FR2760040A1 - CABLE FOR WELL LOGGING COMBINING AN ELECTRICAL CONDUCTOR AND AN OPTICAL FIBER - Google Patents

Abstract

Logging cable comprising first conductive elements 16 each of which consists of a steel wire 16A surrounded by copper twists 16B and covered with an electrically insulating material, and at least one second conductive element 18 comprising at least one incorporated optical fiber in a metal tube, its inner wall comprising a conductive coating, the said tube being covered with an electrically insulating material. The first elements and at least the second element are arranged in a central bundle. The second element at least is positioned in the bundle so as to be surrounded helically around a central axis of the bundle. The bundle is surrounded by reinforcing wires 12, 14 wound helically outside the bundle.

Description

The invention relates to the field of armored electric cables used for

  oil or gas well logging. More particularly, the invention relates to designs of logging cable combining electrical wire and optical fiber. Well logging is known in the art to provide

  measurements of the properties of geological formations crossed by boreholes.

  In well logging, the measuring instruments are lowered into the borehole at one end of an armored electrical cable. Logging electrical cables known in the art typically have at least one insulated electrical conductor which is used to supply electrical power to the instruments and for signal communication between the logging instruments and the control equipment located at the ground surface. The well-known logging cables also have steel reinforcing wires wound helically around the electrical conductor to provide the cable with tensile strength and

abrasion resistance.

  The signals generated by the instruments for transmission to the control equipment are typically electrical signals. The electrical signals can be in the form of analog voltages or digital pulses. A drawback to the use of electrical signals in well logging is that the mechanical requirements imposed on the logging cable, for example, relatively high flexibility, and low weight per unit length, dictate that the cable and the electrical conductor are formed of a small diameter cable. A typical logging cable, for example, includes an electrical conductor consisting of seven twists of 0.0128 inch (0.0325 mm) diameter copper wire covered by plastic insulation with an outer diameter of 0.096 inch (2, 43 mm). This electrical conductor has a resistance of approximately 9 ohms per 1000 feet (304.80 m) of conductor and a capacity of several picofarads per foot (30.48 cm) of conductor. Other cables known in the art may include a plurality of conductors arranged in a central bundle, each conductor having substantially the same structures and the same electrical characteristics as said

conductor used alone.

  Due to the electrical properties of the conductors of typical logging cables, known logging cables cannot effectively transmit electrical signals at frequencies above 100 kiloHertz (kHz). Newer types of logging instruments can generate data at speeds that make use of their electrical signal transmission

difficult and expensive.

  It is known in the art to provide logging cables with optical fibers to allow the use of optical telemetry, which is capable of much higher frequencies and data transmission speeds than the transmission of a electrical signal. For example, US Pat. No. 4,696,542 (Thompson), describes a logging cable with optical fibers arranged substantially axially inside copper conductors wound in a helical fashion. The conductors are themselves covered with two layers of helical steel wire. A disadvantage of the logging cables described in the Thompson '542 patent lies in the fact that the optical fibers are enclosed in a plastic tube. Logging cables can be exposed to hydrostatic pressures and temperatures in the well high enough to

  exclude the possibility of using plastic tubes.

  Other optical fiber cables known in the art include incorporating optical fibers into a steel tube. For example, "Electro-Optical Mechanical Umbilicals", Vector Cable, Sugar Land, TX (date of publication unknown) discloses several so-called "tow and connection cables" which may include steel tubes containing the optical fibers. A drawback to the combination of electrical conductors / optical fiber described in the reference "Vector cable" lies in the fact that the embodiments of the cable described here have electrical conductors of very large diameter, intended to be used only for the transmission of electrical power. . Fiber optics perform almost all of the cable's communication functions. For reasons known to those skilled in the art, the use of large diameter power conductors as described in the "Vector Cable" reference leads to a cable having an external diameter so large that the use of certain

  pressure fluid control equipment is excluded.

  The cables described in the "Vector Cable" reference also have characteristics of transmission of the electrical signal substantially different from those of the logging cables known in the art because of the large size of the power conductors. It is also desirable to provide a fiber optic / electrical conductor combination having electrical conductors capable of maintaining the possibilities of transmission of the electrical signal from the electrical logging cables known in the art, so that existing logging instruments using the

  electric telemetry does not need to be recalculated.

  A borehole logging cable combining an electrical / optical fiber conductor with the optical fiber incorporated in a steel tube is disclosed, for example, in US Pat. No. 4,522,464 (Thompson et al.). The cable disclosed in the '464 patent includes an optical fiber incorporated in a steel tube disposed in the center of a logging cable. A drawback to the cable disclosed in the '464 patent is that the conductive members, positioned outside the central tube containing the optical fiber, are made of copper-clad steel wire to provide non-elastic strength and resistance to the cable. Copper-clad steel wire typically has a different electrical impedance than copper wire of similar electrical conductance. Cable conductors in the '464 patent can be difficult to use

  for electrical signal transmission programs known in the art.

  In another embodiment of the logging cable disclosed in the '464 patent, one or more of the copper-plated steel conductors may be replaced by optical fibers. A disadvantage of directly replacing optical fibers with conductive elements as disclosed in the '464 patent is that some electrical power and the signal transmission capacity of the logging cable will be lost since the substituted conductors are

  replaced by a non-conductive element, namely optical fiber.

  Another disadvantage of the cable disclosed in the Thompson '464 patent lies in the fact that the steel tube used to enclose the optical fiber is subjected to a non-elastic stress and to possible deterioration resulting from repeated applications and loosening of placing. under axial tension of the cable. The tube, positioned in the center of the cable as disclosed in the '464 patent, is subjected to a greater axial elongation under tension than any of the reinforcing wires since the reinforcing wires are wound helically around the axis. of the cable and therefore allow the elongation of the cable by unwinding the helical layer of the wires

  reinforcement under the effect of axial tension.

  A logging cable from another type of drilling combining electrical conductor and optical fiber 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 article by Randall et al. provides an optical fiber

  with a plastic sheath to replace one or more of the electrical conductors.

  A disadvantage of the cable in the article by Randall et al is that the optical fiber is subjected to the pressure of the fluid in the wellbore since it is not pressure tight. Another disadvantage of the cable in the article by Randall et al. lies in the fact that some of the electrical conductors are replaced by optical fibers. The electrical transmission characteristics of a cable constructed according to the realization of Randall et al may not have the properties suitable for transmission

  electric for use with certain logging instruments.

  Another combiantn electrical conductor / fiber optic well drilling cable is disclosed in international patent application WO 94/28450 published by the PCT. The cable disclosed in application WO 94/28450 comprises an optical fiber enclosed in a metal tube. The metal tube can be surrounded by gimped copper twists used to conduct electrical power and electrical signals. One embodiment of the cable described in application WO 94/28450 comprises the direct application of copper guipures to the metal tube. A drawback of the cable described in application WO 94/28450 lies in the fact that the tube is positioned in the center of the cable. The positioning of the tube in the center of the cable, as explained above, can subject the tube and the optical fiber to excessive axial tension under certain conditions. In addition, the cable disclosed in application WO 94/28450 does not describe or suggest a configuration of the metal tube and copper guipures to provide electrical impedance characteristics similar to the insulated copper wires to the well known logging cables. art. In fact, the preferred embodiment of the application cable WO 94/28450 provides a layer of insulating material

  between the metal tube and the copper guipures.

  As will be understood by those skilled in the art, cables for well logging typically include electrical conductors and outer armor wires which are positioned respectively to maintain a substantially round cross-sectional shape of the cable, even after applications and repeated releases of the axial tension of the cable at the same time as the cable is also subjected to significant bending stresses. As understood by those skilled in the art, applications and releases of axial tension and bending stresses occur as a result of the descent of the instruments into the wellbore and subsequently their extraction from the wellbore by winding and unwinding the cable around various pulleys which direct the cable into the wellbore from a winch equipment provided for winding and unwinding the cable. Well logging cables known in the art having only electrical conductors provide good cross-sectional maintenance of the cable because all of the conductors have similar bending and tensile properties. Direct substitution of conductors with optical fibers to provide a logging cable with optical fibers will have asymmetric bending and tensioning properties, and possibly a reduction in resistance to

  deformation of the circular section of the cable.

  Solutions to many drawbacks inherent in the prior art of cables for the logging of fiber optic boreholes have been proposed in US Pat. No. 5,495,547 (Rafle et al.). The cable described in the Rafie et al. '547 requires a compromise in the overall performance of the combination and the fiber optic and electrical conductor elements described therein. These elements include an optical fiber incorporated in a stainless steel tube. Copper wires can be placed outside the steel tube to improve the electrical conductivity of the conductive / fiber optic element. However, when copper wires are used outside the steel tube, it is necessary to reduce the thickness of an insulation applied outside the

  conductor and copper wires to maintain the same outside diameter of the element.

  Reducing the thickness of the insulation wall can excessively limit stresses

  which can be applied to the electrical conductor.

  Consequently, it is an object of the invention to propose an electrical conductor / optical fiber combination for a logging cable with a view to improving the electrical conductivity performance of the cable compared to the cable described by Rafie et al. in '547, without having to sacrifice the insulation capacity of electrical conductors compared to an electrical logging cable

classic.

  The invention provides a logging cable comprising first elements, each of which consists of a steel wire surrounded by copper twists and covered with an electrically insulating material, and at least one second conductive element. The second conductive element comprises at least one optical fiber incorporated in a metal tube, coated on its inner wall with conductive metal and covered by the electrically insulating material. The first elements and at least the second element are arranged in a central bundle. The second element is positioned in the beam so as to be surrounded helically around a central axis of the beam. The bundle is surrounded by reinforcing wires wound helically outside the bundle. In the preferred embodiment, the metal layer

  conductive can be metallic copper.

  Figure 1 is a cross section of a logging cable according to the present invention, Figure 2 is a detailed section of the electrical conductor / optical fiber element of a prior art cable, and Figure 3 is a detailed section of an element

  electrical conductor / optical fiber of the cable according to the invention.

  A cross section of a type of cable 10 for logging according to the present invention is shown in Figure 1. The cable 10 may include seven conductive elements insulated by plastic, generally designated by 16 and 18, which will be explained later . The seven conductive elements 16, 18 are typically positioned in a central bundle 15. The central bundle 15, for this particular type of cable, has substantially a hexagonal configuration of axial symmetry, in which six of the conductive elements surround the seventh conductive element . This symmetrical arrangement of the conductive elements 16, 18 is familiar to those skilled in the art. The symmetrical arrangement of the conductive element is intended to provide significant resistance to deformation of the substantially circular cross section of the cable 10. As will be explained below, the invention does not depend on having seven conductive elements. Other numbers of conductive elements can be used provided that the central beam 15 is arranged in a substantially symmetrical configuration, as will be explained below. Five of the seven conductive elements, referred to here as first elements and generally represented by 16, can be insulated electrical conductors incorporating a steel wire 16A covered with copper which can have a diameter of approximately 0.027 inch (0.6858 mm), surrounded by nine copper wires, represented by 16B, each of which may have a diameter of approximately 0.0128 inch (0.3251 mm). The first elements 16 may comprise an outer insulating sleeve which may be composed of plastic material resistant to heat and humidity such as polypropylene or ethylene-tetrafluoroethylene copolymer ("ETFE") sold under the brand name "TEFZEL "which is a brand name of EI du Pont de Nemours & co. Alternatively, the first elements 16 may consist of twisted copper wires covered with an outer insulating sleeve as described here. The alternative construction of the first elements can be similar to that

  used in conventional electrical logging cables well known in the art.

  According to yet another alternative construction of the cable 10, the first element 16 located at the center of the central bundle 15 may consist of

  twisted copper wires covered with an outer insulating sleeve as described above.

  Four of the first elements 16 located at positions external to the central bundle can comprise the steel wire 16A surrounded by copper twists 16B as described above. This arrangement can reduce the possibility of deterioration of the first element 16 located centrally, as a result of the extension of the cable 10 under

tension constraints.

  In this embodiment, the other two of the seven conductive elements, which are referred to here as second elements and generally represented by 18, can each comprise an optical fiber disposed inside a metal tube The tube has a layer of conductive internal coating, as will be explained later. The second elements 18 are intended to provide the cable 10 with optical fibers and to give it electrical and mechanical properties substantially the same as those of the first five elements 16. The cable shown in FIG. I incorporates two second elements 18 positioned symmetrically, however it is envisaged that the cable 10 of the invention can function as expected with the second elements 18 positioned in any of the six external positions of the symmetrical hexagonal configuration formed by the seven elements

conductors 16, 18.

  The empty spaces inside the hexagonal configuration of the seven conductive elements 16, 18 can be filled with a material generally represented by 17, such as neoprene or ETFE. The filling material 17 maintains the relative position of the seven conductive elements 16, 18 at

  The inside of the cable 10 during tension and bending stresses.

  The conductive elements 16, 18 and the filling material 17 can be covered with galvanized steel reinforcing wires helically wound, shaped into an internal reinforced sheath, generally designated by 14. The reinforced internal sheath 14 is itself even covered externally with galvanized steel reinforcing wire surrounded in a helical fashion and shaped into a reinforced external sheath, as shown in 12. Generally the reinforced external sheath 12

  is wound in the opposite direction to that of the inner frame 14, as is known in the art.

  The construction of the inner 14 and outer 12 sheaths of the frame is intended to provide the cable 10 with a tensile strength and an abrasion resistance

significant.

  As is known in the art, a cable 10 particularly intended for use in the hostile chemical environment of a wellbore such as resulting from the presence of appreciable amounts of hydrogen sulfide, the wires making up the armature 12 , 14 may be composed of a cobalt-nickel alloy such as

  that identified by the industrial code MP-35N, instead of ordinary galvanized steel.

  To compare the cables of the prior art combining optical fiber / electrical conductor, with the logging cable of the invention, FIG. 2 is a sectional drawing of one of the conductive elements of the combination of optical fiber / electrical conductor as as described in patent US-5,495,547 (Rafie et ai) represented by 25. The combined element 25 of the prior art comprises an optical fiber 30 incorporated in a stainless steel tube 32. Ten copper wires 34 of a diameter of 0.1 inch (0.0254 mm) is wound outside the tube 32. The assembly is incorporated

  in an insulator 36 which can be made from a plastic material such as TEFZEL.

  The overall resistance of this element is approximately 8.5 ohms for a length of the element of 1000 feet (304.80 m). The outer diameter of the item is 0.096 inches (2.4384

  m). The thickness of the wall of insulation 34 is about 0.0125 inches (0.3175 mm).

  Figure 3 is a cross-sectional drawing through one of the second elements 18 of the invention. The second element 18 may comprise an optical fiber 40 incorporated in a metal tube 46, which in this form of

  embodiment, is made of stainless steel to provide corrosion resistance.

  Other well known metals and metal alloys having a tensile strength and corrosion resistance similar to that of stainless steel can also be used for tube 46. Tube 46 can have an outer diameter of about 0.046 inch (1.1684 mm) and an internal diameter of about 0.036 inch (0.9144). The tube 46 provides the optical fiber 40 with abrasion and bending protection, and protects the optical fiber 40 from contact with the fluids present in the borehole (not shown) in which extends the cable (10 in Figure 1) when used. The tube 46 can optionally be plated on its interior with a thin layer of conductive metal such as copper to further reduce its electrical resistance. The tube 46 is covered with an insulator 48, which can be composed of a heat-resistant plastic material such as TEFZEL, ETFE or polypropylene. The outside diameter of the insulation 48 on the second element 18 is substantially the same as the outside diameter of the insulation on the first element 16, so that the hexagonal configuration of the seven elements as shown in the cross section in figure I can be substantially symmetrical, without taking into account the relative position of the second element 18 inside the hexagonal configuration of the beam 15. The thickness of the wall of the insulation 48 is approximately 0.0250 inch (0.635 mm), which is about 16 percent greater than the thickness of the insulation (36 in Figure 2) of the conductive element of the combination of the prior art (25 in Figure 2). The tube 46 can be coated with a conductive plated layer, which in this preferred embodiment

  may be metallic copper 44 whose thickness is 0.045 inch (1, 143 mm).

  Typically, the plated conductive layer 44

  will be applied in sheet or layer form to the metal used to form the tube 46.

  During the manufacture of the tube 46, the optical fiber 40 is placed against the interior surface of the tube 46, which in this case would be on the exposed surface of the copper 44 then the tube is rolled and welded to form an envelope with 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 it is not necessary to use the layer of Teflon 42 for the second element 18 operates as required. The second element 18 as shown will have an electrical resistance of approximately 9.4 ohms for an element with a length of 1000 feet (304.80 m). An additional advantage of the second element 18 as shown here is the reduced external diameter of the conductive part of the element. Reducing the outside diameter of the conductive portion of the element can reduce the capacitance between the conductors provided by the cable 10. It should be noted that copper is not the only metal that can coat the interior of tube 46 to form the conductive layer represented by 44. Other highly conductive metals such as silver, gold and copper / beryllium alloys would provide as good a conductive layer 44. Criteria for choosing a metal composition of the layer conductive 44 should include a high conductivity to avoid the requirement of an excessively large metal thickness, and malleability to avoid breakage during tensile and bending stresses

  repeated on the cable 10, during its use.

  It should be understood that the second elements 18 can be positioned in any of the six external positions of the hexagonal structure as shown in FIG. 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 on the external locations are wound helically around the element in the central position. As is understood by those skilled in the art, for reasons such as the lateral reduction of the pitch diameter with an axial stress, the course of the helical configuration and the longer overall length of the external elements wound helically relative to the length of the central element 18, the elements 16, 18 positioned outside benefit from a reduction of the axial stress compared to the axial elongation of the cable (represented at 10 in FIG. 1), thanks to which is reduced the possibility of axial defect induced by the stress of the tube 46 and the optical fiber 40. In this embodiment of the invention, the second elements 18 are positioned in two external locations, opposite one to I ' other, in the hexagonal configuration, as it can be observed by referring to the

figure 1.

  The construction of the tube (46 in FIG. 3) can be better understood with reference to FIG. 4. The metal from which the tube 46 is made can be initially formed in a ribbon 46A. To form the tube 46 having the specified outer diameter as shown in Figures 2 and 3, the tape 46A should have a width of 0.1900 inch (4.826 mm) and a thickness of 0.0050 inch (0.127 mm). The length of the ribbon 46A will depend on the overall length of the cable to be manufactured. The ribbon 46A should be covered with copper 44A on its face which will form the inside of the tube 46. The ribbon 46A can be rolled and laser welded by methods known in the art for forming the

tube 46.

  Although the invention as described here relates to a logging cable having a total of seven of the first elements 16 and second elements 18 in the central bundle (represented by 15 in FIG. 1), it is envisaged that cables having d 'other substantially symmetrical arrangements of the first elements 16 and of the second elements 18 in the central bundle 15 in which the elements 16 and 18 are surrounded helically around the 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 inside the cable so

  such that the axial stress applied to the fiber is minimized.

  Other embodiments of the invention are possible which will be consistent with the spirit of the present invention, its scope of the invention

  being, therefore, limited only by the claims which follow.

he

Claims (14)

  1.- Well logging cable, comprising: first conductive elements (16) each formed of an electrically conductive wire (16), at least one second conductive element (18) comprising at least one incorporated optical fiber (40) in a metal tube (46), the said tube being provided with a conductive layer (44), the said tube being covered by an electrically insulating material (48), in which cables the said first conductive elements and at least the said second conductive element are arranged in a central bundle (15), the said second conductive element at least being positioned in the said central beam wound helically around a central axis of the said beam, the said second conductive element at least having an electrical impedance substantially the same as that of one of the said first conducting elements, and   reinforcing wires (12) wound helically around said beam.   2.- cable according to claim 1, characterized:   in that the electrically conductive wire forming the said first electrically   conductors is a steel wire (16A) surrounded by copper twists (16B) covered   of an electrically insulating material.   3.- Cable according to claim 1, characterized:   in that the electrically conductive wire forming the said first electrically   conductors is a copper wire covered with an electrically insulating material.   4.- Cable according to claim 2, characterized:   in that said insulating material is ETFE.   5.- Cable according to claim 2, characterized in that said insulating material is polypropylene.   6.- Cable according to any one of claims 2 and 3; characterized:   in that said metal tube is stainless steel.   7.- Cable according to any one of claims 2 and 3, characterized:   in that the conductive layer is copper.   8.- Cable according to any one of claims 2 and 3, characterized:   in that the bundle comprises a total number of seven conductive elements comprising the said first conductive elements and at least one of the said second conductive elements.   9. Cable according to claim 8, characterized: in that the said central bundle comprises two of the said second conductive elements and five of the said first conductive elements, the said bundle being arranged in a substantially regular hexagonal configuration such that two of the said second conductive elements are positioned in opposition two by two and are wound helically around one of said first conductors centrally positioned.   10.- A beam according to claim 9 characterized: in that it further comprises a filling material (17) disposed inside the voids of said hexagonal configuration substantially regular.   1 1.- A beam according to claim 10 characterized:   in that the filling material is polypropylene.   12.- Cable according to claim 2 characterized:   in that the said reinforcing wires are galvanized steel wires.   13.- Cable according to claim 2, characterized: in that the said reinforcing wires consist of two contiguous layers   coaxial (12,14) of helically wound wires.   14.- Cable according to any one of claims 2 and 13, characterized:   in that said reinforcing wires consist of a cobalt-nickel alloy.   15.- Cable according to claim 3, characterized: in that the said bundle comprises two of the said second conductive elements and five of the said first conductive elements, the said bundle being arranged in a substantially regular hexagonal configuration such that two of the said second conductive elements are arranged in opposition to each other and are wound helically around one of said first conductive elements centrally positioned.