• Tómasdóttir, Sigrún
• Lévy, Léa
• Jónsson, Jón E.
• Ratouis, Thomas
• Gailler, Lydie
• Byrdina, Svetlana
• Sigfússon, Bergur Sigfüsson
• Bording, Thue Sylvester
• Árnason, Knütur
• Meldgaard Madsen, Line
• Maurya, Pradip Kumar
• Fiandaca, Gianluca

Abstract

The hydrogen sulfur (H2S) emitted from geothermal exploitation is an air pollutant: high concentrations of H2S are corrosive and toxic. A solution to decrease emissions caused by geothermal exploitation is to re-inject geothermal gases (H2S and CO2) back into the basaltic rocks, where the gasses mineralize into pyrite and calcite, respectively. This solution has been applied at the Hellisheiði geothermal plant since 2014: H2S and CO2 are captured from the non-condensable gases stream and injected into basaltic rocks. The success of H2S sequestration, by transformation into pyrite (FeS2), is usually documented by chemical monitoring in wells located downgradient. The GEMGAS research project (Geo-Electrical Monitoring of H2S Gas Sequestration) aims at enhancing this documentation by developing a methodology for geophysical monitoring of H2S mineralization into pyrite after injection. The 3D time-lapse visualization of mineralization processes provided by geophysics should allow constraining flux velocities calculations and solving the monitoring problem with geostatistics instead of discrete well monitoring. The GEMGAS project, started in 2019, focuses on a shallow H2S re-injection project in the Nesjavellir geothermal reservoir, where injection has started in September 2020 in the depth range 200-500 m. With repeated measurements before and after injection, we aim at providing a dynamic view of electrical resistivity and polarization structure changes upon formation of pyrite in basalts. Five complementary methods are combined: surface Direct Current (DC) and Time-Domain Induced Polarization (IP), Self-potential (SP), Transient Electro-Magnetic (TEM), borehole logging, and finally DCIP using two metallic wellbore casings as current electrodes. The two baseline rounds of measurement carried out in 2019 and 2020 illustrate important issues related to the dense metallic and electrical infrastructure buried at the site. We present improvements made to the data quality by changing the acquisition strategy between 2019 and 2020. We also give an overview of the variability between the two baseline rounds before H2S injection, in particular through time-lapse inversions. Finally, we show preliminary results of the geophysical monitoring thanks to the first measurements carried out post-injection in late 2020 (borehole logging and self-potential).

Topics

• impedance spectroscopy
• surface
• resistivity
• Hydrogen
• transmission electron microscopy