Abstract
AbstractAnalysis of past and present stimulation projects reveals that the temporal evolution and growth of maximum observed moment magnitudes may be linked directly to the injected fluid volume and hydraulic energy. Overall 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. Data suggest that magnitudes can grow either in a stable way, indicating the constant propagation of self‐arrested ruptures, or 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. Monitoring and traffic light systems used 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 high resolution for an immediate reaction in injection strategy.
Highlights
The effort to reduce production of greenhouse gases, especially in Europe, has fueled the search for clean and renewable energy sources
Two large‐scale Enhanced Geothermal Systems (EGS) projects in Australia have been operational for years, and their remote location has prevented any acceptance issue arising from induced seismicity (Albaric et al, 2014; Baisch et al, 2006)
For most of the respective stimulation period, all analyzed data sets show an increase of maximum observed moment magnitude Mmax,obs with injected volume that roughly fits to a linear trend in a double logarithmic plot (Figure 1a)
Summary
The effort to reduce production of greenhouse gases, especially in Europe, has fueled the search for clean and renewable energy sources. Nucleation and propagation of earthquake ruptures related to fluid injection pose fundamental questions related to understanding and controlling injection‐induced seismicity. Models such as McGarr (1976) assumed that volume changes induce local changes in deviatoric stresses, which are relaxed by induced earthquakes. Galis et al (2017) found a fracture mechanics‐based scaling relationship between maximum expected magnitude of self‐arrested events and injected volume. These models predicting maximum magnitudes of induced events implicitly assume a stable rupture propagation process (Galis et al, 2017). Our analysis provides insight about how seismic moment and related parameters of induced events evolve during injection and how these parameters are affected by tectonic faulting regime
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