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

Abstract Nuclear magnetic resonance (NMR) is a commonly used method to quantitatively characterize the pore structure of coal. T2 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 T2 cutoff values and multifractal characteristic parameters were discussed. The results show that the pore structure characteristics of CZ, GD and GHS coal samples are basically the same, all of which are mainly micropores, with relatively less content of mesopores, macropores and microcracks, 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 micropores. There is an obvious linear relationship between T2 cutoff values and the multifractal characteristic parameters. Five models of T2 cutoff values changing with the multifractal characteristic parameters are obtained by linear fitting. The error range between T2 cutoff values calculated by the five models and the experimental values are within 0.1 ms, among which T2 cutoff values calculated by model 5 are highly consistent with the experimental values, indicating that model 5 can better predict T2 cutoff values. The results can provide theoretical reference for the characterization of coal pore structure and the prediction of T2 cutoff values.