High-sensitivity tube type fiber grating temperature sensor for oil and gas well

Abstract

A kind of high sensitivity tubular fiber grating temperature sensor for oil and gas well, temperature sensing tube one is equipped with temperature sensing tube two, rectangular notch one is machined on the side wall of the end of temperature sensing tube one, the end of temperature sensing tube two in temperature sensing tube one is located in the middle of rectangular notch one, rectangular notch two is machined on the side wall of the other end of temperature sensing tube two, the other end of temperature sensing tube one is located in the middle of rectangular notch two, optical fiber is fixed in temperature sensing tube one and temperature sensing tube two by four fixed points in axial direction, first grating is engraved on the optical fiber in rectangular notch one and is in suspended state, second grating is engraved on the optical fiber in rectangular notch two and is in suspended state;Temperature sensing tube one and temperature sensing tube two are different in thermal expansion coefficient.The present application is the sensor of tubular structure, simple structure, easy to process, radial direction size can be made very small, while axial direction can be adjusted according to the need of specific measurement sensitivity and range, especially suitable for tubular narrow environment in oil and gas well.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a high-sensitivity tubular fiber optic temperature sensor for oil and gas wells. Background Technology

[0002] Temperature is one of the seven basic physical quantities. Temperature logging is an important parameter in oil and gas field development. It is often used for dynamic evaluation of production formations in production wells, judgment of casing channeling and leakage, and post-fracture assessment of sand fracturing.

[0003] Currently, electrical temperature sensors, such as thermocouples, platinum resistance thermometers, and semiconductor temperature sensors, are still the most commonly used in oil and gas well temperature measurement. These sensors are susceptible to electromagnetic interference, have low sensitivity, and cannot withstand high temperatures for extended periods. Fiber Bragg grating (FBG) temperature sensors, on the other hand, offer numerous advantages in oil and gas well measurement due to their resistance to electromagnetic interference, corrosion, high temperature resistance, high sensitivity, strong multiplexing capability, small size, light weight, and ease of embedding within materials. While FBGs inherently possess temperature-sensitive characteristics, their sensitivity is only around 10 pm / ℃ without any encapsulation or enhancement, which is insufficient for high-precision temperature measurement. Therefore, researchers are continuously exploring methods to improve the temperature sensitivity of FBGs, and various enhancement methods have been reported, including single-metal substrate encapsulation, polymer encapsulation, and bimetallic substrate encapsulation.

[0004] Chinese patent application number 2021112016636 discloses a "dual F-shaped fiber Bragg grating temperature sensor". It consists of two F-shaped structural members made of materials with different coefficients of thermal expansion and fiber Bragg gratings. The two F-shaped structural members are fixed together at the middle in a centrally symmetrical arrangement, and two fiber Bragg gratings are fixed at the ends of the two F-shaped structural members respectively. When the temperature changes, the two F-shaped structural members will experience relative displacement due to their different coefficients of thermal expansion, causing strain changes in the fiber Bragg gratings fixed between them, thus improving the sensitivity of the temperature sensor. Although this structure improves the sensitivity of the fiber Bragg grating temperature sensing, the inherent characteristics of the F-shaped structure result in a relatively large volume. Fixing the fiber Bragg gratings to the ends of the two F-shaped structural members is difficult, and bending is required in the middle for fixation. Furthermore, the optical fibers in this sensor are located on the outer side of the structure, making them easily touched during actual use, which could lead to fiber breakage and sensor failure.

[0005] Chinese patent application CN202010954557.4 discloses a "FBG temperature sensor based on a bimetallic cantilever beam and its application." It constructs a thermally bimetallic cantilever beam and extends a fiber Bragg grating (FBG) between the fixed and free ends of the cantilever beam. Utilizing the thermal deformation characteristics of the bimetallic sheet, it introduces additional deformation into the FBG, thereby improving its sensing sensitivity. However, the sensitivity improvement is limited because the additional deformation of the FBG depends on the thermal deformation of the bimetallic cantilever beam. The patent only describes "the FBG being laid on the thermally bimetallic sheet" or "the FBG being welded to the thermally bimetallic sheet," without specifying the type of cantilever beam or the location where the grating is attached. This could potentially cause FBG chirping, affecting the normal operation of the sensor.

[0006] Chinese patent application CN200810105788.7 discloses a "method for fabricating a high-sensitivity fiber Bragg grating temperature sensor operating at high and low temperatures." It employs a special bimetallic structure, allowing adjustment of the sensor's initial operating temperature by regulating the pre-slack length of the fiber Bragg grating, thus achieving high sensitivity. However, this method is limited by the tensile strength of ordinary fiber Bragg gratings, resulting in a relatively small sensor range. Furthermore, the sensor's displacement adjustment during temperature range setting is measured in 1 / 100mm increments, placing stringent requirements on the packaging process.

[0007] Chinese patent application CN202111201664.0 discloses a "thermal expansion type fiber Bragg grating temperature sensor". This sensor has two fiber Bragg gratings fixed in the gap between two mounting components. Utilizing the difference in thermal expansion coefficients between the mounting components, it ensures that one fiber Bragg grating is always in a stretched state when the temperature rises or falls, avoiding chirp and large initial pre-stress, and exhibiting high sensitivity. However, in this sensor, the fiber Bragg gratings are fixed to the outer surfaces of the first mounting platform and the second mounting boss, making the fiber Bragg gratings susceptible to damage during installation and use. Furthermore, the first mounting platform is larger than the second mounting component, resulting in a large overall sensor size, making it unsuitable for use in the confined spaces of oil and gas wells.

[0008] Overall, existing fiber Bragg grating temperature sensors suffer from problems such as large size, insufficient sensitivity or range, and lack of adequate protection for optical fibers, which cannot meet the needs of large-range, high-sensitivity temperature monitoring in oil and gas wells. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the shortcomings of existing fiber Bragg grating temperature sensors and provide a tubular fiber Bragg grating temperature sensor for oil and gas wells that is simple in structure, small in size, protects the optical fiber, and allows for flexible adjustment of the measurement range and sensitivity.

[0010] The technical solution adopted to solve the above-mentioned technical problems is as follows: a high-sensitivity tubular fiber optic grating temperature sensor for oil and gas wells, wherein a second temperature sensing tube is installed inside a first temperature sensing tube, a rectangular notch 1 is machined on the side wall of one end of the first temperature sensing tube, one end of the second temperature sensing tube is located in the middle of the rectangular notch 1 inside the first temperature sensing tube, and a rectangular notch 2 is machined on the side wall of the other end of the second temperature sensing tube, the second rectangular notch 2 and the first rectangular notch 1 are located on the same side, and the other end of the first temperature sensing tube is located in the middle of the second rectangular notch. Optical fibers are axially fixed inside the first and second temperature sensing tubes through four fixed points. The optical fiber has a first fixing point A on one of the temperature sensing tubes at a rectangular notch, a second fixing point B on one end of the temperature sensing tube, a third fixing point C on the other end of the temperature sensing tube at the second rectangular notch, and a fourth fixing point D on the temperature sensing tube. A first grating is etched on the optical fiber between the first fixing point A and the second fixing point B and is suspended. A second grating is etched on the optical fiber between the third fixing point C and the fourth fixing point D and is suspended. The temperature sensing tubes one and two have different coefficients of thermal expansion.

[0011] As a preferred technical solution, the first grating and the second grating are fabricated directly on the optical fiber without stripped coating using a femtosecond laser.

[0012] As a preferred technical solution, the grating area lengths of the first grating and the second grating are equal, ranging from 1 mm to 10 mm, and the difference in center wavelength is ≥ 3 nm.

[0013] As a preferred technical solution, the difference between the inner diameter of the first temperature sensing tube and the outer diameter of the second temperature sensing tube is 0.1 mm.

[0014] As a preferred technical solution, two encapsulation ports are machined on the other side of the rectangular notch on one side wall of the temperature sensing tube. The two encapsulation ports are respectively aligned with the first fixing point A and the second fixing point B. A U-shaped groove is provided at one end of the temperature sensing tube, which coincides with the encapsulation port corresponding to the second fixing point B. Two encapsulation ports are machined on the other side of the rectangular notch on the side wall of the temperature sensing tube. The two encapsulation ports are respectively aligned with the third fixing point C and the fourth fixing point D. A U-shaped groove is provided at the other end of the temperature sensing tube, which coincides with the encapsulation port corresponding to the third fixing point C.

[0015] As a preferred technical solution, the first temperature sensing tube and the second temperature sensing tube are fixedly connected by a threaded fastening connector.

[0016] As a preferred technical solution, the threaded fastening connector is located in the axial middle of the integral part of temperature sensing tube one and temperature sensing tube two.

[0017] As a preferred technical solution, the material of the first temperature sensing tube is one of aluminum alloy and stainless steel, and the material of the second temperature sensing tube is one of Invar alloy and carbon fiber, or the material of the first temperature sensing tube is one of Invar alloy and carbon fiber, and the material of the second temperature sensing tube is one of aluminum alloy and stainless steel.

[0018] As a preferred technical solution, the thickness of the first temperature sensing tube and the second temperature sensing tube are equal, ranging from 0.1 mm to 0.5 mm.

[0019] As a preferred technical solution, the sensitivity S↑ of the sensor under a heated environment is:

[0020]

[0021] In the formula, L2 is the effective length of temperature sensing tube one, L3 is the effective length of temperature sensing tube two, and P eff L is the effective elastic-optical coefficient of the optical fiber, ξ is the thermo-optical coefficient of the optical fiber material, α is the thermal expansion coefficient of the optical fiber material, α2 and α3 are the thermal expansion coefficients of temperature sensing tube one and temperature sensing tube two, respectively. 12 λ is the effective length of the second grating. B-12 The Bragg center wavelength of the second grating at the calibration temperature and without external stress;

[0022] The sensitivity S↓ of the sensor under cooling conditions is:

[0023]

[0024] In the formula, λ B-11 λ is the Bragg center wavelength of the first grating at the calibrated temperature and without external stress.

[0025] The beneficial effects of this invention are as follows:

[0026] The two temperature-sensing tubes of this invention have a difference in their coefficients of thermal expansion. This allows the relative displacement change of the two temperature-sensing tubes caused by temperature to be converted into a strain change in the grating encapsulated on them, thereby effectively improving the sensitivity of the grating temperature sensor. Both temperature increases and decreases are detected through changes in the center wavelength of a single grating, resulting in a measurement range that is twice that of conventional sensors while maintaining high sensitivity. This invention uses two gratings etched onto a single optical fiber, with each grating fixed at two points. Regardless of temperature increases or decreases, the gratings in operation are always under tension, effectively avoiding the chirp problem of fully bonded fiber Bragg grating temperature sensors.

[0027] This invention relates to a tubular sensor, which is simple in structure and easy to manufacture. Its radial dimensions can be made very small, while its axial dimensions can be adjusted according to specific measurement sensitivity and range requirements, making it particularly suitable for use in the narrow tubular environments of oil and gas wells. During the fiber optic grating encapsulation process, the side with the rectangular notch of both temperature sensing tubes is placed at the bottom. This effectively utilizes the characteristics of the circular tube, allowing the fiber optic grating to be arranged inside the tube and easily parallel to the tube's axis. Under gravity, the adhesive naturally and evenly covers the optical fiber and the inner wall of the tube, ensuring consistent and reliable bonding. Simultaneously, the optical fiber passes through both temperature sensing tubes, effectively protecting both the optical fiber and the two gratings on it, significantly improving the sensor's reliability during well deployment and operation.

[0028] Due to the inherent characteristics of the circular tube, the present invention enables the parallelism of the axes of temperature sensing tube one and temperature sensing tube two under the action of the fastening connector, and the consistent radial strength of the tubes ensures the stability of their relative positions. This overcomes the defect in Chinese patents with application numbers 2021112016636 and 202111201664.0, where the position between the endpoints of the fixed fiber optic grating is easily changed, thus affecting the output of the sensor. The invention features a simple structure and good stability. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the high-sensitivity tubular fiber optic temperature sensor for oil and gas wells according to the present invention.

[0030] Figure 2 This is a schematic diagram of the structure of the temperature sensing tube 2 of the present invention.

[0031] Figure 3 This is a schematic diagram of the structure of the second temperature sensing tube 3 of the present invention.

[0032] Among them: optical fiber 1, first grating 11, second grating 12, temperature sensing tube 1 2, rectangular notch 1 21, U-shaped groove 1 32, temperature sensing tube 2 3, rectangular notch 2 31, U-shaped groove 2 22, encapsulation port 2 4, threaded fastening connector 5, encapsulation port 1 6. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0034] Example 1

[0035] exist Figure 1 , 2In embodiment 3, the high-sensitivity tubular fiber optic temperature sensor for oil and gas wells includes a first sensing tube 2, a second sensing tube 3, a threaded fastening connector 5, and an optical fiber 1. The second sensing tube 3 is housed inside the first sensing tube 2. The difference between the inner diameter of the first sensing tube 2 and the outer diameter of the second sensing tube 3 is 0.1 mm, ensuring the stability of the entire assembly of the two sensing tubes. The lower sidewalls of the first sensing tube 2 and the second sensing tube 3 are fixedly connected by the threaded fastening connector 5, which is located at the axial center of the assembly of the first sensing tube 2 and the second sensing tube 3. A rectangular notch 121 is machined on the upper sidewall of the left end of the first sensing tube 2, with the left end of the second sensing tube 3 located in the center of the rectangular notch 121. A rectangular notch 231 is machined on the upper sidewall of the right end of the second sensing tube 3, with the right end of the first sensing tube 2 located in the center of the rectangular notch 231. The first sensing tube 2 and the second sensing tube 3 are axially connected by... Four fixed points are installed with optical fibers 1, that is, optical fibers 1 are located inside both temperature sensing tube 2 and temperature sensing tube 3. The first fixed point A is set on the left side of the rectangular notch 21, the second fixed point B is set on the left end of temperature sensing tube 3, the third fixed point C is set on the right end of temperature sensing tube 3, and the fourth fixed point D is set on the right side of the rectangular notch 31. The first grating 11 is engraved on the optical fiber 1 between the first fixed point A and the second fixed point B and is in a suspended state. The second grating 12 is engraved on the optical fiber 1 between the third fixed point C and the fourth fixed point D and is in a suspended state. The grating area lengths of the second grating 12 and the first grating 11 are equal and the difference in center wavelength is ≥3nm. Two encapsulation ports 6 are machined on the lower side wall of the temperature sensing tube 2, which are aligned with the first fixing point A and the second fixing point B, respectively. A U-shaped groove 32, overlapping with the encapsulation port 6 corresponding to the second fixing point B, is machined on the left end of the temperature sensing tube 3. This U-shaped groove 32 serves as a clearance groove. Two encapsulation ports 4 are machined on the lower side wall of the temperature sensing tube 3, which are aligned with the third fixing point C and the fourth fixing point D, respectively. A U-shaped groove 22, overlapping with the encapsulation port 4 corresponding to the third fixing point C, is machined on the right end of the temperature sensing tube 2. This U-shaped groove 22 serves as a clearance groove. The four encapsulation ports are specifically used to adjust the position of the two gratings and to apply adhesive to the fixing points.

[0036] Temperature sensing tube 2 and temperature sensing tube 3 are metal tubes of equal thickness but with different coefficients of thermal expansion. In this embodiment, temperature sensing tube 2 is made of Invar alloy, with a coefficient of thermal expansion of 1.2 × 10⁻⁶. -6 The temperature sensor tube 23 has a temperature range of 0.3℃ and a thickness of 0.3mm. It is made of aluminum alloy with a coefficient of thermal expansion of 2.2×10⁻⁶. -5 / ℃.

[0037] In this embodiment, the first grating 11 and the second grating 12 are directly etched onto the uncoated optical fiber 1 using a femtosecond laser. The grating region length of the first grating 11 is 4 mm and the center wavelength is 1525 nm. The grating region length of the second grating 12 is 4 mm and the center wavelength is 1545 nm.

[0038] During the fiber Bragg grating encapsulation process, the side with the rectangular notch machined into temperature sensing tubes 1 and 2 is placed at the bottom, while the two encapsulation ports are located at the top. This effectively utilizes the characteristics of the circular tube, allowing the fiber Bragg grating to be arranged inside the tube and easily parallel to the tube's axis. Adhesive is applied through the encapsulation ports, and under gravity, the adhesive naturally and evenly covers the optical fiber and the inner wall of the tube, ensuring consistent and reliable bonding. Simultaneously, optical fiber 1 passes through temperature sensing tubes 1 and 2, effectively protecting both optical fiber 1 and the two gratings on it, significantly improving the reliability of the sensor during well deployment and operation.

[0039] The working principle of this invention is as follows:

[0040] When the sensor of this invention is lowered into an oil and gas well, the thermal expansion and thermo-optical effects of the fiber optic material will cause a shift in the center wavelengths of the two gratings due to the temperature inside the well. The relative shift is as follows:

[0041]

[0042] In the formula, λ B-n Δλ is the center wavelength of the fiber 1 grating at the calibration temperature and without external stress, where n is the grating number; B-n Let ξ be the amount of wavelength shift of the Bragg center of a certain fiber 1 grating caused by the temperature change ΔT, ξ be the thermo-optic coefficient of fiber 1 material, α be the thermal expansion coefficient of fiber 1 material, and ΔT be the amount of temperature change in the well where the sensor is located relative to the calibration temperature.

[0043] Meanwhile, due to the significant difference in the thermal expansion coefficients of the two temperature sensing tubes (e.g., in Example 1, the thermal expansion coefficient of temperature sensing tube 2 is less than that of temperature sensing tube 3), temperature sensing tube 2 experiences less thermal elongation after temperature rise, while temperature sensing tube 3 experiences greater thermal elongation. Since temperature sensing tubes 2 and 3 are fixed by threaded fastener 5, the distance between the right end of temperature sensing tube 2 and the right side of the rectangular notch 31 of temperature sensing tube 3 increases as the temperature rises, with an increase value ΔL. 12 for

[0044] ΔL 12 = (L3×α3-L2×α2)×ΔT

[0045] In the formula, L2 is the effective length of temperature sensing tube 2, that is, the distance from the right end of temperature sensing tube 2 to the threaded fastening connector 5; L3 is the effective length of temperature sensing tube 3, that is, the distance from the right side of rectangular notch 31 to the threaded fastening connector 5; and α2 and α3 are the thermal expansion coefficients of the materials of temperature sensing tube 2 and temperature sensing tube 3, respectively.

[0046] ΔL 12The increase in temperature will stretch the second grating 12, causing it to be affected by an additional axial strain ε in addition to the effects of temperature changes. The value of this strain is...

[0047] ε=ΔL 12 / L 12

[0048] In the formula, L 12 The distance between the right end of the temperature sensing tube 2 and the right side of the rectangular notch 31 at the calibration temperature.

[0049] Based on the strain sensing principle of fiber optic grating 1, the relative wavelength shift of the second grating 12 caused by this axial strain is:

[0050]

[0051] In the formula, Δλ B-12ε P is the amount of wavelength shift at the center of the second grating 12 caused by the relative displacement of the two temperature sensing tubes. eff is the effective elastic coefficient of fiber 1.

[0052] In summary, when the tubular fiber optic grating temperature sensor is lowered into the oil and gas well, the second grating 12 is in working condition due to the increase in ambient temperature. The relative wavelength shift of the second grating 12 is the sum of equations (1) and (2).

[0053]

[0054] Then the sensor's sensitivity S↑ at this time is

[0055]

[0056] In this way, by optimizing the size and material of temperature sensing tube 2 and temperature sensing tube 33, the initial distance L between their end faces on the same side can be adjusted. 12 This can change the temperature response sensitivity of the second grating 12.

[0057] For the first grating 11, when the temperature rises, the center wavelength of the grating increases due to the temperature. However, at this time, the distance between the left end of the rectangular notch 21 of the temperature sensing tube 2 and the left end of the temperature sensing tube 3 will shorten, causing the prestress of the first grating 11 fixed thereon to be released. When the temperature continues to rise to a certain value, the prestress of the first grating 11 will be completely released. Throughout the process, the first grating 11 is in a non-working state, and the tubular structure has no temperature-sensitizing effect on it, only a simple temperature response. Its sensitivity can be calculated according to equation (1).

[0058] When the ambient temperature decreases, the first grating 11 is in operation. In addition to its normal temperature response, it will also experience additional tensile strain due to the tubular structure. While the cooling will decrease the center wavelength of the first grating 11, the strain applied by the tubular structure is tensile, which will increase the center wavelength of the first grating 11. Considering all factors, the relative wavelength shift of the first grating 11 caused by the decrease in ambient temperature is the difference between equations (1) and (2), i.e.

[0059]

[0060] Then the sensitivity S↓ of the sensor at this time is

[0061]

[0062] In the above formula, the strain response of fiber 1 grating one caused by the decrease in temperature is about an order of magnitude higher than that caused by temperature alone. Therefore, the overall sensitivity of fiber 1 grating one to temperature response is significantly improved, and fiber 1 grating one is always under tension and is not affected by the magnitude of the initial prestress. At this time, fiber 1 grating two is in a non-working state and is only affected by temperature alone; the tubular structure has no sensitizing effect on it.

[0063] By measuring the decrease and increase of temperature using two fiber 1 gratings respectively, and with the same tensile strength of the fiber 1 gratings, the range of the tubular fiber 1 grating temperature sensor is doubled compared to the usual case.

[0064] Example 2

[0065] In this embodiment, the thickness of temperature sensing tube 2 and temperature sensing tube 3 is the same, 0.1 mm. Temperature sensing tube 2 is made of aluminum alloy, and temperature sensing tube 3 is made of Invar alloy. In this embodiment, the first grating 11 and the second grating 12 are directly etched onto the uncoated optical fiber 1 using a femtosecond laser. The grating area length of the first grating 11 is 1 mm, and the center wavelength is 1530 nm. The grating area length of the second grating 12 is 1 mm, and the center wavelength is 1555 nm. Other components and their connections are the same as in Embodiment 1.

[0066] Example 3

[0067] In this embodiment, the thickness of temperature sensing tube 2 and temperature sensing tube 3 is the same, 0.5 mm. Temperature sensing tube 2 is made of stainless steel, and temperature sensing tube 3 is made of carbon fiber. In this embodiment, the first grating 11 and the second grating 12 are directly etched onto the uncoated optical fiber 1 using a femtosecond laser. The grating area length of the first grating 11 is 10 mm, and the center wavelength is 1530 nm. The grating area length of the second grating 12 is 10 mm, and the center wavelength is 1555 nm. Other components and their connections are the same as in Embodiment 1.

[0068] Example 4

[0069] In embodiments 1 to 3, the material of temperature sensing tube 2 is stainless steel, and the material of temperature sensing tube 3 is Invar alloy. The other components and their connections are the same as in the corresponding embodiments.

Claims

1. A high-sensitivity tube optical fiber grating temperature sensor for oil and gas wells, characterized in that: The temperature sensing tube one (2) is sleeved with the temperature sensing tube two (3), a rectangular notch one (21) is processed on the end side wall of the temperature sensing tube one (2), the one end of the temperature sensing tube two (3) in the temperature sensing tube one (2) is located in the middle of the rectangular notch one (21), a rectangular notch two (31) is processed on the other end side wall of the temperature sensing tube two (3), the rectangular notch two (31) is located on the same side with the rectangular notch one (21), the other end of the temperature sensing tube one (2) is located in the middle of the rectangular notch two (31), the temperature sensing tube one (2) and the temperature sensing tube two (3) are fixed with the optical fiber (1) through four fixed points in the axial direction, the first fixed point A is arranged on the temperature sensing tube one (2) at the rectangular notch one (21), the second fixed point B is arranged on the end of the temperature sensing tube two (3), the third fixed point C is arranged on the other end of the temperature sensing tube one (2) at the rectangular notch two (31), the fourth fixed point D is arranged on the temperature sensing tube two (3), the first grating (11) is engraved on the optical fiber (1) between the first fixed point A and the second fixed point B and is in a suspended state, the second grating (12) is engraved on the optical fiber (1) between the third fixed point C and the fourth fixed point D and is in a suspended state; the temperature sensing tube one (2) and the temperature sensing tube two (3) have different thermal expansion coefficients.

2. The high-sensitivity tube optical fiber grating temperature sensor for oil and gas wells according to claim 1, characterized in that The first grating (11) and the second grating (12) are directly engraved on the optical fiber (1) without stripping the coating layer by a femtosecond laser.

3. The high-sensitivity tube fiber grating temperature sensor for oil and gas wells according to claim 1, characterized in that: The grating area length of the first grating (11) and the second grating (12) is equal to 1mm-10mm, and the difference between the center wavelengths is greater than or equal to 3nm.

4. The high-sensitivity tube optical fiber grating temperature sensor for oil and gas wells according to claim 1, characterized in that: The difference between the inner diameter of the temperature sensing tube one (2) and the outer diameter of the temperature sensing tube two (3) is 0.1mm.

5. The high sensitivity tube optical fiber grating temperature sensor for oil and gas wells according to claim 1, characterized in that: Two packaging openings one (6) are processed on the other side of the rectangular notch one (21) on the side wall of the temperature sensing tube one (2) and are aligned with the first fixed point A and the second fixed point B respectively, a U-shaped groove one (32) is arranged on the end of the temperature sensing tube two (3) and coincides with the packaging opening one (6) corresponding to the second fixed point B; two packaging openings two (4) are processed on the other side of the rectangular notch two (31) on the side wall of the temperature sensing tube two (3) and are aligned with the third fixed point C and the fourth fixed point D respectively, a U-shaped groove two (22) is arranged on the other end of the temperature sensing tube one (2) and coincides with the packaging opening two (4) corresponding to the third fixed point C.

6. The high sensitivity tube optical fiber grating temperature sensor for oil and gas wells according to claim 1, characterized in that: The temperature sensing tube one (2) and the temperature sensing tube two (3) are fixed and coupled by a threaded fastening coupling piece (5).

7. The high-sensitivity tube optical fiber grating temperature sensor for oil and gas wells according to claim 6, characterized in that: The threaded fastening coupling piece (5) is located in the axial middle part of the whole composed of the temperature sensing tube one (2) and the temperature sensing tube two (3).

8. The high-sensitivity tube optical fiber grating temperature sensor for oil and gas wells according to any one of claims 1 to 7, characterized in that, The material of the temperature sensing tube one (2) is one of aluminum alloy and stainless steel, the material of the temperature sensing tube two (3) is one of invar alloy and carbon fiber, or the material of the temperature sensing tube one (2) is one of invar alloy and carbon fiber, and the material of the temperature sensing tube two (3) is one of aluminum alloy and stainless steel.

9. The high-sensitivity tube fiber grating temperature sensor for oil and gas wells according to claim 8, characterized in that, The thickness of the temperature sensing tube one (2) and the temperature sensing tube two (3) is equal to 0.1mm-0.5mm.

10. The high-sensitivity tube fiber grating temperature sensor for oil and gas wells according to claim 8, characterized in that, Sensitivity of the sensor in a temperature rising environment Is: wherein L2 is the effective length of the first temperature sensing tube (2), L3 is the effective length of the second temperature sensing tube (3), is the effective photoelastic coefficient of the optical fiber (1), and ξ is the thermal-optic coefficient of the material of the optical fiber (1), is the thermal expansion coefficient of the material of the optical fiber (1), and are the thermal expansion coefficients of the first temperature sensing tube (2) and the second temperature sensing tube (3), respectively, and L 12 is the effective length of the second grating (12), is the Bragg center wavelength of the second grating (12) at the calibration temperature and without an applied stress; Sensitivity of the sensor in a cooling environment Is: wherein is the Bragg center wavelength of the first grating (11) at the calibration temperature and without an applied stress.

Images