Research On Antenna Evaluation And Optimization Method Of Nuclear Magnetic Resonance Logging Tool

Nuclear magnetic resonance logging is the only logging method that can directly obtain the properties of the fluid through the nuclear magnetic resonance phenomenon of the fluid.The development of nuclear magnetic resonance logging methods is crucial to China's logging technology.The probe of the nuclear magnetic logging tool is the core of the design of the nuclear magnetic logging tool.The static magnetic field generated by the magnet part first polarizes the hydrogen nuclei in the fluid,and the antenna part is used to emit radio frequency pulses to cause nuclear nuclei to undergo nuclear magnetic resonance and receive signal.However,not all hydrogen nuclei in the detection area can produce NMR phenomena,which is related to the design performance of the probe.Among them,the matching effect of the magnet and the antenna has an important influence on the detection characteristics and signal-to-noise ratio of the instrument.At present,when designing an NMR logging tool,it is only relying on engineers to manually adjust the probe parameters to achieve a relatively high matching effect,and there is no comprehensive evaluation method.This paper first uses the finite element method to simulate and analyze the static magnetic field and radio frequency field of several nuclear magnetic logging commercial instruments.In the case of the fixed magnet scheme,various antenna schemes are designed,and the radio frequency field and the static magnetic field are simulated to match.The matching degree,sample volume,and signal-to-noise ratio were quantified separately,and the results of several schemes were compared to obtain the optimal antenna structure.Finally,a complete set of evaluation and optimization methods of antenna and magnet matching degree was formed.