Characteristics of Methane Diffusion in Coal Matrix and Its Effect on Gas Production
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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.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.
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Key words:
- coal bed methane /
- high rank coal seam /
- diffusion /
- gas rate /
- coal texture
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图 1 不同努森数的渗透率变化系数
Fig. 1. Variation coefficient of gas permeability at different Knudsen numbers
图 2 常温常压下孔隙大小对扩散系数的影响
Fig. 2. Influence of pore size on diffusion coefficient at normal temperature-pressure
图 4 20 ℃下甲烷分子运动自由程随压力的变化
Fig. 4. Variation of mean free path of methane over pressure at 20 ℃
图 5 不同吸附时间常数煤层产气速率
a.渗透率为0.2×10-15 m2；b.渗透率为5×10-15 m2
Fig. 5. Gas rate of a CBM well at different sorption time constants
表 1 气体在多孔介质中的运移模式
Table 1. Flow regimes of gas through porous media
努森数Kn ＜0.001 0.001~0.1 0.1~10 ＞10 运移模式 粘性连续流动 滑脱流动 过渡型流动 分子自由流动 
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表 2 煤层气藏数值模拟输入参数
Table 2. Parameters used in numeric simulation of coal bed methane reservoirs
参数 指标 煤层埋深(m) 500 煤层厚度(m) 5 绝对渗透率(10-15 m2) 5A、0.2B 渗透率各向异性 1:2:2 滑脱因子(kPa) 13.8 割理压缩系数(1/MPa) 0.25 基质收缩系数 0.012 兰氏体积(m3/t) 40 兰氏压力(kPa) 1 500 原地含气量(m3/t) 18 吸附时间常数(d) 10、1、0.1 裂缝半长(m) 50A、30B 裂缝导流系数(μm2·m) 2 排采范围(m×m) 300×200 注：上角标A、B代表两次模拟的参数.
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表 3 模拟结果统计
Table 3. Results of numerical simulation
渗透率
(10-15 m2)吸附时间常数
(d)产气速率峰值
(m3/d)累计产气量
(m3)0.2 10 257.89 131 086.6 1 306.99 131 428.5 0.1 335.31 131 575.7 5 10 1 403.12 1 871 920.4 1 1 543.16 1 882 621.3 0.1 1 579.93 1 882 613.9
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