Patent Description:
Wellbores for oil drilling and the like typically comprise a circular bore formed through the earth's crust (referred to as the formation) lined with a pipe, formed from a robust material such as steel which is known as the casing.

Once a wellbore has been formed, it is often necessary to seal and abandon the wellbore. This may be because, for instance, the resources accessed through the wellbore have been depleted to the level where further use of the wellbore is not economically viable.

In the sealing and abandonment of a wellbore, a bridge plug (which may, for example, be hydraulic or mechanical) or the like may be set in the wellbore at a desired depth, and the bridge plug may be activated, for example (in the case of a hydraulic bridge plug) by a ball being pumped down the drill string from the surface, and landing in a seat, causing pressure to build up and set the bridge plug.

A quantity of cement or a similar substance may then optionally be displaced on top of the bridge plug to form a cement plug, further sealing the wellbore.

The casing of the wellbore may then be cut, at a position above the plug, so that the casing above the plug can be retrieved and re-used or discarded.

Examples of tools used in the cutting of the casing in a wellbore may be seen, for example, in <CIT> and <CIT>.

<CIT> discloses an expandable downhole tool comprises a tubular body, at least one moveable arm disposed within the tubular body and being radially translatable between a retracted position and a wellbore engaging position, and at least one piston operable to mechanically support the at least one moveable arm in the wellbore engaging position when an opposing force is exerted. A method of reaming a formation to form an enlarged borehole in a wellbore comprising disposing an expandable reamer in a retracted position in the wellbore, expanding at least one movable arm of the expandable reamer radially outwardly into engagement with the formation, reaming the formation with the at least one moveable arm to form the enlarged borehole; and mechanically supporting the at least one moveable arm in the radially outward direction during reaming.

<CIT> discloses a drill bit in which a ball may be received in a seat in an internal piston, causing the piston to be driven downwardly with respect to surrounding components, and in turn to the deployment of cutters.

<CIT> discloses a downhole tool for cutting a wellbore casing. The downhole tool comprises a gripping mechanism for gripping a section of wellbore casing and a cutting mechanism configured to cut the casing. The grip mechanism is configured to grip a range of casing diameters.

It is an object of the present invention to seek to provide an improved tool for use in the process of abandoning wellbores.

Accordingly, one aspect of the present invention provides a cutting tool according to claim <NUM>.

Another aspect of the invention provides a method of sealing and cutting a wellbore, according to claim <NUM>.

Preferred features of the invention are set out in the dependent claims.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying figures, in which:.

<FIG> shows a tool <NUM> embodying the present invention. The tool <NUM> comprises an elongate main body <NUM>, which is generally cylindrical in form and of a suitable size to be run into a wellbore. The main body <NUM> has an inlet end <NUM> at one end thereof and an outlet end <NUM> at the opposite end thereof. In use of the tool <NUM>, it is expected that the tool <NUM> will be oriented such that the inlet end <NUM> is uppermost, and the outlet end <NUM> is lowermost. In this document references to "top", "bottom", "above", "below" and the like are used in terms of this orientation, although it should be understood that these terms are used for convenience and do not rule out use of the tool in any other orientation.

Both the inlet and outlet ends <NUM>, <NUM> have threaded connections <NUM>. In the arrangement shown in <FIG>, the tool <NUM> is attached to a top sub <NUM> and a bottom sub <NUM> by way of these threaded connections <NUM>.

The top sub <NUM> is attached to the inlet end <NUM> of the tool <NUM> at its lower end <NUM>, and its upper end <NUM> comprises a standard female threaded connection.

Similarly, the bottom sub <NUM> is attached to the tool <NUM> at its top end <NUM>, and its bottom end <NUM> comprises a standard male threaded connection.

The combination of the tool and the top and bottom subs <NUM>, <NUM> is therefore able to be integrated into a drill string, using the standard threaded connections, in a straightforward manner. In other arrangements the top and bottom subs <NUM>, <NUM> may be omitted, with the tool <NUM> itself including the standard threaded connections at its ends.

<FIG> shows a more close-up view of the internal components of the tool <NUM>. The tool <NUM> comprises a plurality of cutters <NUM>, positioned at radially spaced-apart positions around the circumference thereof. In the embodiment shown, the tool <NUM> has three cutters <NUM>, which are regularly spaced around its circumference, although other numbers of cutters and/or kinds of angular spacing may also be used. Each cutter may be moved between a retracted position and a deployed position. In the retracted position, each cutter does not, or substantially does not, protrude beyond the outer diameter of the main body <NUM>. In the deployed position, each cutter protrudes outwardly beyond the outer diameter of the main body <NUM>. This will be discussed in more detail below.

In the example shown, each cutter <NUM> includes a connection portion <NUM>, which is rotatably mounted on a mounting pin <NUM>, which is perpendicular or generally perpendicular to the main longitudinal axis of the tool <NUM> itself. The cutter <NUM> further comprises a cutting portion <NUM>, generally taking the form of a blade, which extends away from the mounting portion <NUM>.

Overall, each cutter <NUM> is preferably generally flat in configuration, and arranged so that the plane thereof is substantially perpendicular to, and passes through or close to, the main longitudinal axis of the tool.

It will therefore be understood that, when each cutter is in the deployed position, it protrudes radially or substantially radially outwards from the tool <NUM>.

In the example shown, where the cutters <NUM> are provided the main body <NUM> of the tool <NUM> has a region <NUM> of increased thickness. In line with each of the cutters <NUM> a slot or window <NUM> is provided in the main body <NUM>. In the retracted position, each cutter <NUM> is positioned within one of these slots or windows <NUM>, preferably entirely accommodated within the thickness of the wall of the main body <NUM>, and in the deployed position each cutter <NUM> protrudes outwardly through the slot or window <NUM>.

The main body <NUM> is generally hollow, and has a main cavity <NUM> passing therethrough.

Positioned within the main cavity <NUM> is a piston <NUM>, which is generally hollow and has a cavity <NUM> passing therethrough.

The piston <NUM> has a central region <NUM> which passes through, and preferably is a close fit within, the widened region <NUM> of the main body <NUM> (it will be understood that in this region <NUM>, the internal diameter of the main body <NUM> is reduced, due to the increased wall thickness). The piston <NUM> is of a suitable size that it may slide longitudinally in either direction with respect to the main body <NUM>.

In the region of the mounting portion <NUM> of each cutter <NUM>, the outer surface of the piston <NUM> has a series of spaced-apart teeth <NUM> formed on its outer surface. These teeth <NUM> may extend around the entire circumference of the piston <NUM> or, as shown in the figures, a separate set of teeth <NUM> may be formed to be aligned with each cutter <NUM>.

The mounting portion <NUM> of each cutter <NUM> has corresponding teeth <NUM> protruding therefrom. The teeth <NUM>, <NUM> of the piston <NUM> and the mounting portion <NUM> engage and intermesh with one another, so that linear movement of the piston <NUM> causes rotational motion of the mounting portion <NUM> of the cutter <NUM>.

The skilled reader will appreciate that the interaction between these teeth <NUM>, <NUM> is akin to the operation of a rack and pinion.

In the arrangement shown in <FIG>, one cutter <NUM> is visible, in the retracted position. It will be understood that, starting from this position, if the piston <NUM> moves linearly with respect to the main body <NUM> in the direction towards the outlet end <NUM> thereof, this will cause the mounting portion <NUM> of the cutter <NUM> to rotate so that the cutting portion <NUM> of the cutter <NUM> protrudes outwardly from the main body <NUM>. In this position, the cutter <NUM> is in the deployed configuration.

In the preferred embodiment each cutter <NUM> may rotate through around <NUM>°-<NUM>° to move from the retracted position into the deployed position. However, in other embodiments each cutter <NUM> may move through a greater or lesser angle to move into the deployed position. In some embodiments the cutters <NUM> may move through around <NUM>° or around <NUM>°.

In the initial, retracted position for each cutter <NUM> shown in <FIG>, each cutter <NUM> preferably lies against an outer surface of the piston <NUM>,.

The piston <NUM> has an upper or inlet end <NUM>, which is wider than the middle part <NUM> thereof. Where the upper end <NUM> meets the central region <NUM>, the upper end <NUM> presents a downward-facing shoulder <NUM>. Similarly, at the upper end of the widened region <NUM> of the main body <NUM>, an upward-facing shoulder <NUM> is formed. A cavity <NUM> is formed between the shoulders <NUM>, <NUM>, and a generally cylindrical compression spring <NUM> is provided in this cavity <NUM>, positioned between the downward-facing shoulder <NUM> and the upward-facing shoulder <NUM>. As the skilled reader will understand, this compression spring <NUM> biases the piston <NUM> upwardly with respect to the main body <NUM>.

The upper end <NUM> of the piston <NUM> is open, and a widened recess <NUM> is formed at the opening. In the example shown in the figures, an insert <NUM> is provided in the widened recess <NUM>. This insert <NUM> may be hardened to prevent or minimise damage to the widened recess <NUM>, through fluid flow or contact with other components.

The piston <NUM> further has a lower or outlet end <NUM>, which is positioned below the widened region <NUM> of the main body <NUM>, and is wider than the central region <NUM> of the piston <NUM>. The lower end <NUM> of the piston <NUM> is too wide to fit through the region <NUM> of the main body <NUM> which has a widened wall. The lower end <NUM> of the piston <NUM> is also open.

It is likely to be necessary to form the piston <NUM> in two or more parts, in order to allow the tool <NUM> to be assembled. In the example shown, the widened lower end <NUM> of the piston <NUM> is formed by attaching a generally annular collar <NUM> to the exterior of the piston <NUM>. It will be understood that, in the production of the tool <NUM>, the piston <NUM> will be inserted through the region <NUM> of the main body <NUM> that has a thickened wall, and the collar <NUM> can then be attached to the lower end of the piston <NUM>.

In preferred embodiments the cross-sectional area of the upper surface of the piston <NUM> is equal, or approximately equal to the cross-sectional area of the lower surface of the piston <NUM>. In other words, the upward-facing annular region of the widened upper end <NUM> of the piston is of the same, or approximately the same, area as the downward-facing annular surface of the widened lower part <NUM> of the piston <NUM>.

This means that, when pressurised fluid surrounds the piston <NUM>, the piston <NUM> is substantially balanced and will not be driven in either direction longitudinally with respect to the main body <NUM>.

In the example shown, one or more shear screws <NUM> pass through the main body <NUM>, in the widened region <NUM> thereof, and protrude inwardly into corresponding apertures <NUM> formed on the outer surface of the piston <NUM>. In other embodiments, instead of separate apertures for each shear screw <NUM>, an annular groove may be formed in the exterior surface of the piston <NUM>, as shown in <FIG>, into which one or more shear screws protrude.

It will therefore be understood that, in an initial configuration (shown in <FIG>), the shear screws <NUM> prevent movement of the piston <NUM> longitudinally with respect to the main body <NUM>. However, the shear screws <NUM> may, in operation of the tool <NUM>, be broken (discussed in more detail below), allowing relative longitudinal movement of the main body <NUM> and the piston <NUM>. Other types of frangible connections may also be used instead of shear screws.

The tool <NUM> further comprises a flow regulator <NUM>, which in the illustrated embodiment takes the form of a flotel. The flow regulator <NUM> is positioned closer to the inlet end <NUM> of the tool <NUM> than the piston <NUM>. The flow regulator <NUM> comprises a blocking portion <NUM>, which is provided at its upper end (i.e. closest to the inlet end <NUM> of the tool <NUM>), and completely or substantially completely fills the internal diameter of the main body <NUM>. Fluid entering the inlet end <NUM> of the tool <NUM> therefore cannot flow around the blocking portion <NUM> of the flow regulator <NUM>. The blocking portion <NUM> may have a seal, such as an O-ring, around its perimeter to form a seal against the interior of the main body <NUM>.

The flow regulator <NUM> further comprises a delivery portion <NUM>, which is generally cylindrical, hollow and elongate, and protrudes from the blocking portion <NUM> in the direction towards the outlet end <NUM> of the tool <NUM>. The delivery portion <NUM> has a sealing region <NUM>, which fits closely within the widened recess <NUM> (or the insert <NUM> therein). In some embodiments this close fit completely blocks the recess <NUM> so that fluid cannot flow or pass between the sealing region <NUM> and the interior of the recess <NUM>. However, in preferred embodiments some fluid may pass between the sealing region <NUM> and the interior of the recess <NUM>. This may be achieved, for example, by having a bypass area in the form of one or more grooves or cut-outs formed in the delivery portion <NUM> (in particular, in the sealing region <NUM> thereof) and/or in the interior of the recess <NUM>. In some examples the flow area between the sealing region <NUM> and the interior of the recess may be equivalent to a pipe having a <NUM>/<NUM>" (<NUM>) or <NUM>/<NUM>" (<NUM>) diameter.

The sealing region extends over at least a part of the length of the delivery portion <NUM>. The delivery portion <NUM> also has a narrowed region <NUM> at its distal end, which has a reduced diameter compared to the sealing region <NUM>.

The blocking portion <NUM> has an aperture formed therethrough which is in fluid connection with the delivery portion <NUM>. The delivery portion <NUM> is open at its lower end <NUM>. Its lower end <NUM> is fitted into the widened recess <NUM> at the upper end <NUM> of the piston <NUM>, and the interior of the delivery portion <NUM> is in fluid communication with the interior of the piston <NUM>.

Part way along its length the delivery portion <NUM> has a series of flow apertures <NUM> formed therethrough. Each flow aperture <NUM> passes through the entire thickness of the wall of the delivery portion <NUM>, and is preferably oriented radially or generally radially.

In an initial configuration, as shown in <FIG>, a sleeve element <NUM> (which is preferably cylindrical in form) is positioned within the delivery portion <NUM>, and aligned with the flow apertures <NUM>. In preferred embodiments the sleeve element <NUM> is not a tight fit within the interior of the delivery portion <NUM>, and fluid pressure can communicate through the flow apertures <NUM> between the interior of the delivery portion <NUM> and the exterior region immediately surrounding the delivery portion <NUM>. However, when flow or circulation of drilling fluid through the tool <NUM> occurs, this flow of fluid is not communicated through the flow apertures <NUM>.

The sleeve element <NUM> is initially held in place with respect to the delivery portion <NUM> of the flow regulator <NUM> by one more shear screws <NUM> or other frangible connections.

<FIG> shows the tool <NUM> in an initial configuration.

The internal bore <NUM> of the piston <NUM> is relatively wide. In preferred embodiments, the internal diameter of the internal bore <NUM> is at least <NUM>/<NUM>th of the total external diameter of the main body <NUM>. In more preferred embodiments, the internal diameter of the internal bore <NUM> is at least one quarter of the total overall external diameter of the main body <NUM>.

In preferred embodiments the internal bore is at least around <NUM>" (<NUM>) in diameter, and may be <NUM>" (<NUM>) or at least around <NUM>" (<NUM>). The overall external diameter of the tool <NUM> may be <NUM>" (<NUM>) or therearound, or may be <NUM>" (<NUM>) or therearound. However, the invention is not limited to bores or tools of this size. The tool may be of any other suitable size, for instance <NUM>" (<NUM>) or <NUM>" (<NUM>).

In preferred embodiments, the internal diameter of the flow path through the flow regulator <NUM>, including the internal diameter of the delivery portion <NUM>, is of at least substantially the same diameter as that of the internal bore of the piston <NUM>.

Importantly, in preferred embodiments a flow path is defined through the tool <NUM>, in this initial configuration, which has a wide bore, and includes no significant internal obstacles or restrictions. In preferred embodiments, in the initial configuration the cross-sectional area of the flow path through the tool, at all points along the length of the tool, corresponds to that of a pipe having a diameter of at least <NUM>" (<NUM>). In yet more preferred embodiments, in the initial configuration the cross-sectional area of the flow path through the tool, at all points along the length of the tool, corresponds to that of a pipe having a diameter of at least <NUM>" (<NUM>).

In preferred embodiments the flow path through the tool <NUM>, in the initial configuration, is centrally or substantially centrally disposed within the tool <NUM>. Preferably, a central longitudinal axis of the tool <NUM> passes along the flow path, for at least a majority of the flow path. In more preferred embodiments, the central axis passes along the flow path for at least <NUM>% of its length. In yet more preferred embodiments, the central axis passes along the flow path for <NUM>%, or substantially <NUM>%, of its length.

Use of the tool <NUM> will now be described.

Initially, the tool <NUM> is incorporated into a drill string, which (as the skilled reader will appreciate) may include many other components which are connected together end-to-end.

In preferred embodiments of the invention, a bridge plug (not shown) is attached below the tool <NUM>. The bridge plug may be attached directly to the lower end of the tool <NUM>, or one or more other tools/components may be positioned between the tool <NUM> and the bridge plug.

The drill string, including the tool <NUM> and the bridge plug, is run into a wellbore in a known fashion. As this occurs drilling fluid of any suitable type may be circulated through the drill string, as the skilled reader will understand. The drilling fluid will pass into the inlet end <NUM> of the tool <NUM>, through the flow regulator <NUM> and the interior cavity <NUM> of the piston <NUM>, and out through the outlet end <NUM> of the tool <NUM>. The fluid will not flow through the flow apertures <NUM> of the delivery portion <NUM> of the flow regulator <NUM>. However, as mentioned above, fluid pressure within the delivery portion <NUM> will be communicated to the region immediately surrounding the delivery portion <NUM>. This means that the fluid pressure experienced by the upper surface of the piston <NUM> will be the same (or substantially the same) as that experienced by the lower surface of the piston <NUM>, and the piston will be in a pressure balanced state, and will not tend to be driven longitudinally in either direction with respect to the main body <NUM>.

While (as mentioned above) drilling fluid may be circulated during this phase, this is not essential. The drill string may alternatively be filled with fluid from the surface or above the tool with no or minimal circulation.

When the bridge plug is at a suitable depth, the plug is set, i.e. activated so that it grips onto the inner surface of the casing, for instance through one or more slips, and completely or substantially occludes the wellbore.

The bridge plug may be activated hydraulically, through pressurised fluid within the drill string, mechanically (for instance by dropping a ball or other object through the drill string, including the tool <NUM>, to reach the bridge plug), or in another suitable way.

Once the bridge plug has been set, the integrity of the bridge plug and the casing can be tested, by means of a pressure test. If the integrity is found to be lacking/unacceptable through this pressure test, it may be necessary to set another bridge plug in the wellbore, displace further cement through the drill string to create a further barrier in the wellbore, and/or move the tool to a different depth to cut the casing in a different location. It may even be necessary to remove the initial bridge plug before setting another bridge plug in place.

Once the bridge plug has been set with respect to the wellbore, and any pressure testing has been successfully completed, the remainder of the drill string (along with the tool <NUM>) is disengaged from the bridge plug. This could be done, for example, through rotation of the drill string to disengage a threaded connection between the bridge plug and the remainder of the drill string. In preferred embodiments, the components of the drill string are connected to one another through conventional right-hand threaded connections, but the connection between the bridge plug and the next lowest component (which may be the tool <NUM>) is a left-handed threaded connection. This means that, if the drill string is rotated clockwise, this will tend to tighten the connections between the majority of the components, but to disengage the threaded connection with the bridge plug.

Once the drill string has been disconnected from the bridge plug, the drill string can be lifted upwardly away from the bridge plug.

In preferred embodiments, a quantity of cement is then pumped through the drill string, and out of the open end of the drill string to set and form a cement plug on top of the bridge plug.

It will be appreciated that the relatively wide bore passing through the tool will allow the ready delivery of cement through the tool. By contrast, many known tools for cutting a casing have relatively narrow fluid pathways, which include several bends or turns or are otherwise convoluted, and these known tools are therefore much less well-suited to the delivery of cement.

During this phase of operation the cement may, for instance, be displaced at a rate of around <NUM> to <NUM> litres per minute. Cement used for this purpose may have a specific gravity of around <NUM>.

As cement flows through the tool <NUM>, cement will be prevented from passing through the flow apertures <NUM> of the delivery portion <NUM> of the flow regulator <NUM> by the sleeve element <NUM>.

The displacement of cement through the tool is likely to produce a "surge" effect, and the presence of the compression spring <NUM> and shear pins <NUM> help to maintain the piston <NUM> in its correct position as this process is carried out.

The drill string is preferably raised as the cement is displaced, so that the drill string remains above the cement and does not become fixed by the cement in the borehole.

Once a suitable quantity of cement has been displaced through the tool <NUM>, regular circulating fluid/drilling fluid can once again be introduced through the drill string and to the tool <NUM>. The drill string will be entirely above the cement plug at this stage. Once the cement has set, a pressure integrity test can be carried out to test the integrity of the cement plug, the bridge plug and/or the casing.

The drill string is then raised so that the cutters <NUM> of the tool <NUM> are level or substantially level with the location at which the casing of the wellbore is to be cut.

When the cutting operation is to begin, a ball <NUM> is dropped through the drill string, and is carried by the drilling fluid (which at this stage may be introduced into the drill string with a low pump rate/low circulation rate) along the drill string until it reaches the inlet end <NUM> of the tool <NUM>.

The drill string is then pressurised, without circulation of fluid.

<FIG> shows a close-up view of the flow regulator <NUM>, when the ball <NUM> has arrived at the flow regulator <NUM>.

The ball <NUM> is formed to have an outer diameter which is slightly less than the inner diameter of the delivery portion <NUM> of the flow regulator <NUM>. However, the sleeve element <NUM> has an inner diameter which is less than the outer diameter of the ball <NUM>. The ball <NUM> therefore lands on the upper edge of the sleeve element <NUM>, and entirely or substantially entirely blocks the fluid flow path through the flow regulator <NUM>. Fluid pressure above the ball <NUM> drives the ball downwardly, rupturing the shear pins <NUM> which hold the sleeve element in place. The ball <NUM> and sleeve element <NUM> therefore travel downwardly, with respect to the flow regulator <NUM>, until the sleeve element <NUM> lands on an upward-facing shoulder <NUM> formed within the delivery portion <NUM> of the flow regulator <NUM>.

As can be seen in <FIG>, the upper end of the blocking portion <NUM> of the flow regulator <NUM> may have one or more sloping surfaces, forming a funnel, to guide the ball <NUM> into the delivery portion <NUM> thereof when the ball arrives at the tool <NUM>.

When the sleeve element <NUM> travels downwardly with respect to the delivery portion <NUM>, the flow apertures <NUM> are exposed (i.e. no longer blocked by the sleeve element <NUM>), and fluid may flow from the interior of the flow regulator <NUM> outwardly through the flow apertures <NUM>.

In this configuration, flow of fluid through the lower end of the delivery portion <NUM> of the flow regulator <NUM> to the interior cavity <NUM> of the piston <NUM> is blocked by the ball <NUM> itself, which completely or substantially completely occludes (together with the sleeve element <NUM>) the delivery portion <NUM> of the flow regulator <NUM>.

As can be seen from <FIG>, therefore, fluid delivered to the tool <NUM> through the drill string may no longer flow directly into the piston <NUM> through the delivery portion <NUM> of the flow regulator <NUM>, but instead is diverted out through the flow apertures <NUM> into an annular chamber which surrounds the delivery portion <NUM> of the flow regulator <NUM>.

The fluid is then in contact with the uppermost annular surface of the upper end <NUM> of the piston <NUM>. Since the fluid is pressurised, and this pressure will not be matched by corresponding pressure acting on the bottom surface of the piston <NUM>, this fluid exerts a downward force on the piston <NUM> with respect to the main body <NUM>.

As this occurs, the shear pins <NUM> that initially joined the piston <NUM> to the main body <NUM> will break, allowing longitudinal movement between the piston <NUM> and the main body <NUM>. The piston <NUM> will then be driven downwardly, against the biasing force of the compression spring <NUM>, causing the cutters <NUM> to rotate outwardly towards the deployed position, as discussed above.

<FIG> shows the resulting configuration. It can be seen that, compared to the configuration shown in <FIG>, the piston <NUM> has moved downwardly with respect to the main body <NUM>, thus moving the cutters <NUM> into their deployed position.

The tool <NUM> must be rotated in order for the casing to be cut. In preferred embodiments, rotation of the drill string will be commenced before the cutters <NUM> are moved to the deployed position. The drill string may, at this stage, be rotated at around <NUM> to <NUM> rpm, although different rotational speeds may be used depending on the particular application. The drill string will build up angular momentum during this phase, which will assist the early stages of the cutting operation. In other embodiments, however, the cutters may be moved into, or towards, the deployed position before any rotation of the drill string takes place.

The ball <NUM> is then dropped, with the result that the cutters <NUM> are rotated outwardly towards the deployed position. Rotation of the drill string will continue during the cutting operation, which in some embodiments may take a few minutes.

As the cutters <NUM> begin to cut the casing, they will be progressively rotated outwardly towards the deployed position, as a result of continued fluid pressure acting on the upper surface of the piston <NUM>. As the interior surface of the casing is cut, the cutters <NUM> will be able to rotate outwardly to a greater degree. As the cutters <NUM> rotate outwardly, the piston <NUM> will move progressively further downwardly with respect to the main body <NUM>, further compressing the compression spring <NUM>.

The length of the delivery portion <NUM> of the flow regulator <NUM>, and the position of the flow regulator <NUM> within the main body <NUM>, are set so that, when the piston <NUM> has moved downwardly with respect to the main body <NUM> by a certain amount, the sealing region <NUM> of the engagement portion <NUM> is completely removed from the recess <NUM> in the upper end of the piston <NUM>. This means that fluid can now flow more freely around the lower end of the delivery portion <NUM> and through the central cavity <NUM> of the piston <NUM>. This position is shown in <FIG>.

As discussed above, when the sealing region <NUM> of the delivery region <NUM> is received in the widened recess <NUM> of the piston <NUM>, fluid can preferably flow between the exterior of the sealing region <NUM> and the interior of the recess <NUM>, and the flow area may be equivalent to a pipe having a <NUM>/<NUM>" (<NUM>) or <NUM>/<NUM>" (<NUM>) diameter. When the sealing region <NUM> is removed from the recess <NUM>, the resulting flow area around the narrowed region <NUM> and the recess is greater, and may be equivalent to a pipe having a diameter of <NUM>/<NUM>" (<NUM>).

In preferred embodiments, the flow area after the sealing region <NUM> is removed from the recess <NUM> is at least <NUM> times, and more preferably at least twice, the flow area before the sealing region <NUM> is removed from the recess <NUM>.

At this point, there will be a pressure drop across the piston <NUM>, which will be detectable from the surface. Moreover, the net downward force on the piston <NUM> will be greatly reduced.

It may, for example, be expected that the casing will be cut when the cutters <NUM> reach an angle of <NUM>° to <NUM>° with respect to the main longitudinal axis of the tool <NUM>. The length and/or position of the flow regulator <NUM> may therefore be chosen so that, when the cutters <NUM> reach this angle of rotation, the sealing region <NUM> of the delivery portion <NUM> is completely removed from the piston <NUM>.

When the cutters <NUM> reach the desired angle, the resulting drop in fluid pressure will therefore be detectable from the surface, and operators at the surface will have an indication that the casing has been successfully cut. In some embodiments an O-ring or other type of seal may be formed around the sealing region <NUM> of the delivery portion <NUM>, which will lead to a more distinct and recognisable pressure drop as the sealing region <NUM> of the delivery portion <NUM> is completely removed from the piston <NUM>.

As with conventional cutting tools, the torque that will need to be applied to the drill string to maintain the desired rotational speed during the cutting operation will be relatively high. Once the casing has been cut, however, the resistance experienced by the cutters will drop, and this will lead to a drop on torque which will be detectable from the surface. However, a drop in torque could equally arise from the cutters having broken or failed. Having a drop in pressure, arising from the sealing region <NUM> being removed from the recess <NUM>, provides a direct measurement of the extent to which the cutters <NUM> have been rotated outwardly, and hence gives a valuable second confirmation that the cutting operation has concluded successfully.

This drop in fluid pressure is likely to lead to a decrease in the net downward force on the piston <NUM>. If the operators wish to continue further cutting, the flow rate can be increased, to increase the pressure and continue driving downward movement of the piston <NUM> with respect to the main body <NUM>.

As can be seen in (for example) <FIG>, a downward-facing shoulder <NUM> is formed in the external surface of the piston <NUM>, spaced apart from the widened upper end <NUM> thereof. The spacing of this shoulder <NUM> from the widened upper end <NUM> is such that, when the cutters <NUM> have rotated through <NUM>° or approximately <NUM>° from their initial position (shown in <FIG>), and protrude perpendicularly or substantially perpendicularly with respect to the longitudinal axis of the main body <NUM>, the shoulder <NUM> comes into contact with the upward-facing shoulder <NUM> formed where the region <NUM> of increased thickness of the main body <NUM> begins. This position is shown in <FIG>. The skilled reader will understand that this prevents rotation of the cutters <NUM> beyond this position.

In alternative embodiments, the downward-facing shoulder <NUM> could be placed at a different distance from the widened upper end <NUM> of the piston <NUM>, so the shoulder <NUM> comes into contact with the upward-facing shoulder <NUM> formed where the region <NUM> of increased thickness of the main body <NUM> begins when the cutters are at a different angle, for instance <NUM>° with respect to the longitudinal axis of the main body <NUM>.

While the cutting operation is underway, and if the cutters <NUM> are extended to protrude at <NUM>° or substantially <NUM>° with respect to the longitudinal axis of the main body <NUM>, the drill string may be raised or lowered. This may allow additional regions of casing to be cut in an upward or downward direction, e.g. to create an opening in the casing rather than simply cutting the casing at one depth or level.

To stop the cutting operation, the fluid flow, and thus pressure, in the drill string is reduced or stopped. The compression spring <NUM> will then drive the piston <NUM> upwardly with respect to the main body <NUM>, thus returning the cutters <NUM> to the retracted position.

As described above, when the piston <NUM> moves downwardly with respect to the main body past the sealing region <NUM> of the delivery portion <NUM> of the flow regulator <NUM>, equal or substantially equal fluid pressure will act on the upper end lower surfaces of the piston <NUM>, maintaining the piston in position.

In the embodiments shown the flow regulator <NUM> is fixed in place longitudinally with respect to the main body <NUM>. However, in other embodiments the flow regulator <NUM> may float longitudinally within the main body <NUM>. In these embodiments, there may be a stop member protruding from the inner wall of the main body <NUM> at a suitable location, either formed by a shoulder which is formed as part of the main body <NUM> or, for instance, a snap ring which is installed in a groove in the interior surface of the main body <NUM>.

Before the ball <NUM> is dropped the delivery portion <NUM> of the flow regulator <NUM> may be received in the upper end of the piston <NUM>, as shown in the attached figures. When the piston <NUM> is driven downwardly, the flow regulator <NUM> may initially move with the piston <NUM>, but once the flow regulator <NUM> contacts the stop member, the flow regulator <NUM> will not move downward any further, and as the piston <NUM> continues to move downwardly with respect to the main body <NUM>, the piston <NUM> will clear the sealing region <NUM> of the delivery portion <NUM> of the flow regulator <NUM>, as discussed above.

If it is necessary to cut the casing again in a further position, the drill string can be raised, or lowered (as appropriate), and the cutting sequence begun again, i.e. the drill string is rotated, and the flow and/or pressure in the drill string is increased so that the biasing force of the compression spring <NUM> is overcome and the cutters <NUM> are deployed once more.

This cutting sequence can be repeated as many times as is necessary.

Once the casing has been cut, the drill string, including the tool <NUM>, may be retrieved. The casing itself may then also be retrieved, and this is likely to take place after the drill string has been raised.

Alternatively, a retrieval arrangement can be included in the drill string to allow the casing to be engaged and lifted once it has been cut. For instance, the drill string may include a fishing tool such as a spear, and/or a pack-off arrangement, to grip or otherwise engage the casing and raise the casing along with the drill string itself. The skilled reader will appreciate how this may be achieved, and which kinds of retrieval arrangement will be most suitable for use.

It is envisaged that the retrieval arrangement will be located above the tool <NUM> in the drill string, although this is not essential.

As discussed above, in preferred embodiments of the invention the piston <NUM> is substantially pressure balanced, in that the surface area of the top surface of the piston <NUM> is equal or substantially equal to the surface area of the bottom surface of the piston <NUM>. In other embodiments, however, the piston may not be pressure balanced. For instance, the collar <NUM> that is fitted around the lower end of the piston <NUM> in the illustrated embodiments may be omitted or replaced by one with a smaller diameter. Additionally, or alternatively, the collar <NUM> may be scalloped or otherwise include flow passages/areas, so that it provides support and registration within the central passage <NUM> of the piston <NUM>, but does not present a significant flow restriction. In such embodiments the surface area of the upper surface of the piston <NUM> may be at least <NUM>% greater than that of the lower surface of the piston <NUM>.

The result of this would be that, prior to the ball being dropped to initiate the cutting operation, the piston will move much more readily in response to changes in fluid pressure within the drill string. However, the use of a compression spring <NUM> of suitable properties, and/or the use of suitable shear pins or other frangible connections, will be sufficient to prevent unwanted movement of the piston prior to the commencement of the cutting operation.

It will be understood that tools embodying the invention provide a robust, simple and reliable way for a casing to be cut, in the context of a single-trip operation to seal and abandon a wellbore.

<FIG> shows two different views of a debris catcher <NUM>, for installation in the space around the delivery portion <NUM> of the flow regulator <NUM>. <FIG> shows the debris catcher <NUM> when installed in position in the embodiment shown in <FIG>.

The debris catcher <NUM> has a sleeve portion <NUM>, which is cylindrical or substantially cylindrical, and which in use is positioned in the annular chamber around the delivery portion <NUM> of the flow regulator <NUM>, to lie adjacent or near the flow apertures <NUM>. The sleeve portion <NUM> has a number of holes <NUM> formed therethrough, which are preferably relatively small, and may for example have a diameter of <NUM> (<NUM>/<NUM>"). In the example shown in <FIG>, these holes are arranged into a series of groups <NUM>, one of which will (in use) align with each of the flow apertures <NUM> of the flow regulator <NUM>.

The debris catcher <NUM> also has a flange portion <NUM>, which is preferably wider than the sleeve portion <NUM> and protrudes outwardly from one end of the sleeve portion <NUM>, preferably at an angle to the longitudinal axis of the debris catcher <NUM>. The flange portion <NUM> has a series of apertures <NUM> formed therethrough. These apertures <NUM> are preferably larger, and may be significantly larger, than the holes <NUM> formed through the cylindrical portion <NUM> of the debris catcher <NUM>. In use the flange portion <NUM> is located at the lower end of the sleeve portion <NUM>, so that fluid passing through the sleeve portion <NUM> may then flow through the apertures <NUM> of the flange portion <NUM> to come into contact with the upper end <NUM> of the piston <NUM>.

The debris catcher <NUM> may be fixed in place with respect to the flow regulator <NUM>, or another part of the tool <NUM>. In the example shown, the flange portion <NUM> has attachment points <NUM> on its outer surface, by which the debris catcher <NUM> may be attached to a support sleeve <NUM> (shown in <FIG>) positioned at the outer side of the annular chamber.

The skilled reader will understand that the presence of the debris catcher <NUM> will help to prevent unwanted solids s from passing through the flow apertures <NUM> of the flow regulator <NUM>, and thus to maintain reliable operation of the tool <NUM>. Unwanted solids could include, for instance, swarf debris, which may arise from previous operations in the well bore, such as casing milling operations, or from a casing which is corroded or otherwise in poor condition. Such debris could enter circulation from sources such as surface storage tanks or pipe lines which conduct fluid to the well bore.

As can be seen in <FIG>, when the debris catcher <NUM> is installed in place around the flow regulator <NUM>, there is preferably a gap between the free end <NUM> of the cylindrical portion <NUM> and the underside of the blocking portion <NUM> of the flow regulator <NUM>. This means that if all, or a large proportion, of the holes <NUM> formed in the cylindrical portion <NUM> become blocked, fluid passing through the flow apertures <NUM> of the flow regulator <NUM> can pass through this gap, and hence around the cylindrical portion <NUM> to reach the apertures <NUM> of the flange portion <NUM>. Blocking of these holes <NUM> will therefore not stop operation of the tool <NUM>.

<FIG> shows a variation on the embodiment shown in <FIG>. In <FIG>, a series of ports <NUM> are provided, allowing direct fluid communication between the top end <NUM> of the piston <NUM> and the interior cavity <NUM> of the piston <NUM>, at a location below the flow regulator <NUM>. In the example shown in <FIG>, the ports <NUM> are each set at an angle to the longitudinal axis of the piston <NUM>. The ports <NUM> extend from an inlet <NUM> formed in the top end <NUM> of the piston <NUM>, and slant radially inwardly towards an outlet <NUM> formed in a wall of the interior cavity <NUM> of the piston <NUM>.

Any suitable number of ports <NUM> may be provided, spaced radially around the longitudinal axis of the tool <NUM>. For example, one, two, four, eight or twelve ports may be provided.

When the ports <NUM> are provided, after the ball <NUM> has been dropped and received in the flow regulator <NUM>, fluid can still circulate through the tool <NUM>, by flowing through the flow apertures <NUM> of the flow regulator <NUM>, then through the ports <NUM> and along the interior cavity <NUM> of the piston <NUM>. With the inclusion of the ports <NUM>, it is still possible to maintain a pressure difference across the piston <NUM>, thus driving the piston <NUM> downwardly and moving the cutters <NUM> into a cutting position, by setting a suitable circulation rate. It is expected that the circulation rate will need to be increased in order for this to be possible.

An advantage of including the ports <NUM> is that the cutters <NUM> can be activated, and circulation through the tool <NUM> maintained, so that debris resulting from the cutting of the casing can be carried away by the circulating fluid.

<FIG> shows both the ports <NUM> and the debris catcher <NUM>. It is preferred that the debris catcher <NUM> (or another filtering arrangement) is used when the ports <NUM> are provided, to prevent the ports from becoming blocked or clogged. However, the ports <NUM> may be provided without the debris catcher <NUM> (and vice versa).

<FIG> shows a further variation. In this embodiment, at the top end of the piston <NUM>, the interior cavity <NUM> (i.e. main bore) of the piston <NUM> is offset with respect to the central longitudinal axis of the tool <NUM>. In the view shown in <FIG>, the interior cavity <NUM> is offset towards the bottom of the page. Preferably the distance of the offset is <NUM> (<NUM>/<NUM>"). The result is that the interior cavity is closer to the exterior of the tool <NUM> on one side of the tool <NUM> than on the opposite side of the tool <NUM>.

The flow regulator <NUM> is shaped in an asymmetric manner to fit correctly with the offset interior cavity <NUM>, while still blocking the entirety or substantially the entirety of the wellbore, as the skilled person will appreciate.

In preferred embodiments the interior cavity <NUM> of the piston <NUM> is offset only in a region near the top end of the piston <NUM>, and further down the piston <NUM> the interior cavity <NUM> returns to being centrally or substantially centrally positioned within the tool <NUM>.

If the interior cavity <NUM> is offset away from the longitudinal axis of the tool in a first direction, this allows a single, relatively wide port <NUM> to be provided on opposite side of the interior cavity <NUM>, as shown in <FIG>. As with the ports <NUM> shown in <FIG>, the wide port <NUM> extends from an inlet <NUM> formed in the top end <NUM> of the piston <NUM>, and slants inwardly toward an outlet <NUM> in the wall of the interior cavity <NUM>, at a location below the flow regulator <NUM>.

Forming a single, relatively wide port <NUM> in this manner allows a greater total flow diameter than can be achieved with the smaller, radially distributed ports <NUM> shown in <FIG>. This means that, once the ball <NUM> has been dropped, a higher flow rate can be maintained through the tool <NUM>. Once again, this is likely to mean that the rate of circulation will need to be increased in order to maintain the necessary pressure difference across the tool <NUM> to move the cutters <NUM> to the deployed position, and to maintain the cutters <NUM> in this position. However, the higher circulation rate will allow debris arising from the cutting of the casing to be carried away more effectively.

Circulation through the tool <NUM> may also be desired for other reasons, beside carrying away debris. For instance, circulation may be needed for the operation of one or more other tools or components within the drill string.

<FIG> shows a second debris catcher <NUM>, suitable for use with the embodiment shown in <FIG>. This second debris catcher <NUM> is similar to the debris catcher <NUM> discussed above, having a cylindrical portion <NUM> with a plurality of holes <NUM> formed therethrough, which are preferably relatively small.

The second debris catcher <NUM> also has a flange portion <NUM>, which preferably has a generally circular outer perimeter <NUM>, which is offset with respect to the cylindrical portion <NUM>. The flange portion <NUM> therefore protrudes from one side of the cylindrical portion <NUM> by a greater amount on a first side than on an opposite second side. On the first side, the flange portion <NUM> has a single aperture <NUM> formed therethrough, which is preferably relatively wide. The skilled reader will understand that, when the second debris catcher <NUM> is installed in the tool <NUM> (as shown in <FIG>), the single aperture <NUM> will be aligned or substantially aligned with the inlet <NUM> of the port <NUM>. The second debris catcher <NUM> will function in a similar manner to the debris catcher <NUM> described above, and while not essential is preferred in this embodiment.

With reference to <FIG>, the above discussion mentions a single port <NUM>, and a single aperture <NUM> formed in the flange portion <NUM> of the second debris catcher <NUM>. However, this is not essential and two or more ports <NUM>, and/or two or more apertures <NUM>, may be provided in this embodiment.

It is envisaged that the tool <NUM> may be used to cut the casing "in tension", as will be understood by the skilled reader. If the casing is resting on the bottom of the well, then the casing's own weight will place the casing in compression. This may cause the casing to deform (in a manner known as "belly out"), during the cutting process, because the thinner wall may slump outwardly, as it is no longer able to support its own weight. The wall may form a chicane-type shape, leading to a much larger effective thickness or diameter to cut through.

In embodiments, an anchor may be provided as part of the drill string, and in preferred embodiments the anchor is positioned above the tool <NUM>. Before the cutting operation commences (but preferably after the bridge plug is set, if a bridge plug is used) the anchor is engaged with the casing, and the casing is lifted upwardly, with the result that the region of the casing that is to be cut is in tension. This will, as the skilled person will appreciate, improve the ease and reliability of the cutting process.

<FIG> shows an alternative embodiment. In the example shown in <FIG>, the debris catcher <NUM> is provided, but the ports <NUM>, <NUM> shown in <FIG> and <FIG> are not present.

In this example the delivery portion <NUM> of the flow regulator <NUM> is concentric with the main longitudinal axis of the tool <NUM>. However, the recess <NUM> in the upper end of the piston <NUM> is radially offset with respect to the main longitudinal axis of the tool <NUM>. In the view shown in <FIG>, the recess <NUM> in the upper end of the piston <NUM> is radially offset downwardly, towards the bottom of the page. This means that, on a first side of the delivery portion <NUM> (the top side, in <FIG>) the gap <NUM> between the delivery portion <NUM> and the recess <NUM> has a first width, and on a second side of the delivery portion <NUM> (the bottom side, in <FIG>) the gap <NUM> between the delivery portion <NUM> and the recess <NUM> has a second, greater width.

The result of this is that, once the ball <NUM> has been dropped, if fluid is to flow between the exterior of the delivery portion <NUM> and the interior of the recess <NUM>, to allow flow and circulation during the cutting operation, the gap between the exterior of the delivery portion <NUM> and the interior of the recess <NUM> is less likely to become blocked with particles and/or debris. The wider gap <NUM> on the second side of the delivery portion is more likely to allow particles and debris to pass therethrough.

Providing an offset of this kind allows, for a particular flow area, a relatively wide space which is less likely to become clogged with particles and debris. By comparison, if the delivery portion <NUM> and the recess <NUM> were concentric with one another and the same flow area was provided, this flow area would take the form of an annulus, which would be narrow enough at all points to risk becoming clogged.

In the discussion above, the delivery portion <NUM> is concentric with the main axis of the tool <NUM>, and the recess <NUM> is offset from this axis. However, in other embodiments this may be reversed, or indeed neither of these components may be fully concentric.

As an example, the gap between the delivery portion <NUM> and the recess <NUM> may be <NUM> (<NUM>"), where these components are closest together, and <NUM> (<NUM>") where these components are furthest apart. In an alternative example, the gap between the delivery portion <NUM> and the recess <NUM> may be zero (or substantially zero), where these components are closest together, and <NUM> (<NUM>") where these components are furthest apart. The invention is not limited to these examples, however.

As with other examples discussed above, a seal (which may, for example, take the form of a close tolerance ground finished part) may be provided around the outside of the delivery portion <NUM>.

It is also envisaged that filters may be provided at the surface, to remove particulate matter as fluid is circulated through the drill string.

The discussion above mentions a ball being dropped to change the operation of the flow regulator. However, any other suitable method may be used, for instance use of a dart instead of a ball, or an indexing mechanism which can be controlled from the surface through regulation of fluid supplied to the tool.

In the example shown in the drawings, a seat is formed in the flow regulator to receive a ball (or other activation object). In other embodiments the seat may be provided elsewhere in the tool, for instance in the interior of the piston. The skilled reader will appreciate how the tool may be adapted if the seat is provided in a location other than in the flow regulator.

In the embodiments discussed above the delivery portion of the flow regulator has a sealing region <NUM>, and a narrowed region <NUM>. In other embodiments, the delivery portion may omit the narrowed region, but have a shorter overall length, so that when the piston has moved by a certain amount the delivery portion is entirely withdrawn from the piston. Conversely, the delivery portion of the flow regulator may have three or more regions of different external diameters, so that the flow area around the exterior of the delivery portion changes in a series of steps as the delivery portion is withdrawn from the recess in the upper end of the piston. This will lead to a series of corresponding pressure drops, which will be detectable from the surface.

In general, the configuration of the delivery portion <NUM> of the flow regulator <NUM>, and the recess <NUM> in the upper end of the piston <NUM>, are preferably such that the flow area between these two components changes at two or more different relative positions of the piston <NUM> and the flow regulator <NUM>. This will lead to pressure differences which can be detected at the surface, to provide information to operators about the state of the tool <NUM>. For example, when the cutters <NUM> are in their initial position (shown in <FIG>), in which each cutter <NUM> touches, or lies close to, the outer surface of the piston <NUM>, a relatively wide part of the delivery portion <NUM> may come into contact with the interior of the recess <NUM>, leading to a pressure which is may be interpreted by operators at the surface as a sign that the tool <NUM> is in the initial configuration. As soon as the piston <NUM> moves away from this position, a narrower part of the delivery portion <NUM> may come into contact with, or align with, the interior of the recess <NUM>, leading to a detectably lower pressure at the surface.

In the examples discussed above, the sleeve element <NUM> does not close off the flow apertures <NUM> completely, and allows the communication of pressure through the flow apertures <NUM>. However, it is also envisaged that the sleeve element <NUM> may entirely or substantially entirely block the flow apertures <NUM>, so that fluid pressure is not communicated through the flow apertures <NUM>.

This will provide extra protection against the possibility of cement passing through the flow apertures <NUM> as the cement is displaced through the tool <NUM>.

If this is the case, then before the ball <NUM> is dropped the pressure acting on the bottom surface of the piston <NUM> will be significantly greater than the pressure acting on the top surface of the piston <NUM>, as pressurised fluid within the piston <NUM> will come into contact with the bottom surface of the piston <NUM>, but will be prevented by the sleeve element <NUM> from acting on the top surface of the piston <NUM>. Forces will therefore act on the piston <NUM>, tending to push the piston <NUM> in an upward direction. However, as mentioned above, preferably in the initial configuration each cutter <NUM> lies against an outer surface of the piston <NUM>, and the cutters <NUM> will therefore prevent upward movement of the piston <NUM> with respect to the main body <NUM> - this movement would tend to rotate the cutters <NUM> through the interaction of the teeth <NUM>, <NUM> of the cutter <NUM> and the piston <NUM>, and the piston <NUM> itself blocks this movement.

In this example, the shear screws <NUM> that initially hold the piston <NUM> in place longitudinally with respect to the main body <NUM> may be omitted, since the piston <NUM> will be maintained by fluid pressure in the initial position until the ball <NUM> has been dropped (or fluid flow through the flow regulator <NUM> is somehow otherwise diverted).

It is also envisaged that in other embodiments, i.e. where the sleeve element <NUM> does allow the communication of fluid pressure through the flow apertures <NUM>, the shear screws <NUM> may also be omitted.

In some embodiments of the invention, a ball (or other activation object) may be dropped through the drill string to a location in the tool, to divert flow within the tool (as discussed above), and the ball may then be removed from the location in the tool. This preferably has the effect of returning the tool to its state before the ball was initially dropped (aside, potentially, from the fact that the sleeve element will have been moved from its original position, and the flow apertures will remain uncovered).

One technique for this may make use of a ball (or other activation object) which is at least partly dissolvable. Such a ball may be provided, for example, by Dissolvalloy™. The ball may be dropped through the drill string and into the tool, to allow the cutting operation to commence, and then fully or partly dissolved once the cutting operation is complete, so the ball reduces in size sufficiently to pass through the outlet end of the tool. The ball may dissolve (preferably at a predictable rate) through exposure to regular drilling fluid, or there may be a substance which is added to the drilling fluid, at a time chosen by operators at the surface, to cause the ball to dissolve, or accelerate the rate of dissolution.

Another technique for this may make use of a ball which is deformable, for instance being formed from Urethane. A ball of this kind may be dropped through the drill string and into the tool, to allow the cutting operation to commence, and will remain in position within the tool while the pressure above the ball remains below a threshold. However, once the pressure above the ball exceeds the threshold, the ball will deform sufficiently to pass through the tool and out of the outlet end thereof.

In a further technique for this, the ball may be retrieved magnetically, by way of a suitable tool that is passed down the drill string to the tool.

The skilled reader will be aware of other ways in which a ball (or other activation object) may be dropped through the drill string to a location in the tool, and the ball may then be removed from the location in the tool. Once the ball has been removed, the tool will be placed into a state where the piston may be pressure balanced once more. In addition, a higher flow rate through the tool will be possible, without risk of inadvertently activating the cutters.

A further ball (or other activation object) can be dropped through the drill string to the tool, if it is desired to initiate a further cutting operation.

It will be advantageous, although not essential, to ensure that the cutters are held in place longitudinally within the bore as the cutting operation proceeds. As discussed above, in preferred embodiments an anchor or packer is set in the wellbore below the tool, before the cutting operation starts. A packer may be set in the wellbore below the tool, and a cement plug may be formed on top of this packer. In some embodiments, the tool may be longitudinally fixed or registered with respect to this first packer or anchor during the cutting operation. As an alternative a second anchor or packer may be set in the wellbore during operation of the tool, with the tool being longitudinally fixed or registered with respect to this second packer or anchor during the cutting operation. If this component is positioned above the tool, this component should preferably be an anchor, rather than a packer, to allow circulation during the cutting operation. If the component is positioned below the tool then it can be an anchor or packer. If the component is positioned above the tool, it should be retrievable. Whichever option is employed, the tool (or at least the part of the tool that contains the cutters) will be rotationally mounted with respect to the appropriate anchor or packer, for instance by means of one or more bearings, as the skilled reader will understand. It should also be borne in mind that it may be necessary to displace a relatively large quantity of cement through the second anchor or packer, to allow the setting of a plug on the packer that is set in the well bore below the tool.

As an alternative to this, the drill string may be maintained at the correct depth by using some kind of reference in the well bore, at the surface or at the well head. The skilled reader will be aware of both of these options.

It is also envisaged that the drill string may include a cutting or milling head, below the tool, but above the location of a bridge plug or the like. Once the bridge plug has been set and cement displaced onto the bridge plug, the cutting or milling head can be used, if necessary, to remove excess cement and allow access for the cutters to regions of the casing that would otherwise not be accessible because of the presence of the cement.

Claim 1:
A cutting tool (<NUM>) for cutting a casing of a wellbore, comprising:
an elongate main body (<NUM>) having an inlet end (<NUM>) and an outlet end (<NUM>), a fluid flow path being defined between the inlet and the outlet ends (<NUM>, <NUM>);
a piston (<NUM>) mounted within the main body (<NUM>) and longitudinally movable with respect to the main body (<NUM>), an upper end of the piston being open, a recess being formed at the opening;
one or more cutters (<NUM>), each cutter (<NUM>) being moveable between a retracted position and a deployed position, wherein the piston (<NUM>) and each cutter (<NUM>) engage one another so that longitudinal movement of the piston <NUM>) with respect to the main body (<NUM>) moves each cutter (<NUM>) between the deployed position and the retracted position; and
a flow regulator (<NUM>), operable to divert fluid flowing into the inlet end (<NUM>) of the tool (<NUM>) selectively along a first path, which passes through the piston (<NUM>) to the outlet end (<NUM>) of the tool (<NUM>), and a second path, in which the fluid tends to drive the piston (<NUM>) longitudinally with respect to the main body (<NUM>), wherein:
the flow regulator (<NUM>) comprises a blocking portion (<NUM>), which is provided at its upper end, and completely or substantially fills the internal diameter of the main body (<NUM>); and
a delivery portion (<NUM>) of the flow regulator (<NUM>) has a sealing region (<NUM>), which fits closely within the recess (<NUM>), or within an insert (<NUM>) in the recess (<NUM>), characterised in that:
the delivery portion (<NUM>) has one or more flow apertures (<NUM>) formed therethrough;
a sleeve element (<NUM>) is positioned within the delivery portion (<NUM>);
in an initial configuration of the tool (<NUM>), the sleeve element (<NUM>) is aligned with the flow apertures (<NUM>) so that the flow apertures (<NUM>) are at least partially occluded by the sleeve element (<NUM>);
in a second configuration the flow apertures (<NUM>) are exposed, allowing fluid to flow along the second path; and
in the second configuration, fluid may flow from the interior of the flow regulator (<NUM>) outwardly through the flow apertures (<NUM>).