US3616850A

US3616850A - Cementing plug launching mandrel

Info

Publication number: US3616850A
Application number: US29973A
Authority: US
Inventors: Lyle B Scott
Original assignee: Byron Jackson Inc
Current assignee: Byron Jackson Inc; Baker Hughes Holdings LLC
Priority date: 1970-04-20
Filing date: 1970-04-20
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02
Also published as: FR2086201B1; AR205963A1; GB1304996A; DE2118815A1; FR2086201A1; DE2118815B2; BR7102317D0; AT302933B; GB1304995A; DE2118815C3; AR211923A1; JPS5212121B1; RO61776A; NL7105025A

Abstract

A cementing plug launching mandrel which includes an annular piston chamber and a piston in the chamber operable by fluid pressure in the mandrel to provide a force to shear the pins which support the cementing plug on the mandrel when a closure carried in the fluid closes the opening through the cementing plug. In addition, the mandrel has a surge chamber to absorb the shock pressure wave caused by closure of the opening through the cementing plug.

Description

United States Patent Lyle B. Scott South Gate, Calif. 29,973

Apr. 20, 1970 Nov. 2, 1971 Byron Jackson, Inc. Long Beach, Calif.

inventor Appl. No. Filed Patented Assignee CEMENTING PLUG LAUNCHING MANDREL 13 Claims, 6 Drawing Figs.

US. Cl 166/155, 166/5, 166/70 Int. Cl ..E2lb33/l6, E21b 33/14 Field of Search ..166/.5, 291, 70,153,155,156

[56] llle ferenc es Cited UNITED STATES PATENTS 2,630,179 3/1953 Brown 166/155 X 2,704,980 3/1955 Vincent..... 166/153 X 3,076,509 2/1963 Burns et a1 166/153 X 3,364,996 l/l968 Brown 166/155 X 3,507,325 4/1970 Scott 166/.5

Primary Examiner-David H. Brown Attorneys-Donald W. Banner, William S. McCurry and John W. Butcher PATENTEDNHV 2 ml 3.616.850

SHEEI 2 [IF 3 [NVENTOR CV66 5 SCOTT WA c9.

BACKGROUND OF THE INVENTION In the cementing of the well casing in offshore wells drilled beneath a body of water, and wherein the casing head apparatus is located on the floor of the ocean or other body of water, the cementing plug or plugs may be run into the well bore along with the casing string on a running in string of drill pipe. Such practice and apparatus for accomplishing such practice are more particularly disclosed in my pending application for U.S. Letters Pat. Ser. No. 735,685, filedJune 10, 1968, now US. Pat. No. 3,545,542. In the use of suchapparatus, for example, a bottom cementing plug is launched from its support in the casing when an opening through the bottom plug is closed off by a closure entrained in the cement slurry as it flows downwardly through the running in string-of drill pipe, through the'plug-launching mandrel, and through" the bottomcementing plug. The cement slurry, being substantially heavier than the fluid preceding it in the well bore generally causes the preceding fluid or flushing fluid to be rapidly downwardly displaced in the casing and upwardly in the annulus between the casing and the well bore wall. When this occurs, the well is said to be on vacuum" and the casing below the top cementing plug, which is still retained on its mandrel, as well as the running-instring of drill pipe are devoid of hydraulic fluid or cement. Thereafter, a closure for the opening through the top cementing plug is introduced into the running-in string of drill pipe in advance of a quantity of displacing fluid, which is to be employed to finally displace the cement slurry into the casing well bore annulus. It is desirable that the separation of the top cementing plug from its mand'rel be positively determinable, since generally in this type of cementing operation, the volume of the displacing fluid above the top plug is calculated, based on the known volume of the casing string and the volume of the casing well bore annulus. However, since the top cementing plug is located substantially below the top of the running-in string of drill pipe, where the well is being completed in water which may be hundreds of feet deep, the displacement fluid above the closure for the top plug opening attains substantial velocity during its downward travel towards the top cementing plug. When the closure closes the top plug, there is an instantaneous shock pressure wave or water hammer effect which acts on the top plug, and in the case where the mandrel has a port above the top plug to expose the entire cross-sectionalarea of the plug to displacing fluid pressure to discharge or launch the plug, the plug may be prematurely launched as a result of the shock pressure wave. As a consequence, pressure cannot be developed in the displacing fluid for the purpose of giving a sensible pressure increase at the top of the running-in string of drill pipe, the drop in which pressure would otherwise serve to signify that the top plug has been launched.

While the problem is more or less acute in plugs of different sizes, from large cementing plugs used to displace cement in the cementing of the usual conductor pipe in the well bore, downwardly to the smaller string of casing which is ultimately run into the well and cemented in place, the problem is basically caused by the fact that if the shearpins which connect the top plug to the mandrel are so strong as to prevent premature launching of the plug by the above-referred shock pressure wave acting across the entire cementing plug area, then a very substantial increase in displacing fluid pressure must be relied upon to effectively displace the plug by shearing the pins, such high pressure possibly resulting in the flow of fluid past the outside of the plug without causing shearing of the connection of the plug to the mandrel. On the other hand, while the problem can be somewhat reduced by confining the displacing fluid to the running-in string of drill pipe, thereby reducing the effective area of the plug to which displacing fluid pressure is applied, such solution involves the objectionable reduction of the shear strength of the shear pins which connect the top plug to its mandrel, with resultant uncertainty as to the launching of the plug.

SUMMARY OF THE INVENTION The present invention provides the cementing plug launching apparatus which obviates the foregoing problems.

More particularly, the present apparatus includes a pluglaunching mandrel adapted to be connected to the running-in string of drill pipe for the conduct of cement and other fluids into the casing below the cementing plug apparatus, and wherein the displacing fluid which is to shear the top plug from the mandrel is confined to the mandrel itself, and wherein the mandrel has an annular piston chamber provided with an annular piston responsive to the pressure of displacing fluid, when the passage through the top cementing plug is closed by the usual closure or dart, to provide a supplemental force for shearing the shearpins which connect the plug to the mandrel.

In addition to the foregoing, the present apparatus also includes in the plug-launching mandrel a surgechamber which is evacuated when the cement slurry rapidly moves downwardly in the wall casing placing the wellhead on vacuum, and which enables the dampening of the shock pressure wave occasioned by the closure or dart blocking the flow of displacing fluid through the top plug, so that the water hammer effectwill not inadvertently displace the top plug.

As a-result of the foregoing, applicant's invention provides a means whereby a'predetermined pressure must be applied to the annular'piston as well as to the plug closure in order to displace the plug from its mandrel, and the instantaneous drop in such pressure will suffice to positively signify at the top of the running-in string of drill pipe that the plug has been displaced. Thus, the invention affords greater certainty for the well-cementingcrew.

This invention possesses many other advantages, and has other purposes which may be made more clearly apparent from a consideration of a form in which it may be embodied. This form is shown intlie drawings accompanying and forming part of the present specification. It will now be described in detail, for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense, since the scope of th'einvention is best defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a view partly in elevation and partly in vertical'section illustrating plug-launching apparatus made in accordance with the invention installed in association with underwater wellhead equipment apparatus;

FIG. 2 is a view in vertical section and on an enlarged scale illustrating the plugs and mandrel in the wellhead body shown in the lower portion of FIG. 1;

FIG. 3' is a view in vertical section showing the bottom plug released and providing a barrier between flushing fluid below the bottom plug and cement slurry above the top plug, prior to the release of the top plug;

FIG. 4 is a view in vertical section showing the launching dart for the top plug seated in the top plug, prior to release of the top plug;

FIG. 5 is a view in vertical section showing the release of the top plug; and

FIG. 6 is a view in vertical section showing the top plug nested in the bottom plug following displacement of the cement slurry from the casing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, there is generally illustrated wellhead apparatus H of the type adapted to be located beneath the water adjacent to the floor F or bottom of the water. Cementing and other completion operations are performed through the wellhead apparatus from a vessel or platform at or above the surface of the water, and for cementing operations the surface apparatus includes a cementing head CH. The well includes a large bore W in which an outermost string of easing C is disposed and cemented into place. The upper end of the casing C has a seat 1 for a supporting flange 2 of the tubular wellhead body 3 which supports another string of well casing Cl adapted to extend downwardly into a reduced well bore section and to be cemented in place. Internally thereof the wellhead body 3 may support a casing hanger body 4 for another casing string (not shown) adapted to extend still further into a further reduced well bore and to be cemented in place. Supported on the casing hanger 4 is an innermost casing hanger body 5 connected as at 6 to an innermost casing string C2 which is illustrated as extending downwardly into the reduced well bore W2. The casing hanger body 5 and the casing string C2 are run into the well on a universal running tool T which is threadedly connected as at 7 to the lower end of a string of drill pipe 8, the running tool T being threadedly connected as at 9 to the upper end of the casing hanger body 5.

Such subsurface apparatus also generally includes an annular support member 10 which is affixed to the upper end of the outer pipe or casing C and has vertically extended guide posts 11 slidably receiving guide tubes 12 which are guided on cables R3 extending to the usual platform or vessel above or on the surface of the water. These guide tubes 12 support the usual blowout preventer stack P, only the lower portion of which is shown in FIG. 1, and the blowout preventer P is secured to the top of the wellhead body 3 by suitable latch means 1... Such latch means are well known and include releasable latch dogs 14 shiftable to inner positions to secure the latch body 15 on the wellhead body 3 in response to movement of an annular latch piston 16 downwardly, the latch dogs 14 being releasable upon upward movement of the latch piston 16. Piston chambers l7 and 18 are provided and are adapted to be pressurized by respective conduits l9 and 20 to shift the piston 16 downwardly and upwardly, as desired. The blowout preventer P is connected to the latch body 15 as by fasteners 15a.

The casing string C2 is run into the well on the drill pipe 8 until the casing hanger 5 lands upon the casing hanger 4. At the lower end of the casing string C2 may be a conventional casing float shoe S having the usual backflow-preventing valve 22 which prevents the flow of fluid from the well bore into the casing string C2, but which will permit the circulation of fluid downwardly through the casing string C2 into the well bore W2 and upwardly in the casing well bore annulus.

The wellhead structure described above is more or less conventional and needs no further detailed description herein. The present invention involves the cementing plug launching apparatus generally illustrated in FIG. 1 as comprising a mandrel M which is threadedly connected at 23 to the running tool T, and extends downwardly into the upper end of the casing string C2, the mandrel supporting at its lower end a top cementing plug TP and a bottom cementing plug BP.

As seen in FIG. 2 in greater detail, the mandrel M comprises an elongated tubular assemblage comprising a lower body section 24 and an upper body section 25 providing a central flow passage 26 extending axially through the mandrel. A swivel connection 27 is provided between the

mandrel body sections

24 and 25 to enable making up the threaded connection 23 between the upper end of the mandrel and the running too] T while the plugs TP and HP are disposed within the casing C2, without requiring that the plugs themselves rotate. The swivel connection 27 may take any suitable form but illustratively includes an outwardly projecting annular flange 28 on the upper mandrel section 25 and an outwardly extended annular flange 29 on the lower mandrel body section 24 held in relatively rotatable relation by a channebshaped split ring 30 which is in turn held in assembled relation with the

flanges

28 and 29 by a retainer ring 31. To prevent leakage through the swivel connection, a sealing extension 32 on the mandrel body section 25 is rotatable and sealingly disposed in a bore 33 in the lower mandrel body section 24.

At the lower end of the mandrel M is threaded a support ring 34 to which the top plug TP is connected by a suitable number of circumferentially spaced shearpins 35. The top plug TP, as is customary, comprises a tubular inner body 36 of rigid but drillable material on which is molded an outer elastomeric body 37 having a suitable number of outwardly extended wiper rings 38 slidably engageable with the inside wall of the casing, and preferably provided at its upper end with an upwardly extended sealing lip 39 also slidably engageable with the casing. Internally of the body 36 is an annular groove 40 cooperative with a dart, as will be hereinafier described, to latch the dart in the top plug TP.

Means are provided for releasably connecting the bottom plug B? to the top plug TP. In the illustrated structure, the bottom plug BP supports a tubular connector 41 which is threaded into the lower end of the top plug body 36. At the lower end of the connector 41 is a ring 42 to which the bottom plug BP is connected by a suitable number of circumferentially spaced shearpins 43. In general, the bottom plug BP is like the top plug TP in that the bottom plug has a tubular inner body 44 of rigid but drillable material on which is molded an outer elastomeric body having wiper rings 46 and a top ring or cup 47 slidably engageable in the casing. lntemally of the bottom plug body 44 is a reduced-diameter seat 48, and at the lower end of the body is a catcher 49 for a ball which, as will be hereinafter described, is adapted to cause launching of the bottom plug.

The cementing head CH, generally illustrated in FIG. 1, is adapted to be connected to the upper end of the drill pipe 8 at the deck of the vessel or platform on or above the water. This cementing head contains a ball 51 and a dart 52 which are supported in the head CH but which are selectively releasable to move downwardly through the cementing head and through the plug-launching mandrel M to cause the launching of the plugs BP and TP, respectively. The ball 51 is located in a bore 53 of a ball drop housing 54 which is disposed to one side of the head CH, the bore 53 opening into the passage 55 extending axially through the head CH and into the drill pipe 8. The bore 53 in which the ball 51 is retained is located below a fluid inlet 55a. The ball 51 is retained in the bore by a slide 56 which is reciprocable in the bore by a screw actuator 57, whereby the ball may be moved from the position illustrated, into the passage 55 so as to be dropped from the slide 56. The dart 52 is releasably supported on a pin or slide 58 which extends into the head CH to a position beneath the dart 52 but which is retractable by a screw actuator 59. Such ball drop devices and dart-releasing devices are well known and require no further specific illustration or description.

The ball 51, however, is of such diameter as to readily pass downwardly through the pipe 8 and through the top plug TP and the connector 41, but is too large to initially pass through the seat 48 in the passage through the bottom plug 8?. Thus, fluid pressure may be supplied through the drill pipe 8 to act upon the bottom plug B? to shear the shearpins 43 and launch the bottom plug. In order that fluid pressure may be effective over substantially the entire cross-sectional area of the bottom plug, suitable radial ports 41a may be provided in the connector 41 to expose the outer peripheral portion, and more particularly the cup 47 of the plug BP, to fluid pressure. As will hereinafter be more fully described, the ball 51 is preferably sufficiently rigid as to enable the bottom plug B? to be effectively launched by the shearing of the pins 43, but when the plug is arrested in the casing, say upon contact with the casing shoe S or other stop in the casing, the ball 51 is sufficiently resilient as to be hydraulically deformed and forced through the seat 48. Thereafter, the ball 51 will engage a central stop portion 49a of the catcher 49 but fluid may bypass the ball 51 through suitable ports 49b in the catcher 49.

The dart 52 is adapted to engage in the top plug T? to launch the latter, and includes a rigid body 60, on an upward extension 61 of which is mounted an elastomeric body 62 having wiper rings 63 slidably engageable in the drill pipe 8. The body 60 supports a resiliently deformable lock ring 64 adapted to latch the dart 52 into the top plug Tl upon engagement in the annular groove 40 in the latter. When the dart 52 engages the top plug TP, the pressure of fluid above the dart in the mandrel M and the drill pipe 8 is relied upon to shear the shear pins 35 by which the top plug Tl is releasably connected to the mandrel.

The present invention is more particularly directed to the problems encounted in the launching of plugs in submarine cementing operations employing the apparatus generally described above. customarily the forces required to shear the shearpins ed to release the bottom plug and the forces required to shear the shear pins 35 to release the top plug can be readily calculated so that the bottom plug HP will be launched without difficulty and the top plug T? will be retained on the mandrel. In this connection, in the cementing of the casing C2 in the well bore W2, it would be the usual practice to circulate a flushing fluid downwardly through the drill pipe ll, through the mandrel M and the plugs thereon and thence downwardly through the casing C2 and into the well bore and then upwardly through the casing well bore annulus. In any event, the drill pipe 8 and casing C2 would initially be full of fluid, such as drilling mud. Then a quantity of pumpable cement slurry is introduced into the cementing head, the ball 51 being released substantially simultaneously with the flow of cement through the cementing head, the cement displacing the flushing fluid or mud from the drill pipe as the cement travels downwardly and carrying the ball 51! downwardly. When the ball Ell seats on the seat 48 of the bottom plug BP, the shear pins d3 will be sheared, and the released bottom plug will effectively constitute a traveling barrier between the cement slurry and the preceding flushing fluid or mud. Generally, the weight of the cement slurry is sufficient to displace the preceding fluid, so that the cement moves rapidly downwardly through the drill pipe, and the well goes on vacuum. Thereafter, it is the general practice to release the dart 52 and to force the dart downwardly through the drill pipe with a quantity of displacing fluid which is pumped into the drill pipe and travels rapidly and downwardly therethrough until the dart seats into the top plug TP, effectively closing d the fluid passage through the top plug TP, and resulting in a shock pressure wave which heretofore has been known to cause the instantaneous shearing of the shearpins which connect the top plug to the mandrel. This is a particular problem when the pressure is applied to the entire cross-sectional area of the plug TP. The deeper the water in which the well is being completed, the more aggravated the problem. When thetop plug T? is launched by a shock pressure wave, a pressure signal or indication is not given at the top of the drill pipe to indicate that the top plug is released. Such a signal is important to the cementing crew from the standpoint of determining the subsequent volume of displacing fluid introduced into the casing to displace the cement slurry into the casing well bore annulus.

The present invention obviates these problems by confining the displacing fluid to the mandrel, so that any shock pressure can act only on the pressure-responsive area of the dart, but thereafter the pressure acting to launch the top plug is in creased so as to produce a sensible indication at the top of the drill pipe h caused by the drop in pressure when the plug TP ultimately is launched.

Accordingly, adjacent to the lower end of the mandrel body section 2d there is mounted, as by a threaded coupling 70, a sleeve 71 defining with the mandrel body, an annular piston chamber 72, to the upper end of which fluid may be supplied through a port 73 Reciprocable in the piston chamber 72 is an annular piston 7d having suitable side seals 75 slidably engageable with the opposing cylindrical chamber walls. At its lower end, the piston 7d is in abutting engagement with the upper end of the rigid top plug body 36. The piston 7d and the mandrel body section 24 have ports '76, 77 respectively, which are adapted to normally maintain a balanced pressure condition across the plug assembly, particularly when the well goes on vacuum following the introduction of cement into the drill pipe d. Stop means including a screw 7% carried by the piston 74 and engageable with the lower end 79 of a slot 80 in the sleeve 71 are adapted to limit outward movement of the piston 74 with respect to the chamber 72 prior to assembly with the top plug.

The ports 77 in the mandrel body section 2d are so located relative to the seat 52a for the dart 52 that the elastomeric body, and more particularly the wipers 63 will close the ports 77 when the dart lands in the top plug Tl, as seen in FIG. 4 so that displacing fluid above the dart 52 cannot pass through the ports 77 and is therefore effectively confined to the mandrel, and any shock pressure wave will act only on the relatively minor cross-sectional area of the dart: 5h. The mandrel body section 24 also has a number of ports hl which communicate with the ports 73 leading to the piston; chamber 72, so that as pressure builds up in the mandrel M and the drill pipe 8 above the top plug TP, such pressure will be applied to the annular area of the piston 74 to produce an additional downward force acting on the plug TP to shear the shearpins 35.

For purposes of some wells and some plug sizes, it will be found that confining the displacing fluid to the plug-launching mandrel and the subsequent application of additional pressure to the annular piston 74 is a sufficient solution to the problem of unrecognized launching of the top plug TP. However, the present invention also provides further assurance that the shock pressure wave will not be effectively applied even to the minor cross-sectional area of the dart. This is accomplished by providing an elongated cylindrical extension 32 on the sleeve 71, the extension 82 defining with the outer surface of the mandrel body section 24 an annular surge chamber 83 which communicates through the ports hit with the interior of the mandrel section 24 above the dart 52 when the dart seats in the top plug TP. At its upper end the extension @2 is of reduced outside diameter so that the surge chamber volume may be maximum. in addition, preferably, the ports hi lead upwardly at an angle from the interior of the mandrel section 24 into the surge chamber 83 to minimize the tendency of the cement slurry to enter the surge chamber 83 and the piston chamber 72.

OPERATlON In the use of the apparatus described above, a casing string is run into the well on a running tool T which is supported by a suitable length of the drill pipe 3, capable of extending from the wellhead apparatus H at the floor of the body of water to a vessel or platform on or above the surface of the water. The mandrel M, with the plugs TP and BP thereon, is previously connected to the running tool, with the plugs disposed in the casing. The casing string is supported by the casing hanger.

The casing will be filled with fluid, such as a weighted mud, and if desired, fluid will be circulated for a period of time to flush the casing and the casing well bore annulus. Then, the cement slurry will be admitted to the cementing head CH through the inlet 55a and the ball slide 56 actuated to move the ball 51 into the fluid column, approximately at the interface between the cement slurry and the mud or flushing fluid, the ball traveling downwardly in the fluid stream until it seats on the seat M in the passage. through the bottom plug Bl. Fluid pressure on the bottom plug is admitted to the casing from the ports llla in the connector ll between the two plugs BP and TP, and causes the shearpins did to be sheared. The plug Bl then moves downwardly in the casing, as seen in H6. 3, between the cement and the preceding fluid, acting as a barrier therebetween. When downward movement of the bottom plug BP is arrested, say by the casing shoe S, as seen in FIG. 6, the ball 511 will be forced through the seat db and will be caught in the catcher W, as cement slurry flows downwardly through the shoe S and thence upwardly in the casing well bore annulus.

As also seen in FIG. 3, as the cement moves downwardly in the casing, below the top plug Th, the mandrel M will be devoid of fluid, and the well will be on vacuum, as the relatively heavy cement forces the preceding fluid downwardly through the casing shoe S. At this time, the

ports

73 and 76 in the mandrel and in the piston 74 enable balancing of the pressure below the plug TP and the pressure in the annular space between the mandrel M and the casing above the plug Tl, so that there is no unbalance tending to cause the release of the top plug TIP.

Next, the dart 52 is released by retraction of the dart retainer pin 58, and the dart will fall to a location below the cement head inlet 550, through which a displacing fluid of any suitable type is pumped to force the dart downwardly through the drill pipe, the wipers 63 wiping the drill pipe 8 clean of residual cement.

As the displacement fluid above the dart 52 moves the dart downwardly, the dart lands in the seat 520 of the top plug TP, the latch ring 64 latching into the groove in the passage through the plug TP, and the lower wiper 63 closing the mandrel port 73, so that the displacement fluid is confined to the flow passage in the mandrel. Thus, the instantaneous shock pressure wave caused by the closure of the fluid fiowpath by the dart is limited in its application to the relatively small dart area. In the use of some plug sizes, the shearpins 35 can be calculated to resist such shock pressure, but for other sizes, depending on the depth of the water, the surge chamber 83 in the mandrel M is employed to dissipate the shock pressure wave. In other words, it is contemplated that the confinement of fluid to the mandrel and the use of a supplemental force to launch the top plug derived from the pressure acting on the annular piston 74 may, in some cases, be solely relied on, without a surge chamber 83, to provide a means for causing the buildup in prwsure in the drill pipe to launch the top plug, so as to provide a pressure drop which is recognizable at the top of the drill pipe to indicate the release of the plug.

More particularly, as illustrated in FIG. 4, the instantaneous shock pressure wave caused by the interruption to fluid flow through the top plug Tl is dissipated in the surge chamber 83, when it is employed, since a portion of the displacing fluid above the dart can find access to the chamber d3 through the ports 8! which are slightly spaced above the dart when it finally seats in the top plug. As the pressure of the displacing fluid increases, more of such fluid finds access to the surge chamber 83, as seen in H0. 5, until the pressure acting on the dart area and on the annular piston 74 effects shearing of the pins 35 and the plug '1'? is forced downwardly through the casmg.

Ultimately, as seen in H6. 6, the top plug TI will land on the bottom plug Bl, downward movement of the latter having been previously arrested by the casing shoe, as previously described, and the cement will have been fully displaced into the casing well bore annulus.

While the invention has been described as applicable to the cementing of well casings wherein both a bottom and a top plug are employed, it will be understood that subsurface well installations of the type illustrated involve the cementing of certain of the larger pipe or casing strings, say on the order of or inches in diameter, wherein only a top plug is employed. The features of the invention may also be availed of, therefore, in connection with the launching of such large top plugs.

In any event, the present invention provides plug-launching apparatus wherein the pressure-responsive area of the top plug exposed to the effect of fluid pressure in the running string of pipe is controlled, so that a sensible pressure rise is required to ultimately launch the plug. Such rise affords a recognizable drop in pressure when the plug is released. Such drop may be visually observed on a suitable pressure gauge customarily found in association with the cementing equipment.

lclaim:

1. In cementing plug apparatus for use in cementing a well pipe in a well bore, comprising: a mandrel, means for connecting said mandrel to the well pipe with said mandrel extending longitudinally in the well pipe, said mandrel having a passage therethrough for the flow of fluid into said well pipe through said mandrel, cementing plug means having a passage for the flow of fluid from said mandrel passage into said well pipe, said plug means having means cooperative with a closure in the fluid for closing said passage in said plug means, releasable means connecting said plug means to said mandrel and releasable by the pressure of fluid in said mandrel upon closure of said passage through said plug means, the improvement wherein said mandrel has fluid pressure operated piston means operable upon closure of said passage in said plug means to assist in the release of said releasable means.

2. ln cementing plug apparatus as defined in claim 1, said releasable means comprising shearable means connecting said plug means to said mandrel, said piston means including an annular piston chamber in said mandrel, an annular piston reciprocable in said chamber and extending from said chamber into engagement with said plug means, and fluid inlet means in said mandrel leading from the passage in said mandrel into said piston chamber.

3. In cementing plug apparatus as defined in claim 1, said mandrel also having means defining a surge chamber communicating with said passage in said mandrel for absorbing the shock pressure caused by closure of said passage in said plug means to prevent release of said releasable means by such shock pressure.

4. In well-cementing apparatus as defined in claim 1, said plug means including a top plug and a bottom plug, said releasable means connecting said top plug to said mandrel, and including additional releasable means connecting said bottom plug to said top plug, said bottom plug having a passage for the flow of fluid therethrough into said well pipe from said passage of said top plug, and means in said passage in said bottom plug cooperative with a closure in the fluid flowing through said latter passage to close the same and effect release of said bottom plug from said top plug.

5. In well-cementing apparatus as defined in claim I, said mandrel having port means for balancing the pressure across said plug means until said passage through said plug means is closed.

6. ln well-cementing apparatus as defined in claim 1, said piston means comprising a sleeve disposed about said mandrel in spaced relation thereto and defining with said mandrel an annular piston chamber, fluid inlet means leading from the passage through said mandrel into said piston chamber, and an annular piston in said chamber and projecting therefrom to engage said plug means.

7. In well-cementing apparatus as defined in claim I, said piston means comprising a sleeve disposed about said mandrel in spaced relation thereto and defining with said mandrel an annular piston chamber, fluid inlet means leading from the passage through said mandrel into said piston chamber, and an annular piston in said chamber and projecting therefrom to engage said plug means, said sleeve and said piston having cooperative stop means to prevent displacement of said piston from said chamber prior to assembly of said plug means with said mandrel.

8. ln well-cementing apparatus as defined in claim 1, said piston means comprising a sleeve disposed about said mandrel in spaced relation thereto and defining with said mandrel an annular piston chamber, fluid inlet means leading from the passage through said mandrel into said piston chamber, and an annular piston in said chamber and projecting therefrom to engage said plug means, said sleeve having an elongated extension spaced from said mandrel and having sealing engagement with said mandrel in spaced relation to said piston means to form an elongated surge chamber communicating with said passage in said mandrel through said fluid inlet means to said piston chamber.

9. In cementing plug apparatus as defined in claim 1, said releasable means comprising shearable means connecting said plug means to said mandrel, said piston means including an annular piston chamber in said mandrel, an annular piston reciprocable in said chamber and extending from said chamber into engagement with said plug means, and fluid inlet means in said mandrel leading from the passage in said mandrel into said piston chamber, said mandrel also having means defining a surge chamber communicating with said passage in said mandrel for absorbing the shock pressure caused by closure of said passage in said plug means to prevent release of said releasable means by such shock pressure.

10. In well-cementing apparatus as defined in claim 9, said surge chamber communicating with said passage in said mandrel through said fluid inlet means.

11. In cementing plug apparatus for use in cementing a well pipe in a well bore, comprising: a mandrel, means for connecting said mandrel to the well pipe with said mandrel extending longitudinally in the well pipe, said mandrel having a passage therethrough for the flow of fluid into said well pipe through said mandrel, cementing plug means having a passage for the flow of fluid from said mandrel passage into said well pipe, said plug means having means cooperative with a closure in the fluid for closing said passage in said plug means, releasable means connecting said plug means to said mandrel and releasable by the pressure of fluid in said mandrel upon closure of said passage through said plug means, the improvement wherein said mandrel includes means defining a surge chamber communicating with the fluid passage through said mandrel for absorbing the shock pressure wave caused by closure of said passage through said plug means.

12. In cementing plug apparatus as defined in claim 11, said means defining a surge chamber including an elongated sleeve connected at its opposite ends to said mandrel and defining with said mandrel an elongated annular space, and port means leading into said space from said fluid passage through said mandrel.

13. In cementing plug apparatus as defined in claim 11, said mandrel having port means for balancing the pressure across said plug means, said port means being located to be closed by said closure when said closure closes the fluid passage through said plug means to confine fluid to said passage.

Q i I 0 3,1 3? UNTUCD slums PATENT 'OFIICF CERTIFICATE OF CORRECTION Patent No. 3, 1 5 Dated November 2 97 Inventor(s) LYLE B. SCOTT It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' 001. 2, line 56, "top plug should be bottom p1ug--.

Col. a, line 11, support should be --is supported by---. -Col. 6, line 70, "

ports

73 and 76" should be -ports T6 and 7?- Col. 7, line 11, "port 73" should be port 77".

Signed and sealed this 2nd day of January 1973 [Sh/\L) Attcst:

ROBERT GOTTSCHALK EDWARD M. FLETCHER,JR.

Commissioner of Patents Attesting Officer

Claims

2. In cementing plug apparatus as defined in claim 1, said releasable means comprising shearable means connecting said plug means to said mandrel, said piston means including an annular piston chamber in said mandrel, an annular piston reciprocable in said chamber and extending from said chamber into engagement with said plug means, and fluid inlet means in said mandrel leading from the passage in said mandrel into said piston chamber.

5. In well-cementing apparatus as defined in claim 1, said mandrel having port means for balancing the pressure across said plug means until said passage through said plug means is closed.

6. In well-cementing apparatus as defined in claim 1, said piston means comprising a sleeve disposed about said mandrel in spaced relation thereto and defining with said mandrel an annular piston chamber, fluid inlet means leading from the passage through said mandrel into said piston chamber, and an annular piston in said chamber and projecting therefrom to engage said plug means.

7. In well-cementing apparatus as defined in claim 1, said piston means comprising a sleeve disposed about said mandrel in spaced relation thereto and defining with said mandrel an annular piston chamber, fluid inlet means leading from the passage through said mandrel into said piston chamber, and an annular piston in said chamber and projecting therefrom to engage said plug means, said sleeve and said piston having cooperative stop means to prevent displacement of said piston from said chamber prior to assembly of said plug means with said mandrel.

8. In well-cementing apparatus as defined in claim 1, said piston means comprising a sleeve disposed about said mandrel in spaced relation thereto and defining with said mandrel an annular piston chamber, fluid inlet means leading from the passage through said mandrel into said piston chamber, and an annular piston in said chamber and projecting therefrom to engage said plug means, said sleeve having an elongated extension spaced from said mandrel and having sealing engagement with said mandrel in spaced relation to said piston means to form an elongated surge chamber communicating with said passage in said mandrel through said fluid inlet means to said piston chamber.