Háskóli Íslands

On Wednsday the 19th of April, Sandra Ósk Snæbjörnsdóttir will defend her Ph.D. thesis in Geology.

The theisis is titled Mineral storage of carbon in basaltic rocks.

Opponents are Dr. Jordi Cama, Researcher at the Institute of Environmental Assessmentand Water Research, Barcelona, Spain and Professor Stuart Haszeldine, Professor of carbon capture and storage at the University of Edinburgh.

Advisers are Dr. Sigurður Reynir Gíslason, Research professor, University of Iceland and Professor Eric Oelkers, Professor at the University College London, Director of Research at CNRS, Toulouse, France and Visiting Adjunct Professor at the University of Iceland.

The assessment committee included Professor Martin Stute, professor at Barnard College and Adjunct Senior Research Scientist at Columbia University, New York and Dr. Hjalti Franzson, senior geologist at Iceland GeoSurvey.

Abstract

In-situ carbonation of basaltic rocks could provide a carbon storage solution for the long term. Permanence is essential for the success and public acceptance of carbon storage. The aim of this study was twofold, to evaluate and make a first estimate of the theoretical mineral storage potential of CO2 in basaltic rocks, and to characterise the mineralisation process using geochemical data from the CarbFix test site in Hellisheidi, SW-Iceland, which comprises both injection and monitoring wells.

Studies on mineral storage of CO2 in basaltic rocks are still at an early stage. Therefore, natural analogues are important for gaining a better understanding of the carbon mineralisation process in basaltic rocks at elevated pCO2. The amount and spatial distribution of CO2 stored as calcite in the bedrock of geothermal systems in Iceland indicate a large storage potential for CO2 in basaltic rocks. These natural analogues were used as a guideline for evaluating the theoretical potential of CO2 storage in basaltic formations. The largest storage potential lies offshore, where CO2 may be stored in minerals for the long term in mid-ocean ridges. The theoretical mineral CO2 storage capacity of the mid-ocean ridges exceeds, by orders of magnitude, the amount of CO2 that would be released by the burning of all fossil fuel on Earth. Iceland is the largest landmass found above sea level on the mid-ocean ridges, about 103,000 km2. It is mostly made of basaltic rocks (~90%), which makes it ideal for demonstration of the viability of this carbon storage method.

Two injection experiments were carried out at the CarbFix site where 175tonnes of pure CO2 and 73 tonnes of a CO2-H2S-gas mixture were injected in to basaltic rocks at 500-800 m depth with temperatures ranging from 20-50°C.All gases were dissolved in water during their injection. Extensive geochemical monitoring was carried out prior to, during, and after these injections. Sampled fluids from the first monitoring well, HN-04, showed a rapid increase in Ca, Mg, and Fe concentrations during the injections. Pyrite was identified in water samples from the injection well, which indicates that the H2S was mineralised before it reached the first monitoring well. In July 2013, the fluid sampling pump in the well broke down due to calcite precipitation, confirming the mineralisation of the injected CO2. Calculations indicate that the sampled fluids were saturated with respect to siderite about four weeks after the injections began, and with respect to calcite about three months after each injection. Pyrite was supersaturated prior to and during the mixed gas injection and in the following months. Mass balance calculations, based on the recovery of non-reactive tracersco-injected into the subsurface together with the acid gases, confirm that more than 95% of the CO2 injected into the subsurface was mineralised within two years. Essentially all of the injected H2S was mineralised within four months of its injection.

Data collected prior to, during, and after the CO2 injection was used in an attempt to model the CO2-water-rock interaction during and after the injection. The results suggest that the mineralisation of the second and main breakthrough of the injected carbon is mainly driven by basaltic glass dissolution. The results also point towards dissolution of crystalline basalts during the first breakthrough of the injected solution.

This breakthrough path is dominated by fracture flow, indicating that the fracture transects the more crystalline interiors of the lavas. No carbonates are saturated in the injection fluid, but iron rich carbonates, such as siderite, are predicted to form if the pH exceeds ~4.6. With progressive dissolution of basaltic rock, and a subsequent rise of pH along with a decrease in the dissolved CO2 concentration, more Ca-rich carbonates, such as calcite, are calculated to become saturated. At this stage, carbonates become more abundant, forming along with chalcedony, and later, both zeolites and smectites appear. The efficiency of the carbon injection is limited by the porosity and the availability of cations, both of which are restricted by the formation of zeolites and smectites at pH above ~6.5.

About the Ph.D. student

Born in Reykjavik 29th of December 1983. Started geology studies at the University of Gothenburg in 2006 after one year of studying music at the Academy of Music and Drama. Finished bachelor degree from the University of Iceland in February 2009 and was employed as a geologist by Iceland GeoSurvey by graduation. Finished a MSc degree from the University of Iceland in November 2011 with emphasis on geothermal alteration and clay minerals. Started PhD studies in June 2012.  Her research was carried out at the University of Iceland with three months on secondment at Lamont Doherty Earth Observatory in New York, USA early 2016.

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