Individual or paired arm control in a wellbore tractor

Abstract

The present invention relates to an individual or paired arm control (20) in a wellbore tractor (10) being run in a borehole (30) penetrating a formation (40), as well as a wellbore tractor (10) with such an individual or paired arm control (20).

Description

TECHNICAL FIELD

[0001] The present invention relates to newer generation wellbore conveyance systems, also known as wellbore tractors. The invention relates further to an individual or paired arm control in a wellbore tractor as well as a wellbore tractor with such an individual or paired arm control.BACKGROUND OF THE INVENTION

[0002] A wellbore tractor is a conveyance tool used in a horizontal section of an oil well to “convey” a “payload” further into the well. The “payload” could typically be logging tools for inspection of the well, perforating tools for making radially holes in the well to allow for oil to emerge, setting tools for setting plugs / patches / straddle, clean-out tools for removing debris, or manipulation tools for opening / closing valves, etc.

[0003] The wellbore tractors work in a harsh environment with temperatures downhole typically in the range of 80-200° C. and at pressures ranging up to typically 20 000 or 25 000 psi. And even more extreme wells exist.

[0004] A typical wellbore tractor normally has the shape of a cylindrical base where “arms” with (drive) wheels or belts, at the end, pivot out from the cylindrical body until the wheels / belts reach the inner wall of the well. The wheels or belts on the arms typically face down into the well (also called “downstream”) since if they were facing up towards the surface (also called “upstream”) they would have a higher potential of getting stuck inside the wellbore while pulling out of hole (POOH). The powertrain of the tractor is typically electro-hydraulically or electro-mechanically driven.

[0005] The typical up-to-date wellbore tractor is usually powered from the surface and utilizes telemetry for communication between a surface system and a downhole tool string via an electrically powered wireline winch, also known as e-line. As previously described the purpose of the wellbore tractor is to convey various tools with different functionality to a relevant location in the well where normal tool strings relying only on gravity will not reach.

[0006] The typical wellbore tractor has 2, 4, 6 or 8 wheels, where a pair of wheels are forced in opposite directions to create sufficient traction when pulling a wireline into a deviated / horizontal section of the well. Different pair of wheels can be offset by 90 degrees to improve centralization providing force in both radial axes. A pair of arms will typically either have a common pivot point or individual pivot points axially shifted in near proximity. In both cases a typical tractor would activate all arms simultaneously typically from a common hydraulic source.

[0007] The wellbore tractor could work in an open hole or cased hole environment. A cased hole environment in a deviated well will typically consist of a vertical, cased wellbore with a tapered production tubing, meaning that the wellbore tractor on its way into the well will experience transitions from a larger outer diameter to a smaller outer diameter (OD). In addition, there exists other types of OD restrictions in the well, such as safety valves or other inserts that the wellbore tractor will have to maneuver through.

[0008] First generation wellbore tractors were generally powered by a simple electro-hydraulic drive mechanism where adjusting the power to the electromotor would allow change of speed. All wheels work simultaneously, and if one wheel fails, then all will fail. In order to provide grip towards the wellbore (typically a cased hole production tubing or even an open hole) a single hydraulic power circuit would be typically utilized to force all the arms and wheels towards the wall of the well for the purpose of creating traction for the wheels. The first-generation tractors were fully electro-hydraulic with no or very limited communication via telemetry. The typical electro-hydraulic tractor will only allow tractoring into the well (excluding reverse tractoring).

[0009] Second generation wellbore tractors would typically utilize a similar electro-hydraulic (alternatively electro-mechanical) powered arm to force an individual arm or a pair of arms and wheels towards the wall of the well and use one or multiple electromotors to power either individual, paired or all wheels simultaneously for propulsion in the wellbore. Once multiple electromotors are utilized, the ability to control and get feedback from the different electromotors appears. But still, the arms are controlled simultaneously as one unit, for example via hydraulic circuitry. The second generation wellbore tractors would typically feature a telemetry function that would enable limited sensor feedback and control to surface. Here reverse tractoring is possible but not typical.

[0010] Third generation wellbore tractors are fully electronically controlled robots with a local downhole master controller and higher bandwidth telemetry. The arms are still typically electro-hydraulically (alternatively electro-mechanically) actuated to provide force to the wheels. The arms, however, are typically still operated as a single system, all being activated simultaneously. The wheels are powered for propulsion in the wellbore by electromotors controlled by electronics and sensors. The speed and torque of the motors are setpoint adjusted from the surface but controlled locally by algorithms in the controller in the tractor. Reverse tractoring can be safely implemented by the use of wireline tension feedback and other sensor inputs, preventing backing over the wireline cable.INTRODUCTION TO THE INVENTION

[0011] As mentioned, the wellbore tractor is utilized to convey other tools, via an electrically powered wireline winch, into an oil and gas well. The tools can typically be logging packages, perforation guns or other well intervention equipment. The wellbore will typically be a long vertical wellbore ending up in a deviated and / or horizontal section where propulsion assistance will be required in order to reach a target. When entering the wellbore and on its way into the wellbore the tractor will have to function properly and will also have to pass potential restrictions and transitions from larger outer diameter to smaller outer diameter wellbores. If not successfully negotiated the mission fails. The mission into the wellbore will typically take many hours, so any improvement related to efficiency and risk reduction will be highly beneficial.

[0012] During a mission there are multiple other things that can go wrong which again, depending on severity, could cause a mission abort.

[0013] If for example an individual wheel fails, the wheel would be stuck towards the cased hole preventing efficient propulsion, causing excessive friction and subsequently potential mission abort. Loss of propulsion in general will likely cause mission failure.SUMMARY OF THE INVENTION

[0014] The present invention relates to the latest third generation of wellbore tractors, where the wheels are powered either individually or in pairs of 2 (but also a set of 3 and 4 wheels) and controlled by a local downhole master controller and also featuring / having a higher bandwidth telemetry.

[0015] It is an object of the invention to provide individual or paired arm control, mainly for the purpose and advantage of improved negotiation of wellbore restrictions.

[0016] Another object of the invention is to provide tailored sets of arms for different applications or different wellbore sizes.

[0017] Yet another object of the invention is to allow a failed wheel to be retracted and disabled individually (or in pairs of 2, 3, 4 wheels) for the purpose of being able to continue tractor operation.

[0018] The main features of the present invention are given in the independent claims. Additional features of the invention are given in the dependent claims.

[0019] According to a first aspect of the invention this is achieved with an individual or paired arm control in a wellbore tractor.

[0020] According to a second aspect of the invention this is achieved with a wellbore tractor with an individual or paired arm control.

[0021] The present invention teaches an independent or paired arm control system configured for a wellbore tractor configured to be run in a borehole penetrating a formation. The control system comprises:

[0022] a 3-port / 2-way normally open solenoid valve arranged in a drive module of the wellbore tractor, the drive module having at least one arm with at least one wheel or belt,

[0023] a hydraulic cylinder-piston assembly comprising a hydraulic cylinder with an actuator piston and a retraction spring arranged therein, the hydraulic cylinder-piston assembly arranged within the drive module,

[0024] a hydraulic pressure line connected to a hydraulic pump configured to provide high pressure by means of a hydraulic fluid,

[0025] a return hydraulic line connected to a reservoir configured for containing the hydraulic fluid,

[0026] wherein a control port of the 3-port / 2-way normally open solenoid valve is connected to the hydraulic cylinder of the hydraulic cylinder-piston assembly, a return port of the 3-port / 2-way normally open solenoid valve is connected to the return hydraulic line, and a pressure port of the 3-port / 2-way normally open solenoid valve is connected to the hydraulic pressure line.

[0027] The independent or paired arm control system can further comprise a 3-port / 2-way normally closed fail-safe solenoid valve, wherein a control port of the 3-port / 2-way normally closed fail-safe solenoid valve is connected to the 3-port / 2-way normally open solenoid valve via the pressure port of the 3-port / 2-way normally open solenoid valve; a return port of the 3-port / 2-way normally closed fail-safe solenoid valve is connected to the return hydraulic line; and a pressure port of the 3-port / 2-way normally closed fail-safe solenoid valve is connected to the hydraulic pressure line having the hydraulic pump.

[0028] The hydraulic pump can be driven by an electrical motor that can be controlled by a motor controller.

[0029] The independent or paired arm control system can further comprise a pressure safety valve connected between the hydraulic pressure line and the return hydraulic line.

[0030] The independent or paired arm control system can further comprise a check valve arranged in the hydraulic pressure line and between the hydraulic pump and the 3-port / 2-way normally closed fail-safe solenoid valve.

[0031] The independent or paired arm control system can further comprise a pressure / temperature sensor connected to the hydraulic pressure line and configured to monitor the pressure in the hydraulic pressure line.

[0032] The independent or paired arm control system can further comprise a pressure compensation unit comprising the (oil sump) reservoir (containing the hydraulic fluid), a spring activated piston (i.e. a compensator spring and a compensator piston) and a ventilation port.

[0033] Said at least one arm can be arranged in at least one pair of arms having at least one of: i) a common pivot point and / or ii) individual pivot points axially shifted in near proximity. The common pivot point type or version can have a single actuator (or also called “actuator unit”) for a pair of arms. The axially shifted type or version can have an individual actuator (unit) for each arm.

[0034] In an alternative embodiment, each piston actuator in the system can be linked to a dedicated solenoid valve, allowing individual control of each arm, facilitating precise maneuvering through wellbore restrictions by enabling or disabling individual arms and wheels.

[0035] The invention teaches also a wellbore tractor configured to be run in a borehole penetrating a formation, wherein the wellbore tractor comprises said independent or paired arm control system.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] These and other aspects of the invention are apparent from and will be further elucidated, by way of example(s), with reference to the drawings, wherein:

[0037] FIGS. 1A-1G show the working principle of the present invention.

[0038] FIGS. 2A-2C show a drive module of a wellbore tractor according to the invention under different operational conditions.

[0039] FIGS. 2D-2F show a solenoid valve in the drive module of the wellbore tractor of FIGS. 2A-2C under different operational conditions.

[0040] FIGS. 3A-3B show a hydraulic power module (HPM) of the wellbore tractor according to the invention in two different states.

[0041] FIG. 4 shows a detailed piping and instrumentation diagram (P&ID) of the wellbore tractor according to the invention.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] Below there will be described several scenarios pointing out the advantages of the present invention.

[0043] Scenario 1: The wellbore tractor is struggling to get into a restriction in the wellbore, and traditionally would need to abort mission and pull out. Prior to aborting the mission, the operator has likely performed multiple attempts to succeed. These attempts are associated with high risk of excessive wear, or even damage, to both the tractor / tool string, restriction and wellbore. With an individual or paired arm control according to the invention a first individual arm or first pair / set of arms could be retracted to allow easier and less aggressive access into the restriction. The remaining arms will provide traction into the restriction. Once the first arm or pair of arms is inside or has passed the restriction, the arm(s) can be activated again. The process can then be repeated for the next individual or pair of wheels and so on, until all wheels have passed through the restriction. This has traditionally been solved by running two tractors in “tandem”.

[0044] Scenario 2: If, for some reason the wellbore tractor has a failure that causes a wheel to stop, this would traditionally need to abort the mission and pull out. With the individual or paired arm control according to the invention, the affected wheel and arm or the affected pair of wheels and arms can be retracted and disabled, allowing the remaining wheels a better chance of completing the mission.

[0045] Scenario 3: During the “run-in-hole” period the focus for the wireline tractor is speed and sufficient torque to convey the tool string into the deviated portions of the well. But, for fine adjustments close to target it could be envisioned that the focus should be ultra-fine speed and high torque, below the range of what would be possible with a high-speed motor. This could for example be solved by introducing a gearbox for “high” and “low” gear, but this might not be feasible due to the size constraints (fixed limited diameter, length can more easily be adjusted). It could therefore be envisioned that the individual or paired arm control according to the present invention could be utilized to power for example two different sets of arms with different characteristics, typically “high speed / limited torque” and “low speed / high torque”. A second possibility is that this function could also be utilized for different sizes of the wellbore, where the different arm sets are tailored specifically for different wellbore sizes or a range of wellbore sizes.

[0046] This individual or paired arm control action could be electronically controlled as an algorithm but could also be performed manually via a communication system (e.g., telemetry) from the surface.

[0047] The present invention gives the ability of disabling / enabling any arm from being driven in or out of the tool independently to have the ability to either: disable an arm, in the event of a motor / gear / electrical failure without aborting the tractor operation, or to be able to independently disable / enable arms when overcoming an obstruction in the well which normally would prohibit the tractor from passing or would alternatively require two tractors to run in “tandem” in order to achieve the same result.

[0048] The present invention also opens up to equip a tractor with multiple sets of arms with different characteristics tailored for e.g. different size wellbores, speeds, push / pull force(s).

[0049] The main principle of the present invention will be described below with respect to the first scenario mentioned above and with reference to some of the drawings (FIGS. 1A-1G).

[0050] A wellbore tractor can comprise a system control module (SCM) 24 having electronics, several drive modules (DM) and a hydraulic power module (HPM). The wellbore tractor 10 is connected to the surface via a wireline 60.

[0051] FIG. 1A illustrates a wellbore tractor 10 having, in this example, four drive modules (DM) 13A, 13B, 13C, 13D with drive arms 12A, 12B, 12C, 12D with wheels 11A, 11B, 11C, 11D. The drive arms 12A, 12B, 12C, 12D with drive wheels 11A, 11B, 11C, 11D are illustrated enabled or extracted thus providing grip towards a wall 35 of a wellbore 30 in a formation 40. The wellbore tractor 10 uses an independent or paired arm control 20 (FIGS. 2A-2C) to pass through an obstruction 50, which normally would prevent a typical wellbore tractor from getting through the obstruction 50 as the wheels of the typical wellbore tractor are not able to climb the obstruction 50 by themselves.

[0052] When a lower-most or first wheel 11A of the wellbore tractor 10 hits the obstruction 50 the independent or paired arm control 20 (FIGS. 2A-2C) will disable simultaneously a lower-most or first arm 12A and its neighbor or second arm 12B (i.e. a lower-most or first pair of arms 12A, 12B) and these two arms 12A, 12B will be retracted, as illustrated in FIG. 1B.

[0053] Then the wellbore tractor 10, with the help of the other two drive modules 13C, 13D having arms 12C, 12D with wheels 11C, 11D, will be able to run further into the obstruction 50 with the length of the two disabled drive modules 13A, 13B, as illustrated on FIG. 1C.

[0054] Then the first and second arms 12A, 12B will be enabled, i.e. extracted, to provide grip towards a wall 55 of the obstruction 50, as illustrated on FIG. 1D.

[0055] When the third wheel 11C of the third drive arm 12C hits the obstruction 50 (FIG. 1D) the independent or paired arm control 20 (FIGS. 2A-2C) will disable simultaneously the third arm 12C and the fourth arm 12D (i.e. the second pair of arms 12C, 12D) getting these two arms 12C, 12D retracted (FIG. 1E). This allows the wellbore tractor 10, with the help of the first and second drive modules 13A, 13B, to run further into the obstruction 50 with the length of the two disabled drive modules 13C, 13D (FIG. 1F). Finally, the third and fourth arms 12C, 12D will be enabled / extracted to provide grip towards the wall 55 of the obstruction 50 (FIG. 1G) and the tractor 10 will continue its journey down the wellbore 30.

[0056] In another embodiment of the invention, the independent or paired arm control 20 can be configured to individually control each arm 12A, 12B, 12C, 12D of the wellbore tractor 10, and not a pair of arms as described above. Of course, the independent or paired arm control 20 can be configured to individually control a set of several (more than two) arms of the wellbore tractor 10.

[0057] FIGS. 2A-2C show one of the drive modules 13A, 13B, 13C, 13D of the wellbore tractor 10 under different operations or working conditions.

[0058] The drive module 13A, 13B, 13C, 13D comprises an actuator (also called “an actuator unit” or “a hydraulic cylinder-piston assembly”) comprising a hydraulic cylinder 17 having an actuator piston 36 and a retraction spring 18 arranged therein. The retraction spring 18 can retract the arm 12A, 12B, 12C, 12D when it is disabled, i.e. when it loses hydraulics. Also, in a default position, the retraction spring 18 in the cylinder 17 of the actuator unit keeps the arm 12A, 12B, 12C, 12D retracted (FIG. 2A).

[0059] The individual or paired arm control 20 is making the control by utilizing a set of solenoid valves 14 located in each drive module 13A, 13B, 13C, 13D. These solenoid valves 14 are connected to a tractor / system controller 26 (located in the system control module (SCM) 24 of the wellbore tractor 10). The solenoids / solenoid valves 14 can be controlled based on sensor inputs (such as, but not limited only to, for example motor torque, motor speed, accelerometer, gyro, tension, proximity, pressure, etc.) and / or algorithms and / or manually from the surface via a communication system (telemetry).

[0060] The solenoid valve 14 in each drive module 13A, 13B, 13C, 13D is by default not activated (i.e. without supplied power thereto) as shown in FIG. 2A. The solenoid valve 14 (normally open) not only releases pressure and fluid volume but also individually distributes pressure and fluid volume to the corresponding hydraulic actuator piston 36, enabling precise and independent control of each arm 12A-12D of the tractor 10. Then, in order to facilitate normal operation of the tractor 10, pressure from system hydraulics will go through a hydraulic pressure line 21 to the actuator with the piston 36 and cylinder 17 that will activate the arm 12A-12D as shown in FIG. 2B. The hydraulic cylinder 17 and the actuator piston 36 are thus configured to keep the respective arm 12A-12D activated by means of the high pressure of the hydraulic fluid going, through the hydraulic pressure line 21, to the hydraulic cylinder-piston assembly with the hydraulic cylinder 17 and the actuator piston 36. When any of the solenoid valves 14 is activated (i.e. power is supplied thereto), the valve 14 will then close the access of the system hydraulics to the respective cylinder 17 and the hydraulic fluid will be guided through a return hydraulic line 22 back to a reservoir 19 (FIG. 3A) in the hydraulic power module (HPM) 25. Then the selected cylinder(s) 17 will lose pressure, and the corresponding arm(s) 12A-12D will be deactivated and so retracted by means of the retraction spring(s) 18 back into the drive module(s) 13A-13D as illustrated on FIG. 2C.

[0061] According to one embodiment, the above-mentioned process can be controlled from the surface, where an operator will, after the obstruction 50 is located and / or observed, stop and reverse the tractor 10 a controlled distance up from the obstruction 50. Then the operator will deactivate the first set of arms (e.g. 12A, 12B), either individually or in pairs, in order to drive the tractor 10 into the obstruction 50 with the remaining arms (e.g. 12C, 12D) which are activated. The tractor 10 will proceed further with the same controlled distance plus a selected distance in order to ensure that the deactivated arms (e.g. 12A, 12B) have passed the obstruction 50. Then the arms (e.g. 12A, 12B) that have passed the obstruction 50 are activated by the operator, and the operator will deactivate the next set of arms (e.g. 12C, 12D) that will then pass the obstruction 50. The next set of arms (e.g. 12C, 12D) that have passed the obstruction 50 will then, along with any available activated arms above and / or below (e.g. 12A, 12B) the obstruction 50, be used to drive the tractor 10 through the obstruction 50. See also FIGS. 1A-1G.

[0062] Alternatively, according to another embodiment, the above-described process can be an automated process carried out by use of algorithm(s) based on sensor inputs (such as, but not limited only to, for example motor torque, motor speed, accelerometer, gyro, tension, proximity, pressure, etc.). This can also or alternatively be incorporated as a fully automated process based on sensor inputs and / or wellbore characteristics where the system automatically selects the optimal algorithm for negotiating known or unknown obstruction(s) / feature(s) in the well. A known obstruction can be planned based on wellbore characteristics (and real-time sensor inputs), but an unknown obstruction would rely solely on real-time sensor inputs. The advantages of implementing such a system would allow for a less aggressive (and more gentle) operation resulting in less downtime and wear / damage to the wellbore and tractor / tool string.

[0063] FIGS. 2D-2F show the different states and ports (P: pressure, C: control and R: return) of the solenoid valve 14. FIG. 2D shows the default state (where the solenoid valve 14 is not activated, no pressure, and the arms are retracted-FIG. 2A). FIG. 2E shows a normal operation (where the solenoid valve 14 is not activated, with pressure on the actuator, and the arms are extended-FIG. 2B). And FIG. 2F shows the individual arm control state (where the solenoid valve 14 is activated, the pressure on the actuator is switched off, and the individual arm is retracted-FIG. 2C).

[0064] FIGS. 3A-3B show the hydraulic power module (HPM) 25 of the wellbore tractor 10, where FIG. 3A shows a system 3-way fail-safe valve 23 in a default state (where the fail-safe valve 23 is not activated and the arms are retracted-see also FIG. 2A), and FIG. 3B shows the 23 fail-safe valve in an active state (where the fail-safe valve 23 is activated and the arms are extended (or the individual arms are retracted if the solenoid valves 14 are activated)—see also FIG. 2B (and FIG. 2C)).

[0065] The hydraulic power module (HPM) 25 comprises an electrical motor 28 driving a hydraulic pump 29 creating high pressure to hydraulic high-pressure line 21. The high-pressure line 21 is monitored by a pressure / temperature sensor 31, which again can be used as sensor input(s) for control of the hydraulic system. The electrical motor 28 can be controlled by a motor controller 15. The motor controller 15 can be arranged close to or adjacent to and is connected to the electrical motor 28.

[0066] A pre-set hydraulic-mechanic pressure relief valve (also called a pressure safety valve) 27 is further included as a part of the high-pressure line 21 to allow for a quick bleeding of hydraulic fluid in case of an extraordinary pressure build-up.

[0067] Due to the tractor 10 operating in a high pressure / temperature downhole environment and also due to the tractor 10 also operating over a wide pressure range (from ambient surface pressure / temperature to high pressure / temperature downhole conditions), the hydraulic system needs to be pressure compensated (towards the pressure in the wellbore 30) due to thermal oil expansion and control of differential pressure (between the hydraulic lines in the system and the wellbore pressure). This is achieved with a spring 33 activated piston 34 creating a slightly higher pressure in the oil sump reservoir 19 compared to the pressure in the wellbore 30. A ventilation port 32 allows access to fluid pressure from the wellbore 30 to activate the piston 34.

[0068] FIG. 4 shows a detailed piping and instrumentation diagram (P&ID) of the wellbore tractor 10.

[0069] The hydraulic schematic system diagram (P&ID) for the wellbore tractor 10 illustrates the option to disconnect one or more of the tractor's 10 drive arms 12A, 12B, 12C, 12D without affecting the remaining arms so that the remaining arms can continue the operation of the tractor 10.

[0070] This is done by activating one of the solenoid valves 14, each being 3-port / 2-way normally open solenoid valve 14 (abbreviated as 3 / 2 NO valve), according to the hydraulic schematics diagram (FIG. 4), so that a cylinder 17 loses pressure and contracts to its starting or closed or deactivated position resulting in that the arm 12 retracts. The contraction can be provided by the built-in retraction spring 18, and the hydraulic fluid that is inside the cylinder 17 bleeds off to the reservoir 19 in the hydraulic power module (HPM) 25 (FIGS. 3A-3B). Said in other words, when activated the 3-port / 2-way normally open solenoid valve 14 is configured to independently deactivate its corresponding arm 12A-12D, resulting in that said corresponding arm 12A-12D is retracted by means of its corresponding retraction spring 18. The 3-port / 2-way normally open solenoid valve 14 closes the access of the hydraulic fluid to the respective hydraulic cylinder 17, and the hydraulic fluid is guided through the return hydraulic line 22 back to the reservoir 19.

[0071] The system 3-way fail-safe valve (being 3-port / 2-way normally closed solenoid valve (abbreviated as 3 / 2 NC valve)) 23 can be arranged or placed in the hydraulic power module (HPM) 25 of the tractor 10. The system 3-way fail-safe valve 23 is being activated by default under a normal operation (to activate all arms 12A, 12B, 12C, 12D). If this fail-safe valve 23 is being deactivated or loses power, all arms 12A, 12B, 12C, 12D will retract independently of the position of the individual valve 14 (3 / 2 NO valve) in each arm 12A-12D. This provides as a fail-safe feature in the event of issues or loss of power to the wellbore tractor 10. This fail-safe valve 23 has also ports C, P, R as the solenoid valve 14.

[0072] A check valve 16 can be arranged as a part of the hydraulic pressure line 21 and between the hydraulic pump 29 and the 3-way fail-safe solenoid valve 23. The check valve 16 is configured to maintain the high pressure when the hydraulic pump 29 is not active.

[0073] The pre-set hydraulic-mechanic pressure relief valve (pressure safety valve) 27, included as a part of the high-pressure line 21, allows for a quick bleeding of the hydraulic fluid if a tractor arm 12A-12D unexpectedly should collide with the obstruction 50 in the well 30 so that an extraordinary pressure will be built-up as a result of excessive flow resistance. The pressure safety valve 27 is set to a selected maximum pressure which will open at this selected maximum pressure so that the system pressure will not become very high and will not damage any components thereof.

[0074] The present invention covers also tractors 10 with a common pivot point.

[0075] Two or more wellbore tractors 10 can be run in tandem and controlled in the previously described manner.

[0076] Additional modifications, alterations and adaptations of the present invention will suggest themselves to those skilled in the art without departing from the scope of the invention as defined in the following patent claims.LIST OF COMPONENTSA wellbore tractor 10

[0078] Wheels 11A, 11B, 11C, 11D

[0079] Arms 12A, 12B, 12C, 12D

[0080] Drive modules 13A, 13B, 13C, 13D

[0081] A solenoid valve / 3-port / 2-way normally open solenoid valve (3 / 2 NO valve) 14

[0082] A motor controller 15

[0083] A check valve 16

[0084] A hydraulic cylinder 17 with a retraction spring 18 therein (a part of an actuator (also called an actuator unit))

[0085] A retraction spring 18 (a part of the actuator (unit))

[0086] A reservoir / an oil sump reservoir 19 having a hydraulic fluid

[0087] An independent or paired arm control 20

[0088] A hydraulic pressure line 21

[0089] A return hydraulic line 22

[0090] A 3-way fail-safe solenoid valve 23

[0091] A system control module (SCM) 24

[0092] A hydraulic power module (HPM) 25

[0093] A tractor / system controller 26

[0094] A pressure safety valve 27

[0095] An electrical motor 28

[0096] A pump / a hydraulic pump 29

[0097] A wellbore 30

[0098] A pressure / temperature (PT) sensor 31

[0099] A wellbore ventilation port 32

[0100] A compensator spring 33 (shown in FIG. 3A)

[0101] A compensator piston 34 (shown in FIG. 3A)

[0102] A wall 35 of the wellbore 30

[0103] An actuator piston 36 (a part of the actuator (unit))

[0104] A formation 40

[0105] An obstruction 50

[0106] A wall 55 of the obstruction 50

[0107] A wireline 60

Claims

1. An independent or paired arm control system configured for a wellbore tractor configured to be run in a borehole penetrating a formation, the control system comprising:a 3-port / 2-way normally open solenoid valve arranged in a drive module of the wellbore tractor, the drive module having at least one arm with at least one wheel or belt,a hydraulic cylinder-piston assembly comprising a hydraulic cylinder (with an actuator piston and a retraction spring, arranged therein, the hydraulic cylinder-piston assembly arranged within the drive module,hydraulic pressure line connected to a hydraulic pump configured to provide high pressure by means of a hydraulic fluid,a return hydraulic line connected to a reservoir configured for containing the hydraulic fluid,a control port-S of the 3-port / 2-way normally open solenoid valve is connected to the hydraulic cylinder of the hydraulic cylinder-piston assembly, a return port of the 3-port / 2-way normally open solenoid valve is connected to the return hydraulic line, and a pressure port of the 3-port / 2-way normally open solenoid valve is connected to the hydraulic pressure line.

2. The independent or paired arm control system; according to claim 1, further comprising: a 3-port / 2-way normally closed fail-safe solenoid valve, wherein a control port of the 3-port / 2-way normally closed fail-safe solenoid valve is connected to the 3-port / 2-way normally open solenoid valve via the pressure port of the 3-port / 2-way normally open solenoid valve; a return port of the 3-port / 2-way normally closed fail-safe solenoid valve is connected to the return hydraulic line; and a pressure port of the 3-port / 2-way normally closed fail-safe solenoid valve is connected to the hydraulic pressure line having the hydraulic pump.

3. The independent or paired arm control system according to claim 1, further comprising a pressure safety valve connected between the hydraulic pressure line and the return hydraulic line.

4. The independent or paired arm control system according to claim 1, wherein the hydraulic pump is driven by an electrical motor.

5. The independent or paired arm control system according to claim 1, further comprising a check valve arranged in the hydraulic pressure line and between the hydraulic pump; and the 3-port / 2-way normally closed fail-safe solenoid valve.

6. The independent or paired arm control system according to claim 1, further comprising a pressure / temperature sensor connected to the hydraulic pressure line and configured to monitor the pressure in the hydraulic pressure line.

7. The independent or paired arm control system according to claim 1, further comprising a pressure compensation unit comprising an oil sump reservoir, a spring activated piston and a ventilation port.

8. The independent or paired arm control system % according to any claim 1, wherein said at least one arm is arranged in at least one pair of arms having one of: i) a common pivot point and ii) individual pivot points axially shifted in near proximity.

9. The independent or paired arm control system-according to claim 8, wherein the common pivot point type has a single actuator for a pair of arms and the axially shifted type has an individual actuator for each arm.

10. A wellbore tractor configured to be run in a borehole penetrating a formation, wherein the wellbore tractor comprises an independent or paired arm control system according to claim 1.

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