A fracture mapping and extended finite element scheme for coupled deformation and fluid flow in fractured porous media

This paper presents a fracture mapping (FM) approach combined with the extended finite element method (XFEM) to simulate coupled deformation and fluid flow in fractured porous media. Specifically, the method accurately represents the impact of discrete fractures on flow and deformation, although the individual fractures are not part of the finite element mesh. A key feature of FM‐XFEM is its ability to model discontinuities in the domain independently of the computational mesh. The proposed FM approach is a continuum‐based approach that is used to model the flow interaction between the porous matrix and existing fractures via a transfer function. Fracture geometry is defined using the level set method. Therefore, in contrast to the discrete fracture flow model, the fracture representation is not meshed along with the computational domain. Consequently, the method is able to determine the influence of fractures on fluid flow within a fractured domain without the complexity of meshing the fractures within the domain. The XFEM component of the scheme addresses the discontinuous displacement field within elements that are intersected by existing fractures. In XFEM, enrichment functions are added to the standard finite element approximation to adequately resolve discontinuous fields within the simulation domain. Numerical tests illustrate the ability of the method to adequately describe the displacement and fluid pressure fields within a fractured domain at significantly less computational expense than explicitly resolving the fracture within the finite element mesh. Copyright © 2013 John Wiley & Sons, Ltd.     

某水电站坝基G23挤压蚀变破碎带成因及工程性质分析

G23挤压蚀变破碎带是某水电站坝基的主要工程地质问题。通过对该水电站枢纽区新构造运动及构造体系特征的综合分析,探讨了G23挤压蚀变破碎带的成因机制。基于典型岩样的物相及微形貌试验成果,分析了G23带构造岩的岩性特征,明确了G23带岩体的物理性质及其转变特征,预测了该挤压蚀变破碎带在工程作用下的变化趋势。结合蚀变岩样三轴压缩试验的变形破坏规律,阐述了G23带对工程作用的响应及其对工程的影响。     

非饱和裂隙岩体渗流的统计模型研究方法

常规方法研究非饱和裂隙介质往往采用宏观连续体的概念,然而现场试验证明,非饱和裂隙岩体中的渗流具有相当的非均质性。天然裂隙岩体的非饱和渗流是发生在三维裂隙网络中的多场非等温流,要在不完整的信息基础上刻画和表现介质的不均匀性,通常借助随机模拟的手段。将三维裂隙系统近似为概化的二维非均匀多孔介质的二维平面裂隙。采用模拟退火法将取自各种来源的信息资料通过建立适当的目标函数汇集至模型之中,以此模拟裂隙的空间特征。     

Boundary element analysis of contaminant transport in fractured porous media

A two-dimensional boundary integral method to analyse the flow of contaminant in fractured media having a two- or three-dimensional orthogonal fracture network is presented. The method assumes that the fractures provide the paths of least resistance for transport of contaminants while the matrix, because of its low permeability, acts as ‘storage blocks’ into which the contaminant diffuses. Laplace transform is used to eliminate the time variable in the governing equation in order to facilitate the formulation of a boundary integral equation in the Laplace transform space. Conventional boundary element techniques are applied to solve for the contaminant concentrations at specified locations in the spatial domain. The concentration in the time domain is then obtained by using an efficient inversion technique developed by Talbot. The method is able to analyse the behaviour of waste repositories which have diminishing concentration due to the mass transport of the contaminant into the surrounding fractured media.     

Contaminant migration in a regular two- or three-dimensional fractured network: Reactive contaminants

A semi-analytic solution for the migration of dilute reactive contaminant species in a fractured medium having a one-, two- or three-dimensional fracture network is presented. The application of the technique to practical problems is discussed and illustrated by examples. The effects of sorption in the matrix and on the surface of the fractures are considered for a range of parameters. It is demonstrated that the synergistic interaction of the various parameters is such that the effect of changing a single variable may not be immediately obvious and a designer should perform computations for the range of expected parameters to determine the maximum impact. For example, it is shown that it is not necessarily conservative to consider only one-dimensional or only three-dimensional diffusion into the matrix; which of these cases is most critical will depend on the values of other parameters. It is also shown that an effective leachate collection system may significantly reduce the impact of those contaminants which do escape into the fractured media.     

深井地下灌注数值模型SWIFT

深井地下灌注是一种有效处理工业废液的方法,随着认识的不断深入和技术的不断提高,将成为未来我国对难处理、高毒性废液实行最终处理的重要选择之一。SWIFT数值模型能够同时模拟灌注层中水流运动、热量运移、盐分运移和放射性核素运移过程,并且能够充分考虑深部灌注层高温高压等特殊地质环境。其多方面的模拟功能不但适用于深井灌注的数值模拟,还可用于海水入侵、高放射性核废料填埋的模拟分析。在美国,该数值模型已经被用于多个深井灌注项目的数值模拟。对SWIFT进行了较全面的介绍,并指出该模型使用过程中应特别注意的几个问题。     

断裂多孔岩石中的复杂反应流体动力学

:断裂多孔介质中的流体流动和组分输运过程对于理解金属成矿过程、油气运移、污染物质的输运和分布十分重要 ,近 2 0年来得到了广泛的研究。文中综述了这一领域的进展 ,涉及断裂多孔介质中的流体流动和组分输运的连续介质模型、多孔介质和断裂多孔介质的渗透率关系 ,特别是重点介绍了渗滤理论及其在断裂多孔介质中的流体流动和组分输运中的应用     

交叉裂隙渗流无网格法初探

作为两相介质的裂隙岩体,裂隙渗流是岩体渗流的主体。由于岩体的网状裂隙用传统的方法难以模拟复杂裂隙系统的水流特征。为此,基于岩体随机裂隙三维网络数值模拟,提取裂隙二维网络,并试用基于移动最小二乘近似的最小二乘法之无网格法来模拟岩体中交叉裂隙的渗流规律,通过自行编制的程序进行了简单示例与工程实例的试算,获得了较好的模拟结果,证实了无网格法模拟岩体裂隙流的可行性。     

A finite element method for modeling coupled flow and deformation in porous fractured media

Modeling the flow in highly fractured porous media by finite element method (FEM) has met two difficulties: mesh generation for fractured domains and a rigorous formulation of the flow problem accounting for fracture/matrix, fracture/fracture, and fracture/boundary fluid mass exchanges. Based on the recent theoretical progress for mass balance conditions in multifractured porous bodies, the governing equations for coupled flow and deformation in these bodies are first established in this paper. A weak formulation for this problem is then established allowing to build a FEM. Taking benefit from recent development of mesh‐generating tools for fractured media, this weak formulation has been implemented in a numerical code and applied to some typical problems of hydromechanical coupling in fractured porous media. It is shown that in this way, the FEM that has proved its efficiency to model hydromechanical phenomena in porous media is extended with all its performances (calculation time, couplings, and nonlinearities) to fractured porous media. Copyright © 2015 John Wiley & Sons, Ltd.     

离散裂隙渗流方法与裂隙化渗透介质建模

流体渗流模拟的连续介质方法通常适用于多孔地质体，并不一定适用于裂隙岩体，由于裂隙分布及其特征与孔隙差异较大。若流体渗流主要受裂隙的控制，对于一定尺寸的裂隙岩体，多孔介质假设则较难刻划裂隙岩体的渗流特征。离散裂隙渗流方法不但可直接用于模拟裂隙岩体非均质性和各向异性等渗流特征，而且可用其确定所研究的裂隙岩体典型单元体及其水力传导（渗透）张量大小。主要讨论了以下问题：（1）饱和裂隙介质中一般的离散流体渗流模拟；（2）裂隙岩体中的REV（典型单元体）及其水力传导（渗透）张量的确定；（3）利用离散裂隙网络流体渗流模型研究裂隙方向几何参数对水力传导系数和REV的影响；（4）在二维和三维离散裂隙流体渗流模型中对区域大裂隙和局部小裂隙的处理方法。调查结果显示离散裂隙流体渗流数学模型可用来评价不同尺度上的裂隙岩体的水力特征，以及裂隙方向对裂隙化岩体的水力特征有着不可忽视的影响。同时，局部小裂隙、区域大裂隙应当区别对待，以便据其所起的作用及水力特征，建立裂隙化岩体相应的流体渗流模型。     

非均质裂隙介质中重非水相流体运移

由于裂隙介质具有强烈非均质性,使得重非水相流体(DNAPLs)在裂隙介质中的运移行为较孔隙介质更为复杂.基于指示模拟算法模拟裂隙介质的二元变量(粗糙面接触和裂隙开口),综合模拟退火算法构建裂隙渗透率随机场.采用T2VOC模拟DNAPLs在裂隙介质的运移,探讨粗糙面接触的相关长度、各向异性比、倾角以及非均质性程度对DNAPLs运移分布的影响.数值分析结果表明,粗糙面接触的相关性越差,DNAPLs污染范围越大;粗糙面接触的各向异性比和倾角增大,将导致更多DNAPLs残留;而相关长度和各向异性比增大以及倾角减小,都会引起DNAPLs锋面运移速率增大;随着渗透率非均质性增强,会导致局部粗糙面接触上蓄积的DNAPLs饱和度增大,运移速率减小.     

Effect of chemical reactions on the hydrologic properties of fractured and rubbelized glass media

Understanding the effect of chemical reactions on the hydrologic properties of geological media, such as porosity, permeability and dispersivity, is critical to many natural and engineered sub-surface systems. Influence of glass corrosion (precipitation and dissolution) reactions on fractured and rubbelized (crushed) forms HAN28 and LAWBP1, two candidate waste glass forms for a proposed immobilized low-activity waste (ILAW) disposal facility at the Hanford, WA site, was investigated. Flow and tracer transport experiments were conducted using fractured and rubbelized forms, before and after subjecting them to corrosion using vapor hydration testing (VHT) at 200 °C temperature and 200 psig pressure, causing the precipitation of alteration products. Data were analyzed using analytical expressions and CXTFIT, a transport parameter optimization code, for the estimation of the hydrologic characteristics before and after VHT. It was found that glass reactions significantly influence the hydrologic properties of ILAW glass media. Hydrologic properties of rubbelized glass decreased due to precipitation reactions, whereas those of fractured glass media increased due to reaction which led to unconfined expansion of fracture aperture. The results are unique and useful to better understand the effect of chemical reactions on the hydrologic properties of fractured and rubbelized stony media in general and glass media in particular.     

Analyzing unsaturated flow patterns in fractured rock using an integrated modeling approach

Characterizing percolation patterns in unsaturated fractured rock has posed a greater challenge to modeling investigations than comparable saturated zone studies due to the heterogeneous nature of unsaturated media and the great number of variables impacting unsaturated flow. An integrated modeling methodology has been developed for quantitatively characterizing percolation patterns in the unsaturated zone of Yucca Mountain, Nevada (USA), a proposed underground repository site for storing high-level radioactive waste. The approach integrates moisture, pneumatic, thermal, and isotopic geochemical field data into a comprehensive three-dimensional numerical model for analyses. It takes into account the coupled processes of fluid and heat flow and chemical isotopic transport in Yucca Mountain’s highly heterogeneous, unsaturated fractured tuffs. Modeling results are examined against different types of field-measured data and then used to evaluate different hydrogeological conceptualizations through analyzing flow patterns in the unsaturated zone. In particular, this model provides clearer understanding of percolation patterns and flow behavior through the unsaturated zone, both crucial issues in assessing repository performance. The integrated approach for quantifying Yucca Mountain’s flow system is demonstrated to provide a practical modeling tool for characterizing flow and transport processes in complex subsurface systems.     

裂隙网络非连续介质滲流场与温度场耦合分析研究

在岩体水力学研究中，一个重要问题就是岩体渗流场与温度场的耦合分析。笔者关注了国内外相关的研究，对其发展的趋势进行了展望。系统综述了裂隙岩体渗流一温度耦合的研究情况，阐明了裂隙岩体三种渗流模型的特点、适用范围及裂隙网络非连续介质模型的研究概况，分析了裂隙岩体渗流一温度耦合作用研究及工程应用。最后，提出基于裂隙网络非连续介质渗流温度耦合分析是目前亟待解决的课题，并指出今后需要深入研究的问题。     

裂隙网络非连续介质渗流场与温度场耦合分析研究

在岩体水力学研究中,一个重要问题就是岩体渗流场与温度场的耦合分析。笔者关注了国内外相关的研究,对其发展的趋势进行了展望。系统综述了裂隙岩体渗流-温度耦合的研究情况,阐明了裂隙岩体三种渗流模型的特点、适用范围及裂隙网络非连续介质模型的研究概况,分析了裂隙岩体渗流-温度耦合作用研究及工程应用。最后,提出基于裂隙网络非连续介质渗流温度耦合分析是目前亟待解决的课题,并指出今后需要深入研究的问题。     

Mixing induced reactive transport in fractured crystalline rocks

In this paper the solute retention properties of crystalline fractured rocks due to mixing-induced geochemical reactions are studied. While fractured media exhibit paths of fast flow and transport and thus short residence times for conservative solutes, at the same time they promote mixing and dilution due to strong heterogeneity, which leads to sharp concentration contrasts. Enhanced mixing and dilution have a double effect that favors crystalline fractured media as a possible host medium for nuclear waste disposal. Firstly, peak radionuclide concentrations are attenuated and, secondly, mixing-induced precipitation reactions are enhanced significantly, which leads to radionuclide immobilization. An integrated framework is presented for the effective modeling of these flow, transport and reaction phenomena, and the interaction between them. In a simple case study, the enhanced dilution and precipitation potential of fractured crystalline rocks are systematically studied and quantified and contrasted it to retention and attenuation in an equivalent homogeneous formation.     

Pathline tracing on fully unstructured control-volume grids

The trend toward unstructured grids in subsurface flow modeling has prompted interest in the issue of streamline or pathline tracing on unstructured grids. Streamline tracing on unstructured grids is problematic because a continuous velocity field is required for the calculation, while numerical solutions to the groundwater flow equations provide velocity in discretized form only. A method for calculating flow streamlines or pathlines from a finite-volume flow solution is presented. The method uses an unconstrained least squares method on interior cells and a constrained least squares method on boundary cells to approximate cell-centered velocities, which can then be continuously interpolated to any point in the domain of interest. Two-dimensional tests demonstrate that the method correctly reproduces uniform and corner-to-corner flow on fully unstructured grids. In three dimensions using regular hexahedral grids, the method agrees well with established semianalytical methods. Tests also demonstrate that the method produces physically realistic results on fully unstructured three-dimensional grids.     

Stress-Dependent Fluid Flow and Permeability in Fractured Media: from Lab Experiments to Engineering Applications

Summary. Permeability is a physical property in rocks of extreme importance in energy engineering, civil and environmental engineering, and various areas of geology. Early on, fractures in fluid flow models were assumed to be rigid. However, experimental research and field data confirmed that stress-deformation behavior in fractures is a key factor governing their permeability tensor. Although extensive research was conducted in the past, the three-dimensional stress-permeability relationships, particularly in the inelastic deformation stage, still remain unclear. In this paper, laboratory experiments conducted on large concrete blocks with randomly distributed fractures and rock core samples are reported to investigate fluid flow and permeability variations under uniaxial, biaxial and triaxial complete stress-strain process. Experimental relationships among flowrate, permeability and fracture aperture in the fractured media are investigated. Results show that the flowrate and stress/aperture exhibit “cubic law” relationship for the randomly distributed fractures. A permeability-aperture relationship is proposed according to the experimental results. Based on this relationship, stress-dependent permeability in a set of fractures is derived in a three-dimensional domain by using a coupled stress and matrix-fracture interactive model. A double porosity finite element model is extended by incorporating such stress-dependent permeability effects. The proposed model is applied to examine permeability variations induced by stress redistributions for an inclined borehole excavated in a naturally fractured formation. The results indicate that permeability around underground openings depends strongly on stress changes and orientations of the natural fractures.     

Multiscale mass conservative domain decomposition preconditioners for elliptic problems on irregular grids

Multiscale methods can in many cases be viewed as special types of domain decomposition preconditioners. The localisation approximations introduced within the multiscale framework are dependent upon both the heterogeneity of the reservoir and the structure of the computational grid. While previous works on multiscale control volume methods have focused on heterogeneous elliptic problems on regular Cartesian grids, we have tested the multiscale control volume formulations on two-dimensional elliptic problems involving heterogeneous media and irregular grid structures. Our study shows that the tangential flow approximation commonly used within multiscale methods is not suited for problems involving rough grids. We present a more robust mass conservative domain decomposition preconditioner for simulating flow in heterogeneous porous media on general grids.     

A finite element porothermoelastic model for dual-porosity media

An existing dual-porosity finite element model has been extended to include thermo-hydro-mechanical coupling in both media. The model relies on overlapping distinct continua for the fluid and solid domains. In addition, conductive and convective heat transfers are incorporated using a single representative thermodynamics continuum. The model is applied to the problem of an inclined borehole drilled in a fractured formation subjected to a three-dimensional state of stress and, a temperature gradient between the drilling fluid and the formation. A sensitivity analysis has been carried out to study the impact of thermal loading, effect of heat transport by pore fluid flow and, the effect of parameters of the secondary medium used to represent the fractures. Copyright © 2004 John Wiley & Sons, Ltd.