Developing geoenergy technologies such as Enhanced Geothermal Systems (EGS) requires underground fluid injection operations, which, under certain conditions, can induce large-magnitude earthquakes. To mitigate the seismic hazard, various injection protocols have been proposed to regulate operational parameters. This study evaluates the impact of injection protocol on induced seismic hazard, using theoretical models, numerical simulations and field data. Within the theoretical framework, perturbed rock volume was inferred from the concept of triggering front that serves as a proxy for pressure perturbation, whereas numerical modeling captured the spatio-temporal evolution of pore-pressure. Our results indicate that short-duration injection protocols are likely characterized by lower seismic hazard, as they perturb smaller areas of pre-existing critically stressed faults. This decreases the likelihood of larger ruptures, that might propagate beyond the pressurized rock volume. Given the same (net) injected fluid volumes and geological conditions, the duration emerges as a key factor controlling the extent of the perturbed rock mass. The findings are further illustrated by the 2017 ML 5.4 Pohang earthquake, which was triggered by the hydraulic stimulation of the nearby EGS. Previously in 2006, the injection of roughly similar fluid volume in Basel induced an earthquake of magnitude ML 3.4. This difference in energy release is likely linked to the duration of the injection protocols, being approximately 600 days at Pohang and 6 days at Basel. Our findings highlight the importance of injection protocol, detailed subsurface characterization and real-time seismic monitoring of perturbed rock volumes to mitigate the seismic hazard during EGS developments.
