WO2015036574A2

WO2015036574A2 - Transport device

Info

Publication number: WO2015036574A2
Application number: PCT/EP2014/069555
Authority: WO
Inventors: John Davies; Hans Van Dongen
Original assignee: Maersk Olie Og Gas A/S
Priority date: 2013-09-13
Filing date: 2014-09-12
Publication date: 2015-03-19
Also published as: WO2015036574A3; GB2537483A; GB201604192D0; DK178996B1; GB201316354D0; NO20160585A1; NO347096B1; DK201570284A1; GB2537483B

Abstract

A downhole propulsion apparatus comprising at least one pliant or deformable propulsion wheel, wherein the wheel is configured to engage a passage wall and be rotated to propel the apparatus through a passage and wherein the apparatus is configured to exert a contact pressure or force of less than 1000psi or 7500lbs or 7MPa or 33.4kN per wheel; and an associated system and method of propelling a downhole propulsion apparatus.

Description

Transport Device

FIELD

Embodiments described herein relate generally to a transport device, such as a tractor, for operation in passageways such as wellbores and pipelines. Particularly, embodiments relate to a transport device for transporting other devices, such as sensors, tools and equipment, into and within the passageways.

BACKGROUND

Downhole tractors are commonly used for transporting equipment into and through passageways such as wellbores and pipelines. An example of such a tractor can be found in WO2008/091 157. Such tools are typically provided with some form of propulsion mechanism for propelling the tractor along the passageway and are configured to push, pull or otherwise transport equipment such as sensors, tools and other equipment.

STATEMENTS OF INVENTION According to a first aspect of the invention is a downhole propulsion apparatus comprising at least one wheel, wherein the wheel is configured to engage a passage wall.

The at least one wheel may be pliant and/or deformable. The at least one wheel may comprise a propulsion wheel and may be rotated to propel the apparatus through a passage.

In this way, it may be possible to propel the tractor through soft formations with improved traction.

The propulsion apparatus may be provided with a drive system. The drive system may comprise a motor or other suitable mechanism for rotating the wheels.

The propulsion apparatus may be locatable within the passage. The passage may comprise a bore, a pipeline, a conduit or the like.

The downhole propulsion apparatus may comprise a downhole tractor.

The apparatus may comprise a housing, such as a tubular housing. At least part of the at least one propulsion wheel may be located or locatable radially outwardly of the housing. At least part and optionally the whole of the at least one propulsion wheel may be locatable so as to be aligned with or inwardly of the housing, e.g. such

51961098-1-GMCGLASHAN that it is flush with the housing. The housing may be formed from a composite material, such as carbon fibre.

The at least one pliant or deformable wheel may comprise a pliant or deformable portion, that may be provided on at least a periphery or outward portion of the wheel. The pliant or deformable portion may comprise or be comprised in a tyre, for example. The at least one wheel and/or the pliant or deformable portion may be ductile, elastic, flexible, malleable and/or yielding. The at least one wheel and/or at least the pliant or deformable portion of the at least one wheel may comprise a pliant or deformable material, such as an elastomeric and/or polymeric material, which may be reinforced.

Optionally, the at least one wheel may comprise a solid wheel.

Optionally, the at least one wheel and/or at least the pliant or deformable portion of the at least one wheel may comprise a filled or fillable wheel or portion of the wheel, for example, such that the wheel or portion of the wheel may be filled or fillable with a fluid or liquid such as water or gel. For example, the fluid may be incompressible such that it can accommodate pressure changes without a significant change in volume. In this way, the wheels may more easily withstand high pressures commonly found downhole.

The filled or fillable wheel or portion of the wheel may be partially filled in use or configured to be partially filled in use, for example, to between 70% and 95% and most preferably to between 80% and 90%, e.g. substantially 90%, of its maximum inflation volume. In this way, the traction of the wheels may be improved.

The wheels may be configured to be buoyant or semi-buoyant, in use.

The at least one wheel may comprise one or more bendable or deformable members, such as spokes. The bendable or deformable members may be radially extending in a neutral or unloaded position and/or form or be comprised in a matrix or lattice.

The propulsion wheel may be treaded, grooved or otherwise provided with protrusions at least on a peripheral or outward surface.

The propulsion apparatus may comprise a plurality of propulsion wheels. At least two of the propulsion wheels may be provided on opposite sides of the apparatus. The propulsion wheels may be provided in a staggered arrangement.

For example, the propulsion apparatus may comprise at least a first set of wheels and a second set of wheels, wherein each of the first and second sets of wheels comprise one or more wheels, such as a pair of wheels. At least the second set of wheels may be longitudinally spaced from the first set of wheels. The second set of wheels may be positioned such that they are rotated around a longitudinal axis of the propulsion apparatus with respect to the first set of wheels, e.g. by 90°.

The apparatus may be configured to exert a contact pressure or force of less than "l OOOpsi or 7500lbs or 7MPa or 33.4kN, and preferably 400psi or 3000lbs or 2.76MPa or 13.35kN or less per wheel. The apparatus may be configured to produce a total pull force greater than 6,000lbs or 26.6kN and preferably 19,000lbs or 84.5kN or higher. The contact pressure may be limited as not to exceed the strength of the borehole wall.

Substantial digging in of the wheels into the borehole wall may be avoided to maximize pull force.

The propulsion apparatus may comprise a sensor that is operable to determine a property indicative of borehole wall conditions or damage, such as cracking, deformation. The sensor may comprise a microphone. The microphone may be operable to collect sounds associated with the borehole wall. The propulsion apparatus may be operable to adjust the contact pressure or force of the wheels based on the determination of borehole wall condition or damage, for example, responsive to an identification of sounds indicative of borehole wall condition or damage collected using the microphone.

The propulsion apparatus may be configured to be at least partially propelled by a propulsion member. For example, the propulsion apparatus may be configured to be pushed and/or pulled by the propulsion member. The propulsion member may comprise an elongate member such as a rod, pipe or tubular member. The propulsion member may comprise a push and/or pull rod. For example, the apparatus may comprise a coupling or other receiving means for coupling to and/or otherwise receiving the propulsion member. The propulsion member may comprise a stiff propulsion member. The propulsion member may comprise a flexible propulsion member. For example, the propulsion member may be stiff in a longitudinal direction but flexible in an axial direction. The propulsion member may comprise a composite member, such as a carbon fibre or glass reinforced plastic (GRP) member. The propulsion member may comprise power and/or data transmission, which may comprise, for example, one or more conductive wires or cables, such as copper wires.

In this way, the propulsion member can contribute to the propulsion of the tractor. In addition, since the data and communications can be incorporated in the propulsion member, which can be at least partly supported by a drive mechanism, the requirement for the tractor to pull relatively heavy power and data cables is reduced, which may improve traction of the tractor. The propulsion apparatus may be configured to be at least partially propelled by fluid (e.g. liquid or gas) flow, pressure and/or momentum, such as fluid flow, pressure or momentum in the passage. For example, the propulsion apparatus may be configured to be at least partially propelled by water (or other fluid) injection into the passage in order to at least partially force the propulsion apparatus through the passage.

The propulsion apparatus may comprise, be configured to receive or be configured to propel one or more load items, which may be provided on or in or towed by the propulsion apparatus. The load item(s) may comprise or be comprised in a tool string. The one or more load items, such as sensors or survey tools or logging tools, may be comprised in one or more of the wheels, such as in the tyres. At least part of the one or more sensors or survey tools may be externally and/or outwardly facing from the wheels. The one or more survey tools and/or logging tools and/or sensors may be configured to survey, log and/or sense the environment around the propulsion apparatus, such as the bore walls and/or contents of the bore and/or the geological environment around the propulsion apparatus.

By providing sensors and/or other load items on or in the propulsion apparatus, the drag and/or weight provided by the tool string may be eliminated or reduced. When at least part of the sensors or logging tools are provided in one or more of the wheels, this arrangement may provide advantageous proximity between the sensor and the bore walls, which may improve surveying of the borehole wall and geological structure surrounding the bore walls.

The propulsion apparatus may be provided with at least one mount or coupling for coupling, mounting or receiving the one or more load items.

Examples of the one or more load items, may include, for example, cables, wires, tools, sensors, or other items of equipment or apparatus. Examples of sensors may include accelerometers, callipers or other distance and/or bore geometry measuring devices, flow meters, force meters, detectors for detecting marker or tracer materials, distributed sensing systems such as optical fibres based systems, which may comprise distributed temperature and/or pressure sensing systems, and/or the like. Distributed sensing systems may be embedded in the propulsion member.

For example, the load item may comprise a detector for detecting a marker or other tag in injection water in the passage in which the apparatus is provided, which may in turn be used as a flow profiling device that is configured to determine a time of flight or change in time of flight of the marker or tracer at different points along the passageway. This may provide a simply flow measurement arrangement. The propulsion apparatus may be configured to be buoyant or semi-buoyant in use.

The wheel(s) may be movable radially outwardly and inwardly, for example, by being mounted on pivoting arms or other suitable movement mechanism. The wheel(s) may be biased radially outwardly. The wheel(s) and/or movement mechanism may be configured to space the housing from the bore walls.

The wheel may be configured such that at least a portion of the wheel bulges out when contacting the passage wall. This may make the contact area larger and may make the contact pressure lower (for a given contact force). The wheel may resiliency bulge when in contact with the passage wall. The wheel may return to or stay in its normal or unbulged shape close to the pivoting arm. This may prevent a bulging tyre rubbing against the pivoting arm.

According to a second aspect of the present invention is a downhole propulsion system comprising a downhole propulsion apparatus according to the first aspect and a drive system for at least partially propelling the downhole propulsion apparatus.

The drive system may comprise a rod drive system configured to drive a rod and thereby push and/or pull the downhole propulsion apparatus.

The rod may comprise a stiff rod. The rod may comprise a flexible rod. The rod may comprise a composite rod, such as a carbon or glass reinforced plastic (GRP) rod.

The rod may comprise power and/or data transmission, such as copper or other conductive wires or cables and/or optical fibres for data transmission. The rod may comprise distributed temperature and/or acoustic sensing, which may comprise use of optical fibres.

The rod drive system may comprise at least one spooling mechanism, for spooling the rod. The rod drive mechanism may comprise at least one guide for guiding the rod.

The drive system may comprise a fluid injector, such as a water injector. The downhole propulsion apparatus may be configured to be drivable by pressure or pressure differential in the passage, for example due to the fluid injector.

The drive system may be configured to be located externally to the downhole propulsion apparatus. At least part of the drive system may be located or locatable outwith the passage, such as at the surface of a borehole.

According to a third aspect of the present invention is a method for propelling apparatus in a passage, the method comprising rotating at least one pliant or deformable propulsion wheel of a downhole propulsion apparatus that engages a passage wall to propel the apparatus through the passage. The downhole propulsion apparatus may comprise a downhole propulsion apparatus according to the first aspect.

It will be appreciated that features analogous to those described above in relation to any of the above aspects may be individually and separably or in combination applicable to any of the other aspects.

Apparatus features analogous to those described above in relation to a method and method features analogous to using, producing or assembling those described above in relation to an apparatus are also intended to fall within the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are now described, by way of non-limiting example, and are illustrated in the following figures, in which:

Figure 1 shows a downhole tractor according to an embodiment of the invention in use in a bore;

Figure 2a shows an example of a pliable wheel construction for use with the downhole tractor of Figure 1 ;

Figure 2b shows another example of a pliable wheel construction for use with the downhole tractor of Figure 1 ;

Figure 2c shows a further example of a pliable wheel construction for use with the downhole tractor of Figure 1 ;

Figure 3 is a schematic of propulsion system for a downhole tractor; and

Figure 4 is a schematic of another propulsion system for a downhole tractor.

DETAILED DESCRIPTION OF THE DRAWINGS

Downhole tractors that are configured to provide propulsion and push or pull load items through the bores are known. Such tractors, for example, comprise a plurality of propulsion wheels that are driven by a suitable drive mechanism such as an electric or hydraulic motor. The propulsion wheels are provided on pivoting arms that are biased outwardly to bring them into engagement with the bore walls. The driven wheels are then rotated by the drive mechanism in order to propel the tractor along the bore.

Such tractors are often used to push or pull load items into and through bores.

For example, the tractor can be used to pull a toolstring and/or cables used for power, control and data communication. Typically, downhole tractors are provided with steel wheels in order to withstand the adverse working conditions, such as high pressure and temperature, and for longevity. However, for bores in formations that pass through relatively soft material such as sand or acidized limestone, then traction can become an issue.

Figure 1 shows a downhole tractor 5, advantageously suitable for providing propulsion in environments such as underground boreholes and the like. Such boreholes can comprise rough or uneven bore walls 10 and can be formed through a variety of materials that potentially have a range of firmnesses. For example, the bore may pass through firm regions, such as hard rock, and/or may pass through relatively softer regions such as sand or acidized limestone.

The downhole tractor 5 comprises an elongated tubular housing 15 that houses a drive mechanism (not shown), such as an electric motor, and further comprises a plurality of propulsion wheels 20a-h that are driven by the drive mechanism. The tractor 5 is configured to transport load items, such as cables, tools, sensors or other equipment items into and through passageways such as bores.

The wheels 20a-h are arranged in a staggered configuration. In the particular example shown in Figure 1 , the wheels 20a-d are arranged at 0°, 180°, 90° and 270° rotations around the longitudinal axis of the tractor relative to the first (0°) wheel 20a. The staggering arrangement is then repeated for subsequent wheels 20e-20h. Each wheel 20a-20h is mounted using a mechanism 25 such as a pivoting arm that is operable such that the wheels 20a-20h are movable radially inwardly and outwardly. The wheel mounting mechanisms 25 are provided with biasing apparatus 30 such as springs, pistons or the like that are operable to bias the wheels 20a-20h outwardly, away from the housing 15. In this way, the wheels 20a-20h are forced into engagement with the bore walls 10 in use.

In embodiments, the force applied by the biasing apparatus 30 is enough to keep the housing 15 of the tractor 5 spaced from the bore walls 10 in use.

Advantageously, the wheels 20a-20h are pliable and resiliency deformable. In this way, the wheels 20a-20h are configured to spread and conform to the bore wall under the action of the force applied by the biasing apparatus 30. In embodiments, the pliability is provided by the provision of suitable tyres 35 and/or the pliability can be provided using a suitable wheel construction.

For example, in the embodiment of a wheel 20' shown in Figure 2a, the tyres 35a are solid tyres provided on a hub 45, wherein the tyres are constructed of a suitable pliable and deformable material such as suitable elastomeric or polymeric materials. Logging tools or sensors can optionally be provided in the wheel 20'. In another example of a wheel 20", as shown in Figure 2b, tyres 35b of the wheel 20" comprise a compartment 47 that is filled or fillable, preferably with a liquid such as water or a gel. Using a liquid to fill the tyres 35b, particularly incompressible liquids, allows the wheels 20" more easily withstand high pressures commonly found downhole, for example, by better balancing internal and external pressure and/or minimising volume change with pressure, relative to pneumatic tyres. The tyres 35b are provided on a hub 45. In the case of the filled or fillable tyres 35b, the tyres 35b are configured to be filled at less than their maximum inflation (e.g. 80-90% or less), which may provide improved grip. Logging tools or sensors can optionally be provided in the wheel 20".

In another example, as shown in Figure 2c, a wheel 20"' is constructed in such a manner as to provide the necessary degree of pliability or deformation, for example, by being provided with spokes 40 or other resiliency deformable members between a hub 45 and a deformable outer band 50. In the particular example shown, the spokes 40 are formed from polymeric (e.g. polyurethane) plates, rods or other elongate members. Logging tools or sensors 55 can optionally be provided in the wheel 20"'.

Although the above mechanisms for providing a suitable degree of pliability in the wheels 20a-20h are provided by way of example, it will be appreciated that a person skilled in the art could likely determine other suitable pliable and/or resiliency deformable wheel constructions in view of the teaching of the present application.

By providing pliable and resiliency deformable propulsion wheels 20a-20h that are configured to engage the bore walls 10 and be rotated to propel the tractor 5, it is possible to improve traction through soft formations.

The degree of wheel pliability or deformability required will vary depending on the application and the environment. For example, for a bore extending through acidized limestone at a depth of 1000m, the tractor wheels 20a-20h may be arranged to apply a pulling force required to pull the tractor 5 and any wireline cable, logging tools or other load items through the bore. In such formations, the rock strength can be as low as 400psi (approximately 2.76MPa). In this case, for an example tractor 5 having twelve wheels 20a-20h and a wheel contact area of 8 square inches (approximately 52cm²), the maximum pulling force the tractor 5 can generate will equal rock strength (e.g. 400psi) x a friction factor (e.g. 0.5) x the number of wheels (e.g. twelve) x the contact area (e.g. 8 square inches). The contact area between the tyre and the borehole wall depends on the wheel diameter (for example, between 3 and 5 inches or 7.5 and 13 cm), the wheel width (for example, between 2 and 4 inches or 5 and 10.2cm) and the contact length (for example, between 2 and 4 inches or 5 and 10.2cm). In the above example, the pull force would be 19,000 lbs (84.5kN), which would be enough to pull 38,000 ft (1 1.58km) of cable weighing 1 lbs per foot (14.4N.m^" ¹). Advantageously, the wheels 20a-20h are configured to receive a load of 400psi or 3000lbs (2.76MPa or 13.35kN) or under per wheel.

The wheel is configured such that a portion of the wheel contacting the passage wall bulges out. This may make the contact area larger and may make the contact pressure lower (at a given contact force). The resilience of the wheel is such that the bulged portion of the wheel returns to its normal or unbulged shape close to the pivoting arm, i.e. away from the passage wall. This helps prevent a bulging tyre rubbing against the pivoting arm.

In some embodiments, the tractor 5 is provided with on-board sensors or surveying or logging tools 55 in addition to or instead of pulling or pushing the sensors in a tool string 57. This arrangement can eliminate or reduce the drag and/or weight provided by the tool string. Examples of suitable sensors 55 include flow meters, accelerometers, callipers or other distance measurement tools, marker or tracer detectors and the like.

In some advantageous embodiments, at least part of the sensors or logging tools 55 are comprised in one or more of the wheels 20a-20h. This arrangement can provide advantageous proximity between the sensor 55 and the bore walls 10, for example, improving surveying of the geological structure surrounding the bore walls.

An example of a sensing arrangement is an arrangement for use in a water injector. In this case, the tractor can be provided with a fluid velocity sensor and one or more tools for determining the geometry of the borehole. In another embodiment, the tractor can be provided with sensors for detecting tracer / marker materials and thereby the time of flight and change in time of flight of the tracer/marker material between the injection point and the device to provide flow profiling.

In embodiments, the tractor 5 is configured to be buoyant or semi-buoyant in use, for example, by selecting an appropriate wheel construction and/or material.

In optional embodiments, the tractor 5 is configured to be propelled by additional or alternative drive mechanisms as alternatives or supplemental to the propulsion by the wheels 20a-20h.

One example of such a drive mechanism is a rod drive mechanism 60, as shown in Figure 3, wherein the tractor 5 is pushed or pulled through the bore using a stiff but flexible rod 65. For example, the rod 65 is provided with sufficient stiffness in its longitudinal direction such that it can apply a required pushing and/or pulling force on the tractor 5 but is flexible in the axial direction of the rod 65 such that the rod 65 can flex to accommodate deviations in the bore and/or be deployed using a spooling mechanism 70. The rod 65 can be, for example, formed from a composite material such as carbon fibre or glass reinforced plastic. The rod 65 is drivable using a suitable rod drive mechanism such as the driven rotatable spool 70 for spooling the rod 65 or the like. The rod 65 is guided through a guide 75 provided between the rod drive mechanism 60 and the bore in order to feed the rod 65 down the bore. The end of the rod 65 furthest from the rod drive mechanism 60 is connected to the tractor 5 to push and/or pull the tractor 5 under the action of the rod drive mechanism 60.

Advantageously, the rod 65 can be provided with power, control and/or communication mechanisms 80 for providing power to the tractor 5 and conveying control signals and/or data between the tractor 5 (and/or any on-board or connected sensors, logging/surveying tools 57 or other equipment) and the surface. Examples of suitable communication mechanisms 80 include wires or cables, that can be, for example copper or other suitable flexible conductive material.

By providing such a rod drive mechanism 60 for the tractor 5, the rod drive mechanism 60 can contribute to the propulsion of the tractor 5. In addition, since the data and communications can be incorporated in the rod 65, which is at least partly supported by the rod drive mechanism 60, the requirement for the tractor 5 to pull relatively heavy power and data cables is reduced, which may improve traction and reach of the tractor 5.

Another suitable drive mechanism 85, as shown in Figure 4, comprises using flow of fluid within the bore to force the tractor 5 through the bore. This may involve, for example, providing the fluid to provide a pressure differential (e.g. by at least partially obstructing the bore with the tractor 5 to induce a pressure drop in the fluid flow at the tractor 5) and/or by simply using the motion of the flowing fluid to carry the tractor 5, particularly if the tractor is buoyant or semi-buoyant. For example fluid (e.g. water) injection from a water injector 90 can be used to force the tractor 5 through the bore.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed the novel methods and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms and modifications as would fall within the scope of the invention.

For example, whilst embodiments have been described above in which the pliability of the wheels 20a-20h is provided by use of suitable resiliency deformable materials, fluid filled tyres 35b and/or a wheel construction that imparts compliance in the wheel (e.g. by providing a spoked or lattice wheel), it will be apparent that other mechanisms for providing the required compliance or pliability in the wheel may be provided.

Furthermore, although various specific examples of drive mechanisms are described, it will be appreciated that other drive mechanisms may be used. For example, the motor need not be an electric motor but instead other motors such as hydraulic or pneumatic motors could be used.

Claims

A downhole propulsion apparatus comprising at least one pliant or deformable propulsion wheel, wherein the wheel is configured to engage a passage wall and be rotated to propel the apparatus through a passage; and wherein the apparatus is configured to exert a contact pressure or force of less than 10OOpsi or 7500lbs or 7MPa or 33.4kN per wheel.

A downhole propulsion apparatus according to claim 1 , wherein the at least one pliant or deformable wheel comprises a pliant or deformable portion, that is provided on at least a periphery or outward portion of the wheel.

A downhole propulsion apparatus according to claim 1 or 2, wherein the at least one wheel and/or at least the pliant or deformable portion of the at least one wheel comprises a filled or fillable wheel or portion of the wheel.

A downhole propulsion apparatus according to claim 3, wherein the wheel or portion of the wheel is filled with a liquid or gel.

A downhole propulsion apparatus according to any preceding claim, wherein the at least one pliant or deformable wheel comprises one or more bendable or deformable spokes.

A downhole propulsion apparatus according to any preceding claim, wherein the propulsion apparatus comprises a plurality of propulsion wheels and at least two of the propulsion wheels are provided on opposite sides of the apparatus to each other in a staggered arrangement.

A downhole propulsion apparatus according to any preceding claim comprising at least first and second sets of wheels, wherein the second set of wheels is longitudinally spaced from the first set of wheels and located in positions that are rotated around a longitudinal axis of the propulsion apparatus with respect to positions of the first set of wheels.

8. A downhole propulsion apparatus according to any preceding claim, wherein the propulsion apparatus is configured to be pushed and/or pulled by an elongate propulsion member.

9. A downhole propulsion apparatus according to claim 8, wherein the propulsion member is stiff in a longitudinal direction.

10. A downhole propulsion apparatus according to claim 8 or claim 9, wherein the propulsion member comprises power and/or data transmission.

1 1 . A downhole propulsion apparatus according to any preceding claim, wherein the propulsion apparatus is configured to be buoyant or semi- buoyant in use.

12. A downhole propulsion apparatus according to any preceding claim, wherein one or more of the wheel(s) are movable radially outwardly and inwardly.

13. A downhole propulsion apparatus according to any preceding claim, wherein the wheel(s) are configured to space the housing from the bore walls.

14. A downhole propulsion system comprising a downhole propulsion apparatus according to any of the preceding claims and a drive system for at least partially propelling the downhole propulsion apparatus.

15. A downhole propulsion system according to claim 14, wherein the drive system comprises an elongate propulsion member and a drive system configured to drive the elongate propulsion member to push and/or pull the downhole propulsion apparatus.

16. A downhole propulsion system according to claim 15 wherein the propulsion member comprises power and/or data transmission.

17. A downhole propulsion system according to any of claims 14 to 16, wherein the elongate member is spoolable and the drive system comprises at least one spooling mechanism for spooling the elongate member and/or at least one guide for guiding the elongate member.

18. A downhole propulsion system according to any of claims 14 to 17, wherein at least part of the drive system is located or locatable outwith the passage.

19. A method for propelling apparatus in a passage, the method comprising, using at least one pliant or deformable wheel of a downhole propulsion apparatus, exerting a contact pressure or force of less than 1000psi or 7500lbs or 7MPa or 33.4kN per wheel on a passage wall and rotating the least one pliant or deformable propulsion wheel of the downhole propulsion apparatus that engages the passage wall to propel the apparatus through the passage.

20. The method of claim 19, wherein the downhole propulsion apparatus comprises a downhole propulsion apparatus according to any of claims 1 to 13 or comprised in the system according to any of claims 14 to 18.

21 . A downhole propulsion apparatus comprising at least one pliant or deformable propulsion wheel, wherein the wheel is configured to engage a passage wall and be rotated to propel the apparatus through a passage.

22. A downhole propulsion apparatus substantially as shown and/or described herein in relation to the drawings.