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Reykjavík, Iceland

Alfredsson H.A.,University of Iceland | Alfredsson H.A.,University Paul Sabatier | Oelkers E.H.,University of Iceland | Oelkers E.H.,University Paul Sabatier | And 4 more authors.
International Journal of Greenhouse Gas Control | Year: 2013

The subsurface rocks at the Hellisheidi carbon injection site are primarily olivine tholeiite basalts consisting of lava flows and hyaloclastite formations. The hyaloclastites are low permeability glassy rocks formed under ice and melt water during glaciations that serve as the cap rock at the injection site; the boundaries between hyaloclastites and lava flows and those between individual lava flows boundaries are preferential fluid flow pathways. Some alteration is observed in the hyaloclastite cap rock situated at 100-300. m depth consisting primarily of smectite, calcite, Ca-rich zeolites, and poorly crystalline iron-hydroxides. Alteration increases with depth. These alteration phases lower the porosity and permeability of these rocks. Carbon dioxide injection will be targeted at a lava flow sequence at 400-800. m depth with the main aquifer located at 530. m depth. Loss on ignition suggests that over 80% of the primary rocks in the target zone are currently unaltered. The target zone rocks are rich in the divalent cations capable of forming carbonates; on average 6 moles of divalent cations are present per 1. kg of rock.The water in the target zone ranges in temperature from 15 to 35°C; the in situ pH ranges from 8.4 to 9.8. The partial pressure of CO2 and O2 suggest that the water in this system is isolated from the atmosphere. The concentration of Ca and Mg in these waters are limited by secondary mineral precipitation. All the waters are supersaturated with Ca-zeolite, analcime, Ca-Mg-Fe smectite, calcite, and aragonite, and some are supersaturated with respect to dolomite and Fe-Mg carbonates.Pure commercial CO2 and a 75%-24.2%-0.8% mixture of CO2-H2S-H2 gases will be dissolved into water prior to its injection into this system. The injected water will have a temperature of ~25°C and be equilibrated with ~25bar pressure of the CO2 gas, and ~14bar pressure of the CO2-H2S-H2 mixture. The injected gas will have total CO2 concentrations of ~0.8-0.42mole/kg H2O and a pH of 3.7-4.0, depending on the H2S concentration of the injected gas. All host rock minerals and glass will be strongly undersaturated with respect to the gas charged injection waters. The water will therefore create porosity in the near vicinity of the injection by dissolving primary and secondary minerals. Further from the injection, secondary minerals will precipitate, potentially clogging the system. Reaction path modeling shows that 1-2 moles of basaltic glass are needed to lower the injected CO2 concentration down to natural pre-injection concentrations, but less than 1 mole is needed for the sequestered H2S. Major carbonates formed were Ca-Mg-Fe-carbonate and dolomite at pH <5, whereas ankerite and calcite formed later at higher pH. Associated minerals at lower pH were chalcedony, kaolinite and iron hydroxide, followed by smectite and zeolites at higher pH. Modeling result suggest that the first sulfur bearing phase to precipitate is elemental sulfur, followed by greigite and mackinowite upon further basalt dissolution. © 2012 Elsevier Ltd. Source

Scott S.,University of Iceland | Gunnarsson I.,Reykjavik Energy | Arnorsson S.,University of Iceland | Stefansson A.,University of Iceland
Geochimica et Cosmochimica Acta | Year: 2014

The geochemistry of aquifer fluids of the Hellisheidi geothermal system, southwest Iceland, was studied. Based on samples of vapor and liquid from well discharge fluids, the aquifer fluid compositions at the depth of the geothermal system were reconstructed taking into account the highly variable degree of excess well discharge enthalpy, where the enthalpy of the discharge is significantly higher than that of vapor-saturated liquid at the measured aquifer temperature. Decreasing concentrations of non-volatile components such as Si in the total well discharge suggest that the main cause of elevated discharge enthalpies is liquid-vapor phase segregation, i.e. the retention of liquid in the aquifer rock due to its adhesion onto mineral surfaces. Moreover, the slightly lower than equilibrium calculated concentrations of H2 and H2S in some of the hottest and highest-enthalpy wells is considered to be caused by conductive heat transfer from the rocks to the fluids. Alternatively, the cause may lie in the selection of the phase segregation conditions. The calculated concentrations of volatile species in the aquifer fluid are very sensitive to the assumed phase segregation conditions while non-volatiles are not greatly affected by this model parameter. In general, the level of uncertainty does not contradict previous findings of a close approach to fluid-mineral equilibrium at aquifer temperatures above 250°C. The CO2 concentrations were observed to fall below equilibrium with respect to the most likely mineral buffers, suggesting a possible source control. Elevated H2 concentrations indicate a small equilibrium vapor fraction in aquifer fluids (~0.2% by mass or ~3% by volume). Previous conceptual models of the Hengill volcanic area (e.g. Bödvarsson et al., 1990) have implied a central magmatic heat source underlying the Hengill central volcano. Instead, a new conceptual model of the Hellisheidi system is proposed that features two main regions of fluid upflow heated by a complex of dikes and sills associated with an eruptive fissure active during the Holocene. © 2013 Elsevier Ltd. Source

Galeczka I.,University of Iceland | Wolff-Boenisch D.,University of Iceland | Jonsson T.,University of Iceland | Sigfusson B.,Reykjavik Energy | And 2 more authors.
Applied Geochemistry | Year: 2013

The objective of this study was to design, build, and test a large scale laboratory high pressure column flow reactor (HPCFR) enabling experimental work on water-rock interactions in the presence of dissolved gases, demonstrated here by CO2. The HPCFR allows sampling of a pressurized gas charged liquid along the flow path within a 2.3m long Ti column filled with either rock, mineral, and/or glass particles. In this study, a carbonated aqueous solution (1.2M CO2(aq)) and basaltic glass grains was used. Given the pressure and temperature rating (up to 10MPa at 90°C) of the HPCFR, it can also be used with different gas and/or gas mixtures, as well as for supercritical fluid applications. The scale of the HPCFR, the possibility of sampling a reactive fluid at discrete spatial intervals under pressure, and the possibility of monitoring the evolution of the dissolved inorganic carbon and pH in situ all render the HPCFR unique in comparison with other columns constructed for studies of water-rock interactions. We hope that this novel experimental device will aid in closing the gap between bench scale reactor experiments used to generate kinetic data inserted into reactive transport models and field observations related to geological carbon sequestration. A detailed description and testing of the HPCFR is presented together with first geochemical results from a mixed H2O-CO2 injection into a basalt slurry whose solute concentration distribution in the HPCFR was successfully modelled with the PHREEQC geochemical computer code. © 2012 Elsevier Ltd. Source

Aradottir E.S.P.,Reykjavik Energy | Aradottir E.S.P.,University of Iceland | Sonnenthal E.L.,Lawrence Berkeley National Laboratory | Bjornsson G.,Reykjavik Geothermal | Jonsson H.,University of Iceland
International Journal of Greenhouse Gas Control | Year: 2012

Two and three-dimensional field scale reservoir models of CO2 mineral sequestration in basalts were developed and calibrated against a large set of field data. Resulting principal hydrological properties are lateral and vertical intrinsic permeabilities of 300 and 1700×10-15m2, respectively, effective matrix porosity of 8.5% and a 25m/year estimate for regional groundwater flow velocity.Reactive chemistry was coupled to calibrated models and predictive mass transport and reactive transport simulations carried out for both a 1200-tonnes pilot CO2 injection and a full-scale 400,000-tonnes CO2 injection scenario. Reactive transport simulations of the pilot injection predict 100% CO2 mineral capture within 10years and cumulative fixation per unit surface area of 5000tonnes/km2. Corresponding values for the full-scale scenario are 80% CO2 mineral capture after 100years and cumulative fixation of 35,000tonnes/km2. CO2 sequestration rate is predicted to range between 1200 and 22,000tonnes/year in both scenarios.The predictive value of mass transport simulations was found to be considerably lower than that of reactive transport simulations. Results from three-dimensional simulations were also in significantly better agreement with field observations than equivalent two-dimensional results.Despite only being indicative, it is concluded from this study that fresh basalts may comprise ideal geological CO2 storage formations. © 2012 Elsevier Ltd. Source

Opfergelt S.,University of Oxford | Opfergelt S.,Catholic University of Louvain | Burton K.W.,University of Oxford | Burton K.W.,Durham University | And 8 more authors.
Geochimica et Cosmochimica Acta | Year: 2014

Understanding the biogeochemical cycle of magnesium (Mg) is not only crucial for terrestrial ecology, as this element is a key nutrient for plants, but also for quantifying chemical weathering fluxes of Mg and associated atmospheric CO2 consumption, requiring distinction of biotic from abiotic contributions to Mg fluxes exported to the hydrosphere. Here, Mg isotope compositions are reported for parent basalt, bulk soils, clay fractions, exchangeable Mg, seasonal soil solutions, and vegetation for five types of volcanic soils in Iceland in order to improve the understanding of sources and processes controlling Mg supply to vegetation and export to the hydrosphere. Bulk soils (δ26Mg=-0.40±0.11‰) are isotopically similar to the parent basalt (δ26Mg=-0.31‰), whereas clay fractions (δ26Mg=-0.62±0.12‰), exchangeable Mg (δ26Mg=-0.75±0.14‰), and soil solutions (δ26Mg=-0.89±0.16‰) are all isotopically lighter than the basalt. These compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex. Successive adsorption-desorption of heavy Mg isotopes leads to the preferential loss of heavy Mg from the soil profile, leaving soils with light Mg isotope compositions relative to the parent basalt. Additionally, external contributions from sea spray and organic matter decomposition result in a mixture of Mg sources on the soil exchange complex. Vegetation preferentially takes up heavy Mg from the soil exchange complex (δ26Mgplant-exch=+0.50±0.09‰), and changes in δ26Mg in vegetation reflect changes in bioavailable Mg sources in soils. This study highlights the major role of Mg retention on the soil exchange complex amongst the factors controlling Mg isotope variations in soils and soil solutions, and demonstrates that Mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of Mg that is bioavailable for plants and is exported to the hydrosphere. © 2013 Elsevier Ltd. Source

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