Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 0. ABSTRACT Microseismic activity is an observable manifestation of geomechanical deformation induced by injection. As the pore pressure is increased in a reservoir, effective compressive stress is reduced, causing it to expand. I use a recently developed coupled fluid-flow/geomechanical modelling technique to model this injection-induced deformation. I develop a series of simple, representative models to study how the stress path induced by injection is modulated by the geometry of the reservoir and the material properties of both the reservoir and surrounding non-pay rocks. I find that small reservoirs that are softer than the surrounding rocks are most prone to stress arching effects, while flat extensive reservoirs that are stiffer than surrounding rocks tend to experience only the hydrostatic effective stress decrease induced by pore pressure increase. By computing the changes in deviatoric stress induced by injection, I am able to analyse which types of reservoir will be most prone to brittle failure (fracturing) and where this failure is most likely to occur. I find that small reservoirs are most prone to fracturing - in small soft reservoirs the overburden is most likely to fracture above the injection well, in small stiff reservoirs fracturing is most likely to occur in the reservoir. Changes in the applied stress field will change the seismic properties of rocks in and around the reservoir. Therefore in theory it should be possible to image geomechanical deformation using seismic methods. To do so, a rock physics model is needed to map modelled stress changes into changes in seismic velocities. I have developed a model that uses a microstructural effective medium approach to map triaxial stress changes into anisotropic changes in P and S wave velocity. I have calibrated this model with over 200 sets of stress-dependent ultrasonic velocity data, measured on core samples, taken from the literature. I use this model to show that the differences in stress path caused by differing reservoir geometries and material properties may well generate resolvable variations in the 4-D seismic response of a CO 2 injection site. These responses have the potential to be used as a diagnostic in imaging geomechanical deformation, and thereby be used to contribute a greater understanding of the risk of CO 2 leakage due to fracturing. I have developed a workflow to construct geomechanical models, make seismic and microseismic predictions, and thereby to make comparisons with geophysical observations. In the final part of this thesis I apply this workflow to the Weyburn field. I construct a representative geomechanical model to simulate production from the reservoir, followed by CO 2 injection for storage and EOR. The modelled rock mechanical properties are based on measurements made on core samples from Weyburn. I use the modelled stress changes to make microseismic and seismic anisotropy predictions, finding a poor match between predicted microseismic zones and where microseismicity has been observed at Weyburn, and a poor match between predicted and observed seismic anisotropy. However, I develop a second model that takes into account the softening effect that vugs and fractures will have on the stiffness of the reservoir that will not be included if core sample measurements are used without modification. This new model provides a much better match for both microseismicity and seismic anisotropy. The model indicates that microseismicity observed in the overburden at Weyburn does not represent fluid migration through the caprock but stress transfer through the rock frame. This approach shows how geophysical observations can be used to help calibrate and thereby improve the accuracy of a geomechanical model, providing a greater understanding of the risks that geomechanical deformation can pose to the secure storage of CO 2 . VI
Author’s Declaration I declare that the work in this thesis was carried out in accordance with the requirements of the University’s Regulations and Code of Practice for Research Degree Programmes and that it has not been submitted for any other academic award. Except where indicated by specific reference in the text, the work is the result of the candidate’s own independent research performed at the University of Bristol, Department of Earth Sciences, between September 2006 and September 2010. Work done in collaboration with, or with the assistance of, others, is indicated as such. Any views expressed in the thesis are those of the author and not necessarily those of the University. James P. Verdon Date: September 2010 VII
Page 1: Microseismic Monitoring and Geomech
Page 5: Abstract Capture of CO 2 produced a
Page 9 and 10: Acknowledgments There are many thin
Page 11 and 12: Table of Contents Abstract Author
Page 13 and 14: TABLE OF CONTENTS 5.4 Results . . .
Page 15 and 16: Preface ‘Research is not an end i
Page 17 and 18: 6. D. Angus, J-M. Kendall, J.P. Ver
Page 19 and 20: 1 Introduction A technology push ap
Page 21 and 22: 1.2. CCS OVERVIEW Figure 1.2: CCS s
Page 23 and 24: 1.3. THESIS OVERVIEW for CO 2 to be
Page 25 and 26: 1.3. THESIS OVERVIEW as microseismi
Page 27 and 28: 2 The Weyburn CO 2 injection projec
Page 29 and 30: 2.2. WEYBURN GEOLOGICAL SETTING Fig
Page 31 and 32: 2.2. WEYBURN GEOLOGICAL SETTING N F
Page 33 and 34: 2.4. EVENT TIMING AND LOCATIONS 500
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Page 37 and 38: 2.4. EVENT TIMING AND LOCATIONS Nor
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Page 41 and 42: 2.5. DISCUSSION 500 1000 1100 North
Page 43 and 44: 2.6. SUMMARY 2.6 Summary • CO 2 s
Page 45 and 46: 3 Inverting shear-wave splitting me
Page 47 and 48: 3.2. INVERSION METHOD the additiona
Page 49 and 50: 3.2. INVERSION METHOD 1. P-wave inc
Page 51 and 52: 3.2. INVERSION METHOD 180 160 140 C
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Page 55 and 56: 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
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5.3. NUMERICAL MODELLING MORE FLUID
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5.3. NUMERICAL MODELLING (a) (b) Fi
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5.4. RESULTS 0 500 1000 Depth (m) 1
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5.4. RESULTS 1 0.9 0.8 Soft Med Sti
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5.4. RESULTS Overburden Extension i
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5.4. RESULTS 1z:100x:100y 1z:100x:5
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5.5. SURFACE UPLIFT 1 0.9 0.8 Soft
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5.5. SURFACE UPLIFT Figure 5.17: Ma
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6 Generating anisotropic seismic mo
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6.2. STRESS-SENSITIVE ROCK PHYSICS
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6.3. A MICRO-STRUCTURAL MODEL FOR N
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6.4. CALIBRATION WITH LITERATURE DA
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6.4. CALIBRATION WITH LITERATURE DA
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6.5. COMPARISON OF ROCK PHYSICS MOD
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6.5. COMPARISON OF ROCK PHYSICS MOD
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7 Forward modelling of seismic prop
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7.2. SEISMODEL c⃝ WORKFLOW time s
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7.3. RESULTS FROM SIMPLE GEOMECHANI
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7.4. SUMMARY 7.4 Summary • I have
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8 Linking geomechanical modelling a
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8.2. MODEL DESCRIPTION Layer Thickn
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8.2. MODEL DESCRIPTION Unit E (GPa)
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8.3. RESULTS Marl Vugg 0.8 0.8 Norm
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8.3. RESULTS 15 10 Injector Produce
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8.3. RESULTS 2000 15 2000 15 1500 1
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8.4. A SOFTER RESERVOIR 8.4 A softe
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8.4. A SOFTER RESERVOIR 2000 15 200
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8.4. A SOFTER RESERVOIR 2000 Max SW
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8.5. DISCUSSION pore-pressure being
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8.6. SUMMARY 8.6 Summary • I appl
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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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