Abstract:
An apparatus for launching at least two plugs, said apparatus comprising a mandrel ( 2 ), a lower plug ( 22 ) and an upper plug ( 21 ) releasably disposed with respect to said mandrel ( 2 ), wherein said upper plug ( 21 ) comprises a body ( 23 ) having a flow path ( 24 ) therethrough and a cap ( 25 ) for blocking fluid flow through said flow path ( 24 ) characterized in that said cap ( 25 ) is detachable from said body ( 23 ) in use if the upper plug ( 21 ) is prematurely ejected.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of Great Britain application number 9723581.6, filed Nov. 7, 1997, which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a plug for use in wellbore operations and an apparatus for launching said plug. 
     2. Description of the Related Art 
     During the construction of an oil or gas well a hole is bored in the ground. A string of casing is lowered down the hole and the annular space between the casing and the wellbore filled with cement. 
     In order to inhibit the cement mixing freely with other wellbore fluids it is normal to use two devices which are referred to as “cementing plugs”. When it is desired to cement the casing in place a first (bottom) cementing plug is released into the casing. The cement is then pumped into the casing and pushes the bottom plug downwardly. When the desired volume of cement has been introduced into the casing a top plug is introduced into the casing and pumped downwardly by a convenient fluid, typically sea water or circulating fluid. The cement then travels down the casing trapped between the bottom and top plugs. When the bottom plug reaches the bottom of the casing it lands on a one way valve known as a float collar. Pressure is applied to the top plug until a bursting disk in the bottom plug ruptures and allows the cement to be pushed through a flow bore in the bottom plug, through the float collar and upwardly into the annular space between the casing and the bore. The description thus far relates to normal operation. However, occasionally the bottom plug and the top plug are released simultaneously with the result that a large column of cement is pumped down the casing on top of the top plug. Since the top plug is designed to withstand high pressures it will not rupture and consequently the cement sets and has to be drilled out. 
     A large number of devices have been designed for preventing inadvertent simultaneous release of the bottom plug and top plugs and those which have been designed for use at the surface of a land well are generally satisfactory. However, the problem which does remain is when a casing is extended by a liner and it is necessary to cement the liner in position. In this case the bottom and top plugs have to be released remotely from the surface. Although there are a number of apparatus intended for inhibiting simultaneously release of the top and bottom plugs in these circumstances it is still not unknown for the plugs to be released simultaneously with expensive consequences. 
     SUMMARY OF THE INVENTION 
     The present invention approaches this problem from an entirely new angle and, whilst it is primarily intended for use with liner hanging and drilling equipment, can also be used at the surface. 
     According to the present invention there is provided a plug for use in wellbore operations, which plug comprises a body having a flow path therethrough and a cap for blocking flow of fluid through said flow path, characterized in that said cap is detachable from said body in use. 
     The present invention should not be confused with plugs of the type having hinged caps which are not detachable from the body in use. 
     Preferably, said plug further comprises a restraining device attached to said cap. 
     Advantageously, said restraining device comprises a flexible belt. 
     Preferably, said plug further comprises a sealing ring to provide a substantially fluid tight seal between said cap and said body of said plug. 
     Advantageously, said plug comprises elastomeric material. 
     Preferably, said cap is attached to said body by fingers. 
     The present invention also provides an apparatus for launching at least two plugs, said apparatus comprising a mandrel, a lower plug and an upper plug releasably disposed with respect to said mandrel, wherein said upper plug comprises a body having a flow path therethrough and a cap for blocking fluid flow through said path characterized in that said cap is detachable from said body in use. 
     Preferably, said apparatus further comprises a restraining device attached to said cap and to said mandrel. 
     Advantageously, said restraining device comprises a flexible belt. 
     Preferably, said restraining device is attached to said mandrel via a shear pin. 
     Advantageously, said shear pin is adapted to be sheared after completion of an operation intended to release said upper plug. 
     Preferably, said operation is landing of a dart intended to initiate release of said upper plug. 
     Advantageously, said apparatus further comprises a sealing ring to provide a substantially fluid tight seal between said cap and said body of said plug. 
     Preferably, said cap is attached to said body by fingers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof 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. FIGS. 1 a  to  1   g  show a cross-section of an apparatus in accordance with the invention in use in various stages of normal operation; and FIGS. 2 a  to  2   d  show a cross-section of the apparatus of FIG. 1 in various stages of abnormal operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings there is shown a plug launching apparatus which is generally identified by reference numeral  1 . The plug launching apparatus  1  comprises a hollow mandrel  2  which is connected at its upper end to the lower end of a liner setting tool (not shown). The plug launching apparatus  1  incorporates a displacement member  3 . 
     The displacement member  3  comprises a lip  4 , an upper row of displacement member ports  5 , a middle row of displacement member ports  6  and a lower row of displacement member ports  7 , each row being disposed circumferentially thereabout. The displacement member  3  also comprises a seat  8 . 
     There is provided an annular space  9  in the hollow mandrel  2  which accommodates the displacement member  3 . A first row of mandrel ports  10  is disposed circumferentially about the hollow mandrel  2  and allows fluid communication between the annular space  9  and the exterior of the plug launching apparatus  1 . Upper shear pins  11  are retained in recesses  12  in the hollow mandrel  2  and inhibit the displacement member  3  from longitudinal movement therein. Lower shear pins  13  are retained in recesses  14  in the hollow mandrel  2 . A second row of mandrel ports  15  is disposed circumferentially about hollow mandrel  2  and is initially in line with the lower row of displacement member ports  7 . 
     Upper sealing ring  16  and lower sealing ring  17  act as a fluid tight seal between hollow mandrel  2  and the displacement member  3 . It should be noted the lip  4  does not sealingly engage the hollow mandrel  2  and there are provided small gaps therebetween through which fluid may flow. 
     A plug container  18  is disposed below the hollow mandrel  2  and is integral or otherwise connected thereto. A row of small diameter container ports  19  is disposed at the top and circumferentially around the plug container  18 . A set of container ports  20  are disposed at the bottom of and circumferentially around the plug container  18  in three rows. 
     In use, the plug container is loaded with an upper plug  21  and a lower plug  22 . The upper plug  21  comprises an elastomeric body  23  which has external fins (not shown) arranged circumferentially thereabout. The elastomeric body  23  is provided with a generally concentric flow path  24 . The flow path  24  is initially blocked by a cap  25  which is locked in place by fingers  26 . The cap  25  is provided with a sealing ring  27  to provide a substantially fluid tight seal between the cap  25  and the elastomeric body  23 . Flexible belts  28  are attached to the cap  25  at one end, and to a shear bolt  29  at the other. The shear bolt  29  is located in a shear bolt recess  30  in the hollow mandrel  2 , and projects into a recess  31  between the hollow mandrel  2  and the displacement member  3  and below a shoulder  32  thereon. 
     The lower plug  22  comprises an elastomeric body  33  which also has external fins (not shown) arranged circumferentially thereabout. The elastomeric body  33  is provided with a generally concentric flow path  34 . The flow path  34  is initially blocked by a bursting disk  35 . 
     The plug launching apparatus  1  is then loaded into a liner assembly generally referred to by reference numeral  36 . The liner assembly  36  comprises a liner  37 , a coupling  38  and a liner hanger unit  39 . 
     The plug container  18  is aligned with the coupling  38  in order that an annulus is formed between the plug container  18  and the coupling  38 . The coupling  38  is screwed or otherwise attached to the top of the liner  37 . A liner hanger unit  39  (only part of which is shown) is screwed or otherwise attached to the top of the coupling  38 . 
     The top of the liner hanger unit  39  is attached to a tool string (not shown). The liner assembly  36  is lowered into the cased wellbore (not shown) to the lower end thereof. The liner is hung on the lower end of the casing using the liner hanger unit  39  which is typically activated mechanically or hydraulically. Initially fluid is allowed to flow through the hollow mandrel  2  and out into the liner  37  through displacement member ports  7  which are aligned with mandrel ports  15 . 
     The liner  37  can now be cemented in place. To effect this operation a first dart  40  (FIG. 1 b ) is pumped down the tool string and through the hollow mandrel  2  using cement. The first dart  40  lands on seat  8  of displacement member  3  and substantially blocks the lower displacement member ports  7  and hence the second row of mandrel ports  15 . 
     Pressure builds up in the hollow mandrel  2  and forces displacement member  3  downwardly shearing the upper shear pins  11  (FIG.  1 C). The fluid in the annular space  9  flows through the small gap between the lip  4  and the hollow mandrel  2 . The pressure differential across the lip  4  acts as a damper and helps prevent impact on the lower shear pins  13 . The lower plug  22  is ejected from the plug container  18  by the seat  8  of the displacement member  3 . Middle displacement member ports  6  now align with the second row of mandrel ports  15  and allow cement to flow from the hollow mandrel  2  into the annulus formed by the plug container  18  and into liner  37  hence pushing the lower plug  22  down therethrough ahead of the cement (FIG. 1 d ). The flexible belts  28  extend, however there is provided enough slack in the belts so that full extension of the flexible belts  28  is not reached. A row of small diameter container ports  19  are provided to aid damping i.e. as the upper plug  21  moves downwardly the displaced volume slowly fills with cement through the small diameter container ports  19 . 
     The set of container ports  20  aids separation of the plugs by allowing cement to flow into the plug container  18  before the lower plug  22  has completely ejected the plug container  18 . 
     After a predetermined quantity of cement has been pumped down the tool string a second dart  41  is pumped down the tool string and through hollow mandrel  2  using mud, water or any other suitable fluid. The second dart  41  lands on first dart  40  and substantially blocks the middle displacement member ports  6  and hence mandrel ports  15  (FIG. 1 e ). Pressure builds up in the hollow mandrel  2  and forces displacement member  3  downwardly shearing the lower shear pins  13 . Substantially simultaneously, the shoulder  32  shears shear bolt  29  which releases the flexible belts  28 . Fluid flows through the small gap between the lip  4  and the hollow mandrel  2 . Again, the change in pressure differential across the lip  4  acts as a damper. The upper plug  21  is ejected from the plug container  18  by the seat  8  of the displacement member  3  with the cap  25  remaining attached to the upper plug  21  blocking the flow path  24 , together with the flexible belts  28 . 
     The lip  4  meeting shoulder  42  of the hollow mandrel  2  prevents displacement member  3  from further downward movement. Upper displacement member ports  5  are now aligned with the second row of mandrel ports  15  and allow fluid to flow from the hollow mandrel  2  into liner  37  hence pushing the upper plug  21  down therethrough behind the cement. 
     It should be noted that the internal diameter of the liner hanger unit  39  is slightly larger than the internal diameter of the liner  37  so that the plug container  18  may be raised in relation to the liner hanger unit  39  without the need for collapse of the plug container  18 . However, if the external diameter of the plug container  18  is greater than the internal diameter of the liner hanger unit  39 , the plug container  18  may be allowed to collapse on lifting therethrough. 
     FIGS. 2 a  and  2   d  relate to abnormal operation, wherein both upper and lower plugs are inadvertently launched together. 
     With regard to FIGS. 2 a  and  2   b  the first two stages of operation are identical to that described above with reference to FIGS. 1 a  and  1   b . However, after the first dart  40  is received in the displacement member  3 , the displacement member moves downwardly, shearing shear pins  11  and launching the lower plug  22 . However, upper plug  21  abnormally launches simultaneously. It should perhaps be added that the reasons why an upper plug and a lower plug launch simultaneously are not fully understood although the phenomena itself is well documented. 
     Lip  4  landing on shear pins  13  prevent further downward movement of the displacement member  3 . Thus, shoulder  32  does not shear the shear bolts  29 . The flexible belts  28  reach maximum extension and restrain the cap  25  from downward movement. The upper plug  21  continues downward and parts from the cap  25 . Flow path  24  is now available for the cement to pass through thus enabling the operator to flush the cement from the liner. 
     Various modifications to the preferred embodiment are envisaged, for example the plug launching apparatus could accommodate three or more plugs. The upper shear pins, lower shear pins  13  and shear bolts  29  could be replace by shear rings or any other suitable shear devices. Similarly, the flexible belt  28  could be replaced by any suitable form of attachment means.