The influence of coal pore structure on gas desorption-diffusion-seepage process
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摘要: 充分认识煤层瓦斯运移机制是提升抽采效率的根本前提。而目前针对煤体瓦斯微观运移特性的研究探讨的多是煤微观孔隙瓦斯运移特性,忽略了瓦斯解吸−扩散过程。以焦煤为例,采用压汞测试、纳米级工业CT扫描和数值仿真,精准重构并定量表征了煤孔隙空间结构,从微观角度分析了瓦斯解吸−扩散−渗流的演化过程,初步探讨了煤孔隙空间结构对瓦斯运移的影响。结果表明:① 在孔隙中心位置的瓦斯压力相对较高,解吸−扩散由孔隙中心向边缘进行,不同时间及位置上瓦斯压力分布规律差异明显,造成瓦斯压力分布差异性的原因在于各代表性体积(REV)单元中孔隙与喉道的半径、长度、形状、连通性能不同。② 孔隙结构和拓扑优势拓展了瓦斯解吸−扩散−渗流范围,大尺寸孔隙结构能为气体分子提供多元化运动空间,削弱尺寸效应对扩散广度的影响,促进瓦斯解吸−扩散速率。③ 强非均质连通孔隙结构中,瓦斯渗流分散而高效,能通过广泛沟通煤基质完成气体由扩散到渗流的转变,提升瓦斯传质效率;弱非均质连通孔隙结构中,气体渗流路径单一、流线集中,渗流传质阻力较大,气体分子由扩散到渗流的转变效率低,不利于瓦斯高效运移。研究结果从微观角度丰富了煤体瓦斯运移理论,为瓦斯抽采工程实践提供了理论基础。Abstract: Fully understanding the mechanism of coal seam gas migration is the fundamental prerequisite for improving extraction efficiency. At present, research on the micro migration features of coal gas mostly focuses on the micro pore gas migration features of coal, ignoring the gas desorption-diffusion process. Taking coking coal as an example, the pore space structure of coal is accurately reconstructed and quantitatively characterized using mercury intrusion testing, nanoscale industrial CT scanning, and numerical simulation. The evolution process of gas desorption-diffusion-seepage is analyzed from a microscopic perspective, and the influence of coal pore space structure on gas migration is preliminarily explored. The results show the following points. ① The gas pressure is relatively high at the center of the pore, and desorption-diffusion proceeds from the center of the pore to the edge. The distribution of gas pressure varies significantly at different times and positions. The reason for the difference in gas pressure distribution is that the radius, length, shape, and connectivity of pores and throats in each representative elementary volume (REV) unit are different. ② The pore structure and topological advantages expand the range of gas desorption-diffusion-seepage. The large-sized pore structure can provide diversified movement space for gas molecules, weaken the influence of size effect on diffusion breadth, and promote the rate of gas desorption-diffusion. ③ In the strongly heterogeneous connected pore structure, gas seepage is dispersed and efficient, and the transformation of gas from diffusion to seepage can be achieved through extensive communication with the coal matrix, improving the efficiency of gas mass transfer. In weakly heterogeneous connected pore structures, the gas seepage path is single, the seepage lines are concentrated, the mass transfer resistance of the seepage is high, and the transformation efficiency of gas molecules from diffusion to seepage is low. It is not conducive to efficient gas migration. The research results enrich the theory of coal gas migration from a microscopic perspective and provide a theoretical basis for gas extraction engineering practice.
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Key words:
- gas extraction /
- coal seam gas /
- pore structure /
- gas desorption /
- gas diffusion /
- gas seepage /
- CT scan
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表 1 Bi−PTI模型拟合参数
Table 1. Fitting parameters of Bi-PTI model
煤样 γ1/
104 HUγ2/
104 HUζ1/10−4 ζ2/10−4 $\omega $ 相关
系数Gm/HU 5号煤 5.32 3.81 3.110 0.94 0.44 0.99 10475 6号煤 1.31 2.45 2.100 0.81 0.65 0.99 12923 表 2 数值模拟参数
Table 2. Numerical simulation parameters
参数 5号煤 6号煤 R/(J·K−1·mol−1) 8.314 8.314 T/K 303 303 s/m2 5.6×10−9 5.9×10−9 a/(m3·kg−1) 0.0112 0.0112 b/MPa−1 1.86×10−7 1.86×10−7 Vm/(m3·kg−1) 0.024 0.024 f/m2 1×10−9 1.2×10−9 Nsolid 2.2×1017 2.1×1017 Vvoxel/m3 1×10−18 1×10−18 ρ/(kg·m−3) 1260 1260 Ve/m3 3.1×10−14 3.0×10−14 D/(m2·s−1) 3.60×10−12 3.60×10−12 P/MPa 1.0×10−2 1.0×10−2 -
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