GB2321973A

GB2321973A - Well logging cable with optic fibre in metal tube with conductive liner

Info

Publication number: GB2321973A
Application number: GB9800306A
Authority: GB
Inventors: Arnold M Walkow; James C Hunziker
Original assignee: Western Atlas International Inc
Current assignee: Western Atlas International Inc
Priority date: 1997-02-07
Filing date: 1998-01-07
Publication date: 1998-08-12
Also published as: ID19880A; FR2760040A1; NO976019L; IL123046A0; NO976019D0; GB9800306D0; CA2225153A1

Abstract

A well logging cable includes first conductor elements 16 each consisting of a steel wire 16A surrounded by copper strands 16B and covered in an electrically insulating material, and at least one second conductor element 18 including at least one optical fiber enclosed in a metal tube, conductive cladding bonded to the interior wall of the tube, and the tube being covered by the electrically insulating material. The first elements 16 and the at least one second element 18 are arranged in a central bundle 15. At least one second conductor element 18 is positioned within the bundle so as to be helically wound around a central axis of the bundle. The bundle is surrounded by armor wires 12 helically wound externally to the bundle. Inner armour wires 14 and filler material 17 are shown.

Description

WELL LOGGING CABLES The present invention relates to well logging cables and in particular to armored cables used for logging of oil and gas wells.

Well logging is known in the art for providing measurements of properties of earth formations penetrated by wellbores. In well logging, measuring instruments are lowered into the wellbore at one end of an armored electrical cable. The well logging electrical cables known in the art typically have at least one insulated electrical conductor which is used for supplying electrical power to the instruments and for communication of signals between the logging instruments and control equipment located at the earth's surface. The well logging cables known in the art also have steel armor wires helically wound around the electrical conductor to provide tensile strength and abrasion resistance to the cable.

Signals generated by the instruments for transmission to the control equipment are typically electrical signals. Electrical signals can be in the form of analog voltages or digital pulses. A drawback to using electrical signals in well logging is that the mechanical requirements imposed on the logging cable, for example, relatively high bending flexibility and low weight per unit length, require that the cable and the electrical conductor be formed from small diameter wire.

A typical well logging cable, for example, comprises an electrical conductor consisting of seven strands of 0.0128 inch (0.325mm) diameter copper wire covered by a 0.096 inch (2.44mm) extemal diameter plastic insulator. This electrical conductor has a resistance of about 9 ohms per 1,000 feet (305m) of conductor and has a capacitance of several picofarads per foot (305mm) of conductor.

Other cables known in the art can include a plurality of conductors arranged in a central bundle, each conductor having about the same construction and electrical characteristics as the conductor used in the single conductor cable.

Because of the electrical properties of the conductors in typical well logging cables, the well logging cables known in the art typically cannot effectively transmit electrical signals at frequencies above 100 kilohertz (kHz). Newer types of well logging instruments can generate data at rates which make using electrical signal transmission difficult and expensive.

It is known in the art to provide optical fibers in well logging cables to enable use of optical telemetry, which is capable of much higher frequencies and data transmission rates than is electrical signal transmission. For example, U.

S. patent number 4,696,542 issued to Thompson, describes a well logging cable having optical fibers disposed substantially centrally within helically-wound, copper-clad steel conductors. The conductors are themselves covered by two layers of helically wound steel armor wires. A drawback to the well logging cable described in the Thompson '542 patent is that the optical fibers are encased in a plastic tube. Well logging cables can be exposed to hydrostatic pressures and to temperatures in the wellbore which are high enough to preclude the use of plastic tubes.

Other fiber optic cables known in the art include enclosing the optical fibers in a steel tube. For example, "Electro-Optical Mechanical Umbilicals", Vector Cable, Sugar Land, TX (publication date unknown) discloses several so-called "towing and umbilical" cables which include steel tubes enclosing the optical fibers. A drawback to the combination electricalloptical fiber cables described in the Vector Cable reference is that the cable designs disclosed therein have very large diameter electrical conductors which are intended to be used only for electrical power transmission. The optical fibers perform substantially all the signal communication functions of the cable. For reasons known to those skilled in the art, use of the large diameter power conductors as disclosed in the Vector Cable reference results in a cable having such a large external diameter that use of certain fluid pressure control equipment is precluded.

The cables disclosed in the Vector Cable reference also have substantially different electrical signal transmission characteristics than do well logging cables known in the art because of the large size of the power conductors. It is also desirable to provide a combination electricailfiber optic cable having electrical conductors capable of maintaining the electrical signal transmission capabilities of the electrical logging cables known in the art so that existing well logging instruments using electrical telemetry need not be redesigned.

A combination fiber-optic/electrical well logging cable having the optical fiber enclosed in a steel tube is disclosed for example in U. S. patent no.

4,522,464 issued to Thompson et al. The cable disclosed in the '464 patent provides an optical fiber enclosed in a steel tube disposed in the center of a well logging cable. A drawback to the cable disclosed in the '464 patent is that conductive members, positioned externally to the central tube containing the optical fiber, are constructed of copper clad steel wire in order to provide strength and inelastic strain resistance to the cable. Copper clad steel wire typically has different electrical impedance than does copper wire of similar electrical conductance. The conductor members in the cable of the '464 patent can be difficult to use for the electrical signal transmission schemes known in the art.

In another embodiment of logging cable disclosed in the '464 patent, one or more of the copper clad steel conductors can be substituted by optical fibers.

A drawback to directly substituting optical fibers for conductor elements as disclosed in the '464 patent is that some of the electrical power and signal transmission capability of the logging cable will be lost since the substituted conductors are substituted by a non-conductive element, namely the optical fiber.

A further drawback to the cable disclosed in the Thompson et al '464 patent is that steel tube used to enclose the optical fiber is subject to inelastic strain and eventual failure as a result of repeated applications and relaxations of axial tension to the cable. The tube, positioned in the center of the cable as disclosed in the '464 patent, is subject to greater axial elongation under tension than any of the armor wires since the armor wires are helically wound around the axis of the cable and therefore enable elongation of the cable by unwinding of the helical lay of the armor wires under axial tension.

Another type of combination fiber/optic electrical well logging cable is described in "Manufacturing and testing of armored fiber optic downhole logging cable" by Randall et al, WIre Journal, September 1980. The cable disclosed in the Randall et al article provides plastic-sheathed optical fiber to replace one or more of the electrical conductors. A drawback to the cable in the Randall et al article is that the optical fiber is subject to fluid pressure in the wellbore since it is not pressure sealed. Another drawback to the cable in the Randall et al article is that some of the electrical conductors are replaced by optical fibers. The electrical transmission characteristics of a cable built according to the Randall et al design may not have suitable electrical transmission properties for use with certain well logging instruments.

Another combination fiber opticlelectrical well logging cable is disclosed in international patent application number WO 94/28450 published under the Patent Cooperation Treaty. The cable disclosed in the WO 94/28450 application includes an optical fiber enclosed in a metal tube. The metal tube can be surrounded by braided copper strands which are used to conduct electrical power and electrical signals. An embodiment of the cable disclosed in the WO 94/28450 application includes application of the copper braids directly to the metal tube. A drawback to the cable disclosed in the WO 94/28450 application is that the tube is positioned at the center of the cable. Positioning the tube at the center of the cable, as previously explained, can subject the tube and the optical fiber to excessive axial strain under certain conditions. Furthermore, the cable disclosed in the WO 94/28450 application does not disclose or suggest a configuration of the metal tube and copper braids to provide electrical impedance characteristics similar to the insulated copper wires of the electrical well logging cables known in the art. In fact, the preferred embodiment of the cable in the WO 94/28450 application provides a layer of insulating material between the metal tube and the copper braids.

As is understood by those skilled in the art, well logging cables typically include electrical conductors and external armor wires which are respectively positioned to maintain a substantially round cross-sectional shape of the cable even after repeated applications and relaxations of substantial axial tension to the cable while further subjecting the cable to significant bending stresses. As is understood by those skilled in the art, the applications and relaxations of axial tension and bending stresses occur as a result of lowering the instruments into the wellbore and later removing them from the wellbore by winding and unwinding the cable through various sheaves which direct the cable into the wellbore from winch equipment provided for spooling and unspooling the cable. The well logging cables known in the art having only electrical conductors provide good maintenance of the round cross-section of the cable because all of the conductors have similar tensile and bending properties. Direct substitution of conductors with optical fibers to provide a logging cable having optical fibers will result in the cable having asymmetrical tensile and bending properties, and possibly reduced resistance to deformation of the circular cross-section of the cable.

Solutions to many of the drawbacks inherent in prior art fiber-optic well logging cables have been addressed in U. S. patent no. 5,495,547 issued to Rafie et al. The cable disclosed in the Rafie et al '547 patent requires somewhat of a tradeoff in the overall performance of combination fiber-optic and electrical conductor elements disclosed therein. These elements include an optical fiber enclosed in a stainless steel tube. Copper wires can be placed externally to the steel tube to improve the electrical conductance of the conductor/fiber-optic element. However, when copper wires are used externally to the steel tube, it is necessary to reduce the thickness of an insulator applied to the exterior of the conductor and copper wires in order to maintain the same external diameter of the element. Reduced insulation wall thickness can unduly limit the voltages which may be applied to the electrical conductor.

Various aspects of the present invention are exemplified by the attached claims.

Another aspect of the invention can provide a combination electricaUfiber- optic well logging cable having the improved electrical conductance performance of the cable disclosed by Rafie et al '547 without sacrificing the insulation capacity of the electrical conductors in a standard electrical well logging cable.

A specific embodiment of the present invention is a well logging cable including first elements, each of which consists of a steel wire surrounded by copper strands and covered in an electrically insulating material, and at least one second conductor element. The second conductor element includes at least one optical fiber enclosed in a metal tube, conductive metal clad to the interior wall of the tube, and the tube is covered by the electrically insulating material. The first elements and the at least one second element are arranged in a central bundle. The second element is positioned in the bundle so as to be helically wound around a central axis of the bundle. The bundle is surrounded by armor wires helically wound externally to the bundle. In the preferred embodiment, the conductive metal layer can be metallic copper.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a cross-section of a well logging cable according to an aspect of the present invention; Figure 2 shows a detailed section of a fiber-opticlelectrical conductor element of a cable in the prior art; and Figure 3 shows a detailed section of a fiber-optic/electrical conductor element of the cable according to the invention.

A cross-section of one type of well logging cable 10 embodying the invention is shown in Figure 1. The cable 10 can include seven, plastic-insulated conductor elements, shown generally at 16 and 18, which will be further explained. The seven conductor elements 16, 18 are typically positioned in a central bundle 15. The central bundle 15 for this particular type of cable has a substantially axially symmetric hexagonal pattem, wherein six of the conductor elements surround the seventh conductor element. This symmetric arrangement of the conductor elements 16, 18 is familiar to those skilled in the art. The symmetric arrangement of conductor elements is intended to provide a significant amount of resistance to deformation of the substantially circular cross-section of the cable 10. As will be further explained, embodiments of the invention do not depend on having seven conductor elements. Other numbers of conductor elements may be used provided that the central bundle 15 is arranged in a substantially symmetric pattem, as will be further explained.

Five of the seven conductor elements, referred to herein as first elements and shown generally at 16, can be insulated electrical conductors including a copper covered steel wire 16A which can be about 0.027 inches (0.686mm) diameter, surrounded by nine copper wires, shown at 16B, each of which can each be about 0.0128 inches (0.325mm) diameter. The first elements 16 can include an exterior insulating jacket which can be composed of heat and moisture resistant plastic such as polypropylene or ethylene-tetrafiuoroethylene copolymer (m) ("ETFE") sold under the trade name "TEFZEL';tYhich is a trade name of E. l. du Pont de Nemours & Co.

Alternatively, the first elements 16 can consist of stranded copper wires covered with an exterior insulating jacket as described herein. The altemative construction of the first elements can be similar to that used in conventional electrical logging cables well known in the art.

In still another alternative construction of the cable 10, the first element 16 located in the center of the central bundle 15 can consist of stranded copper wires covered by an exterior insulating jacket as previously described. Four of the first elements 16 located on exterior positions of the central bundle can include the steel wire 1 6A surrounded by copper strands 1 6B as previously described. This arrangement can reduce the possibility of damage to the centrally located first element 16 as a result of extending the cable 10 under tensile stress.

In this embodiment of the invention, the other two of the seven conductor elements, referred to herein as second elements and shown generally at 18, each can include an optical fiber disposed within a metal tube. The tube has a conductive internal cladding layer, as will be further explained. The second elements 18 are intended to provide optical fibers to the cable 10 and to have electrical and mechanical properties substantially the same as those of the five first elements 16. The cable 10 shown in Figure 1 includes two symmetrically positioned ones of the second elements 18, however it is contemplated that the cable 10 of the invention will perform as intended with the second elements 18 positioned at any or all of the six external positions on the symmetric hexagonal pattern formed by the seven conductor elements 16, 18.

The void spaces within the hexagonal pattern of the seven conductor elements 16, 18 can be filled with a material, shown generally at 17, such as neoprene or ETFE. The filler material 17 maintains the relative position of the seven conductor elements 16,18 within the cable 10 under bending and tensile stresses.

The conductor elements 16, 18 and the filler material 17 can be covered with helically-wound galvanized steel armor wires, formed into an inner armor sheath, shown generally at 14. The inner armor sheath 14 is itself externally covered with helically-wound galvanized steel armor wires formed into an outer armor sheath, as shown at 12. Generally the outer armor sheath 12 is wound in the opposite direction to the wind of the inner armor 14, as is known in the art.

The inner 14 and outer 12 armor sheath construction is designed to provide significant tensile strength and abrasion resistance to the cable 10.

As is known in the art, on a particular cable 10 which is intended to be used in a chemically hostile environment such as a wellbore having significant quantities of hydrogen sulfide, the wires making up the armor 12, 14 can be composed of a cobalt-nickel alloy such as one identified by industry code MP35N, instead of ordinary galvanized steel.

To compare the prior art combination fiber opticlelectrical logging cables with the cable of the invention, a sectional drawing through one of the combination electrical fiber-optic conductor elements as described in U. S. patent no. 5,495,547 issued to Rafie et al is shown at 25 in Figure 2. The prior art combination element 25 included an optical fiber 30 enclosed in a stainless steel tube 32. Ten 0.010 inch (0.254mm) diameter copper wires 34 were wound externally to the tube 32. This was all enclosed in an insulator 36 which could be made from a plastic such as TEFZEL. The overall resistance of this element is about 8.5 ohms per 1000 feet (305m) of element length. The external diameter of the element is 0.096 inches (2.44mm). The insulator 34 wall thickness is about 0.0215 inches (0.546mm) A sectional drawing through one of the second elements 18 of the invention is shown in Figure 3. The second element 18 can consist of an optical fiber 40 enclosed in a metal tube 46, which in this embodiment preferably is composed of stainless steel in order to provide corrosion resistance. Other well known metals and metal alloys having tensile strength and corrosion resistance similar to those of stainless steel can also be used for the tube 46. The tube 46 can have an external diameter of about 0.046 inches (1.17mm) and in internal diameter of about 0.036 inches (0.914mm). The tube 46 provides abrasion and bending protection to the optical fiber 40, and excludes, from contact with the optical fiber 40, fluids in the wellbore (not shown) into which the cable (10 in Figure 1) is extended when in use. The tube 46 can optionally be plated on its exterior with a thin layer of conductive metal such as copper to further reduce its electrical resistance. The tube 46 is covered with insulation 48, which can be composed of a heat-resistant plastic such as TEFZEL, ETFE or polypropylene.

The external diameter of the insulation 48 on the second element 18 is substantially the same as the extemal diameter of the insulation on the first element 16, so that the hexagonal pattern of the seven elements as shown in the cross-section of Figure 1 can substantially symmetrical, irrespective of the relative position of the second element 18 within the hexagonal pattern of the bundle 15.

The wall thickness of the insulator 48 is about 0.0250 inches (0.635mm), which is about 16 percent greater than the thickness of the insulator (36 in Figure 2) of the prior art combination conductor element (25 in Figure 2). The tube 46 can be lined with a conductive clad layer, which in this embodiment can be 0.045 inch (1.14mm) thick metallic copper 44.

Typically, the conductive clad layer 44 will be applied in sheet or layer form to the metal used to form the tube 46. During manufacture of the tube 46, the optical fiber 40 is placed on the inner surface of the tube 46, which in this case would be on the exposed surface of the copper 44, and the tube is rolled and welded to form the enclosure for the fiber 40. The tube 46 may also optionally include a layer of teflon 42 applied to the interior surface of the copper 44 to reduce abrasion on the fiber 40, but the teflon layer 42 is not necessary to use for the second element 18 to perform as required. The second element 18 as shown will have an electrical resistance of about 9.4 ohms per 1000 feet (305m) length of the element. An additional benefit of the second element 18 as shown herein is the reduced external diameter of the conductive portion of the element.

Reducing the external diameter of the conductive portion of the element can reduce the capacitance between conductors provided by the cable 10. It should be noted that copper is not the only metal which can be clad to the interior of the tube 46 to form the conductive layer shown at 44. Other highly conductive metals such as silver, gold and alloyed copper/beryllium would also function well as the conductive layer 44. Considerations in selecting a metal composition for the conductive layer 44 should include high conductivity to avoid requiring too much metal thickness, and ductility to avoid breaking under repeated bending and tensile stresses on the cable 10 during use.

It is to be understood that the second elements 18 can be positioned at any or all of the six-external positions of the hexagonal structure as shown in Figure 1. The second element 18 is preferably placed in an external location on the hexagonal structure of the bundle 15 because the elements 16, 18 in the external locations are helically-wound around the element in the central position.

As in understood by those skilled in the art, for reasons such as lateral reduction in pitch diameter with axial strain, unwinding of the helical lay and the longer overall length of the helically wound external elements relative to the length of the central element 18, the externally positioned elements 16, 18 undergo reduced axial strain relative to the axial elongation of the cable (shown in Figure 1 as 10), thereby reducing the possibility of axial strain-induced failure of the tube 46 and the optical fiber 40. In this embodiment of the invention, second elements 18 are positioned at two, external locations opposite to each other in the hexagonal pattern, as can be observed by referring back to Figure 1.

The construction of the tube (46 in Figure 3) can be better understood by referring to Figure 4. The metal from which the tube 46 is made can be initially formed into a ribbon 46A. To form the tube 46 having the specified external diameter as shown in Figures 2 and 3, the ribbon 46A should have a width of 0.1900 inches (4.83mm) and a thickness of 0.0050 inches (0.127mm). The length of the ribbon 46A will depend on the overall length of cable to be made.

The ribbon 46A should have ribbon copper 44A clad to the face of the ribbon 46A which will form the interior of the tube 46. The ribbon 46A can be rolled and laser welded by processes known in the art to form the tube 46.

While the invention as described herein is directed to a logging cable having a total of seven of the first elements 16 and second elements 18 in the central bundle (shown as 15 in Figure 1), it is contemplated that cables having other substantially symmetrical arrangements of first elements 16 and second elements 18 in the central bundle 15, in which the elements 16, 18 are helically wound around a central axis of the bundle 15, will also have the electrical and mechanical characteristics of a cable having only copper wires in the bundle, but will include at least one optical fiber positioned within the cable so as to minimize axial strain applied to the fiber.

Other embodiments of this invention are possible which do not depart from the spirit of the invention as disclosed herein.

Claims

CLAIMS:

1. A well logging cable comprising: first conductor elements including a steel wire surrounded by copper strands, said copper strands covered by an electrically insulating material; at least one second conductor element including at least one optical fiber enclosed in a metal tube, said tube lined with a conductive layer, said tube covered by said electrically insulating material, wherein said first conductor elements and said at least one second conductor element are arranged in a central bundle, said at least one second conductor element positioned in said central bundle so as to be helically wound around a central axis of said bundle, said at least one second conductor element having electrical impedance substantially the same as one of said first conductor elements; and armor wires helically wound around said bundle.

2. A cable as claimed in claim 1 wherein said insulating material comprises ETFE.

3. A cable as claimed in claim 1 wherein said insulating material comprises polypropylene.

4. A cable as claimed in claim 1, 2 or 3 wherein said metal tube comprises stainless steel.

5. A cable as claimed in claim 1, 2, 3 or 4 wherein conductive layer comprises metallic copper.

6. A cable as claimed in any one of claims 1 to 5, wherein said bundle comprises a total number of seven conductor elements consisting of said first conductor elements and at least one of said second conductor elements.

7. A cable as claimed in claim 6 wherein said bundle comprises two of said second conductor elements and five of said first conductor elements, said bundle arranged in a substantially regular hexagonal pattern so that said two of said second conductor elements are positioned opposite each other and helically wound around a centrally positioned one of said first conductor elements.

8. A cable as claimed in claim 7 further comprising a filler material disposed within void spaces within said substantially regular hexagonal pattern of the said bundle.

9. A cable as claimed in claim 8 wherein said filler material comprises polypropylene.

10. A cable as claimed in any one of the preceding claims, wherein said armor wires comprise galvanized steel.

11. A cable as claimed in any one of the preceding claims, wherein said armor wires comprise two coaxial, contiguous layers of helically wound wires.

12. A cable as claimed in any one of the preceding claims, wherein said armor wires comprise a cobalt-nickel alloy.

13. A well logging cable comprising: first conductor elements including copper wires covered by an electrically insulating material; at least one second conductor element including at least one optical fiber enclosed in a metal tube, said tube lined with a conductive layer, said tube covered by said electrically insulating material, wherein said first conductor elements and said at least one second conductor element are arranged in a central bundle, said at least one second conductor element positioned in said central bundle so as to be helically wound around a central axis of said bundle, said at least one second conductor element having electrical impedance substantially the same as one of said first conductor elements; and armor wires helically wound around said bundle.

14. A cable as claimed in claim 13 wherein said metal tube comprises stainless steel.

15. A cable as claimed in claim 13 or 14 wherein conductive layer comprises metallic copper.

16. A cable as claimed in claim 13, 14 or 15 wherein said bundle comprises a total number of seven conductor elements consisting of said first conductor elements and at least one of said second conductor elements.

17. A cable as claimed in claim 16 wherein said bundle comprises two of said second conductor elements and five of said first conductor elements, said bundle arranged in a substantially regular hexagonal pattern so that said two of said second conductor elements are positioned opposite each other and helically wound around a centrally positioned one of said first conductor elements.

18. A well logging cable substantially as hereinbefore described with reference to, and as shown in, Figs. 1 and 3 of the accompanying drawings.