Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 3. INVERTING SHEAR-WAVE SPLITTING MEASUREMENTS FOR FRACTURE PROPERTIES If the subsurface structure being illuminated by the Phase II results is too complicated to be modelled by one set of HTI fractures and a VTI fabric then the inversion may fail. Another possibility is that the Phase II arrivals come from many different locations and travel through different layers, and so are illuminating different zones of the subsurface that have differing properties. A final possibility is that, in lumping together A and B class measurements in order to increase the number of results to an amount worth interpreting I have included too many unreliable SWS measurements and there are not enough A class measurements to conduct the inversion using these alone. The inversion procedure has now been successfully applied to a number of settings, including: the Yibal oil field, Oman (Al-Harrasi et al., 2010); the Valhall oil field, North Sea (Wüstefeld et al., 2010a); Weyburn during Phase IB (this chapter and Verdon et al., 2010b,c); the hydraulic fracture stimulation presented in Chapter 4 (Verdon et al., 2009, 2010a); a hydraulic fracture stimulation in the Cotton Valley field, Texas (A. Wüstefeld et al., in prep., 2010); and during block-collapse mining at Northparkes mine, Australia (Wüstefeld et al., 2010b). This method has also demonstrated the potential to image multiple fracture sets (Verdon and Kendall, 2010). The wide ranging successful application of this approach suggests that the approximations made in the rock physics model are suitable for characterising fractures in many different settings. Therefore, I do not think that it is an excessive complexity of the fracture system that has caused the inversion to fail. It is far more likely to be a combination of the other two reasons given, that the splitting results are of poor quality and that the arrivals may have imaged portions of the subsurface that differ excessively. 3.5 Discussion When interpreting SWS caused by fractures, it is commonly assumed that the fast direction rotated into geographical coordinates corresponds to the strike of the major fracture strike and/or the maximum horizontal principle stress orientation, and that an increase in δV S corresponds to an increase in fracture density. However, in reality, this may be an oversimplification. The presence of fractures, sedimentary layering and other structures all combine to give the overall elasticity of a rock. The respective contributions must all be understood before SWS can be interpreted with confidence. For instance, with the Phase IB data the principal fracture set is not imaged, while the secondary set is. This is because most of the waves have travelled normal to the principal fracture set, and so are not split by them. This highlights the need to consider all the potential contributions to anisotropy when interpreting SWS. It also demonstrates how detailed modelling can be used to infer fracture properties despite an unfavourable source-receiver geometry. As mentioned above, I invert for an orthorhombic symmetry assuming either a single set of vertical fractures and a horizontal sedimentary fabric, or two sets of vertical fractures. Furthermore, the model I use to estimate the fracture compliance is quite simple. These assumptions were made in order to reduce the number of free parameters and therefore simplify the inversion, while being appropriate for the reservoir analysed. They are not necessary conditions. I could certainly conceive of situations where additional fracture sets, dipping fractures or dipping sedimentary structures, or more 50
1 1 1 1 1 1 1 1 1 1 3.5. DISCUSSION 270° 300° 240° 330° 210° 0° 180° 30° 150° 60° 120° Anisotropy [%] 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 90° (a) 180 0.3 0.3 1 160 140 1 1 0.25 0.25 1 120 0.2 0.2 Frac strike 100 80 Gamma 0.15 1 Gamma 0.15 60 0.1 1 0.1 1 40 20 0.05 0.05 1 0 0 0.05 0.1 0.15 0.2 Frac density 0 0 0.05 0.1 0.15 0.2 Frac density 0 0 50 100 150 Frac strike (b) (c) (d) 0.3 0.3 0.25 1 0.25 1 0.2 0.2 Delta 0.15 Delta 0.15 1 0.1 0.1 0.05 0.05 0 0 0.05 0.1 0.15 0.2 Frac density (e) 1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Gamma (f) Figure 3.12: Inversion results for Phase II assuming 1 fracture set is present. The inversion fails to find a suitable minimum misfit, implying that no stable result is possible. 51
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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
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
Page 35 and 36: 2.4. EVENT TIMING AND LOCATIONS 500
Page 37 and 38: 2.4. EVENT TIMING AND LOCATIONS Nor
Page 39 and 40: 2.4. EVENT TIMING AND LOCATIONS Fig
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
Page 53 and 54: 3.2. INVERSION METHOD 2800 2800 260
Page 55 and 56: 3.2. INVERSION METHOD Loop over γ,
Page 57 and 58: 20 10 5 5 1 20 30 40 80 100 60 40 1
Page 59 and 60: 3.4. SWS MEASUREMENTS AT WEYBURN th
Page 61 and 62: 3.4. SWS MEASUREMENTS AT WEYBURN ei
Page 63 and 64: 3.4. SWS MEASUREMENTS AT WEYBURN ξ
Page 65 and 66: 3.4. SWS MEASUREMENTS AT WEYBURN da
Page 67: 4 2 2 3.4. SWS MEASUREMENTS AT WEYB
Page 71 and 72: 3.6. SUMMARY 3.6 Summary • I have
Page 73 and 74: 4 A comparison of microseismic moni
Page 75 and 76: 4.2. EVENT LOCATIONS 2400 Velocity
Page 77 and 78: 4.2. EVENT LOCATIONS 200 150 Northi
Page 79 and 80: 4.3. EVENT MAGNITUDES Pressure (MPa
Page 81 and 82: 4.3. EVENT MAGNITUDES 160 140 120 N
Page 83 and 84: 4.4. SHEAR WAVE SPLITTING 50 −3 E
Page 85 and 86: 4.5. INITIAL S-WAVE POLARISATION In
Page 87 and 88: 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
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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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