湿度对CH_4/CO_2在干酪根中吸附的影响:分子模拟研究

Effect of moisture on CH_4/CO_2 adsorption on kerogen:A molecular simulation study

CH_4和CO_2在不同湿度页岩干酪根中的吸附行为对页岩CO_2埋存具有重要指导意义。本文采用分子动力学(MD)方法构建和优化干燥的干酪根模型,采用巨正则蒙托卡罗(GCMC)方法构建不同湿度含量的干酪根模型,并模拟CH_4、CO_2单组分及其混合气体在干酪根模型中的吸附行为。通过分析湿度对CH_4和CO_2吸附量、等量吸附热和吸附选择性的影响,对页岩CO_2埋存提高气体采收率的潜在可行性进行了探讨。研究发现:(1)水分子在高湿度下易形成笼形团簇结构,恶化CO_2置换CH_4的效果;(2)页岩CO_2埋存提高CH_4采收率在低储层压力条件下开发效果更好;(3)CO_2置换CH_4的效果在低湿度下随湿度含量增加而得到改善,对于干酪根成熟度较高的页岩储层,可考虑在气藏开发后期注入CO_2和一定水蒸气含量的混合物。

The adsorption behaviors of CH_4 and CO_2 on moist kerogen are vital to understand the process of CO_2 sequestration and shale gas exploitation.In this work,a realistic type II dry kerogen model was constructed and optimized by the molecular dynamics(MD) method.Then moist kerogen models with various moisture contents(0.7 wt.%,1.4 wt.%,2.1 wt.% and 2.8 wt.%) were developed by Grand Canonical Monte Carlo(GCMC) simulations,and the adsorption characteristics of CH_4,CO_2 and their mixtures on these kerogen models were investigated.Based on the influences of moisture content on the adsorption capacity,isosteric heat of adsorption and adsorption selectivity of gas molecules,the potential feasibility of CO_2 sequestration and enhanced shale gas recovery was discussed.Results show that water molecules are prone to aggregate into cage cluster structures at high moisture content,worsening the replacement effect of CH_4 by CO_2.The development effect of shale gas reservoirs with CO_2 sequestration is better at low reservoir pressure.Moreover,the development effect for CO_2 replacing CH_4 can be improved by increasing the kerogen moisture content at low moisture conditions.For a shale gas reservoir with higher thermal maturity,it can be beneficial to inject some mixtures of CO_2 and a certain amount of steam at the late development stage in order to enhance the shale gas recovery.