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扩散是煤层甲烷运移的关键环节之一,而目前有关煤层中甲烷扩散特征的认识并不充分.以沁水盆地南部高煤阶煤层气藏为例,应用微纳渗流力学理论分析了煤基质中气体扩散模式及定量表征参数;应用Simed软件开展了扩散性能对不同煤体结构煤层气排采规律的影响数值研究.结果表明:煤层甲烷的扩散受化学势梯度的驱动,产气过程中体相扩散、努森扩散和构型扩散模式并存且呈动态变化;甲烷扩散性能受气体温度、压力、气体种类、水分以及基质孔隙结构共同影响,基质孔隙吸附甲烷会改变微孔孔径并影响扩散路径的空间形态;煤基质中甲烷的扩散是非热力平衡过程,扩散系数是吸附量的函数.基于拟稳态扩散的数值研究表明,扩散性能强弱对于长期累计产气量几乎没有影响,而对短期产气速率具有较大的影响;扩散性能弱的,产气速率峰值较低,但峰值之后的一段时间内产气速率相对较高;与高渗煤层相比,低渗构造煤层的产气速率对吸附时间常数更敏感.
Diffusion is one of the key processes of methane migration in coal seam, and the current understanding about the characteristics of methane diffusion in coal seams is not sufficient.Based on the high rank coalbed methane reservoir in southern Qinshui Basin, the application of micro-seepage mechanics theory to coal matrix The gas diffusion model and the quantitative characterization parameters were studied.The numerical study on the effect of diffusion performance on the coalbed methane extraction law of different coal body structures was carried out by using Simed software.The results show that the diffusion of methane in the coal seam is driven by the chemical potential gradient and the gas- The phase diffusion, Knudsen diffusion and configuration diffusion model coexist and change dynamically. The methane diffusion performance is affected by gas temperature, pressure, gas type, water content and matrix pore structure. The adsorption of methane by matrix pores will change the pore size and affect the diffusion path The diffusion of methane is a function of adsorption capacity.The numerical study based on quasi-steady-state diffusion shows that the diffusion performance has little effect on long-term cumulative gas production, but the short-term production Gas velocity has a greater impact; diffusion performance is weak, gas production rate lower peak, but the peak after a period of time The gas production rate is relatively high. Compared with the high permeability coal seam, the gas production rate of the low permeability coal seam is more sensitive to the adsorption time constant.