Stress magnitudes in the Basel enhanced geothermal system

Valley, Benoît; Evans, Keith F.

doi:10.1016/j.ijrmms.2019.03.008

Cited by 39 publications

(59 citation statements)

References 51 publications

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“…The methodology used in Valley and Evans [] has been the standard procedure for stress measurements; their results were much more reasonable than the stress model used in our previous study. Since our approach was based on observational geophysical data, and stress magnitude from borehole analysis is independent from FPSs, we used the stress magnitude estimated by Valley and Evans [].…”

Section: Estimation Of Critical Pore Pressurementioning

confidence: 55%

“…The stress magnitudes given by equations (6)-(9) as a function of depth from 2569 to 4992 m are shown in Figure 3a, while those of the stress model used by Mukuhira et al [2013] are also shown. As equation (6) shows, S Hmax has a very low gradient to depth, which is because of a decrease in breakout width with depth [Valley and Evans, 2015]. This causes the stress state to transition from a strike slip to normal faulting regime at around 4800 m, showing consistency with the fact that strike slip and normal faulting FPSs were observed .…”

Section: Stress Informationmentioning

confidence: 67%

“…[] are also shown. As equation shows, S H max has a very low gradient to depth, which is because of a decrease in breakout width with depth [ Valley and Evans , ]. This causes the stress state to transition from a strike slip to normal faulting regime at around 4800 m, showing consistency with the fact that strike slip and normal faulting FPSs were observed [ Kraft and Deichmann , ].…”

Section: Estimation Of Critical Pore Pressurementioning

confidence: 86%

“…(a) Depth dependent Mohr stress circles based on the stress model of Valley and Evans []. Depth is in TVD.…”

Section: Estimation Of Critical Pore Pressurementioning

confidence: 99%

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Pore pressure behavior at the shut‐in phase and causality of large induced seismicity at Basel, Switzerland

et al. 2017

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Induced seismicity with unexpectedly large magnitude often occurs after shut‐in or end of stimulation, generating concerns at the end of stimulation. We investigated the physical mechanism of large‐magnitude induced seismicity during shut‐in following the hydraulic stimulation at Basel, Switzerland. Larger postinjection events occurred at the periphery of the seismic cloud. We estimated the pore pressure required to cause shear slip using Coulomb failure criteria from stress information, geometry of the fault planes of microseismic events, and a constant coefficient of friction. Time series analysis of pore pressure distribution indicated that pore pressure migrated to the far field even after shut‐in. Redistribution of pore pressure at shut‐in brought sufficient pore pressure increase to induce seismicity in the peripheral region. After shut‐in, the pore pressure gradient away from the well lessened and eventually pressure became uniform. These observations suggest that the higher pore pressure, which remained in the vicinity of the injection point, shifted to the farthest field. Shut‐in pressure migration caused uniform pore pressure distribution at the edge of the seismic zone. Shut‐in pressure destabilized a large part of the fault located at the edge of the seismic cloud, resulting in the shear slip of a large section of the fault. Meanwhile, during stimulation, only some parts of the fault entered the critical state because of the pressure gradient. The resulting shear slip on that specific part causes moderate magnitude events at most.

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Section: Estimation Of Critical Pore Pressurementioning

confidence: 55%

Section: Stress Informationmentioning

confidence: 67%

Section: Estimation Of Critical Pore Pressurementioning

confidence: 86%

“…(a) Depth dependent Mohr stress circles based on the stress model of Valley and Evans []. Depth is in TVD.…”

Section: Estimation Of Critical Pore Pressurementioning

confidence: 99%

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Pore pressure behavior at the shut‐in phase and causality of large induced seismicity at Basel, Switzerland

et al. 2017

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“…In multiplet analysis, the normal vector of a fault plane is estimated using principal component analysis to the locations of cluster members under the assumption that all members of one cluster are associated with the same fault plane. Meanwhile, the following regional stress information at Basel was inferred based on borehole analysis: azimuth of S Hmax = N144°E ± 14°, S Hmax = 0.00104 z + 115, S hmin (minimum horizontal stress) = 0.01990 z − 17.78, S v (vertical stress) = 0.0249 z , and P h = 0.00981 z ( z : depth from the surface (m), P h : hydrostatic pressure (MPa)) [ Valley and Evans , , ]. These estimates indicate that a transition of stress state from a strike slip to a normal fault stress regime occurred at the deeper part of the reservoir.…”

Section: Field Data and Methodologymentioning

confidence: 99%

Pore pressure migration during hydraulic stimulation due to permeability enhancement by low‐pressure subcritical fracture slip

Mukuhira

Moriya

Ito

et al. 2017

Geophysical Research Letters

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Understanding the details of pressure migration during hydraulic stimulation is important for the design of an energy extraction system and reservoir management, as well as for the mitigation of hazardous‐induced seismicity. Based on microseismic and regional stress information, we estimated the pore pressure increase required to generate shear slip on an existing fracture during stimulation. Spatiotemporal analysis of pore pressure migration revealed that lower pore pressure migrates farther and faster and that higher pore pressure migrates more slowly. These phenomena can be explained by the relationship between fracture permeability and stress state criticality. Subcritical fractures experience shear slip following smaller increases of pore pressure and promote migration of pore pressure because of their enhanced permeability. The difference in migration rates between lower and higher pore pressures suggests that the optimum wellhead pressure is the one that can stimulate relatively permeable fractures, selectively. Its selection optimizes economic benefits and minimizes seismic risk.

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On the b-value Dependency of Injection-Induced Seismicity on Geomechanical Parameters

Mukuhira

Ito

Fehler

et al. 2021

Preprint

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Variability in the b-value, which describes the frequency distribution of earthquake magnitudes, is usually attributed to variations in differential stress in the setting of natural and laboratory earthquakes. However, differential stress is unlikely to explain bvalue variations on the reservoir scale of injection-induced seismicity cases where significant differential stress variability can hardly be expected. We investigate the responsible geomechanical parameters for the b-value reduction observed in injectioninduced seismicity at the Basel EGS field in Switzerland, for which a measured in-situ stress model and fault orientations of numerous microseismic events are available. We estimate the shear and normal stresses along faults, differential stress, pore pressure increase at failure, and the Coulomb failure stress, for each event. Event magnitude and shear stress display the most systematic and clear correlation between each other, while other parameters do not show a clear correlation with event magnitude. We further examine the relationship between the b-value and these geomechanical parameters. We discover that the b-value systematically decreases with increasing shear stress. Again, other geomechanical parameters do not show a clear correlation with the b-value. We conclude that b-value variability is explained by variations in shear stress in the injectioninduced seismicity setting, where near constant differential stress conditions are expected. Furthermore, we observe that b-value reduction with time also correlates with an increasing number of events along faults having high shear stress, which strongly supports our conclusions. Thus, we discovered a profound physical mechanism behind b-value variation in injection-induced seismicity beyond general understandings of b-value variation.

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Stress magnitudes in the Basel enhanced geothermal system

Cited by 39 publications

References 51 publications

Pore pressure behavior at the shut‐in phase and causality of large induced seismicity at Basel, Switzerland

Pore pressure behavior at the shut‐in phase and causality of large induced seismicity at Basel, Switzerland

Pore pressure migration during hydraulic stimulation due to permeability enhancement by low‐pressure subcritical fracture slip

On the b-value Dependency of Injection-Induced Seismicity on Geomechanical Parameters

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