煤的孔隙结构对于瓦斯、水等流体的运移起关键作用，煤储层资源评估和致裂效果评价均有赖于高效的测孔技术。在众多的测孔技术中，低场核磁共振是近年来应用广泛的煤孔隙结构无损、快速、精准测试技术。该技术通过检测多孔介质中的饱和含氢流体（甲烷、水、油等）的核磁共振驰豫信号，反映孔隙结构及孔隙流体分布等信息。通过核磁共振弛豫谱分析可以得到煤的孔隙度、孔径分布、孔连通性、孔隙流体、润湿性和渗透率等物性特征，评价致裂增透效果；通过二维核磁共振可以区分水、甲烷等不同含氢流体及同一含氢流体的不同相态；通过核磁共振成像可以使煤内部的孔隙分布与流体运移可视化。高温高压核磁共振分析仪可以模拟深部原位煤层环境，实现水力压裂、注气驱替、瓦斯吸附/解吸、应力变形等过程煤心的实时分析与成像。介绍了核磁共振弛豫法和冻融法的测孔原理及相应的设备系统，总结了基于核磁共振的煤孔隙结构及流体特性分析体系，涉及了弛豫分布曲线到孔径分布曲线的转换、孔隙流体的划分、孔隙润湿性评价等关键问题，进而阐述了高温高压低场核磁共振测试系统在煤炭开发领域的最新应用进展，未来低场核磁共振技术将向着模拟原位煤层环境、结合大数据与机器学习以及煤矿现场应用的方向发展。

The pore structure of coal plays a key role in the migration of gas,water,and other fluids.The assessment of coal reservoir resources and the evaluation of fracturing effect all depend on some efficient pore measuring technologies.Among the pore measuring technologies,the low field nuclear magnetic resonance (NMR) is an emerging technology for a non destructive,rapid,and accurate testing of coal pore structure in recent years.This technology detects the relaxation signal of hydrogen containing fluids (methane,water,oil,etc.) in porous media to reflect some information such as pore structure and fluid distribution.The parameters of porosity,pore size distribution,pore connectivity,pore fluid,wettability,and permeability of coal can be obtained by NMR relaxation spectrum analysis to evaluate the fracturing and permeability enhancement effect of coal.Magnetic resonance imaging can visualize the pore distribution and fluid migration inside coal.The new high temperature and high pressure NMR analyzer can simulate the deep in situ coal seam environment,and realize the real time analysis and imaging of coal cores in the processes of hydraulic fracturing,gas injection,gas adsorption,and stress deformation.The pore measuring principle and corresponding equipment system of the NMR relaxation method and cryoporometry method are introduced.The analysis system of coal pore structure and fluid characteristics is summarized,which involves the key problems such as the transformation from relaxation time distribution to pore size distribution,the division of pore fluid,and the evaluation of pore wettability.In the future,the low field NMR technology will be developed for simulating in situ coal seam environments,combining big data and machine learning,and coal mine applications.

1 测试原理与设备

   1.1 核磁共振弛豫法

   1.2 核磁共振冻融法

   1.3 设备介绍

2 煤孔隙结构分析体系

   2.1 孔径分布

   2.2 孔隙流体

   2.3 孔隙连通性

   2.4 孔隙度

   2.5 润湿性

   2.6 渗透率

3 应用案例分析

   3.1 致裂效果评价

   3.2 瓦斯吸附与解吸

   3.3 注气驱替

   3.4 应力敏感性

4 结语

5 应用展望