| Flow monitoring is of great significance in modern industrial production,especially in the field of oil and gas exploration.Through the measurement of flow parameters in the well,the flow of fluid in the well can be known,so that the production performance of the oil well can be accurately controlled and the production zone can be evaluated.And analyze the operation of the oil well downhole.Traditional flow monitoring uses mechanical and electromagnetic sensors,which are easily affected by harsh environments and liquid shocks.Therefore,in some special cases,traditional flow monitoring methods cannot meet the measurement requirements.Optical fiber sensing technology is a new external signal detection technology,which has a broad application prospect in the field of flow monitoring because of its advantages of anti-electromagnetic interference,corrosion resistance and long-term stability.Before this,the point optical fiber sensing technology is mostly used in the field of flow monitoring,which has some problems,such as complex sensor preparation,low measurement sensitivity and so on.Distributed optical fiber acoustic sensing(DAS)technology uses Rayleigh scattering interference effect in optical fiber to realize quantitative monitoring of dynamic strain.Compared with point optical fiber sensing technology,this technology has the advantages of simple structure,large dynamic range and high sensitivity.Therefore,the development of a high-performance DAS system and the application of DAS technology in the field of flow monitoring to achieve quantitative analysis and distributed detection of well flow is an important research direction of the current well flow monitoring system.Based on DAS technology,this paper studies the pipeline flow monitoring and velocity quantitative calibration.Based on the distributed optical fiber sensing theory and hydrodynamics theory,the non-invasive dynamic detection of flow is realized by optical fiber interference demodulation technology,and the quantitative measurement of flow is realized by simulation algorithm and signal processing technology.The main research contents and achievements of this paper are as follows:(1)By analyzing the impact of the impact signal generated by the liquid flow in the pipe on the pipe wall,an energy method was proposed to measure the flow rate,and the turbulent vibration of the pipe was simulated by using the k-w model in the COMSOL software.The flow velocity at the elbow of the pipeline and the pressure distribution of the fluid impact on the pipe wall are analyzed,and the flow velocity and the pressure on the pipe wall are numerically fitted,and a positive correlation is obtained,which provides a theoretical basis for the flow monitoring experiment.(2)Combining?-OTDR technology with interference detection technology,the working principle of phase detection in DAS system is explained theoretically,the relationship between pressure and phase is analyzed,and the conclusion of linear correlation is drawn.It is proved that there is a proportional relationship between velocity and phase in theory.Based on the secondary interference reduction theory of optical fiber distributed sound field,a distributed optical fiber spatial differential interference model is established,and the amplitude and phase of external sound field are obtained accurately on the basis of obtaining position and frequency information.(3)The modulation method of distributed optical fiber acoustic sensing signal is analyzed and deduced theoretically.3×3 algorithm is used as the demodulation scheme of DAS system,and Faraday rotation mirror is introduced to eliminate polarization fading in the system.Labview software is used to simulate the influence factors of 3×3demodulation algorithm.At the same time,aiming at the problem of high algorithm complexity and large amount of data caused by long distance and high positioning accuracy of sound field restoration,a more effective orthogonal optimization algorithm is proposed,and the demodulated signal collected in real time is processed by necessary filtering and moving average.compared with the traditional 3×3 algorithm,the signal-to-noise ratio of the system is improved by about 10d B.(4)The components of DAS system are introduced in detail,including optical system and signal acquisition system,and the parameters of each key device are given.Based on each device,the prototype of DAS system is assembled independently,several key performances of DAS system are tested,and the underwater acoustic signal reduction experiment is designed.The measurement of sound pressure sensitivity of151d B(rad/?Pa)@1000Hz is realized,and the minimum detectable sound pressure is6Pa.It is verified that the prototype of the DAS system can restore the external sound waves and acoustic signals with high fidelity.(5)Build a DAS system prototype and a simulated flow monitoring experiment platform to conduct actual tests on the flow at the pipe bend.Use an electromagnetic flowmeter to record the standard flow rate value.Collect the demodulated signals of the DAS system at different flow rates for spectrum analysis,and fit the two data to obtain the mathematical relationship between the real radian value measured by the system and the flow rate in a specific frequency range:y=0.18892x~2-0.34782x+0.17232.The experimental results show that the phase is proportional to the square of the flow rate,the fitting R~2=0.998,and the minimum detectable flow rate is 0.73 m~3/h. |
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