Abstract:

Temperature and pressure are important effect factors of the efficiency and quantity of CO₂ storage in the deep saline aquifers. Molecular dynamics (MD) simulation is applied to investigate the CO₂-NaCl systems in 343-373 K and 6-35 MPa, the interfacial tensions (IFT) which are obtained from the simulations are consistent with experimental results and the pressure balance point p_plateau are observed as well in this paper. Meanwhile, the interfacial tension variations with the temperature and pressure are analyzed and the reasons of p_plateau from the molecular viewpoint are explained. The results show that the pressure rise and temperature decline will increase CO₂ density and decrease IFT before p_plateau, but after p_plateau the IFT will be stable and less affected by temperature. In addition, the changes of CO₂ surface excess and the hydrate quantities with temperature and pressure showed the opposite trend compared with IFT variations, the saturation phenomena of hydrates at the interface under high pressure may be the fundamental reason of p_plateau.

Key words: CO₂-brine system, interfacial tension, molecular simulation, temperature, pressure, hydrates

摘要：

在超临界态CO₂封存于深部盐水层过程中，温度、压强等控制条件是影响封存效率和封存量的重要因素。应用分子动力学模拟的方法对343～373 K和6～35 MPa范围内的CO₂-NaCl盐水系统进行了界面张力（IFT）及界面特性的研究，分析了IFT随温度及压强的变化关系，并观测到了压力平衡点p_plateau；从分子尺度（物质密度、界面过余量、界面水合物密度）分析了IFT随压强、温度的变化，以及p_plateau产生的原因。结果表明，p_plateau前压强升高或温度降低将导致CO₂密度升高，IFT下降，而p_plateau后IFT趋于稳定且受温度影响较小；CO₂的界面过余量及界面处水合物数量随压强及温度变化，与IFT的变化相反；高压下界面水合物密度的饱和现象可能是p_plateau产生的重要原因。

关键词: CO₂-盐水系统, 界面张力, 分子模拟, 温度, 压强, 水合物