Tracking of the origin and history of fluids in the Hellisheiði and Nesjavellir geothermal fields, Iceland

This is the authors' abstract. We don't add key points for this paper.

The geochemistry of geothermal fluids provides critical insights into their provenance and subsurface transport mechanisms, which are essential for exploration and sustainable exploitation of geothermal resources. Prolonged production and reinjection of wastewater might overprint original fluid chemistry, complicating reservoir characterisation. This study uses stable isotopes of water (δD, δ 18 O) and the noble gas composition of production fluids collected during multiple sampling campaigns at the Hellisheiði and Nesjavellir fields, Iceland, to evaluate fluid evolution during continuous production and reinjection. Comparison with pre-commissioning data indicates that water isotopic composition has remained stable, suggesting that production and reinjection have not significantly altered the primary fluid composition over 18 years at Hellisheiði and 39 years at Nesjavellir. The noble gas composition of most samples is consistent with phase separation. The temperature of phase separation constrained by noble gas compositions exceeds surface separator temperatures, suggesting that subsurface boiling has likely occurred. However, the estimated boiling temperatures are 29–145 °C lower than reservoir temperatures, implying that boiling occurs in the shallow subsurface or within the wellbores. This study also provides an evaluation of a Kr tracer test at Nesjavellir that was initiated in 2019, prior to CO 2 injection. Measurements made within 330 days of injection failed to detect excess Kr. However, we record excesses of up to 271% relative to the background baseline. This study reiterates the utility of noble gases as conservative vapour-phase tracers for monitoring subsurface fluid migration and containment of injected fluids.