Search

Induced seismicity observed during Enhanced Geothermal Stimulation at Otaniemi, Finland is modeled using both statistical and physical approaches. The physical model produces simulations closest to the observations when assuming rate‐and‐state friction for shear failure with diffusivity matching the pressure build‐up at the well‐head at onset of injections. Rate‐and‐state friction implies a time‐dependent earthquake nucleation process which is found to be essential in reproducing the spatial pattern of seismicity. This implies that permeability inferred from the expansion of the seismicity triggering front (Shapiro et al., 1997, https://doi.org/10.1111/j.1365-246x.1997.tb01215.x) can be biased. We suggest a heuristic method to account for this bias that is independent of the earthquake magnitude detection threshold. Our modeling suggests that the Omori law decay during injection shut‐ins results mainly from stress relaxation by pore pressure diffusion. During successive stimulations, seismicity should only be induced where the previous maximum of Coulomb stress changes is exceeded. This effect, commonly referred to as the Kaiser effect, is not clearly visible in the data from Otaniemi. The different injection locations at the various stimulation stages may have resulted in sufficiently different effective stress distributions that the effect was muted. We describe a statistical model whereby seismicity rate is estimated from convolution of the injection history with a kernel which approximates earthquake triggering by fluid diffusion. The statistical method has superior computational efficiency to the physical model and fits the observations as well as the physical model. This approach is applicable provided the Kaiser effect is not strong, as was the case in Otaniemi. Plain Language Summary: Around 60,000 earthquakes are recorded during a span of 50 days where large volumes of water were injected underground for the stimulation of a geothermal well at Otaniemi, near Helsinki, Finland. We compare the observations with numerical simulations to analyze the physical processes that have driven these earthquakes. A model based on physics finds that it is important to use a friction law that includes friction's dependence on slip‐rate and state variables to match the observations. In particular, the model allows relating the spatio‐temporal evolution of seismicity with fluid pressure diffusion in the sub‐surface. An empirical statistical model is also developed using the recorded catalog. The statistical model is shown to perform well in the particular case of the Otaniemi stimulations. The models provide insight into the physical processes that govern induced seismicity. The models presented in this study could help safer operations or the design of mitigation and optimization strategies that may help improve the efficiency of geothermal energy extraction. Key Points: We model induced seismicity from a geothermal well stimulation operation near Helsinki, Finland, using physical and statistical approachesHydraulic diffusivity may be misestimated by the triggering front without accounting for nucleation effects from rate‐and‐state frictionNucleation effects are expected to be significant at short‐time scale injections commonly employed in geothermal well stimulation operations [ABSTRACT FROM AUTHOR]

We investigate induced seismicity associated with a hydraulic stimulation campaign performed in 2020 in the 5.8 km deep geothermal OTN‐2 well near Helsinki, Finland as part of the St1 Deep Heat project. A total of 2,875 m3 of fresh water was injected during 16 days at well‐head pressures <70 MPa and with flow rates between 400 and 1,000 L/min. The seismicity was monitored using a high‐resolution seismic network composed of 10 borehole geophones surrounding the project site and a borehole array of 10 geophones located in adjacent OTN‐3 well. A total of 6,121 induced earthquakes with local magnitudes MLHel>−1.9 ${M}_{\mathrm{L}}^{\mathrm{H}\mathrm{e}\mathrm{l}} > -1.9$ were recorded during and after the stimulation campaign. The analyzed statistical parameters include magnitude‐frequency b‐value, interevent time and interevent time ratio, as well as magnitude correlations. We find that the b‐value remained stationary for the entire injection period suggesting limited stress build‐up or limited fracture network coalescence in the reservoir. The seismicity during the stimulation neither shows signatures of magnitude correlations, nor temporal clustering or anticlustering beyond those arising from varying injection rates. The interevent time statistics are characterized by a Poissonian time‐varying distribution. The calculated parameters indicate no earthquake interaction. Focal mechanisms suggest that the injection activated a spatially distributed network of similarly oriented fractures. The seismicity displays stable behavior with no signatures pointing toward a runaway event. The cumulative seismic moment is proportional to the cumulative hydraulic energy and the maximum magnitude is controlled by injection rate. The performed study provides a base for implementation of time‐dependent probabilistic seismic hazard assessment for the project site. Plain Language Summary: We investigate anthropogenic seismicity associated with fluid injection into the 5.8 km deep geothermal OTN‐2 well near Helsinki, Finland, as a part of St1 Deep Heat Project. A total of 2,875 m3 of fresh water was injected during 16 days at well‐head pressures <70 MPa and with flow rates between 400 and 1,000 L/min. The seismicity was monitored using a seismic network composed of 20 borehole geophones located in Helsinki area and in the OTN‐3 well located close by the injection site. A total of 6,121 earthquakes indicating fractures of 1–30 m size were recorded during and after stimulation campaign. Using a handful of statistical properties derived from earthquake catalog we found no indication for earthquakes being triggered by other earthquakes. Instead, the earthquake activity rates, as well as the maximum earthquake size stayed proportional to the fluid injection rate. The spatio‐temporal behavior of seismicity and its properties suggest earthquakes occurred not on a single fault, but in a distributed network of similarly oriented fractures, limiting the possibility for occurrence of violent earthquakes. The performed study provides evidence that the induced seismicity due to injection performed within St1 Deep Heat project is stable and allow to constrain seismic hazard. Key Points: Induced seismicity associated with stimulation campaign in a 5.8 km deep geothermal OTN‐2 well passively responds to injection operationsSeismicity is a non‐stationary Poisson process with seismicity rate and maximum magnitude modulated by the hydraulic energy input rateSeismicity clusters in space and time in response to fluid injection but no interaction between earthquakes is observed [ABSTRACT FROM AUTHOR]

We describe the design and implementation of a traffic light system (TLS) created for the stimulation phase of a 6.1-km-deep geothermal well in the Helsinki area, Finland. Because of the lack of local seismic data to calibrate the TLS, the TLS thresholds have been proposed on the basis of two parameters: acceptable ground motion levels and probabilities to reach these levels, which were then used to establish TLS magnitudes. A peak ground velocity (PGV) of 1 mm/s associated with a ML ≥ 1 event, or a ML ≥ 1.2 event alone, triggered an Amber alert. A PGV of 7.5 mm/s associated with a ML ≥ 2.1 event triggered a Red alert, where ML was a local "Helsinki" magnitude. Specific thresholds based on PGV and peak ground acceleration were gathered for critical infrastructure sites and related to earthquake magnitudes in a probabilistic way. The implementation of the TLS during the stimulation showed that the selected thresholds were reasonably conservative. Some of the Amber events were reportedly felt or heard, but the public opinion remained very favourable to the project. Measured ground motions were too low to have any impact on the built environment and the stimulation was able to proceed without being impaired by the occurrence of a Red event. Seismic data acquired during the stimulation and lessons learnt are presented and used to revisit the TLS thresholds for potential future stimulations. [ABSTRACT FROM AUTHOR]

We show that near-realtime seismic monitoring of fluid injection allowed control of inducedearthquakes during the stimulation of a geothermal well near Helsinki, Finland. The injectionwell, OTN3, was drilled down to 6.1 km-depth into Precambrian crystalline rocks. WellOTN3 was deviated 45∘ from vertical and an open hole section at the bottom was divided intoseveral injection intervals. A total of 18,159 m3 of fresh water was pumped into crystallinerocks during 49 days in June- and July, 2018. The stimulation was monitored in near-realtime using (1) a 12-level seismometer array at 2.20-2.65 km depth in an observation welllocated 10 m from OTN3 and (2) a 12-station network installed in 0.3-1.15 km deepboreholes surrounding the project site. Earthquakes were processed within a fewminutes and results informed a Traffic Light System (TLS). Using near-realtimeinformation on induced-earthquake rates, lo-cations, magnitudes, and evolutionof seismic and hydraulic energy, pumping was either stopped or varied betweenwellhead-pressures of 60-90 MPa and flow rates of 400-800 l/min. This procedure avoidedthe nucleation of a project-stopping red alert at magnitude M2.1 induced earthquake, alimit set by the TLS and local authorities. The stimulation resulted in detection of&#x003E;43,000 earthquakes with -1.2&#x003C;ML&#x003C;1.9. The original catalog was relocated usingdouble-difference technique to improve hypocenter precision. The 4032 relocatedearthquakes were used to investigate the spatio-temporal evolution of seismicity, seismicenergy release, and maximum magnitude in response to injection. We found hypocenterdistribution, Gutenberg-Richter (GR) distribution and relation between hydraulic andradiated energy suggest (re-)activation of size limited network of distributed fractures.The temporal behavior of G-R b-value, as well as a lack of temporal (Omori-type)correlations in a presence of spatial localization of earthquakes suggest very limitedearthquake triggering and stress transfer at low level of ambient stress. The maximumobserved magnitudes scale with stored elastic (=hydraulic) energy, following afracture-mechanics based model of Galis et al. (2017). Our results suggest a possiblephysics-based approach to controlling stimu-lation induced seismicity in geothermalprojects. [ABSTRACT FROM AUTHOR]