孔隙压力和含水量对煤系高岭石吸附甲烷能量影响的分子模拟研究

Molecular simulation of effects of pore pressure and water content on methane adsorption for kaolinite in coal series

采用蒙特卡洛方法模拟计算了高岭石吸附甲烷的吸附能和范德华能及其随孔隙压力和含水量的变化规律,分析了高岭石吸附甲烷的微观机理。结果表明:随着孔隙压力的增大,高岭石吸附甲烷的范德华能和吸附能均呈先增大后趋于平衡的规律,符合Langmuir模型;随着含水量的增大,高岭石吸附甲烷的吸附能呈线性规律降低。高岭石中的水分子以氧原子端靠近高岭石孔壁表面、氢原子端远离孔壁表面的方式吸附,并占据了高岭石吸附甲烷的空间;高岭石对甲烷分子的吸附是典型的物理吸附,对水分子的吸附是物理与化学吸附并存;孔隙压力和含水量影响高岭石和甲烷分子间的距离,进而影响了高岭石和甲烷分子间范德华能的大小。研究结果从微观角度揭示了在松软低渗透煤层条件下,将水力压裂和卸压钻孔置于煤层顶底板岩层中,在富含黏土矿物的岩层中实施水力压裂和卸压有利于释放煤层气,为实现煤及其顶底板岩层中煤系气的一体化强化开采奠定了微观理论基础。

The Monte Carlo method was used to simulate and calculate the adsorption energy and Van Der Waals energy of methane adsorbed by kaolinite and its changing variation with pore pressure and moisture content.The microscopic mechanism of the adsorption of methane on kaolinite was theoretically analyzed.The results show that the Van der Waals energy and adsorption energy of methane adsorbed by kaolinite were increased firstly and then stable with increasing pore pressure,which was agreed with the Langmuir model.The adsorption energy of methane adsorbed on kaolinite decreased linearly with increasing water content.The water molecules in kaolinite were adsorbed in such a way that the oxygen atoms end near the surface of the kaolinite pore wall and the hydrogen atoms end away from the pore wall surface and occupy the space of methane adsorbed by kaolinite.The adsorption of methane molecules on kaolinite was a typical physical adsorption and the adsorption of water molecules was the coexistence of physical adsorption and chemical adsorption.Changes in water content and pore pressure had an effect on the distance between kaolinite and methane molecules,which in turn affects the Van der Waals forces.The research results revealed the mechanism of the effect of moisture on the adsorption of methane energy on kaolinite from the microscopic viewpoint.It is a micro-theoretical foundation for the integration of coal-series gas by placing hydraulic fracturing in the coal seam roof and floor strata under conditions of soft coal seams.