裂隙岩体渗流概念模型研究

裂隙岩体中的渗流和传统的多孔介质渗流在机理上存在本质的差别,这种差别主要表现为裂隙岩体在各种尺度上存在的非均质性。模拟裂隙岩体渗流的主要困难在于描述这种非均质性。目前的概念模型,包括等效连续体模型、离散裂隙网络模型和混合模型使用了不同的技术来预测裂隙岩体中的渗流。这些模型基于不同的假设和概念框架,有着各自不同的优缺点。在实际应用时,应当根据研究域的具体特点和所要解决的问题的要求对其选择,此外,还讨论了单裂隙的概念模型。     

Spatial statistics for predicting fluid flow through a single rock fracture

Steady-state laminar flow through single rock fractures is predicted in terms of spatial statistics computed from the arrangement of voids and contact areas within the fracture. Within the voids, aperture is assumed to be constant. One statistic measures how often pixels alternate from void to contact area in the rows parallel to the flow direction. Two others measure the dispersion of voids in the rows and columns of the pattern. Fractures with complexity typical of observed data are simulated. Flow through patterns with 80% voids is predicted in terms of a linear combination of the three statistics. Using an extended model involving one of the three statistics, flow through patterns with other void fractions is predicted.The author did this work at the Earth Sciences Division at Lawrence Berkeley Laboratory, Berkeley, California. It is part of a Ph.D. thesis which was submitted at Stanford University.     

碳酸盐岩单裂隙渗流-溶蚀耦合模型及其参数敏感性分析

岩溶地下水系统是由碳酸盐岩裂隙含水介质演化形成的,系统初始的裂隙网络介质特征及边界条件决定了其演化过程。为揭示岩溶系统演化过程中裂隙介质特征和边界条件的影响程度,建立了裂隙溶蚀扩展的渗流-溶蚀耦合模型,并对不同边界条件下不同隙宽的单裂隙溶蚀扩展特征进行了模拟分析。结果表明:裂隙溶蚀扩展受水的侵蚀性(CO2分压)、水动力条件(水力梯度)、裂隙介质特征(裂隙初始隙宽)等综合作用影响,Ca2+的平衡浓度、水力梯度以及裂隙初始隙宽等参数的增加均能促进裂隙的快速扩展。在这些参数中,初始隙宽B0对岩溶发育的影响最为敏感,水力梯度J和Ca2+平衡浓度Ceq对岩溶发育具有相同的敏感性;此外,随着各参数值的不断增大,参数变化对岩溶发育的敏感程度越来越低。     

岩体黏弹性蠕变计算的高阶数值流形法研究

数值流形法（numerical manifold method）是一种新型的数值计算方法,已成功应用于岩土工程的诸多领域,但该方法尚未应用于岩土工程蠕变分析。近年来对高阶流形法的研究表明,对复杂的岩土工程问题,使用高阶覆盖函数可明显提高流形法的计算精度。为此,开展了用高阶流形法模拟蠕变的研究,在高阶流形法中引入“时步-初应变”法计算蠕变,以广义开尔文体为基础,推导了相关的计算公式,并编制了相应的计算程序,同时还通过算例,验证了方法的可行性和合理性。结果表明,高阶流形可以方便地与“时步-初应变”法结合用于蠕变计算,可较好地模拟蠕变变形。算例分析表明,在不改变网格密度情况下,仅通过采用高阶覆盖函数,高阶流形法可大幅提高传统流形法的计算精度。     

黏弹性节理岩体的位移不连续模型研究

选用三参数标准线性固体作为岩石本构,提出了一种考虑岩体黏弹性的位移不连续模型;根据一维黏弹性波的特征线法,推导了节理处质点速度、应力和应变递推公式。首先,基于分离式霍普金森压杆（SHPB）对砂层进行试验,得到其应力-应变关系,并换算出砂层节理的法向刚度;接着,通过一维强间断黏弹性波的波速公式、高频波衰减系数以及任一频率下的衰减系数,确定出数值算法中的三参数。最后,基于自制的摆锤装置,探讨了一维应力波在节理岩体中的传播规律,试验中以两根长1 000 mm、直径为68.50 m的岩杆作为入射和透射杆,以3 mm砂层模拟节理。试验和数值结果吻合度良好,进一步验证了该方法的可靠性。     

岩石非线性黏弹性蠕变特性的时温等效效应

岩石的时温等效效应是指岩石在低温、长时间尺度下与在高温、短时间尺度下的流变力学特性等效的效应。利用岩石的时温等效效应,可以通过提高试验温度方法得到岩石在相对低温下的长期流变力学特性。为研究岩石在非线性黏弹性条件下的时温等效效应,基于Schapery单积分型的非线性本构方程,得到了岩石非线性黏弹性蠕变力学特性时温等效效应的数学表示。根据不同应力及温度下花岗岩的蠕变试验结果,设定20 ℃为参考温度,考虑温度改变对岩石瞬时蠕变柔量的影响,得到了垂直移位因子随温度的变化规律及修正后的蠕变柔量的时间对数曲线;对修正后的曲线进行水平移位使得相邻温度曲线之间相交于一点,得到不同应力下蠕变柔量的主曲线及水平移位因子的变化规律;主曲线中相邻温度曲线之间的一致性较好,进一步验证了花岗岩时温等效效应的存在性。通过对不同应力下的垂直移位因子和水平移位因子的变化规律分析,发现应力为40～60 MPa之间的非线性并不明显,但到87 MPa,非线性特性明显增强。     

岩石中的渗透率、流体流动及热液成矿作用

热液成因的固体金属矿床形成过程中都曾拥有一段流动的历史。成矿物质以溶液的形式从矿源地穿过孔隙或断裂达到了现在的栖身之所,在此过程中流体通过的路径决定了它们的走向,而溶液本身也改变了路径的特征。因此,对于此类矿床的研究不能仅仅囿于眼之所见,而应将古流体流动的观念融入其中,并考察它们的跋涉之路。文中总结了近年来国内外在岩石孔隙、断裂介质及与其相关渗透率对热液成矿作用的控制等领域的研究成果和最新进展,对岩石渗透率、渗透率的影响因素、岩石中流体流动的特征等问题进行了阐述,讨论了渗透率对于热液成矿作用深度、成矿规模、矿质沉淀机制等方面的重要影响,藉此在固体金属矿床研究中强化流"体运动"的观念,并促进构造地质学和成矿作用地球化学间一个新研究方向的发展。     

Numerical modelling of fluid flow in microscopic images of granular materials

A program for the simulation of two‐dimensional (2‐D) fluid flow at the microstructural level of a saturated anisotropic granular medium is presented. The program provides a numerical solution to the complete set of Navier–Stokes equations without a priori assumptions on the viscous or convection components. This is especially suited for the simulation of the flow of fluids with different density and viscosity values and for a wide range of granular material porosity. The analytical solution for fluid flow in a simple microstructure of porous medium is used to verify the computer program. Subsequently, the flow field is computed within microscopic images of granular material that differ in porosity, particle size and particle shape. The computed flow fields are shown to follow certain paths depending on air void size and connectivity. The permeability tensor coefficients are derived from the flow fields, and their values are shown to compare well with laboratory experimental data on glass beads, Ottawa sand and silica sands. The directional distribution of permeability is expressed in a functional form and its anisotropy is quantified. Permeability anisotropy is found to be more pronounced in the silica sand medium that consists of elongated particles. Copyright © 2001 John Wiley & Sons, Ltd.     

Mechanical controls on fluid flow during regional metamorphism: some numerical models

The control of fluid flow by plastic deformation during metamorphism is critical to our understanding of metamorphic processes. Various geological observations and field studies demonstrate the consequences of fluid flow control by deformation, so that the concept appears to be accepted, at least for small-scale PUBLIC (for example faults and vein PUBLIC). However, the concept appears to be less well recognized at regional scales. Considered here are examples of simple, conceptual models based on fully coupled mechanical–fluid flow concepts; they include deformation of a section of fluid-saturated crust containing a block or a layer of material of different properties from its surrounds. In particular, rheological and permeability contrasts between rock types during deformation associated with regional metamorphism are sufficient to control the form of fluid flow over the range of a few kilometres. Low contrasts and small strains allow pervasive fluid flow, whereas greater contrasts and increasing strains cause focusing of the flow. Such focusing is generally associated with localization of the deformation, especially for a strongly dilatant elastic–plastic material. However, a rate of fluid flow much greater than the rate of deformation may result in pervasive flow, although for most models pervasive flow is difficult to attain over regional distances. Furthermore, lateral and downward fluid flow may occur, demonstrated here by simple models for folding and for deformation of regions containing plutons. Therefore, such modelling may be used as a means of testing the various hypotheses concerning the volumes of fluid predicted to have passed through some rock volumes. Numerical models of the future will become increasingly complex and powerful, allowing greater coupling of thermal, mechanical, chemical and fluid flow effects, and based more on the physical processes involved. Combined field and laboratory studies will provide correspondingly greater understanding and will permit the determination of the timing of fluid flow and structural controls on fluid flow patterns.     

Transverse and longitudinal fluid flow modelling in fractured porous media with non‐matching meshes

A new discrete fracture model is introduced to simulate the steady‐state fluid flow in discontinuous porous media. The formulation uses a multi‐layered approach to capture the effect of both longitudinal and transverse permeability of the discontinuities in the pressure distribution. The formulation allows the independent discretisation of mesh and discontinuities, which do not need to conform. Given that the formulation is developed at the element level, no additional degrees of freedom or special integration procedures are required for coupling the non‐conforming meshes. The proposed model is shown to be reliable regardless of the permeability of the discontinuity being higher or lower than the surrounding domain. Four numerical examples of increasing complexity are solved to demonstrate the efficiency and accuracy of the new technique when compared with results available in the literature. Results show that the proposed method can simulate the fluid pressure distribution in fractured porous media. Furthermore, a sensitivity analysis demonstrated the stability regarding the condition number for wide range values of the coupling parameter.     

Response of sand ripples to change in oscillatory flow

Ripples take time to evolve to a new equilibrium state in response to a change in wave-generated oscillatory flow. The paper presents results from flow tunnel experiments designed to examine oscillatory flow transient ripple processes under controlled, full-scale laboratory conditions. The experiments include study of the growth of ripples from flat bed and the evolution of existing ripples to new equilibrium ripples in response to a step change in the flow. In general, ripples evolve through a combination of two main processes: (i) from a flat bed or from a bed consisting of ripples that are smaller than the equilibrium ripples through a combination of 'slide' and 'merge'; (ii) from a bed consisting of ripples that are larger than the equilibrium ripples through a combination of 'split' and 'merge'. The experimental results show that equilibrium ripple geometry is independent of initial bed morphology while the time to reach equilibrium is largely independent of the initial bed and the equilibrium ripple size. The time to reach equilibrium depends strongly on the mobility number, and a new empirical equation relating mobility number and the number of flow cycles to equilibrium is proposed. This equation is combined with a simple exponential function for ripple height growth or decay to produce a new empirical model for ripple height evolution, which gives a reasonably good overall agreement with the measurements. The model is based on experiments involving one sediment size only and further work is needed to develop the model for other sand sizes.     

Modelling hydraulic and capillary-driven two-phase fluid flow in unsaturated concretes at the meso-scale with a unique coupled DEM-CFD technique

The goal of the research was to demonstrate the impact of thin porous interfacial transition zones (ITZs) between aggregates and cement matrix on fluid flow in unsaturated concrete caused by hydraulic/capillary pressure. To demonstrate this impact, a novel coupled approach to simulate the two-phase (water and moist air) flow of hydraulically and capillary-driven fluid in unsaturated concrete was developed. By merging the discrete element method (DEM) with computational fluid dynamics (CFD) under isothermal settings, the process was numerically studied at the meso-scale in two-dimensional conditions. A flow network was used to describe fluid behaviour in a continuous domain between particles. Small concrete specimens of a simplified particle mesostructure were subjected to fully coupled hydro-mechanical simulation tests. A simple uniaxial compression test was used to calibrate the pure DEM represented by bonded spheres, while a permeability and sorptivity test for an assembly of spheres was used to calibrate the pure CFD. For simplified specimens of the pure cement matrix, cement matrix with aggregate, and cement matrix with aggregate and ITZ of a given thickness, DEM/CFD simulations were performed sequentially. The numerical results of permeability and sorptivity were directly compared to the data found in the literature. A satisfactory agreement was achieved. Porous ITZs in concrete were found to reduce sorption by slowing the capillary-driven fluid flow, and to speed the full saturation of pores when sufficiently high hydraulic water pressures were dominant.     

不同建模假定下岩石损伤黏弹模型的参数转换关系探讨

将外界因素对岩石变形的影响考虑为瞬间弹性损伤和长期蠕变损伤,并根据加载瞬间弹性模量及蠕变模量的劣化分别对瞬间弹性损伤变量和蠕变损伤变量进行定义,由此建立了考虑损伤的kBurgers模型。在此基础上,通过假定泊松比和体积模量为常量分别得到了对应的模型三维表达式,并得到了不同建模思路下模型参数间的换算关系。最后结合实际算例,对常体积模量假定下模型参数随含水率变化的损伤演化规律进行了分析,获得了常体积模量假定下模型的瞬间弹性损伤和长期蠕变损伤演化方程,检验了模型参数换算关系的有效性。     

粘弹性圆形巷道支护结构上的荷载及其围岩应力的解析解

叙述了采用积分算子解法时弹性解与粘弹性解之间的对应关系;具体求算出H-K体的蠕变核与松弛核;借助带有弹性支护的粘弹性围岩应力解析解,计算分析了支护结构上荷载以及围岩应力的分布形式与变化规律,对围岩的应力调整有了更深一步的认识,以便更好地为衬砌设计服务。     

Review and classification of structural controls on fluid flow during regional metamorphism

Mechanisms for kilometre-scale, open-system fluid flow during regional metamorphism remain problematic. Debate also continues over the degree of fluid flow channellization during regional metamorphism, and the mechanisms for pervasive fluid flow at depth. The requirements for pervasive long-distance fluid flow are an interconnected porosity and a large regional gradient in fluid pressure and hydraulic head (thermally or structurally controlled) that dominates over local perturbations in hydraulic head due to deformation. In contrast, dynamic or transient porosity interconnection and fluid flow accompanying deformation of heterogeneous rock suites should result in moderately to strongly channellized flow at a range of scales, of which there are many examples in the literature. Classification of fluid flow types based on scale and degree of equilibration between fluid and rock, wallrock permeability, and mode of fluid transport contributes to an understanding of key factors that control fluid flow. Closed-system fluid behaviour, with restricted fluid flow in microcracks or cracks and limited fluid–rock interaction, occurs over a range of strains and crustal depths, but requires low permeabilities and/or small fluid fluxes. Long-distance, open-system fluid flow in channels is favoured in heterogeneous rocks at high strains, moderate (but variable) permeabilities, and moderate to high fluid fluxes. Long-distance, broad, pervasive fluid flow during regional metamorphism requires that the rocks are not accumulating high strains and have high permeabilities, low permeability contrasts, and high fluid fluxes. The ideal situation for such fluid flow is in situations where the rocks are undergoing stress relaxation immediately after a major deformation phase. In the mid-crust, fairly specific conditions are thus required for pervasive fluid flow. During active orogenesis, structurally controlled fluid flow (with focused open systems surrounding regions of closed-system behaviour) predominates in most, but not all, regional metamorphic situations, at a range of scales.     

Structurally controlled fluid flow associated with breccia vein formation

A breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein-fill is coarse grained and equigranular, suggesting precipitation in a fluid-filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ¹³C values of -11.1 ± 0.1% and δ¹⁸O values of 6-10%_o, whereas the host metapelite has δ¹³C values of -10.62 and -10.11% and δ¹⁸O values of 14-15%_o. These values are consistent with an igneous-derived, H₂O-dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large-volume fluid flow within the shear zone.     

Competition between topography- and compaction-driven flow in a confined aquifer: Some analytical results

 An analysis is presented of the competition between topography- and compaction-driven flow in a confined aquifer. The aquifer crops out and is unconfined in an area with an elevated water table. In the absence of compaction, the elevated water table drives the flow toward areas with a lower water table, and leakage occurs from the aquifer through an overlying aquitard. Under these conditions, recharge occurs in the outcrop area. When sedimentation occurs above the aquifer away from the outcrop area, compaction tends to reverse the flow direction in the aquifer, causing expulsion of groundwater in the outcrop area. The actual flow field arises from the competition between the two driving mechanisms. An analytical expression for the combined flow field is derived in which the contribution from both mechanisms is apparent. The expression is useful as a practical guideline for estimating whether or not compaction can be safely ignored in regional groundwater-flow studies of confined aquifers. Received, July 1998 / Revised, February 1999 / Acccepted, February 1999     

Contact mechanism of a rock fracture subjected to normal loading and its impact on fast closure behavior during initial stage of fluid flow experiment

Fast closure of rock fractures has been commonly observed in the initial stage of fluid flow experiments at environmental temperatures under low or moderate normal stresses. To fully understand the mechanisms that drive this fast closure, the evolution of local stresses acting on contacting asperities on the fracture surfaces prior to fluid flow tests needs to be evaluated. In this study, we modeled numerically the asperity deformation and failure processes during initial normal loading, by adopting both elastic and elastic–plastic deformation models for the asperities on a real rock fracture with measured surface topography data, and estimated their impact on initial conditions for fluid flow test performed under laboratory conditions. Compared with the previous models that simulate the normal contact of a fracture as the approach of two rigid surfaces without deformations, our models of deformable asperities yielded smaller contact areas and higher stresses on contacting asperities at a given normal stress or normal displacement. The results show that the calculated local stresses were concentrated on the contacts of a few major asperities, resulting in crushing of asperity tips. With these higher contact stresses, however, the predicted closure rates by pressure solution are still several orders of magnitude lower than that of the experimental measurements at the initial stage of fluid flow test. This indicates that single pressure solution may not likely to be the principal compaction mechanism for this fast closure, and that the damages on contacting asperities that occur during the initial normal loading stage may play an important role. Copyright © 2015 John Wiley & Sons, Ltd.     

火成岩中的超临界流体晶及其研究意义

流行火成岩理论中,岩浆被默认为自然熔体,因而火成岩中的矿物晶体都形成于熔体的结晶作用,可称为熔体晶。许多证据表明,火成岩中也可以含有从超临界流体析出的晶体,被称为超临界流体晶(文中简称为流体晶)。根据三个典型实例分析,流体晶既可以从超临界流体直接析出,类似于从热液析出晶体的过程;也可以由超临界流体浓缩产生的熔体结晶形成。不管是哪一种晶出方式,流体晶产生的前提都是岩浆达到流体过饱和态;而满足这一前提的基本条件则是透岩浆流体过程和岩浆快速上升。结合前人关于熔体流体平衡条件的研究进展,以及熔体黏度对挥发分含量和岩浆上升速度对熔体黏度的依赖,发现透岩浆流体过程与岩浆上升过程之间具有耦合关系,这种关系可以用来阐明岩浆系统行为非线性变化的原因。流体晶的研究具有重要意义,可用于：(1)提供一种研究岩浆系统流体条件的新途径;(2)揭示岩浆系统偏离理想态的程度;(3)为反演岩浆系统动力学过程提供新的约束;(4)为识别致矿侵入体和评估侵入体的找矿潜力提供新的矿物学标志;(5)理解火成岩理论中一些长期得不到解决的问题,如岩浆中挥发分溶解度问题、冻结岩浆的活化问题、岩浆成矿专属性问题。