Understanding the fate of H2S injected in basalts by means of time-domain induced polarization geophysical logging

• Main Authors: Lévy, Léa, Ciraula, Daniel, Legros, Bruno, Martin, Tina, Weller, Andreas
• Format: Other/Unknown Material
• Language: unknown
• Published: Authorea, Inc. 2023
• Subjects: Nesjavellir; Iceland
• Online Access: http://dx.doi.org/10.22541/essoar.170143987.77309386/v1

To help meet emission standards, hydrogen sulfide (H2S) from geothermal production may be injected back into the subsurface, where basalt offers, in theory, the capacity to mineralize H2S into pyrite. Ensuring the viability of this pollution mitigation technology requires information on how much H2S is mineralized, at what rate and where. To date, monitoring efforts of field-scale H2S reinjection have mostly occurred via mass balance calculations, typically capturing less than 5\% of the injected fluid. While these studies, along with laboratory experiments and geochemical models, conclude effective H2S mineralization, their extrapolation to quantify mineralization and its persistence over time leads to considerable uncertainty. Here, a geophysical methodology, using time-domain induced polarization (TDIP) logging in two of the injection wells (NN3 and NN4), is developed to follow the fate of H2S re-injected at Nesjavellir geothermal site in south-west Iceland. Results show a strong chargeability increase at +40 days, corresponding to precipitation of up to 1\% in NN4 and 2\% in NN3 according to laboratory-based relationships. A uniform increase is observed along NN4, whereas it is localized below 450 in NN3. Changes are more pronounced with the larger electrode spacing, indicating that pyrite precipitation takes place away from the wells. Furthermore, a chargeability decrease is observed at later monitoring rounds in both wells, suggesting that pyrite is either passivated or re-dissolved after precipitating. These results highlight the ability of TDIP logging to monitor pyrite mineralization and have implications for understanding the fate of H2S upon subsurface storage in basaltic environments.