• Energy Research

With the global challenge to satisfy an increasing demand for energy while at the same time stabilizing or reducing carbon dioxide (CO2) concentrations in the atmosphere, geothermal energy from enhanced geothermal systems (EGSs) increasingly is being recognized as an attractive alternative energy source throughout the world. However, the risks associated with the seismicity necessarily induced during the development of an EGS constitute a significant challenge for the widespread implementation of this technology. This article provides a preliminary overview of lessons learned from an attempt to develop an EGS beneath the city of Basel, Switzerland.

The seismic risk associated with deep fluid injection in Enhanced Geothermal Systems can be mitigated by stopping reservoir stimulation when the seismic risk becomes unacceptable or by reducing production flow rates when seismicity occurs during the operational phase. So far, none of these mitigation measures have been included in the Levelized Cost Of Electricity. A meta-model is introduced that estimates the optimal price of electricity, based on an analytical geothermal energy model, and updates this cost to include the outlay for mandatory seismic risk mitigation measures. The proposed energy model computes both electricity production and heat credit. The costs added during reservoir stimulation are based on the probability of abandoning an injection well, based on a traffic-light system, defined as the ratio of scenarios that exceed a given seismic safety threshold in the risk space. In the production phase, the net energy generated is reduced by clipping the production flow rate so that the reservoir's overpressure does not exceed the regional minimum effective stress. Based on a generic geothermal triplet, we investigate the trade-off between heat credit and seismic risk mitigation cost. The added cost, mostly due to financial risk aversion, shifts the optimal site for a plant from between a few kilometres to tens of kilometres away from populated areas, for increasingly vulnerable building stocks. Finally, using a simple yet realistic optimisation strategy, we study the role that a seismic safety standard plays for determining the number of EGS plants that can be sited in a given region. Applied Energy, 238 ISSN:0306-2619 ISSN:1872-9118

AbstractWe analyze slip distribution and rupture kinematics of a Mw3.3 induced event that occurred in the St. Gallen geothermal reservoir (NE Switzerland) in 2013. We carry out a two‐step procedure: (1) path effects are deconvolved from the seismograms using an empirical Green's function, resulting in relative source time functions at all seismic stations; (2) the relative source time functions are back‐projected to the corresponding isochrones on the fault plane. Results reveal that the mainshock rupture propagates toward NNE from the hypocenter with an average velocity of 2,000 m/s. Spatiotemporal organization of foreshocks and aftershocks shows that the mainshock broke a previously less active portion of the fault and suggests that the aftershock sequence could be mainly driven by stress transfer. Applying this method in an operational environment could enable fast retrieval of seismic slip, allowing assessment of fault asperities and structures involved in the reservoir creation process.

Slip model and Relative Source Time Functions for the the 2013 Mw 3.3 St. Gallen earthquake

AbstractSeismic monitoring of the Basel Enhanced Geothermal System has been running for more than a decade. Yet the details of the long‐term behavior of its induced seismicity remained unexplored because a seismic event catalog with consistent detection sensitivity and magnitudes did not exist. This knowledge is essential for developing guidelines and mitigation procedures on how to safely operate and terminate injection activities. Only few observational data exist that cover all phases of such projects in a consistent manner. Here we describe a method that overcomes these deficiencies based on sensitive matched filter detection and a machine learning approach to remove false detections. With an emphasis on consistency, we create a catalog that contains more than 280,000 events down toMw−1.5. The much higher temporal resolution allows us to analyze induced microearthquakes in great detail and to gain new insights. We resolved temporal variations of seismicity parameters and, in the post‐operational phase, a preferential temporal clustering of events. We find a breakdown in the Gutenberg‐Richter scaling during reservoir stimulation, which may have physical reasons or could be caused by a method‐independent detection limit during high event rates. The scaling breakdown has implications for the design of Adaptive Traffic Light Systems and may limit the potential of real‐time mitigation strategies in future Enhanced Geothermal System projects. Nevertheless, our catalog gives the opportunity to study the temporal evolution of the sequence in unprecedented detail, which will help to better understand the physical processes in a geothermal reservoir, potentially not only in the Basel case.

AbstractInduced earthquakes pose a substantial challenge to many geo-energy applications, and in particular to Enhanced Geothermal Systems. We demonstrate that the key factor controlling the seismic hazard is the relative size distribution of earthquakes, the b-value, because it is closely coupled to the stress conditions in the underground. By comparing high resolution observations from an Enhanced Geothermal System project in Basel with a loosely coupled hydro-mechanical-stochastic model, we establish a highly systematic behaviour of the b-value and resulting hazard through the injection cycle. This time evolution is controlled not only by the specific site conditions and the proximity of nearby faults but also by the injection strategy followed. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.