Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 6. GENERATING ANISOTROPIC SEISMIC MODELS BASED ON GEOMECHANICAL SIMULATION essence means that, although a rock might be initially anisotropic, the change in velocity caused by a particular stress change will be the same regardless of the axis along which this stress is applied. The ability to deal with intrinsic and stress induced anisotropy represents a significant advantage for the Prioul et al. (2004) model. However, as with all of the models discussed above, the observed nonlinear stress-velocity relationship is fitted with a linear trend. Prioul et al. (2004) get around this issue by fitting low and high stress regions separately, an approach that significantly limits the general applicability of the model. Furthermore, the model can only be parameterised with triaxial stress velocity measurements. Such experiments are much less common in the literature, so the kind of extensive calibration that I was able to perform for the micro-structural approach is not as easily performed. Given that the Prioul et al. (2004) model is already less intuitive to grasp, the difficulties in parameterisation mean that this approach does not provide such an intuitive framework within which understand how seismic velocities respond to changes in applied stress. The model I have outlined in this chapter attempts to describe the micro-structural response to stress changes, using this as a route to describe seismic properties via an effective medium model. Real rocks do not contain the idealised, penny-shaped discontinuities that I use as the framework for my modelling. However, it has long been recognised (e.g., Hudson, 1981; Sayers and Kachanov, 1995; Schoenberg and Sayers, 1995; Thomsen, 1995; Hall et al., 2008) that the response of compliant grain boundary discontinuities to the infinitesimal strain, high strain rate deformation induced by a seismic wave can be approximated very closely using such an approach. The additional step that I have made is to assume that the compliant grain boundary discontinuities respond in the same manner to the finite strain, low strain rate deformation induced by geomechanical stress changes as they do to the deformation during the passage of a seismic wave. This is a reasonable assumption to make so long as geomechanical deformation remains elastic. The second assumption that I make is that the size distribution of the grain boundary discontinuities can be modelled with an exponential distribution. This assumption is somewhat more arbitrary in its nature, as there is no physical reason why a power law distribution could not be used instead. However, the exponential distribution provides a good match to velocity observations, and is easily parameterised. The advantage gained by describing stress sensitive velocities using a micro-structural model is that we can move closer to understanding the physics behind the phenomenon. By doing so, it is possible to develop a more intuitive understanding of the processes involved. The model I have developed is far more intuitive in its use than the third-order elasticity approach. For instance it is not intuitive to say how increased core damage will affect the three independent terms of Prioul’s isotropic third order tensor. Furthermore, given the need to limit the third-order tensor to isotropy, this model cannot account for anisotropic rock fabrics as seen in many clay and/or mica rich rocks. In contrast, for the micro-structural model it is intuitive to conceive that damage will increase the initial microcrack density terms, while alignment of platy minerals will increase the crack density along one axis of symmetry alone. Additionally, the improved understanding of the physical processes that a micro-structural analysis provides leads to a model that can account for observed phenomena such as intrinsic and stress-induced anisotropy, and the nonlinear response of velocities to stress (the micro-structural model has no need to fit separate linear portions of the stress-velocity curve). 132
6.5. COMPARISON OF ROCK PHYSICS MODELS Furthermore, the microstuctural model can be easily parameterised and calibrated using any kind of stress-velocity data, as Doug Angus and I have performed for over 200 sample datasets found in the literature. Because it can account for many observed phenomena, including the nonlinear response to a triaxial stress tensor, and because it has been calibrated using a range of lithologies, the model is highly generalised, capable of deployment without modification for a range of scenarios. In the following chapters I will use this model to investigate the effects of geomechanical deformation on seismic observables. 133
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Microseismic Monitoring and Geomech
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Abstract Capture of CO 2 produced a
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Author’s Declaration I declare th
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Acknowledgments There are many thin
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Table of Contents Abstract Author
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TABLE OF CONTENTS 5.4 Results . . .
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Preface ‘Research is not an end i
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6. D. Angus, J-M. Kendall, J.P. Ver
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1 Introduction A technology push ap
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1.2. CCS OVERVIEW Figure 1.2: CCS s
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1.3. THESIS OVERVIEW for CO 2 to be
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1.3. THESIS OVERVIEW as microseismi
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2 The Weyburn CO 2 injection projec
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2.2. WEYBURN GEOLOGICAL SETTING Fig
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2.2. WEYBURN GEOLOGICAL SETTING N F
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2.4. EVENT TIMING AND LOCATIONS 500
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2.4. EVENT TIMING AND LOCATIONS 500
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2.4. EVENT TIMING AND LOCATIONS Nor
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2.4. EVENT TIMING AND LOCATIONS Fig
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2.5. DISCUSSION 500 1000 1100 North
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2.6. SUMMARY 2.6 Summary • CO 2 s
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3 Inverting shear-wave splitting me
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3.2. INVERSION METHOD the additiona
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3.2. INVERSION METHOD 1. P-wave inc
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3.2. INVERSION METHOD 180 160 140 C
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3.2. INVERSION METHOD 2800 2800 260
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3.2. INVERSION METHOD Loop over γ,
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20 10 5 5 1 20 30 40 80 100 60 40 1
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3.4. SWS MEASUREMENTS AT WEYBURN th
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3.4. SWS MEASUREMENTS AT WEYBURN ei
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3.4. SWS MEASUREMENTS AT WEYBURN ξ
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3.4. SWS MEASUREMENTS AT WEYBURN da
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4 2 2 3.4. SWS MEASUREMENTS AT WEYB
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1 1 1 1 1 1 1 1 1 1 3.5. DISCUSSION
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3.6. SUMMARY 3.6 Summary • I have
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4 A comparison of microseismic moni
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4.2. EVENT LOCATIONS 2400 Velocity
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4.2. EVENT LOCATIONS 200 150 Northi
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4.3. EVENT MAGNITUDES Pressure (MPa
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4.3. EVENT MAGNITUDES 160 140 120 N
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4.4. SHEAR WAVE SPLITTING 50 −3 E
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4.5. INITIAL S-WAVE POLARISATION In
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4.5. INITIAL S-WAVE POLARISATION 6
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4.5. INITIAL S-WAVE POLARISATION of
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4.6. INTERPRETATION OF SHEAR WAVE S
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1 1 5 5 30 4.6. INTERPRETATION OF S
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80 4.6. INTERPRETATION OF SHEAR WAV
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2 3 3.5 4 5 2 1.8 1.4 4 4.6. INTERP
Page 99 and 100: 4.7. DISCUSSION 4.7 Discussion The
Page 101 and 102: pressure, P fl σ ′ ij = σ ij
Page 103 and 104: 5.2. EFFECTIVE STRESS AND STRESS PA
Page 105 and 106: 5.3. NUMERICAL MODELLING 5.3.1 Flui
Page 107 and 108: 5.3. NUMERICAL MODELLING MORE FLUID
Page 109 and 110: 5.3. NUMERICAL MODELLING (a) (b) Fi
Page 111 and 112: 5.4. RESULTS 0 500 1000 Depth (m) 1
Page 113 and 114: 5.4. RESULTS 1 0.9 0.8 Soft Med Sti
Page 115 and 116: 5.4. RESULTS Overburden Extension i
Page 117 and 118: 5.4. RESULTS 1z:100x:100y 1z:100x:5
Page 119 and 120: 5.5. SURFACE UPLIFT 1 0.9 0.8 Soft
Page 121 and 122: 5.5. SURFACE UPLIFT Figure 5.17: Ma
Page 123 and 124: 6 Generating anisotropic seismic mo
Page 125 and 126: 6.2. STRESS-SENSITIVE ROCK PHYSICS
Page 127 and 128: 6.3. A MICRO-STRUCTURAL MODEL FOR N
Page 145 and 146: 6.4. CALIBRATION WITH LITERATURE DA
Page 147 and 148: 6.4. CALIBRATION WITH LITERATURE DA
Page 149: 6.5. COMPARISON OF ROCK PHYSICS MOD
Page 153 and 154: 7 Forward modelling of seismic prop
Page 155 and 156: 7.2. SEISMODEL c⃝ WORKFLOW time s
Page 157 and 158: 7.3. RESULTS FROM SIMPLE GEOMECHANI
Page 163 and 164: 7.4. SUMMARY 7.4 Summary • I have
Page 165 and 166: 8 Linking geomechanical modelling a
Page 167 and 168: 8.2. MODEL DESCRIPTION Layer Thickn
Page 169 and 170: 8.2. MODEL DESCRIPTION Unit E (GPa)
Page 171 and 172: 8.3. RESULTS Marl Vugg 0.8 0.8 Norm
Page 173 and 174: 8.3. RESULTS 15 10 Injector Produce
Page 175 and 176: 8.3. RESULTS 2000 15 2000 15 1500 1
Page 177 and 178: 8.4. A SOFTER RESERVOIR 8.4 A softe
Page 179 and 180: 8.4. A SOFTER RESERVOIR 2000 15 200
Page 181 and 182: 8.4. A SOFTER RESERVOIR 2000 Max SW
Page 183 and 184: 8.5. DISCUSSION pore-pressure being
Page 185 and 186: 8.6. SUMMARY 8.6 Summary • I appl
Page 187 and 188: 9 Conclusions Geological storage wi
Page 189 and 190: calibrate. This model has been cali
Page 191 and 192: 9.1. NOVEL CONTRIBUTIONS techniques
Page 193 and 194: 9.2. FUTURE WORK Finally, the compa
Page 195 and 196: Bibliography Abt, D. L., Fischer, K
Page 197 and 198: BIBLIOGRAPHY Gassmann, F., 1951. Ub
Page 199 and 200: BIBLIOGRAPHY Kendall, J.-M., Pilido
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BIBLIOGRAPHY Sayers, C. M., 2002. S
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BIBLIOGRAPHY White, D., 2009. Monit
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A List of Symbols The table below l
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B In Support of Carbon Capture and
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Both China and India are aggressive
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Utsira rock properties vary signifi
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