CH4/H2O/CO2在β-SiO2（100）面吸附的第一性原理研究

First-principles study of CH4/H2O/CO2 adsorption on β-SiO2（100） surface

多孔介质中的吸附直接影响页岩气赋存、运移.基于密度泛函理论从量子力学角度研究CH4/H2O/CO2在页岩储层主体矿物成分SiO2上的吸附构型和吸附特性,计算并分析了吸附能与态密度等特征.研究表明:CH4、H2O和CO2在β-SiO2(100)面的吸附能分布在-0.2 eV^-0.1eV区间内,为物理吸附;最小吸附能大小依次为:CH4>H2O>CO2,即,CO2的吸附能力最强,H2O次之,CH4最弱;各吸附质处于吸附能最大与最小时的键长键角变化均小于1%,最大吸附能对应的吸附质键长键角变化率均大于吸附能最小时的,吸附质的物理结构变化微弱表明其所受作用力微弱;基底处于最稳定吸附位时态密度基本重合,表明各吸附质与β-SiO2表面相互作用相似且差异较小;CH4、H2O、CO2的态密度均出现不同程度偏移,且CO2在能量更低的区域具有态密度分布,更易优先吸附.

Adsorption in porous media has a direct impact on the reserves and migration of oil-gas fluid in shale gas reservoirs.This research is based on Density Functional Theory and from the point of quantum mechanics.Configuration and characteristics are studied,which belong to CH4/H2O/CO2 adsorption on SiO2(main mineral composition of shale reservoir).Adsorption energy and density of states are also calculated and analyzed.These results outline below.The adsorption energies of these systems(CH4,H2O and CO2)are distributed in the range of-0.2 eV to-0.1 eV,which means physical adsorption.The size of the minimum adsorption energy is CH4>H2O>CO2 in turn,that is to say,the adsorption capacity of CO2 is the strongest,followed by H2O and CH4 is the weakest.The bond lengths and bond angles of adsorbates at the limit adsorption sites are all less than 1%.When the adsorption energy is maximal,the change rates of bond length and bond angle are greater than those of minimum adsorption energy systems.The change of physical structure of adsorbates indicates that its force is weak.When the most stable adsorption conditions are achieved,the density of states of adsorbent is basically coincided,indicating that the interaction between adsorbates and the surface is similar and the difference is small.The densities of states of CH4,H2O and CO2 shift with different degrees,and CO2 has the density of state distribution in the lower energy region,which indicates preferential adsorption.