A Source Model for Induced Earthquakes at the Geysers Geothermal Reservoir

Johnson, Lane R.

doi:10.1007/s00024-014-0798-7

Cited by 19 publications

(18 citation statements)

References 23 publications

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“…These observations are consistent with those of Guilhem et al (2014) and Johnson (2014a). The extended crack model proposed by Johnson (2014b) accounts for volume increase through the opening of wing cracks at the ends of a shear crack. Not only does this model account for positive isotropic components, it also suggests that two modes of failure, including shear slip and tensile opening, may be sequential.…”

Section: Discussionsupporting

confidence: 88%

“…Whether or not this could explain the discrepancy between high-frequency first motions, indicative of a normal mechanism for event 10, and the long-period moment tensor solution with a 57% isotropic component would require further analysis. Another relevant point mentioned by Johnson (2014b) is the likelihood that cracks in close proximity may interact. Given the highly fractured subsurface with complex conjugate fault geometries and stress heterogeneity from nearby weakened fault zones, it may be possible that shear slip occurs such that critically stressed fractures, favorably oriented for failure, are reactivated (Majer and McEvilly, 1979;Bufe et al, 1981;McLaughlin, 1981;Eberhart-Phillips and Oppenheimer, 1984;Oppenheimer, 1986;King et al, 1994;Cladouhos et al, 2009;Martinez-Garzon et al, 2013).…”

Section: Discussionmentioning

confidence: 99%

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A Systematic Analysis of Seismic Moment Tensor at The Geysers Geothermal Field, California

Boyd¹,

Dreger²,

Lai³

et al. 2015

Bulletin of the Seismological Society of America

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The Geysers geothermal field is one of the most seismically active regions in northern California. Most of the events occur at shallow depths and are related to stress and hydrological perturbations due to energy production operations. To better understand the relationships between seismicity and operations, better source mechanism information is needed. Seismic moment tensors offer insight into the nature of equivalent forces causing the seismicity. Fifty-three M >3 events located at The Geysers geothermal field were selected from the University of California Berkeley Moment Tensor Catalog for analysis of seismic moment tensor solutions and associated uncertainties. Deviatoric and full moment tensor solutions were computed, and statistical tests were employed to assess solution stability, resolution, and significance. In this study, we examine several source models including double-couple (DC), pure isotropic (ISO; volumetric change), and volume-compensated linear vector dipole (CLVD) sources, as well as compound sources such as DC CLVD, DC ISO, and shear-tensile sources. In general, we find from a systematic approach toward characterizing uncertainties in moment tensor solutions that The Geysers earthquakes, as a population, deviate significantly from northern California seismicity in terms of apparent volumetric source terms and complexity.

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Section: Discussionsupporting

confidence: 88%

Section: Discussionmentioning

confidence: 99%

A Systematic Analysis of Seismic Moment Tensor at The Geysers Geothermal Field, California

Boyd¹,

Dreger²,

Lai³

et al. 2015

Bulletin of the Seismological Society of America

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show abstract

“…Further effort is also required to help establish that moment tensor results obtained in this study of 20 events are truly representative of the over 2,000 other events that occurred in the same study area in the same time interval. Finally, a better understanding of the results could benefit from the development of a physical model of the earthquake source process that is appropriate for these types of induced earthquakes, and that is the subject of a companion study (JOHNSON 2014).…”

Section: Discussionmentioning

confidence: 99%

Source Mechanisms of Induced Earthquakes at The Geysers Geothermal Reservoir

Johnson

2014

Pure Appl. Geophys.

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Abstract-At The Geysers geothermal reservoir in northern California, evidence strongly suggests that activities associated with production of electric power cause an increase in the number of small earthquakes. First-degree dynamic moment tensors are used to investigate the relationship between induced earthquakes and injection of water into a well as part of a controlled experiment in the northwest Geysers. The estimation of dynamic moment tensors in the complex shallow crust at The Geysers is challenging, so the method is described in detail with particular attention given to the uncertainty in the results. For seismic events in the moment magnitude range of 0.9-2.8, spectral moduli of dynamic moment tensors are reliably recovered in the frequency range of 1-100 Hz, but uncertainty in the associated spectral phases limits their use to a few simple results. A number of different static moment tensors are investigated, with the preferred one obtained from parameters of a model fitted to the spectral modulus of the dynamic moment tensor. Moment tensors estimated for a group of 20 earthquakes exhibit a range of source mechanisms, with over half having significant isotropic parts of either positive or negative sign. Corner frequencies of the isotropic part of the moment tensor are about 40 % larger than the average of the deviatoric moment tensor. Some spatial patterns are present in source mechanisms, with earthquakes closely related in space tending to have similar mechanisms, but at the same time, some nearby earthquakes have very different mechanisms. Tensional axes of displacement in the source regions are primarily horizontal, while the pressure axes range from near horizontal to vertical. Injection of water into the well in the center of the study area clearly causes an increase in the number of earthquakes per day, but an effect upon source mechanisms is not evident.

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“…Moreover, Johnson (2014a), in studying source mechanisms for 20 seismic events in the EGS area around P-32 injection, found evidence indicating a substantial opening mode in addition to shear. Johnson (2014b) interpreted the seismic events using a source model of a shearing fracture with wing-cracks, which could explain the substantial volumetric component. According to this model and interpretation, seismic events in the moment magnitude range of 0.9-2.8 around P-32 injection would correspond to shear crack radii in the range of 10-150 m, wing crack lengths in the range of 2-25 m, and maximum slips in the range of 0.3-1.1 cm.…”

Section: Changes In Reservoir Properties With Stimulationmentioning

confidence: 99%

The Northwest Geysers EGS Demonstration Project, California – Part 2: Modeling and interpretation

et al. 2016

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a b s t r a c tIn this paper, we summarize the results of coupled thermal, hydraulic, and mechanical (THM) modeling in support of the Northwest Geysers EGS Demonstration Project, which aims at enhancing production from a known High Temperature Reservoir (HTR) (280-400 • C) located under the conventional (240 • C) geothermal steam reservoir. The THM modeling was conducted to investigate geomechanical effects of cold-water injection during the stimulation of the EGS, first to predict the extent of the stimulation zone for a given injection schedule, and then to conduct interpretive analyses of the actual stimulation. By using a calibrated THM model based on historic injection and microseismic data at a nearby well, we could reasonably predict the extent of the stimulation zone around the injection well, at least for the first few months of injection. However, observed microseismic evolution and pressure responses over the one-year stimulation-injection revealed more heterogeneous behavior as a result of more complex geology, including a network of shear zones. Therefore, for an interpretive analysis of the one-year stimulation campaign, we included two sets of vertical shear zones within the model; a set of more permeable NWstriking shear zones and a set of less permeable NE-striking shear zones. Our modeling indicates that the microseismic events in this system are related to shear reactivation of pre-existing fractures, triggered by the combined effects of injection-induced cooling around the injection well and rapid (but small) changes in steam pressure as far as a kilometer from the injection well. Overall, the integrated monitoring and modeling of microseismicity, ground surface deformations, reservoir pressure, fluid chemical composition, and seismic tomography depict an EGS system hydraulically bounded by some of the NE-striking low permeability shear zones, with the more permeable NW-striking shear zone providing liquid flow paths for stimulation deep (several kilometers) down into the HTR. The modeling indicates that a significant mechanical degradation (damage) inferred from seismic tomography, and potential changes in fracture porosity inferred from cross-well pressure responses, are related to shear rupture in the stimulation zone driven by both pressure and cooling effects.

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A Source Model for Induced Earthquakes at the Geysers Geothermal Reservoir

Cited by 19 publications

References 23 publications

A Systematic Analysis of Seismic Moment Tensor at The Geysers Geothermal Field, California

A Systematic Analysis of Seismic Moment Tensor at The Geysers Geothermal Field, California

Source Mechanisms of Induced Earthquakes at The Geysers Geothermal Reservoir

The Northwest Geysers EGS Demonstration Project, California – Part 2: Modeling and interpretation

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