DE3227083C2 - - Google Patents

Description

The invention relates to an optical transmission device for borehole measurements with at least one laser light source and at least one optical fiber for transmitting laser light from and to an underground site in a borehole.

The largest transfer rate, with the bits over electromechanical Cable from the deepest oil wells of about 10 000 meters can be transmitted is a few tens of kilohertz. On the other hand are currently under development, especially sophisticated and equipped with a large number of sensors Probes require a higher transmission speed. The well-known broadband properties of the optical Fibers in conjunction with those without intermediate reinforcement to reach Large transmission lengths fulfill this purpose. The Fiber must of course be placed in a reinforced cable, thereby noticeable loss of light due to interference to avoid the fiber or microbending.

The problem of using a fiber optic transmission system created by the extremely tough conditions in very deep holes. These are with corrosive brine filled, which often contains hydrogen sulfides. The pressure in the drilling mud may be 2110 kg / cm² (30,000 psi), the Temperature 250 ° C. Other restrictions arise as a result of that electrical force and space in a borehole probe are only limited available. Finally, go too often lost probes in the borehole. The transmission device therefore may not be very expensive.

From US 39 05 010 a condition monitoring system is known, that of monitoring the reservoir pressure and the  Temperature in the borehole is used. The measuring system is within a casing at the bottom of the hole used as a permanent measuring station. The measurements of underground are via an optical fiber from an underground laser light source transmitted to a remote detector arranged. control pulses be from overage with a second laser light source and a second fiber transmitted to underground. It is an optical transmission device for borehole measurements with two laser light sources and two fibers for transmitting laser light from and to one underground place in a borehole, being a thermal and Pressure-tight encapsulated modulator for modulating the laser light with a data signal is present and a demodulator for the demodulation of the data signals of the upper end the cable is received modulated laser light is provided.

A disadvantage of this device is that for data transmission after the laser was required, the laser required and temperature conditions in the borehole is exposed. Further the optical fibers are not against interference, such as Microbending, protected. A continuous measurement while carried out in the borehole Meßfahrt is not provided.

A technically similar solution on a completely different Subject area is described in DE-GM 72 41 689. The device serves for the transmission of measured quantities from one to High voltage potential lying measuring location to a display location by means of light rays as information carrier. The to be modulated Light is from a laser beam source at the display location via a glass fiber cable to a light modulation device guided. From there, the modulated light becomes Display location returned via a fiber optic cable and to  a photodetector converted into an electrical signal. Thus, both the laser light source and the demodulation device arranged at the display location.

Also in this device lacks a transmission mechanism, to continuously measure when moving relative to the display location To allow the measuring location. Special protective measures for the Optical fiber are also not indicated.

A transmission mechanism is described in DE-OS 25 51 527. This device is used for information transmission between two mutually rotating arrangements at the transmission of the guided in optical fibers carrier light beams over opposite ends of the optical fibers takes place, the ends of the axis of rotation of the two arrangements are arranged centric circular surfaces.

The disadvantage is that in this arrangement high leakage losses occur at the opposite ends of the optical fibers.

A similar transmission mechanism is also in the DE-OS 19 54 643 discloses. There is a device for the transmission of measured values between systems rotating against each other, when in the one system an analog-to-digital converter for converting the measured value quantities into digital signals is provided and these signals as light from the one system be transferred to the other system, wherein in the receiving system provided a digital-to-analog converter is. There are several signal transmitter opposite to them passing recipients, each individual signaling device represents a digital value (1 bit).  

A disadvantage of this device is that a data transmission only to discrete rotational positions of the mutually rotating Systems is possible and the transmission speed directly from the expenditure on equipment, namely the number of Signal transmitter and receiver, depends. One for the logging technique required data transfer rate can be with Do not achieve this device.

Starting from the US 39 05 010 it is an object of the invention an optical transmission device for borehole measurements indicate the pressure and temperature conditions in the borehole withstands only small space and energy requirements in the Borehole probe and a continuous data recording in Bohrlochmeßfahrt enabled.

To solve this problem according to the invention for borehole probes optical transmission system usable in deep boreholes created with the features of claim 1. Embodiments of the invention are in the subclaims characterized.

In a practical embodiment, the inventive System a armored cable with one or more, Glass-lined optical fibers within a tubular Moisture barrier, a neodymium laser in the cable drum for irradiating infrared light into one of the fibers, a modulator in the downhole Cable connector for modulating light and its Return to the surface, and a semiconductor detector in the drum for demodulating the data signal the returned light.  

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

Further advantages and features of the invention will become apparent from the claims and from the description below and the drawings in which the invention, for example is explained and illustrated. It shows

Fig. 1 is a schematic representation of a transmission system according to the invention for borehole probe,

Fig. 2 shows a modified embodiment of the invention,

Fig. 3 shows a connecting piece in detail representation,

Fig. 4 shows a cross section through an armored cable, fiber optic strands,

Fig. 5 is an axial section through an axle-a cable drum for transmission of three separate light beams,

Fig. 6 is a section through an optionally to be used embodiment of an optical slip ring device for transmitting light signals from a rotating shaft to a stationary detector, and

Fig. 7 is a section along the line 7-7 of Fig. 6.

A fiber optic transmission system for recorded in borehole probes data, Fig. 1, has a neodymium laser 21 a as a light source and a detector 34 a, which are housed in the actual drum portion 36 a of a cable drum. Electrical connections are made by wires 39 a via slip rings, not shown, to a base plate. The armored cable 58 a leads from the drum via rollers, not shown, in the borehole and ends inside the coupling sleeve 10 a of the cable end. The laser beam is directed through a lens 22 a in the core of the glass-clad fiber 23 a of the cable. At the downhole end, the fiber is connected to a light modulator 31 within a chamber 40 a, which is sealed against the external environment. The modulator receives power via wires 37 a supplied to come from a multi-pole electrical coupling 13 a, which fits into the not shown tool part or the probe or a second coupling half, which is connected to the probe by a short piece of electrical cable. This avoids the problem of accurate transmission of the light through a so-called make-break coupling surface. The cable also includes force and control lines, not shown, which are performed after the coupling part 13 a.

The usual optically operating crystal laser doped with neodymium ions, Nd ³⁺ , oscillates in the wavelength range λ = 1.06 μm. However, if the laser resonator is made lossy at λ = 1.06μm, the laser can be forced to oscillate in the range of λ = 1.32μm. This is desirable because in glass-clad fibers, light is lost due to scattering of inhomogeneities introduced during manufacture. The loss size due to this property varies with λ ^-4 . Therefore, after passing 20 km of fiber down and back up, there is a difference of about 15 dB in the light conduction loss at these two wavelengths. The neodymium laser provides sufficient light output, above 0.1 watts, so that the light goes back and forth and sufficient power is received. This avoids the need to install anything in the borehole probe or coupling part, which would be a viable and space consuming laser at best.

Fig. 4 shows the cross section of a cable provided for the invention. In this case, three glass-clad fibers 51 , one of which corresponds to the fiber 23 a of FIG. 1, are enclosed in a cladding 53 having the following essential properties:

• A) It must be tough and rigid to protect the fibers against bending during subsequent manufacturing steps of the cable, such as the support of the outer reinforcement 58 d. This is essential because "microbending" allows the light to leak out of the fiber cladding, that is, to increase damping. Any bubbles or voids in the first "soft" plastic "buffer" liner 52 around the fibers 51 are compressed causing microbending unless the jacket is sufficiently impermeable that the surrounding pressure is not transmitted.
• B) The jacket 53 must be pinhole-free and resist the diffusion of liquid from the environment. This should not only keep the pressure low, but also protect the fiber and its plastic buffer against chemical attack. The microcracks in the surface of a glass-clad fiber under tension continue in the presence of moisture, causing the fiber to break.

In the example shown in Fig. 4, the three fibers 51 are dipped with a silicone rubber elastomer 52 to form a symmetrical buffered core. An additional buffering can be achieved with an additional plastic shell. During the coating process, the fibers are twisted into a high pitch screw, for example about 3.75 cm. This facilitates flexing of the core, and this helical twisting has the additional advantage that if the finished cable is subjected to stress, the fibers tend to straighten out. This compresses the elastomer and elongates the core without exposing the glass fibers themselves to as much stress as the entire cable. This reduces the possibility of breakage.

The buffered core 52 is wrapped with a hard shell which is resistant to the exiting pressure and has such low diffusibility that the internal components are protected from attack by the wellbore fluid. The jacket can have several layers. For example, a layer 53 may be hard and resistant to breaking forces, while a second layer 54 may have low diffusivity and be resistant to corrosive influences. The two layers 53 and 54 therefore provide in combination the required shell properties. For example, the layer 53 may be a high temperature epoxy polymer filled with longitudinal fiber glass strands. It has been found that this casing material, applied by the known "pultrusion" process, contributes very little to the loss of light in the fibers due to microbending, even at high pressure or stress. Upon curing or polymerization of the liquid epoxide, it will conform exactly to the buffered fibers without causing microbending. If the material is thermally cured, it contracts and compresses the fiber lengthwise. This counteracts to some extent the effect of stress and thermal expansion in the cable reinforcement. The layer 54 may be a fluoroplastic, such as a plastic available under the trade name Teflon. Such plastics are chemically inert and have extremely low permeability to diffusion. Instead, and especially for high temperature applications, the sheath layer 53 may be a water impermeable metal tube. For example, a welded tube made of a nickel-steel alloy having an outer diameter of 2.413 mm and a wall thickness of 0.21 mm has been tested to a load of 1055 kg / cm² without collapse.

For downhole power supply, the fiber protecting jacket 53, 54 is surrounded by a ring containing conductors 55 , each forming groups isolated from each other by spacers 56 . Instead, the wire bundles may each have their own isolation. The conductors are in turn encased d with an applied by the extruder, the insulating resin layer 57, which is a fluorine plastic again preferably, is resistant to chemical attack at high pressure and high temperature. In another embodiment, the layer 57 d may be constructed like the jacket 53, 54 . That is, the fibers 51 and conductors 55 may be included in the hard, pressure resistant and low diffusivity facing jacket.

The layer 57 d is not only a barrier against aggressive well fluid, but also serves to embed the double-layered, counter-screwed and torsion relieved reinforcement 58 d. This reinforcement must be provided on the outside of a probe cable to be used in the borehole in order to protect against the abrasion that occurs during retraction and extension of the probe.

Without limiting in any way the foregoing description, Table I below sets forth the sizes of the various components of the armored fiber optic cable shown in Figure 4 to be used for the transmission of data from well probes.

Glass-clad fiber 51 , diameter of each of the three fibers | 140 μm Silicone rubber buffer 52 , outer diameter 0.813 mm Glass fiber reinforced epoxy layer 53 , outer diameter 1.37 mm Teflon PFA layer 54 , outer diameter 1.63 mm 4 groups of copper wires 55 , diameter 0,226 mm PFA insulation and reinforcing bed 57 d, outer diameter 2.996 mm 2 layers steel reinforcement 58 d, outer diameter 4,699 mm

The armored fiber optic cable, Fig. 1, ends down in the borehole within the cable head coupling sleeve 10 a. The reinforcement 58 a, which forms the main strength element of the cable can be anchored in the cable head in any known manner. For example, it can be bent around the ring 27 a and be squeezed into the conical end of the coupling part 1 a. The low-pressure chamber 40 a is isolated from the flooded chamber 41 a through the barrier 8 a. Spacers and other details are omitted to simplify the drawing. The lock 8 a is sealed against the coupling sleeve 10 a through the O-ring 15 a and against the cable layer 57 a low diffusivity by an elastomeric shoe 7 a. If the chamber 41 a is not previously filled with a protective grease, the shoe 7 a should be made of a fluorine plastic for protection against chemical influences.

FIG. 3 shows, in half scale, the cable head coupling actually made and used for the cable described in FIG. 4 and Table I. FIG . The arrangement and function of the coupling is the same as described with reference to FIG. 1, with the exception of an additionally provided shoe seal 7 e. The shoe seal 7 e is arranged back to back with the shoe seal 7 c to allow a pressure test of the seals of the fiber core 57 c prior to entering the wellbore. The test is carried out by introducing high pressure oil 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, using one of the embodiments shown in FIGS. 1 and 2.

In the example of Fig. 1, the laser light is transmitted through the fiber 23 a down, modulated in the chamber 40 a, transmitted back through a second fiber 28 and focused by the lens 33 a on the detector 34 a. The detector and its amplifier receive electrical power through lines 38 a, which are connected to slip rings, not shown. In a preferred embodiment, the detector is a germanium avalanche photodiode.

The ends of the fibers 23 a and 28 are located exactly at the focal points of the lenses 29 a and 30 , respectively. Thereby, the light emerging from the fiber 23 a, infrared light to the beam 59 a is formed and aligned and passes through the optical elements 35 , 31 and 32 and is then focused again for the return up to the surface in the fiber 28 . The optical elements 35, 31 and 32 are components of a light beam modulator. There are several types of such modulators, for example acousto-optic modulators. Preferably, an electro-optic crystal modulator is used since this type can be made insensitive to temperature changes. The elements 35 are the light polarizers needed for this type of modulator. The electro-optic crystal may be divided into four crystals arranged to allow double compensation, see also pages 17-12 of the "Handbook of Optics" issued by the Optical Society of America. The electro-optic crystals are preferably made of lithium tantalate, which has a high electro-optic coefficient, a high Curie temperature and a low 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 37 a, which connect to the multipolar plug part 13 a and are acted upon by electrical signals from the actual probe, not shown. The prism 32 reverses the travel 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 carrier with a low thermal coefficient, for example of "INVAR", a nickel-iron alloy.

The optical transmission system shown in Fig. 1 operates with direct perception of amplitude modulated light. An advantage of this operation is that multimode fibers having a core or core diameter of 50 μm or more can be used. This facilitates the task of keeping the location of a focused spot on the end of the fiber. A separate fiber 28 is included in the cable for the return of the light to the detector. This allows the optical isolation of the detector end of the fiber 28 from the laser end of the fiber 23 a and avoids the absorption of light, which is scattered back from the laser end of the fiber 23 a. It also avoids the loss of light at a beam splitter, which would be required if a single fiber were used for the downlink and uplink. This system is also insensitive to extension of the cable.

If more than one optical channel is needed, multiple fibers may be provided to return the light from several separate modulators to the earth's surface. However, only one fiber 23 a is required to transmit the light from the laser down to the different modulators. The light from the fiber 23 a can be shared by several beam splitters.

The other embodiment of the invention shown in Figure 2 uses optical homodyne detection rather than direct detection of the modulated light transmitted uphole by the probe. According to the invention, the neodymium laser 21 b is forced to oscillate at a wavelength λ = 1.32 μm. A substantial portion of the exiting beam is directed through the lens 22 b in the soul of the optical fiber 23 b. This light is transmitted down there into the probe coupling, modulated with the measurement signal and transmitted back in the same fiber. The emerging light is partially directed by the beam splitter 60 from the detector 34 b.

In the known homodyne demodulation method, the signal modulated beam is coherently combined with a portion of the unmodulated laser beam, commonly referred to as a local oscillator beam. That is, the two beams are superimposed with parallel wavefronts when they are put on the detector by the lens 33b . This is achieved by a beam splitter 60 and the reflector 61 . Instead, the reflector 61 may be omitted and the local oscillator beam may be obtained as a reflection from the front of the fiber core to ensure coincidence of the wavefronts.

In practice, the shot noise generated by the demodulation of the local oscillator beam exceeds the noise peculiar to the detector. On the other hand, the interference of the signal and the local oscillator waves in the detector produces an electrical signal whose frequency is equal to the difference frequency of these waves and a current which is proportional to the product of their amplitudes. Thus, the data signal is amplified in proportion to the noise. It follows that the ratio of useful to interfering signal may be greater than in direct demodulation, even if the detector is a germanium PIN diode instead of an avalanche photodiode, provided that the two waves are spatially coherent at the detector , This condition means that the fiber 23 must be b a Einzelmode- fiber. That is, the fiber transmits only the two lowest-order net-type optical waveguide types.

The down in the fiber 23 b light exiting in the sealed space 40 b within the coupling sleeve 10 b. The exiting light is collected by the lens 29 b to the beam 59 b, which passes through the single crystal modulator 62 . The light is passed by the modulator 62 is reflected back by means of the retroreflective cube-corner prism 63 back into the core of the fiber 23 b is focused, and then goes to the surface. The core of a single-mode fiber that encloses the light typically has a diameter of about 5 μm compared to 50 μm or more of a multimode fiber. Since 5 microns are only 4 wavelengths of neodymium light, the beam 59 b must be returned from the modulator 62 and focused by the lens 29 b exactly on the core of the fiber 23 b, despite the movements resulting from temperature changes in the device , This is achieved by using a cube corner return reflector 63 as a prism. This prism has the property of reflecting a beam in a direction exactly opposite that of the incident beam.

The modulator 62 may be an acousto-optic crystal modulator or an electro-optic crystal modulator. In the latter case, with a single crystal and without polarizer and analyzer, the light beam is phase modulated with the signal applied by the lead wires 37 b. The returning light can be demodulated by the homodyne demodulation system as opposed to the immediate demodulation.

The embodiment shown in Fig. 2 is similar to that of Fig. 1 in all parts not particularly mentioned here, for example in the design of the cable and the seal of the chamber 40 b through the sealing shoe 7 b, which presses on the jacket 57 b low diffusivity ,

In this preferred embodiment of the invention is the modulated laser light coming through the cable comes up, grasped and demodulated by a Receiver in the rotating drum part of the Cable reel is arranged. The electrical data signal must then on a stationary data processing or Recording device to be transmitted. if the Data transmission speed less than about 1 MHz, the signal can be reliably through conventional Transfer slip rings on the shaft extension of the cable reel become. However, if the speed is greater is, the bit error count becomes unacceptable Heights due to electrical noise and increased crosstalk. This can be avoided by the data flow optically to a stationary detector by an embodiment of those described below Types of "optical slip rings" is transmitted.  

Fig. 5 shows an axial section through the Achsansatz a cable drum, which is adapted for the transmission of three separate light beams by "optical slip rings". In this embodiment of the invention, modulated laser light signals which have been transmitted upwardly through three optical fibers in the cable are demodulated in the drum of the cable reel. The resulting electrical signals are transmitted through coaxial cables 103 in the axis to light emitting diodes (LEDs) or laser diodes 102 and 102a . The light emitted from the light source 102 a in the axis of the rotating axial part signal light is focused by the lens 104 a and focused on the stationary detector 106 a. For the sake of simplicity, carrier parts and a coaxial line emanating from the detectors have been omitted. The light from the two or more sources 102 , which are arranged at different radial distances with respect to the axis, remains focused on the stationary detectors 106 by the nested parabolic reflectors 105 , while the rays pass around the axis. Annular lens sections could also be used to direct the signal light to the detectors, but the parabolic reflectors are preferred because they additionally provide optical and electrical shielding.

In principle, the ends of the optical fibers could be led out of the cable directly to the location of the light sources 102 and 102 a, instead of regenerating the light signals. However, the intensity of the laser light returning to the earth's surface is low and would be further attenuated by the loss that would result from the insertion of optical slip rings. However, this would lead to particularly tight tolerances for all components. Also, rather than small PIN diodes, detectors 106 and 106 would require fairly large avalanche photodiodes.

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

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

Figures 6 and 7 show schematically another embodiment of an optical slip ring for transmitting light signals from the rotating axis to an adjacent stationary detector. Fig. 6 is a view in the direction of the axis, Fig. 7 is a section along the line 7-7. A light source 120 is disposed on the axis and radiates a modulated light signal in all radial directions into a transparent disk 121 having a small bore in the center for the light source. The disc may be made of an acrylic resin or similar transparent plastic. The light is limited by total internal reflection on the disc and exits at the edge of the disc. A stationary transparent sheet 122 of about the same thickness as the disk is fitted to a substantial portion of the disk periphery except for a clearance which permits rotation of the disk with the axle. The outer edge 125 of the sheet reflects the light to a diode detector 123 which is connected to a coaxial line 124 . The shape of the sheet edge 125 is part of an ellipse in which the light source and the detector are in the focal points. Therefore, a significant portion of the light emitted from the source is collected at the detector.

The light source may be an edge-emitting LED, or, if the frequency of the modulated signal greater than about 30 MHz, a laser diode may be used become. As a laser diode in its transition plane does not radiate over full 360 °, the desired Pattern of radiation into the disk thereby be achieved that the laser is placed so that he emits a beam along the axis. The beam is then radially in the disc by a conical Reflector deflected, which is arranged coaxially at 120 with the apex facing the laser. Instead of of which the laser can be in the drum of the reel together arranged with its electronic driver circuit become. The emitted light signal is then through a short section of an optical fiber in the axis guided to the apex of the reflective cone.

For multiple optical fiber channels, transmission is provided by separate optical slip rings, all of which have the embodiment shown in FIG. 6 and which are arranged along the axis of the axle extension of the cable reel. The slots 126 in the light enclosing disk 121 serve to allow any components and electrical or light conductors to be passed through the disk without obscuring more than a small fraction of the light.

Claims (11)

1. Optical transmission device for borehole measurements with at least one laser light source ( 21 a, 21 b) and at least one optical fiber ( 23 a, 23 b, 28 , 51 ) for transmitting laser light from and to an underground place in a borehole, wherein

• a) the optical fiber ( 23 a, 23 b, 28 , 51 ) arranged in a reinforced cable ( 52-58 ) is surrounded by a buffer material ( 52 ) and spirally wound with a large pitch,
• b) a cable head coupling ( 10 a) is provided at the lower end of the cable,
• c) a thermally and pressure-tight encapsulated modulator ( 31, 62 ) for modulating the laser light with a data signal is present in the cable head,
• d) a demodulator ( 34 a, 34 b) is provided for demodulating the data signals of the modulated laser light received at the upper end of the cable and the data signals are regenerated via an optical slip ring.

2. Device according to claim 1, characterized in that the laser light source ( 21 a, 21 b) is a neodymium laser. 3. Device according to claim 2, characterized in that the neodymium laser ( 21 b) is excited to oscillate at a wavelength of about 1.32 microns. 4. Device according to one of claims 1 to 3, characterized in that the optical fiber ( 51 ), which transmits in the cable, the modulated light from the probe to the surface, with respect to the fiber, the light down to the Probe transmits, is separate fiber. 5. Device according to one of claims 1-3, characterized in that the transmission of modulated light from the probe goes to the surface over the same optical fiber ( 51 ) which directs the light to the probe. 6. Device according to claim 5, characterized in that a reflector device ( 63 ) is provided for the return of the modulated light in the fiber ( 51 ). 7. Device according to one of the preceding claims, characterized in that the transmission in the optical fiber ( 51 ) is limited to two optical modes. 8. Device according to claim 7, characterized in that the Means for demodulating the data signal from the modulated Light an optical homodyne recording and demodulation device is. 9. Device according to one of the preceding claims, characterized in that the armored cable ( 51-58 ) contains a water-impermeable metal pipe in which at least one optical fiber ( 51 ) is included. 10. Optical transmission device according to claim 1, for transmitting a plurality of light signals from the end of a rotating reel to stationary detectors, characterized by

• a) separate sources for each light signal in the reel for emitting separate light beams at different radial distances from the reel axis,
• b) separate, stationary, concentric and rotationally symmetrical means for focusing the separate light beams to separate locations, which are arranged along the extension of the reel axis and
• c) separate detectors, which are arranged at the separate locations to receive light signals and convert them into electrical signals.

11. Optical transmission device according to claim 1, characterized characterized in that the laser light source with a wavelength of more than 1 micron swings that a chamber in the Cable head coupling is provided, which is against the immediate Liquid is sealed, in which the cable enters, where the chamber is insulated thermally and pressure tight and the end of the at least one optical fiber the modulator to modulate the laser light with the data signals, and that a multi-pole electrical connector for connection to an instrument probe is provided, through which an optical Fiber connector at the bottom of the cable avoided becomes.