非饱和带裂隙岩体渗流的特点和概念模型

非饱和带裂隙岩体中的渗流和饱和带相比有其不同的特点,包括毛细管流、薄膜流、优先流和裂隙-基质相互作用等。这些特点导致非饱和带裂隙岩体中的渗流具有相当的非均质性。笔者描述了裂隙岩体中非饱和渗流的这些特点,并简单讨论了目前所存在的模拟非饱和渗流的概念模型。     

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。     

基于裂隙连续方法的三维裂隙岩体渗流传热数值模拟

模拟裂隙岩体渗流传热的主要困难在于岩体各种尺度上的非均质性。为了兼顾裂隙岩体渗流传热过程模拟的效率和精度,将二维裂隙连续方法拓展到三维问题中,应用深度优先搜索算法挑出对网格块渗透性有贡献的有效裂隙,综合考虑有效裂隙和岩石基质作用给出网格块的等效渗透率张量,采用Matlab对COMSOL Multiphysics有限元软件进行二次开发,生成由不同渗透率网格块组成的三维裂隙连续模型。数值模拟结果表明：裂隙连续模型结合了随机连续介质模型和离散裂隙模型的特点,既能避免处理裂隙网络的复杂性,又能考虑岩体渗透率的空间变异性,兼顾了模拟效率和精度;当岩石基质渗透率与裂隙渗透率比值的数量级在10^-4～10^-6范围内时,有效裂隙网络模型的流量计算误差会超过5%。     

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。     

基于非均质系数的裂隙岩体表征单元体研究

表征单元体（REV）是岩体力学中的一个基础性概念,其存在性是确定岩体等效参数和应用连续介质方法进行研究的前提条件。为综合反映裂隙大小、产状和密度对岩体REV的影响,本文提出采用岩体非均质系数HI来确定岩体的REV。首先,详细介绍了非均质系数的概念和含义。利用General Block软件建立了中等间距-中等延展性（MS1-MP1）裂隙的网络模型,并完成20次随机实现。从各模型中选取10个不同尺寸的岩体模型计算非均质系数,结果表明:非均质系数具有明显的尺寸效应,当岩体尺寸不小于8 m时,非均质系数的平均值和标准差分别为0.5和0.14,综合确定该岩体的REV为8 m。采用HI对三峡地下电站厂房围岩的研究表明,该岩体的REV为60 m,可作为非连续介质方法适用性的判断标准。HI是从岩体结构角度提出的新指标,适用于确定岩体的REV和统计范围,对岩体的统计分析和等效参数研究具有重要意义。     

非常低延展性裂隙岩体REV存在性研究

裂隙岩体渗流问题一直是水文地质与工程地质的前沿课题之一,等效多孔介质模型是裂隙岩体渗流研究和工程应用的主要方法,此方法的重点在于裂隙岩体渗流模型即典型单元体(REV)的确定。本文采用作者自行开发的离散元软件FractureToKarst,根据国际岩石力学协会对岩石的分类,讨论了非常低延展性裂隙岩体典型单元体(REV)的存在性及大小,并给出确定其存在性的一般方法。通过研究可知,非常低延展性裂隙岩体REV存在的条件就是裂隙平均间距在0.2m以下,即极密间距、很密间距和密间距裂隙岩体存在REV。裂隙平均间距大于0.2 m,即中等间距、宽间距、很宽间距和极宽间距裂隙岩体不存在REV。     

裂隙岩体渗透系数确定方法综述

总结了近年来裂隙岩体渗透系数的确定方法,主要有现场水力试验法、裂隙测量法和离散裂隙网络渗流数值试验法。每种方法都有自己的适用性和测量尺度。裂隙岩体渗透系数存在尺度效应,针对不同尺度的研究对象,应尽量选取与渗流模型网格剖分尺度匹配的测量方法。裂隙的延伸具有方向性,测试点的布设应合理科学,不同测量方法需要结合起来才能得到裂隙岩体真实的渗透系数。     

基于离散裂隙网络法和水流数值模拟技术的地下水封洞库水封性研究

水封性是保证地下水封洞库安全运营的关键,目前研究地下水封洞库水封性的主要难点在于裂隙岩体含水层强烈的非均质性和各向异性.以某地下水封洞库工程为背景,通过实测裂隙几何参数分析裂隙发育特征,建立了研究区离散裂隙网络模型.之后对研究区进行网格化划分计算了各单元体渗透系数张量,基于对裂隙渗流基本规律的理解和假设建立了研究区高精...     

软硬互层状岩体渗透特性研究

叙述软硬互层状岩体中的裂隙发育规律,并由此建立了互层状岩体渗流的离散裂隙网络模型.根据裂隙网络水均衡原理,介绍了裂隙岩体网络渗流数学模型以及模型的求解方法;通过一个数值算例验证了模型的合理性,并且讨论了软岩中的切层裂隙以及软岩层面裂隙开度对渗流场分布和通过软岩流量的影响.     

裂隙岩体中非饱和渗流与运移的概念模型及数值模拟

探讨了裂隙岩体中非饱和地下水渗流与溶质运移的几种概念模型的构造及数值模拟问题 ,如裂隙网络模型、连续体模型、等效连续体模型、双孔隙度 (单渗透率 )模型、双渗透率模型、多组份连续体模型等。在裂隙岩体中 ,非饱和地下水的渗流可能只局限于岩体中的岩石组份、或裂隙网络 ,也可能在裂隙和岩石中同时发生 ;对前一种情形只需考虑单一连续体中的流动 ,而后一种情况则需要包括地下水在岩石和裂隙之间的交换。岩体中的裂隙网络往往是溶质运移的主要通道 ;但当溶质在裂隙与岩石之间的渗透和扩散是重要的运移机制时 ,就需要考虑岩石与裂隙界面处的溶质交换。为了模拟岩石与裂隙之间地下水和溶质的交换 ,就需要了解岩石与裂隙之间相互作用的模式和范围 ,使得这类问题的概念模型较单一连续体模型多了一层不确定性、其数值模拟也变得更为困难。因为在实际问题中不易、甚至根本不能判别非饱和渗流的实际形态 ,具体采用哪种模型主要取决于分析的目的和对现场数据的掌握程度。不论哪种模型都会受到模型及参数不确定性的影响 ,因此必须考虑与其他辅助模型的比较.     

Application of Stochastic Fracture Network with Numerical Fluid Flow Simulations to Groundwater Flow Modeling in Fractured Rocks

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XFEM-based cohesive zone approach for modeling near-wellbore hydraulic fracture complexity

Near-wellbore fracture tortuosity has important impacts on the productivity of fractured oil and gas wells and the injectivity of CO₂ or solids disposal injectors. Previous models for simulating near-wellbore fracture tortuosity usually assume fracture growth in linear-elastic media, without considering the effects of porous features of the rock. In this paper, a 2D fully coupled model is developed to simulate near-wellbore fracturing using the XFEM-based cohesive segment method. The model takes into account a variety of crucial physical aspects, including fracture extension and turning, fluid flow in the fracture, fluid leak-off through wellbore wall and fracture surfaces, pore fluid flow, and rock deformation. The proposed model was verified against two sets of published experimental results. Numerical examples were carried out to investigate the effects of various parameters on near-wellbore fracture trajectory, injection pressure, and fracture width. Results show that near-wellbore fracture behaviors are not only dependent on rock elastic properties and field stresses, but also greatly influenced by porous properties of the rock, such as permeability and leak-off coefficient. Some field implications were provided based on the simulation results. By overcoming some limitations of the previous models, the proposed model predicts more realistic fracture evolution in the near-wellbore region and provides an attractive tool for design and evaluation of many field operations, for which near-wellbore fracture behaviors play an important role on their successes.

     

Fully coupled hydro-mechanical numerical manifold modeling of porous rock with dominant fractures

Coupled hydro-mechanical (HM) processes are significant in geological engineering such as oil and gas extraction, geothermal energy, nuclear waste disposal and for the safety assessment of dam foundations and rock slopes, where the geological media usually consist of fractured rock masses. In this study, we developed a model for the analysis of coupled hydro-mechanical processes in porous rock containing dominant fractures, by using the numerical manifold method (NMM). In the current model, the fractures are regarded as different material domains from surrounding rock, i.e., finite-thickness fracture zones as porous media. Compared with the rock matrix, these fractured porous media are characterized with nonlinear behavior of hydraulic and mechanical properties, involving not only direct (poroelastic) coupling but also indirect (property change) coupling. By combining the potential energy associated with mechanical responses, fluid flow and solid–fluid interactions, a new formulation for direct HM coupling in porous media is established. For indirect coupling associated with fracture opening/closure, we developed a new approach implicitly considering the nonlinear properties by directly assembling the corresponding strain energy. Compared with traditional methods with approximation of the nonlinear constitutive equations, this new formulation achieves a more accurate representation of the nonlinear behavior. We implemented the new model for coupled HM analysis in NMM, which has fixed mathematical grid and accurate integration, and developed a new computer code. We tested the code for direct coupling on two classical poroelastic problems with coarse mesh and compared the results with the analytical solutions, achieving excellent agreement, respectively. Finally, we tested for indirect coupling on models with a single dominant fracture and obtained reasonable results. The current poroelastic NNM model with a continuous finite-thickness fracture zone will be further developed considering thin fractures in a discontinuous approach for a comprehensive model for HM analysis in fractured porous rock masses.     

增强型地热系统（EGS）的裂隙模拟方法

增强型地热系统（EGS）是在干热岩技术基础上提出来的一个清洁能源概念,水力压裂建立人工热储是开采地下干热岩热能的有效方法之一。利用TOUGH2系列软件对增强型地热系统进行模拟,具体介绍了对水压致裂过程中裂隙网络模拟的处理方法。裂隙中的水流可以采用不同的概念模型,最为常见的模型包括双空隙率、双渗透率、多重相互作用连续统一体（MINC）以及有效连续统一体（ECM）,这些模型明确了对离散的裂隙和基质的模拟方法。应根据基质的渗透性和裂隙的性质灵活地选择裂隙处理方法,也可将不同方法结合起来使用。提出了几种有效的混合模拟方案,对将来高温岩体地热开发具有重要意义。     

Review: Mathematical expressions for estimating equivalent permeability of rock fracture networks

Fracture networks play a more significant role in conducting fluid flow and solute transport in fractured rock masses, comparing with that of the rock matrix. Accurate estimation of the permeability of fracture networks would help researchers and engineers better assess the performance of projects associated with fluid flow in fractured rock masses. This study provides a review of previous works that have focused on the estimation of equivalent permeability of two-dimensional (2-D) discrete fracture networks (DFNs) considering the influences of geometric properties of fractured rock masses. Mathematical expressions for the effects of nine important parameters that significantly impact on the equivalent permeability of DFNs are summarized, including (1) fracture-length distribution, (2) aperture distribution, (3) fracture surface roughness, (4) fracture dead-end, (5) number of intersections, (6) hydraulic gradient, (7) boundary stress, (8) anisotropy, and (9) scale. Recent developments of 3-D fracture networks are briefly reviewed to underline the importance of utilizing 3-D models in future research.     

Numerical simulation of two-phase flow in conceptualized fractures

Two-phase flow in fractured rock is an important phenomenon related to a range of practical problems, including non-aqueous phase liquid contamination of groundwater. Although fractured rocks consist of fracture networks, the study of two-phase flow in a single fracture is a pre-requisite. This paper presents a conceptual and numerical model of two-phase flow in a variable fracture. The void space of the fracture is conceptualized as a system of independent channels with position-dependent apertures. Fundamental equations, governing two-phase displacement in each channel, are derived to represent the interface positions and fractional flows in the fracture. For lognormal aperture distributions, simple approximations to fractional flows are obtained in analytical form by assuming void occupancy based on a local capillary allowability criterion. The model is verified by analytical solutions including two-phase flow in a parallel-plate fracture, and used to study the impacts of aperture variation, mobility ratio and fracture orientation on properties of two-phase flow. Illustrative examples indicate that aperture variation may control the distribution of wetting and non-wetting fluids within the fracture plane and hence the ability of the fracture to transmit these fluids. The presence of wetting fluid does little to hinder non-wetting fluid flow in fractures with large aperture variations, whereas a small volume of non-wetting fluid present in the fracture can significantly reduce wetting fluid flow. Large mobility ratios and high fracture slope angles facilitates the migration of non-wetting fluid through fractures.     

Dimensional reduction of a fractured medium for a polymer EOR model

Computational Geosciences - Dimensional reduction strategy is an effective approach to derive reliable conceptual models to describe flow in fractured porous media. The fracture aperture is several...