GB2611416A

GB2611416A - Improvements in or relating to well abandonment and slot recovery

Info

Publication number: GB2611416A
Application number: GB2212335.0A
Authority: GB
Inventors: Wardley Michael; Linklater James; Telfer George; Fairweather Alan
Original assignee: Ardyne Holdings Ltd
Current assignee: Ardyne Holdings Ltd
Priority date: 2021-08-26
Filing date: 2022-08-25
Publication date: 2023-04-05
Anticipated expiration: 2042-08-25
Also published as: GB202112201D0; GB202212335D0; NO20220896A1; GB2611416B

Abstract

A method of removing a section of tubular 16 such as casing in a wellbore. A section mill 34 is run downhole on a work string 26, pumping fluid through a flow path in the bore 72 of the work string to operate the section mill. The return flow path is directed to travel up the annulus 42, the downhole and return fluid paths are switched over a first length. Also independently claimed is a milling apparatus with flow diverter means for milling a tubular.

Description

IMPROVEMENTS IN OR RELATING TO WELL ABANDONMENT AND SLOT RECOVERY

The present invention relates to methods and apparatus for well abandonment and slot recovery and in particular, to methods and apparatus for milling casing.

When a well has reached the end of its commercial life, the well is abandoned according to strict regulations in order to prevent fluids escaping from the well to the environment. In meeting the regulations it has become accepted practice to set a cement plug over predetermined sections of the well. In some wells, the quality of the cement already present in an annulus outside casing is not adequate as a competent barrier to prevent migration of fluids from the wellbore to the environment. In this situation, one solution is to remove the casing and install a new cement barrier over the region of interest.

One method of removing the casing is to mill away the entire casing string from the surface down to the region of interest. This process can be time consuming and results in large quantities of steel swarf requiring processing. Another method is to remove a section of the casing by milling over the region of interest by deploying a section mill. This technique is known as section milling and the milled section is known as a window. Much less swarf is produced by section milling and time may be saved using this method.

Section mills have blades or knives that are deployed to cut the casing when at the desired location. The blades are commonly deployed by pumping fluid, known as mud through the bore of the section mill. The bore of the section mill contains a nozzle and the pressure drop at the nozzle is used to drive a piston that in turn, pushes the knives out into the casing.

Conventionally, during milling, mud is circulated down a drill string and up the annulus outside the drill string. The return flow up the annulus carries the cuttings back to surface.

It may be appreciated that the steel cuttings are relatively dense compared to the density of the mud and so a high annular flow speed is required to avoid the cuttings falling back towards the bottom of the wellbore. It will further be appreciated that for any given flow rate, the flow speed at any point is inversely proportional to the area of the flow path.

During circulation of the mud, pressure losses are experienced along the entire flow pathway. Of particular interest in clearing the swarf is the totality of pressure losses from the milling depth back to the surface. These pressure losses apply a pressure to the formation additional to the hydrostatic pressure already present due to the column of mud above the region of interest. This additional pressure, along with the density of the mud and the depth of the region of interest are combined to produce a parameter known as Effective Circulating Density (ECD). This parameter is well known in the industry and the formula for calculation is readily available.

When the casing section to be removed is the outermost i.e. when there is rock formation outside the casing, controlling the ECD can be extremely important. Under downhole conditions, rock formations have a Pore Pressure defined as the pressure of the fluid contained in the pores in the rock. It may be appreciated that if the bottom hole pressure, due to the hydrostatic column of mud in the well, is lower than the pore pressure, fluids contained in the rock may flow into the wellbore. In an extreme situation, if a large quantity of fluids flows into the wellbore in an uncontrolled situation, then a dangerous situation may occur at the surface resulting in a blow-out. Thus the pore pressure places a lower bound on the density of the mud used in downhole operations.

Another rock parameter is the Fracture Pressure. Fracture pressure is the pressure applied to the formation, above which the bonds between the rock particles will break. If this happens then mud can be lost into the rock formation. This situation is highly undesirable and can have many adverse consequences; including total mud loss to the formation and non-return of cuttings to surface. Thus the fracture pressure places an upper bound on the ECD seen in downhole operations.

Depending on the formation conditions, a particular problem may occur when the difference between the Pore Pressure and the Fracture Pressure is small. In these situations, when flowing conventionally, it may be difficult or impossible to return the cuttings to surface whilst keeping the ECD between the lower and upper bounds. This is because the pressure required to carry the swarf up the annulus is too high.

Pressure losses in the return path can be calculated using formulae provided by American Petroleum Institute standards. These formulae show that for cases where the flow speeds are equal, pressure losses in a pipe are lower than losses in an annulus. A major factor is that pressure losses are related to the surface area over which the fluid flows and that pipe surface area is lower than annulus surface area.

In high angle sections of wells, there is a further problem with returning swarf up the annulus. The drill pipe will preferentially lie on the side of the wellbore. In these areas of contact the flow speed is reduced and swarf can collect, ultimately blocking the flow and potentially resulting in stuck drill pipe.

It is an object of this invention to provide a flow regime that keeps the ECD within the required limits and allows milling to be accomplished under conditions where conventional flow regimes are not practical.

According to a first aspect of the present there is a method for removing a section of tubular in a wellbore comprising the steps: a) providing a tubular work string with a section mill mounted at an end thereof; b) lowering the work string into the wellbore to locate the section mill at the section of tubular to be removed; c) pumping fluid at surface into a bore of the work string; d) circulating the fluid in a fluid path downhole in the bore, out of the work string, up in an annulus outside the work string to return at surface in the annulus; e) the fluid path entering the section mill to actuate the section mill and extend cutter blades; f) rotating the section mill to mill the section of tubular with the cutter blades; and g) bringing cuttings back to surface in cuttings-laden fluid on the returning fluid path up the annulus; characterised in that: the downhole and returning fluid paths are switched over a first length in the well bore.

In this way, the ECD can be kept within the required limits as the returning fluid path is switched to the through bore of the work string providing a higher flow speed than for the annulus to lift the cuttings and preventing cuttings collecting on a low side of the work string. The returning flow path is returned to the annulus at surface for the cuttings to be handled conventionally.

Preferably the downhole and returning fluid paths are switched by: diverting the downhole fluid path to the annulus at a first location relative to the work string; diverting the returning fluid path to reach the bore at a second location, the second location spaced apart downhole from the first location by the first length; diverting the downhole fluid path back from the annulus at the second location to actuate the section mill; and diverting the returning fluid path back to the annulus at the first location to return the cutting-laden fluid at surface in the annulus.

Preferably the first location and the second location are on the work string and the fluid path is diverted directly between the annulus and the through bore of the work string. Alternatively, a dual string is located between the work string and the section mill, the downhole fluid flow path is diverted from the annulus into the dual string annulus leading to the bore of the section mill and the returning fluid path is diverted to the dual string bore leading to the through bore of the work string.

Preferably the method includes the step of creating a seal across the annulus at the first and second locations in the well bore. More preferably the method includes maintain the seal while the work string is moved in the well bore. This will be needed as the section of tubular is milled. Preferably, milling is performed in a downhole direction. More preferably, milling is performed in an upward direction.

At step (f) the method may include rotating the work string to rotate the cutter blades. Alternatively, step (a) may include locating a motor above the section mill and step (f) operates the motor to rotate the cutting blades.

The annulus may be arranged between an outer surface of the work string and a tubular located in the well bore, the tubular including the section of tubular to be removed. More preferably, the section of tubular to be removed is cemented in place as is known in the art. Alternatively, the annulus may be arranged between an outer surface of the work string and a first tubular located in the well bore, the first tubular being arranged within a second tubular and the second tubular including the section of tubular to be removed being located downhole of the first tubular. More preferably, at least a portion of the first tubular and the section of tubular to be removed is cemented in place.

In an embodiment, step (a) includes mounting a dual string between an end of the work string and the section mill and step (b) includes locating the dual string at an end of the first tubular. In this way, the dual string provides an annulus with a smaller cross-section than the annulus at the section of tubular to be removed in the second tubular.

According to a second aspect of the present invention there is provided apparatus for removing a section of tubular in a well bore, comprising: a tubular work string having a through bore on a central longitudinal axis; first flow diverter means located in the work string at a first location, second flow diverter means at a second location spaced apart from the first location by a first length, and a section mill located at an end of the work string, the section mill including cutter blades actuated by fluid flow in the through bore entering a bore of the section mill, to contact the tubular and by rotation of the cutter blades removing a section of the tubular.

Preferably the first and second flow diverter means include a seal to prevent flow passed the outer surface of the work string at the first and second locations.

Preferably, the first flow diverter means comprises a first conduit directing the through bore at a first side of the seal to the outer surface at an opposite second side of the seal and a second conduit directing the through bore at the second side of the seal to the outer surface at the opposite first side of the seal.

Preferably, the second flow diverter means comprises a third conduit directing the through bore at the second side of the seal to the outer surface at an opposite first side of the seal and a fourth conduit directing the through bore at the first side of the seal to the outer surface at the opposite second side of the seal.

The apparatus may include a motor to rotate the cutting blades of the section mill. The motor may be fluid driven. This may be considered as a mud motor as is known in the art.

Preferably the motor is located between the section mill and the end of the work string.

The apparatus may include a dual string located between the section mill and the end of the work string. Preferably the dual string has a first channel being formed of an inner string and having a first end co-linear with the through bore and a second end on an outer surface of an outer string; and a second channel, the second channel formed of an annulus between the inner string and the outer string and having a first end including an input port at an outer surface and a second end including an output port on the central longitudinal axis, the input being arranged above the seal of a flow diverter means to direct fluid from an outer surface of the work string to the bore of the second mill.

The second flow diverter means may be the dual string.

The work string may comprise drill pipe. Alternatively, the work string may comprise coiled tubing.

The apparatus may further comprise an auto-load control sub being a hydraulic tensioning device which maintains a constant load on the section mill during milling.

In the description that follows, the drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is to be fully recognized that the different teachings and elements of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results. Additionally, while relative terms such as 'upper' and 'lower' are used and the drawings indicate vertical wells, the invention finds application in deviated wells.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a schematic illustration of method and apparatus to remove a section of a tubular in well bore according to a first embodiment of the present invention; Figure 2 is a schematic illustration of method and apparatus to remove a section of a tubular in well bore according to a second embodiment of the present invention; and Figure 3 is a schematic illustration of method and apparatus to remove a section of a tubular in well bore according to a third embodiment of the present invention.

Referring initially to Figure 1 of the drawings there is illustrated a bottom hole assembly (BHA) 10 within a wellbore 12 for removing a section 14 of tubular, casing 16, according to an embodiment of the present invention. In this arrangement the tubular is casing 16 in which cement 18 may be located in a first annulus 20 formed between the outer surface 22 of the casing 16 and the rock formation forming the wellbore wall 24. The BHA 10 is mounted at an end of a work string 26 which is tubular to provide a through bore 28 for the circulation of fluid 30, referred to as mud, to the BHA 10. The work string 26 may be drill pipe or coiled tubing as is known in the art. At the end 32 of the BHA 10 is a section mill 34. As described hereinbefore, section mill 34 is of conventional design with blades 36 in the form of knives which, by application of fluid 30 pressure extend radially outwards and contact the tubular 16. The blades 36 are rotated and the section mill 34 raised or lowered so that the blades 36 mill the casing 16 and remove the casing 16 across the distance over which the section mill 34 is moved. The blades 36 can be rotated by use of a mud motor 38 mounted in the BHA 10 above the section mill 34 also driven by the circulating fluid 30 or may be rotated via rotation of the work string 26 from surface 40. Thus fluid 30 circulated down the bore 28 operates the motor 38, if used, and extends the blades 36 of the section mill 34 to remove the casing 16. The fluid 30 exits the end 32 of the BHA 10 and returns up the outside of the section mill 34 in a second annulus 42 formed between the outer surface 44 of the BHA 10 and the inner surface 46 of the casing 16.

This describes section milling as is typically carried out with the returned fluid 30 travelling up the annulus 42 to surface carrying the steel swarf cuttings created during the milling process. In the present invention the downhole fluid path and the return path are switched.

BHA 10 includes a length of drill pipe 48 between the work string 26 and the section mill 34. Drill pipe 48 may be considered as a continuation of the work string 26, having the same diameter and including the bore 28. The BHA 10 includes a first outlet port 50 in the drill pipe 48 arranged between the bore 28 and the outer surface 44 to direct downhole circulating fluid 30 from the bore 28 into the annulus 42. The fluid 30 is directed via a first plate 52 located in the bore 28 so that all fluid 30 exits the bore 28. At a distance 54 below the first outlet port 50, a first inlet port 56 through the drill pipe 48 allows the fluid 30 to return to the bore 28 and reach the section mill 34. A second plate 58 is arranged in the bore 28 to prevent the fluid 30 from returning up the bore 28 and direct it to the bore of the section mill 34 and mud motor 38, if used. Plates 52,58 are arranged at an angle to the central axis of the bore 28, being oppositely directed. At the upper side 60 of the first outlet port 50, there is arranged an upper flow diverter 62. Upper flow diverter 62 acts as a seal across the annulus 42 between the outer surface 44 of the drill pipe 48 and BHA 10, and the inner surface 46 of the casing 16. At the lower side 64 of the first inlet port 56 there is arranged a lower flow diverter 66, identical in shape and nature to the upper flow diverter 62. The flow diverters 62,66 maintain the seal even when the work string 26 and BHA 10 are moved up or down the wellbore 12 during milling. The flow diverters 62,66 create a fluid flow path where the circulating fluid 30 passing from the surface 40 down the bore 28, exits at the first outlet port 50, travels down the annulus 42 and returns to the bore 28 through the first inlet port 56. Below the lower flow diverter 66 is arranged a second inlet port 68 through the drill pipe 48. Of note is that the second plate 58 is located at the lower side 70 of the second inlet port 68. In this way, returning fluid 30 is forced via the lower flow diverter 66 into the drill pipe 48 above the second plate 58. Plates 52,58 form upper and lower walls of a chamber 72 in the bore 28. The fluid 30 passes up the chamber 72 and exits at a second outlet port 74 whose lower side 76 is arranged above the upper flow diverter 62 while its upper side 78 is below the first plate 52. In this way, the fluid path is switched from the bore 28 to the annulus 42 on the forward or downhole path and from the annulus 42 to the bore 28 on the return path. The cross-sectional area of the chamber 72 and bore 28 is smaller than the cross-sectional area of the annulus 42. This results in a higher flow speed in the chamber 72 to lift the cuttings in the fluid 30 and ECD is kept within the working requirements. The flow path returns to the annulus 42 at surface 40 for the cuttings to be disposed of.

The upper flow diverter 62, first plate 52, first outlet port 50 and second outlet port 74 may be considered as first flow diverter means while the lower flow diverter 66, second plate 58, fist inlet port 56 and second inlet port 68 may be considered as second flow diverter means.

In use, as Figure 1, the flow regime is as follows: * Mud 30 is pumped down the drill pipe 48 conventionally from surface 40; * At a selected depth (A) below the wellhead, the flow is switched from the bore 28 to the annulus 42; * At a selected depth (B) above the region of interest, the flow is switched back to the bore 28 of the drill string 26; * The flow then operates the required downhole tools 34,38 conventionally and exits at the lower end 32 of the drill string 26; * The fluid 30 begins its return journey by flowing past the milling knives 36, picking up the cuttings, flowing up the annulus 42 and then entering the drill string 26at depth B; then * Flowing up the bore 28 of the drill string to depth A, where it re-enters the annulus 42 and then flows to surface 40 conventionally.

The flow diverters 62,66 are provided between the drill string 26 and the casing 16 at both depths A and B to guide the fluid in the desired direction.

To reduce pressure losses, the wellbore length between depths A and B are chosen to be a significant proportion of the depth to the region of interest. The region of interest 80 will be that depth over which milling is desired to take place and the section 14 of casing 16 is to be removed. Depth A may be chosen to be in a vertical section of the wellbore 12 relatively close to the wellhead (not shown).

This flow regime is advantageous due to the following features: * The majority of the return flow path is inside the bore 28 of the drill pipe 48 where the flow speed can be high and yet the pressure drop will be lower than the corresponding pressure drop would be if in the annulus 42.

* There are no 'low flow' areas inside the drill string 26 where cuttings may agglomerate.

* By switching the flow back to the annulus 42 at depth A in the vertical section of the wellbore 12, the risk of cuttings agglomerations are avoided and the swarf can be received at surface 40 in conventional manner without passing through surface equipment that is not designed for this purpose.

By adopting this flow regime it may be appreciated that the return pressure loss and ECD are lower than the case for conventional flow.

In a second embodiment of the invention depicted in Figure 2, the flow regime is adapted for 'through-casing' section milling. Like parts to those of Figure 1 have been given the same reference numeral with the addition of 100 to aid clarity. 'Through-casing' section milling is where the casing 116 to be milled is larger than another section of casing 17 that is already in the well. For instance, it may be desired to mill 9-5/8" casing 116 at the depth of interest, but there is 7" casing 17 in the wellbore above the milling depth. Cement 118 may be located between the two casings 116,17.

In this situation, a window 82 may be milled into the 7" casing 17 at the region of interest 180 before milling the 9-5/8" casing 116. The 7" window would be at least as long as the required 9-5/8" section 114 of casing 116 to be removed. The same BHA 110 may be used to mill both casings 116, 17 with the fluid 130 flow rate through the section mill 134 being controlled so that the blades 136 extend only part way to be sufficient to mill the inner casing 17 before they are fully extended to mill the outer casing 116. The BHA 110 and the method operate in the same manner as described hereinbefore with reference to Figure 1.

Reference is now made to Figure 3 of the drawings which illustrates a BHA 210 for use when the flow speed from the section mill 234 up to point B, may be inadequate to transport the cuttings successfully. Like parts to those of Figures 1 and 2 have been given the same reference numeral with the addition of 200 to aid clarity. While the BHA 210 is illustrated in the 'through-casing' wellbore arrangement of Figure 2 it is equally applicable to the single casing arrangement of Figure 1.

In the BHA 210 of Figure 3, a dual string section of pipe 84 is included between the section mill 234 and the lower flow diverter 266. The dual string 84 is one pipe 86 inside another pipe 88 providing an inner bore 90 and an outer annulus 92. The inner bore 90 has a cross-sectional area smaller than the bore 228 and chamber 272. The outer annulus 92 has a smaller cross-sectional area than the annulus 242. The dual string 84 is arranged such that the first inlet port 256 is from the annulus 242 into the outer annulus 92, with the second plate 258 now moved to the bottom of the inner pipe 86, to direct the incoming fluid towards the inlet of the section mill 234. The second inlet port 268 is now at the lower end 94 of the outer pipe 88 of the dual string 84 above the second plate 258, which is ducted to the base of the inner pipe 86 so that the returned fluid passes through bore 90, exits directly into bore 228 and chamber 272 to continue out through second outlet port 274 as described with reference to Figures 1 and 2. The bores 90,228 are colinear and lie axially in the centre of the work string 226. A further annular plate 96 is provided at the upper end 98 of the dual string 84 to separate the bores 90,228 from the annuli 92,242.

In this scenario, the downward flow of fluid is in the annulus 92 between the two pipes 86,88 and the fluid returns up the bore 90 of the inside pipe 86. The inclusion of the dual string 84 overcomes the shortcoming in the second embodiment where the flow speed from the mill to point B is too low.

The length from the upper end 98 of dual string 84 to the end 232 of the BHA 210 should be at least the distance from the bottom of the larger casing window 82 to the top of the smaller casing window, the region of interest 80. This ensures that the return flow speed is high over this wellbore length.

In any of these embodiments, it may be advantageous to mill upwards rather than downwards (which is more conventional). This is because in order to achieve the required flow speed to transport the cuttings upwards in the zone immediately above the milling knives 36, the flow speed would be determined by the area inside the casing 16 to be milled, rather than the larger area of the already milled section.

In any of these embodiments, the section mill 34 may be rotated by including a downhole motor 38 above the section mill 34, or the pipe 48 may be rotated from surface 40, in which case provision for rotatably coupling the flow diverters 62,66 to the drill string 48 will be made.

In any of these embodiments where the direction of milling is upwards, a load control sub may be included above the section mill 34. Such a tool is described in GB2567157 to the applicants, the contents of which are incorporated herein by reference. The load control sub ads to maintain a constant load on the section mill knives 36 and thereby produces a more consistent size of swarf particles.

The principle advantage of the present invention is that it provides a method and apparatus for removing a section of tubular in a wellbore with a flow regime that keeps the ECD within the required limits and allows milling to be accomplished under conditions where conventional flow regimes are not practical.

Those skilled in the art will appreciate that various modifications may be made to the invention hereindescribed without deviating from the scope of the invention as defined in the claims. For example, while only single inlet and outlet ports are described to direct fluid flow, there may be more ports at each location.

Claims

CLAIMS1. A method for removing a section of tubular in a wellbore comprising the steps: a) providing a tubular work string with a section mill mounted at an end thereof; b) lowering the work string into the wellbore to locate the section mill at the section of tubular to be removed; c) pumping fluid at surface into a bore of the work string; d) circulating the fluid in a fluid path downhole in the bore, out of the work string, up in an annulus outside the work string to return at surface in the annulus; e) the fluid path entering the section mill to actuate the section mill and extend cutter blades; f) rotating the section mill to mill the section of tubular with the cutter blades; and g) bringing cuttings back to surface in cuttings-laden fluid on the returning fluid path up the annulus; characterised in that: the downhole and returning fluid paths are switched over a first length in the well bore.
2. The method according to claim 1 wherein the downhole and returning fluid paths are switched by: diverting the downhole fluid path to the annulus at a first location relative to the diverting the returning fluid path to reach the bore at a second location, the second location spaced apart downhole from the first location by the first length; diverting the downhole fluid path back from the annulus at the second location to actuate the section mill; and diverting the returning fluid path back to the annulus at the first location to return the cutting-laden fluid at surface in the annulus.
3. The method according to claim 2 wherein the first location and the second location are on the work string and the fluid path is diverted directly between the annulus and the bore of the work string.
4. The method according to claim 1 or claim 2 wherein a dual string is located between the work string and the section mill, the downhole fluid flow path is diverted from 5. 6. 7. 8. 9. 10. 11.the annulus into the dual string annulus leading to the bore of the section mill and the returning fluid path is diverted to the dual string bore leading to the bore of the work string.
The method according to any one of claims 2 to 4 wherein the method includes the step of creating a seal across the annulus at the first and second locations in the well bore.
The method according to claim 5 wherein the method includes the step of maintaining the seal while the work string is moved in the well bore.
The method according to any preceding claim wherein milling is performed in a downhole direction.
The method according to any one of claims 1 to 6 wherein milling is performed in an upward direction.
The method according to any preceding claim wherein at step (f) the method includes rotating the work string to rotate the cutter blades.
The method according to any one of claims 1 to 8 wherein step (a) includes locating a motor above the section mill and step (f) operates the motor to rotate the cutting blades.
The method according to any one of claims 4 to claim 8 wherein the annulus is arranged between an outer surface of the work string and a first tubular located in the well bore, the first tubular being arranged within a second tubular and the second tubular including the section of tubular to be removed being located downhole of the first tubular and step (a) includes mounting the dual string between an end of the work string and the section mill and step (b) includes locating the dual string at an end of the first tubular.
12. Apparatus for removing a section of tubular in a well bore, comprising: a tubular work string having a bore on a central longitudinal axis; first flow diverter means located in the work string at a first location, second flow diverter means at a second location spaced apart from the first location by a first length, and a section mill located at an end of the work string, the section mill including cutter blades actuated by fluid flow in the through bore entering a bore of the section mill, to contact the tubular and by rotation of the cutter blades removing a section of the tubular.
13. Apparatus according to claim 12 wherein the first and second flow diverter means include a seal to prevent flow passed the outer surface of the work string at the first and second locations.
14. Apparatus according to claim 12 or claim 13 wherein the first flow diverter means comprises a first conduit directing the bore at a first side of the seal to the outer surface at an opposite second side of the seal and a second conduit directing the bore at the second side of the seal to the outer surface at the opposite first side of the seal.
Apparatus according to claim 14 wherein the second flow diverter means comprises a third conduit directing the bore at the second side of the seal to the outer surface at an opposite first side of the seal and a fourth conduit directing the bore at the first side of the seal to the outer surface at the opposite second side of the seal.
Apparatus according to any one of claims 12 to 15 wherein the apparatus includes a motor to rotate the cutting blades of the section mill.
Apparatus according to claim 16 wherein the motor is fluid driven.
Apparatus according to claim 16 or claim 17 wherein the motor is located between the section mill and the end of the work string.
Apparatus according to any one of claims 12 to 18 wherein the apparatus includes a dual string located between the section mill and the end of the work string.
Apparatus according to claim 19 wherein the dual string has a first channel being formed of an inner string and having a first end co-linear with the through bore and a second end on an outer surface of an outer string; and a second channel, the second channel formed of an annulus between the inner string and the outer string and having a first end including an input port at an outer surface and a second end including an output port on the central longitudinal axis, the input being arranged above the seal of a flow diverter means to direct fluid from an outer surface of the work string to the section mill.
21. Apparatus according to claim 19 or claim 20 wherein the second flow diverter means is the dual string. 15. 16. 17. 18. 19. 20.
22. Apparatus according to any one of claims 12 to 21 wherein the work string comprises drill pipe.
23. Apparatus according to any one of claims 12 to 21 wherein the work string comprises coiled tubing.
24. Apparatus according to any one of claims 12 to 23 wherein the work string the apparatus further comprises a load control sub being a hydraulic tensioning device which maintains a constant load on the section mill during milling.