NL2035577B1

NL2035577B1 - Borehole following device

Info

Publication number: NL2035577B1
Application number: NL2035577A
Authority: NL
Inventors: Veenstra Feitze; Van Der Tempel Jan
Original assignee: Green Soccs B V
Priority date: 2023-08-10
Filing date: 2023-08-10
Publication date: 2025-02-24
Also published as: WO2025034114A1

Abstract

Title: Borehole following device Abstract Method of radially expanding a tubular element in a borehole, the method comprising steps of: - bending the tubular element radially outward and in axially reverse direction so as to form an expanded tubular section extending around an unexpanded tubular section, wherein the bending occurs in a bending zone; - increasing the length of the expanded tubular section by pushing the unexpanded tubular section in axial direction relative to the expanded tubular section further into the borehole; wherein a borehole following device is positioned in front of the bending zone such that the borehole following device is pushed further into the borehole as the unexpanded tubular section is pushed further into the borehole, and wherein the borehole passes at least partially through soft formation, for example comprising sand, mud, clay, loam, and/or silt.

Description

P135658NL00

Title: Borehole following device

TECHNICAL FIELD

The aspects and embodiments thereof relate to the field of lining boreholes through soft formation.

BACKGROUND

In the field of transporting hydrocarbons underground, methods are known in which a pipe is positioned underground. Such a pipe can for example be a steel pipe with a coating layer for corrosion protection. The coating layer may comprise asphalt bitumen, concrete, PE , HDPE, or a composite.

SUMMARY

It is an object to position a tubular element in a borehole drilled through soft formation, such as sand, mud, clay, loam, silt, or any other soft formation optionally combined with water, gases, organic matter, and/or any other matter, in any combination thereof. Soft formation may comprise loose particles, the dynamic behaviour of which may resemble that of a fluid.

Contrary to hard formation, such as rock, soft formation is more prone to collapse when a hole is drilled through said soft formation, in particular for non-vertical sections of a borehole through soft formation. As such, a tubular element may be positioned in the drilled hole not only to transport fluids, liquids, gasses, cables, or any other material through, but also to prevent collapsing of the drilled borehole.

W02012095472 discloses a system and method for radially expanding a tubular element. The method comprises the steps of bending the tubular element radially outward and in axially reverse direction so as to form an expanded tubular section extending around an unexpanded tubular section, wherein bending occurs in a bending zone; increasing the length of the expanded tubular section by pushing the unexpanded tubular section in axial direction relative to the expanded tubular section; operating a drill string, which extends through the unexpanded tubular section and is provided with a drill bit at a downhole end thereof, to drill a borehole; and operating directional drilling means, which are coupled to the drill string, to deviate the borehole and direct the borehole along a predetermined path.

In a particular embodiment, WO2012095472 suggest using a guiding sleeve for guiding the bending zone through the borehole. Because

W02012095472 relates to drilling through rock formation, the guiding sleeve is flexible to allow the guiding sleeve to follow a curved shape of the borehole through the hard rock formation. Furthermore, the guiding sleeve of

WO2012095472 has a smaller outer diameter than the inner diameter of the borehole, to allow the guiding sleeve to pass through the borehole without getting stuck in the hard rock formation.

It is an object of the present disclosure to position an expanded tubular section in a borehole drilled through soft formation. Since the known technology of expanding a tubular element in a borehole is not presently optimised or even suitable for soft formation, it is an object to make this technology more suitable for use in a borehole passing through soft formation.

It has for example been observed that the material forming the tubular element typically is not perfectly homogenous. As such, the tubular element may deviate from a centreline of the borehole when the unexpanded tubular section is pushed further into a borehole. This tendency to deviate from the axial direction may not form a problem when the borehole passes through hard formation, such as rock. However, it has been observed that the deviation of the tubular element of the course of the borehole may be problematic as the borehole passes through soft formation, in particular in a curved section of the borehole passing through soft formation.

The tubular element may for example not be homogenous when the tubular element is formed from a sheet of which opposite ends are welded together. Due to the weld, the tubular element may have different material properties at one side of the tubular element, and as a result thereof the bending zone may have a tendency to deviate from the centreline of the tubular element as the unexpanded tubular section is pushed further into the borehole.

To prevent the tubular element from deviating from the centreline of the borehole, the use of a borehole following device is envisioned.

A first aspect provides a method of radially expanding a tubular element in a borehole, the method comprising steps of: - bending the tubular element radially outward and in axially reverse direction so as to form an expanded tubular section extending around an unexpanded tubular section, wherein the bending occurs in a bending zone; - increasing the length of the expanded tubular section by pushing the unexpanded tubular section in axial direction relative to the expanded tubular section further into the borehole; wherein a borehole following device is positioned in front of the bending zone such that the borehole following device is pushed further into the borehole as the unexpanded tubular section is pushed further into the borehole, and wherein the borehole passes at least partially through soft formation, for example comprising sand, mud, clay, loam, and/or silt.

Generally, any method according to the first aspect may further comprise a step of operating a drill using a drill string extending through the unexpanded tubular section to drill the borehole.

Because the length of the expanded tubular section can be increased as the borehole is formed, the borehole can be supported by the expanded tubular section, which decreases the risk of the borehole collapsing due to being formed through soft formation.

When the borehole following device is positioned in front of the bending zone, it may be prevented or at least a chance is reduced, that the bending zone deviates from the course of the borehole.

In general, a majority, more than 75%, or even more than 90% of the borehole passes through soft formation, for example comprising sand, mud, clay, loam, and/or silt. It will be appreciated that the borehole can pass through minor sections of hard or harder formation, for example formed by rocks. It will also be appreciated that pieces of hard formation may be encountered while drilling the borehole through soft formation.

For any method disclosed herein, the borehole may have a substantially horizontal section. A borehole with a horizontal section for example allows underground transport between two locations. In particular, at least part of the borehole may have an upwards section oriented towards the Earth’s surface. As such, the borehole may have an entry point into the

Earth’s surface, but also an exit point at the Earth’s surface, wherein the exit point is located at a distance from the entry point. The distance between the exit point and the entry point may be in the order of metres, but preferably is in the order of kilometres, tens of kilometres, or even hundreds of kilometres.

The borehole can pass below at least partially below a body of water and/or land, for example land supporting farm land, built environment, and/or nature.

When the borehole starts of as and/or ends as a generally vertical borehole, or at least is oriented at an angle relative to horizontal, and the borehole also comprises a substantially horizontal section, or at least a section at a smaller angle relative to horizontal, part of the borehole may be curved at a particular radius. Typically, the bending zone of the tubular element will want to continue in a direction parallel to the orientation of the distal part of the tubular element, and the bending zone will not follow the curvature of the borehole without an additional force or a change in forces being applied to the bending zone. A normal force exerted by a borehole following device may provide for this force or change in force.

Preferably, but not necessarily, as an option for any method and device disclosed herein, an outer diameter of at least part of the borehole following device exceeds an inner diameter of the borehole. When the outer diameter of the borehole following device exceeds the inner diameter of the borehole, friction may be created between the borehole following device and the borehole. This friction may result in the borehole following device 5 following the course of the borehole better. Because the borehole passes through soft formation, the friction does not result in the borehole following device becoming stuck in the borehole, which would have happened in case of a borehole passing through hard formation. Instead of becoming stuck, the borehole following device may radially push the soft formation away sufficiently for the borehole following device to pass further into the borehole.

Preferably, but not necessarily, for any embodiment of the device and method disclosed herein, the borehole following device may comprise one or more resilient elements, such as one or more leaf springs, at an outer perimeter of the borehole following device, which one or more resilient elements in use contact an inner wall of the borehole. The one or more resilient elements may aid the borehole following device in following the course of the borehole. The resilient elements may be resiliently deformed towards a centreline of the borehole hollowing device.

When the borehole following device comprises a rigid tubular device body, in particular essentially without any internal degrees of freedom, the tubular element may follow the curvature of the borehole better while the borehole following device is pushed further into the borehole by the tubular element, in particular by the bending zone of the tubular element.

An outer diameter of the drill used to drill the borehole may be smaller than a diameter of the borehole in which the expanded tubular section is present. This may be the result of the expanded tubular section radially expanding after having been formed, for example due to stresses in the material of the tubular element.

As an option for any method and device disclosed herein, an inner part of the borehole following device may extend into the unexpanded tubular section, beyond the bending zone. Preferably, a sliding seal is formed between the inner part of the borehole following device and the unexpanded tubular section. The sliding seal on the one hand prevents or reduces foreign matter from entering a space between the bending zone and the borehole following device, and on the other hand allows the inner part of the borehole following device to be moved relative to the unexpanded tubular section. This movement is required since the unexpanded tubular section typically moves at approximately two times the speed of the borehole following device.

Foreign matter may for example comprise soft formation drilled away to form the borehole.

Preferably, but not necessarily, the entire bending zone is accommodated in a bending zone reception space of the borehole following device. The bending zone may be defined as ending where the unexpanded tubular section and the expanded tubular section are oriented substantially parallel to one another.

Any method may further comprise a step of positioning one or more rollers in the bending zone between the expanded tubular section and the unexpanded tubular section. The one or more rollers are preferably positioned in the bending zone at or near the entry of the borehole, where the bending zone is best accessible. As an option, at least one of the rollers may have a tapered outer shape towards one or both ends of the rollers. Such a tapered shape may prevent the bending zone from being bent over a sharp edge of a roller if the roller would have a non-tapered end.

Whenever a plurality of rollers is positioned in the bending zone, the rollers may be connected to form a closed loop of rollers. For example, a ball may be positioned in-between adjacent rollers to form ball-and-shoulder joints, and/or rollers may comprise male and female mating parts, wherein a male part of a first rollers mates with a female part of an adjacent second roller. When the rollers form a closer loop of rollers, a form-closed loop is formed, and it may be prevented that one of the rollers moves out of the bending zone.

Any method disclosed herein may comprise an optional step of filling at least part of an annulus between the expanded section and the unexpanded section with a lubricant, in particular wherein the lubricant comprises a mix of a liquid and solid particles. The lubricant may be pressurised, for example to control a spacing between the unexpanded tubular section and the expanded tubular section.

In use, during any of the methods disclosed herein, a radially inward flange or protrusion of the borehole following device may contact an outer wall of the expanded tubular section. Preferably, but not necessarily, the radially inward flange or protrusion is elastically deformed to contact the outer wall of the expanded tubular section, thus creating friction between said radially inward flange or protrusion and the outer wall of the expanded tubular section. Preferably, a plurality of radially inward protrusions is comprised by the borehole following device. Any radially inward protrusion may be hingedly connected to the borehole following device, for example using a living hinge. In an undeformed state, the radially inward protrusions and/or radially inward flange may define a diameter which is smaller than an outer diameter of the expanded tubular section.

In any method disclosed herein, the material forming the tubular element may the same material for the entire borehole, or different sections of the tubular element may comprise different material. As a particular example, for example when the borehole is drilled to halfway the total desired length of the borehole, a different material may be used to form the unexpanded tubular section. For example, the material may be coated and/or provided with one or more centralisers, and may in particular be suitable for transporting fluids and/or cables through. The fluids may for example be gaseous and/or liquid fluids, such as hydrocarbons or hydrogen. This material does not have to radially expanded to form an expanded tubular section when more than half of the borehole is already lined with expanded tubular section, and can thus have different properties.

A second aspect provides a borehole following device for following a borehole passing through soft formation. It will be appreciated that any embodiment according to the second aspect may be used in any method according to the first aspect. Furthermore, it will be appreciated that embodiments of borehole following devices are envisioned with any of the features discussed in conjunction with any of the methods, in any combination thereof.

According to the second aspect, the borehole following device comprises a tubular device body with a passage for accommodating a drill string through said passage. The tubular device body has a front end for guiding the borehole following device through a borehole through soft formation, and the tubular device body further comprises a bending zone reception space positioned at a distance from the front end, the bending zone reception space being arranged for accommodating at least part of a bending zone of a tubular element formed by bending the tubular element radially outward and in axially reverse direction so as to form an expanded tubular section extending around an unexpanded tubular section.

The bending zone reception space may be positioned at a rear end of the tubular device body. In use, the bending zone of the tubular element may thus push the borehole following device forward at the rear end of the tubular device body. Additionally, or alternatively, the bending zone reception space may be formed as a pocket into a rear end plane of the tubular device body. The pocket may as a further option form a curved surface with a constant radius over at least 120 degrees, preferably over approximately 180 degrees. The radius may correspond to a radius of the bending zone of the tubular element. The radius of the bending zone may be determined by theory, for example depending of material and geometric properties of the tubular element.

Preferably, but not necessarily, for any borehole following device disclosed herein, the tubular device body may have an at least partially hollow interior. This may allow the density of the borehole following device to be lowered compared to a borehole following device with a solid tubular device body. A lower density may result in the borehole following device being less prone to sink in the soft formation, and thus to follow the course of the borehole better, in particular in non-vertical sections of the borehole.

For any borehole following device disclosed herein, the materials comprised by the borehole following device an any optional hollow interior may be tuned to achieve an average density between 500-4000 kg/m? in particular between 1000-3000 kg/m.

BRIEF DESCRIPTION OF THE FIGURES

In the figures,

Fig. 1A schematically shows a borehole being formed in the Earth's surface;

Fig. 1B shows a schematic view of a detail of the borehole of Fig. 1A;

Figs. 2A and 2B schematically depict a bending zone; and

Fig. 3 shows another example of a tubular element positioned in a borehole.

DETAILED DESCRIPTION OF THE FIGURES

Fig. 1A schematically shows a well 100 with a borehole 102 being formed in the Earth’s surface 104. To form the borehole 102, a drill 402 is used. The drill 402 is typically connected to a drill string 404 which passes through the borehole 104 to the Earth’s surface 104. To provide torque and/or fluid to the drill 402, via the drill string 404, a drilling unit 406 is provided.

As depicted in Fig. 1A, the borehole 102 can have a vertical section, a curved section, and a generally horizontal section. The horizontal section allows a horizontal distance to be overcome by the borehole 102. The borehole 102 depicted in Fig. 1A may not be in a completed state, as the borehole 102 may have a further curved section and/or vertical section oriented upward towards the Earth’s surface 104. As such, the borehole may for example be used for transporting one or more fluids and/or cables from one point to another underneath the Earth’s surface.

A distance d between the drill 402 and the borehole following device 300 may be small, for example in the order of centimetres, but can conceivably also be one metre or more, or even 10 metres or more, 20 metres or more, or even 30 metres or more. For example, when the soft formation through which the drill 402 is moved is very soft, a smaller distance d the drill 402 and the borehole following device 300 may be preferred.

Fig. 1B shows a schematic view of a detail of the borehole 102 of

Fig. 1A. It is a general aim of the present disclosure to line the borehole 102 with a tubular element 200. In particular when the borehole 102 passes at least partially through soft formation, a borehole following device 300 can be used to guide the tubular element 200 through the borehole 102. Fig. 1B schematically shows a distal end of the tubular element 200 while the tubular element 200 is moved further into the borehole 102, with an example of borehole following device 300.

The tubular element 200 comprises an unexpanded tubular section 204, an expanded tubular section 202, and a bending zone 206 between the unexpanded tubular section 204 and the expanded tubular section 202. The tubular element 200 is in the bending zone 206 bent radially outward and in axially reverse direction so as to form the expanded tubular section extending around the unexpanded tubular section. In use, the expanded tubular section 202 may be forced against an inner wall of the borehole 102. To increase the length of the expanded tubular section 202, the unexpanded tubular section 204 1s pushed further into the borehole 102.

The borehole following device 300 shown in Fig. 1B is an example of only one of the embodiments envisioned for the borehole following device 300. Other embodiments may be provided with less of the optional features than the embodiment depicted in Fig. 1B. Furthermore, optional features discussed in conjunction with Fig. 1B may be readily applied to any other borehole following device 300 disclosed herein. Optional features disclosed herein and not indicated in Fig. 1B may be readily applied to the embodiment of Fig. 1B.

The borehole following device 300 comprises a tubular device body 302, with a passage 304 through the body 302. In use, the drill string 404 may pass through the passage 304 from a proximal end of the body 302 to a distal end of the body 302. As a particular option depicted in Fig. 1B, a drill string guide 316 may be positioned in the passage 304 at a closer radius to the centreline 301 than an inner wall 320 of the body 302. In use, the drill string guide 316 can restrict a radial movement of the drill string 404 relative to the borehole following device 300, in particular compared to another situation wherein the radial movement of the drill string 404 relative to the borehole following device 300 is only limited by the diameter of the passage 304.

The drill string guide 316 may be connected to the inner wall 320 of the device body 302, for example by one or more struts 318. When struts 318 are used, a passage for fluid between the inner wall 320 and the drill string guide 316 can be provided — in a downstream and/or upstream direction. In general, it may thus be preferred to provide a passage for fluid between the inner wall 320 and the drill string guide 316, also by other means than by using struts.

A bending zone reception space 308 is formed as a pocket 310 into the body 302, in particular at a rear end — also referred to as proximal end, proximal relative to the entry of the borehole — of the body 302. In use, the bending zone 206 can be accommodated in the bending zone reception space 308, as schematically depicted in Fig. 1B. The pocket 310 preferably but not necessarily has a constant radius, even more preferably over at least 120 degrees, in particular over approximately 180 degrees. The radius may correspond to the outer radius of the bending zone 206, but conceivably can also be larger or smaller than the outer radius of the bending zone 206.

Additionally, or alternatively, at least part of the outer surface of the pocket 310 — which in use can contact the tubular element 200 — is formed from a low friction material, such as Teflon or a self-lubricating material such as a solid lubricant.

In use, one or more rollers 212 may be positioned in the bending zone 206 between the expanded tubular section 202 and the unexpanded tubular section 204 — thus in an annular space 210 between the expanded tubular section 202 and the unexpanded tubular section 204.

Now referring to Figs. 2A and 2B, two schematic depictions of the bending zone 206 are shown, with the expanded tubular section 202 surrounding the unexpanded tubular section 204, in a cross-sectional view.

Figs. 2A and 2B both depict a set of rollers 212 positioned in the bending zone 206 of the tubular element 200. In the example of Fig. 2A, bearing balls 216 are positioned in-between the rollers, which allow the rollers 216 to rotate around their rotation axis 214. In the example of Fig. 2B, the balls are omitted, but the rollers 216 are still allowed to individually rotate around their rotation axis 214.

As a preferred option, the rollers 212 are not perfectly cylindrical, but instead have a convex outer surface 213. The convex outer surface shape depicted in Figs. 2A and 2B is an example of an outer shape which is tapered towards one or both ends of the rollers.

As another preferred option, additionally or alternatively, the rollers 212 and optional bearing balls 216 form a closed loop of rollers and/or a closed loop of rollers and balls. The closed loop allows forces to be transferred between adjacent rollers 212 and optional bearing balls 216 in the circumferential direction and/or may prevent rollers and/or balls from accidentally escaping from the bending zone 206.

Now referring back to Fig. 1B, wherein the borehole 102 is schematically depicted by dashed lines. It will be appreciated that, in particular when the borehole 102 passes through soft formation, a cross- section of the borehole 102 may not be a perfect circle. Preferably, as schematically indicated in Fig. 1B, an outer diameter of the borehole following device 300 exceeds an inner diameter of the borehole 102. If the borehole 102 would pass through hard formation, such as rock, this would cause the borehole following device 300 to become stuck. However, it has been observed that when the borehole 102 passes through soft formation, the larger borehole following device 300 improves how the device 300 can follow the curvature of the borehole 102.

As an option, one or more resilient elements, such as springs, in particular bow springs 312, may be comprised by the borehole following device 300 and positioned at an outer perimeter of the borehole following device 300. The resilient elements may for example be symmetrically disposed around a centreline 301 of the borehole following device.

Typically, the device body 302 will be formed of material with a higher density than the density of the soft formation. As such, the device 300 may have a tendency to sink in the soft formation. As an option, the average density of the device 300 may be decreased by providing the body with an at least partially hollow interior, for example in the form of one or more pockets 314. The size and location of the one or more pockets 314 may for example be tuned to a particular type of soft formation through which the borehole 102 passes.

As an even further option, schematically depicted in Fig. 1B, the device body 302 may comprise one or more inward protrusions 322 protruding into the bending zone reception space 308 or at least protruding radially inward. In use, the one or more inward protrusions 322 may contact an outer surface 203 of the expanded tubular section 202. In general, one or more inward protrusions 322 may be formed as a single circumferential flange, or as a set of separate protrusions which are circumferentially spaced from one another.

In the example of Fig. 1B, and thus applicable to any other embodiment of the borehole following device 300 disclosed herein, the one or more inward protrusion 322 are connected to the device body 302 in a hinging connection 324 — for example via a living hinge. The hinging connection 324 may allow the one or more inward protrusions 322 to be resiliently hinged radially outward, and thus to exert a radially inward elastic force onto the outer surface 203 of the expanded tubular section 202.

Instead of or next to the hinging connection 324, the one or more inward protrusions may be resiliently deformable in a radial direction, for example when the one or more inward protrusions are or comprise one or more springs.

As an even further option depicted in Fig. 1B, for any borehole following device 300 disclosed herein, the borehole following device 300 may comprise a scraper 330. The scraper is oriented radially outward, and in use contacts an inner wall 205 of the unexpanded tubular section 204. By virtue ofthe scraper 330, it may be prevented that foreign matter enters the bending zone reception space 308 via a pathway between the unexpanded tubular section 204 and the borehole following device 300. Preferably, the scraper 330 comprises or consists of a low friction material, such as Teflon or a self- lubricating material such as a solid lubricant. The scraper 330 is an example of a sliding seal formed between the inner part of the borehole following device and the unexpanded tubular section.

Fig. 3 shows another example of a tubular element 200 positioned in a borehole 102, in particular showing a proximal end 200’ of the tubular element, and a distal end 200” of the tubular element. Part of a borehole following device 300 is also depicted, with the device body 302 positioned in front of the distal end 200” of the tubular element. The tubular element 200 may have any length, for example spanning tens, hundreds, or even thousands of metres.

Schematically depicted in Fig. 3 is a tubular element forming module 502 for forming the tubular element 200. For example, the tubular element 200 may be formed by connecting discrete tubular sub-elements together. Alternatively, the tubular element 200 may be formed in a continuous process, for example by injection moulding or folding a sheet and welding a seam of the sheet to form a closed tubular element. Alternatively, the tubular element 200 may be supplied as a whole, and can for example be unwound from a coil.

Further depicted in Fig. 3 is a tubular element moving module 504, which is typically positioned downstream from the tubular element forming module 502. Using the moving module 504, the unexpanded tubular section 204 can be moved further into the borehole 102. An anchor 506 is also depicted in Fig. 3B, which anchor 506 is used to fixate the proximal end of the expanded tubular section 202. As such, it can be prevented that the expanded tubular section 202 is moved into the borehole 102.

In any method disclosed herein, with or without borehole following device, it is preferred but not essential to fill an annulus 210 between the expanded tubular section 202 and the unexpanded tubular section 204 with a fluid, in particular a lubricating fluid. As such, a lubricating fluid supply 508 may be provided, which is in fluid communication with the annulus 210.

An optional seal 510 may be provided in the annulus through which the lubricating fluid can pass, and which may prevent or reduce leakage of said fluid from the annulus 210.

Claims

Conclusions

1. A method for radially expanding a tubular element in a borehole, the method comprising the steps of: - bending the tubular element radially outwardly and in an axially reverse direction to form an expanded tubular section extending around an unexpanded tubular section, wherein the bending takes place in a bending zone; - increasing the length of the expanded tubular section by pushing the unexpanded tubular section axially relative to the expanded tubular section further into the borehole; wherein a borehole tracking device is positioned ahead of the bending zone so that the borehole tracking device is pushed further into the borehole as the unexpanded tubular section is pushed further into the borehole, and wherein the borehole passes at least partially through soft formation, for example comprising sand, mud, clay, loam, and/or silt.

2. A method according to claim 1, wherein a majority, in particular more than 75%, of the borehole passes through soft formation, for example comprising sand, mud, clay, loam, and/or silt.

3. A method according to claim 1 or 2, wherein at least part of the borehole has a substantially horizontal portion.

4. The method of claim 3, wherein at least part of the borehole has an upward facing portion toward the surface of the Earth.

5. A method according to any preceding claim, wherein an outer diameter of at least part of the borehole tracking device is larger than an inner diameter of the borehole.

6. A method according to any preceding claim, wherein the borehole tracking device comprises one or more resilient elements, such as one or more leaf springs, on an outer periphery of the borehole tracking device, the one or more resilient elements contacting an inner wall of the borehole.

7. A method according to any preceding claim, wherein the borehole tracking device comprises a rigid tubular device body, in particular without any internal degrees of freedom.

8. A method according to any preceding claim, wherein a diameter of the borehole in which the expanded tubular section is located is greater than an outer diameter of the borehole downstream of the borehole tracking device.

9. A method according to any preceding claim, wherein an inner portion of the borehole tracking device extends into the unexpanded tubular section beyond the bending zone, preferably wherein a sliding seal is formed between the inner portion of the borehole tracking device and the unexpanded tubular section.

10. A method according to any preceding claim, wherein the entire bending zone is accommodated in a bending zone receiving space of the borehole tracking device.

11. A method according to any preceding claim, further comprising positioning one or more rollers in the bending zone between the expanded pipe section and the unexpanded pipe section.

12. The method of claim 11, wherein at least one of the rollers has an outer shape that tapers toward one or both ends of the respective roller.

13. A method according to claim 11 or 12, wherein a plurality of rollers are positioned in the bending zone, the rollers being connected to form a closed loop of rollers.

14. A method according to any preceding claim, further comprising filling at least part of an annulus between the expanded tubular section and the unexpanded tubular section with a lubricant, in particular wherein the lubricant comprises a mixture of a liquid and solid particles.

15. A method according to any preceding claim, wherein one or more radially inwardly directed flanges and/or protrusions of the borehole tracking device contact an outer wall of the expanded tubular section.

16. Method according to claim 15, wherein the one or more radially inwardly directed flanges and/or protrusions are pressed against the outer wall of the expanded pipe section.

17. A method according to any preceding claim, wherein a scraper of the borehole tracking device contacts an inner wall of the unexpanded tubular section, preferably over the entire circumference of the unexpanded tubular section.

18. A method according to any preceding claim, further comprising drilling the borehole with a drill positioned downstream of the borehole monitoring device, wherein the drill is fed with fluid and/or torque via a drill string extending through the borehole monitoring device.

19. A method as claimed in claim 18, wherein the drill string extends through a drill string guide of the borehole tracking device, the drill string guide limiting radial movement of the drill string relative to the borehole tracking device.

20. A method according to any preceding claim, wherein a material used to form a first half of the expanded tubular section is different from a material used to form the unexpanded tubular section after the first half of the expanded tubular section has been formed.

21. A borehole monitoring apparatus for monitoring a borehole passing through soft formation, comprising: - a tubular apparatus body having a passage for accommodating a drill string through said passage; wherein said tubular apparatus body has a forward end for guiding said borehole monitoring apparatus through a borehole passing through soft formation, and said tubular apparatus body further comprises a bending zone receiving space positioned a distance from said forward end, said bending zone receiving space being adapted to accommodate at least a portion of a bending zone formed by radially outwardly and axially reversely bending said tubular element to form an expanded tubular section extending around an unexpanded tubular section.

22. A borehole tracking device as claimed in claim 21, wherein the tubular device body has an at least partially hollow interior.

23. Borehole monitoring device according to claim 21 or 22, wherein the borehole monitoring device has an average density between 500-4000 kg/m², in particular between 1000-3000 kg/m².

24. A borehole tracking apparatus according to any one of claims 21 to 23, wherein the bending zone receiving space is positioned at a rear side of the tubular apparatus body 1s.

25. A borehole tracking device according to any one of claims 21 to 24, wherein the bending zone receiving space is formed as a cavity in a rear end face of the tubular device body.

26. A borehole tracking device as claimed in claim 25, wherein the cavity forms a curved surface having a constant radius over at least 120 degrees, preferably over about 180 degrees.

27. A borehole tracking device as claimed in any one of claims 21 to 26, further comprising a radially inwardly directed flange or protrusion extending into the bending zone receiving space.

28. A borehole tracking apparatus as claimed in any one of claims 21 to 27, further comprising a drill string guide positioned in the passage through the apparatus body at a radius closer to a centre line of the body than an internal wall of the body.

29. A borehole tracking apparatus as claimed in claim 28, wherein the drill string guide is connected to an internal wall of the apparatus body via one or more supports.