Downhole fiber grating temperature and pressure sensor, linear array temperature and pressure sensor system

Abstract

The application discloses a downhole fiber grating temperature and pressure sensor, which comprises a sensing cavity, a pressure sensing diaphragm, and a pressure sensing fiber grating and a temperature sensing fiber grating connected in series; the sensing cavity is provided with a rigid cavity, and the rigid cavity is provided with an outwardly extending temperature sensing fixing plate; the rigid cavity of the sensing cavity is in airtight cooperation with the pressure sensing diaphragm to form a cavity in communication with a target fluid; the pressure sensing diaphragm is strained by the fluid pressure and the fluid temperature in the cavity; the temperature sensing fixing plate is strained by the fluid temperature; the pressure sensing fiber grating is arranged on the central surface of the pressure sensing diaphragm, and the temperature sensing fiber grating is arranged on the surface of the temperature sensing fixing plate. The application makes the pressure measurement almost not affected by the temperature, reduces the high-temperature drift, and improves the accuracy of the pressure measurement in the downhole high-temperature environment. The application forms a linear array temperature and pressure sensing system through the mutual connection and isolation sealing of multiple sensors, so that the downhole distributed point measurement is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical fiber sensing, and more particularly relates to a downhole fiber grating temperature and pressure sensor and a linear array temperature and pressure sensing system. BACKGROUND

[0002] Real-time and online monitoring of multiple parameters such as pressure and temperature of oil wells can provide real-time information about the actual situation in the well without stopping production, which is of great significance for optimizing oil production, improving oil and gas recovery and production. Traditional electronic sensors and logging methods have great limitations, and even cannot work normally in harsh environments such as high temperature and high pressure downhole, and have low measurement accuracy.

[0003] Fiber grating is a diffraction grating formed by axial periodic modulation of refractive index, and is a passive filter device. It has the advantages of small size, small fusion loss, full compatibility with optical fiber, ability to be embedded in intelligent materials, high sensitivity, resistance to electromagnetic interference, easy to realize long-distance transmission and distributed measurement, and easy to work in harsh environments, and has been widely used in the field of sensing.

[0004] In recent years, existing fiber grating sensors have also been used for downhole monitoring, but the current sensors are large in size and cannot be used in small size casings. Moreover, either the range is low or the sensitivity is low, and it is difficult to achieve a large range with high sensitivity. For example, the downhole fiber grating sensor provided in Chinese patent CN115452021A uses two independent fiber gratings to measure hydraulic pressure and environmental temperature. However, the fiber grating sensor has high pressure grating sensitivity in high temperature and high pressure environment, is sensitive to temperature, is prone to drift, and cannot accurately monitor pressure and temperature. Especially in the case of severe temperature changes downhole, such as deep oil wells, the temperature rises sharply during the operation of the drill bit, and the temperature drops sharply when the drill bit stops working. With the working law of the drill bit, the fiber grating sensor is repeatedly subjected to the change of sharp rise and sharp drop in environmental temperature, is affected by thermal shock, and results in inaccurate test results. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a downhole fiber grating temperature and pressure sensor and a linear array temperature and pressure sensing system, which aims to simultaneously transmit pressure and temperature by using a sensing cavity, and avoid crosstalk caused by simultaneous changes in pressure and temperature, increase the pressure sensitivity by using a pressure sensing diaphragm, increase the sensitivity, accurately sense the thermal stress of a temperature sensing fixed plate, accurately compensate for temperature, effectively reduce the measurement error caused by the sensitivity of the pressure sensing fiber grating to temperature changes, and improve the reliability, thereby solving the technical problem that the existing downhole pressure sensor cannot accurately compensate for temperature, causing the downhole fiber grating sensor to easily drift and being unable to accurately monitor pressure and temperature.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a downhole fiber grating temperature and pressure sensor is provided, comprising a sensing cavity, a pressure sensing diaphragm, and a pressure sensing fiber grating and a temperature sensing fiber grating connected in series.

[0007] The sensing cavity has a rigid cavity provided with an outwardly extending temperature sensing fixed plate; the rigid cavity of the sensing cavity is in airtight cooperation with the pressure sensing diaphragm to form a cavity in communication with a target fluid.

[0008] The pressure sensing diaphragm is strained by the fluid pressure and the fluid temperature in the cavity;

[0009] The temperature sensing fixed plate is strained by the fluid temperature;

[0010] The pressure sensing fiber grating is arranged on the central surface of the pressure sensing diaphragm, and the temperature sensing fiber grating is arranged on the surface of the temperature sensing fixed plate.

[0011] Preferably, the downhole fiber grating temperature and pressure sensor, the pressure sensing diaphragm is orthogonal to the direction of the temperature sensing fixed plate strained by the fluid temperature.

[0012] Preferably, the downhole fiber grating temperature and pressure sensor, the pressure sensing fiber grating and the temperature sensing fiber grating are in the same direction.

[0013] Preferably, the downhole fiber grating temperature and pressure sensor, the distance between the pressure sensing fiber grating and the temperature sensing fiber grating is greater than the distance between the center of the pressure sensing diaphragm and the temperature sensing fixed plate where the temperature sensing fiber grating is arranged.

[0014] Preferably, the downhole fiber grating temperature and pressure sensor, the rigid cavity of the sensing cavity is a good thermal conductor, preferably having a thermal conductivity greater than 10 W / (m·℃).

[0015] Preferably, the downhole fiber grating temperature and pressure sensor has a rigid cavity of the sensing cavity, and the thermal expansion coefficient of the rigid cavity is equivalent to the thermal expansion coefficient of the sensing fiber grating, and the variation of the thermal expansion coefficient per degree Celsius is less than 0.01x10 -6 ℃, and the elastic modulus is preferably greater than 195GPa.

[0016] Preferably, the downhole fiber grating temperature and pressure sensor has a rigid cavity of the sensing cavity, and the thermal expansion coefficient of the rigid cavity is equivalent to the thermal expansion coefficient of the sensing fiber grating, and the variation of the thermal expansion coefficient per degree Celsius is less than 0.01x10

[0017] Preferably, the downhole fiber grating temperature and pressure sensor further comprises a base, and the base has an L-shaped fluid channel, one end of which is in communication with the cavity, and the other end is open to the side of the sensor.

[0018] Preferably, the downhole fiber grating temperature and pressure sensor comprises a straight bar-shaped shell, and the base is radially embedded in the shell, and the base has a through hole through which the pressure sensing fiber grating and the temperature sensing fiber grating are connected in series, and the two ends of the through hole are respectively led out from the lead-out holes of the corresponding ends of the shell.

[0019] According to another aspect of the present application, a downhole linear array temperature and pressure sensing system is provided, which comprises a plurality of downhole fiber grating temperature and pressure sensors provided by the present application connected in series.

[0020] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0021] 1. The present application effectively increases the sensitivity of the pressure grating by designing the structure of the sensing cavity, and measures the temperature through the temperature sensing fiber grating pasted on the temperature sensing fixed plate, thereby avoiding the inaccuracy of temperature measurement caused by vibration of the fiber, inconsistency between the measurement target and the pressure sensing fiber grating measurement target, and reducing the sensitivity of the pressure sensing fiber grating to the change of the external temperature, so that the pressure measurement is almost not affected by the temperature, the high temperature drift is reduced, and the precision of the pressure measurement in the downhole high temperature environment is improved.

[0022] 2. The present application realizes the downhole distributed point measurement by mutual series connection and isolation sealing of multiple sensors. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a sensing cavity structure diagram of the downhole fiber grating temperature and pressure sensor provided by the present application;

[0024] Figure 2 is a structure diagram of the downhole fiber grating temperature and pressure sensor provided by the present application;

[0025] Figure 3is the finite element simulation result diagram of the pressure sensing diaphragm of the sensing cavity of the downhole fiber grating temperature and pressure sensor provided by the embodiment of the present application;

[0026] Figure 4 is the structural diagram of the downhole linear array temperature and pressure sensing system provided by the present application.

[0027] In all the drawings, the same reference signs are used to represent the same elements or structures, in which: 1 is a base, 2 is a sensing cavity, 201 is a pressure sensing diaphragm, 202 is a temperature sensing fixed plate, 3 is a pressure sensing fiber grating, 4 is a temperature sensing fiber grating, 5 is an optical fiber, 6 is one end of a shell, 7 is the other end of the shell, 8 is the outside of a sealing structure, 9 is the inside of the sealing structure, 10 is a steel pipe, 11 is a groove at both ends of the base, and 12 is a groove at the opening of the base. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0029] The downhole fiber grating temperature and pressure sensor provided by the present application comprises a sensing cavity, a pressure sensing diaphragm, and a pressure sensing fiber grating and a temperature sensing fiber grating connected in series, a base, and a shell.

[0030] The sensing cavity has a rigid cavity provided with an outwardly extending temperature sensing fixed plate; the rigid cavity of the sensing cavity is in airtight cooperation with the pressure sensing diaphragm to form a cavity in communication with a target fluid; the rigid cavity of the sensing cavity is a good thermal conductor, preferably having a thermal conductivity greater than 10 W / (m·℃), and good thermal conductivity can transmit the temperature of the fluid, so that the temperature of the temperature sensing fixed plate can be used to represent the temperature of the fluid, thereby effectively compensating for the temperature, and the outwardly extending temperature sensing fixed plate stably and accurately associates the thermal stress of the temperature sensing fixed plate with the fluid temperature, thereby ensuring the accuracy of the measurement. In a preferred embodiment, the thermal expansion coefficient of the rigid cavity of the sensing cavity is comparable to the thermal expansion coefficient of the optical fiber used by the temperature sensing fiber grating, i.e., the values of the two are in the same order of magnitude, and the difference is less than 5 times; at the same time, the thermal expansion coefficient changes little with the increase of temperature, and the change per degree Celsius is less than 0.01x10 -6 ℃, and the elastic modulus is preferably greater than 195 GPa. Too small elastic modulus will result in that the pressure sensing diaphragm is not sensitive to external pressure, and there will be a loss in force transmission, resulting in that the strain sensed by the pressure sensing fiber grating is too small, thereby limiting the sensitivity and accuracy of the sensor. The rigid cavity of the sensing cavity is made of a fatigue-resistant material with a theoretical fatigue service life of more than 10 years, an oxidation-resistant and corrosion-resistant material with a corrosion-resistant depth of less than 0.005 mm / year.

[0031] The pressure-sensitive diaphragm is strained by fluid pressure in the cavity and fluid temperature; the cavity in communication with the target fluid is a pressure sensing cavity, and the pressure load causes the pressure-sensitive diaphragm to be positively strained,

[0032] The temperature-sensitive fixed plate is strained by fluid temperature;

[0033] The pressure-sensitive diaphragm is strained by fluid pressure in the direction orthogonal to the direction in which the temperature-sensitive fixed plate is strained by fluid temperature, so that the strain sensed by the temperature-sensitive fiber grating is caused by the temperature change of the temperature-sensitive fixed plate and is not affected by fluid pressure.

[0034] The pressure-sensitive fiber grating and the temperature-sensitive fiber grating are in the same direction. The fiber gratings in series are generally processed from the same fiber, and the thermal expansion and the thermal light coefficient of the fiber gratings are consistent. When the thermal expansion and the thermal light coefficient of the two fiber gratings are the same and the gluing method is the same, the wavelength interval change caused by the ambient temperature can be considered to be small and negligible, and the temperature compensation is effectively performed.

[0035] The pressure-sensitive fiber grating is arranged on the central surface of the pressure-sensitive diaphragm, and the temperature-sensitive fiber grating is arranged on the surface of the temperature-sensitive fixed plate. Compared with the existing fiber grating arranged in suspension, the fiber grating fixed on the surface of the temperature-sensitive fixed plate senses the thermal stress expansion of the temperature-sensitive fixed plate caused by temperature, is stable and accurate, does not cause temperature drift due to sharp temperature change, avoids the measurement error caused by the influence of acoustic vibration on the fiber grating, ensures that the actual strain of the temperature-sensitive fiber grating is affected by other factors and can be ignored, and the signal-to-noise ratio is effectively improved.

[0036] The pressure-sensitive fiber grating is close to the surface of the pressure-sensitive diaphragm and is used for sensing the positive strain of the diaphragm, and the temperature-sensitive fiber grating is close to the surface of the temperature-sensitive fixed plate and is used for sensing the temperature signal around. Therefore, the pressure chamber and the temperature-sensitive fixed plate are used to improve the sensitivity and precision of pressure measurement in the downhole high-temperature and high-pressure environment.

[0037] The distance between the pressure-sensitive fiber grating and the temperature-sensitive fiber grating is greater than the distance between the center of the pressure-sensitive diaphragm and the temperature-sensitive fixed plate where the temperature-sensitive fiber grating is arranged, so as to prevent the pitch change of the pressure-sensitive fiber grating from stretching the temperature-sensitive fiber grating.

[0038] The base has an L-shaped fluid channel, one end of which is in communication with the cavity, and the other end is open to the side of the sensor. The shell is a straight strip type, the base is radially embedded in the shell, the base has a through hole, and the pressure-sensitive fiber grating and the temperature-sensitive fiber grating in series pass through the through hole, and the two ends are respectively drawn out from the lead-out holes of the corresponding ends of the shell.

[0039] The well down linear array temperature and pressure sensing system comprises a plurality of series connection of the well down fiber grating temperature and pressure sensors.

[0040] The following is an example:

[0041] As shown in Figure 1 and Figure 2 The well down fiber grating temperature and pressure sensor comprises a sensing cavity 2, a pressure sensing diaphragm 201, and series connection of a pressure sensing fiber grating 3 and a temperature sensing fiber grating 4, a base 1, and a shell 6 and 7.

[0042] The sensing cavity 2 is a cylindrical tube at one end, which passes through the base 1 and is fixed with the base 1, and the cavity formed in the inside thereof communicates with the target fluid on one side of the pressure sensing diaphragm 201, and the pressure sensing diaphragm 201 is used to convert the pressure in the cavity into the strain of the diaphragm. Preferably, the sensing cavity can adopt a material of nickel alloy 718. The sensing cavity 2 has a rigid cavity, and the rigid cavity is provided with an outwardly extending temperature sensing fixed plate 202.

[0043] The pressure sensing fiber grating 3 is arranged on the surface of the pressure sensing diaphragm 201 for sensing pressure, and the temperature sensing fiber grating 4 is arranged on the surface of the temperature sensing fixed plate 202 at the center for sensing temperature and having a temperature compensation function.

[0044] The pressure sensing fiber grating 3 and the temperature sensing fiber grating 4 are written by femtosecond laser transparent coating, and the outer coating material of the optical fiber 5 is polyimide.

[0045] The pressure sensing fiber grating 3 and the temperature sensing fiber grating 4 are series connected and led out through the through hole on the base 1. In order to prevent the pressure sensing fiber grating 3 from stretching the temperature sensing fiber grating 4 due to the change of the degree of perturbation, the distance between the pressure sensing fiber grating 3 and the temperature sensing fiber grating 4 is greater than the center distance between the pressure sensing diaphragm 201 and the temperature sensing fixed plate 202.

[0046] According to the knowledge of mechanics, the wavelength shift caused by the change of temperature and stress can be represented as B

[0047]

[0048] Simplified as

[0049] Δλ B =(K T ΔT+K ε ε)λ B

[0050] ​Wherein, ε is stress, P ij is photo pressure coefficient, v is Poisson's ratio, α is thermal expansion coefficient, ΔT is temperature change; K T is temperature sensitivity coefficient, K ε is pressure sensitivity coefficient.

[0051] According to stress analysis of the pressure sensing fiber grating and temperature sensing fiber grating,

[0052]

[0053] Therefore, from the temperature compensation compensation scheme,

[0054] Δλ B1 -Δλ B2 =(K ε ε)λ B +(K T1 λ B1 -K T2 λ B2 )ΔT

[0055] When the thermal expansion and thermal light coefficient of the two fiber gratings are same and the gluing mode is same, there is K T1 =K T2 =K T

[0056] K T1 ΔTλ B1 -K T2 ΔTλ B2 ≈0

[0057] Therefore, the wavelength interval change caused by ambient temperature is small and can be ignored.

[0058] The pressure sensing diaphragm 201 is circular, the thickness is 1mm, and is integrally processed with the sensing cavity 2, and the stress concentration problem at the connection of the pressure sensing diaphragm is eliminated by rounding the corner, the sensing cavity 2 is fixed with the base 1 through threads and is sealed by laser welding.

[0059] The strain distribution of the sensing cavity under the action of 60MPa pressure is calculated by using the finite element method, as shown in Figure 3 From the figure, the maximum strain of the sensing cavity under the action of 60MPa pressure is 3.558x10-3, that is, the micro-strain is 3558με. The wavelength change corresponding to 1με of the grating is about 1.2pm, and the sensitivity of the sensor is 49.4pm / Mpa.

[0060] The present application can realize flexible adjustment of pressure sensitivity and range by selecting different thickness diaphragms to change the range or pressure sensitivity, and the range and sensitivity can be determined by finite element simulation, as shown in Figure 3The application greatly reduces the temperature sensitivity of the pressure sensing fiber grating and improves the precision of the pressure measurement in the high temperature and high pressure environment in the well by fixing the grating on the fixed plate.

[0061] In conclusion, the application effectively improves the range and sensitivity of the pressure measurement by setting the pressure cavity, and greatly reduces the temperature sensitivity of the pressure sensing fiber grating by fixing the pressure sensing fiber grating on the fixed plate, so that the temperature has little effect on the high temperature and high pressure measurement, and the precision of the measurement in the well is improved; the application realizes the point type measurement in the well by the protection device and the multi-stage sealing structure 8 and 9.

[0062] The base 1 has an L-shaped fluid channel, one end of which is communicated with the cavity, and the other end is opened on the side of the sensor. The shell 6 and 7 are cylindrical, the base is radially embedded in the shell, and the fluorine-containing O-shaped ring is placed in the groove 11 at both ends of the base for sealing. The base has a through hole, and the pressure sensing fiber grating and the temperature sensing fiber grating are passed through the through hole in series, and the two ends are respectively passed out from the lead-out holes of the corresponding ends of the shell and sealed.

[0063] The optical fiber 5 is protected by a high-temperature-resistant polytetrafluoroethylene capillary tube, and when it passes through the lead-out holes of the base 1 and the protective shell 6 and 7, it is sealed by coating high-temperature-resistant glue at the hole, preventing external liquid from entering the sensitive element area of the sensor.

[0064] The downhole linear array temperature and pressure sensing system provided by the embodiment comprises a plurality of series-connected downhole fiber grating temperature and pressure sensors. The series-connected downhole fiber grating temperature and pressure sensors are connected with a fiber temperature / pressure demodulation system, and different positions of the pressure sensing fiber grating or the temperature sensing fiber grating are represented by different wavelengths in the well. The fiber temperature / pressure demodulation system reads the pressure or temperature data measured by the fiber grating through the reflected spectrum.

[0065] The series connection of the sensors and the mutual isolation and sealing of the multi-stage sealing structure 8 and 9 are realized by the double-end lead-out fiber fusion, so as to realize the point type measurement. The steel pipe 10 is used to protect the optical fiber between the sensors, and the material of the steel pipe 10 can be nickel alloy 718 or 316 stainless steel. Specifically, the two adjacent downhole fiber grating temperature and pressure sensors are sealed and connected through the steel pipe 10, or are sealed and connected through the optical fiber fusion protection device, and then are sealed and connected with the optical cable special for oil wells, so as to realize the point type series connection measurement of the sensors.

[0066] The sensor is connected with the external measured object by the flange type straight insertion groove method, and the double-layer high-temperature-resistant sealing ring is placed in the groove 12 at the opening of the base. Preferably, the sealing ring can be a fluorine rubber ring or a nickel-coated metal sealing ring.

[0067] As shown in FIG. 1, the downhole fiber grating temperature and pressure sensor comprises a base 1, a shell 6 and 7, and an optical fiber 5. Figure 4As shown, the downhole fiber grating temperature and pressure sensor of the embodiment is fixed outside the oil pipe, a sleeve is arranged outside the oil pipe, and the plurality of downhole fiber grating temperature and pressure sensors are sealingly connected through the optical cable connecting device, and the uppermost sensor is connected with the optical fiber temperature / pressure demodulation system through the optical cable, wherein, in order to protect and fix the optical cable, the optical cable fixing clamp is arranged between the oil pipe and the optical cable. In actual use, there is a small hole in the wall of the oil pipe which is in communication with the downhole fiber grating temperature and pressure sensor, and the liquid / gas in the measured layer enters the pressure cavity through the small hole, the sensor senses the temperature and pressure change of the measured oil layer, and then transmits the measured data to the optical fiber temperature / pressure demodulation system through the optical cable.

[0068] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A downhole fiber Bragg grating temperature and pressure sensor, characterized in that, It includes a sensing cavity, a pressure-sensitive diaphragm, and a pressure-sensitive fiber grating and a temperature-sensitive fiber grating connected in series; The sensing cavity has a rigid cavity, and the rigid cavity is provided with an extended temperature-sensing fixing plate; the rigid cavity of the sensing cavity is a good conductor of heat; the rigid cavity of the sensing cavity and the pressure-sensing diaphragm are sealed together to form a cavity communicating with the target fluid. The pressure-sensitive diaphragm is subjected to strain by the fluid pressure and temperature inside the cavity. The temperature-sensing fixing plate is subjected to strain due to the fluid temperature. The pressure-sensitive fiber grating is disposed on the central surface of the pressure-sensitive diaphragm, and the temperature-sensitive fiber grating is disposed on the surface of the temperature-sensitive fixing plate; The direction of the pressure-sensitive diaphragm under fluid pressure is orthogonal to the direction of the strain generated by the fluid on the temperature-sensitive fixing plate due to the influence of fluid temperature.

2. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, The pressure-sensitive fiber grating and the temperature-sensitive fiber grating are oriented in the same direction.

3. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, The distance between the pressure-sensitive fiber grating and the temperature-sensitive fiber grating is greater than the distance between the center of the pressure-sensitive diaphragm and the location where the temperature-sensitive fiber grating is set on the temperature-sensitive fixing plate.

4. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, The rigid cavity has a thermal conductivity greater than 10 W / (m·℃).

5. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, The coefficient of thermal expansion of the rigid cavity of the sensing cavity is comparable to that of the optical fiber used in the temperature-sensing fiber grating, and the change in the coefficient of thermal expansion per degree Celsius is less than 0.01 x 10⁻⁶. -6 ℃.

6. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, The rigid cavity of the sensing cavity has an elastic modulus greater than 195 GPa.

7. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, The rigid cavity of the sensing cavity is made of corrosion-resistant material with a corrosion resistance depth of less than 0.005 mm / year.

8. The downhole fiber optic grating temperature and pressure sensor as described in claim 1, characterized in that, It also includes a base; the base has an L-shaped fluid channel, one end of which is connected to the cavity and the other end of which opens to the side of the sensor.

9. The downhole fiber optic grating temperature and pressure sensor as described in claim 8, characterized in that, The device includes a straight-shaped outer shell, with a base radially embedded in the outer shell. The base has a through hole through which a pressure-sensitive fiber optic grating and a temperature-sensitive fiber optic grating connected in series pass, with their two ends exiting from corresponding outlet holes at the outer shell.

10. A downhole linear array temperature and pressure sensing system, characterized in that, It includes multiple downhole fiber Bragg grating temperature and pressure sensors connected in series as described in any one of claims 1 to 9.

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