One for CO 2 Optical fiber sensing structure and three-component detector for reservoir monitoring

Abstract

One for CO 2 The optical fiber sensing structure for reservoir monitoring has two cantilever beam mass block structures symmetrical about the vertical plane of the mounting seat on the upper and lower surfaces of the mounting plate. The cantilever beam mass block structure includes a cantilever beam and a mass block. The cantilever beam is a straight section with an arc section connected to one end. The straight section end of the cantilever beam is connected to the mounting plate, and the arc section end is connected to the mass block. The mass block is pointed at the end away from the cantilever beam. Optical fiber fixing points are set on the outer surfaces of the four mass blocks. The optical fiber is fixed on the four mass blocks through the optical fiber fixing points. Gratings are engraved on the suspended optical fiber between the two mass blocks on the same side of the mounting plate. The present invention can detect low-frequency vibration signals, which can provide information about CO 2 Important information about the reservoir status, realizing CO 2 Clear description of fluid migration and reservoir stability monitoring.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to an optical fiber sensing structure and a three-component detector for CO2 reservoir monitoring. Background Art

[0002] The key to safe and effective CO2 storage is to have an accurate understanding of the geological structure before developing a CO2 storage site. At the same time, an accurate and reliable long-term monitoring plan is also needed to understand the diffusion path of CO2 injected into the deep underground reservoir and the interaction between CO2 and geological formations. Four-dimensional multi-component seismic data combined with rock physical data can more accurately determine possible CO2 leakage risk areas and more reliably evaluate the safety of CO2 geological storage. Through extraction comparison, optimization, consistency analysis and difference calculation, three-dimensional seismic wave differences are obtained to identify the migration direction and distribution range of CO2 in the underground after injection. At the same time, the changes in reservoir fissures and cracks before and during CO2 injection, as well as the stress state of the reservoir and caprock, are effectively determined.

[0003] At present, CO2 reservoir monitoring mostly uses electrical detectors based on piezoelectric, magnetoelectric, and eddy current mechanisms, most of which are downhole "active" devices. They have essential deficiencies in detection sensitivity, response frequency band, dynamic range, spatial resolution, resistance to electromagnetic field interference, multiplexing, and reliability. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an optical fiber sensing structure and a three-component detector for CO2 reservoir monitoring that are not interfered by environmental electromagnetic fields, have good stability and high sensitivity.

[0005] The technical solution adopted to solve the above technical problems is: a fiber optic sensing structure for CO2 reservoir monitoring, wherein the upper and lower surfaces of the mounting plate are provided with two cantilever beam mass block structures symmetrical about the vertical plane of the mounting seat, the cantilever beam mass block structure includes a cantilever beam and a mass block, the cantilever beam is a straight section with an arc section connected to one end, the straight section end of the cantilever beam is connected to the mounting plate, and the arc section end is connected to the mass block, the mass block is pointed at one end away from the cantilever beam, and fiber optic fixing points are provided on the outer surfaces of the four mass blocks, the optical fibers are fixed on the four mass blocks through the fiber optic fixing points, and gratings are engraved on the optical fibers suspended between the two mass blocks on the same side of the mounting plate.

[0006] As a preferred technical solution, the mounting plate and the four cantilever beam mass block structures are integrally formed and the material is metal.

[0007] As a preferred technical solution, a fiber installation groove is provided at the center of the mass block surface, and the distance L between the ends of the two fiber installation grooves on the same side is fThe thickness of the mounting groove is 2 to 3.5 mm, and the depth h of the mounting groove is 0.3 to 0.7 mm.

[0008] As a preferred technical solution, the mass block has a width B of 18 to 22 mm, a length L of 18 to 21.5 mm, and a mass m of 23.95 g.

[0009] As a preferred technical solution, the width of the cantilever beam is the same as the width of the mass block, the thickness T of the cantilever beam is 0.35-0.5 mm, the height H of the straight section is 5-8.5 mm, and the radius R of the arc section is 4-5 mm.

[0010] As a preferred technical solution, the gratings located on the upper and lower sides of the mounting plate have the same grating region length and different central wavelengths.

[0011] As a preferred technical solution, the grating located on the upper side of the mounting plate has a grating region length of 0.8 to 1.2 mm and a central wavelength of 1460 to 1640 nm.

[0012] As a preferred technical solution, the sensitivity S of the optical fiber sensing structure is:

[0013]

[0014] In the formula, A f is the cross-sectional area of ​​the optical fiber, E f is the Young's modulus of the optical fiber, P e is the elastic-optical coefficient of the optical fiber, E s is the Young's modulus of beryllium bronze, Δθ is the bending arc of the cantilever beam, L f is the distance between the optical fiber installation grooves on the two mass blocks on the same side, L1 is the length of one end of the optical fiber installation groove extending out of the mass block, R is the radius of the arc section of the cantilever beam, T is the thickness of the cantilever beam, H is the height of the straight section of the cantilever beam, L is the length of the mass block, B is the width of the mass block, and h is the depth of the optical fiber installation groove.

[0015] As a preferred technical solution, the resonant frequency f of the optical fiber sensing structure is:

[0016]

[0017] Where π is the circumference of a circle.

[0018] A three-component optical fiber detector for CO2 reservoir monitoring, wherein an X-direction sensor, a Y-direction sensor and a Z-direction sensor are arranged in the housing 1 in the X direction, and the X-direction sensor, the Y-direction sensor and the Z-direction sensor all adopt the optical fiber sensing structure described above.

[0019] The beneficial effects of the present invention are as follows:

[0020] 1. The present invention achieves a significant improvement in the sensitivity of the sensing structure through the design of a four-cantilever beam structure and a double short grating. This design enables the sensing structure to detect tiny vibration signals, which is very critical for monitoring seismic wave vibration signals during CO2 injection.

[0021] 2. The present invention utilizes wavelength division multiplexing technology and a double-grating differential sensing structure, and the sensing structure can automatically compensate for the influence of temperature changes on the measurement results, thereby improving the accuracy and reliability of the measurement.

[0022] 3. The design of the four-cantilever beam structure and the mass block of the present invention improves the lateral anti-interference ability of the sensing structure, so that the vibration signal is mainly transmitted in the main axis direction, reducing the influence of lateral vibration on the measurement result.

[0023] 4. The present invention is able to detect low-frequency vibration signals, which is particularly important for CO2 storage seismic monitoring, because these signals can provide important information about the state of the CO2 reservoir, achieving a clear description of CO2 fluid migration and reservoir stability monitoring.

[0024] 5. As a non-invasive "passive" device, the present invention is not affected by environmental electromagnetic fields, and the material is resistant to strong corrosion and high temperature. It is suitable for monitoring under harsh environmental conditions. It has a simple, compact structure and small size, which makes it easy to install and deploy. It is especially suitable for use in environments with limited space. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic structural diagram of a three-component optical fiber detector for CO2 reservoir monitoring according to the present invention.

[0026] Figure 2 It is a structural schematic diagram of the optical fiber sensing structure of the present invention.

[0027] Figure 3 yes Figure 2 Bottom view of .

[0028] Figure 4 It is the amplitude-frequency response diagram of the optical fiber sensing structure.

[0029] Figure 5 It is the linear sensitivity response diagram of the optical fiber sensing structure.

[0030] Figure 6 This is the 30Hz wavelength drift response diagram of the optical fiber sensing structure.

[0031] Wherein: housing 1; X-direction sensor 2; Y-direction sensor 3; Z-direction sensor 4; mounting plate 22; mass block 211; cantilever beam 212; gratings 23 and 26; optical fiber 24; optical fiber mounting groove 25. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below in conjunction with the accompanying drawings and examples, but the present invention is not limited to the following embodiments.

[0033] exist Figure 1 In the three-component optical fiber detector for CO2 reservoir monitoring of this embodiment, an X-direction sensor 2 is fixedly installed in the X direction in the shell 1, a Y-direction sensor 3 is fixedly installed in the Y direction, and a Z-direction sensor 4 is fixedly installed in the Z direction. The X-direction sensor 2, the Y-direction sensor 3, and the Z-direction sensor 4 have the same structure, which are all optical fiber sensing structures, and are used for CO2 reservoir sensing detection in the X direction, the Y direction, and the Z direction, respectively.

[0034] exist Figure 2 , 3 In the optical fiber sensing structure of this embodiment, the upper and lower surfaces of the mounting plate 22 are both installed with two cantilever beam mass block structures symmetrical about the vertical plane of the mounting plate 22, which are used to maintain the balance of the sensing structure, reduce lateral interference, and reduce structural deformation caused by vibration or temperature change. The cantilever beam mass block structure includes a cantilever beam 212 and a mass block 211. The cantilever beam 212 is a straight section with an arc section connected at one end, which is used to optimize the force transmission path and improve the response efficiency of the structure. The straight section end of the cantilever beam 212 is connected to the mounting plate 22, and the arc section end is connected to the mass block 211. The mass block 211 is pointed at one end away from the cantilever beam 212, which helps to concentrate vibration energy and improve the response speed and sensitivity of the sensing structure to vibration. Optical fiber fixing points are set on the outer surfaces of the four mass blocks 211, and the optical fiber 24 is fixed on the four mass blocks 211 through the optical fiber fixing points. The optical fiber 24 suspended between the two mass blocks 211 on the same side of the mounting plate 22 is engraved with gratings 23 and 26.

[0035] The mounting plate 22 and the four cantilever beam mass blocks of this embodiment are integrally formed and made of metal. A fiber mounting groove 25 is machined at the center of the mass block 211 surface. The distance L between the ends of the two fiber mounting grooves 25 on the same side is f The cantilever beam 212 has a thickness T of 0.4 mm, a height H of the straight section of 7 mm, and a radius R of the arc section of 4.5 mm.

[0036] The gratings 23 and 26 of this embodiment have a grating length of 1 mm and central wavelengths of 1540 nm and 1550 nm, respectively. The two gratings 23 and 26 with different central wavelengths respond to different vibrations or strains, respectively, and their wavelength changes can be detected and analyzed independently, realizing multi-parameter detection on a single optical fiber sensor, thereby increasing the functionality and flexibility of the sensor.

[0037] The sensitivity S of the optical fiber sensing structure of this embodiment is:

[0038]

[0039] A f is the cross-sectional area of ​​the optical fiber, E f is the Young's modulus of the optical fiber, P e is the elastic-optical coefficient of the optical fiber, E s is the Young's modulus of beryllium bronze, Δθ is the bending arc of the cantilever beam, L f is the distance between the optical fiber installation grooves on the two mass blocks on the same side, L1 is the length of one end of the optical fiber installation groove extending out of the mass block, R is the radius of the arc section of the cantilever beam, T is the thickness of the cantilever beam, H is the height of the straight section of the cantilever beam, L is the length of the mass block, B is the width of the mass block, and h is the depth of the optical fiber installation groove.

[0040] The resonant frequency f of the optical fiber sensing structure of this embodiment is:

[0041]

[0042] Where π is the ratio of circumference to diameter

[0043] The working principle of the present invention is as follows:

[0044] Under the influence of environmental vibration, the mass block 211 of the optical fiber sensing structure of the present embodiment vibrates in the vertical direction, wherein one of the two gratings 23 and 26 is stretched during the vibration, while the other grating is compressed. The vibration is detected and the vibration signal is restored by measuring the relative change of the central wavelength of the two gratings. At the same time, the influence of temperature change is eliminated because the wavelength change of the two gratings will offset the temperature effect.

[0045] Example 2

[0046] In this embodiment, the optical fiber sensing structure is that the upper and lower surfaces of the mounting plate 22 are both mounted with two cantilever beam mass block structures symmetrical about the vertical plane of the mounting seat, the cantilever beam mass block structure includes a cantilever beam 212 and a mass block 211, the mass block 211 has a width B of 18 mm, a length L of 18 mm, and a mass m of 23.95 g, the cantilever beam 212 is a straight section with an arc section connected at one end, the thickness T of the cantilever beam 212 is 0.35 mm, the height H of the straight section is 5 mm, the radius R of the arc section is 4 mm, and the straight section end of the cantilever beam 212 is 10 mm. It is connected to the mounting plate 22, and the arc segment end is connected to the mass block 211. The mass block 211 is pointed at one end away from the cantilever beam 212. The center of the outer surface of the four mass blocks 211 is processed with an optical fiber installation groove 25. The distance between the ends of the two optical fiber installation grooves 25 on the same side is 3mm, and the depth h of the installation groove is 0.4mm. Optical fiber fixing points are set in the installation grooves. The optical fibers are fixed on the four mass blocks 211 through the optical fiber fixing points. Gratings are engraved on the suspended optical fibers between the two mass blocks 211 on the same side of the mounting plate 22. Other components and the connection relationship of the components are the same as those in Example 1.

[0047] Example 3

[0048] In this embodiment, the optical fiber sensing structure is that the upper and lower surfaces of the mounting plate 22 are both equipped with two cantilever beam mass block structures symmetrical about the vertical plane of the mounting seat, and the cantilever beam mass block structure includes a cantilever beam 212 and a mass block 211. The width B of the mass block 211 is 22 mm, the length L is 21.5 mm, and the mass m is 23.95 g. The cantilever beam 212 is a straight section with an arc section connected at one end. The thickness T of the cantilever beam 212 is 0.5 mm, the height H of the straight section is 8.5 mm, and the radius R of the arc section is 5 mm. The straight section of the cantilever beam 212 The end is connected to the mounting plate 22, and the arc segment end is connected to the mass block 211. The mass block 211 is pointed at one end away from the cantilever beam 212. The center of the outer surface of the four mass blocks 211 is processed with an optical fiber installation groove 25. The distance between the ends of the two optical fiber installation grooves 25 on the same side is 3.5mm, and the depth h of the installation groove is 0.7mm. Optical fiber fixing points are set in the installation grooves. The optical fibers are fixed on the four mass blocks 211 through the optical fiber fixing points. Gratings are engraved on the suspended optical fibers between the two mass blocks 211 on the same side of the mounting plate 22. Other components and the connection relationship of the components are the same as those in Example 1.

[0049] experiment

[0050] In order to verify the beneficial effects of the present invention, the inventors applied the optical fiber sensing structure of Example 1 to conduct the following experiments.

[0051] Fix the fiber optic sensing structure on the vibration table, connect the fiber optic to the si255 demodulator through a jumper, set a certain acceleration size, select different frequency bands for detection, and determine the frequency suitable for detection by the drift of the central wavelength.

[0052] The amplitude-frequency response of FBG1 and FBG2 was measured and recorded, the resonant frequency and the working flat area of ​​the detector were determined, the sensitivity calibration was performed at an operating frequency of 30 Hz, the sensitivity of FBG1 and FBG2 was recorded, and the sensitivity of the accelerometer was calculated through the optical differential scheme.

[0053] Depend on Figure 4 It can be seen that the amplitude-frequency responses of FBG1 and FBG2 are consistent, which indicates that the two FBGs have similar dynamic characteristics under the same test conditions. Through the amplitude-frequency response test after differentiation, the resonant frequency of the fiber optic sensing structure is 57 Hz and the working flat area is in the range of 10 to 40 Hz, which means that within this frequency range, the fiber optic sensing structure can provide stable vibration detection.

[0054] Depend on Figure 5 It can be seen that the sensitivity of FBG1 is 1151.71pm / g, and the sensitivity of FBG2 is 1083.15pm / g. The sensitivity of the accelerometer is doubled to 2234.8pm / g through the optical differential scheme.

[0055] Depend on Figure 6 It can be seen that the resolution of the optical fiber sensing structure is 0.5 mg and the detection threshold is 2.93 mg. These indicators show that the optical fiber sensing structure can detect very small acceleration changes.

[0056] In summary, the optical fiber sensing structure of the present invention has a good response to vibration signals within the designed operating frequency range and has a strong ability to resist lateral interference.

Claims

1. An optical fiber sensing structure for CO2 reservoir monitoring, characterized in that: The upper and lower surfaces of the mounting plate are both provided with two cantilever beam mass block structures symmetrical about the vertical plane in the mounting plate. The cantilever beam mass block structure includes a cantilever beam and a mass block. The cantilever beam is a straight section with an arc section connected to one end. The straight section end of the cantilever beam is connected to the mounting plate, and the arc section end is connected to the mass block. The mass block is pointed at one end away from the cantilever beam. Optical fiber fixing points are provided on the outer surfaces of the four mass blocks. Optical fibers are fixed on the four mass blocks through the optical fiber fixing points. Gratings are engraved on the suspended optical fibers between the two mass blocks on the same side of the mounting plate. The sensitivity S of the optical fiber sensing structure is: S= In the formula, The cross-sectional area of ​​the optical fiber, is the Young's modulus of the optical fiber, is the elastic-optical coefficient of the optical fiber, is the Young's modulus of beryllium bronze, is the bending curvature of the cantilever beam, is the distance between the fiber mounting slots on the two mass blocks on the same side, is the length of the mass block extending out of one end of the optical fiber installation slot, R is the radius of the arc section of the cantilever beam, T is the thickness of the cantilever beam, H is the height of the straight section of the cantilever beam, L is the length of the mass block, B is the width of the mass block, and h is the depth of the optical fiber installation slot.

2. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 1, characterized in that: The mounting plate and the four cantilever beam mass block structures are integrally formed and made of metal.

3. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 1, characterized in that: The center of the mass block surface is provided with an optical fiber installation groove, and the distance between the optical fiber installation grooves on the two mass blocks on the same side is , is 2~3.5mm, and the depth h of the installation groove is 0.3~0.7mm.

4. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 3 is characterized in that: The mass block has a width B of 18 to 22 mm, a length L of 18 to 21.5 mm, and a mass m of 23.95 g.

5. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 4 is characterized in that: The width of the cantilever beam is the same as that of the mass block, the thickness T of the cantilever beam is 0.35-0.5 mm, the height H of the straight section is 5-8.5 mm, and the radius R of the arc section is 4-5 mm.

6. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 1, characterized in that: The gratings located on the upper and lower sides of the mounting plate have the same grating area length and different central wavelengths.

7. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 1, characterized in that: The grating located on the upper side of the mounting plate has a grating region length of 0.8 to 1.2 mm and a central wavelength of 1460 to 1640 nm.

8. The optical fiber sensing structure for CO2 reservoir monitoring according to claim 1, characterized in that: The resonant frequency f of the optical fiber sensing structure is: In the formula, is the ratio of pi.

9. A three-component optical fiber detector for CO2 reservoir monitoring, characterized in that: An X-direction sensor is arranged in the X direction, a Y-direction sensor is arranged in the Y direction, and a Z-direction sensor is arranged in the Z direction in the housing (1); the X-direction sensor, the Y-direction sensor, and the Z-direction sensor all adopt the optical fiber sensing structure according to any one of claims 1 to 8.

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