Patent Publication Number: US-10760364-B2

Title: Milling tool and method

Description:
BACKGROUND 
     In the hydrocarbon exploration and production industry, a milling tool, which is commonly referred to as a section mill, may be used to remove a length of metal tubing within a borehole, by comminuting the tubing to swarf. Merely by way of example, one circumstance in which milling is used is when removing tubing/casing and exposing the rock formation around a borehole in preparation for setting a cement plug to meet regulatory requirements when a well is abandoned. 
     A conventional section mill, such as the K-Master section mill supplied by Schlumberger, is incorporated into a drillstring which extends up to the surface and also extends forwardly beyond the section mill itself. The portion of the drill string which extends forwardly from the section mill includes components which are dimensioned to have an exterior diameter slightly smaller than the interior diameter of the tubing which is to be removed. These components locate the axis of the drillstring relative to the axis of the tubing which is to be milled. 
     In many instances, the cutting blades of a conventional section mill can be expanded outwards relative to the body of the tool. The ability to expand allows the mill to be inserted along the tubing to a desired starting point and then to cut outwards through the tubing before being advanced axially to extend the length of tubing which is being removed. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth. 
     Disclosed now is a tool and method for removing tubing within a borehole. In embodiments of the present disclosure, the rotary tool comprises a plurality of elements that project out from the body of the tool while it is in operation and these elements carry guide surfaces that are able to make sliding contact with an interior surface created on the tubing by a cutter or cutters positioned on the tool axially ahead of the guide surfaces. By creating the interior surface using one or more cutters, the diameter of the surface is controlled and the guide surfaces can then be positioned as a close fit against this surface, giving stable positioning of the tool relative to the tubing being milled. 
     In one aspect of embodiments of the present disclosure there is disclosed here a rotary milling tool for removing tubing in a borehole, where the milling tool comprises a tool body and a plurality of elements projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool body. If the elements are extensible then means are provided for extending the elements from the tool body. At least some of the elements have outward facing guide surfaces that make sliding contact with an inward facing surface of the tubing in the borehole. And at least some of the elements comprise cutters to remove material from the tubing, where at least some of these cutters are configured to produce the inward facing surface that is contacted by the guide surfaces. 
     The cutters and guide surfaces are provided in a configuration in which a plurality of the outward facing guide surfaces are distributed azimuthally around the tool axis. At least one cutter is positioned as a leading cutter at an axial position that is at least as far forward in the direction of advance of the tool to provide that the cutter creates the inward facing surface in front of the guide surface, with respect to the motion of the tool. This cutter extends radially outward from the tool axis to a same distance/extent as the plurality of guide surfaces. At least one further cutter is positioned to follow axially behind the plurality of guide surfaces and this cutter extends radially outwards beyond the guide surfaces to provide for further cutting of the tubing behind the guide surface as the tool is moved axially through the tubing. 
     In some embodiments the cutters and guide surfaces may be disposed on a milling block and the milling tool may comprise a plurality of these milling blocks disposed around a circumference (circumferentially) of the milling tool. The milling blocks may comprise a forward cutter that extends outwards from the surface of the milling block and is configured to cut/mill a surface on the tubing. In use, the milling block may be extended from the tool body to mill the surface on the tubing. The extent of the cut of the cutter into the tubing defines a drilling silhouette. A guide surface extend from the milling block to the same extent as the forward cutter to provide that in use the guide surface extends to the cutting silhouette of the forward cutter. A trailing cutter is positioned behind the guide surface and extends beyond the guide surface so that in use the trailing cutter cuts/mills the tubing beyond the cutting silhouette of the forward cutter. 
     In some embodiments, the plurality of outward facing guide surfaces may be at the same axial position on the tool although distributed azimuthally around it. In some embodiments, the plurality of outward facing guide surfaces are not identical to one another and/or only some of the plurality of outward facing guide surfaces are identical. 
     In some embodiments, the plurality of guide surfaces are provided on three or more elements projecting from or extensible from the tool body and distributed azimuthally around the tool axis. These three or more elements may be disposed at the same axial position on the tool. These elements, which according to embodiments of the present disclosure, each have an outwardly facing guide surface may each have at least one leading cutter ahead of the guide surface and with the same radial extension from the tool as the guide surface, and at least one further cutter located axially behind the guide surface and extending radially outwards beyond the guide surface. In some embodiments, there may be more than one plurality of guide surfaces at different axial positions on the tool, with increasing radial distances from the tool axis and each with at least one leading cutter which extends out from the tool axis to the same distance as the guide surfaces. 
     In a second aspect of embodiments of the present disclosure there is disclosed here a method for removing a length of tubing in a borehole inserting a rotary milling tool into the tubing, where the milling tool comprises a tool body and a plurality of elements which project from or are extensible from the tool body. The plurality of elements are distributed azimuthally around a longitudinal axis of the tool body. At least some of the elements have outward facing guide surfaces for sliding contact with an inward facing surface of the tubing in the borehole and at least some of the elements comprise one or more cutters to remove material from the tubing. At least one cutter is positioned as a leading cutter at an axial position which is at least as far forward in the direction of advance of the tool as the axially leading parts/sections/edges of the guide surfaces and extends radially out from the tool axis to the same distance as the guide surfaces. And at least one further cutter is positioned axially behind the guide surfaces and extends radially outwards beyond the guide surfaces. 
     In embodiments where the elements are extendable from the tool body, the elements may be extended outwards during operation. In the method, the tool is rotated and advanced axially through the tubing. This motion provides for the at least one leading cutter contacting the interior surface of the tubing and removing material from the interior surface of the tubing and expose a new or renewed interior tubing surface with diameter swept out by the at least one cutter. The outward facing guide surfaces on a plurality of the azimuthally distributed elements makes a sliding type contact with the new or renewed interior surface as the tool is advanced through the tubing. Behind the outward facing guide surfaces, the new or renewed interior surface is removed by contacting it and cutting it with the at least one further cutter as the tool advances. 
     Using the tubing, which is about to be removed in the milling process, as a guide for components dimensioned to have an exterior diameter smaller than the interior diameter of that tubing leads to inaccuracy in the positioning of the tool and undesirable vibration. However, an obstacle to using the tubing as a guide when milling tubing which is in a borehole is that the interior of the tubing may not be of uniform diameter because it may have become corroded, have some form of detritus accumulated on it and/or the like. 
     In embodiments of the present disclosure, one or more leading cutters are used to expose a new or renewed inward facing tubing surface which can be contacted by the outward facing guide surfaces to give more accurate positioning of the tool as it is advanced and cuts into the tubing. This surface which is exposed may be a renewed surface at approximately the internal diameter of the tubing when it was new, in other words a refreshed internal surface of the tubing. Another possibility is that the leading cutter(s) may remove some of the original metal from the inside of the tubing, as well as any corrosion or accumulated material, thus reducing the tubing thickness. In this case the newly exposed internal surface of the tubing is a new surface with an internal diameter which is greater than the original internal diameter of the tubing. 
     Furthermore, although this new or renewed internal surface provides a guide for the positioning the tool, it has only a transient existence. As the tool advances, the further cutters which follow axially behind the guide surfaces cut into and remove the remaining metal of the tubing. Further, the configuration provides for separated stages of cutting with a stabilizing section, the guide surface, between cutting stages, which may provide for stabilizing the cutting tool. In some embodiments, different areas of stabilizing sections may be used to provide stabilization. 
     For the tool and method described above, it is possible that some or all of the projecting elements may be fixed relative to the tool body. A tool with fixed elements may be used to mill tubing where it is possible to begin at an end of the tube, which of course would require the end to be accessible to the tool. However, in some forms of the tool, the elements are extensible from the tool body by operation of a drive mechanism. The tool may then be inserted into tubing with the elements retracted and when the tool is at the position where milling is to start, the elements are extended by operation of the drive mechanism and cut outwards into the tubing as they are extended. 
     When the cutters cut into the tubing and remove tubing metal, this tubing metal is reduced to swarf and so the tubing is destroyed by comminuting it. The further cutters may also extend radially outwards beyond the tubing and then they may remove some non-metallic material which was outside the tubing. More specifically they may remove some or all of any cement around the exterior of the tubing and possibly even some formation rock. 
     As stated above, the tool has projecting or extensible elements distributed azimuthally around it. For instance, in some embodiments, there may be three or four elements with guide surfaces at 120° or 90° azimuthal intervals around the tool body. In other embodiments, more elements may be used. Each one of at least three elements at the same axial position relative to the tool may have a guide surface at the same radial distance from the tool axis. It is possible that some elements may comprise cutters while other elements provide guide surfaces. It is also possible that some elements comprise leading cutters and guide surfaces while other elements provide the further cutters axially following the guide surfaces. In some forms of rotary tool all the projecting or extensible elements comprise leading cutters, further cutters and guide surfaces between them. 
     Cutters on the projecting or extensible elements, both leading cutters and further cutters following axially behind the guide surfaces may comprise a hard material. For example 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 1800 or more on the Knoob scale or a hardness of 9 or more on the original Mohs scale (where diamond has a Mohs hardness of 10). Cutters of hard material may be made as ceramic objects, with particulate hard material embedded in a matrix of another material which may be a metal or metal alloy. Cutters of this form may be attached by brazing to a projecting or extensible structure which is steel. 
     When the tool has expandable elements, the drive for their expansion may in some embodiments be powered hydraulically by fluid pumped from the surface. The drive may be arranged to expand a plurality of elements, distributed azimuthally around the tool body, in unison. The travel of the elements as they are expanded may be a motion around a pivotal attachment to the tool body or it may be a motion in which the elements move outwardly without changing their orientation relative to the tool body. The latter may be brought about by constraining each element to be movable along a pathway. More specifically pathways may be angled relative to the tool axis and configured so that when the elements are moved axially they also move outwardly in unison. 
     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 DRAWINGS 
       The present disclosure is described in conjunction with the appended figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1, 2 and 3  schematically illustrate a task performed by a milling tool; 
         FIG. 4  is a cross-sectional elevation view of an expandable tool, in accordance with embodiments of the present disclosure, showing its expandable blades in retracted position; 
         FIG. 5  is a cross-sectional elevation view of the expandable tool of  FIG. 4 , showing the blades in expanded position; 
         FIG. 6  is a perspective view of a cutter block for the expandable reamer of  FIGS. 4 and 5 ; 
         FIG. 7  is a cross sectional elevation showing part of the tool of  FIGS. 4 to 6 , in use to remove tubing; 
         FIG. 8  is a cross section on the line VIII-VIII of  FIG. 7 ; 
         FIG. 9  is a similar view to  FIG. 7 , showing an arrangement with modifications; 
         FIG. 10  is an enlargement of part of  FIG. 9 , showing a different cutter; 
         FIGS. 11A, 11B and 11C  are side views of the outer part of three cutter blocks used on a variation of the tool of  FIGS. 4 to 6 ; 
         FIG. 12  is a cross sectional elevation of part of a different tool in accordance with embodiments of the present disclosure; 
         FIG. 13  is a cross section through the tool of  FIG. 12 ; 
         FIG. 14  is a similar view to  FIG. 12 , with the tool in use to remove tubing; and 
         FIG. 15  is a cross sectional elevation of part of a further tool, in accordance with embodiments of the present disclosure. 
     
    
    
     In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
     DETAILED DESCRIPTION 
     The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the invention. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims. 
     Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments maybe practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
     Also, it is noted that the embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but may have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
     Moreover, as disclosed herein, the term “storage medium” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “computer-readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data. 
     Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium such as storage medium. A processor(s) may perform the necessary tasks. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. 
       FIGS. 1 to 3  show a function which has conventionally been performed with a section mill and which may be performed by a milling tool and method as disclosed herein. 
     As shown by  FIG. 1  an existing borehole is lined with tubing  12  (the wellbore casing) and cement  14  has been placed between the casing and the surrounding rock formation. The tubing and cement may have been in place for some years. It is now required to remove a length of tubing, starting at a point below ground. One possible circumstance in which this may be required is when a borehole is to be abandoned, and regulatory requirements necessitate removal of a length of tubing and surrounding cement in order to put a sealing plug in place. 
     As shown by  FIG. 1  a milling tool  16  is included in a drillstring  18  which is lowered into the borehole. This drill string can be formed from conventional drill pipe which is able to convey drilling fluid down to the milling tool  16  and which is rotatable by a drive motor at the surface. In an alternative arrangement (not shown) the milling tool may be attached to a hydraulic motor attached to coiled tubing inserted into the borehole. 
     When the milling tool  16  has been inserted to the desired depth in the borehole, cutters  20 , represented diagrammatically as rectangles in  FIGS. 1 to 3  are expanded from the tool body as the tool is rotated. The cutters  20  cut outwards, into and through the tubing. The fully expanded cutters extend beyond the tubing into the surrounding cement  14 . 
     Once the cutters have been fully expanded, the rotating drillstring is advanced axially down the borehole, removing tubing and some of the surrounding cement. As the tool is advanced axially downhole, the cavity  22  above it has the diameter swept out by the extended cutters  20  and is larger than the outside diameter of the tubing which is being removed. 
     A milling tool embodying the concepts disclosed here may be implemented using a tool body and expansion system of a type currently used in some under-reamers. Such an embodiment is shown by  FIGS. 4-6 .  FIG. 4  shows the tool body and expandable cutter blocks with the blocks in retracted position.  FIG. 5  is a corresponding view with the blocks in expanded position. 
     This expandable tool comprises a generally cylindrical tool body  106  with a central flowbore  108  for drilling fluid. The tool body  106  includes upper  110  and lower  112  connection portions for connecting the tool into a drilling assembly. Intermediately between these connection portions  110 ,  112  there are three recesses  116  formed in the body  106  and spaced apart at 120° intervals azimuthally around the axis of the tool. 
     Each recess  116  accommodates a cutter block  122  in its retracted position. The three cutter blocks are identical in construction and dimensions. One such cutter block  122  is shown in perspective in  FIG. 4 . The block  122  is formed of a steel inner block part  124  with a projecting lug  125  along its outer surface and an outer block part  126  astride the lug  125  and bolted to the inner part  124  by bolts (not shown) inserted through the apertures  128  into threaded holes in the inner part  124 . Details of the outer part  126  are not shown in  FIGS. 4 and 5  and will be described in more detail below. The outer face  129  of the outer block part  126  is indicated without detail in  FIGS. 4 and 5 . 
     The inner block part  124  has side faces with protruding ribs  117  which extend at an angle to the tool axis. These ribs  117  engage in channels  118  at the sides of a recess  116  and this arrangement provides a pathway which constrains motion of each cutter block such that when the block  122  is pushed upwardly relative to the tool body  106 , it also moves radially outwardly to the position shown in  FIG. 5  in which the blocks  122  project outwardly from the tool body  106 . It will be appreciated that each cutter block is constrained by the ribs  117  in channels  118  to move bodily upwardly and outwardly without changing its orientation (i.e. without changing its angular position) relative to the tool axis. 
     A spring  136  biases the block  122  downwards to the retracted position seen in  FIG. 4 . The biasing spring  136  is disposed within a spring cavity  138  and covered by a spring retainer  140  which is locked in position by an upper cap  142 . In some embodiments, the spring may comprise a complaint/elastic material. A stop ring  144  is provided at the lower end of spring  136  to keep the spring in position. 
     Below the moveable blocks  122 , a drive ring  146  is provided that includes one or more nozzles  148 . An actuating piston  130  that forms a piston cavity  132  is attached to the drive ring  146 . The piston  130  is able to move axially within the tool. An inner mandrel  150  is the innermost component within the tool, and it slidingly engages a lower retainer  170  at  172 . The lower retainer  170  includes ports  174  that allow drilling fluid to flow from the flowbore  108  into the piston chamber  132  to actuate the piston  130 . 
     The piston  130  sealingly engages the inner mandrel  150  at  152 , and sealingly engages the body  106  at  134 . A lower cap  180  provides a stop for the downward axial movement of piston  130 . This cap  180  is threadedly connected to the body  106  and to the lower retainer  170  at  182 ,  184 , respectively. Sealing engagement is provided at  586  between the lower cap  180  and the body  106 . 
     A threaded connection is provided at  156  between the upper cap  142  and the inner mandrel  150  and at  158  between the upper cap  142  and body  106 . The upper cap  142  sealingly engages the body  106  at  160 , and sealingly engages the inner mandrel  150  at  162  and  164 . 
     In operation, drilling fluid flows downwards in flowbore  108  along path  190 , through ports  174  in the lower retainer  170  and along path  192  into the piston chamber  132 . The differential pressure between the fluid in the flowbore  108  and the fluid in the borehole annulus surrounding tool causes the piston  130  to move axially upwardly from the position shown in  FIG. 4  to the position shown in  FIG. 5 . A portion of the flow can pass through the piston chamber  132  and through nozzles  148  to the annulus as the cutter blocks start to expand. As the piston  130  moves axially upwardly, it urges the drive ring  146  axially upwardly against the blocks  122 . The drive ring pushes on all the blocks  122  simultaneously and moves them all axially upwardly in recesses  116  and also radially outwardly as the ribs  150  slide in the channels  118 . The blocks  122  are thus driven upwardly and outwardly in unison towards the expanded position shown in  FIG. 5 . 
     The movement of the blocks  122  is eventually limited by contact with the spring retainer  140 . When the spring  136  is fully compressed against the retainer  140 , it acts as a stop and the blocks can travel no further. There is provision for adjustment of the maximum travel of the blocks  122 . This adjustment is carried out at the surface before the tool is put into the borehole. The spring retainer  140  connects to the body  106  via a screwthread at  186 . A wrench slot  188  is provided between the upper cap  142  and the spring retainer  140 , which provides room for a wrench to be inserted to adjust the position of the screwthreaded spring retainer  140  in the body  106 . This allows the maximum expanded diameter of the reamer to be set at the surface. The upper cap  142  is also a screwthreaded component and it is used to lock the spring retainer  140  once it has been positioned. 
     The outer part  126  of each cutter block is a steel structure to which cutters with faces made of a hard material such as tungsten carbide are attached. Referring to the perspective view which is  FIG. 6 , this steel outer part  126  has a side face  200  which is the leading face in the direction of rotation. An area  204  is slanted back and cutters  211 - 216  are secured within this area  204 . The cutters may comprise a hard material: for instance they may be formed of sintered tungsten carbide. 
     These cutters may take the form of circular tiles made of sintered tungsten carbide (or other hard material) attached to the area  204  by brazing. Another possibility, illustrated by the cross-section which is  FIG. 8 , is that the cutters are cylindrical and are secured by brazing in cylindrical pockets formed in the outer block part  126 . 
     The outward facing surface of the outer block part  126  comprises a part-cylindrical outward facing surface  221  with a radius such that the surface  221  is centered on the tool axis when the cutter blocks are fully extended. The cutter  211  is positioned so that its radially outer extremity is at the same distance from the tool axis as the surface  221 . Thus, the radial extremity of the cutter  211  is aligned with the edge  218  of the surface  221 . There is also a part-cylindrical outward facing surface  222  which is further out from the tool axis and again is centered on the tool axis when the cutter blocks are fully extended. The extremity of cutter  212  is at the same distance from the tool axis as the surface  222 . This pattern of a cutter and a part-cylindrical outward facing surface where the surface and the radial extremity of the cutter are both at the same distance from the tool axis is repeated along the block by cutter  213  and surface  223 , cutter  214  and surface  224  and so on at progressively greater radial distances from the tool axis. Transitional surfaces  228  connecting adjacent surfaces  221  and  222 , similarly  222  and  223  and so on, have the same curvature as, and are aligned with, the curved edges of cutters  211 - 216 . 
       FIG. 7  shows the outer part  126  of a cutter block in use to remove tubing  250  within a borehole. There is cement  251  outside the tubing  250  between it and the surrounding rock formation. The tool has already been placed in the borehole and expanded from the tool body while rotating the tool so as to cut into and through the tubing  250 . An edge of the outer wall of the tool body  106 , exposed at the side of a recess  116 , is indicated  107  in  FIG. 7 . The tool is now advancing axially in the downward direction shown by arrow D. The tubing  250  may have some corrosion and deposited material on its inside surface as depicted schematically at  252 . In the fully expanded position of the cutter blocks, the leading cutters  211  on each cutter block are positioned to remove this material  252  and also remove some material from the inside wall of the tubing  250 , thus exposing a new inward facing surface  254 . 
     It should be appreciated that the expansion of the cutter blocks by the mechanism within the tool body proceeds as far as the drive mechanism in the tool body will allow. If necessary, the amount of expansion is limited by adjusting the screwthreaded spring retainer  140  in the body  106 , using a wrench in the wrench slot  188  while the tool as at the surface so that expansion goes no further than required. The adjustment of expansion is arranged such that when the cutter blocks are fully expanded, the surfaces  221  and the outer extremities of the leading cutters  211  are at a radial distance from the tool axis which is slightly greater than the inner radius of the tubing  250  but less than the outer radius of the tubing. The curvatures of the part-cylindrical outward facing surfaces  221  to  226  are such that each of them is centered on the tool axis when the cutter blocks have been expanded. 
     The new internal surface  254  is at a uniform radius which is the radial distance from the tool axis to the extremities of the leading cutters  211 . Because the part-cylindrical outward facing surfaces  221  of the three blocks have a curvature which is centered on the tool axis and at the same radial distance from the tool axis as the extremities of the leading cutters  211 , they are a close fit to this surface  254  created by the cutters  211 , as is shown in  FIG. 8 , and act as guide surfaces which slide over this new internal surface  254  as the tool rotates. The tool axis is thus positioned accurately, relative to the tubing  250 . This reduces vibration of the tool as it rotates and cuts compared with a conventional tool whose position in the tubing is less accurately controlled by components in the drill string which are undergauge (i.e. dimensioned to provide clearance between their exterior and the inside surface of the tubing). 
     As the tool advances axially, the cutters  212  which extend outwardly beyond the surfaces  221  remove the remainder of the tubing indicated at  256  outside the new surface  254  so that the full thickness of the tubing  250  has been removed. The cutters  213  to  216  cut through any cement or other material which was around the outside of the tubing. As shown, some cement outwardly from cutters  216  remains in place. If it is necessary to remove this and expose formation rock, an under-reamer to do this may be included further up the drill string. Alternatively the dimensions may be arranged such that the outermost cutters  216  contact and cut into the formation rock around the borehole. 
     Because the part-cylindrical surface  221  is centered on the tool axis when the cutter blocks are fully expanded, the tool is configured for removing tubing of a specific internal diameter. However, the tool can be used to remove tubing within a range of internal diameters by preparation at the surface, before it is put into a borehole. The tool is configured by fitting the cutter blocks with outer parts  124  dimensioned so that the radius of curvature of the surface  221  is the same as or slightly larger than the original (i.e. as manufactured) internal radius of the tubing to be removed. Also, at the surface, spring retainer  140  is adjusted, using a wrench in slot  188 , so that expansion of the tool is limited to the extent required, at which the cutter s  211  create the new internal surface on line  254  and the surfaces  221  are a close fit against this surface. 
       FIG. 9  shows an arrangement which is similar to that in  FIG. 7 , but with slightly different dimensions and a modification. The tubing  260  is thicker than the tubing  250  seen in  FIG. 7 . Consequently the cutter  212  removes some metal  256  outwardly from the surface  254  and creates another surface  258 . The surfaces  222  of the cutter blocks make sliding contact with this surface  258  and so the tool is positioned by the surfaces  221  and the surfaces  222  both acting as guide surfaces which make sliding contact with the surfaces  254  and  258  respectively. The remaining tubing  261  outwardly of the line  258  is removed by the cutter  262 . 
     Like the cutters  211 , and  212 , this cutter  262  has a surface which faces forwardly in the direction of rotation. However, it is not circular and has a straight cutting edge  264  extending perpendicular to the tool axis at which the remaining tubing material  261  is cut. This is a precaution to assist removal of the entire thickness of the remaining tubing  261 . It is possible that the tubing  261  which remains after cutting by the cutters  211  and  212  may be thin and easily deformable so that a following cutter with a curved edge, like the edges of the circular cutters  211  to  216 , may push the remaining tubing  261  outwardly rather than cutting it. The straight edge  264  avoids this possibility. A cutter such as cutter  262  may also be used on the cutter block shown in  FIG. 7 , in place of its cutter  212 . 
       FIG. 10  shows an arrangement which is similar to that in  FIG. 9 , but with the cutter  262  replaced by a cutter  362  in which the cutting edge includes two parts  372  and  373  which are inclined and meet at  374 . While the tool is being expanded, the radially outer part of the cutter  362 , which has a curved edge, cuts through the tubing  250  and into the surrounding cement. Thereafter, when the tool has been expanded and is being advanced in the direction of the arrow D, the part  373  of the cutting edge contacts and removes the remaining material  261  of the tubing  250 . Because this part  373  is inclined as shown in  FIG. 10  so that its radially outward end is axially forward from the point  374 , this part  373  of the cutting edge does not push the remaining tubing material  261  outwardly. On the contrary, it opposes outward displacement of the material  261  by guiding the material  261  towards the point  374  where the cutting edge parts  372  and  373  meet. 
     In the tools shown by  FIGS. 4 to 10  all three cutter blocks have outer block parts  126  which are identical to each other. However, they may have some differences and  FIGS. 11A to 11C  show a possible variation from the block shown in  FIG. 7 . The outer parts  126  of the three blocks have the same dimensions but differ in the cutters provided on them. On one outer block part, shown in  FIG. 11A , cutters in the form of square tiles  268  are positioned axially ahead of the cutters  212  and further tiles  268  are positioned between cutters  214  and  215 . The cutter  211  is not present on this block part. 
     On the outer block part shown in  FIG. 11B  there are similar tiles  268  between cutters  212  and  213 . On the third outer block part there are similar tiles between cutters  213  and  214  and between cutters  215  and  216 . Again the cutter  211  is not present on this block part. These cutting tiles  268  function to cut into and through the tubing during the stage schematically illustrated by  FIG. 2  in which the expanding cutter blocks are cutting outwards through the tubing. It is not necessary to provide these tiles  268  at identical positions on all three cutter blocks.  FIGS. 11A and 11C  also illustrates the possibility that the cutters  211 - 216  which remove tubing as the tool advances axially may not be configured to be identical on all cutter blocks. 
     Although the leading cutter  211  is absent from the blocks in  FIGS. 11A and 11C  the removal of some material from the tubing and the creation of the new surface on line  254  as shown in  FIG. 7  is carried out by the cutter  211  on the outer block part shown in  FIG. 11B . Although the cutters  268  are mentioned here is tiles, which may be attached to surface  204  by brazing, it is possible that cutters at these positions may be partially embedded in sockets in the outer cutter block parts  126 . 
       FIGS. 12 to 15  show tools implementing the same general concepts as the tool of  FIGS. 4 to 8 , but using cutters on arms which are expanded by a pivoting motion.  FIG. 13  is a cross-sectional elevation showing part of the tool to the right of chain dotted centre line CL-CL of  FIG. 12 .  FIG. 13  is a schematic cross section looking along the tool axis at the level of the arrow XIII in  FIG. 12 , with plunger head  280  omitted. As shown by  FIG. 13 , the tool has a cylindrical body with an outer wall  270 . Three slots are formed in this body at the same axial position and distributed azimuthally around the tool axis. 
     At either side of each slot there is a plate  272  extending inwardly from the wall  270 . An expandable arm  274 , formed of steel plate, is accommodated within each slot. As can be seen from  FIG. 12 , each arm  274  is pivoted to swing around a pin  276  supported by the plates  272  from a retracted position shown in  FIG. 12  to an expanded position shown in  FIG. 14 . Expansion is brought about by a hydraulic cylinder and piston, not shown, operated by pressure of drilling fluid and connected to drive plunger shaft  278 . Pressure of drilling fluid causes the plunger shaft  278  to move downwardly. A domed plunger head  280  on the end of shaft  278  acts of the inside edges of arms  274 , forcing the arms to pivot outwardly to the position shown in  FIG. 14 . Outward expansion is limited by projections  282  on the arms  274  when these projections abut the inside face of the tool wall  270 . 
       FIG. 14  shows the tool after it has been fully expanded and is being advanced axially downwards. A cutter  284  made of hard material on each arm  274  is removing corrosion and deposited material  252  on the inside surface of tubing  250  and is also removing some metal, creating a new surface on the line  286 . An edge surface  288  (indicated in  FIG. 12 ) on each arm  274  acts as a guide surface making sliding contact with the newly created surface on the line  286 . As before this close contact between the guide surfaces  288  and the newly created surface on the tubing gives good stabilization of the tool axis relative to the tubing. Metal  290  which is outside the line  286  is removed by square cutters  292  also made of hard material and positioned as tiles on the arm  274 . These cutters  292  also remove some of the cement  251  around the tubing  250 . 
       FIG. 15  shows another tool which has similarity to the tool of  FIGS. 12 to 14 , but has the cutters on two sets of expandable arms. Where the component parts are the same as in  FIGS. 12 to 14 , the same reference numerals are used. The three arms in each set are at the same axial position and distributed azimuthally around the tool body. Each arm  302  in the upper set is pivoted around a pin  304  and can be expanded outwardly by a plunger  306  which has a domed end and a cylindrical portion which is longer than in the plunger head  280  of  FIG. 12 . The arm  302  of the upper set carries a number of cutters made of hard material and arranged as an L-shaped array of square tiles with a part  316  extending along an axially forward facing edge of the arm and a part  318  extending along an outward facing edge of the arm  302 . Each arm  312  in the lower set is pivoted around a pin  314  and can be expanded outwardly by the plunger  306  when this descends sufficiently far. As shown in  FIG. 15 , the arm  312  has an edge  288  which provides a guide surface. Alongside this edge there are hard cutters  326  shaped as square tiles. The leading tile is indicated at  327 . 
     Thus, this tool has the guide surface  288  and the leading cutter  327  on the lower set of three expandable arms  312  and the further cutters  316  on the upper set of arms  302 . For use, the tool is initially inserted into tubing with all arms retracted. At the place where the removal of tubing is to begin, the tool is rotated and drilling fluid is pumped down to the tool. The plunger  306  begins to descend and expands the upper set of arms  302 .  FIG. 15  shows these arms partially expanded so that some of the cutters  318  are beginning to cut into the tubing. At the stage shown, the plunger  306  cannot descend beyond the arms  302  because they have not yet been fully expanded. Outward expansion is limited by projections  320  on the arms  302  when these projections abut the inside face of the tool wall  270 . 
     The lower arms  312  therefore remain retracted. When the arms  302  and the cutters  318  on them have cut through the tubing and out into the cement to reach their fully expanded position, the plunger  306  descends further and expands the arms  312 . The cutters  326  on the lower arms  312  cut out into the tubing  250  until the lower arms are fully expanded. Expansion is limited by the projection  322  on each arm  312  abutting the inside face of the tool wall  270 . 
     The tool is next advanced axially downwards and operates in a manner very similar to that illustrated by  FIG. 14 . The leading cutter  327  on each lower arm  302  removes corrosion and deposits  252  from the tubing and also removes some metal from the inside face of the tubing, thus creating a new surface. The guide surfaces  288  on the arms  312  make sliding contact with this newly created surface. Metal outside this surface is removed by the cutters  316  on the upper arms  302 . 
     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. 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.