GaMin’11 – an International Inter-laboratory Comparison for Geochemical CO2 - Saline Fluid - Mineral Interaction Experiments

Due to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies

Colloid/nanoparticle formation and mobility in the context of deep geological nuclear waste disposal (Project KOLLORADO-2) ; final report (KIT Scientific Reports ; 7645)

To assess the relevance of colloidal influences on radionuclide transport for the long-term safety of a radioactive waste repository, the KOLLORADO-2 project integrates the results of geochemical and hydrogeological studies. The results may serve as a basis for an appraisal of the implications of colloid presence in the vicinity of radioactive waste repositories in different deep geological host-rock formations

Engineered materials as potential geocatalysts in deep geological nuclear waste repositories: A case study of the stainless steel catalytic effect on nitrate reduction by hydrogen

International audienceThe reduction of NO3- in natural waters is commonly promoted by biological activity. In the context of deep geological nuclear waste repositories with potentially high H-2 pressure, abiotic redox reactions may be envisaged. Here, the catalytic effect of "inert'' metallic surfaces, in part used for nuclear waste canisters, on NO3- reduction under H-2 pressure is evaluated. The study is focused on stainless steels by testing the 316L and Hastelloy C276 steels. A parametric kinetic study (0 < P(H-2) < 10 bar, 0.1 < [NO3-] < 10 mM, 90 < T degrees < 150 degrees C, 4 < pH(in situ) < 9) reveals that NO3- reduction, in the presence of stainless steel 316L and Hastelloy C276, proceeds via a pH-independent reaction requiring H-2 as an electron donor. No corrosion of these steels is observed indicating a true catalytic process. The reaction is inhibited in the presence of PO43-. Activation energies assuming a first-order reaction in the 90-150 degrees C temperature range are found to be 46 kJ/mol for stainless steel 316L and 186 kJ/mol for Hastelloy C276, making the reaction efficient at lower temperature and on a human time scale. Nitrate sorption at the metallic surface being thought to be the limiting step, sorption and competitive sorption isotherms of several oxyanions were performed at 90 degrees C on 316L. Nitrate and PO43- are more strongly sorbed than SO42-, likely as inner sphere complexes, and in a large pH range, from acidic to pH 9. The Langmuir-Hinshelwood formalism best fits the kinetic data. The nature of the surface complex, and the competition for sorption between NO3- and PO43- account for the macroscopic features of NO3- reduction by H-2 observed at the steel surface. It should be stressed that some engineered materials such as stainless steels should be considered both as geological material and as geocatalysts as they could remain in the environment over an extremely long period of time

Abiotic nitrate reduction induced by carbon steel and hydrogen: Implications for environmental processes in waste repositories

International audienceReducing conditions induced by steel canister corrosion and associated H-2 generation are expected in nuclear waste repositories. Aqueous NO3- present in the aquifers will become thermodynamically unstable and may potentially be converted to N-2 and/or NH4+. However, NO3- reduction by H-2, in the absence of bio-mediators, is generally thought to be kinetically hindered at low temperature, although the reaction may be promoted by the concomitant oxidation of Fe. In this study the reduction rate of aqueous NO3- is quantified in the presence of H-2 and carbon steel surfaces from waste canisters and construction materials, as well as magnetite as their possible corrosion by-products. A parametric study (0 < P(H-2) < 10 bar, 0.1 < [NO3-] < 10 mM, 90 < T degrees < 180 degrees C, 4 < pH(in situ) < 9) reveals that even at 90 degrees C the reaction can occur within hours or days and leads to the formation of NH4+ and pH increase. Different mechanisms may be potentially involved. It is shown that NO3- reduction in the presence of carbon steel does not require H-2, since steel constitutes an electron donor by itself, as does metallic Fe. The reaction rate is strongly pH-dependent. Activation energy in the 90-180 degrees C range is found to be 45 kJ/mol. Magnetite is the main corrosion by-product and specific experimental runs demonstrate that it can serve as a catalyst for the NO3--H-2 reaction. Hydrogen alone, without the presence of steel, is not sufficient to reduce NO3- under the temperature and pressure conditions used in this study

Modeling deep control pulsing flux of native H2 throughout tectonic fault-valve systems

International audiencePulsing emanations of native hydrogen (H2) have been observed at the surface of emitting structures, specifically “Fairy circles” in Minas Gerais State, Brazil. However, the underlying cause of these H2 pulses remains unclear. Possible controlling factors include deep migration processes, interactions with the atmosphere and near-surface conditions. In this study, we examine the mechanisms that may trigger pulsating fluid migration and the resulting periodicity, with particular attention to the influence of deep geological processes. We employed a numerical model to simulate the migration of a constant deep fluid flow. To ensure the accuracy of the model in solving complex fluid flows, we conducted initial simulations to compare the morphology and amplitude of 2D thermal anomalies induced by buoyancy-driven water flow within a fault zone with another numerical model. Following the validation of our model in capturing complex fluid flows, we proceeded to model the H2 gas flow along a 1-km draining fault, intersected by a low permeable rock layer acting as a “pressure relief valve” system. Our objective is to investigate the conditions that give rise to a pulsing regime. Our results indicate that sustained surface bursts in the model only occur if: (I) a permeability with an effective-stress dependency is considered, (II) a significant contrast in permeability exists between different zones, (III) an adequately high initial effective stress state is present at the base of the low-permeability layer, and (IV) the incoming and continuous fluid flow of H2 at depth is sufficiently low to prevent instant “opening" of the low-permeability layer due to overpressure.The predicted periodicity for surface H2 pulses regulated by the fault-valve mechanisms is expected to range from 100 to 300 days, representing a considerably longer duration compared to the measurements observed up to now in Brazilian Fairy circles

Uranyl-chloride speciation and uranium transport in hydrothermal brines: Comment on Migdisov et al. (2018) “A spectroscopic study of uranyl speciation in chloride-bearing solutions at temperatures up to 250 °C”, Geochim. Cosmochim. Acta 222, 130–145

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A new nondestructive X-ray method for the determination of the 3D mineralogy at the micrometer scale

The combination of synchrotron-based X-ray absorption and fluorescence computed tomographies (CT) is a new method allowing a noninvasive and nondestructive determination of the three-dimensional (3D) mineralogy with micrometer resolution of sub-millimeter silicate grains, possibly stored in a silica holder. These CT were performed with beams of a few tens of keV from a third-generation synchrotron source on one olivine grain of the NWA817 Martian meteorite presenting a reddish alteration phase. The reconstructed sections show a network of fractures and a few micrometer-thick layers formed on one grain. The 3D facet orientation and the X-ray attenuation coefficient indicate that this grain is an Fo(44+/-9) olivine crystal. The fluorescence section reveals rims enriched in Fe (a major element) or depleted in Ca (a minor element). This CT combination shows that the micrometer-thick layer is preferentially formed on the (010) olivine face and has a lower density (3.5 +/- 0.4 g/cm(3)) than the olivine, even though it is enriched in Fe. Its complex nano-petrography and the distributions of nanometer-sized voids and fractures in such a micrometer thick layer, first observed by scanning electron microscopy on focused ion-beam cuts, is not shown by CT. The precision presently achieved, although moderate, is sufficient to obtain a 3D semi-quantitative view of the mineralogy consistent with the one previously established by electron probe microanalyses (Sautter et al. 2002)

Experimental simulation of chemomechanical processes during deep burial diagenesis of carbonate rocks

International audienceChemomechanical processes involved in the deep burial diagenesis of carbonate petroleum reservoirs are still poorly understood. To better understand these processes and explain how porosity and permeability can be preserved at the great depth of DBRs (deeply buried reservoirs), we developed an experimental device allowing both the simulation of high-pressure/stresses/temperature conditions (80 degrees C, 60MPa of confining pressure, and differential stress up to 40MPa) of DBR and the circulation of different fluids in rock samples. We tested (triaxial multistep creep tests) four core samples of a cemented limestone and analyzed creep deformations, fluids chemistry, and petrographical and petrophysical properties of samples. Different flow conditions (no flow and flow through) and chemical compositions (natural meteoric water with and without phosphate ions) were considered. Our study showed that the precipitation of calcite on free pore walls of micrites blocks the microporosity between micrite crystals, thus rendering the microporosity inaccessible to fluids. Hence, the connected porosity decreased strongly after experimentation. This is due to the PSC (pressure solution creep) which is the main process implied in the porosity reduction of a carbonate rock during deep burial. The preservation of macropores during PSC allows the preservation of permeability. In addition, calcite solubility is positively dependent on mechanical parameters (axial compaction and axial stress), thus suggesting that calcite can precipitate during decompression of deep basinal fluids, resulting in changes in porosity. A comparison of experimental results with theoretical calculations showed that the integration of the PSC process into calculation databases would greatly improve the modeling of DBR

Short lifespans of serpentinization in the rocky core of Enceladus: Implications for hydrogen production

International audienceThe discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' observed density, and variable water flow rates within the rocky core to constrain duration of active serpentinization, and therefore, the maximum age of the liquid water circulation in the core. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H 2 production rate in all model cases (> 1 ×10 12 moles yr-1) is several orders of magnitude greater than the minimum H 2 release rate calculated from the observed H 2 in Enceladus' plume (1 ×10 9 moles yr-1), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H 2 production for up to ~75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H 2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus developed only very recently-even as recent as within the past 100 Myr