The present invention relates to apparatus for milling tubulars in a wellbore. In particular, aspects of the invention relate to a cutter base for a downhole tubular milling apparatus and a removable functional cassette for a cutter base.

Background to the invention

Downhole milling is the process of removing material from casing in a wellbore. Milling can be performed, for example, in well abandonment operations or to create an opening from which to initiate side-track drilling.

Section milling is the term given to removing an entire section of casing, without disturbing the casing string above. A typical section milling operation consists of cutting out a relatively short entry window in the casing, followed by milling a section of the casing to form a longer casing window in what is known as a mill-ahead operation.

Dedicated tools can be used to form each of the entry window and the longer casing window, or the same tool, configured appropriately, can used for both.

An example of a milling tool that can be configured for both cutting out entry windows and performing mill-ahead operations is shown in Figure 1. The tool, generally shown at 10, has a main body 12 which can be coupled to a workstring, and a pair of cutter bases 14a and 14b attached to the main body 12 by hinged operating arms 16. In Figure 1 , the tool is configured for a mill-ahead operation. The cutter bases 14a and 14b support four banks of mill-ahead blades 18a, 18b, 18c and 18d welded to the outer surface of the cutter bases 14a and 14b. The cutter base 14a is formed with three longitudinally-oriented surfaces 20a, 20b and 20c, which are the outward facing surfaces of the cutter base configured to bear against the inner surface of the tubular being cut or milled, in use.

The milling tool can be used for downward or upward milling. In a downward milling operation, the milling tool 10 is run into a wellbore and positioned such that the milling blades 18a, 18b, 18c and 18d extend into a previously cut entry window in the casing (not shown), with the lowermost row of blades 18a contacting an upward facing cut surface of the casing. The blades extend radially outward into the entry window a sufficient dimension beyond the outermost surface of the cutter bases 14a and 14b to remove material from the casing but to avoid damaging an outer tubular surrounding the casing (if applicable). During milling, stabilising surfaces of the cutter bases 14a and 14b bear against an inner surface of the casing, below the window.

A typical downward mill-ahead operation consists of a first phase in which the tool 10 is rotated whilst being moved in a downhole direction, with the lowermost bank of blades 18a acting on the casing to mill a section of the casing away. Due to the abrasive nature of the mill-ahead operation, the lowermost bank of blades 18a becomes worn. The tool 10 is pulled out of hole and the area of the lowermost bank 18a is redressed, to make it smooth.

In a second phase, the tool 10 is run into the wellbore again. This time, the tool is positioned such that the next bank of blades 18b contacts the upward facing surface of the casing to continue the milling operation. When the second bank 18b becomes worn, the tool can again be pulled out of hole and redressed. The process can be repeated for multiple phases of milling. Throughout the milling operation, the stabilising surfaces of the cutter bases are subject to wear.

Instead of merely redressing the tool to smooth out the worn banks of blades, new blades can be added to the cutter bases 14a and 14b. This involves applying heat to remove the remnants of spent blade and weld material and welding new blades in their place: again, requiring the application of heat. Working on the tool in this way subjects the cutter bases to heat cycling which can have a notable impact on the life span of the cutter bases and/or the tool and which can ultimately result in premature failure.

As noted above, cut-out blades can be mounted on the cutter bases of a milling tool to configure the tool to cut-out the entry window in the casing. Cut-out blades are typically only provided in a single bank and require removal or replacement after a cut-out operation.

The methods describe above involve time-consuming and costly maintenance and repair operations, which increase the overall time taken to complete milling. Heat cycles impact on the operating life of the cutter bases. Operational requirements may require the delivery and storage of multiple milling tools and/or cutter bases on site for a milling operation (or a series or milling operations). For offshore oil and gas wells, where transport and storage space is at a premium, this can be logistically challenging and costly. The provision of on site maintenance facilities and resources can also be challenging.

The applicant’s international patent application number PCT/US2013/053770 discloses a milling tool similar to that which is described above.

Summary of the invention

The terms “upper”, “lower”, “above”, “below”, “up” and “down” are used herein to indicate relative positions in a wellbore and the terms “upward”, “downward”, “uphole” and “downhole” are used herein to indicate directions of movement with respect to a wellbore.

The terms “milling tool” or “milling apparatus” are used herein to indicate an apparatus operable to perform a downhole mill-ahead operation and/or a cut-out operation.

It is amongst the aims and objects of the invention to provide apparatus for milling tubulars which obviates or mitigates one or more drawbacks associated with the prior art.

In particular, one aim of an aspect of the invention is to provide apparatus for milling tubulars which is flexible and adaptable for use in different kinds of milling operations.

A further aim of an aspect of the invention is to provide apparatus for milling tubulars which can be restored for use after wear and/or failure and/or undergo a change of use quickly and efficiently, with minimal need for machining and/or the application of heat (and associated processes).

A further aim of an aspect of the invention is to provide apparatus for milling tubulars which has an improved structure for reducing uneven wear and/or promoting uniform wear, in use.

Other aims and objects of the invention will become apparent from the following description.

According to a first aspect of the invention there is provided a cutter base for a downhole tubular milling apparatus, the cutter base comprising: at least one slot for receiving a removable functional cassette.

The tubular may be casing.

The removable functional cassette may be configured to perform one or more functions selected from the group comprising: providing stabilisation, downhole tubular cutting (cut out operations) and/or downhole tubular milling (mill ahead operations including mill up and mill down operations).

As the cutter base is capable of receiving a removable functional cassette, the function of the downhole tubular milling apparatus can be easily adapted. Different operations - including cutting and milling in uphole and downhole directions and/or stabilisation - can be performed using a single tool by simply removing and replacing the functional cassette, as necessary. This reduces the effort and time taken to perform multiple operations and minimises equipment requirements. This can also reduce project inventory requirements as a single tool for use with various cassettes (or a combination of cassettes) can facilitate a number of milling or cutting operations having different specifications and requirements, replacing the need for multiple separate tools.

Because a functional cassette can be removed from the cutter base and replaced with a another (same or different) functional cassette with minimal disruption or interference, the need to machine and apply heat to a tool or a cutter base for a tool to redress blade wear is minimised, or avoided entirely. The functional cassette may be relatively much smaller in size than the cutter base and/or the downhole tubular milling apparatus as a whole. As such, where there is a desire to repair or work on the cutters or any other part of a functional cassette, this can be done when the cassette is detached from the cutter base and/or the downhole tubular milling apparatus. Working on a single functional cassette instead of an entire cutter base and/or the milling apparatus is easier because the cassette may be lighter, smaller and easier to manoeuvre. In addition, by working on a single functional cassette away from the entire cutter base and/or the milling apparatus the risk of damaging, degrading or impairing the cutter base and/or the milling apparatus by machining or applying heat is avoided.

The removable functional cassette may comprise one or more blades or cutters. The removability and replaceability of the functional cassette means that the precise type, geometry, size, number and arrangement of the cutters can be provided to match specific operational requirements.

The removable functional cassette may comprise one or more milling cutters for a downhole tubular milling operation.

The removable functional cassette may comprise one or more cutting cutters for a downhole tubular cutting operation.

The removable functional cassette may comprise one or more wear pads. The removable function cassette may function as an extension of a surface of the cutter base and may absorb and/or share the load that the cutter base would be subjected to, in use. This advantageously may reduce the wear on one or more parts of the cutter base and/or the downhole tubular milling apparatus as a whole. In addition, wear may be spread more uniformly around the cutter base and/or the downhole tubular milling apparatus as a whole, minimising point loading, single points of failure, the risk of creating snagging surfaces and/or the risk of the milling apparatus becoming stuck in a wellbore.

The at least one slot is preferably configured to receive a removable functional cassette of complimentary shape to the at least one slot.

The at least one slot may comprise a single opening. The at least one slot may be configured to receive the removable functional cassette relative movement in a transverse direction with respect to a longitudinal direction of the cutter base. The at least one slot may be configured to receive the removable functional cassette when it is slid into the slot in a direction which is tangential to a substantially circular cross section defined by the apparatus for milling tubulars into which the cutter base is to be incorporated.

The at least one slot may comprise one or more recesses configured to receive and/or engage with one or projections on the removable functional cassette, or vice versa. The cutter base and/or the at least one slot may comprise one or more sections of material arranged to overlap the removable cassette when it has been received in the at least one slot to retain or to help to retain the removable cassette in place. The removable functional cassette is preferably configured to be positively secured to the cutter base in the at least one slot by a fixing means. The fixing means may comprise a bolt, a screw, a pin, a flange, adhesive, or any other suitable fixing means. The removable functional cassette may be configured to be bolted, screwed and/or pinned to the cutter base in the at least one slot.

Some or all removable functional cassettes may be intended for single use.

The cutter base may comprise a plurality of slots for receiving a plurality of removable functional cassettes.

The plurality of removable functional cassettes may be configured to perform one or more functions, or a combination of functions, selected from the group comprising: providing stabilisation, downhole tubular cutting (cut-out operations) and/or downhole tubular milling (mill ahead operations including mill up and mill down operations).

The cutter base may define an outer profile configured to face outwardly and contact an inner surface of a tubular being cut or milled by a tool comprising the cutter base, in use. The outer profile may comprise one or more surfaces. The one or more surfaces may be substantially planar. The one or more surfaces may be angled with respect to one another to approximate an outwardly curved surface which may share an axis with the tubular milling apparatus and/or which may be configured to substantially match or compliment an inner tubular surface of the tubular to be milled. Preferably, the outer profile comprises a number of surfaces greater than three. The outer profile may comprise eight surfaces.

Forming the outer profile of the cutter base from a greater number of surfaces produces better approximation of a curve. This is beneficial because the outer profile is configured to contact a curved inner surface of the tubular being cut or milled during use. Therefore, the closer the outer profile is to being curved, the more evenly wear resulting from rotation of the tool (and thus the cutter base outer profile) within and with respect to, the tubular being cut or milled will be spread over the outer profile of the cutter base. It is advantageous to provide a cutter base having an outer profile formed from planar surfaces, instead of merely providing a curved surface, because it is simpler, quicker and more cost effective to machine this type of cutter base. Although it may be costlier to form a cutter base having an outer profile formed from a greater number of planar surfaces thank a lesser amount, by doing so can reduce wear and therefore increase the life-span of the cutter base and the milling apparatus into which it is incorporated.

Different sections of the cutter base may define different outer profiles in a longitudinal direction of the cutter base.

According to a second aspect of the invention there is provided a removable functional cassette for a cutter base of a downhole tubular milling apparatus, the removable functional cassette comprising: a body configured to be received in a slot located in or on the cutter base; wherein the removable functional cassette is configured to provide the downhole tubular milling apparatus with the means to perform one or more functions selected from the group comprising: stabilisation, downhole tubular cutting and/or downhole tubular milling.

Embodiment of the second aspect of the invention may include one or more features of the first aspect of the invention, or its embodiments, or vice versa

According to a third aspect of the invention there is provided a downhole tubular milling apparatus comprising a cutter base according to the first aspect of the invention.

The apparatus may be configured to be coupled to a work string, including but not limited to a drill string and/or coiled tubing.

Embodiment of the third aspect of the invention may include one or more features of the first or second aspects of the invention, or their embodiments, or vice versa

According to a fourth aspect of the invention there is provided a downhole tubular milling apparatus comprising a cutter base according to the first aspect of the invention and at least one removable functional cassette according to the second aspect of the invention.

Embodiment of the fourth aspect of the invention may include one or more features of the first to third aspects of the invention, or their embodiments, or vice versa Brief Description of Drawings There will now be described, by way of example only, an embodiment of the invention with reference to the following drawings, of which: Figure 1 is a perspective view of a prior art milling tool; Figure 2 is a side view of a cutter base according to an embodiment of the invention; Figures 3A and 3B are perspective views of the cutter base of Figure 2, provided with functional cassettes according to embodiments of the invention; Figure 4 is a side view of a cutter base according to an embodiment of the invention; Figure 5 is a perspective view of the cutter base of Figure 4, provided with functional cassettes according to embodiments of the invention; Figures 6 is a perspective view of a milling cassette according to an embodiment of the invention; Figures 7 is a perspective view of a cutting cassette according to an embodiment of the invention; Figures 8 is a perspective view of a stabiliser cassette according to an embodiment of the invention; and Figures 9A, 9B and 9c are side, perspective and sectional views respectively of a cutter base according to an embodiment of the invention. Detailed Description Figure 1 shows a prior art milling tool 10 having a pair of cutter bases 14a and 14b with four rows of milling cutters 18a, 18b, 18c and 18d welded to their outer surfaces. The cutter bases 14a and 14b are parallel to the main body 12 of the tool 10 in retracted and extended positions, and during extension. The cutter bases 14a or 14b may require attention during a milling operation as the cutters become worn. The cutter bases 14a and 14b of the tool 10, or the tool itself, may need to be replaced during a milling operation, increasing the time taken and the resources required to complete the milling operation.

Figure 2 shows a cutter base 114 for a milling tool according to an embodiment of the invention. The cutter base 114 is configured to be connected to a body of the milling tool by operating arms (not shown) which are configured to move the cutter base 114 (and an additional cutter base or bases) between retracted and extended conditions on operation of an actuating mechanism. In the retracted condition the cutter bases will be positioned close to the body of the milling tool for running into and pulling out of a wellbore and in the extended condition the cutter bases will be positioned away from the body, for milling. In both the retracted and extended conditions, the cutter base 114 is maintained parallel to a longitudinal axis of the milling tool body (not shown) by the operating arms (not shown). The cutter base 114 comprises a portion 119 which is devoid of cutters and which is provided with a plurality of carbide wear buttons 122. This portion 119 is herein referred to as the stabilising section and is intended to bear against an inner surface of the tubular being milled, to stabilise the milling tool inside the tubular and ensure that it remains central within the tubular throughout operation.

Unlike the prior art arrangements, the cutter base 114 does not have blades (interchangeably referred to as cutters) welded to it. Instead, the cutter base 114 comprises four slots 124a, 124b, 124c and 124d, each for receiving a functional cassette (not shown). The slots 124a, 124b, 124c and 124d have a dove-tail cross sectional shape and are configured to accept functional cassettes (not shown) having complimentary dovetail profiles for snugly fitting inside the slots 124a, 124b, 124c and 124d.

The functional cassettes (not shown) can support the necessary cutters to allow a milling tool comprising the cutter base 114 to carry out a cutting operation or a milling operation. By providing a cassette carrying the necessary cutters for a project in the required arrangement, the same tool can be used to perform different types of cutting operations and different types of milling operations. During a cutting or a milling operation, a cassette can be removed and replaced if the cutters become worn, without having to machine or perform repair work directly upon the tool. Figures 3A and 3B show the cutter base of Figure 2 114 with functional cassettes inserted into each of its four slots 124a, 124b, 124c and 124d. Although not shown, there is a positive bolted connection between the cutter base 114 and the functional cassettes. In Figure 3A, each of the functional cassettes are milling cassettes 125a, 125b, 125c, 125d, each comprising three mill-ahead cutters configured for use in a downward milling operation (in the direction indicated by the arrow A). In this way, the cutters are arranged in rows. When the lowermost row of cutters of cassette 125a become worn during milling, there is no need to perform any restoration work upon the cutter base 114. Instead, the tool can be pulled out of hole and the functional cassette can be removed and replaced. Figure 3B shows the same cutter base 114 following replacement of the milling cassette 125a with a stabiliser cassette 127. The stabiliser cassette 127 is configured to absorb impact and wear during the milling operation and to provide a surface to bear against the inner surface of the tubular milled. By simply replacing the functional cassette, the milling operation can continue using the next row of cutters on cassette 125b, without the need machine the tool or the cutter base or to replace the milling tool entirely.

Figure 4 shows a cutter base 214 for a milling tool according to an alternative embodiment of the invention. The cutter base 214 is similar to the cutter base 114 of Figures 2, 3A and 3B with like features indicated by like reference numerals incremented by 100. The cutter base 214 differs from the cutter base 114 in that it comprises two slots 224a, and 224d, instead of four.

Figure 5 shows the cutter base of Figure 4214 with functional cassettes inserted into each of its two slots 224a and 224d. A cutting cassette 225d comprising three cutting blades is provided in the uppermost slot 224d and a stabilising cassette 225a is provided in the lowermost slot 224a. In this way, the cutter base is configured for use in a downhole cutaway operation. This type of configuration can be used to form an initial window in a tubular, before a larger section of the tubular is milled away by milling cutters.

Although the section of material of the cutter base 214 between the slots 224a and 224d is shown in Figure 5 as being uninterrupted, this section may comprise wear pads or buttons. In alternative embodiments of the invention, this section may comprise additional components or be configured to perform additional functions. For example, it may comprise cutters or recesses for the addition of cutters or wear. Figures 6 and 7 show alternative cutter cassettes. Figure 6 shows a milling cassette 330 and Figure 7 shows a cutting cassette 430. Each cassette 330 and 430 has a body 338 and 438 comprising a substantially trapezoid, dove-tail shaped profile configured to be inserted into a dove-tail slot in a cutter base, like those shown in any of Figures 2 through to 5. The complimentary mating shapes of the cutter base slots and the cassette bodies minimise any sharp or overhanging edges which may result in uneven wear, damage to the milling tool and/or damage to the tubular being cut or milled.

The cassette 330 of Figure 6 comprises three milling cutters and can be used to configure a tool for a milling operation. Although only one row of milling cutters is shown, any number of cutters in any number of rows can be provided on a single cassette.

Dimensions of the cassette - for example, its length - can be adjusted to match a cutter base slot and to provide greater or fewer milling cutters, as required.

The cassette 430 of Figure 7 comprises three cutting cutters and can be used to configure a tool for a cutting operation. In a similar manner, although only one row of cutting cutters is shown, any number of cutters in any number of rows can be provided on a single cassette. Dimensions of the cassette - for example, its length - can be adjusted to match a cutter base slot and to provide greater or fewer cutting cutters, as required.

The cassette 540 of Figure 8 is a stabiliser cassette, configured to absorb impact and wear during a cutting or a milling operation and to provide a surface to bear against the inner surface of the tubular being cut or milled. The cassette 540 comprises a plurality of blind holes 542, each being configured to receive a wear button: a disc-shaped piece of material formed from wear resistant material. Stabiliser cassettes according to alternative embodiments of the invention may comprise greater or fewer wear pads, which may be coupled to the cassette in different ways. Parts of a stabiliser cassette - or any other functional cassette - may also be sacrificial. Depending on the project requirements, a stabiliser cassette may be configured to extend radially outward from the outermost surface of a cutter base. Like the milling and cutting cassettes described above, the dimensions of the stabiliser cassette - for example, its length - can be adjusted to match a cutter base slot and to provide greater or fewer wear pads, as required.

Although four-slot and two-slot cutter bases are described in the foregoing, it will be appreciated that cutter bases according to alternative embodiments of the invention can comprise different numbers of slots in different orientations and arrangements, having different shapes and dimensions. In each case, the functional cassettes configured for use with each cutter base are appropriately dimensioned.

Figures 9A, 9B and 9C show a cutter base 614 for a milling tool according to an alternative embodiment of the invention. The cutter base 614 is similar to the cutter base 114 of Figures 2, 3A and 3B, with like features indicated by like reference numerals incremented by 500. The cutter base 614 differs from the cutter base 114 in that it its main body comprises two T-shaped apertures 622a and 622b for the operating arms to extend into. The operating arms (not shown) are pivotally connected to the body of the cutter base 614 within the apertures 622a and 622b.

The cutter base 614 additionally differs from the cutter base 114 in that it comprises a single slot 624 instead of four. The slot 624 spans a large portion of the cutter base 614. This type of cutter base 614 may support an elongated cassette (now shown) comprising multiple rows of cutters for cutting or milling. Figure 9C is a cross-sectional view through the cutter base 614. The cutter base 614 has an eight-faceted profile shown generally at 626 which comprises eight surfaces 620a, 620b, 620c, 620d, 620e, 620f, 620g and 620h. Although being formed from planar surfaces, the outer profile 626 closely resembles a curve. However, the planar geometry of the profile 626 means that the cutter base 124 is easier and quicker to machine than a cutter base formed with a curved surface would be.

In addition, the cutter base wears more uniformly over its profile 626 in use than the cutter bases shown throughout Figures 1Ato 5.

Uneven wear of a cutter base is an issue because it can lead to premature failure of a milling tool, the need for premature repair work and/or it can cause issues in the way that the tool works. For example, uneven wear can cause the tool to sit off-centre within the tubular. This is problematic because it could result in cutting or milling cutters extending too far in a radial direction, past the outer surface of a cut or milled tubular and into contact with the tubular surrounding the cut or milled tubular. This will ultimately damage the surrounding tubular. Off-centre operation may also reduce operating efficiency of the tool.

For certain projects or project inventories, it may be more convenient to provide a cutter base for receiving a single functional cassette. However, by providing cutters on smaller, individual cassettes - using the cutter base 114 of Figure 2, 3A or 3B, for example - it can be easier and more cost effective to replace individual smaller cassettes with defective, worn or damaged cutters than it is to replace the large-scale cassette required for use with the cutter base of Figures 9A, 9B and 9C.

Although cutter bases and functional cassettes according to aspects of the invention are generally suitable for use in all milling tools, they are particular beneficial for milling tools comprising cutter bases which are expandable to suit different tubular diameters, such as those described in the applicant’s international patent application numbers PCT/US2013/053770, PCT/US2015/062264 and PCT/US2016/051780. Functional cassettes can be tailored to provide optimum contact between a cutter base and an inner tubular to be milled which can be particularly beneficial for milling tools which are not configured to lock their cutter bases into a fixed extended position, but which instead urge the cutter bases into an extended position (hydraulically or otherwise), and maintain application of the urging force during use.

Although the foregoing description describes functional cassettes being inserted into slots in a cutter base, it will be appreciated that alternative means for securely coupling functional cassettes to a cutter base may be used. For example, the cutter base may comprise apertures (instead of slots) for cassettes to extend though. Additionally, or alternatively, cassettes may be bolted or otherwise secured to surface of the cutter base.

Additionally, although the operating arms are described as being pivotally connected to a cutter base it will be appreciated that the arms may be connected to a cutter base in alternative ways and at alternative locations.

The invention provides a cutter base for a downhole tubular milling apparatus comprising at least one slot for receiving a removable functional cassette, a removable functional cassette for a cutter base, a downhole tubular milling apparatus comprising a cutter base and a downhole tubular milling apparatus comprising a cutter base and at least one removable functional cassette. The removable functional cassette comprises a body configured to be received in a slot located in or on the cutter base. The removable functional cassette is configured to provide the downhole tubular milling apparatus with the means to perform one or more functions selected from the group comprising: stabilisation, downhole tubular cutting and/or downhole tubular milling. Various modifications to the above-described embodiment may be made within the scope of the invention, and the invention extends to combinations of features other than those expressly claimed therein.