Design of Well Temperature Measurement System Based on Distributed Optical Fiber Temperature Sensor

In the process of oil exploitation, the measurement of downhole temperature is an indispensable measurement parameter. Accurate downhole temperature measurement plays an important role in geological data interpretation and oil well monitoring. Especially in the heavy oil thermal recovery process, it is necessary to monitor the changes of the downhole temperature field. In the traditional process of measuring well temperature, infrared thermometers, infrared thermal imagers, temperature sensor arrays, etc. are used, but the harsh environment in the well will have a great impact on the testing instruments, which is easy to cause test errors, and for the temperature field measurement has many shortcomings.The modern distributed fiber optic temperature sensor has many measurement points, is lightweight and can withstand the downhole harshness.

introduction

In the process of oil exploitation, the measurement of downhole temperature is an indispensable measurement parameter. Accurate downhole temperature measurement plays an important role in geological data interpretation and oil well monitoring. Especially in the heavy oil thermal recovery process, it is necessary to monitor the changes of the downhole temperature field. In the traditional process of measuring well temperature, infrared thermometers, infrared thermal imagers, temperature sensor arrays, etc. are used, but the harsh environment in the well will have a great impact on the testing instruments, which is easy to cause test errors, and for the temperature field measurement has many shortcomings. The modern distributed optical fiber temperature sensor has the advantages of many measurement points, light weight and can withstand the harsh environment of the well, and can obtain the temperature field information of the entire optical fiber distribution area. At present, the distributed optical fiber temperature sensor has realized the measurement of parameters such as downhole temperature field, and the measurement of temperature field in the process of thermal recovery of heavy oil has broad application prospects.

1 Principle of distributed optical fiber temperature sensor

1.1 The principle of distributed measurement

Distributed temperature sensors enable distributed measurements with the help of Optical Time Domain Backscattering (ODTR) technology. Scattering occurs when a light pulse is injected into the fiber from point O and propagates through the fiber. After a period of time, the backscattered light returns to point O. Assuming that the time of light pulse injection is the time origin, the relationship between the distance L between the scattering point in the fiber and the point O and the return time t of the scattered light at this point is:

 Design of Well Temperature Measurement System Based on Distributed Optical Fiber Temperature Sensor

In the formula: c is the speed of light in vacuum; n is the refractive index of the optical fiber; t is the time it takes for the signal to go from emission to reception.

It can be seen from equation (1) that the echoes at different times correspond to the scattering generated by different distance points, and the location of the temperature measurement point is realized according to the echoes at different times.

1.2 The principle of distributed temperature measurement

The perception and measurement of temperature is based on the principle of back Raman scattering from optical fibers. When the wavelength is λ0. When the laser is injected into the fiber, it continuously generates backscattered light while propagating forward in the fiber, except for the same wavelength λ0 as the incident light. In addition to the central spectral line of , there are also two spectral lines of λ0-Δλ and λ0+Δλ on both sides. The central spectral line is the Rayleigh scattering spectral line, the spectral line with the wavelength of λs=λ0+Δλ on the low-frequency side is called Stokes line; the spectral line with the wavelength of λa=λ0-Δλ on the high-frequency side is called the Stokes line. Anti-Stokes line. It can be found by experiments that the anti-Stokes scattered light is sensitive to temperature and its intensity is modulated by temperature, while the Stokes scattered light is basically independent of temperature. The ratio of the two light intensities is only related to the temperature of the scattered light, namely:

Design of Well Temperature Measurement System Based on Distributed Optical Fiber Temperature Sensor

where h is Planck’s constant; c is the speed of light in vacuum; k is Boltzmann’s constant; T is temperature; Δγ is the offset wave number.

Therefore, taking the anti-Stokes light as the signal channel and the Stokes light as the reference channel, and detecting the ratio of the two light intensities, the temperature information of the scattering region can be demodulated. By combining the measurement of the back Raman scattering signal with the OT-DR technology, a distributed optical fiber temperature measurement system based on the back Raman scattering can be realized.

1.3 Implementation of temperature measurement algorithm

For formula (2), taking the logarithmic function on both sides at the same time, we have:

Design of Well Temperature Measurement System Based on Distributed Optical Fiber Temperature Sensor

By transforming, we can get:

Design of Well Temperature Measurement System Based on Distributed Optical Fiber Temperature Sensor

In the actual test process, the optical fiber sensor is placed in a thermostat with a temperature of T0 for calibration. Through the calibration, we can know:

It can be known from the above formula that after the calibration, the temperature value of each point of the distributed optical fiber can be deduced by measuring the ratio R(T) of the light intensity.[page]

2 Design of temperature field measurement system

2.1 Hardware Design of Measurement System

The distributed optical fiber temperature measurement system is mainly composed of pulsed laser, optical fiber amplifier, optical fiber directional coupler, filter, photoelectric detector, amplifier, data acquisition and processing circuit, information processing Display (computer), sensitive optical fiber, incubator, etc. The block diagram of the system structure is shown in Figure 1.

Design of Well Temperature Measurement System Based on Distributed Optical Fiber Temperature Sensor

The functions of each part are as follows: the main function of the pulse laser is to provide optical pulses for the system; the fiber amplifier amplifies the weak light signal to improve the signal strength; the fiber coupler couples the optical signal into the fiber according to the design requirements, and returns the scattered light The signal is coupled into the optical processing path in a predetermined ratio; the filter separates the Stokes light and the anti-Stokes light, and filters out the Rayleigh backscattered light; the photodetector converts the received optical signal into The corresponding electrical signal provides input for the information processing circuit; the amplifier amplifies the weak electrical signal for A/D conversion; the data acquisition and processing circuit mainly completes the A/D conversion of the optical signal and the related processing of the signal; the computer will The data is analyzed and displayed, and communicated with the data acquisition circuit. Among them, key components such as fiber couplers, filters and photodetectors are placed in a constant temperature box (200°C constant temperature).

The working process of the temperature field measurement system is as follows: under the control of the computer and the data acquisition circuit, the pulsed laser emits an optical pulse signal, and the optical pulse signal is amplified by the fiber amplifier and then coupled to the sensing fiber through the directional coupler, and the sensing fiber in the heavy oil temperature field. For the light pulse propagating in the sensing fiber, the backscattered part of the scattered light (Stokes and anti-Stokes in Raman scattered light) induced at each point during the propagation process is transmitted through the fiber again. The channel enters the directional coupler and is coupled to the receive channel. After optical filtering, the Rayleigh backscattered light with relatively strong energy is filtered out, and the anti-Stokes light and Stokes light carrying temperature information are separated, and sent to the photodetector for photoelectric signal conversion. After amplification, it is sent to the data acquisition and processing circuit for A/D conversion and signal related processing, and the processed information is sent to the computer for analysis, processing and display.

2.2 Software design of measurement system

The software of the system mainly includes data acquisition and processing software and computer software. Among them, the software of data acquisition and processing terminal is mainly responsible for the acquisition of signals. At present, the technology in this area is very mature, and will not be repeated here. The computer-side software of this system is developed by LabVIEW of NI Company, which mainly realizes the functions of temperature field test system control, system setting and temperature field distribution graph display. The software interface is shown in Figure 2.

3 Experimental design of the temperature field measurement system

The measurement system is constructed according to the structural diagram 1. The pulse light source is a domestic large-cavity high-power semiconductor laser with a peak power of 110 W, a pulse width of 50 ns, a center wavelength of 1 550 nm, and an offset of less than 2 nm. The splitting ratio is 50:38:12, the purpose is to reduce the optical loss; the filter adopts a coated optical filter, and multiple pieces are stacked to reduce the interference of the pump light, and the wavelength offset of the filter is less than 5 nm; the photodetector The required bandwidth is 80 MHz, and C30902E/C30724E are selected; the amplifier must be a high-gain, broadband, low-noise amplifier, and the rail-to-rail operational amplifier AD8552 is selected; the optical fiber sensor is composed of a linear distributed optical fiber temperature sensor and a distributed optical fiber point winding temperature sensor ; The A/D converter of the data acquisition circuit is realized by a high-speed 8-bit A/D converter AD9048, and the sampling rate is 35MSPS.

After calibrating the temperature field measurement system designed by ourselves, when the temperature field of heavy oil thermal recovery is carried out, the optical fiber is distributed in the heat insulation pipe, and the temperature field measurement with a length of about 1 400 m can be realized. After verification, it is found that the design method of the temperature field measurement system is unique, the spatial resolution of the system reaches 1 m, and the temperature measurement reaches 0.5 °C, which can meet the requirements of temperature field measurement during thermal recovery of heavy oil.

4 Conclusion

As a new technology in the field of temperature measurement that is currently emerging, the distributed optical fiber temperature measurement system has obvious advantages. It is not only applied in high-end fields, but also rapidly promoted in traditional industrial fields. The system is especially suitable for the continuous and real-time temperature field measurement requirements of large area and multiple points, and can completely replace most of the traditional temperature measurement systems by virtue of its unique material and morphological advantages. At present, the use and research and development of distributed optical fiber temperature measurement systems in my country’s oil industry are in their infancy. more widely used in other fields.

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