First results of monitoring fluid injection in EGS Reservoirs Using Magnetotellurics

Magnetotelluric (MT) data was recorded during a fluid fracture stimulation of an enhanced geothermal system (EGS), at Paralana, South Australia. MT is directly sensitive to the bulk electrical conductivity of the subsurface, specifically hot and electrically conductive, fluids. MT measurements before, during and after fluid injection are compared to estimate the lateral extent of the induced reservoir. 3D forward modelling studies show the residual MT response between a system with and without fluids to be small, of the order of a few degrees in MT phase. It is crucial to obtain accurate and precise noisefree MT responses throughout the surveys in order to produce a reliable model of the subsurface. Presented are MT responses and preliminary inversion models of base surveys in comparison with measurements throughout the fluid injection. After injection temporal and spatial changes of MT response function are visible and correlate with other monitoring data sets from seismic event analysis.