Abstract:

In the geological carbon dioxide (CO₂) sequestration in abandoned oil reservoirs, there usually exists a fluid system including CO₂, oil and brine, where the interplay between them is very important for the successful CO₂ storage. In this paper, molecular dynamics simulations were carried out to investigate the interfacial characteristics of the CO₂, n-hexane and NaCl solution system with different CO₂ fractions at the miscible state. CO₂ and n-hexane formed a miscible phase, and a clear interface was observed to separate the CO₂ and n-hexane mixture with brine. Both the interfacial width and roughness increased with increasing CO₂ composition, indicating the stronger interfacial interactions when water molecules contacted with CO₂, leading to the reduction of interfacial tension. The surface adsorption of CO₂ reached the maximum when CO₂ molar fraction equaled to 65%, as well as the deficiency of n-hexane. This implied the surfactant feature of CO₂ and the hydrophobicity of n-hexane towards the interface investigated. A special micro structure was observed at the interface, where CO₂, water and n-hexane molecules had own preferred arrangement tendency. The special interfacial characteristics found could explain the experimental IFT phenomena and the microstructure variation in the CO₂ injecting process, and could provide some theoretical guidance for CO₂ sequestration.

Key words: supercritical carbon dioxide, CO₂ geological storage, interface, molecular simulation

摘要：

在CO₂以超临界状态封存于油气藏时,储层中流体间的界面性质是影响封存效率和封存量的重要因素。利用分子动力学模拟的方法,对330 K、20 MPa混溶条件下(CO₂+正己烷)/NaCl溶液系统的界面微观性质进行了研究,分析了混溶相中CO₂摩尔分数变化时,界面处CO₂和正己烷的亲水、疏水特性及其影响,为CO₂地质封存提供理论依据。研究发现,随着混溶相中CO₂摩尔分数的增加,界面厚度及粗糙度增大,分子渗透加深,热波动加剧。界面上CO₂与水之间更强的相互作用造成了CO₂注入过程中界面张力的降低。CO₂表现出类似于表面活性剂的性质,并在CO₂摩尔分数为65%(质量分数为50%)时,其界面累积量以及正己烷的驱离量最大。界面处存在特殊的分子微观结构,CO₂、水及正己烷分子呈现特殊的排布方式。

关键词: 超临界CO₂, CO₂地质封存, 界面, 分子模拟