How close to failure is a granite rock mass at a 5 km depth?
Résumé
We discuss in this paper the mechanical significance of microseismicity induced by pore pressure variations. Particular attention is given to identifying whether microseismic events reflect only small effective stress perturbations, in a manner somewhat similar to the so-called Kaiser effect, or whether they outline the onset of large-scale failure. This issue is addressed in the context of the development of microseismic activity observed during large-scale water injections conducted in the 5000 m deep experimental geothermal reservoir at Soultz (France). Results from large hydraulic tests together with analysis of borehole images and induced seismicity are integrated to provide a well-constrained characterization of the complete stress field down to 5 km. It is shown that, for this site, pore pressure increments larger than 10% of the natural minimum principal stress magnitude are required for inducing large-scale shear failures that are characterized by a clear structuring of the microseismic events occurrence. But the onset of induced seismicity occurs for much smaller pore pressure variations that only reflect the “elastic” response to changes in local effective stresses. It is concluded that the linear variation with depth of principal stress magnitudes observed in this granite depends on the long-term rheology of this formation rather than on the frictional characteristics of the main faults that affect the massif.