核磁共振测井受激回波校正方法研究

摘要

The CPMG sequence is the most widely used pulse sequence in NMR logging. However, the formation of stimulated echo will distort the first two echoes acquired with the CPMG sequence, affecting the signal-to-noise ratio (SNR) of logging tools and the outcome of reservoir evaluation. To improve the performance of NMR logging tools, a correction method for stimulated echo was proposed, based on theoretical studies on the how the stimulated echoes are formed and what are their effects on CPMG acquisition. Because the radio frequency (RF) field generated by a RF pulse might vary by the load, a relationship between stimulated echo correction coefficient and the RF field was established to calibrate the CPMG measurement made under different environment. With real logging data of low SNR, the correction method proposed resulted a measurement error of 4. 9%, which was a significant improvement from a 24. 6% measurement error resulted from the traditional cast-out-the-first-two-echoes correction method.%核磁共振测井中,受激回波导致CPMG序列中前2个回波与期望值存在较大差异,严重影响仪器信噪比和储层评价效果.为进一步改善核磁共振测井仪性能,通过探究受激回波产生机理,揭示其对CPMG回波串作用规律,提出了受激回波的校正方法.同时针对仪器负载不同,相同发射功率产生射频场强度各异的特性,建立受激回波校正系数与射频场强的数学关系,在不同应用环境中实现对CPMG测量结果的校正.实验表明,在信噪比较低的应用环境中,采用传统的舍去前2个回波的方法误差达24.58％,而该文设计的校正算法误差仅为4.9％.