Monitoring Reservoir Fluid Flow from Surface Self Potential (SP) Measurements: Application to the Soultz-sous-Forets Hot Dry Rock Site (France)

摘要

We have carried out monitoring of surface electric potential (SP) variations around the Soultz-sous-Forets Hot Dry Rock site (France) during two injection experiments. We have observed long-period (T>1 day) SP variations with a maximum amplitude of several mV well correlated to the injection phases: this anomalous potential increases during the injection and decreases after shut-in. We interpret this behavior as being caused by the electrokinetic effect of the injected water flow in deep fractures (5 km). We found that the steel well casing channels the electric current toward the surface; this is why we can record surface SP variations even though the electrokinetic sources are 5 km below. SP and borehole pressure data are usually well correlated but they sometimes show discrepancies because pressure sensors are sensitive to the flow in the vicinity of the well while SP is sensitive to the overall reservoir flow. Monitoring of surface SP therefore adds significantly to the information obtained by standard downhole sensors and could be used in the future as a low-cost decision-making tool to monitor the overall dynamics of geothermal reservoirs.