The Research Of Echo Excitation And Processing Circuit For Downhole Fluid NMR Analyzer

The traditional fluid analysis technology analyzes the sampled reservoir fluids in the laboratory.Lag results are easily affected by the quality of the sample and the change in fluid characteristics.The downhole fluid nuclear magnetic resonance analyzer(FDT-NMR)real time measure and on line analysis the original reservoir fluid directly,which effectively solved the defects of traditional fluid analysis technology.According to the principle of nuclear magnetic resonance,FDT-NMR needs to solve two problems: one is to provide the RF pulses exciting the echo signals,the other is to get the T2 spectrum of the fluid in the echo signals processing.The emission and echo data processing circuits of the FDT-NMR are studied in the thesis.The function of the emitting circuit mainly provids the energy for fluid nuclear magnetic resonance.Due to the limitation of the maximum voltage and power dissipation in this system,the highly effective class D power amplifier circuit is adopted.Design the RF pulse control signal,dead time regulation and MOS tube drive circuits for the class D power amplifier.In order to prevent the high frequency energy dissipation on the antenna,the bipolar balance filter is designed to get alternating pulse signals.The emitting circuit can provide alternating pulse signals for the antenna and the preamplifier circuit can receive the echo signal with good quality which indicate that the emitting circuit can meet the application requirements.The echo signals acquired from the preamplifier circuit can be effectively extracted after the digital phase sensitive detection algorithm.Because of the limitation of the communication speed in the fluid analysis system,the echo datas can not be uploaded in real time.So an inversion method which is suitable for this hardware platform is designed to get the T2 spectrum in real time.All the original echo datas are stored in the downhole so these datas can be processed accurately after the instrument is raised to the ground.The correlation coefficient of T2 spectrum between downhole and ground is 0.9819,which shows that the downhole inversion algorithm has high precision while satisfying real-time requirements.FDT-NMR and core analyzers are used to test the same samples.The correlation coefficient of T2 spectrum is 0.9925 which shows that the downhole formation fluid nuclear magnetic analysis instrument has good performance.