A brief history of CarbFix: Challenges and victories of the project’s pilot phase

The pilot phase of the CarbFix project ran for over a decade and consisted of the training of students, creating the scientific basis for the fixation of carbon dioxide in the subsurface through the in-situ carbonation of basalts, and the demonstration of this technology by fixing approximately 200 tons of injected CO2 as carbonate minerals during 2012 and 2013. Over the course of this effort numerous parts of this project have been reported in scientific articles, but a number of challenges including that of separating CO2 gas from a H2S-rich effluent gas, the clogging of the original CarbFix injection well and the damage to the project’s gas pipe by a third party that eventually shut down the project’s pilot phase, have yet to be detailed in the scientific literature. This brief manuscript reviews the CarbFix timeline over the past 12 years, describing in detail some of these challenges. CarbFix demonstrates how interdisciplinary collaboration between the green energy industry, academia, engineers and technicians allows for a fast and efficient development of the idea of battling climate change by permanently petrifying otherwise emitted CO2 in subsurface basalt formations into an economic industrial scale process useful to the global economy

The chemistry and potential reactivity of the CO 2 -H 2 S charged injected waters at the basaltic CarbFix2 site, Iceland

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

The geology and hydrology of the CarbFix2 site, SW-Iceland

Injection of CO2 and H2S emissions from the Hellisheidi Geothermal Power Plant, SW-Iceland, as part of the CarbFix project, is currently taking place in the Húsmúli reinjection zone. Here we present detailed descriptions of the geology of the reservoir rock in Húsmúli including descriptions of its intrusions, secondary mineralogy and sources of permeability. We further present preliminary results from a modelling study of the Húsmúli reinjection zone that was conducted to obtain better understanding of flow paths in the area. The model was calibrated using results from an extensive tracer test that was carried out in 2013-2015

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

The CarbFix method was upscaled at the Hellisheiði geothermal power plant to inject and mineralize the plant’s CO₂ and H₂S 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 CO₂ and H₂S then react with the basaltic rocks liberating divalent cations, Ca^{2+}, Mg^{2+}, Fe^{2+}, increasing the fluid pH, and precipitating stable carbonate and sulfide minerals. By the end of 2017, 23,200 metric tons of CO₂ and 11,800 metric tons of H₂S 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

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

The geology and hydrology of the CarbFix2 site, SW-Iceland

Injection of CO2 and H2S emissions from the Hellisheidi Geothermal Power Plant, SW-Iceland, as part of the CarbFix project, is currently taking place in the Húsmúli reinjection zone. Here we present detailed descriptions of the geology of the reservoir rock in Húsmúli including descriptions of its intrusions, secondary mineralogy and sources of permeability. We further present preliminary results from a modelling study of the Húsmúli reinjection zone that was conducted to obtain better understanding of flow paths in the area. The model was calibrated using results from an extensive tracer test that was carried out in 2013-2015

Using stable Mg isotope signatures to assess the fate of magnesium during the in situ mineralisation of CO2 and H2S at the CarbFix site in SW-Iceland

The in-situ carbonation of basaltic rocks could provide a long-term carbon storage solution. To investigate the viability of this carbon storage solution, 175 tonnes of pure CO2 and 73 tonnes of a 75% CO2-24% H2S-1% H2-gas mixture were sequentially injected into basaltic rocks as a dissolved aqueous fluid at the CarbFix site at Hellisheidi, SW-Iceland. This paper reports the Mg stable isotope compositions of sub-surface fluids sampled prior to, during, and after the CO2 injections. These Mg isotopic compositions are used to trace the fate of this element during the subsurface carbonation of basalts. The measured Mg isotopic compositions of the monitoring well fluids are isotopically lighter than the dissolving basalts and continue to become increasingly lighter for at least two years after the gas-charged water injection was stopped. The results indicate that the formation of isotopically heavy Mg-clays rather than Mg-carbonates are the predominant Mg secondary phases precipitating from the sampled fluids. Isotope mass balance calculations suggest that more than 70% of the Mg liberated from the basalt by the injected gas charged water was precipitated as Mg-clays, with this percentage increasing with time after the injection, consistent with the continued precipitation of Mg clays over the whole of the study period. The formation of Mg clays in response to the injection of CO2 into basalts, as indicated in this study, could be detrimental to carbon storage efforts because the formation of these minerals consume divalent Mg that could otherwise be used for the formation of carbonate minerals and because such clays could decrease host rock permeability