Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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CHAPTER 8. LINKING GEOMECHANICAL MODELLING AND MICROSEISMIC OBSERVATIONS AT WEYBURN 8.4.1 Heterogeneity The models I have presented here do not deal with reservoir heterogeneity. Values for porosity, permeability and mechanical properties are constant for each layer in the model. In reality, carbonate reservoirs such as Weyburn are renown for their heterogeneity, across many length scales. That there is heterogeneity at Weyburn is clear from the range in porosity and permeability seen in Table 2.1. The match between 4-D seismic imaging of the CO 2 plume at Weyburn and fluid-flow modelling suggests that such heterogeneity is not having a particularly strong effect on CO 2 distribution at the scale of each pattern. However, where there is variation in flow properties (especially porosity) it is likely that there are also variations in mechanical properties as well. Differences in porosity through a carbonate reservoir imply differences in rock fabrics, as well as possible differences in diagenesis. As I have demonstrated in Chapter 6, differences in grainscale architecture can exert significant influence on elastic stiffness. Furthermore, differing degrees of carbonate cementation will produce different elastic stiffnesses as well. I have not investigated the effects of these variations, so it is difficult to comment with certainty on what their effects might be. Nevertheless, this issue is worthy of discussion. The role of heterogeneity - regions of the reservoir that are stiffer or softer than the mode - is probably dependent on the length-scale of the heterogeneities in question. Small heterogeneities (reservoir thickness seems a suitable length to which to scale these relative terms) will probably not lead to changes in the shape of stress loops around the reservoir. Small scale features such as this are best incorporated using an effective medium approach where the stiffness of the reservoir is scaled according to the fraction of the rock that is made up of these stiffer regions. This is, in essence the approach taken for any finite element modelling approach, where each node is assigned properties representative of the rock surrounding that point. However, larger scale heterogeneous zones may act to change the nature of the geomechanical response of a reservoir. For instance, it is possible to envisage stiff zones within a reservoir that, if of a sufficient scale, could act as ‘pillars’ on which to support stress arches that would otherwise not be capable of supporting the overburden of an extensive reservoir. As I have shown in Chapter 5, whether or not a stress arch can develop has a significant effect on the evolution of stresses in and around an inflating reservoir. Scope for further study exists to investigate this issue. Such a study would involve using a geostatistical model which varies the difference in mechanical properties between the heterogeneous zones and the ‘background’ reservoir material, the proportion of the reservoir made up of the ‘heterogeneous’ material, and, importantly, the characteristic length scale of the heterogeneous zones. By using this geostatistical model as an input for the geomechanical modelling, it should be possible to determine at what length-scales and proportions heterogeneities with a reservoir begin to influence the stress path during production and injection. 8.5 Discussion Event locations at Weyburn indicate that there is microseismicity in the overburden. This observation was a cause for concern, as it was inferred that the events represented either CO 2 leakage, or at least 164
8.5. DISCUSSION pore-pressure being transferred into the overburden. Either would imply that pathways exist for CO 2 to migrate out of the reservoir. Nevertheless, within the resolution available, controlled source 4-D seismic monitoring surveys do not indicate any leakage. However, without geomechanical modelling, there is no alternative explanation for why the events are found where they are. A representative geomechanical model shows that, if the reservoir is softer than measured in core samples, deviatoric stress will increase in the overburden, increasing the chances of shear failure and thereby of microseismic activity, especially above the producing wells. In contrast, if there were porepressure connections, or buoyant fluid leaking into the overburden, I anticipate that microseismicity would be located above the injection well, where pore pressures are highest and buoyant CO 2 is situated. This is what I observed during the hydraulic fracture described in Chapter 4. At Weyburn events are located above the producing wells, suggesting that the former is the case - a softer than anticipated reservoir is transferring stress into the overburden, inducing microseismicity. The anisotropy generated by such stress transfer also matches the observations of anisotropy made at Weyburn. Angus et al. (2010) show that it is changes to the stress state that have the strongest control on the distribution of microseismic events. Fluid migration plays only a secondary role. The microseismicity in the overburden at Weyburn is not an indication that fluids are leaking into the overburden. It is therefore worth asking whether we are putting the hydraulic integrity of the caprock at risk with this microseismicity Unfortunately this question is difficult to answer, as even active faults and fractures do not necessarily act as conduits for fluid flow, and there is no way of knowing how well connected any fractures in the caprock may be. The fact that there are few events, most of which are of low magnitude, suggests that there are not many large-scale fractures in the overburden. Most importantly, the integrated geophysical and geochemical monitoring systems at Weyburn do not indicate any leakage, so it would appear that any fracturing generated by microseismicity in the overburden is not currently providing a pathway for leakage. By continuing to monitor the field it will be possible to ensure that this remains the case. The reduction in stiffness I show to produce the match with observations is large - from 14 to 0.5GPa. This is done to show the changes that a softer reservoir can produce in extremis. In this case the changes to fracture potential and shear wave splitting introduced by a softer reservoir are clear for the reader to see. As the stiffness is reduced from 14GPa, the trends that I have highlighted gradually establish themselves. It is well known that the presence of fractures and vugs will make core sample measurements overestimate the true values. However, an order of magnitude overestimate is perhaps too much to attribute entirely to the presence of fractures and vugs. It is at this point that we should remind ourselves that what we are dealing with here is a simplified representative model, useful for determining the principal controls on reservoir stress changes, and the directionality of stress changes introduced by variations material parameters. In this case, we suspect that the Young’s modulus is overestimated by an unknown amount, and we know that reducing it will produce a stress path closer to that inferred from microseismic observations. However, to determine more exactly how much the Young’s modulus needs to be reduced to get a good match with observation will probably require a more detailed model that provides a better match with the details of the reservoir geology, and a 165
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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
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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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6.3. 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 153 and 154: 7 Forward modelling of seismic prop
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Page 167 and 168: 8.2. MODEL DESCRIPTION Layer Thickn
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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
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Page 181: 8.4. A SOFTER RESERVOIR 2000 Max SW
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
Page 209 and 210: Both China and India are aggressive
Page 211 and 212: Utsira rock properties vary signifi
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