August 17, 2022

Team Profile:
The Potential of Rhenium and Indium as Geothermal Tracers

Lucjan Sajkowski & Bruce Mountain

photo credit:
Lloyd Homer (GNS Science)

Our specialist high temperature-pressure geochemistry equipment was used to conduct an experimental study on the potential of Rhenium (Re) and Indium (In) as geothermal tracers under subcritical, super-hot and supercritical conditions.

Tracer Testing

During operation of a geothermal power station, the geothermal fluid extracted from production wells is used to produce steam to drive turbines to generate electricity. The energy-depleted (and cooler) fluid is then reinjected back underground. This reinjected fluid circulates through the geothermal fracture system of the reservoir, where it is reheated and eventually flows back to the production wells.

Tracer testing is used to determine the connectivity between production and injection wells. The tracer compound is injected into a reinjection well, and its concentration over time is measured at the production wells. In this way, tracers can show the hydrodynamic properties of the reservoir, such as flow paths and fluid velocities.

At supercritical conditions, finding a suitable tracer is even more challenging than in ‘normal’ geothermal conditions.  

Studying Rhenium and Indium

Flow-through experiments were conducted in the temperature range of 200–400°C, and in presence of a New Zealand greywacke rock substrate to evaluate the thermal behaviour of rhenium and indium.

The results show that Rhenium is potentially a suitable candidate for subcritical conditions, but not in supercritical conditions. Indium is not suitable as a geothermal tracer at any temperature, as it reacts with hydrogen sulfide.

As part of this study, a sensitive ICP–MS procedure combined with a resin extraction method has been developed for measurement of In and Re in saline geothermal fluids.

The report can be downloaded from the GNS website.

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experimental geochemistry

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