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

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.