Evaluating injection strategies for EGS from the temporal evolution of the Gutenberg-Richter b-value.

Induced seismicity is a hot topic within geo-applications, however the physical mechanisms driving the induced ruptures is yet to be fully understood. The injection of fluid in the subsurface in particular has been shown to cause changes in the stress field leading to the induction of eqarthquakes. Recent events in Switzerland (Basel, Sankt-Gallen) and Korea (Pohang) have shown that such injection operations can have dramatic consequences. The hazard associated with these earthquakes thus needs to be managed to prevent infrastructure damages and protect both the population and viability of the project.

The Gutenberg-Richter b-value has been used as a proxy for the state of stress in the subsurface. The temporal evolution of the b-value provides statistical tools to estimate the seismic hazard posed by an earthquake sequence. Thus, monitoring and forecasting changes in the b-value could be used as a proxy in a near-real-time mitigation context (Adaptive Traffic Light System). Several studies have looked at the evolution of the b-value both in time and space, for example in Basel, where the observed b-value dropped before shut-in and further away from the injection well.

We present a numerical approach coupling a fluid flow simulator with a geomechanical-stochastic formulation (TOUGH2-Seed) to simulate injection-induced seismicity sequences. We model a Hot Dry Rock-type setting, and we investigate the variation of b-value during injection-induced seismic sequences with different injection scenarios and levels of complexity as to the geological features.