Magnetic field measurement cable and distributed well magnetic field measurement system

Abstract

A magnetic field measurement cable (10) of the present invention comprises a cable (1) arranged in an axial portion, and a peripheral cable (2) arranged outside the cable (1) and formed by spirally winding a plurality of steel wires, and a magnetic field measurement optical cable (3) having an optical cable (3a).

Description

Technical Field

[0001] The invention relates to a magnetic field measurement cable and a distributed in-well magnetic field measurement system. Background Art

[0002] Electrical logging, which is an electrical logging method for oil wells, is a basic technology for oil well measurement. A large amount of historical materials on electrical logging have been collected, and electrical logging has played an important role in oil field development. Recently, instead of electrical logging, a method using optical fiber has gradually become popular (for example, see Patent Documents 1 and 2).

[0003] However, it is generally believed that optical fiber is not affected by electromagnetic properties, and its technology similar to electrical logging has not yet been put into practical use.

[0004] However, recently, a distribution measurement technique focusing on polarization of Rayleigh scattering has been realized (for example, refer to Non-Patent Document 1). In this technique, focusing on the birefringence vector β generated in the optical fiber, Expression (1) is proposed.

[0005] [Mathematical formula 1]

[0006]

[0007] Here, j is a variable representing the axial direction of the optical fiber, the first term on the right side represents linear birefringence, and the second term on the right side represents cyclic birefringence.

[0008] Linear birefringence occurs due to the shape or deformation of the measurement target, while cyclic birefringence occurs due to the twisting of the measurement target or the Faraday effect based on the magnetic field.

[0009] In the above expression (1), β3 is represented by the expression (2) shown below (refer to Non-Patent Document 2).

[0010] [Mathematical formula 2]

[0011]

[0012] Here, g is the photoelastic coefficient (g≈0.15), τ is the torsion angle, ν is the Verdet coefficient, B is the magnetic field strength, and is the rotation angle of light (for example, refer to Non-Patent Document 3).

[0013] Note that ν is expressed as ν = 1.43 / λ 2 (The unit of λ is μm), and is usually (when λ=1.5 mm) 0.6 rad / T / m.

[0014] Regarding the Faraday effect based on a magnetic field, a POFDR (abbreviation of Polarized Optical Frequency Domain Reflectometry with a standard OFDR equipped with a polarization controller) method for realizing current distribution measurement is known.

[0015] In this regard, Palmieri et al. have examined the Faraday effect (change in rotation angle) based on changes in magnetic field while using a current measurement experimental device to change the current intensity up to 2.5kA with a resolution of 100A, and through measurements it was made clear that when the measurement position changes, the rotation angle also changes.

[0016] Here, in the experimental device used by Palmieri et al., two current circuits each having a length of 20 m are arranged in parallel to be spaced 1.2 m apart from each other, and by using the current circuits, connection is achieved so that a current of amplitude I flows from the center conductor 1 to be branched in half with a current of amplitude I / 2 to two conductors (one of which is the conductor 3 to be measured) located outside the center conductor 1. In addition, a measuring cable having a sensing function is spirally wound around the periphery of each of the conductors 1 and 3 at a pitch p. In this experimental device, the above-mentioned POFDR method is applied, thereby measuring the change in the rotation angle with respect to the change in the measurement position, that is, the Faraday effect based on the change in the magnetic field (see non-patent document 3).

[0017] Reference List

[0018] Patent Literature

[0019] Patent Document 1: Japanese Patent Publication No. 2011-27533

[0020] Patent Document 2: Japanese Translation of PCT International Application Publication No. 2005-517105

[0021] Non-patent literature

[0022] Non-patent document 1: Luca Palmieri, “Distributed polarimetric measurement for fiber sensoring”, Optical Fiber Technology 19, December 2013, pp. 720-728.

[0023] Non-patent document 2: Andrea Galtarossa et al., “Distributed polarization sensing”, Proc. SPIE 10323, 25th International Conference on Fiber Optic Sensors, 1032318, April 23, 2017.

[0024] Non-patent literature 3: Zhenyang Ding et al., “Distributed Optical Fiber Sensors Based on Optical Frequency Domain Reflectometry: A review”, Sensors 2018, 18, 1072; doi10.3390 / s18041072. Summary of the invention

[0025] Problems to be solved by the present invention

[0026] The above Verdet coefficient determines the polarization sensitivity of the optical fiber, but its amplitude has not been significantly improved. Specifically, the actual required accuracy is 10 -4 A, however, in the research of the above literature, the current measurement accuracy only reached 0.1KA.

[0027] In addition, as described above, the birefringence vector β depends on "twist", so its influence cannot be ignored. Considering the use conditions in actual wells, such as the need for long-length optical fibers (e.g., 3 km or longer) and the harsh use environment, it is difficult to ensure a condition without twist.

[0028] The present invention is proposed to solve the above problems, and an object of the present invention is to provide a magnetic field measurement cable using optical fiber and a distributed in-well magnetic field measurement system, in which the accuracy problem and torsion problem of the Verdet coefficient are solved.

[0029] Solution to the problem

[0030] A magnetic field measuring cable according to the present invention is a magnetic field measuring cable including an optical fiber for measuring a magnetic field at a desired position, the magnetic field measuring cable comprising: an electrical cable arranged at an axial portion of the magnetic field measuring cable; and a peripheral cable comprising a plurality of cables, and an optical cable consisting of an optical fiber and a plurality of steel wires spirally wound around the optical fiber, the plurality of cables and the optical cable being spirally wound around the periphery of the cable, the magnetic field measuring cable being constructed to detect a frequency shift of scattered light of the optical fiber placed at the desired position.

[0031] The distributed in-well magnetic field measurement system of the present invention is a distributed in-well magnetic field measurement system as follows, comprising: the above-mentioned magnetic field measurement cable; and a computing device, which calculates and stores signals, wherein, based on the relationship between the magnetic field and the frequency shift of the scattered light of the optical fiber calculated in advance by measurement, the computing device calculates the frequency shift signal detected by the magnetic field measurement cable, and calculates the distribution of the magnetic field in association with the placement position of the optical cable placed at the measurement position in the well serving as the measurement target.

[0032] Effects of the Invention

[0033] The magnetic field measurement cable and distributed in-hole magnetic field measurement system according to the present invention can achieve remarkable results, namely, a magnetic field measurement cable and distributed in-hole magnetic field measurement system using optical fiber can be provided, in which the accuracy and torsion problems of the Verdet coefficient are solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 An example of physical characteristics of an optical fiber for explaining the principle of the distributed in-well magnetic field measurement system according to Embodiment 1 is shown.

[0035] Figure 2 The relationship between the magnetic field change and the scattered light change over time is shown to illustrate the principle of the distributed in-well magnetic field measurement system according to Example 1.

[0036] Figure 3 Graphs illustrating the relationship between temporal changes and spatial changes of scattered light and magnetic field according to Example 1.

[0037] Figure 4 An example of a magnetic field measurement cable used in the distributed in-well magnetic field measurement system according to Embodiment 1 is shown.

[0038] Figure 5 is perpendicular to Figure 4 A cross-sectional view of a plane along the axis of the magnetic field measurement cable is shown.

[0039] Figure 6 It is shown Figure 4 An example table of specifications for cables used in magnetic field measurement cables is shown.

[0040] Figure 7 An example of a distributed in-well magnetic field measurement system according to Embodiment 1 is shown. DETAILED DESCRIPTION

[0041] Example 1

[0042] First, the physical properties of optical fiber used for the purpose of the present disclosure will be described below, especially its relationship with the magnetic field.

[0043] Conventionally, it is believed that optical fibers for communication generally do not produce magnetic field reactions as described above. However, through tests conducted using optical fibers located at the axis position of the column of a cylindrical electromagnet, it was found that the optical fibers exhibit magnetic field reactions. This will be described in more detail below with reference to the accompanying drawings.

[0044] Figure 1 An example of the spatial distribution of the frequency shift of Rayleigh scattered light and Brillouin scattered light as backscattered light of the optical fiber in a magnetic field is shown. Figure 1In FIG. 1 , the horizontal axis represents the distance (unit: m), and the vertical axis represents the frequency shift of the Brillouin scattered light (unit: MHz) and the frequency shift of the Rayleigh scattered light (unit: GHz). The electromagnet is set at a position corresponding to the coordinate range of 59.9 m to 60.3 m from the distance to the optical fiber, and the current value flowing through the electromagnet is changed to change the intensity of the magnetic field generated by the electromagnet. In this case, for both the Brillouin scattered light and the Rayleigh scattered light, as shown in FIG. Figure 1 As shown in the graph in , a frequency shift occurs correspondingly at the position where the electromagnet is set. Based on this fact, conversely, it can be expected that the change in the magnetic field can be measured by using Brillouin scattered light and Rayleigh scattered light.

[0045] Therefore, in order to study the quantitative relationship between the change in magnetic field intensity and the frequency shift, Rayleigh scattered light was used to quantitatively measure the relationship between the temporal change in magnetic field intensity (here, the change in magnetic flux density) and the frequency shift (of Rayleigh scattered light). The results are shown in Figure 2 .

[0046] exist Figure 2 In the graph, the horizontal axis represents the time after the start of the measurement in minutes. On the other hand, the vertical axis represents the frequency shift of the Rayleigh scattered light in GHz. In the graph, the curve connecting the points represents the change in the frequency shift of the Rayleigh scattered light. On the other hand, the thick solid line curve represents the time change of the magnetic flux density. Figure 2 As shown, during the 30-minute measurement period, the magnetic flux density increases from 0 to 8 T (Tesla) and then becomes 0 again.

[0047] like Figure 2 As shown in the figure, it is found that the frequency shift of the Rayleigh scattered light changes from 0 to about 10 GHz as the value of the magnetic field (magnetic flux density) M increases from 0 to 8 T (Tesla). In addition, in the time period when the value of the magnetic flux density is 8 T (the time period from about 15 minutes to 28 minutes after the start of measurement), the frequency shift (amount) of the Rayleigh scattered light is about 10 GHz to 12 GHz, which can be considered to be almost constant, although there is a slight change.

[0048] Based on the above results, it can be said that the frequency shift (amount) of Rayleigh scattered light changes with the change of magnetic flux density. That is, it becomes clear that the magnetic field intensity can be measured by using the scattered light of the optical fiber. It is noted that the strain change amount of the optical fiber relative to the Rayleigh scattered light per 1GHz frequency shift is 25 to 30με.

[0049] Figure 3The time and space changes of the magnetic field intensity and the frequency shift of the Rayleigh scattered light relative to it are shown at the same time. In the chart, the horizontal axis represents the distance (unit: m) from the center (position) of the magnetic field, where the center position of the magnetic field is defined as 0. The vertical axis (central axis position) at the center position of the magnetic field represents the magnetic field intensity with a scale of one Tesla from 1 to 9T (Tesla). In addition, the vertical axis on the left side of the chart represents the frequency shift of the Rayleigh scattered light with a scale of 2GHz.

[0050] exist Figure 3 In the graph, the quadrilateral diamond mark shown on the left half relative to the central axis position of the graph represents the spatial variation of the magnetic field strength (magnetic flux density) (the variation in strength due to the change in position), and as shown in the graph, the magnetic field strength varies in a Gaussian distribution shape.

[0051] On the other hand, in this case, regarding the frequency shift of Rayleigh scattered light, each data represented by a continuously connected curve is shown to change with time at each corresponding position on the horizontal axis (here, pairs of data at almost the same value on the central axis correspond to the same magnetic field intensity value, but in terms of time, correspond to data when the magnetic field intensity increases and data when the magnetic field intensity decreases, respectively).

[0052] As described above, the figure simultaneously shows the temporal and spatial variations of the magnetic field intensity and the frequency shift of the Rayleigh scattered light relative thereto, and it can be said that an almost symmetrical distribution about the central axis is obtained for the frequency shift of the Rayleigh scattered light (it can be said that there is almost no lag between the case where the magnetic field intensity is increased to a certain value and the case where the magnetic field intensity is reduced from a certain value).

[0053] Under the above premise, a magnetic field measurement cable 10 that can be used in a distributed in-well magnetic field measurement system has been manufactured. Hereinafter, the magnetic field measurement cable 10 will be described in detail with reference to the accompanying drawings.

[0054] Figure 4 and Figure 5 An example of a magnetic field measurement cable 10 for a distributed in-well magnetic field measurement system is shown. Figure 4 and Figure 5 In the embodiment, at the central axis position of the magnetic field measurement cable 10, an armored cable type cable 1 is arranged, which is a conductor cable composed of a plurality of wires in a spirally wound FIMT (abbreviation of Fiber in Metal Tube) type.

[0055] Immediately outside the cable 1, there is provided an outer peripheral cable 2 formed of two layers, which also serves to protect the cable 1. Hereinafter, the outer peripheral cable 2 will be described in more detail.

[0056] First, a first outer peripheral cable 2a including a plurality of steel wires spirally wound is provided in the inner peripheral portion of the outer peripheral cable 2. The plurality of steel wires constituting the first outer peripheral cable 2a are partially replaced with the temperature measurement optical fiber built-in cable 4, for example.

[0057] Similarly, a second peripheral cable 2b consisting of a plurality of steel wires spirally wound and formed is provided at the periphery of the first peripheral cable 2a (the periphery around the plurality of steel wires and the temperature measurement optical fiber built-in cable 4). In a portion of the second peripheral cable 2b, a magnetic field measurement optical cable 3 for measuring the magnetic field distribution of a measurement object having electromagnetic characteristics is provided to partially replace the steel wires constituting the second peripheral cable 2b.

[0058] Here, the magnetic field measuring optical cable 3 includes an optical fiber cable 3a disposed at its central axis position and used to measure the magnetic field, and an outer peripheral protective layer 3b disposed on the outer periphery of the optical fiber cable 3a to protect the optical fiber cable 3a and composed of a plurality of steel wires spirally wound and formed.

[0059] In the magnetic field measuring cable 10 constructed as described above, the magnetic field measuring optical cable 3 is provided at a peripheral position, and the optical fiber cable 3a for measuring the magnetic field is used in the magnetic field measuring optical cable 3. Therefore, the above reference Figure 1-Figure 3 The frequency shift of the Rayleigh scattered light described above can be measured by the magnetic field measurement optical cable 3, and can be calculated using the distributed in-well magnetic field measurement system 100 including a computing device 20 for calculating and storing the signals obtained by the measurement. Therefore, the distribution of the in-well magnetic field in the oil well to be measured (hereinafter, referred to as the measurement object) can be measured (see Figure 7 ).

[0060] Here, for the magnetic field measuring optical cable 3, since the outer peripheral protective layer 3b is formed spirally on the outer circumference thereof, there is no danger of twisting the cable itself despite its elongated shape and the harsh environment inside the oil well. The steel wires used as described above are all made of non-magnetic shielding materials (hereinafter, may be referred to as non-magnetic shielding materials). For the steel wires, non-magnetic shielding materials other than steel wires may be used.

[0061] Even when temperature affects the magnetic field measurement, since the magnetic field measuring cable 10 has a temperature measuring optical fiber built-in cable 4, the magnetic field in the well can be measured with high precision by correcting the temperature influence (compensating for temperature) at each measuring position in the well using two signals (i.e., the signal measured by the magnetic field measuring optical cable 3 and the signal measured by the temperature measuring optical fiber built-in cable 4).

[0062] In addition, the structure of the magnetic field measurement cable 10 used was checked, and the cable 1 having multiple cores and made of copper can meet the requirements. Figure 6 The specifications are shown in the table. Figure 4 In , OD represents the outer diameter of the magnetic field measurement cable 10, and specifically, in the case where the American Wire Gauge (AWG) 16 specification is adopted for the cable 1, the outer diameter is 7.3 mm. Here, AWG is a standard specification mainly used in North America since 1857 and defines the diameter, cross-sectional area, resistance, etc. of solid non-ferrous wire, and the larger the AWG number, the smaller the outer diameter. For example, in AWG 16, the diameter dn (unit: mm) of the wire can be obtained by substituting n=16 into the expression (3).

[0063] [Mathematical formula 3]

[0064]

[0065] The above standards also apply to strands (e.g., corresponding to wires that are spirally wound around each other). In the AWG standard specification, the total value of the cross-sectional area of ​​each strand is shown (excluding the gaps between the strands). In the case where the spiral strands account for 25% of the wire area, the outer diameter is about 13% larger than the equivalent solid wire. The strand is characterized by three values, namely the total AWG size, the number of strands, and the AWG size of the strands.

[0066] Although the present disclosure is described above based on exemplary embodiments, it should be understood that the various features, aspects and functions described in the embodiments are not limited to their applicability to the specific embodiments in which they are described, but may be applied alone or in various combinations in combination with the embodiments of the present disclosure.

[0067] Therefore, it can be understood that many modifications not illustrated can be designed without departing from the scope of the present disclosure. For example, at least one constituent component can be modified, added or eliminated.

[0068] Description of Reference Numerals

[0069] 1 Cable

[0070] 2 Peripheral cables

[0071] 2a First peripheral cable

[0072] 2b Second peripheral cable

[0073] 3 Magnetic field measurement optical cable

[0074] 3a Fiber Optic Cable

[0075] 3b Peripheral protective layer

[0076] 4 Temperature measurement fiber optic built-in cable

[0077] 10 Magnetic field measurement cable

[0078] 20 Computing Devices

[0079] 100 Distributed Well Magnetic Field Measurement System

Claims

1. A magnetic field measuring cable, used to measure the magnetic field at a predetermined position inside a well, the magnetic field measuring cable comprising: a cable disposed at an axial portion of the magnetic field measurement cable; as well as A peripheral cable surrounding the cable, the peripheral cable comprising: a first peripheral cable disposed on the inner peripheral side, the first peripheral cable comprising a temperature measurement optical fiber built-in cable; as well as a second peripheral cable disposed on the peripheral side, the second peripheral cable comprising an optical cable consisting of an optical fiber and a plurality of non-magnetic shielding steel wires, the plurality of non-magnetic shielding steel wires being spirally wound around the optical fiber, The magnetic field measurement cable is configured to detect a change in a frequency shift of scattered light of the optical fiber caused by the magnetic field at the predetermined position.

2. A distributed well magnetic field measurement system, comprising: The magnetic field measurement cable according to claim 1; as well as A computing device that calculates and stores the signal, wherein Based on the relationship between the magnetic field and the frequency shift of the scattered light of the optical fiber calculated in advance through measurement, the calculation device calculates the frequency shift signal detected by the magnetic field measurement cable, and calculates the distribution of the magnetic field in association with the placement position of the optical cable at the measurement position inside the well as the measurement object.

3. A distributed well magnetic field measurement system, comprising: The magnetic field measurement cable according to claim 2; as well as A computing device that calculates and stores the signal, wherein Based on the relationship between the magnetic field and the frequency shift of the scattered light of the optical fiber calculated in advance through measurement, the calculation device calculates the frequency shift signal and the signal measured by the temperature measurement optical fiber built-in cable, and calculates the magnetic field in which the temperature influence at the placement position of the optical cable is corrected in association with the placement position of the optical cable placed at the measurement position inside the well as the measurement object.

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