DOWNHOLE CUTTING TOOL
A rotary tool for milling tubing in a borehole comprises at least one cutter with a cutter body and a cutting surface on the body. Each cutter is shaped and positioned on the tool so as to reduce tensile stress in the cutter, thereby reducing risk of the cutter becoming chipped or broken in use and produces swarf of reduced rigidity, less likely to form a blockage in the borehole.
The present document is based on and claims priority to GB Non-Provisional Application Serial No.: 1513927.2, filed Aug. 6, 2015, which is incorporated herein by reference in its entirety.
BACKGROUNDOil and gas wells are usually lined with steel tubing which is cemented in place and forms a casing. Other steel tubing may be located inside the casing. Some operations carried out within a well require the removal of a length of steel tubing which has been secured within the borehole. This is customarily carried out using a tool referred to as a mill which is used to mill out a length of tubing at a subterranean position which may be some distance from the surface. The mill cuts into the tubing and comminutes it to swarf.
Various types of mill are used in boreholes. A mill for removing a length of tubing is commonly referred to as a section mill. It has the characteristic that it cuts away tubing as it moves along the tubing. Parts of the tool, or parts of the tool string which incorporates the tool, may extend into the tubing which has not yet been removed and thereby guide the tool to progress axially along the tubing as it is advanced. This functionality contrasts with a window mill whose function is to cut outwardly through tubing and start a new borehole branching from an existing borehole.
A section mill which is able to mill out a length of tubing may have a rotatable body with one or more projecting or expandable parts which may be referred to by various names including blades and knives. These projecting or expandable parts carry cutters of hard material, often tungsten carbide, which cut into the tubing. The cut swarf is customarily entrained in the circulating flow of drilling fluid which carries it to the surface. However, pieces of swarf can becoming entangled within the borehole and form a blockage, sometimes referred to as a “bird's nest”, which can necessitate time-consuming interruption of the milling operation and removal of tools from the borehole in order to clear the blockage.
Another problem which can arise is damage to the cutters fitted to the tool. Wear of cutters during use of the milling tool is normal but it is possible for cutters to become chipped or broken which reduces the efficiency and working life of the tool.
When a hard cutting tool acts on a workplace, the cutting surface may be held perpendicular to the direction of traverse of the tool relative to the workpiece or at an angle to the perpendicular. This angle to the perpendicular is referred to as a rake angle. A rake angle may be referred to as forward or back, positive or negative, and the literature is not consistent in use of this terminology. In the present disclosure, when the edge of the cutting surface which is in contact with the workpiece is trailing behind the remainder of the cutting surface, the cutter is said to have a back rake also sometimes referred to as a positive rake.
SUMMARYThis summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to be used as an aid in limiting the scope of the claimed subject matter.
In a first aspect of the present disclosure, a rotary tool for milling tubing in a borehole comprises at least one cutter with a cutter body and a cutting surface on the body, where the cutter is shaped and positioned 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 with an edge where the cutting surface cuts furthest into the tubing being a trailing edge of the cutting surface relative to the direction of rotation,
at least part of the back raked cutting surface extends from the said edge with a back rake angle which is from 15 to 60 degrees and at the said edge has an angle greater than 90 degrees included between the cutting surface and the surface of the cutter body following the cutting surface.
Because the rake angle between the cutting surface or part of the cutting surface and a perpendicular to the direction of traverse relative to the workpiece (i.e. direction of rotation relative to the tubing) lies in a range from 15 to 60 degrees, the angle between the cutting surface or part thereof and the direction of rotation lies in a range from 30 to 75 degrees.
We have found that a cutting surface with a significant 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.
The angle of the back rake may be such that the rake angle between the cutting surface or part thereof and a perpendicular to the direction of motion relative to the tubing is in a range from 20 or 25 degrees to 45 or 50 degrees. In this case the angle between the cutting surface and the direction of rotation will lie in a range from 40 or 45 degrees to 65 or 70 degrees. The surface of the cutter body trailing back from the edge of the cutting surface may be aligned with the direction of rotation or may be at a small angle to the direction of rotation. Thus it may be at an angle of 0 to 10 or 15 degrees to the direction of rotation. In this case, the angle included between this surface of the cutter body and the cutting surface will be at least 95 degrees.
The cutter body may be dimensioned such that the at least part of the back raked cutting surface extends at least 1 mm 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 3 mm or at least 5 mm back from the said edge.
An individual cutter body may be formed from a hard material other than diamond. The hardness may be defined as a hardness of 1800 or more on the Knoop scale or a hardness of 9 or more on the original Mohs scale (where diamond has a Mohs hardness of 10).
The rotary tool may comprise one or more supporting structures, each having a plurality of cutters partially embedded in one of the supporting structures. A cutter body may have a front which is exposed and a thickness extending into the support behind the exposed front. The thickness dimension may be at least half, or at least three-quarters of any distance across the cutter body transverse to the thickness. The portion of the body which is embedded in the support structure may be greater than the volume of any portion projecting forwardly from the support structure. The cutting surface may extend as a bevel between side and front faces, at an angle (other than a right angle) to both and of course such that the rake angle between the cutting surface or part thereof and a perpendicular to the direction of motion relative to the tubing is in the range from 15 to 60 degrees.
Such a supporting structure may be an element which projects or is extensible outwardly from a central structure of the tool. The rotary tool may be constructed with a central structure for insertion axially into the tubing, and at least one element which carries at least one said cutter and which projects outwardly from the central structure to bring the at least one cutter into contact with the tubing. There may be a plurality of such elements which are distributed azimuthally around a longitudinal axis of the tool.
In some embodiments the at least one element is expandable outwardly, by operation of a mechanism within the tool structure. This can allow the tool to be inserted to a desired depth into a borehole and then expanded to begin cutting into tubing.
In another aspect of the present disclosure, a method of removing a length of tubing in a borehole comprises inserting into the tubing a rotary milling tool which comprises a structure extending axially and at least one element which projects or is extensible from the tool structure and carries at least one cutter as set out above, and then advancing the tool axially while rotating the tool with the at least one cutter cutting into the tubing completely around the tubing. The cutter may cut into a sidewall of the tubing or into an end face created by an initial cut through the tubing.
The cutter body has a flat front face 20 and a side surface 22 which extends backwards relative to the direction of rotation, connected by bevelled face 24 which constitutes the cutting surface. The edge 26 of the cutting surface, cutting most deeply into the tubing 12, is trailing relative to the parts of the cutting surface 24 which are not in contact with tubing 12. The cutting surface 24 is thus positioned with a back rake angle. The rake angle between the cutting surface 24 and the radius 16 is indicated 34. The angle between the cutting surface 24 and the direction of rotation 14 is indicated 32.
The side surface 22 of the cutter body 10, extending back from the edge 26 of the cutting surface 24, is inclined so as to diverge from the newly-cut surface at a small angle 36 to the direction of rotation 16, so that the parts of the cutter body 10 behind the cutting surface 24 do not contact the freshly cut surface 28 on the tubing 12. The overall included angle between the cutting surface 24 and the side surface 22 extending back from the edge 26 is the sum of the angles 34 and 38.
In accordance with the concepts set out above, the angle between the cutting surface 24 and the radius 16 lies in a range from 15 to 60 degrees. The sum of the angles 32 and 34 is a right angle, because the direction of rotation 14 and the radius 16 are perpendicular. Thus the angle 32 lies in a range from 30 to 75 degrees. In the embodiment illustrated the rake angle 34 lies in a narrower range from 20 to 50 degrees and may be approximately 30 degrees.
The behaviour of a cutter made of hard material cutting into steel, with variations in rake angle, was investigated by finite element analysis, which is a computational modelling procedure, assuming a constant depth of cut of 0.25 mm and a constant speed of traverse of the tool relative to the steel workpiece of 1 metre/sec. The analysis ran for a period of 1 millisecond corresponding to a distance traversed of 1 mm which is four times the depth of cut. This analysis predicted that as the rake angle of the tool is increased (and consequently the angle between the cutting surface and the direction of traverse decreases) the force required to drive the tool increases and moreover with angles of 15 degrees or more the force oscillates between upper and lower values. The values predicted by this procedure were:
This analysis also provided visual maps of the workpiece showing internal distribution of the equivalent stress in the direction of traverse. Such maps are shown by
The interpretation of the oscillation in applied force, tabulated above, and the finite element analysis shown by
The predictions of finite element analysis shown by
Finite element analysis was also applied to stress within the cutters, as shown by
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. Manufacturers of sintered tungsten carbide cutters include Cutting and Wear Resistant Developments Ltd, Sheffield, England and Hallamshire Hard Metal Products Ltd, Rotherham, England.
Tungsten carbide 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 (which is other than diamond) may have a hardness of 1800 or more on the Knoop scale or a hardness of 9 or more on the original Mohs scale (where diamond has a Mohs hardness of 10).
Each arm 82 has cutters 86, 87 of the type shown by
For use the section mill is included in a drill string and lowered to the point within the borehole tubing 72 where milling is to begin. The drill string is then rotated and the plunger head 91 is driven downwards forcing the arms 82 outwards towards the position shown by
As the cutters 86 on an arm 82 cut into the tubing 72, their cutting surfaces are at an angle of 30 degrees to the plane of the arm 82. This arm extends axially and the axial direction is perpendicular to the rotational direction of the tool and to the end face of the tubing 72 which is being cut. The cutters therefore are at a back rake of 30 degrees as they cut into the tubing 72. Previously, as the cutters 87 were cutting into the inside face of the tubing 72, they also were at a back rake relative to the inside surface of the tubing, although the back rake angle relative to this surface will vary as the arms swing around their pivots 83.
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 degrees intervals azimuthally around the axis of the tool.
Each recess 116 accommodates a cutter block 122 in its retracted position. The three cutter blocks may be identical in construction and dimensions. One such cutter block 122 is shown in perspective in
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 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 towards the position shown in
A spring 136 biases the block 122 downwards to the retracted position seen in
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 186 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 order to expand the blocks 122, drilling fluid is directed to flow downwards in flowbore 108. It flows 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
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 with side face 200 which is the leading which is the leading face in the direction of rotation. An area 204 of this face is slanted back. This steel outer part 126 incorporates cylindrical pockets which receive the cylindrical bodies of cutters of the type shown in
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 centred on the tool axis when the cutter blocks are fully extended. The cutter 211 is positioned so that its radially outer edge is at the same distance from the tool axis as the surface 221. There is also a part-cylindrical outward facing surface 222 which is further out from the tool axis and again is centred on the tool axis when the cutter blocks are fully extended. The edge 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 edge 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 227 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.
For use as a section mill, the tool is attached to a drill string and lowered into the borehole tubing 68 to the required depth. The drill string is then rotated and the tool is expanded by pumping fluid into flowbore 108 as described above. The radially outer edge of cutter 216 contacts the interior face of the tubing 68 and cuts into it. This allows expansion to continue and the cutters 215 to 211 contact the inside face of the tubing in sequence, cutting into and through the tubing until the fully expanded position of the blocks is reached. The tool is then advanced axially. This is illustrated by
The amount of expansion of the tool 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. 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 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 centred 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
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.
Because the part-cylindrical surface 221 is centred 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 cutters 211 create the new internal surface on line 254 and the surfaces 221 are a close fit against this surface.
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 in particular back raked cutting surfaces may be larger or smaller than shown in the drawings. 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 tool for milling tubing in a borehole, the rotary tool comprising:
- at least one cutter with a cutter body and a cutting surface on the body,
- wherein the cutter is shaped and positioned on the rotary tool such that at least part of the cutting surface is back raked relative to a direction of rotation so that the cutting surface cuts furthest into the tubing 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 edge with a rake angle between the cutting surface and a perpendicular to the direction of rotation which is in a range from 15 to 60 degrees and, at the edge of the cutting surface, has an angle greater than 90 degrees included between the cutting surface and the surface of the cutter body following the cutting surface.
2. The rotary tool of claim 1 wherein the at least part of the back raked cutting surface extends from the edge with a rake angle which is in a range from 20 to 50 degrees.
3. The rotary tool of claim 1 wherein, at the edge, the surface of the cutter body following the at least part of the back raked cutting surface diverges from the cutting surface at an angle of between 0 and 15 degrees to the direction of rotation.
4. The rotary tool of claim 1 wherein the at least part of the back raked cutting surface extends from the edge with a rake angle which is in a range from 20 to 60 degrees and has an angle of at least 100 degrees included between the at least part of the back raked cutting surface and the surface of the cutter body following the cutting surface.
5. The rotary tool of claim 1 wherein the at least one cutter comprises a cutter body of a hard material.
6. The rotary tool of claim 5 wherein the hard material has a Knoop hardness of 1800 or more.
7. The rotary tool of claim 1, further comprising:
- a central structure for insertion axially into the tubing, and
- at least one element which carries the at least one cutter and which projects or is extensible from the central structure to bring the at least one cutter into contact with the tubing.
8. The rotary tool of claim 7 wherein the at least one element is configured to bring the at least one cutter into contact with an internal surface of the tubing to cut radially outwardly into the tubing.
9. The rotary tool of claim 7 wherein the at least one element has the at least one cutter with the cutter body partially embedded therein and partially exposed, such that the embedded portion of the cutter body is of greater volume than the exposed portion.
10. The rotary tool of claim 9 wherein the at least one cutter has an exposed front and a partially embedded thickness following the exposed front with an extent which is at least half the length of any dimension across the cutter body, perpendicular to the partially embedded thickness.
11. The rotary tool of claim 7 wherein the rotary tool has a plurality of elements which each carry at least one cutter formed of hard material, which project or are extensible from the tool body and which are distributed azimuthally around a longitudinal axis of the rotary tool.
12. A method of removing a length of tubing in a borehole, the method comprising:
- inserting into the tubing a rotary tool for milling tubing in a borehole, the rotary tool comprising at least one cutter as defined in claim 1, and
- advancing the rotary tool axially while rotating the rotary tool with the at least one cutter cutting into the tubing completely around the tubing.
13. The method of claim 12 wherein the at least one cutter is carried by at least one element configured to bring the at least one cutter into contact with an internal surface of the tubing, whereby the at least one cutter cuts radially outwardly into the internal surface completely around the tubing.
Type: Application
Filed: Jul 15, 2016
Publication Date: Aug 16, 2018
Inventors: Ashley Bernard JOHNSON (Cambridge), Francesco BATTOCCHIO (Cambridge)
Application Number: 15/750,510