页岩表面甲烷气吸附机理及影响因素研究

The Mechanism and Influencing Factors of Methane Adsorption on Shale Surfaces

页岩表面甲烷气的吸附机理及影响因素研究对于准确预测页岩气藏的可采储量至关重要,为此利用MS软件,采用巨正则蒙特卡罗方法建立了页岩气在泥页岩地层中的吸附模型,对页岩气在地层中的存在状态进行了分子动力学模拟。分别用石墨代替有机质干酪根、二氧化硅晶体代替黏土矿物、甲烷气(CH_4)代替页岩气,模拟了不同大小孔隙石墨和不同极性二氧化硅表面CH_4的吸附过程,分析了吸附状态、吸附能等参数。分子模拟结果表明,CH_4与二氧化硅表面的作用以库仑力为主,吸附能和吸附量随表面极性增强而降低;CH_4与石墨表面的作用以范德华力为主,吸附体系总能量和吸附能均随孔隙直径增大而降低,因此CH_4的吸附厚度和吸附态CH_4的比例也随之减小。研究表明,有机质是CH_4吸附的主要载体,石墨对CH_4的吸附能力远大于二氧化硅,且CH_4在石墨表面的吸附为多层的物理吸附,是影响页岩储层CH_4储量的主要因素。

Understanding the mechanism of methane adsorption on shale surfaces and the key influential factors is very critical for the accurate prediction of shale gas EUR. In this paper, a model for gas ad- sorption in shale was built based on the Grand Canonical Monte Carlo （GCMC） method in MS software. In this model, the molecular dynamics of shale gas were simulated to reflect gas existence state in shale reser- voirs. The process of methane （CH4） adsorption in graphite with varying pore size and also on the surface of silica with different polarity was simulated using graphite to replace organic kerogen, silica crystals to replace clay minerals,and methane to replace shale gas. The purpose was to analyze the adsorption state, adsorption energy, and other parameters. The results of molecular simulation showed that the interaction between CH4 and silica surface is dominated by Coulomb force, in which the adsorption energy and the ad- sorption volume decrease with an increase in surface polarity. On the other hand, the interaction between CH4 and the graphite surface is dominated by van der Waals force,in which both total system energy and adsorption energy decrease with the increase in pore size. The research results in the paper indicates that organic matter is the main carrier of CH4 adsorption and that the CH4 adsorption capacity of graphite is much larger than that of silica. Further, the adsorption of CH4 on the graphite surface is a multi-layer phys- ical adsorption,and it is the major influencing factor in CH4 reserves in shale reservoirs.