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Post-injection seismicity remains a key challenge for the sustainable deployment of enhanced geothermal systems (EGS), as seismic activity may persist or even intensify after injection has ceased. This behaviour was observed at the Basel, Switzerland, and Pohang, South Korea, EGS development sites, where the maximum magnitudes of M3.2 and M5.4, respectively, occurred after reservoir stimulation and ultimately led to project cancellation. We here develop fully coupled hydromechanical simulations to investigate the physical mechanisms controlling delayed fault slip and to evaluate the potential of wellbore bleed-off as a commonly applied mitigation strategy. The model represents stimulation of a fractured reservoir interacting with a nearby fault under conditions of hydraulic connectivity or isolation. We find that poroelastic stress transfer and associated undrained pressure buildup largely govern fault stability during injection. After stopping injection, however, continued pore-pressure diffusion, promoted by dilation-induced permeability enhancement along fractures and the fault, can progressively load critically stressed fault segments, leading to delayed rupture on timescales of weeks to months. While bleed-off efficiently reduces pressure in the near-well region, its influence rapidly decays with distance. Bleed-off may even advance the onset of slip under both hydraulic-connection scenarios by relaxing the stabilising poroelastic stresses and facilitating pressure migration along the dilated fault. Our results show that bleed-off can successfully suppress post-injection seismicity only when the nucleation region is in close proximity to the injection zone. These findings emphasise the importance of accurate subsurface characterisation and optimised design of stimulation and mitigation strategies considering the underlying coupled processes to limit unintended pressure propagation into regions where seismicity is less controllable.
| References | Kivi, I. R., Vilarrasa, V., Kim, K. I., Yoo, H., & Min, K. B. (2025). Bleed-off control on post-injection seismicity in enhanced geothermal systems. Underground Space, 22, 21-38. Kivi, I. R., Vilarrasa, V., Kim, K. I., Yoo, H., & Min, K. B. (2024). On the role of poroelastic stressing and pore pressure diffusion in discrete fracture and fault system in triggering post-injection seismicity in enhanced geothermal systems. International Journal of Rock Mechanics and Mining Sciences, 175, 105673. Kivi, I. R., Boyet, A., Wu, H., Walter, L., Hanson-Hedgecock, S., Parisio, F., & Vilarrasa, V. (2023). Global physics-based database of injection-induced seismicity. Earth System Science Data, 15(7), 3163-3182. |
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| Country | United Kingdom |
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