Sensitivity-enhanced fiber-optic magnetic field sensor and magnetic positioner

Abstract

The application discloses a sensitivity-enhanced optical fiber magnetic field sensor and a magnetic positioner. The strain concentration type optical fiber magnetic field sensor comprises alloy one, alloy two, magnetostrictive material, sensing optical fiber and first spring. The magnetostrictive material is arranged on the alloy two. The spring is arranged between the alloy one and the alloy two. The sensing optical fiber is fixed on the magnetostrictive material. The magnetostrictive material is elongated under the action of external magnetic field, and drives the alloy two to compress the first spring. In turn, the reaction force of the first spring will exert pre-stress on the magnetostrictive material, so as to improve the strain size of the magnetostrictive material to the magnetic field. The mechanical gain structure adopted by the application simultaneously exerts pre-stress on the terbium-dysprosium-iron magnetostrictive material, enhances the sensitivity of the optical fiber sensor, and can realize the purpose of magnetic positioning of the downhole casing coupling. Meanwhile, the application is suitable for casing damage detection.

Description

Technical Field

[0001] This invention specifically relates to a strain-concentrated fiber optic grating magnetic field sensor, belonging to the field of fiber optic sensing technology.

[0002] By designing the sensor's mechanical gain structure and innovating the coupling method between the sensor and the magnetostrictive material, the combined effect significantly improves the sensor's sensitivity, meeting the needs of weak magnetic field detection in downhole applications such as oil well casing joint identification and defect identification. Background Technology

[0003] In oil production, well depth measurement is a crucial step in oil logging and an essential procedure. Well depth is the most fundamental parameter in oil logging; all other logging parameters are calculated and measured based on it. Therefore, well depth is an absolute parameter in oil logging, directly impacting the success or failure of other logging operations. Accurate well depth measurement is crucial to ensure the precision of other logging parameters. Currently, magnetic positioning logging technology stands out in the field of well depth measurement due to its advantages such as temperature and pressure resistance, strong anti-interference capabilities, high detection sensitivity, and low detection cost. This technology works by first magnetizing the casing with a permanent magnet until it is magnetically saturated. Then, a fiber optic magnetic field sensor picks up the changes in the magnetic field at the coupling caused by differences in thickness or material permeability, and subsequently demodulates and analyzes the signal. While traditional electromagnetic field sensors are technologically mature, they often suffer from limitations such as susceptibility to interference, poor tolerance, and difficulties in wiring under extreme environments such as strong electromagnetic interference, high temperature, high pressure, and flammable and explosive conditions. In contrast, fiber optic magnetic field sensors have become a research hotspot in recent years due to their unique advantages such as resistance to electromagnetic interference, intrinsic safety, corrosion resistance, small size, and ease of networking. Currently, existing fiber optic magnetic field sensors mainly employ a structure combining fiber Bragg gratings and magnetostrictive materials. The basic principle is to utilize the mechanical deformation of the magnetostrictive material under an external magnetic field, which directly acts on the fiber grating, causing axial strain in the fiber and resulting in a Bragg wavelength shift, thereby achieving magnetic field measurement. However, existing structures still have the following prominent problems in practical applications: low strain transfer efficiency. In traditional structures, the optical fiber is usually directly bonded or encapsulated on the surface or center of the magnetostrictive material. Due to the difference in modulus between the two and the shear hysteresis effect of the bonding layer, the macroscopic deformation generated by the magnetostrictive material is difficult to be completely and uniformly transferred to the fiber core, resulting in limited sensor sensitivity. For weak magnetic field environments or high-precision detection requirements, the strain generated by conventional uniform deformation structures is relatively small. In complex background noise, small wavelength drift signals are often difficult to demodulate accurately, limiting the sensor's detection limit. Secondly, the structure lacks optimization. Existing technologies mostly focus on improving material properties, while neglecting to actively enhance strain response through mechanical structural design. There is a lack of an effective mechanism to "converge" and "amplify" the deformation generated by magnetostriction.

[0004] Therefore, this invention proposes a novel structure that can effectively concentrate the magnetic field-induced mechanical deformation in the sensitive area of ​​the optical fiber, achieving a "stress concentration" effect, thereby significantly improving the sensitivity and response accuracy of the sensor and solving the key technical problems in the field of weak magnetic field detection in optical fibers. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a sensitivity-enhanced fiber optic magnetic field sensor and a magnetic positioning device that improves the sensitivity of the sensor.

[0006] By innovating the coupling method between a specific mechanical gain structure and the sensor and the magnetostrictive material, the macroscopic deformation of the magnetostrictive material is transformed into microscopic high-pressure strain in the fiber grating region, thus solving the problems of low sensitivity and poor strain transfer efficiency of existing sensors.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: This invention first provides a sensitivity-enhanced fiber optic magnetic field sensor, comprising alloy one, alloy two, a magnetostrictive material, a sensing fiber, and a first spring; the magnetostrictive material is disposed on alloy two; the spring is disposed between alloy one and alloy two; the sensing fiber is fixed on the magnetostrictive material; the magnetostrictive material elongates under the action of an external magnetic field, causing alloy two to compress the first spring, and conversely, the reaction force of the first spring applies prestress to the magnetostrictive material, thereby increasing the strain of the magnetostrictive material to the magnetic field.

[0008] The strain-concentrated fiber optic magnetic field sensor provided by this invention includes a magnetostrictive material and a stress concentration conversion structure composed of a ceramic ferrule, a ceramic ferrule, and an optical fiber. This stress concentration conversion structure features a variable cross-section design (the cross-sections of the ceramic ferrules are larger and smaller than the optical fiber cross-sections, creating a cross-sectional difference). It utilizes the principle of area difference to convert the small displacement of the large cross-section into a high-concentration stress of the narrow cross-section.

[0009] The strain-concentrated fiber optic magnetic field sensor provided by this invention guides the displacement generated by the strip-shaped terbium-dysprosium magnetostrictive material to the fiber optic sensitive region through a nested design of L-shaped alloy one and U-shaped alloy two. Structural constraints achieve stress concentration on a first spring between alloy one and alloy two, applying a stable axial prestress to the magnetostrictive material through spring thrust. This design not only adjusts the sensor's initial operating point but also significantly improves the material's strain response slope to an external magnetic field.

[0010] The present invention also provides a magnetic positioning device, including a magnetic positioning device housing, a magnet, and the strain-concentrated fiber optic magnetic field sensor provided above. The strain-concentrated fiber optic magnetic field sensor is disposed inside the magnetic positioning device housing; the magnet is disposed on both sides of the strain-concentrated fiber optic magnetic field sensor.

[0011] The present invention also provides a magnetic positioning instrument. One side of the fiber optic magnetic field sensor package shell is made of industrial pure iron magnetic material to conduct the magnetic circuit. Combined with neodymium iron boron strong magnets on both sides, it ensures that the sleeve coupling under test can reach the magnetic saturation state, providing a strong original signal source for the sensor.

[0012] The beneficial effects of this invention are as follows: This invention provides a stress-concentrated fiber optic magnetic field sensor to address the challenges of high temperature and pressure in downhole drilling, where high sensitivity is required. This sensor is applicable to various working scenarios. By combining it with a magnetostrictive material and using a ceramic ferrule to suspend the grating region, the strain concentration effect enhances the sensitivity of the fiber optic grating, amplifying the minute strain of the material onto the grating. The employed mechanical gain structure simultaneously applies prestress to the terbium-dysprosium magnetostrictive material, further enhancing the sensitivity of the fiber optic sensor. This enables magnetic positioning of casing couplings in downhole drilling and is also suitable for casing damage detection.

[0013] By designing a strain concentration structure, the strain generated by the magnetostrictive material can be concentrated into the fiber grating, amplifying the strain on the fiber grating and thus improving the sensitivity of the fiber grating magnetic field sensor. The strain in the magnetostrictive material caused by the magnetic field change is uniformly distributed on the material, and therefore the strain is also uniformly transferred to the optical fiber attached to the material surface. Ideally, the strain distributed on the optical fiber is the same as the strain distributed on the magnetostrictive material; therefore, it is necessary to concentrate the uniformly distributed strain on the optical fiber to the location of the fiber grating to achieve strain concentration. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of the fiber optic magnetic field sensor in Embodiment 1 of the present invention.

[0015] Figure 2 This is a schematic diagram of the strain concentration structure of the fiber optic magnetic field sensor of the present invention.

[0016] Figure 3 This is a schematic diagram of the magnetic positioning device of the present invention.

[0017] Among them: spring 1, spring 2, permanent magnet 3, permanent magnet 4, fiber optic magnetic field sensor 5, magnetic positioning device housing 6, alloy 7, alloy 8, magnetostrictive material 9, optical fiber 10, high temperature adhesive 11, ceramic ferrule 12, spring 13. Detailed Implementation

[0018] 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.

[0019] Example 1 This embodiment provides a sensitivity-enhanced fiber optic magnetic field sensor, see [link / reference]. Figure 1The system comprises alloy 7, alloy 8, magnetostrictive material 9, optical fiber 10, high-temperature adhesive 11, ceramic ferrule 12, and spring 13. Alloy 8 is a U-shaped alloy with a groove on one side, having two walls: a high wall and a low wall. The groove is located on the high wall. One side of the magnetostrictive material 9 is embedded in the groove of alloy 8, while the other side of the magnetostrictive material 9 only contacts the surface of the low wall of alloy 8. Spring 13 is positioned between alloy 7 and alloy 8. Optical fiber 10 is bonded to magnetostrictive material 9 through a small hole between alloy 7 and alloy 8. The high-temperature adhesive 11 used is 353ND optical adhesive.

[0020] Alloys 7 and 8 are bonded together with high-temperature adhesive. A spring 13 is placed between alloys 7 and 8. When the spring 13 is relaxed, it only contacts the surfaces of alloys 7 and 8. The magnetostrictive material 9 elongates under the influence of an external magnetic field, causing alloy 8 to move towards alloy 7 and compress the spring 13. The reaction force of the spring 13 applies prestress to the magnetostrictive material 9, thereby increasing the sensor sensitivity. One end of the optical fiber 10 passes through a small hole in alloy 7, and the other end is bonded to the magnetostrictive material and passes through alloy 8, forming the optical fiber's inlet and outlet ports. The sensor also increases sensitivity by employing the stress concentration effect. The optical fiber 10 passes through ceramic ferrules 12 and is bonded to amplify the deformation of the magnetostrictive material to the grating region of the optical fiber. This strain-concentrated fiber optic magnetic field sensor is suitable for detecting oil well casing couplings and pipeline defects.

[0021] When the magnetostrictive material 9 is stretched under the action of an external magnetic field, it causes the alloy 8 to compress the spring. In turn, the reaction force of the spring 13 will apply prestress to the magnetostrictive material 9, thereby increasing the strain of the magnetostrictive material 9 to the magnetic field.

[0022] like Figure 2 As shown, two ceramic sleeves 12 are on both sides of the fiber grating 10. The overall structure changes from two thicker sleeve cylinders to a smaller fiber grating when viewed from both sides towards the middle, and the overall structural radius changes.

[0023] By performing mechanical analysis on the strain-concentrated structure, we can obtain:

[0024] Where F is the magnitude of the force on the optical fiber; K, K f and K C These represent the stiffness coefficients of the overall strain structure, the optical fiber, and the ceramic ferrule, respectively. , and These represent the length changes of the overall strain structure, the optical fiber, and the ceramic sleeve, respectively; these variables satisfy the following relationship:

[0025] Among them, L C L f L and E represent the lengths of the ceramic sleeve, the fiber grating suspended in the middle, and the overall strain concentration structure, respectively; c and E f S represents the Young's modulus of the ceramic sleeve and the optical fiber, respectively; c and S f These represent the cross-sectional area of ​​the ceramic sleeve and the cross-sectional area of ​​the optical fiber, respectively.

[0026] Under a force F, the strain of the overall strain concentration structure and strain of fiber grating It can be represented as:

[0027] The magnification factor on a fiber Bragg grating is defined as... The following formula can be derived:

[0028] It can be deduced that when hour, Therefore, the strain of the fiber grating can be amplified by increasing the cross-sectional area of ​​the ceramic sleeve, and L can also be changed. f The strain amplification of the fiber grating is achieved by adjusting the size of L.

[0029] Example 2 This embodiment provides a magnetic positioning device, such as Figure 3 As shown, the device consists of spring 1, spring 2, magnet 3, magnet 4, fiber optic magnetic field sensor 5, and magnetic positioning device housing 6. Magnets 3 and 4 are located on both sides of the fiber optic magnetic field sensor 5, and are neodymium iron boron strong magnets in a ring-shaped cylindrical form. Springs 1 and 2 on both sides of the magnets provide cushioning to prevent damage to internal components during the well-drilling process. The fiber optic magnetic field sensor 5 is the sensitivity-enhanced fiber optic magnetic field sensor provided in Example 1.

[0030] The magnetic positioning device housing 6 has spiral threads on both sides. Magnets 3 and 4 are fixed by tightening. Because the magnets have magnetic force, they will attract the magnetic ring and the spring. That is, when the springs 1 and 2 are relaxed, they are attracted to magnets 3 and 4. The length of the fiber optic magnetic field sensor 5 is the same as the length of the magnetic positioning device.

[0031] At least four pairs of magnets are fixedly installed on both sides of the fiber optic magnetic field sensor 5. The size of the magnets is determined by the diameter of the magnetic positioning instrument's outer shell. The larger the diameter of the magnetic positioning instrument, the larger the size of the magnets it can accommodate, thus ensuring that the magnetic field generated by the magnets can completely magnetize the sleeve coupling.

[0032] In this embodiment, there are 4 pairs of magnets on each side.

[0033] During testing, the magnetic locator of this invention is placed inside the casing of the oil well to be tested. A magnet is used to magnetize the casing. During internal measurements, the difference in magnetic permeability between the casing coupling and air causes magnetic field lines to refract and "leak" into the air. During external measurements, the magnetic reluctance caused by the thickness variation at the coupling also causes magnetic field lines to refract. The fiber optic magnetic field sensor captures this refracted magnetic field, causing the terbium-dysprosium magnetostrictive material to elongate, thereby stretching the fiber optic grating and causing a shift in its center wavelength. The stronger the magnetic field, the greater the center wavelength shift. The fiber optic magnetic field sensor's optical fiber is connected to a demodulator. The demodulator converts the optical signal collected by the sensor into an electrical signal and transmits it to computer software. After processing and judging the signal, the computer software obtains a relatively regular periodic signal, which is the coupling signal. Therefore, this invention can achieve both internal and external measurements, enabling precise coupling positioning.

[0034] During testing, the magnetic positioner is placed on the push rod, and the speed of the push rod is controlled by a stepper motor. The lift-off value is precisely controlled by a displacement stage, thereby enabling signal measurement.

[0035] Furthermore, the fiber optic magnetic field sensor of this invention, due to its small size, can be used for annular operations downhole. If a ferromagnetic object is present along the detection path, the sensor can collect different signals based on the object's shape and size, thereby locating the object. Therefore, in addition to measuring well depth, if ferromagnetic material is present outside the casing, the sensor can also detect signal changes. Since this signal differs from the coupling signal, it can be distinguished after processing by computer software. For different situations encountered in actual production logging, the fiber optic magnetic field sensor of this invention can also perform internal and external pipeline detection. When defects exist on the casing, leakage magnetic fields are generated. The fiber optic magnetic field sensor captures the leaked magnetic field, and after signal processing and judgment by computer software, the size, shape, and depth of the defects on the tested pipeline, as well as the type of damage, can be deduced. The fiber optic magnetic field sensor of this invention can not only be used in the field of non-destructive testing and pipeline positioning in oil pipelines, but also has wide applications in smart grids, medical and pharmaceutical fields, and chemicals.

Claims

1. A sensitivity-enhanced fiber optic magnetic field sensor, characterized in that, The device includes alloy one, alloy two, a magnetostrictive material, a sensing optical fiber, and a first spring. The magnetostrictive material is disposed on alloy two. The spring is disposed between alloy one and alloy two. The sensing optical fiber is fixed to the magnetostrictive material. Under the action of an external magnetic field, the magnetostrictive material elongates, causing alloy two to compress the first spring. Conversely, the reaction force of the first spring applies prestress to the magnetostrictive material, thereby increasing the strain of the magnetostrictive material to the magnetic field.

2. The sensitivity-enhanced fiber optic magnetic field sensor according to claim 1, characterized in that, The sensing fiber includes an optical fiber and a ceramic ferrule, the ceramic ferrule being bonded to a magnetostrictive material.

3. The sensitivity-enhanced fiber optic magnetic field sensor according to claim 2, characterized in that, The bonding points between the ceramic ferrule and the magnetostrictive material are as follows: the side of the ceramic ferrule closest to the grid region is bonded to the optical fiber, and the side furthest from the grid region is bonded to the magnetostrictive material.

4. The sensitivity-enhanced fiber optic magnetic field sensor according to claim 2, characterized in that, The sensitivity of the sensor can be increased by adjusting the length of the suspended grating area and the magnetostrictive material.

5. The sensitivity-enhanced fiber optic magnetic field sensor according to claim 1, characterized in that, The alloy 2 is a U-shaped alloy with a groove on one side. The U-shaped alloy has two walls, a high wall and a low wall, and the groove is provided on the high wall. One end of the magnetostrictive material is embedded in the groove of the U-shaped alloy, and the other side of the magnetostrictive material is in contact only with the surface of the low wall of the U-shaped alloy.

6. The sensitivity-enhanced fiber optic magnetic field sensor according to claim 1, characterized in that, The alloy is L-shaped.

7. The sensitivity-enhanced fiber optic magnetic field sensor according to claim 1, characterized in that, The magnetostrictive material is a terbium-dysprosium magnetostrictive material.

8. A magnetic positioning device, characterized in that, The device includes a magnetic positioning device housing, a magnet, and a sensitivity-enhanced fiber optic magnetic field sensor as described in any one of claims 1-7, wherein the strain-concentrated fiber optic magnetic field sensor is disposed inside the magnetic positioning device housing; and the magnet is disposed on both sides of the strain-concentrated fiber optic magnetic field sensor.

9. The magnetic positioning device according to claim 8, characterized in that, A second spring is also provided inside the housing of the magnetic positioning device, and the second spring is respectively located on the outside of the magnet.

10. The magnetic positioning device according to claim 8, characterized in that, At least four pairs of magnets are arranged on both sides of the strain-concentrated fiber optic magnetic field sensor.

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