NumericalModellingandAnalysisofFluidFlowandDeformationofFracturedRockMasses0080439314.pdf

1、Numerical Modelling and Analysis of Fluid Flow and Deformation of Fractured Rock Masses Numerical Modelling and Analysis of Fluid Flow and Deformation of Fractured Rock Masses This Page Intentionally Left BlankNumerical Modelling and Analysis of Fluid Flow and Deformation of Fractured Rock Masses Xi

2、ng Zhang and David J.Sanderson Imperial College of Science,Technology and Medicine University of London,London,UK 2002 PERGAMON An Imprint of Elsevier Science Amsterdam-Boston-London-New York-Oxford-Paris San Diego-San Francisco-Singapore-Sydney-Tokyo ELSEVIER SCIENCE Ltd The Boulevard,Langford Lane

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11、 use or operation of any methods,products,instructions or ideas contained in the material herein.Because of rapid advances in the medical sciences,in particular,independent verification of diagnoses and drug dosages should be made.First edition 2002 Library of Congress Cataloging in Publication Data

12、 A catalog record from the Library of Congress has been applied for.British Library Cataloguing in Publication Data A catalogue record from the British Library has been applied for.ISBN:0-08-043931-4)The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992(Permanence of Pap

13、er).Printed in The Netherlands.PREFACE Our understanding of the subsurface system of the earth is becoming increasingly more sophisticated both at the level of the behaviour of its components(solid,liquid and gas)as well as their variations in space and time.The implementation of coupled models(e.g.

14、mechanical-hydraulic,mechanical-thermal and hydraulic-thermal)is essential for the understanding of an increasing number of natural phenomena and in predicting human impact on these.There is growing interest in the relation between fluid flow and deformation in subsurface rock systems that character

15、ise the upper crust.This has led to knowledge becoming increasingly specialized in many branches of earth sciences(structural geology,geophysics,rock mechanics,metamorphic petrology,petroleum geology)and engineering,(including hydrology,waste management,petroleum engineering,civil engineering,enviro

16、nmental engineering and mining engineering).A multidisciplinary subject dealing with deformation and fluid flow in the subsurface system is emerging.Investigating a subsurface system of the Earth requires reliable methods of predicting deformation and fluid flow in natural,heterogeneous rocks.These

17、must incorporate the essential physics,accommodate appropriate material properties and boundary conditions,and allow quantitative estimates of flow parameters,together with uncertainties,at every resolvable scale.The indispensability of numerical modelling of subsurface systems may be justified by t

18、he following:1.An intrinsic characteristic of a subsurface system is the difficulty,or even impossibility,of carrying out meaningful experiments in situ.In other areas of science the validation of models and their tuning may be obtained by means of physical experiments.Numerical modelling often prov

19、ides the only means of testing hypotheses in subsurface systems.2.The properties and parameters of a subsurface system are poorly understood in comparison to artificial materials.For such a data-limited system,sensitivity analysis of parameters using numerical modelling methods is an effective way t

20、o gain an understanding of the relative importance of parameters and the uncertainty of models.3.Mathematical models,incorporating statistical information,fractal concepts,etc.,can be easily employed in a numerical model to optimise the use available information in rock-masses which are heterogeneou

21、s at many scales.Whilst research in the subject area of faulting,fracturing and fluid flow has led to significant progress in many different areas,the approach has tended to be reductionist,i.e.involving the isolation and simplification of phenomena so that they may be treated as single physical pro

22、cesses.The reality is that many processes operate together within subsurface systems,and this is particularly true for fluid flow and deformation of fractured rock masses.The aim of this book is to begin to explore how vi Numerical Modelling of Fluid Flow and Deformation advances in numerical modell

23、ing can be applied to understanding the complex phenomena observed in such systems.This book is mainly based on original research,published by the authors over the last 10 years,but the fundamental principles and practical methods of numerical modelling,in particular distinct element methods,are als

24、o included.The book aims to explore the principles of numerical modelling and the methodologies for some of the most important problems,in addition to providing practical models with detailed discussions on various topics.We hope to provide a roadmap for those readers who want to construct and devel

25、op their own models.In Chapter 1,we will introduce the fundamental principles of numerical modelling and discuss the difference between continuous medium and discontinuous medium modelling methods.The distinct element method is introduced and the reasons why this method is used to model a subsurface

26、 system and the interaction with fluids will be discussed at length.A series of simple models will be presented in Chapter 2 to demonstrate the basic functions.The models start with very simple parameters and then,step by step,more complex models are developed.The distinct element method will be exp

27、lained from the governing equations used to its practical implementation.At this stage,a series of examples with input and output commands,are provided to illustrate the modelling procedures.In this way the reader may repeat similar problems and develop them into more complex models.In Chapters 3 to

28、 8,practical methodologies are provided to show how to tackle some important problems encountered in investigating the fluid flow and deformation of fractured rock systems and their interrelationships.Detailed examples cover a range of concepts from model-based process investigations to site-specifi

29、c simulations and building continuum approximations.These issues include:(a)Evaluation of 2-D permeability tensors for fractured rock systems and the effects of fracture network geometry and applied stresses on fluid flow;(b)A scaling technique is developed to determine if a Representative Element V

30、olume(REV)exists within a fractured rock mass by a scaling-down process.The overall hydraulic conductivity tensor of a large region may also be determined from the modelling of exposure-scale fractured networks by a scaling-up process;(c)Connectivity,permeability and deformation of fractured rock ma

31、sses are shown to display critical behaviour controlled by geometry and stress,which may be described by universal laws on the basis of percolation models;(d)Models of fluid flow and deformation in regions of fractured rock around extensional faults suggest that fault slip has significant effects on

32、 distribution of porosity,fluid pressure fluctuation,stress redistribution and fluid flow;(e)Localised behaviour of fractured rock masses is a common phenomenon in the upper crust.Diffuse flow through fracture networks changes to highly localised flow at a critical stress state.Fractal and multifrac

33、tal techniques have been used to characterise heterogeneity of the flow in order to generalise the results of modelling of discrete systems to a continuum-based description of the flow;and Preface vii(f)A dual permeability model is developed to investigate the initial and propagation of fractures at

34、 grain scale,and associated changes in permeability.In the last part of this book,emphasis is put on some practical applications of distinct element modelling.In Chapter 9,a numerical model of the 3-D permeability tensor for the rock-mass around the ship-locks of the Three Gorges Project,China,is de

35、veloped and calibrated to in situ flow measurement,using an inverse method involving trial and error adjustment of hydraulic parameters in the model to best match the field data.In Chapter 10,the mechanical and hydraulic behaviour of fractured rock in a wellbore is modelled and used to tackle wellbo

36、re instability problems encountered in many oil and gas fields.Funding for much of the work described in this book was provided by the British Natural Environmental Research Council(GR9/915,GR9/1243,GST/02/2311 and GST/03/2311),the European Commission(JOU2-CT93-0377),Chinese Academy,British Geologic

37、al Survey,British Petroleum Ltd,Nirex Ltd(U.K.)and K.C.Wong Education Foundation(Hong Kong).Natural fracture networks were sampled in conjunction with British Geological Survey,Chinese Academy,Beijing University of Science and Technology,Nirex Ltd(UK)and GeoScience Ltd.The mapping of fracture trace

38、maps and the collecting of data involved David Bailey,C.C.Chen,Simon Day,Steven Dee,Paul Jackson,Q.S.Jing,David Jobson,David Johnston and Simon Young.The work in this book was carried out in the Geomechanics Research Group originally set up in the University of Southampton and continued,since 1998,j

39、ointly by Southampton and Imperial College,London.We would like to thank our colleagues Richard Harkness,William Powrie,Nigel Last,Sijin Wang,David Peacock and Andrew Barker for advice and discussion on various aspects of the work in this book.The authors would also like to thank the many reviewers

40、for their constructive comments on the original papers that form the basis of this book;these include I.W.Farmer,Kes Heffer,Lidia Lonergan,Dick Norris,Carl Renshaw,P.G.Roberts,Steve Roberts,Rick Sibson,K.Watanabe and Robert Zimmerman.David Pollard and John Barker are particularly thanked for their h

41、elpful comments and constructive suggestions on an early outline of the book.Most of the modelling presented in this book was carried out using various versions of UDEC(Universal Distinct Element Code).The authors are grateful to Dr Peter Cundall for developing this code and for his association with

42、 the Geomechanics Research Group at Southampton.The code is available commercially from Itasca Consulting Group Inc.,who has provided excellent support and service.This Page Intentionally Left BlankCONTENTS Preface Chapter I Introduction to Modelling Deformation and Fluid Flow of Fractured Rock 1.1.

43、Introduction 1.2.Approaches to modelling rock systems 1.3.Continuum models 1.4.Flow models 1.5.Discontinuum models 1.6.Overview of UDEC 1.7.Summary of numerical modelling Chapter 2 Modelling of Simple Rock Blocks 2.1.2.2.2.3.2.4.2.5.2.6.2.7.2.8.2.9.Introduction Basic components of natural fracture n

44、etworks Model geometry and initial conditions Basic behaviour of deformation and fluid flow Effects of fracture geometry Effects of fracture properties Effects of applied boundary stresses Effects of rock deformation models Summary Chapter 3 Evaluation of 2-Dimensional Permeability Tensors 3.1.Intro

45、duction 3.2.Calculation of components of flow-rates 3.3.Permeability in naturally fractured rocks 3.4.Geometrical effects on permeability 3.5.Effects of stress on permeability 3.6.Conclusions Appendix 3-A 1:Input codes for example one Appendix 3-A2 Derivation of 2-D permeability tensor Chapter 4 Sca

46、ling of 2-D Permeability Tensors 4.1.Introduction 4.2.Development of the previous approach 4.3.Testing the concept of a representative element volume by down-scaling 4.4.Scaling-up of permeability 4.5.Effects of sample number and sample size 1 1 2 4 6 8 9 18 23 23 23 25 25 31 38 42 44 48 53 53 54 59

47、 63 69 80 85 87 91 91 92 94 99 103 x Numerical Modelling of Fluid Flow and Deformation 4.6.Determining the permeability of a region 4.7.Conclusions Chapter 5 Percolation Behaviour of Fracture Networks 5.1.Introduction 5.2.Modelling of 2-dimensional fracture networks 5.3.Density,percolation threshold

48、 and fractal dimension 5.4.Critical behaviour of fractured rock masses 5.5.Conclusions Chapter 6 Slip and Fluid Flow around An Extensional Fault 6.1.Introduction 6.2.Outline of modelling 6.3.Stress distribution and fluid flow in model A:At a shallow depth with a hydrostatic fluid pressure 6.4.Compar

49、ison of model A with a supra-hydrostatic fluid pressure at greater depth 6.5.Effects of irregularities in fault zone 6.6.Discussion of dynamic response of fluid-dilation interactions 6.7.Conclusions Chapter 7 Instability and Associated Localization of Deformation and Fluid Flow in Fractured Rocks 7.

50、1.Introduction 7.2.Numerical determination of instability 7.3.Instability and R-ratio 7.4.Effects of fracture network geometry 7.5.Multifractal description of flow localisation 7.6.Permeability of three natural fracture networks before and at critical stress state 7.7.Effects of loading direction 7.