Unley, Australia
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Peacock J.R.,University of Adelaide | Thiel S.,University of Adelaide | Heinson G.S.,University of Adelaide | Reid P.,Petratherm Ltd.
Geophysics | Year: 2013

Realization of enhanced geothermal systems (EGSs) prescribes the need for novel methods to monitor subsurface fracture connectivity and fluid distribution. Magnetotellurics (MT) is a passive electromagnetic (EM) method sensitive to electrical conductivity contrasts as a function of depth, specifically hot saline fluids in a resistive porous media. In July 2011, an EGS fluid injection at 3.6-km depth near Paralana, South Australia, was monitored by comparing repeated MT surveys before and after hydraulic stimulation. An observable coherent change above measurement error in the MT response was present and causal, in that variations in phase predict variations in apparent resistivity. Phase tensor residuals proved the most useful representation for characterizing alterations in subsurface resistivity structure, whereas resistivity tensor residuals aided in determining the sign and amplitude of resistivity variations. These two tensor representations of the residual MT response suggested fluids migrated toward the northeast of the injection well along an existing fault system trending north-northeast. Forward modeling and concurrent microseismic data support these results, although microseismic data suggest fractures opened along two existing fracture networks trending north-northeast and northeast. This exemplifies the need to use EM methods for monitoring fluid injections due to their sensitivity to conductivity contrasts. © 2013 Society of Exploration Geophysicists.

Albaric J.,NORSAR | Oye V.,NORSAR | Langet N.,University of Strasbourg | Hasting M.,University of Auckland | And 4 more authors.
Geothermics | Year: 2014

In order to understand the development of a fracture network generated during the first large-scale hydraulic stimulation at Paralana, South Australia, we analysed more than 7000 induced microearthquakes. In July 2011, about 3 million litres of water were injected in the Paralana 2 well to create a geothermal reservoir. A 3-D velocity model was built from seismic reflection data and used for absolute location of the events, which cluster at the base of the injection well. Hypocentre relocations were determined by inverting travel-time differences, improved by waveform cross-correlation. The geometry of the seismic cloud and the associated seismic moment vary during the injection experiment. Relocated microearthquakes outline NNE-SSW and ENE-WSW preexisting structures. The main part of the seismic moment is released during stimulation and is dominated by three Mw 2.4 events and one Mw 2.5 event. The largest event was associated with right-lateral reverse faulting on a plane striking N82°E and dipping 39°N. © 2013 Elsevier Ltd.

Peacock J.R.,University of Adelaide | Thiel S.,University of Adelaide | Reid P.,Petratherm Ltd | Heinson G.,University of Adelaide
Geophysical Research Letters | Year: 2012

Enhanced geothermal systems (EGS) are on the verge of becoming commercially viable for power production, where advancements in subsurface characterization are imperative to develop EGS into a competitive industry. Theory of an EGS is simple, pump fluids into thermally enhanced lithology and extract the hot fluids to produce energy. One significant complication in EGS development is estimating where injected fluids flow in the subsurface. Micro-seismic surveys can provide information about where fractures opened, but not fracture connectivity nor fluid inclusion. Electromagnetic methods are sensitive to conductivity contrasts and can be used as a supplementary tool to delineate reservoir boundaries. In July, 2011, an injection test for a 3.6 km deep EGS at Paralana, South Australia was continuously monitored by both micro-seismic and magnetotellurics (MT). Presented are the first results from continuous MT measurements suggesting transient variations in subsurface conductivity structure generated from the introduction of fluids at depth can be measured. Furthermore, phase tensor representation of the time dependent MT response suggests fluids migrated in a NE direction from the injection well. Results from this experiment supports the extension of MT to a monitoring tool for not only EGS but other hydraulic stimulations. Citation: Peacock, J. R., S. Thiel, P. Reid, and G. Heinson (2012), Magnetotelluric monitoring of a fluid injection: Example from an enhanced geothermal system, Geophys. Res. Lett., 39, L18403. © 2012. American Geophysical Union.

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