Monitoring EGS Fluids with Magnetotellurics

Abstract

Development of enhanced geothermal systems (EGS) prescribes the need for novel technology to monitor the enhanced reservoir. Magnetotellurics (MT) is a passive electromagnetic method that characterizes geoelectric structure by measuring the Earth’s electromagnetic response to natural magnetic fields. Measuring the MT response before, during and after fluids are injected provides a tool to delineate reservoir boundaries at depth as MT is sensitive to electrical conductivity changes. This is important as electrical conductivity can change by a few orders of magnitude making a conductive fluid in resistive host rock a viable MT target at depth. Furthermore, the MT reponse can be compared with micro-seismic data suggesting the location of fluid filled fractures. A test case of fluids injected at 3600m from Paralana, South Australia suggests MT to be a viable geophysical method to not only monitor fluids at depth but also monitor in real time the fluid injection. Conductive features computed from the difference between pre-injection and post-injection correlate well with micro-seismic data. Also, real time measurements during fluid injection suggest MT can measure transient changes in subsurface geoelectric structure associated with inclusion of electrically conductive fluids.