[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] This is especially useful in applications at great depth where such pressure fluctuations
are likely because of the length of hydraulic conduit required.
[0016] 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.
[0017] 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:
[0018] 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.
[0019] Alternatively, the tool may be rotated by rotating the drill string at the surface.
[0020] 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.
[0021] Other modifications and improvements may be incorporated without departing from the
scope of the invention.
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.