Energy transition and the challenge of induced seismicity

Induced seismicity is likely a major obstacle in front of the widespread deployment of geoenergy applications, such as geothermal energy or geologic carbon storage (GCS), which are indispensable components of efforts to mitigate the climate change emergency. Induced earthquakes may jeopardize the integrity of subsurface structures and, if felt at the surface, negatively impact the public perception of geoenergy projects. Thus, the effective and safe use of the subsurface to provide clean and sustainable energy and reduce atmospheric carbon emissions needs to properly address the risks and hazards posed by induced seismicity. In this Outstanding Early Career Scientist ‎Award Lecture of the ERE‎ Division, I discuss some important topics of induced seismicity in low-carbon geoenergies. First, I explain the potential mechanisms of seismic events that are unexpectedly induced far away from and/or long after operations related to geothermal energy developments. Such seismic sequences have been found problematic because of partial loss of control over their management. In particular, ‎thermal stress is key in reactivating distant faults from a fluid circulation doublet after several years ‎of operation in hydraulically bounded and unbounded hot deep sedimentary aquifers. The observed delays can be explained by the relatively large characteristic time scales of thermal effects (small thermal diffusivity). In enhanced ‎geothermal systems, a sequence of processes, which can be identified when explicitly including fractures in numerical models, may give rise to post-injection seismicity. The stabilizing effect of poroelastic stress generated during reservoir ‎stimulation rapidly attenuates after stopping injection, while the injection overpressure gradually diffuses ‎away, which could bring distant faults to slip conditions with time delays as long as several months. Interestingly, bleed-off, i.e., ‎flow back to relieve wellbore pressure,‎ as an industrial practice to prevent post-injection seismicity may not effectively work under certain conditions. This is because the ‎stimulated fractures become progressively less responsive to hydraulic perturbations with distance from ‎the wellbore. In the second part of my presentation, I discuss induced seismicity within GCS at the gigatonne scale. Analysis of data from the global, multiphysics database of induced seismicity underscores some similarities between large-scale GCS and massive wastewater disposal that led to a drastic rise in seismic activity in central and eastern US in the 2010s – not to negate fundamental differences between the two technologies. Although GCS at the megatonne scale has been extensively demonstrated, its scale-up could face elevated risk of induced seismicity. We have developed the open-source tool CO2BLOCKSEISM that employs simplified physics models for screening subsurface CO₂ storage resources at regional scales constrained by the risk of induced seismicity. The tool’s application is shown within the Utsira storage unit in the North Sea. Induced seismicity draws a more restrictive and ‎realistic limit to the storage resource use at regional than at single-site scales. I conclude that reliable methodologies for induced seismicity forecasting and mitigation should be developed in light of the underlying physics and continuous characterization of the subsurface during operations to safely unlock the huge potential of the subsurface for a timely approach toward climate targets.