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Fault reactivation and associated microseismicity pose a potential threat to industrial processes involving fluid injection into the subsurface. In this research, fracture criticality, defined as the gradient of critical fluid pressure change to trigger seismicity (Δpc/h), is proposed as a novel reservoir depth-independent metric of fault slip susceptibility. Based on statistics of the fracture criticality, a probabilistic evaluation framework for susceptibility to injection-induced seismicity was developed by integrating seismic observations and hydrogeological modelling of fluid injection operations for faulted reservoirs. The proposed seismic susceptibility evaluation method considers the injection-driven fluid pressure increase, the variability of fracture criticality, and regional fracture density. Utilising this methodology, the probabilistic distribution of fracture criticality was obtained to evaluate the potential for injection-induced seismicity in both fault and off-fault zones at the Hellisheiði geothermal site, Iceland. It has been found that the fracture criticality within both fault and off-fault zones shows natural variability (mostly ranging between 0.001 and 2.0 bar/km), and that fault zones tend to be characterised by larger fracture criticality values than the off-faut zones. Fracture criticality values estimated within each zone roughly follow a Gaussian distribution. Fault zones around five geothermal fluid re-injection wells at the site were estimated to have relatively high probability of seismic event occurrence, and these regions experienced high levels of induced seismicity over the microseismic monitoring period. The seismotectonic state estimated for each zone is generally consistent with the forecasted susceptibility to seismicity based on statistics of fracture criticality. Highlights: A probabilistic seismic susceptibility evaluation framework which integrates seismic observations and hydrogeological modelling is proposed. Fracture criticality is proposed as a novel reservoir depth-independent metric for fracture slip susceptibility. Fracture criticality values estimated at Hellisheiði roughly follow a Gaussian distribution ranging between 0.001 and 2.0 bar/km. Satisfactory probabilistic seismic susceptibility evaluation results were achieved at Hellisheiði. [ABSTRACT FROM AUTHOR]

Understanding injection‐induced microseismicity in geothermal systems can provide insight into reservoir connectedness. However, fault and reservoir complexity are difficult to represent in simple analytical models, which makes it difficult to discern clear relationships from incidental associations. Here, we have used data‐driven models to study how fluid injection and microseismicity are related in the Rotokawa (New Zealand) and Hellisheiði (Iceland) geothermal fields. We tested two classes of model: (a) lagged linear regression of seismicity rate as a function of well injection rates; and (b) systematic extraction of injection time series features that are then evaluated for associations with the seismicity. These models allowed us to determine which wells had the greatest correlation with microseismicity and to explain this association in a reservoir context. Finally, exploring different data types and transformations, we were unable to establish a link between rapid changes in injection rate and seismicity spikes, as suggested by some theoretical models. Plain Language Summary: To better understand the small earthquakes triggered in geothermal fields, we used two kinds of data‐driven models: time series feature engineering (a machine learning method) and the non‐negative linear regression method. We applied these techniques to microearthquakes recorded in the Rotokawa geothermal field, New Zealand, and the Húsmúli reinjection area in the Hellisheiði field, Iceland. We searched for associations and patterns hidden within the noisy injection and seismicity data. These patterns are further complicated by the complex injection schedules that involve several wells that switch off and on over time. We found a stronger association between injection and long‐term microseismicity rate as opposed to short‐term fluctuations driven by noise. We showed that particular injection wells are strongly associated with the microseismicity, which we interpret in terms of reservoir connectedness and stress changes in the rock. We found that injection rate and volume are the most useful data for understanding microseismicity, whereas there was no evidence that rapid injection rate changes triggered seismicity at these two locations. Key Points: Relative associations of geothermal wells with seismicity rate are evaluated using regression analysis and time series feature engineeringReservoir operational changes such as injection transfers between wells can affect the association between wells and microseismicityThere is a stronger correlation between injection and long‐term seismicity rate than short‐term seismicity rate at Rotokawa and Hellisheiði site [ABSTRACT FROM AUTHOR]

The spatial distribution of seismicity in the Hengill region, SW Iceland, is analyzed by relocation and collapsing. The Hengill region is a diffuse triple junction with volcano-tectonic activity associated with rifting, tectonic activity on a transecting transform and induced seismicity due to drilling and injection of fluid into geothermal fields. The Icelandic Meteorological Office has compiled 114,000 events over a 20-year period within an area of approximately 600 km2. The events in their catalog are relocated by application of empirical travel-time tables using a non-linear location strategy. The relocations are then redone applying a Bayesian inversion using the catalog event density as a prior. Finally, they are collapsed using the same catalog density as an attractor. We show that this catalog processing reproduces details of the spatial pattern of seismicity that independently emerges from relative relocations of a small subset of the catalog events (swarm activity). In particular, the predominant faulting orientations are reproduced in different parts of the region and the depth distribution of events resembles that obtained by dense deployments in the area. Its depth extent varies between 5 and 7 km in the northern part of the region, where volcanic processes dominate, and between 7 and 8 km in the southern part, where tectonic deformation is predominant. Induced seismicity is shallower than adjacent natural seismicity. An intriguing lineation emerges in the lateral distribution of inferred depth to the brittle-ductile transition in the northern volcanic part of the region, which is parallel to the strike of the fissure swarms in the area. Associating this transition with an isotherm (650 °C), the Hengill volcanic system and its fissure swarm appear to be considerably cooler than the Hrómundartindur system. This may relate to a recent intrusion into the latter or more efficient cooling in the Hengill fissure swarm due to deeper penetrating permeability. In both cases this has potential consequences for geothermal exploitation in the area. • Spatial distribution of seismicity in Hengill is analyzed by earthquake relocations. • Relocations reproduce details of seismicity pattern that independently emerge from relative relocations. • Depth distribution of relocated events is similar to that based on denser deployments. • Depth to brittle-ductile transition is greater in southern tectonic part of Hengill area than in the northern volcanic part. • The Hengill fissure swarm appears considerably cooler than other volcanic parts of the area. [ABSTRACT FROM AUTHOR]

In autumn 2017 a network of 14 broadband seismic stations was deployed at the Theistareykir high temperature geothermal field (NE Iceland). This experiment was conducted as part of the current efforts to characterize the field's main structures, and possible short and long term stress variations due to the ongoing fluid injection and extraction operations which started in autumn 2017. In this work, we use two years of continuous seismic records (October 2017–October 2019) to compute a 3D shear wave velocity model of the geothermal field and to detect possible crustal stress changes related to the injection and production activities. From phase cross-correlations of the vertical component recordings, we measure the Rayleigh wave group velocity dispersion curves to obtain 2D group velocity maps between 1 and 5 s. Subsequently, we use a neighborhood algorithm to retrieve the 3D shear wave velocity model of Theistareykir. Mainly, two sets of elongated high and low velocity anomalies can be observed oriented in a NW/WNW direction, parallel to the lineaments of the active Tjörnes fracture zone. Velocity reductions west of Ketilfjall and at Baerjafjall could indicate the location of upflow zones of the magmatic reservoir or hydrothermal system. We analyzed the temporal evolution of phase and amplitude of phase auto-correlations using the stretching technique and discuss their behavior in relation to the geothermal field operations. We notice a slightly stronger long-term velocity decrease in the reservoir region compared to outer regions. This could be related to the mass depletion in that area (higher fluid extraction compared to the water reinjection). In summary, our findings show how a monitoring network can be set up to enable a detailed imaging and monitoring of reservoir behavior in general. • The Vs model of the Theistareykir geothermal field was obtained using ambient noise tomography. • Reservoir structures and upflow zones are identified in combination with other collected data. • the temporal evolution of phase auto-correlations can be used to monitor geothermal resources. [ABSTRACT FROM AUTHOR]

• Fluid injection and transient cooling-induced permeability enhancement is proposed as a novel mechanism for induced seismicity. • Seismicity at Hellisheiði is under combined effects of pore pressure, poroelastic stress, thermal stress and permeability enhancement. • Temperature dependence of induced seismicity at Hellisheiði is explained by both permeability enhancement and thermoelastic effect. Both field injectivity and induced seismicity were reported to be inversely correlated with the temperature of re-injected fluids at the Hellisheiði geothermal field in Iceland. This observation has led to a hypothesis that transient cooling-induced permeability enhancement is a novel mechanism for induced seismicity, in addition to elevated fluid pressure, poroelastic stressing, and thermoelastic stressing in geothermal environments. In this work, a 3D calibrated coupled THM model was developed to model the colder fluids re-injection process over a 1-year period and evaluate the potential for induced seismicity in terms of Coulomb stress changes at the Hellisheiði geothermal field. Three modelling scenarios taking into account respectively the poroelastic effect, thermoporoelastic effect, and thermoporoelastic effect with permeability enhancement, were examined and compared to identify the dominant mechanism for the recorded seismicity and examine the contribution from each individual mechanism. Results have shown that, under normal fluid re-injection pressure and temperature conditions, the permeability enhancement effect is the dominant mechanism for induced seismicity at the Hellisheiði geothermal field. Specifically, the contribution to Coulomb stress changes from the permeability enhancement effect is almost twice of that from the thermoelastic stressing, which is in turn two orders of magnitude larger than that from the poroelastic stressing. It has also been noted that, when reducing temperature of re-injected fluids from 120°C to 20°C, the temperature change is increased by 2.1 times at 1,000 m depth, while the amount of mass flow by around 4 times. Thus, the amount of heat transferred can be increased 8.4 times by lowering temperature of the injected fluids, which explains the high sensitivity of induced seismicity to temperature. Outcomes of this work suggest temperature control of injected fluids as a feasible regulation method to mitigate against injection-induced seismic risk in geothermal reservoirs. [ABSTRACT FROM AUTHOR]

• Conceptual model of the subsurface flow processes at the Carbfix Demonstration site. • Field scale model including hydraulic gradients imposed by far-field injection and production wells. • Reproduce the results of field tracer tests at the CarbFix2 project and estimate the fraction of mineralized CO 2 using a non-reactive simulation scheme. The approach described here evaluate the deficiency in measured DIC against the simulated tracer concentrationprovided by the transport model. • Long-term forecast of fluid injection on the maximum size of the storage reservoir and electricity generation potential. The Carbfix 2 project was developed to demonstrate the cost effectiveness and technical maturity of CO 2 and H 2 S direct capture from emission sources and permanent geological storage by in-situ mineralization in basalts. CO 2 and H 2 S emissions of the Hellisheiði Geothermal Power Plant are separated from the other non condensable gases and captured by water dissolution. It is then transported to an injection well where it is co-injected with geothermal brine from the power station. The injection takes place in the Húsmúli re-injection area also known as the Carbfix demonstration site and has been an integral part of the operations of the power station since 2014. Field scale three-dimensional transport models of the injection at Húsmúli are developed to characterize the storage reservoir and constrain the flow paths between the Carbfix re-injection site and the nearby Skarðsmýrarfjall production zone. The model shows a good match to the tracer recovery and enthalpy from the monitoring wells and suggests a strong structural control at Húsmúli associated with faults acting as preferential pathways for the fluid. Implementation of boron, calcium, CO 2 , and H 2 S as solutes in the transport model and comparison between the modelled and the measured concentrations in the samples from the monitoring wells allows to assess the conservative and reactive behavior of each solute. By comparing expected (modelled) and measured CO 2 concentrations we can estimate the fraction of dissolved gas mineralization between the injection well and monitoring wells. The model confirms values recorded in the literature of a mineralization degree in excess of 70% for the well closest to the injection and 95% for the furthest well. This suggests that non-reactive simulation schemes can be used to effectively monitor in-situ mineralization, CO 2 containment and long term storage security of the gelogical reservoir. Modelling the long-term injection of fluids at Húsmúli suggests a drop in electricity generation from the closest well to the injection zone by 2040 and provides a theoretical maximum estimate of the storage capacity of the reservoir of 300 megatons of CO 2 by 2050 if 10% of the space available is filled. [ABSTRACT FROM AUTHOR]

• New Data set of continuous magnetotelluric monitoring over two months and results from seismic monitoring and the related hydraulic data of the RN15/IDDP-2 well in Reykjanes, Iceland. • Changes in the electromagnetic field in the subsurface induced by measures during reservoir engineering or even natural seismicity are observed. • Results show a temporal relation between low resistivity anomalies (considered as induced noise) and the geomagnetic field activity, the fluid losses up to 60 L/s, and mechanic processes occurring in the reservoir prior to clusters of induced seismicity. To date, magnetotelluric monitoring of processes during reservoir engineering of geothermal systems have been carried out only at three sites world-wide. Here, we add a new survey at the Reykjanes peninsular (Iceland). The MT data acquisition at RN-15/IDDP-2 covered the last third of the drilling period and thermo-hydraulic stimulation. Drilling was accompanied by temporal total fluid losses of up to 60 L/s as well as randomly distributed induced seismicity. Our experimental results of a two-months magnetotelluric monitoring during the deepening of the RN-15/IDDP-2 well on the Reykjanes peninsular (Iceland) to 4'659 m are in line with earlier observations on decreasing resistivities at periods of a few s to about 20 s (or up to about 40 s) in conjunction with fluid injection. Simple models indicate however that it is not the fluid volume itself that causes the anomaly. Moreover, here, temporal decreases in the electric resistivities occur at 0.2-20 s with minima at periods of about 0.4 s 1-2 days ahead of clusters of seismic events with magnitudes up to M L < 2. Unlike in the previous studies, where resistivity decreases occur on the component that is parallel to Shmin, here they in both components (XY and YX) of the MT data. At the Reykjanes peninsular, Shmin is oriented N120°E off the XY component (N0°E). Therefore, if we rotate our data in Shmin direction, the difference between both components would be slightly smaller than that for the non-rotated ones. The decrease in resistivity (on both components) extends over > 1 order of magnitudes over a short period range and is thus considered noise induced in the subsurface. This study aims at adding an essential dataset to the general discussion on MT monitoring of reservoir processes. The results show a temporal relation between decreasing apparent resistivity and (i) the geomagnetic field activity, (ii) the fluid losses up to 60 L/s, as well as (iii) mechanic processes occurring in the reservoir before clusters of induced seismicity on the other hand. [ABSTRACT FROM AUTHOR]

• Soft stimulation treatment procedures were successfully applied at field scale in a densely populated area. • Hydraulic performance of well RV-43 was increased by hydraulic stimulation. • Moment magnitudes of induced seismic events were below 0. Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city significantly increased the heat demand. Past experiences in Iceland's capital region showed that hydraulic stimulation of existing geothermal wells is suited to improve hydraulic performance and energy supply. However, fluid injection may also trigger felt or even damaging earthquakes, which are of concern in populated areas and pose a significant risk to stimulation operations. Consequently, soft stimulation concepts have been developed to increase geothermal well performance while minimizing environmental effects such as induced seismicity. In a demonstration project of hydraulic soft stimulation in October 2019, more than 20.000 m³ of water were injected into well RV-43 in Reykjavik in multiple stages and with different injection schemes. The hydraulic performance of the well was improved without inducing felt seismicity. An a priori seismic risk assessment was conducted and for the first time the risk was continuously updated by an adaptive traffic light system supported by a sophisticated realtime microseismic monitoring. Our results confirm that it is possible to improve the performance of geothermal wells in Reykjavik and worldwide with acceptable technical, economic, and environmental risks. Here we provide an overview of the entire stimulation project including site description, stimulation design, zonal isolation, logging, seismic risk assessment and mitigation measures, realtime seismic, hydraulic and chemical monitoring, and stimulation results and challenges. [ABSTRACT FROM AUTHOR]

Induced seismicity is often associated with fluid injection but only rarely linked to surface deformation. At the Hellisheidi geothermal power plant in south-west Iceland we observe up to 2 cm of surface displacements during 2011–2012, indicating expansion of the crust. The displacements occurred at the same time as a strong increase in seismicity was detected and coincide with the initial phase of geothermal wastewater reinjection at Hellisheidi. Reinjection started on September 1, 2011 with a flow rate of around 500 kg/s. Micro-seismicity increased immediately in the area north of the injection sites, with the largest seismic events in the sequence being two M4 earthquakes on October 15, 2011. Semi-continuous GPS sites installed on October 15 and 17, and on November 2, 2011 reveal a transient signal which indicates that most of the deformation occurred in the first months after the start of the injection. The surface deformation is evident in ascending TerraSAR-X data covering June 2011 to May 2012 as well. We use an inverse modeling approach and simulate both the InSAR and GPS data to find the most plausible cause of the deformation signal, investigating how surface deformation, seismicity and fluid injection may be connected to each other. We argue that fluid injection caused an increase in pore pressure which resulted in increased seismicity and fault slip. Both pore pressure increase and fault slip contribute to the surface deformation. • Measurements of surface deformation can contribute to the understanding of episodes of induced seismicity. • Initial pressure increase at the Húsmúli site due to fluid injection caused both deformation and seismicity. • Deformation indicates northward flow from the injection site, whereas the intended flow direction was northeastward. [ABSTRACT FROM AUTHOR]