Updating induced seismic hazard assessments during hydraulic stimulation experiments in underground laboratories: workflow and limitations

Abstract

Advancing technologies to harvest deep geothermal energy has seen backlash related to unacceptable levels of induced seismic hazard during hydraulic stimulations. A thorough analysis of induced seismic hazard before these operations has recently become standard practice in the last decade. Additionally, more process understanding of the underlying causes of induced seismicity as well as novel approaches to develop geomechanical reservoirs are being explored in controlled underground laboratory experiments worldwide. Here, we present a probabilistic analysis of the seismic hazard induced by the ongoing hectometre-scale stimulation experiments at the Bedretto Underground Laboratory for Geoenergies and Geosciences (BULGG). Our workflow allows for fast updates of the hazard computation as soon as new site-specific information on the seismogenic response (expressed primarily by the feedback afb value and the Gutenberg-Richter b value) and ground motion models (GMMs) become available. We present a sequence of hazard analyses corresponding to different project stages at the BULGG. These reveal the large uncertainty in a priori hazard estimations that only reduces once site-specific GMMs and information on the seismic response of specific stimulation stages are considered. The sources of uncertainty are (1) the large variability in the seismogenic response recorded across all stimulation case studies and (2) uncertain GMMs on the underground laboratory scale. One implication for large-scale hydraulic stimulations is that hazard computation must be updated at different project stages. Additionally, stimulations have to be closely accompanied by a mitigation scheme, ideally in the form of an adaptive traffic light system (ATLS), which reassesses seismic hazard in near-real time. Our study also shows that the observed seismogenic responses in underground laboratories differ from large-scale stimulations at greater depth in that the seismogenic response is substantially more variable and tends to be weaker. Reasons may be lower stress levels, but also smaller injected volumes accessing a more limited fracture network than large-scale stimulations. Controlled underground laboratory experiments can contribute to improving our understanding of the the physical reasons leading to such variable seismogenic responses. The presented analysis implied that such experiments may be limited in terms of upscaling but are likely to be safe in terms of induced seismic hazard.