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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. Plain Language Summary: 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‐Forêts 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. Key Points: Homogenization over multiple stimulations at the Soultz‐sous‐Forêts enhanced geothermal system site of the seismic moment estimates of the induced seismicityThe ratio of the radiated seismic energy and the hydraulic energy is shown to be constant during stimulationsThis 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 [ABSTRACT FROM AUTHOR]

Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, which is significant for thermally dynamic environments such as volcanoes and geothermal reservoirs. Compared with other crustal rocks like granite, volcanic rocks have a complex and variable response to temperature; it remains unclear how thermal microcracks form and how they are affected by temperature. We heated and cooled samples of low‐porosity basalts containing different amounts of microcracks and a porous andesite over three cycles, whilst monitoring microstructural changes by acoustic emission (AE) monitoring and measurement of P‐wave velocity (vP; up to 450°C) and thermal expansion coefficient (TEC; up to 700°C). During the second and third cycles, the TEC was positive throughout and the rate of detected AE was low. In contrast to studies on granite, we measured a strong and reversible increase in vP with increasing temperature (by 15%–40% at 450°C), which we interpret as due to microcrack closure. During the first cycle, AE and vP measurements indicated thermal microcracking within the andesite and the basalt with a low initial microcrack density. For these samples, strong inflexions in the TEC indicated stress relaxation during heating, preceding significant thermal microcracking during cooling. The basalt with a high initial microcrack density underwent little microcracking throughout all cycles. Our results and a review of the literature relate the initial microstructure to the occurrence of thermal microcracking and explore the potentially significant influence of temperature on volcanic rock properties. Plain Language Summary: Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, playing an important role within volcanoes and geothermal reservoirs. Volcanic rocks exhibit a complex response to temperature, and it remains unclear how thermal microcracks form and how they are affected by temperature. Here, we repeatedly heated and cooled samples of basalt and andesite with different initial porosities and microcrack content over three heating/cooling cycles, whilst monitoring for laboratory‐scale seismicity and changes in acoustic wave velocity (up to 450°C) and sample length (up to 700°C). During the second and third cycles, we measured a strong, reversible increase in wave velocity with increasing temperature (by up to 40% at 450°C): opposite to measurements on granite, and which we interpret as due to crack closure during heating. During cycle one, we detect thermal microcracking within the andesite and the basalt with a low initial microcrack population. For these samples, anomalous thermal expansion during heating is linked to the significant thermal microcracking during cooling. In contrast, the initially highly‐microcracked basalt underwent little microcracking throughout. We relate the initial microstructure to the occurrence of thermal microcracking, and explore how rock properties may significantly change with temperature. Key Points: One basalt cracked significantly during cooling, following an anomalous thermal expansion, indicative of stress relaxation, when heatedAll samples see an increase in wave velocity with temperature during repeated heating, attributed to microcrack closure during heatingOur data literature study suggest low‐porosity volcanic rocks with high P‐wave velocities are most susceptible to thermal microcracking [ABSTRACT FROM AUTHOR]

Abstract: To monitor both the permanent (thermal microcracking) and the nonpermanent (thermo‐elastic) effects of temperature on Westerly Granite, we combine acoustic emission monitoring and ultrasonic velocity measurements at ambient pressure during three heating and cooling cycles to a maximum temperature of 450°C. For the velocity measurements we use both P wave direct traveltime and coda wave interferometry techniques, the latter being more sensitive to changes in S wave velocity. During the first cycle, we observe a high acoustic emission rate and large—and mostly permanent—apparent reductions in velocity with temperature (P wave velocity is reduced by 50% of the initial value at 450°C, and 40% upon cooling). Our measurements are indicative of extensive thermal microcracking during the first cycle, predominantly during the heating phase. During the second cycle we observe further—but reduced—microcracking, and less still during the third cycle, where the apparent decrease in velocity with temperature is near reversible (at 450°C, the P wave velocity is decreased by roughly 10% of the initial velocity). Our results, relevant for thermally dynamic environments such as geothermal reservoirs, highlight the value of performing measurements of rock properties under in situ temperature conditions. [ABSTRACT FROM AUTHOR]