After a break filled with analyses and preparations, the BedrettoLab team is conducting a new series of stimulations in continuation of the VALTER project and in preparation of the FEAR project. The first stimulation is starting this week with relatively small volumes of water being injected during a few hours.
The main aim of these stimulations is to investigate stress preconditioning. More specifically, the behavior of individual fractures by increasing the fluid pressure will be tested. These investigations are an important prerequisite for the upcoming FEAR experiment. Later stimulations, to be conducted in March and April, will focus on the increase of transmissivity of the host rock which is essential for the generation of a geothermal reservoir.
As for all activities in the BedrettoLab, safety is the top priority, and this also applies to the upcoming stimulations. We have deployed several safety layers to ensure that the seismicity does not reach crictical levels. Since we closely monitor the stimulations with our exceptionally dense and highly sensitive measuring system, we can identify in near real-time changes within the rock volume of interest. This enables continuously updated hazard re-evaluations. If predefined safety thresholds in magnitude or vibration are reached, the injection is halted and bleed off initiated. 28 February 2023
A group of scientists of the BedrettoLab team recently conducted experiments in the Merkers salt mine in Germany with the aim of calibrating a variety of seismic sensors. Calibration of these sensors is required to infer quantitative physical measurements from the recorded ground motions originating from seismic activity at the BedrettoLab. The sensors will later be installed in boreholes for the FEAR project aiming to measure small magnitude earthquakes, so-called “pico-seismicity”, that range between magnitudes of Mw -6 to -2. Special about these small magnitude earthquakes is that they are usually not felt by humans and that they radiate high-frequency energy (i.e., 1 – 100 kHz), a frequency bandwidth that was targeted with the recent experiments. The results gained are allowing us to calibrate the installed sensors and physically quantify pico-seismic earthquakes.
The saltmine offered an ideal setting for in-situ calibration of these sensors as the salt rock has a simple homogenous structure and low attenuation. In general sensor calibration requires an ideal setting meaning that the measurements of specific physical values should work without disturbances. In granite, like the one in the BedrettoLab, the velocity of the waves travelling through the rock volume is disturbed due to fractures and faults which makes calibration very difficult.
For the calibration experiments a variety of sensors were installed around a salt pillar measuring approximately 40 m by 40 m. Different artificial sources were deployed, most informative being a steel ball pendulum were used to initiate seismic waves in the pillar. The recorded seismic waves at a sensor were then compared to a reference laser sensor as well as models that predict the seismic wave field, ultimately leading to the calibration.
The salt mine experiment (SaMiEx) benefited from multiple partners, most notably Gesellschaft für Materialprüfung und Geophysik (GMuG) and the German Research Centre for Geosciences (GFZ) but also the Institute of Mine Seismology (IMS), Kistler Instruments AG and Elsys Instruments AG.
14 February 2023
The drilling campaign for the Bedretto On-Fault Observatory (BOFO) has started last week. The BOFO observatory is going to be part of the monitoring system of the FEAR project allowing us to closely observe any seismic activities in and around the fault zone, which is the study object of the FEAR project. The BOFO forms the first part of the FEAR integrated monitoring system that will ultimately be completed when the new side tunnel is finished and further boreholes can be drilled.
In the next two weeks, eight boreholes will be drilled with a length of 8 to 12 meter each. They are designed to place highly sensitive acoustic emission sensors in the shape of four tetrahedral arrays. 16 January
Last week, the GEO.8 – European Alliance for Earth Sciences had their director’s board meeting and visited the BedrettoLab. 5 December 2022
In the framework of the SPINE project, a team of scientists is conducting hydrotests in the BedrettoLab. These experiments aim at applying the so-called SIMFIP (Step-Rate Injection Method for Fracture In-situ Properties) probe, a device for measuring the stress state in boreholes. The key idea of the SPINE project is that a single measurement of a displacement in the borehole wall can be used to determine the overall stress. Such a displacement occurs in the micrometer to nanometer range and is caused by the injection of water. Detailed knowledge of the state of stress in the subsurface contributes to estimating the seismic risk of a deep geothermal project.
During a period of two weeks, the international SPINE team runs about ten hydrotests for approximately four hours each. Hydrotesting is the term used to describe tests in which a small amount of water is deliberately directed into small fractures found naturally along the borehole. Since only small volumes of water will be injected, the occurrence of seismic events is not expected. Nevertheless, the routine safety protocols will be applied and the experiments would be put on halt if predefined magnitude thresholds are reached.
In these experiments the water flows through the SIMFIP probe which is then able to capture millimeter to nanometer displacements of fractures affecting the borehole. Being part of a test series carried out in various laboratories, these hydrotests at BedrettoLab are contributing to the development of the SIMPFIP technology within the SPINE project. 1 December 2022
Members of the BedrettoLab and the Swiss Seismological Service (SED) at ETH Zurich recently installed a seismic station just 400 m beneath the Pizzo Rotondo. The new station closes an important gap in the seismic background monitoring system of the BedrettoLab. It will improve the hypocentre location estimates of earthquakes in and around the Bedretto experimental volumes.
With this latest addition, the BedrettoLab network consists now of five tunnel and three surface stations allowing to closely observe seismic activities in its surroundings. The new station is the closest surface station to the BedrettoLab; it is ~1.2km above and ~500m to the north-east of the tunnel.
The new station includes a high sensitivity and a strong motion sensor also called accelerometer. Both sensors are part of the standard equipment of seismic stations deployed by the SED. The high sensitivity sensor is capable of recording even very small seismic events (M<2), while the accelerometer is capable of measuring moderate and strong local tremors with great accuracy.
As a novelty, the seismometer has been lowered into a ~70cm deep borehole, drilled directly into the Rotondo granite. This new installation technique can potentially reduce ambient noise levels caused by e.g. wind, melt-water or rain. Because of the low noise levels even very weak seismic signals can be detected and characterised.
The new station was installed as part of the extension of the seismic background monitoring system, motivated by the Fault Activation and Earthquake Rupture project (FEAR). The project aims to investigate how earthquakes start and how they stop by initiating small, non-damaging earthquakes on a natural fault in the BedrettoLab. The FEAR monitoring system should allow to image earthquake rupture processes from unusually short distances, and with unusual detail. November 2022
The BedrettoLab recently moved all its drill cores to a special repository site close to the city of Zurich. Here around 1,600 meters of inventoried cores from the BedrettoLab and the rock laboratory in Grimsel are stored in 662 boxes.
When drilling a borehole with a certain method, a core is extracted. Geologists and geophysicists analyse those to find out more about the stress and porosity conditions as well as about existing faults or fractures, etc. This information later helps them to characterise the surrounding rock. Also, scientists create numerical models of the rock using data drawn from core analyses.
The cores from the BedrettoLab form a geological footprint that not only support current operations but will also support future geological or geophysical research about the Rotondo massif. October 2022
The lab construction for the FEAR experiments is moving into the next phase with a major drilling campaign going on now.
All in all, three boreholes are being drilled until the end of September. The three boreholes start at tunnel metre ~2360, and we use them to verify the extension of the fault and thus the experimental design of FEAR. Additionally, the information from this prospection campaign will inform our decision about the exact starting point of the FEAR tunnel. From this new tunnel, the fault will be stimulated via injection boreholes. The first experimentation phase of the FEAR project is planned to start in summer 2023. September 2022
After a two year break, the local event Mangia e Cammina finally took place yesterday. For the first time, the BedrettoLab had the honor to be part of it and opened its doors once again for the public.
Mangia e Cammina attracted 1’300 visitors who hiked along a route through the Bedretto Valley, where several alps offered specialties from the region. The BedrettoLab was one destination on the Mangia e Cammina parkour and opened its doors for guided tours. Around 50 people walked additional 5 km trough the tunnel to visit the BedrettoLab. In front of the tunnel, the BedrettoLab team and several helpers from ETH Zurich and SUPSI set up several booths where visitors could play a rock game, admire a small rock and mineral exhibition, see how a seismic station works or play an entertaining “snakes & ladders” game.
The event was an excellent occasion after a longer Covid related break to show our laboratory to the public. 8 August 2022
With a rich data set gathered during the two stimulation cycles of the reservoir engineering phase (Phase 2), the VALTER project team now leaves for the summer break. Further stimulations of Phase 2 are scheduled for autumn.
After having conducted experimental stimulations in June and July, the VALTER team is pausing Phase 2 until the end of the summer. This time will be used to continue analysing the wealth of data that has been gathered and also allow team members to get a rest. The VALTER team is particularly interested in gaining a better understanding of the tiny, induced earthquakes that occurred within the reservoir and further away. Such events were expected to happen, however, exploring their occurrence in more detail is exactly what is needed to advance our understanding of the behaviour of the deep underground when stimulated in the context of a geothermal reservoir. July 2022
After a short break for data analysis and planning, the VALTER team resumes with the stimulations that are part of the Bedretto Reservoir Project. This week, the next phase (Phase 2), the actual reservoir engineering phase, is going to start. The main aim is to stimulate approximately four borehole intervals for a period of up to 48 hours to create a connection between two boreholes, namely borehole ST1 and ST2.
Starting from today, the VALTER team is going to carry out hydraulic stimulations of four intervals until September 2022. In the past phase of the VALTER project, the intervals (=borehole segments) in borehole ST1 had been stimulated according to a stimulation protocol with consistent and predefined injection protocols. Based on the data from Phase 1, the fracture systems linked to the different borehole intervals were characterized and the most promising segments selected for Phase 2.
In Phase 2, approximately four intervals that turned out to be most suitable for connecting the boreholes ST1 and ST2 are going to be further enhanced. By stimulating an interval over a period of up to 48 hours, the water creates new small fractures in the rock and ideally connects the boreholes ST1 and ST2. The distance between the boreholes ST1 and ST2 is 35 meters. High-precision temperature sensors will be installed in borehole ST2. If the connection is successful, this will result in subtle changes in the temperatures measured in borehole ST2.
Hydraulic stimulations (injection of water under pressure) will enhance transmissivity through both seismic and aseismic deformation. Transmissivity is an indicator of the amount of water fractures can transmit. While more than 99% of the hydraulic energy is released without creating seismic events (aseismically), through creeping processes, we expect that again numerous tiny earthquakes will accompany the stimulation. Seismologists call earthquakes in the magnitude range -4 to -2 nanoseism.
The network installed at Bedretto is also able to detect events down to a magnitudes of -6, so called picoseism. During the upcoming Phase 2 stimulation numerous pico- and nanoseismic events are planned to be recorded again that are not only needed to create the reservoir but also represent a rich source of information on the reservoir evolution. The largest events observed during the Phase 1 stimulation was a magnitude -3. Slightly higher magnitudes are expected to be observed in Phase 2, since more water will be injected.
To make sure that the operations are safe, the injection is halted and bleed off initiated as soon as predefined safety thresholds in magnitude or vibration are reached. Besides that, the seismologists of the VALTER team are going to monitor the stimulations continuously and watch out for unusual developments.
Between the stimulations, a period of around three to four weeks is needed for data analysis. With this final phase, the scientists expect to gain insights into safe reservoir engineering on a scale that is meaningful for real-world applications, such as the geothermal project planned in Haute Sorne.
With a series of small- to moderate-scale injections in the anticipated VALTER reservoir, key physical properties of the host rock have been investigated. The experiments led to a comprehensive data set, which is currently being analysed by the VALTER Team. The results will form the basis for the next phase of the project, in which a proxy of a geothermal reservoir will be created.
Since February 2022, the multidisciplinary VALTER Team, including roughly 15 researchers as well as engineers and technicians of various backgrounds, worked on a tight schedule to carry out the first suite of multi-stage stimulations of the VALTER project. This stimulation concept is a unique approach, in which a borehole is divided by packers into several segments that can be stimulated separately. Following this methodology, which has been developed and tested at some intervals in the BedrettoLab by Geoenergie Suisse within the Destress project, specific fracture systems can be targeted and characterized in more detail. Additionally, multi-stage stimulations may minimise the level of induced seismicity.
Hydrotests and stimulations
The stimulations were carried out in borehole ST1 (s. image) and set up to obtain more insights on the fractured rock mass and its transmissivity. From the 14 intervals within ST1, nine intervals were probed (the remaining five intervals were stimulated earlier in the framework of Destress). The tests included initial hydrotests, followed by the actual stimulations and another hydrotest after the completion of the stimulations. During the stimulations, water was injected in a controlled fashion into a specific interval, and the resulting pressure, temperature and deformation changes, as well as induced seismicity, were monitored with a dense network of sensors.
Besides acquiring hydraulic and hydromechanical data, a real time-monitoring of the seismicity is essential for such experiments, because the injected water is not only flowing through pre-existing fractures but also creates new fractures of various sizes that then help to enhance the reservoir transmissivity. The latter is required for creating a reservoir and therewith the exploitation of geothermal energy. This process is also associated with induced seismicity. During the VALTER stimulations, seismic events of moment magnitudes ranging from -6 to -3 occurred. These earthquakes are way too small to be felt and can only be measured by very sensitive instruments. However, this seismicity is an essential diagnostic tool during the reservoir creation process, and such small events are thus welcome. In order to avoid earthquakes of larger magnitudes that could be felt or put anyone into danger, a so-called traffic light system has been put into place. When predefined thresholds would be reached and detected by the monitoring system, ongoing procedures would immediately be put on halt. During this first phase of VALTER, none of the experiments triggered events that would have required any change of the injections.
Data analyzation and preparation
First data analyses are currently underway. Initial results show a large variety of the hydraulic, hydromechanic and seismic behaviour of the rock mass volume during the stimulations. For example, some intervals exhibit strong seismic response but little hydraulic changes, whereas other intervals show the opposite. The VALTER team is currently establishing conceptual models that are compatible with all these observations. These models will be of great importance for the next phase of the VALTER project, in which a proxy of a geothermal reservoir should be generated. Here, the most promising structures will be stimulated with larger injections that may extend over longer periods.
A multidisciplinary effort
The complexity of the experimental setup and the multidisciplinary nature of the VALTER project required substantial coordination efforts. Efficient communication between the individual research disciplines and with the technical staff proved to be essential. Here, the project benefitted from the excellent infrastructure in the BedrettoLab. The entire monitoring network is connected to a fast internet connection. This enabled seamless interactions between the onsite team and the various experts located at ETH Zurich. The individual data streams could be checked in real time, and, when necessary, decisions on adjustments of the procedures could be taken very quickly.
The Bedretto Reservoir Project
The currently running VALTER project is part of the Bedretto Reservoir Project, which is an umbrella project for VALTER, Destress and ZoDrEx. While having the complementary targets, the three projects test and develop different techniques for the creation of a test reservoir. In this context, VALTER and Destress investigate the enhancement of the stimulation processes. Thereby, DESTRESS mainly focused on the proof of concept of the multi-stage stimulations whereas VALTER is monitoring the processes in the rock more densely and over a longer period of time. ZoDrEx focuses on the improvement of drilling efficiency, zonal isolation technologies and development of instrumentation for geothermal energy plants.
The FEAR Annual Meeting took place in Airolo and Bedretto this week. The team of almost 50 participants from ETH Zürich, RWTH Aachen, and INGV Rome gathered to share their results and discuss the next steps of the project.
The meeting focused on preparing for FEAR the first experiment. A lot of things need to be done to get there. Boreholes have to be drilled, prospection campaigns carried out, a tunnel designed and constructed, monitoring systems designed and optimized, rock samples have to be tested in the lab, numerical models calibrated, parameter values and their uncertainties estimated, a real-time traffic-light risk mitigation system and remote control for safe operation of the experiment implemented.
The last few days were spent with lively discussions and in-depth breakout sessions on who will do what and when, so that one year from now, we can push the button. After all the hard work, some fun and games were in order. There was a visit to the BedrettoLab with dinner and a concert. May 2022
The website for the Fault Activation and Earthquake Rupture ('FEAR') project is now online. The goal of FEAR is to improve our understanding of earthquake physics, by densely instrumenting and then stimulation an existing, natural fault. The small earthquakes resulting from this stimulation shall provide new insights on how earthquakes originate and evolve. Again, safety is our first priority and the measures taken to ensure safe procedures are described on this page.
The construction work for the new FEAR experimental setup is ongoing. The experiment is situated about 2.5 km and therewith 400 meters beyond the currently used lab at the Bedretto tunnel. Recently, first boreholes have been drilled. They will be instrumented and serve to determine if there is any seismic background activity in the experimental volume, and whether the selected target structure - a north-west dipping natural fault zone with evidence of past seismic activity - is large enough to host the FEAR experiments.
Please check the website to find detailed information about FEAR: http://fear-earthquake-research.org
In addition to the sophisticated sensor network, a large network of fiber optic cables and ultimately data storage, forms the backbone of our research lab. Recently, another 1.2 petabytes of storage has been installed at the ETH Zurich.
Constantly, 800 MB of data per day are being sent from the acoustic emission sensors in the BedrettoLab to the servers in Zurich. On top of that, up to 1TB per day from the ongoing experiments, measured by sensors such as fiber optics interrogators, geophones, temperature, pressure and strain meters, are stored in the raw time series archive. This is a central access point for all data together with consistent timing information. For the researchers, this is the key for any type of synoptic interpretation, e.g. seismic event detection. March 2022
Recently, we had the honour of welcoming a TV crew of RSI, who shot an episode of the science show “Il giardino di Albert”. Two moderators interviewed Prof. Domenico Giardini, board member of the BedrettoLab, who explained the scope of our research activities and presented the lab infrastructure.
The episode of “Il giardino di Albert” provides you with a comprehensive insight into the underground laboratory and can be watched here. The program is in Italian. March 2022
After an intense phase of preparation and first extensive test stimulations in November 2021, the VALTER team recently initiated a next phase of stimulations in the BedrettoLab.
The primary aim of the current phase is to characterize the hydro-mechanical properties of the host rock. For that purpose, the VALTER team will inject relatively small volumes of water at eight locations in the rock volume. Initial results from the first stimulations in November 2021 are promising, demonstrating that the sophisticated monitoring system is capable to identify subtle changes of the hydro-mechanical properties. Furthermore, the associated seismic activities can be well characterized down to magnitudes of -6. The largest events happening during the stimulations reached magnitudes of about -4.6. To have a comparison: Usually earthquakes can be felt from a magnitude of 2.5 on, which is about 45 billion times stronger than a magnitude of -4.6.
The current phase of stimulations is expected to be completed in late spring. For the following phase, the VALTER team will design stimulation protocols appropriate to permanently enhance the transmissivity of the rock volume and create an experimental geothermal reservoir structure. This step is expected to be completed in summer 2022. Again, safety is top priority and ensured by several safety layers. Further information is available in the last news article featuring the VALTER stimulations. February 2022
The BedrettoLab entered a new phase in November 2021 (see news article), in which the Lab is being extended to include a second testbed. The first step of the retrofitting has been completed, including the installation of electricity and lighting to 2900 TM (TM = tunnel meter).
In the following weeks, the ventilation ducts will be extended to 2900 TM and we will begin the first phase of cementing the tunnel passage. Then a series of boreholes will be drilled, one set for long term seismic monitoring and another for geologic exploration of the second testbed. These will be completed late Summer 2022. Afterwards, a second phase of construction will finalize the road extension to 3050 TM and complete additional safety reinforcements in the latter part of the tunnel. 31 January 2022
On Wednesday, 17 November 2021, we will start the stimulations for the VALTER project. We will inject a few cubic meters of water using different injection procedures and following a multi-stage stimulation approach that isolates parts of the injection boreholes to maximise the control on the reservoir creation. Using a high-resolution observational network, it is our goal to monitor stimulation related processes in unprecedent detail at a realistic scale, with the ultimate aim to create a commercially viable underground heat exchanger while minimising the risk of felt earthquakes.
We choose a stepwise approach with different experimental phases, interrupted by periods of data analyses, to establish such a geothermal test reservoir at the BedrettoLab. The first stimulations, taking place on Wednesday, will mainly examine if the monitoring system and stimulation procedures work as foreseen. To this end, we will start small and only stimulate one interval of the injection borehole at a depth of around 220 meters from the cavern. Then, we will pause the stimulation activities, assess the data, and if required, we will improve our systems and procedures.
As for all activities in the BedrettoLab, safety is the top priority, and this also applies to the upcoming stimulations. We have deployed several safety layers to ensure that the seismicity does not reach damaging levels. Since we closely monitor the stimulations with our exceptionally dense and highly sensitive measuring system, we can identify in near real-time changes within the rock volume of interest. This enables continuously updated hazard re-evaluations. The chances are very small that these stimulations will cause damage to the tunnel (a probability of less than 1 in 10’000) and even smaller that they will cause earthquakes large enough to be felt or cause damage in nearby villages (a probability of less than 1 in 10 million).
Read more about the media event we recently had portraying the VALTER project and the project itself here. 15 November 2021
Last week, the first annual meeting of the FEAR (Fault Activation and Earthquake Rupture) project has taken place. 36 people from RWTH Aachen, Instituto Nazionale di Geofisica e Vulcanologia in Rome, EPFL and ETH Zurich met in Bedretto to work on the project and to plan the activities and experiments. After all those online meetings it was nice to finally meet and get to know each other in real life. If you want to learn more about FEAR, have a look at the project description. 3 November 2021
In 2019, the Bedretto tunnel was identified as an ideal place to establish a deep underground laboratory. Shortly after, an agreement with the tunnel owner, the Matterhorn-Gotthard-Bahn, was signed and the preparatory work begun. The tunnel needed to be secured and the infrastructure was installed in the main cavern 2 km deep into the tunnel, to enable conducting outstanding scientific experiments. To gain a better understating of the rock properties, a geological characterization phase took place. At the same time, we tested equipment and the instruments, many of them newly developed for our specific purposes. Finally, a first experimental testbed has been set up, by drilling ten 250 to 400 m long boreholes below the main cavern and equipping them with hundreds of sensors, effectively transforming a large volume of rock in a unique sensing testbed.
Following this extensive preparatory work, the BedrettoLab is now ready for the next phase, where we really want to get to the bottom of things. The three experimental projects FEAR (Fault Activation and Earthquake Rupture, an EU ERC Synergy Project), MISS (Mitigating Induced Seismicity for Successful Geo-Resources Applications, financed by the Werner Siemens Foundation), and VALTER (VALidating of TEchnologies for Reservoir Engineering, financed by the Swiss Federal Office of Energy) are of particular importance for this phase.
Whereas VALTER focuses on different aspects of building and operating a geothermal reservoir in a safe and efficient way, MISS and FEAR are interested in fundamental earthquake processes – how earthquakes start and stop, how faults slip – and use for this purpose the tiny earthquakes induced by water injections as well as fault stimulations. The installation of a second testbed to host the FEAR and MISS experiments is now starting. For this purpose, the BedrettoLab will be extended by retrofitting additional caverns deeper in the tunnel to double the rock volumes available for experiments. Further, an additional side-tunnel of about 100 m length is planned, making it possible to monitor the target fault at close distance and add new monitoring boreholes equipped with hundreds of sensors. Outside the tunnel, this work will not be noticeable. However, the excavated material is high-quality Granite and will thus be transported from the site for further use. Theses extensions are planned to be completed by summer 2022. 14 October 2021
We have added a new feature to the BedrettoLab website: a publicly available list, which shows all detected earthquakes that were registered in the wider Bedretto area and the Lab itself. It contains all earthquakes, natural or induced, that have been detected since 01.01.2019 by the national seismic network operated by the Swiss Seismological Service at ETH Zurich. You can find it here. Additionally, you can find here more information about the monitoring network. 14 October 2021
On 28 September, we invited the press to the BedrettoLab, because the stimulations within the VALTER project are about to start.
The Validating of Technologies for Reservoir Engineering (VALTER) experiment, funded by the Swiss Federal Office of Energy, tests in unprecedented detail on a realistic scale of several hundred metres innovative technologies and processes to create a commercially viable underground heat exchanger while minimising the risk of major earthquakes.
The media reports from this (and earlier events) are listed here.
We have published the first issue of the BedrettoLab newsletter. In this issue, you will take a glimpse into the deep underground, learn more about the Bedretto Reservoir Project and discover how we collect data at the BedrettoLab.
Curious? You can read, download and sign up for the newsletter here. 2 August 2021
After almost two years of intense work to build the stimulation and monitoring boreholes, the drilling campaign has been finished last week by cementing a steel-casing in ST2. By now, nine boreholes with a total length of some 2’500 meters drilled have been completed - an impressive achievement!
For the drilling in Bedretto a completely new drill-rig has been engineered and was transported brand-new on-site in 2020. Now, after successful completion the drill-rig and its equipment were dismantled, moved out of the tunnel and are now being returned to the drilling company's warehouse.
Currently, the new boreholes are being prepared for the instrumentation, which will start in July 2021. 11 June 2021
As part of the ongoing Bedretto Reservoir Project, high-resolution seismic networks have been installed along the Bedretto main tunnel and in dedicated boreholes to monitor the behaviour of the rock volume and the possible occurrence of seismicity.
The water injections performed in the past two weeks by Geo Energie Suisse (GES) induced numerous very small earthquakes in the vicinity of the borehole – an expected and desired effect of the stimulations. The last stimulation activities took place three days ago, and the induced seismicity near the injection borehole has since decreased and ceased.
The seismic network allows to detect events in the whole tunnel area, and on the night of 6 May 2021, a couple of events have been registered at about 500 meters away from the injection area, in the direction of the entrance of the Bedretto tunnel. The largest event had a magnitude of +0.1, according to the Swiss Seismological Service at ETH Zurich. Therewith, this event is considerably lower than the magnitude threshold above which people can usually notice an earthquake. Only the very dense seismic monitoring system made it possible to record and locate such small events. A full analysis of the data recorded over the past weeks enabled us to detect so far nine events with magnitudes down to ML= -1.8, some of which occurring already several months before the start of the water injections.
Although these events are too small to raise any immediate safety concerns, they provide a unique possibility to understand how pressure perturbations connected to activities in the reservoir and to water flow propagate through the rock volume. The BedrettoLab team has initiated a range of investigations to gain a better understanding of the causes of these unusual events and will use this new knowledge to steer future stimulation activities. 7 May 2021
The Bedretto team continues to investigate the micro-earthquakes observed first on May 6th. In the past five days, three events with a maximum magnitude of -0.5 were observed, plus one event at more than 1 km from the tunnel. Using additional reprocessing steps, the team was able to reconstruct the timeline of the earthquakes of the past two years in more detail. Seismologists use an approach called “template matching”: Once a signal is known, they can scan past data for similar patterns that could not be detected before. This analysis revealed that another seismicity episode occurred unknown to the Bedretto team in October 2020 a few hundred meters further into the tunnel as compared to the May 6th events, with a maximum magnitude of about 0.2. However, as early as March 2020, another episode of very small events of magnitude -1.5 and smaller was detected.
So far, the events in March and October 2020 seem to have no obvious connection to drilling or stimulation activities in the Lab. The events in October followed within a few days of extreme rainfall at Bedretto, so it is possible that the seismicity is related to changes in pore pressures induced by rainfall, but other sequences do not so clearly correlate. Rain induced earthquakes have been observed in other parts of Switzerland (see for example here), mainly in Karst areas where water from the surface can quickly reach seismogenic depths of 1 kilometre or more.
The detective work therefore continues: The Bedretto team is installing additional seismic stations, gathering fluid samples and is re-analysing data, and they are considering hydro-mechanic models to build plausible models for the micro-earthquakes. At the same time, activities in the tunnel continue with microdrilling, notching and stress measurements in one of the long boreholes; activities that do not result in any micro-seismicity. Injections are not planned for the next days.
The detective work on the cause of the observed micro-earthquakes has produced some clues, but further analysis is necessary to better understand the seismicity around the BedrettoLab.
One of the clues is that much of the observed seismicity occurred during a period of heavy rainfalls in the Bedretto region. Rainfall-driven microearthquake triggering is a plausible but somewhat debated scientific hypothesis, and our observations form an interesting test case for the theory. We are currently collecting regional precipitation and meltwater data, to do an in-depth study, with recently developed microearthquake detection algorithms.
Another clue is that microearthquake sequences are not at all uncommon in underground operations such as the ones performed in the BedrettoLab. Activities like drilling and tunnelling can change crustal stresses and pore pressures in the host rock, even at some distance from the tunnel itself. Such perturbations are also likely candidates to drive the observed microquake activity.
The Bedretto team is currently expanding the seismic monitoring network in order to be able to better characterize the microseismic events, not just in the immediate vicinity of the laboratory, but also across a larger region around it.
The reservoir stimulation activities are completed for the moment, which will allow the reservoir system to equilibrate. With the expanded monitoring network, we will be able to even better study the microseismic activity and hunt for further clues. With the additional knowledge about the background seismicity in Bedretto, the team is preparing next stimulations starting in autumn.
After first stimulations in November 2020, in the next days, Geo-Energie Suisse (GES) will perform another set of high-pressure injections at the BedrettoLab using a multi-packer-system. Packers are large rubber sleeves that are lowered into the borehole to divide it into several sections. They allow to stimulate the different sections separately with exactly the pressure and the amount of water needed. This method aims at enhancing the permeability per section in a controlled way while minimizing induced seismicity. GES conducts the so far largest stimulation test at BedrettoLab as part of the DESTRESS project.
For these stimulation tests, GES will use a set of 14 packers (see illustration showing the concept) that are mounted at fixed intervals of 12 to 25 meters. This means, that a larger rock volume will be stimulated than in November, when the packers were installed at narrower intervals. The packers are placed at depths of about 250 to 400 meters in the borehole ST1, whereby only the lowest sections will be stimulated. The goal is to observe how the packer system behaves as a whole and if the packers succeed in separating the sections allowing targeted stimulations.
As for all activities in the BedrettoLab, safety is the top priority also for these stimulations. For this reason, GES and ETH Zurich closely monitors the preparation and implementation of the stimulations. The chances are very small that these stimulations will cause damage to the tunnel (a probability of less than 1 in 10’000) and even smaller that they will cause earthquakes large enough to be felt or cause damage in nearby villages (a probability of less than 1 in 10 million).
After many months of preparations, one of the project partners, GeoEnergie Suisse AG (GES), is now ready to start the first full-scale stimulations in the Bedretto Lab. These stimulations are part of a European Research project DESTRESS that aims at demonstrating so-called soft stimulation techniques for creating a heat exchanger in the underground and extracting geothermal energy.
By injecting a few tens of cubic meter of water into the rock under high pressure, GES aims at creating a network of small fractures that connect the two experimental boreholes. This process triggers very small earthquakes. Eventually, water can then circulate through numerous tiny fractures from one borehole to the other and gradually heat up on its way.
GES has chosen the Bedretto Lab to demonstrate the concept of multi-stage stimulations that they are purposing to use for commercial geothermal plants. To conduct multi-stage stimulations, the borehole is temporarily divided into different sections or stages, which are stimulated separately. GES hopes to have more control on the reservoirs properties by applying this method and to limit the seismic risk.
For all activities in the Bedretto Lab, safety is the top priority. For this reason, ETH Zurich has conducted an extensive seismic risk study to evaluate the chance of damaging earthquakes in consequence of these stimulations. The chance of such earthquakes of magnitude 2 or 2. 5 to occur is estimated to be about 1 in 10 Million. A dense monitoring system has been installed to closely monitor the stimulations. Its data will feed into a set of so-called Traffic Light Systems defining criteria to stop any activity in case certain thresholds are exceeded. They take into account the observed vibrations, the magnitudes of induced events, and the pressure build-up on a nearby fault. One such predefined criteria would be an earthquake of a magnitude of 0.5 or bigger, which would lead to an immediate stop of any ongoing activity.
Besides relying on such predefined thresholds used for classical Traffic Light Systems, a specific goal of the project is to demonstrate and test more sophisticated approaches. To this end, Advanced or Adaptive Traffic Light Systems are implemented taking into account real-time data during the stimulation and assessing whether a stimulation can continue (green), has to be paused (yellow) or must be stopped (red). In addition, we will update the risk assessment after having injected 5m3 of water and decide about the continuation of the stimulations. This process will be repeated after each stimulation of one of the borehole stages. 6 November 2020
Monitoring boreholes were instrumented with geophones, high-frequency accellerometer, piezoelectric In-situ Acoustic Emission (AE) sensors, AE-accelerometer sensor chains, ultrasonic transmitters, Fiber Bragg Grating (FBG) sensors, fibre optic cables, rod system with centralizers, tiltmeters, pore-pressure sensors and mulitpacker. These instruments will monitor the rock volume during fracture generation and water circulation.