基于分子动力学方法的致密砂岩储层CO_(2)驱油研究进展

Research progress of CO_(2) flooding in tight sandstone reservoirs based on molecular dynamics method

随着致密砂岩储层CO_(2)-EOR项目的不断增加,亟需对CO_(2)与致密砂岩储层流体在孔隙介质中的相互作用机理进行研究,但室内实验难以观察到CO_(2)与流体在孔隙通道中的动态过程,而分子动力学模拟则可以弥补传统实验和数值方法的不足。为此,围绕该方法在CO_(2)-EOR研究中的2大重点——界面行为与受限空间运移行为,对包括萃取、溶胀、降黏、降低界面张力、最小混相压力、润湿性等6个方面界面行为的相关研究成果进行了系统梳理,结合前人对CO_(2)与致密砂岩储层流体在受限空间中运移行为的研究成果,提出了分子动力学在致密砂岩储层CO_(2)-EOR研究中的关键科学问题。研究结果表明:①分子动力学能够很好地将CO_(2)萃取原油中—轻烃组分的过程进行可视化,建议根据原油实际组分构成分析CO_(2)的萃取能力,从而弄清CO_(2)萃取原油中各组分的优先顺序及效率;②有关CO_(2)分子对原油溶胀降黏作用的研究,大多数成果中的原油组分体系较为单一,建议根据实际储层的原油组分,系统探讨高温高压条件下CO_(2)的溶胀降黏作用;③对于CO_(2)与油水两相之间界面张力的研究更多是在体相中进行展开,而在纳米受限空间界面行为的研究还有待完善;④有关地层水在孔隙壁面的润湿性研究,对不同因素之间的协同效应影响润湿性的机理尚不明确。结论认为,随着量子力学研究的不断深入以及计算性能的日益强大,该方法可以在更大尺度上进行复杂体系的计算,以其为基础的油藏、页岩油气藏CO_(2)-EOR机理等研究将会取得较大的突破。

With the increasing number of tight sandstone reservoir CO_(2)-EOR projects,it is in urgent need to study the interactive mechanisms of CO_(2) and tight sandstone reservoir fluids in porous media.In laboratory experiments,however,it is difficult to observe the dynamic processes of CO_(2) and fluids in pore channels,while molecular dynamics simulation can make up for the shortcomings of traditional experiments and numerical methods.Focusing on the two major points of molecular dynamics simulation in the research of CO_(2)-EOR,i.e.,interface behaviors and flow behaviors in confined spaces,this paper systematically reviews the research results related to interface behaviors from six aspects including extraction,swelling,viscosity reduction,interfacial tension reduction,minimum miscible pressure and wettability.And then combined with the previous research results of the flow behaviors of CO_(2) and tight sandstone reservoir fluids in confined spaces,the key scientific issues of molecular dynamics in the research of tight sandstone reservoir CO_(2)-EOR are pointed out.First,molecular dynamics can visualize the extraction process of medium-light hydrocarbon compositions from crude oil by CO_(2).It is suggested to analyze CO_(2)'s extraction capacity considering the actual crude oil compositions,so as to determine the extraction order and efficiency of each composition by CO_(2).Second,the research results related to the swelling and viscosity reduction effect of CO_(2) molecule on crude oil are mostly involved in the simple crude oil composition system.It is suggested to systematically discuss the swelling and viscosity reduction effect of CO_(2) under high temperature and high pressure based on the crude oil compositions of actual reservoirs.Third,the interfacial tension between CO_(2) and oil-water two phases is studied mostly in gas phase,and it is necessary to further study the interface behaviors in nano-scale confined spaces.Fourth,as for the wettability of formation water on the surface of pore wall,the influential mechanisms of synergistic effects between different factors on wettability have not been figured out.In conclusion,as quantum mechanics is studied further and the calculation capacity gets stronger,this method can be used for the calculation of complex systems in a larger scale,and the researches on CO_(2)-EOR mechanisms in oil reservoirs and shale oil and gas reservoirs based on it will achieve great breakthroughs.