Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

In this study,the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically.We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling.The geomechanical response of the fractured rock is simulated using a hybrid finitediscrete element model,which can capture the deformation of intact rocks,the interaction of matrix blocks,the displacement of discrete fractures and the propagation of new cracks.Under far-field stress loading,the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures.The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment.We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks.A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening,leading to greater rock mass permeability.However,a disconnected fracture network accommodates much less fracture shearing and opening,and has much lower bulk permeability.We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric(i.e.percolation parameter) of fracture networks,which yields an excellent match to the numerical results.We infer that fluid flow through a well-connected system is governed by traversing channels(forming an"in parallel" architecture) and thus equivalent permeability is sensitive to stress loading(due to stress-dependent fracture permeability),whilst fluid flow through a disconnected system is more ruled by matrix(linking isolated clusters"in series") and has much less stress dependency.     

非均匀性对岩石水压致裂过程的影响

使用岩石破裂失稳的渗流应力耦合系统研究带有圆孔的圆形岩石试件水压作用下的开裂、扩展和失稳过程。结果表明,岩石的非均质性导致破裂的初始压力和失稳压力明显不同,水压破裂路径不规则发展。     

Application of a local degradation model to the analysis of brittle fracture of laboratory scale rock specimens under triaxial conditions

Development of brittle fracture and the associated macroscopic behaviour of rock specimens in laboratory tests are simulated using a local degradation model for brittle fracture in heterogeneous rocks. We examine this subject because laboratory uniaxial and triaxial tests are widely used in the rock mechanics community to both characterise rock behaviour and to interpret fracture phenomena observed in natural rock, such as the Earth's crust, and developed around rock engineering structures. In addition, numerous historical efforts related to the detailed study of this subject have made available a great deal of information for use both as model input data and comparison results.A series of numerical experiments have been performed to investigate the influence of a number of parameters on rock fracture. In particular, rock fracture under various confining stresses has been explored. The results show that the degradation algorithm is capable of reproducing many characteristics associated with brittle fracture in heterogeneous rocks, including: the development of fracture from the elemental scale to the macroscopic scale; fracture pattern as a function of confining pressure; variation of fracture plane angle with respect to confining pressure; the complete stress–strain curve and corresponding strain energy dissipation characteristics; dependence of the stress–strain curve on confining pressure; and loading–unloading hysteresis loops.Independent investigations into the effect on rock fracture of (i) the degradation parameter embodied with the model and (ii) the Weibull shape parameter used to introduce heterogeneity distribution are described. The results indicate that the degradation parameter controls the degree of degradation relative to confining pressure. As this parameter increases, and the elemental degradation decreases, the number of failed sites generated prior to the formation of macroscopic fracture plane increases, and both the peak and ultimate strengths of the model increase.The Weibull parameter influences the formation of the final fracture plane. As this parameter increases, reducing the heterogeneity, the number of diffused failed sites and the angle of the eventual fracture plane to the major principal stress tends to decrease, and the brittleness of the resulting stress–strain curve increases. It is suggested that values in the range 2–4 are appropriate for this parameter in representing elemental strength distribution of rock materials.     

Development of a hydro-mechanical local degradation approach and its application to modelling fluid flow during progressive fracturing of heterogeneous rocks

A hydro-mechanical local degradation approach is developed and used to investigate progressive damage and associated flow behaviour in heterogeneous rock. The approach is based on the establishment of an elemental scale hydro-mechanical constitutive model that incorporates simple idealised representations of degradation of strength and stiffness, confining pressure-dependent dilatancy, and deformation-dependent permeability. Through the use of such an idealised elemental behaviour, together with hydro-mechanical heterogeneity at the elemental scale, a finite difference numerical formulation allows both the mechanical and hydraulic behaviour of heterogeneous rocks to be simulated.

A series of numerical experiments that replicate laboratory-scale rock specimens have been performed to simulate fracture development and the associated evolution of fluid flow. Two independent statistical distributions are used to represent mechanical and hydraulic heterogeneity, and an uncoupled hydro-mechanical algorithm is used to investigate local variations of permeability induced by mechanical damage, and how these influence macroscopic flow patterns. The model is shown to be capable of reproducing, and allowing visualisation of, a range of hydro-mechanical responses of rock that, until now, have only been monitored in the laboratory.     

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Rocks in underground works usually experience rather complex stress disturbance. For this, their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions. The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies. However, little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading (FL-CSU) conditions that influence rock failure. In this paper, we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography (CT) scanning technique and the GCTS RTR 2000 rock mechanics system. Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble. The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests, and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests. In FL-CSU tests, as the unloading rate increases, the dissipated energy grows and elastic energy reduces. CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension. It shows that the crack pattern after FL-CSU tests depends on the unloading rate, and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released. The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.     

岩石介质细观非均匀性对宏观破裂过程的影响

运用岩石破裂过程分析系统 ,研究了岩石介质非均匀性对宏观力学行为的影响。通过对均质度系数m =1.5的 10个样本破裂过程的模拟 ,发现样本的宏观力学行为在变形初期只有微弱的统计涨落 ,但在失稳阶段表现出显著的样本个性行为。对 8种不同均质度系数的 80个样本破裂过程的模拟结果表明 ,岩石介质的非均匀性对岩样宏观强度和变形非线性行为有显著影响 ,而且也显著影响试样破裂模式。随着均质度系数的提高 ,宏观强度呈现指数规律上升 ,宏观变形的线性规律增强 ,主破裂呈脆断模式。     

Numerical manifold method modeling of coupled processes in fractured geological media at multiple scales

The greatest challenges of rigorously modeling coupled hydro-mechanical(HM) processes in fractured geological media at different scales are associated with computational geometry.These challenges include dynamic shearing and opening of intersecting fractures at discrete fracture scales as a result of coupled processes,and contact alteration along rough fracture surfaces that triggers structural and physical changes of fractures at micro-asperity scale.In this paper,these challenges are tackled by developing a comprehensive modeling approach for coupled processes in fractured geological media based on numerical manifold method(NMM) at multiple scales.Based on their distinct geometric features,fractures are categorized into three different scales:dominant fracture,discrete fracture,and discontinuum asperity scales.Here the scale is relative,that of the fracture relative to that of the research interest or domain.Different geometric representations of fractures at different scales are used,and different governing equations and constitutive relationships are applied.For dominant fractures,a finite thickness zone model is developed to treat a fracture as a porous nonlinear domain.Nonlinear fracture mechanical behavior is accurately modeled with an implicit approach based on strain energy.For discrete fractures,a zero-dimensional model was developed for analyzing fluid flow and mechanics in fractures that are geometrically treated as boundaries of the rock matrix.With the zero-dimensional model,these fractures can be modeled with arbitrary orientations and intersections.They can be fluid conduits or seals,and can be open,bonded or sliding.For the discontinuum asperity scale,the geometry of rough fracture surfaces is explicitly represented and contacts involving dynamic alteration of contacts among asperities are rigorously calculated.Using this approach,fracture alteration caused by deformation,re-arrangement and sliding of rough surfaces can be captured.Our comprehensive model is able to handle the computational challenges with accurate representation of intersections and shearing of fractures at the discrete fracture scale and rigorously treats contacts along rough fracture surfaces at the discontinuum asperity scale.With future development of three-dimensional(3 D) geometric representation of discrete fracture networks in porous rock and contacts among multi-body systems,this model is promising as a basis of 3 D fully coupled analysis of fractures at multiple scales,for advancing understanding and optimizing energy recovery and storage in fractured geological media.     

裂隙岩体宏观力学参数研究

 裂隙岩体的变形模量及强度特性是工程界关注的焦点,其关键是岩体变形模量及强度的尺寸效应和表征单元尺寸(REV)。根据锦屏二级水电站大理岩裂隙统计分布规律及岩块和结构面力学特性试验成果,确定岩块和结构面的本构模型,建立考虑无厚度裂隙面力学响应的分析模型。研究不同尺寸、不同统计窗裂隙岩体的力学响应特征,并重点研究裂隙岩体变形模量和单轴抗压强度的尺寸效应、REV特征以及各向异性特征。该研究方法为裂隙岩体的宏观力学参数取值提供参考,研究成果也为锦屏二级水电站工程岩体参数取值提供了依据。     

Multiscale hierarchical analysis of rock mass and prediction of its mechanical and hydraulic properties

Engineering geological and hydro-geological characteristics of foundation rock and surrounding rock mass are the main factors that affect the stability of underground engineering. This paper presents the concept of multiscale hierarchical digital rock mass models to describe the rock mass, including its structures in different scales and corresponding scale dependence. Four scales including regional scale, engineering scale, laboratory scale and microscale are determined, and the corresponding scale-dependent geological structures and their characterization methods are provided. Image analysis and processing method, geostatistics and Monte Carlo simulation technique are used to establish the multiscale hierarchical digital rock mass models, in which the main micro- and macro-structures of rock mass in different geological units and scales are reflected and connected. A computer code is developed for numerically analyzing the strength, fracture behavior and hydraulic conductivity of rock mass using the multiscale hierarchical digital models. Using the models and methods provided in this paper, the geological information of rock mass in different geological units and scales can be considered sufficiently, and the influence of downscale characteristics (such as meso-scale) on the upscale characteristics (such as engineering scale) can be calculated by considering the discrete geological structures in the downscale model as equivalent continuous media in the upscale model. Thus the mechanical and hydraulic properties of rock mass may be evaluated rationally and precisely. The multiscale hierarchical digital rock mass models and the corresponding methods proposed in this paper provide a unified and simple solution for determining the mechanical and hydraulic properties of rock mass in different scales.     

裂隙单元修正等效渗透率模型及其验证

 采用裂隙单元表征裂隙网络,引入裂隙单元等效渗透率的概念,按照流量等效的原则计算其大小,然而阶梯状裂隙单元造成渗流流程的增加,同时压差不变导致流量的减少,为解决这一问题,用裂隙在网格中的实际流程长度与裂隙迹长之比来修正裂隙单元等效渗透率,并且针对复杂裂隙网络,对其进行预处理--删除孤立裂隙、死端裂隙、孤立裂隙簇等非连通裂隙。用此修正模型对单裂隙、相交裂隙、复杂裂隙网络进行渗流数值模拟,并与理论解及离散裂隙网络模型方法渗流结果进行比较,结果显示：研究区域下游出口总流量及出口处流量分布均取得较好一致性;同时,此裂隙单元修正等效渗透率模型也能反映出裂隙岩体渗流的非均质和各向异性。     

二维裂隙岩体渗流传热的离散裂隙网络模型数值计算方法

 针对裂隙岩体渗流传热问题,用解析方法,比较2种不同岩石基质与裂隙水界面热交换假设下的计算结果,对一般裂隙岩体,2种假设下的计算结果相同。基于离散裂隙网络模型的思想,在商业有限元软件COMSOL中实现一种计算已知裂隙网络的裂隙岩体渗流和传热过程的数值方法,该方法可以同时计算岩石基质与裂隙中的渗流和传热过程及二者间的交换,并与解析解比较进行验证。用该方法对一随机生成的二维裂隙岩体进行计算,得到的出口温度曲线,可以反映裂隙岩体渗流传热的早期热突破和长尾效应等特点,并分析岩石基质渗透率、热传导系数的不同取值对裂隙岩体渗流和传热过程的影响。     

Analysis of compressive fracture in rock using statistical techniques: Part II. Effect of microscale heterogeneity on macroscopic deformation

We have performed a parameter-sensitivity analysis to evaluate the relative importance of different types of grain-scale heterogeneity on fracture processes and compressive strength in simulated compression tests of brittle, heterogeneous materials such as rock. This was done using a non-linear, rule-based model described in a companion paper. Results presented here indicate that heterogeneity in local stress field (due to grain shape and loading) has a first-order effect on macroscopic properties and is much more important than heterogeneity in site strength or location. In particular, increasing local stress heterogeneity lowers the mean ultimate strength following an inverse power law. Increasing heterogeneity in the lattice-site locations (i.e. irregular lattice spacing) decreases crack localization and decreases normalized crack strain energy. This result is consistent with the postulate that systems with increasing disorder require more energy to break. Heterogeneity in site-strength distribution had a relatively minor effect on macroscopic behavior. Peak strength is dependent on the mean site strength, not on the width of the site-strength distributions. This study also revealed that percolation thresholds are much lower than those predicted from stochastic fracture models. Consequently, statistical models for rock fracture must consider alternative percolation algorithms such as directed-bond percolation, because the standard percolation models may not be appropriate for analyzing systems where crack interaction dominates behavior at a low fraction of sites broken.     

Geomechanical modeling of CO_2 geological storage:A review

This paper focuses on the progress in geomechanical modeling associated with carbon dioxide(CO_2)geological storage.The detailed review of some geomechanical aspects,including numerical methods,stress analysis,ground deformation,fault reactivation,induced seismicity and crack propagation,is presented.It is indicated that although all the processes involved are not fully understood,integration of all available data,such as ground survey,geological conditions,microseismicity and ground level deformation,has led to many new insights into the rock mechanical response to CO_2 injection.The review also shows that in geomechanical modeling,continuum modeling methods are predominant compared with discontinuum methods.It is recommended to develop continuum-discontinuum numerical methods since they are more convenient for geomechanical modeling of CO_2 geological storage,especially for fracture propagation simulation.The Mohr-Coulomb criterion is widely used in prediction of rock mass mechanical behavior.It would be better to use a criterion considering the effect of the intermediate principal stress on rock mechanical behavior,especially for the stability analysis of deeply seated rock engineering.Some challenges related to geomechanical modeling of CO_2 geological storage are also discussed.     

基于数字图像的岩石非均匀性表征技术在流固耦合分析中的应用

岩石的非均匀性对于其破裂及渗流过程具有显著的影响。本文提出了用数字图像处理技术表征岩石非均匀性的基本方法,并通过把该技术实施到岩石破裂过程分析程序RFPA中,来模拟非均匀岩石在外载荷和渗流耦合作用下发生破裂的整个过程。结果表明,基于数字图像的技术是表征岩石细观非均匀性的一种有效方法,为研究非均匀性对于岩石破裂过程及渗流特性的影响提供了一个重要的手段。     

节理岩体表征单元体尺寸确定的数值模拟

从节理岩体表征单元体的力学意义出发,在通过对某物理试验结果进行模拟以验证RFPA程序模拟节理岩体强度及破裂模式具有适用性的基础上,提出一种基于RFPA数值模拟确定节理岩体表征单元体的方法。该方法基于蒙特卡洛法生成二维节理裂隙网格,实现节理裂隙的表征,将其导入岩石破裂过程分析软件中,分析节理岩体弹性模量和单轴抗压、抗拉强度的尺寸效应和各向异性,并据此确定了节理岩体的表征单元体尺寸。最后,针对某采场围岩节理面统计参数,分析讨论节理岩体的尺寸效应和各向异性,通过综合分析确定了节理岩体的弹性模量、抗压强度和抗拉强度等参数,并得出其表征单元体的尺度为6 m×6 m,这为后续的岩石力学研究奠定基础。     

Rôle des fluides dans le comportement hydromécanique des roches fracturées hétérogènes: Caractérisation in situ et modélisation numérique

Hydromechanical coupled processes in a shallow fractured rock mass were investigated in situ through field experiments and numerical simulations. The experimental approach consists of performing simultaneous and multi-frequency measurements of fluid pressures and displacements at different points and on different fracture types within a carbonate reservoir. Two kinds of experiments were conducted at the Coaraze Laboratory Site (France):

1. At the fracture network scale, a global hydraulic loading by groundwater level change shows that the coupling between fluid flow and deformation is simultaneously governed by a dual-permeability hydraulic behaviour and a dual-stiffness mechanical behaviour. The following fluid flow and hydromechanical conceptual scheme was established: first, a transient flow only occurs in faults with high permeability; second, when a steady-state flow is reached in faults, water flows from faults into lower permeability bedding planes. The intact rock matrix is practically impervious but the connectivity between the discontinuities is high. When fluid pressure changes occur within the fracture network, the hydromechanical coupling is direct in the highly permeable faults where a pressure change induces a deformation change. No direct hydromechanical coupling occurs within the lower permeability zones where deformation is not directly correlated with pressure changes. This means that the mechanical deformation of the bedding planes and rock matrix is induced by the fault deformation.
2. At the single fracture scale, the hydromechanical behaviour was evaluated by performing hydraulic pulse injection testing. This test was monitored using high-frequency (f = 120 Hz) hydromechanical measurements conducted with innovative fiber-optic borehole equipment. The hydromechanical response is simultaneously monitored at two measuring points spaced about 1 m apart within the plane of the sub-vertical fracture. Observed fluid pressure versus normal displacement curves shows a characteristic loop-shaped evolution in which the paths for loading (pressure increase) and unloading (pressure decrease) are different. The test was evaluated by coupled hydromechanical modelling using a distinct element technique. By matching the loop behaviour, modelling indicates that the pulse pressure increase portion allows the fracture hydromechanical properties to be determined while the pulse pressure decrease portion is strongly influenced by the hydromechanical effects within the surrounding fractured rock mass. A sensitivity study shows that the key parameters to coupled hydromechanical processes in such fracture systems are the initial hydraulic aperture and normal stiffness of the fracture, the stiffness of the rock matrix and the geometry of the surrounding fracture network.

     

岩体裂隙尺度对其变形与破坏的控制作用

岩体是一种含有大量缺陷的天然材料,其中的断层、裂缝和裂隙称为高层次缺陷,微裂隙、孔隙等称为低层次缺陷.首先分析包含宏观和微观裂隙的岩石裂隙数量与尺度之间的分形关系,并按照裂隙尺度进行分级.然后采用数值试验方法,通过逐级增加裂隙,研究岩体裂隙尺度对其变形与破坏的控制作用.研究结果表明,当裂缝、裂隙、微裂隙、孔隙等不同尺度、不同层次缺陷并存时,高层次缺陷对岩体的变形、稳定、破坏起着主导控制作用.     

Rock mass characterization and assessment of block-forming geological discontinuities during caving of primary copper ore at the El Teniente mine, Chile

The primary copper ore at the El Teniente mine is very competent and massive. It is a rock with almost no open discontinuities. Nevertheless, there is a high frequency network of small scale veins coupled with widely spaced faults. A research investigation was implemented to determine what discontinuities are the most relevant in the rock mass disassembly process during cave mining. Rock mass characterization was undertaken at four different scales within the rock mass, in particular the block-forming discontinuities were characterized by studying a significant number of blocks at the caving draw points. Two relevant discontinuity characteristics have been found. The data suggest that discontinuities having approximately less than 1/3 of hard minerals as infill and thicknesses greater than or equal to 2 mm are weaker and more likely to define blocks during caving and the subsequent comminution process. The infill characteristics have been used to characterize rock mass quality in different sectors, and the results are in accordance with actual observations at the mine site.     

Tectonic structures and commercial compartments in active quarrying: a case history from northern Italy

By studying a quarried site exposed in the Ligurian Alps (northern Italy), this work aims to assess the impact of the tectonic structures on the exploitable rock volume characteristics/properties and, more in general, on the three-dimensional quarry architecture for its future exploitation. A field-based, multidisciplinary dataset has been integrated within a 3D geological model performed in a Geographical Information System (GIS) environment. The spatial distribution, geometry, and internal fabric of the main tectonic structures (mainly represented by faults and fracture networks) have been recognised as responsible for a structural compartmentalisation within the study area. In situ geomechanical measurements (uniaxial strength by using a Schmidt hammer) have been compared with distribution of the tectonic structures. A simulation of the influence of the tectonic structures on the rock mechanical behaviour has been performed in GIS. We conclude that occurrence of tectonic structures determines a first order degree in geodiversity at quarried sites, influencing both intrinsic (lithology and petrography) and secondary (mechanical) properties of the excavated rock volume. Consequently, understanding and reconstructing the deformation style and intensity of the tectonic setting constitute a further geological exploration criterion for assessing strategy of rock excavation and exploitation. The identification of tectonic structures is relevant to face frequently documented problems in active quarrying, such as mass movement and slope failure, reduced rock-mass quality, or non-exploitable quarry domains.     

岩体裂隙网络注浆模拟试验系统研制及应用

为研究浆液在岩体裂隙网络内的扩散迁移规律,研制了可视化裂隙恒压注浆试验系统。该系统由供压设备、恒压出浆设备、裂隙模拟设备以及监测设备组成,可根据试验要求设计特定岩体裂隙网络,模拟不同注浆压力、浆液黏度、裂隙开度等多种参数影响下的浆液流动过程,并可研究裂隙岩体中浆液–水/气驱替作用机制。通过单一裂隙注浆理论与试验结果的对比,验证本试验系统的可靠性。基于此,进一步研究随机裂隙网络注浆扩散机制,结果表明:(1)裂隙内同一点压力随注浆压力的增大而增大,随裂隙开度的增大而减小,不随浆液黏度的变化而变化;(2)浆液在裂隙网络内由总流分散为支流后,压力显著下降,流速放缓,且各支流压力与流量分配系数受交叉(分叉)裂隙夹角影响较大。试验系统的研制及研究成果对岩体工程注浆具有一定指导价值。