烟气从天然气水合物中置换甲烷机理及三维实验研究

Production of methane (CH4) by external stimulation ways of decomposing natural gas hydrate (NGH) might easily destroy the geological structure, resulting in slop geological hazard. Thus, it is necessary to develop a new and more secure way. Production by injecting flue gas into the NGH to displace CH4 is proposed and investigated in this project. Due to the replacement occurs in the hydrate phase, the hydrate structure remains stable. The project includes the studies of thermodynamic mechanism, molecular dynamics (MD) simulation, kinetic model on the replacement, and the studies on three-dimensional experiments. The thermodynamic mechanism is revealed by comparison of decomposition enthalpies between CH4 hydrate and flue gas hydrate, and Gibbs free energy of gas replacement in hydrates calculated by molecular simulation. Microscopic mechanism of the replacement is revealed by the MD simulation. Simultaneously, the thermodynamic model of replacement and the hydrate structure are investigated. The kinetic model of the replacement is developed based on the driving force which is considered as the fugacity difference of gases in phases. By the three-dimensional experiments, the law of the replacement reaction is clarified by the calculations of permeability, rate of replacement and methane recovery. In the experiments, in-site Raman spectroscopy analysis is used for further revealing the mechanism the replacement. The project establishes the experimental and theoretical foundation for developing the technology of producing methane from NGH.

利用外激法分解天然气水合物开采天然气容易破坏地质结构，导致斜坡地质灾害，因此，需要探寻更安全的方法。本研究利用置换开采法，将烟气充入甲烷水合物中，利用气体在固-固相的转化，在水合物相中置换出甲烷气体，维持水合物结构的稳定。研究包括置换热力学、分子动力学模拟、置换动力模型以及三维模拟实验。通过比较甲烷水合物和烟气水合物的分解焓、利用分子模拟计算置换反应吉布斯自由能揭示热力学机理；利用分子动力学模拟揭示置换的微观机理，明确置换的发生方式并考察水合物结构的稳定性；以气体在不同相中的逸度差为驱动力，建立置换动力学模型，并在此基础上，进行三维模拟实验研究，通过计算渗透率、置换速率及效率等参数，揭示了置换反应规律，获得提高置换速率和效率有效方法，利用原位Raman图谱分析在实验研究中进一步揭示置换的机理，为发展天然气水合物开采技术奠定理论和数据基础。

为了避免外激法开采天然气水合物带来的地质灾害，本项目利用烟气从天然气水合物中置换开采甲烷，二氧化碳与甲烷气体在水合物相发生转化，成功置换并开采出甲烷。项目研究的内容主要包括置换开采热力学、动力学、置换开采微观机理以及三维置换模拟实验等。研究结果表明，利用烟气置换开采天然气水合物可行，相比较纯二氧化碳置换，烟气置换开采效率更高；通过拉曼图谱分析，置换过程包括甲烷水合物的变形以及二氧化碳水合物的形成两个过程；气体在水合物相中扩散是置换开采动力学控制因素；利用置换法三维开采天然气水合物可行。项目完全按照计划实施，共发表了SCI/EI论文17篇，申请发明专利3件，培养了2名博士生和3名硕士生，研究成果超出了预期。项目研究结果也为后续置换开采天然气水合物提供了科学和理论依据。