Self-Potential Response in Laboratory Scale EGS Stimulation

Self-potential (SP) response during stimulation of a laboratory-scale enhanced/engineered geothermal system (EGS) was monitored to improve the current understanding of its role in characterization of the stimulation process. Stimulation tests were conducted on 33.0 cm cubic blocks of igneous rocks with one injection well and four nearby producers. The data show excellent correlation between the pressure drop and the SP recorded during the fracturing and circulating phases of the tests. The main direction of fluid flow (and thus the fracture) is identified by the larger coupling coefficient in the main flow direction. The results show that the SP response is mainly controlled by electrokinetic coupling and that thermoelectric coupling is negligible. After fracturing, the coupling coefficient increases even with different saturation and electrical boundary conditions. Injection fluid salinity has been shown to have a great influence on the SP response when the salinity difference between the injected fluid and the formation fluid is large. According to the laboratory results, an SP array can be used to detect and map fluid flow in an EGS both during the fracturing and production stages. However, sufficient liquid saturation and porosity are needed to obtain a strong signal.