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Seismic Fracture Characterization

Concepts and Practical Applications (EET 8)

image of Seismic Fracture Characterization
  • By Enru Liu and Alex Martinez
  • Format: EPUB
  • Publication Year: 2012
  • Number of Pages: 280
  • Language: English
  • Ebook ISBN: 9789073834507

During the last three decades, seismic anisotropy has evolved from a purely academic research topic into applications in the mainstream of applied geophysics. Today, nobody doubts that the earth is anisotropic and most (if not all) hydrocarbon reservoirs are anisotropic. Since shale accounts for 70% of sedimentary basins and fractures exist in all reservoirs, seismic anisotropy may be even more extensive than we think. Taking anisotropy into account in seismic processing has improved the quality of seismic images, even though it makes seismic processing more challenging since additional parameters are needed. At the same time, fracture characterization using the concept of seismic anisotropy has added value in reservoir characterization, reservoir management, and has increased recovery and optimized well locations. This book and the associated course provide an introduction to the fundamental concepts of seismic fracture characterization by introducing seismic anisotropy, equivalent-medium representation theories of fractured rock and methodologies for extracting fracture parameters from seismic data. We focus on practical applications using extensive field data examples.

Three case studies are included to demonstrate the applicability, workflow and limitations of this technology: a physical laboratory 3D experiment where fracture distributions are known, a Middle East fractured carbonate reservoir and a fractured tight gas reservoir. Our ultimate goal is to build discrete fracture network models incorporating all data. These models should not only be geologically consistent but also geophysically and geomechanically consistent, so that the models can be used to forecast the behaviour and performance of fractured reservoirs.

The EAGE’s Education Tour (EET) offers a one-day course delivered by renowned geoscientists at various locations globally. Accompanied by a comprehensive course book, it provides members and others with access to the latest developments in key topics in the Geosciences. The Tour has been a great success since its launch in 2006.

© 2012 EAGE Publications bv

Table of Contents

Acknowledgements
General disclaimer
Preface
1 Introduction

1.1 Purpose
1.2 Target readers
1.3 “All reservoirs should be considered as fractured unless proven otherwise”
1.4 Classification of fractured reservoirs
1.5 Basic geological elements of fractures, joints, faults and microcracks
1.5.1 Fractures
1.5.2 Joints
1.5.3 Faults
1.5.4 Microcracks
1.5.5 Controlling factors for fractures
1.6 Non-seismic methods for fracture characterization
1.7 Seismic fracture characterization technology: A snapshot
1.8 Geophysics as a bridge between geology and physics
1.9 Outline of this book

2 Fundamentals of seismic anisotropy
2.1 Introduction: Definition and symmetry classes
2.1.1 Definition
2.1.2 Symmetry classes
2.2 Characteristics of wave propagation in anisotropic media
2.3 Physical causes of seismic anisotropy
2.3.1 Shale and intrinsic anisotropy due to aligned crystals
2.3.2 Fine layering-induced anisotropy
2.3.3 Direct stress-induced anisotropy
2.3.4 Fracture-induced anisotropy
2.4 Thomsen’s anisotropic parameters
2.4.1 Thomsen’s anisotropic parameters for VTI media
2.4.2 Are Thomsen’s three anisotropic parameters independent?
2.4.3 Notations for general anisotropic media
2.5 Applications of seismic anisotropy in exploration and reservoir geophysics
2.5.1 Anisotropy helps to improve seismic imaging
2.5.2 Anisotropy helps to improve seismic-well tie and AVO analysis
2.5.3 Anisotropy is a basis for seismic fracture characterization
2.6 The ExxonMobil Canyon Lake Experiment
2.7 Anisotropy and heterogeneity
2.8 Clarification on terminology
2.9 Summary

3 Equivalent medium modelling of fractured rock
3.1 Introduction
3.2 Brief review of effective medium theories and fracture modelling
3.3 Definition and parametrizations
3.3.1 Crack density for ellipsoidal inclusions
3.3.2 Fracture density for layer-bounded or bed-limited fractures
3.4 Modelling fractured media
3.4.1 Hudson’s model of cracked media
3.4.2 Equant porosity models: Fractures in porous media
3.4.3 Modelling multi-scale fractures and frequency-dependent anisotropy
3.5 Fracture compliance and Schoenberg-Sayers model
3.5.1 Displacement-discontinuity or slip-interface model
3.5.2 Fracture modelled as a planar distribution of cracks or “a collection of cracks”
3.5.3 Fracture compliance ratio as a fluid indicator
3.5.4 A note on compliance and stiffness expressions
3.6 Modelling complex fracture systems
3.7 Effects of fluids: Static and dynamic behaviours
3.8 Mechanical and hydraulic responses of fracture systems
3.9 Summary

4 Estimation of fracture parameters from azimuth analysis of P-wave data
4.1 Introduction
4.2 Azimuthal variations of NMO velocity and traveltime
4.2.1 NMO velocity ellipse in anisotropic media: Grechka-Tsvankin equation
4.2.2 Azimuthal variation in interval traveltime: Li’s “equation
4.3 Azimuthal AVO analysis in fractured rock: Corrigan-Rüger equation
4.3.1 Determination of anisotropic AVO gradient B1 from a fixed offset
4.3.2 Relating AVO gradient to fracture density
4.4 Other formulations and attributes
4.5 Implementation and practical considerations
4.6 “Fracture substitution” modelling and sensitivity study
4.7 High-order terms and non-HTI effects
4.8 Summary

5 Multicomponent seismology and its application to fracture characterization
5.1 Introduction
5.2 Shear-waves and multicomponent seismology
5.2.1 Vp/Vs as a lithological indicator: Example from Grane field, North Sea
5.2.2 Imaging complex subsurface structures using PS-waves
5.3 Shear-wave splitting as a fracture diagnostic
5.3.1 Shear-wave splitting: Early development
5.3.2 Shear-wave splitting analysis: 2C rotation
5.3.3 Shear-wave splitting analysis: Alford rotation for 2×2C data
5.4 Examples form multicomponent VSP data
5.5 Shear-wave amplitude anomalies
5.6 Converted PS-wave analysis
5.6.1 Gaiser’s method for combining orthogonal lines
5.6.2 PS reflectivity in anisotropic media
5.6.3 Other PS-wave attributes
5.7 Characterization of multi-fracture sets
5.7.1 Shear-wave polarization in media with multiple sets of fractures
5.7.2 PP and PS reflectivities in media with multiple sets of fractures
5.8 Kinematic and dynamic attributes: Effects of fluids
5.9 Summary

6 Fracture detection using 3D seismic data: physical laboratory studies
6.1 Introduction and results
6.2 The physical models
6.3 Data acquisition
6.4 Initial data processing
6.4.1 Model 1 data processing
6.4.2 Model 2 data processing
6.5 Azimuthal variations of P-wave attributes
6.5.1 Model 1 analysis
6.5.2 Model 2 analysis
6.6 Extracting fracture parameters
6.7 Results
6.7.1 Model 1 results
6.7.2 Model 2 results
6.8 Discussion
6.8.1 Choice of attributes
6.8.2 Choice of methods
6.8.3 Effects of acquisition
6.8.4 Effects of structural complexity
6.9 Summary

7 Mitigation of overburden effects in azimuthal AVO analysis of a Middle East carbonate field
7.1 Introduction
7.2 Study area
7.3 Methodology and target-oriented workflow
7.4 Data preparation
7.4.1 Azimuth-offset distribution, superbinning and azimuth sectoring
7.4.2 Usable/effective offset range
7.4.3 Impact of superbinning size on anisotropy estimation
7.5 Acquisition footprint and linear noise mitigation
7.6 Overburden correction/compensation
7.6.1 Evidence of overburden effects
7.6.2 Overburden correction method
7.6.3 Test of overburden correction scheme
7.7 Results and comparison with core data
7.8 Summary

8 Integrated study of fracture characterization of a tight gas reservoir using 3C-3D seismic data at Piceance Basin, Colorado, USA
8.1 Introduction
8.2 Study area and Piceance 3C-3D data
8.3 Evidence of fractures at Piceance from well and VSP data
8.3.1 Log measurements of fractures
8.3.2 VSP measurements of fractures
8.4 Surface 3C-3D seismic programme
8.4.1 Data acquisition
8.4.2 Converted PS-wave data processing
8.5 Evidence of HTI anisotropy and azimuthal analysis
8.5.1 Evidence of azimuthal analysis of P-waves
8.5.2 Azimuth PP-wave attribute analysis
8.6 Preliminary interpretation
8.7 Summary

9 Summary and road ahead
9.1 Summary
9.2 New developments
9.2.1 Frequency- and scale-dependent seismic anisotropy
9.2.2 Attenuation anisotropy
9.2.3 Modelling discrete fractures: Beyond the equivalent medium modelling
9.3 Integration of complementary information
9.4 Road ahead – from seismic “fracture” attributes to reservoir simulation
9.5 Concluding remarks

References
Appendix A. The two-index notation of elastic tensors
Appendix B. Wave propagation in anisotropic media
Appendix C. Effective elastic constants of cracked media – Hudson’s model
Appendix D. Plane wave reflection and transmission coefficients in anisotropic media – Schoenberg and Protazio algorithm
Index

References

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