基于VOF方法的超临界二氧化碳——水两相流动孔隙尺度数值模拟

Pore-scale numerical simulation of supercritical CO_(2)-brine two-phase flow based on VOF method

二氧化碳(CO_(2))捕集与封存技术有利于减少CO_(2)的排放量,近年来针对CO_(2)地质封存形成了从纳米尺度到油气藏尺度的大量研究成果,大多数研究只针对单一维度多孔介质中流动行为开展研究,且物理实验方法受许多不确定性因素影响,十分耗费时间和成本。为了从微观角度深入理解CO_(2)地质封存过程中的渗流行为,提高CO_(2)地质埋存量,基于追踪两相界面动态变化的VOF(Volume of Fluid)方法,分别建立了2D和3D模型,开展了超临界CO_(2)-水两相流动数值模拟研究,对比了不同润湿性、毛细管数、黏度比条件下的CO_(2)团簇分布特征、CO_(2)饱和度变化规律,揭示了孔隙尺度CO_(2)埋存的内在机理。研究结果表明:①随着岩石对CO_(2)润湿性增加,CO_(2)波及范围扩大,同时CO_(2)团簇的卡断频率减少,CO_(2)埋存量增加;②随着毛细管数的增加,驱替模式由毛细指进转变为稳定驱替,CO_(2)埋存量增加;③随着注入超临界CO_(2)黏度逐渐接近水的黏度,两相流体之间的流动阻力降低,促进了“润滑效应”,CO_(2)相的渗流能力提高,CO_(2)埋存量增加;④润湿性、毛细管数、黏度比在不同维度多孔介质模型中对CO_(2)饱和度的影响程度不同。结论认为,基于VOF方法的CO_(2)-水两相渗流模拟研究在孔隙尺度上揭示了CO_(2)地质封存过程中的渗流机理,对CCUS技术的发展有指导意义,也为更大尺度的CO_(2)地质封存研究提供了理论指导和技术支撑。

CO_(2)capture and storage technology is favorable for the reduction of CO_(2)emissions.In recent years,a great number of research achievements have been obtained on CO_(2)geological storage from nano scale to oil/gas reservoir scale,but most researches only focus on the flow behaviors in single-dimension porous media.Besides,the physical experiment method is influenced by many uncertain factors and consumes a lot of time and cost.In order to deeply understand the flow behaviors in the process of CO_(2)geological storage in microscopic view and increase the volume of CO_(2)geological storage,this paper establishes 2D and 3D models by using VOF(Volume of Fluid)method which can track the dynamic change of two-phase interface,to numerically simulate supercritical CO_(2)-brine two-phase flow.Then,the distribution characteristics of CO_(2)clusters and the variation laws of CO_(2)saturation under different wettability,capillary number and viscosity ratio are compared,and the intrinsic mechanisms of CO_(2)storage at the pore scale are revealed.And the following research results are obtained.First,with the increase of rock wettability to CO_(2),the sweep range of CO_(2)enlarges,and the disconnection frequency of CO_(2)clusters deceases,and thus the volume of CO_(2)storage increase.Second,with the increase of capillary number,the displacement mode transforms from capillary fingering to stable displacement,and thus the volume of CO_(2)storage increases.Third,as the viscosity of injected supercritical CO_(2)gradually approaches that of water,the flow resistance between two-phase fluids decreases,promoting the"lubricating effect".As a result,the flow capacity of CO_(2)phase is improved,and thus the volume of CO_(2)storage is increased.Fourth,the influence degrees of wettability,capillary number and viscosity ratio on CO_(2)saturation are different in multi-dimensional porous media models.In conclusion,the CO_(2)-brine two-phase flow simulation based on VOF method reveals the flow mechanisms in the process of CO_(2)geological storage at the pore scale,which is of guiding significance to the development of CCUS technology and provides theoretical guidance and technical support for the study of CO_(2)geological storage in a larger scale.