Title: Latest results from the hybrid micro-gravity monitoring of the Theistareykir geothermal field (North Iceland)

Nolwenn Portier1, Jacques Hinderer1, Florian Schäfer2, Philippe Jousset2, Kemal Erbas2, Vincent Drouin3, Siqi Li4, Freysteinn Sigmundsson4, Ingwar Magnusson3, Gylfi Pall Hersir3, Kristjan Agustsson3, Agrismur Gudmundsson5, Egill Juliusson5, Hreinn Hjartasson5, Anette Mortensen5, Jean-Daniel Bernard6

1ITES France; 2GFZ Potsdam Germany; 3ISOR Iceland; 4University of Iceland; 5Landsvirkjun Iceland; 6EOST France

Event: GeoKarlsruhe 2021

Date: 2021

DOI: 10.48380/dggv-5qp1-ze50

Geothermal energy represents around 30% of the produced electricity in Iceland with a cumulative capacity being equal to 755 MWe (Ragnarsson et al., 2020). In particular, the Theistareykir geothermal plant, which is located on the Mid-Atlantic ridge in North Iceland, produces 90 MWe using two turbines in operation since autumn 2017 and spring 2018, respectively. We will report on the hybrid micro-gravity monitoring and discuss how this technique will contribute to the sustainable management of this renewable energy. Indeed, the gravity method highlights the mass redistribution and, consequently, helps to quantify the recharge/discharge of the geothermal reservoir.

On one hand we show the results of the repetition of the Theistareykir micro-gravity network of 27 stations measured in summer 2017, 2018 and 2019 i.e. before and after the beginning of the geothermal production, with a Scintrex CG5 gravimeter.

On the other hand, we will also show the continuous gravity changes recorded from fall 2017 to summer 2020 at 3 permanent stations with iGrav superconducting gravimeters calibrated with a FG5 ballistic absolute gravimeter.

The combination of these different types of gravimeters defining the hybrid micro-gravity method is then used to investigate the measured gravity changes in relation to geothermal activity parameters like injection and extraction rates.

After correcting the gravity measurements for the effect of the vertical displacements deduced by continuous GNSS measurements at the permanent stations and InSAR analysis by the University of Iceland, we compare the gravity changes due to mass redistribution to what is expected from the injection/extraction rates.

We finally focus on the question of the sustainability of the Theistareykir power plant since the start of exploitation and discuss the discharge/recharge of the geothermal reservoir.

Ragnarsson, Á., Steingrímsson, B. and Thorhallsson, S. Geothermal development in Iceland 2015-2019. Proceedings World Geothermal Congress 2020, Reykjavik, Iceland (2020).

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