GEISER Final Conference · Induced seismicity from anthropogenic sources can be a significant...

GEISER Final Conference

Thursday 30 May 2013 - Friday 31 May 2013

Sala del Capitolo, Convento di San Domenico Maggiore, Napoli

Book of abstracts

GEISER Final Conference / Book of abstracts Friday 30 August 2013

Table of contents

Predicting ground motion from induced earthquakes in geothermal areas 1.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fault tractions, moment tensors, fluid pressure and stress drops during Basel geothermal injection 2.. . . . . . . . . . . . .

The role of the orientation of large scale structures on large magnitudes events during stimulation ofgeothermal reservoirs 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Induced seismicity and injection tests. The Different behaviours observed at the Enhanced Geothermal Systemof Soultz-sous-Forêts. 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

How to image seismic velocity structures in geothermal reservoirs using noise cross-correlation tomographyand VSP tomography. 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Relative importance of natural and man-made stress changes in The Geysers geothermal area 6.. . . . . . . . . . . . . . . . . . .

Identification of faults activated during the stimulation of the Basel geothermal project from cluster analysisand focal mechanisms of the larger magnitude events. 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Three-Dimensional Reservoir Structure of The Geysers Geothermal Reservoir Derived from SeismicTomographic Imaging 7.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Self-induced seismicity due to fluid circulation along fault 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INDUCED MICRO-SEISMICITY MODELLING DURING HYDRO SHEARING IN A ROUGH FRACTURE:ROLE OF A HETEROGENEOUS FRICTION COEFFICIENT 8.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermo-hydro-mechanical coupling in a 3D stochastic fracture network at the site of Rosemanowes (UK):Relationship between induced seismicity and thermal stress 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Modeling Flow and Transport during Enhancement in EGS Reservoirs 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Gutenberg-Richter relation originates from stress fluctuations caused by elastic rock heterogeneity 10.. . . . . . . . . . . .

Induced seismicity risk assessment for the 2006 Basel, Switzerland, Enhanced Geothermal System project: Roleof parameter uncertainty on risk mitigation 10.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

What can we learn from a source parameter analysis at the Basel enhanced geothermal system: earthquakestress drops and full waveform moment tensor inversion 11.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Induced seismicity and seismic tomography in geothermal reservoir: a case study at Paralana, South Australia 12

Simulation of fluid injection and induced seismicity in geothermal reservoir by discrete element modeling 13.. .

Overview of occurrence of induced seismic activity in Iceland 13.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Triggered seismicity during drilling and injection in Húsmúli, Hengill area, SW Iceland 14.. . . . . . . . . . . . . . . . . . . . . . . . . . .

Recommended procedure for mitigating risk of unacceptable induced seismicity during re-injection intoconventional geothermal reservoirs. 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Forward Induced Seismic Hazard Assessment, from Hydro-Geomechanical Model to Seismic HazardAssessment 16.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Correlation between injection rates, pressure data and induced microseismicity 17.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Physical Mechanisms of Induced Seismicity: Modeling and Characterization 17.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Full Moment Tensor Analysis of Large Magnitude Events Using First Motion Data at The Geysers GeothermalReservoir, CA, USA 18.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Analysis of seismic activity in the Reykjanes and Svartsengi geothermal systems, SW-Iceland: December 2008– May 2009 19.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The art of balancing induced seismic hazard and reservoir size 20.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contrasts between deformation accommodated by induced seismic and aseismic processes revealed bycombined monitoring of seismicity and surface deformations: Brady Geothermal Field, Nevada, USA 21.. . . . . . . . .

ROLE OF A FAULTS NETWORK ON MAJOR INDUCED MICROSEISMIC EVENTS DURING THEHYDRAULIC STIMULATION OF A GEOTHERMAL WELL, BASED ON THE SOULTZ-SOUS-FORETS EGSSITE 21.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Improving fault mechanical behavior for a better assessment of induced seismicity during stimulation of ageothermal system 22.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ambient seismic noise cross-correlation techniques: a new tool for geothermal exploration and monitoring 22..

Effective detection capability of a local seismic network 23.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The role of triggering by static stress transfer during geothermal reservoir stimulation 23.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Modeling EGS and Recent Developments of GPU-based Computations of TMHC Systems 24.. . . . . . . . . . . . . . . . . . . . . . . .

A multiple-criteria network optimization 24.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Natural stress and fault controls in induced seismicity: what can we learn from gas depletion in theNetherlands? 25.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Analysis of microseismicity from hydrothermal reservoir at Yanaizu-Nishiyama, Japan 25.. . . . . . . . . . . . . . . . . . . . . . . . . . .

Coupled Continuum Modeling of Fracture Reactivation and Induced Seismicity during Enhanced GeothermalOperations 26.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Habanero 2012 Stimulation - Implications for EGS - 26.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Geomechanical Controls on Fault Reactivation, Fluid Flow and Induced Seismicity in Natural and EngineeredGeothermal Systems 27.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Simulation-based Probabilistic Seismic Hazard and Risk Analysis for Seismicity Induced by Fluid Injection 30.. .

Analysis of induced seismicity at The Geysers geothermal field, California 32.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Time-dependent seismic hazard studies at The Geysers geothermal area 33.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Reinjection strategy for minimizing micro-seismic activity during geothermal exploitation at the EGS Soultzsite 33.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Campi Flegrei Deep Drilling Project: direct measurements of minimum stress and permeability at depth 34

Induced and triggered seismicity: Coulomb stress modeling and statistical evidence 34.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

New Zealand Examples of Geothermal Induced Seismicity: 30 years of experience 35.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Non-stationary seismic hazard evaluation for gas fields in The Netherlands 36.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COMPUTATIONAL INVESTIGATION OF HYDRO-MECHANICAL EFFECTS ON TRANSMISSIVITYEVOLUTION DURING THE INITIAL INJECTION PHASE AT THE DESERT PEAK EGS PROJECT, NV 36.. . . . . .

Hazard Assessment in the EGS Experiment, Berlin Geothermal Field, El Salvador. 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Assessment of Poroelastic effects during an EGS reservoir stimulation treatment 37.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Rate-and-state friction in numerical modeling in the framework of injection induced seismicty 38.. . . . . . . . . . . . . . . . .

Induced seismicity from EGS projects: observations and implications to future modeling efforts 39.. . . . . . . . . . . . . . . . .

WP3 - Introduction to seismic analysis 39.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

WP5 - Overview 40.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

WP6 - Mitigation strategy 40.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Predicting ground motion from induced earthquakes in geothermal areasDr. DOUGLAS, John 1; EDWARDS, Benjamin 2; CONVERTITO, Vincenzo 3; SHARMA, Nitin 4; TRAMELLI, Anna 3; KRAAIJPOEL, Dirk 5;

MENA CABRERA, Banu 2; MAERCKLIN, Nils 6; TROISE, Claudia 3

1 BRGM2 SED-ETHZ3 INGV4 Università degli Studi di Napoli Federico II5 KNMI6 AMRA

Corresponding Author: [email protected]

Induced seismicity from anthropogenic sources can be a significant nuisance to a local population and inextreme cases lead to damage to vulnerable structures. One type of induced seismicity of particular recentconcern, which, in some cases, can limit development of a potentially important clean energy source, is thatassociated with geothermal power production. A key requirement for the accurate assessment of seismichazard (and risk) is a ground-motion prediction equation (GMPE) that predicts the level of earthquake shaking(in terms of, for example, peak ground acceleration) of an earthquake of a certain magnitude at a particulardistance. Few such models currently exist in regard to geothermal-related seismicity, and consequently theevaluation of seismic hazard in the vicinity of geothermal power plants is associated with high uncertainty.

Various ground-motion datasets of induced and natural seismicity (from Basel, Geysers, Hengill, Roswinkel,Soultz, and Voerendaal) were compiled and processed, and moment magnitudes for all events wererecomputed homogeneously. These data are used to show that ground motions from induced and naturalearthquakes cannotbe statistically distinguished. Empirical GMPEs are derived from these data; and, although they have similarcharacteristics to recent GMPEs for natural and mining-related seismicity, the standard deviations are higher.To account for epistemic uncertainties,stochastic models subsequently are developed based on a single cornerfrequency and with parameters constrained by the available data. Predicted ground motions from thesemodels are fitted with functional forms to obtain easy-to-use GMPEs. These are associated with standarddeviations derived from the empirical data to characterize aleatory variability. As examples, we demonstratethe potential use of these models using data from Campi Flegrei and Cooper Basin.

GEISER Final Conference / Book of abstracts

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Fault tractions, moment tensors, fluid pressure and stress drops duringBasel geothermal injectionDr. FISCHER, Tomas 1; Dr. GOERTZ-ALLMANN, Bettina 2; Dr. ZHAO, Peng 2

1 Charles University in Prague2 Norsar


The analyses of the induced earthquakes that occurred during the injection at the Basel geothermal site showan increase in stress drop with distance from the injection and successively with the decreasing pore pressure.We extend this analysis by including the stress-field data and moment tensors of selected events to get insightin the relation of the stress drop and pore pressure with the non-DC components of the source mechanisms.

We find that stress drop increases with effective normal stress, which implies its decrease with pore pressure.Similarly, stress drop appears to increase with shear stress. The non-DC components are found to decreasewith shear and normal stress. We determine the pore pressure using the Mohr-Coulomb criterion and the fluidpressure drop from the moment tensors of tensile earthquakes. Both compressive and extensive events occurat pore pressures up to 10 MPa and pressure drops ranging from -1 to 5 MPa. While the extensive non-DCevents show correlated occurrence of pore pressure and pressure drop, the events occur at the highest porepressures and show negative pressure drop., which is consistent with compressive regime.


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The role of the orientation of large scale structures on large magnitudesevents during stimulation of geothermal reservoirsDr. BAUJARD, Clement 1; Dr. OLLINGER, Dieter 1; Dr. BADOUX, Vincent 1

1 GEOWATT AG


The moment magnitude of seismic events linearly increases with the displacement (or slip) and with the areaof rupture. Thus, it is clear that large scale faults play an important role in the occurrence risk of largemagnitude seismic events that can be observed during geothermal reservoir stimulation or operation. Wepropose to investigate and quantify the risk of triggering large magnitude seismic events in imaginarygeothermal reservoirs showing different geometric configurations of large scale faults.

To that purpose, we computed the hydromechanical response of different synthetic models. The computationsare run with the numerical code HEX-S, developed at GEOWATT AG, in which a generated fracture networkis mapped on a Final Element Mesh. The hydromechanical couplings include the computation of fractureopening in mode I (jacking) and mode II (shearing), using a Mohr-Coulomb criterion. Thus, the stress fieldmagnitude and orientation as well as the fracture orientations play a role in the process. The computation ofthe Coulomb Stress Change, implied by the failure of fractures has also been recently implemented in thecode, in the framework of the GEISER project. Failure of a fracture –or of a fracture part– may propagate toneighbouring fractures because of local change of the stress field, thus simulating an earthquake triggeringmechanism. It has been showed that triggering effects due to Coulomb Stress Change may play a role ininduced seismicity in Basel as well as in Soultz-sous-Forêts, to a lesser extent.

Our model is based on the stress field and on the fracture network of Soultz-sous-Forêts. 4 synthetic modelshave been computed, assuming that a regional fault is localised 400 m of the well open section. Theorientation of this fault is different in each model, in order to cover a broad spectrum of reactivation potentialvalues for this structure, from the lowest possible value (fault preferentially oriented to shear) to very elevatedvalues. The results presented, comparing the computations with and without triggering mechanism effectstend to show that large seismic events are related to faults that are not preferentially oriented to shear, as thecontact surface between the pressure wave front hitting these faults is much wider in that case as for faultsshowing a low reactivation potential value.

Indeed, the works here presented allow drawing the following conclusions:- The faults located in the direct vicinity of the borehole present a lower risk during high pressure injections,as they may fail continuously (a wide pressure wave front can never hit these faults);- The highest risk of triggering large magnitude events is related with faults which are located at distance(100-500m) of the well, as the faults located there are hit by a wide pressure wave front;- Consequently, stimulation injection schemes should be very limited in time (2-3 hours), in order not to reachthe danger zone.


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Induced seismicity and injection tests. The Different behaviours observedat the Enhanced Geothermal System of Soultz-sous-Forêts.Dr. CALO', Marco 1; Dr. DORBATH, Catherine 2

1 EOST, University of Strasbourg2 IRD-UR234 (GET) Toulouse, France


The geothermal power plant of Soultz-sous-Forêts (France) consists of three boreholes (GPK2, GPK3, GPK4)reaching a depth of about 5 km. In order to connect the boreholes to the fracture network efficiently and toimprove the global permeability of the reservoir, GPK2 was stimulated in June/July 2000, GPK3 in May/June2003, and GPK4 in September 2004 and again in February 2005. An acidified water injection experiment wasalso conducted in GPK4 in February/March 2005. During the stimulations, several thousand events weredetected and located.Here we present the results of two time-dependent (4D) seismic tomography studies obtained with the datarecorded in 2000 and 2003 and a detailed analysis of the induced seismicity recorded in 2004 and 2005.The seismic data analysis performed here allowed to describe the behaviors of the geothermal reservoir duringand after the injection periods showing that the repartition of the effective stresses around the wells isstrongly affected by the existing network of faults. Injections tests performed in regions that are initiallypoorly connected to large faults (e.g. the case of GPK2) are characterized by a low anomaly of the P-wavevelocity mainly located around the zone where microseismic activity develops. Seismicity appears as acompact cloud without internal structures. In some specific periods (i.e. the injected flow rate was suddenlyincreased) the velocity anomaly disappears suggesting that the velocity variations within the reservoir (andconsequently the related variations of effective stress) are not associated with simple water diffusion from theinjection well, but rather reflect the occurrence of large-scale aseismic motions in the reservoir.In regions where pre-existing faults are well documented (e.g. for the GPK3 well) the accumulation ofeffective stresses close to the well is avoided probably because the structures represent the main paths of theinjected water. This results in a lack of large low Vp anomalies during the stimulation and in the occurrenceof the induced seismicity located along the major structures.Finally, a comparison between the seismicity recorded during fresh water and acidified injections in GPK4reveals similarities and differences of the two approaches for increasing the injectivity of the well and givesinsight into different behaviors of the reservoir that is related to the stress state at the moment of the injectionand to the nature of the injection adopted.


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How to image seismic velocity structures in geothermal reservoirs usingnoise cross-correlation tomography and VSP tomography.Dr. CALO', Marco 1; Dr. DORBATH, Catherine 2

1 EOST, University of Strasbourg2 EOST


In this work we present two independent approaches that resulted able to image the seismic velocitystructures of the Soultz-sous-Forêts geothermal reservoir.In the first study we assessed the feasibility of the noise-cross correlation tomography method for imaging anddetecting also potential geothermal reservoirs in a wider region than that of the Soultz EGS. We invertedRayleigh waves reconstructed from cross- correlations of 15 months of ambient seismic noise recorded by ninepermanent and thirteen temporary seismological stations installed around the Soultz-sous-Forêts geothermalpower plant. By correlating noise records between 22 receivers, we reconstructed Rayleigh waves withsufficient signal-to-noise ratio for 231 inter-station paths. The reconstructed waveforms were used to measuregroup velocity dispersion curves at periods between 1.0 and 5.0 s. The obtained measurements were invertedfor two-dimensional group velocity maps and finally for a 3-D S-wave velocity model of the Soultz regionfrom 0 to 5.2 km depth. Our results clearly show a crystalline basement marked by low velocity bodies belowthe Soultz power plant at depth of 4-5 km, and at shallower depth (2.5-3.5 km) beneath the Rittershoffen andWoerth towns.The low Vs anomaly associated with the Soultz reservoir is located West of the EGS boreholes, and is 2-4 kmwide.In the second study we performed a P-wave seismic tomography using the travel times collected during aVertical Seismic Profiling (VSP) survey performed in 2007. The experiment was conducted using 3C geophonesplaced at 20 m of interval between 3 and 5 km depth in two wells (GPK3 and GPK4) and a vertical vibratorlocated at 26 surface positions placed at distances between 0.5km and 5km from the wellheads. Thetomograms display a seismic velocity pattern marked by low Vp anomalies West to the wells and high Vp tothe East. Furthermore a thin (200m) high Vp anomaly separates in two blocs the low Vp region. This pattern isin agreement with the positions of the reflecting fault zone revealed by the VSP analysis and, at larger scale,with the seismic velocity distribution observed in the noise cross-correlation tomography.The seismic velocity patterns observed with these two independent approaches suggest that the developmentof the Soultz geothermal reservoir was strongly affected by the main faults present in the regions. Theseobservations highlight the importance of the seismic imaging techniques presented here for detecting andcharacterizing the geothermal reservoirs and for planning improvements of the reservoir performances.


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Relative importance of natural and man-made stress changes in TheGeysers geothermal areaDr. ALTMANN, Johannes 1; Mr. HEIDBACH, Oliver 1; Dr. GRITTO, Roland 2

1 GFZ Potsdam2 Array Information Technology


Quantification of the different processes that contribute to the spatio-temporal stress changes at geothermalsites is critical for the understanding of induced seismicity. Poroelastic coupling and thermal-mechanicalprocesses during hot water extraction and cold water injection are probably the key processes that contributeto the stress changes in The Geysers geothermal field. In particular, with the onset of massive injection ofwastewater into the reservoir the number of induced events with magnitudes M > 4 increased significantly.This raised the question which stress changing process is the key control of the induced seismicity rate. Inaddition to man-made stress changes the tectonic loading from the Pacific plate motion relative to the NorthAmerica plate also alters the stress state as well as the co-seismic stress change of the induced events itself. Inorder to assess the relative importance of the various stress changing processes, we built a 3Dthermo-hydro-geomechanical numerical model of The Geysers geothermal field. The model accounts for thefar-field tectonic loading as well as for the co-seismic stress changes of the M > 4 events. To solve the resultingfully coupled partial differential equations we use the commercial finite element code Abaqus. After theimplementation of an initial stress state that is calibrated against data of the orientations of maximumhorizontal stress and stress regime from earthquake focal mechanism solutions, we apply kinematic far-fieldboundary conditions derived from continuous GPS stations. The results show a good fit to the subsidencerates observed from GPS stations within the reservoir. The main stress changes come from fluid pressuredecrease and from thermal stresses; the tectonic loading rate and co-seismic stress change is comparablysmall. However, at the bounding faults of the reservoir the tectonic loading rate is relatively high compared tothe man-man stress changes within the reservoir.

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Identification of faults activated during the stimulation of the Baselgeothermal project from cluster analysis and focal mechanisms of thelarger magnitude events.Dr. DEICHMANN, Nicholas 1; Dr. KRAFT, Toni 1

1 Swiss Seismological Service - ETH Zurich


High-precision relative location procedures of the stronger seismic events (0.7 < ML < 3.4), based oncross-correlations of signals recorded by a six-sensor borehole array and numerous surface stations in theimmediate epicentral area, show that clustering of hypocenters on different spatial scales is a dominantfeature of the microseismicity induced by the stimulation of enhanced geothermal reservoir in Basel. In linewith the fact that many of the observed earthquakes form clusters of similar events, several focal mechanismsare also nearly identical to each other. A comparison between the high-precision relative locations of theevents within each cluster and the focal mechanisms often shows a good coincidence of the hypocentraldistribution with one of the nodal planes of the focal mechanism. In some cases, the spatial extent of theindividual clusters is limited to a few meters, which suggests that the corresponding events represent repeatedslip with partial stress drop as pore pressures increase with time. In other cases, that include some of thestronger events (ML > 2), the dimension of the individual clusters can amount to several 100 meters, and theactivity within these clusters can extend over several days. Given that the orientation of many fault segmentsidentified in this way deviates significantly from the overall orientation of the seismic cloud, these resultsreveal a complex internal structure of the flow paths in the rock volume stimulated by the water injection.


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Three-Dimensional Reservoir Structure of The Geysers GeothermalReservoir Derived from Seismic Tomographic ImagingDr. GRITTO, Roland 1; Dr. YOO, Seung-Hoon 2; JARPE, Steve 3

1 Array Information Technology2 University of California Berkeley3 Jarpe Data Solutions Inc.


Three-dimensional Vp/Vs-ratio structure is presented for The Geysers geothermal field using seismictravel-time data. The data were recorded by the Lawrence Berkeley National Laboratory (LBNL) using a34-station seismic network. The results are based on 32,000 events recorded in 2011 and represent the highestresolution seismic imaging campaign at The Geysers to date. The results indicate low Vp/Vs-ratios in thecentral section of The Geysers within and below the current reservoir. The extent of the Vp/Vs anomalydeceases with increasing depth. Spatial correlation with micro-seismicity, used as a proxy for subsurface waterflow, indicates the following. Swarms of seismicity correlate well with areas of high and intermediate Vp/Vsestimates, while regions of low Vp/Vs estimates appear almost aseismic. This result supports past observationsthat high and low Vp/Vs-ratios are related to water and gas saturated zones, respectively. In addition, thecorrelation of seismicity to intermediate Vp/Vs-ratios is supportive of the fact that the process of waterflashing to steam requires four times more energy than the initial heating of the injected water to the flashingpoint. Because this energy is dawn from the reservoir rock, the associated cooling of the rock generates morecontraction and thus seismic events than water being heated towards the flashing point. The consequences arethe presence of some events in regions saturated with water, most events in regions of water flashing to steam(low steam saturation) and the absence of seismicity in regions of high steam concentrations where the waterhas already been converted to steam. Furthermore, it is observed that Vp/Vs is inversely correlated to Vs butuncorrelated to Vp, leading support to laboratory measurements on rock samples from The Geysers thatobserve an increase in shear modulus while the core samples are dried out. As a consequence, traditionalporoelastic theory is no applicable at The Geysers geothermal reservoir.

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Self-induced seismicity due to fluid circulation along faultDr. AOCHI, Hideo 1; Dr. RACHEZ, Xavier 1

1 BRGM


In this paper, we build a system of equations describing fluid migration, fault rheology, fault thicknessevolution and shear rupture during a seismic cycle, triggered either by tectonic loading or by fluid injection.Supposing the phenomena predominantly take place on a single fault described as a finite permeable zone ofvariable width, we are able to project the equations within the volumetric fault core on the 2D fault interface.On the basis of this “fault lubrication approximation”, we simulate the evolution of seismicity when the fluidis injected at one point along the fault to model induced seismicity during injection test in a borehole thatintercepts the fault. We perform several parametric studies in order to understand the basic behaviour of thesystem established. Fluid transmissivity and fault rheology are key elements. We generally observe thatseismicity tends to rapidly evolve after triggering, independently of the injection history and end by flowingout the injected fluid toward the model outer end in the case where shear rupturing on a planar fault becomesdominant over the fluid migration process. If there is no healing process within the fault (e.g. decrease inpermeability) after rupture,, and the fluid is completely drained On the contrary, if healing processes takeplace, so that the fluid mass is somehow trapped along the fault, seismicity and fluid migration are stronglycontrolled by the injection rate and are typically stopped at shut-in.


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INDUCED MICRO-SEISMICITY MODELLING DURING HYDROSHEARING IN A ROUGH FRACTURE: ROLE OF A HETEROGENEOUSFRICTION COEFFICIENTDr. BRUEL, dominique 1

1 Mines-ParisTech / Centre de Geosciences


The purpose of the study is to evaluate the role of the friction coefficient on shear displacements on apre-existing fracture plane of any orientation at reservoir scale during a hydraulic stimulation experiment.The friction coefficient which governs the shear strength of a rough surface is not uniform in this model. Thefracture aperture is variable in space, with a lognormal distribution and a spatial (isotropic) correlation length.Zones with apertures less than a given threshold are delineated with a given cohesion and static frictioncoefficient. Out of these zones, the static friction is less. At any time step when transient fluid injections aresimulated, a Mohr-Coulomb stress criterion is applied and in case of failure or in case of jacking at a particularlocation, a stress drop is derived using a dynamic friction coefficient. Assuming the rock mass in betweenfractures behaves elastically, we use a displacement discontinuity method to balance normal andslip-displacements, fluid pressure, far field stress and induced stresses. Patches of connected sheared cells areidentified to calculate the seismic moments upon these areas. Thus the entire surface does not continuouslyslip at once. Depending on the proportion of zones with higher shear resistance, shear ruptures propagate indifferent ways and ‘seismic’ cumulated moments, as well as seismic catalogues that are calculated are notsimilar. This is of prior interest to discuss the probability of an undesired large magnitude event to happen.

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Thermo-hydro-mechanical coupling in a 3D stochastic fracture networkat the site of Rosemanowes (UK): Relationship between induced seismicityand thermal stressDr. MOUSSA, Kayad Ahmed 1; Dr. BRUEL, dominique 2

1 Mines-ParisTech2 Mines-ParisTech / Centre de Geosciences


This study primarily focuses on induced seismicity and thermal stress due to the cooling of the rocks. Thepredictive analysis of delayed events of large magnitude is the first goal but then it is shown that the THMcoupling may be of prior importance for the economic performance of any Enhanced Geothermal Reservoir.To understand the interactions in between physical processes such as rock shrinkage and pressure increase ina reservoir we set up a 3D stochastic fracture network model which geometric fracture properties (density,size and orientation) and petrophysic properties are derived from field data gathered during a long durationcirculation test conducted at the site of Rosemanowes (Cornwall, UK) between August 1985 and December1988. The main objective of the set of numerical simulations is to show that rock cooling does createobservable coupling effects. Seismic activity within a couple of years is obtained at this site but highmagnitude failures may be mitigated using pulses of injection of short duration that would trigger phenomenaearlier. To discuss the temporal rate in shear failure due to thermal stress during the long term hydraulic test,we introduced a new stick-slip algorithm with the objective of better simulating the effects of shear failuredue to a combination of pressure increase and thermal stresses. When the classic Coulomb stress criterion ismet, we have introduced a coefficient of dynamic friction for calculating the amount of stress drop and thecorresponding fracture slip and attached seismic moment.


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Modeling Flow and Transport during Enhancement in EGS ReservoirsMr. KARVOUNIS, Dimitrios 1; Prof. JENNY, Patrick 2; Dr. GISCHIG, Valentin 3; Prof. WIEMER, Stefan 4

1 ETH - Zurich2 Institute of Fluid Dynamics, ETH - Zurich3 Schweizerischer Erdbebendienst, ETH - Zurich4 Schweizerischer Erdbebendienst (SED)


Geothermal energy and Enhanced Geothermal Systems (EGS) in particular are expected to play a vital role infuture energy scenarios.Initially, in order to enhance the efficiency of the power plant, cold working fluid (usually water) is injected toincrease the conductivity of existing fractures and to create new ones.For the modeling of single phase flow and transport in such dynamically changing, highly fracturedreservoirs, a new approach is presented.

It is based on an adaptive hierarchical fracture representation (a-HFR) that results in a network of multipledominant fractures, through which most of the mass flow occurs.These large fractures have a discrete representation, i.e. each fracture is represented by a lower dimensionalcontinuum.A single continuum representation is also employed for the damaged matrix, which consists of many smalland medium sized fractures accounted for by appropriate effective properties.The main advantage of the new approach is that no expensive re-meshing of the domain is required,whenever new fractures are added.Dynamically changing fracture networks can be simulated with a grid that effectively adapts itself to changes.Moreover, tiny volumes that reduce the time-step size are avoided.

Here it is explained how the coupling of flow and transport in discrete and continuous fracturerepresentations in combination with heat conduction in the rock can be modeled.Verification of the modeling approach is presented and simulations from EGS scenarios are discussed.


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Gutenberg-Richter relation originates from stress fluctuations caused byelastic rock heterogeneityLANGENBRUCH, Cornelius 1; Prof. SHAPIRO, Serge A. 1

1 Freie Universitaet Berlin


The analysis of well log data from various sites suggests that the distribution of elastic properties in theEarth's crust is highly heterogeneous and of fractal nature. It has been shown that the fractal scaling behaviorof the heterogeneity derived from well logs is universal. We extract statistical parameters of elastic propertiesfrom sonic logs along the KTB-1 main hole and create an elastically heterogeneous 3D random mediumrepresenting the surrounding rock. Using a finite element program we apply a homogeneous far-field stressand compute stress fluctuations resulting from elastic rock heterogeneity. The heterogeneity causes significantstress fluctuations. We determine the distribution of Coulomb Failure Stress (CFS) as a measure of fracturestrength. The resulting CFS is highly heterogeneous and related to elastic parameters. We assume thatrupturing takes place inside isovolumes of CFS and determine resulting size distribution of fractures. Fracturesizes and correspondingly earthquake magnitudes exhibits power-law scaling according to theGutenberg-Richter law with b close to one. This b-value is universal as long as the fractal scaling of elasticheterogeneity is universal. Elastic heterogeneity in rocks causes significant stress fluctuations of power-lawtype. This suggests that the Gutenberg-Richter relation originates from elastic heterogeneity. Moreover, wefind an inverse relation between differential stress level and b-value. This result coincides with findings oflaboratory studies and observations. Our modeling shows that a relation between b-value and stress stands fora break in self-similarity, which can only occur, if naturally site specific characteristic length scales exist.

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Induced seismicity risk assessment for the 2006 Basel, Switzerland,Enhanced Geothermal System project: Role of parameter uncertainty onrisk mitigationDr. MIGNAN, Arnaud 1; Prof. WIEMER, Stefan 1; Mr. LANDTWING, Delano 1

1 ETH Zurich, Swiss Seismological Service


We present a probabilistic risk assessment analysis of the 2006 Basel Enhanced Geothermal System. Wecombine induced seismicity time-dependent hazard with the European Macroseismic Method and propose alogic tree approach to capture uncertainties. We find that the losses vary over several orders of magnitude forthe tested parameters. A deterministic loss curve can be calculated before the start of hydraulic stimulationproviding information at a first-order level, i.e. at what magnitude one can expect reaching unacceptablelosses. During hydraulic stimulation, using a traffic light system that links short-term induced seismicityforecasts to the deterministic loss curve should facilitate decision-making.


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What can we learn from a source parameter analysis at the Baselenhanced geothermal system: earthquake stress drops and full waveformmoment tensor inversionDr. GOERTZ-ALLMANN, Bettina 1; Dr. ZHAO, Peng 2; Dr. OYE, Volker 1

1 NORSAR2 CGG, formerly NORSAR


During stimulation of an enhanced geothermal system (EGS) a proper estimation of the in-situ mechanicalproperties of the reservoir is necessary for an assessment of both the economics of reservoir treatment as wellas the associated seismic hazard at the surface. In this presentation, we show what the analysis of sourceproperties of seismicity induced by fluid injection in the Basel EGS project in 2006 can tell us about theestimation of the in-situ stress regime and faulting regime, and thus of the mechanical properties of thereservoir. More than 11,000 events were induced since the beginning of stimulation, most of them during theactual 6-day stimulation period. Events were recorded by a six station seismic array installed in fivemonitoring boreholes within 5 km of the injection as well as at surface stations around the injection well.We estimate stress drops of the induced events from the best-fitting corner frequency of the P-wave sourcespectra. We observe significantly lower stress drops near the injection point. Stress drop increases by about afactor of five with radial distance from 10 m to 300 m. Comparison with forward-modeled pore pressureperturbation reveals a correlation with the pore pressure distribution in the reservoir. Stress drop is inverselyproportional to the forward-calculated pore pressure perturbation. The observations can be described byforward-modeling of the pressure-induced stress changes and seismicity triggering based on Coulomb frictionand suggests a link of source parameters with differential stress.We further present results of a moment tensor inversion of the 19 largest microseismic events (ML between 2.0and 3.4). We use the software package 'Kiwi' and adopt a three-step procedure to retrieve point solutionparameters based on full waveform inversion. The inversion is performed by fitting displacement amplitudespectra and time series seismograms in the first and second step, respectively. After the first two steps, weobtain focal solutions of all 19 events assuming a double couple source model. Our results are in agreementwith focal mechanisms from a previous study, which used first-motion polarities, whereas our solutions areachieved using full waveform data recorded by less than 10 surface stations. In the last step, a full momenttensor inversion is performed. The isotropic components of the moment tensor solutions of some events arenot negligible, suggesting source volume changes due to fluid injection. Events with significant isotropiccomponents occurred mainly during the stimulation phase and are distributed close to the injection well. Onthe other hand, events that occurred in the post-stimulation phase are predominantly pure shear failure andlocate further away from the well bore. These spatio-temporal patterns can also be explained by processeslinked to pore pressure variations during and after the hydraulic stimulation at the geothermal site.


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Induced seismicity and seismic tomography in geothermal reservoir: acase study at Paralana, South AustraliaDr. ALBARIC, Julie 1; Dr. OYE, Volker 1; LANGET, Nadege 2; Dr. CALO, Marco 3; Dr. HUSEN, Stephan 4; Dr. LECOMTE, Isabelle 1;

HASTING, Michael 5; MESSEILLIER, Mathieu 6

1 NORSAR2 EOST3 EOST, University of Strasbourg4 ETH5 Hasting Micro-Seismic Consulting6 Petratherm


Paralana is an Enhanced Geothermal System (EGS) that is being developed in South-Australia. The objectiveis to recover the energy generated by the radioactive decay of high heat producing basement rocks, buriedunder several layers of sediments. In July 2011, about 3 million litres of water were injected in the 4-km depthwell (Paralana-2). Induced seismicity was monitored by a network of 20 stations (from surface to 1800 mdepth) and automatically processed. We detected more than 7000 microearthquakes during the five days ofinjection. We computed source parameters and observed that most of the seismic moment was released duringthe injection. Earthquakes were located using a 3D velocity model which was built from well log and seismicreflection data. Most of the seismic events occurred at the base of the well and form a cloud of about 800 mlong growing in the NE direction. Using waveform cross-correlations, we measured travel-time differences forevents having similar waveforms and relocated a subset of 4808 well-located earthquakes. Results of this highresolution microseismic analysis suggest that the two main groups of fractures observed in the well wereactivated. Focal mechanisms of the largest events (Mw 2.4 to 2.5) indicate reverse faulting, which is consistentwith the stress regime in the area. We also present a series of tests we have performed for passive seismictomography using Paralana data. The synthetic tests indicate that small velocity heterogeneities into thereservoir can be resolved at Paralana if high-precision relative arrival times are used, such as provided bywaveform-cross correlation.


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Simulation of fluid injection and induced seismicity in geothermalreservoir by discrete element modelingDr. YOON, Jeoung Seok 1; Dr. ZANG, Arno 1; Prof. STEPHANSSON, Ove 1; Dr. HAKIMHASHEMI, Amir 1; Dr. HEIDBACH, Oliver 1

1 GFZ German Research Centre for Geosciences


This talk presents a geomechanical model designed to simulate fluid injection and induced seismic events in ageothermal reservoir stimulated by means of hydraulic fracturing. We use a discrete element model withbonded particle model that represents a crystalline geothermal reservoir. Fluid flow algorithm is implementedto simulate injection of viscous fluid and its pressure driven migration in reservoir causing tensile (Mode I)and shear (Mode II) failure of reservoir rock matrix and the embedded pre-existing discrete fractures. Themechanical failures in tension and shear are converted to seismic events where seismic information of eachevent is recorded and analyzed in terms of moment magnitude, source mechanisms.A synthetic crystalline reservoir is generated assuming the horizontal reservoir section is located at 4.5 kmdepth in Soultz-sous-Forêts, France with anisotropic in-situ stress (SH = 75, Sh = 60 MPa). Four scenarios arepresented, where the first two scenarios deal with conventional style of fluid injection where rates areincreased step-wise (30, 40, 50 l/s) as similar to the stimulation of the GPK2 in 2000, in intact (withoutembedded discrete fractures) and fractured reservoirs, respectively. In the third and fourth scenarios, the styleof fluid injection is changed to cyclic where ca. 10,000 m3 of fluid is injected and distributed over 112 hours inan intact and fractured reservoir, respectively.Results to be presented and discussed are: (1) how the induced seismic events distribute in time and space, (2)distribution and source mechanisms of induced events with moment magnitude ≥ 1, (3) regional stresschanges near and far away from the injection point, (4) how the results from such forward deterministicmodeling can be linked to probabilistic assessment of induced seismic hazard, (5) how occurrence (rate) ofinduced events with moment magnitude ≥ 1.5 can be mitigated by changing the style of fluid injection.

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Overview of occurrence of induced seismic activity in IcelandMr. ÁGÚSTSSON, Kristján 1; Dr. FLOVENZ, Olafur 1

1 ISOR


Re-injection of geothermal fluid into the geothermal reservoir has been practised in Iceland on regular basissince 1997 but at low wellhead pressure. Altogether re-injection has been made at 10 geothermal sites, 4 lowtemperature areas and 6 high temperature areas. Injection rates vary from 10 L/s to 600 L/s. Until recentlyinduced seismicity was not considered a problem in Iceland due to the high background seismic activity.Induced seismicity has never been observed in the low temperature fields although some of them are locatedin highly seismic parts of the Icelandic crust.The high temperature areas are located in active volcanic centres where the crust is highly altered and weakand tensile deformations are prominent with persistent microseismic activity. However, where the seismicnetworks are sensitive enough some induced activity has been observed in association with injections in theseareas. Areas within and bordering transform zones where much higher strain accumulation can occur arelikely to produce larger triggered events. This has been the case recently when large scale re-injection createdhuge swarms of triggered events at the 303 MWe Hellisheiði geothermal power plant.The characteristics of the induced seismicity in Icelandic geothermal fields will be discussed in thepresentation and how it is related to the tectonic regime.


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Triggered seismicity during drilling and injection in Húsmúli, Hengillarea, SW IcelandMr. ÁGÚSTSSON, Kristján 1; Dr. FLOVENZ, Olafur 1; Mr. GUÐMUNDSSON, Ólafur 2; Mr. GUÐMUNDSSON, Gunnar 3

1 ISOR2 University of Uppsala, Sweden3 Icelandic Meteorologic Office


Hellisheiðarvirkjun, a 303 MWe power plant in the Hengill area in SW Iceland came stepwise into productionfrom 2006 to 2011. It is owned and operated by Reykjavík Energy. The production requires re-injection 500-600L/s. A site south of the power plant that was initial site chosen for the re-injection turned out to be moresuited for production and a new site was therefore constructed in Húsmúli north of the plant. In January 2011the re-injection borehole, HN-17, was drilled in Húsmúli.In 1994 to 1998 an intense seismic activity occurred in the Hengill area which is located at the junction of theReykjanes peninsula, the western volcanic zone and South Iceland seismic zone. The magnitude of theseearthquakes did reach 5.0 Ml. Húsmúli is located in the perifery of this junction but significant activity wasobserved there during this period.During drilling of most of the injection boreholes that were drilled prior to HN-17 seismic swarms wereobserved. This activity was recorded on the SIL-network which is the regional seismic network of Icelandoperated by the Icelandic Meteorological Office. The SIL-station closest to the area is at 15 km distance andconsequently the locations, in particular the depth determination of the earthquakes, could be improved by adenser seismic network. In January 2011 ISOR ÍSOR installed 5 seismic stations in the vicinity of the plannedborehole HN-17 and operated them during the drilling. Simultaneously, pressure gauges were installed inneighbouring boreholes. During this period a seismic project with participation of Uppsala University,Sweden, MIT and LBL, USA and ISOR was in operation in SW Iceland. Altogether an extensive dataset wastherefore achieved.In 11 of February at 1320 m there was total circulation loss during the drilling of HN-17 and simultaneously aseismic swarm started with largest event of magnitude of 2,2 Ml. The presentation will demonstrate therelation between the seismicity and the circulation loss and the extensive triggered seismicity with maximummagnitude of 3,8 Ml that occurred after the initiation of full scale re-injection from the power plant.


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Recommended procedure for mitigating risk of unacceptable inducedseismicity during re-injection into conventional geothermal reservoirs.Dr. FLOVENZ, Olafur 1; Mr. ÁGÚSTSSON, Kristján 1

1 ISOR


There is a basic difference between classical re-injection into geothermal reservoirs and stimulation of EGSwells. The latter needs high injection pressure compared to the natural pressure state in order to createpermeable fractures while the former usually only needs low wellhead pressure to inject the fluid into thereservoir where the pressure sometimes has already been reduced considerably due to the production.Extensive experience of re-injection in Iceland has shown that in most cases there is a very low inducedseismicity associated with re-injection and the recorded events are usually too small to be felt by the public.However, if the fluid is injected into fracture systems that are critically stressed the re-injection can triggermedium to large magnitude earthquakes; the size depending on the pre-existing stress level and the strengthof the rock.Since the experience in Iceland had, until 2011, only shown negligible induced seismicity due to re-injectionno regulations or guidelines existed on how to initiate or operate re-injection and it was even not accountedfor in the environmental impact assessment of the power plants.The Icelandic geothermal industry got a serious warning in the fall of 2011 when repeated swarms of triggeredearthquakes occurred as a consequence of commissioning a new re-injection site for the 303MWe Hellisheiðigeothermal power plant. Thousands of quakes where observed in these swarms with maximum magnitude of3.8 and hundreds of earthquakes where felt in a nearby village of Hveragerði. There were no materialdamages but the shaking was very annoying for the public in Hveragerði who experienced considerabledamages in 2008 caused by a 6.3 magnitude earthquake with epicentre in the outskirts of the village.Reykjavik Energy, the owner and operator of the power plant, created an expert group who analysed thesituation and recommended a procedure for future re-injection programs. These recommendations are likelyto form the basis for future official regulation in Iceland. The recommendations were made with respect toand based on knowledge collected in the GEISER project so the GEISER project has already created a valuablespin-off. The recommendations will be explained in the presentation.


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Forward Induced Seismic Hazard Assessment, fromHydro-Geomechanical Model to Seismic Hazard AssessmentDr. HAKIMHASHEMI, Amir Hossein 1; Dr. YOON, Jeoung Seok 2; Dr. HEIDBACH, Oliver 1; Dr. ZANG, Arno 3; Mr. SCHOENBALL,

Martin 4; Prof. GRÜNTHAL, Gottfried 1

1 GFZ Potsdam2 GFZ German Research Centre for Geosciences3 GFZ German Research Centre for Geosiences4 KIT / GEOWATT


In this presentation, we introduce statistical approaches to assess the hazard of the induced seismicity usingthe outputs of hydro-mechanical-numerical models of geothermal reservoirs. The general approach is calledForward Induced Seismic Hazard Assessment (FISHA), since it connects the results of forward deterministicmodeling of fluid injection induced seismicity with methods of time-dependent seismic hazard assessment.The FISHA workflow has two branches dedicated to two general classes of hydro-mechanical-numericalmodels. One branch is related to the majority of models that deliver only spatiotemporal stress changes. Herethe stress change can be translated into time-dependent seismicity rate using the Rate-and-State law ofDieterich (1994).The other branch of FISHA, which is the main focus of this presentation, incorporates those hydro-mechanicalnumerical models that are capable of simulating dynamic rupture process resulting from fluid injection andinduced seismic events including occurrence time, location and magnitude, resulting in a synthetic catalogueof induced events. The statistical seismic hazard model, i.e. the model to estimate the occurrence rate ofinduced events of a given magnitude threshold using the synthetic seismic catalogues, is then based on thefrequency-magnitude relation with time-dependent parameters of a and b.Using this FISHA branch, different reservoir stimulation scenarios are tested by varying the operable injectionparameter; e.g., injection flow rate, duration of injection, and style of well shut-in. Resulting seismicityrecords, including locations, occurrence times, magnitudes, etc., are monitored for each stimulation scenario.The seismicity records; i.e. seismicity catalogues, are used to calibrate the magnitude completeness (Mc), thefrequency-magnitude distribution a and b parameters, and the seismicity rate as well as the uncertainties foreach scenario in different time intervals, basically for pre- and post-shut-in phases. Using these parameters thetime-dependent occurrence rate of earthquakes with magnitude larger than a given magnitude threshold iscalculated for each stimulation scenario. Finally, we discuss these results to modify a “suitable stimulationscenario” with respect to an acceptable level of these occurrence rates.


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Correlation between injection rates, pressure data and inducedmicroseismicityDr. OYE, Volker 1; Dr. GOERTZ-ALLMANN, Bettina 1; Dr. ZHAO, Peng 2; IRANPOUR, Kamran 1

1 NORSAR2 NORSAR, now CGG


In this paper we investigate the relation between injection rate, fluid pressures and the activity rate ofmicroseismicity. Two case studies are presented, the Longyearbyen CO2LAB on Svalbard and the Krechba, InSalah CO2 injection project in Algeria. The CO2LAB project has so far only injected for short testing periods(maximum of 5 days) and injection fluids were water and brine. One significant microseismic event occurredabout 17 hours after shut of the 5-day water injection, followed by a series of 7 aftershocks that could only beretrieved with master-event cross-correlation methods. Following injection tests with smaller injectionvolumes did not induce any measurable microseismicity, although clear pressure drops accompanied by anincrease in injection rate were observed. This increase in permeability is likely due to aseismic deformation.In the second case study at In Salah, CO2 is continuously injected at about 1.9 km depth in three horizontalwells. Close to one of the injection wells, a microseismic network (at times only consisted of one 3-componentgeophone) recorded significant amounts of microseismic activity with over 5000 microseismic events duringabout two years of observation. The microseismic events mainly occur in clusters and are clearly related to theCO2 injection process.

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Physical Mechanisms of Induced Seismicity: Modeling andCharacterizationDr. MCCLURE, Mark 1

1 University of Texas at Austin


In this talk, I will summarize highlights from two papers I wrote on the subject of induced seismicity. In onepaper, I performed modeling that coupled fluid flow with fracture deformation, friction evolution, andpermeability enhancement on a single, discrete fault. Several interesting mechanisms were identified in themodel results that may have bearing for both the estimation of the initial reservoir hydraulic diffusivity andfor injection strategies involving mitigation of induced events. In the second paper, I reviewed the literaturefor nine projects involving deep fluid injection in crystalline rock. There was a strong correlation between thethickness of faults present in the formation and the maximum magnitude of the induced events. This resultsuggests that characterization of the preexisting fracure network (in the context of the local stress regime)may be a technique that is highly predictive of induced seismic magnitude. In many cases, sufficient data tocharacterize the natural fracture network adaquately is not gathered, and I argue strongly that this type ofcharacterization needs to be routinely performed.


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Full Moment Tensor Analysis of Large Magnitude Events Using FirstMotion Data at The Geysers Geothermal Reservoir, CA, USAMs. BOYD, Sierra 1; Prof. DREGER, Doug 2; Ms. LAI, Voon 1; Dr. GRITTO, Roland 3

1 Berkeley Seismological Laboratory, University of California, Berkeley2 Berkeley Seismological Laboratory University of California, Berkeley3 Array Information Technology


Seismicity associated with geothermal energy production at The Geysers Geothermal Field in northernCalifornia has been increasing during the last forty years. We investigate source models of over fiftyearthquakes with magnitudes ranging from Mw 3.5 up to Mw 4.5. We invert three-component, completewaveform data from broadband stations of the Berkeley Digital Seismic Network (BDSN), the NorthernCalifornia Seismic Network (NCSN) and the USA Array deployment (2005-2007) for the complete, six-elementmoment tensor. Some solutions are double-couple while others have substantial non-double-couplecomponents. To assess the stability and significance of non-double-couple components, we use a suite ofdiagnostic tools including the F-test, Jackknife test, bootstrap, and network sensitivity solution (NSS). The fullmoment tensor solutions of the studied events tend to plot in the upper half of the Hudson source typediagram where the fundamental source types include +CLVD, +LVD, tensile-crack, DC and explosion. Usingthe F-test to compare the goodness-of-fit values between the full and deviatoric moment tensor solutions,most of the full moment tensor solutions do not show a statistically significant improvement in fit over thedeviatoric solutions. Because a small isotropic component may not significantly improve the fit, we includefirst motion polarity data to better constrain the full moment tensor solutions.


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Analysis of seismic activity in the Reykjanes and Svartsengi geothermalsystems, SW-Iceland: December 2008 – May 2009Mr. GUDNASON, Egill Arni 1; Mr. AGUSTSSON, Kristjan 2

1 Iceland GeoSurvey2 ISOR


At Iceland GeoSurvey, data from three geothermal sites in Iceland, Reykjanes, Hengill and Krafla, wereinvestigated as part of the GEISER project. These sites are situated in a comparable volcanic setting, but withdifferent seismic response to injection.

Two geothermal power plants are currently in operation on the Reykjanes Peninsula, in Reykjanes andSvartsengi. In the autumn of 2008, an injection test was planned in the Reykjanes geothermal field by thepower company HS Orka hf. To be able to detect and locate eventual small size induced seismic events, atemporary seismic network of 5 stations was operated on the Reykjanes Peninsula from December 2008 toMay 2009. In addition, data from 4 stations from the regional seismic network in Iceland (SIL) and 3 stationsfrom the University of Wisconsin was available for analysis. The seismic network was mainly focused on theplate boundaries from Reykjanes to Svartsengi.

The injection test was delayed, and some of the seismic stations were dismantled before the test was carriedout. However, some injection was made in Reykjanes during the recording period, but a more extensive one inthe Svartsengi geothermal field. Seismic activity during the recording period was less than average. Theseismic activity was analyzed, with focus on the relationship between injection and production rates and theseismic activity, relocation of earthquakes using the double-difference algorithm and evaluation of focalmechanisms of the earthquakes.

The SIL network recorded 20 earthquakes west of longitude -22.4° (roughly the western boundary of thetemporary network), compared to 122 earthquakes recorded by the temporary network and the 4 SIL stations.A few earthquakes were observed at Reykjanes but the majority of the recorded earthquakes occurred close tothe injection boreholes in Svartsengi, with spatial indications of induced seismicity.


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The art of balancing induced seismic hazard and reservoir sizeDr. GISCHIG, Valentin 1; Prof. WIEMER, Stefan 2; Dr. ALCOLEA, Andres 3

1 Swiss Seismological Service, ETH Zurich2 Swiss Seismological Service, ETH Zürich3 GeoEnergy Suisse


The hazard and risk perspective in research on induced seismicity usually focuses on the question how toreduce the occurrence of induced earthquakes. However, it is also well accepted that shear-dilatancyaccompanied by seismic energy radiation is a required process for reservoir creation in low permeability rock.Assessment of induced seismic hazard for a planned stimulation experiment must be a coupled analysis inwhich the target reservoir size is in balance with the acceptable predefined hazard level. We here present asimple 2D model, in which permeability is a function of the slip that occurs during an induced earthquake.Thus, the model has two coupled components: 1) a flow model that solves the pressure diffusion equations,and 2) a stochastic seismicity model, which uses the transient pressure disturbances to trigger seismic eventsat so-called seed points. At triggering, a magnitude is randomly drawn from a Gutenberg-Richter distributionwith a local b-value that depends on the stress state at the seed point. In the source area of the events thepermeability is increased depending on the amount of slip, but only by a maximum factor of 200. Due to thestochastic nature of the modelling approach, a representative number of 500 model realizations are computed.We then analyse the synthetic earthquake catalogues in combination with the enhanced permeability fields.The results reveal that the observed maximum magnitude, reservoir size, b-value and seismogenic index areaccompanied by considerable variability arising from the intrinsic stochastic nature of induced seismicity,which compromises planning and controlling of reservoir engineering operation. Further the sensitivity toinjection strategy and site-specific parameters is explored. We find that injection volume has the highestimpact on both reservoir size and seismic hazard, while changing injection rate and strategy (e.g. step-wiseincrease or decrease of rate) at constant injection volume has a negligible effect. However, greater than thevolume effect is the sensitivity of seismicity and reservoir size to site-specific conditions such as the initialhydraulic parameters, fracture (or seed) density, or parameters that potentially influence the encounteredb-value (e.g. stress level and regime, fault friction). Thus, reservoir stimulation suffers from great uncertaintyin targeting reservoir size and seismic hazard. To illustrate this, we performed a suite of synthetic injectionexperiments that are controlled with a traffic lights system based on a probabilistic stop criterion for seismichazard. The results are a quantified assessment of the success rate of finalizing a target reservoir size, whichstrongly depends on the level of seismic hazard that is acceptable for a specific site, but also on site-specificconditions.


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Contrasts between deformation accommodated by induced seismic andaseismic processes revealed by combined monitoring of seismicity andsurface deformations: Brady Geothermal Field, Nevada, USADr. WANG, Herb 1; Mr. DRAKOS, Peter 2; Dr. DAVATZES, Nicholas 3; Prof. FEIGL, Kurt 4; Dr. MELLORS, Robert 5; Dr. FOXALL, bill 5

1 U. of Wisconsin-Madison2 ORMAT3 Temple University4 University of Wisconsin-Madison5 Lawrence Livermore National Laboratory


Fluid pressure change accompanying pumping in the Brady Geothermal Field is associated with twocomplimentary deformation responses: (1) surface deformations sensible by InSAR corresponding tothermo-poroelastic volume change at depth and (2) seismic slip on fractures induced by either changes ineffective normal stress or solid stress with minimal impact to volume but potential impact on permeabilitystructure due to dilation or compaction along fractures. We present an integrated data set that compares theimpulse from pumping to these deformation responses in order to investigate their coupling and to constrainthe geometry and rheology of the reservoir and surrounding crust and to clarify the relationship betweeninduced seismicity and pumping. Currently, the dominant impulse is pressure reduction resulting fromon-going production since 1992. Initial modeling indicates that the pattern of subsidence is consistent withfluid extraction along a vertical conduit from shallow depths to approximately 1 km within the fault bend andthen extraction at ~1 km depth along the entire length of the mapped Brady normal fault indicating a reservoirmuch larger than would be expected from the footprint of the production wells (Figure 1a). In the comingmonths, well 15-12ST1, located immediately SSW of the current production area along the direction of thestructural trend and SHmax, will be stimulated to expand the reservoir, providing the opportunity to evaluateinjection dominated behavior.

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ROLE OF A FAULTS NETWORK ON MAJOR INDUCED MICROSEISMICEVENTS DURING THE HYDRAULIC STIMULATION OF AGEOTHERMAL WELL, BASED ON THE SOULTZ-SOUS-FORETS EGSSITEDr. RACHEZ, Xavier 1; Dr. GENTIER, Sylvie 1; Dr. GUILLON, Theophile 1; Dr. BLAISONNEAU, Arnold 1; PETER-BORIE, Mariane 1

1 BRGM


This presentation will deal with the global approach and with the current status of the modelling activities,performed by the Geothermal Department of BRGM, within the frame of the WP4 of the GEISER project.These activities aim at studying the behaviour of faults and faults network during the hydraulic stimulation ofa well in order to explain how the irreversible injectivity/productivity is increased and to estimate the majormicro-seismic induced events. Various tasks have been performed in order to reach this endeavour. Aconceptual model of fault, based on a detailed analysis of its internal structure, has been proposed in order toestimate with adapted modelling approaches the fault hydraulical and mechanical properties. Also, specificnumerical developments have been performed in order to estimate the major microseismic event. Thesemodelling activities are based on the Soultz-sous-Forêts EGS case.


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Improving fault mechanical behavior for a better assessment of inducedseismicity during stimulation of a geothermal systemDr. GUILLON, Théophile 1; Dr. RACHEZ, Xavier 1; Dr. GENTIER, Sylvie 1; Dr. BLAISONNEAU, Arnold 1

1 BRGM


In order to increase the overall productivity, Enhanced Geothermal Systems (EGS) rely on a stimulation phase.Due to its geological background, any geothermal reservoir exhibits faults which are rather impermeable(filled with crushed rock and cement). Stimulation aims at improving the faults permeability. When hydraulicstimulation is used, a water overpressure is applied at well head and results in faults irreversible opening(under combine effect of shearing and dilatancy). Previous results already showed that some faults exhibitinduced seismicity when stimulated. For a finer description of the seismicity, it appears that an accurate jointbehavior law should be used: a peak-residual behavior allows a clear and physically relevant identification ofcoseismic displacements. Moreover, several seismic events may appear on a sheared fault, and would gather aspatches of slipping nodes (i.e., seismically active) in the numerical model. It thus becomes necessary todiscriminate coseismic clusters in order to avoid overestimating the induced seismicity Richter magnitudes.

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Ambient seismic noise cross-correlation techniques: a new tool forgeothermal exploration and monitoringDr. JOUSSET, Philippe 1

1 GFZ Potsdam


The knowledge of structural and fluid dynamics of geothermal areas is fundamental in order to targetinggeothermal reservoir, improving fluid recovery and managing the resource optimally. Micro-seismicity ingeothermal systems is one of the important indicators which give insights in both structural features anddynamical behavior of geothermal systems, i.e., better assess conditions that prevail to trigger LargeMagnitude earthquakes. In order to locate accurately the micro-seismicity, the knowledge of the velocity fieldis of prime importance. However, when no micro-seismicity is occurring, we use ambient seismic noiseanalysis with cross-correlation techniques to get insights in structural features of geothermal systems. Wereview briefly what is ambient seismic noise. We retrieve the Green’s function, characteristic of the seismicvelocity field of rocks, using surface-wave tomography from the cross-correlation of ambient seismic noise. Inorder to monitor the reservoir dynamical features, we infer seismic velocity changes using waveforminterferometry of the ambient noise. We apply cross-correlation of the ambient seismic noise to recover theGreen’s functions for Bouillante, Guadeloupe (French Antilles) exploited geothermal field. The geothermalfield of Bouillante (Guadeloupe, French Lesser West Indies) is the only high temperature site exploited overallthe French territory. Since 2004, a seismological network has enabled classification and analysis of continuousseismic data. In about 8 years or recording, very little micro-seismicity has been detected generated in thegeothermal field, whereas intense seismicity is linked to the subduction processes. Cross-correlation techniqueis also accurate enough to resynchronize data with missing GPS, which then allow us to pick seismicity withaccuracy better than picking error. We locate and characterize the few local micro-seismic events in terms ofspectral frequencies, wave forms, ratios of seismic velocities, magnitudes. We interpret these results in thelight of the structural and dynamical model of the reservoir. We also briefly present results obtained atHengill geothermal field, Iceland and Soultz geothermal field, Alsace, France. These studies demonstrates thepotential of ambient seismic correlation techniques in geothermal areas.


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Effective detection capability of a local seismic networkDr. GAUCHER, Emmanuel 1; Prof. KOHL, Thomas 1

1 KIT


In applications such as geothermal energy, underground storage, mining, hydraulic fracturing, it is more andmore current practice to implement local seismic networks to monitor induced seismicity and to helpmitigating the associated risks. In such contexts, it is crucial to determine the capability of the network todetect a seismic event of predefined magnitude in the target area. Besides, such characteristic is also requiredto properly interpret seismic monitoring results, whether seismicity was recorded or not, since it iscomparable to the magnitude of completeness expected from the network. We propose a method to computethis detection capability for existing kilometre-scale seismic networks, even when no seismicity has beendetected in the target zone yet. The calibration of a local magnitude scale using earthquakes already recordedby the network and listed in a reference seismic catalogue is first necessary. It involves the pre-processing ofthe seismograms recorded at each station as performed during the station triggering sequence. This last mustbe based on amplitude thresholds. Thus, magnitude-distance-amplitude relationships are obtained for eachstation and are combined with the amplitude thresholds to mimic the network detection sequence. This leadsto the probability, for the network, to detect a seismic event of a given magnitude at a given location. Thisobservation-based approach offers an alternative to a fully theoretical detection capability modelling andlikely better handles the issues of the seismic wave attenuation by geometrical spreading and intrinsicattenuation. Moreover, local effects, coupling issues, effective geophone responses are intrinsically taken intoaccount during the calibration. We apply this procedure on the seismic network deployed in Bruchsal(Germany) geothermal field. Since monitoring started, mid-2010, no induced seismicity was identified in thearea despite the good working order of the system. According to our procedure, there is 95% probability thatno seismic event with ML ≥ 0.7 was induced below the network footprint, at 2.4 km depth, which correspondsto the geothermal reservoir level.

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The role of triggering by static stress transfer during geothermalreservoir stimulationSCHOENBALL, Martin 1; Prof. KOHL, Thomas 1

1 KIT


Understanding of induced seismicity during stimulation and production of geothermal reservoirs is a keyaspect towards future large-scale application of deep geothermal energy. During creation of an EnhancedGeothermal System massive fluid injections are conducted to induce shear events which generate reservoirpermeability. At the European research project at Soultz-sous-Forêts (Alsace, France) several well stimulationexperiments have been conducted and a unique dataset has been assembled. In this study we analyzecoseismic static stress transfer by the induced seismicity during a stimulation. For this purpose we developedan efficient method to calculate coseismic static stress changes from an elliptical slip distribution on a circularfracture using superposition of rectangular sources, which enables us to apply an analytical solution for fastcomputation. This method is applied on a dataset of 715 focal mechanisms derived from seismic recordings ofthe stimulation of the well GPK2 to calculate temporal evolution of static stress transfer. We find that theresulting structure of coseismic stress changes can be divided into three parts: a quiet zone where no spreadingof seismicity occurs, an active zone within the created reservoir with ongoing fracturing and a process zonewhere the growth of the reservoir occurs. Static stress changes in the active zone are of the order of 1 MPa,both positive and negative, but may exceed this value considerably on a local scale. Analysis of stress changesfrom a cluster of events that occurred after shut-in lets us conclude, that triggering by coseismic static stresschanges is possible for some events.


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Modeling EGS and Recent Developments of GPU-based Computations ofTMHC SystemsProf. MILLER, Stephen 1; Dr. GALVAN, Boris 1; Mr. HEINZE, Thomas 1

1 University of Bonn


The permeability structure resulting from high fluid pressure stimulation is the most important parametercontrolling Enhanced Geothermal Systems (EGS), yet is the most elusive to constrain. Linear diffusion modelsdo a reasonably good job of constraining the front of the stimulated region because of the square root of timedependence of the perturbation length, but triggering pressures resulting from such models, and thepermeability inferred using the diffusivity parameter, drastically underestimate both. This leads to incorrectinterpretations about the nature of the system, including the degree of fluid pressures needed to enhance thesystem. Using a minimalist approach to modeling, it is shown that all of the observations from BaselGeothermal Experiment are well-matched by a simple model where the dominant control on the system is alarge-scale change in permeability at the onset of slip. The excellent agreement between observations andmodel results indicates that it captures the dominant processes controlling EGS systems. Using this simplemodel as a baseline, we are developing much more sophisticated models designed run on Graphics ProcessingUnit (GPU) computer clusters. The model under development includes pore-elasticity, thermo-elasticity,plasticity, hardening/softening/damage, coupled to a non-linear diffusion model for fluid pressure propagationand flow a two-phase fluid. Comparisons of model results with experimental fluid injection experiments showexcellent agreement, with numerical resolution at the grain size of the specimen and at significantlyaccelerated simulation times.

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A multiple-criteria network optimizationDr. TRAMELLI, Anna 1; Dr. GIUSEPPE, De Natale 1; Dr. TROISE, Claudia 2; Dr. ORAZI, Massimo 2

1 INGV - Osservatorio Vesuviano2 INGV -Osservatorio Vesuviano


Optimizing a seismic network for locating earthquake is a crucial issue in seismology. Precise earthquakelocation is indeed the key factor for most sophisticated seismological analyses and for monitoring activities.Seismic monitoring of tectonic and volcanic areas involves the use of seismic networks which should havehigh efficiency in detecting and locating microearthquakes. Network testing and optimization shouldtherefore be a basic procedure to plan, install and improve a monitoring seismic network. In this paper, wefirstly review the most appropriate methods for network testing, using two different approaches based,respectively, on linearized and Bayesian methods. Furthermore, we developed and tested a completeoptimization technique which allows to take into account different parameters (i.e. cost, transmission linesredundancy, etc.) in addition to the location quality. The new method is based on the direct comparison of allthe possible networks resulting from the permutation of N stations in M sites (with M>N). The locationperformance of a network can be defined using different criteria; in our method we use either the determinantof the covariance matrix, which is generally used in literature, and the condition number of the coefficientmatrix, a different approach which is new in this field. We show that our new procedure is very efficient fornetwork optimization with respect to multiple criteria, and overcomes several problems of the actualprocedures. The method is thus applied to the Campi Flegrei volcanic area, to optimize and improve itsseismic monitoring network.


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Natural stress and fault controls in induced seismicity: what can we learnfrom gas depletion in the Netherlands?Prof. VAN WEES, Jan-Diederik 1; Mrs. WASSING, Brecht 2; Dr. FOKKER, Peter 1; Ms. BUIJZE, Loes 1

1 TNO2 Drs.


In this paper we present a review of controlling geological, tectonic and engineering factors for inducedseismicity associated to gas depletion in the Netherlands and we place experiences from extensive Dutchgeomechanical studies in the past decade in the context of recent models for EGS seismicity. The Netherlandsis in a mature gas production phase, marked by excellent subsurface structural and stratigraphiccharacterization. Over 190 gas fields of varying size have been exploited. No more than 15% of these fieldsshow seismicity, which is being closely monitored. Geomechanical studies show that, similar to EGS, largestseismicity is localized on pre-existing fault structures. However, the prime cause for seismicity in gasdepletion is differential compaction, whereas in EGS pressure build-up and fluid pressure diffusion along thefaults form the prime mechanism. Stress measurements from leak-off pressure and the accumulated pressuredevelopment prior to earthquakes consistently show that most faults showing induced seismicity are generallyfar from critically stressed.

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Analysis of microseismicity from hydrothermal reservoir atYanaizu-Nishiyama, JapanDr. ASANUMA, Hiroshi 1; Mr. ETO, Tai 2; Mr. ADACHI, Masaho 3; Mr. SAEKI, Kazuhiro 4; Mr. AOYAMA, Kengo 4; Mr. OZEKI, Hitoshi 5

1 Institute for Geo-resources and Environment, AIST2 Tohoku Univ.3 INPEX4 Okuaizu Geothermal5 MOECO


Microseismicity from a hydrothermal reservoir at Yanaizu-Nishiyama, where one of the largest geothermalplants in Japan is under operation since 1995, has been monitored by a geothermal operation company. Anumber of microseismic events, including four felt earthquakes with a magnitude MJMA larger than 3.0(estimated by Japan Meteorological Agency: JMA), have been collected by the network.

Authors in AIST and Tohoku University have analyzed a total number of 5,825 microseismic events whichoccurred in this field in a period of 1996 to 2012 to investigate relation between production/injection ofgeothermal fluids and microseismicity using analysis techniques of microseismicity which include highresolution hypocenter determination by multiplet clustering and double difference (DD). We have found thatthere is no clear relation between the activity of microseismicity and the change in the production rate,although increased seismic activity associated with treatment injection around at the bottom of an injectionwell was observed. A seismostatistical analysis also revealed that seismicity around at the treatment injectionpoint has more induced nature. It has been also revealed that the four felt earthquakes had closely similarfault plane solutions (FPSs), which have inclinations around 45˚ to NE, even the orientations of themicroseismic multiplet seismic structures showed various azimuths and inclinations, suggesting that most ofthe accumulated strain energy within the seismically active zone was released from particular weak structureas felt earthquakes.


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Coupled Continuum Modeling of Fracture Reactivation and InducedSeismicity during Enhanced Geothermal OperationsMrs. WASSING, Brecht 1; Dr. FOKKER, Peter 2; Prof. VAN WEES, Jan-Diederik 2

1 Drs.2 TNO


We developed a coupled code to obtain a better understanding of the role of pore pressure changes in causingfracture reactivation and seismicity during Enhanced Geothermal Systems operation (EGS). We implementedconstitutive models for fractures in a continuum approach, which is advantageous because of the ease ofintegration in existing geomechanical codes (FLAC3D), the speed of the calculations and the flexibility of thefracture representation. For modeling the mechanical behavior of the fracture zone the softening ubiquitousjoints model was used with random strength properties for the fracture zone. We implemented a hyperbolicdeformation with effective stress for the reversible tensile fracture opening, and a linear relationship betweenplastic shear strain and irreversible fracture opening. The effective permeability of the fracture network wasdescribed by a cubic dependence on the fracture aperture. We created a model inspired on theSoultz-sous-Forêts GPK3 injection well, where the granite rock mass is intersected by a dominant fracturezone. The model reproduced reactivation of the fracture zone due to injection of water and we observed thegrowth of a zone with large directional permeability. The model was used to perform a sensitivity analysis onparameters like in-situ stress regime, fracture strength, and frictional weakening. This allowed us to evaluatethe trends of their impact on fracture reactivation, including reactivated area, seismic moment and momentmagnitudes. A comparison with a Block-Spring model as reported by Baisch (Baisch, Vörös et al. 2010) yieldedsimilar results.The models were applied on the GPK3 stimulation case that was performed in Soultz in 2003. The basicfeatures of the observed seismicity were reproduced. The 3D implementation and the Block-Spring modelshowed their specific advantages: the Block-Spring model, if calibrated, establishes a fast modeling tool forsensitivity analyses; the FLAC3D implementation allows better understanding as it is based on actual physics.

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Habanero 2012 Stimulation - Implications for EGS -HOLL, Heinz 1; HOGARTH, Robert 1; BARTON, Colleen 2

1 Geodynamics Limited2 Baker Hughes / GMI Geomechanics Services



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Geomechanical Controls on Fault Reactivation, Fluid Flow and InducedSeismicity in Natural and Engineered Geothermal SystemsHICKMAN, Stephen 1; Dr. DAVATZES, Nicholas 2; ZOBACK, Mark 3; DEMPSEY, David 4; KELKAR, Sharad 4; MOOS, Daniel 5;

ZEMACH, Ezra 6; CHABORA, Ethan 7; LUTZ, Susan 8; ROSE, Peter 9; MAJER, Ernie 10; ROBERTSON-TAIT, Ann 7; BARTON, Colleen 5;

BENOIT, Richard 11

1 U.S. Geological Survey2 Temple University, Philadelphia PA 19122 USA3 Stanford University, Stanford CA 94305 USA4 Los Alamos National Laboratory, Los Alamos NM 87545 USA5 Baker Hughes/GMI Geomechanics Services, Palo Alto CA 94306 USA6 0rmat Nevada Inc., Reno NV 89511 USA7 GeothermEx-Schlumberger, Richmond CA 94806 USA8 TerraTek-Schlumberger, Salt Lake City UT 84104 USA9 University of Utah, Salt Lake City UT 84104 USA10 Lawrence Berkeley National Laboratory, Berkeley CA 94720 USA11 Magma Energy Corporation, Reno NV 89511 USA


In fracture-dominated geothermal systems, knowledge of the in-situ state of stress and the mechanical properties of host rocks is needed both to understand tectonic controls on natural permeability and to establish the conditions leading to fault reactivation and induced seismicity during creation of Engineered Geothermal Systems (EGS). Two field studies in the tectonically active northern Basin-and-Range Province, USA, yield results that 1) illustrate the importance of geomechanics in understanding and manipulating fracture permeability (both magnitude and anisotropy) in natural and engineered geothermal systems, and 2) show how knowledge of in-situ stresses (and stress perturbations) in relation to local faults can be used to evaluate the potential for injection- or production-induced seismicity. Dixie Valley Geothermal Field: The first field study was carried out at Dixie Valley, Nevada, where a highly productive natural geothermal reservoir (generating more than 60 MW of electricity for ~25 years) results from deep fluid circulation and upwelling along a system of major, range-bounding normal faults within the Dixie Valley Fault Zone (DVFZ). Dixie Valley lies within the Central Nevada Seismic Belt, and although earthquakes have not ruptured this segment of the DVFZ in historic times, large (M = 6.8 to 7.3) earthquakes have occurred within the past 100 years along range-bounding faults several tens of km to the northeast and southwest. This study used borehole measurements of stress, fracture orientation and hydrologic properties at depths of 1.6 to 3.0 km to characterize the manner in which reservoir permeability is related to the in-situ stress field. Observations of borehole breakouts and drilling-induced tensile fractures in image logs from three wells spanning a distance of ~4 km within the main geothermal reservoir show that the orientation of the least horizontal principal stress, "shmin", is nearly perpendicular to the strike of the DVFZ. Although these image logs reveal pervasive macroscopic natural fractures with a wide range of orientations, temperature-pressure-flowmeter logs conducted during fluid injection and withdrawal indicate that the most permeable fractures strike parallel to the DVFZ and lie within a damage zone extending about 50 m into the hanging wall. Hydraulic fracturing tests indicate that the magnitude of "shmin" is low enough relative to the calculated vertical stress, SV, to lead to frictional failure on these highly permeable fractures under current in-situ fluid pressures. Crack sealing and permeability reduction would be expected within the DVFZ, given thermal and geochemical evidence for up-dip transport of silica-saturated fluids. However, the observation that highly permeable fractures within the damage zone are favorably aligned and critically stressed for normal faulting suggests that dilatancy due to intermittent fault slip counteracts the expected permeability reduction. In one well drilled into a non-productive segment of the DVFZ, borehole breakouts and drilling-induced tensile fractures indicate a ~90° rotation in the azimuth of "shmin" at about 2.7 km depth; similar rotations were observed in a producing well located ~0.6 km to the northeast. This rotation, together with the low "shmin" magnitude measured at 2.5 km depth in this well, is most readily explained through local stress perturbations induced by the occurrence of one or more normal faulting earthquakes in the hanging wall of


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the DVFZ. Similar localized stress perturbations at a variety of scales have been observed from borehole image logs collected elsewhere, including in geothermal wells, and can be used to infer the occurrence and frequency¬magnitude behavior of recent fault slip. When combined with in-situ stress measurements, analyses of localized stress perturbations observed in borehole image logs can help evaluate the potential for injection-or production-induced seismicity, augmenting what might otherwise be a sparse (or non-existent) historical or instrumental earthquake record. Desert Peak EGS: The second study was conducted at depths of 0.9 to 1.7 km in a well penetrating low-permeability rocks on the margins of a producing geothermal field at Desert Peak, Nevada. This investigation used measurements of fracture geometry, stress and rock mechanical properties to develop the geomechanical framework for creating an EGS between well 27-15 and the main geothermal reservoir. Drilling-induced tensile fractures seen in image logs from well 27-15 indicate that the direction of "shmin" is consistent with normal faulting on ESE-and WNW-dipping faults mapped at the surface and seen in the image logs. A hydraulic fracturing stress measurement carried out at a depth of 930 m prior to EGS stimulation indicates that the magnitude of "shmin" is 13.8 MPa, which is ~0.61 of SV. Coulomb failure calculations using these stresses together with measurements on core indicate that dilatant shear failure should be induced on pre-existing fractures once fluid pressures are increased ~2.5 MPa above the ambient, far-field pore pressure. This shearing was predicted to generate a zone of enhanced permeability elongated parallel to the maximum horizontal principal stress, SHmax, in the direction of nearby geothermal injection and production wells ~0.5 and 1 km to the SSW, respectively. Hydraulic stimulation of well 27-15 began in August 2010, and was monitored using a combined surface and down-hole seismic network and inter-well tracer testing. An initial phase of low-pressure stimulation was carried out for 113 days at pressures <"shmin" and injection rates <380 l/min, employing four stepwise increases in pressure to induce shear failure along pre-existing natural fractures. This low-pressure stimulation phase increased injectivity by more than one order of magnitude, consistent with 3-D numerical simulations of shear-enhanced permeability gain conducted for this site using measured stresses, fracture parameters and rock mechanical properties. Although injectivity was successfully increased, little related seismicity was detected during this stimulation phase. High-pressure stimulation was then carried out for 60 days at pressures >"shmin" and injection rates up to 2800 l/min to induce combined tensile and shear failure and promote fluid pressure transfer to greater distances from the borehole. This high-pressure phase resulted in an additional 6-fold increase in injectivity. Numerous microearthquakes were induced during this high-pressure phase, which together with tracer testing demonstrated growth of the stimulated volume and establishment of a strong hydrologic connection between well 27¬15 and active geothermal injection and production wells to the SSW, as predicted by the stress model. The seismic array was then augmented before carrying out additional phases of low- and high-pressure stimulation in late 2011 and early 2013 to further increase permeability in the region around well 27-15, resulting in continued growth of the microseismic cloud in the NNE – SSW direction and a cumulative injectivity gain of 175-fold over the course of the EGS project. The success of the Desert Peak geomechanical model in predicting both the initiation criteria and directional characteristics of injection-induced shear failure indicates that similar studies would be useful in predicting both the onset and spatial distribution of injection-induced seismicity at other sites being considered for EGS stimulation and long-term production. Summary Comments: As the above case studies illustrate, knowledge of the in-situ state of stress is essential for understanding mechanical controls on the magnitude and anisotropy of fracture permeability, both in natural systems and in EGS. This understanding is needed to formulate physically realistic models for the temporal and spatial evolution of fracture permeability and fluid flow during deep well injection, enabling the stimulation of fracture permeability in the most effective manner for increased geothermal power production and prediction of the distribution of seismicity induced by these operations. Also, when geologic mapping and geophysical imaging reveal the existence of large, throughgoing faults, knowledge of the background state of stress and geohydrologic structure can be used to evaluate the extent to which these faults might be pushed closer to failure due to fluid injection or production operations and to design appropriate mitigation measures. These mitigation measures include locating geothermal wells and establishing EGS stimulation and injection/production protocols to minimize the potential for triggering felt or damaging earthquakes on these large-scale tectonic faults, either through direct fluid pressure effects or through poro-thermoelastic perturbations to the in-situ stress field. Although most EGS stimulations, such as that carried out at Desert Peak, require significant, positive wellhead pressures to induce shear failure and the associated microseismicity, faults in extensional stress regimes under conditions of highly sub-hydrostatic background formation fluid pressures can be reactivated even if no wellhead pressure is required for injection. The susceptibility of these types of naturally under-pressured


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faulting environments to fault reactivation during injection at negligible (or zero) wellhead pressures shouldbe taken into account when designing EGS stimulation and long-term injection operations to minimize risksfrom induced seismicity.


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Simulation-based Probabilistic Seismic Hazard and Risk Analysis forSeismicity Induced by Fluid InjectionDr. FOXALL, bill 1

1 Lawrence Livermore National Laboratory


Risk associated with ground motions caused by induced seismicity is a significant factor in the design, permitting and operation of enhanced geothermal, geological CO2 sequestration and other fluid injection projects. The risk of generating ground motions capable of causing structural damage from a properly designed and operated project is generally exceedingly low. However, the risk of causing nuisance to nearby communities and hence creating a negative public perception that impacts the viability of a project can often be much more significant. Therefore, assessment of the hazard and risk from induced seismicity is required both before and during injection and during the post-shut-in or circulation stages of a project. Whereas widely-used conventional probabilistic seismic hazard and risk analysis (PSHA/RA) methods provide an overall framework, they require adaptation to address specific characteristics of induced earthquake occurrence and ground motion estimation and to include assessment of the of nuisance. First, earthquakes induced by a fluid pressure field that evolves in time and space clearly do not conform to the Poisoning occurrence model generally assumed in conventional PSHA to derive frequency-magnitude statistics based on the regional seismicity history. Furthermore, a site-specific catalog of induced events obviously cannot be available for hazard and risk assessment at the design and permitting stage, before injection begins. We approach estimation of frequency-magnitude statistics by using a modified version of the program RSQSim (Richards-Dinger and Dieterich, 2012) to produce synthetic catalogs of earthquakes generated by pressure-induced accelerated shear failure on faults embedded within a volume that is subject to constant-rate tectonic loading. The 3D site model is based on available measurements of site-specific petrophysical and hydrologic properties and in-situ stresses. Pore-pressure histories on the faults are extracted from a precomputed 3D flow simulation driven by injection into the reservoir interval. RSQSim employs a rate-and-state frictional constitutive law to simulate the full earthquake cycle on a set of gridded faults. Stressing-rate changes on all of the grid elements resulting from the coseismic slip on any one element are determined using a 3D boundary element calculation, so that larger events occur as propagating multi-element cascades of failures. Very rapid simulation of catalogs containing the thousands of events required for PSHA is achieved by employing approximate analytical calculations to determine event-driven (rather than short predetermined) time steps. Values of rate-and-state and other fault constitutive parameters are sampled from appropriate generic ranges. The second modification to the conventional PSHA method is dictated by the need to calculate relatively low-amplitude ground motions from small, shallow events that can be felt at distances of a few km. Empirical ground motion prediction equations in general use for PSHA are poorly constrained for small magnitudes (<M4) and short distances. Therefore, we estimate ground motions using physics-based analytical calculations employing synthetic Green’s functions derived from the local site velocity and attenuation structures. As an alternative, we intend to implement the empirical short-distance, small magnitude ground motion prediction equations recently developed under the GEISER project specifically for induced seismicity scenarios (Douglas et al., 2013). The hazard calculations are carried forward to estimate risk of nuisance using “nuisance sensitivity functions” developed following the same general approach used in the HAZUS loss estimation project (FEMA, 2012) to construct structural damage fragility functions. The nuisance sensitivity functions are based on the ANSI and ISO deterministic acceptance criteria for ground motions and vibrations generated by mining and construction activities adopted in DOE’s protocol and best practices for mitigating nuisance risks from EGS induced seismicity (Majer et al., 2012a,b). We discuss demonstration results from application of the PSRA approach to a realistic long-term CO2 storage scenario. In this scenario, CO2 injection takes place over a time interval of 50 years at a rate of 0.6 million


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metric tons per year. We anticipate that the results should scale to EGS operations that take place on timescales of days to months and involve correspondingly lower net injected fluid volumes. REFERENCES Douglas, J., B. Edwards, V. Convertito, N. Sharma, A. Tramelli, D. Kraaijpoel, B. Mena Cabrera, N. Maercklin,and C. Troise, Predicting ground motion from induced earthquakes in geothermal areas, to be published Bull.Seismol. Soc. Am., June, 2013. HAZUS 2013: The Federal Emergency Management Agency’s (FEMA’s) Methodology for Estimating PotentialLosses from Disasters. www.fema.gov/hazus/#3 Majer, E., J. Nelson, A. Robertson-Tait, J. Savy, J., and I. Wong, Protocol for Addressing Induced SeismicityAssociated with Enhanced Geothermal Systems. Report DOE/EE- 0662, DOE Geothermal Technologies Prog.,45 p, 2012a. Majer, E., J. Nelson, A. Robertson-Tait, J. Savy, J., and I. Wong, Best Practices for Addressing InducedSeismicity Associated With Enhanced Geothermal Systems (EGS). Draft report prepared by LawrenceBerkeley National Laboratory for the DOE Geothermal Technologies Prog. 2012b. Richards-Dinger, K., and J. Dieterich, RSQSim earthquake simulator, Seismol. Res. Let., 83, 983-990, 2012.


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Analysis of induced seismicity at The Geysers geothermal field, CaliforniaOREFICE, Antonella 1; EMOLO, Antonio 2

1 University Federico II, Napoli2 University Federic II, Napoli


Fluid injection, steam extraction, and reservoir stimulation in geothermal systems lead to induced seismicity.While in rare cases induced events may be large enough to pose a hazard, on the other hand themicroseismicity provides information on the extent and the space-time varying properties of the reservoir.Therefore, microseismic monitoring is important, both for mitigation of unwanted effects of industrialoperations and for continuous assessment of reservoir conditions. Here we analyze induced seismicity at TheGeysers geothermal field in California, a vapor-dominated field with the top of the main steam reservoir some1-3 km below the surface. Commercial exploitation began in the 1960s, and the seismicity increased withincreasing field development.We focus our analyses on induced seismicity recorded between August 2007 and October 2011.Our calibrated waveform database contains some 15000 events with magnitudes between 1.0 and 4.5 andrecorded by the LBNL Geysers/Calpine surface seismic network. We associated all data with events from theNCEDC earthquake catalogue and re-picked first arrival times. Using selected events with at least 20high-quality P-wave picks, we determined a minimum 1-D velocity model using VELEST. A well-constrainedP-velocity model shows a sharp velocity increase at 1-2 km depth (from 3 to 5 km/s) and then a gradient-liketrend down to about 5 km depth, where velocities reach values of 6-7 km/s. The station corrections showcoherent, relatively high, positive travel time delays in the NW zone, thus indicating a strong lateral variationof the P-wave velocities. We determined an average Vp-to-Vs ratio of 1.67, which is consistent with estimatesfrom other authors for the same time period. The events have been relocated in the new model using anon-linear probabilistic methods. The seismicity appears spatially diffused in a 15x10 km2 area elongated inNW-SE direction, and earthquake depths range between 0 and 6 km. As in previous seismicity studies of thisgeothermal field, we find that events occurring in the NW sector are on average deeper than in the SE area.To infer the present stress regime, we computed focal mechanisms of a large event data set with M> 2, usingP-wave first-arrival polarities. The found fault-plane solutions show a dominant strike-slip and normalfaulting mechanisms, with P and T axes coherently oriented with expected regional stress field for the area.We also determined the main seismic source parameters from a multi-step, iterative inversion of P-wavedisplacement spectra, assuming a four-parameters spectral model and a constant-Q attenuation mechanism. Inparticular, we computed seismic moments, source radii and stress drops. We observe a self-similar scaling ofsource parameters in the whole investigated magnitude range, with a nearly constant stress-drop of 20 and 120MPa depending on the use of Brune (1970) or Madariaga (1976)’s source model respectively.


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Time-dependent seismic hazard studies at The Geysers geothermal areaSHARMA, Nitin 1; EMOLO, Antonio 2

1 University Federico II, Napoli2 Unversity Federico II, Napoli


The continuous industrial operations like fluid injection and extraction alters the stresses in upper earth crust.As the consequence, induced seismicity has gained considerable attention among seismologists, worldwide.The Geysers is the largest vapor-dominated geothermal field in the world, situated about 120 km north of SanFransisco, California. No ground-motion prediction equations and seismic hazard maps were availablespecifically for this area before our study. Ground-motion prediction equations (GMPEs) are estimated for TheGeysers geothermal area using data recorded at 29 stations of the Berkley-Geysers network during the periodSeptember 2007 through November 2010. Non-linear mixed effect regression method is used to computeGMPEs for peak ground velocity (PGV), peak ground acceleration (PGA), and 5% damped spectral accelerationSA(T) at T = 0.2s, 0.5s, and 1.0s. We demonstrated that the inter-event residuals represents misfit due to theaverage source effects and also reflects various factors like stress drop and/or variation of slip in time andspace, which are not captured by including magnitude, focal mechanism and source depth. On the other hand,intra-event residual represents misfit due path and site effects (crustal heterogeneity, geological structure andnear surface layering) which modify the waveform (in terms of amplitude and frequency) and are notcaptured by distance metric and site classification based on the average shear wave velocity. The two-stepapproach used to compute GMPEs, demonstrates that even if information about local geology is not available,the method of introducing site/station effect correction is effective and shows significant improvement in themodel. Results from time-dependent probabilistic seismic hazard analysis (PSHA) demonstrated that hazard isnot constant with time and space. We also propose that time-dependent probabilistic seismic hazard analysiscan be used to monitor the effect of ongoing field operations. The variations in seismic hazard with time canbe a consequence of the variation of both seismicity rate and Gutenberg-Richter b-value which can be relatedto the rate of fluid injection. We concluded that, for the exposure time taken into account (i.e., 2 months), as aconservative limit, peak-ground acceleration values corresponding to the lowest probability of exceedance(e.g., 30%) must not exceed to ensure the safe field operations. We also suggested that the proposed techniquecan be tested at other geothermal areas or in regions, where seismicity is induced for example by hydrocarbonexploitation or carbon dioxide storage.

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Reinjection strategy for minimizing micro-seismic activity duringgeothermal exploitation at the EGS Soultz siteDr. GENTER, Albert 1; CUENOT, Nicolas 2

1 GEIE Exploitation Minière de la Chaleur2 GEIE Exploitation Minière de la Chaleur, Kutzenhausen, France


Research activity of the EGS Soultz power plant has been achieved during geothermal exploitation from theend of 2009 to mid-2013. Several hydraulic circulation tests have been performed that involve one productionwell, GPK-2 and several reinjection wells, GPK-1, GPK-3 and GPK-4 (Figure 1). During those hydrauliccirculation tests, induced seismic activity was seriously decreased by re-injecting simultaneously in severalinjection wells with various flow-rate and/or reinjection pressure.


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The Campi Flegrei Deep Drilling Project: direct measurements ofminimum stress and permeability at depthCARLINO, Stefano 1; GIUSEPPE, de Natale 1

1 INGV-OV, Napoli


A new procedure of Leak Off Test was performed to asses the in situ permeability and stress, in theframework of the Campi Flegrei Deep Drilling Project, during the drilling of the pilot hole,. This test wascarried out within the active caldera of Campi Flegrei (Southern Italy), one of the most hazardous volcanicareas in the World, hosting part of the large City of Naples. This caldera is characterized by significantepisodic uplift, a phenomenon generally considered as one of the most reliable precursors of eruptions, despiteuplifts without eruptive activity is also well documented. The models for caldera unrest consider magmaintrusion or disturbances in geothermal fluids within the shallow crust as the source of ground deformation.The latter is very sensitive to the rocks permeability. A major goal is to determine the relative contribution ofeach process, because the potential for eruption is significantly enhanced if magma movement emerges as theprimary component. Thus, the evaluation of actual in situ permeability, and stress, is fundamental to validatethe reliability of the solutions. Permeability at depth is generally dominated by fluid propagation alongfractures, rather than by fluid diffusion in pores. This makes laboratory measurements of permeability onintact rock samples generally underestimated and so meaningless. On the contrary, borehole permeabilitymeasurements give insight in the real rock permeability. The results, here obtained for the first time, putstrong constraints in understanding volcanoes behavior and to evaluate the hazard and scenario of the futureeruption.

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Induced and triggered seismicity: Coulomb stress modeling and statisticalevidenceDE NATALE, Giuseppe 1

1 INGV-OV, Napoli


Fluid injection and withdrawal in deep wells is a basic procedure in mining activities and deep resourcesexploitation, i.e. oil and gas extraction, geothermal exploitation, geothermal permeability enhancement andwaste fluid disposal. All these activities have the potential to favour seismicity, as demonstrated for instanceby the 2006 Basel earthquake of magnitude ML=3.4. Despite several decades of experience, the mechanism ofinduced/triggered seismicity is not known in detail, preventing an effective assessment and/or mitigation. Inthis work, we point out the main statistical features distinguishing the induced and triggered seismic activity,by analyzing seismic catalogues of both tectonic and geothermal areas. We will show that induced seismicitycan be characterized by the very small number of aftershocks and that, generally, induced seismicity is unableto generate a cascading process in which aftershocks generate other aftershocks. This characteristic can beused to provide an operative definition of the difference between triggered and induced seismicity.We further give an interpretation of induced seismicity based on the computation of Coulomb stress changesresulting from fluid injection/withdrawal at depth, mainly focused to interpret induced seismicity due toEnhanced Geothermal System (EGS) reservoir stimulation. Seismicity is in fact, theoretically, more likelywhere Coulomb stress changes are larger. For modeling purposes, here we simulate the thermodynamicevolution of the system after fluids injected/withdrawn. The retrieved changes of pressure and temperatureare subsequently considered as sources of incremental stress changes, which are then converted to Coulombstress changes on favored faults, taking into account also the background regional stress. Numerical results arethen applied to simulate the water injection used to create the fractured reservoir at the Soultz-sous-Forets(France) EGS site. The obtained results show that our approach provides a very good description of inducedseismicity, and gives a natural explanation to the different impact between fluid injection and withdrawal.


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New Zealand Examples of Geothermal Induced Seismicity: 30 years ofexperienceMr. BROMLEY, Chris 1

1 GNS Science


Several geothermal power projects in New Zealand (now totalling 950 MWe running capacity) have been inoperation since the late 1950’s, although reinjection of separated fluids only commenced in the mid 1980’s(Wairakei). Here, initial injection trials were shallow (0.4km) and triggered some local low-magnitudeseismicity, but only when high effective pressure (~5 MPa) was used. Other reinjection projects soon followed:Ohaaki (1989), Kawerau (1992), Rotokawa (1997), Ngawha (1998), Mokai (2000), and Ngatamariki (2013).Although all geothermal sites are in high-temperature volcano-tectonic settings, the background levels ofnatural seismicity are locally quite variable, and triggered seismicity effects from geothermal operations havealso been very mixed.At Ngawha and Ohaaki, background seismicity rates are relatively low, and local induced seismicity (M>2)has never been detected, despite 100% peripheral injection to ~1km depth at Ngawha, and 70% peripheralinjection (0.4 to 1km depth) at Ohaaki. At Kawerau, natural rates of seismicity are high (average of 2 feltevents per month), but there is no evidence of induced or triggered events associated with production orinjection changes, including a transition from shallow (0.4km) infield to mostly deep (2km) peripheralinjection in 2008 (for 100 MWe expansion). At Wairakei, natural seismicity rates are also high, but untilrecently, reinjection at up to 2km depth had not triggered significant seismicity. Since 2011, development of anew deep injection zone (Karapiti South) has seen a small local increase in events (M<2.5). At Mokai, 100%shallow (0.4km) injection to the NW of the production bore-field for the first 8 years triggered no obviouslocal seismicity, but a switch to deep (2km) reinjection to the north in 2008 resulted in an increased number ofseismic events (M<2.9) located along a fault zone trending SW about 1 - 2.5 km from the injection bore.Cooling contraction has also increased the injectivity of this bore over time. This is inferred to indicate apreferred flow-channel for injected deep fluid, parallel to a deep permeability barrier, and oblique to thenorth-south gradient implied from pressure gradients.Perhaps the most notable example of induced seismicity in New Zealand has been at Rotokawa. Here, naturalbackground seismicity was at a relatively low rate (several events per year), and remained so during the first 9years of production (35 MWe) with 100% infield reinjection at shallow depth (~0.5 km). Deep injection trialscommenced in 2006 west of the production bore-field. This triggered seismicity on a NE trending fault zone(relocated using the double-difference method). Tracer tests confirmed rapid flow of injected fluid along thisfault towards the production zone. Seismicity was inferred to be associated with injection stimulation, eventhough no direct temporal relationship with injection flow-rate could be established. Expansion by 140 MWein 2010 accompanied a switch in reinjection strategy into deep wells on the SE periphery. The triggeredseismicity followed this switch and the event rate doubled. Events were mostly clustered 0.1 to 1km NW ofinjection bores, at 1.5 to 3 km depth, and along a deep NE trending fault zone midway between injection andproduction sectors. This passes through the centre of the system and is oriented perpendicular to the pressuregradient. The seismicity has included an event of magnitude M3.5 but all other events are M<2.8. As with theother New Zealand geothermal projects, felt seismicity effects have not been an issue with the localinhabitants, who are familiar with similar felt events of natural origin.In conclusion, it appears that these New Zealand examples of induced seismicity favour a mechanismassociated with the secondary effects of increased fluid flow. These are driven by pressure gradients through afracture network, but trigger seismic failure only on pre-existing favourably-oriented fracture-networks,through local chemical, temperature or pressure transients, or by local stress perturbations unlockingasperities on stressed fractures.


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Non-stationary seismic hazard evaluation for gas fields in TheNetherlandsDr. CACCAVALE, mauro 1

1 KNMI


The seismic hazard due to induced seismicity was evaluated for gas fields where a time-depended seismicitywas identified. This analysis was performed using a classical Probabilistic Seismic Hazard Analysis (PSHA)(Cornell, 1968) approach. The PSHA allows defining the state-of-art of current knowledge with regard toabout 20 years of induced seismicity. On the other hand a classical PSHA approach is based on a stationaryPoisson model probably not appropriate to describe the time varying induced seismicity. To overcome thislimitation it is possible to define different time windows in which the stationary hypothesis is still valid or usea more general/complex approach using a non-stationary Poisson model. The second option is substituting theconstant activity seismicity rate (λ) with a time dependent variable (λ(t)). Both approaches were investigatedusing seismicity data from exploration fields in The Netherlands.

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COMPUTATIONAL INVESTIGATION OF HYDRO-MECHANICALEFFECTS ON TRANSMISSIVITY EVOLUTION DURING THE INITIALINJECTION PHASE AT THE DESERT PEAK EGS PROJECT, NVBENATO, Stefano 1

1 Desert Research Institute, Reno, NV, USA


The low-flow-rate injection phase of an Engineered Geothermal System (EGS) experiment in Desert Peak well27-15 produced increased injectivity at wellhead pressures less than the minimum principal stress, consistentwith hydraulically induced mechanical shear failure in the surrounding rock. We use statistical fractureanalysis and hydro-mechanical modeling to simulate the observed pressure response during this shearstimulation, to explore one possible conceptual framework for the overall Desert Peak EGS experiment. This ispart of a long-term study to simulate the complete Desert Peak EGS stimulation, including both shearing andhydraulic fracturing (tensile) failure.Discrete fracture network simulations, based on fracture/fault attributes measured downhole and at thesurface, were used to derive equivalent permeability tensors for comparison with preferred fluid migrationdirections observed in hydraulic and tracer tests. FLAC3D, a hydro-mechanical simulator, was used toinvestigate changes in stress and displacement according to a Mohr-Coulomb frictional model subjected toperturbations in pore pressure. Although almost all of the seismicity observed during the EGS stimulationoccurred during the high-flow-rate tensile stimulation phase, we use this seismicity to illuminate thegeometry of large-scale geologic structures that could also have served as preferential flow paths during shearstimulation. This analysis shows that conditions for shear failure during the low-flow-rate shear stimulationcould occur in locations consistent with locations of micro-seismicity seen during the tensile phase of the EGSexperiment, providing a possible hydrologic connection between EGS well 27-15 and injection/productionwells further south-southwest. This FLAC3D hydro-mechanical model will next be coupled to TOUGHREACTto investigate the near-field evolution of reservoir transmissivity associated with thermal, hydraulic,mechanical and chemical processes during all phases of the Desert Peak EGS stimulation.


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Hazard Assessment in the EGS Experiment, Berlin Geothermal Field, ElSalvador.Mrs. BARRIOS, Luz 1

1 35


An HFR project was executed in a joint venture between Shell and LaGeo to extract heat from the rock byusing a brine to inject into one well and recover it with a higher temperature in another nearby well in BerlínGeothermal field. To control the impacts that may occur, several monitoring strategies on reservoir pressure,temperature, chemistry and seismicity were undertaken. The induced seismicity related to the hydraulic tests,did not exceed the natural levels. Negative impact on the pressure, temperature and chemical estimated waslimited. The HFR test demonstrated that hot injection at 180°C, at elevated pressure could be executed in areliable and safe manner.

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Assessment of Poroelastic effects during an EGS reservoir stimulationtreatmentMr. URPI, Luca 1; Dr. ZIMMERMANN, Günter 2

1 GFZ2 GFZ Potsdam


Enhanced Geothermal System (EGS) are characterized by low natural permeability, that must be enhanced toenable commercial flow rates, which are high enough to achieve the target set for heat extraction and/orpower production. Stimulation of the target reservoir with hydraulic fracturing and hydraulic shearing havebeen performed in various geological settings, marked by different stress regimes.A relevant role in evaluating the effectiveness of the stimulation is played by the "cloud" of microseismicevents recorded during the stimulation treatment. More than 10'000 microseismic events have been recordedfor a single multi-day stimulation treatment in natural fractured rocks, in different worldwide sites. Maximummagnitude recorded for events triggered by geothermal stimulation treatment ranged from -3 up to >3, frompurely instrumental seismicity to human felt but not damaging seismicity.Seismic response to the treatment is therefore a powerful tool to understand the correct development of theoperations and at the same time a potential harm to the project. Understanding why this response showdistinct temporal and spatial patterns, requires to investigate the complex rock fluid interaction taking place.In this study the focus is on the evaluation of poroelastic response of the rock mass to a tensile fracturecreated by injecting cold fluid at a pressure high enough to overcome minimum in-situ stress and tensilestrength of the rock. Correctly forecasting the rock mass response to the stimulation treatment, in term oflocal stress and pore pressure change, allows to design effective treatment while mitigating unwantedseismicity.

SOME OBSERVATIONS FROM GROSS SCHÖNEBECK RESEARCH WELLIn August 2007, Groß Schönebeck well was stimulated with a six days lasting stimulation treatment withalternating low and high injection rate up to 150 L/s.Two faults located close to the well were expected to show microseismic activity due to the treatment, butonly one of them showed low microseismic activity, 30 events were recorded with average and maximummoment magnitude respectively -1.3 and -1. The silent fault is located between the stimulated well and amonitoring well. Sudden increase of water level associated with increase in injection rate have been observedin the monitoring well, suggesting the presence of a geological feature hydraulically connecting the two wells.

A coupled hydro-mechanical model is proposed to discuss the presence of this kind of feature.


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Rate-and-state friction in numerical modeling in the framework ofinjection induced seismictyBUIJZE, Loes 1

1 TNO/Utrecht University


Geomechanical modeling of induced seismicity provides a method to make detailed computations of the stresschanges due to fluid injection and the reactivation potential on faults resulting from this. Friction lawscontrolling slip on faults in these models are often quite simple and limited to Mohr Coulomb failure using asingle coefficient of friction or a strain-dependent friction coefficient. However, these fail to describeimportant dynamic characteristics of fault behavior, which influence the nucleation and propagation ofearthquakes and thus also the fault area that can be reactivated. From laboratory experiments constitutiveequation describing fault friction have been developed, the so-called rate-and-state friction laws. These areable to adequately model fault behavior such as coseismic weakening and healing, interseismic creep,aseismically moving faults. In this study the influence of rate-and-state friction laws on rupture propagation isinvestigated in the framework of fluid injections in Enhanced Geothermal Systems. A quasi-dynamicboundary element methods implementing rate-and-state friction is used to model fault slip and investigatecharacteristics of the earthquake cycle. A simplified stress perturbation function is implemented to simulatedthe stress changes due to fluid injection.


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Induced seismicity from EGS projects: observations and implications tofuture modeling effortsProf. KOHL, Thomas 1

1 GEOWATT Zurich / KIT Karlsruhe


Induced seismicity is often seen as an uncontrollable consequence for the fluid injection in fracturedgeothermal reservoirs. The larger the experience is and the more detailed investigations of these phenomenaare performed, an insight in the physical processes associated to the massive injection and circulation of fluidsin fractured host rock becomes possible. Like tectonic seismicity, the origin of micro-seismicity can be inferredto an accumulation of elastic energy in subsurface. Under failure conditions in brittle rock, the release of theassociated mechanical stresses yields thermal or acoustic (seismic) energy. However, geothermal systems offerthe advantage of controlling the energy source that allows quantifying the associated processes. A thoroughinvestigation is getting quickly challenging since the source energy of tectonic earthquakes can only bebudgeted from indirect observations.

The observation of seismicity at the geothermal site at Soultz-sous-Forêts (France) shows typical mechanicalrelated phenomena. Clearly, the more data are investigated the more evidence appears on the importance ofstructural heterogeneity in the host rock. There are strong indications that even the large seismicity at shut-inseems to be influenced by the physical processes in a complex subsurface. The operation conditions at Soultzprovide the background for the onset of seismicity during several years of monitoring. It is clearly linked todistinct pressure levels that seem to vary at different reservoirs even at the same site. Such key observations inEGS systems, especially shear and tensile failure of rock requires therefore a characterization by advancednumerical modeling. Physics-based models are, in theory, capable to predict the effect of stress changes as afunction of fluid injection and circulation in such an environment. However, these need to integratewell-chosen boundary conditions of the pre-existing natural stress, fault and fracture geometry and rheology,rock matrix characteristics and a proper coupling of geomechanical, fluid flow and heat transport processesrelevant to induced seismicity in geothermal applications.

There are a large number of numerical models which are used to simulate the seismic response of a reservoirto fluid injection and production. Often they rely on very different basic phenomena for the same process. E.g.fluid flow is commonly modeled as either a diffusive process in a 3D rock matrix or as flow confined to anetwork of 2D or even 1D fractures or a mixture of these two end members. Resulting fluid pressure fieldsmay thus be fundamentally different. Typically, most of these models can be tuned to match the seismicresponse observed in stimulation experiments. This demonstrates the huge gap we still have in the knowledgeof hydro-mechanical interaction in fractured geothermal reservoirs and the urgent need for a betterunderstanding of the physical processes, beyond their phenomenological description. To overcome this gap, asuite of observations from EGS has to be identified or needs to be measured in future. This should includegeometrical, statistical and physical data. It is proposed that future modeling efforts will merge existingstatistical and deterministic approaches.

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WP3 - Introduction to seismic analysisDr. ZANG, Arno 1

1 GFZ German Research Centre for Geosiences


WP3 Introduction


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WP5 - OverviewDr. WIEMER, Stefan 1

1 ETH Zurich


WP5 overview

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WP6 - Mitigation strategyProf. VAN WEES, Jan-Diederik 1

1 TNO


WP6 introduction


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