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

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-For & ecirc;ts 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. Monitoring and predicting fluid-injection-induced seismicity is a major concern for the development of the geothermal energy. Several geothermal sites have shown a linear relationship between the radiated seismic energy and the hydraulic energy, suggesting that these two quantities are useful for seismic monitoring and prediction. Here we study the relationship between these two quantities for different fluid injections carried out in the Soultz-sous-For & ecirc;ts enhanced geothermal systems reservoir to see if it remains similar despite the changes and setting in the injection strategy (flow-rate, pressure of the injected fluid) and depths. Based on a harmonized database, we show that the ratio between the radiated seismic energy and hydraulic energy is at first order constant in time during stimulations and of the same magnitude independently of the stimulation strategy and depth. This ratio can then be seen as an intrinsic large-scale reservoir property that can be estimated from the early stages of the first stimulation. Using this reservoir property, we propose a method for predicting the maximum magnitude of induced seismic events given the maximum hydraulic energy planned. Homogenization over multiple stimulations at the Soultz-sous-For & ecirc;ts enhanced geothermal system site of the seismic moment estimates of the induced seismicity The ratio of the radiated seismic energy and the hydraulic energy is shown to be constant during stimulations This ratio can be seen as a reservoir property describing its seismic response to fluid injection and is used to predict the largest magnitude of induced earthquakes