Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A reactive percolation experiment was conducted by injecting seawater into a permeable olivine aggregate at 190 °C and 19 MPa to explore the relationships between hydration reactions and hydrodynamic properties during the onset of serpentinization in the ultramafic lithosphere exposed at mid-ocean ridges. The experiment was stopped after 23 days when the sample became impermeable. The initial flow rate was 0.2 mL/h and then was decreased down to 0.06 mL/h after 8 days. Permeability decreased continuously throughout the experiment. The analyses of fluid chemistry time series and of the mineralogy and structure of the reacted sample showed olivine dissolution and precipitation of proto-serpentine, brucite and Fe-oxides. These reactions are controlled by coupled hydrodynamic and chemical processes interacting at different time and spatial scales. Their first characteristic is the production of silica rich outlet fluids in disequilibrium with the observed reacted mineral assemblages. These compositions are interpreted as resulting from a suite of coupled dissolution-precipitation reactions, controlled at the pore scale by surface kinetic processes, that rapidly reaches steady-state (constant fluid composition independent of fluid flux). Hence, the effective rate of serpentinization was controlled, to the first order, by the transport of reactants during the experiment. Mass balance calculations show that the rate of olivine conversion was fast (0.2-0.5 wt%/day), yet only ∼8 wt% of the olivine sample reacted, because the permeability drop limited fluid circulation. Porosity varied little during the experiment compared to permeability changes: the decrease of permeability was explained by the structure of the newly formed serpentine favouring the clogging of fluid paths. The rate at which permeability decreased was the fastest at low flux conditions. This suggests that permeability changes were not controlled simply by the kinetics of the serpentinization reactions, but mainly reflected the development of low Peclet pore scale zones forming micro-environments that control the serpentinization reaction-paths. These results outline the need to take into account the coupling of hydrodynamic and chemical processes when modelling the effects of reactive transport within the hydrating oceanic lithosphere.
ISSN: 0012-821X

hal-00855043
https://hal.archives-ouvertes.fr/hal-00855043
DOI : 10.1016/j.epsl.2013.03.052

Éditeur(s) : HAL CCSD
Résumé : In order to minimize CO2 atmospheric concentration, a solution consists in sequestrating CO2 in geological reservoirs. To estimate long term risks, it is necessary to quantify the couplings between reaction processes as well as structural and hydrodynamical modifications. We realised two experimental benches enabling injecting CO2-enriched-brine in conditions corresponding to in situ storage (T < 200 oC and P < 200 bar) and developed
an experimental protocol using X-Ray microtomography and fluid and rock analyses in order to measure the variations of physical and chemical parameters.
The study of carbonated reservoirs near the injection well, allows quantifying different k-phi relationships depending on the dissolution processes and triggered by the local fluid chemical composition and initials conditions. Away from the injection well, we observe carbonate precipitation decreasing the permeability. The study of fracturated caprock samples shows that alternative percolation of CO2-enriched-brine and CO2 gas increases the fracture permeability.
The study of silicated rocks indicates carbonate precipitation in zeolite sandstone and sintered dunite grains. Nevertheless, in zeolite sandstone we also observe the precipitation of clay particles located in the uid pathways which decrease strongly the permeability.
Afin de minimiser la concentration en CO2 dans l'atmosphère une solution consiste à le séquestrer dans les réservoirs géologiques. Pour évaluer les risques à long terme, il est nécessaire de quantifier les couplages entre les processus réactionnels et les modifications structurales et hydrodynamiques. Dans cette optique, nous avons construit deux dispositifs expérimentaux permettant d'injecter des saumures chargées en CO2 dans les conditions de stockage (T < 200 °C et P < 200 bar) et mis en oeuvre un protocole expérimental utilisant la microtomographie RX et l'analyse des roches et des fuides pour quantifier les variations des paramètres physiques et chimiques. Dans le cas des réservoirs carbonatés, on observe, près du puits d'injection, une forte variabilité des relations k-phi en fonction du régime de dissolution contrôlé par la chimie locale du fluide, ainsi que par les conditions initiales. A plus grande distance, on observe des processus de précipitation diminuant fortement la perméabilité. L'étude des roches de couverture fracturées (argilite) a montré qu'une percolation alternée d'une saumure chargée en CO2 et de CO2 gaz augmentait la perméabilité de fracture. Dans le cas des roches silicatées, la précipitation de carbonates est mise en évidence aussi bien dans les grès à zéolites que dans les frittés de dunite de San Carlos. Cependant, dans les grès à zéolites, la précipitation d'une phase argileuse est observée dans les chemins d'écoulement et engendre une forte diminution de la perméabilité.
https://tel.archives-ouvertes.fr/tel-00416408

tel-00416408
https://tel.archives-ouvertes.fr/tel-00416408
https://tel.archives-ouvertes.fr/tel-00416408/document
https://tel.archives-ouvertes.fr/tel-00416408/file/These_Luquot.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A set of four reactive flow-through experiments at temperature T = 100 degrees C and total pressure P=12 MPa was performed in limestone reservoir samples. By using various ranging from 0.7 to 10 MPa, these experiments mimic mass transfers occurring (1) near the injection well, where the brine is almost saturated with CO2 (i.e. P-CO2 approximate to P), and (2) at increasing distances from the injection well, where the fluid displays lower values and higher divalent cation concentrations due rock dissolution along the fluid pathway. Results for P-CO2 = 10 MPa show non-uniform dissolution features associated with transport-controlled mass transfer, while reaction-controlled uniform dissolution is observed for P-CO2 = 2.5 MPa. The experiment with P-CO2 = 6 MPa allows investigating in detail the transition from transport-to reaction-controlled dissolution. Conversely, the experiment reproducing conditions far from the injection well (P-CO2 = 0.7 MPa), shows a decrease of porosity triggered by the precipitation of Mg-rich calcite. For all the dissolution experiments, the time-resolved porosity phi(t) can be modeled by a simple non-linear equation including parameters that characterise the dissolution regime triggered by the reactivity of the inlet fluid (measured by the Damkohler number, Da). Furthermore, all dissolution experiments display power scaling between permeability (k) and porosity (phi) with distinctly different scaling exponents characterising the reactivity of the fluid percolating the sample, independently from the decrease with time of the reactive surface area. It is shown also that dissolution at moderate positive values of Da seems the most efficient to increase permeability and promote a rapid spreading of the reaction front, while inducing minimal modification of the porosity in the vicinity of the CO2 injection well. These results can be used to parameterize the k-phi function for modeling the earliest dissolution processes occurring in the vicinity of the reaction front.
ISSN: 0009-2541

hal-00445277
https://hal.archives-ouvertes.fr/hal-00445277
DOI : 10.1016/j.chemgeo.2009.03.028

Éditeur(s) : HAL CCSD
Résumé : This study provides additional constraints on the relations between deformation, hydration and percolation of fluids and melts in the subcontinental lithospheric mantle beneath a craton and a rift, as well as their implication on its geodynamical behaviour. I have analysed the microstructures, the CPOs, and the hydrogen content of mantle xenoliths from the Kaapvaal craton, and two sets of xenoliths from different localities along the East African Rift (North Tanzanian Divergence and SE Ethiopia).The coarse-granular microstructures and the well-defined CPOs in Kaapvaal peridotites suggest a deformation followed by a long quiescence time. Orthorhombic olivine CPOs predominates, but axial-[100] and axial-[010] are also measured. Cratonic peridotites record multiple metasomatic episodes, leading to a significant compositional heterogeneity, which cannot be imaged by seismic studies. Olivine hydrogen contents are variable, but tend to increase until 150 km depth, reaching up to 50 ppm wt. H2O. The deeper samples are almost dry. Piston-cylinder experiments on hydrogen diffusion between a volatile-rich kimberlitic melt and forsterite suggest that the presence of CO2 in the system could significantly decrease water fugacity and thus forsterite hydration. These experimental results indicate that the hydrogen contents measured in olivine were acquired during a metasomatic event rather than during xenolith extraction by kimberlites. However, this metasomatism was not followed by remobilization of the cratonic root.In the North Tanzanian Divergence, localities within the rift axis and the volcanic transverse belt (Lashaine and Olmani) show significant differences in microstructures and olivine CPO patterns. In Lashaine, coarse-granular microstructures and orthorhombic to axial-[100] CPO patterns in olivine can be explained by transpressional deformation during the formation of the Mozambique belt, or by the occurrence of a remnant of a cratonic domain embedded within the Mozambique belt. Within the rift axis, porphyroclastic to mylonitic microstructures suggest a recent rift-related deformation accompanied by syn-kinematic melt-rock reactions, and followed by variable annealing. The strong heterogeneity in microstructures and olivine CPO suggests that this deformation was acquired during multiple tectonic events probably linked to episodic magma percolation, separated by quiescence episodes. The axial-[100] patterns in olivine and the oblique fast directions reported by SKS studies are coherent with transtensional deformation within the lithospheric mantle beneath the rift.The peridotites from SE Ethiopia are less recrystallized than the rift-axis Tanzanian peridotites, displaying coarse-porphyroclastic microstructures. Microstructures and orthorhombic CPOs in olivine suggest syn- to post-metasomatic deformation. S-waves polarization anisotropies calculated for these samples cannot explain alone the delay times reported by SKS studies in this part of the East-African Rift.
Les travaux réalisés durant cette thèse apportent de nouvelles contraintes sur les relations entre déformation, hydratation et percolation de fluides et/ou de magmas dans le manteau subcontinental sous un craton et sous un rift, et leurs implications sur son comportement rhéologique. Il repose sur l'analyse des microstructures, des OPRs et des teneurs en hydrogène de xénolites mantelliques du craton du Kaapvaal, et sur deux séries de xénolites provenant de différentes localités le long du rift Est-Africain (Divergence Nord Tanzanienne et SE de l'Ethiopie). Les microstructures granulaires à gros grains et les OPRs bien définies des péridotites du craton du Kaapvaal sont cohérentes avec un épisode de déformation suivi d'une longue période de quiescence. Les OPRs de l'olivine sont majoritairement à symétrie orthorhombique, mais des symétries axiale-[100] et axiale-[010] sont aussi mesurées. Les péridotites cratoniques enregistrent de multiples épisodes métasomatiques, ayant entraîné une hétérogénéité de compositions à petite échelle ne pouvant être détectée par les études sismiques. Les teneurs en hydrogène mesurées dans l'olivine sont variables, mais ont tendance à augmenter jusqu'à 150 km de profondeur, atteignant alors jusqu'à 50 ppm wt. H2O. En dessous de cette profondeur, les échantillons montrent des teneurs en hydrogène très faibles. Les expériences réalisées en piston-cylindre sur la diffusion de l'hydrogène issue d'un liquide kimberlitique vers de la forstérite suggèrent que la fugacité en eau pourrait fortement être diminuée par la présence de CO2, empêchant l'hydratation de l'olivine durant extraction des xénolites par les kimberlites. Ces résultats expérimentaux suggèrent que les teneurs en hydrogène dans l'olivine des péridotites du craton du Kaapvaal ont été acquises durant un épisode métasomatique en profondeur et non pendant leur extraction par les kimberlites. Ces teneurs n'ont toutefois pas à ce jour entraîné de remobilisation de la racine cratonique. Enfin, le calcul des propriétés sismiques des péridotites cratoniques révèle que les anisotropies générées par les OPRs de ces échantillons sont suffisantes pour expliquer les anisotropies mesurées par les ondes SKS et les ondes de surface.Les xénolites de la Divergence Nord-Tanzanienne, montrent des variations significatives de microstructures et d'OPR de l'olivine entre les péridotites des localités dans l'axe du rift et celles de la chaîne volcanique transverse (Lashaine et Olmani). A Lashaine, les microstructures granulaires à gros grains et les OPRs de type orthorhombique et axial-[010] peuvent être expliquée par une déformation en transpression liée à la formation de la chaîne Mozambique ou par la présence d'une relique d'un domaine cratonique à l'intérieur de la chaîne Mozambique. Dans l'axe du rift, les microstructures porphyroclastiques à mylonitiques suggèrent une déformation plus récente, accompagnée de réactions magma-roche sous des conditions proches du solidus, suivie d'un recuit variable. L'hétérogénéité des microstructures enregistrées par les échantillons du rift suggère de multiples épisodes de déformation localisée, probablement liés à l'injection percolation épisodique de magmas, espacés de périodes d'accalmie. Les OPRs de l'olivine de type axial-[100] et l'orientation des directions de polarisation des ondes SKS suggère que le rift s'est formé en régime de transtension Les péridotites du Sud-Est de l’Éthiopie présentent des microstructures porphyroclastiques à gros grains moins recristallisées qu'en Tanzanie. Les microstructures et les OPRs principalement de type orthorhombique suggèrent une déformation syn- à post-métasomatisme. Les anisotropies de polarisation des ondes S calculées pour ces échantillons sont insuffisantes pour expliquer à elles seules les déphasages des ondes SKS dans cette partie du rift.
https://tel.archives-ouvertes.fr/tel-01684759

Droits : info:eu-repo/semantics/OpenAccess
NNT : 2014MON20139
tel-01684759
https://tel.archives-ouvertes.fr/tel-01684759
https://tel.archives-ouvertes.fr/tel-01684759/document
https://tel.archives-ouvertes.fr/tel-01684759/file/43443_BAPTISTE_2014_archivage_cor.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Underground CO2 sequestration is highly recommended as an effective means of significantly decreasing CO2 concentration in the atmosphere. Mineral storage is the more secure technology, but requires the presence of high concentration of divalent cations in the pore-fluid. Results from CO2 percolation experiments through chlorite/zeolite-rich sandstone samples from the Pretty Hill Formation (Otway Basin, Australia) are presented. The dissolution of the laumontite (7 wt.%) and chamosite (7 wt.%) are the potential sources of calcium, iron and magnesium required for carbonate precipitation. The percolation experiment was setup to reproduce, at laboratory scale, the in situ temperature and pressure conditions (T = 95 degrees C and P = 10 MPa). The fluid injected at constant flow rate is a rock-equilibrated brine subsequently enriched in CO2 up to partial pressure of 6 MPa. We observe feldspars, laumontite and chamosite dissolution, kaolinite and silica precipitation and a noticeable sink of CO2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of CO2 and Fe-rich carbonate. Permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths, while porosity increases. The high reactivity of this sandstone makes this reservoir a valuable target for CO2 mineralization, but the associated permeability decrease may limit the injection rate and the spreading of the CO2 in the reservoir.
ISSN: 0009-2541

hal-00712829
https://hal.archives-ouvertes.fr/hal-00712829
DOI : 10.1016/j.chemgeo.2011.11.018

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
In situ mineralization of CO2 in ultramafic rock-hosted aquifers is one of the promising solutions for decreasing CO2 concentrations in the atmosphere. Naturally altered ultramafic rocks suggest that carbonation processes are controlled by local heterogeneities in the structure of the rock and fluid transport at the water-rock interfaces. We studied the role of rock crystallographic anisotropy relative to the global fluid flow direction on the mineralization of CO2 by means of electron microscope analyses from the macro- to the micrometer scale (EBSD-FIB). The sample used for the measurements was a hot pressed olivine core percolated by water enriched in CO2 (pCO2 = 10 MPa) at 180 °C. During the percolation experiment, olivine was dissolved and two types of carbonates, dolomite, and magnesite, were precipitated on olivine surfaces. The results showed that the dissolution of olivine is controlled by its crystallographic properties as shown by the development of etch-pits only on the (010)ol planes and with elongated shapes parallel to the [010]ol axes. In contrast, the precipitation of carbonates is governed by hydrodynamic properties. Carbonates are heterogeneously distributed in the percolated rock. They are mainly located along the moderate (for dolomite) and the minor (for magnesite) flow paths, both oriented parallel to the principal fluid flow direction, which allow carbonates to be supplied with divalent cations (e.g., Ca2+, Mg2+, and Fe2+). In these flow paths, carbonate growth is systematically oriented normal to the flow that facilitates the development of chemical gradients with cationic supersaturation conditions for carbonate precipitation near the walls.In natural systems, the (010)ol planes are parallel to the Moho and the (100)ol planes are vertical; our study suggests that flow of CO2-rich fluids will induce precipitation of carbonates localized along, and preferentially clogging, vertical flow paths while favoring olivine dissolution along horizontal fluid pathways. This dual control of structure and fluid flow on carbonation mechanisms could be an important parameter allowing sustainable CO2 storage in peridotites, while limiting the risks of leakage toward the surface.
ISSN: 0003-004X

hal-01143393
https://hal.archives-ouvertes.fr/hal-01143393
DOI : 10.2138/am-2015-4913

Éditeur(s) : HAL CCSD
Résumé : In coastal carbonate aquifers submitted to a saline intrusion, water - rock interactions processes at the interface between freshwater and saltwater are a cause for heterogeneities development within the porous media. The dissolutions and precipitations induced by the mixing zone play a major role in the transport properties of the reservoir and so in the long term evolution of the saltwater intrusion. The characterization and the in situ observation of a current saline intrusion in the Llucmajor carbonate platform at the experimental site of Ses Sitjoles (South-Easth of Mallorca Island, Spain), allowed to study the temporal evolution of the saltwater intrusion and the biogeochemical processes active in the mixing zone. The influence of microstructure, characterized using X-Ray microtomography, on the petrophysical and flow properties was investigated in laboratory and flow-through experiments were also conducted to study the modifications of the microstructures when submitted to water - rock interactions processes. The mixing zone at the Ses Sitjoles site appears as quite thick and not very sensitive to climate and anthropic stress and is proved to be the locus of active microbiological activity. Laboratory studies highlight a strong control of the microporosity on transport processes and of pCO2 on water - rock interactions. The percolation experiments of slightly reactive waters reveal calcite dissolution together with a decrease in permeability due to a rearrangement of micro-grains within the microporous network.
Les processus d'interactions fluide-roche à l'interface eau douce-eau salée, dans les aquifères carbonatés côtiers soumis à une intrusion saline, sont une des causes du développement d'hétérogénéités dans la structure du milieu poreux. Les dissolutions et/ou précipitations engendrées par la présence de cette zone de mélange jouent un rôle déterminant pour les propriétés de transport du réservoir et donc l'évolution à long terme de l'intrusion d'eau salée. En prenant l'exemple de la plateforme carbonatée de Llucmajor au niveau du site expérimental de Ses Sitjoles (Sud-Est de Majorque, Espagne), un travail de caractérisation et d'observation in situ à moyen terme de l'intrusion saline actuelle de ce réservoir permet de décrire son évolution temporelle et les processus biogéochimiques actifs dans la zone de mélange. L'influence de la microstructure du milieu poreux, caractérisée par microtomographie RX, sur les propriétés pétrophysiques et hydrodynamiques est étudiée en laboratoire et des expériences de percolations d'eaux réactives (mélanges eau douce - eau salée et eau pure chargée en CO2) sont réalisées afin d'étudier les modifications de ces microstructures lorsqu'elles sont soumises à des interactions eau - roche. La zone de mélange eau douce - eau salée rencontrée au niveau du site de Ses Sitjoles est large et peu sensible aux contraintes extérieures (climatologiques et anthropiques) et constitue un lieu d'activité microbiologique particulièrement actif. En laboratoire, on observe un fort contrôle des phénomènes de transport par la microporosité et des processus d'interactions eau - roche par la pCO2. Dans le cas d'une percolation de fluides moyennement réactifs, les expériences mettent en évidence une dissolution de calcite accompagnée par une diminution de la perméabilité, liée à un réarrangement des micro-grains à l'intérieur du réseau microporeux.
https://tel.archives-ouvertes.fr/tel-00767152

tel-00767152
https://tel.archives-ouvertes.fr/tel-00767152
https://tel.archives-ouvertes.fr/tel-00767152/document
https://tel.archives-ouvertes.fr/tel-00767152/file/thesegaring.pdf

Éditeur(s) : HAL CCSD
Résumé : International audience
In real world situations, each person is generally in contact with only a small fraction of the entire population and exchange information through these interactions. Their number and their frequency vary from one to another individual and may be much depending on mobility of individuals. The objective of this article is to better understand how human mobility may have an impact on mobile social networking systems. This should help to answer a question as: "How might an information, a rumor, a pathogen, etc., driven by physical proximity, spread through a population?". We present a first stage of our work in which we focus on percolation processes as information flow mechanisms. We propose a synthetic mobility model and we define an artificial world populated by heterogeneous agents who differ in their mobility. Simulations are conducted on a multi-agent programmable environment. Our experimental results clearly demonstrate positive correlations between agent mobility factors and percolation thresholds.
Procedia CS

hal-00767043
https://hal.archives-ouvertes.fr/hal-00767043
DOI : 10.1016/j.procs.2012.06.007

Éditeur(s) : HAL CCSD
Résumé : The main purpose of this thesis was to characterize and to model the hydrodynamic and thermochemical processes leading to the alteration of the wellbore cement materials under borehole conditions. Percolation experiments were performed on fractured cement samples under CO2 storage conditions (60°C and 10MPa). Injection flow rate was dictated by the fracture aperture of each sample. CO2 enriched brine was flowed along the fracture aperture, and permeability changes as well as chemical evolution of major cations were continuously acquired during the experiment time. Reaction paths developed by the alteration of the cement were characterized using microtomography and ESEM images. The experiments conducted using samples presenting large fracture apertures during 5h showed that permeability was maintained constant during the experiment time. Three reacted layers were displaying by the alteration of portlandite and CSH. Long term experiment (26h) conducted with large initial fracture aperture showed a decrease of the permeability after 15hours of CO2 exposure. Otherwise, experiments performed on samples presenting narrow apertures indicated the conversion of portlandite and CSH to calcite leading to the permeability reduction and the fracture clogging. Assemblages of phases and chemical changes were modelled using GEMS-PSI speciation code. We studied also using a coupled transport-reactive model the conditions leading to the cement alteration and the formation of associated layers.
L'objectif de cette thèse était de modéliser à partir des expériences de percolation-réactive, les processus hydrodynamique et réactionnel qui gouvernent l'altération des ciments de puits. Différentes expériences ont été réalisées dans des conditions représentatives de celles du stockage du CO2. Des échantillons fracturés ont été utilisés pour injecter une saumure enrichie en CO2 à 60°C et 10MPa et à différentes pressions partielles de CO2. Le débitd'injection variait en fonction des propriétés hydrauliques de l'échantillon exposé. L'injection d'une saumure enrichie en CO2 à débit constant à travers une fracture supposée plane a permis d'étudier les modifications des propriétés hydrodynamiques et ces conséquences sur la géochimie et la microstructure du ciment altéré. L'impact dynamique de l'évolution microstructurale a été mis en évidence. Les expériences conduites sur des échantillons présentant de larges ouvertures, réalisées sur une durée de 5 h, ont montré que la perméabilité était maintenue constante le long de l'expérience. Trois couches d'altération se sont développées consécutivement à la dissolution de la portlandite et la décalcification des CSH. L'altération a entrainé la précipitation des carbonates et de la silice amorphe à proximité de la fracture. Dans le cas d'une expérience longue durée appliquant les mêmes conditions que précédemment on a observé que la croissance de la silice amorphe a entrainé la diminution de la perméabilité. Par ailleurs les expériences effectuées sur des échantillons présentant de faibles ouvertures, ont indiqué que la conversion de la portlandite en calcite conduit au colmatage de la fracture. L'évolution des assemblages de phases conduisant à la formation des carbonates et de la silice amorphe a été modélisée à partir du code géochimique GEMS. Les mécanismes de diffusion et les processus de mise en place des couches d'altération ont été étudiés à partir d'un modèle analytique et d'un modèle de transport réactif à partir du code géochimique PHREEQC.
https://tel.archives-ouvertes.fr/tel-01038034

tel-01038034
https://tel.archives-ouvertes.fr/tel-01038034
https://tel.archives-ouvertes.fr/tel-01038034/document
https://tel.archives-ouvertes.fr/tel-01038034/file/theseabdoulghafour.pdf

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The upward oil migration in a transparent and rough fracture containing a liquid of higher density is studied experimentally using a light transmission technique. The aperture heterogeneity is also measured by light transmission. The injection of oil in a fracture containing a heavier fluid produces a gravitational fingering instability. Depending on the injection velocity (capillary number, Ca) and the density difference between the two fluids (Bond number, Bo), different patterns are observed during the oil displacements. The width of the finger and its velocity are measured as a function of Bo and Ca. A stability analysis of the interface, based on a generalised Darcy equation in a two-dimensional fracture, allows an accurate description of the experimental results. The order of magnitude of the finger width and its velocity are in good agreement with the calculations, with small disparities due to various unknown fluid flow parameters: capillary pressure, finger thickness in the gap of the fracture, finger tortuosity and oil velocity field into the finger. The theoretical approach allows constructing a (Ca, Bo) phase diagram of stability of the interface.
ISSN: 0169-3913

hal-00429329
https://hal.archives-ouvertes.fr/hal-00429329
DOI : 10.1007/s11242-009-9340-2

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Carbonation of peridotite is a widespread process in nature, with emerging societal and environmental implications through geological storage of CO2. We studied the micro- and nano-scale structures associated with the carbonation of peridotite pervaded by a CO2-rich water and the induced changes of its hydrodynamic properties. Focused ion beam (FIB) nanotomography was coupled with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. An experimental sample of dunite core percolated by a CO2-rich fluid (pCO2 = 11 MPa) at 160 °C was compared with a naturally carbonated sample from the Atlantic Ridge (Exp. IODP 304). Results of the 8-days percolation experiment show continuous alteration of olivine into magnesite locally associated with a poorly crystalline serpentine-type mineral referred to as proto-serpentine. The experiment is also characterized by an overall decrease in rock porosity (from 7.7 to 6.4%) and permeability (from 8.5 × 10− 16 to 1.7 × 10− 16 m2). The primary inter-granular porosity (1 to 10 μm) is progressively filled with magnesite, leading to the overall decrease of permeability. At the same time, a secondary small-scale porosity (10 nm to 1 μm) is created at the tip of the dissolution features along the (010) cleavage planes of olivine grains. Carbonate crystals grow within these etch-pit structures behind the dissolution front. Dissolution and precipitation occur at a relative rate that maintains the secondary porosity over the timescale of the experiment (i.e. carbonate growth has to be the slowest one). This process is interpreted as an interface coupled dissolution-precipitation mechanism of olivine carbonation. Proto-serpentine is only formed in highly reduced flow areas, such as small olivine etch-pits (10 to 500 nm wide), which are more isolated from CO2 input. SEM-FIB analyses show the 3D preferential orientation of the dissolution-precipitation planes within a given olivine grain. It suggests the formation of lateral connections between the etch-pits (infra-micrometric “channels”) providing access to reactants and removing reaction products at the reaction sites. The experimental results show that olivine dissolution and magnesite precipitation converge to a quasi-steady state resulting in a constant carbonation rate controlled by reaction kinetics and not by mass transfer for this flow regime. The system self-regulates, independently of the porosity change at least during this incipient stage of olivine carbonation. Similar textures are observed in natural samples collected at 170 m-depth in the vicinity of veinlets driving late fluids down into the cooling oceanic lithosphere. This suggests that similar mechanisms are active in nature and can play an important role in the late, low temperature alteration stages of fractured peridotite. These results also highlight the critical role of nm-scale spatial arrangement of dissolution and precipitation features on mass transfers and on the sustainability of olivine carbonation through coupled dissolution-precipitation processes.
ISSN: 0009-2541

hal-01684733
https://hal.archives-ouvertes.fr/hal-01684733
DOI : 10.1016/j.chemgeo.2017.11.013

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
The dissolution of carbonate rocks usually leads to both porosity (ϕ) and permeability (k) increase. We present experimental evidences and physical-based models of positive and anti-correlated dynamics of k and ϕ observed during dissolution experiments of carbonate rocks. We study the way the rate of change of ϕ and k is controlled by the degree of undersaturation of the percolating solution for two different types of carbonate rocks. We document the occurrence of an anti-correlated k−ϕ trend when the flowing fluid (deionized water) has a weak capacity of dissolution. A positive correlation is found when CO2 is added to the deionized water to increase the potential dissolution rate. Detailed analyses of the microstructures of the rock performed by X-ray microtomography reveal that low dissolution rate favors detachment of solid particles and their subsequent accumulation at the pore-throat inlet. Particles are detached from the rock matrix due to the differential dissolution rate of the indurated grains and the microporous cement. We then propose a simple phenomenological model to interpret the effect of the pore-throat clogging by the accumulation of partially dissolved carbonate particles. We conjecture that permeability is controlled by the decrease of the effective hydraulic radius and the increase of the tortuosity due to partial and localized obstruction of the pore network. Conversely, increasing the level of undersaturation of the flowing solution leads to an augmented potential of dissolving most of the transported particles before they reach the throats. In this case, both k and ϕ increase and display power-law correlations.
ISSN: 0169-3913

hal-01174145
https://hal.archives-ouvertes.fr/hal-01174145
DOI : 10.1007/s11242-015-0456-2

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report concentrations of hydrogen (H) in upper mantle minerals of peridotites (olivine and pyroxenes) transported by alnöitic lavas, which erupted on the southwestern border of the Ontong Java Plateau (Malaita, Solomon Islands, West Pacific). Unpolarized FTIR analyses show that olivine, orthopyroxene, and diopside contain 2–32 ppm, 162–362 ppm and 159–459 ppm wt H2O, respectively. In the studied lherzolites, garnets are anhydrous. The concentration of hydrogen within individual olivine and pyroxene grains is almost homogeneous, indicating no evidence of dehydration or hydration by ionic diffusion. In the lherzolite, the concentration of hydrogen in olivine tends to increase weakly with depth (based on geothermobarometry), consistent with the increase of water solubility with increasing water fugacity as a function of pressure, but concentrations remain well below water-saturation values determined experimentally. The highest concentration of H in olivine (32 ppm wt H2O) is, however, found in refractory spinel harzburgites, which equilibrated at depths of 85 km., while deeper specimens as the high-temperature spinel harzburgites, and some of the garnet lherzolites, contain less hydrogen in olivine. Olivines from pyroxene- or pargasite-rich peridotites have also lower hydrogen concentrations. We interpret the high hydrogen concentrations in olivine from the refractory spinel harzburgites as due to (1) simultaneous hydration and metasomatism of the lithospheric mantle by a water-rich silicate melt/fluid, during which hydrogen follows MREE and where spinel harzburgite have experienced ‘stealth’ metasomatism, and/or (2) to a late ‘fleeting’ hydrogen metasomatism, which would hydrate the rock after this first ‘stealth’ metasomatism event. In the second case, the composition of the ‘fleeting’ percolating fluid (small volume fraction of very evolved fluids, with high volatiles concentration and transient properties) is likely to be linked to the decrease of the plume activity and resulting in the downwelling of the lithosphere asthenosphere boundary. The difference in hydrogen concentration between harzburgites and lherzolites could be linked to variation in trace elements in olivine induced by the first ‘stealth’ metasomatism alone or associated to the late ‘fleeting’ hydrogen metasomatism, triggered by the lack of secondary crystallization of clinopyroxenes in the harzburgite. To conclude, H concentrations in upper mantle minerals may thus be correlated to metasomatic events, with the later yielding variation of the mineral assemblage and trace element composition that constitute the peridotites.
ISSN: 0024-4937

hal-01143385
https://hal.archives-ouvertes.fr/hal-01143385
DOI : 10.1016/j.lithos.2014.11.005

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The minerals constituting the Earth's upper mantle are nominally anhydrous silicates (NAMs). However they do contain hydrogen as a trace element, decorating point defects in their crystalline structure. Experimental petrology and mineralogy have quantified the maximum concentration under several compositional and thermodynamic conditions, but systematic studies on the hydrogen concentration in minerals from mantle-derived rocks have only recently been carried out. Here, we have compiled the distribution of hydrogen in upper mantle peridotite xenoliths, from which several conclusions can be drawn. NAMs from peridotite xenoliths contain a few ppm wt. H2O in their structure. From the current database, the hydrogen concentrations in olivine regularly increase with increasing depth. The amount of hydrogen in NAMs from peridotite xenoliths from subduction contexts is not higher than in other geological context for similar temperature and pressure conditions. The highest hydrogen concentrations is found in peridotitic olivines from cratonic mantle, and are likely due to the depth of origin. The increasing hydrogen concentration in olivine with increasing depth is likely controlled by the increase of H partitioning into olivine at the expense of orthopyroxene as imposed by a decrease in Al content in opx with depth. However, the sparse data could also indicate that the bulk hydrogen concentration slightly increases with depth > 150 km. In this case, it would suggest, locally (Udachnaya for example), a possible increase in water fugacity due to fluid saturation. Even if the most abundant mineral in mantle rocks is olivine, the bulk hydrogen concentration in peridotites is controlled by the amount of hydrogen stored in pyroxenes. However, hydrogen concentration in olivine remains crucial for consequences on physical properties such as rheology and electrical conductivity. Kinetics of hydrogen transport is reviewed and hydrous melt/fluid percolation appears necessary to homogenize the hydrogen distribution at km-scale. Sampling of natural rock specimens is currently biased (e.g., in favor of optically attractive samples) and needs improvement, which will be achieved by increasing sample diversity (all type of grain sizes, lithologies, geological settings) and size of rock samples, as well as advances in analytical techniques. Acquisition of high quality data will be achieved by studying the co-existing minerals in mantle specimens, exploring each sample by linear and mapping measurements, and using appropriate FTIR calibrations for polarized and unpolarized radiation.
ISSN: 0024-4937

hal-01306676
https://hal.archives-ouvertes.fr/hal-01306676
DOI : 10.1016/j.lithos.2015.11.012