Monitoring EGS Reservoirs Using Magnetotellurics

Abstract

A novel time-lapse magnetotellurics (MT) method is proposed to monitor and estimate areal extent of enhanced geothermal systems (EGS). Being directly sensitive to subsurface electrical conductivity, MT has the capability to measure fluids at depth which are both thermally and electrically conductive. MT measurements before, during and after fluid injection are compared to estimate the lateral extent of the induced reservoir at Paralana, South Australia. 3D forward modelling shows the residual MT response between a system with and without conductive fluids to be small, of the order of a few degrees in MT phase. It is crucial to obtain accurate and precise MT responses throughout the surveys in order to produce a reliable model of the changing subsurface. Presented are MT responses before fluid injection and after the first fluid injection. Furthermore, time series measurement of the electric and magnetic fields during the fracturing process may allow studies of EM disturbances generated by the seismic waves, also known as the seismoelectric effect, which can provide information about pore fluid inclusion.