Casing expander

Abstract

A static casing expander tool (10) is provided for forming a 360° expansion of a casing (32) of an underground wellbore. The static casing expander tool (10) comprises: a drill pipe connector (16) for joining to a drill pipe (17); an expander section (14); and a power section (12). The expander section (14) comprises a tapered actuator (42) for deflecting fingertips (40) of a finger collet device (34) in a radially outward direction in order to form the 360° expansion of the casing (32). The power section (12) can provide an axial force in order to drive relative axial movement of the tapered actuator (42) and the finger collet device (34) during an expansion operation. The tapered actuator (42) has a narrow end (44) facing toward the finger collet device (34) and is configured to move into an open end (46) of the finger collet device (34) to thereby deflect the fingertips (40) radially outward. The tool (10) is configured such that forces between the drill pipe (17) and drill pipe connector (16) are transmitted to the expander section (14) so that during the expansion operation the radially outward force at the fingertips (40) can be increased by controlling surface tension of the drill string (8).

Description

[0001] CASING EXPANDER

[0002] TECHNICAL FIELD

[0003] The present invention relates to a static casing expander tool for 360° expansion of a casing of an underground wellbore as well as to a method for using such a device to form a 360° expansion in a casing. Also disclosed is a disconnection device for enabling a lower part of a well tool, such as the casing expander part of the casing expander tool, to be easily disconnected and left in the wellbore whilst upper parts of the tool can be lifted out of the wellbore, as well as a method for disconnecting a part of a tool using such a disconnection device.

[0004] BACKGROUND

[0005] Wellbores for use in the oil and gas industry typically have a generally cylindrical hole in which, during normal use, a cement sheath fills an annulus that surrounds a tubular metal casing. In a plug and abandonment operation it is commonplace to perform a perforation, washing and cementing operation in order to wash the annulus and remove old cement before restoring a cement barrier ready for the wellbore to be subsequently plugged. For such operations it is of interest to be able to provide an adequate foundation for the new cement, e.g. by ensuring that there is a sealed region that extends across the annulus. This can be done by locally expanding the casing so that it extends across the cement free annulus, after which the new cement is introduced above the deformed casing portion.

[0006] This kind of a local expansion is also of benefit in other situations. One example can be seen in W02018083069A1, where they discuss corrective action in order to seal an existing cement sheath surrounding the well casing to ensure that there is no unwanted flow of fluids in the cured cement sheath. This can be needed, for example, if the cement sheath is poorly formed and / or there are unanticipated issues causing poor sealing such as unexpected factors relating to the surrounding geological formations.

[0007] A known casing expander tool for the purpose of dealing with cavities in a cured cement sheath can be seen in W02018083069A1. A radial set of hardened knucklebent fingers is pushed against the internal wall of the casing in order to deform the casing outward in a 360° bulge, thereby also deforming and compressing the cured cement sheath outside of the casing. A cone / wedge piece is used for expansion of the fingers, with the base of the cone initially within an open space provided at the base of the fingers, and the point of the cone toward the tip end of the fingers. When the cone is moved upward relative to the fingers then an increasing diameter part of the cone moves in between the fingertips so that the fingers are pushed outward against the casing. The degree to which the cone is moved relative to the fingers determines the outward displacement of the fingers. When the required expansion has been completed then the cone is withdrawn and the fingers, which are elastically deformable cantilever elements, move back inward to their starting position.

[0008] This type of casing expander has fingers that can be moved by a variable displacement distance according to the amount of movement of the cone, allowing for different degrees of expansion at different locations along the wellbore. This system can hence be used to seal small gaps as well as to introduce large deformations of the casing in regions where both the casing and the cement sheath require more extensive plastic deformation to create a seal and prevent gas flow. The process can be repeated at different points along the wellbore, e.g. to ensure redundancy by allowing for two or three expanded sections and thereby allow for a high degree of confidence in the sealing of the cement sheath.

[0009] Such systems for local expansion, with a static tool, should be differentiated over other types of casing expander that are used for installing or modifying the casing by expanding it along its length, i.e. for dynamic expansion rather than local, static expansion. This is typically done by sliding an expansion device along the length of a tubular casing, sometimes with the expansion device placed into the wellbore prior to installation of the tubular so that there is no need for a deployable expansion device. In contrast a static casing expander for local expansion of a casing of an underground wellbore is designed to expand the casing at a single location and typically must therefore be able to change in size between an expanded configuration for deforming the casing and a collapsed configuration for transport in the unexpanded casing. In addition, the static casing expander must be able to apply enough radial force, whilst static, to deform the casing. This places different loads on the tool and generally not means that a dynamic casing expander tool is not suitable for use as a static casing expander tool.

[0010] SUMMARY OF THE INVENTION

[0011] Viewed from a first aspect, the present invention provides a static casing expander tool for forming a 360° expansion of a casing of an underground wellbore, the static casing expander tool having a central longitudinal axis that is aligned with the longitudinal axis of the casing when the tool is in use, and the static casing expander tool comprising: a drill pipe connector for joining the tool to a drill pipe at a lower part of a drill string assembly; an expander section; and a power section for providing a force to the expander section; wherein the expander section comprises: a finger collet device having a plurality of elastically deflectable fingers with fingertips located in a circular array about the central longitudinal axis of the tool, wherein the finger collet device has an open end; and a tapered actuator for deflecting the fingertips in a radially outward direction during an expansion operation to thereby expand the circular array in order to form the 360° expansion of the casing when the fingertips are deflected inside the casing; wherein the radially outward deflection of the fingers occurs when the tapered actuator and the finger collet device move axially relative to one another in a direction parallel to the longitudinal axis of the tool; and wherein the power section can provide an axial force in order to drive the relative axial movement of the tapered actuator and the finger collet device; characterized in that: the tapered actuator has a narrow end facing toward the finger collet device and is configured to move into the open end of the finger collet device during the relative axial movement to thereby deflect the fingertips radially outward; and in that: the tool is configured such that forces between the drill pipe and drill pipe connector are transmitted to the expander section so that during the expansion operation the radially outward force at the fingertips can be increased by controlling surface tension of the drill string and thereby increasing the axial force in the direction of the relative axial movement of the tapered actuator and the finger collet device.

[0012] With the above static casing expander tool it is possible to expand the casing to provide a seal across the annulus, e.g. before introducing new cement in a plug and abandonment operation, whilst optimizing the use of force from the drill pipe as well as minimizing the load on the finger collet device. After the tool is run in hole to the target depth then the power section may be activated to exert an axial force between the finger collet device and the tapered actuator. In effect the power section can pull the collet over the tapered actuator so that the outer parts of the fingertips engage with the casing inner surface. At this point the collet fingers will begin to deform the casing radially outwards. The surface tension (i.e. overpull at the surface mounting point of the drill string assembly, e.g. at a derrick) will increase as the power section will exert a pulling force between the surface holding point and the downhole deformation / expansion point. This surface tension would increase linearly with increasing pull force from the power section, but as explained above the tool is configured so that by controlling surface tension then the force at the fingertips can increase. This may advantageously be done by maintaining surface tension at a stable value while increasing the force and / or displacement from the power section, i.e. the force / displacement occurring between the finger collet device and the tapered actuator. To keep surface tension stable at a certain value / range (i.e. a value in the range 50 to 100 tons), the drill string may be “slacked-off” or “lowered”. What is meant by this is a “slacking off’ on surface that will cause a slight movement of the drill string downwards (e.g. reducing stretch in the drill pipes) and thus aid in moving the tapered actuator relative to the finger collet device. This creates a direct force transfer from the tapered actuator through the fingertips into the casing without particularly increasing the force through the axial length of the fingers. This is because the forces may effectively include downward pressure (or reduced tension pull) at the tapered actuator, which does not include a force path along the axial length of the fingers, rather than only relying on a “pull” of the finger collet device toward the tapered actuator, which necessarily involves transmission of forces along the axial length of the fingers. This allows for optimized force transfer with benefits including a reduced risk of buckling the fingers and / or reduced friction forces between the finger collet device and the tapered actuator.

[0013] Having the narrow end of the tapered actuator facing toward the finger collet device and configured to move into the open end of the finger collet device results in a considerably different operating principle compared to prior art such as WO2018083069 where a tapered actuator faces the opposite way, with its wide end inside the finger collet device. The inverse configuration of the prior art cannot make use of the same principles to transfer forces from the drill pipe to the expander section.

[0014] In some examples the open end of the finger collet device faces upward toward the drill pipe connector and the narrow end of the tapered actuator hence faces downward in order to enable it to be moved into the open end. In this case the tapered actuator is located above the finger collet device when the tool is in use. This aids in ensuring that the fingertips are locked in place during the transmission of forces from the drill pipe, with the tapered actuator moving axially downwards whilst the fingertips move radially outwards.

[0015] The tool is configured such that forces between the drill pipe and drill pipe connector are transmitted to the expander section. This may be done by having a rigid mechanical link between the drill pipe connector and the expander section. In cases where the tapered actuator is above the finger collet device, as discussed above, then there may be a rigid mechanical link between the drill pipe connector and the tapered actuator. The rigid mechanical link may be a fixed part of the power section, such as an outer housing thereof. Thus, the tapered actuator may be rigidly connected to a fixed part of the power section (for transfer of axial forces from the drill pipe connector) whilst the finger collet device may be connected to an actuatable part of the power section, i.e. an actuatable part that provides the axial force and / or displacement from the power section to the expander section. The static casing expander tool is static in the sense that it is configured to be used to expand the casing without movement of the expander section along the length of the casing. Thus, it creates a local 360°expansion which may be envisaged as a 360° bulge / outward dent in the diameter of the casing at the point of expansion. The 360° expansion may involve an increase in the diameter about the full circumference of the casing, typically with a similar degree of expansion at all points around the circumference, i.e. avoiding uneven expansion as might be cause by applying pressure at discrete / separated locations, rather than using elastically deflectable fingers located in a circular array. The static casing expander tool is capable of applying adequate radial force to outwardly deform a casing whilst the tool is held in the required position, i.e. without expanding whilst sliding. The radial force may for example be a total outward force of at least 1000 tons or optionally at least 1300 tons, e.g. about 1500 tons or more. In contrast to dynamic casing expansion tools movement of the tool is not used during the main expansion step and in fact the static casing expansion tool may be incapable of operation whilst sliding, e.g. because the finger collet system would jam and / or because the force transfer between the tapered actuator and the fingertips would be disrupted and / or because the force transfer to the expander section from the power section or the drill pipe would be disrupted.

[0016] Each finger has a fingertip at the open end of the finger collet device. Each finger may be a finger length extending from the fingertip to a base portion, wherein the fingers are each connected to the tool at the base portion. The circular array of fingers preferably includes at least 10 fingers, optionally at least 12 fingers. There may for example be 10 to 24 or 12 to 20 fingers. The fingers may be symmetrically located about the central longitudinal axis of the tool and advantageously each have a similar form, e.g. a similar or identical profile, length and / or stiffness. The fingertips may protrude outwardly from the finger collet device so that when deflected outward, i.e. with the expander section in an expanded configuration, the fingertips extend outward of the maximum diameter of the remainder of the tool and thereby allow for deformation of the casing. The fingertips may have a V shape profile, e.g. a knuckle formation, protruding outward from the main part of the fingers. The protrusion may extend by at least 20mm from the outer surface of the main part of the finger, e.g. an extension of 20-40mm, or more. The total depth of the fingertips, i.e. the axial extent of the most protruding part of the fingertips, may be at least 40 mm, e.g. 40-60 mm.

[0017] The fingers are each connected to the tool at their base portions and may form elastically deflectable cantilever structures with the base portions integrally connected with a base end of the finger collet section, which may for example comprise a solid form (e.g. an outer ring or a disc) that extends around the full circumference of the expander section. As noted above the strength / cross-sectional size of the fingers (e.g. the cantilever structures) may be reduced compared to finger collet devices in which the fingers must bear compression forces as they are pulled over the tapered actuator. The main part of the length of the fingers (i.e. the part aside from the protrusion at the fingertips) may have a depth of 30mm or less, optionally a depth of 15-25 mm, and / or a width of 60-80 mm, wherein the depth is the distance along the radial direction of the tool and the width is the distance around the circumference of the tool. The fingers may be 700-900 mm long.

[0018] The finger collet device may be configured so that when in a non-expanded configuration, e.g. with the tapered actuator withdrawn from the open end so that no radial force is applied to the fingertips, then the adjacent fingers may be in contact with one another at least at the fingertips in order to form a continuous ring of fingertips. There may be no structures in between adjacent fingers either in the non-expanded or the expanded configuration. Thus, in the expanded configuration there may be open spaces extending circumferentially between the fingers and between the fingertips. In the un expanded configuration there may be slits extending axially between adjacent fingers from the base portions to the fingertips. Stress relieving holes, e.g. circular holes larger than the gaps between the fingers, may be provided in between the adjacent base portions, e.g. at the base ends of the slits.

[0019] The expander section may have an unexpanded configuration where the fingertips are located at a narrower point of the tapered actuator (e.g. at a narrower end thereof) and an expanded configuration where the fingertips are located at a wider point of the tapered actuator (e.g. at a wider end thereof), wherein the tapered actuator moves axially into the open end of the finger collet device in order to go from the unexpanded configuration to the expanded configuration. The expanded configuration may place the outermost part of the fingertips at a diameter that is at least 20% greater than the diameter in the unexpanded configuration, optionally a diameter at least 25% greater that the outer diameter of the fingertips in the unexpanded configuration. The expanded configuration may place the outermost part of the fingertips at a diameter of at least 450 mm, optionally at least 475 mm, whereas the unexpanded configuration may place the outermost part of the fingertips at a diameter of less than 400 mm, optionally less than 375 mm. It will of course be appreciated that the size of the tool may be varied in different versions dependent on the size of the casing for which the tool is intended to be used.

[0020] The tapered actuator may have a wedge shape, e.g. a linearly varying wedge shape, when viewed in side profile. It increases in size from its downhole end to its upper end, so that the taper points downward into the open end of the finger collet device when the tool is in use in the wellbore. The tapered actuator may for example have a generally conical shape, such as a faceted cone with multiple wedge surfaces corresponding to the multiple fingers of the finger collet device. The tapered actuator is configured to move the fingers from their unexpanded configuration to their expanded configuration. The tapered actuator may be designed to increase the outermost diameter of the fingertips by at least 20% as they change from the unexpanded configuration to the expanded configuration, optionally increasing the diameter by at least 25%. The wider end of the tapered actuator may have a diameter that is at least 30% greater than the diameter at the narrower end, optionally at least 35% greater. In some examples the narrower end of the tapered actuator may have a diameter of less than 300 mm, optionally less than 275 mm, whereas the wider end may have a diameter of at least 350 mm, optionally at least 375 mm. The tapered actuator may have a wedge shape at an angle of 4-8 degrees, optionally 5-7 degrees. The tapered actuator may have a wedge shape at a slope of between 1 :8 and 1 : 12, e.g. a slope of about 1 : 10 in some examples. The relative axial displacement of the tapered actuator and the finger collet device may be at least 350 mm or optionally at least 400 mm This provides a force multiplier that can increase the radial force of the fingertips against the casing compared to the applied axial force between the tapered actuator and the finger collet device.

[0021] The tapered actuator may be an end profile of an upper part of the expander section with the finger collet device coupled to the tapered actuator via a sliding central shaft, e.g. an inner tubing, that is coupled to and / or forms a part of the power section, e.g. an actuatable part thereof.

[0022] The power section may be configured to provide an initial pull force in the range 50-100 tons during an initial movement of the finger collet device relative to the tapered actuator, whereby the fingertips may be engaged with the casing so that the tool is fixed in the required location for expansion. The power section may also be configured to increase the pull force up to more than 600 tons or more than 700 tons, e.g. about 800 tons, whilst the surface tension is controlled to remain at the initial pull force of 50-100 tons (e.g. about 75 tons or about 100 tons).

[0023] The power section may comprise a hydraulic power unit, preferably a hydraulic power unit comprising multiple piston sections each having a piston and pressure chamber. For example, there may be at least three piston sections or at least four piston sections, such as a power unit with five piston sections (or more). This allows for the power section to provide the required force and displacement for the relative movement of the tapered actuator and the finger collet device. The hydraulic power unit may comprise an actuatable part such as a shaft driven by hydraulic forces, e.g. via the piston sections, wherein the shaft can move axially to provide axial force and / or axial displacement to the expander section. The shaft may be an inner tubing that also transfers fluid pressure to each of the piston sections. The shaft may have several parts with each part spanning between adjacent piston sections, wherein the shaft diameter varies in each part with stepwise increases in diameter (and hence stronger shaft parts) for parts closer to the expander section. This allows the shaft (or inner tubing) to hold increasing compressive forces as the axial load from each subsequent piston section is added to the load from the preceding piston sections.

[0024] It will be understood from the discussion above that the static casing expander tool should be deployed using a drill pipe / drill string and therefore the invention also may extend in another aspect to a drill string assembly including the static casing expander tool at a lower part thereof. The drill string assembly may comprise a drill string extending downward into a wellbore from a surface mounting point, e.g. a mounting point at drill rig, which may be on a derrick, with the static casing expander tool at a lower part of the drill string, optionally at a bottom part thereof. The drill string may comprise interconnected drill pipes of a conventional type with the lowermost drill pipe coupled to the drill pipe connector of the static casing expander tool. In example embodiments the static casing expander tool is mounted to the lower part such that drill string tension forces can be transferred to the expander section, and in particular so that drill string axial forces may be transferred to the tapered actuator. Thus, there may be a direct mechanical link between the drill string and the tapered actuator, e.g. a rigid mechanical link as discussed above. The drill string assembly includes a surface mounting point and is configured for control of surface tension, e.g. via the control features of the drill rig, in order that during the expansion operation the surface tension can be controlled as discussed above, for example so that it is maintained at a steady value whilst the power section of the static casing expander tool is actuated. This advantageously allows for direct transfer of forces between the drill string and the expander section, with optimal use of force to move the fingertips radially outward without increasing the overp ull / surface tension of the drill string assembly. The drill string assembly may be configured to be controlled in accordance with the method aspects of the invention as set out below and may hence comprise a control system adapted to perform steps as in the method.

[0025] The drill pipe connector may be configured to connect the power section to a drill pipe of the drill string. The power section may be connected to the expander section by a transition piece.

[0026] The static casing expander tool may include a disconnection device for allowing separation of the expander section from the power section of the static casing expander tool, e.g. via disconnection at the transition piece (if present). This may be useful to allow removal of the upper parts in the event that the expander section is jammed, for example so that the power section and the upper parts of the drill string can be recovered whilst a jammed expander section is left behind. The disconnection device may thus provide an emergency release feature. When in use for a plug and abandonment operation then the jammed expander section may be left in place and utilized as a cement base. The disconnection device may include an actuatable device, e.g. a hydraulic device as a means to enable a disconnection operation. In example embodiments a ball drop activation system is used as the actuatable device. The actuatable device may trigger release of one or more locking element(s) to enable decoupling features to be used, in particular to enable decoupling of the expander section from the upper parts. The decoupling features may comprise one or more threaded connector(s), so that releasing the locking element(s) may permit relative rotation to decouple the threaded connector(s). In some examples the actuatable device is configured to release the locking element(s) by overcoming one or more retention device(s). This may be done by a sliding component, such as a sliding sleeve that is driven by hydraulic pressure upon activation of the actuatable device, e.g. by use of the ball drop activation system to block a hole in the sliding component. The sliding component may be configured to overcome the retention device(s) in order to move to a position where the decoupling features can be used. This may be done by permitting movement of locking elements in the form of locking pins, e.g. resiliently biased locking pins that are held in a locking position until the sliding component has moved, after which they move to an unlocking position driven by a resilient bias, which may for example be provided by a spring.

[0027] The disconnection device is considered novel and inventive in its own right and therefore, in a further aspect, the invention provides a disconnection device for a well tool, the disconnection device comprising: decoupling features for decoupling lower parts of the tool from upper parts of the tool, wherein the decoupling features comprise one or more threaded connector(s); one or more locking element(s) that are releasable to enable the decoupling features to be used; and an actuatable device that is configured to release the locking element(s) by overcoming one or more retention device(s) using a sliding component, wherein the sliding component is configured to overcome the retention decvice(s) in order to move to an unlocking position that releases the locking element(s) and thereby permit rotation to disconnect the threaded connector(s).

[0028] Such a disconnection device can be used for any well tool, especially tools with parts that are at risk of becoming jammed. For example the disconnection device might be used with expanders of other types than the static casing expander, including dynamic / moving casing expanders. The disconnection device might also be used with any anchor / slip based system, with casing jacks, with perforation tools and / or with cutting or spearing tools. One known tool that might usefully be combined with the proposed disconnection device is the Sumo® casing jack system produced by Archer Oiltools AS of Norway.

[0029] The use of the sliding component to overcome the retention device(s) may comprise using a sliding movement of the sliding component and / or using a force generated / transmitted by the sliding component, e.g. a force in the direction of a sliding movement of the sliding component. Overcoming the retention device(s) may involve moving and / or breaking one or more parts of the retention device(s), typically by application of a force via the sliding component. The retention device(s) may be a shearable device(s), such as a shear pin or a shear screw. As such, overcoming the one or more retention device(s) may entail shearing one or more shearable devices(s), e.g. one or more shear pin(s). Alternatively, the retention device(s) may be a biasing device(s). As such, overcoming the one or more retention device(s) may entail overcoming a biasing force of the biasing device(s). The biasing device may, for example, comprise a spring-loaded element that engages a profile. When a certain threshold is reached, e.g. by applying hydraulic pressure to the sliding component, the biasing force of the spring is overcome by contact with the profile, releasing the spring-loaded element from the profile. Consequently, the sliding component may move to a position where the decoupling features can be used. In some examples, the sliding component may comprise the spring-loaded element(s). In other examples, the sliding component may comprise the profile(s).

[0030] The disconnection device of this aspect may include additional features as discussed above. Thus, the actuatable device may be a hydraulic device and in example embodiments a ball drop activation system is used. The one or more retention device(s) may be one or more shearable device(s), such as shear pins, that are sheared by sliding of the sliding component. The sliding component may be a sliding sleeve that is driven by hydraulic pressure upon activation of the actuatable device, e.g. by use of the ball drop activation system. In this case the sliding component may be configured to shear the shear pins in order to move to a position where the decoupling features can be used. The locking elements may be in the form of locking pins that may be resiliently biased locking pins that are held in a locking position until the sliding component has moved to its unlocking position, after which the locking pins move to an unlocking position driven by a resilient bias.

[0031] The disconnection device (whether alone or combined with the casing expander) may be rotationally symmetrical. The device may be centered on the central axis of tool, such as by having the sliding component and the ball drop activation system centered on the central axis. The locking pins may be housed within a first part (e.g. a lower part) of a transition piece that is located radially outward of the sliding component. This can be the transition piece of the static casing expander tool when the disconnection device is combined with this tool, e.g. a transition piece that connects the power section to the expander section. The locking pins may have a locking position in which they extend outward from the first part of the transition piece into a second part (e.g. an upper part) of the transition piece, wherein the first part and second part of the transition piece are connected together via the one or more threaded connector(s). Thus, moving the sliding component may permit the resilient bias to drive the locking pins inwardly so that they no longer extend into the second part of the transition piece, thereby permitting relative rotation of the first part and second part of the transition piece in order to disconnect them from each other.

[0032] With or without the disconnection device then the transition piece of the static casing expander tool may connect the expander section to the power section, with the lower part of the transition piece extending through a lower part of the tapered actuator and being fixedly connected to the finger collet device at the base of the fingers, whilst the upper part of the transition piece extends through an upper part of the tapered actuator and is fixedly connected to an actuatable part of the power section, e.g. a shaft of a hydraulic power unit. In some examples there are two threaded connectors, with a first threaded connector that couples the lower part and upper part of the transition piece together and a second threaded connector that couples the tapered actuator to a fixed part of the power section. The two threaded connectors may be spaced apart both axially and radially. The fixed part of the power section, e.g. an outer housing thereof as mentioned above, may transfer axial force to the tapered actuator. The transition piece may be configured to move axially driven by the power section (e.g. a shaft of a hydraulic power unit) and to slide within the tapered actuator to provide axial force and / or axial displacement of the finger collet device relative to the tapered actuator. Such a transition piece may be present on the static casing expander tool without the disconnection device, or it may be combined with a disconnection device having a different configuration to that discussed above.

[0033] With this transition piece and the static casing expander tool of the first aspect, then axial forces at the drill pipe connector may be transmitted via rigid parts of the power section, e.g. an outer housing thereof, to the tapered actuator. When the power section is used then the transition piece transmits axial forces and / or axial displacement to the finger collet device. As explained above, by control of the surface tension then the compression / buckling forces in the finger collet device can be minimized whilst optimal force is applied for radial expansion of the fingertips.

[0034] Viewed from a yet further aspect, the present invention provides a method for expanding a casing of a wellbore using a static casing expander tool at a lower part of a drill string assembly, wherein the static casing expander tool is as defined above in relation to the first aspect or optional variations thereof, the method comprising: running the tool in hole to a target depth; activating the power section in order to exert an axial force between the finger collet device and the tapered actuator so that the fingertips apply a radial force on the casing; controlling surface tension of the drill string in order to increase the axial force between the finger collet device and the tapered actuator; and deflecting the fingertips in a radially outward direction to thereby form a 360° expansion of the casing.

[0035] The step of controlling surface tension may comprise maintaining the surface tension at a required level, or within a certain range. Thus, for example, the step of activating the power section may be done first resulting in increased surface tension, e.g. a tension in the range 50-100 tons, with the surface tension then being controlled to remain within that range or in a narrower range whilst the power section continues to be activated for axial displacement of the tapered actuator relative to the finger collet device.

[0036] This method may advantageously be carried out in a wellbore with an annulus that is free of cement, such as a washed annulus that has been prepared by a perforation and washing operation. The method may be performed as a part of a plug and abandonment operation, in particular to provide a casing expansion that is suitable as a foundation for cementing during plug and abandonment. The method may alternatively be performed for the purpose of sealing cavities in or adjacent to a cured cement sheath surrounding the well casing. The method may be repeated at several depths in order to be sure of obtaining the required effect.

[0037] When the collet fingertips reach the casing and start biting into the casing, beginning to deform the casing radially outwards, then the surface tension will increase as the power section will exert a pulling force between the surface holding point and the downhole deformation point. The surface tension may be controlled even while increasing the force / displacement from the power section. To keep surface tension stable at a certain value / range (i.e. a value in the range 50 to 100 tons, such as 75 tons), then the drill string may be slacked off (or lowered), e.g. by suitable control of the drill rig such as by control of the surface holding point. The method may comprise suitable control of the surface tension simultaneous with increasing the forces at the power section, e.g. increasing hydraulic pressure for a hydraulic power unit.

[0038] The method may include deflecting the fingertips radially outward until a required deformation of the casing is achieved or until a maximum outward displacement is reached.

[0039] Any of the features discussed above in connection with the tool of the first aspect may be present in relation to the method. One particular example of this is the use of the disconnection device described above, e.g. a disconnection device for allowing separation of the expander section from the power section of the static casing expander tool such as via disconnection at the transition piece, where present. This can be done to allow a later step of removal of the upper parts, e.g. in the event that the expander section is jammed. Thus, the method may include determining that there is a jammed expander section, separating the expander section from the power section, and recovering the power section and the upper parts of the drill string whilst the jammed expander section is left behind. If the method is a part of a plug and abandonment operation then the jammed expander section may be left in place and utilized as a cement base.

[0040] A method for operating a well tool using such a disconnection device is considered novel and inventive in its own right and therefore, in a further aspect, the invention provides a method for disconnecting a lower portion of a well tool using a disconnection device for a well tool, the disconnection device comprising: decoupling features for decoupling lower parts of the tool from upper parts of the tool, wherein the decoupling features comprise one or more threaded connector(s); one or more locking element(s) that are releasable to enable the decoupling features to be used; and an actuatable device that is configured to release the locking element(s) by overcoming one or more retention device(s) using a sliding component, wherein the sliding component is configured to overcome the retention device(s) in order to move to an unlocking position that releases the locking element(s) and thereby permit rotation to disconnect the threaded connector(s). The method may comprise: activating the actuatable device to thereby overcome the retention device(s) using the sliding component; moving the sliding component to the unlocking position to thereby release the locking elements; and rotating upper parts of the well tool, e.g. by rotating the drill string, to disconnect the threaded connector(s) and thereby decouple the lower parts.

[0041] The disconnection device used in the above methods may include any of the features of the disconnection device that are discussed above for the preceding aspects.

[0042] Certain embodiments of the invention will be described in further detail below, by way of example only, and with reference to the accompanying drawings.

[0043] LIST OF FIGURES

[0044] Figure 1 shows cross section of a lower part of a drill string with a static casing expander tool;

[0045] Figure 2 is a close-up cross section view of a hydraulic piston B in a power section of the static casing expander tool of Figure 1;

[0046] Figures 3A and 3B are cross sections showing unextended and extended configurations of a part of the power section including two hydraulic piston sections B and C; Figures 4A and 4B shows an expander section of the tool with Figure 4A being an axial cross section as marked I-I in Figure 1 and Figure 4B showing an enlarged side cross section of the region G in Figure 1;

[0047] Figures 5 A and 5B each show a similar cross section to Figure 4B, with the expander section in an expanded configuration, where Figure 5A shows a disconnection device not yet activated and Figure 5B shows the disconnection device activated;

[0048] Figure 6 shows the expander section of the static casing expander tool whilst it is in an unexpanded configuration;

[0049] Figure 7 shows an expanded configuration of the expander section;

[0050] Figure 8 illustrates the forces applied during a first phase of expansion using a cross section and a schematic diagram;

[0051] Figure 9 illustrates the forces applied during a second phase of expansion using a cross section and a schematic diagram;

[0052] Figures 10A to 10C are schematic diagrams showing the operation of the static casing expander tool in a typical offshore wellbore;

[0053] Figure 11 shows a part of the expander section in a first cross section with a ball drop activated to trigger the disconnection device;

[0054] Figure 12 shows the same parts as in Figure 11 in a second cross section at 90 degrees to the first cross section of Figure 11;

[0055] Figure 13 is a close-up cross section view showing more detail of parts of the disconnection device as seen in Figure 11; and

[0056] Figure 14 is a close-up cross section view showing more detail of parts of the disconnection device after activation.

[0057] DESCRIPTION OF EXAMPLES

[0058] Figure 1 shows a static casing expander tool 10 in full, with key parts being shown in greater detail in the later Figures. In Figure 1 the upper left is the uphole direction relative to a wellbore and the lower right is the downhole direction. The static casing expander tool 10 comprises two main parts, a power section 12 and an expander section 14. The power section 12 is connected at its upper end to a drill pipe connector 16 for onward connection to the main part of a drill string 8 (e.g. by connection to a drill pipe 17), which is used to deploy the static casing expander tool 10 as depicted in later Figures. The function of the power section 12 is to provide an axial force to the expander section 14. The power section 12 comprises one or more piston sections 18 configured to translate axially in the up-hole direction as a result of fluid pressure being increased inside the tool 10. Each piston 18 thus provides a hydraulic power unit. The power section 12 of Figure 1 includes five different piston sections 18 (labelled B, C, D, E and F). Figure 2 illustrates details of one piston section, B, and it will be appreciated that all the piston sections 18 are similar. As seen in Figure 2 the piston section 18 comprises a piston 20, an inner tubing 22 connected to the piston 20 and a fluid port 24 that allows fluid communication between the inner tubing 22 and a piston chamber 26. Thus, when fluid pressure is increased in the inner tubing 22, fluid enters the piston chamber 26 through the fluid port 24 and pushes the piston 20 towards the left in the Figure. The inner tubing 22 moves to the left together with the piston 20. An outer housing 28 of the power section 12 and a piston base 30 remain static. The piston base 30 is rigidly fixed to the outer housing 28 and has a central bore that allows the inner tubing 22 to slide in the axial direction.

[0059] Figures 3 A and 3B below show a close-up view of two of the piston sections 18 of Figure 1 (B and C), with Figure 3A showing the power section 12 in an inactivated state (as in Figure 2) whilst Figure 3B shows an activated state. Thus, in Figure 3B, fluid pressure in the piston chambers 26 has been increased and the two pistons 18, together with the inner tubing 22, have moved to the left in the Figure (i.e. towards the uphole direction of the wellbore). This displacement moves the expander section 14 to an expanded state, as shown in later Figures and as explained in more detail elsewhere herein. The inner tubing 22 transfers fluid pressure to each of the piston sections 18 as well as transferring axial forces from each of the pistons 20 to the subsequent piston sections 18 and eventually to the expander section 14. The inner tubing 22 is in several parts with each part spanning between adjacent piston sections 18, wherein the inner tubing 22 diameter varies in each part with stepwise increases in diameter (and hence stronger tubing) for parts closer to the expander section 14. This allows the inner tubing 22 to hold increasing compressive forces as the axial load from each subsequent piston section 18 is added to the load from the preceding piston sections 18.

[0060] Fluid pressure is supplied to the inner tubing 22 from the surface through drill pipe 17 of a drill string and to the static casing expander tool 10 via the drill pipe connector 16. It is important for the static casing expander tool 10 that drill pipe is used since this allows for advantageous use of controlled surface tension when moving the expander section 14 from the un expanded to the expanded configuration.

[0061] The function of the expander section 14 is to expand a casing 32 radially upon activation, as depicted in later Figures. Thus, the expander section 14 translates the axial force provided by, and through, the power section 12 to a radial force acting against the casing 32. Figures 4A and 4B illustrate two orthogonal cross-sectional views of the expander section 14 in its inactivated state. Figures 5 A and 5B show the expander section 14 an activated state, i.e. an expanded configuration in cross section. In Figure 5A the normal situation is shown, where a disconnection device 35 has not been activated. Typically the tool would be used like this with the expander section 14 being activated / deactivated as needed. In Figure 5B another configuration is shown where in addition a disconnection device 35 has been activated with movement of a sliding sleeve 37 (a ball 74 that may be used to activate the disconnection device 35 is omitted from this Figure). The use of the disconnection device 35 is discussed in further detail below. Figures 6 and 7 show external views of the expander section in the inactive (unexpanded) and activated (expanded) configurations respectively.

[0062] The expander section 14 comprises a finger collet device 34 that includes a plurality of fingers 36 with respective V-shaped protrusions 38 (shown well in Figures 6 and 7) at fingertips 40. The fingers 36 are elastically deflectable at their fingertips 40, and supported in cantilever fashion from a collet base section 50, to which a base 52 of each of the fingers 36 is connected. The fingers 36 are separated by slits 54 which terminate at the base end with a stress relieving hole 56. Upon activation of the tool, the finger collet device 34 is pulled by the power section 12 towards the uphole direction (i.e. towards the left in these Figures) and over a tapered actuator 42, which in this example takes the form of a faceted cone shape. The tapered actuator 42 has its narrower end 44 directed downwards into an open end 46 of the finger collet device 34 and its wider end 48 directed upwards toward the power section 12. The open end 46 of the finger collet device 34 is surrounded by the fingertips 40 that hence form a circular array.

[0063] When the expander section 14 moves to the expanded configuration, which is shown in Figure 5A, the tapered actuator 42 and the finger collet device 34 move axially relative to one another, with the tapered actuator moving from the position as in Figure 4B, where the narrower end 44 is at the start of the open end 46 of the finger collet device 34, toward the position seen in Figure 5A, where the wider end 48 has moved toward and into the open end 46 of the finger collet device 34. In this way the fingers 36 are deflected with the fingertips 40 and their V-shaped protrusions 38 being forced radially outwards. Also visible in Figures 4A, 4B, 5A and 5B is the inner tubing 22, which is connected to the pistons 20 of the power section 12. The finger collet device 34 is connected to the inner tubing 22 via the collet base section 50, to which the base 52 of each of the fingers 36 are connected. This is done through a transition piece 58, 60, which moves together with the inner tubing 22.

[0064] As explained above in relation to the power section 12, the inner tubing 22 moves in the up-hole direction together with the pistons 20 upon an increase in fluid pressure within the tool 10.

[0065] The transition piece 58, 60 of the static casing expander tool 10 connects the expander section 14 to the power section 12 via the inner tubing 22. A lower part 60 of the transition piece 58, 60 extends through a lower part of the tapered actuator 42 and is fixedly connected to the finger collet device 34 at its base 52. An upper part 58 of the transition piece 58, 60 extends through an upper part of the tapered actuator 42 and is fixedly connected to the inner tubing 22. In some examples there are two threaded connectors that join the transition piece 58, 60 and the tapered actuator 42 to the power section 12. There can be a first threaded connector 62 that couples the lower part 60 and upper part 58 of the transition piece together and a second threaded connector 64 that couples an upper end of the tapered actuator 42 to a fixed part of the power section 12, such as the outer housing 28. The outer housing 28 of the power section 12 transfers axial force from the drill pipe connector 16 to the tapered actuator 42. The transition piece 58, 60 is configured to move axially driven by the power section 12 due to movement of the inner tube 22. The transition piece 58, 60 hence slides within the tapered actuator 42 to provide axial force and / or axial displacement of the finger collet device 34 relative to the tapered actuator 42.

[0066] As best seen in Figures 6 and 7 the effect of the expander section 14 is to increase the diameter of the circular array formed by the fingertips 40. When in use the result is that a casing 32 that is around the tool 10 will be expanded around its full circumference upon activation of the tool 10. In other words, activating the tool will result in a 360° bulge in the casing 32. Once the required deformation of the casing 32 has been completed then the expander section 14 can be moved back to the unexpanded configuration, with a reversal of the relative axial movement of the tapered actuator 42 and the finger collet device 34 resulting in reversal of the deflection of the fingers 36 as they return to their original position due to elasticity in the material of the finger collet device 34. If desired the tool 10 can be moved and the process repeated, e.g. to provide for redundancy when creating a seal across the annulus by deformation of the casing 32 at multiple points.

[0067] With this static casing expander tool 10 axial forces at the drill pipe connector 16 are transmitted via rigid parts of the power section 12, e.g. the outer housing 28 thereof to the tapered actuator. When the power section 12 is activated then the transition piece 58, 60 transmits axial forces and axial displacement of the inner tubing 22 to the finger collet device 34. In addition, as explained further below with reference to Figures 8 to 10, by control of surface tension then compression / buckling forces in the finger collet device 34 can be minimized whilst optimal force is applied for radial expansion of the fingertips 40. Figure 8 shows the forces in a first phase of a method of using the static casing expander tool 10, in which after the tool 10 is run in hole to the target depth then the power section 12 is activated to exert an axial force between the finger collet device 34 and the tapered actuator 42 by a pulling force on the transition piece 58, 60 applied by the inner tubing 22 (not visible in Figure 8). This allows the power section 12 to pull the fingers 36 up over the tapered actuator 42, i.e. via relative axial movement, so that the protruding outer parts 38 of the fingertips 40 engage with the inner surface of the casing 32. At this point the protruding outer parts 38 will begin to deform the casing 32 radially outwards. The tension in the drill string 8 above the tool 10 will increase as the power section 12 exert a pulling force between the surface holding point and the downhole tool. This surface tension tends to linearly increase with increasing pull force from the power section 12, but in a second phase of the method, as seen in Figure 9, the tool 10 is configured so that by controlling surface tension then the force at the fingertips 40 can increase.

[0068] This is done by maintaining surface tension at a stable value while increasing the force and / or displacement from the power section 12. The result is shown in the schematic drawing at the right in Figure 9, which illustrates a direct force transfer from the tapered actuator 42 through the fingertips 40. This allows for a force transfer into the casing 32 without particularly increasing the compressive force through the axial length of the fingers 36. This results in optimized force transfer with a reduced risk of buckling the fingers 36 and reduced friction forces between the finger collet device 34 and the tapered actuator 42.

[0069] Figures 10A to 10C show an example using a typical scenario in which a derrick 66 supports a drill string 8 that holds the static casing expander tool 10. As seen in Figure 10A the tool 10 is run in with the expander section 14 in the unexpanded configuration, so it can easily pass through the casing 32. The surface tension 68 is steady at a low value. Then, as shown in Figure 10B, expansion is initiated by the power section 12 using the process described above in relation to Figure 8. The force applied by the power section 12 increases the surface tension 68, e.g. to a value in the range 50-100 tons. Finally, as shown in Figure 10C, the surface tension 68 is controlled, e.g. by partly “slacking off’ the drill string 8, so that it remains at the same or a similar value to that of Figure 10B whilst the power section 12 continues to displace the moving parts of the expander section 14. During this phase, as discussed above in connection with Figure 9, there is increased force transfer / reduced increase in tension between the drill string 8 and the tool 10, so that forces at the expander section 14 are optimized without excessive force through the fingers 36 of the finger collet device 34. The power section 12 may apply a pull force of at least 600 tons, e.g. perhaps 800 tons, and this is converted into radial force with minimal friction losses giving an optimized radial deformation force of perhaps 1500 tons. Deformation of the casing 32 can be continued until the required degree of expansion has been achieved, e.g. to form a seal across the width of the annulus 70.

[0070] The static casing expander tool 10 also comprises a disconnection device 35 which allows a part of the tool 10 to be retrieved from the well in case the tool gets stuck in its activated state. This disconnection device 35 can also be used for other tools that may need disconnection in similar circumstances. The disconnection device 35 may be referred to as an emergency disconnect or downhole disconnection device. It is visible in some of the earlier Figures and its features are seen in more detail in Figures 11 to 14. In this example the disconnection device 35 allows the upper part 58 of the transition piece 58, 60 to be disconnected from the lower part 60 of the transition piece 58, 60, whilst the tapered actuator 42 is disconnected from the outer housing 28 of the power section 12. After the disconnection is completed then the drill string 8 with the power section 12 and the upper part 58 of the transition piece may be retrieved from the well, while the lower part 60 of the transition piece is left in the well along with the tapered actuator 42 and the finger collet device 34. When the tool 10 is used as a part of plug and abandonment operation then the jammed parts of the expander section 14 can be left in place and utilized as a cement base.

[0071] As seen in Figures 11-14 the disconnection device 35 comprises a sliding sleeve 37 with a ball seat 72 configured to receive a ball 74. Figures 11 and 14 show the ball 74 being held by the ball seat 72. Figures 12 and 13 show the ball seat without the ball 74 being present. The sliding sleeve 37 acts as a retainer for locking pins 76 that connect the upper part 58 of the transition piece to the lower part 60 of the transition piece. The locking pins 76 are best seen in Figures 13 and 14, which also show disc springs 78 that bias the locking pins 76 inwards toward the sleeve 37. In this example the disconnection device 35 uses a retention mechanism in the form of a shear pin and hence the sleeve 37 with ball seat 72 is initially kept in position by at least one shear pin 80, as seen in Figure 12, which shows a cross section where the tool 10 is sliced at 90 degrees compared to the other. Thus, the shear pin 80 is located a quarter turn around the circumference of the sliding sleeve 37 compared to the location of the two locking pins 76. The locking pins 76 are housed in the lower part 60 of the transition piece 58, 60 and when the sliding sleeve 37 is in its inactivated position, as seen in Figures 11 and 13, the locking pins 76 are forced radially outwards into engagement with the upper part 58 of the transition piece 58, 60. The locking pins 76 ensure that there can be no rotation between the transition pieces 58, 60 and as such that they remain securely connected via the first threaded connector 62.

[0072] When it is decided to disconnect the expander section 14, e.g. in a situation where the casing expander tool 10 gets stuck in its activated state as seen in Figure 5A, then the ball 74 is dropped to initiate the disconnect. Figure 11 shows the ball 74 being received in the ball seat 72 with the sliding sleeve 37 in its normal position. Figure 14 shows the sliding sleeve 37 moved in the downhole direction, i.e. to the right in the Figure, due to pressure increase in the tubing above the ball 74. This tubing is a part of the inner tubing 22 of the power section 12 and so it can receive hydraulic fluid. After the ball 74 has landed in the ball seat 72 then the pressure inside the tool 10 is increased, thereby increasing the axial force on the sliding sleeve 37, until the shear pin 80 is sheared. Thus, the force from the sliding sleeve 37 overcomes the retention device (shear pins 80). When the shear pin 80 is sheared, the sliding sleeve 37 is free to move further downwards, and this movement releases the locking pins 76 as seen in Figure 14.

[0073] When the sleeve 37 is no longer supporting the locking pins 76, the disc spring 78 pushes the locking pins 76 toward the center of the tool 10 and out of engagement with the upper part 58 of the transition piece 58, 60. Consequently, the upper part 58 and lower part 60 are free to rotate relative to each other. At the same time, the outer housing 28 of the power section 12 can rotate relative to the tapered actuator 42. This means that by rotating the drill string 8 it is possible to disconnect the first threaded connector 62 between the upper part 58 and lower part 60 and simultaneously disconnect the second threaded connector 64 between the outer housing 28 and the tapered actuator 42. Subsequently the drill string 8 with the power section 12 and the upper part 58 of the transition piece may be retrieved from the well.

[0074] In the example transition piece shown in the Figures a smaller diameter lower part 60 sits inside a larger diameter upper part 58 and the disconnection device 35 stays within the lower part 60. It will be understood that the configuration of the transition piece could be varied so that the upper and lower parts are switched, with the upper part being a smaller diameter part that sits inside a larger diameter lower part. With that alternative configuration the disconnection device 35 would be held in the upper part so that it can be removed therewith when the upper part is disconnected from the lower part. In that case when the sleeve 37 is moved and no longer supports the locking pins 76 then the locking pins 76 would move inward to disengage from the lower part in place of disengaging from the upper part.

Claims

CLAIMS:

1. A static casing expander tool for forming a 360° expansion of a casing of an underground wellbore, the static casing expander tool having a central longitudinal axis that is aligned with the longitudinal axis of the casing when the tool is in use, and the static casing expander tool comprising: a drill pipe connector for joining the tool to a drill pipe at a lower part of a drill string assembly; an expander section; and a power section for providing a force to the expander section; wherein the expander section comprises: a finger collet device having a plurality of elastically deflectable fingers with fingertips located in a circular array about the central longitudinal axis of the tool, wherein the finger collet device has an open end; and a tapered actuator for deflecting the fingertips in a radially outward direction during an expansion operation to thereby expand the circular array in order to form the 360° expansion of the casing when the fingertips are deflected inside the casing; wherein the radially outward deflection of the fingers occurs when the tapered actuator and the finger collet device move axially relative to one another in a direction parallel to the longitudinal axis of the tool; and wherein the power section can provide an axial force in order to drive the relative axial movement of the tapered actuator and the finger collet device; characterized in that: the tapered actuator has a narrow end facing toward the finger collet device and is configured to move into the open end of the finger collet device during the relative axial movement to thereby deflect the fingertips radially outward; and in that: the tool is configured such that forces between the drill pipe and drill pipe connector are transmitted to the expander section so that during the expansion operation the radially outward force at the fingertips can be increased by controlling surface tension of the drill string and thereby increasing the axial force in the direction of the relative axial movement of the tapered actuator and the finger collet device.

2. A static casing expander tool as claimed in claim 1, wherein the open end of the finger collet device faces upward toward the drill pipe connector and the narrow end of the tapered actuator hence faces downward in order to enable it to be moved into the open end.

3. A static casing expander tool as claimed in claim 1 or 2, being configured such that forces between the drill pipe and drill pipe connector are transmitted to the expander section by having a rigid mechanical link between the drill pipe connector and the expander section.

4. A static casing expander tool as claimed in claim 3, wherein when the tool is in use the tapered actuator is above the finger collet device, wherein there is a rigid mechanical link between the drill pipe connector and the tapered actuator via a rigid connection to a fixed part of the power section, and wherein whilst the finger collet device is connected to an actuatable part of the power section.

5. A static casing expander tool as claimed in any preceding claim, wherein the fingers are each connected to the tool at their base portions and form elastically deflectable cantilever structures with the base portions integrally connected with a base end of the finger collet section.

6. A static casing expander tool as claimed in any preceding claim, wherein the finger collet device is configured so that when in a non-expanded configuration then adjacent fingers are in contact with one another at least at the fingertips in order to form a continuous ring of fingertips.

7. A static casing expander tool as claimed in any preceding claim, wherein there are no structures in between adjacent fingers either in a non-expanded or an expanded configuration, wherein in the expanded configuration there are open spaces extending circumferentially between the fingers and between the fingertips, and wherein in the unexpanded configuration there are slits extending axially between adjacent fingers from the base portions to the fingertips.

8. A static casing expander tool as claimed in any preceding claim, wherein the power section comprises a hydraulic power unit comprising multiple piston sections each having a piston and pressure chamber.

9. A static casing expander tool as claimed in claim 8, wherein the hydraulic power unit comprises an actuatable part in the form of an inner tubing that provides a shaft driven by hydraulic forces as well as transferring fluid pressure to each of the piston sections, wherein the inner tubing may have several parts with each part spanning between adjacent piston sections, and wherein the inner tubing diameter varies in each part with stepwise increases in diameter for parts closer to the expander section.

10. A static casing expander tool as claimed in any preceding claim, comprising a disconnection device for allowing separation of the expander section from the power section of the static casing expander tool.

11. A static casing expander tool as claimed in claim 10, wherein the disconnection device comprises: decoupling features for decoupling lower parts of the tool from upper parts of the tool, wherein the decoupling features comprise one or more threaded connector(s); one or more locking element(s) that are releasable to enable the decoupling features to be used; and an actuatable device that is configured to release the locking element(s) by overcoming one or more retention device(s) using a sliding component, wherein the sliding component is configured to overcome the retention device(s) in order to move to an unlocking position that releases the locking element(s) and thereby permit rotation to disconnect the threaded connector(s).

12. A static casing expander tool as claimed in any preceding claim, comprising: a transition piece that connects the expander section to the power section, with a lower part of the transition piece extending through a lower part of the tapered actuator and being fixedly connected to the finger collet device at the base of the fingers, whilst an upper part of the transition piece extends through an upper part of the tapered actuator and is fixedly connected to an actuatable part of the power section; and two threaded connectors, with a first threaded connector that couples the lower part and upper part of the transition piece together and a second threaded connector that couples the tapered actuator to a fixed part of the power section.

13. A static casing expander tool as claimed in claim 12, wherein axial forces at the drill pipe connector are transmitted to the tapered actuator via rigid parts of the power section, and wherein the transition piece is arranged to transmit axial forces and / or axial displacement to the finger collet device when the power section is used.

14. A drill string assembly comprising a drill string extending downward into a wellbore from a surface mounting point, with a static casing expander tool as claimed in any preceding claim at a lower part of the drill string, wherein the static casing expander tool is mounted such that drill string axial forces are transferred to the tapered actuator, and wherein the drill string assembly includes a surface mounting point and is configured for control of surface tension in order that during the expansion operation the surface tension can be controlled whilst the power section of the static casing expander tool is actuated.

15. A drill string assembly as claimed in claim 14, comprising a control system adapted to: run the tool in hole to a target depth; activate the power section in orderto exert an axial force between the finger collet device and the tapered actuator so that the fingertips apply a radial force on the casing; control surface tension of the drill string in order to increase the axial force between the finger collet device and the tapered actuator; and thereby deflect the fingertips in a radially outward direction to thereby form a 360° expansion of the casing.

16. A method for expanding a casing of a wellbore using a static casing expander tool at a lower part of a drill string assembly, wherein the static casing expander tool is as claimed in any of claims 1 to 13, the method comprising: running the tool in hole to a target depth; activating the power section in order to exert an axial force between the finger collet device and the tapered actuator so that the fingertips apply a radial force on the casing; controlling surface tension of the drill string in order to increase the axial force between the finger collet device and the tapered actuator; and deflecting the fingertips in a radially outward direction to thereby form a 360° expansion of the casing.

17. A method as claimed in claim 16, wherein the step of controlling surface tension comprises maintaining the surface tension at a required level or within a certain range.

18. A method as claimed in 16 or 17 wherein the wellbore has an annulus that is free of cement.

19. A method as claimed in claim 16, 17, or 18, wherein the method is performed as a part of a plug and abandonment operation in order to provide a casing expansion that is suitable as a foundation for cementing during plug and abandonment.

20. A method as claimed in any of claims 16 to 19, including: determining that there is a jammed expander section, separating the expander section from the power section, and recovering the power section and upper parts of the drill string whilst the jammed expander section is left behind.

21. A method as claimed in claim 20 wherein the step of separating the expander section from the power section comprises disconnecting a lower portion of a well tool using a disconnection device as claimed in claim 11 by: activating the actuatable device to thereby overcome the retention device(s) using the sliding component; moving the sliding component to the unlocking position to thereby release the locking elements; and rotating upper parts of the well tool to disconnect the threaded connector(s) and thereby decouple the lower parts.

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