Downhole cable

Abstract

 A downhole cable (10) for use in an oil or gas well is described as having a pair of conductors (12, 14) for transmission of power and/or data, and a separate load bearing member (16). A non-conductive outer sheath (18) is also preferably provided.

Description

[0001] The invention relates to a downhole cable, particularly for use in oil or gas wells for conveying data sensors into and out of the wellbore, and for communication between surface equipment and sensors and other apparatus downhole, particularly logging tools and drive units for inserting logging tools into a well, such a drive unit typically being a downhole tractor.

[0002] Conventionally, braided wire cables are used downhole in oil and/or gas wells or water injector wells (which are used to inject water into an oil or gas well to increase the pressure within that well and hence the production rates therefrom) to run in electrically operated logging tools and other electrically operated tools. These conventional braided wire cables comprise one or more conductors laid side by side, each surrounded by an insulating material, and which together form the core of the cable. The insulated conducting core is then surrounded by two layers of steel wires which act as all of the following features:-

a) the strength bearing element of the cable;

b) the electrical return path for the one or more conductors in the core of the cable; and

c) the armour to protect the core of the cable; i.e. to provide abrasion resistance and crush resistance.

[0003] Such conventional braided wire cables suffer from several disadvantages however. Firstly, since the outer surface of the cable is braided, the cable must be inserted into, and pulled from, the well through a grease injection unit and stuffing box. The grease injection unit basically provides a chamber filled with grease at relatively high pressure, such that the grease works its way into the interstices between the braids so that the pressure of the well is contained. This pressure control arrangement necessarily requires a large amount of grease, and is relatively time consuming. In addition, the fact that the outer steel wires carry the electrical current return is an inherent safety risk, particularly since the environment of use of the braided wire cable contains high pressure hydrocarbons, and hence has great implications for Health and Safety which is a particularly relevant consideration in the Norwegian oil and gas sector.

[0004] In addition, this conventional braided wire cable is relatively heavy, in the region of 120kg/1000m for a typical 7/32" (5.6mm) braided monoconductor cable.

[0005] Thus, relatively large and heavy duty deployment apparatus such as wireline apparatus is required to run in the conventional braided wire cable.

[0006] According to the present invention there is provided a downhole cable for use in an oil or gas well or a water injector well, the downhole cable comprising a pair of conductors for transmission of power and/or data, and a load-bearing member which is separate from the pair of conductors.

[0007] Preferably, the downhole cable further comprises a non-conductive outer sheath. The outer sheath is typically non-conductive in the sense that it substantially prevents conduction of electricity.

[0008] The downhole cable of the invention can incorporate a fibre optic cable. The pair of conductors preferably comprise a first conductor having an electricity conducting core, formed from a metal material such as copper, and a second conductor formed from an electricity conducting sheath, typically formed from a metal material such as copper. The pair of conductors are preferably capable of transmitting power to and from downhole sensors.

[0009] The pair of conductors are preferably arranged co-axially. The conductors are preferably also capable of transmitting data, such that they act as telemetry conduits. Additional conductors or telemetry conduits can optionally be provided in the cable of the invention for transmitting electrical and optical data and instructions to and from sensors in the well.

[0010] The cable may comprise more than two conductors.

[0011] The load-bearing member preferably comprises a sleeve surrounding the conductors and, if present, additional telemetry conduits. A preferred material for the load-bearing member is a polymer fibre or yarn, and preferably a fibre such as Zylon (™) PBO (poly(p-phenylene-2,6-benzobisoxazole)), polyamide or polybenzimidazole, which is woven or wound around the inner core, optionally as it is being made.

[0012] Alternatively, an aramid fibre such as Kevlar (™) may be utilised as the material for the load bearing-member. Aramid materials are aromatic polyamides (in particular poly(p-phenylene terephthalamide)).

[0013] It should be noted that Zylon (™) is similar to aramid material in that it is also classed as a liquid crystalline polymer.

[0014] The strength of the cable can be increased by increasing the quantity of fibre in the cable. Any suitable polymer fibre can be used, for example, any polymer fibre with a chemical structure based on repeating aromatic structures such as aramids derived from aromatic acids and amines with linkages on the para position of the aromatic ring structure, and derived from terephthalic acid and p-phenylene diamine or p-aminobenzoic acid.

[0015] The outer sheath preferably comprises a plastics material such as PEEK (™) (Polyether ethyl ketone) or FEP (™) (Fluorinated Ethylene Propylene).

[0016] Alternatively, the outer sheath comprises a material such as polyethelene which typically has been treated, for example by electron beam irradiation, to render it more inert chemically, if necessary. The outer sheath is non-conducting so as to insulate the cable from the environment. The outer sheath can also protect the cable from damage by external abrasion, and can also reduce internal damage to tubulars through which the cable is deployed, when compared to the damage produced by conventional braided wire cables. As well as protecting the cable from the environment, the outer sheath can insulate the environment from the cable by resisting electrical discharge from the cable to the environment. The outer surface of the outer sheath is preferably smooth and typically comprises a low friction surface.

[0017] In certain embodiments of the invention such as polyethelene, the outer sheath can be colour coded to indicate wear and the need for replacement or re-coating.

[0018] The load-bearing member comprising the Zylon (™) sleeve preferably also provides wear and crush protection, and can also insulate the cable from adverse effects of pressure differentials experienced downhole.

[0019] In certain embodiments of the invention the fibre optics telemetry cables can be included in the woven sleeve.

[0020] The two or more conductors and, if present, further telemetry conduits are preferably insulated from one another by sheaths or other insulating members. A first electrical insulation material, which is preferably ETFE (™) (a fluoropolymer), is arranged between the first and second conductors, such that the first electrical insulation material surrounds the first (inner) conductor. The second conductor, which is preferably in the form of the conductive braid, is itself preferably surrounded by a second electrically insulating material, preferably formed from ETFE (™), and which typically has an outer diameter in the region of ∼2.5mm.

[0021] Preferably, the cable may be manufactured by arranging the conductors and, if present, the further telemetry conduit(s) co-axially.

[0022] Alternatively, the cable of the invention may be manufactured by arranging the conductors and if present further telemetry conduits side by side and weaving or winding the (preferably Zylon (™)) sleeve over the assembled conductors and, if present, further telemetry conduits during production, so as to obtain a continuous length of finished cable.

[0023] Internal sheaths can be moulded or fitted onto the outer surface of the conductors as required during assembly, and likewise, the outer sheath can be extruded over the outer surface of the (preferably Zylon (™)) sleeve.

[0024] Other yarn materials (such as other synthetic polymeric yarns) can be used in place of the Zylon (™) sleeve, such as Kevlar (™) etc.

[0025] An embodiment of the invention will now be described with reference to the accompanying drawing in which:-

Fig. 1 is a cross sectional view of one embodiment (Example 1) of a downhole cable in accordance with the present invention.

Example 1

[0026] A downhole cable 10 in accordance with the present invention is shown in Fig. 1, and comprises two nickel coated copper conductors 12, 14 arranged co-axially. The first copper (inner) conductor 12 may be a single strand or may comprise a plurality of strands, and the first (inner) conductor 12 is in the region of ∼1mm outer diameter and the second copper (outer) conductor 14 is preferably in the form of a conductive braid 14 (i.e. many relatively thin strands). A first electrical insulation material 13, which is preferably ETFE (™), is arranged between the first 12 and second 14 conductors, such that the first electrical insulation material 13 surrounds the first (inner) conductor. The second conductor 14 in the form of the conductive braid 14 surrounds the first electrical insulation material 13. The second conductor 14 in the form of the conductive braid 14 is itself surrounded by a second electrically insulating material 15, preferably formed from ETFE (™), and which has an outer diameter in the region of ∼2.5mm.

[0027] The first (inner) conductor 12, the first layer of electrical insulation coating 13, the second (outer) conductor 14 and the second layer of electrical insulation coating 15 together form the core of the cable 10.

[0028] As shown in Fig. 1, the coaxial core of the cable 10 is surrounded by a strength bearing member 16 of Zylon (™) yarns 16, which is typically in the region of ∼4mm outer diameter. Thereafter, the layer of Zylon (™) yarns 16 is in turn surrounded by a low friction, smooth, and hard wearing (abrasion resistant) and electrically insulating outer sheath 18 which typically has an outer diameter in the region of ∼5mm. The outer sheath 18 is preferably formed from PEEK (™) or FEP (™).

[0029] One example of how the downhole cable 10 of the present invention may be manufactured is as follows. The core of the cable is typically formed into extremely long lengths such as 22,000 ft lengths, and is then passed through weaving apparatus which weaves the Zylon (™) coating 16 over the core as it is passing through the weaving apparatus. The PEEK (™) or FEP (™) sheath 18 is extruded over the woven Zylon (™) layer 16 further down the production line. Alternatively or additionally, a PETP (™) tape 17 can be wound around the Zylon (™) yarn 16 after it is woven over the conductors 12, 14, such that the PEEK (™) or FEP (™) sheath 18 is extruded over the PETP (™) tape. The presence of the PETP (™) tape 17 provides the advantage that the PETP (™) tape 17 holds and binds the Zylon (™) fibres 16 together, thus increasing their strength and useable lifespan. The PETP (™) tape 17 comprises polyamide which is a polymer of amides and is the base material in the tape 17 known as Kapton (™) (or its equivalent Apical (™)), and although PETP (™) tape 17 is preferred, any suitable polymer tape 17, such as polyamide, for example, could be used.

[0030] The first (inner) conductor 12 preferably comprises 16awg nickel coated copper.

[0031] The weight of the downhole cable 10 is critically important, since the weight must not be so great that the cable 10 would break under its own weight if a very long length is free hanging. The approximate weights and diameters of each element are as follows:-

Element	Weight (g/m)	Diameter (mm)
Centre conductor 12	5.4	1.00
Insulation 13	1.7	1.50
Metal braid 14	6.2	2.03
Second insulation 15	3.7	2.59
Zylon (™) yarns	9.8	4.00
PETP (™) tape	0.3	4.10
Outer sheath	9.7	5.10

[0032] The cable has the following technical characteristics:-

Conductors 12,14

Co-axial core: Nickel coated copper 12,14 with fluoropolymer ETFE (™) insulation 13, 15 or silver plated annealed copper or plain copper.

Current rating

2 Amps D.C.

Voltage rating

500 VDC

Capacitance copper

Approximately 52 pf/ft

Cde Resistance cop

Approximately 5.2 ohms/kft

Length

22,000 ft continuous

Diameter

5.1mm

Yield strength

Own weight plus 5,000 N

Tensile rating

8,000 N at 0.75% elongation

Elongation

With loading up to the yield, elongation is not limited so long as no permanent deformation or damage occurs to any components

Stretch to yield limit

<7.5%

Pressure rating

20,000 psi

Temp rating

200°C

Sheave wheel cycles

6,000 over 14" reverse bend

H₂S resistant

4% Volume (for minimum operating conditions of pressure 10,000 and temperature 150°C)

CO₂ resistant

10% by volume (for minimum operating conditions of pressure 10,000 and temperature 150°C)

Ovality

The cable preferably has a high degree of conformity to a circular cross-section

Example 2

[0033] Two nickel coated copper conductors are arranged co-axially. The first copper (inner) conductor may be a single strand or may comprise a plurality of strands, and the second copper (outer) conductor is preferably in the form of a braid (i.e. many relatively thin strands). A fluoropolymer insulation coating is arranged between the first and second conductors. A second insulation coating is provided around the outer circumference of the second (outer) conductor. One or more optical fibres are wound in a spiral about the second insulation coating. The first (inner) conductor, the electrical insulation coating, the second (outer) conductor, the second insulation coating and the one or more optical fibres form the core of the cable. The optical fibre(s) is/are suitable for data communication and temperature profiling. The conductors and optical fibres are formed separately into 22,000 ft lengths and gathered together in the required configuration before being passed through weaving apparatus which weaves a Zylon (™) coating over the core as it is passing through the weaving apparatus. A polyethylene sleeve is moulded over the woven Zylon (™) further down the production line. Alternatively or additionally, a PETP (™) tape can be wound around the Zylon (™) yarn after it is woven over the conductors and optical fibres.

[0034] The approximate weights and diameters of the downhole cable of Example 2 are broadly the same as for Example 1.

[0035] The cable has the following technical characteristics:-

Conductors

Co-axial core with one or more optical fibre(s). Nickel coated copper with fluoropolymer insulation or silver plated annealed copper or plain copper.

Optic Fibres

One or more fibres, suitable for data communication and temperature profiling.

Current rating

2 Amps D.C.

Voltage rating

500 VDC

Capacitance copper

Approximately 52 pf/ft

Cde Resistance cop

Approximately 5.2 ohms/kft

Length

22,000 ft continuous

Diameter

5.1mm

Yield strength

Own weight plus 5,000 N

Tensile rating

8,000 N at 0.75% elongation

Elongation

With loading up to the yield, elongation is not limited so long as no permanent deformation or damage occurs to any components

Stretch to yield limit

<7.5%

Pressure rating

20,000 psi

Temp rating

200°C

Sheave wheel cycles

6,000 over 14" reverse bend

H₂S resistant

4% Volume (for minimum operating conditions of pressure 10,000 and temperature 150°C)

CO₂ resistant

10% by volume (for minimum operating conditions of pressure 10,000 and temperature 150°C)

Ovality

The cable preferably has a high degree of conformity to a circular cross-section

Example 3

[0036] A cable is constructed as described in Example 1, but the Zylon (™) sleeve is formed by winding the yarn around the inner core during assembly rather than weaving it around the core.

Example 4

[0037] A downhole cable is constructed as described in Example 1, but the core of the downhole cable differs in that the two nickel coated copper conductors are arranged side by side with fluoropolymer insulation coating around each of the two conductors. However, it should be noted that this is a less preferred embodiment than Example 1 since the core of Example 4 will not be perfectly circular, which means that the application of the Zylon (™) layer may be more difficult.

Example 5

[0038] A downhole cable is constructed as described in Example 4, but the core of the downhole cable differs in that the two nickel coated copper conductors are arranged side by side with fluoropolymer insulation coating around each of the two conductors, and one or more, such as two, optical fibres are arranged to nest between the side by side conductors. However, it should also be noted that this is a less preferred embodiment than Example 1 since the core of Example 5 will also not be perfectly circular, although it is likely to be more circular than the core of Example 4, and hence as with Example 4, the application of the Zylon (™) layer for the downhole cable of Example 5 may be more difficult.

[0039] The number of conductors and optical fibres can clearly be varied as can the characteristics, for example three or more optical fibres can be included although normally two conductors will be sufficient. In certain embodiments, the protective outer sheath can be replaced if worn. In certain embodiments, the Zylon (™) sleeve can be woven in advance and wrapped or passed over the cores etc.

[0040] Certain embodiments of the cable can be used for powering tractors (devices which can move along the horizontal sections of oil wells to deploy tools or measurement devices) enabling an extended reach into the well by the tractors as a direct consequence of the cables lightweight construction. Furthermore the including of a fibre optic cable would enable concurrent tractor deployment and measurement of well parameters.

[0041] Certain embodiments of the invention provide a lighter easier to handle cable capable of supporting greater loads over its own weight than conventional slick line or electric line cables. Other benefits arise in safety, environmental, operations and logistics areas. The embodiments of the downhole cable described herein differ from existing braided wire line cable technology in that all electric signals (telemetry and power) are conveyed on the central co-axial core and not on the external armouring, thus making the cable intrinsically electrically safe. This has implications for Health and Safety which is a particularly relevant consideration in the Norwegian sector.

[0042] The embodiments of the downhole cable described herein have similar strength capabilities to a conventional braided wire steel cable but are capable of achieving the same strength at a far lower weight (7kg/1000m in water compared to 120kg/1000m for a typical 7/32" braided monoconductor cable). The main reason for this weight reduction is the Zylon (™) fibre yarns, which although being relatively light, provide the load bearing capability to the downhole cable. In addition, the low friction surface provided by the PEEK (™) or FET (™) outer sheath provides the advantage that the embodiments of the downhole cable described herein are more suited to horizontal or highly deviated wells or to deep wells than the conventional braided wire cables, since the tension pulled at surface is closer to the tension pulled at the tool string with the present embodiments. This is an important consideration during logging operations since there is a limit as to the tension which can be applied at surface (50% of the breaking load at surface is considered the safe working limit). It also means the downhole cable of the present embodiments is more suited to wells where the tools will be deployed using a tractor since there is less weight and also less drag for the tractor to pull along the interior of the wellbore.

[0043] In addition, since the outer surface of the downhole cable of the present embodiments is substantially smooth, more complex pressure control apparatus such as grease injectors are not required, and more simplified pressure control apparatus, such as an annular rubber seal arrangement, can be utilised to control the wellbore pressure.

[0044] Modifications and improvements can be incorporated without departing from the scope of the invention.

Claims

1. A downhole cable (10) for use in a well, the downhole cable (10) comprising a pair of conductors (12, 14) for transmission of power and/or data, and a load-bearing member (16), characterised in that the load-bearing member (16) is separate from the pair of conductors (12, 14).

2. A downhole cable (10) according to claim 1, further comprising a non-conductive outer sheath (18).

3. A downhole cable (10) according to either of claims 1 or 2, further comprising a fibre optic cable.

4. A downhole cable (10) according to any preceding claim, wherein the conductors (12, 14) comprise electrically conducting wires, and are capable of transmitting power to and from downhole sensors.

5. A downhole cable (10) according to any preceding claim, wherein the pair of conductors (12, 14) comprise a first conductor (12) having an electrically conducting core (12), and a second conductor (14) formed from an electricity conducting sheath (14).

6. A downhole cable (10) according to any preceding claim, wherein the pair of conductors (12, 14) are arranged co-axially.

7. A downhole cable (10) according to any preceding claim, wherein the conductors (12, 14) are also capable of transmitting data, such that they act as telemetry conduits.

8. A downhole cable (10) according to any preceding claim, wherein additional conductors or telemetry conduits are provided in the downhole cable (10) for transmitting electrical and/or optical data and/or instructions to and from sensors located downhole in the well.

9. A downhole cable (10) according to any preceding claim, wherein the load-bearing member (16) comprises a sleeve (16) surrounding the pair of conductors (12,14).

10. A downhole cable (10) according to any preceding claim, wherein the load-bearing member (16) is a woven material such as a polymer fibre or yarn.

11. A downhole cable (10) according to claim 10, wherein the woven load-bearing member comprises a fibre such as Zylon (™) PBO (poly(p-phenylene-2, 6-benzobisoxazole)), polyamide or polybenzimidazole, which is woven or wound around a core of the downhole cable (10).

12. A downhole cable (10) according to either of claims 10 or 11, wherein the strength of the downhole cable (10) can be increased by increasing the quantity of fibre in the downhole cable (10).

13. A downhole cable (10) according to claim 2, or to any of claims 3 to 12 when dependant on claim 2, wherein the outer sheath (18) comprises a plastics material.

14. A downhole cable (10) according to claim 2, or to any of claims 3 to 13 when dependant on claim 2, wherein the outer sheath (18) is non-conducting so as to insulate the downhole cable (10) from the environment.

15. A downhole cable (10) according to claim 2, or to any of claims 3 to 14 when dependant on claim 2, wherein the outer sheath (18) also protects the downhole cable (10) from damage by external abrasion, and also reduces internal damage to coated tubulars through which the downhole cable (10) is deployed.

16. A downhole cable (10) according to claim 2, or to any of claims 3 to 15 when dependant on claim 2, wherein the outer sheath (18) insulates the environment from the downhole cable (10) by resisting electrical discharge from the downhole cable (10) to the environment.

17. A downhole cable (10) according to claim 11, or to any of claims 12 to 16 when dependent on claim 11, wherein the load-bearing member (16) comprising the Zylon (™) sleeve (16) also provides wear and crush protection, and also insulates the downhole cable (10) from adverse effects of pressure differentials experienced downhole.

18. A downhole cable (10) according to any preceding claim, wherein the two or more conductors (12, 14) are insulated from one another by an electrically insulating sheath (13).

19. A downhole cable (10) according to any preceding claim, wherein the cable (10) comprises at least the following materials, in order from the centre of the cable (10) outwardly:-

a) the first conductor (12) having an electrically conducting core (12);

b) an electrically insulating sheath (13);

c) the second conductor (14) formed from an electricity conducting sheath (14);

d) a second electrically insulating sheath (15);

e) the load-bearing member (16) comprising a sleeve (16);

f) a non-conductive outer sheath (18).

Drawing