Hydraulic stimulation of geothermal reservoirs: fluid flow, electric potential and microseismicity relationships

We have observed surface electric potential variations of several mV during a reservoir stimulation experiment run in 2000 at the Soultz-sous-Forets (France) Hot Dry Rock site that we have interpreted as deep electrokinetic processes. Using data from another experiment run in 2003 at the same site, we show here that the electrically conducting steel casing makes possible the measurement of surface SP anomalies even 5 km above the injection zone, but obliterates any information about the geometry of the electrokinetic sources. We also observed a slow temporal SP decay after the end of the that we interpret as large fluid flow persisting long after shut-in at the edge of the stimulated area. This persisting deep fluid flow explains why large induced microseismic events are recorded in this area long after the end of the stimulation experiment, as fluid overpressures diffuse far away from the injection area. This interpretation is consistent with the diffusive pattern of seismicity analysis of Shapiro et al.