Differences in desorption rate and composition of desorbed gases between undeformed and mylonitic coals in the Zhina Coalfield, Southwest China
Differences in desorption rate and composition of desorbed gases between undeformed and mylonitic coals in the Zhina Coalfield, Southwest China
复制标题
西南智那煤田未变形煤与糜棱岩煤解吸速率及解吸气体成分差异
DOI：
10.1016/j.fuel.2018.11.085
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发表时间：
2019-03-01
期刊：
影响因子：
7.4
通讯作者：
Qu, Zhenghui
中科院分区：
文献类型：
--
作者：
Chen, Yilin;Qin, Yong;Qu, Zhenghui
解吸通常用于评估煤层气资源的可采性。然而，解吸行为受煤结构的强烈影响。对从中国西南地区志那煤田钻孔采集的未变形煤和糜棱岩煤进行罐解吸实验，以研究解吸速率和瓦斯成分随时间的变化。低压N-2和CO2吸附试验表明，与未变形煤相比，糜棱岩煤的微孔、中孔和大孔具有更大的孔体积和更大的比表面积。糜棱岩煤的孔隙结构以“墨瓶状”孔隙为主，而未变形煤则呈缝状孔隙发育，连通性良好。本研究中对于未变形煤和糜棱岩煤的详细解吸实验分别持续了 61 d 和 110 h。对于未变形煤，CH4和C2H6浓度随着解吸时间的延长而稳定增加，而N-2和CO2浓度逐渐下降。糜棱岩煤在整个解吸过程中的气体成分变化可分为两个阶段：初始阶段，解吸气体中CH4和C2H6浓度增加（前12 h内）；后期阶段，解吸气体中CH4和C2H6浓度迅速下降； N-2 和CO2 浓度呈现相反的趋势。糜棱岩煤中两阶段气体成分的变化可能与其孔隙结构有关。未变形煤的“平行板状”孔隙中的气体具有流动性，而糜棱岩煤的墨瓶状孔隙中的气体流动性受到限制。这是因为CO2和N-2可以进入墨瓶状孔隙的狭窄喉道(直径为3.3～3.8埃)，而CH4和C2H6因直径较大而无法进入，导致糜棱岩煤后期解吸气体中CO2和N-2相对富集。糜棱岩煤的初始解吸速率远大于未变形煤，这与糜棱岩煤微裂缝发育密切相关。未变形煤的解吸速率是时间的幂函数，而糜棱岩煤的解吸速率可分为两个阶段，解吸速率与时间之间呈幂函数关系。结果表明，两级解吸速率归因于糜棱岩煤中存在大量墨瓶状孔隙而产生的分子筛效应。
Desorption is routinely employed for assessing the recoverability of coalbed gas resources. However, desorption behavior is strongly affected by coal structure. Canister desorption experiments on undeformed and mylonitic coals collected from drill holes in the Zhina Coalfield (Southwest China) were conducted to study temporal desorption rates and gas compositional shifts. Low-pressure N-2 and CO2 adsorption tests revealed that the micropores, mesopores, and macropores in mylonitic coal have larger pore volumes and greater specific surface areas compared to those of undeformed coal. The "ink-bottle-shaped" pore is the main type of pore structure in mylonitic coal, whereas undeformed coal shows well-developed slit-shaped pores with good interconnectivity. The detailed desorption experiments in this study lasted 61 d and 110 h for undeformed and mylonitic coal, respectively. For undeformed coal, CH4 and C2H6 concentrations steadily increased with desorption time, whereas N-2 and CO2 concentrations progressively decreased. The gas compositional shift of mylonitic coal during the entire desorption process could be divided into two stages: an initial stage, when CH4 and C2H6 concentrations in the desorbed gas increased (within the first 12 h), and a later stage, when they decreased rapidly; N-2 and CO2 concentrations exhibited the opposite trend. The two-stage gas compositional shifts in mylonitic coal are likely linked to its pore structure. Gases in the "parallel-plate-shaped" pores of undeformed coal are mobile, whereas those in ink-bottle-shaped pores of mylonitic coal have restricted mobility. This is because CO2 and N-2 can enter the narrow throats (with diameters of 3.3-3.8 angstrom) of ink-bottle-shaped pores, which are inaccessible to CH4 and C2H6 because of their larger diameter, causing the late-desorbed gas of mylonitic coal to be relatively enriched in CO2 and N-2. The initial desorption rate of mylonitic coal is much greater than that of undeformed coal, which is closely related to developed microfractures in mylonitic coal. The desorption rate of undeformed coal is a power function of time, whereas that of mylonitic coal can be divided into two stages in which the relationship between desorption rate and time is a power function. The results suggest that the two-stage desorption rate is attributable to the molecular sieve effect due to the presence of massive ink-bottle-shaped pores in mylonitic coal.