The Design Of Probe And Tuning System For Downhole NMR Formation Fluid Analyzer

Compared with the conventional stratigraphic fluid identification method,the downhold NMR formation fluid analyzer is an effective means to explore oil and gas resources which can real-time sample and analyze the formation fluid and can continuously evaluate the fluid pollution of the formation,also can obtain the fluid properties fluid properties such as the viscosity of the fluid and the gas-oil ratio in real time.The probe and tuning system are important parts of the formation fluid analyzer,which play main role in generating and receiving nuclear magnetic resonance signals.This thesis will start from the following three aspects of specific research and design:First of all,based on the requirements of the static magnetic field in the fluid analyzer,the static magnetic field is designed in the subregion.Aiming at the difference of magnetic field strength and uniformity in different regions of the probe,two permanent magnet structures are proposed to design: A region with a uniform degree of uniformity is used in a region with a uniform degree of uniformity and a halbach structure is used for a region having a high degree of uniformity.At the same time,the saddle-shaped antenna is designed to launch high-voltage pulses to produce alternating magnetic fields and the solenoid antenna is designed to receive nuclear magnetic resonance echo signals.Secondly,analyzing the coupling mode between the antenna and the transmitter and the receiver,the necessity of antenna tuning is determined and a tuning circuit is designed.Aiming at the requirements of high Q value and tuning range of antenna resonance in this thesis.four kinds of reconnaissance methods are analyzed,and a kind of segmented tuning method is designed.The structural design and parameters of tuning circuit are proved rational by data analysis and experiment.Finally,Aiming at the variation of the resonant frequency of the hydrogen nucleus caused by the difference of the logging depth,a sweep circuit was designed to determine the Larmor frequency at ambient temperature.The design idea and method of the sweep circuit accumulator is introduced from the aspects of hardware design and software design.The key issue-Using DSP and FPGA joint control DDS to achieve frequency synthesis were introduced in detail.The critical parts and overall performance of the designed circuit were tested with silicone oil samples.The results show that in the joint operation of the sweep circuit and the tuning circuit,the tuning system can complete the determination and tuning of the Larmor frequency at different working temperatures,so as to obtain the echo signal with high signal-to-noise radio.