Insights from Subsurface Monitoring for Engineering of the Stimulation Pattern in Fractured Reservoirs

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.