DE3227083A1 - OPTICAL TRANSMISSION SYSTEM FOR HOLE HOLE PROBE - Google Patents

Description

Hamburg, July 16, 1982 2592 82

Priority; July 20, 19 81, U.S.A. Pat. Ann. No. 285,146

Applicant;

Chevron Research Company

5 25 Market Street

San Francisco

CaI. 94105

UNITED STATES.

Optical transmission system for borehole probes

The invention relates to an optical transmission system for borehole probes, in which data from a probe body in the borehole is carried out at high speed transmitted to the surface of the earth.

The greatest speed with which bits are extracted from the deep oil wells via electromechanical cables about 10,000 meters can be transmitted is • tenths of a kilohertz. That’s what they’re doing against it at the moment

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under development, particularly sophisticated and equipped with a large number of sensors Probes require a higher transmission speed. The well-known broadband properties of optical fibers in conjunction with those without intermediate amplification to meet the long transmission lengths to be achieved this purpose. The fiber must of course be placed in an armored cable without being noticeable Causing light loss due to fiber disruption or microbending.

The problem of using a fiber optic transmission system arises from the extremely harsh operating conditions in very deep bores. These are filled with corrosive brine, which is often hydrogen

Contains 2 sulfides. The pressure in the drilling mud can be 2110 kg / cm (30000 ρ si), the temperature 25O ° C. Other limitations arise from the fact that electrical The power and space available in a borehole probe are limited. After all, probes often go in the Borehole lost. The transmission device must therefore not be very expensive.

The components of common optical transmission systems can only work satisfactorily with cooling under the conditions prevailing below in the borehole. Semiconductor laser and LEDs (Ii ent inducing diodes) do not work above 100 ° C. High pressure couplings that There is no such thing as a reliable contact for the optical path from the end of the cable to the probe. All Plastics lose their cohesion under the extreme conditions in deep boreholes. Self

Fluorine compounds, which are chemically inert, tend to flow under tension. An additional The problem is the water that penetrates the plastic and promotes stress corrosion of the glass fiber.

US Pat. No. 4,156,104 is to be considered as prior art. The cable dealt with in this document is referred to as watertight per se, but it could not subjected to abrasion or repeated flexing without fatigue failure.

In order to avoid the above difficulties According to the invention, an optical transmission system which can be used for borehole probes in deep boreholes with dan features that claim 1 created. Refinements of the invention are set out in the subclaims marked.

In a practical embodiment, the invention contains System an armored cable with one or more glass-clad optical fibers inside a tubular moisture barrier, a neodymium laser in the cable drum for irradiation of Infrared light into one of the fibers, a modulator in the cable connector located in the bottom of the borehole for modulating light and sending it back to the surface, and a semiconductor detector in the Drum for demodulating the data signal from the returned light.

NACHeEREIOHTj

According to a further feature of the invention, the neodymium laser is sensitive to vibrations in one wavelength limited by about 1.32 µm. At this wavelength the Rayleigh scattering losses are minimal in comparison to shorter wavelengths, and on the other hand the thermal disturbance in the detector is small compared to the one at longer wavelengths.

Further advantages and features of the invention emerge from the claims and from the following description and the drawings in which the invention is illustrated and illustrated by way of example. Show it: Fig. 1 is a schematic representation of a he

inventive transmission system for Borehole probes,

Fig. 2 shows a modified embodiment of the Er

finding,

Fig, 3 shows a connector in more detail Dar

position,

4 shows a cross section through an armored Ka

bel with fiber optic conductor,

5 shows an axial section through an axle extension

a cable drum for the transmission of three separate light beams, Fig. 6 is a section through an optional to

using embodiment of an optical slip ring device for transmission of light signals from a rotating axis to a stationary detector, and

Fig. 7 is a section along line 7-7 of

Fig. 6.

A fiber optic transmission system for use in borehole probes recorded data, Fig. 1, has a neodymium laser 21a as a light source and a detector 34a, which is located in the actual drum part 36a of a cable drum are housed. Electrical connections are made by wires 39a via slip rings, not shown manufactured according to a base plate. The armored cable 5 8a leads from the drum via not shown Rolls into the borehole and ends inside the coupling sleeve 10a of the cable end. The laser beam will directed through a lens 22a into the core of the glass-clad fiber 23a of the cable. On the one in the borehole located end, the fiber is connected to a light modulator 31 within a chamber 40a, the is sealed against the external environment. The modulator receives power via wires 37a, which are fed by a Multipole electrical coupling 13a come from, which is in the instrument part, not shown, or the probe or a second coupling half fits which is connected to the probe by a short length of electrical cable is. This eliminates the problem of an accurate transmission of the light through a so-called make-break coupling surface avoided. The cable also contains power and control lines (not shown) that follow the coupling part 13a are performed.

The usual, optically working crystal laser, which is doped with neodymium ions, Nd, oscillates in the range of the wavelength X = 1.06. However, if the laser cavity is made lossy at λ = 1.06 µm, the laser can be ^{forced to oscillate in the range of A =} 1 »32, un. This is desirable because in glass cladding

de te η fibers light by scattering at inhomogeneities lost during manufacture. The size of the loss due to this property

t -4
fluctuates with / ·,. Therefore, after traversing 20 km of fiber downwards and back up again, there is a difference of about 15 dB in the light transmission loss at these two wavelengths. The neodymium laser provides sufficient light output, above 0.1 watt, so that the light travels back and forth and sufficient power is received. This avoids the need to install something in the borehole probe or in the coupling part, which in the best case would be a less useful and space-consuming laser.

Fig. 4 shows the cross section of a cable provided for the invention. In this case three fibers 51 clad with glass, one of which corresponds to the fiber 23a of FIG Bending during the subsequent η _w manufacturing steps of the cable, such as the support of the outer armouring 58d. This is important because "microbends" allow light to leak out of the fiber cladding, that is, increase attenuation. Any bubbles or voids in the first soft plastic "buffer" coating 52 around the fibers 51 will be compressed causing microbending unless the jacket is incompressible enough that the surrounding pressure is not transmitted.

I FOLLOWED

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B The coat 5 3 must be na & e high-free and the Resist diffusion of liquid from the environment. This is not just about the pressure be kept low, · but also the fiber and its plastic buffer against chemical Attacks are protected. The micro-cracks in the surface of a glass-coated fiber under tension continue in the presence of moisture and cause the Fiber breaks.

In the example shown in Fig. 4, the three fibers 51 are coated with a silicone rubber elastomer 52 by dipping, to form a symmetrical, buffered core. Additional buffering can be achieved with an additional plastic cover. While During the coating process, the fibers are twisted into a screw with a steep pitch, for example about 3.75 cm. This makes it easier to bend the core, and this screw twist has the added benefit of Advantage that, if the finished cable is exposed to tension, the fibers to incline towards a straight line. This compresses the elastomer and elongates the core, without the glass fibers themselves being subjected to as much stress as the entire cable. In order to the possibility of breakage is reduced.

The buffered core 52 is covered with a hard jacket, which is resistant to the exiting pressure and has such a low diffusivity that the internal components are protected against attack by the borehole fluid. The coat can have several

NAO HEREIOHT

Have layers. For example, a layer 53 can be hard and resistant to breaking forces while a second layer 5 4 has low diffusivity and is resistant to corrosive influences is. The two layers 53 and 54 in combination therefore provide the required jacket properties. To the For example, the layer 5 3 may be a high temperature epoxy polymer made with glass fiber strands running in the longitudinal direction is filled. It has been shown that this casing material, by the known "pultrusion" process contributes very little to the loss of light in the fibers due to microbends, even with high pressure or tension. When the liquid epoxy cures or polymerizes, it fits precisely to the buffered fibers without causing microbends. If the material is thermally hardened it contracts, compressing the fiber lengthways. This works to a certain extent the effect of stress and thermal stress Expansion in the cable armouring against. the Layer 5 4 can be a fluoroplastic, such as one available under the trade name Teflon Plastic. Such plastics are chemically inert and have an extremely low permeability for Diffusion. Instead, and especially for applications at the highest temperatures, the cover layer 53 can be a be water-impermeable metal pipe. For example is a welded tube made of a nickel-steel alloy with an outer diameter of 2/413 mm and a wall thickness of 0.21 mm up to a load of 1055 kg / cm without it collapsing is.

'NACHQEKEICHTI

For the power supply in the borehole below that is the Fiber-protective jacket 53, 54 surrounded by a ring containing conductors 55, which each form groups that are isolated from one another by spacers 56. Instead, the wire bundles can each also have their own have their own isolation. The conductors are in turn covered with an insulating plastic layer applied by an extruder 57d encased, which in turn is preferably a fluorine plastic that is resistant to chemical Attacks at high pressure and high temperature is. In another embodiment, the layer 57d be constructed like the jacket 53, 54. That is, the fibers 51 and the conductors 55 in the hard, against Pressure-resistant and poorly diffusible sheath can be included.

The layer 57d is not only a barrier against aggressive borehole fluid, but also serves for embedding the double-layer reinforcement 58d, screwed in opposite directions and relieved of torsion. This reinforcement Must be used on the outside of a downhole Probe cables are provided to protect against abrasion that occurs when retracting and extending the probe occurs.

Without in any way the preceding description To restrict, the sizes of the various components of the in Fig. 4 armored fiber optic cable shown for the transmission of data from borehole probes should be used.

Table I.

Glass clad fiber 51,

Diameter of each of the three fibers 140 µm

Silicone rubber buffers 52,

Outside diameter 0.813 mm

Glass fiber reinforced epoxy layer 53,

Outer diameter 1.37 mm

Teflon-PFA layer 54,

External diameter 1.63 mm

4 groups of copper wires 55,

Diameter 0.226 mm

PFA insulation and reinforcement bed 57d,

Outer diameter 2.996 mm

2 layers of steel reinforcement 58d,

Outside diameter 4.699 mm

The armored fiber optic cable, FIG. 1, ends at the bottom in the borehole within the cable head coupling sleeve 10a. the Armouring 58a, which forms the main strength element of the cable, can be in the cable head in any known manner be anchored. For example, it can be bent around the ring 27a and inserted into the conical end of the coupling part la be constrained. The low pressure containing chamber 40a is from the flooded chamber 41a through the barrier 8a isolated. Spacers and other details are omitted to simplify the drawing. the Lock 8a is against the coupling sleeve 10a through the O-ring 15a and against the cable layer 5 7a low diffusivity sealed by an elastomeric shoe 7a. If the chamber 41a is not previously with a protective fatt is filled, the shoe 7a should be made of a fluoroplastic to protect against chemical influences getting produced.

AS

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Fi g. 3 shows the cable head coupling on half scale, which was actually manufactured and used for the cable described in FIG. 4 and Table I. the The arrangement and function of the coupling is the same as described with reference to FIG. 1, with the exception of one additional provided shoe seal 7e. The shoe seal 7e is back to back with the shoe seal 7c arranged to a pressure test of the seals of the fiber core 5 7c before driving into the borehole enable. The test is carried out by introducing oil at high pressure through the holes, which are later closed by screws 6.

The light transmitted into the borehole is modulated with the data stream and transmitted back to the earth's surface, one of the embodiments shown in Figures 1 and 2 being used.

In the example of FIG. 1, the laser light is through transmit the fiber 23a downward, modulated in the chamber 40a, transmitted back up through a second fiber 28 and focused on detector 34a through lens 33a. Preserved the detector and its amplifier electric power through lines 38a which are connected to slip rings, not shown. In a preferred In the embodiment, the detector is a germanium avalanche photodiode.

The ends of the fibers 23a and 28 are precisely positioned at the focal points of the lenses 29a and 30, respectively. This will the infrared light emerging from the fiber 23a is shaped and aligned to the beam 59a and passes through the

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optical elements 35, 31, and 32 and is then used again for return up to the surface in the Fiber 28 focused. The optical elements 35, 31 and 32 are components of a light beam modulator. It are different types of such modulators, for example acousto-optical modulators. Preferably will an electro-optical crystal modulator is used, as this type is made insensitive to Tampe raturä nde runge η can be. The elements 35 are the light polarizers required for this type of modulator. Of the electro-optic crystal can be divided into four crystals arranged to form a enable double compensation, see also page 17-12 of the "Handbook of Optics", published by Optical Society of America. The electro-optical crystals are preferably made of lithium tantalate, that has a high electro-optic coefficient, a high Curie temperature and a low one Loss tangent at a high modulation frequency. The connection of electrical voltages to the electrodes of the elements 31 is shown schematically by the wires 37a shown, which connect to the multipole connector part 13a and acted upon by electrical signals from the actual probe, not shown. The prism 32 reverses the path of the light beam and back into the optical fiber 28. The various components of the modulator and the fiber ends are arranged on a support with a low thermal coefficient, for example from "INVAR", a nickel-iron alloy.

The optical transmission system shown in Fig. 1 operates with direct perception of amplitude modulated Light. One advantage of this way of working is that

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that multimode fibers with a core or core diameter of 50 µm or more can be used. That makes the task easier, the location of a focused Keep light spot on the end of the fiber. A separate fiber 28 is in the cable for the return line of the light after the detector. This allows optical isolation of the detector end of the fiber 28 from the laser end of the fiber 23a and avoids the reception of light emanating from the laser end of the Fiber 23a is scattered zmrück. It also avoids the loss of light at a beam splitter that is required would be if a single fiber were used for the downlink and uplink. This The system is also insensitive to stretching of the cable.

If more than one optical channel is required, several fibers can be provided to convey the light from several separate modulators to the earth's surface. However, only one fiber 23a is required, to transmit the light from the laser down to the various modulators. The light off the fiber 23a can be divided there by several beam splitters.

The other embodiment of the invention shown in FIG. 2 uses optical homodyne detection instead of direct detection of the modulated light transmitted up the borehole from the probe. According to the invention, the neodymium laser 21b is forced to vibrate at a wavelength J ^ = 1.32 µm. A substantial part of the exiting beam

is directed through the lens 22b into the core of the optical fiber 23b. That light is going down there transmitted into the probe coupling, modulated with the measuring signal and transmitted back in the same fiber. The exiting light is partially directed through the steel splitter 60 out of the detector 34b.

In the known homodyne demodulation method, the beam modulated with the signal becomes coherent with a Part of the unmodulated laser beam, usually called a local oscillator beam referred to as. That is, the two beams with parallel wavefronts are superimposed when they can be applied to the detector through the lens 33b. This is done by a beam splitter 60 and the reflector 61 reached. Instead, the reflector 61 can be omitted and the local oscillator beam as Reflection from the front of the fiber core can be obtained to ensure coincidence of the wavefronts.

In practice, the shot noise exceeds that the demodulation of the local oscillator beam generates the noise peculiar to the detector. On the other hand, the interference of the signal and the local oscillator waves generates in the detector an electrical signal, the frequency of which is equal to the differential frequency of these waves, and a current proportional to the product of their amplitudes. Accordingly, the data signal becomes in proportion amplified to the sound. It follows that

I NACKG Z

the ratio of useful to interference signal can be greater than with direct demodulation, even if the Detector is a germanium PIN diode rather than an avalnche photodiode, provided that the two Waves at the detector are spatially coherent. This condition means that the fiber 23b is a single mode fiber have to be. This means that the fiber only supports the two different types of optical wave guide η of the lowest Order transmits.

That in the. Light traveling downwardly fiber 23b emerges in the sealed space 40b within the coupling sleeve 10b. The emerging light is through the Lens 29b collected to beam 59b that passes through single crystal modulator 62. The light is reflected back through the modulator 62 by means of the retro-reflecting η cube-corner prism 63, focussed back into the core of fiber 23b, and then goes to the surface. The core of a single mode fiber, which encloses the light is typically about 5 µm compared to 50 µm in diameter or more of a multimode fiber. Since 5 to only 4 wavelengths of neodymium light, the beam 5 9b must be returned from modulator 62 and accurate through lens 29b be refocused on the core of the fiber 23b, despite the movements caused by temperature changes in originate from the establishment. This is accomplished by using a cube corner back reflector 63 as a prism will. This prism has the property of reflecting a beam in a direction that is exactly opposite to that of the incident ray.

The modulator 62 can be an acousto-optic crystal modulator or an electro-optic crystal modulator

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be. In the latter case, the light beam is generated with a single crystal and without a polarizer and analyzer phase modulated with the lead wires 37b applied signal. The returning light can be opposed by the homodyne demodulation system demodulated to the immediate demodulation.

The embodiment shown in Fig. 2 is similar to that of Fig. 1 in all parts not specifically mentioned here, for example in the execution of the cable and the seal the chamber 40b through the sealing shoe 7b, which presses on the jacket 5 7b of low diffusivity.

In this preferred embodiment of the invention, the modulated laser light passing through the cable comes up, detected and demodulated by a receiver located in the rotating drum part of the Cable reel is arranged. The electrical data signal must then be on a stationary data processing or - be transferred to the recording device. if the Data transfer speed slower than approx 1 MHz, the signal can be reliably transmitted through conventional slip rings on the axle attachment of the cable reel will. But if the speed is greater the bit error count is reduced to unacceptable levels due to electrical noise and Crosstalk increased. This can be avoided by opting the data flow onto a stationary detector is transmitted through an embodiment of the types of "optical slip rings" described below.

Fig. 5 shows an axial section through the axial extension of a cable drum for the transmission of three separate light beams through "optical slip rings" is set up. In this embodiment of the invention are modulated laser light signals that are transmitted upwards through three optical fibers in the dam cable have been demodulated in the drum of the cable reel. The resulting electrical signals will be through coaxial cables 103 in the axis to light emitting diodes (LEDs) or laser diodes 102 and 102a transfer. That from the light source 102a in the axis of the Signal light emitted by the rotating axial part is converged by the lens 104a and onto the stationary Detector 106a in focus. For the sake of simplicity, there are support members and one emanating from the detectors coaxial lightness omitted. The light from the two or more sources 102 referenced with respect to the axis in FIG Different radial distances are arranged, remains due to the nested parabolic Reflectors 105 are focused on the stationary detectors 106 while the rays are focused around the axis to run. Annular lens sections could also be used to direct the signal light onto the detectors, but the parabolic reflectors are preferred, because they also provide optical and electrical shielding.

In principle, the ends of the optical fibers from the cable could be directly up to the point of the light sources 102 and 102a, instead of regenerating the light signals. The strength of the However, laser light is returning to the earth's surface low and would be further dampened by the loss,

■ ',:. ","' [PostponedJ

which would result from the insertion of optical slip rings. However, this would apply to all components lead to particularly tight tolerances. Instead of small PIN diodes for detectors 106 and 106a requires quite large avalanche photodiodes.

The light source 102a and the detector 106a can also be swapped to work in the opposite direction to transmit and route command signals down into the probe.

The optical slip ring assembly is against the external environment protected by a stationary housing 107, which is closely connected to the extension 108, the rotates with axis 100.

FIGS. 6 and 7 schematically show another embodiment of an optical slip ring for transmission of light signals from the rotating axis onto an adjacently arranged stationary detector. Fig. 6 is a view in the direction of the axis, Fig. 7 is a section along the line 7-7. A source of light 120 is arranged on the axis and emits a modulated light signal in all radial directions into a transparent disk 121, which has a small hole in the center for the light source. the Disc can be made of an acrylic resin or a similar ' be made of clear plastic. The light is limited to the pane by total internal reflection and exits at the edge of the disc. A stationary transparent sheet 122 of about the same

Thickness like the disc is adapted to a noticeable part of the disc circumference except for a gap, which allows the disk to rotate with the axis. The outer edge 125 of the sheet is reflective the light to a diode detector 123, which is connected to a coaxial line 124. The shape of the Sheet metal edge 125 is part of an ellipse in which the light source and the detector lie in the focal points. Therefore, a significant part of the light emitted by the source is collected at the detector.

The light source can be an edge-emitting LED, or, if the frequency of the modulated signal is greater than about 30 MHz, a laser diode can be used will. Since a laser diode does not radiate over a full 360 in its transition level, the desired Patterns of radiation into the disc can be achieved in that the laser is positioned so that it emits a beam along the axis. The jet is then radially into the disc through a conical Deflected reflector placed coaxially at 120 with the apex directed towards the laser. Instead of of this, the laser can be placed in the drum of the reel along with its electronic driver circuit will. The emitted light signal is then in-axis through a short section of optical fiber led to the apex of the reflective cone.

One transmission is required for several optical fiber channels provided by separate optical slip rings, all of which have the embodiment shown in FIG and along the axis of the axle extension of the cable

SUBSCRIBED

reels are arranged. The slots 126 in the light enclosing disc 121 serve that any components and electrical or light conductors can be passed through the pane, without covering more than a small fraction of the light.

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Claims (12)

Claims 1.J Optical transmission device for a one Borehole probe containing a measuring instrument inserted into a borehole at the lower end of an armored cable is arranged, characterized by a laser light source (21b) with a wavelength of more than 1 to oscillates through at least one optical fiber (51) arranged in the armored cable (51 to 5 8), which the laser light from the light source arranged above ground down into the borehole and back, transmits, by means for modulating the laser light with a data signal, in the on the cable hanging probe and means (34b) for demodulating the data signals from the at the top of the cable received, modulated laser light. 2. Device according to claim 1, characterized in that the laser light source is a neodymium laser. 3. Device according to claim 2, characterized in that that the neodymium laser (21b) is excited to vibrate at a wavelength of about 1.32 µm. 4. Device according to one of claims 1 to 3, characterized characterized in that the optical fiber (51) which in the cable follows the modulated light from the probe above days, one separate with respect to the fiber that transmits the light downward to the probe Fiber is. 5. Device according to one of claims 1 to 3, characterized in that the transmission of modulated Light from the probe after days passes through the same optical fiber (51) that the light goes to the probe directs. 6. Device according to claim 5, characterized in that that a reflector device (63) is provided for the return of the modulated light into the fiber (51). 7. Device according to one of the preceding claims, characterized in that transmission in the optical fiber (51) is limited to two optical modes. 8. Device according to claim 7, characterized in that that the means for demodulating the data signal from the modulated light is an optical homodyne pickup and -Demodulation device is. 9. Device according to one of the preceding claims, characterized in that the armored cable (51 to 5 8) contains a metal tube impermeable to water, in which includes at least one optical fiber (51). 10. Device according to one of the preceding claims with a coupling part at the end of the cable on the borehole side, characterized in that the coupling part (10a; 10b) contains a chamber (40a; 40b) which is sealed against the ingress of liquid and in which the cable enters, wherein a modulation device (31; 62) for the laser light is arranged in the chamber is. 11. Device according to one of the preceding claims with a reel for unwinding and winding up the armored cable containing a plurality of optical fibers, characterized in that different for the transmission Light signals from the end of the rotating axis of the reel for stationary detectors a device is provided with the following components: a. for each light signal separate sources arranged in the axis of the reel for emission separate light rays at different radial distances from the geometric axis of the Axis, b. separate, stationary, concentric and rotationally symmetrical means (105) for focusing separate ones Rays of light on separate spots that run along the extension of the geometric Axis of the reel axis are spaced apart and on each of which a detector (106) is arranged which picks up the light signals and converts them into electrical signals. 12. Device according to one of claims 1 to 9 with a reel for unwinding and winding the a plurality of cables containing optical fibers by a device for the transmission of light signals from the rotating axis of the cable reel to stationary detectors via separate optical Channels, each containing the following funds: a. at least one radiation device (120) arranged in the axis close to the geometric axis of light signals, b. Means for directing the light on transparent ones ? 7 η 8? ^::: ·· I naohgereicht I Paths in essentially all radial directions over the circumference of the axis, a stationary, concave reflection device, which has a substantially elliptical shape in section and is arranged next to the rotatable axis is to the light emerging from the transparent light path to a stationary spot to focus, and a detector arranged at the stationary spot for converting the light signals into electrical signals (Figures 6 and 7).