Nuclear magnetic resonance T_2 cutoffs of coals: A novel method by multifractal analysis theory

摘要

Nuclear magnetic resonance (NMR) has been widely used in petrophysical characterization of coals. For NMR experimental data analysis, transverse relaxation time (T-2) cutoff value is a key parameter to identify the form of movable and irreducible fluids, and to evaluate permeability and full-scale pore size distribution (PSD). Conventionally, the T-2 cutoff value is procured by a series of centrifugal experiments, which is much complicated and time consuming, and is also hard to be used in fields such as well logging. Thus, a convenient and practical method is needed for T-2 cutoff value prediction. Based on series of centrifugal experiments, this study firstly determined an optimal centrifugal pressure of 1.38 MPa for T-2 cutoff value calculation. The results from centrifugal experiments show that the T-2 cutoff values of bituminous coals and anthracite coals in the range of 0.62-15.11 ms. Then, the multifractal analysis theory is introduced into the estimation of T-2 cutoff values of coals. The results showed that the NMR T-2 distribution of 100% water-saturated coal is multifractal, and the multifractal parameters of multifractal dimension (Dq) and singularity strength range (Delta alpha) can be used to evaluate the T-2 cutoff value of coals. Finally, a new multifractal-based NMR T-2 cutoff calculation model was provided, and the model was verified by centrifugal experimental data of six coal samples. It is concluded that the provided multifractal analysis method is effective, convenient and independent of any other experiments. The model derived from the study of coal, is also applicable for researches of other rock types such as shales and so on.