Multifractal analysis of coal pore structure based on NMR experiment: A new method for predicting T2 cutoff value

Nuclear magnetic resonance (NMR) is a commonly used method to quantitatively characterize the pore structure of coal. T-2 cutoff value is closely related to the microscopic pore structure of coal reservoirs, which directly affects the calculation of porosity, permeability, movable and irreducible fluid saturation. In this study, the pore structure of four types of coal samples were studied based on NMR experiments and the multifractal characteristics of saturated coal samples were analyzed with multifractal theory. Furthermore, the correlation between T-2 cutoff values and multifractal characteristic parameters were discussed. The results show that the pore structure characteristics of CZ, GD and GUS coal samples are basically the same, all of which are mainly micropores, with relatively less content of mesopores, macropores and micmcracks, while PDS coal samples have relatively more content of mesopores, macropores and microcracks. Different types of coal samples have similar multifractal characteristics and all coal samples are dominated by micmpores. There is an obvious linear relationship between T-2 cutoff values and the multifractal characteristic parameters. Five models of T-2 cutoff values changing with the multifractal characteristic parameters are obtained by linear fitting. The error range between T-2 cutoff values calculated by the five models and the experimental values are within 0.1 ms, among which T-2 cutoff values calculated by model 5 are highly consistent with the experimental values, indicating that model 5 can better predict T-2 cutoff values. The results can provide theoretical reference for the characterization of coal pore structure and the prediction of T-2 cutoff values.

Automated summary

Nuclear magnetic resonance (NMR) was used to study the pore structure of four types of coal samples and analyze the multifractal characteristics. The results showed that all coal samples were dominated by micropores and had similar multifractal characteristics, with a linear relationship between T-2 cutoff values and multifractal characteristic parameters. Models were developed to predict T-2 cutoff values with high accuracy.