A distributed fiber optic pressure sensor, method of manufacture, and sensing system

Abstract

The application belongs to the field of optical fiber sensing, and specifically discloses a distributed optical fiber pressure sensor, a preparation method and a sensing system. The distributed optical fiber pressure sensor comprises a polarization maintaining optical fiber, an inner metal tube and an outer metal tube. A weak grating array is arranged on the polarization maintaining optical fiber. The weak grating array comprises at least two weak gratings which are distributed at intervals along the axial direction of the polarization maintaining optical fiber. The inner metal tube is sleeved outside the polarization maintaining optical fiber. At least one glue injection hole is formed in the tube wall of the inner metal tube at the position corresponding to each weak grating. The polarization maintaining optical fiber is fixed by being bonded with the inner metal tube through high-temperature-resistant glue injected from the glue injection hole at the position of the weak grating. The outer metal tube is wrapped outside the inner metal tube. An array of sensitizing structures is arranged on the outer wall of the outer metal tube at the position corresponding to each weak grating. Through the application, the mechanical strength, the measurement sensitivity and the measurement accuracy of the sensor can be effectively improved, and the sensor is suitable for high-temperature and high-pressure complex working conditions such as downhole.

Description

Technical Field

[0001] This application belongs to the field of fiber optic sensing, and more specifically, relates to a distributed fiber optic pressure sensor, its preparation method, and a sensing system. Background Technology

[0002] In the exploration and development of oil and gas wells, obtaining real-time and accurate pressure distribution information throughout the entire wellbore is crucial for optimizing production plans, improving recovery rates, and ensuring safe production. Traditional electronic pressure sensors suffer from drawbacks such as susceptibility to electromagnetic interference, poor high-temperature resistance, and difficulty in achieving distributed measurements.

[0003] Fiber optic sensing technology, with its advantages of resistance to electromagnetic interference, corrosion resistance, and distributed measurement capabilities, has shown great potential in downhole monitoring. Among these, distributed fiber optic sensing systems based on weak grating arrays can achieve continuous measurements over distances of several kilometers with millimeter-level spatial resolution. However, applying fiber optic sensors to downhole pressure monitoring still faces the following challenges: First, the poor mechanical strength of the encapsulation structure makes it unable to withstand the high temperatures, high pressures, and complex mechanical environments downhole, resulting in a short service life. Second, the low pressure transmission efficiency and insufficient sensitivity of the sensing structure make it difficult to capture minute pressure changes downhole. Third, it is difficult to balance spatial resolution and measurement range in distributed monitoring, failing to meet the requirements for accurate monitoring throughout the entire well section. Fourth, most sensors do not achieve effective decoupling between pressure and temperature, and temperature drift can easily lead to pressure measurement errors.

[0004] To address the aforementioned issues, some existing technologies employ methods such as directly coating sensitive materials onto bare optical fibers, but the sensitivity enhancement effect is limited, and mechanical protection is insufficient, making it difficult to withstand the harsh downhole environment. Others use simple steel tube encapsulation, but the pressure sensitivity is low, and it cannot effectively transmit external pressure signals to the optical fiber, nor can it effectively decouple temperature effects. Some use fiber optic Fabry-Perot (FBP) sensors for sensitivity enhancement, but this only enables single-point monitoring and cannot be reused. Still others use fiber Bragg grating (FBG) sensors for sensitivity enhancement; while these sensors can be connected in series, the number of reused sensors is limited, making distributed pressure monitoring impossible. Summary of the Invention

[0005] In response to the deficiencies or improvement needs of existing technologies, this application provides a distributed optical fiber pressure sensor, its fabrication method, and a sensing system, aiming to solve the problems of poor mechanical strength, low sensitivity, and large measurement errors in existing sensors.

[0006] Firstly, this application provides a distributed optical fiber pressure sensor, specifically comprising:

[0007] A polarization-maintaining fiber, wherein a weak grating array is disposed on the polarization-maintaining fiber, the weak grating array comprising at least two weak gratings spaced apart along the axial direction of the polarization-maintaining fiber;

[0008] An inner metal tube is fitted over the polarization-maintaining fiber, and at least one injection hole is provided on its tube wall corresponding to each of the weak grating positions. The polarization-maintaining fiber is bonded and fixed to the inner metal tube at the weak grating positions by high-temperature resistant adhesive injected from the injection hole.

[0009] An outer metal tube is wrapped around the inner metal tube, and an array of sensitive enhancement structures is provided on its outer wall corresponding to each of the weak grating positions.

[0010] As a further preferred embodiment, the polarization-maintaining fiber is a stress-induced high birefringence fiber, wherein stress is applied to a portion of its core.

[0011] As a further preferred embodiment, the polarization-maintaining fiber has a core diameter of 8 μm to 10 μm, a relative refractive index difference between the core and cladding of 0.3% to 0.5%, a polarization crosstalk of no more than -25 dB / 100 m at a wavelength of 1550 nm, and a birefringence of no less than 3.0 × 10⁻⁶. -4 .

[0012] As a further preferred embodiment, the weak grating is written on the non-stress region of the polarization-maintaining fiber core using a femtosecond laser, the reflectivity of the weak grating is less than 1%, and the spacing between adjacent weak gratings is ≥5mm.

[0013] As a further preferred option, the operating bandwidth of a single weak grating covers 1525nm~1625nm.

[0014] As a further preferred embodiment, the outer diameter of the inner metal tube is 1.8mm to 4.4mm, and the wall thickness is 0.5mm to 2.0mm.

[0015] As a further preferred embodiment, the outer diameter of the outer metal tube is 4.4mm to 12mm, and the wall thickness is 0.5mm to 2.5mm.

[0016] As a further preferred embodiment, the inner and outer metal tubes are made of stainless steel.

[0017] As a further preferred embodiment, the sensitizing structure array includes multiple sensitizing structures, which are uniformly distributed along the circumference of the outer metal tube.

[0018] As a further preferred embodiment, the sensitizing structure is a pit structure, wherein the depth of the pit structure is no greater than half the thickness of the outer metal tube wall and the width is no greater than half the diameter of the outer metal tube.

[0019] As a further preferred embodiment, the recess structure is an arc-shaped recess, and the width of the arc-shaped recess is 10% to 50% of the outer diameter of the outer metal tube.

[0020] As a further preferred embodiment, the high-temperature resistant adhesive has a temperature resistance range of -40℃ to 300℃, a compressive strength ≥70MPa, and a difference between its coefficient of thermal expansion and that of the inner metal tube ≤1×10⁻⁶. -5 / ℃.

[0021] Secondly, this application provides a method for fabricating a distributed optical fiber pressure sensor, which specifically includes the following steps:

[0022] S1. Prepare at least two weak gratings on the polarization-maintaining fiber;

[0023] S2. Process a glue injection hole at the position of the weak grating in the inner metal tube, insert the polarization maintaining fiber into the inner metal tube, and align the weak grating with the corresponding glue injection hole.

[0024] S3. Inject high-temperature resistant adhesive into the gap between the inner metal tube and the polarization-maintaining fiber through the injection hole, and allow it to cure.

[0025] S4. The outer metal tube is wrapped around the outside of the inner metal tube, and an enhanced sensitivity structure array is fabricated on the outer surface of the outer metal tube at the position corresponding to the weak grating.

[0026] As a further preferred option, the injection pressure of the high-temperature resistant adhesive is 0.5MPa~2MPa, and it is cured at a temperature of 80℃~150℃ after injection.

[0027] As a further preferred option, in step S4, the outer metal tube is made of stainless steel strip, and the stainless steel strip is first wrapped around the outside of the inner metal tube by a coiling machine, and then seamless welding is performed; the welding power is 1000W~3000W, and the welding speed is 0.5m / min~2m / min.

[0028] As a further preferred embodiment, the sensitivity-enhancing structure array includes multiple sensitivity-enhancing structures, which are recessed structures formed by pressure riveting with a pressure head, and the pressure riveting pressure is 5MPa~30MPa.

[0029] As a further preferred option, during the riveting process, a positioning fixture is used to ensure the alignment accuracy between the sensitizing structure array and the weak grating, with an axial positioning error ≤ ±0.2mm and a circumferential distribution uniformity error ≤ ±5°.

[0030] Thirdly, the distributed optical fiber pressure sensing system provided in this application specifically includes a demodulation module and the distributed optical fiber pressure sensor. The demodulation module is connected to a polarization-maintaining optical fiber and is used to decouple temperature and pressure based on a weak grating signal, and output pressure data.

[0031] As a further preferred embodiment, a temperature-pressure decoupling algorithm is used to decouple temperature and pressure, including:

[0032] S1. Obtain the center wavelength offset of the two orthogonal polarization states of each weak grating, including the fast axis offset Δλ1 and the slow axis offset Δλ2.

[0033] S2. Establish a coupled model of temperature T, pressure P, and center wavelength offset:

[0034] Δλ1=k 11 T+k 12 P;

[0035] Δλ2=k 21 T+k 22 P;

[0036] Where, k 11 k 12 These are the temperature coefficient and pressure coefficient along the fast axis, respectively, k 21 k 22 These are the temperature coefficient and pressure coefficient in the slow axis direction, respectively;

[0037] S3. Solve the coupled model to separate temperature T and pressure P:

[0038] P=(k 21 Δλ1-k 11 Δλ2) / (k 12 k 21 -k 11 k 22 );

[0039] T=(k 12 Δλ2-k 22 Δλ1) / (k 12 k 21 -k 11 k 22 ).

[0040] In summary, compared with the prior art, the technical solutions conceived in this application have the following main technical advantages:

[0041] 1. This application utilizes the design of the injection hole to bond and fix the polarization-maintaining fiber to the weak grating array position with the inner metal tube using high-temperature resistant adhesive. Combined with the double-layer metal tube structure design, this effectively ensures tight coupling between the weak grating and the metal tube, significantly improving the sensor's mechanical strength, corrosion resistance, and sensitivity to external load signal transmission. It can effectively adapt to the high-temperature and high-pressure working environment downhole and has a long service life. At the same time, combined with the sensitivity-enhancing structure array set at the corresponding weak grating array position on the outer metal tube, the pressure sensitivity can be greatly improved through the stress concentration effect.

[0042] 2. This application effectively improves the spatial resolution and measurement range of the sensor by combining the design of weak grating reflectivity and spacing with the design of a sensitivity-enhancing structure.

[0043] 3. This application achieves temperature and pressure decoupling without the need for an additional temperature sensor by combining the structural design of a distributed fiber optic pressure sensor with a temperature-pressure decoupling algorithm. This significantly reduces the impact of temperature error on pressure measurement, resulting in small pressure measurement error and high measurement accuracy. Attached Figure Description

[0044] Figure 1 This is a cross-sectional view of the distributed optical fiber pressure sensor provided in an embodiment of this application;

[0045] Figure 2 This is a schematic diagram of a polarization-maintaining fiber with a weak grating array inscribed on it, provided in an embodiment of this application.

[0046] Figure 3 This is a schematic diagram of the sensitivity enhancement structure array provided in the embodiments of this application;

[0047] Figure 4 This is a partial cross-sectional view of the sensitivity-enhancing structure array provided in the embodiments of this application;

[0048] Figure 5 This is a finite element simulation result diagram of the distributed optical fiber pressure sensor provided in the embodiments of this application.

[0049] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0050] 1-Polarization-maintaining fiber, 2-Inner metal tube, 3-Sensitization-enhancing structure array, 4-Outer metal tube, 5-Injection hole, 6-High-temperature adhesive filling layer, 7-Weak grating, 31-Sensitization-enhancing structure, 71-Grate. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0052] like Figure 1 As shown in the figure, this application provides a distributed optical fiber pressure sensor, which includes, from the inside out, a polarization-maintaining fiber 1, an inner metal tube 2, and an outer metal tube 4. Specifically, this distributed optical fiber pressure sensor adopts a core structure of a double-layer metal tube (preferably stainless steel) + a polarization-maintaining fiber weak grating array. The inner metal tube 2 serves as the direct encapsulation carrier for the polarization-maintaining fiber 1, and the outer metal tube 4 is tightly integrated with the inner metal tube 2, improving overall compressive strength and corrosion resistance. Utilizing the birefringence characteristics of the polarization-maintaining fiber 1, the weak grating generates two orthogonal polarization reflection peaks, providing a physical basis for temperature and pressure decoupling, avoiding the use of an additional temperature sensor, simplifying the system structure, and improving temperature compensation accuracy.

[0053] like Figure 2 As shown, a weak grating array is provided on the polarization-maintaining fiber 1. The weak grating array includes at least two weak gratings 7 that are spaced apart along the axial direction of the polarization-maintaining fiber 1. Each weak grating 7 is uniformly distributed along the axial direction of the polarization-maintaining fiber 1. Each weak grating 7 includes multiple grids 71, which are uniformly distributed along the axial direction of the polarization-maintaining fiber 1.

[0054] In a preferred embodiment, the polarization-maintaining fiber 1 is a stress-induced high birefringence fiber (with a birefringence coefficient B on the order of 10 or greater). -4 For example, polarization-maintaining polyimide fiber 1 has a core containing at least a stress-applied region symmetrically distributed along a predetermined direction to ensure stable linear birefringence in the core. The predetermined direction can be designed according to actual needs and is not limited in this application. The beat length of polarization-maintaining fiber 1 is no greater than 5 mm at the operating wavelength of the femtosecond laser to ensure effective spatial separation of polarization modes within the microscale of femtosecond laser processing, thereby supporting precise polarization-selective characterization.

[0055] In a preferred embodiment, the core diameter of the polarization-maintaining fiber 1 is 8 μm to 10 μm, the relative refractive index difference between the core and cladding is between 0.3% and 0.5%, the polarization crosstalk of the polarization-maintaining fiber 1 at a wavelength of 1550 nm is no greater than -25 dB / 100 m, and the birefringence is no less than 3.0 × 10⁻⁶. -4 This ensures that the weak grating has a high sensitivity response to specific polarized light (linearly polarized light whose polarization direction is consistent with the direction of the inherent birefringence principal axis (i.e., slow axis or fast axis) of the polarization-maintaining fiber).

[0056] In a preferred embodiment, femtosecond laser etching technology is used to progressively etch gratings 71 into the non-stress region of the polarization-maintaining fiber core 1, forming periodically distributed weak gratings 7. The reflectivity of the weak gratings 7 is less than 1%, thereby reducing superposition loss and increasing the length and number of multiplexed optical cables. The more weak gratings that can be reused in the optical cable, the more pressure monitoring points can be established, and the longer the distance can be covered. Figure 2 As shown, the spacing L between adjacent weak gratings 7 is ≥ 5 mm, and the operating wavelength of weak gratings 7 covers 1525 nm to 1625 nm, thus covering all C+L bands. The weak gratings 7 are fabricated in the non-stress region of the polarization-maintaining fiber 1 core using a femtosecond laser writing system to ensure the grating's spectral quality.

[0057] like Figure 1As shown, the inner metal tube 2 is fitted onto the outside of the polarization-maintaining fiber 1. At least one injection hole 5 is provided on the tube wall corresponding to each weak grating 7. The injection hole 5 is used to inject high-temperature resistant adhesive, which tightly bonds and fixes the portion of the polarization-maintaining fiber 1 with the weak grating 7 to the inner metal tube 2 (inner wall), ensuring efficient transmission of the downhole high-pressure signal to the weak grating. Simultaneously, the adhesive's high-temperature resistance is suitable for the high-temperature environment downhole. Specifically, by controlling the amount of high-temperature resistant adhesive injected, it is ensured that only the portion of the polarization-maintaining fiber 1 with the weak grating 7 is bonded and fixed to the inner metal tube 2, while the portion of the polarization-maintaining fiber 1 without the weak grating 7 is loosely fitted to the inner metal tube 2. The high-temperature resistant adhesive injected through the injection hole 5 fills the space between the portion of the polarization-maintaining fiber 1 with the weak grating 7 and the inner metal tube 2, as well as the injection hole 5 (i.e., the injection hole 5 is also filled with high-temperature resistant adhesive), forming a high-temperature adhesive filling layer 6. The distribution length of the high-temperature adhesive filling layer 6 along the axis of the polarization-maintaining fiber 1 is greater than the distribution length of the corresponding single weak grating 7 along the axis of the polarization-maintaining fiber 1, so that the high-temperature adhesive covers the corresponding weak grating 7 and ensures the connection strength.

[0058] Specifically, each weak light grating 7 is provided with multiple injection holes 5, which are evenly distributed around the inner metal tube 2 in the circumferential direction (circumferential direction, the same below) to ensure that the high-temperature resistant adhesive is injected from multiple directions and to ensure consistent flow. Specifically, the diameter of the injection hole 5 can be set as needed, for example, 0.3mm~1mm, to ensure that the high-temperature resistant adhesive can flow smoothly.

[0059] In a preferred embodiment, the high-temperature resistant adhesive has a temperature resistance range of -40℃ to 300℃, a compressive strength ≥70MPa, and the difference between the thermal expansion coefficient of the high-temperature resistant adhesive and the thermal expansion coefficient of the inner metal tube is ≤1×10⁻⁶. -5 / ℃, thus ensuring that the stress difference between the high-temperature adhesive filler layer 6 and the inner metal tube 2 is small at high temperatures, avoiding shearing, peeling or breakage of the high-temperature adhesive filler layer 6, and ensuring connection strength.

[0060] See Figure 1 and Figure 3 The outer metal tube 4 covers the outer surface of the inner metal tube 2, and an enhancement structure array 3 is provided on its outer wall at the position corresponding to the weak grating 7 (which is also the position corresponding to the glue injection hole). Specifically, each weak grating 7 corresponds to a set of enhancement structure arrays 3, and the distribution length of a set of enhancement structure arrays 3 along the axial direction of the outer metal tube 4 is preferably greater than the distribution length of the corresponding weak grating 7 along the axial direction of the polarization-maintaining fiber 1. Each set of enhancement structure arrays 3 includes multiple enhancement structures 31, which are evenly distributed circumferentially along the outer metal tube 4. For example, 10 to 50 enhancement structures 31 are evenly distributed circumferentially around each weak grating 7. Specifically, the enhancement structures 31 are integrally formed by a press-fitting process.

[0061] Furthermore, such as Figure 4 As shown, the sensitizing structures 31 in each array of sensitizing structures 3 can be the same or different. In a preferred embodiment, the sensitizing structure 31 is a pit structure, which utilizes the stress concentration effect of the pit structure to amplify the effect of pressure on the weak grating and significantly improve pressure sensitivity. Specifically, the depth h of the pit structure is no greater than half the wall thickness of the outer metal tube 4, and the width s of the pit structure (the length distributed along the axial direction of the outer metal tube 4) is no greater than half the diameter (outer diameter) of the outer metal tube 4. This improves stress sensitivity while ensuring that the tensile strength of the optical cable is not excessively reduced due to the pit structure being too large. Figure 4 As shown, the longitudinal cross-sectional shape of the recess structure can be arc-shaped, square, triangular, or rectangular, etc. The depth of the recess structure can be the same or different, and the width of the recess structure can be the same or different.

[0062] In a preferred embodiment, such as Figure 3 As shown, the preferred recess structure is an arc-shaped recess. The width m of the arc-shaped recess is 10% to 50% of the outer diameter of the outer metal tube 4, and the depth is no more than half the wall thickness of the outer metal tube 4. This ensures both sensitivity and optical cable strength while maintaining uniform stress.

[0063] Specifically, the recessed structure is preferably formed by press riveting with a press head, with a press riveting pressure of 5MPa~30MPa, so as to ensure the formation of the required recessed structure.

[0064] In a preferred embodiment, the inner metal tube 2 is preferably a stainless steel tube with an outer diameter of 1.8mm to 4.4mm and a wall thickness of 0.5mm to 2.0mm, and the outer metal tube 4 is preferably a stainless steel tube with an outer diameter of 4.4mm to 12mm and a wall thickness of 0.5mm to 2.5mm. The above design makes the size of the sensor match the size of the oil well tubing or casing, making it suitable for downhole operations.

[0065] In a preferred embodiment, the inner metal tube 2 and the outer metal tube 4 are formed by seamless laser welding of stainless steel strip coils. The outer metal tube 4 is tightly bonded to the inner metal tube 2, improving the overall compressive strength and corrosion resistance. The stainless steel strip is made of 316L or Inconel alloy.

[0066] In another embodiment, this application also provides a method for fabricating the distributed optical fiber pressure sensor, characterized by comprising the following steps:

[0067] S1. Fabrication of a weak grating array

[0068] At least two weak gratings 7 are fabricated on the polarization-maintaining fiber 1. Specifically, the weak gratings 7 are spaced out and inscribed in the non-stress region of the core of the polarization-maintaining fiber 1 using femtosecond laser inscription technology.

[0069] S2. Machining injection holes

[0070] A glue injection hole 5 is processed at the position corresponding to the weak grating 7 in the inner metal tube 2. The polarization maintaining fiber 1 is inserted into the inner metal tube 2, and the weak grating 7 is aligned with the corresponding glue injection hole 5. The specific alignment method can adopt the existing technology in this field, and is not limited without application.

[0071] Specifically, the inner metal tube 2 can be made by laser welding of stainless steel strip edge wrapping and then processing the glue injection hole, and then the polarization maintaining fiber 1 with weak grating array engraved is inserted into the inner metal tube 2.

[0072] The inner metal tube 2 can also be made of 316L stainless steel. Cut to the preset length, and use laser drilling to process the glue injection hole on the tube corresponding to the position of the weak grating 7. After the glue injection hole is processed, it is deburred, cleaned and dried.

[0073] S3, potting encapsulation

[0074] High-temperature resistant adhesive is injected into the gap between the inner metal tube 2 and the polarization-maintaining optical fiber 1 through the injection hole 5 and then cured.

[0075] Specifically, the position of the polarization-maintaining fiber is adjusted to align each weak grating with its corresponding injection hole. High-temperature resistant adhesive is then injected through the injection hole into the gap between the inner metal tube 2 and the polarization-maintaining fiber 1 using an adhesive injection machine. The injection pressure of the high-temperature resistant adhesive is 0.5 MPa to 2 MPa, and the curing temperature is 80℃ to 150℃. This process ensures smooth injection and curing of the high-temperature resistant adhesive, preventing cracking or delamination and guaranteeing connection strength. Furthermore, before injection, the inner wall of the inner metal tube 2 and the surface of the polarization-maintaining fiber 1 undergo plasma cleaning treatment to improve the adhesion strength between the adhesive and the contact surface.

[0076] S4, Processing Sensitivity Enhancement Structure

[0077] The outer metal tube 4 is wrapped around the outside of the inner metal tube 2, and the sensitivity enhancement structure array 3 is processed on the outer surface of the outer metal tube 4 at the position corresponding to the weak grating 7.

[0078] Specifically, the outer metal tube 4 is made of stainless steel strip. First, the stainless steel strip is wrapped around the outside of the inner metal tube 2 using a coiling machine, and then seamlessly welded. The welding power is 1000W~3000W, and the welding speed is 0.5m / min~2m / min. This welding process ensures the welding quality of the stainless steel strip, resulting in a smooth, pore-free weld, achieving a seamless weld and thus guaranteeing the overall strength of the sensor. After welding, the weld can be inspected for flaws.

[0079] Then, the welded double-layer steel pipe is fixed on the riveting equipment. At the outer ring position of the outer metal pipe 4 corresponding to each glue injection hole, the riveting process is performed by the pressure head. The riveting pressure is 5MPa~30MPa to form a sensitive array structure.

[0080] Preferably, during the riveting process, a positioning fixture is used to ensure the positional alignment accuracy between the sensitizing structure array and the weak grating, with an axial positioning error ≤ ±0.2mm and a circumferential distribution uniformity error ≤ ±5°.

[0081] In another embodiment, this application also provides a distributed optical fiber pressure sensing system, which includes a demodulation module and a distributed optical fiber pressure sensor designed in this application. The demodulation module is connected to a polarization-maintaining fiber 1 and is used to decouple temperature and pressure based on the signal acquired by the weak grating 7, i.e., to separate the coupling effect of temperature and pressure, and output pressure data. Specifically, the demodulation module is connected to the polarization-maintaining fiber 1 through an optical fiber connector. The demodulation module can perform photoelectric conversion, converting the optical signal of the weak grating into an electrical signal, and then, combined with a temperature-pressure decoupling algorithm, demodulates the optical signal into temperature and pressure.

[0082] In a preferred embodiment, a temperature-pressure decoupling algorithm is used to decouple temperature and pressure, specifically including:

[0083] S1. The demodulation module obtains the center wavelength offset Δλ1 (fast axis) and Δλ2 (slow axis) of the two orthogonal polarization states (fast axis and slow axis) of each weak grating.

[0084] S2. Establish a coupled model of temperature T, pressure P, and center wavelength offset:

[0085] Δλ1=k 11 T+k 12 P;

[0086] Δλ2=k 21 T+k 22 P;

[0087] Where, k 11 k 12 These are the temperature coefficient and pressure coefficient along the fast axis, respectively, k 21 k 22 These are the temperature coefficient and pressure coefficient in the slow axis direction, respectively, which can be obtained through experimental calibration.

[0088] For example, the middle 100m section (including 20 weak grating measuring points) of the distributed pressure sensor (sensing optical cable) is placed in the temperature and pressure integrated calibration chamber, and the calibration temperature range is set to 25℃~200℃ (25℃ interval), and the calibration pressure range is 0MPa~60MPa (5MPa interval).

[0089] Under each temperature and pressure combination condition, after holding the temperature and pressure for 30 minutes, record Δλ1 and Δλ2 for each weak grating;

[0090] The coupling model coefficients are obtained by fitting using the least squares method, as shown in the example below (for a typical weak grating measurement point): k 11 =10.2 pm / ℃, k 12 =1.8 pm / MPa, k 21 =8.5 pm / ℃, k 22 =0.3pm / MPa, calibration complete.

[0091] S3. Solve the coupled model using matrix inversion or multiple linear regression algorithms to separate temperature T and pressure P, thus achieving decoupling between temperature and pressure:

[0092] P=(k 21 Δλ1-k 11 Δλ2) / (k 12 k 21 -k 11 k 22 );

[0093] T=(k 12 Δλ2-k 22 Δλ1) / (k 12 k 21 -k 11 k 22 ).

[0094] Specifically, the demodulation module is a demodulator used to demodulate the polarization state wavelength shift signal of the weak grating, and separates the coupling effect of temperature and pressure through a temperature and pressure decoupling algorithm to output distributed pressure monitoring data.

[0095] The following is a specific example of a method for fabricating a distributed fiber optic pressure sensor, which includes:

[0096] 1) Fabrication of weak grating array: Polarization-maintaining polyimide fiber was selected as the polarization-maintaining fiber, and a weak grating array was written on the fiber (specifically on the non-stress region of the fiber core 1) using a femtosecond laser writer. The reflectivity of a single weak grating is 0.5%, and the spacing between adjacent weak gratings is 5m.

[0097] 2) Inner layer stainless steel tube processing: Select 316L stainless steel strip, use a coiling equipment to coil the steel strip into a steel tube, and then laser weld the steel strip. The outer diameter is 2.8mm and the wall thickness is 0.89mm. Use pulsed laser drilling technology to process glue injection holes (diameter 0.5mm) on the steel tube. Two glue injection holes are set for each weak grating. The two glue injection holes are evenly distributed along the circumference of the steel tube. After processing, remove burrs by ultrasonic cleaning and dry for later use.

[0098] 3) Fiber Packaging and Glue Injection: The polarization-maintaining fiber with a weak grating array engraved is threaded into the inner stainless steel tube during the winding process. The fiber position is adjusted by optical positioning fixtures to align the center of each weak grating with the center of the glue injection hole (axial error ≤ ±0.2mm). High-temperature resistant glue (temperature range -40℃~300℃, compressive strength 70MPa) is injected from the glue injection hole at a pressure of 1MPa using a glue injection machine. After glue injection, high-temperature curing is performed.

[0099] 4) Outer layer stainless steel tube forming: Select 316L stainless steel strip (outer diameter 6.35mm, thickness 0.89mm after coiling and welding, length adapted to the inner layer steel tube length), and use a coiling machine to tightly wrap the stainless steel strip around the outside of the inner layer steel tube to form the outer layer steel tube; use a fiber laser welding machine (power 2000W, welding speed 1m / min) for seamless welding, and after welding, use X-ray flaw detection to inspect the weld quality to ensure that there are no pores or cracks;

[0100] 5) Sensitizing structure processing: First, such as Figure 5 As shown, the pit-enhancing structure (pit depth, number, and arrangement) was simulated and designed using finite element simulation software. The double-layer steel pipe was fixed on a CNC riveting machine. Based on the simulation results, a hemispherical pressure head with a radius of 1mm was used to perform riveting at the outer ring position of the outer steel pipe corresponding to the glue injection hole. The machine was calibrated in real time using its built-in vision positioning system to ensure that 30 arc-shaped sensitive pit structures were uniformly formed circumferentially at the position corresponding to each weak grating. After riveting, the surface of the sensitive structure was lightly polished to remove edge burrs and avoid scratching the cable or sleeve during underground installation.

[0101] Overall, the design of this application can achieve high-sensitivity distributed pressure measurement, effectively decouple temperature interference, and has high-strength mechanical protection. It has the following characteristics: 1) Low temperature interference: Utilizing the birefringence characteristics of polarization-maintaining fiber and the temperature-pressure decoupling algorithm, temperature-pressure decoupling can be achieved without an additional temperature sensor. The impact of temperature error on pressure measurement can be significantly reduced to within ±0.1%FS; 2) High pressure sensitivity: The arc-shaped sensitivity-enhancing pit structure of the outer metal tube can significantly improve pressure sensitivity through stress concentration effect, increasing it by 3-5 times compared to traditional metal-encapsulated sensing optical cables. 3) Strong environmental adaptability: The double-layer metal tube (preferably stainless steel tube) structure design gives the sensor excellent compressive strength (≥70MPa) and corrosion resistance. The high-temperature resistant adhesive can adapt to extreme environments of 300℃, meeting the needs of extreme downhole working conditions in oil and gas wells; 4) Distributed measurement: The spacing of the weak grating array can be flexibly adjusted to realize distributed pressure monitoring, covering the monitoring needs of the entire well section of oil and gas wells; 5) Good long-term stability: The glue injection process ensures tight coupling between the weak grating and the metal tube. At the same time, the laser seamless welding and riveting process ensures structural integrity, and the service life of the sensor can reach more than 5 years.

[0102] It should be understood that expressions such as "comprising" and "may include" as used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as "comprising" and / or "having" may be interpreted as indicating a specific characteristic, number, operation, constituent element, component, or combination thereof, but should not be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0103] It should be understood that the terms “center,” “upper,” “lower,” “front,” “rear,” “left,” “right,” “vertical,” “horizontal,” “inner,” “outer,” “clockwise,” “counterclockwise,” “axial,” “radial,” and “circumferential” indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0104] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0105] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0106] Those skilled in the art will readily understand that the above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A distributed optical fiber pressure sensor, characterized in that, include: A polarization-maintaining fiber (1) is provided with a weak grating array, the weak grating array including at least two weak gratings (7) spaced apart along the axial direction of the polarization-maintaining fiber (1). The inner metal tube (2) is fitted on the outside of the polarization-maintaining fiber (1), and at least one glue injection hole (5) is opened on its tube wall corresponding to each of the weak gratings (7). The polarization-maintaining fiber (1) is bonded and fixed to the inner metal tube (2) at the position of the weak grating (7) by high-temperature resistant glue injected from the glue injection hole (5). The outer metal tube (4) covers the outside of the inner metal tube (2), and an enhancement structure array (3) is provided on its outer wall corresponding to each of the weak gratings (7); the enhancement structure array (3) includes multiple enhancement structures (31), which are evenly distributed around the outer metal tube (4), and the enhancement structure (31) is a pit structure.

2. The distributed fiber optic pressure sensor as described in claim 1, characterized in that, The polarization-maintaining fiber (1) is a stress-induced high birefringence fiber, and stress is applied to a portion of its core.

3. The distributed fiber optic pressure sensor as described in claim 2, characterized in that, The polarization maintaining optical fiber (1) has a core diameter of 8-10 μm, a relative refractive index difference between the core and the cladding of 0.3-0.5%, a polarization crosstalk of no more than -25 dB / 100 m at a wavelength of 1550 nm, and a birefringence coefficient of no less than 3.0×10 -4 .

4. The distributed fiber optic pressure sensor as described in claim 2, characterized in that, The weak grating (7) is written on the non-stress region of the core of the polarization-maintaining fiber (1) using a femtosecond laser. The reflectivity of the weak grating (7) is less than 1%, and the spacing between adjacent weak gratings (7) is ≥5mm. And / or, the operating bandwidth of a single weak grating (7) covers 1525nm~1625nm.

5. The distributed fiber optic pressure sensor as described in claim 1, characterized in that, The outer diameter of the inner metal tube (2) is 1.8mm~4.4mm, and the wall thickness is 0.5mm~2.0mm; And / or, the outer diameter of the outer metal tube (4) is 4.4mm~12mm, and the wall thickness is 0.5mm~2.5mm; And / or, the inner metal tube (2) and the outer metal tube (4) are made of stainless steel.

6. The distributed optical fiber pressure sensor as described in claim 1, characterized in that, The depth of the recess structure is no greater than half the wall thickness of the outer metal tube (4), and the width of the recess structure is no greater than half the outer diameter of the outer metal tube (4). And / or, the recess structure is an arc-shaped recess, and the width of the arc-shaped recess is 10% to 50% of the outer diameter of the outer metal tube (4); And / or, the temperature resistance range of the high-temperature resistant adhesive is -40℃ to 300℃, the compressive strength is ≥70MPa, and the difference between the coefficient of thermal expansion of the adhesive and the coefficient of thermal expansion of the inner metal tube is ≤1×10⁻⁶. -5 / ℃.

7. A method for fabricating a distributed optical fiber pressure sensor as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. At least two weak gratings (7) are fabricated on the polarization-maintaining fiber (1). S2. Make a glue injection hole (5) at the position of the weak grating (7) in the inner metal tube (2), insert the polarization maintaining fiber (1) into the inner metal tube (2), and align the weak grating (7) with the corresponding glue injection hole (5). S3. Inject high-temperature resistant adhesive into the gap between the inner metal tube (2) and the polarization-maintaining fiber (1) through the injection hole (5) and cure it. S4. The outer metal tube (4) is wrapped around the outer side of the inner metal tube (2), and a sensitive enhancement structure array (3) is fabricated on the outer surface of the outer metal tube (4) at the position corresponding to the weak grating (7).

8. The preparation method according to claim 7, characterized in that, In step S3, the injection pressure of the high-temperature resistant adhesive is 0.5MPa~2MPa, and it is cured at 80℃~150℃ after injection. And / or, in step S4, the outer metal tube (4) is made of stainless steel strip, and the stainless steel strip is first wrapped around the outside of the inner metal tube (2) by a coiling machine, and then seamless welding is performed; the welding power is 1000W~3000W, and the welding speed is 0.5m / min~2m / min; And / or, the enhanced sensitivity structure array (3) includes multiple enhanced sensitivity structures (31), the enhanced sensitivity structure (31) is a recessed structure, which is processed by press head riveting, and the riveting pressure is 5MPa~30MPa; And / or, during riveting, the positioning fixture is used to ensure the alignment accuracy of the sensitive enhancement structure array (3) and the weak grating (7), with an axial positioning error ≤ ±0.2mm and a circumferential distribution uniformity error ≤ ±5°.

9. A distributed optical fiber pressure sensing system, characterized in that, It includes a demodulation module and a distributed optical fiber pressure sensor as described in any one of claims 1-6. The demodulation module is connected to a polarization-maintaining fiber (1) and is used to decouple temperature and pressure based on the signal of the weak grating (7) and output pressure data.

10. The distributed optical fiber pressure sensing system as described in claim 9, characterized in that, A temperature-pressure decoupling algorithm is used to decouple temperature and pressure, including: S1. Obtain the center wavelength offset of the two orthogonal polarization states of each weak grating, including the fast axis offset Δλ1 and the slow axis offset Δλ2. S2. Establish a coupled model of temperature T, pressure P, and center wavelength offset: Δλ1=k 11 T+k 12 P; Δλ2=k 21 T+k 22 P; Where, k 11 k 12 These are the temperature coefficient and pressure coefficient along the fast axis, respectively, k 21 k 22 These are the temperature coefficient and pressure coefficient in the slow axis direction, respectively; S3. Solve the coupled model to separate temperature T and pressure P: P=(k 21 Δλ1-k 11 Δλ2) / (k 12 k 21 -k 11 k 22 ); T=(k 12 Δλ2-k 22 Δλ1) / (k 12 k 21 -k 11 k 22 )。

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