Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 6. GENERATING ANISOTROPIC SEISMIC MODELS BASED ON GEOMECHANICAL SIMULATION Sample Crack set a 0 ξ 0 Sample Crack set a 0 ξ 0 α 11 0.0014 0.165 α 11 0.0006 0.590 1784 α 22 0.0018 0.155 2194 α 22 0.0005 0.635 α 33 0.0012 0.440 α 33 0.0007 0.740 α 11 0.0019 0.315 α 11 0.0009 0.085 1788 α 22 0.0044 0.280 Bere α 22 0.0009 0.085 α 33 0.0014 0.475 α 33 0.0009 0.140 α 11 0.0007 0.345 α 11 0.0005 0.150 1909 α 22 0.0008 0.295 Penr α 22 0.0005 0.180 α 33 0.0009 0.300 α 33 0.0005 0.210 α 11 0.0014 0.190 1950 α 22 0.0004 0.150 α 33 0.0008 0.195 Table 6.2: Best fit initial average crack aspect ratios (a 0 ) and densities (ξ 0 ) for the Clair and literature samples used to calculate the velocities as a function of stress shown in Figure 6.8. The effects of non-hydrostatic stresses on anisotropy also are expected to be important. For example, the effects of uniaxial stresses on seismic anisotropy have been documented (e.g., Scott and Abousleiman, 2004; Sayers and Schutjens, 2007). When the applied stress is uniaxial, cracks with faces perpendicular to the principle stress axis will close, while those parallel will open or remain unaffected. As a result, velocities will be faster in the direction parallel to the maximum stress. Since the stress field within and around reservoirs is likely to be non-hydrostatic, it is important that any model used to estimate seismic velocities is capable of incorporating these effects. For example, Herwanger and Horne (2005) model seismic anisotropy due to a triaxial stress field based on 3rd order elasticity theory (Prioul et al., 2004) to explain shear wave splitting observations from the Valhall and Ekofisk fields. Here I consider non-hydrostatic stresses by resolving the in-situ stress field in terms of stresses normal to the modelled crack faces. This is shown in Figure 6.9. In Figure 6.9a, the results of a hydrostatic compression test on a sample of Berea sandstone (Scott and Abousleiman, 2004) are shown, and best fit ξ 0 and a 0 values computed to back-calculate velocities. Scott and Abousleiman (2004) then perform a uniaxial strain test on a similar core sample. The details of the uniaxial test are shown in Figures 6.9b and 6.9c, and the results plotted in Figure 6.9d. The ξ 0 and a 0 values calculated for the hydrostatic case (given in Table 6.3) are then used to predict velocities for the uniaxial case. It can be seen that upon application of this uniaxial stress, the velocity of the P-waves along the main axis increase rapidly with pressure, while those perpendicular to the main axis increase more slowly. These effects are predicted by our model, and the fit is particularly good for the faster P-waves (V P x ) and fast S-waves (V Syz ), as well as the P-waves at 45 ◦ . The model does not accurately predict the slower P (V P z ) and S (V Sxz ) wave velocities above a confining pressure of 20MPa. This means that the model underestimates the magnitude of shear wave splitting. Since the model predicts 122
6.3. A MICRO-STRUCTURAL MODEL FOR NONLINEAR ELASTICITY a 0 ξ 0 α 11 0.00031 0.250 α 22 0.00061 0.135 α 33 0.00061 0.140 Table 6.3: Best fit initial average crack aspect ratios (a 0 ) and densities (ξ 0 ) calculated from hydrostatic stress test shown in Figure 6.9a, and used to calculate the velocities for the uniaxial test shown in Figure 6.9d. that as confining pressure increases, the crack sets orientated parallel to the main axis will gradually close and lead to increasing velocity of the slower waves. What is observed is that the anisotropy becomes ‘locked in’ (Scott and Abousleiman, 2004) and the velocities do not increase further. The reason for the locking in mechanism remains unclear and so any improvements to our analytical model will require understanding of this mechanism. It is possible that this failure arises partly due to my assumption that all deformation occurring is elastic. Scott and Abousleiman (2004) observe significant amounts of acoustic emissions when the confining stress exceeds 20MPa during the triaxial stress test, indicating that inelastic deformation is indeed occurring. It may also be possible that crack-crack interactions are affecting deformation in the manner similar to that described by Batzle et al. (1980). Time-lapse seismic data can show an asymmetry in the P-wave velocity/effective stress (V P /σ) relationship between stress up (compaction or pore pressure depletion) and stress down (extension or pore pressure increase) effects (e.g., Hatchell and Bourne, 2005). Observations indicate that the increase in V P due to an increase in σ is smaller than the decrease caused by an equivalent σ decrease. The nonlinear nature of my stress-velocity model means that these effects are accounted for to an extent. However, it could be argued that the modelled asymmetry between stress up and stress down effects are not as large as those observed by Hatchell and Bourne (2005), particularly at higher stresses, where the rate of change of the velocity/stress gradient (d 2 V P /dσ 2 ) is lowest (Sayers, 2007). If a degree of irreversible deformation such as cement breakage occurs when the rock is moved from its initial stress state then this will increase the asymmetry, as the decrease in compliance due to a stress increase will be cancelled out by the additional compliance induced by inelastic deformation. In the following section we consider (in a qualitative sense only) how we might deal with inelastic damage within the framework outlined above. 6.3.7 Coring and damage It is becoming increasingly clear among rock physicists that using velocities measured on cored samples may not be representative of the velocities of in situ reservoir rocks. Tests involving synthetic sandstones (e.g., Holt et al., 2000), and comparison of cored samples with well log measurements (e.g., Furre et al., 2007), indicate that in situ rocks generally have higher velocities and a lower stress sensitivity. The explanation forwarded for this is that coring of the sample causes large differential stresses that create permanent damage in the sample. While this effect is compensated to some extent 123
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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
Page 89 and 90: 4.5. INITIAL S-WAVE POLARISATION of
Page 91 and 92: 4.6. INTERPRETATION OF SHEAR WAVE S
Page 93 and 94: 1 1 5 5 30 4.6. INTERPRETATION OF S
Page 95 and 96: 80 4.6. INTERPRETATION OF SHEAR WAV
Page 97 and 98: 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
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Page 145 and 146: 6.4. CALIBRATION WITH LITERATURE DA
Page 147 and 148: 6.4. CALIBRATION WITH LITERATURE DA
Page 149 and 150: 6.5. COMPARISON OF ROCK PHYSICS MOD
Page 151 and 152: 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
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9.1. NOVEL CONTRIBUTIONS techniques
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9.2. FUTURE WORK Finally, the compa
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Bibliography Abt, D. L., Fischer, K
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BIBLIOGRAPHY Gassmann, F., 1951. Ub
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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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