核磁共振测井仪探头设计中的数值方法

Numerical method of designing nuclear magnetic resonance logging device sensor

核磁共振测井仪探头的优化设计能够增强仪器的探测特性,提高仪器的信噪比,而探头设计中的数值方法对设计结果至关重要.本文利用电磁场有限元方法对贴井壁型核磁共振测井仪探头静磁场和射频场进行了2D和3D的数值模拟,深入分析了数值模型形状、模型尺寸、单元形状对数值模拟结果的影响,并将有限元数值模拟结果与实测数据做了对比.结果显示:数值模拟结果与实测数据符合.在设计核磁共振测井仪探头结构时,选取与井眼形状一致的圆形模型,模型尺寸范围在10—15倍探头外径,并采用三角形单元可以有效提高数值模拟方法精度,增强优化设计结果的可靠性.

Optimal design of nuclear magnetic resonance（NMR） logging device sensor can enhance its detection performance and improve signal-to-noise ratio,and the accuracy of results from numerical method is absolutely critical for optimal design.In this paper,an eccentric NMR logging device static field distribution and radio frequency field distribution are simulated by the electromagnetic finite element method,and the influences of model shape,model size and element shape on simulation results are analyzed.A comparison between the measurements and simulation data indicates that they are in good agreement with each other.In the design process of NMR logging device sensor,by choosing a circinal model which is similar to the shape of borehole,setting model size to be 10-15 times the sensor diameter,and adopting triangle element,the accuracy of numerical simulation can be improved and the reliability of optimization design can also be enhanced.