Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 6. GENERATING ANISOTROPIC SEISMIC MODELS BASED ON GEOMECHANICAL SIMULATION where c r = 2(1 − νr ) πµ r a 0 (6.38) and ξ 0 is the crack density at a defined initial pressure (usually 0 MPa). As discussed in the previous section, by making the scalar crack assumption, we treat the overall crack distribution as three mutually orthogonal aligned sets, each contributing to one of the nonzero components of α. For each set, an initial crack density and average aspect ratio is defined; hence, for any applied stress field, α is calculated using equations 6.37 and 6.38 to give ⎛ ⎞ ξ 1 (σ c(n1) )/h 1 0 0 α ij = ⎜ ⎝ 0 ξ 2 (σ c(n2) )/h 2 0 ⎟ ⎠ . (6.39) 0 0 ξ 3 (σ c(n3) )/h 3 6.3.5 Results Figure 6.8 shows the results of modelling the P- and S-wave velocities using equations 6.37 to 6.39 for the samples discussed in the previous section. Table 6.2 shows the best fit initial average aspect ratios and crack densities used to produce these models. The fit between observed and modelled velocities is reasonable. Furthermore, the initial aspect ratios range between 5 × 10 −4 < a 0 < 5 × 10 −3 , which is a reasonable range of values expected for a distribution of flat, penny shaped cracks (Kuster and Toksoz, 1974). The results from Figure 6.8 and Table 6.2 indicate that the nonlinear elastic behaviour can be modelled based on the assumption that it is made up of stiff, non-deforming mineral grains and displacement discontinuities in the form of flat, penny shaped cracks with physically reasonable initial aspect ratio distributions. 6.3.6 Anisotropy A benefit of my approach is the treatment of anisotropy. This model is capable of considering intrinsic anisotropy as well as stress induced anisotropy. Most rocks are intrinsically anisotropic. This intrinsic anisotropy is derived from two sources: alignment of minerals and alignment of fabrics. The alignment of mineral grains due to depositional, deformation or diagenetic processes (crystal preferred orientations, CPO) has been well studied as a cause of anisotropy (e.g., Blackman et al., 1993; Rümpker et al., 1999; Barruol and Hoffmann, 1999; Kendall et al., 2007; Valcke et al., 2006). Elongate or platy minerals, such as micas and clays will tend to become aligned during deposition. The elasticities of these minerals can be highly anisotropic, with the principle axes of the elastic tensor aligned with the grain shape. By using the geomathematical model developed by Kendall et al. (2007) to evaluate the background compliance S r , we are able to assess the contribution of CPO to the anisotropy of a sample based on detailed petrofabric analysis. Equation 6.11 limits us to cases where the principle axes of the compliance tensor and of α are aligned. This should not pose a problem for VTI systems so long as one of the principle stress axes to be aligned vertically; however, more complicated anisotropic symmetries will still need to be dealt with carefully. 120
6.3. A MICRO-STRUCTURAL MODEL FOR NONLINEAR ELASTICITY 5000 1784 4000 1788 4500 1909 5500 1950 4500 3500 4000 5000 Velocity (m/s) 4000 3500 3000 Velocity (m/s) 3000 2500 Velocity (m/s) 3500 3000 Velocity (m/s) 4500 4000 3500 3000 2500 2000 2500 2500 2000 0 20 40 Pressure (MPa) 1500 0 20 40 Pressure (MPa) 2000 0 20 40 Pressure (MPa) 2000 0 20 40 Pressure (MPa) 4500 2194 4200 Bere 4500 Penr 4000 4000 3800 4000 3500 3600 Velocity (m/s) 3000 2500 Velocity (m/s) 3400 3200 3000 2800 Velocity (m/s) 3500 3000 2000 2600 2500 2400 1500 0 20 40 Pressure (MPa) 2200 0 50 100 Pressure (MPa) 2000 0 20 40 60 Pressure (MPa) Figure 6.8: Modelled stress dependent velocities calculated using equations 6.37 - 6.39 for the Clair and literature samples (lines), shown with observed velocities (symbols). Red - V P x, blue - V P y , green - V P z , black - V P 45 , cyan - V Sxy , magenta - V Sxz , yellow - V Syz . Shape preferred orientation (SPO) anisotropy is also related to alignment of fabric during sedimentary deposition and/or diagenesis. If platy or elongate grains are deposited in a manner such that there is a preferential alignment between grain contacts, then there will be an increase in displacement discontinuities in this direction, and hence an increased compliance. This effect is best demonstrated by the Clair samples 1784 and 1788 (Hall et al., 2008). These samples are mica rich, and these platy grains are orientated with normals parallel to the z-axis. With this the case, we expect to find that there are a greater number of grain boundaries with normals parallel to the z-axis than to the x- or y-axis. As a result, V P z is greatly reduced (Figure 6.2), and the inversion for α indicates that α 33 is larger than α 11 and α 22 . Since the preferred orientation of mineralogical axes and grain boundaries will not be greatly affected by in situ reservoir stresses (unless these are of sufficient magnitude to cause deformation or failure of the mineral grains), I refer to the anisotropy that they generate as static anisotropy. 121
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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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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
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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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