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Featured researches published by Qinghua Lei.


Journal of Geophysical Research | 2015

A new approach to upscaling fracture network models while preserving geostatistical and geomechanical characteristics

Qinghua Lei; John-Paul Latham; Chin-Fu Tsang; Jiansheng Xiang; Philipp Lang

A new approach to upscaling two-dimensional fracture network models is proposed for preserving geostatistical and geomechanical characteristics of a smaller-scale “source” fracture pattern. First, the scaling properties of an outcrop system are examined in terms of spatial organization, lengths, connectivity, and normal/shear displacements using fractal geometry and power law relations. The fracture pattern is observed to be nonfractal with the fractal dimension D ≈ 2, while its length distribution tends to follow a power law with the exponent 2 < a < 3. To introduce a realistic distribution of fracture aperture and shear displacement, a geomechanical model using the combined finite-discrete element method captures the response of a fractured rock sample with a domain size L = 2 m under in situ stresses. Next, a novel scheme accommodating discrete-time random walks in recursive self-referencing lattices is developed to nucleate and propagate fractures together with their stress- and scale-dependent attributes into larger domains of up to 54 m × 54 m. The advantages of this approach include preserving the nonplanarity of natural cracks, capturing the existence of long fractures, retaining the realism of variable apertures, and respecting the stress dependency of displacement-length correlations. Hydraulic behavior of multiscale growth realizations is modeled by single-phase flow simulation, where distinct permeability scaling trends are observed for different geomechanical scenarios. A transition zone is identified where flow structure shifts from extremely channeled to distributed as the network scale increases. The results of this paper have implications for upscaling network characteristics for reservoir simulation.


Geophysical Research Letters | 2016

Tectonic interpretation of the connectivity of a multiscale fracture system in limestone

Qinghua Lei; Xiaoguang Wang

This paper studies the statistics and tectonism of a multiscale natural fracture system in limestone. The fracture network exhibits a self-similar characteristic with a correlation between its power law length exponent a and fractal dimension D, i.e., a ≈ D + 1. Contradicting the scale-invariant connectivity of idealized self-similar systems, the percolation state of trace patterns mapped at different scales and localities of the study area varies significantly, from well to poorly connected. A tectonic interpretation based on a polyphase fracture network evolution history is proposed to explain this discrepancy. We present data to suggest that the driving force for fracture formation may be dissipated at the end of a tectonic event when the system becomes connected. However, the “effective” connectivity can successively be reduced by cementation of early fractures and reestablished by subsequent cracking, rendering a variable “apparent” connectivity that can be significantly above the percolation threshold.


Rock Mechanics and Rock Engineering | 2016

Implementation of an Empirical Joint Constitutive Model into Finite-Discrete Element Analysis of the Geomechanical Behaviour of Fractured Rocks

Qinghua Lei; John-Paul Latham; Jiansheng Xiang

An empirical joint constitutive model (JCM) that captures the rough wall interaction behaviour of individual fractures associated with roughness characteristics observed in laboratory experiments is combined with the solid mechanical model of the finite-discrete element method (FEMDEM). The combined JCM-FEMDEM formulation gives realistic fracture behaviour with respect to shear strength, normal closure, and shear dilatancy and includes the recognition of fracture length influence as seen in experiments. The validity of the numerical model is demonstrated by a comparison with the experimentally established empirical solutions. A 2D plane strain geomechanical simulation is conducted using an outcrop-based naturally fractured rock model with far-field stresses loaded in two consecutive phases, i.e. take-up of isotropic stresses and imposition of two deviatoric stress conditions. The modelled behaviour of natural fractures in response to various stress conditions illustrates a range of realistic behaviour including closure, opening, shearing, dilatancy, and new crack propagation. With the increase in stress ratio, significant deformation enhancement occurs in the vicinity of fracture tips, intersections, and bends, where large apertures can be generated. The JCM-FEMDEM model is also compared with conventional approaches that neglect the scale dependency of joint properties or the roughness-induced additional frictional resistance. The results of this paper have important implications for understanding the geomechanical behaviour of fractured rocks in various engineering activities.


Rock Mechanics and Rock Engineering | 2014

A Morphological Visualization Method for Removability Analysis of Blocks in Discontinuous Rock Masses

Zixin Zhang; Qinghua Lei

Block theory has been successfully applied to solve practical problems in rock engineering associated with the stability of discontinuous rock masses. Traditional methods of block theory, which have not proposed detailed schemes for characterizing geometric features of arbitrary blocks with complex geometries, are difficult to simultaneously visualize the locations of blocks. To overcome the shortcomings of these traditional methods, this paper introduces the concept of block morphology to systematically represent the morphological characteristics of blocks. Morphological visualization analysis involves the classification, characterization and visualization of various blocks and the determination of their removability using the stereo-analytical method. The proposed morphological visualization method provides a basis for computer modeling and mechanical calculations of large-scale rock mass systems. Furthermore, examples are presented in this paper to verify the validity and demonstrate the exceptional performance of this method.


Geophysical Research Letters | 2018

Correlation Between Fracture Network Properties and Stress Variability in Geological Media

Qinghua Lei; Ke Gao

We quantitatively investigate the stress variability in fractured geological media under tectonic stresses. The fracture systems studied include synthetic fracture networks following power law length scaling and natural fracture patterns based on outcrop mapping. The stress field is derived from a finite-discrete element model, and its variability is analyzed using a set of mathematical formulations that honor the tensorial nature of stress data. We show that local stress perturbation, quantified by the Euclidean distance of a local stress tensor to the mean stress tensor, has a positive, linear correlation with local fracture intensity, defined as the total fracture length per unit area within a local sampling window. We also evaluate the stress dispersion of the entire stress field using the effective variance, that is, a scalar-valued measure of the overall stress variability. The results show that a well-connected fracture system under a critically stressed state exhibits strong local and global stress variabilities. Plain Language Summary Knowledge of the stress state in the Earth’s crust is of great importance for many scientific and engineering problems, such as earthquake prediction, petroleum recovery, groundwater management, underground excavation, and deep geological disposal of radioactive/toxic waste. However, it is very challenging to fully understand the stress state, because it is highly variable in the subsurface. One of the key factors is the presence and complexity of natural fractures in rock. Then, a fundamental question is raised: is the stress field correlated with these fractures? How? To answer this question, we build numerical models to calculate the stress state in geological media embeddedwith realistic fracture networks. We utilize a new mathematical approach to process the simulation data. We report that the local stress fluctuation is strongly related to the local fracture distribution, and the overall stress variability is greatly controlled by the fracture network connectivity.


Hydrogeology Journal | 2017

Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer

Qinghua Lei; Xiaoguang Wang; Jiansheng Xiang; John-Paul Latham

A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for large-scale assessments.RésuméUne étude sur l’influence des tensions polyaxiales (triaxiales véritables) sur la perméabilité d’une couche de roche fracturée en 3D est. présentée. Le système de fractures 3D est. construit en extrayant le motif d’affleurement 2D d’un niveau calcaire qui présente une structure en échelle consistant en un ensemble de joints parallèles contigu à des fractures courtes d’un stade ultérieur. Le comportement géomécanique de la roche fracturée en 3D en réponse aux contraintes in-situ est. modélisé par la méthode des éléments finis discrets, qui peut saisir la déformation des blocs de matrice, les variations du champ de contraintes, la réactivation de fractures préexistantes rugueuses et la propagation de nouvelles fissures. Une série de simulations numériques est. conçue pour imposer des contraintes à une roche fracturée en utilisant différentes tensions in situ polyaxiales et les propriétés d’écoulement dépendantes des tensions sont ensuite calculées. La couche fracturée a tendance à présenter une localisation plus forte des flux et une perméabilité équivalente supérieure lorsque le rapport des contraintes du champ éloigné augmente et que le champ de contraintes pivote de telle manière que les fractures sont orientées préférentiellement en faveur du cisaillement. La dilatation par cisaillement de fractures préexistantes a des effets dominants sur la localisation du flux dans le système, tandis que la propagation de nouvelles fractures a des impacts mineurs. Le rôle de la contrainte liée à la surcharge suggère que l’analyse conventionnelle 2D qui néglige l’effet de la contrainte hors plan (perpendiculaire à l’interface de stratification) peut fournir des approximations indicatives mais ne traduit pas entièrement le comportement du réseau de fractures soumis à des tensions polyaxiales. Les résultats de cette étude ont d’importantes implications pour comprendre l’écoulement hétérogène de fluides géologiques (par ex. eau souterraine, pétrole) en milieu souterrain et pour améliorer la perméabilité pour des évaluations à grande échelle.ResumenSe presenta un estudio sobre la influencia de los esfuerzos poliaxiales (triaxiales verdaderas) sobre la permeabilidad de una capa tridimensional (3D) de rocas fracturadas. El sistema de fractura en 3D se construye mediante la extrusión de un patrón de afloramiento 2D de un lecho de caliza que exhibe una estructura de escalera que consiste en un conjunto articulado limitado por fracturas cortas de una etapa posterior. El comportamiento geomecánico de la roca fracturada en 3D en respuesta a los esfuerzos in situ es modelado por el método de elementos finitos y discretos, que puede capturar la deformación de bloques de matriz, la variación de campos de esfuerzos, la reactivación de fracturas irregulares preexistentes y la propagación de nuevas grietas. Una serie de simulaciones numéricas está diseñada para cargar la roca fracturada utilizando diferentes esfuerzos poliaxiales in situ y se calculan además las propiedades de flujo dependientes del esfuerzo. La capa fracturada tiende a exhibir una localización de flujo más fuerte y una permeabilidad equivalente más alta a medida que se incrementa la relación del esfuerzo en el campo lejano y se hace girar el campo de esfuerzo de tal manera que las fracturas se orientan preferentemente para el cizallamiento. La dilatación por cizalladura de fracturas preexistentes tiene efectos dominantes sobre la localización del flujo en el sistema, mientras que la propagación de nuevas fracturas tiene impactos menores. El papel del esfuerzo de la sobrecarga sugiere que el análisis 2D convencional que descuida el efecto del esfuerzo fuera del plano (perpendicular a la interfase del lecho) puede proporcionar aproximaciones indicativas pero no capturar completamente el comportamiento de la red de fractura dependiente del esfuerzo poliaxial. Los resultados de este estudio tienen importantes implicancias para la comprensión del flujo heterogéneo de fluidos geológicos (por ejemplo, aguas subterráneas, petróleo) en el subsuelo y en el escalado de la permeabilidad para evaluaciones a gran escala.摘要本文论述了多轴(真三轴)应力对三维裂隙岩层渗透率影响的研究成果。该石灰岩层具有阶梯结构,早期裂缝贯穿整个系统,后期较短的裂缝毗连早期裂缝。通过对两维露头 的几何沿岩层方向进行延申,建立了三维裂隙系统的几何模型。通过有限-离散元方法模拟了现场应力作用下三维裂隙岩体的力学特性,该方法能捕获岩块变形、应力场变化、自然裂缝的变形以及新裂隙的扩展。本文 设计了一系列的数值模拟实验,模拟加载多种多轴现场应力,进一步分析了渗透率对应力场的依赖性。随着远场应力比的 增加以及应力场的旋转,有些裂缝表现出很强的滑移,岩层也因此表现出很强的局部流动及很高的渗透率。研究表明,已有裂缝的滑移及剪涨效应对渗透率有非常重要的影响,而新裂缝的扩展影响较小。本文中发现的纵向应力效应表明,忽略平面外应力(垂直于层理界面)影响的传统二维分析可能提供了象征性的近似值,但无法完全捕获取决于多轴应力的裂缝网络的特性。本研究的结果对于了解地质流体(如地下水、石油)的不均匀流动、预测大尺度渗透率等具有重要的意义。ResumoUm estudo é apresentado sobre a influência de esforço poliaxial (verdadeiramente triaxial) sobre a permeabilidade de uma camada de rocha fraturada tridimensional. O sistema fraturado 3D é construído extraindo-se um padrão de afloramento 2D de uma camada de calcário que exibe uma estrutura de escada consistindo de um conjunto de juntas interceptadas posteriormente por fraturas pequenas. O comportamento geomecânico 3D da rocha fraturada em resposta aos esforços locais é modelado pelo método de elementos discretos finitos, que pode capturar a deformação da matriz dos blocos, variação do campo de esforços, reativação de fraturas pré-existentes e propagação de novas rachaduras. Uma serie de simulações numéricas é desenvolvida para sobrecarregar a rocha fraturada usando diversos esforços poliaxiais locais e as propriedades de fluxo dependente dos esforços são posteriormente calculadas. A camada fraturada tende a mostrar forte fluxo localizado e maior permeabilidade equivalente assim que a taxa de esforço do campo externo é aumentada e o campo de esforço é rotacionado de forma que as fraturas são preferencialmente orientadas para cisalhamento. A dilatação por cisalhamento de fraturas pré-existentes tem efeitos dominantes no sistema de fluxo localizado, enquanto que a propagação de novas fraturas tem um impacto menor. O efeito do esforço do sobrecarregamento sugere que a análise 2D convencional que negligencia os efeitos dos esforços de fora do plano (perpendicular à interface do acamamento) pode fornecer indicações aproximadas, porém não captar integralmente o comportamento a rede de fraturas dependente do esforço poliaxial. Os resultados deste estudo possuem importantes implicações para entender o fluxo heterogêneo de fluidos geológicos (ex. águas subterrâneas e petróleo) em subsuperfície e aumentar a escala da avaliação da permeabilidade em grande escala.


Pure and Applied Geophysics | 2018

Influence of Landscape Coverage on Measuring Spatial and Length Properties of Rock Fracture Networks: Insights from Numerical Simulation

Wenzhuo Cao; Qinghua Lei

Natural fractures are ubiquitous in the Earth’s crust and often deeply buried in the subsurface. Due to the difficulty in accessing to their three-dimensional structures, the study of fracture network geometry is usually achieved by sampling two-dimensional (2D) exposures at the Earth’s surface through outcrop mapping or aerial photograph techniques. However, the measurement results can be considerably affected by the coverage of forests and other plant species over the exposed fracture patterns. We quantitatively study such effects using numerical simulation. We consider the scenario of nominally isotropic natural fracture systems and represent them using 2D discrete fracture network models governed by fractal and length scaling parameters. The groundcover is modelled as random patches superimposing onto the 2D fracture patterns. The effects of localisation and total coverage of landscape patches are further investigated. The fractal dimension and length exponent of the covered fracture networks are measured and compared with those of the original non-covered patterns. The results show that the measured length exponent increases with the reduced localisation and increased coverage of landscape patches, which is more evident for networks dominated by very large fractures (i.e. small underlying length exponent). However, the landscape coverage seems to have a minor impact on the fractal dimension measurement. The research findings of this paper have important implications for field survey and statistical analysis of geological systems.


International Journal of Rock Mechanics and Mining Sciences | 2014

Effects of geomechanical changes on the validity of a discrete fracture network representation of a realistic two-dimensional fractured rock

Qinghua Lei; John-Paul Latham; Jiansheng Xiang; Chin-Fu Tsang; Philipp Lang; Liwei Guo


Geomechanics for Energy and the Environment | 2015

Polyaxial stress-induced variable aperture model for persistent 3D fracture networks

Qinghua Lei; John-Paul Latham; Jiansheng Xiang; Chin-Fu Tsang


Computers and Geotechnics | 2017

The use of discrete fracture networks for modelling coupled geomechanical and hydrological behaviour of fractured rocks

Qinghua Lei; John-Paul Latham; Chin-Fu Tsang

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Ke Gao

University of Toronto

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J.-P. Latham

Imperial College London

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Philipp Lang

Imperial College London

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