【作者】 王婷；

【导师】 陈益峰； 胡冉；

【作者基本信息】 武汉大学 ， 水工结构工程， 2021， 博士

【摘要】 岩体裂隙的溶蚀广泛存在于自然界和工业过程中,如岩溶地貌的形成、可溶岩区高坝工程长期防渗安全、CO₂地质封存以及石油注酸开采等。无论是自然过程还是工业过程,一旦岩体裂隙中的水流具有溶蚀特性,持续的水流作用将逐渐溶蚀裂隙介质中的可溶性矿物,形成不同的溶蚀模式,直接决定了渗透特性的演化特征。另一方面,介质渗透特性的改变将进一步影响反应物质的运移,进而影响溶蚀模式的形成及其转变。由此可见,岩体裂隙溶蚀模式与渗透率演化规律,是开展可溶岩区裂隙介质渗流与溶蚀过程分析的基础。受限于可视化实验技术的制约,以往研究主要针对光滑平行板或多孔介质,聚焦流速等单个因素对溶蚀过程的影响,复杂条件下粗糙裂隙溶蚀模式与渗透性演化规律的研究,还不够深入。为此,本文以可溶岩粗糙裂隙为研究对象,以溶蚀模式及渗透率演化为关键科学问题,采用可视化实验、理论分析、数值模拟相结合的方法,深入研究流速、流体性质和介质特性等复杂条件下粗糙裂隙溶蚀形态的动态特征、溶蚀模式的理论判据、考虑溶蚀模式的渗透性演化模型等。主要创新成果如下:（1）研发了粗糙裂隙溶蚀可视化技术,实现了对溶蚀过程的高精度、实时动态以及可重复观测可视化观测是粗糙裂隙溶蚀机理研究的关键技术。然而,目前广泛应用的CT扫描技术存在成像时间长,无法开展实时动态观测,而光透射方法则存在粗糙裂隙重复制备难的问题。为此,研发了粗糙裂隙溶蚀可视化实验装置,实现了成像精度达25μm/像素、10帧/秒的实时动态观测,研发了粗糙裂隙样品的高精度、可重复制备技术（重构精度达10μm、相对误差小于5%）,形成了集动态观测、样品制备和图像后处理为一体的溶蚀可视化实验技术,克服了传统可视化技术难以实现实时、可重复观测粗糙裂隙溶蚀过程的缺陷,为复杂条件下裂隙溶蚀模式研究提供了一种有效的研究手段。（2）揭示了复杂条件下粗糙裂隙溶蚀形态的动态特征,首次发现了裂隙溶蚀热点新现象岩体裂隙溶蚀的影响因素众多,溶蚀形态极为复杂。受到可视化实验技术的制约,目前有关溶蚀的研究主要集中在光滑平行板或者多孔介质,复杂条件下粗糙裂隙溶蚀形态及其动态特征的研究,极其不深入。为此,基于自主研发的溶蚀可视化实验平台,开展了不同流速、不同溶蚀速率、不同开度、不同粗糙度等复杂条件下溶蚀形态的实验研究,揭示了紧凑溶蚀、虫洞溶蚀、均匀溶蚀等三种溶蚀模式的时空分布特征;在此基础上,首次发现了粗糙裂隙溶蚀热点新现象,阐明了流速和裂隙开度对溶蚀热点发生位置的重要影响。研究成果为复杂条件下溶蚀模式理论判据的构建提供了关键实验支撑。（3）建立了考虑复杂影响因素的溶蚀模式理论判据,提出了溶蚀热点发生位置的理论预测模型溶蚀模式的形成及其转变决定了渗透特性的演化。因此,溶蚀模式理论判据是可溶岩裂隙介质渗流和溶蚀过程分析的基础。传统的判据主要针对圆管、光滑平行板等简单的几何结构,粗糙裂隙溶蚀模式的理论判据研究,还不够深入。为此,基于成果（2）中对溶蚀形态特征的认识,通过对溶蚀形态开展特征长度分析,建立了考虑流速、溶蚀速度、裂隙开度和裂隙粗糙度等复杂影响因素的溶蚀模式理论判据,揭示了复杂条件下紧凑溶蚀、虫洞溶蚀、均匀溶蚀等溶蚀模式形成的临界条件;在此基础上,揭示了重力效应引起的浮力对流是溶蚀热点形成的本质原因,进而提出了预测溶蚀热点发生位置的理论模型。研究成果为溶蚀过程中裂隙介质渗透率演化规律研究奠定了基础。（4）建立了考虑溶蚀模式的渗透率演化模型,确定了不同溶蚀模式对应的模型参数取值范围溶蚀过程中裂隙介质的渗透特性显著偏离经典的立方定律模型,亟需构建考虑溶蚀效应的裂隙介质渗透率演化模型。为此,基于（3）中对溶蚀模式及其转变规律的认识,并结合基于纳维-斯托克斯方程和对流扩散方程耦合的溶蚀过程数值模拟方法,分析了复杂条件下（流速、裂隙开度、裂隙粗糙度以及重力等）裂隙介质渗透性演化特征,建立了考虑溶蚀模式的裂隙介质渗透率演化模型,揭示了渗透率变化率与开度变化率之间的非线性关联,进而确定了不同溶蚀模式对应的模型参数取值范围,为可溶岩裂隙介质的渗流与溶蚀过程分析提供关键参数。研究成果揭示了复杂条件下粗糙裂隙的溶蚀模式及其渗透率演化规律,对岩溶地区坝基防渗安全评价、CO₂地质封存泄露风险评价、注酸油气增采措施优化等具有重要意义。更多还原

【Abstract】 The dissolution of fractures widely exists in nature and industrial processes,such as karst formation,the long-term safety of high dams in karst areas,CO₂ geological storage,and acid-injection enhanced oil recovery.Whether in a natural process or an industrial process,if the flowing fluid is reactive,such fluid injection into fractured reservoirs would expand the fracture aperture and produce complex dissolution patterns,which will change permeability of rocks significantly.On the other hand,the change of permeability of fractures will further affect the transport of reactive fluid and then affect the formation and transition of dissolution patterns.Therefore,the study of the dissolution patterns and permeability evolution of fractures are the basis of analyzing the seepage and dissolution of fractures in soluble areas.Limited by visualization of experiments,previous studies mainly focused on the influence of single factors such as flow rate on the dissolution process in a Hele-Shaw cell or in porous media,and the study on the dissolution patterns and evolution of permeability in rough fractures under complex conditions is not sufficiently studied.In this paper,we investigate the coupling effect of flow and dissolution using the soluble rock fissures by the combination of experiments,theoretical analysis and numerical simulation.We deeply studied the dynamic characteristics of dissolution patterns,proposed the theoretical criterion of dissolution patterns and permeability evolution model considering dissolution patterns under complex conditions such as flow rate,fluid properties and medium properties.The main innovations are as follows:（1）The visualization technology of dissolution in rough fractures is developed to realize real-time and repeatable observation of the dissolution process with high precisionVisual observation is the key technology for quantifying the dissolution patterns in dissolving fractures.However,CT scanning technology which is widely used at present,usually need a long imaging time and is unable to carry out real-time dynamic observation.Further,the light transmission method has difficulty in repeated experiments of soluble cracks.To this end,a flow-visualization experiment device for dissolution of rough fractures has been developed to realize real-time dynamic observation with an imaging accuracy of 25μm/pixel and 10 frames/second.A repeatable preparation technology with a high precision of soluble fracture samples（a reconstruction accuracy of 10μm and a relative error less than 5%）has been developed.So far,a visualization technology for dissolution of rough fractures integrating dynamic observation,sample preparation and image post-processing has been formed.It overcomes the shortcoming of traditional visualization technology,which is difficult to realize real-time and repeatable observation of the dissolution process of rough fractures.This visualization device provides an effective research method for studying dissolution patterns of fractures under complex conditions.（2）The dynamic characteristics of dissolution morphology in rough fractures under complex conditions are revealed,and a new phenomenon of dissolution hotspots in fractures are observed for the first timeThere are many factors affecting the dissolution of fractures,and the dissolution morphologies are very complex.Limited by visualization of experiments,previous studies mainly focused on the influence of single factors such as flow rate on the dissolution process in a Hele-Shaw cell or in porous media,and the study on the dissolution patterns and evolution of permeability in rough fractures under complex conditions is not sufficiently studied.Therefore,based on the independently developed visualization system,experimental studies on dissolution morphology under complex conditions such as different flow rates,different dissolution rates,different opening degrees and different roughness were carried out to reveal the spatial and temporal distribution characteristics of the three dissolution patterns,namely compact,wormhole and uniform dissolution.On this basis,a new phenomenon of dissolution hotspots in fractures is observed for the first time,and the important influence of flow rate and aperture on the location of dissolution hotspots is illustrated.This research provides key experimental support for the establishment of a theoretical criterion of dissolution patterns under complex conditions.（3）A theoretical criterion of dissolution patterns considering complex factors is established,and a theoretical prediction model of the location of dissolution hotspots is proposedThe formation and transition of dissolution patterns determine the evolution of permeability characteristics.Therefore,the theoretical criterion of dissolution patterns is the basis for the analysis of flow and dissolution process in soluble fractures.The traditional criterion mainly focuses on simple geometrical structures such as a circular pipe and a Hele-Shaw cell.The theoretical criterion for dissolution patterns in rough fractures has not been studied thoroughly.Therefore,based on the results（2）of the identification of dissolution morphologies,a theoretical criterion for dissolution patterns is established by the analysis of characteristic lengths.The theoretical criterion considers complex factors,such as the flow rate,the dissolution rate,the aperture,and the roughness.It then determines the critical conditions for the formation of compact,wormhole and uniform dissolution under complex conditions.On this basis,it is revealed that buoyancy convection caused by gravity is the essential factor of the dissolution hotspots.And a theoretical model for predicting the location of dissolution hotspots is put forward.The research results lay a foundation for the study of the permeability evolution of fractures in the process of dissolution.（4）The model for permeability evolution considering the dissolution patterns is established,and the range of parameters in the permeability evolution model under different dissolution patterns is determinedIn the process of dissolution,the permeability evolution of fractures deviates significantly from the classical cubic law,so it is urgent to establish a permeability evolution model of fractures considering dissolutions.Therefore,based on the result（3）about the dissolution patterns and their transitions,the permeability evolution characteristics in fractures under dissolutions is investigated by combining the numerical simulation method of dissolution process based on navier-stokes equations,A theoretical model of permeability evolution is established considering dissolution patterns under the complex conditions（flow rate,aperture,roughness of fracture and gravity）.The nonlinear relation between the rate of change of permeability and change of aperture is revealed,and the value ranges of model parameters corresponding to different dissolution patterns are determined.This research provides key parameters for the analysis of flow and dissolution processes in the fractured media.The research reveals the mechanics of dissolution patterns and permeability evolution in rough fractures under complex conditions.The results are of great significance to the engineering applications,such as the safety evaluation of high dams in karst areas,the risk assessment of CO₂ geological storage,and the optimization of enhanced oil recovery.更多还原