PHYSICS OF FLUID-INDUCED SEISMICITY AND ITS MAGNITUDE DISTRIBUTION

Pore fluids in rocks and pore pressure perturbations can trigger earthquakes. Sometimes fluid injections into boreholes are able to induce potentially damaging seismic events. This was the case by stimulations at such Enhanced Geothermal Systems like the ones at Basel, in Cooper Basin and at Soultz. This is rarely the case by hydraulic fracturing of hydrocarbon reservoirs. The seismicity triggering is controlled by a process of relaxation of a stress- and pore-pressure perturbation that was initially created at the injection source. This relaxation process can be approximated by a pressure diffusion (possibly a non-linear one). Recently we have found that under rather general conditions the number of fluid-injection-induced earthquakes with a magnitude larger than a given one increases approximately proportionally to the injected fluid volume. Using the seismicity rate of induced events and the fluid-injection rate, we derive a parameter (the seismogenic index) that quantifies the seismotectonic state at an injection location. This index is independent of injection parameters and depends only on tectonic features. Comparing temporal seismicity rates of statistically well represented small-magnitude events to the seismicity rate of large-magnitude events we found that the last ones are systematically underrepresented. We modelled statistics of induced events by statistics of randomly distributed thin flat discs representing rupture surfaces in a finite volume. We found that the factor limiting the seismicity rate of large-magnitude events is the minimal principal axis of the stimulated volume. It controls the order of a largest possible magnitude. This conclusion is in a well agreement with real data on induced seismicity at geothermal and hydrocarbon reservoirs. We observe a significant influence of the geometry of a stimulated volume on the frequency-magnitude distribution. This geometrical control indicates that a necessary condition of a large-scale earthquake is a stress-state perturbation on a large part of a potential rupture plane. This also explains why hydraulic fracturing of hydrocarbon reservoirs induces seismicity of magnitudes significantly lower than stimulations of geothermal reservoirs. The minimal principal axis of the stimulated volume is usually many times smaller in the former case than in the latter one.