CN115324566A - Underground distributed temperature measurement system and method based on weak reflection fiber bragg grating - Google Patents
Underground distributed temperature measurement system and method based on weak reflection fiber bragg grating Download PDFInfo
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Abstract
The invention provides a system and a method for measuring underground distributed temperature based on a weak reflection fiber grating. The invention meets the spectrum range requirement of the single fiber grating string reflected light signal with more than 100 points, and can ensure that the return signals between two adjacent gratings do not overlap in time, thereby establishing a distributed fiber grating temperature sensing network and obtaining each temperature information of the grating connected in series with more than 100 points; the two wavelength gratings are alternately connected in series at equal intervals, so that the limitation of the bandwidth of a light source on the number of sensing nodes is eliminated, the limitation of the spectral shadow effect on the number of the sensing nodes is also eliminated, and a distributed fiber grating temperature sensing network is completely established.
Description
Technical Field
The invention belongs to the technical field of oil, gas and water well logging, and particularly relates to a weak reflection fiber grating-based underground distributed temperature measurement system and method.
Background
In recent years, the optical fiber temperature sensing technology has the technical advantages of high accuracy, easy formation of distributed multipoint temperature measurement, long signal transmission distance, stable performance and the like, and is widely regarded in the petroleum logging technology. At present, a plurality of Fiber Bragg Gratings (FBGs) are connected on one optical fiber in series, so that a quasi-distributed temperature sensor is formed. The distributed temperature sensor has the advantages of high measurement spatial resolution, high measurement accuracy, stable and reliable performance, long service life and the like.
However, most of the existing FBG distributed sensors are formed by strong reflection FBGs, which has the advantages of simple signal demodulation and accurate, stable and reliable measurement. The disadvantage is that the number of sensing nodes formed on one optical fiber is very limited due to the limitation of the bandwidth of the light source, so that a large-scale distributed temperature measurement network is difficult to form. This greatly limits its application in the oil logging industry.
Disclosure of Invention
The invention aims to provide a downhole distributed temperature measurement system based on a weak reflection fiber grating, and the downhole distributed temperature measurement system based on the weak reflection fiber grating overcomes the technical problems in the prior art.
The invention also aims to provide a downhole distributed temperature measurement method based on the weak reflection fiber bragg grating, and the whole borehole temperature measurement accuracy is high.
Therefore, the technical scheme provided by the invention is as follows:
a downhole distributed temperature measurement system based on a weak reflection fiber grating comprises a broadband light source, an optical isolator, an optical circulator, a multi-point weak reflection fiber grating string, a wavelength division multiplexer, an optical coupler, a slope filter, a photoelectric detector, a collection card and an upper computer, wherein the multi-point weak reflection fiber grating string is an optical fiber formed by alternately connecting two kinds of gratings with reflection center wavelengths in series at equal intervals;
the broadband light source is characterized in that pulsed light emitted by the broadband light source passes through the optical circulator in a single direction and reaches the multipoint weak reflection fiber grating string, central wavelength signals reflected by the multipoint weak reflection fiber grating string are output to the wavelength division multiplexer through the other end of the optical circulator, the central wavelength signals are divided into two paths through the wavelength division multiplexer and are divided into two paths through the corresponding optical couplers, one path reaches the photoelectric detector through the slope filter, the other path directly reaches the photoelectric detector, the optical signals are converted into electric signals through the photoelectric detector and are output to the acquisition card and the upper computer, data processing and analysis are completed on the upper computer, the ambient temperature where the measuring point is located is calculated and displayed.
The broadband light source has the wavelength range of 1525nm-1625nm and the pulse width of 5-8ns.
The gratings with two reflection center wavelengths in the multipoint weak reflection fiber grating string are alternately arranged, the number of the gratings is more than 100, the interval between weak reflection fiber gratings with the same wavelength is 1m, the interval between adjacent gratings is 0.5m, and the reflectivity is consistent and is 1 percent.
The slope filter is a strong coupling multi-core optical fiber, and seven fiber cores are contained in the same cladding.
A downhole distributed temperature measurement method based on weak reflection fiber bragg gratings comprises the following steps:
step 1) connecting the set optical pulse into an optical isolator for integration to obtain an optical pulse signal with a certain bandwidth;
step 2) the optical pulse signal enters a multipoint weak reflection fiber grating string through one end of an optical circulator to obtain a plurality of specific optical signals with different time sequences and different positions, which are weakly reflected by the grating string;
step 3) the specific optical signal enters a wavelength division multiplexer through the other end of the optical circulator and is divided into two paths according to the difference of the wavelength of the specific optical signal, and two paths of optical signals are obtained;
step 4), introducing the two optical signals into the same reference optical path, and simultaneously obtaining the demodulated optical signals by adopting a slope filter;
and 5) establishing a relation between the reference light path and the demodulated optical signal to obtain an accurate temperature value of each grating position.
The specific process of step 2) is as follows:
the multipoint weak reflection fiber grating string is arranged at intervals by weak reflection fiber gratings with two wavelengths, the interval between every two adjacent weak reflection fiber gratings is the same, the time interval returned by the two adjacent weak reflection fiber gratings with the same wavelength is calculated, each grating on the grating string sequentially reflects specific optical signals with corresponding different time sequences according to 1% of reflectivity, and each specific optical signal carries temperature information of a corresponding position.
The specific process of step 4) is as follows:
the pulse light signals reflected by each weak reflection fiber grating are divided into two light paths through a point coupler, one light path is provided with a slope filter, light intensity signals are obtained, and the other light path directly obtains wavelength signals.
The specific process of step 5) is as follows:
one optical path is provided with a slope filter, a wavelength signal is converted into a light intensity signal, the light intensity signal passes through a photoelectric detector to obtain a voltage U1, the other optical path passes the wavelength signal through the photoelectric detector to obtain a voltage U2, the U2 is used as a reference signal to suppress system noise, and accurate information of the measured temperature is obtained by establishing a relation between the U1/U2 and the reflection wavelength of the weak reflection fiber bragg grating;
wherein U1= f' (λ) × Pi × Δ λ, U2= Pi
f (lambda) is a function of the output power of the slope filter along with the change of the wavelength, f' (lambda) is the slope, pi is the power of the input coupler, and delta lambda is the change quantity of the reflection wavelength of the weak reflection fiber grating under the change of the temperature;
Δλ＝1/f'(λ)×(U1/U2)
f' (lambda) is a known quantity, and delta lambda can be obtained by measuring U1/U2, so that the measured temperature information can be obtained.
The invention has the beneficial effects that:
the underground distributed temperature measurement system based on the weak reflection fiber bragg grating selects a high-power broadband pulse light source with the wavelength range of 1525nm-1625nm and the pulse width of 5-8ns, meets the spectral range requirement of a single fiber bragg grating string reflected light signal with more than 100 points, can ensure that return signals between two adjacent gratings do not overlap in time, thereby establishing a distributed fiber bragg grating temperature sensing network, obtaining each temperature information of the series gratings with more than 100 points, and the highest spatial resolution can reach 0.2m; the multipoint weak reflection fiber grating string is simultaneously connected with two wavelength gratings in series alternately at equal intervals, so that the limitation of the bandwidth of a light source to the number of sensing nodes is eliminated, the limitation of the spectral shadow effect to the number of the sensing nodes is also eliminated, and a distributed fiber grating temperature sensing network is completely established.
After the double-wavelength optical signals are separated by the wavelength division multiplexer, an optical path is formed respectively, the double wavelengths are transmitted without interference, and the synchronous measurement of the temperature of each grating is achieved. The introduction of the double-optical-path detection method effectively inhibits the noise of the common optical path, improves the signal-to-noise ratio of the system and further improves the demodulation precision. The requirement of underground full-wellbore temperature measurement is met.
The method can simultaneously extract and analyze the sensing signals of the multipoint weak reflection grating temperature sensor on one optical fiber, thereby accurately obtaining the fixed-point temperature value of the whole underground shaft. The temperature measurement range is-20-250 ℃, the precision is +/-0.1 ℃, the spatial resolution of the sensor is 0.5m, and more than 100 single optical fibers are connected with the gratings in series. The temperature value of each temperature measuring point is not interfered by a light source, a light path and the like, and the accuracy of the temperature value is far higher than that of the existing system.
This will be described in further detail below with reference to the accompanying drawings.
Drawings
FIG. 1 is a schematic diagram of the present invention;
fig. 2 is a noise suppression optical path diagram of the present invention.
1. A broadband light source; 2. an optical isolator; 3. an optical circulator; 4. a multipoint weak reflection fiber grating string; 5. a wavelength division multiplexer; 6. an optical coupler; 7. a ramp filter; 8. a photodetector; 9. a collection card and an upper computer.
Detailed Description
The following description of the embodiments of the present invention is provided for illustrative purposes, and other advantages and effects of the present invention will become apparent to those skilled in the art from the present disclosure.
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein, which are provided for complete and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, the same units/elements are denoted by the same reference numerals.
Unless otherwise defined, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Further, it will be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
Example 1:
the embodiment provides a downhole distributed temperature measurement system based on a weak reflection fiber grating, which comprises a broadband light source 1, an optical isolator 2, an optical circulator 3, a multipoint weak reflection fiber grating string 4, a wavelength division multiplexer 5, an optical coupler 6, a slope filter 7, a photoelectric detector 8, an acquisition card and an upper computer 9, wherein the multipoint weak reflection fiber grating string 4 is an optical fiber formed by alternately connecting two gratings with reflection center wavelengths in series at equal intervals;
pulse light emitted by the broadband light source 1 passes through the optical isolator 2 and then passes through the optical circulator 3 in a single direction to reach the multipoint weak reflection fiber grating string 4, a central wavelength signal reflected by the multipoint weak reflection fiber grating string 4 passes through the other end of the optical circulator 3 to be output to the wavelength division multiplexer 5, the central wavelength signal is divided into two paths by the wavelength division multiplexer 5 and is divided into two paths by the corresponding optical coupler 6, one path passes through the slope filter 7 to reach the photoelectric detector 8, the other path directly reaches the photoelectric detector 8, the optical signal is converted into an electric signal by the photoelectric detector 8 and is output to the acquisition card and the upper computer 9, data processing and analysis are completed on the upper computer, the ambient temperature of a measuring point is calculated and displayed.
According to the underground distributed temperature measurement system based on the weak reflection fiber bragg grating provided by the embodiment, the broadband light source 1 provides light signals for the grating distributed temperature sensor network; the optical isolator 2 is used for avoiding the damage of stray light and reverse injection light to a light source and the interference of optical signals; the optical circulator 3 is used for constructing optical paths, so that each path of light is led to pass through a respective channel; the multipoint weak reflection fiber grating string 4: the number of the weak reflection fiber gratings with two wavelengths is more than 100 on a single optical fiber in series; the wavelength division multiplexer 5 is an optical filter, and separates light reflected by the two wavelength gratings to realize wavelength division multiplexing of the system; the optical coupler 6 is a one-to-two coupler and divides an optical signal into two paths, one path is transmitted to the ramp filter 7 and then to the photoelectric detector 8, and the other path is directly transmitted to the other photoelectric detector 8; the slope filter 7 converts the wavelength information of the reflected light into optical power information, so that wavelength scanning is avoided, and the demodulation speed is increased. The photodetector 8: and combining a conditioning circuit, and supplying the output voltage signal which is in direct proportion to the optical power and meets a certain range to a subsequent sampling processing system. Acquisition card and host computer 9: the output voltage signal is collected, the upper computer performs data communication with the upper computer through a USB interface, data processing and analysis are completed on the upper computer, the ambient temperature of the measuring point is calculated, and the ambient temperature is displayed and stored.
Example 2:
on the basis of embodiment 1, the present embodiment provides a downhole distributed temperature measurement system based on a weak reflection fiber grating, where the wavelength range of the broadband light source 1 is 1525nm to 1625nm, and the pulse width is 5 ns to 8ns.
The broadband optical signal emitted by the broadband light source 1 can meet the requirement of the spectral range of the single optical fiber grating string reflected optical signal with more than 100 points, so that the requirement of underground whole shaft temperature measurement is met.
Example 3:
on the basis of embodiment 1, this embodiment provides a downhole distributed temperature measurement system based on a weak reflection fiber grating, where two gratings with reflection center wavelengths in the multi-point weak reflection fiber grating string 4 are alternately arranged, the number of the gratings is more than 100, the distance between the same-wavelength weak reflection fiber gratings is 1m, the distance between adjacent gratings is 0.5m, and the reflectivities are consistent and are all 1%.
The multipoint weak reflection fiber grating string 4 is an optical fiber with two gratings reflecting central wavelengths alternately connected in series at equal intervals, the total length is within 10 kilometers, and the two gratings reflecting central wavelengths alternately connected in series at equal intervals, so that the establishment of a distributed fiber grating temperature sensing network is realized.
Example 4:
on the basis of embodiment 1, the present embodiment provides a downhole distributed temperature measurement system based on a weak reflection fiber grating, where the slope filter 7 is a strongly-coupled multi-core fiber, and seven cores are included in the same cladding.
As shown in fig. 1, one path of the optical coupler 6 is connected to the ramp filter 7 and the photodetector 8 in sequence, and the other path is directly connected to the photodetector 8. The reflection wavelength of the weak reflection fiber grating is converted into light intensity information through the slope filter 7 for fast demodulation.
Example 5:
the embodiment provides a downhole distributed temperature measurement method based on weak reflection fiber bragg grating, which comprises the following steps:
step 1) connecting the set optical pulse into an optical isolator 2 for integration to obtain an optical pulse signal with a certain bandwidth;
step 2) the optical pulse signal enters a multipoint weak reflection optical fiber grating string 4 through one end of an optical circulator 3 to obtain a plurality of specific optical signals with different time sequences and different positions, which are weakly reflected by the grating string;
step 3) the specific optical signal enters a wavelength division multiplexer 5 through the other end of the optical circulator 3, and is divided into two paths according to the difference of the wavelength of the specific optical signal to obtain two paths of optical signals;
step 4), introducing the two optical signals into the same reference optical path, and simultaneously obtaining a demodulated optical signal by adopting a slope filter 7;
and 5) establishing a relation between the reference light path and the demodulated optical signal to obtain an accurate temperature value of each grating position.
The method comprises the steps that pulse light emitted by a broadband light source 1 passes through an optical isolator 2, then passes through an optical circulator 3 in a single direction, reaches a multipoint weak reflection fiber grating string 4, double-wavelength optical signals reflected by the multipoint weak reflection fiber grating string 4 are output to a wavelength division multiplexer 5 through the other end of the optical circulator 3, and are separated through the wavelength division multiplexer 5 to form optical paths respectively, the double wavelengths are transmitted without interference, and the synchronous measurement of the temperature of each grating is achieved. The dual wavelengths respectively travel a light path, each light path is divided into two paths, one path passes through the slope filter 7, the other path does not pass through the slope filter 7 and serves as a reference light path, and each slope filter 7 converts the reflection wavelength of the weak reflection fiber grating into light intensity information for fast demodulation. The demodulated optical signal obtained by the slope filter 7 is subjected to photoelectric conversion and then output voltage U1, the wavelength signal of the reference optical path is subjected to photoelectric conversion and output voltage U2, and the relationship between the reference optical path and the demodulated optical signal is established to obtain the accurate temperature value of each grating position.
Example 6:
on the basis of embodiment 5, the embodiment provides a method for measuring downhole distributed temperature based on weak reflection fiber bragg grating, and the specific process of step 2) is as follows:
the multipoint weak reflection fiber grating string 4 is arranged at intervals by weak reflection fiber gratings with two wavelengths, the interval between two adjacent weak reflection fiber gratings with the same wavelength is calculated, the time interval returned by the two adjacent weak reflection fiber gratings with the same wavelength is calculated, each grating on the grating string sequentially reflects specific optical signals with corresponding different time sequences according to 1% of reflectivity, and each specific optical signal carries temperature information of a corresponding position.
The dual-wavelength optical fiber grating is alternately arranged, so that the limitation of the light source bandwidth on the number of the sensing nodes is eliminated, the limitation of the spectrum shadow effect on the number of the sensing nodes is also eliminated, and a distributed optical fiber grating temperature sensing network is completely established.
Example 7:
on the basis of embodiment 5, the embodiment provides a method for measuring downhole distributed temperature based on weak reflection fiber bragg grating, and the specific process of step 4) is as follows:
the pulse light signals reflected by each weak reflection fiber grating are divided into two light paths through a point coupler, one light path is provided with a slope filter 7, light intensity signals are obtained, and the other light path directly obtains wavelength signals.
The dual wavelengths respectively travel one light path, each light path is divided into two paths by the coupler, one path passes through the slope filter 7, the other path does not pass through the slope filter 7, and each slope filter 7 realizes that the reflection wavelength of the weak reflection fiber bragg grating is converted into light intensity information for fast demodulation.
Example 8:
on the basis of embodiment 5, the embodiment provides a method for measuring downhole distributed temperature based on weak reflection fiber bragg grating, and the specific process of step 5) is as follows:
one optical path is provided with a slope filter 7, the wavelength signal is converted into a light intensity signal, the light intensity signal obtains a voltage U1 through a photoelectric detector 8, the other optical path obtains a voltage U2 through the photoelectric detector 8, the U2 is used as a reference signal to inhibit system noise, and accurate information of the measured temperature is obtained by establishing the relationship between the U1/U2 and the reflection wavelength of the weak reflection fiber bragg grating; as shown in fig. 2;
wherein U1= f' (λ) × Pi × Δ λ, U2= Pi
f (lambda) is a function of the output power of the slope filter 7 along with the change of the wavelength, f' (lambda) is the slope, pi is the power of the input coupler, and delta lambda is the change quantity of the reflection wavelength of the weak reflection fiber grating under the change of the temperature;
Δλ＝1/f'(λ)×(U1/U2)
f' (lambda) is a known quantity, and delta lambda can be obtained by measuring U1/U2, thereby obtaining measured temperature information.
Because the two paths of light pass through the same light path before entering the electric coupler, the light intensity change caused by the light source power fluctuation, the environmental noise interference and the like is completely synchronous with the two paths of light signals. Namely, the system optical path noise carried by the U1 and the U1 is completely consistent. Therefore, the system noise can be corrected in real time by taking U2 as a reference signal, and accurate information of the measured temperature can be acquired in real time by establishing the relationship between U1/U2 and the reflection wavelength of the weak reflection fiber grating.
The above examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention, which is intended to be covered by the claims.
Claims (8)
1. The utility model provides a based on distributed temperature measurement system in weak reflection fiber grating pit which characterized in that: the multi-point weak reflection fiber grating string is an optical fiber formed by alternately connecting two gratings with reflection center wavelengths in series at equal intervals;
the broadband light source is characterized in that pulsed light emitted by the broadband light source passes through the optical circulator in a single direction and reaches the multipoint weak reflection fiber grating string, central wavelength signals reflected by the multipoint weak reflection fiber grating string are output to the wavelength division multiplexer through the other end of the optical circulator, the central wavelength signals are divided into two paths through the wavelength division multiplexer and are divided into two paths through the corresponding optical couplers, one path reaches the photoelectric detector through the slope filter, the other path directly reaches the photoelectric detector, the optical signals are converted into electric signals through the photoelectric detector and are output to the acquisition card and the upper computer, data processing and analysis are completed on the upper computer, the ambient temperature where the measuring point is located is calculated and displayed.
2. The downhole distributed temperature measurement system based on the weak reflection fiber bragg grating as claimed in claim 1, wherein: the broadband light source has the wavelength range of 1525nm-1625nm and the pulse width of 5-8ns.
3. The downhole distributed temperature measurement system based on the weak reflection fiber bragg grating as claimed in claim 1, wherein: two kinds of gratings with reflection center wavelengths in the multipoint weak reflection fiber grating string are alternately arranged, the number of the gratings is more than 100, the interval between the same-wavelength weak reflection fiber gratings is 1m, the interval between adjacent gratings is 0.5m, and the reflectivity is consistent and is 1%.
4. The downhole distributed temperature measurement system based on the weak reflection fiber bragg grating as claimed in claim 1, wherein: the slope filter is a strong coupling multi-core optical fiber, and seven fiber cores are contained in the same cladding.
5. A downhole distributed temperature measurement method based on weak reflection fiber bragg gratings is characterized by comprising the following steps:
step 1) connecting the set optical pulse into an optical isolator for integration to obtain an optical pulse signal with a certain bandwidth;
step 2) the optical pulse signal enters a multipoint weak reflection fiber grating string through one end of an optical circulator to obtain a plurality of specific optical signals with different time sequences and different positions, which are weakly reflected by the grating string;
step 3) the specific optical signal enters a wavelength division multiplexer through the other end of the optical circulator and is divided into two paths according to the difference of the wavelength of the specific optical signal, and two paths of optical signals are obtained;
step 4), introducing the two optical signals into the same reference optical path, and simultaneously obtaining the demodulated optical signal by adopting a slope filter;
and 5) establishing a relation between the reference light path and the demodulated optical signal to obtain an accurate temperature value of each grating position.
6. The method for measuring the downhole distributed temperature based on the weak reflection fiber bragg grating as claimed in claim 5, wherein the specific process of the step 2) is as follows:
the multipoint weak reflection fiber grating string is formed by arranging weak reflection fiber gratings with two wavelengths at intervals, the interval between every two adjacent weak reflection fiber gratings is the same, the time interval returned by the two adjacent weak reflection fiber gratings with the same wavelength is calculated, each grating on the grating string reflects specific optical signals with corresponding different time sequences according to 1% of reflectivity in sequence, and each specific optical signal carries temperature information of a corresponding position.
7. The underground distributed temperature measurement method based on the weak reflection fiber bragg grating as claimed in claim 5, wherein the specific process of the step 4) is as follows:
the pulse light signals reflected by each weak reflection fiber grating are divided into two light paths through a point coupler, one light path is provided with a slope filter, light intensity signals are obtained, and the other light path directly obtains wavelength signals.
8. The method for measuring the downhole distributed temperature based on the weak reflection fiber bragg grating as claimed in claim 5, wherein the specific process of the step 5) is as follows:
one optical path is provided with a slope filter, a wavelength signal is converted into a light intensity signal, the light intensity signal passes through a photoelectric detector to obtain a voltage U1, the other optical path passes the wavelength signal through the photoelectric detector to obtain a voltage U2, the U2 is used as a reference signal to suppress system noise, and accurate information of the measured temperature is obtained by establishing a relation between the U1/U2 and the reflection wavelength of the weak reflection fiber bragg grating;
wherein U1= f' (λ) × Pi × Δ λ, U2= Pi
f (lambda) is a function of the output power of the slope filter along with the change of the wavelength, f' (lambda) is the slope, pi is the power of the input coupler, and delta lambda is the change quantity of the reflection wavelength of the weak reflection fiber grating under the change of the temperature;
Δλ＝1/f'(λ)×(U1/U2)
f' (lambda) is a known quantity, and delta lambda can be obtained by measuring U1/U2, so that the measured temperature information can be obtained.
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| CN117072155A (en) * | 2023-10-16 | 2023-11-17 | 克拉玛依市红都有限责任公司 | Temperature monitoring method, system, equipment and medium applied to petroleum exploitation |
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|---|---|---|---|---|
| CN117072155A (en) * | 2023-10-16 | 2023-11-17 | 克拉玛依市红都有限责任公司 | Temperature monitoring method, system, equipment and medium applied to petroleum exploitation |
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