Monitoring the in-situ temperature is key for the characterization of a seasonal geothermal energy storage. Distributed fiber-optic temperature sensing (DTS) systems provide temporally and spatially continuous measurement data in near real-time that captures borehole temperature dynamics. In the presented project, three boreholes of a seasonal geothermal energy storage with a vertical depth of down to 500 meters were instrumented with distributed fiber-optic sensors. For this purpose, a standard armored sensor cable was modified to allow for combined DTS, distributed acoustic sensing (DAS) and distributed strain sensing (DSS) at temperatures up to 120 °C. The cable was installed in a loop configuration to allow for temperature calibration using a temperature matching approach. A “Mini-Bend” solution was selected to cope with the limited space between formation and casing and the single cable feedthrough at the wellhead of the pressurized system. Using specially designed centralizers, the fiber-optic cable was installed and hold in place successfully in the 2” annulus outside the casing. Within the present paper, the focus lies on the DTS system and the data acquired during cementation. This continuous temperature data showcases the ability of DTS systems to detect small temperature changes in high detail. During the whole cementation process, the rising level of cement as well as setting temperatures were captured by rising temperature values in the range of a few degrees Celsius. As next project steps, not part of this paper, spatiotemporally temperature data will be recorded during stimulation treatments of the seasonal geothermal energy storage. The authors believe that the experience made with this fiber-optic monitoring system can be a good demonstration of the capabilities of fiber-optic sensing in deep borehole environments.

ABSTRACT Enhanced Geothermal Systems (EGS) offer the best potential for meeting the U.S. Department of Energy's objective of 90,000 MWe by 2050. The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) was established to de-risk tools and technologies required for commercialization of EGS. Six deep wells have been drilled into the low permeability granitic and metamorphic rocks hosting the reservoir. One is highly deviated and will serve as the injection well during reservoir creation. The deepest well, drilled to a depth of (2,896 m (9500 ft) has an estimated bottom-hole temperature of 241°C (465°F). Injection tests indicate a minimum horizontal stress gradient of 16.74 kPa/m (0.74 psi/ft) and a permeability of ∼20 micro-Darcies. A three-stage stimulation of the injection well was performed in April 2022. In the first stage, slickwater was pumped into the open hole section of the well at 50 bpm (7.95 m3/min). The second and third stages were performed in the cased portion of the well using slickwater and a viscosified fluid. Flow rates of 35 bpm (5.56 m3/min) were achieved in both zones. Microseismic events recorded during the stimulation ranged from −2.3 to +0.5 Mw. Drilling of the production well is planned for 2023. INTRODUCTION Enhanced Geothermal Systems (EGS) offers the greatest potential for meeting the U.S. Department of Energy's (DOE) goals of 90,000 MWe by 2050 and reducing the cost of electricity by 90% to $45 per MW-hour. Since the late 1970s, more than a dozen EGS projects have been conducted throughout the world in an effort to create geothermal reservoirs where none exist naturally. None of the projects have achieved commercial scale levels of production. The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) was developed to de-risk and test techniques for creating, sustaining and monitoring EGS reservoirs. The ultimate goal of the Utah FORGE field-scale project is to demonstrate to the public, stakeholders and the energy industry that EGS technologies have the potential to contribute significantly to future power generation (Moore et al., 2020, 2021).

The increased interest in subsurface development (e.g., unconventional hydrocarbon, engineered geothermal systems (EGSs), waste disposal) and the associated (triggered or induced) seismicity calls for a better understanding of the hydro-seismo-mechanical coupling in fractured rock masses. Being able to bridge the knowledge gap between laboratory and reservoir scales, controllable meso-scale in situ experiments are deemed indispensable. In an effort to access and instrument rock masses of hectometer size, the Bedretto Underground Laboratory for Geosciences and Geoenergies (“BedrettoLab”) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the BedrettoLab, its general setting and current status. Combined geological, geomechanical and geophysical methods were employed in a hectometer-scale rock mass explored by several boreholes to characterize the in situ conditions and internal structures of the rock volume. The rock volume features three distinct units, with the middle fault zone sandwiched by two relatively intact units. The middle fault zone unit appears to be a representative feature of the site, as similar structures repeat every several hundreds of meters along the tunnel. The lithological variations across the characterization boreholes manifest the complexity and heterogeneity of the rock volume and are accompanied by compartmentalized hydrostructures and significant stress rotations. With this complexity, the characterized rock volume is considered characteristic of the heterogeneity that is typically encountered in subsurface exploration and development. The BedrettoLab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses.

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) is located in central Switzerland and serves as a test bed for geothermal energy research. Several boreholes were drilled from the laboratory section (ca. 1.1 km overburden) to serve as injection boreholes for stimulation and geophysical monitoring boreholes. During a hydraulic stimulation injection in winter 2020 into injection borehole ST2 interval ranging from 313 to 320 m, we observe a thermal perturbation using distributed fiber optic temperature sensing in a neighboring open borehole (MB1) at a depth of 275 m to 295 m. Prior to injection, there is a thermal anomaly in MB1 at about 289 m due to natural fracture fluid flow. Below this depth the temperature is approximately 1.5 °C higher than above. During injection there is a gradual upward movement of the thermal anomaly to ca. 278 m depth. After injection is stopped, the thermal signal gradually recovers to the original depth. The cause for such a temperature change is potentially due to increased warm water flow reaching the base of MB1 from deeper ST2 or poro-elastic fracture closure of the cold-water conducting fractures at 278 and 289 m depth in MB1 during stimulation.

The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS.

In this study, we investigate numerically the hydro-mechanical behavior of fractured crystalline rock due to one of the five hydraulic stimulations at the Pohang Enhanced Geothermal site in South Korea. We use the commercial code FracMan (Golder Associates) that enables studying hydro-mechanical coupled processes in fractured media in three dimensions combining the finite element method with a discrete fracture network. The software is used to simulate fluid pressure perturbation at fractures during hydraulic stimulation. Our numerical simulation shows that pressure history matching can be obtained by partitioning the treatment into separate phases. This results in adjusted stress-aperture relationships. The evolution of aperture adjustment implies that the stimulation mechanism could be a combination of hydraulic fracturing and shearing. The simulated extent of the 0.01 MPa overpressure contour at the end of the treatment equals to similar to 180 m around the injection point.

Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design.

In Switzerland the large geothermal potential for electrical power production can only be used if Enhanced Geothermal Systems (EGS) will be technically feasible because 90% of the deep underground consist of low permeable crystalline rocks. Based on the data of the Deep Heat Mining Project Basel a new multi-stage stimulation concept has been developed which reduces the risks of induced seismicity and promises a better return of energy. The mitigation plan for induced seismicity includes three major points. First, induced seismicity will be reduced by limiting the areas of the hydraulically stimulated fracture planes. This can only be achieved with a borehole completion allowing the hydraulic isolation of up to 30 individual borehole sections within the reservoir. Secondly, the site selection criteria lead to avoid densely populated areas, areas with high natural seismic activity and placing the geothermal reservoir at a safety distance of at least one kilometer from regional or major fault zones. Third, in addition to deterministic risk studies we have applied a probabilistic approach recently developed by the Swiss Seismological Service to show that the multistage stimulation of smaller fracture zones has a better risk profile than the massive stimulation concept of the Basel project. 1. INTRODUCTION In Switzerland the large geothermal potential for electrical power production can only be used if Enhanced Geothermal Systems (EGS) will be technically feasible. Based on the data of the Deep Heat Mining Project Basel a new concept has been developed which reduces the risks of induced seismicity and promises a better return of energy. In addition the acceptance by the local population and the authorities play an important role. Geo-Energie Suisse Ltd. the following organization of Geopower Basel Ltd. has started permitting procedures for EGS pilot projects at five sites in Switzerland in order to create a portfolio of projects sites. The sites are located in the communities of Haute-Sorne (Canton Jura), Avenches (Canton Vaud), Etzwilen (Cantons Thurgau and Zurich), Pfaffnau and Triengen (both canton Luzern). Hydrothermal systems for an electricity production from deep geothermal energy in a larger extent are in Switzerland exceptional areas like fault zones and permeable aquifers in depth of 4000 to 5000 m. A large-scale extension of such systems like in the Munich region could not be proven up to now. In Switzerland for sedimentary systems in greater depths, whose temperatures also allow an electricity production, only a small zone in the foreland of the Alps can be taken into account. Additionally the situation complicates, as the very good aquifers in the Bavarian Molasse Basin become to the West less permeable with lower flow rates of the groundwater. Based on these considerations the Geo-Energie Suisse Ltd. decided to focus their activities on the development of EGS-pilotprojects (petrothermal systems) because the great potential for electrical power production lies within the crystalline basement (Fig. 1). If it is possible to install in the crystalline rocks of the basement an artificial heat-exchanger, that technology can be used nearly everywhere in Switzerland. For the first time such a big permeable system could be created in the well Basel-1 in 5000 m depth, however, the unrequested side effect of induced seismicity occurred.

We investigate attenuation (Q-1) of sediments of 2.53.5 km thickness underneath the city of Basel, Switzerland. We use recordings of 195 induced events that were obtained during and after the stimulation of a reservoir for a Deep Heat Mining Project in 2006 and 2007. The data set is ideally suited to estimate Q as all events are confined to a small source volume and were recorded by a dense surface network as well as six borehole sensors at various depths. The deepest borehole sensor is positioned at a depth of 2.7 km inside the crystalline basement at a mean hypocentral distance of 1.8 km. This allows us to measure Q for frequencies between 10 and 130 Hz. We apply two different methods to estimate Q. First, we use a standard spectral ratio technique to obtain Q, and as a second measure we estimate Q in the time domain, by convolving signals recorded by the deepest sensor with a Q operator and then comparing the convolved signals to recordings at the shallower stations. Both methods deliver comparable values for Q. We also observe similar attenuation for P- and S- waves (QP similar to QS). As expected, Q increases with depth, but with values around 3050, it is low even for the consolidated Permian and Mesozoic sediments between 500 and 2700 m.

Theoretical considerations and empirical regressions show that, in the magnitude range between 3 and 5, local magnitude, M-L, and moment magnitude, M-w, scale 1:1. Previous studies suggest that for smaller magnitudes this 1: 1 scaling breaks down. However, the scatter between M-L and M-w at small magnitudes is usually large and the resulting scaling relations are therefore uncertain. In an attempt to reduce these uncertainties, we first analyze the M-L versus M-w relation based on 195 events, induced by the stimulation of a geothermal reservoir below the city of Basel, Switzerland. Values of M-L range from 0.7 to 3.4. From these data we derive a scaling of M-L similar to 1.5 M-w over the given magnitude range. We then compare peak Wood-Anderson amplitudes to the low-frequency plateau of the displacement spectra for six sequences of similar earthquakes in Switzerland in the range of 0.5 <= M-L <= 4.1. Because effects due to the radiation pattern and to the propagation path between source and receiver are nearly identical at a particular station for all events in a given sequence, the scatter in the data is substantially reduced. Again we obtain a scaling equivalent to M-L similar to 1.5M(w). Based on simulations using synthetic source time functions for different magnitudes and Q values estimated from spectral ratios between downhole and surface recordings, we conclude that the observed scaling can be explained by attenuation and scattering along the path. Other effects that could explain the observed magnitude scaling, such as a possible systematic increase of stress drop or rupture velocity with moment magnitude, are masked by attenuation along the path.

ECOS-09 (Earthquake Catalog of Switzerland 2009) is the current earthquake catalog of Switzerland. It integrates the following basic information: The Macroseismic Earthquake Catalog of Switzerland, with events from AD 250, revised and supplemented to 2008; Yearly reports of the Swiss earthquake commission since 1879; Earthquake locations of the instrumental networks of the SED since 1975; Supplemented by 12 earthquake catalogs from neighboring and international agencies. The macro-seismic part of the catalog is based on the analysis of observed and historical impacts of earthquakes on people and the environment. The epicenters and earthquake magnitudes are estimated from this information. The instrumental part of the catalog is based on the analysis of measured ground-motion, from which the location of the epicenter and the magnitude can be directly computed. ECOS-09 covers the period AD 250 to the end of 2008. The available data covers Switzerland and bordering regions (Swiss coordinates (km): 460-882 / 20-350; geographical coordinates approx.: 5.6-11.1E / 45.4-48.3N). Hours are given in UTC (Coordinated Universal Time).

We estimate moment magnitudes M-w for earthquakes in Switzerland recorded between 1998 and 2009 using three different spectral methods. The M-w estimation in Switzerland is extended to lower magnitudes (local magnitude M-L 0.1), and scaling relations between M-L and M-w are investigated. Above M-L 4, the obtained M-w estimates are consistent with the previously obtained scaling relation of M-w = M-L -0.3 at the Swiss Seismological Service (SED). Below M-L 4, all three methods indicate that a 1:1-type relationship is inappropriate. Therefore, we propose a new piecewise empirical scaling relation for earthquakes in Switzerland. The scaling is linear below M-L 2 and above M-L 4. To obtain a smooth transition between the two linear scales we fit a quadratic relation in between (2 <= M-L <= 4). This scaling relation is also consistent with M-w estimates from moment-tensor (MT) solutions based on broadband waveform fitting of local earthquakes with M-L > 3.0. We have tested all three methods carefully to ensure that the observed break in scale at around M 3 cannot be attributed to bias in the M-w determination. However, we cannot determine with certainty from the dataset at hand whether the break in scaling is due to bias in the routine determination of M-L or to physical properties of the source.

We estimate moment magnitudes MW for earthquakes in Switzerland recorded between 1998 and 2009. Compared to previous studies the MW determination in Switzerland could be extended to lower magnitudes and scaling relations can be investigated from ML = 0 to 5.4. Three different spectral methods are applied to estimate MW and the scaling relations resulting from the different methods are compared with one another. Above ML = 4, the obtained estimates are consistent with the previously used scaling relation of MW = ML−0.3. BelowML = 4, all three methods indicate that a 1:1-type relationship is inappropriate. Therefore, w propose a new empirical piecewise ML to MW scaling relation for earthquakes in Switzerland. The scali ng is linear belowML = 2 and aboveML = 4. To obtain a smooth transition between the two linear scales we fit a quadratic relation in between ( 2 ≤ ML < 4). This scaling relation is also consistent with MW estimates from moment tensor solutions based on broadband waveform fit ting of local earthquakes with ML > 3.0.