In the context of deep reservoir exploitation, it is necessary to enhance reservoir permeability before exploitation. One method to achieve this is by conducting stimulations through fluid injection, which increases pore pressure, reduces the effective normal stress and allows dilatant shear and porosity increase along small fractures in the reservoir, in the vicinity of the injection well. The pore pressure diffuses throughout the reservoir and can also sometimes reach distant faults that are critically stressed, with a risk to trigger seismicity along these. The model we propose aims to decrease the effective normal stress reduction caused by pressure disturbance along such distant faults, which can cause the rupture of critically stressed distant faults and induce seismic activity. This work investigates an alternative pumping method to stimulate a reservoir without triggering distant faults. To achieve this, a numerical model based on the finite difference method has been developed to solve the diffusion equation of pressure disturbances. The simplifying assumption is that the domain is isotropic and homogeneous. The 2D domain represents the fault plane and permeable damaged zone embedded in less permeable rock. To validate the numerical model, the numerical distant pressure disturbances are compared to analytical solutions developed from the Green's function of the diffusion equation. The numerical model investigates the impact of a time-dependent oscillating injection strategy on near-well and distant pressure disturbances, in comparison to other tested methods, to minimize induced seismicity. The results suggest that the oscillating pumping strategy has the potential to significantly reduce induced seismicity on distant faults. Future research will involve developing mitigation strategies using more complex models that incorporate realistic fault geometries and operational conditions.

How to cite: Vallier, B., Toussaint, R., Fahs, M., Baujard, C., Genter, A., Flekkøy, E. G., and Måløy, K. J.: What are the alternative pumping strategies to stimulate areservoir without triggering distant faults?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6443, https://doi.org/10.5194/egusphere-egu24-6443, 2024.

To date, only several small-scale CO₂ storage projects, injecting about 1 Mt of CO₂ per year, exist worldwide. Induced seismicity has been recorded during operation. Overall, magnitudes of the seismic events are small (below M=3). Nevertheless, there is the concern that future large-scale projects will induce significantly larger magnitudes, like it is observed for basin-scale waste-water disposal. For instance, large-scale water disposal in Oklahoma and Kansas (USA) induced thousands of widely felt M3+ earthquakes with a maximum magnitude of M=5.8. We analyze seismicity and injection data from CO₂-storage projects including Quest (CA), Decataur (USA), In-Sahla (Algeria), Otway (AUS) and Gorgon (AUS). We compute the normalized seismic response of the subsurface to injection of a unit volume of CO₂ using the Seismogenic Index (SI) and compare it to water-disposal case studies. We find that the SI at CO₂-storage sites is smaller compared to water-disposal cases. It indicates a lower seismic hazard per injected volume of CO₂. We discuss physical processes that could explain our observations and show how earthquake magnitude probabilities can potentially be upscaled, considering CO₂ storage volumes needed in the future.

How to cite: Langenbruch, C. and Shapiro, S.: Comparing water-disposal and CO2-storage induced earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20416, https://doi.org/10.5194/egusphere-egu24-20416, 2024.

High-pressure fluid injection into subsurface is often carried out to enhance the permeability of deep geothermal reservoirs. The operation sometimes triggers induced earthquakes that may be as large as natural earthquakes. Novel injection protocols such as cyclic injection schemes have been proposed to mitigate the risk of inducing larger events. Currently, the of impact cyclic injection schemes on the maximum magnitude M_max is not fully understood. Here, the working hypothesis  is that the pore-pressure diffusion is the dominant triggering mechanism of induced events and the maximum induced earthquake scales with the pressure-perturbed fault size. We developed a first-order hydrogeological model and simulated the fluid injection into a porous rock with an embedded large-scale fault zone. Different injection scenarios were implemented, and the pressure-perturbed fault size was computed and translated to the maximum induced earthquake magnitude. The numerical models showed that the duration of the injection protocol plays an important role and likely controls the occurrence of larger-magnitude events. Our numerical models can provide significant insight into the effectiveness of mitigation strategies during the engineering of Enhanced Geothermal Systems and underground storage reservoirs.

How to cite: Moein, M. J., Langenbruch, C., and Shapiro, S.: The duration of injection protocol likely controls the maximum magnitude of induced earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18907, https://doi.org/10.5194/egusphere-egu24-18907, 2024.

The conventional understanding of tectonic faults primarily categorizes them based on frictional behavior: stable due to velocity-strengthening (VS) behavior, or unstable owing to velocity-weakening (VW) that lead to seismic ruptures. This classification has traditionally led to the assumption that VS faults are unlikely candidates for earthquake nucleation. However, emerging evidence from recent laboratory experiments and field studies is challenging this simplistic view, pointing towards a more complex mechanism. In this study, we utilize a hydro-mechanically coupled fault model, which integrates VS friction governed by rate-and-state friction laws with dynamic weakening influenced by poroelastic effects. A key aspect of our findings is the impact of fluid injection on the mechanical state of the fault. This process decreases the effective normal stress and frictional resistance, initially paving the way for the propagation of an aseismic, slow-slip event. The transition from aseismic to seismic slip on VS faults hinges on the balance between shear-induced dilation and compaction. These opposing mechanisms respectively lead to a decrease and an increase in pore-fluid pressure, dictating the balance between fault stability or instability. Our results show that when the effect of compaction-induced pressurization surpasses the initial dilatancy phase, it enables the propagation of dynamic rupture as a solitary pore-pressure wave. Conversely, when dilation predominates over compaction, an aseismic slow-slip event propagates through the fault, maintaining stability and preventing rapid seismic activity. These findings advance our understanding of seismic risk associated with VS faults. They are especially relevant in the context of fluid injection practices in geothermal energy production and CO2 storage, demonstrating how such activities might activate faults that are considered nominally stable. Additionally, our results underscore the critical need for more experimental and theoretical investigations into shear-induced compaction as an efficient mechanism for fault self-pressurization, which plays a key role in leading to seismic instabilities.

How to cite: Dal Zilio, L., Selvadurai, P., Gerya, T., Ampuero, J.-P., Tinti, E., Cocco, M., Cappa, F., Wiemer, S., Giardini, D., and FEAR team, T.: Fluid-induced earthquake nucleation controlled by shear-induced compaction and dilation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11558, https://doi.org/10.5194/egusphere-egu24-11558, 2024.

The exploration of geo-resources in complex geological provinces poses significant challenges, often resulting in severe geological disasters with economic losses and fatalities. Fractured rocks, pre-existing fractures, and excavation-induced fractures contribute to the complexity of these disasters. Despite numerous studies, understanding the coupling mechanism of induced seismicity and geological deformations in such complex mining environments remains unclear. This study introduces the "Acousto-Frac Model," a novel, cost-effective, and robust geophysical approach designed to comprehensively understand experimental microseismicity and reveal such a mechanism. Traditionally, physical models and numerical simulations have been employed for dynamic disaster prediction in underground coalmines. However, these methods are often neither cost-effective nor robust. The Acousto-Frac Model offers an innovative methodology for mapping induced fracture networks through Acoustic Emission (AE) experiments conducted on coal and rock samples. This approach tracks each AE event, constructs networks of induced fractures, identifies geological lineaments, and predicts zones prone to failure/disasters.

To implement the model, AE and rock mechanics testing systems were utilized to conduct experiments on coal and rock samples under uniaxial loading. The proposed model successfully identified weak zones, predicting general deformation propagation directions. Moreover, the 3D crack growth theory and the criterion for microcrack density were employed to analyze the fracture transformation process, ranging from small-scale microfractures to large-scale microfractures and from local deformation to complete damage for the coal and rock samples subjected to uniaxial loading.

The study further leverages Single Link Cluster (SLC) simulations and b-value theory to characterize the spatiotemporal response of microearthquakes, including b-value, spatial correlation length (ξ), and information entropy (H). Notably, the results indicated that at the onset of initial loading (15%), the spatial correlation length (ξ) exhibited an upward trend, while the b-value remained comparatively stable. These parameters showed a significant change trend before the buckling failure of coal and rock samples, suggesting that, in combination with the proposed model, spatial correlation length (ξ), b-value, and information entropy (H) provide a new and robust method for complete deformation evaluation and the prediction of geo-material failure. This innovative method, while a panacea for imaging the entire fracturing phenomenon, provides insights with widespread implications for academic researchers and industry practitioners. It serves as a valuable tool for predicting geological failures in global underground engineering excavations, offering a comprehensive and cost-effective solution for the mapping of induced seismicity and geological deformations in underground mines.

How to cite: Khan, M., He, X., Song, D., Li, Z., Tian, X., and Khan, U.: Presenting a new holistic robust approach for predicting geo-failures in complex underground engineering projects, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8044, https://doi.org/10.5194/egusphere-egu24-8044, 2024.

The burgeoning development of hydraulic fracturing (HF) for unconventional resource extraction has been paralleled by a rise in injection-induced earthquakes (IIEs), posing significant seismic hazards. A critical challenge in mitigating these hazards is the accurate assessment of the seismogenic potential and earthquake productivity of individual HF pads. We addresses this challenge by analyzing over 35,000 earthquakes in the Southern Montney Play (SMP), Western Canada, from 2014 to 2022, and associating them with 357 HF pads.

We employed the eXtreme Gradient Boosting (XGBoost) machine-learning algorithm, integrating fifteen geological and operational factors to evaluate their influence on IIE occurrence and intensity. We also utilized Shapley Additive Explanations (SHAP) values for a nuanced interpretation of the model outputs, providing insights into the relative importance and interaction of these factors.

Our analysis reveals that the cumulative injected volume and the location of HF pads within the Fort St. John Graben (FSJG) are the primary determinants of seimogenic potential (occurrence of IIE). In contrast, the number of HF stages targeting the Lower Middle Montney formation, cumulative volume from preceding injections, and the HF pad's location within the FSJG predominantly influence the seismogenic productivity (number of IIE). These findings suggest that both operational and geological factors are critical in determining the seismogenic productivity of HF pads. The XGBoost model demonstrated high predictive accuracy (R2 ~0.90), although its performance is constrained by the dataset's size and potential overfitting issues.

The study challenges the conventional understanding that proximity to known faults is a major factor in IIE occurrence, instead highlighting the significance of cumulative injection volumes and specific geological settings. The analysis also underscores the complex interplay between various factors, such as the correlation between the location fo the HF pads and the targed formation during HF stimulations, which may influence seismogenic patterns.

Overall, our result provides a comprehensive assessment of the factors influencing seismogenic behavior in HF-related IIEs, paving the way for more accurate forecasting of IIE activity levels for individual HF pads in the SMP. The findings have significant implications for seismic hazard assessment and risk mitigation strategies in regions undergoing HF operations. The application of machine learning in this context not only enhances our understanding of induced seismicity but also demonstrates the potential of such techniques in addressing complex geoscientific challenges.

How to cite: Wang, B., Kao, H., Yu, H., Li, G., M.H. Dokht, R., and Visser, R.: Deciphering Seismogenic Patterns in Hydraulic Fracturing: A Machine Learning Approach in the Southern Montney Play, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14139, https://doi.org/10.5194/egusphere-egu24-14139, 2024.

A thorough understanding of geological and hydraulic fracturing aspects can offer significant insights into the physical mechanisms governing earthquakes. In this study, we conducted seismic monitoring near a hydraulic fracturing well-pad in the southern Sichuan Basin. The monitoring adopts a dense seismic array that consists of 60 three-component stations, and last for a duration of 53 days. Accordingly, we resolved and located over 1,000 events (-1.43<M<2, Mc=-0.72). Most events (~ 70%) distributed near the southwest direction of the injection wells, delineating a series of NE-SW trending structures. Our high-resolution hypocenter locations and statistical analysis reveal two distinctive clusters: (i) one linearly-distributed cluster characterized by larger magnitudes, deeper focal depths and a b value (1.09) comparable to tectonic earthquakes; (ii) one relatively scattered cluster with smaller magnitudes, shallower depths and a higher b value (1.29). We speculate that deeper events are more consistent with seismicity occurring on pre-existing fault(s), whereas shallower events occur within a fracture network.

The detailed structures are further evaluated with resolved focal mechanisms and 3D seismic reflection imaging. The deeper events unanimously support a right-lateral, steep strike-slip fault, consistent with the fault geometry depicted by high-resolution hypocenter locations. In comparison, focal mechanisms of the shallower earthquakes are more complex and diverse, showing a mixture of normal fault and strike-slip events. In the vicinity of the two clusters, seismic reflection data indicates a ~3 km-length fault that strikes in approximately north-south (NS) orientation. Therefore, we suggest that the damage zone along the NS fault enhanced the connectivity and provided additional hydraulic channel for fluid migration during shale gas extraction. Overall, the distinct characteristics of the two earthquake clusters could be well-explained by their spatial proximity to the fault zone: shallower earthquakes occur on dense fractures near the main fault, whereas the deeper cluster occur on a distant small-scale fault. This study sheds light on the complex relationship between hydraulic fracturing, geological factors, and earthquake occurrence, and may assist strategy development toward risk mitigation of HF-induced seismicity.

How to cite: Zhang, F., Wang, R., Chen, Y., and Yu, H.: Pre-existing Fault Regulates the Distribution and Behavior of Hydraulic Fracturing-Induced Seismicity in southern Sichuan, China , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8812, https://doi.org/10.5194/egusphere-egu24-8812, 2024.

The Hverahlíð geothermal field, located in the fissure swarm of Hengill volcano, SW Iceland, has been producing steam and geothermal fluids for the Hellisheiði geothermal power plant (303 MW_e, 210 MW_th), since late 2016.  A total of 7 geothermal wells in the field (HE-21, -26, -53, -54, -60, -61 and -66) have been producing up to 150 kg/s of steam and 60-70 kg/s of separated liquid.  The combined extraction from the wells is limited by the size of the steam pipeline connecting the field to the power plant.  

Within months of the initiation of production an increase in seismicity was noted within the field.  This is in contrast to the nearby Hellisheiði geothermal field, ~2 km away, which has experienced very little induced seismicity since commission in 2006, despite more than twice as high mass extraction rates and higher deformation rates.  Since early 2018 a total of 8 events with M> 2.5 have occurred in the field.  The latest of these events occurred in November 2022 with M 3.2. The seismicity largely does not line up on faults and is relatively evenly distributed throughout what is considered the top of the geothermal reservoir (Kristjánsdóttir et al 2019).

Currently a second pipeline connecting the geothermal field with the power plant is in construction and with the commission, planned for fall 2024, an increase in mass extraction rates from the Hverahlíð geothermal field  of ~30% is expected.

In this presentation we compare the observed location, seismicity rates and M_max values to those expected from different models of earthquake triggering.  We furthermore predict the expected increase in seismicity rates due to the increase in production rates and the increase in seismicity that will be felt by the neighboring community of Hveragerði.

How to cite: Hjörleifsdóttir, V., Jónsdóttir, K., Geirsson, H., Benediktsdóttir, Á., and Kristjánsdóttir, S.: Predicting observed induced seismicity due to production in the Hverahlid, SW Iceland, geothermal field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3259, https://doi.org/10.5194/egusphere-egu24-3259, 2024.

The Hengill geothermal field, located in southwest Iceland, is host to the Hellisheiði power plant, with its 40+ production wells and 17 reinjection wells. Located on a tectonically active area, the field experiences both natural and induced seismicity associated to the power plant operations. To better manage the risk posed by this seismicity, the development of robust and informative forecasting models is paramount.

In this study, we compare the forecasting performance of a model developed for fluid-induced seismicity (the Seismogenic Index model) and a class of well-established statistical models (Epidemic-Type Aftershock Sequence). The pseudo-prospective experiment is set up with 14 months of initial calibration and daily forecasts for a year. In the timeframe of this experiment, a dense broadband network was in place in Hengill, allowing us to rely on a high quality relocated seismic catalogue. The seismicity in the geothermal field is characterised by four main clusters, associated with the two reinjection areas, one production area an area with surface geothermal manifestations but where no operations are taking place. We show that the models are generally well suited to forecast induced seismicity, despite some limitations, and that a hybrid ETAS  model accounting for fluid forcing has some potential in complex regions with natural and fluid-induced seismicity.

How to cite: Ritz, V., Mizrahi, L., Clasen Repollés, V., Hjörleifsdóttir, V., Rinaldi, A. P., and Wiemer, S.: Pseudo-prospective forecasting of induced and natural seismicity in the Hengill geothermal field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8026, https://doi.org/10.5194/egusphere-egu24-8026, 2024.

In April 2022, a three-stage hydraulic stimulation was performed in a deep granite heat reservoir of low permeability at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE). During the stimulation, around 1600 m³ pressurized fluids were injected into the target reservoir of ~2.4 km depth aiming at creating fracture networks and improving reservoir permeability for heat extraction. Microseismic monitoring is required to assess the stimulation efficiency and manage the induced earthquake risk during the stimulation. We perform near-real-time microseismic monitoring in a playback mode at the third stage of the stimulation where three deep monitoring boreholes equipped with three-component geophone chains were in operation. We apply machine learning (ML) techniques in combination with waveform back projection approaches to automate the microseismic event detection, increase microseismic event location accuracy, and promote the real-time capability of the monitoring workflow. Due to a lack of labeled datasets for model training or transfer learning, we devise a rescaling technique to tune the continuous microseismic recordings of high sampling rates that enables the application of existing ML models pre-trained on tectonic earthquakes. Our benchmark tests show that the proposed rescaling approach achieves high precision and accuracy in detecting microseismic events and picking their phase arrivals.

With the proposed workflow, we compiled a high-resolution microseismic catalog containing around 36, 000 microseismic events with magnitudes of –3.0 to 0.5. Detected events are relocated using a double-difference relocation method and waveform cross-correlation-based arrivaltime refinement. We cluster the detected microseismic events according to their spatial distributions and identify the dominant stimulated fracture planes with principle component analysis of the different event clusters. The spatial distribution of the detected events nicely depicts the stimulated fracture networks which can be used to design the trajectory of the future production well. We analyze the spatio-temporal evolution of the induced microseismic events during and after the stimulation to illuminate the rupturing mechanisms responsible for the induced fracture networks. Induced microseismic events are analyzed together with the injection data to quantify the induced earthquake hazard and the hydraulic stimulation efficiency. The proposed microseismic monitoring workflow and the corresponding analysis provide more insights into the fracturing dynamics and the potential induced earthquake hazard in the Utah FORGE geothermal site, and would benefit the operation of other enhanced geothermal systems.

How to cite: Shi, P., Schultz, R., Lanza, F., Scarabello, L., Ermert, L., and Wiemer, S.: Machine learning based real-time microseismic monitoring and stimulated fracture characterization at the Utah FORGE Geothermal site, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15621, https://doi.org/10.5194/egusphere-egu24-15621, 2024.

The risk of induced and triggered seismicity is often present in deep geothermal resources exploitation. These facilities allow exploiting practical and green energy resources.
There are many regions with high geothermal capacity, such as Alsace, France. In Vendenheim, north of Strasbourg, the Geoven plant project was expected to extract geothermal energy from the Robertsau fault by circulating fluid at depth.
Two clusters of humanly perceivable seismicity occurred in 2019-2020, one of them close to wells and other one at the Robertsau area at 5km to the south. A question was raised about a possible link between these seismic events and wells activities of Geoven site. A large distance with no earthquakes between the injection wells and the southern cluster was reason for disagreement between scientific experts and the company in charge about the link between the injection and the seismicity on this cluster.
Our objective is studying such a possible connection with numerical modeling through a simple methodology based on fluid/solid deformation and mechanical coupling. In addition, we aim at modelling the pressure perturbation during the time resulting from the history of the injection flux and comparing it with measured data.
The methodology has 3 steps: 1. Structural plan: extracting the geometry of the fault and tectonic stresses  2. Mechanical stability: the stresses on the fault are evaluated and the risk of earthquake triggering is analyzed based on Mohr-Coulomb  criterion. 3. Pore Pressure: a quasi 2D pressure diffusion equation with the proper injection parameters is solved for modeling pressure perturbation in the area due to water injection/extraction.
According to the results, the fault is strong enough in the northern cluster area and slip can happen only by high activation pressure. However, slip and micro-earthquakes resulted from large pressure increase near the wells, which is necessary for permeability increase to improve the transmissivity of  the reservoir. On the other hand, the fault is in the weakest state around the southern cluster, because the pressure required for sliding drops sharply. Indeed, with low amount of pressure increase, slip occurs. Our simulation shows that, the triggered earthquake is expected at this point, due to large enough pressure increase. But between these two clusters, not only is the fault resistant based on its orientation, but also the pore pressure increase is notlarge enough for slip. This explains, the distance of 5km between the two clusters, and absence of earthquakes in between.
Also, we simulated the pressure perturbation in the wells resulting from real injection regime data, during 85 days of operation in Geoven site in 2020, and compare it with real pressure change.
In conclusion, the lowest activation pressure in comparison to other parts can be observed around the southern cluster, which is coherent with the fault and stress tensor geometry implying a weak state in this location. Also, we identify a physical mechanism showing that earthquakes in that zone were possibly triggered by pore pressure perturbation resulting from the Geoven operation.

How to cite: Dodangeh, A., Toussaint, R., Fahs, M., Flekkøy, E., and Måløy, K. J.: Modelling of Strasbourg 2019-2020 seismicity crisis induced by geothermal operations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10650, https://doi.org/10.5194/egusphere-egu24-10650, 2024.

Enhanced Geothermal Systems (EGS) are effective means of developing hot dry rock (HDR) type geothermal resources, transforming low porosity and permeability rock masses deep underground into artificial geothermal reservoirs with high permeability through reservoir stimulation. EGS systems can economically extract a considerable amount of thermal energy over the medium to long term to be utilized for power generation. The development and research of EGS has been ongoing internationally for over 40 years. However, at present, the development of EGS is still in the stage of on-site experimental research and development, and its commercial development still faces many challenges. China, like many other countries, is in need of developing deep geothermal resources to meet its energy demands. In 2017, a well named GR1 with a temperature of 236°C was drilled to a depth of 3705m in the Gonghe Basin of Qinghai Province. Recognizing the potential of HDR resources, the China Geological Survey launched an exploration and production project in Gonghe Basin in 2019. Following several critical technological breakthroughs, the first power generation test of HDR was successfully conducted in the Gonghe Basin in 2022. This study offers a detailed introduction to the localization of microseismic events that occurred during thermal reservoir stimulation at different stages of development. By analyzing these microseismic events, we can evaluate the effectiveness and volume scale of artificial thermal reservoir transformation. In addition, we assess the development of natural fractures utilizing data from 3D seismic attributes of the granite, imaging logging, etc. We conclude by discussing implications for successful geothermal development of the specific geological conditions present at the Gonghe HDR field, based on the localization results of microseismic data and the distribution of natural fractures in the field.

How to cite: Zhang, H. and the Hao Zhang, Institute of Geomechanics, Chinese Academy of Geological Sciences: Microseismic monitoring and insights of rupture mechanism from China's pilot EGS project in Gonghe, Northwestern China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15394, https://doi.org/10.5194/egusphere-egu24-15394, 2024.

Managing induced seismicity risk is an absolute must to enable the widespread use of deep geothermal technologies, and thus contribute to the transformation towards a sustainable and low-carbon energy sector. A full-scale application of real-time monitoring and forecasting of induced seismicity was tested in April 2022, during a three-stage hydraulic stimulation in a deep granite heat reservoir of low permeability at the Utah FORGE EGS site. During the stimulation, a total of ~1600 m³ pressurized fluids were injected into the target reservoir of ~2.4 km depth to generate fracture networks and improve reservoir permeability for heat extraction. Stage 3 had the most complete monitoring network and thus was used to test the components of an Adaptive Traffic Light System (ATLS). The test produced very positive results, although the data stream was lost early into the stimulation.

Here, we further characterize and perform a retrospective forecasting of post-processed data related to the induced seismicity recorded during stage 3 of the 2022 stimulation at FORGE site. We first investigate the geometrical distribution of the seismicity and discuss it in the context of the stress field at FORGE injection site. The statistical inference indicates that the distribution of the seismicity lies on a plane sub-parallel to the S_v - S_Hmax and orthogonal to S_Hmin, and with strike orientation rotated 10^o counterclockwise with respect to the N25^oE average orientation of S_Hmax. The analysis seems to indicate that seismicity is induced by a tensile fracture, although we cannot completely rule-out seismic activity on a pre-existing fault. Gutenberg-Richter b-value variations in space are compatible with large magnitude events occurring at the edges of the earthquake propagating front. We further investigate the possible fracturing mechanisms triggered by the injection operation by fitting three plausible physical models: (1) a high-pore pressure diffusion model, (2) an aseismic crack model, and (3) a penny-shaped tensile crack model as the causative process of the recorded seismicity. The analysis of the seismicity evolution alone allows us to fit the three considered scenarios to the data independently, however we cannot exclude a combination of the three processes acting together. Through pseudo-retrospective forecasting, we then replay the induced seismicity as it was happening in real-time. We demonstrate that even if the physical processes are complex and likely difficult to disentangle using the seismicity alone, a simple empirical statistical seismicity rate forecasting model has stable predictability of hydraulic fracturing.

How to cite: Lanza, F. and Wiemer, S. and the DEEP Team: Forecasting and characterizing induced seismicity at the Utah FORGE EGS site, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12860, https://doi.org/10.5194/egusphere-egu24-12860, 2024.

The location and focal mechanism of microseismicity induced during fluid injection experiments in geothermal wells can be used to infer the extent of fracturing and the orientation of the local stress field. Passive seismic instrumentation is typically deployed at the surface and in boreholes around the injection site to monitor microseismic activity. Recent methodological advancements enable locating the often thousands of seismic events in a timely fashion. However, the determination of focal mechanisms is often limited to a selected number of larger-magnitude events.

Time-Reverse Imaging (TRI) exploits the time-invariancy of the elastic seismic wavefield to propagate the seismic wavefield backwards in time from seismic stations through an adequate velocity model. Under ideal conditions, the wavefield will converge on the initial source location at the origin time. The excellent location accuracy for events with signal-to-noise ratios smaller than one and the capability of determining the moment tensor for each locatable event has been demonstrated in controlled synthetic and real studies. Numerous improvements and adaptations have been proposed to augment the resulting image volume used to identify individual seismic events. TRI is a promising one-stop solution for analyzing microseismicity but has two major disadvantages: (1) the computation time needed to simulate the high-frequency elastic wavefield prohibits the analysis of continuous hours or days of microseismic recordings, and (2) the identification of individual seismic events from TRI image volumes is susceptible to overestimating the number of seismic events due to noisy images.

Alterations of the TRI concept based on pre-computed Green’s functions exist and provide a near-real time solution but require compromises in terms of location accuracy and minimal signal-to-noise ratio. The focal mechanism cannot be identified yet with these types of methods. Thus, the main challenge of applying TRI is reducing the needed computational time, while retaining most beneficial capabilities. This balancing act requires a careful analysis of possible compromises through careful scaling of simulation parameters.

Here, we apply TRI to synthetic seismic recordings created with the sensor setup deployed during the injection experiment in April 2022 at the UtahFORGE test site. A combined elastic velocity model of the complex site geology is used with a network including real locations of fiber optic cables, deep and shallow borehole sensors, and nodal seismic sensors. This synthetic data is used to demonstrate the accuracy gain of using multiple types of sensors and the speed gain of using characteristic functions applied to the seismic recordings prior to back propagation. Accuracy and speed are compared for synthetic and real test cases to optimize their trade-off. Finally, instead of individually picking seismic events, we demonstrate the usefulness of interpreting the spatio-temporal evolution of hypocenters and moment tensors directly from the TRI results.

How to cite: Finger, C., Tuinstra, K., Niemz, P., Shi, P., Ermert, L., and Lanza, F.: Spatio-temporal evolution of hypocenters and moment tensors derived from time-reverse imaging, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14828, https://doi.org/10.5194/egusphere-egu24-14828, 2024.

Microseismic monitoring plays a crucial role in assessing the effectiveness and integrity of Carbon Capture and Storage (CCS) projects. By the detection of microearthquakes we can gain real-time insights into the pressure and stress perturbation due to injection operations, aiding in the detection of potential leakage and ensuring the long-term viability of carbon sequestration efforts.

At the Quest CCS site in Alberta, Canada, CO2 injection into a 2 km depth saline reservoir is ongoing since 2015 at a rate of one million tonnes per year.  Several hundreds of small-magnitude seismic events have been located in the Precambrian basement below the reservoir.  A spatio-temporal analysis of seismicity reveals clustered as well as more diffuse distributions of events.  At the Quest site various microseismic monitoring technologies are in place including a downhole 8-level 3-component geophone string, temporary surface nodes arranged in mini-arrays, and downhole optical distributed acoustic sensing (DAS) fiber. The site offers an ideal opportunity to compare and combine the different setups with respect to event detection thresholds and location uncertainties. We demonstrate the importance of advanced signal and array processing techniques and highlight the advantages and disadvantages of different sensor technologies.

How to cite: Goertz-Allmann, B. P., Langet, N., Baird, A., Iranpour, K., Kühn, D., Vernier, J., Rebel, E., and Oates, S.: Microseismic event analysis using multi-technology sensors at the Quest CCS site, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8850, https://doi.org/10.5194/egusphere-egu24-8850, 2024.

Hydropower facilities utilize the potential energy of water to generate electricity, with maximum efficiency achieved when there is a significant topographic gradient between reservoir and turbines. Therefore, high dams are typically built in regions with rugged topography, often associated with (frequently combined) erosion, folding or displacement along fault zones, leading to juxtaposed different material properties.

At the Enguri Arch Dam in Georgia, extensive limestone formations from the Cretaceous and Jurassic were thrust southward, resulting in a topography difference exceeding 1000 m between the southward-extending Rioni Basin and the contiguous mountain ranges. The reservoir extends about 25 km to the north. There, nearby mountains reach heights of 3000 m and more. As part of the ongoing crustal shortening process, multiple fault systems have emerged, including prominent SW-NE trending thrust faults, steep strike-slip faults, and to a minor extend normal faults.

The Enguri valley carves into the surrounding mountains, reaching an elevation of 280 m above sea level at the dam site. These substantial topographic variations between hilltops and valleys establish a variable initial stress field characterized by lateral heterogeneity in both magnitude and orientation. The initial stress conditions were determined using borehole imaging data and hydraulic fracturing tests, while the mechanical properties of the subsurface materials were evaluated using mechanical tests on core samples.

The Enguri high-head Dam has a construction height of 271 m and the Jvari-reservoir reaches at full level more than 510 m above sea level. Geodetic GNSS and seismic stations were installed to evaluate the impact of the annual water level changes of about 100 m on the surrounding area.

The subsurface information on stress conditions and material properties was used to create an elastic 3D model of the area. The modelling results were compared with field observations to gain a better understanding of the dynamic processes in the area. In a first step the initial stress field was simulated. Loads were applied to simulate the water level changes. Modelled and observed displacements indicate that rising water level causes the west bank to move north-west, while the east bank moves south-east. Furthermore, both banks of the valley show a downward movement. Conversely, when the water level decreases, the effect is reversed. Variations in water level induce changes in the shear stress and changes in Coulomb Failure Stress (ΔCFS) calculated for different fault orientations. They reveal an increased seismic potential during low water levels, aligning with first seismic observations.

How to cite: Niederhuber, T., Müller, B., Westerhaus, M., Rietbrock, A., Weisgerber, J., Röckel, T., Karamzadeh, N., Tsereteli, N., Tugushi, N., Kalabegishvili, M., Svanadze, D., and Schilling, F.: Stress and Strain Changes in Response to Reservoir Water Level Variations – A Case Study of the Enguri High-Head Arch Dam in the Caucasus, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16938, https://doi.org/10.5194/egusphere-egu24-16938, 2024.

Under critical conditions where fault slip exhibits self-sustained oscillation in experiments, effects of normal stress oscillation (NSO) on fault strength and stability remain uncertain, as do potential effects of NSO on natural and induced seismicity. In this study, we employed double direct shear testing to investigate the frictional behavior of a synthetic, near velocity-neutral (VN) fault gouge (characterized by self-sustained oscillation under quasi-static shear loading), when subjected to NSO at different amplitudes and frequencies. During the experiment, fault displacement and gouge layer thickness were measured. Transmitted ultrasonic waves were also employed to probe grain contact states within the gouge layer. Our results show that fault weakening and unstable slip can be readily triggered by oscillations, depending on oscillation frequency and amplitude. Interestingly, an amplified shear stress drop and weakening effect were observed when the oscillation frequency fell in a specific range (0.01–0.1Hz). No such effects were seen in a velocity-strengthening gouge. Analysis of transmitted ultrasonic waves in the test on the VN gouge reveals the presence of fault dilatation, accompanied by unstable slip and weakening. By extending an existing microphysical model (the "CNS” model), to account for elastic effects of NSO on gouge microstructure and grain contact state, the mechanical and wave data obtained in our experiment on the VN gouge was reproduced. Assisted by the microphysically-based friction model, resolving the instability criterion of a velocity-neutral fault under perturbation is crucial for understanding and thus predicting the fault behaviors of certain scenarios, like periodic gas storage in deep reservoirs.

How to cite: Yu, B., Chen, J., Spiers, C. J., and Ma, S.: Frictional Properties of Simulated Fault Gouges subject to Normal Stress Oscillation and Implications for Induced Seismicity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12822, https://doi.org/10.5194/egusphere-egu24-12822, 2024.

We performed a series of hydraulic stimulations at 1.1 km depth in the Bedretto underground laboratory in the Swiss Alps. The goal was to achieve an unprecedented detailed and profound understanding of hydromechanical and seismic processes during hydraulic reservoir development with a dense multi-sensor monitoring network. With our seismic network that includes various sensor types with different sensitivities, we succeeded in characterizing induced seismicity down to the pico-seismicity level (Mw<-4), thus illuminating details of a complex fracture network more than 100 m from the injection locations. Here, we present the experiments and seismic catalogs as well as a comparative analysis of event number per injection, magnitudes, b-values, seismogenic index and reactivation pressures.

During a first-order data analysis, we could make the following observations: 

-        We find that the ultra-high frequency seismic network with custom-made AE sensors, allows us to observe seismicity over 3 orders of magnitude scale. Thanks to collocated accelerometers and acoustic emission sensors, AE sensors could be calibrated in-situ and adjusted moment magnitudes could be implemented into the seismic catalog. 

-        The volume impacted by the stimulations in different intervals differs significantly with a lateral extent from a few meters to more than 150 m. Most intervals activated multiple fractures. Only during the stimulation of an interval located next to a dominant shear zone, an extended single fracture was activated, which is likely attributed to the dominant shear zone in this area. The seismic clouds typically propagate upwards towards more permeable, shallow depth on parallel dipping planes that are consistent with the stress field and seem to a large extent associated with preexisting open fractures.

-        It is worth noting the strong correlation between the propagation patterns observed in the seismic events and the hydromechanical observations, specifically in terms of the strain and pressure data obtained from Distributed Strain Sensing (DSS), the Fiber Bragg Grating (FBG) and the pore pressure sensors that form part of the multi-component borehole monitoring system. 

-        This experiment confirms the diversity in seismic behavior independent of the injection protocol. Some intervals showed rapidly increasing seismicity that is spatially restricted to the volume in direct vicinity of the injection point, while others have seismicity extending as far as 150 m away from the injection point.

-        The reactivation pressures hint at hydraulic shearing as the dominant process, since the elastic fracture opening appears to be mostly aseismic.

-        The seismicity shows no distinct deviation from “normal” behavior with regard to Gutenberg Richter or McGarr. 

We have the opportunity to analyze the seismic data jointly with a multitude of other geophysical observables, such as strain and pressure, to allow more insights into the correlation of slow fracture opening and aseismic deformation processes. In future studies, the existence of these multi-disciplinary observations will allow us to put more constraints on the processes responsible for the diversity observed in seismicity.

How to cite: Obermann, A., Rosskopf, M., Durand, V., Plenkers, K., Bröker, K., and Gholizadeh Doonechaly, N.: Picoseismic response of hectometer-scale fracture systems to stimulation under the Swiss Alps, in the Bedretto Underground Laboratory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5696, https://doi.org/10.5194/egusphere-egu24-5696, 2024.