scCO2-brine多相流体在岩石孔隙内流动特性的研究

This project is planned to investigate the flow characteristics of scCO2-brine multiphase fluids in rock nanopores. The scCO2 leakage along the caprock pores (diameter less than 50 nm) in the geological sequestration is a typical multiphase flow process which accompanies with the ion exchange between the flow phase and solid phase. Due to the ion exchange, which happens in the process of dissolution and mineralization reactions after the CO2 hydration, has significant influence on the pore surface morphology and system internal force, these effects cannot be ignored in the nanoscale flow. Therefore, in this project, we plan to utilize molecular dynamics (MD) simulations to investigate the interfacial characteristics, the mechanisms of ion exchanging reactions on the rock surface and the multiphase flow process coupling with the ion exchange. Specifically, we plan to research the variations of microstructure, viscosity and flow characteristics (velocity distribution, and flow pattern) of the phases caused by the ion exchange. Meanwhile, the influence of the flow state on the ion migration is also planned to be analyzed in this project. Moreover, for the multiphase flow process with the ion exchange, we plan to observe the interfacial phenomena and system forces, and then probe the causes of their variations. Finally, the vital parameters (temperature, pressure, salt concentration and CO2 injection, etc.) of flow control will be determined in detail. This study is expected to explain and probe the flow mechanism and interfacial phenomena in the CO2 leaking process up through rock pores, which is a fundamental guideline to the relative research on CO2 leakage prevention methods.

本项目拟开展耦合离子交换的scCO2-brine多相流体在纳米尺度岩石孔隙内流动特性的研究。地质封存中scCO2沿盖层岩石孔隙（直径小于50nm）的泄漏是伴随着流－固相间离子交换的多相流动过程。离子交换与CO2水合所引发的岩石表面溶解、矿化反应相关，其过程改变了孔隙表面形态和体系内受力，在纳米尺度流动中不可忽略。本项目拟采用分子模拟方法，对scCO2-brine-岩石系统界面特性、scCO2-brine与岩石表面间离子交换机理、耦合离子交换的多相流动过程及影响因素进行研究。从微观角度分析离子交换所引发的各相微观结构、相间作用力、流体粘性、流型、流速分布等流动特性变化；探究含离子交换的流动系统相间界面特性及相间作用力的变化规律及原因；确定调控流动的关键参数（温度、压强、盐浓度和CO2注入量等），以期对地质封存中CO2岩石孔隙渗流机理及相关界面现象进行阐述，为防止CO2泄漏的研究提供理论指导。

地质封存中scCO2沿盖层岩石孔隙（直径小于50nm）的泄漏是伴随着流－固相间离子交换的多相流动过程。本项目采用分子模拟方法，对scCO2-brine-岩石系统界面特性、scCO2-brine与岩石表面间离子交换机理、耦合离子交换的多相流动过程及影响因素进行了研究。通过本项目研究的开展，构建了scCO2-brine-岩石多相系统分子动力学模拟计算模型，并适配建立了可描述地质封存中岩石表面碱金属离子键断裂、成键过程的分子模拟反应力场。主要分析了镁橄榄石表面在不同盐溶液中的溶解活化能和溶解规律。结果表明，镁橄榄石表面的溶解取决于Mg-O键的强度。镁橄榄石表面溶解时，溶液中的Mg2+、Ca2+、H3O+倾向与镁橄榄石表面的镁原子发生离子交换，CO32-倾向与镁橄榄石表面的镁原子形成MgCO3；进一步研究了孔隙宽度、作用力、温度和压强、CO2注入量、盐离子浓度、表面结构破坏等参数对scCO2-brine-岩石系统流动的影响。结果表明，孔隙宽度小于15 nm时，密度震荡使得H2O的平均密度随孔隙宽度的减小而增加；作用力的增加、温度的升高、盐离子浓度的提高均能促进H2O分子间氢键网络的致密性；温度的升高、盐离子浓度的提高有利于增加镁橄榄石表面对H2O的流动阻力；氢键网络和流动阻力的综合作用影响了H2O的粘度；此外，表面结构的破坏增加了H2O的流动阻力，减小H2O的流动速度；H2O和CO2多相流动时，H2O吸附在镁橄榄石表面形成H2O层，CO2被隔离处于孔隙的中间区域；随CO2注入量的增加，H2O分子间的氢键数目不断减少，镁橄榄石和H2O分子间的作用力不断增强。