Downhole temperature sensing

Abstract

Disclosed is a system and method for measuring temperature in a subterranean wellbore wherein the system comprises an elongate optical fibre having at least one bragg reflector etched thereinto at a known location therealong, a light source providing an input light into the elongate optical fibre, an interrogator adapted to receive back reflected light from the at least one bragg reflector and backscattered light from the optical fibre and a processor adapted to receive information from the interrogator.  The method comprises locating the fibre optic strand within the wellbore, transmitting incident light down the fibre optic strand, receiving the reflected wavelengths at a receiver and comparing the reflected wavelength bands against a calibrated profile to determine a temperature at each FBG as well as performing Raman spectrometry on the backscattered light to determine a distributed temperature profile in the fibre optic strand. 

Description

Downhole temperature Sensing BACKGROUND1.Technical FieldThis disclosure relates generally to temperature measuring and in particular to a method and system for measuring the temperature of a subterranean well at a plurality of locations.  2.Description of Related ArtIn many areas of petroleum exploration and production it is necessary to measure and monitor the temperature with a downhole well. In particular, during the production of heavy crude oil or bitumen such as during steam-assisted gravity drainage (SAGD) operations it is necessary to monitor and control the temperature of the reservoir to ensure optimal flow of the petroleum products to the production well. The use of discrete thermocouples is often impractical for such locations as each sensor is on its own disparate line such that an installation requires a capillary loaded with a plurality of separate electric lines. Many of these installations are remote providing a large amount of potential for difficult repairs or calibration activities. Furthermore, such probes commonly fall out of calibration and generally when analyzing temperature data, it is the change across the well that is most important and lacking accurate temperatures can lead to uncertainty when analyzing reservoir conditions. Current methods of measuring the temperature within the wellbore and therefore the surrounding reservoir have been limited in their speed, accuracy and detail. In particular, a common method of measuring downhole temperature comprise distributed temperature scanning (DTS) 200 as illustrated in Figure 1 which utilizes a light source 202 and an optical fibre 204 to measure of the backscattered light 208 along the fiber 204 to determine temperature changes along the length. Such backscattering shifts in relative intensity are known in the art as raman anti-stokes and raman stokes signals, the interpretations of which are known for determining temperature along the fibre. Disadvantageously, such DTS systems may be subject to drift over time of the accuracy of conventional DTS systems due to cyclical heating and cooling of the system along with other factors. Additionally, such DTS may be subject to a reduced signal to noise ratio under the influence of hydrogen darkening.  SUMMARY OF THE DISCLOSUREAccording to a first embodiment of the present disclosure, there is disclosed a system for measuring temperature in a subterranean wellbore comprising an elongate optical fibre having at least one bragg reflector etched thereinto at a known location therealong, a light source providing an input light into the elongate optical fibre, an interrogator adapted to receive back reflected light from the at least one bragg reflector and backscattered light from the optical fibre and a processor adapted to receive information from the interrogator and determine a temperature at the at least one bragg reflector.  The at least one bragg reflector may comprise a plurality of brag reflectors, each at a unique known location along the optical fibre. Each of the plurality of bragg reflectors may have a uniquely tuned bragg wavelength. Each bragg reflector may be tuned under known conditions outside of the wellbore.  The light source may be operable to provide a broad spectrum light. The input light may be selected to have a frequency range between 1528 and 1568 nm. The input light may be provided in pulses.  According to a further embodiment of the present disclosure, there is disclosed a method for measuring a temperature at a plurality of locations along a wellbore comprising locating a fibre optic strand within the wellbore transmitting incident light down the fibre optic strand, reflecting wavelengths at at least one location along the fibre optic strand, receiving the reflected wavelengths at a receiver and comparing the reflected wavelength bands against a calibrated profile to determine a temperature at each FBG as well as performing Raman spectrometry on the backscattered light to determine a distributed temperature profile in the fibre optic strand.  The fibre optic strand may comprise a single fibre optic strand. The incident light may be pulsed. The incident light may comprise a broad spectrum light. The incident light may be selected to have a frequency spectrum selected between 1528 and 1568 nm. The at least one location may comprise a plurality of locations. The plurality of locations may include bragg reflectors.  The processor may be configured to determine a best fit line for each of the plurality of bragg reflectors and the backscattered light. The processor may be configured to compare the best fit lines for the plurality of bragg reflectors and backscattered light. The processor may be configured to perform an analysis to find a corresponding best fit line between the plurality of bragg reflectors and backscattered light.  Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings constitute part of the disclosure. Each drawing illustrates exemplary aspects wherein similar characters of reference denote corresponding parts in each view,Figure 1 is an illustration of a prior art system for distributed temperature sensing. Figure 2 is an illustration of a downhole wellbore having a system for sensing the temperature therealong according to a first embodiment of the present disclosure. Figure 3 is an illustration of the system for sensing the temperature within the well bore of Figure 2. Figure 4is a schematic of the system of Figure 3. Figure 5is an illustration of raman stokes and anti-stokes return wavelength as utilized in the system of Figure 2.  DETAILED DESCRIPTIONAspects of the present disclosure are now described with reference to exemplary apparatuses, methods and systems. Referring to Figure 2, a wellbore 10 is drilled into the ground 8 to a production zone 6 by known methods. The production zone 6 may contain a horizontally extending hydrocarbon bearing rock formation or may span a plurality of hydrocarbon bearing rock formations such that the wellbore 10 has a path designed to cross or intersect each formation. As illustrated in Figure 2, the wellbore may include a vertical section 12 and a bottom or production section 14 which may be horizontal or angularly oriented relative to the horizontal located within the production zone 6. The wellbore may optionally include production tubing 16 or any other tool string therein formed of a plurality of alternating liner or casing. The wellbore 10 further includes an elongate single mode optical fibre strand 20 extending therethrough to the production zone 6. The optical fibre is configured to measure temperature profile along the wellbore as provided to and interpreted by a signal processing unit 32 through a combination of distributed temperature scanning and fibre bragg grating interrogation as will be more fully described below. In some embodiments, a steam injection 18 or other heating conduit may be provided to the production zone to assist with recovery. In the present method, the rate of heating or steam injection may be controlled or influenced by the sensed temperature within the wellbore 10.  Referring to Figure 3, the optical fibre strand includes a plurality of sets of fibre bragg grating reflectors 22, 24, 26 and 28 distributed along the length of the optical fibre strand within the production zone 6 of Figure 2. Although four fibre bragg grating reflectors are illustrated in Figure 3, it will be appreciated that more or less may be utilized depending on the length of the optical fibre strand within the production strand as well as the level of detail desired for the temperature profile therealong. The fibre bragg grating reflectors may be selected to be of any type and may be formed by any known method within the optical fibre strand. Each of the fibre bragg grating reflectors 22, 24, 26 and 28 are selected to have a unique bragg wavelength.  As illustrated in Figure 3, the system includes an optical light source 30 which may comprise a broad band laser source selected to provide a source of incident light encompassing all of the fibre bragg wavelengths. By way of non-limiting example the optical light source may be selected to be a broad band or tuneable laser source although it will be appreciated that other sources capable of providing a known spectrum of wavelength to the fibre 20 may also be utilized. By ways of non-limiting example it has been found that a light source operable to output a wavelength between 1528 and 1568 nm may be useful, although it will be appreciated that other ranges may also be useful.  In particular, each of the fibre bragg grating reflectors may be tuned to reflect a particular wavelength of light within the range provided by the light source. It is known that such reflected wavelength is referred to as the bragg wavelength. It is further known that the bragg wavelength is influenced primarily by the physical strain experienced by the fibre 20 and secondly by the temperature encountered by the fibre and the resulting strain on the fibre due to such temperature change. It will therefore be appreciated that the optical fibre strand may advantageously be installed in a relaxed state to prevent physical strain on the fibre bragg grating reflectors. By being tuned to a unique light wavelength, each FBG may be adapted to transmit a unique temperature value as measured by that sensor so as to indicate to the signal processor 32 thereby providing a more detailed temperature distribution along the fibre. It will be appreciated that FBG may be tuned to a wavelength sufficiently distinct from the other gates so as to prevent overlap. By way of non-limiting example it has been found that tuning each FBG to approximately 4 nm from each other has been found to be useful.  The general makeup of an FBG sensor is a single mode optical fibre that has been inscribed holographically with a series of gratings which act as an interferometer. These inscribed gratings are written to a single mode fiber as an array in periodic or semi-periodic positions. When the fibre is optically scanned, each FBG returns a specific wavelength, which are further affected by induced strain caused by temperature perturbations surrounding the sensor with each degree shifts the signal by approximately 10pm. Unlike DTS where specific wavelengths are pulsed and raman backscattering is averaged over time to used to estimate temperature, FBG arrays are scanned across a broad spectrum to acquire instantaneous data. Under such conventional systems, the use of both DTS and FBG measurements would require at least two separate optical fibres or performing the DTS and FBG measurements separately so as to not permit the signals from one to interfere with the measurements of the other.  The system further includes a sensor 32 selected to receive the plurality of reflected wavelengths from each of the fibre bragg reflectors 22, 24, 26 and 28. In operation, the Bragg wavelength at each of the fibre bragg reflectors 22, 24, 26 and 28 may be calibrated or tuned at the surface under known conditions. Thereafter upon insertion into the wellbore, thermal variations within the wellbore will cause a slight shift in the Bragg wavelength for each of the fibre bragg reflector. The amount of each shifted wavelength will be utilized by the processor to determine the temperature at each optical bragg reflector corresponding to the tuned wavelength for the bragg reflector 22, 24, 26 or 28 and therefore the temperature profile therealong as set out below as represented by those bragg reflectors. Furthermore, the sensor 32 may be configured to measure both the reflected FBG wavelengths as well as the Raman backscattering within a single fiber. By way of non-limiting example, a line of best fit may be computed for both the FBG and DTS data every time they are sampled. The FBG line of best fit herein referred to as the control line and the DTS line of best fit herein referred to as the sample line are then compared against each other to determine the difference in slope and offset (in reference to a simple linear regression formula y = mx + b with m being the slope and b being the offset). The processor 40 may modify the DTS parameters after each sample is taken to achieve a match between the slopes and intercepts of the control line and sample line that falls within pre-configured tolerances. If a match is achieved, this iterative optimization process will halt until such time as the control and sample lines differentials are no longer within pre-configured tolerances. A subset of the DTS data is taken where points are within a predetermined physical proximity to each FBG and are then used in calculating the sample line. DTS samples are also adjusted to accommodate losses introduced by each FBG sensor before being used in temperature and subsequent sample line calculations. In particular, the DTS loss compensation may makes use of gaussian smoothing then a FIR (finite impulse response) filter may be used to partition the sample after which each segment is adjusted (slope, offset) so that a re-constituted curve can be provided to the DTS interrogator for temperature calculation. Optionally, an exponential decay curve may be used.  Turning now to Figure 4, the system comprises the processor 40, and memory 42 that stores machine instructions that, when executed by the processor 40, cause the processor 40 to perform one or more of the operations and methods described herein. The processor 40 may optionally contain a cache memory unit for temporary local storage of instructions, data, or computer addresses. The system further includes a database 440 or data storage of any conventional type operable to store information as set out above and may optionally include an input device 46, display other output device 48 for receiving and displaying inputs from an operator or user and an optional network interface 50, such as by way of non-limiting example wired, wireless or cellular networks as are commonly known. As outlined above, the processor 40 is adapted to receive sensed intensity from the sensor from various wavelengths output from the optical source and to determine the temperature profile along the optical fibre strand.  More generally, in this specification, the term “processor” is intended to broadly encompass any type of device or combination of devices capable of performing the functions described herein, including (without limitation) other types of microprocessors, microcontrollers, other integrated circuits, other types of circuits or combinations of circuits, logic gates or gate arrays, or programmable devices of any sort, for example, either alone or in combination with other such devices located at the same location or remotely from each other. Additional types of processor(s) will be apparent to those ordinarily skilled in the art upon review of this specification, and substitution of any such other types of processor(s) is considered not to depart from the scope of the present invention as defined herein. In various embodiments, the processor 40 can be implemented as a single-chip, multiple chips and / or other electrical components including one or more integrated circuits and printed circuit boards. Computer code comprising instructions for the processor(s) to carry out the various embodiments, aspects, features, etc. of the present disclosure may reside in the memory 42. The code may be broken into separate routines, products, etc. to carry forth specific steps disclosed herein. In various embodiments, the processor 40 can be implemented as a single-chip, multiple chips and / or other electrical components including one or more integrated circuits and printed circuit boards. The processor 40 together with a suitable operating system may operate to execute instructions in the form of computer code and produce and use data. By way of example and not by way of limitation, the operating system may be Windows-based, Mac-based, or Unix or Linux-based, among other suitable operating systems. Operating systems are generally well known and will not be described in further detail here. Memory 42 may include various tangible, non-transitory computer-readable media including Read-Only Memory (ROM) and / or Random-Access Memory (RAM). As is well known in the art, ROM acts to transfer data and instructions uni-directionally to the processor 40, and RAM is used typically to transfer data and instructions in a bi-directional manner. In the various embodiments disclosed herein, RAM includes computer program instructions that when executed by the processor 40 cause the processor 40 to execute the program instructions described in greater detail below. More generally, the term “memory” as used herein encompasses one or more storage mediums and generally provides a place to store computer code (e.g., software and / or firmware) and data. It may comprise, for example, electronic, optical, magnetic, or any other storage or transmission device capable of providing the processor 40 with program instructions. Memory 42 may further include a floppy disk, CD-ROM, DVD, magnetic disk, memory chip, ASIC, FPGA, EEPROM, EPROM, flash memory, optical media, or any other suitable memory from which processor 14 can read instructions in computer programming languages. While specific embodiments have been described and illustrated, such embodiments should be considered illustrative only and not as limiting the disclosure as construed in accordance with the accompanying claims.What is claimed is:1. A system for measuring temperature in a subterranean wellbore comprising:  an elongate optical fibre having at least one bragg reflector etched thereinto at a known location therealong;  a light source providing an input light into the elongate optical fibre;  an interrogator adapted to receive back reflected light from the at least one bragg reflector and backscattered light from the optical fibre; and  a processor adapted to receive information from the interrogator and determine a temperature at the at least one bragg reflector.  2. The system of claim 1 wherein the at least one bragg reflector comprises a plurality of brag reflectors, each at a unique known location along the optical fibre.  3. The system of claim 2 wherein each of the plurality of bragg reflectors has a uniquely tuned bragg wavelength.  4. The system of claim 3 wherein each bragg reflector is tuned under known conditions outside of the wellbore.  5. The system of claim 1 wherein the light source is operable to provide a broad spectrum light .  6. The system of claim 2 wherein the input light is selected to have a frequency range between 1528 and 1568 nm.  7. The system of claim 1 wherein the input light is provided in pulses.  8. A method for measuring a temperature at a plurality of locations along a wellbore comprising:  locating a fibre optic strand within the wellbore;  transmitting incident light down the fibre optic strand;  reflecting wavelengths at at least one location along the fibre optic strand;  receiving the reflected wavelengths at a receiver;  comparing the reflected wavelength bands against a calibrated profile to determine a temperature at each FBG as well as performing Raman spectrometry on the backscattered light to determine a distributed temperature profile in the fibre optic strand.  9. The method of claim 8 wherein the fibre optic strand comprises a single fibre optic strand.  10. The method of claim 8 wherein the incident light is pulsed.  11. The method of claim 8 wherein the incident light comprises a broad spectrum light.  12. The method of claim 11 wherein the incident light is selected to have a frequency spectrum selected between 1528 and 1568 nm.  13. The method of claim 8 wherein the at least one location comprise a plurality of locations.  14. The method of claim 13 wherein the plurality of locations include bragg reflectors.  15. The method of claim 8 wherein the processor is configured to determine a best fit line for each of the plurality of bragg reflectors and the backscattered light.  16. The method of claim 15 wherein the processor is configured to compare the best fit lines for the plurality of bragg reflectors and backscattered light.  17. The method of claim 16 wherein the processor is configured to perform an analysis to find a corresponding best fit line between the plurality of bragg reflectors and backscattered light.    ABSTRACT Disclosed is a system and method for measuring temperature in a subterranean wellbore wherein the system comprises an elongate optical fibre having at least one bragg reflector etched thereinto at a known location therealong, a light source providing an input light into the elongate optical fibre, an interrogator adapted to receive back reflected light from the at least one bragg reflector and backscattered light from the optical fibre and a processor adapted to receive information from the interrogator. The method comprises locating the fibre optic strand within the wellbore, transmitting incident light down the fibre optic strand, receiving the reflected wavelengths at a receiver and comparing the reflected wavelength bands against a calibrated profile to determine a temperature at each FBG as well as performing Raman spectrometry on the backscattered light to determine a distributed temperature profile in the fibre optic strand.  

Claims

1. A system for measuring temperature in a subterranean wellbore comprising:  an elongate optical fibre having at least one bragg reflector etched thereinto at a known location therealong;  a light source providing an input light into the elongate optical fibre;  an interrogator adapted to receive back reflected light from the at least one bragg reflector and backscattered light from the optical fibre; and  a processor adapted to receive information from the interrogator and determine a temperature at the at least one bragg reflector.  2. The system of claim 1 wherein the at least one bragg reflector comprises a plurality of brag reflectors, each at a unique known location along the optical fibre.  3. The system of claim 2 wherein each of the plurality of bragg reflectors has a uniquely tuned bragg wavelength.  4. The system of claim 3 wherein each bragg reflector is tuned under known conditions outside of the wellbore.  5. The system of claim 1 wherein the light source is operable to provide a broad spectrum light .  6. The system of claim 2 wherein the input light is selected to have a frequency range between 1528 and 1568 nm.  7. The system of claim 1 wherein the input light is provided in pulses.  8. A method for measuring a temperature at a plurality of locations along a wellbore comprising:  locating a fibre optic strand within the wellbore;  transmitting incident light down the fibre optic strand;  reflecting wavelengths at at least one location along the fibre optic strand;  receiving the reflected wavelengths at a receiver;  comparing the reflected wavelength bands against a calibrated profile to determine a temperature at each FBG as well as performing Raman spectrometry on the backscattered light to determine a distributed temperature profile in the fibre optic strand.  9. The method of claim 8 wherein the fibre optic strand comprises a single fibre optic strand.  10. The method of claim 8 wherein the incident light is pulsed.  11. The method of claim 8 wherein the incident light comprises a broad spectrum light.  12. The method of claim 11 wherein the incident light is selected to have a frequency spectrum selected between 1528 and 1568 nm.  13. The method of claim 8 wherein the at least one location comprise a plurality of locations.  14. The method of claim 13 wherein the plurality of locations include bragg reflectors.  15. The method of claim 8 wherein the processor is configured to determine a best fit line for each of the plurality of bragg reflectors and the backscattered light.  16. The method of claim 15 wherein the processor is configured to compare the best fit lines for the plurality of bragg reflectors and backscattered light.  17. The method of claim 16 wherein the processor is configured to perform an analysis to find a corresponding best fit line between the plurality of bragg reflectors and backscattered light.    

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