Abstract:
An hydraulic lock alleviation device a well cementing stage tool with such a device, and related methods for their use. The device has a channel for transmitting trapped fluid which causes a hydraulic lock from the trapped area to another area. The channel is blocked by a pressure responsive member which ruptures, moves, breaks, or is punctured to permit trapped fluid to glow from the area of hydraulic lock. In one embodiment of a stage tool with such a device, a puncture apparatus can be moved to puncture the pressure responsive member. In other embodiments, a movable piston or spool is mounted in the channel for movement to permit the flow of trapped fluid.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to wellbore cementing operations and apparatuses and methods for use in such operations and particularly to devices for alleviating hydraulic locks encountered in a wellbore and to cementing tools called stage tools and methods of their use. 
     2. Description of Prior Art 
     A drilled wellbore hole is prepared for oil or gas production by cementing casing, liners or similar conduit strings in the wellbore. Cementing is the process of mixing a composition including cement and water and pumping the resulting slurry down through the well casing and into the annulus between the casing and the wellbore. Cementing provides protection from the intermixing of the contents of various production zones which could result in undesirable contamination of produced oil or gas or in contamination of the producing strata. 
     In the early days of the oil field industry, the shallower wells allowed cementation to be accomplished by pumping a cement slurry down the well casing, out the casing bottom, and back up the annular space between the bore hole and casing. As wells were drilled deeper, the cementing process was accomplished in two or even three stages. Cementing tools, stage tools, or ported collars equipped with internal valving, were needed for multi-stage cementation. 
     Typically, the internal valving of cementing tools, or stage tools, consist of one or more sliding sleeve valves for the opening and closing of the cement ports before and after a cement slurry has passed through the ports. A variety of plugs are used to aid multi-stage cementing to open and close the correct sleeve valve at the correct time. 
     Problems have been encountered when two sleeve valves are employed to open and close the cement ports. The sleeve valves are shear-pinned in an upper position with the lowermost sleeve sealing the ports closed for running in the wellbore hole. When stage cementing is desired, an opening plug is moved, or dropped and gravitated, to seat and seal off the lowermost sleeve. Pressure applied at the surface applies enough downward force on the plug and seat arrangement to break the shear pins and shift the lower sleeve valve down, thus opening ports which allow cementing solutions or slurries to flow down the interior of the casing and then through the ports into the annulus between the exterior of the casing and the interior of the wellbore. Cement is pumped down the casing, through the ports and back up the annulus. 
     As the tail end of the cement slurry is pumped down the casing, a second plug often called a &#34;closing plug&#34; is placed into the casing behind the cement. This plug moves down to seat and seal off the uppermost sleeve valve until sufficient surface casing pressure is applied to break the shear pins holding the sleeve. The upper sleeve and plug shift downward to cover and seal off the ports so that no more solution or slurry passes either into the annulus or back from the annulus. An engaging mechanism can be used to lock the closing sleeve in position. 
     A problem has been encountered in this operation due to the creation of a hydraulic lock when a seal is established across the ports. When a portion of the solution or slurry ahead of the closing plug is pushed downwardly and the ports close off, this small portion of fluid becomes trapped between the plugs within the stage tool and can flow nowhere. The nearly incompressibile nature of the trapped material does not allow the upper sleeve valve to travel sufficiently downward to engage a positive locking mechanism to prevent a reopening of the ports. If the engaging mechanism does not engage on the upper sleeve valve, internal casing pressure must be held until the cement sets. 
     In accordance with §1.56 of 37 C.F.R. the following references are disclosed and copies thereof are submitted herewith: 
     (a) U.S. Pat. Nos. 2,602,510; 2,928,470; 3,811,500; 3,824,905; 3,948,322; 3,768,556; and 4,487,263; 
     (b) publications including: Dowell Schlumberger, CEMENTING TECHNOLOGY, (1984) &#34;Primary Placement Techniques&#34;, Chapter 10, pp. 1-20; Chapter 13, &#34;Cementing Equipment&#34;, pp. 11-12; Halliburton Services Sales &amp; Service Catalog 43, COMPOSITE CATALOG, pp. 2440-2451 (1986-1987); Weatherford Cementing Program, especially pp. 36-37 (1986) 
     U.S. Pat. Nos. 2,928,470 and 2,602,510 disclose a device used in a two-step operation for closing off cementing ports that allows a lower sleeve valve to shift in unison with an upper sleeve valve the instant before a hydraulic lock is effected. This is accomplished by introducing a third shear means to release the lower sleeve valve as the upper sleeve valve lands on its upper side. This configuration has the inherent disadvantage of prematurely shearing the third shear means (and thus fouling it) when the lower sleeve valve impacts with too much force while opening the ports or by excessively high cement pump pressures exerting a sufficient downward force before closing is desired. U.S. Pat. No. 2,602,510 discloses the use of bleeder holes which provide fluid communication between the casing interior and the annulus. Since the holes are relatively small and are always open, they can plug up with cementing particles or well mud particles. Also, because of the design of the ported cementing apparatus and the disposition of its seals, the holes must be small (and hence pluggable), otherwise the seals which are required for proper operation of the tool will not be able to properly seal off the holes. U.S. Pat. No. 2,602,510 discloses a device which has seals which can be permitted to move past the ports. Because of the flow of fluid under pressure through the ports and its impact on a seal, the seal can be damaged in the areas adjacent the ports. This same problem can happen in the prior art devices of U.S. Pat. No. 3,811,500. 
     U.S. Pat. Nos. 3,811,500 and 3,842,905 disclose a device which uses an opening plug to shift a lower sleeve valve open. As the upper sleeve valve slides to cover and seal flow ports, the closing plug, used to shift the upper sleeve valve closed, imposes a downward force on a rod extending through the opening plug which breaks shear pins holding the rod in place and opens a passage through the opening plug for trapped fluid to exit. This configuration is pressure sensitive to excessively high cement pump pressure which can break the shear pins and cause undesirable premature activation; i.e. the rod is pushed out during the cementing operation rather than at its completion. Also, there is no guarantee the mechanism will be aligned correctly upon the seating of the opening plug, due to the loose fitting characteristics of such a plug and the requirement that the plug go down the casing in a properly aligned configuration. If the plug becomes misaligned the device will not work properly. Because of the sensitivity of the shear pin used to hold the rod in place, it is difficult if not impossible to use a hammer means such as a drill pipe joint to jar a stuck plug--since such jarring will cause premature release of the rod or the plug may become damaged. 
     U.S. Pat. Nos. 3,768,556 and 3,948,322 disclose a device similar to that of U.S. Pat. No. 2,928,470, but with a metal locking device for holding the various elements of the tool in fixed relation to each other, including holding the opening and closing plugs in fixed relation to each other so that the closing plug at some point no longer pushes down on fluid trapped beneath it, eliminating further hydraulic locking effects. 
     U.S. Pat. No. 4,487,263 discloses a device which has small apertures which permit flow from the interior of the device, where cementing fluid may be trapped, into a chamber in the device. These holes can be plugged up by cementing fluid or mud and the chamber may not be large enough to hold all the fluid. If the chamber has filled and there is still more fluid trapped between the plugs, there can still be an unwanted hydraulic lock. 
     The listed publications generally describe well cementing operations and stage tools. 
     There has long been a need for an effective and efficient cementing stage tool and methods for its use. There has long been a need for a device for alleviating hydraulic locking in a wellbore. There has long been a need for a stage tool which does not activate prematurely, which has ports not subject to unwanted plugging, and which does not require complex engaging mechanisms. Also there has long been a need for such a tool and device which do not damage seals used therein. The present invention recognizes, addresses, and satisfies these long-felt needs. 
     SUMMARY OF THE INVENTION 
     The present invention teaches an apparatus for alleviating hydraulic locking; a stage tool with such an apparatus; a stage tool which does not damage seals as do the prior art devices; and methods for using the apparatuses and tools. 
     An apparatus for alleviating hydraulic locking according to this invention includes a fluid conducting mechanism having a fluid channel which is closed off by a puncturable or rupturable disc or by a movable sealing pin disposed in the channel. The disc is made so that it will rupture in direct response to the pressure of fluid trapped above and in the fluid channel or so that it is punctured by a puncture device positioned adjacent the disc. The puncture device can be acted upon by a portion of an upper sleeve in a stage tool moving to contact and push the puncture device through the disc. In another embodiment a movable spool or sealing pin can be used which can move to permit flow of the trapped fluid. The apparatus can be disposed to permit the trapped fluid to flow from an entrapment space (including but not limited to the space in a stage tool between an opening plug and a closing plug in a well cementing operation) into an adjacent but separately defined space (e.g. the space below an opening plug in a well cementing operation). 
     A stage tool for cementing operations can advantageously utilize such an apparatus. In one embodiment of such a tool the apparatus can be disposed on a lower opening sleeve where it can be acted upon by an upper closing sleeve. It can be emplaced so that trapped fluid between an opening and closing plug flows into the casing interior below the cementing ports. A diffuser groove can be provided on the tool so that the deleterious effects of fluid flowing to and/or through the cementing ports are reduced or eliminated, thereby preserving O-ring seals and preventing damage to them. 
     It is therefore an object of the present invention to provide a novel, unobvious, efficient and effective device for alleviating hydraulic locking. 
     Another object of this invention is the provision of a novel, unobvious, efficient and effective stage tool for well cementing operations. 
     A further object of this invention is the provision of a stage tool which protects certain of its O-ring seals from damage by high velocity pressurized fluid flow. 
     An additional object of this invention is the provision of methods of use of such apparatuses and tools. 
     To one of skill in this art who has the benefit of this invention&#39;s teachings other and further objects and advantages will be clear from the following description of preferred embodiments given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1A is a longitudinal section of a tool according to this invention; FIG. 1B is an end view of the tool of FIG. 1A; FIG. 1C is another longitudinal section of the tool of FIG. 1A; FIG. 1D is an end view of the tool of FIG. 1C; FIG. 1E is an exploded view of the tool of FIG. 1A. 
     FIG. 2 is an enlarged view of a portion of the tool of FIG. 1A. 
     FIG. 3A is a view of the tool of FIG. 1A in a wellbore, FIG. 3B is a view of the tool of FIG. 1A with an opening plug in it. 
     FIG. 4 is a view of the tool as shown in FIG. 3 with a closing plug. 
     FIG. 5 is a view of the tool shown in FIG. 4 with the plugs in closer proximity. 
     FIG. 6 is a crossectional view of an hydraulic lock alleviation device according to this invention. 
     FIGS. 7A, 7B, and 7C are crossectional views of a device according to this invention. 
     FIGS. 8A and 8B are crossectional views of a device according to this invention. 
     FIGS. 9A and 9B are crossectional views of a device according to this invention. 
     FIG. 10 is a crossectional view of a device according to this invention. 
    
    
     For convenience in reviewing the drawings, the following legend is given: 
     
         ______________________________________10    stage tool      65     recess on 61 for 6411    upper case      66     snap ring12    lower case      67     recess on 61 for 6613    threads, upper  68     diffuser groove14    threads, lower  69     seal15    shear plugs     70     recess on 6116    cementing ports                 71     lower lip of 6017    recess on 11    72     pin 41 top18    wellbore        73     bore in 6019    recess on 11    74     threads (w/39)20    recess on 11    75     recesses in 60 w/7821    recess on 11    76     recesses in 8022    hole on 11      78     shoulders on 80 w/7523    plugs           79     recess in 85 for 10124    bore of 10      80     lower sleeve25    casing          81     body member26    casing          82     central circular opening27    annulus         83     ridge of 8028    interior surface                 84     ridge of 80 of 18           85     exterior surface29    portion of 16   86     interior cylindrical30    upper seat             surface of 8031    exterior surface                 87     O-ring seal32    interior surface                 88     recess on 80 for 8733    bore of 30      89     groove34    cylindrical surface                 90     stepped channel35    ridge           91     top opening36    ridge           92     top shoulder37    shear balls     93     mid portion38    recesses for 37                 94     mid shoulder39    threads (w/74)  95     intermediate portion40    hydraulic lock  96     intermediate shoulder alleviation device                 97     lower portion41    puncture pin    98     lower shoulder42    puncturable disc                 99     bottom opening43    gland nut      100     groove on 81 w/6344    step on pin 41 101     seal45    seal           102     lip on lower sleeve46    rim on 41      103     head of 1547    threads on 97  104     recesses48    threads on 43  105     seal on 5149    interior bore of 43                106     anti rotation pins50    opening plug   107     closing plug51    plate of 50    108     bore of60    upper sleeve body                109     space in 1161    exterior of 60 110     area between plugs62    interior of 60 112     recompression angle63    downwardly extending                113     area member of 60 w/100                114     groove for 6664    seal           115     space210   hydraulic lock 429     shoulder alleviation device                430     shear pin242   rupturable disc                431     shear pin243   gland nut      440     channel in 420245   seal           441     a, b holes247   interior threads                442     recess of 297         481     edge248   threads on 243 490     channel in 80249   bore of 243    510     hydraulic lock250   space                  alleviation device290   stepped channel                520     solid spool294   shoulder of 290                527     end297   lower portion  531     shear pin of 290         590     channel in 80310   hydraulic lock 610     hydraulic lock alleviation device     alleviation device311   threaded portion                630     lock member312   fracture point 631     edge of 636313   recess in plug 632     break off rod of 630314   end            633     hole in 630 for 640315   groove of 320  634     side of 638316   seal           635     lower arm of 630317   edge           636     extension of 630318   thread relief  637     side of 632 recess         638     upper extension of 635319   side           639     corner of 632320   knock-off plug 640     pivot pin321   bore of plug   641     shaft of 640390   channel        642     head of 640393   shoulder of 390                650     valve spool397   threads of 390 651     side of 650410   hydraulic lock 652     groove of 651 for 653 alleviation device                653     seal420   valve spool    654     groove in 650421   groove         655     side of 654422   seal           681     recess423   hole for 430   682     side of 681424   hole for 431   683     side of 681425   edge           684     hole in 80427   end            690     channel through 80428   surface of 420______________________________________ 
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1A-1E and 2, a stage tool 10 is shown with a hydraulic lock alleviation device 40. The stage tool 10 has an outer case including an upper case 11 and a lower case 12 which is shown as threadly connected to the upper case 11, but which may be welded or otherwise secured. Threads 13 are provided on the upper case and threads 14 are provided on the lower case for mating with standard connections on casing or other tubulars. Preferably the cases are made from a grade of steel compatible with typical casing. 
     Within the cases 11, 12 are disposed an upper seat 30, an upper sleeve 60, and a lower sleeve 80. With appropriate action each of these items can be moved within the case 13. Shear plugs 15 are partially disposed in a tight sliding fit in cementing ports 16. These plugs prevent the flow of cementing fluid to the exterior of the tool 10 until the lower sleeve 80 has been moved downwardly to shear the plugs 15 thereby opening the ports (as will be described in detail below). 
     The lower sleeve 80 has a generally circular lower sleeve body member 81 with recesses 76 for threadedly receiving and holding a threaded portion of the shear plugs 15. The body member 81 has a central circular opening 82 with an interior cylindrical surface 86 at the termination of ridges 83 and 84 which extend inwardly from the exterior surface 85 of the body member 81. An O-ring seal 87 is disposed in recess 88 in the exterior surface 85 of the body member 81. An O-ring seal 101 is disposed in recess 79 in the exterior surface 85 of the body member 81. A stepped channel 90 at an angle of 15° to the longitudinal axis of the lower sleeve 80 is provided in and through the body member 81. 
     As shown in FIG,. 2, the stepped channel 90 has a top opening 91, a top shoulder 92, a mid portion 93, a mid shoulder 94, an intermediate portion 95, an intermediate shoulder 96, a threaded lower portion 97, a lower shoulder 98, and a bottom opening 99. A groove 89 on the bottom of the body member 81 is provided for anti-rotational locking of the body member 81 onto anti-rotation pins 106. A groove 100 on the top of the body member 81 is provided for anti-rotational locking with a downwardly extending member 63 of the upper sleeve body 60; conversely shoulders 78 on the lower sleeve 80 extend into recesses on the sleeve 60 to prevent rotation. Although the channel 90 may be at any angle so long as flow through the body 81 is permitted and other parts do not restrict flow through the tool 10, it is preferred that the channel be at an angle of 15° tilted away from the tool&#39;s longitudinal axis as indicated for alignment between the top 72 of the puncture pin 41 and an angled lower lip 71 on the upper sleeve body 60. 
     The recess 17 in the interior wall of the case 12 is provided for clearance for the seals 87 and 64 as they move past the ports 16 upon downward movement of the lower sleeve 80. The recess 19 in the interior wall of the case 12 is provided for receiving a lock ring 66 for holding the upper sleeve 60 in locked position. The seals 101 and 87 assist in sealing off flow to port 16. 
     The hydraulic lock alleviation device (including the stepped channel 90) includes a puncture pin 41, a puncturable disc 42, a seal 45, and a gland nut 43 disposed in the lower portion 97 of the stepped channel 90. It is preferred that the puncture pin 41 be made from a drillable material such as brass or aluminum. The pin 41 has a step 44 which abuts the shoulder 92 to prevent the pin 41 from falling out inadvertently through the top opening 91. A seal 45 abuts the shoulder 94 and the puncturable disc 42 abuts the seal 45. The disc 42 effectively seals off the channel 90 to the flow of fluid therethrough and the disc is fabricated to withstand a certain amount of pressure and to be rupturable in response to a certain amount of pressure. The pin 41 can have a pointed rim 46 to assist in puncturing the disc 42. The lower portion 97 of the channel 90 has interior threads 47 which can mate with threads 48 on the gland nut 43. The gland nut 43 retains the disc 42 and the seal 45 in position. The gland nut 43 has an interior bore 49 which can receive the pin 41 and through which the pin 41 can pass. 
     The hollow upper sleeve body 60 (preferably made from steel) is generally circular and has an exterior surface 61 , an interior surface 62, a generally circular bore 73, and a downwardly extending member 63. The member 63 is disposable in groove 100 of the lower sleeve 80 for antirotation locking. O-ring seal 64 is disposed in a recess 65 in the exterior surface 61. O-ring seal 69 is disposed in recess 70 in the exterior surface 61. A snap ring 66 is disposed in recess 67 in the exterior surface 61. A diffuser groove 68 formed in the exterior surface 61 interrupts or diffuses the flow of fluid flowing to the ports 16 which, if uninterrupted, could damage the seal 64. The upper sleeve body 60 has a lower lip 71 which, as shown in FIG. 2, can be configured at an angle to meet flush and parallel with a top 72 of the pin 41 for more accurate pushing on the pin 41. 
     The upper seat 30 has an exterior surface 31, an interior surface 32, a generally circular bore 33, and an interior cylindrical surface 34 at the termination of ridges 35 and 36. The lower portion of the upper seat 30 is threadedly connected in the top of the upper sleeve body 60 by means of threads 39 on the upper seat 30 and threads 74 on the upper sleeve 60. Closing shear balls 37 rest in and are partially disposed in recesses 38 in the exterior surface 31. The shear balls 37 are also partially disposed in and held by recesses 21 in the case 11. The balls are initially inserted by lining up a recess 38 with the hole 22, dropping a ball into a recess. The upper seat 30 is then rotated until another recess 38 appears under the hole and another ball 37 is inserted. The balls 37 move in a recess 21 in the interior surface of the case 11. After the balls are inserted, a plug 23 is placed in the hole 22 to seal it. The balls 37 are fabricated so that they will shear at a desired pressure releasing the upper seat 30. 
     FIGS. 3B, 4, and 5 illustrate various stages in the operation of a tool according to this invention such as the tool 10 shown in between two casings 25, 26 in a wellbore 18 in FIG. 3A. As shown in FIG. 3 an opening plug 50 has been inserted into the tool 10 which is emplaced in a wellbore (not shown). A plate 51 of the plug 50 has contacted the ridge 83 of the lower sleeve 80 and pushed the lower sleeve 80 downwardly with force sufficient to break the shear plugs 15, thereby freeing the lower sleeve 80 so that it can move downwardly to a point where its motion is stopped by the abutment of a lip 102 of the lower sleeve against a shoulder 24 of the lower case 12. Rotation of the lower sleeve 80 is prevented by antirotation pins 106 which are received in and held by recesses 104 in the exterior surface 85 of the lower sleeve 80. 
     Once the shear plugs 15 are broken and the sleeve 80 moves, the ports 16 are open to the flow of cementing fluid from the interior of the tool 10 to the annulus 27 (FIG. 3A) between the wellbore&#39;s interior surface 28 and the tool&#39;s exterior. A seal 105 on the plate 51 helps the plate 51 to seal against the ridge 83 so that cementing fluid is prohibited from flowing downwardly beyond the plate 51. As shown in FIG. 3 a head 103 of the broken shear plug 15 has fallen out of and away from the port 16. Because the head 103 is larger in diameter than a portion 29 of the port 16, the head 103 cannot fall into the tool 10. 
     With the cement ports 16 open, circulation may be established to prepare the wellbore annulus for cementing or cementing through the port 16 may begin immediately. As cementing progresses to the final stages, a considerable hydrostatic pressure differential is realized across the lower sleeve 80 and the opening plug 50 due to a relative increase in density of the cement column in the annulus 27 with respect to the static displacement fluid in the area below the plug 50. This pressure differential is prevented from equalizing with the pressure below the plug 50 by: the bridge and seal of the plug 50 in the sleeve 80; the gland nut 43, the disc 42 and the seal 45 arrangement in the sleeve 80; and the seals 101 and 87 in the sleeve 80. The disc 42 is designed to sustain extremely high pressures that would not even be expected in cementing, and these pressures have no effect on the pin 41 to cause premature activation of the volume relief mechanism. 
     Referring now to FIGS. 3 and 4, a closing plug 107 displaces final quantities of cement slurry through the ports 16, the plug 107 has landed on the seat 30 and a pressure tight bridge has been formed across the bore 24. As surface casing pressure increases, a sufficient downward force is imparted to the shear balls 37 to break them and allow the upper sleeve 60 and the upper seat 30 to shift downward. At this moment, a recess 20 in the interior surface of the case 11, under the seal 69 in FIG. 3, has equalized pressure in the small space 109 (between the seals 69, 64, the sleeve 60 and the case 12) with cement slurry pump pressure in the area 110 between the two plugs. This pressure equalization prevents the seal 64 from being pressure energized in to the port 16 and cut as the sleeve 60 slides past the port. 
     When the lip 71 slides past the recompression angle 112, cement slurry inside the area 110 begins to create high velocity jets in space 115 as it exits through the space 115 between the sleeve 60, the snap ring 66 and the bore 24 in proximity to the ports 16. These jets of fluid are extremely small in volumetric flow rate as compared to the volumetric flow rates of the ports 16. This decrease in volumetric flow rate and the incompressible nature of the cement slurry imparts a braking force to the upper sleeve 60 causing it to slow down in its remaining travel as fluid slowly meters out of the space 115 through the port 16. The high velocity flow of these fluid jets close to ports 16 would impart a sudden pressure differential that would lift the seal 64 out of the recess 65 and into the port 16 and cut it as the sleeve 60 slides close. A diffuser groove 68 in the sleeve 60 causes a more even flow pattern around the circumference of the sleeve 60 that disrupts the lifting force and prevents or inhibits seal damage. 
     As entrapped fluid continues to flow slowly from area 110 into space 115 and then to port 16, the upper sleeve 60 slowly moves into position as shown in FIG. 4. The lip 71 of the upper sleeve 60 comes into contact with the end of the pin 41. Downward force that is continuously being applied to the upper sleeve 60 by the upper seat 30 through the plug 107 from surface casing pressure above the plug is now imparted to the puncture pin 41. The pointed rim 46 of the lower end of the pin 41 is driven through the disc 42 as it is held in place by the gland nut 43. This action takes place before the seal 64 reaches recompression angle 112 to seal off the ports 16. Further movement of the upper sleeve 60 drives the pin 41 into the gland nut interior bore 49. As the seal 64 reaches the recompression angle 112, a hydraulic lock will be effected and inhibit or freeze any further travel of the upper sleeve 60 until entrapped fluid in the area 110 helps to push the pin 41 out of the bore 90 and into the area 113. As the pin 41 enters the area 113, cement slurry from the area 110 may flow freely into the area 113 to relieve the hydraulic lock and allow the upper sleeve 60 to travel to the position shown in FIG. 5. 
     As the entrapped fluid exits through the channel 90 of the lower sleeve 80, the sleeve 60 travels downward until it abuts the lower sleeve 80. The snap ring 66 is now adjacent the groove 114 in the upper case 11 and springs outwardly, since it is biased as such, to permanently lock the upper sleeve 60 in place. The seals 64 and 69 are positioned across ports 16 to effect a pressure tight seal. 
     Closing the ports 16 completes the cementation process of this stage. Other stages may be cemented or the drill out of the plugs 50 and 107 and the seats 30 and 80 may be performed. This device according to the present invention is a significant improvement over apparatuses as disclosed in U.S. Pat. Nos. 3,811,500 and 3,842,905. The devices in these patents use a single shear device to secure a release rod in place. This single shear device must withstand loads placed on it by cementing pressures pushing on the rod, so it must have a relatively high resistance to shear to prevent premature activation. But this same shear device must also shear when desired to release the rod and hence high shear resistance is not desirable; i.e., the easier it can be for the shear device to shear and release the rod, then the easier it is to design the shear device and have the rod-release action accomplished correctly. In our device there is a rupturable disc which can withstand the high cementing pressures, but which also has a puncture pin which is movable by a relatively small force from the lower sleeve. The puncture-pin mode does not disrupt sleeve travel or tool operation. So there is the protection or desired aspects of resistance to high pressure via the disc, but also the ability to release the trapped fluid in response to a relatively low force on the puncture pin. This same concept is present in the knock-off plug, crush plug, a spool-with-separate-locking-and-releasing-means embodiments described below. 
     Placing the hydraulic lock alleviation apparatus in the lower sleeve and related methods have proven to be most reliable (i.e. puncture pin 41, disc 42); however, other methods can be applied within the scope of the present invention, including, e.g., ceramic crush plugs; rupturable discs; plastic knock-off plugs; and a valve spool with a lock released by an upper sleeve. 
     A hydraulic lock alleviation device 210 shown in FIG. 6 as disposed in the lower sleeve 80 of FIG. 2 in place of the device 40 includes a rupturable disc 242, a seal 245, a stepped channel 290, and a gland nut 243 disposed in stepped channel 290. An O-ring seal 245 abuts the shoulder 294 and the rupturable disc 242 abuts the seal 245. The disc 242 effectively seals off channel 290 to the flow of fluid therethrough, and the disc is fabricated to withstand a certain amount of pressure and to be rupturable at a certain amount of pressure. The lower portions 297 of the channel 290 has interior threads 247 which can mate with threads 248 on gland nut 243. The gland nut 243 retains the disc 242 and the seal 245 in position. The gland nut 243 has an interior bore 249 through which fluid may flow. A pressure increase in the space above the device 210 will be transmitted through the channel 290 to the disc 242 until the rupture pressure of the disc is reached; thereby rupturing the disc and allowing fluid to flow from above the device, through the channel 290 and into a second space 250 below the device. As this occurs, an upper sleeve such as sleeve 60 in FIG. 2 will be permitted to move to abut the lower sleeve 80 and any hydraulic lock will be relieved. 
     A hydraulic lock alleviation device 310 as shown in FIGS. 7a, 7b, and 7c includes a channel 390 and a knock-off plug 320. It is preferred the knock-off plug 320 be made from plastic so that the plug may break in the proper manner as described below. The plug 320 has a threaded portion 311 which can mate with threads 397 of channel 390. A groove 315 of the plug 320 receives a seal 316 that abuts with a shoulder 393 of the channel 390 to effectively seal off the channel 390 to the flow of fluid therethrough. The plug 320 is secured in place by the threads 311 and is designed to sustain extremely high pressures that would not even be expected in cementing operations. The plug 320 has a recess 313 that extends into an edge 317 past the groove 315 but not through an end 314. A thread relief recess 318 has a fracture point 312 that is made to break if sufficient force is put on end 314 or side 319. The device 310 can be disposed, for example, in a lower sleeve such as the sleeve 80 (FIG. 2) in place of device 40. With this structure, before the seal 64 hits the recompression angle 112, the lip 71 of the upper sleeve 60 comes into contact with the end 314 o±the knock-off plug 320. Downward force that is continuously being applied to the upper sleeve 60 by the upper seat 30 through the plug 107 is now applied to the plug 320. The plug 320 breaks at the fracture point 312 as the plug is forced down and channel 390 is open to area 110 through bore 321 through the plug. Cement slurry from area 110 may flow freely into area 113 to prevent a hydraulic lock, and upper sleeve 60 may travel until it hits lower sleeve 80. The plug 320 may be a crushable plug made from a suitable crushable material (e.g. ceramics or glass) which instead of being forced down is crushed or shattered. 
     A hydraulic lock alleviation device 410 as shown in FIGS. 8A and 8B includes a channel 490, a valve spool 420, shear pins 430 and a seal 422. It is preferred that the spool 420 be made from a drillable material such as brass or aluminum. The spool 420 has a groove 421 in a surface 428 that accepts the seal 422. The spool 420 has shear pins 431 and 430 pressed into holes 424 and 423, respectively, to prevent the spool 420 from inadvertently falling out or shearing out of sleeve 80 under cement pressures. The spool effectively seals off the channel 490 to the flow of fluid therethrough and the pins 431 and 430 are made to withstand a certain amount of shear force to hold the spool 420 in place. The spool 420 has a channel 440 that consists of a recess 442 and holes 441a, b. The recess 442 in an end 427 extends axially into the spool 420 until it intersects the holes 441 a, b. The holes 441 a, b are in close proximity to the groove 421 on the opposite side of groove 421 as the hole 423. The device 410 can be disposed in a sleeve of a stage tool such as the sleeve 80 of the tool of FIG. 2. With this structure, before the seal 64 hits the recompression angle 112, the lip 71 of the upper sleeve 60 comes into contact with the end 427 of spool 420. Downward force applied to the spool 420 breaks the pin 431. Further movement of the upper sleeve 60 abuts an edge 425 of a shoulder 429 onto an edge 481 of sleeve 80 and causes the holes 441 a, b to travel into the bottom opening 99. Cement slurry now flows freely from area 110 to area 113 to prevent a hydraulic lock from occurring. The sleeve 60 may travel uninterrupted until abutting with the sleeve 80. 
     The device 510 shown in FIGS. 9A and 9B is like device 410 but a spool 520 is solid and has no shoulder like shoulder 429, no end 425 and no channel through it. The device 510 can be disposed in a sleeve 80 as in FIG. 2 in place of the device 40. With this structure before the seal 64 hits recompression angle 112, the lip 71 of the upper sleeve 60 comes into contact with an end 527 of spool 520. Downward force is now imparted to the spool 520 to break a pin 531. Further movement of the sleeve 60 causes the seal 64 to hit the recompression angle 112, a hydraulic lock will be effected and inhibit or freeze any further travel of the sleeve 60 until entrapped fluid in area 110 helps to push the spool 520 out of channel 590 in sleeve 80 and into the area 113. As the spool 520 enters area 113, cement slurry from the area 110 may flow freely into the area 113 to relieve the hydraulic lock. 
     An hydraulic lock alleviation device 610 as shown in FIG. 10 includes a valve spool 650, a seal 653, and a lock member 630. It is preferred that the valve spool 650, the lock member 630 and a pivot pin 640 be made from a drillable material such as aluminum or brass. The lock member 630 has a hole 633 into which the pivot pin 640 slides. This device 610 may be used in place of device 40 in sleeve 80 of FIG. 2. The sleeve 80 has a hole 684 in side 682 of recess 681 into which a shaft 641 of the pivot pin 640 is pressed. The pivot pin 640 has an upset or head 642 that cannot pass through hole 633 of member 630. Therefore, member 630 is free to rotate on the pivot pin 640, and is captured on the shaft 641 by the head 642 on one side and by the side 682 of sleeve 80 on another. The spool 650 has a groove 654 in a side 651 that accepts an upper extension 638 of a lower arm 635 of the member 630. A groove 652 on the side 651 accepts the seal 653. The valve spool 650 effectively seals off the channel 690 to the flow of fluid therethrough and the upper extension 638 is designed to withstand a certain amount of shear force to withstand a certain amount of cement pressure that pushes the spool 650 until the side 655 of the recess 654 hits a side 634 of the upper extension 638. The member 630 with the pin 640 is designed in proximity to the spool 650 such that no rotational motion is imparted to member 630 by the spool 650 from cement pressures in the area above it which would prematurely release it. 
     The member 630 has break-off rod 632 whose side 637 abuts with a side 683 of a recess 681 of the sleeve 80 to prevent any clockwise rotation of member 630. The break-off rod 632 is designed to break at a fracture point at corner 639 at a certain amount of force. It can be fashioned to break in response to fluid pressure or in response to a member pushing down on it. Once broken at corner 639, member 630 may rotate to let the upper extension 638 retract from groove 654. With the device 610 in the sleeve 80 of the structure of FIG. 2, before the seal 64 hits the recompression angle 112, the lip 71 of the sleeve 60 hits the edge 631 of an extension 636 of the member 630. Downward force is now imparted to the member 630. The corner 639 is broken as the edge 637 is forced against edge 683 to impart breading stresses to corner 639. Break-off rod 632 is broken off of member 630 and allows member 630 to rotate around pin 640 to retract extension 638 from groove 654. As the seal 64 reaches the recompression angle 112, a hydraulic lock will be effected and inhibit or freeze any further travel of the sleeve 60 until entrapped fluid in the area 110 helps to push the spool 650 out of bore 690 and into area 113. As the spool 650 enters area 113, the cement slurry from area 110 may flow freely into the area 113 to relieve the hydraulic lock and allow the sleeve 60 to travel untill it abuts with sleeve 80. 
     As will be apparent to ordinarily skilled artisans, there are many tool configurations and methods which can utilize this invention and the invention is equally applicable to such tools and methods. Skilled artisans could conduct minor routine experimentation to determine precisely the best combination of the aspects described herein and yet still be within the scope of this invention. While there have been described various embodiments of the present invention, the methods and apparatus described are not intended to be understood as limiting the scope of the invention. It is realized that changes therein are possible and it is further intended that each step or element recited in any of the following claims is to be understood as referring to all equivalent steps or elements for accomplishing substantially the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized.