Oil and gas well optical cable design method and oil and gas well optical cable

Abstract

The present invention relates to the field of design and preparation of special optical cables. Disclosed are an oil and gas well optical cable design method and an oil and gas well optical cable. By designing the structural selection of an oil and gas well optical cable, designing the optical cable having an outer crush-resistant layers, an airtight shielding layer and an optical unit, and respectively designing and calculating the collapse yield strength of the outer crush-resistant layer, the strain of the optical cable caused by the weight of the optical cable and an external mechanical stress, and the strain of the optical cable caused by the temperature of the optical cable, the selection design of the outer crush-resistant layer and the design of an excess fiber length of the optical cable can be accurately completed, and the design of oil and gas well optical cable can be accurately completed in light of the selection design of the airtight shielding layer, thereby ensuring that the designed oil and gas well optical cable can correspond to actual application scenarios, meeting the application requirements of the optical cable in oil and gas wells under different environmental conditions, improving the use reliability of the oil and gas well optical cable, ensuring the signal transmission and use precision of the oil and gas well optical cable, and prolonging the service life of the oil and gas well optical cable.

Description

Oil and gas well optical cable design method and oil and gas well optical cable Technical Field

[0001] The present invention belongs to the field of design and preparation of special optical cables, and in particular relates to an oil and gas well optical cable design method and an oil and gas well optical cable. Background Art

[0002] In recent years, with the development of optical fiber and cable research and manufacturing technology, specialized optical cables that meet the needs of different special environments have been increasingly used, and the ultimate performance of optical cables has also been greatly improved. Specifically, the maximum operating temperature of optical fiber cables has been significantly increased compared to traditional optical cables, from 85°C to 300°C or even higher.

[0003] As the operating temperature of optical fiber cables increases, combined with the current sensing capabilities of DTS, DAS and related technologies to read temperature, pressure and vibration signals, it is possible to use optical fiber cables to monitor relevant parameters in oil and gas wells.

[0004] However, due to the high temperature and high pressure conditions typically found underground in oil and gas wells, the fact that optical cables are subject to the effects of their own weight and mechanical forces during service, coupled with performance differences between the materials inside and outside the cable, can easily cause strain in optical fiber cables, resulting in significant additional attenuation. For DAS and DTS sensing applications, even small attenuation changes can lead to significant drift in test results, affecting actual test accuracy and precision. This significantly limits the application of existing optical fiber cables in oil and gas well environments. Furthermore, due to the complex conditions within oil and gas wells, the service life of optical cables used in oil and gas wells, both domestically and internationally, is typically short, resulting in high application costs. Summary of the Invention

[0005] In response to one or more of the above-mentioned defects or improvement needs in the prior art, the present invention provides an oil and gas well optical cable design method and an oil and gas well optical cable, which can accurately implement the design of oil and gas well optical cables, fully consider the impact of various factors on the performance of oil and gas well optical cables, improve the performance of oil and gas well optical cables in actual application environments, and extend the service life of oil and gas well optical cables.

[0006] To achieve the above objectives, one aspect of the present invention provides a method for designing an oil and gas well optical cable, which includes the following steps:

[0007] (1) Based on the use requirements of optical cables in oil and gas wells, the structure of the optical cable for oil and gas wells is designed, and the optical cable is designed to include, from the inside to the outside, an optical unit, an airtight shielding layer, and an outer pressure-resistant layer; wherein the optical unit includes a casing and an optical fiber disposed in the casing;

[0008] (2) According to the environmental pressure P of the optical cable use environment and the collapse yield strength P of the outer compressive layer YDesign and select the outer pressure-resistant layer; among them, P Y >P, and the collapse yield strength P Y Calculated by the following formula:

[0009]

[0010] Where, P Y is the collapse yield strength of the outer pressure-resistant layer; δ is the yield strength of the material used for the outer pressure-resistant layer; D is the outer diameter of the outer pressure-resistant layer; t is the thickness of the outer pressure-resistant layer;

[0011] (3) Design and select the airtight shielding layer according to the operating environment parameters of the optical cable;

[0012] (4) Select the type of optical fiber according to the use requirements of the optical cable, and complete the design of the optical fiber excess length according to the following formula:

[0013]

[0014]

[0015]

[0016] Where, is the excess length of the optical fiber; the optical fiber is calculated in a spiral curve in the casing, L is the pitch of one cycle of the spiral; D0 is the inner diameter of the casing; ε Cmax is the maximum stress strain per unit length of optical cable; ε Tmax is the maximum temperature strain per unit length of optical cable; L is the vertical length of the optical cable in service; is the cable density; g is the acceleration of gravity, M C is the composite elastic modulus of the optical cable; C c and C f Represent the thermal expansion coefficients of optical cables and optical fibers respectively; It is the temperature difference between the actual operating temperature of the optical cable and the room temperature of the optical cable.

[0017] As a further improvement of the present invention, in process (3), the design and selection of the airtight shielding layer is determined by calculating the diffusion rate of the gas in the outer pressure-resistant layer, which is calculated using the following formula:

[0018]

[0019] Where J is the diffusion rate of gas through a material layer per unit thickness; A is a constant related to the material; P is the ambient pressure; K is a known constant related to diffusion; and T is the ambient temperature.

[0020] As a further improvement of the present invention, in process (3), a fitting curve diagram of the diffusion rate of different materials with conventional design dimensions as a function of temperature is pre-designed; thus, after the calculation of the diffusion rate of the gas in the outer pressure-resistant layer is completed, the material selection of the airtight shielding layer is directly determined by the fitting curve diagram.

[0021] As a further improvement of the present invention, in process (1), an outer sheath with a corresponding cross-sectional size is designed on the periphery of the outer pressure-resistant layer according to the actual use requirements of the optical cable, and the material selection of the outer sheath is determined according to the actual use environment of the optical cable.

[0022] As a further improvement of the present invention, in process (4), taking into account the complexity of the application environment of oil and gas well optical cables, the excess length of the optical fiber is characterized by the following formula:

[0023] .

[0024] Another aspect of the present invention provides an oil and gas well optical cable designed using the oil and gas well optical cable design method.

[0025] The oil and gas well optical cable comprises an optical unit, an airtight shielding layer and an outer pressure-resistant layer arranged in sequence from the inside to the outside;

[0026] The optical unit includes a sleeve and an optical fiber which is inserted into the sleeve and has a certain excess length.

[0027] As a further improvement of the present invention, the outer pressure-resistant layer is made of 316L stainless steel, 625 alloy or 825 alloy;

[0028] and / or

[0029] The maximum operating temperature of the oil and gas well optical cable is lower than 180°C, and the airtight shielding layer is made of modified polypropylene, modified nylon, perfluoroethylene propylene copolymer FEP, ethylene-tetrafluoroethylene copolymer ETFE or polytetrafluoroethylene PFA; or, the maximum operating temperature of the oil and gas well optical cable is greater than 180°C, and the airtight shielding layer is made of aluminum material.

[0030] As a further improvement of the present invention, the sleeve is made of stainless steel;

[0031] and / or

[0032] The sleeve is filled with a buffer material; the buffer material is a temperature-resistant grease or a fiber material.

[0033] As a further improvement of the present invention, the excess length of the optical fiber is between 0.2% and 0.9%;

[0034] and / or

[0035] The optical fiber is a carbon-coated optical fiber.

[0036] As a further improvement of the present invention, an outer sheath is further provided on the periphery of the outer pressure-resistant layer.

[0037] The above-mentioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0038] In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

[0039] (1) The oil and gas well optical cable design method of the present invention designs an optical cable having an outer pressure-resistant layer, an airtight shielding layer and an optical unit by designing the structural selection of the oil and gas well optical cable, and then combines the design and calculation of the collapse yield strength of the outer pressure-resistant layer, the strain of the optical cable caused by the influence of the optical cable's own weight and external mechanical stress, and the strain of the optical cable caused by the influence of temperature on the optical cable, so as to accurately complete the selection design of the outer pressure-resistant layer and the design of the optical fiber excess length of the optical cable. In combination with the optimal design of the airtight shielding layer, the selection design of the oil and gas well optical cable can be accurately completed, thereby ensuring the reliability of the optical cable in oil and gas wells under different environmental conditions, improving the accuracy of signal transmission and use of the oil and gas well optical cable, and extending the service life of the oil and gas well optical cable.

[0040] (2) The oil and gas well optical cable design method of the present invention can quickly complete the calculation of the diffusion rate of hydrogen in the outer pressure-resistant layer by introducing gas diffusion theory and pre-designing a fitting curve diagram of the diffusion rate of different materials changing with temperature. Based on the calculation results and the pre-designed fitting curve diagram, the material selection of the airtight shielding layer can be quickly determined, and the airtight shielding layer can be accurately designed to ensure the reliability of the oil and gas well optical cable in a hydrogen-rich environment and reduce the additional attenuation of the optical fiber caused by hydrogen penetration.

[0041] (3) The oil and gas well optical cable of the present invention has a simple structure and accurate design and selection. It can change the corresponding structural selection conditions according to the requirements of different application environments, ensure that the optical cable is adapted to the actual application scenario, improve the accuracy of the design, preparation and use of the oil and gas well optical cable application, avoid the occurrence of redundant functions when the oil and gas well optical cable is actually used, reduce the preparation and application costs of the oil and gas well optical cable, and meet the use requirements in different application environments.

[0042] (4) The oil and gas well optical cable design method of the present invention has simple steps and is easy to operate. It can fully consider the application characteristics of the oil and gas well optical cable in the oil and gas well use environment, accurately realize the design of the oil and gas well optical cable, provide an accurate basis for the preparation and use of the oil and gas well optical cable, ensure the accuracy and stability of the oil and gas well optical cable when used in actual application scenarios, extend the service life and use accuracy of the oil and gas well optical cable, and has good practical value and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0044] FIG1 is a schematic structural diagram of an oil and gas well optical cable in a specific embodiment 1 of the present invention;

[0045] FIG2 is a schematic structural diagram of an oil and gas well optical cable in a specific embodiment 2 of the present invention;

[0046] FIG3 is a fitting curve diagram of the permeation rate-temperature change of the outer pressure-resistant layer in a specific embodiment of the present invention. Modes for Carrying Out the Invention

[0047] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0048] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0049] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0050] In the present invention, unless otherwise specified or limited, the terms "installed," "connected," "connect," "fixed," etc. should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integration; mechanical connection, electrical connection; direct connection, or indirect connection through an intermediate medium; internal communication between two components, or interaction between two components, unless otherwise specified. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0051] In the present invention, unless otherwise expressly specified or limited, when a first feature is "above" or "below" a second feature, it may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediary. Furthermore, when a first feature is "above," "above," or "above" a second feature, it may mean that the first feature is directly above or diagonally above the second feature, or simply means that the first feature is at a higher level than the second feature. When a first feature is "below," "below," or "below" a second feature, it may mean that the first feature is directly below or diagonally below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0052] The technical solutions in the preferred embodiments of the present invention are intended to realize the design of oil and gas well optical cables and provide a reliable theoretical basis for the preparation and use of oil and gas well optical cables.

[0053] Since oil and gas well optical cables have been in service in high temperature, high pressure and hydrogen-rich environments for a long time, they are exposed to the influence of high ambient temperature T, high ambient pressure P and hydrogen atom penetration in hydrogen-rich environments.

[0054] For this reason, when actually setting up oil and gas well optical cables, it is necessary to set up at least an outer pressure-resistant layer and an airtight shielding layer for the optical unit in the middle of the cable that can withstand high temperature, high pressure and hydrogen atom penetration to ensure the service reliability of the cable.

[0055] At the same time, since oil and gas well optical cables are immersed in the oil and gas well environment for a long time during use, the temperatures inside and outside the optical cable tend to be the same or have little difference. Therefore, when actually designing oil and gas well optical cables, it is necessary to select high-temperature resistant optical units as the core components in the middle of the optical cable.

[0056] In addition, in order to ensure the practical use of oil and gas well optical cables, an outer sheath is preferably provided on the periphery of the outer pressure-resistant layer to meet the requirements of clamping, fixing and laying of the optical cable.

[0057] More specifically, the outer sheath of oil and gas well cables primarily serves to meet construction requirements (to secure the cable), mitigate external impact, and protect the outer pressure-resistant layer from direct corrosion by corrosive gases. In practical designs, the outer sheath of oil and gas well cables is preferably designed with a square, circular, or polygonal cross-section to facilitate cable clamping and fixation.

[0058] Secondly, the outer sheath material can be modified to suit the specific application environment, as long as it meets the maximum operating temperature of the cable. For example, depending on the actual application scenario (oil and gas well depth, environmental pressure, and environmental issues), outer sheath materials with different operating temperature ranges can be selected, such as modified PP (<135°C), modified nylon (<145°C), FEP (<200°C), and PFA (<250°C) for the outer sheath of different oil and gas well cables.

[0059] In addition, to ensure the reliability of the optical cable fixation, the material hardness of the outer sheath is preferably not less than 92 A. When the cross-section of the outer sheath is designed to be circular, the outer pressure-resistant layer can preferably replace the function of the outer sheath.

[0060] Furthermore, the outer pressure-resistant layer in the preferred embodiment is primarily used to provide mechanical properties to the optical cable, preventing strain on the cable due to its own weight and mechanical stretching. Simultaneously, the outer pressure-resistant layer can also provide a certain degree of gas shielding.

[0061] In actual settings, the cross-sectional area and thickness of the outer compression layer directly determine the cable's resistance to lateral pressure, impact, and tensile strength. In addition, oil and gas well optical cables are subject to the environmental pressure P during actual use.

[0062] Therefore, when designing the structural parameters of the outer pressure-resistant layer, it is preferred to mainly consider the influence of the environmental pressure P on it. In a preferred embodiment, the collapse yield strength P is used. Y To indicate the performance of the outer compression layer, in order to ensure the normal operation of the optical cable, it is necessary to ensure that P Y >P.

[0063] More specifically, in the preferred embodiment, the following formula (1) is used to characterize the collapse yield strength P of the outer compressive layer: Y ,

[0064] Formula (1)

[0065] Where, P Y is the collapse yield strength of the outer pressure-resistant layer; δ is the yield strength of the material used for the outer pressure-resistant layer; D is the outer diameter of the outer pressure-resistant layer; t is the thickness of the outer pressure-resistant layer.

[0066] In actual design, depending on the application and production requirements of the optical cable, the value of D typically ranges from 3mm to 12mm, with typical diameters including 6.35mm (1 / 4 inch) and 9.8mm (3 / 8 inch). Therefore, in actual design, the outer diameter of the outer pressure-resistant layer can be determined by selecting commonly used production sizes.

[0067] In addition, considering the actual application scenarios and functional requirements of the outer pressure-resistant layer, the outer pressure-resistant layer in the preferred embodiment is preferably made of metal material. After the material type of the outer pressure-resistant layer is selected, its corresponding yield strength can be obtained when selecting the material. In this way, according to the above formula (1), the minimum design thickness t of the outer pressure-resistant layer can be calculated accordingly. min On this basis, the thickness of the outer pressure-resistant layer can be designed accordingly.

[0068] In this way, the material type, design outer diameter and design thickness of the outer pressure-resistant layer can be determined according to the above-mentioned design process.

[0069] Obviously, in actual design, if the material type of the outer pressure-resistant layer changes, the theoretical calculation of the design thickness of the outer pressure-resistant layer can also be completed based on its corresponding yield strength.

[0070] When the outer pressure-resistant layer is actually manufactured from a metal material, the difficulty of forming it increases as the thickness of the outer pressure-resistant layer increases. Therefore, in a preferred embodiment, the value of t is preferably within the range of 0.4 mm to 1.25 mm. More preferably, the outer pressure-resistant layer in the preferred embodiment is manufactured by welding and drawing.

[0071] At the same time, for the outer compression-resistant layer made of metal material, the yield strength δ of the material is preferably in the range of 200 MPa to 1300 MPa.

[0072] According to the calculation of formula (1), the relationship between the service environment pressure P of the optical cable and the outer diameter D and thickness t of the outer pressure-resistant layer can be determined.

[0073] In a typical preferred embodiment, the standard yield strength δ of the outer pressure-resistant layer is 800 MPa, the outer diameter D is 6.35 mm, and the thickness t of the outer pressure-resistant layer is 1.25 mm. At this time, according to formula (1), the outer pressure-resistant layer can withstand an environmental pressure of up to 252 MPa. Under these conditions, the outer pressure-resistant layer can meet the application requirements of most oil and gas well operating environments and meet the design requirements of the outer pressure-resistant layer of oil and gas well optical cables.

[0074] More preferably, the material of the outer pressure-resistant layer can be further preferably 316L stainless steel, 625 alloy, or 825 alloy. After the arrangement is completed, the outer pressure-resistant layer forms a steel pipe structure of a certain thickness.

[0075] In addition, in applications where the requirements for lateral pressure resistance of optical cables are relatively low, optical cables can also be reinforced by twisting multiple layers of steel wires. Of course, the mechanical strength of optical cables can also be improved by setting up a dual structure of steel wire twisting and steel tubes.

[0076] Furthermore, because optical cables are exposed to high-temperature, high-pressure, hydrogen-rich environments for long periods of time, the diffusion of hydrogen into the cable interior can increase the additional attenuation of the optical fibers. Therefore, to reduce the impact of hydrogen-rich gas diffusing through the outer pressure-resistant layer into the cable interior on the cable's performance, in a preferred embodiment, an airtight shielding layer is provided inside the outer pressure-resistant layer to provide hydrogen-rich gas shielding for the cable.

[0077] Specifically, the hydrogen damage resistance of optical cables used in oil and gas wells is primarily reflected in two aspects: the cable's gas shielding performance and the optical fiber's resistance to hydrogen damage in hydrogen-rich environments. Furthermore, depending on the cable's design dimensions, the thickness of the gas-tight shielding layer is typically set between 0.5 and 1.0 mm.

[0078] Since the outer pressure-resistant layer has a certain shielding effect on gas penetration after actual setting, in the preferred embodiment, the first consideration in the design of the airtight shielding layer is the gas penetration resistance of the outer pressure-resistant layer, and the gas penetration rate in the outer pressure-resistant layer is used as the basis for the selection and design of the airtight shielding layer.

[0079] In a preferred embodiment, the gas diffusion theory is introduced to characterize the diffusion rate of hydrogen in the outer pressure-resistant layer, and the calculation process is preferably completed by the following formula:

[0080] Formula (2)

[0081] Where J is the diffusion rate of gas through a material layer per unit thickness; A is a constant related to the material; P is the ambient pressure; K is a known constant related to diffusion; and T is the ambient temperature.

[0082] According to the above formula (2), the diffusion rate of gas permeating through the airtight shield is proportional to the square root of the pressure P; and as the temperature increases, the diffusion rate and the temperature-related parameter 1 / (T+273.15) are exponentially related.

[0083] More specifically, take the case where 316L stainless steel is used as the outer pressure-resistant layer and the pressure is less than 70 MPa as an example.

[0084] According to formula (2), when the operating temperature T of the optical cable is lower than 180°C, the diffusion rate of the gas in the outer pressure-resistant layer is low, less than <10 -6Atm / h. Under this condition, assuming the diffusion rate remains unchanged, the internal pressure of the outer pressure-resistant layer of the optical cable will not exceed 0.1 atm after five years of service. Under this condition, the impact of hydrogen loss on the transmission quality of optical fiber is limited.

[0085] In the above case, it is determined that the outer pressure-resistant layer can effectively shield hydrogen-rich gas. In this case, when making an airtight shielding layer, the requirements for the airtight shielding layer are relatively low. The corresponding airtight shielding layer can preferably be made of a polymer material that meets the temperature requirements of the product, such as modified polypropylene, modified nylon, perfluoroethylene propylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), or polytetrafluoroethylene (PFA).

[0086] However, when the operating temperature T of the optical cable is between 180℃ and 300℃, the temperature of the optical cable operating environment is relatively high, and the higher temperature is generally accompanied by a greater increase in gas pressure, and the gas diffusion rate is faster, usually greater than 10 -6 atm / h, and the diffusion rate increases rapidly with increasing temperature and pressure. In this case, the outer pressure-resistant layer alone cannot meet the hydrogen-rich gas shielding function of the optical cable. In this case, it is necessary to select a material with better anti-diffusion properties to prepare the airtight shielding layer.

[0087] In a preferred embodiment, aluminum material is further preferably used as the preparation material of the airtight shielding layer. The permeation rate of hydrogen-rich gas in aluminum material is 1 to 3 orders of magnitude lower than that of 316L stainless steel, which can greatly improve the performance of the optical cable in resisting the diffusion of hydrogen-rich gas.

[0088] Furthermore, in actual design, in order to simplify the selection process of the airtight shielding layer, it is preferred to pre-design a fitting curve diagram of the diffusion rate of different materials under conventional design dimensions (outer diameter, wall thickness) as a function of temperature. In this way, after calculating the corresponding diffusion rate, the selection of the shielding material can be quickly completed.

[0089] For example, the hydrogen permeation performance of an outer pressure-resistant layer made of 316L material with an outer diameter of 6.35mm and a wall thickness of 0.85mm was tested. The hydrogen permeation rate of the outer pressure-resistant layer was tested at 150°C, 200°C, 225°C, and 250°C under 75MPa. The test results are shown in Figure 3. No obvious hydrogen permeation was detected at 150°C. The permeation rates at 200°C, 225°C, and 250°C are shown in the figure, and the permeation rate curve of the outer pressure-resistant layer under these conditions was obtained by fitting.

[0090] According to the above method, a fitting curve diagram of the hydrogen permeation rate of the outer pressure-resistant layer as a inverse of temperature under different materials, different size parameters and different air pressure environments can be obtained respectively, which can be used as the basis for the subsequent selection and preparation of the airtight shielding layer, thereby realizing the rapid selection of the airtight shielding layer.

[0091] Furthermore, as the most important component of the optical cable, the optical unit provided in the middle of the optical cable needs to consider not only the influence of temperature but also the stress and strain during the actual use of the optical cable.

[0092] In a preferred embodiment, the optical unit is the smallest protection unit in the optical cable, which preferably includes a sleeve and an optical fiber passing through the sleeve and having a certain excess length of optical fiber.

[0093] Because oil and gas well optical cables are primarily used in high-temperature and high-pressure environments, the casing is generally made of stainless steel, a popular material in the industry. Based on the molding process requirements of the optical unit in the cable, the outer diameter of the casing is typically between 1.5mm and 3.2mm, and the thickness is between 0.20 and 0.30mm.

[0094] In more detail, when actually setting up the optical unit, it is preferred to provide a buffer material between the optical fiber and the sleeve to achieve buffering of the optical fiber when the optical cable is subjected to stress, thereby reducing the stress on the optical fiber under temperature or mechanical shock conditions.

[0095] In a preferred embodiment, the above-mentioned buffer material may be preferably a temperature-resistant grease filled in the sleeve, or may be preferably a fiber material filled in the sleeve.

[0096] Furthermore, in the design of optical cables, the excess length of optical fiber is an important indicator, which usually refers to the difference between the length of optical fiber per unit length of optical cable and the length of optical cable. In actual characterization, the excess length of optical fiber can be calculated using the following formula:

[0097] Formula (3)

[0098] Where, is the excess length of optical fiber; L c is the length of the optical cable, L f The length is L c The length of the optical fiber in the cable.

[0099] In the preferred embodiment, the design of the excess length of the optical fiber mainly considers the following two aspects: one is the stress strain ε caused by the weight of the optical cable and the external mechanical stress. C ; Second, the temperature strain ε caused by the temperature of the optical cable under different temperature conditions T .

[0100] Among them, the stress strain ε caused by the weight and external mechanical stress of the optical cable per unit length isC , in the preferred embodiment, the following formula (4) is used for characterization:

[0101] Formula (4)

[0102] Where, L is the vertical length of the service optical cable in the well; is the cable density; g is the acceleration of gravity; F is the mechanical tensile force on the cable other than its own weight; M C It is the composite elastic modulus of the optical cable. When taking the actual value, since the outer compressive layer of the optical cable accounts for more than 90% of the mass and serves as the tensile design layer of the optical cable, the elastic modulus of the outer compressive layer is preferably used instead.

[0103] At the same time, for the temperature strain of the optical cable per unit length caused by temperature conditions under different operating temperature conditions, the following formula (5) is used in the preferred embodiment to characterize it:

[0104] Formula (5)

[0105] Where C c and C f Represent the thermal expansion coefficients of the optical cable and optical fiber respectively; Since the mass of the optical cable's compression layer accounts for more than 90%, in actual design, It is preferred to directly use the thermal expansion coefficient of the metal material used to prepare the outer pressure-resistant layer as a substitute; is the temperature difference between the actual operating temperature of the optical cable and the room temperature of the optical cable. , T0 is generally preferably 25℃.

[0106] More specifically, for vertical wells, the strain caused by gravity generally decreases as the depth of the cable increases, but the downhole temperature increases with depth. Therefore, after the cable is lowered into a specific oil and gas well, the ε of the vertical section of the downhole cable is C As the depth increases, the maximum strain per unit length of the optical cable decreases. Cmax Appears at the top of the optical cable; ε T As the depth increases, the maximum strain per unit length of the optical cable ε Tmax Appears at the bottom of the fiber optic cable.

[0107] Considering the influence of both, in a preferred embodiment, the maximum value of the two is taken as the design reference. That is, the sum of the maximum values ​​of the two strains of the optical cable is used as the design basis for the excess length of the optical fiber, so that the excess length of the optical fiber is not less than the sum of the maximum values ​​of the two strains, as shown in the following formula:

[0108] Formula (6)

[0109] Where, is the excess length of optical fiber; L c is the length of the optical cable, L f The length is L c The length of the optical fiber in the optical cable; ε Cmax is the maximum stress strain per unit length of optical cable; ε Tmax is the maximum temperature strain per unit length of optical cable.

[0110] In addition, it is not difficult to understand that if the excess length of the optical fiber is too small, the optical fiber will be in a tensile stress state under the action of force strain and temperature strain, affecting the performance of the optical fiber.

[0111] However, considering the actual arrangement of the optical fiber in the casing (in an approximately spiral wave-shaped arrangement), too much excess length will cause the wave peaks formed by the optical fiber layout to abut the wall of the casing, thereby causing the optical fiber to be affected by the wall stress and accelerated temperature, resulting in failure of the optical cable.

[0112] Therefore, in actual design, it is preferred to combine the helical pitch of the optical fiber and the inner diameter of the sleeve for calculation, and take the case where the bending radius of the optical fiber is the minimum bending radius and just abuts the inner wall of the sleeve as the maximum reference size of the optical fiber excess length. It is further preferred to approximate the maximum value of the optical fiber excess length by the following formula:

[0113] Formula (7)

[0114] Where, is the maximum value of the excess length of the optical fiber; the optical fiber is calculated in a spiral curve in the casing, where L is the pitch of one cycle of the helix; D0 is the inner diameter of the casing;

[0115] That is, in actual design, it is preferred to control the excess length of the optical fiber within the following range:

[0116] Formula (8)

[0117] Where, is the excess length of the optical fiber; the optical fiber is calculated in a spiral curve in the casing, L is the pitch of one cycle of the spiral; D0 is the inner diameter of the casing; ε Cmax is the maximum stress strain per unit length of optical cable; ε Tmax is the maximum temperature strain per unit length of optical cable.

[0118] More specifically, in a preferred embodiment, based on the characteristics of the optical cable and the features of actual application, and in combination with the calculations of formulas (3) to (8), it can be determined that the excess length of the optical fiber of the optical cable in the preferred embodiment is preferably controlled to be above 0.1%.

[0119] However, considering the complexity of the actual application environment of the optical cable and the local mechanical and environmental impacts to which the optical cable is subjected during use, the minimum value of the actual excess fiber length is preferably increased by 0.1% on the basis of the theoretically calculated value, that is, the excess fiber length of the optical cable in the preferred embodiment is preferably controlled to be above 0.2%, and the excess fiber length satisfies the following relationship:

[0120]

[0121] Furthermore, in actual use, the periodic pitch L of the optical fiber should be no less than 60 mm. If the pitch is less than this, the optical fiber will be subjected to significant stress and exhibit a large additional attenuation. At the same time, in actual preparation, the inner diameter of the optical unit sleeve is generally 2.2 mm or 2.4 mm. For a 2.2 mm sleeve, according to formula (7), the excess length of the optical fiber should be no more than 0.66%; for a 2.4 mm sleeve, according to formula (7), the excess length of the optical fiber should be no more than 0.78%.

[0122] Therefore, in a preferred embodiment, the excess fiber length of the optical cable is preferably between 0.2% and 0.8%.

[0123] In actual setup, the actual downhole ambient temperature is not a fixed value but varies with depth. This means that different cable lengths will experience different operating temperatures. Therefore, the strains caused by the weight and temperature mentioned above need to be analyzed and calculated based on the actual situation.

[0124] Specifically, when the optical cable is used in oil and gas wells at a depth of 6000~8000m, the excess fiber length is preferably 0.45~0.6%; when the optical cable is used in oil and gas wells at a depth of 4000~6000m, the excess fiber length is preferably 0.35~0.45%; and when the optical cable is used in oil and gas wells at a depth of 2000~4000m, the excess fiber length should preferably be controlled between 0.25~0.35%.

[0125] Furthermore, considering the actual use environment of the optical cable (high temperature, high pressure, hydrogen-rich environment), the optical fiber in the optical unit needs to be selected according to the actual use environment.

[0126] In a preferred embodiment, carbon-coated optical fiber is further preferably used; under the same conditions, the additional attenuation of hydrogen loss at 1550nm of the carbon-coated optical fiber is 2 orders of magnitude lower than the additional attenuation of hydrogen loss at 1550nm of the conventional optical fiber, and can better resist the attenuation loss caused by hydrogen penetration.

[0127] In addition, since optical fibers need to withstand high temperatures, high pressures, long-term stress, hydrogen loss, and other effects in oil and gas wells, the actual maximum operating temperature of the selected optical fiber is preferably lower than the theoretical maximum operating temperature obtained from optical fiber tests, and further preferably 15 to 30°C lower.

[0128] Of course, when the optical cable is used in a relatively benign environment, for example, when the maximum temperature in the well is no more than 150°C, the carbon-coated optical fiber can be replaced with an optical fiber coated with a high-temperature resistant acrylic resin or silicone rubber. When the maximum temperature in the well is greater than 150°C but the hydrogen pressure is relatively low, an optical fiber coated with a polyimide layer is preferred. Furthermore, when the operating temperature is greater than 150°C and the hydrogen pressure is higher, a carbon-coated optical fiber with better hydrogen damage resistance is preferably selected based on actual usage conditions.

[0129] In a preferred embodiment, through the combination setting of the above design principles, by utilizing the comprehensive consideration and design of the optical cable strain, external stress and hydrogen penetration during the actual use of the optical cable, the combined design of the outer sheath, outer pressure-resistant layer, airtight shielding layer and optical unit of the optical cable is accurately completed, so that the obtained optical cable is more in line with the application requirements in the oil and gas well use environment, thereby obtaining an oil and gas well optical cable with strong service performance and long service life, ensuring the stability of the optical signal during the service of the optical cable, reducing temperature drift during temperature measurement and errors in vibration testing.

[0130] In general, the design method of the oil and gas well optical cable in the preferred embodiment preferably includes the following design process:

[0131] (1) Designing the structure of an oil and gas well optical cable based on the use requirements of the optical cable in oil and gas wells, and designing an optical cable comprising, from the inside to the outside, an optical unit, an airtight shielding layer, and an outer pressure-resistant layer; wherein the optical unit comprises an optical fiber disposed in a casing;

[0132] (2) According to the environmental pressure P of the optical cable use environment and the collapse yield strength P of the outer compressive layer Y Design and select the outer pressure-resistant layer; among them, P Y >P, and the collapse yield strength P Y Calculated by the following formula:

[0133]

[0134] Where, P Y is the collapse yield strength of the outer pressure-resistant layer; δ is the yield strength of the material used for the outer pressure-resistant layer; D is the outer diameter of the outer pressure-resistant layer; t is the thickness of the outer pressure-resistant layer;

[0135] (3) Design and select the airtight shielding layer according to the operating environment parameters of the optical cable;

[0136] Preferably, the design and selection of the airtight shielding layer is realized based on the anti-permeation capability of the outer pressure-resistant layer. The diffusion rate of the gas in the outer pressure-resistant layer is calculated using the following formula, and the design and selection of the airtight shielding layer is determined based on the calculation result.

[0137]

[0138] Where J is the diffusion rate of gas through a material layer per unit thickness; A is a constant related to the material; P is the ambient pressure; K is a known constant related to diffusion; and T is the ambient temperature.

[0139] (4) Select the type of optical fiber according to the use requirements of the optical cable, and complete the design of the optical fiber excess length according to the following formula:

[0140]

[0141]

[0142]

[0143] Where, is the excess length of the optical fiber; the optical fiber is calculated in a spiral curve in the casing, L is the pitch of one cycle of the spiral; D0 is the inner diameter of the casing; ε Cmax is the maximum stress strain per unit length of optical cable; ε Tmax is the maximum temperature strain per unit length of optical cable; L is the vertical length of the optical cable in service; is the cable density; g is the acceleration of gravity, M C is the composite elastic modulus of the optical cable; C c and C f Represent the thermal expansion coefficients of optical cables and optical fibers respectively; It is the temperature difference between the actual operating temperature of the optical cable and the room temperature of the optical cable.

[0144] As another aspect of the present invention, according to the aforementioned design method, an oil and gas well optical cable is designed and prepared, which includes an optical unit, an airtight shielding layer and an outer pressure-resistant layer arranged in sequence from the inside to the outside; preferably, an outer sheath is further provided on the outside of the outer pressure-resistant layer for the actual use of the optical cable.

[0145] For the aforementioned oil and gas well optical cable, the structural selection from the inside to the outside is determined according to the aforementioned method.

[0146] The oil and gas well optical cable in the preferred embodiment is further supplemented with the following two specific embodiments.

[0147] Specific embodiment 1:

[0148] In this embodiment, the environmental parameters for oil and gas well optical cables are as follows: the downhole ambient temperature increases by 2°C for every 100-meter increase in depth, and the downhole depth is no more than 4,500 meters. This means that the maximum actual service temperature of the optical cable is 115°C. Furthermore, the maximum downhole pressure is no more than 90 MPa. Based on the application and installation requirements, the outer sheath of the optical cable needs to be 11 x 11 mm.

[0149] On this basis, according to the above-mentioned design method, the structure of each layer of the oil and gas well optical cable is designed.

[0150] Specifically, the outer sheath is made of 11×11mm high-temperature resistant PP material.

[0151] At the same time, the optical cable uses 316L material as the molding material of the outer pressure-resistant layer, and its physical performance indicators are as follows: thermal expansion coefficient 16.8×10 -6 / ℃ (0~200℃), elastic modulus 193GPa, density 8.0g / cm 3 , the yield strength after molding is not less than 500Mpa. The standard size of 6.35mm is selected as the outer diameter of the outer compression layer. In order to ensure that the collapse yield strength is not less than 90MPa, the wall thickness t of the outer compression layer of the optical cable should be not less than 0.636mm according to formula (1).

[0152] Since the service temperature is lower than 180°C, the diffusion rate of gas in the outer pressure-resistant layer is low, and a polymer material that meets the temperature conditions can be used as the preparation material for the airtight shielding layer.

[0153] Furthermore, the thermal expansion coefficient of optical fiber is 5.8×10 -7 / ℃, the maximum strain value of the optical cable is calculated by formula (3) and formula (4): ε c +ε T , and the excess length of the optical fiber is not less than 0.367%.

[0154] Based on the above design process and the selection of corresponding structural parameters during the actual preparation of optical cables, an oil and gas well optical cable with the following structural parameters was prepared:

[0155] This oil and gas well optical cable uses single-mode or multimode GI62.5 high-temperature-resistant optical fiber as the optical transmission medium. The optical unit's casing is a steel tube with an outer diameter of 2.8mm and a thickness of 0.20mm. A high-temperature nylon airtight shielding layer with an outer diameter of 4.6mm is used between the optical unit and the outer pressure-resistant layer. The outer pressure-resistant layer is made of 316L material with an outer diameter of 6.35mm and a wall thickness of 0.70mm. The excess fiber length of the cable is controlled to 0.4% to 0.45%. The structure of this optical cable is shown in Figure 1.

[0156] The above optical cables were actually lowered into oil and gas wells, and the optical fibers in the cables were monitored for up to one month. No stress effects were found on the optical fibers, and the test results of the cables were stable in DTS and DAS tests.

[0157] Specific embodiment 2:

[0158] In this embodiment, the oil and gas well optical cable is used in an environment where the cable needs to monitor temperature in gas injection wells. The vertical depth of the well is no more than 1100 meters, the horizontal depth is no more than 800 meters, the maximum downhole pressure is no more than 110 MPa, and the maximum downhole gas injection temperature is no more than 260°C. To meet the application and installation requirements, the optical cable outer diameter should be a standard 6.35mm.

[0159] On this basis, according to the above-mentioned design method, the structure of each layer of the oil and gas well optical cable is designed.

[0160] First, in order to ensure the installation requirements of the outer diameter of the optical cable, the design of the outer sheath is cancelled, and the outer diameter of the outer pressure-resistant layer is 6.35mm.

[0161] At the same time, the optical cable uses 316L material as the molding material of the outer pressure-resistant layer, and its physical performance indicators are as follows: thermal expansion coefficient 16.8×10 -6 / ℃ (0~200℃), elastic modulus 193GPa, density 8.0g / cm 3 , the yield strength after molding is not less than 500Mpa. To ensure that the yield collapse strength is not less than 110MPa, the wall thickness of the outer compression layer of the optical cable should be not less than 0.799mm according to formula (1).

[0162] Since the service temperature is higher than 180°C, the diffusion rate of gas in the outer pressure-resistant layer is relatively fast, and aluminum material is used as the preparation material for the airtight shielding layer.

[0163] In addition, the thermal expansion coefficient of the selected optical fiber is 5.8×10 -7 / ℃, the maximum strain value of the optical cable is calculated by formula (3) and formula (4): ε c +ε T , and the excess length of the optical fiber is not less than 0.429%.

[0164] Based on the above design process and the selection of corresponding structural parameters during the actual preparation of optical cables, an oil and gas well optical cable with the following structural parameters was prepared:

[0165] This oil and gas well optical cable utilizes two single-mode or two multimode GI62.5 optical fibers coated with a carbon layer, meaning they are carbon-coated fibers. More preferably, the fibers are also coated with a PI layer. The optical unit's casing is a steel tube with an outer diameter of 2.8mm and a thickness of 0.20mm. An airtight shielding layer made of aluminum with an outer diameter of 4.4mm is installed on the outside of the optical unit. The outer pressure-resistant layer is made of 316L material with a thickness of 0.85mm. The excess fiber length in the cable is controlled to 0.5% to 5.5%. The structure of this optical cable is shown in Figure 2.

[0166] The above-mentioned oil and gas well optical cable was laid in a gas injection well and tested at 260°C for 45 days. The well was then converted to a production well. The long-term operating temperature was distributed between 100°C and 170°C. In actual use, the optical fiber attenuation index was stable, and the additional attenuation in the 1550nm band was no more than 0.10dB / km.

[0167] The oil and gas well optical cable design method of the present invention has simple steps and is easy to operate. It can fully consider the application characteristics of oil and gas well optical cables in the oil and gas well use environment, accurately realize the design of oil and gas well optical cables, provide an accurate basis for the preparation and use of oil and gas well optical cables, ensure the accuracy and stability of oil and gas well optical cables when used in actual application scenarios, extend the service life and use accuracy of oil and gas well optical cables, and has good practical value and application prospects.

[0168] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A design method for an oil and gas well optical cable, characterized in that, It includes the following processes: (1) According to the usage requirements of the optical cable in the oil and gas well, design the structure of the optical cable for the oil and gas well. The designed optical cable sequentially includes an optical unit, an airtight shielding layer, and an outer compressive layer from the inside to the outside. Among them, the optical unit includes a sleeve and optical fibers arranged in the sleeve; (2)Select the design type of the outer compressive layer according to the environmental pressure P of the optical cable usage environment and the collapse yield strength P of the outer compressive layer; among them, P Y is Y > P, and the collapse yield strength P Y is calculated by the following formula: Where P Y is the collapse yield strength of the outer compressive layer; δ is the yield strength of the material used for the outer compressive layer; D is the outer diameter of the outer compressive layer; t is the thickness of the outer compressive layer; (3) According to the usage environment parameters of the optical cable, conduct the design and selection of the airtight shielding layer; (4) According to the usage requirements of the optical cable, select the type of optical fiber, and complete the design of the fiber slack length according to the following formula: In the formula, is the surplus length of the optical fiber; the optical fiber is calculated in a spiral curve manner in the sleeve, L is the pitch of one cycle of the spiral; D0 is the inner diameter of the sleeve; ε Cmax is the maximum stress strain of the optical cable per unit length; ε Tmax is the maximum temperature strain of the optical cable per unit length; L is the vertical downhole length of the service optical cable; is the optical cable density; g is the acceleration due to gravity, M C is the composite elastic modulus of the optical cable; C c and C f represent the coefficients of thermal expansion of the optical cable and the optical fiber, respectively; is the temperature difference between the actual operating temperature of the optical cable and the temperature at room temperature of the optical cable.

2. The design method of the oil and gas well optical cable according to claim 1, wherein In process (3), for the design and selection of the airtight shielding layer, it is judged by calculating the diffusion rate of the gas in the outer compressive layer, and the following formula is used for the calculation: In the formula, J is the diffusion rate of the gas through the material layer with a unit thickness; A is a constant related to the material; P is the ambient pressure; K is a known constant related to diffusion; T is the ambient temperature.

3. The oil and gas well optical cable design method according to claim 2, characterized in that In process (3), a fitting curve graph of the diffusion rate of different materials varying with temperature under the conventional design dimensions is pre-designed. Thus, after completing the calculation of the diffusion rate of the gas in the outer compressive layer, directly determine the material selection of the airtight shielding layer through the fitting curve graph.

4. The design method of the oil and gas well optical cable according to any one of claims 1 to 3, characterized in that In process (1), according to the actual usage requirements of the optical cable, an outer sheath with corresponding cross-sectional dimensions is also designed on the outer periphery of the outer compressive layer, and the material of the outer sheath is determined according to the actual usage environment of the optical cable.

5. The design method of the oil and gas well optical cable according to any one of claims 1 to 3, characterized in that, In process (4), considering the complexity of the application environment of the optical cable for the oil and gas well, its fiber slack length is characterized by the following formula: 。 6. An optical cable for an oil and gas well, which is designed by using the optical cable design method described in any one of claims 1 to 5, and is characterized in that the optical cable for the oil and gas well sequentially includes an optical unit, an airtight shielding layer, and an outer compressive layer from the inside to the outside; the optical unit includes a sleeve and optical fibers passing through the sleeve and having a certain fiber slack length.

7. The optical cable for oil and gas wells according to claim 6, wherein The outer compressive layer is made of 316L stainless steel, 625 alloy, or 825 alloy material; and / or the maximum operating temperature of the optical cable for the oil and gas well is lower than 180 °C, and the airtight shielding layer is made of modified polypropylene, modified nylon, fluorinated ethylene propylene copolymer FEP, ethylene-tetrafluoroethylene copolymer ETFE, or polytetrafluoroethylene PFA; or, the maximum operating temperature of the optical cable for the oil and gas well is greater than 180 °C, and the airtight shielding layer is made of aluminum material.

8. The optical cable for oil and gas wells according to any one of claims 5 to 7, characterized in that, The sleeve is made of stainless steel material; and / or, a buffer material is filled in the sleeve, and the buffer material is a temperature-resistant oil paste or a fiber material.

9. The optical cable for oil and gas wells according to any one of claims 5 to 7, characterized in that The fiber slack length of the optical fiber is between 0.2% and 0.9%; and / or, the optical fiber is a carbon-coated optical fiber.

10. The optical cable for oil and gas wells according to any one of claims 5 to 7, characterized in that, An outer sheath is also provided on the outer periphery of the outer compressive layer.

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