Induced Seismicity in Geothermal Reservoirs : Physical Processes and Key Parameters

In order to reach Europe’s 2020 and 2050 targets on greenhouse gas emissions, geothermal resources have to contribute substantially to carbon-free energy needs. Deep geothermal developments, however, are often accompanied by induced seismicity due to stimulation. The induced seismicity can be a threat for the development of future large scale application of deep geothermal power plants. Therefore, understanding the physical processes at the origin of the seismicity induced by forced fluid circulation in geothermal fields is essential, and this paper reviews the current knowledge in connection with field cases. The driving force of a seismic event is a change of the stress state in the crust. To asses this quantitatively, one needs to know the initial stress state, the spatio-temporal stress changes, the failure criterion, and the rupture dynamics that describes how a seismic event is produced. Several existing geomechanical-numerical models are, in theory, capable of predicting the spatio-temporal changes of the stress state and few of their effects on induced seismicity. They consider coupling between geomechanical, fluid flow, and heat transport processes, with different levels of complexity. The characteristics of the recorded seismicity induced in geothermal fields play a major role to calibrate and to assess the models. The Soultz-sous-Forets enhanced geothermal system in France, where thousands of seismic events were induced during stimulations, is an example representative for fields developed in deep crystalline rocks. In such formations, induced seismicity mainly occurs on a network of pre-existing faults and fractures oriented in accordance with the stress field, and shearing on these structures is apparently the dominating failure mechanism. Several physics-based models have been tested on this well-documented field to reproduce the observations. By contrast, the hydraulic fracturing operations carried out in Gros Schonebeck (Germany) geothermal reservoir, which is located in sedimentary formations at a depth similar to Soultz-sous-Forets, induced very few and weak seismic events. This behavior is consistent with a less seismogenic tensile fracture opening as being the dominant failure mechanism. Interestingly, the few recorded and located seismic events at this site likely occurred on a pre-existing fault. To characterize a geothermal field with regards to the expected induced seismicity, the following key factors are proposed: natural seismicity at a field scale, stress field, structural fracture/fault characterization, rock type, history of pressure and injection/circulation rates, and past induced seismicity. To mitigate induced seismicity during major hydraulic stimulations, and to prevent large-magnitude event occurrence once injection stopped, early-warning systems and decision support systems are required. To feed these systems, we advocate the application of hybrid methods and the development of fast models. Hybrid methods would combine the best a priori knowledge of the expected behavior of the field underground, inherited from geomechanical-numerical modeling, with a statistical approach based on the real-time observation of the induced seismicity. Fast models would capture the essential physics while minimizing computing time. The quantitative understanding of induced seismicity, however, remains a challenging and complex matter. Only an integration of all current research and development efforts, in the fields of modeling, measuring, monitoring, and matching, will make a chance on success.