118 / 2023-06-19 22:20:34

Study on Accessibility Characteristics of Pore Structure of Hard-to-drain Coal Based on NMR-MnCl2 Method

Nuclear magnetic resonance,Diffusion of manganese ions,Pore connectivity,Hard-to-drain coal,Classification of Pore Accessibility

In order to achieve a fine characterization of the connectivity characteristics of the coal internal pore structure, this paper proposes an evaluation method for the connectivity characteristics of the coal pore structure based on NMR-MnCl₂. Since the transient magnetic ion Mn²⁺ diffuses into the pores, the water signal of the pores will be shielded, which will be reflected in the change of the peak area of the T₂ spectrum. Therefore, this paper inverts the Mn²⁺ diffusion characteristics by analyzing the change rate of the NMR T₂ spectrum peak area, so as to characterize the access characteristics of the pore structure in coal. At the same time, according to the change characteristics of NMR T₂ spectrum distribution at different times, the pores in coal are divided into four types: easy-to-reach pores, relatively difficult-to-reach pores, difficult-to-reach pores and isolated pores. The results show that the proportion of easily accessible holes in hard-to-drain coal is only 39.75%, which is far lower than 61.69% in easy-to-drain coal; and the proportion of isolated holes is 33.60%, which is much higher than 12.21% in easy-to-drain coal. In addition, the pore size distribution range of the isolated pores of hard-to-drain coal is wider, and both nanometer and micrometer pore diameters are developed. In addition, through the observation of pore morphology and development characteristics, it is found that the overall pore development of hard-to-drain coal is poor, the mineral pores are relatively developed, and the matrix pores are less developed. At the micron scale, the distribution of pores in the pore development area is chaotic, and the proportion of connected pores is only 32.41%, and the distribution of zonal pore groups is present. More pore sizes are smaller than 5 μm and develop in isolation in the form of micro-nano pore groups, resulting in the inaccessibility of the overall pore structure.