Research And Design Of Electromagnetic Transmitter In EM-MWD System

In the process of air drilling,foam drilling and atomization drilling,the Electromagnetic Measurement While Drilling(EM-MWD)system is an important technical means to solve the drilling information transmission and an indispensable technical support for drilling information intellectualization.The downhole electromagnetic transmitter is the core subsystem,which can transmit the measured data of the directional probe pipe to the surface through the formation and the drill pipe with extremely low-frequency electromagnetic wave as the carrier signal.However,the formation conductivity,drill pipe conductivity,excitation mode of transmitting source,signal frequency,digital modulation mode and other factors will seriously lead to functional degradation of transmitter.Furthermore,the low power consumption,reliability and stability of the EM-MWD system remain key constraints to the development of EM-MWD system in the downhole environment of high temperature,high pressure and strong vibration.Therefore,the research of underground electromagnetic transmitter has high practical application value for the development of EM-MWD system.Firstly,in order to optimize the continuous working time of the downhole electromagnetic transmitter and improve the depth and accuracy of petroleum exploration,a novel downhole electromagnetic transmitter topology for electromagnetic MWD system is designed.Aiming at the complex characteristics of transmission channel of transmitter,an approximate equivalent transmission line model of EM-MWD system is established with drill string,formation,insulation gap and the grounding electrode being taken into account.The model is verified by using COMSOL Multiphysics software,providing theoretical basis for the design of the downhole electromagnetic transmitter.Secondly,considering the complex and changeable load impedance,severe working environment and limited working time of underground electromagnetic transmitter,the hardware platform of underground electromagnetic transmitter is constructed based on TMS320F2812,which mainly includes the design of minimum system,power amplifier and driver module,directional probe communication module and conditioning circuit.The topological structure of the electromagnetic transmitter was designed,and a double closed-loop real-time feedback control scheme based on load voltage feedback loop and capacitor current loop was proposed to optimize the output dynamic characteristics and continuous working time of the electromagnetic transmitter.Finally,the working state of the power module of the electromagnetic transmitter,the SPWM modulation process of the control system and the double closed-loop control strategy are modeled and simulated based on MATLAB/Simulink software.The theoretical analysis is verified by the spectrum analysis of the output waveform of the transmitter,the frequency change of the output signal and the dynamic load test.The simulation and experimental results show that :(1)the low frequency excitation signal has small attenuation coefficient,large skin depth and strong penetration ability in the formation.(2)Low conductivity formation,high conductivity drill pipe and large size insulation sub are more conducive to the transmission of electromagnetic waves.(3)The double closed-loop real-time feedback control can reduce the output harmonic distortion of the electromagnetic transmitter,ensure that the system has enough phase margin,and optimize the working stability and dynamic characteristics of the system.