GB2257447A

GB2257447A - Stabiliser for a drill string

Info

Publication number: GB2257447A
Application number: GB9114885A
Authority: GB
Inventors: Garrigh John Young
Original assignee: STEELE GEOFFREY
Current assignee: STEELE GEOFFREY
Priority date: 1991-07-10
Filing date: 1991-07-10
Publication date: 1993-01-13
Also published as: GB9114885D0

Abstract

A stabiliser for a drill string comprises a substantially cylindrical body member (1) having an axial bore (2) and, at its ends, means (3, 4) for connecting said stabiliser to collars of a drill string, a sleeve (5) concentric with said body member, provided with blades (6), and adapted for limited axial motion between a central position and upper and lower stop positions, body member when in said central position and being said sleeve being capable of axial rotation with respect to said engageable with upper and lower end members (7, 8) when respectively in said upper and lower stop positions, each of said end members having resilient means (9, 9') for urging said sleeve towards its central position and a driving member (10, 10') adapted to engage a drivable member (11, 11') at a respective end of said sleeve when said sleeve is in a respective stop position. When the drill string is under tension or compression, the sleeve will automatically be forced towards one of its stop positions, so that the driving and drivable means engage, thereby providing rotation of the sleeve, with a corresponding reaming action, as the drill string rotates. <IMAGE>

Description

STABILISER FOR A DRILL STRING This invention relates to a stabiliser for a drill stringjand to a method of drilling a well using such a stabiliser. More specifically, this invention relates to a stabiliser for a drill string which comprises a body member and a sleeve around the body member. The sleeve is adapted to be moveable between a position in which it is rotatable with respect to the body member, and positions in which it is not rotatable with respect to the body member.

In drilling technology, stabilisers are provided in order to ensure that a drill string is maintained centrally within a bore, and generally comprise a series of blades, which generally form a spiral with respect to the central axis of the body and have abrasion-resistant outer edges. A drill string will normally comprise a series of lengths of drill pipe (otherwise known as collars), with a drill member at the base of the well. The drill string will itself generally be in tension, and the drill collar will itself generally be in compression. The stabilisers are provided to ensure that the drill string is centralised in the well, to prevent buckling and to provide directional stability.

In general, there are two main types of stabiliser, the integral blade stabiliser and the sleeve type stabiliser.

Integral blade stabilisers have blades which are fixed with respect to the body. Such stabilisers are robust and can assist in reaming in and out of the hole when conditions are tight. The disadvantage of such stabilisers is that they create a high torque in the drill string. Wells are frequently not vertical and it is common, in an offshore drilling rig, for a series of wells to be drilled outwards from the rig at angles which even approach the horizontal. Oil-based drilling fluids have a degree of lubricity which helps to reduce the problems of torque in such wells, but the use of oil-based fluids is being phased out because of environmental considerations. It has been known to provide captive balls at the outer edge of an integral blade stabiliser.This can reduce the effect of torque, but the balls themselves are subject to a high degree of wear and thus rapidly lose their effectiveness.

Sleeve type stabilisers are sleeves provided with blades which fit over the drill collars but are free to rotate independently. While this provides a lower drilling torque, such stabilisers are not commonly used because of their fragility, and the impossibility of carrying out a reaming operation using them.

The object of this invention is to provide a stabiliser which, when in use, does not lead to excessive torque, but which can nevertheless be used for reaming in and out.

According to the invention there is provided a stabiliser for a drill string, which stabiliser comprises a substantially cylindrical body member (1) having an axial bore (2) and, at its ends, means (3,4) for connecting said stabiliser to collars of a drill string, a sleeve (5) concentric with said body member, provided with blades (6), and adapted for limited axial motion between a central position and upper and lower stop positions, said sleeve being capable of axial rotation with respect to said body member when in said central position and being engageable with upper and lower end members (7,8) when respectively in said upper and lower stop positions, each of said end members having resilient means (9,9') for urging said sleeve towards its central position and a driving member (10,10') adapted to engage a drivable member (11,11') at a respective end of said sleeve when a said sleeve is in a respective stop position.

This invention also relates to a method of drilling a well in a formation using a drill string having one or more such stabilisers. The present invention will be described more particularly with respect to the accompanying drawings in which: Figure 1 is a vertical section through a stabiliser according to the invention; and Figure 2 is a horizontal section along the line II...II' of Figure 1.

Referring now to the drawings, the stabiliser comprises a body member (1) having an axial bore (2). This body member is adapted to be compatible with conventional collars of a drill string. It is provided with end fittings, for instance a female joint (3) and a male joint (4), which engage with corresponding joints on normal lengths of drill pipe.

A sleeve (5) is provided coaxial with the body member (1).

This sleeve has blades (6) distributed symmetrically around its circumference, in generally conventional manner. They will be provided with a surface of a hard-wearing material, such as tungsten carbide, for abrasion resistance and to assist in effective reaming out of a well.

The sleeve is provided on its inside with bearings (12) so that it is capable of rotation about the axis of the body member (1) in circumstances which will be described in more detail below.

The sleeve (5) is capable of limited axial movement with respect to the body member (1') but is prevented from more than limited axial movement by means of a pair of end members (7,8) which are themselves coaxial with the body member (1) and are fastened to the body member by means of a suitable fastening means, such as a pin (13) or bolt, or other convenient means which will be apparent to those skilled in the art.

In the embodiment shown in the invention, each end member is formed with an annular recess (15), coaxial with the linear axis of the body member (1), while the sleeve member (5) has an annular recess (14) at each end. Consequently, each end member has an outer shroud (16) projecting coaxially with the main axis of the body member (1) and towards the other end member and each end of the sleeve (5) has an inner cylindrical projection (17) separated from an outer shroud (18) by the annular recess (14).

Cooperation of the respective shrouds and recesses of the sleeve and end members thereby produces a labyrinthine seal, whereby fluid within a well bore is prevented from passing through the recess (14) and (15), over the outer shroud (18), the shroud (16) and projection (17). Suitable seals such as O-rings or chevron seals (19) are provided so that fluids cannot pass through the labyrinth, although mutual rotation between the body member (1) and sleeve (5) is possible.

The end members (7,8) are each provided with resilient means (9,9'), for instance constituted by springs or compression disks. These act to urge the sleeve into a central position in which it is capable of rotation with respect to the body member (1).

Each end member is provided with a driving member (10,10') which is capable of engaging a corresponding driveable member (11,11') when the sleeve (5) is urged from its central position into one or other of stop positions determined by steps (20,20') on the end member and body member, respectively.

These driving and driveable members (10,11) can be, for example, intermeshing gear members, or friction plates.

The operation of the stabiliser according to the invention is as follows. When the diameter of a well bore is adequate, the sleeve will be in its central position and able to rotate freely with respect to the body member, thereby minimising torque.

When the stabiliser is moved into a tighter hole, however, so that the string comes under tension or compression, the blade member will undergo relative movement into one of the stop positions, whereby the driving and driveable members (lo,ll;lO',ll') come into engagement with one another. When this happens, the sleeve is forced to rotate as the body member rotates with the rest of the drill string, whereby the blades (6) rotate and produce the required reaming action. Once this reaming action is complete, the effect of the resilient means (9,9') will automatically return the sleeve (5) into its central, non-driven position so that the sleeve and blades no longer rotate with the drill string, and act only to centralise the string in the well bore.

Claims

1. A stabiliser for a drill string, which stabiliser comprises a substantially cylindrical body member (1) having an axial bore (2) and, at its ends, means (3,4) for connecting said stabiliser to collars of a drill string, a sleeve (5) concentric with said body member, provided with blades (6), and adapted for limited axial motion between a central position and upper and lower stop positions, said sleeve being capable of axial rotation with respect to said body member when in said central position and being engageable with upper and lower end members (7,8) when respectively in said upper and lower stop positions, each of said end members having resilient means (9,9') for urging said sleeve towards its central position, and a driving member (10, 10') adapted to engage a drivable member (11,11') at a respective end of said sleeve when said sleeve is in a respective stop position.

2. A stabiliser according to claim 1 wherein the sleeve (5) has bearings engaging the outer surface of the body member (1).

3. A stabiliser according to claim 1 or 2 wherein said driving members (10,10') and drivable member (11,11') comprise gear members which engage when the sleeve is in a respective stop position and do not engage when the sleeve is not in said position.

4. A stabiliser according to claim 1 or 2 wherein said friction plates (10,10') and drivable members (11,11') comprise friction plates which engage when the sleeve is in a respective stop position and do not engage when the sleeve is not in said position.

5. A method of drilling a well in a subterranean formation making use of a drill string having one or more stabilisers, wherein the stabilisers are in accordance with any one of claims 1 to 4.

6. A stabiliser as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

7. A method as claimed in claim 5 and substantially as hereinbefore described.