Research On Electromagnetic Low-frequency Flexural Monopole Acoustic Logging Transmitting Transducer

Formation permeability is of great significance for the development and evaluation of oil and gas reservoirs.Some experts and scholars have pointed out that the attenuation of Stoneley wave amplitude and the shift of center frequency are related to formation permeability,and when the frequency is low,the correlation is large.Therefore,obtaining pure low-frequency Stoneley waves is significant for the evaluation of formation permeability.At present,the transmitting resonant frequency of the monopole piezoelectric ceramic transmitting transducer commonly used in acoustic logging is more than 10 k Hz.At this transmitting frequency,all kinds of mode waves in the receiving full wave are mixed,and it is difficult to separate the pure Stoneley wave.Therefore,because of the demand for low-frequency transmitting transducers in permeability evaluation,this study developed a low-frequency acoustic transmitting transducer through the finite element simulation software COMSOL Multiphysics combined with the actual wellbore environment.The transducer is driven by electromagnetic force,and designed by using the working characteristics of low frequency,small volume and high power of the flextensional transducer.The main contents of this study include the simulation analysis of the magnetic circuit structure,vibration shell structure,and the design of the transducer excitation circuit.The working characteristics of the transducer are verified based on simulation calculation and pool experiment.In the magnetic circuit simulation study,several magnetic circuits that meet the downhole application are proposed,and the influence of the structure and size of the permanent magnet,soft iron and air gap on the magnetic flux density in the air gap is analyzed respectively.Based on the analysis,this study chooses the internal magnetic circuit as the magnetic circuit of the transducer.The variation law of the air gap flux density of the internal magnetic circuit is as follows :(1)The increase in the width of the permanent magnet has a great influence on the improvement of the air gap flux density;(2)The increase in the height of permanent magnet can improve the air gap flux density,but when the height reach a certain extent,the flux density will no longer increase.(3)When the soft iron appears magnetic saturation,increasing the volume of the soft iron can improve the air gap flux density;(4)With the air gap width increasing,the air gap flux density decreases.When the air gap height increases,the air gap flux density decreases,but the height of the air gap uniform zone increases.(5)The magnetic flux density distribution in the center of the air gap is the most uniform;(6)The change of magnetization direction of the permanent magnet does not affect the air gap flux density.In the simulation study of the vibrating shell,the concept of optimal vibration mode of shell vibration is proposed.In the multi-order vibration mode,the shell radiation energy is the largest under the optimal vibration mode,and the energy circumferential distribution is the most uniform.The vibrating plate is the most important structure that affects the vibration characteristics of the shell.In this study,by changing the structural characteristics of the vibrating plate,the shell is simulated and the law of shell vibration under the optimal vibration mode is summarized :(1)When the thickness of the vibrating plate increases,the resonant frequency of the shell increases,and the acoustic radiated energy also increases.(2)When the height of the vibrating plate increases,the resonant frequency of the shell decreases,and the acoustic radiation energy also decreases.(3)When the curvature of the vibrating plate decreases,the resonant frequency and radiation energy of the shell increase first and then tend to be stable.In the design of the excitation circuit,FPGA is employed as the main controller,and the power supply circuit,peripheral circuit,isolation drive circuit,and full bridge circuit are designed.Based on the principle of SPWM,a sinusoidal excitation current with a peak value of 40 A is emitted to the load coil through the hardware circuit,and the excitation efficiency is improved by impedance matching.Through the simulation analysis results,the transducer and the matching excitation circuit are designed and processed.The experimental results show that the resonant center frequency of the transmitting transducer is 900 Hz,which meets the requirements of low frequency acoustic transmitting transducer.