EP0298663A2

EP0298663A2 - Downhole cutting tool

Info

Publication number: EP0298663A2
Application number: EP88305962A
Authority: EP
Inventors: Thomas Doig
Original assignee: Tri State Oil Tool UK Ltd; Tri State Oil Tools Inc
Current assignee: Tri State Oil Tool UK Ltd
Priority date: 1987-07-08
Filing date: 1988-06-29
Publication date: 1989-01-11
Also published as: NO883016L; NO883016D0; GB8716040D0; EP0298663A3

Abstract

The tool, for example an underreamer for descaling a casing (46), has an upper body (10) telescoped within a lower body (12) on which are pivoted operating arms (22). The arms (22) are deployed from a retracted to an extended position by the hydraulic pressure of drilling mud pumped through a through-bore (18) against a shoulder (26) causing the upper body (10) to move within the lower body (12), which movement pivots the arm (22) via camming surfaces (28,29).

In the fully extended position, an outer surface (44) of the arm (22) acts as a locating pad, and the arm is mechanically locked by mating surfaces (34,36).

Description

This invention relates to a downhole cutting tool particularly, but not exclusively, in the form of an underreamer for use in descaling pipes downhole in an oil well.
Many areas of oil production have the problem of pipe bores becoming clogged up with a scale build up which subsequently reduces production. Difficulties arise because, in order to remove the scale in situ, the tool must first pass through a restricted bore then open out into a larger bore.
Previous underreamers have been capable of passing through a restriction and opening out to a larger hole size but the arms on such equipment are not suitable for cutting inside the steel pipe forming the wellbore casing.
One aspect of the present invention provides a downhole cutting tool comprising a body, at least one operating arm pivotally mounted on the body about an axis perpendicular to the longitudinal axis of the body for pivotal movement between a retracted position substantially within the body and an extended position projecting from the body, arm extension means for selectively driving the arm from the retracted to the extended position, and mechanical locking means arranged to lock the arm in the extended position during operation with weight on bit.
According to another aspect of the present invention there is provided an underreamer for use in descaling pipes comprising a body, an operating arm pivotally mounted on said body about an axis perpendicular to the longitudinal axis of the body for pivotal extension from the body, the arm having a leading edge having cutting means adapted for descaling a surface on rotation of the body and having a stabiliser arranged at an angle to the leading edge such that the stabiliser engages the surface at a predetermined extension of the arm to prevent the cutting means cutting the surface.
An embodiment of the invention will now be described, by way of example, with reference to the drawings, in which:-

Fig. 1 is a side view, partially in cross-section, of an underreamer embodying the invention;
Fig. 2 is a detail plan view of one arm of the underreamer of Fig. 1;
Fig. 3 is a side view corresponding to Fig. 2; and
Fig. 4 is an end view of the arm taken on arrow A of Fig. 3.

Referring to Fig. 1, the underreamer comprises an upper body 10 which is telescopically received within a lower body 12. The upper body 10 and lower body 12 are provided with pin and box connectors 14 and 16 respectively for connection in a drill string. A through bore 18 is provided for passage of drilling mud. A coil spring 20 trapped between facing shoulders on the upper body 10 and lower body 12 biases these parts outwardly.
The underreamer is provided with three cutting arms, one of which is seen at 22 in Fig. 1 (the others being identical and spaced at 120° intervals). The arm 22 is mounted to the lower body 12 by pivot 24 for movement between a retracted position shown chain-dotted and an extended position shown full-line.
In use, the underreamer is included in the drill string and pased downhole with the arms 22 retracted. Once it has passed the limiting restriction and is in the area to be descaled, the mud pump is started to pressurise mud through the drill string. The bore 18 is formed with a restriction 26 on which a force is exerted by the pressurised mud to urge the upper body 10 inwardly of the lower body 12 against the bias of spring 20. During this movement, an external flange 28 on the upper body 10 cams against a curved inner face 29 on the arm 22, driving the arm 22 into the extended position. The inward relative movement of the upper body 10 is limited by contact of a resilient buffer 30 carried thereby with a shoulder 32 on the lower body 12. At this point, the arm 22 is mechanically locked in the extended position by mating of faces 34 and 36 on the flange 28 and arm 22 respectively. The underreamer is then maintained in this condition by either or both of mud pump pressure and weight on bit during descaling.
Removal of the underreamer past an obstruction simply requires the mud pump to be stopped and the drill string pulled, the tool then reverting to its original condition. Additionally, to ensure retraction of the arm 22 if some resistance is present (e.g. caused by fouling of the pivot), a notch 38 (Fig. 3) formed on the base of the arm 22 is engaged by a stud (not seen in the drawings) on the upper body as it returns to its rest position to provide a positive retraction force.
Turning to Figs. 2 to 4, the arm 22 is shown retracted at B and extended at C. The arm 22 has a first cutting surface 40 which forms a leading surface in the circumferential direction, and a second cutting surface 42 which forms a leading surface in the axial direction. A further surface 44 forms a stabilising pad.
When used for descaling, the operation is as follows. The underreamer is brought to the desired location, the mud pump is brought into operation to begin extension of the arms 22 as described, and the tool is rotated. The cutting surfaces 40 engage scale lining the wellbore casing 46 and cut into the scale as the arms extend. In the fully extended position, the stabilising pads 44 engage the interior surface of the casing 46 as shown at C in Fig. 3. This prevents the arms 22 from opening further and thus the cutting surfaces 40 are prevented from cutting into the casing 46.
The engagement of the arms 22 with the casing 46 via the stabilising pads 44 also centralises the underreamer in the bore, and subsequent movement of the underreamer down the bore causes the second cutting surfaces 42 on the leading faces of the arms 22 to mill away the scale lining the casing. The descaling operation can thus be performed without damaging the pipe forming the casing.
The provision of a mechanical locking mechanism for the arms 22 to retain them in the extended position avoids the problem whereby pressure fluctuations in the hydraulic actuating circuits could cause the underreamer tool to lose gauge.
This is especially useful in applications at great depth where such pressure fluctuations are likely because of the length of hydraulic conduit required.
The underreamer illustrated has a 2 7/8" (73 mm) outer diameter body in which the arms open to 5 1/2" (140 mm). However, the arms can be adjusted to open further and still collapse flush with the body by adjusting the angle of the leading face of the arm. The stabilising pad 44 is parallel with the bore in which the arm 22 is cutting when the arm is full open. The second cutting surface 42 has a slight negative angle of 3° which assists the arm to open out.
The tool may be rotated by a downhole motor. In this case, a procedure for determining when the arms are actually locked out has been developed since, unlike conventional underreaming, the pumps cannot be switched off and testing carried out for drilling weight as the motor would stop rotating. The system is as follows:
As the parameters of the motor and the tubing are known, before entering the tool into the cement it is cycled at 40 gallons per minute (180 l/min) in the lubricator and the pressure in the control unit is noted. This gives a free rotating pressure. Taking this pressure as a starting point, the tool is entered into the cement and pumping is begun. As torque on the motor can be monitored in relation to the pressure, as the pressure rises from the initial free rotating pressure, this can be equated to torque being applied to the motor by the underreamer arms as they open out. Once the pressure has equalised and, in some cases, actually begins to fall back off, this provides an indication that the arms are fully out.
Alternatively, the tool may be rotated by rotating the drill string at the surface.
Although described above with specific reference to descaling, the invention may also be used in other applications where a downhole cutting tool must be passed beyond a bore restriction and then deployed to an extended position, for example a tool for cutting casing.
Other modifications and improvements may be incorporated without departing from the scope of the invention.

Claims

1. A downhole cutting tool comprising a body, at least one operating arm pivotally mounted on the body about an axis perpendicular to the longitudinal axis of the body for pivotal movement between a retracted position substantially within the body and an extended position projecting from the body, arm extension means for selectively driving the arm from the retracted to the extended position, and mechanical locking means arranged to lock the arm in the extended position during operation with weight on bit.

2. A tool according to Claim 1, in which the arm extension means comprises hydraulic means actuated by pressurised mud flow.

3. A tool according to Claim 2, in which the body comprises a first body part mounting the operating arm, a second body part telescoped within the first body part, resilient means biasing the second body part to a rest position with respect to the first body part, a mud conduit passing through the second body part, a restriction in said mud conduit such that pressurised flow of mud through said conduit acts on the restriction to produce a force moving the second body part relative to the first body part against the bias of the resilient means, and cam means formed between the second body part and the arm by means of which said relative movement causes pivotal movement of the arm towards the extended position.

4. A tool according to Claim 3, in which the cam means comprises a radial projection on the second body part engaging a curved face on the arm.

5. A tool according to Claim 4, in which said locking means comprises an end face on said radial projection which, in the extended position of the arm, is seated against a mating face formed on the arm at the end of said curved face.

6. A tool according to any preceding claim, in which there is a plurality of operating arms spaced around the body, and each arm includes a face at an angle to the axis of the arm such that, when the arm is in its extended position within a pipe of predetermined bore, the face engages the pipe wall to act as a stabilising pad.

7. The tool of Claim 6, in which each arm further includes a first cutting face and a second cutting face, the first cutting face being disposed to cut into scale during pivotal extension of the arm, and the second cutting face being disposed to mill away scale during axial movement of the tool.

8. An underreamer for use in descaling pipes comprising a body, an operating arm pivotally mounted on said body about an axis perpendicular to the longitudinal axis of the body for pivotal extension from the body, the arm having a leading edge having cutting means adapted for descaling a surface on rotation of the body and having a stabiliser arranged at an angle to the leading edge such that the stabiliser engages the surface at a predetermined extension of the arm to prevent the cutting means cutting the surface.