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The simplified view of two-phase flow, such as oil and gas, in a fracture is often assumed to occur in a stratified behavior. However, recent studies and production practices have revealed that two-phase flow in fractures exhibits diverse flow patterns. This paper investigates the control of the fracture aperture, fluids viscosity, and wettability on two-phase flow in a 2D cross section of a 3D Berea fracture. Lattice Boltzmann Method (LBM) simulations are used to model the impact of these properties on relative permeability curves. Notably, in strongly wet fractures, two distinct permeability regimes emerge. High aperture values exhibit behavior resembling parallel planes, while low aperture values lead to a linear decrease in permeability due to fluid interactions between fracture surfaces. Conversely, anomalous behavior of the relative permeability curves is identified in weakly wet fractures within specific aperture ranges. This behavior is associated with the occurrence of specific flow patterns within the fracture. Results also emphasize that changes in viscosity ratio do not affect the presence or the saturation range of the anomalous behavior but do influence its intensity for each fluid. Comparisons with Poiseuille profile equations reveal the limited impact of the fracture roughness. These findings enhance our understanding of the interactions between aperture, viscosity, and wettability and how they control the shape of the relative permeability curves. These curves are pivotal parameters for the continuum scale modeling (reservoir models) in oil and gas application, for instance. • Two distinct permeability (k r e l) regimes emerge in strongly wet fractures. • High apertures act like parallel planes; low apertures cause linear k r e l decrease. • Anomalous k r e l curves are linked to specific flow patterns in the fractures. • The viscosity ratio affects the intensity but not the presence of the anomaly. [ABSTRACT FROM AUTHOR]

Summary In this paper we solve the Navier Stokes’ equation using finite difference method, on a simulated porous rock structure in 2-D, to study the velocity distribution of fluid flowing through it under a constant pressure gradient. A reactive solute carried through the fluid is allowed to interact with the minerals in the rock. Depending on the rock composition, both dissolution and precipitation reactions may occur. However precipitation occurs only through the cations that are released in the solution due to dissolution. These combined dissolution–precipitation reactions change the porosity, permeability and pore geometry of the sedimentary rock. We study the temporal changes of these properties as functions of Peclet number, concentration of the reactive solute and ratio of Damkholer numbers of dissolution to precipitation. The final flow property is decided by a combination of these parameters. [ABSTRACT FROM AUTHOR]

Abstract: A set of four reactive flow-through experiments at temperature T =100 °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 ≈ 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 ϕ(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 Damköhler number, Da). Furthermore, all dissolution experiments display power scaling between permeability (k) and porosity (ϕ) 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–ϕ function for modeling the earliest dissolution processes occurring in the vicinity of the reaction front. [Copyright &y& Elsevier]

Abstract 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 °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 (aMg2+/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 SiO 2 (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. Highlights • Flow defines length and time scale of fluid-rock reactive domains • Interplay between transport and reaction kinetics controls local chemical fluxes • Local chemical fluxes control fluid composition thus serpentinization reaction paths • Kinetics- and transport-controlled reaction domains can coexist in heterogenous rocks • Serpentine precipitation buffers pH and cationic activity in outlet fluids [ABSTRACT FROM AUTHOR]

Many countries are planning to host long-lived high- and intermediate-level radioactive waste in disposal facilities sited in deep argillaceous formations. Bentonite, as a swelling material, is often chosen as engineered buffers due to its low permeability and high swelling capacity. However, its properties can be influenced by the hydraulic/gas conditions prevailing in-situ that are expected to change throughout the duration of the repository. The objective of this work is to estimate the effect of partially water-saturated conditions on the mobility of water and ions in compacted MX-80 bentonite in relation to the evolution of its swelling pressure. For that purpose, (i) diffusion of water and ions (I− and Na+) and (ii) the swelling pressure of compacted bentonite samples partially water-saturated were studied by means of the osmotic technique that allows controlling the suction from 9 to 0 MPa. Results showed that (i) water diffusivity remained unchanged independently of the degrees of saturation (70, 84 and 100%) and (ii) iodide diffusivity decreased by a factor of 12 (from 100 to 70%), while (iii) Na+ diffusion was reduced by a factor of more than 70 (from 100 to 70%), associated to a drop of the distribution factor by a factor of 16. Comparison of these results with diffusion data already acquired in clayey materials whose porous structure evolves little with suction (i. e. compacted kaolinite and intact Collovo-oxfordian clay-rocks) underlined how the bentonite porous structure whose organization evolved with the degree of saturation is a key parameter. This statement is supported by mechanical tests, which indicated a distinct evolution of the swelling pressure curves depending on the degree of saturation. At 70% saturation, mechanical tests indicate (i) a preservation of the large inter-aggregate porosity where water can diffuse in both vapor and liquid phases, and (ii) a final swelling pressure 1.5 to 2 times lower than at full water saturation, meaning that a part of swelling clay particles was not accessible to water ingress and so to 22Na+. Conversely, at 84 % and 100 % saturation, change in swelling pressure curves was explained by a collapse of bentonite inter-aggregate porosity, in favor of expanding the inter-particle and interlayer porosities where 22Na + diffusion is enhanced by surface diffusion phenomenon. • Diffusive rate of water tracer is similar in bentonite for water saturation from 1 to 0.7 • Strong reduction of diffusive rate and adsorption of sodium by 73 and 16, resp. in bentonite for water saturation from 1 to 0.7 • The tortuosity of iodide diffusive paths in bentonite increases by only 1.4 times for water saturation from 1 to 0.7 • Diffusive behavior is linked to the bentonite saturation-dependent microstructure changes • Mechanical data are in agreement with the diffusion findings [ABSTRACT FROM AUTHOR]

Experiments were conducted to assess the potential impact of fractured well-cement degradation on leakage rate. Permeability was monitored while CO2-enriched reservoir-equilibrated brine was flowed at constant rate through a single fracture in a class G cement core under conditions mimicking geologic sequestration environments (temperature 60 °C, pressure 10 MPa). The results demonstrate that, at least for the conditions used in the experiment, an initial leakage in a 42 μm aperture fracture (permeability = 1.5 × 10-10 m²) can be self-mitigated due to the decrease of the fracture hydraulic aperture after about 15 h. This decrease results from the development of continuous highly hydrated amorphous Si-rich alteration products at the edge of the fracture and the dense carbonation of the bulk cement that mitigate the penetration of the alteration front. [ABSTRACT FROM AUTHOR]

Abstract: Several studies, both theoretical and experimental, show that sedimentary rocks have fractal pore–grain interface. The real rocks are 3-d structures with highly tortuous and often fractal pore spaces. Before attempting simulation of this daunting geometry, we present here, as a preliminary study, a simpler 2-d version. In this paper, a computer simulated 2-d sedimentary rock structure is generated by the relaxed bidisperse ballistic deposition model. Grains of two different sizes are dropped ballistically on a linear substrate. By changing the fraction of the two types of particles, the porosity of the rock structure can be tuned. The structure undergoes compaction through the relaxation of possible unstable overhangs. The micro structure of the pore space is investigated. The pore mass and the rock–pore interface show a fractal behaviour with the same fractal dimension indicating that the pore volume is a fractal. Our simulation results indicate that the process of compaction of grains during the deposition process seems to erase the dependency of the fractal dimension on the grain size distribution. The two point density correlation is measured for the pore space. It shows anisotropy which is an outcome of the growth rule. X-ray tomography of two-dimensional sections of real sedimentary rocks obtained from Mallorca Island is subjected to the same study and the results compared with those obtained from simulation. The simulation results agree qualitatively with the real rock sample. We also study diffusion on the pore space. Diffusion is found to be anomalous as is expected in fractal spaces. It also bears the signature of anisotropy of the structure. Diffusion studies on the real rock sample could not yield conclusive results as the system size is not large enough. [Copyright &y& Elsevier]

Carbonation of ultramafic rocks in geological reservoirs is, in theory, the most efficient way 10 trap CO2 irreversibly; however, possible feedback effects between carbonation reactions and changes in the reservoir permeability must be considered to realistically assess the efficiency and sustainability of this process. We investigated changes in the hydrodynamic properties of sintered dunite samples by means of percolation experiments, under conditions analogous to that of in situ carbonation. Our results show that carbonation efficiency is controlled by the local renewal of the reactants and the heterogeneity of the pore structure. Preferential flow zones are characterized by the formation of magnetite and of a silica-rich layer at the olivine surfaces, which eventually inhibits olivine dissolution. Conversely, sustainable olivine dissolution together with coprecipitation of magnesite, siderite, and minor Mg-TOT- phyllosilicates, occur in reduced-flow zones. Thus carbonate precipitation only decreases porosity in zones where diffusion-controlled transport is dominant. Consequently, while high flow rates will decrease the carbonation efficiency of the reservoir and low flow rates may reduce the permeability irreversibly close to the injection point, moderate injection rates will ensure a partial carbonation of the rock and maintain the reservoir permeability. [ABSTRACT FROM AUTHOR]

Abstract: Among the radionuclides that can migrate from radioactive wastes buried within consolidated argillaceous rocks, 129I poses a particular problem due to its high mobility in geomedia and its very long half-life of 1.6 107 years. Previous studies carried out on such rocks showed the existence of a low iodide retardation regarding the other anion species. The purpose of the present study was two-fold. It was hoped (1) to confirm this retardation by means of radial diffusion experiments and (2) to estimate the influence of the initial iodide concentrations towards the iodide uptake. The results obtained from in-diffusion stage show that (1) no iodide uptake occurs at high initial concentrations and (2) an uptake at low initial concentrations cannot be clearly evidenced. Finally, the most singular behaviour of iodide was highlighted for all the initial iodide concentrations, from the out-diffusion stage, during which some amount of iodide, available for out-diffusion, were not involved in the diffusion process. Although the origin of such phenomenon remains unclear, a kinetic effect for the iodide uptake was proposed, assuming that the period of 55days (corresponding to the in-diffusion stage duration) would be necessary for inducing the iodide uptake. This kinetic effect, already shown elsewhere by (Descostes, M., Blin, V., Bazer-Bachi, F., Meier, P., Grenut, B., Radwan, J., Schlegel, M.L., Buschaert, S., Coelho, D, Tevissen, E. Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones. In press for Appl. Geochem.) on similar materials, needs to be deepened. [Copyright &y& Elsevier]

Abstract Reactions caused by the diffusion of reactants from different sources can alter rock diffusivity and are therefore the critical mechanisms for evaluating short and long-term behavior of low-permeability rocks used as confinement layers for underground storage, for instance. This paper presents and discusses a set of two diffusion-driven reaction experiments focusing on precipitation of two end-member types of sulfate minerals (gypsum: CaSO 4. 2 H 2 O and barite: BaSO 4) in low-permeability chalk. The time-resolved changes in porosity and effective diffusion coefficient (D e) were evaluated along the duration of the experiments (~140 days), by analyzing the behavior of passive tracers and evaluating the amount of precipitated gypsum or barite from measuring the reactant concentration evolution in the reservoirs at both ends of the sample. Then SEM-EDS and X-ray microtomography (μCT) imaging were used to characterize the initial rock structure and the precipitated materials. Results showed that the change in porosity (from 45% to about 43%) corresponding to the volume of sulfate precipitated, are similar for gypsum and barite. Conversely, the precipitation impact on diffusion properties of the water tracers that were injected 70 days after the beginning of the precipitation step is distinctly different for the each of the studied sulfate mineral. The precipitation of barite generated a more significant impact than gypsum: D e intact = 4.15 × 10−10 m2·s−1 vs. D e barite = 1.1 × 10−10 m2·s−1 and D e gypsum = 2.5 × 10−10 m2·s−1. Post-mortem imaging revealed a thin precipitated zone (~250 μm) in the center of the sample for the barite precipitation experiment, whereas isolated quasi-spherical clusters resulted from the gypsum precipitation. The μCT images at higher resolution showed that the precipitation of barite is heterogeneous at small scale, which explains the HTO diffusion curve. For gypsum, the post mortem imaging around the quasi-spherical clusters showed a significant presence of initial macropores of the connected porosity that were still unfilled. These intact chalk zones allowed HDO to diffuse through the precipitated zone lowering the impact on water tracer diffusivity. These experimental results indicate that the morphology and the distribution of barite precipitates is mainly controlled by homogeneous and heterogeneous nucleation phenomena, whereas gypsum precipitation is mainly controlled by the spatial variability of the initial porous system properties (reactive surface area, tortuosity, pore network structure). Highlights • Diffusion-driven clogging experiments of gypsum and barite were carried out in chalk. • At similar amount of precipitates in chalk, contrasting impact on water tracer diffusivity was observed. • Spatial variability in chalk and intrinsic properties of precipitating mineral controlled the evolution of reacted zone. [ABSTRACT FROM AUTHOR]

Abstract The Wadi Dima area (Oman Ophiolite) exposes partially altered to highly serpentinized harzburgites that are cross-cut by intense (>20 Vol%) carbonate veining. We identified a sequence of 3 types of carbonate veins with compositions ranging from calcite to dolomite (Mg/Ca = 0-0.85). Type 1 carbonates occur as a fine diffuse vein network, locally replacing olivine cores, penetrative into the serpentinized harzburgites. They have depleted trace elements abundances (e.g., Yb < 0.2 × C1-chondrite) relative to other Wadi Dima carbonates, exhibit negative Ce and positive Y, U anomalies and a broad range in δ13C V-PDB (-5 to -15‰) and δ18O SMOW (18 to 31‰). These compositions are consistent with precipitation after seawater-derived fluids and/or fluids in equilibrium with mantle rocks and serpentines during cooling of oceanic lithosphere (110 to 15°C). Type 2 carbonates are localized in veins, which acted as main flow paths for fluids interacting with peridotites in the exhumed Oman mantle lithosphere (50°C-10°C). The orientation of these veins is controlled by the crystallographic anisotropy of Oman mantle peridotites. Type 2 carbonates record two stages. The first involved the formation of large calcite crystals of composition similar to Type 1 carbonates (trace element depleted; δ13C V-PDB B = -4 to -9‰ and δ18O SMOW = 26 to 30‰), which during the second stage were recrystallized to form dolomite and calcite microcrystals (trace element enriched; δ13C V-PDB = -7 to -13‰ and δ18O SMOW = 29 to 32‰), after fluids sampling different sources including contributions of sediment-derived components. They were most likely formed at shallow depths and record the transition from oceanic to continental settings during late Cretaceous ophiolite obduction. Type 3 veins reactivate Type 2 veins. They comprise dominantly calcite and dolomite microcrystals (Light REE enriched patterns) with isotopic compositions (δ13C V-PDB B ~ -7 to -8‰; δ18O SMOW ~ 28 to 32‰) consistent with precipitation at low temperatures (T°<30°C) from surface/meteoric fluids. Type 3 veining is probably triggered by ophiolite uplift during the Oligocene to early Miocene. Our study presents new insights into the role of the initial mantle anisotropy in the orientation of the vein network and of principal flow paths during serpentinization and carbonatization of mantle peridotites. It also highlights the highly variable carbon isotope composition of carbonates and suggest different origins for these heterogeneities: the carbon isotope composition of the early Type 1 carbonates dispersed in the poorly connected peridotites is locally modified by serpentinization reactions whilst the carbon isotope compositions of Type 2 and 3 carbonates record mixing of fluids from different sources in high flow veins. Highlights • Successive episodes of carbonatization from mid-ocean ridge to ophiolite. • Onset of carbon trapping is related to low-temperature oceanic hydrothermalism. • Inherited mantle fabric governs the field scale orientation of carbonate veins. • Ca- then Mg-rich carbonate-bearing fluids use the same flow paths. • Carbonates precipitate from pervasive to focused fluid flow paths. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of flow through two millimeter-scale rock samples of limestone and sandstone are performed using three diverse fluid dynamic simulators. The resulting steady-state velocity fields are compared in terms of the associated empirical probability density functions (PDFs) and key statistics of the velocity fields. The pore space geometry of each sample is imaged at 5.06− μm voxel size resolution using X-ray microtomography. The samples offer contrasting characteristics in terms of total connected porosity (about 0.31 for the limestone and 0.07 for the sandstone) and are typical of several applications in hydrogeology and petroleum engineering. The three-dimensional fluid velocity fields within the explicit pore spaces are simulated using ANSYS® FLUENT® ANSYS Inc. (), EULAG Prusa et al. (Comput. Fluids 37, 1193-1207 ), and SSTOKES Sarkar et al. (). These computational approaches are highly disperse in terms of algorithmic complexity, differ in terms of their governing equations, the adopted numerical methodologies, the enforcement of internal no-slip boundary conditions at the fluid-solid interface, and the computational mesh structure. As metrics of comparison to probe in a statistical sense the internal similarities/differences across sample populations of velocities obtained through the computational systems, we consider (i) integral quantities, such as the Darcy flux and (ii) main statistical moments of local velocity distributions including local correlations between velocity fields. Comparison of simulation results indicates that mutually consistent estimates of the state of flow are obtained in the analyzed samples of natural pore spaces despite the considerable differences associated with the three computational approaches. We note that in the higher porosity limestone sample, the structures of the velocity fields obtained using ANSYS FLUENT and EULAG are more alike than either compared against the results obtained using SSTOKES. In the low-porosity sample, the structures of the velocity fields obtained by EULAG and SSTOKES are more similar than either is to the fields obtained using ANSYS FLUENT. With respect to macroscopic quantities, ANSYS FLUENT and SSTOKES provide similar results in terms of the average vertical velocity for both of the complex microscale geometries considered, while EULAG tends to render the largest velocity values. The influence of the pore space structure on fluid velocity field characteristics is also discussed. [ABSTRACT FROM AUTHOR]

Diffusion is the main transport process of water and solutes in clay-rich porous media owing to their very low permeability, so they are widely used as barriers against contaminant spreading. However, the prediction of contaminant mobility can be very complicated when these media are partially water-saturated. We conducted diffusion experiments for water (HTO and HDO) and ions (22Na+ and 125I−) through partially water saturated compacted kaolinite, a weakly charged clay material, to quantify the distinct diffusive behavior of these species. The osmosis method was used to set kaolinite samples at 67, 86 and 100% saturation. The results showed that desaturation led to a sharp decrease in diffusive rates by factors of 6.5, 18 and 35 for HTO, 125I− and 22Na+, respectively, from 100 to 67% of the degree of saturation. Thus, to interpret water diffusivities, we proposed a model taking into account the diffusion of water in both gas and liquid phases, using diffusion data obtained for ions, considered as inert species. This model was capable of properly predicting water diffusive flux, especially at a low degree of saturation (67% saturation), for which the assumption made for the occurrence of air phase continuity throughout the sample appears to be more relevant than at 86% saturation. • Decrease of water and ion diffusion in partially-water saturated kaolinite. • Significant discrepancy of water and ion diffusivity in unsaturated conditions. • No significant increase of surface diffusion when dehydrating. • A double diffusion model was developed for predicting water diffusion data. [ABSTRACT FROM AUTHOR]

Highlights: [•] We monitor a saltwater intrusion at long-term and with combined methods. [•] The wide mixing zone is not at thermodynamical equilibrium despite very slow flow. [•] Calcite undersaturation and dissolution features are observed within the mixing zone. [•] The data highlight a pH acidification correlated with microbiological activity. [•] Bio-activated mass transfers are likely to be localized and increase heterogeneity. [Copyright &y& Elsevier]

Abstract: Diffusion parameters for HTO, 36Cl− , and 125I− were determined on Upper Toarcian argillite samples from the Tournemire Underground Research Laboratory (Aveyron, France) using the through diffusion technique. The direction of diffusion was parallel to the bedding plane. The purpose of the present study was 3-fold; it was intended (i) to confirm the I− interaction with Upper Toarcian argillite and to verify the effects of initial I− concentration on this affinity, as previously observed by means of radial diffusion experiments, (ii) to highlight any discrepancy between Cl− and I− diffusivity, and (iii) to investigate the effect of an increase of the ionic strength of the solution on the anionic tracers’ diffusive behaviour. The results show that the effective diffusion coefficient (De ) and diffusion accessible porosity (εa ) values obtained with an ionic strength (I.S.) synthetic pore water of 0.01eqL−1 are: De =2.35–2.50×10−11 m2 s−1 and εa =12.0–15.0% for HTO, and De =14.5–15.5×10−13 m2 s−1 and εa =2.5–2.9% for 36Cl−. Because of anionic exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than HTO, taken as a water tracer. The associated effective diffusion coefficient (De ) and rock capacity factor (α) obtained for 125I are: De =7.00–8.60×10−13 m2 s−1 and α =4.3–7.2%. Such values make it possible to calculate low 125I distribution ratios (0.0057< RD <0.0192mLg−1) which confirm the trend indicating that the 125I rock capacity factor increases with the decrease of the initial I− concentration. Additional through-diffusion experiments were carried out with a higher ionic strength synthetic pore water (I.S.=0.11eqL−1). No evolution of HTO diffusion parameters was observed. The anionic tracers’ effective diffusion coefficient increased by a factor of two but no clear evolution of their accessible porosity was observed. Such a paradox could be related to the particularly small mean pore size of the Upper Toarcian argillite of Tournemire. The most significant finding of this study is the large discrepancy (factor of two) between the values of the effective diffusion coefficient for 125I and 36Cl. Whatever the ionic strength of the synthetic solution used, 125I exhibited De values two times lower than those of 36Cl. A detailed explanation for this difference cannot be given at present even if a hypothesis based on ion-pairing or on steric-exclusion cannot be excluded. This makes questionable the assumption usually made for quantifying 125I sorption and postulating that 36Cl and 125I would diffuse in the same porosity. In other terms, at Tournemire, 125I sorption could be more pronounced than previously indicated. [Copyright &y& Elsevier]

Abstract: GenMolTM code is used to simulate Atomic Force Microscopy (AFM) tests at a kaolinite/brine/kaolinite contact, the confined fluid in sub-nanometre interspaces being in equilibrium with an external multi-species solution. The attraction/repulsion effort, i.e. the derivative versus the interspace aperture h of the interaction energy between both kaolinite faces, is computed versus the variable composition of the confined fluid (see for the method Part I of this work). Two external solutions are tested. Solution S1 is a neutral brine (pH=7.5) leading to a possible attraction for apertures lower than 7Å. Solution S2 is an acidified brine (pH=3.2) leading to repulsion whatever may be the aperture h. These two AFM simulations prove the existence of a critical pH value (3.2

Abstract: Predicting the impact of underground engineering on the environment requires the knowledge of natural media at different scales. In particular, understanding basic phenomena controlling the properties of rocks in the presence of complex fluids necessitates a detailed atomic description of the solid/fluid/solid contacts, the subject of Part I of the present study. First, building the solid interspace between two different crystals in a non-periodic situation is achieved using the ab initio and molecular mechanics code GenMolTM. A description of the fluid confined within the interspace is then derived from the original genetic iterative multi-species (GIMS) algorithm implemented in the same code. This approach consists of equilibrating chemical potentials, cycle after cycle and species after species, between the confined fluid and the free natural fluid. An elementary iteration for a species k consists of different steps incrementing the number N k of particles k, with other numbers N k'' remaining constant. At each step, an optimum fluid composition is obtained by a genetic process distributing the fluid particles on a grid by stochastic shots, followed in fine by a refining process. The effectiveness of the GIMS approach is demonstrated in the case study of a fluid confined between two (001) kaolinite faces, with apertures h varying between 4 and 10Å, connected to a 9-species external solution [H2O, Cl−, Na+, CO2(aq), NaCl(aq), Ca2+, Mg2+, , H3O+] where concentrations are ranging from 55 to 10−4 mol/L. The results show a drastic variation in the solute/solvent and cations/ions ratios in the confined fluid when aperture h is lowered to less than 1nm. These results obtained with a very rapid convergence of the iterative algorithm combined with a very competitive genetic optimizer are validated with high precision on a free solution. This description of contacts between crystals is original and unattainable by standard crystal interface approaches. It opens the way to understanding and/or predicting phenomena between crystals in a complex natural environment, such as adhesion or repulsion investigated in the Part II of this study. [Copyright &y& Elsevier]

Summary: One of the main difficulties encountered when characterizing the hydrodynamic properties of a fractured aquifer is to identify the preferential flow paths within it. Different methods may be applied to determine the variability of the permeability at the borehole scale and to image the structure of the main flow zones between boreholes. In this paper, we compare the information obtained from different measurement techniques performed in a set of three 100m depth wells (well-to-well spacing: 5–10m) in a fractured crystalline rock setting. Geophysical logging and borehole-wall imaging are used to identify open and closed fractures intersecting the boreholes and their orientation. The comparison with flowmeter and single packer tests shows that few of the fractures interpreted as open from geophysical logs are significantly transmissive. Cross-borehole connectivity is first investigated from single packer tests with pressure monitoring in adjacent boreholes. To determine fracture zone connectivity, we propose a methodology simply based on the variation with packer depth of the ratio of the drawdown in the observation well and the drawdown in the pumping well. The results are compared to the analysis of cross-borehole flowmeter tests. We show that both methods provide consistent results with a similar level of information on connectivity. [Copyright &y& Elsevier]