Patent Application
20090067776
This invention concerns optical fibers, and in particular relates to a method of reducing the rate at which the optical properties of an optical fiber degrade in an elevated temperature environment, and to an optical fiber cable with enhanced stability against such degradation.
It is been known for some time that optical fibers can be used as temperature and pressure transducers, and particularly as distributed temperature sensors. Optical fibers may also be used in other sensing systems measuring other parameters, including acoustic and motion measurement systems. These abilities are exploited in a number of technologies, and particularly for making downhole measurements in oil and gas fields or for monitoring temperature in other remote and difficult to access locations including boreholes and tunnels of all kinds.
When used for temperature and pressure monitoring, optical fibers, which are likely to be of glass, based on silica, are typically deployed in metal tubes for protection. The tubes in which they are deployed are commonly referred to in the art as control lines, since they are essentially similar to the tubing used for hydraulic control lines in downhole applications in the oilfield services industry.
In oil and gas fields, fiber optic based distributed temperature measurement systems can extend for up to 30 km. Frequently a control line will include two or more fibers, for different purposes. While it is possible to manufacture control lines with the fibers installed, it is generally preferable to form the tubing of the control line first, and to place the fiber in the tubing afterwards. This is typically achieved by pumping a fluid through the tube, and using the drag effect of the fluid flow to take a fiber off a reel and carry it through the line as far as may be required. For convenience, a return loop may be included at the remote end of a control line so that a fiber can be pumped down a well through one line and back to the surface through a second parallel line in one continuous loop of fiber which after deployment has two ends each accessible at the wellhead.
Control lines are typically deployed in a well taking advantage of support provided by fixed parts of the completion. It is often convenient to clamp the control line to the outside of steel production tubing.
Normally, distributed temperature measurement systems have a long life. They typically operate in oil and gas wells at temperatures up to not more than about 100° C. It has been found that in wells working at elevated temperatures, the performance of the optical fiber may quickly degrade at a significantly accelerated rate, leading to an unacceptable decline in performance and an early requirement to replace the fiber, with considerable inconvenience and interruption to normal well operations. The temperatures in some wells which rely on thermal recovery methods can reach 300° C. Examples of such wells are those using steam assisted gravity drainage (SAGD) for the extraction of bituminous hydrocarbon reserves. Elevated temperatures in the context of this invention may be considered to be temperatures above about 100° C. and typically above about 150° C.
This invention addresses the rate of degradation of optical performance of fibers in elevated temperature environments and seeks to mitigate the problems caused thereby.
According to one aspect of the invention there is provided a method of reducing the rate of optical degradation of an optical fiber deployed within a metal tube in an elevated temperature environment, which comprises electrically insulating a region of the tube that is to contain the fiber and be exposed to elevated temperature, and deploying the fiber in the tube in the elevated temperature environment.
A further aspect of the invention provides an optical fiber cable comprising a bundle of substantially parallel metal tubes, at least one optical fiber deployed within each tube, and electrical insulation covering a substantial length of each tube containing the optical fiber.
In another aspect of the invention an optical fiber cable comprises a metal tube, at least one optical fiber deployed within the tube, electrical insulation covering a substantial length of the tube containing the fiber, and a protective sheath over the electrical insulation.
It will be seen that a common feature of these aspects of the invention is the provision of electrical insulation around the metal tube containing the optical fiber. It is considered that this has an unexpected beneficial effect on the optical performance of the fiber in an elevated temperature environment.
Without intending to be bound by any mechanism or theory, it is believed that insulating the tube carrying the optical fiber prevents or substantially diminishes the opportunities for electrical contact between the tube and other metallic conductors external to the tube. In normal circumstances, this would be of little consequence. But we have found that the effects of elevating temperature can seriously compromise the longevity of the optical fiber, and that electrically insulating the control line from its environment has beneficial consequences in mitigating these deleterious effects.
Control lines used in downhole applications are normally made of corrosion resistant alloy. Various such alloys are used in oilfield installations. They tend to be alloys with high proportions of metals other than iron, including chromium and nickel in particular. While one purpose of the control line is to provide means for deployment of the optical fiber over distances of thousands of meters by transporting it through the internal capillary bore entrained in a flow of pumped fluid, a second purpose is to provide physical protection between the fiber and the aggressive downhole environment. Corrosion resistance is generally beneficial in order to ensure a long working life for the tube once it has been installed in a borehole.
However, the specialist alloys used for these components of the well completion have different compositions from most other metal components in that environment. For example, production of tubing is likely to be made of a regular grade of steel. Metals of different alloy compositions have different electrochemical properties. Accordingly, when placed in electrical contact in a suitable environment, the difference in electrochemical potential between the different metals can be realized as chemical activity. In respect of the optical fibers deployed along a control line, this activity will of course be external to the control line and would not be expected to influence the condition of the fiber internally of the control line.
However, our studies indicate that in elevated temperature environments, electrically conductive contact between control lines and dissimilar metals can result in remarkable and unexpected increases in the generation and diffusion of molecular hydrogen into the interior of the fiber carrying tubes, with consequent deleterious effects, both reversible and irreversible, on the optical properties of the glass. As a result of increased attenuation in the optical fiber, the performance of downhole temperature sensing systems used for monitoring the thermal profile of a well and the temperature of equipment within the well is substantially decreased, affecting reliability and limiting runtimes.
Control lines have incidentally been simply enclosed in the past for mechanical reasons, but not for the purpose of suppressing electrochemical corrosion leading to hydrogen diffusion and optical degradation of optical fibers and elevated temperatures.
In accordance with the invention, the electrical insulation is preferably further protected against hostile environments by means of a protective sheath. This sheath may comprise a fluid resistant sealing layer over the electrical insulation. The sheath may also comprise a mechanical armor, generally on the outermost layer of the fiber optic cable. In a particularly preferred embodiment, the sheath comprises a fluid resistant sealing layer over the electrical insulation and a mechanical armor over the sealing layer.
The invention is illustrated by way of non-limiting example in the accompanying drawings, in which:
In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.
Oil wells using thermal recovery techniques such as steam assisted gravity drainage may use heating applied by means of high temperature steam injected at up to 300° C. In accordance with the invention, degradation of the optical properties in the optical fiber used for temperature monitoring, due to diffusion of hydrogen into the fiber, with consequences ranging from the presence of absorbing peaks due to hydrogen itself to the creation of absorption peaks due to chemical reaction is retarded by electrically isolating the metal control line inside which the optical fibers themselves are deployed. The elevated temperature environments in which the invention is particularly useful are those above 100 or 150° C., and especially those above 200° C.
The optical fibers 22 in the control line 20 are shown as being three in number. Actual numbers will vary, but there will be at least one fiber forming part of a sensing system. Two or three are typical. The fibers are used as extensive transducers for temperature and pressure monitoring by sending pulses of laser light down the optical fiber from a facility at the surface, and detecting the weakly reflected signals and interpreting them to create a distributed temperature log at, typically, one meter separations along the length of the fiber. For these purposes, the optical fibers are typically based on silica, SiO2, with or without various dopants. Among the deleterious irreversible changes brought about by hydrogen in silica fibers, the formation of hydroxide species, especially as silicon hydroxide, may in particular be mitigated by the invention.
The fibers are located within control line 20, which is a tube of corrosion resistant metal alloy. A variety of metal compositions may be used, and the latter will be chosen according to the well conditions that are expected. A long life is required. The alloy may be a stainless steel, or an alloy with a higher proportion of nickel, such as Alloy 825 which contains 38-46 weight percent nickel, 19-24 weight percent chromium, and more than 22 weight percent iron, with minor amounts of molybdenum, copper and titanium. The balance is made up of incidental elements below 1 weight percent each, and impurities.
Typical control line dimensions are an overall outside diameter of a quarter of an inch (6.35 mm), with a wall thickness of 1.25 mm. This provides a long lifetime and physical protection of the optical fiber.
The fiber or fibers can be positioned in the tube during manufacture of the tube, after the tube has been formed, or after insulating the tube, or after protecting the tube, but preferably the optical fiber is deployed in the tube after the tube has been formed.
The next layer of the optical fiber cable is electrical insulation 24. The insulating material should remain effective at the temperatures to which the cable is like to be exposed in the environment in which it is to be put to use. A preferred insulating material is EPDM rubber (ethylene propylene diene monomer rubber). Other insulators that may be suitable include paints, varnishes, and in particular numerous families of polymers including polyimides, fluoropolymers including polytetrafluoroethylene and modified ETFE (ethylene-tetrafluoroethylene) polymers, and polyether ether ketones (PEEK). The electrically insulation layer 24 is coated over the tube 20 using techniques appropriate for the particular nature of the insulator, and will typically be applied by an extrusion process. Insulation thicknesses may vary, but from 0.5 to 0.75 mm may be suitable.
The conditions in which the optical fiber cable is to be put to use will influence the design and application of any subsequent layers. As a practical matter, it can be assumed that the electrical insulation 24 will require protection from the environment. This protection might be required to defend the insulating properties against the aggressive actions of fluids in the environment of an oil production well, for example. EPDM rubber is an effective insulator but vulnerable to the effects of hydrocarbons at conventional and elevated temperatures. Furthermore, the electrical insulation should be able to withstand impact and abrasion and other mechanical assaults within a well bore. The fluid resistant sealing layer 26 and mechanical armor 28 form a combination protective sheath for these purposes.
Fluid resistant layer 26 is formed of an extruded layer of lead or a lead alloy. The particular requirement is for integrity of cover to prevent ingress of environmental fluids, in particular to keep the well fluids separated from the insulation. A wide variety of other materials, including metals and polymers, may be used for this purpose. The thickness of sealing layer 26 may suitably be from 0.2 to 1.5 mm, typically from 0.5 to 0.75 mm.
When the insulation layer 24 is protected by a sealing layer 26, the outer mechanical armor 28 more immediately protects the sealing layer rather than the insulation layer. In the case of the sealing layer 26 being of a soft material, such as lead, it is suitably provided with a hard protective outer covering. In the example shown, this covering is provided by a steel strap 28 wound helically along and around the surface of the lead, and succeeding turns of the helically wound strap overlap preceding turns. The strap is preferably shaped with complementary edge portions which inter-engage so that overlapped edges of the helically wound strap are resistant to mutual separation. The strap is preferably a formed metal tape. Other configurations of armor may be used; for example, a narrow elongate strip of metal sheet may folded lengthwise over the sealing layer 26 and its edges seam welded together. Steel is a suitable material for the outer armor, and a corrosion resistant steel or galvanized steel may be chosen. Alternatively, corrosion resistant alloys including Monel metal may be used. Non-metallic materials such as plastics with suitable thermal and mechanical properties may also be used.
The tube 30 and fiber 32 correspond to the tube 20 and fiber 22 in
It may be convenient to utilize two tubes in a bundle, since this permits the use of a duplex control line extending from a surface termination to a remote location in a bore where a turnaround sub provides a loop connecting the remote ends of the two lines; accordingly, by strapping the two lines together in a bundle with the outer protective strap 38 shown in
Reverting to
The region of elevated temperature may be of varying length. In portions of the fiber optic cable not expected to be operated in an elevated temperature region, electrical isolation is not required, but may optionally be applied.
The lower curve 40 illustrates the type of attenuation spectrum found in the fiber before deployment. The upper curve 44, on the other hand, is illustrative of the attenuation spectrum that can develop after deployment and operation for several months in a high temperature well, such as an SAGD well operated at around 200° C. The attenuation increase is quite severe (an attenuation of 3 dB means that half the power is lost). Also it can be observed that the attenuation increase is non-uniform with wavelength, creating variation in differential loss over the spectrum. This alone can contribute to the loss of calibration for distributed temperature sensor systems if there is no compensation for differential loss. By means of this invention the development of elevated attenuation levels such as those shown at 44 can be delayed very substantially, allowing considerably longer lifetimes and longer service intervals for the fiber optic cables.
From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a novel and unobvious optical fiber cable and system and method for reducing the rate of optical degradation of an optical fiber has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow.
