Molecular mechanism of adsorption/desorption hysteresis: dynamics of shale gas in nanopores

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• 作者：Jie Chen ; FengChao Wang ; He Liu ; HengAn Wu
• 关键词：shale gas ; adsorption/desorption hysteresis ; numerical simulation ; capillary condensation ; pore throat
• 刊名：SCIENCE CHINA Physics, Mechanics & Astronomy
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：60
• 期：1
• 全文大小：3,052 KB
• 刊物类别：Physics and Astronomy
• 刊物主题：Physics
  Chinese Library of Science
  Mechanics, Fluids and Thermodynamics
  Physics
  
• 出版者：Science China Press, co-published with Springer
• ISSN：1869-1927
• 卷排序：60

文摘

Understanding the adsorption and desorption behavior of methane has received considerable attention since it is one of the crucial aspects of the exploitation of shale gas. Unexpectedly, obvious hysteresis is observed from the ideally reversible physical sorption of methane in some experiments. However, the underlying mechanism still remains an open problem. In this study, Monte Carlo (MC) and molecular dynamics (MD) simulations are carried out to explore the molecular mechanisms of adsorption/desorption hysteresis. First, a detailed analysis about the capillary condensation of methane in micropores is presented. The influence of pore width, surface strength, and temperature on the hysteresis loop is further investigated. It is found that a disappearance of hysteresis occurs above a temperature threshold. Combined with the phase diagram of methane, we explicitly point out that capillary condensation is inapplicable for the hysteresis of shale gas under normal temperature conditions. Second, a new mechanism, variation of pore throat size, is proposed and studied. For methane to pass through the throat, a certain energy is required due to the repulsive interaction. The required energy increases with shrinkage of the throat, such that the originally adsorbed methane cannot escape through the narrowed throat. These trapped methane molecules account for the hysteresis. Furthermore, the hysteresis loop is found to increase with the increasing pressure and decreasing temperature. We suggest that the variation of pore throat size can explain the adsorption/desorption hysteresis of shale gas. Our conclusions and findings are of great significance for guiding the efficient exploitation of shale gas.