今天是2017年11月18日

煤基质中甲烷扩散特征及其对气井产能的影响

DOI:10.3799/dqkx.2017.537

作者:李国庆,孟召平,刘金融

联系地址:中国地质大学构造与油气资源教育部重点实验室,湖北武汉 430074

摘要: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.

关键字:煤层气;高煤阶煤;扩散;产气速率;煤体结构.

Characteristics of Methane Diffusion in Coal Matrix and Its Effect on Gas Production

Author:Li Guoqing, Meng Zhaoping, Liu Jinrong

Author Address:Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, China University of Geosciences, Wuhan 430074, China

Abstract:Diffusion is one of the key steps of methane transport in coal seam, yet our understanding of it is still insufficient. Taking the high rank coal bed methane (CBM) reservoir in the southern Qinshui basin, China as the study area, the patterns and quantitative characterization of methane diffusion in coal seam were analyzed based on microflows and nanoflows mechanics theory; the influence of diffusion property on gas production in coal seams of different coal textures was studied by using a numerical simulation software (Simed) in this study. Results show that the diffusion of methane in coal is driven by a chemical potential gradient and the diffusion modes include bulk diffusion, Knudsen diffusion and configurational diffusion. Various diffusion modes coexist and vary during the extraction of coalbed methane; the diffusion coefficient is influenced by temperature, gas pressure, gas type, moisture and pore structure of coal matrix, and the size of micropore varies due to the adsorption of methane and thereby the diffusion path will be transformed; the dynamic adsorption of methane on the coal matrix determines that methane diffusion in coal is a non-equillibrium process and that the diffusion coefficient is a function of adsorbed gas concentration; the results of numerical simulation based on quasi-steady diffusion show that the diffusion property has a slight influence on the long-term cumulative gas production while it exerts a significant effect on the short-term gas rate; if the diffusion coefficient is low, that is, the sorption time constant is relatively high, the peak gas rate will be relatively low while the gas rate will be relatively high in a period of time after reaching the peak gas rate; the gas production is more sensitive to sorption time constant for the low-permeability tectonically deformed coal seam than for the high-permeability one.

Keyword:coal bed methane; high rank coal seam; diffusion; gas rate; coal texture.

 

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作者简介:李国庆(1980-),男,副教授,主要从事煤层气地质、矿井地质灾害研究.ORCID: 0000-0002-3463-6715. E-mail: ligq@cug.edu.cn

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