【作者】 王超；

【导师】 苏小四；

【作者基本信息】 吉林大学 ， 地质工程， 2024， 博士

【摘要】 石油行业快速发展的同时,也引发了严重的土壤和地下水环境污染。包气带是污染物从地表进入含水层的关键地带,研究石油烃类污染物在包气带中的迁移机制对评估其对土壤的污染程度,预测其对地下水污染的潜在风险有重要意义。季节性冻土在我国广泛分布,面积约占我国国土总面积的一半,集中分布了我国重要油田开采区和石油化工区。受季节性冻融影响,以LNAPL(Light Non-Aqueous Phase Liquid)为主要成分的石油烃污染物在季节性冻土区包气带非饱和多孔介质中的迁移过程是一个包括水-冰-气-LNAPL在内的典型四相系统的动态变化过程。然而,由于难以设定与自然条件相一致的冻结/融化条件,以及缺乏合适的非破坏性LNAPL无损可视化定量实验研究方法,目前对冻融条件下LNAPL在非饱和多孔介质中迁移过程的研究仍处于探索阶段,对于季节性冻融期间LNAPL在包气带中的迁移规律和驱动机制尚不完全清楚,限制了LNAPL在土壤-地下水系统中传质过程的深刻理解。基于此,论文依托国家自然科学基金联合基金项目“寒区地下水有机污染溯源辨识、传质过程与原位修复及在线监测技术研究(U19A20107)”,选择东北季节性冻土区石油烃污染场地为研究区,在监测野外污染场地冻融前/后石油烃污染物迁移变化特征的基础上,开发可用于冻融条件下的无损可视化定量非饱和多孔介质四相系统(水-冰-气-LNAPL)中LNAPL饱和度的监测技术,通过室内冻融模拟实验,刻画季节性冻融过程作用下包气带内水分和LNAPL的动态迁移规律,探究冻结/融化过程中包气带内LNAPL再迁移的动力学过程、驱动机制和关键影响因素。为季节性冻土区土壤-地下水系统中石油烃污染物迁移预测和修复提供科学依据。通过本论文研究,主要获得以下结论和认识:(1)基于野外调查、动态监测和样品采集与测试等手段,分析了东北某典型石油烃污染场地包气带冻融特征和冻融期石油烃迁移特征。典型石油污染场地最大冻结深度130 cm,冻融期长达147天。包气带冻结融化过程中包括了稳定冻结阶段、不稳定冻结阶段、不稳定融化阶段和稳定融化阶段四个阶段。包气带石油烃污染物组成主要以烷烃类为主;经过一个年冻融循环后,包气带中石油烃污染物具有明显的向下运移趋势;冻融过程中水分迁移和水的相态改变是驱动包气带石油烃发生再迁移的重要因素。(2)提出了基于改进光透射法的刻画冻融过程中非饱和多孔介质内LNAPL迁移特征的无损可视化定量观测的新方法。在对传统光透射法改进的基础上,提出了可计算多孔介质中三相(水-气-LNAPL)和四相(水-冰-气-LNAPL)系统中LNAPL饱和度的数学模型,建立了冻融条件下无损可视化定量非饱和多孔介质中LNAPL的实验装置,并对改进光透射法的精度进行了效果验证。验证结果表明LNAPL实际添加量与计算量之间存在较强的相关性(R~2>0.996),改进光透射法具有较高的计算精度,可为冻融条件下非饱和多孔介质中LNAPL再迁移规律的实验研究提供技术支撑。(3)基于室内一维砂柱动态模拟实验,探究了季节性冻融过程中包气带中水分和LNAPL的动态迁移规律。实验结果表明,冻融作用对LNAPL迁移有明显的抑制作用,冻融组LNAPL的湿润锋最终迁移深度比非冻融组降低了33.6%;水分的入渗作用导致LNAPL沿砂柱纵向剖面呈“弓”形的分布趋势,冻结作用会使LNAPL在包气带垂向上分布更均匀,并沿冻结方向深处聚集。利用Green-Ampt模型分别模拟了冻融/非冻融两种条件下包气带中水分入渗和LNAPL入渗锋面深度随时间的变化过程,计算了不同冻融阶段内包气带中的LNAPL累积入渗量,冻融过程中的不稳定冻结阶段是LNAPL的主要入渗阶段,其入渗量总贡献率可达93.4%。(4)基于二维砂槽动态模拟实验,探究了季节性冻融循环对包气带中LNAPL再迁移的影响特征及机制。实验结果表明,在冻结作用下,LNAPL沿着冻结锋面向包气带深部再迁移,在融化后向包气带浅层迁移,最终在多次冻融循环后,LNAPL沿着冻结锋面自冻结区向未冻结区迁移;冻结锋面与LNAPL迁移的零通量面并不同步变化,表明LNAPL的迁移存在位置上的超前性。冻融过程中LNAPL再迁移主要受冻结诱导驱动力的控制,而非以往研究所指出的冻结微裂隙。(5)基于2.5D微流芯片技术实验技术,从孔隙微观尺度上探究了冻融过程中LNAPL在包气带内的再迁移机制。冻融过程中,LNAPL再迁移主要发生在冻结前期和融化后期。冻结诱导驱逐和冻结诱导截断是不连续离散状LNAPL(Ganglia)的再迁移机制。LNAPL的再迁移主要受冻结势、冻结诱导压力和毛细力共同作用。势能进行量化结果表明,毛细力受LNAPL性质、多孔介质孔隙(孔喉)半径控制;冻结势受介质类型、比表面积和溶质盐分控制;冻结诱导压力受初始LNAPL饱和度和冻结速率控制。更多还原

【Abstract】 With the rapid development of petroleum industry,environmental pollution of soil and groundwater is increasing.Unsaturated zone is the key area where pollutants enter the aquifer from the surface.It is important to study the mobilization mechanism of petroleum hydrocarbon pollutants in the unsaturated zone to assess the degree of soil pollution and predict the potential risk of groundwater pollution.The area of seasonal frozen soil accounts for about half of China’s total land area,and it is concentrated in the important oil field exploitation area and petrochemical industry area of China.Under the influence of seasonal freeze-thaw,the transport process of petroleum hydrocarbon pollutants with LNAPL(Light Non-Aqueous Phase Liquid)as the main component is a dynamic process,which is a four-phase system including water-ice-gas-LNAPL.However,it is difficult to set freezing/thawing conditions consistent with natural conditions,and suitable non-destructive visualization quantitative experimental research methods for LNAPL are still lacking.At present,the research on the migration process of LNAPL in unsaturated porous media under freezing and thawing conditions is still in the exploratory stage.The migration regularity and driving mechanism of LNAPL in the unsaturated zone during seasonal freeze-thaw are not fully understood,which limits the deep understanding of the mass transfer process of LNAPL in soil-groundwater system.Based on the above,this paper relies on the joint fund project of National Natural Science Foundation of China"Research on Source Identification,mass transfer process,in-situ remediation and online monitoring technology of groundwater Organic pollution in Cold Areas(U19A20107)",The site polluted by petroleum hydrocarbon in the seasonal frozen soil area of northeast China was selected as the study area,and on the basis of monitoring the migration and change characteristics of petroleum hydrocarbon pollutants in the field polluted site before/after freezing and thawing.The objective is to develop a non-destructive visualization and quantitative LNAPL saturation monitoring technology and 2.5D microfluidic technology in the four-phase system consisting of water-ice-gas-LNAPL under freeze-thaw conditions.Through freeze-thaw simulation experiments,the dynamic migration rules of water and LNAPL in the unsaturated zone under the seasonal freeze-thaw process are described.To explore the dynamic process,driving mechanism and key influencing factors of LNAPL remobilization in the unsaturated zone during freezing/thawing.so as to provide a scientific basis for the migration prediction and remediation of petroleum hydrocarbon pollutants in the soil-groundwater system in the seasonal permafrost area.Based on this research,the following conclusions and understandings are obtained:1.Based on field investigation,field dynamic monitoring,sample collection and laboratory testing,the freeze-thaw characteristics and the migration characteristics of petroleum hydrocarbons during freeze-thaw period in a typical petroleum hydrocarbon polluted site in Northeast China were analyzed.The maximum freezing depth of a typical oil-contaminated site is 130cm,and the freeze-thaw period is as long as 147 days.The freeze-thaw process of the unsaturated zone includes four stages:stable freezing stage,unstable freezing stage,unstable melting stage and stable melting stage.The oil hydrocarbon pollutants in the unsaturated zone are mainly alkanes.After an annual freeze-thaw cycle,petroleum hydrocarbon pollutants in the vadage zone have an obvious downward migration trend.The migration of water and the change of water phase state during freeze-thaw process are the important factors driving the remobilization of petroleum hydrocarbons in the unsaturated zone.2.A new lossless visual quantitative observation method for LNAPL migration characteristics in unsaturated porous media during freeze-thaw is proposed based on improved light transmission method.Based on the improvement of the traditional light transmission method,a mathematical model for calculating the saturation of LNAPL in three-phase(water-gas-LNAPL)and four-phase(water-ice-gas-LNAPL)systems in porous media is proposed,and an experimental device for non-destructive visualization and quantitative LNAPL in unsaturated porous media under freeze-thaw conditions is established.The accuracy of the improved light transmission method is verified.The experimental results showed that there was a strong correlation between the actual amount of LNAPL and the calculated amount(R~2>0.996).The improved light transmission method has a high calculation accuracy,and can provide technical support for the experimental study of LNAPL remobilization in unsaturated porous media under freeze-thaw conditions.3.Based on the indoor one-dimensional sand column dynamic simulation experiment with improved light transmission method,the dynamic migration law of water and LNAPL in the vespa zone during seasonal freeze-thaw process was explored.The final migration depth of LNAPL in the freeze-thaw condition was33.6%lower than that in the unfreeze-thaw condition.Water infiltration results in the distribution of LNAPL in a"bow"shape along the longitudinal section of the sand column,and freezing changes the distribution trend of LNAPL,making it more evenly distributed in the vertical direction,and accumulates deep along the freezing direction.Green-Ampt model was used to simulate the changes of water infiltration and LNAPL infiltration front depth with time under freeze-thaw/unfreeze-thaw condition,and the model was used to calculate the cumulative infiltration amount of LNAPL in different freeze-thaw stages.The unstable freezing stage in the freeze-thaw process was the main infiltration stage of LNAPL.The total contribution rate of infiltration can reach 93.4%.4.Based on two-dimensional sand tank dynamic simulation experiment,the influence characteristics and mechanism of seasonal freeze-thaw cycles on LNAPL remobilization in the unsaturated zone were explored.The experimental results show that LNAPL migrates to the depth along the freezing front under the action of freezing,migrates to the shallow layer of the unsaturated zone after melting,and finally migrates from the frozen zone to the unfrozen zone along the freezing front after several freeze-thaw cycles.The frozen front does not change synchronously with the zero flux surface of LNAPL migration,which indicates that LNAPL migration is advanced in position.The remobilization of LNAPL during freeze-thaw process is mainly controlled by freeze-induced driving forces,rather than frozen micro-cracks as previously suggested.5.Based on 2.5D microfluidic chip technology experimental technology,the remobilization mechanism of LNAPL in the vadicle zone during freeze-thaw process was explored from the pore micro-scale.During the freeze-thaw process,LNAPL remobilization mainly occurred in the early freezing period and the late thawing period.Freeze-induced expulsion and freeze-induced truncation are the remobilization mechanisms of discontinuous discrete LNAPL(Ganglia).The remobilization of LNAPL is mainly influenced by freezing potential,freezing induced pressure and capillary force.The quantization of potential energy shows that the capillary force is controlled by the LNAPL property and the radius of pore throat of porous media.Freezing potential is controlled by medium type,specific surface area and solute salinity.Freezing induced pressure is controlled by initial LNAPL saturation and freezing rate.更多还原