The chemistry and potential reactivity of the CO2-H2S charged injected waters at the basaltic CarbFix2 site, Iceland

Publisher's version (útgefin grein)The CarbFix2 project aims to capture and store the CO2 and H2S emissions from the Hellisheiði geothermal power plant in Iceland by underground mineral storage. The gas mixture is captured directly by its dissolution into water at elevated pressure. This fluid is then injected, along with effluent geothermal water, into subsurface basalts to mineralize the dissolved acid gases as carbonates and sulfides. Sampled effluent and gas-charged injection waters were analyzed and their mixing geochemically modeled using PHREEQC. Results suggest that carbonates, sulfides, and other secondary minerals would only precipitate after it has substantially reacted with the host basalt. Moreover, the fluid is undersaturated with respect to the most common primary and secondary minerals at the injection well outlet, suggesting that the risk of clogging fluid flow paths near the injection well is limited.This publication has been produced with support from Reykjavik Energy and the European Commission through the projects CarbFix (EC coordinated action 283148) and CO2-REACT (EC Project 317235).Peer Reviewe

Assessment of in-situ weathering of an histic andosol-microcosm to field scale study

Soil pore water carbon from a Histic Andosol from Western Iceland was studied at three different scales; in the field, in undisturbed outdoor mesocosms and in laboratory repacked microcosms. Pore water was extracted using suction cup lysimeters and hollow-fibre tube Rhizon sampler devices. There were significant differences in all measured variables, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and pH values between the scales of the experiment. Concentrations of DIC and pH values varied between sampling devices used. Gaseous constituents of soil solution and pH were more susceptible to changes in scale and difference of sampling devices used. DOC concentrations were significantly different near the surface but differences were diminished below 35 cm depth. Studies considering long term changes in pedogenesis or the role of anthropogenic impacts on the pedosphere require considerable experimental duration though they can be conducted with confidence in leaching experiments with micro- and mesocosms. Nearly twenty percent of the organic carbon bound annually in the soil surface horizon under field conditions was lost by leaching of DOC and decomposition to DIC in disturbed non-vegetated microcosms. This percentage increased to 38 % in undisturbed vegetated mesocosms underlining the importance of surface vegetation on the turnover of carbon in soils. Increased influx of nutrients will increase growth and photosynthesis and subsequent carbon sequestration in surface horizons. Increased influx of nutrients and pollutants, such as fluorine, will further mobilise carbon in deep horizons

The influence of weathering process on riverine osmium isotopes in a basaltic terrain

This study presents Os isotope and comprehensive major and trace element data for the dissolved load, suspended particulates and bedload for Icelandic rivers, draining predominantly basaltic catchments that range in age from historic to ca. 12 Ma. Hydrothermal waters and precipitation have also been analysed. Both Os and Re concentrations are greater in the suspended load than the bedload, while Re / Os ratios are lower, suggesting that both elements are concentrated in weathering resistant minerals. Despite this elemental fractionation the suspended particulates and bedload for each river yield indistinguishable 187Os / 188Os isotope compositions that range from 0.136 to 0.292. In contrast, the dissolved load (< 0.2 μm filtered) often possesses a significantly more radiogenic Os isotope composition than the corresponding suspended or bed load with 187Os / 188Os ratios ranging from 0.15 to 1.04. The isotope and elemental data for the dissolved load can be explained in terms of an unradiogenic contribution from congruent basalt weathering (and/or hydrothermal input) and a radiogenic contribution that arises from two distinct processes. For the glacier-fed rivers there is a covariation between 187Os / 188Os and the extent of glacial cover in the catchment, and this is most readily explained by the entrainment of seawater aerosols into precipitation and subsequent glacial melting. While for direct-runoff (and spring-fed rivers) there is a covariation between 187Os/188Os and the age of the bedrock in the catchment, that cannot be explained by congruent weathering of old basalt. Calculations indicate that those direct-runoff rivers with radiogenic 187Os/188Os values are also undersaturated with respect to the primary basalt minerals olivine, pyroxene and plagioclase, indicating that these phases are unstable and prone to preferential dissolution. Published Re–Os isotope data indicate that the same phases possess exceptionally high 187Re / 188Os ratios and thus evolve to radiogenic 187Os/188Os compositions in very short time intervals. Taken together, these results indicate that incongruent (preferential) weathering of certain primary basalt minerals can impart a radiogenic Os isotope composition to the dissolved riverine load. Nevertheless, overall the Os isotope signal to the Oceans from Icelandic rivers is little affected because rivers with unradiogenic 187Os/188Os values and a high discharge dominate the Os flux

Author Correction: Carbon dioxide storage through mineral carbonation

International audienceAn amendment to this paper has been published and can be accessed via a link at the top of the paper

CarbFix2: CO2 and H2S mineralization during 3.5 years of continuous injection into basaltic rocks at more than 250 °C

International audienceThe CarbFix method was upscaled at the Hellisheiði geothermal power plant to inject and mineralize the plant's CO 2 and H 2 S emissions in June 2014. This approach first captures the gases by their dissolution in water, and the resulting gas-charged water is injected into subsurface basalts. The dissolved CO 2 and H 2 S then react with the basaltic rocks liberating divalent cations, Ca 2+ , Mg 2+ , and Fe 2+ , increasing the fluid pH, and precipitating stable carbonate and sulfide minerals. By the end of 2017, 23,200 metric tons of CO 2 and 11,800 metric tons of H 2 S had been injected to a depth of 750 m into fractured, hydrothermally altered basalts at >250°C. The in situ fluid composition, as well as saturation indices and predominance diagrams of relevant secondary minerals at the injection and monitoring wells, indicate that sulfide precipitation is not limited by the availability of Fe or by the consumption of Fe by other secondary minerals; Ca release from the reservoir rocks to the fluid phase, however, is potentially the limiting factor for calcite precipitation, although dolomite and thus aqueous Mg may also play a role in the mineralization of the injected carbon. During the first phase of the CarbFix2 injection (June 2014 to July 2016) over 50% of injected carbon and 76% of sulfur mineralized within four to nine months, but these percentages increased four months after the amount of injected gas was doubled during the second phase of CarbFix2 (July 2016-December 2017) at over 60% of carbon and over 85% of sulfur. The doubling of the gas injection rate decreased the pH of the injection water liberating more cations for gas mineralization. Notably, the injectivity of the injection well has remained stable throughout the study period confirming that the host rock permeability has been essentially unaffected by 3.5 years of mineralization reactions. Lastly, although the mineralization reactions are accelerated by the high temperatures (>250°C), this is the upper temperature limit for carbon storage via the mineral carbonation of basalts as higher temperatures leads to potential decarbonation reactions

Carbon dioxide storage through mineral carbonation

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The rapid and cost-effective capture and subsurface mineral storage of carbon and sulfur at the CarbFix2 site

One of the main challenges of worldwide carbon capture and storage (CCS) efforts is its cost. As much as 90% of this cost stems from the capture of pure or nearly pure CO2 from exhaust streams. This cost can be lowered by capturing gas mixtures rather than pure CO2. Here we present a novel integrated carbon capture and storage technology, installed at the CarbFix2 storage site at Hellisheiði, Iceland that lowers considerably the cost and energy required at this site. The CarbFix2 site, located in deeper and hotter rocks than the original CarbFix site, permits the continuous injection of larger quantities of CO2 and H2S than the original site. The integrated process consists of soluble gas mixture capture in water followed by the direct injection of the resulting CO2-H2S-charged water into basaltic rock, where much of the dissolved carbon and sulfur are mineralized within months. This integrated method provides the safe, long-term storage of carbon dioxide and other acid gases at a cost of US $25/ton of the gas mixture at the CarbFix2 site and might provide the technology for lower CCS cost at other sites

Rapid CO2 mineralisation into calcite at the CarbFix storage site quantified using calcium isotopes

The engineered removal of atmospheric CO2 is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the case of Iceland’s CarbFix experiment, precipitates dissolved CO2 as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO2 injection waters to quantify the amount of carbonate precipitated, and hence CO2 stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO2 were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means