Optical fiber acoustic wave circumferential sensitization structure and method

Abstract

The invention relates to the technical field of cased well logging, and discloses an optical fiber acoustic wave circumferential sensitization structure and method.The structure is characterized in that a hollow thin-walled cylinder is annularly sleeved with a plurality of metal spring rings, a plurality of sheets circumferentially surround the hollow thin-walled cylinder and are attached to the metal spring rings, and a gap is formed between every two adjacent sheets to form an opening; the plurality of groups of fiber gratings are annularly wound on the plurality of sheets; a cylindrical structure is selected, one or more groups of sheets, metal spring rings and a hollow thin-walled cylinder are utilized to form an optical fiber acoustic wave circumferential sensitization structure, and an optical fiber is wound on the optical fiber acoustic wave circumferential sensitization structure, so that the purpose of improving the acoustic wave measurement sensitivity is achieved, and a stable and reliable optical fiber acoustic wave circumferential sensitization method is formed. The problem that the sensitivity of the bare fiber grating for measuring the sound wave signal is too low is solved.

Description

Technical Field

[0001] The invention relates to the technical field of cased well logging, and in particular to an optical fiber acoustic wave circumferential sensitivity enhancement structure and method. Background Art

[0002] Distributed fiber optic sensing technology has the advantages of small size, high temperature resistance, long-distance sensing, safety and environmental protection. It can provide physical information such as temperature, stress, sound waves, vibration, etc. along the fiber optic line, and can achieve long-term, real-time and effective monitoring of oil and gas wells, optimize oil production, and improve recovery rate. It has shown great advantages and value in the field of oil exploration. Among them, distributed acoustic sensing (DAS) technology has broad application prospects in the field of oil exploration. DAS technology usually uses single-mode optical fiber as a sensor element. Due to the high Young's modulus of the optical fiber itself, the sensitivity of direct detection of acoustic pressure is low.

[0003] Fiber Bragg grating (FBG) sensitization is a technology that improves the ability of bare fiber Bragg grating to receive sound waves by optimizing the design of fiber Bragg grating structure. Good sensitization can greatly increase the sensitivity of fiber Bragg grating acoustic wave sensing. Due to the complex downhole environment, high-precision acoustic wave monitoring is required. The fiber Bragg grating sensitization structure is reasonably designed to achieve the purpose of acoustic well logging. The existing fiber Bragg grating sensitization technology cannot fully meet the requirements of downhole acoustic wave monitoring due to complex structure, poor sensitization effect or delayed dynamic response of polymer.

[0004] In recent years, researchers have explored a variety of external sensitization methods for fiber Bragg gratings, that is, fixing the existing optical fiber on external sensitization structures such as films, thin plates, and cylinders. Compared with flat structures, cylindrical structures are easier to wind long optical fibers. Fiber winding sensitivity enhancement has received widespread attention in fiber optic hydrophones based on Michelson interferometers. This method can also be applied to distributed acoustic wave sensing technology. However, standard single-mode optical fibers are used in existing fiber optic distributed acoustic wave sensing systems. The acoustic wave sensitivity of single-mode optical fibers is not high enough to fully meet the requirements for downhole acoustic wave monitoring. Summary of the invention

[0005] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to provide a fiber optic acoustic wave circumferential sensitivity enhancement structure and method to solve the technical problems that the standard single-mode optical fiber used in the existing fiber optic distributed acoustic wave sensing system has insufficient acoustic wave sensitivity and weak detection of acoustic wave signals.

[0006] The present invention is achieved through the following technical solutions:

[0007] A fiber acoustic wave circumferential sensitivity enhancement structure comprises a plurality of thin sheets, a plurality of groups of metal spring coils, a hollow thin-walled cylinder and a plurality of groups of optical fiber gratings; the plurality of groups of metal spring coils are annularly sleeved on the hollow thin-walled cylinder, a plurality of thin sheets circumferentially surround the hollow thin-walled cylinder and are attached to the plurality of groups of metal spring coils, wherein gaps are provided between adjacent thin sheets to form openings, and a plurality of groups of optical fiber gratings are annularly wound around the plurality of thin sheets.

[0008] Preferably, the diameters of the groups of metal spring coils are smaller than the outer diameter of the hollow thin-walled cylinder.

[0009] Preferably, a plurality of arc-shaped grooves are provided on the hollow thin-walled cylinder along the circumferential direction from top to bottom, and a plurality of groups of metal spring rings are annularly sleeved in the corresponding arc-shaped grooves.

[0010] Preferably, the number of the plurality of sheets is an even number.

[0011] Preferably, the plurality of thin sheet structures are of the same size, and the plurality of thin sheets are curved, circumferentially surrounding the hollow thin-walled cylinder to form a cylindrical structure.

[0012] Preferably, a groove is provided on one side of the plurality of thin sheets close to the metal spring ring, and the outer surface of the metal spring ring is embedded in the annular groove formed by the plurality of thin sheets.

[0013] A method for increasing the sensitivity of optical fiber acoustic waves in the circumferential direction, based on the optical fiber acoustic wave circumferentially increasing sensitivity structure described above, comprises the following process:

[0014] A plurality of metal spring rings are sleeved on the outside of a hollow thin-walled cylinder, and a plurality of thin sheets are arranged on the outside of the metal spring rings, wherein the openings between adjacent thin sheets ensure that they generate a vibration response under the action of sound waves, thereby forming a cylindrical structure with plane symmetry; a fastening structure is applied to the outside of the plurality of thin sheets and a certain pre-tightening force is applied to ensure that the thin sheets, the metal spring rings, and the hollow thin-walled cylinder remain in a fixed position when the optical fiber is wound; the positions of the plurality of thin sheets are adjusted so that they are completely fitted with the metal spring rings and form a standard cylindrical optical fiber acoustic wave circumferential sensitization structure; the cylindrical structure with a fastening device is fixed on a fiber winding machine, and the optical fiber grating is wound on the outer surface of the cylindrical structure with a constant tension, and after the optical fiber grating is wound, the fastening structure located on the outer surface of the plurality of thin sheets is removed; the deformation caused by the metal spring rings under the action of sound waves is converted into the axial strain of the optical fiber grating, resulting in a periodic change of the optical fiber grating and its own elastic-optical effect, and the reflection wavelength of the optical fiber grating is displaced, thereby realizing high-sensitivity acoustic wave signal detection.

[0015] Preferably, the metal spring coil is made of metal material, including austenitic stainless steel SS304 or austenitic stainless steel SS316.

[0016] Preferably, the sheet and the hollow thin-walled tube are made of high molecular polymers, including polyetheretherketone, polyimide or polytetrafluoroethylene.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects:

[0018] The present invention provides a fiber acoustic wave circumferential sensitivity enhancement structure, wherein a plurality of groups of metal spring coils are annularly sleeved on a hollow thin-walled cylinder, a plurality of thin sheets circumferentially surround the hollow thin-walled cylinder and are attached to the plurality of metal spring coils, wherein gaps are provided between adjacent thin sheets to form openings, and a plurality of groups of optical fiber gratings are annularly wound on the plurality of thin sheets; a cylindrical structure is selected, and the fiber acoustic wave circumferential sensitivity enhancement structure is formed by utilizing one or more groups of thin sheets, metal spring coils and the hollow thin-walled cylinder, and the optical fiber is wound around the fiber acoustic wave circumferential sensitivity enhancement structure to achieve the purpose of improving the acoustic wave measurement sensitivity, thereby forming a stable and reliable fiber acoustic wave circumferential sensitivity enhancement method, and solving the problem of too low sensitivity of bare fiber grating in measuring acoustic wave signals.

[0019] Furthermore, a plurality of arcuate grooves are provided on the hollow thin-walled cylinder from top to bottom along the circumferential direction, and a plurality of groups of metal spring rings are annularly sleeved in the corresponding arcuate grooves, thereby improving the stability of the metal spring rings in assembling the hollow thin-walled cylinder.

[0020] Furthermore, the number of thin sheets is an even number, the structures of the thin sheets are the same in size, and the thin sheets are all curved, circumferentially surrounding the hollow thin-walled cylinder to form a cylindrical structure. The openings between the thin sheets ensure that they produce a vibration response under the action of sound waves, forming a plane-symmetrical cylindrical structure.

[0021] Furthermore, a groove is provided on one side of the plurality of thin sheets close to the metal spring ring, and the outer surface of the metal spring ring is embedded in the annular groove formed by the plurality of thin sheets, thereby ensuring the stability of the assembly of the plurality of thin sheets on the metal spring ring.

[0022] The present invention also provides a method for circumferential sensitization of optical fiber acoustic waves. In terms of performance, the combined structure of the arc-shaped thin sheet, the metal spring coil and the hollow thin-walled cylinder not only increases the effective area of ​​the sound pressure, but also converts the radial pressure of the sound wave into the axial strain of the optical fiber grating, thereby greatly increasing its sensitivity to the acoustic wave signal. In addition, in well logging applications, the acoustic wave signal detected by the circumferential sensitization method of optical fiber acoustic waves described in the present invention has excellent consistency and stability. Its small size can be combined with other well logging methods to measure multiple downhole parameters at the same time. It is easier to form a distributed optical fiber acoustic wave measurement array based on wavelength division multiplexing technology, which is suitable for various application scenarios such as production profiles, injection profiles, wellbore integrity, fracturing monitoring, CCUS, etc., and can achieve higher sensitivity in combination with high-resolution wavelength detection technology. Therefore, the present invention is of great significance to improving the acoustic wave detection sensitivity and dynamic acoustic wave signal response characteristics in DAS technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the optical fiber acoustic wave circumferential sensitivity enhancement structure in the present invention;

[0024] Figure 2 A top view of the optical fiber acoustic wave circumferential sensitivity enhancement structure of the present invention;

[0025] Figure 3 These are the test results of the optical fiber acoustic wave circumferential sensitivity enhancement structure in the present invention under continuous and pulsed acoustic wave fields.

[0026] In the figure: 1- thin sheet; 2- metal spring coil; 3- hollow thin-walled cylinder; 4- fiber Bragg grating. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0028] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0029] The purpose of the present invention is to provide a fiber acoustic wave circumferential sensitivity enhancement structure and method to solve the technical problems of the existing fiber distributed acoustic wave sensing system using standard single-mode optical fiber, the acoustic wave sensitivity is not high enough, and the detection of acoustic wave signals is relatively weak. By utilizing the excellent dynamic frequency response characteristics of the metal spring coil, the delay of the conventional cylindrical structure in the dynamic response of acoustic waves can be overcome, and the response sensitivity of the fiber grating to acoustic waves can be significantly improved. The fiber acoustic wave circumferential sensitivity enhancement structure has the advantages of simple manufacturing process, short production cycle, low cost, stable performance, and good consistency. The method can realize accurate measurement of acoustic wave signals of oil and gas wells, and is suitable for various application scenarios such as production profiles, injection profiles, wellbore integrity, fracturing monitoring, CCUS, etc., while meeting the high temperature, high pressure, and strong corrosion working environment underground.

[0030] See also Figure 1 In one embodiment of the present invention, a fiber acoustic wave circumferential sensitivity enhancement structure is provided, comprising a plurality of thin sheets 1, a plurality of groups of metal spring coils 2, a hollow thin-walled cylinder 3 and a plurality of groups of optical fiber gratings 4; the plurality of groups of metal spring coils 2 are annularly sleeved on the hollow thin-walled cylinder 3, a plurality of thin sheets 1 circumferentially surround the hollow thin-walled cylinder 3 and are attached to the plurality of groups of metal spring coils 2, wherein gaps are provided between adjacent thin sheets 1 to form openings, and a plurality of groups of optical fiber gratings 4 are annularly wound around the plurality of thin sheets 1.

[0031] Specifically, the diameters of the groups of metal spring coils 2 are smaller than the outer diameter of the hollow thin-walled cylinder 3 .

[0032] Specifically, a plurality of arc-shaped grooves are provided on the hollow thin-walled cylinder 3 along the circumferential direction from top to bottom, and a plurality of groups of metal spring rings 2 are annularly sleeved in the corresponding arc-shaped grooves.

[0033] Specifically, the number of the plurality of thin sheets 1 is an even number; the structure and size of the plurality of thin sheets 1 are the same, and the plurality of thin sheets 1 are all curved, and circumferentially surround the hollow thin-walled cylinder 3 to form a cylindrical structure. A groove is provided on one side of the plurality of thin sheets 1 close to the metal spring ring 2, and the outer surface of the metal spring ring 2 is embedded in the annular groove formed by the plurality of thin sheets 1.

[0034] In the present invention, the fiber Bragg grating 4 must ensure that at least two gratings are wound around a fiber acoustic wave circumferential sensitivity enhancement structure to ensure the stability and sensitivity of acoustic wave detection.

[0035] The present invention also provides a method for increasing the sensitivity of optical fiber acoustic waves in the circumferential direction, based on the optical fiber acoustic wave circumferentially increasing sensitivity structure described above, comprising the following process:

[0036] A plurality of metal spring coils 2 are sleeved on the outside of the hollow thin-walled cylinder 3, and a plurality of thin sheets 1 are arranged on the outside of the metal spring coils 2, wherein the openings between adjacent thin sheets 1 ensure that they produce a vibration response under the action of sound waves, forming a cylindrical structure with plane symmetry; a fastening structure is applied to the outside of the plurality of thin sheets 1 and a certain pre-tightening force is applied to ensure that the thin sheets 1, the metal spring coils 2, and the hollow thin-walled cylinder 3 remain in a fixed position during the winding of the optical fiber; the positions of the plurality of thin sheets 1 are adjusted so that they are completely fitted with the metal spring coils 2 and form a standard cylindrical shape The invention discloses a fiber acoustic wave circumferential sensitivity enhancement structure; a cylindrical structure with a fastening device is fixed on a fiber winding machine, and a fiber Bragg grating 4 is wound on the outer surface of the cylindrical structure with a constant tension. After the fiber Bragg grating 4 is wound, the fastening structure located on the outer surface of the plurality of thin sheets 1 is removed; the deformation of the metal spring coil 2 caused by the acoustic wave is converted into the axial strain of the fiber Bragg grating 4, resulting in the periodic change of the fiber Bragg grating 4 and its own elastic-optic effect, and the reflection wavelength of the fiber Bragg grating 4 is displaced, thereby realizing high-sensitivity acoustic wave signal detection.

[0037] Example 1

[0038] like Figure 1 and Figure 2 As shown: 4 thin sheets 1, 2 metal spring coils 2, a hollow thin-walled cylinder 3 and a grating optical fiber 4, wherein the thin sheet 1 is a π radian thin sheet.

[0039] The outer diameter of the hollow thin-walled cylinder 3 is 30 mm, and it serves as the support body of the entire sensitivity enhancement structure. The outer diameter of the metal spring coil 2 is slightly smaller than the outer diameter of the hollow thin-walled cylinder 3. At the same time, in order to ensure that the metal spring coil 2 does not move axially in the hollow thin-walled cylinder 3 but only undergoes radial deformation, the outer wall of the hollow thin-walled cylinder 3 has two circumferential arc grooves.

[0040] The four thin sheets 1 form a hollow cylinder with an outer diameter of 50 mm. The four thin sheets 1 are symmetrical structures and four openings are formed between the thin sheets 1. At the same time, there is no adhesive or welding between the metal spring ring 2 and the thin sheet 1 to ensure the axial deformation of the optical fiber grating 4. Before winding the optical fiber grating 4, the cylindrical structure composed of the hollow thin-walled cylinder 3, the metal rubber ring 2 and the thin sheet 1 is fixed by a fastening structure. By adjusting the position of the thin sheet 1, it is ensured that the openings between the thin sheets 1 are consistent and a standard cylinder is formed. The optical fiber grating 4 is wound on the surface of the cylinder composed of the four thin sheets 1 with a constant tension. It is necessary to ensure that at least two gratings are wound on a fiber acoustic wave circumferential sensitivity enhancement structure. After winding, the fastening device is removed to form a fiber acoustic wave circumferential sensitivity enhancement structure.

[0041] The metal spring ring 2 is made of metal materials with high elastic modulus and excellent corrosion resistance, including but not limited to austenitic stainless steel SS304 or SS316. The thin sheet 1 and the hollow thin-walled tube 3 are made of high molecular polymers with high temperature resistance, corrosion resistance and high mechanical strength, including but not limited to polyetheretherketone, polyimide, polytetrafluoroethylene, etc.

[0042] Example 2

[0043] The acoustic wave signal is detected by using the fiber acoustic wave circumferential sensitization method. Four thin sheets 1, two metal spring coils 2, and a fiber Bragg grating 4 with two fiber FBG gratings are selected to prepare a distributed fiber acoustic wave sensitization structure. The sensitization structure is placed in a water pool. The output frequency of the signal source is 300Hz to 2000Hz, and the source distance is 400mm. The test results after phase demodulation are as follows: Figure 3 shown.

[0044] The results of the fiber optic circumferential acoustic wave sensitivity enhancement method show that a stable sinusoidal wave signal is obtained under a continuous acoustic wave field of 115V and 500Hz, and the signal-to-noise ratio of the system reaches 15dB under a pulse acoustic wave signal of 60V and 1200Hz. By comparing with the standard distributed optical fiber acoustic wave measurement system, the sensitivity of the sensitivity enhancement structure is close to the requirements of practical applications, and has good stability and consistency.

[0045] In summary, the present invention provides a fiber acoustic wave circumferential sensitivity enhancement structure and method, which can overcome the delay of the fiber Bragg grating to the dynamic response of downhole acoustic waves and is suitable for distributed fiber acoustic logging technology.

[0046] In terms of structural design, the fiber acoustic wave circumferential sensitivity enhancement method of the present invention does not require welding, drilling and other steps. The cylindrical structure is more suitable for winding long-distance fiber Bragg gratings, and the circumferential metal spring coil is used for radial sensitivity enhancement. It has the characteristics of simple structure, simple manufacturing process, short production cycle and low cost. The π-radian thin sheet, metal spring coil and hollow thin-walled cylinder are made of materials with high temperature resistance, corrosion resistance and strong mechanical properties, which meet the logging requirements of fiber acoustic wave logging and can quickly realize engineering applications.

[0047] In terms of performance, the combined structure of the thin sheet, metal spring coil and hollow thin-walled cylinder not only increases the effective area of ​​the sound pressure, but also converts the radial pressure of the sound wave into the axial strain of the fiber grating, thereby greatly increasing its sensitivity to the sound wave signal. In addition, in well logging applications, the sound wave signal detected by the fiber acoustic wave circumferential sensitization method described in the present invention has excellent consistency and stability. Its small size can be combined with other well logging methods to simultaneously measure multiple downhole parameters. It is easier to form a distributed fiber acoustic wave measurement array based on wavelength division multiplexing technology, which is suitable for various application scenarios such as production profiles, injection profiles, wellbore integrity, fracturing monitoring, CCUS, etc., and can achieve higher sensitivity in combination with high-resolution wavelength detection technology. Therefore, the present invention is of great significance to improving the acoustic wave detection sensitivity and dynamic acoustic wave signal response characteristics in DAS technology.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A fiber optic acoustic wave circumferential sensitivity enhancement structure, It is characterized in that The invention comprises a plurality of thin sheets (1), a plurality of groups of metal spring coils (2), a hollow thin-walled cylinder (3) and a plurality of groups of optical fiber gratings (4); the plurality of groups of metal spring coils (2) are annularly sleeved on the hollow thin-walled cylinder (3); the plurality of thin sheets (1) circumferentially surround the hollow thin-walled cylinder (3) and are attached to the plurality of groups of metal spring coils (2); gaps are provided between adjacent thin sheets (1) to form openings; and the plurality of groups of optical fiber gratings (4) are annularly wound on the plurality of thin sheets (1).

2. The optical fiber acoustic wave circumferential sensitivity enhancement structure according to claim 1, It is characterized in that The diameters of the plurality of groups of metal spring rings (2) are smaller than the outer diameter of the hollow thin-wall cylinder (3).

3. The optical fiber acoustic wave circumferential sensitivity enhancement structure according to claim 1, It is characterized in that The hollow thin-walled cylinder (3) is provided with a plurality of arc-shaped grooves along the circumferential direction from top to bottom, and a plurality of groups of metal spring rings (2) are annularly sleeved in the corresponding arc-shaped grooves.

4. The optical fiber acoustic wave circumferential sensitivity enhancement structure according to claim 1, It is characterized in that The number of the plurality of sheets (1) is an even number.

5. The optical fiber acoustic wave circumferential sensitivity enhancement structure according to claim 1, It is characterized in that The structures of the plurality of thin sheets (1) are all the same in size, and the plurality of thin sheets (1) are all curved, surrounding the hollow thin-walled cylinder (3) in the circumferential direction to form a cylindrical structure.

6. The optical fiber acoustic wave circumferential sensitivity enhancement structure according to claim 1, It is characterized in that A groove is provided on one side of the plurality of thin sheets (1) close to the metal spring ring (2), and the outer surface of the metal spring ring (2) is embedded in the annular groove formed by the plurality of thin sheets (1).

7. A method for increasing the sensitivity of optical fiber acoustic waves in a circumferential direction, based on the optical fiber acoustic wave circumferentially increasing sensitivity structure according to any one of claims 1 to 6, It is characterized in that The process includes the following: A plurality of metal spring rings (2) are sleeved on the outside of a hollow thin-walled cylinder (3), and a plurality of thin sheets (1) are arranged on the outside of the metal spring rings (2), wherein the openings between adjacent thin sheets (1) ensure that they generate a vibration response under the action of sound waves, thereby forming a plane-symmetrical cylindrical structure; a fastening structure is applied to the outside of the plurality of thin sheets (1) and a certain pre-tightening force is applied to ensure that the thin sheets (1), the metal spring rings (2), and the hollow thin-walled cylinder (3) remain in a fixed position when the optical fiber is wound; and the positions of the plurality of thin sheets (1) are adjusted so that they are completely fitted with the metal spring rings (2) and form a standard A quasi-cylindrical optical fiber acoustic wave circumferential sensitivity enhancement structure; fixing the cylindrical structure with a fastening device on an optical fiber winding machine, winding an optical fiber grating (4) on the outer surface of the cylindrical structure with a constant tension, and removing the fastening structure located on the outer surface of a plurality of thin sheets (1) after the optical fiber grating (4) is wound; converting the deformation of the metal spring coil (2) caused by the action of acoustic waves into the axial strain of the optical fiber grating (4), resulting in periodic changes of the optical fiber grating (4) and its own elastic-optical effect, so that the reflection wavelength of the optical fiber grating (4) is displaced, thereby realizing high-sensitivity acoustic wave signal detection.

8. A method for circumferentially enhancing the sensitivity of optical fiber acoustic waves according to claim 1, It is characterized in that The metal spring ring (2) is made of metal material, including austenitic stainless steel SS304 or austenitic stainless steel SS316.

9. A method for circumferentially enhancing the sensitivity of optical fiber acoustic waves according to claim 1, It is characterized in that The thin sheet (1) and the hollow thin-walled tube (3) are made of high molecular polymers, including polyetheretherketone, polyimide or polytetrafluoroethylene.

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