Near-realtime seismic monitoring of fluid injection allowed control of induced earthquakes during the hydraulic stimulation of a 6.1 km deep geothermal well near Helsinki, Finland in the frame of the St1 Deep Heat project. The stimulation was monitored in near-real time using a deep seismic borehole array and series of borehole stations located nearby the injection well. Earthquakes were processed within a few minutes and results informed a Traffic Light System. Using near-realtime information on evolution of seismic and hydraulic energy, pumping was either stopped or varied, following the theoretical predictions from a physics-based model of maximum magnitude. This procedure effectively avoided the nucleation of a project-stopping red alert at magnitude M2.1 induced earthquake, a limit set by the TLS and local authorities. The presented results suggested a possible physics-based approach to controlling stimulation-induced seismicity in geothermal projects.
In this study we discuss evolution of different parameters derived from seismic catalogs with injection parameters from several hydraulic stimulation projects including St1 Deep Heat, and conduct laboratory fluid injection experiments on permeable sandstone samples containing a critically stressed fault at different fluid pressurization rates. This includes relations between seismic energy release, operational parameters, and structural inventory of the reservoir, to shed the light on potential physics-based early identification of increased seismic hazard associated with fluid injection.
We found the majority of field stimulation campaigns investigated reveal a clear linear relation between injected fluid volume, hydraulic energy and cumulative seismic moments suggesting extended time-spans during which induced seismicity evolution is pressure-controlled. The evolution of seismic moment seems independent of the tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory (e.g. limited fracture network, existence of major faults). For most projects studied, the observations are in good agreement with existing physical models that predict a relation between injected fluid volume and maximum seismic moment of induced events. However, some stimulation projects reveal unbound increase in seismic moment suggesting that for these cases evolution of seismicity is mainly controlled by stress field, the size of tectonic faults and fault connectivity.
Similar to field studies, laboratory fluid-induced experiments reveal that the total seismic energy release does relate to the total injected volume, independent of actual macroscopic fault slip behavior (including a mixture of creep, stable slip, and stick-slip behavior), while the seismic energy rate scales with measured fault slip velocity. The cumulative seismic energy shows a linear scaling with injected volume for stable slip (steady slip and fault creep) while we find a cubic relation for dynamic slip (stick-slip).
Both laboratory and field studies suggest that observed fluid-induced deformation is dominantly aseismic. Data suggest that magnitudes can grow either in a stable way, indicating the constant propagation of self-arrested ruptures, or be generally unbound for which the maximum magnitude is only limited by the size of tectonic faults and fault connectivity. Transition between the two states may occur at any time during injection, or not at all, as evidenced by high resolution data from St1 Deep Heat. Monitoring during stimulations need to account for the possibility of unstable rupture propagation from the very beginning of injection by observing the entire seismicity evolution in near‐real time and at the maximum resolution for an immediate reaction in injection strategy.