Study on Adsorption Behavior and Mechanism of Shale Gas by Using GCMC Molecular Simulation
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摘要: 揭示页岩气的吸附机理是阐明页岩气的吸附规律及转化条件、建立具有普适意义的定量评价模型的基础.采用GCMC（Grand Canonical Monte Carlo）分子模拟方法，对不同温压条件下CH4和CO2在不同孔径的伊利石狭缝形孔隙中的吸附行为进行模拟，结果表明，分子模拟与实验所得的吸附量归一化到单位表面积才具有相同的内涵和比较的意义.在此基础上进行的对比表明，分子模拟与实验结果相近，奠定了由分子模拟考察页岩气吸附行为和机理的基础：气体吸附于矿物表面的内因（机理）是气-固分子之间的范德华力和库仑力，伊利石表面对CO2的吸附能力比其对CH4的吸附能力强是其结合能更高的反映；CH4和CO2在伊利石表面的吸附虽然并非严格的单分子层吸附，但以一个强吸附层为主；孔径减小到微孔后吸附相密度将发生叠加，形成微孔填充，也是其结合能叠加的结果.Abstract: The mechanism of shale gas adsorption is the theoretical foundation for elucidating the adsorption and transformation conditions, and establishing a universal quantitative evaluation model. The adsorption behavior of CH4 and CO2 in illite slit pores with different sizes under various temperature and pressure conditions was simulated by GCMC (Grand Canonical Monte Carlo) method. It is found that adsorption capacities obtained from both molecular simulation and experiments have the same connotations and are comparable when being normalized to the surface area, under which conditions the molecular simulation results are consistent with the experimental measurements. In this way, the basis for the study of adsorption behavior and mechanism of shale gas is established by molecular simulation:the internal cause (mechanism) of gas adsorption on the mineral surface is van der Waals force and Coulomb force in gas-solid molecules, the larger adsorption capacity of CO2 than CH4 on the surface of the illite is the reflection of a higher binding energy; the adsorption of CH4 and CO2 on the illite is not rigorous monolayer-adsorption, but a strong adsorption layer mainly; the adsorption phase density will overlap when pore size reduced to micropore, and microporous filling is thus formed, which is also a result from the superposition of their binding energy.
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图 1 Ⅰ型(a)和Ⅱ型(b)模拟单元剖面
孔隙空间被气体分子和K+充填.其中，气体和钾离子为球状模型，钾伊利石骨架为球棍模型.色标：氧，红色；氢，白色；硅，黄色；铝，粉色；钾，紫色；碳，灰色
Fig. 1. A snapshot of type Ⅰ (a) and type Ⅱ (b) simulation cells
图 2 363 K(90 ℃)时CH4和CO2气体分子在3 nm孔径的钾伊利石不同类型模拟单元中加载量对比
过剩吸附量以单位质量吸附剂的吸附能力表示
Fig. 2. Comparison of the total loading number of CH4 and CO2 in both type Ⅰ and Ⅱ simulation cells with the pore size of 3 nm at the temperature of 363 K (90 ℃)
图 3 模拟与实测所得CH4(a)和CO2(b)过剩吸附量对比
Fig. 3. The comparison of excess adsorption amount of CH4 (a) and CO2 (b) in illite by both molecular simulation and experiment
图 4 模拟与实测所得CH4(a)和CO2(b)过剩吸附量对比
过剩吸附量用单位吸附剂比表面积的吸附能力表达
Fig. 4. The comparison of excess adsorption amount of CH4 (a) and CO2 (b) in illite by both molecular simulation and experiment
图 5 温度为90 ℃和压力为30 MPa条件下，CH4和CO2在孔径为3 nm的钾伊利石孔隙中的密度分布曲线
Fig. 5. The gas distribution of CH4 (a) and CO2 (b) in K-illite pore with the size of 3 nm at 90 ℃ and 30 MPa
图 6 CH4和CO2分别于钾伊利石表面之间平均结合能对比图
Fig. 6. The interaction energy between K-illite surface and gas molecules
图 7 温度为90 ℃和压力为30 MPa条件下，CH4(a)和CO2(b)气体在不同孔径的钾伊利石孔隙中的密度分布曲线
Fig. 7. The gas distribution of CH4 (a) and CO2 (b) in K-illite pore with various sizes at 90 ℃ and 30 MPa
图 8 不同孔径的伊利石孔隙表面与CH4(a)和CO2(b)分子之间的结合能
Fig. 8. The interaction energy between the surface of illite pores with various sizes and gas molecules, including both CH4 (a) and CO2 (b)
表 1 Ⅰ型、Ⅱ型钾伊利石模拟单元比表面积以及CH4和CO2气体分子动力学直径统计
Table 1. Kinetic diameters of both CH4 and CO2 molecules and specific surface areas of types Ⅰ and Ⅱ K-illite simulation cells
吸附质 动力学直径(nm) 模拟单元类型 比表面积(m2/g) CH4 0.38 Ⅰ 997.73 Ⅱ 529.60 CO2 0.33 Ⅰ 989.79 Ⅱ 536.22 
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