Magnetotelluric Monitoring of Hydraulic Fracture Stimulation at the Habanero Enhanced Geothermal System, Cooper Basin, South Australia

Abstract

Magnetotelluric (MT) data were collected across the Habanero Enhanced Geothermal System project in the Cooper Basin, South Australia. A baseline regional MT survey consisting of two profiles was collected to delineate the pre-injection resistivity structure. Two dimensional inversions of the MT data reveal three main resistivity structures to a depth of 5 km. The low resistivity surface layer (about 1.5 km thick) is interpreted as poorly consolidated sediments of Lake Eyre and Eromanga Basins. Below the conductive layer, a zone with relatively high resistivity with thickness of 2 km can be correlated to consolidated Cooper Basin sediments. A high resistivity zone below depths of 3.5 km is interpreted as the hot intrusive granodiorite (granite) of the Big Lake Suite with low porosity and permeability. This deep structure is also related to the Habanero EGS reservoir.

The second MT survey was conducted during stimulation of Habanero-4 well by Geodynamics Ltd, where 36.5 ML of water with a resistivity of 13 Ωm (at 25°C) was injected at a relatively continuous rate of between 27–53 L/s over 14 days at a depth of almost 4 km. Analysis of pre- and post-injection residual phase tensors for periods greater than 10 s indicate conductive fractures oriented in a N/NNE direction. Apparent resistivity maps also revealed that injected fluids possibly propagated towards N/NNE direction. This result is in agreement with the micro-seismic events with an area of 4 km2 observed during fluid injection, as well as orientation of pre-existing N-S striking sub-horizontal fractures susceptible to slip due to stimulation. The MT responses close to injection show on average 5% decrease in apparent resistivity at 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 (about 3.6 km thick) and the presence of pre-existing saline fluids with resistivity of 0.1 Ωm (equivalent to salinity of 16.1 g/L at 240°C) in the natural fractures. Overall, the MT monitoring at Habanero EGS highlights the need for favourable geological settings to measure significant changes in resistivity in EGS reservoirs.