GB2134162A - Directional drilling - Google Patents

Abstract

A stabiliser for use in directional drilling and having its effective outside diameter adjustable by alteration of axial forces applied to the stabiliser during drilling.

Description

SPECIFICATION Drilling apparatus This invention relates to drilling apparatus.

In the technique of directional drilling, a number of subsurface targets are drilled at angles to the vertical so as to obtain access to a wide spread of wells from a single platform. Several problems are associated with directional drilling, for example the difficulty in keeping the borehole directed towards the target area, as the weight of the drill bit tends to make it drop below the intended drilling line.

At the present time the drilling direction is adjusted by withdrawing the entire drill string from the borehole, replacing stablisers on the string with stabilisers designed to effect drilling direction changes, and reinserting the string down the borehole. This can be extremely timeconsuming and expensive, especially as directional drilling by its very nature is intended to provide a very long borehole from a remote point.

Further, this operation has to be-performed each time the drilling direction is to be adjusted.

In the past it has been necessary from time to time to withdraw the string from the borehole to replace the drill bit in view of the high degree of wear occurring at the bit, and advantage has been taken of these opportunities to adjust or replace the stabilisers in order to correct the drilling direction. Recently, however, much harder and more wear-resistant drill bits have been developed, for example those sold under the Trade Name STRATAPAX, and this has resulted in entire strings being withdrawn from the borehole and then replaced for the sole purpose of replacing the stabilisers; the drill bits now have the capability of being used for much longer periods without replacement.

An object of this invention is to provide a stabiliser which is adjustable downhole to provide an adjustment of the drilling direction.

According to the present invention there is provided a stabiliser for use in directional drilling, comprising a body having a side wall at least a portion of which is adjustable in its dimension normal to the intended drilling direction in response to the magnitude of a force applied to it in said intended drilling direction.

Preferably the stabiliser has one or more projections which are adjustable in their radial extent thereby to alter the effective diameter of the stabiliser. Such projections may for example be in the form of fins which project radially from the central body of the stabiliser.

Preferably also the projections form abutment members for engaging the wall of a borehole in use, and these abutment members are movable laterally of the body into and out of apertures in the side wall of the body.

In the case of cam control the stabiliser may have its body in two parts which are axially movable relative to one another under the effect of a force applied to the stabiliser in the drilling direction. The movement of the shaft may be against the action of a spring or other resilient means. The said parts of the body may be telescopically mounted, the opposite ends of the body having means for connection to respective parts of a drill string.

The invention also provides a drill string having spaced stabilisers for bearing against the wall of a borehole, wherein at least one of the stabilisers is generally cylindrical and is adjustable in its effective diameter in response to the magnitude of a force applied to it in the drilling direction.

The apparatus of the invention can be applied particularly effectively in combination on the drill string with STRATAPAX or other long-life drill bits, as it may allow the drilling to continue, controlled in direction from the surface, without having to withdraw the string from the borehole for replacement or adjustment of the stabiliser before the drill bit has worn out.

Means may be provided for stepwise movement of the abutment members in response to a graduai change in the applied force, so that the shaft moves in quantum jumps reflecting specific intervals of increase or decrease in the applied force. Such an effect can be produced for example by providing rollers or other stop members on one of the parts of the stabiliser body engaging in spaced recesses along the other part of the body, the rollers being spring-biassed into the recesses.

The spring between the first and second parts of the stabiliser body may be a single unit capable of resisting the high working forces exerted on it, or may be a number of springs acting in conjunction.

An embodiment of this invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a side view, partly in section, of a stabiliser of the present invention; Fig. 2(a) is a side view of a preloading and stepwise transfer assembly of the stabiliser of Fig.

1: Fig. 2(b) is an exploded perspective view of part of the assembly of Fig. 2(a); Fig. 3 is an end sectional view of the assembly of Fig. 2(a); and Fig. 4 is an end view of the stabiliser of Fig. 1.

Referring to the drawings, the stabiliser of this embodiment of the invention has a body which is in two main parts, namely first and second hollow cylindrical shaft assemblies 1 and 2. The assembly 1 comprises a shaft 4 having a pack-off end portion 3 which has a screw-threaded bore 4 for engagement with a complementary threaded end portion of part of a drill string (not shown) in which the stabiliser is disposed. The shaft 4 carries four fixed parallel projections 5 disposed at right-angles to one another around the shaft 4, these projections 5 all increasing in height along the shaft 4 so that their outer faces provide identically-inclined cam surfaces.

Screw-threaded onto the shaft 4 co-axially with it is a shaft 6 having a pair of stepped abutment faces 7, 8 against which bear one end of respective compression springs 9, 1 0, the spring 9 being of greater strength than the spring 10. (In other embodiments of the invention the compression springs 9, 10 can be replaced by disc springs). The shaft 6 has longitudinal recesses 11 diametrically opposed in its outer face, one of the recesses 11 being shown in more detail in Fig. 2(b). Each of the recesses 11 has a side wall which has ten semi-circular cut-outs 12 equally spaced along it.

A free-floating piston 43 is disposed in the bore of the shaft assembly 2 to allow for equilibrium of pressure at both ends of the tool.

The piston 43 has a pair of circumferential grooves 1 3 in which O-rings 14 are located, the O-rings 14 providing seals between the piston 43 and a shaft 15 of the assembly 2 disposed coaxially around it.

The free end of the shaft 1 5 is externally screw-threaded at 1 6 to provide means for connection to a complementary threaded bore of a part of a drill-string in which the stabiliser is to be disposed in use. The shaft 1 5 is screwed to one end of a short housing 1 7 forming part of the assembly 2, the housing 1 7 being generally cylindrical and having a stepped inner face providing a portion of smaller internal diameter 18 and a portion of larger internal diameter 19.A cylindrical liner 20 extends along the smaller diameter portion 18, terminating at one end by abutment against a preload setting ring 21 (see Fig. 2(a)) keyed or otherwise fixed against rotation on the external surface of the housing 1 7 and having an aperture which receives and anchors one end of a torsion spring 22 extending around the housing 17. The opposite end of the spring 22 is anchored on a cylindrical roller support 23 which carries four rollers 24. The support 23 is biassed by the spring 22 in such a manner that the rollers 24 are held in engagement with four of the cut-outs 12 of the shaft 6, thereby preventing relative axial movement of the shaft assemblies 1 and 2 until the force of the spring 22 is overcome.

The preload setting ring 21 can be adjusted by means of selectors 25 to alter the force exerted by the spring 22 on the roller support 23.

The housing 17 is secured through a screw connection 26 to a shaft 27 having on its inner face a shoulder 28 against which the spring 9 bears. The spring 10 bears against an abutment 29. At its other end the shaft 27 is screwed at 30 to a further cylindrical housing 31 which is recessed at four locations around its outer wall to receive slidable fins 32 sealed against the wall of the housing 31 by O-rings 33. Passing through the housing wall and screwed into each fin 32 is a cam follower 34 whose free-end bears against the cam surface of the corresponding projections 5.

A pair of posts 35 are secured within the wall of the housings 31 and extend into corresponding recesses 36 in the fins 32. The posts 35 carry coilars 37 against which bear compression springs 38 biassing the fins 32 inwardly of the housing 31 against the projections 5.

The housing 31 and the shaft 4 are held against relative rotational movement by splines 42 around their mating faces, while relative axial movement remains possibie.

A sleeve 39 is screwed to the housing 31 and a seal is formed between it and the shaft 4 by Orings 40. Low-friction bearing pieces 41 are also provided between the sleeve 39 and shaft 4.

In use, the stabiliser of this embodiment of the invention is screwed into a drill string by means of the bore 4 and screw-thread 1 6 at a distance from the drill bit, and rotational drive is transferred to the drill bit through the stabiliser from the shaft assembly 1 to the shaft assembly 2 through the splines 42. During drilling an axial force is also applied to the drill bit through the stabiliser, and this axial force can be increased or decreased as desired.

An increase or decrease in the axial force to which the stabiliser is subjected tends to cause the shaft assembly 1 to slide within the assembly 2 reducing or increasing telescopically the overall length of the stabiliser, and this movement causes the cam followers 34 to move along the cam surfaces of the projections 5, resulting in lateral extension or retraction of the fins 32. In this way an alteration in the applied axial force to the stabiliser produces an alteration in the effective diameter of the stabiliser body across the surfaces of the fins. As the fins 32 extend further outwards they engage more readily the side walls of the borehole in which the drill string is located, lessening the degree of curvature of the string.

Retraction of the fins 32 reduces the overall diameter of the stabiliser and allows greater curvature of the string in the borehole. The direction of deviation of the drill bit can thus be controlled by applying a greater or lesser force to the string at the surface, the effect of this force being to control the diameter of the stabiliser.

Thus if the drill bit is found to be acting below the desired line of drilling the axial force applied to the string from the surface is increased, causing the shaft assembly 1 to move to the right relative to the assembly 2, with the result that the cam follower 34 engages a portion of the cam surfaces of lesser height, thus causing the fins 32 to retract. The curvature of the drill string therefore increases, forcing the drill bit upwards, as the stabiliser's effective diameter decreases.

Similarly the curvature of the string can be reduced, with consequent dropping of the drill bit, by reducing the axial force on the stabiliser, with resulting increase in the projection of the fins 32 from the wail of the stabiliser.

Stepwise transfer of alterations in the applied force to the stabiliser diameter is achieved by means of the roller arrangement between the housing 1 7 and shaft 6. As the applied force increases but is insufficient to overcome the force exerted by the torsion spring 22, axial movement of the shaft assembly 1 relative to the assembly 2 is prevented by the rollers 24 engaging in the cutouts 12. Incerase of the applied force to a level sufficient to overcome the effect of the torsion spring 22 causes the rollers 24 to leave the cutouts 12 and the shaft assembly 1 to move within the assembly 2 until the rollers 24 engage in the next adjacent cut-outs 12, again locking the shaft assemblies against further movement.

Incremental movement of the shaft assemblies 1, 2 is thus provided in response to predetermined levels of increase or decrease in the axial force applied to the stabiliser. As a result, the fins 32 extend and retract in stepwise fashion also.

Modifications and improvements may also be made without departing from the scope of the invention.

Claims (11)

Claims

1. A stabiliser for use in directional drilling, comprising a body having a side wall at least a portion of which is adjustable in its dimension normal to the intended drilling direction in response to the magnitude of a force applied to it in said intended drilling direction.

2. A stabiliser according to Claim 1, wherein the side wall of the body is apertured to receive an abutment member which is laterally of the body movable into and out of the aperture.

3. A stabiliser according to Claim 2, wherein the abutment member is movable out of the aperture by engagement with a cam within the body, the cam being movable relative to the abutment member in response to the magnitude of a force applied to the body in the intended drilling direction.

4. A stabiliser according to Claim 3, wherein the body has a first part axially movable within a second part, one of said parts having the cam mounted on its and the second part retaining the abutment member against movement relative to it in the intended drilling direction.

5. A stabiliser according to Claim 4, wherein the first part of the body extends from one end thereof and has means for connection to a first part of a drill string, and the second part extends from the opposite end of the body and has means for connection to a second part of a drill string.

6. A stabiliser according to Claim 4 or 5, wherein the cam is in the form of an elongate projection increasing in depth along its length and extending laterally of the body on the first part thereof.

7. A stabiliser according to any one of Claims 2 to 6, wherein a pluraiity of abutment members are provided equispaced around the body.

8. A stabiliser according to Claim 4, 5 or 6, wherein the first and second parts of the body are biassed apart by a spring, towards a position in which the abutment member is fully retracted into its aperture in the side wall of the body.

9. A stabiliser according to any one of Claims 4 to 8, including means for providing stepwise movement of the abutment member into and out of its aperture in response to gradual alteration in the magnitude of a force applied to the body in the intended drilling direction.

10. A stabiliser according to Claim 9, wherein the means for providing stepwise movement comprises a stop member on one of said parts of the body spring-biassed into engagement with a recess in the other of said parts of the body, an increase or decrease in the magnitude of the force applied to the body in the drilling direction having the effect of overcoming the spring bias and causing the stop member to leave the recess.

11. A stabiliser according to Claim 10, wherein a plurality of recesses are provided in sequence, the stop member moving from one to another when the spring bias is overcome.

1 2. A stabiliser substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

1 3. A drill string having one or more stabilisers according to any one of the preceding Claims.