Downhole under-reamer and associated methods

1. A downhole under-reamer for use in reaming a downhole bore, the under-reamer comprising a body and a plurality of cutters mounted on the body, wherein the cutters are mounted on the body so as to be extendable from a retracted position to at least two extended positions, the at least two extended positions comprising a first extended position and a second extended position corresponding to reaming positions at respective first and second reaming diameters such that the under-reamer is capable of reaming a bore to a plurality of gauges; wherein the cutters are formed on cutter blocks located in windows of corresponding shape in the wall of the body.

2. The downhole under-reamer of claim 1 , wherein the under-reamer is configurable to ream at more than two reaming diameters, the cutters being extendable to at least a third extended position corresponding to a third reaming diameter.

3. The downhole under-reamer of claim 1 , wherein the under-reamer is configurable to ream multiple passages or sections at similar or different diameters, with the multiple passages or sections comprising one or more of: contiguous passages or sections; and discontiguous passages or sections.

4. The downhole under-reamer of claim 1 , wherein the cutters are moveable between the retracted and first extended positions and between the retracted and second extended positions and between the first extended and second extended positions by an axial movement of an activation member, the retracted position corresponding to a first axial position of the activation member, the first extended position corresponding to a second axial position of the activation member, and the second extended position corresponding to a third axial position of the activation member.

5. The downhole under-reamer of claim 4 , wherein at least one of:

the activation member is fluid-actuated by fluid in the throughbore;

the activation member comprises an activation piston;

the under-reamer is configured to selectively axially support the activation member at one or more of: the first axial position, the second axial position, and the third axial position.

6. The downhole under-reamer of claim 4 , wherein the under-reamer comprises a limiter to define the second axial position of the activation member, corresponding to the first extended position.

7. The downhole under-reamer of claim 6 , wherein at least one of:

the limiter may limit the travel of the activation member in a single direction such that the limiter substantially prevents movement of the activation member in a first axial direction beyond the second axial position corresponding to the first extended position of the cutters;

the limiter prevents further movement of the activation beyond the second axial position corresponding to the first extended position, whilst allowing selective movement of the activation member from the second axial position back to the first axial position, such that the cutters are selectively returnable from the first extended position to the retracted position;

the limiter supports the activation member at the second axial position such that the cutters are supported at the first extended position by the activation member; and

the limiter supports the activation member at the second axial position up to a maximum force threshold, the maximum force threshold at the second axial position being greater than a force generated across the limiter with the apparatus in a first configuration.

8. The downhole under-reamer of claim 6 , wherein at least one of:

the limiter is fixed relative to the body in the first configuration to define the second axial position of the activation member;

the limiter remains fixed to the body up to the maximum force threshold, whereby the limiter is selectively released to allow movement of the activation member beyond the second axial position by reconfiguring the under-reamer from the first configuration to a second configuration when the maximum force threshold is exceeded, such as to allow movement of the activation member beyond the second axial position to the third axial position.

9. The downhole under-reamer claim 6 , wherein the limiter is movable relative to the body directly in response to fluid conditions.

10. The downhole under-reamer of claim 9 , wherein the limiter is operatively associated with an indexing mechanism, the indexing mechanism defining the limiter at the second axial position according to a first indexing position, and the indexing mechanism being indexable from the first indexing position corresponding to the first under-reamer configuration to a second under-reamer configuration, with the activation member permitted to travel beyond the second axial position to the third axial position, by indexing the indexing mechanism to a second indexing position.

11. The downhole under-reamer of claim 6 , wherein in response to a signal the under-reamer is reconfigurable from the first configuration to the second configuration, the signal comprising a remote signal from the surface.

12. The downhole under-reamer of claim 6 , wherein at least one of:

the axial location of the limiter is selected to provide a predetermined clearance between the limiter and the activation member at the first axial position, corresponding to the retracted position, to allow a corresponding stroke or axial length of travel of the activation member from the first axial position to the second axial position;

the axial location of the limiter and/or the cam member may be varied between downhole operations to provide different reaming diameters for different operations.

13. The downhole under-reamer of claim 6 , wherein the under-reamer comprises a control mechanism for at least one of: selectively effectively locking the activation member in the first axial position corresponding to the retracted position; and selectively effectively preventing locking in the first axial position thus allowing movement of the activation member between the first and second axial positions.

14. A downhole toolstring comprising the downhole under-reamer of claim 1 .

15. A method of under-reaming comprising:

running an under-reaming tool comprising a plurality of extendable cutters into a bore;

extending the cutters from a retracted position to a first extended position;

reaming a first section of bore at a first diameter corresponding to the first extended position of the cutters;

extending the cutters to a second extended position;

reaming a second section of bore at a second diameter corresponding to the second extended position of the cutters,

wherein the cutters are formed on cutter blocks located in windows of corresponding shape in the wall of the body.

16. The method of claim 15 , wherein at least one of:

the first section comprises a lined or cased section and the first extended position is selected to ream inside the liner or casing; and

wherein the first section comprises one or more of: a previously-reamed section and a cement sheath.

17. The method of claim 15 , wherein the method comprises at least one of: reaming at least two diameters with a single under-reamer according to claim 1 .

18. The method of claim 15 , wherein the method comprises reaming at least two diameters with the same cutters in a single run.

19. The method of claim 15 , wherein the first and second sections are contiguous, or wherein the first and second sections are discontiguous.

20. The method of claim 15 , wherein at least one of:

the method comprises reaming a single passage or section at successive or sequential diameters, such as at a first diameter during a first pass of the under-reamer and at a second diameter during a second pass of the under-reamer; and

the method comprises retracting the cutters in between successive downhole reaming operations; and re-extending the cutters for the successive downhole reaming operation.