Application and prospect of low field nuclear magnetic resonancetechnology in accurate characterization of coal pore structure

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.