FASTENER FOR USE IN AN OIL TOOL AND OIL TOOL PROVIDED WITH SUCH A FASTENER

Abstract: A fastener for use in an oil tool between a first member (20) and a second member (30), said fastener comprising an outer member (50) which, in use, extends between said first member (20) and said second member (30), characterised in that said outer member (50) is provided with an inner recess (51).

Description: Fastener for use in an Oil Tool and Oil Tool provided with such a Fastener This invention relates to a fastener for use in an oil tool and to an oil tool provided with such a faste- ner. During the construction of oil and gas wells a borehole is drilled in the ground. The borehole is then lined with a casing string. One of the problems which sometimes arises is that the ground subsides as the oil and/or gas is extracted. This compresses and distorts the casing. This is highly undesirable since it is often desired to re-enter the casing to extend the well either vertically or horizontally. In order to help reduce this problem it is know to incorporate casing slip joints into lengths of casing. Such joints comprise two sections which are intended to telescopically contract if the ground subsides. Typically, casing slip joints are maintained in a fully extended position by shear pins as the casing is urged down hole. However, the same shear pins must shear before the casing deforms if the ground subsides. It will be appreciated that the ideal solution would be some form of fastener which was initially relatively strong but which would subsequently become relative weak. Such fasteners would also find application in a wide variety of oil tools including cementing stage tools and packers where similar properties would be desirable. The present invention aims to help achieve this desiderata. According to the present invention there is provided a fastener for use in an oil tool between a first member and a second member, said fastener comprising an outer member which, in use, extends between said first member and said second member, characterised in that said outer member is provided with an inner recess. In its simplest form the present invention simply envisages allowing a fluid in the recess to structurally weaken the fastener over a period of time. Such fluid may be introduced into the recess as, for example a paste immediately prior to use of the fastener or may be found native in the wellbore, for example brine. Preferably, said fastener further comprises an inner member which can be introduced into said inner recess. Advantageously, said inner member and said outer member are correspondingly threaded. Preferably, said inner member is provided with a head to facilitate rotation thereof. Advantageously, said head comprises a socket. Preferably, said inner member and said outer member are made of dissimilar materials, for example the inner member could be made of aluminium and the outer member of steel, so that upon contact, for example by a well fluid such as brine, a galvanic cell is formed that produces stress corrosion cracking in the outer member that weakens it and/or destroys it. The propagation of such cracks is facilitated by placing one or more notches or recesses in the outer member. Advantageously, said fastener includes an electrolyte between said inner member and said outer member. Preferably, said inner member exerts a force on said outer member in the range of from 50% to 80% of the yield strength of said outer member. Advantageously, said outer member is provided with one or more notches or recesses. The present invention also provides an oil tool comprising a first member and a second member secured together by at least one fastener in accordance with the present invention. The present invention further provides a casing slip joint comprising an outer casing and an inner casing secured together by a fastener in accordance with the present invention. For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: - Fig. 1 is a side view, partly in elevation and partly in section, of a prior art casing slip joint with conventional shear screws utilised as fasteners; Fig. 2 is a side view, partly in elevation and partly in section, of one embodiment of a casing slip joint secured with fasteners according to the present invention; Fig. 3 is a side view in cross-section of another embodiment of a casing slip joint secured with fasteners according to the present invention; Fig. 4A is a section through part of the casing slip joint shown in Fig. 2; Fig. 4B is a cross-section through an outer member which forms part of the fastener of Fig. 4A; Fig. 4C is a side view of an inner member which forms part of the fastener of Fig. 4A; and Fig. 4D is an end view of an inner member of the fastener of Fig. 4A. Referring now to Fig. 1, a prior art casing slip joint A has an outer casing C and an inner casing B. Conventional shear screws D in channels E releasably hold the outer casing C to the inner casing B. Fig. 2 shows a casing slip joint 10 according to the present invention which has an outer casing 30 (which can be any desired, appropriate length) and an inner casing 20. Fasteners 40 in channels 42 through the outer casing 30 releasably attach the inner and outer casings together. Each fastener 40 has an outer member 50 and an inner member 60. As shown in Figs. 4A and 4B, the outer member 50 has a body 52 which is generally cylindrical and an internally threaded inner recess 51 for receiving the inner member 60, the recess having a lower end 53. Preferably the outer members 50 are sufficiently tightened (e.g. about 7 foot pounds) in place so that they are under tension. As shown in Figs. 4C and 4D, the inner member 60 has a hexagonal recess 62 into which a correspondingly shaped tool can be inserted to threadedly engage threads 61 (indicated by dashed lines) on the inner member 60 with the inner recess 51 of the outer member 50. In one aspect the outer member 50 and inner member 60 are made of different metals so that, with electrolytic well fluid, they set up an electrolytic cell to induce damage, e.g. but not limited to stress cracking, to the outer member 50. In another aspect the outer member 50 and the outer casing 30 are made of different metals so that the cell is created. The inner member 60 may be inserted into the outer member 50 and torgued to such an extent that the lower end 63 pushes against the lower end 53 of the recess 51 of the outer member 50, stressing the outer member 50 to facilitate crack propagation. In one case an aluminium outer member 50 is anodic to a steel casing 30. In certain aspects, the inner member 60 is made of steel, iron, copper, brass or aluminium, e.g. but not limited to aluminium alloy 2011- T3. Any electrolytic well fluid may be used including but not limited to brine and salt water. It is within the scope of this invention to adjust the pH of such a well fluid, e.g. by adding acid, e.g. acetic acid. In one aspect the pH is adjusted to about 5.5. Prior to running a casing slip joint into a wellbore, the inner member 60 and/or the outer member 50 can be treated with an acid, with salt water, or with an acid-salt water mixture to facilitate initiation of the electrolytic cell effect although this may not be necessary in many wells. Stress corrosion cracking and/or weakening of the shear screws, in one aspect, is facilitated when the wellbore temperature at the casing slip joint is 66 β C (150 β F) or higher and the tensile strength applied on the outer member by the inner member is about 50% to about 80% of the yield strength of the outer member. A notch 54 across a top portion of the outer member 50 and/or at least one notch 55 around the outer member 50 provide a weakened area from which stress corrosion cracking may propagate. Fig. 3 shows a casing slip joint system 70 with a casing 71 (shown partially), a coupling 72 threadedly connecting the casing 71 and a casing 73, fasteners 80 (e.g. like the fasteners 50) which releasably connect the casing 73 and a casing 74, and a coupling 75 connecting the casing 74 and a casing 76 (shown partially) - all disposed in a wellbore (not shown) as part of a casing string casing the wellbore and cemented therein (cement not shown). Upon destruction of the fasteners 80, (either by electrochemical action and/or by shearing) the casing 73 may be moved downwardly up to a distance d with respect to the casing 74, for example to accommodate subsidence. The outer members 50 may be installed with a tool that is placed in the notch 54 for turning or by a tool placed in the hex opening 62 of the inner member 60. In another aspect the inner member does not occupy the entire recess 51 and an erodeable container containing an electrolyte or an acid is emplaced therein. Alternatively, no inner member is used and such a container is used. Fasteners in accordance with the present invention are not limited to use in casing slip joints. In particular, they may find application in other oil tools such as stage cementing tools and packers. In the preferred embodiment herein described it is envisaged that the fasteners will lose their structural strength over a number of days, typically 1 to 2 weeks. However, it is envisaged that by correctly designing and selecting the materials the required degradation in strength could take place in a few hours or such time as may be necessary to run the oil tool into position.

Claims: CLAIMS 1. A fastener for use in an oil tool between a first member ( 20 ) and a second member ( 30 ) , said fastener comprising an outer member (50) which, in use, extends between said first member ( 20 ) and said second member (30), characterised in that said outer member (50) is provided with an inner recess ( 51 ) . 2. A fastener as claimed in Claim 1, wherein said fastener further comprises an inner member (60) which can be introduced into said inner recess ( 51 ) . 3. A fastener as claimed in Claim 2, wherein said inner member ( 60 ) and said outer member ( 50 ) are correspondingly threaded. 4. A fastener as claimed in Claim 3, wherein said inner member (60) is provided with a head to facilitate rotation thereof. 5. A fastener as claimed in Claim 4, wherein said head comprises a socket. 6. A fastener as claimed in Claim 2, 3, 4 or 5, where- in said inner member ( 60) and said outer member ( 50) are made of dissimilar materials. 7. A fastener as claimed in Claim 6, including an electrolyte between said inner member ( 60 ) and said outer member ( 50) . 8. A fastener as claimed in any of Claims 2 to 7, wherein said inner member (60) exerts a force on said outer member (50) in the range of from 50% to 80% of the yield strength of said outer member ( 50) . 9. A fastener as claimed in any preceding Claim, wherein said outer member ( 50 ) is provided with one or more notches (54,55) or recesses. 10. An oil tool comprising a first member (30) and a second member ( 20 ) secured together by at least one fastener as claimed in any preceding claim. 11. A casing slip joint comprising an outer casing (30) and an inner casing (20) secured together by a fastener as claimed in any of Claims 1 to 9.

Inventors: ERIKSEN ERIK PETER VESTERGAAD, HAUGEN DAVID, MAHMOUD SALAH, ERIKSEN, ERIK, PETER, VESTERGAAD, HAUGEN, DAVID, MAHMOUD, SALAH
Applicants: LUCAS BRIAN RONALD, WEATHERFORD LAMB, LUCAS, BRIAN, RONALD, WEATHERFORD/LAMB, INC.
Application year: 1998
Main classification: E21B17
IPC codes: E21B17