GNGTS 2014 - Atti del 33° Convegno Nazionale

Appropriate boundary conditions on the electrical potential or the electrical current density has been considered. Results and discussion. The effects of the fluid injection sketched in Fig. 1b has been analyzed reconstructing the electric potential changes on the ground surface at six distinct time (indicated as a-f) spanning the whole stimulation process. SP are characterized by a typical dipolar trend, with a general intensity of order of mV (Fig. 2a). The electric potential grows up when the rate of fluid injection increases, at times a-b-c-d, after decreasing in correspondence of reduction of the fluid injection rate (or well shut-off), at times e and f. The dipolar trend reconstructs the pattern of groundwater flow showing a privileged direction of fluid flows, aligned to the regional tectonic load. The electric signal has been compared with experimental data recorded at Soultz-sous-Forets. A good agreement has been found, firstly for intensity of signal (Fig. 2b). In effect, the synthetic potential presents the same order of magnitude of typical electric potential recorded. Both numerical and experimental signal show a linear trend for small injection rate, falling sharply in correspondence of wells shut-off. Successively, a strong increase of electric potential is present, induced by residual circulation of groundwater flow in the geothermal reservoir. It is worth noting how this persistence of fluid flows explains the occurrence of seismic events also several days after the end of wells stimulation. To the aim of evidencing such correlation between SP anomalies and induced seismicity, the obtained distribution of electric potential has been compared with the density of seismic events recorded at Soultz-sous-Forets during the wells stimulation, retrieving a good analogy (Fig. 3). Fig. 2 – a) Typical trend of the SP anomalies related to the stimulation cycle. b) Comparison between numerical (green line) and real (blue line) SP anomalies related to the stimulation cycle of Fig. 1b. 228 GNGTS 2014 S essione 1.3 Fig. 3 – Comparison between induced seismicity density (blue line, left panel) and numerical SP anomalies (green line, right panel) related to the stimulation cycle of Fig. 1b.