Cable for downhole tractor deployment

Abstract

The invention concerns a power cable suitable for providing power to and from a downhole tool situated within a borehole. The cable comprises at least one inner conductor comprising at least one first electrically conductive material, at least one inner insulating layer surrounding the inner conductor(s), comprising at least one electrically insulating material, an armour sheath surrounding the inner insulating layer(s) comprising at least one second electrically conductive material and at least one outer conducting layer surrounding, and electrically contacting, the armour sheath, comprising at least one third electrically conductive material. The armour sheath further comprises at least one inner radial layer comprising a plurality of armouring wires with a diameter D and at least one outer radial layer electrically contacting the inner radial layer(s), the outer radial layer(s) comprising a plurality of armouring wires (6c) with a diameter d the diameter d being dissimilar to the diameter D, and wherein said armouring wires are radially arranged, in a closed packed structure in order to maximize the armour sheath density.

Claims

1. A power cable for providing power to a downhole tool situated within a borehole, comprising: an inner conductor comprising a first electrically conductive material, an inner insulating layer surrounding the inner conductor, comprising an electrically insulating material, an armour sheath surrounding the inner insulating layer comprising a second electrically conductive material and an outer conducting layer surrounding, and electrically contacting, the armour sheath, comprising a third electrically conductive material, wherein the armour sheath further 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, and wherein said armouring wires are radially arranged in order to maximize the armour sheath density, wherein the diameter D is larger than the diameter d.

2. The power cable in accordance with claim 1, wherein the inner conductor is a solid conductor.

3. The power cable in accordance with claim 1, wherein 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, wherein the diameter D′ is larger than the diameter d.

4. The power cable in accordance with claim 1, wherein, in the radial direction, the outermost surface positions of the armouring wires defining the outer periphery of the armour sheath constitute positions on a circle.

5. The power cable in accordance with claim 1, wherein the second electrically conductive material has higher tensile strength than at least one of the first and third electrically conductive material.

6. The power cable in accordance with claim 1, wherein at least one of the first electrically conductive material is identical to at least one of the third electrically conductive material.

7. The power cable in accordance with claim 1, wherein at least one of the first and third conductive material comprises mainly copper.

8. The power cable in accordance with claim 1, wherein the conductivity per unit length at 20° C. of the first and third electrically conductive material is higher than the conductivity per unit length at 20° C. of the second electrically conductive material.

9. The power cable in accordance with claim 1, wherein the second electrically conductive material comprises mainly steel.

10. The power cable in accordance with claim 1, wherein at least the majority of interstices within the armour sheath are filled with a pressure compensating filling material of petroleum jelly comprising an elastic material, and wherein an outer insulating layer, separate from said pressure compensating material surrounds the outer conducting layer.

11. The power cable in accordance with claim 1, wherein the outer insulating layer comprises mainly a fluorine based polymer.

12. The power cable in accordance with claim 1, wherein the outer insulating layer 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).

13. The power cable in accordance with claim 11, wherein the outer insulating layer 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).

14. A downhole tool assembly for drilling a borehole for hydrocarbon production, comprising a downhole tool and a power cable constructed in accordance with claim 1, being in one longitudinal end electrically connected to the downhole tool.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a cross-sectional view of a power cable in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) A cross section of a power cable 1 in accordance with the invention is shown in FIG. 1. In this particular embodiment the power cable 1 comprises an inner core 2,3 composed of one or more insulated conductors 2, preferably of solid copper, surrounded by one or more electrically insulating sheaths 3. The inner core is surrounded by an armour sheath 6 comprising a plurality of stranded steel wires 6a,6b,6c. The interstices 4 formed between the steel wires 6a,6b,6c are preferably filled with a pressure compensating filling compound such as a petroleum jelly that may block undesired gas migration and/or ensure sufficient pressure compensation during operation. Particularly the latter effect may reduce the risk of crack formation. The armour sheath 6 is further surrounded by a conducting tube 7, preferably of copper, that may be act as a main return conductor for power transmission from the downhole tool/tractor. The tube 7 is surrounded by an outer insulating layer 8 made of an electrically insulating material, thereby acting as an outermost sheath for the power cable 1. The layer 8 may for example be made of a fluropolymer such as ETFE (ethylene tetrafluoroethylene).

(3) The above described configuration provides a power cable 2 having a main return conductor 7 compactly arranged within the cable's 1 cross section. This relatively simple cable design makes the production of power cables of long length (i.e. several kilometres) easier while allowing accommodation of a larger power transmission compared to prior art solutions.

(4) The main purpose of the armour sheath 6 is to protect the inner insulated conductor(s) 2 and give the cable 1 high longitudinal strength, i.e. at strength that at least corresponds to a strength necessary for the cable 1 to carry its own weight. This is often a critical requirement for cables employed at large sea depths such as depths of more than four kilometres. For this reason the armour sheath 6 preferably exhibits higher tensile strength than both the inner core 2,3 and the tube 7. Relevant examples of conductive materials with high tensile strength may be various steel types, tungsten, titanium alloys and aluminium alloys, or a combination thereof. In the embodiment of FIG. 1 this armour 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. FIG. 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 a 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 FIG. 1 represents points on a perfect, or near perfect, circle having the inner core 2,3 as a centre.

(5) The armour sheath 6 and the tube 7 are preferably electrically connected along at least the major part of the cable's longitudinal length in order to maximise the radial cross section in which electrical power may flow during return from the downhole tool.

(6) Note that the direction of the power flow may be interchanged as convenient. For example, in an alternative embodiment of the invention armour sheath 6 and/or the tube 7 may act as an conductor for the power flow into the downhole tool, in which case the one or more insulated conductors 2 of the inner core 2,3 act as the conductor for the power flow from the downhole tool,

(7) Typical dimensions of the inventive power cable 1 are a solid conductor 2 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 a conductive tube 7 of diameter within the range of 7-10 mm, for example 8.65 mm and an outer insulating layer 8 of diameter within the range 10-20 mm, for example 15 mm.

(8) The above mentioned radial arrangement is typically arranged in order to support a cable weight of at least 4 km sea depth, for example 5 km sea depth. The weight of the inventive power cable 1 may be within the range 0.4-0.8 kg/m, for example about 0.6 kg/m.

(9) The power cable 1 may be used as part of a downhole tool arrangement such as a cable transmitting necessary power to a downhole tractor within a hydrocarbon producing well.

LIST OF REFERENCE NUMERALS

(10) Power cable 1

(11) Insulated conductor 2

(12) Electrically insulating sheath 3

(13) Interstices (between armour wires) 4

(14) Armour sheath 6

(15) Armouring wire with diameter D 6a

(16) Armouring wire with diameter D′ 6b

(17) Armouring wire with diameter d′ 6c

(18) Inner radial layer 6′

(19) Outer radial layer 6″

(20) Conducting tube 7

(21) Outer insulating layer 8