Uncertainty in the reactive transport model response to analkaline perturbation in a clay formation

The mineral alteration in the concrete barrier and in the clay formation around long-lived intermediate-level radioactive waste in the French deep geological disposal concept is evaluated using numerical modeling. There are concerns that the mineralogical composition of the surrounded clay will not be stable under the high alkaline pore fluid conditions caused by concrete (pH {approx} 12). Conversely, the infiltration of CO{sub 2}-rich groundwater from the clay formation into initially unsaturated concrete, at the high temperature (T {approx} 70 C) produced from the decay of radionuclides, could cause carbonation, thereby potentially affecting critical performance functions of this barrier. This could also lead to significant changes in porosity, which would affect aqueous diffusive transport of long-lived radionuclides. All these processes are therefore intimately coupled and advanced reactive transport models are required for long-term performance assessment. The uncertainty in predictions of these models is one major question that must be answered. A mass-transfer model response to an alkaline perturbation in clay with standard model values is first simulated using the two-phase non-isothermal reactive transport code TOUGHREACT. The selection of input parameters is thereafter designed to sample uncertainties in a wide range of physico-chemical processes without making a priori assumptions about the relative importance of different feedbacks. This 'base-case' simulation is perturbed by setting a parameter to a minimum, intermediate or maximum value or by switching on/off a process. This sensitivity analysis is conducted using grid computing facilities of BRGM (http://iggi.imag.fr). Our evaluation of the preliminary results suggests that the resaturation and the heating of the near-field will be of long enough duration to cause a limited carbonation through all the width of the concrete barrier. Another prediction is the possibility of self-sealing at the concrete/clay interface. Further research is however required to discuss the effect of such evolution on the desirable performance function of both barriers

Simulation of the degradation of a concrete/clay interface: influence of temperature, unsaturated conditions and porosity variations

For long-lived intermediate-level radioactive waste, the use of concrete as engineering barrier and Callovian-Oxfordian clay as geological barrier at a depth of 500 m is considered in the French disposal concept (ANDRA, 2005). Upon emplacement, initially unsaturated concrete is expected to experience coupled processes involving heating, re-saturation with groundwater from the clay formation, gas exchanges and geochemical reactions. After an early period of re-saturation, solute transport is supposed to be diffusion-controlled because of the extremely low permeability of the two media. These coupled processes may lead to changes in the porosity of the concrete or clay barriers. In the present paper, a fully coupled Thermo-Hydro-Chemical (THC) response of a two-phase (gas and solution) mass-transfer model was evaluated and tested by a sensitivity analysis. This study is an extension of a previous model applied to an isothermal and fully saturated concrete/clay interface (Burnol et al., 2005); it investigated the coupled effect of temperature and unsaturated conditions assuming no production of H2(g). The system was simulated for a 2000-year period, which covers the most predominant thermal perturbation

Experimental Measurement of CO2 Solubility in Aqueous NaCl Solution at Temperature from 323.15 to 423.15 K and Pressure of up to 20 MPa

ACLInternational audienceInterest in CO2 solubility in brine at high pressure and high temperature has grown in the last few decades. Solubility data are especially important in petroleum geology, carbon capture and geological storage, and geothermal reservoir engineering. Nevertheless, for the CO2 + NaCl + H2O system there are fewer solubility data available in literature, particularly at high salt molality. A high-pressure experimental device was designed to perform measurements for carbon dioxide solubility in a complex aqueous solution. The apparatus was first validated from experiment on the CO2-pure water system at 323.15 K by comparison with literature data. Thirty-six new experimental solubility data point were obtained in the pressure range between 5 and 20 MPa at three temperatures (323.15, 373.15, and 423.15 K) and at three molalities of NaCl (1, 3, and 6 moles per kilogram of water). Solubility measurements were obtained by potentiometric titration after sample trapping in a sodium hydroxide solution. The experimental solubility data generated in this work were consistent with literature data, and four original isotherms were obtained at high salinity. \textcopyright 2016 American Chemical Society

Experimental Measurement of CO2 Solubility in Aqueous CaCl2 Solution at Temperature from 323.15 to 423.15 K and Pressure up to 20 MPa Using the Conductometric Titration

ACLInternational audienceIn the framework of the efforts of the scientific community developed for the reduction of CO2 emissions, the geological storage of CO2 in deep saline aquifers is under focus. An increase of salinity decreases the potential of CO2 solubilization into the water. In salty waters, the salinity is not only due to NaCl but also to others ions and in particular Ca and Mg. Experimental solubility data of CO2 in calcium chloride solution available in the literature at conditions relevant to carbon storage are particularly scarce. In this work, a new analytical method was developed for experimental measurement of CO2 solubility in calcium chloride solutions (1, 3, and 6 mol/kg) at high pressures (5-20 MPa) and temperatures (323.15, 373.15, and 423.15 K). This method is based on conductometric titration coupled with classical pH titration. The conductimetry shows sharper curves than the pH titration allowing a higher precision. Thirty-six new experimental data are reported in this paper. These data presented an experimental average uncertainty of 2.1% with the ANOVA calculation method based on repeatability and reproducibility experiments. The CO2 solubility in CaCl2 solutions is noticeable lower than in NaCl solution increasing the salting out effect. Considering our previous work on NaCl solutions and this paper for CaCl2 solutions, estimations of the real quantity of CO2 that may be dissolved in saline aquifers can be made with a significantly better precision

Water-rock interactions and self-remediation: Lessons from a hydraulic fracturing operation in the Vaca Muerta formation, Argentina

International audienceIn order to analyze the effect of a new gelling agent for hydraulic fracturing, fluid samples from different stages of the operation (hydraulic fracturing fluid, coil tubing, flowback and produced waters) were collected from a well in the Vaca Muerta formation in Argentina. Collected samples were analyzed for major and trace elements, first within a few days after sampling, then reanalyzed 6 months later and again 2 years after sampling. Results show that the salinity of samples increased quickly with time, from 2000 mg/L up to 43,000 mg/l a month later, due to the mixing of hydraulic fracturing fluids with formation water. No evidence of water-rock reactions was observed. Results from the later analyses showed that the composition of the samples evolved with time with a sensible decrease of concentration for most trace elements over the course of these two years (e.g. Ba from 137 mg/L to 55 mg/L, Mn from 8 mg/L to 5 mg/L) and heavy metals (e.g. As /L to 1 f/L, Co /L to /L, Cr from /L to /L). Interpretation of the results shows that delayed, post-sampling, precipitation of barite in the preserved samples is the reason for such a decrease. This opens a very interesting option for mitigation and remediation of wastewaters from hydraulic fracturing as natural or even triggered precipitation of barite could involve most of the dissolved heavy metals and decrease strongly their concentrations

Synsedimentary to early diagenetic rejuvenation of barite-sulfides ore deposits: Example of the Triassic intrakarstic mineralization in the Lodève basin (France)

International audienceThe exhumed Lodève Basin (Hérault, France) provides a rich suite of outcrops showing diagenetic Ba–Pb–Fe–Cu fronts trapped in karst system in Cambrian dolomites during the Triassic post-rift exhumation of the basin. The sedimentological analysis on 10 sites in the basin reveals that barites-sulfides fronts formed during humid-arid climate fluctuations and the emplacement of a shallow lake environment. The fabric of ore deposits, the microthermometry of fluid inclusions entrapped within barites and the strontium/sulfur isotopic compositions of barite-sulfides associations indicate two distinct groups of mineralizations, Type I and Type II, which are contemporaneous but resulting from different processes. The synsedimentary mineralization of the Type I, the presence of only primary single-phase liquid fluid inclusions within barite crystals and the gradual increase of δ34S for both barites and chalcopyrites with depth (from −7 to +18.9‰ V-CDT) suggest ore precipitation close to the vadose zone under bacterial sulfate reduction (BSR) in a confined sulfate-rich playa lake aquifer. The similar 87Sr/86S ratios between barites and the overlying Triassic evaporites indicate that the barium and strontium derived directly from the overlying sulfate-rich lake. For the Type II, the high homogenization temperature of fluid inclusions entrapped within barite (modal Th between 60 and 80 °C) and the association with hydrocarbon markers, confirm the participation of deeper basinal brines in addition to downward percolating sulfates derived from the lake environment. The high positive values of δ34S for both barites and sulfides are typical of a precipitation linked to the combined action between anaerobic oxidation of methane and sulfate reduction (AOM-SR) at the sulfate-methane transition zone (SMTZ) during hydrocarbon migration. Similar 87Sr/86Sr ratios between Middle Triassic barites and previous Late Permian barites confirm that the source of metals precipitated at the SMTZ originated from the dissolution of anterior ore deposits located in the sulfate-depleted zone. This study links very shallow metallogenesis processes to reworking of MVT ore deposits by the action of sulfate-reducing bacteria around hydrocarbon seeps in a karstic environment

Mineralogical and geochemical study of serpentinized peridotites from the North-Western Pyrenees: New insights on serpentinization along magma-poor continental passive margins

International audienceThe peridotite massifs from the NorthWestern Pyrenean belt are fragments of a short-lived mid-Cretaceous hyper-extended rift inverted by the Pyrenean orogeny. We studied the petrology, mineralogy and geochemistry of 32 hydrothermally altered peridotites from the Montaut, Turon and Urdach massifs. This study aims to bring new constraints on the nature of serpentinization processes from domains having experienced various degrees of crustal hyper-extension and mantle exhumation in a context analogous to non-volcanic ocean-continent transitions. The three massifs studied are mainly composed of spinel-lherzolites and sample the same sub-continental mantle lithosphere as Eastern Pyrenean refertilized-peridotite massifs. They display contrasted degrees of hydrothermal alteration and serpentinization during cooling (e.g., serpentine contents ranging from 1 to 100 wt%) as well as variable reaction paths. The Montaut massif, located below an extremely thinned Paleozoic upper crust, records cooling from high temperature hydrothermalism with incipient serpentinization affecting mostly pyroxenes (serpentine, tremolite, talc, clinochlore assemblage; ~350-450 • C) to the formation of lizarditechrysotile and magnetite after olivine (~200 • C-350 • C). The Turon peridotites, which remained below the detached Mesozoic pre-rift cover after complete removal of the crust, are little serpentinized and have no magnetite, suggesting serpentinization during the latest stages of cooling (< 200 • C). The Urdach massif comprises magnetite-bearing ophicalcites as well as magnetite-poor serpentinites formed concurrently when mantle was exhumed to the sub-sea!oor at the margin toe (down to ~200 • C). Each massif shows selective enrichments in !uid mobile elements, in particular, Cs, Sb and Li, indicative of hydrothermal interactions with !uids deriving from neighboring sediments and continental crust, likely mixed with seawater-derived !uids for the Urdach peridotites. Our results show that the formation of magnetite in the studied samples is not tied solely to temperature, and point to a role of !uid composition in the development of the different serpentinization pathways observed from one massif to the other, and within a same massif in the case of Urdach. The development of complex sequences of temperature-, and composition-controlled reaction paths distinguish serpentinization in the Western Pyrenees peridotites from that commonly observed in the oceanic subsea!oor. It results in variable redistribution of iron between magnetite and serpentine minerals, with the predominance of Fe-rich lizardite in magnetite-poor serpentinized peridotites. It likely controls local redox conditions and H 2 production; suggesting that the impacts of serpentinization on the production of dihydrogen will vary from passive margins to oceanic sub-sea!oor

Origins of elements building travertine and tufa: new perspectives provided by isotopic and geochemical tracers.

18 pagesInternational audienceFluid/rock interaction represents a major process in the formation of calcitic or aragonitic travertine and tufa (CATT). In most cases, CATT is associated to limestone dissolution somewhere along the hydrogeological pathway. However, a wide array of other substratum (basalts, rhyolites, carbonatites, ultramafics, granites, dolomites, evaporites) can act as potential source of elements involved in the formation of CATT. This study reports on the evaluation of potential geochemical tracers linking CATT to their substratum, and unravelling the origin of elements. A large database was established from available literature data as well as new data acquired in the frame of this study for a set of Modern to Recent CATT (Ligurian ophiolites, Italy; the Chaine des Puys, Limagne graben and Paris Basin, France; Reunion Island, Indian Ocean; Jebel Oust, Tunisia). Four most reliable tracing methods are identified (1) δ13C and δ18O cross-plot allows distinguishing epigean (minδ13C = − 27.2 ‰, maxδ13C = 0.9 ‰, meanδ13C = − 12.3 ‰ for N = 314) from hypogean systems (minδ13C = − 4 ‰, maxδ13C = 11.7 ‰, meanδ13C = − 2.87 ‰ for N = 198). Very low δ13C values (− 4 ‰ associated to negative δ13C values are specifically indicative of an ultramafic source rock. (2) Barium and strontium cross-plot helps to discriminate different groups of source rocks amongst the hypogean CATT: (i) source rocks composed of mixed limestones, evaporites, and dolomites are characterised by low barium ( 400 ppm) contents, (ii) mafic and granitic source rocks are undifferentiated and display similar barium (from 15 to 930 ppm) and high strontium (> 200 ppm) contents, (iii) the carbonatite group is characterised by its exceptional high barium and strontium values. In epigean CATT, a pure limestone source rock usually relates to very low barium and strontium contents ( 50 ppm). (3) Relatively high beryllium content (> 30 ppm) in CATT seems to indicate a pure granitoid source. (4) High chromium concentrations (> 20 ppm) are systematically documented in Modern CATT located on an ultramafic substratum. The definition of diagnostic compositional fields for actively forming or recently formed CATT is influenced by many factors including water composition, water temperature, dissolved gas composition and concentration, biological activity, position in the sedimentary body and early diagenesis, in addition to substratum lithology. However, the results of this study illustrate that, despite these many factors, the combined use of Ba, Sr, Be, Cr, δ13C, and δ18O may be valuable to discriminate the rock lithology prevailing in the hydrogeological or palaeo-hydrogeological reservoir of CATT

Geochemical and sulfate isotopic evolution of flowback and produced waters reveals water-rock interactions following hydraulic fracturing of a tight hydrocarbon reservoir

International audienceAlthough multistage hydraulic fracturing is routinely performed for the extraction of hydrocarbon resources from low permeability reservoirs, the downhole geochemical processes linked to the interaction of fracturing fluids with formation brine and reservoir mineralogy remain poorly understood. We present a geochemical dataset of flowback and produced water samples from a hydraulically fractured reservoir in the Montney Formation, Canada, analyzed for major and trace elements and stable isotopes. The dataset consists in 25 samples of flowback and produced waters from a single well, as well as produced water samples from 16 other different producing wells collected in the same field. Additionally, persulfate breaker samples as well as anhydrite and pyrite from cores were also analyzed. The objectives of this study were to understand the geochemical interactions between formation and fracturing fluids and their consequences in the context of tight gas exploitation. The analysis of this dataset allowed for a comprehensive understanding of the coupled downhole geochemical processes, linked in particular to the action of the oxidative breaker. Flowback fluid chemistries were determined to be the result of mixing of formation brine with the hydraulic fracturing fluids as well as coupled geochemical reactions with the reservoir rock such as dissolution of anhydrite and dolomite; pyrite and organic matter oxidation; and calcite, barite, celestite, iron oxides and possibly calcium sulfate scaling. In particular, excess sulfate in the collected samples was found to be mainly derived from anhydrite dissolution, and not from persulfate breaker or pyrite oxidation. The release of heavy metals from the oxidation activity of the breaker was detectable but concentrations of heavy metals in produced fluids remained below the World Health Organization guidelines for drinking water and are therefore of no concern. This is due in part to the co-precipitation of heavy metals with iron oxides and possibly sulfate minerals