Optical cable, optical cable monitoring system, and well monitoring method

Abstract

The optical cable (10, 10a) is configured to include an optical fiber (1) that senses the deformation of a hydrocarbon-absorbing resin (2) that has the property of absorbing hydrocarbons in oil and expanding, a twisted wire (20) arranged to cover the outer periphery of the optical fiber (1), and hydrocarbon-absorbing resin (2) filled in the space between the optical fiber (1) and the twisted wire (20). The optical cable (10, 10a) configured in this manner is laid over the entire depth direction of the borehole (100) to be measured, and the frequency shift signal of the backscattered light from the optical fiber (1) is measured by a backscattered light measuring device (40) to detect the presence or absence of oil leakage from the borehole (100) over the entire depth direction of the borehole (100).

Description

[Technical field]

[0001] The present application relates to an optical cable, an optical cable monitoring system, and a method for monitoring a well. [Background technology]

[0002] Traditionally, after an oil well reached the end of its useful life, the disposal of the oil well was completed by cutting the casing, which is a structural component of the well, at a certain depth (approximately 50m) that does not affect land-based buildings, and welding a steel cap attached to the top end of the casing to seal the well's outlet, and then filling the space between the casing and the outlet with sand. This method of decommissioning was established approximately 80 years ago, and to date, approximately 50,000 oil wells per year have been decommissioned using this method, totaling more than 4 million oil wells.

[0003] However, in California, USA, contamination of cooking water was found to be caused by abandoned oil wells (hereafter referred to as abandoned oil wells), which has had an impact on daily life, and the situation now requires the adoption of more stringent disposal methods. In addition, oil leaks from abandoned oil wells off the coast of Malaysia are increasing day by day, making the problem of marine pollution impossible to ignore.

[0004] Now, about 100 years after the start of large-scale oil development, the disposal of abandoned oil wells and the like is becoming an increasingly important issue due to the increase in the number of oil wells and the large-scale development of offshore oil.

[0005] On the other hand, there have been very few examples to date that have examined the treatment of abandoned oil wells such as those described above. Most of the studies have focused on only some of the related elements rather than the treatment of the abandoned oil wells themselves, as can be seen in the invention of a temporary drilling sealant used in drilling wells to produce oil or natural gas, etc. (see, for example, Patent Document 1), and the invention related to the treatment of produced water (fracturing water) that is produced at the same time when oil or natural gas is extracted from oil wells or natural gas wells (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2015 / 072317 [Patent Document 2] JP 2018-43221 A [Patent Document 3] International Publication No. 2014 / 181617 [Non-patent literature]

[0007] [Non-Patent Document 1] Wu, Qian, et al., “Advanced Distributed Fiber Optic Sensors for Monitoring Poor Zonal Isolation with Hydrocarbon Migration in Cemented Annui”, Society of Petroleum Engineers. 2016, September 14. doi:10.2118 / 180329-MS Summary of the Invention [Problem to be solved by the invention]

[0008] In order to clarify the problem to be solved by the present application, the principle of oil leakage from an abandoned oil well will be explained with reference to Fig. 14. Fig. 14 is a diagram for explaining the principle of oil leakage from an abandoned oil well.

[0009] As shown in this figure, underground carbon dioxide (CO2), air (O2), hydrogen sulfide (H2S), and other components induce the occurrence of cracks between the casing and the cement surface (upper part) and cracks inside the cement (lower part) in the casing that constitutes the well below the ground or seabed, or in the cement surrounding the casing, which also induces corrosion of the external casing. The occurrence of these cracks is also induced by stress due to geological movements. In this way, the cracks that occurred around the well grow over time, forming channels from the oil-bearing layer to the underground drinking water layer or seawater, leading to accidents in which leaks occur. Another cause is when an internal corrosion hole occurs in the casing from the inside of the well, and this internal corrosion hole connects to the cracks mentioned above, causing a leak.

[0010] In the case of California mentioned above, water containing radioisotopes was injected into the abandoned oil well from above ground, and if radiation was detected in the groundwater, it was determined that there was a leak in the oil well. In this California case, if corrosion damage to the casing could be identified by some method, and if the identified part was in the upper part of the oil well, the corresponding part was repaired by construction.

[0011] Also, if the leak path can be identified, as shown in Figure 15, a cheap and effective repair method is to use an underground tool to drill a hole in a specific location of the casing, and then inject a special adhesive called grout into the gap where the crack that caused the leak occurred. In this figure, the dashed line S1 indicates the movement path of the underground tool, and the tip of the arrow indicates the specific location of the casing. The part repaired with grout is the area S2 surrounded by a dashed line in a trapezoid, which is the part where the cracks that occurred in the cement outside the oil well are located on the side closest to the oil well. However, with this method, it is difficult to confirm the effectiveness of the repair, that is, to verify that no leaks have occurred. The symbol P indicates the ground surface or sea level.

[0012] Recently, it has been announced that by applying a special overcoat to optical fiber, the overcoat has the property of selectively absorbing and expanding the hydrocarbons in oil, i.e., hydrocarbons (HC), and that this deformation can be sensed by optical fiber to indicate the presence of hydrocarbons (see, for example, Non-Patent Document 1). In addition, when the hydrocarbons disappear, the expanded deformation may disappear, indicating the possibility of confirming the effect after cutting the leak path, but the method for confirming the effect of the repair remains unsolved.

[0013] Furthermore, the size of oil wells has become larger and larger in recent years, which has caused two major problems. The first problem is that there was insufficient understanding of the corrosion resistance of the steel and cement used at the beginning, and the carbon dioxide (CO2) concentration in the oil wells at the end of the life of the wells was not taken into account, reaching 77% or more. The second problem is that the costs incurred at the time of development of the oil wells are not enough to deal with these problems. As explained above, the possibility of detection using optical fiber has been confirmed, but the method of implementing it in oil wells has not yet been verified.

[0014] The present application discloses technology for solving the above-mentioned problems, and aims to provide an optical cable, an optical cable monitoring system, and a well monitoring method that can promote the regulation and dissemination of petroleum technology that takes corrosion resistance into consideration, and that can verify the effectiveness of oil well repair methods by detecting and identifying leak paths from oil wells, as well as enable long-term monitoring of oil wells. [Means for solving the problem]

[0015] The optical cable disclosed in the present application comprises: An optical fiber; a plurality of cables provided in a radial direction of the optical fiber are twisted to cover an outer periphery of the optical fiber and arranged in a circular ring; a hydrocarbon-absorbing resin that absorbs hydrocarbons and fills the gaps between the optical fiber and the strands; It is equipped with the following:

[0016] In addition, the optical cable disclosed in the present application is An optical fiber; a plurality of cables provided in a radial direction of the optical fiber are twisted to cover an outer periphery of the optical fiber and arranged in a circular ring; a second stranded wire, which is provided in a radial direction of the optical fiber and in which a plurality of cables having an outer diameter larger than that of the stranded wire cable are twisted and arranged in a circular shape so as to cover an outer periphery of the stranded wire; an optical fiber for measuring a physical property, which is arranged by replacing one cable of the second stranded wire and measures a physical property of an object to be measured, such as temperature, strain, or pressure; a third stranded wire, which is provided in a radial direction of the optical fiber and in which a plurality of cables having an outer diameter larger than that of the second stranded wire are twisted and arranged in a circular shape so as to cover an outer periphery of the second stranded wire; a hydrocarbon-absorbing resin that absorbs hydrocarbons and fills spaces between the optical fiber, the strand, the second strand, and the third strand; It is equipped with the following: Effect of the Invention

[0017] The optical cable, the optical cable monitoring system using the optical cable, and the well monitoring method disclosed in the present application can promote the regulation and dissemination of petroleum technology that takes corrosion resistance into consideration, and by detecting and identifying the leak path from an oil well, it is possible to obtain the remarkable effect of verifying the effectiveness of an oil well repair method and enabling long-term monitoring of an oil well. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a diagram for explaining an example of the optical cable monitoring system according to the first embodiment. [Diagram 2] FIG. 2 is a diagram for explaining an example of a basic structure of an optical cable according to the first embodiment. [Diagram 3] 3 is a schematic diagram of a cross section perpendicular to the axis of the optical cable of FIG. 2. [Figure 4] 4 is a diagram for explaining another example of the basic structure of the optical cable according to the first embodiment. FIG. [Diagram 5] 5 is a schematic diagram of a cross section perpendicular to the axis of the optical cable of FIG. 4. [Figure 6] 1 is a schematic configuration diagram of an optical cable oil immersion test device used in the optical cable monitoring system according to the first embodiment. [Figure 7] 7 is a diagram for explaining an optical cable sample used in the oil immersion test of FIG. 6. FIG. [Figure 8] FIG. 7 is a table showing the specifications of a measuring device for measuring the characteristics of the optical cable used in the oil immersion test of FIG. [Figure 9] 9 is a diagram showing an example of center frequency characteristics of an optical cable sample in the characteristic measurement test shown in FIG. 8. FIG. [Figure 10] FIG. 7 is a diagram showing an example of measurement of strain distribution of two optical cable samples in the oil immersion test shown in FIG. 6. [Figure 11]7 is a diagram showing an example of a change over time in strain distribution of an optical cable sample in the oil immersion test shown in FIG. 6. FIG. [Figure 12] 7 is a diagram for explaining the effect of a filler in strain distribution measurement of an optical cable sample in the oil immersion test shown in FIG. 6. FIG. [Figure 13] 2 is a schematic diagram showing an example of monitoring data of the optical cable monitoring system according to the first embodiment. FIG. [Figure 14] FIG. 1 is a diagram for explaining a problem with an optical cable monitoring system. [Figure 15] FIG. 1 is a diagram illustrating an example of a conventional optical fiber cable monitoring system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Embodiment 1 FIG. 1 is a diagram illustrating an example of an optical cable monitoring system 50 according to the first embodiment. As shown in this figure, in order to monitor a borehole 100 provided on the seabed, a monitoring center 110 is equipped with a hybrid type backscattering light measuring device 40 that uses both Brillouin scattering and Rayleigh scattering to process signals from an optical fiber. An optical cable 10 having an optical fiber with a hydrocarbon detection function is laid along the seabed from the monitoring center 110 to the location where the borehole 100 is installed, and after reaching the location where the borehole 100 is installed (the top of the borehole 100), it is further laid along the periphery of the borehole 100 so as to reach the bottom (bottom) of the borehole 100. In the figure, the symbol Q indicates the sea surface.

[0020] In this case, the backscattered light measuring device 40 is connected to the optical cable 10 having a hydrocarbon detection function, and the backscattered light measuring device 40 measures the frequency shift of the scattered light corresponding to the installation position of the optical cable, and based on this measurement value, the optical cable monitoring system 50 detects the location of oil leaks from the borehole. That is, the optical cable monitoring system 50 comprises the optical cable 10 and the backscattered light measuring device 40 as its main components.

[0021] In this way, by deploying the optical cable monitoring system 50, it is possible to monitor for a long period of time, such as 50 years or more, whether or not there is a leak of oil or the like, which is the object of monitoring, from the top position of the borehole 100, which is at a height equivalent to the seabed, to the bottom position of the borehole 100. Details of the optical cable monitoring system 50 will be described below, focusing on the optical cables used in the optical cable monitoring system 50.

[0022] First, the optical cable 10 used in the optical cable monitoring system 50 will be described with reference to the drawings. Figures 2 and 3 are diagrams showing an example of the basic structure of an optical cable 10 that can be used in the optical cable monitoring system according to the first embodiment. Of these, Figure 2 is a schematic diagram for explaining the structure of the optical cable 10, and shows the structure of this optical cable three-dimensionally. Also, Figure 3 is a diagram showing a cross section in a direction perpendicular to the axis of this optical cable.

[0023] As shown in Figures 2 and 3, an optical fiber 1 is provided at the central axis of a stranded wire 20, which is made up of a plurality of twisted steel wires 3 arranged in an annular shape. A hydrocarbon-absorbing resin 2a, which is a resin that absorbs hydrocarbons, is filled around this optical fiber 1. On the outside of this stranded wire 20, a second stranded wire 21 is arranged, which is made up of a plurality of steel wires 4 having a larger outer diameter than the steel wires 3 and twisted in the opposite direction to the stranded wire 20 to form an annular shape.

[0024] The stranded wire 20 and the second stranded wire 21 are usually constructed as an armored cable, and by placing an optical fiber inside it, it can be used for so-called universal purposes such as communications or data transmission.

[0025] In the optical cable 10 of the first embodiment, the above-mentioned hydrocarbon-absorbing resin 2b is filled in the gaps between the stranded wires 20 (consisting of a total of six cables in Figs. 2 and 3) and the second stranded wires 21. Usually, the optical fiber 1, the stranded wires 20, and the second stranded wires 21 are assembled together, and then the above-mentioned hydrocarbon-absorbing resin 2 is filled in the gaps therebetween to manufacture the optical cable 10.

[0026] The hydrocarbon-absorbing resin 2 is filled in such a manner that all of the voids are filled, and an optical cable 10 that is typically 3 km or longer is produced. In addition, the hydrocarbon absorbent resin 2 may be filled intermittently in the axial direction of the optical cable 10, and in this case, the pitch of the positions at which the hydrocarbon absorbent resin 2 is filled is determined by the accuracy of detecting the position of leaks of oil, etc.

[0027] Generally, when unconstrained, the above-mentioned hydrocarbon-absorbing resin 2 has the property of expanding in three axial directions (the directions of each of the three axes) when it absorbs hydrocarbons, and as a result, due to the influence of the resin that has absorbed hydrocarbons, distortion due to tensile stress occurs in the optical fiber (for example, see Non-Patent Document 1). However, in the optical cable 10 of this embodiment, when the hydrocarbon-absorbing resin 2 absorbs hydrocarbons, the presence of, for example, the twisted wire 20 restrains the deformation of this hydrocarbon-absorbing resin 2, and as a result, compressive distortion (distortion due to compressive stress) occurs in the optical fiber 1.

[0028] In the above, an optical cable 10 having a double (two-layer) twisted wire on the outside of the optical fiber 1 has been described, but the present invention is not limited to this, and the same effect can be obtained with an optical cable consisting only of the optical fiber 1 and a twisted wire 20 arranged on the outside of the optical fiber 1. Regarding the relationship between the twist direction of the first stranded wire and the twist direction of the second stranded wire, as shown in FIG. 2 of the first embodiment, an optical cable having a structure in which the twist direction of the second stranded wire is opposite to that of the first stranded wire provides an optimal structure for preventing fraying of the stranded wires.

[0029] Embodiment 2 In the optical cable monitoring system 50 according to the second embodiment, the configuration of the optical cable 10 used in the above-mentioned first embodiment is modified to employ an optical cable that can extract only the signal of pure hydrocarbons contained in an oil well by compensating for the effects of temperature, strain, pressure, etc. Hereinafter, an optical cable 10a having a modified configuration of this optical cable 10 will be described.

[0030] In the optical cable 10a of the second embodiment, a part of the steel wires 4 constituting the second stranded wires 21 of the above-mentioned optical cable 10 is modified, and an optical fiber sensor for sensing temperature, strain, pressure, etc. is provided in addition. By providing optical fiber sensors capable of sensing temperature, strain, pressure, etc. in addition according to the purpose, it is possible to use the measurement results obtained by these optical fiber sensors to extract a signal from the optical fiber 1 that is due only to the effect of absorbing hydrocarbons, with the effects of temperature, strain, pressure, etc. compensated for.

[0031] That is, when it becomes necessary to adopt a cable having the functions of DPATS (abbreviation of Distributed Pressure, Acoustic, Temperature and Strain Sensing), the optical cable 10a of this embodiment 2 can be used. In other words, if temperature compensation is required, a separate optical fiber for temperature measurement can be prepared, if strain compensation is required, a separate optical fiber for strain measurement can be prepared, and if pressure compensation is required, a separate optical fiber for pressure measurement can be prepared.

[0032] A specific example of such an optical cable having a DPATS function will be described below with reference to the drawings. Fig. 4 is a diagram showing an example of the basic configuration of an optical cable 10a having a physical property measuring optical fiber 6 for measuring the physical property of a measurement target object as a component. Fig. 5 is a schematic diagram of a cross section perpendicular to the axis of the optical cable 10a in Fig. 4.

[0033] As shown in Fig. 4 and Fig. 5, the optical cable 10a is characterized in that a third stranded wire 22 composed of a plurality of steel wires 5 is newly disposed at a position corresponding to the outermost periphery of the optical cable 10 described in the first embodiment, and a physical property measuring optical fiber 6 for measuring the temperature, strain, pressure, etc. of a measurement object is newly provided inside the third stranded wire 22 in a form replacing a part of the second stranded wire 21. The spaces between the optical fiber 1, the stranded wire 20, the second stranded wire 21, and the third stranded wire 22 are filled with the hydrocarbon absorbing resin 2, as in the case of the optical cable 10 described above. The oil enters the area filled with the hydrocarbon absorbing resin 2.

[0034] The reason for this configuration is that, normally, in an optical cable with multiple strands, even if the optical fiber 6 for measuring physical properties (here, this particularly means the optical fiber for measuring temperature) is located somewhat inside, the external temperature is transmitted and can be detected, and also, it is advantageous to install it inside in terms of protecting the optical fiber 6 for measuring physical properties itself. Note that the optical cable itself cannot become a path for oil leakage.

[0035] In addition, in multi-layer optical cables, the gap between the outer and inner layers of the optical cable is usually filled with a protective material, so it is possible to place a cable wire for the purpose of detecting leaks of oil, etc. around the outermost periphery of the optical cable, which is even more advantageous in terms of improving detection performance.

[0036] In the above, an optical cable has been described in which the hydrocarbon absorbing resin 2 is manufactured so as to fill the gaps between the optical fiber 1, the stranded wire 20, and the second stranded wire 21, or the gaps between the optical fiber 1, the stranded wire 20, the second stranded wire 21, and the third stranded wire 22. However, the present invention is not limited to this, and the same effect can be obtained even if the resin is manufactured in the form of an overcoat on the surface of the optical fiber 1. Moreover, an optical cable filled with this hydrocarbon absorbing resin 2 has a lifespan of at least 50 years in an environment of 200°C. If this optical cable is used in an environment with a low temperature and a shallow depth, a semi-permanent lifespan can be expected.

[0037] Next, verification experiments conducted on the hydrocarbon detection function of the above optical cable and the results thereof will be described below with reference to the drawings. In particular, as explained in the above embodiment 1, verification was conducted using an optical cable with a structure consisting of only an optical fiber and the innermost (single) layer of twisted wire just outside the optical fiber, which is the basic structure for confirming the action and effect of filling with resin that detects hydrocarbons.

[0038] Figure 6 shows the schematic configuration of the experimental equipment used in this verification experiment. One test cable (optical fiber cable with hydrocarbon detection function) was immersed in each of two diesel containers, and the shift in backscattered light that occurs when these cables absorb the hydrocarbons in the diesel was measured using a hybrid backscattered light measurement device, DTSS (Distributed Temperature and Strain Sensing), which is connected to these cables and can measure two types of frequency shifts with a single device.

[0039] The two types of frequency shifts mentioned here are frequency shift analysis using PPP-BOTDA (Pulse Pre-Pump Brillouin Optical Time Domain Analysis) for Brillouin scattered light, and frequency shift analysis called TW-COTDR (Tunable Wavelength Coherent Optical Time Domain Reflectometry) for Rayleigh scattered light. Therefore, even if temperature and strain change simultaneously, the amount of change in both can be analyzed.

[0040] Figure 7 shows the detailed specifications of the test cable used in this verification experiment. An optical fiber with a sensing section in which a resin with hydrocarbon detection capabilities is filled around the optical fiber is held inside the armored cable. The 9 cm long part shown as the sensing section in this figure is the sensing section that detects hydrocarbons.

[0041] A cross-sectional view taken at position AA, which represents this sensing unit, is shown below the sensing unit as section AA. As shown in this cross-sectional view, the optical fiber located in the center is filled with a resin having hydrocarbon detection capabilities, indicated by a multi-dot pattern. Meanwhile, a cross-sectional view taken at position BB of the armored cable is shown to the left of section AA as section BB. As shown in this cross-sectional view, at position BB, the optical fiber is not filled with the resin having hydrocarbon detection capabilities. As described above, while the sensing unit is filled with a resin having hydrocarbon detection capabilities, the parts of the armored cable other than the sensing unit are not filled with the resin having hydrocarbon detection capabilities.

[0042] In addition, a reference fiber (not in the form of an armored cable) with a length of 1 m was connected to both ends of the optical fiber. The entire armored cable portion of the test cable constructed in this way was immersed and held in a container filled with light oil (containing hydrocarbons).

[0043] As described above, since the length of the sensing portion is shorter than other portions, the change in strain due to temperature and pressure changes is small compared to the change in strain due to the absorption of hydrocarbon components and can be considered to be negligible.

[0044] Figure 8 is a table showing the specifications of the hybrid backscattered light measurement device. Since it has two major measurement functions, the PPP-BOTDA and TW-COTDR mentioned above, the specifications are shown separately for each of these functions. As shown in this table, the distance range, distance resolution, etc. are the same, but the frequency characteristics are significantly different.

[0045] Next, FIG. 9 shows the center frequency distribution in each component part of the two sample cables when the frequency shift analysis of the above-mentioned sample cables is performed by the PPP-BOTDA method. This figure shows the characteristics of the test cable before it was immersed in light oil. It can be seen that the armored cable part being measured has a center frequency of about 11.1 GHz, and the reference fiber part has a center frequency of about 10.8 GHz.

[0046] Next, Fig. 10 shows examples of strain distribution measurements for two sample cables. For both samples, the occurrence of strain (the strain distribution for each sample has a depression in the center) is observed, which is thought to be due to the absorption of hydrocarbons caused by being immersed in light oil.

[0047] Next, FIG. 11 shows the change in strain distribution over time when the strain distribution of sample cable #2 was measured for about one month. The numbers in the figure indicate the dates when the strain distribution was measured. As shown in this figure, the strain distribution changes significantly, which is believed to be the result of the gradually increasing content of hydrocarbon compounds in the filler.

[0048] Finally, Figure 12 shows the change over time in strain distribution with and without a filler, which is a resin with hydrocarbon detection capabilities, with the measurement date and time on the horizontal axis. In this figure, the downward arrows (2 locations in total) indicate the time when the test cable was immersed in diesel, and the upward arrows (2 locations in total) indicate the time when the test cable was removed from the diesel. The double-arrow section Ts indicates the period when measurements were stopped. It can be seen that by immersing the test cable (with filler) in diesel, the filler absorbs hydrocarbons, resulting in a clear change in strain.

[0049] Next, the case where the optical cable 10 or 10a is used for long-term (approximately 50 years) monitoring of oil wells or the like will be described with reference to FIGS.

[0050] The presence or absence of oil or other leakage is monitored based on whether the optical cable 10 detects hydrocarbons, which are the main components of oil. That is, when the resin provided in the optical cable 10 detects hydrocarbons somewhere between the top position of the borehole 100 shown in FIG. 1 (the height position corresponding to the seabed in FIG. 1. In FIG. 13, the point with a depth value of 0 (zero) is the height position corresponding to the seabed) and the bottom position of the borehole 100 (the height position corresponding to the depth value of 1 in FIG. 13), the amount of scattered light shift at the detected position is different from the amount of shift at other positions and is output as a leak signal (see FIG. 13). The point (depth position) where this different output is output is the position where oil or other leakage has occurred. Also, in FIG. 13, it can be seen that the leak signal was detected for the first time 50 years after the installation of the oil well. It can also be seen that the leak position is slightly below the central height position of the borehole 100.

[0051] Here, the amount of scattered light shift that has changed due to the hydrocarbons absorbed by the resin is not affected by the depth direction of the borehole 100, and is considered to change only depending on the amount of the absorbed hydrocarbons, whereas temperature, pressure, strain, etc. are highly likely to change depending on the depth direction, and when the influence of these factors is large, it is necessary to correct the amount of scattered light shift due to these factors. In such a case, the optical cable 10a having the above-mentioned DPATS function can be used.

[0052] As described above, according to the optical cable monitoring system 50 using the optical cable 10 or 10a, the leak depth position of oil or the like in the inspected borehole can be detected. By carrying out local repairs at the position where the leak was detected, the borehole can be repaired, and the lifespan of the borehole can be extended without having to discard the entire borehole.

[0053] Furthermore, after the local repair, by checking the presence or absence of a leak signal from the optical cable monitoring system 50, if no leak signal is detected, it can be confirmed that the leak repair has been effective.

[0054] Although the present application describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless modifications not exemplified are assumed within the scope of the technology disclosed in the present specification, including, for example, modifying, adding, or omitting at least one component, and further, extracting at least one component and combining it with a component of another embodiment. [Explanation of symbols]

[0055] 1 optical fiber, 2, 2a, 2b Hydrocarbon absorbing resins, 3, 4, 5 Steel wire, 6 Optical fibers for measuring physical properties; 10, 10a optical cable, 20 strands, 21 second strand, 22 third strand, 40 Backscattered light measuring device, 50 Optical cable monitoring system, 100 wells

Claims

1. An optical fiber; a plurality of cables provided in a radial direction of the optical fiber are twisted to cover an outer periphery of the optical fiber and arranged in a circular ring; a second stranded wire, which is provided in a radial direction of the optical fiber and in which a plurality of cables having an outer diameter larger than that of the stranded wire cable are twisted and arranged in a circular shape so as to cover an outer periphery of the stranded wire; an optical fiber for measuring a physical property, which is arranged to replace one cable of the second stranded wire and measures a physical property of an object to be measured, such as temperature, strain, or pressure; a third stranded wire, which is provided in a radial direction of the optical fiber and in which a plurality of cables having an outer diameter larger than that of the second stranded wire are twisted and arranged in an annular shape so as to cover an outer periphery of the second stranded wire; a hydrocarbon-absorbing resin that absorbs hydrocarbons and fills spaces between the optical fiber, the strand, the second strand, and the third strand; An optical cable comprising: The optical fiber for measuring the physical properties is structurally arranged in the optical cable so as to be able to compensate for the frequency shift of the signal from the optical fiber caused by the physical properties of the object to be measured, so that the frequency shift of the compensated signal is affected only by the expansion of the hydrocarbon absorbing resin caused by the absorption of hydrocarbons.

2. 2. The optical cable according to claim 1, wherein the hydrocarbon-absorbing resin is intermittently filled in the longitudinal direction of the stranded wires.

3. The optical cable according to claim 1 ; a backscattering light measuring device for measuring the frequency shifts of the Brillouin scattered light and the Rayleigh scattered light; Equipped with An optical cable monitoring system characterized by distinguishing and measuring frequency shift changes in the optical fiber caused by the hydrocarbon absorption resin absorbing hydrocarbons contained in the object being measured, and frequency shift changes in the optical fiber caused by changes in pressure, strain or temperature, which are physical properties of the object being measured.

4. The optical cable monitoring system according to claim 3, characterized in that the frequency shift change in the optical fiber due to absorption of hydrocarbons contained in the object to be measured is determined by distinguishing it from the amount of frequency shift change in the optical fiber due to changes in pressure, strain or temperature, which are physical properties of the object to be measured, and measuring the corresponding position in the optical fiber where the frequency shift change in the optical fiber due to absorption of hydrocarbons contained in the object to be measured occurs.

5. A method for monitoring a well, comprising: using the optical cable monitoring system according to claim 4; laying an optical fiber for detecting hydrocarbons along a depth direction at an outer position of the well; and monitoring for oil leakage from the well, comprising the steps of: A time-dependent change in a leak signal, which is a signal in which the amount of shift in scattered light due to hydrocarbons detected by the optical fiber is different from the amount of shift in scattered light at other positions due to the oil leak, is recorded with respect to the position in the well; A method for monitoring a well, comprising the steps of repairing the well at the location where the leak signal is detected and checking whether or not the leak signal is detected after the repair, thereby determining whether or not the well has been repaired properly.

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