal. 
     In one or more embodiments disclosed herein, the casing floatation system also includes an upper bottom latch-in plug comprising a pressure seal. 
     In one or more embodiments disclosed herein, the casing floatation system also includes a top latch-in plug having a transitionable seal. 
     In one or more embodiments disclosed herein, the transitionable seal is an expendable cap. 
     In one or more embodiments disclosed herein, the lower bottom latch-in plug pressure seal releases at a first pressure; the catch mechanism releases at a second pressure; the upper bottom latch-in plug pressure seal releases at a third pressure; the transitionable seal is triggerable by a pressure signal at a fourth pressure; and the first pressure is less than the second pressure, which is less than the third pressure. 
     In one or more embodiments disclosed herein, the third pressure is less than the fourth pressure. 
     In one or more embodiments disclosed herein, the catch mechanism releases in response to a pressure signal. 
     In one or more embodiments disclosed herein, upon release, the catch mechanism does not obstruct an interior of the casing at the pre-load collar. 
     In one or more embodiments disclosed herein, the casing floatation system also includes a plurality of bottom latch-in plugs. 
     In one or more embodiments disclosed herein, the casing floatation system also includes a float shoe with a check valve. 
     In one or more embodiments disclosed herein, the casing floatation system also includes one or more toe sleeves. 
     In one or more embodiments disclosed herein, the lower bottom latch-in plug pressure seal blocks a bore of the lower bottom latch-in plug when sealed. 
     In one or more embodiments disclosed herein, the upper bottom latch-in plug pressure seal blocks a bore of the upper bottom latch-in plug when sealed. 
     In one or more embodiments disclosed herein, one or more of the latch-in plugs has an anti-rotation feature. 
     In an embodiment, a method of well completion includes floating a casing in a wellbore, wherein the casing includes a pre-load collar located uphole from a landing collar, the floating the casing comprising: disposing the casing in the wellbore; disposing buoyancy fluid in the casing between the pre-load collar and the landing collar; and sealing the buoyancy fluid in the casing by engaging a lower bottom latch-in plug with the pre-load collar; discharging the buoyancy fluid from the casing; releasing the lower bottom latch-in plug from the pre-load collar; and engaging the lower bottom latch-in plug with the landing collar. 
     In one or more embodiments disclosed herein, the floating the casing further comprises moving the casing further downhole in the wellbore. 
     In one or more embodiments disclosed herein, the method also includes pumping cement downhole through the casing to supply cement between the casing and the wellbore; sequentially engaging a top latch-in plug with the bottom latch-in plug and the landing collar, wherein the top latch-in plug includes a transitionable seal sealing a bore of the top latch-in plug; pressure testing the casing; and triggering the transitionable seal to unseal the bore of the top latch-in plug. 
     In one or more embodiments disclosed herein, the method of also includes creating a first downhole pressure to discharge the buoyancy fluid from the casing. 
     In one or more embodiments disclosed herein, the lower bottom latch-in plug includes a pressure seal, and the first downhole pressure releases the pressure seal of the lower bottom latch-in plug. 
     In one or more embodiments disclosed herein, the method also includes, after discharging the buoyancy fluid from the casing and before releasing the lower bottom latch-in plug from the pre-load collar, engaging an upper bottom latch-in plug to the lower bottom latch-in plug. 
     In one or more embodiments disclosed herein, the method also includes creating a second downhole pressure to release the lower bottom latch-in plug from the pre-load collar. 
     In one or more embodiments disclosed herein, the lower bottom latch-in plug includes a catch mechanism, and the second downhole pressure releases the catch mechanism of the lower bottom latch-in plug. 
     In one or more embodiments disclosed herein, the catch mechanism includes a collet with a shear ring, and the second downhole pressure shears the shear ring. 
     In an embodiment, a method of assembling a latch-in plug includes obtaining a casing having a pre-load collar and a landing collar; disposing buoyancy fluid in the casing between the pre-load collar and the landing collar; catching a forward portion of a latch-in plug with the pre-load collar, thereby sealing the buoyancy fluid in the casing; and securing an aft portion of the latch-in plug to the forward portion. 
     In one or more embodiments disclosed herein, the forward portion has a landing mechanism that is compatible with the landing collar. 
     In one or more embodiments disclosed herein, the aft portion has a retaining mechanism to latch-in with other latch-in plugs. 
     In an embodiment, a method of well completion includes causing a casing to be floated in a wellbore, wherein: the casing includes a pre-load collar located uphole from a landing collar, and floating the casing comprises: disposing the casing in the wellbore; disposing buoyancy fluid in the casing between the pre-load collar and the landing collar; and sealing the buoyancy fluid in the casing by engaging a lower bottom latch-in plug with the pre-load collar; discharging the buoyancy fluid from the casing; causing a lower bottom latch-in plug to be released from the pre-load collar; and engaging the lower bottom latch-in plug with the landing collar. 
     In one or more embodiments disclosed herein, the floating the casing further comprises moving the casing further downhole in the wellbore. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.