Carbon trapping in ultramafic (UM) and basaltic basements is one of the options explored to mitigate industrial CO2 emissions in the Earth’s atmosphere. UM rocks and basalts comprise silicates rich in divalent cations (Mg, Ca, Fe) that are dissolved to form carbonates when in contact with CO2-rich fluids, thus trapping CO2 over geological time scales. UM rocks have the highest concentrations in divalent cations and thus they have the highest potential for carbon trapping by CO2-mineralization. Nevertheless, because of their low permeability, UM basements have been overlooked for possible in situ CO2 storage in favor of basaltic basements. Recent research shows that CO2-mineralization is active and efficient in UM basements, and that it is associated to potential benefits, such as the production of H2. However, the hydrodynamic, physical and chemical mechanisms driving CO2-mineralization whilst sustaining fluid flow are still poorly understood and numerous scientific and technological challenges remain before implementing industrial CO2 geological storage in UM basements. Here we present an overview of our recent results on CO2-mineralization in UM rocks combining (i) laboratory experiments, and (ii) field studies of carbonated UM basements with a focus on the Semail ophiolite (Sultanate of Oman), in relation to the recently completed ICDP (International Continental Scientific Program) Oman Drilling Project.

Ophiolitic formations play a critical role in the groundwater resource of numerous countries and areas. Previous studies show that the structural heterogeneities of these rocks, coming from the presence of both different lithological units and multi-scale discontinuities, result in complex hydrogeological features that are not well characterized yet. In particular, there is a need for understanding how these heterogeneities impact the hydrodynamic properties of ophiolitic aquifers and the highly variable chemical composition of the water. To this end, we conduct various kinds of pumping experiments between two boreholes 15 m apart in the ophiolitic formation of the Batin (BA1) site in the wadi Tayin massif of the Sultanate of Oman. Cross-borehole open pumping experiments, as well as multi-level pumping and monitoring hydraulic tests, are performed in conductive zones that were identified from temperature and flowmeter data, but also in low-permeability zones requiring to manage very low pumping flow rates. The collected data are interpreted with a model implementing non-integral flow dimension, leakage and time-dependent pumping flow rates. The considered modeling concepts and the estimated hydrogeological properties show that the multi-directional structural heterogeneities of ophiolitic aquifers are key features that must be considered in future hydrogeological models because they drive the hydraulic responses of these systems.

The objective of this paper is to quantify the mass transfers involved in the hydrothermal alteration of olivine-rich peridotites in the presence of CO2-enriched waters, and to determine their effects on the rock hydrodynamic properties. Three flow-through experiments were performed at a temperature of 185 degrees C and a total pressure of 22.5 +/- 2.5 MPa. They consisted in injecting a hydrothermal fluid with different concentrations of carbon dioxide (CO2 = 6.26, 62.6 and 659.7 mmol.L-1 i.e. pCO(2) = 0.1, 1 and 10 MPa, respectively) into cylinders of sintered San Carlos (Arizona, USA) olivine grains. The results show that for low pCO(2) conditions (from 0.1 to 1 MPa), olivine is mainly altered into hematite and Mg(Fe)-rich phyllosilicates. Such iddingsitic-type assemblages may clog most of the rock flow paths, resulting in a strong decrease in permeability. Rare Ca-Fe-carbonate minerals also precipitated under these conditions despite the initial Mg-rich system. For higher pCO(2) conditions (similar to 10 MPa), olivine is more efficiently altered. A greater amount of poorly crystallized Fe(Mg)-rich phyllosilicates and magnesite is produced, and the carbonation rate of olivine is 3-11 times higher than when the pCO(2) is 10-100 times lower. Interestingly, the changes in porosity caused by the formation of carbonated and hydrous minerals are small while a strong decrease in permeability is measured during the experiments. The formation of reduced carbon is also observed. It is located preferentially at the inlet, where pH is the lowest. This testifies to a competition between reduction (probably associated with the oxidation of ferrous iron) and carbonation; two processes involved in the fixation of CO2 in a mineral form. One may speculate that the formation of reduced carbon can also be a significant mechanism of CO2 sequestration in olivine-dominated basements. (C) 2019 Elsevier Ltd. All rights reserved.

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This paper presents the results of 4 reactive percolation experiments set up for investigating the impact of flow rate on serpentinization reaction paths for conditions relevant of the oceanic peridotite sub-seafloor during the initial stages of its hydrothermal alteration. The experiments consisted in injecting artificial seawater into porous compressed olivine powder cores at constant flow rates Q: 0.24, 0.48,1.14 and 5.21 mL.h(-1). The experiments were conducted at constant temperature (170 degrees C) and pressure (25 MPa) and lasted 11 to 28 days. At the end of the experiments, the outlet fluids composition displayed similar compositions, buffered by the formation of serpentine (aMg(2+)/a(H+)(2) = 9.7-10; aSiO(2) = -3.9 to -5.2; pH (in situ) = 6.1). These values were achieved in a few to up to 300 h for the high flow rate experiment suggesting that they corresponded to a steady-state regime of mass transfer which depended on flow rate. Differences in the composition of fluid versus time and in the structure of reacted samples during and after the four reactive percolation experiments suggested also various incipient serpentinization reaction paths. The low Q experiments produced SiO2(aq) enriched outlet fluids and nodular aggregates were identified covering the reacted olivine surfaces. During high Q experiments, fibrous filaments of proto-serpentine were formed on the olivine surfaces and the fluids progressively achieved steady state compositions similar to the other experiments. These results together with those of previously published reactive percolation experiments lead us to propose two end-member reaction paths for incipient serpentinization of olivine-dominated permeable rocks infiltrated by seawater derived hydrothermal fluids: (1) a transport-controlled reaction path occurring in diffusion dominated zones is characterized by transient brucite precipitation, which produces Mg trapping and Si release in solution, followed by serpentine precipitation and (2) a kinetics-controlled reaction path occurring in advection dominated zones where transport conditions are favorable to Mg leaching and where serpentine precipitates first. The occurrence of these two end-member reaction paths is determined locally by the composition of the fluid, which varies along flow paths. Thus, both reaction paths can coexist in the sample depending on the local pore geometry. Our study shows that the interplay between fluid transport and reaction kinetics controls the chemical fluxes between the mineral surface and the bulk solution, and the incipient serpentinization reaction paths. In natural systems, the scale and distribution of these reaction domains will depend on the complex structure of the ultramafic basement. Our results suggest that the precipitation of serpentine and silica rich phases will be favored in fluid focusing zones such as faults and fractures, whilst formation of brucite will preferentially occur as part of pervasive background serpentinization. (C) 2018 Elsevier B.V. All rights reserved.

Mantle peridotites have the potential to trap CO2 as carbonates. This process observed in ophiolites and in oceanic environments provides a long term and safe storage for CO2. It occurs as a part of a complex suite of fluid–rock reactions involving silicate dissolution and precipitation of hydrous phases, carbonates and minor phases that may in turn modify the hydrodynamic properties and the reactivity of the reacted rocks. The efficiency and lastingness of the process require the renewal of fluids at the mineral-fluid interface. Fractures are dominant flow paths in exhumed mantle sections. This study aims at better understanding the effect of CO2-enriched saline fluids on hydrodynamic and chemical processes through fractured peridotites. Experiments were performed using the reactive percolation bench ICARE Lab 3 – Geosciences Montpellier. It allows monitoring the permeability changes during experiments. Effluents are recurrently sampled for analysing cation concentration, pH and alkalinity. Reacted rock samples were characterized by high resolution X-ray microtomography (ESRF ID19, Grenoble, France) and SEM. Experiments consisted in injecting CO2-enriched brines (NaCl 0.5 M) at a rate of 6 mL.h-1 into artificially fractured cores (9 mm diameter × 20 mm length) of Oman harzburgites at T=170°C and Ptotal = 25 MPa for up to 2 weeks. Fractures are of few µm apertures with rough walls. Three sets of experiments were performed at increasing value of [CO2] (0, 0.1 and 1 mol/kg). All experiments showed a decrease in permeability followed by steady state regime that can be caused by a decrease in the roughness of fracture walls (dissolution dominated process), thus favouring fracture closing, or by the precipitation of secondary phases. Maximum enrichments in Mg, Fe and Ca of the effluent fluids occur during the first 2 hours of the experiments whereas Si displays a maximum enrichment at t = 20 h, suggesting extensive dissolution. Maximum enrichments are observed with the highest values of the [CO2]. After one day, effluent fluid concentrations decrease and become constant. By analysing both the permeability and the outlet fluid concentration one can investigate the coupling processes controlling the transport and the reaction mechanisms that in turn act at maintaining the circulation in the fractures.

We investigate experimentally the alteration of fractured class-G cement flowed by CO2-rich brine. The experiment mimics a mechanically damaged rough-walled fractured cement annulus at temperature 60 degrees C and pressure 10 MPa. The experiment consists of flowing a reservoir-equilibrated brine mixed with CO2 (partial pressure of 2.3 MPa) through the fracture of average aperture 14 mu m at constant flow rate (100 mu L min(-1)). This flow rate corresponds to pressure gradient representative of an average in situ hydrodynamic condition. Results indicate an intense alteration of the cement with a large removal of mass at the scale of the sample. However, the fracture alteration patterns are triggered by the initial heterogeneity of the fracture aperture; the aperture of the low aperture zones tends to decrease due to calcite precipitation whereas preferential paths develop in the zones of higher aperture associated. Nevertheless, the expected large permeability increase triggered by the mass removal is mitigated by the precipitation of a low density Si-rich amorphous material. The alteration rate will decrease with time because of the increasing distance of diffusion between the fracture where the reactants are actively renewed by advection and the portlandite and C-S-H dissolution fronts. The different zones of reaction can be adequately modeled by a simple 1D diffusion-reaction model using published kinetics coefficients and extrapolation to larger times than the experiment time can be drawn. Altogether, and in addition to the previous studies of the alteration of fractured well cement annulus, this study shows that the leakage potential is strongly controlled by the initial distribution of the aperture along the fracture: low aperture zones will tend to self-heal while localized flow in connected high aperture paths will be perennial. (C) 2016 Elsevier Ltd. All rights reserved.

An experimental setup for shallow subsurface hydrogeophysical monitoring has been installed at the Maguelone site, located along the Mediterranean lido of the Gulf of Lions near Montpellier, France. This experimental site was developed in the context of MUSTANG EC project (FP7). SIMEx (Shallow Injection Monitoring Experiment) is a unique opportunity to test in a cost effective manner a full suite of coordinated monitoring techniques, either from surface or downhole. The field spread includes an injection hole, a logging hole, a downhole hydrodynamic observatory based on a pore fluid sampling completion from WestBay (SWS), a downhole seismic observatory, plus surface seismic observatories. This coordinated set of observatories should lead to the design of integrated sensors and methods for the monitoring of gas injection in deeper reservoirs. More recently, nitrogen injection was undertaken to measure the site response to gas injection. Nitrogen was chosen because of the reducing nature of the in-situ environment present in the shallow subsurface at Maguelone, precluding oxygen injection to avoid massive bacterial developments. The next phase of SIMEx will be that of CO2 injection using similar surface and downhole hydrogeophysical monitoring.

Contaminant migration in aquifers is one of the most debated issues in hydrogeology, as most experimental results display large deviations from the standard (asymptotic) Fickian dispersion theories. Multi-scale investigation and high-resolution sensors are required to determine the origin of non-asymptotic dispersion and validate models. For this, a set of multi-scale Single-Well Injection-Withdrawal (SWIW) tracer tests using a new dual-packer probe CoFIS, including a high resolution optical sensor TELog, are presented. When compared to standard techniques such as salinity measurements, it is shown that high-resolution optical measurements allow an improved characterization of the long-lasting non-asymptotic dispersion mechanisms. To cite this article: P Gouze et al., C. R. Geoscience 341 (2009). (C) 2009 Published by Elsevier Masson SAS on behalf of Academie des sciences.

We describe an experimental system including monitoring of temperature, pressure, pH, oxidation reduction potential and optical density at 600 nm, designed for studying the role of microorganisms on the geological sequestration Of CO2 and its transformation into solid carbonate phases. Measurements were performed in an artificial ground water (AGW) supplemented with urea (2 g.l(-1)) and equilibrated at controlled temperatures with a gaseous phase before bacterial inoculation. We used the ureolytic strain Bacillus pasteurii as a model carbonate precipitating bacteria and showed that it can successfully promote strong pH increases by ureolysis in the AGW equilibrated with CO2 pressures of up to I bar. Increasing salinities (5.8,13.5 and 35.0 g.l(-1)) have a positive effect on the rate of pH increase, whereas the effect of increasing temperatures (30,35 and 38 degrees C) is less important. Calcium is also shown to have a specific positive influence on the rate of ureolysis. The number of viable cells present in solution decreases greatly during the carbonate precipitation event but the population partially recovers once precipitation is over. (C) 2009 Elsevier B.V. All rights reserved.

We present a set of single‐well injection withdrawal tracer tests in a paleoreef porous reservoir displaying important small‐scale heterogeneity. An improved dual‐packer probe was designed to perform dirac‐like tracer injection and accurate downhole automatic measurements of the tracer concentration during the recovery phase. By flushing the tracer, at constant flow rate, for increasing time duration, we can probe distinctly different reservoir volumes and test the multiscale predictability of the (non‐Fickian) dispersion models. First we describe the characteristics, from microscale to meter scale, of the reservoir rock. Second, the specificity of the tracer test setup and the results obtained using two different tracers and measurement methods (salinity‐conductivity and fluorescent dye–optical measurement, respectively) are presented. All the tracer tests display strongly tailed breakthrough curves (BTC) consistent with diffusion in immobile regions. Conductivity results, measured over 3 orders of magnitude only, could have been easily interpreted by the conventional mobile‐immobile (MIM) diffusive mass transfer model of asymptotic log‐log slope of −2. However, the fluorescent dye sensor, which allows exploring much lower concentration values, shows that a change in the log‐log slope occurs at larger time with an asymptotic value of −1.5, corresponding to the double‐porosity model. These results suggest that the conventional, one‐slope MIM transfer rate model is too simplistic to account for the real multiscale heterogeneity of the diffusion‐dominant fraction of the reservoir.

A b s t r a c t : Carbonate dissolution of tensile fractures during percolation by CO2-enriched fluid is studied using synchrotronic XRay Computed Micro Tomography as a tool to quantify the changes of the fracture walls geometry and extend. The experimental dissolution of a pure-calcite sample confirms the process of homogeneous chemical “erosion” classically implemented in the numerical simulators, but with a reactive surface coefficient much larger than unity. In opposition, the dissolution of a non-pure calcite sample, representative of the average micritic carbonate composition, displays very heterogeneous dissolution patterns associated with an increase of the fractal dimension. The initial topographic surfaces of the fracture walls evolve rapidly toward “non-topographic” interfaces displaying multiple overhangs due the preferential dissolution of the carbonate grains. In this case the classical 2Dprofilometric methods fail. The specific surface coefficient increases strongly, more than 5 times the planar surface, and probably faster than the reactive surface

The changes of fracture surfaces geometry and extend are studied using X‐ray tomography during aperture increase due to CO2‐rich fluid percolation. Dissolution experiments were conducted on two micritic rock samples; one pure calcite end‐member and one with typical composition for marine carbonates (85% calcite). High‐resolution digital images of the fracture geometry allow quantifying the surface properties changes over four spatial scales with a resolution of 4.91 μm. Fracture surfaces are self‐affine with an initial dimension of 2.5. Dissolution of the pure‐calcite sample is clearly a process of homogeneous chemical “erosion” of the surface elevation: fractal dimension and specific surface remains constant (1.5 times the planar surface). Conversely, for the 85% calcite sample, initial topographic surfaces of the fracture walls evolve rapidly toward “non‐topographic” interfaces displaying overhangs due the preferential dissolution of the carbonate grains. In this case, the conventional definition of the effective aperture must be revisited. Such structures can only be assessed from 3D observations. As dissolution progresses, the specific surface increases strongly, more than 5 times the planar surface, and probably faster than the reactive surface.