Doctoral defence in Geophysics - Daniel Anthony Ciraula

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Doctoral candidate: Daniel Anthony Ciraula

Title of thesis: Understanding Hydrogen Sulfide Mineral Storage: Joint Geophysical Surveying and Geochemical Modeling at the Nesjavellir Geothermal Site, Iceland

Opponents:
Dr. Adrian Mellage, Assistant Professor of Hydrogeology, Department of Civil & Environmental Engineering, University of Kassel, Germany.
Dr. Mariette Wolthers, Professor of Aquatic Geochemistry, Utrecht University, Netherlands

Advisor: Dr. Léa Lévy, Scientist at CNRS-INSU, Geosciences Rennes UMR, France

Faculty supervisor: Dr. Halldór Geirsson, Professor at the Faculty of Earth Sciences, University of Iceland

Other members of the doctoral committee:
Dr. Barbara Kleine-Marshall, Professor at the GeoZentrum at Friedrich-Alexander-Universität, Germany
Dr. Gianluca Fiandaca, Professor at the Department of Earth Sciences “Ardito Desio”, University of Milano, Italy
Dr. Samuel Scott, Researcher at the Institute of Earth Sciences, University of Iceland

Chair of Ceremony
Dr. Andri Stefánsson, Professor and Head of the Faculty of Earth Sciences, University of Iceland 

Abstract

To reduce atmospheric emissions, hydrogen sulfide (H2S) gas from geothermal energy production can be reinjected into the subsurface, where it interacts with basalt to form pyrite. However, the underlying field-scale processes controlling mineralization remain uncertain, and monitoring methods are limited.

This thesis aims to improve the monitoring and understanding of H2S mineralization near the Nesjavellir geothermal area, SW Iceland, by integrating time-lapse geophysics with reactive transport geochemical modeling through petrophysical relationships. The direct current/induced polarization (DCIP) geophysical method is sensitive to the volumetric content of pyrite and is explored as a monitoring tool.

Reactive transport simulations couple fluid flow and transport with geochemical reactions between H2S-rich water and basalt to constrain the processes controlling mineralization and support geophysical interpretations. The reactive transport models were calibrated to borehole temperature measurements and utilize fluid and rock chemistries from samples collected at Nesjavellir.

Time-lapse DCIP data were collected in boreholes and at the surface to assess mineralization across different scales, capturing high-resolution changes near the injection wells and mineralization throughout the reservoir. The high-resolution borehole DCIP survey recovers changes consistent with pyrite mineralization, while the surface DCIP survey does not.

Additional synthetic tests show that cross-hole DCIP surveying between two boreholes has the potential to resolve H2S mineralization.The reactive transport models reveal underlying processes that challenge the ability of surface DCIP surveys to monitor H2S mineralization. The simulations predict that most of the injected H2S mineralizes, but the pyrite is too dispersed and deep to be resolved by the surface DCIP survey. Additionally, the models show that the disposal of conductive geothermal wastewater and smectite mineralization lowers the electrical resistivity, reducing DCIP signals.

Comparison of electrical geophysical data from 1985 to data collected in 2020 confirms the decrease in electrical resistivity. Lastly, the reactive transport simulations show that variable flow and transport processes between the fractures in the basalt reservoir and the rock matrix strongly control the degree of basalt alteration and subsequent distribution of pyrite mineralization. The alteration of iron-bearing minerals within the basalt is critical to pyrite formation.

Overall, this thesis highlights the complexities of field-scale H2S mineralization and the value of interdisciplinary approaches to better evaluate and monitor hydrogeological systems. 

About the doctoral candidate

Daniel Ciraula was born in 1997 in Round Rock, Texas, in the United States. He grew up in Colorado with his parents and two brothers. He attended the Colorado School of Mines from 2015-2019 and received a BS in Geophysical Engineering. Daniel continued his studies at the University of Wyoming where he researched geyser structure and eruption dynamics and graduated with a MS in Geophysics in 2021. He started his PhD at the University of Iceland as part of the NordVulk fellowship program in October 2021.