Understanding the Fate of H₂S Injected in Basalts by Means of Time‐Domain Induced Polarization Geophysical Logging

Abstract

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 as a complementary tool to follow the fate of H2S re‐injected at Nesjavellir geothermal site (Iceland). Results show a strong chargeability increase at +40 days, interpreted as precipitation of up to 2 vol.% based on laboratory relationships. A uniform increase is observed along NN4, whereas it is localized below 450 m in NN3. Changes are more pronounced with 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 that a sequence of overlapping reactive processes (pyrite precipitation, passivation, pore clogging and possibly pyrite re‐dissolution) results from H2S injection and that TDIP monitoring is sensitive to this sequence.

Advanced Monitoring of H2S Injection through the Coupling of Reactive Transport Models and Geophysical Responses

“…While RTMs help to understand trends in the IP response, discrepancies between the RTM and geophysical IP responses and the lack of a quantitative link between the SVF and chargeability changes highlight challenges in the current application of this joint monitoring approach. Furthermore, this study does not address the long-term stability of sulfides following H 2 S injection as assessed with subsequent monitoring data presented in Levy et al (2024) 101 To strengthen the joint use of IP and RTM as a monitoring technique and assess the long-term stability of H 2 S mineralization, more complex models are required that incorporate, e.g., measured dissolved oxygen levels to understand how leakage impacts the system, interactions with oxidizing microbes, 102,103 and retroaction between flow parameters and mineral dissolution/precipitation. Additionally, constraining water flow parameters through direct measurements would reduce uncertainty in the RTM.…”