Diffusion coefficient is usually used to evaluate the methane (CH₄) diffusion properties in the coal matrix and is vital to coalbed methane (CBM) development. Although extensive literature on the CH₄ diffusion coefficient can be obtained, most of them aim at the whole coal or coal rank instead of the macrolithotype. Additionally, the primary structure of coal was destroyed with the common determination technologies (e.g., the particle, steady-state, and inverse diffusion methods) which could result in great errors. In this work, to avoid the shortcomings of the above methods, nine flake coal samples from six coal mines in the Qinshui and Ordos Basins were prepared to determine the CH₄ diffusion coefficient with the slab calculation model. Meanwhile, the effects on the diffusion from the gas pressure, temperature, water saturability, and coal pore structure, and the gas adsorption capacity controlled by the coal rank and macrolithotype, were analyzed to reveal the diffusion mechanism (mode) at the CBM reservoir and laboratory conditions. Results show that the CH₄ diffusion coefficient, at an order of magnitude of 10^–10 m²/s measured with the flake coal sample, is more truthful. High temperature and gas pressure, low water saturability, developed pore structure, and high gas adsorption capacity contribute to large CH₄ diffusion coefficient. Although the higher rank coal has the larger gas adsorption capacity, the CH₄ diffusion coefficient exhibits a “U” shape (first decreasing and then increasing) with the increase of coal rank due to more micropores in low- and high-rank coals than the middle-rank coal. From the bright to dull coals at the same coal rank, the decreasing development of pore structure and gas adsorption capacity causes the decreasing CH₄ diffusion coefficient. But compared to the coal rank, the influence of coal macrolithotype on CH₄ diffusion coefficient is weaker. In addition, the CH₄ diffusion modes in coal mainly are transitional and Fick diffusions in the CBM reservoir and laboratory.

中文翻译：

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沁水和鄂尔多斯盆地煤中甲烷（CH ₄）扩散特征及其影响因素

扩散系数通常用于评估煤基质中甲烷（CH ₄）的扩散特性，对于煤层气（CBM）的发展至关重要。尽管可以获得有关CH ₄扩散系数的大量文献，但大多数文献都针对整个煤或煤等级，而不是宏观石版岩型。另外，煤的基本结构被通用的测定技术（例如颗粒，稳态和逆扩散法）破坏了，这可能会导致很大的误差。在这项工作中，为避免上述方法的缺点，准备了沁水盆地和鄂尔多斯盆地六个煤矿的九个片状煤样品来确定CH _4。平板计算模型计算扩散系数。同时，分析了气压，温度，水饱和度和煤孔结构对气体扩散的影响，以及受煤级和大岩性控制的瓦斯吸附能力，揭示了煤层气储层的扩散机理（模式）。实验室条件。结果表明，用片状煤样品测得的CH ₄扩散系数约为10 – ¹⁰ m ² / s，更为真实。高温和高压，低水饱和性，发达的孔结构以及高的气体吸附能力有助于形成较大的CH ₄扩散系数。尽管较高等级的煤具有更大的气体吸附能力，但由于低等级和高等级煤中的微孔更多，CH ₄扩散系数随煤等级的增加而呈现“ U”形（先降低后增加）。中阶煤。从相同煤阶的光亮煤到无光煤，孔隙结构和气体吸附能力的降低导致CH ₄扩散系数降低。但是与煤阶相比，煤岩性对CH ₄扩散系数的影响较弱。另外，煤中CH _4的扩散方式主要是煤层气储层和实验室的过渡扩散和菲克扩散。