| Title | Time-lapse Micro-gravity Monitoring of the Theistareykir and Krafla Geothermal Reservoirs (Iceland) |
|---|---|
| Authors | Nolwenn PORTIER, Jacques HINDERER, Vincent DROUIN, Freysteinn SIGMUNDSSON, Florian SCHAFER, Philippe JOUSSET, Kemâl ERBAS, Andreas GÃœNTNER, Ingvar MAGNUSSON, Gylfi PALL HERSIR, Kristján ÃGUSTSSON, Elske DE ZEEUW VAN DALFSEN, ÃsgrÃmur GUðMUNDSSON, Egill JULIUSSON, Jean-Daniel BERNARD |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | gravity changes, monitoring, geothermal energy, Theistareykir, Krafla |
| Abstract | We use the micro-gravity time-lapse method to monitor the subsurface mass changes induced by the geothermal energy exploitation at the Krafla and Theistareykir geothermal fields. The two sites are located on the path of the Mid-Atlantic ridge in northeastern Iceland. The Theistareykir plant produces 90 MWe since 2017 thanks to a geothermal fluid of around 300°C extracted from 13 wells at 2 to 2.5km depth. The Krafla geothermal operation began in 1977 and reached a power production capacity of 60 MWe in 1997. We study both the short and long-term impact of the geothermal reservoir production. We carried out time-lapse relative micro-gravity surveys in summer 2017 and 2018 using a Scintrex CG5 gravimeter at both sites. The stations were chosen according to previous gravity campaigns by ISOR (Magnússon, 2012, 2014, 2016) and De Zeeuw-van Dalfsen et al. (2006). After tidal and instrumental drift corrections, we calculated the double differences in gravity, which show the gravity variation at each measurement point compared to a reference time and station. The stability of the reference station was investigated by repeated absolute measurements with the Micro-g solutions FG5#206 ballistic absolute gravimeter. Since December 2017, continuous gravity measurements were performed by GFZ Potsdam, using three superconducting gravimeters and one gPhone, to obtain high-resolution time series of gravity changes at the geothermal fields (see WGC2020 paper of Erbas et al., n°13124). To determine the mass redistribution, gravity data are corrected for the ground vertical motion by using GPS measurements performed by the University of Iceland (see WGC2020 papers of Drouin et al., n°13143 and Sigmundsson et al., n°13174). Considering the gravity variations corrected for the vertical displacements, we observe a gravity increase close to the injection area at the Krafla site. On the contrary, we notice a general decrease in gravity at the Theistareykir geothermal area with a maximum value in vicinity to the production zone, between the 2017 and 2018 surveys. |