Abstract Modeling gas flow behaviors in coal is the basis for quantitatively predicting gas production and planning the life cycle for coalbed methane recovery. Air leakage is a tricky problem encountered in underground borehole gas drainage, which seriously affects gas extraction quality. However, seldom studies consider the air flows in coal associated with drainage air leakage, which may yield some misleading guidance for in-situ drainage operation. This work first presents two theoretical models (single-phase flow model (S-model)/compositional flow model (C-model)) to characterize gas flow behaviors through the fracture-matrix system of coal. The S-model only regards the methane flows, while the C-model couples the methane flows and air flows in coal to model drainage borehole leakage. Then the C-model is numerically solved using the finite difference method and validated to be reliable for evaluating actual gas drainage. The results reveal: (a) The S-model will overestimate methane production, while the C-model can accurately predict drained gas concentration, methane production and drainage air leakage; (b) Air leakage flux is mainly dominated by the fractures gas flux, and slightly affected by the matrix flows; (c) Poor gas deliverability of coal matrix incurs a slow diffuses/releases rate of adsorbed methane, forming a weak gas source for the fracture flows, which is the main culprit of rapid attenuation of methane production; (d) the drained gas concentration is collaborative determined by the fracture flow flux and the matrix flow flux; (e) Sealing up the coal wall around the borehole will linearly reduces air leakage flux and promote the drainage concentration.

中文翻译：

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一种表征煤层内排水钻孔周围裂隙基质煤中气体流动行为和漏气机制的新方法

摘要 煤中气体流动行为建模是定量预测瓦斯产量和规划煤层气回收生命周期的基础。漏气是地下钻孔瓦斯抽放中遇到的棘手问题，严重影响瓦斯抽采质量。然而，很少有研究考虑与排水漏气相关的煤中空气流动，这可能会对原位排水操作产生一些误导性的指导。这项工作首先提出了两个理论模型（单相流模型（S 模型）/组合流模型（C 模型））来表征通过煤的裂缝-基质系统的气体流动行为。S-模型只考虑甲烷流，而C-模型将煤中的甲烷流和空气流耦合以模拟排水钻孔泄漏。然后使用有限差分法对C模型进行数值求解，并验证其对实际瓦斯抽采评价的可靠性。结果表明： (a) S-模型会高估甲烷产量，而C-模型可以准确预测排放气体浓度、甲烷产量和排放空气泄漏量；(b) 漏气通量主要以裂缝气体通量为主，受基质流动影响较小；(c) 煤基质输气能力差，吸附甲烷扩散/释放速度慢，形成裂缝流的弱气源，是造成甲烷产量快速衰减的罪魁祸首；(d) 排出的瓦斯浓度由裂缝流动通量和基质流动通量共同决定；