NO160955B - OPTICAL COMMUNICATION SYSTEM FOR DRILL LOGGING. - Google Patents

Description

The invention relates to an optical communication system

for borehole logging, comprising a laser light source, an armored cable with at least one optical fiber for transmitting laser light down through and up through the borehole, a data-collecting logging probe connected to the cable at the lower end of the borehole, a device for modulating data signals on the light, and a device at the surface for demodulating the data signals from the modulated light received at the upper end of the cable.

A system of the above type is known from US patent document 4,162,400. A similar system is known from US patent document 3,903,496 which shows an optical communication system where a light source is placed in a surface unit, and where the light is transmitted in an optical fiber down to and up from a deeper-lying, data-collecting probe, the probe comprising a device for modulating the data signal on the light at the optical fiber's turning point in the probe, as well as a demodulator at the fiber's upper end.

The fastest bit rate that can be transmitted through electromechanical cables from the deepest oil wells (10,000 m) is a few tens of kilohertz. In contrast, the increasingly sophisticated multi-sensor probes under development have created a need for higher transmission rates. The well-known broadband characteristics of optical fibers, together with the long lengths that can be used for transmission without amplifiers, make this possible. The fiber must of course be built into an armored cable without the addition of significant light loss as a result of irregularities in the fiber ("micro-bend").

The problem with realizing a transmission system with optical fibers is due to the very inhospitable environment encountered in deep boreholes. These are filled with corrosive salt water, often with dissolved hydrogen sulphide. The pressure in drilling mud must be as high as 2,100 kg/cm 2. The temperature can be as high as 250°C. Other limitations are that electrical energy and space are at a premium in the probe down the borehole. It must be convenient to connect and disconnect the cable and the probe. Finally, the probes are often lost down the borehole. The transmitter cannot thus be disproportionately expensive.

No component in the conventional optical transmission systems can function satisfactorily in the downhole environment without cooling. Semiconductor lasers and LEDs (light-emitting diodes) do not work above 100°C. High pressure connectors that provide an optical make/break path from the cable head to the probe do not exist. All plastic materials lose their integrity in the extreme environment down the borehole. Even fluorinated compounds, which are chemically neutral, tend to flow under stress. A further problem is that water penetrates plastic materials and promotes stress corrosion of the glass fibre.

Related prior art is shown in US Patent 4,156,104 (Mondello). Although the cable described in this patent is "watertight", it cannot withstand abrasive impact or repeated bending without fatigue failure.

The purpose of the invention is to provide an optical communication system which solves the problems mentioned above.

This purpose is achieved with an optical communication system of the type indicated at the outset which, according to the invention, is characterized by the fact that the laser light source is arranged to oscillate with a wavelength greater than 1 µm, that the system comprises a cable head-connector cover which is connected to the cable at its lower end and comprising a chamber sealed against the surroundings which accommodates the modulation device, and an electrical multi-pin connector adapted to mate with the logging probe for electrical connection thereof to the modulation device,

and that the demodulation device comprises a device for transmitting the received light signals to detectors.

In that the laser light source is designed to oscillate

with an optimal wavelength (greater than 1 pm), Rayleigh scattering losses are minimized. By means of the arrangement of a cable head connector cover with an electrical multi-pin connector for connecting the logging probe, it is achieved that the transformation of the electrical signal takes place in the connector cover and not in the logging probe, and that therefore only electrical signals pass through the logging probe and the cable head connector. In this way, a fiber optic coupling piece at the lower end of the borehole is eliminated, which is a

significant advantage.

The invention shall be described in more detail below

in connection with design examples with reference to the drawings, where fig. 1 shows a first embodiment of a fiber optic communication system according to the invention, fig. 2

shows an alternative embodiment of a communication system according to the invention, fig. 3 shows a half-scale view of an embodiment of a cable head connector for use in the system according to the invention, fig. 4 shows a cross section of an armored fiber optic cable for use in the system according to the invention, fig. 5 shows an axial section through an extended shaft portion of a cable reel which is adapted for the transmission of three separate light beams, fig. 6 shows a cross-sectional view of an alternative optical slip ring assembly for transmitting light signals from a rotating shaft to a stationary detector, and FIG. 7 shows a section along the line 7-7 in fig. 6.

Fig. 1 schematically shows a configuration of the components in the fiber optic communication system according to the invention. A neodymium laser source 21a and a

detector 34a is placed in the drum 36a in a cable reel. Electrical connections are made by means of lead wires 39a via slip rings (not shown) to the mounting platform. An armored cable 58 extends from the coil over washers (not shown) down into the borehole and is terminated within a cable head connector jacket 10a. The laser beam is focused into the core of a glass-clad fiber 23a in the cable by means of a lens 22a. At the downhole end, the fiber is connected to a light modulator 31 (described below) inside a chamber 40a which is sealed against the external environment. Lead wires 37a supplying drive power to the modulator come from a multi-pin electrical connector 13a that mates with the instrument probe (not shown) or with a mating connector half, which is connected to the probe by a short length of electrical cable. In this way, a precise transmission of light through the make/break contact piece's interface is avoided. The cable also contains power and control conductors (not shown) which lead to the connecting piece 13a.

The conventional/ optically pumped crystal laser which

is doped with neodymium ions, Nd 3+ , oscillates near the wavelength Å = 1.06 ym. However, by making the laser resonator lossy at X = 1.06 ym, the laser can be forced to oscillate near X = 1.32 ym. This is desirable because the glass-clad light. fibers are lost due to dispersion due to inhomogeneities introduced during manufacture. The loss percentage as a result of this

-4

intrinsic mechanism varies proportionally with X . When laying back a fiber length of 20 km down and up through the borehole, there is thus a difference in light energy loss of approx. 15 dB at these two respective wavelengths. The neodymium laser provides sufficient light energy (up from 0.1 watts) to perform the round trip with received power to spare. In this way, the need to install what would at best be an inefficient and space-consuming laser in the probe or connector down the borehole is avoided.

Fig. 4 shows a cross-section of a cable constructed in accordance with the invention. In this embodiment, three glass-clad fibers 51 (one of which corresponds to the fiber 23a in Fig. 1) are enclosed in a mantle or sheath 53 which has the following essential properties: A. It must be hard and rigid to protect the fibers against bending under later cabling operations, such as the application of the outer reinforcement 58d. This is important as "microbending" allows light to leak out of the fiber cladding - i.e. the attenuation is thereby increased. Any bubbles or voids in the first soft plastic "buffer" layer 52 around the fibers 51 will be compressed and thereby produce microbends unless the sheath is sufficiently incompressible so that the ambient pressure is not transmitted.

B. The cover 53 must be without small holes and resist diffusion

of the surrounding fluid. This is not only to keep the pressure down, but to protect the fiber and its plastic buffer from chemical attack. Microcracks in the surface of a glass-clad fiber under tension will propagate in the presence of moisture and cause the fiber to break. In the illustrative embodiment in fig. 4 is three fibers

51. by immersion coated with silicone rubber elastomer 52 to form a symmetrical, buffered core. Additional buffer material can be provided by means of an additional plastic cover. During this coating process, the fibers are twisted into a helical line with a long pitch, e.g. 3.8 cm long. Next to that

it facilitates bending of the core, this helical line has the additional advantage that when the completed cable is subjected to a tensile stress, the fibers will tend to straighten so that the elastomer is compressed, and the core will elongate without so much stress on the glass fibers themselves stress as the total cable. This reduces the risk of breakage.

The buffered core 52 is enclosed in a hard jacket which is able to withstand the pressure, and which has little diffusivity to protect the internal components from attack by salt water. The jacket may comprise more than one layer. E.g. a first layer 53 can be hard and pressure-resistant, while a second layer 54 has little diffusivity and is resistant to corrosive attack. Thus, the two layers 53 and 54 in combination provide the necessary covering qualities. By way of illustration, layer 53 may be a high temperature epoxy polymer filled with longitudinal fiberglass strands. This coating material, which is applied using the well-known "pultrusion" technique, has been shown to contribute very little to the light loss in the fibers as a result of microbend, even at high pressure or tension. As the liquid epoxy cures or polymerizes, it conforms precisely to the buffered fibers without causing microbends or microbends. If it is thermally hardened, it also contracts and compresses the fibers in the longitudinal direction. This tends to counteract the effect of tensile stress and thermal expansion in the cable reinforcement. Layer 54 may be a fluorine-added composition, such as one of Duponf's Teflon materials. These plastic materials are chemically neutral and have little diffusivity. Alternatively,

and also preferably for operation at the highest possible temperature, the jacket layer 53 can be a metal tube which is impermeable to water. E.g. a welded nickel-steel alloy tube with an outer diameter of 2.2 mm and a wall thickness of 0.21 mm has been

2

tested to 1050 kg/cm without crushing.

In order to provide an effect down the borehole, the fiber protective jacket 53, 54 is surrounded by a ring of conductors 55 which are divided into groups which are isolated from each other by means of spacers 56. Alternatively, the bundles of conductor wires can each have their own insulation. The conductors are in turn covered by an extruded, insulating plastic layer 57d which is again preferably a fluorine-added composition to resist chemical attack at high pressure and high temperature. In an alternative embodiment, the layer 57d can be constructed in the same way as the sheath 53, 54. This means that both the fibers 51 and the conductors 55 can be contained in the hard, pressure-resistant sheath with little diffusivity.

In addition to being a saltwater barrier, the layer 57d also serves as a substrate for the counter-helical, torque balancerté double layer reinforcement 58d. This reinforcement must be located on the outside of any operating borehole logging cable to resist the abrasion resulting from raising and lowering the instrument probe.

Without in any way affecting the general applicability of the foregoing description, the following Table I sets forth the dimensions of the various elements of the armored fiber optic cable manufactured for borehole logging data transmission and shown in FIG. 4.

Referring now again to fig. 1, the armoured, fiber optic cable is terminated at the bottom of the drill hole inside the cable head connector cover 10a. The armature 58a, which forms the main strength part of the cable, can be held in place in the cable head in any of several conventional ways. E.g. it can be bent around a ring 27a and wedged in the conical end la of the copying piece. The low-pressure chamber 40a is isolated from a filled chamber 41a by means of a barrier 8a. Spacers and other details are omitted for clarity. The barrier 8a is sealed against the connector cover 10a by means of an O-ring 15a and against the cable layer 57a, which has little diffusivity, by means of an elastomer boot protection 7a. If the chamber 41a is not pre-filled with a protective lubricating grease, the boot guard 7a should be made of a fluorine-added elastomer to resist chemical attack.

Fig. 3 shows a half-scale view of a cable head connector constructed for use with the cable shown in Fig. 4 and indicated in Table I. The assembly and function of the copying piece is described in connection with fig. 1 except that a further boot seal 7e is arranged. The boot seal 7e is placed back to back with a boot seal 7c to allow pressure testing of the seals on the fiber core 57c prior to immersion in the borehole.

The test is carried out by introducing oil at high pressure through the holes shown, which are later sealed using screws 6.

The light sent down through the borehole is modulated

with the data stream and sent back to the surface using one or the other of the alternative designs shown in fig. 1 and 2.

In the embodiment shown in fig. 1, the laser light is sent down through the fiber 23a, modulated in the chamber 40a, sent back up through a second fiber 28 and focused by means of a lens 33a onto a detector 34a. The detector and its amplifier draw power through wires 38a which are connected to slip rings (not shown). In a preferred embodiment, the detector is a germanium avalanche photodiode.

The ends of the fibers 23a and 28 are precisely located in the focal points of the lenses 29a and 29a respectively. 30. The infrared light emerging from the fiber 23a is then collimated into a beam 59a and passes through the optical elements 35, 31 and 32,

and is then refocused into the fiber 28 for repeated transmission back to the surface. The optical elements 35,

31 and 32 are components of a light beam modulator. The modulator can be any of several types, e.g. an acousto-optic modulator. However, it is preferably an electro-optical crystal modulator as this type can be made insensitive to temperature changes. The elements 3 5 are the light polarizers that are necessary for this type of modulator. The electro-optical crystal may be divided into four crystals arranged to provide double compensation, as shown on pages 17-12 of the "Handbook of Optics" sponsored 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 angle at a high modulation frequency. Supply of electrical voltages to the electrodes of the elements 31 is indicated by lead wires 37a from the multi-pin connector 13a which are driven by electrical signals from the instrument probe (not shown). The prism 32 reverses the path of the light beam back to the optical fiber 28. The various components of the modulator and the fiber ends are mounted on a chip with a low coefficient of thermal expansion, such as "INVAR" which is an alloy of nickel and iron.

The optical transmission system shown in FIG. 1, utilizes direct detection of amplitude modulated light. An advantage of this operating mode is that multimode fibers with a core diameter of 50 um or more can be used. This facilitates the maintenance of the position of a focused spot of light at the end of the fiber. A separate fiber 28 is included in the cable to transmit light to the detector. This allows optical isolation of the detector end of the fiber 28 from the lasing end of the fiber 23a

and thus avoids reception of light that is scattered back from the fiber's 23a lasing. Furthermore, loss of light is avoided by the beam splitter that would be necessary if a single fiber were used

was used for both the up and down transfers. This system is also insensitive to cable stretching.

If more than one optical channel is required, multiple fibers can be included to return the light to the surface from multiple separate modulators. However, only one fiber 2 3a is needed to transmit light from the laser down to the multiple modulators. Light from the fiber 23 can then be divided using

of several beam splitters.

The alternative embodiment of the invention shown

on fig. 2, uses optical homodyne detection instead of direct detection of the modulated light transmitted up from the downhole probe. In accordance with the invention, the neodymium laser 21b is forced to oscillate at a wavelength * = 1.32 µm. A substantial part of the outgoing beam is focused by means of a lens 22b into the core of an optical fiber 23b. This light is transmitted down to the instrument probe's connector, modulated with the data signal and transmitted up again via the same fiber. The outgoing light is partially directed to a detector 34b by means of a beam splitter 60.

In the well-known homodyne detection process, the signal-modulated beam is coherently combined with a portion of the unmodulated laser beam, commonly referred to as the "local oscillator" beam. This means that the two beams are superimposed with parallel wavefronts as they approach the detector through the lens 33b. This is achieved by means of the beam splitter 60 and a reflector 61. Alternatively, the reflector 61 can be looped and the local oscillator beam obtained as a reflection from the front surface of the fiber's core, to ensure coincidence of the wave fronts.

The hail noise generated by detection of the local oscillator beam in practice exceeds the noise inherent in the detector. Interference of the signal and the local oscillator waves in the detector, on the other hand, generates an electrical signal with a frequency equal to the difference frequency of these waves, and a current proportional to the product of their amplitudes. The data signal is thus amplified in proportion to the noise. The result is that the signal/noise ratio can be greater than with direct detection 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 requirement means that the fiber 2 3b must be a single-mode fiber. This means that the fiber will transmit only the two degenerate, lowest order, optical waveguide modes.

The light that travels downwards along the fiber 23b comes out

in the sealed space 40b inside the connector cover 10b.

The emerging light is collimated by means of a lens 29b

a beam 59b which passes through a single crystal modulator 62. The light is reflected back through the modulator 62 by means of a back-reflecting cube corner prism 63 and refocused into the core of the fiber 23b, and then travels up to the surface. The core of a single-mode fiber, in which the light is limited, is typically approx. 5 µm in diameter compared to 50 µm or more in a multimode fiber. As 5 ym is only four wavelengths of neodymium light, the beam 59b must be returned from the modulator 62 and, with the help of the lens 29b, refocused precisely into the core of the fiber 23b, despite the movements resulting from temperature changes in the equipment. This is achieved by using a cube-corner back-reflector such as prism 63. This prism has the property that it reflects a beam back in a direction exactly opposite to the direction of the incident beam.

The modulator 62 can be an acoustic-optical crystal modulator or an electro-optical crystal modulator. In the last-mentioned case, with a single crystal and without polarizer and analyzer, the light beam will be phase modulated with the signal supplied by means of the lead wires 37b. The returning light can be demodulated using the homodyne detection system, as opposed to direct detection.

The embodiment of the invention shown in fig. 2, is similar to that of fig. 1 showed embodiment in all aspects not described above, e.g. the cable construction and the sealing of the chamber 40b by means of the boot seal 7b pressing on the low diffusivity jacket 57b.

In the preferred embodiment of the invention, the modulated laser light coming up through the cable is detected and demodulated by means of a receiver located in the rotating drum of the cable coil. The electrical data signal

must then be transferred to a stationary data processing or recording equipment. If the data transfer frequency is less than approx. 1 MHz, the signal can be transmitted reliably via conventional slip rings on the extended shaft of the cable reel. However, if the frequency or speed is higher, the bit error rate will increase to unacceptable levels such as

due to electrical noise and crosstalk. This can be avoided by repeatedly transmitting the data stream optically to a stationary detector via one or the other of two alternative "optical slip rings" to be described below.

Fig. 5 shows a longitudinal section through the axis of the extended shaft of the cable reel. The design is adapted for the transmission of three separate light beams via "optical slip rings".

In this embodiment of the invention, modulated laser light signals which are transmitted up through three optical fibers in the cable are demodulated in the drum of the cable reel. The resulting electrical signals are routed through coaxial cables 103 in the shaft 100 to light-emitting diodes (LEDs) or laser diodes 102 and 102a. It is obvious that signal light emitted by the light source 102a

on the axis of the rotating shaft 100, is collimated by means of the lens 104a and then focused on the stationary detector 106a. Supporting structures and a coaxial line from the detectors are omitted for clarity. It is not so obvious that light from the two (or more) sources 102 located at different radial distances from the axis will remain focused on the stationary detectors 106 by means of the stacked paraboloidal reflectors 105 as the beams rotate about the axis . Annular sections of lenses could be used to focus the signal light onto the detectors, but the paraboloid reflectors are preferred as they also serve the function of optical and electrical shielding.

In principle, the ends of the optical fibers from the cable could be placed directly at the location of the light sources 102 and 102a instead of regenerating light signals. The effect of the laser light that returns to the surface, however, will be small and will be further attenuated due to the insertion loss of the optical slip rings. This would impose ultra-precise tolerances on all components. Inconveniently space-consuming avalanche photodiodes would also be required instead of very small PIN diodes such as detectors 106 and 106a.

The light source 102a and the detector 106a can be switched to transmit in the opposite direction to provide descent command signals to the probe.

The optical slip ring assembly is protected against it

external environment by means of a stationary housing 107 which fits snugly around a pipe assembly 108 which rotates with the shaft 100.

Figs. 6 and 7 show an alternative "optical slip ring" for the transmission of light signals from the rotating shaft to an adjacent, stationary detector, as Figs. 6 shows a view along the axis of the axle and fig. 7 shows a section along the line 7-7 in fig. 6. A light source 120 is placed on the axis to radiate a modulated light signal in all radial directions into a transparent disc 121 with a small hole in the center of the light source. The disc can be made of an acrylic resin or a similar, transparent plastic material. The light is limited to the disc by means of total internal reflection and makes its way out of the edge of the disc. A stationary transparent plate 122 of approximately the same thickness as the disk is joined along a substantial portion of the disk's circumference except for a clearance to allow the disk 121 to rotate with the shaft. The outer edge of the plate 125 reflects the light to a diode detector 12 3 which is connected to a coaxial line 124. The shape of the plate edge 125 is part of an ellipse with focal points at the light source 120 and at the detector 123. A significant part of all the light emitted by the source , are thus collected at the detector.

The light source can be an edge-emitting LED diode, or a laser diode can be used if the modulated signal frequency is higher than approx. 30 MHz. As a laser diode does not radiate over the entire circle (360°) in the plane of its transition layer, the desired radiation pattern into the disc 121 can be realized by positioning the laser so that it radiates a beam along the axis. The beam is then deflected radially into the disc by means of a conical reflector which is coaxially placed at 120 with its apex directed towards the laser. Alternatively, the laser can be placed in the coil's drum together with its electronic drive unit, and the emitted light signal is guided through a short length of optical fiber through the shaft to the apex of the reflecting cone.

Multiple optical fiber channels (multiple channels) are transmitted via separate optical slip rings (of the type shown in Fig. 6) which are separated along the axis of the extended shaft of the cable reel. Slits 126 are provided in the light limiting disc 121 to allow structural elements and electrical conductors or light conductors to pass through the disc without obscuring more than a small fraction of the light.

Claims (7)

1. Optical communication system for borehole logging, comprising a laser light source (21a; 21b), an armored cable (58) with at least one optical fiber (23a, 28; 23b; 51) for transmitting laser light down through and up through the borehole, a data collecting logging probe connected to the cable at the lower end of the borehole, a device (31; 62) for modulating data signals on the light, and a device (33a, 34a, 38a; 33b, 34b, 38b, 61, 62) at the surface for demodulation of the data signals from the modulated light received at the upper end of the cable (58), CHARACTERIZED IN THAT the laser light source (21a; 21b) is arranged to oscillate at a wavelength greater than 1 pm, that the system comprises a cable head connector jacket (10a; 10b; 10c) which is connected to the cable (58) at its lower end and which comprises an environmentally sealed chamber (40a; 40b; 40c) which accommodates the modulation device (31; 62) and an electrical multi-pin connector (13a; 13b) ; 13c) which is designed to fit together with loggings the probe for electrical connection of this to the modulation device (31; 62), and that the demodulation device comprises a device for transmitting the received light signals to detectors (34a). 2. Optical communication system according to claim 1, CHARACTERIZED IN THAT the laser light source (21a; 21b) is a neodymium laser which is forced to oscillate with a wavelength that is approx. 1.32 p.m. 3. Optical communication system according to claim 1 or 2, wherein the optical fiber (23b) transmitting light up from the probe is the same fiber as that transmitting light down to the probe, CHARACTERIZED IN THAT it comprises a retro-reflector (63) to redirect the light passing through the modulation device ( 62), back into the optical fiber (23b). 4. Optical communication system according to claim 3, CHARACTERIZED IN THAT the optical fiber (23b) is arranged to transmit only two optical modes. 5. Optical communication system according to claim 3 or 4, CHARACTERIZED IN THAT the device for demodulating the data signals from the modulated light is an optical homodyne detection device (60, 61, 33b). 6. Optical communication system according to one of the preceding claims, comprising a coil for winding the armored cable (58), and where the transmission device is capable of transmitting several separate light signals from the end of the coil's rotating shaft (100) to the detectors which are stationary , CHARACTERIZED IN that the transmission device comprises separate sources (102, 102a) for each light signal located in the shaft (100) to emit separate light beams at different radial distances from the axis of the shaft, separate, stationary, concentric, rotationally symmetric devices (105) to focusing the separate light beams to separate locations spaced along the elongated axis of the shaft (100), and separate detectors (106, 106a) located at the separate locations to capture and convert the light signals into electrical signals. 7. Optical communication system according to one of claims 1-5, comprising a coil for winding the armored cable (58), and where the transmission device is capable of transmitting one or more separate light signals from the rotating shaft of the coil to the detectors which are stationary . , concave reflective device (122) having a substantially elliptical cross-sectional shape disposed near the rotating shaft to focus the light signal emanating from the transparent paths to a stationary location, and a detector (123) at the stationary location for conversion of the light signals into electrical signals.