Geothermal Engineering Integrating Mitigation of Induced Seismicity in Reservoirs
Geothermal energy will play an important role in this future energy mix as it is independent of season, day, time and weather and thus is capable of supplying a base-load energy. Geothermal resources have been used successfully and economically in some locations in Europe where geological conditions are exceptionally favourable (e.g. Italy and Iceland), but they can play a much more important role at the European scale, if they can be made accessible at other places. However, the need for Community support is essential to overcome initial barriers. The exploitation of these resources can become economically viable and competitive, if a number of problems are solved. One of the key issues addressed by this call and the project is the need to inject fluids to enhance productivity of a geothermal well (Enhanced Geothermal Systems, EGS), which often induces seismicity. Earthquakes induced by human activities occur in most mining-related operations, depletion of oil and gas reservoirs, fluid injection in the subsurface and dam impoundment and they often reduce public acceptance of such ventures. Two promising geothermal projects jeopardized by this problem are at Soultz-sous-Forêts and, most recently, at Basel. In the latter case, repeated seismic events, although not destructive, were felt by the local population and prompted the authorities to halt operations. To avoid these problems, action has to be taken in order to better understand and mitigate induced seismicity in the development of geothermal reservoirs.
The aim of the project is to contribute to the improvement of the concept of Enhanced Geothermal Systems by addressing the need to investigate the role of induced seismicity, which is twofold:
- as an instrument to image fluid pathways induced by hydraulic stimulation treatments, which has been done to some extent in previous projects; and
- as an implication of such treatments to potential seismic hazards.
The project GEISER is broken down into seven workpackages:
WP 1: Project Management.
WP 2: Compilation of induced seismicity data from geothermal sites.
WP 3: Analysis of Induced Seismicity.
WP 4: Understanding the Geomechanical Causes and Processes of Induced Seismicity.
WP 5: Seismic Hazard Assessment.
WP 6: Strategies for EGS operations with respect to Induced Seismicity (Mitigation).
WP 7: Dissemination.
Our contribution to the project GEISER is defined under the framework of the WP5 and is dedicated to “the assessment of seismic hazard associated to EGS triggered seismicity”. Using the outputs of work packages WP2, WP3, and WP4, we
- develop an approach to quantify the probability of triggering a larger earthquake (M>4) ahead of its natural time of occurrence, either during the stimulation phase or during the long-term EGS operation, on the basis of the statistical analysis of seismic sequences already occurred in similar geological and stimulation conditions (assembled in WP2 and WP3) and of the results of synthetic catalogues and geomechanical models (developed in WP4),
- consider the uncertainties of the above mentioned model assumptions to produce a representative suite of models, to be built as epistemic uncertainties in a logic tree approach,
- take into account all relevant information and seismotectonic indicators on the proximity of active faults, density of faults, stress field etc.,
- quantify the variation in seismic activity rates due to enhanced probability of triggered earthquakes, with related uncertainties
- identify exclusion zones and exclusion rules as condition for the EGS licensing, such as maximum borehole depth and minimum distance of boreholes and of induced seismicity from known seismogenic faults.