Induced Seismicity Controlled by Injected Hydraulic Energy: The Case Study of the EGS Soultz‐Sous‐Forêts Site

Abstract

How induced seismicity in deep geothermal project (enhanced geothermal systems, EGS) is controlled by fluid injection is of central importance for monitoring the related seismic risk. Here we analyze the relationship between the radiated seismic energy and the hydraulic energy related to the fluid injection during several hydraulic stimulations and circulation tests at Soultz‐sous‐Forêts geothermal site. Based on a harmonized database, we show that the ratio between these energies is at first order constant during stimulations and of the same magnitude independently of the stimulation protocol and injection depth. Re‐stimulations are characterized by a sharp evolution of this ratio during injection which ultimately converges to the characteristic value of the reservoir. This supports that the seismicity is caused by the relaxation of the pre‐existing strain energy in the stimulated volume, rather than by the deformation generated from fluid injection. The ratio appears as an intrinsic large‐scale property of the reservoir that can be assessed at the very beginning of the first stimulation. Based on this property, we suggest a way to predict the largest magnitude of the induced seismic events knowing the maximum targeted hydraulic energy of the injection.