We performed a series of hydraulic stimulations at 1.1 km depth in the Bedretto underground laboratory, Switzerland, as part of an overall research strategy attempting to understand induced seismicity on different scales. Using an ultra‐high frequency seismic network we detect seismic events as small as M_w < −4, revealing intricate details of a complex fracture network extending over 100 m from the injection sites. Here, we outline the experimental approach and present seismic catalogs as well as a comparative analysis of event number per injection, magnitudes, b‐values, seismogenic index and reactivation pressures. In our first‐order seismicity analysis, we could make the following observations: The rock volume impacted by the stimulations in different intervals differs significantly with a lateral extent from a few meters to more than 150 m. In most intervals multiple fractures were reactivated. The seismicity typically propagates upwards toward shallower depth on parallel oriented planes that are consistent with the stress field and seem to a large extent associated with preexisting open fractures. This experiment confirms the diversity in seismic behavior independent from the injection protocol. The overall seismicity patterns demonstrate that multi‐stage stimulations using zonal isolation allow developing an extended fracture network in a 3D rock volume, which is necessary for enhanced geothermal systems. Our stimulations covering two orders of magnitude in terms of injected volume will give insights into upscaling of induced seismicity from underground laboratory scale to field scale.

Stimulation operations enhance the performance of geothermal reservoirs by enhancing fluid flow and heat transfer. Predicting stimulation outcomes is challenging due to the complexity of reservoir properties and limited observations given by operational conditions. The stress state, natural geological structures, pressure distribution, and injection protocols play crucial roles in the engineering of a stimulation operation. This study provides in-depth observations from a hectometer-scale stimulation experiment conducted at the Bedretto Underground Laboratory for Geosciences and Geoenergies within a densely monitored crystalline rock volume with an overburden of more than 1 km. We found that hydraulic connectivity, pressure compartments, and the geomechanical characteristics of existing geological structures play important roles in the propagation patterns of seismic events. Notably, the initiation and distribution of seismicity are markedly influenced by the zonal pressure isolation and the propagation of nominal pressure diffusion fronts across the reservoir. Our findings highlight the necessity of adapting stimulation strategies according to the unique geomechanical and geological characteristics of the reservoir. This claim is supported by the distinct activation patterns observed between the first and second injection cycles in the current case study. The spatial extent of the stimulated volume can be partially controlled by the number of stimulation cycles and injection pressure level, as farther structures are more likely to be activated in the subsequent cycles. The results also indicate that the Kaiser effect can be attenuated due to changes in the flow pathway and stress caused during stimulation, consistent with a proposition from a recent study. Our findings underscore the critical importance of the interplay between hydraulic pressures and stress states to optimize the stimulation of fractured reservoirs.

Although Aquifer Thermal Energy Storage (ATES) systems are widely researched, Fractured Thermal Energy Storage (FTES) systems are comparatively underexplored. This study presents a detailed numerical model of a fractured granitic reservoir at the Bedretto underground laboratory in Switzerland, developed using COMSOL Multiphysics. Energy efficiency was evaluated across different flow rates and well configurations, including single-well and doublet systems, as well as for two different temperatures, namely 60 °C and 120 °C. The doublet configuration at an injection temperature of 60 °C with a flow rate of 2 kg/s demonstrated the highest energy efficiency among the cases studied. Potential applications for the stored heat are discussed, with scenarios including district heating for the nearby village and greenhouse heating. The results show that although FTES is associated with unique challenges, it has significant potential as a reliable thermal energy storage method, particularly in regions without suitable aquifers. It can also be considered as a cost-effective and competitive approach for climate mitigation (assuming the system is solely powered by solar-PV). This study provides insights into the viability and optimization of FTES systems and highlights the role of fracture/fault properties in enhancing energy efficiency.

Fractures can greatly impact fluid flow and pore pressure distribution in the upper crust. By conducting borehole hydraulic experiments, we can determine the ability of fractures to transport fluids and affect pressure diffusion under in situ conditions. We performed a series of periodic hydraulic tests in a borehole at the Bedretto Underground Laboratory for Geosciences and Geoenergies (Switzerland). We investigated the period‐ and pressure‐dependent response of a fractured granite by controlling the flow rate of harmonic and non‐harmonic oscillations, covering a wide range of periods as well as mean‐ and amplitude‐pressure values. By analyzing the phase and amplitude relation between the flow rate and fluid pressure in the injection interval, we found that the intersected fracture zone exhibits a period‐dependent hydraulic response close to radial flow perturbed by fracture‐related hydromechanical coupling effects. The main features of the interval's hydraulic response were reproduced by an uncoupled diffusion solution for radial flow, which was used to derive apparent hydraulic properties. The largest deviations from this model occur for the phase shifts and correlate with the pressure amplitude. This non‐linear expression of hydromechanical coupling associated with deformable fractures was interpreted in terms of a pressure‐dependent effective compliance of the fractures that affects their effective storativity. The period dependence is likely related to the spatial heterogeneity expected in fractured rocks and the varying sensitivity of the hydraulic response for different periods. Accounting for these effects is necessary for a correct hydraulic characterization in fractured environments and is valuable for inferring fracture mechanical behavior.

Stimulation operations enhance the performance of geothermal reservoirs by boosting fluid flow and heat transfer. Predicting stimulation outcomes is challenging due to complexity of reservoir properties and limited observations given by operational conditions. Factors like stress state, natural structures, pressure distribution, and injection patterns play crucial roles in engineering of a stimulation operation. This study provides in-depth observations from a hectometer-scale stimulation experiment conducted at the Bedretto Underground Laboratory for Geosciences and Geoenergies within a densely monitored crystalline rock volume with an overburden of more than 1 km. We found that hydraulic connectivity, pressure compartments, and the characteristics of existing geological structures play pivotal roles in the propagation patterns of seismic events. Notably, the initiation and distribution of seismicity are markedly influenced by the zonal isolation and the structured propagation of pressure front across the reservoir. The research highlights the necessity of adapting stimulation strategies to the unique geomechanical as well as geological characteristics of the reservoir, as evident from the distinct activation patterns observed between the first and second injection cycles. The spatial extent of the stimulated volume can be partially guided by the number of stimulation cycles and injection pressure level, as farther structures are increasingly likely to be activated in the subsequent cycles. The results also indicate that the Kaiser effect is more pronounced in regions closer to the injection borehole. However, this effect can be attenuated due to stress changes caused by stimulation, consistent with a proposition from a recent study. Our findings underscore the critical importance of understanding the interplay between hydraulic pressures and stress states to optimize the stimulation of geothermal reservoirs effectively.

Fractures can significantly impact fluid flow and pore pressure distribution in the subsurface. Understanding the mechanisms and conditions influencing their ability to transport fluids and to promote pore pressure diffusion is key for many activities relying on fracture-controlled flow such as, for example, enhanced geothermal systems. In situ characterization of these properties is typically done by performing hydraulic tests in selected intervals of a borehole and their interpretation relies on the solution of a linear pressure diffusion equation. However, it has been shown that the hydraulic behavior of fractures as well as the associated near borehole flow regimes can be largely affected by the coupling between the solid deformation and fluid pressure upon injection/production. In this work, we explore these effects by performing a series of harmonic injection tests (HIT) as well as non-harmonic production tests (NHPT) in a packed-off interval of a borehole containing multiple natural fractures. The borehole is located in the Bedretto Underground Laboratory for Geosciences and Geoenergies in Switzerland and penetrates granitic rock mass. The two kinds of tests consist of a periodic repetition of the same injection or production protocol. Flow rates, interval pressures as well as pressures above and below the double-packer probe are recorded at the surface. An important advantage of periodic testing is that it permits a continuous tracking of hydraulic changes during the test. For our study, we conducted a so-called injectivity analysis, in which the phase-shift (time delay) and amplitude ratio between flow rate and interval pressure are used to infer effective hydraulic properties. We performed over 200 periodic tests including both HIT and NHPT with a large range of periods (7.5 s to 1800 s) as well as varying mean interval pressures (~1300 kPa to 2100 kPa) and flow oscillation amplitudes. As a result, we obtained a robust constraint of the radial flow regime prevailing in the fractures. Overall, we found that results from HIT and NHPT are in very good agreement despite the remarkably different injection protocols. For all cases, a prominent and consistent period dependence of phase shifts and amplitude ratios of flow rates and interval pressure was observed, in which both increase as the oscillatory period decreases. Amplitude ratios showed almost no variation with mean interval pressure regardless of the injection protocol. In contrast, a prominent pressure dependence of the phase shifts is captured by the HIT but not the NHPT data. Using the pressure-independent NHPT results, we reconstruct the general hydraulic response of the tested fractured section, which can be well represented by an analytical solution of the pressure-diffusion equation. This general trend explains the HIT data as well, although evidence of significant variations that are correlated with the amplitude of the pressure oscillations points to the predominant role of hydromechanical coupling effects on the fluid pressure diffusion process.