GB2287731A

GB2287731A - Drillstring assembly with torque decoupler

Info

Publication number: GB2287731A
Application number: GB9405236A
Authority: GB
Inventors: Roger J Tailby
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor ASA
Priority date: 1994-03-17
Filing date: 1994-03-17
Publication date: 1995-09-27
Anticipated expiration: 2014-03-17
Also published as: GB2287731B; GB9405236D0

Abstract

A drillstring assembly for coiled tubing drilling comprising a coiled tubing drillstring (11), an inhole motor (13), a drill bit (14) and a torque decoupler (12) connected between the drillstring (11) and the motor (13). The decoupler (12) includes a lower section connected to the motor (13) and an upper section connected to the drillstring (11). The lower and upper sections have mutually engageable teeth and are relatively moveable between a locked position and a released position. In the lock position, relative rotation between the drillstring (11) and the motor (13) is prevented while in the released position, relative rotation is permitted. <IMAGE>

Description

DRILLSTRING ASSEMBLY AND TOROUE DECOUPLER THEREFOR The present invention relates to a drillstring assembly and torque decoupler particularly but not exclusively for use in drilling with continuous coiled tubing.

Coiled tubing (CT) drilling is a relatively new technology. Conventional underground drilling, for example, borehole drilling in oil exploration, employs a string of short straight pipes connected together for the drillstring. Similarly, conventional smaller scale drilling operations, such as milling, use straight drillstrings.

With CT drilling, coiled tubing is used as the drillstring in spite of problems caused by the phenomenon of buckling of a constrained curved tube. Coiled tubing has a lower stiffness and slenderness ratio than conventional straight drillstrings and these characteristics tend to exacerbate already inherent difficulties.

The purpose of the torque decoupler in the present invention is to de-couple the drillstring from the buttomhole assembly (motor drill bit etc), for example, when jamming occurs, and to allow accumulated tension to be released safely.

In order to be able to drill, for example, downwards, a downward force must be applied to the bit, usually referred to as the weight-on-bit. The presence of this weight-on-bit results in compression in the drillstring for a significant distance above the bottomhole assembly. This tendency is amplified when, as is often the case with CT drilling, the borehole is horizontal. The coiled tubing will then be under compression for the whole horizontal length and for a short way into the deviated section at least past the point where the deviation is 80 C.

While straight pipe in a cylindrical borehole will buckle when the compressive force exceeds a certain limit, coiled tubing, will always be buckled under compression because of the residual bend remaining from its storage time on the pipe drum at the surface.

Moreover, due to this residual bend, there will be torque in the coiled tubing cyclically distributed along its length, equal parts left-handed and right-handed, following the reversing spiral buckling pattern.

It is under these circumstances that drilling takes place. Drilling fluid under pressure, supplied through the CT bore, drives the mud motor in the bottomhole assembly and rotates the bit to the right. Since neither the coiled tubing nor the bottomhole assembly is fixed to the borehole wall, an equal and opposite (left-handed) torque will be applied to the upper part of the bottomhole assembly and thence to the coiled tubing. In its already buckled condition, the coiled tubing initially has very little resistance to torque and very soon the reversing spiral and torque distribution will be converted to a complete left-hand spiral and evenlydistributed left hand torque. However, this torque is not high enough to cause failure under these conditions.

A serious problem arises if the bit or bottomhole assembly should become stuck. This would normally be accompanied by a stalled motor, and the natural reaction then would be to shut down the mud pumps on surface and pick up off the bottom. The result in this case would be a straightening of the coiled tubing and conversion of the left-hand spiral to additional left-hand torque.

This increased torque combined with the tension needed to pull loose a bottomhole assembly which is stuck may be sufficient to cause the coiled tubing to yield and twist off. This would probably occur close to the bit in a normally deviated well, but may occur nearer the top of a horizontal wall section where compression is highest and buckling most severe. It is an object of the present invention to provide a torque decoupler which can avoid this difficulty. It will be appreciated that, unlike a conventional drillstring, coiled tubing cannot rotate at the surface to compensate for accummulated torque.

According to one aspect of the invention, there is provided a drillstring assembly comprising a drillstring, an in-hole motor, and a drill bit and a torque decoupler connected between the drillstring and the motor, the decoupler comprising a lower section connected to the motor and an upper section connected to the drillstring; the upper and lower sections having mutually engagable formations and being axially relatively moveable between a locked position in which the formations interengage and relative rotation between the drillstring and motor is prevented, and a released position in which the formations do not interengage and relative rotation between the drillstring and motor is permitted; the drillstring and torque decoupler defining a bore for conveying driving fluid to the motor.

This arrangement is particularly applicable to a CT drillstring. During drilling, there will be a significant spiral buckling and torque in the drillstring and if the bit should jam there would be an increased danger of drillstring fracture. Under these conditions fluid circulation is stopped and the motor shuts down.

If the drillstring is then withdrawn relatively slowly, the upper sections of the decoupler is separated from the lower section which is held by the jammed bit. The engaged formations disengage and the upper section of the decoupler is permitted to rotate relative to the lower section. In this way, the drill string can unwind, reducing the accummulated spiral buckling. This reduces the likelihood of drillstring twist-off.

Preferably, the lower section of the decoupler is directly connected to the motor and preferably, the upper section of the decoupler is directly connected to the drillstring. However, there may be respective intermediate components.

Preferably, the relative movement between the upper and lower sections of the decoupler is achievable by compression induced by applying weight-on-bit, but also by means of the pressure of driving fluid in the bore.

Thus, when the mud pumps are in operation and fluid in the bore is at a high pressure, there is preferably a tendency to move the decoupler sections to the locked position. Correspondingly, a reduction in the bore fluid pressure caused by shutting down the pumps will allow the decoupler sections to attain the released position.

According to another aspect of the invention, there is provided a torque decoupler for connection between the drillstring and an in-hole motor and drilling bit, the decoupler comprising a lower section arranged to be connected to the motor and an upper section arranged to be connected to the drillstring; the upper and lower sections having mutually engagable formations and being axially relatively moveable between a locked position in which the formations interengage and relative rotation between the two sections is prevented, and a released position in which the formations do not interengage and relative rotation between the two sections is permitted; the decoupler having a bore arranged to be aligned with a corresponding bore in the drillstring; the two sections of the decoupler being moveable to the locked position by the pressure of fluid in the bore.

Preferably, the upper section of which comprises an upper housing and, connected to it, a bearing member and preferably, the lower section of which comprises a lower connector and an insert mandrel. These components are preferably co-axial and the insert mandrel abuts a shoulder on the upper housing when the two sections of the decoupler are in the locked position and abuts the bearing section in the released position. In a preferred embodiment, the insert mandrel abuts a shoulder on the upper housing when the two sections of the decoupler are in the locked position and abuts the bearing section in the released position. The bearing section may include a ball race, which is abutted by the insert mandrel in the released position.

Preferably, the mutually engagable formations comprise respective intermeshing teeth on the bearing section and the lower connector. The teeth are preferably chamfered.

Preferably, a first upper fluid chamber is formed between the insert mandrel wall, the upper surface of a radially outwardly extending formation on the insert mandrel, the upper housing and a radially inwardly extending surface on the upper housing, the first chamber being in fluid communication with the outside of the upper housing. Preferably, a second fluid chamber is formed between the insert mandrel wall, the lower surface of a radially outwardly extending formation on the insert mandrel, the upper housing and the bearing member, the second chamber being in fluid communication with the bore. Preferably, the insert mandrel is moveable to cause the two sections of the decoupler to attain the locked position by the application of fluid pressure to the second fluid chamber through fluid in the bore.

The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic sketch of a coiled tubing drillstring and bottomhole assembly, incorporating a torque decoupler according to the invention; Figure 2 is a axial cross-section through the torque decoupler when coupled; and Figure 3 is a view similar to Figure 2 but showing the decoupler uncoupled.

As shown in Figure 1, the drillstring assembly includes a coiled tubing drillstring 11, a torque decoupler 12 a motor 13 and a drill bit 14. These are shown in the working position in a vertical bore 15 in rock strata 16. The spiral nature of the drillstring 11 is shown exaggerated in the vertical direction for reasons of clarity.

Referring now to Figure 2, the torque decoupler 12 includes an upper housing 21 and a lower connector 22.

The upper housing 21 has an internal screw thread 23 for connection to the drillstring 11 while the lower connector 22 has an external screw thread 24 for connection to the motor 13. At its lower end, the upper housing 21 is joined internally to a bearing section 25.

A piston mandrel 26 is slidably and rotatably located within the bearing section 25 and is connected at its lower end to the lower connector 22.

The upper housing 21 has an internal annulus 27 beneath the screw thread 23 and beneath the annulus 27, a region of increased wall thickness 28 terminating in a downwardly facing shoulder 29. The piston mandrel 26 has an outwardly extending flange 31 with an upper section 32 above this. An annular recess 33 is formed in the undersurface of the flange 31. The upper section 32 is slidably received within the thick wall portion 28 of the upper housing 21 while the flange 31 is located beneath the shoulder 29. The annulus 27, the piston mandrel 26 and the lower connector 22 together define a continuous axial bore 30 through the decoupler 12 to allow driving fluid passing along the drillstring 11 to reach the motor 13.

The lower connector 22 and the bearing section 25 having interlocking teeth 34,35 respectively which extend generally axially. At its upper end, the bearing section 25 has an upwardly extending radially inner collar 36.

A ball race 37 is located between the collar 36 and the inner surface of the upper housing 21.

A first small diameter 0being seal 40 is located in a groove 38 in the thick wall portion 28 and seals against the upper portion 32 of the piston mandrel 26.

A second small diameter 0-ring seal 39 is located in a groove 41 in the inner surface of the bearing section 25 and seals against the lower outer surface of the piston mandrel 26. A first large diameter 0-ring seal 42 is located in a groove 43 in the outside face of the flange 31 and seals against the inside surface of the upper housing 21. A second large diameter 0-ring seal 44 is located in a groove 45 in the bearing section 25 and also seals against the inside surface of the upper housing 21.

The dispositions of the shoulder 29, the flange 31 and the top of the bearing section 25 are such that two chambers are formed: an upper chamber 46 between the shoulder 29 and the flange 31 and a lower chamber 47 between the flange 31 and the bearing section 25. An external opening 48 in the upper housing 21 connects the upper chamber 46 to the wall bore 15. An internal opening 49 connects the lower chamber 47 to the internal bore 30. The chambers 46, 47 are separated by the first large diameter 0-ring seal 42.

Figure 2 shows the decoupler in its disposition for drilling. The weight on the bit closes the decoupler 12 and the teeth 34, 35 intermesh. The upper chamber 46 is closed up as the flange 31 abuts the shoulder 29 and the lower chamber 47 is opened. This interengagement of the lower connector 22 and the bearing section 25 is enhanced when the mud pumps (not shown) are started-up and drilling fluid is supplied to the motor 13 since the pressure in the bore 30, and hence the chamber 47, will increase. A condition for this is that the area swept by the 0-ring seal 42 is more than twice the area swept by the first and second small diameter 0-ring seals 39,40.

When drilling is commenced, the reactive left-hand torque from the drilling process is conveyed to the coiled tubing drillstring 11, and initially, a constant left-hand torque is established along the lower portion of the drillstring 11. The torqued drillstring 11 then acts as a fixed support for the motor 13 and subsequent torque generation by the motor is transmitted fully to the bit 14 for the drilling operation.

When the mud pumps are shut down and drilling ceases, the left hand torque will remain in the drillstring 11. The pressure in the lower chamber 47 is removed, allowing the teeth 34, 35 to be disengaged as the drillstring 11 is withdrawn as shown in Figure 3.

The excess left-hand torque stored in the drillstring 11 can then be relieved as the upper housing 21 and bearing section 25 rotate relative to the lower connector 22 motor 13 and drill bit 14. This relative rotation is assisted by the engagement of the lower rim of the flange 31 with the ball race 37, allowing rotation to proceed even if there is tension between the various components.

Thus, provided the drillstring is not withdrawn or picked up too rapidly, tendency to twist and break the drillstring 11 will be eliminated.

As an added precaution, a "fishing neck" profile may be formed in the piston mandrel 26 to aid retrieval if, for some reason, a breakage does occur. One or more disconnect tools (not shown) may be included in the bottom hole assembly.

Although the present invention has been described with reference to vertical borehole drilling it is also applicable to milling operations in casing or production tubing, for example for retrieving packers or removing scale deposits.

Claims

1. A drillstring assembly comprising a drillstring, an in-hole motor, and a drill bit and a torque decoupler connected between the drillstring and the motor, the decoupler comprising a lower section connected to the motor and an upper section connected to the drillstring; the upper and lower sections having mutually engagable formations and being axially relatively moveable between a locked position in which the formations interengage and relative rotation between the drillstring and motor is prevented, and a released position in which the formations do not interengage and relative rotation between the drillstring and motor is permitted; the drillstring and torque decoupler defining a bore for conveying driving fluid to the motor.

2. An assembly as claimed in Claim 1 in which the drillstring is a coiled tubing.

3. An assembly as claimed in Claim 1 or Claim 2 in which the lower section of the decoupler is directly connected to the motor.

4. An assembly as claimed in any preceding Claim, in which the upper section of the decoupler is directly connected to the drillstring.

5. An assembly as claimed in any preceding Claim, in which the relative movement between the upper and lower sections of the decoupler is achievable by means of the pressure of driving fluid in the bore.

6. A torque decoupler for connection between the drillstring and an in-hole motor and drilling bit, the decoupler comprising a lower section arranged to be connected to the motor and an upper section arranged to be connected to the drillstring; the upper and lower sections having mutually engagable formations and being axially relatively moveable between a locked position in which the formations interengage and relative rotation between the two sections is prevented, and a released position in which the formations do not interengage and relative rotation between the two sections is permitted; the decoupler having a bore arranged to be aligned with a corresponding bore in the drillstring; the two sections of the decoupler being moveable to the locked position by the pressure of fluid in the bore.

7. A decoupler as claimed in Claim 6, the upper section of which comprises an upper housing and, connected to it, a bearing member.

8. A decoupler as claimed in Claim 6 or Claim 7, the lower section of which comprises a lower connector and an insert mandrel.

9. A decoupler as claimed in Claim 8, in which the insert mandrel extends through the bearing member and into the upper housing.

10. A decoupler as claimed in Claim 9, in which the insert mandrel abuts a shoulder on the upper housing when the two sections of the decoupler are in the locked position and abuts the bearing section in the released position.

11. A decoupler as claimed in Claim 10, in which the bearing section includes a ball race, which is abutted by the insert mandrel in the released position.

12. A decoupler as claimed in any of Claims 6 to 11 in which the mutually engagable formations comprise respective intermeshing teeth on the bearing section and the lower connector.

13. A decoupler as claimed in Claim 12, in which the teeth are chamfered.

14. A decoupler as claimed in any of Claims 6 to 13 in which a first upper fluid chamber is formed between the insert mandrel wall, the upper surface of a radially outwardly extending formation on the insert mandrel, the upper housing and a radially inwardly extending surface on the upper housing, the first chamber being in fluid communication with the outside of the upper housing.

15. A decoupler as claimed in any of Claims 6 to 13, in which a second fluid chamber is formed between the insert mandrel wall, the lower surface of a radially outwardly extending formation on the insert mandrel, the upper housing and the bearing member, the second chamber being in fluid communication with the bore.

16. A decoupler as claimed in Claim 15 in which the insert mandrel is moveable to cause the two sections of the decoupler to attain the locked position by the application of fluid pressure to the second fluid chamber through fluid in the bore.

17. A decoupler constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 2 and 3 of the accompanying drawings.

18. A drillstring assembly as claimed in any of Claims 1 to 5, in which the decoupler is a decoupler as claimed in any of Claims 6 to 17.

19. A drillstring assembly constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 1, 2 and 3 of the accompanying drawings.