Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 7. FORWARD MODELLING OF SEISMIC PROPERTIES 1z100x100y 1z100x5y 1z5x5y ∆V (ms −1 ) ∆V (ms −1 ) ∆V (ms −1 ) −300 −200 −100 0 100 200 300 2200 ∆V ∆t 2300 −300 −200 −100 0 100 200 300 2200 2300 −300 −200 −100 0 100 200 300 2200 2300 2400 2400 2400 Soft Depth (m) 2500 2600 2700 2800 Depth (m) 2500 2600 2700 2800 Depth (m) 2500 2600 2700 2800 2900 2900 2900 3000 3000 3000 3100 3100 3100 −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) ∆V (ms −1 ) ∆V (ms −1 ) ∆V (ms −1 ) −300 −200 −100 0 100 200 300 2200 −300 −200 −100 0 100 200 300 2200 −300 −200 −100 0 100 200 300 2200 2300 2300 2300 2400 2400 2400 Medium Depth (m) 2500 2600 2700 2800 Depth (m) 2500 2600 2700 2800 Depth (m) 2500 2600 2700 2800 2900 2900 2900 3000 3000 3000 3100 3100 3100 −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) ∆V (ms −1 ) ∆V (ms −1 ) ∆V (ms −1 ) −300 −200 −100 0 100 200 300 2200 −300 −200 −100 0 100 200 300 2200 −300 −200 −100 0 100 200 300 2200 2300 2300 2300 2400 2400 2400 Stiff Depth (m) 2500 2600 2700 2800 Depth (m) 2500 2600 2700 2800 Depth (m) 2500 2600 2700 2800 2900 2900 2900 3000 3000 3000 3100 3100 3100 −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) −5 −4 −3 −2 −1 0 1 2 3 4 5 ∆t (ms) Figure 7.1: Changes in the normal incidence P-wave velocity (red) and travel time (black) through the centre of the geomechanical models developed in Chapter 5. The reservoir interval is marked by the green dotted lines. to the two-way travel time (TWTT) for reflections coming from the overburden and reservoir. These are plotted in Figure 7.1. Parameter Value Parameter Value β w 1.0 a 0 0.0005 ξreservoir 0 0.1 ξ f 0.0 ξoverburden 0 0.05 Table 7.2: User-defined input parameters to compute changes in seismic velocities for the simple reservoir models. For all cases in Figure 7.1 there is a decrease in velocity in the reservoir. No fluid saturation changes have been included in this model, so this decrease is caused solely by stress and pore fluid pressure changes. The presence of CO 2 would cause further velocity decreases, and so disentangling the contributions to velocity slowdown from pressure and saturation changes will be difficult without some form of rock physics and geomechanical model. If the slowdown is assumed to be caused by saturation changes alone (as is often the case) then CO 2 saturation could be overestimated. In Chapter 5 I noted that the small, soft reservoirs were most prone to stress arching and compression in the overburden. This overburden compression leads to increases in vertical P-wave velocity. 140
7.3. RESULTS FROM SIMPLE GEOMECHANICAL MODELS This can be seen for the soft 1z100x5y and 1z5x5y models (Figure 7.1). The accumulated TWTT change is at most 1ms, a small but detectable shift. At present no CCS project has looked for overburden travel time-shifts, so we cannot know whether such shifts really do occur above CCS reservoirs, but I noted in Chapter 5 that reservoirs where stress arching develops are at a greater risk of failure in the overburden. TWTT shifts in the overburden might provide a tool to image whether or not this is happening. However, the pore pressure changes simulated in the geomechanical model were large, as were the contrasts between material properties in the reservoir and overburden, and yet the TWTT shift is at the lower end of what is detectable. Therefore it is conceivable that stress arching can occur without generating detectable overburden TWTT shifts. That said, more complicated and realistic models may produce larger time-shifts. Further work is needed both in terms of better measurements of overburden material properties for modelling, and in looking more diligently at changes in TWTT from overburden reflections from real datasets. 7.3.2 Shear wave splitting As we have seen in earlier chapters, shear wave splitting can be a useful tool for identifying nonhydrostatic stress changes. Yet SWS has rarely been used to link seismic observations with geomechanical deformation. This is in part because splitting is rarely measured in or above reservoirs as part of industry standard practise, and partly because few rock physics models, such as Prioul et al. (2004) and Verdon et al. (2008), exist that translate triaxial stress changes into variations in anisotropy. At Valhall, a producing reservoir in the North Sea that is experiencing significant subsidence, Olofsson et al. (2003) noted a characteristic ‘ring’ of SWS in the overburden marking the extent of the region of depletion in the reservoir. Herwanger (2007) generated a geomechanical model of the Valhall reservoir, and used the Prioul et al. (2004) model to predict the anisotropy, and thereby the SWS, in the reservoir and overburden, finding that it is subsidence over the depleting reservoir that has generated the SWS pattern. The good match between SWS observations and geomechanical modelling found by Herwanger (2007) is promising, yet attempts to link seismic observation with geomechanical modelling are still generally limited to overburden P-wave travel times. In this section I will generate predictions about splitting generated by my simple models. In Figure 7.2 I plot the fast directions and splitting magnitudes for a shear wave travelling vertically through the 3 different shaped medium stiffness reservoirs (flat and extensive, long and thin, and small). The resulting pattern is that at the centres of the reservoirs there is no anisotropy. SWS is generated at the edges of the reservoir. Inside the reservoir, the fast direction is orientated parallel to the edge of the reservoir, while outside the reservoir the fast direction is orientated perpendicular to the edge. The explanation for this, in terms of stress redistribution around an expanding reservoir, is illustrated in Figure 7.3. At its centre, the reservoir expands uniaxially, so no splitting develops. However, moving towards the edges, the reservoir can expand preferentially into the sideburden, where no pore pressure changes occur, meaning that the stresses perpendicular to the edge can be released in comparison to the stresses parallel. Therefore, with larger horizontal stresses parallel to the reservoir edge, the fast splitting direction becomes orientated in this direction. The sideburden is compressed 141
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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
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4.7. DISCUSSION 4.7 Discussion The
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pressure, P fl σ ′ ij = σ ij
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5.2. EFFECTIVE STRESS AND STRESS PA
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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 and 150: 6.5. COMPARISON OF ROCK PHYSICS MOD
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Page 153 and 154: 7 Forward modelling of seismic prop
Page 155 and 156: 7.2. SEISMODEL c⃝ WORKFLOW time s
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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
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Page 177 and 178: 8.4. A SOFTER RESERVOIR 8.4 A softe
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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
Page 201 and 202: BIBLIOGRAPHY Sayers, C. M., 2002. S
Page 203 and 204: BIBLIOGRAPHY White, D., 2009. Monit
Page 205 and 206: A List of Symbols The table below l
Page 207 and 208: 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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