NMR-based characterization of pore structure and nitrogen-water percolation in coal

The coal mining gradually goes deeper, and the deep coal-rock exhibits complex pore structure and low permeability characteristics.For this reason, it is of significance to characterize the transportation constitutive of methane and pore water in the low-permeability coal rock and the pore fracture network.NMR has fast and non-destructive characteristics in studying the pore structure and seepage path of coal-rock.In order to study the fluid migration and distribution of deep coal in the process of nitrogen flooding and seepage, and understand the distribution of pores in the state of deep coal and the coal under confining pressure, the cylindrical coal sample at the working face No.31030 of the Ji-15 coal seam in Pingdingshan Coal Mine No.12, Henan Province, China, was collected and subjected to a NMR CPMG sequence test and a MRI (Magnetic Resonance Imaging) test under the conditions of weighing, drying, saturated water, nitrogen flooding and seepage under different flooding pressures.The transverse relaxation T₂ spectrum, one-dimensional frequency code and NMR imaging of coal samples were obtained.Also, the saturated sample was subjected to a nuclear magnetic resonance CPMG sequence test under confined pressure, and analyzed the results.The results show that the porosity of the coal does not change monotonously with the increase of confining pressure, but increases and then decreases and finally tends to remain unchanged.The variation of the number of adsorption pores approaches the change of porosity.Nitrogen flooding can drive out the water in the transport pores, which has little effect on the total water content below the percolation level.The porosity evolution of coal was calculated by using NMR CPMG sequence.The MRI shows that the coal sample is different from other rock masses, with no large pores scattered, and the distribution of small and medium pores is relatively uniform.