核磁共振测井仪CPMG脉冲射频场相位研究

Study on the Phase of CPMG Pulse Radiofrequency Field of Nuclear Magnetic Resonance Logging Tool

核磁共振测井仪通过CPMG脉冲时序测量地层中质子弛豫时间得到地层孔隙度、孔隙结构、流体特性等信息。为提高核磁共振测井仪的信噪比,通过电磁场理论方法研究CPMG脉冲射频场初始相位与井下核磁共振自旋信号幅度关系,由理论计算和实验验证相得出CPMG时序第1个90°脉冲射频电场初始相位为π/2,后续180°镜像脉冲射频电场初始相位为π时核磁共振回波幅度最大,为提高信噪比提供了理论依据。

NMR(Nuclear magnetic resonance)logging tool records proton relaxation time,from which porosity,pore structure,fluid properties,etc.,are derived.In order to improve the signal-to-noise ratio of the NMR logging tool,the relationship between the initial phase of CPMG pulsed radiofrequency field and the amplitude of downhole NMR spin signal based on electromagnetic field theory was investigated.Based on theoretical calculation and experimental verification,it is concluded that when the initial phase of the first 90°pulsed RF field isπ/2 and the initial phase of the 180°mirror image RF field isπ,the amplitude of NMR echo is the largest.This conclusion provides a theoretical basis for improving signal-to-noise ratio.