Abstract Nuclear magnetic resonance (NMR) severs as a nondestructive and relative new technique that has been widely used in characterizing reservoir fluids and pore size distribution (PSD) of coals. Conventionally, pore fluids in coals are classified into movable fluid and irreducible fluid based on a single NMR T2 cutoff value (T2C). However, the single NMR T2 cutoffs model has some apparent defects in pore fluid/size classification, and few researches have reported the limitation of the single T2 cutoffs model. In contrast, the dual T2 cutoffs model may provide an accurate quantified model to classify different pore fluid types in coals. In this study, fifteen coal samples with different ranks were conducted in systematic NMR and centrifugal experiments to investigate the characteristics of pore fluid typing and PSD. Results show that when tried applying the single NMR T2 cutoffs model to classify the pore fluid typing, there are still some movable fluids when T2 T2C, there is remaining some irreducible fluid in pores after high pressure centrifugal experiments. These results indicated the limited application of single NMR T2 cutoffs model in pore fluid classification. By introducing a novel pore fluid classification method (i.e. the dual T2 cutoffs model), a typical T2 spectrum under fully-saturated condition, the absolute irreducible fluid T2 cutoffs (T2C1) and absolute movable fluid T2 cutoff (T2C2) can re-divide the pore fluid typing of coal into three types: absolute irreducible fluid (T2 T2C2). The results show that the T2C1 is in the range of 0.10–0.32 ms, while the T2C2 has a wider range from 36.12 ms to 89.07 ms. Finally, a conceptional model were proposed to clarify a full-scale PSD classification that includes the absolute irreducible fluid pores, partial movable fluid pores and absolute movable fluid pores. The model established in this study, can also be applicable for other rock types (e.g., sandstones, carbonates and shales).

中文翻译：

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煤的孔径分级新方法：基于核磁共振弛豫的研究

摘要 核磁共振（NMR）作为一种无损的、相对较新的技术，已广泛应用于储层流体和煤的孔径分布（PSD）表征。通常，煤中的孔隙流体根据单一的 NMR T2 截止值 (T2C) 分为可动流体和束缚流体。然而，单一的NMR T2 cutoffs模型在孔隙流体/尺寸分类方面存在一些明显的缺陷，很少有研究报道单一T2 cutoffs模型的局限性。相比之下，双 T2 截止模型可以提供准确的量化模型来对煤中的不同孔隙流体类型进行分类。本研究对 15 个不同等级的煤样进行了系统的核磁共振和离心实验，研究了孔隙流体类型和 PSD 特征。结果表明，当尝试应用单一NMR T2截止模型对孔隙流体类型进行分类时，T2 T2C时仍有一些可动流体，高压离心实验后孔隙中仍有一些束缚流体。这些结果表明单一 NMR T2 截止模型在孔隙流体分类中的应用有限。通过引入一种新的孔隙流体分类方法（即双 T2 截止模型），完全饱和条件下的典型 T2 谱、绝对束缚流体 T2 截止（T2C1）和绝对可动流体 T2 截止（T2C2）可以重新划分煤的孔隙流体分为三种类型：绝对束缚流体（T2 T2C2）。结果表明，T2C1 的范围在 0.10-0.32 ms 之间，而 T2C2 的范围更广，从 36.12 ms 到 89.07 ms。最后，提出了一个概念模型来阐明包括绝对束缚流体孔、部分可动流体孔和绝对可动流体孔的全面PSD分类。本研究中建立的模型也适用于其他岩石类型（例如，砂岩、碳酸盐岩和页岩）。