GNGTS 2013 - Atti del 32° Convegno Nazionale

Being the induced seismicity risk correlated to fluid circulation stimulated in an area exceeding the well of several hundreds of meters, the wellbore pressure values are totally uncorrelated to seimic hazard. However, SP anomalies, being generated from pressure gradient in the whole area where fluids flow, has an interesting potential as induced earthquake precursor Detectability of changes in the SP values respect to the natural background values has been investigated in our study. Method. Our method of analysis consists of a two-step procedure. In the first step, injection of water is simulated (Pruess, 1991) in a homogeneous medium, approximating a crystalline granite basement compatible with the deep structure of the Soultz-sous-Forets (France) EGS site. The modeled 3D physical domain and the imposed initial con- ditions are shown (Fig. 1). Water at ambient condition is injected at a 100 kg/s rate for 10 days in a point located at -5 km depth. In such a way we obtain the pressure and temperature changes at each point in the medium, at the final time. Such P and T changes at any point are subse- quently considered as elementary sources, heterogeneously distributed in the whole discretized volume, which generate an incremental stress tensor field estimated by using the Comsol Mul- tiphysics finite element code (Troiano et al. , 2011, Troiano et al. , 2013). Once the complete field of stress changes is computed, a conceptual model linking induced stress and permeability modification in orthogonally fractured media is adopted, incorporating the influences of both normal strain and shear dilation on the effect of fluid flow, in order to reconstruct k’ , the new medium permeability after fluid stimulation. Histogram of k’ show a gaussian distribution of permeability values around the volume (Fig. 2). The gaussian result centered around the k 0 value, with a standard deviation of about k 0 /3. To give an idea of the stimulation effects, it has to be considered that permeability at the in- jection point has been imposed as enhanced of two orders of magnitude. Furthermore, if values of k’ , are selected exceeding the mean of the gaussian more than 1 standard deviation, the cor- responding points in the stimu- lated volume can be considered as the zone where permeabili- ty is effectively enhanced due to fluid injection (Fig. 3). The mean value of k’ in this zone re- sults of 4.2*10 -16 m 2 , leading to a permeability about three times greater than k 0 in a spheric vol- ume of about 0.5 km 3 . Fig. 2 – Histogram of k’ , the new medi- um permeability after fluid stimulation. The histogram show a gaussian distri- bution of permeability values around the volume, centered around k 0 , the ini- tial permeability value. Mean value and standard deviation are represented with the red lines. Fig. 3 – Points in the stimulated vol- ume corresponding to values of k’ exceeding the mean of this gaussian distribution more than 1 standard de- viation. This area is considered as the zone where permeability is effectively enhanced due to fluid injection. The enhancement is estimated considering the mean value if this point. 287 GNGTS 2013 S essione 1.3