WO2002071178A2

WO2002071178A2 - Improved bandwidth wireline data transmission system and method

Info

Publication number: WO2002071178A2
Application number: PCT/US2001/017594
Authority: WO
Inventors: Viswanathan Raman
Original assignee: Baker Hughes Incorporated
Priority date: 2000-06-02
Filing date: 2001-05-31
Publication date: 2002-09-12
Also published as: GB2382454B; CA2411411C; GB0229609D0; GB2382454A; NO333396B1; NO20025764D0; CA2411411A1; WO2002071178A3; NO20025764L; WO2002071178A9

Abstract

A well logging apparatus is provided having downhole well data acquired by a sensor (406), transmitted to the surface via complementary modems (410 and 418), and conveyed to the surface modem (418) via a data cable (412) linking the modems, the cable (412) having at least one pair of insulated conductors (102 and 104) wound in a substantially helical twist, an insulation sheath (108) surrounding the twisted pair of conductors (102 and 104) and a tensile load carrier (112) surrounding the insulation sheath (108), the load carrier comprising a sheath of tensile load carrying filaments.

Description

IMPROVED BANDWIDTH WIRELINE DATA TRANSMISSION SYSTEM AND

METHOD

BACKGROUND OF THE INVENTION

1. Related Application

This application is related to a U.S. provisional application titled "Improved

Bandwidth Wireline Data Transmission System and Method" filed on March 30, 2000, serial number 60/193,098, and from which priority is hereby claimed for the present application.

2. Field of the Invention

This invention pertains to data communications and particularly to data communications on a wireline such as one employed in an oil or gas well borehole application.

Description of the Prior Art

It is common in an oil or gas well borehole application to transmit and receive electrical digital data and control signals between surface electronics and downhole electronics package via a wireline of one or more conductors connecting the two. Such signals are typically used to remotely control the functions of various downhole devices such as sensors for detecting borehole parameters as well as tools and devices for performing functional operations in the borehole such as setting equipment or operating testers, motors, directional drilling equipment or the like, which may be operable in stages and in any event requiring a plurality of differing control signals at different times. Likewise, it is desirable to transmit information indicative of the operation of the downhole devices or parameters detected or measured downhole, to the surface over the same conductor path. It is customary in such downhole operations to utilize a sheathed or armored cable which includes either a single conductor or multiple conductors. A single conductor armored cable typically includes a single insulated conductor as a core, and a protective conductive sheathing surrounds the insulated core. The core and sheathing form an electrical circuit path for transmitting electrical power and data. The standard multi-conductor armored cable is a 7-conductor armored cable used for multiple channel tools. Such so called single conductor wireline cables, or similarly constructed multi-conductor cables, are almost exclusively used to operate downhole electrical devices because of a variety of reasons associated with the space limited and rigorous environment of a borehole. In such oil and gas borehole operations, a borehole depth of many thousands of feet is not uncommon. In communicating between the surface and downhole in a borehole over a wireline cable, control signals and data signals are normally converted to digital signals transmitted by a transmitter at rates up to a maximum of 20 Kbits/second. A receiver on the other end of the cable receives the signals, and a processor decodes the signals for further use. The transmission and receiver scheme described above operates well when the rate of transmission does not exceed about 20 Kbits/second or the wireline is relatively short. However, the wireline transmission medium does cause a problem when the transmission is over a relatively long length or as the data rate increases.

That is, the detection and distinguishing of the two voltage levels associated with the digital signal is impaired by distortions caused by the medium. Distortions become more acute for faster bit rates, where the periods at each of the two voltage levels are very short. For example, the frequency characteristic of a typical single conductor wireline used for downhole application has a loss of about -20 db at 5.6 Khz for a 30,000 foot length. At higher frequencies, the loss is significantly greater.

Often, multi-conductor cables are used when multiple channels to several sensors are used. The most commonly used cable today is a 7-conductor armored logging cable. For comparison purposes, a cable of at least 30,000 feet in length wherein the cable is a 7-conductor cable provided within an armored logging cable having a nominal size of 7/16 inches has a frequency bandwidth of 90 to 270 Khz. Bandwidth is defined as the frequency at which an input signal is attenuated to the point where the signal cannot be effectively recovered by the receiving device. Typically, and for the purposes of this disclosure, the attenuation is -60 db. Today, while the wells become deeper, the measuring devices have also become more complex. That is, they provide data at a much greater rate. Moreover, the advent of digital computers installed at the well head measuring equipment have enabled the handling of greater volumes of data in a more effective fashion. All of this has occurred simultaneously increasing the requirements on the logging cable. The cables have become more complex i.e., they have added conductors, and the band pass requirements for the conductors have been increased. Still, the cables used today are unable to provide bandwidth in deep wells matching the transmission capabilities of the instrumentation.

There are several factors affecting the bandwidth of a particular cable configuration including resistance (R), capacitance (C), inductance (L) and conductance (or leakage.) Typically gains to be achieved in inductance and conductance are small since these factors are negligible. The most straightforward correction for high resistance of a cable, which is proportional to the diameter cable conductors, is to have larger diameter cables. This correction is opposed by the need to balance cable size with borehole parameters. Parameters such as borehole diameter and fluid pressure lead designers to smaller diameter cables. Capacitance of logging cables has been minimized, thereby increasing bandwidth, by adding conductors or by using a coaxial cable.

As discussed earlier, the coaxial cable is used by referencing a signal to the shield (or armor.) Although capacitance is improved, the capacitances of typical coaxial and multi-conductor cables are still around 40 to 60 pF/ft. To address some of the deficiencies described above, the present invention provides a load bearing cable having improved bandwidth and lower capacitance per foot for use in wireline applications. This invention also provides a multi- conductor load bearing cable used in a single conductor mode with lower capacitance than the typical single conductor cable used today.

Although increasing the bandwidth of a cable is necessary to improve data rate transmission, it should also be appreciated that the efficient use of the bandwidth is also required. As discussed earlier, instruments now have the capability to transmit data at rates far beyond cable capabilities. Methods of encoding data for transmission used in the telecommunication industry include Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase (CAP) modulation, and Discrete Multi-Tones (DMT) modulation. CAP is a modified QAM method, and DMT is the method in digital subscriber line (DSL) applications currently marketed mainly as an enhancement to internet connections. At this time, the well logging community has not taken advantage of the state of the art encoding methods. The primary driver being that the cables in current use cannot provide the bandwidth necessary to utilize these encoding methods efficiently.

To meet the demand for higher data rates, the present invention provides a system utilizing telecommunication data encoding methodologies in conjunction with a load bearing data cable having enhanced bandwidth to increase transmission data rate. This invention also provides a method of well logging data transmission having a higher data rate.

SUMMARY OF THE INVENTION

In general, the present invention provides a logging data transmission method and apparatus. The apparatus includes a logging cable having improved bandwidth characteristics.

In one embodiment, a logging cable has a twisted pair of signal conductors, each of the conductors being separately insulated. An insulation sheath surrounds the twisted pair of conductors, and a tensile load sheath surrounding the insulation sheath, the tensile load sheath comprising a plurality of filaments provides the support necessary for downhole applications.

In an alternate embodiment, a cable is provided having at least 6 twisted pairs of conductors disposed around a center conductor, all conductors being within the insulation sheath. This configuration may have twisted pair conductors operating in a single conductor mode or in differential mode.

A system having an improved data transmission rate is provided comprising a downhole well data sensor and a downhole data transmitter such as a modem and an encoding method of QAM, CAP or DMT. Included in the system is a surface data receiver complementary to the downhole transmitter. A data transmission cable linking the transmitter and the receiver, the cable having at least one pair of insulated conductors wound in a substantially helical twist, an insulation sheath surrounding the twisted pair of conductors and a tensile load carrier surrounding the insulation sheath, the load carrier comprising a sheath of tensile load carrying filaments.

Also provided is a method of transmitting data from a well borehole to a surface location comprising transmitting the signal with a downhole data transmitter and conveying the signal on a data transmission cable linking the transmitter and to a surface receiver, the cable having at least one pair of insulated conductors wound in a substantially helical twist, an insulation sheath surrounding the twisted pair of conductors and a tensile load carrier surrounding the insulation sheath, the load carrier comprising a sheath of tensile load carrying filaments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross section view of a cable according to the present invention.

Figure 2A is a simulation showing attenuation as a function of frequency using the dimensional and material specifications of a cable according to the present invention as a starting point for the simulation. Figure 2B is a simulation showing attenuation as a function of frequency for a cable in accordance with the present invention using measured values of capacitance as the simulation input.

Figure 2C is a simulation showing attenuation as a function of frequency using correction factors due to the effects of armor surrounding the conductors of a cable according to the present invention.

Figure 3 is a cross section view of a 7-conductor cable configuration according to the present invention.

Figure 4 is a schematic representation of a wireline system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a cross section view of a cable according to the present invention. A cable 100 includes a twisted pair of insulated conductors 102 and 104 helically twisted together and about a central axis of the cable. Each of the insulated conductors 102 and 104 comprises a group of electrically conductive stranded wires 106 encased by a tightly fitted, tubular sheath of insulating material 108. The stranded wires may be copper or any other suitable metallic material, and the insulating material 108 is preferably an extrudable plastic, which maximizes electrical insulation and temperature characteristics while minimizing the insulation thickness and dielectric constant. For downhole applications, a preferred insulating material 108 is a fluorinated ethylene propylene (FEP) plastic such as Teflon®. It may also be a combination such as Teflon®/Tefzel®, both of which are well known insulators. If FEP insulation is used for a downhole data transmission application, a thickness of 0.0125" (.32mm) is recommended. Power applications may require more insulation. A protective elastomer bedding 110 is disposed around the twisted pair to provide protection from abrasions and other damage due to rough handling and harsh environments.

The cable 100 includes a tensile load bearing tubing 112 comprising an inner layer 114 and an outer layer 116 of wires. The inner layer of wires 114 is a plurality of stranded structural steel wires with 0.025" (.64 mm) total outer diameter helically wound around the elastomer bedding 110. The outer layer 116 is a plurality of stranded structural steel wires with 0.0345" (.88 mm) total outer diameter helically wound around the inner layer 114.

The overall outer diameter of a cable built to these dimensions would be 0.025" (6.35 mm). The relationship between resistance and diameter of a conductor is inversely proportional and the load bearing capability is directly proportional to the diameter. These relationships would normally lead one to larger cable designs. However, the overall diameter of a cable should be minimized in a downhole application, because the pressure of the fluid in the well may force a cable out of the well if the diameter is too large. Referring now to Figure 1 and Figures 2A through 2C showing bandwidth plots based a twisted pair load bearing cable as described above and shown in Figure 1. Figure 2A is a simulation using dimensional and material specifications of a cable as a starting point for the simulation. Figure 2B is the same simulation 5 using values from measurements with a capacitance meter. Figure 2C is a simulation using correction factors due to the effects of armor 112 surrounding the conductors 102 and 104.

The most useful capacitance to know is the effective capacitance per foot 10 (C_eff) of the cable. This is the effective capacitance between the conductors 102 and 104. To determine C_Θf_f, equations are used that require measured values between the conductors 102 and 104 (designated as Cι_2m) and between each conductor and the armor 112 (designated as Cι_3m and C₂₃m respectively.) The computation is initiated with an experienced based empirical value of 1 F for the 15 same parameters, C-i₂, C-₁₃ and C₂₃. To determine the actual Cι₂ or C_eff, equations are then set up as follows:

The equations are then iteratively solved for the correct values of C-|₂, C₁₃, and C₂₃ yielding:

Ci2 = 2.999 x 10-11 F/m; C₁₃ = 8.999 x 10-11 F/m; and

C₂₃ = 8.999 x 10-11 F/m.

Therefore, since 1 m = 3.28084 ft, the C_etf of C-ι₂ for the cable described is actually 9.144 pF/ft. Compare this to the typical cable values of 40-60 pF/ft as stated above. The capacitance and conductor configuration of a cable according to the present invention results in a bandwidth of about 350 KHz.

There are two modes of operation or configuration modes useful for the twisted pair cable described above. These are the single conductor mode and the twisted pair or differential mode. In the single conductor mode, the ends of the conductors 102 and 104 are tied together electrically. A signal transmitted on the cable is then sensed with reference made to the armor 112. In the differential mode, the conductors 102 and 104 are each used independently for signal transmission, and the signal is sensed as a differential between the conductors 102 and 104. The bandwidth of either configuration is larger than the bandwidth of current single conductor load bearing cables used in well logging systems.

Figure 3 is a cross section view of a 7-conductor cable configuration 300 according to the present invention. In this configuration, a core or center conductor 302 is covered in an insulation material 304 such as the extrudable Teflon or Teflon/Tefzel combination as described above. Six twisted pair wires 306, each comprising twisted pair insulated conductors 308 and 310 as described above with respect to Figure 1, are disposed around a circumference of the center conductor 302. The twisted pairs are also insulated as described in Figure 1 with a protective cover 312. The center 302 and surrounding twisted pair conductors 306 are encased in an insulating dielectric material 314, several of such materials being well known in the art. Also well known in the art and not shown separately here is a plurality of fiber cords running axially the length of the cable and disposed in the dielectric material 314. These cords provide internal strength and stability to the cable to ensure the conductors are substantially fixed with respect to the internal distance between each other. Disposed circumferentially around the dielectric material 314 is an elongated tubular sheath 316, which may be a conductive paste, a plastic tape or an insulation material like well known in the art. A tensile load bearing covering comprised of an inner layer of wires 318 and an outer layer of wires 320 is disposed about the sheath 316. The inner layer of wires 318 is a plurality of stranded wires with helically wound around the sheath 316. The outer layer 320 is a plurality of stranded wires helically wound around the inner layer 318.

In this configuration, center conductor 302 is shown as a single conductor. However, the intent is not to exclude the use of a twisted pair for the center conductor. Also, the preferable mode for the twisted pair wires is the single conductor mode where the ends are electrically connected, but the differential mode may be preferable in a particular application. As known in the art, any conductor may carry both data and power simultaneously.

Figure 4 is a schematic representation of a wireline system 400 according to the present invention. A tool 402 disposed in a well borehole 404 includes one or more sensors 406 for measuring parameters such as pressure, temperature, flow rate, etc.. A processor 408 is located within the tool 402 for processing and encoding data received from the sensor 406. The processor 408 is connected to a downhole modem 410. The modem 410 can be of any high data rate type used in two-conductor communication using an encoding method such as quadrature amplitude modulation (QAM), carrierless amplitude and phase (CAP) modulation, or discrete multi-tones (DMT) modulation. The tool 402 is supported by a load bearing communication cable 412 as described above in Figure 1 or Figure 3 depending on the application needs.

At the surface the cable is carried by a sheave and winch assembly 414, and the end of the cable 412 is connected to a surface control unit 416 comprising a surface modem 418, a processor 420, an output/storage device 422. The surface modem is complementary to the downhole modem 410, and the processor 420 is connected to the surface modem 418 to receive, decode and process the data transmitted to the surface. The processor 420 is also used to send commands to the instruments downhole via the modem-cable-modem connection. An output device/storage 422 such as a display screen, printer, magnetic tape, CD, or the like is connected to the processor for display and/or storage of the processed data. The output device 422 may also include a transmitter 424 for relaying the processed data to a remote location.

In operation, a well engineer or user deploys the tool 402 supported by the cable 412 in the well 404 to a desired depth using the winch and sheave mechanism 414. Commands generated by user input, algorithm, or a combination are encoded at the surface using one of the methods described above. The encoded commands are then transmitted by the modem 418 through the cable 412 to the tool 402 disposed in the well. The downhole modem 410 receives the command which is then decoded for downhole operation of the tool.

When sensors 406 are activated to sense a desired parameter, the sensed parameter is delivered to the downhole processor 408 for pre-processing or sent directly to the surface. In either case, the data is encoded using one of the methods described above and transmitted by the downhole modem 410 through the cable 412 to the surface control unit 416. At the surface, the surface modem 418 receives the data. The processor 420 decodes the signal, performs further processing of the data, and the data is then displayed on a screen, printed on a printer, stored on magnetic tape, CD, or the like. The data may also be relayed to any remote location using a transmitter 424.

The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.

Claims

We claim: 1. A well logging system comprising: (a) a downhole well data sensor; (b) a downhole data transmitter; (c) a surface data receiver; and (d) a data transmission cable linking the transmitter and the receiver, the cable having at least one pair of insulated conductors wound in a substantially helical twist, an insulation sheath surrounding the twisted pair of conductors and a tensile load carrier surrounding the insulation sheath, the load carrier comprising a sheath of tensile load carrying filaments.

2. A well logging system as described by claim 1 wherein the transmitter and receiver each includes a signal modem complimentary to each other.

3. A well logging system as described by claim 2 wherein the modems utilize data encoding and decoding methods selected from the group consisting of (i) QAM, (ii) CAP, and (iii) DMT.

4. A well logging system as described by claim 1 wherein the filaments are distributed about a perimeter of the load carrying sheath in radial layers.

5. A well logging system as described by claim 2 wherein wire size respective to filaments in outer radial layers of the sheath are greater than those of interior layers.

6. A well logging system as described by claim 1 wherein the cable has seven twisted pairs of insulated conductors within the insulation sheath.

7. A well logging data cable comprising : (a) a twisted pair of signal conductors, each of the conductors being separately insulated; (b) an insulation sheath surrounding the twisted pair of conductors; and (c) a tensile load sheath surrounding the insulation sheath, the tensile load sheath comprising a plurality of filaments.

8. A data cable as described by claim 7 comprising at least 6 twisted pairs of conductors disposed around a center conductor, all conductors being within the insulation sheath.

9. A data cable as described by claim 7 wherein the filaments are distributed about a perimeter of the tensile load sheath in radial layers.

10. A system as described by claim 1 wherein the sensor is selected from the group consisting of (i) a pressure sensor, (ii) a temperature sensor and (iii) a flow sensor.

11. A data cable as described by claim 7 having an effective capacitance between the twisted pair of conductors of less than 30 pF per foot of cable length.

12. A method of transmitting a signal from within a well borehole to a surface location comprising: (a) transmitting the signal with a downhole data transmitter; (b) conveying the signal on a data transmission cable linking the transmitter and to a surface receiver, the cable having at least one pair of insulated conductors wound in a substantially helical twist, an insulation sheath surrounding the twisted pair of conductors and a tensile load carrier surrounding the insulation sheath, the load carrier comprising a sheath of tensile load carrying filaments.

13. A method according to claim 12 wherein the transmitting and receiving the signal are accomplished using complimentary signal modems.

14. A method according to claim 13 wherein the signal is encoded and decoded using decoding methods selected from the group consisting of (i) QAM, (ii) CAP, and (iii) DMT.