GB2543847A

GB2543847A - Rotary Milling Tool

Info

Publication number: GB2543847A
Application number: GB1519329.5A
Authority: GB
Inventors: Bernard Johnson Ashley
Original assignee: Schlumberger Technology BV
Current assignee: Schlumberger Technology BV
Priority date: 2015-11-02
Filing date: 2015-11-02
Publication date: 2017-05-03
Anticipated expiration: 2035-11-02
Also published as: GB2543847B; GB201519329D0; WO2017079092A1

Abstract

A rotary milling tool for comminuting 12 in a borehole comprises a tool body 70 and a plurality of cutting assemblies projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool wherein at least one cutting assembly comprises a supporting arm 74 with a plurality of hardcutters 94 attached. A first cutter or group of cutters provide cutting surfaces at positions whose distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases. The tool may feature a second group of cutters arranged to form a v-shaped recess 88 with cutters on either side. This reduces the risk that part of a coupling 14 will become detached and impede the milling operation by blocking access of cutters to the tubing.

Description

ROTARY MILLING TOOL

BACKGROUND

[0001] There are occasions when it is necessary to remove a length of tubing which has been fixed in place in a borehole. This tubing may be borehole casing which is surrounded by cement. Sometimes such removal of a length of tubing is done in preparation for setting a cement plug when a well is being abandoned. Removing a length of tubing which has been fixed within a borehole is customarily done with a rotary milling tool, customarily referred to as a section mill or casing mill, which comminutes the tubing to swarf.

[0002] A difficulty can, however, arise from couplings on the exterior of lengths of tubing. Standard couplings have an internal thread which engages an external thread on end portions of drill pipe. It is possible that the milling tool will cut into the tubing within such a coupling before completely destroying the coupling. The result is that part of the coupling becomes detached and is then a ring which encircles the tubing but is no longer attached to it. Such a ring can slide axially along the tubing and can rotate relative to the tubing. Eventually such a ring is pushed against an obstacle, possibly the next coupling, which prevents further axial movement. It may then block the advance of the tool. As the rotary tool continues to turn, the ring itself is not cut because it is rotated with the tool. It also prevents the tool's cutters from contacting tubing which is fixed in place and so the tool turns but does not cut anything. This can lead to considerable waste of time.

SUMMARY

[0003] This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to be used as an aid in limiting the scope of the subject matter claimed.

[0004] Disclosed now is a tool and method for removing tubing within a borehole.

[0005] A first aspect of the present disclosure provides a rotary milling tool for comminuting tubing in a borehole comprising a tool body and a plurality of cutting assemblies projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool body wherein at least one cutting assembly comprises a supporting structure and a plurality of hard-surfaced cutters attached thereto, with the shape of the supporting structure and the positions of the cutters thereon being such that a first cutter or group of cutters provide cutting surfaces at positions whose distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases.

[0006] A cutter or cutters with cutting positions arranged in this way can mill a coupling or other item projecting from tubing progressively from its outside inwards towards the tubing. This avoids creating a separate ring by destroying attachment to tubing before destroying the ring which was attached.

[0007] A second aspect of this disclosure provides a method of removing tubing in a borehole comprising comminuting the tubing with a rotary tool which comprises a tool body and a plurality of cutter assemblies projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool body; wherein at least one cutting assembly comprises a supporting structure and plurality of hard-surfaced cutters attached thereto with a first cutter or group of cutters positioned to cut into material projecting outwardly beyond the exterior of the tubing at cutting positions arranged so that distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases, whereby removal of projecting material progresses inwardly towards the tubing as the tool advances.

[0008] The statements above may refer to cutters and/or structure of a cutting assembly in "as new" configuration with which it was manufactured, before use. Some milling tools change shape as they wear, but the change of shape and the timing have been unpredictable. A cutting assembly as set out above may have the stated features when it is inserted into the borehole and so will reliably have these features when milling through couplings begins. The cutters which are used may be sufficiently durable that there is no significant change in shape during use.

[0009] In addition to the cutter or group of cutters which can remove material projecting outwardly from tubing, a cutting assembly may also have a second cutter or group of cutters which provide cutting surfaces for milling the tubing itself, located radially inwardly from the cutting surfaces of the first cutter or group of cutters. This second cutter or group of cutters may have cutting positions whose distance from an axially leading end of the rotary tool increases as radial distance from the tool axis increases, so that removal of tubing may progress outwardly from the inside wall of the tubing as the tool advances.

[0010] For the tool and method stated above, it is possible that some or all of the cutting assemblies could be fixed to the tool body and project from it. A tool with fixed cutting assemblies could be used to mill tubing where it is possible to begin at an end of the tubing, which of course would require the end to be accessible to the tool. However, in some forms of the tool, the cutting assemblies are extensible from the tool body by operation of a drive mechanism. The tool may then be inserted into tubing with the cutting assemblies retracted and when the tool is at the position where milling is to start, the cutting assemblies are extended by operation of the drive mechanism and cut outwards through the tubing as they are extended.

[0011] An extensible cutting assembly may have a third cutter or group of cutters which provide cutting surfaces radially outwardly from the cutting surfaces of the first cutter or group of cutters and serve for cutting outwardly through the tubing as the cutting assemblies are extended from the tool body.

[0012] The supporting structure of a cutting assembly may have (and may have been manufactured with) one or more edges arranged so that distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases and the first cutter or group of cutters may be adjacent such an edge or edges.

[0013] Such an edge may lie at one side of a recess in the support structure. A cutting assembly may be shaped to extend radially outwards across the upper end of the tubing which is to be milled while also including a region which projects axially into an end portion of the uncut tubing and a region which projects axially forward outside the end portion of the uncut tubing. These portions which project axially forwardly will then lie at either side of a recess extending axially back from the leading edge of the cutting assembly. The uncut end of the tubing will project into this recess and the cutting positions for cutter(s) milling couplings from the outside and cutter(s) milling tubing from the inside may be located at either side of the recess.

[0014] The rotary tool may have at least three cutting assemblies distributed azimuthally around it at the same axial position. For instance there may be three cutting assemblies at 120° azimuthal intervals around the tool body, or four at 90°intervals or six at 60°intervals.

[0015] When the tool has expandable cutting assemblies, the drive for their expansion may be powered hydraulically by fluid pumped from the surface. The drive may be arranged to expand a plurality of cutting assemblies, distributed azimuthally around the tool body, in unison. The travel of the cutting assemblies as they are expanded may be motion around a pivotal attachment to the tool body or it may be a motion in which the cutting assemblies move outwardly without changing their orientation relative to the tool body. The latter may be brought about by constraining each cutting assembly to be movable along a pathway. More specifically pathways may be angled relative to the tool axis and configured so that when the cutting assemblies are moved axially they also move outwardly in unison.

[0016] The cutters have hard faces and may be bodies of a hard material. Tungsten carbide is a material which is commonly used for cutters because it is very hard and also has good thermal stability. Other hard materials which may be used are carbides of other transition metals, such as vanadium, chromium, titanium, tantalum and niobium. Silicon, boron and aluminium carbides are also hard carbides. Some other hard materials are boron nitride and aluminium boride. A hard material may have a Knoop hardness of 1300,1600,1800 or even more.

[0017] One or more of the cutters may have a shape of cutting surface and a position on the tool such that at least part of the cutting surface is back raked, that is to say it is inclined relative to the direction of rotation such that an edge where the cutting surface cuts furthest into the tubing, coupling or other outward projection is a trailing edge of the cutting surface relative to the direction of rotation and extends from the said edge with a back rake angle which is from 15° to 70° (possibly between 30° and 60°) and at the said edge has an angle greater than 90° included between the cutting surface and the surface of the cutter body following the cutting surface.

When there is such a rake angle in a range from 15° to 70° between at least part of the cutting surface and a perpendicular to the direction of traverse relative to the workpiece, the angle between the cutting surface or part thereof and the direction of rotation lies in a range from 20° to 75°.

[0018] As disclosed in a currently unpublished GB patent application, we have found that a cutting surface with a large back rake angle leads to the formation of swarf with less rigidity. It may be in the form of short pieces weakly connected together, or sometimes not connected at all. Changing the nature of the swarf reduces the risk of entangled swarf forming a "birds nest" blockage in the borehole. A significant back rake may require the cutter to be pressed against the tubing with more force than would be required with less back rake or none. In a machine-shop context, a requirement for increased force between a cutting tool and workpiece would be a disadvantage, but we have recognized that when operating a cutting tool in a wellbore, a requirement for greater force is beneficial. More force can be provided by increasing the weight on the tool and control of the cutting speed by varying the weight on the tool becomes easier. Increasing the included angle between the cutting surface and a surface of the body behind the cutter surface makes the cutter more robust and reduces the risk of the cutter being chipped or broken.

[0019] The cutter body may be dimensioned such that the at least part of the back raked cutting surface extends at least 2mm from the said edge where the cutting surface cuts furthest into the tubing and the cutter body's surface trailing back from the said edge extends at least 2 mm possibly at least 3mm or at least 5mm back from the said edge.

[0020] The length of tubing which is removed by the tool and method above may be considerable. It may for example be a length which is many times (for instance more than 10 times) greater than the axial length taken up by of the cutters and guide surfaces of the tool itself. The length of tubing removed may be 5 metres or more. The removal of tubing may be carried out for various reasons, but in some instances it may be done before plugging and abandoning the borehole.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Fig 1 is a schematic axial view of a rotary milling tool with fixed cutting assemblies, positioned to mill tubing from one end; [0022] Fig 2 shows a rotationally leading face of one cutting assembly of the tool of Fig 1 when the tool is milling tubing; [0023] Fig 3 is a cross section on line B-B of Fig 2; [0024] Fig 4 shows the rotationally leading face seen in Fig 2 when the tool is milling a coupling; [0025] Fig 5 is a face view of the leading end of a cutter; [0026] Fig 6 is a side view of a cutter in contact with a workpiece; [0027] Fig 7 is a perspective view of another rotary milling tool; [0028] Fig 8 is a sectional elevation of the tool of Fig 7 with the extensible cutting assemblies retracted; [0029] Fig 9 is a sectional elevation of part of the tool of Fig 7 with a cutting assembly partially extended; [0030] Fig 10 is a sectional elevation of part of the tool of Fig 7 with a cutting assembly fully extended and milling tubing; [0031] Fig 11 is a perspective view of one cutting assembly; [0032] Fig 12 is an enlarged underneath view of the cutting region of a cutting assembly; [0033] Fig 13 diagrammatically shows the radial and axial layout of cutters of a cutting assembly of Figs 7 to 12; and

[0034] Fig 14 is a side view of parts of a cutter block used in another rotary tool. DETAILED DESCRIPTION

[0035] Figs 1 to 6 show a rotary milling tool with fixed cutting assemblies used for milling tubing when it is possible to access an upper end of the tubing. For example, casing milling downwards from the top of a borehole may be carried out when it is required to place a sealing plug at a modest depth below the surface, such as within 700 metres of the surface as part of the process of abandoning a well.

[0036] As shown, an existing borehole is lined with lengths of tubing 12 (wellbore casing) which are screw-threaded at each end and joined end to end by correspondingly threaded couplings 14 (seen in Figs 2 and 4). Although standard couplings have an internal thread to engage an external thread on the end portions of drill pipe, that thread does not extend fully to each end of a coupling. Cement 15 has been placed between the casing and the surrounding rock formation. The tubing 12, couplings 14 and cement 15 may have been in place for some years.

[0037] Fig 1 schematically illustrates the tool and borehole looking axially from above. The tubing 12 is shown with hatching. The tool has a central hollow cylindrical body 16 which can be attached to the bottom end of a drill string. This body 16 defines a through passage 17 for drilling fluid pumped down the drill string. The fluid flows out of the bottom end of the tubing and returns up the annulus around the drill string in conventional manner. The direction of rotation is indicated by arrow A.

[0038] Six cutting assemblies 18 are rigidly attached to the central body 16 and project radially out from it at 60 degree intervals azimuthally around the axis of the body. Figs 2 and 4 show the rotationally leading face of one cutting assembly 18. Each cutting assembly comprises a supporting structure and cutters attached to it. The supporting structure is a steel block 20 rigid with the body 16. The cutters 22-28 are generally cylindrical and secured in cavities in the block 20 so that they are partially embedded in block 20 with their leading ends exposed and facing in the direction of rotation. These cutters are made of a hard material which may be tungsten carbide. This hard material may be provided as a powder which is compacted into the shape of the cutter and then sintered giving a Knoop hardness greater than 1600. Manufacturers of sintered tungsten carbide cutters include Cutting and Wear Resistant Developments Ltd, Sheffield, England and Hallamshire Hard Metal Products Ltd, Rotherham, England.

[0039] Tungsten carbide is a material which is commonly used for cutters because it is very hard and also has good thermal stability. Other hard materials which may be used are carbides of other transition metals, such as vanadium, chromium, titanium, tantalum and niobium. Silicon, boron and aluminium carbides are also hard carbides. Some other hard materials are boron nitride and aluminium boride. A hard material may have a hardness of at least 1300, or at least 1600 and possibly at least 1800 or more on the Knoop scale. By contrast, steel or other metal used for a supporting block 20 is likely to have a Knoop hardness below 1000.

[0040] The cutters 22-28 are secured in cavities in the block 18 by brazing, but other methods of securing cutters may be used if desired.

[0041] The block 20 has an upper part which extends outwardly across the tubing 12 and an outer part 30 which extends down at the exterior of the tubing 12. Thus there is a recess extending axially upwards into the block 20 between its inner part 31 and outer part 30. Cutter 27 is at the upper end of this recess and cutters 24, 25 and 26 are attached to the outer part 30 at the radially outer edge of the recess which is angled such that its distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases.

[0042] Fig 2 shows action of the tool while it is cutting tubing 12 but has not yet reached a coupling 15.

[0043] A radially outward facing surface 32 on the inner block part 31 is a part-cylindrical outward facing surface 32 with a radius such that the surface 32 is centered on the tool axis. The cutter 22 is positioned so that its radially outer extremity is at the same distance from the tool axis as the surface 32. Thus, the radial extremity of the cutter 22 is aligned with the surface 32 as shown by Fig 3. There is also a part-cylindrical outward facing surface 33 centred on the tool axis at larger radius from the tool axis. The extremity of cutter 23 is at the same distance from the tool axis as the surface 33 and so is aligned with it.

[0044] The rotating tool advances axially in the downward direction shown by arrow D. The tubing 12 may have some corrosion and deposited material on its inside surface as depicted schematically at 36. The axially leading cutter 22 on each block 20 is positioned to remove this material 36 and also remove some material from the inside wall of the tubing 12, thus creating a new inward facing surface on the tubing 12. This surface is indicated 42 in Fig 3.

[0045] Because the part-cylindrical outward facing surfaces 32 are centered on the tool axis and aligned at the same radial distance from the tool axis as the extremities of the leading cutters 22, they are a close fit to the inward facing surface 42 created on the tubing by the cutters 22 as is shown in Fig 3, and slide over this new inward facing surface 42 as the tool rotates. The cutters 23 remove a further thickness of tubing 12, creating a fresh inward facing surface on which the surfaces 33 slide. This close fit of surfaces 32, 33 to surfaces created on the tubing 12 positions the tool axis accurately relative to the tubing 12.

[0046] As the tool progresses downwardly, the cutter 26 removes some thickness from the exterior of the tubing 12 and the remainder is cut by the cutter 27. The cutters 24 and 25 both cut through cement 15 around the tubing 12. The radially outermost cutter 28 also cuts through cement 15.

[0047] Fig 3 shows function of the tool when it comes to coupling 14. The cutters 22, 23 continue to cut tubing 12 as described above. The cutter 24 is the first cutter to contact the coupling 14. It removes part of the thickness from the exterior of the coupling. As the tool advances, cutter 25 removes a further thickness and finally the cutter 26 removes the remaining thickness of the coupling 14 as well as removing a portion of the thickness from the exterior of the tubing 12. Thus the coupling 14 is milled to swarf, working inwards from its exterior, greatly reducing opportunity for part of the coupling to become a detached ring.

[0048] Figs 5 and 6 show the shape of cutters 22, 23, 24, 25 and 27. Each of these cutters has a cylindrical body 44 and a shaped leading end in which a front face 46 with smaller diameter than the body 44 is surrounded by an annular surface 48 at an angle of 45° to the front face 46. The angle included between the side wall of the cutter body 44 and the annular surface 48 is 135°, as shown. When the cutter is mounted on a tool, part of the annular surface 48 is the cutting surface. With this geometry, the back rake angle between the cutting surface 48 and a perpendicular to the substrate 52 (tubing or coupling) which is being cut is approximately 45°. We have discovered that cutting with this substantial back rake angle leads to swarf with much less mechanical strength and rigidity than swarf produced by cutters without any bake rake. This reduces the risk that pieces of the swarf will hook together and clog the path of flow back to the surface.

[0049] The cutter 26 is slightly different. It is also cylindrical with a shaped leading end including an annular cutting surface. However, the angle of this cutting surface is such that the back rake angle is greater, approximately 60°. With such a large back rake angle the remnant of coupling 14 which is being cut is also pushed inwardly with a force greater than the cutting force. This pushes the remnant of the coupling 14 very forcefully against the partially cut tubing 12 and friction and plastic deformation can act to hold the uncut remnant of the partially milled coupling 14 in place while it is being milled.

[0050] The cutter 28 is provided to assist with cutting cement in the path of the outer part 30 of the cutting assembly. It does not cut the metal of the tubing and couplings and may have a flat leading face as its cutting surface. It is possible that this cutter 28 could be a polycrystalline diamond cutter which has sintered diamond at its leading surface.

[0051] The six projecting cutting assemblies 18 shown in Fig 1 may all be identical to each other and constructed as described above with reference to Figs 2 to 6. However, it is possible that the cutting assemblies will differ from each other. One possibility is that the cutting assemblies 18 all have blocks 20 and cutters 22-28 with the general configuration shown but with slight differences in the axial and radial positions of cutters so that corresponding cutters on successive assemblies cut to progressively increasing radius. Another possibility is that only three of the six cutting assemblies 18 extend outwardly beyond tubing and have cutters 24, 25, 26 for cutting the couplings. The other three cutting assemblies could then have only cutters such as 22, 23 and 27 for cutting tubing from the inside.

[0052] Figs 7 onwards show a tool which uses the same principles for milling tubing but is expandable downhole. This allows the tool to be inserted to a chosen depth through existing tubing which is not going to be removed, then expanded to cut outwardly through the tubing before being made to advance axially to remove a length of tubing. One circumstance in which this is required is when removing tubing and exposing the rock formation around a borehole, in preparation for setting a cement plug when a well is abandoned.

[0053] Figs 7 to 10 show the general layout and function of the expansion mechanism of an embodiment of rotary milling tool. This expansion mechanism is of a type already in use for expandable reamers. As seen in perspective view in Fig 7, the tool has a tubular main body 60 with upper end 62 and lower end 64. In a central section there are three longitudinal slots 66 distributed at 120° intervals around the tool axis.

[0054] The tool can be incorporated into a drill string. As shown in Fig 8, the upper and lower end regions include portions 68 which are threaded to enable connection to standard drill pipe.

[0055] A central tube 70 is a sliding fit within the main body 60. Axial movement of the tube 70 is guided by the body 70 and sleeves 71 fixed to the body 70. This tube 70 is urged upwardly by a return spring 72. Each slot 66 houses an arm 74 which can swing through 90° around pivot 75 from the retracted position shown in Fig 8 to the extended position shown in Fig 10. The inner end of each arm 74 is formed with projections 76 which function as gear teeth. These mesh with projections 78 from the tube 70.

[0056] When the tool is in its retracted condition as shown in Fig 8, drilling fluid pumped down the drill string can flow downwardly through the tube 70 and out of the lower end 64 of the main body 60. When the tool, included within a drill string, has been lowered to the desired depth, a ball is dropped down the drill string. This ball is dimensioned to block the tube 70 at the restriction 80. Pressure of the drilling fluid then forces the tube 70 to slide downwards against the force of return spring 72, thereby compressing that spring. As the tube 70 moves downwards, the projections 78 on the tube meshing with the teeth 76 urge the arms 74 to rotate around their pivots 75 towards their fully extended position shown in Fig 10 when the surfaces 81 of the arms 74 abut stop blocks 82 bolted to the main body 60. Downward movement of tube 70 allows some drilling fluid to flow out through opening 84, into chamber 85 and out through nozzles 86.

[0057] Each arm 74 carries a number of cutters which each have the general configuration shown by Figs 5 and 6, with a cylindrical body which is partially embedded in the arm 74 and an exposed leading end shaped so that the annular cutting surface is at a back rake. These cutters may be sintered tungsten carbide. The cutters are shown in Figs 7 to 10 but their positions are shown in more detail by Figs 11 and 12.

[0058] As shown by Fig 10, each arm 74 extends radially outwardly beyond the tubing 12 which is being cut. An outer portion 87 of the arm projects axially forwards at the exterior of the tubing and a recess 88 extends into the arm between this outer portion 87 and the remainder of the arm 74 which is within the tubing 12. The radially outer edge of this recess, which is also an inner edge of the outer portion 87 is such that its distance from the axially leading end of the rotary tool decreases as radial distance from the tool axis increases.

[00591 However, the axial extent of an arm 74 is limited by the space available for it within a slot 66. Consequently only some of the cutters on each arm are exposed at the leading face of the arm. This is shown by perspective view Fig 11 and by Fig 12 which is an enlarged view of the outer part of an arm seen from below. The radially outward end face of the arm incorporates a channel 89 which continues as channel 90 inwardly some distance along the underside of the arm. Cutters 92, 94, 96, 98,100 and 102 have their leading ends exposed at the leading face 77 of the arm 74. Cutters 91, 93, 95 and 97 have their leading ends exposed in the channel 90. The radial and axial positions of the cutters are shown diagrammatically in Fig 13 in which the cutters are all shown in the plane of the diagram but their circumferential positions are not shown.

[0060] For use the tool is attached to a drill string and lowered to the depth at which milling out of section of casing tubing 12 is required to start. The drill string and tool are rotated but their axial positions are kept constant. Drilling fluid is pumped down the drill string and a ball is dropped to lodge at restriction 80 and start expansion of the arms 74. Initially each arm extends until the cutter 102 on the arm begins to cut into the tubing 12 as shown in Fig 9.

[00611 As the arm cuts into the tubing 12, it expands further. After the cutter 102 cuts through the tubing, expansion continues with cutter 100 and then cutter 98 cutting the tubing. When the fully extended position shown in Fig 10 is reached, weight is applied to the tool so that axial advance of the tool begins. The cutting action at this stage is generally analogous to that shown by Figs 2 and 4. Tubing 12 is progressively cut from the interior working outwards. The first cut is made by cutter 91, the second by cutter 92 which is exposed at the leading face 77 and then further cuts by cutters 93 and 94. It may be noted that cutter 94 is positioned slightly differently from cutter 27 in that the centre of cutter 94 is slightly inward from the exterior of the tubing 12.

[0062] The steel structure of arm 74 includes surfaces 111, 112 and 113, seen as edges in Fig 12, which are aligned with extremities of cutters 91, 92 and 93 so that these surfaces slide on new metal surfaces cut on the tubing by the cutters 91, 92 and 93 respectively and thereby position the tool in the tubing 12. As can be seen from Fig 13, when the tool reaches a coupling 14, the coupling will initially be cut by cutter 95, then by cutter 96 followed by cutter 97. The cutter 97 has a back rate of 60° like the cutter 26 described earlier. This very large back rake enables the cutter to push the remnant of the coupling 14 hard against tubing 12. The remnants of the coupling and tubing are finally removed by cutter 94. Cutters 95, 96, 97 and 94 constitute a group of cutters for cutting the couplings 14, These cutters are adjacent the outer edge of recess 88 which is such that its distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases. Thus, these cutters are also arranged such that distance from the axially leading end of the rotary tool decreases as radial distance from the tool axis increases, and so the couplings 14 are milled progressively inwards from the outside and are comminuted to swarf without becoming detached from the tubing 12.

[0063] The three arms 74 which are distributed at 120° intervals around the body 60 are similar to each other in the number and layout of cutters. However, they may vary slightly in the axial and radial positioning of cutters. For instance the cutters 91, 92 and 93 on one arm 74 may be positioned at slightly greater radius and axially slightly above the corresponding cutters on the preceding arm 74. Cutters on the next arm 74 may be at greater radius still, but further above axially. With such an arrangement all the cutters 91, 92 and 93 on the three arms 74 can cut helices as they rotate and advance so that the work of cutting tubing is shared by all the cutters on all three arms.

[0064] Other mechanisms may be used to expand cutters to mill tubing, and concepts disclosed here may be used with such mechanisms. US2003/0155155 is one of several documents in which the expansion of three cutting assemblies from a cylindrical tool body is brought about by a mechanism which uses the pressure of drilling fluid to drive cutter blocks upwardly. The cutter blocks have protruding splines which are at an angle to the tool axis and fit into matching channels which are part of the cutter body. Consequently when the blocks are pushed upwardly in unison, the splines slide in the matching channels and guide the blocks to expand radially in unison. In this prior document the tool is an under reamer for enlarging a borehole.

[0065] Fig 19 illustrates use of such a mechanism used for a section mill. A cutter block has an inner part 120 with angled splines 122 and an outer part 124. This block is one of three blocks distributed azimuthally around the body of a rotary tool as shown and described in US2003/0155155. The splines 122 correspond to those shown at 650 in Figs 7 and 8 of US2003/0155155. The mechanism shown and described in that document is used to push the blocks upwards and outwards while the tool is rotating within tubing which is to be removed. The outer part 124 of each block is largely the same as a cutting assembly shown in Fig2, with hard cutters 22-27 whose function is the same as described with reference to Figs 2-4, but it has a row of cutters 126 for cutting outwardly through tubing as the block is expanded outwardly from the tool body.

[0066] It will be appreciated that the embodiments and examples described in detail above can be modified and varied within the scope of the concepts which they exemplify. Proportions may be varied and may not be as shown in the drawings which are schematic and intended to explain layout and action in the embodiments shown. Features referred to above or shown in individual embodiments above may be used together in any combination as well as those which have been shown and described specifically. More particularly, where features were mentioned above in combinations, details of a feature used in one combination may be used in another combination where the same feature is mentioned. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

Claims

1. A rotary milling tool for comminuting tubing in a borehole comprising a tool body a plurality of cutting assemblies projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool wherein at least one cutting assembly comprises a supporting structure and a plurality of hard-surfaced cutters attached thereto, with the shape of the supporting structure and the positions of the cutters thereon, as manufactured prior to use in a borehole, being such that a first cutter or group of cutters provide cutting surfaces at positions whose distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases, for removal of projecting material to progress inwardly towards the tubing as the tool advances.

2. A tool according to claim 1 wherein a second cutter or group of cutters on the at least one cutting assembly provide cutting surfaces radially inwardly from the cutting surfaces of the first cutter or group of cutters at positions whose distance from an axially leading end of the rotary tool increases as radial distance from the tool axis increases, for removal of tubing to progress outwardly as the tool advances.

3. A tool according to claim 1 or claim 2 wherein the cutting assemblies are extensible from the tool, body and wherein a third cutter or group of cutters on the at least one cutting assembly provide cutting surfaces radially outwardly from the cutting surfaces of the first cutter or group of cutters for cutting outwardly through the tubing as the cutting assemblies are extended from the tool body.

4. A tool according to any one of the preceding claims wherein the cutting surfaces of the first cutter or group of cutters are positioned adjacent at least one edge of the supporting structure, which is an edge arranged so that its distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases.

5. A tool according to any one of the preceding claims wherein the supporting structure of the at least one cutting assembly is shaped to extend radially outwards from the tool body, and cutting surfaces of the first cutter or group of cutters extend within a recess axially back from an axially leading edge of the support structure at a position spaced radially outwardly from the tool body and the are at positions within the recess.

6. A tool according to any one of the preceding claims wherein the cutters are bodies with hard cutting faces, partially embedded in the supporting structure with the hard cutting faces exposed as rotationally leading faces of the cutters.

7. A tool according to claim 6 wherein the cutters are partially embedded within cavities in the support structure.

8. A tool according to any one of the preceding claims having at least three cutter assemblies projecting from or extensible from the tool body and distributed azimuthally around the tool axis at the same axial position.

9. A tool according to any one of the preceding claims wherein at least one cutter is shaped and positioned on the cutting assembly such that at least part of its cutting surface is back raked relative to the direction of rotation so that the cutting surface cuts deepest at an edge which is a trailing edge of the cutting surface relative to the direction of rotation and wherein at least part of the back raked cutting surface extends from the said edge with a rake angle between the cutting surface and a perpendicular to the direction of rotation which is in a range from 30° to 70°.

10. A method of removing tubing in a borehole comprising comminuting the tubing with a rotary tool which comprises a tool body and a plurality of cutter assemblies projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool body; wherein at least one cutting assembly comprises a supporting structure and plurality of hard-surfaced cutters attached thereto with a first cutter or group of cutters positioned to cut into material projecting outwardly beyond the exterior of the tubing at cutting positions arranged so that distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases, whereby removal of projecting material progresses inwardly towards the tubing as the tool advances.

11. A method according to claim 10 wherein material projecting outwardly beyond the exterior of the tubing comprises couplings connecting lengths of tubing together.

12. A method according to claim 10 or claim 11 wherein the cutters are bodies with hard cutting faces, partially embedded in the supporting structure with the hard cutting faces exposed as rotationally leading faces of the cutters.

13. A method according to claim 12 wherein the cutters are partially embedded within cavities in the support structure.

14. A method according to any one of claims 10 to 13 wherein the supporting structure is manufactured with at least one edge arranged so that its distance from an axially leading end of the rotary tool decreases as radial distance from the tool axis increases, and the first cutter or group of cutters is positioned adjacent the at least one edge.

15. A method according to any one of claims 10 to 14 wherein each cutting assembly also comprises a plurality of cutters positioned to cut into the tubing, with the cutting positions of these cutters arranged so that distance from an axially leading end of the rotary tool increases as radial distance from the tool axis increases, whereby removal of tubing progresses outwardly as the tool advances.

16. A method according to claim 15 wherein the supporting structure of each cutting assembly has a radially outward facing guide surface at the same radial distance from the tool axis as the extremity of cutter, positioned to slide over a surface created on the tubing interior by that cutter.

17. A method according to according to any one of claims 10 to 16 wherein at least one cutter is shaped and positioned on the cutting assembly such that at least part of its cutting surface is back raked relative to the direchon of rotation so that the cutting surface cuts deepest at an edge which is a trailing edge of the cutting surface relative to the direction of rotation and wherein at least part of the back raked cutting surface extends from the said edge with a rake angle between the cutting surface and a perpendicular to the direchon of rotation which is in a range from 30° to 70°.

18. A method of removing tubing in a borehole comprising comminuting the tubing with a rotary tool as defined in any one of claims 1 to 9 with the a hrst cutter or group of cutters cutting into material projecting outwardly beyond the exterior of the tubing.

19. A method according to any one of claims 10 to 18 wherein the tool has expandable cutter assemblies, the method comprising inserting the tool into a borehole, lowering it to a chosen depth, extending the cutter assemblies from the tool body while rotahng the tool so as to cut outwardly through the tubing, and then advancing the tool axially while continuing to rotate the tool.