Magnetotelluric monitoring of permeability enhancement at enhanced geothermal system project

Magnetotelluric (MT) data were collected across the Habanero Enhanced Geothermal System (EGS) project in the Cooper Basin, South Australia. A baseline regional MT survey consisting of two profiles were collected to delineate subsurface resistivity structure. An MT monitoring survey was conducted during stimulation of the Habanero-4 well. Inversions of the MT data in 2-D reveal three main resistivity layers to a depth of 5 km. The surface layer is ≤6 Ωm, 1.5 km thick, and composed of poorly consolidated sediments of Lake Eyre and Eromanga Basins. The second layer is ≤25 Ωm, 2 km thick, and correlated to consolidated Cooper Basin sediments. A high resistivity zone below depths of 3.5 km is interpreted as the hot intrusive granodiorite of the Big Lake Suite related to the Habanero EGS reservoir. The second MT survey was conducted during stimulation of Habanero-4, where 36.5 million liters (ML) of water with a resistivity of 13 Ωm was injected over 14 days. Analysis of pre- and post-injection residual phase tensors show possible conductive fractures oriented in a N-S direction for periods greater than 10 s. Apparent resistivity maps also revealed that injected fluids possibly propagated towards N-S direction. This result is in agreement with micro-seismic events observed at the Habanero EGS during fluid injection. The MT responses close to injection show on average 5% decrease in apparent resistivity for periods greater than 10 s. The main reasons for observing subtle changes in resistivity at Habanero EGS is the screening effect of the conductive thick sedimentary cover. Analysis of time-lapse models indicate an increase in total conductance of about 25 S in the N-S direction, which likely indicate anisotropic permeability generated by hydraulic stimulation. Overall, the MT monitoring at Habanero EGS highlights the need for favorable geological settings and/or controlled source and downhole EM methods to measure significant changes in resistivity in EGS reservoirs.