PLUG FOR USE IN WELLBORES

Abstract: A plug (10) has a first member (14) which is flexible enough to bypass breakaway units (52,54) in a casing string (56) and a second member (18) which is rigid enough to break off the bypassed breakaway units (52,54). In use, pieces broken off the breakaway units (52,54) by the second member (18) are trapped between the first member (14) and the second member (18).

Description: This invention relates to plugs for use in wellbores. Plugs are used for various purposes in wellbore operations, for example to act as a barrier between cement and displacement fluid to inhibit the displacement fluid and cement intermingling; to wipe off fluid such as drilling mud or cement from the interior casing wall; to provide means for indicating when cement has been displaced from within the casing; and to break off frangible breakaway units which protrude into the casing. A very serious problem created by prior art plugs is the forward migration of broken breakaway units. These units can become lodged inside check valves, landing baffles or other operative members in the wellbore thereby rendering them inoperative. US-A-3 102 595 discloses a plug for use in cementing operations. The plug comprises a head end which is provided with a deformable ring followed by a flange and a series of wiper blades. When lowered into a wellbore, the deformable ring can be forced past a stop ring mounted in the wall of the casing whilst further downward travel is prevented by the flange casing into contact with the stop ring. Any attempt to use this plug in a wellbore with frangible breakaway units protruding into the casing would result in the breakaway units being broken away by the deformable ring ahead of the plug. The present invention is directed to an improved wellbore plug which, at least in its preferred embodiments, mitigates this problem. According to the present invention there is provided a pump down plug suitable for use in wellbore operations, said plug having a head end and comprising at least one flexible wiper blade remote from the head end of the plug, characterized by radially extending members comprising a first member located adjacent the head end of the plug and sufficiently flexible to by-pass a breakaway unit extending inwardly from the inner walls of the tubes of a string, and a second member which is relatively rigid in comparison with said first member, is located between said first member and said wiper blade(s), and is capable of breaking away said breakaway unit. Preferably, the first member and the second member are of circular cross-section. Advantageously, the diameter of the second member is substantially equal to the diameter of the first member. In one form of the invention, the diameter of the second member is smaller than the diameter of the first member. In such an embodiment there is preferably provided a third member having a diameter substantially equal to the first member, the first member and the third member being disposed one to either side of the second member. Preferably, the first member is segmented. Advantageously, means are provided for reinforcing the second member. Preferably, the first member and the second member are substantially conical and are arranged to taper away from one another. The second member can be made of any inherently rigid material such as metal, rubber or plastic or it can have a portion made from a relatively hard or rigid material to enhance the member's ability to break the breakaway units. For a better understanding of the invention reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of one embodiment of a plug according to the present invention; Fig. 2 is an end view of the first member of the plug of Fig. 1; Fig. 3 is a cross-sectional view of the plug within a packer in a wellbore at the point at which the first member is passing breakaway units; Fig. 4 is a cross-sectional view of the plug of Fig. 3 as the second member breaks off the breakaway units; and Fig. 5 is a cross-sectional view of another embodiment of a plug according to the present invention. Referring to Figures 1 and 2 of the drawings, there is shown a plug 10 for use in cementing operations. The plug 10 has a head 12 to which is threadedly connected a first member 14. An axial spacer 16 is threadedly connected to the first member 14 on one end and to a second member 18 on the other end. A connector 20 is threadedly connected at one end to the second member 18 and at the other end to a second rear cup 22. The second rear cup 22 is threadedly connected to a first rear cup 24. The head 12 is made from an easily drillable material such as aluminium. It has a threaded female recess 26 for threadedly receiving the male nose 28 of the first member 14. The nose 28 is formed integrally with the shaft 30 of the first member 14. A threaded female recess 32 in the shaft 30 accommodates the nose 40 of the axial spacer 16. The first member 14 comprises a flexible cone 34 formed circumjacent the shaft 30. As shown in Fig. 2, the flexible cone 34 has a plurality of radially extending slits 36 therethrough dividing the flexible cone 34 into segments 33. The relatively thin cross section of the flexible cone 34 and the radial extending slits 36 render the cone 34 relatively more flexible so that it does not break breakaway units in use. The axial spacer 16 is threadedly connected to the female recess 42 of the second member 18. The cone 44 of the second member 18 is made thick enough, rigid enough, and/or strong enough to break selected frangible breakaway units which the plug 10 will encounter in use. The length of the axial spacer 16 is chosen so that sufficient movement of the plug 10 allows the flexible cone 34 of the first member 14 to pass a breakaway unit before the cone 44 of the second member 18 breaks it off. After the breakaway unit has been broken the broken piece(s) are inhibited migrating forward of the head 12 by the flexible cone 34 and from migrating rearward by the cone 44. Figs. 3 and 4 illustrate the operation of the plug 10. As shown in Fig. 3 the plug 10 has entered a packer 50 in a casing string 56 and the flexible cone 34 of the first member 14 has encountered the breakaway units 52 and 54 of the packer 50. Segments 33 in contact with the breakaway units 52 and 54 have bent upwardly, the second member 18 has not yet contacted the breakaway units 52 and 54. The breakaway units 52 and 54 seal off packer inflation passages during running of the casing string. As shown in Fig. 4 the flexible cone 34 has passed the breakaway units 52,54 and the cone 44 has contacted and broken the breakaway units. The segments 33 of the flexible cone 34 have resumed their normal position and prevent the broken-off plug parts from falling ahead of or migrating ahead of the plug 10. Also in the embodiment shown in Fig. 4, the broken off parts are maintained in the space between the leading face of the cone 44, the rear of the flexible cone 34, the interior wall of the packer 50 and the surface of the axial spacer 16 so that they cannot move to the area at the rear (top) of the plug or to the area in front of the plug 10. The embodiment of the plug 110 shown in Fig. 5 has a second member 118 which has a cone of lesser diameter than the cone 44 of the plug 10. Nevertheless, the cone 118 is sufficiently large that it can effectively contact breakaway units such as the breakaway units 152,154 of the packer 150. The cone 118 also has a rigid inner member 119 made of metal or plastic which strengthens the cone and enhances its ability to break breakaway units. The plug 110 also includes a third member 121 which is of the same diameter as the first member 114 and which prevents broken pieces of breakaway units 152,154 from migrating upwardly beyond the plug 110.

Claims: A pump down plug (10) suitable for use in wellbore operations, said plug (10) having a head end and comprising at least one flexible wiper blade (22) remote from the head end of the plug (10), characterized by radially extending members comprising a first member (14) located adjacent the head end of the plug (10) and sufficiently flexible to by-pass a breakaway unit (52, 54) extending inwardly from the inner walls of the tubes of a string, and a second member (18) which is relatively rigid in comparison with said first member (14), is located between said first member (14) and said wiper blade(s) (22), and is capable of breaking away said breakaway unit (52, 54). A plug as claimed in Claim 1, wherein said first member (14, 114) and said second member (18, 118) are of circular cross-section. A plug as claimed in Claim 2, wherein the diameter of the second member (18) is substantially equal to the diameter of said first member (14). A plug as claimed in Claim 2, wherein the diameter of said second member (118) is smaller than the diameter of said first member (114). A plug as claimed in Claim 4, including a third member having a diameter substantially equal to said first member (114), said first member (114) and said third member being disposed one to either side of said second member (118). A plug as claimed in any preceding Claim, wherein said first member (14, 114) is segmented. A plug as claimed in any preceding Claim, including means (119) for reinforcing said second member (118). A plug as claimed in any preceding Claim, wherein said first member (14, 114) and said second member (18, 118) are substantially conical and are arranged to taper away from one another.

Inventors: SCHNEIDER, DAVID E.
Applicants: WEATHERFORD U.S. INC.
Application year: 1987
Main classification: E21B23
IPC codes: E21B29, E21B33, E21B23, E21B27