A multifractal-based method for determination NMR dual T2 cutoffs in coals

Abstract

An accurate classification of pore fluid typing in coals is critical for understanding and assessing fluid flow characteristics during coalbed methane production, but the widely used nuclear magnetic resonance (NMR) single T2 cutoff (T2C) model remains some apparent defects. In this study, we proposed a novel NMR dual T2 cutoffs model for fluid typing classification based on the multifractal theory. Firstly, the NMR and high-speed centrifugal combined measurements were performed for twenty-one coals with strikingly different ranks. Secondly, the multifractal analysis theory was introduced to investigate multifractal characteristics of the fully water-saturated NMR spectra. Thirdly, the correlations between dual T2 cutoffs and multifractal parameters were systematically analyzed. Fourthly, a multifractal-based model was established to evaluate the NMR dual T2 cutoffs values in coals. Results show that the pore fluid typing was re-divided into: absolute irreducible- (T2 < T2C1), absolute movable-(T2 > T2C2), and partial movable-fluid (T2C1 < T2 < T2C2) based on dual T2 cutoffs. The values of T2C1 and T2C2 were in the range of 0.049–0.455 ms and 36.12–102.34 ms, respectively. Multifractal numerical simulations indicate the coals have typical multifractal characteristics. The minimum multifractal dimension (Dmin), singularity strength subtraction (αmax-αmin), and singularity strength proportion (αmax/αmin) were linearly correlated with dual T2 cutoffs, that can be further used for model-building. The dual T2 cutoffs calculated from the proposed multifractal-based model qualitatively agree with those obtained from complicated laboratory experiments, indicating the accuracy of the established multifractal-based model in this study – from the validations of six non-modeling coals. It can be concluded that the proposed multifractal-based model is accurate and convenient, also be scalable in the applications of other rocks such as shales.