Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 6. GENERATING ANISOTROPIC SEISMIC MODELS BASED ON GEOMECHANICAL SIMULATION 6.6 Summary • A calibrated rock physics model is required to compute the effects of stress and/or strain on seismic velocities. This model should include empirically observed effects such as nonlinear elasticity and stress induced anisotropy, but should not be unduly complex or require excessive numbers of parameters that are difficult to constrain. • I develop a micro-structural model that fulfils these requirements, treating grain boundaries and microcracks as displacement discontinuities, the number density of which changes with the applied stress. The overall compliance of the medium can be given as the sum of its parts - the background matrix and the additional compliance introduced by the presence of the discontinuities. • I have developed an inversion procedure that computes the 2nd and 4th order additional compliance tensors based on ultrasonic velocity measurements. Unlike previous inversion approaches, this procedure does not make any a priori assumptions about the relative magnitudes of the 2nd and 4th order tensors. • The change in number of displacement discontinuities can be computed by treating them as penny-shaped features. The number density at a given triaxial stress is calculated using an initial crack density and crack aspect ratio at a reference stress state. These parameters can be computed from empirical observations of ultrasonic velocity changes with stress. • I find that the model provides a good match with observation for many core samples, and does a good job of incorporating anisotropy both inherent in a sample and induced by non-hydrostatic stress changes. • Over 200 datasets from the literature have been used to calibrate these crack density and aspect ratio parameters. I find a remarkable consistency in the aspect ratio term, while crack density appears to correspond to the degree of damage and the amount of stress sensitivity of the sample. By providing rules of thumb and typical parameter ranges, the calibration results can be used as a tool to facilitate model population. 134
7 Forward modelling of seismic properties 7.1 Introduction In Chapter 5 I generated geomechanical models to simulate the effects of pore pressure changes on the stress field in and around a reservoir. I wish to model the seismic properties of these models, and developed in Chapter 6 a rock physics model capable of mapping changes in stress into changes in seismic velocity. In this chapter I develop a workflow to generate elastic models based on the geomechanical simulations. These elastic models can then be used to make predictions about changes to seismic properties using seismic modelling tools such as ray tracing or finite difference simulation. This work was conducted as part of the IPEGG project. Doug Angus and I developed a workflow, SeisModel c⃝ , specific to the IPEGG modelling tools capable of reading the output from the MORE- ELFEN simulator and computing the seismic properties on a regularised grid. I will begin this chapter by outlining this workflow. A significant development in the linking of geomechanical simulation with seismic observation was made by Hatchell and Bourne (2005), who match changes in seismic travel times through the overburden with reservoir compaction and stress arching. I use ray tracing to compute the changes in travel time in the overburden above the models generated in Chapter 5, assessing whether travel time changes can distinguish between the different stress paths already identified. Such a tool would be very useful in determining the stress being experienced by a reservoir as CO 2 is injected, and thereby the risk of caprock failure. While undoubtedly useful, overburden travel times provide information about only one aspect of geomechanical deformation - how much the overburden is being stretched/compressed along a vertical axis. Can other seismic attributes be used to image other aspects of geomechanical deformation One potential option is to use shear-wave splitting, which is, as we have seen, sensitive to stress changes. Therefore I also compute splitting for the simple models developed, and assess how useful such measurements would be for imaging such things as reservoir compartmentalisation. 7.2 SeisModel c⃝ workflow 7.2.1 Input of results and parameters The first stage of the SeisModel c⃝ workflow is to import the geomechanical results from the MORE- ELFEN simulations. During the simulation a number of parameters are written to file at specified time steps. These values are then used to compute the seismic properties. The values are output on an element-by-element basis, and are given in Table 7.1. The first stage of SeisModel c⃝ is to read in these parameters from the specified geomechanical output file. A number of required inputs are not provided by the geomechanical modelling. These are read by SeisModel c⃝ from a user-defined input file. These values can be specified separately for the overburden and the reservoir, and are also given in Table 7.1. 135
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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
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
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Page 119 and 120: 5.5. SURFACE UPLIFT 1 0.9 0.8 Soft
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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 149 and 150: 6.5. COMPARISON OF ROCK PHYSICS MOD
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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
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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
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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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