Study On The Detection Characteristics Of The Acoustoelectric Logging Detector

The basis of acoustoelectric logging is the double layer effect,which is directly related to parameters such as porosity and permeability of the formation;therefore,acoustoelectric logging has potential application value in permeability evaluation and fracture identification.The acoustoelectric logging detector studied in this thesis is a composite detector that can detect the acoustic and electrical charcateristics at the same time,it can measure the formation parameters that can not be obtained by single logging method in the past.This paper explores the instrument performance and detection effect of the new generation of acoustoelectric logging detector,studies the waveform characteristics of acoustoelectric logging through numerical simulation,and carries out a series of studies on other key technologies helpful to improve the detection performance.By building experimental test platform,the acoustic system radiation and reception characteristics of the acoustoelectric detector,and the performance of the phased linear array radiator are studied.The results show that the radiated acoustic pressure and the emission voltage response produced by the detector emission acoustic system can meet the logging requirements;the main lobe angle of the radiation beam of phased linear array can also deflect with the increase of the excitation time of array element;the receiving acoustic system has good consistency.Based on the finite element method,the numerical analysis model of the detector electrode system is established.According to the simulated electrode system coefficient,the measured data of instrument electric-emission and electric-reception are selected,and the formation apparent resistivity is calculated,which verified the accuracy of the finite element method.Aiming at the waveform response characteristics of the acoustoelectric effect of the fluid-solid interface,a set of acoustoelectric experimental device is built to study the acoustoelectric interface response characteristics of the single-interface sandstone model.And the information of the signal waveform,amplitude and travel time are got.In addition,the measurement method of analyzing the acoustoelectric effect in the experiment gives the key point of measuring weak signal,which can effectively improve the repeatability of the acoustoelectric measurement of the instrument.By studying the actual logging data collected by the detector,it is found that the interface mode acoustoelectric conversion wave generated by the pulse acoustic wave in the borehole wall propagates to each receiving electrode at the speed of electromagnetic wave,the arrival time of the signal is almost only related to the distance from the radiator to the borehole wall,and is not affected by the source distance.The characteristics of the actually measured interface acoustoelectric conversion signal can be consistent with the theory and experiment.Based on the theory of acoustoelectric coupling,the acoustic and acoustoelectric field waveforms generated by the monopole acoustic source in the borehole are calculated using the instrument parameters.The effects of porosity,salinity,permeability on the received signal are studied.And the result shows that when the porosity increases,the amplitude of refracted P-wave and S-wave decreases slightly,while the amplitude of Stoneley wave decreases significantly.The waveform of axial electric field is different,the amplitude of interface mode acoustoelectric conversion wave and other accompanying mode waves increases when the porosity increases.The amplitude of Stoneley wave decreases significantly,while the refracted P-wave and S-wave are not when the permeability increases.For the full waveform of the acoustoelectric conversion electric field,the amplitude of each accompanying mode wave of the electric field decreases with continuous increase of the permeability,and the acoustoelectric accompanying Stoneley wave has the most serious attenuation,indicating that it is the most sensitive to the permeability.Analyzing the mode wave amplitude of the acoustic and acoustoelectric field,the Stoneley wave is most affected by the permeability,and the Stoneley wave amplitude attenuation in the acoustoelectric field is larger than that in the acoustic field.The acoustoelectric interface converted wave is more sensitive to the salinity than other mode waves.When the salinity of formation water and the salinity of mud reach a certain difference,the amplitude of interface mode acoustoelectric conversion wave will be larger than that of other mode waves.To improve the detection performance of the acoustoelectric detector,a multichannel receiving system with high sensitivity and high signal-to-noise ratio based on FPGA(Field Programmable Gate Array)and ADS1278(Analog-to-digital converter)is designed.After testing in high temperature environment,the signal-to-noise ratio of the receiving system can be stabilized above 40 d B.The excitation technology of high-power sound source is studied,and the pulse transformer is designed with the help of finite element software.The excitation voltage amplitude is increased by 18.5 %,and the excitation intensity of the emission probe is further enhanced.The sinusoidal pulse excitation system is designed with CPLD(complex programmable gate array)as the main control;the whole system consists of emission control circuit,inverter circuit,driving circuit and other modules,which provides an optional excitation mode for the detector.Through the research work in this thesis,it is helpful to deeply understand the detection performance and acoustoelectric logging response of the acoustoelectric logging tool,and provides a reference for the subsequent performance improvement of the instrument prototype and the interpretation of the acoustoelectric logging data.