Induced Seismicity in Geothermal Settings: Historical Perspective, Enhanced Geothermal Systems, Setbacks, and Future Directions

ABSTRACT: Geothermal energy exploration and production has a long history of inducing seismicity, both beneficial and disastrous. Seismicity is common in naturally occurring geothermal systems as they often are co-located in areas with high strain rates, low permeability but fractured rocks, and a shallow brittle-ductile transition. These conditions generally limit the maximum magnitudes of earthquakes, whether tectonic or induced. Furthermore, seismicity often aids in the identification of prominent fluid and heat pathways within the geothermal resource. Engineered or Enhanced Geothermal systems (EGS) need not be located in geologic settings akin to conventional geothermal resources and, thus, may have unknown fault and fracture locations and extents owing to lower tectonic seismicity prior to exploration and thicker crust increasing the likelihood of larger magnitude seismicity. Several EGS projects have experienced earthquakes during the exploration portion of the projects that caused significant damage, e.g. Basel, Switzerland (2006) and Pohang, South Korea (2017). Other more recent EGS projects have experienced little seismicity of concern for the safe operation of reservoir development, e.g. Milford, Utah (2022) and Blue Mountain, Nevada (2022). Both sites benefited from experience gained in previous EGS projects, extensive site characterization, and adherence to Best Practices developed in response to earlier unsuccessful projects. In all cases, probabilistic seismic hazard analyses (PSHA) are crucial steps during planning and stimulation. These generally include a variety of models that can range from simple relationships between expected seismicity and injected volume to detailed geomechanical models of resulting seismicity. Aseismic deformation is seldomly accounted for, although recent research suggests it plays a significant role in the deformation in geothermal resources. Here I will discuss the inclusion of deterministic reservoir models in standard PSHA models and focus on the effects of aseismic deformation in geothermal and EGS settings. Continual improvement in PSHA and reservoir exploration and stimulation that are part of EGS development can ensure that the vast potential of EGS can be unlocked safely.