Various tools have been developed for downhole cutting or severing of casing strings in wellbores, and for cutting or milling window sections in casing strings. Generally, such tools have comprised a main body with multiple hinged arms or blades, which are rotated outwardly into contact with the casing (by hydraulic or other means) when the tool is in position downhole. Usually, fluid is pumped down through the drillstring and through the tool to actuate the mechanism and rotate the blades outward. Once the blades are rotated outwardly, rotation of the drillstring (and tool) causes the cutting surfaces on the blades to cut through the casing string. Fluids are pumped through the system to lift the cuttings to the surface. Known tools, however, cannot efficiently cut or sever multiple, cemented-together casing strings, and in particular cannot efficiently cut "windows" in such strings; by the term "window" is meant the cutting or milling of a section (e.g. 60.1m [20 feet]) of the casing string, as opposed to simply severing same. In addition, known tools tend to form long, connected metal shavings which must be lifted from the wellbore by the fluid flow, else same become nested together downhole and potentially cause the drillstring to become stuck.

WO 2014/025763 A2 discloses an apparatus for cutting wellbore casing downhole comprising the features of the preamble of claim 1.

US 4 809 793 A discloses an enhanced diameter clean-out tool and method.

According to a first aspect of the present invention, there is provided an apparatus for cutting wellbore casing according to Claim 1.

According to a second aspect of the present inventio, there is provided a method of milling a casing string having a cement sheath on an interior wall thereof according to independent Claim 10.

• Fig. 1 is a side view of an exemplary tool embodying the principles of the present invention, particularly the cutter base/cutter combination, with the cutter bases in their retracted position.
• Fig. 2 is another side view of an exemplary tool embodying the principles of the present invention, corresponding to the tool in Fig. 1, showing the cutter bases in their extended position.
• Fig. 3 is a side view of the tool with cutter bases extended, and the tool in position to mill a section of casing.
• Figs. 4 - 7 are side views of the tool, similar to those shown in Figs. 1 - 3, with the tool in successive positions of opening.
• Fig. 8 is a side view of the tool in an open position, and in position in a casing string, and removing a cement sheath in the lowermost casing string.
• Fig. 9 is a side view in partial cross section of the tool, showing additional detail of the operating mechanism, cutter bases, linkage arms, and cutters.
• Fig. 10 is a view similar to Fig. 9, with the cutter bases in a partially open position.
• Fig. 11 is another view similar to Fig. 9, with the cutter bases in a fully open position.
• Fig. 12 is a cross section view of the operating mechanism of the tool, showing detail of the operating piston, fluid flow paths, and uppermost linkage arms.
• Fig. 13 is another cross section view of the operating mechanism of the tool, showing detail of the operating piston, fluid flow paths, and uppermost linkage arms.
• Fig. 14 is a cross section view, looking down the bore of the tool, showing additional detail regarding the flow path through the linkage arms, with the linkage arms in an open position.
• Fig. 15 is a cross section view, looking down the bore of the tool, showing additional detail regarding the flow path through the linkage arms, with the linkage arms in a closed position.
• Fig. 16 is a cross section view of an alternative embodiment of the positioning arm operating mechanism not forming part of the invention,

While a number of embodiments are possible, within the scope of the invention, with reference to the drawings some of the presently preferred embodiments can be described.

As shown in Fig. 1, the cutting tool 10 comprises a main body 20, typically having a means for connection to a tubular string, referred to herein as a drillstring 100, said means for connection preferably being a threaded connection 22 at the upper end of the tool. Preferably, a safety joint SJ 110 (shown) is installed above cutting tool 10, to provide a means for detachment from the tool should it get stuck. As is well known in the art, cutting tool 10 is run downhole into a tubular or casing string on a drillstring. Main body 20 has a bore 26 (which can be seen in Fig. 9) which runs through at least a portion of the length of main body 20, sufficiently far down to route fluid to the positioning arm area. By forcing fluid to exit the tool in the vicinity of positioning arms 50 and the recesses 28 in main body into which cutter bases 30 retract, fluid flow tends to keep these surfaces flushed and relatively free of cuttings and debris, described in more detail below.

As can be seen in the figures, especially Figs. 9 - 13, attached to main body 20 by a plurality of linkage or positioning arms 50 are cutter bases 30. In the embodiment shown in the drawings, cutting tool 10 has two cutter bases 30, but other numbers are possible within the scope of the invention. Positioning arms 50 are substantially of equal length, so it is understood that when cutter bases 30 are in an extended position as in Fig. 2, cutter bases 30 are substantially parallel to the longitudinal axis of main body 20. Positioning arms 50 are hingedly attached to both main body 20 and to cutter base 30. It is to be understood that the invention encompasses different numbers of positioning arms; generally, a minimum of two are required (one actuated arm and at least one additional arm), but a greater number may be used depending upon the particular tool dimensions.

Cutting tool 10 comprises a means for moving cutter bases 30 from a first, retracted position, generally within main body 20 and not protruding significantly therefrom, as shown in Figs. 1 and 4; to a second, extended position, wherein cutter bases 30 are partially or fully extended from the body, as seen in Fig. 2. This means for moving cutter bases may comprise an operating mechanism generally utilizing fluid pumped down the bore of the drillstring and main body 20 to actuate said operating mechanism. While not confining the current invention to any particular operating mechanism, one suitable mechanism is that disclosed in USP 7063155, owned by the assignee of this invention. Referring also to Figs. 9 - 13, generally, suitable operating mechanisms employ a piston 21 disposed in the bore of main body 20. The piston itself has a bore 21A of smaller diameter than the bore 26 in which it is disposed; therefore, fluid pumped down bore 26 of main body 20 forces the piston downward, pushing on a heel portion of an positioning arm 50 and causing it to rotate about a pin 52. It is understood that only one of positioning arms 50 per cutter base 30 need be actuated; generally the uppermost of positioning arms 50 on each cutter base 30 is actuated. For clarity, cutter bases 30 and some of the plurality of positioning arms 50 are omitted; the internal operating piston and a pair of operating arms 50 are shown, with heel portions 50A noted.

An alternative embodiment not forming part of the invention of the operating mechanism, shown in Fig. 16, comprises meshed gear teeth 200, 300 on piston 21 and positioning arms 50, respectively, in lieu of piston 21 bearing on heel portions 50A of positioning arms 50. As can be readily understood, movement of piston 21 causes positioning arms 50 to rotate around pins 52, causing cutter bases 30 to move inwardly and outwardly as previously described.

Referring to the drawings, cutter bases 30 comprise a plurality of cutters 40 mounted thereon (for space and clarity, not all of cutters 40 are so annotated). While various embodiments of cutters maybe used, one suitable embodiment uses a metal base or cutter plate which is attached to cutter base 30 by welding or similar means; on the cutter plate is attached a plurality of metal cutting surfaces, such as carbide buttons or inserts, or hardened buttons of other materials, or other means known in the art; alternatively the cutter plates may be covered with carbide or other suitable hardened surface, or a combination of hardened material buttons and carbide or similar materials. A variety of cutting surfaces are suitable, as long as they present a hardened surface to the upward-facing casing edge to permit milling of same. Further detail regarding acceptable cutting surfaces is set forth below.

As can be seen in Figs. 2 and 3, cutters 40 are preferably arranged in a plurality of vertically spaced apart rows along the length of cutter base 30. To facilitate milling in a downward direction, with conventional right-hand rotation of the drillstring, cutters 40 may be angled or inclined, wherein an upper end of cutters 40 is inclined in a direction of rotation of cutting tool 10. The number, position, and spacing of cutters 40 maybe varied to suit particular applications. With cutters 40 positioned in a plurality of vertically spaced apart, horizontally aligned rows, as shown in the figures, it can be appreciated that as milling progresses, and a row of cutters wears out, the diameter of the cutters decreases such that the next row of cutters above moves downward into contact with the casing surface. In this manner, a fresh cutting surface is presented to the casing edge being milled. It can be appreciated that the multiple rows of cutters permit the tool to remain in the hole for an extended period, thereby greatly reducing time spent in pulling and re-dressing the cutting tool tool. By way of example, each row of cutters may be approximately 2.54 cm (1") apart (vertically) from the adjacent row.

Generally, cutter bases 30 are sized so as to fit generally within the radius of main body 30 when retracted, as in Figs. 1, 4. The dimensions of positioning arms 50 and cutter bases 30 yield sufficient outward radius to position cutters 40 over the edge of casing 70 in order to mill same, as can be seen in Fig. 3. Dimensions of cutter base 30 are therefore dependent upon the size of casing 70 being milled, and upon the dimensions of main body 20 and positioning arms 50. Likewise, the dimensions of cutters 40 in a radially outward direction may be adjusted as necessary to suit particular jobs.

Fig. 3 shows cutting tool 10 in an operating position. A section of casing 70 is shown in which a window section 72 has already been milled. Cutter bases 30 are fully extended on positioning arms 50, so as to bring the outer surface of cutter bases 30 to or nearly to the inner wall of casing 70, and the lower, cutting surface of cutters 40 against the edge of casing 70. It is understood that, as well known in the art, Fig. 3 shows cutting tool 10 in a downhole position, run downhole on a drillstring (not shown), and being rotated in a conventional, right hand direction. Fluid is also being pumped through the drillstring and through cutting tool 10, and circulated back uphole.

With fluid circulation ongoing, thereby extending cutter bases 30 and cutters 40 to the position shown in Fig. 3, cutting tool 10 is lowered so that cutters 40 engage the upper surface of casing 70. The drillstring and cutting tool 10 are rotated while weight is applied to cutting tool 10, resulting in casing 70 being milled away. Milling continues as cutters 40 are gradually worn away, since as described above once a given row or set of cutters is sufficiently worn to move down inside the casing inner diameter, the next set of cutters moves into cutting position and cutting continues.

Yet another attribute of cutting tool 10 is the centering and stabilizing aspect of cutter bases 30 in conjunction with the positioning arms 50. Preferably, a section of cutter bases 30 has no cutters 40 mounted thereon, as noted in certain of the figures as stabilizing section 32. As is readily understood with reference to Fig. 3, when cutter bases 30 are in their extended position, placing them into or nearly into contact with the inner wall of casing 70, then main body 20 is centered within casing 70 and stabilized therein, and cutters 40 are properly positioned over the edge of casing 70 for optimum cutting. A second stabilizing section 32 may be provided at the upper end of each of cutter bases 30, in order to stabilize and centralize the tool while pulling it in an uphole direction.

Another preferred attribute of cutting tool 1 0 is that the dimensions of positioning arms 50 and cutter bases 30 are such as to enable cutter bases 30 to bear against and be supported by main body 20, when cutter bases 30 are in their second, extended position; this is shown at the location noted as 31 in Figs. 2 and 3. This attribute provides significant support to cutter bases 30, and consequently cutters 40, as weight is applied to cutting tool 10 during the cutting tooling process. A jetted sub, as seen in Fig. 5, maybe provided above cutting tool 10 to direct fluid flow in a desired direction and onto desired parts of the tool.

Figs. 1 - 8 show an attribute of the cutting tool which yields additional capability to its use. Cutting tool 10, and more specifically cutter bases 30, comprise a tapered nose section 34 at their lower end. The lower end of main body 20 is formed in a rounded point to roughly correspond to the shape of tapered nose section 34. A hardened cutting surface, represented by 34A as labeled in Fig. 1, is provided on tapered nose section 34. Hardened cutting surface 34A may be of one or more suitable materials, for example tungsten carbide, hardened steel, polycrystalline diamond compact buttons, etc., all as known in the relevant art.

Those skilled in the art will recognize that tapered nose section 34 yields significant added utility to cutting tool 10, as the tool is capable of cutting and/or milling through obstacles disposed below it in a wellbore. The particular shape and configuration of the nose section taper can be modified to suit particular needs. The type, location, and placement of hardened cutting surface 34A can likewise be modified to suit particular applications.

Fig. 8 illustrates one possible application or use of the tapered nose sections 34. In Fig. 8, cutting tool 10 is shown in a partially open state; that is, cutter bases 30 are expanded from their initial, retracted position, but are not at the maximum expansion due to cutter bases 30 contacting the inner wall of casing 70. A common situation is one wherein a cement sheath 74 is present on the inner wall of casing 70, typically as a result of the cementing of a smaller casing string which has already been removed from within casing 70. By positioning cutting tool 10 within casing 70 as in Fig. 8, and commencing fluid flow (to expand cutter bases 30 to the position shown) and lowering cutting tool 10, tapered nose section 34, along with hardened cutting surface 34A, can remove substantially all of cement sheath 74. This enables cutting tool 10 to be properly centered within casing 70, and for cutters 40 to cut/mill casing 70 as desired.

In a presently preferred embodiment, the heel portions 50A of positioning arms 50 are modified so as to direct fluid flow in a desired direction, depending upon the operating position of cutting tool 10. In Fig. 13, cutting tool 10 is in a first, retracted position, wherein cutter bases are retracted. Positioning arms 50 are rotated to the position shown in Fig. 13. Piston 21 is in an upper position, as fluid flow through bore 26 has not commenced. Piston 21 itself has a bore 21A, through which fluid flows. A seal element 21B (seen in Fig. 12) provides a seal so as to force piston 21 downward with fluid flow. Fluid flow through piston bore 21A may be split as can be seen in Fig. 13, with a portion flowing substantially straight down (the Directly Downhole Flow) and a portion being diverted through flow passages 26C (the Diverted Portion). As can be recognized, depending upon the size and shape of heel portions 50A of positioning arms 50, fluid flow in a directly downhole direction can be stopped or diminished. In a preferred embodiment, the shape of heel portion 50A is modified, whether in the initial manufacture or post-manufacture by grinding, etc., to remove the inner corner sections, to produce an angled surface, depicted as 50B in Fig. 14. As can be understood from Fig. 14, which shows positioning arms 50 in an expanded position (i.e. cutting tool 10 is open, as in Fig. 12), a fluid path substantially directly downhole and through positioning arms 50 is created, denoted by the circle 50C. This enables fluid circulation down through the lowermost end of cutting tool 10, which is particularly beneficial when cutting with the tapered nose sections 34. Fig. 15 shows the tool in a closed position (as in Fig. 13), with positioning arms 50 retracted.

Piston 21 is disposed in bore 26, so as to move longitudinally in the bore in response to fluid flow. While piston 21 is aligned in bore 26 to an extent by its shape, and by seal assembly 21B (which provides a fluid seal around piston 21 in bore 26), additional alignment is desired for the smaller diameter section 21D of piston 21. Earlier known designs utilized alignment blocks positioned within bore 26. The presently preferred embodiment of the present invention uses a pair of pins 21C (only one pin shown for clarity), inserted through holes 21E in main body 20. Pins 21C are positioned so as to closely constrain piston 21, and more specifically smaller diameter section 21D, from side to side movement. Pins 21C are more easily fitted, removed and replaced than are alignment blocks.

An exemplary method of use of cutting tool 10 with expandable cutter bases 30 can now be described. A main body 20, cutter bases 30, and positioning arms 50, with multiple cutters attached to each cutter base 30, are selected with dimensions appropriate for the size casing that is to be cut. A relatively short downhole window is first cut in the tubular in interest, with a two-arm casing cutter or conventional cutting tool, or with cutting tool 10 when configured for that task. As seen in Fig. 3, a window 72 of sufficient length that cutter bases 30 can fit therein is generally desired.

The next step is to locate cutting tool 10 within window 72. Although various methods are possible, one preferred method is to lower cutting tool 10 to a depth known to be slightly below window 72. Fluid circulation is then started, which will move cutter bases 30 (and cutters 40) outward, into contact with the casing wall. Cutting tool 10 is then pulled uphole, while cutters 40 are in contact with the casing wall. When cutting tool 10 is positioned within casing window 72 such that the lowermost cutters are above the casing edge, cutter bases 30 can fully extend and multiple indicators will be noted at the surface, including a decrease in drag, change in pump pressure, decrease in torque, etc. Now, the stabilizing section 32 of cutter bases 30 will be positioned against the wall of the casing, and cutters 40 will be positioned over the casing edge; this is the position seen in Figs. 3 and 5. Fluid circulation continues so as to maintain the proper positioning of the cutter bases and cutters. Rotation of the cutting tool 10 is commenced, and a desired amount of weight is applied to the cutting tool, to force the lowermost cutter edges against the upward-facing casing edge and consequently commence cutting or milling of the casing. It is to be understood that the sequence of steps set forth above is only one possible method of use; same may be changed as required, including but not limited to the sequence or order of the different operations, additional steps maybe added, steps maybe omitted, insofar these changes fall within the scope of claim 10.

While the preceding description contains many specificities, it is to be understood that same are presented only to describe some of the presently preferred embodiments of the invention, and not by way of limitation. Changes can be made to various aspects of the invention, without departing from the scope thereof, as defined by the appended claims. For example, dimensions of the various components of the tool can be varied to suit particular jobs; the number of cutter bases can be varied; the number and positioning of cutters per cutter base can be varied; size and shape of the cutters can vary; and methods of use can ve varied.

Therefore, the scope of the invention is to be determined not by the illustrative examples set forth above, but by the appended claims.