Coiled tubing power cable for deep wells

Abstract

 The invention concerns a power cable for providing power to a downhole tool situated within a deep well. The power cable comprises a plurality of stranded cable cores (2). Each cable core comprises an inner conductor (5) of a first conductive material, an armour core layer (6) surrounding the inner conductor of a second conductive material having higher tensile strength than the first conductive material and a conducting core layer (7) surrounding the armour core layers of a third conductive material. The inner conductor, the armour core layer and the outer conducting core layer are electrically connected to each other along the cable core's longitudinal length.

Description

Technical Field:

[0001] The present invention relates to a power cable as defined in the preamble of claim 1 and an intervention system using such a power cable.

Background and prior art:

[0002] During installation and service, the deep water power cable will be exposed to large tensile forces, and dynamic motion which will induce fatigue problems. Existing coiled tubing technology normally consist of a steel pipe having a three core power cable suspended within the pipe in order to supply power to an electrical submersible pump (ESP) fixed to the end of the steel pipe. As the wells are getting deeper, for example above 5-6000 meters, these types of power cable are unable to carry their own weight beyond a certain depth. Therefore, there is a need in the field for power cables that are able to withstand the enormous tensile strains set up due to its own weight, also at large depths.

[0003] An example of a prior art cable used in deep water applications, and conceived to withstand high tensile forces, may be found in EP 2'521'139A disclosing a power cable comprising a plurality of insulated conductors and an armour package surrounding the conductors. The cable comprises elastic materials as center elements and polymer materials arranged between the insulated conductors. In this way the elastic material will function as soft bedding for the insulated conductors and thus allow the conductors to move towards the center due to radial forces applied from the armour package and axial tensile loads in the insulated conductors themselves.

[0004] An example of a power cable used in coiled tubing may be found disclosed in WO 2011/146353 where each cable core comprises one or more stranded insulated conductors surrounded by one or more polymer layers. These cable cores are then stranded and surrounded by one or more polymer bonded strength members. The metallic conducter(s) is in this prior art cable fully isolated, significantly limiting all currents through these conducter(s) only. However, the main purpose with the particular configuration of the power cable in WO 2011/146353 is not to provide a cable that may withstand high tensile strains due to its weight, but to provide a cable for use with a downhole motor that is capable of withstanding the extreme environment downhole such as corrosion from well fluids.

[0005] It is thus an object of the present invention to design a power cable that may support the high tensile stress set up along the cables longitudinal length due to the cables own weight and at the same time to avoid significant reconfiguring of existing coiled tubing cables.

Summary of the invention:

[0006] The present invention is set forth and characterized in the main claim while the dependent claims describe other characteristics of the invention.

[0007] In particular, the invention concerns a power cable suitable for providing power to a downhole tool situated within a deep well. The power cable comprises a plurality of stranded cable cores, where each cable core comprises one or more inner conductors comprising one or more first conductive materials, one or more armour core layers surrounding the inner conductor(s) comprising one or more second conductive materials having higher tensile strength than the one or more first conductive materials and one or more outer conducting core layers surrounding the one or more armour core layers, comprising one or more third conductive materials, wherein the one or more inner conductors, the one or more armour core layers and the one or more outer conducting core layers are electrically connected to each other along a major part, preferably all, of the cable core's longitudinal length. Hereinafter, conductive material signifies any material or combination of materials (e.g. mixture / alloys) that exhibits conductivity per unit length (σ) of more than 1x10⁴ S/m at 20°C (293 K) along at least part of the power cable, preferably along the whole length of the power cable. The conductivity per unit length of the first and third conductivity materials is preferably more than 1x10⁶ S/m at 20°C, for example more than 1x10⁷ S/m, at 20°C.

[0008] In an advantageous embodiment at least one of the materials constituting the one or more first conductive materials is identical to at least one of the materials constituting the one or more third conductive materials.

[0009] In another advantageous embodiment more than 50 % of the conductive material(s) within at least one of the first and third conductive material is copper.

[0010] In another advantageous embodiment the conductivity per unit length at 20°C of the first and third conductive material is higher than the conductivity per unit length at 20°C of the second conductive material.

[0011] In another advantageous embodiment more than 50 % of the conductive material(s) within the second conductive material is composed of an iron based alloy such as steel.

[0012] In another advantageous embodiment at least the majority of the interstices formed within the armour core layer is filled with a pressure compensating filling material comprising an elastic material such as petroleum jelly inter alia in order to avoid crack formation.

[0013] In another advantageous embodiment the armour core layer comprises a plurality of radial layers of armouring wires, the wires being mutually arranged in the radial direction in order to maximize the armour core layer density.

[0014] In another advantageous embodiment the armour core layer comprises an inner radial layer comprising a plurality of armouring wires with a diameter D and an outer radial layer electrically contacting the inner radial layer, the outer radial layer comprising a plurality of armouring wires with a diameter d, the diameter d being dissimilar to the diameter D, preferably smaller.. As an example, the ratio between the diameter D and diameter d may be at least 1.25.

[0015] In another advantageous embodiment the outer radial layer further comprises a plurality of armouring wires with diameter D' arranged at least partly between the armouring wires with the diameter d and at least partly between the armouring wires with the diameter D of the inner radial layer 6', wherein the diameter D' is larger than the diameter d.

[0016] In another advantageous embodiment, in the radial direction, the plurality of armouring wires constituting the radial layers is mutually arranged in a two-dimensional closed packed structure.

[0017] In another advantageous embodiment, in the radial direction, the outermost surface positions of the armouring wires defining the outer periphery of the armour core sheath constitute positions on a circle.

[0018] In another advantageous embodiment an outer insulating core layer surrounds the outer conducting core layer, which outer insulating layer preferably comprising mainly a fluorine based polymer, for example a fluorine based polymer within the group poly/ethane-co-tetrafluoroethene (ETFE), fluorinated ethylene propylene (FEP), perfluoroethers (PFA), ethylene-fluorinated ethylene propylene (EFEP).

[0019] The invention also concerns an intervention system suitable for pumping hydrocarbon from a hydrocarbon production well. The intervention system comprises a downhole tool such as a pump situated within the well, a pipe extending to the downhole tool and a power cable in accordance with any of the configurations mentioned above, which power cable extends along the longitudinal direction of the pipe and is electrically connected to the downhole tool.

[0020] In the following description, specific details are introduced to provide a thorough understanding of an embodiment of the claimed power cable. One skilled in the relevant art, however, will recognize that this embodiment can be practiced without one or more of the specific details, or with other components, systems, etc. In other instances, well-known structures or operations are not shown, or are not described in detail, to avoid obscuring aspects of the disclosed embodiments.

Brief description of the drawing:

[0021] 

Figure 1 is a cross-sectional view of a three conductor cable in accordance with an embodiment of the invention.

Detailed description of the invention

[0022] A cross section of a power cable 1 in accordance with the invention is shown in figure 1. In this particular embodiment the power cable 1 comprises three stranded cable cores 2 surrounded by a filling material 3 such as a pressure filling compound and an outer cable sheath 4 providing outer protection of the cable 1. The outer cable sheath 4 may be a polymer sheath such as a fluorine based plastic, for example of type ethylene tetrafluoroethylene (ETFE). An optical fibre 9 may optionally be arranged in the centre of the power cable 1 for transmitting optical signals.

[0023] Each of the cable cores 2 (or power phases) comprises one or more inner conductors 5 of high conductive material(s) such as copper.

[0024] These centrally located core conductors 5 are surrounded by an armour core sheath 6 of a conductive material, which armour core sheath exhibits higher tensile strength than the inner core conductor(s) 5 in order to, inter alia, achieve the strength necessary for the cable 1 to carry its own weight at large sea depths, i.e. depths of several kilometres, for example more than 5 kilometres. In the embodiment of figure 1 this armour core sheath 6 comprises radial layers 6',6" made of a plurality of steel armouring wires 6a,6b,6c which are mutually arranged to reach highest possible, or close to highest possible, density. One way to achieve such an maximum packing density is to stack the wires 6a,6b,6c radially in a closed packed structure (cps), or near closed packed structure, where at least some of the wire diameters D, D', d are dissimilar. Figure 1 shows an inner radial layer 6' of armouring wires with a wire diameter D 6a arranged in contact with the insulating sheath 3, and an outer radial layer 6" of armouring wires 6b, 6c surrounding the inner radial layer 6', wherein wires of a small wire diameter d 6c alternates with wires of a larger diameter D' 6b, for example equal to the wire diameter D. Further, the wires 6b, 6c of the second layer 6" are arranged within the outer valleys or recesses set up by the wires 6a of the inner radial layer 6'. With this particular configuration of the armour sheath 6, the outermost radial position of each armouring wires 6b,6c constituting the outer radial layer 6" in figure 1 represents points on a perfect, or near perfect, circle having the inner core conductor(s) 5 as a centre. Relevant examples of conductive materials with high tensile strength may be various steel types, tungsten, titanium alloys and aluminium alloys. A filling compound of an elastic material, typically a petroleum jelly, may be inserted within the interstices of the armouring wires to ensure sufficient pressure compensation during operation, and consequently reduce the risk of crack formation.

[0025] The armour core sheath 6 is in the embodiment of figure 1 surrounded by an outer conducting core layer 7. This layer may be of identical material(s) as the inner conductor(s) 5, for example copper.

[0026] This outer conducting core layer 7 is surrounded by an outer insulating core layer 8 preferably of a flour-based polymer such as ETFE (ethylene tetrafluoroethylene).

[0027] The inner conductor(s) 5, the armour sheath 6 and the outer conducting core layer 7 are all electrically connected along at least the major part of the cable core 2 to maximise the radial cross section in which electrical power may flow.

[0028] Typical dimensions of the cable core 2 are

• An inner conductor 5 having diameters within the range of 2-3 mm, for example 2.45 mm,
• armouring wires 6a of the inner layer 6' having diameters (D) within the range of 1-2 mm, for example 1.52 mm,
• armouring wires 6b of the outer layer 6" having large (D') and small (d) diameters within the range of 1.3-1.6 mm, for example 1.52 mm, and within the range of 0.96-1.16 mm, for example 1.06 mm, respectively
• an outer conductive core layer 7 of diameter within the range of 7-10 mm, for example 8.65 mm,
• an outer insulating layer 8 of diameter within the range 20-50 mm, for example 34 mm.

[0029] The power cable 1 with three of the cable cores 2 stranded together is typically arranged in order to support a cable weight of at least 3 km sea depth, for example 4 km sea depth. The weight of the power cable 1 may be within the range 1.5-2.5 kg/m, for example about 2 kg/m.

[0030] The power cable 1 may be used as part of a coiled tubing system for providing power to a downhole pump or any other power demanding downhole tools, situated within a deep hydrocarbon producing well.

List of reference numerals:

[0031] 

Power cable

1

Cable core / power phase

2

Filling material

3

Outer cable sheath

4

Inner conductor

5

Armour core sheath / armour core layer

6

Inner radial layer (of armour core sheath)

6'

Outer radial layer (of armour core sheath)

6"

Armouring wire with diameter D

6a

Armouring wire with diameter D'

6b

Armouring wire with diameter d

6c

Outer conducting core layer

7

Outer insulating core layer

8

Optical fibre

9

Claims

1. Power cable (1) for providing power to a downhole tool situated within a deep well, comprising a plurality of stranded cable cores (2),
characterized in that each cable core (2) comprises

- an inner conductor (5) comprising a first conductive material,

- an armour core layer (6) surrounding the inner conductor (5) comprising a second conductive material having higher tensile strength than the first conductive material and

- an outer conducting core layer (7) surrounding the armour core layer (6), comprising a third conductive material,

wherein the inner conductor (5), the armour core layer (6) and the outer conducting core layer (7) are electrically connected to each other along a major part of the cable core's longitudinal length (2).

2. The power cable (1) in accordance with claim 1, characterized in that at least one of the first conductive material is identical to at least one of the third conductive material.

3. The power cable (1) in accordance with claim 1 or 2, characterized in that at least one of the first and third conductive material comprises mainly copper.

4. The power cable (1) in accordance with one of the preceding claims, characterized in that the conductivity per unit length at 20°C of the first and third conductive material is higher than the conductivity per unit length at 20°C of the second conductive material.

5. The power cable (1) in accordance with one of the preceding claims, characterized in that the second conductive material comprises mainly steel.

6. The power cable (1) in accordance with one of the preceding claims, characterized in that interstices within the armour core layer (6) is filled with a pressure compensating filling material comprising an elastic material.

7. The power cable (1) in accordance with one of the preceding claims, characterized in that the armour core layer (6) comprises a plurality of radial layers (6',6") of armouring wires (6a,6b,6c), the wires (6a,6b,6c) being mutually arranged in the radial direction in order to maximize the armour core layer density.

8. The power cable (1) in accordance with one of the preceding claims, characterized in that the armour core layer (6) comprises an inner radial layer (6') comprising a plurality of armouring wires (6a) with a diameter D and an outer radial layer (6") electrically contacting the inner radial layer (6'), the outer radial layer (6") comprising a plurality of armouring wires (6c) with a diameter d, the diameter d being dissimilar to the diameter D.

9. The power cable (1) in accordance with claim 8, characterized in that the outer radial layer (6") further comprises a plurality of armouring wires with diameter D' (6b) arranged at least partly between the armouring wires with the diameter d (6c) and at least partly between the armouring wires with the diameter D (6b) of the inner radial layer (6'), wherein the diameter D' is larger than the diameter d.

10. The power cable (1) in accordance with any one of claims 7 to 9, characterized in that in the radial direction, the plurality of armouring wires (6a,6b,6c) constituting the radial layers (6',6") is mutually arranged in a two-dimensional closed packed structure.

11. The power cable (1) in accordance with any one of claims 7 to 10, characterized in that, in the radial direction, the outermost surface positions of the armouring wires (6b,6c) defining the outer periphery of the armour core sheath (6) constitute positions on a circle.

12. The power cable (1) in accordance with one of the preceding claims, characterized in that an outer insulating core layer (8) surrounds the outer conducting core layer (7).

13. The power cable (1) in accordance with claim 12, characterized in that the outer insulating core layer (8) comprises mainly a fluorine based polymer.

14. The power cable (1) in accordance with claim 13, characterized in that the outer insulating core layer (8) comprises mainly a fluorine based polymer within the group poly/ethane-co-tetrafluoroethene (ETFE), fluorinated ethylene propylene (FEP), perfluoroethers (PFA), ethylene-fluorinated ethylene propylene (EFEP).

15. Intervention system for pumping hydrocarbon from a hydrocarbon production well, comprising

- a downhole tool situated within the well,

- a pipe extending to the downhole tool,
characterized in that
the intervention system further comprises

- a power cable (1) in accordance with any one of claims 1-14, which power cable (1) extends along the longitudinal direction of the pipe and is electrically connected to the downhole tool.

Drawing