甲烷在页岩黏土矿物中吸附行为的分子模拟

为从微观角度研究页岩气在页岩黏土矿物中的吸附行为,利用分子模拟软件Materials studio构建了蒙脱石、伊利石及伊/蒙混层3种黏土矿物的模型,采用巨正则蒙特卡洛(GCMC)方法对页岩气主要成分甲烷在黏土矿物上的吸附行为进行模拟。研究表明,甲烷在不同孔径蒙脱石、伊/蒙混层和伊利石上的平均等量吸附热均小于42 kJ/mol,说明甲烷在黏土矿物表面发生物理吸附。甲烷在3种黏土矿物表面的吸附等温线呈现出相同的规律,均符合I型吸附曲线特征,Langmuir模型的拟合精度较高。3种黏土矿物对甲烷的吸附能力强度顺序为:蒙脱石伊/蒙混层伊利石。在伊/蒙混层矿物中,层间离子比甲烷分子更靠近黏土片层,蒙脱石片层一侧吸附的甲烷密度大于伊利石一侧,表明蒙脱石片层的吸附能力强于伊利石,但伊/蒙混层矿物中黏土片层的吸附作用对甲烷吸附层密度的影响随着压力升高而减弱。本文分别构建了孔径为1,2和4 nm的3种黏土矿物模型,从微观角度研究孔径对甲烷吸附的影响。模拟结果表明,黏土矿物孔径越小,甲烷吸附的等量吸附热越大,证明吸附更加牢固,对甲烷的吸附能力更强,但受到孔内有限空间的限制,甲烷在黏土矿物内的绝对吸附量及饱和吸附量随孔径减小而降低。

Molecular simulation of methane adsorption behavior on clay minerals in shale

To investigate factors that influence the adsorption behavior of shale gas in clay minerals from a microscopic perspective,the molecular models of smectite,illite and mixed layer I/S were built using molecular simulation software Materials studio.Grand Canonical Monte Carlo method was used to simulate methane’s adsorption behavior on clay minerals.The research results found that the adsorption isotherms of methane on three clay minerals are lower than 42 kJ/mol,demonstrating that the adsorption of methane on clay minerals belongs to physical adsorption.The adsorption isotherms of methane also show the same trend,which conform to the characteristics of type I adsorption curve.The fitting accuracy of Langmuir model is high.The adsorption capacity of methane is in the sequence of smectite > mixed layer illite/smectite>illite.In mixed layer I/S,interlayer cations are closer to clay layer than methane.The density of methane adsorbed on the side of smectite is higher than that on the illite side,demonstrating that smectite has stronger affinity to methane compared with illite,but the effect of adsorption on density decreases with the increase of pressure.The authors constructed the clay models with the pore sizes of 1 nm,2 nm and 4 nm,respectively.As pore width decreases,the isosteric heat of methane increases,which means that the adsorption becomes stronger,the adsorption capacity of clay minerals increases,but the absolute and saturated adsorption amount decrease with pore size declining.