REAMER

Abstract

A reaming tool for enlarging an underground borehole comprises a plurality of cutter assemblies distributed azimuthally around a longitudinal axis of the tool, wherein each cutter assembly includes a supporting structure bearing a sequence of cutters which have hard surfaces exposed and facing in a direction of rotation of the tool. The outer surface of the support structure on each cutter assembly includes zones which each face towards an end of the assembly and is configured such that as it extends circumferentially relative to the tool axis in the direction opposite to rotation of the tool, it also extends away from an end of the assembly.

Claims

1. A reaming tool for enlarging an underground borehole, comprising: a plurality of cutter assemblies distributed azimuthally around a longitudinal axis of the tool, wherein each cutter assembly includes a supporting structure bearing a sequence of cutters which have hard cutting surfaces facing in a direction of rotation of the tool, and the outer surface of the supporting structure of each cutter assembly comprises at least one zone which faces towards an end of the tool such that notional lines perpendicular to the zone surface are at no more than 45° to the tool axis; wherein the zone is configured such that as it extends circumferentially relative to the tool axis in the direction opposite to rotation of the tool, it also extends away from an end of the assembly.

2. The reaming tool according of claim 1 wherein the zone is located between two adjacent cutters at different axial distances from an end of the assembly and different radial distances from the tool axis and extends circumferentially back from the hard surfaces of the cutters.

3. The reaming tool according of claim 1 wherein the zone is configured such that as it extends circumferentially relative to the tool axis in the direction opposite to rotation of the tool, it extends away from an end of the assembly with an angle between the zone and the tool axis which is not more than 95°.

4. The reaming tool of claim 1 wherein each cutter assembly has at least three said zones.

5. The reaming tool of claim 1 wherein a configuration of supporting structure bearing a plurality of cutters and comprising at least one zone is present on each of the cutter assemblies but is positioned at differing axial distances from an end of the tool such that on assemblies which follow one another in succession during rotation of the tool, corresponding zones in the sequences are at progressively increasing distances from the end of the tool.

6. The reaming tool of claim 5 wherein corresponding zones on the cutter assemblies lie on a helix around the axis of the tool.

7. The reaming tool according of claim 5 wherein the configuration of supporting structure comprises a plurality of said zones and is positioned at differing radial distances from an end of the tool such that on assemblies which follow one another in succession during rotation of the tool, corresponding zones in the sequences are at progressively increasing distances from the end of the tool and progressively increasing distances from the tool axis.

8. The reaming tool of claim 7 wherein the zones on the cutter assemblies lie on a helix of increasing diameter around the axis of the tool.

9. The reaming tool of claim 8 with a spacing of between 3 mm and 10 mm between adjacent turns of the helix.

10. A reaming tool for enlarging an underground borehole, comprising: at least three cutter assemblies distributed azimuthally around a longitudinal axis of the tool, wherein each cutter assembly includes a supporting structure bearing a sequence of cutters which are spaced axially along the cutter assembly and which have hard cutting surfaces at least partially exposed as leading surfaces facing in a direction of rotation of the tool, the outer surface of the support structure of each cutter assembly comprises a plurality of zones which each face towards an end of the tool such that notional lines perpendicular to the zone surface are at no more than 45° to the tool axis, each said zone is configured such that as it extends circumferentially relative to the tool axis in the direction opposite to rotation of the tool, it also extends away from an end of the assembly, and a configuration of supporting structure bearing a plurality of said cutters and including a plurality of said zones is present on a first of the cutter assemblies and is repeated on the following assembly at greater distance from the end of the assembly and greater radial distance from the tool axis and repeated again on the third assembly at even greater distance from the end of the assembly and even greater radial distance from the tool axis.

11. The reaming tool of claim 10 wherein the zones on the cutter assemblies lie on a helix of increasing diameter around the axis of the tool.

12. The reaming tool of claim 8 with a spacing of between 3 mm and 10 mm between adjacent turns of the helix.

13. The reaming tool of claim 1 wherein the outer face of the support structure of an assembly includes surfaces at the same radial distance from the tool axis as extremities of cutters in the sequence, where the sequential cutters are at different radial distances from the tool axis.

14. The reaming tool of claim 1 wherein the cutter assemblies are expandable radially from the tool axis.

15. A method of enlarging a borehole by rotating a reaming tool as defined in claim 1 in the borehole and advancing the tool axially.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a schematic, cross-sectional view of a drilling assembly in a borehole;

[0020] FIG. 2 is a cross-sectional elevation view of one embodiment of expandable reamer, showing its expandable cutter blocks in collapsed position;

[0021] FIG. 3 is a cross-sectional elevation view of the expandable reamer of FIG. 2, showing the cutter blocks in expanded position;

[0022] FIG. 4 is a perspective view of a cutter block for the expandable reamer of FIGS. 2 and 3;

[0023] FIG. 5 is a schematic, cross-sectional view of the reamer expanded in a pre-existing borehole;

[0024] FIG. 6 is a detail view of a PDC cutter;

[0025] FIG. 7 is a cross section on line A-A of FIG. 4;

[0026] FIG. 8 is an isometric drawing of the lower cutting portion of the outer part of a cutter block, with the axial direction of the tool extending horizontally;

[0027] FIG. 9 is a side view of the lower cutting portion shown in FIG. 8, also with the axial direction of the tool extending horizontally;

[0028] FIG. 10 is a cross section on the line K-K of FIGS. 8 and 9;

[0029] FIG. 11 is a diagrammatic enlarged view showing one cutter of FIG. 9;

[0030] FIG. 12 is an enlarged radial view onto the end portion of a cutter block in the direction of arrow R in FIG. 9;

[0031] FIG. 13 is a radial view onto the lower cutting portions of three cutter blocks;

[0032] FIG. 14 is a radial view onto the lower cutting portion of a cutter block with the axial direction of the tool extending vertically;

[0033] FIG. 15 diagrammatically illustrates positioning on a helix;

[0034] FIG. 16 shows the outer parts of three cutter blocks in three-quarter view; and

[0035] FIG. 17 is a section on line K-K of any of the three cutter blocks of FIG. 16.

DETAILED DESCRIPTION

[0036] FIG. 1 shows an exemplary drilling assembly which includes an expandable under-reamer 122. A drill string 112 extends from a drilling rig 110 into a borehole. An upper part of the borehole has already been lined with casing and cemented as indicated at 114. The drill string 112 is connected to a bottomhole assembly 118 which includes a drill bit 120 and an under-reamer 122 which has been expanded beneath the cased section 114. As the drill string 112 and bottomhole assembly 118 are rotated, the drill bit 120 extends a pilot hole 124 downwards while the reamer 122 simultaneously opens the pilot hole 124 to a larger diameter borehole 126.

[0037] The drilling rig is provided with a system 128 for pumping drilling fluid from a supply 130 down the drill string 112 to the reamer 122 and the drill bit 120. Some of this drilling fluid flows through passages in the reamer 122 and flows back up the annulus around the drill string 112 to the surface. The rest of the drilling fluid flows out through passages in the drill bit 120 and also flows back up the annulus around the drill string 112 to the surface. The distance between the reamer 122 and the drill bit 120 at the foot of the bottom hole assembly is fixed so that the pilot hole 124 and the enlarged borehole 126 are extended downwardly simultaneously.

[0038] As shown in FIG. 5, it would similarly be possible to use the same reamer 122 attached to drill string 112, although without the drill bit 120 and the part of the bottom hole assembly 118 shown below the reamer 122 in FIG. 1, to enlarge a borehole 125 which had been drilled previously. In FIG. 5, the initial expansion of the reamer has created a fairly short section where the borehole has enlarged diameter. This enlarged portion of the borehole can then be elongated downwardly by advancing the drill string 112 and reamer 122 downwardly.

[0039] Referring now to FIGS. 2 and 3, one embodiment of expandable reaming tool is shown in a collapsed position in FIG. 2 and in an expanded position in FIG. 3. The expandable tool comprises a generally cylindrical tool body 510 with a central flowbore 508 for drilling fluid. The tool body 510 includes upper 514 and lower 512 connection portions for connecting the tool into a drilling assembly. Intermediately between these connection portions 512, 514 there are three recesses 516 formed in the body 510 and spaced apart at 120° intervals azimuthally around the axis of the tool.

[0040] Each recess 516 accommodates a cutter support element 140 in its collapsed position. This support element has the general form of a block to which cutters are attached. One such cutting block 140 is shown in perspective in FIG. 4. The block 140 has an outer face 144 which confronts the wall of the borehole and side faces with protruding ribs 142 which extend at an angle to the tool axis. These ribs 142 engage in channels 518 at the sides of a recess 516 and thus provide a guide mechanism such that when the block 140 is pushed upwardly relative to the tool body 510, it also moves radially outwardly to the position shown in FIG. 3 in which the blocks 140 extend radially outwardly from the tool body 510. The blocks move in unison and so are all at the same axial positions relative to the tool body. Details of the outer face 144 of a block 140 have been omitted from FIGS. 2 and 3.

[0041] A spring 540 biases the block 140 downwards to the collapsed position of FIG. 2. The biasing spring 540 is disposed within a spring cavity 545 and covered by a spring retainer 550 which is locked in position by an upper cap 555. A stop ring 544 is provided at the lower end of spring 540 to keep the spring in position.

[0042] Below the moveable blocks 140, a drive ring 570 is provided that includes one or more nozzles 575. An actuating piston 530 that forms a piston cavity 535 is attached to the drive ring 570. The piston 530 is able to move axially within the tool. An inner mandrel 560 is the innermost component within the tool 500, and it slidingly engages a lower retainer 590 at 592. The lower retainer 590 includes ports 595 that allow drilling fluid to flow from the flowbore 508 into the piston chamber 535 to actuate the piston 530.

[0043] The piston 530 sealingly engages the inner mandrel 560 at 566, and sealingly engages the body 510 at 534. A lower cap 580 provides a stop for the downward axial movement of piston 530. This cap 580 is threadedly connected to the body 510 and to the lower retainer 590 at 582, 584, respectively. Sealing engagement is provided at 586 between the lower cap 580 and the body 510.

[0044] A threaded connection is provided at 556 between the upper cap 555 and the inner mandrel 560 and at 558 between the upper cap 555 and body 510. The upper cap 555 sealingly engages the body 510 at 505, and sealingly engages the inner mandrel 560 at 562 and 564.

[0045] In operation, drilling fluid flows along path 605, through ports 595 in the lower retainer 590 and along path 610 into the piston chamber 535. The differential pressure between the fluid in the flowbore 508 and the fluid in the borehole annulus surrounding tool 500 causes the piston 530 to move axially upwardly from the position shown in FIG. 2 to the position shown in FIG. 3. A small amount of flow can pass through the piston chamber 535 and through nozzles 575 to the annulus as the tool 500 starts to expand. As the piston 530 moves axially upwardly, it urges the drive ring 570 axially upwardly against the blocks 140. The drive ring pushes on all the blocks 140 simultaneously and moves them all axially upwardly in recesses 516 and also radially outwardly as the ribs 142 slide in the channels 518. The blocks 140 are thus driven upwardly and outwardly in unison towards the expanded position shown in FIG. 3.

[0046] The movement of the blocks 140 is eventually limited by contact with the spring retainer 550. When the spring 540 is fully compressed against the retainer 550, it acts as a stop and the blocks can travel no further. There is provision for adjustment of the maximum travel of the blocks 140. The spring retainer 550 connects to the body 510 via a screwthread at 551. A wrench slot 554 is provided between the upper cap 555 and the spring retainer 550, which provides room for a wrench to be inserted to adjust the position of the screwthreaded spring retainer 550 in the body 510. This allows the maximum expanded diameter of the reamer to be set at the surface. The upper cap 555 is also a screwthreaded component and it is used to lock the spring retainer 550 once it has been positioned.

[0047] FIG. 4 is a perspective view of a cutter block 140 showing the outer face of the block and the side face which is the trailing face in the direction of rotation. There is a conventional arrangement of cutters on the outer face. The block is formed of an inner part 145 and an outer part 146 bolted to the part 145 by bolts (not shown). The inner part 145 is steel and incorporates the protruding ribs 142. The outer part 146 of the block 140 is also steel and has polycrystalline diamond (PDC) cutters secured to it.

[0048] As shown in FIG. 6 such cutters have a sintered disc 150 of diamond crystals embedded in a binder material. This disc is at one end of a cylindrical body 152 which may be a sintered mass of tungsten carbide particles and a binder material. The bodies 152 of cutters are secured, for example by brazing, to the outer part 146 of the block 140 so that the hard faces 154 of the cutters are exposed. Although the cutter shown in FIG. 6 has a hard surface 154 which is a flat face, other shapes including cones can be used for the hard surface.

[0049] The outer part 146 of the block 140 has upper and lower cutting portions 160, 162 on which PDC cutters are arranged in a leading row of cutters 164 and a following row of cutters 166. It will be appreciated that the upper and lower cutting portions 160, 162 are inclined (they are curved as shown) so that the cutters in these regions extend outwards from the tool axis by amounts which are least at the top and bottom ends of the block 140 and greatest adjacent the middle section 168 which includes stabilising pad 170.

[0050] When a reamer is advanced downwardly within a hole to enlarge the hole, it is the curved lower cutting portions 162 which do the work of cutting through formation rock. This takes place in FIGS. 1 and 5 as the drill string is advanced. The enlarged portion of the borehole can also be extended upwardly using the cutting portions 160 on the blocks 140 to remove formation rock while pulling upwardly on the drill string 112. The leading row of cutters has the cutters positioned side by side and spaced axially apart. The following row of cutters 166 also has the cutters spaced apart but the cutters in this following row are positioned circumferentially behind the spaces between adjacent cutters in the front row. If a portion of the rock to be cut passes between cutters of the leading row, it is cut by a cutter of the trailing row.

[0051] The stabilising pad 170 does not include cutters but has a generally smooth, part-cylindrical outward surface positioned to face and slide over the borehole wall. To increase resistance to wear, the stabilising pad 170 may have pieces 172 of harder material embedded in it and lying flush with the outward facing surface.

[0052] FIG. 7 is a section on line A-A of FIG. 4 showing one front row PDC cutter 164 mounted to the outer part 146 of the block 142. The cutter 164 is partially embedded in the outer part 146 and is oriented so that the hard face 154 will be facing forwards when the reamer is rotated. The direction of rotation is indicated by arrow 180. This hard face extends outwards to an extremity 156 which is at the maximum radius swept by the rotating reamer (i.e. its full gauge). The extremities of the other PDC cutters secured to the middle region 168 are also at the maximum radius swept by the rotating reamer. The outer surface of the support structure is indicated at 176.

[0053] The reamer as described above, referring to FIGS. 1 to 7, is of a conventional construction. FIGS. 8 onwards show parts of expandable reamers which utilise much of this conventional construction but have cutter arrangements and cutter blocks in accordance with the novel concepts disclosed here. Specifically, the reamers of FIGS. 8 to 17 utilise the expandable block construction shown in FIGS. 2 and 3 and have cutter blocks with inner and outer parts as in FIG. 4. However, the construction of the outer parts of the cutter blocks and the arrangement of the cutters on the blocks is different from that shown in FIG. 4 and is in accordance with novel aspects of the present disclosure.

[0054] As with the conventional construction, the outer part of each cutter block is a steel support structure for PDC cutters. FIGS. 8 to 10 show the lower cutting portion of the outer part of a cutter block. In these figures the tool axis is shown as horizontal. The block has a side face 200 which is the leading face in the direction of rotation and it has a lower axial end face 202. For part of its length indicated 203, the side of the block has an area 204 which is slanted back as shown by FIG. 10. The trailing face of the block is indicated 207 in FIG. 10.

[0055] A row of PDC cutters 211-216 is positioned with the hard surfaces of the cutters exposed within the slanted area 204 of the leading face of the block. The cutters are fitted into sockets in the steel supporting structure and secured by brazing so that they are embedded in the supporting structure. The cutters 211-215 are positioned at progressively increasing radial distances from the tool axis. The next cutter 216 is at the same radial distance from the tool axis as cutter 215.

[0056] These cutters 211-216 arranged in a single sequence are the only cutters on the lower portion of the cutter block. In contrast with FIG. 4, there is no second row of cutters behind.

[0057] This length 203 of the block with the slanted area 204 and cutters 211-216 adjoins a length 205 which does not include cutters and provides a stabilising pad with a part-cylindrical outward facing surface 220 which includes a leading region 221 which extends forwardly (in the direction of rotation) of the cutter 216. The leading side surface 200 of the block extends outwards to meet the region 221 of surface 220 at an edge 222 with the consequence that there is a surface 224 facing axially at one end of the slanted area 204. As best seen in the cross-section which is FIG. 10, the edge 222 is a curved transition between the surfaces 200 and 220.

[0058] The outer surface 220 of the stabilising pad is at the full gauge of the reamer and so when the cutter blocks are fully expanded, the outer surface 220 is part of a cylinder which is centred on the tool axis and lies on the notional surface swept out by the rotating tool. The outer extremities of the cutters 215 and 216 are also at the full gauge of the reamer and also lie on this notional surface. This notional surface is akin to a surface of revolution, because it is the surface swept out by a rotating body, but of course the reamer may be advancing axially as it rotates.

[0059] The outer surface 220 extends axially over the cutter 216 and over half of cutter 215. Thus, as shown by the cross-section in FIG. 11, the cutter 216 (and also cutter 215) has its extremity 218 aligned with outwardly facing surface area which is behind the leading faces of these cutters 215, 216 and follows these leading faces as the reamer rotates. The block thus has a surface 220 which faces outwardly at full gauge and is larger than the surface area within the length 205 of the stabilising pad.

[0060] The shape of the block inhibits any pivoting around the extremities of cutters during rotation. If the extremity 218 snags on the borehole wall, any pivoting around the extremity 218 in the sense seen as clockwise and denoted by arrow 182 in FIG. 10 is limited by the leading region 221 of surface 220 abutting the borehole wall. Pivoting in the opposite sense is less likely but is limited by the trailing part of surface 220 abutting the borehole wall. The leading edge 222 is formed as a smooth curve so as to inhibit this leading edge from snagging on the borehole wall during rotation.

[0061] The cutters 211-214 are embedded in the outer part of the block in a manner similar to the cutters 215, 216. The outer face of the block includes part-cylindrical surfaces 231-234 which extend behind the leading faces of cutters 211-214 respectively and which are aligned radially with the extremities of the respective cutters. Each of the part-cylindrical surfaces 231 -234 has a radius which lies on the tool axis when the cutter blocks are fully expanded.

[0062] These surfaces 231-234 act as secondary gauge areas: the surface 231 slides over rock which has just been cut by the action of cutter 211, surface 232 slides over rock cut by cutter 232 and so on. Of course, the rock surfaces created by cutters 211-214 have only a transient existence. They are cut away by cutters at a greater radius as the reamer advances. Nevertheless, this provision of secondary gauge areas contributes to stabilisation of the position of the rotating reamer.

[0063] The outer face of the block includes portions connecting the part cylindrical surfaces 231-234. Referring to FIG. 11, from the surface 232 towards surface 231 the outer face of the block curves through an arc (indicated by angle 242) where it is aligned with the perimeter of cutter 232. It then curves in the opposite sense, as seen at 244, to join the part cylindrical surface 231. There is a similar arrangement between surfaces 234 and 233, between 233 and 232 and also between surface 231 and a part cylindrical surface 240 located between cutter 211 and the axial end of the block. This geometry allows small areas of the cylindrical surfaces of the cutters to remain visible as for example indicated at 246. The surface 220 is connected to surface 234 by a small tapered face 221.

[0064] These connecting portions of the outer face of the block have zones, such as between the chain lines 248, which face in a generally axial direction and so face towards formation rock which is to be cut away as the reamer advances axially. In order that contact between these zones and the rock does not prevent axial advance of the reamer, these zones are configured in accordance with an aspect of the present disclosure and is illustrated by FIGS. 12 and 13.

[0065] On each of the three cutter blocks of the reamer, the zones of the outer face which face axially forwards are not positioned exactly orthogonal to the reamer axis. Instead, they are shaped and oriented so that they extend away from the axial end 202 of the reamer as they extend back from the leading faces of the cutters. Secondly the three cutter blocks of the reamer are not identical. They have similar appearance but differ in dimensions. FIG. 12 is an enlarged view, looking radially inwards as indicated by arrow R in FIG. 9, onto the cutter block of FIGS. 8 to 11. Directions orthogonal to the axis of the reamer are shown by notional lines 249. The lines 250 aligned with edges of cutters 211-213 in FIG. 12 are the inflection where curvature through arc 242 changes to curvature through arc 244. The portions of outer surface which face generally axially are shaped to taper away from the end of the reamer. Thus the lines 250 are at an angle to the orthogonal direction indicated by the lines 249.

[0066] By way of illustration, a reamer with an expanded diameter of 150 mm may have angle of slightly less than 1 degree between the lines 250 and 249 and advance by 6 mm in each revolution. The axial spacing between the cutters may then be approximately equal to this distance of 6 mm. A reamer may have a diameter larger than 150 mm, for instance up to 600 mm or even more with the same designed rate of advance of 6 mm.

[0067] FIG. 13 shows the lower cutting portions of the three cutter blocks of the reamer. The ends 202 of the blocks are aligned axially as indicated by a chain-dotted line. The block shown in FIGS. 8 to 11 is block 251 at the bottom of the diagram. The lower cutting portions of the other two blocks are indicated at 252 and 253. These follow block 251 as the reamer is rotated and of course block 251 follows block 253. The axial positions of the cutters 211-216 relative to each other as described above with reference to FIGS. 8 to 10 for block 251, is reproduced on blocks 252 and 253. However, the axial distances to the end of the blocks differs from one block to another. Moreover, since the blocks are aligned and move in unison, the axial distances to the end of the tool, or any other reference point on the tool, likewise differ from one block to another. As indicated by the arrows 254, 255, 256 the axial distances from the end of each block to the edge of cutter 211, and likewise the distances to the other cutters, increase in the order: block 251, block 252, block 253. However, the distance indicated by arrow 256 to the edge of cutter 211 of block 253 is not as great as the distance 257 to the edge of cutter 212 of block 251.

[0068] The radial positions of the cutters 211-213 relative to each other is the same on all three cutter blocks, but the cutters 211-213 on block 252 are positioned radially slightly further from the axis of the tool than the corresponding cutters of block 251. Similarly the cutters 211-213 of block 253 are positioned slightly further from the axis of the tool than the corresponding cutters 211-213 of block 252. Thus the cutters 211-213 and the support structure around them has a configuration in which both axial and radial positions are the same, relative to each other, on all three cutter blocks, but this configuration of cutters and associated support structure is positioned slightly differently both axially relative to the ends of the blocks and radially relative to the tool axis. The cutters 214 are at progressively increasing radial distances from the tool axis on the blocks 251, 252 and 253, but for these cutters 214 the increase in distance is smaller than in the case of the cutters 211-213. The support structure around blocks 214-216 is similar in shape and appearance on all three cutter blocks but the cutters 215 and 216 are all at the same radial distance from the tool axis.

[0069] The tapering of the axially facing surface zones as described with reference to FIG. 12 and the differing positions of cutters and supporting structure on the three blocks, as described with reference to FIG. 13, are arranged so that, when the blocks are expanded, the axially facing surface zones lie on and are aligned with an imaginary helix of progressively increasing diameter around the tool axis.

[0070] This is illustrated by FIGS. 14 and 15. FIG. 14 shows the lower end portion of block 252 with the tool axis vertical and FIG. 15 shows the path of the imaginary helix as a solid line 265. This helix has progressively increasing radius as it winds upwards around axis 267. The block 252 is positioned so that when expanded the longitudinal edges of the block are on the lines 269 and the axially facing zones are on the helix 265 In FIG. 14 the chain lines 265 which coincide with the lines 250 are portions of the imaginary helix. The block 251 is also positioned so that their axially facing zones are aligned with and lie on the helix. Block 251 is positioned between lines 270. The position of block 253 is not shown because it is at the back of the helix as the latter is depicted in FIG. 15.

[0071] With this arrangement of the axially facing surface zones on a helix, the axially facing zones will not impede axial advance of the tool if it rotates, so long as the amount of advance in one revolution does not exceed the distance between turns of the helix, which may be in the range of 3 to 10 mm and in some embodiments is 6 mm. When the amount of advance in a revolution reaches the distance between turns of the helix, each axially facing zone will slide on, and be aligned with, a rock surface formed by the cutter at the leading edge of the axially facing zone.

[0072] The inventors have found that a rate of advance which is controlled in this way can be approximately the same as the rate of uncontrolled advance achieved with a conventional reamer construction.

[0073] FIG. 16 shows the whole of the outer parts of the three cutter blocks of another reamer. These use a number of features already shown by FIGS. 8-13 and the same reference numerals are used where appropriate. There are also some differences. As before the general structure of the reamer and the mechanism which expands it are as shown by FIGS. 2, 3 and 4. FIG. 17 shows a section, which could be on any of the lines K-K of FIG. 16.

[0074] The blocks 301, 302, 303 have cutters 211-215 at their lower cutting portions as in FIGS. 8 to 13. At the upper cutting portion, which is used to enlarge a borehole when pulling up on a drill string, there are a group of cutters 306 mounted conventionally, similarly to those in upper cutting portion 160 of FIG. 4.

[0075] A middle section between these two ends has an outer surface 320 which is a part-cylindrical surface at full gauge. Within this middle section, each block includes a length 305 without cutters which is a full gauge stabilising pad. As in FIG. 8, within the lengths 305 which are the stabilising pads, the outer surface 320 has a leading region 221 which extends to a leading edge 222 which is ahead, in the direction of rotation, of the leading surfaces of the cutters and is curved as shown in FIG. 15.

[0076] As disclosed in copending GB patent application GB2520998A, these lengths 305 which provide stabilising pads are at different axial positions on the blocks in order to provide stabilisation without preventing expansion of the reamer. As the reamer is expanded, each stabilising pad presses on the borehole wall. The pads cannot cut into the wall but the other two cutter blocks have cutters at the corresponding axial position and these do cut into the wall. This arrangement avoids placing three stabilising pads at the same axial position on the reamer, which does prevent expansion.

[0077] The remainder of each middle section of each block is provided with a row of cutters which are embedded so that their faces are exposed in a slanted area 304 and their radial extremities are aligned with the outer surface 320. However, these cutters are made with a truncated cylindrical shape and are secured to the support structure such that, as seen in FIG. 15, their extremities are an area 312 which is flush with surface 320. It will be appreciated that the cutters on each block form a single sequence of cutters distributed axially along the block with each cutter alongside another.

[0078] As can be seen from the drawing, the cutters in the lower cutting portions of blocks 302, 303 are positioned axially further from the end of the block than the corresponding cutters on block 301.

[0079] Near the trailing edge of surface 320, each block has a row of hard inserts 324 which are set flush with the surface 320 and are harder than the surface 320 of the steel outer part of the block, so as to resist wear. These hard inserts may be made of tungsten carbide particles sintered with a binder. There are also hard inserts 326 embedded to be flush with surfaces 231-234.

[0080] Modifications to the above embodiments are possible, and features shown in the drawings may be used separately or in any combination. The geometrical arrangements of supporting structure and cutters could also be used in a reamer which does not expand and instead has cutter blocks at a fixed distance from the reamer axis. Other mechanisms for expanding a reamer are known and may be used. Cutters may be embedded or partially embedded in supporting structure. They may be secured by brazing or in other ways. The hard faces of the cutters will of course need to be exposed so that they can cut rock, but the radially inner part of a cylindrical cutters' hard face may possibly be covered or hidden by a part of the support structure so that the hard face is only partially exposed.